No title

ADVANCES IN IMAGING AND ELECTRON PHYSICS SIR CHARLES OATLEY AND THE SCANNING ELECTRON MICROSCOPE

A tribute published to coincide with the centenary of the birth of Charles William Oatley O. B. E., F. R. S. 14 February 1904 – 11 March 1996. VOLUME 133

EDITOR-IN-CHIEF

PETER W. HAWKES CEMES-CNRS Toulouse, France

Advances in

Imaging and Electron Physics Sir Charles Oatley and the Scanning Electron Microscope Edited by

BERNARD C. BRETON Engineering Department University of Cambridge, England

DENNIS McMULLAN Cavendish Laboratory University of Cambridge, England

KENNETH C.A. SMITH Engineering Department University of Cambridge, England

VOLUME 133

Elsevier Academic Press 525 B Street, Suite 1900, San Diego, California 92101-4495, USA 84 Theobald’s Road, London WC1X 8RR, UK

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CONTENTS

Contributors . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . Foreword . . . . . . . . . . . . . . Congress and Other Abbreviations Acknowledgments . . . . . . . . . Future Contributions . . . . . . .

PART I 1.1

1.2

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. xi . xv . xvii . xix . xxi . xxiii

INTRODUCTION

Charles Oatley: Father of Modern Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . K. C. A. Smith and D. McMullan The Early History of the Scanning Electron Microscope . . . C. W. Oatley

PART II

3 7

THE SCANNING ELECTRON MICROSCOPE AT THE CAMBRIDGE UNIVERSITY ENGINEERING DEPARTMENT

2.1A The Development of the First Cambridge Scanning Electron Microscope, 1948–1953 . . . . . . . . . . . D. McMullan 2.1B An Improved Scanning Electron Microscope for Opaque Specimens . . . . . . . . . . . . . . . . . . D. McMullan 2.2A Exploring the Potential of the Scanning Electron Microscope . . . . . . . . . . . . . . . . . K. C. A. Smith 2.2B The Scanning Electron Microscope and its Fields of Application . . . . . . . . . . . . . . . . . . . . . . K. C. A. Smith and C. W. Oatley

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vi 2.3

CONTENTS

Building a Scanning Electron Microscope . . . . . . . . . O. C. Wells 2.4A Contrast Formation in the Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . T. E. Everhart 2.4B Wide-Band Detector for Micro-microampere Low-Energy Electron Currents . . . . . . . . . . . . . . . . . . . . . T. E. Everhart and R. F. M. Thornley 2.5 A Simple Scanning Electron Microscope . . . . . . . . . P. J. Spreadbury 2.6 New Applications of the Scanning Electron Microscope . R. F. M. Thornley 2.7A A. D. G. Stewart and an Early Biological Application of the Scanning Electron Microscope . . . . . . . . . . . A. Boyde 2.7B Investigation of the Topography of Ion Bombarded Surfaces with a Scanning Electron Microscope . . . . . . A. D. G. Stewart 2.8 The Scanning Electron Microscopy of Hot and Electron-Emitting Specimens . . . . . . . . . . . . . . . . H. Ahmed 2.9A Towards Higher-Resolution Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . R. F. W. Pease 2.9B High Resolution Scanning Electron Microscopy . . . . . . R. F. W. Pease and W. C. Nixon 2.10 The Application of the Scanning Electron Microscope to Microfabrication and Nanofabrication . . . . . . . . . . A. N. Broers 2.11 Scanning Electron Diffraction: A Survey of the Work of C. W. B. Grigson . . . . . . . . . . . . . . . . . D. McMullan

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CONTENTS

PART III THE DEVELOPMENT OF ELECTRON PROBE INSTRUMENTS AT THE CAVENDISH LABORATORY AND THE TUBE INVESTMENTS RESEARCH LABORATORY 3.1

The Development of the X-ray Projection Microscope and the X-ray Microprobe Analyser at the Cavendish Laboratory, Cambridge, 1946–60 . . . . . . . . . . . . V. E. Cosslett 3.2A The Contributions of W. C. Nixon and J. V. P. Long to X-ray Microscopy and Microanalysis: Introduction . . . P. Duncumb 3.2B X-Ray Projection Microscopy . . . . . . . . . . . . . . W. C. Nixon 3.2C Microanalysis . . . . . . . . . . . . . . . . . . . . . . . J. V. P. Long 3.3A Development of the Scanning Electron Probe Microanalyser, 1953–1965 . . . . . . . . . . . . . . . . P. Duncumb 3.3B Micro-Analysis by a Flying-Spot X-Ray Method . . . . V. E. Cosslett and P. Duncumb 3.4 Tube Investments Research Laboratories and the Scanning Electron Probe Microanalyser . . . . . . . . . D. A. Melford

PART IV

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COMMERCIAL DEVELOPMENT

4.1A Commercial Exploitation of Research Initiated by Sir Charles Oatley . . . . . . . . . . . . . . . . . . . . . . . 311 K. C. A. Smith 4.1B AEI Electron Microscopes—Background to the Development of a Commercial Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . . . 317 A. W. Agar 4.2A Microscan to Stereoscan at the Cambridge Instrument Company . . . . . . . . . . . . . . . . . . . . . . 321 M. A. Snelling

viii 4.2B 4.3

4.4 4.5

CONTENTS

A New Scanning Electron Microscope . . . . . . . . . . A. D. G. Stewart and M. A. Snelling Memories of the Scanning Electron Microscope at the Cambridge Instrument Company . . . . . . . . . . D. J. Unwin From Microscopy to Lithography . . . . . . . . . . . . B. A. Wallman Commercial Electron Beam Lithography in Cambridge, 1973–1999: A View from the Drawing Board . . . . . . J. M. Sturrock

PART V 5.1 5.2

5.3 5.4

5.5 5.6

5.7

5.8

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EPILOGUE

Charles Oatley: The Later Years . . . . . . . . . . . . The Editors The Detective Quantum Efficiency of the Scintillator/ Photomultiplier in the Scanning Electron Microscope . C. W. Oatley Professor Oatley Remembered . . . . . . . . . . . . . E. Munro Recollections of Professor Oatley’s Reincarnation as a Research Student . . . . . . . . . . . . . . . . . . . . G. Owen My Life with the Stereoscan . . . . . . . . . . . . . . B. C. Breton Research at the Cambridge University Engineering Department Post-Stereoscan . . . . . . . . . . . . . . K. C. A. Smith The Development of Biological Scanning Electron Microscopy and X-ray Microanalysis . . . . . . . . . P. Echlin From the Scanning Electron Microscope to Nanolithography . . . . . . . . . . . . . . . . . . J. R. A. Cleaver

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APPENDICES Appendix I

Sir Charles William Oatley, O. B. E., F. R. S. (Royal Society Biographical Memoir) . . . . . . . . . . . . . . . . . . 503 K. C. A. Smith

CONTENTS

Appendix II

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A History of the Scanning Electron Microscope, 1928–1965 . . . . . . . . . . . . . . . . . . . . . . . . 523 D. McMullan Appendix III The Cambridge Instrument Company and Electron-Optical Innovation . . . . . . . . . . . . . . 547 P. Jervis Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557

CUED WEBSITE: SUPPLEMENTARY MATERIAL (www.eng.cam.ac.uk/to/oatley) Research at the Cambridge University Engineering Department Post-Stereoscan (Chapter 5.6 in full) K. C. A. Smith, B. C. Breton and N. H. M. Caldwell Bibliography of Scanning Electron Microscopy, CUED, 1951–2004 K. C. A. Smith, B. C. Breton, N. H. M. Caldwell and D. McMullan A Brief History of the Cambridge Instrument Co. D. J. Unwin V. E. Cosslett, F.R.S (The Oxford Dictionary of National Biography) D. McMullan

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CONTRIBUTORS

Numbers in parentheses indicate the pages on which the authors’ contributions begin.

Alan W. Agar (317), Hinds Cottage, Sproxton, Helmsley, York YO62 5EF, Email: [email protected] Haroon Ahmed (179), Corpus Christi College, Trumpington Street, Cambridge CB2 1RH, Email: [email protected] Alan Boyde (165), Hard Tissue Research Unit, Biophysics, Centre for Oral Growth and Development, St Barts & The London School of Medicine & Dentistry, Queen Mary, University of London, Turner St., Whitechapel, London E1 2AD, Email: [email protected] B. C. Breton (449), 47 Church Street, Great Shelford, Cambridge, Email: [email protected] Lord Alec Broers (207), Flat 429, 10 St George Wharf, London SW8 2LZ Email: [email protected] John R. A. Cleaver (485), Fitzwilliam College, Cambridge CB3 0DG, Email: [email protected] V. Ellis Cosslett (237), (Deceased) Peter Duncumb (251, 269), 5a Woollards Lane, Great Shelford, Cambridge CB2 5LZ, Email: [email protected] Patrick Echlin (469), 65 Milton Road, Cambridge CB4 1AX, Email: [email protected] Thomas E. Everhart (137, 147), California Institute of Technology, Pasadena, CA 91125, Email: [email protected] Paul Jervis (547), Cylla’s Rill, Stowhill, Childrey, Wantage, Oxon OX12 9XQ, Email: [email protected]

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CONTRIBUTORS

James V. P. Long (259), (Deceased) Dennis McMullan (3, 37, 59, 221, 523), Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, Email: [email protected] David A. Melford (289), Ryders, Strethall, Saffron Walden, Essex. CB11 4XJ, Email: [email protected] Thomas Mulvey, 3 Earlwood Drive, Sutton Coldfield, West Midlands B74 2NG, Email: [email protected] Eric Munro (437), Munro’s Electron Beam Software Ltd., 14 Cornwall Gardens, London SW7 4AN, Email: [email protected] William C. Nixon (195, 253), Peterhouse, Cambridge CB2 1RD Sir Charles W. Oatley (7, 111, 415, 419), (Deceased) Geraint Owen (445), 3265 Greer Rd, Palo Alto, CA 94303, Email: [email protected] R. Fabian W. Pease (187, 195), CISX 314, Stanford University, Stanford, CA 94305-4075, Email: [email protected] Kenneth C. A. Smith (3, 93, 111, 311, 467, 499), Fitzwilliam College, Cambridge CB3 0DG, Email: [email protected] M. A. Snelling (321, 335), 44 Broad Lane, Haslingfield, Cambridge CB3 7JF, Email: [email protected] P. J. Spreadbury (153), Emmanuel College, Cambridge CB2 3AP, Email: [email protected] J. M. Sturrock (387), Nether Rumgally, Kemback, by Cupar, Fife, KY15 5SY, Email: [email protected] R. F. M. Thornley (147, 155), 4150 Eutaw Drive, Boulder, CO 80303-3625, U.S.A, Email: [email protected]

CONTRIBUTORS

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D. J. Unwin (339), 2 Redfern Close, Cambridge CB4 2DU, Email: [email protected] B. A. Wallman (359), 190 Cambridge Road, Great Shelford, Cambridge CB2 5JU, Email: [email protected] Oliver C. Wells (127), IBM Research Division, P.O. Box 218, Yorktown Heights, NY 10598, Email: [email protected]

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PREFACE

An occasional feature of these Advances is historical material, of which the present volume is a fresh example. In the past, biographical articles about Ernst Ruska, Bodo von Borries and Jan le Poole have appeared, as well as two entire volumes: The Beginnings of Electron Microscopy (Supplement 16) and Growth of Electron Microscopy (vol. 96). Here, the beginnings of the scanning electron microscope are traced in more detail than has been attempted before and its subsequent penetration into many areas is described. The whole volume is centred on Sir Charles Oatley and is timed to coincide with the centenary year of his birth. Although Oatley was not the first person to champion the scanning principle, it was his enthusiasm and persistence that overcame the widespread indifference to the idea of such an instrument in the post-war years and led to the first commercial exploitation. Two of the guest-editors, D. McMullan and K. C. A. Smith are pioneers of the instrument; with B.C. Breton, who worked on the SEM alongside Sir Charles for many years, they have succeeded in gathering contributions from the majority of Oatley’s SEM research students as well as other workers, both in university and industry. Much of the new material is complemented by early articles by the same authors, reprinted entire or, in a few cases, confined to excerpts. I am very pleased that the guest editors agreed to publish their centenary celebration collection in these Advances and am confident that it will awake many memories among older readers from the SEM world and attract considerable interest among younger generations. Peter Hawkes

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FOREWORD

The present volume originated with a suggestion from Bernie Breton that the research students working on the scanning electron microscope in the Cambridge University Engineering Department during the 1950s under the supervision of Charles Oatley should each write a personal account of their experiences. From subsequent discussions it became clear that the ramifications of this early work made a much broader approach desirable. In particular, the experiences of those engaged in the parallel development of the scanning X-ray microanalyser at the Cavendish Laboratory, and the subsequent commercial exploitation of both instruments by the Cambridge Instrument Company, needed to be recorded if the story was to be complete. As a consequence, the work has expanded considerably from its original concept: it now embraces accounts of the early instrumental research and development undertaken in the University, at the Tube Investments Research Laboratory and at the Cambridge Instrument Company. It also covers many of the subsequent developments that have emerged over the past half-century at the Engineering Department and at the Cambridge Instrument Company. These accounts are accompanied in some cases by reproductions of papers published at the time or excerpts from such papers. Supplementary material, particularly concerning later developments, are to be found on the Cambridge University Engineering Department website (www.eng.cam.ac.uk/to/oatley), which has been established to coincide with the publication of this volume. With regret the Editors have to record that some of those who had intended to contribute have been prevented from doing so by ill health or death. Bill Nixon suffered a stroke with consequent impairment of faculties, and Jim Long died in February 2003 [Obituary: Mineral. Mag. 67, 592–3 (2003), by Stephen Reed]. In their place, Peter Duncumb introduces papers published by them describing their early work in the Cavendish Laboratory on, respectively, X-ray projection microscopy and X-ray microanalysis. Nixon’s later work on the SEM is described in several supporting papers by his colleagues. The pioneering work on scanning electron diffraction undertaken by Chris Grigson, who died in February 2001 [Obituary: The Independent, 25 April 2001, by Sir Douglas Faulkner], is surveyed by Dennis McMullan. Gary Stewart has been unable to contribute, but his work on ion beam etching in the SEM and later work on the Cambridge Stereoscan is represented by contributions from Alan Boyde, Mike Snelling and others. xvii

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FOREWORD

In approaching potential contributors to this volume it was stressed that the Editors were not looking merely for a repetition of technical work already published but rather for personal accounts of how contributors came to be involved in the subject, the difficulties they encountered, the people they worked with, the failures as well as the successes. Those who worked with Charles Oatley or who had occasion to meet him, were asked to provide any interesting and relevant reminiscences. Thanks are due to everyone who took up this wide-ranging and challenging brief, which has inevitably produced a great variety of responses. It is hoped that readers will agree that, as a result, a broad picture has emerged of the beginnings of modern scanning electron microscopy and of the man who initiated it. Finally, the Guest Editors wish to thank Peter Hawkes for his help and encouragement in the preparation of this volume, Tom Mulvey for his valuable comments, Oliver Wells for his helpful suggestions concerning the structure and content of the volume, and Sheila Smith for reading and commenting on all of the manuscripts. Bernie Breton Dennis McMullan Ken Smith

CONGRESS AND OTHER ABBREVIATIONS

The various European and International Conferences on electron microscopy are referred to so frequently that we merely give place and date in the individual lists of references. Full publishing details are given below. The abbreviations CUED (Cambridge University Engineering Department) and CIC (Cambridge Instrument Company – with its various successor companies) are often used, as are the conventional abbreviations SEM (scanning electron microscope), TEM (transmission electron microscope) and STEM (scanning transmission electron microscope).

References London (1949). Metallurgical Applications of the Electron Microscope, Royal Institution, London 16 November 1949 (organized by the Institute of Metals and 7 other Learned Societies); IoM Monograph and Report Series, No. 8 (Institute of Metals, London 1950). Cambridge (1956). X-ray Microscopy and Microradiography. Proceedings of a Symposium held at the Cavendish Laboratory, Cambridge, 16–21 August 1956 (Cosslett, V.E., Engstro¨m, A., and Pattee, H.H., eds.; Academic Press, New York 1957). Stockholm (1959). X-ray Microscopy and X-ray Microanalysis, Proceedings of the Second International Symposium, Stockholm, 1959 (Engstro¨m, A., Cosslett, V.E., and Pattee, H., eds.; Elsevier, Amsterdam, London, New York & Princeton 1960). Delft (1960). The Proceedings of the European Regional Conference on Electron Microscopy, Delft, 29 August–3 September 1960 (Houwink, A.I. and Spit B.J., eds.; Nederlandse Vereniging voor Elektronenmicroscopie, Delft n.d.), 2 Vols. Philadelphia (1962). Electron Microscopy. Fifth International Congress for Electron Microscopy, Philadelphia, Pennsylvania, 29 August–5 September, 1962 (Breeze, S. S., ed.; Academic Press, New York, 1962) 2 Vols. Stanford (1962). X-ray Optics and X-ray Microanalysis, Proceedings of the Third International Symposium, Stanford University, Stanford, CA, 22–24 August 1962 (Pattee, H.H., Cosslett, V.E., and Engstro¨m, A., eds.; Academic Press, New York & London 1963).

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CONGRESS AND OTHER ABBREVIATIONS

Toronto (1964). Proceedings First International Conference on Electron and Ion Beam Science and Technology, Toronto, 28 April–2 May 1964 (Bakish, R., ed.; Wiley, New York & London 1965). Prague (1964). Electron Microscopy 1964, Proceedings of the Third European Regional Conference, Prague, 26 August–3 September 1964 (Titlbach, M., ed.; Publishing House of the Czechoslovak Academy of Sciences, Prague 1964) 2 Vols. Kyoto (1966). Electron Microscopy 1966. Sixth International Congress for Electron Microscopy. Kyoto, 28 August–4 September 1966 (Uyeda, R., ed.; Maruzen, Tokyo, 1966) 2 Vols. St Paul (1969). Proceedings of the 27th Annual Meeting Electron Microscopy Society of America, St Paul MN, 26–29 August 1969 (Arceneaux, C.J., ed.; Claitor, Baton Rouge 1970). Chicago (1969). Second Annual Scanning Electron Microscopy Meeting, Chicago, 1969 (Illinois Institute of Technology (ITT), Chicago 1969). EMAG (1975). Developments in Electron Microscopy and Analysis. Proceedings of EMAG 75, Bristol, 8–11 September 1975 (Venables, J.A., ed.; Academic Press, London and New York, 1976). Lausanne (1981). Microcircuit Engineering 81. Ecole Fe´de´rale Suisse de Technologie, Lausanne, 28–30 September 1981. Proceedings (Oosenbrug, A., ed.) issued by the Swiss Federal Institute of Technology, Lausanne. Manchester (1992). X-ray Optics and Microanalysis 1992. Proceedings of the Thirteenth International Congress, UMIST, UK, 31 August–2 September 1992 (Kenway, P.B., Duke, P.J., Lorimer, G.W., Mulvey, T., Drummond, I.W., Love, G., Michette, A.G., and Stedman, M., eds.; Institute of Physics, Bristol and Philadelphia 1993) Conference Series No. 130. Cambridge (1997). The Electron, Proceedings of the International Centennial Symposium on the Electron, Churchill College, Cambridge 15–17 September 1997 (Kirkland, A. and Brown, P.D., eds.; IoM Communications, London 1998).

ACKNOWLEDGMENTS

The Editors wish to thank the following publishers for their permission to reproduce in this volume the papers, or extracts of papers, cited. The American Institute of Physics for: ‘‘The Early History of the Scanning Electron Microscope’’ by C. W. Oatley (Chapter 1.2). The Institution of Electrical Engineers for: ‘‘An Improved Scanning Electron Microscope for Opaque Specimens’’: by D. McMullan (Chapter 2.1B). The Institute of Physics for: ‘‘The Scanning Electron Microscope and its Fields of Application’’ by K. C. A. Smith and C. W. Oatley (Chapter 2.2B). ‘‘Wide-band Detector for Micro-microampere Low-energy Electron Currents’’ by T. E. Everhart and R. F. M. Thornley (Chapter 2.4B). ‘‘High Resolution Scanning Electron Microscopy’’ by R. F. W. Pease and W. C. Nixon (Chapter 2.9B) ‘‘X-Ray Projection Microscopy’’ by W. C. Nixon (Chapter 3.2B) ‘‘Microanalysis’’ by J. V. P. Long (Chapter 3.2C) Academic Press for: ‘‘Investigation of the Topography of Ion Bombarded Surfaces with a Scanning Electron Microscope’’ by A. D. G. Stewart (Chapter 2.7B) Czechoslovak Academy of Sciences for: ‘‘A New Scanning Electron Microscope’’ by A. D. G. Stewart and M. A. Snelling (Chapter 4.2B) Taylor and Francis for: ‘‘The Development of Electron Microscopy and Related Techniques at the Cavendish Laboratory’’ by V. E. Cosslett (Chapter 3.1) Macmillan for: ‘‘Micro-analysis by a Flying-Spot X-Ray Method’’ by V. E. Cosslett and P. Duncumb (Chapter 3.3B) The Royal Microscopical Society for: ‘‘The Detective Quantum Efficiency of the Scintillator/photomultiplier in the Scanning Electron Microscope’’ by C. W. Oatley (Chapter 5.2) The Royal Society for: ‘‘Sir Charles William Oatley O. B. E., F. R. S.’’ by K. C. A. Smith (Appendix I) The Foundation for Advances in Medicine and Science for: ‘‘A History of the Scanning Electron Microscopy 1928–1965’’ by D. McMullan (Appendix II).

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ACKNOWLEDGMENTS

Elsevier for: ‘‘The Cambridge Instrument Company and Electron-Optical Innovation’’ by P. Jervis (Appendix III) In addition the Editors would like to thank Mr. Michael Oatley for permission to reproduce the papers by C. W. Oatley in Chapters 1.2 and 5.2, and Mrs. Margaret Long for permission to reproduce the paper by J. V. P. Long in Chapter 3.2C.

FUTURE CONTRIBUTIONS

G. Abbate New developments in liquid-crystal-based photonic devices S. Ando Gradient operators and edge and corner detection C. Beeli Structure and microscopy of quasicrystals G. Borgefors Distance transforms A. Buchau Boundary element or integral equation methods for static and time-dependent problems B. Buchberger Gro¨bner bases T. Cremer Neutron microscopy H. Delingette Surface reconstruction based on simplex meshes R. G. Forbes Liquid metal ion sources E. Fo¨rster and F. N. Chukhovsky X-ray optics A. Fox The critical-voltage effect P. Geuens and D. van Dyck S-matrix theory for electron channelling in high-resolution electron microscopy G. Gilboa, N. Sochen, and Y. Y. Zeevi Real and complex PDE-based schemes for image sharpening and enhancement

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FUTURE CONTRIBUTIONS

L. Godo and V. Torra Aggregation operators A. Go¨lzha¨user Recent advances in electron holography with point sources K. Hayashi X-ray holography M. I. Herrera The development of electron microscopy in Spain D. Hitz Recent progress on HF ECR ion sources H. Ho¨lscher and A. Schirmeisen Dynamic force microscopy and spectroscopy D. P. Huijsmans and N. Sebe Ranking metrics and evaluation measures K. Ishizuka Contrast transfer and crystal images K. Jensen Field-emission source mechanisms G. Ko¨gel Positron microscopy T. Kohashi Spin-polarized scanning electron microscopy W. Krakow Sideband imaging B. Lencova´ Modern developments in electron optical calculations R. Lenz Aspects of colour image processing W. Lodwick Interval analysis and fuzzy possibility theory M. Matsuya Calculation of aberration coefficients using Lie algebra L. Mugnier, A. Blanc, and J. Idier Phase diversity

FUTURE CONTRIBUTIONS

K. Nagayama Electron phase microscopy A. Napolitano Linear filtering of generalized almost cyclostationary signals S. A. Nepijko, N. N. Sedov, and G. Schon¨hense Measurement of electric fields on the object surface in emission electron microscopy M. A. O’Keefe Electron image simulation N. Papamarkos and A. Kesidis The inverse Hough transform R.-H. Park and B.-H. Cha Circulant matrix representation of feature masks K. S. Pedersen, A. Lee, and M. Nielsen The scale-space properties of natural images E. Rau Energy analysers for electron microscopes H. Rauch The wave-particle dualism E. Recami Superluminal solutions to wave equations J. Rehacek, Z. Hradil, and J. Pervina Neutron imaging and sensing of physical fields G. Schmahl X-ray microscopy G. Scho¨nhense, C. M. Schneider, and S. A. Nepijko Time-resolved photoemission electron microscopy R. Shimizu, T. Ikuta, and Y. Takai Defocus image modulation processing in real time S. Shirai CRT gun design methods K. Siddiqi and S. Bouix The Hamiltonian approach to computer vision N. Silvis-Cividjian and C. W. Hagen Electron-beam-induced deposition

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FUTURE CONTRIBUTIONS

T. Soma Focus-deflection systems and their applications W. Szmaja Recent developments in the imaging of magnetic domains I. Talmon Study of complex fluids by transmission electron microscopy I. J. Taneja Divergence measures and their applications M. E. Testorf and M. Fiddy Imaging from scattered electromagnetic fields, investigations into an unsolved problem R. Thalhammer Virtual optical experiments M. Tonouchi Terahertz radiation imaging N. M. Towghi Ip norm optimal filters Y. Uchikawa Electron gun optics K. Vaeth and G. Rajeswaran Organic light-emitting arrays J. Valde´s Units and measures, the future of the SI D. Vitulano Fractal encoding D. Walsh The importance-sampling Hough transform D. Windridge The tomographic fusion technique C. D. Wright and E. W. Hill Magnetic force microscopy M. Yeadon Instrumentation for surface studies

PART I INTRODUCTION

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

1.1 Charles Oatley: Father of Modern Scanning Electron Microscopy K. C. A. SMITH AND D. McMULLAN ,1 1

Cavendish Laboratory, University of Cambridge

When Charles Oatley took up his Lectureship at the Cambridge University Engineering Department together with a Fellowship at Trinity College, immediately following World War II, he was ideally placed to undertake an ambitious programme of research. His life and career is set out in the Royal Society Biographical Memoir contained in Appendix I, and it is evident that he had acquired a strong theoretical and practical background in electron physics at King’s College, London, and that his work during the war at the Radar Research & Development Establishment, Malvern, had provided him with an unrivalled knowledge of a broad spectrum of the very latest techniques in electronics. His initial choice of research topics not only embraced the field of vacuum and electron physics but included such diverse subjects as mass spectrometers, microwave generators, electron conduction in crystals, high-power microwave amplifiers and electroluminescence. However, it was scanning electron microscopy that was to emerge as the field for which he was to become famous. His remarkable prescience in choosing the scanning electron microscope (SEM) as a mainstream research project can only be understood if the climate of opinion regarding the SEM among microscopists at that time is fully appreciated. The German electron microscope pioneer Manfred von Ardenne had published his work on the scanning transmission microscope in 1938 and, in a book that appeared in 1940 during the war, had described many of the principal features of a surface imaging SEM, but for the reasons outlined in the history of the development of the SEM contained in Appendix II, he was unable to bring his ideas to fruition. A team at the RCA Laboratories in the United States led by Vladimir Zworykin and James Hillier had in 1942 constructed an SEM that delivered promising results, but again other factors had led to the termination of the project. Oatley 

Formerly at: Engineering Department, University of Cambridge 3 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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learned of Zworykin’s work soon after the war, and on reading von Ardenne’s pre-war publications was convinced that the scanning principle held considerable potential. (He was at that time unaware of von Ardenne’s book on the subject.) His personal account of the reasons that led to his decision to take up research on the SEM is contained in a paper published in 1982 in the Journal of Applied Physics. This paper is reproduced in Chapter 1.2. At the time, and for many years afterwards, the consensus among electron microscopists was that the SEM represented a dead-end. The reasons for this are examined in more detail in Appendix II, but the principal one was the invention of the replica process by H. Mahl in Germany in 1940: he showed that a very thin replica of a metal surface, for example an oxide film from aluminium, could be imaged in a transmission electron microscope at close to its full resolving power. Consequently, the much inferior resolution obtainable with, for example, the RCA SEM was treated with disdain, regardless of the fact that direct imaging of a surface was possible without the complicated procedures required to produce a replica free of artefacts. The then prevailing view of the SEM is made abundantly clear in the proceedings of a conference on ‘‘Metallurgical Applications of the Electron Microscope’’ held in London in 1949 under the auspices of the Institute of Metals. In this conference, which brought together many eminent microscopists for the first time since the war, the SEM appears as little more than an historical curiosity (see Appendix II). It was against this unpromising background that Charles Oatley made his decision to take up and pursue research on the SEM. However, he had one considerable advantage over those gathered in London in 1949: he was not an electron microscopist. From an entirely diVerent background, he was able to bring a fresh mind to the problems holding back the development of the SEM. Furthermore, as the people who worked with him on the project will testify, he had an unerring, almost intuitive, grasp of how to guide the research into the most fruitful and productive channels. The reader will find in the following chapters numerous examples of this remarkable faculty. If, as Oatley himself acknowledged, ‘von Ardenne was the true father of the scanning electron microscope’, then Oatley was most certainly the father of modern scanning electron microscopy. Even so, the indiVerence, even antipathy, with which the SEM was regarded meant that for much of the 1950s the work of Oatley and his students was considered literally to be a waste of time. After the Institute of Metals London Conference, a full decade and a half was to pass before Oatley’s judgement was completely vindicated with the launch by the Cambridge Instrument Company (CIC) of the first series production instrument—the Stereoscan.

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The story of this early work is recounted in Part II of this volume, which comprises the personal accounts of nine of the research students who worked under Oatley’s supervision from 1948 to 1960. One student from these years, Garry Stewart, has unfortunately been unable to contribute; but a colleague, Alan Boyde, has described the work on which they collaborated. Related work undertaken by another of Oatley’s students, Christopher Grigson, who pioneered scanning electron diVraction, is also included in Part II. There follows in Part III a complementary account by V. Ellis Cosslett, originally published in Contemporary Physics in 1981, on the development of the scanning X-ray microanalyser by his group in the Cavendish Laboratory. This development, which had stemmed from the original work of Cosslett and W.C. Nixon on the point projection X-ray microscope, and which was inspired by the investigations of R. Castaing and A. Guinier in Paris, was begun in 1954. It culminated in 1960 with the launch of the Microscan, the world’s first production scanning electron/X-ray analyser. This instrument played a crucial part in the SEM story, since, although its development at Cambridge started later than that of the SEM, it went into production at CIC earlier. Thus the company had already attained considerable expertise in the manufacture of this type of instrument by the time Oatley oVered the SEM for production. Without this incentive it is unlikely that CIC would have risked taking on such a speculative venture, and the commercial development of the SEM would almost certainly have moved to Japan (see Appendix II). Cosslett’s paper is followed by contributions from Peter Duncumb, his first research student on the microanalyser, and by David Melford of Tube Investments Research Laboratories who developed the first engineered version of the instrument. Also included are representative papers by Bill Nixon and Jim Long, both members of Cosslett’s group in the 1950s, who have made important contributions to the field of microanalysis. Unfortunately, Bill Nixon has been unable to contribute directly to this volume owing to illness, and Jim Long has died during its preparation. Their papers, both of which were presented at a conference held in Cosslett’s honour in 1992, are introduced by Peter Duncumb. Dennis McMullan’s short biography of Cosslett may be found on the Cambridge University Engineering Department (CUED) website that has been established to accompany the publication of this volume. Production of the Stereoscan and the Microscan at CIC revived the flagging fortunes of the company, and created a new industry in Cambridge necessitating new skills and new marketing techniques. An account of this early work at the company by some of those directly involved, Michael Snelling and Donald Unwin, is given in Part IV. A further important development—electron beam microfabrication (lithography)—that arose

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out of later work at the CUED and was taken up by CIC, is described by Bernard Wallman and John Sturrock. The manner in which university research was exploited in this instance had wider implications for British industry, and gave rise to an academic study by Paul Jervis at Sussex University, extracts from which are included in Appendix III. Part V, the Epilogue, covers broadly the period following Charles Oatley’s appointment to the University Chair of Electrical Engineering in 1960. His direct involvement in research on the SEM then largely ceased for a number of years, but after his oYcial retirement from the University in 1971, he returned once more to spend a further productive period on the SEM. His last paper, published in 1985, is reproduced in the Epilogue. Articles from Gerry Owen and Eric Munro exemplify the influence he exerted on a later generation of research students. The Epilogue also contains a brief introduction to the research undertaken at the CUED from the early 1960s onwards, and of the fruitful collaboration between the CUED and CIC that began after the launch of the Stereoscan. (This research is described in greater detail on the associated CUED website.) Although the company has changed ownership and name several times, that collaboration continues to the present day. The Epilogue concludes with articles by Patrick Echlin and John Cleaver relating their experiences in applying the SEM in, respectively, the biological sciences and in the field of microelectronics.

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

1.2 The Early History of the Scanning Electron Microscope C. W. OATLEY University Engineering Department, Trumpington Street Cambridge, CB2 1PZ, England

The Editors have encouraged me to write this account of the early history of the scanning electron microscope from a very personal point of view: to reminisce about the work that was carried out in the Engineering Department of the University of Cambridge from 1948 onwards and to try to explain not only what was done, but also why it was done and the conditions under which it was done. First, however, it is necessary to give a brief account of earlier research carried out elsewhere. The story of the scanning microscope and, for that matter, of every other electron-optical instrument, must begin with H. Busch (1926) who studied the trajectories of charged particles in axially-symmetric electric and magnetic fields. In 1926 he showed that such fields could act as particle lenses and thus laid the foundations of geometrical electron optics. Following this discovery the idea of an electron microscope began to take shape and, in Berlin, two teams set out to test this possibility; one with Knoll and Ruska at the Technische Hochschule, and the other with Bru¨che and his collaborators at the A. E. G. Laboratory. The success of their eVorts attracted other workers to the field and, in due course, a successful transmission electron microscope was built. The German scientists were aware that, in principle, it should be possible to construct a quite diVerent type of electron microscope, which has since become known as a scanning microscope. The principle of such an instrument is indicated diagrammatically in Fig. 1. A narrow beam of electrons from a cathode C is accelerated to high velocity and then passes through lenses L1, L2, and L3. These reduce it to an extremely fine probe, which is focused on to the surface of the specimen S. Current from a sawtooth generator B passes through coils D1D1, to deflect the electron probe, and also through coils D2D2 to deflect the beam in a separate cathode-ray tube. With two such sets of coils causing deflections at right angles to each other, 

Reprinted from J.Appl. Phys. 53, R1–R13 (1982). 7 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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FIG. 1. Principle of the scanning electron microscope (Oatley 1972).

both the focused electron spot on the specimen and the spot on the screen of the cathode-ray tube traverse zig-zag rasters in synchronism. Secondary and/ or backscattered electrons which leave S are collected at P and the resulting current, after amplification at A, is used to control the potential of the grid G of the cathode-ray tube and hence the brightness of the picture formed on the face of the tube. Because of the one-to-one correspondence in the positions of the electron spot on S and the light spot on the face of the tube, the picture built up by the latter must, in some sense, be an image of the surface of S. Moreover, by control of the currents in the various deflecting coils, it can be made a highly magnified image. It is a fortunate circumstance that, apart from colour, the appearance of this image is rather similar to one which would be produced by an optical microscope. The first instrument operating on the above principles was built by Knoll (1935) and further work was published by Knoll and Theile (1939). However, these investigators were primarily interested in studying various properties of the surface such as secondary emission. They used no demagnifying lenses to produce a very fine probe and resolution was limited by the diameter of the focused spot on the specimen to something of the order of 100 mm. The principles underlying the scanning microscope as we know it today were clearly enunciated in a theoretical paper published by von Ardenne (1938a). In this he proposed to obtain a suitably fine electron probe by demagnifying the crossover with two magnetic lenses, between which the scanning coils would be placed. He examined the electron-optical aberrations and calculated the currents to be expected in probe spots of various diameters, assuming particular current densities leaving the cathode. These calculations led him to the conclusion that, with noise-free detectors, fluctuation noise in the probe would limit resolution to about 1 mm at television scan rates and to a few nanometers with scan times of the order of ten minutes. He envisaged that the microscope might be used to examine thin

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9

specimens, using transmitted electrons with either bright-field or dark-field illumination, or the surfaces of thick specimens, using electrons which had been backscattered or secondarily emitted. He even foresaw the possibility of using an electron probe to fabricate submicroscopic structures, such as grids on photographic plates. His discussion of possible detectors for the very small electron currents leaving the specimen showed that, if a metal collector were followed by a thermionic valve amplifier of the type available at that period, recording times were likely to be unacceptably long, even if the input capacitance could be reduced to the very low value of 3 pF. He noted that matters would be greatly improved if an electron multiplier could be used but, at that time, these devices contained dynodes coated with cesium, which could not be exposed to air. Finally, he showed that recording times could be reduced by a factor of about one hundred if the electron probe, after passing through a thin specimen, were allowed to fall directly on to a photographic film. Von Ardenne’s experimental work was published in a second paper in 1938 (von Ardenne 1938b) and further information is given in a recent historical note (von Ardenne 1978). His apparatus produced an electron probe giving a focused spot 50–100 nm in diameter and this was first used to examine the surface of a specimen using direct collection of secondary and backscattered electrons. The display was on a cathode-ray tube with afterglow. This experiment was not continued for any length of time because the apparatus was needed for the main work with transmitted electrons through thin-film specimens. Photographic recording could then be used and, as indicated earlier, better resolution was to be expected. A schematic diagram of von Ardenne’s apparatus is shown in Fig. 2. A narrow beam of electrons from the gun A passed successively through magnetic lenses B and C to form a focused spot on the specimen D. Electrons passing through the specimen fell on a photographic film attached to a cylindrical drum E which was caused to rotate and to progress axially by means of a screw thread. The electron probe was deflected by currents through a pair of coils F1F2 and a similar pair at right angles to the plane of the diagram, and the currents through these coils were controlled by potentiometers attached to the rotating drum. It was thus possible to arrange for the focused electron probe to describe a small zig-zag raster over the surface of the specimen, while the electrons passing through the specimen fell on a small area of the photographic film. The position of this area eVectively moved over the film in a corresponding raster some thousands of times as large. The potential advantage of this type of microscope lay in the fact that electrons were not required to pass through a lens after traversing the specimen. The spread of velocities caused by absorption in the specimen

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FIG. 2. Principle of von Ardenne’s 1938 microscope.

would not, therefore, give rise to chromatic aberration, as it does in the ordinary transmission microscope, and it should be possible to examine thicker specimens. Experiments along these lines were continued for some years but did not lead to a commercial instrument, presumably because the results obtained did not compete with those given by the transmission microscope. The experimental scanning microscope was destroyed in an air raid in 1944. Von Ardenne was the true father of the scanning electron microscope, who had all the right ideas. His misfortune was to have worked at a time when experimental techniques had not advanced quite far enough to enable him to bring those ideas to full practical fruition. Details of a new scanning microscope were published a few years later by Zworykin, Hillier, and Snyder (1942). By this time it must have been clear that, for thin transparent specimens, competition with the transmission microscope was unlikely to be very profitable, so the new instrument was designed for the examination of opaque specimens, from which thin sections could not readily be cut. At that time replica techniques were in their infancy. The essential features of one form of the new microscope are shown in a greatly simplified form in the schematic diagram of Fig. 3. Electrons from a tungsten filament F passed through a controlling grid G, where the current was chopped by the application of a 3 kHz square-wave voltage. The

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FIG. 3. Schematic diagram of the microscope of Zworykin, Hillier, and Snyder (1942).

electrons were then accelerated by a potential diVerence of 10 kV applied between F and the anode A. The electron beam so formed passed through electrostatic lenses L and M to form on the specimen S a focused spot with a diameter of about 0.01 mm. S was maintained at a positive potential of about 800 V with respect to F, so that the electrons struck it with a velocity favourable to the production of secondaries. The secondaries were attracted back through the lens M and were brought to a focus near the lens. Thereafter they spread out to strike a fluorescent screen K, in which there was a hole to permit the passage of the primary beam. Light from K was focused on to the cathode of a photomultiplier P and the output from this provided the signal from which the final image was built up. This signal consisted of a 3 kHz square-wave carrier, modulated by variations in the secondary emission from the specimen. After amplification and filtering it was applied to a facsimile printer, where the final image was recorded over a period of the order of ten minutes. Since this arrangement provided no simple means of focusing the instrument, an oscilloscope was added to display the waveform of the output signal. The microscope was judged to be in focus when the highest frequency components in the waveform had their maximum amplitude: defocusing caused an increase in the diameter of the incident probe and thus reduced the amplitudes of these components. Scanning in the original instrument was carried out by mechanical displacement of the specimen through links controlled by the recorder. At a later stage, greater precision was attained by using magnetic deflection of the electron probe, the sawtooth circuits being triggered by the recorder. With the final instrument a resolution of about 0.05 mm was obtained.

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This microscope was the work of a highly skilled team, backed by the resources of a very large industrial laboratory. It embodied sophisticated electronic techniques and it deserved to succeed. Yet, in the end, it was a disappointment. It was expensive; recording times were unacceptably long; focusing was not easy and the final image was marred by excessive noise. Moreover, replication techniques were being perfected at about this time and they seemed likely to have every advantage over the scanning microscope for the examination of opaque specimens. So the project was discontinued and it must have appeared that the scanning principle had finally been tested and found wanting. In 1946 some interest in the possibilities of scanning electron microscopy was shown in France and Brachet (1946) published a note suggesting that, with a noise-free detector, a resolution of 10 nm should be achieved. The instrument constructed by Le´aute´ and Brachet during 1949 and 1950 used secondary electrons collected by a metal electrode. The resulting current was amplified by thermionic vacuum tubes, so high resolution was hardly to be expected. The above account of the earliest work on the scanning microscope leaves me with a question to answer: Why, in the face of the discouraging results that had hitherto been obtained, did I think it worthwhile to reopen the matter in 1948? To give a convincing reply to this question I must go back a few years, to explain the conditions in the University Engineering Department at that time and to say something about my own views on university research. Before the war, graduates from the Cambridge University Engineering Department normally left immediately after obtaining their first degree and went into industry to take a graduate apprenticeship which would later qualify them for membership of a professional Institution. Very few remained in the Department and such research as was done was carried out largely by members of the teaching staV. In light-current electrical engineering it was confined almost entirely to problems relating to circuits. By 1945 there was general agreement that the prewar pattern could no longer meet the needs of rapidly advancing technology and that a considerable research eVort should be built up in the Cambridge Engineering Department. To assist with this program I was appointed to a lectureship in engineering. At the time of my appointment I was in charge of the army Radar Research and Development Establishment, in which I had worked for the past six years. Prior to that I had been a lecturer in a university physics department so it was natural that, in thinking of possible future research projects, I should look with favour on those which could be broadly classified as applied physics.

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From a diVerent point of view, a project for a Ph.D. student must provide him with good training and, if he is doing experimental work, there is much to be said for choosing a problem which involves the construction or modification of some fairly complicated apparatus. Again, I have always felt that university research in engineering should be adventurous and should not mind tackling speculative projects. This is partly to avoid direct competition with industry which, with a ‘‘safe’’ project, is likely to reach a solution much more quickly, but also for two other reasons which are rarely mentioned. In the first place, university research is relatively cheap. The senior staV are already paid for their teaching duties and the juniors are Ph.D. students financed by grants which are normally very low compared with industrial salaries. Thus the feasibility or otherwise of a speculative project can often be established in a university at a small fraction of the cost that would be incurred in industry. So long as the project provides good training and leads to a Ph.D., failure to achieve the desired result need not be a disaster. (The Ph.D. candidate must, of course, be judged on the excellence of his work, not on the end result.) The second reason is rather similar. A Ph.D. student stays at the university for about three years and his departure provides a convenient point at which the promise of his project can be reviewed. If it seems unlikely to succeed, it can be discontinued without the dissatisfaction and discouragement which sometimes attends similar action in industry. These, then, were the thoughts at the back of my mind, when I came to Cambridge in 1945. I had already begun thinking about electron optics as a possible field of research although, at that time, I knew hardly anything about the subject. Furthermore, in 1946, V. E. Cosslett started work in the Cavendish Laboratory on transmission electron microscopy. Clearly, it was important to avoid trespassing on his ground but, in the outcome, this has never caused any diYculty. It gives me great satisfaction to put on record that, over a period of twenty-five years or so, until both Cosslett and I reached the retiring age, the two teams worked side by side in harmony. They helped us a great deal and I like to think that we helped them. Moreover, there was interchange of senior staV in both directions. When I joined the Engineering Department it soon became clear that research could not begin at once. Research students were not immediately available and my own time was fully occupied with the preparation of lecture and laboratory courses and with the acquisition of equipment. By 1948 I had been able to do a good deal of reading and had come to the conclusion that a fresh attempt at the construction of a scanning electron microscope would form a project meeting the conditions I have outlined above. So far as I can remember, my reasons for reaching this decision were roughly as follows.

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Although the work of Zworykin, Hillier, and Snyder had not led to an instrument that was likely to be commercially viable, it was by no means a failure. They had shown that the scanning principle was basically sound and could give useful resolution in the examination of solid surfaces. If only one could collect a much larger fraction of the electrons leaving the specimen and make them contribute to the output signal, there would be a corresponding reduction in noise in the final image and it might be possible to shorten the recording time to the extent that cathode-ray tube presentation became feasible. Collection of more of the electrons from the specimen might be achieved if these electrons went directly to the detector, instead of having to travel back through the final lens, although the specimen surface might then have to be included at an angle to the incident beam. This idea led to further consideration of the detector. I was aware of some work being carried out in the Cavendish Laboratory by A. S. Baxter (1949) on secondary-electron multipliers. Such multipliers had been known for more than a decade, but had commonly used cesiumcoated dynodes, which could not be exposed to air. Baxter had been experimenting successfully with beryllium-copper dynodes which could be used in a demountable vacuum system and he very kindly gave me one of his multipliers. I thought it might make an excellent detector in the scanning microscope. Other factors favouring a new attack on the problem were the improvements in electronic techniques and components that had resulted from work during the war, particularly in cathode-ray tubes with longpersistence screens. The above considerations obviously did not add up to any kind of certainty that we could build a successful scanning microscope and I think they would not have justified a new industrial attempt. However, viewed as a university Ph.D. project, the proposition was much more attractive. The design and construction work would provide an excellent training in research; we should learn a great deal about the practical side of electron optics, which would be useful in other projects; and we should almost certainly end up with a microscope which gave results of some kind, which might or might not justify further work. I decided to go ahead. The research student to whom this project was assigned in 1948 was D. McMullan and in this I was doubly fortunate. Quite apart from his ability as an experimenter, which was very great, he had, since graduating from Cambridge in 1943, spent five years in industry working on radar, cathoderay tubes, and analog computers. For the job in hand, he could hardly have had better experience. It is unnecessary to give details of the microscope which he built, since these have already been published (McMullan 1953a and 1953b) but the extent of his achievement cannot be appreciated without some account of conditions in the Engineering Department at that time.

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When I joined the Department, no electronic apparatus of any kind was to hand. Shortly after the war, the Government made available to universities surplus equipment of all kinds and we were able to acquire a virtually unlimited supply of valves, cathode-ray tubes, capacitors, resistors, meters, and many other components. We were given some vacuum pumps and I had an initial grant of 1000 to buy some essential measuring apparatus, but this was the whole of our initial stock. Moreover, our annual income to cater for three or four diVerent research projects was very small.1 On the credit side we had free access to a large and well-equipped instrument workshop presided over by A. A. K. Barker whose vast knowledge of mechanical techniques and enthusiasm for taking on diYcult jobs were of the utmost value. Under the above conditions, McMullan realized that almost everything he needed would have to be constructed on the spot. He set out to make a transmission electron microscope, to which scanning facilities were added later. On the way, he designed and built a 40 kV stabilized supply unit, electron lenses, a cathode-ray tube display unit and so forth, much of it with his own hands. Electrostatic unipotential lenses were used because powersupply stability was thereby eased and the construction of the microscope column was somewhat simplified. It was appreciated that magnetic lenses would have lower aberration coeYcients but, in this early work, it was not thought likely that lens aberrations would be the factors limiting the overall performance of the instrument. Figure 4 shows a schematic diagram of McMullan’s microscope, Fig. 5 is a photograph of the instrument, and Fig. 6 is a micrograph of etched aluminum, at a magnification of 5500, which he obtained with it. A rereading of his Ph.D. dissertation leaves one amazed at the amount of ground that he was able to cover. As well as building the microscope, he laid the foundation of a theoretical analysis of its behaviour and made quantitative measurements of its performance. He compared his micrographs with optical micrographs of the same specimens since, at that time, there was no certainty that the two techniques would give pictures looking alike. He devoted a great deal of thought to the mechanism by which contrast might be produced and was influenced by results being obtained elsewhere with the reflection electron microscope. In this instrument electrons which have struck the specimen at a very small glancing angle are brought to a focus by a further lens. McMullan concluded that, in the scanning microscope, good contrast should be achieved if fast electrons were allowed to strike the specimen at a much larger glancing angle of about thirty degrees and if electrons backscattered 1

Later on, Government policy towards university research changed completely and grants of a diVerent order of magnitude became available.

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FIG. 4. Schematic diagram of McMullan’s microscope (McMullan 1953a and b).

over a fairly wide solid angle entered the electron multiplier. The foreshortening of the resulting image would then be tolerable (which was not the case with the reflection microscope) and the tilted specimen would greatly facilitate the passage of electrons to the multiplier. This was the arrangement that he put into practice. For the metal specimens, with which he was primarily concerned, he thought that more reliable results would be obtained if the image were formed from fast backscattered electrons and if the slow secondaries were excluded from the detector. This was because he expected secondary emission to be aVected by adsorbed films on the surface of the specimen and thus to be uncharacteristic of the underlying metal. He therefore made careful measurements of the distribution in angle of backscattered electrons from several metals and of the way in which the distribution was aVected by the inclination of the incident beam to the surface of the specimen. Above all, he confirmed that, with careful attention to the collection and amplification of electrons leaving the specimen, it was possible to use a cathode-ray-tube display with a frame period of a second or two. In a subsidiary experiment he

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FIG. 5. Photograph of McMullan’s microscope (McMullan 1953b).

obtained images by collecting the light emitted from grains of phosphor scanned by the electron beam. By the time his work at Cambridge was finished there was no doubt that we had an interesting new instrument which warranted further development. My next research student was K. C. A. Smith. He joined in 1952, a year or so before McMullan left, and began by making improvements to the microscope which had been suggested by the earlier work; the incorporation of a stigmator, the use of double-deflection scanning coils (suggested by McMullan), an improved final lens, and means for centering the final aperture. He then introduced a much more eYcient system for collecting electrons from the specimen which, for a given probe current, increased the output signal by a factor of about fifty. Moreover, by biassing the collector

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FIG. 6. Micrograph of etched aluminum (16 kV; 300 sec; 10 12 amp; glancing angle 25 ; field of view measures 15 mm from left to right; McMullan 1953a and b).

positively with respect to the specimen, slow secondary electrons as well as the fast backscattered ones could be made to contribute to the output signal. Smith then carried out a detailed investigation of the relative usefulness of the two contributions. This showed that the secondaries normally provided more than ninety per cent of the total signal and that, contrary to what McMullan had expected, they gave rise to a picture showing excellent contrast, which was not spoiled by surface contamination. Smith had also redesigned the specimen stage so that rotation and tilt could be varied from outside the chamber and, on the theoretical side, he had extended McMullan’s quantitative assessment of the performance of the instrument and had given a good deal of thought to the factors aVecting contrast. He now had a microscope giving a resolution of about 30 nm and was ready to use it for the examination of a wide range of specimens (Smith and Oatley 1955). These included metals, textile fibers, and a number of diVerent biological objects. In another direction he showed that the microscope could be used for the continuous observation of specimens that were changing with time. Thus the decomposition of a crystal of silver azide, heated at one end, was recorded (McAuslan and Smith 1956) and there was a detailed

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FIG. 7. Point contact between tungsten-molybdenum whisker and germanium surface: (a) Before forming; (b) After forming; (field of view measures 50 mm from left to right; Smith 1956).

investigation of the changes taking place during the electrical forming of a germanium point-contact microwave mixer (Fig. 7). Finally, Smith showed that it was quite possible to obtain a micrograph of a biological specimen when the specimen itself was surrounded by water vapor at a pressure great enough to prevent desiccation. At the end of Smith’s first year, in 1953, he was joined by O. C. Wells whose major task was to design and build a new microscope, similar to the earlier instrument, but incorporating a number of improvements which had been shown to be desirable. This occupied about three-quarters of his available time but, in his last year, he was able to examine some new techniques in the use of the microscope. An attempt to observe synthetic fibers under tension was hindered by the cracking of the evaporated metal coating which is normally applied to prevent electrical charging of an insulating specimen. This led to an investigation of other possible methods of preventing charging: by irradiation with positive ions, by the addition of antistatic sprays, or by collecting only high-energy electrons. The use of high-energy, backscattered electrons involved a study of the diVerent eVects to be obtained by collecting electrons leaving the specimen in diVerent directions. In another part of this work, Wells wished to examine the interior surface of the very fine bore in a spinneret through which synthetic fibers are extruded. The normal method of collecting electrons could not be used, but he showed that satisfactory micrographs could be obtained by allowing the incident probe to fall on the surface after entry through one end of the bore

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FIG. 8. Platinum spinneret for nylon (diameter ¼ 50 mm; Wells 1959).

and by collecting electrons which left through the opposite end after multiple collisions with the walls (Fig. 8). Wells also made two very useful theoretical contributions to scanning microscopy. He showed how quantitative information about the topography of a surface could be obtained from stereoscopic pairs of micrographs and he provided a detailed theory of the way in which resolution would be likely to be aVected by the penetration and subsequent scattering of the incident electrons within the specimen (Wells 1959; Wells 1960; and Everhart, Wells, and Oatley 1959). T. E. Everhart joined the team as a research student in 1955 and took over the original McMullan microscope, as modified by Smith. I asked him to extend our knowledge of the factors aVecting contrast and this work led to two important advances in technique. Hitherto we had used the electron multiplier almost exclusively for the detection of electrons from the specimen and, without it, the earlier work could not have been done. Nevertheless it had two major defects; it was bulky and could not conveniently be placed close to the specimen, and it provided an output signal at a high potential with respect to earth. From the outset, the possibility of using a scintillator and photomultiplier had been considered, since this technique had been used by Zworykin, Hillier, and Snyder (1942). McMullan experimented along these lines but showed that the inorganic phosphors then available to him did not decay suYciently rapidly to give satisfactory images at the faster scan rates that he was using. With these phosphors the decay of light normally results from a bimolecular reaction and is thus not exponential; the lower the brightness,

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the slower the decay. In the scanning microscope the brightness level is very low and the decay unacceptably slow. To overcome this diYculty, Wells had built a detector suggested by McMullan in which the electrons first entered a six-stage electron multiplier to raise the signal level. Electrons leaving the multiplier were then accelerated through a potential diVerence of 2 kV to fall on a screen coated with a zinc-oxide phosphor. The light so produced was conveyed by a light pipe to a photomultiplier to provide further amplification of the signal. Although this arrangement did not get rid of the electron multiplier, it did produce a signal at ‘‘earth’’ potential. It was then possible to use dc amplification throughout and thus to avoid the necessity for dc restoration at the end of each scan line. A little later I became aware that organic scintillators with very fast decay times were being produced for counting nuclear particles. I obtained a sample of this material and Smith showed that, coupled to a photomultiplier, it formed a very convenient detector for the fast backscattered electrons leaving the specimen. He used a light-pipe, with the scintillator mounted close to the specimen, so as to subtend a large solid angle. Everhart then took the next step and designed a detector for slow secondary electrons, based on the scintillator/photomultiplier combination. This was a more complicated matter since it was necessary to attract the electrons to the detector by a relatively weak electrostatic field which would not distort the primary beam, and then to accelerate them so that they struck the scintillator with energies of the order of 10 keV. A knowledge of trajectories was clearly needed and this was before the days when a computer could provide the answers. However, as a quite separate development, we had in the laboratory a highly accurate trajectory tracer in which a large electrolytic tank was coupled to a homemade computer (Sander and Yates 1953). At the time this trajectory tracer was being operated by M. R. Barber, who very kindly gave expert assistance in producing a number of traces that were needed for the scanning microscope. Using these, Everhart designed the detector shown in Fig. 9. With relatively little modification it has remained the standard detector since that time. With his new detector Everhart went on to other work, to be discussed later. Detailed measurements on the detector itself were left to R. F. M. Thornley, who came to us in 1957. He showed that the eYciency of the scintillator was seriously impaired if it were allowed to become overheated during polishing or if unsuitable materials were used in this process. He measured the eYciency of light pipes, extended the work on electron trajectories and examined the way in which they were aVected by potentials applied to diVerent parts of the detector. In short, he put the design of the detector on a sound quantitative basis. His results were published in a joint paper with Everhart (Everhart and Thornley 1960).

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FIG. 9. Everhart’s detector (Everhart and Thornley 1960).

Meanwhile Everhart was attacking a quite diVerent problem. Some time earlier Smith had noticed that the strength of the output signal from his electron multiplier was influenced quite markedly by the potential of the specimen. This suggested to me that, if the specimen were a reverse-biassed p–njunction diode, the image should show sharp contrast between the p and the n regions. The experiment was tried and the contrast was found (Oatley and Everhart 1957). Everhart now used his new detector to examine this eVect in much greater detail. Once more the trajectory tracer was used to determine the best position for the detector, its optimum aperture, and the best potential diVerence between specimen and detector to give rise to maximum contrast at the junction. Figure 10 shows the state of the art at this time. Thornley had taken over the microscope constructed by Wells, had improved it in various ways, and had added an electrostatic stigmator. He also showed that focus modulation of the final electrostatic lens could be used to avoid defocusing during the examination of an inclined flat specimen. In the

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FIG. 10. Germanium-indium p–n junction: (a) Reverse bias ¼ 3 V; (b) Reverse bias ¼ 1 V. (Everhart 1958; Everhart, Wells, and Oatley 1959).

use of the microscope he was the first to point out that charging of an insulating specimen can often be overcome by operating with a low beam voltage, where the secondary-emission coeYcient exceeds unity, and he obtained some excellent micrographs of ceramics with beam voltages as low as 1 kV (Fig. 11). He added a refrigerating specimen stage to the microscope and examined the surfaces of biological materials in either the frozen or the freeze-dried condition (Thornley 1960). He realized that neither of the microscopes constructed in the Department had achieved the resolution to be expected on theoretical grounds and this led him to make a careful quantitative assessment of the extent to which each component of the two instruments had fallen short of perfection. This analysis pointed the way towards further improvements, though Thornley himself did not have time to carry out the necessary experimental work. Another research student who contributed to the improvement of components was P. J. Spreadbury, who joined the team in 1956. As an oYcer on leave from the Regular Army, his time with us was limited to two years and I suggested that he should build a very simple scanning microscope, using an ordinary cathode-ray oscillograph for the display unit. In the course of this work he designed and constructed an excellent 0–20 kV stabilized E.H.T. supply, which was used by several of his colleagues. He also made careful measurements of the performance of the electron guns that we were then using and showed how the size, brightness, and position of the crossover were related to conditions of operation.

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FIG. 11. A fault in opaque crystalline alumina (field of view measures 25 mm from left to right; 1.5 keV; Thornley 1960; Thornley and Cartz 1962).

In an endeavor to excite more general interest in the scanning microscope, I was now seeking projects which would demonstrate that the instrument could provide information which could not be obtained by the use of replicas. Thus, to A. D. G. Stewart, who joined us in 1958, I gave the problem of using the microscope to investigate the sputtering of materials by positive ions. Initially he used the microscope that had been built by Spreadbury but, at a later stage, he took over and completed a more ambitious instrument that I had designed and was constructing for my own use. To this he added an ion gun, so that the specimen could be bombarded while under observation. This work provided a great deal of information about the basic mechanism of sputtering, which need not be described here, but Stewart’s major contribution to the development of the microscope was made some time afterwards, and will be related later. In 1959 a rather similar project was undertaken by H. Ahmed, supervised by A. H. W. Beck, who was primarily interested in the activation of thermionic dispenser cathodes and took over one of our microscopes as a useful

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tool for his purpose. The investigation involved special problems since the specimen had to be heated to temperatures exceeding 1300 K, while all light was excluded from the photomultiplier following the scintillator. Again emission currents up to 20 mA had to be drawn from the specimen, while the microscope was operating with a probe current of 10 11 A. That these diYculties were successfully overcome is shown by the micrograph reproduced in Fig. 12. My last two research students were R. F. W. Pease, who came in 1960, and A. N. Broers in 1961. I chose their projects and started them on their work but, in 1960, I was elected to the Chair of Electrical Engineering and became Head of the Electrical Division in the Engineering Department. There were new laboratories to be built and new courses to be planned and it soon became clear that I should no longer have time for the detailed supervision of research students. Fortunately W. C. Nixon, who had worked for several years in V. E. Cosslett’s electron-microscopy group in the Cavendish Laboratory, had joined our staV in 1959 and I was able to hand over to him the supervision of Pease and Broers. Pease had been asked to design and build a new microscope in an attempt to obtain resolution close to the theoretical limit. By this time we had more money to spend on equipment and the new instrument was to have magnetic lenses and properly stabilized power supplies. Under Nixon’s guidance the resolution finally achieved by

FIG. 12. Surface of dispenser cathode (field of view measures 20 mm from left to right; Ahmed and Beck 1963).

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Pease was about 10 nm (Pease and Nixon 1965a). The instrument is still in use in the Department and, in the hands of a skilled operator, its performance bears comparison with that of modern microscopes, though the latter have adjuncts which make them very much easier to use. The project which I assigned to Broers was to take over Stewart’s apparatus and continue the investigation of sputtering. To this end he constructed a mass analyzer for the positive ions, so that the eVects produced by unwanted ions, particularly oxygen, could be removed. In the course of this work Broers noticed that, if the surface of the specimen were scanned for any length of time by the electron probe of the scanning microscope, the rate of subsequent sputtering was greatly reduced by the film of contamination that had been built up. Since this contamination is produced by decomposition of residual vapors by the electron probe, at the surface of the specimen, it can be deposited in any desired pattern. Nixon, who had now taken over the supervision of the project, encouraged Broers to study this eVect and to use it to produce extremely small structures. Thus a film of gold was evaporated onto a single-crystal silicon substrate and a pattern of contamination, in the form of a grid, was then deposited on the gold. Subsequent sputtering removed gold which had not been protected by contamination, as well as the pattern of contamination itself. It thus left on the silicon a grid of gold bars whose width was about 0.05 mm. This was one of the earliest successful attempts at microfabrication. Further work was carried out by Pease on the deposition of protective layers by the electron probe, using known pressures of particular organic vapors, rather than the residual vapor of pump oil which had been the most probable active constituent in Broers’ experiments (Broers 1965, Pease and Nixon 1965b). Broers also investigated the electronbeam exposure of photoresist (Fig. 13). The direction of our research was now changing. We were concerned less with the development of the microscope as an instrument and more with its adaptation to various fields of investigation. For the next few years this program was largely supervised by Nixon, but he was later joined on the teaching staV by Ahmed in 1963 and by Smith in 1965. Thus electron-optical research in the Department has continued to flourish, but this is another story and it is not my story. My account of the early history of the microscope would be sadly deficient without mention of one other person who has contributed so much. Leslie Peters joined me as a young assistant in 1946. Already a skilled instrument maker and photographer he soon became familiar with the details of our work. Year after year he has acted as mentor to successive generations of new research students, has helped them to modify designs so that it was possible to construct the required apparatus, and has then made it in a surprisingly short space of time. I, and I think they, would wish to pay

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FIG. 13. Grid wires of width 250 nm formed from 400 nm of gold-platinum alloy on a silicon slice; protected with photoresist; exposed with electron beam; developed; baked; and ion-etched (Broers 1965).

tribute to the tremendous help that he has so willingly given us and to rejoice that he is still carrying on this invaluable work. I must now go back in time to explain how the scanning microscope came to be manufactured commercially. The story begins in 1956 when Smith had just completed his Ph.D. research. The development of the instrument was at an exciting stage and I was anxious to retain Smith’s services for another year or two. There was, however, no post for him in the Department and his research grant had come to an end. At this point fate came to our aid in the guise of the Pulp and Paper Research Institute of Canada. An oYcer of the Institute, D. Atack had been spending sabbatical leave in Cambridge and had learned of Smith’s work. It seemed to him that the microscope would be a valuable tool for the research undertaken by the Institute and, at his request, Smith prepared a montage of micrographs of a spruce fiber (Fig. 14). This greatly impressed the President of the Institute, L. R. Thiesmeyer, who, on his next visit to England, called on us and expressed a wish to buy a scanning microscope. It was not immediately obvious how this request was to be met, since the microscope was not in commercial production. At an earlier stage, Associated Electrical Industries (A.E.I.) had given us some financial help and it was understood that, if the microscope appeared to be commercially viable, they would take it up. However, no

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FIG. 14. Montage of spruce fibers (field of view measures 750 mm from left to right; Atack and Smith 1956).

steps in this direction had yet been taken. It was finally agreed that a microscope for the Institute would be built in the Engineering Department, under Smith’s direction. To assist with the work, A.E.I. made available an obsolete EM 4 transmission microscope, from which lenses were taken. Since there was no shortage of money for the project, Smith was able to build a properly engineered instrument with magnetic lenses, which gave excellent results (Figs. 15 and 16). The work was completed in 1958 and the microscope was shipped to Canada by A.E.I. Smith also went to Canada to work at the Institute for two years (Smith 1959–1961). This microscope gave excellent service for about ten years, when it was replaced by a more modern instrument. It is now in the Canadian National Museum of Science in Ottawa. Following this episode, we hoped that regular commercial production of the microscope might take place but A.E.I. were not convinced that there was a real market for instruments of this kind and, at the time, there was no other company in England to which we could have turned. If A.E.I. are thought to have been lacking in foresight, the same criticism can be levelled against other firms throughout the world. Our results had been freely reported at conferences and published in the scientific journals, and there were no patents to deter manufacture of the microscope by

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FIG. 15. Microscope described by Smith (1960).

FIG. 16. Micrographs obtained with Smith’s microscope: (a) The simple eyes (ocelli) at the vertex of a fly’s head. (b) The compound eye of the same fly. (Fields of view measure 400 mm and 40 mm from left to right; by courtesy of A. Rezanowich and the Pulp and Paper Research Institute of Canada.)

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anyone. For whatever reason, there was a somewhat frustrating period of about six years when no one was interested and it is perhaps worthwhile to speculate why this should have been so. The first obvious point to make is that putting into manufacture an instrument of this complexity is an expensive operation. Apart from the work involved in settling the final design, detailed drawings have to be prepared, sales literature composed and arrangements made to service microscopes which have left the factory. Moreover, there will usually be other projects on the stocks to compete for limited eVort and finance and the choice between them will depend on estimates of future sales. In estimating the probable market for scanning microscopes, the crucial question was ‘‘what will it do that other instruments cannot do?’’ The answer generally given was that it was serving the same purpose as the transmission microscope with replicas and that, at no time, was the resolution obtained with the scanning microscope superior to that produced by replicas of appropriate specimens. What was overlooked was that there are many specimens of which replicas cannot be made and that replication is a time-consuming business. Finally, that the image obtained with a replica is a transmission image through a thin film, which does not begin to produce the striking threedimensional appearance obtained with the scanning microscope. All this is obvious today but, looking back, I can understand why industry did not jump at the opportunity of manufacturing scanning microscopes. The deadlock was broken in a rather roundabout way. In the Cavendish Laboratory, Cosslett was interested in the microanalysis of specimens using characteristic x-rays produced by a fine electron probe incident on a very small area of the specimen—an idea described in a patent by Hillier (1947) but first put into practice by Castaing and Guinier (1949). Mechanical scanning of the specimen had been tried but Cosslett, aware of our work on the microscope, thought it worthwhile to attempt electronic scanning. In 1953 the project was given to a new research student, P. Duncumb, who made excellent progress and eventually produced the scanning x-ray microanalyzer. When Duncumb left, he joined the research laboratory of Tube Investments Ltd. and there built a properly engineered version of his microanalyzer. Unlike the scanning microscope, the microanalyzer gained ready acceptance because it gave information which could not be obtained from any other instrument, and because this information was needed by a great many investigators. Arrangements were therefore made for Tube Investments to give details of their design to the Cambridge Instrument Company, which could manufacture the microanalyzer. S. A. Bergen, who was Chief Development Engineer of the Company, knew of our work and was persuaded by Nixon and Smith that it would be a

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good idea to manufacture the scanning microscope as well as the microanalyzer, particularly since the two instruments could have many components in common. Eventually, in 1961, A.E.I. agreed that my obligation to this Company had been discharged and that I was free to look elsewhere for production of the microscope. I was then able to approach H. C. Pritchard, the managing Director of the Cambridge Instrument Company and it was agreed that the Company would build a scanning microscope. Bergen immediately set to work and an experimental model was put together in about six months. It was displayed at the Institute of Physics and Physical Society Exhibition in January 1962 and aroused a good deal of interest. Shortly afterwards my research student, A. D. G. Stewart, joined the Instrument Company, at first to work on the microanalyzer, but later to take charge of all development work on the microscope. By late 1962 the Company had constructed a second prototype and, at this time, a firm order for a microscope was received from the DuPont Company in Canada. This Company knew of our work in the Engineering Department and had already been able to make use of Smith’s microscope at the Pulp and Paper Research Institute. It now decided that it wanted an instrument of its own. Another potential purchaser was J. Sikorski who, since 1958, had been strongly advocating the use of the scanning microscope for research on fibers (Sikorski 1960). However, the Instrument Company had no real idea of the market for a scanning microscope and did not feel that it could go into production on the basis of one firm order. There was therefore a delay while the situation was being assessed and this delay was the more protracted because development eVort in the Company had to be split between the microscope and the microanalyzer which, at that time, seemed likely to be the more profitable product. During this period Pritchard approached the Government Department of Scientific and Industrial Research and obtained an assurance that, if the microscope came on the market, a reasonable number of grants would be made available to those British universities which wished to purchase models. The Company then decided to make a batch of five microscopes to test the market and these became available in 1965 (Fig. 17). Meanwhile, the second prototype had been sold in 1964 to du Pont, which had been pressing for delivery. Many people contributed to this satisfactory outcome, but it is appropriate to mention particularly the foresight of Pritchard, the drive of Bergen and the great technical skills of Stewart. The first four production models, sold under the trade name ‘‘Stereoscan,’’ were delivered respectively to P. R. Thornton of the University of North Wales, Bangor, to J. Sikorski of Leeds University, to G. E. PfeVerkorn of the University of Mu¨nster, and to the Central Electricity Research

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FIG. 17. Cambridge Instrument Company ‘‘Stereoscan’’ Mk 1 prototype (Stewart and Snelling 1965).

Laboratories. By this time the Company had launched a publicity campaign and orders began to roll in. An additional batch of twelve microscopes was put in hand; and then a further forty . . . . . . The scanning microscope had come of age.

References H. Ahmed and A. H. W. Beck, (1963): ‘‘Thermionic emission from dispenser cathodes,’’ J. Appl. Phys. 34, 997–998. M. von Ardenne, (1938a): ‘‘The scanning electron microscope: Theoretical fundamentals’’ (in German), Z. Phys. 109, 553–572. M. von Ardenne, (1938b): ‘‘The scanning electron microscope: Practical construction’’ (in German), Z. Tech. Phys. 19, 407–416. M. von Ardenne, (1978): ‘‘The history of scanning electron microscopy and of the electron microprobe’’ (in German with English abstract), Optik 50, 177–188. D. Atack and K. C. A. Smith, (1956): ‘‘The scanning electron microscope. A new tool in fiber technology,’’ Pulp Pap. Mag. Can. (Convention issue) 57, 245–251. A. S. Baxter, (1949): ‘‘Detection and analysis of low energy disintegration particles.’’ Ph.D. Dissertation, Cambridge Univ., England. C. Brachet, (1946): ‘‘Note on the resolution of the scanning electron microscope’’ (in French), Bull. Assoc. Tech. Marit. Aeronaut, No. 45, 369–378.

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A. N. Broers, (1965a): ‘‘Combined electron and ion beam processes for microcircuits,’’ Microelectron. Reliab. 4, 103–104 and 1 plate. A. N. Broers, (1965b): ‘‘Micromachining by sputtering through a mask of contamination laid down by an electron beam,’’ First International Conference on Electron and Ion Beam Science and Technology, edited by R. A. Bakish. 191–204. H. Busch, (1926): ‘‘Calculation of the path of cathode rays in the axially symmetric electromagnetic field’’ (in German). Ann. Phys. 81, 974–993. R. Castaing and A. Guinier, (1949): ‘‘Application of electron probes to metallographic analysis’’ (in French), in Proceedings of the 1st International Conference on Electron Microscopy, Delft, pp. 60–63. T. E. Everhart, (1958): ‘‘Contrast formation in the scanning electron microscope.’’ Ph.D. Dissertation, Cambridge Univ., England. T. E. Everhart, O. C. Wells, and C. W. Oatley, (1959): ‘‘Factors aVecting contrast and resolution in the scanning electron microscope,’’ J. Electron. Control 7, 97–111. T. E. Everhart, (1960): ‘‘Simple theory concerning the reflection of electrons from solids,’’ J. Appl. Phys. 31, 1483–1490. T. E. Everhart, K. C. A. Smith, O. C. Wells, and C. W. Oatley, (1960): ‘‘Recent developments in scanning electron microscopy,’’ in Proceedings of the Fourth International Conference on Electron Microscopy, Berlin, Sept. 1958. Physics, Vol. 1, edited by W. Bargmann, et al. (Springer Verlag, Berlin, 1960), pp. 269–273. T. E. Everhart and R. F. M. Thornley, (1960): ‘‘Wide-band detector for micro-microampere low-energy electron currents,’’ J. Sci. Instrum. 37, 246–248. J. Hillier, (1947): ‘‘Electron probe analysis employing x-ray spectrography,’’ U. S. Patent 2 418 029. M. Knoll, (1935): ‘‘Static potential and secondary emission of bodies under electron irradiation’’ (in German), Z. Tech. Phys. 11, 467–475. M. Knoll and R. Theile, (1939): ‘‘Scanning electron microscope for determining the topography of surfaces and thin layers’’ (in German), Z. Phys. 113, 260–280. J. H. L. McAuslan and K. C. A. Smith, (1956): ‘‘The direct observation in the scanning electron microscope of chemical reactions,’’ in Electron Microscopy: Proceedings of the Stockholm Conference, Sept. 1956, edited by F. S. Sjostrand and J. Rhodin (Academic, New York 1957), pp. 343–345. D. McMullan, (1953a): ‘‘An improved scanning electron microscope for opaque specimens,’’ Proc. IEE II 100, 245–259. D. McMullan, (1953b): ‘‘The scanning electron microscope and the electron-optical examination of surfaces,’’ Electron. Eng. (England), 25, 46–50. C. W. Oatley and T. E. Everhart, (1957): ‘‘The examination of p–n junctions with the scanning electron microscope,’’ J. Electron. 2, 568–570 and 1 plate. C. W. Oatley, W. C. Nixon, and R. F. W. Pease, (1965): ‘‘Scanning electron microscopy,’’ Adv. Electron. Electron Phys. 21, 181–247. C. W. Oatley, (1969): ‘‘Isolation of potential contrast in the scanning electron microscope,’’ J. Sci. Instrum. (J. Phys. E), 2, 742–744. C. W. Oatley, (1972): ‘‘The scanning electron microscope. Part I. The instrument.’’ (Cambridge University Press, England). C. W. Oatley, (1981): ‘‘Detectors for scanning electron microscope,’’ J. Phys. E: Sci. Instrum. 14, 971–976. R. F. W. Pease and W. C. Nixon, (1965a): ‘‘High resolution scanning electron microscopy,’’ J. Sci. Instrum. 42, 31–35. R. F. W. Pease and W. C. Nixon, (1965b): ‘‘Microformation of filaments,’’ in First International Conference on Electron and Ion Beam Science and Technology, Toronto, May 1964, edited by R. A. Bakish (Wiley, New York), pp. 220–230.

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K. F. Sander and J. G. Yates, (1953): ‘‘The accurate mapping of electric fields in an electrolytic tank,’’ Proc. IEE II 100, pp. 167–183. J. Sikorski, (1960): ‘‘Studies of fibrous structures,’’ in Proceedings of the Fourth International Conference on Electron Microscopy, Berlin Sept. 1958. Physics Vol. 1, edited by W. Bargmann et al. (Springer Verlag, Berlin, 1960) pp. 686–707. K. C. A. Smith and C. W. Oatley, (1955): ‘‘The scanning electron microscope and its fields of application,’’ Br. J. Appl. Phys. 6, 391–399. K. C. A. Smith, (1956): The scanning electron microscope and its fields of application, (Ph.D. dissertation, Cambridge Univ., England). K. C. A. Smith, (1959): ‘‘Scanning electron microscopy in pulp and paper research,’’ Pulp Pap. Mag. Can. 60, T366–T371. K. C. A. Smith, (1960): ‘‘A versatile scanning electron microscope,’’ in Proceedings of the European Regional Conference on Electron Microscopy, Delft 1960, Vol. 1, edited by A. L. Houwink and B. J. Spit (De Nederlandse Vereniging Voor Electronenmicroscopie, Delft, 1961), pp. 177–180. K. C. A. Smith, (1961): ‘‘Scanning,’’ in Encyclopedia of Microscopy, edited by G. L. Clark (Reinhold, New York), pp. 241–251. A. D. G. Stewart and M. A. Snelling, (1965): ‘‘A new scanning electron microscope,’’ in Electron Microscopy: Proceedings of the Third European Regional Conference, Prague, Sept. 1964, edited by M. Titlebach (Czechoslovak Academy of Sciences, Prague), pp. 55–56. R. F. M. Thornley, (1960): ‘‘Recent developments in scanning electron microscopy,’’ in Proceedings of the European Regional Conference on Electron Microscopy, Delft 1960, Vol. 1, edited by A. L. Houwink and B. J. Spit, (De Nederlandse Vereniging Voor Electronenmicroscopie, Delft, 1961), pp. 173–176. R. F. M. Thornley and L. Cartz, (1962): ‘‘Direct examination of ceramic surfaces with the scanning electron microscope,’’ J. Am. Ceram. Soc. 45, 425–428. O. C. Wells, (1959): ‘‘Examination of nylon spinneret holes by scanning electron microscopy,’’ J. Electron. Control 7, 373–376. O. C. Wells, (1960): ‘‘Correction of errors in stereomicroscopy,’’ Br. J. Appl. Phys. 11, 199–201. V. K. Zworykin, J. Hillier, and R. L. Snyder, (1942): ‘‘A scanning electron microscope,’’ ASTM Bull. No. 117, pp. 15–23.

PART II THE SCANNING ELECTRON MICROSCOPE AT THE CAMBRIDGE UNIVERSITY ENGINEERING DEPARTMENT

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2.1A The Development of the First Cambridge Scanning Electron Microscope, 1948–1953 D. McMULLAN Cavendish Laboratory, University of Cambridge Formerly at: Engineering Department, University of Cambridge

I. Introduction The previous chapters describe how Charles Oatley returned to Cambridge in 1945 as a Fellow of Trinity College and Lecturer in the Engineering Department and how he decided that the scanning electron microscope would be a suitable topic for a research student in the Electronics Laboratory of the department. I was lucky enough to be chosen by him for this project. I had studied in the department during the war and took the Mechanical Sciences Tripos examination in 1943 after a two-year course. I was then directed into industry and spent the next three years in London working in the design laboratory of Bush Radio Ltd, a company of the Rank Organisation. Here I was initiated into the secrets of radar, as the main activity was the design and manufacture of airborne centimetric radar sets for the Fleet Air Arm. After the war, I moved to another Rank company, Cinema-Television Ltd (later Rank Cintel), which was engaged in the development of cathode-ray tubes for TV film scanners and for large-screen projection in cinemas; this provided me with excellent experience in highvacuum technology. I moved again in 1948, to the Sperry Gyroscope Company to work on analog computers but by then I was not so keen on a life in industry and was anxious to return to university and take a research degree. However, enquiries to the Ministry of Education about obtaining a grant were not encouraging: five years had elapsed since I graduated and in any case I had been awarded only second-class honours.

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A. Charles Oatley It was only by chance that I was put in touch with Oatley. My college at Cambridge was Clare, and in June 1948 the Clare Association held a dinner in London, which I decided to attend. The Master of the College, Sir Henry Thirkill, was present and I ventured to ask him whether there was any possibility of returning to Cambridge to do a PhD. He immediately said that Charles Oatley at the Engineering Department was looking for students and suggested that I contact him. This I did and went for interview the following week. My main worry at the interview, having read the University regulations governing courses in research, was that I would be asked to propose the subject of the research and I had no idea of anything concrete that might be suitable. I was soon relieved of this naive belief when, after a preliminary discussion of my work in industry, Oatley suggested that I should work on developing a scanning electron microscope (SEM). Knowing nothing of electron microscopy except having seen the first RCA Type B transmission electron microscope (TEM) at the Cavendish in 1943, my immediate (private) reaction from experience with cathode-ray tubes, where one was having diYculty in getting spot sizes of the order of tens of micrometres, was that it would be an impossible job. Later I was much encouraged by reading Denis Gabor’s 1946 monograph The Electron Microscope (ref. 2 )1 which included a short section on the work of Zworykin, Hillier, and Snyder at RCA (mentioned by Oatley in Chapter 1.2) and which was fairly optimistic about its possibilities. I do not recollect that there was any detailed discussion of the project except that Oatley told me that K. F. Sander, who was then a research student in the laboratory and was about to become a Demonstrator, had started to build a two-stage electrostatically focused TEM but had abandoned it. Instead he was working on an electron trajectory plotter using the Bush mechanical diVerential analyser in the Mathematics Faculty. The completed parts of the microscope would be available for me to use if I wished. He also told me of an electron multiplier that had been developed in the Cavendish by A. S. Baxter (1949), which he believed would be very suitable for the SEM because low signal-to-noise ratio had limited the performance of earlier instruments. In addition, he pointed out the possibility of using slow-scan radar-type displays, with which I was very familiar. There was no possibility of obtaining a grant, but again I was lucky because my father was able and willing to support me for the three years it 1

The asterisk indicates references, sections and figures in the paper ‘An improved scanning electron microscope for opaque specimens’ that is reproduced as Chapter 2.1B.

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would take; Oatley said that I could start that October. I have at times wondered why Thirkill was in such close touch with Oatley who was a fellow of Trinity and only recently appointed, but there is a simple answer. Thirkill was a Lecturer at the Cavendish in the 1920s when Oatley was an undergraduate there, and with Edward Appleton (Oatley’s college supervisor), organized the advanced practical class.

B. Oatley’s Electronics Laboratory I started at the Engineering Department early in October 1948, initially under the supervision of Sander but very soon under Oatley himself, and immediately set about completing the TEM. Sander had designed some of the component parts, which Leslie Peters, a young technician in the laboratory had then made; major items included the microscope column and the specimen stage. Sander had also settled the geometry of the electron lenses, which was based on work by Bru¨ck and Romani (ref. 30 ), and the magnification was to be fixed at 10 000 times; there seemed to be no reason to change these design parameters. There was much work to be done since virtually everything had to be made in-house, including such items as high-voltage power supplies and vacuum gauges. However, as described in Chapter 1.2, Oatley had organized the laboratory in such a way as to provide ideal conditions for this kind of work: one wall, some 25 feet long and 10 feet high, had been fitted with shelving and stocked with a vast variety and quantity of exgovernment electronic components. The shelves were screened from the rest of the room by a wire mesh, known as ‘the cage’, but the door to this was kept unlocked during working hours and virtually every kind of component needed was immediately to hand—although not always of the exact type one would have chosen, and sometimes rather obsolete devices were pressed into service. If what was required was not there, one had to make it because there was very little money available for outside purchases. I remember using a bicycle wheel to twist up long lengths of 55 strands of 38 SWG wire to make litz cable for the radiofrequency transformer of the 40-kV highvoltage supply. However, I believe that these arrangements were one of the reasons why it was possible to build a lot of apparatus in a rather short time. We were also extremely fortunate in the technical support provided in the laboratory by Les Peters and Phil Woodman, and in the main workshop by the Superintendent, J. H. Brooks, and his deputy, A. A. K. Barker. Unfortunately, Peters was not available to continue the construction of the microscope because he was needed for other projects, but V. Claydon in the main

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workshop, working from the most elementary kind of sketches, was allocated full-time to make most of the mechanical components. There were also other factors that contributed to the eYciency of the laboratory but which at the time seemed to be impediments. Oatley insisted that working hours must be limited to the times that the technical staV were present: 8.30 a.m. to 6.00 p.m., Saturday mornings, and no Sundays. This was certainly very diVerent from the popular conception of the university researcher working through the night and also from the practice in other laboratories of the university both then and now. However, I believe that this rule (which was presumably enforced mainly for safety reasons) enabled one to keep a balance between thought and action, to plan in detail what to do the next day, and get down to it immediately without distraction. An 8.30 a.m. start was, though, practically unknown—research students then were no better at getting up in the morning than they are now! A more irksome restriction was that equipment was not allowed to be left running overnight; this was extremely frustrating when one was trying to obtain a good vacuum, as anyone who has worked with such systems (particularly of that vintage) will appreciate. This ordinance was brought in because, some time previously, the laboratory had been flooded by a broken water pipe, but no doubt fire was an even more serious hazard. Electron microscopy was only one of several research projects in Oatley’s laboratory. When I arrived there were six research students: J. E. Curran who was in his final year, working on the generation of microwaves of <1 cm wavelength; D. M. Taub and I. M. Ross (later President of Bell Telephone Laboratories), who were studying noise in thermionic valves; H. I. Pizer, who continued Sander’s work on electron trajectory plotting and built a relay computer; I. Lowe, whose subject was EBIC in diamond and other crystals; and J. B. Gunn (later famous as the discoverer of the Gunn eVect), who stayed for only one year. Another contemporary of mine was C. W. B. Grigson, who was building an electron diVraction camera (see Chapter 2.11). The academic staV in the Electronics Department included L. B. Turner (Reader), who was on the point of retirement, and J. G. Yates (Lecturer) who had been with Oatley at the Radar Research and Development Establishment, Malvern, and who was to die in 1957 at a tragically early age. All the research was done in two large rooms in a part of the Engineering Department overlooking Coe Fen (Fig. 1), which had great advantages for the interchange of advice and helped to create a happy and lively atmosphere. But as far as scanning electron microscopy was concerned, the message from outside was, at the best, scepticism summed up by the phrase ‘gifted amateurs’. Oatley has written (Oatley et al., 1985) that in 1948 ‘several experts expressed the view that [the construction of a SEM] would be a complete waste of time’ and I remember Professor Otto Klemperer of

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Figure 1. The Engineering Department building facing Coe Fen (demolished and rebuilt in the 1960s). The windows of the laboratory where SEM1 was constructed are between the trees on the left.

Imperial College, one of the leading authorities on electron optics, asking me, when I was showing him around the laboratory (in Oatley’s absence and before the SEM was working), ‘Why on earth have you resurrected that old idea?’ It was diYcult to make a convincing answer as at that time it was far from clear how we could hope to approach what Zworykin and his brilliant team (including James Hillier) at RCA had accomplished a decade earlier. I must record that there was an exception to this general attitude: V. E. Cosslett and his group at the Cavendish were always very helpful, although I feel sure that they did not believe, at that time justifiably, that we would accomplish anything useful.

II. Completing the TEM The plan of action was for the electrostatically focused TEM to be completed and then converted into a SEM. Little or no thought was given at this stage to the question of how such a SEM would diVer from earlier instruments (apart from the use of the Baxter electron multiplier), but the requirements of the future SEM were kept in mind. For example, the voltage stability needed for an electrostatically focused TEM with einzel lenses is

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not critical for voltages below about 50 kV, but for a SEM a relatively high degree of stabilization is essential. I started by building a 40-kV stabilized power supply, which had a voltage quadrupler fed from an 80-kHz oscillator and power amplifier, with a feedback stabilizer circuit using a potential divider. The gun filament current, the voltage for biasing the gun Wehnelt electrode, and a voltage for adjusting the focus of the objective einzel lens were generated by supplies floating at the full negative accelerating potential. The power for these was fed in through radiofrequency transformers working at 250 kHz. After some experimentation, the gun designed by Sander was found to have too large a beam angle and hence low current density at the specimen (there was no condenser lens); the geometry was therefore changed to that shown in Fig. 6 . This was based on papers by Grivet (ref 31 ) and Bru¨ck and Bricka (ref. 29 ), the developers of the CSF electrostatically focused microscope. The specimen stage, which was of course just above the TEM objective lens, is not shown in Fig. 6 . It was a simple but eVective arrangement designed by Sander, with the block holding the specimen mounted on flat springs that constrained the block to X and Y parallel motions. As already mentioned, the electrostatic lenses were based on data published by Bru¨ck and Romani (ref. 30 ). The lens is shown in Fig. 6 . Much time was spent in trying to prevent electrical breakdown across the surface of the insulators: various materials (glass, Perspex, porcelain, etc.) and surface treatments (e.g. ferric oxide) were tried, but 35 kV was the highest voltage that could be used reliably. The original brass electrodes were changed to aluminium with beneficial results; stainless steel was ruled out because it was said to be ‘diYcult to machine’, which seems strange nowadays. Looking back, the trouble was that the diameter of the column was rather small and the space for the electrodes and the insulators was too constricted; consequently, the potential gradient across the insulators was nonuniform, leading to breakdown. In a separate test on a porcelain insulator between parallel electrodes, the voltage had been taken to 45 kV without any sign of breakdown; one wonders why one did not appreciate the cause of the trouble at the time. The projector lens had the same geometry as the objective, and an intermediate phosphor screen was mounted above it. The final electron image was observed on a phosphor screen mounted on the lid of the recording 35mm film holder. All very conventional, but subject to disturbing eVects, such as electrical discharges across the film, that were not mentioned in the literature of the time. Micrographs of shadowed polystyrene latex particles were obtained early in 1950, which confirmed that the magnification was the expected 10 000

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˚ . The latter was limited by the astigtimes; the resolution was about 200 A matism of the objective lens owing to ellipticity of the central holes in the lens electrodes, and considerable eVorts were made by Claydon to reduce the ellipticity by lapping. The solution would of course have been to incorporate a stigmator, but the development of these was still at an early stage. For us, improving the TEM performance was not a priority and all eVorts had to be devoted to converting the microscope into a scanning instrument.

III. Converting the TEM to a SEM In about February 1950, work commenced on converting the TEM to a SEM. Two major developments were needed: alterations to the column so that a specimen could be placed immediately below the projector lens (which would then become the objective lens); and the design and construction of electronic circuits for driving scan coils, amplifying signals from the electron multiplier, and displaying an image on a monitor. I was still unclear how the specimen should be placed relative to the lens, but it was obviously going to be a problem because of the very short working distance of the lens (1 mm) which would make the collection of the signal electrons diYcult. A new specimen chamber was built containing the objective lens, deflection coils and the specimen stage, and mounted in place of the TEM phosphor screen and camera (Fig. 6 ); the electron multiplier was attached below the chamber and could be pulled down for changing the specimen. A photograph of the microscope is shown in Fig. 2. The scanning circuits are described in Section 3.8 ; Woodman gave invaluable assistance in the construction of this equipment and suYcient of it was ready by August 1950 for initial tests. A. Initial Failure The eight-stage electron multiplier lent to us by Baxter was mounted and, with a transparent specimen in position, switched on. It quickly became evident that its gain was very low: in fact, I measured it to be only 100 (gain per stage ¼ 1.78). There followed a rather fraught period: had I damaged the multiplier in some way? was the vacuum inadequate so causing electrodes to be poisoned? or was it something silly that I had overlooked? Baxter took it back and said he would let us have another, but at the time there was never any explanation of what might be wrong with it. It later became clear that there was an insuYcient number of stages; the replacement, when it arrived

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Figure 2. The first SEM adapted from the two-stage electrostatically focused TEM (1951). The column had not yet been shortened. (McMullan, 1952.)

several months later, had 16. It had only a slightly higher gain per stage (2.05) but a total gain of about 105. In the meanwhile, an obvious detector to try was a phosphor screen and photomultiplier, as had been used by Zworykin et al. With zinc sulphide the gain was over 105, and our spirits rose, only to be dashed the next day when the long decay time became evident. ZnS[Ni] was then being used successfully in TV flying-spot scanners (decay time <1 ms) and it was some time

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before low current density was identified as the cause (ref. 25 ). Unfortunately, I did not appreciate the significance of the diVerence between mono- and bi-molecular decays, and did not pursue the search for alternative phosphors or scintillators. (Actually, Zworykin’s microscope must also have been limited by this eVect, but as he did not obtain a directly viewed image and recorded for 10 minutes using a facsimile machine, it would not have been apparent.) Whether I would have found a suitable alternative phosphor at that time I do not know, but if successful it would have certainly led to a more rapid development of the SEM. Instead, for the following six months or so, while waiting for the new multiplier, I switched to building a compact TEM using part electron optical magnification (200) and part optical (50). This arose out of some work Oatley was himself doing on the electrical properties of evaporated zinc sulphide films: these showed an extremely fine grain structure under electron bombardment and it seemed worthwhile to try them as the phosphor screen of a TEM. Partly as an insurance against the failure of the SEM project (I was in my third, and what I thought was my last, year), the compact TEM was built and produced some rather low-resolution images (a few hundred angstroms). These were even poorer when recorded on Kodak maximum resolution plates, covered necessarily with aluminium foil to prevent charging due to the high current density in the small electron image. In retrospect it is probable that I would have better used the time for the investigation of the properties of available phosphors. To my great relief, in the spring of 1951 Baxter supplied us with the 16-stage electron multiplier and it was then possible to continue with the SEM project.

B. Success Initially, some tests were made with transparent specimens, using the microscope as a STEM. This avoided the knotty problem of how surfaces were to be imaged. I had given a lot of thought to this and felt that the main lesson apparent from Zworykin’s work in 1942 was that a high-energy beam should be used. The results achieved by Zworykin et al. (refs. 1 , 15 ) were not very good, the main defects being a limited contrast range and poor signal-tonoise ratio in the micrographs. The reasons for these deficiencies were not identified by the authors, but the most likely explanation was the low incident electron energy, 800 eV, which apart from other disadvantages limited the beam current. According to Zworykin et al. (ref. 1 ), ‘this relatively low voltage is favourable for obtaining a large diVerence in the secondary emission currents from diVerent parts of the object’. However, since an untrapped oil diVusion pump was being used, the surfaces of the

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specimens must have been heavily contaminated and no meaningful Z contrast could have been expected. Indeed, only a few years previously, Bruining and de Boer (ref. 19 ) had demonstrated conclusively that the emission ratio with low-energy primaries depends on the state of the surface. Actually, all the micrographs in Zworykin et al. (ref. 15 ) were of etched or abraded specimens, contrast being produced by the topography of the surfaces. The authors did not comment on this fact. Incidentally, Knoll (ref. 12 ) was no doubt successful in obtaining true secondary electron images because he used sealed-oV glass envelopes which could be baked to give a good vacuum; but he did discuss whether contrast was being produced by adsorbed layers of gas rather than by the underlying metal. Even comparatively recently, Howie (1995) wrote that ‘there is continuing scope for improved vacuum conditions and surface cleanliness if the full potential of SE imaging in electron microscopy is to be realized’. It therefore seemed essential that a high incident electron energy should be used, although it was unclear how the contrast in the image would be produced; I thought that back scattering from a normally incident beam would not have a large enough variation. The way ahead occurred to me during a seminar at the Cavendish by James Menter (Menter, 1952) of the Cambridge Department of Physical Chemistry. He described the work he  was doing on imaging metal surfaces at grazing incidence (1 ) in a TEM, the  method first used by von Borries (ref. 7 ) in 1940. It then seemed obvious that, in a SEM, if the surface of the specimen were placed at an angle of, say,  45 to the beam, the elongation of the image would be much reduced with little loss of resolution because the back-scattered electrons do not have to be focused. This arrangement also circumvented the problem of the short working distance of the electrostatic lens and the obstruction of signal electrons if the specimen were placed normal to the beam. The first SEM specimen to be imaged in this way was the surface of a piece of mild steel that had been roughly ground. It was mounted in the microscope at 30 to the beam axis, as shown in Fig. 6 . The slow-scan time-base had not yet been completed and the micrograph in Fig. 3 is of the visible image taken using the afterglow of the monitor screen. This result, although poor by any standard, was most encouraging. The potential of the microscope became evident later when an etched surface of aluminium was imaged (Fig. 4), which showed the three-dimensional impression that is the hallmark of a SEM micrograph. As can be seen in Fig. 6 , the arrangement for getting the back-scattered electrons from the specimen surface to the electron multiplier was very rudimentary. Plainly the multiplier needed to be moved so that the back-scattered high-energy electrons entered it unimpeded, but this would have involved major alterations to the specimen chamber and I was nearing the end of my third year;

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Figure 3. The first image recorded of the surface of a metal specimen: roughly ground  mild steel mounted at 30 to the beam axis; beam energy 25 keV; current 1.5  10 10 A; horizontal field width ¼ 40 mm. (McMullan, 1952.)

obtaining more results from the existing system and writing-up took precedence. The low-energy secondary electrons would also have been detected, but at the time I had the erroneous belief, based on Zworykin’s results, that they would be deleterious to the image. This was another reason why I did not make the necessary alteration by building a completely new specimen chamber that could have later replaced the original one. In Chapter 2.2A, Ken Smith describes this important step and how it immediately brought a very large improvement in the performance of the microscope.

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Figure 4. One of the early images (etched aluminium surface) produced with SEM1.  Angle of incidence of 16 keV electrons 25 : (a) visible image, 0.95 frames/s; beam current 10 A; horizontal field width ¼ 37 mm. (b) 5 min recording; 10 13 A; horizontal field 1.5  10 width ¼ 13 mm (McMullan, 1952.)

C. Elucidation of Contrast Mechanisms After obtaining the images of etched aluminium, the most important task seemed to me to be the elucidation of the formation of contrast with a specimen at an angle to the beam. It was clear that it was mainly topographic, but we needed to know the variation of electron emission with angle and with atomic number: measurements were made on a scaled-up model outside the microscope, see Fig. 11 , and Appendices 8 . Figure 5 is reproduced from my dissertation (McMullan, 1952), Chapter 6, and the following extract describes it. Although the formation of contrast with oblique specimens in the scanning electron microscope has been briefly discussed . . . it is interesting to compare it with the optical microscope and with replica techniques. In the Figure, (a) represents the cross-section of a metal specimen with large irregularities but otherwise smooth and polished. In (b) is shown the brightness in diVerent parts of the image formed by an optical microscope perpendicular  to the surface which is lit at an angle of 30 from direction A. If the optical  microscope was also at an angle of 30 (in direction B), and the specimen lit  by a source subtending an angle A1A2 (30 ), the brightness variations would be as in (c). The variation in image brightness with the scanning electron microscope is  shown in (d). The scanning beam is at an angle of 30 to the specimen (from direction B) and the electrons are collected from the specimen over the angle

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Figure 5. Elucidation of contrast from an angled specimen (McMullan, 1952.)

A1A2. As can be seen, the variations in brightness are almost identical to those in (c), and it can be said that the image is of the same form as would be  obtained if the specimen were lighted at an angle of 30 and viewed from  the other direction at 30 . The micrographs . . . confirm this. It should be noted that the graphs (b), (c) and (d) are only approximate and that the sloping portions are not necessarily linear. (e) represents the brightness variations using a negative replica.

In the following chapter, I looked forward to the possibilities of improving the resolution:

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It should be possible, with modern magnetic lenses, to obtain a spot diameter of the same order as the resolution of the conventional transmission electron microscope. With oblique scanning, as a result of the penetration of the electrons into the specimen it is not certain whether the resolving power of the scanning microscope can be as high as the spot diameter if this is of the order of tens of Angstrom units. The penetration of course decreases with the reduction of the accelerating voltage and it may be necessary to work with very low voltages to obtain high resolution. DiYculty may then be experienced with the contamination of the specimen by oil vapour. The results already obtained provide some indication of the resolutions that may be obtained with various metals and accelerating voltages.

There followed some estimates of the depth of electron penetration and the eVect on resolution and it was concluded that, if this is the limiting factor, ˚ the accelerating voltage would have to be lowered to 7 kV to resolve 100 A with an aluminium specimen (see also Chapter 2.1B, p. 80). In practice, it is doubtful whether it would be necessary to work at such a low voltage since the bean from the specimen could be restricted to the electrons which have lost only small amounts of energy and which have travelled only small distances through the specimen.

It was not possible then to try out this idea because there was no space between the specimen and the electron multiplier for an energy filter, and in any case I was having to write my dissertation and would not have had the time. However, about 20 years later, it was taken up by Oliver Wells at IBM, Yorktown Heights, and he published the first paper (Wells, 1971) on low-loss electron imaging. This was followed by many others (e.g. Wells, 1986). When considering the ways in which contrast can be formed in a SEM using high-energy back-scattered electrons, it was evident from a curve published by Palluel (Fig. 2 ) that atomic number contrast might be used. However, the back-scattering measurements I had made (Fig. 3 ) indicated  that with the surface at 25 to the beam there was only a small diVerence between aluminium (Z ¼ 13) and tungsten (Z ¼ 74). With normal beam incidence, Palluel’s curve showed that the emission ratio for tungsten is over 3 times higher than for aluminium. Because of the small working distance of the objective lens, only a minute signal could be obtained from a specimen mounted normal to the scanning beam. With a large beam current, and hence large probe, images could be recorded, see for example Fig. 6. I did not think of trying a specimen with areas of two diVerent metals to demonstrate Z contrast, albeit at low resolution. This was done some years later by Oliver Wells with a specimen of silver and brass (see Fig. 19 in Oatley et al., 1985). During the period when an electron multiplier had not been available I did some calculations on the signal-to-noise ratio required in a scanning electron

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Figure 6. SEM image of a polished mild steel specimen mounted normal to scanning beam; 15 keV; frame period 300s; horizontal field width ¼ 54 mm. (McMullan, 1952.)

microscope; these were based on a recently published analysis of television pick-up tubes by Rose (ref. 23 ). They confirmed that, with the expected beam currents, low-noise visible images and recordings should be obtainable in reasonable times. This is described in Sections 2.5 , 3.6 and 3.7 . D. Further Improvements My three years as a research student had come to an end in September 1951, but Oatley applied for a Senior Maintenance Grant from the Department of Scientific and Industrial Research and this was awarded to me starting in

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October. I was allowed to spend the first half of 1952 mainly writing my dissertation and I submitted it in July. During this time I made some improvements including shortening the column and increasing its diameter to accommodate additional magnetic shielding and also a specimen airlock. Beam deflection blanking was also introduced at this time. The next step should have been the extensive rebuilding of the specimen chamber (mentioned above) to enable the electron multiplier to be positioned so that a larger proportion of the back-scattered electrons could be collected, but there was now only about a year remaining before I would have to leave Cambridge and I felt that there was not suYcient time for me to do this. Also Ken Smith was starting as my successor in October 1952 with a year’s overlap and it was going to be for him to take the whole project forward when I left. The year was spent by him becoming acquainted with what had been done and making plans; he also introduced certain improvements including a new interface to the electron multiplier output.

E. Publication In the autumn of 1951 Oatley urged me to write a paper for publication in the Proceedings of the Institution of Electrical Engineers; this was submitted in December, and was subjected to very rigorous reviewing, presumably by electron microscopists who took exception to the original title ‘A new scanning electron microscope . . .’. It is the final version with the revised title that is printed as Chapter 2.1B. Oatley gave me a lot of advice on the writing but he was adamant that he should not be a co-author: he had an inflexible principle that only those who had made a major contribution should appear on publications. The fact that he had proposed the project in the first place, and made it possible, was not in his view suYcient. A few months before I read the paper at the IEE I attended a conference in the H. H. Wills Physics Laboratory, University of Bristol, organized by the Electron Microscopy Group of the Institute of Physics (16–19 September 1952), and gave a talk about the SEM work in the Engineering Department. It did not seem to arouse much interest among the audience but Ellis Cosslett included a quite detailed summary in his report of the meeting published in Nature (Cosslett, 1952). He wrote: ‘The new instrument is a great advance both in resolution and lack of damage to the specimen. . . . Both methods [reflection in TEM, and the SEM] open up new prospects of high resolution microscopy, especially for metallurgy, and the full exploration of their potentialities will be awaited with great interest.’ Actually, it so happened that Cosslett was the external examiner for my Ph.D. exam in that October so he must have seen my Dissertation at about that time.

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In January 1953 I read the IEE paper to an audience in London that included a number of leading electron physicists: the report of their comments following the paper makes interesting reading. I also wrote an article for Electronic Engineering (McMullan, 1953b); the editor, H. G. Foster, had been an enthusiastic supporter and had taken the trouble to visit Cambridge to see the SEM. For this article the Radio Industry Council gave me their 1953 Award for Technical Writing.

F. Move to Scroope House and Smith Takes Over At the end of 1952 the part of the laboratory where the microscope was situated was needed for other projects, but a room was available in Scroope House, which had been the administrative centre of the Engineering Department before the opening of a new building the previous year. The microscope had therefore to be moved and I took the opportunity to tidy everything up, but without making any really major changes. Figure 7 shows the microscope after the move. At about this time I also introduced

Figure 7. Photograph of SEM1 taken in 1953 after move to Scroope House.

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double-deflection scan coils (Oatley et al., 1985), something that I thought was original, but later it turned out that it had been anticipated by von Ardenne in a patent specification (von Ardenne, 1937). My purpose in making this innovation was to reduce the build-up of insulating layers in the region immediately before the objective aperture. The beam angle and total beam current could be much reduced and drift due to electrostatic charging was no longer a problem. For reasons I do not remember, but probably because Smith was working on the electron multiplier signal chain, I decided to look at some phosphor crystals in the SEM using a photomultiplier as the detector. The crystals were the same ZnS[Ni] that I had used unsuccessfully as an electron detector two years earlier. Because of the high current density in the scanning probe there was now no diYculty with afterglow and I produced the first SEM cathodoluminescence images (Fig. 8). It was now time for me to leave the Engineering Department. There was still no interest in scanning electron microscopy in industry, or indeed anywhere else; I was therefore rather discouraged and did not write up the work I had done since the IEE paper. It was plain that I would have to switch to another field and in fact I went to Canada and worked on air-to-air

Figure 8. Cathodoluminescence image of zinc sulphide crystals taken in 1953 (McMullan, unpublished; Smith and Oatley, 1955).

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Figure 9. SEM1 replica in the Science Museum (1986).

guided missiles. I had been extremely lucky and privileged to have spent five years in a virtually green field of research with no competitors anywhere in the world, but as I have mentioned, this had its down side because hardly anyone outside the department was interested and there was virtually no

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dialogue with other workers in electron microscopy. We had visitors, including the Duke of Edinburgh and Marshall Tito (the then ruler of Yugoslavia), but electron microscopists were rare. Two that I recall were M. E. Haine of AEI Ltd and, of course, Ellis Cosslett, who was the external examiner for my PhD examination (Oatley was the internal one).

G. Postscript Some 35 years later, a replica of the first Cambridge SEM, now known as SEM1, was built for the Science Museum, South Kensington, London, by Leslie Peters, who is mentioned above and elsewhere in this volume and who devoted most of his professional life to the development of the SEM. Hardly any of the parts of the original microscope had survived but even so the replica is very realistic, although it is not a working one. However, the cathode-ray tube display is arranged to produce an image, using one of the original micrographs, which is virtually identical to that seen in 1951. The replica (Fig. 9) went on view in 1986 and remained in the Museum until about 1993 when it was withdrawn and stored, because of the re-arrangement of the adjacent gallery (McMullan, 1986).

References (Those references marked in the text with an asterisk are in the Bibliography, Section 7 of the McMullan, 1953a, which is reprinted as Chapter 2.1B). Baxter, A. S. (1949). ‘Detection and analysis of low energy disintegration particles’, Ph.D. Dissertation, University of Cambridge. Cosslett, V. E. (1952). Electron microscopy of solid surfaces. Nature 170, 861–863. Howie, A. (1995). Recent developments in secondary electron imaging. J. Microsc. 180, 192–203. McMullan, D. (1952). ‘Investigations relating to the design of electron microscopes’, PhD Dissertation, University of Cambridge. McMullan, D. (1953a). An improved scanning electron microscope for opaque specimens. Proc. IEE 100, Part II, 245–259. McMullan, D. (1953b). The scanning electron microscope and the electron-optical examination of surfaces. Electron. Eng. 25, 46–50. McMullan, D. (1986). Replica of the first Cambridge SEM for the Science Museum. Proc. R. Microsc. Soc. 21, 203–206. Menter, J. W. (1952). Direct examination of solid surfaces using a commercial electron microscope in reflection. J. Inst. Met. 81, 163–167. Oatley, C. W., McMullan, D., and Smith, K. C. A. (1985). Development of the scanning electron microscope. Adv. Electron. Electron Phys. (Supplement 16), 443–482.

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Smith, K. C. A., and Oatley, C. W. (1955). The scanning electron microscope and its fields of application. Br. J. Appl. Phys. 6, 391–399. von Ardenne, M. (1937). Improvements in electron microscopes. British Patent No. 511204, convention date (Germany) 18 Feb. 1937. Wells, O. C. (1971). Low-loss image for surface scanning electron microscope. Appl. Phys. Lett. 19, 232–235. Wells, O. C. (1986). Low-loss electron images of uncoated photoresist in the scanning electron microscope. Appl. Phys. Lett. 49, 764–766.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.1B* An Improved Scanning Electron Microscope for Opaque Specimensy D. McMULLAN Engineering Department, University of Cambridge

(1) INTRODUCTION As is well known, the conventional transmission electron microscope1,2,3,5 can be used only for specimens which are thin enough to allow the passage of the electron beam, and the examination of metals by such an instrument is ruled out unless they are in the form of thin foils. The usual procedure for observing the structure of metals in the electron microscope is to make a replica of the metal surface in the form of a thin film, e.g. of formvar or polystyrene, and to examine it by transmission.3,4 The contrasts in the image are formed by the variations in thickness of the replica corresponding to the elevations and depressions in the surface of the metal specimen. Attempts have been made to obtain images by reflection in the conventional electron microscope. The specimen is illuminated with electrons at grazing incidence, and the electrons leaving the specimen are focused by the objective lens. Electrons leaving the specimen both normally6 and at a grazing angle7 have been used, but neither method has given really satisfactory results. This is because the electrons from the specimen have lost considerable energy, and up to the present it has not been possible to make electron lenses suYciently achromatic to focus them with high resolution. The intensity of the image with Ruska’s method6 is also very low, and von Borries’ arrangement,7 which has looked more promising, introduces considerable geometric distortion in the image, whilst the resolution in one direction is only one-fourteenth of the resolving power of the instrument used. Other instruments used for the direct examination of metal surfaces rely on the emission of electrons from the specimen and their imaging on the screen. The

*Reprinted from: Proc. IEE. 100, Part II, 245–259 (1953). y The paper was first received 13th December, 1951, and in revised form 4th June, 1952. Proofs were made available to the public 15th September, 1952, and the paper was read before a joint meeting of the I.E.E. Measurements and Radio Sections 6th January, and the North-Eastern Radio and Measurements Group 2nd March, 1953.

59 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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emission may be thermionic,8 photo-electric,9 secondary10 or field,11 each method being suitable for a few metals only under special conditions, and useless for a general-purpose instrument. Another instrument, the scanning electron microscope, was first proposed by Knoll,12 and microscopes of this type were constructed by von Ardenne13,14 and Zworykin.15

LIST OF PRINCIPAL SYMBOLS A¼ B¼ C¼ Ct ¼ D¼ d¼ dc ¼ e¼ F¼ df ¼ G1, G2 ¼ Ie ¼ Im ¼ In ¼ Is ¼ K, K0 ¼ k¼ l¼ M¼ N¼ ne ¼ np ¼ ns ¼ T¼ te ¼ ts ¼ V¼ a¼ y¼ f¼ i¼

Area of displayed picture, ft2. Brightness, ft-lamberts. Spherical-aberration constant. Threshold contrast. Length of side of picture point, ft. Gaussian diameter of spot, cm. Diameter of circle of confusion, cm. Electronic charge, 1
6  10 19 coulomb. Focal length, cm. Bandwidth, c/s. Conversion gains. Electron current, amp. Electron current for maximum resolution, amp. Noise current, amp. Collected signal current, amp. Constants. Boltzmann’s constant. Length of side of specimen, cm. Magnification. Number of lines forming picture. Number of electrons per picture point. Number of photons per picture point. Number of retinal stimuli. Temperature of cathode, deg K. Exposure time of eye, sec. Scanning time, sec. Velocity of electrons, volts. Beam semi-angle, radians. Specimen angle, deg. Angle subtended by pupil of eye, radians. Current density, amp/cm2.

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61

(2) THE DESIGN OF SCANNING ELECTRON MICROSCOPES (2.1) General Principles The schematic of a scanning electron microscope is shown in Fig. 1. An electronoptical system focuses an intense electron spot of small diameter on the specimen. The electron beam is deflected by electric or magnetic fields just above the second lens, and the electron spot is moved across the specimen in parallel straight lines, as in a television raster. For transparent specimens contrast is produced as in the conventional electron microscope by absorption and scattering of electrons. Opaque specimens may be arranged normal to, or at an angle to, the axis of the beam, contrast being produced by the variation over the surface in the intensity either of the secondary low-velocity electrons or of the electrons reflected elastically from the specimen. The electron current leaving the specimen is collected and amplified and modulates some type of display or recording device. This may be a cathode-ray tube or facsimile recorder scanned in synchronism. Scanning generators produce voltages or currents suitable for the deflection circuits of the microscope and of the display. The ratio of these deflections gives, of course, the magnification of the instrument. (2.2) The Electron-Optical System The electron-optical system comprises an electron gun and one or more electron lenses. The design of the gun follows standard practice, the requirements being those

Fig. 1.—Schematic of scanning electron microscope.

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D. MCMULLAN

of the conventional electron microscope; these are a monochromatic beam of electrons diverging from a small area with a high current density per unit solid angle. As is well known, the diameter of the smallest electron spot that can be obtained with an electron lens is limited by the aberrations of the lens and by diVraction. If the semi-angle, a, of the beam is large and the beam is monochromatic, the diameter of the disc of confusion dc is limited by the spherical aberration, and dc ¼ CF a3

2,5

ð1Þ

It has been shown that when both spherical aberration and diVraction are limiting factors there is an optimum value of a to give a disc of confusion of minimum diameter dc0 . This condition is reached when the spherical aberration and diVraction errors are equal, and this occurs when 1

a ¼ 0:014ðCF Þ 4 V

1 8

radians

ð2Þ

8

ð3Þ

and 1

dc0 ¼ 750ðCF Þ4 V

3 8

 10

cm

In order to obtain a large beam current the diameter, d, of the Gaussian image should be no smaller than the disc of confusion, the highest value for any given spot size being obtained when they are equal. The demagnification of the lenses and the ˚ and the electron source has an size of the electron source controls d. Thus, if dc0 is 50 A eVective diameter of 50 microns (a typical value) the demagnification required is 10 000 times. The current in an electron beam is restricted, as has been shown by Langmuir,16 and has a maximum value given by pd 2 2 eV a i amp ð4Þ 4 kT Although this gives only the maximum current, the actual value obtainable is not much less, because of the high eYciency of electron guns.17 If a has the value for optimum dc0 and d ¼ dc0 , then by substituting the values of a and dc0 given by eqns. (2) and (3) in Langmuir’s equation (4) it is found that C, F and V vanish from the equation and Ie ¼

Im ¼ 10

10

i=T amp

ð5Þ

From the above it can be seen that for the smallest spot size obtainable with any electron lens the maximum current in the beam depends only on the properties of the cathode (assuming that space-charge eVects are negligible, which is the case with cathodes at present used in electron microscopes).  For a tungsten cathode at 3 000 K, i ¼ 10 amp/cm2 and therefore Im ¼ 3
3  10 13 amp. In practice, a higher beam current may be required. If the aperture of the lens is kept at the value given by eqn. (2) the current (and, of course, the spot size) can be increased by reducing the demagnification of the lenses. The spot size with any given

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63

current is found from eqn. (4) by substituting the value of a given by eqn. (2), provided, of course, that d is large compared with dc0 . But to obtain the minimum spot diameter with any given current a should be increased until the diameter of the disc of confusion is equal to d. Since a is greater than the optimum, the disc of confusion will result mainly from spherical aberration, and its diameter dc will be given by eqn. (1). Substituting the value of a from eqn. (1) (with d ¼ dc) into eqn. (4) then gives #38 2 Ie TðCF Þ3 d¼ cm 9  103 V i "

ð6Þ

(2.3) The Deflecting System Although mechanical scanning of the specimen has been used,15 it cannot be very reliable, because of the small movements required. Thus, if the final picture is 10 cm square and the magnification is 10 000 times, the required movement of the specimen is only 10 microns. It is therefore necessary to deflect the beam with a magnetic or electrostatic field. Since the focal length of the final lens is only a few millimetres and the working distance is smaller still, there is no space for the deflecting field between lens and specimen. The most convenient position for the deflecting field is just before the final lens. (2.4) The Specimen The scanning microscope is most useful for opaque specimens, and although transparent specimens can be examined,14 the conventional electron microscope is generally more suitable for this purpose. For the sake of completeness, however, brief mention will be made of possible advantages of using the scanning microscope for transparent specimens. The main advantage of the scanning electron microscope for transparent specimens is that the resolution is not aVected by energy losses of the electrons in the specimen, which in the conventional electron microscope give rise to chromatic aberration. Von Ardenne13 has pointed out that it should therefore be possible to examine very thick specimens. Although from theoretical considerations dark-field operation of the conventional electron microscope should improve the resolution, in practice it has been found inferior18 because of chromatic aberration. The scanning microscope should permit the theoretical improvement to be obtained through its freedom from this aberration. (2.4.1) Opaque Specimen. With opaque specimens contrasts are formed by variations in the intensity of reflected or emitted electrons over the surface. If the velocity of the primary electrons is less than a few kilovolts that of the majority of the secondary electrons will be less

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Fig. 2.—Electron-emission ratio as a function of atomic number; 20-kV primary electrons (after Palluel).

than a hundred volts. This is the normal secondary-emission phenomenon which has been investigated by many workers.19 The ratio of the number of secondary to primary electrons depends almost entirely on the state of the surface, and this will be modified by the condensed oil-film which is almost inevitable with a demountable vacuum system. Zworykin’s scanning microscope used a beam accelerating voltage at the specimen of only 800 volts, and the image contrasts obtained depended on secondary emission, an unetched pure metal giving no contrast. If the primary electrons have velocities of the order of tens of kilovolts some lowvelocity secondaries are still produced, but with increasing voltage there is a larger proportion of reflected* electrons. It seemed probable that variations in the intensity of the high-velocity reflected electrons would be less dependent on the surface state of the specimen, since the penetration of these electrons will be large compared with the thickness of any surface film. Several papers have been published on this phenomenon, which is discussed in the Appendix. Briefly, if the primary electrons strike the surface normally the intensity of the reflected (high-velocity) electrons is a function of the atomic number of the specimen. Palluel20,21 has published a curve (Fig. 2) showing this relationship. Some measurements made by the author with normal incidence were repeatable, even in a poor vacuum and with oil diVusion pumps. This phenomenon, then, would be suitable for forming image contrasts in a scanning microscope, but again the structure of an unetched pure metal would not be observable. Measurements were also made on high-velocity electrons bombarding metals at an  angle. From these it was found that with a grazing angle of about 25 the intensity of  the reflected beam collected by an electrode subtending an angle of 30 at the metal surface is practically independent of the density of the metal. The intensity, however, *In the paper the term ‘‘reflected’’ is applied to all the emitted electrons, other than the secondaries which have energies of only a few electron-volts. Some writers have called these ‘‘rediffused’’ and have applied the term ‘‘reflected’’ only to those electrons which have suffered elastic encounters and have lost no energy.

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65 

is very sensitive to changes in the angle of the incident beam, a change in angle of 1 from the mean causing a change of about 8% of the reflected beam. For a grazing  angle of 25 the intensity is about 10% that of the primary beam. Curves illustrating this eVect are shown in Fig. 3. When a metal surface is polished and etched, as for examination with an optical metallurgical microscope, faces of the crystals are exposed and will be orientated in various planes. Image contrasts are generally formed with an optical metallurgical microscope by light being specularly reflected by the crystal faces into or outside the aperture of the objective lens. Similarly, contrasts will be formed with an etched specimen at an angle in the scanning electron microscope by the reflection of electrons into the collecting system, the intensity

Fig. 3.—Variation of electron emission with specimen angle, measured with the apparatus shown in Fig. 11. ——At 40 kV. – –At 25 kV. (a) Aluminium (electrolytically polished). (b) Tungsten (polished). (c) Zinc (polished).

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D. MCMULLAN

depending on the orientation of the crystal faces. A pure metal can, of course, be examined by this method if suitably etched. Fractures and scratches on the surfaces of materials also produce large contrasts. It must be noted in passing that, with the specimen at an angle, the scanning spot will be elongated in one direction, with consequent deterioration of the resolution in this dimension. With a specimen angle  of 25 and a spot diameter d the spot will become an ellipse with a major axis of d  cosec 25 , i.e. 2
37d, and the resolution will thus be only two-fifths as high in this direction. For the same reason there will also be some geometrical distortion, but this can be corrected in the recording system. Again, surface contamination will have no eVect if only high-velocity electrons are accepted by the recording system. Thus, to sum up, to avoid the eVects of surface contamination with opaque specimens a scanning beam of high velocity must be used and only the high-velocity reflected electrons must be recorded. The specimen may be scanned at right angles, when contrasts will be produced by variations in the constitution of the specimen,  etching being unnecessary. When the specimen is scanned at an angle of about 25 image contrasts depend only on the topography of the surface and it is necessary to polish and etch the specimen to show its structure. Fig. 4 shows diagrammatically the arrangement of transparent specimens for light-field and dark-field pictures and of opaque specimens with normal and oblique scanning.

Fig. 4.—Arrangement of specimens between final lens and electron multiplier. (a) Transparent specimen, light field. (b) Transparent specimen, dark field. (c) Opaque specimen, oblique incidence. (d ) Opaque specimen, normal incidence.

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67

(2.5) The Detector, Display and Recording System The detector includes the means of collecting the electron beam leaving the specimen and devices for amplifying the variations in the intensity of the beam. The signal from the detector is reassembled by the display to form a visible picture, and by the recording system as a permanent record. Before describing the devices which may be used it is necessary to consider the fundamental processes involved. (2.5.1) Fundamental Processes.22,23,24 Electrons will leave the specimen at randomly spaced intervals, i.e. they follow the shot-eVect law, and if the electron current is Ie the mean-square noise current is In2 ¼ 2edfIe It is more convenient in this discussion to work in terms of the mean-square fluctuations in the number of electrons arriving in unit time, or better still in terms of the number of electrons per picture point. The number of picture points is assumed to equal the square of the number of lines forming the picture. 1 If ne is the number of electrons per picture point the r.m.s. fluctuation in ne is n2e. If all the electrons are collected and the electron current is amplified by a wide-band noise-free amplifier and the picture displayed on a cathode-ray tube with a screen of negligible persistence, there will be ne pulses of light per picture point, and G1ne photons per picture point, where G1 is the conversion gain of the amplifiers and cathode-ray tube. If the amplifying system has a narrower bandwidth than that required to pass pulses due to the individual electrons, but is wide enough to pass without distortion pulses corresponding to picture points, there will still be G1ne photons per picture point. Rose23 has pointed out that an amplifying or converting system (e.g. electron beam into light) is noise free if at no stage in the system is information conveyed by fewer quanta than are involved in the input signal. This, of course, assumes that there is zero noise output for no signal. That is, with the scanning microscope, in no part of the system from scanning beam to the brain of the observer must there be fewer than ne quanta per picture point. G1 must be greater than unity. If the screen of the cathode-ray tube is viewed by the naked eye a small proportion of the photons leaving the screen will enter the eye and stimulate the retina. Let np photons per picture point reach the retina. Then np ¼ G1 ne sin2 f ¼ 1
3  1016 BD2 ðsin2 fÞte

ð7Þ

where 1
3  1016 is the number of photons per second per lumen of white light. If G2 is the conversion gain of the retina (i.e. 1/G2 photons are required to produce one stimulus from the retina to the brain) the number of stimuli is given by ns ¼ G2 np ¼ G1 G2 ne sin2 f

ð8Þ

As pointed out by Rose, the picture will appear to be noise free to the eye if ne is appreciably greater than ns; if ne is appreciably less than ns the fluctuation of the

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D. MCMULLAN

microscope scanning beam will be visible on the screen. Obviously, ne should not be greater than ns or information would be lost. It is also undesirable for ne to be less than ns, since noise on the picture is distracting to the observer; ne should therefore be made equal to ns. The optimum brightness on the viewing screen is then given by eqns. (7) and (8), and ne ¼ G2 ð1
3  106 BD2 sin2 fÞte where G2 is of the order of 0
01 and te is of the order of 0
2 sec, or B¼

ne 1
3 

1016 G2 te D2

sin2 f

ft-lamberts

ð9Þ

An important quantity is the value of the smallest change, dne, in ne that will be observable by the eye; dne depends on the probability demanded by the eye of 1 1 detecting the change in the presence of the fluctuation n2e. We may write dne ¼ Kn2e. If K ¼ 2, mathematically there would be a 95% probability of detection, but it is found that the eye demands a considerably higher probability corresponding to a value of K ¼ 4 or even higher under some conditions.123,24 The threshold contrast Ct ¼ dB=B ¼ dne =ne ¼ Kne 2 . If the picture is recorded photographically similar considerations apply, but the problem is complicated by the variation in contrast and gamma obtainable with the photographic process. When ne is small the contrast of the print should be considerably larger than is necessary to show all the gradations present, otherwise the print 1 will appear very flat. The eVective threshold contrast in the print will be about Kne 2 and some noise will be visible. The value of ne is given by ne ¼ Is ts =eN 2

ð10Þ

From the foregoing equations the basic characteristics of a scanning microscope can be determined, and it is then possible to design suitable amplifiers and recording instruments. (2.5.2) Modes of Operation. There are three modes of operation for a scanning microscope. First, the provision of a visible picture not necessarily of the maximum resolution for the selection of interesting parts of the specimen. Secondly, the recording of the picture at highest magnification with the maximum resolving power of the electron-optical system. Thirdly, recording at a magnification less than the maximum, when the beam current can be greater and the recording time shorter. (2.5.2.1) Visible Picture. The fixed parameters are N, which is limited by the resolving power of the display; ts, which must be short enough to give a continuous picture; B, the peak brightness of the displayed picture; and A, the size of each picture element on the screen.

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69

From eqns. (8) and (9) Is ¼

BeN 2 D2 G2 te sin2 f  1:3  1016 amp ts ¼

BeAG2 te sin2 f  1
3  1016 ts

where G2 ¼ 0
01 and te ¼ 0
2 sec. So that

Is ¼

BA sin2 f  4  10 ts

6

amp

ð11Þ

From this value of Is the required value of Ie, the scanning beam current, may be calculated: Is ¼ K0 Ie, where K0 ¼ 1 for a transparent specimen and 0
1 for an opaque specimen with oblique scanning (assuming 100% collection). The diameter of the scanning spot may be found from eqn. (6), and hence also the maximum useful magnification.

(2.5.2.2) Recording with Maximum Resolution. In this case Ie, and hence Is, is fixed by eqn. (5), which gives the value of Ie for maximum resolution (Im), and N is limited by the recording instrument. Thus, from eqn. (10)

ne ¼

K 0 Im t s eN 2 1

and since the threshold contrast Ct = K/n2e the scanning time is

ts ¼

K 2 eN 2 sec K 0 Im Ct2

ð12Þ

Eqn. (3) gives the diameter of the scanning spot.

(2.5.2.3) Recording at Lower Magnifications. When recording with a resolution less than maximum the scanning-spot size is made equal to l/N, where N is the number of lines and an area of the specimen l centimetres square is being scanned. The maximum beam current is found from eqn. (6) to be I ¼ 9  103

d 8=3 V i

amp TðCF Þ2=3 l 8=3 V i ¼ 9  103 TN 8=3 ðCF Þ2=3

and the scanning time ts is found from eqn. (12).

ð13Þ

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D. MCMULLAN

(2.5.3) Von Ardenne’s Method. Methods of recording the picture will now be considered. The simplest in principle is that due to von Ardenne.13 The electrons leaving the specimen are focused to a spot on a photographic plate moved in synchronism with the scanning spot. The disadvantage of this arrangement is the impossibility of observing and focusing the picture before recording. Long recording times are also necessary, because of the relative insensitivity of photographic plates compared with amplifying systems. (2.5.4) Detectors and Amplifiers. All other proposed means of recording employ amplifiers for the beam current from the specimen and separate recording devices. The bandwidth of the amplifier must be at least N2/2ts c/s and can with advantage be twice this figure if the noise level of the amplifier is low. A collecting electrode and valve amplifier have been used,15 but, as is well known, noise from the input resistor and first amplifying stages is considerable at the high gain required (about 106). Thus, with this method it was necessary to use beam currents far in excess of the optimum, or else to reduce the bandwidth and record for an impracticable length of time to obtain a picture with a reasonable signal/noise ratio. An electron beam can be amplified in an electron multiplier without much increasing its original fluctuation. Although the output from the multiplier fluctuates considerably the eVective number of quanta per picture point is unchanged, provided that the gain of each stage of the multiplier is high and the output current is zero for no input. Since electron multipliers suitable for use in a demountable system were not available at the time, Zworykin15 used a fluorescent screen and photomultiplier. About three hundred photons are liberated by each electron striking the screen and approximately only one in a hundred photons liberates a photoelectron in the multiplier. If a wide-aperture optical system is used to focus the light from the screen to the photomultiplier, the electron current from the photocathode will not be less than the specimen beam current and the conversion will be free from noise. The dark current of the multiplier is a source of noise and in some types it is a limiting factor, but its eVects may be reduced by suitable circuits. A more serious diYculty is the poor frequency response of fluorescent screens at low excitations. Even zinc sulphide, silver-activated and nickel quenched, has a decay of about a millisecond when the beam-current density is of the order of 10 8 amp/cm2, compared with a few microseconds at high excitations.25 Although the frequency response is adequate for recording it is too poor for the relatively high rates of scanning necessary for visual observation. Electron multipliers are now available which are insensitive to the relatively poor vacuum conditions in a demountable system.26,27 The electron beam can be directed straight into the multiplier, and as the spurious signal (corresponding to dark current in a photomultiplier) can be as low as one electron per minute (referred to the input), the performance approximates to the theoretical maximum. Unless the multiplier has very many stages it will be necessary to follow it by a valve amplifier, but since this will be working at a relatively high level it will contribute an insignificant amount of noise.

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71

(2.5.5) The Display. For visual observation the signal from the amplifiers may be used to modulate a cathode-ray tube. To keep the scanning time long a tube with a long-persistence screen is necessary, one frame per second being a typical scanning speed; but the eVective value of ts may be three or four times this if the fluorescent screen integrates several consecutive scans. A number of screens which were developed for use in radar systems have this property. A dark-trace tube (Skiatron)28 can also be used. (2.5.6) Recording Apparatus. The picture on the cathode-ray tube may be recorded by photography with the scanning speeds normally used for visual observation, but any slight movement of the specimen during the exposure of several minutes would cause blurring. It is therefore better to scan the picture once, slowly, when the eVect of a small specimen movement would be negligible and large movements would cause not blurring but geometrical distortion. A facsimile recorder can also be used15 and is capable of producing a higher-quality picture than is possible with a cathode-ray tube of convenient size. It may be necessary to include a gamma-control amplifier before the recording device if the latter is not linear. Most cathode-ray tubes have a gamma of between two and three, and the gamma of the signal must be reduced if reasonable reproduction of the black parts of the picture is to be obtained. On the other hand, since it may be advantageous to have a high gamma for specimens with poor contrast the gamma control should be made variable. Means must be provided for the accurate maintenance of the black level (no electron current from specimen), and unless d.c. amplifiers are used the black level must be restored at the cathode-ray tube. This is especially important when gammacorrecting circuits are employed. To fix the black level either the scanning beam or the electron multiplier may be switched oV during the flyback periods.

(2.6) The Scanning Circuits It is unnecessary to discuss the circuit arrangements in this section, since an actual design is described in Section 3.8. The speed of the time-bases depends, of course, on the number of lines and the number of frames per second. The lines of alternate frames may be interlaced to increase the permissible frame-scanning speed. When the specimen is scanned at an angle y the magnification in one direction will be multiplied by sin y (see Fig. 5). To compensate for this the scanning amplitude in this direction must be reduced if a plan view of the specimen is required. It may also be worth while to intermodulate the time-bases, in order to correct for trapezium distortion. Furthermore, if a large number of lines are used the focal length of the final lens may have to be varied, to keep the spot focused whilst scanning the inclined surface of the specimen. Referring to Fig. 5, if the specimen is being scanned over a square of side l cm and there are N lines, the width of a scanning line is l/N cm.

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D. MCMULLAN

Fig. 5.—Illustrating depth of focus with inclined specimen.

The point A is closer to the lens by 12l cos y, and if the beam semi-angle is a the outof-focus spot diameter will be 2a12l cos y—assuming that the beam is in focus at B, the centre of the specimen. The out-of-focus spot diameter must be no larger than one scanning line; thus al cosy < l=N or N < 1=a cosy 

If a ¼ 0
002 radian and y ¼ 25 , cos y ¼ 0
906, so that N must be less than 550 if the necessity for focus modulation is to be avoided. The scanning circuits must also supply suitable pulses actuating the black-level setting circuits. (2.7) Power Supplies The design of the power supplies for the scanning electron microscope follows standard practice. With magnetic lenses the stabilities required are the same as for a conventional microscope with equal resolving power. It is unnecessary to stabilize the beam-accelerating voltage in a conventional microscope with unipotential electrostatic lenses, the focal lengths depending only on their dimensions (neglecting relativity eVects). However, the scanning electron microscope with this type of lens needs stabilized supplies, since the beam deflection depends on the velocity of the electrons in the beam. It is easily shown that the allowable fluctuation DV in the accelerating voltage V is related to the number of lines N in the picture by the following expressions:

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73

For magnetic deXection DV =V < 2=N

ð14Þ

For electrostatic deXection DV =V < 1=N

ð15Þ

Thus, with N ¼ 500 a stability of the order of 0
4% is required with the electrostatically focused and magnetically deflected scanning microscope. The fluctuation in the beam-accelerating voltage with magnetic lenses must be about a hundred times less than this. The high-voltage supply to the recording cathode-ray tube must also be stabilized, and again the stability required depends on the type of deflection; eqns. (14) and (15) are applicable.

(3) A SCANNING ELECTRON MICROSCOPE A conventional electron microscope has been modified for scanning. This microscope is a two-stage instrument with electrostatic lenses. The geometry of the electron gun and lenses is based on papers by Grivet and Bruck.29,30,31,32 Astigmatism correction33 has not yet been incorporated, and for this reason the resolution of electron micrographs taken with the instrument has been rather inferior to other similar micro˚. scopes, being of the order of 200 A When used as a scanning microscope the projector lens is removed and the final fluorescent screen is replaced by a special assembly comprising deflection coils, electrostatic lens, specimen stage and electron multiplier. A cross-sectional elevation of this assembly and also of the electron gun and first lens is shown in Fig. 6, the interposing part of the microscope column being omitted. A block schematic of the complete microscope is shown in Fig. 7. (3.1) The Electron-Optical System The electron gun gives an electron source of 50-microns eVective diameter and the current density at the aperture of the first lens is 10 mA/cm2. The aperture of the first lens must be no larger than necessary to allow the specimen to be fully scanned at minimum magnification. In this instrument the aperture is 200 microns in diameter. The focal length of the first lens is adjusted to give the required demagnification by taking the central electrode to a potential divider connected across the high-voltage supply. Another aperture is placed just before the final lens and limits the beam angle at the specimen. This aperture is 25 microns in diameter. Focusing of the beam on the specimen is obtained by small variations in the potential of the central electrode of the final lens which is approximately at cathode potential. The focal length of this lens is 0
5 cm. Since with the present lenses aberrations limit the diameter of the electron spot to ˚ , a maximum demagnification of about 2 500 times is required for about 200 A recording and the focal length, F1, of the first lens is set to 1
5 cm. As shown later,

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D. MCMULLAN

Fig. 6.—Elevation through electron-optical system, one-half full size. *Length between gun and first lens ¼ 812 in. Length between first and second lenses ¼ 40 in. *Reproduced at about one-third full size (eds.).

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75

Fig. 7.—Block schematic of scanning microscope.

for visual observation a beam current of about 1
5  10 10 amp is required, and to obtain this the demagnification is reduced to 500 times (F1 ¼ 5 cm), giving a spot size ˚. of about 1 000 A Double Mumetal shielding extends over most of the length of the microscope column. Unfortunately, the diameter of the lower part of the column is only suYcient to accommodate a single tube in the vicinity of the final lens. This is a serious deficiency which is to be remedied, as at present stray magnetic fields limit the ˚. resolution to about 500 A

(3.2) Deflecting Coils The beam is deflected by the magnetic field from two pairs of coils, each coil having 1 200 turns wound on a porcelain bobbin. The dimensions of the coils can be seen in Fig. 6 (marked C). A current of 15 mA through the coils gives a deflection of 30 microns at the specimen when the beam accelerating voltage is 25 kV.

76

D. MCMULLAN

(3.3) The Specimen The specimen is placed immediately below the final lens. For metal specimens it  can be the end of a rod 3 mm in diameter mounted with its axis at an angle of 65 to the axis of the beam, as shown in Fig. 6. The specimen S is mounted in a holder H which screws into a block B, the multiplier being lowered for the purpose, the microscope, of course, having previously been opened to the air. The block B is moved laterally by the two specimen stage-controls. The high-velocity electrons leaving the specimen strike the electrode E mounted on  the specimen holder and subtending an angle of 30 at the specimen. Some of the electrons from this plate pass down the tube of the specimen holder and are accelerated into the electron multiplier, the input of which is at a positive potential of 1 kV. This arrangement is obviously ineYcient, and the actual current entering the multiplier is only 20% of that leaving the specimen. Very few of the low-velocity electrons leaving the specimen reach the multiplier, so that contrast is formed mainly by variations in the emission of the high-velocity electrons. Scanning with the beam perpendicular has not yet been tried with opaque specimens. When transparent specimens are scanned normally the electrons passing through the specimen enter the electron multiplier directly. The eVective aperture of the multiplier is in the form of a slot only 1
5 mm wide and 1
5 cm long, so that extra stops are not entirely necessary, although provision is made for them on the transparent-specimen holder (not shown in Fig. 6). (3.4) The Electron Multiplier The electron multiplier was designed and constructed by Dr. Baxter27 of the Cavendish Laboratory. It has beryllium-oxide-coated dynodes which enable it to work under relatively poor vacuum conditions and to be opened to the air without damage. There are sixteen stages of secondary multiplication (only the first few are drawn in Fig. 6) and with 300 volts per stage the total gain is 105. The multiplication per stage is rather low for this type of multiplier, which is almost certainly due to the contamination of the surfaces of the dynodes by oil from the pumps. However, it is adequate for the present purpose. The input of the multiplier is at a potential of 1 kV positive to earth and the output electrode of the multiplier must therefore be at a high positive potential (about 6 kV) relative to earth. This complicates the design of the head amplifier, the output signal from the multiplier being only about 50 mV. Capacitive coupling to a head amplifier at earth potential would not be satisfactory without elaborate precautions to prevent the random discharges across the insulation and in the coupling condenser from masking the signal. (3.5) The Video Amplifiers The head amplifier has therefore been designed to be insulated to 6 kV, and it is mounted with its own power pack at the top of an isolating transformer. Fig. 8 shows

AN IMPROVED SEM FOR OPAQUE SPECIMENS

77

Fig. 8.—Video channel, showing simplified circuits of head and gamma-control amplifiers.

a simplified circuit diagram of the video channel. V1 is a cathode follower, the screening surrounding the signal lead from the multiplier being connected to the cathode of V1, in order to reduce the capacitative loading on the multiplier. The input resistor, R, is switched to 200 kilohms for wide bandwidth and to 10 megohms for narrow bandwidth and high gain. Valves V2 and V3 amplify the video signal 250 times with a bandwidth 3 db down at 150 kc/s. The peak video signal is now 12
5 volts and is fed to a cathode follower V4 at earth potential through condenser C. The d.c. level is now restored and the gamma reduced to about 0.5 by the non-linear amplifier (V5, V6 and V7), which is of a type described by Nuttall.34 The video signal with the d.c. level restored and the gamma reduced is fed from the cathode follower V7 to the cathode-ray tubes. (3.6) The Display The cathode-ray tube used for observation of the picture has a double-layer screen of zinc sulphide and zinc cadmium sulphide, with a long afterglow.35 A screen of this type also integrates pulses occurring at intervals of the order of seconds and about 5 scans at 1-sec intervals are required for the brightness to build up to its maximum, which is about 0
75 ft-lamberts. The screen will comfortably resolve 405 lines and a scanning speed of 1
8 sweeps/sec with alternate lines interlaced has been found satisfactory. The picture is 7 in square. Eqn. (11) gives the beam current required from the specimen, assuming that the multiplier collects all the current, but since the multiplier used has a very small input aperture and the directing system is still relatively crude, collection eYciency achieved is only about 20%; since only approximately 10% of the incident beam is reflected the beam current required is about 50 times that given by eqn. (11). Is ¼

BA sin2 f  4  10 ts

6

amp

78

D. MCMULLAN

If B ¼ 0:75 ft-lamberts:

A ¼ 0
4 ft2 : ts ¼ 5 sec:

f ¼ 4  10 3 radian (dark-adapted eye 40 in from the screen).

then I ¼ 3
3  10

12

amp

and if K0 ¼ 0.02 the beam current must be 1.6  10 10 amp. As mentioned in Section ˚. 3.1, the spot diameter with this beam current is about 1 000 A (3.7) The Recorder The recording tube (type 5FP7) also has a double-layer screen. The long persistence is, in fact, not entirely necessary, although it is sometimes useful for photographing pictures at lower magnifications. This type of tube has a flat face which is easily photographed without distortion. The beam current for recording at maximum resolution is given by eqn. (5) and is 3
3  10 13 amp for a tungsten filament with i ¼ 10 amp/cm2. A slow-speed frame time-base is used and the specimen is scanned in 300 sec. Eqn. (12) gives the threshold contrast in a photograph recorded with this beam current. Substitution of the values K ¼ 4, K0 ¼ 0
02, ts ¼ 300 sec, N ¼ 500, and I ¼ 3
3  10 13 amp in the equation (Ct2 ¼ k2e N 2 =KI0 T) yields Ct ¼ 0
5. Although with high-contrast specimens the maximum resolution will apparently be attained, fine gradations will not be resolved. With more eYcient beam-current collection K0 could be as high as 0
1, when Ct would become 0
2. Two methods are used for recording at intermediate magnification. First, the afterglow of the recording tube may be photographed when the threshold contrast in the photograph is given by eqn. (12) with ts ¼ 5 sec. Secondly, the picture may be recorded using a slow-speed time-base and a scanning time of 300 sec. As the magnification of the instrument is varied the demagnification of the electron optical system is also adjusted to keep the scanning spot size equal to N/l. It has not been convenient with the present instrument to vary the aperture diameter of the final lens, so the maximum possible beam currents are not reached with the smaller demagnifications. (3.8) The Time-Bases The time-bases are driven from a pulse generator at frequencies of 375 pulses/sec and just over 1
85 pulses/sec. The ratio of these frequencies (202
5) is maintained by counting-down circuits, so that the number of lines in the picture will be constant and alternate scans are interlaced. Thus there are 405 lines and approximately 0
9

AN IMPROVED SEM FOR OPAQUE SPECIMENS

79

Fig. 9.—Simplified circuit of slow-speed time-base.

complete pictures per second. The line time-base is also maintained at 7
5 times the mains-supply frequency, to minimize the eVect of 50-c/s ripple on the power supplies and of stray magnetic fields alternating at mains frequency. The time-bases employ boot-strap circuits.36 A simplified diagram of the slowspeed time-base (1
85 sweeps/sec) is shown in Fig. 9. D.C. couplings are used throughout; V1 and V2 form the boot-strap circuit and V3 is an output cathodefollower. In order to economize in current consumption the cathode load of V3 is a constant-current pentode, V4. The output of the time-base is from 75 volts to +75 volts at 15 mA to +15 mA, the deflection coils of both cathode-ray tubes and microscope being connected in series. A variable resistor is connected across the microscope coils to control the magnification. The high-speed time-base has a similar boot-strap circuit, but capacitive couplings are used and the output valve is connected as an amplifier with the deflection coils in series in the anode circuit. For recording, the first time-base continues to run at 1
85 sweeps/sec, but switches are incorporated in the second time-base to enable it to make one sweep in 300 sec. When switched the circuit of the time-base is similar to that shown in Fig. 9, but C1 and C2 are increased to 10 mF and R to 16 megohms. The recorded picture has 550 lines. The pulse generator also supplies suitable pulses for beam blanking in the microscope and cathode-ray tubes, and for black-level setting in the amplifiers. (3.9) Power Supplies The design of the power supplies follows standard practice all the d.c. supplies to the instrument being stabilized by feedback amplifiers; since only 0
4% stability is required, neon reference tubes have been found satisfactory. The beam-accelerating voltage can be varied between 5 and 40 kV.

80

D. MCMULLAN

(4) PRELIMINARY RESULTS Typical electron micrographs taken with the instrument are shown in Fig. 10(a), (b) and (c). The specimen—annealed aluminium—was electrolytically polished and etched with hydrofluoric, nitric and hydrochloric acids. Fig. 10(a) gives a fair indication of the quality of the picture visible on the cathode-ray tube when scanning at the rate of 0
95 pictures/sec and with a beam current of 1
5  10 10 amp. Fig. 10(b) was recorded over a longer period with a smaller spot diameter and beam current. The magnifications are about 1 000 and 3 000 respectively, and the same part of the specimen has been used in each Figure. The micrograph shown in Fig. 10(c) is also of etched aluminium at a magnification of 1 000. The sharpness of this micrograph is limited by the coarseness of the cathode-ray-tube screen. The specimen was mounted  at an angle of about 25 and, as can be seen, there is a well-marked stereoscopic eVect in the micrographs, giving the impression that the specimen is being viewed at this angle. The scanning-beam voltage in each case was 16 kV. A higher voltage (25 kV) was used initially, but a worse resolution was obtained with aluminium specimens. This eVect is due to the increased depth of penetration of the higher-velocity electrons, which may pass right through the crystals near their edges. Under these conditions the electron micrograph will not show the highest possible resolution except in a direction at right angles to the scanning beam. This eVect may prove to be the limiting factor on the resolution of the scanning microscope with opaque specimens. Even with 16 kV penetration is still noticeable, as can be seen from Figs. 10(b) and (c), in which light bands extend along the far edges of the crystals. The range of 16-kV electrons in aluminium is of the order of one micron and the width of these bands is also of this order, but in spite of this the blurring of some of the edges of the crystals is only a small fraction of a micron, even on the far sides. The lack of sharpness is partly due to the imperfect magnetic shielding, which, as  ˚ mentioned above, limits the resolution to about 500 A .



Fig. 10.—Micrographs of etched aluminium specimens. Angle of incidence of 16-kV electrons ¼ 25 . (a) Exposure, 5 sec; beam current, 1.5  10 10 amp; magnification, 1 000. (b) Exposure, 5 min; beam current, 10 12 amp; magnification, 3 000. (c) Exposure, 5 min; beam current, 10 12 amp; magnification, 1 000.

AN IMPROVED SEM FOR OPAQUE SPECIMENS

81

With the lowering of the beam voltage the scanning spot diameter will increase and there is therefore an optimum beam voltage which depends on the specimen and on the electron lenses. With the present instrument and an aluminium specimen this is about 16 kV, but with better lenses it may well be only a few kilovolts. It will be noticed that the micrographs are very similar to those of aluminium made by replica techniques.3,4 No trouble has as yet been experienced with contamination of the specimen under electron bombardment. There has been no visible change even when an area of the specimen has been exposed to the beam for several hours. The sharpness of the electron micrographs obtained with the instrument at 1 000 times magnification has been limited by imperfections in the recording system and the relatively small number of lines. To obtain sharply focused micrographs of reasonable dimensions at least 1 000 lines would be required, and consequently a cathoderay tube with a very small spot diameter and fine-grain screen would be necessary. The cathode-ray tube at present used for recording is hardly adequate, even for 405 lines.

(5) CONCLUSIONS Preliminary results with the relatively ineYcient instrument described in the paper are very promising. Various modifications could be made to the instrument, including better magnetic screening, astigmatism correction, more-eYcient beam collection and an improved recording system. ˚ should then be expected, but the actual A spot size of between 50 and 100 A resolution that would be obtained with opaque specimens is still uncertain, because of the penetration of the electrons. Wagner37 showed that the reflection of the electrons ˚ ngstro¨m units, depending on the density of occurs at depths of up to a few thousand A the metal. For gold the maximum depth is less than 0
2 microns at 40 kV, for silver it is 0
2 microns at 20 kV and for aluminium it is 0
5 microns at 20 kV. The resolution of ˚ . If the penetration of the the micrograph in Fig. 10(b) is estimated to be 500 A ˚ with aluminium electrons is assumed to be the limiting factor, then to resolve 100 A the beam voltage would have to be reduced to about 7 kV, since the penetration of electrons depends upon the square of the beam voltage. The resolution achieved with heavier metals would be considerably better. In practice, it is doubtful whether it would be necessary to work at such a low voltage, since the beam from the specimen could be restricted to the electrons which have lost only small amounts of energy and which have therefore travelled only short distances through the specimen. With the present instrument electrons of all energies above a few hundred electron volts are detected. The specimen could also be mounted in a rotatable holder, so that micrographs could be taken with the beam striking the specimen from diVerent directions. The optimum resolving power could then be obtained in several directions on the specimen in turn. It is expected that normal scanning will have advantages for some types of specimen, and this method is to be tried in the near future. Until the instrument has been improved and many more metals have been examined and the micrographs

82

D. MCMULLAN

compared with those obtained by optical and replica processes, it will be diYcult to assess all the advantages and disadvantages of the scanning microscope. However, there are three clear advantages, namely the direct observation of the surface of the specimen, the simple preparation of the specimen by standard metallurgical etching methods and the reproducibility of the results. (With some replica techniques it has been necessary to carry out the process several times so as to be certain that a spurious structure has not been introduced.) The complexity and cost of the auxiliary electronic equipment may be held to be a disadvantage. However, quite complicated circuits are already used in the stabilized supplies of most electron microscopes, and the cost of a scanning electron microscope should not be more than 25% greater than that of a conventional electron microscope.

ACKNOWLEDGEMENTS The author wishes to express his gratitude to Mr. C. W. Oatley for suggesting the investigation into the possibilities of the scanning microscope and for his continued help and encouragement, and also to the other members of the Electronics Laboratory of the Engineering Department for many useful suggestions during the course of the work. The author is greatly indebted to Dr. A. S. Baxter of the Cavendish Laboratory for the loan of one of his electron multipliers, without which the microscope could not have reached its present stage of development. He would also like to thank the Department of Scientific and Industrial Research for grants towards the cost of the instrument.

(7) BIBLIOGRAPHY (1) Zworykin, V. K., Morton, G. A., Ramberg, E. G., Hillier, J., and Vance, A. W.: ‘‘Electron Optics and the Electron Microscope’’ (Wiley, New York, 1945). (2) Gabor, D.: ‘‘The Electron Microscope’’ (Hulton, London, 1946). (3) Cosslett, V. E.: ‘‘Practical Electron Microscopy’’ (Butterworth, London, 1951). (4) Drummond, D. G. (Editor): ‘‘The Practice of Electron Microscopy,’’ Journal of the Royal Microscopical Society, 1950, 7, p. 1. (5) Haine, M. E.: ‘‘The Design and Construction of a New Electron Microscope,’’ Journal I.E.E, 1947, 94, Part 1, p. 447. ¨ ber Fortschritte bei der Abbildung Elektronenbestrahlter (6) Ruska, E., and MU¨ller, H. O.: ‘‘U Oberfla¨chen,’’ Zeitschrift fu¨r Physik, 1940, 116, p. 366. (7) von Borries, B.: ‘‘Sublichtmikroskopische Auflo¨sungen bei der Abbildung von Oberfla¨chen im ¨ bermikroskop,’’ ibid., p. 370. U ¨ ber das elektrostatische Emissions-u¨bermikroskop,’’ ibid., 1942, 120, p. 21. (8) Mecklenburg, W.: ‘‘U (9) Pohl, J.: ‘‘Elektronenoptische Abbildungen mit Lichtelektrische ausgelo¨sten Elektronen,’’ Zeitschrift fu¨r Technische Physik, 1934, 15, p. 579. (10) Meshter, E.: ‘‘Electron Microscope for studying Thermal and Secondary Emission,’’ Review of Scientific Instruments, 1938, 9, p. 12.

AN IMPROVED SEM FOR OPAQUE SPECIMENS

83

(11) MU¨ller, E. W.: ‘‘Oberfla¨chenwanderung von Wolfram auf dem eigenen Kristallgitter,’’ Zeitschrift fu¨r Physik, 1949, 126, p. 642. (12) Knoll, M.: ‘‘Aufladepotentiel und Sekunda¨remission elektronenbestrahlter Ko¨rper,’’ Zeitschrift fu¨r Technische Physik, 1935, 16, p. 767. (13) von Ardenne, M.: ‘‘Das Elektronen-Rastermikroskop, Theoretische Grundlagen,’’ Zeitschrift fu¨r Physik, 1938, 109, p. 553. (14) von Ardenne, M.: ‘‘Das Elektronen-Rastermikroskop, Praktische Ausfu¨hrung,’’ Zeitschrift fu¨r Technische Physik, 1938, 19, p. 407. (15) Zworykin, V. K., Hillier, J., and Snyder, R. L.: ‘‘A Scanning Electron Microscope,’’ Bulletin of the American Society for the Testing of Materials, 1942, 117, p. 15. (16) Langmuir, D. B.: ‘‘Theoretical Limitations of Cathode-Ray Tubes,’’ Proceedings of the Institute of Radio Engineers, 1937, 25, p. 977. (17) Haine, M. E., and Einstein, P. A.: ‘‘Characteristics of the Hot-Cathode Electron-Microscope Gun,’’ British Journal of Applied Physics, 1952, 3, p. 40. (18) Hall, C. E.: ‘‘Dark-Field Electron Microscopy,’’ Journal of Applied Physics, 1948, 19, p. 198, 271. (19) Bruining, H., and de Boer, J. H.: ‘‘Secondary Emission,’’ Physica, 1938, 5, p. 17, ibid., 1939, 6, p. 941. (20) Palluel, P.: ‘‘Composante rediVuse´e du rayonnement e´lectronique secondaire des me´taux,’’ Comptes Rendus de l’Acade´mie des Sciences, 1947, 224, p. 1492. (21) Palluel P.: ‘‘Sur la me´canisme de la rediVusion e´lectronique par les me´taux,’’ ibid., p. 1551. (22) de Vries, H. L.: ‘‘The Quantum Character of Light,’’ Physica, 1943, 10, p. 553. (23) Rose, A.: ‘‘Television Pick-up Tubes and the Problem of Vision,’’ Advances in Electronics, I. (Academic Press, 1948), p. 131. (24) Schade, O.: ‘‘Electro-Optical Characteristics of Television Systems,’’ RCA Review, 1948, 9, p. 5. (25) Nelson, R. B., Johnson, R. P., and Nottingham, W. B.: ‘‘Luminescence During Intermittent Electron Bombardment,’’ Journal of Applied Physics, 1939, 10, p. 72. (26) Allen, J. S.: ‘‘An Improved Electron Multiplier Particle Counter,’’ Review of Scientific Instruments, 1947, 18, p. 739. (27) Baxter, A. S.: ‘‘Detection and Analysis of Low-Energy Disintegration Particles,’’ Dissertation, Cambridge University, 1949. (28) King, P. G. R.: ‘‘The Skiatron or Dark-Trace Tube,’’ Journal I.E.E., 1946, Part IIIA, 93, p. 289. (29) Bruck, H., and Bricka, M.: ‘‘Sur un nouveau canon e´lectronique pour tubes a` haute tension,’’ Annales de Radioe´lectricite´, 1948, 3, p. 339. (30) Bruck, H., and Romani, L.: ‘‘Sur les proprie´te´s de quelques lentilles e´lectrostatiques inde´pendantes,’’ Cahiers de Physique, 1944, 24, p. 1. (31) Grivet, P.: ‘‘Le microscope e´lectrostatique C.S.F.,’’ Le Vide, 1946, 2, p. 5. (32) Bruck, H., and Grivet, P.: ‘‘Improvements in the Electrostatic Microscope,’’ Annales de Radioe´lectricite´, 1947, 2, p. 242. (33) Rang, O.: ‘‘Der elektrostatische Stigmator, ein Korrektiv fu¨r astigmatische Elektronenlinsen,’’ Optik, 1949, 8, p. 518. (34) Nuttall, T. C.: ‘‘Phase Correction and Gamma Correction,’’ Journal of the Television Society, 1949, 5, p. 257. (35) Jesty, L. C., Moss, H., and Puleston, R.: ‘‘Wartime Developments in Cathode-Ray Tubes for Radar,’’ Journal I.E.E., 1946, Part IIIA, 93, p. 149. (36) Chance, B., Macnichol, E. F., and Williams, F. C.: ‘‘Wave-forms’’ (M.I.T. Radiation Laboratory Series, McGraw-Hill, New York, 1948). (37) Wagner, P. B.: ‘‘Secondary Electrons of High Velocity from Metals Bombarded with Cathode Rays,’’ Physical Review, 1930, 35, p. 98. ¨ ber sekunda¨re Kathodenstrahlung,’’ Physikalische Zeitschrift, (38) Baltruschat, M., and Starke, H.: ‘‘U 1922, 23, p. 403. ¨ ber die Geschwindigkeit der sekunda¨ren Kathodenstrahlung,’’ Annalen der Physik, (39) Becker, A.: ‘‘U 1925, 78, p. 228. (40) Trump, J. G., and van de Graaf, R. J.: ‘‘The Secondary Emission of Electrons by High-Energy Electrons,’’ Physical Review, 1949, 75, p. 44.

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(8) APPENDICES (8.1) Measurements on the Reflection of High-Velocity Electrons (8.1.1) Previous Measurements. Measurements on the electron emission from surfaces bombarded with high-velocity electrons have been published by Baltruschat and Starke38 (1922), Becker39 (1925), Wagner37 (1930), Palluel20,21 (1947) and Trump and Van de Graaf40 (1949). Baltruschat and Starke measured the intensity and velocity distribution of the electrons emitted from various metals with bombarding (or primary) electron voltages of   up to 30 kV and with angles of incidence of 0 and 70 . Wagner measured the velocity distribution of the electrons with primary-electron voltages between 16 and 40 kV  and an angle of incidence of 45 . Trump and Van de Graaf’s results were for normal incidence and primary-electron voltages of between 40 and 300 kV; they measured the total electron emission and also the proportion of electrons having energies greater than 800 eV. Palluel measured the electrons emitted with velocities greater than 40 volts from metals bombarded at normal incidence with electrons having energies of up to 20 keV. There are some diVerences in the numerical results given in these papers, but the qualitative results are similar and are summarized in the following paragraphs. The velocities of the emitted electrons vary over a wide range, the majority of those with high velocities having energies greater than three-quarters of the primary electron energy and the slow electrons having energies less than 40 eV. Becker classified the emitted electrons according to their energies. Electrons with energies near that of the primary electrons he called ‘‘reflected,’’ electrons which had suVered a number of encounters with nuclei and had lost energy he named ‘‘rediVused’’ and slow electrons knocked out by primary and rediVused electrons he called ‘‘secondary.’’ The maximum energy of electron of the latter class is about 40 eV. Not all writers have followed this method of classification, and in this paper only two classes of electrons are distinguished: ‘‘secondary’’ electrons includes all those emitted with energies less than 40 eV and ‘‘reflected’’ electrons all those energies greater than this value. The proportion of the electrons reflected increases with primary-electron energy, but the total emission falls. The emission is greater with metals with high atomic number and it also increases with the angle of incidence of the primaries. Palluel published a curve connecting the intensity of the reflected electrons (or ‘‘rediVused’’ component in his terminology) with the atomic number of metals. (8.1.2) Measurement of the Variation of Emission with Angle. 

The current collected by an electrode subtending an angle of 30 at the specimen was measured with the apparatus shown in Fig. 11. S is the specimen bombarded by the electrons from the gun G and the primary beam current is measured by withdrawing the specimen and allowing the beam to enter the Faraday cage C. The brass electrode E collects the electrons reflected by S

85

AN IMPROVED SEM FOR OPAQUE SPECIMENS



Fig. 11.—Measurement of current collected by electrode subtending an angle of 30 at the specimen, for diVerent specimen angles. Diameter of cylinder ¼ 3
5 cm. Length of cylinder and electrode ¼ 3 cm. 

over an angle of 30 . The specimen can be turned through an angle y and is biased 100 volts positive with respect to the electrode E and the earthed cylindrical screen. Thus only the reflected electrons can reach E. These will cause electron emission from E, and the emitted electrons will travel back to the specimen, causing the current measured by ME to be somewhat lower than the actual current in the reflected beam. The electrons from S strike E normally, and it is therefore possible to calculate the necessary correction from a knowledge of the emission ratio with normal incidence. About 45% of the electrons hitting E are reflected and the current measured by ME must be multiplied by 1
8 to give the true beam current from S. The results of these measurements are plotted in Fig. 3 (p. 66) for aluminium, zinc and tungsten at 25 and 40 kV. The curves are very similar and it appears that the density of the metal has little eVect on the intensity of the reflection. The diVerences between the curves for the diVerent metals could be caused by variations in the surface  finish. The values of the slope of the curves for a mean value of y0 of 25 , and the percentage of the primary-beam current reflected for this angle, are given in Table 1 for these three metals. There was no variation in the emission with time, even when a visible film of decomposed pump oil had appeared on the surface of the specimen. DISCUSSION BEFORE THE MEASUREMENTS AND RADIO SECTIONS, 6TH JANUARY, 1953. Dr. J. Thewlis: My interest in electron microscopy is that of a user, and my first reaction to the paper was, How can the scanning microscope help me? One of the troubles with the conventional electron microscope has always been that it is restricted to the examination of specimens whose thickness must be only a small fraction of a micron. There has therefore been a demand for many years for some kind of microscope that would permit the examination of thicker specimens, but for

86

D. MCMULLAN

Table 1 Specimen

Voltage

Current to electrode E y ¼ 25

Tungsten

..

Zinc

..

..

Aluminium

..

kV 40 25 40 25 40 25

% 12
2 10
0 11
5 9
5 12
5 11
0





Variation with y (y0 ¼ 25 ) % per deg 6
5 7 7 7
5 9 10

many years nothing eVective has been done about it. Within the last year, however, no fewer than four reflection-type electron microscopes have been described, and now we have the author’s, which is another variety although working on a somewhat diVerent principle. The next question I asked was, What are the advantages and disadvantages of the two types of reflection microscope? The author states that the diYculties of resolution have not been overcome in the conventional reflection electron microscope, but I was under the impression that the resolution was, in fact, somewhat better than that which he obtained with the scanning microscope. One starts with a bias in favour of the usual type of reflecting microscope, because it involves less additional complexity than the scanning type. With that bias in mind one feels the need for it to be shown that the scanning type is more than equal to the reflecting type, and I should therefore be interested to know the answers to one or two further questions. First, what are the best resolving powers expected from reflecting and scanning microscopes, and how will they be achieved in the latter? One of the diYculties is the eVect due to penetration of the specimen corners, but the author thinks this can be overcome by restricting the collection of electrons to those which have suVered a small loss of energy. I should like to know how much this will limit the sensitivity of the apparatus. I should like the author’s view on the diYculties which might be introduced by the presentation. In the conventional type one gets the whole picture on a small plate about 2 in square and then uses photographic enlargement in order to obtain the final picture, which gives the advantage of a small record containing a complete field of view. To obtain such a field of view in one shot without photographic enlargement, as in the scanning microscope, assuming a magnification of, say, 50 000 or 100 000 times, we should need a cathode-ray tube screen of 10 or 20 in square. We must bear in mind, therefore, that however great the electron-optical resolution we obtain with the scanning microscope, we cannot make use of it to the same extent as in the conventional electron microscope unless we have a cathode-ray screen capable of giving a magnification of 50 000–100 000 times. So it seems necessary to develop some means of producing a suYcient number of lines to the inch for presentation on the cathode-ray tube and a suYciently fine grain in the cathode-ray tube screen. Even so, the keeping of records will not be easy.

AN IMPROVED SEM FOR OPAQUE SPECIMENS

87

I should be interested to know how the results obtained by the scanning microscope and by the reflection microscope compare with the results obtained by replica techniques, and to know how easy it is to examine specimens of, say, biological materials from which it is quite diYcult, if not impossible, to obtain replicas. Mr. A. W. Agar: This microscope should be useful for the high-resolution examination of surfaces with deep structure. Such surfaces cannot normally be reproduced successfully by replicas for examination in the conventional electron microscope; nor can they be observed in reflection because of the very small angle of viewing. Another interesting feature is the scanning of the specimen at normal incidence, where the response is in terms of atomic number. In the transmission microscope the scattering at the specimen is dependent on the mass thickness, and low-angle reflection examination shows only the topography of the surface. I should like to know whether the author has operated the microscope in this mode and whether he considers that the sensitivity of operation can be made great enough to distinguish between diVerent materials in a smooth surface. The experience of most workers with the reflection microscope is that the rate of contamination of the specimen is very high, a visible layer being formed in a few seconds. The current density is about a thousand times greater than that employed in the scanning microscope, but it illustrates that trouble might be experienced if the current density were materially increased. The use of magnetic lenses instead of electrostatic ones would simplify the problem of adequate screening and of astigmatism correction, and would probably be preferable if it were not intended to employ focus modulation. Dr. D. Gabor: The paper contains a passage which can be easily rounded oV into a powerful argument against the scanning microscope as a high-resolution instrument. The author states that high resolution can be obtained only if the beam current is kept at a value of 10 10i /T amp. This is only about three times the so-called ‘‘coherent current’’:* Icoh ¼ ðph2 =32mkTÞi which is the current at which the electron beam is capable of producing interference fringes of any high order. This is a very small current, of the order of 10 13 amp or less; one is forced to use it in diVraction microscopy, where the information about the object is extracted from the interference fringes. In a high-resolution scanning microscope one has therefore the disadvantage of diVraction microscopy; the very small current, entailing very long exposures but without its compensating advantages; the high contrast, in particular the phase contrast produced by coherence. The author apparently subscribes to an old error due to von Ardenne, which ought not to be left uncontradicted. This is the statement that with the scanning microscope it ought to be possible to investigate very thick specimens, as the scanning instrument is free from chromatic error. If this were true it would be an enormous advantage of the scanning microscope, as in the opinion of James Hillier, one of the best experts in this field, the microscopy of thick specimens is the point where improvements are *Gabor, D: Proceedings of the Physical Society, B, 1951, 64, p. 464.

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most urgently needed. But the statement is wrong in two essential points. The first is that, although the chromatic error does not appear in the scanning microscope, the electron diVusion in thick specimens is strong enough to prohibit resolutions better ˚ . The second is that the chromatic error in the conventional than perhaps 200 A transmission microscope has no appreciable adverse eVect on the resolution; on the contrary it improves it by enhancing the contrast.y It may sound incredible that an aberration is beneficial. In the last resort this is due to the curious fact that for the most part electrons in solids do not lose their energy gradually and almost continuously, but in rather large discrete steps of 15–25 eV. Thus the electrons which have suVered losses will form, owing to the chromatic aberration, a rather broad fairly even fringe around any scattering object point, which itself is clearly shown up by the missing electrons. The situation is quite diVerent in dark-field illumination; here the scanning microscope really has an advantage. I want to ask also why the author has preferred the dynode arrangement shown in Fig. 6, which he himself characterizes as of low eYciency, and why he has not tried to focus the whole beam of secondary electrons into the multiplier? Mr. M. E. Haine: This instrument should not be considered as a competitor to the normal transmission-type instrument. It represents a new method in so far as the contrast mechanism employed is diVerent from that used by von Ardenne and Hillier. It is important that such new methods should be explored very thoroughly. Up to the present, we can judge from the paper that the instrument has been successfully made to operate and the contrast mechanism proved. The resolving power is, however, far from the theoretical values suggested. The replica method, although having its limitations, is hardly likely to be replaced by either the scanning or the reflection microscope. Both these instruments are more likely to play their part in investigations of a few specialized problems. If comparisons between the various methods are to be made, it would be wise to include both the elegant method of Castaing, who makes use of the X-rays produced at the specimen surface to produce contrast in a scanning microscope, and the ion-induced secondary-electron emission microscope under development in Germany. Castaing’s instrument has the very important advantage that it enables an analysis of the surface composition to be made point by point over the surface under examination. The author seems to suggest that the resolving power of the scanning microscope will be limited mainly by the electron spot size. Electron penetration is mentioned late in the paper. It appears very probable, in fact, that electron penetration will provide much the most serious limitation. No doubt this could be reduced by collecting electrons of reduced energy spread, but only at the cost of image brightness and hence exposure time. It is of interest that electron penetration and energy loss occur also as limitations in the reflection electron-microscope. Energy loss and penetration are to a certain extent mutually related. In the reflection microscope the energy loss provides the limitation to resolution, in the scanning instrument, the penetration. The final resolving powers would appear about equal and much inferior to those of the transmission instrument. y

Gabor, D: ‘‘The Electron Microscope’’ (Hulton, London, 1946), p. 32.

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The author’s description of the noise limitation is surely more complicated than need be. The essential facts are relatively straightforward. If the detector collects n electrons from each picture point, statistical fluctuations of the order of the fraction pffiffiffi 1= n will occur. For the eye to discriminate a fractional contrast diVerence C between adjacent picture points, the contrast ratio must exceed these noise fluctuations by a factor of about 3. Hence n > 9=C 2 For a contrast discrimination of 5%, ne ¼ 3 600, i.e. at least 3 600 electrons must be collected from each picture point. Following this, of course, is the condition that no noise greater than the input noise must subsequently be introduced into the system. This requires a quanta density level always greater than at the input and no spurious noise of non-quantum nature. The author suggests that the quanta density at the eye, ns, should be made equal to the primary signal density, ne, so that no noise will be visible to the eye. This is surely not a desirable thing to do. It means that for a low value of ne, i.e. a value where any detectable noise is present, the screen brightness is reduced until the noise is not visible. Even for 10% noise amplitude this means a very low brightness level on the cathode-ray tube. A better solution is simply to defocus the cathode-ray tube or reduce the picture size on it, but the more logical procedure is to increase the spot size and beam current of the microscope itself until no noise is present. The eVect is strictly analogous to overmagnifying a picture of any sort until the resolution is greater than that of the eye. It is rather a serious disadvantage not to be able to observe a picture giving the full resolution, and I suggest it would be worth while to incorporate an arrangement limiting the total field very severely so that higher resolution over a restricted area could be obtained for accurate focusing. This would necessitate reducing the number of scanning lines and the length of each line for visual observation. I should like to suggest that the ultimate aim in this type of instrument should be to combine the scanning electron microscope, Castaing’s X-ray method, glancingincidence electron microscopy by the scanning method, point-projection X-ray microscopy (with or without scanning), microbeam electron and X-ray diVraction. All these methods utilize a micro-electron probe, and several of them the same type of amplifiers and recorders. The same electron-optical system would be used, with adaptors on the bottom end for the diVerent techniques. In this way an instrument of great versatility would be produced. Dr. H. G. Lubszynski: I should be interested to know whether the scanning microscope of the transparent type will enable us to observe living objects better than the conventional electron microscope. Borries has made calculations based on the sensitivity of photographic plates; for a given density and 50% mortality of the specimens observed, he found that in the best case for the conventional electron ˚ resolution can be expected. This is roughly microscope no more than 500 to 2 000 A the same resolution as that obtained with the scanning electron microscope, and I should like the author to state how the eYciency of the transparent scanning

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microscope, so far as signal/noise ratio and intelligibility are concerned, compares with that of the conventional type, and whether there is any hope that we may get away with fewer electrons per picture point on the specimen than with the conventional electron microscope. One point which makes me doubtful is the rather poor eYciency of collection, because as we have seen, the reflected primaries are used for generating the video signal and the number of secondaries produced by them in the multiplier must be rather small; there is also the angle limitation. Signal/noise ratio depends on the ratio of the number of primary electrons in a picture point of the specimen to the number of secondary electrons leaving the first dynode of the multiplier. I think that the use of modern types of storage tube might be emphasized more in connection with the scanning microscope, because tubes are now available which will give high-intensity displays with high frame-repetition rates and with storage times varying from a few seconds up to half an hour or more. The corresponding integration of noise will probably help the scanning microscopes considerably. Is there any hope that the tolerance of biological specimens for very short and intense exposures may be greater than expected from simple integration? Cooling on a solid base may allow a greater amount of radiation to be used than in the conventional electron microscope. Being interested in television techniques, I think that the use of the scanning technique involving an amplifier chain is very important, because of the possibility of introducing gamma correction. We can thus alter the contrast law so as to magnify the contrast over part of the range as desired. Thus one can increase an originally small contrast either in the dark or light parts of the picture at the expense of the rest. This should be very useful. On the assumption that there may be gains in intelligibility obtainable by applying television techniques, there is, of course, the other possibility of looking at the fluorescent-screen image in the conventional electron microscope with a pick-up tube and camera. In this case all the possibilities of gamma control, etc., apply, and I wonder which is likely to be the more sensitive method. Dr. R. Feinberg: There is no doubt that the scanning electron microscope represents a very interesting development in instrument technique, and I wish to support Mr. Haine’s view that we do not know yet all the potentialities of this instrument, and that future development and further work will show what can be achieved with it. The instrument as described by the author implies that only metal specimens can be examined. Here, the high-velocity electrons from the specimen are used as a signal for the electron multiplier. It would appear that the scanning electron microscope could also be used to examine the surface condition of insulator specimens if the electron collector were suitably modified to collect the low-velocity electrons emitted from the specimen surface. The collector could, for example, be a thin metal film covering the specimen surface. Since the beam electrons impinging on the surface may aVect its charge and hence its potential, it will probably be necessary to use a single-frame scan of the specimen surface. With regard to nomenclature, I think that the term ‘‘scanning electron microscope’’ is very appropriate. It should be adhered to in contrast to the term for the

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other type of electron microscope, the flying-spot microscope, which in general structure is similar to the scanning electron microscope but has a fundamental diVerence in its operation. THE AUTHOR’S REPLY TO THE ABOVE DISCUSSION Mr. D. McMullan (in reply): In reply to Dr. Thewlis, a figure for the best possible resolving power to be expected from the reflecting microscope has been given by ˚ Haine:* by using a very small aperture in the objective lens a value less than 50 A might be attained, but exposure times would become inconveniently long. For the reasons given in Section 5 of the paper the limiting resolving power of the scanning electron microscope would appear to be of the same order as that of the reflection microscope, but the scanning electron microscope has the considerable advantage of freedom from gross geometrical distortion and of approximately equal resolving powers in both dimensions. Since there are only about 500 lines in the micrographs it is no use enlarging small areas after recording. The interesting area must be selected beforehand and recorded at the full magnification required. This does not appear to be a major disadvantage. Mr. Agar questions scanning at normal incidence: I have operated the microscope in this mode, and find that diVerent materials can be distinguished if there is a large diVerence between their atomic numbers. I agree with Dr. Gabor that the scanning electron microscope is not a highresolution instrument. Its advantages lie in its ability to magnify opaque specimens. For transparent specimens the conventional electron microscope obviously has the advantage, except under dark-field illumination and also for one particular class of thick specimen, typified by absorbing particles lying on a thick film. I have been able to obtain sharp micrographs of particles lying on 2
5-micron aluminium film using an accelerating voltage of only 25 kV. This would be impossible in the conventional microscope. Better beam collection would, of course, greatly improve the performance of the microscope, but mechanical diYculties have so far prevented this from being carried out. Mr. Haine suggests increasing the spot size and beam current of the microscope until no noise is present in the picture, the brightness being kept high. This will obviously prevent the best resolving power from being obtained. His analogy is a useful one, but increasing the brightness beyond the point where noise becomes visible is analogous to overmagnifying a picture, and it is well known that usually no purpose is served in doing so. He seems to imply the reverse. In reply to Dr. Lubszynski, it is probable that with the scanning microscope specimen dosage will be less than with the conventional type. Nearly every electron passing through the specimen can be recorded, and the contrast will be limited by the fluctuation of the electron beam, which is, of course, a fundamental limitation.

*Cosslett, V.E.: Nature, 1952, 170, p. 861.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.2A Exploring the Potential of the Scanning Electron Microscope K. C. A. SMITH Formerly at: Engineering Department, University of Cambridge

I. Introduction Until Charles Oatley invited me, at the end of the Easter Term 1952, to consider the possibility of joining his group, I had never entertained the idea of doing research at Cambridge. I had just completed Part II D (Electrical Option) of the old Mechanical Sciences Tripos, an unclassified examination—pass or fail—and although I knew I had passed, the order of merit was unknown. My academic performance in Prelims and Part I (first- and second-year examinations) had hardly been distinguished, so I saw no reason to suppose that I had achieved anything other than an average performance in Part II. My plan, therefore, was to spend a fourth year at Cambridge reading for the Chemical Engineering Tripos, for which I had already been accepted. Prior to coming up to Cambridge I had served an industrial apprenticeship at the British Thomson-Houston Company in Coventry, studying parttime for the Higher National Certificate in electrical engineering. This was not the ideal preparation for the Fast Course of the Mechanical Sciences Tripos with its multiplicity of non-electrical subjects, and although I felt more at home concentrating on electrical subjects in the third year, subjects such as aerial theory (taught by Professor E. B. Moulin) still presented considerable diYculties. What had lifted my examination results in Part II (D) to the level necessary to be considered for research was, I subsequently realized, due largely to Oatley’s lectures in physics, described briefly in my Royal Society Biographical Memoir (Appendix I). It was the first real physics course I had attended; the clarity and style of lecturing were inspiring, far superior to anything I had encountered previously at Cambridge. For a change I was able to absorb the material with ease, which undoubtedly stood me in good stead in the examination. 93 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Oatley’s method of allocating projects at that time from among the list of aspiring research students was to give them all a tour of the laboratory, explaining each of the available projects in detail, and then allowing choice according to order of merit in the Tripos. Most of the projects seemed to be quite interesting, all stemming directly from Oatley’s background in physics, and with my newly acquired taste for the subject I felt that I could have had a reasonable shot at them. But the moment I saw the flickering green screen of Dennis McMullan’s SEM I knew that this was what I wanted to do. One reason, I confess, for this immediate aYnity was that some parts of it looked remarkably like the television set I had constructed shortly after the war using war-surplus components and an old RAF radar display, the design for which was published in one of the electronics magazines of the time. I was to discover that the first SEM was likewise constructed largely from war-surplus components—no wonder it looked familiar! Unfortunately, from my point of view, the chance of getting onto this project seemed pretty remote since by then I had discovered that I was fifth on the list. I was, therefore, astonished when Oatley called me in and said that the SEM was still available and would I like it. I could not believe my luck that the others had turned down what seemed to me to be the outstanding project. But, of course, no one realized at the time just how big an opportunity the SEM presented. The second piece of luck, I quickly discovered, was that Dennis McMullan was staying on for another year. He initiated me into the mysteries of scanning electron microscopy and taught me a great deal of general electronics besides. Without his presence during that first year I am quite certain that the project would have failed; the complexity of the instrument and the primitive nature of much of the equipment was simply too great for a raw, inexperienced research student to handle alone. Oatley appreciated this, of course, and had taken good care to ensure that Dennis was around to pass on his vast knowledge and experience. Under his tutelage my knowledge of practical vacuum physics, electronics and, of course, electron microscopy, was transformed. II. Preliminary Work From the outset there was plenty of practical electronics work to do. Dennis had hooked up the amplifier chain very quickly, so my first task was to construct a new head amplifier and associated power supplies. As explained by Dennis in Chapter 2.1A, the output of the electron multiplier has to be directly coupled to the head amplifier, which means the amplifier floats at a

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potential of several kilovolts above earth. The major problem with this arrangement is supplying power to the valve amplifier without at the same time injecting spurious signals on the low-level signal at the output of the multiplier. I chose to supply power by means of a double-screened RFtransformer using a test tube as a former, metalized inside and out with a paint known as Johnsons’s silver. The construction of this transformer turned out to be unexpectedly diYcult; it stands out in my memory as the first of many challenges. The amplifier chain itself presented few problems: the head amplifier was based on a design developed for the CPS Emitron TV camera; a new gamma correction circuit using a germanium diode as the nonlinear element was incorporated, which worked well, and a 50 Hz amplitude/phase compensating network took care of stray mains pick-up. The result was a good, clean signal at the grids of the display tubes; I felt I had made a reasonable start. While this constructional work was proceeding, I had frequent discussions with Oatley and McMullan concerning the direction my research should take. There were two obvious electron-optical problems with the microscope that clearly required attention: astigmatism and contamination of the final aperture. Dennis suggested that the latter might be overcome by the use of a heated aperture assembly. I therefore started thinking about these problems and doing some preliminary design and construction. However, it was not possible to proceed with any experimental work on the microscope itself since at that time, as previously explained, the column was undergoing modification and the move to Scroope House was under way. Probably the most important factor influencing our thinking at this stage was the fact that the future of the instrument as a viable method of microscopy was still far from certain. On the other hand, it was clear that as a piece of experimental apparatus it had great potential, and it seemed prudent to look at possible lines of research from this angle. It was about this time that Dennis produced the first cathodoluminescence pictures of phosphor crystals, and Oatley suggested that I might like to follow this work up by making measurements of the excitation and decay characteristics of single phosphor grains. The idea was to pulse the electron beam on and oV at diVerent regions of the grain and observe the decay of the light output. Accordingly, I set to work to design and construct the necessary apparatus. The design of the pulse circuitry was based on a wartime development, used in radar equipment, known as the ‘sanatron’. (It was given this name by its inventor, F. C. Williams, whose favourite description of a well-behaved circuit was ‘sanitary’.) Although I derived a good deal of satisfaction constructing this circuit and making it work, more pressing events intervened, and this work never saw the light of day.

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During this initial period I spent a substantial amount of time learning about various aspects of electron microscopy and getting to know the people in other laboratories engaged on electron-optical projects, particularly those in Dr V. E. (Ellis) Cosslett’s group in the Cavendish where Bill Nixon and Peter Duncumb were working. I also got to know Dr J. W. (Jim) Menter who was working on the reflection microscope in F. P. Bowden’s group in the Department of Physical Chemistry. At the time, the prevalent view, expressed by M. E. Haine in the discussion on McMullan’s paper (Chapter 2.1B) was that for the study of surfaces ‘the replica method, although having its limitations, is hardly likely to be replaced by either the scanning or the reflection microscope’. It was, therefore, evident that I needed to find out as much as possible about these techniques, and for replicas I was advised by Ellis Cosslett to consult Sheila Vernon-Smith who was then working in the Plant Virus Unit in Cambridge but had previously been with the British Non-Ferrous Metals Research Association. She was an acknowledged expert on the use of replicas in metallurgy, particularly of the artefacts produced in the process (Bailey and Vernon-Smith, 1950), and I subsequently received much help from her (later, she became my wife). She also put me in touch with Miss M. K. B. (Peggie) Day of the British Aluminium Research Laboratories, who kindly supplied a steady stream of etched aluminium specimens throughout the research and gave unstintingly of her time and advice. I also followed closely the work of two of my contemporary research students engaged on projects related to my own: Chris Grigson, building an electron diVraction camera (Grigson, 1955; see also Chapter 2.11), and Adrian March, building a scanning microdensitometer (March 1957). Also of relevance was the electron trajectory tracer, which supported several research students, among them Mark Barber, who subsequently used the tracer to determine trajectories of secondary electrons in the SEM (see Chapter 2.4A). Occasionally, Oatley would invite his research students to tea at his home in Gilbert Road and to meet his wife, Enid. Their semi-detached house always struck me as being surprisingly modest for such a distinguished university academic. (Some years were to elapse before I came to appreciate that a university salary does not necessarily match the distinction of the recipient.) Matters improved in this respect, however, when in 1955 or thereabouts the Oatleys moved into a new house built to their own specification. Their architect was A. H. Chapman, who was the Secretary of the Engineering Department at the time. The house was in Porson Road; a new road opened up by Trinity College, which had sold the land for housing development. College Fellows were oVered first choice of plots; consequently, many of Oatley’s neighbours were Trinity dons.

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A. Initial Problems My first year passed smoothly but all too quickly and soon the time came for Dennis McMullan to leave the Engineering Department, an event I viewed with trepidation. I was not at all confident that I could keep the instrument going in the absence of its creator. My worst fears were quickly realized. First, the counting circuits, keeping line and frame time-bases in step, kept going out of synchronization in spite of all my tweaking. Eventually, I abandoned the locked time-base and used a free-running time-base instead, which worked satisfactorily since it was used only for direct visual observation of the picture and did not aVect recording. Next, electrostatic discharges associated with the electron multiplier, giving rise to ‘snow’ on the image, were increasingly troublesome until image recording became virtually impossible. This phenomenon had started before Dennis left, and he had eVected a temporary cure by blowing hot air onto the junction box at the output of the multiplier. The eVect was worst in the mornings when the instrument was first switched on, and starting the hot-air blower first thing had become a daily routine. I eventually traced the trouble to the porcelain insulators on which the junction box was mounted; replacing these cured the fault.

III. Electron-Optical Modifications A rather more serious problem was the deteriorating condition of the final electrostatic lens, in which discharges were occurring at progressively lower accelerating voltages, requiring ever more frequent cleaning of the lens insulators. This was ascribed to contamination from pump oil and from the liberal coating of Apiezon grease that had to be used on the flat neoprene-rubber vacuum seals (see Fig. 6 in McMullan, 1953 (Chapter 2.1B)). I finally decided that the time had come to build a completely new objective lens unit incorporating a heated aperture stage, a stigmator and a new lens. I decided also to replace the double-deflection scan coil assembly with a more eYcient unit incorporating ferrite cores. Wherever possible, O-ring seals (then representing a ‘new’ technology) were used to replace the old flat seals. Little did I realise what an ambitious programme this represented, and many months were to elapse before construction was completed and the unit was ready for testing. Then began the most diYcult and frustrating experience of my time as a research student. With no great expectation that the new components would work, I initially assembled the complete unit on the microscope. My

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expectation was quickly confirmed, but at least the vacuum was good and the lens withstood the highest voltage available. Image resolution was, ˚ however, of the order of a micrometre or so (compared with around 500 A previously achieved). Embarking on a gradual process of elimination, first the heated aperture was removed, and then the stigmator assembly; but this made no diVerence—still the resolution remained stubbornly round the 1 mm mark. At that stage the configuration appeared to be exactly the same as McMullan’s original configuration apart from the lens itself (McMullan, 1952). This also was replaced without success. At this point I had a microscope that appeared to my eyes optically and mechanically identical to that of the original McMullan set-up, but with a resolution worse by at least a factor of 20. I was completely stumped! Mechanically, the only diVerence that I could perceive between my arrangement and that of the original was that the copper aperture disc (18 inch diameter) was mounted at the end of an aluminium tube of the same diameter and about 2 inches long. This had been installed as a temporary measure after removing the heated aperture and stigmator. In McMullan’s arrangement the aperture was mounted at the end of a tube having an internal diameter of about 38 inch. How could this diVerence possibly be the cause of the trouble? Contamination of the interior of the tube was an obvious possibility, but this was eliminated following repeated cleaning and its final replacement. Eventually, in desperation, I assembled the microscope for the umpteenth time with the original aperture carrier tube, and much to my astonishment, and not a little relief, the original performance was immediately restored. The explanation for this anomalous behaviour turned out to be remarkably simple. In the original arrangement the hole in the fluorescent screen above the scan coil assembly, being of smaller diameter than the tube carrying the aperture, prevented the beam from striking the walls of the aperture carrier tube, whereas in my arrangement this was not the case. Thus, electrons reflected from the walls of the narrow-bore tube were able to pass through the aperture, producing a huge disc of confusion in the plane of the specimen. This elementary electron-optical error cost several months of precious research time. Each change to the final lens assembly, however trivial, necessitated dismantling and re-assembly of the entire column. My abiding memories of this era are the endless wait for the system to pump down, and the seemingly never-ending routine of changing the phosphorus pentoxide trap on the backing pump. However, if it was not time well spent, it was not time wasted: I learned a lot about practical electron optics along the way, and even more about the virtues of dogged persistence. After the redesign of the aperture carrier and installation of a shielding (spray) aperture above the

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scan coil assembly, all the new components worked satisfactorily. The heated aperture system, almost certainly suVering from the same fundamental design fault, was abandoned, however. I found that although changing the aperture when it became contaminated necessitated stripping down the whole column, I had had by that time so much practice that it no longer presented any problem. Throughout this diYcult period I received great support from Leslie Peters and from Oliver Wells, who had started his research at the beginning of my second year.

IV. New Specimen Chamber Entering upon my third year, I could at long last get down to the most pressing business of the research which, as indicated by Dennis McMullan in Chapter 2.1A, was to improve the electron collection eYciency. This obviously required the input aperture of the electron multiplier to be positioned as close as possible to the specimen, and this meant the construction of a new specimen chamber. Investigation of new applications that would exploit the unique imaging characteristics of the SEM was becoming a matter of urgency. At about this time Charles Oatley had suggested that I should look into the possibility of investigating point-contact rectifiers, which would involve placing a tungsten whisker at precise locations on the surface of a germanium or silicon crystal while it was under observation—an ideal dynamical application for the SEM. The specification for the new chamber therefore included a micro-manipulator for this purpose. It was also obvious that facilities for rotating and tilting the specimen were required. With these requirements in mind, I designed a large brass chamber with lots of ports and electrical and mechanical lead-throughs (Fig. 1). The chamber was constructed with remarkable rapidity in the main Departmental Workshop by an extremely skilled craftsman, Stanley Lawrence. Testing of the new chamber was equally rapid; there were no leaks, and within an hour of switching the beam on I knew we had reached a turning point. The signal level was up by a factor of 50 or so, and the picture observed was clearly characteristic of a true secondary electron image— dominated by what is today known as SE contrast. A little while later I was able to produce the curve shown in Fig. 2, which demonstrated the role of the low-energy secondary electron component. The shape of this curve was explained in my dissertation (Smith, 1956) as follows: The fact that only small changes in the potential of the specimen cause a large change in the signal intensity indicates that the major part of the signal is attributable to true secondary emission. The shape of the curve in the figure

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Figure 1. New specimen chamber with simple goniometer stage and point-contact rectifier manipulator. (From Smith, 1956.)

may be explained on the assumption that secondaries originate from two sources. Some will be produced at the surface of the specimen by the direct action of the incident beam, these may be termed ‘direct’ secondaries; others will be produced at the face of the lens and other surfaces in the vicinity of the specimen by the action of the reflected and diVuse [backscattered] components, these may be termed ‘indirect’ secondaries. As the potential of the specimen is increased positively, the direct secondaries are prevented from entering the multiplier and the curve falls steeply . . . . Those indirect secondaries produced near to the surface of the specimen will be attracted towards it as the potential is raised and so will contribute towards the initial fall in the curve. Above +20 V the curve flattens out and there is an appreciable response even when the potential of the specimen exceeds that of the multiplier input. No direct secondaries can possibly enter the multiplier in this case so that the residual response must be due to indirect secondaries produced nearer to the input of the multiplier. The slow fall in the curve is then attributable to the specimen attracting an increasing proportion of these indirect secondaries.

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Figure 2. Curve of relative signal level versus specimen potential. (From Smith, 1956.)

The part of the curve for negative specimen potentials may also be explained in terms of secondary emission. For negative potentials of a few volts, many of the direct secondaries strike the face of the lens and do not enter the multiplier, hence the sharp fall in the curve. However, on further increase of the potential in the negative direction, the higher energy direct secondaries will begin to produce other secondaries at the face of the lens; the conditions being favourable for the production of secondaries because the incident electrons strike the surface of the lens at grazing incidence. At a potential of 15 V this process causes the curve to rise again. The curve falls at high negative potentials because the field becomes strong enough to deflect electrons away from the multiplier. The experiment shows that only a small proportion, about 6%, of the total signal intensity is due to the reflected component entering the multiplier directly. Indirect secondaries produced at some distance from the specimen by the reflected component, account for possibly, from the figure, another 5 to 10 per cent of the signal intensity. Indirect secondaries produced near to the

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specimen must contribute to the total signal intensity, but the proportion of these must be small, since only a small fraction of the primary beam which is reflected hits the face of the lens near to the specimen, and of this only a fraction will produce secondaries. It may be concluded that, at a rough estimate, between 80% and 90% of the signal is made up of secondary electrons which are produced at the surface of the specimen and which then enter the multiplier without further collision.

A. Results The new specimen chamber also enabled me to explore a wide range of applications, some of which are described in the paper reproduced as Chapter 2.2B (Smith and Oatley, 1955). At the Institute of Physics Electron Microscope Conference held in Glasgow in July 1955 (Challis, 1956), I delivered a paper in which some of the results obtained were presented. In particular, the examination of fibres and various biological specimens, and the experiments on the point contact rectifier were described. During the discussion following the paper, the opinion was expressed that the replica technique was capable of much higher resolution than the SEM and could accomplish all that was necessary in electron microscopy. This left me with the uncomfortable thought that maybe the SEM would always remain the province of just a few specialized applications, such as the point-contact rectifier. It demonstrated that much still remained to be done in 1955 in order to establish the merits or otherwise of the technique. There were, however, a few microscopists who by this time had recognized the advantages of the SEM: Jan Sikorski of Leeds University was an early enthusiast who gave us much encouragement and, many years later, he was a recipient of one of the first batch of Stereoscans produced by the Cambridge Instrument Company (Sikorski and Hepworth, 1969). B. New Applications Towards the end of my third year of research, two events occurred that were to have profound consequences for the future development of the SEM. The first of these involved some experiments concerning the examination of biological specimens at water vapour pressure. In the initial experiments the specimen was enclosed in a double-walled cell and observed in transmission, the beam passing through the cell with the electron multiplier placed on the exit side of the cell to detect the transmitted beam. Charles Oatley took a keen interest in this work, and one morning he came into the laboratory with the suggestion that the transmitted electrons could be detected by means of a

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plastic scintillator and photomultiplier, the scintillator forming one wall of the cell. This arrangement worked well, and although the experiments on the water vapour cell were inconclusive, Oatley’s suggestion regarding the use of the plastic scintillator marked the beginning of a major line of detector development, details of which will unfold in subsequent chapters. The second of these events occurred when Dr Douglas Atack, then on sabbatical leave from the Pulp and Paper Research Institute of Canada (PPRIC), where he was Director of the Applied Physics Division, came into Scroope House and asked if he could try some of his pulp and paper specimens in the SEM. These experiments were highly successful (Atack and Smith, 1956) and resulted in my being employed to construct a new microscope for the PPRIC. The arrangements Oatley made with the PPRIC in this connection, and the understanding he reached with AEI regarding possible commercial development, are described in Part IV of this volume. Part of the deal involved my setting up the microscope and training personnel in its use at the Montreal Laboratories of the PPRIC. At about the same time, another ideal application for the SEM turned up in the shape of silver azide crystals. The thermal decomposition of these crystals was the subject of an investigation being undertaken by Dr (later Professor) F. P. Bowden and Dr James McAuslen (on sabbatical leave from Imperial Chemical Industries Ltd) in the Department of Physical Chemistry. Their attempts at using the reflection electron microscope had failed owing to premature ignition of the crystals under the intense illumination necessary in this mode of operation. In the SEM, with the crystals mounted on a small hot-plate, the decomposition process could be readily controlled and observed without diYculty (Bowden and McAuslen, 1956). In the autumn of 1955 McMullan’s microscope, now designated SEM1 (Oliver Wells had by then embarked on the construction of a second microscope, SEM2) was placed in the capable hands of Tom Everhart.

V. Post-Doctoral Work: SEM3 Design of the new SEM for the PPRIC, designated SEM3, was commenced in the summer of 1956, although prior to this a good deal of thought had already gone into the specification of the new instrument. (I combined this work with writing up my dissertation, submitted in September 1956, my examiners being V. E. Cosslett and J. G. Yates.) Both SEM1 and SEM2 were electrostatically focused instruments, but the electron-optical and practical advantages of electromagnetic lenses for probe forming had earlier been demonstrated by Bill Nixon, Peter Duncumb and others and were fully

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appreciated when it came to the design of SEM3. It was a fortunate coincidence at the time that G. Liebmann, working at the AEI Aldermaston Laboratory, had just published a theoretical study of the magnetic pinhole lens (Liebmann, 1955). It was evident to me that this was an ideal configuration for the SEM objective lens since it combined an advantageous ratio of working distance to spherical aberration while allowing the specimen to be situated in a substantially field-free region. From Liebmann’s paper it was possible to decide on the pole-piece bore and gap dimensions for a scanning magnetic objective of this type. AEI, at the time, had just ceased production of a small TEM, the EM4, and as part of the cooperative arrangements with them, they made available a spare microscope. Although this was an old model and not fully functional, I was able to utilize many of its components in the construction of SEM3. Richard Page, designer of the EM4, generously gave of his time in making me conversant with the EM4, and he also assisted with the design of the iron circuit of the Liebmann pinhole lens. When oVered the EM4, my first thought was to strip out a couple of the lenses and use them for the demagnifying section of the new column, but on further consideration it seemed mechanically simpler and quicker to retain the whole of the EM4 column intact (with the exception of the gun which, being air insulated, had to be scrapped). I then decided to retain transmission facilities in the new instrument by building a simple conversion section housing the transmission viewing screen and camera, which could be bolted on to the top of the instrument in place of the scanning section. (All our experience at that time favoured a microscope orientation with the gun at the bottom and specimen chamber uppermost at desk-top level (the arrangement adopted by Wells for his SEM2)). Following M.E. Haine’s suggestion during the discussion on McMullan’s paper on SEM1 (Chapter 2.1B), I decided to incorporate also a facility for operating the instrument as an X-ray projection microscope. This was based on Bill Nixon’s work in Cosslett’s Group at the Cavendish Laboratory (see Chapter 3.2B). While the construction of SEM3 was still underway, Tom Everhart perfected the scintillator-photomultiplier secondary electron detector (Everhart, 1958). I was thus able to incorporate this ingenious and relatively simple device rather than the electron multiplier detector, with its complex associated electronics, saving months of design and construction work and adding immeasurably to the reliability of the finished instrument. Through this fortuitous timing, many of the important elements contributing to the success of later commercial instruments—pinhole magnetic objective, Everhart–Thornley detector, double-deflection scanning, goniometer specimen stage—were brought together in a single scanning instrument for the first time.

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Construction of SEM3 was carried out in the Engineering Department by a team of people already well versed in the technology required to build an SEM. Chief among these was Leslie Peters, who was ably assisted by H. Asplin, P. Woodman, J. Brown, S. H. Lawrence, and H. Stutters. A. A. K. Barker, then in charge of the Department’s Instrument Workshop, provided invaluable advice concerning the machining of components. He suggested machining the specimen chamber from a solid block of steel; the specimen chambers of the two previous instruments had been fabricated in brass, necessitating extensive mu-metal magnetic screening. The 50-kV EHT set from the EM4 was not functional, and I failed to repair it in time to fit it to the new instrument. I therefore utilized an excellent 20-kV unit designed by Peter Spreadbury (see Chapter 2.5). This voltage was too low for satisfactory operation of the microscope in the transmission mode, and I fully intended to build a 50-kV unit when the installation of the microscope in the Montreal Laboratories of the PPRIC had been completed. This intention was never realized; the heavy workload on the instrument in its scanning mode precluded any further development of its ancillary operating modes. Construction of SEM3 was completed around February 1958. The console was found to be too wide to go through the laboratory door, but after appropriate surgery on the doorframe, the component parts of the instrument went oV to AEI Manchester where they were packed and shipped to Canada. My wife, Sheila, and I sailed for Canada at the beginning of April. A. Installation at PPRIC When SEM3 arrived at the PPRIC, it was substantially complete and preliminary tests had been conducted in Cambridge, but some work remained to be done on the lens control circuitry; PPRIC personnel quickly completed this. A more substantial problem was the lack of engineering drawings and adequate documentation on the instrument since much of its construction had been based on engineering sketches and direct verbal communication among the team building the instrument at Cambridge. Also, no operating manual existed, and no documentation was available concerning the associated laboratory techniques required to prepare specimens and maintain and service the instrument; indeed, a complete electron microscope laboratory had to be set up from scratch at the PPRIC, and people had to be trained in all aspects of the work. All of this entailed a tremendous commitment on the part of the institute and its staV to what was, it must be remembered, still a relatively untried and untested technique. It says much for the enthusiasm, and powers of persuasion, of Dr Lincoln R. Thiesmeyer, President of the PPRIC, that financial backing for this project was always forthcoming.

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The manuals for SEM3 were produced at PPRIC during the first year or so of operation. Among the staV involved with this work were Bob Lindsay and Sheila Smith, who had been engaged to help set up the microscope laboratory. Initially, the main users of the microscope were myself (Smith, 1959) and Doug Atack, who continued with his study of groundwood fibres begun in Cambridge and John Buchanan who initiated a study of papermaking wood chips (Buchanan and Smith, 1960; Buchanan and Lindsay, 1961). John Buchanan became one of the heaviest users of the SEM in those early days (Buchanan and Washburn, 1964; Washburn and Buchanan, 1964). Soon, the microscope became increasingly used by other members of the institute working in many diVerent areas (McGrath et al., 1962) and during this early period they pioneered many of the techniques now routine in scanning electron microscopy. Towards the end of my time with the PPRIC, I was invited to contribute to an encyclopaedia of microscopy that was being compiled and edited by George L. Clark of the University of Illinois. When the encyclopaedia was published in 1961, I found my article (Smith, 1961) sitting only two pages removed from one on replica and shadowing techniques. In it there appeared the statement: ‘Large specimens cannot be examined directly except by means of reflection electron microscopy.’ After two years of immensely rewarding work at the PPRIC, I left SEM3 in the expert hands of Bob Lindsay who continued with the development and improvement of the instrument, assisted by Owen Washburn and Alex Rezanowich. SEM3 was finally retired in 1967, being replaced by a new Stereoscan. The console of SEM3 is now stored in the National Museum of Science and Technology, Ottawa. Thanks to the initiative and enterprise of Peter Sewell, an early enthusiast of scanning electron microscopy, this saw the light of day once more when it was exhibited in 1995 on the occasion of the 22nd Annual Meeting of the Microscopical Society of Canada. B. Postscript The microscope was first described briefly in a paper presented at the European Conference on Electron Microscopy at Delft in 1960 (Smith, 1960). At that time a drawing of the column existed only in the form of a full-scale blueprint, and it was not possible in the circumstances to produce a version suitable for inclusion in the paper. However, 37 years later, with the encouragement and help of a number of colleagues, including Paul Brown, Alan Agar and Tom Mulvey, I finally got round to remedying this omission with a more detailed description of the instrument in a paper presented at the International Symposium on the Electron held in Cambridge in 1997 (Smith, 1997). I am deeply indebted to Paul Brown for turning the old and now

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Figure 3. Cross-section of SEM3 column. (From Cambridge, 1997, p. 554. Reproduced by kind permission of the Institute of Materials.)

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faded blueprint of the column into the beautifully executed computer-drawn rendition of the instrument that is reproduced in Fig. 3. Acknowledgements I am greatly obliged to Alan Agar, Doug Atack, John Buchanan, David Clayton, Bob Lindsay and Sheila Smith for their help in preparing this article. References Atack, D., and Smith, K. C. A. (1956). The scanning electron microscope—a new tool in fibre technology. Pulp Pap. Mag. Can. 57(3), 245–251. Bailey, G. L. J., and Vernon-Smith, S. (1950). The structure of some non-ferrous alloys as revealed by the electron microscope. London 1949, 43–55. Bowden, F. P., and McAuslan, J. H. L. (1956). Slow decomposition of explosive crystals. Nature (London) 178, 408–410. Buchanan, J. G., and Smith, K. C. A. (1960). Preliminary studies of damage in papermaking wood chips using the scanning electron microscope. Delft 1960, 547–550. Buchanan, J. G., and Lindsay, R. A. (1961). A note on the structure of paper as revealed by the scanning electron microscope, in The Formation and Structure of Paper, Transactions of the Oxford Symposium Technical Section of British Paper and Board Makers Association, pp. 101–108. Buchanan, J. G., and Washburn, O. V. (1962). The surface and tensile fractures of chemical fibre handsheets as observed with the scanning electron microscope. Pulp Pap. Mag. Can. 63, T485–T493. Buchanan, J. G., and Washburn, O. V. (1964). The surface and tensile fractures of ground wood handsheets as observed with the scanning electron microscope. Pulp Pap. Mag. Can. 65, T52–T60. Challis, C. E. (1956). Summarized proceedings of a conference on electron microscopy— Glasgow, July, 1955. Br. J. Appl. Phys. 7, 89–93. Everhart, T. E. (1958). ‘Contrast formation in the scanning electron microscope.’ PhD Dissertation, University of Cambridge. Grigson, C. W. B. (1955). ‘An electron diVraction study on the growth of ionic crystals.’ PhD Dissertation, University of Cambridge. Liebmann, G. (1955). The magnetic pinhole electron lens. Proc. Phys. Soc. Ser. B 68, 682–685. March, A. A. C. (1957). ‘The design and construction of a scanning microdensitometer and the structure of composite films of gold and bismuth oxide.’ PhD Dissertation, University of Cambridge. McGrath, J. T., Buchanan, J. G., and Thurston, R. C. A. (1962). A study of fatigue and impact fractures with the scanning electron microscope. J. Inst. Met. 91, 34–39. McMullan, D. (1952). ‘Investigations relating to the design of electron microscopes.’ PhD Dissertation, University of Cambridge. McMullan, D. (1953). An improved scanning electron microscope for opaque specimens. Proc. Inst. Electr. Eng. 100, Part II, 245–259.

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Sikorski, J., and Hepworth, A. (1969). The place of the scanning electron microscope in textile research and industry. Chicago 1969, 249–259. Smith, K. C. A. (1956). ‘The scanning electron microscope and its fields of application.’ PhD Dissertation, University of Cambridge. Smith, K. C. A. (1959). Scanning electron microscopy in pulp and paper research. Pulp Pap. Mag. Can. 60, T366–T371. Smith, K. C. A. (1960). A versatile scanning electron microscope. Delft 1960, 177–180. Smith, K. C. A. (1961). Scanning, in ‘Encyclopaedia of Microscopy’, edited by G. L. Clark. Princeton, New Jersey: Van Nostrand-Reinhold, pp. 241–251. Smith, K. C. A. (1997). The origins and history of a versatile electron optical instrument combining facilities for SEM, TEM and X-ray point projection microscopy. Cambridge 1997, 552–557. Smith, K. C. A., and Oatley, C. W. (1955). The scanning electron microscope and its fields of application. Br. J. Appl. Phys. 6, 391–399. Washburn, O. V., and Buchanan, J. G. (1964). Changes in web structure on pressing and drying. Pulp Pap. Mag. Can. 65, T400–T407.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.2B* The Scanning Electron Microscope and its Fields of Applicationy K. C. A. SMITH AND C. W. OATLEY Formerly at: Engineering Laboratory, University of Cambridge

1. introduction In the scanning electron microscope a beam of electrons, which is focused to a very small spot, is caused to move in turn over each point of the object. The electron current leaving the object is collected and amplified and is then used to modulate the brightness of a cathode-ray tube whose spot is moving over the screen in correspondence with the motion of the electron beam over the object. If, then, any property of the object causes the electron current which leaves it to change from point to point, the picture built up on the cathode-ray tube will constitute a record of the variation of this property over the area of the object which is scanned. It does not follow that this picture will bear any resemblance to the one which would be seen through an optical microscope, although it may do so. A microscope based on the above principles was first proposed by Knoll(1) in 1935 and a few years later an actual instrument, in which mechanical scanning was employed, was described by von Ardenne.(2,3) In 1942, Zworykin, Hillier and Snyder(4) gave an account of a scanning microscope which is much more closely related to the present work. This instrument made use of the variation of secondary emission over the surface of the specimen and was employed chiefly for the examination of metals. One of the greatest diYculties encountered in this work was the very low signal-to-noise ratio that it was found possible to achieve. Soon after the end of the war, electron multipliers with beryllium-copper dynodes came into general use. Unlike earlier multipliers, these devices could be used in demountable vacuum systems and it seemed probable that they could profitably be employed for the detection of electrons in a scanning microscope. Work along these lines was therefore put in hand in this laboratory and in 1953 McMullan(5) was able to report the construction of a successful instrument. He also showed that it is unnecessary to rely on variation of secondary emission to provide contrast. If the  mean plane of the surface under examination is set at an angle of about 25 to the incident electron beam and the scattered electrons (including secondaries) are collected by the multiplier, it is found that the multiplier current depends strongly on the

*Reprinted from: Brit. J. Appl. Phys. 6, 391–399 (1955). y

Paper received 3 August 1955.

111 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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angle between the incident beam and the portion of the surface on which it is falling. This angle normally varies from point to point of the surface, so the picture finally built up on the cathode-ray tube is related to the topographical structure of the surface rather than to its secondary-emission properties. The scanning electron microscope constructed by McMullan has now been in use for several years and suYcient experience has been gained with it to make possible an appraisal of the probable fields of usefulness of instruments of this kind. The original apparatus has been modified in several ways but the modifications are not of a fundamental nature and will not be considered in any detail here. In the present paper an attempt is made to compare the performance and possibilities of the scanning electron microscope with those of the conventional transmission electron microscope (with or without the use of replicas) and with the reflexion electron microscope. At the present time the conventional transmission instrument is firmly established as the most useful type of electron microscope for general work. Furthermore, it is usually possible to modify such microscopes quite simply to allow examination of opaque specimens to be carried out by reflected electrons. Thus the conventional instrument is likely to be used whenever it can be made to give satisfactory results. By comparison, the scanning electron microscope involves the use of a good deal of additional apparatus and is inherently a more complicated instrument. Its use can therefore be justified only in cases where ordinary electron microscopes are unsatisfactory, and it is one object of the present paper to indicate where such cases may arise.

2. formation of the image in the scanning electron microscope As has been indicated above, contrast in the image formed by a scanning electron microscope can be made to depend either on variation of secondary emission over the surface of the object or on the surface topography of the object. In the former case the range of objects that can be examined is severely limited, since for most surfaces the secondary emission coeYcient is unlikely to show suYcient variation. Furthermore, this coeYcient almost always falls to a low value when the primary electrons have energies of a few ten thousands of electron volts so that, to obtain suYcient secondary emission, it may become necessary to work with a primary energy of about 1 keV. This in turn brings other diYculties; surface contamination of the object is likely to be serious and it is not easy to obtain good resolution with electrons of such low energy. For these reasons the work described below was done under conditions such that contrast depended on the surface topography of the object and the angle  between the incident beam and mean surface was usually about 25 , though angles up  to 45 have been found satisfactory. The energy of the incident electrons was about 20 keV; this is suYcient to penetrate thin contaminating films on the object and no serious trouble from such films has been encountered. Since the number of electrons scattered from the object varies with the local angle of incidence of the primary beam, the final image built up on the cathode-ray tube will be related to this variation, but it is not obvious that the image will be a

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recognizable picture of the surface. However, this proves to be the case as is clearly shown by Fig. 1 which is an image of a portion of a grid from a television camera tube. The marked three-dimensional eVect is particularly noteworthy and is also found in the examples to be discussed later. In the present instrument the collection of scattered electrons is performed in a relatively crude manner. The first dynode of the electron multiplier is placed as close to the object as is convenient and is maintained at a positive potential of about 500 V with respect to the object. Of the electrons leaving the object, some have nearly the full energy of the primary beam, while others are relatively slow. Because the secondary emission coeYcient of the first dynode falls to a low value for high primary energies, the slow electrons may have a disproportionately large eVect in the formation of the image. This is suggested by the fact that the signal strength is considerably reduced if the potential of the first dynode is made equal to that of the object. This change would be unlikely to alter the number of swift electrons collected but it might seriously reduce the number of slow ones. Relatively little work has so far been done on the investigation of these factors and it seems unlikely that the present arrangement of the collector is the best that can be devised. It is, however, good enough to give very satisfactory results. Scintillation counters have now been brought to a high state of eYciency and they might have numerous advantages for the detection of the scattered electrons in a scanning microscope. Work is in hand to test this possibility. A rather special

Fig. 1. Portion of aluminized copper grid from a television-camera tube.

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Fig. 2. Grains of silver-activated zinc sulphide phosphor.

example of this technique is shown in Fig. 2 (obtained by Dr. McMullan), where the object itself consisted of grains of a silver-activated zinc sulphide phosphor and the normal electron multiplier was replaced by a photomultiplier. In this particular phosphor most of the light comes from active regions which occupy only small portions of the grains within which they occur.

3. specimens for which the scanning electron microscope offers advantages The most obvious field of application for electron microscopes of the scanning or reflexion types is for the examination of opaque objects when replica techniques cannot be used. For example, the object might have indentations which were undercut so that a replica would be keyed to the surface. Again it might be desired to examine the surface at a temperature at which the replica would be destroyed; or to watch the changes in structure as the surface was heated. Although relatively little work along these lines has so far been done, there are clearly many situations in which a scanning or a reflexion microscope is the only instrument that can be used. A rather diVerent case occurs with many biological specimens where the object is too thick to be viewed by transmission, but too small for the preparation of replicas to be at all easy. Two examples of results obtained with the scanning microscope in such cases are shown in Fig. 3 and 4(a, b). They represent respectively an amoeba and

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Fig. 3. Amoeba, fixed with osmic acid (no metal coating) on nickel surface.

part of the skin of a meal-worm grub. A somewhat similar situation arises in the microscopy of fibres and Fig. 5 shows the result obtained with a fibre of Orlon. With specimens of this kind which are insulators, diYculties may be caused by charging of the object. These troubles can usually be overcome by coating the object with a thin evaporated layer of metal. Although the above photographs were obtained with a scanning electron microscope, the objects belong to a class for which the use of a reflexion microscope might be contemplated (e.g. Holdgate, Menter and Seal(6)) and it is therefore necessary to consider whether the former instrument has any advantages to justify its greater complexity. From this point of view the chief diVerence between the microscopes lies in the angles at which the electrons strike the object and are collected from it. In the  reflexion microscope a glancing angle of incidence of 5 or less has hitherto proved essential and most workers have collected electrons lying in a narrow pencil which  makes an angle of 10 or less with the mean surface. An obvious disadvantage of such a small glancing angle of reflexion is the very large foreshortening of the final picture, and a ratio of maximum to minimum magnification of ten or more is not uncommon. An even more serious diYculty lies in the fact that some asperities on the surface cast large shadows and the deeper depressions are not illuminated at all. Thus, on rough surfaces, much of the detail is concealed. Recent work in France by Fert(7) suggests that it may be possible to work with  glancing angles of reflexion up to 35 and thus lessen the troubles due to foreshortening.

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Fig. 4. (a) Surface of meal worm grub, Tenebrio molitor (silver coated). (b) Bristle on meal worm grub.

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Fig. 5. Orlon fibre (gold-palladium coated).

However, unless the glancing angle of incidence can also be increased, the other diYculties will remain. The scanning electron microscope is inherently free from the above disadvantages since large glancing angles of incidence and reflexion can be used without diYculty. In fact, careful examination of some of the photographs (e.g. those shown in Figs. 1 and 6) shows that electrons must have been reaching the collector by indirect paths since detail is revealed at some points from which there was no direct path to the multiplier. This eVect is likely to prove extremely valuable and further investigation of it is planned. The ability of a scanning electron microscope to operate with very irregular surfaces, coupled with the large depth of focus which is characteristic of most electron microscopes, makes it possible to carry out certain types of manipulation on a specimen while it is actually under observation. An example of this is shown in Fig. 6, which shows a tungsten point contact on a germanium crystal (a) before and (b) after the discharge of a condenser through the contact. The process was carried out while the contact was under observation and the electrical rectification characteristics could also be measured.

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Fig. 6. Tungsten-germanium point contact. (a) Before discharge of condenser through contact. (b) After discharge.

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4. fundamental limitations There are certain fundamental principles which set a limit to the performance of any electron microscope, but their application to the scanning microscope is rather diVerent from what it is with conventional electron microscopes. Before making further comparisons between the diVerent types it is therefore necessary to establish some basic equations which are of general application. Suppose a square area of the object of side D cm to be under observation and let d cm be the smallest distance that it is desired to resolve. For the time being, electronoptical aberrations of the instrument are assumed to be negligible. Then the object may be divided into D2/d2 square elements, each one of which must be resolved from its neighbours. Following the principles laid down by Rose,(8) there must fall on each element of the object a certain minimum number N of electrons, which can be calculated as follows. Because the arrival of electrons is a random will pffiffiffiffiffi process,pthere ffiffiffiffiffi p ffiffiffiffiffi be associated with N a fluctuation of r.m.s. magnitude N and N= N ¼ N represents the basic signal-to-noise ratio from the element. Subsequent stages in the production of the final picture cannot improve this basic ratio, though they may make it worse. Consider two adjacent elements such that all the electrons striking the first go to form the corresponding bright area of the image, while those striking the second are removed by absorption or scattering so that the corresponding area in this case is completely black. This represents the most favourable case for resolution of the two elements and experiment shows that, in the final picture, the eye will be unable to distinguish the black area from the adjacent area, which is speckled as a result of the random fluctuations, unless N exceeds a value of about 5. Thus in this case a minimum of twenty-five electrons per element would be needed. In general it will be desirable for the eye to be able to distinguish much smaller changes of contrast in the final picture than the complete change from ‘‘white’’ to ‘‘black’’ in adjacent elements. If, for example, an element of brightness B is to be distinguishable from one of brightness B  DB, the relevant equation becomes pffiffiffiffiffi N  5B=DB

ð1Þ

so that for 5% contrast to be detectable a minimum of about 104 electrons per element is required. So far it has been assumed that randomness in the primary electron stream is the limiting factor, but this may not be the case. For example, in the conventional electron microscope used visually, electrons leaving the object strike a fluorescent screen where each electron produces, on the average, several quanta of light. A fraction of these quanta enter the eye of the observer and are absorbed by the retina. Several quanta are required to produce a visual stimulus, so the number of stimuli conveyed to the brain is less than the number of quanta absorbed. Thus the number N0 of independent events per element varies from point to point of the apparatus. So long as N0 is, on the average, always greater than N, no additional noise is introduced since complete randomness was already associated with the original number N. If, however, N0 falls below N at any stage, the stage with the lowest value of N0 will

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become the limiting one and this value of N0 should replace N in equation (1). This point will be discussed further in Section 5. Let j A/cm2 be the useful electron current density arriving at the object, i.e. that portion of the total current density which is contained in a solid angle small enough to allow it to pass through the complete electron-optical system of the instrument, and let t be the eVective time during which electrons from each element are collected. Then, if e coulombs is the electronic charge, N ¼ jd 2 t=e

ð2Þ

and from equation (1) j  25B2 e=d 2 tDB2

2

ð3Þ

However, j is limited by the maximum emission density j0 A/cm of the cathode from which the electrons originate and according to Langmuir’s theorem j  j0 sin2 að1 þ Ve=kTÞ

ð4Þ

where k J/degree is Boltzmann’s constant, T the absolute temperature of the cathode, V volts the potential diVerence between object and cathode and a radians the semiangle of the cone within which lies the useful electron current striking a point of the : object. Since Ve/kT  1 and sin a ¼
a, equations (3) and (4) can be combined to give j0  25ðB=DBÞ2

kT A=cm2 Vtd 2 a2

ð5Þ

5. resolution Equation (5) is valid for the conventional transmission microscope as well as for the scanning electron microscope and for the purpose of comparison it will be convenient to use the following typical values B=DB ¼ 20



T ¼ 3000 K

j0 ¼ 10 A=cm2 V ¼ 30 kV to give

2

k ¼ 1
38  10

a  1
38  10

23 21

J=degree

=td 2

ð6Þ

In the conventional transmission microscope used visually, t is determined by the properties of the eye, and experiment shows that the time over which eVective integration takes place is about 0.2 s. Haine and Einstein(9) have shown that practical electron guns approach quite closely to the maximum theoretical eYciency indicated ˚ is required, equation (6) merely by equation (4) so that, even if a resolution of 5 A

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requires a to be greater than 1.7  10 3 rad. However, the designer would in any case wish to exceed this limit to obtain the best compromise between spherical aberration and diVraction eVects. If the image is recorded photographically, t can readily be made much greater than 0.2 s, and the minimum value of a is correspondingly smaller. Thus, with this type of instrument, the random nature of the beam current does not impose any practical limitations on design. For the scanning microscope the case is quite diVerent. Here the elements of the object are scanned sequentially and t is found by dividing the total scan period T by the number of elements (D/d)2. To cover a reasonable field of view D/d must be of the order of 1000 and for visual work T cannot be much greater than 2 s, giving, for a ˚, resolution of 5 A a  0
53 rad:

ð7Þ

For photographic work T can, in principle, be increased without limit, but the practical problem of ensuring that the object does not move sets a limit of, say five minutes, A longer period would, in any case, be extremely inconvenient. This gives, ˚, for a resolution of 5 A a  4
3  10

2

rad:

ð8Þ

The limit for a set by equation (7) is impossibly high and even that given by equation (8) is much larger than that required to give the best compromise of aberrations in the electron-optical system. The above figures represent limiting cases and are given merely by way of example since the equations cannot, in the nature of things, yield an exact limit for a. For example, it has been tacitly assumed that the collection of electrons scattered from a ‘‘white’’ element of the object is 100% eYcient, and this is certainly not the case. Again, many of the electrons striking the first dynode of the collector have such high velocities that they contribute little to the secondary emission. On the other side of the picture it would be possible, by accepting a smaller field of view, to reduce the number of elements very considerably and thus increase the value of t. Similarly, for some specimens, a value of B/DB less than twenty might be acceptable. A great deal more work is needed to provide further information on these various factors but it is clear that, for visual work, the random nature of the beam will be an important factor in setting a limit to the resolution that can be obtained. It may also limit the resolution in photographic work. A quite diVerent factor which may set a limit to the attainable resolution is the penetration of the incident electrons into the object. Suppose, for example, the object to consist of a rectangular solid and let the incident beam fall on the top surface and be moving towards the edge formed by the top and one of the sides. If the top surface is perfectly smooth and the beam is not near the edge, electrons will penetrate the surface to various depths, will be deflected by collision and a proportion of them will re-emerge from an area of the surface much larger than the original spot. However, under the conditions stated above, the electron current flowing to the multiplier will have a definite constant value. Similarly when the incident beam has passed the edge by a considerable distance and is falling on the side of the object, the current to the

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multiplier will have a diVerent constant value. In the vicinity of the edge, however, it is possible for electrons to enter the surface and emerge from the side or vice versa, and the transition of the multiplier current from the value characteristic of the surface to that characteristic of the side will thus be gradual and the edge may appear blurred in the final image. The extent to which this eVect will limit resolution would be expected to vary with the nature of the object and the energy of the incident electrons. Once again, detailed information on these points is not yet available. The scanning microscope at present in use is not very suitable for tests on ˚. resolution, since it was designed to give a Gaussian spot-diameter of about 150 A Now that a stigmator has been added and the eYciency of collection of the scattered electrons greatly improved, a reduction of the diameter of the spot would be advantageous and the electron lenses will shortly be modified to make this change possible. With the aperture at present in use in the second demagnifying lens, the calculated ˚ and the diVraction aberration to about spherical aberration, Csa3, amounts to 70 A ˚. 30 A A photograph of latex particles taken with the microscope used as a scanning transmission instrument (the latex being mounted on a collodion film) is shown in ˚ , i.e. approximately the Fig. 7(a), and suggests a resolution of between 200 and 250 A theoretical value. It was hoped that, with this arrangement and with such small objects, penetration of the primary electrons into the object would not have such serious consequences as in the more normal arrangement. However, a photograph of similar particles taken with the instrument used as a scanning reflexion microscope, is shown in Fig. 7(b) and the resolution is about the same as before. At the present time it is diYcult to hazard a guess at the ultimate resolution that it ˚ will be possible to attain with the scanning microscope, but a value less than 100 A would seem to be within reach.

6. bombardment of the object There are many objects which are partially or totally destroyed by excessive electron bombardment, so it is of interest to compare diVerent types of electron microscope from this point of view. The reflexion microscope is known to be very much worse than the ordinary transmission microscope because, in the former instrument, only a very small proportion of the electrons striking the specimen pass through the object lens and thus contribute to the final image. In what follows, therefore, a comparison will be made between the transmission microscope and the scanning microscope. In the light of the principles laid down in Section 4, consider first what the situation would be if a perfect instrument of each type were available. In the case of the transmission microscope, every electron striking a ‘‘white’’ element of the object would subsequently enter the object lens and, in the scanning microscope, every electron striking a white element would be collected by the multiplier. Furthermore, in each

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Fig. 7. Latex particles. (a) Transmission micrograph. (b) Reflexion micrograph.

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instrument the number of independent events per white element of the object, at each subsequent stage of observation, would be greater than the number of electrons striking that element. Under these conditions it should be possible to obtain an image, in each case, when the number of electrons per element was as great as N in equation (1). Thus the performances of the two instruments would be equal from the point of view of the bombardment of the specimen. Practical instruments may fall short of the ideal in two respects. In the first place the collection of electrons from a ‘‘white’’ element is not perfect. Quantitative measurements are lacking but it is probable that the eYciency of collection could be made to approach 100% in both types of instrument. In any case, the diVerence between them is unlikely to be very large. Consider next the assumption that the number of individual events per element, at each stage in the production of the image, is greater than the number of electrons per element. In the scanning microscope this will be true if the average secondary emission coeYcient at the first dynode is greater than unity, since enough dynodes can be used to counteract ineYciency in any subsequent process. In the existing instrument the average secondary emission coeYcient probably does not fall short of unity by a large factor, and there is certainly room for improvements in the design so that electrons shall strike the first dynode with the optimum energy. Thus the scanning microscope approaches the ideal quite closely. For the transmission microscope used visually the case is quite diVerent. Each electron entering the object lens strikes the fluorescent screen and, in a good screen, rather less than 10% of the energy of the electron is converted into light so that a 50 keV electron might produce about 2000 quanta. Only about 2 in 104 of the total number of quanta enter the eye of the observer; furthermore, according to the work of Hecht(10) about 100 quanta must enter the eye to produce a single visual signal to the brain. Thus the processes subsequent to the entry of the electrons into the object lens reduce the number of individual events per element by a factor of about 250 and the initial bombardment of the object must therefore be increased in this ratio. An additional factor of ten in favour of the scanning microscope arises from the fact that the eVective integration period of the eye is about 0.2 s. Thus if, as is usually the case, the image is to be viewed continuously for at least a few seconds, the required number of electrons must fall on the object in the transmission microscope every 0.2 s. In the scanning microscope, however, the integration is carried out by a long-persistence screen on the cathode-ray tube and the eVective integration period may easily be as high as 2 s. It is interesting to note that the poor performance of the transmission microscope is not fundamental. In principle it could be greatly improved by the use of a longpersistence screen and/or an image intensifier, but this would entail considerable additional complication. Again, the performance is greatly improved when photographic recording with suitable plates is used. There remains, however, the practical diYculty of selecting and focusing the image if it cannot be observed visually. It appears, therefore, that the scanning microscope oVers material advantages when bombardment of the object is to be kept to a minimum.

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7. conclusions It will be apparent from what has been written that relatively little is yet known about many of the factors aVecting the performance of the scanning electron microscope and it is reasonable to expect that, when these factors are better understood, an improved instrument will result. Even now, however, there is a wide range of objects with which the scanning microscope will yield results that cannot be obtained in any other way.

Acknowledgements We wish to express our thanks to the Admiralty for a grant to cover the cost of apparatus. One of us (K. C. A. Smith) is also indebted to the Department of Scientific and Industrial Research for a maintenance grant.

references

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

Knoll, M. Z. Tech. Phys., 16, p. 767 (1935). von Ardenne, M. Z. Phys., 109, p. 553 (1938). von Ardenne, M. Z. Tech. Phys., 19, p. 407 (1938). Zworykin, V. K., Hillier, J., and Snyder, R. L. Bull. Amer. Soc. Test. Mater., 117, p. 15 (1942). McMullan, D. Proc. Instn Elect. Engrs, 100(II), p. 245 (1953). Holdgate, H. W., Menter, J. W., and Seal, M. International Electron Microscopy Conference, London, (Royal Microscopical Society, 1954). Fert, C. Conference on Recent Progress in Corpuscular Microscopic Techniques, Toulouse, (1955). Rose, A. Advances in Electronics, Vol. 1, p. 131 (New York: Academic Press, 1948). Haine, M. E., and Einstein, P. A. Brit. J. Appl. Phys., 3, p. 40 (1952). Hecht, S. J. Opt. Soc. Amer., 32, p. 42 (1942).

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2.3 Building a Scanning Electron Microscope O. C. WELLS IBM Research Division, Yorktown Heights, New York Formerly at: Engineering Department, University of Cambridge

I. Introduction I first met Charles Oatley when I was about 12 years old—not in person but as the author of his book Wireless Receivers, which my father gave to me at about that time (Oatley, 1932). My education in electrical engineering had begun when I was about six or seven years old and watched my father assemble gramophone amplifiers by screwing the valve sockets, resistances and condensers (as they were called in those days) and transformers on to a piece of wood. He then connected wires between the various terminals. Some of the components (such as the mains transformer) were technically more advanced in that the wires were soldered to metal tags instead of being fixed mechanically to terminals. In due course I spent many hours building radios and high-fidelity amplifiers in a similar way. Oatley’s book was a source of authority on this subject in those days. My education in electronics took a step forward when I spent a year as a radar instructor during military service (1949–50) before coming to Cambridge. This involved vacuum-tube pulse circuits, amplifiers, power supplies, cathode-ray tube displays and so on. Little did I realize that I would soon enough be facing the identical electrical components that had been bought as government surplus by the then Mr Oatley (later Professor) in the Cambridge University Engineering Department, to be used in his PhD projects such as the development of the SEM. Undergraduate studies in Cambridge lasted for three years. After two years studying mathematics, I moved to the Engineering Department to take the electrical option of the Mechanical Sciences Tripos in which lectures by Oatley included wartime developments in electronics and some applied physics. Teaching such subjects to engineers was a new idea at that time.

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II. SEM Development in Cambridge When I started my PhD project in 1953, there was only one working SEM in the world. This had been started by Ken Sander as a transmission electron microscope, converted to a SEM by Dennis McMullan, and then further improved by Ken Smith (McMullan, 1953a,b; Smith and Oatley, 1955). As an introduction into the mysteries of the SEM, I was sent on a visit to a wellknown electron microscopist of the day, who explained to me in quite some detail the importance of the better resolution of the TEM. When I asked him about the SEM he replied ‘If I thought that the scanning microscope was any good I would have made one’. On another occasion I asked an electron microscopist in Cambridge whether the SEM would be good for looking at sintered materials, he replied ‘You should look at sintered material with replicas. Bradley can take a replica from a pollen grain. The scanning microscope does not have the resolution’. Of course, the SEM is not unique in being criticized by the experts of the day. For example, in an editorial comment about the origin of computers in the Aug./Sept. 2002 issue of APS News we find: ‘It’s diYcult to believe, in this Internet age of laptop and handheld personal computers, that such machines and the ideas from which they developed were once strenuously resisted, and even scientists and engineers were slow to grasp the implications of the technology . . .’. As opposed to these negative opinions, when I visited Dr. D. G. Drummond of the Shirley Institute to learn about fibres, he was much more positive about the SEM (as also were many of our colleagues in Ellis Cosslett’s TEM group in the Cavendish Laboratory half a mile away). Later I recognized that the negative attitude of some members of the TEM community towards the SEM was part of a general phenomenon. That is to say, if you are to make any impact on your assigned project, then usually you must concentrate your mind on it to the exclusion of all else. If you are not very careful, competing ideas can then become distractions that must be swept aside. Many years later I was on the other side of such an argument when I became convinced that there would be little if any future for the scanning tunnelling microscope (STM)—but fortunately no-one took any notice of my opinion in that case. Meanwhile, back in the Engineering Department, work was continuing with a high level of enthusiasm. I was given the task of designing and building the new SEM2, and with hindsight it is clear that I should first have spent more time than I did in learning about the existing SEM1. One of the new ideas that had diVused from the Cavendish Laboratory to the

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Engineering Department was the use of elastomer O-rings in place of the Wilson seals (with a flat rubber gasket) that had been used by Sander and by McMullan. Another idea that was current at that time (and with which I complied) was to move large sections of the column from side-to-side with hand-operated screws instead of centring the beam with deflection coils. Also, it was the custom to centre the tungsten hairpin filament in the electron gun mechanically in those days. A source of frustration was the use of electrostatic lenses instead of the magnetic lenses that are almost always used today. Oatley preferred them because they were simple and easy to understand and had the advantage that we did not have to build the high-stability current supplies that are needed by lenses of the magnetic variety. (High-stability transistorized power supplies for magnetic lenses were developed later.) One of the many positive aspects of our lives in the Engineering Department was being invited to the monthly meetings of Ellis Cosslett’s TEM group in the Cavendish Laboratory (referred to above) in which recent publications and events at conferences were reviewed together with regular presentations on electron microscopy. Three years was the period allotted for a PhD programme, but in my case it expanded into four years because of the time taken to build the SEM together with my tendency to assemble and test equipment for cave diving and related underwater activities that occupied much of my time and energy in those days. (We had anticipated by about 30 years the present fashion of using mixed gas re-breathers for cave diving, and for some reason I thought that this sort of thing was worthwhile.) Later I learned that Oatley had himself done less well than expected in the Part II Physics Tripos at Cambridge in 1925 because of his interest in competitive swimming, and perhaps this explains his tolerant attitude towards these activities on my part. Dr Christopher Grigson, who took over from Oatley as my supervisor in my final year, was not happy with my underwater activities. He took the view that it interfered with my PhD work. However, he was himself carrying out research into ways for transporting oil across the oceans in flexible containers, and in so doing suVered the misfortune of dropping an expensive piece of equipment into one of the deeper and muddier parts of the River Ouse. I made use of my diving equipment to get this back for him and this improved the general atmosphere considerably. As a result of the shortage of time, I had diYculty in getting the current supply for the filament in my SEM to work properly. I therefore supplied it from a rechargeable lead–acid motorcycle battery and a variable resistor balanced on the high-voltage box at the top of the rack of electronic

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equipment—luckily we did not have industrial-type safety inspections in those days. The problems that were involved in designing this sort of apparatus were considerably reduced by the helpfulness of the workshop Superintendent, A. A. K. Barker, who would look at your diagrams and then say ‘What is this supposed to do?’. By the time that he had gone over it with you, the design would be much simpler and easier to make than when you had taken it to him. Oatley’s assistant Leslie Peters was extremely helpful also.

III. Research with the SEM When my SEM was finally working I was able to proceed with my research work. Right from the beginning I was fascinated by questions of image formation as shown in the micrographs that had been obtained by McMullan and by Smith. Discussions were taking place as to whether it was the faster back-scattered electrons (BSE) or the slower secondary electrons (SE) that were contributing high-resolution information to the image. McMullan (1953b) had compared the image formation process in the SEM with that in the reflection EM with a tilted solid target in which the image is formed by focusing the forward-scattered electrons (FSE) that leave the specimen with a low takeoV angle and relatively small loss of energy in the forward direction. This is related to the SEM by reversing the direction of electron flow in the system (reciprocity principle). He also described quite clearly the diVerence between (1) the FSE image in the SEM with a tilted sample and with a low detector takeoV angle similar to the above, and (2) the BSE image in which the sample is generally at right angles to the electron beam and the BSE detector is above. McMullan (1953b) also proposed what later became known as the low-loss electron (LLE) image in which only the fastest FSE or BSE are collected and used to form the image. (This became one of the central ideas in my later work.) Smith then showed that you can obtain a high-resolution image from a solid specimen in the SEM by collecting secondary electrons; indeed, much of the present-day usefulness of the SEM when operated in the SE mode follows directly from his work on this subject (Smith and Oatley, 1955; Smith, 1959). Later workers made many significant contributions, such as in SE detection and with the low voltage SE image, but to me it is Smith’s work that dominates this part of the subject. Later it became clear that you can obtain high-resolution SEM images by collecting the BSE, the FSE or the SE in diVerent ways depending on the

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nature of the sample and on what sort of information is required—surface topography, compositional variations, high-Z inclusions in a low-Z matrix, subsurface structures, local potentials with either biased or unbiased p–n junctions, local magnetic fields, changes in crystal orientation, crystal defects, or anything else. Also involved in the SE discussions were William Nixon (who later transferred to the Engineering Department from the Cavendish Laboratory) and Thomas Everhart, who started his PhD work in 1955 on SEM improvement, SE collection, electron scattering from a solid target, and voltage contrast from a reverse-biased p–n junction. An important question was why Ken Smith’s SE images were so sharp when the penetration of the incident electrons into the target was a micrometre or more. So I developed the idea that it is the SE excited at the point of entry of the incident electrons into the specimen that are the source of the high-resolution information. These are now called SE-I following a detailed discussion by Peters (1982). I was very pleased by this idea of how the high-resolution SE image was formed, so you will understand how I felt many years later when I read Dennis McMullan’s 1988 article ‘Von Ardenne and the scanning electron microscope’ telling that von Ardenne had described precisely this same idea rather better than I had done (and other important ideas such as the low-voltage SEM), in his book published in 1940 (von Ardenne, 1940; Liniger and Liniger, 1990). With reference to the above paragraph, one of the fascinations of the SEM and similar projects is to see the way in which ideas that were described perfectly clearly by pioneers such as Knoll, von Ardenne, Zworykin, Hillier, Oatley and others are rediscovered by later workers and then described as being ‘new’, on the other hand, you must get on with things, so I suppose that this is inevitable. Without any question, one of the better things that I did with reference to the SEM occurred many years later when I was one of three editors who persuaded Sir Charles to write an article describing the development of the SEM (Oatley, 1982) (see Chapter 1.2). The other two editors were Kurt Heinrich and Dale Newbury from what was then called the National Bureau of Standards in Washington, DC. Concerning my own PhD work (Wells, 1957), Sir Charles was kind enough to describe it as follows (see Chapter 1.2): At the end of Smith’s first year, in 1953, he was joined by O.C. Wells whose major task was to design and build a new microscope, similar to the earlier instrument, but incorporating a number of improvements which had been shown to be desirable. This occupied about three-quarters of his available time but, in his last year, he was able to examine some new techniques in the use of the microscope.

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An attempt to observe synthetic fibres under tension was hindered by the cracking of the evaporated metal coating which is normally applied to prevent electrical charging of an insulating specimen. This led to an investigation of other possible methods of preventing charging; by irradiation with positive ions, by the addition of anti-static sprays or by collecting only high-energy electrons. The use of high-energy, back-scattered electrons involved a study of the diVerent eVects to be obtained by collecting electrons leaving the specimen in diVerent directions. In another part of this work, Wells wished to examine the interior surface of the very fine bore in a spinneret through which synthetic fibres are extruded. The normal method of collecting electrons could not be used, but he showed that satisfactory micrographs could be obtained by allowing the incident probe to fall on the surface after entry through one end of the bore and by collecting electrons which left through the opposite end after multiple collisions with the walls [see Fig. 8 in Chapter 1.2]. Wells also made two very useful theoretical contributions to scanning microscopy. He showed how quantitative information about the topography of a surface could be obtained from stereoscopic pairs of micrographs and he provided a detailed theory of the way in which resolution would be likely to be aVected by the penetration and subsequent scattering of the incident electrons within the specimen (Wells, 1959; Wells, 1960; Everhart, Wells and Oatley, 1959).

A. Metallurgical Research The reference to stereoscopic imaging in the final quoted paragraph above is to joint work with Dr Constance Tipper during my final month in Cambridge. She is known to metallurgists as the inventor of the ‘Tipper test’ for determining the brittleness of stel used in the construction of ships. At this time she was breaking single crystals of iron at the temperature of liquid nitrogen to give flat fracture surfaces with asperities and cavities of various kinds. These were the ideal samples for showing the way in which the SEM can be used to examine and compare the corresponding features on two such opposing surfaces. We calculated the geometry of each half of the fracture from pairs of images taken from two directions at right angles (Tipper et al., 1959; Wells, 1960). Dr Tipper therefore became one of the earliest metallurgists to support the study of fractures in the SEM. Stereoscopic methods in the SEM as applied to biological samples were later refined by Alan Boyde at University College London, who was also one of the first to point out that surface topography can be seen more clearly in the SE image with a lower beam energy (Boyde and Wood, 1969).

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IV. Professor Sir Charles Oatley The editors have asked for my recollections of Professor Oatley. The first is how he emphasized simplicity. On one occasion when I proposed something rather complicated, he replied ‘Why don’t you put together something simple that works?’ Second is the way in which he would think things through for himself: his confidence in the SEM and in electrostatic lenses is described above. Another example is in his book Wireless Receivers, where he does not describe the superhet receiver (Oatley, 1932). He told us that this was because he had believed that the ‘straight’ set would be the winner, which of course it was not. The final recollection is his habit of saying exactly what he thought. For example, he told us that after leaving the university he worked for a while for a company (now defunct) that manufactured vacuum tubes. His employer sent a group to receive inspiration by paying a visit to Albert Einstein, who insisted on playing his violin to them. On being asked how well Einstein did this, he replied ‘Not very well’. In my own case, I asked Sir Charles for a recommendation when I came to America in 1959. Such things are totally confidential. When we arrived in New York the Immigration OYcer handed me my folder and said ‘Can you please take this to the oYcer over there?’ So I was able to open it and see that Sir Charles had described my work in Cambridge as being ‘competent but not brilliant’. This came as a surprise to me, but at least I am now willing to tell people about it. To summarize this account of my time at Cambridge, it is not my purpose to describe in any great detail my own work with the SEM since Sir Charles was kind enough to do this for me in the above quotation. Rather, I have tried to give some idea of how he started such a remarkably successful series of PhD projects that led (with others) to the commercial production and proliferation of the SEM, and to express my appreciation of his many contributions to the development of the scanning electron microscope and to engineering education in general.

V. Research in North America A. Westinghouse The SEM has played a very important part in my life because, after leaving England in 1959, I spent five years at the Westinghouse Research Center in Pittsburgh where we designed and built an SEM and applied it to the study

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of microcircuits and to electron beam fabrication in collaboration with John Coltman, Thomas Everhart, Richard Matta and Harvey Nathanson (Everhart et al., 1963, 1964a,b; Wells et al., 1963, 1965). When I arrived in America there was only one SEM on that side of the Atlantic. This was Ken Smith’s SEM3 at the Pulp and Paper Research Institute at Montreal in Canada. In due course we paid several visits to Alex Rezanowich, who was then in charge of it, with transistors from the Westinghouse Research Center. Using the voltage contrast method as described by Oatley and Everhart (1957) we showed the positions of the p–n junctions (Wells, 1962). The way in which these were not quite where they were supposed to be in an early transistor made a great impression. Interest in the SEM then grew at Westinghouse until an SEM project was started there also. B. IBM The SEM3 in Montreal also received visitors from the IBM Research Center at Yorktown Heights, NY, where there was an increasing interest in the SEM and in related techniques. Michael Hatzakis writes: It was the visit to Ken Smith’s SEM in Montreal that triggered the idea for cross-sectioning the wafer in order to observe electron penetration profiles in resists. This in turn led to the development of a number of significant processes including the electron beam fabrication of transistors [Thornley and Hatzakis, 1967] and the lift-oV metallization technique [Hatzakis, 1969]. It also provided a considerable boost to the study of fast electron energy deposition in solids both experimentally in the SEM and using computer simulation (Monte Carlo), a science that has generated many hundreds of papers and PhD theses over the years.

For the past 38 years I have worked with the SEM, and/or have been Emeritus, in the IBM Research Division where I was lucky enough to be able to collaborate for many years with Conrad Bremer (who was the first microscopist to obtain a low-loss electron image) and with Alec Broers in the demonstration of in-lens low-loss electron imaging (Wells et al., 1973). As a family matter, I had the good fortune to derive a fundamental theorem about magnetic contrast in the SE image in the SEM with the help of my sister (Wells and Stoye, 1986). My present interests include the application of the in-lens low-loss electron image to the study of silicon wafers during the fabrication process (Wells et al., 1990, 2001; Postek et al., 2001; Wells, 2002). The SEM has now become a part of everyday life, with SEM images being published in newspapers and in scientific journals without any reference being made to the existence of the SEM. One of my sons has told me that

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you cannot regard yourself as being a successful composer of great music until you hear your life’s greatest masterpieces being played over the loudspeakers in the supermarket. Based on this criterion, we can certainly judge that Professor Oatley’s work has been a great success. I would like to thank Michael Hatzakis, Kurt Heinrich, Richard Kelisky, Gerry Owen, Peter Price, Michael Rooks, Andras Vladar, James Wells and the Editors for their very helpful comments.

References Boyde, A., and Wood, C. (1969). Preparation of animal tissues for surface scanning electron microscopy. J. Microsc. 90, 221–249. Everhart, T. E., Wells, O. C., and Oatley, C. W. (1959). Factors aVecting contrast and resolution in the scanning electron microscope. J. Electron. Control 7, 97–111. Everhart, T. E., Wells, O. C., and Matta, R. K. (1963). Evaluation of passivated integrated circuits using the scanning electron microscope. Extended Abstracts of Electronics Division, Electrochemical Society 12(2), 2–4 (New York Meeting, Oct. 1963). Everhart, T. E., Wells, O. C., and Matta, R. K. (1964a). Evaluation of passivated integrated circuits using the scanning electron microscope. J. Electrochem. Soc. 111, 929–936. Everhart, T. E., Wells, O. C., and Matta, R. K. (1964b). A novel method of semiconductor device measurements. Proc. IEEE. 52, 1642–1647. Hatzakis, M. (1969). Electron resists for microcircuit and mask production. J. Electrochem. Soc. 116, 1033–1037. Liniger, W., and Liniger, E. (1990). Private communication: translation into English of the section ‘The intensity relationships in sideview with the scanning microscope’ from von Ardenne (1940). McMullan, D. (1953a). The scanning electron microscope and the electron-optical examination of surfaces. Electron. Eng. 25, 46–50. McMullan, D. (1953b). An improved scanning electron microscope for opaque specimens. Proc. IEE. 100, Pt. II, 245–259. McMullan, D. (1988). Von Ardenne and the scanning electron microscope. Proc. RMS. 23, 283–288. Oatley, C. W. (1932). ‘‘Wireless Receivers. The Principles of their Design.’’ London: Methuen. Oatley, C. W. (1982). The early history of the scanning electron microscope. J. Appl. Phys. 53, R1–13. Oatley, C. W., and Everhart, T. E. (1957). The examination of p–n junctions with the scanning electron microscope. J. Electron 2, 568–570. Peters, K. R. (1982). Conditions required for high quality high magnification images in secondary electron-I SEM, in ‘SEM 1982/IV’ edited by O. Johari, pp. 1359–1372. Postek, M. T., Vladar, A. E., Wells, O. C., and Lowney, J. L. (2001). Application of the low-loss (LLE) scanning electron microscope (SEM) image to integrated circuit technology. Part 1. Applications to accurate dimension measurements. Scanning 23, 298–304. Smith, K. C. A. (1959). Scanning electron microscopy in pulp and paper research. Pulp Pap. Mag. Can. (Techn. Sect.) 60, T366–T371. Smith, K. C. A., and Oatley, C. W. (1955). The scanning electron microscope and its fields of application. Br. J. Appl. Phys. 6, 391–399.

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Thornley, R. F. M., and Hatzakis, M. (1967). Electron-optical fabrication of solid-state devices, in ‘Record IEEE 9th. Annual Symposium on Electron, Ion and Laser Beam Technology,’ edited by R. F. W. Pease. Berkeley: San Francisco Press, pp. 94–100. Tipper, C. F., Dagg, D. I., and Wells, O. C. (1959). Surface fracture markings on alpha iron crystals. J. Iron Steel Inst. 193, 133–141. ¨ bermikroskopie’ Berlin: Springer [Edwards, Ann Arbor, von Ardenne, M. (1940). ‘Elektronen-U 1943]. Wells, O. C. (1957). ‘The construction of a scanning electron microscope and its application to the study of fibres.’ PhD Dissertation, Cambridge University. Wells, O. C. (1959). Examination of nylon spinneret holes by scanning electron microscopy. J. Electron. Control 7, 373–376. Wells, O. C. (1960). Correction of errors in electron stereomicroscopy. Br. J. Appl. Phys. 11, 199–201. Wells, O. C. (1962). Electron beams in microelectronics, in Proceedings of the Fourth Symposium on Electron Beam Technology Boston, MA, 29–30 March 1962, pp. 354–381 (Bakish R., ed.; Alloyd Electronics Corporation, Cambridge MA 1962). Wells, O. C. (2002). Imaging of samples with shallow surface topography in the scanning electron microscope (SEM), in ‘Proceedings of the 3rd International Symposium on Atomic Level Characterization for New Devices and Materials (ALC’01)’, (Nov. 2001, Nara, Japan), pp. 142–147. Japan Society for the Promotion of Science. Wells, O. C., and Stoye, C. A. (1986). Relationship between type-1 magnetic contrast in the scanning electron microscope and the vector potential of the magnetic field. J. Microsc. 144(1), RP1–RP2. Wells, O. C., Everhart, T. E., and Matta, R. K. (1963). Automatic positioning of device electrodes using the scanning electron microscope. Ext. Abstr. Electrochem. Soc., Electron. Div. 12(2), 5–10. Wells, O. C., Everhart, T. E., and Matta, R. K. (1965). Automatic positioning of device electrodes using the scanning electron microscope. IEEE Trans. Electron Devices ED-12, 556–563. Wells, O. C., Broers, A. N., and Bremer, C. G. (1973). Method for examining solid specimens with improved resolution in the scanning electron microscope (SEM). Appl. Phys. Lett. 23, 353–355. [The oxide step height in Fig. 3 should be 220 nm. The magnification is correct.] Wells, O. C., LeGoues, F. K., and Hodgson, R. T. (1990). Magnetically filtered low-loss scanning electron microscopy. Appl. Phys. Lett. 56, 2351–2353. Wells, O. C., McGlashan-Powell, M., Vladar, A. E., and Postek, M. T. (2001). Application of the low-loss (LLE) scanning electron microscope (SEM) image to integrated circuit technology. Part 2: Chemically-mechanically planarised samples. Scanning 23, 366–371.

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.4A Contrast Formation in the Scanning Electron Microscope T. E. EVERHART California Institute of Technology, Pasadena, California Formerly at: Engineering Department, University of Cambridge

I. Introduction The three research students who preceded me working on the scanning electron microscope were all English, and had a good deal of Cambridge in their blood. I, an American, arrived as a research student with a much diVerent background, and considerably less practical experience in vacuum tube electronics. As an undergraduate at Harvard, I performed laboratory experiments but had not constructed circuits. My Master’s degree from UCLA was entirely theoretical; I had worked at various odd jobs during the summers while a college student, including carpentry, trucking freight and farming, but not until the summer before my senior year had I done anything technical, and that summer was spent in the aerodynamics department of Cessna Aircraft Company in my home town of Wichita, Kansas. I had worked as a Master’s Cooperative student in the Microwave Tube Laboratory of Hughes Research Laboratories while attending UCLA, and had picked up some experience in machine shop, mechanical drawing, vacuum techniques and microwave measurements as well as a passing knowledge of electron optics, primarily as used in travelling-wave tubes. I must have appeared naive indeed when I arrived in Cambridge, speaking English with a mid-west accent. I owe a great deal to Ken Smith, who helped me understand his scanning electron microscope, which I inherited, and to Michael Forrest, a MSc student who was the acknowledged electronics guru of the laboratory. The technicians also were most helpful: Les Peters, Henry Asplin, Joe Brown, and others. I learned a lot of practical information from them and from other fellow students. The sociology of the electronics group in the Cambridge University Engineering Department was most interesting to me, coming as I had from an industrial position in the United States where one started at 8 a.m. and worked at least until 5 p.m. In 1955, starting times for students at the CUED 137 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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varied from 9 to 11 a.m., and people could work no later than 7 p.m. in the laboratories, which were locked at that time! Interactions with fellow students and members of staV were easiest at coVee time (about 11 a.m.) and tea time (about 4 p.m.). StaV and students had coVee together, and this was the best time to ask a question, have a requisition signed, or get some advice from your supervisor. Students had tea separately from staV in the afternoon, and this was a good time to discuss ideas with colleagues. They generally could destroy an idea if there were errors in it, and if they couldn’t, it was reasonably safe to invest some more time on it, until you were able to either reject it yourself or prove it useful. My background was physics, and I most naturally would have studied for an advanced degree in the Cavendish. When my application was provided to that laboratory by the Marshall Aid Commemoration Commission, I was told that they were full, but that Charles W. Oatley had an interesting project in the Engineering Department that might prove suitable. Although I didn’t know it at the time, this was an extraordinary stroke of good luck. The then Mr. Oatley had come from physics himself, and his background was excellent to supervise the project he set for me in October 1955: continue work on the scanning electron microscope. Ken Smith had just rebuilt much of McMullan’s original instrument (SEM1), as he has described, including the specimen chamber. He was starting to write his dissertation, which was submitted a year later. He was in the process of finishing some measurements on SEM1, and I was able to help him with these, gaining valuable experience about an idiosyncratic instrument, rather diVerent from the commercial microwave instruments I had been using at Hughes. In fact, I felt I had moved from a post-war environment at Hughes to a pre-war environment at Cambridge, since the tubes (electron valves, sorry) we were using were mainly war-surplus, and the amplifiers in the oscilloscope used to measure waveforms were uncalibrated. Oliver Wells, who shared our room in Scroope House, was busy constructing SEM2 with a modified design.

A. Secondary and Reflected Electron Components McMullan had worked out a theory of contrast based on high-energy reflected electrons, but Ken Smith’s new electron-multiplier collection system indicated that low-energy secondary electrons were forming most of the electronic signal that produced an image. Oatley suggested that I determine how much of the video signal was due to secondary electrons and how much to the higher-energy reflected electrons. Accordingly, I built a system of three grids that could be mounted on the electron multiplier and used to determine the energy of the electrons that were contributing to the

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video signal. Using this system, I found that about 3% of the video signal in the existing setup was due to high-energy electrons, and the rest was from secondary electrons (Everhart, 1958, pp. 11–13). Having determined that secondary electrons produced the images we were observing, it then became a challenge to produce the reflected electron images that Dennis McMullan had postulated, and compare them with the secondary images. To accomplish this, I was able to use a piece of plastic scintillator held close to the sample so that a large fraction of the reflected electrons could be collected. On striking the scintillator, they produced light, which could be collected with a photomultiplier tube and amplified as the video signal. Using the electron multiplier to observe an area of the sample conventionally, and then moving the plastic scintillator into position, I was able to observe the same area using first the secondary electron signal and then the reflected electron signal. The diVerences in contrast were appreciable, and led to the title of my dissertation (Everhart, 1958, pp. 13–18).

II. A New Detector System The instrumental problems that had plagued Ken Smith did not disappear when he turned SEM1 over to me. In particular, I had problems with the high-voltage power supply, and the video-signal head amplifier that floated at the output voltage of the electron multiplier. The input of the electron multiplier had to be slightly positive with respect to the grounded sample, so the output signal voltage was a small video signal from 2 to 6 kV above ground potential. Another problem with the electron multiplier was that it was bulky and, being fixed in position, could not be moved around to determine the best position to collect the signal. I needed something that was small and reliable and, if possible, had an output video signal at earth potential. The contrast experiments described above using a scintillator and photomultiplier combination had been successful, and suggested that if a scintillator could be held at high voltage with respect to the sample, the secondary electrons would produce enough light to provide a noise-free amplification of the collected secondary electron signal. Because the signal was carried by photons of light, the electrical continuity needed with the electron multiplier system could be broken, obviating the head amplifier at an elevated potential. I proceeded to design such a collector. My design incorporated a hemispherical piece of scintillator in order to reflect all the light towards the photocathode of the photomultiplier. I decided to carefully machine such a hemisphere and, using a jeweller’s lathe, made light cuts with a sharp tool, followed by careful polishing with

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emery paper. This produced a suitable test sample on which I deposited a thin coating of aluminium over the surface of the hemisphere. This ensured that its surface would be an equipotential, and also that the light produced in the scintillator would be reflected down the light-pipe, on which it was mounted, to the photocathode. I was ready to test the system: it worked! The image was at least as good as before, and the problem of the floating head amplifier was solved (Everhart, 1958, pp. 19–28). After I had used this system for some time, a colleague tried to fabricate a similar scintillator. He had great diYculty in getting any signal. After some weeks of frustration, he asked me how I had done it, and said he had followed my directions explicitly. He challenged me to make a scintillator he could use, and prove to him it could be done. I accepted the challenge, machined up a hemispherical scintillator, aluminized it, and he installed it. Lo and behold, it worked as I had predicted. We then very carefully analysed the diVerence in our fabrication methods. He had been advised to polish the scintillator just before the aluminization step with a chemical polish compound we had in the laboratory, called Brasso. We determined that this destroyed the light-producing property of the scintillator at the polished surface, which was the only region the electrons could penetrate to create light. I learned a valuable lesson about the importance of process and realized how fortunate I had been in my original procedure. Had I used Brasso the first time, it might have taken weeks or months of valuable research time to get the scintillator/light-pipe/photomultiplier collection system to work well. Subsequently, Richard Thornley, who inherited SEM2 from Oliver Wells, made more quantitative measurements on the scintillator/light-pipe/photomultiplier collector, and we published a paper (Everhart and Thornley 1960) on its properties. That paper is reproduced as Chapter 2.4B. Having determined the diVerent character of images that were produced by backscattered electrons on the one hand and low-energy secondary electrons on the other, I felt I needed to explain the diVerences. To accomplish this, a more careful analysis of the nature of the electrons producing the image was required. This led to a series of experiments which determined that, with the new scintillator/light-pipe/photomultiplier detector in the position that gave good contrast, the secondary electrons generated at the sample produced not less than 65% of the video signal that formed the image: reflected electrons from the sample entering the collector produced about 5% of this video signal, and secondary electrons produced by reflected electrons striking the exit face of the lens at grazing incidence formed not more than 30% of the signal. Hence, the contrast could be termed secondaryelectron produced, but the reflected electrons played a larger role than might

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have been suspected (Everhart, 1958, pp. 28–31). My dissertation discussed in qualitative terms why this might be so, especially taking into account ‘surface modulation’ and ‘specimen collection’ contrast produced by roughness on the sample surface, which can modulate the number of reflected electrons that strike the exit surface of the final lens.

III. A Theory of Reflected Electrons Since both secondary electrons and back-scattered (reflected) electrons were involved in the image formation and the contrast could be altered depending upon which was allowed to play the dominant role, it seemed important to try to understand the physics of both secondary and reflected electrons. A useful theory would show not only where the electrons producing the signal might emerge from the sample (a subject related to the ultimate resolution of the SEM), but also how reflected electrons might produce low-energy secondary electrons as they emerged, which would aVect the resolution of the secondary electron image, as well as its contrast. Accordingly, I tried to learn as much as I could about how electrons were scattered through large angles, both elastically and inelastically, and how energetic electrons lose energy as they penetrate through solids and gases. While I was able to find a simple theory of how low-energy secondary electrons were produced by an incoming beam, there seemed to be no similar simple theory concerning high-energy reflected electrons. Using an arbitrary definition that secondary electrons had energies less than 50 eV, and reflected electrons were all electrons escaping the sample with energies between 50 eV and the primary beam energy, I proceeded to devise a simple theory of reflected electrons. The assumptions were: an empirical formula for electron-energy loss as energetic electrons penetrate matter (the Thomson–Whiddington law); that scattering was determined by the Rutherford elastic scattering formula; and that if electrons were scattered through less than a right-angle, they were not scattered at all. This simple theory (Everhart, 1958, Chapter 4) provides a good deal of physical insight into the way that electrons are reflected from matter. It correctly predicts that the fraction of electrons reflected depends primarily on the atomic number Z, and explains how the reflected electrons should increase as the primary beam becomes more inclined to the sample surface. It also provides insight into the area of the sample surface from which the reflected electrons might emerge. Much of this theory was subsequently published (Everhart, 1960).

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A. Electron Trajectories and Contrast Formation To understand the performance of the SEM in more detail, and how it might be optimized further for future designs, it seemed important to learn more about the secondary electron trajectories in the sample chamber we were using. This geometry was not susceptible to mathematical analysis, but, because it did have planar symmetry, it could be modelled in an electrolytic tank. Fortunately, a fellow graduate student, Mark Barber, was working with such a tank connected to a computer to automatically trace electron trajectories. This apparatus seemed ideal for tracing the electron trajectories between the sample and the secondary electron collector, and we proceeded to do so. The resulting secondary-electron trajectories in the sample chamber showing the influence of diVerent voltages on the collector, the influence of exit angle of the secondary electrons (Fig. 1) and the influence of their initial energy of emission from the sample (Fig. 2), etc., are summarized in my doctoral dissertation (Everhart, 1958, Chapter 7). This improved understanding of the secondary electron trajectories was very helpful in explaining voltage contrast, which was easily detected when a few volts of reverse bias were applied across a sectioned p–n junction in the SEM (Oatley and Everhart, 1957). Getting good reproducible samples of p–n junction diodes, let alone transistors, was not easy in Cambridge during the late 1950s. The best samples I was able to obtain came from Dr J. R. Tillman and Dennis Baker at the Post OYce Research Station. These were made by alloying an indium pellet to a single-crystal germanium surface that had been carefully ground, lapped and etched to a mirror-like surface along the (111)

Figure 1. 4-eV electron trajectories, exit position and angle varied. (Fig. 7.2 from Everhart, 1958.)

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Figure 2. 4-eV electron trajectories, specimen voltage variable. (Fig. 7.10 from Everhart, 1958.)

plane. The resulting diode was carefully ground and polished so one could have access to the cross-section of the p–n junction. Scanning electron micrographs of this cross-sectioned p–n junction showed that it followed a straight line, presumably a crystallographic plane, to the resolution of the SEM (a few hundred Angstrom units). Voltage diVerences across the junction as low as a volt could be easily detected as contrast diVerences on a micrograph (Fig. 3). In addition, as the beam scanned across the junction, a large increase in the reverse-biased current could sometimes be observed, due to the hole–electron pairs that were created in the depleted junction region, and swept across it (Everhart et al., 1959). These experiments provided valuable background for future work at Westinghouse Research Laboratories with Dr O. C. Wells a few years later, when excellent integrated circuits became available (Everhart et al., 1964a,b).

IV. Cambridge Recollections As indicated in my opening paragraphs, my study at Cambridge University was not limited to science and engineering. Interaction with other students, who came from many lands as well as from England, and with citizens of the

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Figure 3. Voltage contrast demonstrated by reverse-bias at a germanium-indium p–n junction. (Figs. 9.2 and 9.3 from Everhart, 1958.)

town were also important for both me and my wife. Married students in Cambridge in the late 1950s were definitely a small minority. My wife and I especially appreciated the thoughtfulness of Mrs Oatley (later Lady Oatley), who stopped by our flat occasionally to make sure we were faring all right. She and Mr Oatley (later Sir Charles) personally touched our lives, particularly after the birth of our first daughter. We remember very fondly many friends from both town and gown from the three years we spent in Cambridge. Research students could take part in club sports, and I was able to be a member of the Cambridge University Basketball team; it was a great thrill

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when we won our annual game with Oxford for the first time in history. It is also quite satisfying to have been a member of a research group who brought an important instrument, the scanning electron microscope, to the stage where it could be produced and sold commercially, benefiting many areas of engineering, science and medicine. It is a pleasure to thank the Marshall Aid Commemoration Commission for financial support that made my three years as a research student at the University of Cambridge possible.

References Everhart, T. E. (1958). ‘Contrast formation in the scanning electron microscope.’ PhD Dissertation, University of Cambridge. Everhart, T. E. (1960). A simple theory concerning the reflection of electrons from solids. J. Appl. Phys. 31, 1483–1490. Everhart, T. E., and Thornley, R. F. M. (1960). Wide-band detector for micro-microampere low-energy electron currents. J. Sci. Instrum. 37, 246–248. Everhart, T. E., Wells, O. C., and Oatley, C. W. (1959). Factors aVecting contrast and resolution in the scanning electron microscope. J. Electron. Control 7, 97–111. Everhart, T. E., Wells, O. C., and Matta, R. K. (1964a). Evaluation of passivated integrated circuits using the scanning electron microscope. J. Electrochem. Soc. 111, 929–936. Everhart, T. E., Wells, O. C., and Matta, R. K. (1964b). A novel method of semiconductor device measurements. Proc. IEEE. 52, 1642–1647. Oatley, C. W., and Everhart, T. E. (1957). The examination of p–n junctions with the scanning electron microscope. J. Electron. 2, 568–571.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.4B* Wide-Band Detector for Micro-microampere Low-Energy Electron Currentsy T. E. EVERHART AND R. F. M. THORNLEY Engineering Department, University of Cambridge

Introduction The properties of the detector described in this paper were largely determined by the instrument for which it was designed, the scanning electron microscope. In this instrument, a fine electron probe is scanned in a rectangular raster across the surface to be examined. Secondary electrons leaving the surface are collected, amplified and used to control the brightness of a cathode-ray tube scanned in synchronism with the probe. Details of the instrument and the contrast mechanism have been published by McMullan,(1) Smith and Oatley,(2) and Everhart, Wells and Oatley.(3) For the full capabilities of the instrument to be realized, relatively noise-free amplification of the modulated secondary electron current is required. As this current ranges from 10 13 to 10 10 A, and contains frequency components up to 200 kc/s, conventional thermionic amplifiers cannot comply with the low-noise and bandwidth requirements. Early workers used secondary emission electron multipliers, but these were not very satisfactory, electrically or mechanically.

General description The new detector (shown in Fig. 1) consists of a cylindrical brass shield which is closed at the end facing the specimen by a grid of copper gauze (etched in nitric acid to increase the grid porosity) and is biased positively in order to attract the lowenergy electrons. Once through the grid, the electrons are accelerated toward the ˚ layer of hemisphere of plastic scintillator, the surface of which is covered with a 700 A aluminium (maintained at 7 to 12 kV positive). The intense electrostatic field, shaped by the focusing electrode, causes most electrons to strike the hemisphere near its apex. The light generated in the scintillator is guided by a Perspex light pipe to a commercial photomultiplier tube which then converts the light back to an electron *Reprinted from: J. Sci. Instrum. 37, 246–248 (1960). y

Paper received 25 January, 1960.

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Fig. 1. Diagram showing collector geometry.

signal and provides most of the required amplification.

Theoretical performance For such a system to be reasonably noise-free, each electron entering the collector must liberate, on the average, at least one electron from the photomultiplier cathode. The number of photoelectrons produced by one zero-energy secondary electron entering the collector is given by: n ¼ fðEs

Ea ÞCp C0 Ce f ðvp Þg=Ep

ð1Þ

where Es ¼ energy of the electron striking the scintillator surface (eV), Ea ¼ energy lost by the electron in passing through the aluminium film (eV), Cp ¼ energy conversion eYciency into photons of average energy Ep, C0 ¼ eYciency of the optical system, Ce ¼ conversion eYciency of the photocathode, f(v) ¼ spectral response of the photocathode to photons of energy Ep (eV). Ea can be calculated from the Thompson-Whiddington law: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ea ¼ ðEs2 axdÞ ð2Þ

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where x is the film thickness in cm, a is 4  1011 (eV)2 cm2/g, and d is the density in g/cm3. In the early work on the collector, the manufacturer’s figure for Cp was used and C0 was assumed to be unity. This gave n equal to 2.2 for Es ¼ 7 kV, indicating that under these conditions no noise was introduced. It was found in practice that noise was being introduced and a detailed investigation of Cp and C0 was carried out.

Scintillator eYciency Several workers(4–6) have reported that the conversion eYciency of scintillators falls as the incident particle energy is reduced below 50 keV, but the losses quoted vary considerably. It was therefore necessary to measure Cp at the energy levels in question. Two makes of plastic scintillator were investigated—Pamelon, made by Isotopes Development Ltd.; and Naton 11, made by Nash and Thompson Ltd. No significant diVerence in performance was detected. A 10 kV electron beam carrying about 10 12 A was arranged to strike the scintillator surface, the potential of which could be varied between þ10 and 10 kV. It was found that the response varied approximately linearly with incident energy for energies above a threshold value which varied between 2 and 10 kV. Typical curves are shown in Fig. 2. Further investigation showed that the method used to prepare the scintillator surface was of critical importance in determining the performance for low incident energies. Scintillators machined under water-cooled conditions and polished with French chalk in water showed threshold energies of 2 to 2.5 keV. This energy is that required to ˚ aluminium film used to make the scintillator surface conducting. penetrate the 700 A Dry machining and/or polishing with proprietary brands of metal polish raised the

Fig. 2. Scintillator performance. Curve (a), wet machined, French chalk polished; curve (b), dry machined, polished with metal polish.

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threshold into the 5 to 10 keV range. Any slight overheating during the metallizing had a similar deleterious eVect on the performance. Measurements with a calibrated photomultiplier showed that the conversion eYciency of a well prepared scintillator was 0.02  0.005 for 7 keV electrons. This figure agrees, within the limits of experimental error, with the manufacturer’s value. The scintillator did not show any saturation eVects until the input current density reached about 1 A/cm2. Under normal operating conditions, the input current density is less than 1 mA/cm2 so saturation is not a limiting factor in the performance of the collector. Optical system eYciency The optical eYciency of the scintillator-light-pipe system is estimated by assuming that all electrons entering the collector shield (see Fig. 1) strike the scintillator near the apex of the hemisphere. The critical angle yc of the scintillator material (measured  from a surface normal to the ray path) is 40.5 . If no light is absorbed by the scintillator, and no loss occurs at surface reflexions, then by using a simple geometrical argument, over 90% of the generated light is found to enter the light-pipe. Further, all these light rays entering the light-pipe subtend an angle to the surface normal greater than the critical angle of the light-pipe material, Perspex. Thus in the absence of absorption in the light-pipe, all light entering the pipe should reach the photomultiplier. If it is assumed that a 10% loss of intensity occurs at each surface reflexion in the scintillator, about 80% of the generated light enters the light-pipe. The transmission eYciency of the light-pipes used was measured with a special neon light source having an intensity distribution similar to that expected from a scintillator hemisphere. The light-pipes were made from 38 in. diameter Perspex rod cut   in lengths varying from 9 to 11 cm. The ends were polished. Bends of 45 and 90 with a radius of 2.5 cm were made on some light-pipes by gently warming the rod in a low bunsen flame and then bending to the desired shape. It was found that in no case did the transmission exceed 65%, and if extreme care was not taken with the surface finish, the transmission fell below 40%. The bends did not increase the loss by more than 10% in any case. This loss, which is rather surprising in view of the high transparency of Perspex, is believed to be mainly caused by reflexion losses on entering and leaving the pipe. A drop of vacuum oil placed in a joint between two pipes increased the transmission by about 5% provided the oil did not spread along the ferrule supporting the joint. If this occurred, the loss was greater than for a dry joint. Measured performance Putting C0 ¼ 0.4, Cp ¼ 0.02 and n ¼ 1 in the transfer equation gives Es ¼ 8 keV. This analysis depends on the accuracy of the photomultiplier calibration which was not better than 20%. An independent check on the result was made by observing the signal/noise ratio of the photomultiplier output for a constant input current, with the

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Fig. 3. Signal/noise ratio variation with scintillator potential.

scintillator voltage varied to give values of Es between 0 and 20 keV. This gave the curve shown in Fig. 3, indicating that the noise is eVectively constant for scintillator potentials greater than 10 kV. (The 18 dB signal-to-noise ratio is determined by the shot noise in the primary electron beam plus the noise introduced in the secondary emission process.)

Conclusions A wide-band detector for micro-microampere secondary electron currents has been constructed and tested. Noise-free current gains ranging from 105 to 108 have been observed with a bandwidth exceeding 200 kc/s. The minimum signal which can be detected is set by the dark current of the photomultiplier used. For the tube used in this investigation, an E.M.I. type 6094B, this dark current is less than 10 15 A, and it could be reduced further by cooling the photocathode. The maximum bandwidth is set by the decay time of the scintillator or the transit time spread of the photomultiplier, or both. The decay time for Pamelon is stated to be less than 10 8 s, and the transit time spread for the photomultiplier used is about 10 8 s. Thus the bandwidth of the present system is estimated to be greater than 10 Mc/s. As used in the scanning electron microscope, this detector oVers important advantages. The mechanical flexibility is much better than that of the old secondary electron detector. A movement of the detector input requires only a new piece of light-pipe bent to the proper shape with the new detector, while previously an entire new brass bottom plate for the specimen chamber had to be constructed. Previously, the input of the electron multiplier was necessarily near ground potential, requiring the output to be approximately 6 kV positive. This in turn required that the initial stages of valve amplification float at 6 kV positive. With the new detector, the output of the photomultiplier can be at ground potential, making a direct-coupled video-amplifier possible.

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Acknowledgements We wish to express our thanks to our supervisor, Mr. C. W. Oatley, for much useful advice and encouragement. One of us (T. E. Everhart) was supported by a Marshall Scholarship, while the other (R. F. M. Thornley) was in receipt of a D.S.I.R. Research Studentship.

References (1) (2) (3) (4)

McMullan, D. Proc. Instn Elect. Engrs, 100, Part II, p. 245 (1953). Smith, K. C. A., and Oatley, C. W. Brit. J. Appl. Phys., 6, p. 391 (1955). Everhart, T. E., Wells, O. C., and Oatley, C. W. J. Elect. & Control, 7, p. 97 (1959). Taylor, C. J., Jentschke, W. K., Remley, M. E., Eby, F. S., and Kruger, P. G. Phys. Rev., 84, p. 1034 (1951). (5) Birks, J. B., and King, J. W. Phys. Rev., 86, p. 568 (1952). (6) Belcher, E. H. Brit. J. Radiol., 30, p. 103 (1957).

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.5 A Simple Scanning Electron Microscope P. J. SPREADBURY Formerly at: Engineering Department, University of Cambridge

In October 1956, returning to Cambridge after a relaxing summer spent as a ‘courier’ for a holiday company in the mountains of Austria, I was faced with the somewhat open-ended suggestion from Charles Oatley of building a simple scanning electron microscope. I was personally most able at circuit development, and I suppose that my laboratory course work had been noticed by him in my final year as an undergraduate, for he had taken the initiative in approaching me. At the time, I was a regular army oYcer in the Royal Electrical and Mechanical Engineers (REME), and had been given paid leave to fill a place that I had already secured at Cambridge. Taking up Oatley’s suggestion meant that I had to serve at least as many years back in the army as I had taken as leave, so a two-year MSc, rather than a three-year PhD, was all that was planned. It had long been Oatley’s contention that for a wide range of applications the great depth of field and ease of specimen preparation and manipulation aVorded by the SEM was more important than high resolution. He believed that an SEM of the simplest possible construction operating at a moderate magnification, but which was capable of matching the resolution of the best optical microscope, would be of considerable utility to microscopists. Building such an instrument became the aim of my research. A schematic diagram of the simple scanning electron microscope (SSEM) is shown in Fig. 1 (from Fig. 6.1 of my dissertation, Spreadbury, 1958). Following established practice in Oatley’s laboratory at the time, it was decided to build an electrostatically focused instrument, thereby avoiding the complication and expense of lens current supplies. Electrostatic deflection of the electron beam was used, and utilizing a standard laboratory oscilloscope for the display eVected further simplification. A major part of the work involved the design and construction of the various supplies for the electron gun and the circuitry associated with the deflection system, the electron detector and the amplifier chain. The ‘Brandenburg’ 30-kV units used for supplying the EHT for the some of the electron-optical instruments previously constructed in the laboratory had proved to possess insuYcient 153 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Figure 1. Schematic diagram of the simple scanning electron microscope. (From Spreadbury, 1958.)

stability and were rather expensive, so my first task was to design a replacement. Bipolar transistors were becoming widely used at the time, but they were electrically fragile; field-eVect transistors costing about 10 each were available as development samples, but had the reputation of being even more fragile. About half of new equipment was still valve-based design. Oatley was careful with money and would have been very unhappy with

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experiments risking ‘blown’ devices. In fact, among my fellow research students, only Mike Forrest was brave and skilled enough to be using these devices at the time. I decided to stick to valves. An output of 20 kV at 1 mA suYced for the SSEM, so work was started on a 30-W, 25-kHz power amplifier coupled to a four-stage ladder rectifier multiplier through a step-up transformer with an output voltage of 5 kV. The amplifier utilized EL821 compact tetrodes, widely employed in highfidelity audio work at the time. The step-up transformer was designed with a ferrite core and low self-capacitance, and was potted in epoxy resin to provide the requisite insulation. This was one of the many occasions I needed to call on the experience of Les Peters, since the epoxy setting process was exothermic, which could, if care was not taken, result in strains and cracks in the resin. Stabilization of the supply was achieved by means of a resistor divider network connected to compare the output with an 85-V neon discharge tube. All the delights of instability in the feedback loop were encountered before a satisfactory performance was attained. My dissertation (Spreadbury, 1958)—which I confess to not having opened for 42 years—contains the following record for the performance of the supply: Change in output for 0 – 1 mA load change: 0.1% Change in output for 20% change in mains supply: 0.2% Output stability over an 8 hour period: 0.01%. At full load: 25 kHz ripple on the output was 0.02% rms. 100 Hz hum on the output was 0.005% rms. Other noise on the output was 0.005% rms. I see clearly the hand of Oatley in these results; he always stressed the importance of careful measurements. He was also concerned about the safety of apparatus constructed in his laboratory. The last sentence of the chapter in which the supply is described reads: ‘The supply should be non-lethal; the output condensers store 1/5 joule at full voltage and the maximum current of the supply at any voltage is less than 1.5 mA.’ Components used extensively in the supply were ex-war stock highstability resistors of 1-W rating. By chance I came across a sample of such a resistor recently, and to my amazement found that it was about 50% higher than the nominal value; we forget how components have improved nowadays. As indicated by Oatley in his 1982 paper (Chapter 1.2), a major part of my research constituted an investigation of the performance of the electron gun that I was to use on the SSEM. The electron gun was one made to a design by Ken Smith that was then becoming standard for the microscopes in the laboratory. Measurements of the size and position of the gun crossover and

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beam brightness were made using a test rig incorporating a single electrostatic lens, arranged to obtain a ten times magnified image of the crossover on a phosphor screen. The screen contained three small apertures, and the current passing through these was collected in a Faraday cage coupled to an amplifier and oscilloscope display. By scanning the beam falling on the screen across the apertures, the beam current profile could be displayed. It was convenient to present the beam profile in the form of a Y-modulation (scan-modulation) display on a two-dimensional raster. Thus the display showed three contours as the beam was swept across each of the apertures in turn. Knowing the precise distances between the apertures enabled the display to be calibrated and thus the beam diameter to be determined. A prior calibration of the magnification of the imaging lens enabled the diameter of the beam at the screen to be related to the diameter of the gun crossover. These and other measurements showed that the gun could be set up to achieve substantially the theoretical (Langmuir) brightness. One important factor on which the operation of the gun depended was the actual shape of the tungsten filament, and the following extract from my dissertation indicates that filament manufacture was far from straightforward. It is necessary to standardise the way of forming the hairpin filaments in order to obtain consistent results from the gun: this is done as follows. A piece of tungsten wire, 0.1 mm diameter and 1 inch long [note the mixed units!], is pressed in its centre onto a rubber surface with the edge of a sharp razor blade. A suitable surface is provided by a rubber ‘cork’ used in chemical apparatus. The wire is formed with even pressure into a very sharp hairpin and examination under a microscope shows that a smooth tip is obtained.

The dissertation goes on to explain how the filament is mounted, cleaned electrolytically in potassium hydroxide and flashed at 3000 K. Typically, the operating lifetime of filaments was about eight hours. Without suYcient care ‘the even pressure of the razor blade’ could result in two half-inch pieces of wire instead of the desired filament, which was embarrassing since Oatley expected his students to be careful and resourceful, particularly if money could be saved thereby. Here again I recall that Les Peters was a source of skilled advice on how to improve one’s success rate. Today’s researchers can simply order a box of pre-formed plug-in filaments for their microscopes; the diYculties confronting the early workers in scanning electron microscopy are often forgotten. My final year of research was spent building the SSEM (Fig. 2). The twostage electrostatic lens system was designed with the help of Chris Grigson, using data published by Archard (1956). Focusing was achieved by varying the potential of the final lens electrode, relative to the gun supply, by means

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Figure 2. Photograph of the completed simple scanning electron microscope. (From Spreadbury, 1958.)

of a high-voltage potentiometer. Beam deflection plates, located between the two lenses, required plate potentials of 430 V to obtain a minimum magnification of 75 on the 3-inch square oscilloscope display at a gun voltage of 10 kV. The scanning system was operated at near television rates of 25 frames per second and between 200 and 400 lines per frame, although the oscilloscope could resolve only 250 lines. Because of the high scanning frequencies used, both for observation and recording, the video amplifier chain could be AC coupled, which appreciably simplified its design. It was fortunate at the time I was doing my research that Tom Everhart had recently perfected his scintillator/photomultiplier detector system, which he described in his dissertation (Everhart, 1958), and I was able to incorporate

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one of these in the SSEM. Without this innovation it would hardly have been feasible to build anything that could be described as ‘simple’. Construction and testing of the microscope was finished by about July of my second year. Using the ‘edge-sweep’ test devised originally by Dennis McMullan (McMullan, 1952) I was able to demonstrate a scanning probe of ˚ diameter at a working distance of 18 mm, thus achieving a major aim 1500 A of the research. However, owing to frequent intermittent shifts of the image when operating at high magnification, I was unable to record micrographs demonstrating an equivalent resolution. This fault was attributed to charging of surfaces in the vicinity of the deflection plate assembly in the column, but owing to lack of time I was unable to eliminate the trouble. I submitted my dissertation in November 1958, and the oral examination took place very soon afterwards, as I was to report almost immediately in uniform as a regular Captain in REME. On leaving Cambridge I found myself in West Germany acting as workshops oYcer in a mobile workshop, mending army equipment ranging from pistols to tanks, and from radio sets to binoculars. The change from research came as a considerable shock, particularly as it also involved my absence on manoeuvres, living in tents and sometimes in the snow for months at a time. Some 10 years after leaving Cambridge, I received a telephone call from Oatley at SheYeld University, where I was then a lecturer. He asked why I had expressed no interest in a vacant lectureship at Cambridge, for which apparently none of the applicants had been found suitable after interview. So I was added as a late applicant and had a most friendly interview with Oatley and J. L. Reddaway; the latter had been one of my supervisors in my undergraduate days and was now the Secretary of the Faculty Board of Engineering. The upshot was that I rejoined the Engineering Department and commenced research in the field of precision electrical measurements, an interest stimulated by my previous work with Oatley. ‘Prof’ was a little oldfashioned but immensely careful and experienced; he was an example to us all and a pleasure to have worked with. References Archard, G. D. (1956). Focal properties and chromatic and spherical aberrations of three electrode electron lens. Br. J. Appl. Phys. 7, 330–332. Everhart, T. E. (1958). ‘Contrast formation in the scanning electron microscope.’ PhD Dissertation, Cambridge University. McMullan, D. (1952). ‘Investigations relating to the design of electron microscopes.’ PhD Dissertation, Cambridge University. Spreadbury, P. J. (1958). ‘Investigations relating to the design of a simple scanning electron microscope.’ MSc Dissertation, Cambridge University.

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2.6 New Applications of the Scanning Electron Microscope R. F. M. THORNLEY Formerly at: Engineering Department, Cambridge University

I. Introduction I first met Charles Oatley when I was an undergraduate. After completing Part I of the Natural Sciences Tripos, I transferred to the Mechanical Sciences Tripos because I felt that physics was becoming closer to pure mathematics than I liked. I also preferred the hands-on satisfaction of actually building things. Oatley was the representative of the Cambridge University Engineering Department who interviewed me. Since it was well known that scientists usually are bad engineers, he required me to take two summer courses to check my suitability for transfer. One was a welding course, taken in Cambridge, which I enjoyed while expanding my knowledge of construction methods; the other was a machine tool familiarization course, which was taken at Ferranti in Edinburgh. Although I enjoyed Edinburgh, I learned nothing new or useful from the course. Several classes in Part II of the Tripos were taught by Oatley. I found him to be a low-key, patient lecturer whose classes were both enjoyable and eVective. On completion of Part II in 1957, I asked him about the chances of doing research in Cambridge but I had no idea what I wanted to work on or how I would survive while doing it. He gave me a quick tour of the facilities and suggested that I might continue the work of Oliver Wells. Personal funding was with a grant from the Department of Scientific and Industrial Research, and the laboratory fees were mysteriously arranged by Oatley to be covered by the Navy. I never did fully understand the connection between scanning microscopy and the Navy, but I think it had something to do with work done on the laboratory’s electron trajectory plotter on the transit-time spreads in photomultipliers, a topic in which the Navy was very interested.

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II. Research I believe that Oatley used the sink-or-swim method of supervising his students. With hindsight, this opinion may be overly unkind since, at the time, the Professor of Electrical Engineering, E. B. Moulin, was having serious health problems, forcing Oatley to do both his own and Professor Moulin’s work. Wells’ microscope was located on the ground floor room in Scroope House, which I shared with Tom Everhart who was in his third year when I started my research. I shall always be grateful to Tom for introducing me to the practical aspects of the world of scanning electron microscopy and for his patience with the new student sharing his space and asking innumerable questions. Oliver Wells gave me a rapid review and demonstration of his SEM and disappeared to write his thesis, leaving me to fill in the blanks. Like all the microscopes built by Oatley’s students, Wells’ microscope (SEM2) was a work in progress. This is not a criticism of Wells’ work but a recognition of the reality that a student does not have the time, knowledge or resources to optimize his creations. My previous experience in the British army (Royal Electrical and Mechanical Engineer) on radar and computer systems for the control of anti-aircraft guns had given me an extensive knowledge of nonlinear vacuum tube circuits. After struggling with my inherited scanning circuits for several weeks, I realized that a rebuild was in order. I ran the idea by Oatley, who eVectively said ‘If it needs rebuilding, go and do it’. Oatley’s great contribution at this time (although I did not appreciate it until I worked in other laboratories) was his establishment of a wonderful supply of electronic components. This enabled myself and others to design, build and test new circuits very rapidly. At this time (1957), the laboratories used by the research students were divided. Everhart and myself were in the ground floor of Scroope House; students working on theoretical topics used the upper floors. The other students had laboratory space in the Coe Fen building. Measurement equipment such as multimeters and oscilloscopes were, in principle, shared between the two locations. In practice, competition for some of the equipment, particularly the Model 8 Avometer (which had a 50-mA movement) and the Cossor oscilloscope (which had a high-sensitivity, DC-coupled input) was vigorous. Oatley initially rejected any suggestions that duplicate equipment would lead to faster work, citing the cost of such duplication and what he saw as only a small gain in productivity. Since financial matters were not shared with the students, we were unable to judge the merits of his argument. Later, the availability of higher-performance oscilloscopes and meters in the Coe Fen laboratory permitted the Scroope House students to monopolize

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the Model 8 Avo and the Cossor oscilloscope. A particularly expensive piece of equipment, a vibrating capacitor electrometer that could measure currents down to 1 fA, was never duplicated; this caused very little friction because it was only rarely needed but was indispensable when it was.

A. Improving the SEM A major problem with the earlier microscopes and with the Wells microscope was interference from the 50-Hz power supplies. In the Wells microscope, several power supplies were stacked at odd angles on the floor as far away as possible from the microscope itself to minimize 50-Hz deflection of the electron beam. Wells had lived with the problem by synchronizing his scan rates to the 50-Hz power line; my improvements in the quality of the scanning circuits showed that the problem had many separate manifestations that, being all synchronized to 50 Hz, could combine constructively or destructively. These were identified and resolved to the point at which they did not limit instrumental performance, although they could be detected under the right conditions (Thornley, 1960). Earlier microscopes used radio-frequency current to heat the tungsten filament in the electron gun because it allowed the filament to operate at 50 kV relative to ground with very simple insulation. One of the changes I made was to rebuild the video amplifier for the signals from the Everhart collector system. The primary reason for the rebuild was to improve the DC stability for the long recording scans; another reason was to increase the bandwidth to enable the visual scan rates to be increased. The improved bandwidth showed that the electron beam was flapping around at the frequency used to heat the filament because the crossover in the electron gun itself was being displaced by the heating current. This problem was resolved by replacing the RF supply by a DC supply fed from an AC line stabilizer and a Variac transformer to provide a stable, variable 50-Hz power source. The power was delivered through an air-insulated 50-Hz transformer, a selenium rectifier (which could survive the occasional spark-over transients much better than the alternative silicon or germanium diodes) and an LC smoothing circuit to the microscope filament. Imaging of the gun crossover by scanning across the aperture in the objective lens showed that it was very elliptical in shape. It also varied with the skill of the person (usually myself) who had made the filament, the radius of the bend at the top of the filament, and the accelerating voltage. This problem was resolved by placing a physical aperture in the gun at the crossover to define its size and shape. The downside of this solution was that the filament had to be located precisely in the shield of the gun and any

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movement of it could change the beam current significantly. The addition of a vertical adjustment to the existing X–Y adjustment for the filament solved this problem. The original electron gun utilized the Bru¨ck and Bricka design with a radial gap of 58 inch allowing operation up to 50 kV. Given the radial symmetry of this design, the gap could not be altered to maintain gun eYciency at low voltages: below about 12 kV, the combination of enlarged crossover size and low current density eVectively prevented operation with any useful resolution. B. The Everhart Detector The Everhart electron collection system using a plastic scintillator and a sealed photomultiplier had already replaced the open multiplier system using oxidized beryllium copper dynodes when I started my research. I inherited a scintillator system with the Wells microscope and this was where the ability to compare microscope performance against another microscope became invaluable. Tom Everhart, working on the other side of the room, was consistently seeing much less noise than I was when using the same beam current, accelerating voltage and specimen. In addition, his photomultiplier voltages were much lower than I was using to achieve adequate picture contrast. These diYculties were traced to poisoning of the scintillator surface during machining and polishing, and incorrect wiring of an Aralditeencapsulated divider chain for the photomultiplier. A model of the collector was built and tested in the electron trajectory plotter in the laboratory. After several iterations, a collector design was developed that could use up to 12 kV to accelerate the electrons on to the scintillator at the optimum point for the generated light to enter the light-pipe to the photomultiplier (see Chapter 2.4B, which reproduces the paper of Everhart and Thornley, 1960). C. Low-voltage Scanning Electron Microscopy Operation of a scanning electron microscope at low voltages was one of the new areas of investigation (Thornley, 1961). Potential advantages were increased sensitivity to thin surface films, less loss of resolution caused by penetration eVects, and more signal amplitude because the secondary emission coeYcient of most materials increases as the accelerating voltage decreases. OVsetting these factors are the decrease in beam brightness, the increase in the diVraction disc diameter, and the increase in sensitivity to stray magnetic fields. All of these eVects were verified without diYculty. The advantage that turned out to be the most significant was the increase in the

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secondary emission coeYcient. When the accelerating voltage is lowered enough to make the secondary emission coeYcient greater than unity, the surface of insulating samples tries to charge up positively instead of negatively. Because the energy of many of the secondary electrons is less than 10 eV, the sample surface never charges up to more than a few volts above the grounded specimen chamber and there is no distortion of the primary beam. There is a predictable change in the secondary electron collection process because the total secondary emission coeYcient from the insulator surface is eVectively unity, so that high-energy electrons contribute more to the final image. The ability to image insulating surfaces was used on nylon fibres and fractured ceramic specimens (Thornley and Cartz, 1962). The ability to sense very thin surface films enabled contamination areas to be examined and, rather surprisingly, removed during extended low-voltage viewing. Sensitivity concerns ruled out the possibility of using this mechanism for data storage, but it undoubtedly helped in securing my first job at IBM after leaving Cambridge. D. Biological Specimens Biological materials present perhaps the greatest challenge to electron microscopists (Thornley, 1961). The relatively low density of most biological samples makes contrast and resolution poor. The high vacuum inside the microscope damages samples with volatile constituents and, when dried, many samples become good insulators. Techniques in use at the time relied on staining with heavy metals or freeze-drying, followed by deposition of a thin conducting metal skin, usually of gold, to prevent charging eVects and improve contrast. Two additional approaches were shown to be feasible in the scanning electron microscope. Freeze-drying followed by low-voltage microscopy avoided the charging artefacts usually observed with beam voltages above 6 kV. The very low power loading in the scanning microscope enabled wet samples to be flash-frozen (to minimize cell damage) and viewed while keeping the sample very cold to prevent loss of water vapour. E. Magnetic Materials Magnetic materials present particular problems to electron microscopists because the primary interest is in viewing magnetic states rather than physical topography or chemical variations. Transmission electron microscopy has been used very successfully on thin ferromagnetic samples but cannot help on solid specimens. Wells had previously tried to show magnetic

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contrast on a cobalt single crystal, with marginal results: I tried as well (on the same crystal, which was greatly revered and carefully guarded by all the microscopists) with similarly disappointing results (Thornley, 1960). Use of Bitter colloid on the crystal certainly revealed the domain boundaries in the scanning microscope but oVered no advantage over optical microscopy. III. Conclusions Two important things that I learned under Oatley were: 1. Get on with doing the work—you don’t really know what may turn up and thinking about it cannot substitute for experiments. 2. Do everything you can to facilitate the work. At Cambridge, the availability of parts, machine shop and the daily coVee break with everyone—staV, students and visitors—motivated all. After leaving Cambridge to work in industrial laboratories, I saw innumerable opportunities lost because the organizations lacked these support facilities. Ideas are the easy part—following them up takes time, money, motivation and, regrettably, politics. References Everhart, T. E., and Thornley, R. F. M. (1960). Wide-band detector for micro-microampere low-energy electron currents. J. Sci. Instrum. 37, 246–248. Thornley, R. F. M. (1960). ‘ New applications of the scanning electron microscope.’ PhD Dissertation, University of Cambridge. Thornley, R. F. M. (1961). Recent developments of the scanning electron microscope. Delft 1960, 173–176. Thornley, R. F. M., and Cartz, L. (1962). Direct examination of ceramic surfaces with the scanning electron microscope. J. Am. Ceram. Soc. 45, 425–428.

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2.7A A. D. G. Stewart and an Early Biological Application of the Scanning Electron Microscope A. BOYDE Centre for Oral Growth & Development, Barts & The London School for Medicine & Dentistry, University of London Formerly at: University College London

I have spent most of my working lifetime pursuing very enjoyable research activities based upon the SEM. How did this come about, when I came from a clinically oriented biological background? Graduating in 1958, and after a six-month ‘house job,’ I took up a position in 1959 as a Junior Demonstrator in the Anatomy Department of my Alma Mater, the London Hospital Medical College, and was initially paid by unemployment benefit. Richard Harrison, then Head of Department, told me that he hoped I had private money, since it was the only way that one could survive in science. Actually, I had none. In a little while, I received a salary of 400 p.a.; riches indeed. But, much more than that, I benefited from the vision of my teacher and friend Ronald Fearnhead, a wily bird who had survived as a prisoner of war of the Germans, escaped, and walked across Europe to Odessa. Ron Fearnhead had contacted Ellis Cosslett at the Cavendish Laboratory, University of Cambridge, with the result that he had the privilege of working with Roy Switsur on the Duncumb–Cosslett Scanning Electron Probe Microanalyser (see Chapter 3.3A) on a once-a-week basis from 1959 (Boyde and Switsur, 1962; Boyde et al., 1961, 1962; Switsur and Boyde, 1962). Backscattered electron SEM imaging was used to monitor the position of the beam on the sample, if not exactly where the X-ray emission emanated. I came to learn that the Microanalyser was partly based on the separate development of scanning electron microscopy per se at the Cambridge University Engineering Department (CUED) under Charles Oatley. Morphology being the queen of the sciences, I was very much for the SEM imaging aspect. Roy Switsur introduced me to Garry Stewart, one of Oatley’s research students, who had built an SEM that was upside-down by today’s standards (Figs. 1 and 2). The gun was at the bottom, and the specimen chamber at the top, the latter surrounded by a space into which one could slosh liquid nitrogen, a brilliant vacuum pump. This permitted the 165 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Figure 1. Diagram of the layout of Garry Stewart’s SEM at Scroope House, Engineering Department, Cambridge.

operation of an argon ion source with which one could hammer the sample even while it was being imaged by back-scattered electrons in the SEM (Stewart, 1962; see Chapter 2.7B; Boyde and Stewart 1962a,b). He was encouraging from the first meeting. Did I have a specimen in my pocket? Of course. It happened to be a slice of a kangaroo tooth, of great interest to me, if only to a very few others before or since, because, as a marsupial (and other than a wombat), it has tubules in its dental enamel. Tubules are spaces

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Figure 2. Coordinate system at the specimen in the Stewart SEM. Considerable attention at that time was paid to the correct interpretation of orientation of the sample in 3D space.

left by bits of the formative cells trapped in the tissue when it is forming (Boyde and Lester, 1967). Enamel is, of course, a dead tissue and has no living cell component in the adult, functional condition. I was a raw recruit, and knew nothing of SEM sample preparation, but then, for that matter, neither did anyone else, since hardly anyone in biology was aware of the SEM. The first thing that we discovered, or uncovered, was that dental enamel underwent plastic deformation: we showed the formation of a Beilby or smeared layer for the first time using SEM. Another less optimistic way of describing this is to declare that we had found that polished samples, as such, were useless (Figs. 3 and 4). Luckily, Garry’s ion beam etching device rescued us. We could erode the deformed layer away and develop an etched surface that told us something about the tissue structure (Figs. 5–10). We pointed out that the surface topography was determined by that which had been removed, not by what

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Figure 3. BSE scanning electron micrographs of a transverse section of kangaroo cheektooth, before and after argon ion etching. Stereo-pair of polished enamel surface with a scratch mark placed for orientation purposes while studying ion beam erosion phenomena in the SEM. This image is prior to ‘etching.’ No recognizable structure is present, owing to the formation of superficial ‘smeared’ layers (Field width 33 mm).

Figure 4. BSE scanning electron micrographs of a transverse section of kangaroo cheektooth, before and after argon ion etching. Stereo-pair of polished enamel surface with adherent nickel dust to prevent ion beam erosion of the underlying surface in the SEM (Field width 250 mm).

was still there. Never mind, people still etch away and happily proclaim that they are looking at structure. Revisiting these back-scattered electron images after 43 years has shown me that they are as perfect as any produced since, and that we should have paid more attention to the gradient of the ion beam etching process. The beam had a width of about 500 mm, but with a wide

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Figure 5. BSE scanning electron micrographs of a transverse section of kangaroo cheektooth, before and after argon ion etching. Stereo-pair of polished enamel surface with adherent nickel dust to prevent ion beam erosion of the underlying surface in the SEM. This image shows light etching in the lower right side of the field of view, with relief due to the underlying ‘prismatic’ structure in which orientation of 100 nm diameter, extremely elongated, hydroxyapatite crystals varies across 5 mm diameter bundles, the ‘prisms’ (Field width 250 mm).

Figure 6. BSE scanning electron micrographs of a transverse section of kangaroo cheektooth, before and after argon ion etching. Ion-etched dentine, with peritubular (more densely mineralized) regions standing proud of the etched surface (Field width 250 mm).

skirt, and a closer study of the less bombarded regions would have been most rewarding. I have selected some of these images for this note. I was completely hooked on SEM as the method for hard-tissue structure research: hard tissues are those that have very little water, and therefore do

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Figure 7. Dentine (below) has been eroded away more rapidly than enamel and intertubular dentine more than peritubular. These diVerences in ‘sputtering’ rate are related to diVerences in composition (degree of mineralization).

Figure 8. Argon ion-etched surface of transverse section of enamel. The prismatic structure can be visualized because diVerential erosion has occurred related to diVerences in ‘sputtering’ rate of diVerent crystal faces of hydroxyapatite.

not suVer terribly from dehydration. Deep frustration came from the inaccessibility of this instrumentation, but gratification came from Garry’s disregard for academic protocol. He wrote his thesis but did not submit it, instead joining the Cambridge Instrument Company to help design the

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Figure 9. Same surface as in Fig. 8 at lower magnification. Only a limited degree of diVerential erosion has occurred in the true surface zone of the enamel. This implies that the composition and structure of this surface zone is uniform, contrasting with the diVerences in crystallite orientation in the underlying bulk enamel.

Stereoscan. I, as a friend, could later use the prototype—a case of both who you know and what you know. We used it to make the first SEM images of the surface of developing mammalian dental enamel (Boyde and Stewart, 1963; Stewart and Boyde, 1962). The influence of other CUED students had had its impact in the conception of the Stereoscan, and thus it came about that the Everhart–Thornley biased scintillator secondary electron (SE) detector was all that was oVered with the first Stereoscans. Only later would the primacy of fast-electron imaging be re-established. With SE imaging in SEM of biological nonconductors, one is, in truth, mainly examining the conductive surface coating that has been applied. So I thought to myself, let us remove this and examine it in a TEM as a replica until such time as we can purchase the real thing, and so we did for several years (Boyde, 1967a). As a mere lad, I applied to both the Wellcome Trust and the Medical Research Council (MRC) for funding for a Stereoscan, only to be informed that they would not oblige, because electron microscopes were well known and there were plenty about—they had simply failed to understand the diVerence between a TEM and an SEM. Relief came after a third application to the Science Research Council (SRC): they, unlike the others, employed more than two reviewers per grant application and probably kept well away from clinicians. In awarding me the grant, SRC stated that I had established

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Figure 10. Dentine (below) and Araldite adhesive (above). Peritubular dentine stands proud of the more eroded intertubular dentine. The spikes in the etched Araldite patch were initially formed as protrusions where the surface had been protected from erosion by nickel dust particles (Field width 78 mm).

beyond doubt that SEM was suited for hard tissues, but that we should now investigate the possibilities for soft tissues, which we did. Having a grant for such novel capital equipment made me a valuable property at the time, and my appointment to the Anatomy Department at University College London by J. Z. Young was precipitously accelerated as a consequence. In the waiting period prior to this first grant, I accepted an invitation to visit the Institute of Medical Physics at the University of Mu¨nster, Westfalen, Germany to use the Stereoscan purchased by Gerhard PfeVerkorn and Ludwig Reimer, which opened up another set of important contacts for my future career, and got me on the SEM road before my own instrument was delivered. Again I could work on enamel development (Boyde, 1967b). While Gary was still a PhD student, I had met his successor, Alec Broers. Later, I was able to use Alec’s SEM with much improved resolution for work on dental enamel (Boyde and Broers, 1971).

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Acknowledgements My special thanks are due to Ken Smith and Dennis McMullan for persisting in giving me the opportunity of stating how immensely and intensely grateful I am to the CUED for both the fun and the opportunities that were opened to me in the SEM domain. I would also like to thank the SRC, and later the MRC, the Wellcome Trust and the Horserace Betting Levy Board for financial support. Thanks are also due to many long-term colleagues at UCL who joined in the fun, notably Sheila J. Jones, Peter G. Howell, Roy Radcliffe and Maureen Arora.

References Boyde, A. (1967a). A single-stage carbon replica method and some related techniques for the analysis of the electron microscopic image. J. R. Microsc. Soc. 86, 359–370. Boyde, A. (1967b). Direct visualization of the site of development of enamel prism sheaths. Naturwissenschaften 54, 252. Boyde, A., and Broers, A. N. (1971). High resolution surface scanning electron microscopy of mineralised tissues. J. Microsc. 94, 253–257. Boyde, A., and Lester, K. S. (1967). The structure and development of marsupial enamel tubules. Z. Zellforsch. 82, 558–576. Boyde, A., and Stewart, A. D. G. (1962a). Investigations of the erosion of tooth sections with an argon ion beam. Philadelphia, 1962, Paper QQ9. Boyde, A., and Stewart, A. D. G. (1962b). A study of the etching of dental tissues with argon ion beams. J. Ultrastruct. Res. 7, 159–172. Boyde, A., and Stewart, A. D. G. (1963). Scanning electron microscopy of the surface of developing mammalian dental enamel. Nature 198, 1102–1103. Boyde, A., and Switsur, V. R. (1962). Problems associated with the preparation of biological specimens for microanalysis. Stanford, 1962, pp. 499–506. Boyde, A., Switsur, V. R., and Fearnhead, R. W. (1961). Application of the scanning electronprobe X-ray microanalyser to dental tissues. J. Ultrastruct. Res. 5, 201–207. Boyde, A., Switsur, V. R., and Stewart, A. D. G. (1962). An assessment of two new physical methods applied to the study of dental tissues. ‘Proceeding of the 9th ORCA Congress.’ Oxford: Pergamon Press, pp. 185–193. Stewart, A. D. G. (1962). Investigation of the topography of ion etched surfaces with a scanning electron microscope. Philadelphia, 1962, Paper D12. Stewart, A. D. G., and Boyde, A. (1962). Ion etching of dental tissues in a scanning electron microscope. Nature 196, 81–82. Switsur, V. R., and Boyde, A. (1962). A consideration of some design features of a scanning microanalyser for biological applications. Stanford, 1962, pp. 495–497.

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2.7B* Investigation of the Topography of Ion Bombarded Surfaces with a Scanning Electron Microscopey A. D. G. STEWART Engineering Department, Cambridge University

The microscope (Fig. 1), which was modified from an instrument originally built by Prof. C. W. Oatley, had three electrostatic lenses and was prealigned except for the gun anode and the final aperture. The final lens had a working distance of 1 cm and its focal length could be modulated in synchronism with the scans, so that the whole of a sloping surface could be kept in focus; the focus modulation amplitude control was ganged with the magnification control. A preset rotation was used with a total  movement of 8.4 , because: 1) It ensured that all the stereo-pairs in a sequence were taken with the same perspective. 2) It was necessary to be able to relocate previously observed areas solely by resetting the specimen traverse controls; since a small rotation of the specimen control caused a large movement of the image on the display screen, when observing those parts of large inclined specimens which were well oV the axis of rotation. The specimen could also be moved in three orthogonal directions and rotated about the axis of view. The ion beam direction was exactly parallel to the direction marked on the single micrograph, and was as shown in the stero-pair. The average foreshortening due to the tilt of the specimen can be estimated from the magnification symbol, which shows the scale in the directions of maximum and minimum magnification. There were three independent pumping systems, which could be isolated from each other; cold traps were fitted on the pumps associated with the ion lenses and the specimen chamber. There was also a cold baZe in the specimen chamber where the pressure was 1–2.10 6 Torr; this pressure did not increase when the ion source was turned on. No trouble was experienced from contamination. The ion source was run at 5 kV. When the ion beam was focussed, the ion current density distribution (Aþ) had a peak greater than 20 ma/cm2 and half the peak value at radius of 150 m. Defocussing the beam gave lower current densities and a smoother distribution. The total ion current was 10–100 mA. The diameter of the bombarded area was usually about 2 mm. For most experimental runs it was found preferable to bombard for short periods and then view the surface at leisure. To observe the surface while it was under bombardment, only the high energy reflected electrons from the scanning probe *Reprinted from: Proceedings of the 5th International Congress for Electron Microscopy, Philadelphia, PA, edited by S. S. Breeze. Academic Press, New York, 1962. y

Presented by the author, August 1962.

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Fig. 1. Schematic diagram of the microscope and ion source.

(12–16 kV) were used to form the picture. The reflected electrons were detected using an unbiased scintillator, and the low energy particles due to the ion beam were filtered out with a layer of aluminium on the face of the scintillator. It was usual to put small shielding particles on the surface before each run; these preserved part of the original surface and allowed measurements to be made of the thickness which had been removed from the neighbourhood of each particle. The mechanism of the formation of spikes on a surface under ion bombardment has been investigated in detail. These spikes were caused either by inclusions with a lower sputtering coeYcient than the surrounding material, or by particles resting on the surface. When the shielding particle was eroded away, the sides of the spikes

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Fig. 2. Zone refined, electropolished polycrystalline aluminium. Etched with 5 kV, argon ions. Total bombardment time ¼ 2280 seconds.

Fig. 3. Spike on the surface of (110) tungsten crystal after bombardment with 4.9 kV argon ions. Original surface was electropolished. Some nickel particles were dusted on the surface before bombardment. The slight hollows in the surface are the remains of spikes which have otherwise disappeared. Tip of the spike marked B corresponds to the original surface level. Total bombardment time ¼ 1700 seconds.

˚ . With continuing became smooth, and the tip radius usually became less than 1000 A bombardment the spikes decreased in size and eventually disappeared. A spike may be much larger than the inclusion (or particle) which caused it to be formed; the ratio of the sizes being approximately equal to the ratio of the sputtering coeYcients. It is

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therefore possible to determine the position and depth of certain types of inclusion, even when the method of observation does not allow these inclusions to be observed directly. Fig. 3 shows spikes at various stages of development. Fig. 2 shows an eVect obtained by bombarding an electro-polished, polycrystalline, aluminium specimen. ˚ . This was On grain A there was a ridge system with a mean ridge spacing of 1600 A the result of an early experiment and it is possible that oxide formation may have influenced the etching of the grain boundary. This work was supported by a contract from A.E.R.E. Harwell.

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.8 The Scanning Electron Microscopy of Hot and Electron-Emitting Specimens H. AHMED Cavendish Laboratory, University of Cambridge Formerly at: Engineering Department, University of Cambridge

I. Introduction I came to Cambridge in 1959 with a studentship awarded by King’s College, to do a PhD degree at the Department of Engineering. My intention was to continue the research on transistor modelling I had started as an undergraduate at Imperial College and had continued, on semiconductor rectifiers, at the GEC Hirst Research Centre. The then Mr Oatley, who interviewed me, explained that these topics were not available and that the subject suggested for my research was the study of electron emitters from which high current densities could be obtained at relatively low operating temperatures. At the time, there was a great deal of interest in these emitters because of their potential applications in high-power microwave tubes. After some initial studies of these cathodes under the supervision of Mr A. H. W. Beck (later Professor Beck), it was suggested by Professor Oatley, then the Head of the Electrical Engineering Division, that I might examine the surface of the emitter in the scanning electron microscope to try to deduce the relationship between the surface conditions and the work function of the emitting surface. At the time of this suggestion, I was contemplating the abandonment of research on cathodes and a return to the Hirst Research Centre to work on semiconductor devices, but the introduction of the SEM to my research changed my views entirely and there followed a lifelong research interest in the application of electron beams to the microfabrication of semiconductor devices and a 25-year-long association with Professor Oatley. At this point also, I came under the informal supervision of Oatley who introduced me to Richard Thornley, then concluding his research, and I started to use the scanning electron microscope (SEM2) he had been working on. The scanning electron microscope that I inherited would hardly be recognized as such by a user of a current instrument. 179 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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II. The SEM2 Microscope The machine was the second built by Oatley’s students, Oliver Wells and Richard Thornley (see Chapters 2.3 and 2.6), and its electron optics were based on two electrostatic lenses and a tungsten-hairpin thermionic emitter. The other strikingly diVerent feature of the SEM compared with present-day machines was that its column was hung from a frame so that the gun was at floor level, with the specimen chamber at a convenient working height. It may be of some interest to describe in detail a few of the unusual features of this machine and my experience of operating it. The electrostatic lenses were three-electrode einzel lenses, constructed with insulating pillars separating the electrodes, the adjustment of the lens strengths being carried out by means of a potential divider. At the time the microscope was built, for operation at 30 kV, no high-voltage variable potential dividers were available that could withstand the voltage levels while remaining relatively noisefree. Consequently, a somewhat unusual device was installed in the SEM as the potential divider. It consisted of a spherical ball at high potential, mounted well above head height; a wooden dowel passed through the ball to act as the resistive divider. To adjust the lens voltage, the lower end of the wooden dowel had to be gripped and the dowel moved carefully up or down to change its eVective length. This operation did not always make one feel entirely secure and on the very first occasion that I was left to adjust the dowel by myself there were sparks at the high voltage end. Filled with apprehension and not daring to touch the dowel, I approached Professor Oatley, rather nervously suggesting that the adjustment might not be safe. He came along, took one look at the discharge and said, ‘Oh, this is not a very serious matter. The resistance has gone rather high. One needs to have a small current through this system to make it work stably. I suggest you dismantle the dowel and put it in a bucket of water to try and reduce its resistance—it is just a rather dry day.’ This remedy worked a treat and the dowel operated perfectly, although I never quite lost my apprehension when adjusting the voltage. This was just one example of Oatley’s approach to research—pragmatic and direct. The other classic piece of design in this machine concerned the adjustment of the electron gun. The emitter was a tungsten-hairpin filament and we made the hairpins ourselves by taking tungsten wire, first stretching it hard so that it was strained and did not coil when released. Pieces of the right length were formed into hairpins, making the sharp bend by forcing the tungsten wire into the surface of a rubber bung using a razor blade. Once the filament was fitted to the gun, careful adjustments were needed to the highvoltage section using three long rods of insulating material. The whole of the

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gun was pushed by these rods over well-greased O-rings and the gun was eventually aligned by trial and error. Although this was a crude system by modern standards, it was extremely eVective. The final lens of this SEM was beautifully machined and very carefully constructed in all its aspects. The problem for me was that it was easily contaminated by evaporation of materials during the experiments I was carrying out, and needed regular cleaning. Unfortunately, this was a very long procedure as the whole lens had to be taken apart and carefully cleaned and put together. The process took a whole working day and there was always the danger that some tiny piece of insulating material had been left behind in the lens, which in consequence would continue to behave astigmatically; then the whole process would have to be repeated. The number of times this lens had to be cleaned and put together to take a series of micrographs would drive a saint to distraction. On the other hand, when the SEM worked eVectively it provided results that were unique and made all the eVort worthwhile. Users of present-day scanning electron microscopes will find it hard to believe the time and trouble taken in aligning and adjusting the SEM of the 1960s before even a single micrograph could be taken. The satisfaction was that every micrograph was unique as there were no other instruments anywhere else in the world except Smith’s SEM3 in Canada. A. Modifications to SEM2 The examination of cathodes under their normal operating conditions required changes to the detector system and to the specimen chamber of the SEM. A redesign was carried out and a chamber with better vacuum performance was constructed. It was decided to put in a number of ancillary ports to enable in-situ experiments to be carried out; perhaps the first example of using scanning electron microscopy for imaging a sequence of complex and dynamic changes in a surface structure. These ports had facilities for pyrometers to be focused on to the hot specimen so that its temperature could be measured, and electrical connections so that not only could the emitter be heated but the emission current could also measured. Ports were added also for the installation of a novel electron-detection system. The inverted form of the instrument was particularly suited to this work, as it facilitated the incorporation of measuring equipment from the top. This provided a great deal of flexibility in carrying out unusual experiments, and several versions of chamber lids were made. The most demanding part of the work was to design and construct a new electron-collection system for the scanning electron microscope to image the high-current-density cathodes; even though their work functions were low,

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they nevertheless operated with surface temperatures well over 1000 C, with heaters at even higher temperatures. The conventional Everhart–Thornley system was unsuitable because the plastic scintillator and Perspex light-pipe could not withstand the heat from the specimen and also allowed light from the specimen to enter the photomultiplier. The new system needed to withstand the high temperature of the specimen and also to separate out and eliminate the electron emission from the cathode while collecting the secondary electrons that were needed to form the picture of the surface. The light from the specimen and its heater was also a problem and it was essential to prevent light from reaching the photomultiplier that formed part of the detector system. To meet the temperature requirements, the conventional plastic scintillator was replaced with a lithium-doped glass scintillator, and the Perspex light-pipe was replaced with a glass rod. The whole glass system was coated to reflect the stray light from the specimen. In addition, optical filters with a passband in the ultraviolet allowed the light produced by scintillation to reach the photomultiplier while cutting out the longer-wavelength light from the specimen. The electron emission from the specimen was prevented from reaching the scintillator by using a negatively-biased grid in front of the emitter. The negative bias voltage was suYcient to block the electron emission but not large enough to prevent the secondary electrons from reaching the scintillator.

B. Results The detector worked after some development, and it was possible to perform complex in-situ experiments on cathodes in the scanning electron microscope (Ahmed, 1962). The evolving surface of cathodes as they were activated was seen for the first time, and it was possible to speculate that the low work function was created by the formation of crystallites and low-workfunction chemical compounds on the surface. Figure 1 shows the surface of a cathode with chemical contrast clearly delineated by the change in contrast caused by the diVerent secondaryelectron coeYcients of the areas which had been changed by chemical processes. It was believed that the low-work-function areas were found over much of the cathode surface. In a diVerent cathode system, a large increase in microcrystallites over the surface became evident, as shown in Fig. 2. These crystallites eVectively increased the low-work-function area of the cathode surface. Finally, Fig. 3 shows the surface of a cathode at high temperature and emitting electrons. This picture is made largely by the higher-energy and back-scattered electrons and lacks the contrast produced

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Figure 1. The surface of an impregnated L-cathode showing chemical contrast (From Ahmed, 1962).

Figure 2. The surface of a bariated-nickel cathode after activation showing the formation of crystallites (From Ahmed, 1962).

by low-energy secondaries. These experiments were early examples of the dynamic surface evolution experiments that are now carried out easily in SEMs with special environmental chambers. Throughout the work Oatley was not only very supportive as a supervisor but made many useful comments and suggestions when diYculties arose. He was committed to establishing the success of the SEM not only in terms of the improved resolution over the optical microscope and the novel contrast mechanisms that the SEM oVered but also in terms of the applications of the instrument. During the period of my research the microscope was also used to take pictures of surfaces for other research projects and many useful results were obtained (Ahmed and Beck, 1963; Beck and Ahmed, 1963). In

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Figure 3. The surface of a cathode imaged while operating at high temperature and emitting electrons (Field width 23.5 mm). (From Ahmed, 1962.)

particular, the first pictures of spark-machined surfaces were produced for the Hughes Company in the United States. The results were much appreciated and enabled a better understanding of the spark-machining process, then in its infancy; I believe these micrographs eventually led to the sale to the company of a commercial microscope when these became available a few years later.

III. Professor Sir Charles Oatley In conclusion, I might add a personal note. As I have already mentioned, I met Charles Oatley on my first day in Cambridge in 1959, when I came for my studentship interview. He accepted me into his laboratory and later supported my appointment to a university post, and I worked in close proximity to him for 25 years. His commitment to research and to the teaching of electronics was an inspiration to all of us who worked in the Engineering Department over three decades, including a period after his retirement. The enormous regard in which he was held by his former students was apparent when I organized events to mark his 80th and 90th birthdays; these were attended by almost all his former students, some of whom came back from distant parts to acknowledge their debt to him. On his 90th birthday (see Frontispiece) he spoke eloquently of his life in research and the experience of working on radar, which formed a fit background for his subsequent research in the SEM.

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References Ahmed, H. (1962). ‘Studies on high current density thermionic cathodes.’ PhD Dissertation, University of Cambridge. Ahmed, H., and Beck, A. H. W. (1963). Thermionic emission from dispenser cathodes. J. Appl. Phys. 34, 997–998. Beck, A. H. W., and Ahmed, H. (1963). Activation process in dispenser cathodes. J. Electron. Control 14, 623–627.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.9A Towards Higher-Resolution Scanning Electron Microscopy R. F. W. PEASE Stanford University, Stanford, California Formerly at: Engineering Department, University of Cambridge

I. Introduction My first view of scanning electron microscopy was during my final year (1960) as an undergraduate while rushing between classes in the Department of Engineering at Cambridge. Between the main buildings of the department stood Scroope House a dilapidated Victorian structure (I have never found out who Scroope was). In the window of Scroope House were visible some posters containing micrographs that were unlike anything I had seen before. They looked so ‘real’; with tremendous depth of focus showing, for example, crystallite needles decomposing from one end, and the inside of spinneret apertures. The title included the words ‘electron microscopy’ but these looked nothing like the electron micrographs I had seen elsewhere—lattice images showing near-atomic resolution. Indeed the magnification values of these ‘scanning electron’ micrographs, especially the more striking ones, were mostly not much more than 1000  or 2000 . Towards the end of that academic year, Mr (soon to be Professor) Oatley, the member of staV who seemed to have the most interest in our careers, invited us on a tour of the research laboratories and gave us a description of the projects therein. He described the scanning electron microscope and pointed out that its chief merit from his point of view was that it was a great educational vehicle, in that to master it the research student had to be familiar with a wide variety of disciplines ranging from electron optics to mechanical and vacuum engineering, to circuit design and noise and communication theory. Its use was rather limited because of its poor resolution, about 30 nm at best (i.e. less than an order of magnitude better than that of a good optical microscope), but it was useful for examining bulk samples whose surfaces were undergoing some process (e.g. crystallites decomposing). At the end of the year, Professor Oatley oVered me a position as a research student in his group to work on the scanning electron microscope and 187 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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I gladly accepted. It was one of the best decisions I have ever made, although at the time some of my mentors were critical of the choice on the grounds that the scanning electron microscope had been ‘around for a decade and wasn’t going anywhere’. A. Getting Started When I started my research studentship I found the challenges every bit as daunting as the naysayers had predicted. Like those before me, I wondered why the resolution of the SEM failed to achieve the 10 nm that the theory promised. The first problem was that good experimental data were hard to come by. The two instruments in Scroope House (SEM2 that I inherited from Richard Thornley, who had inherited it from Oliver Wells; and SEM4 that Garry Stewart had taken over from Les Peters and Professor Oatley), both relied (as had SEM1, which no longer existed) on electrostatic lenses and it was a real art to keep them clean to avoid arcing and be reasonably free of astigmatism. And Oatley had been right: one needed to learn an awful lot just to fix the myriad subsystems that were constantly demanding attention. Consequently, pushing resolution experimentally with these instruments was very frustrating. B. Resolution Measuring resolution was, and still is, surprisingly inaccurate. But, roughly speaking, those existing SEMs on a good day gave a sharp picture at 10,000 times with, occasionally, a still sharp one at 20,000 times. This translated to claimed resolution of about 50 nm and 30 nm, respectively, but 10 nm should 1 be possible. The conventional electron microscope (CEM) also failed to reach its predicted limit, but the gap between theory and results was closer than with the SEM. My real challenge was to come up with an acceptable PhD project. I could push resolution or I could find a ‘niche’ application. And of course I had conflicting advice: some (notably my supervisor W. C. Nixon) recommended that I abandon electrostatic lenses and build a magnetic lens SEM and at 1

‘Conventional’ refers to the optical system being analogous to a light microscope in that the illumination floods the sample and the emerging radiation is focused to an image. Since the CEM can also be used in reflection, the term ‘transmission’ electron microscope (TEM) can be misleading. The scanning electron microscope can also be used in transmission (STEM), in which case its resolution can be (and is) every bit as good as that of the CEM. It is the reflection aspect that leads to poorer resolution, not the scanning.

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least try to narrow the gap. But K. C. A. Smith and AEI had already done that with apparently no better results. Others advised that I didn’t have time to build a new SEM and, despite the diYculties, it would be better to use the existing SEM2 either to find out why it didn’t deliver the predicted resolution or find an interesting application. After six months I decided to build a new SEM; I had originally switched from Physics to Electrical Engineering because my first love was building things and it was still taking all my skill to keep the electrostatic lenses in good order, let alone do anything useful near their limit of performance. By that time I had read all the previous dissertations and Garry Stewart had become my chief mentor and I sensed he wanted me to build a new instrument. Ken Smith had just returned from Canada where he had installed his magnetic lens instrument (SEM3) at the Pulp and Paper Research Institute and made available to me the design documentation. I also visited the AEI engineers then installing the magnetic lens SEM at the PCS division of the Cavendish Laboratory and, partly through them, established contact with Tom Mulvey at the AEI laboratories at Aldermaston, who was very helpful. So I had two oYcial advisors and several unoYcial ones. From discussions with Ken and Tom it appeared that neither magnetic lens SEM had been built to optimize resolution. So, after six months of awful uncertainty (and fielding the eternal question ‘What are you going to do?’), this became my quest. At the time I was acutely embarrassed by the trouble I had had in coming up with a plan of what to do, and I really envied those who were given an idea to work on by their advisors and got straight into their research instead of spending months ‘doing nothing’. Later I found out that many others had an even worse time than I did trying to come up with a project, and some gave up. Much later, when I found myself directing the research of others, I realized that this initial time was far from ‘wasted’ and that devising projects that are both significant and do-able within a time limit is very important and tough, and having been exposed to the challenge early on is, even today, a great help.

II. A New SEM For High Resolution Resolution depends on beam diameter and beam penetration of the sample (Pease, 1965). The main uncertainty about the former appeared to be the axial astigmatism resulting from mechanical imperfections that gave rise to asymmetry in the focusing fields, and it was not clear that the stigmators provided complete correction (i.e. were orders higher than the first

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significant?). However, the CEM suVered from the same problem and if we could close the gap to that of the CEM we should still be able to demonstrate 10 nm beam diameter. The eVect of penetration was even more uncertain. Existing SEMs had been chosen to work at 15 kV, well below the voltage used for CEMs, to reduce penetration. But even at 15 kV the range of electrons in most materials greatly exceeded (by at least an order of magnitude) the observed resolution. So it was not clear how the penetration aVected resolution and I decided that this was something that needed more investigation. This was arranged by designing the lenses very conservatively so that they could operate up to 100 kV without water-cooling and above 250 kV with. Such high voltages would ensure that the beam diameter could be well below 10 nm so that in the event that the observed resolution failed to reach the promised 10 nm we could more confidently blame penetration. In the accompanying paper reproduced as Chapter 2.9B, Nixon and I outline the technical aspects of the design that led to reaching sub-10 nm beam diameters and an observed resolution of about 10 nm. One key question was the required tolerances of the lens pole-pieces to avoid astigmatism. This included not only careful selection of the iron and its machining, but also the heat treatment required prior to final honing to the required (we estimated) 1 mm roundness error. Another was acquiring suitably stable power supplies for the lenses and gun; drifts of less than 10 ppm/5 min were needed. A further problem was the spurious deflection of the beam, caused by ambient 50-Hz magnetic fields, which was magnified by the long focal length of the final lens (10–20 mm, compared with 1–2 mm for a CEM objective lens) and the lower voltage of the beam. I estimated that the replacement of electrostatic lenses with magnetic lenses would provide enough magnetic shielding for an ambient field of 1 milligauss pk-pk (10 7 T). I was able to complete the design, construction and demonstration of this microscope (SEM5) (Pease, 1963) with the above performance (sub-10 nm beam diameter) in just under two years thanks to a lot of excellent help and advice, and it is appropriate to point out those that were more significant. First, we had access to an excellent machine shop manned by extraordinarily skilled machinists; I have never since enjoyed such excellent service from a machine shop. I was able also to get access to the new Taylor Hobson ‘Talyrond’ for measuring roundness to 25 nm (30 years later this instrument was still the best tool for measuring roundness). Dr Nixon put me in touch with the management at Brandenburg who made available to me a prototype high-stability, high-voltage supply and a filament supply for the gun. Professor Oatley not only pointed out some flaws in my early lens designs but also kept me focused on the main task. For example, at one point, as a

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result of a conversation with a semiconductor circuit designer, I toyed with the idea of designing new scanning circuitry that replaced valves with transistors; Professor Oatley tartly pointed out that I would be judged by the resolution of SEM5 not by the trendiness of its scan circuitry. So I copied the scan circuits engineered for SEM2 by Richard Thornley. Indeed, I copied everything that would suYce; not only the scan circuitry but the video circuitry (Thornley), the gun (Smith), the electron collector (Thornley), stigmator control (various), scan coils (various), and the photomultiplier supply (Forrest).

A. Measurement of Beam Diameter The beam diameter was measured by recording a scanning transmission micrograph of a sample edge (at the regular sample position) staying within the linear range of the recording system and measuring the edge slope across two orthogonal edges with a scanning microdensitometer. This method reduced contamination of the edge and revealed any astigmatism and spurious beam deflection due to vibration or interference. Beam diameters of less than 10 nm were recorded at both 15 kV and at 30 kV. Observing 10 nm resolution in reflection was a much rarer event and depended critically on the sample. The best samples were those of an experimental bariated-nickel cathode structure then being researched by H. Ahmed. This may well have been because the barium particles provided a strong-scattering source on a weaker-scattering substrate. Even today the preferred sample to show oV good resolution is a suspension of gold particles on a carbonaceous background, and there are numerous modelling studies that show how the scattering aVects image quality. None the less, having a machine that could routinely give (most easily at 30 kV rather than 15 kV) sharp pictures at 30,000 times and occasionally a useful one at 90,000 times represented a significant improvement and my advisors and I were happy. So why had the earlier SEMs failed to approach in practice their theoretical resolution? When I wrote up my account I ascribed this gap to the chromatic aberration of the electrostatic lenses. Earlier workers had been concerned with chromatic aberration caused by power supply instability and had overcome this with the use of a potential divider (the famous stick) to slave the strength of the electrostatic lenses to the same power supply used for the accelerating potential; and they appeared to have neglected the eVect of the thermionic spread. Even without the Boersch eVect (whose existence was publicly debated), I estimated that the energy spread of the thermionically emitted electrons was about 1 eV and hence was suYcient to cause a

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serious disc of confusion. This explanation seemed plausible because we had all noted that in practice the optimum aperture diameter was always smaller than the theoretical optimum even when astigmatism had been corrected, and from my observation that 30-kV operation seemed necessary to get the best resolution. It turned out that my estimate was too high and the correct value was 0.75 eV (depending on how beam diameter is defined). This was still enough to cause trouble, but it was not until several years later when it became clear that the Boersch eVect was very real and the likely energy spread was in excess of 3 eV that the explanation was accepted. B. Applications As the machine approached completion, I revisited the problem of what to do with it. Dynamic processes, catalysis and oxidation, came to mind (Pease and Nixon, 1964, 1965). When it was complete and working, there was no shortage of people wanting to use it and, during my last year, we made another three and these continued to be used at least five years after I had left and commercial SEMs were available. I thought it was because with these instruments we could routinely get sharp pictures at 30,000 times (i.e. with a resolution more than an order of magnitude better than that of an optical microscope) that they were popular, and indeed that may have helped as we always want to look as closely as possible at our samples. But I suspect that a reliable SEM would have proved popular even if 10,000 times had been the highest useful magnification. A few years after I had left Cambridge, it was pointed out to me (by Tom Hayes of the Donner Laboratory at the University of California, Berkeley) why the SEM was so powerful and why other electron microscopes with similar resolution (e.g. the photoemission electron microscope) were never popular. It is because the SEM’s contrast mechanism (in secondary or backscattered electron mode) matches so well our macroscopic experience and because of the depth of focus. The pictures can be instantly interpreted by our brain and have far more (in-focus) picture content than do most high-magnification light micrographs. Both the optical microscope near its resolution limit and the CEM give essentially 2D images because of depth of focus of the former and the specimen geometry requirements of the latter; but the SEM image of, say, a fly’s eye, ‘looks like a textbook drawing’. Whoever named the Cambridge Instrument Company’s SEM the ‘Stereoscan’ had doubtless spotted this before it was pointed out to me, but I finally understood why those pictures in the Scroope House window had looked so appealing so many years before.

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References Pease, R. F. W. (1963). ‘High Resolution Scanning Microscopy.’ PhD Dissertation, Cambridge University. Pease, R. F. W. (1965). The determination of the area of emission of reflected electrons in a scanning electron microscope. J. Sci. Instrum. 42, 158–159. Pease, R. F. W., and Nixon, W. C. (1964). Microformation of filaments. Toronto, 1964, pp. 220–230. Pease, R. F. W., and Nixon, W. C. (1965). High resolution scanning electron microscopy. J. Sci. Instrum. 42, 81–85.

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2.9B* High Resolution Scanning Electron Microscopyy R. F. W. PEASE AND W. C. NIXON Engineering Laboratory, University of Cambridge

1. Introduction The principles and some applications of scanning electron microscopy have been described by Smith and Oatley (1955). Their microscope employed two electrostatic lenses to focus a fine electron probe on to one surface of a bulk specimen. The probe was scanned in a rectangular raster across the surface of the specimen and the electrons emerging from the specimen were collected and amplified in an electron multiplier. The signal so derived was used to control the brightness of the display cathode-ray tube whose spot was scanned in synchronism with the electron probe on the specimen. The magnification of such an instrument is given by the ratio of the scan amplitude on the display tube to that on the specimen and is varied by attenuating the current in the coils used to scan the electron probe. The resolution is largely dependent on the probe diameter d. Focusing was carried out by directly viewing the image on the long-persistence screen of the display cathode-ray tube, using a scan frequency of 1 frame per second. For photographic recording a single frame scan lasting a few minutes was generally used. This practice has been adopted in later scanning microscopes and also in most scanning x-ray microanalysers. The micrographs obtained in general represent the surface topography of the specimen although variations in secondary emission coeYcient and reflection coeYcient also give rise to contrast. From these micrographs, published in 1955, the minimum ˚ , and in a few cases was as electron probe size d appears to have been about 500 A ˚ . Between 1955 and 1961 three more scanning electron microscopes were low as 300 A built in this laboratory but in spite of many design improvements, notably in the electron collection system of Everhart and Thornley (1960), there was no reduction in the observed minimum electron spot size. After reviewing the theory of operation and the limitations of previous instruments a new scanning electron microscope employing three magnetic electron lenses was designed and constructed. In operation this instrument demonstrates a resolution of ˚ and a minimum probe size of 50 A ˚. 100 A

*Reprinted from: J. Sci. Instrum. 42, 81–85 (1965). y

MS received 25 August 1964.

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2. Theory of operation A concise derivation of the formula relating the probe diameter d, the probe current i and the semi-angular aperture a at the point of focus has been given by Smith (1956 Ph.D. Dissertation, University of Cambridge). In this derivation the starting-point was the Langmuir formula for the brightness of the image of an electron gun source:   eV JA ¼ JC þ 1 sin2 a kT where JA (a cm 2) is the peak axial intensity at the image, JC (a cm 2) the cathode emission current density, e (c) the electronic charge, V(v) the potential at the image  with respect to the cathode, k (j deg k 1) Boltzmann’s constant, and T( k) the cathode temperature. (a is the semi-angular aperture at the image. The semi-angular aperture at the cathode is assumed to be 12 p.) It is then possible to determine the Gaussian diameter d0 of an electron probe carrying a given current i. The eVects of diVraction, spherical and chromatic aberration and astigmatism were taken into account by adding in quadrature the relevant disks of confusion to d02. The electron probe diameter d defined as the distance between opposing points where the current density is one-fifth of the maximum value, is then given by the following expression. d2 ¼ a4opt ¼

P þ Ca6 þ Qa2 a2

ðQ2 þ 12CPÞ1=2 6C

ð1Þ Q

ð2Þ

where P¼ B ¼ 0
62



i þ ð1
22lÞ2 B



p eV V JC ¼ 5
65 JC  103 4 kT T

and l is the electron wavelength due to the accelerating voltage and is given by ˚ where V is in volts. l ¼ 12
4=V 1=2 A  2 1 Cs C¼ 2 and Cs is the spherical aberration constant,   dV 2 þ ZA 2 Q ¼ Cc V and Cc is the chromatic aberration constant; ZA is the distance between two line foci when the lens is astigmatic.

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The theoretical limits to probe current and probe diameter in the scanning microscope were then found by considering only spherical aberration and diVraction, which lead to:  3=8 iT dmin ¼ 1
29Cs1=4 l3=4 7
92  109 þ 1 ð3Þ JC imax

JC ¼ 1
26 T

0
51d 8=3

aopt ¼

2=3

Cs l2



d Cs

1=3

!

1 10

10

ð4Þ ð5Þ

In the limit, when i ¼ 0 and there is no current in the electron probe, equation (3) reduces to dmin ¼ 1
29Cs1=4 l3=4 ði¼0Þ

ð6Þ

which can be recognized as being similar to the formula for the limit of the resolving power of a conventional transmission electron microscope. Therefore, using this definition of dmin, the resolving power of a scanning electron microscope with zero current is then the same as a conventional microscope with an objective lens where the spherical aberration constant is also Cs. Owing to the particulate nature of the incident and secondary electron beams it is necessary that i be appreciably greater than zero to form a useful image in a finite time. This is one reason why dmin is greater than the resolution of the conventional transmission electron microscope. Smith and Oatley (1955) considered the eVect of shot noise of the incident beam, from which it is possible to derive the formula  1=2 eP Ct ¼ k ð7Þ iT where Ct is the threshold contrast of the image, k is a constant for which a value of 5 is generally used (Rose 1948), T is the recording time and P is the number of picture points. Equation (7) shows that for a 400-line picture (P ¼ 16104) to be recorded in 5 minutes, the required current i ¼ 9  10 13 a, Ct ¼ 0
05. Since 1 pa ¼ 6
24  106 electrons per second and the recording rate is 533 picture points per second, there are approximately 104 electrons per picture point.

3. Performance limitations With the operating values of Smith and Oatley substituted in equation (3) a ˚ might have been expected. (V ¼ 15 kv, i ¼ 10 12 a, T ¼ resolution of about 50 A  2 ˚ .) However, the smallest d observed 3000 k, Jc ¼ 10 a cm , Cs ¼ 20 cm, dmin ¼ 70 A with their microscope and the three succeeding instruments built in this laboratory

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˚ and the finest resolution 350 A ˚ (Smith, 1956 Ph.D. Dissertation, was about 300 A University of Cambridge). In practice trouble had been caused both by stray a.c. magnetic fields moving the electron probe and by mechanical vibration of the specimen. These drawbacks could be avoided by careful design. Chromatic aberration due to the thermal velocity spread within the electron beam appears to have been neglected in earlier instruments. Zworykin et al. (1945) show a curve of the Maxwellian distribution of velocities of electrons emitted from a cathode  at 2900 k which suggests that the energy spread DV for 99% of the emitted electrons was 1
2 v. Thus for V ¼ 15 kv, DV =V ¼ 8  10 5 which, for the electrostatic final lenses used in the earlier scanning electron microscopes, would give a disk of confu˚ and hence may well have been a significant factor in limiting their sion of about 150 A performance. Such an eVect would be much less evident with a magnetic final lens exhibiting a lower Cc but comparable focal length. Magnetic lenses were used throughout for the scanning electron microscope reported in this paper, because: (i) they give lower spherical and chromatic aberration constants in general compared with electrostatic lenses; (ii) they aVord greater ease of operation and reliability since the magnetic focusing field is less sensitive to the surface properties of the field forming materials; (iii) suYciently stable e.h.t. supplies were becoming commercially available.

4. Design of the new instrument The electrons to be collected from the specimen are low-energy secondaries and for eYcient collection the specimen must be outside the magnetic field of the lens, which means that a relatively long focal length (1 cm instead of 2–3 mm), and hence relatively large abberations, are unavoidable. Another factor that leads to poorer resolution in the scanning electron microscope is that the beam voltage is approximately 15 kv compared with 80–100 kv in the transmission microscope. Apart from the greater chromatic aberration mentioned in the previous section, the diVraction limit is also higher. In addition the lower energy beam is more sensitive to stray a.c. magnetic fields and to any dirt or contamination films in the microscope column. A sectional view of the microscope column is shown in figure 1. Three magnetic lenses are used to de-magnify the electron source and hence focus an electron probe on to the specimen surface. The traversable elements are gun anode, first (spray) aperture, lens 1, lens 2 and the final aperture. The final lens was designed from Liebmann’s (1955a) data with radius of lower bore R1 ¼ 18 mm, radius of upper bore R2 ¼ 2 mm, gap S ¼ 3 mm, working distance from inside of final pole piece to specimen l ¼ 5 mm, spherical aberration constant Cs ¼ 10 mm and chromatic aberration constant Cc ¼ 8 mm. These values were checked experimentally; Cc had the design value but Cs ¼ 20 mm (5 mm) which, although it is twice the design figure, makes negligible diVerence to

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Figure 1. Cross section of the three magnetic lens scanning electron microscope column.

the minimum probe size. Substitution of these values into equation (1) and neglect of ˚ for i ¼ 10 12 a and V ¼ 15 kv. astigmatism (Za ¼ 0) gives dmin ¼ 52 A In order to reduce astigmatism to negligible proportions, great care was taken with the making of the final lens. The corrugation of the pole faces was less than 2
5 mm and the alignment of the bores better than 15 mm. In particular, the bores of the final

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lens had a systematic ellipticity (rmax rmin) of less than 0
5 mm, measured on a Taylor Hobson ‘Talyrond’. As an additional precaution an 8-pole electrostatic stigmator was incorporated but it was found to be unnecessary when the column had been carefully cleaned. From Liebmann’s (1955b) data the magnetic field at the specimen for a lens of the above geometry and for V ¼ 15 kv is less than 3 oersteds, which should allow eYcient collection of secondary electrons with energies of a few electron volts. In practice it was found possible to achieve eYcient collection up to V ¼ 30 kv with the collector biased to þ350 v. Mechanical vibration was kept to a minimum by the extensive use of antivibration mountings and careful design of the specimen stage. The eVects of a.c. magnetic fields were minimized with mumetal shielding around the more vulnerable regions; even so it was found necessary to use a d.c. supply for the diVusion pump heater situated 3 ft from the final lens. A commercial e.h.t. generator with a stability of 5 parts in 105 over five minutes was used for the electron accelerating voltage and a transistor lens current supply was developed to give 3 a into a 2 ohm load with a measured stability of better than 1 part in 105 over 10 minutes.

5. Performance of the new instrument The probe diameter d was measured by recording a picture of a contrasty specimen such as a fine grid, and measuring the sharpness of the edges with a microdensitometer. It was essential that throughout the video channel and photographic recording no ‘clipping’ took place, as otherwise too low a value for d would result. A ˚ is shown in figure 2(a) test grid viewed in transmission at 15 kv and with i ¼ 4  10 13 A ˚ . Contamination at 130 000  from which it was estimated that dmin ¼ 75  15 A 13 ˚ A, is shown in figure 2(b), 90 000 , growth viewed in reflection at 30 kv, i ¼ 8  10 ˚ . Although these are the best results from which it was estimated that dmin ¼ 50  15 A ˚ are fairly regularly obtained. that have yet been achieved, values of d < 100 A Results of point-to-point measurement of resolution have not been as satisfactory as the above values of d would suggest. A micrograph of the surface of oxidized iron, ˚ is shown in figure 3, 90 000 . The main V ¼ 15 kv, with a resolution about 150 A diYculty in demonstrating a high resolution is in finding a suYciently contrasty specimen. Specimens which should have shown strong ‘chemical’ contrast due to variations in secondary emission coeYcient d almost always gave disappointing micrographs as a result of a contaminating film of constant d which formed on the specimen surface. A barium carbonate particle on a bariated nickel cathode which demonstrates ˚ point to point resolution is shown in figure 4, 90 000 , taken at 30 kv. 100 A Comparison between these two micrographs in figures 3 and 4 is not strictly valid as they were taken at diVerent times and using diVerent specimens. However, it was observed in general that there was an improvement in picture quality on increasing the accelerating voltage from 15 to 30 kv. This is almost certainly due to the increased

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Figure 2. (a) A test grid in transmission at 15 kv, i ¼ 4  10 13 a, at 130 000 , giving an electron probe ˚ . (b) Contamination viewed in reflection at 30 kv, i ¼ 8  10 13 A ˚ at 90 000 . The estimated size of 75  15 A ˚ . The small black streaks near the centre of the photographs and identical in electron probe size is 50  15 A each are burn marks on the cathode-ray tube fluorescent screen and not part of the specimen image.

˚. Figure 3. The surface of oxidized iron at 15 kv, 90 000 , showing a resolution of about 150 A

probe current density which is possible with the higher voltage as shown in equation (1). In addition it appears that although penetration of the primary electron beam may cause a loss in picture quality, points separated by only 1/100 of the penetration depth of the primary electron beam are being resolved, since figures 3 and 4 are about 1 mm square, which is approximately the ‘range’ of 15 kv electrons for the elements present. The electrons emitted from the specimen, when struck by the primary beam, may be divided into three classes. Firstly, a number of low energy secondary electrons are

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Figure 4. A barium carbonate particle on a bariated nickel cathode at 30 kv and 90 000  showing point˚ (0.9 mm at this magnification). to-point resolution in reflection of 100 A

produced by the primary beam and the area of emission is probably only slightly larger than the cross-sectional area of the primary beam, owing to the low range, about 12 atomic layers, of secondary electrons (Bronshtein and Fraiman, 1961). Secondly, a proportion of primary electrons are backscattered with energies comparable with their primary energy. The area of emission of these electrons might be expected to be as large as 1 mm in diameter for V ¼ 15 kv and a copper specimen. Thirdly, low energy secondary electrons are produced by the backscattered electrons and these will emerge from the same area as the backscattered electrons. At low magnification, say 200 , each picture element of a 400-line picture with an image size 3 in. square is 1 mm square when referred to the specimen. Hence all emitted electrons emerge from inside the picture element and contribute usefully to the signal. At high magnification, say 20 000 , the size of each picture element is ˚ square and only the primary excited secondaries contribute usefully to the only 100 A signal. The backscattered electrons and the back-scattered excited secondaries emerge from outside the picture element and in general appear as spurious signal. The results of Kanter (1961) and Bronshtein and Fraiman (1962) indicate that for most metals the primary excited secondaries account for between one-third and onehalf of the total number of low energy secondaries. Everhart (1958 Ph.D. Dissertation, University of Cambridge) carried out an analysis of the electrons collected in a scanning electron microscope and found that only about 67% of the signal was contributed by the low energy secondaries emerging from the specimen. Thus, at best, only one-third of the signal collected at high magnification is useful signal. This means that to achieve the same picture quality as at low magnification it is necessary to increase the recording time by a factor of 9 (cf. equation (7)). Everhart (1958 Ph.D. Dissertation, University of Cambridge) suggested another cause of poor picture quality since noise is introduced in the secondary emission

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process due to variation in the number of secondary electrons emitted by each incident electron. Everhart gave a theoretical modification to equation (7) so that   ePið1 þ bÞ 1=2 Ct ¼ k ð8Þ iT where b is a constant associated with secondary emission noise and was given a tentative value of 15 at V ¼ 15 kv, varying linearly with voltage. The secondary current entering the collector was measured in this instrument with a Vibron electrometer as well as the mean and peak-to-peak signal resulting from the measured current and it was found that the noise was indeed greater than that predicted solely by shot noise considerations. However, the noise level only corresponded to a value of b ¼ 3 in equation (8). Moreover, b did not increase significantly as V was raised from 7
5 to 30 kv. An iron oxide surface is shown in figure 5(a) at 10 kv, 16 500 , and figure 5(b) at 30 kv and 33 000 . The voltage was lowered again to 15 kv for figure 5(c), 23 000 , and raised to 30 kv for figure 5(d), 33 000 , in order to eliminate the possibility of contamination confusing the changes seen when the voltage is raised. The picture quality is undoubtedly improved on increasing V; the higher voltage pictures appear both sharper and less noisy. The greater penetrating power of the high voltage beam is just evident by examination of the detail behind the ‘blades’ on the surface. One undesirable eVect of the greater penetration is the ‘flaring’ that occurs on ridges. This is due to the incident beam penetrating the ridge and causing the emission of secondaries on the exit side of the ridge (Smith and Oatley, 1955). This flaring can be seen to be more extensive in the higher voltage pictures although the increase is not as much as would be expected from the calculated penetration at these voltages and a slight loss of picture detail may be seen. 6. Conclusions A scanning electron microscope has been built which can give a probe diameter of ˚ which is probably near enough to the theoretical limit not to warrant further 50 A eVort along conventional lines. The best point-to-point resolution yet demonstrated ˚ . Reasons for this relatively poor resolution when working in is limited to 100 A reflection have been suggested and arise mainly from penetration of the primary beam causing most of the emitted electrons to come from an area much larger than that covered by the probe. However, it appears likely that in spite of the greater penetration a value of beam voltage V ¼ 30 kv is in general more suitable than the previously popular 15 kv owing to the increased probe current density which is obtainable at the higher voltage. It was found that contrast due to changes in secondary emission were quickly masked by contamination. As the specimen chamber of the scanning electron microscope is located at one end of the electron optical column it should be possible to have the specimen in a region of ultra-high vacuum, separated from the normal vacuum in the column by the 100 mm final aperture. The contamination rate would then be lower

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Figure 5. An iron oxide surface at various voltages, taken in sequence. (a) 10 kv, 16 500 ; (b) 30 kv, 33 000 ; (c) 15 kv, 23 000 ; (d) as for (b) but taken after (c).

and higher resolution may be possible with specimens that have not so far shown chemical contrast.

Acknowledgements The authors would like to thank Tube Investments Ltd., Hinxton Hall, for the use of some of their workshop facilities, and Taylor, Taylor and Hobson for the use of the ‘Talyrond’ for checking the roundness of lens bores. One of us (R.F.W.P.) was in receipt of a Department of Scientific and Industrial Research maintenance grant during the course of this work.

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References Bronshtein, I. M., and Fraiman, B. S., 1961, Soviet Physics-Solid State, 3, 995. —– 1962, Soviet Physics–Solid State, 3, 2337. Everhart, T. E., and Thornley, R. F. M., 1960, J. Sci. Instrum., 31, 246. Kanter, H., 1961, Phys. Rev., 121, 681. Liebmann, G., 1955a, Proc. Phys. Soc. B, 68, 682. —– 1955b, Proc. Phys. Soc. B, 68, 737. Rose, A., 1948, Advances in Electronics, Vol. 1 (New York: Academic Press), p. 131. Smith, K. C. A., 1960, Proc. European Regional Conf. on Electron Microscopy, Vol. 1 (Delft: Dutch Institute for Electron Microscopy), p. 177. Smith, K. C. A., and Oatley, C. W., 1955, Brit. J. Appl. Phys., 6, 391. Zworykin, V. K., Morton, G. A., Ramberg, E. G., Hillier, J., and Vance, A. W., 1945, Electron Optics and the Electron Microscope (New York: Wiley), p. 207.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

2.10 The Application of the Scanning Electron Microscope to Microfabrication and Nanofabrication A. N. BROERS Royal Academy of Engineering, London Formerly at: Engineering Department, University of Cambridge

I. Introduction I came to Cambridge from Australia in 1960 to explore the opportunities for research in radio astronomy, hopefully with Martin Ryle. I had completed an undergraduate degree in physics at Melbourne University and had followed it with an additional ‘final year’ in electronics, a new course introduced in 1958. My fascination with electronics had been triggered by my father’s interest in the subject and I had started a successful small business in Melbourne, building high-quality audio systems, but I was determined to learn more and to seek a career that involved the design and construction of state-of-the-art electronic systems. I was also interested in astronomy, so the combination that radio astronomy oVered seemed ideal. When I arrived in Cambridge, Martin Ryle was kind enough to meet me but told me that his group had recently finished building a new radio telescope and that work for the next few years would mainly be theoretical. He suggested that I explore possibilities in the Engineering Department. This I did but was told that I should complete a Part II before starting research. This was still the era when Cambridge was reluctant to recognize degrees from universities other than Oxford. In retrospect, though, I was pleased that I was required to become an undergraduate again as it gave me the opportunity to spend a year enjoying a broad spectrum of extracurricular activities, especially singing, sailing, skiing and tennis, in addition to my studies. Despite or perhaps because of these, I completed the Part II and started as Charles Oatley’s student in 1961, to work on electron microscopes rather than telescopes. My task was to take over the scanning electron microscope system that had been designed and brought into operation by Garry Stewart. Before the end of my first year, Oatley decided that he had too much to do as he was not only

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Head of the Electrical Division, but was also putting in place the new Electrical Sciences Tripos, so Bill Nixon became my supervisor. This was a crucial time for the SEM as Garry Stewart had moved to the Cambridge Instrument Company (CIC) and was leading the eVort to build the first commercial instrument. My first task, at his suggestion, was to take stereo images in his SEM. He had been studying the topography of ionetched surfaces and needed to know the height of the features he was observing. It is not surprising that the name Stereoscan was given to the instrument developed at the Cambridge Instrument Company (CIC), as Garry was the project’s chief engineer. Nor was it surprising, with all of the expertise that had been established in the Engineering Department, that the Stereoscan was an excellent instrument and a significant commercial success. I have always been pleased to be a part of this process as some of the images used to sell the first instruments had in fact been obtained by me on the SEM in Scroope House. My work on electron beam fabrication with the SEM began in 1962. The idea for the first process arose while I was trying to assess anomalies in my observations of fine structure on ion-bombarded surfaces. I was able to use this process to fabricate elementary metal structures that were smaller than could be made by other methods and this opened up the possibility that the newly emerging integrated electronic devices could be miniaturized beyond the capability of optical lithography. Although we did not use the prefix ‘nano’ until the late 1970s, these structures could legitimately have been referred to as nanostructures. On completing my PhD in 1965 I continued this work at the IBM Research Laboratory in New York, eventually succeeding in making operating devices with dimensions of only a few nanometres. By then electron beam systems were being used widely in the semiconductor industry for the making of masks for optical cameras, and for the development of future generations of devices. In 1984 I returned to Cambridge and continued research on the limits of electron beam fabrication using a 400-kV transmission electron microscope which I modified so that it could produce a beam with a diameter of 0.5 nm. On Charles Oatley’s intervention, I had become a Fellow of Trinity College and enjoyed his company there as well as in the department where we all drank coVee together every morning, as we had 20 years before when I was a student. There were lively discussions about the direction that research should take in the booming microelectronics industry, with which he remained in touch, and he instructed me on the peculiarities of academic society in Cambridge, providing me with insights which were to prove invaluable during my years as Vice-Chancellor that began in 1996. Sadly this was to be his last year, but I spoke with him about my new position at some length when I visited him at his Porson Road home, only a few weeks

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before he died. He remained vitally in touch with the world around him right up to the end, and made careful and detailed plans about what he wanted done with his possessions, with which he asked my help. II. Electron Beam Fabrication Studies at Cambridge In situ studies can be carried out easily in a SEM because the sample is placed at the end of the electron-optical column. This is in contrast to the transmission electron microscope, where the sample is placed between the pole-pieces of the objective lens and is not readily accessible. Charles Oatley realized this early in the development of the SEM and proposed an experiment in which ion-bombarded surfaces would be examined in a SEM without removing the sample from the microscope. The apparatus for this experiment, which was assembled and brought into operation in 1959 by Garry Stewart, consisted of a SEM and a system that focused an ion beam on to the sample. The SEM originally had three electrostatic lenses, and a beam size of about 30 nm. The ion beam was produced by an RF-excited ion source and brought to a focus on the sample with two electrostatic lenses. The beam, which could be positioned with a set of electrostatic deflection plates, had a diameter at the sample of about 1 mm and a current density of several mA/cm2. Argon ions of 5 keV energy were generally used as the purpose was to study physical sputtering without the added complication of chemical reactions. Garry Stewart carried out a large number of experiments with this apparatus and, in particular, his work led to an explanation of how cones form on ion-etched surfaces (Stewart, 1962; Stewart and Thompson, 1969). I took over the equipment in 1961 and was fortunate that he remained in Cambridge and was able to help me learn how to cope with the four, 6-foot-high, racks of electronics, the complex control console itself laden with electronics, and the SEM electron-optical column and its adjacent ion probe. After using the system for several months I decided to carry out a series of modifications. The first was to replace the electrostatic lens of the SEM with a magnetic lens to take advantage of the lower aberrations that were possible with magnetic lenses. The second was to redesign the vacuum system and fit new and larger pumps to shorten pumping times and improve the vacuum level in the sample chamber. The third was to fit a mass filter in the path of the ion beam to remove unwanted species from the beam. As Ken Smith discovered in the modifications he made to his SEM (Chapter 2.2A), this programme of modification proved more ambitious than I imagined and took me more than a year to complete. The design of the magnetic lens owed much to the earlier designs of Ken Smith and Fabian Pease, although this

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lens had to be considerably smaller than their lenses in order to fit into the space previously occupied by the electrostatic lens. The design proved successful, however, and the new lens reduced the beam diameter from about 30 nm to 10 nm, which was close to the theoretical value as predicted by Ken Smith’s formulae (Smith, 1956). The lens was also much easier to maintain than its electrostatic predecessor. The main purpose of the magnetic filter was to remove oxygen ions from the ion beam. These oxygen ions had led to pitting of metal surfaces undergoing etching, due to nonuniform formation of oxide, which had obscured the finer structures arising from the physical sputtering process. The problem had been particularly severe for aluminium and was eliminated with the filter. Because the magnetic circuit of the filter enclosed the polepieces, there were no fringe fields to interfere with the alignment of the electron beam in the SEM. The new vacuum system reduced the cycle time for specimen changing from 40 minutes to less than 10 minutes, and the pressure in the chamber was reduced by more than an order of magnitude to below 10 6 mm Hg. I used the apparatus in its new form (Fig. 1) to explore the fine structure of a variety of single-crystal metal surfaces undergoing ion etching (Broers, 1965). I was able to follow the surfaces through a succession of etching steps (see, for example, Fig. 2). It had been proposed that the spacing of the parallel ridges (Hayman, 1962), observed on etched single crystals, was a multiple of a characteristic distance that in turn was related to the range over which the momentum of bombarding ions could be transferred to remove surface atoms. The ridge spacings were in fact observed to increase progressively as etching continued, and no characteristic spacing was identified. I observed that it was the surfaces with lowest etch rates that emerged naturally as etching continued. In the course of these experiments I began to suspect that the SEM electron beam was ‘contaminating’ the surface and reducing the etching rate in the areas under examination. Ridges were sometimes observed that were aligned with the line-scan direction and propagated out from a position that corresponded to the start of the scan. The beam paused at this position to recover from ‘fly-back’, and the ridges were the result of the thicker contamination formed at this position on the sample shielding the surface from the ions. I carried out a simple experiment to exaggerate the eVect of the contamination. I left the beam scanning a series of single lines for several seconds to build up a relatively thick layer of contamination. I then etched the sample to a depth of approximately 50 nm, and re-examined it. As I had hoped, the contamination had selectively protected the surface and there were ridges where the beam had been scanning. The high definition of the ridges led me

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Figure 1. SEM - ion probe system used to explore the surface of ion-etched samples and to carry out the first electron beam microfabrication experiments.

to think that this technique might be useful for microfabrication, and I repeated the experiment on a sample, prepared by Les Peters, on which a thin layer of gold had been deposited. The contamination pattern used in this instance is shown in Fig. 3(a), and the sample after etching is shown in Fig. 3(b). The moment I turned on the SEM after etching the sample, and observed the bright 70-nm wide and 250-nm thick gold wires against the darker silicon surface, was surely one of the most exciting moments in my research career. The contamination process was capable of producing very high-resolution structures, but the writing rate was relatively slow, as a charge density of about 1 C/cm2 was required adequately to protect the gold layer. Bill Nixon pointed out that photoresist should be more sensitive, and he used his

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Figure 2. Aluminium sample at successive stages of ion etching.

contacts in industry to help me obtain some KPR (Kodak Photo-Resist), which was the standard photoresist in use at the time. After designing and building a spin-coating system and learning about the baking and development of the resist, I repeated the experiments using KPR and produced the 0.25–0.5 mm lines shown in Fig. 4. The photoresist required a dose of only about 10 4 C/cm2, which made it more practicable, especially when complex structures were needed, but the resolution was not as high as with the contamination, or vapour resist, process. These were the first experiments in which it was demonstrated that it was possible to fabricate useful structures with dimensions below a micrometre,

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Figure 3. (a) Contamination pattern written on the top of gold film. (b) Gold wires formed by etching away the unprotected gold film.

and showed that electron beams could potentially be used to reduce greatly the ultimate size of the components of the integrated circuits that had first appeared in the early 1960s. At the time it had been thought that dimensions would be limited to about 1 mm by the resolution of the optical lithography methods that were the only methods thought to be practicable. In fact it took almost 20 years before 1 mm dimensions were reached in commercial microcircuits (see Fig. 5, which shows the size of the devices reported at the International Solid State Circuits Conferences over the last 20 years [IEEE, 1982–2003]) because many other capabilities had to be improved, including device design, material deposition (epitaxial films, oxides, silicides, etc.) and the processes for transferring resist patterns into real structures (ion implantation, dry etching, metallization, etc.). Following the work at Cambridge, individual 1-mm transistors were soon made at IBM (Thornley and Hatzakis, 1967) using electron beam methods. This showed that it would be possible to scale transistors to dimensions more than ten times smaller than those used in the early microcircuits. It was at about this time that Gordon Moore proposed his law predicting accurately the rate at which this progress would be made. As is still the case today, the high cost and relatively low speed of electron beam methods precluded their use in manufacturing, but in the laboratory they were proving invaluable in establishing the validity of new designs, and their use as mask writers was soon to emerge. With optical cameras, the pattern for the devices does not have to be generated afresh every time, as it does with a scanning electron beam: it is replicated in a fraction of a second.

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Figure 4. Gold wires produced using KPR negative resist and ion etching.

Despite intense research and development eVorts, especially at IBM, the problems of the low throughput and high cost of electron beams in the direct exposure of wafers have never been solved. Optical methods have remained more economical and are thought now to be extendable at least to 0.1 mm dimensions. The only applications where electron beams are cheaper are those where the economic advantages of quick turnaround time oVset the high exposure cost. The most important of these is the fabrication of masks for optical cameras, but there have also been cases where the importance of

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Figure 5. Minimum dimensions used in devices reported at the International Solid State Circuits Conferences (IEEE, 1982–2003).

fast turnaround in the design of logic circuits has justified the use of direct electron beam exposure. Electron beams are uniquely valuable for making structures smaller than 0.1 mm for scientific exploration.

III. Continuation of the Work on Electron Beam Fabrication at IBM A. The Ultimate Resolution of Electron Resists My personal research at IBM concentrated on establishing the ultimate limits of electron beam fabrication processes and in demonstrating that these processes could be used to make useful operating devices. Following the

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development of the lanthanum hexaboride cathode (Broers, 1967), I built a new SEM with a beam size of 3 nm (Broers, 1969), and continued experiments using the polymethyl methacrylate (PMMA) resist developed by Haller et al. (1968). Several devices were made including a 3.5-GHz acoustic surface wave transducer with 0.15-mm wide metal fingers (Fig. 6; Lean and Broers, 1970), and superconducting microbridges with dimensions of 60 nm (Mayadas and Laibowitz, 1972). Devices up until this time had been fabricated on bulk substrates. In 1972, working with Tom Sedgwick, a new thin substrate was developed (Sedgwick and Broers, 1972) that allowed the deleterious eVects of back-scattered electrons to be eliminated. The substrate, which is shown diagrammatically in Fig. 7(a), is a 60-nm thick Si3N4 membrane over a hole in a silicon wafer (Molzen et al., 1978). The silicon was etched away with the newly developed anisotropic etching techniques. An optical micrograph of an actual window substrate is shown in Fig. 7(b). Another important advantage of the new substrate was that the sample could be examined with transmission electron microscopy at higher resolution than was available with the SEM. Figure 8 shows such a TEM micrograph of a niobium nanobridge in a four-terminal configuration that allows accurate determination of the current-versusvoltage characteristic of the nanostructure. Experiments with contamination resist and PMMA were repeated on the new substrate (1975–77) with a new 0.5-nm beam size scanning transmission electron microscope that I had built over the previous couple of years

Figure 6. 3.5-GHz surface wave transducer fabricated with PMMA resist and lift-oV.

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Figure 7. (a) Diagrammatic view of Si3N4 membrane substrate that allows nanostructures to be fabricated and examined without the deleterious eVects of back-scattered electrons. Contact pads extend from the membrane area to the bulk substrate where standard bonding procedures can be used to provide electrical contact to the samples. (b) Optical micrograph of actual membrane substrate with superconducting quantum interference devices.

(Broers, 1973). Eight-nanometre AuPd lines were produced with contamination resist (Broers et al., 1976), and 25-nm lines with PMMA (Broers et al., 1978a). This was a significant advance that we thought justified attaching the prefix ‘nano’ to lithography for the first time in order to distinguish these

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Figure 8. TEM micrograph of a niobium nanobridge in a four-terminal configuration.

techniques from those used in the fabrication of chips. The metal structures were considerably smaller than had previously been reported, but they were still much larger than the 0.5-nm diameter electron beam, showing that it was not the beam size that limited resolution, but electron beam interaction phenomena in the resist. To explore the resolution limit further, a series of experiments were carried out to measure accurately the resolution, or what I called the resist contrast function, for electron beam exposure of PMMA (Broers, 1980). The test pattern used in these experiments (Fig. 9(a) and (b)) is a TEM micrograph of a portion of such a test exposure after resist development and shadowing with a 3-nm layer of AuPd. The pattern contained lines that ranged in size from those much narrower than the previously observed minimum linewidths of 25 nm, to those considerably wider than 25 nm. Exposure doses ranged from those too small to produce any observable eVect, to those at which the resist developed through to the substrate in the location of the narrowest lines. The dose at which the centre of the largest shapes first developed through to the substrate was carefully measured as were the doses at which each of the narrower lines first developed through to the substrate. From these data the eVective exposure distribution in the resist could be calculated. The distribution turned out to be approximately Gaussian with a sigma of about 10 nm. The beam size and the pixel resolution of the pattern generator were both below 1 nm, allowing the distribution to be determined with a precision of better than 1 nm.

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Figure 9. (a) Test pattern used to determine the resolution contrast function for PMMA. (b) TEM micrograph of a portion of a developed and shadowed resist sample exposed with such a test pattern.

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Figure 10. Contrast versus linewidth for 50-kV electron beam exposure of thin PMMA on a thin substrate, and on a bulk substrate, compared with the modulation transfer function for an optical system with NA of 0.4 operating at a wavelength of 405 nm. Contrast is calculated for an infinite array of equal lines and spaces (Broers, 1980).

This distribution was then used together with data (range and fraction of exposure) for the exposure due to electrons back-scattered from the bulk silicon substrate (Grobman and Speth, 1978), to calculate the contrast versus linewidths for 50-kV electron beam exposure of PMMA on the bulk substrate shown in Fig. 10. The double Gaussian approximation for electron beam exposure, first proposed by Philip Chang (Chang, 1975) in connection with the Proximity EVect, was used in these calculations (Broers, 1980). As I discussed in that paper, the Resist Contrast Function is similar to the Modulation Transfer Function used to describe the performance of optical systems and allows electron beam lithography to be modelled accurately. The MTF for an optical system with a Numerical Aperture (N.A.) of 0.4, and operating at a wavelength of 405 nm, is also shown in Fig. 10 for comparison. B. Useful Nanostructures While many nanostructures have been fabricated with polymeric resists, it proved easier in several cases to use contamination resist and ion etching. The dose required to produce an adequate thickness of contamination resist (>1 C/cm2) is much higher than the doses delivered during microscopical examination, which allows the sample to be examined before and after resist

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formation without aVecting it. This makes it easy to position the structures accurately with respect to contact pads, or other device layers, and to examine the resist pattern after it has been formed. The build-up of resist can also be monitored by observing the decay in the transmitted signal. A variety of devices have been made with this method including microbridges (Laibowitz et al., 1979), SQUIDs (superconducting quantum interference devices: Voss et al., 1980), and fine wires for exploring localization eVects (Chaudhari et al., 1980).

IV. Back in Cambridge Exploring Sub-Ten-Nanometre Processes On returning to Cambridge in 1984 I joined with a number of colleagues to build a new clean room, which we equipped for the exploration of nanometre-size devices and techniques. My main interest was to find fabrication methods that would have higher resolution than the vapour and polymer resists. There were a number of candidates oVering this possibility and I felt that all of them would be more practicable at higher electron energies. I considered building a new instrument myself that would operate well above 100 kV, the maximum I had used so far, but decided instead, for the first time in my career, to purchase an instrument. I obtained a JEOL 4000EX transmission electron microscope and modified it so that it could produce a focused beam for fabrication studies. The 4000EX had a resolution in conventional transmission mode of about 0.2 nm, but this was only obtained if the microscope was operated in an environment free of interference from vibration and stray electromagnetic fields. To eliminate interference due to external vibration, Arthur Timbs (CUED Design Engineer) designed a servo-controlled air-suspended isolation system housed in a pit 2 m deep, beneath the 3500-kg microscope. The arrangement reduced the sensitivity of the microscope to external impulses by more than an order of magnitude. Without it, deflections of the beam of the order of 1 nm to 1.5 nm were clearly visible as vehicles passed up and down Fen Causeway. With the suspension system active, no interference was visible. To reduce electromagnetic field interference, Dennis Spicer’s system of field correction coils was used to minimize the AC fields at the microscope column. With the TEM, the probe system consists of the condenser lenses normally used to illuminate the sample, and the upper half of the objective lens which acts as the final lens. For beam writing, these lenses were adjusted so that the Gaussian image of the source at the sample had a nominal diameter of about 0.2 nm. The condenser lenses were also adjusted so that the entrance pupil of

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the objective lens was filled in a manner that provided the optimum illumination aperture where the diVraction, aberration and image discs were matched and the minimum total beam diameter was produced. The pole-piece dimensions of the objective lens were not ideal for a final probe lens because the lens was designed for transmission electron microscopy with the lower half of the lens field acting as the objective lens; however, the aberration coeYcients (Cs ¼ 2.6 mm, Cc ¼ 2.8 mm) were still low enough to produce a beam diameter of 0.5 nm for a current of 10 12 A with the LaB6 cathode electron gun from which I measured a brightness of 2  107 A/cm2 sr at 350 kV. The objective lens was adjusted so that the beam was focused at the electron-optical centre of the lens. When this was done, the lower half of the electron optical column, which comprises the projection system, could be adjusted to produce a highly magnified image on the projection screen of the microscope. This allowed the beam to be observed directly at very high magnification (>2 million times with the 4000EX) and made it easy to correct for astigmatism and to verify that the pattern generator was working correctly. As already mentioned, the resolution achievable with vapour or polymer resists is not determined by electron-optical constraints. Even with a beam diameter of 0.5 nm, the smallest features that can be written in standard resists are 5–20 nm in size and the minimum spacing for closely spaced lines is about 40 nm. With vapour resist the minimum linewidth can be smaller, but if well-defined structures are needed the minimum linewidth is still close to 10 nm; see for example the gold ring shown in Fig. 11, which was fabricated with contamination resist and exposure with the 0.5-nm diameter beam at 350 kV followed by ion etching. The resolution contrast function for PMMA was re-measured in the new instrument at 350 kV with a series of experiments similar to those previously carried out at IBM. The reason for this loss of resolution has never been clearly resolved. I have always thought that it was due to exposure by the cloud of secondary electrons that surround the beam as it passes through a sample. These electrons are excited by inelastic Coulomb interactions between the beam electrons and those in the resist molecules, and they straggle away from the beam into the resist by distances of about 10 nm. It is possible, however, that the loss of resolution may also be due to the size (high molecular weight) of the resist molecules, or due to the mechanism of the development process, although experiments with PMMA samples of variable molecular weight, developed in a variety of ways, all exhibited similar resolution. The eVect due to secondary electrons would be eliminated if resist exposure were insensitive to such low-energy electrons. If this were the case, only

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Figure 11. AuPd ring made on a silicon nitride membrane using contamination resist and ion etching.

the beam electrons would have suYcient energy to eVect the exposure and the resolution would correspond to the beam size; that is, at least ten times smaller. With PMMA, for which there are more data than for other resists, the energy required for exposure is less than 5 eV (it can be exposed by deep-UV light), so this is not the case. Several methods have been discovered that appear to be insensitive to lowenergy electrons and therefore oVer this possibility, two of which I will describe briefly here. The first is the direct sublimation of ionic crystals such as NaCl, MgF2 (Broers et al., 1978b) and the patterning of Langmuir– Blodgett films. The second is the enhancement of the etch rate of SiO2 induced by electron bombardment. An example of the direct sublimation process is illustrated in Fig. 12, which shows holes formed in a NaCl crystal by a 1-nm diameter 50-kV electron beam. The crystal was estimated to be about 0.25 mm thick. The convergence half-angle of the beam was 10 2 rad, and the beam therefore formed a conical-shaped hole with the base of the cone being about 5 nm in diameter, assuming the beam is focused on one face of the crystal. This suggests that the resolution of the process is better than 5 nm. Isaacson and Muray (1981) confirmed this by writing structures as small as 1.5 nm in thinner NaCl films. Our attempts with the new high-voltage system produced some remarkable features, verifying that the resolution was 5–10 times better than for PMMA, but unfortunately we were unable to find eVective methods to transfer these structures into materials that would be ‘useful’ for electronic devices. The ionic materials have little etch resistance

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Figure 12. Holes of 5 nm diameter ‘drilled’ in 0.25-mm thick NaCl crystal.

and attempts to use them as etch masks for either chemical or ion etching failed. We were somewhat more successful with the SiO2 process. Electron bombardment enhances the etch rate of SiO2 in buVered hydrofluoric acid by a factor of about 3. Again a much heavier exposure dose is required than for polymer resists, about a thousand times higher, but the ultimate resolution is about three times better than PMMA. We were able to produce arrays of lines on 12.5-nm centres compared with a minimum of about 40 nm for PMMA (Allee and Broers, 1990). The process was discovered in the 1960s but it was only in the new system in Cambridge that we were able to discover its ultra-high resolution. The resolution is not as high as that of the direct sublimation process, but SiO2 is used in many semiconductor devices, so the method can potentially be used directly in the fabrication of devices. For example, by patterning the gate oxide for field-eVect transistors it should be possible to produce gate-lengths with dimensions under 10 nm. We assume that the resolution is better because the photon energy required to eVect exposure is higher than it is with PMMA.

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A. Final Comment The potential of electron beams for fabricating devices and masks for electronics emerged in a scanning electron microscope experiment initiated by Charles Oatley. Oatley had recognized the power of the instrument for in situ studies of surfaces and it was during observation of surfaces at high resolution that the eVects of the beam on the surface were first observed. We could scarcely have realized at that time how important this was going to be in the evolution of chip technology. But it was justification, once again, for Oatley’s belief that projects involving the building and use of SEMs were ideal for PhD students in electrical engineering. Acknowledgements I would like to acknowledge my many colleagues who have contributed to the work described in this review, in particular co-authors in the references cited. References Allee, D. R., and Broers, A. N. (1990). Direct nanometre scale patterning of SiO2 with electron beam irradiation through a sacrificial layer. Appl. Phys. Lett. 57, 2271. Broers, A. N. (1965). ‘Selective ion beam etching in the scanning electron microscope.’ PhD Dissertation, University of Cambridge. Broers, A. N. (1967). Electron gun using long-life lanthanum hexaboride cathode. J. Appl. Phys. 38, 1991–1992. Broers, A. N. (1969). A new high resolution reflection scanning electron microscope. Rev. Sci. Instrum. 40, 1040–1045. Broers, A. N. (1973). High resolution thermionic cathode scanning transmission electron microscope. Appl. Phys. Lett. 22, 610–612. Broers, A. N. (1980). Measurement of the ultimate resolution of the electron resist PMMA using a STEM, in Proceedings of the 38th Annual Meeting of the Electron Microscopy Society of America, edited by D. Wittry. San Francisco, CA: Claitor, Baton Rouge, pp. 236–237. Broers, A. N., Molzers, W. W., Cuomo, J. J., and Wittels, N. D. (1976). Electron beam ˚ metal structures. Appl. Phys. Lett. 29, 596–598. fabrication of 80-A ˚ linewidths with PMMA Broers, A. N., Harper, J. M. E., and Molzers, W. W. (1978a). 250 A electron resist. Appl. Phys. Lett. 33, 392–394. Broers, A. N., Cuomo, J., Harper, J. M. E., Molzen, W. W., and Pomerantz, M. (1978b). In Electron Microscopy 1978, edited by J. M. Sturgess. Toronto: Microscopical Society of Canada, MSC5, pp. 343–354. Chang, T. H. P. (1975). Proximity eVect in electron beam lithography. J. Vac. Sci. Technol. 12, 1271–1275.

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Chaudhari, P., Broers, A. N., Chi, C. C., Laibowitz, R., Spiller, E., and Viggiano, J. (1980). Phase-slip and localization diVusion lengths in amorphous W-Re alloys. Phys. Rev. Lett. 45, 930–932. Grobman, W. D., and Speth, A. J. (1978). In Proceeding of the 8th International Conference on Electron and Ion Beam Technology, Seattle, Washington, edited by R. Bakish. Proceedings of the Electrochemical Society, PV78-5, Princeton NJ, p. 276. Haller, I., Hatzakis, M., and Srinivassen, H. (1968). High resolution resists for electron beam exposure. IBM J. Res. Dev. 12, 251–256. Hayman, P. (1962). PhD Dissertation, University of Paris. IEEE (1982–2003). ‘Proceedings of the International Solid State Circuits Conferences.’ New York: IEEE. Isaacson, M., and Muray, A. (1981). In situ vaporization of very low molecular resists using 1/2 nm diameter electron beams. J. Vac. Sci. Technol. 19, 1117–1120. Laibowitz, R., Broers, A. N., Yeh, J. T. C., and Viggiano, J. M. (1979). Josephson eVect in Nb nanobridges. Appl. Phys. Lett. 35, 891–893. Lean, E. G., and Broers, A. N. (1970). Microwave surface acoustic delay lines. Microwave J. 13, 97–101. Mayadas, A. F., and Laibowitz, R. B. (1972). One-dimensional superconductors. Phys. Rev. Lett. 28, 156–158. Molzen, W., Broers, A. N., Cuomo, J. J., Harper, J. J. E., and Laibowitz, R. B. (1978). Materials and techniques used in nanostructure fabrication. J. Vac. Sci. Technol. 16, 269–272. Sedgwick, T. O., and Broers, A. N. (1972). A novel method for the fabrication of ultrafine metal lines by electron beam. J. Electrochem. Soc. 119, 1769–1771. Smith, K. C. A. (1956). ‘The scanning electron microscope and its fields of application.’ PhD Dissertation, University of Cambridge. Stewart, A. D. G. (1962). Investigation of the topography of ion bombarded surfaces with a scanning electron microscope. Philadelphia, 1962, Paper D-12. Stewart, A. D. G., and Thompson, M. W. (1969). Microtopography of surfaces eroded by ion bombardment. J. Mater. Sci. 4, 56–60. Thornley, R. F. M., and Hatzakis, M. (1967). Electron-optical fabrication of solid-state devices, in Record of the IEEE 9th Annual Symposium on Electron, Ion and Laser Beam Technology, edited by R. F. W. Pease. San Francisco: San Francisco Press, pp. 94–100. Voss, R. F., Laibowitz, R. B., and Broers, A. N. (1980). Niobium nanobridge d.c. SQUID. Appl. Phys. Lett. 37, 656–658.

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2.11 Scanning Electron Diffraction: A Survey of the Work of C. W. B. Grigson D. McMULLAN Cavendish Laboratory, University of Cambridge Formerly at: Engineering Department, University of Cambridge

I. Introduction Christopher Grigson joined Charles Oatley’s laboratory in the Cambridge University Engineering Department (CUED) in 1950 as a postgraduate research student, having done well in both parts of the Mechanical Sciences Tripos. He was the third of Oatley’s students after K. F. Sander and D. McMullan to undertake the building of an electron-optical instrument and it is probable that Oatley would have liked to have him work on the SEM. However, it was too soon to know whether McMullan’s research would bear fruit and merit further work, and instead Oatley gave him the task of building an electron diVraction camera. II. Electron Diffraction Camera Quoting from the introduction to Grigson’s dissertation (Grigson, 1955): The subject selected for research was the design of an electron diVraction camera and its use for the investigation of thin films important in electrical engineering . . . A survey of the literature soon showed that it was possible to build instruments of far higher resolving power than that of the simple single lens cameras made before the war; it also became clear that a high resolving power would be desirable for the study of the size of the film particles. The first two and a half years were taken up with the design and development of the instrument. The experiments on thin films were done during the summer of 1953. The interpretation of the diVraction patterns and of the behaviour of the films took a year more, though the work was no longer carried on full-time [because of teaching duties, eds.]. A diVraction instrument with an electrostatic optical system has not previously been described. The instrument and the associated apparatus . . . were 227 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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designed subject to the specifications laid down for the camera by Mr C. W. Oatley. One of these, that evaporation must be done outside the main body of the camera, turned out to be very important for the solid state research for it meant that there should be no ambiguity as to whether changes in the film took place as a consequence of migration in some unknown way of the film material. . . . Some of the results of this study are thought to be new: namely for a twin lens instrument the resolution attainable is nearly independent of the specimen to plate distance; that the voltage stability requirement for instruments designed for particle size determination is stringent; and the special requirements for guns. The correct position of the lenses for a twin lens instrument was worked out independently and used for design. It has since been confirmed by Cowley and Rees (1953).

In the last chapter of his dissertation some of his results are summarized, including: This survey has revealed the important features of the way alkali halides LiF, NaCl, and KCl grow in thin films. Namely that growth does not take place as it does in metals by the gradual appearance of myriads of crystals which increase in size at one another’s expense, but by the appearance of ‘full grown’ crystals here and there in the melt, and the gradual change of the melt to the crystalline phase as a result.

It is clear from his dissertation, which he submitted at the beginning of 1955, that he had not taken part in the SEM research. In 1953 he had been appointed University Demonstrator, so that teaching was taking up a fair proportion of his time and even more so when he was promoted to Lecturer in 1956 and elected to a Fellowship at Trinity College. Although not documented, it appears that he was involved in some way with research on the ‘Dracone’ project that was started in 1955 by Professor W. R. Hawthorne, the Head of the CUED. This was the investigation of the possible use of tubes of flexible plastic filled with oil and towed by ships for transporting the oil across the oceans. Oliver Wells relates in his contribution (Chapter 2.3) that he helped Grigson to salvage, from the river, part of an instrument for measuring the speed of a boat through water and also that he mentioned the use of thin plastic tubes. Although there does not appear to be any other confirmation, it is of interest in view of his later work on fluid dynamics. III. Scanning Electron Diffraction In 1956 he took over from Oatley the supervision of Oliver Wells, who was in the fourth year of his SEM research project. This may well have fired his interest in scanning techniques because some time later he started work on a

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fast scanning electron diVraction system (SEDS) following the publication of the description of such an instrument by Bagdyk’yants and Alekseev (1958) in the USSR. His first SEDS began operating in May 1961, and he reported this in a letter to Nature the following November (Grigson, 1961): An electron diVraction system for the quantitative investigation of rapid phase changes in solids has recently been completed. The system will record an electron diVraction pattern of the Debye–Scherrer ring type in 50 msec., giving the electron intensity at the peaks of strong reflexions to an accuracy of 2 per cent. The system gives an improvement in speed of direct recording of about 4 orders of magnitude (Bagdyk’yants and Alekseev, 1958), and is thus attractive for investigating quantitatively rapid changes in solids such as polymorphic transformations, transitions from the amorphous to the crystalline state, diVusion in alloys. It is also suitable for investigating controversial questions in electron diVraction such as the transition from geometrical to dynamic diVraction conditions.

There follow two examples of its application to a polycrystalline gold film and to a high-speed amorphous-to-crystalline change in a film of silver in response to sudden heating. The ultimate speed of recording in such a system is set by the shot noise in the diVracted beams. The limitations are the same as those of the scanning electron microscope. Measurements show that the intensities at diVracted peaks are about 10 9 amp. Thus, for an accuracy of 1 per cent at such a peak, a picture point may be recorded in about 2 msec. One may expect the recording times for a complete diVraction pattern of 1 msec are a question merely of improvements in technology.

This letter was followed about 6 months later by a paper with a full description of the instrument (Grigson, 1962), and during the next six years some 20 papers were published reporting results and several describing improvements to the instrument, for example: Grigson (1965); Denbigh and Grigson (1965); Grigson and Tillett (1968). An important development was the incorporation of post-specimen electron energy filtering to prevent inelastically scattered electrons reaching the detector. An example of the applications reported by Grigson was an electron diVraction study of the growth of thin films of Ni, NiFe, Fe, LiF and Al to name a few: The main features of the observations taken during specimen growth in a scanning electron-diVraction system are: (i) The diVraction profile of the substrate is run. (ii) The source vapour is allowed to strike the substrate and successive diVraction profiles appear as the specimen grows. Clear intensity changes can be seen from average thickness increments equal to atomic monolayers, whether the structure of the film is amorphous, consists of nuclei, or is

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polycrystalline. (iii) Intensities rapidly increase with thickness until thickness equals the mean free path length for all types of collision, then more slowly diminish as the film becomes opaque. (iv) Growth may be stopped at any stage to look for grain-orientation with a two-dimensional scan, or to remove the specimens for electron microscopy. (Grigson, 1966)

Besides the two research students, Denbigh (1964) and Tillett (1969), mentioned above as co-authors, Grigson had two others, M. F. Tompsett (1965) and M. B. Heritage (1968). All were involved with the development and building of the SEDSs and their application to the study of thin polycrystalline films (Grigson, 1968). These films had to be evaporated in situ and it was becoming clear to Grigson as well as to other workers, e.g. Poppa (1965), that the vacuum in a conventional TEM, typically > 10 5 Torr, did not approach the 10 10 Torr that is required. At the end of 1964 the CUED was contacted by the Royal Radar Establishment (RRE) wishing to place a contract for the modification of a TEM to permit the in situ growth of thin films in ultra-high vacuum (UHV). A draft proposal was therefore drawn up by Professor Oatley with Grigson and Dr W. C. Nixon for the modification of an AEI EM6 TEM. It was aimed to have a vacuum of 10 9 to 10 10 Torr, maintainable throughout the growth of a film (Tothill, 1969); the UHV chamber was designed by Grigson. Unfortunately, a competing bid was received by RRE from Dr P. B. Hirsch at the Cavendish Laboratory and they decided to fund both projects with reduced budgets. Nixon and his research student, Tothill, went ahead (as explained below, Grigson left CUED at the end of 1966) but the funds were insuYcient to build a proper bakeable UHV system with ion pumps, Conflat flanges, etc., and the pressure achieved at the specimen in the final instrument, reported by another research student about five years later (Bunting, 1971), was little better than 10 6 Torr. A. Bell Telephone Laboratories In 1964/65 Grigson spent a sabbatical year at Bell Telephone Laboratories, Murray Hill, NJ, where An improved system for scanning electron diVraction has recently been developed in this laboratory. The main improvements on the earlier system of Grigson are greater stability and sensitivity of the diVraction instrument and a more accurate display; moreover provision is made for evaporating specimens, so that intensity profiles from a growing film are obtained; and the diVraction patterns may be scanned in two dimension. (Grigson et al., 1965)

He was very impressed by his experiences there and it seems that he returned to Cambridge rather dissatisfied with the much lower levels of funding at

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CUED. This feeling was, no doubt, intensified by the decision of RRE to reduce the funding for the TEM conversion: as shown by his later paper (Grigson, 1968) he was conversant with the pioneering work of A. V. Crewe at the Argonne Laboratory and was convinced that modern UHV techniques would have to be used if the specification was to be met. He seriously considered moving to the United States, but in any event he went to Norway. IV. Norway A. Shipping Business Grigson left the CUED in January 1967. He spent the next one and a half years learning about the shipping business at Kockums mekaniska Verkstad AB in Malmo¨, Sweden; H. Clarkson & Co in London; and Denholm Shipping Services in Glasgow. In 1968 Grigson moved with his family from Cambridge to Norway, where he was appointed Technical Director of A/S Athene, an oil-tanker company run by his father-in-law, Jørgen Bang. Grigson ran the company himself after Jørgen Bang’s death in 1974, until it had to close because of the oil crises of the 1970s. B. Agder University College From then on until his retirement, Grigson worked as an independent hydrodynamics consultant; he published nearly 20 papers in the Transactions of the Royal Institution of Naval Architects (London) and was elected a Fellow of the Institution. In 1992 he moved from Kristiansand to Grimstad, where the Agder University College Department of Engineering is situated, and there he taught hydrodynamics and basic physics. At that time he helped Professor Douglas Faulkner in the investigation of the sinking of the bulk ore carrier Derbyshire in the Pacific. He maintained his contact with Sir Charles Oatley and was present at the celebrations marking Oatley’s 80th and 90th birthday anniversaries (see Frontispiece). V. Epilogue A. Scanning Electron DiVraction in Scanning Transmission Electron Microscopes Reverting to 1966, as recorded above, Grigson’s work on scanning electron diVraction was continued at the CUED under the supervision of

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W. C. Nixon for about five years after his departure. Otherwise there appears to have been little interest in it elsewhere, as was shown by a history of electron diVraction published in 1981 entitled Fifty Years of Electron DiVraction (Goodman, 1981) in which there was only one minor reference to Grigson’s work. The main reason for this was no doubt because scanning diVraction cameras were not on the market and to build one would have required a major investment. But it may well be that the prejudice against the scanning electron microscope up until 1965, mentioned elsewhere in this volume, was also a factor. However, in about 1975, scanning electron diVraction was incorporated in the scanning transmission electron microscopes (STEMs) that were being developed and marketed by VG Microscopes Ltd of East Grinstead. These were UHV instruments with field-emission cathodes, and scanning diVraction was a very useful technique that could be incorporated relatively easily. This was first done at the request of Dr (later Professor) L. M. (Mick) Brown of the Metal Physics Group in the Cavendish Laboratory, University of Cambridge. The group had ordered the second VG HB5 STEM that was made by the company, and first results were reported in 1975 (Brown et al., 1976). Similar scanning arrangements were included in another UHV STEM, built in the Department of Metallurgy of Oxford University at about the same time (von Harrach et al., 1976). The use of scanning diVraction proved to be very fruitful and nearly all the STEMS sold by VG Microscopes up until they closed after a takeover in 1998 incorporated ‘Grigson coils’. Christopher Grigson died of cancer on 19 February 2001, aged 74.

Acknowledgements I am very grateful to Christopher Grigson’s wife, Hella, for her very helpful comments; and to John T. Conway, Professor of Aeronautics at Agder University College, Grimstad, who provided detailed information about Grigson’s life and work after he moved to Norway in 1968.

References Bagdyk’yants, G. O., and Alekseev, A. G. (1958). Measurement of the scattered electron intensity and elimination of the background in electron diVraction studies. Bull. Acad. Sci. USSR Phys. Ser. 23, 766–768.

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Brown, L. M., Craven, A. J., Jones, L. G. P., Ward, P. R., and Wilson, C. J. (1976). Preliminary investigations with a high resolution scanning transmission electron microscope, in ‘Developments in Electron Microscopy and Analysis’, Proceedings of EMAG75, Bristol, Sept. 1975, edited by J. A. Venables. London: Academic Press, pp. 3–6. Bunting, C. D. (1971). ‘Scanning electron diVraction and transmission electron microscopy.’ PhD Dissertation, University of Cambridge. Cowley, J. M., and Rees, A. L. G. (1953). Design of a high-resolution diVraction camera. J. Sci. Instrum. 30, 33–38. Denbigh, P. N. (1964). ‘Scanning electron diVraction with energy analysis.’ PhD Dissertation, University of Cambridge. Denbigh, P. N., and Grigson, C. W. B. (1965). Scanning electron diVraction with energy analysis. J. Sci. Instrum. 42, 305–311. Grigson, C. W. B. (1955). ‘An electron diVraction study on the growth of ionic crystals.’ PhD Dissertation, University of Cambridge. Grigson, C. W. B. (1961). High-speed direct recording system for electron diVraction. Nature 192, 647–648. Grigson, C. W. B. (1962). On scanning electron diVraction. J. Electron. Control 12, 209–232. Grigson, C. W. B. (1965). Improved scanning electron diVraction system. Rev. Sci. Instrum. 36, 1587–1593. Grigson, C. W. B., Dove, D. B., and Stilwell, G. R. (1965). Some applications of an improved scanning electron diVraction system. Nature 205, 1198–1199. Grigson, C. W. B. (1966). Scanning electron diVraction studies of nucleating films. J. Phys. Chem. Solids H4 (Supplement 1), 611–616. Grigson, C. W. B. (1968). Studies of thin polycrystalline films by electron beams. Adv. Electron. Electron Phys. (Supplement 4), 187–289. Grigson, C. W. B., and Tillett, T. I. (1968). On scanning electron diVraction. Int. J. Electron. 24, 101–138. Goodman, P. Ed. (1981). ‘Fifty Years of Electron DiVraction.’ D. Reidel, Dovdrecht. Heritage, M. B. (1968). ‘Growth studies on thin films using scanning electron diVraction.’ PhD Dissertation, University of Cambridge. Poppa, H. (1965). Progress in the continuous observation of thin film nucleation and growth processes by electron microscopy. J. Vac. Sci Technol. 2, 42–48. Tillett, P. I. (1969). ‘Scanning electron diVraction at cryogenic temperatures.’ PhD Dissertation, University of Cambridge. Tompsett, M. F. (1965). ‘Reflection scanning electron diVraction with energy analysis.’ PhD Dissertation, University of Cambridge. Tothill, F. C. S. M. (1969). ‘Ultra high vacuum in a transmission electron microscope for observing thin film growth.’ PhD dissertation, University of Cambridge. von Harrach, H., Lyman, C. E., Verney, G. E., Joy, D. C., and Booker, G. R. (1976). Performance of the Oxford field-emission scanning transmission electron microscope, in Developments in Electron Microscopy and Analysis, Proceedings of EMAG75, Bristol, Sept. 1975, edited by J. A. Venables. London: Academic Press, pp. 7–10.

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PART III THE DEVELOPMENT OF ELECTRON PROBE INSTRUMENTS AT THE CAVENDISH LABORATORY AND THE TUBE INVESTMENTS RESEARCH LABORATORY

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

3.1 The Development of the X-ray Projection Microscope and the X-ray Microprobe Analyser at the Cavendish Laboratory, Cambridge, 1946–601 V. E. COSSLETT Formerly at: Cavendish Laboratory, University of Cambridge

I. Introduction Historically viewed, it might seem inevitable that the Cavendish Laboratory should become a leading centre for electron microscopy, building and evaluating instruments (the microprobe analyser, high-voltage electron microscope, X-ray projection microscope, energy-loss analyser) as well as developing theoretical electron optics. There were, nevertheless, major elements of chance about how and by whom it all happened, in particular that I myself became involved with electrons long before coming to the Cavendish. It was Maxwell, the first Cavendish professor, who laid the foundations of electromagnetism. His successor, Rayleigh, put forward the first clear and quantitative criterion of image resolution (in light optics). J. J. Thomson, next in the chair, discovered the electron and established its properties. Then Rutherford showed us the inner structure of the atom, including the role of electrons in it and, by proxy, how their energies are related to the production of X-rays. But it was W. L. Bragg, son of the father of X-ray spectroscopy, who brought electron microscopy into the Cavendish, after succeeding to the chair on Rutherford’s death in 1937. 1 The Editors have selected extracts from the paper by the late Dr V. E. Cosslett entitled ‘The development of electron microscopy and related techniques at the Cavendish laboratory, Cambridge, 1946–79, Part I 1946–60’ (Contemporary Physics 22, 3–36, (1981)) to describe the development of the X-ray projection microscope and the X-ray microprobe analyser at the Cavendish.

237 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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In the middle (for us) of World War II, the beginning of it for the United States, a complicated agreement was made concerning bases and equipment (including warships) between the United Kingdom and the United States known as Lend-Lease, or alternatively Lease and Lend. Under this arrangement and probably at the instigation of Sir Charles Darwin, then head of the National Physical Laboratory (NPL), half-a-dozen electron microscopes of the first model made by RCA, the EMB, came to be included. One of them was allotted to the Cavendish, where it was delivered late in 1942. It was this instrument which came into my hands when I arrived there in 1946, with negligible experience of electron microscopy. Meanwhile, since everyone else was busy with war work, it had been in the charge of George Crowe, previously Rutherford’s personal research assistant. Until the beginning of World War II, the Cavendish Laboratory had been almost totally concerned with nuclear physics, under Rutherford’s leadership. Although, by modern standards, the laboratory was run on a shoestring, it produced research that was rewarded by a string of Nobel prizes. In 1937, W. L. Bragg succeeded Rutherford and began to build up a school of X-ray crystallography. But the interruption of the war prolonged the life of nuclear research in the Cavendish, which continued in directions of national importance after the main eVort to produce the atom bomb had been transferred to the United States. In the middle of these preoccupations, the RCA electron microscope arrived at the Cavendish. Cambridge was an obvious candidate to receive one of the Lend-Lease batch, both as a great concentration of scientific research and from Bragg’s personal interest. He had had a hand, along with Darwin and G. P. Thomson, in getting and allocating the microscopes. Also, he was keenly interested in new forms of microscopy, having put forward ideas for an X-ray microscope a few years earlier. The immediate problem was who should take charge of the new acquisition, all senior researchers being fully occupied. Bragg decided that the best man for the job would be George Crowe, who had been Rutherford’s personal assistant. Although he had little knowledge of electronics, he had considerable experimental competence, being a product of the do-it-yourself, sealing-wax-and-string era. This experience gave him confidence in tackling any new practical problem, in spite of the lack of one and a half fingers due to radiation burns from the days when safety precautions were nonexistent or lightly ignored. He it was who became my partner and mentor when I finally came face to face with an electron microscope. In a high degree my entry into the Cavendish was a matter of luck. In the spring of 1946 I wrote to Bragg about the possibility of a job in Cambridge, knowing that the RCA instrument was in active use there. He replied,

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advising me to apply for one of the ICI research fellowships founded specifically to enable mature scientists to bridge the gap between wartime duties and academic work. I put in my application, although doubtful that at 38 I might be too old. Not having had a reply after some weeks, I wrote to ask when a decision could be expected since I was short-listed for a job elsewhere. Bragg replied that somehow my application had gone astray, the ICI Fellowships had already been awarded and that he would nevertheless do what he could for me. Would I come for interview as soon as possible? Armed with the page proofs of my ‘Introduction to Electron Optics’. I took the train to Cambridge, where Bragg and his right-hand man, J. A. RatcliVe, interviewed me. Taking a favourable view of my usefulness to the laboratory, they conjured up an extra ICI Fellowship. So I came to the Cavendish in September 1946 on a three-year appointment, and stayed 30 years and more. II. Development of EM Research in the Cavendish 1946–1960 In describing the progress of this work it is convenient to divide the past thirty-odd years into two periods: 1946–60 and 1959–75. Although my own interest and activity in the problems of high resolution were continuous throughout, there was a sharp break in 1959–60 in both personnel and direction of work. Before that date the story concerns the X-ray projection microscope and its metamorphosis into the microprobe analyser, the development of techniques of electron microscopy, in particular for the study of metals, and some theoretical work on lens aberrations. In the same period, but independently, the scanning electron microscope was being developed into a practical instrument under C. W. Oatley at the Engineering Department down the road. Towards the end of it, the Metal Physics and the Surface Physics sections of the Cavendish began their own adventures in transmission electron microscopy. III. The X-Ray Projection Microscope A. Origins The idea of forming enlarged images by placing an object close to a point source of X-rays was put forward very soon after Ro¨ntgen’s original discovery. A suYciently small source could only be obtained at that time by means of a pinhole in front of the large source of a primitive X-ray tube. The intensity was so weak, however, that only a very low magnification was

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possible within acceptable exposure times. The use of a fine focal spot was suggested by Malsch in 1933 and a few years later von Ardenne and Marton independently proposed forming such a ‘spot’ with electron lenses (for historical details see Cosslett and Nixon, 1960). My attention was first drawn to the possibilities of the method by a short note by von Ardenne in almost the last issue of Naturwissenschaften to reach England before the outbreak of World War II. He gave a design for a pointfocus tube using an electrostatic lens. The close connection with my project for making a microfocus X-ray tube for crystallographic use was obvious. It would simply be necessary to use a thin X-ray transparent foil as anode instead of a massive metal block. The experimental tube on which S. S. D. Jones and D. A. Taylor worked with me at Oxford had a fluorescent screen as target, on which we endeavoured to measure the spot size. It was later replaced by a thin foil of copper thick enough (3 mm) to withstand atmospheric pressure. The use of a magnetic lens made alignment much easier than with the electrostatic lens. The ultimate aim was to image biological specimens in the wet state and perhaps even living. It was already appreciated that that it would be diYcult, if not impossible, to do so in the electron microscope (EM). Indeed, there was considerable scepticism about getting useful information from biological material with the EM even when dehydrated, because of radiation damage. We forged ahead, like good experimentalists, without a clear understanding of the physical limitations. As already mentioned, I soon became aware of the spherical aberration problem. To get a reasonably small focal spot the electron lens had to be stopped down, thereby radically reducing the intensity in the spot. It was only much later that a proper analysis showed how diYcult it would be to do better than the resolving power of the UV microscope (0.1 mm). The Clarendon Laboratory willingly let me move our original set-up from Oxford to Cambridge. It included a Holweck rotary pump as the essential element of the vacuum system, an early form of turbomolecular pump, tricky to start up and slow in pumping speed, but devoid of mercury. David Taylor, who came with it as technical assistant, proceeded to rebuild the apparatus in a corner of the Old Cavendish Laboratory, whilst I was mainly occupied with the problems and users of the RCA and Siemens electron microscopes. By 1948 we were getting some rather poor X-radiographs of test grids, but in November he returned to Oxford and the project languished for a year. B. Development In October 1949 a research student from Canada presented himself in W. C. Nixon. His professor at Queen’s University, Ontario (J. K. Robertson), recommended him to the Cavendish and Bragg relayed him to me. Bragg was encouraging the development of an Electron Microscope Section and

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I was glad to acquire a research student with experimental ability, as Nixon proved to be. He had already had experience in electronics and vacuum physics in his Master’s degree at Queen’s and was acquainted with the development of the first EM in North America at Toronto University (Burton and Kohl, 1942). He was at first somewhat overawed by the reputation of the Cavendish. In fact he turned out to be a skilful and persistent practical man, good at designing, building and trouble-shooting apparatus. His first year was occupied with making and testing a more refined version of the original point-focus tube, vertical instead of horizontal, with a single magnetic lens. The results were promising, so he went on to construct a twostage model, similar to the condenser system of an electron microscope but with the second lens the stronger of the two. An electron-optical demagnification of 50 in the first lens and 100 in the second lens allowed a maximum of 5000, suYcient to reduce a 50 mm electron source to a spot ˚ at the X-ray target. Usually we operated with a spot size of 1 mm of 100 A and in the voltage range 5–15 kV. Exposure times became longer (up to 10 minutes) as spot size was decreased (and thereby target current) in order to obtain better resolution. The target foil thickness had to be reduced pari passu to limit broadening of the region of X-ray production by electron scattering, and this caused a further reduction in intensity (Nixon, 1952). With an improved version of the tube, Nixon (1955) obtained a resolution approaching 0.1 mm. The ultimate limit on image resolution was in practice set by mechanical and electrical instability during long exposure rather than by diVraction and penumbra. It had been hard work keeping the project afloat financially. How we managed to do so, and the diYculties encountered in getting the projection X-ray microscope into production, is a prime example of the gap between innovation and manufacture. Nixon returned to America (to GE) in July 1952 at the end of his grant. To keep things going I appealed to the National Research Development Corporation (NRDC) recently set up (in 1949) by the government for the very purpose of developing inventions made in university and other research laboratories. Backed by my former professor, P. M. S. Blackett, who was one of the directors, it was persuaded to bend the rules (we had then no patent rights) and make a grant to support a research student (N. A. Dyson) for one year. Recognizing a moral commitment, I ultimately found means to pay it back. C. Manufacture After discussions with the patent experts of NRDC and others—I found that the University opted out of such commercial dealings—a patent was applied for in October 1952. With this in hand, I began negotiations with a series of

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firms, local, national and foreign. All were exceedingly cautious, probably after sizing up the market for such a new product. Eventually it was the biggest corporations that decided it was worthwhile to have an option on future possibilities. Agreement was reached with Philips in October 1953, and two years later with Metropolitan-Vickers, to buy rights in the projection X-ray microscope. This money was most helpful in keeping our work going, but in the meantime I obtained a grant from Department of Scientific and Industrial Research (DSIR) to pay two additional research assistants, C. K. Jackson and J. V. P. Long, who started with us early in 1954. Jackson worked with Nixon (back from America) for two years. Long began to investigate the microanalysis of inorganic elements in biological materials. He modified a projection tube for X-ray fluorescence analysis (Long and Cosslett, 1956) and later developed the prototype of the Geoscan microprobe analyser. Microradiographs obtained, in collaboration with colleagues in Cambridge, of a variety of biological, geological and metallurgical specimens had meanwhile attracted a certain amount of attention. A number of enquiries came in from laboratories interested in acquiring a projection X-ray microscope. Since commercial negotiations were hanging fire, we decided to go into production in our own workshops. The instrument had been redesigned in a more compact and mechanically stable form, making it also easier for an inexperienced user to operate (Cosslett and Pearson, 1954). The first unit was supplied in June 1953 to Al Baez (father of Joan) at Stanford University, on a grant from the US National Research Council and American Cancer Society. Under Cambridge University’s status as a charitable institution we were not allowed to make a profit. We certainly did not do so at what now seems the ridiculously low price of 350 (about $1000 at that time), which simply covered the cost of materials and bought-in components. We made two other X-ray microscopes for the United States, for R. W. G. WyckoV (National Institutes of Health) and P. Kirkpatrick (Stanford), but the work was beginning to take up too much of the resources of the workshop. In spite of all our eVorts we had not been successful in persuading any firm to produce them; Philips and Metropolitan-Vickers simply provided money for research and development. Our problem was solved in the most unexpected way, by probably the only amateur X-ray microscopist in the world. As a result of describing the microscope and its uses in the literature and at meetings, we came to know Raymond Ely. He had become interested in the subject through contact with the firm Hilger and Watts, makers of X-ray diVraction apparatus, who had incorporated the electrostatically focused tube of Ehrenberg and Spear (1951) in a microdiVraction unit. He himself was an electrical engineer and

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at home had his own laboratory with a number of optical microscopes as well as an Ehrenberg and Spear tube that he used for microradiography. It was not diYcult to persuade Ely to take up the production of our magnetically focused projection microscope, the first three of which were supplied in 1956 to J. W. Menter (Tube Investments Research Laboratory), Arne Engstro¨m (Karolinska Institute, Stockholm) and Richard Saunders (Dalhousie University, Nova Scotia). Ely later set up the firm of Electron Physical Instruments, where he continued to produce successively improved versions for many years, operating at voltages up to 50 kV. This four-year struggle over the projection X-ray microscope introduced me for the first time to the diYculty of getting a specialized scientific instrument into production. Even when a prototype had been constructed and its potentialities for scientific research and technology demonstrated, major industrial concerns showed great reluctance to take the risk of producing it. In the end it required a small firm willing to manufacture for a limited market. I was at pains to raise this innovation/production problem in memoranda to NRDC (1957) and DSIR (1958); it is considered in a broader context by Jervis (1971/ 72). The same problem arose later in connection with both the X-ray microanalyser and the high-voltage electron microscope.

IV. The X-Ray Microprobe Analyser A. Origins Some time in 1951–52 an urgent medical problem was brought to my attention by Professor Policard of the Centre d’Etudes et Re´cherches des Charbonnages, Paris, and also by pathologists from the Pneumoconiosis Unit of the Medical Research Council. They brought us sections of human lung for examination. Heavy deposition of dust particles was usually observable, and they wanted to identify which were silica. This was impossible by simple density discrimination or by microchemical tests on the grid-mounted specimens, but I recalled that a method of X-ray microspectrometry had been described by Castaing and Guinier (1949) at the First International EM Conference in Delft. Castaing had modified the condenser system of a CSF electrostatic microscope so as to direct a fine electron beam onto the specimen, the X-rays from which were analysed in a Bragg spectrometer. The region under the electron probe was observed through an optical system built into the side of the microscope, and the specimen could be searched by means of a mechanical scanning system. The technique had originally been proposed and investigated by Hillier and Baker in 1943, but not followed up.

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It was obvious that we could do the same in a magnetic EM if the objective lens were to be modified to allow X-rays to emerge through a thin, vacuumtight window. More than that, it occurred to me that we might borrow electronic scanning techniques from Oatley’s laboratory, where McMullan was already getting their first SEM into operation. In this way the scanning microprobe analyser was born, in response to a medical-biological need. X-ray wavelengths being characteristic of the element bombarded, especially in the K-series, we had no doubt that silicon could be positively identified even in very small particles. The problem was to find financial support and manpower to build an experimental instrument. My first approach was to the British Iron and Steel Research Association (BISRA), the Metallurgy Division of which had an advisory subcommittee on electron microscope and electron diVraction methods. I put forward proposals for an experimental instrument to its New Techniques Committee in October 1952, and M. E. Haine tabled a scheme on behalf of Associated Electrical Industries Limited (AEI). He proposed to build a more complicated instrument that would ‘combine the scanning electron microscope, Castaing’s X-ray method, point-projection X-ray microscopy (with or without scanning), microbeam electron and X-ray diVraction’. My project was less ambitious, involving the adaption of our point-focus X-ray tube for scanning and adding a crystal spectrometer. Although this was much the cheaper alternative, BISRA finally decided to support the AEI scheme. In association with three other research associations, financial support was given for a microanalyser to be built at the AEI Aldermaston Research Laboratories. Its development was monitored by an advisory committee of the four research associations, which naturally wished it to be designed with metallurgical applications in mind. In the event, therefore, it became a much simpler instrument than Haine had envisaged, a straight development from Castaing’s, using magnetic instead of electrostatic lenses and lacking a scanning display, an optical microscope being the only means of selecting areas of the specimen for analysis. The prototype was constructed between 1954 and 1956, and a commercial model was produced in 1958–59. Some forty of this AEI microanalyser were sold during the next decade, a scanning system being added to it in 1961. B. Development Meanwhile, having failed to get any help from BISRA, we had gone along on our own at the Cavendish. ‘Having no money, we had to think’, as Rutherford is supposed to have said. At that time the budget for the whole of the EM section was 550 per annum. But by a combination of good luck and first-class experimentation, we made better progress than AEI. With

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later interest from Tube Investments Research Laboratory and then the Cambridge Instrument Company (CIC), the scanning microprobe analyser, as now known, was successfully built and marketed.

C. Peter Duncumb The key step in this direction was the decision of Peter Duncumb to become a research student in my section of the Cavendish. He was rather more mature than the normal PhD candidate, having done service in the Royal Air Force, and then taken an Honours degree in physics. He turned up providentially just at the moment when it was clear that we would not get even a minimal apparatus grant from BISRA, so I put to him my scheme for building a scanning microprobe analyser. With hindsight I shudder at the idea of launching a raw research student into such a radically new constructional project, starting almost from scratch. True, we had by then a fair amount of experience of making point-projection X-ray tubes and making them work, but none of scanning techniques nor of X-ray spectrometry. We had no money to buy more than the cheapest apparatus, though the resources of the Cavendish workshop were available, in which there were a number of skilled instrument makers. Duncumb was supported by one of the DSIR maintenance grants for PhD students from October 1953. To build a completely new instrument was out of the question, so we decided to modify the original RCA EMB electron microscope. The necessary electronic gear was gathered together from wherever we could locate it. Some was begged and borrowed within the Cavendish. Some came from a war surplus centre at Harwell (near the Atomic Energy Research Establishment) from which university researchers could buy components or complete units at give-away prices, reckoned per pound or per cubic foot! With the ready help of the Cavendish workshop (at that time the Electron Microscopy Section did not have its own machines), Duncumb planned and constructed the first scanning microprobe analyser. As we knew little about scanning, and in particular the signal strength and signal-to-noise ratio to be expected, exploratory experiments were first made in the conditions of projection X-ray microscopy. A scanning microscope of this type had already been suggested by Nixon. A 20-kV electron probe, diameter about 1 mm, was scanned across a copper target fixed in the projector lens of the RCA microscope (Duncumb and Cosslett, 1956). The X-rays so generated passed through the specimen point by point, the transmitted intensity being recorded with a scintillation counter, the signal from which modulated the brightness of a cathode-ray tube scanned in synchronism with the probe. The screen thus displayed an image showing X-ray

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absorption contrast. Pattee in 1953 had independently made similar experiments in a modified Metropolitan-Vickers microfocus X-ray tube: he rightly concluded that a small enough probe for high-resolution imaging in reasonable exposure time would require the high intensity of a field-emission source, which he set about building. While Pattee continued with X-ray absorption microscopy, we followed our original aim of imaging by characteristic X-ray emission. Duncumb replaced the projector lens with a lens designed to allow the detection of both the X-rays and back-scattered electrons from a specimen placed between the pole pieces. We did not realize that such a simple modification could be patented. Soon after it was mentioned in a lecture in Colorado, an American firm took out a patent on a similar system, on which the CIC was forced to pay royalties when the Microscan was put on the market. The scanning electron image served to locate areas of interest which could then be analysed with the aid of an X-ray spectrometer set to the wavelength emitted by each element in turn, or by means of a proportional counter and pulse analyser. If the characteristic X-ray was strong enough, an image of the distribution of an element could be displayed for comparison with the SEM picture. Since the X-ray intensity is weaker than that from backscattered electrons, exposure times are slow compared with the SEM. The deflection coils used were similar to those in the early versions of the latter instrument by D. McMullan and K. C. A. Smith. They were DC coupled to the spot on the display tube, which could thus be used to position the probe for analysis at a selected point. The new microprobe was working within two years and the first results were published (Cosslett and Duncumb, 1956; Duncumb, 1956). The metallurgical examples, particularly the identification of impurities (Ca, Mn, Ni) in beryllium foil, attracted the attention of J. W. Menter in the Tube Investments Research Laboratory near Cambridge. He had close contacts with the Cavendish, having taken his PhD in the Physics and Chemistry of Surfaces Laboratory under F. P. Bowden. So it came about that 8 August 1957 was a crucial date in the life-history of microprobe analysis. On that day D. A. Melford of Tube Investments obtained (with Duncumb) analyses of some steel specimens that convinced him of the unique capabilities of the scanning X-ray analyser for localized composition studies. Melford convinced Menter that their laboratory must have such an instrument, and Menter engaged Duncumb, initially as a consultant, to work with Melford on constructing a version specially designed for metallurgical investigations. Drawings were ready by the end of the year, assembly began in July 1958 and the instrument was in operation by December (Duncumb and Melford, 1959).

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D. Manufacture The story of how their machine was taken up by the CIC as the basis of the first commercial scanning microprobe analyser has been told elsewhere as a case study in innovation (Jervis, 1971/72) [see the following chapters and Appendix III, eds]. A vital link was that the Tube Investments research director (T. P. Hughes) and the newly appointed managing director of the Cambridge Instrument Company (H. C. Pritchard) were old friends, having previously worked in the same government laboratory. A prototype ‘Microscan’ was completed by CIC and delivered to the Atomic Weapons Research Establishment by May 1960. A three-cornered agreement was made between CIC, Tube Investments and the Cavendish, covering royalty payments and future development. It provided a fund that was extremely valuable to us over the next decade, for financing research and paying assistants, beyond what DSIR were willing to support. The Microscan hit a market much readier for it than anyone had supposed. My own rough estimate had been a sale of 20 or so over a period of years. In fact 78 of this first model were sold in five years, of which 70% were exported, mainly to Germany and the United States. The rival Metropolitan-Vickers microanalyser had a much smaller sale and was soon dropped from production. Jervis discusses in some detail why the CIC was more successful and pinpoints two factors. First, the Tube Investments prototype was designed in close collaboration with the metallurgist user, whereas AEI’s Aldermaston team were electron-optical experts working to a general specification drawn up by a committee of the interested research associations. Second, the Metropolitan-Vickers production model was separately engineered in Manchester, on the basis of drawings supplied from AEI-Aldermaston 200 miles away, and apparently with little man-to-man contact. Duncumb and Melford were in regular consultation with the engineering staV at CIC, 10 miles down the road. In operation, the display of a scanning image was an important advantage in their instrument, at first overlooked by AEI. A similar moral can be drawn from the history of the scanning electron microscope, which Jervis (1971/72) has also investigated. Oatley had made an agreement with AEI in 1956 for production of the surface scanning model developed at the Engineering Department in Cambridge, but AEI failed to put major eVort into a prototype instrument or to sound the market. In 1961 Oatley turned to the CIC, which had gained experience of the techniques involved (and commercial courage) from the successful production of microanalysers. A Microscan was readily converted into a scanning microscope and shown at the Physics Exhibition in January 1962. Its sales soon

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outstripped those of the Microscan itself: over 100 were sold by the end of 1967 and a similar number in the single year 1968. The basic reason for CIC’s success compared with AEI was again one of personal relations. AEI simply bought a set of plans from Oatley and made little or no attempt to consult his engineer (K. C. A. Smith) who knew most about the scanning microscope: another instance of the Not-Invented-Here prejudice.2 On the other hand, not only was there all along close contact between Oatley’s laboratory and the CIC, but the latter took on to its staV his latest PhD student, A. D. G. Stewart, who had the know-how and dynamism to see the prototype into production. It is a significant comment on the innovation–manufacture problem to recall that all the major developments in electron-optical instrumentation, practically without exception and in all countries, have come from university laboratories and not from industry. The original electron microscope in Germany (Ruska), the first EMs in North America (Hillier) and in Holland (Le Poole), the original microprobe analyser in France (Castaing), as well as the pioneering work described above (Duncumb, McMullan, Smith, Stewart) were all PhD projects. The scanning transmission EM was first investigated by Duncumb in 1956, although finally made actual by Crewe at the University of Chicago. The first HVEM to produce results was built at the Technical University of Delft in 1947 and the first such in England at the Cavendish Laboratory in 1963–65. The value of these academic innovations to industry, as well as to science and technology generally, requires no underlining. E. Further Developments The microanalyser had been successfully developed by Duncumb and Melford for metallurgical purposes, but it had a serious limitation in its range of application. For elements with Z < 15, characteristic X-ray emission is of long wavelength and low intensity, resulting in poor detection eYciency in the usual spectrometer system. When Ray Dolby arrived from Stanford in Autumn 1957, he fortunately took to this problem rather than to a research opening in Oatley’s laboratory. He involved himself at once with methods of deconvoluting mixed signals, of which he had some experience already (and much more since). He devised and built a network system comprising a wide-angle proportional counter followed by three identical pulse analyser channels (Dolby and Cosslett, 1959), with which he was able to display 2

Agar’s later assessment (Chapter 4.1B) places a rather different complexion on the reasons for AEI’s lack of success. (eds.)

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separately the Mg, Al and Si distributions in a complex specimen, and later to discriminate between Be, C and O. Dolby was one of the most inventive research students I ever had, being fertile in ideas and adept at experimental realization, as his later career bears witness. One idea of his had a considerable influence on the development of electron guns. David Swift was investigating the properties of the newly invented point cathode for use in a conventional triode gun (Swift and Nixon, 1961). He came up against the problem of beam stability, vital in electron probe microanalysis. Dolby made a mathematical analysis of negative feedback action and together they devised an improved regulating circuit that incorporated a high-amplification triode and a large cathode-bias resistor (Dolby and Swift, 1960). The requirements of mineralogy and geology were again of a diVerent nature, owing to the multiplicity of components in most specimens. J. V. P. Long became interested in these applications through enquiries from the Mineralogy Department in Cambridge and, after preliminary experiments, he went on to design a more elaborate instrument that was more compact and had an optical microscope built into the side. It could be used on transparent as well as opaque specimens, but primarily on petrological sections (Agrell and Long, 1959). The initial results were so promising that Long soon moved over to the Mineralogy Department, where he constructed a more elaborate instrument with the column horizontal, which was developed into the Geoscan by the CIC. Biological applications also engaged our attention. They involve two special problems: risk of losing substances in the preparation procedure, and damage in the electron beam. The need for thorough investigation of these eVects became apparent in the first attempts to analyse biological material (Boyde and Switsur, 1962). This was done by Roy Switsur in a new scanning microprobe, which he designed and built with a grant from the Medical Research Council. Materials examined included foreign particles in lung sections, tissue from hips containing steel implants, iron in rat liver, rat kidney sections stained for enzymatic activity, and hard tissue such as tooth and bone. When Switsur left in 1961, the biological research was taken over by Ted Hall, who joined us from the Sloan-Kettering Institute in the following July. From then until the present he has devoted himself to establishing microprobe analysis as a reliable technique in biology and medicine. Early on in this period we realized the need to make these X-ray methods known more widely. In 1956 we organized the first international symposium on ‘X-ray Microscopy and Microradiography’, which attracted some 65 contributions. Similar and even larger symposia were held in Stockholm in 1959, at Stanford University in 1962, and every three years since under the title ‘X-ray Optics and Microanalysis’. Also, to meet a need

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for instruction in specialized techniques of preparation, observation and treatment of data, we held Summer Schools on the subject in 1961 and 1963.

References Agrell, S. O., and Long, J. V. P. (1959). The application of the scanning X-ray microanalyser to mineralogy. Stockholm, 1959 391–400. Boyde, A., and Switsur, V. R. (1962). Problems associated with the preparation of biological specimens for microanalysis. Stanford, 1962 499–516. Burton, E. F., and Kohl, W. H. (1942). ‘The Electron Microscope.’ Princeton, NJ: Van Nostrand-Rheinhold. Castaing, R., and Guinier, A. (1949). Application of electron probes to metallurgical microanalysis. Delft, 1949 60–63. Cosslett, V. E., and Duncumb, P. (1956). Microanalysis by a flying spot X-ray method. Nature 177, 1172–1173. Cosslett, V. E., and Nixon, W. C. (1960). ‘X-ray Microscopy.’ Cambridge: Cambridge University Press. Cosslett, V. E., and Pearson, H. E. (1954). An improved X-ray shadow projection microscope. J. Sci. Instrum. 31, 255–257. Dolby, R. M., and Cosslett, V. E. (1959). A spectrometer system for long wavelength X-ray emission microanalysis. Stockholm, 1959 351–357. Dolby, R. M., and Swift, D. W. (1960). The bias network and electron gun stability. Delft, 1960 114–118. Duncumb, P. (1956). Microanalysis with a scanning X-ray microscope. Cambridge, 1956 617–622. Duncumb, P., and Cosslett, V. E. (1956). A scanning microscope for X-ray emission pictures. Cambridge, 1956 374–380. Duncumb, P., and Melford, D. A. (1959). Design considerations of an X-ray scanning microanalyser used mainly for metallurgical applications. Stockholm, 1959 358–364. Ehrenberg, W., and Spear, W. E. (1951). An electrostatic focusing system and its application to a fine focus X-ray tube. Proc. Phys. Soc. B 64, 67–75. Jervis, P. (1971/72). Innovation in electron-optical instruments—two British case histories. Res. Policy 1, 174–207. Long, J. V. P., and Cosslett, V. E. (1956). Some methods of X-ray microchemical analysis. Cambridge, 1956 435–442. Nixon, W. C. (1952). ‘An Experimental X-ray Shadow Microscope.’ PhD Dissertation, University of Cambridge. Nixon, W. C. (1955). High resolution X-ray projection microscopy. Proc. R. Soc. A 232, 475–485. Swift, D. W., and Nixon, W. C. (1960). The behaviour of a point cathode in a triode electron gun. Delft, 1960 69–72.

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3.2A The Contributions of W. C. Nixon and J. V. P. Long to X-ray Microscopy and Microanalysis: Introduction P. DUNCUMB Formerly at: Cavendish Laboratory, University of Cambridge and Tube Investments Research Laboratories, Hinxton Hall, Cambridge

The Editors have asked me to provide an introduction to the two following Chapters (3.2B and 3.2C) by W. C. Nixon (1993) and J. V. P. Long (1993), which are reprinted from the ‘Cosslett Symposium’ held at the 13th International Conference on X-ray Optics and Microanalysis (ICXOM), Manchester, 1992. Bill Nixon and the late Jim Long were part of the backbone of Ellis Cosslett’s group in the Cavendish Laboratory at the time of the first ICXOM, held in Cambridge in 1956. Both made major contributions to the development of electron microprobe techniques, and it is entirely appropriate that their view of the scene in Cambridge should be reproduced here. I also oVered a paper at the 1992 Symposium. Although our brief was to focus on Ellis Cosslett’s leadership in the Cavendish, we had all had close links with Charles Oatley and the Engineering Department, as described elsewhere in this volume, and the following chapters reinforce the spirit of cooperation that prevailed during the period of rapid growth from 1950 to 1990. Bill Nixon relates the history of the X-ray projection microscope, which depends on the creation of a point source of X-rays of high intensity. This focused attention on the benefits of using magnetic over electrostatic lenses and provided an essential complement both to Castaing’s work in microanalysis and to later work on the scanning electron microscope in the Engineering Department. It also showed the way to X-ray microscopy in 3D, and greatly enhanced the possibility of revealing the distribution of a chosen element over a sample surface in 2D—the subject of my own PhD project. Jim Long, in his early period in the Cavendish, explored the possibilities of microanalysis by X-ray absorption and fluorescence, eventually identifying the immense potential for the application of X-ray emission microanalysis to mineralogy. Fortunately, his interests included new techniques in X-ray 251 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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counting, and his innovations in X-ray proportional counters and crystal spectrometry went far beyond their applications to mineralogy. It is no exaggeration to suggest that his early work with the gas flow proportional counter (extended later by Ray Dolby) set the scene for energy-dispersive analysis as it is widely practised in the SEM today. In 1959 Jim moved from the Cavendish to the Department of Mineralogy and Petrology, and Bill to the Engineering Department. Both were natural teachers, as several generations of research students will attest. I include myself in those who benefited, both as a research student and much later when I returned to Cambridge to pick up the threads after a career in industry. The chapters that follow illustrate these points and complement other views in this volume of the developments that took place in Cambridge up to the 1990s.

References Long, J. V. P. (1993). Microanalysis, in ‘X-Ray Optics and Microanalysis 1992’, Inst. Phys. Conf. Ser. 130, edited by P. B. Kenway et al. Bristol: IOP Publishing Ltd. Nixon, W. (1993). X-ray projection microscopy and transmission electron microscopy, in ‘X-Ray Optics and Microanalysis 1992’, Inst. Phys. Conf. Ser. 130, edited by P. B. Kenway et al. Bristol: IOP Publishing Ltd.

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3.2B* X-Ray Projection Microscopyy W. C. NIXON Peterhouse, Cambridge University, U.K.

INTRODUCTION This contribution to the Cosslett Symposium is written from a personal viewpoint. We first met in October 1949 in Cambridge when I commenced research in the Electron Microscopy Group of the Cavendish Laboratory. We had very diVerent backgrounds prior to our meeting and these earlier experiences have a bearing on our work from October 1949 onwards. I was 12 years old at the outbreak of the Second World War in September 1939, with my home in Toronto, Canada. I entered Queen’s University, Kingston, Ontario, Canada at the age of 17, in September 1944, to study Physics and Mathematics. On my eighteenth birthday I interrupted my University studies and joined the British Royal Navy Fleet Air Arm as a Pilot Cadet and travelled to England. Flying training occurred over Salisbury Plain, the Malvern Hills and Stonehenge with all the excitement of solo flying in a Tiger Moth. I also attended a 2-week course in Balliol College, Oxford, which turned my attention to the possibility of graduate research in England. At the end of the war, I took up my interrupted University course again in September 1945 and with accelerated teaching graduated (First Class) in May 1948 (and married in June 1948). It was decided that research in Physics at the Cavendish Laboratory in Cambridge would be a suitable next step but that a 1 year Master’s Degree in Physics with a written dissertation would be good preparation. I stayed on for this further year while negotiations were carried out with the Cavendish and with Trinity College. In addition, full funding was raised in Canada to cover the 3 years from 1949 to 1952 in Cambridge for all fees and living and travel costs. My wife and I travelled from Canada to Cambridge to arrive at the beginning of October 1949, to start as a Research Student in the Cavendish Laboratory and as a Graduate Student of Trinity College, while already married, a war veteran of flying in the Royal Navy Fleet Air Arm in England, with a Canadian BA and MA and 22 years old.

*Edited extract reprinted from ‘X-ray Optics and Microanalysis 1992’ (Inst. Phys. Conf. Ser. 130). y Read by the author to the Cosslett Symposium at IXCOM 13, UMIST, Manchester, 31 August–4 September, 1992.

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The background of Ellis Cosslett is fairly well known. Tom Mulvey has introduced this Symposium in the previous paper with an outline of Cosslett’s research career and I will only record a few additional facts. Ellis Cosslett was 19 years older than I and he was 41 years of age when we met in October 1949. He had received his Ph.D. from Bristol University in 1932, following work in Berlin. He was fluent in German with contacts in Germany and Austria. He had married in 1936; the marriage was dissolved in 1939. He married Anna in 1940. He spent the war years in teaching and research in Oxford and moved to the Cavendish Laboratory, Cambridge, as an ICI Fellow in 1946. He became a University Lecturer in Physics in Cambridge in October 1949 at the same time as that of our first meeting.

X-RAY PROJECTION MICROSCOPY IN THE 1950s The University of Toronto had a reputation in both low-temperature studies at liquid helium temperatures and in transmission electron microscopy, due to links with Britain and Europe through the movement in both directions of Faculty members. I considered both areas and chose electron microscopy—the fact that I brought all my personal funding with me, meant that I could also choose within electron microscopy. Ellis Cosslett also oVered the use of electrostatic lenses for an attempt at correcting spherical aberration using Scherzer’s methods. I declined that topic which was later taken up by Jack Burfoot. The X-ray projection microscope was included in that Bible for work at the time by Zworykin et al. published in 1945. I had purchased this book in Canada and read it through as background for work in Cambridge. Hillier was one of the many research students at the University of Toronto in the 1930s and early 1940s working with Professor Burton, who had visited Ruska in Germany in the early 1930s. Burton started a big electron microscopy activity with Hillier, Hall, Ellis and several others who made their careers in the field. Hillier had joined the RCA Group and made the EMB, one version of which had been supplied to the Cavendish Laboratory under the Lend/Lease scheme to help the war eVort—maybe because it contained an electron gun! That instrument and an old 1939 Siemens ‘bathtub’ electron microscope were the only two instruments in Cosslett’s group when I arrived. The interest in X-ray microscopy had a short period in Oxford with Taylor but there were no publications, almost no equipment and no work had been done by anyone for at least a year before my arrival. Coming from Toronto I was surprised at how small the eVort was in Cambridge. However, I quickly convinced myself that there was a great deal that could be done and rapidly got on the job. As noted above I brought all my necessary financial resources with me—but there was one important condition. I would only get my Canadian Scholarship money for the second year by submitting a fully written up report on the first year’s work, and the same rule for the money for the third year only after a second year report. This necessity meant that

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I kept detailed notes and laboratory books from the very first day and produced full reports as required (Cosslett and Nixon, 1951). With a report also from the first two terms of my third year I could write my Ph.D. dissertation in the Easter vacation of the third year by using these three reports, submit the bound volume by the first week of the Easter Term, be examined and take the Ph.D. degree in the Senate House, all within less than 2 years and 9 months of starting in October 1949. Ellis was my University appointed Internal Examiner and Denis Gabor of Imperial College was my External Examiner, for the written report, the dissertation, and for the joint oral examination. Gabor immediately put me at my ease, as he was very satisfied with the work. He stressed that I had made a much bigger advance than I had claimed as there had been almost no previous real research results. This left time to discuss holography in all its aspects—Gabor’s first thoughts, early ideas, electron and light combinations, X-ray methods and much else—all in 1952, 40 years ago! Ellis was pleased that the examination had gone so well with such a distinguished external examiner. Afterwards, we discussed the future work that could be undertaken with my equipment, both by Norman Dyson who started in October 1952 and in the light of Castaing’s work in Paris on a static electron probe system using electrostatic lenses for X-ray microanalysis. I compared the current density for the same probe size, say one micrometre, available in Castaing’s instrument and my X-ray microscope with magnetic lenses. My magnetic lens system could produce some thousand times more current—one nanoampere with electrostatic lenses could be raised to one microampere with magnetic lenses. This electron optical experience and expertise was continued in all the subsequent instruments in the Cavendish Laboratory and by myself in the Engineering Department of Cambridge University after being appointed there as Assistant Director of Research from October 1959, 10 years after arriving in Cambridge. Having myself supervised 26 research students in Cambridge University I have seen the following phenomena at close hand, which I experienced and worked out for myself when Ellis Cosslett’s first research student in Cambridge. In the first year of research, the student thinks that his supervisor knows everything because if any topic is raised or new proposal suggested, then the supervisor already can quote a publication giving all the details of work already done. In the second year of research, the student thinks that his supervisor knows nothing because by that time the equipment is operating, new results are being produced by that equipment and the student is the first in the world to know. Therefore the supervisor is, at best, only the second! In the third year of research, the student then realizes that the supervisor does not know everything but the supervisor also knows much more than nothing and that indeed there is far more work to do than the student will be able to finish and write up in his dissertation given the time constraints. Ellis Cosslett fitted well into all three categories; with his world-wide contacts he always had a good reference or bit of relevant news to help out. By this time Ellis did not do any experimental work himself and so all the instrumental design, construction, testing and interpretation on the X-ray projection microscope was done by myself. We would discuss the results in detail and ensure that full and prompt

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publication occurred, both in subscription journals and by attending conferences. He spent a great deal of time, even in the early days, on the organization of Electron Microscopy on a national and then international basis with a legacy that persists today. After graduating in 1952 I worked for about 1 year with the American General Electric Company in Schenectady, New York State, building an X-ray microscope. We made both magnetic lens and electrostatic lens versions and the latter was marketed as the company had already produced a small electron microscope from the work of Simon Ramo. In the meantime, a copy of my X-ray microscope, with some slight mechanical changes for ease of use, was being constructed in the Cavendish Laboratory and shipped to Stanford University, California, by the summer of 1953. The agreement included an extended visit to Stanford by myself to commission the instrument and instruct those who would use it from then on in the Department of Physics. The two people involved were Albert Baez and Paul Kirkpatrick who had been investigating X-ray imaging and magnification with grazing incidence mirrors. The X-ray projection microscope could provide a suitable illuminating source for such a system. By this time other groups were interested in acquiring an X-ray microscope and I returned to the Cavendish Laboratory in October 1953, by which time Peter Duncumb had just started as a research student. Further improvements were made and other instruments built, some in association with Raymond Ely. In the autumn of 1955 I was sponsored by a National Science Foundation Fellowship (U.S.A.) to visit Ralph WyckoV at the National Institutes of Health, Bethesda, Maryland; Albert Baez at the University of Redlands, California; and Paul Kirkpatrick at Stanford University, California. By this time we had supplied both Ralph WyckoV and Albert Baez with X-ray microscopes and the one at Stanford had been used for 2 years (Nixon, 1955). The first X-Ray Microscopy and Microanalysis International Conference was organized for Cambridge in the summer of 1956; I returned to Cambridge for that meeting with publications from the work abroad. I had been elected to an Associated Electrical Industries Research Fellowship at the Cavendish Laboratory for 3 years from October 1956 and helped in the expansion of the EM Group to a very much larger size than the few members of October 1949. A natural break came in October 1959 when three of us moved on at the same time. I moved to the Cambridge University Engineering Department as Assistant Director of Research to Charles Oatley; Peter Duncumb and Jim Long also left Ellis Cosslett’s group at that time. Their papers follow later in these proceedings and give their own version of their time in the Cavendish. We all agreed that it was the end of an exciting era but also the beginning of new fields for the three of us and the next chapter for Ellis Cosslett. Shortly afterwards, in 1960, Cambridge University Press published the book X-Ray Microscopy by V. E. Cosslett and W. C. Nixon. This book summarized 10 years of work in Cambridge and in other laboratories and was the first in a field that is still active today.

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REFERENCES Cosslett, V. E. and Nixon, W. C., 1951. X-ray shadow microscope. Nature, 168, 4262. Cosslett, V. E. and Nixon, W. C., 1960. X-ray Microscopy, Cambridge University Press, Cambridge. Nixon, W. C., 1955. High-resolution X-ray projection microscopy. Proc. R. Soc., A, 232, pp. 475–485. Zworykin, V. K., Morton, G. A., Ramberg, E. G., Hillier, J., and Vance, A. W., 1945. Electron Optics and the Electron Microscope, Wiley and Sons, New York, pp. 112, 127, 268, 631 and 684.

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3.2C* Microanalysisy J. V. P. LONG Department of Earth Sciences, Bullard Laboratories, University of Cambridge

INTRODUCTION In attempting to set the contribution of the Cavendish to microanalysis into perspective it is appropriate, at a distance of 40 years, to recall the state of development of the relevant science and technology when work in Ellis Cosslett’s group was gathering momentum. Many techniques for investigating the solid state, which today are established fields of research, some with their own specialist journals and certainly all with individual acronyms, were in the post-war decade either as yet unidentified or, at best, still at the stage of being interesting physical phenomena whose application remained to be developed. Microscopy, whether with light, electrons or X-rays, was primarily concerned with imaging, with only the first dawn of the subsequent developments in localised analysis upon the horizon. There were already important exceptions to this generalisation, notably in the work of Engstro¨m (1946, 1957) and his school at the Karolinska Institute in Stockholm where densitometry of contact microradiographs was developed both for the determination of tissue dry-weight, and by using diVerential absorption across characteristic absorption jumps, for the measurement of specific elements such as calcium (Lindstro¨m, 1955, 1957). The outstanding portent of the future was, of course, Raymond Castaing’s ‘‘Sonde Electronique’’ described in his doctoral dissertation to the University of Paris (Castaing, 1951). However, it was perhaps inevitable that in the early years, many of those concerned with the physics of microanalysis did not appreciate the almost infinite range of analytical problems awaiting solution in the sciences concerned with materials, geology and biology, and conversely, that workers in those disciplines were, in general, less aware of the potential value of physical techniques than would be their present-day successors. Indeed, one of the remarkable features of the 1954 London E.M. Conference was the scant interest displayed by the assembled electron microscopists in the account of recent work presented by Castaing. After all, the ˚ : real imagination was transmission microscope of the time could resolve some 10 A required to envisage any application for a technique that could, at best, achieve 1 mm! *Reprinted from X-ray Optics and Microanalysis 1992 (Inst. Phys. Conf. Ser. 130), P.B. Kenway et al. eds. (1993). y Read by the author to the Cosslett Symposium at IXCOM 13, UMIST, Manchester, 31 August–4 September, 1992.

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Against this general background, the Electron-Microscope Section, and Ellis Cosslett in particular, can now be seen to have played an important role in bringing together the problems and the physical methods of solving them. The basic technology of the electron microprobe was identical with that of the X-ray microscope developed independently in the group, so that it was a natural step to encompass X-ray emission techniques along with those based on absorption. The long-standing tradition of the Cavendish in the field of instrument development and the association with the Cambridge Instrument Company were also to play an important part in the subsequent rapid emergence of commercial instruments based on work in the group.

ANALYSIS WITH THE COSSLETT–NIXON X-RAY SOURCE As a new recruit to the group in the Spring of 1954, I experienced the cultural shock of a transfer from the comparative aZuence and security of the Scientific Civil Service, to a somewhat impoverished but dynamic environment where exciting new developments occurred, it seemed, on a time-scale measured in days rather than the accustomed months. My primary task, to set up techniques for quantitative spectrometry of low-intensity beams of relatively soft X-rays, was initially directed towards diVerential absorption measurements for the study of mineralisation in biological tissue—the projection-microscopy equivalent of the contact techniques of Lindstro¨m in Stockholm. Early attempts to use proportional counters to select suitable energy bands from the continuum transmitted through small areas of the specimen proved abortive, partly due to the limited energy resolution of the detector, partly to the relative crudity of the single-channel recording technique, and not least to a failure to realise what might be achieved with appropriate techniques of spectrum deconvolution. Resort to Bragg crystal spectrometers, unfettered at the time by any knowledge of the work of Johann and Johansson two decades earlier, led to the development of the simple but eVective ‘‘semi-focusing X-ray spectrometer’’ (Long and Cosslett, 1957). This device was peculiarly suited to our needs and served well in a number of instruments including the Cambridge Instruments ‘Microscan I’, the commercial development of the Duncumb–Melford (1960) design. In connection with these early essays into Bragg spectrometry it is a pleasure to record the many occasions when we drew upon the experience and advice of E. F. Priestley of the Royal Armaments Research Establishment at Woolwich in the treatment of monochromators (Priestley, 1959). The point-projection absorption technique (Long, 1958) was eVective, if slow, and a long collaboration with Hans Ro¨ckert of the Department of Histology in Go¨teborg led to the publication of his doctoral thesis on the distribution of calcium in monkey teeth (Ro¨ckert, 1958). The close relationship of the physical sciences in Cambridge (which was partly instrumental in bringing about the fruitful collaboration of David Melford of the Tube Investments Research Laboratories with Peter Duncumb) led also to a link with Mineralogy and Petrology, specifically with Dr Stuart Agrell but

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also with Desmond McConnell among whose interests was the formation of hydrated calcium silicates in natural conditions closely approximating to those obtaining in setting cement. This thermally-sensitive material was not well suited to examination in the electron probe but appeared to be a good candidate for the absorption technique and a study of 10 mm diameter areas in the relatively coarse-textured natural material (Fig. 1) yielded useful data on the calcium concentration of the hydrogel (Long and McConnell, 1959). It is an interesting commentary on the advances of the intervening years to record the results of the same type of measurement made recently by Morrison and colleagues (1990) using synchrotron radiation, the resolution now being such that the texture and calcium distribution among filamentous growths in commercial cement may be studied in detail within an area equal to that within which single concentration measurements were originally performed (Fig. 2). The high specific loading (100 kW/mm2) of the Cosslett–Nixon tube was also exploited to produce fine, collimated beams to excite fluorescence in thin specimens, the secondary radiation being observed in transmission. Detection of potassium in single nerve cells was achieved with a very simple experimental arrangement (Long and Ro¨ckert, 1963) and an extrapolation of these results suggested that sensitivities of the order of 10 10 to 10 11 g would be possible in areas of the order of 10–20 mm diameter. The practical limitation was the low intensity of the signal and just as with the diVerential absorption method, a re-emergence of the technique had to await the development of synchrotron radiation sources and solid-state detectors which together now give mass sensitivities in the region of 10 15 g.

Fig. 1. Optical micrograph showing part of a cavity in which larnite, L, (b-Ca2SiO4) is hydrated in a topochemical reaction to produce a hydrated gel (G). Measurements of mass thickness of Ca show that the gel is Ca-rich equivalent to CaO:SiO2 ¼ 1.5 (Long and McConnell, 1959).

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Fig. 2. Images taken with the scanning transmission X-ray microscope using synchrotron radiation showing filamentous growths from a single cement grain after 48 hr hydration in a 5:1 mixture of water and cement. (a) Image taken at 353 eV where Ca absorbs strongly, (b) image at 355 eV, above Ca L-edge resonance (Morrison et al., 1990). Reproduced by courtesy of Dr Graeme Morrison.

ANALYSIS WITH THE ELECTRON PROBE Thus it was inevitable that the electron microprobe, with its versatility and ability to measure and map the distribution of a wide range of elements, should emerge as the dominant technique. It was not, however, without its deficiencies, notably in the analysis of light elements below Z ¼ 11 (Na) where rapidly-falling spectrometer eYciency, rising absortion corrections and uncertainties as to the applicability of accepted formulae for X-ray generation eYciency, all conspired to deter serious application. It was left to a new research student, Raymond Dolby, to point out that the high collection eYciency of the proportional counter could be exploited in the soft X-ray region, and used to oVset its poor energy discrimination (Fig. 3). Cosslett (1960) himself made an important contribution by demonstrating that the reputedly very low generation eYciency of the characteristic radiation of elements such as carbon was in fact only real when expressed in terms of energy conversion: because the photon energies were very small, the quantum eYciency was relatively high, with that for carbon K radiation no worse than 5% of the value for copper at the same overvoltage. With high count rates available at probe currents of only a few nanoamps, an important component of the success of Dolby’s network analysis technique (Dolby and Cosslett, 1960) for the on-line deconvolution of proportional counter spectra was the full analysis of the behaviour of these detectors at count rates in excess of 106/sec. (Dolby, 1961). By 1960, he had demonstrated scanning ˚ or 109 eV, this work being images taken with the K radiation of Be at 110 A continued by Ian Wardell (Wardell, 1965; Wardell and Cosslett, 1966). It is impossible here to summarise the full development of non-dispersive (or energy-dispersive) X-ray spectrometry, but in describing the Cambridge scene, it is

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Fig. 3. K spectra of Be, C and O (a) recorded with a gas proportional counter (Dolby, 1961), (b) recorded with a modern Si–Li detector with very low-noise FET (Nashashibi and White, 1990). Reproduced by courtesy of Oxford Instruments.

relevant to note that some twelve years later a very significant step forward in quantitative microprobe analysis resulted from work carried out in the Department of Mineralogy and Petrology by another research student, Peter Statham. We were exceptionally fortunate at that time in establishing contact with K. Kandiah and his collegues of A.E.R.E., Harwell, whose research in g-ray spectrometry and the design of pulse-processing circuitry (Kandiah, 1971; Kandiah et al., 1975) with accurately predictable behaviour at high counting rates, presented opportunities for the recording and rigorous deconvolution of the spectra from lithium-drifted silicon detectors that were not available in less well-constrained systems. A prototype of this equipment was installed at Cambridge on a small microprobe constructed originally by Stephen Reed and the author in 1961 and with Statham’s (1975, 1976) software, remained in almost continuous operation until 1987. During its lifetime, this equipment performed over 200,000 complete mineral analyses, mostly for 10 or more elements, the data appearing in more than 200 papers published by members of the department and by visiting scientists. On the rare occasions when the pulse-processor failed, it was placed in the boot of a car, driven to Harwell, restored to health by its designers, and was normally back in action within a day, the occasion having provided, as an incidental bonus, the opportunity for discussion between the two groups. It is also satisfying to record that this very informal collaboration played its part in the successful commercial development of energy-dispersive X-ray spectrometry by Link Systems Ltd., now the Microanalysis Group of Oxford Analytical Instruments. The scientific and commercial costeVectiveness of this operation, and many others like it, which were unimpeded by the benefits of ‘eVort returns’ and other trappings of modern business eYciency now being imposed upon the scientific community, is something upon which our accountant masters may care to ponder.

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THE SCIENCE It is relatively easy to set down the contribution of Cosslett’s group to the design of analytical instruments; much less so to pursue fully the science derived from the application of those instruments. In addition, we need to recognise the advances made in the physics of X-ray production, for while the basic mechanisms were qualitatively well understood, there remained serious gaps in the quantitative knowledge required for relating observed characteristic intensities excited in compound targets to those in pure elements. N. A. Dyson (1956) had already completed a study of the intensity and angular distribution of the continuum; so in turn, Martin Green (1963a,b) was given the task of making new experimental measurements of the eYciency of characteristic X-ray production and target absorption. Noel Thomas studied electron scattering (Cosslett and Thomas, 1964a,b, 1965), and later, H. Bishop (1966a) made measurements of electron backscattering coeYcients, work that was extended to low energies by Darlington and Cosslett (1972). Until 1962, the process of electron retardation and X-ray production in the target was described by models of the average behaviour of many electrons. Martin Green (1963c), using the new EDSAC II computer constructed in the University Mathematical Laboratory, was the first to employ Monte-Carlo techniques in which the intensity of the emergent X-rays was obtained by summing the contributions of a large number of electrons whose individual paths and energy losses were modelled in discrete steps by a computer programme. The results of these calculations could be compared with experimental data and used to explore the eVect of specific parameters; for example, Hugh Bishop (1966b) who extended Green’s work, was able to predict very accurately the variation of backscattering coeYcient with atomic number, and the technique was employed by many subsequent workers. It would be misleading to represent these studies as the main stream of the intense activity of the early 60s in evaluating correction procedures for microanalysis: the work of many individuals in the U.K., France, the U.S.A. and Japan are to be found in the proceedings of the 1962 and 1965 conferences in the present series and are also reviewed by Reed (1975). The Cambridge programme, extending over 20 years, was however particularly distinguished by a strong interaction of theory and measurement. A large body of new data was produced with equipment designed and assembled by individual students. The early application of microprobe analysis in Cambridge and the U.K. was greatly influenced by Duncumb’s scanning technique: the study of the microsegregation of copper and tin at the surface of mild steel, started in the Cavendish and pursued at the Tube-Investments Research Laboratories (Melford, 1960) illustrates very well the advantages of a two-dimensional presentation of the element distribution. Cosslett and Switsur (1963) also demonstrated the value of scanning images in revealing the distribution of metallic elements in biological tissue, while Stuart Agrell and the author (1960) applied the technique to the study of mineral intergrowths. There is no doubt too, that the Cambridge Instruments Microscan I owed much of its success to this facility. Scanning was also a standard feature of the Metropolitan Vickers microprobe (Page and Openshaw, 1960) which originated in the work of

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Tom Mulvey (1960). In France, on the other hand, the greater development of the static probe technique in conjunction with fully-focusing spectrometers is reflected in experiments yielding more quantitative data, for example, the analysis of intermetallic diVusion couples. The development of electron-probe analysis in the Department of Mineralogy and Petrology, where a collaboration between the author and the Cambridge Instrument Company led to the production of the ‘‘Geoscan’’ in 1965, was influenced by the fact that rocks, unlike metals, can be crushed and separated into their component minerals which, in turn, may be subjected to bulk analysis. As a result, petrologists had for decades been able to correlate distinctive optical properties with chemical composition. The probe provided the means of analysing minerals in situ and of studying variations in zoned minerals. The primary need, therefore, was for accurate multielement quantitative analysis and this theme, in addition to application to specific problems in petrology, dominated the research eVort for many years. Stephen Reed, who joined as a research student in 1961, has made important contributions in this area, while Rex Sweatman, a mature research student 3 years older than his supervisor, performed an invaluable service in the rigorous experimental evaluation of matrix correction procedures (Sweatman and Long, 1969). The analysis of biological material presented its own problems: owing to the very variable density of the matrix, it was diYcult to express the measured intensity of a characteristic line in terms of a meaningful concentration or mass thickness. Ted Hall, working first in the E.M. Section, and later in the Department of Zoology, solved this problem by recording also the intensity of the continuum and using this as a measure of the total mass of material in the excited volume (Hall and Gupta, 1983).

EXPERIMENTAL EQUIPMENT It is impossible to describe adequately the work of Cosslett’s group in microanalysis without reference to the development of experimental equipment, some part of which has been covered by other contributors to this symposium. In addition to the microanalysers constructed by Duncumb, Dolby, Switsur and the author, there were the absorption and fluorescence spectrometers and the special rigs set up by Green, Thomas and Bishop for the study of X-ray production and electron scattering, together with their supporting electronics. The apparatus itself bore the unmistakeable hallmarks of the Cavendish: sealing wax had had its day by the 1950s, but brass and soft solder were still in the ascendant. Such was the financial stringency and the need for economy, that durable secondhand components were much prized; my first microprobe used a lens inherited, I think, from Dyson’s equipment and subsequently remobilised in the mineralogical instrument where it was in use until 1987. Pirani gauge heads were constructed by a simple glass-blowing operation on 15-watt lamp bulbs; the need for potentiometric chart recorders was met by a combined design eVort by Peter Duncumb and myself, the result being suYciently successful for a total of 18 to be built by the main workshop for use in other groups. I still possess the worm-and-wheel mechanism of

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a gunsight from the 1914–1918 War that was adapted for the y-drive of a Bragg spectrometer. This continual evolution was only possible as a result of the assistance received from the Cavendish main workshop, the student’s workshop (research students were expected to become proficient in the mechanical arts) and above all, from the E.M. Section workshop. Here, every new challenge was greeted by Ron Pryor and his colleagues with a fund of experience and an enthusiasm which is a pleasure to recall.

CONCLUSION This very selective account of the development of microanalysis is inevitably biassed towards the work of the E.M. Section and its siblings in the Cambridge area. It has been in no sense the intention to ignore the extensive work in other laboratories but rather to present a picture of the wide-ranging research that Ellis Cosslett initiated. Central to his own interests was the underlying physics, as evinced by the long line of experimental studies on X-ray production and electron scattering in which he was certainly more directly involved than in microanalytical technique. An equally strong driving force, however, was his desire to see the application of the physics, particularly in the biological field. However, here his control was very flexible and for example, when I myself found it diYcult to enthuse about the analysis of biological material and turned to minerals, I received nothing but encouragement. Nevertheless, the objective was not abandoned: Roy Switsur was steered towards biology and Ted Hall, already experienced in the field, was brought into the group. In some ways it was a hard school, with the Cavendish sink-or-swim approach to new research students much in evidence: ‘‘. . .you might like to look at this problem; go away and think about it and meanwhile, build yourself an electron probe . . .’’. It says much for the confidence that V.E.C.—and Bill Nixon—engendered within the group that noone drowned.

REFERENCES Agrell, S. O. and Long, J. V. P., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 391–400. Bishop, H. E.., 1966a. In: Optique des Rayons X et Microanalyse. Castaing et al. pp. 153. Bishop, H. E., 1966b. ibid. pp. 112. Castaing, R., 1951. Ph.D. Thesis, University of Paris. Castaing, R., Deschamps, P., and Philibert, J. (eds.), 1966. Optique des Rayons X et Microanalyse, Herman, Paris. Cosslett, V. E., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 346–350. Cosslett, V. E. and Switsur, V. R., 1963. In: X-ray Optics and X-ray Microanalysis, Pattee, H. H., Cosslett, V. E. and Engstro¨m, A., (eds.). Academic Press, New York, pp. 507–512. Cosslett, V. E. and Thomas, R. N., 1964a. Br. J. appl. Phys., 15, 883–907. Cosslett, V. E. and Thomas, R. N., 1964b. Ibid. 15, 1283–1300.

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Cosslett, V. E. and Thomas, R. N., 1965. Ibid. 16, 779–796. Darlington, E. F. and Cosslett, V. E., 1972. J. Phys. D, 5, 1969–1981. Dolby, R. M., 1961. Ph.D. Thesis, University of Cambridge. Dolby, R. M. and Cosslett, V. E., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 351–357. Duncumb, P. and Melford, D. A., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 358–364. Dyson, N. A., 1956. Ph.D. Thesis, University of Cambridge. Engstro¨m, A., 1946. Acta Radiologica, (Suppl.) 63. Engstro¨m, A., 1957. In: X-ray Microscopy Microradiography, Cosslett, V. E., Engstro¨m, A. and Pattee, H. H., (eds.). Academic Press, New York, pp. 24–33. Green, M., 1963a. In: X-ray Optics and X-ray Microanalysis, Pattee, H. H., Cosslett, V. E. and Engstro¨m, A., (eds.). Academic Press, New York, pp. 185–192. Green, M., 1963b. Ibid. pp. 361–377. Green, M., 1963c. Proc. Phys. Soc., 83, 204–215. Hall, T. A. and Gupta, B. L., 1983. Quart. Rev. Biophys., 16 (3), 279. Kandiah, K., 1971. Nucl. Instrum. Meth., 95, 289–300. Kandiah, K., Smith, J. and White, G., 1975. In: Proc. 2nd ISPRA Nucl. Elec. Symp. (EUR 5370e) Commission of Eur. Comm. Directorate, Luxembourg, pp. 153–160. Lindstro¨m, B., 1955. Acta Radiologica (Suppl.) 125. Lindstro¨m, B., 1957. In: X-ray Microscopy Microradiography, Cosslett, V. E., Engstro¨m, A. and Pattee, H. H., (eds.). Academic Press, New York, pp. 443–447. Long, J. V. P., 1958. J. Sci. Instrum. 35, 323–329. Long, J. V. P. and Cosslett, V. E., 1957. In: X-ray Microscopy Microradiography, Cosslett, V. E., Engstro¨m, A. and Pattee, H. H., (eds.). Academic Press, New York, pp. 435–442. Long, J. V. P. and McConnell, J. D. C., 1959. Mineral. Mag. 32, 117–127. Long, J. V. P. and Ro¨ckert, H. O. E., 1963. In: X-ray Optics and X-ray Microanalysis, Pattee, H. H., Cosslett, V. E. and Engstro¨m, A., (eds.). Academic Press, New York, pp. 513–521. Melford, D. A., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 407–415. Morrison, G. R., Beswitherick, J. T., Browne, M. T., Burge, R. E., Cave, R. C., Charalambous, P. S., Duke, P. J., Foster, G. F., Hare, A. R., Michette, A. G., Morris, D., Potts, A. W., and Taguchi, T., 1990. In: Japan. Sci. Soc. Press Tokyo, Shinohara et al., (eds.). Springer-Verlag, Berlin. Mulvey, T., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 372–378. Nashashibit, T. and White, G., 1990. I.E.E.E. Trans on Nucl. Sci. 37, 452–456. Page, R. S. and Openshaw, I. K., 1960. In: X-ray Microscopy and Microanalysis, Engstro¨m, A., Cosslett, V. E. and Pattee, H. H., (eds.). Elsevier, Amsterdam, pp. 385–390. Pattee, H. H., Cosslett, V. E., and Engstro¨m, A. (eds.), 1963. X-ray Optics and X-ray Microanalysis, Academic Press, New York. Priestley, E. F., 1959. Br. J. appl. Phys., 10, 141–142. Reed, S. J. B., 1975. Electron Microprobe Analysis. Cambridge University Press. Ro¨ckert, H. O. E., 1958. Acta odont. scand. 16 (Suppl. 25). Statham, P. J., 1975. Ph.D. Thesis, University of Cambridge. Statham, P. J., 1976. X-ray Spectrometry, pp. 16–28. Sweatman, T. R. and Long, J. V. P., 1969. J. Petrol. 10, 332–379. Wardell, I. R. M., 1975. Ph.D. Thesis, University of Cambridge. Wardell, I. R. M. and Cosslett, V. E., 1976. In: The Electron Microprobe, McKinley, T. D., Heinrich, K. F. J. and Wittry, D. B., (eds.). Wiley, New York, p. 23.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

3.3A Development of the Scanning Electron Probe Microanalyser, 1953–1965 P. DUNCUMB Formerly at: Cavendish Laboratory, University of Cambridge, and Tube Investments Research Laboratories, Hinxton Hall, Cambridge

I. Background It was my brother Ken who stimulated my first interest in wireless and photography, which were his hobbies during the 1930s. When he came back from the war, I was old enough to use a soldering iron, and we built various pieces of equipment together, including radio receivers, amplifiers and even an oscilloscope. There were no kits but there was a plentiful supply of warsurplus gear, and some excellent designs to be had in magazines such as Wireless World, so that our ‘boYn room’ (as our mother called it) quickly became full. We were indeed fortunate to have somewhere we could spread out. It also aVorded space for me to carry out some photography, while my brother concentrated on movie making, using a 16 mm camera he had carried round Burma during the war. Thus electronics and imagery proceeded side by side, predisposing me to interest in the electron microscope, when I came to hear of it, and eventually to the scanning electron microprobe. After National Service in the RAF, my undergraduate years in Cambridge followed a predictable course of Natural Sciences with a Part 2 in Physics, and the time came to embark on a career. I was tempted by the wealth to be obtained from working in industry (the going rate for a new graduate was 500 per year) but eventually decided to try for a research studentship in the Cavendish. My interest was sparked by the work going in V. E. Cosslett’s Electron Microscope Group, but I was advised by an older research student that there was no further development to be done in electron microscopy and, if I joined Cosslett’s group, I would only be operating the instrument for other people. I didn’t know at the time that he was quite mistaken, so I applied instead to join the Radio Ionospherics Group headed by J. A. RatcliVe. Fortunately, as it happened, he turned me down when my exam results came through, as my Class 2.1 was not good enough. While I was 269 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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wondering what to do next, I was invited by Cosslett to come and see around his group properly, and immediately became convinced that this was what I wanted to do. A. Early Days in the Cavendish 1953–1954 The first year was one of learning—reading papers, looking over shoulders and talking to people. Some seeds were sown but no great results were achieved. However, I enjoyed it immensely. My ‘boYn room’ was now much bigger, and the Cavendish Main Store was well stocked with government surplus components on free issue. When I needed a 2-kV power supply, I had to make it, and in so doing learned a lot. This practice of building rather than buying continued through all my time in the Cavendish, even to the extent of designing and building (with Jim Long) a pen recorder, which later served the needs of several generations of research students. We were blessed with an excellent main workshop and could turn for local advice to experienced workshop and photographic assistants in the group—‘experienced’ not only technically but also in dealing with the assortment of research students who each thought their own work was the most important. It was a happy group and Ellis Cosslett’s wife Anna, who held a part-time position, contributed in no small measure to the pleasant atmosphere throughout. My first task was to learn something about X-rays. I had heard of the development of the X-ray projection microscope by Bill Nixon, but he was away for a few months in the United States and his instrument was in use by Norman Dyson for his work on the X-ray continuum. However, I was allowed to operate it and to try out an X-ray proportional counter in an attempt to measure the overall eYciency of X-ray production. This experiment was rather like a final-year undergraduate project and lasted four months, at the end of which I had added little to science but learned a lot of the frustrations of making other people’s equipment work—all useful experience. Then Bill Nixon returned to the group and built a new X-ray microscope, with which he produced some spectacular high-resolution stereo images—an encouragement to get on to imaging as I had originally intended. During this early period I also did a lot of reading and one of the first texts that Cosslett gave me was the thesis of Raymond Castaing, which had been published in Paris the previous year. My schoolboy French was only just up to the task but I quickly came to think that little remained to be done in microprobe analysis. However, one idea that did not seem to be there was the notion that the electron probe could be scanned over the sample, so that the surface could be imaged in terms of the characteristic X-ray emission of a selected element. In principle one could thus build up a picture of the surface in terms of the distribution of its constituent elements, and this was the idea

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that Cosslett suggested I should investigate as my main project. But first I had to learn what I could of the work going on in the Engineering Department just down the road. I had made the acquaintance of Mr Oatley (as he was then known)—a person I held in awe—and his research student Dennis McMullan soon after I first joined the Cavendish. Earlier in the year Dennis had received his PhD for his work on the first scanning electron microscope and he was soon to leave Cambridge, but I remember clearly his enthusiasm for the almost three-dimensional images he showed me of etch pits in aluminium, and the apparatus itself, with its billiard-cue-like control for electrostatic focusing. One point of similarity with my own project was, of course, the scanning principle, but another was the fundamental limit to image quality set by the quantum intensity, in his case of secondary electrons and in my case of detected X-rays. The low eYciency of X-ray production meant that the Xray scanning microanalyser, as I called it at that time, could never approach the resolution of the SEM, and the X-ray resolution was in any case limited to about 1 mm by the penetration of electrons into the sample. Nevertheless, the ability to discriminate between diVerent elements in the periodic table seemed to be a point in its favour and fortunately the engineers had no interest in X-ray physics. So there were no demarcation disputes and the way was clear for friendly relations with Engineering, which continued with Dennis’s successor Ken Smith and others who followed him. With Castaing’s (1951) work on the static microprobe, Nixon’s (1952) on the X-ray projection microscope and McMullan’s (1953) on the scanning electron microscope, I could draw on a rich fund of experience to start on the X-ray scanning microanalyser (see Fig. 1) and I began this task in April 1954. Fortunately, at that time an electron microscope came free in the group: an RCA EMB that came over in late 1942 under the Lend-Lease scheme, and was now obsolete. Cosslett suggested I should use it as a basis for the new instrument, rebuilding it as a two-lens probe and adding suitable detection and scanning electronics. Before doing this, however, I tried it out as a transmission electron microscope, and then, by inserting a copper foil into the projection lens, as an X-ray projection microscope. It was then time to cut oV the lower part of the column and support the gun and remaining two lenses on a ‘Dexion’ framework, so that I could introduce a counter under the sample to detect the transmitted radiation. Meanwhile I was planning a new lens to allow the X-rays from solid samples to be collected in the backward hemisphere, but I had not yet taken thought for the spectrometer that would be needed to analyse the emission. My needs in this direction were greatly helped by the arrival in April 1954 of Jim Long. He came from the Chemical Research Laboratory at Teddington with experience in radioactive materials and measurement techniques,

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Figure 1. Interactions in Cambridge between the Engineering Department, the Cavendish, Mineralogy, Tube Investments Research Laboratories and the Cambridge Instrument Company, tracing the inspiration of the scanning electron microscope and X-ray scanning microanalyser through to commercial realization.

and became interested initially in absorption edge spectroscopy using the X-ray microscope, moving later to the microprobe analysis of minerals. He was to be of great help throughout my PhD and beyond, but the immediate problem he solved was that of the X-ray detector. While we could make sealed-oV counters and have them filled by a skilled assistant in the Cavendish, they did not seem to last very long, and there came a limit on the number of times one could repeat this request. Then we read about a gas-flow proportional counter built by Uli Arndt (Arndt et al., 1954) at the Royal Institution and paid him a visit in May 1954. He explained how easy it was to prevent oxygen contamination of the gas by simply keeping a flow of the argon–CO2 mixture passing slowly through the counter. The gas was cheap and vacuum sealing was no longer required. Back in the Cavendish, Jim promptly tried this out using a cocoa tin for the counter body and found it would count up to 104 quanta per second with excellent energy resolution (though the term ‘excellent’ was redefined 15 years later with the advent of the Si-Li detector). While Jim was putting the flow counter to good use for his own work, I concentrated on the scanning electronics needed for mine, leaving Jim to sort out the principles of energy-dispersive analysis, which he freely passed on to me later.

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II. Towards the First X-ray Scanning Pictures, 1955–1956 The first task was to get some sort of scanning image from the RCA before the new lens was ready. I talked to Ken Smith, who by this time had succeeded McMullan, and learned how to build a double-deflection system (Smith and Oatley, 1955). This allowed the deflection coils to be positioned in the space between the condenser and objective, away from the bulk of the objective, as shown schematically in Fig. 2. Ken was using electrostatic lenses but the double-deflection system was even more necessary with the extra bulk of a magnetic objective. The surprise was that as well as focusing the beam the magnetic lens rotated the direction of scan, so that the beam  deflection after the lens was about 90 away from the deflection before it. All this was predictable from the main source of wisdom on lens design at that time—the paper by Liebmann and Grad (1951) at the AEI Laboratories at Aldermaston Court—but I had not yet made personal contacts there. Another highly useful reference for the electron-optic designer was the paper by Haine and Einstein (1952), also at AEI, on the design and operation of the electron gun. Between them, all the data existed for the design of an eVective two-lens system whose performance was limited only by the lifetime of the tungsten filament and the aberration of the electron lens, which in turn

Figure 2. Schematic diagram of the first X-ray scanning microanalyser, showing the double-deflection system copied from Smith’s SEM. With this system it was possible to scan an area of 0.4 mm square with a focal length of only 4 mm, delivering suYcient current to drive the spectrometer to its maximum of 104 counts/s.

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could be calculated from the lens geometry. These were remarkable pieces of work at a time before digital computers were widely available, depending on arguments from first principles supplemented by experiments with a resistance analogue network. My first thought towards the scanning electronics was to copy the circuitry that Ken Smith by that time had developed for the SEM, using the same exradar display tubes. He was quite agreeable to this, but when I asked Oatley if this would be in order, he discouraged me, saying that I should learn much more building it myself, and in any case the supply of radar tubes was running out. There was also a technical reason for doing so: for point analysis I should need to stop the beam at a precise point in the scan and Ken’s circuits were not immediately suitable for this. So I set to with ‘Schmidt triggers’ and ‘cathode followers’ and, with the help of an electronics assistant, Chris Jackson, built a 19-inch rack full of electronics including two display tubes, which lasted me until I left four years later. The display area was variable from the full screen down through postage-stamp size to a selected point, which could be set in position on the sample surface from the afterglow of the screen. The completed instrument is shown in Fig. 3. All this took time and I was desperate to obtain some sort of scanning pictures using X-rays, which no one had yet actually done. So I borrowed a scintillation detector to record the total X-ray intensity, giving up the possibility of energy discrimination until I had established the beam conditions needed to form an image of reasonable quality. The first X-ray scanning picture was obtained on 19 August 1955, showing copper and silver test grids in transmission. While the 6-mm grid bars were visible, the picture quality was very noisy—much worse than could be explained by the quantum noise criterion set out by Dennis McMullan and developed further by Ken Smith. I also had a suspicion it was slowly getting worse. It was several weeks before I realized that the sodium iodide scintillator had been degraded by ingress of moisture before I started and I was watching a further slow deterioration. Re-preparing it made a wondrous improvement; the noise criterion was quickly verified and it seemed that reasonable images should be obtainable with intensities of a few thousand counts per second, slow enough to change over to the proportional counter and to try to make use of its energy discrimination properties. It was also time to concentrate on the new lens and get to work on some real samples. By January 1956, Jim Long had finished with his first proportional counter and kindly presented it to me for some experiments on energy discrimination in the transmission mode. These showed clearly the ability to image crossed copper and silver grids, using Cu K and Ag L radiations, which were far enough apart in energy to be completely separated even by a gas counter. The results were highly encouraging. I think it was Bill Nixon who suggested

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Figure 3. RCA electron microscope converted as an X-ray scanning microanalyser, with the display electronics to the left of the column and X-ray spectrometer to the right. Residual magnetism in the iron-framed chair allowed the operator to trim the probe current by leaning gently backwards, a benefit that was not part of the intended design.

combining the separate images photographically in diVerent colours to give a single image, and of course the colours could be as bright as one chose. I chose red for copper and green for silver and, after some prolonged work in the darkroom, I showed the image at the first International X-ray Symposium in Cambridge later in the year (Duncumb and Cosslett, 1956). The slide I used is reproduced in Fig. 4. Nowadays the technique is known as colour mapping and the colour combination is carried out at the click of a mouse.

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Figure 4. X-ray colour map of crossed silver and copper grids, obtained by superimposing through red and green filters the images taken with Cu K and Ag L radiation, respectively. In places the X-ray continuum from the silver has overflowed into the copper channel giving yellow. The spacing of the silver grid is about 32 mm. (See Colour Insert.)

But these were not real samples. The first was a piece of beryllium foil containing embedded particles rich in manganese, nickel and calcium. Again these were easily separated with the proportional counter and shone like stars in the firmament in the colour picture. Cosslett decided the results should be published and we wrote a joint letter to Nature (Cosslett and Duncumb, 1956), acknowledging the help received from Bill Nixon and Jim Long. By this time, the new lens was in operation and I could look at solid surfaces, collecting X-rays in the backward hemisphere. This immediately brought about the need for better energy discrimination using a crystal spectrometer. Little was known of the intensities to be expected and Jim suggested that we should try mounting a piece of lithium fluoride in the

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beam to see how much intensity it reflected. The best source of this was said to be Professor R. V. Jones at Aberdeen University (of Secret War fame) and we were flattered and delighted when a package arrived from him for our researches. There was enough to cleave several pieces for each of us and I mounted one on a simple goniometer stage in front of the counter window, making a very compact spectrometer. We had also heard from Eric Priestley at Fort Halstead that lithium fluoride was easy to curve plastically into a focusing configuration, using a tennis ball to squeeze a flake on to a heated former. This we did for our respective instruments, again saving development eVort by a combined approach. The result for me was a ‘semifocusing’ spectrometer, which served me well for the rest of my time in the Cavendish. There was another reason for not aiming for geometric precision in the spectrometer focusing: with the beam scanning up to 0.3 mm oV-axis on the sample, the reflected intensity would fall because of spectrometer defocusing. For a long time the French were opposed to any form of beam scanning because of this eVect, and the Cameca instrument deriving from Castaing’s work was only equipped to scan the sample mechanically under the beam. This was slow, to say the least, though it did allow users to take full advantage of the fully focusing spectrometers which were a feature of this instrument. Thus, their work concentrated initially on quantitative point analysis, well illustrated by Castaing and co-workers at ONERA and by Philibert at IRSID (Castaing et al., 1957). It was some years before the Cameca instrument was fully equipped for both purposes. Meanwhile in the Cavendish, I concentrated on qualitative scanning images with occasional semi-quantitative analysis to show that it could be done. By the spring of 1957 I had looked at a variety of samples, including the lung of a tin miner (finding particles of tin!), an archaeological whetstone (finding neither iron nor bronze), iron carbides extracted from steel (finding particles down to 1 mm), and chromium plating on steel (finding cracks and variation in thickness). All these results were unsurprising but were just about enough to illustrate my thesis, which was due in September, and I resolved to start writing without further delay. Then in August there occurred one of those chance events that altered the course of my universe.

III. Useful Applications at Last, 1957–1959 During the summer there had been a steady stream of visitors to see the instrument. Among them was Dr. J. W. Menter, who was a key figure in setting up the new Research Laboratories of Tube Investments (TI) at

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Hinxton Hall near Cambridge. As an electron microscopist, he quickly assimilated the capability of the instrument and made the connection with a problem currently engaging TI’s steelworks and tube mills. As the tube was pierced it was, under certain conditions, developing tears in the surface—a phenomenon known as ‘hot shortness’ (as in pastry)—resulting in the production of a great deal of scrap. The scientist to whom this problem had been given was David Melford, whom I had known slightly as a research student, and who was deputed to come and see what the ‘X-ray scanning microanalyser’ could show. He came at a time when I was in the thick of thesis writing and my first reaction was to say I was too busy. However, something made me pause as he turned to leave (I can see it now) and I agreed to spend one day looking at a sample in the hope of a quick solution. By a further streak of luck, this hope was fulfilled and my lab notebook for 7 August 1957 reads: Worked with Dr DA Melford TI on segregation of Cu, Ni in steel. Finished in one day.

We were able to obtain scanning images showing unexpectedly high enrichment of these elements, which was suYcient to produce a molten phase at the grain boundaries, causing them to lose their cohesive strength and fall apart. These images are well illustrated in the companion Chapter 3.4 in this volume by David Melford. Later, Melford worked out the science in much more detail and was able to establish limits on the levels of copper and tin to be allowed at the start of the steel-making process (Melford, 1962). This one days’ work had several consequences: 1. It allowed TI to improve the quality of their steel. 2. It convinced TI that it should build an instrument of its own. 3. This instrument stimulated Cambridge Instrument Company to enter the field of electron-optical instruments, building the Microscan as a direct copy of the TI instrument and later their scanning microscope, the Stereoscan. 4. These events resulted in 28 years of employment at TI for myself, and an interest in the subject that continues to this day. How does one reflect on the chance element in research? Perhaps to echo the belief that although you cannot plan for ‘lucky’ events themselves, you can create a climate in which they happen. In Cosslett’s group and in Oatley’s this environment existed, though we did not specially recognize it at the time, deriving from some optimum mixture of discipline with freethinking, of planning with reaction to events. Morning coVee, afternoon tea and weekly seminars all helped to achieve this balance. I was indeed fortunate to have been in this environment when the subject was young.

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All this took time to mature, however, and it was another two years before I joined TI in October 1959. Part of this time was filled with work directly resulting from that one day: analysing more samples, gaining credibility with Melford’s colleagues and then joining with him in designing the instrument TI decided to build (Duncumb and Melford, 1959). But the main reason for staying on in the Cavendish was to develop the scanning microanalyser further in the direction of finer and finer analyses; the oft-quoted spatial resolution of 1 mm was simply not good enough for many of the metallurgical and biological samples we encountered. There were two ways this could be done: (1) by reducing the beam voltage so that electrons penetrated a shorter distance into the sample; and (2) by preparing the sample in the form of a thin film so that electrons passed through it before spreading sideways. Unfortunately, both these techniques involved a serious loss of X-ray intensity and I needed to redesign the instrument to mitigate this loss. I started by exploring the eVect of cutting the beam voltage, first to 10 kV and then to 6 kV. The immediate eVect was to be unable to excite many of the K lines and some of the L lines, and those that were excited were of greatly reduced intensity. The crystal spectrometer was useful only for point analysis, and it was necessary to revert to the gas proportional counter, with its higher collection eYciency, for producing scanning pictures. One idea that occurred to me was to replace the wire anode with a hemispherically tipped needle, so that the whole counter could be pushed right into the lens close to the sample. This increased the X-ray collection eYciency by an order of magnitude, and gave some clear pictures of a test sample of tin needles in aluminum down to 0.3 mm in diameter (Duncumb, 1959). The energy discrimination was only slightly worse than that of the wire counter, but neither was able to resolve the many L and M lines that were generated in this energy region. Thinning the sample oVered more promise and set the scene for development of the scanning transmission electron microscope in the 1960s and 1970s. After some initial experiments with carbide particles mounted on a carbon film replica, I was able to show a resolution somewhat better than 0.3 mm, but at energies up to 30 keV (the highest I could go). Moreover, the scanning image was easily correlated (in principle) with the image obtained by normal transmission electron microscopy. I say ‘in principle,’ because I had to transfer the sample to the TEM next door to do so, and found it diYcult to locate the right grid square and field of view. But the principle was demonstrated; sub-micrometre particles could be analysed individually, and with suYcient intensity and choice of lines to be applicable to a wide range of problems, not the least in steel making and tube making within the interests of TI. Figure 5 illustrates the analysis of such particles, taken on the instrument built at TI to be described below (Duncumb, 1966).

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Figure 5. Ti Ka peaks from carbide particles of various sizes in a steel extraction replica from steel, viewed by electron transmission and back scatter. The scanning technique allowed individual particles less than 0.2 mm in diameter to be analysed by crystal spectrometer. Nowadays a silicon energy-dispersive detector would extend this size limit to well below 0.1 mm.

IV. The Next Generation of Instruments, 1960–1965 As a result of the Cavendish work, the idea was born of building an EMMA—a combined electron microscope and microanalyser—and it was this prospect that finally gained me the oVer of a job at the TI Research Laboratories, which I joined in October 1959. I had learned a lot in the Cavendish from my colleagues Jim Long and Bill Nixon (see Long, 1992; Nixon, 1992), from later research students Ray Dolby (later of noise-reduction fame), Martin Green and Hugh Bishop (both key to development of the theory), and particularly from Ellis Cosslett himself (see Cosslett, 1981). I had also made contacts overseas: first with Raymond Castaing, later with Jean Philibert at IRSID in Paris and with LaVerne Birks at NRL in Washington. Known to us all for his wisdom in electron optics was Tom Mulvey of the AEI Laboratories, a microprobe designer himself (Mulvey, 1959).

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Figure 6. Combined electron microscope microanalyser (EMMA) built at TI Research Laboratories 1960–62 and used for the analysis of carbide particles below 1 mm diameter (now at the Science Museum, London).

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Meetings on the microprobe in the early 1960s were memorable for vigorous debate. What was new and exciting to me at TI was the prospect of full-time assistance to take over the running of the newly constructed microprobe, while I was able to concentrate on the design of EMMA. The finished instrument is shown in Fig. 6. While this was being developed, I was able to interact with the metallurgists who would make use of the technique, without losing contact with the university. Over a period of two years I acquired three further assistants to provide support in instrument design and operation, electronics and X-ray physics and I learned how much more productive a small group could be than a single individual. This phase of the work drew to a close in about 1965, and is referred to in the present account as evidence of the seminal influence that the development of the scanning electron microscope and microprobe in the university had had on a large manufacturing company. By this time, over 80 other Microscans were in existence, paralleled by a number of Geoscans designed by Jim Long for mineralogical use and also made by Cambridge Instruments (see Fig. 1). Both Microscan and Geoscan were descended partly from the original Oatley–McMullan scanning microscope, and it was fitting when they were joined by the Stereoscan as a scanning microscope in its own right. Thereafter the connection fades. EMMA was marketed several years later by Associated Electrical Industries as EMMA-4, using as a basis one of their 100-kV transmission microscopes, but the scanning facility was largely replaced by the TEM image. To a large extent the role of the scanning microanalyser is now taken by the scanning microscope, equipped with an energy-dispersive detector for general use. Where the high resolution aVorded by the crystal spectrometer is essential, the preferred solution is for a multispectrometer (and very expensive) instrument that can only be justified by relatively few centres of excellence. So the SEM lives on, and will continue to do so for many years. Indeed, I have just learned that my old school has acquired one, and I have volunteered to make sure that they are properly aware of the history, and of some of the physics and engineering behind it. Charles Oatley is well remembered.

References Arndt, U. W., Coates, W. A., and Crathorn, A. R. (1954). A gas flow X-ray diVraction counter. Proc. Phys. Soc. B 67, 357–359. Castaing, R. (1951). ‘Application des sondes e´lectronique a` une me´thode d’analyse ponctuelle chimique et cristallographique.’ Dissertation, University of Paris, 1952.

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Castaing, R., Philibert, J., and Crussard, C. (1957). Electron probe microanalyzer and its application to ferrous metallurgy. J. Met. (April) 1, 389–394. Cosslett, V. E. (1981). The development of electron microscopy and related techniques at the Cavendish Laboratory, Cambridge, 1946–79 (Parts I and II). Contemp. Phys. 22, 3–36, 147–182. Cosslett, V. E., and Duncumb, P. (1956). Microanalysis by a flying spot X-ray method. Nature (London) 177, 1172. Duncumb, P. (1959). Improved resolution with the X-ray scanning microanalyser. Stockholm, 1959, pp. 365–371. Duncumb, P. (1966). Precipitation studies with EMMA—a combined electron microscope and X-ray microanalyser, in ‘The Electron Microprobe’ edited by T. D. McKinley, K. F. J. Heinrich, and D. B. Wittry. New York: Wiley, pp. 490–499. Duncumb, P., and Cosslett, V. E. (1956). A scanning microscope for X-ray emission pictures. Cambridge, 1956, pp. 374–380. Duncumb, P., and Melford, D. A. (1959). Design considerations of an X-ray scanning microanalyser used mainly for metallurgical applications. Stockholm, 1959, pp. 358–364. Haine, M. E., and Einstein, P. A. (1952). Characteristics of the hot cathode electron microscope gun. Br. J. Appl. Phys. 3, 40–47. Liebmann, G., and Grad, E. M. (1951). Imaging properties of a series of magnetic electron lenses. Proc. Phys. Soc. B 64, 956–971. Long, J. V. P. (1992). Microanalysis. (Extract from Cosslett Symposium: See Chapter 3.2C, this volume). McMullan, D. (1953). An improved scanning electron microscope for opaque specimens. Proc. IEE 100, Part II, 245–259. Melford, D. A. (1962). A study of segregation at grain boundaries in mild steel by means of electron probe microanalysis. Stanford, 1962, pp. 577–589. Mulvey, T. (1959). A new X-ray microanalyser. Stockholm, 1959, pp. 372–378. Nixon, W. C. (1952). ‘An experimental x-ray shadow microscope.’ PhD Dissertation, University of Cambridge. Nixon, W. C. (1992). X-ray projection Microscopy. (Extract from Cosslett Symposium: See Chapter 3.2B, this volume). Smith, K. C. A., and Oatley, C. W. (1955). The scanning electron microscope and its fields of application. Br. J. Appl. Phys. 6, 391–399.

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3.3B* Micro-Analysis by a Flying-Spot X-Ray Methody V. E. COSSLETT AND P. DUNCUMB Cavendish Laboratory, University of Cambridge

The point-by-point investigation of a surface by analysis of the characteristic X-ray line emission has been initiated by Castaing1. He obtained an electron spot of the order of 1 micron in diameter with an electrostatic lens system and moved the specimen under the fixed spot; the point examined was identified by means of an optical viewing system. Analysis is greatly facilitated if the electron spot is scanned across the specimen and if a counter is used to collect part of the emitted X-rays. The signal from it can be transferred to a cathode-ray tube scanned in synchronism, so that a picture is displayed of the part of the surface under investigation. Such a system is similar to the electron scanning microscope.2 It diVers appreciably from that proposed by Pattee,3 in which an electron spot scans a thin target of a pure metal next to which is placed the specimen to be examined, so that image contrast is due not to emission but to diVerential absorption, as in the normal form of X-ray projection microscope.4 We have built and operated successfully a flying-spot X-ray microscope of the emission type. Magnetic lenses have been used, so as to obtain a smaller electron spot or alternatively a greater beam current in a spot of the same size, as compared with electrostatic lenses. By employing a proportional counter, a particular X-ray line can be selected as the imaging signal, so that in the picture only those parts of the surface appear bright which contain a given element. Fig. 1A, B, C, D shows the degree of diVerentiation obtained from an impure sample of beryllium. Fig. 1A is obtained when all quanta are recorded, and Fig. 1B, C, D when those of certain energies only are passed by the pulse analyser. The magnification and contrast of the image can be controlled electronically and the area of specimen scanned may be varied at will. By stopping the electron probe over a selected point on the surface, the spectrum of X-rays omitted by it may be mapped by varying the band accepted by the pulse analyser. In this way the bright areas in Fig. 1B, C, D were identified as manganese, nickel and calcium respectively. In quantitative analysis the ability to discriminate one element from another is limited to approximately DZ  4; the natural width of a characteristic line is considerably broadened by the proportional counter. The accuracy can be greatly improved by using a simple crystal spectrometer for initial analysis of the X-rays. A new

*Reprinted from: Nature, 177, 1173 (1956). y

Letter to Nature sent 24 May 1956.

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Fig. 1. X-ray emission micrograph of impurities in beryllium foil.  130. A: All constituents recorded. B, C, D: Characteristic emission from diVerent impurities, separately selected—magnesium, nickel and calcium respectively.

magnetic lens has been designed and constructed which allows this to be done through one exit port, while through a second port another portion of the X-rays emitted from the surface is accepted by a scintillation counter for the purpose of image formation. Direct quantitative results for the concentration of an element can thus be obtained; it will be possible to separate neighbouring elements in the periodic table. The concentration sensitivity and accuracy of location are now the subject of detailed investigation. We wish to thank Dr. W. C. Nixon and Mr. J. V. P. Long for many useful discussions and to the Department of Scientific and Industrial Research for a maintenance grant to one of us (P. D.). V. E. Cosslett P. Duncumb Cavendish Laboratory, Cambridge. May 24.

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Castaing, R., thesis, Paris, 1951. Castaing, R., and Descamps, J., J. de Phys., 16, 304 (1955). McMullan, D., Proc. Inst. Elect. Eng., Pt. 1, 100, 245 (1953). Smith, K. C. A., and Oatley, C. W., Brit. J. App. Phys., 6, 391 (1955). Pattee, H. H., J. Opt. Soc. Amer., 43, 61 (1953). Cosslett, V. E., and Nixon, W. C., J. App. Phys., 24, 616 (1953). Nixon, W. C., Proc. Roy. Soc., A, 232, 475 (1955).

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3.4 Tube Investments Research Laboratories and the Scanning Electron Probe Microanalyser D. A. MELFORD Formerly at: Tube Investments Research Laboratories, Hinxton Hall, Cambridge

I. Introduction How did it come about that a Birmingham-based manufacturer of steel, tubes, domestic appliances and bicycles became involved in the development of electron probe instruments in Cambridge in the late 1950s? The best answer to this question appears to be ‘as the result of a sequence of fortuitous and apparently unconnected events.’

A. A Snap Decision It was almost by accident that I became a metallurgist in the first place. I had arrived in Cambridge in 1949 with a serious ambition to undertake research, a deep interest in things mechanical and insuYcient mathematics to read the Engineering Tripos. Even the Preliminary Examinations for the Natural Sciences Tripos had not gone well and, in the rooms of the Senior Tutor, Nick Hammond, in the Old Court of Clare College on a fine morning in early October 1950, I was warned that I would be sent down if there was not a significant improvement in the second year. This did not augur well for an ambition to stay on and do research for which first-class honours were preferred. To make matters worse, Nick Hammond then pointed out that an extra half-subject must be read. He reeled oV a bewildering list of possibilities, none of which I had encountered before: mineralogy, biochemistry, metallurgy and several more. As I had recently acquired a lathe as a birthday present, it struck me that it would be interesting to understand more about the metals that I machined on it. I opted for metallurgy. From this snap decision many consequences flowed. Metallurgy turned out to be a good choice and in 1952, having duly acquired the necessary qualifications and a Department of Scientific and 289 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Industrial Research (DSIR) research grant, several possible topics were suggested for research in this field. One involved thinning aluminium alloy specimens in various stages of heat treatment for examination in the Metallurgy Department’s newly installed Siemens electron microscope. This oVered little or no opportunity for designing and building apparatus and I turned it down in favour of an investigation into the surface tension of molten metals by a novel capillary method because this promised more scope for the design and construction of new experimental equipment. Such measurements are extremely sensitive to contamination by parts per million of surface-active elements such as sulphur. The presence on the other side of the room of an experiment designed to explore the smelting of sulphides stimulated the development of a certain degree of patience and perseverance that came in very handy later on.

II. The Early Days of Hinxton Hall In 1955 it became necessary to find more permanent (and rewarding) employment and so I wrote to Dr (later Professor) Phillip Bowden, who was then involved in setting up the embryonic Tube Investments Research Laboratories (TI) at Hinxton Hall, to enquire whether there was a vacancy for a metallurgist. He replied eVectively ‘No, thanks, we already have one,’ which at first sight seemed a modest establishment for an organization with a turnover of several hundred million pounds made entirely out of manipulating metal. However, this was the era in which basic research was deemed to be good for industry and no blue chip corporation was complete without its Ivory Tower. Among Hinxton Hall’s preoccupations at the time were irradiated plastics (Arthur Charlesby), the properties of metallic and ceramic whiskers (Jim Gordon) and the basic structure of materials (Jim Menter, later Sir James). Bowden had mitigated his refusal to take on a second metallurgist by oVering a two-year TI Fellowship tenable in his group, the PCS at the Cavendish Laboratory. This was eVectively a holding operation until it became clear whether TI needed another metallurgist but, at 600 a year, it was a distinct improvement on the 325 DSIR grant. The PCS had recently been the PCRS, standing for Physics and Chemistry of Rubbing Solids, a reflection of Bowden and Tabor’s classic studies of friction. Its new and broader remit was the Physics and Chemistry of Surfaces, hence PCS. It was the nearest thing to an autonomous fiefdom in the university, due largely to Bowden’s genius for obtaining industrial and government funding. The hub of the group was undoubtedly the workshop and the skill of the

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instrument makers who operated there. Many of the group’s significant achievements depended critically on their remarkable abilities. This point is made because the resulting culture was exported wholesale to Hinxton Hall. JeV Courtney-Pratt, an almost exclusively nocturnal creature who was later head-hunted by Bell Telephone Laboratories, was a brilliant designer of instrumentation and Bowden’s main lieutenant in overseeing the alterations required to turn a country house into a research laboratory. His first priority was to roof in the stable yard and fill it with the best machine tools and ancillary equipment that his generous budget would buy. Among his purchases was a Swiss jig borer which could position a workpiece with an accuracy of about 2 mm over a range of about 30 cm on any of three axes. He was equally discerning in his recruitment of instrument makers, several of whom had been trained by the Cambridge Instrument Company (CIC) and/or had previously worked in university departments. As a member of the PCS, I enjoyed for about 18 months the distinction of being the only scientist on the New Museums Site with an entire laboratory building to himself. It was a recently vacated coal shed immediately behind what is now the Metallurgy Tower, where I wrestled with the problem of determining the mechanical properties of molybdenum at temperatures  above 2000 C. Then, having been duly interviewed by Dr Menter in his oYce, a former maid’s bedroom-cum-linen closet on the top floor of the old hall at Hinxton, I formally joined TI Research Laboratories on 1 July 1957, reporting to their ‘founder metallurgist,’ Dr John Sawkill. Looking back on it from the next century, events then moved with an extraordinary swiftness not apparent at the time. Accountants were already beginning to gnaw at the foundations of the Ivory Tower and a series of visits by Hinxton scientists to the TI operating companies was being encouraged to discover whether there were any ways in which the resources of the laboratory could actually be used to help the business. Iron and steel were the materials with which the company was most involved and yet there was no great understanding of either at Hinxton Hall at that time. A Cambridge metallurgist with a PhD was regarded (to some extent justly) as a distinctly alien life-form by most of the operating companies and one wholly ignorant of the real world in which they functioned. The new laboratory had a huge credibility hill to climb with those whose eVorts ultimately funded it. A. Surface Hot Shortness in Mild Steel Howells was a typical SheYeld tube company whose heyday had been in the age of the steam locomotive, for which they had supplied boiler tubing. Their method for steel tube manufacture consisted of punch-piercing a billet to form a closed-end hollow and then passing this on a mandrel through a

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push bench (a series of roller dies) to reduce the wall thickness. Only a week or two after I joined TI, John Sawkill and I went to visit the company. They were complaining of an increasing problem with circumferential surface cracking occurring at the piercing stage, which was later exacerbated during  reduction. Piercing took place at 1180–1200 C and high-temperature cracking of this kind is known as ‘hot shortness’—‘short’ as in shortbread, a corruption of the Swedish word ‘sko¨rt’, meaning fragile. This form of hot shortness was known to be associated with the presence of the residual or ‘tramp’ elements in steel, particularly copper and tin (Pfeil, 1929). Residual elements are those that do not oxidize in the presence of iron and hence are not removed in the steelmaking process. When the steel is heated, they become enriched in the layer of metal adjacent to the surface oxide scale as the iron in which they were once dissolved is increasingly converted into oxide. Whether other such elements—nickel, arsenic or antimony for example—were important was not known, nor was the mechanism of the failure fully understood. The presence of such residuals in the steel arises from the use of imperfectly sorted scrap. Bronze components introduce copper and tin, for example, and white metal bearings introduce antimony and tin. The economics of steelmaking in the 1950s and 1960s increasingly favoured the recycling of scrap and so here was a problem likely to impact on the TI Group to an increasing extent in the years ahead. Two or three days after the visit, a sample of steel cut from the surface of a pierced billet that exhibited this problem arrived at Hinxton from Howells. When it was sectioned, polished, etched and examined under the microscope, there were clearly small regions at the surface, 40–50 mm across, that had resisted the etchant. Was this because they were enriched in copper or tin? One such region is shown in the photograph in Fig. 1 which, at John Sawkill’s suggestion, was taken up to the linen closet for Jim Menter’s advice on what physical methods were available for determining the composition of such small regions. His immediate suggestion was for me to go and see a certain Peter Duncumb, then a research student in the Cavendish laboratory, who had apparently developed an instrument for just this purpose. It happened that Peter had also been an undergraduate at Clare (and a pupil of the Senior Tutor, Nick Hammond). I certainly knew of him, although he was a year below me, because he was readily distinguished by his great height.

B. A Second Snap Decision With the perspective aVorded by hindsight, this whole chain of circumstances appears to have held together as a consequence of one or two pivotal events that appeared random and trivial at the time. If the first had been the

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snap decision to read metallurgy as an extra half-subject, then the second was about to take place. The setting was the Old Cavendish in Free School Lane–no stranger to pivotal events–since this was where Peter Duncumb had been undertaking the research for his PhD. Having duly made an appointment, I went to see him, armed with a description of the hot shortness problem and clutching the photograph of Howells’ piece of steel. He listened with patience and interest and thought that the specimen might well be one that it would have been interesting to look at with the instrument he had developed. Unfortunately, I was several weeks too late. He had finished experimental work and was now fully occupied in writing his thesis. Disappointed that progress in this direction was not possible, I retrieved the photograph and the rest of my papers, packed my briefcase, said goodbye and got up to leave. My hand was on the door handle when Peter had second thoughts. He called out that perhaps, after all, his thesis would benefit from another example of an application and he would look at the specimen if I brought it along. From this decision also, many consequences flowed, including a collaboration lasting some 30 years. Peter’s instrument, which he describes more fully elsewhere (Cosslett and Duncumb, 1956), consisted essentially of the electron-optical column from an old RCA EMB Lend-Lease electron microscope to which he had fitted a semi-focusing X-ray spectrometer of his own design, arrangements for detecting back scattered electrons and, most importantly, scanning coils. It accommodated a specimen 18 inch diameter and the immediate task was therefore to trepan out such a sample from the mounted cross-section of the surface with the region of interest located precisely in the centre. This involved making a special trepanning tool which, as it revolved, would cut tough steel for half a rotation and relatively soft plastic mounting medium for the other half. This is where the Swiss jig borer in the Hinxton workshops proved invaluable, the rigidity of the quill preventing any drift of the tool towards the softer medium. A slitting saw then parted oV a section about 1 8 inch long. It is not entirely fanciful to suggest that this crumb of steel eventually brought about the whole subsequent involvement of the Cambridge Instrument Company with electron optics. It is diYcult to overestimate the importance to a metallurgist of the understanding that he gains by looking at a polished and etched sample under the optical microscope. Seventy years earlier, in 1887, Henry Clifton Sorby (prompted originally by his interest in metallic meteorites) had started his systematic evaluation of the microstructure of iron and steel with a reflecting microscope of his own design and eVectively founded the science of metallography (Sorby, 1887). Nowadays, an experienced metallurgist examining a steel sample in this way can usually assess the carbon content to within a tenth of a per cent, comment on the nature and quality of the

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steelmaking process, describe the thermal history of the sample since the steel was cast, and even predict the mechanical properties with a fair degree of accuracy. The value to him of any new information about the sample is enormously enhanced, therefore, if he can relate it to this optical image.

C. The Cavendish Laboratory Experiment The experiment took place on 7 August 1957. In the green glow of the display screens in the bowels of the Cavendish Laboratory there appeared a backscattered electron image (Fig. 1) immediately reconcilable with the optical micrograph and then X-ray images revealing the distribution of copper and nickel (tin could not be detected at this time because there was no provision for a vacuum in the spectrometer). The nickel distribution image shown in Fig. 2 confirms that there is indeed significant nickel enrichment in the unetched region visible in the optical image and even more around the edges of the two small surface cracks. The importance of these images was not simply that this was previously unattainable information about fine-scale variations in composition, but that it was presented in a manner that correlated so readily with the optical image that metallurgists know and love. Metallography would never be the same again. Impressed by the results achieved in an hour or two’s examination of the Howells sample, I rang Jim Menter at home that evening to tell him that this technique was in eVect God’s gift to metallurgists and that our best chance of making progress with the problem of surface hot shortness was to acquire it forthwith. In making this plea, I was preaching to the converted, since few

Figure 1. Cross-section of surface of a mild steel billet exhibiting surface hot shortness. Optical micrograph; field width ¼ 500mm.

Figure 2. Ni Ka X-ray emission image of the field of Figure 1 obtained in the Cavendish experiment on 7 August 1957.

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scientists at the time were more persuaded of the value of the application of advanced physical techniques to the study of metals. This particular technique, if appropriately developed, seemed to oVer considerable scope for rapid diagnostic trouble-shooting on production problems and would put the laboratory in a position to supply the TI companies with information that they could obtain nowhere else—a huge asset in closing the credibility gap. The Director of Hinxton Hall at this time was Dr T. P. Hughes, a chemist and one of Phillip Bowden’s former students. He had previously worked at the Royal Aircraft Establishment and later become Director of the rocket engine research centre at Westcott. Strangely, these facts also have relevance as will appear later. Jim Menter readily persuaded him of the potential importance of acquiring an instrument at Hinxton, aided considerably by the photographs taken in the Cavendish experiment, which Peter Duncumb had quickly developed. Negotiations to this end were then commenced. A familiar problem soon emerged. The cannibalized RCA EMB microscope had brilliantly demonstrated the viability of the technique. What Hinxton Hall now needed, however, was an instrument purpose-built for metallurgical work that could be operated by a technician rather than a postdoctoral physicist. A lot of development work was needed that the Cavendish was not in a position to undertake, nor would it have been a proper activity for that laboratory. Similarly, no commercial instrument manufacturer would take on such a large development project for an as yet unproven market, nor would they be likely to have the necessary metallurgical input. The logic of the situation decreed that the instrument would have to be constructed by the Hinxton Hall laboratories.

III. Designing an Instrument for the Metallurgist Arrangements were therefore made to engage Peter Duncumb as a consultant. Sir James Menter has kindly allowed access to his papers relating to this period, the earliest of which is a memo from me, dated 20 August 1957, pointing out the modifications to the instrument that were needed and the fact that completing and submitting the Duncumb thesis by the deadline of 30 September was the highest priority at present. There would, however, be a brief availability window between 23 September, when his thesis had to go to the binders, and the 3 October, when he left for a holiday. Advantage was taken of this window to convene the first exploratory meeting between the interested parties. The minutes are entitled ‘Discussion on the prospect of constructing an X-ray microanalyser at Hinxton Hall.’

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Those present included Dr Cosslett (Peter Duncumb’s supervisor), Dr Menter, Dr Sawkill, Peter Duncumb and me. Dr Menter proposed ‘that TI should pay a sum of money for the use of dimensioned drawings and the expert guidance of Dr Cosslett and/or Mr Duncumb during the construction period.’ One instrument maker (Peter Mizen) and one electronics expert (Henry Strauch) were to be assigned full time to the project and ‘something like one visit a week by either Dr Cosslett or Mr Duncumb would be necessary to supervise this eVort. Responsibility at the Hinxton end would be assigned to one person, possibly Dr Melford.’ The use of the instrument (preferably capable of operation by a technician) was forecast to be mainly metallurgical, and the most important modifications were: 1. To extend the present atomic number range of elements detectable (22–37 and 50–92) to give coverage if possible as far down as aluminium (14) and over the ‘dead’ range 38–49. (This would involve designing a vacuum spectrometer with several readily interchangeable monochromator crystals.) 2. To increase the specimen size to a minimum of 14 inch diameter. (This would involve redesigning the objective lens so that the focus occurred outside the lens body but would bring other major advantages such as facilitating the introduction of pure element standards for calibration, and the possibility of incorporating an optical microscope.) Other, later, improvements forecast at this meeting were the provision of hot stage facilities for dynamic diVusion studies and the improvement of ˚ so that the instrument could also be used as a electron resolution to 200 A scanning electron microscope. Dr Cosslett thought that the cost of the whole exercise should not exceed 3000 and undertook to check the legal position with the university and with DSIR. If no problems emerged, a second meeting would be held in four weeks. The second meeting took place at the Cavendish on 25 October 1957. Dr W. C. Nixon was also present because he had experience of operating a pinhole-type lens and it was suggested that this might be helpful in finalizing the design of the one that Peter Duncumb now had in mind. Duncumb produced a sketch of such a lens with a re-entrant pole piece and a working  distance of 1 cm. This allowed an X-ray take-oV angle of 20 (the minimum desirable) and a specimen surface location 0.6 mm below the bottom of the lens body. A third meeting five days later at Hinxton between Peter Duncumb and Hinxton staV was concerned with drawing up a general bill of materials and with identifying commercially obtainable components such as ratemeters, photomultipliers, etc., that were either already available in the laboratory or needed to be placed on order as soon as possible. The meeting

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also generated a block diagram of the instrument that gave the Hinxton team a first glimpse of the complexity involved. Interestingly, this shows the specimen (in a very schematic form) mounted on a vertical drum, which was the method currently used by Tom Mulvey at AEI Research Laboratories, Aldermaston Court, for his fixed-probe prototype Metropolitan-Vickers instrument (Mulvey, 1959). Paul Jervis (Jervis, 1972) has made the point that the easy interaction between a number of individuals in Cambridge with valuable experience to contribute (Bill Nixon and the pinhole lens for example) greatly facilitated these developments. A discussion with Peter Duncumb on 18 November records that he favoured an electron gun design based on that in the RCA EMB microscope but using a Siemens filament and a length of Pyrex pipeline as the gun insulator. This insulator had been used with success by Dr J. V. P. Long in the Cavendish and was readily available with reproducible dimensions. Another of Jim Long’s suggestions was that there were war-surplus ‘gun sights’ available in Silverman’s junkyard in Cambridge with highly accurate worm and wheel mechanisms, ideal for crystal spectrometers. They were apparently selling like hot cakes and the meeting quickly adjourned to the junkyard so that the purchase could be made. In the event they turned out to be depression rangefinders as used by the coastguard to pinpoint the position of aircraft or pilots shot down in the English Channel. A 10-inch diameter phosphor bronze wheel meshed with a steel worm and was capable of reproducible settings to a minute of arc. The price was 6.10s.0d. At this meeting also, it is clear that a 3-inch diameter drum, removable through the bottom of the specimen chamber, was still under consideration to mount specimens and standards. It was arranged that I should report directly to Jim Menter for the duration of the instrument’s construction. There was a great deal to be getting on with to identify and order all the commercially available components. It was only when Peter Duncumb’s first dimensioned sketches of the electron gun arrived that a penny began to drop. The key dimensions were all there—the relative filament, grid and anode distances, grid and anode aperture diameters—but an exoskeleton was needed to support it all. There had to be ways of pumping out the column eYciently, traverses to align the components, and above all a specimen stage. This last needed to provide X and Y traverses and, if possible, a rotation about the centre of any field-of-view together with means for rapid interchange of specimens and their examination under an optical microscope. There needed to be a console for all this to stand on. Someone had to design and draw all this and, as the probable end-user of the instrument, it was evidently going to have to be me. The prospect would have been daunting but for the fact that the laboratory appeared peculiarly well equipped to tackle it. On the other hand,

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it was a challenge: it would give me the opportunity to immerse myself in matters mechanical and there was the huge advantage of knowing exactly what the instrument was going to be required to do. The third of these early meetings, and the last one to be singled out for mention, was again chaired by Jim Menter and held at Hinxton Hall on 22 December 1957. I submitted a progress report containing the following paragraph: The Specimen Chamber is an undoubted tease if one wishes two traverses, rotation of any field of view about its centre and the rotation from optical to electron axis. The choice seems to lie between drum and table. The former gives one two easy traversing movements if you don’t mind the specimen going out of focus on one of them as it travels through an arc. (Peter tells me Tom Mulvey has increased his drum diameter to twelve inches to minimise this.) The rotation of the field of view is still a problem. I prefer the table even if it does involve slightly more mechanism. Peter explained my first idea to Tom Mulvey who was sceptical as he said Metro Vicks had experienced considerable trouble with any but the simplest mechanisms when working in a vacuum. I have taken this point seriously and, while still preferring a table which gives me all the movements I require, I have arranged a box of air for the mechanism to work in (see attached drawing). [Fig. 3]

This was another example of valuable outside input and also an indication that metallurgical toes were being dug in. There were three other highly desirable user requirements to be catered for in the design of this area: the incorporation of an optical microscope, rapid specimen changing and the ability to accommodate a number of pure element standards. As Fig. 4 illustrates, there were 13 mounts for standards plus the specimen itself. The table could be indexed so that the specimen, or any of the standards, lay under the electron beam, the optical microscope, an observation port or the specimen changer. This last took the form of a ‘caisson’ tube which could be lowered to seal onto the base of the screwed boss that held the specimen cartridge. It can be seen to the right of the optical microscope. Most of the tube was filled with a brass cylinder to minimize the volume of air admitted when the sample was changed. This cylinder was attached to a standard air-admittance valve. Changing samples and restoring an operating vacuum took only three or four minutes.

A. The ‘‘Christmas Day’’ Sketch It was at this third meeting that Jim Menter emphasized the need, as soon as possible, for an ‘artist’s impression’ of the complete instrument. Block diagrams were all very well, but what was the instrument layout going to

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Figure 3. Specimen table with provision for X and Y movements plus rotation about the centre of the field of view, rapid specimen changing and optical microscopy.

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Figure 4. Completed table withdrawn from specimen chamber.

look like, what was its footprint going to be and how easy could it be made to operate? Factors such as the need to operate the gun controls from a sitting position while at the same time being able to get knees under the console and have support for elbows while focusing an optical microscope were all highly relevant issues. The operation of any instrument becomes more eYcient if it can be made eVectively ‘transparent’ and simply an extension of the senses. Much thought was therefore given to grouping the most frequently used controls so that they came readily to hand in the dark or near-dark conditions in which the instrument would be operated. It was this meeting which gave rise to what seems to have become known as the Christmas Day Sketch. It was produced on Christmas Day 1957, provoking a certain amount of domestic disapproval, at a time when only the electron gun had been drawn in detail (Fig. 5). It closely resembles the console eventually constructed (Fig. 6) and demonstrates that many of the key design features had been arrived at by then. In the Hinxton workshop, Peter Mizen had by now started work on the electron gun. Don Unwin (Chapter 4.3, this volume) has shrewdly identified two quite distinct cultures among instrument makers: the slow and hugely precise contingent and the faster, less precise but adequately accurate brigade. It is to the second of these that one turns when there is a need to make a rapid lash-up to explore a concept or finalize a design. Peter Mizen combined both of these abilities. He would look at a dimensioned sketch, ask about the function of the component and the constraints in design, then point out diYdently that there was an easier way to do it, or suggest a change in design that would save material, time or labour and end up making something that was often an improvement on the original. He also very gently explained that it was perfectly possible to design something that could

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Figure 5. The ‘Christmas Day’ sketch, 1957.

Figure 6. Completed console, 1958.

not be assembled—a fact that had simply never occurred to me. He taught me a great deal and it was a privilege to work with him. The first four months of 1958 were spent in a corner of the old dining room in Hinxton Hall making what seemed to be endless dimensioned sketches on graph paper (actually about 80 in total) and in constant discussion over the telephone with Peter Duncumb in the Cavendish to seek his advice, to make sure his designs were not being misinterpreted, and to agree certain key dimensions that were important to us both. As an early example of university/industry collaboration, it was remarkably eVective. The demand for drawings increased as more instrument makers became available from other projects, until at one time there were five engaged in making various pieces of microanalyser and drawing production was only about 45 minutes ahead of them. Radiused corners inside a vacuum system are important for ease of cleaning. On one occasion a column component had been drawn up without the internal corners being specified as having the usual 14 inch. radius. On the way over to the workshop to correct this omission I was buttonholed by another scientist and held in conversation for some ten minutes. By then it was too late—the corners had already been machined. Fortunately, it was no great disaster but at this stage the project had gathered such breathless momentum that there was seldom much time for second thoughts. Peter Duncumb provided all the drawings for the new X-ray spectrometer. This was of the semi-focusing type, similar to the one he had devised for the

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Cavendish instrument but with two important diVerences. It was a vacuum spectrometer and it was equipped with a turret holding four monochromator crystals, interchangeable by pushing a button. Three were usually LiF curved to diVerent radii, and the fourth was either gypsum or mica. There was also a vacant position to enable the X-ray beam to be passed directly into the proportional counter. This had a great virtue when examining a sample containing regions of wholly unknown composition as any characteristic radiation showed up immediately on the pulse monitor and could then be rapidly identified. The design was compact, easy to use and capable of identifying all elements heavier than magnesium (Z ¼ 12). It was ideal for the purpose and yet attracted a certain amount of criticism from some competitors who even urged the view that accurate quantitative analysis was not possible unless the full Rowland circle spectrometer geometry was used. They were usually the advocates of nonscanning instruments, since scanning would defocus their spectrometer at the edges of the scan. Few metallurgists, however, were likely to favour a nonscanning instrument if the alternative was available. Meanwhile, Henry Strauch was constructing stabilized lens supplies and a 30-kV HT set and generally laying out the electronics. He was an interesting character with a somewhat colourful past. During the war he had worked at the German research centre at Peenemu¨nde, where his particular project was the creation of a radar-controlled searchlight network. The radar would track an incoming Allied plane and steer the (masked) searchlights until the target was suYciently pinpointed for all the searchlights in the network to unmask simultaneously and illuminate it for fighters or anti-aircraft guns. As the Americans and Russians closed in on Peenemu¨nde, Henry took a searchlight and the radar tracker and drove towards the American lines to surrender to them rather than to the Russians. Unhappily, the Americans shot up the prizes he was bringing them as he approached, but fortunately Henry survived unharmed. His expertise was soon recognized by Major-General Belchem, a member of General Montgomery’s staV who was a keen model railway enthusiast. Once the war was over, he employed Henry in a private capacity to construct what must have been one of the first electronically controlled model railway layouts. Henry possessed a photograph of this, which he would show with justifiable pride since, apart from Henry and Major-General Belchem, Generals Eisenhower and Montgomery could also be seen playing trains with evident enthusiasm. The intensive, indeed hectic, design period lasted until about mid-April 1958. To add to the pressure, there was a rumour (later shown to be quite unfounded) that a group at ICI Billingham were also in the final stages of constructing a scanning instrument. Assembly of our instrument, however,

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could not start until the console with its vacuum system had been finished and the specimen chamber (which supported the column) had been mounted on it. The customary optimistic view of progress had been taken in early January when a target date for full operation of 1 June had been agreed. This would have been impossible to achieve in any event because of delays in building the new laboratory. When TI bought Hinxton Hall they obtained permission to build a major new block to house the laboratories. It was built to the same design as the Metallurgy building at Harwell which had been erected at a time when structural steelwork was in short supply. As a result, all the rooms tended to be rather small, a disadvantage with which we had to live for the next thirty years.

B. Assembly and Commissioning With the start of the assembly phase, a research diary was commenced and this records that at 4.30 p.m. on Tuesday, 8 July 1958, Room B14 in the New Laboratory became available and the console was moved in and positioned on its antivibration mat. Two days later the diVusion pump was fitted and Peter Duncumb arrived to assist with the assembly of the column. The following day the vacuum pumps were switched on for the first time and the process of assembly merged imperceptibly with that of commissioning. As with any complex system, the initial diYculty seemed to be to get all of it to function properly at the same time. The next few months were characterized mainly by interminable fault-finding on apparently quite unconnected faults. A failure of the time-bases would be corrected, only to be followed within minutes by a vacuum failure due to flux-corroded bellows, and then a fault in the photomultiplier circuitry or a short in the HT supply. The time-bases, which fed both scanning coils and display screens, were a particular source of diYculty until Peter Duncumb as a last resort rebuilt them himself. Eventually, on 3 September, a focusable beam was evident halfway down the column and on 15 October the first, highly astigmatic, electron image was obtained. The astigmatism proved to be caused by minute traces of dirt in the slits of the collet holding the objective lens aperture, which were charging up and distorting the beam. Once this was cleaned, it began to seem that the home-straight had been reached. On 3 December the first X-ray image was achieved, and on the 22nd of that month Jim Menter wrote to the Director to declare the instrument construction essentially completed (Fig. 7) (Duncumb and Melford, 1959). His estimate of the total cost to the Laboratory was 12 000—four times Dr Cosslett’s prediction at the first exploratory meeting.

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This meant that my secondment as instrument designer and general progress chaser was at an end and it was time to get back to the metallurgy and the problem of surface hot shortness. There was still a great deal to do on the instrumental side in general optimization, preparing good monochromator crystals and learning the whole art of quantitative analysis. Nevertheless, on 19 January 1959, some 17 months after the Cavendish experiment, the Howells specimen that had started it all was sampled once more and examined, this time for copper, nickel and tin, with gratifying results. A paper proposing the mechanism by which interaction between residual elements caused this problem was submitted 8 months later (Melford, 1962a). IV. Awakening Commercial Interest Meanwhile, interest in this collaboration between the Cavendish Laboratory and Hinxton Hall was beginning to snowball. A paper based on the experiment of 7 August 1957 was published in March 1958 (Melford and Duncumb, 1958). On 10 April 1958, T. P. Hughes wrote to Dr Cosslett noting that either the Cavendish or Hinxton had received approaches from SheYeld University, the British Welding Research Association, Aeon Laboratories and the Safety in Mines Establishment enquiring about the possibility of obtaining an instrument. He hoped that the Cavendish would soon be able to find a commercial manufacturer to whom all such enquiries could be referred. Failing this, he proposed that Hinxton should refer all enquiries to the Cavendish in the first place, and if use was to be made of any design drawings that Hinxton had originated, these could be made available free to universities and research associations but companies would have to pay a small charge. Perhaps the Cavendish would like to consider some similar arrangement. There would have been no diYculty in distinguishing our respective contributions since all the Hinxton drawings were annotated either ‘DAM’ or ‘Designed by PD. Drawn by DAM.’ The real problem facing many such organizations was that they lacked the electronic and, most particularly, the workshop resources to undertake such a task. Their number increased considerably during the rest of 1958. The diary records visits from the Bureau of Naval Research, US Steel, Republic Steel and Reynolds Aluminium in the United States, AB Atomenergi in Sweden, and from BISRA, United Steels and Metropolitan-Vickers in the United Kingdom. Among TI companies, the Park Gate Iron and Steel Co., Round Oak Steelworks and the Chesterfield Tube Company all paid visits to observe progress. In March 1959, Jim Menter estimated an immediate market for perhaps 50 such

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instruments if they were made available at a cost of, say, 12 000, pointing out that the only one currently available commercially was produced in France based on Castaing’s original design, did not scan, and was priced at 35000. In the United Kingdom, a production prototype of the Metropolitan-Vickers instrument based on Tom Mulvey’s design (also nonscanning at that time) was predicted to be completed by July with series production scheduled for 18 months’ time and a possible price of 10 000.

A. The Cambridge Instrument Company, a Coincidence and Commercial Production This active and increasing interest was now becoming an embarrassment to both parties. Demonstrating to visitors occupied a great deal of time and neither the Cavendish nor Hinxton could contemplate the multiple consultancies that might be requested if other organizations decided on the do-ityourself approach. Just maintaining the Hinxton instrument in perfect working order for each VIP visit was a constant strain. Fortunately, a solution was at hand due to a third fortuitous event. In 1957, the Cambridge Instrument Company (CIC) had acquired a new Managing Director, H. C. Pritchard. He had, quite coincidentally, previously worked at the Royal Aircraft Establishment with T. P. Hughes and, like Hughes, had gone on to be Superintendent of a rocket research establishment (Woomera). He decided that CIC needed a new, high-tech product to reinvigorate its fortunes but it had to be one that did not require much development work and that would be available quickly. On 20 March 1959, Hughes invited his former colleague to see the Hinxton instrument and a few weeks later the board of the CIC agreed to undertake commercial production. Now at last there was somewhere to

Figure 7. Hinxton instrument, 1959.

Figure 8. Cambridge Instrument Company Microscan I, 1960.

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direct enquiries from those interested in acquiring an instrument. The 80 dimensioned sketches were converted into something like 600 production drawings and the first Microscan I instrument (Fig. 8) was demonstrated at their headquarters in Grosvenor Place during the Institute of Physics meeting in London in January 1960, some two years and five months after the experiment in the Cavendish. It was now apparent that Hinxton Hall, by building an instrument for its own use had, as a side eVect, played the role of midwife, taking up an innovation from the university and developing it to the point at which it became a realistic commercial proposition. Orders were received at the rate of about one per month in the first year and altogether about 83 instruments were sold worldwide, one even going to China. For the whole of 1959 and half of 1960, I was almost the only metallurgist with access to this type of instrument and there were a great many applications to explore. The awareness that each new image on the display screen constituted a hitherto unachievable insight into the material being studied was a source of continual excitement. As the CIC instruments percolated into industry, there were invitations to visit the many major research laboratories in the United States and elsewhere that had purchased them. This was an asset in benchmarking the work going on at Hinxton against that of our competitors. It was also helpful to be able to confront TI’s raw-materials suppliers with persuasive evidence of unsatisfactory quality that they were in no position to confirm or refute, unless they acquired an instrument themselves. Although CIC now fielded the initial enquiries, they naturally wished to bring potential customers to Hinxton to see an instrument in operation until such time as they had their own demonstration model available. This was undoubtedly the most stressful period because should the instrument not be operational or, worse still, should it break down in the presence of customers, one was uncomfortably aware that an order might be lost. To have had Peter Duncumb’s expertise on hand would have been an enormous asset. In October 1959, Peter Duncumb, whose DSIR Fellowship had now come to an end, joined Hinxton Hall, to my great relief. We had no vacancies for physicists at the time and so, with the connivance of all parties, he had to be smuggled in disguised as a metallurgist. Considering the great contribution he had already made to the profession, this was wholly appropriate. One of his first achievements was to fit a point proportional counter of his own design down one of the spare access ports in the objective lens. This enabled, in May 1960, a demonstration of the first X-ray image of a metallurgical specimen (graphite flakes in cast iron) using carbon K radiation (Melford and Duncumb, 1960). Although the characteristic radiations from the light ˚ ) and are thus heavily elements have very long wavelengths (carbon K ¼ 44 A absorbed in iron, these elements are of great importance in ferrous metallurgy. Analysing for them became a continuing priority and, once we had

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learned to make lead stearate monochromators by the Langmuir–Blodgett technique, we were later even able to form X-ray images with the carbon emission from cementite plates (6.7% C) in pearlite. Peter Duncumb’s main task on joining the laboratory, however, was to design an instrument that combined the technique of X-ray microanalysis with transmission electron microscopy—an Electron Microscope and Microanalyser or EMMA in fact, but that is another story (Duncumb, 1962). EMMA may now be found sharing a display case in the Science Museum, South Kensington, with the Hinxton microanalyser. B. Further Developments Of the two later developments forecast in the very first meeting between Hinxton and the Cavendish (fitting a hot stage and improving the resolution ˚ for scanning electron microscopy), only the former came to pass. to 200 A Any hot stage capable of working at temperatures of interest to ferrous  metallurgists (say 900–1100 C) would have to be small if thermal radiation was not to damage the nearby plastic phosphor and light-pipe necessary for forming the electron image. In the spring of 1962, a platinum resistance furnace was fitted inside a 3.0-mm diameter steel sample to which a thermocouple was welded. Electron images were taken over a period of time at   950 C and 1000 C and there was evidence of thermal etching and grain boundary migration (Melford, 1962b). Unfortunately, a focused probe, by causing some form of activated surface diVusion, quickly dug a crater in the surface and quantitative, dynamic diVusion measurements, if possible at all, were clearly not going to be straightforward. ˚ , on the one hand there was a As for improving the resolution to 200 A great deal of work for the instrument to do in the role for which it had been designed, and on the other, towards the end of 1961 it became apparent that the CIC was going to undertake commercial production of the scanning electron microscope developed in the Engineering Laboratories in Cambridge. They now had a certain amount of relevant expertise in house and there was also the possibility that they could use some of the component parts from the Microscan as a useful start to the manufacture of what they eventually marketed as the Stereoscan. It made no sense therefore for Hinxton Hall to develop its own scanning microscope. C. Reflections with Hindsight To complete the circle of curious events, in February 2001 Clare College decided to give a lunch (which both Peter and I attended) in honour of the former Senior Tutor, Nick Hammond, who was now 95. Because it is rare at

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the end of a career to be able to report back to someone who had a hand in its formation, I wrote a letter to remind him about our discussion of fifty years and four months ago. I reassured him that the decision we had arrived at (that I should read Metallurgy as an extra half-subject) had worked out all right and had also impacted favourably on another of his students, one Peter Duncumb. He replied courteously that he was glad to hear his advice had been sound; but then, sadly, some four weeks later, he died. I shall always be glad I wrote that rather belated thank-you letter. As a footnote, it is worth reflecting that, if the objective is to set up, staV and equip a basic research laboratory from scratch, Phillip Bowden was amazingly successful. The first Siemens Elmiskop I in the country went to the Cavendish Laboratory, but the second came to Hinxton Hall a few months later—late in 1954, I think. The following year, Jim Menter used it to achieve the first direct imaging of atomic planes in a crystal lattice. He ˚ resolved the 201 planes of platinum phthalocyanine, a distance of 12.0 A apart, and succeeded in demonstrating the presence of dislocations (Menter, 1956). This microstructural coup was eVected in the butler’s pantry at Hinxton where the extra thickness of the walls, originally intended to protect the family silver, was found to produce the lowest ambient magnetic field on the site and hence the best location for installing the electron microscope. The further successes of his group are well illustrated by a quotation from Walter J. Moore’s review of the Proceedings of the 1959 Lake George Conference on ‘The Structure and Properties of Thin Films’: If anyone has not yet seen the exciting results of the Tube Investments Group (Menter, Bassett, Pashley) on the direct electron micrography of crystal structures and their imperfections, here is a good place to look. . . . almost 100 pages of Tube Investments research are reported. Maybe future scientific historians will ponder how and why at mid-twentieth century an English bicycle-maker was contributing more than the entire US steel industry to basic research on metals.

This did much less than justice to the work of Bob Fisher and his colleagues at US Steel’s Edgar C. Bain Fundamental Laboratory at Monroeville, near Pittsburgh. Nevertheless, it was an indication of how far the new Laboratory at Hinxton Hall had come in a relatively short time. Since, by then in mid1959, the instrument that has been the subject of this paper had also been designed, constructed, commissioned and licensed to the CIC for production, it could reasonably be claimed that the first five years of the new laboratory’s existence were an exciting and fruitful time. It had also established the principle that the best way to solve a production problem was to ensure that the underlying science was properly understood. This invaluable ethic was to stand the laboratory in good stead on many occasions in the years to come.

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In the event, my first two years at TI Research Laboratories had turned out to be highly challenging, very exhausting, formidably educational and great fun. The Birmingham-based manufacturer of steel tubes and bicycles had acquired an instrument that was later to prove invaluable when investigating problems with either its raw materials or its products, and CIC had its new, high-tech product. Nevertheless, it is diYcult to escape the conclusion that, but for one or two extremely chancy events, the whole thing might never have happened this way at all.

References Cosslett, V. E., and Duncumb, P. (1956). Microanalysis by a flying-spot X-ray method. Nature 177, 1172–1173. Duncumb, P. (1962). X-ray microanalysis of elements in the range Z ¼ 4–92, combined with electron microscopy and electron diVraction. Stanford, 1962, pp. 431–439. Duncumb, P., and Melford, D. A. (1959). Design considerations of an X-ray microanalyser used mainly for metallurgical applications. Stockholm, 1959, pp. 358–364. Jervis, P. (1972). Innovation in electron-optical instruments—two British case histories. Res. Policy 1, 174–207. Melford, D. A. (1962a). Surface hot shortness in mild steel. J. Iron Steel Inst. 200, 290–299. Melford, D. A. (1962b). A study of microsegregation at grain boundaries in mild steel by means of the electron probe microanalyzer. Stanford, 1962, pp. 577–589. Melford, D. A., and Duncumb, P. (1958). The metallographic application of X-ray scanning microanalysis. Metallurgica 57(341), 159–161. Melford, D. A., and Duncumb, P. (1960). The application of X-ray scanning microanalysis to some metallurgical problems. Metallurgica 61(367), 205–212. Menter, J. W. (1956). The direct study by electron microscopy of crystal lattices and their imperfections. Proc. R. Soc. London A236, 119–135. Mulvey, T. (1959). A new X-ray microanalyser. Stockholm, 1959, pp. 372–377. Pfeil, L. B. (1929). The oxidation of iron and steel at high temperatures. J. Iron Steel Inst. 119, 501–547. Sorby, H. C. (1887). On the microscopical structure of iron and steel. J. Iron Steel Inst. (i), 255–288.

PART IV COMMERCIAL DEVELOPMENT

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

4.1A Commercial Exploitation of Research Initiated by Sir Charles Oatley K. C. A. SMITH Formerly at: Engineering Department, University of Cambridge, UK

Despite the general scepticism with which the SEM was regarded in the early 1950s, Oatley’s growing conviction that it would prove to be a valuable method of microscopy stimulated his eVorts to find a manufacturer willing to exploit the research undertaken in his group. The first avenue that opened up this possibility came in 1955 when Dr Douglas Atack, Director of the Applied Physics Division at the Pulp and Paper Research Institute of Canada (PPRIC, now Paprican), then on sabbatical leave in the Cambridge Department of Physical Chemistry, learned of the work in the Engineering Department. He brought some specimens along to the department for examination in SEM1. The results obtained were suYciently encouraging to persuade the PPRIC that they should acquire an instrument for their Montreal laboratories. Since no commercial instrument was available, Oatley oVered to have a new SEM built in the Engineering Department for the PPRIC. At the same time he came to an arrangement with the Scientific Apparatus Department of AEI (formerly Metropolitan Vickers), whereby the new microscope would be supplied through that company on the understanding that, if successful, it would serve as a prototype for a production model. This complex multipartnership agreement was a prime example of Oatley’s acumen and persuasive powers. The construction of this microscope (SEM3) has been described in Chapter 2.2A. It took a little under two years to complete and was packed and shipped to Montreal early in 1958 by AEI, which company also handled all the paperwork and invoicing. Although originating in the CUED, it was sold by AEI, and in this respect was the first commercial SEM. The SEM3 installation at the PPRIC, when fully operational in the early 1960s, helped to generate a widening interest in scanning electron microscopy. The DuPont Company hired time on the microscope and, as a result, this company eventually decided to acquire an instrument of its own (the second of two prototypes manufactured by the Cambridge Instrument Company (CIC) prior to the 1965 Stereoscan Mk1 production run (see Snelling, 311 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Chapter 4.2A). Oliver Wells also used SEM3, after he had left Cambridge, to carry out some experiments on a semiconductor device (see below and Wells, Chapter 2.3). At this time also, the JEOL Company installed a TEM at the PPRIC, and their engineer was able to see SEM3 in operation; which stimulated an interest within that company concerning the future commercial possibilities of scanning electron microscopy (see Appendix II, this volume). Soon after SEM3 was shipped to Montreal, AEI received an order for a SEM from Dr F. P. Bowden, Head of the Physics and Chemistry of Surfaces Group in the Department of Physical Chemistry at Cambridge. The stimulus for this order had arisen from some original work undertaken by Dr James McAuslan, an employee of Imperial Chemical Industries, also on sabbatical leave in the Department of Physical Chemistry at the same time as Atack. McAuslan was working with Bowden on the thermal decomposition of silver azide crystals, and they were attempting to use the reflection electron microscope as a means for observing the decomposition process directly, replicas of course being out of the question. The high beam intensity in this instrument was, however, causing premature ignition of the crystals. Examination of the crystals at the CUED with SEM1 had demonstrated that the scanning microscope was ideal for this application. Bowden’s order came at an inopportune moment for AEI, as is made clear in the following private communication from Alan Agar, who joined the company in 1962, not long after the events in question took place: From the point of view of the company the order from Bowden came at a very unfortunate time, since no engineering drawings of SEM3 existed, there was no publication describing the instrument, and its designer, Ken Smith, was still busy in Canada commissioning the instrument. The engineering resources of AEI were then at full stretch, and the most economical use of engineers appeared to be to base a new SEM on the existing AEI microanalyser. In retrospect, this appears to have been a poor decision, and it was compounded by mistakes in the engineering work, resulting in the delivery of a malfunctioning instrument. [The diYculties under which AEI laboured at that time are described by Agar in Chapter 4.1B.]

By late 1960, it was becoming apparent that the AEI programme for manufacture and marketing of the SEM was stalling: little of the expertise available in CUED had been utilized in their SEM, and delivery of the Bowden order was considerably overdue. As a consequence Bill Nixon, with the recent successful launch of the Microscan in mind, decided to explore the possibility of involving CIC in the commercial exploitation of the CUED work. He was friendly with Steve Bergen, the company’s Chief Development Engineer, and he arranged a lunchtime meeting with Bergen and Smith (then working with Cosslett in the Cavendish) that took place at the Cambridge

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Garden House Hotel. Production of the CIC Microscan was by then well under way, and the discussion centred on the commonality of the electron optical and electronic components of the microanalyser and the SEM. The outcome was that Bergen was persuaded that CIC could, from the technical point of view at least, manufacture the SEM alongside the Microscan. It was ventured at that meeting that if CIC took up manufacture of the SEM it would outsell the Microscan. Oatley has described the subsequent events leading to the development and production of the Stereoscan (see Chapter 1.2). By the end of 1971, 520 Stereoscans had been shipped compared with 150 X-ray microanalysers (Stewart, 1985). Meanwhile, the AEI SEM was delivered to Bowden’s department in 1961, but in spite of the best eVorts of the engineers involved it failed to meet the agreed specification. The reasons for this have never been fully resolved. An investigation conducted long after the event by Agar, Brown, Mulvey and Smith (1998) concluded that the electron-optical design was certainly sound. But the investigation also revealed that on a sketch of the Everhart– Thornley detector used by the engineer in charge of construction and installation, the scintillator voltage could be misread as 2 kV instead of 7 kV. If 2 kV was indeed the actual voltage used, then it would certainly account for the substandard performance of the instrument. How such a mistake arose, and why it remained undetected, the investigation failed to uncover. The instrument was eventually returned to AEI, and this signalled the end of the company’s involvement in scanning electron microscopy. Bowden later purchased a SEM of the Pease–Nixon design (see Chapter 2.9A) directly from the CUED, which gave excellent service for many years. Several of these instruments were also sold to other departments of the university. It will be clear from the above account that the failure of AEI to continue in the SEM market had a profound long-term eVect on the fortunes of both AEI and CIC. Some years later a move was made to bring the AEI Scientific Apparatus Department and CIC together to form an integrated business better able to compete in the commercial market, but nothing came of the proposal. Four years after the introduction of the Stereoscan, an enterprising research student in the CUED, Barry Drayton, left the department to set up his own company, Cambridge Scanning, to manufacture scanning electron microscopes. His initial design concept recalled some of Oatley’s early ideas concerning the merits of a ‘simple’ SEM having a relatively modest resolution but capable of handling a wide range of specimens at magnifications beyond the limits of the light microscope. It featured a horizontal column with a large, easily-accessible specimen chamber. Marketing of the instrument was placed in the hands of Bausch and Lomb, and the instrument achieved a considerable measure of success. Later, under

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the name CamScan, the company’s products evolved to become relatively sophisticated instruments, which found a niche market for specialized applications, particularly in the forensic sciences. Since it was founded, several CUED students have joined CamScan, including John Catto, Electronics Designer, and Dick Paden, currently Managing Director. The company has experienced financial vicissitudes and changes of ownership, but at the time of writing it remains a flourishing manufacturer of SEMs in the Cambridge region. The story of the growth of the commercial market in Europe for SEMs, following the launch of the Stereoscan, has been admirably chronicled by Agar (1996). The final phase of the exploitation of the research, which had been undertaken largely under the direction of Charles Oatley during the 1950s, came in 1967 when Philip Chang moved from the CUED to take up employment with CIC. In his PhD work Chang had developed a method of controlling the electron beam of the SEM using a flying-spot scanner, by means of which he was able to write complex patterns on to the surface of a suitable substrate. This work had followed directly from that of Broers a few years earlier (see Chapter 2.10). At CIC, Chang continued with this work and in 1971 succeeded in writing patterns in the then recently discovered PMMA photoresist, the basis of silicon-wafer microcircuit production. This initiated a major development at CIC devoted to the production of electron beam lithography systems, and which led eventually to the company becoming the world’s largest supplier of such machines. Bernard Wallman and John Sturrock describe these developments in Chapters 4.4 and 4.5. The Cambridge Instrument Company no longer exists as a separate entity, but two successor companies continue to manufacture electron beam instruments in Cambridge: Leo Electron Microscopy Ltd, which manufactures SEMs, and Leica Cambridge Ltd, which supplies electron beam lithography systems. The preceding account has dealt with those aspects of the commercial exploitation of research in the CUED that have led directly to the marketing of instruments worldwide, but two other developments should also be mentioned. The first of these was initiated by Wells, who, after leaving the CUED in 1959, moved to the Westinghouse Research Center in Pittsburgh, where he was engaged for five years in the design, construction and application of a new SEM (see Chapter 2.3). Everhart took industrial leave in 1962–63 from Berkeley, University of California, to join him in this enterprise. The instrument they constructed, the ‘Micro-Scan’, was used to investigate methods for the nondestructive inspection of semiconductor microcircuits by means of the voltage-contrast technique developed earlier at the CUED by Oatley and Everhart (1957). It was employed also for investigations on semiconductor device fabrication, and for the development of an electron-beam

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induced current technique to produce outlines of the volumes enclosed by a reverse-biased p–n junction. Although the Micro-Scan was not placed on the market, it must be regarded as one of the first commercial applications in the United States of the research begun by Charles Oatley. A description of the Micro-Scan was published at the time in The Engineer (American Editor, 1965). The second development worthy of mention was initiated by Nixon, who had developed at CUED a portable electron-optical column for the purpose of teaching the principles of electron optics, transmission microscopy and electron diVraction. This was used very successfully in the third-year electrical sciences teaching laboratory at the CUED. With additional components, it could be readily converted to a simple SEM. In 1968, John Banbury, Nixon’s research student, completed his PhD and, together with Nixon and Smith, formed Cambridge Western Ltd to exploit the teaching electronoptical column, principally as a scanning instrument. It was developed and marketed, but sales were insuYcient to keep the company afloat and it finally went into voluntary liquidation.

Acknowledgements My thanks to Alan Agar, Doug Atack, John Buchanan, David Clayton, Tom Everhart, Bob Lindsay, Tom Mulvey, Sheila Smith, and Oliver Wells for supplying information and help in preparing this chapter.

References Agar, A. W. (1996). The story of European commercial electron microscopes, in ‘The Growth of Microscopy’, edited by T. Mulvey. Adv. Imaging Electron Phys. 96, pp. 415–584. Agar, A. W., Brown, P. D., Mulvey, T., and Smith, K. C. A. (1998). The curious history of the first SEM produced independently by AEI Manchester. Cambridge 1997, pp. 266–269. American Editor. (1965). The American scene: Scanning electron microscope for inspection of semiconductor microcircuits. The Engineer, March 12, 492–495. Oatley, C. W., and Everhart, T. E. (1957). The examination of p–n junctions with the scanning electron microscope. J. Electron. 2, 568–570. Stewart, A. D. G. (1985). The origins and development of scanning electron microscopy. J. Microsc. 139, 121–127.

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4.1B AEI Electron Microscopes—Background to the Development of a Commercial Scanning Electron Microscope A. W. AGAR Formerly at: Agar Scientific Ltd, Stansted, UK

In the 1950s, Metropolitan-Vickers Electrical Company (‘Metro-Vick’, later to be called AEI), was primarily concerned with heavy engineering products (power stations, switchgear, large transformers and motors). There was a considerable number of smaller departments, but the top management were almost all elevated from the heavy engineering side and the organization was oriented in that direction. Export business was handled by oYces in each territory, and these were naturally staVed by men mainly used to negotiating large contracts at government level, not to marketing small specialist items such as electron microscopes directly to university customers. This greatly limited their exploitation in export markets. The Scientific Apparatus Department was set up to cover work on vacuum equipment, mass spectrometers, electron microscopes, linear accelerators, nuclear magnetic resonance and large vacuum systems. Almost the only common denominator in this range was the need for a high vacuum, which was triggered by the development of the low-vapour-pressure oils by C. R. Burch, permitting the use of high-performance oil diVusion pumps. (Burch was a very ingenious man who not only researched these oils while with Metropolitan-Vickers but also, on moving to Bristol University, developed reflecting microscopes, grinding the lenses himself. He later was intrigued by the men panning for gold, and managed to design a machine to reproduce the required motion). The availability of diVusion pumps using the low-vapour-pressure oils gave a significant boost to any instrument dependent on reliable high vacuum. However, these diverse instruments all needed a significant engineering eVort if they were to be commercially successful. In the mid-1950s, John Waldron, in charge of the smaller scientific instruments, recognized the inadequate engineering eVort on such products, and recommended that the production of large equipment (mainly one-oV orders) should be phased out. This suggestion was not accepted by the 317 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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division in which he worked and of course contributed to the diYculties that followed. It was only in 1964, when Waldron assumed the post of General Manager of the department, that he could carry out his plan. It should be interjected here that in 1948, Mike Haine had moved from Metro-Vick to join the new AEI Fundamental Research laboratories established at Aldermaston. He headed the Electron Physics section, and instituted research into electron lenses, magnetic circuits, high-voltage supplies and their stabilization, and contamination build-up in a vacuum. These studies led to the series of papers by Liebmann on lenses and lens design, and on magnetic circuits by Mulvey. There was also work on high resolution based on the ideas of Denis Gabor for using holography. Haine and Einstein worked on electron guns, and Ennos on contamination. Bradley produced the important breakthrough of using carbon films for supports and replicas. These results were taken up by Page in Manchester, and incorporated in later microscopes in production. After reading of the brilliant work of Castaing in building a microanalyser, Mulvey was commissioned to develop one using magnetic lenses. The Aldermaston instrument was shown at conferences, and was then passed to Manchester for production. In the mid- to late-1950s, production of transmission electron microscopes was running at 12–15 instruments per year, which turnover did not justify a very large engineering expenditure. This very restrictive view in relating engineering eVort to turnover in an early stage of development failed to recognize that a significant extra investment in engineering eVort (and indeed sales eVort) was needed to match the very large potential market. This need should really have been recognized very much earlier because the competing companies in Germany, The Netherlands and Japan, had already devoted much investment into electron microscopes. As a result of the narrow view of earlier management in AEI, there was a sales force of one man for electron microscopes, plus some oYce assistance. The engineering team had Dick Page in overall control and responsible for all forward thinking for the product. He also had to keep a close watch on the quality of the factory output, and help to sort out the more diYcult problems involving some knowledge of electron optics. His team included John Owen, handling detailed engineering changes to successive batches of the EM6 instruments produced from 1956 onwards; Ken Heathcote dealing with the electronics; and Frank Bancroft who dealt with mechanical design for all the instruments in the department and liased with the drawing oYce. The recently recruited Ian Openshaw was responsible for getting the Aldermaston microanalyser into production. When the order for the Bowden SEM was received, this was also given to Openshaw to handle. This team was pathetically inadequate to manage all these tasks successfully (see Chapter 4.1A; Agar et al., 1998).

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It is evident that, at this time, electron microscopes were regarded as just one small component in the overall eVort in Scientific Apparatus, and that they suVered from having no one person with overall responsibility for marketing and engineering development, and for business strategy. Thus when the Bowden order was received, it was happily accepted by the sales team as additional turnover, but they totally ignored the lack of resources to complete it successfully. There was apparently noone around who realized that an order such as this must have arisen from a new customer demand that might have great significance for future business, and which needed immediate evaluation. The complaint of the former Chief Engineer of the division was that he never knew of any market for the SEM, and eVectively blamed Professor Oatley for not pointing it out. However, Oatley himself did not know what the market might be, but was convinced that demand would be substantial. If AEI had sent a microscope user to visit Bowden, asking for the background of his need for the SEM, he would certainly have found that there were problems unsuitable for TEM that were easily dealt with by the SEM. There seems to have been a significant lack of communication about the consultation agreement between the Cambridge University Engineering Department and AEI. It was apparently quite informal; indeed, Ken Smith knew enough about it to consult Dick Page when designing the lens for the SEM3 microscope, but it appears that neither Page nor Openshaw fully appreciated that they could call on the Engineering Department for assistance. All the same, the fundamental failure lay in the lack of engineering and commercial resources devoted to electron microscopy in AEI at that time. As far as can be ascertained, the forward thinking depended heavily on Page himself and very informal conversations with the sales team. According to Openshaw, when he joined Page in 1956, he was first given the job of developing an improved version of the EM4, to be called the EM4A. Page apparently envisaged that this might form a basic design, which could be adapted to a microanalyser or a scanning electron microscope. The EM4A never did reach production, but Page certainly continued the general idea when he decided to copy the basic microanalyser design, used on the Aldermaston instrument, for the SEM ordered by Bowden. This decision might seem perverse, remembering the very successful SEM3 designed and being used by Ken Smith in Canada, but though Page had collaborated with Smith in the design of the objective lens, he had no details of the design of the rest of the SEM3. No detailed drawings existed, there was as yet no scientific publication, and at the time the SEM order was received from Bowden there were no results from Canada. It might have been possible to obtain some outline information from Oatley, but Page’s decision would, in any case,

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probably have been heavily influenced by the savings to be made by adapting an existing microanalyser; he would have had a very limited budget, and he already had in mind the possible versatility of a common basis for a range of instruments. It seems at least possible that had Page, as a very ingenious and perceptive engineer, had early knowledge of the performance of the Canadian instrument, he might have been able to use that as the basis of the instrument for Bowden, and would undoubtedly have incorporated improvements of his own. If this had happened, AEI would have had an excellent SEM to oVer the world, and its subsequent history would have been very diVerent.

Reference Agar, A. W., Brown, P. D., Mulvey, T., and Smith, K. C. A. (1998). The curious history of the first SEM produced independently by AEI Manchester. Cambridge 1997, 266–269.

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4.2A Microscan to Stereoscan at the Cambridge Instrument Company M. A. SNELLING Formerly at: Cambridge Instrument Co. Ltd.

I. Introduction My modest involvement in the development of the Stereoscan had origins further back in my career than might have seemed likely. Towards the end of my ten years or so with the Mullard Research Laboratories, (now the UK Philips Research Laboratories), near Redhill, Surrey, I transferred to their magnet group to design the instrumentation for a major new magnet venture. Mullard had a long-standing interest in general permanent-magnet applications utilizing their ‘Ticonal’ range of materials whose product uses included bicycle dynamos, loudspeakers, oil-well broken drill extraction tools and semi-precision permanent magnets. Mullard saw in the postulated use of permanent magnets for nuclear magnetic resonance (NMR) work, a new and possibly lucrative field for Ticonal sales. At this time Varian Associates’ electromagnet 30- and 40-MHz NMR equipment had been on the market for some years and had demonstrated the advantages and disadvantages of electromagnets in this application. There was a school of thought that permanent magnets with adequate environmental temperature control would provide better field stability than electromagnets, although there could be some diYculty in obtaining magnetic fields of adequate strength for the required spectral resolution. The ability to determine fine structure detail in a NMR response depends, in part, on the absolute field strength across the sample, as well as on the field uniformity over the sample. Mullard embarked on a not very well funded programme to design and build some exploratory precision permanent-magnet systems aimed at a gap-field of about 10 kilogauss (using the units of the time; i.e., 1 tesla). This was reasonably successful, largely through the single-minded drive and energy of the project leader, Brian Evans, and his close contact with Oxford Clarendon Laboratory’s Dr Rex Richards. Eventually, there was funding for the design of a complete NMR magnet control and spectrometer equipment as well as the basic magnet. 321 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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The first such instrument was built under semi-prototype conditions and sold to Birmingham University. Thereafter it was decided to put in hand a production batch of two or three further equipments, but the overall project was probably fated to be unsuccessful. Varian Associates had, meanwhile, successfully introduced a 60-MHz system with an electromagnet, and a number of companies (including the UK branch of Perkin-Elmer) were pressing Mullard to sell them Ticonal permanent magnets for inclusion in their own NMR equipment. Also, AEI Ltd had paralleled the permanentmagnet work at Mullard with their own electromagnet systems, which now became a market distraction. (In the event AEI were no more successful in their NMR ambitions than in their attempts some years later to enter the fields of X-ray microanalysis and scanning electron microscopy!) At about this time I decided that, after ten years at Mullard, it was appropriate for me to move elsewhere. My wife-to-be, Deborah, and I drew-up a short list of places where we would like to go, and both of us were attracted to Cambridge. Rather cheekily, I made a direct approach to the Cambridge Instrument Company and was interviewed at their London oYce by the recently appointed Managing Director, Harold Pritchard, together with Steve Bergen, his personal assistant (destined later to be the Cambridge-based Chief Development Engineer). They immediately made me an attractive oVer to join the company. It was only later that I realized why my rather naive approach to CIC had been so well received. Unknown to me, Pritchard was actively seeking a prestige high-technology product that would utilize CIC’s undoubted skills and knowledge and take it into the twentieth century with an improved market image. For some time, NMR had been the preferred choice for this glamorous future, and was still the favoured candidate when I approached CIC with what must have seemed to Pritchard and Bergen an almost unbelievably relevant background. I eventually joined the expanding R&D department in January 1959, by which time the decision had been taken that CIC would not pursue NMR. No doubt the reasons for this were similar to those of Mullard: the quality and experience of the potential competition.

II. Employment with CIC Rather unexpectedly, on arriving at CIC I found myself involved in various aspects of gas analysis, a field in which the company had been foremost almost from its founding by Horace Darwin. There was some scope for re-examining the company’s use of technologies that it had largely developed and pioneered many years previously. Joining CIC from part of a large

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multinational company, I was well placed to appreciate the strengths and versatility of what was still largely a ‘family’ business in many senses of the word. My background had been in a large industrial laboratory complex, one of several within a world-renowned industrial conglomerate. As such, and with a vast product range, the Philips group had many strengths but was bedevilled with uncertain and varying policies, often expressed as dictates from on-high with apparently little awareness of shop-floor and market conditions and trends. By contrast, CIC was still staVed by people at all levels who rightly felt themselves to be an essential part of the company’s assets, abilities and strengths. It was not at all unusual for someone on the ‘shop floor’ to observe some aspect of a product or procedure that could be changed with advantage, and to have their opinion respected and taken note of. While realising how refreshing and valuable these characteristics were, I could not know how, some years ahead, they would largely be swept away by successive changes in CIC management and the many attempts by ‘new brooms’ to involve management consultants and other ‘modern practices’. The arrival of my wife and myself in the village of Haslingfield, which was to be our home for over forty years, coincided with the completion of the rather splendid new CIC R&D building on Chesterton Road, Cambridge, equipped to an extent that must have seemed to older company employees, and to some longer-standing directors, as grossly extravagant. The formal opening ceremony was to be a very grand aVair. VIPs included Sir Keith Joseph, then Minister for Technology, Col. Hurrell (the Cambridgeshire Lord Lieutenant) and the University Vice-Chancellor (Lord Adrian). I was fortunate enough to be among the welcoming group in the new building’s entrance hall and was well placed to observe an incident that could only have taken place in Cambridge. Every eVort had been made to ensure that a parking space was retained in the road outside to accommodate Lord Adrian’s limousine when it arrived. There was great consternation when a rather frail little elderly gentleman rode up on his bicycle, which he carefully lent against the parking space’s kerb. It was only when the cyclist removed his bicycle clips that someone realized it was Lord Adrian himself, who had decided to travel to the ceremony as he (and most others in Cambridge) always did—by bike! Pritchard had been brought into CIC in January 1958, as a prote´ge´ of Dr Percy Dunsheath (of high-voltage cable fame) then company Chairman, who was therefore the real originator of the ‘breath of fresh air’ that Pritchard was to exemplify in the company. It was Pritchard who had caused the new R&D block to be built at CIC’s Cambridge site. His aim of injecting new life into CIC, while utilizing to the full the company’s strengths—its highly skilled instrument makers and considerable electromechanical experience—had led him to consider another, more local, product option.

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A. Tube Investments X-ray Microanalyser South of Cambridge, at Hinxton Hall, Tube Investments (TI) had established their main R&D unit under Dr T. P. Hughes as Research Director. Coincidentally, Pritchard and Hughes had been colleagues at the Royal Aircraft Establishment earlier in their careers. The growing liaison between Pritchard and Hughes, and later between Pritchard and Dr Jim Menter, is described elsewhere in this volume, as is the partnership of Drs David Melford and Peter Duncumb. What was to become the Microscan Scanning X-ray Microanalyser was the subject of a classic ‘back-of-envelope’ sketch of Melford’s, dated Christmas 1957 (Chapter 3.4). By the time Pritchard first saw the TI instrument on 20 March 1959, it was clearly not only a very interesting piece of equipment but it had also generated enough interest to warrant commercial production. Hinxton Hall had attracted many visiting scientists who, on seeing Duncumb and Melford’s working microanalyser, appeared to be potential customers for a commercial version. Once Pritchard had seen the instrument he was quick to ensure that CIC could manufacture it under licence, believing it to be their sought-after prestige product, with the advantage of a local origin accompanied by traditional university/CIC close contact to which TI was now added. Arrangements with TI and the Cavendish were negotiated and agreed by the end of June 1959, and a suYcient initial range of CIC publicity material was available at the 1959 Stockholm International Conference on X-ray Microscopy and Microanalysis. CIC used Melford’s TI sketches to create production drawings and, making almost unbelievable progress, completed a working prototype in not much over six months. This production prototype was exhibited at CIC’s London oYce, in conjunction with the June 1960 Institute of Physics and Physical Society’s annual scientific instrument exhibition in London. That instrument was subsequently delivered to AWRE (the Atomic Weapons Research Establishment) at Aldermaston later that year. Production of the Microscan grew apace as an unprecedented world market materialized, with more than 150 instruments being delivered over the next few years. B. The Scanning Electron Microscope At a series of meetings arranged by Bergen with Drs W. C. Nixon and K. C. A. Smith, various possible extensions of CIC’s participation in the field of electron-optical equipment were explored (see Chapter 4.1A). An internal, June 1961, CIC report of mine summarized the conclusions of these discussions and outlined a speculative programme for developing a viable commercial scanning electron microscope by evolution from the most recent

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Microscan variant. Another conclusion of these discussion meetings was that there was need to build-up the company’s own electron-optical expertise, preferably by transferring someone with electron-optical knowledge from the university to the company. Within the Cambridge University Engineering Department A. D. G. (Garry) Stewart was coming to the end of his postgraduate work and appeared to be a very suitable candidate for such a transfer. Stewart eventually joined the company oYcially in April 1962, following a period of informal liaison. Bergen, as CIC’s Chief Development Engineer, set as a first work target, the creation of a ‘lash-up’ model demonstrating the basic features of scanning microscopy. Work on such an instrument had already started in late 1961, with Stewart’s involvement. Although in part evolving from my June 1961 proposals, the latter were largely overtaken by Garry Stewart’s arrival. His many subsequent original contributions to what was to become the Stereoscan ensured the eventual emergence of a truly novel instrument with pioneering features, which led the way and set the standard in the new technology for many years before inevitably encouraging the development worldwide of many second-generation competitive instruments. The first ‘lash-up’ instrument was basically a Microscan stripped of its spectrometer and counting equipment, and fitted with an extra lens and a scintillation detector. Attempts to align a multilens column were key to ensuring that later instruments were designed and built to tolerances that allowed the use of a prealigned column. Steve Bergen, in a flash of inspiration, gave the trade name ‘Stereoscan’ to this new product. C. The Geoscan At about this time, CIC began to experience the first of many organizational diYculties attending its intention to develop complex new products. The company had for sometime been assisting Dr J. V. P. (Jim) Long, of the university’s then Department of Mineralogy and Petrology, with the construction of a microanalyser designed primarily for geological applications. Key features of this new instrument were the inclusion of two fully-focusing X-ray spectrometers operating at a high take-oV angle. The final elegant and complex structure of these spectrometers owed much to CIC’s many years experience in mechanisms, largely embodied in Don Unwin’s expertise in the realm of classic semi-kinematic design. The programme for building-up Long’s own instrument almost inevitably led to the company’s decision to manufacture completed instruments and market them in addition to the continuing production of Microscans. The new instrument was to be marketed initially under the name Geoscan, to indicate its prime application and to distinguish it from the Microscan

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family, although there were some reservations about this choice of name from those who felt it implied too restrictive and narrowly specialized an instrument. Work on the Geoscan, and on the as yet unnamed scanning microscope, went in parallel during 1962–64, with growing concern within CIC that the two projects ‘clashed hideously over everything’. The production prototype Geoscan was to have been shown at the January 1963 Institute of Physics and Physical Society’s annual exhibition of scientific instruments, but was not ready in time. In the event an ‘improved’ Microscan incorporating changes proposed by Stewart was shown instead. At the same time, a further version of the scanning microscope was shown at the company’s London head oYce. The Geoscan was eventually launched sometime later in 1963 or 1964, and overlapped in production with the Microscan, until production of the latter ceased around the end of 1964. Continuing improvements to the Geoscan caused its marketing price to rise from an initial £20 000 to around £35 000 after 18 months. Sophisticated and versatile as the instrument was, it seems its price became too high for the market. Later instruments were ‘re-badged’ as higher marks of Microscan.

D. Injecting New Skills at CIC Early in the evolution of CIC’s scanning electron microscope, I felt that the injection into the company of electron-optical expertise by recruitment of Garry Stewart needed to be accompanied by other members of staV acquiring knowledge of electron-optical hardware and its ‘folklore’ that was no doubt common in companies already active in this field. Using the Institute of Physics listing of the Electron Microscopy and Analysis Group (EMAG) membership, I drew up a list of representative UK installations of the major commercial transmission electron microscopes of the time, such as Siemens, Philips, RCA and AEI. I wrote directly to ‘owners’ of sample instruments asking if an informal visit to their installation would be possible. Without exception this approach produced open invitations. Don Unwin, Head of the Mechanical Laboratory, and I embarked on a series of visits to representative installations and attempted to immerse ourselves in the practicalities of electron-optical vacuum equipment design and construction. Invariably such contacts proved informative and very stimulating. On one such occasion we were returning from some Midlands laboratory by train. Having changed at Peterborough onto a local train we were crossing the Fens, covered with their characteristic evening mist, and fell to discussing speculative forms of microscope specimen chamber. The ‘drawer’ format used on the Microscan had long been regarded as awkward

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and restrictive, causing the design of specimen chamber mechanisms to be overly complex. We mused that something like a ‘cruciform’ drainpipe junction would provide a multiplicity of orthogonal access ports for various optional accessories. Continuing discussion recognized that there would be practical disadvantages in actually using a real drainpipe component made of cast semi-porous material. Our ideas evolved towards machining, from a solid metal block, an octagonal or hexagonal chamber allowing the provision of many access ports and flat faces around a central clear volume. This became the basis for its eventual design. CIC staV visits to other relevant establishments were not confined to the acquisition of electron-optical knowledge. A visit to Mr Eric F. Priestley at the Fighting Vehicles Establishment, Fort Halstead, was arranged to seek his guidance on spectrometer crystal-making techniques in which he was a recognized master. My colleague, John Gibbons, who was much involved in the final development stages of the Geoscan’s fully-focusing spectrometers, and I had a prolonged and valuable discussion with Priestley during which he passed on to us an immense wealth of experience and knowledge. Towards the end of this lively talk, Priestley modestly asked if we would like to see ‘his’ microanalyser. I was intrigued because a Fort Halstead installation had not featured in any of the lists I had examined. In a neighbouring room we were shown an unconventional-looking instrument that Priestley shyly confessed he had designed and built himself in his spare time from various salvaged units and components and apparently without a budget! We realized that Priestley’s reputation as an accomplished scientist and engineer was very well deserved. Alongside the evolution of the Geoscan and the Stereoscan were other distractions and uses of the company’s overstretched resources. Attempts were being made to update and extend other spheres of traditional CIC activity—notably in medical and in industrial instrumentation. There were even some other ‘spin-oV’ electron-optical projects, such as the creation of a ‘three-spectrometer’ Microscan monster! This was a one-oV development for Rio Tinto Zinc (RTZ) Mining Corporation. The contract was placed by an RTZ executive, Oscar Weiss, who recruited another Cambridge graduate, Dr Roy Switzur, to spearhead the use of the new instrument. Strangely, RTZ and Oscar Weiss were already known to me. Many years previously, while at Mullard, another development team in the next laboratory to mine had been working on an RTZ project, also involving Oscar Weiss: the development of an airborne magnetometer. There, as at CIC years later, Weiss proved to be a very dynamic and forceful taskmaster, whose extreme and focused enthusiasm proved rather overpowering for his subordinates. The increasing reputation of CIC as a designer and manufacturer of electron-optical instruments was to have some strange side eVects. One such

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event took place long before the spin-oV of the company’s work into electron-beam microfabrication (EBMF), although almost foreshadowing this. Two unexpected visitors arrived in my oYce one day having refused to tell our reception staV why they had come and what their business was. The two men, both in immaculate suits and with a somewhat ex-military bearing, seemed strangely reluctant to explain their presence. Eventually one of them asked whether these ‘new electron-optical devices’ could be used for reading or, better still, creating microscopic writing. It emerged that the two visitors were from some part of the intelligence services and I was firmly told I must never talk to anyone about this visit or their interests. I was uneasy how best to respond to their questions, but thought to mention that our managing director had formerly been Superintendent of the Australian Woomera Rocket Range, and was no doubt well versed in how best to liase with the security forces. They noticeably relaxed when given this information, and became even more so when I oVered to find out if Pritchard would see them. I knew that this day was one of the many during which he visited the company’s Cambridge site, where it was his custom to stroll informally around departments, especially the mechanical laboratory, where he and Don Unwin appeared to have achieved a special rapport. A quick phone call made the arrangements and I took the two visitors up to Pritchard’s oYce. Yet another example of the flexibility of a small ‘family’ company and the then informality of its structure. I guess that the passage of some forty years or so allows me now to tell this tale without risk of charges relating to oYcial secrets legislation! Early in the Stereoscan’s development it was felt that the inclusion of a conventional optical microscope, allowing preliminary visual viewing, would aid manipulation and recognition of the specimen prior to viewing at a much higher electron-optical magnification. An initial design by the company that had been CIC’s previous optical collaborator proved disappointing. The problems of a conventional optical microscope ‘piercing’ the wall of a high-vacuum chamber had not been recognized or addressed. An alternative optical collaborator was sought and we were immensely fortunate to find Mr Bates in R. & J. Beck’s R & D team. His inspired lateral thinking combined with Don Unwin’s considerable mechanism design experience produced a lovely concept in which only the final objective lens assembly of an optical microscope was mounted within the SEM’s vacuum. The lens was modified with ‘vent-holes’ between components to avoid glass fracture due to pressure diVerentials, and was mounted on an elegant classic strip-hinge assembly to allow focus adjustment from outside the vacuum chamber wall.

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III. Selling the Stereoscan The startling initial success of the Stereoscan owed a lot to the unprecedented wave of publicity that was put out by the company to the technical and lay media. This extraordinary campaign was largely the personal creation of Mike Cassidy, then involved in company publicity at CIC’s London head oYce. His approach to publicizing the possibilities of what was a new technology was inspirational. He made extensive use of early Stereoscan results, arguing (rightly) that here was a device that could sell itself by use of its intricate, and often very beautiful, micrographs. The wide-ranging campaign included articles in the major broadsheet newspapers, the New Scientist, the Illustrated London News, the Stock Exchange Gazette, and others. I think television coverage included the BBC’s Tomorrow’s World programme. The very first Stereoscan installation preceded the availability of actual production instruments and arose from pre-marketing interest. In November 1962, during early development of what was to become the Stereoscan, we were visited by Dr Sloane of DuPont at Wilmington, Delaware, USA. I think he had become aware of our activities and intentions via contact with the Pulp and Paper Research Institute of Canada where Ken Smith’s pioneering experimental instrument, SEM3, was still in use. Dr Sloan’s presence in the company’s R&D department contributed one more story to in-company folklore. His company was looking for improved methods for studying artificial fibres such as nylon. CIC staV were shocked when he cut a piece from the sleeve of the nylon shirt he was wearing to provide a suitable specimen. This was at a time when ‘drip-dry’ shirts were a highly prized novelty possession in the United Kingdom. I suspect that, as a result of the sacrifice of his shirt, Dr Sloan’s reports back to DuPont, during his period at CIC changed tone rather abruptly from ‘could be very interesting and useful’ to ‘We must have one’. For the outcome of his visit was prolonged and intense pressure from DuPont to supply an instrument without delay, which led to a much-debated decision to sell the first fully engineered prototype to them. Careful arrangements were made for the transport of this precious equipment to DuPont, but when the installation engineer (Jim Culpin) arrived, he was appalled to find the microscope badly damaged. After it was returned to the United Kingdom, Don Unwin examined the remains and was able to deduce from the distortion of certain components that the crate had been allowed to slide down a ramp without restraint then stopped abruptly. Subsequent investigation by the insurers revealed that this was indeed the case, the trucking company transporting the instrument from the airport to the DuPont site being responsible. It was some six months after the original target date of June 1964 that the microscope was eventually commissioned at DuPont.

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Prior to the arrival in CIC’s very extensive instrumentation range of electron-optical products, the company had been accustomed to worldwide selling and had an extensive range of overseas oYces, agents and service centres, and even a number of overseas subsidiaries, some of which embraced manufacturing plants. However, these new, expensive multitechnology additions to the company’s range were going to necessitate radical changes in philosophy, especially as regards after-sales support. The more traditional products, such as medical and industrial instruments, had generated a lucrative after-sales market for spares and accessories. It sometimes seemed that selling recorder charts and pens, replacement thermocouples and microtome accessories became an end in itself, and was thought to generate appreciable income from these deliberately (?) nonstandard items. The eventual arrival in the market of competitive electron-optical manufacturers caused a major rethink of this attitude. Notably, when Japanese Electron-Optical Ltd (JEOL) started to make inroads into the SEM field, it was found that a typical JEOL-installed instrument was accompanied by a large steel cabinet containing a wealth of spares and consumables: gun filaments, apertures, vacuum seals, specimen holders and sometimes major assemblies as spares or for ‘owner’ customizing. Furthermore, the extensive, versatile and competent teams of JEOL service engineers allowed technical support to arrive quickly and eYciently worldwide. At one time there was a black-humour joke within CIC that the infamous JEOL steel cabinet included a miniscule JEOL service engineer ready to leap out and obey the instrument owner’s every command!

A. Third European Regional Conference on Electron Microscopy, Prague, 1964 Garry Stewart and I attended the 1964 Third European Regional Conference on Electron Microscopy in Prague, at which the Stereoscan was first introduced to the scientific community via three papers: one on the instrument itself (Stewart and Snelling, 1964), and two on applications (Culpin and Dunderdale, 1964; Drummond and Thornton, 1964). The papers presented at Prague originated from some of the first uses of the experimental Stereoscan at CIC, which had already demonstrated the wide diversity of possible applications that were to characterize its versatility. The paper Gary and I gave, which is reproduced as Chapter 4.2B, included a stereo-pair micrograph of Dr Sloan’s ‘soiled shirt material’. Vigorous interest in fibre and fabric specimens was also evident in the paper by K. F. Dunderdale, of the Textile Physics Laboratory in the Leeds University Department of Textile Industries, and M. J. Culpin of CIC

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(Culpin and Dunderdale, 1964). Dr Jan Sikorski, Head of the Textile Physics Laboratory at Leeds, although not lending his name to this paper, was also actively involved in the work described. Sikorski’s studies of textiles extended back over many years, and his laboratory’s acquisition of one of the first production Stereoscans probably represented the culmination of this work. Sadly, despite this involvement and motivation, prior Polish/Russian history during and after World War II, meant that Dr Sikorski felt unable to risk a personal appearance in what was then still a Russian-dominated hard-line communist state. A very diVerent SEM application that eventually led to another early Stereoscan installation was featured in the paper by Drummond and Thornton (1964), again based on work done on the experimental instrument at CIC. The direct examination of voltage diVerences across semiconductor junctions had been demonstrated some years earlier by Oatley and Everhart (Chapter 2.4A). Dr Pat Thornton, of the Services Electronics Research Laboratories at Baldock, a few miles from Cambridge, had followed this work closely and was keen to apply the SEM to his own research on semiconductors. Later, working at the University College of North Wales, Bangor, he received one of the first production instruments, on which he and his students carried out pioneering work. Participation in an international conference usually provides opportunities to experience and absorb the local culture, partly through an associated social programme. Prague was no exception and, perhaps uniquely, gave one a chance to see an imposed communist culture in action. This was long before the thawing of east/west relationships and the end of the cold war. While we were in Prague, the then Russian Prime Minister Nikita Khrushchev visited the city, perhaps to help stifle the relaxed policies of the Czech communist secretary Alexander Dubce´k, and during one of our ‘social’ intervals exploring Prague’s beauty, his limousine cavalcade swept past us at great speed. It was while on one of our walkabouts exploring Prague’s older quarters that we were intrigued to find by the Charles Bridge across the river Vltava what seemed to be monumental masonry steps leading up to a very large plinth on which there was absolutely nothing! As participants at what the Czechoslovakian authorities regarded as an opportunity to ‘sell’ their regime, we had been split into groups for the social extramural activities. Our particular group had been allocated a pair of charming Englishspeaking middle-aged Czech ladies as guides. One evening we asked them to explain the riverside stone steps and plinth. They said that was where the obligatory Stalin statue stood before being toppled when he fell from favour. They were remarkably casual and resigned about such an imposed edifice, and one realized that perhaps much of Central Europe takes a long view of

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the arrival and departure of successive overlords. No doubt Stalin, compared with the Austro-Hungarian Empire, was just a momentary aberration to our Czech friends.

IV. Company Upheavals The oYcial launch of the Stereoscan in 1965 was one of the most significant events in the history of the company, for it marked the beginning of the decline of the old family style of business that had been its hallmark, and of upheavals that would lead to the eventual break-up of the company. In spite of the great technical success of the Microscan and Stereoscan, their development had engendered considerable strains in the company, which were reflected at board and management level by the departure in the same year of the two people most responsible for their introduction: Pritchard and Bergen. In their history of the company, Cattermole and Wolfe (1987) write of the Pritchard era: Thus, in 1962 the Company had parallel design work in hand on three major projects: an updated Microscan, the Geoscan, and the Stereoscan. These projects absorbed nearly all the Company’s research and development budget—and the Geoscan and Stereoscan competed with each other for the largest share of the resources available. The Stereoscan was, however, destined to be another best seller, sales reaching 100 per year in 1968. But the first instrument was not delivered until 1965, and by then Pritchard had left the Company. He had been unable to couple the technological breakthrough which he had achieved for the Company with an increase in profits. Despite a significant rise in the value of the sales turnover in the early 1960s the pre-tax trading profits remained persistently at or below the £0.5 million figure. In 1963 they dropped to under £0.4 million and the results for 1964 were worse still due to increased manufacturing costs and the money being spent on research and development. In 1965 the board decided it could no longer concur with Pritchard’s policies. Although he was oVered the post of Executive Vice-Chairman he eventually decided to leave the Company.

As the electron probe programme continued to grow in scope and pace, my own involvement in it was overtaken by events. CIC’s purchase in the 1960s of other UK instrument companies included Electronic Instruments Limited (EIL) of Richmond (1960), and H. W. Sullivan of Orpington (1967). These and other intended mergers and takeovers, some consummated and others aborted (like the hoped-for amalgamation with Hilger and Watts) are summarized in an article by Don Unwin on the CUED website.

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It was the acquisition of EIL that was eventually to have the most impact on myself and many others. The founder and managing director of EIL was Mr Paul Goudime, who was to continue in the latter role for some time despite a rather uneasy relationships with CIC’s board in general and with Pritchard in particular. One of Goudime’s intentions was that the company’s one-time high profile in so-called industrial instruments should be restored. I was to head up a Cambridge-based R&D team to support this aim, possibly because of my earlier 1959/1960 involvement in the company’s gas analysis work. The industrial instruments R&D team included M. J. G. (Mike) Cattermole and Bernard Wallman; the former was later to make many original design contributions to such instruments, and later still to co-author the definitive book on the history of the company (Cattermole and Wolfe, 1987). A. Metals Research Ltd. Relations between Paul Goudime and myself did not flourish, and turned distinctly sour when one day he arrived in my oYce and announced he had decided that my team and I were to relocate to the refurbished CIC site at Muswell Hill. That evening my wife and I resolved that this was not for us. I started to look around locally and was fortunate in 1967 to obtain a post as Development Manager with a truly green-field young company at nearby Melbourn, Metals Research Ltd. In leaving many good friends at CIC, I could not know that some years later I would be meeting them again as employees of the same company. After many further upheavals at CIC, Metals Research Ltd in 1975 took over the remaining scientific instrumentation part of CIC. However, the takeover was not to prove as successful as the Metals Research board (including brothers Michael and David Cole) had hoped. What seemed an ever-repeating sequence of departmental unification and division left many of us disillusioned and inclined to begrudge the personal eVort needed to encourage and retain staV motivation in the face of demoralizing reorganization. Consequently, I took up a post in 1980 as Administrator to the newly expanded Medical Research Council centre on the new Addenbrookes Hospital site. There I was privileged to provide a service to a number of independent Cambridge MRC units, including the prestigious Laboratory of Molecular Biology, home for a veritable galaxy of Nobel prize winners. Henceforth, I was to be aware of CIC electron optical activities as an observer, albeit with occasional lapses into nostalgic memories.

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References Cattermole, M. J. G., and Wolfe, A. F. (1987). ‘Horace Darwin’s Shop, A History of the Cambridge Scientific Instrument Company, 1878–1968’, Bristol: Adam Hilger / IOP Publishing Ltd. Culpin, M. J., and Dunderdale, K. F. (1964). Studies of fibre surfaces in a scanning electron microscope. Prague 1964, pp. 421–422. Drummond, I. W., and Thornton, P. R. (1964). Studies of electrical high-field regions in high resistivity GaAs with the scanning electron microscope. Prague 1964, pp. 287–288. Stewart, A. D. G., and Snelling, M. A. (1964). A new scanning electron microscope. Prague 1964, pp. 55–56.

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4.2B* A New Scanning Electron Microscopey A. D. G. STEWART AND M. A. SNELLING Cambridge Instrument Co. Ltd., Cambridge, England

The principles and applications of scanning electron microscopy have been discussed elsewhere e.g: (1), (2). The instrument to be described has been designed so that it can easily be modified for special research work as well as being suitable for routine observations. The specimen stage and the electronic modules have therefore been designed as replaceable sub-units. A general view of the microscope is shown in Fig. 1. The column, which contains three electromagnetic lenses, is prealigned except for the gun filament and final aperture. The final lens is designed to work with the specimen 1 cm below the bottom pole piece and is fitted with an 8-pole magnetic stigmator and an aperture changer. The probe is deflected by a double deflection system with scanning coils mounted in the bore of the final lens. The minimum magnification is 30, corresponding to a scanned area 3.5 mm square, the maximum ˚ . The useful magnification being set by the resolution which is better than 500 A electron collection system is similar to that described by Everhart & Thornley (3). A specimen stage is shown in Fig. 2, in the withdrawn position used for specimen changing. X, Y and Z traverses are provided with total movements of 1.6, 1.3 and 1.0  cms respectively. The specimen can be rotated through 90 about an axis perpendicular to the electron beam, this motion being used for taking stereo-pair micrographs. The maximum specimen size is 1 cm square, and 3 mm thick. This specimen stage can be quickly replaced by other stages for special experiments. The specimen chamber interior is 23 cm diameter by 12 cm high, and includes two spare access ports, each 12.7 cm diameter, for the attachment of additional apparatus, or for access to experimental work inside the chamber. The lens windings are tapped, the switch for the taps being ganged with the accelerating voltage selector switch. This allows the accelerating voltage to be changed without having to alter the lens settings, except for slight refocusing, and also reduces the range of currents which the lens supplies must provide. A meter is connected in series with the final lens winding showing the true magnification at all working distances. The accelerating voltage is variable between 1 and 20 kV. The power supplies are housed in two remotely sited racks. The scanning coils of the display and recording cathode ray tubes are connected in series with the input to the magnification control attenuator which feeds the column scanning coils.

*Reprinted from ‘‘Electron Microscopy 1964’’, Proceedings of the Third European Regional Conference on Electron Microscopy, Prague 1964, pp. 55–56. y

Presented at the Conference August 1964.

335 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Fig. 1. Prototype Cambridge Stereoscan scanning electron microscope.

Fig. 2. A specimen stage, partially withdrawn.

The vacuum system includes two diVusion pumps, one pumping the column and one the specimen chamber. Isolation valves are fitted between the specimen chamber and the column, and between the gun and the column. These permit rapid specimen changing and filament replacement without loss of the column vacuum. Most of the vacuum valves are power operated and controlled from a schematic panel. The valves normally operate automatically in the correct sequence but they can be individually controlled if required. Less than 2 minutes are required to pump down the specimen chamber after specimen changing. The column is supported separately from the main display console, thus allowing the operator to work at the console desk without transmitting vibration to the column. To reduce the sensitivity of the instrument to

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Fig. 3. Stereo-pair micrograph, soiled shirt material (Stereo angle 4 , gun potential 15 kV).

vibration the column and specimen chamber are supported above the vacuum system by anti-vibration mounts and connected to it by bellows. When the specimen chamber is under vacuum the column and specimen chamber are resiliently mounted, but when air is let into the specimen chamber the whole column assembly rises and locks rigidly. A stereo-pair micrograph taken with this instrument is shown in Fig. 3. Further examples of work done with the instrument are presented in the papers by I.W. Drummond and P.R. Thornton (4), and M.J. Culpin and K.F. Dunderdale (5) at this Conference. Thanks are due to a large number of people for the loan of specimens and for the useful discussions which have arisen. Particular acknowledgement is due to Professor C.W. Oatley and his co-workers at Cambridge University, who were responsible for much of the original work on scanning electron microscopy.

References

(1) (2) (3) (4)

Smith K.C.A. and Oatley C.W., Brit.J.Appl.Phys. 6, 391–394, (1955). Smith K.C.A., contribution to ‘‘Encyclopaedia of Electron Microscopy’’ – Reinhold, New York, (1961). Everhart T.E. and Thornley R.F.M., Journ.Sci.Instrum. 37, 246, (1960). Drummond I.W. and Thornton P.R., Third European Regional Conference on Electron Microscopy, Prague, (1964). (5) Culpin M.J. and Dunderdale K.F., Third European Regional Conference on Electron Microscopy, Prague, (1964).

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4.3 Memories of the Scanning Electron Microscope at the Cambridge Instrument Company D. J. UNWIN Formerly at: Cambridge Instrument Co. Ltd.

I. Beginnings Ever since an early age the name ‘Scientific’ has been known to me. When my mother was referring to our next door neighbour, Will, she would point out that he had a good job as he worked at ‘The Scientific’, the name by which older local people knew the Cambridge Instrument Company. From the age of eight, on my way to Milton Road School I used to pass that short side-road, once called Camford Road, oV Chesterton Road, and see the big double gates with ‘Cambridge Instrument Company’ in large letters across the top. Of course it did not mean much to me except that it was where ‘Will’ worked. When you walked up Carlyle Road, an intriguing hum could be heard emanating from the double-storey building on the right, while at the gates there were always horse-drawn railway lorries being loaded with wooden boxes. The environment in which I grew up was one of making things, as my father, who was an engineer of the old school, would tackle anything from building a house, setting up a 16-ton machine, to mending a watch or clock. In fact, he was the ‘King of Do It Yourself’ (DIY) before the term was invented. I remember seeing the tiny picture of a woman on the Baird television set he made in 1928! He had a well-equipped home workshop with a treadle lathe in addition to a comprehensive kit of woodworking, building and engineering tools. From an early age every encouragement was given to me, including being taught how to use all this equipment, supplemented by an ever-increasing amount of that greatest of engineering toys, Meccano, which I still have and use for rig-ups, trials of ideas and many other tasks. Reading the several practical journals he took, and the Meccano Magazine for me, backed this up. However, being a self-taught man, father did not consider it necessary for me to go to university. To make up for this I have continued private study throughout my life. 339 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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The Central School followed Milton Road School, and then the Technical School in Collier Road for the engineering course, something I thoroughly enjoyed. The nearest I ever got to the Cambridge Instrument Company (CIC) was when Ikey Buckerfield, engineering instructor at the Tech, sent me to collect some silver solder from Joe Wilderspin who worked there. It was to enable me to hard-solder the boiler of the model locomotive I was building. I got no farther than the entrance room next to the telephone switchboard, but I could hear the tantalizing hum of machines.

A. Starting Work My great friend from early school days, Denis Gravestock, and I both went to the Tech, but when we left he joined the CIC drawing oYce and I went as an apprentice to Unicam Instruments. When I went for my interview, I took the 2 12-inch gauge steam locomotive I had just built, which suitably impressed them. Incidentally I had been awarded Highly Commended for it at the London Model Engineering Exhibition—not bad for a 16-year-old. Unicam was a small outfit, with only about 20 employees, run by the boss Sidney W. J. Stubbens, an ex-CIC Shop H foreman, and his two brothers, Ernest and John, also ex-CIC. Other ex-CIC men were Bill Varley and Ernie Marsh, who always started the answer to a question by ‘When I was at the Company we used to . . .’. He did this for many years until he cottoned on that we were making fun of it; then he attempted to vary it a bit. The directors of Unicam were a Mr Pretty, a director of Robert Sayles, Winton Smith the pork-pie maker, Mr Slater a solicitor, and W. G. Collins, ex-works manager at CIC, who had obviously helped Sid considerably when he was starting up. When I first joined them, I noticed that many of the tools were stamped CIC, so I wrongly assumed that it must be associated in some way! My starting rate was a half-penny per hour more than the normal starting rate of one penny per hour because I had been to the Tech—4 shillings and 8 pence less deductions that included one penny for the hospital! The workshop was in Barrett’s yard next door to the Bun Shop in St Tibbs Row, a rickety building, equipped with mostly old machines, all driven by line shafting powered by an old electric motor which, when starting up, had to be helped by pulling on the belt. I had been there only about a year, during which time I learnt a lot, including gas welding, when Sid Stubbens asked me to go to his new works site in Arbury Road to help fit it out. When I got there it was just a roof, walls still being built and a dirt floor! However, standing on two paving slabs was a Seneca Falls 6-inch centre lathe, secondhand of course but in good order. My first job was to fit up the countershaft and a motor on to the girder work of the building structure. I was completely

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on my own, with only the builders for company, although Sid used to come about once a day to see how I was getting on. After the lathe was operational, I had to make all the fittings for the line shafting and then erect it; fortunately, he sent a young lad to help with the lifting. By this time the walls, windows and floor were completed and Sid and the secretary, his niece, had taken up residence in an already existing timber building on the site. John Mellenby, the electrician who was wiring up the site, also had his store and workshop there. Eventually, the place was ready for occupation and I got involved in moving and installing the plant. I received considerable advice in this from my father, who was the foreman erecting engineer for Chivers at Histon and who considered 15 tons just a nice weight to handle! From this time on I never had production jobs, always specials, usually for Sid’s many university clients. The Seneca Falls lathe became ‘my lathe’, a machine I liked very much indeed. The business must have been doing quite well, as it was not long before an extension was announced. However, soon after the two steel erectors had started work, one of them put his hand through a window and then there was only one! He asked Sid if he could borrow a couple of men to help; guess who was one of them detailed to help. Yes, I was one of them, climbing up stanchions, drilling 3/4-inch holes in girders by hand using a ratchet brace, oxy-acetylene cutting and welding, all in freezing cold weather. I thoroughly enjoyed every minute of it and it was a wonderful and valuable experience. After a spell installing equipment in the extension it was back to instrument making, involving experimental work this time. As most of this was directly for Sid Stubbens, my bench was just outside his oYce and he gave me a young apprentice to help although I was also still an apprentice myself! He also gave me, in addition to the Seneca Falls lathe, a drilling machine and a new 5-inch Atlas lathe. After a period working on various special projects, he obtained several defence contracts as a result of the period of rearmament and it was this work that increasingly occupied my time. Not all was experimental work: much involved special tooling, fixtures and even simple special-purpose machines, most of which I had to scheme out myself. Then the war started and I received my call-up only to have it cancelled due to being in a reserved occupation. Soon after I was in Sid’s oYce and told that, to increase production, two shifts were to be operated in the machine shop, and he wanted me to take charge of one of them. I was transferred to staV and allocated a bright chap to be toolmaker, setter and anything else that needed doing. All this was just after my 21st birthday. We were to work 12-hour shifts, alternate weeks of days and nights. On the night shift we were the only two non-productive people, and I was totally responsible for everything. The operatives were from every walk of life, male and female, some quite young, others old enough to be my grandparents, some very

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intelligent, some dim, mostly unfamiliar with machine work and working in a factory. It was a traumatic but valuable experience for me and I learnt a great deal from it, not least how to deal with people. Sid Stubbens was a very good governor and gave every encouragement to any young person who showed promise. He would support new ideas, suggestions and innovations for production, designs, procedures or organization that resulted in a forward-looking manufacturing unit. I must have been one of these ‘bright boys’ as during the 12 years working at Unicam I had 14 diVerent assignments following that first staV appointment. He would call me into his oYce and say ‘I want you to start up a tool room’, or ‘We have got to make our own optical components, I want you to set this up’. Then as I was leaving the oYce he would say ‘I’ll see that your money is all right’—and he did! During my whole time with Unicam I never had to ask for a rise: it always came after every new assignment.

II. A Job at the Cambridge Instrument Company Then one day early in 1946, as I was cycling home, my friend Denis Gravestock, who had just returned from the forces, met me outside CIC. He said ‘Stallen is retiring and Dr Marsh, Head of Research, would like you to come in and have a chat.’ This I did and was oVered a job as experimental research engineer, to take over the work of William Stallen. It appears that Dr Marsh had been following my activities over the years and decided that I was the person for the job. Of course I had to see the managing director but the interview was very short and painless. This turned out to be a most fascinating job, involving not only engineering but a great deal of physics as well: Dr Marsh, who was an eminent physicist, provided me with encouragement and excellent tuition. I started on 1 April 1946. Compared with Unicam the place seemed to me somewhat old-fashioned in certain areas: machine tools, many quite elderly, were still being driven by belts from overhead shafting, a system we had abandoned at Unicam six years before. However, on press work and the use of die-castings CIC were more advanced. It wasn’t long before I became interested in the history of the company and started to build up my own collection of archive material, and although I was headhunted away in 1974, the process has continued to the present day. The company dealt with such a huge range of instruments: over 2000 diVerent types, not including detail variations, were manufactured at the Cambridge works alone. Products were made at other sites such as Muswell Hill in London and, after 1949, at the Finchley works as well.

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My 28 years at CIC were full of interest and challenges: there were few of the 2000 product lines in which I did not in some way become involved, either on design, development or in a problem-solving role. I also had considerable contact with the other sites as well. Perhaps the work that gave me the most satisfaction was helping young people to develop their potential skills. My first taste of this came when Dr Marsh asked me to start up a ‘first year oV-the-job training’ school for the apprentice intake. It was to be additional to my normal work and expected to absorb about 10% of my time. I was to set the curriculum, choose the plant, fix the layout of the working area, arrange for lectures by various members of the staV and find a suitable instructor, in fact everything except the daily running of the school. Right from the start I decided that all the training workpieces made by the apprentices were to be of use to them after they left the school and went into the factory—no filing of squares to fit holes and suchlike useless and demoralizing activities. Then came my first problem. Albert Barker, the Tool Room Foreman thought he knew all about training apprentices and said that his was the way it should be done. Needless to say, it was done my way and later I had the satisfaction of advising the newly set up Engineering Industries Training Board on devising their curriculum, and they included several of our exercises in their programme. One day, while I was talking to the instrument shop foreman about the new school, he suddenly stopped and pointed to one of the older instrument makers who was passing by. ‘There’s the man you want as instructor, a good craftsman and a fatherly type of person’, he said. That is how the first Instructor, Harry Speechley, was chosen and what a good choice it turned out to be. He was excellent and well respected by everybody. An apprentice committee, chaired by Dr Marsh and of which I was a member, interviewed all new applicants. It must have been quite a traumatic experience for the interviewee confronted by three or four old fogies asking silly questions such as what their hobbies were! Later I took over chairmanship from Dr Marsh and continued for many years. At the time the Physical Society held a yearly competition for apprentice work and we decided to send an entry from the first year of the school. We chose the lad showing the most promise, Michael Willis, and got him to make a simple dividing-head for use in the school. We were delighted when he was awarded ‘Very Highly Commended’ and we continued to enter every year until 10 years later when the competition was stopped. The reason it stopped was that every year after that first year, a Cambridge apprentice gained the First Prize! In later years this interesting and satisfying activity developed into dealing with sandwich students and their tutors, and with new young graduates entering the Research Department. My section of the department was known as the Experimental Department. Although at first my team included only two skilled

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instrument makers, it was soon augmented by an apprentice, each staying for six months. As the workload increased, the number of skilled men and the amount of machinery increased.

III. H. C. Pritchard—the New Broom In 1958, a new managing director, H. C. Pritchard, was appointed and the construction of the new four-floor research building facing on Chesterton Road was started. I was asked to act as the company representative engineer to liase with the contractors for all the R&D services. The building was occupied in 1959; on the top floor was the Drawing OYce and Board Room, the next floor down the Chemistry/Gas Analysis Department. Below this was the Physics Laboratory with electronics, and the ground floor area was designated the Mechanical Laboratory, but known colloquially as the ‘Mech Lab’. In addition to a very well-equipped workshop it had facilities for a wide variety of mechanical research and development projects, a controlledtemperature room and vibration testing equipment, for example. It provided a service to all the other research departments including a well-stocked store. A. Fields New—Microscan and Stereoscan Pritchard was looking for new projects to update the company image and which could be made under licence. The first of these was the Ultra Microtome designed by Dr A. F. Huxley. Then by a chance coincidence he found what he was looking for. At the Tube Investments Research Laboratories at Hinxton Hall, near Cambridge, in March 1959 he was shown an X-ray Microanalyser that had been developed by Dr D. A. Melford and Dr P. Duncumb as a research tool (see Chapters 3.3A and 3.4). One week after Pritchard’s visit to view the instrument in March 1959, Steve Bergen, the CIC Chief Development Engineer, also visited Hinxton. By that summer, the company was publicly declaring its intention to manufacture the ‘‘Microscan’’. Dr Melford’s design sketches were used to prepare some 700 production drawings. In parallel, the research workshop made a prototype instrument in about six months—in time in fact, to be demonstrated at a private exhibition at the company’s London oYce while the Physical Society Exhibition was being held in January 1960. By the end of that year, customers’ instruments were already being delivered. Dealing with the prototype of the Microscan was my first experience of electron-probe technology, and it involved many new technologies such as vacuum systems and mechanical components unfamiliar to us. Although it was really only a ‘copy’ job, this

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work enabled us to develop many new manufacturing techniques, some of which stood us in good stead when we started work on the development of the next project, the scanning electron microscope. Not long after the new research building was opened, Pritchard had made a tentative arrangement with Professor C. W. Oatley of the Cambridge University Engineering Department (CUED) to develop for commercial manufacture the SEM he and his team were working on. This was a very diVerent project from the Microscan, as much more development work was to take place at the CIC. As a result, the Physics Laboratories under Michael Snelling, and the Mechanical Laboratories under myself, became deeply involved. By this time the staV of the Mech Lab had grown considerably and included graduate engineers, physicists and hand-picked men who had a flair for design work, able to develop processes and lead other instrument makers. It seemed to us that the obvious point at which to start development of the SEM was to use the basic Microscan column as this provided a readymade electron optical system, though it needed some modifications to suit the diVerent requirements of the SEM. The Physics Laboratory under Mike Snelling was responsible for the overall planning, with the Mech Lab and the Drawing OYce closely involved also. My particular responsibility was the mechanical design. There was considerable liaison between the CIC staV and Professor Oatley’s team at CUED, in particular Garry Stewart who later joined CIC. The research department was able to make a practical start in 1961 after the appropriate parts had been extracted from the Microscan production. Modifications were made to the final lens that enabled a satisfactory resolution of 50 nm to be achieved by the experimental instrument, which was completed in 1962. One of the pieces of equipment in the Mech Lab was the evaporating plant. During the SEM development it was used to evaporate a conducting film on to nonconducting specimens to enable them to be examined. One member of the Mech Lab team was a bright young applied physics graduate, Anthony Randall, and he took this work under his wing. I well remember him coming into my oYce excitedly waving a photograph of a diatom, a tiny crustacean rather like a little rowing boat, the first picture taken on the experimental SEM. Although the resolution was not all that good, it was better than that of an optical microscope and the great depth of focus possible was very apparent. Not long after he was seen catching flies, wasps, butterflies and the like to enable the techniques for examining biological specimens to be developed. Needless to say, there were a number of serious problems to be solved, of which vibration was one. Any relative movement between the column and the specimen degrades the resolution. The rigidity this demands, together

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with support for a specimen 12 mm in diameter having X, Y and Z linear movements plus rotation, posed considerable design problems on the specimen stage. An experimental stage was designed and constructed in the Mech Lab, but while it was adequate for experimental work it was quite unsuitable for a commercial instrument, partly due to the limitations imposed by the design of the Microscan specimen chamber we were using. The vacuum system consisted of a backing pump and oil diVusion pump which, although coupled to the column by a flexible bellows, was fitted within the plinth on which the instrument was mounted. Both pump and ground vibrations were transmitted to the column and badly degraded the performance. The Microscan specimen chamber was rectangular and, being of brass, provided inadequate screening. Other designs that provided more convenient side access ports were considered: one of my original free-hand sketches made at a design discussion is reproduced in Fig. 1(a). The final form of the

Figure 1. Reproductions of original ‘blunt pencil’ free-hand sketches made at early design discussions. (a) Proposal for Stereoscan specimen chamber. (b) Proposal for Stereoscan final lens.

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specimen chamber was made from a square slab of mild steel flame-cut to shape and machined all over. It had a large circular port in each face and  each of the four corners was angled oV at 45 , with two smaller round, ports one above the other. It was immediately given the nickname ‘threepenny bit’, a name which, like the design, remained unchanged throughout the production life of the instrument. This specimen chamber design was larger than our equipment could deal with, so production was subcontracted at first to Pope and Meads at Ware. We gave them an order for one only, to enable us to assess their capabilities. They proved to be excellent and were chosen later to be the supplier of production components. However, to reduce delays we soon installed a larger lathe and milling machine so that we could make our own large components. During the early development stages of the SEM, we in the Mech Lab had to devise new methods to provide the accuracy needed. One of the objections to the Microscan column was the need to realign the components each time it had been taken down for cleaning. There was a fairly frequent occurrence of deposits, such as from the oil vapour from the pumps, that contaminated the bores and caused deterioration in performance. The realigning operation was irritating and time-consuming and required a considerable knack on the part of the operator. Experience with the experimental instrument showed us that the more stringent requirements of the SEM made it essential that we find a solution to this problem. Designing the lenses involved a considerable amount of calculation, all executed on a desk calculator as computers did not become available until some years later. The physicists defined the requirements that enabled us to fix the tolerances of the components of the prealigned column. Because there were three or four interfaces, the dimensional and circularity tolerances on each component had to be very small. As the pole pieces of the lenses formed part of an electron-optical system, the quality of the spot was dependent upon the accuracy of the components in the same way as in a light-optical system. This imposed geometric tolerances on machined components comparable with those used on glass optical components, and the soft-iron parts had to be designed in a form that would enable the desired accuracy to be achieved. An example is the final lens: the sketch in Fig. 1(b) is another of my infamous blunt pencil eVorts. This design continued to be used on the SEM or Stereoscan, as it was named, until the mid-1970s. The central zone of the bottom plate needed to be flat to within a few Newton rings, while the bore had to be less than 0.3 mm out of round. Similarly, the upper pole-piece needed to be of the same order and also mounted precisely with respect to the lower plate. When I approached various manufacturers for advice about lapping surfaces to these degrees

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of accuracy, we were inundated with representatives all anxious to show us how to do it. Unfortunately, they were all very experienced in producing surfaces of this type on hard materials, but electron-optical lenses have to be made of soft iron and their techniques would not produce the desired results. They all departed despondent and unable to help us. Left on our own we were able to devise a means of producing a lapping technique on the soft iron by using carefully chosen laps and certain types of lapping compounds. However, producing the finish was only half the problem as this had to be combined with circularity, straightness and flatness. Measuring the flatness of the small central zone of the plate was relatively easy using glass proofplanes, although the larger surfaces were a bit of a problem, but measuring the circularity of the bores was more diYcult. At that time the Rank Taylor Hobson (RTH) ‘Talyrond’ was the best tool available for checking circularity, but it was expensive and had an extended delivery time. It was a precision metrological instrument designed and made by RTH for accurately measuring circularity and flatness of a surface to the order of a millionth of an inch. The nearest machine to Cambridge was at the RTH works at Leicester, so we had to take our work 80 miles each time we needed to check the result of a test lapping. This made progress slow and tedious. When Mr Pritchard, who incidentally always gave me a great deal of backing and encouragement, heard of the problem he told me to go to RTH with an order in my pocket to try to twist their arm and see if I could get a Talyrond quickly, and if so to order it on the spot. It is amazing what the attraction of a firm immediate order will do! They soon found a machine to ‘divert’ to us and delivered it within a few days. After delivery it was quickly installed and commissioned in the Mech Lab constant-temperature room. The value of being able to observe frequently the eVects of a change of method was immediate: the problem of lapping both circular bores and flat surfaces in soft iron was solved within a week! When the trade got wind of the fact that we had solved the problem, they came back to try to find out how it was done: needless to say, we were not telling! To enable us to solve the prealigned column problem, the physicists calculated the maximum error that could be tolerated between the top and final lens. We then calculated the maximum tolerance that could be permitted at each component interface. By choosing very tight geometric tolerances for column components and the mating faces, it was possible to design a prealigned column that would need no adjustment after reassembly. The clamping rings of the Microscans were unnecessary as the column was held rigid when under vacuum by the atmospheric pressure. Simple clamps were fitted to the outside to hold the joints suYciently close to seal the O-rings during pump-down. Production instruments were always despatched in a partially pumped-down condition to ensure that the column was rigid. To

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maintain the geometric concentricity, parallelism, circularity and flatness of the components, the development of special manufacturing techniques in the laboratory workshop was required for the first prototype. The lathes used for this work were first checked to make sure that they turned circular and flat, that is that the mandrel was free of cam action or end movement, by testing specially made test pieces on the Talyrond. Before any new machine was purchased the manufacturer was required to allow us to turn a test piece and check it on our Talyrond. It was interesting to discover that it was not the most expensive lathes that were best in this respect. When I asked one manufacturer of a high-class lathe what the circular error was and the amount of cam action, he replied ‘negligible’. My reply was that we would decide what was ‘negligible’ and required a test piece turned to our specification. Although good, the lathe was not good enough. Another managing director was most insulted when his lathe was rejected: ‘Nobody has ever criticized my machines like this before’ he stormed. However, his chief designer was more realistic and agreed with my suggested modifications that would make the lathe acceptable to us. They incorporated the modifications and we purchased what proved to be an excellent machine. It became obvious to us that the machining accuracy and surface finishes we had found to be necessary, and had developed methods of achieving, would eventually be needed in production. These requirements were far ahead of anything being undertaken in the factory, and it would certainly be said that the requirements were impossible to meet. To prove that others than the picked Mech Lab team could meet the demands, we decided to have a set of parts manufactured by an outside subcontractor: Pope and Meads, the firm who had previously allowed us to check their machines before placing an order for the early specimen chamber, was chosen. Before this could happen, however, the drawing oYce had to learn a lot about precision dimensioning, specifying circularity, parallelism and flatness tolerances and surface finishes. After the drawings had been produced they were sent to Pope and Meads, who made a set of machined components well within specification, showing that the choice had proved an excellent one. They continued to be the main supplier of column components for many years with very few rejects, all of which they corrected without question. Ever since the inception of CIC it had been the practice to dimension drawings in metric units as far as possible. However, metric-size materials were not always available, so the imperial dimension was used on the drawing, resulting in hybrid dimensioning such as 1/2-inch diameter  1-mm pitch! As all our Mech Lab machinery had imperial scales, we made the components to imperial dimensions. When they were being drawn in the drawing oYce, all the imperial dimensions we had used were converted to metric. Then it was found that the production workshops were converting back to imperial

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because most of their machines had imperial scales. This was disastrous as two lots of conversion errors resulted in unacceptable departures from the accuracy required. As a result, all the original SEM designs were made to imperial dimensions. It quickly became apparent to us that vibration isolation of the column from ground and pump vibrations was essential. To isolate the column and specimen chamber, the whole assembly was mounted on the top plate of the plinth. This plate, which was very stiV, was not coupled directly to the plinth framework but by large low-natural-frequency coil springs. These were compressed to the correct working position by the downward force exerted by the flexible vacuum connection located in the centre of the bottom of the specimen chamber. To design this system, knowledge of the amplitude and frequency of the disturbing vibrations was needed. For many years CIC had been making a successful recording universal vibrograph that was well suited to measuring frequencies up to about 100 Hz, the range in which we were interested. The instrument was used to provide data necessary to design the isolation system. Later, when the instruments were in production, the service department engineers used a vibrograph to test customers’ proposed sites and advise on the suitability for the installation of an electron-probe instrument. To assist with the vibration problems of the SEM a 125-lb thrust vibration generator was installed and the behaviour of almost all subsections of the instrument, including specimen stages, was investigated. This work became the province of another young graduate engineer, Neil Dunlop, who became an expert on vibration isolation. He later cooperated with the Computer Aided Design Centre on Madingley Road to introduce a computer terminal to the research department. Development of specimen stages with traverses of the specimen in X–Y–Z, able to work in a vacuum and free from vibration problems, became one of the areas of expertise of Mech Lab experimental engineers. Learning from the mistakes of the experimental specimen stage, we designed and made a new one to fit the steel chamber. To allow the specimen to be changed, the stage had to be easily withdrawn from the chamber, and orthogonal traverses of 12 mm in each direction with 10-mm adjustment vertically were required. In addition, the specimen had to be able to be tilted from horizon tal to vertical and have 360 rotation about its axis—all of these movements to be operated from the outside while the stage was under vacuum. Backlash-free movements were essential and large-diameter micrometer heads enabled the specimen to be positioned to 0.01 mm. The specimens were mounted on simple stubs 12.5 mm in diameter which slipped into a socket with kinematic locations on the tilting cradle of the stage. To ensure that there was no relative movement between the stage and the final lens, the

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specimen supporting assembly was clamped directly to the underside of the lens. After the first batch of instruments, it was found that the new stiV specimen chamber made this an unnecessary complication, so it was dropped in favour of a much simpler form of clamping to the bottom plate of the chamber. Since Horace/Darwin’s time, the company had used for precision apparatus Maxwell’s principles of kinematic design and controlled constraints (Unwin, 1990). I soon became an ardent disciple of the system, encouraging its use and frequently lecturing about it to design staV. Maxwell’s rule stated that the number of degrees of freedom plus the number of constraints must equal 6. If more, then there would be strain. When discussing a design with anyone, my first action, therefore, was to ensure that constraints plus degrees of freedom conformed to Maxwell’s rule. Some of the Mech Lab team were academically relatively unqualified but had the ability to visualize a solution to a mechanical problem and, more importantly, were able to guide others to make the parts, so reducing the development time. One group became very skilled in stage design and continued to develop the basic stage to incorporate many extra degrees of freedom for special requirements. I remember one experiment with a sad end. We were trying a ball slide with vee tracks running on sapphire balls. It worked well until it was put down rather sharply and all the balls were found to have sheared across a diameter. We had discovered that sapphire is very weak in shear! One of the problems that consumed a lot of eVort was the assessment of bearing materials suitable for use in a vacuum. I got some very useful advice from NASA, but there were significant diVerences between the space application and the electron-probe system. The probe system has limited pumping capacity; outgassing can increase pump-down times unacceptably and contamination degrades the beam, involving unnecessary cleaning of the column. Space applications have none of these limitations: they have a vacuum ‘pump’ of infinite capacity and contamination is not a problem. Some of the materials they suggested proved successful but were new to us and the techniques of machining, moulding and manipulation had to be mastered, and the methods transferred to the production departments. All brass components in the vicinity of the electron beam had to be nonmagnetic. This needed the introduction of special brasses with no ferrous inclusions. We had to develop processes to clean the finished components of ferrous particles picked up from the tools during machining, and devise tests for the initial acceptance of the raw material and acceptance of the finished parts. An interesting development was the aperture holder. Apertures consisted of three small platinum discs 3 mm in diameter with central holes a few micrometres in diameter. They were located in the small space between the

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upper and lower pole pieces of the final lens. In addition to the three positions, the slide had a fourth, blank, position that provided means to seal oV the column so that it was only necessary to let the specimen chamber up to atmosphere when changing specimens. The slide was precisely located in any of the four positions and moved by an external lever. Two external knobs provided a small amount of orthogonal movement to the aperture discs, which were held in kinematic recesses in the holder slide. The whole assembly had to be easily withdrawn from the column to enable apertures to be replaced. Platinum apertures were originally purchased from outside suppliers but, as the numbers required increased, supply became a problem. We devised a method of coining these from sheet platinum, thus easing the supply. Fortunately, the scrap price of the platinum waste was almost as high as the new sheet, so that oV-cuts and work spoilt during the experimental activities could be sold at little loss. The phosphor tip of the scintillator required some development to produce the shape and polish, and work was also required to select the adhesive to fix the tip to the Perspex light-pipe. Adhesives used had to be suitable for working in a vacuum and have a refractive index to match that of the lightpipe material. Due to the extreme sensitivity to light of the photomultiplier and the high voltages used, the housing involved elements of design not familiar to us. This, as in many other areas of the work, meant that the team had to learn many new ‘tricks of the trade’. It quickly became clear that our concepts of light traps had to be abandoned and the photomultiplier housing designed as a hermetically sealed chamber. Laboratory involvement did not finish when full production of the instrument had started. Electroplating or painting could only be used as protection on nonfitting surfaces because of thickness variations. The need to keep the unprotected mild steel and soft iron surfaces scrupulously clean permitted corrosion to take place. We were able to find and introduce a phosphornickel chemical plating system that enabled us to overcome the problem on many of the surfaces. Unlike electroplate, the thickness deposited was uniform over all surfaces to within 10 per cent and the plater could guarantee a deposit thickness to within 10 per cent of that specified. This allowed closetolerance surfaces to be machined a specified amount over or undersize, then plated up the correct amount to bring the surface to the required dimension and still maintain the tolerance. All mild steel and soft iron components except a few pole pieces were subsequently plated in this manner. Another associated production problem we had to solve involved corrosion arising from the finger marks of assemblers. We found that the perspiration of young men up to the age of about 24 and of women at certain times was very corrosive. Providing people in these groups with cotton gloves to wear when assembling critical column components cured the trouble.

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As the electron-optical performance of the instrument was improved, the slag inclusions in the mild-steel column components began to become significant. The lens bodies were machined from billets of commercial-quality mild steel in which quite large, randomly distributed, slag inclusions occurred. A component could not be verified as satisfactory until it was completely machined, by which time it was expensive to scrap. I found that ultrasonic or X-ray examination did not prove satisfactory in revealing the smaller inclusions. Discussions with Firth Brown, a specialist steel producer at SheYeld, enabled us to get a sample billet of a vacuum-remelted steel in which the inclusion size did not exceed 25 mm. We were able to make a trial lens body of this that seemed perfectly satisfactory. It was considerably more expensive than ordinary mild steel and less easy to machine, but its use stopped the costly scrapping of finished lens bodies used on the production instruments. It was only possible to adopt this steel when the quantities required were suYcient to justify the purchase of a complete billet of steel. Somewhat later, when quantities were even larger, the steel maker suggested using shaped drop-forged billets of vacuum-remelted steel. Not only did the process of drop forging break up any inclusions, the billets were shaped so that the amounts of machining and scrap generated were significantly reduced. An interesting example of the combined eVorts of the physicist and a Mech Lab experimental engineer, Colin Nordon, was the development of the three coil assemblies incorporated in the column. These consisted of beam-bending coils, stigmator coils and scanning coils. All the completed assemblies had to be encapsulated for use in the vacuum. It was important that the encapsulating material exerted no distorting forces during curing, or subsequently, and had no plasticizer in it to leach out when under vacuum. The experimental engineer devised and made jigs, tools, fixtures and methods to machine from solid the specially shaped ferrite formers, wind them with the wire coils and encapsulate the whole assembly. Scanning coils were wound and preformed on a custom made tool. The core was a very thin nonmagnetic brass tube with the coil-locating lugs of epoxy resin cast on the circumference in a specially developed mould. Again after mounting and fitting connectors the whole was encapsulated to prevent any movement of the wire coils.

B. The Geoscan While the Stereoscan work was going on, microanalyser development was also proceeding. Pritchard had come to an arrangement with Dr J. V. P. Long of the Department of Mineralogy and Petrology, University of

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Cambridge, for the company to work with him on the development of a microanalyser designed primarily for geological specimens. This instrument became known as the Geoscan. It was a very diVerent instrument from the relatively simple Microscan, having two programmable precision fully focusing X-ray spectrometers, prealigned column, variable-speed specimen movements, automatic standard selection and other advanced facilities. The specimen chamber volume was small, which permitted rapid pumpdown after changing the large interchangeable combined specimen and standard holder. The first prototype was completed in the Mech Lab during 1964. Some of the mechanical development work was of a similar nature to that of the Stereoscan and went on in parallel, often being undertaken by the same experimental engineers. Possibly the most demanding project was the fully focusing spectrometer. The required Bragg angles had to be preselectable, repeatable to within 6 seconds of arc and within 2 minutes of arc absolute accuracy. A precision angle-measuring tool called the Angle Dekkor was purchased for the purpose. Three crystals had to be quickly interchangeable without letting the chamber vacuum up to atmosphere. I suggested to Dr Long that the way to provide the movements of the counter carriage was in the same manner that the level luYng crane maintains its load at a constant height while luYng. Various movements of the two carriages and the crystal, which maintain the linear and angular relationships, were controlled by a number of interconnected flexible beryllium–copper tapes. Kinematic or semi-kinematic design principles were used throughout to ensure continued accuracy during long use. While the first experimental model spectrometer components were fabricated of brass, light alloy castings were used for the first prototype and subsequent instruments. To maintain the necessary accuracy, the castings had to be stabilized after rough machining, then finish-machined at a constant known temperature to ensure precise dimensions. As accurate positioning of the movements was needed, a 400-Hz AC servo-system was used, a positional encoder driven from a precision-geared quadrant providing angular position. Checking component parts, straightness of tracking, repeatability and absolute angles involved the adaptation and development of optical measuring devices as lasers were not available at that time. A spectrometer command unit of comparable precision was also needed. This had means to preset Bragg angles, and sweep- or step-scanning over a range of angles and speeds. While the first experimental models were designed and made in the Mech Lab using in-house cut gearing, the spectrometer command units were ultimately manufactured by a firm specializing in high-precision gearing who also provided the precision quadrant for the spectrometer. An interesting problem came to light some years later when the Geoscan was being upgraded to the Microscan Mk5. To permit computer control we

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were replacing the spectrometer analog positioning system by a digital system using a circular grating and stepper motors. When modified, the spectrometer would no longer repeat to within the 6 seconds of arc that had been easily achieved with the AC servo-system. We found that the 400-Hz servomotors had induced just suYcient vibration in the mechanical system to overcome static friction and so reduce hysteresis. This was solved by artificially introducing a very small high-frequency vibration. The orthogonal traverses of the specimen stage were to be operated in both the manual and automatic modes covering the relatively large area of 45 mm  80 mm. As the automatic system had to have constant- and highvelocity modes and positional mode, a DC servo-system with tachofeedback and precision screws was used. Hysteresis had to be kept to an absolute minimum, so considerable ingenuity went into the anti-backlash mechanical design. The experimental engineer responsible for this work, Ralph Kerley, was originally a watch and clock maker but had a flair for designing complicated gearing. He eventually moved to the design oYce and became an authority on specimen stages in addition to gearboxes. The proportional counters were fabricated of stainless steel with Kovar glass-to-metal seals at each end. To avoid problems with flux residues we installed a vacuum brazing furnace. This was later transferred to the production department in the same manner as much of the other specialized equipment we had obtained for development purposes. One counter was gas-filled and sealed while the other had gas passing through it. Both had thin Mylar windows that had to be vacuum-sealed—more of a problem than had been anticipated. The gas supply tubes to the flow counter had to be of very flexible plastic to avoid biasing the spectrometer position. No plasticizer could be permitted in the tube material as this would leach out when under vacuum, causing stiVening. This happened on one occasion when an incorrect batch of tube had been supplied. Unlike the Microscan and Stereoscan, which had relatively small vacuum chambers, the greater size of the Geoscan main frame and spectrometer chamber necessitated the use of light alloy castings. Finding an economical method of making them vacuum tight and with a smooth surface on the vacuum side proved to be diYcult. Vacuum impregnation was not wholly reliable; certain solvent-free epoxy paints were more successful. The Geoscan oil-filled electron gun was of diVerent design from that used on the Microscan and Stereoscan, which were air-insulated. It used a large ceramic insulator designed in cooperation with the ceramics manufacturer Worcester Porcelain. Providing the accurate surfaces for the O-ring seals proved diYcult and required a considerable amount of development work. Problems of oil leakage were also experienced with the filament holders, which were a casting of filled epoxy resin. These were cast in moulds and

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then a groove was machined to receive the O-ring seal. The leak occurred where the filler particles were exposed by the machining operation. The problem was not cured until we had found means to accurately mould the O-ring groove and eliminate all machining. All the guns used a standard tungsten wire filament or ‘hairpin’. For many years these were a bought-in item, but supply diYculties prompted us to investigate the possibility of manufacturing our own. A Kovar seal base was designed and a source was located while bending jigs were made to shape the tungsten wire to form the filament. Spot welding of wires of special materials had been a process used at CIC for many years and the equipment was adapted to weld the ‘hairpins’ to the base wires. Some years later, the Mech Lab became involved in developing techniques for machining lanthanum hexaboride pins, which were to replace tungsten filaments in a new generation of guns. The Royal Radar Establishment (RRE) Malvern provided considerable assistance with this work. They also supplied us with some specialized equipment that they had designed. After the laboratory technique had been developed, the experimental engineer then had to transfer it into a production process for use in the factory. In addition to examining the specimen under the influence of the electron beam, a sophisticated light-optical viewing system was developed, working in cooperation with R & J Beck Ltd. This provided means of viewing both sides of transparent specimens, and also viewing opaque specimens from the beam side, all with the choice of high- or low-power objectives. Simultaneous viewing of the specimen surface for probe-induced luminescence was also provided. The high- and low-power objective lenses were carried on a slide mounted between the final electron lens and the specimen. This slide, which could be operated from outside the chamber, had a third position that carried the scanning coils for use in the probe mode. Limitations of space, long light paths and diYculties of vacuum sealing contributed to a very demanding project ultimately carried to a very satisfactory conclusion. An amusing incident occurred in the Mech Lab when the first experimental instrument was being assembled and tested. Unlike the Stereoscan, the Geoscan beam is horizontal, about 0.75 m from the ground. The partially assembled but working instrument was being demonstrated to a party of international company representatives who were gathered around. As an aside the technician demonstrating said that exposure to the beam in the region of the groin was believed to cause males to become sterile. It was interesting to observe those of the party who moved away and those who moved in closer! Actually there was no risk as the column was adequately screened. One of the experimental engineers was involved in devising methods of preparation of the special large crystals for the fully focusing spectrometer.

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These had to be cut, bent and cemented to the curved holder, then ground to a precise curvature. Some of the crystal materials were very diYcult to manipulate, requiring considerable expertise. After we had been working on the project for a while, we asked an expert from the Warren Spring Laboratory for advice. When he left us, he complimented our engineer and said that he felt that there was little he could teach us. In fact we had shown him a trick or two. Special crystal cutting equipment was obtained, and we designed and made a machine for grinding the curved surfaces of the holder and the mounted crystal in the Mech Lab. The equipment and techniques were ultimately transferred to the production departments. Making the stearate crystals was a challenge to both the chemist and the experimental engineer, who had to devise a special bath for producing the film and depositing it on the mount. Dr Long used the first Geoscan to examine some of the rock brought back from the first Moon landing. After the successful launching of the Stereoscan and Geoscan MkI, electron probe development work continued. However, the work was one of improvement and devising new versions that were variants of the original designs, so lacking the challenges and problems of breaking new ground and the satisfaction of creating the first series production commercial scanning electron microscope in the world. Around 1973/1974 saw another period of redundancies and reorganization within the company. At the same time, the decision was made to move from the Chesterton Road site to a leased site at Rustat Road, on the other side of Cambridge. I was asked to lead a Feasibility Group to look at new design possibilities. Although the work was of considerable interest, I was not very happy and so was delighted to be invited to consider three posts from other organizations, two in Cambridge and the other in Hertfordshire. As both of my children had graduated and left home, my wife and I decided to take the Hertfordshire oVer and see another part of the country at someone else’s expense. IV. A New Career The post was that of Chief Engineer of the testing laboratories of the Consumers Association (CA) at Harpenden, Hertfordshire and at Gosfield, Essex. I started on 1 September 1974 and it proved to be a most demanding, varied and fascinating job in which I was able to use almost everything I had ever learned in the past and learn a lot more besides. Changing to this new job was probably the best day’s work I had ever done; I enjoyed every minute of the time I worked for CA.

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A. Retirement I retired from CA at 65 in October 1983 on a Friday but started back again on the following Wednesday. From my point of view the arrangement was excellent, as I was able to shed all the uninteresting administration and concentrate on the interesting technical work. While with CA I represented them on several British Standards technical committees, a duty I continued to carry out until complete retirement and moving back to Cambridge in January 1996, shortly after my wife had died of cancer. Since retirement, I have built replicas of the planetarium of Giovanni de Dondi (1347); the 14th-century astronomical clock of Richard of Wallingford, Abbot of St Albans; an 18th-century Grand Orrery; and the sea clock H3 made by John ‘Longitude’ Harrison in 1757. These are now on public display in the Manor House Museum, Bury St Edmunds. In 1947 the first of my published articles appeared in the Model Engineer and I continue to write for various journals; also contributed a chapter to a book of physicists’ reminiscences (Unwin, 1990). Recently I have written a brief history of the Cambridge Instrument Company entitled ‘The Scientific’ (Unwin, 2002). All this amounts to a pretty full and interesting life.

References Unwin, D. J. (1990). Development of mechanical parts of electron probe instruments, in Physicists Look Back, edited by J. Roche. London: Adam Hilger Ch. 14. Unwin, D. J. (2002). ‘ ‘‘The Scientific’’ The Story of the Cambridge Instrument Company.’ 2nd ed. Cambridge, England: Cambridge Industrial Archaeology Society.

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

4.4 From Microscopy to Lithography B. A. WALLMAN Formerly at: Cambridge Instrument Co. Ltd.

I. Introduction Earlier chapters have described how the many SEM system concepts researched by Charles Oatley, later Sir Charles, and his illustrious dynasty of research students led to the development and manufacture of the Stereoscan at the Cambridge Instrument Company (CIC). Gary Stewart’s arrival at CIC provided a person who possessed not only a comprehensive grasp of the required system technology but also the vision and driving enthusiasm to define a complete system and see it through to a highly successful product. His attention to detail and quest for perfection were key to the birth of the Stereoscan product. Another benefit that came with Gary was the close liaison that subsequently developed between CIC and the Cambridge University Engineering Department (CUED). This liaison took many forms, both formal and informal, with not the least being the succession of research students who started their industrial careers with CIC.

II. Philip Chang My own involvement in this story started in 1969, about a year or so after the arrival at CIC of one such prote´ge´, T. H. P. (Philip) Chang. At this time Philip had already completed his PhD research (between 1963 and 1966) and successfully submitted his dissertation ‘Combined microminiature processing and microscopy using a scanning electron probe system’ (Chang, 1967). With this work Philip had built his first lithography system (Fig. 1) and demonstrated some of the earliest examples of using electron beams to define micro-patterns—a process sometimes referred to as ‘putting the SEM into reverse’. I believe I am right in saying that Oatley played a major role in organizing Philip’s move to CIC using his long-standing relationship with 359 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Figure 1. Philip Chang’s research apparatus at the Cambridge University Engineering Department. Reproduced by permission of T. H. P. Chang (1967).

the Malvern site of what was then known as the Royal Radar Establishment (RRE). A key defence technology under development at Malvern at the time was the surface acoustic wave (SAW) device. These devices were being used for signal processing applications in radar systems. They consisted essentially of metallized wave launcher and receiver patterns formed on quartz substrates. The required patterns were interdigitated finger arrays with their line/space dimension of the order of 1 mm for the most advanced device requirements. It seems unbelievable now that such dimensions were well beyond the capabilities of the best available light-optical lithography methods of the day. Oatley had been made aware of this problem and saw the opportunity for Philip to continue his work locally, as well as potentially starting a new scanning beam application technology. At that time CIC had many calls on its R&D funds as the full application potential of the SEM was unfolding and its many ancillary modular options were being developed. Newer models were also under development and, as in any other new product field, commercial competition had started to hot up. The solution to this problem was to gain external funding for the work. After discussions involving RRE, the management of CIC and Oatley, a research and provisioning contract was negotiated with the UK government department then known as the Ministry of Technology. The requirement was to develop an experimental lithography system and provide one system each to the RRE site in

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Malvern (Worcestershire) and the sister Services Electronics Research Laboratory (SERL) at Baldock (Hertfordshire). During the early part of the contract, Philip was limited to a very small project team; however, what they lacked in numbers was more than made up by the engendered enthusiasm well known to anyone who has ever had the pleasure of working with Philip. Although not a part of the team at that time, I well remember the busy activity of Philip, his two technicians (the late Dave Hills and Richard Newman) and his mechanical designer, the late Monty Cousins. Philip was extremely fortunate to get Monty on to the project: he had recently retired from his position as head of the CIC Drawing (Design) OYce and brought with him a lifetime’s experience of reliable mechanical design. The group very quickly modified a Stereoscan model SII SEM to perform simple lithographic patterning. The modifications consisted essentially of adding a specimen current amplifier and beam blanking capabilities to the SEM column (standard SEM options by then) and developing a ‘pattern replicating’ flying spot scanner (FSS) as an input device. The unit was, in concept, very similar to that which was developed by Philip during his research work in the Engineering Department. Wherever possible it used standard Stereoscan modules to quicken development and reduce costs.

A. Flying Spot Scanner The overall system concept was quite straightforward; the FSS consisted of a high-resolution, short-persistence phosphor CRT scanned with a blank raster, the scanning being performed in synchronism with the beam in the SEM column. The spot on the CRT faceplate was imaged on to a photographic glass plate containing the binary image of the required patterns many hundred times the required final size. A condenser lens collected the light transmitted through the non-opaque regions of the pattern, directing it into the entrance window of a photomultiplier tube. The resulting signal was then further processed electronically to amplify and ‘square’ it, making it suitable for driving the SEM column-blanking unit. Thus, by setting the scanning area of the SEM raster (magnification setting) the pattern could be reproduced demagnified on the ‘specimen’ surface at the required final size. The major diVerence in the design of Philip’s CIC/FSS, compared to that on which he did his PhD research, was in its purpose-designed layout as a free-standing unit. In his earlier CUED model, the FSS was simply adapted from a high-resolution recording display unit where the required mask pattern was placed adjacent to the CRT faceplate, the recording camera lens was used for light collection, and the camera-back had been modified to take the photomultiplier tube (Fig. 2).

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Figure 2. Philip Chang’s flying spot scanner arrangement. Reproduced by permission of T. H. P. Chang (1967).

B. Vector Pattern Generator During his PhD research, Philip had also demonstrated a very early form of vector (rather than raster) pattern generation. This he achieved using a modified X–Y pen recorder. With its servoloops disconnected, he manually moved the pen carriage around a specially constructed stencil on the recorder writing platen using a stylus to replace the pen. Processed outputs from the recorders X–Y potentiometers provided drive signals for the SEM column deflection coils. The SEM probe therefore traced out a demagnified copy of the stencil pattern on the specimen surface (Fig. 3). Although quite primitive, being capable of scanning only lines, possessing quite limited pattern resolution, and with dose variability caused by manual ‘tracing’ of the stencil, it was probably one of the earliest examples of vector scan electron beam (e-beam) lithography. In all of this work it was necessary of course to utilize some form of imagecreating mechanism on the substrate being patterned. Potentially there were many possible mechanisms that might be used, including: 1. Direct thermal machining with a high power beam to modify conduction or insulating layers on the substrate or form isolated micro-features.

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Figure 3. Philip Chang’s first vector-scan apparatus. Reproduced by permission of T. H. P. Chang (1967).

2. Direct polymerization of ‘contaminant gases’ within the chamber (a phenomenon well known to microscopists with early diVusion-pumped systems), the resultant patterns then being used in subsequent processing steps such as an etch resist barrier. 3. Creating crystallographic damage to the substrate, which could subsequently be etched at a diVerential rate to the unexposed regions. 4. The application and exposure of a thin electron-sensitive layer on the surface. After exposure and some form of development, these might be used simply to resist subsequent etching processes or might contain doping materials for thermal diVusion or metallic deposition on the substrate in subsequent processing. Philip had reviewed all of these various methods in his dissertation (Chang, 1967) and performed many experimental results using both the thermal and nonthermal techniques. He had also investigated and predicted some of the electron-scattering eVects that occur when energetic electrons enter a material. These can cause significant exposure away from the point of entry, and can result in serious loss of shape-fidelity in closely spaced, arbitrarily shaped pattern elements. He was later to name this phenomenon the ‘proximity

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eVect’, the compensation or alleviation of which has subsequently engaged the attention of many engineers and physicists over long periods. C. Resists Ultimately, however, it was found that, in order to obtain high resolution, be compatible with the techniques evolving in the growing semiconductor industry and achieve at least some degree of adequate throughput, it was necessary to use a low-dosage, non-thermally-induced image technique. For these reasons, much research attention was being paid to polymer materials with ‘resist-like’ properties where either chain scission or cross-linking predominated upon irradiation to low electron dosage. Suitable materials should therefore yield a latent image that could subsequently be developed out using compatible solvents. Virtually all the early work (and much of our later endeavours) was based around the material polymethyl methacrylate (PMMA) known by the trade names ‘Perspex’ in the United Kingdom or ‘Lucite’ in the United States. Work at IBM Research in Yorktown by Mike Hatzakis (Hatzakis, 1969) had already identified the usefulness of this material in their e-beam lithography eVort. Alec Broers, a close friend and Engineering Department contemporary of Philip was leading this work. D. Initial Lithography Using a Modified Stereoscan Thus it was in 1969 that the initial practicality of the technique was demonstrated using the lightly modified Stereoscan with a few of its standard options, the flying spot scanner and a rudimentary PMMA resist process. However, it became quite apparent that much more work would be needed to turn this laboratory curiosity into a usable tool for practical applications. There were many problems to solve and many of these were not at all related to what was then seen as the mainstream of development at CIC, the scanning electron microscope. It has often been said that there are only three fundamental properties of any form of lithography tool: 1. The minimum linewidth it can deliver with the required linewidth control throughout its image field 2. The placement accuracy with which pattern features can be positioned in the image field (or overlaid when exposing on a prepatterned substrate) 3. The speed with which the overall exposure task can be completed, often referred to as throughput.

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Only the first of these is fundamental to the SEM (in the sense of its resolving power), and of course it was for this well-known SEM advantage that these types of tools were first developed. In all other respects the required properties of an e-beam lithography tool are quite diVerent from the SEM. We soon came to appreciate that ‘resolution came for free’ but everything else had to be hard won. Even with resolution there is a critical freedom within the mode in which the SEM operates that does not exist in the lithography tool. At maximum magnification in the SEM, the probe is being scanned over an area of only a micrometre or so on a side. Under these maximum resolution ‘axial’ conditions, deflection aberrations are virtually nonexistent. On the other hand, when working at low magnification, the scanned area is several millimetres on a side and the probe can (and will) defocus and become astigmatic to micrometre sized dimensions. This will not generally be noticed in the SEM image as the raster line spacing on the specimen at low magnification is approximately the same magnitude as the aberrated probe. Blurring of the image will therefore not be seen. It is one of the few examples in my experience of technology where ‘Mother Nature’ is helping rather than hindering one’s eVorts! Contrast this fortuitous situation with the lithographic case where the need is to write features with the same minimum linewidth and control wherever they appear in the image field (or indeed wherever they are required on the substrate, which may be many inches in extent and far from perfectly flat.) In addition, to achieve useful throughput, the deflection-field size needs to be of the order of a millimetre on a side, or more, in order to reduce stage movement overhead times to a minimum. E. The DiVering Requirements of a Lithography Tool Compared with the SEM Various other critical diVerences were well appreciated by Philip and his team early into the project. 1. The SEM has its optics optimized for a short working distance (a few millimetres below the lens) in order to minimize objective lens aberrations and gain the very best axial resolution. Under such high-resolution conditions the beam deflection is minimal. Lithography requires a reasonably sized deflection field; ideally, the working distance should be increased, enabling the use of a reduced beam deflection angle to limit deflection aberrations. Using the SEM lens design in this mode reduces minimum spot size capability through greatly increased chromatic and spherical aberration eVects. The larger, aberration-dominated, spot reduces beam

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current density and therefore has a negative eVect on the potential system throughput. Clearly a diVerent, non-SEM-style objective lens, better optimized for long working distance, would produce a better overall lithographic performance. 2. The tungsten hairpin electron source common to all SEM systems of the day was low in brightness and a fundamental limit to throughput, especially when using the convenient but relatively poor-sensitivity PMMA resist. Its service life was also quite limited (a few tens of hours), especially if run for best brightness. Broers (1969) was already reporting work on longlife, higher-brightness emitters, with the lanthanum hexaboride (LaB6) source showing particular promise. This gave about an order of magnitude increase in current density compared to the conventional tungsten source and a life of several hundred hours. Clearly the development of such an electron gun source was highly desirable. 3. The operation of a SEM to take a micrograph is basically a manually driven optimization process. It involves much sequential ‘knob twiddling’ to orientate the specimen with the stage relative to the collector, to focus the probe and to set up the required video signal collection/processing conditions. It is a process requiring much skill and practice; top operators pride themselves on being able to coax just that little bit more out of a system than those less experienced. Conversely, with a well-designed lithography tool, far more of the system operation needed to be automated, both to define and improve the reliability of operational sequences as well as to reduce operator delays and fatigue. 4. The SEM predominately uses secondary electrons to generate its image: these emanate from the surface of the specimen. However, in lithography the surface of the substrate is covered with a resist layer several hundreds of nanometres thick. When overlaying subsequent levels of patterning, as in the direct writing mode of operation, it is necessary to detect registration marks formed on or within the underlying substrate. Their registration by the beam enables relative positional errors to be measured and compensated, resulting in accurate overlay. Under these conditions the conventional Everhart–Thornley detector, a key enabling technique in the SEM for low-noise secondary electron detection, is of little use and a more eYcient back-scattered electron detector (BSD) is needed to detect markcontrast through the resist. In addition, an alignment scheme is required to adjust the writing-field position based on the error information returned from detected marks. 5. The SEM is a very versatile apparatus usually designed to operate over a wide range of diVering applications. For example, the beam energy in early Steroscans could be varied from 1 to 20 keV but generally only the highest beam energy was of value for lithography. The specimen stage usually had a

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minimum of four or more degrees of freedom with very limited range and manually driven travel. Conversely, lithography requires only X–Y motion but with much larger range, motor-driven travel and high-accuracy positioning. The typical specimen ‘stub’ mounting technique was also quite incompatible with the much larger semiconductor substrates of the day (1- to 2-inch wafers and 3-inch maskplates). They needed to be held normal to the beam axis without incurring any form of damage. 6. Provisioning suitable interfaces for optional, SEM application-specific subsystems complicated much of the electronic and mechanical system design. Such versatility is not required for the dedicated lithography tool and its very presence could compromise the design freedoms needed to achieve lithographic performance. Philip began to address all of these issues within the very tight budget constraints in operation at the time and with the diYculty of being seen to be outside of the mainstream of SEM development. This by then had captured most of management’s daily attention and financial will. However, he was helped somewhat in this task by the then CIC Sales Director, Peter Charman, whose previous career had been in the semiconductor industry. He appreciated the potential for the technique and helped to promote Philip’s eVorts. A gun test rig was designed and built and a new member, Gordon Miller, was hired to do the day-to-day experimental work on the LaB6 source. With much of the design concept input from Philip and the detailed mechanical design by Monty Cousins, this small team developed a source that was practical to use, albeit with some operational diYculties generally related to vacuum integrity. It was reliable enough, however, to become also an option for the Stereoscan product, where its increased brightness improved the signal-to-noise and hence contrast in adverse imaging situations. The design was based on Broers’ indirectly heated ‘high-eYciency’ mode principles and was manufactured for about 10 years before more reliable directly heated units became available from specialist vendors. An experimental dual BSD detector was developed based on the use of a pair of channel electron multiplier devices (‘Channeltrons’). These were  mounted at 180 to each another and positioned with their ground-potential entrance opening close to the beam–substrate region. Their high-potential exit openings were arranged to be close to standard Stereoscan scintillator/ PMT units. With the scintillators biased at much higher potential, the multiplier electron output stream would thereafter be accelerated, detected and processed in much the same manner as with the Everhart–Thornley detector. Using this type of unit, with a simple manual alignment procedure (including a micrometer rotation control to the scan coil), X–Y overlay of

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separately written and processed pattern levels was successfully demonstrated (Fig. 4). It was around this time (1969) that I first became involved in Philip’s group. Until then I had worked on a range of electromechanical instrumentation projects in various development groups. Most recently I had been working on the system design of digital servos for stage and spectrometer mechanisms for a proposed new X-ray microanalyser that became a victim of budget cut-backs. I was immediately put to work on a self-contained control system for a modified 50-mm travel Stereoscan ‘large’ stage.

Figure 4. The fully aligned and processed Marconi MOST device. Reproduced by permission of Leica Microsystems Ltd.

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Philip and Monty had ‘stripped down’ most of the substage motions of the unit, retaining principally the main X–Y traverses. The stage, by necessity, used the Stereoscan rail insertion technique through the front of the SEM specimen chamber. Access for substrate mounting was by venting the chamber using a column isolation valve associated with the final aperture mechanism. The micrometer barrels were removed and a rather ingenious motor carriage technique was designed to carry the driving stepping motors, which were geared to the micrometer shaft. The gearing produced a 1-mm movement for each motor step. For my part, I set about designing a step-and-repeat control unit that would allow the stage to be moved in predefined amounts in X or Y. The step size was set up on number switches with the position of the stage held in binary-coded decimal counters using miniature ‘Nixie’ tubes for positional readout. There was also a tracker ball to ease stage manoeuvring. All of the digital circuitry was built with small-scale integration TTL logic, which seemed to work well except when an arc-over occurred. This always caused the counter contents to be lost, but most of the time none of the TTL circuits got zapped! The group appeared pleased with the resulting facility, which was probably the first piece of ‘automation’ applied to the system.

III. Vector Scan Digital Pattern Generator Full automation was what was really needed. We then set about the very diYcult task of persuading our masters (both at CIC and at RRE) that we wished to build a vector scan digital pattern generator unit and interface its control to a small computer. I think we persuaded those involved of the technical advantages of digital pattern generation: 1. Complete freedom from master artwork generation. 2. Increased throughput by dispensing with raster scan (needing 100% coverage of the image field) and using vector scanning that addresses only the proportion of the pattern needing exposure. 3. Improved pattern definition (initially a 12-bit deflection scheme equivalent to a 4096-line scan compared with the 2000 lines resolvable with the FSS design current at the time). 4. Ease of automating many other system functions once a computer was available. It seemed such a momentous step for our managers to take at the time—to actually buy a computer! I know how frustrated we all felt, especially Philip, who could clearly see the potential of what this step could lead to.

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We investigated all the available minicomputers of the day, wrote several reports and finally decided that we should buy the new Digital Equipment Corporation (DEC) PDP11 system. John Somers, who had been working in the university’s Mathematical Laboratory was recruited by CIC and joined me in this work. Our government-funded project source seemed under pressure always to buy British, but there was nothing in the price range that was anything like what was needed. So it was that in 1970 John Somers and I bought the first computer that Cambridge Instruments had ever purchased, the PDP11/20. We had also managed to obtain (after a lot of extra justification) increased memory capacity from the basic 4K words up to a heady 8K words! After we had planned the system in detail, John started on some rudimentary software design for controlling the system while I designed and built the digital pattern generator. By this time TTL logic was reaching the mediumscale integration level and the digital design became much simpler. This unit was interfaced to the PDP11 and basically generated scanning signals to expose rectangles provided from a stored pattern list within the PDP11. It drove the standard Stereoscan beam deflection electronics through (non‘deglitched’) 12-bit digital-to-analog converters (DACs) under the control of an external pulse generator that could be used to set the scanning speed (dosage). Looking back now, it was all remarkably primitive with very little sophistication for true automation. It did not have the ability to set the deflection field size automatically and so the manually switched SEM magnification system had to be used; nor could it control the scanning rate dosage. It could trigger stage moves through the step-and-repeat unit but not monitor and feed back beam current readings to the computer. There were no field calibration algorithms and there was certainly no dynamic compensation for field distortion, beam focus or substrate image plane variation with traverse (a cause of field-size errors). In addition, those banes of all widebandwidth precision analog electronics, noise and interference, were clearly visible in the exposed resist (especially at the major state changeovers in the non-deglitched DACs). In spite of these shortcomings, the overall impression that the system (Fig. 5) gave in controlling itself to expose an array of patterns was very powerful and created much interest in those that came to see it. We had made the first big step and knew many of the things we wanted to do next. Most of these improvements have been mentioned above but one in particular was the need to get away from the constraints of the modified SEM approach. We were convinced we needed to design and build a new plinth, chamber, stage and column, for use as a new platform purpose-designed for the lithography-tool application.

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Figure 5. The RRE-contract system based on the StereoscanIIa. Reproduced by permission of Leica Microsystems Ltd.

Philip and I were then directed to write a very detailed report of the progress achieved on the contract work to date, and include a feasibility study of what we thought we could go on to achieve. This was published in November 1970 and circulated among the contract principals. Almost immediately we were being asked to write another report intended for wider circulation, which we entitled ‘An automated electron beam microfabrication machine’. This was published in January 1971. It had a wider readership than the earlier version, mainly within the UK semiconductor industry, although I believe it was shown to several US and European organizations. I guess the report was intended to attract industry investment or, at the very least, a favourable endorsement for further government support. However, the money was not forthcoming for what we wanted to do, and it looked as though the present phase of the project would be completed and the group disbanded. It was also at this time that Philip’s activity and published work had attracted the interest of other participants who were actively engaged in the research of the e-beam lithography technique. It should have been to no-one’s surprise that, after five years of frustration working under the relatively poorly resourced situation at CIC, Philip decided to move on to pastures new. He accepted a position oVered at IBM Research in Yorktown

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Heights and joined Alec Broers’ group there to continue his endeavours under far more favourable conditions. He did, however, discharge one last duty for CIC by giving a paper describing our achievements at the 11th Symposium on Electron, Ion and Laser Beam Technology in Boulder, Colorado in May 1971 (Chang and Wallman, 1971). [It was the continuing Electron, Ion and Photon Beam Technology (EIPB) Symposia (later renamed EIPBN and colloquially known as the ‘BEAMS’ meeting), that became the international high point forum of the year for all advanced lithography system researchers.] So it was left to David Hills and me to define and manage the completion of the outstanding work on the project. The system was finally packed, shipped and reinstalled by the team at what was by then known as the Royal Signals and Radar Establishment (RSRE), Malvern, in 1971/72. IV. Reorganization at CIC It was just before and during this period that the commercial success of the Stereoscan (and the wealth of factory real estate owned by CIC), started to attract the business predators of the day. The resulting battle to gain ownership of CIC between the Rank Organization and George Kent Ltd was finally tipped in favour of Kent through the direct financial support of Harold Wilson’s government. This was, I think, my first personal experience of politicians meddling in things they didn’t really understand. It quickly resulted in a property asset-stripping operation by Kent. Kent hived oV the SEM and medical businesses of CIC on to the leased Clifton Road, Cambridge, site as a separate organization. Similar moves were made with operations on other sites in the London area, freeing up attractive real estate. The large factory site at the rear of Chesterton Road, Cambridge, was subsequently used for housing development, while the prestigious R&D building fronting Chesterton Road ( judged too expensive for an industrial technology company to operate) is now the UK government’s local Department of Social Security oYce, handing out unemployment benefits! It was certainly a real eye-opener in the ways of big business for this junior engineer of the time. Cast adrift with limited funding under the holding company Scientific and Medical Instruments (SMI), Cambridge Scientific Instruments (CSI), as we had then became known, was indeed in a very stormy sea. Increasingly strong SEM competition, by other companies with newer products, severely dented CSI’s profitability. The inertia of SEM production, ramped-up on previous years’ sales forecasts and the reality of declining orders, meant that CSI soon fell into cash flow and ‘overstaYng’ problems. It has to be

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remembered there was little of the flexibility of today’s ‘outsourcing’ and ‘just in time’ credos in operation in those days. If it could be done in-house, then it was. Heavy recruitment in the early success days of the Stereoscan and increased procurement based on optimistic sales forecasts turned out to be very heavy millstones. So began the succession of ‘downsizing’ (a name more acceptable to directors and business owners than redundancies) and the several additional cycles of new ownerships that were to follow. Notwithstanding this rather gloomy scenario, the previous successes of Philip’s group and the worldwide growing interest in advanced novel lithography systems had stimulated growing commercial interest in what CSI might be able to oVer. It was to the credit of those senior managers who were trying to steer the troubled ship at that time (mainly Bob Davies, the Managing Director and David Kynaston the Technical Director), that a major reversal of previous policies was made. So it was that in 1973 David Kynaston, wishing to restart the lithography project, assigned me the position of Project Leader ‘Microfabrication’. My brief was to quickly regenerate the technology we had previously developed in the form of a demonstration system. We were also given the go-ahead to start and address some of the design improvements, which had been identified in the past. V. Microfabrication Project To assist with this task, John Somers, David Halliday and Mike Butler joined me within the project group. We also had access to both electronic and mechanical design resources, our nominated engineers being Mike Penberth and John Sturrock (not only two of the most competent engineers I have ever had the pleasure of working with, but also possessing the personal qualities to operate under very turbulent, adverse conditions without losing either their sanity or sense of humour!). We quickly modified an S4–10 Stereoscan (Fig. 6) to reproduce the type of digital pattern generator system previously developed and then set about some improvements. The first of these was to design a lens with longer working distance, which was optimized for 5-cm. With guidance from Dick Paden and Robin Taylor in the SEM group, Mike Butler ran a number of lens simulations using an early version of Eric Munro’s modelling software in use within CSI at the time. Gradually, with John Sturrock’s help in the mechanical design (see next chapter), an optimized practical lens was evolved that interfaced with the rest of the column, vacuum lines and chamber. This gave us the benefits of better current density and smaller deflection angles, but the inadequacies of the SEM double-lever pre-lens deflection system were still very evident. Linewidth control and positional errors were quite unacceptable beyond a

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Figure 6. The Stereoscan S4-10-based EBMF-1 prototype system. Reproduced by permission of Leica Microsystems Ltd.

500-mm field size. However, the lens design itself stood the test of time, with over 80 units being manufactured and sold in various product models. A. Deflection System We were also suVering from shape distortion and misplacement problems associated with the electromagnetics of the deflection coil. The return path of its deflection flux was free to link with the surrounding final lens pole materials, causing both eddy currents and hysteresis. Within the rasterscanned SEM their eVect is masked as a simple oVset of the image, which generally goes unnoticed. With vector scan, long time-constant settling and positional oVsets occur that are intolerable in pattern writing. Some simple ferrite screening was applied, which greatly benefited the eddy current eVect, but the coil had to be retracted away from its designed location within the lens pole region in order to reduce the disturbing eVect of the ferrites on the lens flux distribution. This caused the beam levering position to be incorrect, introducing the further problem of limiting the area that could be scanned

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before the beam vignetted on the final aperture. Nevertheless, it was an improvement on what we had before. We did consider some form of dynamic deflection-correction system having measured the magnitude of the corrections needed to refocus and restigmate the beam throughout the field. These functions were quite smooth and reproducible but had the added complication of field rotation with focus change. We therefore decided to take a more pragmatic, lower-cost approach. With the increased clearance beneath the lens it was possible to install a single-lever, post-final-lens deflection coil that should have intrinsically lower aberration eVects. Although this increased the beam deflection angle, the system would be completely free from oV-axis, final-lens-induced aberrations that were an additional problem with the pre-lens deflection scheme. About this time Chris Armstrong was recruited to the group, and together we designed such a coil empirically, using the ‘best practice’ guidelines we could find in the published literature. Chris, applying great precision to accurately place each turn in the coil former slots, wound the prototype ‘saddle’ coil pairs with ‘tender loving care’. The first practical results were very encouraging, showing we could cover several millimetre-sized fields with far less defocusing and astigmatism. Disappointingly, we found that we still suVered with eddy current problems. These were alleviated using a ferrite enclosure and the resulting field coverage was considerably better than our previous pre-lens arrangement. In a 1-mm field size we were able to scan the beam with less than 100-nm positional error and produce 100-nm linewidth showing less than 10% variation throughout the field. All this was achieved without any form of dynamic compensations. By comparison, the modified SEM pre-lens arrangement had micrometre-sized errors both in position and spot size at such field corners. Having reached this design empirically we were keen to see if there was any chance of theoretically modelling the performance. At the time, Gerry Owen was completing his doctoral research at the Engineering Department, which included work on post-final-lens deflection systems. In a chance conversation at a conference we discussed the issue and Gerry was persuaded to run a simulation of our coil geometry using the software he had developed. This was probably a first experience of ‘consultancy’ for both of us: it showed some reasonable agreement with our practical results but was probably more agreeable to Gerry who had earned a nominal fee of 100 for his work. Not insignificant ‘pocket money’ for a struggling PhD student in the mid 1970s! In parallel with this work, Mike Penberth and another e-beam electronics stalwart Armin Javer, were starting to make real progress in providing a far more comprehensive, purpose-designed electronic deflection subsystem. This included a new 13-bit pattern generator with ‘deglitched’ DACs and a

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programmable clock unit for dose control. Additionally, new multirange deflection amplifiers were designed including programmable gain and interaxes summing units (multiplying DACs). This at last enabled us to adjust field size and rotation under computer control, a very major step forward at that time. With another engineer, John Kelly, a new stage control system was designed that enabled the computer to act as the servo-controller monitoring position through a register that accumulated motor steps, and with a motor driver that had the added sophistication of accelerate/decelerate techniques to reduce overhead times. B. Early Customers In the commercial world in which we were resident, it was necessary to demonstrate what we had to oVer to prospective customers, although it was not always an easy task when trying to sort out the bugs in fledgling equipment. Noteworthy is the part played by the few early customers who showed enough confidence in our designs to place orders for what was still relatively experimental equipment. Without the likes of Rockwell International’s magnetic bubble group in Anaheim (John Archer, John Reekstin and Joe Kenty); the Fujitsu Laboratories group in Kawasaki (Nakamurasan, Inagaki-san and Furukawa-san); and Professor Froeschle’s group at the RWTH Aachen (with his students Dieter Stephani and Ernst Krachmer) it is unlikely that we would have been able to continue. They all purchased the first real commercial systems CSI built, then known as the EBMF-1 (electron beam microfabricator model 1). By this time (1973–74), John Sturrock and I had started work on an improved stage system, planning a design based on the use of the Hewlett Packard laser interferometer system for positional measurement and a 4inch traverse precision stage manufactured by a small West Coast company called Yosemite Laboratories. As always in the commercial world, we were encouraged to oVer for sale what was still only evident on the drawing board and all of the three early customers ended up purchasing the larger system as retro-fit upgrades to their earlier installed EBMF-1 systems, changing them to what became known as EBMF-2. C. More Reorganization However, just as we thought we were really on the road to a successful product, another major corporate event occurred: SMI’s money ran out and a takeover of CSI was the only lifeline. So it was in 1974 that Metals Research (MR) (in Melbourn near Royston, Hertfordshire) were aided by

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government funding to take over CSI. In the process they gave up their own (MR) trade name, with the new combination simply being known as Cambridge Instruments (CI). With their Melbourn site situated some 10 miles from Cambridge, there were many operational diYculties. These finally convinced the CI directors to concentrate their R&D facility at Melbourn and the production facility at Clifton Road. Apart from the upheaval of such a move, the move itself was of little consequence to me personally (I was then living at Harston, conveniently about midway between the two sites). It did, however, pose major travel problems for many who lived near the Clifton Road site. The solution was a morning and evening bus between the two sites. To me, as Project Leader, it had the major negative eVect of losing nearly all my staV exactly at 4.15 p.m. when the bus left, whatever the current situation regarding equipment status, customer demonstrations or any other panics. I guess the best things that can be said about the Melbourn era were the key contributions made by several people. First there was the late Colin Fisher, who was the MR/CI Technical Director and, I believe, a contemporary of Gary Stewart when they were both PhD students at the CUED. Colin had almost single-handedly invented the technique of image analysis, I have been told, almost incidentally to his main endeavours, when needing such a tool for his research work. Upon completion of his PhD, Colin joined Michael Cole at Metals Research to commercialize his image analysis technology. In his Technical Director role with the new CI, Colin Fisher quickly reviewed the various projects inherited from CSI. As soon as John Sturrock and I described the Yosemite laboratories proposals for the large stage and chamber, he immediately appreciated several weaknesses of their design, and proposed the basis for the ring/strut system described by John Sturrock in Chapter 4.5. Modestly, Colin had the names of John and myself put on the resulting patent application raised to cover the concept. Sadly, Colin later died at a relatively early age as a result of a brain haemorrhage; he had had a brilliant conceptual ability and quickly grasped the key elements of any system design whatever the technology involved. He was very direct and demanding in style, but always positive and very supportive; I remember working for him with great fondness.

D. The Early Customers Again Having got the first EBMF-2 operational, we were then faced with discharging the upgrade programme to the three early customers. Most of the team spent many weeks ensconced in foreign parts integrating the new stage and

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chamber with the earlier EBMF-1 systems. It was not an easy process, but I personally came to understand far more about what the customer really wanted, rather than what we thought they should be provided with. It was also the beginning of a change in direction of my own career, from design to technical marketing. I also began to appreciate the various lithography needs of emerging device technologies and how these must influence the detail design of our system. It was not enough just to provide the system with bland ‘mask making’ and ‘direct-writing’ facilities; instead, applicationspecific facilities were required (especially within the control software) if the tool was to be of real value. These could only be realized by working very closely with the customers, particularly the operators of the tool. Another person who made a major impact on the direction of the project at this time (1976) was the late Ian Cruttwell. The ex-MR management of CI was finding very demanding the workload of controlling a company of over twice the size that they were used to. One approach they instigated was to appoint Ian as business manager to take over the day-to-day responsibility for the lithography product. Ian, who gained his MA degree at Cambridge in mathematics, had briefly worked for MR as a design engineer on the image analysis product. He had more recently been working with the Burroughs Corporation in Scotland and brought a much-needed larger company approach to our activities. It was agreed that, apart from his other responsibilities, he would direct the design group and I would concentrate more on the customer interface of proposal writing, system acceptance testing, and applications and support of systems in the field. E. Further Progress Much was still to be done to create a system with a well-rounded performance and our life was being further complicated by the obsolescence of the S4 Stereoscan and introduction of the S150 Stereoscan. We still needed to utilize the SEM upper electron column and drive supplies as well as the control and console electronics of this model for use in newer systems. Again John Sturrock threw himself into the task and churned out hundreds if not thousands of detail drawings to accommodate the changes. It did, however, give us our first chance to design a purpose-built plinth frame, with vacuum and isolation systems not limited to the constraints of a SEM plinth frame. Despite this progress we still had not produced an eYcient back-scattered electron collector system compatible with the new mainframe. Philip Chang’s earlier dual Channeltron scheme was experimental, and not easily interfaced to computer control. A number of alternative schemes were investigated for producing an annular collector with omnidirectional sensitivity in order to avoid biasing the apparent mark-edge positions. It also had

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to have a very thin profile so that it would fit under the post-lens-deflection coil without interfering with the substrate and chuck beneath. There were then no compatible solid-state diode detectors available commercially like those available today, so we finally decided to use an annular channel plate multiplier (CPM) unit manufactured by Mullard. A rather ingenious drive and signal retrieval system was conceived by Mike Penberth and others and the overall scheme was described at the ‘BEAMS’ meeting in May 1979 held in Boston (Penberth and Wallman, 1979). The system delivered the video signal at ground potential from the high-voltage collector plate on the exit side of the CPM, making it simple to interface with the processing system. With this design we were for the first time able to truly register on a wide range of diVerently formed and shaped marks covered by resist. Although the unit had a few ‘peculiarities’ of its own, it reliably provided the video signal for the over 80 EBMF systems that we later shipped. This period also saw our first attempt to upgrade the control software to take advantage of the file management and operational flexibilities provided within the DEC RSX 11 operating system. Colin Ashurst, Peter Ross, and others who had been working on a recently terminated image analysis project, joined us in this activity. After what seemed an age of system analysis, Colin Ashurst came up with a very versatile program that he named QSYS. Like the earlier MFSYS software conceived by John Somers, it was a run-time interpreter enabling the user to code machine control instructions directly in a relative intuitive mnemonic language. And being a run-time interpreter it gave the freedom to quickly modify sequences without the need to recompile source-to-object code files. This gave us a very powerful user advantage over our Japanese competitor who had opted for the compiler approach. A very comprehensive, yet flexible, mark detection scheme was also conceived. This had the ability to locate arbitrarily shaped and positioned marks and to provide a wide range of signal sampling and averaging facilities. Combined, these capabilities enabled the user to ‘tune’ mark acquisition for a wide range of diVerent mark geometries and signal-to-noise situations. The program QSYS also enabled the system to calibrate and set field size automatically to the accuracy of the stage positioning system. To achieve this, a target mark located on the chuck was first located axially at its known position and then moved in sequence close to each of the four sides of the field. At each position the beam was deflected to locate the mark and return any error between the observed and expected positions of the mark. The resultant error vectors were used to adjust deflection gain and rotation to cancel the errors. The resulting product was probably for the first time a true lithography tool and not just experimental apparatus. There was still much we knew we

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could do to improve performance, but we had reached our first relatively stable product platform from which to sell and develop. We were helped commercially by a UK government Department of Industry pre-production order scheme that saw systems placed at the GEC Wembley site and the Plessey Company Caswell site. Although this gave us two very important potential commercial sales, it also set demanding specifications, acceptance testing, reliability and operational support requirements from which much was learnt and digested. Both the product and several team members were to gain in stature from this encounter. We described the details of the system and its early performance at the Microcircuit Engineering Conference (MCE) held in Cambridge in September 1979 (Wallman and Armstrong, 1980). In a similar manner to the US-based ‘BEAMS’ meeting, the MCE (later renamed yMNE) meetings were to become major annual European events for the advanced lithography fraternity.

VI. The Gooding Era Yet again corporate fragility, brought about through unprofitable trading and mounting losses, led to another change of company ownership. With Margaret Thatcher’s sweep to power, government support to industry was not to be in a continuous ‘baling out’ mode. A deal was organized whereby Dr Terry Gooding, a former nuclear physicist turned entrepreneur, acquired Cambridge Instruments and a new management team was installed. Again, much radical upheaval ensued, new working relationships needed to be established and a leaner and far more focused approach to business was engendered within the company. This very soon led to the closure and sale of the Melbourn site and the return of all activities to Clifton Road, which caused a considerable disruption to progress on the project at the time but was overall for the longer-term benefit. The next major product enhancement steps were to increase the maximum scanning rate from 1 to 6 MHz and implement a full deflection compensation scheme. With this design, digital resolution was also increased to 15 bits and the main field was divided into a square array of 32  32 subfields. Following a calibration procedure, each subfield was assigned its own set of compensation coeYcients. The set provided drive settings for hardware that corrected for subfield centre position, subfield deflection range and rotation as well as for focus and astigmatism corrections. As the beam was addressed into each subfield so its unique correction settings would be y

Micro- and Nano-Engineering.

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loaded into the drive registers. We later described the details of this scheme at the MNE Meeting held in Lausanne in 1981 (Penberth et al., 1981). The system enabled quality 250-nm linewidth lithography within a 3.2-mm square field, and at 1.6 mm better than 100 nm was routine. This much larger field size was a major benefit to throughput, especially for the increasing number of microwave device manufacturers who needed to pattern 100-nm ‘T-gate’ structures formed by the ‘lift oV’ metallization process on compound semiconductor devices (Fig. 7). The unique vector-scan ability to vary the resist dosage within a pattern exposure proved to be a key enabling technique for such device lithography. Over 20 EMBF-6 and, later, EBMF10 systems were sold to such users and produced a much-needed revenue stream for the company. Another key system component developed for the EBMF-6 programme was a ‘height sensing’ transducer. With such relatively large deflection fields, the deflection angle induced appreciable field-size errors if the position of the image plane varied from one area of the substrate to another within the stage traverse region. Although the stage design maintained image plane deviations to better than a few micrometres, this induced errors that were unacceptable; especially when building up extended areas of lithography by ‘stitching’ (abutting) fields together. A novel scheme was designed using a capacitive sensor placed on a motor-driven retractable arm mounted under

Figure 7. An electron-beam-defined ‘T-gate’ of a GaAs microwave transistor. Reproduced by permission of Leica Microsystems Ltd.

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the final lens. This could be swung under the post-lens deflection coil and BSD detector and used in a pre-mapping mode. The stage was driven in sequence to a relatively sparse array of positions within the substrate area. After mapping the height at each of these, QSYS software would interpolate the height for each field position and apply the corrections needed. This greatly improved stitching accuracy and was an essential facility especially when exposing featureless mask plate substrates. As our customer list grew, so did the need for better management of the interfaces between field service support, design of the product and the user. One of the most successful periods occurred when we grasped the nettle and started to hold regular User Group meetings. With groups established in mainland Europe, the United Kingdom and the United States, we very quickly got to hear of problems experienced by the users. The forum was an immediate and very powerful display of any lack of customer satisfaction. The ‘purse string’ managers of the day (whom we had especially persuaded to attend the meetings) could not ignore such input! Gradually we were able to address many of the problems being reported and come up with improvements in design. Much, but not all, of this was to do with the versatility and reliability of the software, and this interaction greatly improved not only the product but also our vendor image. I feel I should mention the contribution made by the long-time Chairman of the US User Group, John Bass, who was a user providing ‘T-gate’ lithography for the organization then known as Comsat Laboratories. He conducted their meetings very skilfully, using a good balance of ‘stick’ and ‘carrot’ to ensure that real progress on user satisfaction was achieved. Unfortunately, not all of the company’s subsequent senior managers appreciated the value of such meetings. During the Gooding era at Cambridge Instruments there were grand new design initiatives on several product fronts including e-beam lithography. (John Sturrock describes much of this work in Chapter 4.5.) Sad to say, much precious money and time was spent on these endeavours but they did not result in any new major revenue earners. One such initiative was a new ebeam system to be named Chiprite: it was quite nicely styled but in the most part failed to improve the three key parameters of minimum linewidth, placement accuracy and throughput. Unfortunately the directors and the most senior managers, who conducted much of the early decision-making on such programmes, did so in isolation without any formal marketing input. They were in general well removed from practical knowledge of what users really needed and how long design and development of complex interactive systems really take. However, from Chiprite’s ashes we were able to make use of a slightly faster (10 Mhz) pattern generator and with other incremental improvements (including a 5-inch traverse stage) we evolved and launched the next product model called EBMF-10.5 (Fig. 8).

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Figure 8. The EBMF-10.5 lithography system. Reproduced by permission of Leica Microsystems Ltd.

This turned out to be one of the most profitable Gaussian beam lithography tools ever produced, with over 30 systems sold in six years. It was a real ‘cash cow’ and did much to sustain the profitability of the company right up to and beyond the time when Leica was formed. This happened in 1990 when Terry Gooding merged CI with the German optical company Wild Leitz of Wetzlar and very soon thereafter sold out his interest in the enterprise (see Chapter 4.5).

VII. Leica Microsystems Up to this time there had been three main competitors in the high-resolution, Gaussian beam lithography tool field: Cambridge Instruments in the United Kingdom, Philips in Holland and JEOL in Japan. All three had migrated into the product field from SEM manufacture with varying degrees of commercial success. The Philips and JEOL products had tended to concentrate on higher resolution, but at the expense of field of coverage and throughput. Combined, their sales were much fewer than the number of

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systems Cambridge Instruments had shipped. Low sales volume, high inventory and a much higher manufacturing cost base finally led the Philips corporate management to reappraise their position. Ultimately they decided to get out of the product field and sold their complete business to CI, coincidentally more or less at the same time as the Gooding–Leitz deal was taking place. Thus another sea-change occurred: new owners and a new general manager who up to then had little experience with e-beam lithography or the semiconductor industry. The immediate problem facing Leica was the integration of the ex-Philips product organization and the high value of their inventory of completed and near-to-completed systems. For various reasons a decision was made to discontinue the EBMF product range and concentrate on the Philips Beamwriter EBPG product in future. This also used the Gaussian beam, vector scan, step-and-expose writing strategy and was in concept quite similar to the EBMF product. It did, however, have some technical advantages regarding resolution, which made it more suitable for the growing range of nanotechnology applications. Probably the most important of these was its capability to operate at 100-keV beam energy: not only does higher beam energy provide improved resolution, it also greatly reduces pattern-fidelity errors caused by the proximity eVect. The relative dosage caused by scattered electrons, locally around the point of entry of the primary beam, is much reduced as the primary electrons penetrate to a greater depth, with their resulting scattering taking place over a much wider range. With one less player in the market place, a specification more suited to the growing interest in nanotechnology and (at least initially) quick delivery from inventory, the early 1990s were a relatively easy time for system sales. A new product was started based on the strengths of the Beamwriter hardware and the control software from the EBMF product. This current system, Vectorbeam VB6 (Fig. 9), also has the added benefits of a larger traverse stage, faster pattern generator (currently 50 MHz) and a very much brighter electron gun using a thermal field-emission source (Chisholm et al., 1995). Slow at first to come on stream, it has nevertheless proved to be a very comprehensive tool producing exceptional resolution and accuracy. Along with its wealth of software capability, it is today a major enabling tool for the nanotechnology revolution. In October 2002, Leica Microsystems moved from Clifton Road to a new purpose-built facility in Coldhams Lane, Cambridge. The university’s ViceChancellor, Sir Alec Broers, formally opened this new facility. At last, the production clean rooms and all aspects of the business have been optimized for the task in hand. Leica Microsystems is now in a very strong position to continue the evolution of high-resolution e-beam lithography started over 30

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Figure 9. The Vectorbeam VB6 lithography system. Reproduced by permission of Leica Microsystems Ltd.

years ago by a small band of very dedicated people. The story will, I am quite sure, continue. There is, of course, much more that could have been said and many more people who were involved, especially in production, sales, service and factory support functions. If I have forgotten to mention anyone who feels they made a major contribution, I apologize unreservedly. Looking back, I remember and appreciate most of all the friendship and support of some very dedicated people without whose intellect, skill, endeavour and perseverance none of this would have been possible.

References Broers, A. N. (1969). Some experimental and estimated characteristics of the lanthanum hexaboride rod cathode electron gun. J. Sci. Instrum. 2, 273–276. Chang, T. H. P. (1967). ‘Combined microminiature processing and microscopy using a scanning electron probe system.’ PhD Dissertation, University of Cambridge. Chang, T. H. P., and Wallman, B. A. (1971). A computer-controlled electron beam machine for microcircuit fabrication. IEEE Trans. Electron Devices ED-19(5), 629–635. Chisholm, T., Wallman, B. A., and Romijn, J. (1995). Thermal field emission sources and optics for gaussian beam lithography. SPIE Microlithography Conference, San Diego, 1995. Proc. SPIE 2522, 31–42.

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Hatzakis, M. (1969). Electron resists for microcircuit and mask production. J. Electrochemic. Soc. 110, 1033–1037. Penberth, M. J., and Wallman, B. A. (1979). Low profile electron collector system. J. Vac. Sci. Technol. 16, 1719–1722. Penberth, M. J., Price, G. G., and Lawson, P. J. (1981). Automatic dynamic correction for microfabricators, hardware and software. Lausanne, 1981, pp. 160–170. Wallman, B. A., and Armstrong, C. J. (1980). Pattern composition using an interferometrically controlled stage and precision electron beam deflection system, in Microcircuit Engineering, edited by H. Ahmed and W. C. Nixon. Cambridge: Cambridge University Press, pp. 367–383.

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

4.5 Commercial Electron Beam Lithography in Cambridge, 1973–1999: A View from the Drawing Board J. M. STURROCK Formerly at: Cambridge Instrument Co. Ltd.

I. Introduction My involvement with electron optics was entirely fortuitous. I had very little hearing after the age of 8, and education, in an age before the availability of help and support, was a struggle. I graduated from St Andrews University in 1962, but then faced the problem of finding an employer. Unknown to me, the Professor of Mechanical Engineering, J. Dick, observed this and wrote to Harold Pritchard, the Managing Director of the Cambridge Instrument Company (CIC)1, recommending me for interview. I joined the company as a Design Engineer in August 1962, three weeks before mechanical design work started on the Geoscan X-ray microanalyser and the Stereoscan SEM, on both of which I worked. After five years I moved to Beckman Instruments, but returned as a Principal Engineer in 1973. My involvement with electron beam lithography (EBL) at CIC, and its successors, began with the delivery in March 1975 of a motorized X–Y coordinate stage to CIC from an American company called Yosemite, based in San Francisco. Philip Chang had succeeded in using a modified Stereoscan SEM to ‘write’ a pattern on a substrate, before leaving for IBM in 1971, as described in Chapter 4.4 by Bernard Wallman. The major mechanical limitation to EBL progress at that time was the standard SEM specimen stage, which lacked the necessary flatness of traverse; freedom from pitch, yaw and roll; orthogonality of axes; and position feedback. Accordingly, Yosemite was contracted to design, and build, a precision X–Y coordinate stage for the purpose. The resulting stage was not very sophisticated, 1

In this chapter the abreviation CIC is used throughout as an abreviation for the Cambridge Instrument Co. and its successor companies (see Appendix to this chapter). 387 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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although the designer, Robert Mizrahi, had had a diYcult brief. It had to enter the original 1963 Stereoscan specimen chamber, which had been designed to accommodate samples that were a then unimaginably large half inch in diameter, and move them half an inch. The new stage had to accept samples 3 inches in diameter and move them, with far greater precision, 3 inches! When delivered, no provision had been made for loading or mounting a substrate, either by Mizrahi or by CIC, and so there was initially no means of using the new stage for lithography. I, as a mechanical engineer, was therefore added to the team by the new Technical Director, David Kynaston. Thus began a 25-year association with EBL. It was already apparent, however, that the new stage was not the way forward; the world, in 1975, wanted to write on 4- or 5-inch masks or wafers, and move them rather more than 4 or 5 inches. Yosemite had been apprised of this, and, in July 1975, Mizrahi arrived at CIC with a roll of blueprints under his arm. He proposed that Yosemite not only provide CIC with one of their ‘standard’ 4-inch stages, but also mount it in a Yosemite vacuum chamber fitted with a complete laser interferometer measurement system, newly invented by Hewlett-Packard, to monitor the position of the stage. The whole would be ‘tuned’ as a unit in San Francisco and shipped, already working, to Cambridge. Mechanically, CIC would have little more to do than stand it on a Stereoscan S4-10 SEM plinth, and place an S4-10 SEM column on top, a suggestion greeted with acclamation by the management of the day. II. Colin Fisher and Metals Research Fate, however, took a hand at this point, when CIC went bust—again (see the Appendix to this chapter for changes of company name). The company ended up in the hands of the Department of Industry, who passed it, like a hot potato, to the National Enterprise Board (NEB) newly set up by the 1974 Labour ‘White Heat of Technology’ government. The NEB’s solution was to inject taxpayer’s cash to tide the company over, and merge it with the then smaller, but faster growing, Metals Research (MR) company located at Melbourn (a village to the southwest of Cambridge). Metals Research, perhaps the first ‘Cambridge Phenomenon’ company, conducted business in a number of diVerent fields, none of them involving electron optics. A. Ring/Strut System Almost everyone had to move somewhere else ‘to get the benefits of the merger’, and fledgling ‘Lithography’ found itself at Melbourn—a fact of great significance for the future. The Technical Director there was Colin

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Fisher (a PhD from the Cambridge University Engineering Department (CUED) who had been supervised by Professor Beck); he quickly recognized the commercial possibilities of EBL, and promoted this in his plans for the new company’s future. Pondering his inheritance, he sent for the Yosemite proposal, and promptly (and correctly) tore it to shreds. Yosemite had drawn a simple fabricated vacuum chamber, and tacked everything required onto it at its handiest point. The X–Y stage, therefore, sat on the floor of the chamber, with the column perched on the roof, and the laser interferometer units parked round appropriate sides. It all looked very neat and pretty but, as Fisher pointed out, you don’t want something neat and pretty, you want something that works. The Achilles heel was that a vacuum chamber is not a rigid ‘static exhibit’ but a living, breathing and moving mechanism. It both expands and contracts with changes in temperature, and its walls flex with changes in barometric pressure. Anything attached to a chamber wall moves perpetually with respect to anything attached to another of the chamber’s walls. With various critical parts of the system attached to each of all six chamber walls, success was not very likely. A way would have to be found such that, while accepting that all vacuum chambers expand, contract and flex, the key system elements remained fixed relative to each other. To achieve this, Fisher proposed the basis of what became known as the ring/strut system (patentees: Sturrock and Wallman, 1978). In the ring/strut system everything of any consequence to successful EBL was attached to a massive mild steel ring, 3 inches thick, extending both above and below the chamber lid. The lid was deliberately weakened around the join area, so that chamber flexure could not be translated into ring deformation. The electron-optical column was mounted on the top of the ring, and the X–Y stage and laser mensuration components were attached to struts beneath the ring. At the time, the thought occurred that the ring and struts might have been made of Invar (a metal having a very low coeYcient of thermal expansion), or similar, but this was not actually carried out through lack of time and money.2 Fisher, at the same time, also insisted that the entire substrate environment must be precisely temperature controlled, and that this, plus vacuum pumping, must extend to the next and previous substrates while the current substrate was being written. Despite the greatly increased complexity that 2

This (nagging) thought resurfaced in 1992, prompting a second patent (Sturrock and Dean, 1996). There was still neither time nor money to make the whole thing of Invar, but there was time to fool the machine into thinking that it was. A much smaller, thinner, cheaper Invar ring was inserted into a recess in the main ring, on strictly kinematic principles. This enabled the main ring to expand and contract without disturbing, or stressing, the Invar insert. All the key mensuration components were then attached to the insert.

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resulted, he set tight deadlines of 1 August 1976 for completion of the first mechanical assembly, and 24 December 1976 for shipping the first working system to Aachen University in Germany. Like all great leaders, Fisher could inspire fear, as well as respect, and so began an adventure that would be unthinkable today: years, for some, of 14-hour days for bare wages. No paid overtime. No bonuses. No stock options. No nothing. All the pain of a successful Silicon Valley start-up, for a net gain, in monetary terms, of zero. Those who took leading parts, however, still glow with quiet pride at what was achieved in such unfavourable circumstances. Fisher vetoed the Yosemite plan in March 1976, with the 1 August deadline then little more than four months distant. Instead, he decided, Yosemite would send to Melbourn a standard oV-the-shelf 4-inch X–Y coordinate stage. Hewlett-Packard would send to Melbourn a set of modules suYcient to build an X–Y laser interferometric displacement measuring system. CIC would supply everything else. In view of what was to happen four years later, it is of interest to record that Fisher refused CIC permission to make their own X–Y coordinate stage, rather than buy one from Yosemite, with the words, ‘It would take you two years just to get their precision.’

B. Summer in Melbourn The summer of 1976 holds many memories. Though long, hot summers became the norm later, that of 1976 followed 20 years, or more, of very average-ish summers, and so the contrast was much greater. The sun came out in April, and stayed out pretty well until the autumn. Had the company still been at either Chesterton Road or Clifton Road in Cambridge, this would have been very welcome. But it came for that first summer at Melbourn. Metals Research resembled a farm more than a factory: the original country house, which formed the nucleus of the site at Melbourn, had been extended several times, until it could extend no more. Agricultural huts, sheds, Portakabins, Banburys, and so on, then sprouted northeastwards towards Cambridge. The general eVect was not unlike wartime Bletchley Park. On arrival, Lithography found themselves in a huge tin shed called ‘Phase 3’—the sort of thing you would put up today for an out-oftown retail store, but without the heating and ventilation services that would  now be the norm. In June, the shade temperature topped 80 F every day for three straight weeks, the Phase 3 temperature, under that monstrous tin roof, regularly exceeding the century mark. This was an unwelcome background to the task; when you are performing miracle, you don’t really want to perform it in a muck sweat. The scattered nature of the sheds caused other

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problems. Normal working hours were insuYcient to tackle more than the fringes of the task, and much of the real work was done at night and over the weekends. There were perpetual problems with the keys that gave access to the various shanties. It is vexing when one cannot get access to a tool, a drawing, a photocopier or a screw, at 1 a.m., especially when one has, eVectively, worked the last seven hours unpaid. The Mechanical Laboratory, once the fount of all mechanical innovation when CIC was at Chesterton Road in Cambridge, became the de facto Lithography Factory at Melbourn, without being given the opportunity to become anything else. The machine tools were moved into a wooden hut called ‘Banbury 1’, and set to work to churn out all the smaller parts for four EBL systems. Fisher was so confident of success that he had decided to dispense with any form of prototype, and build four saleable machines on an improvised production line! That the myriad of unproven parts might not even fit together, let alone work, did not seem to have been taken into consideration. The larger parts were, in the main, made by Pope and Meads, of Ware, Hertfordshire. Much credit must go to S. V. Brown, the workshop supervisor, who organized the manufacture, plating, painting, inspection, cleaning and assembly—often at plants a hundred miles apart—of many thousands of parts, in phased quadruplicate. Almost certainly a computer in 2000 would have been hard pressed to keep track of everything as successfully as he did in 1976 in his head. Miraculously, the first set of mechanical parts came together just after 7.10 p.m. on 1 August 1976 when the vacuum chamber returned from electroplating in Manchester. The problems had been unceasing—and not just the obvious ones like the hundreds of new drawings required before materials could be ordered or manufacturing could start. CIC lacked equipment of suYcient size, strength, cleanliness and precision for almost everything. There was no lifting equipment capable of dealing with the larger parts. There were no tanks big enough to plate those parts. None of the tool room inspection equipment, 11 miles away in Cambridge, was large enough, or precise enough, to deal with what was being attempted. There was a total lack of any form of clean room or temperature control at Melbourn. The laser interferometer system was the first to be shipped by Hewlett-Packard, and it came with a single A5 sheet of hand-typed instructions; it was so new that HP had not had time to write an instruction manual! CIC had to work out, overnight, how to set it up, how to align it, and how to make it work. The whole had had to be designed to sit on an S4-10 SEM plinth, and provide mountings for an S4-10 SEM column, its accessories, and its pumping equipment. The additional post-lens coil, beam blanking coils, etc, required for lithography, and their associated electronics, were, for the most part, those previously devised for the 1975 3-inch machine.

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C. Teething Problems—the One-piece Mirror Block Sadly, the first ‘write’, a few days later, was very disappointing. In theory, such was the precision and resolution of the HP measuring system, and such was the stability of the ring/strut system, and such were the proven results from the 3-inch machine, that writing errors of the order of 0.1 mm were to be expected. Instead, they were between 1 and 2 mm; ten times worse! One mechanical scapegoat was found, eventually, to be the expensive Yosemite stage. Each revolution of the X-axis drive shaft caused the X-table to flex, microscopically, twice. The X- and Y-mirrors, used by the Hewlett Packard system to monitor the positions of the X- and Y-axis stage tables, were mounted separately on this table, and therefore also flexed twice, both with respect to the stage and to each other. They were mounted on long struts magnifying the diVerential flexures. Perhaps my most significant contribution to electron beam lithography was to accept that flexure of a Yosemite stage was inevitable, and to mount both the mirrors and the substrate on a one-piece mirror block that allowed all three to float together, serenely, above the turmoil beneath. The writing errors from purely mechanical sources were thereby reduced to an acceptable level for those days, and the MicroFabricator, as it was christened, became a useable entity. The mirror block concept was refined later by making the whole of it, including the mirror faces themselves, from one piece of Zerodur, a zero-expansion glass, and applying thin metallic coatings to provide mirrors, or electrically conducting faces, where needed. The two mirror faces were lapped flat within 0.1 mm and orthogonal within 1 second of arc, by a specialist contractor, and the residual errors of each block were measured at the National Physical Laboratory, Teddington, and stored in the system-writing software, so that they could be compensated for.

D. First MicroFabricator Delivered The first MicroFabricator left for Aachen University, on time, on Christmas Eve 1976, nine months after the start of the project. There was, however, to be no happy ending. The MicroFabricator broke loose in the van in transit, doing a fair amount of damage to itself. Dick Smith and I successfully rebuilt the damaged machine, at Aachen, during early February 1977. The MicroFabricator was not an entirely new machine. Two-thirds of it already existed in the form of the S4-10 SEM. Although this was long obsolete, three more machines destined for upgrading still existed, and thus the planned four MicroFabricators could all be built in the same way. Two of the four machines were to prove important to the history of commercial

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electron beam lithography: the first to Aachen University, and the second to Fujitsu in Japan. Unknown to all at the time, the Aachen machine was later to play an important part in the future of both Leica (St Gallen) and Philips (Eindhoven). Aachen had not insisted on receiving a working lithography system, they were quite happy to obtain a sort of semi-static ‘exhibit’ to experiment on. One of the staV at Aachen later became the first Head of Lithography at Philips in Holland, and was responsible for the Philips EBPG (Electron Beam Pattern Generator) series of machines. It may be that Aachen’s main interest lay in seeing how someone else had tackled the problems, in eVect ‘sizing the job up’, before doing it themselves. It is always many times easier to be second, and it is usually the second mouse that gets the cheese! Although a successful series of machines resulted, the Philips lithography business never really became the money-spinner that Philips had hoped for.3 E. EBL Comes of Age The Fujitsu machine was to be a very diVerent case. Unlike Aachen, Fujitsu thought that they had bought a working system, and that in return for their cheque they could use it to lay down patterns on masks or wafers! The machine was very far from ready for that, having been thrown together by exhausted staV working on much else besides. The machine left for Japan after only the most basic testing, with no real provision for training, manuals, spare parts, servicing, or the hundred and one things that are taken for granted with, say, a new car. The machine then had to be made to work 10 000 miles away in Japan, involving several months’ residency for Chris Armstrong, John Somers, Amin Javer and Mike Penberth among others. It was here that commercial EBL really came of age, and those were the four who made the machine actually perform lithography, writing real patterns for a real customer. Their achievement was a very significant one. F. Fisher and the Demise of the S4-10 Stereoscan Back at CIC, there was to be no let-up. Fisher, elated by the first shipping, proposed to churn MicroFabricators out like jelly beans, apparently unable to comprehend that two-thirds of each machine was from an S4-10 left over 3

Philips sold fewer machines than CIC, and the loss-making business was sold to Leica, the successor of Cambridge Instruments, in 1990. The technically superior Philips electron beam column, though not the electron gun above it, nor anything else below it, then became the mainstay of the new Leica VB (Vector Beam) series of lithography machines from 1993 onwards.

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from the 1960s. What was to happen when the supply of old SEMs dried up? ‘Make some more S4-10s,’ said Fisher. This was easier said than done since they were packed with obsolete proprietary components and systems no longer obtainable. Not until September 1977 did the penny drop that next time three thirds of each machine would have to be designed, made and assembled by CIC, but by then it was too late; Fisher had promised MicroFabricators right, left and centre, to Blue Chip customers, notably to Motorola in Phoenix, Arizona, for delivery by February 1978. On 8 September 1977, with the last S4-10 cannibalized, not a single line existed on a single sheet of paper for an EBL system that was to be working in Arizona on 28 February 1978. To achieve this, 173 days remained available, the concept of weekends and Christmas being, by now, just a vague memory for the staV. The Arizona delivery would, in itself, have been bad enough. The shattered staV who would have to do it had scarcely seen either their homes or their families for 18 months. But Fisher had two other arrows in his quiver. Some Blue Chips, notably British Telecom at Martlesham Heath, had baulked at his price list. To accommodate them, he proposed a MicroFab-lite—a halfsize MicroFabricator, for a half-size cheque. The broken staV thus found themselves committed to invent not one but two totally new machines, simultaneously, against the clock. At the same time, Fisher decided ‘to do something about the final lens,’ not only of the new systems but the four already shipped. The S4–10 final lens was still, mechanically, the one I had laid out on my drawing board for the first Stereoscan in 1963. It had been intended to focus on things no more than half an inch away from the lens pole-piece. In the Cambridge EBL system, to accommodate the postlens pattern generating coil, the substrate to be written was positioned 2 inches away from the pole-piece—four times that for which it was designed! Accordingly, Fisher commissioned six new long-working-distance lenses, and all the tools and adapters required to install them, to be designed, detailed, made, assembled, set up, tested, calibrated, packed, shipped, installed, documented, etc., by the same few staV who would have to invent two new lithography systems during the next few days . . . . One hundred and seventy-three days might sound ample time to design, manufacture, build, test and install a new EBL system, but because the tasks must be, in the main, sequential, it was in reality very little. Long-lead-time items, of which there were hundreds, needed to be ordered by the end of day 1 to give a sporting chance of success, and knowing what to order was quite a challenge! Around 1500 mechanical drawings were required, the most critical 10% of them by day 17. That meant completing the design of the whole machine—all the major layout drawings—by the end of day 3. In the 50 hours that could be sequeezed out of those three days, many times 50

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major decisions were taken, setting the concept in tablets of stone that had to be lived with for 20 years or more. Two comings and goings stand out in the memory from that bleak period. The coming concerned Ian Cruttwell, the going Colin Fisher. Bernard Wallman was not only managing all this, but also had to do his own work and that of others as well. A professional manager was accordingly hired for EBL. This was Ian Cruttwell, an ex-Metals Research graduate trainee, and an old friend of Fisher’s. Cruttwell assumed command on 7 November 1977 and remained in charge of Lithography until April 1982, when he became Technical Director (UK). He was promoted to Technical Director (World) of the enlarged group in March 1990, moving to St Gallen, Switzerland, to take up this post, where he died in 1994. The going was the death of Colin Fisher, at a similarly early age, in 1979. Fisher, in many ways the driving force of commercial EBL in the United Kingdom, thus did not live to see the fruition of his vision. The Motorola (EBMF-2) and British Telecom (EBMF-1) machines came together during early 1978 at Melbourn. Both machines had their problems. Fisher had oVered Motorola a choice of either a 2-chuck or a 10-chuck airlock for their new machine, as if a pile of both was sitting ready on a shelf. Motorola chose the nonexistent 10-chuck airlock, so this was one more complicated machine that had to be invented by the end of day 3. The mechanics of both machines were 90% new: only the gun from the then S150 SEM and the first and second condenser lenses from the S4-10 SEM being carried forward. The fact that the S150 and S4-10 columns were from diVerent eras, of diVerent diameters, held together in diVerent ways, and pumped diVerently, and not necessarily what one would have chosen for lithography, was not allowed to enter into any considerations. The sole criterion for using them was that they, at least, were available from a shelf. It would be hard to think of a more inappropriate way of designing a key part of a 1 million, state-of-the-art machine. The simultaneous low cost EBMF-1 caused huge resentment over the labour involved. Only two were ever built (the second went to the Hughes Aircraft Corporation in America) though many hundreds of drawings and thousands of parts had to be specially prepared. EBMF-1 drained the very life-blood out of the staV at a time when every sinew should have striven to design the mainstream EBMF-2. Though the mechanics of both the first EBMF-1 and the first EBMF-2 were assembled, and under vacuum, by their appointed dates, both were delivered late and both required much on-site attention from CIC before the customers accepted them. The mechanics of both machines later attracted adverse criticism from bystanders who had not been much in evidence with their knowledge during the three-day design phase.

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Late 1979 saw the first formal management structure for the Lithography Division headed by Ian Cruttwell. Bernard Wallman became Product Manager. Successive production managers, one of whom drove a completed EBMF-2 oV the edge of a loading dock on a forklift truck, and another of whom supervised the dropping of a second completed EBMF-2 from a crane at CIC, came and went, although all the actual production organisation continued to be done by Sam Brown. Like Gaul, Lithography Engineering divided into three parts: Electronics under Mike Penberth, Software under Colin Ashurst and Mechanics under myself. Ashurst soon left, leaving Penberth undisputedly in change of ‘Engineering.’ When Penberth was on holiday, I oYcially occupied that position—the closest I ever got to the top!

III. New Management—the Gooding Era Throughout all of the above, CIC had been losing a steady 3 million per year, made good by the odd 3 million handout from the National Enterprise Board, and was, eVectively, bust again. In the nick of time, the company was bought by the entrepreneur Terry Gooding. Gooding had gone to America as a Fulbright Scholar, become a naturalized American citizen, and, like so many others—from Hewlett/Packard to Jobs/Woziak—started his own business in his garage. This grew to become the quoted scientific instrument company, Kratos, which Gooding sold on his retirement in 1978. In September 1979, however, he secured a 75% stake in CIC for 1 million of his own money; he also gained a cash injection of a further 6 million from a UK government that had become aware of the strategic importance of the company. The motivation behind this must remain a matter of conjecture. It might be that the bait was the lithography business, and that the 6 million was in recognition of the importance of the means of production of integrated circuits, for defence purposes, if for nothing else. (The Cold War was still very real, and the fall of the Berlin Wall was still 11 years in the future.) The lure might have been the SEM business, where CIC had been first in the field. Or the still young entrepreneur, bored in retirement, might simply have relished the thought of doing new deals. He may, or may not, have foreseen CIC as the key to the global empire he eventually created. Gooding set about a rationalization of the company’s aVairs, which included large-scale redundancies and a return to Clifton Road in Cambridge. The Melbourn site was closed and sold. Fully three months work was lost in the move, but by the spring of 1980, Lithography Engineering was free to set about improvements, launching first the EBMF-6.5, and then the EBMF-10.5. (The ‘6’ and the ‘10’ referred to the

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writing speed of the machines in megahertz, and the ‘5’ to the 5-inch travel of a new Cambridge Instruments lithography stage.)

A. The Monoblock Chuck and Other Improvements Three major improvements followed shortly afterwards. The first of these was the height-sensing technique, described by Bernard Wallman in Chapter 4.4. The second major step forward was the introduction of the Monobloc chuck following a proposal by Mike Penberth. It was unfortunate that there was very little agreement between early customers as to what size, shape and material their substrate should be. Everybody wanted to load something diVerent into their machine, and over 150 diVerent chucks resulted. Each chuck represented an enormous investment in time and money, and the tendency was to make the simplest chuck possible, hope that it would do, and dash on to the next. Penberth pointed out that better results might be obtained from a more sophisticated approach, sculpting chucks out of a single piece of material, and gluing the substrate and target location points to it, using a single master set of precision quartz rods and a master optical flat. With the use of the same rods and flat to make every chuck, all CIC chucks would be identical, and interchangeable. The idea was later refined by reverting to more than one piece, to make use of the diVering masses, magnetic properties, coeYcients of expansion, etc., available from diVering materials, but the single set of master quartz rods remains in use today. The Monobloc chuck was the first serious attempt at mounting the substrate in a repeatable, invariant position with respect to the X–Y stage mirrors, and in similarly mounting a ‘datum target’ for registration, field sizing, and other purposes. Coupled with the use of the datum target came the use of improved writing strategies whereby the system performed regular calibration checks on the datum target during writing, making appropriate corrections for any detected thermal or electrical drifts. These changes made possible the steady reduction in linewidths and critical dimensions required by the semiconductor industry. The third important innovation of 1980 was forced on the company during the summer, when Yosemite went out of business. Since 1976, Yosemite had been the sole supplier of their standard 4-inch X–Y coordinate stages for every MicroFabricator. CIC was thus left in the embarrassing position it had feared: orders coming in for EBMFs, but no X–Y stages available around which to build them. It was decided that the only course open, with Fisher’s ghostly voice echoing down the years ‘it will take you two years’, was to invent a replacement at CIC. Starting on blank sheets of paper on Monday, 11 August, a complete set of manufacturing drawings

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was delivered to Sam Brown on Friday the 15th that same week. Brown had the stage manufactured, assembled, set up, calibrated and running within specification, by the first week of November. That this was achieved inside three months depended on a large number of factors. From the outset, no-one had been impressed with the Yosemite stage. It consisted of pairs of home-made ball-and-veeway rails permanently fitted, side by side, to home-made tables. No provision of any kind was made for adjustments or compensations of any description. It had to be used ‘as is.’ It is believed Yosemite achieved ‘as is’ by hand scraping the rails individually until specification was achieved. All mechanisms begin to wear out from the minute they are first used. Many have some form of compensation to maintain precision during wear. The Yosemite had none. Many have some form of compensation to maintain precision during diVerential expansion of the various materials employed, the Yosemite had none. CIC had thus bought something for which the precision was known only for the day of manufacture in San Francisco, at whatever the temperature happened to be at that particular time. The precision deteriorated rapidly in high vacuum, and could not be regained. It was also unlikely that it was shipped from San Francisco to Melbourn without irreversible damage occurring from the varying seasonal temperatures encountered during transit; airport tarmac can be a cold place in mid-winter and a very hot place in mid-summer! Musing on this in an idle moment in March 1979, it occurred to me that as the rails so closely resembled standard Schneeberger rails, why not just buy Schneeberger rails? But why have the pairs of rails side by side? Why not put them one above the other? Why have all four rails, per axis, with vees? Why not have three vees and one flat? Why use umpteen bolts to screw each rail to the tables? Why not use just two bolts per rail, sited at the rails’ ‘Airy points?’ Why have a full complement of balls for every pair of rails? Why not just have three balls per axis, arranged to form a permanently stable, permanently self-adjusting tripod? All of this was embodied in a proposal in March 1979. It was decided to build one and test it over an extended period of time, in high vacuum, at varying temperatures. Very fortunately, the job was given to Colin Norden, the last person still operating as a member of the ‘Mech Lab’ would have operated in the 1950s and 60s. The original Mechanical Laboratory had been founded to provide ingenious solutions to diYcult mechanical problems. The staV had been hand-picked by Don Unwin from those craftsmen who had shown unusual initiative or ingenuity elsewhere. Their job was not simply to do, but to think before they did. Such an approach is, of course, complete anathema to industry today, where, apparently, patentable ideas can be invented to order, to budget and to timescale. In reality, really new ideas come when they come, irrespective of what budgets, timescales or even managers, may say. It is

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Norden-type mavericks, working outside ‘the system’, who make the diVerence and who give a company a competitive advantage. Norden machined the parts to make the new stage, then added two crucial refinements. By the carving out of an integral spring around one Airy point bolt per rail, the aluminium table and steel rails could expand and contract at their diVerent coeYcients without bending the rails. Norden proved this by heating the stage to 40  C and cooling the stage to 4  C, under observation. By adding little jacking screws pressing on each rail, he could pull and push a slightly bent rail straight. Norden proved he could obtain lithography precision from standard commercial rails by doing this. CIC thus had the basis of a stage that was impervious to changes in temperature and wear, and which could be periodically adjusted ‘as good as new’. Norden ran the stage, 24 hours a day, in high vacuum, observing it weekly for signs of wear. Pitch, yaw and roll all remained within 1 arc second, or so, over nine months, before the rig was abandoned through boredom. When the Yosemite emergency arose, some preparatory work had thus been done, but the opportunity was taken to increase the travel of each axis of the stage from 4 inches to 5 inches, which customers wanted, and to make many improvements to the drive system, end-of-traverse sensors and many other details. The demise of Yosemite was a blessing in disguise for CIC, triggering a much-improved high tech stage. It cost less to make; its availability was now within the control of the company; it was more precise; it retained precision at diVerent temperatures; it was transportable to the ends of the earth; it compensated for wear; and it could be periodically refurbished as good as new. And, for the first time, it no longer flexed twice per revolution of the X-axis drive shaft! November 1980 was therefore something of a high-water mark for the mechanics needed for successful EBL. CIC had now added an eVective onepiece mirror block (1977), height sensor (1980), Monobloc chuck (1980) and stage (1980) to the original patented ring/strut system (1976), giving a machine mechanically capable of doing what customers required. All five, that November, were technically better than those of any competition. With hindsight, it is a pity that the company did not now turn its attention to areas where the machine was weak. Two major shortcomings, betraying the 1963 origins, were the vacuum system and the vibration isolation system. In 1963, all high-vacuum machines were pumped by rotary pump/diVusion pump combinations, and this had been retained, despite their lack of suitability for electron optics. Most competitors, starting a little later, used much more advanced turbo pumps, ion pumps or cryo pumps. All CIC EBMFs relied on spring suspension, because the 1963 Stereoscan had sat on springs. Most competitors, starting a little later, used air suspension systems, which had become commercially available in the meantime. These were not, however,

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the main drawback, which was that CIC lacked a column designed for EBL. It would seem axiomatic that if one were to design a state-of-the-art EBL machine, one would start by designing a state-of-the-art column. Everyone else did so, particularly Philips in Holland, Etec and IBM in America, and various firms in Japan, France and Germany. Remarkably, as has been outlined earlier, CIC began lithography with an elderly SEM column, and continued to use a hotch-potch of column sections from diVerent microscopes, of diVerent vintages, until buying the Philips lithography business 21 years later. The Philips electron-optical column was then appropriated for use on CIC machines. Despite these three shortcomings, however, the mechanics proved suYciently robust to sell essentially the same machine right through the 1980s. Between 1980 and 1990, around 60 lithography machines were sold, at a fairly steady six per year. By 1990, the price of each had grown to around 1.25 million. During the 1980s, improvements to the electronics and software and writing strategies moved centre stage, with major contributions coming from Mike Penberth, Amin Javer and John Somers. Electronic improvements included increases in pattern generating speed, stability and reliability, reductions in thermal and electrical drifts, and better shielding from stray magnetic fields. Software improvements included those with regard to system speed, robustness and flexibility. Improved writing strategies included minimizing ‘dead time’, automating registration and calibration, maximizing substrate throughput, etc. Not a great deal was required from the mechanical side of lithography during this period, but the 1980s were as busy as the 1970s. Long hours were devoted to new electron beam projects, including successes such as the Linetester projects for IBM, which brought in much-needed cash at critical moments for the company. But the 1980s also brought failures such as Chiprite, a 7-inch stage lithography system; the S209 SEM, intended to inspect 8-inch wafers; the Electron Beam Tester project, which was to have tested ‘chips’ in their fully operational state; and the Alvey project.

IV. The Alvey Project Of them all, the one with the most relevance to EBL at CIC was the Alvey project, begun in 1986. Integrated circuits were seen as a key to the 21st century. The investment required to produce them, however, whether as a manufacturer of the circuits themselves or as a manufacturer of the necessary equipment on which to make them, was by now outstripping the resources of any single firm or government, except possibly in the United States or Japan. The Berlin Wall still stood; the Cold War was still real.

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Europe foresaw a situation whereby it would lack the resources to make integrated circuits of suYcient sophistication inside Europe, yet be refused the opportunity to buy those required from outside Europe. The Alvey project was a Pan-European initiative to harness appropriate European semiconductor companies and European governments to ensure Europe a future source of supply. The plan was to involve semiconductor companies such as Siemens, Philips, Thomson-CSF and CIC; universities such as Grenoble and Cambridge; and research establishments such as the Rutherford Laboratory, near Oxford. Much of it was to be funded by the governments of the participating countries, and the work would be parcelled out among all the collaborators. The area involving CIC was to be that of EBL, although this in itself was only a part of Alvey. Europe already possessed three EBL machines: those of Philips, CIC, and Thomson-CSF. The last had been selected to form the heart of the collaboration. Why this should be so is unknown, but there may have been two main reasons. The first was that, unlike the others, the Thomson was built round the ‘shaped beam’ concept. In orthodox EBL, the aim is to generate as small a round (or square) spot as is possible at the substrate end, and move it as fast as is possible to complete the tasks in the shortest possible time. In shaped-beam technology, the aim is to generate a variably sized rectangular spot at the substrate end, the exact size depending on the task of the moment. A good analogy is the use of various sizes of brushes to paint a house—broad rollers for large featureless surfaces, thin liner brushes for narrow window frames, medium brushes for skirting boards, and so on. In theory, by exactly matching the spot shape to the task, the overall pattern should be written in a shorter time, and the machine should be capable of performing more useful work in a given period, thus bringing down the cost of a unit of work and maximizing the profit for the machine’s owner. In practice, shaped beams are very diYcult to create and control, particularly via the method adopted by Thomson-CSF, which was two L-shaped electromechanical stencils mounted half way up the electron-optical column. The second reason may have been that it was planned that the stage would not move in discrete steps, as did the CIC and Philips stages, but travel continuously, writing ‘on-the-fly’ and aVording a further saving in time.

A. The Thomson-CSF EBL Machine The Thomson-CSF machine might have been designed specifically to impress—especially those unaware of the history of EBL. It was as diVerent as anything could possibly be from the Philips and the CIC machines, and that,

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in itself, was a powerful recommendation to some. The best description is probably of an upside-down EBL machine. The electron gun hung down, only a few inches from the floor, instead of being right way up, 8 or 10 feet above the floor, as on other systems. A miniature forklift was used to remove the gun from the electron optical column for changes of filament, with an absence of the usual fuss and bother. At that point the machine had practically sold itself to impressionable young bystanders. This was not, however, a new idea. In the 1950s, there had been O. C. Wells’ upside-down SEM at Cambridge University, followed by Smith’s SEM3, and the Pease/Nixon series of SEMs. AEI attempted the same concept commercially a little later, and VG Microscopes used it very successfully for their STEMs. However, while having certain advantages for SEM experimental applications, it is mechanically less attractive for EBL. This was only one of many weaknesses in the Thomson system, although each, like the inverted column, had initially been perceived as a strength. The feature that first attracted every eye, was a huge block of aluminium called the ‘platine’. This was about 1 metre square, and 4 inches thick. It was obviously exceedingly stiV, and obviously the ideal base for lithography. Every item connected with electron beam lithography was screwed directly to this single block of aluminium: the X–Y stage, stage motors, electronoptical column, laser interferometers, vacuum pumps, airlock, vibration isolation, etc. Aluminium has a high strength-to-weight ratio, but it is not a very ‘stiV’ material. Large amounts of material had been scooped out to make way for the X–Y coordinate stage, stage lead-screws, stage motors, etc. So much material had been removed in some places that there was not a great deal of material left to be stiV! Aluminium also has an undesirably high thermal coeYcient of expansion and high thermal conductivity. As it expands or contracts, everything screwed to it moves with respect to everything else screwed to it, linearly along all three principal axes, and rotationally about all three principal axes. It was right back to where Yosemite were in 1975, only much, much worse. The platine did not just expand and contract. It bent under the 7 tons of atmospheric pressure applied on pumpdown, since it formed the base of the specimen chamber. A hollowed-out block of aluminium, even 4 inches thick, is poor defence against 7 tons of applied load, and it assumed a whale-back contour that varied continuously with the atmospheric pressure. The stage tables and laser transducer measuring system were assembled, tested, collimated and calibrated on a flat platine in air, but the stage was used for lithography on a whale-backed platine in vacuum. Possibly through drafting error, the machine had a pronounced Abbe oVset error of around 6 mm in the laser transducer measuring system, whereas CIC and Philips had always striven to keep this to a matter of micrometres. The platine was fitted with two impressively

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large motors bolted directly to it, one to drive each axis of the X–Y coordinate stage. Large electric motors, however, generate large amounts of heat as a by-product, substantially altering the size of the platine in use. No attempt had been made at thermal control. The Alvey project was thus based on a machine whose mechanics were flawed in many ways. Those features were by no means the full extent of the problem. Among others, the Thomson had neither height-sensor nor any vacant space in which to fit one. By Thomson’s own admission, the French laser transducer system installed had inadequate resolution for lithography. But the most serious problem was the chuck. Like the machines of Philips and CIC, that of Thomson had been designed to write on 4-inch wafers. When the world required first 5-inch and then 6-inch wafers and masks, Philips and CIC had responded by making their chucks a little bit bigger to suit, as was usually possible when an EBL machine was ‘right way up’. When it is inverted, however, and a platine approach is adopted, the chuck must be inverted too, and lowered face-down through aligned holes in the stage tables. The Thomson chuck size was therefore constrained by the stage-hole size. Make the holes bigger, and the stage would end up more hole than stage, and be unusable. Despite these manifest shortcomings, work proceeded in Cambridge as if nothing untoward were happening. Something like 40 people worked full time on Alvey. At the same time, a cryptic paragraph appeared, in an obscure trade paper, to the eVect that Thomson-CSF had installed an (American) Aeble 150 electron beam lithography system in France, which did not say much for their faith in their own creation. Feeling that enough was enough, I issued a paper on the mathematical consequences of the eVect of atmospheric pressure, and changes of both pressure and temperature, on the platine. Two weeks later, I was asked to confirm the predictions by practical measurements. A terrible silence then fell on Alvey. The project staggered on for almost exactly 12 months, but that year was marked by the sounds of rats leaving sinking ships, alibis being rehearsed, evidence being hidden under carpets, scapegoats being lined up, and all the other familiar manifestations of the death knell of a project. The project never quite seemed to have an oYcial end, and large numbers remained nominally employed on it. There may, of course, have been much of which the workers were unaware, particularly the exact conditions of the government funding. It may have been necessary, for instance, to maintain an illusion of activity to maintain cash-flow, or fulfil a legal contract. Finally, CIC announced in September 1987 that an unfortunate loss of 1.4 million had been incurred on Alvey, which project was now at an end. (Later it was noted that there was a provision for a 3.7 million loss in the accounts for the financial year ended 31 March 1988.) Neither is likely to be the true figure, since there is

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also the matter of all the useful work 40 people might otherwise have been doing. The ending of Alvey was marked by the mass redundancy of 70 employees in October 1987. This included all of the faithful Mech Lab, who had hand-built the first lithography machines. Their most prominent casualty was probably Eddie Bainbridge (40 years’ service), but many other immensely valuable employees went with him. They included the two foremen, Bob Wood and Ted Jobson, who had given the most unstinting service since the first day of commercial EBL at CIC. Every trace of the Thomson FEPG-HR was removed from Clifton Road by Christmas 1987. Oddly enough, the episode had a curious sequel. It was announced in June 1989 that Thomson-CSF had placed an order for two CIC EBMF-10.5s, for installation in France!

B. More New Managers If the 1970s were characterized by constant changes of premises, the 1980s were marked by continual changes of management at every level, in every department. Throughout the decade Terry Gooding ran the company by remote control from San Diego. He appeared to possess but one business belief: that most firms, but particularly British science-based firms, are overmanned and undermanaged. To ‘turn them round’, therefore, it was only necessary for someone to eliminate the overmanning and install ‘strong management’. There have been instances in Britain where that approach was successful, but the key to those successes generally lay not so much in the easy bit—sacking large numbers of staV—as in the harder bit—installing strong managers. Strong managers do not grow on trees and, like every other department, Lithography Engineering had to accept a constant flow through the revolving door of engineering management. Gooding was quoted in the press of the day as saying: ‘It takes years to learn and understand the intricacies of putting optics, mechanics, high vacuum and electronics together’, which makes it all the stranger that so many uncomprehending strong managers were hired during the 1980s. While the new regime was generally unsuccessful in attracting the sort of management required, Gooding’s passion for good housekeeping did bring a transformation to the factory, and existing products began to leave in a satisfactorily increasing flow. This left him with time to pursue his ambitions for CIC. The Cambridge Instrument Company had obtained its original stock market quotation in 1959, and lost it in 1967 when it was sold to George Kent. From the beginning, Gooding may have envisaged a return to the stock market for a new Cambridge Instrument Company. The company had cost him 1 million; a successful stock market floatation might net, say,

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100 million. He thus embarked on a series of acquisitions and disposals designed to position CIC for such a launch. The first big purchase occurred in May 1986, when Reichert-Jung was acquired. This more than doubled the size of CIC overnight, giving the entrepreneur more credibility in the City, and he prepared to launch the company on both the British and American stockmarkets. The British prospectus was issued in March 1987. This listed group achievements in many fields including over 50 EBL installations, including CNET (France), Comsat (USA), Cornell (USA), GEC (UK), Honeywell (USA), IMEC (Belgium), Plessey (UK), Westinghouse (USA), Motorola (USA), Mitsubishi (Japan) and Fujitsu (Japan). The price of 130 pence per share valued the firm at around 127 million. The press was, at best, lukewarm about the whole thing. The most favourable comment came from the Cambridge Evening News. The Daily Telegraph was unenthusiastic, and the Financial Times sceptical. The best headline came from the Daily Mail: ‘CAMBRIDGE HUMS THE GOODING TUNE’. Despite the lack of enthusiasm, over 30 000 people clipped the entrepreneur’s coupon from their daily newspapers, and the oVer was oversubscribed. The timing was brilliant, as it is unlikely that that could have been achieved except in April or May 1987, at the peak of the ‘privatisation’ frenzy, and just before the crash of 1987. The stock opened for dealing in April 1987, but never traded at more than a few pence above its issue price. In October 1987, Gooding bought the microtome manufacturer LKB of Sweden. Less than three weeks later, he paid $37 million for the Optical Systems Division of Bausch and Lomb. Finally, in August 1989, came the announcement that Gooding was preparing to merge CIC with Wild Leitz. Should he pull it oV, he would create a group to be spoken of in the same breath as the Big Three—Zeiss, Olympus and Nikon. The combined sales would exceed 500 million per annum. He would command a force of 11 500 employees, in 17 countries, and, perhaps most important of all, he would own one of the best-known, most valuable brand names in the world: Leica.

V. A New Era Gooding’s ambitions were realized in March 1990. The remnant of the 30 000 coupon clippers had only a few days to accept the stock oVer, take it or leave it, of 70 pence for their 130-pence shares. They accepted and, on 2 April 1990, Leica plc was duly formed through a formal merger of CIC and Wild Leitz. The Chairman was announced as Terence Gooding and the President as Dr Markus Rauh of Wild Leitz. Only five months later, in

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September 1990, the resignation of Gooding from Leica was announced. The Gooding era thus lasted almost exactly 11 years, from September 1979 to September 1990. Gooding’s purchase of CIC in 1979 was probably the defining moment for EBL equipment in the United Kingdom. The National Enterprise Board had indicated that it was withdrawing all further financial support, and the company was probably only days away from receivership, two years short of its centenary. Had that happened, the production of EBL equipment in Britain would most probably have come to an abrupt end, and Philips, in Eindhoven, would have become the main European manufacturer. It is possible that Philips might have bought CIC, but it is unlikely, as there would have been no compelling motive in 1979. The reason for Gooding’s initial purchase must remain speculation. The attraction might have been any one of several businesses, particularly the SEM business, but none of them had the potential for profit that the lithography equipment business had, and it was Colin Fisher, the lithography champion, that Gooding had been preparing to make co-director at the time of Fisher’s death. If so, lithography was probably the seed from which post-1990 Leica sprang, since without that first crucial purchase there would have been no base from which to create the subsequent (electron) optical empire that spanned the world. If so, much is owed to those who made EBL possible at CIC, particularly those who put their lives ‘on hold’ between 1976 and 1979 to launch it. A. Cambridge (Leica) Takes Over Philips EBL While the events of 1990 outlined above took place in public, a remarkable development took place elsewhere in near secrecy: Philips decided to abandon the manufacture of EBL equipment in Holland and sold the business to Leica! A large team was sent from CIC to learn how to build and service Philips machines in preparation for the transfer of the manufacturing operation to Cambridge. There, it was proposed, both CIC and Philips machines would be built side by side. Visits to Eindhoven, before the transfer, were most interesting, as they had been labouring in the same vineyard since 1977. Philips had not had the constant movement between premises that CIC had suVered, and so there was suYcient in the way of prototypes from the earliest days still on the premises to trace the history of their machine. These revealed that the Philips EBPG had indeed initially not only been a CIC clone, but also how vigorously Philips, unlike CIC, had developed the original machine over the years. As each new technology had become available, for example turbo-pumping, Philips had adopted it. Moreover, they had begun by designing a column specifically for EBL. It had many

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telling little touches that CIC had already engineered into their SEMs (column liner tubes for example) but had never got round to in the supposed flagship lithography machines. Philips had clearly worked hard, and they had also had a number of advantages denied to CIC: more money, more staV, more space, more equipment, greater stability, greater permanency, a stronger brand name and a lack of the ‘niggles’ that beset CIC, which, for example, had been on ‘account stop’ with many suppliers in Britain for most of the previous 15 years because of unpaid bills. CIC had occupied four diVerent sites and had eight changes of ownership during the EBL years; despite that, CIC had sold more systems than Philips, and it was CIC which bought the Philips lithography business, and not, as might have been expected, the other way round. This may, in part, be due to the lithography results actually achieved by 1990 at Eindhoven. At 20 kV, stitching, mask overlay, and registration were all failing to meet Philips’ own (modest) specifications. Tests were only just beginning at 50 kV, and a working 100 kV system was little more than a remote chink of light at the end of a distant tunnel.

B. Alvey Revisited—Esprit Remarkably, during the same few months, and despite the unfortunate nature of the Alvey adventure, a new Grande Euro-Project, ‘Esprit’, was announced. The same three candidates as before would compete for large amounts of funding from the governments of the countries involved. A rambling 120-page document was circulating purporting to show how, in a series of upgrades, any one of the three might be converted into a major force in the lithography world. The document had clearly had many authors, from many disciplines, with many diVerent viewpoints, and had clearly been revised many times. What it had not had was any input from a mechanical engineer. At the heart of the proposal was the requirement that the Esprit machine should handle 8-inch wafers. To think that any of the three existing machines could be ‘upgraded’ to do this was sheer fantasy. All three had been designed for the 4-inch wafer era, now 15 years in the past. The Thomson FEPG-HR was, by its very nature, as explained above, unstretchable. The CIC and Philips machines had already been stretched, involving thousands of hours of the brightest minds, to their limit years earlier. A plan was found at Eindhoven, that summer, proposing a third, despairing, stretch to the Philips machine, but analysis of this showed that it would not have worked, and that in any event, it would not have enabled Philips to cover the surface of even a 6-inch wafer, the theoretical new stage travel being limited to 140 mm.

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As with Alvey, in July 1990 I wrote a paper on Esprit, from the purely mechanical viewpoint, pointing out the impossibility of what was being proposed. As before, this appeared to provoke zero reaction, but no further word was heard of Esprit within Leica. The same paper, however, also proposed new mechanics that would meet the Esprit requirements. This became the blueprint for the VB (vector beam) series of machines. In April 1992 the first line was drawn of the VB6 (Vector Beam/6-inch stage)— Europe’s first, and still the only new commercial EBL machine since the 1970s. This series of machines was able to put to good use all the mechanical experience, good and bad, that had been gained in CIC during the previous 15 years. Technology was borrowed from many projects, even failures like the 1983 Chiprite EBL machine providing worthwhile input. It accepted 8-inch wafers, but stopped well short of being a true 8-inch machine because of the prohibitive size and cost of the vacuum equipment necessary to support one.

C. Maturing Technology At the heart of this machine was an X–Y coordinate stage with rather more than 6 inches of travel. This did not enable it to present the whole surface of an 8-inch wafer to the column, but it did enable a very substantial part of the area to be covered. By fitting the co-planar targets for registration, beam current measurement, and so on, in the ‘missing corners’, they could be visited by the electron beam despite the size of the wafer. The machine stood on a completely new plinth, fitted with self-levelling air suspension and pumped by the latest turbo pumps. The machine retained scaled-up versions of the patented 1976 mechanics in the substrate area, improved as explained above, and used scaled-up versions of the one-piece mirror block, Monoblock chuck and so on. Having bought Philips EBPG, and its lithographyspecific column, this was an ideal time to rationalize the column area, and the Philips column became the standard for both the Leica VB6 and the ex-Philips VB5 range of machines. However, the column was improved in Cambridge in a number of important respects. A completely new 100 kV electron gun was designed and installed, and a completely new laser height sensor was integrated by Fred Strickland into the Philips final lens. This was a huge improvement over anything that had gone before at either CIC or Eindhoven, enabling real-time measurement, and correction, for very small variations in working distance between the final lens and the substrate. A scaled-up 10-chuck airlock became a standard part of the VB6, but the (patented) means of moving selected chucks between the airlock and the

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stage was much improved, using a commercial robotic arm, manufactured by Genmark, USA, and two temporary ‘parking bays’. The latter were at exactly the same vacuum pressure and temperature as the writing area; this enabled incoming substrates to be acclimatized to the system before use, and outgoing substrates to be stabilized before removal from the system. With the 10-chuck airlock also maintained at the same pressure and temperature, substrates could be conditioned for many hours, both before and after writing, without interrupting the maximal flow of substrates through the machine. Among the main contributors to these improvements were Andrew Dean, David George and Maurice King. Simultaneously, the speed, precision and stability of the electronics were steadily improved, and new software was regularly issued, most of the main contributors being those involved from the earliest days of lithography at CIC named above. The price of a complete system was now approaching 3 million, but a steady flow of sales resulted, just as with the earlier machines. The VB6 was an attractive, practical machine, capable of handling the largest substrates of the time in a costeVective manner, yet it was also capable of dealing with new fields such as the manufacture of SCALPEL X-ray masks at Motorola, and nanotechnology (see Chapter 4.4, Fig. 9, for a picture of the VB6). In 1997, one of the leaders in the field of nanotechnology, Cornell University, fabricated in crystalline silicon the then ‘world’s smallest structures’ on their VB6, including a complete guitar 10 mm long, with six strings each 50 nm (i.e. 100 atoms!) in diameter. One limiting factor to the new machine was that the final lens of the column was still, essentially, the 1979 lens designed by Philips in Eindhoven, without any of the advances in electron optics made since then. Accordingly, a new ultra-high-resolution lens was planned at CIC, based on input from several contributors including Tom Chisholm (ex-Philips), and E. Munro (a contributor to this volume). Work began in May 1995, and was completed in 1997, when a working version was fitted to a VB6. This was not received very favourably, and the project was shelved, but six years later, in 2003, essentially the same design began to be fitted to VB machines, although all the original contributors had long since left the company. While this might be thought to have been quite enough work for the 1990s, Leica had had a reversion to earlier thinking, despite the problems this had invariably caused in the past. Some institutions simply could not aVord the cost of a full-scale EBL system, yet were very keen to get involved in EBL technology, particularly nanotechnology. As before, they were promised a ‘half-price’ system, and the result was the EBL40 and the EBL100 machines, the number referring to the voltage kV) available in

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the electron gun. As before, these resulted in an enormous amount of work on top of the regular work, cobbling together systems from bits, pieces, modules and assemblies of electron-optical projects from the previous 30 years. Essentially they were built round the short-lived, but still very attractive, field-emission SEM specimen chamber of 1970. This was made of cast iron, and provided a very stable, screened, environment for a new specimen stage and laser interferometer measuring system, sitting on a new, small air suspension plinth with the latest turbo-pumping. All the mechanical refinements of the past were retained, although now scaled down, rather than scaled up. The column was the ex-Philips Lithography column, with the CIC refinements. These were certainly half-price machines, but they still cost an enormous amount of money to handle substrates of only limited size. As a result, they were not very attractive and, as before, few were made or sold. D. Another New Owner Changes of ownership continued to dog the company (see the appendix to this chapter). In 1998, Leica Microsystems Lithography Ltd was formed and, only six days later, on the 7 April, this was bought by Schroder Ventures Ltd, a move that was to have far-reaching consequences, not least for many of the ‘names’ that have appeared in this chapter. Schroder, as a venture capitalist, could aVord to spare little thought for history, continuity, experience or knowledge regarding EBL. Within a few months, many of the most prominent contributors to lithography had been made redundant, including myself. Schroder apparently took the view that ‘the work had all been done’ and that now a diVerent sort of employee was required, in a diVerent sort of premises, to produce the machines in a diVerent sort of way. In November 2002, Leica Microsystems moved from Clifton Road to a new factory in Coldhams Lane, Cambridge, to make and sell the lithography machines designed during the 1990s. While, on my part, there are no regrets, there is some degree of sadness that the company could not have managed its aVairs better given the circumstances. Electron beam lithography was (and is) an important process, helping to shape 21st-century life, with a worldwide market (almost) prepared to beat a path to the CIC door. The product was suYciently good for CIC to sell more lithography systems than any other manufacturer— some 127 systems by the end of the 20th century. Despite this success, the company made very little, if any, profit, presided over countless rounds of redundancy, and never scratched more than a temporary living for the few survivors. While wishing Leica/Cambridge every success, one hopes that the new EBL generation, in their 1-acre clean room, remain aware of

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the contribution of their predecessors who laboured in far less glamorous circumstances.

Appendix Company Changes of Name during the EBL Era 1967 1968 1974 1974 1975 1976 1979 1980 1984 1985 1985 1987 1990 1997 1998 1998

The Cambridge Instrument Company Ltd George Kent Ltd Brown Boveri-Kent Ltd Cambridge Scientific Instruments Ltd Metals Research Ltd The Cambridge Instrument Company Ltd Gladecrown Ltd The Cambridge Instrument Company The Cambridge Instrument Company Ltd Unotech Ltd Cambridge Instruments Cambridge Instrument Company plc Leica plc Leica Lithography Systems Ltd Leica Microsystems Lithography Ltd Leica Microsystems Lithography Ltd (bought by Schroder Ventures Ltd) 2001 Leica Microsystems Ltd

Companies Forming Leica plc in 1990 The dates given are the founding date of the original firm. 1819 Kern, Aarau, Switzerland 1847 American Optical, BuValo, NY, USA 1849 Ernst Leitz, Wetzlar, Germany 1853 Bausch & Lomb, Rochester NY, USA 1872 R. Jung, Heidelberg, Germany 1876 C. Reichert, Vienna, Austria 1881 Cambridge Instruments, Cambridge, UK 1914 Leica, Wetzlar, Germany 1921 Wild, Heerbrugg, Switzerland 1928 Zett, Braunschweig, Germany

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1987 Cambridge Instrument Co plc, Cambridge, UK 1986 Wild Leitz AG, Wetzlar, Switzerland 1990 Leica plc, St Gallen, Switzerland

References Sturrock, J. M., and Wallman, B. A. (1978). Electron beam lithography machine. US Patent No. A 4 103 168. Sturrock, J. M., and Dean, A. (1996). Electron beam lithography machine. US Patent No. WO 96/00978.

PART V EPILOGUE

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5.1 Charles Oatley: The Later Years THE EDITORS

On his retirement from the university in 1971, Charles Oatley realized a longheld ambition: to undertake research on the SEM on his own accord. He applied for, and was allocated, a room in the Engineering Department in which he set up a small laboratory-cum-oYce, and was quickly at work. The royalties flowing from the Cambridge Instrument Company, and a grant of 50 000 from the Wolfson Foundation for his own personal use, ensured that he was able to finance his research on a modest scale without recourse to university funds. The first task he set himself was the construction of a new SEM, and for most of the following year he was to be seen in the workshop, turning small parts at the lathe, winding coils and, relieved of all administrative and teaching duties, generally enjoying himself immensely. By this time a substantial number of magnetic-lens SEMs had been constructed in the department, and these designs were used to provide a basis for the new instrument. With the help of Les Peters, it was assembled and was soon producing pictures. It was fitting that it should fall to Oatley to build the last of a long line of in-house SEMs, stretching back to the original begun in 1948. This was not, however, the last SEM in which Les Peters was to be involved. For in 1986 he completed a replica of SEM1, the instrument he had worked on some 35 years earlier, for the Science Museum in South Kensington, London, where it was displayed alongside many other early electron microscopes (see Chapter 2.1A for details). Concurrent with this practical work, Oatley was also busy completing a book on the fundamentals of the SEM, which was published by Cambridge University Press in 1972. This is now out of print, but the following extract from the preface will give some idea of the approach he took at the time: The modern scanning electron microscope is a relatively simple instrument to use and an operator with the minimum of training can obtain excellent micrographs of a wide range of specimens. Nevertheless, he has under his control a considerable number of variables. He must select an appropriate accelerating voltage for the electron beam, lens currents to provide the required demagnification, and an aperture size to give a reasonable compromise between exposure time and resolution or depth of field. He can vary the angle at which the 415 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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electron beam strikes the specimen and the distance of the latter from the final lens. The position and orientation of the collector can be changed and so, also, can the voltage applied between collector and specimen. Finally, he may wish to modify the instrument itself, to adapt it to a particular piece of research. It is clear that, if he is to make the best possible use of his microscope, an operator should have a knowledge of the principles on which the design of the instrument is based and an understanding of the facts relating to the interaction of electrons with solid specimens. In the following pages I have attempted to provide this information in the simplest possible form. No prior knowledge of electron optics is assumed, nor of the theory of electron/solid interactions.

During these final, productive years Oatley published the following papers on the SEM, the last of which is reproduced as Chapter 5.2 in this Epilogue: 1975 1975 1981 1982 1983 1985

1985

The tungsten filament gun in the scanning electron microscope, J. Phys. E. 8, 1037 Sixty-sixth Kelvin Lecture. The scanning electron microscope and other electron-probe instruments. Proc. Inst. Electr. Eng. 122, 942 Detectors for the scanning electron microscope, Phys. E. 14, 971 The early history of the scanning electron microscope, J. Appl. Phys. 53(2), R1. (Reproduced in Chapter 1.2) Electron currents in the specimen chamber of a scanning microscope, J. Phys. E. 16, 308 The development of the scanning electron microscope (with D. McMullan and K. C. A. Smith). In ‘The Beginnings of Electron Microscopy’, ed. P. W. Hawkes, Academic Press. The detective quantum eYciency of the scintillator/photomultiplier in the scanning electron microscope, J. Microsc. 139, Pt. 2, 153

Whether by accident or design, Oatley’s small laboratory was conveniently situated immediately next to the area where members of the research students’ ‘CoVee Club’ assembled every morning. He had been a continuous member of the club since the 1940s and was to continue this tradition for almost the next decade and a half. Here he would mingle with students and staV, talk about research and indulge in the occasional argument. While most of the conversation concerned technical matters, one topic never failed to engage his attention, and that of his audience, completely: the pleasures of travel and holidays in Britain. His experience of foreign holiday venues was limited, but he had an encyclopaedic knowledge of the British countryside, there being few places of note that he had not visited. In his younger days he had walked over most of the hill and mountain regions in Britain, particularly those of the Scottish Cairngorms and the hills of Pembrokeshire where he and his wife, Enid, owned a holiday cottage. CoVee Club members drew freely upon this mine of information and not a few have followed an Oatley itinerary on their travels in vacations. It was at one of these morning coVee sessions (in 1969) that Oatley produced, for all to see and handle, his very

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large, solid, gold medal, which he had just received from the Royal Society. The chapter by Gerry Owen and Eric Munro in this Epilogue reflect some of the atmosphere prevailing in the laboratory in these closing years. To mark his 80th birthday in 1984, a one-day symposium was organized by Haroon Ahmed with the help of former students and colleagues, at which Oatley presented a paper on electron detectors for the SEM (Fig. 1). At a splendid dinner held at Corpus Christi College that same evening, he was presented with a silver goblet engraved appropriately with a reproduction of Brunel’s ship, the Great Eastern. It was at this juncture that Oatley’s research life began to wind down. He eventually vacated his room in the Engineering Department to spend the remaining years tending his garden at Porson Road in the same meticulous fashion that he had conducted his research. But a final academic accolade came in 1990 when he received an honorary Doctorate of Science from the university. That same year, Les Peters was awarded an honorary Master of Arts in recognition of his services to the university and the role he had played in the development of the SEM. A photograph of Les Peters and Charles Oatley together, taken at a gathering

Figure 1. Sir Charles Oatley presenting a paper on ‘‘Detectors for the SEM’’ at a symposium held in his honour to mark his 80th birthday.

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Figure 2. Sir Charles Oatley and Leslie Peters (left) at a gathering in the board room of the university Engineering Department to mark the occasion of Leslie Peters’ retirement.

in the board room of the Engineering Department, is shown in Fig. 2. Another stalwart member of Oatley’s team, Joan DuYeld, who was his secretary for 18 years, provided secretarial support for most of those involved in the SEM project until her retirement in 1990. On Oatley’s 90th birthday, Haroon Ahmed, together with students and colleagues (see Frontispiece), again organized a one-day symposium in his honour, which was attended by electron microscopists from around the world. In an autobiography written for the benefit of his family Oatley concludes with the following ‘Final Word’: ‘I have had a very happy life, and in writing about it have been impressed by the extent to which much of my happiness has followed from opportunities which arose without any action on my part. I have had more than my fair share of good luck.’

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

5.2 The Detective Quantum Efficiency of the Scintillator/Photomultiplier in the Scanning Electron Microscopey C. W. OATLEY Engineering Department, University of Cambridge

summary The eYciency of a scintillator for use with a photomultiplier in a scanning electron microscope has, in the past, been specified by its detective quantum eYciency (DQE). It is shown that this is likely to give misleading results since the DQE is the product of two factors, each of which may be influenced by the properties of the photomultiplier. It is further shown that there is reason to mistrust the theoretical basis on which the measurement of DQE rests. It is suggested that a better method of specifying the merit of a scintillator/photomultiplier system is by the mean number of electrons per pulse reaching the first dynode of the photomultiplier. A description is given of the way in which this quantity, f, can be determined. Results of measurements of DQE and of f for three commonly used scintillators are recorded.

1. introduction In an electron current of mean value I, if the arrival of electrons is completely random, the current is subject to fluctuation DI of r.m.s. value given by DI 2 ¼ 2eI Df

ð1Þ

where e is the electronic charge and Df is the eVective frequency bandwidth over which DI 2 is measured. If I1 is the signal current falling on a scintillator/photomultip plier detector, I1 = ðDI 12 Þ is the input signal/noise ratio S1/N1 and, with a perfect detector, it would also be the output signal/noise ratio S2/N2. The eYciency of practical detectors is commonly specified by the detective quantum eYciency (DQE) defined by  y

Reprinted from: J. Microsc. 139, Pt 2, 153–166 (1985).

Received 13 February 1984; accepted 1 June 1984.

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DQE ¼

ðS2 N2 Þ2 for practical detector

ð2Þ

ðS2 =N2 Þ2 for perfect detector

Assuming equation (1) to be true, this gives DQE ¼

ðS2 =N2 Þ2 I1 =2eDf

ð3Þ

Under normal conditions of use in a scanning electron microscope (SEM) or scanning transmission microscope (STEM) the noise generated by background noise in the photomultiplier is negligible and the gain of the photomultiplier may be made suYciently high for any subsequent amplifier noise to be negligible also. Then, if each electron entering the detector produced an output pulse of current and if all of these pulses were of the same amplitude, the detector would be perfect with a DQE of unity. Neither condition is satisfied in practice. It has been shown by Shockley & Pierce (1938) that, if DI12 is the mean square input noise to a detector and if each input electron produces s output electrons, where s may have any integral value from zero upwards, the mean square output noise is given by DI22 ¼ s2 DI12 þ 2eI 1 ðs2

s2 ÞDf

ð4Þ

If the input noise is given by equation (1), equation (4) reduces to DI22 ¼ 2eI 1 Df s2

ð5Þ

and, since the mean output current is sI1 , the DQE of the detector becomes Measurements made on the output of a detector do not enable one to determine s and s2 since they give no information about the number of input electrons which have produced no output pulses. Let a be the fraction of input electrons which have produced output pulses and let s0 and s20 be the mean and the mean square numbers of electrons in these pulses, whose amplitudes we can measure. Then s ¼ as0 and s2 ¼ as20 , so s=s2 .

DQE ¼ s2 =s2 ¼ as20 =s20 ¼ ab ðsayÞ

ð6Þ

We have now divided the DQE into two factors, of which a arises from the complete loss of output pulses and b from their varying size. The method of measuring the DQE which has commonly been used was introduced by Pawley (1974). An unfocused pencil of electrons was directed on to a scintillator and the current was collected by a surrounding box and measured. The output direct current (signal) and the r.m.s. noise over a known bandwidth were measured. Pawley was primarily interested in comparing diVerent scintillators and in investigating their degradation during use rather than in obtaining absolute values of DQE. In summarizing this and other earlier work, DQE values will not be quoted because it will be shown later that variations in experimental conditions are likely to render comparisons misleading. Comins et al. (1978) published a valuable paper on the preparation of P-47 scintillators and gave figures for DQE as a function of the surface density of the phosphor. In their measurements the normal arrangement of an SEM was retained,

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

421

with an electron beam striking a copper specimen and secondaries being attracted to a detector grid before acceleration by a potential diVerence of 10 kV applied between grid and scintillator. The magnitude of the secondary electron current was determined in a separate experiment, but no allowance was made for back-scattering and this led to errors in absolute DQE values. However, their main conclusion, that the optimum surface density of P-47 phosphor with 10 keV electrons is about 15 g/m2, was sound and has been confirmed by more recent work. More recently Comins & Thirlwall (1981) have reported further measurements in which the incident electron beam fell directly on to the scintillator as in Pawley’s experiments. They confirmed their previous result concerning the optimum surface density of phosphor for P-47 and, following Breitenberger (1955), provided an extremely detailed analysis of each stage of the chain from scintillator to output, to give an expression for DQE. This, however, was of limited use since accurate values of several of the constants were not available. Baumann & Reimer (1981) give a rather similar analysis of the components of the chain and provide a great deal of experimental information about the performance of these units. Their figures from which DQE values can be deduced appear to have been derived from a combination of results for separate parts of the chain, rather than from overall measurements. Autrata et al. (1978) report measurements on single-crystal YAG:Ce3+ in various forms, in comparison with P-47 and plastic scintillators. These results appear to relate to light output only and no mention is made of signal/noise ratio. EYciency of light output for YAG:Ce3+ is stated to be between 2 and 3 times as great as for P-47. 2. apparatus and experimental methods For the measurement of DQE by the usual method the apparatus was arranged as in Fig. 1, where L is the final lens of an SEM with a 100 mm aperture at A. After passing through this aperture, the electron probe was brought to a focus and then allowed to diverge to strike the scintillator B in a circular patch. The scintillator was located in the recessed lid of a cylindrical metal box C which had a hole in its base for the admission of the electron probe. This box was fastened to, but insulated from the lid D of a large specimen chamber. C was surrounded by an earthed metal shield E, with a hole in its base which was coaxial with, but smaller than the hole in C. A perspex window F and a photo-multiplier G were mounted above the scintillator, to collect as much light as possible: details of fixing arrangements and vacuum seals are not shown. The photomultiplier was surrounded by a light-tight box H which also contained the voltage divider for the dynodes. The output from the photomultiplier passed to an operational amplifier J with f.e.t. input and 1 MO feedback resistor. The output from J was connected to an oscilloscope K and, after passage through filter M, to a ‘true r.m.s.’ digital voltmeter N (Fluke Multimeter 8010A). The box C, which collected electrons striking the scintillator, was connected to a picoammeter P. All connections were made by

422

C. W. OATLEY

Fig. 1. Apparatus for DQE measurement.

concentric cable and, when measuring the characteristics of the filter M, a signal from a variable-frequency, high-resistance source was injected at point Q and its magnitude was measured at this point. The eVective bandwidth of the system measured in this way included the characteristics of both J and M, as well as the eVects of stray capacitance. Readings of the output signal and noise from N and of the input current, from P, enabled the DQE to be determined from equation (3). Before making any measurement on a scintillator, H and G were removed so that the scintillator could be viewed directly. The lens currents were then adjusted to give a bright spot about 3 mm in diameter and the spot was centred by movement of the aperture in L. For a further series of measurements on the pulse-height distribution of the output signal, the arrangement of Fig. 2 was used. The overall bandwidth of the system limited the count rate to about 15,000/s, so input currents of the order of 10 15 A were needed. Electrons passing through the aperture in L diverged over an area of the lid of box S, which contained a further 25 mm aperture. This not only reduced the current by the appropriate factor, but also shielded the scintillator from light from the distant electron gun.

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

423

Fig. 2. Apparatus for measurement of pulse-height distribution.

The direct output voltage from J was measured by V1 and was also passed to a comparator T. A level-setting voltage from W, measured by V2, was also passed to T. Pulses from J, whose amplitude exceeded this voltage, were counted by U. Thus the number of pulses between predetermined levels could be found by subtraction, and a pulse-height distribution obtained. A small pea-lamp, controlled externally, was mounted in the specimen chamber, so that the photomultiplier could be activated by a random sequence of single photons, rather than by pulses of light from the scintillator, containing arbitrary numbers of photons. In all measurements allowance was made for background pulses caused by thermionic emission from the photocathode or dynodes of the photomultiplier itself. 3. scintillators Three commonly used scintillators were investigated; plastic (Koch Light KL 236), single-crystal YAG:Ce3+, and P-47 crystalline powder. The first two were coated on the surface exposed to the electron beam with an evaporated aluminium layer about

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C. W. OATLEY

20 nm thick. This serves the dual purpose of reflecting light towards the photomultiplier and of providing a leakage path for the incident electron current so that the scintillator does not become charged. Previous investigators (Pawley, 1974; Comins et al., 1978; Comins & Thirlwall, 1981; Volbert & Reimer, 1979) have all worked with P-47 phosphor layers to which a conducting aluminium coating has been applied, using various techniques. This is standard practice in many commercial cathode-ray tubes and in a sealed-oV device is quite satisfactory. In a demountable system such as an SEM, where the scintillator is frequently exposed to air and is then re-pumped, it is found that the aluminium film disintegrates with time, so the technique is rarely used in commercial SEMs. If no metal film is applied, the loss of reflected light is unlikely to be serious, since phosphor layers of the appropriate thickness are almost opaque. The lack of a leakage path is more serious since, under the conditions in an SEM detector, the electric field will prevent the escape of secondary electrons and all but the fastest of back-scattered electrons, so that incoming charge can only be dissipated by conduction. The phosphor coating may itself provide a leakage path, whose eVectiveness may be influenced by the nature of the binder used, but the resistance of the path is likely to be very high. Mustoe (1979) solved this problem by depositing an evaporated layer of carbon on to a glass disc, before allowing the phosphor to settle on top of it. Since the carbon layer is optically transparent, adequate conductivity can be obtained without loss of light. In the present experiments a similar technique was used, but the transparent conducting coating on the glass disc was provided by a layer of tin oxide deposited from stannic chloride vapour on to the hot glass substrate. Various thicknesses of phosphor layer were used. Although plastic scintillators have been largely displaced in SEMs by more eYcient materials, they have advantages for experimental work. They are very uniform in texture and are almost transparent, so disc thicknesses of 1 mm or so may be used. This is important when measurements are to be made over a wide range of incident electron energies, with varying depths of penetration. If the electron probe is focused on to the plastic, deterioration sets in quite rapidly, even with currents as low as 10 10 A. No trouble of this kind was experienced when the electrons were spread over a circular area of about 3 mm diameter, as they would be in a normal SEM detector. Two specimens of YAG were available and one was found to be considerably more eYcient than the other. Results given below are for the more eYcient specimen.

4. pulse-height measurements Variations in the heights of output pulses from the photomultiplier can arise either from variations in the input photon pulses or from variations in the gains of the multiplier dynodes. To investigate the latter, the photocathode was irradiated with photons from a lamp, so that no input photon pulse could produce more than one electron. Under these conditions, variation in the magnitude of output pulses is caused entirely by the photomultiplier itself.

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

425

For each setting of the photomultiplier voltage, the reference voltage applied to the comparator was varied in equal steps of a size to give about ten values of mean pulse-height over the range of heights observed. From these values, the mean height s0 and the mean square height s20 were calculated. Then, from equation (6), b ¼ s20 =s20 . In studies of this kind it has sometimes been assumed that the pulse-height distribution at various points along the chain corresponds to a Poisson distribution Px ¼ lk exp ð lÞ=k!

ð7Þ

where Px is the fraction of pulses in the interval k of pulse heights. There appears to be little theoretical justification for this assumption, since statistical theory relates to a group of almost identical events and the ejection of secondary electrons from a dynode does not conform to this picture. An incident electron, moving in a particular direction with particular energy may eject a secondary: thereafter it moves in a diVerent direction with diVerent energy and may eject another secondary at a diVerent distance from the surface. Thus the two events are in no sense identical and it cannot be assumed that statistical theory will apply and this is equally true for the generation of photons in a scintillator. Nevertheless, the overall cascade process in a photomultiplier may lead to a Poisson distribution of output pulses and this possibility was tested as follows. If equation (7) applies, it can readily be shown (Appendix equations A.2, A.3 and A.13) that s0 ¼ l; s20 ¼ l þ l2 and b ¼ l=ð1 þ lÞ

ð8Þ

For each set of readings, a curve was plotted, joining the discrete values given by equation (7), with l equal to the mean pulse height calculated from the readings. The fractions of pulses in each height interval were then plotted to test coincidence with the theoretical curve. The results of three such tests are shown in Fig. 3, where curves A, B and C are for photomultiplier voltages of 950, 1050 and 1130 respectively. It is clear that the experimental points are represented quite accurately by the Poisson curves. Moreover, the values of b deduced from equation (8) are, for the three curves, 0
8, 0
81 and 0
78, while the values calculated directly from the experimental readings are 0
80, 0
81 and 0
79. It can be shown (Appendix, equations A.9 and A.11) that, if s1 ; s2 . . . are the mean gains produced by successive dynodes in a photomultiplier when a single electron strikes the first dynode, then the variance in the output signal s is given by var s var s1 1 var s2 1 var s3 ¼ þ þ þ ... 2 2 2 s1 s2 s1 s2 s23 s s1

ð9Þ

Also b¼

  var s 1þ 2 s

1

ð10Þ

so the first dynode is the dominant agent in producing changes in b. In the present measurements the first dynode voltage was kept constant by a Zener diode in the

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C. W. OATLEY

Fig. 3. Comparison of measured pulse-height distributions with Poisson curves when the photomultiplier is illuminated with light from a pea-lamp.

divider chain, so we should expect b to be almost independent of the total photomultiplier voltage, as was found to be the case. From the fact that the output pulses from the photomultiplier had height distributions given by Poisson’s formula, we cannot deduce that this would be true for each dynode. We may note, however, that the mean gain at the first dynode, measured in a separate experiment, was 5
4: if we assume equal gains for the other dynodes and Poisson distributions for each, equation (8) would lead to an overall value of 0
82 for b, which is very close to the value actually measured. Measurements were next made on pulse-height distributions when the photomultiplier was activated by photon pulses from a scintillator rather than by single photons from a lamp. Typical results are plotted in Fig. 4 for a plastic scintillator with incident electrons of 15 keV and 20 keV respectively. Departures from Poisson curves are greater than before, but the curves still give reasonable approximations to the distributions. The fact that the pulses leaving the photocathode no longer contain only one electron each, aVects the output noise in two ways which act in opposite directions. On the one hand, the input signal now has added noise which was not previously present: on the other, the mean number of electrons per pulse is greater, so the additional fluctuations introduced by the dynodes are less. Equations (9) and (10)

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

427

Fig. 4. Comparison of measured pulse-height distributions with Poisson curves when the photomultiplier is illuminated with light from a scintillator.

can still be used, but s1 must now be set equal to the mean number of electrons per pulse leaving the photocathode, s2 will be the mean gain at the first dynode, s3 the mean gain at the second dynode and so on. If we assume Poisson distributions for the pulses leaving the photocathode and each of the dynodes, we can calculate the way in which b is aVected by the height variation of the input pulses. Setting s2, s3 . . . each equal to 5
4 as before, we obtain the following values of b for diVerent values of s1 s1 b

2 0
62

4 6 0
77 0
83

8 10 0
87 0
89

5. the effect of photocathode sensitivity on the value of a If the number of photons per pulse reaching the photocathode is too small, some pulses will be lost and the value of a will thereby be reduced. Useful information about the magnitude of this eVect can be obtained from a knowledge of the mean number of electrons per pulse, f, reaching the first dynode. If the gain of this dynode is five or more, the chance of losing pulses beyond this point is negligible.

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C. W. OATLEY

f can be measured in the following manner. Keeping the gain of the photomultiplier constant throughout, the photocathode is first weakly illuminated with light to give n1 single-electron pulses, which are then amplified to give a direct-current output I1 from the operational amplifier. The photocathode is next irradiated with light from a scintillator to give n2 pulses leading to direct-current output I2. Then, for this scintillator and operating voltage, f ¼ I2 n1 =I1 n2

ð11Þ

Alternatively, pulse-height distributions can be used to determine the mean pulse heights in the two cases and f found from their ratio. Both methods have been used, with good agreement between them. Results for three types of scintillator are shown by the full-line curves in Fig. 5. Curve A is for P-47 with a surface density of 20 g/m2, while curve B represents both plastic and YAG; the circles being the measured points for plastic and the crosses those for YAG. We now make the assumption that the photon pulses falling on the photomultiplier have a pulse height distribution represented by Poisson’s formula, equation (7). There is no direct justification for this assumption, but earlier results suggest that it is unlikely to be too far from the truth. In any case, it is being used only to get a rough idea of the importance of photocathode sensitivity. It can be shown (Appendix: equation (A.14) ) that, if the incident photon pulseheights have a Poisson distribution with mean value l, and if p is the average eYciency of conversion from photon to electron, the resultant electron pulse-heights will have a Poisson distribution with mean value pl. The terms in this distribution cover pulse-heights having all possible integral values including zero, so the first term P0 represents the fraction of pulses in the mathematical distribution which have no

Fig. 5. Measured values of f for diVerent scintillators and a photomultiplier of average sensitivity (fullline curves). Calculated values of f for photomultipliers of higher and lower sensitivities (broken-line curves).

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

429

physical existence. For the incident photon pulses it will appear that l is unlikely to be less than 5, so P0 = exp ( g) is negligible and the whole mathematical distribution has physical existence. However, p will not be greater than about 0
25 and may be considerably less so, for the electron pulses, P00 = exp ( pl) is not always negligible. It represents the fraction of the incoming pulses which have been lost in the photon/ electron transition. The quantity f plotted in Fig. 5 is not equal to pl, since f is the mean of those pulses which have survived the transition, while pl is the mean of the whole mathematical distribution, including P00 . The relation between the two quantities is f ¼ pl=½1

exp ð plފ

ð12Þ

Thus, from the measured value of f, pl can be calculated and hence P00 , the fraction of incoming pulses that has been lost at the photocathode. Before taking these calculations any further, it is appropriate to consider the values of p likely to be encountered in practice. The manufacture of photocathodes is not an exact science and, in production, sensitivities obtained for nominally similar cathodes may vary quite markedly. Thus manufacturers specify a minimum sensitivity which all tubes will satisfy and a rather higher average sensitivity. For special purposes they may be prepared to select tubes with sensitivities almost twice as great as the specified minimum. Conversely, a tube which has been in use (or misuse) in an SEM for a year or two, may have a sensitivity which has fallen to perhaps half of the specified minimum. Such a tube may nevertheless be giving acceptable results in the microscope, so long as high resolution at high magnification is not required. The full-line curves A and B in Fig. 5 plot experimental results obtained with a new photomultiplier, which had not been specially selected: they represent normal good performance. With a selected tube the sensitivity might have been higher by a factor of 1
7 (say) and, using the relations developed above, the corresponding values of f can be calculated. These are represented by the broken curves A1 and B1 respectively. Similarly curves A2 and B2 give values of f for a tube whose sensitivity had fallen to half the value of that corresponding to curves A and B. For each of these curves, values of pl and hence of P00 can be calculated. P00 is the fraction of incoming photon pulses which have not produced corresponding electron pulses. It is convenient to write g¼1

P0 0

ð13Þ

where g is now the fractional eYciency of conversion from photon to electron pulses. Values of g are plotted in Fig. 6, to correspond to the f-curves of Fig. 5. For negligible loss of eYciency f should not be much less than 4.

6. measurements of dqe In view of the results recorded in the preceding section, it was decided to make measurements of DQE using three diVerent photomultipliers with widely diVering sensitivities. All three had photocathodes of type S-11. Tube D had not previously

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C. W. OATLEY

Fig. 6. Values of g calculated from the curves of Fig. 5.

been used and, from the manufacturer’s test sheet, had a sensitivity of 105 mA/lumen. Direct comparison with this tube gave 49 mA/lumen for tube E and 34 mA/lumen for F. E was at the lower limit of sensitivity specified for the type and F, which had been in use for some years, was well below this limit. The measured gains at the first dynode for D, E and F respectively were 4
5, 5
4 and 3
4. The DQE values obtained with these tubes are summarized in Figs. 7 and 8. The curves for P-47 confirm earlier measurements that the optimum layer density is in the region of 20 g/m2, the exact thickness depending on the voltage at which the detector is to be operated. Curves for YAG do not substantiate the claims for high eYciency which have sometimes been made for this material—possibly because some of these claims have been based on measurement of light output rather than DQE. Large output pulses are of no avail if many pulses have been lost. Perhaps it should be said that YAG is a relatively new detector material and samples may vary. The results given here are for the better of the two samples available. The fact that plastic, which is less eYcient than P-47 at low voltages, is nevertheless better at high voltages, arises from the fact that plastic is almost transparent, while P-47 is almost opaque. Thus, if the layer density of P-47 were increased to make full use of the electron energy at high voltages, some of the pulses would be lost by absorption. In equation (6), a is the fraction of electrons entering the detector which produce output pulses from the system. a may fall short of unity because some of the incident electrons produce no photon pulses in the scintillator, or because some of the pulses are subsequently lost. It has been shown above that the photon/electron transition at the photocathode is likely to be a major source of loss, so we may expect the DQE curves to show some similarities with the curves for g in Fig. 6, since b in equation (6) does not vary very greatly. The similarities do occur and it is tempting to use comparisons between the two sets of curves to obtain information about other

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

Fig. 7. DQE values for P-47.

Fig. 8. DQE values for plastic and YAG.

431

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C. W. OATLEY

factors contributing to a reduction in a. However, it is shown below that any such comparison is likely to lead to gross error.

7. the absolute value of the dqe The results described above are believed to provide an accurate picture of the relative properties of the three types of scintillator and of the way in which the overall eYciency of the detector depends on these properties and on the eYciency of the photocathode. However, when we come to absolute values of DQE, a diYculty arises. If a were equal to unity in equation (6) the DQE would be equal to b and we have seen that b can hardly be greater than 0
85 and is more likely to be about 0
75. Yet the curves for plastic, Fig. 8, show the DQE rising to unity, and so must be incorrect. A detailed investigation was therefore made of possible experimental errors. Some of the electrons entering the box C in Fig. 1 might fail to strike the scintillator, but this would lead to too low a value of DQE, as would loss of light from the scintillator. The meter measuring the input electron current, of the order of 10 10 A, was checked against two high-grade commercial picoammeters and the ‘true r.m.s.’ digital voltmeter N was compared with a Fluke thermal meter and with a home-made bolometer. None of these comparisons suggested an error greater than about 1%. It was suggested that the pulses from the photomultiplier might be exceeding the slewing rate of the FET operational amplifier, so a 50 pF capacitor was connected across the amplifier input to keep within bounds the rise-time of a pulse. Lastly, it seemed possible that the current pulses were causing variations in dynode voltages, leading to non-linearity of gain. A new divider chain, of lower resistance, was constructed to ensure that the signal current was not more than about 1% of the divider current and capacitors were connected across the last four resistors. When all the precautions had been taken, there remained only a possible uncertainty of bandwidth Df. Using simple R-C filters, a set of readings for three diVerent bandwidths was taken, with the results shown in Table 1. For each reading, two values of input current were used and, in general, the resulting DQE values diVered by less than 2%, so non-linearity of the amplifier appears to be ruled out. In Table 1, the bottom line gives the values of b estimated from measured values of f, using equations (9) and (10). An almost identical set of values was obtained after an interval of a month. It is always diYcult to be certain that some source of error has not been overlooked, but the conclusion has been reached that the above measurements are accurate and that there must therefore be some unsuspected flaw in the method. If we accept that the direct-current signal is unlikely to be in error, this means that either the input noise is not represented by equation (1), or that some factor is reducing the output noise. The four suggestions which follow have not been investigated: they are set down in the hope that they may stimulate further research. Space-charge reduction of noise is a well-known phenomenon in thermionic valves, but has usually been assumed to be negligible in the high electric fields near

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

433

Table 1 DQE for plastic

Measured bandwidth (kHz)

20 kV

25 kV

29 kV

2
78 4
60 8
72 b (estimated)

0
81 0
82 0
83 0
75

0
90 0
91 0
86 0
78

0
94 0
94 0
95 0
84

the cathode of an electron microscope. However, it has been shown by Davey (1973) that the normal condition of operation of a tungsten filament gun is one in which space-charge causes a proportion of the emitted electrons to return to the cathode. Perhaps space-charge smoothing in the crossover should also be considered. So far as is known, equation (1) has been tested experimentally only for pure thermionic emission. It is diYcult to calculate the field at the tip of a hairpin filament in a microscope, but it appears that a considerable portion of the emission may be caused by Schottky eVect. Whether the equation is valid under this condition is not known. The output pulses from a photomultiplier are broadened by transit-time spread, to give a pulse-width of the order of 20 ns. Although very short, this time period is of the same order as the mean interval between the arrivals of successive electrons with currents of about 10 11 A. It is known that, in some scintillators, the initial very rapid decay is followed by a much slower tail and this could cause an eVective reduction in frequency bandwidth.

discussion As a measure of scintillator eYciency the DQE has never really lived up to its name, since it is the product of the two quantities a and b, which both depend on the characteristics of the photomultiplier as well as on those of the scintillator itself. The foregoing results now cast doubt on the theoretical basis of DQE measurements. They also show that, unless a photomultiplier of high sensitivity is used in the comparison of scintillators, DQE measurements will give misleading results, to the detriment of the less eYcient scintillator. Thus, at best, the DQE can only give meaningful results when used to compare complete scintillator/photomultiplier systems. It has no absolute significance. It is suggested that measurement of the quantity f, the mean number of electrons per pulse reaching the first dynode, is a simpler and more satisfactory test of the eYciency of a detector than measurement of the DQE. The f determination requires no modification of a SEM to permit collection and measurement of the small electron current striking the scintillator, nor does it involve knowledge of the noise bandwidth. The only instruments needed are a simple pulse counter and a d.c. meter.

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C. W. OATLEY

appendix Some of the equations used in the preceding article are most readily derived by the use of generating functions. This technique is treated in relatively few books on the theory of probability, so a concise proof of the relevant equations is given below. A more complete account of this useful method may be found in, for example, An Introduction to Probability Theory and its Applications, Volume 1, by William Feller (John Wiley, New York). The impact of an electron on a scintillator may produce no light or it may generate pulses containing 1, 2, 3 . . . s photons. Let the corresponding probabilities for these scores be P0, P1 . . . Ps . . . The generating function G(x) for this event is defined by the equation GðxÞ ¼

s¼x X s¼0

Ps x s ¼ P0 þ P1 x þ . . . Ps xs . . .

ðA:1Þ

The symbol x has no physical significance and is inserted purely for purposes of calculation. Its value will be restricted to regions where the series is convergent and can be diVerentiated term by term. Then, if primes indicate diVerentiation with respect to x, G0 ðxÞ ¼ P1 þ 2P2 x þ 3P3 x2 . . . and G0 ð1Þ ¼ P1 þ 2P2 . . . þ sPs . . . ¼ s

ðA:2Þ

G00 ð1Þ þ G0 ð1Þ ¼ s2

ðA:3Þ

Similarly

and variance ¼ G00 ð1Þ þ G0 ð1Þ

½G0 ð1ފ2 ¼ var s

ðA:4Þ

When an electron strikes a scintillator, the number of photons produced is subject to statistical variation and statistical fluctuations occur during the generation of photoelectrons and their subsequent multiplication at each of the following dynodes. At a particular stage in the chain, let the number of units per pulse (photons or electrons) be represented by the generating function A(x). Let B(x) be the generating function representing the change which each unit undergoes in passing to the next stage. Then it can be shown that the generating function G(x) representing conditions after this next stage is given by GðxÞ ¼ A½Bðxފ

ðA:5Þ

G0 ðxÞ ¼ A0 ½BðxފB0 ðxÞ

ðA:6Þ

Then

DQE OF THE SCINTILLATOR/PHOTOMULTIPLIER IN THE SEM

435

and G00 ðxÞ ¼ A00 ½Bðxފ½B0 ðxފ2 þ A0 ½BðxފB00 ðxÞ

ðA:7Þ

Substituting these results in (A.2), (A.3) and (A.4) and remembering that A(1) ¼ B(1) ¼ 1, we find for the variance in the next stage var s ¼ s2B var A þ sA var B

ðA:8Þ

var s var A 1 var B ¼ 2 þ
2 sA s2 sA sB

ðA:9Þ

or, since s ¼ sA
sB

For a cascade of n events, induction yields s ¼ s1 ; s2 ; s3 . . . sn var s var s1 1 var s2 1 ¼ þ þ ... þ s1 s22 s1 s2 . . . sn s2 s21

ðA:10Þ

1

var sn s2n

ðA:11Þ

An alternative derivation of (A.11) has been given by Shockley & Pierce (1938). The above results are independent of the forms of the generating functions, but two forms are of importance for our purpose. When a photon arrives at the photocathode of a photomultiplier it can generate 1 electron or 0 electrons. If p is the probability of generating an electron we have GðxÞ ¼ ð1

pÞ þ px

ðA:12Þ

If, at any stage, the number of units per pulse has a Poisson distribution GðxÞ ¼

x X lk exp ð lÞ k x ¼ exp ð lÞ exp ðlxÞ ¼ exp ½lðx k! 0

1ފ

ðA:13Þ

Finally, if light pulses with photon counts represented by (A.13) fall on a photocathode with sensitivity indicated by (A.12), the numbers of electrons in the resulting electron pulses will be distributed in accordance with the generating function GðxÞ ¼ expfl½ð1

pÞ þ px

1Šg ¼ exp½lpðx

1ފ

ðA:14Þ

which represents a Poisson distribution with mean value lp. references

Autrata, R., Schauer, P., Kvapil, Ji., & Kvapil, Jo. (1978). A single-crystal of YAG—new fast scintillator in SEM. J. Phys. E. 11, 707–708. Baumann, W. & Reimer, K. (1981). Comparison of the noise of diVerent electron detection systems, using a scintillator/photomultiplier combination. Scanning 4, 141–151. Breitenberger, E. (1955). Scintillation spectrometer techniques, Progress in Nuclear Physics (ed. by O. R. Frisch), pp. 56–94. Pergamon Press, London.

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Comins, N. R., Hengstberger, M. M. E., & Thirlwall, J. T. (1978). Preparation and evaluation of P-47 scintillators for a scanning electron microscope. J. Phys. E. 11, 1041–1046. Comins, N. R. & Thirlwall, J. T. (1981). Quantitative studies and theoretical analysis of the performance of the scintillation detector. J. Microsc. 124, 119–133. Davey, J. P. (1973). Grid control mechanisms in the hairpin filament electron gun. Optik. 38, 123–137. Mustoe, G. (1979). Preparation of P-47 crystalline phosphor scintillator discs. Scanning 2, 41. Pawley, J. B. (1974). Performance of SEM scintillation materials, Scanning Electron Microscopy 1974 (ed. by O. Johari and I. Corwin), pp. 27–34. I.I.T. Res. Inst., Chicago. Shockley, W. & Pierce, J. R. (1938). A theory of noise for electron multipliers. Proc. Inst. Rad. Eng. 26, 321–332. Volbert, B. & Reimer, L. (1979). Preparation of P-47 scintillators. Scanning 2, 107–109.

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5.3 Professor Oatley Remembered E. MUNRO Munro’s Electron Beam Software Ltd., London Formerly at: Engineering Department, University of Cambridge

I. Introduction I owe a very great debt of gratitude to Professor Sir Charles Oatley, as he helped me in many ways with his wise advice at many stages in my career. He aVected the direction of my life at several critical points. Looking back, times at which he particularly helped me were guiding me in my PhD project at Cambridge, helping me with my application for a fellowship at Trinity College, helping me to arrange my first visit to the United States and to get a job at IBM, helping me to return to Cambridge and subsequently to get a job at Imperial College. He helped me with all these things, but more of that later. My first memory of Professor Oatley is during my final undergraduate year at Cambridge, in 1967–68, when I was a student in the Engineering Department, studying for the Electrical Sciences Tripos. During that year, I attended Prof. Oatley’s course on electromagnetic theory. Thirty-five years later, I still have the handwritten notes that I took at his lectures, and I think they are the clearest and simplest explanation I have seen of Maxwell’s equations to this day, clearer than in most undergraduate textbooks. His lectures were always presented with a style and clarity that most lecturers now would find hard to emulate. In the autumn of 1968, I started a PhD project in the Engineering Department at Cambridge. At that time, Prof. Oatley was the Head of the Electrical Division. Originally, my PhD topic was to be on the computer simulation of 3D discontinuities in microwave cavities, but because of the great interest in scanning electron microscopy in Professor Oatley’s research group at that time, I decided to change my topic to the computer modelling and design of electron microscope lenses. I thought it would be a very worthwhile thing if the resolving power of electron microscopes could be improved, and I have indeed been very fortunate to be still working essentially on this topic up to the present time; I owe the initial inspiration for this work in large measure to Prof. Oatley. 437 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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II. Mud—An Introduction to Finite Elements Soon after I started working on my project on electron lens design, I realized that what was needed to improve the performance of electron lenses was to make them stronger by having higher magnetic fields, and that this would involve high excitations and magnetic saturation. Only a few weeks into my PhD project, while puzzling about how to design saturated magnetic lenses, my supervisor Ron Ferrari suggested that I should apply a technique called the finite-element method to the computer simulation of electron lens fields. Ron suggested that I should attend a guest lecture that was about to be given in the Engineering Department by Professor Zienkiewicz, of the University of Wales at Swansea, who at that time was probably the world expert on the finite-element method. I attended the lecture, which was about the application of finite-element methods in soil mechanics. He was discussing the material properties of mud, in particular the nonlinear stress–strain curves for it, and was extolling the virtues of the finite-element method in this application. While he was still talking it suddenly struck me that the nonlinear stress–strain curves he was showing for mud looked exactly like the nonlinear magnetization curves for ferromagnetic materials, and that therefore the same mathematical formalism that he had used to predict the behaviour of mud could almost certainly be used for saturated electron lens design. Ron Ferrari and Prof. Oatley greatly encouraged me in this endeavour and helped me to turn the idea into a reality. After I had come up with a new design for a lens with a very small final bore, Prof. Oatley said he would like to make this lens and try it out in a new experimental SEM that he was in the process of building. I think he was the very first person to test experimentally a new lens I had designed. This sort of hands-on approach was a great confidence-builder for young research students who had the privilege of interacting with him. Even after he retired from being Head of the Division, we would often see him, with his sleeves rolled up, working on the lathes and milling machines, making his own parts for his new microscope. In this way, he continued to be a leader and source of inspiration to generations of research students. Prof. Oatley was always ready, if asked, to oVer wise advice, as illustrated by the following anecdote. One of my friends, Colin Sheppard, who started his PhD project at the same time as myself, told me about a conversation he had had with Oatley. Part of his project involved building an electron diVraction apparatus. At the start of his project, Colin was discussing with Oatley the design of this equipment. Colin said to Oatley, ‘Do you think I should build my electron diVraction column with its axis aligned horizontally or vertically?’ Without hesitation, Oatley replied, ‘I would definitely say

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vertically.’ ‘Why?’ said Colin. ‘Well, people always build electron microscopes with their axis vertical.’ Colin replied, ‘But I’ve been looking through the literature, and most people seem to build electron diVraction equipment with the axis horizontal.’ ‘Well,’ replied Oatley, ‘surely you don’t want to be influenced by what other people do.’ I don’t remember whether Colin told me what transpired, but I imagine that Oatley’s logic probably prevailed. It was a great privilege working in Oatley’s group as young graduate students. As soon as I joined it, I immediately had the feeling that I had to try to live up to the tradition of research that he had created in the group. I remember him once telling me that when he first started working on developing the scanning electron microscope, he was quite uncertain whether it could ever be made to work, but he believed that there was a lot of interesting physics in it that would provide useful research training for graduate students even if the microscope itself never functioned. I have always remembered this, and it has encouraged me in my subsequent research to try to work on things that might seem somewhat speculative at the outset. I think it is a good attitude that leads to scientific advances. We had a good time as research students in those days especially at the 11 o’clock coVee breaks every morning when we would meet with Prof. Oatley and the other staV members, and Les Peters and the workshop assistants. Prof. Oatley would always be there with his coVee mug labelled ‘Prof Only’ and would tell us lots of interesting anecdotes. I always remember him in those days as being very dignified yet approachable.

III. Trinity College In 1971, at the end of the third year of my PhD project, I submitted a dissertation for a fellowship at Trinity College. Each fellowship candidate was assigned an ‘Elector,’ who was a Fellow of the College and would represent him in the competition. Prof. Oatley, being the most senior Fellow in electrical engineering in Trinity at that time, was appointed as my Elector, so it was largely through his support that I got a fellowship. For the fellowship admission dinner, the tradition at that time was that the father of each new Fellow was invited, but as my mother was a widow, Prof. Oatley, Ron Ferrari (my research supervisor) and Dennis Marrian (my personal tutor) arranged that my mother could attend instead. At that time, Trinity was still an all-male institution, and I believe that it was the first time that a candidate’s mother had ever been invited to a fellowship admission dinner, so it was a great honour for her, and mainly due to Prof. Oatley. I will

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always remember the great kindness with which he and his colleagues treated my mother on that occasion and whenever she visited Cambridge. As a Fellow at Trinity I got to know Professor Oatley quite well, and I was now oYcially supposed to call him just plain ‘Oatley’ rather than ‘Professor Oatley.’ I was told that that was the correct protocol for addressing the other Fellows. However, in the case of Prof. Oatley, I didn’t feel very comfortable doing that, as I was still rather in awe of him, so I think I still used to mainly call him ‘Sir.’ He didn’t seem to mind this, even though this was before he received his knighthood (in 1974). I think most likely he felt that it was appropriate and fitting to his dignified status. This was quite diVerent from my personal tutor, Dennis Marrian, who on one occasion when, as an undergraduate, I had called him ‘Sir,’ had said to me, ‘If you call me Sir again, I shall undoubtedly beat you.’ I soon came to realize what a senior status Prof. Oatley enjoyed in the Trinity College hierarchy. For example, while more junior Fellows were on minor committees such as the Admissions Committee, Prof. Oatley was a member of both of the two most prestigious committees in Trinity—the Wine Committee and the Garden Committee. In the College Council minutes, news of members of the college receiving Nobel prizes always took second place to more important items, such as news about the college gardens and the appointment of new gardeners. I was present on one occasion when Prof. Oatley, in his oYcial role as a member of the Garden Committee, was overseeing the planting of a young sapling to replace one of the magnificent trees that had sadly been lost to Dutch elm disease; after the new tree was planted I remember Oatley brushing the soil oV his hands and saying ‘Well, that’s that taken care of for another two hundred years.’ It was quite typical of the timescale on which Trinity operates. Membership of the Wine Committee was considered equally important, because of the need to select good port now for consumption in twenty years’ time. Oatley was an expert at that too. In the spring of 1972, after submitting my PhD thesis (Munro, 1971), I had my PhD oral examination. Like all PhD students, I remember entering the examination room very nervously. There were three examiners, and Prof. Oatley was one of them. They conferred about who should start the questioning and it was agreed that Prof. Oatley should begin. He turned to me and said ‘My first question is about something you wrote on page 127.’ This immediately put me at my ease, as I thought ‘Oh, well, at least he must think the first 126 pages are all right then.’ Later in the examination he asked me ‘Why haven’t you included your computer program listings in your thesis?’ At first, I didn’t quite know how to answer this, but then I recalled that Prof. Oatley was always saying we ought to support British industry, so I replied

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‘Well, I didn’t want the Japanese to get hold of them,’ and Oatley replied, ‘Oh, well that’s fine then,’ or words to that eVect.

IV. North America Towards the end of the first year of my fellowship at Trinity, in the summer of 1972, Oatley said to me, ‘I think you should spend a few weeks in the United States and visit the Massachusetts Institute of Technology, so that you can see how much more advanced our laboratories are than theirs. I remember remarking on this fact when I visited there in 1947.’ Oatley was so kind: he personally provided all the funding for me to make my first visit to North America, supplying half the money from one of his funds at the Engineering Department and the other half from a fund he had in Trinity College. It was a very memorable trip, arranged jointly by Oatley and my supervisor, Ron Ferrari. I spent three weeks working at McGill University in Montreal, with Peter Silvester, a colleague of Ron, and a world expert on finite-element methods; and three weeks at Berkeley with Professor Tom Everhart, a former student of Professor Oatley and the joint inventor of the Everhart–Thornley detector, who was at that time an Associate Professor at Berkeley and who later became President of the California Institute of Technology. I never did get to visit MIT on that trip, so I never discovered ‘how much more advanced our laboratories are than theirs.’ It was a wonderful trip though, all thanks to Prof. Oatley. While I was at Berkeley, I received a phone call from the IBM Research Center in Yorktown Heights, New York, and the voice said ‘This is Marc Heritage from IBM—I heard you are in California; on your way back to Cambridge would you like to visit New York and be interviewed for a job at IBM?’ I agreed to visit him, and was subsequently oVered a post-doc at IBM Yorktown. I never heard how Marc Heritage knew I was in California at that time, but I am absolutely certain that it was Oatley who phoned and asked him to contact me. As a result of that good turn done to me by Oatley, I went to IBM on a one-year post-doc and ended up staying in New York for four and a half years. Oatley was a great patriot and always said that we should support British industry, so it was especially good of him to encourage me to go and work at IBM, as it was such a wonderful work experience. However, much as I loved living in New York, after about four years I began to get quite homesick for England and during the spring of 1977 I came back to Cambridge for a visit and talked to Oatley and others at Trinity about the possibility of returning to work there again for a while. They very generously

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agreed that I could come back to Trinity for a time and I returned in October 1977. I always remember my first evening back in Trinity after the absence of four and a half years, and going into the parlour in my black gown to assemble for dinner with the other Fellows, and one of whom, in his eighties, greeting me by saying ‘Ah, Munro, have you been on holiday? I’ve not seen you for a few weeks.’ I thought that this was a wonderful embodiment of the Trinity College motto Semper Eadem (‘always the same’). Oatley himself was a wonderful embodiment of that tradition too—always the same, in his temperament and in the even-handed and fair way in which he treated his friends and his students alike.

V. Imperial College After a year back at Trinity, Oatley said to me one day, ‘Professor Dan Bradley, who is the head of the Optics Section at Imperial College (London), has been in touch with me saying he is looking for a new member of staV in electron optics. Would you be interested in applying?’ I went for an interview and was oVered a job there, again thanks to Prof. Oatley. I was initially unsure whether to take it as I still had an option to return to IBM in New York. So, as on many previous occasions, I went to ask Oatley for his advice, which I always respected. He said ‘I’d advise you to take the job at Imperial, because if you want to go back into industry later, it’s relatively easy to go from a university post to industry, but it’s harder getting a university post when you’re in industry. Also, in IBM, unless you get to be CEO you’ll always be reporting to someone else, whereas in the university environment you’ll be much more your own boss.’ I took Oatley’s advice, and I never regretted it, although I did greatly enjoy the time that I had spent previously at IBM.

VI. Final Memories My last memory of Oatley was at his 90th birthday celebration, which his colleagues organized for him at Churchill College, when nearly all his former PhD students came back to Cambridge for a one-day scientific celebration meeting in his honour and presented talks about their work, combined with some personal reminiscences about him. I remember how happy Oatley was on that day and that is how I will always remember him. Looking back, I realize more and more what a key role Prof. Oatley played in shaping the direction of my life on so many occasions. Apart from

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my parents, he has probably been the single most influential person in terms of my career path. It is a wonderful thing to be able to look back with such gratitude at the fond memory of a friend who has helped me so much.

Reference Munro, E. (1971). ‘Computer-aided-design methods in electron optics.’ PhD Dissertation, University of Cambridge.

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5.4 Recollections of Professor Oatley’s Reincarnation as a Research Student G. OWEN Agilent Technologies, Palo Alto, California Formerly at: Engineering Department, University of Cambridge

I became aware of Professor Oatley during my first year as an engineering undergraduate at Cambridge in 1968. I was finding the transition from being a pupil at a school to a student at a university diYcult, and one of the contributing factors was the trouble I was experiencing in understanding the lectures. At the time, I assumed the root cause lay within me, although now, 30 years later, I can think of an alternative explanation. At all events, the course on elementary electromagnetic theory, which started with the integral forms of the laws of electrostatics and magnetism, and which culminated in Maxwell’s equations, was a revelation. The lecturer was, of course, Professor Oatley, and the logic and clarity of his presentation of this material was masterful; quite unlike anything to which I had previously been exposed. His calm, ‘common-sense’ delivery, together with the simple but utterly clear diagrams he drew on the blackboard made this subject seem easy. Moreover, the sense of understanding I attained during the lectures remained during my attempts to solve the set examples papers, a pretty sure indication that it was no mirage but a real gain in my knowledge. I know I was by no means the only student who had this wonderful experience. Later, Professor Oatley wrote a book (1976) based on this lecture course, to which I still regularly refer. Subsequently, in my third year as an undergraduate, I took three more courses from Professor Oatley: one on statistical mechanics, one on semiconductor physics and one on semiconductor devices. These, too, were quite exceptional in their clarity of exposition. Professor Oatley’s oYcial retirement occurred at the end of my time as an undergraduate. After obtaining my BA, I stayed on at the Engineering Department to carry out research for a PhD, starting as a research student in October 1971, my supervisor being Dr William Nixon. During the next three years, I benefited greatly from Dr Nixon’s unobtrusive but invaluable 445 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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help and encouragement. In addition, one of the greatest debts I owe to him is the trouble he took to introduce me to established workers in the field of electron optics while I was still a research student. This provided me with many long-lasting friendships, not to mention some invaluable job opportunities! At almost exactly the same time as I was embarking on my PhD research, Professor Oatley started to re-live the life of a research student (as I heard him say), and to this end he set up a small laboratory near the coVee-break area where students and staV met every work day at 11.00 am. As a result, I saw Professor Oatley regularly over the next few years, although I did not have many conversations with him. In fact, only two remain clearly in my memory, but what he said on those occasions has had a lasting eVect on me. During the first of these, he said to me with a twinkle in his eye, something along the lines of ‘So I see you are an iconoclast!’ I cannot remember the context that provoked this remark, or my reply to it. However, what I do remember is that as soon as I returned to my desk, I reached for my dictionary. I was a little surprised when I found out what an iconoclast actually is, but then I felt rather flattered. I must admit that my iconoclastic tendencies have sometimes got me into trouble, but in general, they have made my life a lot of fun, and increased my productivity as a researcher. I am profoundly grateful for Professor Oatley pointing out this facet of my character to me, and in such a way that I was encouraged to use it to my advantage. My second snippet of conversation with Professor Oatley was also extremely brief, but it, too, had an important eVect on my development as an engineer. At the time I was in my second year as a research student and was having tremendous fun. The research project on which I had embarked was the design of low-aberration deflection coils; these were for use in electron lithography systems, which were then just beginning to be developed. The university computer was an IBM 370, and I was using this to calculate the trajectories of electron beams through magnetic deflection fields, and the resulting aberrations. At the time computing seemed to me an amazing adventure, fraught with uncertainty and punched cards. In the context of today’s world of software packages such as ‘Mathematica’, the calculations I was carrying out were quite trivial, although they certainly did not seem so then. Nevertheless, I am very glad that I did this work in the early 1970s, since the available comparatively primitive computational tools forced me to write my own numerical methods programs, rather than just calling up canned subroutines. This was both highly educational, and extremely interesting. One coVee break, Professor Oatley asked me what I had chosen as my research topic, and how it was coming along. I gave him an enthusiastic

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account of my theoretical and computational activities. At the end of this, he looked at me fairly sternly and told me, in no uncertain terms, that I would not get my degree unless I had obtained experimental evidence to prove my designs worked. A dissertation based just on modelling simply would not do, and I distinctly remember the use of the phrase ‘good practical electronics’. Although experimental verification of my calculations had been at the back of my mind, Professor Oatley’s statement brought it right up to the front, and ultimately, two-thirds of my PhD dissertation (Owens, 1974) was experimental. I am now even more convinced than I was then that Professor Oatley’s advice hit the nail squarely on the head. The centrepiece of the experimental apparatus I used was an electronoptical column that had been used for a variety of purposes by several generations of research students before me, including M. E. Barnett, who had completed his PhD dissertation in 1966. I used this column to deflect an electron beam through a distance of a few millimetres with deflection coils of my own design and construction. Such deflection always has the unwanted eVect of increasing the diameter of the beam, possibly by many micrometres if the coils are not optimally designed. The object of my experiments was to measure these increases in diameter and, in particular, demonstrate that they were comparatively small for my deflection coils. Fortunately, this did indeed turn out to be the case! Accomplishing this required me not only to master practical electron optics and vacuum technology, but also to build electronic power supplies, drivers and amplifiers; to design electron lenses and other components; and to become a fairly competent (though still amateur) mechanical engineer. On top of this, the measurements were not easy to make, so I gained a good deal of experience in experimental and data reduction methods. Thus, my practical work was highly multidisciplinary, which made it extremely interesting at the time and extraordinarily useful in my subsequent career. My direct contacts with Professor Oatley were very few in number but, nevertheless, they had a great eVect on me. Of course, in addition to the contacts, he exerted influence on the research students by the very nature of the organization he had built up over the years, and in which we were privileged to work. I eventually ended up spending six years working at the Engineering Department (three of them as a graduate student and three as a Research Fellow), after which I left for the United States as part of the ‘brain drain’. I joined the IBM Thomas J. Watson Research Center at Yorktown, New York, working for Philip Chang on ‘Vectorscan’ lithography systems. At the beginning of my stay, I overlapped briefly with Eric Munro, so Alec Broers’ department at that time contained Philip, Eric and me simultaneously. Oliver Wells was then also at Watson.

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Professor Oatley stayed on as a ‘research student’ for many more years, and I visited him occasionally at the department on my trips back to the UK. One of my last memories is of him operating a lathe, turning a brass cylinder for a piece of apparatus he was making. That memory of the practical scientist at work, in conjunction with the memory of the clarity of his lectures, and the memories of his helpfulness to students such as myself, forms a picture in my mind of someone to whom I owe more than I can express, and whom I will never forget.

References Oatley, C. W. (1976). ‘Electric and Magnetic Fields: An Introduction.’ London: Cambridge University Press. Owen, G. (1974). ‘The analysis and design of probe forming systems for microfabrication.’ PhD Dissertation, University of Cambridge.

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5.5 My Life with the Stereoscan B. C. BRETON Engineering Department, University of Cambridge

I. Introduction My introduction to the scanning electron microscope took a very diVerent route from that of most of the preceding contributors and as such I shall begin at the beginning! The Stereoscan SEM entered the US market in 1965, and a company that was in the business of selling and manufacturing industrial diamond products was selected by the Cambridge Instrument Company (CIC) as the agents for North America. This was the Engis Equipment Company, which was located on the northwest side of Chicago in a suburb called Morton Grove. It was privately owned by two brothers, who had made their money in diamonds and subsequently set up the company to sell all types of industrial diamond powders and pastes for polishing, etc. They also marketed scientific equipment, including optical microscopes. The Stereoscan was sold by sales representatives, rather than directly by Engis, which had agreements with a select number of sales organizations around the United States and Canada. The representatives were resident in California, New York, the Carolinas and Texas, in addition to Illinois. The support for installation, training and servicing the Stereoscan was by a service engineering team, all based in Chicago. When I arrived, this comprised a manager and about four field engineers. We were given a couple of weeks training and then travelled with a senior engineer for a further two weeks before tackling any jobs by ourselves.

II. The Stereoscan in North America I started with Engis in 1968. I had moved to the United States from Canada a few years previously and had been teaching radar principles and operation for a company that supplied engineers for the Distant Early Warning Line, in Northern Canada and Alaska. This early-warning system required engineers 449 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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who could work under pressure and in extreme isolation in the Arctic. It proved to be an excellent training ground for what was to come in servicing and maintaining the Stereoscan! As already mentioned, the Stereoscan had been on the market for about three years, and sales were beginning to increase at a rapid pace. The need for service engineers who had the ability to be creative and enterprising and work independently was a necessary prerequisite of the job and I was taken on and worked under the service manager, Norman Burns.

A. In at the Deep End Bell Laboratories in Murray Hill, New Jersey, had taken delivery of their very first Stereoscan, which was bought by Dr Robert Heidenreich, but they were told that it could be some months before it could be installed. They were beginning to put some pressure on Engis to try to get an earlier date, and hence I was thrown into the field a bit earlier than I had expected. Bob Heidenreich was one of the internationally known electron microscopists at the time and I was somewhat nervous to say the least. I arrived at Bell Laboratories one morning and was met by Bob, as he preferred to be called. We introduced ourselves in the foyer and then he led me up a few flights of stairs to his lab. On the way up, he said quite casually, ‘We did have a picture on the screen for a short period’. I looked at him in a bemused way, and we continued up the stairs. When the lab door opened, I could see open boxes and crates all over the place. OV to the side through another door was the Stereoscan, all assembled, and some people standing around looking at it. They were anxious to find out what I was going to do to make it work. Bob then continued to say more about what they had accomplished. Upon hearing that there was going to be a long delay, they took the manuals out of the crate, read through them, and decided they could assemble it and so they did, and actually got it running. However, it wasn’t running now and they were busy trying to tell me what had happened. It was known that there was a problem with the Miles–Hivolt power supply that fed the scintillator and cathode-ray tubes. In transit, a cable would sometimes break, causing the power supply to fail. I knew about this, of course, and after listening to Bob and taking a quick look, I concluded that it could be the problem. Without even opening my tool case, I asked someone to heat up a soldering iron, and while that was happening I took out the Miles–Hivolt, lifted the cover, and sure enough, there was the broken cable. I soldered it back in place and said to them ‘OK turn it on!’ They

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flipped the switches and the Stereoscan burst into life, imaging and scanning as expected. Smiles all around and me the hero—and I hadn’t even taken my jacket oV! That was my very first service call and one of many that I made to Bell Laboratories. However, the story about this Stereoscan doesn’t end there. About a year later I was called back to try to solve a serious problem with resolution. By this time they were very familiar with the workings of the Stereoscan and could do much of the maintenance and alignments themselves, but the resolution problem had them stumped. I arrived one day and listened to all the things they had tried, and it certainly seemed that they had been very thorough. I concluded that it had to be something in the column and we took it apart again and inspected all the apertures and lens bores, etc. We thoroughly cleaned everything, putting it all carefully back together, only to discover that it hadn’t made the resolution significantly better. While I was searching my brain for ideas, Gu¨nter, one of Bob’s scientists, said to me, ‘Bernie, we have these boxes up there and we are not sure what they are’. I asked ‘Boxes, of what?’ He took a large box oV the top shelf and inside there were several smaller boxes; I opened them and when I saw them, I nearly fainted. They were the mu-metal screens from inside the column! They had never installed them because they didn’t know what they were, and so they had put them on the shelf. When I had taken the column apart, I didn’t notice anything wrong. We had quite a good laugh about it. Needless to say, we installed the screens and the resolution problem disappeared; in fact they were amazed at what good resolution they could now achieve.

B. Increased Responsibility DuPont bought the very first prototype Stereoscan, which was installed in Kinston, North Carolina. They also had another research facility with a Stereoscan, in Wilmington, Delaware. This Stereoscan developed a very diYcult problem, and it was beginning to cause tensions in relations between Engis and DuPont, with the possibility of a loss in future sales. By this time, I was a senior service engineer and regarded by some as the most experienced. Several engineers had visited this Stereoscan and were unable to solve the problem. Thus I was sent to it, with the understanding given to DuPont that the best engineer in the company was coming and would be able to solve their problem. I went to the site and had a look at the Stereoscan. It was quite an early one and lacked the full electrical interlocks for the gun and column. I had to take the gun oV to clean the anode and align the filament. It was always the

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practice with this vintage of Stereoscan to short the gun assembly to the case before touching it, and so they had a big shorting bar available for this purpose. I touched the bar to the outer case and then to the gun assembly and there was a mighty crack; everyone, including me, jumped. I said aloud, ‘Good grief, we’d better do that again’, and so I did, and again there was a mighty crack. It was then that one of them said ‘Bernie, do you want to turn oV the high voltage now?’ I was so embarrassed, because I had forgotten that this machine was not interlocked and that the high voltage could be left on with the gun lid open. Here was I, supposed to be the expert; I think I slid down the pecking order slightly with that episode. However, I did fix the problem, and restored my dignity to some extent. During my Engis days, I met several of the people who had been involved with the development of the SEM at the Cambridge University Engineering Department. One of them was Philip Chang and my first meeting with him was interesting to say the least.

C. Philip Chang and the LaB6 Gun Bell Laboratories had just purchased another Stereoscan, and one of the accessories they ordered was a LaB6 gun unit. We, in the United States, had had no training or knowledge about this gun at all. However, the system was delivered to Bell—I think it was Stereoscan 5 or 6 by then—and I was sent to install it. The people at Bell had little or no idea that I had not seen one before. These things were usually kept quiet; they would eventually know that my familiarity with it was, shall we say, limited! The installation was quite a slow process and it took most of one day and part of the next, but at last we had it all assembled and ready to start. I had no idea what vacuum pressure it needed, other than some guidelines, and so, when it reached the normal safe level we turned the gun on and began to heat the filament. It was not long before it arced over and we lost vacuum and had to wait for it to pump down again. This process went on for a couple of hours and eventually we got a beam down the column, but it soon disappeared. Numerous phone calls to the oYce resulted in only verbal support, but no technical assistance, as there was none to be had. However, it materialized that the Illinois Institute of Technology Research Institute (IITRI) Conference, which was about to start in Chicago, would be attended by Philip Chang, who we all knew to be involved with the LaB6 gun at Cambridge Instruments in the United Kingdom. It was decided I should leave the Bell system and meet up with Philip at the conference to seek his advice about what could be going wrong.

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I met Philip one evening and we had a long chat about the gun and how it worked, the vacuum pressures, etc. This was all very welcome information and the first I had had. He also added that at the meeting there was a Stereoscan with LaB6 on it; we could take it apart and he would show me how to set it up and make it work. This all seemed the proper thing to do. One slight problem: the Stereoscan was intended to be on show all day, and no-one was allowed into the area either before or after the show hours. This was a strict rule to prevent tampering with the exhibits, as sometimes had been known to happen in the past. This left us with a slight dilemma, as without first-hand knowledge I was going to be no better oV, with the exception that I now had had verbal instructions on the process. With that in mind, Philip and I decided that we would chance going in early and try to avoid the security guards. We also had a plan that if we were caught we would explain why we needed to be there and hope they would be sympathetic. We got up early, went to the conference hall, and spent some time monitoring how the guards went round and where they spent most of their time. They were not anticipating anyone coming so early in the morning. Also the layout of the conference area meant that they could be in other rooms. Philip and I picked our way to the Stereoscan and, while we kept a lookout for the guards, Philip took the gun oV and we sat down behind some boxes and proceeded to take it all apart. By this stage I was very impressed with Philip and he gave me considerable confidence in demonstrating what he was doing. He took the gun completely to bits, and I did think about the consequences if something went wrong and he was not able to get it back together in time. He explained the correct procedure for aligning the tip inside the grid and how far the cap should be screwed down. He aligned it and demonstrated how it should be set up, and then explained the vacuum procedure and what would happen if that was not adhered to. Slowly he reassembled the gun and, looking over our shoulders, we repositioned it on the column and pumped the system down. Because it was only the gun, it did not take long to do this. By now, the time was approaching when the doors would be opening for the day, and we had to be very careful not to be seen. We kept a low profile until a few exhibitors arrived and then we emerged from behind the boxes and proceeded to finish the job. Once the vacuum had reached the safe level, Philip slowly ran the gun up and the bright, nearly noiseless image appeared. I was so impressed, because I had never seen a LaB6 gun work, and with so much intensity. I felt quite confident that I could return to Bell Laboratories and not only demonstrate the system but also explain how it could be made better. Some of the staV came along and were a little surprised to find everything already running and tuned up so well. We never mentioned

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anything to the sales staV, as they would have been nervous about the whole process. But we did let the service management know what we had done and that everything was working as before, if not better. To conclude this story, I returned to Bell Laboratories, and with great confidence, I aligned everything and reinstalled the gun. Only this time I waited for the vacuum to achieve a better pressure, and then slowly heated the filament while keeping the vacuum levels at their best. The beam hit the sample and a nearly perfect image was formed. I should also say at this point that the Stereoscan at the IITRI Conference had an additional ion pump on the back of the gun. It was Philip’s suggestion that it would not run eYciently without that, because if the vacuum deteriorated in the vicinity of the LaB6 tip, it would be poisoned and the emission would fall oV. I explained to the people at Bell, that they could improve the performance by adding an ion pump. They did not hesitate and in fact added a larger one than was recommended, with the result that in the following years the performance of that Stereoscan was more than satisfactory.

D. Bell Laboratories and Etec There is a postscript to this story, which involves the group at Bell. A few months later, I received a call from their purchasing department; they explained to me that they wanted to buy a set of parts for another Stereoscan, but they wanted to assemble it themselves. I was confused, and asked why did not they want to talk to a salesman? Their answer was that they needed detailed technical descriptions, which they would not get from him. So on with the process; we discussed all the necessary details and finally the order was placed. The boxes of parts arrived at Bell and disappeared into a room! Yes, disappeared. The Stereoscan with the LaB6 on it was adjacent to this ‘secret’ room as it turned out. The story was even curiouser because occasionally they had cause to call me and ask some advice. As the mystery Stereoscan had a warranty period, they were entitled to receive support, so I used to get involved over the telephone with technical queries about a Stereoscan that I couldn’t see. Most unusual. The mystery was solved some years later. In 1974, I resigned from Cambridge Instruments, moved to California and joined Etec Corporation. I then learned that Bell Laboratories had been working on the development of the MEBES (Manufacturing Electron Beam Exposure System); Etec was granted a licence to produce and sell these machines. The Stereoscan that had been kept a secret was the initial development prototype for the MEBES. When I visited Bell as an Etec employee, they

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took me into that room and I finally saw the Stereoscan that I had been helping with for all those years. I was quite surprised at the sight. Except for the column, there was hardly anything else that could be identified as a Stereoscan. Bell took delivery of several Stereoscans over a short period, and one of them was purchased for Fabian Pease. I came along and installed Fabian’s Stereoscan but had little need to return very often as, naturally, they were very capable of maintaining it themselves. I moved to New York in 1970 and established an Engis service and sales centre in Ossining. We had a Stereoscan and used it for demonstrations to prospective customers and as a test tool for repairing modules for other Stereoscans. It was during this time that I met Alec Broers who was at the IBM Thomas J. Watson Research Center just a few miles away. I called upon Alec one day as I wanted to know more about the LaB6 gun and how it worked. When he was finished, I could have designed one! Alec and I used to meet quite often over the years and he always had something useful to say, and I was eager to listen. It was especially interesting for me in later years, when we were both at the Cambridge University Engineering Department (he as Head of the Electrical Division); we used often to have coVee in his oYce and chat about the old days and about things that were happening with his students. I spent a couple of years in Ossining and then moved back to Chicago to take up the service management and demonstration position. During this time, I travelled back and forth to England and the Cambridge Instrument Company many times. It was also during this period that I met my wife to be, Jane Killingworth, who was head of the demonstration and applications facilities in England and came to the United States to help us with sales and marketing. Two years later, we were married and she came to live in Chicago. However, we did not stay with Engis but instead moved to California and the Etec Corporation. Etec designed and manufactured an SEM and were highly successful with it.

III. England About 1979, I began to think about a new direction: I thought computing was an area that I could be interested in. I resigned my position with Etec and sat back and took a long hard look at where I wanted to go. I had enjoyed my visits to England and I decided it was there that I would like to live. Also I always had an interest in visiting Europe, and I thought that England would be a good base. Jane and I packed up, sold everything we did not want to take, and moved to Cambridge in October 1980. After the New

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Year, I began to fill in applications for various jobs; one was a temporary position with Cambridge University.

A. Cambridge University Engineering Department Bill Nixon, whom I had often met at conferences in the United States, found out that I had applied for this university job and called to ask what I was doing in England. He then suggested we meet at his college, Peterhouse. The first thing we did was to have a glass of sherry and then we chatted about what I had been doing for the past five years. I told Bill I wanted to get into computers and that I didn’t want anything more to do with SEMs. We concluded that meeting and set a date for another in two weeks’ time. Along I went and the first thing we did was to have a glass of sherry. Apparently this was the way business was done in Cambridge! Then he took me to the Engineering Department and gave me a tour of what was happening there. I had never been to the department before and I was quite excited about seeing the place. On the third floor I saw SEMs upside-down, sideways and in almost every other position. Many students were doing research on SEM-related projects. It was very impressive. Bill told me there was a position open that might lead to my objective of getting into computers. The project needed someone with my skills to work on interfacing a computer to a microscope. It didn’t take me too long to consider that this must be the way forward. The project involved interfacing a computer to the Cambridge University High Resolution Electron Microscope (HREM), a joint project between the Engineering Department and the Cavendish Laboratory. Bill was the Principal Investigator on the engineering side, supported by Ken Smith and Haroon Ahmed. The HREM was at the time located in the Old Cavendish Laboratory, Free School Lane, and I was given laboratory space there. However, my main base was in the Engineering Department, and it was then that I discovered it was adjacent to the laboratory Professor Oatley used. I had, of course, read much about Oatley and his work but I had never met him. When I was introduced to him, he became instantly interested to hear what I had actually been doing with the SEM and the people I had met along the way. He was pleased to learn that I had met some of his students and, in fact, had installed one or two of their Stereoscans. I was delighted to find myself working beside the great man himself, as I had grown to have such an appreciation for what he had started. We all used to meet for coVee at around eleven in the morning and Professor Oatley was always keen to pick up on threads of conversation or meet new people who had come along, particularly new students.

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IV. Return to the Stereoscan The HREM project took me back and forth between Engineering and the old Cavendish, and the interfacing of the computer went more or less according to plan. About six months before the project was to end, Bill asked me if I would be prepared to stay on to work on another he had been negotiating. This was with a research student, Simon Garth, and would be funded by Texas Instruments (TI), Bedford. The project was to build a prototype system for the electron-beam testing of operating integrated circuits. At the heart of this system was to be a new SEM. So, although on coming to England I had firmly decided I did not want anything more to do with SEMs, I found myself about a year later sitting in front of a brand new Cambridge Instruments S-200 SEM. My life with the Stereoscan had begun all over again! A. Microelectronics In addition to the Stereoscan, Texas Instruments had funded all the other ancillary accessories for the electron beam tester, including a voltagecontrast subsystem from Lintech Instruments Ltd, of Cambridge. The latter company was the brainchild of Graham Plowes, a former research student with Bill Nixon. He had set up his own company, to develop and produce equipment that could be interfaced to the SEM to quantify device behaviour. The Lintech product was the state-of-the-art at that time. With all this equipment, we had the capability of analysing complex defects. We were given the specifications of devices from TI, so we knew where to measure and what we should be expected in terms of voltage, gate delays, etc. It was extremely exciting research. In the course of the project Simon developed a new type of detector, termed a ‘magnetostatic’ detector, in which the sample is located in the field of a single-pole lens (Fig. 1). With this arrangement, device voltages could be measured with much greater precision than hitherto (Garth, 1985) . We kept in close touch with the TI engineers at Bedford and, at an advanced stage of the project, two of them brought along a new device with which they were having some diYculty in determining why it was failing. They had with them all the circuitry that was necessary to analyse which particular area they wanted to look at. They explained that, in this particular case, they were having an excessively long delay between a couple of the 

In this chapter an asterisk indicates that the research is described in more detail on the associated CUED website.

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Figure 1. The ‘magnetostatic’ detector. (a) Principle of operation (solid lines indicate lines of magnetic flux, dashed lines indicate electrostatic equipotentials). (b) Design implementation. (From Garth, 1985.)

gates but they couldn’t determine exactly where it was. With the equipment Simon had designed, this was entirely possible. Our method was to find the area, position the beam on the gate in question, measure the rise times, and then go to the output side of that gate and do the same thing. It was a manual sequence but highly eVective. Our visitors sat starry eyed, unable to believe what they were seeing—that we were capable of dynamically measuring these delays in real time and recording the results. It wasn’t very long before we discovered the exact area where the delay in their device was occurring. The rise-times were completely oV specification. We had indeed found the problem. The project ended shortly thereafter and Simon was awarded his PhD (Garth, 1985). In my opinion he should have received two! On completion of his PhD, Simon moved to TI at Bedford taking with him all the equipment he had designed including the Stereoscan. In its place TI provided a new S-200 for the Engineering Department to use. At TI

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Simon helped to build a powerful new electron-beam testing SEM system. During the course of this project, TI invited Cambridge Instruments to take on these systems as a special product. TI oVered to give CIC all the drawings, assist with the development, and buy the first ten systems they built. However, CIC simply said it was not in the company’s interest to take on such a project. Throughout the research project with Texas Instruments, Professor Oatley was always present to discuss ideas and was a mountain of support and knowledge. He was rather pleased that the technology was an extension of the voltage-contrast technique he and Tom Everhart had introduced a number of years previously. He was also encouraged that the SEM was being used to research ways to broaden its application base, something he was very eager to promote. When we got our new Stereoscan, I invited him to try it out. He found it somewhat diYcult because the controls were all in the wrong places, but he appreciated the new imaging screens and the TVrate scan. One thing he felt very strongly about, however, was that the specimen chamber should be upside-down so that you could reach in and see your experimental set-up that much more easily.

B. High-speed Beam Chopper The next student to come along was John Thong, who did his third-year undergraduate project with me on voltage contrast and was subsequently oVered a PhD place in 1995. This overlapped with Simon in that John was doing the project while Simon was finishing oV his PhD, so they knew each other. Subsequently their names appeared on a joint paper (Thong et al., 1987). Bill Nixon proposed that John should continue the work on the electron-beam tester with the aim of increasing the acquisition rate of the information from the specimen. To do this, it would be necessary to increase the rate of beam blanking in the column. John Thong’s approach to research was unusual. He disappeared one day and I didn’t see him for about two weeks. When he came back, he said that he felt he hadn’t been able to type well enough so he took the time oV to learn. Subsequently, his speed typing had to be seen to be believed. He vanished again when he discovered he would have to write a lot of C code, again with a similar result. Not all John’s activities were, however, to have such a happy ending. He had designed an interface between the Stereoscan and the column to allow him to drive the scan coils and lenses from a computer. John wired up this device and we switched on. There were loud pops, sparks and mild explosions for an appreciable time. The result: a dead Stereoscan. It took us about two weeks to repair it.

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John’s main research project progressed rapidly, allowing us to demonstrate high-speed beam blanking and acquire voltage-contrast images at an astounding frequency of 64 GHz. This was made possible by a novel method  of beam chopping using a rotating beam and a multislot aperture (Fig. 2) (Thong et al., 1987, 1988). John was awarded his degree (Thong, 1989) and then went to the National University of Singapore, where he continued his research in the SEM field. C. Dynamic Stereo Imaging During the time John and I were getting used to the replacement Stereoscan that Texas Instruments had provided (an updated version of the S-200), we discovered that, while aligning the gun, the image seemed as if it was being viewed from diVerent angles. Experiments confirmed that the beam was indeed being tilted at the specimen and that, as a result, it was possible to generate stereo-pairs by this means. A review of the technique confirmed that this was something that had never been done before, so we filed for a patent and got it. We then developed the capability of electrically ‘tilting’ the beam to produce real-time 3D imaging. The technique impressed Cambridge Instruments and we formed an agreement with them to market the system as an accessory on their range of Stereoscans. Later we also discovered how to extract height profile information and patented that technique as well  (Breton et al., 1986, 1987) . I should add that none of this would have been possible without John Thong’s technical expertise. D. An Intelligent SEM About this time, Bill Nixon retired and David Holburn took over the direction of research on scanning electron microscopy. His first research student was Nicholas Caldwell, a computer scientist, who came along in 1995 as a result of an idea we had of developing an ‘intelligent’ SEM. Nicholas had no working knowledge of SEMs, which seemed to me to be ideal, for I didn’t want anyone to cloud my ideas of how an intelligent SEM should be implemented; I only wanted someone to be able to write the software, and Nicholas seemed a perfect fit. By this time Cambridge Instruments had become part of the Leica group. Our old Stereoscan S-200 was unsuitable for the work we were proposing to do, which required a computer controlled SEM. We therefore approached Leica with a request to support the work and they agreed to exchange our S-200 for a new Stereoscan 440. Later, the Leica SEM business was sold and the company became known as Leo, but support for our work continued and in 2002 the 440 was replaced

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with a Stereoscan 1430VP. Thus, my life with the Stereoscan has included almost every model, from the prototype delivered to DuPont to the latest variable-pressure SEM from Leo.

Figure 2. High-repetition-rate beam-chopping system. (a) Principle of operation. (b) SEM micrograph of 64-slot aperture. (From Thong, 1989.)

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We started by developing a technique to automatically saturate a tungsten filament (Caldwell et al., 1996a); this was very successful, in fact so successful that Leica incorporated it into its software. David Holburn had another student, Gopal Chand, doing a research project (on TEMs) and mentioned something to him about being able to access the SEM over the Internet. Even then, Gopal was an expert on Internet techniques and protocols and it wasn’t long before we had a prototype system operational. It was the beginning of a very exciting time. Nicholas was busy writing software that would allow service engineers to diagnose SEM faults according to sets of rules, while simultaneously Gopal was helping me to work out how to operate an SEM over the Internet. The two techniques merged into one and soon we had a Stereoscan that we could control and diagnose remotely (Caldwell et al., 1996b, 1998b; Chand et al., 1996, 1997). Nicholas carried on with his research and developed the software that would make our SEM ‘intelligent’ (Caldwell et al., 1997a,b). Nicholas’  intelligent system was called XpertEze (Fig. 3) . The Stereoscan could start itself from cold and set key parameters to sensible values on the basis of general sample type (such as conductor, semiconductor, etc.), desired magnification and imaging mode, and the program’s reasoning about the state of the Stereoscan. Then it would apply filtering operations depending on the intended purpose of the final picture. All of this was done in a minimum of time and with a handful of keystrokes. The results were as of good if not better than most operators’, gaining Nicholas his PhD (Caldwell, 1998). In parallel with this research we started to develop a ‘virtual SEM’, a software-only model. The key idea was to use this as a teaching tool for new or inexperienced operators. We called it VSEM and have demonstrated it at several conferences. Its potential benefits for the training and education of microscopists have been widely appreciated (Holburn et al., 2000; Caldwell et al., 2002).

E. High-Flying Remote Control Much of our work in this area has been published, but one story remains. David Holburn and I had gone to a microscopy conference in Atlanta, Georgia, in 1998, and after the conference we were to fly up to Boston for some further meetings. We were flying with United Airlines and at about 39 000 feet I brought out my laptop. David connected its modem to the onboard phone system, and we dialled into my CompuServe account to get on to the Internet. Once that link was established, I connected to the Stereoscan back in the Engineering Department, ran the SEM up, aligned the beam, and took five pictures. Everyone around us was watching with

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Figure 3. Knowledge-based (expert) system, XpertEze, for SEM fault diagnosis and optimal operation. (a) Architectural schematic. (b) XpertEze main window. (From Caldwell, 1998.)

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some nervousness. The modem cable wasn’t quite long enough, so David had to hold it up and I had to keep the laptop from falling oV the table. It all looked as if it was going to fall apart at any moment, but it didn’t. When we had the five images, I sent a couple of emails, and one in particular to Alec Broers, who was now Vice-Chancellor of the University of Cambridge. His reply was short and to the point: ‘Congratulations on your high-flying SEM demonstration, and by the way, how much did it cost!’ V. Epilogue Over my years in the Engineering Department I have enjoyed the interaction and stimulation of the students, and my background with the SEM has proved to be useful. I have, with a great deal of help from the students, also achieved my desire to learn computing. Professor Oatley was in the lab next door for most of the time and, on the occasions when his SEM stopped functioning, I have been privileged to help fix the problem. I think it was through these circumstances that I have had more opportunities than most to converse with him. He was such a modest man and one felt comfortable talking with him as he had so much experience and understanding in this field. It was a pleasure and an honour to know him as I did. As I have mentioned, I shared a laboratory next to him for many years, and when he was over 80 and decided to give it up, I felt as though I had lost a close friend. He was there every day, doing his research and writing papers, a model for us all to follow. He was a true gentleman in every way. I never heard him get angry or shout, or talk about anyone except in high regard. He was immensely proud of all his students and what they had accomplished; he felt that it was they who had earned their place not him, and that he had played only a small part in their success. It was because of these interactions that I eventually came up with the idea that led to this volume. I was curious to know why the students he had supervised had become involved with the SEM as a research project, and what had been their interactions with him. It was thoughts such as these, which I discussed with Les Peters and ultimately with Ken Smith, that led to the creation of this volume. Ken oVered his encouragement and considerable support from the earliest days and as time has gone by has borne an everincreasing share of the burden of production. His knowledge of the people, events and science that created the SEM is surely without parallel; both I and you, the reader, owe him and Dennis McMullan, who produced the first SEM, an immense debt of gratitude. Professor Oatley was a keen gardener and it was his custom to invite visitors to his home in Porson Road to tour his garden. On one occasion,

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when Les Peters and I and our wives had been invited for afternoon tea, my wife, Jane, commented on how beautiful the dahlias were, and Professor Oatley went on to tell her how he had acquired them and how much he enjoyed them. The next day when he arrived in the laboratory he was carrying a small bag with some dahlia bulbs for Jane. Thirteen years later, one of those dahlias still blooms in our garden. We call it ‘the Prof.’

References Breton, B. C., Thong, J. T. L., and Nixon, W. C. (1986). A dynamic real time 3-D measurement technique for IC inspection. Microelectron. Eng. 5, 541–545. Breton, B. C., Thong, J. T. L., and Nixon, W. C. (1987). Contactless 3-D measuring technique for IC Inspection. Proc. SPIE Integrated Circuit Metrology, Inspection and Process Control Conference, Santa, Clara CA. SPIE 775, 109–117. Caldwell, N. H. M. (1998a). ‘Knowledge-based engineering for the scanning electron microscope.’ PhD Dissertation, University of Cambridge. Caldwell, N. H. M., Breton, B. C., and Holburn, D. M. (1996a). Automated filament saturation in the SEM. Proc. R. Microsc. Soc. 31(2), 126. Caldwell, N. H. M., Breton, B. C., and Holburn, D. M. (1996b). An expert system for fault diagnosis in the scanning electron microscope, in Applications and Innovations in Expert Systems IV. Cambridge: British Computer Society, SGES Publications, pp. 177–188. Caldwell, N. H. M., Breton, B. C., and Holburn, D. M. (1997a). ‘Electron Microscopy and Analysis 1997’. Towards the intelligent SEM. Inst. Phys. Conf. Ser. No. 153, IOP. Caldwell, N. H. M., Breton, B. C., and Holburn, D. M. (1997b). XpertEze: A knowledge-based approach to scanning electron microscopy, in Applications and Innovations in Expert Systems V. Cambridge: British Computer Society, SGES Publications, pp. 127–140. Caldwell, N. H. M., Breton, B. C., and Holburn, D. M. (1998b). Remote instrument diagnosis on the Internet. IEEE Intell. Syst. 13(3), 70–76. Caldwell, N. H. M., Breton, B. C., Holburn, D. M., and Robertson, R. P. (2002). VSEM: From technology demonstrator towards integrated educational tool. Microsc. Microanal 8(Suppl. 2), 1566–1567. Chand, G., Breton, B. C., Caldwell, N. H. M., and Holburn, D. M. (1996). World Wide Web controlled SEM. Scanning: J. Scann. Microsc. 18(3), 201–202. Chand, G., Breton, B. C., Caldwell, N. H. M., and Holburn, D. M. (1997). World Wide Web controlled SEM. Scanning: J. Scann. Microscop. 19(4), 292–296. Garth, S. C. J. (1985). ‘Electron beam testing of operating integrated circuits.’ PhD Dissertation, University of Cambridge Holburn, D. M., Breton, B. C., Robertson, R. P., Thompson, J. T., and Caldwell, N. H. M. (2000). VSEM: An interactive simulation and virtual reality model of the scanning electron microscope. Microsc. Microanal. 6(Suppl. 2), 1156–1157. Thong, J. T. L. (1989). ‘Electron beam testing technology for high-speed device characterisation.’ PhD Dissertation, University of Cambridge. Thong, J. T. L., Garth, S. C. J., Breton, B. C., and Nixon, W. C. (1987). Ultra high speed electron beam testing system. Microelectron. Eng. 6, 683–688. Thong, J. T. L., Breton, B. C., and Nixon, W. C. (1988). High speed beam testing using an electron-optical phase shift element. Electron. Lett. 24(23), 1441–1442.

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5.6 Research at the Cambridge University Engineering Department Post-Stereoscan K. C. A. SMITH Formerly at: Engineering Department, University of Cambridge

I. Introduction and Summary Charles Oatley’s appointment in 1960 to the Chair of Electrical Engineering, and the decision taken in the early 1960s by the Cambridge Instrument Company to manufacture the SEM, marked a turning point in the pattern of research on the SEM in the department, which hitherto had been directed by Oatley alone. It also began an era of close collaboration between the Engineering Department and the company (and its successors) that has lasted to the present day. Many PhD students have joined the company to continue work on the development of the SEM and allied instruments, or to follow other career paths. In this chapter we record the research projects related to the field of scanning electron microscopy undertaken in the department from the 1960s onwards. Some of this research has been exploited directly by the company, or has influenced the direction of its research and development policy. Spin-oV from the research has also led to the formation of several small companies within the Cambridge region. The research undertaken in the department during this period was directed largely by Bill Nixon, Haroon Ahmed and Ken Smith, all of whom, as will be evident from the accounts given in previous chapters, were involved in the events leading to the commercial manufacture of the SEM. Nixon joined the department in 1959, and took over the direction of the research on the SEM. Smith, on his return from Canada in 1960, filled the position vacated by Nixon in Cosslett’s group at the Cavendish, but rejoined the department in 1965. Ahmed was appointed Demonstrator in the department in 1963 and Lecturer in 1966. Leslie Peters continued to assist with work on the SEM until his untimely death in 1995; Bernie Breton joined the 

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team in 1982. One of Smith’s research students, David Holburn, was appointed to a Lectureship in 1986, and he and Breton have carried on aspects of research on the SEM to the present time. With the launch of the Stereoscan in 1965, Oatley began to receive a steady stream of royalties from sales, and with this he set up what became known within the department as the ‘Stereoscan fund’. A substantial proportion of this was allocated annually to the triumvirate working on the SEM and, until he left the department in 1966, to Chris Grigson working on scanning electron diVraction. Oatley also made donations to the research students who took part in the development of the early SEMs. When Oatley retired in 1971, the fund enabled him to set up a small laboratory to conduct his own individual research on the SEM (see Chapter 5.1). Throughout most of the period concerned he kept a fatherly eye on the proceedings, and his daily attendance and conversations at the morning coVee break were a source of inspiration to generations of students. The research undertaken was extremely varied, but was mainly instrumental in nature. Nixon was concerned first with improving the resolving power of the SEM and developing new methods of contrast formation. Later he turned his attention to instrumentation for the testing and manufacture of microcircuits. Ahmed investigated methods of microfabrication, research that led to the founding of the Microcircuit Engineering Laboratory as an out-station of the CUED on the Cambridge Science Park and, ultimately, to the establishment of the Microelectronics Research Centre at the Cavendish Laboratory (see Chapter 5.8). Smith initially studied the application of field emitters to the SEM, and later took up the application of computers and computer image storage and processing. Grigson’s work is described in Chapter 2.11. Holburn and Breton were concerned with image processing, automation and later, with Nicholas Caldwell, knowledge-based (expert) systems to perform instrument fault diagnosis and optimal operation (see Chapter 5.5). In addition to the mainstream research on the SEM, a number of other electron-optically related projects are described briefly on the associated CUED website. These include Munro’s work on computer-aided design methods, and a major collaborative project with Cosslett’s Group in the Cavendish Laboratory on the Cambridge University High Resolution Electron Microscope.

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5.7 The Development of Biological Scanning Electron Microscopy and X-ray Microanalysis P. ECHLIN Formerly at: Department of Botany, University of Cambridge

I. Introduction I was asked that my contribution to this collection of papers should focus on the impact of the introduction of the SEM on the biological sciences. It is a near impossible task to compress into a few pages what started nearly fifty years ago and is still continuing. I was fortunate enough to be involved in some of the early events and, along with other people both here and abroad, was impressed with the potential of this new instrument. What now follows is a personal assessment of the importance of the SEM for the biological sciences generally, and the small part I may have played here in Cambridge in making the SEM more useful for a better understanding of living material. II. A Difficult but Interesting Biological Problem During the summer and autumn of 1965, Harry Godwin, then Head of the Botany School in Cambridge, and I were studying the fine structure and development of the pollen grain cell wall in Morning Glory (Ipomea purpurea). The light-microscope studies showed that the outer wall was a complex structure that appeared to be composed of large protruding spines on a flat background, but there was insuYcient resolution to work out the fine details. A through-focus series using a binocular microscope gave a little more information, but the fine details were still missing. The way forward was to use the transmission electron microscope and I embarked on the long process of sample fixation followed by ultramicrotomy to obtain serial sections, which were then imaged and photographed. Attempts were made to reconstruct the series of photographs to obtain a three-dimensional image of what was a complex structure. It was rapidly becoming a rather daunting task. 469 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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A. A Ray of Hope As the sole electron microscopist in the Botany School, I used to go to the weekly seminars and Tea Club meetings held in Ellis Cosslett’s group in the Cavendish and I mentioned the problems I was having. Someone told me about a ‘three-dimensional electron microscope’ that was being developed in the Cambridge University Engineering Department and at the Cambridge Instrument Company (CIC). I cycled past the Instrument Company every day to and from the Botany School, so it was easy to go in and make a date to see if this new microscope could help me with my pollen grain problem. On 3 December 1965 I was introduced to the CIC Stereoscan Mark 1, which had gone into production earlier in the year. I spent the afternoon under the able tuition of Jane Killingworth looking at images of fresh Morning Glory pollen grains. Figure 1 is my very first scanning electron micrograph. It was a sobering realization that the SEM could, in an instant, provide answers to

Figure 1. The first scanning electron micrograph taken by the author. The SE image is of a pollen grain of Morning Glory (Ipomoea purpurea), 10 kV. Coated with 30 nm of gold. Marker bar 1 mm.

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the problem that had engaged us for the previous six months. The next day I persuaded Harry Godwin to come and look at the images and his first words were ‘Good grief ’, for he too saw the information we were seeking. B. My Damascene Conversion That winter afternoon in 1965 opened up a brand new imaging technique and for the next six months I spent as much time as possible at CIC looking at a wide variety of pollen grains. My colleagues and I presented some of these images at the International Electron Microscopy meeting in Kyoto the following summer (Echlin et al., 1966). The audience appeared very impressed by these images and Ellis Cosslett, who was chairing the session, stopped the talk and asked people not to photograph the pictures I was showing.

III. The Earliest Applications of SEM to Biological Samples Although the first commercially available SEM did not appear until early 1965, a number of people were using the developing technology much earlier with prototype instruments at the Engineering Department and at CIC in Cambridge. Images of an amoeba and meal worm grubs can be found in the paper reproduced as Chapter 2.2B (Smith and Oatley, 1955) ten years before the first commercial instruments were available. There were sporadic attempts to use the SEM to look at biological materials during the following years (Thornley’s work on frozen biological specimens, referred to in Chapter 2.6, was a notable contribution), but it was not until the early 1960s that the SEM was being used seriously to solve biological problems. The early papers by Alan Boyde and his colleagues on dental and bone tissue provide a good example of this type of work. A summary can be found in the textbook by Oliver Wells and colleagues (Wells et al., 1974). A. Why Did the Scanning Electron Microscope Have Such an Impact on Biology? First and foremost, the images are invariably instantly recognizable as magnifications of the world we live in, illuminated from above with familiar shadows and bright regions. Unlike with working the TEM, it is easy to follow the transition from the low magnifications familiar to the unaided eye

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through the additional magnification provided by the binocular light microscope to the wide range of magnification of the SEM. Our awareness of the world around us is gained largely by the way our eyes interpret surfaces, and the SEM provides an additional dimension to our acuity. In addition, the diVerent signals generated in the SEM can provide structural, physical and chemical information about the sample. It was only later that we appreciated that the SEM could readily cause beam damage to biological samples for, unlike the thin sections studied in the TEM, a substantial part of the primary beam energy stayed in the bulk sample. B. From Pollen Grains to Plants My experience with the SEM was too good to be true, for together with my research assistant, Brian Chapman, we extended our investigations to plant seeds, leaves, stems, wood and roots. It seemed that, unlike the TEM, all we had to do was to stick a fresh sample on the specimen support, pump it down and look at the images at low magnification. I was lucky to start with hard biological tissue because as soon as I started to examine the images more critically and extend our studies to wet samples, the familiar problems of sample preparation became apparent. In addition to sample damage due to shrinkage, collapse and destruction, there were the new artefacts associated with specimen movement, poor signal-to-noise ratio, and charging. The SEM demanded a re-think of the way we prepared our samples (Echlin, 1968). Initially this was not a problem because we simply used the chemically invasive procedures that for the previous 25 years had worked with the TEM. All we had to do was to convert the soft, wet biological material into sturdy, dry inorganic material by first marinating the samples in heavy-metal salt solutions and then removing all the water by pickling them in alcohol. In addition, I learnt how to emulate the wishes of King Midas and coat the sample surfaces with gold until they shone like jewellery. C. Looking Below the Surface At the same time as we were looking at the natural surfaces of biological material, we were seeking ways to expose their interior features by using diVerent fracturing and sectioning techniques. In addition to these methods, we took advantage of the work being carried out at CIC by David Kynaston who was using a demountable high-energy ion beam, attached directly to the microscope column, to selectively remove parts of specimens. We used the technique to progressively expose the internal components of a number of diVerent biological specimens. As Figs. 2a and 2b show, at a gross level,

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Figure 2. (a) SEM image of a freeze-dried red blood cell ion-beam etched for 30 s, 20 kV. Coated with 25 nm of gold. Picture width 6 mm. (b) SEM image of the same freeze-dried red blood cell ion-beam etched for 2 min, 20 kV. Coated with 25 nm gold. Picture width 6 mm.

progressive ion beam etching did indeed reveal diVerent parts of the sample. It was not entirely clear whether the ion beam removed the organic material by a charring and sputtering process, or whether the mechanism was one of true vaporization. Another diYculty was that when the ion beam hit the

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non-conducting specimens it caused a positive charge to accumulate, which in turn deflected further positive ions away from the surface, while noncharging areas continued to be eroded. A paper (Echlin et al., 1969) showed that although we could demonstrate that soft parts of the specimen, i.e. cytoplasm, etched faster than hard parts, i.e. cell walls, we were never convinced that the technique would give us any new information about the structure and chemical make-up of biological material.

IV. Improvements to Sample Preparation With the continuing support of CIC, I explored other ways the SEM could be used to solve biological problems. Rather suprisingly, there was little contact with the people in the Engineering Department. In retrospect, it was daring enough for a botanist to make contact with the Cavendish Laboratory let alone with engineers! I began to think long and hard about how we might overcome the nagging problem of sample preparation. We wanted to use the SEM to examine wet, low-atomic-number material that existed at atmospheric pressure. In contrast, the SEM is designed to operate best at high vacuum on very dry, electrically conductive, high-atomic-number material. There were two paths we could follow. We could either modify the chemically invasive techniques used for sample preparation or alter the internal environment of the SEM so that it would only be necessary to make small changes to the specimen. The absolute requirement of liquid water in living material was the problem, and there appeared to be three answers: we could freeze the samples and remove the ice by sublimation at low temperatures and pressures (freeze drying); we could enclose the wet, living material in a small chamber inside the microscope; or we could first convert the liquid water to ice and study it at low temperatures inside the microscope. The first option was already being used in the TEM and a number of groups were beginning to adapt this procedure for the SEM. The freezefractured and freeze-dried images gave useful information. A few people took up the challenge of the second option and after much trial and error this approach has given rise to the now-familiar environmental, or variable pressure, SEM in which it is possible to see paint drying, table salt crystals dissolving in water and organic material undergoing hydration changes. The hope of being able to view and image wet biological material was realized only a few years later. I chose to take the third option because it oVered a way not only to obtain high-resolution images but, with the advent of X-ray microanalysis, to perform in-situ chemical analysis of the light elements that play such an important role in biological material.

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A. A Long, Cold Look at the Third Option The challenges of low-temperature SEM were daunting and I went on a sharp learning curve to understand better the physical chemistry of the interconversion of liquid and solid water, in order to diminish the artefacts of ice crystal damage. I still had no instrumentation because the funding agencies gave no support for the development of new techniques for biological research. CIC again came to the rescue. The company had developed a temperature-controlled stage that operated well below the recrystallization temperature of water, and I was invited to see what the new equipment would do for biological material: the initial experiments were very promising. Subsequently, together with David Kynaston and Dick Paden, I spent many evenings and weekends looking at a wide range of frozen samples and began to establish a protocol for handling the material. We described our findings at a meeting in Chicago (Echlin et al., 1970) and Fig. 3 shows one of the first images taken by low-temperature SEM. I was hooked, and during the subsequent years my colleagues and I have sought to understand better the vagaries of low-temperature microscopy. There were two main problems with the Cambridge cold stage. Although it

Figure 3. SEM image of a frozen hydrated mouse L tissues culture at 160 K, 10 kV. Uncoated. Picture width 20 mm.

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would operate at 100 K, the cooling rate was not fast enough to ensure the hydrated material inside the microscope had no ice crystals. The necessary high cooling rates could readily be achieved by plunging small samples into melting isopentane, but it was not clear how to get this now frozen material onto a precooled cold stage without it melting or becoming contaminated with atmospheric water vapour. I developed a simple, and surprisingly eVective, transfer device involving large plastic bags, string and rubber bands and an inordinate supply of liquid nitrogen. Figure 4 shows the later final version developed together with Albert Saubermann and Tony Burgess, a young engineer from CIC. These were heroic beginnings that led to others taking up the challenge, so that we now have a number of diVerent integrated low-temperature sample preparation and cold stages. The book I published ten years ago (Echlin, 1992) provides an account of these early developments and shows that low-temperature microscopy and analysis are important investigative techniques.

Figure 4. Device used to load a precooled specimen on to the Cambridge cold stage on an S4 scanning electron microscope. The frozen sample, placed at the end of a brass rod, was passed through the hole in the lead glass window and screwed onto the precooled cold stage. During transfer, the inside of the microscope was kept at a slightly positive pressure using cold nitrogen gas and the outer transfer rod was kept inside a large plastic bag containing a small amount of boiling liquid nitrogen. Once the cold sample was in position, the transfer device was withdrawn, the internal nitrogen gas was turned oV, the window hole was sealed with a rubber bung and the microscope chamber was pumped down to working vacuum. The glass window was covered with a metal shield during microscopy.

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V. X-ray Microanalysis It had been long appreciated by biologists that dispersed within the organic matrix of biological structures there is a variety of other elements whose local concentration could, in principle, be measured in situ by X-ray spectroscopy. The physics, development and application of this technology are discussed elsewhere in this volume. The X-ray microanalysis of highly hydrated, beam-sensitive, low-density materials in the SEM was a challenge. Not surprisingly, much of the early work was largely qualitative and was centred on mineralized tissue in which the local concentrations of elements such as calcium and phosphorus were stable and the elements were present in relatively high concentrations. The methods designed to faithfully retain the structural architecture of the biological material, caused, alas, most of the elements of interest to disappear down the sink during sample preparation. Thirty-five years ago, biological X-ray microanalysis had a poor reputation that was dryly summarized by a distinguished scientist who stated, somewhat unwisely as it turned out, that all the technique could do was to show that bones contained calcium. A. Work at the Cavendish At the same time as my colleagues and I in the Botany School were making use of the SEM to complement our TEM studies, T. A. (Ted) Hall, working in the Cavendish Laboratory, was developing the technology, experimental protocols and understanding of what was referred to as the microbeam assay of chemical elements. The breakthrough came in a publication in 1968 in which Ted and his colleagues devised a novel algorithm that would provide analytical chemical data from sections of biological material. Like all good scientific ideas, the concept was disarmingly simple and combined elements of the classical Castaing equation with an extension of the Kramer relationship. The Castaing equation assumed that the intensity of the characteristic radiation is in proportion to the concentration of the elements of interest in the sample. The Kramer relationship describes the proportionality between the mass thickness and the intensity of continuum radiation as a function of the matrix atomic number. Since nearly all biological materials have approximately the same mean atomic number, Ted realized that measuring the continuum radiation in a region of the spectrum away from the characteristic emission lines of the elements of interest would provide a builtin monitor of changes in density and thickness from point to point in the sample. In this way one has a measure of both the mass of the analysed

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specimen region and the mass of a particular element in the same region from which local concentration may be derived. Ted made further refinements to this algorithm which is now known as the continuum-normalization method (or, more colloquially, the Hall technique). This is one of the most important algorithms for quantitatively analysing the many diVusible ions and bound elements that form an integral part of living material. A good summary of this pioneering work may be found in Hall (1979). The contribution Ted Hall made to biological X-ray microanalysis cannot be overestimated. B. Low-Temperature Microscopy and the SEM Three hundred metres away from the Cavendish, we in the Botany School were continuing to explore further applications for low-temperature microscopy in general and scanning electron microscopy in particular. This is not to suggest that the two groups were unaware of each other’s existence, Tea Club meetings and seminars ensured we kept in touch. It would probably be true to say that we at this stage were more concerned with imaging, whereas they were more concerned with analysis. By 1971 we were able to routinely image biological material at close to liquid nitrogen temperatures, although the problems associated with freezing damage remained a stumbling block to obtaining high-resolution images (Echlin, 1971). Water is the most abundant building block of biological material, and when converted to a solid state it should provide the perfect matrix in which to study the structure and in-situ chemistry of biological material. By using low-temperature preparation techniques we proposed to circumvent the conventional wet chemical preparative methods, which invariably resulted in massive losses of the diVusible elements (Z ¼ 11–20) that one might have wished to analyse. Coincidentally, and quite separate from the work we were doing, Tim Appleton in the Department of Physiology had developed techniques for cutting 100–200-nm frozen hydrated serial sections from soft biological material. Such sections were then freeze-dried prior to X-ray microanalysis at room temperature in the AEI EMMA 4 instrument (see Chapter 3.3A).

VI. The Biological Microprobe Laboratory In 1971, Ted Hall and I, together with Roger Moreton and two other colleagues from the Department of Zoology, made a successful application to the then Science Research Council for funds to establish a laboratory

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in Cambridge to study the distribution of electrolytes in undisturbed frozenhydrated cells and tissue. The grant provided a CIC S4-10 scanning electron microscope with a cold stage and, at a later date, a fully functional scanning microprobe. The laboratory was in full operation in early 1972 and we were joined by Tony Burgess from CIC, and in 1973 by Albert Saubermann, a National Institutes of Health (NIH) Post-Doctoral Fellow from Harvard. While Ted Hall and colleagues concentrated on the analytical aspects, Albert Saubermann and I concentrated on establishing the procedures to prepare frozen-hydrated samples for both SEM imaging and microanalysis. A year later, a short paper (Moreton et al., 1974) reported that we had demonstrated the feasibility of the technique: Fig. 5 shows one of our early images. With the benefit of hindsight, we were wrong in our supposition that we were dealing with fully frozen-hydrated material. Sadly, diVerences of opinion began to surface in the laboratory concerning experimental protocols, image interpretation, assurance of the fully frozen-hydrated state and hence the validity of the analytical results. During the following turbulent year and a half, Albert and I continued to work together very productively and set in place a rigorous experimental procedure for imaging and

Figure 5. SEM image of a frozen hydrated 2-mm section of blow fly larva salivary gland tissue at 120 K. Image used for energy-dispersive X-ray microanalysis, 20 kV. Uncoated. Picture width 60 mm.

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analysing frozen-hydrated biological material. Albert returned to Harvard at the end of 1975, where he set up his own successful microprobe laboratory based on the procedures he and I had established in Cambridge. I resigned from the Biological Microprobe Laboratory and continued experimental work elsewhere in Cambridge and in the United States. Ted Hall continued to refine the continuum-normalization algorithm that by now was a fully quantitative procedure and, together with colleagues, successfully applied the method to a variety of biological problems. The Biological Microprobe Laboratory continued to function until the late 1970s, at which time it received no further funding and, sadly, it closed. The 1980s were a hive of activity all over the world for using the SEM to study biological material. As new and improved instruments appeared, we would develop new and improved preparation techniques to use the SEM to solve biological problems. During this time I used the SEM in the Department of Anatomy in Cambridge. In addition I spent time developing and using the techniques of low-temperature scanning electron microscopy and X-ray microanalysis with Albert Saubermann in Boston and Tom Hayes in Berkeley, and spent several summers at the Philip Morris Research Centre in Virginia. There was even a fleeting contact with Alec Broers, who helped me to understand better how to improve the procedures for sputter-coating metal films (Echlin et al., 1980). My collaborators and I spent happy and productive hours solving problems in relation both to sample preparation and analysis, and the form and function of plant material.

VII. Cambridge and the Multi-Imaging Centre It is necessary to fast-forward to the early 1990s to come to the latest involvement of Cambridge with biological scanning electron microscopy and microanalysis. In 1993, Raymond Lund from the Department of Anatomy and I, together with contributions from the other departments in the School of Biological Sciences, were awarded a substantial grant from the Wellcome Trust to establish the first interdepartmental centre in Cambridge for multiple imaging and in-situ analysis at the cellular and subcellular level. We were joined by Tony Burgess, and the Multi-Imaging Centre was opened by Aaron Klug in 1994. It has a many-faceted mission including transmission, scanning and confocal microscopy, X-ray microanalysis, and naturally, low temperature techniques. Figures 6 and 7 show how lowtemperature scanning electron microscopy can be used for high-resolution imaging and as a basis for X-ray microanalysis.

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Figure 6. High-magnification SEM image of a frozen hydrated fracture face of the inner cell membrane of a yeast cell at 100 K, 5 kV. 2-nm platinum coating layer. Each of the particles making up the protein arrays is 12 nm in diameter. Picture width 0.7 mm.

Figure 7. Low-magnification SE image of a frozen hydrated fracture face of a young leaf of the tea plant (Camillia sinensis) used for quantitative X-ray microanalysis of Mg, Al and Si, 5 kV. 3-nm chromium coating layer. Picture width 400 mm.

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A. The Rest of the World This brief summary is incomplete, for in the early days there were very few of us involved in using and developing the SEM as a useful technique for biological investigations. The early band of pilgrims was soon eclipsed by the large number of biologists both here in Cambridge and abroad. Investigators from all over the world allowed us to better understand the interaction of the primary beam with fragile biological material, extend the usefulness to biological investigations by using the diVerent signals generated by the SEM, and interpret the images and chemical information that this marvellous instrument continues to provide. The early success of Cambridge in generating a wide interest in biological SEM is due to many people, a few of whom are mentioned in this chapter. Nearly all the early reports on SEM were on nonbiological material although it is significant to remember that the first commercial instrument (SEM3) was used in Canada to study paper and pulp–albeit an ‘honorary’ biological sample. As frequently happens in the development of analytical technology, the biologists follow the engineers and physicists. We should be grateful for their lead and also for two additional developments that took place more than thirty years ago. The annual SEM meetings in Chicago, which started in 1968, provided a unique forum where we could discuss and learn more about the instruments and the diVerent things they could do. The Lehigh Microscopy School of SEM and Microanalysis was started in 1970 and continues to teach people from all over the world how to understand and use the instruments critically. The third edition of our textbook (Goldstein et al., 2003) reveals that none of us have lost our fascination and commitment to scanning electron microscopy and X-ray microanalysis.

VIII. The Future Before we leave this brief account of biological scanning electron microscopy, it is appropriate to see where our past activities are leading. Thirty years ago, a colleague from the Science Research Council and I were asked to assess the future of electron microscopy in biology. The predictions in our report (Echlin and Fendley, 1973) turned out to be surprisingly accurate, and it seems appropriate to finish with another look to the future. The advent of instruments with 1-nm spatial resolution, operating at well below 1 keV and a beam current of a few picoamperes, will allow the SEM to do nearly everything the TEM can achieve with biological material. The recent special issue of the Journal of Microscopy (Vol. 212, Part 1, October

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2003) shows that high-pressure freezing techniques now permit one to obtain subnanometre spatial resolution in biological material without any freezing damage. Provided continued attention is paid to the problems of lowtemperature sample preparation, beam damage and specimen charging, it will be possible to routinely obtain simultaneous high resolution structural and chemical information about any type of biological material. Immunocytochemical labelling methods will continue to play an important role in these analytical studies. Although conventional X-ray microanalysis has moved from centre stage as an in-situ analytical technique, electron energy spectroscopy is now beginning to provide chemical information about small groups of light atoms in biological material. Although we will still use the SEM to obtain high-resolution images, we will move towards molecular biology as a means of obtaining chemical information about cells and tissues. The combined forms of carbon, hydrogen, oxygen and nitrogen as molecules and macromolecules are generally of more interest to biologists than the individual elements. The techniques of light microscopy immunocytochemistry, fluorescent and confocal microscopy, ratio imaging and 3D live-time analysis, are providing a wealth of chemical data about living biological samples, albeit at lower spatial resolution. In some cases, these new techniques are more sensitive than X-ray microanalysis. For example, it is now possible to monitor, in real time, very small changes in calcium levels in cells using calcium-sensitive dyes in combination with laser scanning confocal microscopes. The need for high spatial resolution is considered less important than understanding the nature of the dynamic processes. Further advances in the improvement of biological SEM await the involvement of future Cambridge scientists and engineers.

References Echlin, P. (1971). The application of scanning electron microscopy to biological research. Phil. Trans. R. Soc. Lond. B 261, 51–59. Echlin, P. (1992). Low Temperature Microscopy and Analysis. New York: Plenum Press. Echlin, P., and Fendley, J. A. (1973). Future of electron microscopy in biology. Nature 244, 409–414. Echlin, P., Godwin, H., Chapman, B., and Angold, R. (1966). The fine structure of polyvesicular bodies associated with cell walls in the developing anther of Ipomoea purpurea L (Roth). Kyoto, 1966, pp. 315–316. Echlin, P. (1968). The use of the scanning reflection electron microscope in the study of plant and microbial material. J. Roy. Microsc. Soc. 88, 407–418. Echlin, P., Kynaston, D., and Knights, D. (1969). Ion beam etching of biological material in the scanning electron microscope. St Paul, 1969, pp. 10–11.

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Echlin, P., Paden, R., Dronzek, B., and Wayte, R. (1970). Scanning electron microscopy of labile biological material under controlled conditions, in Scanning Electron Microscopy/1970/ 1. SEM Inc., AMF O’Hare, Chicago, pp. 49–56. Echlin, P., Broers, A. N., and Gee, W. (1980). Improved resolution of sputter coating metal films, in Scanning Electron Microscopy/1980/1. SEM Inc., AMF O’Hare, Chicago, pp. 163–171. Goldstein, J. I., Newbury, D. E., Joy, D. C., Lyman, C. E., Echlin, P., Lifshin, E., Sawyer, L., and Michael, J. E. (2003). ‘Scanning Electron Microscopy and X-ray Microanalysis.’ 3d Ed. New York: Kluwer Academic/Plenum Publishing. Hall, T. A. (1979). Biological X-ray microanalysis. J. Microsc. 117, 145–163. Moreton, R. B., Echlin, P., Gupta, B. L., Hall, T. A., and Weis-Fogh, T. (1974). Preparation of frozen hydrated tissue sections for X-ray microanalysis in the scanning electron microscope. Nature 247, 113–115. Smith, K. C. A., and Oatley, C. W. (1955). The scanning electron microscope and its fields of application. Br. J. Appl. Phys. 6, 391–399. Wells, O. C., Boyde, A., Lifshin, E., and Rezanowich, A. (1974). ‘Scanning Electron Microscopy.’ New York: McGraw-Hill.

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5.8 From the Scanning Electron Microscope to Nanolithography J. R. A. CLEAVER Cavendish Laboratory, University of Cambridge Formerly at: Engineering Department, University of Cambridge

I. Introduction After the field-emission work for my PhD (described by K. C. A. Smith on the associated CUED Website), I spent about seven years on the Cambridge University High Resolution Electron Microscope (HREM). With this system complete and in regular use, it was time to find a new area of activity—it was very clear that further development of TEM systems would take many years and many millions of pounds to enhance resolution by a ˚ ngstrom, and I was seeking a faster-moving very small fraction of an A project with greater novelty. The opportunity arose to join Dr (later Professor) H. Ahmed’s group in the Engineering Department, which had several programmes on electronbeam systems; they were often based on lanthanum hexaboride electron guns (Ahmed, 1973) which, inter alia, had been adopted for the Cambridge University HREM. The twin emphases were on systems for lithography, seeking to improve both the resolution and the writing speed, and on highpower electron-beam systems for material modification—rapid thermal processing for silicon annealing and recrystallization, enabling multiple-layer circuits and material enhancements such as the crystal-defect reduction in silicon-on-sapphire material. However, my role was to establish a new project area in focused ion beams. This had arisen from contacts with the Culham Laboratory of the United Kingdom Atomic Energy Authority, where pioneering work on ion sources was in progress. It was also a time of change for Ahmed’s group. For many years based on the fourth floor of the CUED Baker Building, it had increasing problems with shortage of space as more major research contracts were undertaken. In particular at that time, one of the first EEC Esprit contracts had been won, on electron-beam silicon processing, with British Telecomm (BT) and GEC as industrial partners (in those days both GEC and BT had substantial 485 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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interests in materials and device research, although later their priorities changed substantially, moving away towards systems activities). To meet the needs of these programmes, we took a unit in the Cambridge Science Park, which had been opened by Trinity College six years previously, and established the Microcircuit Engineering Laboratory. And it was there that I began work on focused ion beams.

II. Scanning Ion Beams for Microscopy and for Microstructure Fabrication The invention of the liquid-metal ion source, with its submicrometre eVective source size and high brightness, provided the opportunity to develop focused ion beam (FIB) systems with a range of potential applications. The solid-needle ion sources were developed at the Culham Laboratory, and for several years work was undertaken in collaboration with them and with Dubilier Scientific Ltd. A grant was obtained by Ahmed from the Paul Instrument Fund of the Royal Society and I was appointed to a postdoctoral position to carry out a programme of research. Following work at the Hughes Aircraft Corporation in California, I designed and built the first high-voltage, high-resolution FIB system in Europe (Cleaver et al., 1982) (Fig. 1) and used it to investigate microstructure fabrication processes such as focused ion-beam lithography in ion-sensitive resists (Cleaver, 1983). The system was predominantly electrostatic because of the high mass-to-charge ratio of the ions (so bringing back high-voltage and construction issues that electron-beam system designers had escaped 15 years previously, when magnetic-lens systems became dominant). However, it incorporated a magnetic ion-species filter (Cleaver et al., 1983) to accommodate alloy ion sources capable of providing a wider range of ions than available from single-metal sources. Thus semiconductor dopant ions could be used, enabling the system to be employed for direct localized implantation into semiconductors. For the fabrication of conventional-scale devices, the novel technique of grading the implant dose within devices and between circuit regions by varying the dwell time while scanning the ion beam (Evason et al., 1988a,b; Hussain et al., 1994) was particularly applicable; the system served also as a versatile tool for generating nanoelectronic structures (Blaikie et al., 1990, 1991, 1993). Scanning ion-beam systems derive much of their importance from the strong interactions between the incident ions and the target, which form the basis of several imaging modes and a wide range of sample-modification procedures. In the scanning ion microscope, the ion analogue of the SEM,

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Figure 1. Focused ion-beam system incorporating a four-sector magnetic filter for selection of ions from an alloy liquid-metal ion source. (Cleaver et al., 1983.)

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images are formed by generation of secondary electrons close to the surface of the target (Evason et al., 1985). If the sample is crystalline, the ion incidence direction with respect to the crystal orientation aVects the depth of ion penetration and can give rise to strong channelling contrast (Fig. 2) (Franklin et al., 1988). The first practical use of ion-beam modification was for the repair of the masks used for light-optical lithography, by sputtering unwanted areas of the chromium metal pattern (Cleaver et al., 1985); this is now a very major application of focused ion-beam technology as it is indispensable to the lithography mask industry for the production of defect-free mask sets. An important application for the combination of imaging and sample modification was the preparation of sectional cuts in samples for subsequent examination either in the scanning ion microscope or, at higher resolution, in the SEM. Focused ion-beam systems have two unique advantages over other sectioning methods, particularly for the investigation of samples (such as microcircuits) in which a very specific site must be examined: the sample can be imaged prior to the ion beam being used for ‘cutting’ the section, enabling the cut to be made at a precise location in the sample (Kirk et al., 1987a,b); and cutting by sputtering permits sections to be produced with no dependence on crystal orientation. Since cutting is local to the region to be observed, other parts of the sample are unaVected; voltage-contrast imaging was demonstrated in transverse-sectioned but still operational

Figure 2. Polycrystalline gold sample showing channelling contrast in a secondary-electron image taken with Gaþ ions at 30 keV. The image width is 140 mm. (Franklin et al., 1988.)

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semiconductor devices (Fig. 3). In support of this work, the technique of gasassisted FIB etching was developed. The introduction of reactive gases adjacent to the sample allowed the speed of the etching to be increased very substantially while, at the same time, enabling very clean cuts to be made; residues produced by the ion-beam sputtering process are removed by chemical interaction with the gas (Young et al., 1990, 1993) and do not form deposits that would obscure the sample surface. The preparation of thinly sliced samples for TEM by cutting was also reported (Kirk et al., 1989); for transmission imaging, as for surface imaging, the production of sections at precise locations and with free choice of orientation is very useful. The gasassisted cutting procedure can be applied also to microstructure generation (Ochiai et al., 1991). In later work the FIB system was used to deposit very small quantities of material to make nanodots of gold and other materials composed of only a few tens of ions and enabled very small features to be fabricated. Operation of a gold-source ion-beam system with the target biased electrically enabled the ion landing energy to be controlled and thereby determined the metaldeposit morphology; the structure of the gold nanodots was optimized to provide nanometre-scale island structures for single-electron device studies (Woodham and Ahmed, 1994, 1999; Hori et al., 1999), (Fig. 4). The technique was combined with silicon-on-insulator technology to produce single-electron transistors (Ford and Ahmed, 1999). The combination of focused ion-beam lithography and electron-beam lithography was used to make three-dimensional structures in resist. A high-voltage electron beam penetrates several micrometres into a resist layer with very little lateral scattering, while an ion beam gives shallow exposure, to a depth dependent on the ion species and the ion energy. The gold– silicon–beryllium liquid-metal ion source, which was studied jointly with

Figure 3. Voltage contrast in a micro-sectioned metal-oxide semiconductor field-eVect transistor. In (a) the gate is biased 5 V with respect to the remainder of the circuit; in (b) the gate is at þ5 V. (Kirk et al., 1987a.)

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Figure 4. Coulomb-blockade device structure: the main micrograph shows the metal contact structures with 45 nm width and 30 nm gap; inset is the gap region showing goldnanocluster islands. (Woodham and Ahmed, 1994.)

the Department of Materials Science and Metallurgy in Cambridge (Reich et al., 1986) is particularly suitable for this purpose, in conjunction with a focused ion-beam system incorporating an ion species filter, since silicon and beryllium are emitted predominantly as the doubly charged species Si2+ and Be2+ which, when accelerated through 70 kV, give 140-keV ions with respective ranges about 0.3 mm and 0.6 mm in a typical polymer resist. This formed the basis for a novel process for the fabrication of T-profile and G-profile gates for metal–semiconductor field-eVect transistors (MESFETs) and high electron mobility transistors (HEMTs) used as microwave devices, in which for good high-frequency performance it is necessary to provide short gates that nevertheless have a large metal cross section to keep the parasitic resistance small (Woodham et al., 1992). A single thick layer of polymethyl methacrylate resist was patterned to form a mould for lift-oV metallization; exposure was with electrons that gave complete penetration where the stalk of the gate was to be situated, while ion exposure defined the upper bar of the gate (penetration of the ions through the resist was negligible, preventing contamination of the semiconductor substrate). The resulting gate structures are shown in Fig. 5.

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Figure 5. End view of T-gate electrode with 0.22 mm gate length in contact with GaAs substrate.

A novel system was proposed whereby the electron and ion beams were both available through co-linear focusing optics and were scanned independently over the surface to provide additional flexibility in microstructure and nanostructure examination and generation (Cleaver and Ahmed, 1984). III. Nanolithography Whilst the ion-beam work was in progress, there were substantial developments in other parts of Ahmed’s group. Initially, the electron-beam lithography programme was directed to advancing the instrumental aspects, but as the technology became more established the emphasis changed to processes and to the structures that could be generated. The nature of the collaborations changed, with the particular consequence of building up links with the Department of Physics. So the Science Park laboratory changed from being an out-station of the Department of Engineering to an out-station of the Department of Physics. Old collaborations faded as GEC and BT moved away from device physics and fabrication, and a very major collaboration with Hitachi was established. It was therefore appropriate to move the group to West Cambridge, and funds were raised to make possible the construction of a new building on the Cavendish site. The building for the Microelectronics

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Research Centre was designed for the purpose, specifically to meet the needs of its research programmes in novel microfabrication and nanofabrication techniques and in the physics of advanced and highly speculative structures such as the single-electron transistor. A. Electron-beam Technology Electron-beam technology remained at the heart of our research, and some of the ion-beam methods continued. It was essential to achieve very high resolution in electron-beam lithography, and to follow this through with all associated processes so as to be able to produce nanometre-scale devices. The conventional SEM is intended to produce images that appear to be of good quality when assessed visually; such images may be composed of about a thousand lines and have residual distortions. The combination of high resolution and high electron-beam current (for acceptable image recording times and convenient setting up) is of great importance and necessitates a short objective-lens working distance to keep the lens aberrations low. The consequent large scanning angles introduce distortion even with the doubledeflection pre-lens scanning arrangement traditionally used in the SEM. The scanned area at low magnification is limited, and the field size decreases as the magnification is increased. Generally the image is considerably distorted for large fields. Research on large-area scanning with low distortion was started by Dr W. C. Nixon (Rao and Nixon, 1981). The considerations for large-area imaging have much in common with the requirements for electron-beam lithography, in which it is essential that the placement of the electron beam is precise; the tolerable overlay errors between the diVerent layers in an integrated circuit pattern can be allowed to be only a very small fraction of the minimum feature size in the pattern. The general development of electron-beam lithography systems, which depend on multiple scan systems and precise mechanical stage movements, is not discussed here but only the work that has led to lithography at the highest of resolutions for very advanced devices and structures is described. B. Mesoscopic Electron Devices In the 1980s, research in the Cavendish Laboratory was moving towards the study of mesoscopic electron devices under the direction of Professor M. Pepper. A collaboration began between Ahmed and Pepper in which shared research students fabricated mesoscopic device structures, using electron-beam lithography, and studied the physics of their operation. Initially, a number of research students worked on improving a conventional

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Figure 6. Distributed-feedback laser grating, with pitch 0.23 mm, patterned by electronbeam lithography and anisotropically wet-etched into InGaAsP. Completion of the laser requires overgrowth of the etched grating. (Fice, 1988.)

electron-beam lithography system towards ultra-high resolution. Some of the most significant devices made in this system were the split-gate structure on GaAs (Thornton et al., 1986), the Aharonov–Bohm electron-interference ring structure (Ford and Ahmed, 1987), free-standing semiconductor wires (Hasko et al., 1988) and metal wires, used for instance on phonon-transport studies (Smith et al., 1987). This work led to some remarkable physics results, with the consequence that a whole new research activity was created and much interest was shown in advancing electron-beam lithography to achieve resolutions well beyond the needs of conventional integrated circuits. The system was used also to produce microstructures for more applied projects, and generated the earliest first-order gratings for distributedfeedback lasers (Fice et al., 1987) (Fig. 6) as part of a collaboration with Standard Telecommunications Laboratories, Harlow. C. The Nanowriter This interest in ultra-high-resolution lithography led to the research project on constructing an electron-beam system with a spot size <5 nm and a beam voltage of 100 kV. It had been recognized during our research that to reduce the fabricated size not only was there a need for the electron spot size to be

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reduced but also there was a need to increase the electron-beam voltage. The higher beam voltage reduced scattering and proximity eVects in the immediate vicinity of the point on the substrate at which the electron beam was directed; electrons penetrated several micrometres into the substrate and back-scattered electrons re-emerged over a large area, so that the resist exposure suVered a small loss of contrast but no loss of resolution. The electron-beam machine, called the Nanowriter, was reported by Chen et al. (1998) and used to make a large number of nanostructures (Chen and Ahmed, 1998). Several novel electron-optical features were included: apart from the high-voltage operation of the system, it is arranged with the target within the magnetic field of the final lens, and incorporates a special deflection system that enables resolution better than 5 nm to be maintained over a 250-mm wide scan field. It has been used not only to make structures in which new physics may be explored but also to probe the limits of electron-beam lithography. The electron-optical aspects have been complemented with studies in resists and their processing, using modern resist materials such as callixerines as well as the highly traditional polymethyl methacrylate (PMMA), which has been in use since the very dawn of electron-beam lithography. With PMMA resist and ultrasonically assisted resist development, the system held the record for resolution in polymer resist of below 10 nm (Chen and Ahmed, 1993) and currently has reached about 4 nm (Yasin et al., 2001) (Fig. 7). At present, the machine has been used to show that nanostructures of 5 nm in size can be fabricated. This electron-beam machine has been used recently to fabricate singleelectron devices as part of our extensive research collaboration with the Hitachi Cambridge Laboratory. Initially, structures in GaAs materials were fabricated, leading to the first single-electron memory structure (Nakazato et al., 1993), while later single-electron structures were produced in siliconon-insulator materials for better compatibility with normal silicon technology (Stone and Ahmed, 1998) (Fig. 8). This work required not only a very high resolution but also a comparatively large field size to cover useful areas of nanofabricated structures. Nanowriter represents the state-of-the-art in lithography and is only now, after more than ten years, being matched in performance by commercial systems. The most recent application of electronbeam nanolithography is in making devices for quantum computing (Ardavan et al., 2003). IV. The SEM—Universal Tool for Research in Nanotechnology Electron-beam lithography has enabled microelectronic devices to be scaled down, eventually reaching the nanoscale. Consequently, conventional

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Figure 7. Resist image in PMMA, showing 4 nm line width. (Yasin et al., 2001.)

Figure 8. Silicon single-electron memory structure. (Stone and Ahmed, 1998.)

methods for inspection of structures based on optical microscopy ceased to be adequate, and scanning electron microscopy became the indispensable method for inspecting nanostructures. The microscope resolution must be substantially better than the minimum size of a fabricated feature, so that field emission machines with resolution of 1 nm or less became necessary. The high depth of field of the SEM also has been essential for inspecting

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structures that are inherently three-dimensional. Today the SEM is the universal tool in any laboratory carrying out research in nanotechnology. At the recent conference on Micro and Nano Engineering held in Cambridge (Cleaver and Ahmed, 2004) almost all papers showing microstructures and nanostructures included micrographs taken with the SEM. In the semiconductor industry as in nanostructure research, the SEM has become an essential tool, with applications ranging from inspection of masks, to inspection of stages in the processing sequence and to measurement of critical dimensions to a high level of accuracy. In some cases the SEM is used in device testing, using voltage measurement, on operating circuits. Here too there has been very substantial evolution, so that what began as visual observations in a general-purpose scanning electron microscope has developed into high-resolution voltage measurement in instruments designed around their energy-analyser detectors. Dynamic measurements by stroboscopic voltage-contrast imaging have been extended to the picosecond-scale time range (Thong, 1993). Finally, the wheel has come full circle and the electron-beam lithography techniques have come to the support of imaging. Today the use of a largearea scanning system together with a field-butting strategy enables highresolution imaging of samples of any size. In a further convergence, I have a programme in which the system has been specifically applied to the inspection of semiconductor chips, which are growing larger in size while simultaneously incorporating ever smaller features (Weaver et al., 2002). In conclusion, one can assert today that the symbiosis is complete; just as electron-beam lithography was an outcome of the scanning electron microscope, so the products of electron beam lithography can be observed only by the use of the high-resolution SEM.

References Ahmed, H. (1973). Some characteristics of boride emitters. Inst. Phys. Conf. Ser. 18, 32–34. Ardavan, A., Austwick, M., Benjamin, S. C., Briggs, G. A. D., Dennis, T. J. S., Ferguson, A., Hasko, D. G., Kanai, M., Khlobystov, A. N., Lovett, B. W., Morley, G. W., Oliver, R. A., Pettifor, D. G., Porfyrakis, K., Reina, J. H., Rice, J. H., Smith, J. D., Taylor, R. A., Williams, D. A., Adelmann, C., Mariette, H., and Hamers, R. J. (2003). Nanoscale solid-state quantum computing. Phil. Trans. R. Soc.: Math. Phys. Eng. Sci. 361, 1473–1485. Blaikie, R. J., Cleaver, J. R. A., Ahmed, H., Nakazato, K., and Tanoue, T. (1990). Electron transport in quantum wires and point contacts fabricated in GaAs/AlGaAs heterostructures using focused ion beam implanted gates. Proc. Mater. Res. Soc. EA-26, 75. Blaikie, R. J., Nakazato, K., Fraboni, B., Hasko, D. G., Cleaver, J. R. A., and Ahmed, H. (1991). Fabrication of quantum wires and point contacts in GaAs/AlGaAs heterostructures using focused ion beam implanted gates. Microelectron. Eng. 13, 373–376.

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Blaikie, R. J., Fraboni, B., Cleaver, J. R. A., and Ahmed, H. (1993). A low-temperature EBIC study of quantum wires fabricated in GaAs/AlGaAs heterostructure using ion-implanted gates. Inst. Phys. Conf. Ser. 134, 463–466. Chen, W., and Ahmed, H. (1993). Fabrication of sub-10 nm structures by lift-oV and by etching after electron-beam exposure of poly(methyl methacrylate) resist on solid substrates. J. Vac. Sci. Technol. B11, 2519–2523. Chen, W., and Ahmed, H. (1998). Nanofabrication for electronics. Adv. Imaging Electron Phys. 102, 87–185. Chen, Z. W., Jones, G. A. C., and Ahmed, H. (1998). Nanowriter: a new high-voltage electronbeam lithography system for nanometer-scale fabrication. J. Vac. Sci. Technol. B6, 2009–2013. Cleaver, J. R. A. (1983). Microscopy and lithography with liquid-metal ion sources. Inst. Phys. Conf. Ser. 68, 461–466. Cleaver, J. R. A., and Ahmed, H. (1984). A combined electron and ion beam lithography system. J. Vac. Sci. Technol. B3, 144–147. Cleaver, J. R. A., and Ahmed, H. (eds.) (2004). Micro- and Nano-Engineering 2003. Proceedings to be published by Elsevier as a special issue of Microelectronic Engineering. Cleaver, J. R. A., Heard, P. J., and Ahmed, H. (1982). Fabrication of sub-micron structures by scanning ion beam lithography. Grenoble, 5–8 October, 1982. Proceedings issued by comite´ du Colloque International sur la Microlithographie, Grenoble, Microcircuit Engineering 82, 148–153. Cleaver, J. R. A., Heard, P. J., and Ahmed, H. (1983). Scanning ion beam lithography with a magnetic ion species filter, in Microcircuit Engineering 83, edited by H. Ahmed, J. R. A. Cleaver, and G. A. C. Jones. London: Academic Press, pp. 135–142. Cleaver, J. R. A., Ahmed, H., Heard, P. J., Prewett, P. D., Dunn, G. J., and Kaufmann, H. (1985). Focused ion beam repair techniques for clear and opaque defects in masks. Microelectron. Eng. 3, 253–260. Evason, A. F., Cleaver, J. R. A., Heard, P. J., and Ahmed, H. (1985). Registration mark detection for scanning ion beam lithography. Electron. Lett. 21, 629–630. Evason, A. F., Cleaver, J. R. A., and Ahmed, H. (1988a). Fabrication and performance of GaAs MESFETs with graded channel doping using focused ion beam implantation. IEEE Electron Device Lett. EDL-9, 281–283. Evason, A. F., Cleaver, J. R. A., and Ahmed, H. (1988b). Focused ion implantation of gallium arsenide MESFETs with laterally-graded doping profiles. J. Vac. Sci. Technol. B6, 1832–1835. Fice, M. J. (1988). The application of high-voltage electron-beam lithography to the fabrication of semiconductor lasers. PhD Dissertation, University of Cambridge. Fice, M. J., Ahmed, H., and Clements, S. (1987). Fabrication of 1st-order gratings for 1.5 mm DFB lasers by high-voltage electron-beam lithography. Electron. Lett. 23, 590–592. Ford, C. J. B., and Ahmed, H. (1987). Fabrication of GaAs heterojunction ring structures. Microelectron. Eng. 6, 169–174. Ford, E. M., and Ahmed, H. (1999). Control of coulomb blockage characteristics with dot size and density in planar metallic multiple tunnel junctions. Appl. Phys. Lett. 75, 421–423. Franklin, R. E., Kirk, E. C. G., Cleaver, J. R. A., and Ahmed, H. (1988). Channelling ion image contrast and sputtering in gold specimens observed in a high resolution scanning ion microscope. J. Mater. Sci. Lett. 7, 39–41. Hasko, D. G., Potts, A., Cleaver, J. R. A., Smith, C. G., and Ahmed, H. (1988). Fabrication of sub-micrometer free-standing single-crystal gallium-arsenide and silicon structures for quantum-transport studies. J. Vac. Sci. Technol. B6, 1849–1851. Hori, M., Goto, T., Woodham, R. G., and Ahmed, H. (1999). Control over size and density of sub-5 nm gold dots by retarding-field single-ion deposition. Microelectron. Eng. 47, 401–403.

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Hussain, T., Cleaver, J. R. A., and Ahmed, H. (1994). Fabrication and performance of GaAs MESFETs with step-graded striped FIB doping in the channel region. J. Vac. Sci. Technol. B12, 158–160. Kirk, E. C. G., Cleaver, J. R. A., and Ahmed, H. (1987a). In-situ microsectioning and imaging of semiconductor devices using a scanning ion microscope. Inst. Phys. Conf. Ser. 87, 691–696. Kirk, E. C. G., Cleaver, J. R. A., and Ahmed, H. (1987b). Observation of voltage contrast in scanning ion microscopy of integrated circuits. Electron. Lett. 23, 585–586. Kirk, E. C. G., Williams, D. A., and Ahmed, H. (1989). Cross-sectional transmission electron microscopy of precisely selected regions from semiconductor devices. Inst. Phys. Conf. Ser. 100, 501–506. Nakazato, K., Blaikie, R. J., Cleaver, J. R. A., and Ahmed, H. (1993). Single-electron memory. Electron. Lett. 29, 384–385. Ochiai, Y., Young, R. J., Cleaver, R. J. A., and Baba, T. (1991). Fabrication of microstructures for quantum devices using focused ion beam gas-assisted etching. Microelectron. Eng. 13, 399–402. Rao, V. R. M., and Nixon, W. C. (1981). A computational and experimental analysis of 5thorder deflection aberrations. J. Vac. Sci. Technol. 19, 1037–1041. Reich, D. F., Fray, D. J., Evason, A. F., Cleaver, J. R. A., and Ahmed, H. (1986). Metallurgy and microfabrication applications of gold–silicon–beryllium liquid-metal field-ion sources. Microelectron. Eng. 5, 171–178. Smith, C. G., Ahmed, H., and Wybourne, M. N. (1987). Fabrication and phonon-transport studies in nanometer-scale free-standing wires. J. Vac. Sci. Technol. B5, 314–317. Stone, N. J., and Ahmed, H. (1998). Silicon single-electron memory cell. Appl. Phys. Lett. 73, 2134–2136. Thong, J. T. L. (1993). Electron Beam Testing Technology. New York: Plenum Press. Thornton, T. J., Pepper, M., Ahmed, H., Andrews, D., and Davies, G. J. (1986). Onedimensional conduction in the 2D electron gas of a GaAs–AlGaAs heterojunction. Phys. Rev. Lett. 65, 1198–1201. Weaver, D. J., Cleaver, J. R. A., Avery, L., and Ahmed, H. (2002). Substrate dopant imaging for layout reconstruction of integrated-circuit layers. Microelectron. Eng. 61–62, 1063–1067. Woodham, R. G., and Ahmed, H. (1994). Fabrication of atomic-scale metallic microstructures by retarding-field focused ion beams. J. Vac. Sci. Technol. B12, 3280–3284. Woodham, R. G., and Ahmed, H. (1999). Combined focused ion beam deposition system and scanning probe microscope for metal nanostructure fabrication and characterization. J. Vac. Sci. Technol. B16, 3075–3079. Woodham, R. G., Cleaver, J. R. A., Ahmed, H., and Ladbrooke, P. H. (1992). T-gate, G-gate and air-bridge fabrication for monolithic microwave integrated circuits by mixed ion-beam, high-voltage electron-beam and optical lithography. J. Vac. Sci. Technol. B10, 2927–2931. Yasin, S., Hasko, D. G., and Ahmed, H. (2001). Fabrication of <5 nm lines in polymethylmethacrylate resist using a water:isopropyl alcohol developer and ultrasonicallyassisted development. Appl. Phys. Lett. 78, 2760–2762. Young, R. J., Cleaver, J. R. A., and Ahmed, H. (1990). Gas-assisted focused ion beam etching for microfabrication and inspection. Microelectron. Eng. 11, 409–412. Young, R. J., Cleaver, J. R. A., and Ahmed, H. (1993). Characterization of gas-assisted focused ion beam etching. J. Vac. Sci. Technol. B11, 234–241.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

Appendix I* SIR CHARLES WILLIAM OATLEY, O.B.E. 14 February 1904–11 March 1996 Elected F.R.S. 1969 K. C. A. SMITH Engineering Department, University of Cambridge

Charles Oatley made three outstanding contributions to the engineering sciences: he was one of the brilliant team that developed radar in Britain during the Second World War; he revolutionized the teaching of electronics at Cambridge University; and he developed the scanning electron microscope. It is for the last of these that he will be chiefly remembered. He stands with Manfred von Ardenne as one of the two great pioneers of scanning electron microscopy. His involvement with the instrument began shortly after the war when, fresh from his experience in the development of radar, he perceived that new techniques could be brought to bear which would overcome some of the fundamental problems encountered by von Ardenne in his pre-war research. Oatley’s work led directly to the launch of the world’s first series production instrument—the Stereoscan—in 1965. Thousands of scanning electron microscopes have since been manufactured and are to be found in practically every research laboratory in the world. The striking three-dimensional images of microscopic organisms produced have been used to illustrate countless newspaper and magazine articles, as well as scientific research papers, giving the general public a new perspective and appreciation of the world that lies beyond the resolution of the human eye. The scanning electron microscope is, arguably, the single most important scientific instrument of the post-war era. Early life and education Charles Oatley was born in Frome, Somerset, in 1904, coincidentally the year that mains electrical power was brought to the town. His father, William, the enterprising owner of a flourishing bakery business, installed electrical power in his bakery immediately it became available. Although lacking any formal scientific education, William Oatley was intensely interested in scientific matters and passed this enthusiasm on to his son. He gave Charles an electric motor on his sixth birthday—not a common toy in 1910—and he possessed a fine Watson Royal microscope, which *Reprinted from: Biographical Memoirs of Fellows of the Royal Society, London, 44, 329–347 (1998).

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he taught his son to use. Charles was thus surrounded by electrically powered machinery almost from birth, and his natural scientific bent was nurtured from the earliest years. He began his education at a small ‘dame’ school, close to the family home. In a family memoir (1)* he recounts how at the age of five, accompanied by an older pupil from the school, he ‘set forth, armed with a slate, some slate pencils and a small sponge (to clean the slate) in an empty Lyle’s Golden Syrup tin’. Between the ages of 6 and 12, Charles attended a local council school, which he later acknowledged provided a very good preliminary education. During these early years he also learnt to swim at the town swimming baths, a legacy from Queen Victoria’s Jubilee celebrations. He eventually became a fine water polo player, which contributed not a little towards shaping the pattern of his life and career. After this local education, he proceeded to Bedford Modern School as a boarder, where he excelled in scientific subjects and rose to become head boy and captain of the swimming and Rugby Fives teams. It was a fixture with the St John’s College Rugby Fives team that introduced him to Cambridge, for it was then that he saw for the first time the colleges of Cambridge which, as he later wrote, ‘bowled him over’. It was already clear that he would proceed to a university; from then on he decided that he would try for Cambridge. His Headmaster, A.C. Powell, a Trinity man, advised him to attempt the Trinity entrance scholarship examination but this was unsuccessful; however, a second attempt, this time at St John’s, resulted in the award of an exhibition, which was later supplemented with a second exhibition. He entered the University in 1922 to read for the Natural Sciences Tripos. At St John’s he met the two men who were to have a profound influence on his life and career: E.V. Appleton, his supervisor, and J.D. Cockcroft, a fellow freshman whom he got to know quite well, although Cockcroft was seven years older and reading for the Part II Mathematics Tripos. Oatley’s first two years at Cambridge were very successful: a First in Part I of the Tripos and a half-blue for swimming. His third year began equally well; he was appointed captain of the University swimming team and he had an interview with Rutherford, who agreed to take him on to do research in nuclear physics; however, things did not go according to plan. Appleton had left to take the Wheatstone Chair of Physics at King’s College, London, and his supervision and guidance was greatly missed by Oatley. For whatever reason, the expected first class in Part II (physics) failed to materialize, and with it went the chance of gaining a research grant to work with Rutherford. The possibility of an academic career at Cambridge appeared to be closed. Industrial experience With Appleton’s advice and encouragement, Oatley obtained a job with Radio Accessories, a small company in Willesden manufacturing radio valves, for which Appleton was acting as a consultant. Here Oatley gained valuable experience of manufacturing techniques and, as the only graduate in the company, was called on *Numbers in this form refer to the bibliography at the end of the text.

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to tackle a wide variety of physical problems. This work led him to spend a lot of time in the evenings at the Patent OYce library, which helped him to gain a good deal of specialized knowledge. Towards the end of 1926, new valves from GEC and Mullard caused a serious decline in sales of Radio Accessories’ valves, and in order to compete, the company reached an agreement with the Raytheon Company in America to manufacture under licence a new type of gas-filled rectifier. Oatley was given the task of producing the first samples of these new valves, which resulted in a trip to the Raytheon factory in Boston, his first to the USA. Unfortunately, the company’s fortunes continued to decline, and in the summer of 1927, Appleton, aware of the situation at Radio Accessories, invited Oatley to return to academic life by oVering him a demonstratorship in the Physics Department at King’s College. Radio Accessories went into liquidation shortly afterwards.

King’s College, London The next 12 years at King’s were spent largely in teaching and examining. According to Oatley’s own account he found it an exciting time to be teaching physics: The wave-like nature of matter was being confirmed experimentally and the foundations of modern quantum theory were being laid. None of this had been included in my lectures at Cambridge and it took me a long time to get to grips with it.

He recalls going to the Royal Institution to hear Erwin Schro¨dinger lecture on his famous equation. External examining both for London University and for school certificate examinations occupied a great deal of his time, and he travelled round the country invigilating and marking practical physics examinations for the Oxford and Cambridge Joint Board. He records that in each of the years 1938 and 1939, by which time he had been appointed to a lectureship, he marked 1500 scripts. In addition to this work he took on a consultancy for six years with a small firm, Lissen Ltd, manufacturers of radio sets and dry batteries, which had purchased the factory equipment belonging to Radio Accessories on its liquidation. As he later wrote: ‘in this period I was learning the elements of my trade’. These activities left him with less time for research than he would have wished, but were to some extent necessary to supplement his salary, which for a college demonstrator in the 1920s and 1930s was not large. Nevertheless, he produced some useful papers on a variety of subjects. Already his interests in radio and electronics and the art of electrical measurement were becoming evident. With the publication of his 1932 Methuen monograph, Wireless receivers (2), read widely by professionals and amateur enthusiasts alike, he became an acknowledged expert in the field of radio receivers. (Appleton also produced a companion volume, Thermionic vacuum tubes, in the same series.) This publication almost certainly had a bearing on the direction Oatley’s career was to take. In 1928, while attending an Old Boys’ reunion at Bedford Modern, he was entertained to dinner by his former science master, Mr West, whose daughter, Enid, was also present. The outcome of this was that Charles and Enid were married at

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St Peter’s Church, Bedford, in 1930. Their two sons, John and Michael, were born in 1932 and 1935, respectively.

Wartime work in radar Oatley had met Cockcroft on only a few occasions since their time together at Cambridge, and he was therefore somewhat surprised in the summer of 1939 to receive a letter from Cockcroft, acting on behalf of the Air Ministry, inviting him to join a small party of university physicists and spend a few weeks learning about scientific developments which would be important in the event of war. In the following September, members of Cockcroft’s party were assembled at Bawdsey Manor, where much of the initial work on radar (then known as radiolocation or radio direction finding (RDF)) had been carried out, but they found the place had been hurriedly evacuated for fear of air raids. Cockcroft then drove his party over to Rye, where they were initiated into the secret work on radar being carried out under the direction of R.A. Watson-Watt, and were briefed on the Chain Home stations then under construction around the east and south coasts of Britain, one of which was at Rye. Cockcroft later recalled that occasion: I arranged hurriedly with Watson-Watt to go to Rye instead, for Bawdsey was evacuated on 1st September. I had with me Oatley, Kempton, Shire, Latham, Ashmead and Dunworth; a very good party. We sat in the sun at Rye and taking the sacred handbook of Chain Home (CH) to pieces, learnt our RDF chapter by chapter from it (Cockcroft 1985).

Oatley needed no persuasion that this work was of vital importance and arranged with King’s to take unpaid leave for the duration of the war. Shortly after war had been declared on 3 September, Oatley was to be found, with others of Cockcroft’s party, back at the Cavendish in the old High Voltage Laboratory engaged in modifying Pye TV receivers for use in coastal defence (CD) radar sets designed to detect submarines, work which was done in collaboration with the Pye TV Company in Cambridge. Various pieces of test apparatus were also constructed, including a number of 200 MHz wavemeters manufactured in the Cavendish workshop to Oatley’s design, which he calibrated using an open transmission line—‘a calibration found later to be accurate to 0.1%’. In February 1940, Oatley followed Cockcroft to the newly formed establishment at Christchurch, later to become known as the Air Defence Research and Development Establishment (ADRDE), where all radar work for the army was to be carried out. Cockcroft was appointed Chief Superintendent in April 1941. Oatley’s first task at the ADRDE was to investigate ways of improving the receivers used in the CD sets, which operated at the (then) relatively high frequency of 200 MHz. The detection of weak radar echoes depended crucially on the noise performance of the first stage of amplification in these receivers. The method then in use to test the receivers was to aim the CD set at the Needles, oV the Isle of Wight, and judge the extent to which the echoes displayed on the cathode-ray tube display were noise-free; hardly a quantitative measure. No commercial test equipment was

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available at this frequency, so he set about building a signal generator, which incorporated for the first time at this high frequency, a piston attenuator, a technique which later in the war he extended to centimetric wavelengths. With this apparatus he was able to compare, quantitatively, the noise performance of the various types of valves available for the critical input stage of the receiver. The tests produced spectacular results, showing that one of the new valves produced by the GEC specifically for this purpose was virtually useless: the GEC engineers had tested the valve for gain with large signals but without taking account of noise. This work brought home to everyone in the Establishment the importance of accurate quantitative testing of all components and equipment, and it led to the formation of Oatley’s ‘Basic Group’, which served all other groups in the Establishment throughout the war. As the work of the ADRDE grew, the need for improved liaison with other radar establishments and with the industrial groups designing and supplying equipment became increasingly evident, and Oatley, with his extensive knowledge of the work being undertaken at the ADRDE, became heavily involved in many of the committees that were formed to further cooperation, including the Communication Valve Development (CVD) organization. He had been greatly impressed with the pre-war contract work on the Chain Home stations undertaken by Metropolitan Vickers on the transmitters and by Cossor on the receivers, and he did much to promote a spirit of cooperation with the people involved; in particular, with C.C. Paterson of the GEC Research Laboratories at Wembley. Paterson kept a diary covering the events of the war years, which was later published; in it Oatley receives a very favourable mention. In this connection Oatley has pointed out how fortunate it was that the BBC introduced a television service in 1936. Not only did this mean that when war came there were already in existence many of the components required for radar but, more important, there was a trained body of design engineers within industry familiar with high-frequency techniques. These were the people who were able to turn the projects generated at the ADRDE into the robust equipment required by the armed forces. Oatley’s industrial contacts made during these war years were to endure throughout his subsequent career. The importance of this work, and recognition of his considerable administrative abilities, led to his appointment in 1943 as Deputy Superintendent to Cockcroft, and when in 1944 Cockcroft moved to take charge of the Canadian Chalk River Nuclear Project (a few days after the opening of the Second Front), Oatley became Acting Superintendent. In eVect, he was in charge of the organization which by then numbered about 1000 personnel, had moved to Malvern and had been renamed the Radar Research and Development Establishment (RRDE). By the end of the war Oatley possessed an unrivalled knowledge of the work being undertaken in radar and electronics, both in the government establishments and in industry; he was familiar with much of this work at a detailed technical level, and he knew all the key people involved; all of which would stand him in good stead for the next phase of his career. He was oVered the post of Superintendent at the Establishment, but his heart lay in university life. One option open to him was to return to King’s College, where, it was made clear, he could expect a readership, but he had other ideas. Earlier in the war Professor Willis Jackson, Head of the Electrical

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Engineering Department at Manchester University, had remarked to him during a visit to the Establishment: ‘When this war is over, we are going to need people like you in electrical engineering departments.’ This had helped to steer Oatley towards the conclusion that his cumulative experience fitted him better for a university career orientated rather more towards electronics than pure physics. At that time, university electrical engineering departments were almost exclusively the preserve of heavycurrent electrical engineers (Manchester being an exception); hence opportunities were limited. The situation was, however, quickly resolved. In 1943, J.F. Baker (later Lord Baker) had been appointed as Head of the Engineering Department at Cambridge, with a mandate to build up an important school of research, and he had obtained the agreement of Trinity College to make a Fellowship available for someone appointed to a lectureship in electrical engineering in his Department. Through Cockcroft, Baker learned of Oatley’s wish to return to university life and of his intention to switch to electronic engineering, and he immediately arranged for Oatley to be appointed to the new lectureship. Before leaving the Establishment for Cambridge, Oatley had the satisfaction of seeing, on VE Day, one of the groups under his command drag searchlights to the top of the Worcestershire Beacon where, by means of mirrors, they shone an enormous rotating V-shaped beam into the night sky. Shortly after the war the Institution of Electrical Engineers held a Radiolocation Convention, the proceedings of which were published (3). Oatley was responsible for the section on ‘Measurements’ in this Convention.

Return to Cambridge Prior to Baker’s appointment, engineering undergraduates at Cambridge read for the Mechanical Sciences Tripos, a single-part Tripos without specialization in any particular branch of engineering. Light current electrical engineering was represented in the Tripos by a course designated ‘electrical signalling’, which took the subject little beyond the elements of the triode valve. Baker was clear that this arrangement could no longer continue and, by the time Oatley arrived in 1945, he had obtained the agreement of the University for the introduction of a two-part Mechanical Sciences Tripos in which students in their third year could choose from among a number of specializations; the four-paper electrical option, designated Part II D, contained a suYcient choice of questions to allow candidates to specialize in either heavy-current engineering or electronics. Baker had also secured agreement for the establishment of a Chair of Electrical Engineering, to which E.B. Moullin was elected in 1945. J.G. Yates, who had been with Oatley at Malvern, was appointed to a demonstratorship in the same year. A fourth member of the staV, L.B. Turner, a distinguished radio engineer, shared the responsibility for teaching in electronics. Two years later, K.F. Sander, who was Oatley’s first research student in electron optics, was also appointed to a demonstratorship. Yates became a Lecturer in 1947 and Sander a Lecturer in 1952. Oatley’s first task in the Department was to prepare lecture and laboratory courses for the new Tripos, which was to be introduced in 1947. Very little money was

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available for the purchase of laboratory equipment, but he had two valuable assets at his disposal: access to large quantities of war surplus equipment, and an excellent and well-equipped departmental workshop under the able direction of J.H. Brooks and his deputy, A.A.K. Barker. Before leaving RRDE, Oatley had been involved in setting up the scheme for disposal of surplus electronic equipment to universities; he therefore knew exactly where the best equipment was located and how to get it. So, on his arrival in Cambridge, he was asked to represent the whole University in formulating requests for equipment, a task occupying a great deal of his time until 1951, but which was of enormous benefit to the University. The operation was conducted on a very large scale, with huge quantities of electronic components regularly transported to Cambridge by the lorry load. Brooks and Barker conducted a parallel exercise to acquire machine tools and workshop equipment. With the aid of these excellent facilities, almost everything necessary for the new Tripos was designed and constructed within the Department. This included not only the special apparatus required for each experiment, but also standard items such as power supplies, oscillators and oscilloscopes, which were built in large numbers and used additionally to equip the research laboratory that Oatley was already building up. Yates played a major part in this work, and the ‘Yates Scope’ became a standard tool used for teaching and research in many other University departments throughout the 1950s. The course on electron physics that Oatley introduced in Part II D of the new Tripos was an entirely novel one for engineers, drawing heavily on his teaching experience at King’s College. At the outset there was Schro¨dinger’s equation and elementary quantum theory, kinetic theory of gases and vacuum technique, thermionic and field emission, gas discharges, lens systems, photometry and radiation pyrometry. A little later electron optics was introduced, and when the invention of the transistor was announced, semiconductor theory and statistical mechanics displaced some of the earlier material. This course laid the foundations of modern electronics in the Department, and was an important factor in the subsequent growth of the Electrical Group to become one of the foremost schools of electronic engineering in the country. Also notable was the clear, crisp style with which his lectures were delivered. For the following decade Oatley was involved in attempts to secure more lecture time for electronics teaching in Part I of the Tripos, but in this he was only partly successful. There was strong resistance to any kind of specialization in Part I, and members of other groups in the Department were unwilling to give up lecture time in their own subjects in an already overcrowded syllabus. A small advance was, however, made in 1959, when he was able to introduce a long vacation course in electronic instrumentation as an alternative option to courses in surveying and metrology. Alongside his teaching work Oatley began in 1945 to build up his research group, and to this end he succeeded in recruiting a number of students who were returning from war service. The research projects he set in train in the latter half of the 1940s included: mass spectrometer; microwave amplifiers; electron trajectory analogue plotter; digital electron trajectory plotter; noise in valve amplifiers; flicker noise; electron beam-induced conductivity in diamond; and the scanning electron microscope. At this stage Oatley fully expected that for the future he would be able to

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recruit a small number of research students each year from among the Part II D graduates, but in this he encountered resistance from Moullin. One student, I.M. Ross, whom Oatley wished to recruit, was strongly advised by Moullin to turn down a research grant and instead take up a graduate apprenticeship in industry. Ross, declining this advice, later became President of Bell Telephone Laboratories. Before the war Moullin had led a distinguished research school at Oxford, but his views concerning the place of university research had in the meantime changed. He was firmly of the opinion that the rapid pace of industrial research in electronics made it impossible for universities to compete, and that engineering graduates who wished to do research should go straight out into industry. He believed that postgraduate courses and research in electrical engineering in universities were no longer appropriate. His views were expressed in his inaugural address as President of the Institution of Electrical Engineers (Moullin 1950). This divergence of opinion between Oatley and Moullin concerning the future direction of teaching and research in the department came to a head with Turner’s retirement in 1951. Oatley wanted a replacement who could take on Turner’s Part II D lectures, and who was suYciently experienced in research work to help with the supervision of research students in electronics. Moullin, on the other hand, preferred someone with more general experience in electrical subjects mainly for teaching in Part I; in his view research experience was not essential. Following several full discussions with Moullin, Oatley, with his customary clarity and precision, set out their respective views in letters to both Moullin and Baker. These letters, which mark a turning point in the teaching of electronics in the Department, were later published (after the death of Moullin) in a Departmental report in which Oatley recounted how the subject came to be established in the Department (8). What subsequently transpired between Moullin and Baker is not known, but the outcome of this exchange of views was that Moullin agreed not to oppose the course that Oatley wished to follow. Their warm personal relations remained unimpaired by this incident. In due course, a former research student in Oatley’s Group, C.W.B. Grigson, who had done pioneering work on electron diVraction, was appointed to a demonstratorship to take over Turner’s teaching load. From then on the research eVort grew steadily until there were about a dozen students engaged in electronics research by the end of the decade. Grigson, later appointed to a lectureship, played a significant part in this growth. In parallel with these developments Professor J.F. Coales was building up a group in control engineering, which added considerably to the strength of teaching and research in electrical subjects.

The scanning electron microscope Oatley’s interest in the scanning electron microscope (SEM) was first aroused when he learned of the work of M. Knoll and M. von Ardenne in pre-war Germany and that of V.A. Zworykin, J. Hillier and R.L. Snyder in the USA in the early 1940s. Although in 1935 Knoll had produced the first scanned electron image of a surface, it was von

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Ardenne who, in a relatively short burst of inspired activity just prior to the war, laid the foundations of both transmission and surface scanning electron microscopy. However, this work had produced inconclusive results and the consensus of opinion among microscopists and metallurgists after the war was that the scanning instrument could not compete with the conventional transmission microscope using the replica technique. But, fresh from his wartime experience in radar, Oatley appreciated that new techniques and methods were available that could be applied to the scanning concept. In particular, he was aware of work by A.S. Baxter in the Cavendish Laboratory on a new type of electron multiplier incorporating beryllium–copper dynodes. This could be repeatedly exposed to the atmosphere and thus used in a demountable vacuum system, and he saw that it could, at least in principle, solve the major technical problem encountered in the earlier instruments; namely, the noisefree detection of the picoampere secondary electron currents obtaining in the SEM. Of equal importance to Oatley was that the topic appeared to oVer the ideal avenue through which to exploit his strong background in electron physics and to stretch the minds of his research students. In his 1982 paper ‘The early history of the scanning electron microscope’ (7) he posed the question: ‘Why, in the face of the discouraging results that had hitherto been obtained, did I think it worthwhile to reopen the matter in 1948?’ He answered the question by describing the conditions in the Engineering Laboratory at that time, and providing his own views on university research. A project for a PhD student must provide him with good training and, if he is doing experimental work, there is much to be said for choosing a problem which involves the construction or modification of some fairly complicated apparatus. I have always felt that university research in engineering should be adventurous and should not mind tackling speculative projects.

He decided, against the advice of many experts in the field, some of whom declared it to be a complete waste of time, that the SEM should be included among his research projects. His first research student, D. McMullan, commenced in October 1948 and a working instrument (designated SEM1) was demonstrated by 1951 (figure 1a). This incorporated three features which diVerentiated it from all its predecessors: electrons scattered from the specimen surface were detected by means of an electron multiplier with beryllium–copper dynodes; the specimen was placed at a large angle (25 degrees) to the electron beam; and a high primary-beam energy (10–25 kv) was used to reduce the eVects of surface contamination on secondary emission. In addition, images were viewed directly on a slow-scan radar-type cathode-ray tube (CRT) display, and recorded on film using a short-persistence high-resolution CRT. This instrument from the very first produced the striking ‘three-dimensional’ images characteristic of the modern-day SEM (figure 1b). The results obtained convinced Oatley, even at this early stage, that the SEM would prove to be an important scientific laboratory tool; however, the new instrument was met with indiVerence and even ridicule in some quarters of the electron microscope establishment. The reasons for this have been discussed by Oatley (7) and by P. Jervis, who made the development of the SEM the subject of a case history in instrumental innovation (Jervis 1971–72). They conclude that the primary reason was

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Figure 1. (a) Scanning electron microscope SEM1 by McMullan and Oatley. (b) Early micrograph (etched aluminium) obtained with SEM1.

the fact that electron microscopy was synonymous with the attainment of very high resolution; in this respect the SEM, particularly in its early stages of development, fell far short of the transmission electron microscope (TEM). In 1953, McMullan read a paper on SEM1 to the Measurements and Radio Section of the Institution of Electrical Engineers, at which several of the leading figures in UK electron microscopy were present (McMullan 1953). From the reported discussion on the paper it is apparent that the prevailing view was that the SEM was ‘hardly likely to replace the replica method’, and that by itself was unlikely to become a viable method of microscopy. There was, however, a prescient suggestion that it ‘should be useful for the high-resolution examination of surfaces with deep structure’, and that it might usefully be combined with other facilities, such as X-ray microanalysis, in a multifunction instrument. Jervis suggests also that because it was outside the mainstream of electron microscopy, the work of Oatley’s group was not at first taken seriously. Undeterred, Oatley embarked on a wide-ranging and imaginative programme of research and development with his students, which explored almost every facet of the instrument’s capabilities: secondary electron imaging and micromanipulation (K.C.A. Smith); atomic number contrast and stereomicroscopy (O.C. Wells); voltage contrast in semiconductor devices (T.E. Everhart); simplified SEM (P.J. Spreadbury); low-voltage observation of non-conducting specimens at very low temperatures (R.F.M. Thornley); in situ etching of surfaces by ion bombardment (A.D.G. Stewart); observation of specimens at very high temperatures (H. Ahmed); high-resolution SEM (R.F.W. Pease); and electron beam microfabrication (A.N. Broers). By the end of this initial phase of development, which lasted until the early 1960s, it had been demonstrated conclusively that this was one of the most powerful and

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flexible methods of microscopy yet devised. During the course of the development, many technical innovations were made, one of which greatly reduced the complexity of the instrument and deserves special mention. Although the electron multiplier appeared to oVer an elegant solution to the problem of detection in the SEM, it proved to be extremely diYcult to implement in practice since it involved a head amplifier floating at a high voltage. This diYculty was removed with the introduction in 1956 of a detector that subsequently became known as the ‘Everhart–Thornley’ detector. This detector evolved as a result of an original suggestion by Oatley concerning the use of a plastic organic scintillator coupled to a photomultiplier to detect the electrons transmitted through an environmental cell in the SEM. It subsequently became the standard used in the majority of commercial SEMs. The importance of the work Oatley’s group had accomplished was later brought to the attention of the public when the Science Museum commissioned the Engineering Department to build a replica of SEM1. This was placed in the electron microscope gallery of the Museum in July 1986 (McMullan 1986). The replica was constructed by L.R. Peters, chief technician in Oatley’s group, who had worked on the original instrument almost 40 years earlier. On Oatley’s recommendation Peters was awarded an honorary MA of Cambridge University for his work in the Department and for his long association with the development of the SEM. The Stereoscan Until the mid-1950s there was little interest in the work of Oatley’s group, but a turning point in attitudes towards scanning electron microscopy came when D. Atack, a member of the Pulp and Paper Research Institute of Canada (PPRIC), and J.H.L. McAuslan, with Imperial Chemical Industries, learned of the work on the SEM. Both were on sabbatical leave at the time in F.P. Bowden’s group in the Department of Physical Chemistry, and they decided to explore the potential of the instrument for their work. Using SEM1, Atack examined a range of pulp and paper specimens, while McAuslan studied the thermal decomposition of silver azide crystals. This work generated strong interest at the PPRIC, and Oatley subsequently arranged with the President of the PPRIC, L.R. Thiesmeyer, that the Institute would finance the construction of a new instrument in the Engineering Department. This instrument, designed by K.C.A. Smith as a postdoctoral research project, was designated SEM3, being the third microscope constructed in the Department. The two previous instruments had used electrostatic lenses; the use of magnetic lenses in SEM3, together with the new detector, produced a greatly improved performance and set the pattern for subsequent commercial instrumental development. It was shipped to the Montreal Laboratories of the PPRIC in 1958, where it was applied to a range of problems arising in the Canadian paper industry. The instrument was also used by other organizations, including the DuPont Chemical Company, which later became one of the first purchasers of a commercially manufactured instrument. At this stage Oatley persuaded Associated Electrical Industries (AEI) (formerly Metropolitan Vickers), a company then manufacturing transmission microscopes and developing electron probe microanalysers, to take an interest in the SEM. An

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understanding was reached that if the microscope (SEM3) appeared to be commercially viable, AEI would take it up. In the following year, 1959, AEI received an order from Bowden, but at the time the engineering resources at AEI were overstretched and it was decided to modify a pre-production microanalyser rather than build a copy of SEM3. The resulting instrument delivered to Bowden failed to meet its specification and was not successful. This terminated the involvement of AEI in SEM manufacture, and Oatley had to look elsewhere for a commercial developer. In 1961, W.C. Nixon (who had in 1959 moved from V.E. Cosslett’s group in the Cavendish to assist Oatley with direction of the research) and K.C.A. Smith made an informal approach to S.A. Bergen, Chief Development Engineer of the Cambridge Instrument Company, suggesting that the company should consider manufacture of an SEM as well as the microanalyser, which they were marketing at the time. Shortly afterwards, Oatley reached a formal agreement with H.C. Pritchard, the Managing Director, and arrangements were made for the manufacture of two prototypes, one of which eventually went to the DuPont Company in the USA. In 1962, a former research student, A.D.G. Stewart, joined the Cambridge Instrument Company to take over development of the new SEM, and with government backing, a batch of five microscopes was manufactured in 1965. Oatley’s 1982 paper (7) concludes with the following paragraph—a testimony to his skill and persistence (Everhart 1996): The first four production models, sold under the trade name ‘Stereoscan,’ were delivered respectively to P.R. Thornton of the University of North Wales, Bangor, to J. Sikorski of Leeds University, to G.E. PfeVerkorn of the University of Mu¨nster, and to the Central Electricity Research Laboratories. By this time the Company had launched a publicity campaign and orders began to roll in. An additional batch of twelve microscopes was put in hand; and then a further forty . . . The scanning microscope had come of age.

In 1960, Oatley was appointed to the Chair of Electrical Engineering and Head of the Electrical Division in the Department; thereafter his direct involvement with the research programme declined and supervision of his current students (Pease and Broers) was handed over to Nixon. On his retirement in 1971, however, Oatley again took up the research and continued to make contributions to the field well into his eighties. His book on the SEM was published in 1972 (4). The Electrical Sciences Tripos Part I of the Mechanical Sciences Tripos, introduced by Baker in 1945, was a broadly based course covering all the major branches of engineering, permitting of no specialization, and led to an honours degree if taken after three years of study. Rather more than half of all engineering students took only this part of the Tripos, only the more able students (those on the so-called ‘fast course’) proceeded to the specialized Part II. Throughout the 1950s Oatley had become increasingly of the opinion that the Part I course, by itself, no longer provided an adequate basic education for the prospective electrical engineer; moreover, he considered that the course was overcrowded and that the continuing advance of engineering science would make options

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of some kind essential. Accordingly, he put forward proposals for reforming Part I, which would allow an amount of specialization. These proposals were discussed at group meetings throughout the Department and received a good deal of support, but a final vote against them was decisive, though not overwhelming. On his appointment to the Chair of Electrical Engineering, he decided to return to the attack. Since there was strong feeling in the Department against any change in Part I of the Mechanical Sciences Tripos, he proposed the introduction of a new Part I Electrical Sciences Tripos, which would have much of its course work and examination papers in common with the Mechanical Sciences Tripos, but which would allow for additional electrical subjects. There would also be a Part II of the Electrical Sciences Tripos, in place of the existing Part II D of the Mechanical Sciences Tripos. At a meeting of the Faculty Board on 22 May 1961, it was unanimously agreed that arrangements be made to establish an Electrical Sciences Tripos at the Part II level; however, there was only a very slender majority in favour of an Electrical Sciences Tripos at the Part I level. In view of this, some compromise was clearly desirable and, after further discussion, Oatley agreed to withdraw his proposals for Part I, as long as the Part I Mechanical Sciences Tripos was modified to include an electrical option (with an alternative examination paper in electrical subjects), which would allow suYcient course time for teaching the extra electrical material he wished to include. These arrangements were finally approved at a meeting of the Faculty Board on 24 July 1961. Reflecting Oatley’s belief in the importance of practical laboratory work, one-half of the Part I electrical option paper consisted of questions based on the Part I laboratory courses; in the Electrical Sciences Tripos marks gained in the laboratory accounted for one whole paper. One of the four written papers in the Electrical Sciences Tripos consisted of questions drawn from the so-called ‘selective courses’— courses given by lecturers on topics of their own choosing—another innovative feature Oatley introduced at this juncture. Apart from providing the extra lecture time required for electrical subjects, these changes had other advantages. They gave the electrical representatives on the Faculty Board almost complete autonomy with regard to subject matter and examination for the electrical option Paper in Part I and for the Electrical Sciences Tripos. A second advantage was that, for the first time, the word ‘electrical’ appeared in the Tripos: schools no longer had any reason to believe that teaching at Cambridge was confined to mechanical subjects. Finally, after discussion with the Physics Department, it was agreed that undergraduates who had taken physics in Part I of the Natural Sciences Tripos, and whose interests were primarily in electronics, should be encouraged to change to the Engineering Department, to read for the Electrical Sciences Tripos in their third year. Similar arrangements were extended to undergraduates reading mathematics. As a result of these initiatives, the Electrical Sciences Tripos examination was introduced in 1963, and the numbers of undergraduates reading electrical subjects at Part II level rose from an average of about 24 in the Mechanical Sciences Tripos (Part II D) to about 60 in the Electrical Sciences Tripos, of whom roughly one-half had previously read natural sciences. These structural changes to the Tripos introduced by Oatley were, shortly afterwards, reinforced by the physical expansion of the Engineering Department. Baker’s

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grand post-war plan for the main building, which bears his name, on the Trumpington Street site was completed in 1964 by the addition of the North Wing (Hilken 1967); this provided spacious accommodation for a new Electrical Sciences Tripos laboratory. Originally designated simply as the EST laboratory, its name was later changed to the Oatley laboratory in honour of its founder, and his portrait, painted by Hughes-Hallet, hangs on the wall there. One whole floor of this wing was also allocated to electrical research, and all subsequent work on the SEM was conducted in this area. Three years later the new Inglis building was completed, providing accommodation for the Part I electrical teaching laboratory and additional electrical research laboratories. This expansion was underpinned by government grants totalling over 250 000. Additionally, Oatley received a grant of 50 000 from the Wolfson Foundation specifically for research under his direction, and royalties began to come in from the Cambridge Instrument Company, which had started production of the Stereoscan in 1965. These royalties, amounting to 20 000, enabled Oatley to set up the ‘Stereoscan Fund’, income from which was used mainly to seed new lines of research on the SEM, and to support some of his personal research in the field. By the time Oatley retired in 1971, the Electrical Sciences Tripos occupied a premier position among university electrical engineering courses, and Cambridge was recognized as one of the foremost schools of electronic engineering in the country. After he retired, he wrote a text on the subject of electromagnetic fields, based on a course he had given at undergraduate level (6). It fell to Oatley to eVect one more significant alteration in the way the Department was organized. Shortly before Professor Baker was due to retire, he took sabbatical leave and, in his absence, Oatley became Acting Head of Department. During this time he called a meeting of professors to discuss what organization of the Department would be appropriate when Baker left. Baker had been appointed in 1943 with the title Professor of Mechanical Sciences and Head of the Department of Engineering. At that time there was only one other professor in charge of a small subdepartment; during Baker’s tenure, the size and scope of the Department had increased about fourfold and there were now a number of professors eVectively in charge of their own groups reporting directly to him. Oatley believed this structure to be no longer appropriate and suggested two changes: first, that there should be a clearly defined divisional structure, with heads of division appointed by the Head of Department; second, the Head of Department should be appointed for a limited period of five years in the first instance, with the possibility of extension for a further five years. These changes were agreed by the other professors and by Baker when he returned from leave, and the necessary changes of Ordinance were set in train. The influence Oatley exercised in determining the direction of post-war teaching and research in the Department and its organization was second only to that of Baker. College work and other interests In his family memoir Oatley records: ‘It was Trinity College that brought me to Cambridge and my relations with the College have always been very happy ones.’ For his first ten years at Trinity he was a Teaching Fellow, supervising engineering

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undergraduates, after which he was appointed as Adviser to Research Students. This was a newly created post entailing the admission and general academic welfare of the large number of students entering the College after graduating elsewhere; it also involved social gatherings in College and entertaining at his home. He relinquished this post after a spell of five years on his appointment to the Chair of Electrical Engineering. During his 51 years as a Fellow he served on the College Council for a total of ten years, becoming in the process very much an elder statesman. He chaired numerous College and advisory committees, including the Wine Committee—a subject on which he was expert. But perhaps his most significant contribution to the College came about through his involvement in the creation by Trinity College of the Cambridge Science Park, which was stimulated by the 1969 report of the Mott Committee—a Cambridge University Report on the relationship between the University and science-based industry. Oatley, with his extensive experience spanning academia, government and industry, was able to make a major contribution to the work of this Committee whose Chairman was Sir Nevill Mott. The Committee’s principal recommendation was that there should be a moderate increase in science-based industry in Cambridge, possibly in the form of a Science Park with 15–25 units for science-based start-up firms or branches of firms located elsewhere. It further recommended that the units should be held on a leasehold basis in order to retain control of the development, the aim being to encourage innovation rather than production, and that they should be located in reasonably close proximity to University departments to foster cooperation and sharing of facilities and equipment. Avoidance of the fate of Oxford in becoming a heavily industrialized city was a prime consideration. The deliberations of this Committee concerning the appropriate forms of academic/industrial cooperation and of planning control, constituted the first analysis in the UK (or indeed anywhere outside the USA) of the basic nature and philosophy of a science park, and Oatley played a pivotal role in its formulation. The findings of the Mott Committee drew an enthusiastic response from the Senior Bursar of Trinity, J.R.G. Bradfield, who conceived the idea of creating a Cambridge Science Park on land belonging to the College on the outskirts of the city. In this he benefited much from discussions with Oatley, a close friend and colleague. Bradfield’s report to the College Council in 1970 proposing the creation of a Science Park was accepted without reservation. An essential element of his scheme was that the College should itself retain ultimate control of the development and of the nature of companies that would occupy the Park. The Science Park presently comprises some 130 acres of land occupied by 65 science-based companies with around 4000 employees. It remains the largest in the UK, and has had a profound impact on the Cambridgeshire economy. Oatley took a keen interest in the Park throughout the remainder of his life; it owes much to his seminal contributions to the deliberations of the Mott Committee and of Trinity College. Oatley’s wartime experience, coupled with the influence of Cockcroft, had led him to attach great importance to establishing and maintaining links with industry and government. For 20 years he was on the Board of Directors of the English Electric Valve Company. He also had a number of consultancies with electrical companies

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including the G.E.C. Hirst Research Centre at Wembley and Igranic Ltd, of Bedford. Immediately after the war he joined the Institution of Electrical Engineers and was active at local and national level, serving as Chairman of the Radio Section and becoming a full member of the Council of the Institution. In 1975, he was invited to deliver the 66th IEE Kelvin Lecture (5). On his election to the Royal Society in 1969, he served on the Council for two years and was also active for many years on the Paul Fund Committee. As a result of his wartime work on measurement he became the British representative for that subject on the Union Radio Scientifique Internationale, a position that took him to meetings in many countries. Although essentially a quiet and modest man of few words, Oatley was, nevertheless, an excellent communicator. Nowhere was this more evident than at the daily meetings of students and staV in the laboratory at coVee time, a tradition that extended throughout the four decades he was associated with the Engineering Department. This was when some of the most productive exchanges of ideas and opinions occurred, and when one often learned something of his personal interests and beliefs; for instance, his great knowledge and love of Britain and its countryside. He was a strong supporter of the Council for the Protection of Rural England. His holidays were spent touring and walking, or at the family cottage in Pembrokeshire, and at these morning coVee meetings he introduced many of his students to the delights of the Cairngorms, the Loch Torridon area of the Western Highlands, and less well-known corners of the English and Welsh countryside. Students and colleagues frequently enjoyed hospitality at his home in Porson Road, where he and Enid Oatley extended a warm welcome to everyone.

Honours and tributes Oatley received many honours and awards during his career, chief among which were the Royal Medal of the Royal Society, the Faraday Medal of the IEE, the Distinguished Scientist Award of the Microscopy Society of America, the Howard N. Potts Medal of the Franklin Institute and the Duddell Medal of the Institute of Physics. He was a founder member of the Royal Academy of Engineering, an Honorary Fellow of the Royal Microscopical Society and Foreign Associate of the National Academy of Engineering (USA). For his wartime work on radar he received the O.B.E. He was knighted in 1974. He was held in great esteem by his colleagues and those who worked closely with him, not least by his research students by whom he was known aVectionately as ‘uncle’—an appellation of which he was not unaware and which he regarded with wry amusement. He inspired loyalty from those who worked with him and returned it in full measure: the contribution of everyone was fully recognized, and he refused to put his name on any paper unless he himself had made a substantial contribution to its content. A symposium was held in his honour on his 80th birthday at which he presented a paper on a meticulous investigation of electron detectors for the SEM, which he had conducted after his retirement. Both at this meeting and the one held ten years later at

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Figure 2. Sir Charles Oatley with former research students and colleagues at a symposium held in his honour on the occasion of his 90th birthday. Standing (left to right): A.N. Broers, W.C. Nixon, R.F.W. Pease, T.E. Everhart, D. McMullan, K.C.A. Smith, O.C. Wells, C.W.B. Grigson, A.D.G. Stewart, P. Chang and H. Ahmed. Photographed by Kelvin Fagan (Cavendish Laboratory) and reproduced with the kind permission of Professor H. Ahmed.

Churchill College for his 90th birthday there was a large attendance, with ex-students coming from all parts of the world. He did not present a paper at the latter meeting, but at a dinner at Queens’ College in the evening he gave an absorbing and highly amusing impromptu account of the many eventful episodes in his long career. Figure 2, a photograph taken at this symposium, shows him with several of his exresearch students and colleagues who were engaged in research on the SEM in the years 1948–65. In 1990, Oatley was presented with an Honorary Doctorate of Science from the University of Cambridge. The Oration concluded with these words: ‘A scientist who, seeking no personal fame or glory, cultivated and brought forth the best in others, now in his retirement cultivates his marvellous garden in Porson Road, where the plants and flowers, like his students of old, respond to his guidance and give forth their best’ (Diggle 1994). 1956 1966 1969 1969 1969 1970 1970 1973

O.B.E. Achievement Award, Worshipful Company of Instrument Makers. Duddell Medal, Institute of Physics. Fellow of The Royal Society. Royal Medal, Royal Society. Faraday Medal, Institution of Electrical Engineers. Honorary Fellow, Royal Microscopical Society. Mullard Award, Royal Society.

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Hon. D.Sc., Herriot-Watt University. Knight Bachelor. Fellow of The Royal Academy of Engineering. Fellow King’s College, London. Hon. D.Sc., Bath University. Foreign Associate, National Academy of Engineering (USA). James Alfred Ewing Medal, Institution of Civil Engineers. Distinguished Scientist Award, Microscopy Society of America. Howard N. Potts Medal, The Franklin Institute. Hon. Sc.D, University of Cambridge.

Acknowledgements The assistance of Lady Oatley and of Mr Michael Oatley in preparing this Memoir is gratefully acknowledged. I am also indebted for contributions from the following friends, colleagues and students of Sir Charles: Mr A.W. Agar, Professor H. Ahmed, Dr J.R.G. Bradfield, Mr B.C. Breton, Mrs J. Breton, Professor Sir Alec Broers, Professor J.F. Coales, Miss J. Duffield, Professor T.E. Everhart, Mr R.L. Ferrari, Dr D. McMullan, Professor T. Mulvey, Dr W.C. Nixon, Mrs D. Peters, Professor Sir Brian Pippard, Dr L.M.V. Smith, Mrs S.V. Smith, Mr P.J. Spreadbury and Dr O.C. Wells. The frontispiece photograph, taken in 1969, is reproduced with the kind permission of the Godfrey Argent Studio. The photograph and micrograph in figure 1 are reproduced with the kind permission of Dr Dennis McMullan, in whose PhD dissertation they appear. The photograph in figure 2 was taken by Kelvin Fagan, and is reproduced with the kind permission of Professor H. Ahmed. References to other authors Cockcroft, J. D. 1985 Memories of radar research. Proc. Instn. Elect. Engrs. 132, Pt. A, 6, 327–329. Diggle, J. 1994 Cambridge orations 1982–1993. Cambridge University Press. Everhart, T. E. 1996 Persistence pays oV: Sir Charles Oatley and the scanning electron microscope. J. Vac. Sci. Technol. B14 (6), 3620–3624. Hilken, T.J.N. 1967 Engineering at Cambridge University 1783–1965. Cambridge University Press. McMullan, D. 1953 An improved scanning electron microscope for opaque specimens. Proc. Instn. Elect. Engrs. 100, Pt II, 245–259. (see Chapter 2.1B this volume.) McMullan, D. 1986 Replica of the first Cambridge SEM for the Science Museum. Proc. R. Microsc. Soc. 21, 203–206. Moullin, E. B. 1950 Presidential address. Proc. Instn Elect. Engrs. 97, Pt. I, 1–6. Jervis, P. 1971–72 Innovation in electron-optical instruments—two British case histories. Res. Policy 1, 174–207. (For extract see Appendix III this volume.) Report of the Mott Committee 1969 Relationship between the University and science-based industry. Cambridge University Reporter 22 October, 370.

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Bibliography The following publications are those referred to directly in the text. A more complete bibliography appears on the accompanying microfiche. A copy is available from the Royal Society Library at cost. (1) (37) (—) Family memoir with supplement on wartime work. Churchill College Archives, Cambridge. (2) (4) 1932 Wireless receivers. Methuen. (3) (10) 1946 Radiolocation Convention (Measurements Section). Proc. Instn. Elect. Engrs 93, Pt IIIA. (4) (27) 1972 The scanning electron microscope. Cambridge University Press. (5) (28) 1975 Sixty-sixth Kelvin Lecture. The scanning electron microscope and other electron-probe instruments. Proc. Instn. Elect. Engrs 122, 942–946. (6) (30) 1976 Electric and magnetic fields. Cambridge University Press. (7) (32) 1982 The early history of the scanning electron microscope. J. Appl. Phys. 53, R1–R13. (8) (36) 1986 The establishment of electronics in the Cambridge University Engineering Department. Departmental report: CUED/BELECT/TR74.

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Appendix II A History of the Scanning Electron Microscope, 1928–1965y D. McMULLAN Cavendish Laboratory, University of Cambridge Formerly at: Engineering Department, University of Cambridge

I. Introduction From the very beginning of electron microscopy the imaging of solid samples was an important goal, particularly as the methods for producing thin samples were only developed later. The first attempt was by Ernst Ruska (1933), with the sample surface normal to the viewing direction and illumination by an electron beam at grazing incidence to the surface; he obtained images of copper and gold surfaces but at a magnification of only 10 times. A few years later he made a second attempt (Ruska and Mu¨ller, 1940) with the same geometry and with only marginally better results. Bodo von Borries (1940) was much more successful with his grazing incidence method in the transmission electron microscope, where the sample surface is at a few degrees both to the viewing direction and to the illuminating beam. A breakthrough in the microscopic imaging of surface topography in the TEM was the introduction of replicas by H. Mahl (1941) and these set the standard for the next 25 years, although they were tedious to make and could be subject to serious artefacts. An example is shown in Fig. 1. During the 1930s a very diVerent way of imaging solid samples—scanning electron microscopy—was invented by Max Knoll (1935) for the study of the targets of television camera tubes. Two years later Manfred von Ardenne (1938a,b) built an electron microscope with a highly demagnified probe for scanning transmission electron microscopy and also tried it as a SEM; he was followed soon afterwards by V. K. Zworykin et al. (1942a), who developed a dedicated SEM. The beginning of the general use of the SEM can be accurately dated to 1965 when the Cambridge Instrument Company in the United Kingdom marketed their Stereoscan 1 SEM (to be followed about six months later by JEOL in Japan). This was thirty years after the y

Based on ‘Scanning electron microscopy 1928–1965,’ Scanning 17, 175–185 (1995). 523 Copyright 2004, Elsevier Inc. All rights reserved. ISSN 1076-5670/04

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Figure 1. TEM image of an early oxide replica of etched aluminium (Mahl, 1941); horizontal field width ¼ 9 mm.

initial developments in Germany and the United States, but it was the research project started in 1948 by Charles Oatley at the Cambridge University Engineering Department that led directly to the Stereoscan (Oatley, 1982; Oatley et al., 1985). The purpose of this Appendix is to trace the development of the SEM up to the sale of the first commercial instruments in 1965, but excluding the details of the developments in Charles Oatley’s laboratory 1948–1965, which are fully covered in Part II of this volume. Incidentally, it will be seen that many of the ideas put forward by the early workers were well ahead of their time, becoming technologically practicable only much later. A. Invention of Scanning The 1928 date in the title of this Appendix is somewhat arbitrary and it was chosen because the first mention of scanning applied to microscopy was made in that year. But it is relevant to start nearly 100 years earlier with the invention by Alexander Bain, a Scottish clockmaker, of the principle of dissecting an image by scanning, and the granting of a British patent (Bain, 1843) for the first fax machine (McMullan, 1990). At the transmitter, a stylus mounted on a pendulum contacts the surface of metal type forming the message, thus closing an electrical circuit, and at the receiver a similar stylus, also on a pendulum, records electrochemically on dampened paper.

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Following each swing of the pendulums, the type and the recording paper are lowered one line; means for starting the pendulums swinging simultaneously and synchronizing magnetically are described in the patent. B. Scanning Optical Microscopy The first proposal in print for applying scanning to microscopy was made by Synge (1928), in Dublin. This was for a scanned optical microscope and his aim was to overcome the Abbe limit on resolution by what is now called ‘near-field microscopy,’ that is the production of a very small light probe by collimation through an aperture smaller than the wavelength of the light. Synge was a scientific dilettante who had original ideas in several scientific fields but did not attempt to put them into practice (McMullan, 1990). However, he considered some of the problems that would be encountered with a scanning microscope and he proposed the use of piezoelectric actuators (Synge, 1932), as are now used with great success in the scanning tunnelling microscope and other probe instruments, including of course the near-field optical microscope itself. He envisaged fast scanning of the sample so that a visible image could be displayed on a phosphor screen, and he also pointed out the possibility of contrast expansion to enhance the image from a low-contrast sample—probably the first mention of image processing by electronic means, as distinct from photographic. C. Charged Particle Beams A proposal for using an electron beam in a scanning instrument was described by Stintzing (1929), of Giessen University, in German patents. These patents were concerned with the automatic detection, sizing and counting of particles using a light beam or, for those of sub-light microscopic size, a beam of electrons. Focusing of electrons was at that date unknown to him, as to most others, and he proposed obtaining a small-diameter probe by using crossed slits. The sample was to be mechanically scanned in the case of a light beam, and electric or magnetic fields would deflect an electron beam. Suitable detectors were to be used to detect the transmitted beam, which would have been attenuated by absorption or scattering. The output was to be recorded on a chart recorder so that the linear dimension of a particle would be given by the width of a deflection and the thickness by the amplitude; the production of a two-dimensional image was not suggested. Stintzing did not apparently attempt the construction of this instrument and there are no drawings accompanying the patent specifications.

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II. Knoll’s Electron Beam Scanner Knoll, the co-inventor of the TEM with Ruska, was the first to publish images from solid samples obtained by scanning an electron beam (Knoll, 1935). In 1932, very soon after the building of the first TEM at the Berlin Technische Hochschule, he had moved to the Telefunken Company to work on television camera tubes. There he developed an electron-beam scanner for studying the targets of these tubes: the sample was mounted at one end of a sealed-oV glass tube (Fig. 2) and an electron gun at the other with an accelerating potential in the range 500–4000 V. The electron beam was focused on the surface of the sample and scanned by deflection coils in a raster of 200 lines and 50 frames/s. The current collected by the sample (the diVerence of the incident and secondary emitted currents) was amplified by a thermionic tube amplifier and the resulting signal was used to intensitymodulate a cathode-ray tube that was scanned by deflection coils connected in series with those on the electron-beam scanner. By changing the ratio of the scan amplitudes, the magnification could be varied, a principle demonstrated by Zworykin (1934, 1942a) on an optical microscope fitted with a TV camera. Knoll used unity magnification most of the time but he could increase it to about 10 times before the resolution was limited by the diameter of the scanning probe.

Figure 2. Schematic diagram of Knoll’s (1935) electron-beam scanner. (Labels translated by T. Mulvey.)

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This apparatus had virtually all the features of an SEM but, surprisingly in view of his earlier work on the TEM, Knoll did not use additional electron lenses to reduce the size of the probe below 100 mm; however, the resolution he obtained was entirely adequate for his purpose. The beam current was relatively high, of the order of microamps, and therefore thermionic tubes could be used to amplify the signal current in spite of the fast scan rate. Similar images were produced by others working on the development of TV cameras in the 1930s (including von Ardenne, 1985), but Knoll was the only one at the time who looked at samples other than camera tube targets, for example silicon iron (Fig. 3), and he also elucidated the contrast mechanisms: secondary electron coeYcient and topography. The images were true secondary electron images because the electron gun and sample were enclosed in the highly evacuated and baked glass envelope and there was therefore little or no contamination of the surface. It is only comparatively recently that UHV SEMs have been available that can work in this imaging regime. Knoll continued using his electron beam scanner, which he named ‘der Elektronenabtaster,’ for a number of purposes including the study of oxide layers on metals (Knoll, 1941).

Figure 3. Electron-beam scanner image of silicon iron showing electron channelling contrast; horizontal field width ¼ 50 mm. (Knoll, 1935.)

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III. Von Ardenne’s Scanning Electron Microscope The first scanning electron microscope with a sub-micrometre probe was developed by von Ardenne, a private consultant who had his own laboratory in Berlin, over the very short period of about two years; he also had had experience in the development of TV camera tubes (von Ardenne, 1985). In 1936 he was contracted by Siemens and Halske AG to investigate the possibility of using a scanned electron probe to avoid the eVects of objective lens chromatic aberration with thick samples in TEM. In the course of this work he laid the foundations of electron probe microscopy by making and publishing a detailed analysis of the design and performance of probe-forming electron optics using magnetic lenses (von Ardenne, 1938a,b). The analysis covered the limitations on probe diameter due to lens aberrations and the calculation of the current in the probe. He also showed how detectors should be placed for bright-field and dark-field STEM and for imaging a solid sample in a SEM, and considered the eVects of beam and amplifier noise on imaging. To fulfil the Siemens contract, he built the first STEM and demonstrated the formation of probes down to 4 nm diameter. But in the short time available he was limited to employing existing technology, and because there was no suitable low-noise electronic detector he used photographic film; consequently there was no immediately visible image. A schematic of the microscope column is shown in Fig. 4: a demagnified image of the crossover of the electron gun was focused on the sample by two magnetic lenses, and X–Y deflection coils were mounted just above the second of these. Immediately below the sample was a drum around which was wrapped the photographic film. The image was recorded by rotating the drum and simultaneously moving it laterally by means of a screw, while the currents in the deflection coils were controlled by potentiometers mechanically coupled to the drum mechanism. The intensity of the beam was very low (about 10 3 A) and it was necessary to record the image over a period of about 20 minutes. Since the image was not visible until the film had been developed, focusing could only be accomplished indirectly by using the stationary probe to produce a shadow image of a small area of the sample on a single-crystal ZnS screen, which was observed through an optical microscope and prism system. The recordings were inferior to those from the TEM that was being constructed by Ruska and von Borries at Siemens, and the hoped-for advantages of STEM with thick samples were not fulfilled. He spent a short time trying to use the instrument in the SEM mode on bulk samples, but only low-resolution images could be obtained because of the detector problem: the sample current was amplified by thermionic tubes and a large probe current was needed. He did not publish any images.

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Figure 4. Cross section of the column of von Ardenne’s (1938b) STEM. (‘Strahlerzeugungssystem’ ¼ electron gun; ‘Verkleinerungsoptik’ ¼ reducing lens; ‘magnet. Ablenksystem’ ¼ deflection coils; ‘Objekt’ ¼ sample; ‘Registriertrommel’ ¼ film recording drum.)

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In total, von Ardenne worked for less than two years on scanning electron microscopy before concentrating on the development of his universal TEM (von Ardenne, 1985), and then, with the start of the war, on building a cyclotron and isotope separator for nuclear energy projects. If he had been able to continue, there is little doubt that he would have built an eYcient SEM within a year or two: this is evidenced by a patent (von Ardenne, 1937) that included a proposal for double-deflection scanning, two papers (von Ardenne, 1938a,b) and a book (von Ardenne, 1940). Two of the chapters in the book were on scanning microscopy and were based on the 1938 papers but included additional material relating to imaging the surfaces of solid samples. Most importantly, he proposed a detector using an electron multiplier with beryllium copper dynodes (see Fig. 5) that could be opened to the atmosphere and would work eYciently under poor vacuum conditions. Measurements of the secondary emitting ratio of beryllium copper and its stability when exposed to the atmosphere were first reported only in 1942 by Matthes (1942) of the AEG Research Institute in Berlin, but von Ardenne was probably aware of this research a year or two before. In his book he also discussed the interaction between the beam electrons and the sample and suggested that back scattering would cause a loss of resolution, illustrating this with a diagram that has a quite modern look (Fig. 6). He argued that the incident beam electrons produce secondary electrons at or near the surface from an area approximately equal to the beam

Figure 5. Electron multiplier with beryllium copper dynodes proposed by von Ardenne (1940) as a secondary electron detector for an SEM. The drawing shows the first three stages of the multiplier and its position relative to the objective lens and sample.

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Figure 6. Diagram illustrating von Ardenne’s (1940) discussion of secondary electron imaging of a surface.

diameter and give a high-resolution image (‘nutzbare Strahlung’); the beam electrons then penetrate the sample and a proportion of them are backscattered and reach the surface where they produce further secondaries. These two signals are now generally referred to as SE-I and SE-II respectively (Drescher et al., 1970; Peters, 1982). The back-scattered electrons are emitted from an area of diameter comparable with the penetration depth, and the secondaries they produce (‘scha¨dliche Strahlung’) may impair the resolution; however, he did not consider the case of a sample with small inclusions below the surface. He concluded that good resolution might be obtained either with a very low-energy beam (1 keV), or with one having a high energy (50 keV). In the first case the back-scattered electrons would emerge from an area of the surface little larger than the incident beam and the resolution would be unaVected. On the other hand, with a 50-keV beam the secondary electrons would be produced by the back-scattered electrons over a very much larger area; moreover, they would be evenly distributed so that their main eVect would be to increase the background (reduce the contrast) rather than to aVect the resolution.

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Von Ardenne’s scanning microscope was destroyed in an air raid on Berlin in 1944, and after the war he did not resume his work in electron microscopy but researched in other fields: first in Russia and from 1955 in Dresden, which was then in the DDR. Additional information about von Ardenne’s scientific work is given in his autobiography (von Ardenne, 1972) and by McMullan (1988). IV. The RCA Scanning Electron Microscope Meanwhile in America, V. K. Zworykin, Director of Research at the RCA Camden, New Jersey, laboratories, had in 1938 initiated a development programme on SEM (Zworykin et al., 1942a, 1945) that continued until about 1942. This work was done in parallel with the development of a TEM and by the same staV, in particular Hillier, Ramberg, Vance and Snyder as well as by Zworykin himself. Although Zworykin had every microscope paper from Germany translated as soon as it was received (Reisner, 1989), he was apparently not influenced by von Ardenne’s work on the SEM. Instead he started by, in eVect, repeating Knoll’s beam scanner experiments using a ‘Monoscope’: this was a pattern-generating cathode-ray tube that had been developed by RCA for television use (Burnett, 1938) and was very similar to Knoll’s apparatus. He then built an SEM based on the Monoscope but with two magnetic lenses to produce a very small focused probe, and a demountable vacuum system so that the sample could be changed (Zworykin et al., 1942a). The scan rate was the US TV standard, 441 lines and 30 frames/s, and the signal was amplified by a thermionic tube video amplifier. For a signal-to-noise ratio of 10 the signal current had to be 3  10 8 A, which could only be reached if the probe diameter was at least 1 mm. He next tried to obtain a high current in a smaller probe by the use of a field-emission gun with a single-crystal tungsten point (Zworykin et al., 1942a), presumably based on experience with the point projection microscope that had been built in the RCA Laboratories by Morton and Ramberg (1939). To achieve a suYciently high vacuum he had to return to having the gun and the sample in a glass envelope, which was baked and sealed oV. A single magnetic lens was used and fleeting images were obtained at 8000 times magnification with scanning at TV rate and a thermionic tube amplifier. Stable images could no doubt have been achieved, but at that time a practical microscope would not have resulted because demountable UHV techniques had not yet been developed. To overcome the noise problem, Zworykin therefore decided to build an SEM with an eYcient electron detector and a slow scan. The detector was

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the combination of phosphor and photomultiplier that Everhart and Thornley used nearly twenty years later in an improved form. To bring the secondary electrons to it, he designed an electrostatic immersion lens that retarded the beam electrons and accelerated the secondaries. Figure 7 shows the final electron-optical arrangement: electrostatic lenses were used to produce a demagnified image of the source on the sample, which was held at þ800 V relative to the grounded gun cathode. The electron beam leaving the gun was accelerated to 10 keV in the intervening electron optics. The accelerated secondary electrons diverged as they passed through the fourth electrostatic lens and hit the phosphor screen with an energy of 9.2 keV.

Figure 7. The electron optics of the SEM built by Zworykin et al. (1942a).

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In the first instrument the scanning was done by moving the sample relative to the beam electromechanically using loudspeaker voice coils and later hydraulic actuators; it was only in the final version that magnetic scanning of the beam was employed. The scan time was fixed at 10 minutes by the facsimile recorder that was used for image recording and which also controlled the microscope scans. There was no provision for a faster scan and the production of a visible image on a TV monitor; this seems strange remembering Zworykin’s TV background, but it may have been because the signal bandwidth was limited by the decay time of the phosphor, as was found in later work at Cambridge (McMullan, 1952, and see Chapter 2.1 in this volume). The focus setting was found by maximizing the high-frequency components in the video waveform observed on an oscilloscope, a method that was originally proposed by von Ardenne (1938b). Although the intention was to produce contrast by diVerences in the secondary emission ratio of the surface constituents of a polished specimen, and the incident beam energy of 800 eV was chosen with this in mind, contamination of the surface in the rather poor vacuum prevented meaningful compositional contrast being obtained. It is strange that Zworykin did not foresee this since he was an experienced vacuum physicist and only two years earlier secondary emission measurements and the eVects of contamination had been discussed by Bruining and deBoer (1938). Actually, all of Zworykin’s published micrographs were of etched or abraded samples, and contrast was clearly topographic (Zworykin et al., 1942b), for example in etched brass (Fig. 8). The quality of the recorded images was rather disappointing and, together with the lack of a visible image, must have been a factor in RCA deciding to discontinue the project. Another reason was undoubtedly the excellent results that were, as mentioned in the introduction, being obtained with replicas in TEMs. In the event, all available technical eVort had to be directed to the highly successful RCA EMB TEM, which was then coming into production (Reisner, 1989). Apart from a theoretical analysis of resolving power by a French author (Brachet, 1946), no other work on the SEM had been reported by 1948; but see Section VI.A later in this Appendix.

V. The Cambridge Scanning Electron Microscopes: Historical Background In 1948, as he himself describes in Chapter 1.2, Charles Oatley at the Engineering Laboratories of Cambridge University decided that another look at the SEM might be worthwhile: he was very impressed by von

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Figure 8. Micrograph of etched brass produced by the SEM of Zworykin et al. (1942a); horizontal field width ¼ 18 mm.

Ardenne’s 1938 papers and he did not subscribe to the general feeling among electron microscopists that the RCA SEM was not worth further consideration because, if such an experienced team was unsuccessful, it was very unlikely that anyone else could produce an eVective instrument. A notable exception to this general opinion was Denis Gabor, who devoted a chapter to the scanning electron microscope in his book The Electron Microscope (Gabor, 1946). In this he described the RCA instrument, and was clearly impressed with its potential. Evidence that Oatley was almost alone in recognizing the significance of this early work may be gathered from the proceedings of a conference on ‘Metallurgical Applications of the Electron Microscope,’ held in London in 1949 under the auspices of the Institute of Metals. This conference brought together for the first time since the war many of the leading workers in the field from Europe and the United States. The conference commenced with three review papers presented by N. P. Allen, P. Grivet and H. Mahl. These contained a comprehensive account of all of the known techniques of electron microscopy and their potential for solving metallurgical problems. As might be expected, much of the material in these papers concerned various aspects of the replica technique, then the most widely used method for the examination of metallurgical specimens; but in a section devoted to ‘direct methods’ Allen (1949) describes the reflection method and the

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principle of the SEM, citing the work of Knoll and of Zworykin, Hillier and Snyder. He remarks that ‘the instrument of Zworykin et al. achieved a ˚ , which does not reach the standard attainable with resolution of 500 A replicas, and was in addition complicated and expensive.’ He ends with the cryptic comment: ‘This work is in abeyance.’ Although Allen was familiar with von Ardenne’s book on the electron microscope, since he uses a figure from it to illustrate his paper, he does not mention von Ardenne’s work on the SEM, and was evidently unaware that Oatley and McMullan were intending to build one at Cambridge. Grivet (1949) in his paper describes both the direct reflection method and the scanning principle, but it is clear from his remarks that he sees the potential of the electron probe in terms primarily of an X-ray microanalytical technique: The device [SEM] suVers only from a lack of intensity, which prohibits the use in practice of as high a magnification as seems possible from the theoretical resolving power. Nevertheless, at low magnification (10 or 100 times) the device gave very interesting results as regards the properties of secondary electron emitters, and contributed to the clarification of the complex problem of high-emission alloys. An original version of the scanning principle is being studied at the present time in France by Guinier and Castaing, who have shown that the intensity of X-rays produced in the sample is suYcient to give a current in an ionization chamber. They propose to use this current instead of the secondary emission current in the scanning microscope technique.

Grivet’s assertion that the SEM would be useful for investigation of secondary emission at magnifications of the order of 10–100 times, seems rather wide of the mark in the light of present-day SEM performance! But it is symptomatic of how even well-known authorities on electron microscopy were ignorant of what had been accomplished by von Ardenne and at RCA 10 years before; Grivet was of course quoting from Knoll’s results using an electron beam scanner that was not provided with a short-focus lens. Mahl’s paper (1949) is concerned almost entirely with variants of the replica techniques and their application, the exception being a short reference to the direct reflection method, about which he comments: Finally, mention must be made of the oblique-reflection technique in which the metal surface is reproduced directly by reflected electrons. To obtain an exact picture the angle between the beam and the surface of the object must be very small, of the order of four degrees; the picture therefore shows a pronounced foreshortening (more than 10 times) in one direction. The method is therefore less suitable for the investigation of microstructures. On the other hand, it proves very sensitive for the detection of small surface irregularities, the height of which can be determined from the lengths of the shadows.

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Mahl must have been aware of von Ardenne’s papers and of his book, published in 1940, so it is apparent that he had dismissed the scanning technique to the extent that it was not even worthy of mention. However, it is of interest to note that the first images produced with a SEM at Cambridge (see Chapter 2.1A) were with a metal specimen at an angle to the electron beam, a configuration directly influenced by the reflection microscope, although with a much large angle (30 ). The very full discussion of these three review papers was led by V. E. Cosslett (1949) and his comments are revealing. It [the electron microscope] can give very high resolution, much better than that of the optical microscope, and also much better than the available techniques of preparation permit. It cannot do what most metallurgists would like it to do, namely, give a direct picture of the metal surface. Replica methods are necessary, and unfortunately these are not always easy to apply, and they introduce all sorts of disturbances themselves, such as strains, artefacts, and breakages, and in addition a self-limitation of resolution, arising mainly from the size of the molecules of the replica.

Cosslett then goes on to say: Before leaving the machine, I should like to stress a point that is mentioned in Dr Mahl’s paper but not elaborated, namely that the reflection electron microscope is not without hope. It is possible to get pictures from a solid block of metal which are interpretable and which give resolutions better than the optical microscope. Relief of the surface stands out very much better than in most replica methods. This early work of von Borries was promising, if limited in resolution and range of applicability. The work is worth taking up again, as we are doing at Cambridge.

It is significant that von Ardenne’s work on the SEM is not mentioned either in the review papers or in the discussion. This may be attributed to the fact that he did not actually produce any images, the main reason being that he did not have a suitable detector. As mentioned earlier, he described in his book a possible detector, essentially a demountable electron multiplier developed in another Berlin laboratory (Matthes, 1942); similar ones were produced by Baxter (1949) in the Cavendish Laboratory some years later, and obtained by Oatley for the construction of the first SEM in the CUED. Although Cosslett referred only to the work on the reflection microscope at Cambridge, he was probably aware of Oatley’s intention to work on the SEM, but this was at too early a stage to mention. Three years later he wrote a report on a meeting in Bristol organized by the Institute of Physics at which McMullan gave a talk on the progress of SEM development in the CUED. Cosslett (1952) wrote ‘The new instrument is a great advance both in

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resolution and lack of damage to the specimen.’ (Further details about the meeting are given in Chapter 2.1A of this volume.) To sum up, it must be concluded from the proceedings of the 1949 conference that the SEM was then regarded as being irrelevant to the field of metallurgy. Moreover, it is clear that the majority, if not all, of those present were convinced that the idea of the SEM was not worth pursuing. Any conference held in the 1940s on biological applications of the electron microscope would undoubtedly have arrived at a similar conclusion. At that time microscopists were concerned with resolution to the exclusion of almost ˚ and appeared every other consideration; a technique that oVered only 500 A to be overly complex stood little chance of gaining acceptance. Notwithstanding the micrographs published by Zworykin et al., there appeared to be no conception that the SEM oVered a means of revealing surface structure directly, with all its attendant advantages; instead, the only serious alternative to replicas considered was the reflection microscope, a technique that has been relatively little used. The full story of the development of the SEM in Oatley’s group in the CUED is told in Part II of this volume, which is followed in Parts III and IV that cover the Scanning Microprobe Analyser at the Cavendish Laboratory and the manufacture and marketing of both by the Cambridge Instrument Company.

VI. Other SEMs Developed Outside Germany and the United Kingdom up to 1965 SEM developments in other laboratories up to 1965, as evidenced in scientific publications, included the following. A. In France During the 1939–45 war some preliminary experiments on scanning electron microscopy were carried out by Andre´ Le´aute´ at the Ecole Polytechnique in Paris. These were reported at a meeting in Paris, organised in 1945 by Louis de Broglie and intended to present to the world the work on electron microscopy and related instruments that had been accomplished in France during the wartime years. The paper by Le´aute´ (1946) appears to have been overlooked ever since, but it was discovered and reported by Peter Hawkes (Personal communication) following the 80th anniversary Celebration (2003) in Paris of de Broglie’s theory of the wavelength of charged particles.

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Le´aute´’s experiments on SEM were at a very elementary level. At the time, it seemed useful to employ a fine probe, scanned over the specimen, in order to obtain a transmission image from specimens too thick to yield an image in the TEM mode. His instrument was therefore conceived as a scanning transmission microscope; it did not occur to Le´aute´ to collect a signal in reflection. Even so, it is of historical interest and is discussed in a forthcoming publication (Hawkes and McMullan, 2004). In 1946 a theoretical paper on the resolving power of the SEM was presented to a scientific meeting in Paris by C. Brachet (1946), a Principal Engineer in Naval Artillery. He was hopeful that his calculations would lead to a better understanding of the conditions for optimising the performance of a SEM and said that some experimental work towards this end was in progress. This was apparently a reference to Le´aute´, but there were no further details and nothing more was published then or later. An SEM was built in Lyon by Bernard and Davoine (1957) at the National Institute of Applied Science: it had a probe size of the order of 1 mm and was used over a period of years mainly for cathodoluminescence studies (Davoine et al., 1961). B. In the United States In the early 1960s, at the Westinghouse Laboratories in Pittsburgh, Pennsylvania, an advanced SEM was built for semiconductor studies and microfabrication. Two of the senior members of the team, Oliver Wells and Tom Everhart, had been in Oatley’s laboratory; and the microscope worked well, as might be expected. One of the uses of the microscope that were successfully demonstrated was the automatic positioning of the gate electrode of a field-eVect transistor (Wells et al., 1965). The SEM was named the MicroScan and a description of it was published at the time in The Engineer (American Editor, 1965), but for reasons that are now unclear, little further was heard about it. C. In the USSR In the then USSR there was an SEM at Moscow University from about 1960 (Kushnir et al., 1961, 1963). This instrument (Fig. 9) was designed for both microscopy and microanalysis and used two magnetic lenses, with stigmator, to focus the beam. An electron multiplier with beryllium copper dynodes was used as the beam detector. It appears from the publications that the emphasis was on microanalysis and the smallest probe diameter expected to be useful was 0.1 mm, allowing a magnification of 5000. No

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Figure 9. Block diagram of the Russian scanning electron microscope and X-ray microanalyser (Kushnir et al., 1963): 1, high-voltage supply; 2, lens power supply; 3, preamplifier; 4, 5 and 6, diVractometer recording equipment; 7, scan unit; 8, video amplifier assembly; K1 and K3, CR tubes for visual observation; K2, tube for photographic recording; EG, electron gun; L1, first lens; L2, second lens; DS, reflecting system; O, object; K, analyser crystal; C, gas counter; SEM, secondary electron multiplier.

doubt a year or two later they realized the possibilities of working at higher magnifications. D. In Japan The electron microscope company, JEOL, started the development of an electron probe microanalyser in 1958 and marketed it as the JXA-3 in 1962. At about that date they decided also to produce a SEM, which reached the market in 1966 (Fujita, 1986). Recent evidence (M. Kawasaki and O. C. Wells, Personal communications) shows that that their decision was influenced by SEM3, built by K. C. A. Smith in Oatley’s Group at CUED for the Pulp and Paper Research Institute of Canada (PPRIC), in Montreal, (see Chapter 2.2A), and which was installed there by Smith in 1958–59. In 1962, about two years after Smith returned to England, a JEOL JEM6A transmission electron microscope (TEM) was installed in PPRIC by Mr Yoshio Noguchi, a JEOL engineer from Tokyo. In 2004 he was interviewed by Dr Masahiro Kawasaki of JEOL and said:

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When the JEM-6A was being commissioned various specimens provided by the Institute were looked at, including an attempt to obtain a reflection image from a carbon-coated pulp sample. Not surprisingly it was far from satisfactory.

Noguchi was surprised when one of the Institute staV, Owen Washburn, showed him the image produced by SEM3 from a similar pulp sample (J. G. Buchanan, Personal communication). He had no previous experience of what could be accomplished with a SEM because at that time SEM3 was the only scanning electron microscope in North America, apart from the experimental one at Westinghouse in the US (see paragraph above: In the United States). There were certainly none in Japan. Noguchi continued . . .. . . we were shown the SEM images of the pulp surface that had been obtained with Dr Smith’s SEM. The SEM images that we saw for the first time were clear, sharp, and solid. We had never seen anything like that before. Some of the images, especially those in which numerous fibres intertwined in complicated ways, were truly artistic and impressive. I can still picture them. Needless to say, I spent that night, well into the small hours, writing to my superior to say that Cambridge had installed their first SEM unit at Pulp and Paper in North America, and that the system produced superior results. I immediately sent my report with SEM images to JEOL Ltd. Later, I accompanied Dr Kimoto when he, upon reading the report, came to investigate the SEM . . ..

Kawasaki has also interviewed Dr Y. Kimoto and reports: He was responsible for the X-ray microprobe analyser (XMA) development project that had been started in late 1958. He had almost completed it when he went to see the SEM in PPRIC after receiving Mr. Noguchi’s report. He had not expected to see such a good SEM resolution because of his experience with a probe size of about 1 mm in his XMA which is similar to the resolution of a light microscope. The image he saw on SEM3 thoroughly changed his mind about whether a SEM would be able to show a surface structure at high resolution. Soon after the completion of the XMA, a request for a SEM was received from Dr Watanabe of Musashino Research Institute of Telecommunication, Nippon Telegraph and Telephone Co. (now NTT). This gave Dr Kimoto the opportunity to develop a SEM, based on his XMA, for observing the secondary electron contrast that occurs when an electrical potential is applied between the diVerent parts of the specimen surface. The resolution was at the 1-mm level but this was enough for that project. The development of the JSM-1 then started, and was completed in 1966.

The JEOL JSM-1 was marketed about six months after the Cambridge Instruments Company Stereoscan I. It was at that time the only SEM manufactured in Japan and a few years elapsed before there was a competitor.

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VII. Electron Probe X-Ray Microanalysers The salient events in the development of the scanning electron probe X-ray microanalyser will now be briefly summarized. Originally proposed by Hillier in 1947, a static probe instrument was developed by Castaing and Guinier in Paris in 1949 and marketed by the French firm Cameca in 1956. It is interesting to note that at the Institute of Metals 1949 London Conference, Grivet reported (in the Discussion) some results obtained by Castaing: Another idea, due to Guinier, which is being tried by Castaing, is to direct an extremely fine electron beam (dia. 1/2 mm) on to a specimen, and to observe the secondary X-rays. By traversing the specimen, X-rays emitted by the diVerent phases on which the beam impinges are radiated in succession. The figure [published in the Discussion] shows the X-ray intensities as a specimen of cast iron was traversed past the electron beam.

However, in fact, Castaing did not include a scanning facility in his microanalysers until many years later. As described in Part III of this volume, Cosslett (Chapter 3.1) at the Cavendish Laboratory in Cambridge started a research programme in 1953 that led two years later to a scanning microanalyser built by Duncumb (see Chapters 3.1 and 3.3A). This was further developed at the Tube Investments Research laboratories near Cambridge and was marketed as the Microscan by the Cambridge Instrument Company from 1960 until 1964 when it was superseded by the Geoscan (Chapter 4.2A). During the same period, AEI in the United Kingdom also developed and marketed X-ray microanalysers, while in the United States microanalysers were developed by Birks and Brooks (1957) among others. Firms in several countries were marketing microanalysers by 1965; the probe sizes were generally around 1 mm and electron imaging was only an adjunct. A few years later SEMs, were being equipped with the newly introduced energy-dispersive silicon diode spectrometers and X-ray microanalysis on samples in a SEM quickly became routine.

Acknowledgements I would like to thank Ken Smith for drawing my attention to the Institute of Metals Conference on the ‘Metallurgical Applications of the Electron Microscope’, held in London in 1949, at which N. P. Allen, P. Grivet and other well-known physicists expressed their views on whether the SEM would be of use in metallurgy, and for writing the relevant part of this Appendix.

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Thanks are due to those engineers at JEOL who helped with information about the early work on SEM in the company and particularly to Dr Masahiro Kawasaki who interviewed them, and to JEOL for permission to publish. Much of this Appendix is based on a talk I gave at the 51st Annual Meeting of the Microscopy Society of America in Cincinnati, Ohio, 1–6 August 1993, published in Scanning (McMullan, 1995).

References Allen, N. P. (1949). The application of the electron microscope in metallography. London, 1949, pp. 1–18. American Editor. (1965). The American scene: Scanning electron microscope for inspection of semiconductor microcircuits. The Engineer (March 12), 492–495. Bain, A. (1843). Electric time pieces and telegraphs. British Patent No. 9745, filed 27 May 1843.. Baxter, A. S. (1949). Detection and analysis of low-energy disintegration particles. Ph.D. Dissertation, University of Cambridge. Bernard, R., and Davoine, F. (1957). The scanning electron microscope [in French]. Ann. Univ. Lyon Sci. Sect. B[3] 10, 78–86. Brachet, C. (1946). Note on the resolution of the scanning electron microscope [in French]. Bull. Assoc. Tech. Marit. Aeronaut. 45, 369–378. Birks, L. S., and Brooks, E. J. (1957). An electron probe X-ray microanalyzer. Rev. Sci. Instrum. 28, 709–712. Bruining, H., and deBoer, J. H. (1938). Secondary emission. Physica (Amsterdam) 5, 17–30. Burnett, C. E. (1938). The Monoscope. RCA Review 2, 414–420. Cosslett, V. E. (1949). General Discussion. London, 1949, pp. 139–144. Cosslett, V. E. (1952). Electron microscopy of solid surfaces. Nature 170, 861–863. Davoine, F., Bernard, R., and Pinard, P. (1961). Fluorescence of alkali halides observed with a scanning electron microscope [in French]. Delft, 1960, pp. 165–168. Drescher, H., Reimer, L., and Seidel, H. (1970). Back-scattering coeYcient and secondary electron yield from 10–100 keV electrons in the scanning electron microscope. Z. Angew. Phys. 29, 331. Fujita, H. (1986). The history of electron microscopes. Published in commemoration of the 11th International Congress on Electron Microscopy, Kyoto, Japan, by the Japanese Society of Microscopy, Tokyo. pp. 187–193. Gabor, D. (1946). ‘The Electron Microscope’ London: Hulton Press. Grivet, P. (1949). Electron microscopy in metallurgy. London, 1949 19–36. Hawkes, P. W., and McMullan, D. (2004). A forgotten French scanning electron microscope and a forgotten text on electron optics. Proc. R. Microsc. Soc. (to be published Dec. 2004). Knoll, M. (1935). Static potential and secondary emission of bodies under electron irradiation [in German]. Z. Tech. Phys. 16, 467–475. Knoll, M. (1941). Detection of attached oxide layers on iron with the scanning electron microscope [in German]. Phys. Z. 42, 120–122. Kushnir, Yu. M., Fetisov, D. V., Raspletin, K. K. et al. (1961). Scanning electron microscope and X-ray microanalyser. Bull. Acad. Sci. USSR. Phys. Ser. (Engl. Transl). 25, 709–714.

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Kushnir, Yu. M., Fetisov, D. V., Der-Shvarts, G. V. et al. (1963). Scanning electron microscope—X-ray microanalyser with magnetic electron optics. Bull. Acad. Sci. USSR. Phys. Ser. (Engl. Transl). 27, 1146–1151. Le´aute´, L.(sic) (1946). Applications of the electron microscope to metallurgy (in French), in L’Optique Electronique, Re´unions d’Etudes et de Mises au Point, edited by L. de Broglie. Editions de la Revue d’Optique, Paris 1946. pp. 209–220. Mahl, H. (1941). On the plastic replica for electron microscopic investigation of surfaces [in German]. Z. Tech. Phys. 22, 33–38. Mahl, H. (1949). The use of the electron microscope in metallurgical research in Germany during and since the war. London, 1949, pp. 37–41. Matthes, I. (1942). Investigation of the secondary electron emission from various alloys [in German]. Z. Tech. Phys. 22, 232–236. McMullan, D. (1952). ‘Investigations relating to the design of electron microscopes.’ Ph.D. Dissertation, Cambridge University. McMullan, D. (1988). Von Ardenne and the scanning electron microscope. Proc. R. Microsc. Soc. 23, 283–288. McMullan, D. (1990). The prehistory of scanned image microscopy, Part 1: scanned optical microscopes. Proc. R. Microsc. Soc. 25, 127–131. McMullan, D. (1995). Scanning electron microscopy 1928–1965. Scanning 17, 175–185. Morton, G. A., and Ramberg, E. G. (1939). Point projector electron microscope. Phys. Rev. 56, 705. Oatley, C. W. (1982). The early history of the scanning electron microscope. J. Appl. Phys. 53, R1–R13. Oatley, C. W., McMullan, D., and Smith, K. C. A. (1985). The development of the scanning electron microscope, in ‘The Beginnings of Electron Microscopy.’ edited by P. W. Hawkes. Adv. Electron. Electron Phys., Supplement 16, pp. 443–482. Peters, K.-R. (1982). Generation, collection and properties of an SE-1 enriched signal suitable for high resolution SEM on bulk specimens, in ‘Electron Beam Interactions with Solids’. Chicago: SEM (Inc), pp. 363–372. Reisner, J. H. (1989). An early history of the electron microscope in the United States. Adv. Electron. Electron Phys. 73, 134–231. Ruska, E. (1933). The electron microscopic imaging of surfaces irradiated with electrons [in German]. Z. Phys. 83, 492–497. Ruska, E., and Mu¨ller, H. O. (1940). Progress on the imaging of electron irradiated surfaces [in German]. Z. Phys. 116, 366–369. Stintzing, H. (1929). Method and device for automatically assessing, measuring and counting particles of any type, shape and size [in German]. German Patents Nos. 485155, 4851556. Synge, E. H. (1928). A suggested method for extending microscopic resolution into the ultramicroscopic region. Phil. Mag. 6, 356–362. Synge, E. H. (1932). An application of piezo-electricity to microscopy. Phil. Mag. 13, 297–300. von Ardenne, M. (1938a). The scanning electron microscope. Theoretical fundamentals [in German]. Z. Phys. 109, 553–572. von Ardenne, M. (1938b). The scanning electron microscope. Practical construction [in German]. Z. Tech. Phys. 19, 407–416. von Ardenne, M. (1937). Improvements in electron microscopes. British Patent No. 511204, convention date (Germany) 18 Feb. 1937. von Ardenne, M. (1940). ‘‘Electron Microscopy’’ [in German]. Berlin: Springer-Verlag. von Ardenne, M. (1972). ‘‘A Happy Life in Engineering and Research’’ [in German]. Munich and Zurich: Kinder-Verlag.

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von Ardenne, M. (1985). On the history of scanning electron microscopy, of the electron microprobe, and of early contributions to transmission electron microscopy, in ‘The Beginnings of Electron Microscopy.’ edited by P. W. Hawkes. Adv. Electron. Electron Phys., Supplement 16, pp. 1–21. von Borries, B. (1940). High resolution images from the electron microscope used in reflection [in German]. Z. Phys. 116, 370–378. Wells, O. C., Everhart, T. E., and Matta, R. K. (1965). Automatic positioning of device electrodes using the scanning electron microscope. IEEE Trans. Electron Devices. ED-12, 556–563.  Zworykin, V. A. (1934). Electric microscope, in ‘1 Congresso Internazionale di Electroradiobiologia’, Vol. 1, pp. 672–686. Zworykin, V. A., Hillier, J., and Snyder, R. L. (1942a). A scanning electron microscope. ASTM. Bull. 117, 15–23. Zworykin, V. A., Hillier, J., and Snyder, R. L. (1942b). A scanning electron microscope [Abstract]. Proc. Inst. Radio Eng. 30, 255. Zworykin, V. A., Morton, G. A., Ramberg, E. G., Hillier, J., and Vance, A. W. (1945). ‘Electron Optics and the Electron Microscope.’ New York: Wiley.

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ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 133

Appendix III* The Cambridge Instrument Company and Electron-Optical Innovation PAUL JERVIS Formerly at: Science Policy Research Unit, University of Sussex, Brighton, England

INTRODUCTION This paper is concerned with the invention, development and commercial introduction of two scientific instruments, the scanning electron microscope and the X-ray microanalyser. They are both close relatives of the electron microscope, and have become important and widely used research tools. Events in the series which led to innovation are described, with particular reference to those involving British Universities, research establishments and firms, for this is an area in which much of the basic research and applications work has been done in the United Kingdom. The information on which this paper is based was gathered during Project SAPPHO, a study of success and failure in innovation. SAPPHO was funded by the British Science Research Council and carried out at the Science Policy Research Unit of the University of Sussex. The SAPPHO method consisted of the investigation of ‘pairs’ of attempts at innovation, each pair containing two examples of innovations designed to fill the same market outlet or user need. In each pair one attempt had been successful, and the other relatively unsuccessful or a failure, the criteria of success and failure being formulated in commercial terms. SAPPHO gathered data from two industry sectors, chemicals and scientific instruments, and a total of twenty-nine paired studies (seventeen in chemicals, twelve in instruments) were completed during the first stage of the project. Data was collected about each of the many stages of the innovation sequence, from the scientific research that demonstrated the feasibility of the innovation through to the production and marketing phases, and a large number of measures relating the performance of the success firm to that of the failure firm were made for each paired study. An analysis of the twenty-nine sets of data was designed to indicate consistent patterns of diVerence between success and failure, as well as identifying those factors which were seen to be common to all attempts at innovation, regardless of outcome. In this way it was hoped to construct a set of ‘necessary and suYcient’ conditions for

*Extracts reprinted from ‘‘Innovation in Electron-Optical Instruments – Two British Case Histories’’, Research Policy 1, 174–207 (1971/72).

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innovatory success. The methodology of the project, and its results, have been discussed elsewhere[1]. It was not the aim of the project to draw conclusions about the nature of the innovation process from isolated examples, but many of its findings can best be illustrated by reference to individual case studies. The majority of this paper is devoted to a description of the two case studies, and in the short concluding section, some of the main points which arise are discussed in the light of the statistical results. CAMBRIDGE INSTRUMENT’S SCANNING MICROANALYSER The history of the Instrument Company goes back to the founding in Cambridge in 1881 by Horace Darwin, a son of Charles Darwin, of a company to provide wellengineered instruments for scientists at the University. Through the years the Company grew, and other manufacturing sites and a London head oYce were acquired. The Cambridge factory, however, continued to operate under the name of Cambridge Scientific Instruments Ltd. The Instrument Company has a long history of close liaison with departments of the University of Cambridge, and its product line has always contained items manufactured under licence from various laboratories. Noted for its skilled craftsmen, apprentice training scheme and high quality precision engineering, the Company was one of the founders of the British instrument industry. As with its competitors, after the Second World War the Company found itself in a seller’s market, and the management seem to have been lulled into a false sense of security. The balance sheet for 1957 showed that turnover was approximately 1.6 m. with profits before taxation of over  12 m, and a return on the net assets employed of 35%. Depreciation on tools and equipment was 18,000 and the budget for R&D, such as there was, was not much higher. The Company had introduced no significant new products for a number of years, the average age of the board of directors was near 70, and three board members were former managing directors. In January 1958, H. C. Pritchard was brought in to the Company. He was an Oxford mathematician with experience in both the Civil Service and Industry. He had worked at the Royal Aircraft Establishment (RAE), been Director of the rocket testing range at Woomera in Australia, and immediately before joining Cambridge was with Elliotts. Pritchard’s task was to inject new life into the Company and its products, and from the first realised that the Company’s strength lay in its highly skilled and under-employed instrument makers. There was also capital available for investment. Pritchard knew that he could not hope to maintain the profit/turnover ratio at its existing high level and realised that the Company did not have the expertise to do large scale R&D on new products. Accordingly he started to look for products which could be brought in under licence and which would utilise the Company’s skills while creating new turnover with a chance of profit. The first such product was a microtome with a well-proven design. Pritchard also wanted a ‘‘prestige product’’ which would take Cambridge Instruments into an advanced technology and help its market image. For some time nuclear magnetic resonance was under consideration but the decision was eventually made not to pursue this, probably because of the quality and

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experience of the potential opposition. Another of his early moves was to have a new R&D laboratory built at the Cambridge factory, and he made S. A. Bergen, who had previously been a technical advisor to the London sales department, the Chief Development Engineer of Cambridge Scientific. Pritchard and T. P. Hughes (the Director of Tube Investments Hinxton Hall Research Laboratory) had been colleagues at the RAE., and when Pritchard was invited to see Duncumb and Melford’s instrument it coincided with his search for a new product. Hinxton Hall had a high throughput of visiting scientists, and in the time that the microanalyser was in operation many potential customers were able to see it. By the time that Pritchard first saw the instrument, on 20 March 1959, it was clear that not only was it a very interesting piece of apparatus but that there was enough interest to warrant commercial production. Once Pritchard had seen the instrument, Cambridge wasted no time in deciding to licence it. Arrangements with Tube Investments and the Cavendish were negotiated by the end of June, and leaflets and other publicity material were available from the Company at the June conference on X-ray microscopy and microanalysis in Stockholm. The Company’s intention to manufacture was also mentioned in Melford’s New Scientist article (Melford, 1959). Cambridge Scientific took Melford’s dimension sketches for the microanalyser and used these to produce production drawings. The Company made progress with the instrument and completed a prototype in not much more than six months. The production prototype was exhibited at the London OYce in conjunction with the Institute of Physics and Physical Society exhibition of January 1960 and it was delivered to the Atomic Weapons Research Establishment (AWRE) at Aldermaston in May of that year. From the start, the microanalyser, under the trade name of Microscan, sold far better than Cambridge Instruments had expected. Recollections of the expected market vary, but it seems likely that the first estimate of selling price was around 12,000 with an annual market of between six and twelve instruments. In fact, in less than five years more than seventy Microscans were sold, with approximately 70% being exported. The price of the instruments varied with the number of attachments ordered, and in particular with the number of spectrometers. The average price per installation was about 16,500, but the actual range was from 11,000 (presumably for the basic electron optical and electronic equipment) to 24,500. Most exports in the first three years production went to the United States, but over half the later deliveries went to Germany. In all, approximately 25% of the Microscan production went to Germany and just under 20% to the United States. It seems clear that the arrival of the Microscan adversely aVected the sales of A.E.I.’s instrument. It was mentioned earlier that, although the Company and the Advisory Committee had both tended to dismiss the importance of scanning, as soon as Duncumb and Cosslett’s instrument became widely known interest in their method grew. A.E.I. decided to oVer scanning as an optional extra facility on their microanalyser, but found that its development was very costly. Both in man-hours and money, the work on scanning at Manchester probably cost as much as all the previous development work. The first A.E.I. microanalyser with a scanning probe was installed in 1961, and sales were never as good as those of the Microscan. A list of

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microanalyser installations in the United Kingdom compiled in February 1963 by the Electron Microscopy and Analysis Group of the Institute of Physics recorded twentyfive Microscans, thirteen A.E.I. instruments, one CAMECA apparatus and nine experimental or privately made instruments.

The Second Commercial Scanning Microscope Pritchard and Bergen were well aware from the start of production of the Microscan that, although the first generation instrument had been produced relying almost entirely on outside sources (Hinxton Hall and the Cavendish) for the electron optical know-how, the Company would have to build up its own expertise to tackle any further redesigning and up-dating. One of them, probably Bergen, thought that the best way to learn about a technology was to find a team to work on a specific project. The suggestion was made that the Company should build a scanning microscope. The precise source of the idea is hard to identify, since both Pritchard and Bergen had extensive contacts with University scientists and so both almost certainly knew of Oatley’s instrument. Bergen, in particular, knew Dr. W. C. Nixon, who had worked for some years with Cosslett and was at that time in Oatley’s group. However the idea was generated, Bergen drafted a three-stage plan for developing a commercial scanning microscope. The process he envisaged was first to build a ‘‘lash-up’’ demonstration instrument using all the useful parts of a Microscan, aiming at a resolution of ˚ . In the second stage this would be heavily modified with a target perhaps 2000 A ˚ , and the third stage would consist of the embodiment of all the resolution of 500 A resulting ideas in an instrument of best possible resolution. In the event the third stage of Bergen’s plan was not needed, because the instrument resulting from the second stage had commercial viability. Before the Company could start work on an instrument there was one obstacle to be removed. Oatley’s agreement with A.E.I. Oatley was disappointed by A.E.I.’s apparent inability to find customers for the scanning microscope, and also by the trouble F. P. Bowden was having with the P.C.S. instrument [at the Cavendish, see Chapter 4.1B, (eds.)]. In March or April 1961, Oatley visited Manchester to discuss with members of the Company the status of the agreement between them, and as a result of the conversation, A.E.I. agreed that the Engineering Department had discharged any obligation imposed by the know-how payment, and accepted that another Company could undertake manufacture. It is probable that A.E.I. took the view that with the considerable pressure on finance and manpower caused by the success of other products, and with little indication of a significant market for the scanning microscope, they could not justify the very heavy expenditure which would have been necessary to improve and engineer an instrument based on the model which had been delivered to Bowden. Even before Oatley had obtained A.E.I.’s release from the agreement there had been discussions between Bergen and members of Oatley’s team, the first of a number of meetings being on 21 November 1960. Cambridge Instruments realised that it

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would have to strengthen its research team to work on the new instrument and on 11 January Bergen wrote to Nixon: Further to our telephone conversation on the subject of scanning electron microscopy,. . . we are very conscious of the fact that a man of the right calibre must be attracted to the Company to lead work in this field, and we are starting another recruiting drive in this direction. . . these appointments tend to be made by personal contact rather than broadcast advertising and if you come across anyone suitable we would be very grateful for your good oYces.

As a result of the frequent meetings between the Company and members of the Engineering Department, the idea gradually emerged that the best way to help the Company to build up its electron optical expertise and develop the scanning microscope would be to transfer a man from the University. A. D. G. Stewart was coming to the end of his postgraduate work at the Engineering Department and appeared the obvious man to make the transfer. Stewart finished his three years at the Department in the Autumn of 1961 and oYcially joined the Company in April 1962. Work began on Bergen’s first objective . . . a ‘‘lash-up’’ demonstration model, during the late spring or early summer of 1961 and it was working in not much more than six months. This experimental prototype was displayed at the Company’s London showroom in conjunction with the Institute of Physics and Physical Society Exhibition of January 1962 and aroused considerable interest. Photographs of this instrument show clearly its experimental nature, and indicate how a Microscan was stripped of its spectrometer and counting equipment and an extra lens and a detector were added on. It was in the interim between the launching of the Microscan and the launching of the scanning microscope that Cambridge Instruments came face to face with the diYculties of developing complex new products and in fact nearly lost the advantage they had gained with the Tube Investments instrument. The company had for some time been assisting Dr. J. Long, of the Department of Mineralogy and Petrology at Cambridge, with the construction of a microanalyser designed primarily for geological applications. This co-operation eventually led to a decision to add to the Company’s product range a microanalyser based on Long’s design. It was decided to market this instrument under the name ‘‘Geoscan’’, the rationalisation of the name being that a new designation was needed since it was intended to continue marketing the existing microanalyser, the Microscan. The name Geoscan was disliked by some members of the Company as they thought it implied a narrowly specialised geological instrument. This was not intended to be the case although the Geoscan could cater very well for the particular requirements of geologists. Research and development expenditure on electron-optical instruments grew rapidly and created cash flow problems. How much the decision to market the Geoscan was made in the expectation of a large market, and how much was due to a desire, conscious or subconscious, to recoup some of the resources already put into the instrument by the R&D Department is not clear. Work on the Geoscan and the scanning microscope went on in parallel during the years from 1962–64 and in the words of one of the participants, ‘‘the two projects clashed hideously over everything’’. There was a constant struggle for resources between the two instruments, in which the Geoscan had the advantage

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P. JERVIS

at first but which finally favoured the scanning microscope. The handbook for the January 1963 Institute of Physics and Physical Society exhibition describes the production prototype Geoscan, but this was written some time in advance of the exhibition and in fact the instrument was not ready in time. When Stewart joined the Company he did not start work on scanning microscopy straight away, but was given the task of improving the Microscan while he became used to the Company and its environment. When the Geoscan was not ready for the 1963 exhibition, Stewart’s improved Microscan and a set of attachments was shown in its place. At the same time the next stage of the scanning microscope was on display at the Company’s London showroom. The Geoscan was launched some time later in 1963 or 1964, and overlapped in production with the Microscan for a few months, production of the latter ending in December 1964. Although the Geoscan was initially marketed for under 20,000 within eighteen months its price had risen to an astonishing 35,000. Although it was a more sophisticated and versatile instrument than the Microscan its price seems to have been far too high for the market. The Microscan had some disadvantages, the greatest perhaps being its semi-focussing spectrometers, but many people believe that it could have been marketed successfully for some time after it was discontinued. By late 1962, the Company had constructed a second experimental prototype scanning microscope, and it was then that their first customer appeared. The head of DuPont in Canada knew the President of the Pulp and Paper Research Institute, and through this contact the American company heard of Smith’s scanning microscope. Accordingly, one of their scientists visited the Instrument Company in November 1962, saw the instrument and decided that he wanted one. An order was placed and from that time until the instrument was finally dispatched DuPont pressed the Company for early delivery. Cambridge Instruments found it extremely diYcult to assess the market for the scanning microscope and for some time the view was that sales would be small. Even the most optimistic forecast was a sale of twenty instruments a year, and when, early in the development stage, Cosslett stated the opinion that the scanning microscope would outsell the microanalyser no-one believed him. Eventually Pritchard was able to satisfy himself that if the instrument was marketed a number of people in British universities would apply to the Department of Scientific Industrial Research (DSIR) for funds to buy it. The conflict of interests between the scanning microscope and the Geoscan slowed down the development of the former, and DuPont, became increasingly impatient. In the end Pritchard and Bergen decided to let the Americans have the production prototype, which had been built in the Development Department using some production staV. Some members of the development team objected strongly to this course of action but they were over-ruled. The prototype was shown at the January 1964 Institute of Physics exhibition and shipped to DuPont early in the year. Unfortunately it was extremely badly damaged in transit and had to be returned and virtually rebuilt. It was sent a second time to America in September 1964. At this stage in its development it was decided to make a batch of five production instruments. The first production model of the ‘‘Stereoscan’’, as the Company named the scanning microscope, was delivered to Dr. P Thornton of the University College

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of North Wales, Bangor, in June 1965, the next to Dr. J Sikorski of Leeds University in August, the third to Professor PfeVerkorn of the University of Munster in October, and the fourth to the Central Electricity Research Laboratories at Leatherhead in December. The instrument was selling at around 20,000. The Company then launched an intensive publicity campaign which made use of the applications studies done on the early production instruments and on the research instruments at the Engineering Department. They were successful in obtaining coverage in most scientific journals, many national newspapers and magazines, and on television. Over 1000 was spent on material for the campaign, especially on high quality reproductions of photographs taken with the Stereoscan, but no commercial advertising was undertaken at that stage. So successful was this campaign that by February 1966, over 250 firm enquiries had been received. Two Stereoscans were delivered in March 1966, to the Electricity Council and Corning Glass, and three more in April, to Rolls Royce, Derby, the Max Planck Institute, West Germany, and IBM, Yorktown. By the end of 1966 a total of over 30 units had been installed, and more than 100 by the end of 1967. In 1968 the Company achieved the remarkable performance of selling more than a hundred instruments. In 1967 Cambridge Instruments received the Queen’s Award for technological innovation in recognition of the scanning electron microscope, and this was followed by a further Award, for export performance, in 1970. While the Stereoscan was in development in the United States, Westinghouse were working on a similar instrument which it appears never came to market. The Instrument Company knew of this work, but the knowledge does not seem to have altered either the scale or the pace of their development programme. They were taken much more by surprise when the Japanese firm JEOL marketed a scanning microscope early in 1966, within six months of the Stereoscan launch. The Instrument Company claim that the Japanese instrument was unable to match the Stereoscan’s performance until more than two years after its launch, and in terms of overall marketing operations the English firm had perhaps eighteen months lead. These claims do not seem unreasonable but it does appear that at the time of writing there is very little to choose between the two instruments. Much of the success of the Cambridge Instrument’s scanning microscope is due to Stewart, who was in an excellent position to appreciate the user requirements, and who was responsible for many of the additions and sophistications built into the production version. From the middle of 1963 Stewart had been in charge of the Stereoscan development and in 1968 his work was recognised when he was placed third in the New Scientist Award for contributions to technology from younger scientists. A Different Picture Before considering any light that these events might throw on established theories about innovation one qualification should be made. From the events discussed above it might be thought that Cambridge Instruments was a remarkably successful innovating company. While there can be no question of the Company’s technical

Table 1 Cambridge Instruments Company Ltd. – Performance 1955–1967. Year

Sales ()

% age change over previous year

Profit before taxation ()

% age change over previous year

Net profit after taxation ()

% age change over previous year

Net assets assets employed ()

Return on net assets Rate before taxation

1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967

1,401,479 1,514,876 1,592,377 1,595,933 1,693,164 2,100,750 2,928,943 3,162,063 3,101,934 3,408,684 3,752,938 4,077,170 5,284,950

– + 8.1 + 5.1 + 0.2 + 6.1 + 24.1 + 39.4 + 8.0 1.9 + 9.9 + 10.1 + 8.6 + 29.6

532,491 510,009 536,874 461,973 469,890 527,197 551,328 551,631 506,664 374,244 499,395 495,074 535,807

– 4.2 + 5.3 14.0 + 1.7 + 12.2 + 4.6 + 0.1 8.1 26.1 + 33.4 0.9 + 8.2

235,526 220,503 222,152 216,515 287,545 307,243 269,366 282,722 306,739 213,898 325,274 308,150 420,966

– 7.4 + 1.0 2.5 + 32.8 + 6.9 12.3 + 5.0 + 8.5 30.3 + 52.1 5.3 + 36.6

1,537,542 1,625,264 1,716,941 1,834,795 2,169,938 2,584,903 2,675,523 2,778,103 3,051,476 3,065,791 3,350,966 3,272,856 3,409,681

34.6% 31.4% 31.3% 25.2% 21.7% 20.4% 20.6% 19.9% 16.6% 12.2% 14.9% 15.1% 15.7%

CIC AND ELECTRON-OPTICAL INNOVATION

555

achievements, it must be pointed out that if those criteria which are normally used to indicate financial success are applied then the results are significantly diVerent. Examination of the Company’s Annual Reports, which relate to the total performance of the four operating companies and which are abstracted in Table 1, shows a depressing picture. The lack of research and development expenditure during the 1950s is clearly indicated by the very high return on net assets employed, and as might be expected this figure steadily declined from 1955 to 1964. Even then the return on assets was by no means a bad one, and it was the other aspects of Cambridge’s performance which caused concern. Although their sales increased almost continuously from 1955 onwards, the profit before taxation remained static. In fact, while the 1964 turnover figure showed a 240% increase over the 1955 value, pre-tax profits over the same period showed a 29% drop. 1964 was the nadir of the Company’s performance for well over a decade, and the extremely poor results for the year resulted in many changes within the Company. Cambridge’s management problems, and the steps which they took to remedy them, have been described elsewhere, notably by Winsbury (1967, 1968). It is not suggested that the electron optical work was the only cause of these poor results. There were many other projects under development during this period which contributed to the high rate of expenditure. There is evidence that in its first eVorts to modernise, the Company suVered from the not uncommon inability to cost its development projects, and allowed enthusiastic scientists and engineers to over-commit resources to projects without assuring a suYcient return. Whatever the causes of Cambridge’s financial troubles, it is clear that when the whole range of the Company’s activities are considered it appears in a less favourable light than when its electron optical developments are studied in isolation. The same people who were ultimately held responsible for the poor financial performance brought Cambridge into the field of electron optics and it is clear that the Company owed much to their foresight and imagination. At the time of the takeover by the Kent Group a very significant proportion of Cambridge Instruments’ turnover was due to one instrument, the Stereoscan. Without this, and the other electron probe instruments, the balance sheet would have made far diVerent reading. However, in hindsight it may appear that the Company did not receive an adequate return from its big R&D expenditure and that too much of this expenditure was concentrated in one product area.

References [1] B. Achilladelis, P. Jervis and A. Robertsen. Project SAPPHO: A study of success and failure in innovation. Science Policy Research Unit, 1971. [2] Melford, D. A. (1959). The X-ray scanning microanalyser. New Scientist, p. 746. [3] Winsbury, P. (1967). New readings from Cambridge Instrument. Management Today, January 1967, p. 50. [4] Winsbury, P. (1968). Company profile: Cambridge Instrument Co. Measurement and Instrument Review, November 1968, pp. 813–814.

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Index

microprobe analyser development and, 244–245, 283 NMR ambitions of, 321, 322 Astigmatism, 191, 365 corrections for, 380 electron-optical system and, 95, 188, 302 SEM power supplies and, 190 Atomic number BSE and, 141 contrast and, 48, 87 x-ray microprobe analyser and, 248, 260, 261, 285, 296 Atomic Weapons Research Establishment (AWRE), 539 Automation/computer control MSYS/QSYS software and, 379–380 vector scan digital pattern generator and, 369–370 virtual SEM software and, 462

A Accelerating voltage, 335 ADRDE. See Air Defence and Research Development Establishment AEI. See Associated Electrical Industries Aharonov-Bohm electro-interference ring structure, 493 Air Defence and Research Development Establishment (ADRDE), 506–507 Alvey project for EBL/semiconductor production, 400–405, 407–408 Esprit requirements, 407–408 Amplification system bandwidth of, 67 multiplier connection to, 94–95 noise-free, 139–140 for SEM, 70 video, 76–77 Aperture discs, 351 multislot, 460, 461 Aperture stage, 97, 98, 122, 166, 177 in x-ray projection microscope, 239–240 Associated Electrical Industries (AEI), 189 commercial development of SEM, 29–31, 103–105, 311–313, 317–320 competition, 318 lens design and, 318 Metropolitan Vickers and, 311, 317

B Back-scattered electron detectors (BSD), 366 Back-scattered electrons (BSE). See also Reflected electron current atomic number and, 141 contrast and, 15–17, 46, 142–143, 144, 202 DQE and, 421 high energy, 175–176, 182–183 resolution and, 130, 140 theory of, 141–143, 144 Bandwidth in amplification systems, 67 of electron beams, 70, 161, 419–420 557

558 Bausch and Lomb commercial development of SEM and, 313–314 Baxter electron multiplier, 41 Beam blanking in beam chopper, 460, 461 Beam chopper, high speed, 459–460, 461 beam blanking and, 460, 461 multislot aperture and, 460, 461 Beam current electron beam and, 70, 91, 141, 482, 492 Beam diameter resolution dependence on, 189–190, 191–192 Beam penetration contrast and, 200–201, 202 operating voltage and, 190 resolution dependence on, 189–190, 203 SEM operating voltage and, 190 Bell Telephone Laboratories, 40, 230–231 Etec Corporation and, 450–451, 452, 453, 455 MEBES at, 454–455 Stereoscan installation at, 450–451, 452, 453 Biological Microprobe Laboratory, 478–480 Biological SEM/x-ray microanalysis, 469 Biological Microprobe Laboratory and, 478–480 early applications, 471–474 ion beam etching with, 472–474 low-temperature SEM, 475–476, 478–480, 481 Multi-Imaging Centre, 480–481 sample preparation, 471–476 specimen stage, temperaturecontrolled for, 475, 478 specimen transfer device for, 476

INDEX

TEM v., 471–472 water content and, 472–476 Biological specimens. See Biological SEM/x-ray microanalysis; Specimen considerations; Specimens, biological Boersch effect, 191 Boltzmann’s constant, 196 Bragg angles and in crystal spectrometer, 354 Bragg crystal spectrometer Bragg angles and, 354 microanalysis and, 260, 266, 279–280 British Aluminium Research Laboratories, 96 British Iron/Steel Research Association (BISRA), 244, 245 British Non-Ferrous Metals Research Association, 96 British Telecomm (BT), 485–486 British Thomson-Houston Company, 93 Broers, A. N., 25 microfabrication and, 26–27, 207–225 sputtering and, 25, 26–27 sub-ten-nanometre processes and, 221–225 BSD. See Back-scattered electron detectors BSE. See Back-scattered electrons

C California Institute of Technology, 441 Cambridge Instrument Company (CIC), 4, 5, 6, 31–32, 387. See also Geoscan company name changes and, 410–412 corporate changes at, 332–333, 372–373, 404–405 design/engineering challenges at, 345–347

INDEX

EBL at, 387–411 funding for, 360, 388 Geoscan, 325–326, 332, 353–357 Metals Research takeover of, 388 Microscan, 5, 247–248, 260, 264, 282, 305, 313–315, 332, 344 NMR ambitions of, 322 Stereoscan, 4, 5, 32, 33, 170–171, 311, 313, 332 working environment of, 322–323, 339–344 Cambridge Instruments (CI), 377 Cambridge Scientific Instruments (CSI), 372–373 Cambridge University Engineering Department (CUED), 5, 6, 7, 12, 39–41, 137, 165, 227, 325, 345, 456, 467–468 electron-optical research at, 468 Microcircuit Engineering Laboratory founding at, 468 CamScan commercial development of, 313–314 Canadian National Museum of Science, 29 Castaing, R., 259, 277, 318 Castaing equation and, 477 microprobe thesis of, 270 Cathode ray tube, 7, 16, 38 brightness level in, 89 long persistence, 14 as recorder for SEM, 71 Cavendish Laboratory, 5, 13, 25, 31, 38, 41, 189 metal distribution experiments at, 294, 295 Microelectronics Research Centre founding at, 468 Chang, T. H. P. FSS and, 361 at IBM, 447 LaB6 electron gun and, 452–454, 485 on lithography, 359–369

559 troubleshooting at Bell Laboratories and, 452 Channel plate multiplier, 379 CI. See Cambridge Instruments CIC. See Cambridge Instrument Company Coherent current in SEM, 87 Commercial development AEI and, 29–31, 103–105, 311–313, 317–320 Bausch and Lomb and, 313–314 CamScan, 313–314 CIC and, 29–31, 103–105, 321–331, 387–412 EBMF models, 376, 375–378, 382, 383 Geoscan and, 325–326 Microscan and, 312–313, 314–315 PPRIC and, 313–314 SEM and, 28–33, 311–315 Stereoscan and, 313, 329–330 Contamination electron-optical system and, 95, 97 from pump oil, 347 in vacuums, 318 Contamination process with electron gun, 210–213, 272 Continuum radiation in specimens, 477–478 Continuum-normalization algorithm for diffusible ion analysis, 478 Contrast, 18, 19–22, 23, 220 atomic number and, 48, 87 back-scattered electrons and, 15–17, 46, 142–143, 144, 202 beam penetration and, 200–201, 202 electron glancing angle and, 15, 16, 46, 48, 49, 51, 59, 111–113, 115, 117 in high performance SEM, 200–203, 204 magnetic, 134 SEM mechanisms for, 48–51

560 Contrast (Cont. ) voltage contrast method and, 134, 459, 488–489 Z contrast and, 46, 51 Contrast mechanisms of SEM, 48–51 Conventional electron microscope (CEM) resolution limitations of, 188, 192 SEM v., 192 Coordinate system for ion gun, 166–167 Cosslett, V. E., 5, 25, 41, 259, 261, 264, 265 Electron Microscopy Group collaborators and, 269, 272 electron probe and, 262 quantum efficiency and, 262 SEM and, 52–53, 96 TEM and, 13 X-Ray Microscopy (Cosslett & Nixon), 256 x-ray sources and, 260–261, 265–266 Crystal spectrometer, 260, 266, 279–280, 327 CUED. See Cambridge University Engineering Department

D DAC. See Digital-to-analog converters Deflection field, 365 Deflection systems double, 273 eddy currents in, 374 low-aberration coils and, 446 of SEM, 63, 75, 129, 153, 157, 373, 374 Defocussing, 175, 365 Demagnification, 104, 241 Design/engineering challenges, at CIC alignment/reassembly and, 348 aperture discs and, 351

INDEX

deflection coils and, 344–385 electroplating and, 352 final lens resolution and, 345, 346, 375 hysteresis and, 355 lens pole-piece circularity and, 346, 347–348, 351, 352 machining specs and, 349 metal purity specs and, 351, 353, 355 metric v. english dimensions and, 350 oil leakage and, 355 o-rings and, 348 scintillators and, 352 in SEM, 345–347 space limitations and, 356 specimen chamber and, 346–347 specimen stage and, 350–351 spectrometer crystals and, 356–357 vector scan settling and, 344–385 vibration and, 345–346, 350 Detection systems, 139–141 biased scintillator SE, 171 low detector takeoff angle and, 130 negative bias voltage and, 182 for SEM, 67, 70 Detective quantum efficiency (DQE). See also Scintillator/ photomultiplier combinations, in SEM absolute value of, 432–435 electrons per pulse v., 419 mathematical basis of, 419–421 measurement of, 421–423, 429–432 photocathode sensitivity in measurement of, 427–429, 430 Poisson distributions in measurement of, 425–427 probability mathematics for measurement of, 434–435 pulse-height in measurement of, 423, 424–427 in scintillator/photomultiplier combinations, 419–435

INDEX

scintillators types in measurement of, 423–424 Detectors back-scattered electron, 366 Everhart-Thornley, 366, 367 silicon/lithium-drifted, 263, 272 wide-band, 147–152, 171 Detectors, for low-energy electron currents, 147–152 general description, 147–148 optical system efficiency, 150–151 theoretical performance, 148–149 Digital-to-analog converters (DAC), 370, 375–376 Display system for SEM, 71, 77–78 DQE. See Detective quantum efficiency Duncumb, P. scanning techniques and, 264 X-ray microprobe analyser and, 245–246 DuPont Stereoscan installation at, 451–452 Dynamic stereo imaging for SEM, 460 Dynamic surface evolution, 182–183 Dynodes, 14, 111, 113, 124, 162

E EBIC, 40 EBL. See Electron beam lithography EBMF. See Electron beam microfabricator EBPG. See Electron beam pattern generator Einzel lenses, 41–42, 180 Electron beam, 21, 61, 492 bandwidth and, 70, 161, 419–420 beam current and, 70, 91, 141, 482, 492 brightness and, 156–157

561 characteristics of, 61–63 diameter of, 447 generator filaments for, 156, 161 high energy, 45 low energy, 132 pulse circuitry and, 95 voltage optima for, 24 Electron beam lithography (EBL), 491–492, 494–495. See also Lithography, and SEM; Microfabrication; SEM to nanolithography; Thomson-CSF EBL machine 3D structure inspection and, 495–496 Aeble 152, 403 Alvey project and, 401–405, 407–408 company name changes and, 410–412 first MicroFabricator for, 392–393 Fisher and, 388–396 Gooding and, 380–383, 392–393, 405–406 improvements to, 408–410 Leica/Philips takeover, 406–407 maturing technology and, 408–410 Metals Research and, 388–396 MicroFab-lite for, 394 Monobloc chuck and, 397–400 one-piece mirror block for, 392 overlay errors and, 492 placements of, 405 resolution and, 492 ring/strut system for, 388–390 scanning angles and, 492 Thompson-CSF and, 401–404 Electron beam microfabrication, 5–6, 26–27, 134, 208–209, 228, 492 height sensing transducers in, 381 for semiconductors, 371

562 Electron beam microfabricator (EBMF) Channeltron models, 378–379 EBMF models, 376, 377–378, 382, 383, 392–393 EBMF placements, 395–396 electron beams and, 492 Electron beam pattern generator (EBPG) from Philips, 393 Electron current density, 120 Langmuir’s theorem, 120, 196 Electron diffraction camera in SEDS, 227–228 Electron emissions field, 59–60 noise level contribution by, 201–202 photo-electric, 59–60 ratios of, 63–64 reflected, 138–139 secondary, 59–60, 96, 138–140, 201–202 thermionic, 59–60 Electron glancing angle, 63–66 contrast and, 15, 16, 46, 48, 49, 51, 59, 111, 113, 115, 117 emission variations with, 84–85, 140 reflected electron current and, 85, 86, 87 specimen considerations and, 15, 16, 46, 48, 51, 87 trapezium distortion and, 71–72 Electron gun, 42, 61–62, 73, 154, 155–156, 367 contamination process, 210–213, 272 for etching, 210–212, 213 heated filament, 161–162, 180, 190, 366 LaB6 model, 452–454, 485 Langmuir fomula and, 196 photoresist process, 211–212, 214, 215–221 scanning electron probe analyser in, 297, 299–300 Stewart SEM, 165, 166

INDEX

Electron, Ion, Photon Beam Technology Symposia, 372 Electron microscope (EM), 318, 456, 457, 485 EM4/A model, 319 EM6 model, 318 Electron microscope/microanalyser (EMMA) biological specimens and, 478 Electron probe, 8, 9, 195, 262–263 theoretical considerations of, 196–197 Electron reflection depth in SEM, 81 Electron retardation efficiency, 264 Electron trajectories in SEM, 142–143, 144 Electron-emission ratio in SEM, 64 Electron-optical system, 61–63, 73–75, 95, 97–99 aberrations of, 8–9, 62, 189, 196–197, 198 astigmatism in, 95, 188, 302 contamination in, 95, 97 design considerations of, 97–99, 181–182 filters in, 95, 97, 182 post-specimen filters in, 229 wide-aperture, 70 The Electron Microscope, (Gaboi), 38, 88, 535 Electro-optical columns for teaching, 315 Electrostatic unipotential lenses, 23, 188. See also Lenses aberration in, 15 power stability and, 15 EM. See Electron microscope Emission current, 181 Emission ratio, 46, 85 Emissions, x-ray, 248 Emitter, thermionic, 180. See also Electron gun

563

INDEX

EMMA. See Electron microscope/ microanalyser Engis Equipment Company as Stereoscan North American agent, 449, 452, 455 Etching with biological SEM/x-ray microanalysis, 472–474, 473 crystallographic, in lithography, 363 with electron guns, 210–212, 213, 492–493 with ion guns, 167–168, 175–178, 208, 472–474 Etec Corporation Bell Telephone Laboratories and, 454–455 Everhart, T. E. contrast and, 19, 137–145 detectors and, 21, 139–141, 162, 171 Everhart-Thornley detectors and, 366, 367 wide-band detectors, 147–152, 171

F FIB. See Focused ion beam Fifty Years of Electron Diffraction, (Goodman), 232 Finite-element method in lenses, 438 Fisher, C. EBL and, 388–396 Flying-spot scanner (FSS) lithography with, 361, 362 Flying-spot x-ray microscope in microanalysis, 284–286 Focused ion beam (FIB) systems advantages of, 488 MESFETs and, 490–491 metal deposition with, 489–490 voltage contrast imaging and, 488–489

Focusing, 73 electrostatic, in early SEM, 38, 103, 153, 188, 240 electrostatic, in TEM, 38 focus point in SEM and, 11 Forward-scattered electrons (FSE) energy loss with, 130 low detector takeoff angle and, 130 FSE. See Forward-scattered electrons FSS. See Flying-spot scanner

G Gamma correction circuit, 95 GEC, 485–486 Geoscan, 282, 332 commercial development of, 325–326 electron gun, oil-filled in, 347–348, 355–356 for geological specimens, 353–357 proportional counters in, 355 in SEM history, 542 spectrometer crystals and, 356–357 x-ray spectrometry and, 354 Gooding, T. lithography and, 380–383, 392–393, 405–406 Grazing angle. See Electron glancing angle Grigson, C. W. B. scanning electron diffraction and, 227–232 Gunn effect, 40

H HEMT. See High electron mobility transistors High electron mobility transistors (HEMT), 490–491 High resolution electron microscope (HREM), 456, 457, 485 High-pressure freezing for biological specimens, 482–483 High-velocity electron reflection in SEM, 84–85, 138–139

564 Hot shortness in metals and, 278, 291–292, 294–295, 303 Hysteresis, 355, 374

I IBM, 441–442 Thomas J. Watson Research Center, 134–135, 447 Illinois Institute of Technology Research Institute (IITRI), 452 Imaging LLE and, 130 low beam energy in, 132 low-loss electrons in, 130 secondary electrons in, 138, 140–141, 201–202, 488 in SEM, 112–114, 124 voltage contrast, 488–489 Immunocytochemical labelling, 483 Imperial College Oatley and, 442 Incident electron penetration resolution and, 121–122 Institute of Physics, 326 International Conference on X-ray Microscopy/Microanalysis, 324 International Symposium on the Electron, 106, 108 Ion beam etching with biological SEM/x-ray microanalysis, 472–474 Ion current, 175 Ion gun coordinate system for, 166–167 for etching, 167–168, 175–178, 208, 472–474 for metal sputtering, 25, 176–177, 489

J Japanese Electron-Optical Limited (JEOL), 312, 330, 383, 553

INDEX

K Knoll, M. electron beam scanner and, 526, 527 Knoll’s electron beam scanner, 526, 527 Kramer relationship continuum radiation and, 477–478

L LaB6 electron gun, 452–453, 485 with ion pump, 454, 455 Lanthanum hexaboride. See LaB6 Laser interferometer Yosemite company and, 388 The Lehigh Microscopy School of SEM, 479–480, 481 Leica Microsystems, 383–385 Leica/Philips takeover, 406–407 Vectorbeam VB6, 384–385 Leica Microsystems Lithography Limited creation, 410–412 Lenses. See also Electrostatic unipotential lenses; Magnetic lenses einzel, 41–42, 180 electron, 61, 438 electrostatic, 15, 156, 180, 188, 209, 240 final lens resolution and, 345 finite-element method in, 438 lens pole-piece circularity and, 346, 347–348, 351, 352 lens voltage and, 180 magnetic, 8, 9, 103, 189, 190, 194, 198, 209 magnetic pinhole, 104 resolution with, 189–190 saturated magnetic, 438 thermionic spread in, 191–192

565

INDEX

Lithography, and SEM, 485–486. See also Electron beam lithography; Microfabrication; Microscopy to lithography; SEM to nanolithography beam energy needs for, 366 Chang thesis and, 359–369 Channeltron models, 378–379 early lithography, 364–365 EBMF models, 376, 377–378, 382, 383, 392–393 electron beams in, 492 electron emission needs for, 366 electron source needs for, 366 flying-spot scanner in, 361, 362 fundamental properties of, 364–365, 382 Gooding and, 380–383 hardware upgrades for, 380–381 ion beams for, 486–491 Leica Microsystems and, 383–385 Leica/Philips takeover and, 406–407 maturing technology in, 408–410 mesoscopic electron devices and, 492–493 Metals Research and, 388–396 minimum linewidth in, 364–382 Monobloc chuck and, 397–400 nanolithography, 491–494 Nanowriter and, 493–494, 495 placement accuracy in, 364, 382 ring/strut system for, 388–390 stitching in, 381, 382 T-gate structures in, 381, 382, 490–491 throughput in, 364, 382 LLE. See Low-loss electrons Long, J. V. P. x-ray microscopy and, 255–256 Low-loss electrons (LLE) in imaging, 130 Low-temperature SEM, 475–476, 478–480, 481

M Magnetic lenses, 8, 9, 103, 104, 195, 198, 209. See also Lenses aberration in, 15 advantages of, 198 beam-voltage fluctuation in, 73 resolution with, 189–190 x-ray analysis with, 285–286 Magnetostatic detector SEM accessory, 457–459 Magnification definition in SEM, 198 Manufacturing electron beam exposure system (MEBES), 454–455 Mass filter for ion removal, 209 Massachusetts Institute of Technology, 441 Maxwell’s rules, 351, 437, 445 McMullan, D., 14–17, 18, 19, 21, 94 TEM and, 15, 16, 17, 18 MEBES. See Manufacturing electron beam exposure system Memory structures from nanolithography, 494, 495 MESFET. See Metal-semiconductor field-effect transistors Mesoscopic electron devices, 492–493 Metal deposition with FIB systems, 489–490 Metals identification x-ray microprobe analyser and, 246 Metals Research Limited, 388–396 Cambridge Instruments formation and, 377 CIC relationship and, 333, 376 EBL and, 388–396 ring/strut system, 388–390 takeover of CIC, 388 working environment at, 390–391 Metal-semiconductor field-effect transistors (MESFET) FIB systems and, 490–491

566 Metropolitan Vickers AEI and, 311, 317 Microanalysis. See also Biological SEM/x-ray microanalysis; X-ray microprobe analyser biological specimens and, 477–480 Bragg crystal spectrometer and, 260–266, 279–280 cement and, 260–261, 262 with electron probe, 262–263 flying-spot x-ray method of, 285–287 mineralogy/petrology in, 260–261, 263 Monte-Carlo techniques in, 264 petrology, importance of, 265 quantitative spectrometry in, 260, 292, 294–295 science of, 264–265 x-ray sources for, 260–261 Microcircuit Engineering Conference (MCE), 380 Microelectronics Research Centre at Cavendish Laboratory, 491–492 MicroFab-lite for EBL, 394 Microfabrication, 5–6, 26–27, 208. See also Electron beam lithography; Lithography, and SEM; SEM to nanolithography electron beam and, 5–6, 26–27, 134, 208–209, 328, 492 fast electron energy deposition and, 134 FIB systems in, 486–487 lift off metallization process in, 381 lithography projects and, 373–380 metal deposition and, 489–490 microcircuit junctions and, 134 microcircuits and, 314–315 Nixon and, 26–27 optics needs in, 365–366 photoresist process and, 211–212, 214, 215–221

INDEX

point-contact rectifier application and, 99, 102 proximity effect in, 364–365 resists in, 364 RRE and, 360 SEM v. lithography needs and, 365–369 semiconductors and, 179, 224, 331 silicon wafers and, 134, 314–315 software upgrades for, 379–380 stitching in, 381–382 sub-ten-nanometre processes and, 221–225 TEM and, 221 T-gate structures in, 381, 382, 490–491 vector pattern generator in, 361–364 vector scan digital pattern generator in, 369–372 vector scan electron beam in, 362, 363 MicroFabricator first production model, 392–393 Micrographs, 80 BSE, 168–169 metallurgy, use in, 132, 242 stereoscopic, 132, 337 Microradiographs, 259 in tissue dry-weight, 259 Microscan from CIC, 5, 247–248, 260, 264, 282, 305, 313–315, 332, 344 commercial development of, 312–313, 314–315 in SEM history, 549–553 Microscopy to lithography Chang thesis and, 359–369 CIC reorganization, 372–373 early customers, 376, 377–378 early lithography, 364–365 electron beams in, 492 flying-spot scanner in, 361, 362 Gooding and, 380–383 hardware upgrades, 380

567

INDEX

Leica Microsystems and, 383–385 microfabrication project, 373–380 resists in, 364 SEM v. lithography needs, 365–369 software upgrades, 379–380 vector pattern generator in, 361–364 vector scan digital pattern generator in, 369–372 vector scan electron beam in, 362, 363 Mineralogy/petrology microanalysis and, 260–261, 263, 265 Monobloc chuck in EBL, 397–400 Motorola SCALPEL x-ray masks, 409 MSYS automation/computer control with, 379–380 Multi-Imaging Centre biological SEM/x-ray microanalysis and, 480–481 Multipliers, 52, 70, 76, 90, 111, 139. See also Photomultipliers; Scintillator/photomultiplier combinations, in SEM amplifier connection to, 94–95 Baxter electron multiplier, 14, 16 eight stage electron, 43–44 Everhart and, 19, 21–23 four stage ladder rectifier, 155 secondary electron, 14, 16 six stage electron, 21 sixteen-stage electron, 45 Multislot aperture beam chopper, high speed and, 460–461

N Nanolithography. See Lithography, and SEM; Microfabrication; SEM to nanolithography

Nanowriter SEM to nanolithography and, 493–494, 495 National Physical Laboratory (NPL), 238 National Research Development Corporation (NRDC) manufacturing and, 241–243 Nixon, W. C., 25, 31, 96 microfabrication and, 26–27 resolution and, 25–26 X-Ray Microscopy (Cosslett & Nixon), 256 x-ray microscopy and, 251–252 x-ray sources and, 260–261 Noise level, 370 electron emissions contributing to, 201–202, 274–275 Noise-free amplification of secondary electron current, 147 Noise-free detectors, 8, 12, 67–68, 70 NPL. See National Physical Laboratory Nuclear magnetic resonance (NMR), 321

O Oatley, C. W., 3–6, 8, 37, 38–39 ADRDE and, 505–507 electrical science tripos and, 513–515 electrostatic lenses and, 132 honours of, 517–520 Imperial College and, 442 Kings College and, 504–505 knighthood of, 440 later years of, 415–418, 442–445, 445–448 Maxwell’s equations and, 437, 445 papers published by, 416 retirement and, 445–446 RRDE and, 505–507, 508 SEM and, 509–512

568 Oatley, C. W. (Cont. ) SEM commercial development and, 311–315 Stereoscan and, 512–513 Trinity College and, 439–441 Oils oil leakage and, 355 Oils, low pressure SEM, importance for, 317 O-rings SEM and, 129

P Palluel’s curve, 50–51 Philips EBPG airlock and, 408 Beamwriter, 383, 384 electron gun and, 408 Phonon-transport studies in nanolithography, 493 Phosphors, 95 short-persistence, 361 surface density, DQE and, 420 zinc oxide, 21–23 zinc sulfide, 44–45, 54, 70, 77, 112 Photocathode sensitivity in DQE measurement, 427–429, 430 Photomultipliers, 19, 21, 44, 54, 114 scintillator combination with, 21–22, 25, 26, 102–103, 104, 139, 157–158, 162 Photoresist process ion-sensitive resists and, 486 microfabrication and, 211–212, 214, 215–221 PMMA and, 216–220, 222–224, 314, 364, 490, 494, 495 Pirani gauge, 265 PMMA, and photoresist process, 216–220, 222–224, 314, 364 in nanolithography, 490, 494, 495 p-n junctions, 315 diodes, 23, 134, 142–143, 144

INDEX

Poisson distributions in DQE measurement, 425–427 Power supplies deflection system and, 72–73 interference from, 161 in SEM, 72–73, 79, 95 PPRIC. See Pulp/Paper Research Institute of Canada Pritchard, H. C. at CIC, 342, 345, 348, 548–549 Proportional counters, 270, 272, 279 in Geoscan, 355 Pulp/Paper Research Institute of Canada (PPRIC), 28, 31–32, 103, 105–106, 134, 189 commercial development of SEM and, 311 Pulse circuitry electron beam and, 95 Pulse-height in DQE measurement, 423, 424–427

Q QSYS software automatic field position height with, 382 automatic field size calibration with, 379–380 automation/computer control with, 379–380 Quantitative spectrometry in microanalysis, 260, 292, 294–295

R Radar Research/Development Establishment (RRDE), 12, 42, 506 lithography and, 360, 371 RCA Laboratories, 3, 4 Recording apparatus for SEM, 71, 78

569

INDEX

Reflected electron current electron glancing angle and, 85, 86, 87 Reflected electrons. See Backscattered electrons Reflection electron microscope, 103 Reflexion electron microscope, 112, 114, 115 Replica process, 4, 81, 88, 96, 102 opaque objects and, 114 Resists. See Photoresist process Resolution actual v. theoretical, 188–189, 241 beam diameter importance in, 189–190, 191–192 beam penetration importance in, 189–190, 203 BSE and, 130 combined BSE, FSE, SE data in, 130–131 electron probe diameter and, 195 final lens, 345, 346 incident electron penetration and, 121–122 with magnetic lenses, 189–190 Nixon and, 25–26 poor resolution sources and, 188–189 scanning electron probe analyser v. SEM and, 271, 279, 280 secondary electrons and, 130, 138–140, 201–202, 488 SE-I and, 131, 138–139 in SEM, 11, 12, 19, 63, 86–89, 91, 112, 119 SEM operating voltage and, 190 spacial, 482 in TEM, 19, 21, 25–26 TEM v. SEM and, 30–31 visible microscope v. SEM and, 120–121 Ring/strut system in EBL, 388–390 Rio Tinto Zinc (RTZ) SEM and, 327

Rise-times magnetostatic detectors and, 458 Royal Aircraft Establishment (RAE), 548 Royal Radar Establishment (RRE), 230 RRE. See Radar Research/ Development Establishment Rutherford elastic scattering formula, 141

S SAPPHO project, 547–548 Sawtooth generators, 7, 11 Scanning coils, 8, 45, 76, 122, 124–125, 335 double deflection, 53–54, 104–105, 273 Scanning electron diffraction system (SEDS), 231, 232 advantages of, 229–230 electron diffraction camera in, 227–228 Grigson, and, 227–232 phase changes in solids and, 229 post-specimen electron filters and, 229 thin films and, 227–230 Scanning electron microscope (SEM), 25, 46, 80, 128, 336. See also Transmission electron microscope advantages/disadvantages of, 85–91, 114–118, 119–122 alignment/reassembly and, 348 amplification system for, 70 aperture stage of, 97, 98, 122, 123 applications of, 93–108, 111–125, 159–164, 192 beam chopper, high speed, 459–460, 461 CIC and, 324–325 coherent current and, 87

570 Scanning electron microscope (SEM) (Cont. ) commercial development of, 28–33, 321–315, 317–320 configuration and, 456 contrast mechanisms of, 48–51, 137–145 deflection system for, 63, 75, 104–105 Scanning electron microscope (SEM) design/engineering challenges and, 345–357 detection system for, 67, 70, 113, 114, 139–141 detection/low-energy electron currents in, 147–152 display system for, 71, 77–78 dynamic stereo imaging for, 460 early history, 3–6, 37–41, 93–97 early images from, 47, 48, 51 EBMF models, 376, 377–378, 382, 383, 392, 393 electron diffraction and, 227–232 electron multiplier in, 76, 111 electron probe in, 9, 195 electron reflection depth in, 81 electron trajectories in, 142–143, 144 electron-emission ratio of, 64 electron-optical aberrations of, 8–9, 62, 191, 196–197, 198 electron-optical system of, 61–63, 73–75, 95, 97–99 electron/specimen bombardment in, 76, 122, 124–125 fabrication, micro/nano and, 207–226 father of, 3–6, 10 flying-spot beam and, 314 flying-spot scanner and, 314, 361, 362 focus point of, 11 goniometer specimen stage in, 99–102, 104–105, 277 high-resolution developments of, 187–204

INDEX

high-velocity electron reflection and, 84–85, 138–139 image formation in, 112–114 improvements to, 161–162 ion bombarded surfaces and, 175–178, 209–215 low-temperature, 475–476, 478–480, 481 low-voltage advantages of, 162–163 magnification definition in, 195 mathematical symbols used in, 59 micrographs from, 80 Microscan, 5, 247–248, 260, 264, 278, 305, 313–315, 332, 344 noise output, DQE and, 420 opaque specimens and, 59–81, 89–90, 91, 114 operation modes of, 68–69 with optical microscope, 328 o-rings and, 129 power supplies in, 72–73, 79, 95 principle of, 7–12, 61, 111–114 recording apparatus for, 71, 78 reflected electron theory and, 141–143 remote control XpertEze for, 462, 463, 464 resolution and, 11, 12, 19, 63, 86, 88, 89, 91, 112 scanning circuits in, 71–72 SEM1 model, 53–54, 55 SEM2 model, 104 SEM3 model, 104–107, 108, 128, 311–312, 319, 329, 482 signal amplification in, 43 specimen chamber and, 346–347 specimen positioning in, 76 specimen stage in, 43, 99–102, 327, 335, 336, 350–351 spot size, 62–63, 75, 81, 91, 238 Stereoscan, 4, 5, 32, 33, 170–171, 311, 313, 332, 361, 371 Stereoscan S4-10, 393–396, 479 structural considerations of, 43 sub-ten-nanometre processes, 221–225

INDEX

time-bases and, 78–79 transparent v. opaque in, 10, 59, 63, 66, 89–90, 91 vacuum importance in, 46, 76, 98, 210 vibration and, 345–346, 350 videoamplifiers, 76–77 Scanning electron microscope, high-resolution, 187–193, 195–205 contrast in, 200–203, 204 design considerations, 198–200 operations theories, 196–197 performance, 200–203 performance limitations, 196–197 Scanning electron probe analyser, 165. See also X-ray microprobe analyser applications of, 277–280, 291–292, 293–294 design considerations for, 295–302 electron gun and, 297, 299–300 EMMA and, 280–282 first x-ray pictures from, 270–272, 273, 275, 276, 302–303 later generations of, 280–282, 306 market development of, 303–308 origins, 270–272, 290–291 SEM resolution v., 271 specimen chamber and, 271, 297–298, 299, 327 specimen size and, 276 TI and, 289–308 x-ray colour mapping with, 275–276 Scanning generators, 61 Scanning modes, of SEM lower magnification recording and, 69 maximum resolution recording of, 69 visible picture, 68–69 Scanning transmission electron microscope (STEM), 232 noise output, DQE and, 420 Scanning tunnelling microscope (STM), 128

571 Scanning x-ray microanalyser, 31, 271 CIC and, 324 TI and, 324 Schmidt triggers, 274 Science Research Council (SRC), 171, 547–548 Scientific and Medical Instruments (SMI), 372 Scintillator/photomultiplier combinations, in SEM, 21–22, 25, 26, 102–103, 104, 139, 157–158, 162 DQE and, 419–435 Scintillators, 352 in detectors, 113–114, 139 efficiency of, 149–150 hemispherical, 140 P-47, 420–421, 423, 424, 428, 430 photomultiplier combination with, 21–22, 25, 26, 102–103, 104, 139, 157–158, 162 plastic, 421, 424, 428, 430, 431 types used in DQE measurement, 423–424 unbiased, 176 YAG, 421, 423, 424, 428, 430, 431 SE. See Secondary electrons Secondary electron multipliers, 14, 16 Secondary electrons (SE) coefficient of, 182 DQE and, 421 exit angle of, 142–143 in imaging, 138, 140–141, 201–202, 488 ion penetration depth and, 488 low energy of, 140 multipliers for, 14, 16 resolution and, 130, 138–140, 201–202, 488 Secondary emission, 8, 60, 64, 138–139, 201–202 coefficient of, 112, 113, 124, 163, 200 direct v. indirect, 99–101 properties of, 112 signal intensity and, 99–102

572 Secondary incident electrons (SE-I) resolution and, 131, 138–139 SEDS. See Scanning electron diffraction system SEM. See Scanning electron microscope SEM history, 523–524 charged particle beams and, 525 CIC business performance in, 553 CUED SEMs in, 534–540 ex-UK placement of SEMs in, 540–542 Geoscan in, 552 Knoll’s electron beam scanner and, 516–517, 527 Microscan in, 549–553 scanning invention and, 524–525 scanning microanalyser and, 548–549 scanning microanalyser in, 548–549 scanning optical microscopy and, 524–525 Stereoscan in, 552–553 von Ardenne and, 528, 529, 530–532 Zworykin and, 532–534, 535 SEM to nanolithography, 485–486 See also Lithography, and SEM; Microfabrication; SEM to nanolithography electron beams in, 492 memory structures, 494 mesoscopic electron devices and, 492–493 nanolithography, 491–494 Nanowriter and, 493–494, 495 phonon-transport studies, 493 Semiconductors Alvey project and, 400–405, 407–408 electron beam microfabrication and, 371 metal-semiconductor field-effect transistors, 490–497

INDEX

microfabrication and, 179, 224, 331 Services Electronics Research Laboratory (SERL) lithography and, 360 Shaped-beam technology for variable size electron s pots, 401 Shot-effect law, 67 Signal amplification, 43, 140 Signal-to-noise ratio SEM and, 50–51, 119 TEM and, 38, 45 Silicon wafers. See Microfabrication Simple scanning electron microscope (SSEM), 153–159 electrics/safety features of, 156 Smith, K. C. A., 14, 16, 17–19, 20, 49 applications of SEM and, 93–108 commerical development of SEM and, 28–31, 311–315 stigmator and, 17–18, 122 Specimen considerations biological materials, 24, 87, 102–103, 114–115, 163, 165–173, 238, 247, 472 charging and, 132, 453–454 dessication and, 19 diameter, 335 electron bombardment in SEM and, 76, 122, 124–125 electron glancing angle and, 15, 16, 46, 48, 51, 87 electron scattering and, 19 hardness and, 169–172 hot shortness in metals and, 278, 291–292, 294–295 magnetic materials, 163 non-conductors and, 171 positioning and, 76 potential, 99–101 for sputtering, 25 surface modulation, 140–141 TEM v. SEM, 122, 124–125

573

INDEX

temperature and, 25, 181–182 thickness and, 114–115, 265, 335, 477 topography, 140–141 transparent v. opaque in, 10, 59, 63, 66, 89–90, 92 vacuum damage, 163 water content and, 472–476 Specimen potential, 99–101 Specimen stage in SEM, 45, 99–102, 327 Specimens, biological, 240, 472 amoeba, 114–115, 473 fibres/textiles, 330–331 fly eyes, 30 fly larvae, 479 freeze drying of, 474, 478 frozen-hydrated samples and, 479–480, 481 high-pressure freezing of, 482–483 interior features of, 472–474 meal worm grub, 114–115, 116, 471 paper pulp, 103, 482 pollen grains, 469, 470 preparation for SEM, 471–474, 474–476 spruce fibers, 28 teeth, 166–169, 170, 171, 471 water content and, 472–474 x-ray microprobe analysis and, 243, 249, 264, 477–478 Specimens, metal, 16, 18, 20, 21, 46, 47, 48, 51, 80, 242 electropolishing of, 156, 176–178 hot/electron emitting, 182–184 steel, 277–278, 294–295, 305 Specimens, mineral, 263 cement and, 260–261, 262 crystal lattices and, 307 Spot size in SEM, 62–63, 75, 81, 91, 340

Spreadbury, P.J. SSEMs and, 153–158 Sputtering Broers and, 26–27 coefficient of, 176–177 ion gun and, 25, 176–177, 489 SRC. See Science Research Council Stereoscan 1430VP, 460–461 from CIC, 4, 5, 32, 33, 170–171, 332, 361, 371 commercial development of, 311, 313, 329–330 first, 502 S4-10, 393–396, 479 SEM1 model, 54–55, 56 SEM2 model, 103 SEM3 model, 103–106, 107, 128, 311–312, 319, 329, 482 Stereoscopy in biological specimens, 132 low beam energy and, 132 micrographs and, 132, 337 Stewart, A. D. G. biological specimens and, 165–173 coordinate system and, 166–167 Stigmator, 97 Smith and, 17–18, 122 Thornley and, 23–24 Stitching in lithography, and SEM, 381, 382 Surface acoustic wave (SAW), 360 Surface modulation of specimens, 140–141

T Take-off angle in X-ray microanalyser, 296 Target absorption efficiency of, 264 TEM. See Transmission electron microscope Texas Instruments (TI) integrated circuit testing and, 457

574 Texas Instruments (TI) (Cont. ) SEM S-200 and, 457, 458 T-gate structures in microfabrication, 381, 382, 490–491 Thermionic dispenser cathodes, 19, 26 Thermionic spread in lenses, 191–192 Thin films and SEDS, 227–230 UHV and, 227–230 Third European Regional Conference on Electron Microscopy, 330–332 Thomson FEPG-HR machine, 401–404 Thomson-CSF EBL machine, 401–404. See also Electron beam lithography aluminium disadvantages of, 402 inverted structure of, 402 optical errors of, 402 thermal control in, 403 x-y coordinate stage and, 402 Thornley, R. F. M., 22, 23, 24 Everhart-Thornley detectors and, 366, 367 stigmator and, 23–24 wide-band detectors and, 147–152, 171 TI. See Texas Instruments; Tube Investments Research Laboratories Time-bases, 302 in SEM, 78–79 Transmission electron microscope (TEM), 88. See also Scanning electron microscope with biological specimens, 469 charge prevention in, 19 compact version of, 45 contrast in, 15, 16, 18, 19–22 Cosslett and, 13

INDEX

electrostatic focusing in, 38 McMullan and, 15, 16, 17, 18, 38 microfabrication and, 221 physical structure of, 42–43 resolution and, 19, 21, 25–26, 216, 221 SEM conversion from, 43–56, 128 SEM v., 10–11, 112 signal-to-noise ratio in, 38 vibration and, 221 Wells and, 19 Trinity College Oatley and, 439–441 Tube Investments Research Laboratories (TI), 5, 31, 246, 264 Cavendish laboratory experiments for, 294, 295 scanning x-ray microanalyser development and, 278, 344 steel quality and, 278, 293–294

U UHV. See Ultra high vacuum UHV SEM, 527 Ultra high vacuum (UHV), 230 Unicam, 342 United Kingdom Atomic Energy Authority, 485

V Vacuum, 362, 363 high, 230, 317 importance of, in SEM, 46, 76, 98, 210 Varian Associates NMR instruments from, 321, 322 Vector scan digital pattern generator advantages for lithography, 369

INDEX

automation/computer control and, 369–370 DACs and, 370 in lithography, 369–372 Vector scan electron beam lithography, 362, 363 crystallographic etching in, 363 direct polymerization in, 363 direct thermal machining in, 362, 363 resist/doping in, 363 Vectorscan and, 447 Vectorscan vector scan electron beam lithography and, 447 Virtual SEM (VSEM) automation/computer control software, 462 Voltage contrast imaging FIB systems and, 488–489 Voltage contrast method, 134, 459 Voltage contrast system SEM accessory, 457 von Ardenne, M., 3–4, 8, 9–10, 54, 63, 87, 509 recording method of, 70 SEM history and, 526, 527, 528, 530 VSEM. See Virtual SEM

W Wehnelt electrode, 44 Wells, O. C., 23, 50, 133–135 SE-I and, 132 stereoscopic micrographs, 132 topography improvement, 19 Westinghouse Research Center, 133–134

X XpertEze remote control for SEM, 462, 463, 464 X-ray colour mapping scanning electron probe analyser and, 275–276

575 X-ray crystallography, 238 X-ray fluorescence analysis, 242 X-ray microanalysis biological SEM/x-ray microanalysis and, 477–478 X-ray microprobe analyser, 195. See also Microanalysis atomic number and, 248, 262, 263, 285, 296 biological specimens and, 243, 249, 477–478 commercial development of, 313 developments in, 244–245, 248–250 Duncumb and, 245–246 emissions and, 248 manufacture of, 247–248 metals identification and, 246 Monte-Carlo techniques in, 264 SEM technology in, 244, 269–283 sensitivity in, 261 versatility of, 261, 262 voltage problems and, 313 x-ray sources for, 260–261 X-ray microscopy, 5 X-Ray Microscopy (Cosslett & Nixon), 256 X-ray microscopy and Long, and, 251–252 X-ray projection microscope, 237, 253–254 collaborations and, 254–257, 302 development of, 240–241 manufacture of, 241–243 origins of, 239–240, 254–257 X-ray proportional counter, 270, 272, 279 X-ray spectrometry command unit, 354 commercial development of, 263 in Geoscan, 354 semi-focusing, 293 vacuum in, 301

576 X-ray take-off angle, 296 X-Y coordinate stage CIC replacement of, 397 for large scale traverse, 387 Yosemite company and, 387, 389, 397

Y Yosemite company, 390 laser interferometer and, 388 X-Y coordinate stage and, 387–389

INDEX

Z Zerodur zero-expansion glass, 392 Zinc oxide phosphors from, 21–23 Zinc sulfide phosphors from, 44–45, 54, 70, 77, 112 Zworykin, V., 3–4, 11, 14, 38, 41, 44, 45–47, 509 multiplier use of, 70 RCA SEM and, 532–534, 535 scanning methods of, 70

Duncumb, Figure 4. X-ray colour map of crossed silver and copper grids, obtained by superimposing through red and green filters the images taken with Cu K and Ag L radiation, respectively. In places the X-ray continuum from the silver has overflowed into the copper channel giving yellow. The spacing of the silver grid is about 32 mm.

Frontispiece. Sir Charles Oatley with former research students and colleagues at a symposium held in his honour on the occasion of his 90th birthday. Standing (left to right): A.N. Broers, W.C. Nixon, R.F.W. Pease, T.E. Everhart, D. McMullan, K.C.A. Smith, O.C. Wells, C.W.B. Grigson, A.D.G. Stewart, T.H.P. Chang and H. Ahmed. Photographed by Kelvin Fagan (Cavendish Laboratory) and reproduced with the kind permission of Professor H. Ahmed.