Advances in Electronics and Electron Phisics. Vol. 73

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS ASPECTS OF CHARGED PARTICLE OPTICS

VOLUME 73

EDITOR-IN-CHIEF

PETER W. HAWKES Laboratoire d’Optique Electronique du Centre National de la Recherche Scientifique Toulowe, France

ASSOCIATE EDITOR

BENJAMIN KAZAN Xerox Corporation Palo Alto Research Center Palo Alto, California

Advances in

Electronics and Electron Physics Aspects of Charged Particle Optics EDITED BY PETER W. HAWKES Laboratoire d’Optique Electronique du Centre National de la Recherche Scientifigue Toulouse, France

VOLUME 73

ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers Boston San Diego New York Berkeley London Sydney Tokyo Toronto

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COPYRIGHT 01989 BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED on TRANSMITCED IN ANY FORM O R BY ANY MEANS, ELECTRONIC O R MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

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89 90 91 92

9 8 7 6 5 4 3 2 1

CONTENTS CONTRIBUTORS TO VOLUME 73. . . . . . . . . . . . . . . . . . . . . . . . PREFACE ..................................

vii ix

Ion Optics D . IOANOVICIU

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Ion Beam Focusing in Space . . . . . . . . . . . . . . . . . . . . . 111. Ion Focusing in Time . . . . . . . . . . . . . . . . . . . . . . . . . . IV . Possible Developments and Refinements . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I.

11. 111.

IV . V. VI . VII .

..

Proton Microprobes and Their Applications J . S . C . MCKEEAND G . R . SMITH Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proton Microprobes and Their Current Capabilities . . . . . . . Factors Directly Affecting the Quality of Microbeams . . . . . . Possible Future Developments in Microprobes Systems . . . . . Sample Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Applications of Proton Microprobes . . . . . . . . . . . Advantages of High Energy Microprobes . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 74 90 91

93 97 105 114 117 123 128 129

An Early History of the Electron Microscope in the United States JOHN H . REISNER

I . Author’s Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. The Earliest Electron Microscopes in the United States . . . . . IV . The Legacy from Toronto . . . . . . . . . . . . . . . . . . . . . . V . Electron Microscope Development at the General Electric Company (GE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI . Development of the Electron Microscope at RCA . . . . . . . . VII . The Farrand Optical Company’s Electron Microscope . . . . . VIII . Marton Builds His Fifth Microscope . . . . . . . . . . . . . . . . V

134 135 139 149 154 163 215 220

vi

CONTENTS

IX . Assimilation of the Electron Microscope . . . . . . . . . . . . X . Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . XI . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

222 224 229 230

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Various Methods of Electron Beam Testing . . . . . . . . . . . . 111. Voltage Contrast by Electron Probe . . . . . . . . . . . . . . . . IV . Electron Irradiation Effects . . . . . . . . . . . . . . . . . . . . . V. Electron Optical Column of Electron Beam Testing . . . . . . . VI . Automatic Control System of Electron Optical Column . . . . VII. EB Tester System . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII . Measurement of Microstructures . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

234 236 247 258 270 291 299 307 311

INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

319

Electron Beam Testing K . URAAND H . FUJIOKA

CONTRIBUTORS The numbers in parentheses indicate the pages on which the authors' contributions begin.

H. Fujioka (133), Electron Beam Laboratory, Faculty of Engineering, Osaka University, Yamada-Oka, Suita, Osaka 565, Japan. D. Ioanoviciu (l), NSCL/Cyclotron Laboratory, Michigan State University, East Lansing, Michigan. J. S . C. McKee (93), Department of Physics, Cyclotron Laboratory, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada. John H. Reisner (1 33), 671 Euclid Avenue, Haddonfield, New Jersey.

G .R. Smith (93), Department of Physics, Cyclotron Laboratory, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada.

K. Ura (233), Electron Beam Laboratory, Faculty of Engineering, Osaka University, Yamada-Oka, Suita, Osaka 565, Japan.

vii

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PREFACE The four chapters included here are all concerned with some aspect of the optics of charged particles, a subject that has recently passed an important landmark, the fiftieth anniversary of the electron microscope, but which shows no signs of losing momentum. Ion optics, with which D. Ioanoviciu opens the volume, has taken a new lease of life recently with the increasing interest in time-of-flight spectrometers, and the development of new algebraic tools for obtaining higher order transfer matrices. Ioanoviciu surveys all the various optical elements encountered in ion optics, in isolation and in combination, and relates the theoretical results to practical designs. This survey forms a useful complement to the recent book on Optics of Charged Particles, by H. Wollnik (Academic Press, 1987), and we plan to cover the algebraic methods in a future volume. The second chapter, by J. S. C. McKee and G. R. Smith, is concerned with the use of high-energy proton beams for microanalysis. There has been a rapid increase in the number of proton microprobe facilities, partly because a number of accelerators have become available for new uses, partly because there is no doubt that heavy ions are very suitable for analysing certain types of sample. The authors examine the whole subject. They first survey the systems currently in use, comparing their optical properties and the nature of the results obtained, after which they examine the general problems of designing proton optical systems for microanalysis and possible ways of improving them in the future. The last two sections are concerned with the sample itself, the first with data collection and analysis, the other with a variety of applications. The latter range from locating copper in cirrhotic livers and cadmium in cancerous prostates to a study of the paper and ink in a Gutenberg Bible, by way of many more mundane but important examples. The third chapter, an account of the development of the electron microscope in the USA by J. Reisner, is in a sense a consequence of Supplement 16 of the Advances, on the Beginnings of Electron Microscopy. I was very conscious that the volume did not do justice to the American contribution to the subject but I could not then find a survivor of the early days willing to undertake the task of writing such an account. Dr. Reisner has agreed to fill this gap and he has unearthed a great deal of fascinating information, which he presents with a wealth of anecdotal detail to add colour to the history. The pioneers and their early instruments emerge clearly, and future historians now have an authoritative account on which to base their studies, written by someone who knew many of those who figure in his pages. ix

X

PREFACE

The charged particles of the concluding chapter are again electrons, but these are used for a very modern purpose, the testing ofminiaturized circuits or devices. This is a natural task for the scanning electron microscope, and beam testing equipment is indeed based on the traditional SEM with modifications to ensure high throughput and facilitate computer control. K. Ura and H. Fujioka first survey the different signals that are employed for testing and then examine voltage contrast in some detail. Although electron-beam testing is relatively non-destructive, the beam does have some effect on the specimen, and a section is devoted to this. The remainder of the review is concerned with the design of suitable columns and both the electron optics and the automatic control system are fully described. The coverage of the Japanese literature is particularly good. As usual, we conclude with a list of forthcoming contributions. Peter W. Hawkes

J. K. Aggarwal Parallel Image Processing Methodologies

H. H. Arsenault Image Processing with Signal-Dependent Noise M. Bertero Inverse Problems H. Bley Pattern Recognition and Line Drawings 0. Bostanjoglo Electron Microscopy of Very Fast Processes

A. Bratenahl and P. J. Baum Magnetic Reconnection J. L. Brown Sampling Theory

J. M. Churchill and F. E. Holmstrom Electrons in a Periodic Lattice Potential J. M. Coggins The Artificial Visual System Concept H. G. Craighead High-Resolution Electron Beam Lithography

PREFACE

R. L. Dalglish Corrected Lenses for Charged Particles G . Donelli The Development of Electron Microscopy in Italy

J. Fink Energy-Loss Spectroscopy W. Fuhs Amorphous Semiconductors N. C. Gallagher and E. Coyle Median Filters

J. J. Gagnepain Resonators, Detectors and Piezoelectrics S. and D. Geman Bayesian Image Analysis

E. Hahn Aberration Theory J. Huggett SEM and the Petroleum Industry G. H. Jansen Statistical Coulomb Interactions in Particle Beams M. Kaiser Systems Theory and Electromagnetic Waves

K. Kano et al. Phosphor Materials for CRTs H. Van Kempen The Scanning Tunnelling Microscope H. Kobayashi and S. Tanaka Multi-Colour AC Electroluminescent Thin-Film Devices K. Koike Spin-Polarized SEM

M. Mellini HREM and Geology. S. Morozumi Active-Matrix TFT Liquid Crystal Displays

xi

xii

PREFACE

C. Mory and C. Colliex Image Formation in STEM J. Pawley Low-Voltage SEM

R. H. Perrott Languages for Vector Computers G . A. Peterson Electron Scattering and Nuclear Structure F. H. Read and I. W. Drummond Electrostatic Lenses

T. Sakurai Atom-Probe FIM G. Schmahl X-Ray Microscopy

J. Serra Applications of Mathematical Morphology

T. Soma et af. Focus-Deflection Systems and Their Applications Y. Uchikawa Electron Gun Optics A. M. Wittenberg Thin-Film Cathodoluminescent Phosphors.

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS. VOL. 73

Ion Optics D. IOANOVICIU NSCL/Cyr/oiron Lahnruiory Mic hryan Stare Unrnerrrtv EaTr Lanuny. Mrc hryun

I . Introduction

. . . . . . . . . . . . . . . . . . . . . . . .

11. Ion Beam Focusing in Space . . . . . . . . . . . . . . . . . A. G e n e r a l . . . . . . . . . . . . . . . . . . . . . . . . .

B. Elements of Ion Optical Systems . . . . . . . . . C. Systems with Ion Optical Elements in Tandem. . . . . 111. Ion Focusing in Time . . . . . . . . . . . . . . . A. G e n e r a l , . . . . . . . . . . . . . . . . . . B. Time Focusing Methods . . . . . . . . . . . . C. Temporal Matrix Description of the Ion Optical Elements D. Instruments with Focusing in Time . . . . . . . . IV. Possible Developments and Refinements . . . . . . . . References . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . .

.

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I 2 2 24 69 74 75 80 X3 89 90 9I

I . INTRODUCTION Ion optics, in the usual meaning, covers in part the larger field of charged particle optics. By analogy with the study of light rays, the physics of the movement through electric and magnetic fields was called “optics”. Ion optics deals with charged particles lying on the mass scale from the proton to the heavy ions. Ion optics incorporates a large area of particular fields such as beam formation in ion sources, beam transport to the target or collector, ion beam or packet analysis, focusing to avoid losses along the flight path. The design and achievement of the systems intended for ion beam analysis or transport require a good knowledge of the features of the ion optical elements included in such systems. Ion optics studies the elements that may constitute these systems, the combinations of the elements in systems with high performances, by inventing new elements and systems, by capitalizing and adapting elements used in other related fields such as electron optics and beta ray spectroscopy. Here we include deflecting elements, analyzers or focusing device components of instruments now in use. as well as elements with promising features. 1 Copyright @ I Y X Y by Aciidrrnic Prcs,. liic All rights 01 reproduction i n dn) lorin reserved ISBN o-ixii.w73-n

2

D. IOANOVICIU

We will consider the ion optics of the beams and that of the ion packets formed by continuous or pulsed ionization or source fields, in all these cases the ions being transported or analyzed by stationary or almost stationary fields (their variation rate being small related to the ion flight time through the instrument). We will, at first, investigate the focusing properties in space, that means, the possibilities to gather ions in a narrow area on the target or collector. We will consider also the focusing in time of the ion packets, to compress them at their arrival to the flight path end.

11. IONBEAMFOCUSING IN SPACE

A . General 1. General Laws The ion movement on the trajectory is completely determined (classical physics) by the knowledge, at any time, of the three position coordinates and of the momentum projections on the coordinate axes. Ion coordinates and momentum components determine a representative point in the phase space (I

(Y

t

Source slit M FIG.1. Radial phase subspace representation of a divergent ion beam: (a) at the source slit, (b) after a field free space travel.

ION OPTICS

3

as outlined by Bannenberg (1980).An ion packet in the real space gives a cloud of representative points in the phase space. These points move in the phase space while ions move in the real space. If ions move in stationary fields, the volume including the representative points in the phase space remains constant in time (Liouville’stheorem). Generally difficult to imagine, the phase space representation simplifies when the ion movements on different coordinate axes are uncoupled. Such a phase subspace may be represented in plane, as in the paper of Wollnik (1976), the coordinate on the abscissa, the irrespective momentum component in ordinate. Liouville’s theorem is valid for such a particular case too. If no accelerating fields act on this coordinate, the momentum projection remains constant and the corresponding inclination angle may be used instead. In systems with a plane of symmetry XOZ, the interesting phase subspaces are (x, a) and ( y, /?).In Figs. 1 and 2 these planes are represented for an ion beam in a field free space. Here x and a describe the ion trajectory projection in the median plane with respect to the beam axis (x the distance to, o! the angle with this axis), y gives the ion distance to the median plane, /? the angle made by the ion trajectory with this plane (Fig. 3). The index “zero” means “maximal value”. If we apply Liouville’s theorem to a subspace, it results that for a deflector with the entry and exit limits at the same electric potential, the determinant of the transfer matrix is unity (see Section I.A.9) fact demonstrated by Wollnik (1967a). The same theorem states the invariance of the small phase space volume expressed as dS dR U (the Helmholtz-Lagrange law) with

- -

1 slit M FIG.2. Axial phase subspace representation of a divergent ion beam: (a) at the source slit. (b) after a field free space travel.

4

D. IOANOVICIU

FIG.3. Definition of the symbols for a rectangular cross section ion beam.

dS the beam cross section, dR the solid angle, U the energy. The phase space volume conservation in a subspace for two conjugate, object and image points gives: x ~ ( U ) ”=~constant (Lagrange’s law) that indicates the emittance conservations. Then, the product of the linear magnification Mx = xf/x, by the angular magnification M = uf/a0, depends only on the ion energy at the object and image:

where f indicates final image, o object maximal value. Ions that travel across a plane separating two space regions at two different constant potentials will be refracted. The refraction angle 8, and the incidence angle 8, of the ion trajectory, satisfie the Snell’s law: sin &/sin dl = ( U , / U , ) ” 2 U , and U , being the ion energy inside the first and the second region.

ION OPTICS

5

2 lon Beam Characterization In the (x,a) phase space the ion beam emittance is given by ( 4 / ~ ) ~ , x , ( U ) ' ~ ~ , definition from the field of charged particle accelerators. Analogously, the acceptance of a transport system may be defined by the phase space volume occupied by the representative points of those ions that are allowed to go through the vacuum chamber without wall collisions. To characterize analyzers the following quantities were defined in the paper of Steckelmacher (1973): (i) the Ctendue, product of the emissive area, multiplied by the beam solid angle, both at the instrument entry slit, (ii) the transmission, ratio of the collected ion number to the number of ions entering into the analyzer, (iii) the luminosity, product of the above two, (iv) the normalised luminosity, the preceding divided by the ion beam path length square.

The sensitivity of a mass spectrometer is given by the collected ion current intensity, for a specified ion type, divided to the gas pressure in the ionization space (or by the sample quantity introduced in the source). Sometimes it is also related to the intensity of the ionizing factor. Finally, the sensitivity must be proportional to the instrument's luminosity. 3. Correlation of the Recorded Currents with the Emitted lon Properties

In an ion transport device, the major interest is to minimize ion losses on the path from the source to the target or detector. For ion implanters the ion beam purity at the target is of prime importance. In a mass spectrometer, from the collected ion currents, the nature of the molecules ionized in the source can be derived. A n analyzer correlates the collected ion currents with the properties of the ions at the emissive object. An ideal analyzer would enable derivation from the collected currents, the mass, charge, velocity and direction of each emitted ion group. The ion movement in the electric and magnetic fields depends on the mass-to-charge ratio and not individually on these quantities. However, this fact does not induce any doubt in the single charge ion mass determination. Often, the ion initial direction is of minor analytical interest. Practical analyzers give the mass and velocity (in module) for each analysed ion group. In fact, the ion mass rn and velocity u being determined, its kinetic energy U and momentum module p are also known (or conversely). In mass spectrometry, only the mass must be assigned to each analysed ion group and its relative intensity. The complexity of the analytical task depends upon the properties of the emissive object. For an a particle (He") source, an energy or velocity analysis,

6

D. IOANOVICIU

as well as an analysis of momenta, will allow the determination of the ion velocity of each group. The following relativistic (or unrelativistic, in parentheses) relations are to be used:

mv p=

JW

p2c; = U ( U

+ 2mc;)

(U=&)

(3)

c1 being the velocity of light.

For an ct particle, from a p or U measurement v results from Eqs. (1) and (2) respectively. From the same formulae the m and v of some ion group result if a) the ion source produces ions of almost the same energy U, the momentum p (or velocity u) must be measured with a momentum (or velocity) analyzer, and b) the ion source generates ions of almost the same momentum p , the kinetic energy U (or velocity v ) measurements being performed by an energy (or velocity) analyzer. If ions of different masses and velocities are produced, their analysis may be performed with a single analyzer (momentum selector for instance) if some additional relation exists between the involved quantities. If such a relation is absent, two analyzers of different kinds, in tandem allow to define m and u. In some cases two analyzers are put together in tandem to improve the instrument overall ion optical quality.

4. Resolution An analyzer selects ions according to their physical quantity D (0being the kinetic energy, momentum, velocity or mass). The capability of the analyzer to distinguish ions having the physical quantity D ACTfrom those with D is given by the analyzer's resolution W.It is defined through the relation:

+

g=-0

A0

7

ION OPTICS

To eliminate misunderstandings, the level where the separation takes place must be specified. At first, we observe that the ion analysis after c7 gives a spectrum displayed on an oscilloscope screen, recorded on a paper sheet or accumulated in a computer memory. A real analyzer gives for each ion species a collected current “peak” (the current increases to and decreases from a maximal value) rather than a straight line segment. The a A a and a peaks may have various relative positions depending on A a and the instrument performance. If the two peaks have no superimposed parts, they are completely separated (resolved at the basis). If they are superimposed in part, we speak about a “valley” between the two peaks or about the contribution of one peak to another. This neighborhood peak mixing is related to the peak width. Therefore we define the resolution at v percent valley. That means between two peaks of the same height the collected current lowers to v/100 of its maximal value. In practice, v values of 10 or 50% are often used. Of course, v = 100 indicates the limit when the valley disappears, the peaks being not more separated. Alternatively, the resolution is specified by the peak width measured at some level. The peak width results from the image (beam) width at the collector. To derive an approximate peak shape and express the resolution by the analyzer dispersion coefficient and the image width, we assume a constant current density across a rectangular image and include some derivations from Baril(l970). When the beam width I is narrower than the collector slit width A, the peak is a trapeze of height H , proportional to the total collected ion beam current (Fig. 4).The peak sizes, a proportionality constant disregarded, are: the base width I + A, the flat top A - I and the width at half height A. The flat top disappears for I = A (triangular peak). When A < I, for a constant current density in the image, the peak is also a trapeze but its height is A l l fold smaller (Fig. 5). The flat top is absent for a real, unconstant ion current density and the peak looks like the pointed curve in Fig. 5. To relate the resolution to the beam width, we consider the position of the a and a + A a beams when the collected ion current is on the bottom of its valley (Figs. 6 and 7). The distance between the two beams, measured between the two axes, is gU A a/a, where is the analyzer dispersion coefficient for the physical quantity a. If the valley has v/100, each beam contributes to it with v/200. A sheet of I/(2 x 100) width from each beam falls inside the collector slit. Therefore we have:

+

A0 go.-= c7

a

”

I

+ A - vI/lOO

7 = - c=

A0

gu

[A

and

+ l(1 - ~/100)]

(4)

8

D.IOANOVICIU

FIG. 4. Ideal collected ion current peak shape, constant current density in the image, collector slit width greater than the image extent.

A + I FIG.5 . Collected current peak shape, collector slit narrower than the image.

ION OPTICS

FIG.6. Position of the ion beams when the collector current is at the bottom of the valley.

FIG. 7. Symbols involved in the derivation of the resolution.

9

10

D. IOANOVICIU

For a given instrument with variable collector slit width, we increase the resolution by narrowing A. This is possible without sensitivity loss only when A 2 I . The best resolution at the highest sensitivity is obtained for A = I , triangular peaks, when a, = gU/1(2- v/lOO). The limit of this when the beams can not be more distinguished is given by the resolution for 100% valley, that in the triangular peak case is W,oo = !BU/I. To find the resolution of some particular analyzer, we must substitute in the above formulae appropriate gUvalues (for the energy, momentum, velocity or mass dispersion) and the image width.

5. Image Width and Aberrations We consider spectrometers having a geometry of the pole pieces and electrodes with a symmetry plane (median plane). The only exceptions are the instruments with deflectors bending the ion beam in two normal planes (as the parabola focusing spectrometers). We obtain the image from an ion emiting object that extends from -x, to +x, in the median plane, from - y o to + y o , normal to this plane, delivering ions in a solid angle limited by -a, and +ao in a section parallel with the median plane, -Do and +Po normal to this. The ions are delivered with energies between U and U(l + 6), where 6 is a relative energy spread. For identical mass ions, 6 may be related to t the momentum relative spread and to fl the velocity relative spread by: 6 = 2 t + t 2 = 28 + p2 resulted from the unrelativist energy formula. In the median plane of the instrument a radial image is formed in the point where the ion distance to the beam axis is independent of a, the radial angle, at the first power (Fig. 8). In this chapter, we calculate the radial image extent in a second order approximation. Being less interested to know precisely the image height, we calculate it in a first order approximation. This is justified by the resolution formula (4) where only the image width is involved.

FIG.8. Object, image, aberrations of an analyzer.

ION OPTICS

11

An ion with mass in, kinetic energy U,starting from the point x = y = 0 of the object plane at a = fi = 0, draws the beam axis. Let be xf the distance between the projection in the median plane of an arbitrary ion collection point and the beam axis. It may be expressed by the initial position, direction and energy difference:

Only second order terms in yi and fii are present, from symmetry reasons. The image boundaries result by calculating (xf),,, and (xf)minaccounting for all the values of xi, yi, ai, pi, 6 allowed by the emitted beam. Finally the image width is:

Here A , is the analyzer radial magnification, A, the first order energy aberration coefficient (as well as the energy dispersion coefficient), A,, the distortion. A,, the radial angular aberration coefficient of the second order, A,, the mixed energy-angular aberration coefficient, Ad, the energy aberration coefficient of the second order, A,, the image curvature, A,, the mixed axial aberration coefficient, A,, the axial angular aberration coefficient. The coefficient of a is missing because A , = 0 at the radial image. To obtain high sensitivity, increased angular aperture beams may be used. In this case, a second order radial angular focusing (with A,, = 0) is useful. Then the radial angular aberration is calculated with the third order coefficient A,,, (the contributing term 21Aaa,la2).The smallest beam width in a spectrometer dominated by third order radial angular aberration does not coincide with the image. Generally, the most important contributions to the image width proceed from the first order terms. Such an important term is A,. If it is eliminated the resolution is increased for a given energy spread or the sensitivity may be improved accepting a beam with larger energy differences. Simultaneous radial angular and energy focusing is called double focusing by mass spectrometrists. The successful design of high resolution, high sensitivity ion optical devices depends on the ability to cancel most of the second order and even superior order aberration coefficients.

12

D. IOANOVICIU

An ion is collected at a distance yr from the median plane: yf = A y Y i

+ ApPi

the total image length being 21A,Jyo+ 21A,Iflo. It is possible to obtain axial focusing too. Such a focusing hinders the beam spread normal to the median plane, reducing the losses by scattering with the vacuum system walls. We speak about axial angular focusing if A, = 0 and now A , is the axial magnification. The simultaneous axial and radial focusing in the same point (double focusing in the beta spectroscopy) produces a stigmatic image. To calculate the resolution we still need the dispersion coefficient, that if 6 is not the energy, will generate an additional term in Eq. (5). This term will take the form: A, Ao/u where A , = 9,. 6. Figure of Merit

For a given spectrometer, the resolution may be increased until some limit, deteriorating its sensitivity or conversely. Therefore the comparison of various spectrometers needs some supplementary criterion. A quality factor was defined by Wollnik (1971)as the product of the area in the radial phase subspace with the resolution approximated disregarding aberration terms: Q = 4~,~(,8),= C(Ai/pi)

where C sums on the ratios Ailpi, Ai being the radial area occupied by the beam in the deflector indexed i, pi the radius of deflexion in the irrespective deflector. Spectrometers are also characterized by the transmission to the resolution ratio, defined in Steckelmacher’s paper (1973), or by the reduced dispersion divided to the ion path length inside the instrument, as proposed by Ioanoviciu (1982). The reduced dispersion, at its turn, is defined as the ratio between the dispersion coefficient and the magnification. 7. Optimum Conditions

We consider a spectrometer with known ion optical parameters. We want to use it at a resolution 8,.How to select u0,Po, xo,yo and 6 to obtain the best sensitivity? The optimum values for the beam parameters will be calculated with the method of the Lagrange’s multipliers used by Baril and Kerwin (1965). We search for the extrema of the function B = L , + A/&?,, where L , is the luminosity and A the Lagrange’s multiplier. We assume that the luminosity is proportional (multiplication by a constant C,) to the beam section, to its solid angle and to its energy spread: L , = C,xoyoaoflo6.The

ION OPTICS

13

proportionality with 6 is evident for the sources where the ion beam intensity depends on the depth of the extraction region, the extraction being ensured by an electric field. We cancel the derivatives of 9’performed with respect to the beam parameters:

as a 9 - a 9 a 9 - as = o ax, ay, aa, ap, as

-=----

To these conditions we associate the resolution formula written in the form: 1

-=

-9,

a,xo

+ a,6 + axxx,2+ a,,x,a, + axdxoh+ a,aoZ + aa,ao6

+ a d d s 2 + “YYYoZ + a y p y o P 0 + ap,R where the coefficients ai and ajkresult from the coefficients A , , Ajkthrough the relations of the form:

A is assumed equal to I in 9,. This system of six equations with six unknowns (the optimum beam parameters and A) has generally only numerical solutions. However, in some particular cases, analytic expressions may be derived. In a first order radial focusing instrument the following coefficients play a secondary role, and their contribution to the image widening will be neglected: a,,, axora x 6 ,and,add.In this case, the optimum beam parameters are:

If the spectrometer gives second order radial angular focusing aaa= 0 and the third order coefficient A,,, must be known:

14

D. IOANOVICIU

The optimum beam parameters in this case are:

8. Movement Equations, Ion Trajectory Calculation

The ion movement equations in stationary electric and magnetic fields We write the Lagrangian 9of an ion may be derived from the lagrangian 9. of mass ma, energy V,. The electrostatic potential is V, the magnetic potential vector components being A,, A , and A,. Therefore: gJ=m"(

2

i'

+ Z' + r'@)

- eV - e(A,i

+ AerO + A z A )

The movement equations on the three coordinate axes are:

We perform the partial and total derivations keeping in mind that V and A depend on t through the spatial coordinates only. The derivatives of V, A,, A,, A , were substituted by the electric E and magnetic B field components. The movement equations are:

+ e(ZEe - reE,) = 0 mar(2i8 + r e ) - eE, + e(iB, - ZE,) = 0 m,Z - eE, + e(rbB, - ;Be) = 0 m,(Y- re') - eE,

(6)

(7) (8)

If V and A do not depend on 8: mar26 - erA, = const., if Ae = 0, m,r28 becomes a constant of the movement. The validity of the above general movement equations in cylindrical coordinates may be extended to Cartesian coordinates by using the substitutions. z=ep,

r=p+x,

Z = y

where pis the circular main path radius, x and y the radial and axial distance to

15

ION OPTICS

the beam axis. The movement equations become (1

&[( + 1

;)g+

2

3 - eE,

+ +BY

+ f)iB,,] = 0

(9)

-jBx) =0

(10)

In this set of exact equations we must put l/p = 0 to pass in Cartesian coordinates. Sometimes it is advantageous to write directly ion trajectories. The ion trajectories result selecting from all the ion paths those that satisfy the extremum condition (from the Fermat-Maupertuis variational principle) as given by Nakabushi et al. (1983).

6

1;

Nds

=0

where the integral on the ion-optical refractive index N is taken along the ion paths located between two points of coordinates s1 and s2, ds being a small ion path element. The ion optical refractive index, in stationary fields, for relativistic ions is given by the generalized momentum component tangent to the trajectory:

where is is the unit vector of the direction tangent to the trajectory. We outline that here ma is the rest mass of the arbitrary ion. Now, to locate the ion along its trajectory we choose a coordinate 5. We rewrite the variational equation in the form:

6

jcr

Fdl =0

r2

where tl and are the values of the coordinate for the trajectory ends, F resulting from N through the relation F = N ds/d(. We select two coordinates: u in the median plane and Z off this plane to localize the position of an arbitrary ion. From the variational condition result the following EulerLagrange equations: =0

the radial equation,

16

D. IOANOVICIU

the axial equation.

-0

Here “prime” denotes derivative in 5. In cylindric coordinates (r,8, Z ) with are:

r

= 6’ the

Euler-Lagrange equations

The ion-optical index function F for a relativist ion becomes: F = rn,c,[(q,

+ w,)’

- (r2 + rt2 + z ’ ~ ) ~ / ~

-

+ e(r‘A, + rA, + Z’A,) with the symbols: q, relativist formula:

=

1

+ U,/rn,c:,

w,

= eV/rn,c:. u,

was substituted from the

We introduce F into the Euler-Lagrange equations. Result the following exact relativist trajectory equations: (r” - r)(r2+ Z t 2 )- r’(2rr’ + Z‘Z”)

+ e(q, + w,)[E,(r’ + Z t 2 )

-

E,r’z’ - Eerr”](r2 + r’2 + zt2)

~,c:C(% + W J 2 -

+ e(rB,

-

11

Z’B,)(r2 + rr2 + Z’2)3/2

+ W,l2

~ac,C(%

=o

-

for the radial movement, Z”(r2 + r’2)- Z’r’(r + r”)

+ e(q, + w,)[E,(r’ + r”) - E,r’Z’

+

m,c:C(rl, e(r’& - rBr)(r2+ r” +

+ W A 2 - 111”

~aCICha

+

~ , r z ’ ] ( r+~rI2 + 2”)

-

-

=o

11 (13)

for the axial movement. If we put r = p + x, 8 = z / p and Z = y , these equations may be generalized for both cylindric (see Fig. 9) and Cartesian coordinates (circular and

17

ION OPTICS

FIG.9. Ion trajectory in a cylindric coordinate system.

straight ion main path):

+

mac,C(ro+ w,)’

-

radial equation,

I’”

=O

(14)

18

D. IOANOVICIU

e[xY?.

- (1

+

+

;).I[(+ ;y + + y.’l”2 1

m,c,C(II,

x‘2

+ W,l2 -

=0

(15)

axial equation. The substitution l / p = 0 gives the trajectory equations in Cartesian coordinates. 9. Space Transfer Matrix

In the following paragraphs, we describe the ion trajectories through x, a, y, 6, y and 8. We define y as the relative mass difference y = (m, - m)/m, where marm are the rest mass of certain ion and of the reference ion respectively. A first order description in y gives the mass dispersion coefficient. A second order description in y would be needed to find the angle between the main path and the plane where ions of different masses are focused. This second order feature being important only in some particular applications (as isotope separators) here will omit second order coefficients in y 2 and 76, as well as other cross terms in y. We consider an ion optical element with a determined beam axis. The parameters of a paraxial ion trajectory at some reference plane “f” may be connected with those at another reference plane “i”, traversed before, if the ion movement is known: Xf

+ (;)ai y+);(

= ():xi

+p.i);( +

+

(3 (3 -

d2+

+ (G)Yibi +

+ ($6 +xl);(

+ (;)a; + (;)aid

- yi

(G)P~

19

ION OPTICS

a, =

(+ +

+ (:)ai +y);(

- x.a.

''

+

(:*)

+ ($6 + (;)x?

- x.6

'

) (:@

+

- a:

+6):(

-

Mi6

+ (5)yiPi + (&)P. in a second order approximation, Yf = (')Yi = (!)yi

+ (')Bi + (')bi

in a first order approximation. ( i / j )and ( i / j k )are coefficientsthat depend on the element nature and on the positions of the two reference planes. We construct transfer matrices describing radial and axial movement (the first use of such a treatment was reported by Brown et al., 1964).The formulae (16) and (I 7) give the first and the second row of the radial matrix. We include only in the first order matrix a row (the third) with a single nonvanishing element (y/y) = 1. The meaning of this element is that y is the same for any reference plane. The next row, included in both the first order (where it is the fourth) and the second order (where third) radial matrices, has also one nonvanishing element: (6/6) = 1. This holds when the relative energy spread is the same at the two considered reference planes (if they lie at the same electric potential). The second order rows result from those of first order, of the radial and axial transfer matrices after the following mathematical manipulations:

+

+

(i) x; by squaring the row xf = (x/x)xi (x/a)ai (x/6)6, (ii) xfaf by multiplying the x, row with the a, row, (iii) x,6 by multiplication of the row x f by 6 and so on. The first order axial matrix rows ( I 8) and ( 1 9) must be also used to derive the second order matrix elements in the radial matrix rows in yz, y b and P2, by squaring and cross multiplication. After these manipulations, the space radial and axial matrices look as in Figs. lo,] 1 and 12. A complete space-time matrix description may be given for some ion optical elements (see Section 1I.C.l). For some ion optical elements even third order radial and second order axial transfer matrices are available (as for the multipoles described by Matsuo

20

et al., 1982). The third order treatment was not yet generalized but considerable effort was done. The third order matrix description assumes the use of substantially increased number of matrix elements, each third order element having much more complicate expressions than those of second order. Anyhow, to include the available third order matrix elements would lengthen excessively this chapter.

10. Ion Optical Parameters of Individual Analyzers

We anticipate the derivation, although extremely simple, of the field free space transfer matrix, to derive ion optical parameters for a single analyzer. It is assumed that the analyzer includes an ion optical element described by the transfer matrix elements (x/i),(x/jk),(u/i),(cc/jk), (ylrn),(fi/n), located between two field free spaces, one of length L , on the source side, another L, on the image side. By simple matrix multiplications we obtain for the whole analyzer: the radial magnification M = A , = (x/x)-+ (a/x)L,, the first order radial angular focusing condition

the dispersion coefficient (mass for o = y or energy o = 6) gg=A =

*

(:) + (:) -

the radial angular aberration coefficient:

-

L

Fir;.

I I . Second order radial space ( I 2 x 12)and time (last column and row) transfer matrix.

22

D. IOANOVICIU

FIG.12. First order axial space transfer matrix.

the mixed radial angular-energy aberration coefficient:

);([

+ ( 5 ) L 2 ] L 1+

=

(2)+

(-$L2,

the second order energy aberration coefficient:

the image curvature:

the mixed axial aberration coefficient:

the axial angular aberration coefficient:

The beam axial extent at the radial image result from the following total coefficients: A,=

(3+ (3 -

-

L2 and

A , = A,L1 + ( ; ) + ( ! ) L 2 .

To calculate ion optical coefficients for systems with tandem analyzers without intermediate image, we need also the coefficients:

23

ION OPTICS

Aia

=

(G)+ (i)

2 ~ 1

A &a =x):(- L 2

1

+I‘);(

+

(G)?

AL = (;)‘I

+

)(,;

where the “primed” quantities belong to the analyzer’s second row. We consider now a reversed geometry, as defined by Ioanoviciu and Cuna (1974),denoted by the index “r” below. Its results from the above “direct” geometry by inverting the object and image locations (Fig. 13). In the reversed

Reversed FIG. 13. Source and collector places for analyzers with direct and reversed geometries.

24

D. IOANOVICIU

geometry the ions traverse at first, the field free space L,. The ion optical parameters of a reversed geometry result from those of a direct geometry through the set of formulae:

A,,,

= (Ay,Af - Ay,A;Ab

A,,,

=

+ A,,A;)/M,

[2AyyA,Ab - Ay,(A/A,

A,,, = (AyyAi

A,, = A;

-

and

AybAyA, A,,

+ AyAb) + 2A,,AyA/I/M,

+ A,,A,2)/M,

= A,.

The coefficients A,, and A,, are of interest when A, is canceled (double focusing in mass spectrometry). In this case we have: Am,, = -2MA,,Ah

+ A,,,

+ A,,Ai

Adar = -MA,,Ak2

- A,,/M.

B. Elements of Ion Optical Systems

Here we list the ion optical properties of the elements to be used alone or in combination. A matrix description is tried for all these in the approximation retained in this chapter (second order radially, first order axially). Short considerations are given for some analyzers using particular ion optical elements. Finally systems with tandem analyzers are also quoted. 1. Field Free Space

The field free spaces ensure much freedom in the choice of the ion optical systems. In the field free space the ion trajectory is a straight line and the nonvanishing matrix elements of the first two rows are:

k)= 1,

(;)

=

1,

(+ (:)= ($) L, (g) L,

=

=

1,

1,

with L the length of this space. 2. Superimposed Electric and Magnetic Fields Constant electric and magnetic superimposed fields are used in ion analysis with the following configurations: crossed fields with circular or straight ion path, homogeneous crossed fields with trochoidal ion beam and parallel homogeneous fields (in “parabola” spectrometers).

ION OPTICS

25

a. Crossed Fields, Circular Main Path A system of electrodes and pole faces that produces in each point of a circle of radius p a constant radial electric field E (in the circle’s plane) and a constant magnetic field B (normal to this plane) constitutes an ion optical element with crossed fields (Fig. 14). These conditions are satisfied by toroidal electrodes combined with axially symmetric pole faces generated by a rotation around a common axis. The simplest geometry that still satisfies these conditions results for l/p = 0. It is the homogeneous Wien filter. In practice, geometries with homogeneous magnetic field are to be prefered associated with toroidal electrodes that ensure more flexibility in the ion optical parameter choice, or with cylindrical electrodes if ease in electrode machining is the most important. The crossed field sector has three qualitatively different regions: the fringing fields at the ion beam entry into the element, the main fields and the fringing fields at the ion beam exit. It is assumed that the electric and magnetic field distributions inside the main field region in the actual system are identical with those generated by closed ideal electrodes and pole faces with axial symmetry. Now, we describe the ion movement in the main part of the crossed fields. In the modified cylindrical coordinate system built around the ion main path of radius p, we write the general expressions of the magnetic B and electric E

S

FIG. 14. Inhomogeneous electric and magnetic fields, crossed fields, circular main path.

26

D. IOANOVICIU

fields, as well as of the electric potential V resulting from Ioanoviciu (1974):

(++ 6 1

-

- ce)$}

The field distributions are defined by the coefficients re, r,, ce and c, at the approximation level requested by the second order radial and first order axial treatment. These coefficients are defined on the beam axis: re the axial curvature radius of the electrostatic equipotential surface, r, axial curvature radius of the magnetic force line, c, = 8 / 8 x ( l / r e ) ,c, = 8 / d x ( l / r , ) . These quantities may be related with others used before: re = p / c , c, = c ’ / p 2 , r, = p / n , c, = [X(l - n) - n ] / ( 2 p 2 ) ,c and c’ being here the electric field “index” and its derivative (if for the reader these last symbols are more familiar). The substitution of the potential and of the field components in the radial and axial trajectory equations (14)and (15) leads through series developments to the differential equations: x”

y”

+ k2x = b,y + b26 + (al - k2a3)x2+ (a,b2 + b3)x6 + ( a 3 / 2 ) a 2 + b,d2 + a,y2 - ( a 3 / 2 ) p 2 and + p 2 y = 0.

In the radial equation use was made of the zeroth order condition:

27

ION OPTICS

The involved symbols are: Pm

=d

m / e B ,

(

+-

pe = (1

+ q)U/qeE,

q=1

+ U/rnc:,

---

Pq 2m P m 1

b4 = 2(1 + q ) 2

r:)]’ [ 2 W . + 3 ) -~ 2q2+ 1 1 Pm

The second order solution results from successive approximations or from the Laplace transform. With the above symbols the formulae given by Ioanoviciu (1974) are updated to give relativistic transfer matrix elements as those of Nakabushi et al. (1983) particularized for dipoles. The initial conditions, at the beginning of the main fields must be specified to write the transfer matrix elements. At first order we have:

s and c are defined as follows:

(i) if k 2 > 0, s = sin kZ, c = cos kZ, Z being the element length measured

28

D. IOANOVICIU

along the main path, between the effective boundaries, Z = p#~ if l/p # 0, 4 being the deflexion angle, (ii) if k 2 < 0, k must be replaced by i(- k2)”’, s = isinh(-k’)’/’Z, c = cosh( - k2)1’2Z,i = ( - 1)lI2. Analogously we define sy and c,: (iii) if p 2 > 0, sy = sinpz, cy = cospz, (iv) if p 2 < 0, p becomes i ( - p 2 ) ’ ” , cosh( -pz)’i2Z.

sy = i

sinh(-p2)11’Z

’

cY =

As results from these notations, the element with crossed fields is radially focusing when k Z > 0, diverging when k 2 < 0 and dispersing when k 2 = 0. The second order matrix elements derived from Ioanoviciu (1974)are: S

(5) Q(G = b2

);(

-

2%

-

-(-

+

u 3 ) ~ (c 1) - 1 2a1b2 - u3b2 b 3 ) ( CZ2k2 k2

Caz(k2 - 2P2) + a3p41 (1 - c ) k2(kZ- 4p2)

(6)

= Ca3(2P2 - k 2 ) - 4azI

2k2(k2- 4p2)

(;)=$(l-Zc)+

(1 - c ) +

--a3k

:(

+ 2%) 2 2(k2 - 4p2) S y r

@3P2

(2% + a 3 p 2 ) 2p2(k2- 4p2)

sc+-sc, k

)

p

sy21

i),

29

ION OPTICS

(5)=&

1 - Q)s(I 2a,

b,

- 2c)

+

k

1 2a,b2 + - --

2

(

k2

+b3)(cz+i)5

+!(a ) k4

+ bF2 b 3+ b 4 s + yb!s ( c k P

11,

P(2a2 + a3P2)s Ca2(k2- 2P2) + a3P41 Sk(k2 - 4 p 2 ) k2-4~2 Y

(+)

= (2a2 + a 3 p 2 ) ( 1 - c

(&)

k 2 - 4p2

Y’

- 2s;)

+

[a3(2p2- k 2 )- 4a, J (2a, a 3 p 2 ) S+ p(k2 - 4 p 2 ) = 2k(k2 - 4 p 2 )

It is easy to see that the above formulae can not be used to calculate /BB) elements when k 2 = 4p2. They must be replaced by:

( /yy), ( / y B ) and (

A

c)

+ -sz, B 4k

($)

=

5 (f

- cz),

B

B

A

with the abbreviations: A

= a,

- a 3 k 2 / 8 ,B

= a,

+ a3k2/8.

30

D. IOANOVICIU

To describe dispersive ion optical crossed field elements the limit of the general expressions must be calculated for k -+ 0. We find:

+ (a3b2 +2 b3)Z2

a1b2Z4

+ a1biZ6

+ b2

+

( ~ 3 b 2 b3/2)Z4 12

(2a2 - a , p 2 ) Z 2

8

(:)

=

(;)

1,

8P4 = blZ,

(5)= a,Z, (5)= a1Z2 - -, 1 xx

+

a3b2

(

(;)=-i-

+’

’

(i)

(5)

P

XU

alb2Z4

Y

’

( 2 4 - a3P2)Z2 - (a3p2 4-2U2)s;

= 0,

3

+-b4Z2 2 ’

+ (a3p28P+ 2a,)s3

4P2

():

9

= b2Z,

-

3

-%)z2, 2

2p

The potential difference and the magnetic field induction to be applied

31

ION OPTICS

to a crossed field ion optical element may be calculated with the formulae:

where A V ,and V,, are the applied potential difference and the ion accelerating voltage respectively (in volts), d the gap between the electrodes (in cms as p , ) ,

B = 1.44

x lo2

Jy

Pm

m being given in mass units, p , in crns, V', in volts. If the p , and p e value thus calculated satisfies the zeroth order condition, the main path ion will turn on the p radius circle. The above given general description of the crossed field elements envelopes some particular cases of major interest: (i) toroidal electrostatic deflectors when we must put l/p, = 0, Pe

=

- P 9

(ii) magnetic deflectors when l/p, = 0, pm = p, (iii) Wien filters with l/p = 0, p e = p,.

For nonrelativist ions U << rnc: and q = 1. We consider now the fringing field transfer matrix. In the neighbourhood of the electrode and pole piece ends the fields deviate from the ideal values. They become neglibible at some distance outside the ion optical element. The field distribution part located between a plane where the real fields differ notably from the ideal values and another plane where the fields may be neglected define the fringing field region. For both magnetic and electric fields, effective boundaries may be found. We take a strip A x (respectively A y) wide, parallel to Oz, directed at a small angle (to be specified later) to the ion beam axis outside. Let z 2 be (Fig. 15) a point where the measured and the ideal fields coincide, z 1 another where the real field vanishes. If this assertion is correct for the magnetic field, it is also true for the electric field (that extends less in space, being easier to shield). We seek for a point of the effective boundary. We denote by z , (or z,) its coordinate. We equate the flux of the real field By(or E x ) through the strip, between z1 and z 2 with that of the ideal field on the beam axis B (or E), between z, (or z,) and z 2 . Explicitly: Ax

[:

Bydz = A x

(or

Ay

from this

[:

Exdz = A y

jI

Edz)

32

D. IOANOVICIU

t

\ FIG. IS. Eflective boundary and fringing field distribution.

For the strip around the z-axis we select z, = 0 (origin on the magnetic field boundary). To account for fringing field effects on ion trajectories, two methods were applied, as results from the paper of Wollnik (1967b): a) the numerical calculation of the trajectories through specified fringing field distributions, b) the analytic calculation of the ion path in fringing field distributions expressed by series developments with coefficients functions of By(or E x ) and its derivatives with respect to the z coordinate, as explained by Matsuda (1983) and in the references therein. The second method will be given in detail below. At first, convenable expressions must be obtained for the fringing field components. A fringing field distribution that is logically acceptable must satisfy the Maxwell’s equations

33

ION OPTICS

(a)

(b)

FIG. 16. Coordinate system and boundary curvature radius: (a) electric field, (b) magnetic field.

and the following conditions: the field components must vanish outside the ion optical element and they must coincide with the ideal field components inside. Complicated fringing field distributions can be expressed by the simple, wellknown distributions of the homogeneous magnet and of the plane condenser, both with infinite straight boundaries as was demonstrated by Nakabushi et al. (1983/4). Let By = B(z) or Ex = E ( z ) be the fringing field of the homogeneous magnet or condenser, at the y = 0 or x = 0 plane, Oz being normal to the boundary. Well inside the gap B(z) = B and E(z) = E. We account for the electric field effective boundary curvature (radius Re, in the axial sectionFig. 16a) if we substitute z by z - y2/(2Re1),then develop in series E(z). For a magnetic field effective boundary of a curvature radius R,,, tilted with (Fig. 16b) we must put in B(z) a more complex expression instead z : z - xtanEl

tan2 el - 1 tan 2Rm1cos~, -)X2r,

tan 1 - R , , c o s ~ ~ -)xz rm

(

+

+

+

-3”””

The field components and the electric potential result: Ex=

[

1-

-

-+-

(;e

[1’ + -

;) ]

(Ie2

x E(z)+

+ z:,

[(;fR.j;2)

(31d:jz)9 -

-

d2

34

D. IOANOVICIU

E =Z

P

x 2 z d2E(z)

2p

dz2

Rml COSE,

p1

+

t)

I,,

(

E(z) + - x2-d 2dz2 '

'>

tan E ,

-

r,

By = B*(x) - -

d2B(z)

xyz,

dz --

~

tan E~ R,, cos E,

r,

]

tan' - 1 I 2 tan E,) dB(z) (x2 - Y2) -&2R,1cos~1 r, R,,COSE, B,

= y-

d BdX * ( X )-

-(-+

:(

r,

d2B(z) dz2 '

tanE1(x2- y 2 ) z -

)

tanE1 dB(z) -XY - R,, COSE, dz

1

xyz where tan E ,

tanE1 - C O S ~ E , W z ) ,z2, -a

dz

x=-

Z

cos E l

B*(z) being the fringing field along the z axis for a curved boundary. The derivatives d"E(z)/dz" and d"B(z)/dz" vanish for z = z , and z = z 2 while E ( z , ) = B ( z , ) = 0, E ( z , ) = E, B(z,) = B. We give the field integrals later, all together. We substitute the field components and the electric potential in the trajectory Eqs. (14) and (15) in Cartesian coordinates (1/p = 0). After some rearrangements they take the form:

35

To calculate the ion trajectories by successive approximations from the above equations we use the relations: a = a,

+

x = x1

+

:IJ:

x” dz,

p = B1 +

I:,

y” dz,y

= y,

+

I:,

p dz

adz,

Here the quantity without index refers to z, while that indexed “1” to zl. The ion trajectory thus obtained must coincide inside the main fields with some trajectory of an ion moving in the ideal fields. This last field may be continued back to the z = 0 plane, as the ideal fields would remain unaltered until this plane. We assume that this ideal trajectory has at z = 0 plane the parameters: xi,,, yin,ainrpinrelated to the beam axis inside the fields. If we follow the ion back along its real trajectory we find a straight line well outside the fields.Now we extrapolate this straight line to intersect the z = 0 plane and we obtain two coordinates xex,y e , , the angles being the same as in the field free space a,, = a l , Be, = PI. All these quantities are related to the beam axis outside the fields.If the reference system origin is on the internal beam axis, the external beam axis has the ordinate in origin:

Finally, the fringing field effect changes the ion trajectory parameters x,, , ye,, a,,, p e x into xi,,,yin,mi,,, pi,,(Fig. 17). This is performed by the matrix with

36

D. IOANOVICIU

FIG. 17. Ion trajectory through the fringing fields, trajectory parameters before (index “ex”) and after (index “in”) the reference plane.

the following nonvanishing elements:

31

ION OPTICS

tan +-2PePm (tan2 E,

E,

- 3)(lc0 - j

1

-

I), 1

+ 2R,,cos~J

+-tan 2p;

E~

(1

+ 2 tan2&,)

+ j - 1)

- tanE1(Ic0

2PePmCOS2~1 u]

,);(

=

’

tan E ,

tan2 E ,

+ u])’

-p,(l

The following convention was adopted for j : j = 1 if ze 2 0 , j = 0 if ze < 0. We detail the field integrals:

I,,

=

j:; [ yl;;

Fdz]dz

1:; [y]

-

(z: - 2222, 2

+ jzf)

2

I,,

=

dz - z 2 ,

The exit fringing field matrix elements result from the inverse transformation of the entry fringing fields. The entry fringing field transfer matrix transforms the quantities with the index “ex” into those with the index “in”. At the exit fringing fields we want to transform the quantities inside before the exit referencee plane (index “IN”) into those in the field free space after the ion emergence (index “EX”). In the first set of transformations we must substitute: x e x by x E X , Y e x YE,, aex -@EX, Bex ~ P E X Xi n ~ X I N , Y i n YIN, ain - alNand Pin -,-PIN. By successive approximations we draw out the transfer +

-+

+

-+

+

+

An element with the index “2” results from an element with an index “l”, written for the entry fringing field, by substituting E , , R e , , R,, with c2, Re2, and Rm2respectively. c2 is the emergence angle. Re, and Rmzare the curvature radii of the electric and magnetic effective boundaries. The field integrals involved in the elements with the index “2” must be performed on the exit fringing field distributions. At the exit reference plane, the beam axis starts with the ordinate in origin x A X and slope tlAx, both related to the internal axis: XAX

= -xax,

the integrals being performed on the exit fringing fields. b. Crossed Fields, Trochoidal Ion Beam A homogeneous magnetic field directed normally to a homogeneous electric field is an ion optical element with interesting focusing properties. It is difficult to connect such an element with others (sometimes a drawback). Its use is restricted by the problems connected with the production of homogeneous electric fields in an extended volume. The fields being directed as in the Fig. 18, the movement equations are: d2y d 2 z eB dy _ -0, m, d t ’ dt2 dt2 - ma dt The integration gives the ion coordinates as function of the time t: d2x dt2

eE

(E + uasinca)[

x =3 eB B

z = z a + - tE

B

Y = Y,

+ vyat

eB dz

ma

-+

ma ( E --

eB B

1 - c o s (mga) ]

)

+ %eBc o s c a s i n ( z ) ,

. )sin :. e ( u,sin~,

maua

+-COSE,

eB

ION OPTICS

39

4

E

t

FIG.18. Trochoidal ion beam in homogeneous crossed fields.

Here E, is the angle between the ion direction and the field boundary, negative on the figure (defined as for the magnets), u, the velocity projection onto the median plane, uy4its y component. For other quantities the index “a” refers to the initial conditions when x = 0 and t = 0. An ion returns to the x = 0 plane after a delay: t = 2mn,/(eB) in a point with the coordinate z = z, + 2nrnaE/eB2. Therefore the collection point coordinate is independent of E, and u p , the element ensures double focusing. The homogenous fields do not act axially and the ion collection point will be displaced in this direction by 2am,uy,leB.

c. Parallel Homogeneous Fields The parallel homogeneous electric and magnetic fields were used early in ion analysis. Recently, such analyzers were used in collective acceleration and plasma focusing studies, as reported by Schneider et al. (1985). We consider the two homogeneous constant fields E and B as acting from

40

D. IOANOVICIU

Y

FIG. 19. Ion distributions along constant mass parabolae produced by parallel electric and magnetic fields.

z =O

to z = L (Fig. 19). The following movement equations must be integrated: d2x dt2

eE ma’

-- --

d2y e B dz dt2 - m, dt

d2z e B dy and -= - - dt2 ma dt

Including the initial velocity components, index “i”, the solutions are:

We assume that the element with parallel fields is located between two field

41

ION OPTICS

free spaces, one of the length L o on the source side, another L , before the observation plane (screen). We introduce the notations: pa = mauZi/eB,ct, = uXi/uZi and 8, = uyi/uzi. Only first order terms must be retained because the device is not focusing. From (22) we obtain t , the time spent by the ion inside the fields:

We substitute this expression in (20) and calculate dx/dz as (dx/dt)/(dz/dt). The observation point abscissa will be x,:

PE

with p E = m,u?JeE. The values of y and dy/dz at the ion exit from the element may be found easily because the projection on the median plane of the ion trajectory is the circle: (Y - ~

o Po

yo - p a l 2

+ ( Z + paPo12

=

~ i (+1Pt)

For a fixed mass m and velocity u,: p = mu,/eB and pa = p(1 - T), the momentum spread z = (6 + y)/2. The ordinate of the observation point y , results :

+ [ L . + J r n

(L

+'-)]Po p 2 - - 2

+ yo

We select the ion beam axis along the trajectory of the ion having x, = yo = ct, = Po = 0. If L / p << 1 the impact point coordinates are: xf = a,/uj, y, = ay/uzrwith a, = ( L , L/2)(eEL)/mand ay = eBLL,/m. The ions with

+

different I I ; but with same m/e hit the screen in different points of the same parabola: xf = (a,/ai)yf. The source slit (or aperture) and the beam divergence transform the parabolae of each mass in strips. Two strips are

42

D. IOANOVICIU

resolved on the screen or plate if the y between the central parabola traces go A a/a is at least equal to the strip width I measured also along the y axis. The resolution is:

-

Momentum, energy or mass resolution is obtained when in this formula the adequate dispersion coefficient for momentum gT, energy g8or mass gYis put. These coefficients are:

The strip width I can be calculated from the formula: I = 2y0

+ 28, [Lo + d

m

(.+%)].

the resolution changes along the parabolae being function of p (oz). The ions of a given mass are focused along parabolae in beam separators as Lohengrin, this including a magnetic deflector with a deflexion angle 4,,, =45', followed on the ion path by an electrostatic deflector with 4, = 35.35'. The deflectors bend the beam normal each other as described by Mott et a!. (1977). 3. Electric Analyzing Fields

Three kinds of ion optical devices analyze ions with static electric fields: electrostatic deflectors, mirrors and prisms. In an electrostatic deflector, the ion beam axis lies at the same potential while in mirrors ions are repelled. In the electrostatic prisms the ion trajectories are refracted on planes separating different electric potential regions. a. Electric Toroidal DeJlectors The toroidal deflector transfer matrix elements result from those of the crossed field element by the substitutions: l / p , = l/r, = c , = 0 and pe = - p (in cylindric condensers I/r, = 0, for spherical condensers re = p). Recently. the ion optical properties of the spherical condenser in the median plane were reinvestigated by Nishigaki and Kanai (1986). The fringing field was computed under the assumption that the potential increases (in absolute value) linearly from zero to the potentials applied to the deflecting electrodes. The movement equations, derived from a Hamiltonian in polar coordinates, were integrated numerically. The calculations lead to a geometry

ION OPTICS

43

with the source and image located in the same plane, that of the Herzog shields. The deflecting electrodes subtend an angle of 174.2', the effective boundary of the electric field being at 0.0562' from the plate end, inside. These first order results, obtained by numerical calculations, may be compared with those of the general theory. This last result considers the spherical deflector as having a deflexion angle defined by the effective field boundaries, of 174.1'. Then the object and the image must be located at a distance L, = p/tan(4,/2). The substitution gives L, = 0.051~.This value agrees well with L , = 0.0502~ calculated from the slit position given in the above mentioned paper. The properties of the cylindrical condenser with a deflexion angle near 127" were also recalculated by Oshima et al. (1985). Some of these results desagree with the effective boundary draft. The trajectories were obtained by numerical integration in an electric potential expressed by harmonic series. The Herzog shields, the object and image slits were located symmetrically with respect to the cylindrical deflector plates. The effective boundary coincides with the plate end. The Herzog shields are put at an angle of 0.265 ( p M / p p / p M ) radians from the plates. pM, pm are the outer and inner plate radius respectively, while the main path radius p is connected to the before two by p= The numerical calculations indicate a correct focusing, at the Herzog shield when p M / p = 1.25 (wide electrode gap with extended fringing fields) as well as for p M / p = 1.04, the effective deflexion being 126.5' in both cases. It happens as if the image would be formed at L, = 0.1 19p in the first case, at 0.0209~for the second (both symmetric arrangements). From these numerical results we should conclude that the object and image distances depend not only on the deflexion angle, but also on the gap width between the electrodes (may this alter the field index?). b. Spiral Electric Dejector An electric deflector may be built from two cylinders generated by logarithmic spirals. Both spiral cylinder and toroidal condenser plates are more difficult to machine than circular cylinder plates. To machine a toroidal surface the metal piece must be cut along circles with a continuously variable radius. For a spiral condenser, plate machining proceeds along an unusual curve but the profile remains constant for any cross section. Thus, both the spiral cylinder and the toroidal condenser feature an additional parameter (p- to be defined below- and re respectively)if compared to circular cylinders. Such an additional parameter helps to improve the ion optical design, but its price is the more sophisticated machining. The ion trajectories in the main field of the spiral deflector result from the differential equations in cylindric coordinates (12) and (13), where the substitutions B, = Bo = B, = 0, 9 = 1 and I/ = E R ( p 8 - lnr/R) must be made. Here E is the radial electric field component for 0 = 0, r = R. The beam axis is described by the equation r = Re" (Fig. 20). To move along this spiral,

D. IOANOVICIU

44

FIG.20. Coordinate system to describe ion trajectories in spiral electrode deflectors.

the main path ion must have the energy U = -eER(l

+ p2)/2.

This is the zeroth order condition that defines also the parameter p. We work with the following small quantities:

5

= (;)e-@

-

z

1 and [ = -. R

With the help of the zeroth order condition we transscribe the differential equations through the variables 4; and 5:

& p

+

+ ( 3 p 2 + 7)@ - 3p5'6 - (1 + p2)d2

+ (1 + p 2 ) e - 2 @ [ ' 2 + pe-2'y'"f,

['f- p" = 0

These differential equations can be solved by the conventional successive approximation method. The resulted matrix elements relate the ion trajectory

ION OPTICS

45

parameters at the plane 8 = $, with those at 8 = 0. It must be outlined that the beam axis crosses the 8 = constant planes obliquely. The main field transfer matrix elements are given by Ioanoviciu (1982):

46

D. IOANOVICIU

B

(y) = 0, ($) = 1 where so = sin k04,, c, = cos ko40, k, = (8 - p2)'/'/2 with the condition p < 2& 90 = exp (P40/2)* The fringing field effects were calculated for effective boundaries normal to the beam axis. The coordinate system was selected with Ox lying along the boundary. The fringing field components and the electric potential are:

We solve the trajectory differential equations in Cartesian coordinates by successive approximations. The entry fringing field effects are described by the following matrix elements (they include the oblicity at the main field boundary):

)(:

=

(5)

(;)

- [ R 2 (21e1 1 + p')]'

P

=R J W '

(h)

=

1,

1 =( 2 R R e , J m )

At the entry reference plane, the beam axis changes its ordinate with - l e 2 / R and its slope with Ie2/R'(1 p'). The effect of the exit fringing fields may be derived as the inverse transformation of that at the entry. Care must be taken to substitute

+

47

ION OPTICS

additionally R by Repoo,E by we have:

Ee-”$O

(”

and p by -p. Thus for the exit boundary

Rep@o

)’ (2)

P2

+

= RePoo(l p 2 )

(E)=-R2e2poo(1+p2) 1

At the exit reference plane the beam axis suffers a displacement of the ordinate le2f R e p o o J w and a deviation

-Z,2p/R2e2poo(1+ p 2 ) . The potential difference A V ,that applied between two spiral plates having a gap d at r = R deflects ions accelerated by V,, satisfies the relation:

2d AV, -V,, R(1 + P 2 )

c. Plane Condenser Dejector The plane condenser is very attractive by its simplicity. It has the disadvantage of the deflexion angle limitation for the usual gaps between the plates. The integration of the movement equations: d2x - _ eE --

dt2

m,’

d 2 y d2z dt2 - dt2 -

is almost trivial (the coordinate system is given in Fig. 21 as well as other details). Series development must be made around the beam axis that is directed at an angle to the equipotential surfaces. For a plane condenser of effective length Z , the transfer matrix contains the following elements calculated by Matsuda (1975):

K3Z2

K2Z2 tan&, 2

(2)=2,

K2Z2

48

D. IOANOVICIU

I

Z FIG.21. Ion beam configuration in a deflecting plane condenser.

(i)=ZtanE-

:), (5)

(

K2Z2 t a n 2 & + -

);( );( -4, (i) =2, );( (%) -2+ =

=%tan&,

=-K3Z(1

KZ

KZ

=

= -KZtanE,

--I-tane), KZ

2 tan E ( 1 - 2KZ tan E), 1

(:)

2 ’

=

(2)

= K Z tan ~ ( 2 K Z tan E ) ,

($)

-tan,), KZ

=

-2 KZ

where K = eE/U. The already known methods are employed to determine fringing field effects near the plate ends. At the entry different from zero are the elements:

);( 2, (): COSE) (G) m7 (;)= (;)=-=-1

K

=

=

=

- K21,, COSE

2

’

(5)F.

(:)=1,

K

K’tanE 8 ’

=

At the entry plane, the beam axis is displaced radially with K1,,/2 and continue undeviated.

49

ION OPTICS

The exit fringing field effect is described by:

();

(5)

= COSE,

=

K~I,,COS~E

2 K cos2 E

'

-(;)cosE,

(:)

(k)

= sine,

= 1,

K 2 cos2E tan E 2

At the emergence plane, the beam axis is displaced also radially with K1,,/2 (the beam axes outside the condenser, before the beam entry and after the exit, make the angle 180" - 2 6 ) . The deflexion angle of such a condenser is 2.5 z 20" or less. The potential difference applied between two plates d apart A V,, deviates the ions accelerated by the V,, potential difference in accordance with the formula:

d V,/V,, = d K where 1/K is given in the same units as d. 4. Electrostatic Mirrors The electrostatic mirrors are less spread as ion optical analyzers than the electrostatic deflectors presented before. However, the electrostatic mirrors were studied carefully and their application to ion optics is in progress. The most general geometry of an electrostatic mirror results from the use with this purpose of two toroidal plates. Numerical methods were developed to determine their focusing properties by Bolduc and Baril(l974) and by Des Celles (1 974). The simplest electrostatic mirror is the plane condenser while the cylindric mirror has intermediate complexity. a. Cylindrical Electric Mirror A cylindric condenser having the inner electrode to the ground potential, the cylinder axis in the plane of the ion beam bending, the field directed towards the inner electrode works as a cylindric electrostatic mirror (Fig. 22). Less complex as structure than the toroidal mirror, the cylindrical mirror retains axial focusing properties. To derive the total transfer matrix of a cylindric electrostatic mirror we write the transformation between the two slits, both located on the inner cylinder. The transformations at the entry and at the exit due to the slit inclination with respect to the incident and emergent beam directions will be also included.

50

D. IOANOVICIU

FIG.22. Coordinate system to relate ion movement in a cylindric mirror.

We write the Lagrangian in cylindric coordinates. The z velocity component is constant always. Therefore we transform the movement equation following Sar-el (1967): r"

+ (constant)/(m,r) = 0

into a radial trajectory equation:

d Z r / d z 2+ l/(p,rsinZcicos2/3i)= 0 where pa = 2U,/[eVJln(b/a)], V, being the potential difference applied between the cylindric plates of a and b radii. ci is the ion trajectory incidence angle defined as for magnets, Pi the angle between the trajectory and the median plane. The first integration of the above equation gives: A s i n ci -dz= T dr Jp0 cos2 ci - 2 In(r/a)

(23)

(initial conditions included). The next integration needs a quadrature:

Here rmax= ae(ra/2)Cos2E* results from the condition dr/dz = 0. We perform

51

ION OPTICS

the substitutions pa cos2 ei = KZ and K ; - 2 In(r/a) = u2. We find: z

= zi

-

sf

2 a G s i n cieK2I2

e-u2/2 du

The matrix elements for the transformation between the two slits result by series developments around the main path ion parameters: E, U, with zi = 0. We connect the initial values for r = a with the final values for the same r. For an arbitrary ion, at the entry slit we have: E~ = E - cli while at the exit Ef = E + af.From (23) it results:

(g)‘

=

-tan(& + clf) = -tan ei = -tan(& - cli) and af = - ai.

The only nonvanishing element of the second row is

(t)=

-

1

The first row elements will be derived from the expression of z where we observe that if the ray lies over the beam axis zi must be positive. The series development around E and U in (24) gives a zeroth order term: the distance A, between the main path entry and exit points: K

A, = - 2 a h s i n ~

e9- ~with ~ 9- = /

~e-”’I2 du Jo

Here p and K are pa and K, for the main path ion. The other first and second order terms allow us to write the matrix elements:

(t)= 2aK tant[FeKZl2(K2tan&- cotanc) + K tan&],

(t)=- 1, (t)=-aK (“)= an

- aK

tan&[FeK2/2(1 + K 2 ) + K], tan&[9-eK212(K2 tan2&+K4tanzc-3K2- 1 ) - 3 K + K 3 tan2&],

(%)= - U K tan & [ T e K ” 2 ( 3~~ - tan E - K~ tan &+cotanE + K~ cotan E )

+ K ( -2

tan & - K 2 tan &+cotanE ) ] ,

(&)= -aK t a n ~ [ Y e ~ ~ ’ ~ ( K ~ + 2 K ~ - l +K2)]/4, )+K(l

(:)=

-1

For the off median plane ion movement, the calculation of the matrix elements (first order for the axial, second order for the radial matrix) may be performed by integrating numerically the appropriate differential equations. The axial angle transformation is derived easily. /? = ue/ui = l/m,uir with I = m,r2d a constant of the movement. Because r = a at the ion entry and exit: bf = Pi. From this the second row of the axial matrix has the elements:

(BIB) = 1 and (Ply) = 0,

52

D. IOANOVICIU

At first order: y

= Or

and dO/dz = - l/mar2(z)visin Ei. Then:

e, = -

1 mavi sin ei

loAz+ -

rdi)

ei

This may be obtained by numerial intergration of r(z). The function r(z) is where f = p cos2 E - 2 ln(r/u). Now we

the inverse of z(r) = - K tan E

have two more elements for the axial matrix:

To derive the radial elements due to the off median plane ion movement we use the radial equation with 8 # 0:

The derivative in t is replaced by the derivative in z with the help of the relation: dldt = - v sin E cos pi d/dz. The radial equation becomes:

-

1 +/I; d2r -+---dz2

rpsan2E

V2

r3sin2e

-0

where v 2 = 12/2Um. We invert the roles of the variables r and z. In a second and v 2 being retained) the first order approximation (only terms in integration gives: dr After the second integration we arrive to:

For r = a, dzldr = tan& and the contribution to a, vanishes. The only nonvanishing element in the radial matrix will be:

(z/pB) = -a&sinEeKZ/2

[ K 2 - u2 - p

+ pe(uz

- K2)

du le-”2/2 U2

The total transfer matrix of the cylindric electric mirror must account for the oblique ion beam entry and exit (Fig. 23). In both cases the angle between

53

ION OPTICS

Flci.

a 23. Ion trajectory parameters a ) at the entry into b) at the exit from the cylindrical

mirror.

the ion direction and the z axis is conserved. Trigonometric relations give:

2+

at the entry

zi =

at the exit

x f = cos Ezf + sin Ezfaf.

(s)xiai,

The matrix elements at the entry are:

while at the exit:

(2)

= cosE,

(i)(:) = sine,

= 1.

Axially both quantities are conserved at first order. The total cylindric electrostatic mirror matrix results from the multiplication of the above three. The potential difference to be applied V, to the cylindric mirror if eK, energy ions are allowed is related to the above K and E through the relation:

As an example of cylindrical mirror analyzer we consider an arrangement with ion source and collector located on the cylinder axis, given by Sar-el

54

D. IOANOVICIU

(1967). Now the source collector distance A,, is: A,, = -2a tan&(1

loK

+ KeK212

e-u2/2 du)

and first order radial angular focusing is obtained when:

In this case the energy dispersion coefficient, normal to the beam axis is QJ= - a ( t a n e / c o s ~ ) ( Ktan2 ~ E - 1). In the special case when E = -47.8' second order radial angular focusing is attained, the source collector distance being A,, = 6.12 a as given by Steckelmacher (1973). For this case, the energy resolution may be approximated with the formula: 93 = 0.92/(0.926 5a,3 3.4a06).

+

+

b. Electric Plane Mirror A plane condenser with two slits cut in a plate is the simplest electric mirror structure. It is simple to build if the electrodes are long enough to make end effects negligible in the volume where ions move, and the slits are thin enough to neglect the electric field penetration through them. Semiconducting film coated surfaces may help to eliminate end effects. The ion trajectories in a retarding electrostatic field are parabolae. They result from the movement equations integration where we substitute derivation in t with derivation in z because the velocity component along this direction is constant. As always, we choose a beam axis and consider ions with slightly different parameters. The nonvanishing transfer matrix elements were given by Baril(l970):

(t)= -1,

(z)+ - 4 K '

(&) = 4K-'cos3&, (E) = - 1;

we add

S ~ ~ E C O S ~(3)E=, 2 K - ' s i n ~ s i n 2 ~ , ($) = - 2 K - ' ( 2 s i n ~ c o s 2 ~+ coscsin2~),

(6) =2K' S ~ ~ ~ E C O S E

with K = e E / U as for the plane condenser operated in the deflecting mode (Section I.B.3.c where the potential difference to be applied may also be found. We mention some particular analyzer configurations that include a parallel plate condenser mirror given also by Steckelmacher (1973). If the ion source and collector are located in the slits, they are A,, = -(2/K) sin 28 apart. Focusing takes place if E = -45" and As, = 2/K. The energy resolution may be approximated by the relation: 9 = 1/(2x0/Asc+ a:). Second order radial angular focusing condition is satisfied for E = - 60". Now two field free spaces are to be added on both sides. In this case A,, = 3$3/2K and 9 = 1/(2x0/,hK 4.6~~2).

+

55

ION OPTICS

5. Magnetic Fields Magnetic field sectors are usual momentum analyzers in nuclear charged particle selection as well as in mass spectrometry. The inhomogeneous magnetic field sectors with field index p r , < 1feature high momentum dispersion and axial focusing properties. The homogeneous magnetic field sectors with oblique incidence and exit may also ensure stigmatic focusing at high dispersion with the additional advantage of less vulnerability to magnet saturation effects. a. Homogeneous Magnetic Field The main advantage of a homogeneous magnetic field analyzer over those with an inhomogeneous field are: simple pole piece machining and wide momentum (or mass) range. The machining of the plane parallel pole faces is easier than that of curved or even conical pole faces. The saturation effects arise inside the pole gap where field is greater than about 1.5 T (the yoke material quality and its sizes play also a role). By increasing the magnetic field intensity, saturation is first attained where the pole gap is narrower (higher local fields). For even higher fields, the field distribution in all the gap is more or less distorted. Therefore the highest useful magnetic field value on the ion main path for an inhomogeneous field analyzer is lower than that of a homogeneous field analyzer. Then a reduced explorable momentum or mass range results for the inhomogeneous magnetic field analyzer. Saturation effects arise, of course, in homogeneous field analyzers too. To push them to higher field values, the pole piece corners must be rounded (with Rogowski profiles, for example). Homogeneous magnetic field analyzers are the most widely used. This is the reason that the matrix elements of the homogeneous magnetic main field, for unrelativistic ions, will be given here explicitly. A bit more consumed space will save the reader’s time needed to familiarize himself with the general crossed field elements. We have:

(2)

’

- PS(12-

(;)=

-p(l - c)(2 8

($)=-($ ,+,( ):( (5) 0, (K) =

Xc(

=

x6

= -, S

2p

()i ;( ) = 0,

=

9

(;)=;,

(+,

-S

xx

+ c)

(;)=;,

(2) 2’ (5) =

-S

= 0,

56

D. IOANOVICIU

FIG.24. Symmetric, oblique incidence and exit, homogeneous magnetic field analyzer.

(?)=(?)=o, xx

);(

XU

8,

= - 3s

(;)=(;)=I,

(A)=%, S

=

(;)=p4,

(;)=o,

aa

X6

(p,) );(

-S

= 0,

);(

=-

(3;

(f)=O

where s = sin 4, c = cos 4, $J the deflexion angle. The particularization of the fringing field elements is simple and straightforward. We do not perform it here. A symmetric homogeneous field analyzer (Fig. 24), with field free spaces of equal length L on both sides, same incidence and emergence angles E, has the following basic ion optical parameters (the field integrals neglected): magnification: M = -1 mass dispersion coefficient: g7= p + L tan E angular aberration coefficient: A,, = L3/(R,pcos3 E ) - 3L tan E - p where R, is the common curvature radius of both entry and exit field boundaries; field free space length: L = p/[tan(4/2) - tan E ] (26) For normal incidence analyzers E = 0:

57

ION OPTICS

The most often are encountered the geometries with 4 = 180", 90" and 60", for the two last A,, = 0 if R, = L3/pz(4= 90", R, = p and = 60°, R, = 3Gp). If we cancel A,, we increase other aberration coefficients (for 4 = 90", A,, doubles). The ion optics of the Browne-Buechner, large momentum and energy range (0.5 to 1.2 of the nominal energy) spectrometers is based on the normal incidence 4 = 90" geometry, as described by Enge (1979). For these spectrometers A,, remains small on all the range. The A,, coefficient may be canceled with straight boundaries and negative E angles (the boundaries defocus axially, and the mass dispersion coefficient is somewhat lower than for normal entry). Oblique incidence was used to increase sensitivity, through axial focusing and dispersion. Stigmatic focusing is attained if the condition tan E = tan(4/2)/2 is fulfilled. Such systems were thoroughly studied by Amadori and Wollnik (1971). For this kind of geometries the basic ion optical parameters are: g1, = 2p,

L = p/tanE,

A,,

= 4p

[

]-

2 p cotan3(4/2) R, cos3 E

1,

Symmetric, oblique incidence geometries were studied by accounting for fringing field integrals, by Matsuda (1981a) Small aberration, high dispersion structures were found. The interest of the oblique incidence was early recognized in nuclear reaction product spectrometry by Elbek who used oblique incidence at 35" for

58

D. IOANOVICIU

a system corrected for second order aberration in ctz on an extended mass range (as reported by Enge, 1979). Homogeneous magnetic sectors were used in tandem to reduce gas scattering effects in low abundance measurements (mass spectrometry). Split pole spectrometers ensure the cancelation of A,, and Ass. on the entire energy range by shaping the boundaries of the two successive pole piece pairs (geometries also presented by Enge, 1979). Nuclear reaction spectrometers with refined design use three magnetic deflectors, magnetic quadrupoles (to compensate the kinematic effect) and multipoles (to correct aberrations). It is difficult to find a more intensively investigated field than that of the homogeneous magnetic analyzers. However some comments can not be avoided: one about the accuracy of the aberration coefficient calculations, another concerning the needed magnetic field area. Customarily, for symmetric magnetic deflectors with oblique incidence, the second order aberration coefficients are obtained by successive matrix multiplication. The multiplication is performed from the collector to the ion source: collector field free space matrix with the exit fringing field, with the main field, with the entry fringing field and with the source field free space. The same aberration coefficients may be computed considering this deflector as a composite system, divided in two symmetric halfs. We can calculate the transfer matrix elements of one half and express the entire system aberration coefficients through the elements of this half. We substract the aberration coefficients thus obtained from those derived with the first method. The differences look as superior order quantities and vanish if the incidence angle or/and the field integrals vanish. For E 2 20" and field integral values reported in literature, these differences can not be neglected if compared with the irrespective coefficient value (as points out Ioanoviciu, 1986a).The error made in the calculation coefficient must be at least as great as (in absolute value) the corresponding above mentioned difference. At a first look, the deflectors with greater mass dispersion coefficients would be advantaged. It must be observed that for a given main path radius and a fixed radial angular beam aperture the area occupied by the ion trajectories inside the magnetic field (the useful magnetic field region) is identical. Really, the area A occupied by the ion beam in the field (symmetric analyzer), in the median plane is fourfold that included by the ion axis, the outmost trajectory, the pole boundary and the 4/2 plane (see Wollnik 1971)

+

(y+

1)( 1 - cos;)]

= 4pct,q,

59

ION OPTICS

where the relations (25), (26) and

were used. To ensure homogeneity along a central strip, pole pieces must be made somewhat wider than the area where ions are moving. Then small deflexion angle analyzers need less bulky magnets.

b. Magnetic Wedge Field The magnetic “wedge” field is the inhomogeneous field easiest to generate. It is produced by inclined pole faces. The “wedge” field poses intrinsic axial focusing properties. It may serve as multiple gap ion analyzer accepting wide solid angle beams (orange slice structures). In the following, we use basically the results of Ruedenauer (1970) and Ioanoviciu (1975). The ion trajectories through the main “wedge” field will be described in a cylindric coordinate system with the help of a parameter $. From the Lagrangian, after the substitution of the B components B, = B, = 0, BB= Blrl/r, we obtain the movement equations: d dt

.

.B1r1

-(maz) = er-

r

d -(m,r2B) = o dt We define the parameter $ by:

i = 0, cos p cos

*

(30)

and the initial quantities ri, I(li, pi. We integrate Eq. (28) once and equate the z expression then obtained with (30).It results:

with the notations: Ka= eBlrl/mau,, a = rie-KaCosSicos*i. From (29) rn,r28 = 1 = movement constant, we express p: j = r8/va = Ve-Ka cos , where we used r in the zeroth order approximation. Here v = I/(rn,u,a). Finally:

*

60

D. IOANOVICIU

The another median plane coordinate results from the relations: z - zi =

1(i/i)dr = 1drltan II/ where we put dr resulted by differentiating (3 1): z - z i = -uK,[$:cos$[I

--e-2Kacos$(l V2 -Kacos$)]d$ 2

(32)

For 6 we have: 8 = u,ve-KaCoS~/r and 9 = (9'/z')i. We equate these, then integrate:

The main path starts from the point (rl,zl), in the median plane, with the angle (Fig. 25). Its momentum is defined by K : K = eB,r,/rnv. This ion arrives in a point (r2,z2), where the main field ends, with an angle &. r2 and z2

FIG.25. Definition of the reference planes and trajectory parameters in the main part of the magnetic "wedge"field.

ION OPTICS

61

are connected to rl and z1 through the relations: r2=

I

e K ( d12 ~ - C~O S~*

I)

,

z 2 = z1 - rl Ke-‘

cos

cOs*l

*cos t+h dt+h.

Some paraxial trajectory will be determined at the plane normal to the main path in the point ( r l , z l ) by the following initial quantities (index ‘7’’): =

t+hl + a, + s1c1y:2r:,

rr = r l

v = (Dl - Yrsl/rl)eKC1,

6, = .yl/rl,

+ c l x , + yf/(2r,)

and

z, = z1

-

slx,

where s1 = sin &, c1 = cos 11/1 This same trajectory arrives to the plane normal to the beam axis in the point (r2,z 2 )with the parameters (index ‘7”): I

J/,

=

rf

= r2

$1

+ a, + ~lcIyt/2r:,

0, = yfh-2,

+ c2xf + y;/(2r2),

zf = z 2 - s2xf

v = (r2Df/r1

- yfs2/rl)eKC1,

where s2 = sin t+hz, c2 = cos t j Z . With the help of (31) and (32) we relate the final to the initial quantities. As usually, we perform series developments. In (32) we develop with respect to the integration limits and the integrand. We arrive to the matrix elements that connect the trajectory parameters at the final reference plane (main field end) with those at the initial reference plane (main field beginning). The nonvanishing elements are:

(z) = sls2 + c1cz(r2/r1)+ s 2 ~ l e ~ K r 1 K J 1 , (f) = K(r2slc2 - rls2cl + rls,s2e-Kc1KJl), ):(

= (I)= K { r 2 c 2 ( c 2- c,) =

($1

+ r l ~ 2 e - K C r [ ( l Kcl)Jl + KJ2]}/2, -

($1 = cl(g)/rl - r 2 K ( : ) - r2K(3 -(f),

-r2K(a/xY/2,

=C I ( W 1

(k) = KCrl(Z) + %(f) - r2(321/2, ($) = ( 3 2

+ K s , ( % )- r,K(t) - ($1,

(&) = ( 3 ( K c 2 - K c , - 1)/4 ($) = [sic (A) y/J =

-

r2K($)2/2

+ rlKs2E,

+ slcl(f) + czr2 + rls2e-Kc1J1]/(2r:) - ~ ~ ( f ) ~ / 2 r ~ ,

- s,Clr1

-

c 2 ( 3 4$)/r2,

(6) = G/2 - c2($)2/(2r2), ):(

= (s2c,r2/rl - slc2 - c

~ c ~ ~ - ~ ~ ~ K J ~ ) / ~ ~ K ,

62

D. IOANOVICIU

with the notations:

JJ~I E = Ke-Kc1[(Kclc2- c1 - c,)Jl

+ (2 - K c , - Kc2)JZ+ KJJ8,

G = rzK[s,e-Kc2(Kc,J, + F) + c2(c1 - c2r:/ri)],

H

+ F).

= s2(c1- czr:/ri) - c2e-KC2(KclJl

The ion trajectories are calculated also inside a fringing field structure derived in the usual manner. In the Cartesian coordinate system of Fig. 26 the field components are:

ION OPTICS

63

X

FIG.26. Symbols and coordinate systems at the entry boundary of a magnetic “wedge” field sector.

We integrate the trajectory equations in Cartesian coordinates (14) and (15), approximate successively. The integration is made accounting that well

inside the ideal wedge field at zeroth order:

3 dz = (:)sinO,,

dz2 d’B(z) = ()sin2el

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D. IOANOVICIU

The entry boundary fringing field matrix elements are: (L) X = (=) U =

(5) = -($)/2

1,

(5) = -2($)

=

= ($)/2 =

P

(”)YY

($) = 1/(2rlKcos2el),

-(y) = tanel/(rlK),

(&) = [l/Rm, c0s3 el - sin el(2 cos2 8,

- tan2 el/(2r1K),

+ cos 81 tan2 c1

+ sin8, tane1)/r1]/(2rlK), = [sin 81 + cos 19,tan el + sin el(l + sin2el)/(K cos2 el) - rI/(Rmlcos2e1)]/(2r:K P

(f) = (8) = 1

where

cose,),

el = el - t,hl,

B, = (Bo)r=rl.

The beam axis is shifted at the entry reference plane with -Im2/(r1K cos’ el). We reverse the beam movement sense through a reflected system to calculate the exit boundary fringing field effects. The quantities that refer to the exit boundary were noted with the index “2”. The exit boundary elements are: P

(”) x = (“) a = (E) Y = (8) = 1,

(&) = -(&-)/2 = -($)/2

= tan2e2/(2r2K)

= -1/(2r2Kcos2e2), (5) = -2(“)Xd = -(y)P = tane2/(r2K), (5) (5)= [ 1/(Rm2c0s3 e 2 ) - sin &,(sin82 tan e2 + cos 8 2 tan e2 + 2 cos B2)/r2 -

tan3 &2/(r2K)1/(2r2K),

($) = [sin 8,

+ cos d2 tan c2 + sin e2(1 + sin2e 2 ) / ( Kcos2c 2 )

- r2/(Rm2cos2 e 2 ) ] / ( 2 r : ~C O S E ~ )- tan3 ~ -

~ / ( r t ~ ~ )

tane2/(2r;K2cos2 e 2 )

with 0, = e2 - tj2. At the exit reference plane, the beam axis shifts with Im2/(r2Kcos2e 2 ) . The magnetic field induction needed by a wedge field analyzer is given by the formula of Ruedenauer (1970): B

=

1.44 x 1 O 2 m / ( K r )

where B is expressed in gauss, rn in mass units, V,, in volts, r in cms. A particular wedge field analyzer geometry deflects with 180” and uses a value K x 0.75 (Ioanoviciu et al., 1973, and references therein). It ensures radial angular focusing claimed of third order and stigmatic focusing. Its mass dispersion is 0.405ASc(for momentum 0.9 lAsc), Asc being the sourcecollector distance. The resolution may be estimated from the formula: W = 1/(2x,/0.405As, + 6).

ION OPTICS

65

6. Focusing and Image Corrector lon Optical Elements

Here we consider multipoles that produce on the ion beam effects of first order (quadrupoles), second order (hexapoles), third and superior order (octupoles and superior pole number elements). u. Electric and Magnetic Quadrupoles The electric or magnetic quadrupole structure determines two planes normal each other: the focusing and the defocusing plane. Ions are submitted to focusing forces in the first, to defocusing forces in the second. Quadrupoles combined in doublets or triplets may ensure focusing in both directions. At least one quadrupole must focus on a direction. The quadrupole applications and properties are extensively discussed in the book of Hawkes (1970). The quadrupole transfer matrix elements are included in those given by Matsuo et al. (1982) and Nakabushi et al. (1983). Here we derive quadrupole second order transfer matrix in a simple way. The electrostatic potential of an electric quadrupole field V, is given by the relation:

the geometry and the coordinate system being presented in Fig. 27. We detail qe for quadrupole fields produced by circular wires located inside a vacuum pipe with the formula of Matsuo el al. (1982):

where '/ap is the potential applied to the wires, D the vacuum pipe radius, r, the wire-tube axis distance, d-r, the wire radius. For a magnetic quadrupole (Fig. 28), the magnetostatic potential is:

v, = 4mxY We substitute the field components Ex = -qex, E, = q,y or B, = -qmy, By = q,x in the relativist trajectory Eq. (14) and (15) in Cartesian coordinates. In both cases, a differential equation of the same form results if third order terms are neglected: X"

+_ k : x = 0

The plus sign refers to the convergence plane, minus for the another. For electric quadrupoles: k i = q,erl,/V,( 1 + q,), for magnetic quadrupoles: k: = qmr/[m,U,(l + 4,)]1'2. The second order matrix results from the first order solution: x = xicos k,z

+ (a,/k,)sin k,z

Y

FIG.27. Electric quadrupole, geometry and coordinate system. Y

\

/

/

/

I=

/

FIG. 28. Magnetic quadrupole, geometry and coordinate system. 66

67

ION OPTICS

by the developments: k, = k,(l - p,@, sin k,z

= sin k,z -

cos k,z

= cos k,z

Gp,k,z cos k,z

+ Gp,k,z

and

sin k,z,

where for the electric quadrupole k," = e q , q / U ( l + q), pq = (1 + q2)/2q(1 + q), while for a magnetic quadrupole k," = eq,/(mU(l + q))"', pq = q/2(1 + q). The matrix elements are: (") = (") = cq,

(f) = ~ , / k , ,

(5)= ($1 = PgSsk,Z, (5)= k,p,(s, + c,k,Z).

(f)= - k

($) = P&,

qsq9

-c,k,~)/k,,

In the convergence plane sq = sin k,Z, c, = cos k,Z, Z being the quadrupole sq by effective length. In the divergence plane k, must be replaced by im, i s i n h G Z , cq by c o s h m Z .

b. Electric Hexapoles An electric hexapole uses six parallel electrodes (Fig. 29) to produce the electric field potential V,:

V, = h,X(x2 - 3 y 2 ) / 3 . If the electrodes are wires of radius d - ro, located in a conducting cylindric tube of radius D, the wires being located at ro from the beam axis, he results also from the formulae of Matsuo et al. (1982):

y-[I he =

In

-(;>"I

( d 3 - r:)'(D6 + r,3d3)' (d3 + r,j)2(D6- r,3d3)'

This field has only a second order effect on the ion trajectories. Hexapoles located on the field free spaces in transport systems or spectrometers can serve to correct second order aberrations. In the discussed approximation, the fringing field effect is negligible. The substitution of the field components Ex = he($ - x') and E , = 2hexy in the relativist trajectory equations, in rectangular coordinates allow us to write: the radial second order differential equation x"

+ eqhe(x2 - y2)/U(l + q) = 0

and the first order axial equation y" = 0. In the first order approximation, the hexapole is a field free space of the

68

D. IOANOVICIU

FIG. 29. Hexapole geometry and coordinate system.

same length. Only second order matrix elements differ:

(2));(- );( -(+) 3, z4 j;( j;(- 1x2 ’(%> -(); -xz -xz3

=

=

=

=

=

=

-

with the abbreviation length.

x = eqh,/U(l + v]),

=

=

Z being the hexapole effective

c. Higher Order Multiples The octupoles were studied for their third order properties, useful to compensate aberrations of this order. Dodecapoles were investigated as versatile correcting elements that may work in different modes, having the possibility to superimpose simultaneously many functions as demonstrated by Boerboom et al. (1985). In this case, the electrodes must be fed with multiple potential differences (not only by L V&).

69

ION OPTICS

C . Systems with Ion Optical Elements in Tandem It is difficult to satisfy many focusing and dispersing demands by a single element ion optical system. Therefore, often the high dispersion, aberration free ion optical instruments incorporate two or even more ion optical elements in tandem. 1. System Matrix and Aberrations

The aberrations and other ion optical parameters of a system of analyzers in tandem can be expressed by the elements of the individual analyzers. Let [ / 3 be the transfer matrix elements of the first analyzer, [ / l2those of the second analyzer (Fig. 30). The total transfer matrix elements (of the whole system) are denoted by C. We distinguish two kinds of tandem systems: with intermediate radial image (actual or virtual) and without intermediate image when the ion beam is parallel between the two analyzers. If the system has an intermediate radial image [x/a], = [x/a12 = 0 because both analyzers are angular focusing. The other system parameters are:

I

I

FIG.30. Example of transfer matrix element notations for tandem systems.

P

70

D. IOANOVICIU

G,, = C f l 2 [$I1 + C%11 + &as, G,, = C412 - [&I1 + q9,, GYP = C412 [$I1 + x,,, Gy = I:[ 2 * [;I 1 + [$I 2 * [$I 1 G, = [;I2 [ill + [;I2 - [$Il where -["2 ' [%I1' [$I2 + [%11[%12 dd - 8 1 1 [&I2 + [%I1 + cts: * [&I2 + Ctll * [%I2 + c2T-32, c,, = [$I2 * [;I: + [$I2 * Cfll * Cfll + [&I2 - [$I:, x,, = 2C$12 Cfll. [ill + C%12{C;11 .[$I1 + [$I, - [fll>+ 2C612 * [$I1 * Cfll, x,, = [GI2 [%I: + [*I2 * [ill * [$I1 + [&I2 * [$I:. Gas = C

3 2 *

*

%u

1

-

*

In a system without intermediate image the analyzer located on the collector side must have a zero magnification, because the first analyzer transforms the divergent ion beam delivered by the source into a parallel beam. That means: [x/x12 = [a/.] = 0. The other ion optical parameters of such a system are:

G,and G, remain the same as for the preceding case, no particularization being made in the initial derivation. 2. Systems with Successive Electric and Magnetic Sectors

Tandem electric and magnetic sectors are included in double focusing mass spectrometers (G, = G, = 0). Always widely used, the old Mattauch-Herzog design ensures double focusing for all the masses on the magnet exit boundary with +e = 31.8"(cylindric),+,, = 90" (homogeneous). Its basic parameters are: M = - P e / P m , 9 7 = pml2,

71

ION OPTICS

(for l/Rm1 = 0, G,, = 0 in the plate points where pm = J10/3p,). “e” and “m” indexes indicate electric and magnetic deflector respectively. To accept ions with a wide energy spread, a double focusing mass spectrometer with plane mirror and profiled magnet boundary was designed by Wagli (1979). The magnetic sector boundary y = f ( x ) allows ions of defined mass but different energy to arrive in the same point of ordinate y(m)in the image plane x = a (Fig. 31).The origin of the x axis was selected at the level

a b

\ Fie. 31, Geometry and ion paths for a wide energy spread focusing mass spectrometer.

72

D. IOANOVICIU

of the ion entry in the mirror. Then the ions leave the mirror with a direction parallel to Oy with the condition that this axis make with the slit plane the same angle. For an ion beam launched parallel to Ox E = -45". Then x i= f i U / e E . The trajectory curvature radius inside the magnetic field is related to xi by the formula: p,,, = (bxi)'/' where b = a m E / e B ' . We derive the entry boundary profile f. The circle with the radius p,,,, center in the point (xi pm, - j ) , intersects Ox at A,,, = a - xi - p,,, + ( p i - f ' ) ' I 2 . Simple geometric considerations give: f / ( p i - f ')l/' = A,/y(m). Finally:

+

f=

(a - xi)(bxi)'l'

Y(m)

(a - xi)' -

2bxi

(Jbx, u

-

xi

+

1>]

A mass spectrometer was built with a = 12 cm, y(m) = 28 cm, mass range 4/ 1 , energy range 5 / 1, resolution 30. The electrostatic plane mirror-normal entry homogeneous magnet geometry was investigated and used also for double focusing. Geometries with G,, = G,, = 0 were found. Such a geometry, investigated by Baril (1970), has E = -58.74", 4,,, = 60", M = 0.53, QY = - 0 . 7 6 ~ ~ . Very often used, the Nier-Johnson double focusing mass spectrometer design is specified by 4, = 90°, #,, = 60" (original version) or 4,,, = 90" (modified), p,/p,,, = 1.238, M = 0.676, GBY = 0 . 8 3 7 ~ Its ~ . aberration coefficients are small even with fringing field effects included in the calculations as wasreported by Matsuda(1983): Gab= O.13pm,Gad= -O.06p,,,,G6, = 0 . 3 3 ~ ~ . In mass spectrometry the improvement of ion optics goes in two main directions: to increase reduced mass dispersion thus allowing the use of wide ion source slits, to reduce and cancel the most important aberration coefficients. This last way will be somewhat detailed below, the used means and a few representative geometries being given. The possibilities of the cylindric deflector-homogeneous magnet design are limited to the cancellation of G,,, Gaa,C,, while G,, is small, G,, % 2p, and G,, % 5p,. The introduction of the electric hexapoles improves the optics: G,, = 0 with a hexapole, G,, = G,, = 0 if two are used. Even the complete second order focusing, discussed by Matsuda (1 983), that means G,, = Gas= Gad = Gyy= G,, = G,,. = 0 may be attained by using toroidal deflector-homogeneous magnet combinations. We give two representative examples:

1) converging toroidal field, complete second order focusing instrument with 4, = 85", pelre = 0.525, I$,,, = 90", = 30", e2 = - 10.7", R,, = -2p,,,, p,,, = 20 cm, p,/p,,, = 1.077, M = 0.53, gY= 1.02pm, = 83000 (v = lo%), 2) diverging toroidal field, partial second order focusing, 4, = 1 ] g o , p,/ re = 2.8, 4,,, = 60", R,, = 1.514pm,p,/p,,, = 1, M = -0.254, GBY = 1 . 0 6 5 ~ ~ .

ION OPTICS

73

Also complete second order focusing is possible in cylindric deflectorhomogeneous magnet systems with an electric quadrupole located between the analyzers, as for example in the instrument having 4e = 80", +,, = 70.8", p e / p m = 1.314, M = 0.43, gY= 0 . 8 5 ~ ~ . In despite of these improvements, the best attained resolution, over a million, belongs to an earlier instrument whose parameters we just remember here: toroidal field of 4e = 118.7", pelre = 1.936, p e = 30 cm, first magnet q5,,,l = 198.1",pml= 22cm,pml/r, = 1 (fieldindex),4,, = 30",p,, = 120cm (homogeneous), as reported by Matsuda (1981b). Recently, small magnification ( M = O.l), gYz pm, geometries with two sticked electric sectors (divergent + convergent), followed by a homogeneous magnetic field were investigated by Matsuda (1985). A possible way to increase mass dispersion and therefore resolution is the use of electric prisms combined with oblique incidence magnets, as suggested by Ioanoviciu (1986b). Tandem electric field-magnetic deflector geometries are also used in beam separators for nuclear reaction heavy products. The recoil mass selector at MIT-ORNL, described by Enge (1981), uses a 4m= 20" magnetic deflector, located between two 4e = 10" plane condensers, overall vanishing deflexion for the selected ions, magnetic quadrupole doublets to ensure beam focusing. The Rochester University recoil mass spectrometer (also mentioned by Enge, 1981)is a symmetric structure with a #,, = 36.2" magnet in centre, a 4e = 15" cylindric deflector (opposite deflexion sense to &,) and a magnetic quadrupole triplet on each side.

3. Systems in Tandem with a Crossed Field Element A crossed field element with straight main path, the Wien filter (index "w" for the involved quantities) may be combined with a magnet, as indicated by Ioanoviciu and Cuna (1975) or with an electric deflector (as proposed by Ioanoviciu and Cuna, 1974) to give double focusing mass spectrometers. The second design poses the following advantages: 1) needs a single magnet, 2) features electrically tunable ion optical properties. The first order Wien filter focusing and dispersive properties can be varied through pw(pw= p e = p,,,). It must be fitted with a variable re cylindric deflector foreseen with Matsuda plates. Built mass spectrometers of this kind are: 1) the stigmatic focusing spherical condenser and oblique incidence Wien filter instrument described by Taya et al. (1978), 2) the diverging Wien filter-variable re electro= static deflector apparatus with virtual intermediate image having 0.03873 cm-', pw = 12.25 cm, 2, = 30 cm, 4e = 31.8", p e = 20 cm, M = -0.25, 9*= 20.4 cm, W = 5600 with a source slit width of 0.01 cm, built by Cuna and Ioanoviciu (1983).

74

D. IOANOVICIU

Wien filters were associated with magnetic deflectors in nuclear reaction product recoil spectrometry (as reported by Enge, 1981). In the Michigan State University recoil mass selector, the Wien filter and the magnetic deflector are located between two quadrupole triplets. Some quadrupoles posses hexapole components to correct a6 radial and 6p axial aberrations. The Daresbury instrument has a Wien filter split in two parts to allow primary beam extraction without Wien filter plate stricking. Two quadrupole triplets, located on both Wien filter sides and a quadrupole put after the magnet complete the instrument's optics. The MIT-Brookhaven National Laboratory's mass and energy spectrograph performs velocity analysis in a Wien filter and momentum selection in a split pole magnetic spectrometer. The magnetic field direction inside the Wien filter is normal to that in the magnetic spectrometer. Therefore the ion deviation produced by velocity dispersion y = aau is normal to the magnetic momentum dispersion x = a,mv (aa, a, instrument constants). The ions of a given mass having different velocities are collected on a straight line y = (ap/a,)x/m(see Enge, 1981). A ionospheric research mass spectrometer built by Hahn et al. (1981) embodies an electrostatic deflector followed by a crossed field, circular main path analyzer (index "c" below) bending in the same sense. After the electric deflector, a 3400 volt postacceleration is applied. The electrostatic deflector first used was a cylindric one ($e = 112", field free spaces 1 cm long) pe = 6.92 and 8.92 cm-two channels being used), later replaced by a spherical condenser ($e = 96") to increase the polar viewing angle. The apparatus includes a homogeneous magnetic field-cylindric condenser crossed field element with $c = 123" (however magnetic fringing fields extend the magnetic deflexion angle to 128.7" for 3000 eV H + ions), pE = 6 and 8 cm for the two channels, field free spaces 1.5 cm long. The instrument's resolution was over 10 in mass, 15 in energy.

IN TIME 111. ION FOCUSING

Time of flight systems operate with ion packets (pulses). These packets, formed in the source, separate during the flight in groups according their masses. These groups induce at their arrival to the collector, quickly time variable signals. Ion analysis and focusing in space is performed on stationary beams or on beams with slowly changing position (when the spectrum is explored). Time analysis is made on short ion packets. These packets are ion populated corks moving along the empty way where ion transport takes place. While for space analysis the ions may be generated continuously, time analysis needs pulsed ion sources.

15

ION OPTICS

A . General 1. Temporal Description of Ion Movement in

Homogeneous Electric Fields

To evaluate the length of an ion packet at the source, a temporal description of the movement inside the source electric fields is needed. For simplicity, these fields are assumed to be homogeneous. We consider the ions moving in a homogeneous electric field of intensity E directed along the positive sense of the z axis (Fig. 32). From the Lagrangian of the movement result the following simple equations: Z = eE/m, x = y = 0. We take the time origin at the moment when the ion reaches the z = 0 plane. The integration gives: i = e E t / m + uZi and z = e E t 2 / 2 m vzit. Here uzi is u, for t = 0. We distinguish two cases:

+

a) The ion goes beyond the homogeneous field limit z = d . In the second equation we put z = d when t = td and we find the time spent by the ion in the field: t , = - - +muzi eE

J m T -+-= e2E2

d&

with

X

z = d 1 = td

FIG.32. Ion trajectories in a homogeneous electric field:the ion goes beyond the accelerating or decelerating field (trajectory of type a), the ion trajectory is reversed by the retarding electric field towards the entry limit (trajectory type b).

76

D. IOANOVICIU

The relation is always correct if E > 0. If E < 0, the quantity under square root will be positive for UZi> ed( - E). b) The retarded ion ( E < 0) stops in the field at a distance z = d , < d , after a time t,. It is then accelerated towards the plane z = 0, where it arrives after t = 2t,. The ion stops when i = 0. From this it results: t

= 2t, =

-2J2m~,, eE

The ion penetrates inside the field to the depth d , = Uzi/(- e E ) 2. Ion Packet Formation, Its Size In a stationary beam ion source the ionizing factor (electrons, laser photons, thermal ionization) operates continuously, the extracting and the accelerating fields being constant. The spectrum exploration by accelerating voltage variation is now less used. The time of flight spectrometer ion source generates packets by continuous ionization but with pulsed ion extraction or conversely, by pulsed ionization while draw out fields remain constant. If the ions leave the ion source with constant energy or constant momentum depends on the accelerating voltage. If constant in time, the accelerating voltage imparts the same energy to all the ions of the same electric charge. To acquire constant momentum, all the ions must be accelerated during the same time interval, therefore by a pulsed accelerating voltage. The space and initial velocity structure of the ion packet depends on the ion formation mode and on the space charge densities at the formation place. The initial velocity distribution of the ions formed from gas molecules is Maxwellian. The initial velocity distribution is practically unaltered for pulse ionization. If the ionization is continuous, performed by an electron beam, a potential well builds up. Between two consecutive extraction pulses the ions whose velocities are smaller than d m -are retained in the well of depth V,. The ion packet thus formed has a heart of ions with initial energies less than eV, surrounded by a rarefied cloud of ions with higher energies. This cloud spreads during the time delay between two consecutive extracting pulses, each ion moving according to its initial direction modified only by refraction at the electron beam surface. The ion packet, leaving the source in the form of a coin, grows up along the flight path. We assume that the highest initial velocity component along the extraction field direction is v,,. Because the ion packet contains ions of any velocity, the selection of the value of the v,, is somewhat arbitrary. This velocity would be so selected that ions with higher velocities do not contribute significantly to the packet electric charge. Selecting an upper limit for the initial ion velocity,

77

ION OPTICS

FIG.33. Fields and ion initial velocities, ion packet length determination

we simplify the packet length calculation but we obtain an erroneous image of the charge density distribution inside the packet, on its evolution during the flight, as well as about the charge density distribution effect on the peak shape. The ionizing factor produces ionizations in a region of thickness As, located at so from the extraction space limit, during the time interval t o . In the extraction region the electric field intensity is E,. A field E, accelerates the ions on a space s,. We evaluate the ion packet length A t , at the exit from E , A t , begins when the exit plane is traversed by the first ion produced at the right limit of the ionizing region (Fig. 33) with the velocity uzo. It ends when the exit plane is reached by the last ion produced at the left limit of the ionizing region with a velocity - u,,. Thus:

.

+

Sa

Sa

m+Jcz-m+m+

with the notations: U,,

= mu;,/2,

Us-= U,,

Us+= U,,

+ eE,so,

U,,

+ e(E,s, + E,s,).

=

U,,

U,-

=

U,,

+ eEo(As + so),

+ e E , ( A s + so) + eE,s,,

The first term, inside the brackets, is due to the “turn a r o u n d time of an ion with the velocity u,, directed at the left. The ion packet expands in all the

78

D. IOANOVICIU

directions. Its flight along a field free space Lo will lengthen the packet to the value A t,:

The ions contained in the packet feature two kinds of energy spread: ones due to the initial velocities of the molecules from that ions originate and ones due to the different potentials of the formation places.

3. Resolution Let investigate the resolution in time with respect to the physical quantity o.We must know the delay at the collector t d between an ion with o A ITand another with o, both leaving the source simultaneously. We consider ion packets of length A t at their arrival to the collector, having constant ion density inside. The packet of the o A o ions will be resolved (seen as distinct signal) from the packet of CJ ions if td = ToA o/ois greater or at least equal to A t (Fig. 34). Here Fuis the analyzer dispersion coefficient in time for the physical quantity o.The resolution in time is given by the formula:

+

+

.

It is easy to calculate Yofor a time of flight analyzer where the reference ion path, of length L, lies at the ground potential. To this class belong the deflectors discussed in the space focusing part. Let u+ be the velocity of the o A o ion, u that of the o ion considered as reference particle, t = L / u the time interval needed by the reference ion to travel from the source to the collector. Then T u A o/o = L(l/u+ - l/u), and we can detail o. For the velocity dispersion u+ = u(l + fi), Fp= t while for the momentum dispersion u+ = (p/m)(l + z) = u(l + T) and = t too. All the quantities p , U, m, V, here and below belong to the reference ion. The energy dispersion

+

I

-

I

~-

FIG.34. Resolution definition for a time of flight analyzer.

* I

79

ION OPTICS

coefficient is only half this value Fa = t / 2 , because u, = v(1 + 6/2). The mass dispersion coefficient is Y7= - t / 2 for constant energy ions as it may be seen from the formula: v,

=E(1

+;)="(l-;)

This coefficient increases twice and also the resolution, if constant momentum ions are used Fy= - t . Now u, = ( p / m ) / ( l + y ) or v + = u(l - y). The increased mass resolution advantage does not compensate always the difficulties related with the constant momentum ion production. 4. Optimum Conditions

We assume that the obtained signal at the collector is proportional to the electric charge transported by the ion packet. We suppose that the slit in front of the detector is thus sized that all the ions focused in space to the detector are collected. The number of the ions contained in the packet will be proportional to the source slit or aperture surface and with the emission angles, with the etendue, that is with x,, y o , a,, go.The ion quantity must be also proportional with the volume where the ionization is performed, with the thickness of the region where ions are formed. Since the ion energy spread in the packet depends on the ionization region thickness (if we neglect the initial velocities), packet charge is proportional to the energy spread. The ion quantity is still dependent on the time interval when ionization takes place. We write the packet charge as: Q = atx,yoaoflohto,a, being a proportionality constant. We detail the packet length A t in the resolution formula. It may be written in the form: 1

+ a,x, + a,a, + a,6 + terms of second and superior orders

- = aoto

9

a, are coefficients resulting from the packet length coefficients divided by the dispersion coefficient Fg.By using Lagrangian multipliers we build the function 9 = Q + A/%'. The partial derivatives with respect to the packet parameters must vanish:

6 9 dt,

-6

9-s

sx,

9 - 6 9 - s 9 - 6% = o

sx,

6yo

sg,

6s

They give six relations. They must be associated with the resolution formula that is function of the packet parameters through the coefficients a,. From the seven relations we eliminate A and the optimum values of x,, y o , a,, Po,6 and to result. Analytic expressions may be obtained only in some particular cases. In

80

D. IOANOVICIU

general, a system of equations with unknowns at first, second and even third degree must be solved. It is possible numerically. Because to,the ion formation time interval, enters only at first order, its optimum value is easy to derive. It is to = 1 / ( 3 a 0 9 ) ,if all x,, yo, a,, /I,, 6 and to participate with coefficients to the packet extent. B. Time Focusing Methods

Time focusing may be attained by various methods, including the use of field free spaces of appropriate length, electrostatic mirrors and deflectors. 1. Focusing on Field Free Space

The simplest time of flight analyzer is the field free space. The ion packet longitudinal spreading, produced by ion extraction from different equipotential surfaces can be eliminated by the so-called “space focusing”, used by Wiley and Mc Laren (1955).With this purpose the independence of the time of flight from the ion formation point must be ensured. The flight time is:

+

with the symbols Us = eEosor V, = eE,so eE,s, and C = ASIS,. In this formula the initial kinetic energies U,, were neglected. We develop in series after X. By cancelling the coefficient of X,focusing in time after space, at first order results. If the coefficient of X2 also vanishes second order focusing of this kind is ensured. From the first order condition we have:

The particular cases s, = 0 leads to U, = Usand p = 1 . The condition becomes purely geometric Lo = 2s0. It is of no interest for the construction of an instrument based on this principle. For second order focusing the following relation must be added:

The initial molecular movement, before ionization, contributes to the These are: the “turn around” time interval packet spreading by terms in Uzo. 2(2mU,,)”2eE, and a term that contains also A s considered thus of second order. The time aberration induced by U,, can not be eliminated in a time of flight device with a field free space. It may be reduced by the application of

81

ION OPTICS

intense Eo fields, but this method may violate the space focusing in time condition. 2. Impulse Field Focusing

The reduction of the packet spread due to the initial velocities by extraction field increase and keeping “space focusing” is possible in two ways: 1) by using an additional electrode, slit or grid, in the ion source or, 2) by the variation of the extracting field during ion packet formation, stronger at the beginning, reduced later, by pulsing the extraction electrode (method used by Marable and Sanzone, 1974).We consider the system with four parts (Fig. 35), three with homogeneous fields, one field free, E , > E , . By summing the partial flight times, by developing in series after A s and cancelling the first order coefficient, we obtain the “space focusing” in time condition for this geometry: S2P

s1p=-+v(1

+ v)

sa

v(v

L O

+ p) 5p +

where p = (UC/UA)’”,v = (UB/UA)1’2, UA = e E , s , , U, = UA + e E , s 2 , Uc = V, + eE,s,. If s, is absent UA = U,, v = 1 and the relation reduces to that of the preceding paragraph. We may eliminate the grid between s1 and s2, apply El for a time t , greater than the “turn around” time for the ions major part, then reduce it to E , . The preceding formulae must be used if we substitute s1 by the space travelled by the ion during t,, that is: s1 = eEIri/(2m), what depends upon the extracted ion mass. A time of flight system with impulse field focusing discriminates against mass.

3. Energy Focusing with Electrostatic Mirrors Energy focusing in time can be obtained with homogeneous retarding fields, as related in the paper of Frey et al. (1985)and in the references therein.

A

lB

c‘

FIG.35. Definitions of fields, energies and distances, for time focusing in space and impulse field focusing.

82

D. IOANOVICIU I

I

I

I

A

I I I

I I

I

I z1

-+

Ea

~

I_

z2

I

+

I I -

,

Eb

FIG.36. Ion focusing in time: energy focusing by a mirror with two retarding fields.

We consider the geometry given in Fig. 36. The retarding device has two stages: the first decelerates the incident ions in the field - E,, the second with the help of the field - E , returns the ions towards the entry boundary. The field free space projections on the field direction are L for the source-electric field limit space, L , for the field limit-collector part. We write the total source-

ION OPTICS

83

collector flight time:

+

where U , = UJ1 6), Ue = r n ( v c o s ~ , c 0 ~ ~ , ) ~U/ 2, ,= U , - eE,Z,, U, = eEbZ2. The following terms contribute to t,: the flight on the field free spaces (the first), penetration and emergence time through E, (second term), the “turn around” in Eb (last term). We perform series developments of U , and U , in 6. The energy focusing is accomplished when the terms in 6 are cancelled: the first order term vanishes, first order focusing being obtained, when:

while for second order focusing we must satisfy simultaneously also the condition:

where q = Jv,/v,,1’ = u b / u e , U, = U, - eE,Z,. If we are satisfied with a first order energy focusing in time, a single retarding field suffices. Then U, = U,, q = 1, 2, = 0 and s / i 2 = (L, + t 2 ) / 4 . C . Temporal Matrix Description of the Ion Optical Elements

The matrix method can be extended to the description of the ion movement in time. It is possible to calculate specific matrix elements for each kind of optical analyzer. 1 . Temporal Matrix Elements

As it is usual, we consider two reference planes, one at the entry-the “zero” plane, another at the exit-the “z” plane, of the ion optical element. Temporal matrix calculations give the delay, or its correspondent in distance, at the exit plane between an arbitrary ion and a reference ion, both arriving simultaneously at the entry reference plane. Such a description was derived in a second order approximation, for deflectors and quadrupoles by Matsuda et al. (1982). The ion path through the ion optical element can be divided in small parts ds, the ion velocity u,, being almost constant on each of these. The ion flight time between the two selected reference planes is the sum of the flight times

84

D. IOANOVICIU

dt = ds/u, on these small path pieces:

For the reference ion li, = u and ds lies on the beam axis ds = dz. For this particular ion: t = z/u. The time delay t , - t may be converted in a reduced distance I what results by multiplying ( t , - t ) by u:

In this expression ds/dz must be substituted with: ds/dz = (I + x“ + Y”)”~ in Cartesian coordinates, here “prime” having the meaning “derivative in z”, or ds/d9 = (r2 + rI2 + Z 2 ) l I 2 in cylindrical coordinates with “prime” for d/dd, or still dsldz = [(l + x/p)2 + x ’ + ~ y’2]1/2for both if the the beam has a circular or straight main path. The temporal matrix elements result by integrating the expressions obtained after series developments of ds/dz and u, in (34). The complete spacetime description of the ion movement is accomplished with the help of the matrices presented in Fig. 10 and Fig. 11. At first, to the space transfer matrices established before, we add a column with the elements (l/v),where v = x, a, y, 6, 1 (Fig. 10) or v = x , a, 6, x 2 , xa, x6, a2, a6, a2, y 2 , yp, p2, 1 (Fig. 1 1 ) ; in fact, the only nonvanishing element being ( l / I ) = 1. We put also a new row under the radial space matrix with elements (l/v) that generally are different from zero (see Fig. 10 and Fig. 11). 2. Field Free Space Temporal matrix elements may be derived for a field free space of length L, by simple substitution in the general formula (34) of the u, development, c1 = p = constant, for Cartesian coordinates. A simple integration, as demonstrated by Matsuda et al. (1982) gives:

(;) -(:) =

=

(i) (h) ?, (A) =

=L

= 3L/8.

The other temporal matrix elements vanish. In the second order approximation, the temporal transfer matrix elements of a hexapole or other superior order multipole do not differ from those of a field free space of the same length.

3. Crossed Electric and Magnetic Fields a. Circular Ion Beam To obtain a final spectrum by time of flight analysis, the ion packet must arrive to the collector without before-hand dispersion on mass, energy, momentum or velocity in space. Otherwise, only a small part

85

ION OPTICS

of the spectrum, depending on the resolution in space, will be time analysed. Therefore, to perform a wide range time of flight analysis, only pure electric field deflectors are allowed for constant energy ions, pure magnetic fields for constant momentum ions. The isochronous magnetic systems have mirror symmetry about a plane normal to the ion path. Recently, such a system was studied by Wouters et al. (1985). In systems with a structure of this kind, the final dispersion in space is absent and time of flight analysis is allowed. The condition to collect various ion species is that the radial extent of the vacuum chamber and the magnetic field area ensure a flight without losses of the radially dispersed ions along their path. Generally speaking, crossed fields can not be used to perform wide range ion packet time of flight analysis. However, the temporal matrix elements of these crossed fields describe in a compact form the electrostatic deflector, as well as the magnetic field properties as particular cases. We put in the general formula (34), x, x’. y, y’ from the space transfer matrix elements, u, from the relativistic relation that includes U and V. The integration is made so that second order terms are retained. The temporal matrix elements quoted below result from those given by Nakabushi and Sakurai (1983), by using some slightly different notations: );(

($1 = fl(1

= flS/k,

6,= b*fl(Z

-

s/k)/k2

-

- c)/k2,

z/rl(l

($1

= blfl(Z - s/k)/k2

+ rl)?

2k

($) $ [ =

fl

(q

[

+T k b2fi

+$

a3

+ !)(I P

- c)

-

3)

, - z]:

-f

”;) +

--

I::(

[ ($ $) fl

-

+ z/rt(l + rl),

+ $[jl(; + j$

b3f1 ~

2

- f,

+

[

b,

s)+

rf 3)

-

-

+ 2b,f, + f3k2]r

;]

sc,

f2

-:Is’,

-~ ] C Z

86

($)

D. IOANOVICIU

1 4a1

=4

@bZf1

+ j$

[b,($

(&)

+ b3fi + 2bzfz + f 3 k 2 -

$)

- %]SZ

)

+ $ [fl($

+ $) + bzb, + b4k2

= ${fl[b:($

(1 - 4 -

$) +:

1

+fl(az

)=(;

+$)

kZ-4p2

'

k3(k2 - 4P )

- fZ]sz,

b. Trochoidal Ion Beam From the equations derived in Section I.B.2.b it results that the ions return to the x = 0 plane after t = 2nm/eB. This delay is independent of the initial angle or energy. The ion trajectories are isochronous. The time mass dispersion coefficient is: ( l / y ) = 2nmu/eB.

4. Electrostatic Mirrors a. Cylindrical Electrostatic Mirror The ion velocity component along Oz is constant and t = (Zf- Zi)/U,i

(35)

The calculation of l/uzi gives: 1 ---&&[I uZi - sin E

- c o t a n c - a + -Y- - +6 -+cotan'& a2 2 2 ( ;

E . c r G + - 632 +-cotan 2 8

+q

)

2

We substitute zf - zi = Az + (z/a)a and other second order terms in (39, and we multiply by u = - s i n ~ , / m . The temporal transfer in a cylindric

88

D. IOANOVICIU

mirror is described by the elements: ($) = I/sin E,

(h)= - 1/2 sin E,

(6) = cotan &/sinE,

(f)= [(E) - A, cotan &I/(- sin E), ($) = - Az/(2 sin E),

($) = [ ( f )- Az/2]/( - sin E ) ,

(6) = [(k)(t)cotan E + A,(cotanz E + *)I/( - sin E), ( L )= [($) - (%)cotan E - (:)/2 + A, cotan &/2]/(- sin E), (h)= [Az/2 + (&)I/(-sin&). (L) 66 = [ -($)/2 + i A , + (&)]/(-sin&), -

a6

The other elements vanish. b. Plane Electrostatic Mirror and Condenser The time of flight through a plane condenser working in an electrostatic mirror may be calculated by using the following matrix elements: ($) = l/sin E,

(f) = 2/sin 2&[Acos E cotan E ,3-(

(S) = (l/sin 24[A cos E - 2(%)],

(A)= -cotan

(&)= (2/sin 2~)[($)(cotanE - tan E )

-

($) = -A/2 sin E,

&/sinE , ( f ) = - 1/2 sin E,

($) - ~ A C OE S- A cos E cotan’ 61,

(f) = (l/sin 2&)[2(%)(cotan E tan E ) - 2(%)

+ (f) - A cos E cotan E],

(&)= (l/sin 2~)[($)- $A cos E],

(h)= -(

l/sin 2~)[2(&)+ A cos E ] ,

where

A

= 2K-’

sin 28.

In an electrostatic plane condenser with beam entry and exit between the plates, the ion transit time results from the z movement described by the equation: d2z/dt2= 0. This time is t , = Z/uZi.The before derived formula for l/vzimust be used. The time matrix elements are:

t = -2(&) = z tan ~ / c o E,s $ = - (f) = ($)(&)

(6)= Z ( 1 + 2 tan’ ~ ) / cos 2 E

= (&) = 2/(2 cos &).

5. Quadrupoles a. Electric Quadrupoles V

-=

1

In the general formula (34) we put:

+ (y - 6 + el/,/U)/2 +

0,

V, being defined in Section I.B.6.a. We substitute also the expressions of the radial and axial angles. The integrations performed by Matsuda et al.

89

ION OPTICS

(1982) give:

(;) -(;) );( );( =

(k) -(-!-) =

=

=

= k;Z/2,

z’

=Z

(A)

=

($z.

b. Magnetic Quadrupoles We use again the general formula (34) where the velocity is constant now. We substitute the expressions needed for a 2 and B2, then integrate. The result obtained by Matsuda et al. (1982), is detailed in the following matrix elements:

k,(sh,ch, - k,Z) 4

with sq = sin k,Z, cq = cos k,Z, sh, = sinh k,Z and ch, quantities being defined in Section I.B.6.a.

= cosh k ,

Z other

D. Instruments with Focusing in Time A mass resolution in time of 6500 was reported by Frey et al. (1985). It was attained with an instrument using resonance laser ionization. The instrument incorporates a reflector (mirror) grid less analyzer. The laser ionization is performed with 5 ns pulses in a 0.01 cm ionization region. The 6500 resolution, value measured at peak half height, was obtained at the mass 96, for an ion energy U = 700 eV, the 5 ns length ion packet being collected after a 65 ps flight. A resolution of 10000* is estimated as easy of access for m = 400 u. The use of electrostatic deflectors for time focusing was proposed by Poschenrieder (1972) as the symmetric geometries having $e = 201 pelre = 0.985, respectively q& = 160°, pelre = 0.276. To keep time of flight instuments sensitivity high enough, focusing in space must be also ensured. Symmetric structures offer additional possibilities. O,

* Value outrun in the paper by K. Walter, V. Boesl, E. W. Schlag, Int. J. Mass Spectrom. Ion Proc 71,309 (1986).

90

D. IOANOVICIU

Systems with two electrostatic deflectors located symmetrically with respect to a point in the deflexion plane were studied by Sakurai et al. (1985a). By coupling two such electrostatic condenser pairs many coefficients of the aberrations may be cancelled. Each pair has in this case a point symmetry and only one intermediate image on the flight path between the deflectors. It is possible to have T, = T, = & = 0 (T being global time dependent coefficients) and G, = G, = Gd = 0 simultaneously if the coefficients of the individual deflectors satisfy the conditions:

[g]

= 0,

[b]

- 2[f]

[ f ] [ f ] {2[f][;]

- [%I

=0

and

l} = 0. The geometry of the mass spectrometer reported by Sakurai et al. (1985b) is based on these principles. It features four electrostatic condensers, each with electrically variable r e , having a field free space L , on one side, L, on the another. Along the flight path the field free spaces succede in the order L 2L,, 2L 2L,, L while successive deflexion senses alternate. The instrument encloses a 172.7 cm ion path in a box of 41 cm diameter. Each individual analyzer has 4e= 269", pelre = 0.015, pe = 5 cm, L , = 11.9 cm, L, = 7.8 cm. The spectrometer ensures multiple focusing with ''7 = T, = 5 = q,,= qll.= G, = Gd = G, = 0. The source produces, by pulsed electronic ionization, ion packets of 17 ns length with U = 3000 eV. Ion packets of 18 ns length were collected at W = 730. Charged particle identification in nuclear physics needs the simultaneous measurement of the flight time and energy or momentum. This is done in isochronous systems as the GANIL and the instrument of the Washington University (both described by Enge, 1981). Recently a time of flight isochronous spectrometer was designed by Wouters et al. (1985), with the purpose to identify exotic nuclei. It minimizes aberrations, the calculated mass resolution being 2000. -

,,

,,

IV. POSSIBLE DEVELOPMENTS AND REFINEMENTS In this contribution, except a very few cases, an analytic approach was used to derive transfer matrix elements for ion optical devices. An exhaustive inventory of the unsolved problems in ion optics is a very hard task. Therefore, we quote only some of them that received a limited attention in the literature. The additional, mechanical aberrations, arising in ion optical systems with slightly deformed geometries, were investigated only in a few ion optical elements and systems. The transfer matrix element calculations were limited, almost exclusively, to ion optical elements with circular, straight or spiral main path. Transfer

ION OPTICS

91

matrix elements would be obtained however by numerical calculations for ion optical elements with more complicate beam axis (as in the case of the electrostatic prism, for instance). Even for “classic” analyzers, some points remain unclear, such as the accuracy of the aberration coefficient calculation for oblique incidence magnets. Only a limited number of attempts were made to account for fringing field effects in electrostatic mirrors. A systematic, comparative study of the field penetration effect, through slits and grids, on the ion trajectories would give more safety in the incorporation of these mirrors in the ion optical designs. More comprehensive criteria, including axial focusing properties, would lighten the choice of an analyzer intended for some concrete application. To obtain more realistic resolution values, the effects of the ion scattering on residual gas molecules, as well as on slit edges should be included. Such effects alter peak shape especially near the basis. In the time of flight systems, a more complete investigation of the factors limiting the maximal attainable resolution would be of particular interest. The possibility to collect simultaneously ions formed at different moments by using time variable extracting fields seems attractive but it must be analysed in correlation with other time focusing conditions.

REFERENCES Amadori, R., and Wollnik, H. (1971). Inr. J . Mass Spectrom. lon Phys. 6, 347. Bannenberg, J. G . (1980). Inst. Phys. Conf. Ser, 54, I . The Institute of Physics. B a d , M . (1970). Canad. J . Phys. 48,2487. Baril, M., and Kerwin, L. (1965). Canad. J . Phys. 43, 1657. Boerboom, A. J. H., Stauffer, D. B.,and McLafferty, F. W. (1985).lnt. J . Mass Spectrom. Ion Proc. 63. 17. Bolduc, L., and B a d , M. (1973). J . Appl. Phys. 44, 757. Brown, K. L., Belbeoch, R., and Bounin, P. (1964). Rev. Sci. Instrum. 35,481. Cuna, C., and loanoviciu, D. (1983). Int. J . Mass Specfrom. Ion Proc. 54, 333. Des Celles, M. (1974). Nucl. Insfrum. Methods 114, 557. Enge, H. (1967). In “Focusing of Charged Particles” (A. Septier, ed.) 2, 203. Academic Press, New York. Enge, H. A. (1979). Nucl. Instrum. Merhods 162, 161. Enge. H. A. (1981). Nucl. Instrum. Methods 186,413. Frey. R., Weiss, G., Kaminski, H., and Schlag, E. W. (1985). Z . Naturforsch. M a , 1349. Hahn, S. F.. Burch, J. L., and Feldman, W. C. (1981). Reu. Sri. Instrum. 52,247 Hawkes, P. W. (1970). “Quadrupoles in Electron Lens Design.” Academic Press, New York. loanoviciu, D. (1974). lnt. J . Mass Spectrom. lon Phys. 15.89. loanoviciu, D. (1975). Int. J . Mass Spectrom. Ion Phys. 18,289. loanoviciu, D. (1982). Int. J . Mass Spectrom. lon Phys. 41,229. Ioanoviciu, D. (1986a).In “Advances in Mass Spectrometry”(J. F. J. Todd,ed.) 10,857. John Wiley and Sons Ltd.

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Ioanoviciu, D. (1986b).In “Advances in Mass Spectrometry”(J. F. J. Todd, ed.) 10,899. John Wiley and Sons Ltd. loanoviciu, D., and Cuna, C. (1974).Int. J . Mass Spectrom. lon Phys. 1 5 7 9 . Ioanoviciu, D., and Cuna, C. (1975). Vacuum 24,245. loanoviciu, D., Mercea, V., Cuna, C., and Ardelean, P. (1973). J. Phys. E : Sci. Instrum. 6, 129. Marable, N. L., and Sanzone, G . (1974). Int. J. Mass Spectrom. Ion Phys. 13, 185. Matsuda, H. (1975). Int. J. Mass Spectrom. Ion Phys. 18, 367. Matsuda, H. (1981a). Mass Spectrosc. (Japan) 29, 161. Matsuda, H. (1981b).Nucl. Instrum. Methods 187, 127. Matsuda, H. (1983). Muss Spectrom. Reviews 2,299. Matsuda, H. (1985). Int. J. Mass Spectrom. Ion Proc. 66,209. Matsuda, H., Matsuo, T., Ioanoviciu, D., Wollnik, H.,and Rabbel, V. (1982). Int. J. Mass Spectrom. Ion Phys. 42, 157. Matsuo, T., Matsuda, H., Nakabushi, H., Fujita, Y., and Boerboom, A. J. H. (1982). lnt. J. Mass Spectrom. Ion Phys. 42,217. Mott, E., Schrader, H., Seigert, G., Hammers, H., Asghar, M., Bocquet, J. P., Armbruster, P., Ewald, H., and Wollnik, H. (1977). Kerntechnik 19,374. Nakabushi, H., and Sakurai, T. (1983). Int. J. Mass Spectrom. lon Phys. 50, 275. Nakabushi, H., Sakurai, T., and Matsuda, H. (1983). Int. J . Mass Spectrom. Ion Proc. 52, 319. Nakabushi, H., Sakurai, T., and Matsuda, H. (1983/4). Int. J. Mass Spectrom. Ion. Proc. 55,291. Nishigaki, S., and Kanai, S. (1986). Rev. Sci. Instrum. 57,225. Oshima, C., Souda, R., Aono, M., and Ishizawa, Y. (1985). Rev. Sci. Instrum. 56,227. Rev. Sci. Instrum. 56,227. Poschenrieder, W. P. (1972). Int. J . Mass Spectrom. Ion Phys. 9,357. Ruedenauer, F. G . (1970). Int. J. Mass Spectrom. Ion Phys. 4, 181, 195. Sakurai, T., Matsuo, T., and Matsuda, H. (l985a). Int. J . Mass Spectrom. Ion Proc. 63,273. Sakurai,T., Fujita, Y., Matsuo, T., Matsuda, H., Katakuse, I., and Miseki, K.(1985b). lnt. J. Mass Spectrom. Ion Proc. 66,283. Sar-el, H. 2. (1967). Rev. Sci. Instrum. 38, 1210. Schneider, R. F., Luo, C. M., and Rhee, M. J. (1985). J. Appl. Phys. 57, I . Steckelmacher, W. (1973). J. Phys. E : Sci. Instrum. 6, 1061. Taya, S., Tokiguchi, K., Kadnomata, I., and Matsuda, H. (1978) Nucl. Instrum. Methods 150, 165. Wigli, P. (1979). Rev. Sci. Instrum. 50, 165. Wiley, W. C., and McLaren, I. H. (1955). Rev. Sci. Instrum. 26, 1150. Wollnik, H. (1967a). Nucl. Instrum. Methods 52,250. Wollnik, H. (1967b). Proc. Third Int. Conf. Atomic Masses, p. 779. Wollnik, H. (1971). Nucl. Instrum. Methods 95,453. Wollnik, H. (1976). Nucl. Instrum. Methods 137, 169. Wouters, J. M., Vieira, D. J., Wollnik, H., Enge, H. A., Kowalski, S., and Brown, K. L. (1985).Nucl. Instrum. Methods in Phys. Res. A240, 77.

Proton Microprobes and Their Applications J . S. C. McKEE AND G . R . SMITH Unirersity of

1. Introduction

M<JflilOhU. Depurtment Winnipeg. Maniroha

of' Physics

. . . . . . . . . . . . . . . . . . . . . . . . .

A . The Generation of Proton Microbeams and the Minimization of Spot Size on Target . . . . . . . . . . . . . . . . . . . . . . . . . . B . Some Problems Conerning Microprobe Analysis. . . . . . . . . . . . I1 . Proton Microprobes and Their Current Capabilities . . . . . . . . . . . A . The Variety of Microprobe Systems and A Comparison of Performance . . . B . A Review of Microprobes Currently in Operation or Under Construction . . . C . The Measurement of Spot Size . . . . . . . . . . . . . . . . . . D . A Microprobe as a Scanning Proton Microscope . . . . . . . . . . . 111. Factors Directly Atrecting the Quality of Microbeams . . . . . . . . . . . A . General Considerations . . . . . . . . . . . . . . . . . . . . B . Aberrations . . . . . . . . . . . . . . . . . . . . . . . . C. General Comments on Microprobe Design . . . . . . . . . . . . . D . Removing Chromatic Aberrations . . . . . . . . . . . . . . . . . E. Electrostatic Focussing Elements . . . . . . . . . . . . . . . . . IV . Possible Future Developments in Microprobe Systems . . . . . . . . . . . A . Practical Considerations . . . . . . . . . . . . . . . . . . . . B . Future Prospects . . . . . . . . . . . . . . . . . . . . . . V . Sample Analysis . . . . . . . . . . . . . . . . . . . . . . . . A . Sample Mounting . . . . . . . . . . . . . . . . . . . . . . B. Data Registration . . . . . . . . . . . . . . . . . . . . . . C . Data Analysis . . . . . . . . . . . . . . . . . . . . . . . D . Radiation Damage . . . . . . . . . . . . . . . . . . . . . . VI . Typical Applications of Proton Microprobes . . . . . . . . . . . . . . A . Applications to Biological Systems . . . . . . . . . . . . . . . . B . Mineralogical Applications . . . . . . . . . . . . . . . . . . . C . Antiquarian Applications. . . . . . . . . . . . . . . . . . . . D . Industrial Applications . . . . . . . . . . . . . . . . . . . . VII . Advantages of High Energy Microprobes . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . .

93 95 96 97 97 98 101

104 105 105 106 106 1ox 112 114

114 115 117 119 121 122 122 123 124 125 127 127 128 129

I . INTRODUCTION A microprobe is an instrument for the microanalysis of a sample in which a beam of particles is focussed onto an area less than a micrometer in diameter . A proton microprobe therefore is. or should be. a device capable of producing 93 Cop)riplii &> IYXY b) Ac.idemiL Pre\\ Inc 411 right\ (11 rcproduclion in .my form rewived ISBN 0 I?-014671-X

94

J. S. C. McKEE AND G . R. SMITH

beam spots of sub-micrometer dimensions. Such facilities are only now becoming available, and the drive toward nanometer resolution is barely underway. It is therefore the aim of Sections I1 to IV of this review paper to summarize the theoretical and technical problems to be overcome in the generation of sub-micron beams of fast protons and to identify the most recent developments in the field of ion optics as they relate to this problem. Prior to 1987, some fifty proton microprobe facilities had been developed, as enumerated by Legge (1984), Vis (1986) and Prakash and McKee (1985). The rapid growth in the number of these devices is a function of both the sudden availability of nuclear physics facilities for analytical work in the late seventies and early eighties and the now proven advantages of heavier ions in the chemical analysis of environmental and scientific samples. It is still, however, almost unknown for a microprobe to be designed and constructed as a complete unit from source to sample. In virtually every case, an existing accelerator has been used as the proton source, and the microprobe facility has then been custom-made to match the available input. Factors such as the brightness of the source and the beam emittance have not usually been negotiable. Many microprobes are attached to small low energy single-ended Van de Graaff accelerators. Others use linear accelerators or cyclotrons as the sources of particles. The availability of the accelerator has in each case determined the nature of the concomitant microprobe facility. This overall situation is reviewed in Section 11. Once the source of protons has been determined, the beam on target has traditionally been produced either by collimation or by focussing in a lens system designed specifically to minimize aberrations in the proton beam (Cookson, 1979). Section 111 of this review discusses the variety of such lens systems, the minimization of aberrations in focussing systems, and the precision of measurement likely to be attained in the foreseeable future. Section IV discusses the relative merits of various sources as beam input to a proton microprobe and the appropriate energies for which such microprobes might be designed. As the variety and nature of microprobe facilities is extended to higher beam energies and the precision of measurements improved, the addition of high energy transmission to low energy stopping target systems is seen to be a valuable development. Section V of this paper reviews methods of mounting and scanning samples, registering and analysing data, and investigating the effect of sample deterioration. Section VI will review typical applications of this microanalytical technique, and Section VII will discuss some of the experimental advantages that a 30 to 50 MeV transmission microprobe may have over existing low energy devices.

PROTON MICROPROBES AND THEIR APPLICATIONS

95

A . The Generation of Proton Microbeams and the Minimization of Spot Size on Target

The first focussing proton microprobe to become operational, and still at the forefront of microprobe development, is that at Harwell (Cookson et al., 1972; Cookson and Pilling, 1976). This versatile tool with a beam diameter of a few microns and a typical current of 10 pA/pm2 has been used in a wide range of applications which have been reviewed in two comprehensive publications (Cookson, 1979; 1982). The Harwell microprobe was historically followed by the commissioning of several systems with beam widths of around 10 pm optimum (Cho et al., 1974; Bonani et al., 1978) in which quadrupole lenses with long focal lengths, previously used in nuclear physics experiments, found a new role in microbeam production. The beam energies of most of the early microprobes were naturally in the MeV region, and the magnetic rigidity of such beams (in comparison to electrons, for example) required that lens field strength be high in order to ensure that the overall dimension of each instrument was kept small. As Nobiling (1986) has reminded us, gas scattering at a pressure of less than lop6 torr alone can lead to unreliability in microprobe operation when the total length of the system is in excess of several metres. In addition, there is concern with mechanica! stability in overlarge and cumbersome devices. Because the early microprobes were installed at nuclear physics laboratories where familiarity with quadrupole lens systems was an indigenous skill, the microprobes themselves incorporated either existing or new quadrupole lens systems into each design. Quadrupoles cause focussing and defocussing in perpendicular planes, so two such elements at least must be combined to give a focussing lens system. A doublet of quadrupole lenses is perhaps the simplest focussing array, and the microprobes at Amsterdam, Heidelberg and Karlsruhe among others have used this simple recipe. The demagnification factors for the x and y directions in such systems have however been found to differ by a factor of 5 or 6, but this can readily be compensated for by the use of crossed slits as the source aperture. Such non-stigmatic images are typical effects from the incorporation of a quadrupole doublet into the microprobe system. Perhaps the most popular form of microprobe incorporates a design due to Dymnikov et al. (1965) in which four singlet lenses are combined into a quadruplet of quadrupoles. Here the demagnification factors for horizontal and vertical planes are equal, and if properly aligned most beam imperfections can be neglected to third order. This “Russian quadruplet” design has been adopted by the Harwell, Namur, Melbourne, Studsvik, Lund and Manitoba groups (Nobiling ,1983; Al-Ghazi and McKee, 1982). It attempts to use strong

96

J. S. C. McKEE AND G . R. SMITH

focussing lenses to produce a round image of a round object. The idea has always been an attractive one, but aligning four quadrupole lenses along a common magnetic axis, and in perfect rotational alignment, is not a simple task. Triplet lens systems on the other hand, have been adopted by various laboratories including Darmstadt (See Proc. Namur, 1982; Fischer, 1977)and Murray Hill (Augustyniak et al., 1978) in order to avoid the asymmetry in demagnification factors observed with quadrupole doublets. In these cases, a pinhole source of protons can be used, the resulting facility becoming somewhat simpler to use. It transpires, however, that microprobes incorporating conventional quadrupoles only, can rarely reach the 1 pm acceptable limit for the focussed spot dimensions of a microbeam (see, for example, Nobiling, 1983;Table I). In order to reach a higher resolution, it becomes necessary to invoke a new concept or more recent technology, such as that suggested by Martin and Goloskie (1982) and now effectively used by them in the development of a microbeam of 500 nm width (Martin, 1987). Their design incorporates an achromatic quadrupole lens which has been demonstrated to remove beam aberrations caused by velocity spread in the beam from the source. As chromatic aberrations are the principle source of beam spread in a lens focussing system, the design yields a major improvement in the quality of beam delivered by the microprobe and opens up many possibilities for future development. Other lens combinations, including the use of a superconducting final element at Los Alamos will be discussed later on in this review paper, in Section 111. In Section IV, future plans and prospects are discussed. B. Some Problems Concerning Microprobe Analysis

The microanalysis of a sample using a proton microprobe involves mapping out elemental concentrations in the various regions of the sample that are comparable in size to or larger than the effective microbeam spot size. This mapping may be carried out point by point along a linear scan of the beam or in a two-dimensional rastered scan of the sample. Elements are usually and most easily identified by the characteristic X-rays which follow the ejection of a K or L electron from an atom by an energetic proton passing through the region of interest. This technique is usually referred to as Particle Induced X-ray Emission (PIXE) analysis. In order to obtain a useful mapping of a sample, several experimental problems require detailed attention. These are the registration of the proton microprobe beam on the sample, the effect of sample thickness upon ionization and data collection, the variation with time of the beam intensity from the particle source, and the deterioration of the

PROTON MICROPROBES AND THEIR APPLICATIONS

97

sample under bombardment by energetic protons. In addition, data acquisition must be under computer control, because so many experimental quality factors must be monitored simultaneously with the registration of positional data and energy data from the X-ray detector. These problems are addressed in Section V.

AND THEIR 11. PROTONMICROPROBES CURRENT CAPABILITIES

A . The Variety of Microprobe Systems and Comparison of Performance

Because of the wide acceptance of the proton microprobe as an analytical tool, it is now becoming extremely difficult to keep the rapidly expanding list of microprobe facilities available worldwide up-to-date. Single-ended Van de Graaff accelerators, tandem accelerators, cyclotrons and dynamitrons are in use as sources for microbeam production. The brightest proton sources currently available are however not usually suited to installation in a high voltage Van de Graaff terminal. In the case of cyclotrons, much of the brightness is lost in the ion optical systems external to the accelerator at injection or extraction. Higher beam stability and higher vacuum throughout an assembly can however improve the present situation considerably. Cyclotrons with significant energy spread can still be used for precise analytical work when the beam profile is not varying with time. Deconvolution of the energy distribution of the beam may then be used to advantage. Highly analyzed high energy beams can also be used when the provision of significant current to a micro-area is not the primary design consideration. The prospect of using a space charge or plasma lens to generate high current beams of positive ions has been investigated by Booth and Lefevre (1978). This lens consists of an axial electron trap produced by means of biased ring electrodes and an axial magnetic field which inhibits radial motion of the electrons. The trapped electrons then focus positive ions which traverse the lens. Booth has designed such a simple system and operated it in a 400 keV-25 mA deuteron beam transport system. Such a system could be equally possible for protons. The performance of the lens at a base pressure below torr was stable throughout. Legge et al. (1982) feel that further work on such lenses is necessary in order to determine whether the plasma presents a real choice to the experimenter requiring a high resolution microprobe. The best spatial resolution obtained by this system was 20 pm.

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J. S. C. MCKEE AND G . R. SMITH

As Legge (1984) has ably pointed out, a bewildering range of lens configurations is currently incorporated into existing microbeam systems. The majority of microprobes however still incorporate quadrupole lenses either in doublet or multiarray designs. The ability to estimate accurately the aberrations due to such systems depends upon the use of computer programs which can determine the optical trajectories of proton beams in these assemblies and display typical characteristics of each facility. Programs such as TRANSPORT, IONBEAM, PRAM and OXRAY are applicable to magnetic lens systems, and others are becoming available. The most extensive set of published measurements (Legge, 1984) is still that for the Oxford microprobe which shows good agreement in general with predicted values of its parameters. Nobiling (1983) however points out that the calculations indicate that the most serious aberrations in existing microprobes are the chromatic aberrations and errors due to misalignment of lens systems. The predictions of Grime et al. (1982) seem to be, on average, pessimistic and seem to underestimate the performance of some systems. Because the aberration coefficients in the Grime paper appear in terms of object coefficients, they are not independent of the proton source/accelerator system. Legge believes that these aberrations should be expressed in terms of image coefficients. He believes the wide variation in values which appear in the Grime paper will not occur, as aberrations are determined only by convergence angles at the image. While this position is wholly defensible, the fact remains that the experimentalist has control only over the object space, and for that reason, the Grime data continue to be useful. As it is possible in principle to build all microprobe systems with the same total length, focal length or working distance; and as the quantity CJf can be used as a dimensionless criterion of performance, Legge (1984) has displayed the quantity C;/f,f, as a measure of each lens system and demonstrated that there is no significant difference between the capabilities of doublets and quadruplets in microprobe assemblies, although triplet systems seem to exhibit higher degrees of chromatic aberration. In Table I (reproduced from Legge, 1984), the most significant quantities in practice are taken to be the working distance WD and the values for C:/WD2 which give a figure of chromatic merit for each design of microprobe. Other, more recent microbeam systems such as those at Guelph and Manitoba are not yet operational, and as a result, values for these systems are not currently available. B. A Review of Microprobes Currently in Operation or Under Construction

Because of the rapid increase in the number of microprobe assemblies known to be in existence worldwide, a comprehensive list seems impossible to

TABLE I hERRAnON

Coefficient"

COEFF~CIENTS (IN IMAGE COORDINATES)OF

SOME

PUBLISHED MAGNEnC MULTIWLEMICROPROBE SYSTEMS (1984)

Heidelberg doublet

Karlsruhe doublet

Oxford triplet

Hanvell triplet

2.030 0.1 10 0.071 0.257 - 26.4 - 4.6 0.07 0.43 0.03 1.6 2.5 -0.17 - 2.9 -11.3 - 2.5 26 1400 2100

3.115 0.125 0.380 0.083 - 2.3 - 29.0 1.53 0.10 0.16 5.1 10.3 - 160 - 5.0 -0.16 - 16.9 298 9500 19100

6.450 0.170 -0.09 1 0.566 67.8 - 15.2 -0.19 2.58 0.49 9.5 - 16.9 0.28 7.1 - 125 -6.1 -404 7900 - 14Ooo

4.590 0.199 -0.069 0.361 32.1 - 10.2 -0.23 2.16 0.50 19.9 - 12.5 0.43 7.4 - 78.3 - 8.0 -402 16200 - 10100

Harwell quadruplet

Melbourne quadruplet

4.590 0.199 0.642 0.642 - 5.3 - 5.3 0.60 0.98 0.59 1.4 14.8 - 7.4 - 39.0 - 30.9 - 37.6 2886

8.605 0.236 0.403 0.403 - 20.4 - 20.4 0.36 0.44 0.16

7000

72600

1.o

2.8 - 17.2 - 58.4 -25.3 - 50.7 5682 35000 102000

Units m m

m m

m/rad 6 m/rad 6 (m/rad 6)' (rad a)-' (rad a)-' m/rad m/rad3 m/rad m/rad3 (m/rad 3 ) 2 rad-6 rad-6

'

In this table a L = overall length, WD = working distance, f, and f , = focal lengths in x and y planes. D, and D, = demagnification factors, C, = ( x 1 0 6 ) ( y 1 + 6 ) , where 6 = A E / E , C, = [(x1O3) ( X ~ O I $ ~ ) ( ( X ~ O ~ ) / ( ~ ~ ~ ~ ) ) ~ ]+X [ ( ~ ~ ~ ~ ) ( y I 0 2 + ) ( ( y I @)/(x I 0~5))~].The aberration coefficients (I) are transformed from those given in object coordinates in Grime et al. (1982).

+

I00

J. S. C. McKEE AND G. R. SMITH

produce. By the end of 1987, new Canadian microprobes at the University of Guelph, Ontario operating in the MeV region and a high energy 50 MeV transmission microprobe at the University of Manitoba in Winnipeg will become operational, and add to the overall proliferation and variety of such facilities. As Cookson (1979) originally pointed out, the simplest way of producing a small beam spot is to use a very fine collimator to select a small part of the beam from a particle accelerator. In this way, the first microbeam was generated by Zirkle and Bloom (1953). The fine collimator used was an approximately triangular groove scratched on one of a pair of optically flat jaws which when clamped formed a 2.5 pm spot from the incident beam falling upon it. Not surprisingly, only a few hundred protons per second reached the sample, but this pioneering work pointed the way to future development. The limiting factor in such a design lies in the ratio between transmitted current through an aperture, assumed circular, and scattering from the edges of the aperture which goes as r / 2 , where r is the radius of the hole. Clearly for a spot less than 1 pm in diameter, the ratio of real beam to scattered beam is already a severe problem for the designer. Collimated microprobes however were the earliest microanalytical tools to use protons. They are listed in Table 11. The use of H - beams rather than protons can be advantageous in reducing the scattering problem. However the overall beam current is lower than in the proton case. Focussing proton beams to micrometer dimensions was originally seen as a difficult task. The problems associated with bending a beam of subatomic particles with an energy of 1 MeV for example were seen as inhibiting (Cookson, 1979). For a given focal length of lens system, a proton beam requires a magnetic field strength 230 times greater than that for an electron beam in a typical 30 keV electron microprobe, and to non-nuclear scientists, this requirement seemed prohibitive. The first proton microprobe using TABLE I1 ~

~~

Year

Laboratory

Reference

1953 1966 1966 1973 1975 1975 1976 1978 1979

Chicago Harwell Lucas Heights Munich Heidelberg M.I.T. Tallahassee Brookhaven Kingston

Zirkle & Bloom Pierce et a/. Mak et a/. Schmid et a/. Nobiling et al. Horowitz & Grodzins Gentry et al. Schroy et al. MacArthur et al.

Minimum spot 2.5 25 60 <4 1

> 25 30 25 10

Cornment s protons per second

protons per second <<0.1nA tandem accelerator carbon collimator protons

PROTON MICROPROBES AND THEIR APPLICATIONS

101

focussing lenses was assembled in Harwell (Cookson and Pilling, 1976) and used four singlet quadrupole lenses in the arrangement proposed by Dymnikov et al. (1965) now known as a Russian Quadruplet. Oxford, Namur, Melbourne, Studsvik, Lund and Manitoba microprobes followed basically a similar design. The first focussing microprobe at Harwell used a single ended Van de Graaff generator as the proton source, but tandem accelerators were found to have adequate ion optical properties also and with a gas stripper they can compete well with other microprobe facilities, as the Oxford group have shown (Watt et al., 1982).Cyclotron laboratories at Hamburg, Eindhoven and Manitoba now also have microprobe facilities either in operation or almost so, enabling microanalysis with higher energy proton beams to be carried out when appropriate. The Manitoba microprobe in particular is a high-energy probe for the analysis of thin transmission samples, and adopts the proton induced K X-ray emission technique in the measurement of concentrations of medium and heavy elements in samples of many kinds. K X-rays from heavier elements can only be excited with high probability when protons of up to 50 MeV are available to a microprobe system. The Eindhoven and Hamburg accelerators have 30 MeV as maximum proton energy. The Manitoba 50 MeV microprobe has an intermediate design figure for spatial resolution of 20 pm with a current of 250pA on target. The A.V.F. cyclotron at Amsterdam has already become a useful tool in analytical work and uses a focussed beam of area 42 pm2 with typical current densities of the order 1 pA/pm2. The optimum spatial resolution available here is around 6 pm. Cyclotrons were long believed to be unsuitable as proton sources for analytical microprobes. The myth was that K X-rays for example would be swamped by background from secondary Bremsstrahlung and y induced processes. This turned out not to be the case, and indeed processes at a level lo’ times lower than normal K X-ray production have already been studied (Al-Ghazi et al., 1982). This fact is relevant to much of Section VII of this review paper. Table 111 gives an inventory of presently active proton microprobe facilities including a few collimator-based systems that currently generate useful microbeams. It provides a condensed guide to most existing and commissioned systems and presents sufficient data to ensure that the reader can obtain a contemporary picture of facility performance. Further detailed information can, of course be requested directly from the users of each assembly. C . The Measurement of Spot Size One of the unsung problems of microbeam analysis lies in the precise determination of the dimensions of and distribution across small images.

102

J. S. C. McKEE AND G. R. SMITH

TABLE 111 INVENTORY OF ACTIVE PROTONMICROPROBE FACILITIES WITH LOCATION AND OPTIMUM SPATIAL RESOLUTION OF OPTICAL LENSSYSTEMSIDENTIFIED

Laboratory Amsterdam Bell Labs., Murray Hill Birmingham Univ. (England) Bochum Rhur Univ. Brooklyn College Bruyeres Le Chatel Cambridge Massachusetts Darmstadt Delaware, Bartol E. T. H. Zurich Eindhoven EUT Guelph Hamburg Harwell Harwell Heidelberg Karlsruhe Lower Hutt Los Alamos Lund Manitoba Melbourne Montreal Namur

Oxford

Type of microprobe

Spatial resolution (pm)

(den Ouden 1981; Bos 1984) (Augustyniak 1978)

MD

6

ET

12

(Earwacker 1987)

RQ

(Wilde 1978; Proc. Namur 1982)

4M

6

COIL RQ

20 2 10

Reference

(Engelmann 1983) (Horowitz 1976)

sc

(Fischer 1977) (Legge 1982) (Suter 1974, 1975; Bonani 1978) (Legge 1982)

4M

20

(Campbell 1986) (Legge 1982)

MD

40

(Cookson 1970, 1972) (Cookson 1976) (Nobiling 1977; Bosch 1977, 1978) (Heck 1985) (Coote 1978; Legge 1982) (Martin 1980) (Proc. Namur 1982) (Al-Ghazi 1982) (Legge 1978, 1980; Sealock 1983) (Hinrichsen 1987) (Deconninck 1979; Proc. Namur 1982) (Watt 1982)

MT Coll. 2MD

8 50 11 x 26

Accelerator energy (protons) 2.4- > 28 MeV (AVG cyclotron) 2 MeV 3 MeV (dynamitron) 4 MeV

3.75 MeV 4 MeV single-ended Van de Graaf > 1 MeV > 1 MeV 2 MeV 6 MeV (tandem)

RQ

1

3.5 MeV (cyclotron) 3 MeV 10-30 MeV (cyclotron) 3 MeV

MT MD

2 1->2

3 MeV 2 MeV; 6 MeV

MD RQ

1->2 10

3.75 MeV 3 MeV

sc

5 6 20 1.5

6.5 MeV 3 MeV (pelletron) 35-SO MeV 5 MeV

RQ RQ ET RQ

2MD

+

5

4.5 MeV 3.2 MeV

1

6 MeV (tandem)

-

PROTON MICROPROBES AND THEIR APPLICATIONS

103

TABLE 111 (Continued) Queen’s, Kingston Sandia, Albuquerque Studsvik SUNY, Albany Surrey, Guildford Tokyo, IMS UCLA Uppsala-Studsvik Worcester Polytech. Key AD Coll. E ET M

(MacArthur 1977)

Cell.

-

4 MeV

(Doyle 1983)

AD

2

5 MeV

(Brune 1977) (Morns 1984) (Hemment 1978) (Prakash and McKee 1985) (Cho 1974; Singh 1976) (Lindh et. nl 1985) (Legge 1982)

RQ AD RQ MD

25 1.5 30 60

2 MeV 25 MeV

2MD

8

2 MeV

RQ AD

2 4

1-4 MeV 2 MeV

Achromatic doublet Collimator defines the beam Electric quadrupole Electrostatic triplet Magnetic quadrupole

MD MT RQ SC

5.5 MeV

Magnetic quddrupole doublet Magnetic quadrupole triplet Russian quadruplet Superconducting coll.

Devices for monitoring both low and high energy charged and neutral particle beams are abundant (e.g., Haddock er al., 1982). In the early days of proton microprobe development, optical objective lenses were used to magnify microbeam spots so that their dimensions could be studied by measurement on a project screen. Such methods were however highly expensive as the lifetime of the glasses used was short in this mode of operation and laboratory activity could be measured in terms of the number of destroyed objectives lying discarded by the beam line. Measurement of microbeam spot dimensions still presents difficulty. A method developed by Lapointe and McKee (1980)is being used to study 10-20 pm spots at Manitoba. The K X-rays from an array of thin wires are useful in the measurement of spot position. As Legge et al. (1982) have pointed out, the necessity of identifying full width at half height in a Gaussian profile is often now met by scanning the beams of protons across a sharp edge, and this procedure must be carried out in two, usually perpendicular, directions. It is also important to make each measurement quickly so as to determine the values of aberrations present in that particular configuration of optical components. Using a thin glass cover slip over a microscope can enable measurements down to a few microns to be made with ease when the particle beam is of low energy. If the magnification of the lens is X200 or more and the depth of focus

104

J. S. C. McKEE AND G. R. SMITH

short, then, for less penetrating beams, adequate spatial resolution is available. The images of scattered beams and of scattered light will not be in focus and can be ignored in this measurement. Gonsior of Rhiir Universitat, Bochum has measured spot dimensions with crossed Walliston wires of 2pm diameter, and in the case of high velocity beams, crossed-wire scanning techniques seem appropriate, as in the case at Manitoba. Secondary electron emission techniques are also useful, and assemblies of objects with dimensions smaller than the beam spot will always be preferable in establishing the spatial resolution of a microbeam. According to Watt et a/. (1982),gold shadowed 0.1 pm spheres could be used in an array, as is the case in several electron microprobes. If this technique is indeed possible there can be no better method of measuring both profile and halo for existing and projected microbeams (Legge et al., 1982). The sputtering of balls with adequate amounts of metal presents a problem however, and forming a suitable matrix of 0.1 pm spheres is not a simple task. It goes without saying that a two dimensional scanning and imaging system is a concomitant necessity for many of these techniques for generating a self-consistent and full profile of the beam spot. D. A Microprobe as a Scanning Proton Microscope

In transforming a microprobe to a scanning proton microscope there are two principal options open to consideration: either the beam scans across the target sample, or the sample moves through the fixed geometry microbeam. It seems to the authors of the present paper that scanning the target through the beam is preferable, particularly for high energy beams and relatively low scanning frequencies. Here the microbeam is fixed with respect to detectors and monitors and the spot is always focussed to the same reference coordinates throughout a run. Alternatively, the beam itself can be scanned across the target. There are some problems here in that the aberrations caused by perturbing potentials, whether in the case of magnetic coils or electrostatic systems, are complicated to calculate, and the lever arms may be long for fast cycling of high energy beams. Pre-lens scanning is invoked in the Karlsruhe design, by analogy with electron microscopes. Post-lens scanning may be preferable, but space is needed for this. Cookson (1982) has concerns about the coupling of fields between the lens system and deflection coils or plates. In summary, the scanning target seems preferable to the scanning beam unless high speed scanning is necessary, in which case a quite different technique to those discussed here is probably necessary.

PROTON MICROPROBES AND THEIR APPLICATIONS

105

111. FACTORS DIRECTLY AFFECTING THE QUALITY OF MICROBEAMS A . Generul Considerations

Most microprobes currently in use consist of a series of quadrupole lenses designed to produce beams of micrometer dimensions. The characteristic properties of such systems usually determine the minimum spot size obtainable, and therefore a theoretical understanding of the optics of proton beams in passage through magnetic or electric lenses is of great importance. Hawkes (1966), Steffen (1965) and Klemperer and Barnett (1971) have contributed substantially to detailed knowledge of this area, and Grime et al. (1982) have attempted to deal specifically with the design problems associated with quadrupole based probe arrays and with real and parasitic aberrations associated with them. A typical quadrupole assembly of course usually gives a demagnified image of a small object aperture located several meters from the lens system. Specially designed collimator slits are used to define the divergence of the proton beam entering the system. However, because quadrupoles focus in one plane and defocus in the other a minimum of two lenses, a quadrupole doublet, is necessary to ensure stigmatic imaging. According to the precise properties, divergence, etc., of the beam from the source, a triplet or a quadruplet lens system may be the more appropriate in a particular case. An incident beam highly divergent in one plane may be a natural candidate for a specially designed triplet array. Alternatively, various combinations of quadrupoles, such as the previously mentioned Russian quadruplet, may have particular advantages in minimizing aberrations of multilens systems. The outer doublet in a Russian quadruplet, that is to say the first and fourth lenses, have equal but opposite fields. The same is true of the second and third lenses, the inner doublet, which are rotated 90" relative to the outer pair in the vertical plane. Imperfect alignment of the quadruplet can of course, lead to serious twist problems, but such a system can in principle minimize distortions to third order and, indeed the first order properties have been well studied by Dymnikov et a!. (1965). For a full discussion of the properties of quadrupole lenses and lens systems, see Grime and Watt (1984). The identification and minimization of aberrations is essential to the production of particle beams of sub-micron dimensions. The following two sections will discuss the nature and importance of such imperfections and the methods for dealing with them effectively.

106

J. S. C. McKEE AND G . R. SMITH

B. Aberrations The laws of optics as applied to charged beams in paraxial geometry are valid only when limiting conditions are met. In general, a real image will deviate from the perfect expectation because of aberrations, and indeed the total aberration of any particular ray can be seen as the sum of several possible independent components. Such components may be purely geometrical and depend only upon the properties of the charged particle (i.e. proton) beams. Others arise from what are often described as “electronic errors”. Klemperer and Barnett (1983) have classified errors as: a) asymmetry errors arising from deviations from circular or rectangular symmetry, b) geometric errors arising from image aperture and field size, c) chromatic errors arising from inhomogeneity in proton beam velocity. d) space charge errors arising from high current density, and e) diffraction errors arising from small scale aperture use. Most of these aberrations can be minimized in the design of focussing lens systems, although the cause of an observed asymmetry for example can be difficult to identify. Misalignments and departures from perfect circular symmetry in lens construction can be difficult problems to deal with. Astigmatism, the existence of coma and anisotropic aberrations, all exist as geometric-optical errors of one kind or another. Of the aberrations that arise from inhomogeneities in, or other properties of the proton beam, undoubtedly the most significant relates to the range of emission velocities from the proton source. A truly monochromatic proton beam cannot be generated, so it becomes imperative to remove this velocity spread as far as possible in the microprobe lens system. C. General Comments on Microprobe Design

Nobiling (1986)has reached several conclusions in his review of operating experience with ion optical elements in microprobe systems to this date. Firstly, a rather simple design incorporating an accelerator beam, source aperture, slits and a simple quadrupole doublet can be shown to produce an MeV beam on target of dimensions between 1 and 3 pm. The provision of triplet lens systems, and octupole trimming fields can of course improve this situation, but micron beams can, with care, be generated from relatively unsophisticated facilities. Examples of simple one-stage microprobes are listed in Nobiling (1983). It has been shown that for specific situations, sub-micron beams can be produced by essentially uncomplicated lens arrangements, (Proc. Namur, 1982). The optimum resolution however of a simple quadrupole doublet-based microprobe arrangement seems to lie within the 0.5 - 1 pm range. Indeed, in order to improve upon this performance, aberrations of the chromatic and

PROTON MICROPROBES AND THEIR APPLICATIONS

107

\ C

dxs= $ 0

I

FIG.la.

dx,= 2C,a&

E

FIG.lb.

spherical kind require precise attention, and the provision of additional different and/or higher multipole lens elements seems essential. A schematic diagram illustrating the nature of spherical and chromatic aberrations is shown in Fig. 1 (from Maggiore, 1982). In the case of spherical aberrations a lens focusses ions far from the optical axis more strongly than those near the optical axis. Because of these spherical aberrations, the area of the focussed spot can be no smaller than the “disk of least confusion” of diameter tixa. The angle u is the divergence half-angle of the focussed spot (see Fig. la). In the case of chromatic aberrations however, a lens focusses ions with higher energy less strongly than those with lower energy. These chromatic aberrations produce a focussed spot with a diameter dxc.This is shown schematically in Fig. lb. The minimization of both spherical and chromatic aberrations presents a major challenge to ion-optical lens design and will form the focus of much of the discussion in the paragraphs which follow. These aberrations limit the usefulness and ultimate precision of most existing microprobe systems, and their reduction or removal presents an important challenge. Mechanical stability and vibration-free operation also become vitally important in the provision of precision 500 nm beams, however compact the microprobe facility may be. The need for high vacuum throughout is an additional but important feature of the design of microprobe systems with high spatial resolution.

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J. S. C. McKEE AND G. R. SMITH

D . Removing Chromatic Aberrations Although both spherical and chromatic aberrations are important, chromatic aberrations have become the limiting factor in conventional microprobe design. Such aberrations result from the inability of the lens system to focus ions of different velocities to the same image spot. Slow protons are bent more effectively than faster particles and are therefore focussed at a shorter distance from the final lens element. Spot diameter due to chromatic aberrations is of first order in the lens aperture and can be written as d x , = 2Cca A E / E

where Ccis the chromatic aberration coefficient and A E / E is fractional energy spread in the beam, and c1 is the half-angle subtended by the aperture as seen in the image plane. It is in fact the semi-divergence angle of the beam. d x , is the diameter of the image spot due to chromatic aberration. Martin (1984) has pointed out that in the case of a thin lens with purely magnetic properties forming an image of an object at long distance, the coefficient C, yields a value of f / 2 where f is the focal length of the lens. In a compact microprobe system with focal length of 0.5 m and A E / E = 0.001 and with c1 = 0.01 radians d x , becomes 5 pm which is a dominating and totally unacceptable value. This simple calculation emphasizes the need for minimization or, if possible, removal of chromatic inhomogeneities in the beam. The fact that chromatic aberrations present a formidable hurdle to the attainment of high resolution microprobe beams was first addressed in an aggressive fashion by Martin (1981). He observed that a combined electric and magnetic lens system with a chromatic aberration coefficient of zero had been described some time ago by Kelman and Yavor (1962), and that such a combination is to preferred to any purely magnetic assembly. Indeed, Hawkes (1970) has shown that when the converging component of the electric quadrupole lens is coincident with the diverging component of the magnetic lens, and the magnetic force is arranged everywhere to be double the electric force in magnitude, then the focal length is independent of particle energy to the first order. With this fact in mind, Martin (198 1) has designed and evaluated a doublet lens system for use in a microprobe system. Figure 2 shows a cross-sectional drawing of the central region of such a lens (reproduced from Martin 1981). It is worth repeating here the technical details of this unique lens, incorporating some specific and unusual materials. The four magnetic poles were constructed from an alloy, 27%cobalt and 73% iron. These poles were glued to Macor glass-ceramic spacers in a vacuum-tight rigid assembly. The electric poles were constructed of Inconel 600,16% Cr, 8%

PROTON MICROPROBES AND THEIR APPLICATIONS

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FIG.2. Cross sectional scale drawing of the central region of an achromatic quadrupole lens. The magnetic poles have tip radius h. The four circles of radius b, are the cross sections of the electrodes. Glass-ceramic spacers are bonded to the magnetic poles in a vacuum-tight, rigid assembly. The spacers also support the electrodes.

Fe, 76% Ni alloy wire; all three materials had matching thermal expansion coefficients. The magnetic poles were cylindrically ground with less than 3pm taper and the distance between opposing poles was set to within 2 7 pm Separation between adjacent poles was accurate to + 8 pm. The principal lens dimensions are given in Table IV (from Martin, 1981),and the windings produced a 1 tesla pole field for a total of 10 W power. The small bore lenses give high field-gradients and can focus protons of energies up to 39 MeV. This lens system was tested with a 600 keV beam from the Van de Graaff accelerator at Worcester Polytechnic Institute. Achromatic operation was tested by measuring the beam width as a function of beam energy E over a range of energies AE. The increase in width caused by chromatic aberrations, in this case, Ax’, is given by the formula,

where u is the angle of convergence of the beam at the focus as measured from the z-axis to the extreme value in the x-plane.

110

J. S. C. McKEE AND G. R. SMITH TABLE IV

DIMENSIONS OF THE CENTRAL REGION OF AN ACHROMATIC QUADRUFQLE LENS Electric poles Distance of pole tips from lens axis Radius of cylindrical pole surface Lens lengths ~~~

~

~

a, = 523 b,

=

63.0"

18 pm

559 pm 0.05 m m

Magnetic poles a = 1702 f 4 pm

b = 1920 f 8 pm 59.89b f 0.02 mm

~

Electrodes are cylindrical wires with hemispherical ends. Pole ends are plane surfaces perpendicular to lens axis.

In the tests, data obtained were consistent with the focal lengths of the achromatic lenses varying quadratically with AE. A value of 1.2 cm was obtained for C:'), but it was impossible to extract a precise value for C:') from the data. When the same doublet lens system was operated in the purely magnetic mode, a value of 20 cm was obtained for C:') indicating an order of magnitude reduction in chromatic aberration when the coupled electric/ magnetic field combination is used. In these tests, a 4 pm beam was used in each of the cases just described. Another method of dealing with chromatic aberrations has recently been used by the Los Alamos Group (Maggiore, 1982). They take a proton beam from the Los Alamos Van de Graaff accelerator which then passes through entrance slits, a bending magnet, energy selecting slits, a magnetic quadrupole triplet, object slits, and finally a superconducting solenoid lens which focusses the beam to a small image on a target. The arrangement is such that if the beam energy changes, the beam position is shifted with respect to the slits and current is lost to the downstream beamline. The beam crosses over at the object slits which essentially define object size, and 95% of the beam is discarded at this point. The final component of the system is the superconducting lens which then brings the beam to a focus. The proton beam on entering this lens is in fact highly divergent, which leads to a correspondingly small spot size as shown in Fig. 3 (schematic details from Maggiore, 1982). Chromatic aberrations in any lens system are of course normally difficult to calculate (Maggiore, 1982),but the solenoid lens in this case has a magnetic field which approximates to a Glaser field, and is well represented by the expression B(z) = B, 1 +(:)2

Vertical ]Van de Graoffl

10 pA Accelerated Beam

-

2 5OOpm-!-Entrance

Slits

L

I

2500pm

50pm FIG.3.

112

J. S. C. McKEE AND G . R. SMITH

where Bo is the field strength at the center of the lens, z is the distance in centimeters along axis from the center of the lens, and a has the measured value of 6.5 cm. For this reason, the aberrations are readily calculable and are found to be largely responsible for the large diameters of the microbeam spot. It appears therefore that energy stability of one part in ten thousand is necessary in order to obtain sub-micron spatial extension of the image using this system. Maggiore notes that by changing the focal length of the final superconducting lens, the beam spot should be able to be kept in focus and chromatic effects eliminated. The problem lies in modulating the solenoid rapidly enough in order to respond to the short term fluctuations in energy of the beam. Plans for a small electrostatic quadrupole triplet between object slit and final lens which could compensate adequately for such fluctuations are however in place. This dynamic focussing component which responds to feedback signals from the energy regulating slits seems to have the potential for solving the energy stability problem and reducing the effect of chromatic aberrations. Such a modified microprobe is not yet complete. Returning then to the achromatic lens system suggested by Martin (1981; 1984),it is encouraging to note that a 500 nm beam at 1.2 MeV proton energy has already been obtained by this method (Martin and Goloskie, 1987). As chromatic aberration gives the ultimate limit to microprobe beam spatial resolution on target, it is important to assess the performance of achromatic lens assemblies in practice. Martin used lenses of 1.04 mm diameter bore, length 6.0 cm and width 7.0 cm separation. The beam spot was produced at a distance 10.0 cm from the second lens. Parameters such as focus and lens rotation were altered so as to compensate first order aberrations, and additional sextupole perturbing fields were added to compensate second order parasitic aberrations. Measurements of beam width were made by examining secondary electron production as the beam grazed a 4.2 pm carbon filament. The 500 nm spot was larger than originally anticipated and chromatic aberration in the earth’s magnetic field was suspected as the cause. The authors concluded that such was not the case by experimentally perturbing the value of the vertical component of that field. It transpired however that residual gas scattering in the microprobe made a significant contribution to beam spot width and accounted completely for the magnitude and energy resolution of the final spot. E. Electrostatic Focussing Elements It is perhaps worthwhile to comment briefly on the topic of electrostatic focussing elements for proton beams and their performance. There is some

PROTON MICROPROBES AND THEIR APPLICATIONS

113

feeling that the general techniques for aberration correction in visible light optics have not yet been adequately adapted to the proton optical situation. If reliable lens systems with negative power are available (Gabor, 1946) then complete lens assemblies with a sufficient number of elements must eventually enable all aberrations to be corrected for simultaneously. At the University of New South Wales, a particular lens geometry named ELCO, standing for “electrostatic coaxial” element has been investigated recently. Here a thin wire is suspended on the axis of a cylinder allowing for the provision of either positive or negative power by holding the centre wire at ground potential and applying a positive or a negative potential to the cylinder. According to Dalgleish (1981), two such elements in tandem can reduce spherical aberrations over a range of entry radii and produce an on-axis focus beyond the dimensions of the cylinder. A disadvantage of this system however is to be found in the requirement of an annular entry aperture. Unless the beam is hollow, most of the beam is rejected on the centre element. It is suggested nonetheless that ELCO is well suited for high energy work, perhaps at GeV energies (Krejcik, 1980), but cooling and shielding of the aperture can present problems. Good alignment of multi-element ELCO systems is difficult to obtain but is essential because the focussing field is in fact radius-dependent. On the grounds that the coaxial lens system is not entirely suitable, Dalgleish (1981) attempted to design a more suitable element. The one chosen was a twin-transverse rod configuration in which the x-z plane is the symmetry plane of the element and the origin is at the centre point of that element. The advantage of this system is that because the lens potential is small compared to accelerating voltages, the effect of the lens on ion path should also be small. The deflecting field is fairly uniform, normal to the path of the proton beam, and therefore is considerably improved from the earlier ELCO design. A collimated beam passing through the lens converges on one side of the symmetry plane and diverges on the other. Two-element pairs in a Caledonian Quadruplet assembly have been investigated in detail. In this arrangement to the first order, the behavior of the lens system is given by B ( Y ) = A(u,

+ a,Y2),

where Y = y/c, is a generalized coordinate scaled to rod separation c, and the right hand side of the equation is an “even” power series function in Y, cut off at the u4 term. This quadruplet system has been studied in detail and as a result, the author suggests that two such quadruplets configured at 90” to each other would produce focussing in two dimensions and a high resolution microbeam. Such a system is now under development at New South Wales as a possible new direction for charged particle optical lens design, but as yet has not become operational.

114

J. S. C . McKEE AND G . R. SMITH

IV. POSSIBLE FUTURE DEVELOPMENTS IN MICROPROBE SYSTEMS A. Practical Considerations

Paraxial and laminar beams represent idealizations of the practical situation. In reality, thermal effects in the ion source, aberrations in the transport system and other factors always result in non-laminar behaviour of a beam. A qualitative measure of the extent to which a beam is laminar is given by a figure of merit known as the “emittance”. This is closely related to the projection on a plane of the volume in phase space occupied by particles that comprise the beam. In practice, most beams have either two planes of symmetry or are axially symmetric. For a beam symmetrical about the xz and yz planes, the x-plane emittance E, is usually defined as 1/71 times the area in xx’ space occupied by the points represented by the particles of the beam for a given value of z. z is distance along the beam axis. It is often acceptable to specify the area within which 90% of the observed points lie, but an “emittance plot” displays not only the area, but the distribution of points within it, and it is the emittance plot that is of most value in understanding beam properties. In a real situation, it may be important to measure beam emittance in the yy’ plane, for example. Experimentally what happens is that a slit is scanned across the beam, and the density distribution of the transmitted beam is then measured by means of a second slit. This method is analyzed in detail by Steenbergen (1967).For high energy proton beams the method is not without complications as particle range in material is long; the slits are therefore thick, and slit scattering is substantial. Such problems are addressed on an ad hoc basis. According to Liouville’s Theorem, if the momentum vector P, does not vary with z, the emittance is an invariant quantity. If on the other hand P, does vary with z, as in accelerating or decelerating particle beams, the emittance is inversely proportional to P,, and the invariant “normalized emittance” is given by E , = fly&. Most accelerator beams used in microprobe facilities are not accelerating or decelerating through the focussing systems. The emittance of a beam is closely related to another quantity called the “brightness” of the beam. Brightness is defined as the current per unit area per unit solid angle.

B

= dl/dAdR

Generally, B varies across a beam so an average value of B is used. For a uniformly confined beam within limits I and I’ the emittance diagrams are rectangles and the average value of brightness is given by: B = q 1 / n 2 ~ * (Lawson, 1977),where q is a form factor of approximately unity.

PROTON MICROPROBES AND THEIR APPLICATIONS

115

In plotting emittance measurements, contours of constant brightness can be displayed on a graph. As Martin (1987) has pointed out, suitable equipment for producing submicron beams with reasonable currents does not exist. Production of a small beam spot demands a small aperture.

B. Future Prospects In looking toward the future and the provision of sub-micron and eventually sub-100 nm beams of reasonable intensity, bright sources are required and aberrations must be minimized. Achromatic lenses offer a means of removing chromatic effects, and future concentration on vertical microprobes may be appropriate for microanalysis with few MeV beams. Nobiling (1986) has discussed these matters, as has Martin (1984). Microprobes currently in operation are approaching the limit of resolution set by chromatic aberration. Other aberrations associated with rotational misalignment can be compensated by lens rotation, and second order aberrations appearing as “bananas” and “new moons” can be compensated for by dipole excitation of the quadrupole structure (Martin, 1984). Hexapoles can also be specifically useful in counteracting other inconvenient second order effects. Legge (1984) has considered the importance of residual third order aberrations. Superconducting lenses may be helpful, but only in association with stable accelerators, and even then, according to Martin they may suffer from parasitic aberrations related to axial current flow in a real solenoid. Good voltage stability in the accelerator used is essential to the successful development of more powerful microprobe systems. In considering the development of high energy proton microprobes, Martin and Goloskie (1987) have commented on the capability of the existing Oxford microprobe. They state that with a source 10,000 times brighter than the Van de Graaff they use, spatial resolution for a 100 pA beam would run around 100 nm. However, it is the chromatic aberration that determines this figure. If aperture aberration in their triplet lens system was the only limiting problem, spot size from this microprobe could approach 1 nm. For the moment we can note that achromatic lenses exist and seem to work well, although new bright proton sources are being developed and may soon open up new possibilities (Martin, 1978; Martin and Goloskie, 1987). Construction of vertical microprobes should improve mechanical stability, and high vacuum assemblies reduce gas scattering broadening to a minimum. Low brightness sources can only produce reasonable currents at present if object slits are opened up, with resultant chromatic aberration of the beam and deterioration of the image. For transmission of 100 pA current to a

116

J. S. C. McKEE AND G . R. SMITH

sample using existing accelerators as sources, the aberrations due to energy spread in the object contribute at least a micron to broadening of the spot. Brighter sources, perhaps 1O4 times brighter would improve the situation greatly, and field ion sources specifically designed for microprobe use may eventually contribute a solution to this problem. Grime and Watt (1987) comment on this possibility with reference to the work of Hanson and Siege1 (1981). Of more immediate usefulness is a recent plan which incorporates proven technology to improve brightness of a microprobe beam by means of thermally cooling protons from the University of Manitoba cyclotron. This idea was first suggested by Oh (1979) and seems likely to meet many of the criteria peculiar to the efficient creation of microbeams. The technique of electron cooling of protons was first conceived by Budker (1967) and was later demonstrated by a team at Novosibirsk (Budker, 1976). The idea here is that protons circulating in a storage ring with a dense and cold electron beam of the same velocity repeatedly interact with each other, and both the longitudinal and transverse components of the proton momentum spread are rapidly transferred to the electrons. At the same time, the space-charge field of the electron beam reduces the radial dimension of the proton beam. A more recent experiment at CERN (Bell et al., 1979),repeated at Fermilab, has used an initial beam of 46 MeV protons with a horizontal emittance of 60 n mm mrad and a vertical emittance of 30 n mm mrad. The intensity of the proton beam was 3 x lo8 particles per pulse, stored in a storage ring of 74 m circumference. Electron cooling reduced momentum spread in the The cross section of the beam to 4 x beam (FWHM) from 2.5 x after cooling was only 0.5 mm. The most interesting fact was that the brightness of the beam increased (in six dimensional phase space) by a factor of a million, of which a 60-fold improvement lay in the longitudinal momentum space component (ie. the energy spread). The improvement in the remaining four dimensional phase space was therefore 16,000, a figure of brightness aspired to by many current microprobe designers. In summarizing their work, Bell et al. (1979)consider residual gas scattering as the final limitation to cooling of the proton beam. The measurements were carried out at a pressure of 2 x low9torr, but further improvement in the base pressure to torr is expected to result in an energy spread below 1 keV in a 46 MeV beam. As several cyclotrons are currently being used as proton sources for microprobes, and because K X-ray analysis is a particularly useful tool in the 30-50 MeV proton energy range, we anticipate that spilling a stored proton beam over a sufficiently extended time period can result in a current on target sample in excess of 100 pA per pm2 where the spot size is indeed of sub-micron dimensions.

PROTON MICROPROBES AND THEIR APPLICATIONS

117

The day of the complete microprobe, in which all components from source to spot are custom-designed for a particular function, may be just around the corner. From proton source to sub-micron beam spot, the future looks bright. V. SAMPLE ANALYSIS A number of different types of data registration techniques can be used with a proton microprobe to measure elemental concentrations in a sample (Cookson, 1979). Proton Induced X-ray Emission (PIXE) (Johansson et al., 1970), Rutherford Backscattering (RBS) (Themner and Malmqvist, 1986), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA) are such techniques, and each has advantages relevant to a specific experimental situation. PIXE is perhaps the easiest of these techniques to apply and enables the simultaneous collection of data on most elements in the Periodic Table to be made. In particular, an energetic proton passing through matter can remove an electron from the innermost K or L shells of an atom. Readjustment of the remaining electrons in the atom will follow resulting in the emission of characteristic X-rays, and the detection of such X-rays unambiguously identifies the element. The X-rays are detected by a solid state detector (Si(Li) or intrinsic Ge, for instance), and the resultant signals are energy analyzed. The resulting spectrum contains information on all the elements contained in the sample. Because the total charge of the protons passing through the sample can be measured accurately, the yield of X-rays from the sample can be determined by comparison to standard samples of known composition and a reasonably accurate quantitative analysis of the sample can be made. This technique is sensitive to elements with Z > 13, and it is capable of detecting concentrations at the parts per million (ppm)level or better. The PIXE method is inherently non-destructive. NRA is used to detect the lighter elements, Z < 13, when the incident particle has sufficient energy to penetrate the Coulomb barrier surrounding the nucleus of an atom within the sample. The incident particle undergoes a reaction within the nucleus of the atom, and a neutron, another charged particle or a gamma ray may be emitted from the resultant nucleus. Energy analysis and identification of the emitted particle, along with a knowledge of the incident particle energy will allow the identification of the atom within the sample to be made. Concentrations of 10 to 1000 ppm can be measured, depending on the nuclear reaction under observation. RBS occurs when the incident particle does not have sufficient energy to penetrate the Coulomb barrier surrounding the nucleus of the atom in the

118

.I.S. C . McKEE AND G . R. SMITH

sample but is scattered elastically from that nucleus. The energy and angle of the scattered particle are observed and the identity of the scattering atom then determined. The energy of the scattered particle will in general be less than the incident energy, due to energy loss in traversing the sample. The depth at which scattering occurs within the sample can be deduced, allowing the sample to be scanned in depth for elements of different kinds. For efficient scattering of the incident particle, the 2 of the scattering atom should be large, and for efficient propagation of the incident particle to and from the scattering site, the energy lost in the sample should be low. A low mass matrix about the scattering atom is therefore desirable. This condition is easily met in studies of heavy trace elements in biological material. Concentrations of ppm are measurable by this technique. In ERDA the energy of the recoiling nucleus is measured, rather than the energy of the scattered particle. Since the scattered nucleus has a very small range within the material of any sample, this technique is only effective in measuring atoms on or very close to the surface of the sample. The incident particle beam must strike the surface at a grazing angle so that the scattered nucleus can emerge from the surface under investigation. The three techniques based on nuclear processes have the disadvantage of very low probability of occurrence as compared to PIXE. Because of this fact the PIXE technique is the method of choice for a simple fast accurate quantitative analysis of elements within samples of material. The complimentary techniques are used only when they hold an advantage over PIXE analysis due to the need for additional information or to low efficiency of the PIXE technique in a particular instance. A review of applications of these techniques can be found in Cookson (1979). Figure 4 shows the energy dispersive part of a PIXE experimental computer-controlled data acquisition system, based on a Si(Li) detector. The four data channels of the system that register X-ray energy data are identified, as are also several data quality monitors. Sample thickness is monitored by registering scattered protons, beam intensity by measuring Faraday-cup current, and the surface of the sample by secondary electron emission measurement. The method of data collection is event by event, with all measures of data quality recorded along with the X-ray energy data. Monitoring the Faraday-cup current allows normalization to proton beam current. Registering a signal proportional to the emission of secondary electrons from the sample surface facilitates the identification of X-ray signals and relates them to significant topographical features of the sample. The computer removes data from the ADCs and also halts the data flow if a monitor of data quality indicates that such a course of action is advisable. The systems controlling the microprobe as a proton source are added to the data acquisition and control system to complete control of the facility.

PROTON MICROPROBES AND THEIR APPLICATIONS

119 to

from

Protons \,0

L

Computer

Farad ay CUP

1

l

FIG.4. A typical data computer controlled registration and control system including Nuclear Backscattering (NBS) detector, Secondary Electron Emission Detector (SEED), a solid state x-ray detector (Si(Li)) and a Faraday-cup beam current monitor connected to a current to voltage converter (C to V).

A. Sample Mounting

The proton microprobe can operate in three different modes which relate to the method of microprobe beam formation. A collimated microprobe beam requires a sample that can be positioned such that a particular feature of interest lies in the microprobe beam. When a collimated microprobe is used to scan a sample, the stage holding the sample must in practice move the sample through the beam. A focussed microprobe beam can additionally be deflected either electrically or magnetically across a stationary sample. The amplitude of such a scan can be as large as 2 mm. The samples are mounted on a stage which can be moved, either manually, or under computer control along three perpendicular axes. Significant features of the sample can be positioned in the proton microprobe beam with the aid of an optical microprobe, which can be coupled to a TV camera for high energy microprobe studies (e.g. 20 to 50 MeV). The same system can be used to locate the proton microbeam if a thin quartz plate replaces the sample on the stage of the microprobe. This method is acceptable as long as the microbeam spot is not too small, and the proton energy is not too large.

J. S. C. McKEE AND G . R. SMITH

120

Samples can be scanned by the proton microprobe beam in one of two ways, either by moving the sample stage through the proton beam, or rastering the beam across a stationary sample. Several benefits accrue from scanning the beam across the sample: a. Local heating of the sample by the proton beam is reduced. b. Erosion of the sample under proton bombardment is distributed over the whole sample region scanned. c. Local ionization induced radiation damage is reduced. d. Mapping the location of elements in the sample, by secondary emission electron imaging of the sample becomes possible. The first three advantages are of particular importance for low energy proton accelerators as large amounts of energy are deposited by the low energy protons in and near the surface of the sample. The choice of scanning method, i.e. moving the sample (den Ouden et al., 1981) or moving the beam (Heck, 1979; Grime et al., 1984), is a matter of choosing between different types of experimental facilities. A moving sample ensures that no change in microbeam spot size or divergence occurs as different regions of the sample are irradiated. The technique is limited in scanning frequency by the mechanical linkages

I

I

,Stage

Protons’ Y

X

/

Magnets

A i

Faraday CUP

Raster Drive

to

Computer

FIG.5. Block diagram indicating computer control by two different means for dimensional (x.y) scanning of a sample. The raster drive controls magnets which deflect the beam across the

sample, while the motor controller steps the sample through the beam. Either system can be found in operation at microprobe facilities.

PROTON MICROPROBES AND THEIR APPLICATIONS

121

causing the motion. A moving beam spot, in the case of magnetically focussed microbeams, can be deflected either before or after the image forming magnets. Since little working space exists between the magnets and the sample stage, choosing to deflect the beam before the magnets is the easiest technique to adopt experimentally. However the size of the beam spot changes with deflection when deflections on the order of Imm or larger are required. It has been shown (Heck, 1982) that if the beam is deflected twice before the image forming magnets so that the beam crosses the optical axis in the principle plane of the lens, then this type of beam distortion can be minimized. A scanned sample has the final advantage in that secondary electrons are emitted from the surface as protons strike it and can be used to form an image of the sample (Traxel and Mandel, 1984). Monitoring the secondary emission electron current aids in positioning and focussing the beam (Younger and Cookson, 1979; Kneis et al., 1982), and has the possibility of solving the problem of associating X-ray features in the sample with physical coordinates of the sample. In either case, two more channels can be added to those of Fig. 4, as shown in Fig. 5. Readouts for positional information come from either the stage motor controllers in the case of a moving sample, or the beam deflection voltages for a scanned beam and locate the x and y positions of the beam relative to the sample stage. B. Data Registration A large amount of data can be collected from any one sample. In the simplest operating mode, where a feature of the sample is positioned in a stationary microprobe beam, an X-ray energy spectrum is collected along with a measure of the total irradiation of the sample from the Faraday-cup data channel. If the beam is scanned relative to the sample, an X-ray spectrum, along with data quality information is collected for each pixel. A pixel (picture element) is identified from a combination of the beam resolution, the available computer memory, and the size of the sample region being scanned. If a 10 micrometer diameter microprobe beam is scanned across a 1 square millimeter area, a square containing 100 x 100 pixels could be generated. If an X-ray spectrum containing 1,000 channels is accumulated for each pixel (plus a small amount of extra information), a large amount of computer memory is consumed to store this information at a relatively low density. However, the data can be written to a mass storage device (magnetic tape or disk) event by event including all the information indicated in Figs. 4 and 5 and be analyzed off line. The result of this analysis can include maps of elemental distributions in one, two or possibly three dimensions (Heck, 1984),or simply the concentrations of elements in one pixel.

122

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C. Data Analysis

The analysis of PIXE spectra is an example of a mature mathematical technique, Several laboratories have developed computer codes which incorporate linear and non-linear least squares fits to different models of peaks superimposed on a mathematically expressed background, for X-rays coming from thick or thin targets. The analysis generally involves the creation of a database of K and L X-ray relative intensities, the energy calibration of the Xray detector, and finally, the deconvolution of the observed spectrum (Rogers et al., 1987). A summary of eight different codes has been prepared (Watjen, 1987)and an intercomparison of five of the eight codes (Campbell et al., 1986) has been made. The five codes produce good agreement ( < 1%) except in cases where X-ray energies were below 3 keV, or where weak X-ray peaks resided on the low energy side of more intense nearby peaks. The latter information has generated a study of peak lineshapes (Campbell, 1987) with particular emphasis on modelling the low energy tails of the X-ray peaks and a proposal for studying the interference of neighboring peaks for specially prepared samples (Watjen, 1987). The calibration process also requires an understanding of the homogeneity of specially prepared calibration samples. If the microbeam analysis is to be self-calibrated as to elemental concentrations, some kind of labelling of the sample with known concentrations of given elements must be carried out. The uniformity of the deposition technique has to be guaranteed to dimensions smaller than the microbeam diameter. Two recent studies (Rogers et al., 1987; Themner et al., 1987) have verified the homogeneity of different commercially available elemental standards down to microbeam spot areas of 200 pmZand 400 pm2. These, unlike evaporated labels that have uneven elemental distribution can be used to label microprobe samples.

D. Radiation Damage The passage of energetic protons through a sample can cause several different types of radiation damage. The damage depends sensitively upon the energy of the proton passing through the sample. Low energy protons of a few MeV or so do not travel very far into most samples before losing all their energy. Often only a few tens of microns are involved. The resulting deposit of energy, close to the surface of a sample, causes considerable local heating and can lead to erosion of the sample. Differences in volatility of the elements making up the sample lead to a differential disappearance of the elements of the sample. Additionally, the local microstructure of materials (alloys, for example) could suffer damage from such local heating. Radiation damage can also occur due to ionization induced by the protons. Here the local chemical

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structure of a sample can change because of the breaking of chemical bonds or a change in the chemical compounds involved. In such cases, the loss of some elements is experienced relative to others. A number of studies have been performed relating to damage caused by sample heating (Legge and Mazzolini, 1980; Talmon and Thomas, 1977) and by beam current effects (Slatkin and Jones, 1977; Slatkin et al., 1985). In their 1985 study, Slatkin et al. investigated the deterioration of air dried blood cells resulting from bombardment by H',2,3 and He4 at an energy of 1.7 MeV and identified beam fluences that caused similar damage to the cells, as judged by visual inspection. Because the particle fluences for similar damage were proportional to the stopping power of the samples for the particle in question, they concluded that the observed damage was proportional to the ionization rate. Thus the damage was caused by radiation, rather than by heating of the sample. They calculate, for a typical proton microprobe, that severe damage to cells is to be expected. Earlier work on specimen damage has been reviewed by Kraner and Jones (1981). VI. TYPICALAPPLICATIONS OF PROTON MICROPROBES The proton microprobe represents an analytical tool that is in the process of discovering its own set of applications. As such, many current investigations are exploratory in nature, and not all have been successful. Several have generated extensive additional study, and a number of reviews (Malmqvist, 1986; Blank and Traxel, 1984; Campbell and Cookson, 1984) of the applications of the proton microprobe/PIXE analysis technique have been produced. The purpose of this particular section is to indicate where recent work has taken place and to discuss several selected applications in detail. The technique of PIXE operation, data registration and analysis combined with the proton microprobe as a particle source has proven to be a powerful analytical tool. Applications that require the unique properties of the proton microprobe because of sample size or microstructure within the sample are of interest. For the purpose of this review, the applications are separated into four categories: biological, mineralogical, antiquarian, and industrial. Investigating trace element concentrations in biological materials has the advantage that the matrix which contains areas of interest is mostly low Z material (Z < 13). Here the combined processes of ionization and emission of X-rays and their propagation through the sample, followed by their detection by means of a solid state detector, have a low overall efficiency. As a result, these materials in the matrix do not contribute significantly to the X-ray energy spectrum recorded by the detector. In mineralogical, or similar

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materials, the elements of the matrix containing the objects of interest (grains of an element or chemical compound, for instance) are detected as readily as the elements being investigated. Large peaks appear in the observed spectrum due to the large concentrations of the elements of the matrix material interfering with the detection of elements appearing in trace concentrations. Special techniques (Duffy et al., 1987), such as a) limiting the region of the energy spectrum under study to an area remote from large interfering peaks, b) making peak identifications on the basis of L rather than K X-rays, and c) invoking deconvolution methods to extract several K and/or L X-rays from an extended peak, limit the overall sensitivity of this microanalytical technique in some instances. A . Applications to Biological Systems

Small samples arise in biological systems from the desire to investigate trace element concentrations in the various regions of a single cell. Malmqvist et al. (1 986) have recently reviewed the use of proton microbeams in biological areas, separating human medical applications from studies of plant and animal tissues. Some interrelationship exists between the human and animal studies in that laboratory animals are used to determine the toxicity of materials to which humans may be exposed. The role of trace elements in the regulation of bodily processes has become of increasing interest in the last 15 years. Similarly, the observation is made that certain elements, either in excess concentration or in deficiency, accompany specific illnesses. When such an effect is known to exist, the proton microprobe can be used to investigate the region in the affected organ where those differences occur. Watt et al. (1984) have succeeded in identifying the region of the liver where copper occurs in excess concentrations in primary biliary cirrhosis of the liver. They have also found that the location of the copper was correlated with the location of sulphur in the liver cells. Heck et al. (1987) have similarly found that iron, copper, zinc and bromine, which are distributed uniformly in the normal liver, are associated with regions of inflammation and scarring within the cirrhotic liver. Use of the proton microprobe allows the investigation of hypotheses relating to the effect of the presence or absence of trace elements and their concentrations in normal and diseased organs. Kayama-Ito et al. (1984) have investigated the possibility that excess concentrations of trace elements accompany cataracts in eye lenses. They observed very small differences in trace element distributions associated with different types of cataracts. In this work, the lenses were scanned in two dimensions. Subsequently, Kayama-Ito et al. (1987) identified areal concentrations of sulphur, potassium and calcium associated with the opacity in the eye of a mouse with a hereditary cataract.

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The connection between trace elements and cancerous cells yields results showing both some and no correlation. While no significant differences from healthy organs were found for gastric cancer samples (Heck and Rokita, 1983), excess cadmium has been associated with prostate cancer. While the cadmium was distributed throughout the organ, excesses were found to be associated with certain structures within the organ (Vis et al., 1985). The suspected association of excess calcium with arteriosclerosis was investigated by Chichocki et al. (1985). Calcium was found to be associated with bromine, iron, and zinc in the artery walls and localized in a way that indicated that crystals of these elements were formed. Chichocki et al. ( 1 987) have continued their work, concentrating on calcium-phosphorous deposits in the artery walls. Localization of the deposits with a proton microprobe was possible, but the chemical structure of the compounds could not be determined.

B. Mineralogical Applications The mineralogical applications of the proton microprobe were recently reviewed by Blank and Traxel(l984). Minerals occur in complex patterns of interweaving veins or in small grains within some matrix. If a microanalysis of these structures is to be made, only small volumes of rock need be sampled. This avoids contamination of results by elements contained in nearby but perhaps unrelated structures. For this situation, a proton microprobe is an ideal tool. Prepared samples are thick enough so that a low energy microbeam is stopped and the thin sample analytical techniques used for biological specimens are inapplicable. Campbell and Cookson (1984) have reviewed the various modifications of analysis needed to accommodate thick targets for PIXE analysis, and their summary applies to proton microprobe targets also. The proton microprobe has the potential to determine trace element concentrations down to ppm, as long as interference from major constituents of the specimen is not present. Here the search is for peaks from minor and trace constituents in regions of the X-ray energy spectrum well away from the peaks from the major constituents. The large penetration depth of the protons creates the possibility of a different type of interference, that from other structures within the matrix that lie behind the grain of interest. Analysis of the sample in depth requires careful interpretation of observed data but, as already indicated in Section VI, this is a technique that is possible with the proton microprobe. The microstructure of minerals and the trace elements contained in a sample can yield information on the processes of formation of the rock under consideration as well as the influence of factors like temperature and diffusion during the formation period. At the same time,

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the presence of different trace elements within a sample can determine the economic feasibility of removing, or purifying some other valuable element from the ore. This type of investigation of the microstructure and trace element concentration of certain minerals has been carried out recently by Kwiatek et al. (1986) with reference to limestone from Poland, and by Cabri et al. (1984; 1985) in a study involving sulfide ores from several different locations. The first investigation identified correlations and anticorrelations between different trace elements and the microstructure of the limestone. The second study searched for valuable trace elements that might accompany and also be extracted from the ores, in particular the platinum group elements, germanium, arsenic, selenium, cadmium, indium, tin and antimony. A microprobe was used to isolate grains of the mineral within its matrix and a comparison of the trace element content of the grains for several sulfide ores was made. These studies demonstrated the applicability of the proton microprobe technique by identifying trace elements present in economic concentrations and also indicated how the technique helps discriminate between elements in compounds and elements in an ore solution. More study is required to establish a database for evaluating the economic feasibility of removing trace elements from various other ores. Kraner et al. (1 982) investigated the trace element content of coal and coal ash, with particular emphasis on determining the arsenic content of fly ash. Arsenic can form a major pollutant in ground water near fly ash disposal areas. Minkin et al. (1982) studied trace element concentrations in coal as a means of identifying those coals that when burned would produce the least adverse environmental impact and then related that fact to the region of production. An incidental result from the latter study was that the trace element content of coal was found to depend on its vertical position in the bed from which it was extracted. Another type of mineralogical investigation with implications for the history of the solar system has been the study of lunar rocks (Blank et al., 1984) and meteorites (Jones et al., 1986; Burnett et al., 1986; Wollum et al., 1987; Vis et al., 1987). Burnett et al., Wollum et al. and Vis et al. were interested in verifying that carbonaceous chondrite elemental abundances represented the average primordial composition of the solar system. In particular, they proposed to test the “smoothness” of odd mass heavy elements (Z > Fe) as a function of mass number, as that is the argument linking these meteorites with the primordial composition. They found the smoothness to be only approximate, with deviations up to as much as 50%. In general, mineralogical applications of the proton microprobe/PIXE technique are just beginning to demonstrate their usefulness. The next step

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requires the creation of a suitable database containing trace element concentrations of minerals in ores from many places around the world. C. Antiquarian Applications

The possibility of using the proton microprobe/PIXE technique to categorize historical and archeological objects such as books and pottery and jewelry is advanced in two recent papers (Kusko and Schwab, 1987; Swann and Flemming, 1987). These investigations involve study of the microstructure of antiquarian objects for the purpose of discovering the process of manufacture, smelting, or production of the objects and concentrate on measuring the elemental concentrations of common materials such as ink, paper, solder and alloys. Printer’s ink, and the paper it sits upon, can be studied separately with microbeams of sizes above 50 to 100 microns. Kusko and Schwab (1987) have studied the Gutenberg Bible, paper and ink, with this technique. Small inclusions, resulting from the assembly of ancient gold and silver jewelry by soldering or brazing, have recently been studied by Demortier and Hackens (1982). The presence of cadmium in the gold jewelry has suggested that a brazing process for gold including only yellow metals was available in ancient times. This process has recently been rediscovered by Demortier et al. ( 1984). D. Industrial Applications

Three opportunities for the use of the proton microprobe in modern manufacturing techniques have been discussed in the literature. Fine carbon filaments, 7 micrometers in diameter, and coated with a superconducting NbCn layer were studied by Heck (1984) in the analysis of problems (breaking of the wires for no apparent cause) relating to the manufacture of the wires. Bodart and Donnelly (1983) also used a proton microbeam to study helium implanted in an aluminum surface. The uniformity of the implantation of the helium was in question. In the analysis, helium was found to be present in small spheres in the aluminum material of size requiring the microprobe for study. Hanson et al. (1981) used a proton microprobe to study the migration of fission fragments in reactor grade carbon. The porous microstructure of carbon employed in high temperature gas reactors allows fission products (molybdenum was selected in this study) to diffuse through the material. This microstructure can be studied in detail by the proton microprobe which has an additional advantage in that it can sample concentrations at varying depths in

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the graphite if required. These examples indicate that the proton microprobe is useful in most modern industrial processes involving the creation of microscopic devices.

v11. ADVANTAGES OF HIGHENERGY MICROPROBES A proton microprobe is currently under construction at the University of Manitoba Accelerator Laboratory which will use the laboratory’s cyclotron as a particle source (Al-Ghazi et al., 1982).The cyclotron produces protons with energies between 20 and 50 MeV, and the microprobe is optimized to operate efficiently at energies between 30 and 50 MeV. This device is designed to address a number of problems that are inherent in the use of low energy machines (producing particles with energies up to several MeV) as particle sources for microprobes. First, low energy protons have difficulty ionizing the innermost electrons of heavier elements so that the identification of these elements must rely upon identifying L, rather than K X-rays from the elements under study. The L Xrays are not only of lower energy, but also overlap K X-rays from lighter elements thus over-populating the low energy end of the observed energy spectrum. These features may require complicated deconvolution techniques to isolate the X-ray energies and identify the elements, and yet may not yield extra information in proportion to the extra analytical work required. The higher energy protons can excite the K X-rays with sufficient efficiency so that they may be readily detected. This fact should help in identifying the lanthanide elements that are at present virtually undetectable by low energy microprobes. Secondly, the range of low energy protons in biological materials is in general less than 50 microns, a value approximately 200 times smaller than the range of the higher energy protons. Also, as the density of material under analysis increases, the range of protons in the sample decreases dramatically. This restricts the low energy microprobe to measuring elemental concentrations in the near surface regions of thick samples, and requires that biological specimens be prepared in very thin sections. Complicating the situation is the fact that the low energy protons lose energy at a rate roughly ten times higher than the high energy protons so that the actual energy deposited near the surface of the sample can be much larger when low energy protons are in use. This results in more radiation and heat-induced damage near the surface of samples irradiated by low energy protons. The high energy protons have a range on the order of centimeters in biological materials, and proportionately larger in more dense matter. Thus for mineralogical and

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similar samples the high energy proton microprobe when used as a transmission device is often capable of surveying elemental concentrations throughout a sample. In particular, coupling the PIXE technique with registration of quasi-elastically scattered protons from atoms within the sample offers the possibility of examining elemental concentrations as a function of depth to an extent not currently available. In summary then, the decrease in near surface radiation and heat induced damage, the possibility of exciting many K X-rays simultaneously and the simplification in data analysis are important advantages. The particular sensitivity of a high energy K X-ray emission technique to rare earth elements, and the ability to probe samples to considerable depth indicates that the high energy proton microprobe can be an analytical tool of significant utility in the future and can complement the information already available from low energy proton microprobe facilities. REFERENCES Al-Ghazi, M. S. A. L. and McKee, J. S. C. (1982).Nucl. Instr. and Meth. 197, 117. Al-Ghazi, M. S. A. L., Birchall, J. and McKee, J. S. C. (1982). Phys. Rev. A 25, 3072. Augustyniak, W. M., Betteridge, D. and Brown, W. L. (1978). Nucl. Instr. and Meth. 149,669. Bell, M., Chaney, J., Cittolin, S., Herr, H., Koziol, H., Krienen, F., Lebee, G., Meller Petersen, P., Petrucci, G., Poth, H., Sherwood, T., Stefanini, G.. Taylor, C., Tecchio, L., Rubbia, C., Van der Meer, S. and Wikberg, T. (1979). Phys. Letts. 87B, 275. Blank, H. and Traxel, K. (1984).Scanning Electron Microscopy 111, 1089. Blank H., El Gorsey, A,, Janicke, J., Nobiling, R. and Traxel, K. (1984). Earth and Planetary Science Lett. 68, 19. Bodart, F. and Donnelly, S. E. (1983).Nucl. Instr. und Meth. 218, 529. Bonani, G., Suter, M., Jung, H., Stroller, C . and Wolfli, W. (1978). Nucl. Instr. and Meth. 157, 55. Booth. R. and Lefevre, H. W. (1978).Nucl. Instr. and Meth. 151, 143. Bos, A. J. J. (1984). Thesis, Vrije Universiteit, Amsterdam, The Netherlands. Bosch, F., El Coresy, A,, Martin, B., Povh, B., Nobiling, R., Shwalm, D. and Traxel, K. (1978). Nucl. Instr. and Meth. 149, 665. Bosch. F., El Goresy, A,, Martin, B., Povh, B., Nobiling, R., Shwalm, D. and Traxel, K. (1977). Science 199,765. Brune, D., Lindh, U. and Lorenzen, J. (1977). Nucl. Instr. and Meth. 142, 51. Budker, G. 1. (1976). Part. Accel. 7, 197. Budker, G. 1. (1967). At. Energ. 22,346. Burnett, D. S., Woolum, D. S., Benjamin, T. M.. Rogers, P. S. Z., Duffy, C. J. and Maggiore, C. J. (1986). Paper presented to the Seventeenth Lunar and Planetary Science Conference, Houston, Texas. Cdbri, L. J., Campbell, J. L., LaFlamme, J. H. G., Leigh, R. G., Maxwell, J. A. and Scott, J. D. (1985). The Canadian Mineralogist 23, Part 2, 133. Cabri, L. J . , Blank, H., El Gorsey, A,, Laflamme, J. H. G . , Nobiling, R., Sizgoric, M. 9. and Traxel, K. (1984). The Canadian Mineralogisi 22, 521. Campbell, J. L. (1987). Nucl. Instr. and Meth. in Physics Research B22, 13.

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Morris, W. G., Bakhru, H. and Haberl, A. W. (1984). I n “Microbeam Analysis”, p. 327. San Francisco Press. Nobiling, R. (1986). Nucl. Instr. and Meth. in Physics Research B14, 142. Nobiling, R. (1983). Nucl. Instr. and Meth. 218, 197. Nobiling, R., Traxel, K., Bosch, F., Civelekoglu, Y.,Povh, B. and Schwalm, D. (1977). Nucl. Instr. and Meth. 142,49. Nobiling, R., Civelekoglu, Y.,Povh, B., Schwalm, D. and Traxel, K. (1975). Nucl. Instr. and Meth. 130,325. Oh, S. (1979).University of Manitoba, Private communication. Pierce, T. B., Peck, P. F. and Cuff, D. R. A. (1966). Nature 211,66. Prakash, R. and McKee, J. S . C. (1985). Nucl. Instr. and Meth. in Physics Research B10,679. “Proc. 2nd Int. Cod. on Chemical Analysis”, Namur, (1982).Ed. G. Demortier and in Nucl. Instr. and Meth. 197, 1-255. Rogers, P. S. Z., DuRy, C. J. and Benjamin, T. M. (1987).Nucl. Instr. and Meth. in Physics Research B22, 133. Schmid, K., Muller, H., Ryssel, H. and Ruge, I. (1973). Thin Film Solids 199, 313. Sealock, R. M., Mazzolini, A. P. and Legge G. J. F. (1983). Nucl. Instr. and Meth. 218,217. Shroy, R. E., Kraner, H. W. and Jones, K. W. (1978). Nucl. Instr. and Meth. 157, 163. Singh, M., Melvin, J. and Cho, Z. H. (1976). I E E E Trans. on Nucl. Sci. NS-23-1,657. Slatkin, D. N., Shroy, R. E. and Jones, K. W. (1985). Nucl. Instr. and Meth. in Physics Research B9, 66. Slatkin, D. N. and Jones, K. W. (1977). Nucl. Instr. and Meth. 142, 589. Steenbergen, A. van (1967).Nucl. Instr. and Meth. 51,245. SteRen, K. G . (1965). “High Energy Beam Optics, Monographs and Texts in Physics and Astronomy, xvii.” Wiley, New York. Suter, M., Wolfli, W., Bonani, G., Jung, H. and Stoller, C. (1974). E T H Zurich Prog. Report 263; (1 975). Helu. Phys. Acta 48, 5 11. Swann, C. P. and Flemming, S. J. (1987). Nucl. Instr. and Meth. in Physics Research B22,407. Talmon, Y . and Thomas, E. L. (1977). J. of Microscopy 111, 151. Themner, K, Lovstem, N. E. G., Tapper, U. A. S. and Malmqvist, K. G. (1987). Nucl. Instr. and Meth. in Physics Research B22, 126. Themner, K. and Malmqvist, K. G. (1986). Nucl. Instr. and Meth. in Physics Research B15,404. Traxel, K. and Mandel, A. (1984). Nucl. Instr. and Meth. in Physics Research B3, 594. Vis, R. D., VanderStap, C. C. A. H. and Heymann, D. (1987). Nucl. Instr. and Meth. in Physics Research B22,380. Vis, R. D. (1986). “The Proton Microprobe: Applications in the Biomedical Field”, CRC Press, Inc., Boca Raton. Vis, R. D., Lenglet, W. J. M. and VanderStap, C. C. A. H. (1985). Nucl. Instr. and Meth. in Physics Research B10/11,683. Watjen, U. (1987). Nucl. Instr. and Meth. in Physics Research B22,31. Watt, F., Grime, G . W., Takacs, J. and Vaux, D. J. T. (1984). Nucl. Instr. and Meth. in Physics Research B3,599. Watt, F., Grime, G. W., Blower, G. D., Takacs, J. and Vaux, D. J. T. (1982). Nucl. Instr. and Meth. 197, 65. Wilde, H. R., Roth, M., Uhlhorn, C. D. and Gonsior, B. (1978). Nucl. Instr. and Meth. 149, 675. Wollun, D. S., Burnett, D. S., Benjamin, T. M., Rogers, P. S. Z., DufTy, C. J. and Maggiore, C. J. (1987). Nucl. Instr. and Meth. in Physics Research B22.376. Younger, P. A. and Cookson, J. A. (1979). Nucl. Instr. and Meth. 158, 193. Zirkle, R. E. and Bloom, W. (1953). Science 117,487.

A D V A N C E S IN E L E C T R O N I C S A N D F-LEC-TRON PHYSICS . V O L 7 3

An Early History of the Electron Microscope in the United States J O H N H . REISNER 1. Author's Preface . . . . . . . . . . . . . . . . . . . . . . . . 11. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .

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A . The Debt to German Electron Microscope Technology . . . . . . B. Basic Technology 1930-1950 . . . . . . . . . . . . . . . C. Economic and Political Restraints . . . . . . . . . . . . . The Earliest Electron Microscopes in the United States . . . . . . . A . The Fitzsimmons- Anderson Electron Microscope at Washington State University 1934- I938 . . . . . . . . . . . . . . . . . . B. The Newberry-Scott Microscope at Washington University of St . Louis 1935-1940 . . . . . . . . . . . . . . . . . . . . . . I . The Emission Microscopes . . . . . . . . . . . . . . . 2. The Transmission Microscope . . . . . . . . . . . . . . The Legacy from Toronto . . . . . . . . . . . . . . . . . . A . Cecil Hall and the Kodak Microscope . . . . . . . . . . . . B. Albert Prebus and the Ohio State University Microscope . . . . . C. William Ladd and the Columbian Carbon Company Microscope . . Electron Microscope Development at the General Electric Company ( G E ) A . Early Decisions . . . . . . . . . . . . . . . . . . . . . B. Manufacturing and Marketing a Microscope . . . . . . . . . . C. The Second Try . . . . . . . . . . . . . . . . . . . . . Development of the Electron Microscope at RCA . . . . . . . . . A . From Idea to Instrument (1932-1942) . . . . . . . . . . . . 1. Zworykin Initiates a Program . . . . . . . . . . . . . 2. Marton and the EMA . . . . . . . . . . . . . . . . 3. Vance and the Power Supply Revolution . . . . . . . . . . 4 . Hillier builds the EMB . . . . . . . . . . . . . . . . 5. Manufacturing the EMB . . . . . . . . . . . . . . . 6. Impact of the EMB . . . . . . . . . . . . . . . . . . 7. The High Voltage Electron Microscope . . . . . . . . . . 8 . The Scanning Electron Microscope . . . . . . . . . . . 9. James Hillier . . . . . . . . . . . . . . . . . . . . 10. RCA Research Personnel Who Made Major Contributions to the Development of the Electron Microscope . . . . . . . . . 1 1. Research Moves to Princeton . . . . . . . . . . . . . B. The Era of Advanced Development 1942-1950 . . . . . . . . . I . A New Team of Engineers . . . . . . . . . . . . . . . 2 . The EMU Electron Microscope . . . . . . . . . . . . . 3. The EMC Console Microscope . . . . . . . . . . . . . 4. The RCA Service Company . . . . . . . . . . . . . .

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I. AUTHOR’S PREFACE Although the electron microscope is unquestionably recognized as one of the most important scientific instruments of the 20th century, the technical achievement involved in its development has not been so well recognized. The very tardy award of a Nobel prize to Ernst Ruska more than fifty years after his successful demonstration of the electron microscope attests to this lack of recognition. Denis Gabor observed that looking at it after the fact, the electron microscope seems to many to have been an obvious invention (Marton 1968). Fortunately, much history on the microscope has been written since Gabor’s comment in 1968, that has helped to correct such a misunderstanding, but more needs to be written. For example, there has been no comprehensive account of the development of the electron microscope in the United States prior to the one presented here. Like most of the authors who have written historical articles to date, I have concentrated primarily on the development of the electron microscope itself and have provided a comparatively small amount of information on the development and achievements of what is properly called electron microscopy. Far more effort has gone into developing the uses of the microscope than went into developing the instrument. There is need for a history of how microscopists learned to use the instrument and how it influenced their science. It is not a task that I, a physicist/engineer, am qualified to undertake. I have somewhat arbitrarily chosen the time frame for my historical account as 1935 to 1955. There was n o microscope activity in the United States

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prior to 1935, and after 1955, microscopical activity in the United States was no longer separable from the international scene. How this transition took place is the subject of this article. Some readers may be disturbed that microscope development in the rest of the world seems to have been ignored. However, it is a fact of history that scientific communication with Europe and Asia was cut off or, at best, severely attenuated for considerably more years than just the period of open hostilities of World War 11. During this time, the United States did experience a separate and unique scientific development. Many times while engaged in writing this article, I have pondered why I was writing it, and for whom. My experiences as editor of the feature “Reflections” in the “EMSA Bulletin” for the past six years have made me acutely aware that time is running out for having first hand contact with the pioneers who developed the electron microscope. Even now it is late to put their stories in writing. The second reason for writing an article is to put some chronological order into the history we do know. There were over nine different successful microscopes built between 1936 and 1946, and without some perspective in time, their histories become confused and the people forgotten. Ostensibly I have written my article for microscopists and engineers who have an interest in their historical antecedents. However, I can not dismiss so lightly the question of for whom was I writing. In the forty years I have been active in the field of electron microscopy I have known many people, and in the early years, I knew most of the early workers. I was not writing about strangers. I find that I often seemed to sense the presence of the people, about whom I was writing, looking over my shoulder. I found myself writing for them. That state of mind may decrease objectivity, but it does wonders for the sense of responsibility to be correct in what one writes. John H. Reisner Haddonfield, N.J. 11. INTRODUCTION

The past is a foreign country: they do things diflerently there.--. P Hartley American Historical Review A . The Debt to German Electron Microscope Technology

The decade of the 1920s might well be called the decade of the electron. As physicists began to understand the electron, engineers began to create a

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technology for it. This was the decade when vacuum tubes, cathode-ray tubes and photo-cells made their entrance and created what is now termed electronics. Germany was the center of activity in the research on how to control electron motion which became known as electron optics. German scientists developed analytical expressions for electron lenses and other elements. The possibility of building an electron microscope became an occasional item of conversation for engineers and scientists, particularly in Berlin, and the first electron microscope was demonstrated by Max Knoll and Ernst Ruska in 1931. By 1933, an electron microscope with better resolving power than the light microscope was operating in Ruska’s laboratory. The early history of the electron microscope in Germany is well documented (Mulvey, 1962; Marton, 1968). The German successes did not stimulate much activity in the United States scientific community. The major champions for developing the electron microscope were biologists and chemists who were feeling the pressure for better resolving power. Prior to 1939, when Siemens started to manufacture electron microscopes and Marton completed his prototype for RCA, there were no visible prospects for a commercial source of electron microscopes. Faced with the fact that availability of a commercial instrument was indefinitely remote in time, a very few institutions embarked on their own microscope building programs. Anyone starting to build an electron microscope in the United States before 1939 could find no prior art locally. However, by that time a large amount of information on the theory and design of the electron microscope was readily available in German scientific publications that could be found in the better science libraries aound the country. This source of information had a profound effect on the development of electron microscopy in the United States.

B. Basic Technology 1930-1950 To understand the significance of what the pioneers in electron microscopy achieved one must have some idea of the technical resources, or the lack of them, that existed when they did their work. One of the most illuminating sources concerning the technical status quo of physics laboratories in the 1930s under one cover is John Strong’s “Procedures in Experimental Physics”, first printed in 1938. It was an immensely popular book and went through five printings by 1943 (Strong, 1943). It reveals the primitive nature of vacuum technology at the time construction of the first electron microscopes was being attempted. Glass mercury-diffusion pumps were still the rule. One of the early electron microscope experimenters actually used a gas heated glass mercurydiffusion pump. Oil diffusion pumps made their appearance about 1929,

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eventually leading the way to the wide use of the metal diffusion pump. In the mid-1930s Octoil and Apiezon pump fluids came into use and marked a new era in vacuum technology. Demountable vacuum systems are necessary for an electron microscope, yet the O-ring did not appear until the early 1940's. Prebus and Hillier at Toronto used greased metal flanges to permit lateral motion of components under vacuum. It was not until oil resistant elastomers, such as neoprene, were first available for cut flat gaskets and later for O-rings, that demountable vacuum systems became reliable. Vacuum valves were ordinary plumbing valves where the packing gland was replaced with a sylphon bellows. Such valves were usually homemade, because they were scarce and expensive. Vacuum-tight castings were a myth. Their surfaces had to be coated with glyptal or picein or shellac, or if the leaks were big, with beeswax and rosin. The difficulty of working with the primitive systems was aggravated by the lack of good vacuum measuring equipment and leak locators. Before 1945, a leak locator was a person who sprayed ether on a suspected leak area and watched the vacuum indicator for a telltale drop in pressures that indicated a leak. The state of electronic technology was somewhat more advanced than vacuum technology during the 1930s, but it was not very available to the people interested in developing the electron microscope, partly for reasons of cost. Battery banks were usually available in laboratories to power magnetic lenses. Unfortunately, batteries are constant voltage devices and give rise to current drift as lens coils heat up. High voltage supplies used existing X-ray technology and components, which was a fairly acceptable solution for electrostatic lens instruments, but not for magnetic lens instruments which require high stability for accelerating voltages. Electronically stabilized power supplies were being developed for the radio and television industry, but these were too sophisticated and expensive for the financially limited physics laboratories of that era. The first electron microscopes had to use inefficient willemite fluorescent materials for their viewing screens. Toward the end of the 1930s and through the 1940s, great improvement was made in fluorescent materials for kinescopes for military and video purposes. Again about the end of the 1930s magnetic materials of very high permeability became available for use in shielding sensitive apparatus from stray magnetic fields. These two examples of new materials are part of a great number of specialized glass, ceramic, plastic and metal materials that made old ideas feasible and new ideas possible. It will be pointed out again in this article that around 1940 the technology necessary to support an electron microscope was finally available, and essentially what the early builders of electron microscopes did was to pull that technology together to make a workable system.

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C . Economic and Political Restraints

The early workers in the field of electron microscopes had more restraints on them than the technical state of the art. Economic and political factors imposed serious limitations on the ability and the freedom of scientists and technologists to pursue their work. The great depression which began in 1929 threatened the very existence of academic and industrial institutions, and research programs were expendable budget items. Physics laboratories had to survive by their own resources. After 1935, the economic situation improved considerably, but it was not until the very end of the decade that research received real support. The primary stimulus for such support was political, the increasing threat of war in Europe and the idea that the United States was to be the “arsenal of democracy”. Academic and industrial laboratories were mobilized, funds were made available, and workers recruited so that by the time the United States entered the war in December, 1941, research was flourishing. The price of support was loss of freedom. The purpose of research support was the winning of the war, and all available effort was directed to that end. To insure compliance with this goal, the War Production Board was set up by the government. One of its responsibilities was to ration available materials and resources according to priorities based on the importance of a project to the war effort.This control of materials gave the government almost total control over which research programs were to be pursued or rejected. Little or no federal funds were ever expended on the development of the electron microscope, although large amounts of federal funds were expended to purchase electron microscopes for research projects utilizing them. Eastman financed C. E. Hall’s microscope, Columbian Carbon Co. financed W. A. Ladd‘s microscope, and General Electric and RCA funded their own programs. This does not mean that the government was not interested in the development. In fact, it was very much behind the effort, as indicated in the following letter of March, 1944 by C. H. Kerr, Chief Laboratories and Technical Equipment Section of the War Production Board to M. C. Banca, Sales Manager of RCA Electron Microscope Sales Department. I am very pleased to learn from you that you expect to make delivery of the first electron microscope, Model EMU-1, in April, certainly in early May.. . . In my opinion, its importance to the war effort can not be overemphasized and it is for that reason that I am writing to urge that everything possible be done to produce the instrument.. . . As you know, every order you now have for Model EMU-1 carries a AA-1 rating and no future application for the right to purchase will be authorized with any rating other than AA- 1. Therefore, in your manufacture, the electron

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microscope should take its place as having importance equal to anything you are producing.. .. I hope you will do everything possible to impress upon your organization the urgency of the situation. This letter dealt with the introduction of the new EMU-I microscope, that followed the earlier EMB series, most of which were also rationed. With the intensification of activity under war time pressures, trying to make every moment count and restricting one’s efforts to essentials, the climate was not conducive to writing papers or disseminating information. Some of the work carried on between 1940 and 1946 on the development of the electron microscope was never reported in the technical press as it might have been in a peacetime situation. Immediately after the conclusion of the war all restrictions were lifted. However, conditions did not return to the prewar situation. Big physics had caught the imagination of the physics community with nuclear power and high energy particles. Few people in the United States were interested in working on the further development of the electron microscope with such exciting and popular opportunities open to them. The successful manufacture of the instrument by RCA had made it seem like an engineering field rather than a physical field of activity; and nobody wanted to compete with RCA with its head start and technical resources. During the war, the government support of academic science as a technical resource had been so successful that it was continued in many areas by the defense department after the conclusion of the conflict. The funding of research in academic and not-for-profit institutions by the National Institutes of Health also expanded greatly. By any prewar standards, science had become rich. Where necessary, it could afford electron microscopes to open up the fields of materials science and biology, where researchers were getting excited and revolutionary discoveries were being made. The political support of science after the war made the rapid expansion of electron microscopy possible. 111. THEEARLIEST ELECTRON MICROSCOPES IN THE UNITED STATES

A , Fitzsimmons- Anderson Electron Microscope Washington State University 1934-1938

“In the middle of the 1930s, in one of the most remote farming regions of the United States and in a college known chiefly for work in plant pathology and cereal genetics, two men built and operated an electron microscope.”

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These are the words with which Arthur Cohen and R. G. E. Steever, Jr., summarized a talk on one of the most amazing episodes in the history of electron microscopy, and one that would have drifted into oblivion except for a quirk of fate (Cohen and Steever, 1971). The bold idea to build an electron microscope at Washington State University came from Paul A. Anderson, a then newly appointed professor of physics at that institution. Anderson was born in Chicago in 1897 and attended public schools there. During World War I he served in the Naval Air Force. He received a B. A. degree from the University of Illinois in 1920. From 1920 to 1923 he pursued graduate studies in Physical Chemistry at University College, London, for one year and at Harvard University from which he received a Ph.D. in Physical Chemistry. He was employed by Eastman Kodak Laboratories in Rochester from 1923 to 1925. Hejourneyed to Peking, China, where he was appointed by the Rockefeller Foundation to head the Physics Department of Yenching University. He returned to the United States in 1929 and became a National Research Council Fellow in Physics at Harvard and the Physikalisch-Technische Reichsanstalt in Berlin. From 193 1 until his retirement in 1963 he was professor of Physics at Washington State University and was chairman of the department from 1931 to 1960. Anderson was an experienced scientist well embarked on a distinguished career when he became interested in the electron microscope. In a letter to the author dated September 30,1987, Paul Anderson recounts his recollections of how the electron microscope development at Washington State University got started. In 1930, after spending the first year of a National Research Council Fellowship at Harvard, I went to the Low Temperature Laboratory of the P. T. Reichsanstalt in Berlin to carry out some work-function measurements on tungsten at liquid helium temperatures. The tube design involved the electrostatic focusing of an electron beam on a distant metal target, and so a self-taught course in electron optics. In going through the literature, I ran across the Ruska-Knoll papers and was impressed .... I did not take time to visit their laboratory, an omission I have regretted. When I returned in August of 1931 and took, at Washington State, the only academic post in sight (made so by the sudden death of the department chairman) I knew nothing about the college. I found there a small department with no facilities for research, a budget of $750 per annum for equipment and supplies, and heavy teaching loads. The assets were a supportive dean and staff which included Kenneth Fitzsimmons, who had taken his BS and MS degrees at WSU and held the post of laboratory instructor. By limiting expenditures to the purchase of

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materials and making our own equipment (General Electric donated a used high-voltage transformer, Kenotron parts, and other components) we got two productive research programs going. At this point Kenneth asked me to suggest a research program to work on. I suggested several projects suited to our resources, but he had heard me talk about Ruska’s work, and it was evident that this heart was set on building a microscope. We had acquired a new lathe with the proceeds of a radium needle hunt at a hospital, and Kenneth was an excellent machinist, but the magnitude of the project, in terms of the time required to complete it, was such that it was only his unquenchable enthusiasm that led me to agree to it. I had a long-term program of work-function measurements under way but, no less enthusiastic than Kenneth once the decision had been made, joined him in getting the project started. We did the designing and ordered the materials; then Kenneth took over the construction singlehandedly while I went back to my own research. We had frequent discussions, and I contributed some ideas for improving our original design, but the hard work was his. Progress was slow. Fitzsimmons and Anderson had little free time to devote to the effort, and, because their financial support was negligible, they had to make everything themselves, including the diffusion pump. Only a few items such as a fore pump and electrical meters were purchased. The instrument was completed by the end of 1935. Years later, when the existence of the instrument became known, Anderson suggested that the instrument should be known as the Fitzsimmons-Anderson Electron Microscope, because Fitzsimmons had fabricated considerably more of the instrument than did Anderson. The last recorded work with the instrument was June 4, 1938. Anderson explains their decision to terminate the project in the letter quoted above. By the time we were getting low magnification imaging, other groups had moved well ahead of us, and their lead was increasing rapidly. To us, the construction of the microscope was a means to an end rather than an end in itself and we agreed that there was no point in continuing our work on it. There were no regrets. To Kenneth the satisfaction of having accomplished so much was reward enough, and he now was ready to forge ahead on his own. It is important to note that from the beginning the primary end in building the microscope was to acquire an instrument that could do microscopy of biological materials. Building the microscope was merely a means to that end. Anderson continued his research on work functions and high vacuum, and Fitzsimmons turned to mass spectrometry, while their electron microscope,

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more remarkable and successful than they realized, was relegated to the attic of the Physics building, where it was subjected to the indignity of cannibalization of its parts for use on other projects. The Fitzsimmons-Anderson electron microscope operated at 30 kv, took photographs at 1250 times magnification, and had a resolving power equivalent to the best light microscope. It was a very sophisticated instrument for its time, and one that merits admiration for the boldness of its concepts. It had a vertical column with three iron-jacketed water-cooled lenses, including a condenser. The objective and projector had external Helmholtz coils to provide beam alignment. A variable bias electron gun with a thermionic filament provided illumination. There were viewing ports at a point intermediate between the objective and projector lenses, and at the final screen. The contrast in the image was greatly increased by apertures in the anode and the projector lens. There was also a photochamber that held six 34 x 4inch plates, not to mention a photometer that read the beam current striking the fluorescent screen. There was even an attempt to stabilize the high voltage power supply that used a full-wave rectifier and a capacitor filter. The first recorded photographic exposure was made December 1937, although directions for operating the photographic system are dated February 16, 1937. The first notebook entry is dated January 16, 1936, and describes operating conditions for the projector lens. The ultimate problem that they did not solve was the making of specimens that could survive in the beam. Apparently Fitzsimmons and Anderson never got beyond fine wires and wire mesh for specimens. One suspicions that the apparent difficulty of surmounting the specimen problem had some influence on their decision to end their microscope program. From a historical perspective fifty years later it seems unfortunate that nothing, with the exception of an article on the plate camera that did not reference the microscope, was ever published about the project, nothing in spite of the fact that there had been no publications on the subject in the scientific press of the United States prior to the shut-down of the project in mid-1938. It is unfortunate because Anderson and Fitzsimmons had more to give than encouragement, which in itself is important. They had creative new ideas that did not reappear for several years and experience to share when experience was at a premium. Without significant documentation, the project, by 1963, had been lost in the flow of history. Anderson, who saw the first draft of this paper, responded to the author’s comments in the preceding paragraph, as follows: In answering this apparently justified and oft repeated criticism I would point out that our sights were set on a completed instrument capable of high magnifications, and it did not occur to us to report details of its

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design until they could be described in context. (The paper on the camera was submitted because of its more general usefulness.)The responsibility for that omission, which stemmed from my tendency to publish when a project has been completed, was mine. It is the practice in most physics laboratories to store experimental apparatus that is no longer in use in any available storage space, where it may be cannibalized for use in current research projects. Occasionally storage areas become so filled with experimental memorabilia that a clean-up is necessary. That is what R. G. E. Steever, Jr., was doing in the attic of the Physics building one day in 1963 when he discovered what he recognized to be the column of an old electron microscope. At that time Steever was an undergraduate assistant in the Physics Department and also in the Electron Microscope Laboratory (now the Electron Microscope Center). He excitedly passed the news on to Dr. Arthur L. Cohen, head of the Electron Microscope Laboratory, who reports that he could hardly believe Steever. It was indeed a microscope, and a notebook found with it identified the experimenters who built it and some significant dates between January 16, 1936, and June 4, 1938. The first 33 pages provide performance data, diagrams and photomicrographs and pictures of the microscope (Fig. 1). Five plates with images of wire specimens were found with the microscope. The power supplies, meters, and pumps were missing, presumably removed years ago for other projects. At the time of the discovery Dr. Anderson confirmed the authenticity of what was found. With the permission of the Physics Department, the microscope was moved to the Electron Microscope Laboratory and in 1965 Steever and Dan Marlow (an undergraduate assistant in the Electron Microscope Laboratory) cleaned and restored the microscope, carefully preserving its original appearance, The restored Fitzsimmons-Anderson microscope (Fig. 2) is now on permanent exhibit at the Electron Microscope Center at Washington State University, Pullman, Washington. The author is indebted to Dr. Arthur Cohen for copies of the material referenced herein. Readers interested in the Fitzsimmons-Anderson microscope can find additional material and pictures in a paper by Zensaku Yoshii (1970).

B. The Newberry-Scott Microscope at Washington University of St. Louis, 1935- I940

A significant early attempt in the United States to develop an electron microscope took place at Washington University of St. Louis. Sterling Newberry was a graduate student in physics at the university (1938-1940) and was assigned to and became an important member of the electron microscope

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FIG. 1. The Fitzsimmons-Anderson electron microscope as photographed in 1937 (courtesy of A. Cohen, 1971).

project. He has published an important and detailed memoir (Newberry, 1985a) of that project, which has been the author’s principal source of information. Washington University is a large private university with a long standing record of outstanding accomplishments in research. It had the attitudes and resources in personnel and financial support, though limited, to be able to undertake as uncertain a project as microscope development. The actual impetus to embark on such an undertaking came from the Department of Anatomy of the Medical School, at that time headed by E. V. Cowdry, rather than from the Physics Department. The central figure of the undertaking was Professor Gordon H. Scott, also of the Department of Anatomy. Scott was interested in the role of trace elements in muscle, nerve and other tissues and felt that the electron microscope might help give chemical identification, on a microscopic scale, of the metallic elements whose location was revealed by microincineration and whose average composition

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FIG.2. Restoration of the Fitzsimmons-Anderson microscope column on exhibition at the Electron Microscopy Center of Washington State University, Pullman, Washington (courtesy of A. Cohen, 1971).

was given by optical spectroscopy without relation to structure. He was influenced in this direction by Howard McMillen who had been a research associate in the Physics Department from 1930 to 1933 and was informed on the German electron microscope work. McMillen left the university in 1933 on a two-year stint as a National Research Fellow at Princeton. At the end of that time, Scott called him back to the university as a post doctoral fellow to help develop an electron microscope. Scott and McMillen began work on the project in 1935 and continued together until McMillen left the university in 1937 to accept an associate professorship at Kent State University. It is important not to convey the impression that the Physics Department was indifferent to the project. While the instrument was being built in the medical school under the auspices of the Anatomy Department, members of

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the Physics Department were following progress closely and consulting and taking part in the program. Dr. Arthur L. Hughes, head of the Physics Department included a large portion of electron optics in one of his courses in support of those interested in the electron microscope. This has to have been one of the earliest formal courses on the subject in the United States. 1. The Emission Microscopes By the time of McMillen’s departure, a working instrument had been achieved and reported in the literature (McMillen and Scott, 1937). It was a simple emission microscope. It had one magnetic lens and a large Helmholtz coil mounted on gimbals to provide beam alignment. These elements were external to a brass tube which was mounted horizontally. A fluorescent screen, cut from a commercial cathode ray tube, was attached to one end of the brass tube and served as a viewing screen. A flange was soldered to the other end of the brass tube to provide a mounting for the gun and emission specimen assembly. The gun assembly had a mating flange, and the joint between the two flanges was made vacuum tight by a ring of Apiezon “Q” compound. A glass mercury diffusion pump was used to evacuate the copper tube. There were no vacuum valves. Access to the specimen could only be made by cooling down the diffusion pump and removing the “ Q compound seal. The magnification of this instrument was only 50 x yet it was encouraging enough to keep going. McMillen left the project in 1937. He was followed by Donald Packer who was Scott’s physics assistant. Packer added a second magnetic lens to increase the magnification to 150 x . He continued to use the instrument in an effort to distinguish metal elements with the emission images. The research was not very fruitful. Some results were published during Packer’s tenure. He also constructed a second instrument with two significant ideas. He put the objective lens gap inside the vacuum and modified his cathode holder so that it could take transmission specimens. This microscope was also horizontally mounted, and on a table with wheels which caused mechanical instability. The performance was disappointing, and before he had a chance to solve the problems, Packer left, after one year on the project, for a new job. Packer’s replacement was a newly enrolled graduate student in the Physics Department, Sterling P. Newberry. He was chosen partly because he had an unusually large number of course credits in biology. Newberry took to the project with enthusiasm. The first instrument was by this time dedicated to research, so he involved himself with improving the second instrument that Packer had built. By now Scott was having serious doubts as to whether the emission microscope was going to be the way to go. He had heard reports of high resolution. Newberry (1985a)comments: “It was during this time that our horizons were pushed beyond a device to further the current research area to

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the desirability of producing the design for an American version of the Siemens-type instruments.” With this idea in mind, Scott and Newberry started a third microscope, cannibalizing the second instrument for useable parts. Newberry did most of the building, and Scott was responsible for finding financial support. Starting in the fall, a new transmission microscope was assembled by spring 1939. 2. The Transmission Microscope The new microscope (Fig. 3) was a far cry from the two earlier emission microscopes. It had a vertical column. It had a concrete slab over a thick layer of cork for a base. Its high voltage supply was housed in a tub at the top of the column, reminding one of the early Siemens designs, although Newberry had not seen a picture of a Siemens microscope at that time. The whole assembly stood seven feet in height. It had a viewing chamber by which it was possible to view the back side of the fluorescent screen, but photography was still external. The vacuum system was quite good for 1939. It utilized a flexible metal bellows in the gun traverse. Metal parts were silver soldered. It did have some reminders of earlier times, a 160 L/sec, glass diffusion pump, designed by Professor Hughes, and a Tesla coil to indicate vacuum conditions. The great limitation in this microscope, and in almost all other very early microscopes, was the electrical system. The high voltage supply was limited to 15 k v and was noisy. Lens currents were supplied by battery banks and drifted as the coils warmed up and changed resistance. The third instrument showed a magnification of about 2000 times and resolving power better than a light microscope. One can not help wondering what could have been achieved with a better power supply. The two researchers were aware of the limitation they could not overcome for financial reasons. The microscope program came to an abrupt end in the fall of 1939 when the goal was almost in sight. The Medical school was offered a grant by the Rockefeller Foundation to build a medical research cyclotron facility. This offer was on the basis of the university raising matching funds. To accomplish this the university pooled all available research funds including those supporting the microscope project. Without financial support there was no way that program could proceed. Newberry, unlike the others on the project, had seen the microscope development as an end in itself, rather than as a means to an end. For the next year, until he received his degree in late spring 1940, he experimented with simple electron microscopes. Some of the work he financed out of his own limited funds. He experimented with magnetic and electrostatic lenses, which stood him in good stead a few years later when he worked on the General Electric electrostatic electron microscope. It was also out of this time that he acquired the desire to design a low cost electron microscope with better than an order of magnitude more resolution than the

FIG. 3. Newberry-Scott transmission electron microscope at Washington University, St Louis, Missouri, 1939 (courtesy S. P. Newberry, 1985a). 148

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light microscope. This period of experimentation also kindled his interest in the x-ray microscope. Nothing was published in the scientific literature about the third microscope. Scott and Newberry were not daunted by the news in April, 1939 that Prebus and Hillier at the University of Toronto had achieved outstanding performance with their microscope. Rather, they were encouraged. However when their project was suddenly aborted they were in no mood to report results in light of the Toronto publications. The important legacy of the Washington University electron microscope program was two scientists who went on to further contributions to the field. Sterling Newberry developed a production prototype of a low cost electrostatic microscope, an x-ray microscope at General Electric, and a fly’s eye electron beam memory at the MicroBit Corporation. He was active at scientific meetings and traveled widely visiting numerous laboratories to everyone’s profit. Gordon Scott, on a leave of absence, went to the Johnson Foundation at the University of Pennsylvania, where he was involved in electron microscopy. In 1942, he became head of the Department of Anatomy at the University of Southern California School of Medicine, where he took an active interest in the establishment of electron microscopy in the Medical School. After 1948, when he became Dean of the Medical School of Wayne State University, he did not have time to participate actively in electron microscopy. It is interesting to note that Dr. F. 0. Schmidt of the Zoology Department at Washington University was supportive of the electron microscope effort; and, after he went to MIT, quickly entered the field of electron microscopy when instruments became available.

Iv. THE LEGACY PROM TORONTO A . Cecil Hall and the Kodak Microscope

Five Canadian graduate students were primarily responsible for introducing the electron microscope into the United States. They were students of Professor E. F. Burton, Chairman of the Physics Department of the University of Toronto, who was interested in developing and using the electron microscope. There the students C. E. Hall, J. Hillier, A. F. Prebus, W. A. Ladd and J. H. L. Watson achieved Burton’s dream and caught a vision of what the electron microscope could do; they were fired with enthusiasm for electron microscopy. With the exception of Watson, all found sponsors who lured them to the United States to construct microscopes. One must keep in mind that until mid-1941 when the R.C.A. microscope actually became available, the only sure way to get an instrument was to have one made. Thus Hall went to Kodak in 1937, Hillier to RCA in 1940, Prebus to Ohio

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State University in 1940 and Ladd to Columbian Carbon Company in 1941. Watson, who had been deeply involved in the design, construction, testing and use of the Toronto microscopes, never became seriously involved in instrumentation after he left Toronto in 1943. He also pursued his professional career in the United States, but exclusively in research with the microscope. The first Toronto graduate to move to the United states was Hall. He joined the research staff of the Eastman Kodak Company on February 1, 1937. Amazingly, although he had been hired to build and to use an electron microscope, he was given other assignments. Actually Dr. C. E. K. Mees, famed director of the Kodak Research Laboratory, wanted to start on the microscope immediately, but his senior staff members were reluctant to expend effort on so questionable a project. The old bugaboo fear that the electron beam would destroy the specimen, and especially photo-chemicals, had caused his staff members to became skeptical about the microscope’s ultimate usefulness. It was not until the Toronto microscope constructed by Prebus and Hillier started to demonstrate its usefulness that Hall was permitted to start building an instrument of his own design. Even then, Mees had to appeal to the Board of Directors for a special grant separate from the Research Laboratory appropriation. Two years delay at that particular phase of microscope history was a long time, and one is tempted to conjecture as to what might have happened if Hall had not been delayed by executive myopia. Hall stated (Hall, 1985): “The time from writing requisitions and making construction drawings to production of micrographs of the specified quality was about four months”. The microscope that Hall built reflected his great gift for reducing experimental work to its simplest form. He possessed an innate sense for economy of design. He did not like frills and gimmicks, and he never compromised on fundamentals. The cost, excluding Hall’s time and the high voltage transformer, was $4,000, which looks small today but must have seemed large to Hall who had learned to get along on next to no financial support for his research at Toronto. Hall estimates that the microscope had a resolving power in the range of 10 to 20 Angstroms. It had a variable high voltage up to well over 100 kv. Hall had originally expected to use the microscope part of the time to improve its operation and performance. However it worked so well from the start that it was used exclusively for research, and the improvement was never undertaken as long as Hall remained at Kodak. The overall design was related to that of E. Ruska and also that of Prebus and Hillier and showed the strong influence of Hall’s creativity and originality. He was particularly concerned with maintaining lens current and high voltage stability, and his success in this area was an important factor in achieving excellent resolving power. Electron tube

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feedback regulators were used to stabilize lens currents. The high voltage supply was more of a brute force system. A good line voltage stabilizer with good wave form was used ahead of the high voltage transformer. A rectifierdoubler developed the dc high voltage, and an RC filter with high capacitance reduced the ripple, which was further reduced by operating at low beam current. The instrument had some useful sophistication, such as a plate camera that held 20 2 x 2 in. photographic plates and required only 5 minute pumping time. There was a specimen air lock with a 1 to 2 minute cycle and a separately adjustable objective aperture. It was an instrument with which one could do research. Interested as he was in the instrument itself, Hall was even more interested in using it for research. For him, the microscope was not an end in itself but a means to do microscopical research. Thus it is understandable why he welcomed the invitation to join the staff of Dr. Francis 0.Schmitt who was the new Chairman of the Biology Department at MIT, where one of the specific areas of activity was to be a study of the potentialities of the electron microscope in research on biological materials. In July, 1941, Hall moved to MIT where one of the first commercial RCA EMB microscopes was installed in September of the same year. Hall relates that on his arrival at MIT he was surprised to discover that G . G. Harvey of the Physics Department had built a working electron microscope but did not know how to use it. Early in his tenure at MIT, Hall completed his Ph.D. His dissertation became the basis for his famous book “Introduction to Electron Microscopy”, first printed in 1953 with a second edition in 1966. A reprint edition was printed in 1983. The book became immensely popular and was the one book that every microscopist owned. The Biology Department of MIT became a remarkably productive institution for research and for education, and in this activity Cecil Hall was one of the star performers. Hall will probably be best remembered for his research, and secondarily for his work as an educator and text book writer. However recognition of his outstanding work in microscope development at Kodak will probably not fare so well, particularly because the work was never published. While Hall never did further hands-on work on instrumentation, he maintained his contacts in the field through consultation with manufacturers. RCA retained him as a consultant from 1954 to 1968 and profited greatly from his unusual ability as a problem solver and from his understanding of the needs and feelings of electron microscopists. He was always interested in the physics relevant to the electron microscope. He showed early that the scattering actually encountered in a specimen was about a third of that predicted for Rutherford scattering. He was instrumental in turning over a superceded EMB microscope to

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Prof, R. Ogilvie of the Metallurgy Department for the development of electron microprobe analysis. B. Albert Prebus and the Ohio State University Microscope

Early in their electron microscope project at Toronto, Albert Prebus and James Hillier decided on a bold course of action. They elected to build their transmission microscope in a single jump, avoiding a slower and much more cautious step-by-step building and testing of its several components. The first operational test of the individual parts was as part of a completely assembled instrument. Their decision and their very able implementation of it saved microscopy at least a very important year of development. Thus when World War I1 started for the United States on December 7,1941, commercial electron microscopes were in the field and were a real scientific presence. Had the program been a year later the first RCA microscope would only have been in the process of installation, and the microscope program at RCA would have faced review as to whether it should be continued. Today, a posteriori, any decision to drop the microscope looks preposterous, but at that time the pressures on manufacturers of military equipment were enormous, and the electron microscope did not look important to people who did not understand it. As one might expect, Prebus and Hillier considered the possibility of undertaking the manufacture of electron microscopes themselves. They rejected the idea in a decision that obviously had major consequences for electron microscopy. In 1940, Prebus and Hillier left the University of Toronto. Hillier took a position at the RCA Research Laboratories in Camden, N. J. (See section VI.A), and Prebus joined the faculty of Ohio State University. Prebus immediately undertook the building of an electron microscope which was in operation by the spring of 1941. It is reported to have performed well. Prebus was very much interested in the analytical use of the electron beam for which he saw great potential and a bright future, and he tried to promote interest in that area. For a number of years his microscope was used by his physics students for research and educational purposes. An article on the microscope is given in a Ohio State University publication (Prebus, 1942). With the passing of time Prebus lost his active interest in the instrument itself and turned his efforts to other areas of physical science. The importance of his contribution to the practical realization of the electron microscope will not be forgotten. At a special convocation of the University of Toronto held in conjunction with the Ninth International Congress on Electron Microscopy in August, 1978 and on the 40th anniversary of the Prebus-Hillier microscope,

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the University awarded Hall, Hillier and Prebus honorary doctoral degrees for their contributions to the development of electron microscopy. C. William Ladd and the Columbian Carbon Company Microscope

The Columbian Carbon Company was one of the first commercial establishments to show interest in the Toronto electron microscope. Finely divided carbon such as carbon black is used as a filler for rubber and imparts to the rubber a wide range of properties that depend on the characteristics of the carbon black. The ability to characterize carbon blacks, which was essential for understanding their function, was limited by the resolving power of the light microscope. Thus, the ability of the electron microscope to work beyond the limits of the light microscope was of immediate interest to Columbian Carbon. During 1939, the Toronto group did studies of carbon specimens for them that convinced them that they needed an instrument in their own laboratory in Brooklyn, N. Y., and so they contracted with the university to have the Physics Department construct a microscope for them. In the fall of 1939, William A. Ladd, a graduate student in the physics department, was assigned to the microscope group. He was soon working on the new instrument by virtue of Hillier's departure from the group in February, 1940, and Prebus' leaving later in the year. Ladd was equal to the task and the new instrument represented a substantial improvement over its prototype. It was delivered to the Columbian Carbon Company and put in operation in July, 1941. At the same time the company hired Ladd to operate the instrument. A real measure of the quality of that instrument is the fact that it served as Ladd's research microscope until 1946, and all of Ladd's important early work on carbons was done on that microscope. In 1946, he put in operation a second microscope of his own design, embodying a number of changes which improved performance. He used the new instrument for his research work until he left Columbian Carbon. The first instrument was kept in the laboratory mostly as a historical momento for a number of years. It was later sent to EMSA Archives where it is currently on display with other historical instruments at the Medical Museum at the Armed Forces Institute for Pathology in Washington, D.C. In 1954, Ladd left Columbian Carbon to found his own consulting firm. The new enterprise was named Ladd Research Industries, and both Ladd and his wife Margaret worked on the business. It was housed in the basement of their residence on Long Island. They had a Philips 100 microscope and hoped to do ultrastructure studies for institutions which were without electron

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microscopes. When adequate business did not materialize, they began to market materials and accessories for microscope laboratories. This gave Ladd the opportunity to use his skill in instrument design to his great satisfaction and to great benefit of others. What was the legacy from Toronto? In 1939, there was one microscope in the United States, 23 years later, at the time of the first electron microscope conference in Chicago, there were twenty-five. By 1944, there were fortyeight. The design philosophy and practice developed for the electron microscope at Toronto by Hillier, Prebus and Hall was brought to the United States by Hillier to a commercial institution which was ready and eager to build microscopes, and the result was an explosive growth of instruments, and the equally explosive growth of knowledge of how to use them. The others who came to the U.S. from Toronto were witnesses to the possibilities and practicality of the electron microscope. They were confident and built microscopes to prove it. When the commercial designs overwhelmed their own, they turned their attention to learning how to make the instrument realize its potentialities for research with the same skill and attention that they had used when designing their own instruments. Their legacy was the first step toward practical electron microscopy. V. ELECTRON MICROSCOPE DEVELOPMENT GENERAL ELECTRIC COMPANY (GE)

AT THE

A. Early Decisions

Around 1930, and perhaps even before, there was interest in the electron microscope in GE; however, on the surface it seemed too far from practice to pursue its development at that time. It was not until 1938 that the stimulus to undertake a program of development was sufficient to get things started. The stimulus was the announcement that Siemens in Germany was preparing to market an electron microscope. This is a very similar time line to that followed by RCA. However the approaches taken by the two companies were very different, reflecting the difference in corporate attitudes and organization. The G.E. laboratories were very large for that time (1938) and highly organized. Decision making was a well-defined procedure. GE's first step was a market survey which was completed in August of 1939. The prognosis for sales potential was encouraging enough that H. B. Marvin in the Electronics Division was selected to initiate a development program. One finds this an interesting contrast with what happened at RCA, where things were on a much smaller scale and comparatively casually organized. When Zworykin decided he wanted to start an

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electron microscope program he had only two people to convince and he had no committees to pursuade and few well-defined rules to follow. The flexibility provided Zworykin was an asset for moving rapidly and decisively. The initial plan was for GE to design and manufacture a magnetic electron microscope like the Siemens microscope which had reached the field in the spring of 1939. Before starting however, the decision was made to consult with AEG (Allgemeine Elektricitats-Gesellschaft) which had constructed an electrostatic microscope for sale in Germany. G E and AEG had some commercial and technical information exchange agreements at that time. As a result of this contact, G E was sold on the electrostatic microscope as the way to go. G E apparently had three options: to import the whole AEG microscope for sale in the U.S., to import the components and assemble the instrument in the US., or to use the AEG drawings and build it from scratch in the U.S. The political situation in Europe had become very precarious by 1939, and GE rejected the options which involved importing anything from Germany as being too risky. Actually it finally did not avail itself of the third option either, which was to copy the AEG design. Instead, G E opted for a simplification of the AEG design which led to a very different design of the electron microscope. Early in 1940, C. H. Bachman and Simon Ram0 were selected to work on the electron microscope project. Together they wrote three papers summarizing their work from 1940 to 1943 (Bachman er al., 1943a,b,c).In these papers, they express interesting philosophies about their work that appear to be contradictory. In the first paper (Bachman et al., 1943a), they show a strong research orientation as they state: The authors.. . do not take the view that the investigations reported here constitute a proof that one or another microscope is superior. They feel rather that the field is young and important advances may be expected in each type of instrument, so that investigations of all microscope attacks should be continued and an open minded attitude should be used in making comparisons between these various attacks. In their final paper (Bachman et al., 1943c) published a few months later, they are product oriented as they write about their objectives which led to the choice of design parameters: It was to construct an instrument which would give a particular resolving power range with the maximum of simplicity in manufacture, operation, and maintenance. The range chosen was about ten times superior to the light microscope or a resolving power of 200 Angstroms. This was regarded as a field of microscopy of tremendous importance and one whose secrets

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are not more likely to be discovered quickly by a wider range instrument (which entails many complexities in achieving its performance) than by a radically simplified instrument whose operation and construction are everywhere balanced for this particular range. Unlike previously described electron microscopes, no element of simplification was ruled out because it might affect the resolving power. Simplification and resolving power were balanced in an effort to have no part of the instrument over-designed. They were product oriented to the point where they lost sight of the ultimate purpose of the electron microscope and made decisions based on

FIG.4. Bachman-Ramo experimental electrostatic transmission electron microscope constructed at the General Electric Laboratories in 1941 (courtesy s. P. Newberry, 1985b).

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assumptions about the practice of electron microscopy that were questionable then, and have since been proven wrong in the laboratory and the market place. One suspects that their premature concept of a microscope of deliberately limited resolving power was one reason a promising effort by capable people ultimately failed to yield a useful instrument. The idea of deliberately compromising resolving power for other ends seems to have been anathema to the early microscopists who could not themselves define the limits of resolving power required in their own developing fields of microscopy. That is a major reason why the so-called simple economy microscopes, regardless of who made them, were not well-received in the early days of electron microscopy. Early in their development program, Bachman and Ram0 constructed an experimental electron microscope (Fig. 4) with which they studied the lenses they had designed and tested out the design of a second instrument which was to be a prototype for a future production model. By 1942, Bachman, Ramo, Marvin, et al., had the second electron microscope ready for demonstration to the public. The stage for the debut was the Second National Chemical exhibition sponsored by the Chicago Section of the American Chemical Society held in Chicago in November, 1942.It was this meeting that sponsored the now famous first symposium on electron microscopy out of which the Electron Microscope Society of America developed. RCA had been invited to show its EMB microscope, and it had a non-operating one on display. G E took a suite in the Palmer House which was next to the headquarters hotel and set up an operating microscope which was demonstrated to selected individuals until Ramo, speaking at the symposium, offered an open invitation to everyone to see the instrument. This was the first demonstration of an operating electron microscope at a scientific meeting (Fig. 5). The so-called “war model” which was shown at Chicago did reflect some of its AEG heritage. It also showed creative ideas by the G E staff. One of these ideas which greatly simplified the instrument was to connect the high voltage electrodes of the lenses internally. The column of optical elements was only two inches in diameter and 20 inches long. The vacuum system was pumped by a small 100 liter/sec, air-cooled glass oil diffusion pump. The optical elements were permanently aligned by a housing that also acted as vacuum container and magnetic shield. The magnetic shield consisted of 30 alternating layers of mu-metal and copper to provide both AC and DC shielding. The image was formed on a fluorescent screen which was viewed by transmission. The electron optical magnification at the screen was 1000 times, and light optical eyepieces were provided to raise the total magnification to 7000 times. Photography was done by attaching a camera in place of the eyepiece. The accelerating potential was limited to 40kv. The second G E microscope was indeed a simple instrument.

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Fic;. 5. GE “war model’’ electron microscope first shown at the electron microscope conference in Chicago in November 1942. Note the horizontal column and the end-on viewing. Left to right, Bachman and Ram0 (courtesy S. P. Newberry).

B. Manufacturing and Marketing a Microscope

It was never intended that the electron microscope shown at Chicago would be manufactured. It was a prototype that served its purpose to test out a design. In 1944, product design on a commercial microscope was started in earnest. The styling concept was changed in a major way. A cabinet in the shape of a secretary’s desk supported a horizontal microscope column with its axis left and right. Thus the viewing screen was not viewed end-on but a prism was used with the eyepiece to present it to the operator at a comfortable viewing position. A subtle change from the experimental microscope to the prototype with serious consequences was the reduction of the diameter of the electron optical lens barrel from three inches to two. The three inch barrel had incorporated an internal connection of the high voltage electrodes of the lenses to the electron source. Such a scheme does cause some distortion in the lens fields, which becomes more serious as the radial dimensions are scaled down. Perhaps more serious is the reduction of the clearances from high voltage surfaces to ground

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and the reduction of insulator dimensions which limit the operating voltage of the microscope. The reduction in barrel diameter also put serious demands on some of the mechanical parts by reducing their dimensions below good design practice causing lenses to tilt with respect to the optical axis. Evidence (Newberry, 1985) indicates that changes incorporated late in the design phase to correct difficulties brought on by use of the two inch column were not adequately tested with a prototype before going into production. The fact that the best micrographs to come from the early GE microscopes were taken on the experimental microscope corroborates the opinion that the 2 inch column was too small for the functions it had to fulfill. The first order by the marketing department was for ten microscopes (Fig. 6 ) . These were all disposed of with enough facility that a second order

FIG.6. Artist’s rendition ofthe GE production model electron microscope. Note the side viewing and the horizontal column. A number of these instruments were sold (courtesy of S. P. Newberry, 1985b).

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FIG. 7. Portable electron microscope designed by Igor Benson using the 2 inch diameter column. Covers were removed to show placement of parts. This instrument was never manuractured (courtesy S. P. Newberry, 1985b).

of twenty was manufactured. The second order did not sell well and most of the instruments were declared salvage and sold, at $20 each, to G E employees who wanted the cabinets. As with most radically new products, there were many production problems. Outside vendors did not understand quality requirements, such as the finish on lens electrodes. Manufacturing costs turned out to be higher than estimated. The program was a financial disaster. An interesting side light on the G E microscope story is the portable microscope designed by Igor Benson, who did much of the mechanical engineering for the microscope program. At the first annual Electron Microscope Society of America meeting in New York City, January, 1944, Benson gave a talk on his “portable” (Fig. 7) which housed the microscope column in one medium sized suitcase and a fore pump in a matching suitcase. GE never manufactured the portable, but it did cause some concern in RCA, as possible competition. RCA did build a model of a portable microscope using the column from the EMC table model which was marketed in 1944. It required three large suitcases and was portable in name only. Except for the portable, there was very little sense of competition between G E and RCA because their products were directed at different markets. In mid 1946, discouraged with the program, Ram0 departed for the west coast for reasons of his wife’s health, and Bachman took a professorship at Syracuse University. At the end of that year, the unsold microscopes were

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scrapped and the microscope program was in limbo. Such was the situation at GE in May 1947 when Sterling Newberry joined the General Engineering and Consulting Laboratory.

C . The Second Try The fact that a large expenditure of funds had been made on the BachmanRam0 microscope program without yielding a product hung over any further effort to develop an instrument. While, remarkably, some interest remained in trying to develop a microscope, available funding was very scarce. Newberry was eager to undertake the rescue of the effort. He had maintained an intense interest in the electron microscope since the days at Washington University, St. Louis when he and Gordon Scott had built an early microscope; he saw the situation at G E as a rare opportunity to pursue that interest. He was given other assignments during regular hours and worked on the microscope after hours. He did have enough funds to maintain shop help during regular hours. When Newberry started to investigate the earlier program, he was able to find the model shown at Chicago in 1942, the research instrument, and the portable prototype. The production model was missing. In analyzing the production model from drawings and notebooks he came to the conclusion that the majority of the problems stemmed from the 2 inch barrel and the strain it put on the design of the rest of the system. He discarded the Bachman-Ram0 approach and proposed building a modestly shielded, mechanically adjustible design of larger diameter. He was given a developmental shop order and completed the instrument in eight months. It worked as anticipated and the sale price was estimated at $5,000. Unfortunately the appearance was too crude and experimental for the sales department. An industrial designer was called in to provide an attractive and efficient appearance and the target sale price was raised to $10,000 to offset a higher manufacturing cost. The new styling of the microscope caused Newberry to redesign a number of components including the vacuum housing which also held and aligned the lenses. The new microscope was finished a few hours ahead of a deadline for approval and received the go-head to fund the construction of three preproduction prototypes (Fig. 8). U p to this point, Newberry had been working almost alone. For the task of constructing the three instruments, Selby S. Summers joined him to help in assembly and test, and in the very important task of finding qualified vendors and cost estimates. The power supplies were designed by M. J. Columbe, the designer of the power supply for the X-ray microscope. Three prototypes were completed and performed satisfactorily. The outside vendor who built the G E electron diffraction camera quoted a manufacturing price of $3,500 a microscope in lots of 20. The design mission had been

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FIG.8. Sterling Newberry changing film in the GE microscope of his own design (courtesy of S. P. Newberry).

I

accomplished, yet no more were ordered and the microscope project was terminated. There are a number of reasons why, at the moment of success, the program was ended. Internal reorganizations in GE about 1949 produced a climate hostile to special products such as mass spectrometers and electron microscopes. There was a question of timeliness of introduction that troubled the Operating Department at GE. Had the microscope been ready earlier, that is, when the first GE microscope was introduced, it would have been better received than in 1949-1951. Finally, and perhaps the last straw, an agreement with Metropolitan Vickers to have GE build and market the Metvick EM6 to round out the line with the GE electrostatic microscope fell through. While Newberry’s electron microscope was never put into production, it did serve to assure his good reputation in the field of electron optical instrumentation. Subsequently at GE, he developed the first commercial model of the X-ray microscope and was involved in a variety of electron optical projects, including the esoteric fly’s eye lens. (See Section 111. B for Newberry’s role at Washington University).

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The author is indebted to Sterling P. Newberry for the photographs used here and for much of the information included in this article. The Newberry references in the bibliography provide substance and flavor to the history of the GE microscope program.

VI. DEVELOPMENT OF THE ELECTRON MICROSCOPE AT RCA A . From Idea to Instrument (1932-1942) 1. Zworykin Initiates a Program

The first recorded mention by an RCA employee of the possibilities of making an electron microscope is an article in the Journal of the Franklin Institute by Dr. V. K. Zworykin (1933). It is probable that he was aware of the work of Knoll and Ruska. It is true that he was an experienced experimenter with electron optical devices including the iconoscope, which earned him the title of the “Father of Television.” It is also true that he was not only a scientist but also an engineer. It was characteristic of him that he could visualize the practical aspects of his science well ahead of everyone else. In the 1930’s, the RCA research laboratories were heavily engaged in television development. Meanwhile, without losing any interest in television, Zworykin was getting more and more interested in the electron microscope. Word from Germany was encouraging and exciting. He had every microscope paper from Germany translated as soon as it was received. By 1937, he could see the field getting away from him unless he acted to start an electron microscope development program of his own. While Zworykin was the Director of Electronics Research, he did have to report to Ralph Beale who was Director of the Laboratories. When Zworykin proposed to Beale that RCA initiate an electron microscope development program, the proposal was strongly rejected because RCA was already heavily committed to the television program. Never one to take “no” for an answer, Zworykin went to David Sarnoff, the President of RCA, who was usually his great supporter and received the same negative answer. How Zworykin got the decision reversed is a matter of conjecture. T. A. Smith, who later was the first sales manager for electron microscopes (1940-1944) and later became an executive vice-president of the firm, proposed the following story. (Smith, 1984) Zworykin telephoned a friend at Amtorg, the Russian Trading Agency in the United States. Zworykin was on good terms with the agency, being Russian himself and having done them some speaking favors in the past. Zworykin asked his friend to inquire from RCA the cost of some electron

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microscopes. His friend protested that they didn’t want any such instruments. Zworykin responded that he did not need to worry because RCA didn’t have any. The friend obliged, and in due time, the request for microscopes reached Sarnoff. He in turn called Beale who responded that they did not have any electron microscopes, to which Sarnoff replied, “Let’s get some; if the Russians want them they must be important”. Smith makes the comment about the story that Zworykin always denied it, but that, if it wasn’t true, he must have used some equally devious scheme to cause Beale and Sarnoff to change their decision on whether to start an electron microscope program. With permission to go ahead with the development of an electron microscope, Zworykin set out to man the program. Because he was looking forward to a marketable instrument as soon as possible, he went outside his own talented group for someone with actual experience in building an electron microscope. In 1938, there were not many people with such a qualification and certainly none in the United States. He did look at the German scientists but found little interest among them in moving to the United States, especially when the action was in Germany. Coincidentally, the Belgian scientist Ladislaus Marton, who had built three generations of experimental microscopes in Belgium was looking for some sort of financial support with which to continue his work in electron microscopy. His achievements in specimen preparation, especialy with biological materials, had earned him a reputation in the field. He had actually left Belgium for Great Britain taking his third instrument with him, when he received an invitation from Zworykin to come to RCA in Camden. 2. Marton and the E M A Marton joined the research staff of RCA in the fall of 1938 bringing with him his third instrument, which he set up in his laboratory. What appeared to be a fortunate arrangement at the outset gradually turned into a difficult and disappointing situation. Zworykin was interested in a commercial product. Marton was interested in continuing his research. Both men were strongminded, impatient people, used to having their own way. Marton did set to work on what was intended to be a prototype for future production. Essentially he copied his third microscope’s column and enclosed it in a metal bell jar. There was a measure of logic to this move. The large diameter enclosure for the column was intended to provide stability against environmental disturbances, and the magnetic shielding of the column was to be improved. The lenses were sealed assemblies so the windings would not have to be pumped out in the bell jar. Although there were air-locks for specimen and photographic materials, other access to the column was very difficult. As a commercial product it would have been a disaster. It was a slow instrument

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with a long pumping time because there was so much useless volume to pump out and surface to out-gas, and diffusion pumps were still slow. Because of the vacuum system design the ultimate pressure was high, causing a very high rate of contamination and concomitant rapid loss of resolving power. After a cleaning, the performance of the microscope was reasonably good, but after a very short period of operation, it became so contaminated that cleaning was needed again. Disassembly and reassembly were very cumbersome procedures because of the bell jar construction. The instrument was known as the EMA (Fig. 9). It was put in operation in the spring of 1939 and a number of papers were written describing its general design and indicating its resolving power (Marton et a!., 1940).It was obvious to Zworykin that the EMA was not marketable. It was too slow, difficult to use, contaminated excessively, too large and too costly to manufacture. More than anything else, the decision to use the bell-jar concept doomed the EMA

FIG.9. EMA constructed at RCA by L. Marton and put in operation in 1939. The entire column including lense coils was inside the tall cylindrical “belljar”(courtesyAm. Soc. Microbio.; Marton, 1941).

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design. It would not be correct to say that the building of the EMA had been a fruitless effort. Valuable experience was gained by a number of people who were to stay with the project. It generated interest in the electron microscope, and RCA had said enough about the instrument that it had effectively committed itself to go on with the program. However, its limited success merely whetted Zworykin’s desire for an electron microscope. It also ended any confidence that Zworykin might have had in Marton’s ability or willingness to design what Zworykin wanted.

3. Vance and the Power Supply Revolution One facet of the EMA electron microscope development was a brilliant success-the power supplies. When the project began in earnest in the fall of 1938, Zworykin recognized that all earlier microscopes had been limited by their power supplies, particularly the high voltage. He was aware that the state of the art in television was electronically stabilized voltage and current. He assigned to Arthur Vance, a brilliant electronics engineer, the task of providing stable lens currents and stable high voltage for the microscope to come. Vance’s success in this task should have earned him much higher recognition and esteem than he has been accorded. It is apparent that Zworykin had opted for the magnetic lens when he set Vance to work on the development of electronically stabilized power supplies. To attain 10 resolving power with an electrostatic electron microscope the voltage stability needs to be only about O S % , which is easily attainable with brute-force techniques. To attain the same resolving power with a magnetic lens microscope requires an accelerating voltage stability of 0.011% .and a stability of 0.0055% in objective lens current. Such stringent requirements were well beyond the capabilities of the then current state-of-the-art microscope supplies which utilized a rectified X-ray voltage supply and an RC filter. To provide suitable power to a magnetic microscope was a formidable engineering problem which had to be solved as early as possible. Vance’s first power supply used an X-ray high voltage transformer and operated at 60 Hertz frequency. He used a resistor-capacitor bridge to sample the high voltage and compare it against a dry battery reference. Variations in high voltage showed up as an error signal, which was amplified to change the effectiveresistance of vacuum tubes across the primary of a transformer whose secondary was in series with the X-ray high voltage transformer. Thus an error signal caused a change in the impedance of the series transformer, in turn changing the voltage from the X-ray transformer in a direction which would offset the original variation of the high voltage. This first stabilized high voltage supply was very successful. It was used with the EMA, and it was a major factor in the relatively good limits of performance achieved with that

a

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instrument. Because of the size of the power supply and the strong 60 Hertz magnetic field it produced, the power supply was housed in a separate room and was connected to the instrument with 50 feet of high voltage cable. Although the supply performed well enough, it fell far short of the size and cost goals which Zworykin had in mind. Vance, realizing that he could not achieve the size and cost objectives if he stayed with electrical power engineering practice, turned to electronics for his technology. From radio transmitter design he got his high-frequency highvoltage power. He replaced step up transformers with resonant LC circuits. He used radio and transmitter tubes. He used inverse feedback to regulate the high voltage. The high frequency, 20 kHz, power provided performance advantages such as faster response to line fluctuations, less power to dissipate in arc-overs. From the standpoint of instrument design, the electronic approach was revolutionary. It was possible for Vance to contain his whole generator-rectifier supply in an oil tank only 30 x 50 x 50 centimeters in size. This was small enough to house in a steel cabinet located behind the microscope column. Since there were no line frequency fields, and since the high frequency fields were attenuated before they could reach the column, the proximity of the power supply caused no interference problems. Vance completed the compact high voltage supply (Fig. 10) in the early part of 1940. Its high voltage was selectable from 20 to 60 kv. It met the performance requirements and the size and cost goals for a commercial product. It was ready for Hillier when he started on his ”crash” program in February of 1940,and it is one of the reasons why the EMB prototype could be achieved in such a remarkably short time. Prior to Vance’s time, most current supplies, gun filament and lens, employed lead-acid automobile batteries. These are very stable, relatively cheap and available sources. When used for filament heating, they were relatively large and had to be floating at the high voltage level, causing difficult structural problems in the high voltage supply. Vance turned to high frequency radio techniques for coupling the filament circuit to oscillators at ground. Battery banks for energizing lenses were subject to drift as the coils warmed up from ohmic heating. Prebus and Hillier had dealt with this problem at Toronto by water cooling the lenses. Vance used inverse feed-back circuits to stabilize lens currents with respect to battery reference sources to keep currents steady in spite of changes in either source or load. As a result of having regulated supplies for the EMB, Hillier was able to dispense with a lens cooling system which simplified the design of lens spools. The EMB electron microscope was a great achievement. Along with its success it also brought with it a large share of instrumental problems, a portion of which were caused by the high voltage supply. Tubes and capacitors were particularly susceptible to failure under the high stresses at which they worked. Repair required access to the oil tank which was 6 feet off the floor.

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FIG. 10. 60 kV oil-insulated, radio-frequency,high-voltage supply developed by A. W. Vance for the EMB microscope. Coils L5 through L9 and C9 were the High-Q resonant circuit that generated the high voltage. Picture taken from an EMB instruction book.

Working in or removing oil was a messy procedure that everyone dreaded. By 1942, it was obvious that the EMB had taught so much about microscope operation that it was time to start the design ef another microscope that embodied the lessons that had been learned from the EMB. Everyone who had suffered with the high oil tank said that it had to be eliminated. Again Vance developed an answer. In a slightly larger volume he used a tripler to attain 50 kV in an air-insulated high voltage supply. While this was 10 kV, less than the EMB could provide, it seemd a fair price for getting rid of the oil tank, particularly in light of the fact that most microscopists operated their EMBs at less than 60 kV. This air insulated tripler became the high voltage supply for the EMU series of microscopes introduced in 1944. More complete technical information is to be found (See, Vance, 1941; Zworykin et a/., 1945). 4. Hillier Builds the E M B

Almost from the beginning of his interest in the electron microscope, Zworykin had been convinced that it was feasible to build such an instrument.

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When he invited Marton to Camden it was not for pursuing a research program but rather for designing a reasonably manufacturable and useable instrument. Zworykin had no illusions about the need for further research but he knew from his own experience, from reports from Germany and from the work at Toronto that existing technology could support the design of a useful electron microscope. When it became obvious that Marton was not going to accomplish this, Zworykin looked for another person who shared his oneness of purpose. James Hillier recalls

I had written Zworykin indicating our (Hillier and Prebus) availability. We had some glimmerings of troubles between Marton and Zworykin. After several weeks we received an invitation from Zworykin to come to Camden, followed immediately with instructions to go to Montreal for an interview. Within a day or so we received a communication from RCA Montreal telling us that their personnel people would be in Toronto and would interview us in their hotel there. They did this and interviewed Prebus and me separately. Shortly after the interviews Zworykin invited me to come to Camden.” Hillier says that at this point Zworykin asked him only one question, “How long will it take you to build an electron microscope?” Hillier answered “Six months.” Hillier continues his recollection: “It is interesting that later, Zworykin confided in me that the reason he decided to invite me instead of Prebus was that the Montreal personnel report indicated that I had better eyesight than Prebus.” At the time Hillier came to Camden, New Jersey, the Research Division of the Radio Corporation of America was housed in three old factory buildings of the old Victor Talking Machine Corporation, romantically named “Buildings 5, 6, and 7.” The buildings were brick and concrete with wood floors on furring strips which could not be cleaned and were a constant source of dirt. The windows did not fit well and were another source of dirt from a foundry, a soap factory, and coal-burning switch engines in a railroad yard only two blocks away. The Benjamin Franklin Bridge, a very large suspension bridge between Philadelphia and Camden, stood a hundred meters from the # 5 building. It carried subway trains that shook and clattered along the bridge. It was before air conditioning, and open windows in the summer and old iron steam radiators in the winter were the means for making life bearable. This was hardly a desirable area in which to build and operate an instrument as sensitive to environmental conditions as an electron microscope. Hillier was given an office and laboratory area in # 5 Building just up the hall from where Marton and the EMA microscope were housed. He shared the

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room with M. C. Banca who was assigned to assist him in designing the new microscope. Banca was an electrical engineer who knew the inner workings of the research laboratory. In addition, he had been working with Marton on the EMA and was thus somewhat experienced in the principles of design and operation of the electron microscope. Immediately upon Hillier’s arrival in Camden, he and Banca started to turn out drawings. They followed them through the Model Shop and assembled the instrument in the astoundingly short time of 140 days (43 months). Hillier laughingly recounts that he joined the Laboratory on Valentine’s Day and turned on the new instrument on the Fourth of July (Independence Day). The new instrument (Fig. 11) was given the model designation EMB. It was to be the prototype of 57 more instruments to come by the end of 1943. The inevitable question raised by the very short construction time is “How did they do it?’. In a conversation with Hillier in March of 1987 the writer posed that very question, with the following response: I look back on the whole thing as being absolutely a miracle of timing.. . to do what Banca and Vance and I did in the first four months I was at RCA.. . and for $10,000.. .(laughter).. . , You couldn’t touch it today even with the allowance for inflation. I came along and got interested (in the microscope) just at the time the technology was almost there to support it.. . . By pulling the technology together and doing a little bit ourselves, we made it work, which couldn’t be done ten years earlier. There were a number of circumstances that contributed to the speed with which the design and construction were carried out. First, before Hillier started to work in Camden he had a relatively definite idea of what he wanted. His experience at Toronto left him with a clear understanding of what the next steps should be in the evolution of the microscope. As a result of this experience and his self-confidence,Hillier was given a large degree of freedom. Second, operating prototypes of the high voltage and the lens power supplies that Arthur Vance had designed for the microscope project were available for incorporation in the design. Third, V. K. Zworykin supported the project providing a high priority for the fabrication of components. Fourth, Hillier was an uncommonly creative, energetic, rapid, and effectiveworker who raised the pace and level of work of those associated with him. One of the goals that Zworykin set for the commercially produced electron microscope was that it be enclosed in a single cabinet. He also wanted the whole microscope to be compact so that it would fit into a normal laboratory room. While such principles reflect some sensitivity to esthetics, they really

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FIG.1 I . The EMB microscope with Hillier (seated) and Zworykin.

deal with cost and marketability. Such goals become feasible when Vance developed the regulated vacuum tube power supplies, which were small enough to fit in conventional electronic equipment racks. The cabinet of the EMB was designed by the well-known professional industrial designer, John Vassos, who was retained by RCA for designing cabinets for electronic equipment. It is not surprising then that the EMB had sheet metal cabinets rather than the thin-wall castings that European microscopes employed. The prototype remained in Hillier’s laboratory until December, 1940 when it was moved to the Stamford laboratory of the American Cyanamid

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Corporation. Why Cyanamid got the first instrument rather than the second is an interesting story illustrative of how Zworykin operated from time to time. When Zworykin’s research budget for 1940 was approved, there was no provision for Hillier who was not yet in the picture. Thus, when Hillier was added to the staff in February, 1940, Zworykin had to divert funds from other projects to meet the added expense. This expedient plan worked well until the approaching end of the year threatened to disclose the subtrafuge. R. Bowling Barnes, who was director of research at Cyanamid at that time, had been pressuring Zworykin for some time for an electron microscope. To solve his financial problem, Zworykin sold the prototype to Barnes, leaving Hillier without a microscope from December to April, when the first production instrument was completed. From the first the quality of performance of the prototype EMB was very encouraging. In October of 1940 Hillier reported seeing bright fringes around slightly out-of-focus images. These he identified as Fresnel fringes (Hillier, 1940). This observation was almost coincident with a similar report by Boersch a month later. While the performance of the prototype was a major step forward, the reliability was a problem as in most radically new instruments and especially one that had been built so rapidly; there was little time to test its components before it was assembled and put in operation. The first production model was delivered to Hillier’s laboratory where it was used in the multiple functions of instrumental development, demonstration to the scientific public, development of specimen preparation techniques, and new applications for electron microscopy. For the last function, Thomas Anderson, a physical chemist, was chosen by the National Science Foundation in a special arrangement with RCA. 5. Manufacturing the E M B

Hillier operated his EMB prototype for the first time on July 4th. Good operation was achieved almost at once, and on the strength of that performance, manufacturing was started within two weeks on the first group of electron microscopes. There were seven instruments in the first group. So small a number says something about the times. According to T. A. Smith, who was product manager and in charge of sales in 1940,there was no great rush at the outset to buy the microscopes. The cost of $9,600 was out of sight for a mere microscope, when a good light microscope sold for only a few hundred. Before the EMB was field tested it was an unknown quantity, and people were reluctant to risk buying one. RCA management had no idea how many to build in the initial group. Once there were a few in the field performing reasonably well, finding customers was not a serious problem. There is a story that T. A. Smith or V. K. Zworykin or both surveyed the directors of a number of research laboratories with a response from twelve that they would consider

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the purchase of a practical electron microscope if RCA could make one. There was also a story about an independent survey which reported that twenty-five microscopes would saturate the market. Within a very short time after the EMB microscopes reached the field they were back-ordered, which was a situation that was to persist for the next ten years. After the United States entered World War I1 in December 1941, electron microscopes could only be purchased under priorities issued by the War Production Board. It took the highest priority, AA-1, to obtain a microscope, which also meant that RCA had the same high priority to obtain the materials and components with which to build the instruments. Even so, priorities did no good if materials were unavailable, and production in war times was often difficult. Actually RCA was in a pretty good position in this regard because at that time it made many of its own components such as capacitors, transformers, and tubes. As soon as the factory in Camden was informed it was to build the EMBs, it set about to bring up to factory drawing standards the sketches used to make the prototype. Neither Hillier nor Banca had the time to make detailed drawings when they drew up the prototype nor were they versed in the art of making production type drawings that contain all the information required by a manufacturing facility. Complete and accurate drawings are essential to assure quality and standardization. An electron microscope is mostly an assembly of mechanical parts. However these parts required special techniques and tests to assure vacuum tightness, and in the early 1940s the tests were crude. Vacuum leak testing could be done at the research laboratory in Building # 5. The most popular test facility was a large alcohol tank in which a part, sealed off and filled with hydrogen under pressure, was immersed and observed for telltale bubbles indicating the presence of a small leak. The microscopes were fabricated and assembled in a building three blocks from the laboratory, and in the early days the parts were carried back and forth for testing and frequently retesting, until the factory got its own testing tank. The mechanical performance, particularly of the vacuum system, was more of a concern in the first instruments than the electrical or optical performance. Arnold Wilson started as a technician in the factory machine shop of RCA in Camden in January, 1941, and as a part of his job he carried the microscope components to and from the vacuum test facility. He soon became involved in testing more and more of the instrument until the factory was finally testing optical performance. Factory management did not share the same feeling for the importance of the microscope that the War Production Board held. The dollar value and the magnitude of the problems of the microscope project were infinitesimal compared to the defense projects which were pouring in due to the war effort. It is somethng of a wonder that the whole microscope

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program was not swamped. If it had not been so far along by the start of the war it might have been dropped. Wilson relates that a corner of the factory production floor was curtained off as an electron microscope assembly and test room. Unfortunately testing, which requires darkness, and assembly, which requires light, could not be done simultaneously. Also overlooked was the fact that an operating instrument, as it must be for testing, puts out several kilowatts of energy as heat. This made the curtained area almost unbearable in summer. Wilson was appointed Foreman of Microscope Testing in 1946. A man of good technical ability, conscientiousness, and exceptional integrity, he played a major role in maintaining the quality of the RCA electron microscopes for over twenty years. 6. Impact of the E M B The impact of the EMB on the scientific scene was enormous. It was greatly enhanced by the rapidity with which the EMB reached the field. It effectively created a considerable community of electron microscopists all starting out at about the same time, which provided a good environment for information exchange and healthy competition. About 30 EMBs were in customers’ laboratories by November, 1942, the date of the famous first conference on Electron Microscopy in Chicago. The experiences with those EMBs provided most of the substance for the meeting and assured its success. The remaining twenty-two were completed in 1943. In the short space of two and one half years (April, 1941 to fall, 1943) the number of electron microscopes in the United States jumped from one to 48. (10 of the EMBs were shipped out of the country.) Thus by 1943, a large number of adventurous people, the kind who seek out new challenges, were developing techniques for handling specimens that two or three years earlier were not believed to be

TABLE I AREASOF APPLICATION OF THE EMB ELECTRON MICROSCOPES Number of instruments Area of application

First group

Second group

Total

Academic institutions Chemical industry Industrial research U.S. government Exported on lend-lease

8 7 6 1 I

6 5 10 5 3

14

29

29

58

Total

12 16 6 10

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subjects for electron microscopy. They were also providing valuable information on how microscopes should be designed and operated. In the investigation of early documents, a complete list of purchasers of the EMB microscopes surfaced in Arnold Wilson’s papers in a notebook of factory test procedures for the EMB. It is illuminating to see who bought those early instruments. Table I shows, in broad categories, where the 58 microscopes went. Two groups of microscopes are shown. The first group of 29 was delivered in the period from roughly April, 1941 to October, 1942, the second group of 29, the period from November, 1942 to late 1943. There is not much difference in the disposition of the two groups except that purchases by the government for its own use rose sharply in the later group. The ten instruments shipped outside the country went to England and Russia, and inexplicably, one went to Guatamala. Thirty percent of the microscopes went to academic institutions. 7. The High Voltage Electron Microscope In the fall of 1940 Marton left Camden. As soon as his laboratory was vacated, Zworykin had the EMA junked and moved Vance into the area, with instructions to build a stable, very high voltage supply for an electron microscope. Zworykin felt that there might be major advantages for microscopy at voltages above the 60kV used in the EMB. The attraction was better electron penetration and the use of thicker specimens. One should recall that in 1940 ultra-thin sectioning of biological specimens, or any materials, was still a decade away. Zworykin was not the only early pioneer with questions about the value of higher voltages nor was the RCA high voltage microscope the only one attempted. Zworykin did not want to get too deeply into microscope manufacturing without knowing the optimum voltage for microscopy, or at least, for the best compromise for a general purpose instrument. No single factor in microscope design has more influence on the final instrument design than the high voltage. Vance and Morgan built the high voltage power supply. It was a quadrupler with a 75kV, 19 kHz, input voltage. The rectifier assembly was housed in a metal tank 2 meters high and 1i meters in diameter filled with oil. Hillier obtained microscope components from production to provide the column. The cabinet was extended vertically to support a three stage electron gun. It was braced to reduce vibration and supported on rubber pads. Lead shielding was used to reduce X-radiation. The total weight was so great that the floor had to be reinforced. The high voltage microscope operated up to 300 kV and attained resolutions as small as 100 A. The instrument (Fig. 12) was put in operation in the late summer of 1941 (Zworykin et al., 1941). It served its purpose by indicating that to have any real advantage, voltages of over 150 k V

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FIG. 12. High voltage microscope. It was so heavy with the huge oil tank on the left and lead (not shown) around the column for X-ray shielding that the floor had to be reinforced (courtesy Am. Ins. Phys.; Zworykin et a/., 1941).

would have to be used. The size and cost of such a high voltage instrument was obviously prohibitive in 1941. Zworykin was convinced that higher voltages would not be a significant factor in the market for many years. History has proven this to be a correct prognosis. While Hillier carried out the high voltage experiments conscientiously and with great interest, he felt, from the outset of the experiments, that very high voltage operation was not a necessary or practical goal at that time. In the end, the electron penetration problem was essentially solved by electron microscopists through their development of ultra-thin sectioning and etching techniques. Once the questions about comparative microscope performance at low and high voltage had been adequately answered, the high voltage instrument did not get much use. R. F. Baker did publish a paper on the properties of photographic materials at different voltages. When the RCA Research Laboratories moved to Princeton in 1942, the high voltage microscope went along and was set up there by Vance and Hillier. It found little use, and a few years later, the power supply was modified for use at 50 to 150 kV with an experimental microscope column.

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8. The Scunniny Electron Microscope During the years 1938 to 1942 while development of the transmission microscope was going on in the RCA laboratories, a parallel program was also being conducted on the scanning microscope. The project involved a considerable number of the workers in the laboratory. It is not surprising that there should have been interest at RCA in applying scanning techniques to microscopy since RCA was heavily involved in video research. In 1934, Zworykin had reported the construction of an “electric microscope”. Essentially, it projected the image from a conventional light microscope with quartz optics upon the screen of an iconoscope, which converted the image into a corresponding video signal. The video signal was applied to a kinescope which displayed the image from the light microscope. The primary purpose of the device was to use the microscope with ultra-violet and infra-red sources beyond the range of the human eye. While this was not a scanning microscope, it did indicate Zworykin’s interest in using electronics to improve microscopy. In 1938, C. E. Burnett replaced the iconoscope on the “electric microscope” with a monoscope which, unlike the iconoscope, had a variable length scan. Thus it was possible to change magnification, although it did nothing to change resolving power. This was followed by a true scanning microscope, one where a fine beam was scanned across the specimen, the secondary electrons were collected and their signal displayed on a kinescope. The system had a magnification of up to two thousand times and a resolving power of one micron. It was encouraging but not nearly good enough. Another scanning microscope was built which was a major departure from the previous one. The microscope is described in detail in an article (Zworykin ef ul., 1942).This scanning microscope (Fig. 13) had a number of unusual features, including an inverted column and electrostatic lenses. The signal was generated by collecting the secondary electrons on a phosphor screen where the emitted light was detected by a photocell multiplier. The secondaries were even accelerated to the fluorescent screen to increase light output. The image was recorded on a facsimile recorder to permit very slow scanning which increased signal-to-noise ratio. The initial scanning device moved the specimen, while the beam was fixed. It used loudspeaker voice coils synchronized with the motion of the facsimile recorder to accomplish the motion. This system was too sensitive to vibration, so a hydraulic system was tried. Finally a magnetic beam deflector was used for scanning. The instrument had a resolving power of 500 Angstroms. The technology that was needed to make a higher resolving power scanning microscope did not seem to be near at hand. The technological deficiency was the magnitude of the secondary electron signal and its complicated relationships with the signal-to-noise ratio, resolving power and the required image scanning time. There was little enthusiasm to engage in the

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FIG. 13. The scanning microscope. It was many years ahead of the technology needed to make it a viable commercial product. Note the inverted vertical column with the stage at the top. Left to right, Snyder, Zworykin and Hillier.

development of an instrument where the single frame exposure time was ten minutes, particularly when the possibilities of the transmission microscope looked so attractive, and so the project was dropped. It was at this point that technology was to be stymied for nearly two decades. One must be impressed with the number of bold projects that Zworykin and his relatively small group of about twenty engineers and scientists were engaged in. Nearly all had multiple assignments. In the early work on the scanning microscope, A. W. Vance and L. E. Flory had been active. The final model was primarily the responsibility of Hillier and Snyder. E. G . Ramberg

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provided valuable assistance in connection with the mathematical problems involved in the design of the instruments. Zworykin characteristically followed his programs closely and made technical contributions to them. 9. James Hillier

The preeminent figure in the development of the electron microscope in the United States was James Hillier. He was the right person, in the right place, at the right time. Hillier himself refers to it as a “miracle of timing”. The timing was set by the evolution of technology to the point where it was nearly ready to support a system as demanding as an electron microscope. The place was set by Vladimir Zworykin, who generated the interest and support within RCA necessary to undertake and succeed in the development of an electron microscope. Without Zworykin it is quite likely that RCA would never have undertaken such a daring development. The right time and right place were a promise of possibility not a guarantee of success. It required a unique person to exploit the opportunity they presented. Hillier was just such a person. Hillier was a hands-on engineer/physicist. He gained the ability to work with hands in his father’s shop in Brantford, Ontario, Canada. His father was a mechanical engineer who specialized in the design of equipment for the baking industry. Hillier had little if any formal training in engineering. He was graduated from the University of Toronto in 1937 with a B.A. degree in mathematics and physics. He continued as an assistant in the Physics Department as he pursued a master’s degree in physics. He and another graduate student, Albert Prebus, undertook the building of a transmission electron microscope as a research project, following in the path of Cecil E. Hall who had built two emission microscopes a year earlier. Prebus and Hillier in the fall of 1937 gained valuable experience rehabilitating Hall’s two instruments. They actually designed their transmission microscope during the Christmas break. They then constructed the instrument themselves, machining the components, and begging and borrowing what they did not have time or equipment to build. The instrument was operational before mid 1938 and was performing well in a short time. In 1939, they published a paper on their results (Prebus and Hillier, 1939).Hillier had about a year and a half to use and further refine the Toronto Microscope before he was called to RCA in Camden in February, 1940. The fact is that Hillier was a brash college kid whom Zworykin trusted with the responsibility for designing a commercial electron microscope where an experienced scientist had failed. As a physicist, Hillier understood the principles under which the electron microscope operated, and he recognized that satisfactory operating conditions were going to be achieved by engineering. While not educated as an

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engineer, Hillier understood engineering. It was the engineer in him, and particularly the mechanical engineer, that made the EMB microscope a success. Most of his first year at RCA was occupied in engineering design and testing the EMB. After his prototype was shipped to American Cyanamid in December, 1940, he was without a microscope until April, 1941 when the first of the production instruments was delivered to him. During this time he worked on constructing the high voltage microscope, another major engineering undertaking. The physicist in him asserted itself in 1941 as he became absorbed in experimentation with two types of electron beam instruments, the scanning electron microscope and the microanalyzer. The scanning microscope experiments were ended in 1942 in Camden. They had whetted Hillier’s longstanding desire to use the interaction of an electron beam with matter to achieve an analysis of a material of microscopical size, to complement the geometrical information provided by the electron microscope. After the move of the Research Laboratories to Princeton, in September of 1942, he constructed a microanalyzer (Hillier, 1943; Hillier and Baker, 1944) with which he was able to demonstrate analytical capabilities with an electron beam. This proved to be another instrument which neither the technology nor the field was ready to support. Hillier’s vision has been fully vindicated by the number of modern analytical instruments using free electrons as a means for analysis. He rates his work on the microanalyzer as one of his most significant contributions to science. Hillier was involved in a major engineering role in the design of the prototype table microscope which led to the commercial model EMC console microscope. This was his last major instrument design. Separated from the commercial aspects of the electron microscope business as he was at the laboratory, he was able to devote himself to very timely research on the understanding and improvement of the microscope. During his tenure at the RCA Research Laboratories, he maintained a constant flow of information to Camden on new ideas and improvements for the microscopes, as is often pointed out in this article. Throughout his career in electron microscopy, he stressed the need to educate microscopists in the art as well as the science of microscopy. Hillier did a lot of experimentation on specimen preparation and authored many papers on the subject. In his fourteen years at RCA he wrote over 150 papers and was awarded 41 patents. In 1953, to the surprise and sorrow of many of his associates, he left RCA to take a management position at Melpar Inc. He returned to RCA after a short time in a management role and rose to the position of executive vice-president for research and engineering in 1969. He retired in 1977.

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10. R C A Research Personnel Who Made Major Contributions to the Development of the Electron Microscope

The principals in the early development of the electron microscope at RCA, in the order of their appearance, were Zworykin, Marton, Vance, and Hillier. They in turn had a strong support group of very wide capability, mostly made up of the members of Zworykin’s research staff. Of these the best known to microscopists is probably Dr. Edward G. Ramberg, recognized as an outstanding theoretical physicist, particularly in electron optics. He joined RCA in 1935, and for forty years acted in the capacity of a consultant and advisor to a research staff deeply involved in light and electron optical problems. He is best known to electron microscopists for a paper he coauthored with Hillier on Fresnel fringes (Hillier, 1947).He collaborated with Morton on building and testing a simple point projection electron microscope in 1939. In 1941, he co-authored with Zworykin a paper reporting techniques for studying surfaces, and in 1942, he, R. F. Baker, and J. Hillier published a study of the photographic action of electrons in the range between 40 and 212 kilovolts. Ramberg was bilingual in German and English. He translated the major papers on electron microscopy coming from Germany in the period from 1935 to 1946 and circulated them among the research staff.As with everything he did, Ramberg’s translations were masterpieces. His subtle and considerable contributions to the microscope program were a significant factor in its success. Hillier reports that the book “Electron Optics and the Electron Microscope” (Zworykin et al., 1945) is a true collaboration of the authors. However, Ramberg was distinctly the major contributor. In addition, he did almost all of the actual writing. During the war, he was involved in an accident in which he suffered a broken leg. I t was that broken leg that made the completion of the book possible. Dr George Morton was a more senior member of the early microscope group, He was an authority on photoelectricity and on photo cells, and had much practical experience in the laboratory. He worked with Zworykin in the mid-thirties on image tubes and multiplier photo cells, and co-authored a book on television with him. He also worked with Marton on some aspects of the EMA microscope. Richard F. Baker was working on ultraviolet microscopy at Johns Hopkins University Medical School in 1941 when Hillier invited him to join the RCA Laboratory staff, to help carry out the large number of projects in which Hillier was involved. Baker accepted and for the next six years he was a close associate of Hillier and assisted him in research on the 300 kV microscope, astigmatism correction, illuminating systems and the energy-loss spectrometer. During that time, he co-authored ten papers, including six with

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Hillier. In 1947, he left RCA to take up an academic carrier at the University of Southern California School of Medicine. There he published 71 papers, mostly dealing with microbiology. He is probably best known for his work with D. Pease on techniques for thin sectioning. M. C. Banca is remembered, not as an engineer, but rather as a salesman. This is somewhat ironic in that he was an able engineer and played a significant part in the technical development of the EMA and EMB electron microscopes. In 1942, when the research laboratories moved to their new quarters in Princeton, Banca chose to be transferred to the Sales Department in Camden where he was made sales manager for electron microscopes. His first-hand knowledge of the microscope was helpful not only in sales but also in solving early field failure problems. His great visibility in the field was largely responsible for his being elected the first secretary-treasurer of the Electron Microscope Society of America, It was he who wrote, edited, and collected the material for the “EMSA 1943 Year Book”, which is by all criteria the most important record of the birth of the EMSA and the status of electron microscopy in the United States in 1942 to 1944. He was not particularly happy in the role of a salesman, and when R. Bowling Barnes, the first buyer of an EMB microscope, left American Cyanamid to form his own corporation in the late forties, Banca joined him as an electrical engineer. Neither Banca nor Barnes had further contact with the electron microscope. During the time he was developing the power supplies for the microscopes, Vance had the help of a young technician Jerome P. Morgan. Although Morgan did not have an engineering degree, he demonstrated a high level of engineering proficiency and was moved to Princeton with the research group when it occupied their new quarters in 1942. After Vance left the project, Morgan provided electrical engineering support and designed the high voltage supplies for the EMC console microscope and a small quadrupler supply for a portable electron microscope that never materialized. 11. Research Moves to Princeton

In September of 1942, the entire research laboratory left Camden to occupy specially built new quarters just outside of Princeton N. J. While the majority of the research had been carried on at Camden, there were smaller groups scattered around the country in other RCA factories. In order to unify the research effort, the company set up the new facility to get the research program under one roof. Such a laboratory, separated from the obligations to be concerned with the problems of manufacturing and marketing, and free to pursue research objectives, had long been a dream of David Sarnoff, who was then the Chairman of the Board of Directors of RCA. In justice to Sarnoff’s memory he should be credited with having nurtured the electron microscope

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by providing the kind of environment in which such developments could flourish on the basis of their social and scientific significance as well as on the basis of their potential for turning a profit. Even after the microscope had become relatively commonplace, Sarnoff used to cite its development by RCA as an example of corporate social responsibility. This sensitivity of corporate leadership was felt throughout the whole company and made the Sarnoff years great for engineers and scientists and left a legacy of outstanding achievement. The move to Princeton was a good thing for the microscope program because it freed Hillier to pursue his research full-time on the instrument as well as on allied projects. Thus the entire electron microscope research and engineering team left Camden, with the exception of M. C. Banca, who remained to take responsibility for microscope sales and field support. The manufacturing departments remained. The move made it necessary to create a factory support and design engineering group in Camden. DEVELOPMENT B. THEERAOF ADVANCED 1942-1950 1. A New Team of Engineers

The microscope engineering group that was set up in Camden to replace the personnel of the research laboratory who had moved to Princeton was made a part of the Broadcast Engineering Section to provide operating structure. Perry C. Smith was appointed manager, with the responsibility for pulling the group together. Smith was a charismatic leader and a good organizer. He was a bright man, but because of the Great Depression, he had not been able to complete his technical education. He had extensive field experience with switching systems and telephone equipment in the New York Stock Exchange, but his lack of technical training was a handicap in the decision making which his position required. As manager of the engineering group he was a prime contact point between RCA and the microscope users. He became widely known by microscopists in the years between 1942 and 1950 and was elected president of the EMSA. In 1950, he was transferred into the sales department of a different product line and had no further contact with the microscope. He deserves to be remembered for organizing and steering the microscope group through the war years and for guiding the design of the EMU-1, -2 microscopes. The group that Smith gathered together in 1942 comprised Frank Runge, an exceptionally gifted mechanical engineer, Edmund G. Dornfeld, a mechanical draftsman and the only person in the company who could work with Runge, Samuel M. Zollers, a recently graduated electrical engineer, and G. F. Burger, an instrument maker of remarkable ability whose objective apertures

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were once in demand worldwide and who occasionally got the department in trouble because he could fabricate devices which no one could duplicate. It was soon obvious to everyone that, given the number of physical problems encountered daily, a physicist was needed to round out the microscope group in Camden. Fortunately Smith was able to locate and hire Robert G. Picard, who had received a Ph.D. in physics at the University of Michigan in 1943. The university had received one of the first EMBs in 1941, and Picard had used it to do research on thin evaporated metal films with Duffendack (Picard and Duffendack, 1942). O n completion of his degree work, Picard took a position with the U.S. Rubber Company in Passaic, N. J. to do research with their newly acquired EMB. Shortly thereafter, Smith invited him to Camden during the winter of 1943-44. Whereas physicists had difficulty getting a job in 1940, by 1943, there were five jobs to every graduate in physics. John H. Reisner was awarded a Ph.D. in Physics in 1943 and elected to accept an offer of a position with RCA in Camden. By virtue of having taken a course in crystallography in graduate school, he was immediately assigned to the crystal manufacturing department which was having serious trouble orienting quartz crystals used for maintaining frequency in radio equipment. The crystal engineering laboratory was on the fourth floor and microscope engineering was on the second, and Reisner, who was very much interested in the microscope, was a frequent visitor and soon became well acquainted with everyone in the group. Smith was interested in setting up an advanced development and research function in his group and negotiated a transfer into the microscope group for Reisner as soon as the latter had completed his assignment in the Crystal Department. The move took place in 1945. Thus, from 1945 to 1949, the group of fulltime people comprised two physicists, two mechanical engineers, an electrical engineer, an instrument maker, several draftsmen, and a manager who was basically an electrical engineer. The engineering group was greatly benefited by a strong support system. The transfer of the original research group to Princeton did not end their support of the electron microscope program. They were available for advice and technical support. Hillier, freed to do research, provided the Camden group with numerous improvements such as the stigmators for the objective lens. Specialized support was also available from the very large staff of outstanding engineers and scientists in Camden. An excellent model shop and a pool of draftsmen and technicians could be called on when needed. During the years from 1945 to 1952 the Camden group retained the consultative services of Ralph W. G. Wyckoff, the noted crystallographer and physicist who was one of the pioneers in the use of the electron microscope in crystallography, and with R. C. Williams, the first to demonstrate metal shadowing to enhance image detail. He was at that time a commissioned

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captain in the US. Health Service and was headquartered at the NIH in Bethesda where he had an electron microscope laboratory. He was invaluable in providing a critical user’s viewpoint on contemporary instruments and in making assessments of future needs and possibilities. This fruitful relationship was terminated in 1952 when he became Scientific Attache to the U S . Embassy in Great Britain.

2. The E M U Electron Microscope This new engineering group had to familiarize itself with the electron microscope so as to be able to support a manufacturing group and the people doing the field service. In addition, it had to pick up an in-progress design project for an instrument to replace the EMB, which in spite of its comparative success, had many problems and unnecessary cost and complexity. The research laboratories had almost finished a new prototype called the model “F” (Fig. 14). It essentially got lost in the shift of design responsibility from the research people to the new team of engineers in Camden. Zollers reports that he remembers seeing, prior to the start of the EMU design, an unnamed experimental microscope that had lever actuated vacuum valves. Partly due to the experience with the EMB, which by 1943 numbered about 40 instruments in the United States, partly due to advances in technology, and partly due to the change in engineering personnel, the EMU microscope had major changes from the EMB. The primary change dealt with the vacuum system. A faster diffusion pump was available on the market so the whole interior of the microscope could be pumped in less than a minute. This meant that specimen and photographic air locks could be eliminated and with them, the troubles that they occasioned. This also eliminated some valves so that Runge was able to centralize the valving system in a single valve block actuated by a single small crank on the side of the console. Four crank positions provided: a neutral position, where all valves were closed for shut down; a load position, where the microscope interior was at atmospheric pressure so that specimens and photographic plates could be changed; a rough pump position, where the mechanical pump reduced pressure to where the diffusion pump could operate; and an operate position, where the diffusion pump was used to pump the column and the fore pump backed the diffusion pump. The simplification of vacuum operation was a great boon to the operator and more reliable because the sequencing of operations was automatic. Even the rugged Philips discharge gauge was available to improve vacuum measurement over what had been available for the EMB. Vance and Morgan had already designed and built an air insulated high voltage supply, to be used in a new instrument, by the time Vance left for Princeton. Field experience with the EMB had dictated that its heavy,

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FIG. 14. Model F electron microscope was an intermediate engineering step between the model EMB and the EMU series. It was never manufactured.

inaccessible and messy oil-insulated supply should be replaced by an airinsulated high voltage supply. Zollers was responsible for the construction of the electronics. He reports that he built the high voltage supply from Morgan’s sketches without ever seeing the original prototype. His initial design called for all the copper corona rings and fittings to be highly polished and chromiumplated to avoid corona and voltage breakdown. Since chromium was in very short supply the plating was changed to gold. This gave rise to much comment, and even when chromium was available after the war, the gold power supply had become such a trade mark for the EMU that the plating was never changed back to chromium. Runge and Dornfeld designed a mechanical traversing and tilting system

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for the electron gun. To make operation and viewing the image easier, the column was raised and shortened so the operator could sit on a normal sized chair. The electrical controls were all mounted on a small console just above lap height. The viewing chamber was changed to provide three large viewing windows and the photo system was simplified, although the 2 x 10 plate used on the EMB was retained. At the same time its volume was reduced. The cabinet design was primarily the work of Robert Holley, who was an industrial designer. The war, particularly in the aviation equipment designed for the military, had emphasized the importance of designing equipment that matched human needs and human capabilities. This was called “human engineering” and received much attention in engineering journals and meetings. Holley was a good practitioner of this technology as the design of the EMU testifies (Fig. 15). The EMU series of microscopes were very successful. The fundamental design was sound and adapted to change well. The first E M U left the factory in June, 1944. 299 were to follow in the next nine years. It was manufactured in

FIG.15. EMU microscope. It was simpler and more compact than the EMB microscopes and much more comfortable to operate.

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eight lots of twenty five and two lots of fifty. Each time a lot was started it would incorporate improvements developed since the previous lot was started. Each lot had its own designation; EMU-1, EMU-lA, EMU-2A etc., to EMU-2F. The change from - 1 to -2 was the largest systemic change. It occurred when the valving system was changed from a mechanical to an electrically actuated system. Keeping abreast of the modifications and improvements was a task that involved everyone in the engineering group. Many of the improvements came from Hillier, others came from the service company and from microscopists. The first EMU was priced at $12,000 in 1944, the last at $18,500 in 1953. A feeling for the actual level of performance of electron microscopes as of 1946 is given by a survey made by the Committee on Resolution of the Electron Microscope Society of America (Kinsinger et al., 1946). The survey was extended to the entire membership of the society, and 30 responded. At that time there were probably about 48 EMBs and 80 EMUS in the field. The survey questioned its recipients as to the best resolution they had achieved on their microscopes. The results were displayed on a bar graph which was printed in the committee report and is reproduced in Fig. 16. Since no criteria for measuring resolution were prescribed by the committee there was bound to

RCSOLVINC KIWLR-ANGSTROM UMltS

FIG.16. Graph copied from the report of the EMSA Committee on Resolution shows the estimated best resolution achieved with 30 electron microscopes as of 1946 (courtesy Am. Inst. Phys.; Kinsinger et d.,1947).

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be a spread in the results. What was not recognized at the time was that the variability in resolving power from instrument to instrument and from time to time on a given instrument was frequently caused by astigmatism. At the time of the survey, the EMU microscope was guaranteed to have a resolving power of 100 A, which is considerably poorer than the mean value of 40 A shown in the graph. 3. The E M C Console Microscope

The EMC was a small console microscope that had been conceived by the research group while still in Camden. Later a model (Fig. 17) was constructed under Hillier’s guidance at Princeton. It came about as a response to the very widely held belief that a small microscope was the wave of the future. The sales department felt that there would be a good demand for the smaller microscope, and even the War Production Board encouraged RCA to undertake the manufacture of such an instrument because they were getting more requests for the electron microscope than RCA could meet. Accordingly, the technology on the console microscope was transferred to the Engineering group in Camden, where it was incorporated into a product design. It was a large load to add to the group doing a design on the EMU. Nevertheless, the first EMC (Fig. 18) was delivered at the end of 1944.

FIG.17. Prototype of the small microscope. I t was completely self-contained (courtesy Am. Inst. Phys.; Zworykin and Hillier, 1943).

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FIG. 18. Production model of the EMC console microscope. Left to right, Zworykin, P. Smith, Hillier.

The EMC had a desk size console with a short column tilted about twenty five degrees from the horizontal. It had a transmission type viewing screen, facing the operator, and could take one picture on a 2 x 2 plate at each loading. It had a single coil which energized two gaps in a long pole piece. These two gaps formed the objective and projector lenses. There was no condenser lens, a small aperture in a platinum disc between the gun and object served to limit the beam. It had a 30 kV high voltage and a single magnification of 5000 x . The pumps and high and low voltage supplies were housed in the cabinet, so that the microscope was a completely self-contained unit. It also had a valve-block which did all the valving with a turn of a crank. It sold for $7,000 compared to $12,000 for the EMU. With a good pole piece its resolving power could be better than 100 Angstroms. Eighty-five EMCs were manufactured and sold in the period from 1944 to 1948. It was a relatively troublefree instrument and served well where it was used in a limited situation such as monitoring particle size. It should be pointed out that quite a number of electron microscopes were purchased to clean up process and quality control problems, which they often did. At the same time they provided sufficient understanding of the processes, so that engineers were able to devise other

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means that were better for providing control and were simpler and less expensive than the microscope. Thus the microscope did itself out of a job in many of these cases, and was wrongly branded of no use to the purchaser. The EMC had several technical drawbacks that hurt its performance. The combination of no condenser lens and low accelerating voltage was very damaging to specimens and limited the usefulness of the microscope to rugged specimens. The single magnification was too limiting. The EMC could not compete against the EMU at the price differential that existed between the two instruments and was discontinued in 1948. The EMC was the first RCA instrument to use the self-bias gun. One can only conjecture why the discovery of the self-bias gun was as late as it was in the development of the electron microscope. Hillier reported that its discovery was a matter of serendipity. He was working with a microscope when there was suddenly a large jump in the intensity of illumination. He investigated and found that a failure of a part in the high voltage filter had caused the high voltage to develop a negative self-bias on the grid cap. He pursued the matter further and suggested that self-bias be used on the EMC gun to improve image intensity. A short time later he applied it to the EMU with such an improvement in brightness that all new production was modified to use selfbias, and kits were made available to modify EMUS already in the field. There were not many times that RCA felt a competitive threat from General Electric concerning electron microscopes, because by 1942, RCA had a huge head start. However G E s talk about making a portable electron microscope did cause some concern, especially since the electrostatic microscope could be packaged in a considerably smaller and lighter assembly than was possible for a magnetic microscope. Engineering in Camden was asked to design a portable microscope with the same capabilities as the EMC. The plan was to use the same column as the EMC. This would take one suitcase by itself. The power supplies were modified in shape so as to fit in a second suitcase together. This required a new much smaller 30 kV supply using a quadrupler designed by Morgan. The third suitcase housed the fore jump, miscellaneous cables and accessories. The microscope was portable in name only. The suitcases were large and made of aluminum, and really needed two people to carry each of them. A model (Fig. 19) was constructed in 1946, and it operated as well as an EMC. However, the deficiencies of the console microscope were becoming obvious, and being portable would not cure or offset them. The portable microscope project was allowed to die, but the hope for a small and inexpensive microscope did not die with it. 4. The R C A Service Company

From the very inception of the electron microscope program Zworykin had as a goal the marketing of a microscope that could be set up anywhere and

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FIG. 19. The portable electron microscope was essentially an EMC microscope housed in three large aluminum suitcases. It was too large and too heavy to be called portable.

that almost any scientist could run. For a first try, the EMB came close. However, as the instruments entered the field, they began to require technical support beyond that required for their installation, particularly in cases where the user was not comfortable with the new technologies involved. The early users were a remarkable lot. They were adventurers welcoming a challenge. They showed courage and some times foolhardiness, pride and self confidence. When they needed technical help they would first get it by phone and try to solve the problem themselves. It was not unusual in those very early days to hear the comment, “I spend more time working on the instrument than I do working with it”. There was good rapport between the microscope group at RCA and the microscope users, to the great mutual advantage of both groups. However, the instrumental problems often could not be solved by remote control over the telephone, and it became necessary to send trained people to care for the field problems. This need for field support was not unexpected. RCA had acquired the Photophone business in 1928. Photophone was a sound-on-film system that made sound movies possible. Operational failures could not be tolerated at

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the theater box office. To provide maintenance of the Photophone equipment in off hours, and emergency service at all times, a field service organization was set up with offices all over the country with technicians trained in service problems. When radio transmitters became numerous they were provided service by the service organization. Service after the warranty on the equipment expired was usually sold as a one year contract entitling the customer to a specified number of maintenance visits and unlimited emergency calls. As microscopes began to accumulate in the field, similar contracts were extended to their owners in 1943. This arrangement helped greatly in providing field service for microscope users and giving neophyte operators confidence that RCA wouldn’t abandon them to the problems they were sure to encounter with an electron microscope. The service contract, which is so common today even on household appliances, was an innovation in 1943. It represented a continuing commitment by the manufacturer to support the customer. In the early days there were serious problems with microscope service. Photophone and transmitter technicians were usually adequate with the electrical problems of microscopes, but often they knew less about dealing with vacuum and operational problems than the customer. It soon became obvious that microscope specialists were needed for field service. During the war years, such personnel were impossible to find. For a time RCA tried to train quasi-scientific people such as a chiropractor, but the results were generally unsatisfactory. Field service technicians were often seen and often acted as advocates for the customers. The first electron microscope field service man, Chester Davis, a converted photophone technician, carried this role to extremes, as the following true story will show. “Chet” had been having a difficult time trying to restore adequate vacuum to the EMB microscope at the American Cyanimid Laboratory at Stamford, Connecticut. He finally decided that the problem had to be in the specimen chamber. It was a weekend and he could not contact Camden for instructions on what he should do. He solved the problem by removing the suspected specimen chamber getting on his motor cycle-his standard mode of transportation-and driving the 150 miles to Camden where he exchanged the specimen chamber from Cyanamid for the one in Hillier’s microscope. He then motorcycled back to Stamford and installed the pilfered chamber in the Cyanamid microscope. The ill-advised action created some interpersonal tensions. Zworykin had a fit, and Wilson reports that, had it not been for Hillier’s intervention, Zworykin might have ended Davis’ career at that point. It was common practice for field servicemen to work very long hours and on weekends and holidays. The existence of a large and experienced RCA Service Company at the time the EMB reached the field played an important role in the early effectiveness of the instrument.

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5. Electron Optical Research and the Optical Bench Having succeeded in making an electron microscope system work the group turned considerable attention to studying and understanding the individual elements of the system. The microscope itself can be used to study, in a limited way, some of its components; and on the early instruments this was the way the electron optical elements where characterized. However, testing as part of a microscope is usually much more restrictive and cumbersome than desirable. Experimenters with light optical elements have long utilized the optical bench, to provide access and adjustability to the optical elements under study. Because of the necessity for maintaining the electron optical paths under vacuum, an electron optical bench is a much more difficult device to achieve than its light optical counterpart. In 1945, Reisner and Picard designed and had constructed a unique electron optical bench which proved to be extremely useful in studying electron gun characteristics and in designing the permanent magnetic lenses (Reisner and Picard, 1948). The electron optical bench was a lathe bed from an obsolete metal turning lathe, housed in a rectangular steel box (Fig. 20). It had an inside length of 61 cms and could accommodate optical elements of 17 crns diameter or width, and weighing ten kilograms or more. It had an EMC gun, a 100 l/sec oil diffusion pump, and an EMU-1 valve block. It was equipped with an EMC 30 kV high voltage supply, modified to provide voltages down t o 10 kV and a lens supply providing up to 500 mA of stabilized current. A large carriage, riding on the lathe bed could be positioned both longitudinally and transversely, while a smaller carriage could be positioned longitudinally. All three of these adjustments were separate and could be made from outside the vacuum. It was equipped with a cathetometer to measure transverse distances and set up initial alignment. The pumping system was fast enough to pump a wire-wound lens in less than an hour. The vacuum chamber was covered with a 2.5 cm thick slab of Lucite over which a pane of glass was placed to limit X-ray emission. The use of a transparent cover permitted accurate measurement of longitudinal distances. The electron optical bench was used for about ten years until analytical methods began to supercede experimental techniques in electron optical design. The time and money that went into building the optical bench in 1945 is witness to how seriously RCA took its responsibility to improve the microscope. 6. The EMD Electron Diflraction Camera The demonstration of electron diffraction by Davisson and Germer caught the imagination of scientists as few experiments have. To have the wave properties of the electron so obviously demonstrated brought a sense of reality to the concept of the dual nature of the electron as wave and particle.

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FIG.20. The electron optical bench viewed from above the front window. The carriage is supporting an EMC double lens assembly. A vacuum tight transparent cover (not shown) made it possible to evacuate the enclosure to good operating levels (courtesy Am. Inst. Phys.; Reisner and Picard. 1948).

In the early days of electron microscopes people who watched a demonstration of the instrument for the first time were often more impressed with the diffraction pattern than with the image of a sample. Electron diffraction is almost a byproduct of the electron microscope, needing only a fine beam, a crystaline specimen and a viewing screen. In the pressure of making the prototype EMB as rapidly as possible there were no provisions for doing electron diffraction. This was not an oversight, and in 1942 a diffraction stage was introduced (Hillier and Baker, 1942). The EMB projector lens was replaced by a new assembly in which an externally adjustable stage was located just below the projector coil. This stage was removable through air-locks for the change of specimens. It could be used with

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both reflection and transmission specimens, and provided tilt and transverse motions. There was also a second coil below the stage to focus a cross-over just below the objective lens onto the viewing screen. The camera length was small, and the lens following the specimen reduced it further. It was a useful device and saw a good deal of use. When the EMU series of microscopes was designed, a similar diffraction stage was designed by Runge. Since there were no airlocks on the EMU, more space was available and a somewhat more versatile stage was achieved. The microscopy group at American Cyanamid in Stamford bought an EMU microscope in 1944 to replace their overworked EMB, which had been the prototype of the EMB series and subject to all the problems subsequently solved for later instruments. They then turned the EMB into a diffraction camera in which the projector pole piece was removed to provide a wide bore, thus increasing the field width. The wide bore lens was then used to magnify a diffraction pattern from a specimen in the regular specimen holder. This made it possible to produce effective camera lengths of up to 200 cm. Picard was very much interested in electron diffraction and, with Reisner, experimented with alternative ways of doing diffraction on the EMU microscopes, including the magnified diffraction patterns of the Cyanamid group (Picard and Reisner 1946). It was soon apparent that, while a microscope was useful for doing transmission diffraction, it was restrictive on reflection diffraction. There was not space in the column for an adequate reflection stage. The relatively small bore of the lens spools limited the size of patterns, as did the photoplates. Two lenses above the specimen were necessary to provide small spot sizes and high resolution (Hillier et al., 1946). Interest in electron diffraction had been slowly growing. G E was marketing a simple electrostatic lens diffraction camera, although it had withdrawn from the microscope market. The decision was made in 1947 to proceed with the design of an instrument specifically for doing all kinds of electron diffraction. The new design (Picard et al., 1949) was named the EMD (Fig. 21). It used as much of the EMU system as possible, such as the high voltage supply, lens supplies, vacuum system etc. The gun was borrowed from the EMC microscope. The key innovation was a stage capable of supporting a 2.5 x 2.5 cms specimen and providing three degrees of translation and two angular degrees of freedom. A charge neutralizer and a hot stage were also included. Four identical lens spools with a 3.8 cm. bore were provided, two above and two below the stage chamber. There was a strong lens pole piece in the top spool and a large-bore moderate-strength pole piece in the spool below the specimen chamber. The photo system of the EMU employed 2 x 10 inch plates, giving five exposures a plate. The EMD used three 4 x 5 inch plates. The camera length, specimen to plate distance was 50 cms and with the magnification mode the effective length was over 200 cm.

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FIG.21. The E M D electron diffraction camera was dedicated to doing diffraction. Its design borrowed heavily from the EMU. R. G . Picard is operating the instrument.

The EMD was reasonably successful.Twenty-five were manufactured and sold. Its price was $20,000 compared to $18,000 for an EMU-2 in 1950. The instrument was phased out in 1956, because the radically new design for the EMU-3 series included an opening in the column between the objective and the projector to accommodate either a lens or a diffraction stage assembly very much like that in the EMD. The lens permitted selected area diffraction from a microscope specimen, as Le Poole had demonstrated, or an extra stage of image magnification. Electron diffraction, particularly the reflection mode, is a difficult technique as indicated in what experimenters have jokingly called the first rule in electron diffraction: “Electron diffraction should never be used except as a last resort.”

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I . The Objective Pole Piece

The objective pole piece was recognized very early as an important but unpredictable influence on the resolving power of the electron microscope. Supposedly identical pole pieces behaved very differently in the same instrument. A necessary precondition to improving the objective pole piece was the achievement of stable high voltage and stable lens currents. Without stability contour effects from Fresnel diffraction are washed out. Using the EMB with its stabilized power supplies and improved mechanical stability, Hillier had first observed and identified these contour effects as Fresnel fringes in 1940. Mechanical axial symmetry of pole pieces was a basic postulate for good performance and great care was exercised in machining pole pieces. To insure maximum accuracy, Hillier designed a pole piece where the upper and lower pole pieces and the spacer were turned from a single piece of iron at a single lathe setting. This was accomplished by leaving a thin walled tube of iron joining the top and bottom poles of the lens. While this thin-walled connection shorted the gap, it actually saturated in the magnetic field and worked essentially as if it had not been there. The one piece construction was fragile and difficult to fabricate and was abandoned in the EMU microscope for a three piece design, where the iron pieces were threaded to fit on a brass spacer which also acted as pilot for aligning the iron pieces. The assumptions were made that the iron used for pole pieces must be of high permeability, low retentivity, and fine grain structure. Armco iron was the material best meeting these requirements. At that time it had a total impurity of approximately 1%. It was believed that the unpredictability of performance of essentially identical pole pieces was due to inhomogeneities resulting from large-size grains frequently encountered in the iron. Since iron purity and large grain size go hand-in-hand, the iron was forged by Armco to reduce the grain size. The billets of forged iron was 4 x 4 inches in cross section and three feet long. The billets were then cut lengthwise into 2 x 2 inch bars from which the pole pieces were turned. The reason for cutting the four bars from the billet was to avoid using the center section of the billet where faults from forging were most likely to occur. By 1946 the problem of unpredictable pole piece performance had become nearly intolerable. The factory was encountering a fifty percent rejection rate for objective pole pieces. Some would have a resolving power of 20 Angstroms, others had trouble reaching 100 Angstroms. Good pole pieces would sometimes lose resolving power after a few cleanings, and poor pole pieces would sometimes improve. It was possible to guarantee a resolving power of only 100 Angstroms. The engineering department at Camden was trying to solve the problem by improving the iron, a long term solution, and by lapping

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which seemed uncertain at best. A request was made to the RCA Laboratories in Princeton for help with the problem. Hillier responded with a remarkable solution. He started where most interested people were working, lapping the pole pieces on the faces and in the bores. The author recalls visting Hillier’s laboratory in Princeton and finding it full of lapping plates, jigs, abrasives, and lapping paraphernalia. Hillier was keeping records of pole piece performance as lapping progressed. A number of times he thought that he saw a pattern of improvement but in a short time he concluded that better machining was not the answer. He observed that the amounts of material he removed would not account for the changes he saw in lens assymetry. Abruptly one afternoon he decided to abandon the lapping approach and to attack the magnetic field assymetry directly. Hillier went to his instrument maker with a pole piece spacer and asked him to drill and tap eight evenly spaced radial holes in it at the center of the gap. He also asked for eight screws made out of soft iron welding rod to fit the holes in the spacer. He put the screws in the spacer, assembled the pole piece and inserted it into the microscope. Immediately he saw changes in the contours of the image that told him he had the cure for anisotropic astigmatism. In a few subsequent adjustments he had a corrected pole piece. In one of the most important papers in all of electron microscope literature (Hillier and Ramberg, 1947) describe the theory and practice of stigmation. Hillier called the process “compensation” after the process of correction with shims. The term prevailed for about fifteen years, until the influx of foreign instruments brought in the term stigmation which relates to the change of a lens property rather than a magnetic field property. Surprisingly, it took many months before the new compensable pole piece was put into production and made available in the field. The delay was not because of technical difficuties. For a variety of reasons, now forgotten, P. C. Smith, Manager of Electron Microscope Engineering, rejected Hillier’s invention and would not adopt the compensable pole pieces. Picard and Reisner had a spacer modified and found it to work well. They made a recommendation to Smith to adopt it for current production, but Smith was still adamant. The event that turned Smith around was itself a surprise. With the success of his first compensated pole piece Hillier modified several more pole pieces with which he had been experimenting with the same excellent results. He took one of these to Bethesda, Maryland. where R. W. G. Wyckoff had a laboratory at the NIH, and demonstrated the compensated pole piece to him. Wyckoff was at that time a consultant to RCA on electron microscopy and had a pole piece specially chosen from factory production for its quality of performance. The compensated pole piece was obviously the better of the two. Wyckoff was convinced and pursuaded Smith to adopt the idea. It soon went into production, and shortly after that, the guaranteed resolving power of

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the EMU was changed to 30 Angstroms. It proved to be a great benefit to manufacturing. Factory personnel became very adept at correcting lenses. It was much easier to adjust the shim screws to correct astigmatism than it had been to lap polepieces, and there were very few rejected. Even revolutionary ideas have drawbacks. It became evident that the shims did not provide a correction completely independent of lens current. At high lens current the required correction was a little different from that at low current. A lens corrected at one lens current would show slight astigmatism at another current. More annoying was the fact that cycling the current to a high value and returning it to the original focus current would produce an assymetry in the starting field, resulting in the appearance of some astigmatism. In a single voltage instrument such as the EMU this did not turn out to be a serious problem because the objective current was never changed by large amounts. Hillier showed that turning the objective lens current on and off a few times would produce a standard configuration for the field and the compensation would be correct. The importance of these hysteresis and saturation effects was that they were a warning that on later microscopes it would be necessary to provide externally adjustable stigmation for the objective lens. In 1952 Reisner developed an electrostatic stigmator that was controlled by analog potentiometers separating the control of the strength and azimuth of the stigmation. Two sets of potentiometers were used to provide correction of the objective at both 50 and 100 kV. They were switched automatically with the high voltage. The electrostatic system was chosen because it was simpler and smaller, and with a relatively long 2.5 mm objective focal length, the specimen holder was shielded from the electrostatic field by the pole piece. Although stigmation provided a good solution for the problem of anisotropic astigmatism, a program to improve the iron continued at a low level of activity for many years. S . G. Ellis attempted to fabricate pole pieces by electrodeposition without success. For several years the metallurgical division of Westinghouse supplied a variety of pure irons and alloys without coming up with anything significantly better than the forged Armco iron. 8. The E M T Tuble Model Microscope

A secondary but very persistent theme of electron microscope history has been the dream of achieving a small, inexpensive microscope with performance close to that of the large instruments. This dream was behind the development of the console microscope and the portable microscope. From the beginning of these developments Hillier and his associates seriously entertained the thought of using permanent magnets to energize the magnetic lenses. Alnico V was available to provide adequate field strength. E. G.

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Ramberg had filed a patent (Ramberg, 1943) on a pair of axial cylindrical magnets in bucking relation so that the ends of the assembly were at the same magnetic potential. This provided two highly energized gaps along the axis. At the same time an external cylindrical shield was used to join the two magnet ends at equal magnetic potential. The assembly had negligible stray magnetic field. Magnets were acquired, but under the pressure of other developments, they were never used. Ultimately they were sent to Camden. In the spring of 1946, Reisner undertook a study of permanent magnetic lenses, using the optical bench which he and Picard had recently completed. It was ideally suited to such a task, and in a relatively short time, Reisner had examined several different magnet configurations and had established design criteria and performance figures. He filed a patent application on these configurations in October, 1946, for which a patent was granted in 1950. The good results with permanent magnet lenses aroused renewed interest by the marketing department in a small microscope, with the result that Reisner was not permitted to publish a paper on his results until five years later in 1951 after a permanent magnet microscope was marketed by RCA (Reisner 1951). The team that designed the EMT had Reisner as the project leader, Dornfeld as the mechanical engineer and Zollers as the electrical engineer, and, once it had a prototype going, Stewart Pike joined the group as an industrial designer. While the EMT was small, its electrical supplies were not. The 50 k V rectifier unit was taken from the EMU and housed in a metal box that usually sat on the floor behind the table on which the microscope column was sitting. Another metal cabinet housed the driver and regulator of the rectifier unit and had the four electrical controls for the system on its front panel. This driver/regulator unit sat on the table beside the microscope column. The electron gun used was originally designed for the EMC and had proven itself at 50 kV on the EMD diffraction camera. The column, however, was all new. A pole piece assembly had the projector lens in one end, energized by the lower gap of the permanent magnets, and the conical lower pole of the objective was at the other end. The upper half of the objective lens had a flat face and was mounted on the flat upper face of the lens spool. It was centerable from outside the column, and provided, with the gun traverse, the means for optical alignment. The specimen was inserted by a rod-type specimen holder which was also a stage. The translation of the rod provided one specimen movement, and rotating the rod provided a motion perpendicular to that. The specimen could be changed without turning off the beam. Pictures were taken on a standard 5 x 5 cm glass plate. The plate was housed in a light-tight cassette which could be loaded, pumped, and exposed by actuating a single lever that moved a combination valve and viewing screen (Reisner and Dornfeld, 1950). In 1951, Dornfeld designed a 35 mm film camera that would take 30 exposures per loading. The appearance of the table model microscope was unique

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FIG.22. Table model microscope EMT. The high voltage supply and mechanical pump are not shown. The spun copper sphere safely enclosed the gun and cable terminations. J. Reisner is shown changing a photo cassette.

(Fig. 22). This was primarily the work of W. S. Pike who later did the styling and industrial design of the EMU-3 microscope. The tilted-back column, something which the large instruments can not do, made the instrument easy and comfortable to view. The copper sphere that enclosed the gun was made from two spun copper hemispheres, soldered together at an equitorial flange. The flange was ground off on the first prototype and the copper sphere made the instrument look top heavy. It was shown to Pike, who pointed out that he wanted the flanges of thin copper visible so that people would instinctively know that the big sphere was not solid copper. The instrument was designed and tested in 1948 and 1949 and introduced for sale in 1950. It had a resolving power of 100 Angstroms and worked at either 3000 or 6000 times magnification, depending on which of two objective pole pieces was used. The EMT had a good reliability record. It served well in industrial quality control, and in some educational situations. It cost about

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407:) of the price of the EMU microscope. 100 of the EMT microscopes were sold between 1950 and 1954, but like the EMC, it could not compete against the EMU. Its lower cost could not offset the EMU’S superiority in performance. The engineering department proposed and demonstrated a version of the EMT that had a variable magnification range from 1500 to 20,000 and a condenser lens; but after two attempts to launch simple low-cost electron microscopes with only marginal success, the sales department was no longer interested. The simple instrument was a specialized type of product for which there was limited interest in a then still new and relatively small market for electron microscopes. 9. 100 K V Electron Microscope Prototype

By 1948, experience had taught RCA enough about the very successful EMU microscopes to indicate that it should begin planning for a successor to them. Additionally, for the first time serious foreign competition was appearing from several directions. In the summer of that year, J. Hillier and S. G. Elliis wrote a report to management on the subject of new or improved instruments. Two interesting comments are quoted from the report as follows: The RCA Model EMU electron microscope is, in our opinion, still basically the best electron microscope being offered commercially. It is however, deficient in a large number of ways which are trivial individually but taken as a whole, provide good ammunition for our competitors. In addition to this, competitive instruments now include a number of features which make good selling points even though in the final analysis their value is found to be little, if any. In the report they discuss features of competitive instruments: The Use of 100 k V Accelerating potentials-It has been, and still is, our opinion that accelerating potentials of 50 kV are satisfactory for over ninety percent of the known applications of the electron microscope and that the remaining applications are not of sufficient importance to justify the extra cost of supplying 100kV. It does not seem like good business to saddle the majority of the users with unnecessary extra cost. It appears more logical to us-to have two models, particularly since this would involve-extra design work for only the gun and the higher voltage supply. The report went on to suggest adding a double condenser and sticking to the diffraction stage rather than adopting selected area diffraction. The report

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was quite comprehensive and was helpful in creating impetus to make changes on the EMU. The marketing department accepted the recommendation to continue updating the EMU, but felt very strongly that it would be desirable, from a marketing standpoint, to have 100 kV operation as soon as possible. In due time, Engineering was authorized to start the design of a 100 kV microscope. The design team was the same group that was designing the EMT table model microscope augmented by H. E. Reeber and C. Felheimer, both experienced mechanical engineers, and A. A. Litwak a laboratory technician. By 1948, the drafting department was experienced in microscope design and was an important factor in the ability to conduct the two design programs simultaneously. J. H. Reisner was the project engineer. R. G. Picard was close to the design project but he was leading work with radiation detectors and did not become personally involved in detailed design. P. C. Smith was still the manager of the Electron Microscope Engineering which had two years earlier been expanded to include other scientific instruments, such as radiation meters, vacuum evaporation units, leak detectors, etc. The expanded operation was called the Scientific Instruments Department. Smith, who became the manager in 1942, had lived through the exasperation of servicing the EMB’s oil-filled high voltage supply. The oil was not the only problem. The location of the supply at the top of the instrument had greatly aggravated the service problems because of the difficulty of access to it. Smith swore that he would never again build another oil supply, and the good record of the air-insulated EMU high voltage supply was taken as proof that he was right. Zollers and Reisner were predisposed toward the oilinsulated supply but thought that a small enough supply might be feasible and conscientiously undertook the design of an air-insulated 100kV supply. It, like its predecessors, was a radio frequency supply, and used a quadrupler rectifier system. It was essentially two concentric 50 kV supplies, so that the components at 100 kV would not “see” ground, thus enabling the design to use smaller clearances. This minimized the overall size of the supply. However it was large, much too large to be acceptable, and too expensive. After more than its share of troubles, it did work adequately and provided the high voltage necessary to permit the testing of electron guns and lenses for use at 100 kV. Had the insulating gas sulfur hexafluoride been available at that time the story might have been different. Realizing fairly early in the project that the so-called air-insulated supply was probably not going to be suitable as a product, Reisner, Zollers and Picard agreed to bring along in parallel an oil-insulated supply. Knowing that Smith would probably not approve such a project at that date, it was drawn up in secret and parts were accumulated to build a model. When assembly was started, it was no longer possible to keep the secret. Smith was understandably

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irate, and there were some tense moments. Fortunately the new supply worked well from the start, and, to Smith’s credit, it must be said that he was objective about the matter and accepted the oil-insulated supply as the better choice. The 100 kV oil-insulated supply was about fifty percent larger in volume than the 50 k V air-insulated supply of the EMU. It might have been smaller except for the decision to use commercially available components such as x-ray rectifiers, transformers and capacitors. Only the high Q oscillator coil and the filament coupling coils were made by RCA. A doubler circuit was used and the oscillator frequency was 17 kHz. This supply with minor modifications became the high voltage supply for the entire sequence of EMU-3 and EMU-4 microscopes. The microscope design that was authorized in 1948was termed the EMU3. It was essentially a bigger and better EMU-2, designed to work with the new 100 k V power supply. The gun still used the stand-off type of insulator that was used on the EMU-1, -2 at 50 kV. Having a 100 k V electrode at the end of the gun meant that there must be a protective housing around it. Stewart Pike came up with a metal cover that looked like an ash can (Fig. 23). The EMU-3 was completed and in operation the summer of 1950, and Reisner presented a paper on it at the EMSA meeting that year in Detroit. (At the same meeting he also presented a paper on the EMT, for which a design program had been running concurrently.) Only two EMU-3 microscopes were built. One stayed in Camden for research and study. The other was installed at Wyckoff’s laboratory at the NIH in Bathesda. The EMU-3 was extremely useful in learning about 100 kV operation and in providing valuable design information. However, it satisfied no one at RCA. It was not bold enough or creative enough. The model in Camden was later dismantled for parts. C . The Era of Competition- After 1950 1. The Tenth Anniversary of the E M B

1950 marked the tenth anniversary of the first EMB electron microscope. During that time RCA had sent into the field 58 EMBs, about 180 EMU-1, -2s, and about 50 EMCs, a total of about 290 microscopes. They were literally state-of-the-art, because they were all that existed. With so many instruments in the field, the art of specimen preparation expanded explosively, and in so doing, many demands were made on the manufacturer RCA. The first demands were for reliability, and as reliability improved the demand was for better performance and resolving power. As performance improved the demand was for more speed and versatility, and finally, ease and comfort of operation. The early microscopists, 1941- 1944, begged for any instrument that could get them into a sub-micron region of visibility, and they would put

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FIG.23. The microscope that was the developmental link between the EMU-I and EMU-2 series of microscopes and the EMU-3A to EMU-3H series. The air-insulated 100 kV supply may be seen at the rear. The large sheet-metal cylinder at the top of the column is the shield around the gun and cable termination.

up with major inconveniences with those instruments just to get there. By 1950, microscopists had become more sophisticated, and the sense of partnership between user and manufacturer, that had been necessary to keep the early instruments running, was seriously diluted. The microscope had become just another commercial product. The technology important to the electron microscope had also changed. Nowhere was this more evident than in the realm of high vacuum. O-rings of oil-resistant elastomers had replaced cut flat rubber gaskets. RCA could buy diffusion pumps instead of making them as it did for the EMBs. Diffusion pump oils were far more durable than they had been a decade earlier. Vacuum hardware such as valves and bellows had become available in a wide variety

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and were much improved in reliability. Helium leak locators had become factory test equipment to assure vacuum tightness of the elements of the microscope vacuum system. What was true of vacuum technology was true also of electronic components. There were brighter phosphors, a byproduct of television, and new materials, especially plastics. Some of these technological achievements, such as a brighter phosphor, could be quickly adapted to improve existing products; but many, such as a diffusion pump, could only be incorporated with very major changes in design. The obsolescence of its components can discredit a product as fast as the obsolescence of its basic design, particularly if it is manufactured over a number of years. This is what ultimately happened to the EMU-1, and -2 series of microscopes manufactured from 1944 to 1952. Although the electron microscope in 1950 was reasonably reliable and performed quite well, everyone knew that it had a very long way to go in its logical development. For several years there had been a general feeling among users and customers that the lack of competition was an unfortunate situation, and they looked forward to the end of the virtual monopoly that RCA had held since the first EMB was produced in 1940. There is ample evidence that RCA acted responsibly, given its dominance in the field. The changeover of production from the EMB to the EMU in 1944, during the middle of the war, was a creative exercise of responsibility. There was no competitive challenge to meet, except perhaps the desire to support fully the war effort. During this period there was very little sales activity, users got their instruments as a matter of War Production Board priorities, rather than by any activity of an RCA salesman. This meant that the product philosophy that dominated the microscope enterprise was that of scientists and engineers. They were productcentered and their basic impulse was to improve the product. At the conclusion of the war, the product-centered attitude carried over, and improvements to the microscopes found their way into the factory and field. However, as peace time sales began to be more difficult, commercial factors began to influence the operation of the microscope business strongly. By 1950, competition had materialized in the American market in the form of the Philips EM 100, made in the Netherlands; and it was common knowledge that Siemens was planning to introduce a new instrument in the not too distant future. There were numerous rumors of commercial activity in the production of electron microscopes in the U.K., France, Switzerland and Japan. There was no competition from manufacturers in the United States. GE and Farrand had withdrawn from the field during 1951. The thrust of American technology was into solid state, nuclear physics, television, etc., and any interest in pursuing electron microscope production had passed. For Europe, recovering from the ravages of the war, the microscope was a welcome step on the way to industrial recovery. In the long run, technical competition

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was a salutory influence for everyone including RCA, for it is questionable how long RCA would have continued to develop the microscope without the challenge of competition. Changes were taking place at the same time in the personnel of the engineering department at Camden. M. C. Banca left the sales department in 1948. P. C. Smith was transferred in 1949 to a market research position in a different product line. Picard succeeded him as manager of the Scientific Instruments Engineering Section, which responsibility he held until he resigned in 1957 to take a position at the Central Scientific Corporation in Chicago. Reisner was made Group Leader for the Electron Microscopy Group in 1950, a position he held for twenty years until 1970 when he moved to the RCA Laboratories in Princeton after RCA divested itself of the electron microscope business. Thus RCA’s second decade of building microscopes started out with an experienced leadership and technical staff. At the same time Hillier, at the Research Laboratories, was supporting Camden with a steady flow of information and new developments. He felt that too many instruments in the field were not being used effectively and associated himself with several biological and medical laboratories in a consultative position in order to learn at first hand what the field problems were. During the several years he did the consulting he was effectively doing two jobs, which required great stamina and energy and ultimately was an influence in his leaving research for technical administration in 1954. It was during this period that he realized that the biological work, which was so difficult in the early days because of specimen preparation difficulties, would become by far the most active area of electron microscopy. He also investigated ways in which microscopes might be designed to improve the performance of biological electron microscopy. For example, he systematically studied the positioning of apertures to reduce wall scattering and improve image contrast. August of 1952 was an important date for RCA’s electron microscope activity, because it marks the setting up of policies that resulted in the manufacturing of the EMU-3A thru-3H series of microscopes ultimately amounting to six-hundred instruments. A planning meeting was held in Camden including Zworykin, Hillier, and Halma from Research, Picard and Reisner from Engineering, and representatives from Sales, Service, Manufacturing and Management, all of whom had a stake in what the design of a new electron microscope would be. The writer has reports on the meeting written by Hillier and by Picard which give some idea of the status of electron microscopes in 1952. Hillier moderated the meeting, and in his opening remarks, cited four basic points which he felt were fundamental to any high quality instrument: [11 30 Angstrom resolution; [2] Insensitivity to vibration; [3] Insensitivity to magnetic fields; and [4] Good instruction books. One

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should note that the last three points dealt with field problems. High voltage of 100 kV was not a question, but the number of voltage steps was. Magnification range was to be 1,OOO to 20,000 times. A large direct-view viewing screen was to be used with a plate camera below it and a 35 mm film camera above it. Externally adjustable apertures were to be available in the condenser and objective lenses, and the objective aperture was to be located at the rear focal plane of the lens. No decision was made about how to stigmate the lens. There is no record of the electrostatic stigmator being mentioned. Styling of the instrument came up for serious discussion. Industrial design, of which styling is a facet, was taken very seriously at RCA. It was more than achieving good looks. It was a systems concept in which the structure accommodates the components in the optimum way for function and for cost. The human operator is one of the components and must be accommodated in such a way as to assure the operator’s maximum efficiency and comfort. The appearance comes from good systems design and honest expression of its function by its exterior. Everyone agreed that the industrial design group should be called in as soon as possible including RCA’s highly regarded consultant John Vassos. However, the meeting posed the restrictions that the column should be vertical, the table considerably larger than that on the EMU-2, the panels moved to the back of the table, and all controls should be on the panels. Zworykin did not fail to remind everyone that he wanted the entire microscope, pumps and high voltage supply included, in a single cabinet. 2. A Definitiue Electron Microscope The first of the EMU-3A electron microscopes left the factory in the spring of 1954, a year and a half after the specification planning session of August, 1952. Such a short time cycle was possible because the experimental 100 kV microscope, which had been authorized in 1948 and was first operative in 1950, had been available for two years for study and testing. It is an interesting exercise to trace the evolution of the models in an individual microscope family. Figure 24 shows the RCA microscope family tree. There are however several missing links which were short-lived and are now extinct. They were intermediate steps between the models that did survive. Between the EMB and the EMU there was a model F (Fig. 14). Between the EMB and the EMC there was the small microscope shown in Fig. 17. Between the EMU and the EMU-3 and the EML there was the experimental microscope shown in Fig. 23. There was also a link between the EMC and the EMT that is memorialized by only a few sketches. These transitional links are the most interesting part of the evolutionary process. They are usually highly experimental systems embodying creative changes in

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FIG.24. Paths of development through which the several commercial models of RCA electron microscopes evolved.

design, while the models which survived as a product represent an engineering consolidation of the useful features generated in the link. The new microscope was designed by essentially the same team that had built and worked with the experimental microscope. Very little had to be done with the electrical system to adapt it to the new design other than fit parts on different size chassis. An oil-insulated high voltage supply and a metalsheathed gun were designed and ready. However, there were three areas where major design changes were made from the experimental prototype-the column, the automation, and the styling. The column of the experimental EMU-3 microscope had been provided with a double condenser lens system, as was the EMD diffraction camera. As late as 1953 the group saw no advantage in its use for microscopy commensurate with its cost, and it was not carried over into the EMU-3A.

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From the first of the EMU-1 microscopes, all the large RCA microscopes had been built with an intermediate section, between the objective and projector lenses. In addition to providing a support structure for the objective lens spool, it was a means of access to the objective and projector pole pieces. The intermediate section provided an opening in front through which the bottom of the objective spool and the top of the projector spool could be reached for insertion and removal of the pole pieces. During operation the space between the spools was sealed off by an expandable bellows and a cylindrical spacer. This convenient access to the pole pieces made cleaning easy and changing objective apertures simple. This idea was carried over to the EMU-3A, but instead of an intermediate section an aluminum barrel was used to support, center, and align the condenser lens with the projector. In addition, a shelf in the barrel supported the objective lens so that its axis was parallel to the condenser-projector axis. At the same time an opening in the wall of the barrel provided access to the intermediate area of the column. On the EMU-3A a thin, spring-loaded, vacuum-tight, pressure plate, on the bottom of the objective spool sealed off the intermediate section when a smaller diameter intermediate lens was slid into the intermediate opening. It was intended at the time of design that the opening would also be used to insert a versatile diffraction stage similar to that used in the E M D diffraction camera. The stage was made available as an accessory in 1957. The barrel was an excellent idea in many ways. It was a casting and was machined to align accurately the optical elements. The only adjustments possible were the objective and intermediate traverses. It was securely bolted to the viewing chamber, and it made the column highly immune to vibration and shock. It was covered with a skin of mu-metal for attenuation of external magnetic fields. The barrel was originally cast aluminum to keep it light and to avoid magnetic interaction of the lenses, both of which were useless precautions. In 1957, the material was changed to cast iron to improve shielding. In order to simplify the operation of the microscope the decision was made to automate the two major mechanical systems, the vacuum valving and the photo plate loading and positioning systems. The vacuum system had motordriven valves so there would be no sudden changes in pressure to destroy specimens when valves opened, such as sometimes occurred on the EMU-2 microscopes. The user was provided three push buttons on the panel, load, operate and neutral. Vacuum gauges controlled the switching of the fore pump between roughing the column, and backing the diffusion pump. In the neutral position all valves were closed and the pumps were turned off. The photo plate system was entirely a mechanical device driven by an electric motor. The photo plates were held in light-tight cassettes for daylight loading in the microscope. They could accommodate three 34 x 4 inch plates or one 2 x 10

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inch plate. The 10 inch plate was a carry-over from the EMU-1 and -2 instruments. It provided five 2 x 2 exposures per plate and was a kind of “de facto” standard for the RCA microscopes. The 3$ x 4plates were an answer to biological microscopists who wanted wider fields. Once the plate cassette was fed into its carriage in the microscope and the photochamber door was closed, the automatic cassette drive would position a plate for exposure, and after each exposure, would automatically move the plates into position for the next exposure, taking into account the size of the plates with which it was dealing. With the EMU-3A, Zworykin got his wish for an electron microscope which was a single unit. Even the mechanical fore pump was inside the cabinet. The form of the EMU-3A was generic. As one reviews the appearance of the many models of the electron microscope that have been built world wide, one is struck by how frequently the appearance bears a strong similarity to the EMU-3A (Fig. 25). Stewart Pike broke the microscope’s functions down into

FIG.25. The EMU-3 (A through F) series all looked like the EMU-3D shown above. A total of 600 EMU-3 series were manufactured.

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four categories: [11 high voltage; [2] low voltage-including control circuits for the HV; [3] vacuum apparatus; [4] electron optical column. To this he added a fifth, the human operator. The focus of the whole system was the viewing screen, whose height from the floor was determined by the ability and comfort of the operator to view it while sitting in a normal position. The column was tall and heavy and needed radial support, which was supplied by two radially disposed low and shallow racks housing the equipments in categories 2 and 3. The high voltage tank was placed radially at the back of the assembly. It was on wheels for easy separation from the rest of the instrument for servicing and did not participate in the column support function. The early commercial electron microscopes made in Europe looked more massive than the RCA instruments of the same period. The difference stems from the methods of cabinet construction used on opposite sides of the Atlantic. The European countries were far ahead of the U.S. in the practice of casting large extended thin metal surfaces and shapes. It was common practice in Europe to use such castings to make cabinets for electrical equipment. Cabinets made of cast sections are usually characterized by large rounded corners and edges and a massive look. On the other hand, cabinets made in the United States were made of thin sheet steel welded to steel frames. Corners and edges are relatively sharp and the cabinets look lighter, boxier, and also more precise. The EMU-3A provided an opportunity to design what are sometimes called “frills”. These are adjuncts to the basic instruments which are not essential to its operation but which are used to make the operator’s functions easier and more effective. Automation is often considered to be a frill, especially when it fails to work properly. The EMU-3A was equipped with a photometer which measured the light generated by the viewing screen. It had an exposure timer that operated a magnetic shutter. However, the most exotic feature was the back-lighted panels which came from RCA’s Defense Electronics Division. Back-lighted panels were developed during the war for military electronic equipment that had to be operated in the dark. They were certainly nice devices. The sales department gave them the poetic name “Glow-Dark Panels”. Even before the design of the EMU-3A was started in earnest, it was obvious that the new microscope was going to cost a lot more than the EMU-2F that it was to replace. When the EMU-3A did reach the market its price was about $30,000 compared to the approximately $20,000 for the EMU-2F. The sales department was very apprehensive about their ability to sell a microscope at the higher price in spite of its obvious advantages and capability. They asked engineering to design concurrently a simpler 50 kV version of the EMU-3A. It would use the same cabinet and vacuum system as the EMU-3A, but would have a new 50 kV high voltage supply and use the EMD diffraction camera’s electron gun. Being a single voltage instrument, it would need no externally

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adjustable stigmator and would have no frills such as a magnetic shutter, exposure meter, automatic timer, film camera and numerous other attractive accessories. Its sale price turned out to be about $20,000 as desired. Originally, the Sales Department planned to order equal numbers of EMLs and EMU3As but at the last moment they became frightened and changed the order to 30 EMLs and only 13 EMU-3As. These instruments became available in 1954. The EMU-3A was immediately in demand, and the EML drew little response. It took a long time to sell out and was discontinued after the first order was completed. The EMU-3A suffered from the usual problems of any radically new design. It is one thing to do laboratory testing of a single prototype, trying to anticipate possible problems; and it is quite another thing to have the production instruments undergo the testing imposed by customer use in the field. The latter is far more thorough, and frequently, what it turns up is very unexpected. The EMU-3A had three problem areas, the automatic mechanisms that operated the vacuum system and the photoplate transport, the overheating of the high voltage tank, and the occasional breakage of the gun insulator. In the early days of the microscope the simpler construction of the instrument permitted the users to service their instruments for most problems. As the instruments became more complex and sophisticated, trained technicians were required to deal with problems such as the three cited above. The valving and plate drive mechanisms used cams to control plate and valve positions. With time, the cams would work out of position and cause the mechanisms to jam. The geared-down drive motors produced so much torque that they would bend their mountings when the mechanism jammed. Repair by the user was not possible. The solution was to pin the cams to the shaft. The presence of a field service engineer who could make the needed changes was essential to solving the problem. The overheating high voltage oil tank was caused by using steel for the tank instead of aluminum or copper. A conducting enclosure for an inductor acts as a constant current shorting turn. The high voltage generating coil was the inductor. Given a constant current, it was important to minimize ohmic heating by using a low resistance material for the tank walls. The problem was solved by coating the inside of the steel tank with copper and adding cooling coils in the oil. The gun problem persisted for several years and its was common practice for field service people to carry a spare electron gun in the trunk of their car for quick gun replacement. The EMU-3 series went through eight shop orders ending with the model EMU-3H in 1964 and involving a total of 600 instruments. Each new model was significantly improved over its predecessor. Only one major cabinet change was involved. This was for the EMU-3G when the cabinet was mod-

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ified to provide more panel space, which was brought about by the increase in functions as the microscopes were improved. Very few changes were made during a production run. Such changes are inordinately expensive and cause standardization difficulties. The EMU-3 microscopes were never written about in the scientific press as were the earlier microscopes, but they were very well documented in detailed instruction books and advertising literature. There were reams of factory manufacturing and test procedures, not to mention volumnous field service notes. Many, probably most, of these materials have disappeared. With the merger between RCA and G E in 1986, the electron microscope field service was terminated. Jack Vees, who was supervisory field engineer for electron microscopes for the RCA Service Company for the period from 1966 to 1986, estimates that in 1987 two thirds of the EMU-3 series microscopes are still in use-twenty to thirty years after their manufacture. That is the ultimate criterion of success.

VII. THEFARRAND OPTICALCOMPANY’S MICROSCOPE A . The Start of u Program

C. L. Farrand first appeared on the electron microscope scene at the first national meeting of the EMSA held at Columbia University in January, 1944. At about this same time he put two men to work on an electron microscope development program in his firm which bore his name and of which he was the head. He next appeared on the scene as a member of one of the government sponsored investigative teams sent to Germany during and after the collapse of Germany at the end of World War 11 in mid 1945. The teams were made up of civilians who were recognized scientists and engineers, and their purpose was to ascertain the status of German science and industry at the war’s end. The team on optics included R. Bowling Barnes of American Cyanamid, M. C. Banca of RCA and C. L. Farrand of Farrand Optical. For reasons of security the team members were given the temporary rank of colonel without authority. They referred to themselves as “capon colonels.” They brought back useful information about the state of electron microscopy in Germany at the end of the war. The author recalls that they were surprised at the progress that had been made under the stressful war conditions in Germany. They were not supposed to bring back equipment, but Barnes did liberate an electrostatic lens from an AEG electron microscope, which he showed at a lecture, and later circulated among interested parties. The author had it in his possession for a short period of time for study. Another factor that raised Farrand’s interest

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in electron microscopy was his contact with Reinhold Rudenberg, who had emigrated from Germany and was a professor of electrical engineering at Harvard University. He had been awarded United States patents on the electron microscope before the war. However, the Alien Property Custodian had taken over Rudenberg’s patents during the war. Following the end of hostilities Farrand helped to get the patent rights restored to Rudenberg for a share of the royalties to be paid on the patent. Farrand also retained Rudenberg’s services as a consultant.

B. An Experimental Microscope is Successful Early in 1945,Dr. Gertrude Fleming Rempfer and her husband Dr. Robert Rempfer joined the staff of the Farrand Optical Company. Gertrude Rempfer had demonstrated her ability as a physicist at the University of Washington at Seattle where she had done work in field and thermionic emission. Robert Rempfer was a mathematician. The opportunity to work on the microscope came about because the two men who had been put to work on the project earlier were transferred to defense-related projects to protect them from the draft. By the time the Rempfer’sjoined the project, an electron optical bench had been constructed. They used this for testing lens design and carried out a study of unipotential lenses, With the lens data obtained they designed an electron microscope with a vertical column. This was the experimental microscope that ultimately was modified to the point where it gave the excellent results shown to the public (Fleming et al., 1948). Little information on the technical aspects of the microscope development at Farrand has ever been released, although it was obvious from the quality of the micrographs shown publically in 1947 that they had been taken on a microscope of excellent quality. The secrecy restriction imposed by Farrand may well have been for competitive reasons because the experimental microscope embodied a number of novel ideas that he wished to use on the projected commercial model he hoped to market. Gertrude Rempfer recently wrote a short note about the experimental microscope at Farrand which is quoted here with permission.

’,

We used an extension of the Spangenberg and Field two-grating method for studying the lenses. Besides studying a number of special lenses, we carried through a systematic investigation of unipotential lenses with a wide range of geometrical and voltage parameters. During these tests

’ Private communication from Gertrude F. Rempfer to John H. Reisner, July 12, 1987.

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we were joined by a bright young physicist, Edith Asherman. From the optical bench data on focal lengths and focal distances and their aberrations, we selected optimum parameters for the objective lens (minimum Cs for the given accelerating voltage), and for the projector lens (zero distortion at full magnification). The conditions for the condenser and intermediate lenses were less stringent. Besides the regular machine shop, there was an excellent model shop at Farrand’s, in which our lenses were made. In addition, a machinist from the regular shop, Louis d’Alessandro, was assigned to help us put together a laboratory system with a vertical column to test the lens system at magnifications higher than could be used in the optical bench. This laboratory system became our research electron microscope. We used a triode gun with adjustable positioning of the filament and an adjustable bias voltage. A condenser stop was used but not an objective stop. The accelerating voltage was 30 to 40 kV. To begin with, the maximum electronic magnification in the laboratory microscope was about 10,000 (later, about 18,000).At this magnification, spurious disturbances due to vibrations and magnetic fields were diagnosed and dealt with, disclosing the presence of astigmatism. The roundness of the center electrode aperture proved to be too sensitive a parameter to adjust in correcting astigmatism. Instead we chose a less critical dimension, the spacing between electrodes. Our first attempt at correcting astigmatism involved melting two small spots of solder onto the inner face of the rear electrode, in opposite azimuths near the aperture, and filing them down to about .001”. The solder spots were oriented in the azimuth of the minimum focal distance. The correction turned out to be close to perfect, and dramatically improved the image quality. In December 1947, we presented some of our micrographs in a report to the Philadelphia meeting of EMSA. Farrand did not allow us to disclose any details about the construction of the instrument. This was a disappointment to us, but the micrographs were enthusiastically received. One of the micrographs appeared on the cover of the Journal of Applied Physics for January, 1949. During the following 8 months we added a beam wobbler with a square-wave voltage as an aid to focussing, and a field stop in front of the intermediate lens for diffraction studies. These components along with a pair of deflection plates just below the intermediate lens also made it possible to operate the microscope in an on-screen stereo mode. The square-wave voltage was synchronously applied to the wobbler plates and the deflection plates to produce two separated stereo images on the final viewing screen. With improvements in alignment and astigmatism correction, a resolution of about 15 A was attained during this period.

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The period referred to ended in the summer of 1948. A remarkable and successful instrument had been achieved, and it was never described in the scientific literature. C. The Product Design Ends in Failure

The steps from an experimental instrument to a prototype to a production model are notoriously difficult. It requires what is called the transfer of technology, which is almost invariably a transaction complicated by the pride and prejudices of the people involved. Farrand was emotionally involved with the electron microscope project. It was his favorite undertaking. He had been greatly impressed by the Philips EM100, with its nearly horizontal column and its transmission viewing screen, and he decided that such a format should be used on the Farrand microscope. The Rempfers did not favor that format, but more importantly, they did not feel that the design would be well served to simply lay the experimental microscope, which had been designed for vertical operation, on its back and attach a transmission screen to its end. However, Farrand directed that this should be done, and the performance of the column in this position should be investigated. Experiments with the horizontal format continued for about six months, but the best performance achieved was never better than 100 A. These experiments went on for about six months to the end of 1948.They were repeated a second time in 1949,for a period of three months. In between the interruptions to try the horizontal format, the Rempfers’ continued to work on what they hoped would lead to a commercial electron microscope. Their efforts were to no avail. Gertrude Rempfer recalls this to have been a “demoralizing experience.” Farrand was adamant on the matter of the horizontal column and transmission viewing screen. He personally took over the design of an instrument of this type about the first of 1950 and chose R. G. Alexander to assist him. The design never worked, primarily because they made drastic departures from the electrode and insulator design of the experimental microscope, with the inevitable result that the design had serious voltage breakdown problems. At one point in his design project Farrand became optimistic enough to release advertising material which showed a picture of the instrument (Fig. 26) and included micrographs taken on the Rempfers’ original experimental microscope. Unfortunately, the design failed and nothing further was heard from Farrand on the electron microscope. The Rudenberg name usually arouses interest. His involvement with the electron microscope was almost entirely theoretical. In his position as consultant to Farrand he had some influence in the beginning but this dissipated as the work got into the experimental realm. The Rempfers were somewhat

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FIG.26. Artist’s drawing of the Farrand electrostatic electron microscope as it appeared on an advertising sheet released to the public about 1950.

obligated to devote some time to investigating his ideas. Gertrude Rempfer recounts one of these investigations with an unexpected result: Rudenberg had the idea that the hyperbolic potential field was a perfect lens field, without aberrations, because the radial component of the electric field was proportional to the distance from the axis. We were unable to convince him that the time during which the force acts is also important in the deflection of the electrons, and that the converging hyperbolic field was uncorrected like any ordinary lens field. Later, our analysis of the overcorrected properties of a diverging hyperbolic field led to a patent on a method of correcting spherical aberration in electron lenses. At the close of World War I1 several microscopes in Germany were brought to the United States. This included both Siemens (magnetic)and AEG

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(electrostatic) instruments. The Naval Research Laboratory in Washington, D.C. took possession of an AEG which Bruche had assembled from parts he had collected, mostly from the Kaiser Wilhelm Laboratory. The instrument was not in working shape when it left Germany. Early in 1947 Gertrude Rempfer was asked to put it in operating condition. There were a number of missing and broken parts which had to be replaced or repaired in the Farrand factory in N.Y., while the assembling and testing had to be done at the NRL in Washington. It took Gertrude Rempfer aided by two co-workers two months to get it going properly. Compared with her experimental microscope the restored AEG did not show outstanding performance. There was no astigmatism correction and the projection lens was uncorrected for distortion. Only three magnifications were available. The Farrand Electron Microscope experience is now largely forgotten. Like all other history, the history of science is full of forks in the road, and in the Farrand story after a series of fortunate decisions, the wrong turn was made when a non-scientific choice of styling was made over a scientific recommendation that precluded such styling. Gertrude Rempfer sums up her feelings philosophically: In spite of the frustration which we felt at times, and the disappointment of not being able to publish, or to see a successful commercial instrument materialize, for me personally this was still a very productive period. I had the opportunity of delving deeply into both the theoretical and experimental aspects of electron optics. What I learned has been a valuable asset ever since. I acquired a number of patents, including one dealing with aberration correction with a foil window. And I became acquainted with a number of wonderful people whose friendship I still treasure. Robert Rempfer left the Farrand Optical Co. in 1950, and Gertrude left in 1951. Gertrude Rempfer never lost her interest in the design of electron microscopes. She went on to design a state-of-the-art electrostatic transmission microscope which was completed in 1971.It was built by Elektros, Inc. (Rempfer, 1972). VIII. MARTONBUILDSHIS FIFTHMICROSCOPE Ladislaus Marton was one of the great figures in the development of the Electron Microscopy. In twenty-two years of active participation in the laboratory, he built five microscopes, each of which represented the state-of-

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the-art at the time it was put in service. He had what one could call “scientific courage” combined with great persistence. When he came to RCA in 1938 to design a commercial electron microscope, he brought his third instrument with him for a starting point. (An account of his two years at RCA is given in Section VI.A.2) While there, he built his fourth microscope, called the EMA. In the fall of 1940, he left RCA for a short sojourn at the University of Michigan, in Ann Arbor. While there he negotiated a position on the Stanford University faculty and support from the Rockefeller Foundation for a program to develop an electron microscope for biological specimens. Marton moved to Palo Alto California in 1941. There he designed and constructed a large microscope of advanced design. He was able to use the machine shop facilities of the university and had the services of R.G.E. Hutter, a post-doctoral assistant, who designed the electrical system. The instrument is well-described in two papers (Marton and Hutter, 1944; Marton, 1945). The instrument was highly innovative but its performance was not correspondingly outstanding. Marton was an excellent scientist, but he was not a very good engineer, so that good ideas were often inadequately implemented. The Stanford microscope utilized five lenses. Two served as a double condenser, and two were projector lenses. Apparently Marton was not then aware of the selected area diffraction capability of the two projector lenses. However, he did utilize the two projector lenses to provide wide-field low-magnification images, which was not done by RCA until several years later. Marton was very much concerned about vibration from both the instrument and from the environment. The bell-jar construction of his EMA was intended to provide stability. Unfortunately, it caused other problems that were even more serious. He abandoned that idea in the Stanford microscope. Instead he used a very strong vacuum manifold in back of the column to support and brace the column. The specimen chamber was rigidly attached to the manifold providing a very rigid structure. At the same time, the objective lens used part of the specimen chamber to complete its magnetic circuit and thus the objective pole piece was fixed in space and was the alignment reference for the column. Most microscopes supported their columns on the projector lens which thereby became the alignment reference. The other pole pieces were aligned by transverse adjustments in the lens spools. These were apparently not external adjustments. It was a difficult instrument to set up and to operate. The Stanford microscope was a very complex instrument, perhaps too complex for the technology of its day. The resolving power was not particularly outstanding. The resolution in published micrographs was definitely not as good as that of the EMU microscopes at that time. The instrument was not used much for research and probably never achieved its full potential. When Marton left to take a position at the National Bureau of

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Standards in Washington, D.C. in 1946, the microscope project at Stanford lost its motivating spirit and gradually closed down. It is not easy to point out specific areas where the Stanford microscope directly influenced microscope design. The author remembers going over the Marton publications about the instrument with the members of the RCA microscope engineering staff when the papers first came out. They were unique because there had been no papers on magnetic electron microscopes in the American scientific press other than those by RCA personnel. They provoked some examination of what RCA's design philosophy was. The papers probably did not motivate any direct changes, but they did contribute to the store of useful technological experience.

IX. ASSIMILATION OF THE ELECTRON MICROSCOPE A. Training and Education

The process by which a new scientific instrument is assimilated into the practice of science can probably be described as a series of evolutionary steps which, taken as a whole, do not make interesting history and for this reason are usually overlooked. These steps include training, education, testing, research and evaluation, which have to be taken by its users before an instrument can be said to have been assimilated-that is, before its function can be taken more or less for granted. In the case of the electron microscope there are a number of interesting and unique events that did speed the process of assimilation significantly. When a new microscope reached a laboratory that had previously had none, which was nearly always the case in the early days, some training was necessary for operating the instrument, and some education about the scientific principles by which it operated was desirable. (The first institution to acquire a second electron microscope was the Biology Department of MIT in 1943.) Such training was particularly important when the instrument was to be operated by several people. Most of the very early operators were self-taught. They were given a brief instruction when the microscope was installed, and then they were pretty much on their own. It was a tribute to the talent of the pioneer operators that they did as well as they did. Training in operation of the instrument was not the only need. Knowledge of how to prepare specimens and interpret micrographs was just as important, and one of the important functions of the EMSA was to help disseminate such knowledge among its members. Until 1945, an institution had to work out its own training program for employees who were to use its electron microscope.

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Finally, in 1945, at least three courses on electron microscopy were offered to the scientific public by Pennsylvania State University, Illinois Institute of Technology, and the University of Texas. At that time there were about 100 microscopes operating in the United States. In each of the next few years one or two universities joined the list, and some dropped out. By 1951, according to R. L. Weber who initiated the course at Penn State, there were about ten universities offeringsolid courses on electron microscopy, which had provided instruction to 500 to 700 people(Weber, 1952).Also, by 1951, there were about three hundred and fifty microscopes in the field. The courses ranged from intensive one or two week sessions, often in the summer, to courses providing one or two hours credit which were part of the regular curriculum. To argument the faculty at such times wellknown microscopists were retained to provide special lectures or demonstration. Several governmental agencies, and particularly the NIH, regarded such educational ventures to be sufficiently important to the growth of electron microscopy in the United States that they provided a measure of their financial support in some cases. Such special study groups have remained to this day, often in a more sophisticated form. They fulfilled a very important need during the early years when the field was rapidly growing and changing. After the mid- 1950s the commercial suppliers of microscopes provided three to five day study and training sessions without cost to customers, similar to the study groups North American Philips conducted for their x-ray customers. B. T h e E M S A

Nothing speeds the assimilation of a new instrument like having a good number of them in the hands of the users. Then if there is a medium for the exchange of information among the users, everyone profits. Such was the case in the United States. RCA built and delivered a good number of microscopes in a short period of time, and the Electron Microscope Society of America, often called the EMSA, furnished the means for information exchange among the users. (A number of years ago the EMSA changed the word “microscope” in its name to “microscopy” to agree with the shift of its emphasis from the instrument to the use of the instrument.) However, at the time of the society’s founding the microscope itself was the subject of most interest to the EMSA. By September 1942, there were about 20 EMB microscopes in the field. Professor G. L. Clark at the University of Illinois, Professor 0.S. Duffendack at the University of Michigan and R. L. A. Matheson at the Dow Chemical Co. Each had one of them and all felt a need for a conference on electron microscopes at which users could share their experiences. Professor Clark and the other two organized the first National Conference on the Electron

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Microscope which was held in Chicago on November 27, 1942. The only microscopes then available were the RCA EMBs, and about twenty of these were represented at the meeting. Most EMBs had their share of problems as well as success. As a consequence the main subject of the discussions was how to keep the EMBs operating. Actually a wide range of microscopical subjects was discussed. The conference was a very great success. On the spot a committee was set up to draft a plan for a permanent organization to be adopted at a second meeting to be held within a year. It turned out to be a little more than a year later on January 14-15,1944, that the EMSA met at Columbia University in a joint meeting with the American Physical Society, a move to minimize travel during war time. The EMSA was voted into being at that meeting. It was an effective society. It affiliated with the American Institute of Physics and arranged for publication of the abstracts of the EMSA annual meeting in the Journal of Applied Physics and also preprinting them in pamphlet form to be used as programs before and at the meetings. This fortunate arrangement started with the second annual meeting December, 1944, and has preserved the abstracts of all the papers given in the early years. The EMSA organized, publicized and ran the annual meeting from its beginning in 1944 until the 1960s when it became so large an undertaking that an outside firm was retained to run it. In 1945, EMSA appointed a Committee on Resolution to develop some standards, which brought some order to the subject. It sponsored a bibliography on Keysort cards that became too numerous to handle and keep up to date with such a limited sorting method. Each year EMSA published a membership list which facilitated communication among the members. The EMSA was an important force in bringing about assimilation of the microscope, but it was much more than that in the early days. It was a group of exciting, imaginative people sharing a pioneering scientific adventure. There was an inspirational quality that energized its members. Extensive information on the origin and growth of the EMSA has been published (Reisner, 1981,1982, 1983).

X. ELECTRON MICROSCOPY Microscopy is the use of a microscope, or better, investigation with a microscope and it involves sample, specimen preparation, manipulation of the microscope and interpretation of the image or micrographs. The United States became the leader in the development of electron microscopy in 1941. It came about as the result of World War I1 which isolated the United States from Europe thus establishing a scientific enclave of electron microscopy in the United States. The large number of good microscopes reaching the field in the years of isolation, 1941 to 1948, created the necessary condition for

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activity in microscopy. Developing procedures for making specimens suitable for observation in the electron microscope meant inventing entirely new approaches, because most methods applicable for light microscopy were useless for the electron microscope. The man-hours invested in specimen preparation alone far exceed those expended in developing or producing microscopes; and if and when the history of specimen preparation is traced historically, it will take far more space to record it than will the story of the development of the microscope itself. By the time the microscopists of Europe had recovered from the severe curtailment of their activities during the war and recovery years, microscopy in the United States had made great progress, particularly in the life sciences. Along with the early dreams of an electron microscope, the specter was raised of the electrons destroying any material under examination thus making the device useless. This apprehension carried over to the early 1940s. It is interesting to note that when the early promoters of the electron microscope concept spoke of its possibilities they usually emphasized its hoped-for application in biology. Rudenberg even went so far as to say his interest in the instrument was motivated by his reaction to his son’s paralysis from polio. He saw the microscope as a means to identify the causitive agent of the disease. The strongest prophet for biological microscopy was Marton. He showed the earliest electron micrographs of biological materials and pointed out that the mechanism of contrast was electron scattering, which in thin specimens resulted in minimal loss of energy in the specimen. With such a situation, biological specimens should be proper subjects for electron microscopy. When Marton built his EMA microscope at RCA he brought with him his interest in microscopy of biological materials. It is highly probable that Marton’s enthusiasm for the possibilities for biological microscopy influenced Zworykin to take this aspect of electron microscopy very seriously. After Marton’s EMA was operating in 1939, it received considerable publicity. A number of visitors were received and micrographs were made of bacterial specimens. Mudd and Lackman (1941) published a paper on bacterial morphology with micrographs taken on this instrument. When Hillier’s EMB microscope started operating well in the summer of 1940, Zworykin began to invite scientists to see and use the new instrument. The situation soon threatened to get out of hand. A couple of visitors took the pictures of their specimens and published questionable claims. Scientists with a real interest were losing out to those who were merely curious. T. A Smith (Smith, 1984) recounts the following event. Zworykin became concerned that there might be unscientific conclusions drawn from the pictures and that the instrument might become the center of controversy. He determined that a more orderly and authoritative program should be set up to use the device. By now it was

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considered a major achievement at RCA and he had no difficulty in securing funds. As a result, Zworykin and I went to Washington and met with the National Research Council and offered to fund a fellowship to use RCA’s electron microscope in Camden to take pictures for the scientific community, under their aegis. Our offer was accepted and Dr. Thomas Anderson was appointed as the fellow. A committee of ten eminent biologists was appointed to administer the fellowship, Dr. Stuart Mudd (bacteriology), University of Pennsylvania was the chairman. The others were: Dr. M. Demerec, Carnegie Institution of Washington; Dr. Caryl Haskins, Union College; Dr. Michael Heidelberger, College of Phvsicians and Surgeons, N. Y.; Dr. J. H. Kempton, Bureau of Plant Industry, Washington, D. C.; Dr. C. W. Metz (genetics), University of Pennsylvania; Dr. Katherine Polevitsky (bacteriologist), University of Pennsylvania; Dr. Gordon Scott, Washington University, St. Louis; Dr. Wendell Stanley (virology), Rockefeller Institute for Medical Research, Princeton; and V. K. Zworykin, RCA. The first task of the committee was to select from among the applicants for the fellowship the research worker best qualified for the position (Morton, 1941). The Committee chose a young physical chemist who had been doing research and teaching at the University of Wisconsin, Thomas F. Anderson. He had been reluctant to apply for the fellowship but his associates had encouraged him to try for it. The appointment was for a two year period, from mid 1941 to mid 1943. Anderson’s role was to do the microscopy, essentially specimen preparation and instrument operation, in collaboration with the biologists who brought materials to be examined. His was a scientific function, not an educational or promotional one. During this time Anderson coauthored 3 1 papers with distinguished biologists representing a wide range of interests. Most importantly for microscopy, Anderson’s work created a highly visible and relatively bias-free witness to the usefulness of the electron microscope in biological science. From the standpoint of timing, Anderson’s pioneering work was important because, although RCA was sending microscopes into the field during the time of his tenure, only a very few were going to biologists. Without the National Research Council Fellowship, the development of biological electron microscopy might have proceeded more slowly. At the conclusion of his fellowship Anderson joined the staff of the Johnson Foundation of the University of Pennsylvania in Philadelphia. There he continued his research with the microscope, using a newly acquired EMB. He is probably best known for his development of the critical point method of drying specimens. He is well known wherever electron microscopy is practiced. He is a past president of IFEMS, the International Federation

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of Societies for Electron Microscopy, and was co-chairman, with the author, of the 5th International Congress on Electron Microscopy held in Philadelphia in 1962. Anderson has written a charming memoir of his career which is filled with history and is well worth reading (Anderson, 1975). As mentioned in Section IV.A, the Biology Department of MIT was ahead of the field in developing techniques for the electron microscopy of biological materials. They acquired a microscope in the summer of 1941 and a second microscope in 1943. The team of Schmitt. Jackus, Hall and, after the war, many others who studied in the department, produced a constant flow of new techniques and opened up a wide range of applications for biological electron microscopy. Another “head start” situation came about fortuitously in the chemical industry when American Cyanimid received the prototype EMB several months before the regular production instruments began to reach the customers, and they exploited it with great ability in various applications before anyone else was in the field. The microscope group at Cyanamid was a very talented and very productive group for most of the nearly years of microscopy. The microscope operator was a physicist, Charles J. Burton. He was able to keep the rickety prototype going and at the same time take 2,800 electron micrographs during the period from the receipt of the instrument December 9,1940 to June 17, 1941 (Rochow, 1983). The chemical industry was the largest group of users of the early microscopes, which included the Interchemical Corp., DuPont, Dow Chemical and Columbian Carbon Co. A number of the industrially owned electron microscopes were used, on a part-time basis, to do microscopy for research for medical institutions which had not been able to obtain a microscope. Some of the outstanding scientists in the field such as K. R. Porter started that way. For a more detailed picture of the state of microscopy a decade after electron microscopes became available (see Wyckoff, 1949). Many and perhaps most of the specimen preparation techniques in use today originated in the decade from 1940 to 1950, and by the end of that period, many were highly developed. The technique that elicited the most immediate and excited response from electron microscopists was shadow casting, first demonstrated by Williams and Wyckoff in 1944. It burst forth like a revelation. It was so simple and made such a dramatic difference in the appearance of a micrograph. The technique was immediately accepted. Every laboratory had to have a metal evaporation system. Since its inception very extensive research has been done to optimize shadow casting and extend its usefulness to include a wide variety of specimens. Everyone who read the first paper and saw the first micrographs of shadow casting knew that electron microscopy would never be the same. The development of thin sectioning was a very different story. It took the

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whole decade from 1940 to 1950 and involved many different people and several different ideas. The need for producing biological sections thin enough to permit examination in an electron microscope was painfully obvious to everyone. The conventional microtome used for light microscopy was utterly inadequate. Attempts to cut wedge sections such as made by Richards, Anderson and Hance (Richards et al., 1942) were occasionally moderately successful but never an answer to the sectioning problem. In 1943, the technology of ultra-thin microtomy took a strange turn, which now looks more like a detour. The idea was proposed that thinner sections could be cut by a knife moving at very high speeds e.g. 700 ft/sec. The first use of such an idea was reported by O’Brien and McKinley (1943),followed by W. A. Ladd and H. A Braendle in 1945 and E. F. Fullam and A. E. Gessler in 1946.The high speed microtome, as the device was called, looked sufficiently promising to prompt a business venture to manufacture high speed microtomes for sale. The high speed knife was achieved by mounting a sharp blade on a rotor which was spun at very high speed. The specimen was advanced into the blade which cut the sections which flew off and were caught by slides or screens. These machines were formidable. The driving motors were taken from commercial grinding equipment and ran at speeds up to 40,000 rpm or 700 rps. The noise from the machine thus had a fundamental near 700 Hertz and was very loud. More importantly the machine was dangerous and inadequately protected. It did provide some sections suitable for electron microscopy, but it was a temperamental process, not very predictable, somewhat brutal to specimens, and serial sections were not possible. People instinctively felt that the high speed microtome was not an answer, and their transitory interest in it merely witnessed to how desperately a reliable means to cut thin sections was needed. The solution to thin sectioning came through modification of the conventional microtome. Pease and Baker (1948) demonstrated the importance of knife sharpness and the accuracy and rigidity of the feed system to achieving reliable thin sectioning. The Pease and Baker work received extensive publicity because their laboratory was a part of the University of Southern California. That university, being close to the center of motion picture industry had a school of Motion Picture Arts, or something similar, that used the work of the university as a subject for the motion pictures they were incessantly taking. They made a picture of the sectioning procedures Pease and Baker were using, such as sharpening the knives, etc. The movie was called “The Thinnest Slice” and was very well done. It was circulated among microscopists and others and loudly proclaimed that thin sectioning had at last arrived. Pease and Baker received much good natured kidding, but the net result was useful. Newman et al. (1949)used thermal expansion to advance the specimen at a low rate with great uniformity and used a new embedding material. They infiltrated the specimen with a monomer of butyl methacrylate

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and polymerized the mass in situ. When microscopists realized that the means for thin sectioning was not out of reach, great activity ensued in research on fixatives, embedding materials and processes, knives and sharpening equipment. In the 1950s, there was a lot of art involved in cutting thin sections, and numerous courses were available to those who wanted to master the art. The practical ultra- microtome opened up whole areas of biology to the use of the electron microscope and contributed to the rapid growth of biological electron microscopy in the 1950s. As early as 1953, sixty percent of the papers given at the EMSA meeting were on biological subjects. Metallurgical electron microscopy has been from the beginning a very active and highly sophisticated research activity. Yet for the first fifteen years of electron microscopy, replicas were the only method available for studying metallurgical specimens. This is not to deprecate the creative achievements of those who have made replication techniques versatile and very useful technical resources. Replicas are still the only way for dealing with some materials. The thin foil techniques which have revolutionized metallurgy came into use after 1955 which is the arbitrary end of the historical period covered by this article. As pointed out at the beginning of this section on microscopy the effort involved in learning to exploit the inherent great resolving power of the electron microscope far exceeds the considerable effort required to develop the microscope and deserves an historical treatment where the emphasis is focussed on the microscopy and not on the microscope. The two aspects are inseparable and developed together. The lack of penetration of the electron beam required sampling techniques that provided thin specimens. The microscopists developed the ultra-microtome, and the engineers developed higher voltages. Samples were destroyed or damaged by the beam. The microscopists developed stronger specimens, and the engineers provided less damaging sources like biased guns and double condensers. Contrast in images was low. Microscopists developed metal shadowing and heavy metal stains and engineers came up with improved aperturing systems. For the first decade and more of the era of the electron microscope, the engineers and microscopists were in close communication, which contributed to the amazing progress of electron microscopy in the early years.

XI. ACKNOWLEDGEMENTS

The wide scope of this historical essay has made it necessary to request information and comment from a number of people who were participants in the early development of the electron microscope. This provided enriching opportunities for me to renew old friendships and also to meet people with

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whom I had not had direct contact in the early days. The author wishes to acknowledge the important help he has received from many more sources than those specifically listed here. Sterling P. Newberry has provided much important information and pictures on the developments at Washington University in S . Louis, and at the General Electric Company, as well as information about historically important events in the whole field of microscopc development. I am indebted to Paul Anderson, Arthur Cohen and Gertrude F. Rempfer for detailed information about what transpired in their respective areas of activity. I wish to thank my former colleagues at RCA, Edmund Dornfeld, Arnold Wilson and Samuel Zollers with whom I had long talks, and who reviewed my text and provided helpful documentation. Thanks are due James Hillier for the insights only he can provide, and for reading the section of the paper on RCA and suggesting changes where desirable. Finally, and most importantly, I acknowledge the essential encouragement, good humor, and help of my wife Carol who was my consultant, critic, and proof reader.

REFERENCES Anderson, T. F. (1975). “Annual Review of Microbiology” 29, Annual Reviews, Palo Alto, California. Bachman, C. H. and Ramo, S. (1943a).J. Appl. Phys. 14,8-18. Bachman, C. H. and Ramo, S. (1943b).J. Appl. Phys. 14,69-77. Bachman, C. H. and Ramo, S. (1943~).J . Appl. Phys. 14, 155-160. Cohen, A. L., and Steever, R. C. E. Jr. (1971).“29th Ann. Proc. EMSA.” (C. J. Arcenaux, ed.), p. 4. Fleming (Rempfer),G., Rempfer, R., Asherman, E., and Nolan, P (1948). J. Appl. Phys. 19, 125 (abstract) Hall, C. E. (1985).“The Beginnings of Electron Microscopy.” (P. W. Hawkes, ed.) pp. 275-296, Academic Press, Orlando. Florida. Hillier, J. (1940).Phys. Reo. 58, 842. Hillier, J., Baker, R. F., and Zworykin, V. K. (1942).J . Appl. Phys. 13, 571. Hillier, J. (1943).Phys. Reo. 64, 318-319. Hillier, J., and Baker, R. F. (1944).J. Appl. Phys. 15,663-675. Hillier, J., and Baker, R. F. (1946). J. Appl. Phys. 17, 12. Hillier, J., and Ramberg, E. G. (1947). J. Appl. Phys. 18,48. Kinsinger, W. G., Hillier, J., Picard, R., and Zieler, H. W. (1946).J. Appl. Phys. 17,989. Marton, L (1940).Phys. Rev. 58, 57. Marton, L., Banca, M. C., and Bender, J. F. (1940b).R C A Reo. 5,232. Marton, L. (1941).Jour. Bact. 41,402. Marton, L., and Hutter, R. G. E. (1944).Phys. Rev. 65, 161-167. Marton, L.(1945).J. Appl. Phys. 16, 131-138. Marton, L. (1968). “Early History of the Electron Microscope”, San Francisco Press, San Francisco, California. NcMillen, J. H., and Scott, G. H. (1937).Rev. Sci. lnst. 8, 288. Morton, G. A.(1941). R C A Rev. 6,131-166.

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Mudd, S., and Lackman, D. B. (1941). Jour. Bact. 41,415-420. Mulvey, T . (1962). Brit. J . Appl. Phys. 13, 197. Newberry, S. P. (1985a). E M S A Bull. 15-1, 39. Newberry, S. P. (1985b). EMSA Bull. 15-2, 39. Newberry. S. P. (1986~).EMSA Bull. 16-1,44. Newman, S. B., Borysko, E., and Swerdlow, M. (1949). Science 110,66. OBrien, H. C., and McKinley, G. M. (1943). Science 98,455. Pease, D. C., and Baker R. F. (1948). Proc. Soc. Exp. Biol. Med. 67, 470. Picard, R. G., and Dufiendack, 0.S. (1943). J . Appl. Phys. 14,291. Picard, R. G., and Reisner, J. H. (1946). Reo. Sci. Inst. 17, 484. Ricard, R. G., Smith, P. C., and Reisner, J. H. (1949). J. Appl. Phys. 20,601. Prebus, A. F., and Hillier, J. (1939). Can. J. Research 17,49-63. Prebus, A. (1942). Ohio State Univ. Eng. Exp. Sta. News 14, 6-32. Ramberg, E. G. (1943). U.S. Patent 2,369, 796. Reisner, J. H., and Picard, R. G. (1948). Rev. Sci. Inst. 19, 556. Reisner, J. H. (1950). U.S. Patent 2,503, 173. Reisner, J. H., and Dornfeld, E. G. (1950). J. Appl. Phys. 21, 1131. Reisner, J. H. (1951). J. Appl. Phys. 22, 561. Reisner, J. H. (1981). E M S A Bulletin 11-1, 13; 11-2, 16. Reisner, J. H. (1982). E M S A Bulletin 12-1, 11. Reisner, J. H. (1983). EMSA Bulletin 13-2, 12. Rempfer, G., Connell, R., Mercer, L., and Louiselle, 1. (1972). American Laboratory, April, p. 40. Richards, A. G. Jr., Anderson, T. F.,and Hance, R. T. (1942). Proc. Soc. Expt. Bio. and Med. 51,148. Rochow, T . G. (1983). E M S A Bulletin 13-1, 10-18. Smith, T. A. (1984). E M S A Bulletin 14, 19. Vance, A. W. (1941). RCA Reoiew 5,293. Weber, R. L. (1952). Am. Jour. Phys. 20,301-304. Wyckoff, R. W. G. (1949). “Electron Microscopy”, Interscience Publishers Inc., N. Y. Yoshii, Z. (1970). Bull. Yamaguchi Med. School 17, 191. Zworykin, V. K. (1933). J . Frank. Inst. 215. 535. Zworykin, V. K., Hillier, J., and Vance, A. W. (1941). J. Appl. Phys. 12, 738. Zworykin, V. K., Hillier, J., and Snyder, R. L. (1942). ASTM Bulletin 117, 15-23. Zworykin, V. K., and Hillier J. (1943). J. Appl. Phys. 14, 658. Zworykin, V. K., Morton, G. A,, Ramberg, E. G., Hillier, J., and Vance, A. W. (1945). “Electron Optics and the Electron Microscope” p. 200., John Wiley & Sons, Inc., New York.

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ADVANCES I N ELECTRONICS A N D ELECTRON PHYSICS VOL . 73

Electron Beam Testing K . URA AND H . FUJIOKA

. .

(v'

Faculty Engineering Osaka Uniuersir y Yuma(la.Okl1. Sutra Osaktr. Japan

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . I 1 . Various Methods of Electron Beam Testing . . . . . . . . . . . . . . A . Real-Time Waveform Mode and Voltage Coding Mode . . . . . . . . . B. Stroboscopic Image Mode and Stroboscopic Waveform Mode (Sampling Mode) . . . . . . . . . . . . . . . . . . . . . . C. Logic State Mapping and Tracing . . . . . . . . . . . . . . . . D . Frequency Mapping and Tracing . . . . . . . . . . . . . . . . E . Current Feeding . . . . . . . . . . . . . . . . . . . . . . 111 . Voltage Contrast by Electron Probe . . . . . . . . . . . . . . . . . A . Fundamentals of Voltage Contrast . . . . . . . . . . . . . . . . B. Address Error of the Primary Electron . . . . . . . . . . . . . . . C . Transit Time Effect on Voltage Contrast . . . . . . . . . . . . . . D . Voltage Contrast in Passivated Devices . . . . . . . . . . . . . . IV . Electron Irradiation Effects . . . . . . . . . . . . . . . . . . . . A . Beam-Induced Specimen Contamination . . . . . . . . . . . . . . B . Direct Contribution to Device Current . . . . . . . . . . . . . . . C. Characteristics Variation of MOS FET's by a Non-Penetrating Beam . . . . D . Charging-Up of a Floating Electrode . . . . . . . . . . . . . . . E . Charging of Passivated Devices by Non-Penetrating Electron Beam Irradiation V . Electron Optical Column of Electron Beam Testing . . . . . . . . . . . A . Low Accelerating Voltage Electron Gun . . . . . . . . . . . . . . B. Pulse Gate . . . . . . . . . . . . . . . . . . . . . . . . C. Secondary Electron Detectors . . . . . . . . . . . . . . . . . . D . Peripheral Equipment for Specimen Chamber . . . . . . . . . . . . VI . Automatic Control System of Electron Optical Column . . . . . . . . . . A . ControlSystem . . . . . . . . . . . . . . . . . . . . . . . B. Procedure for Control . . . . . . . . . . . . . . . . . . . . C . Experimental Performance . . . . . . . . . . . . . . . . . . . D.Prospects. . . . . . . . . . . . . . . . . . . . . . . . . V I I . EB Tester System . . . . . . . . . . . . . . . . . . . . . . . A . EB Testers of the First Generation . . . . . . . . . . . . . . . . B. EB Testers of the Second Generation . . . . . . . . . . . . . . . C . EB Testers of the Third Generation . . . . . . . . . . . . . . . . D. Peripheral Equipment and Techniques . . . . . . . . . . . . . . . V I I I . Measurement of Microstructures . . . . . . . . . . . . . . . . . . A . Linewidth Measurements . . . . . . . . . . . . . . . . . . . B. Three-Dimensional Measurements . . . . . . . . . . . . . . . . C . Pattern Inspection. . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .

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I. INTRODUCTION The term “Electron Beam Testing” means the testing of electrical properties of a circuit or an electron device by using the electron beam. The testing system is called “Electron Beam Tester” compared to “LSI Tester” (Large Scale Integration). It is difficult to pinpoint the inception of the electron beam testing. It is based on the scanning electron microscope (SEM), which had been improved and updated by Oatley and his school (Oatley, 1982). Voltage contrast in the SEM was discovered by Oatley and Everhart (1957) and the stroboscopic SEM by Plows and Nixon (1968).These are undoubtedly the roots of electron beam testing. Early works to about 1980 are reviewed by Feuerbaum (1979), Wolfgang et al. (1979), Menzel and Kubalek (1981). Electronics owes its recent development to semiconductor devices, especially LSIs and VLSIs. As the speed and density of these devices become higher, their function testing and fault diagnosis (failure analysis) for both design verification and quality control become more important and more difficult. The usual LSI tester uses the correspondence between a sequence of the input signals and that of output ones. If one wishes to localize the defect points in the device, a sequence of special signals must be devised for it; detailed knowledge about the concerned device and sophisticated skill are needed. Success, however, is not always assured because of the complexity and complication. There are two approaches to directly localize the defect points of a circuit or a device. The first is: a sequence of the signals are fed through the input terminals, the internal voltage is detected with a probe which is shifted toward output terminals sequentially, and the defect points of electrical properties are localized. The second is: a sequence of the signals are fed via a movable current feeder which is shifted from the output terminals to the input terminals sequentially, the response at the output terminals is compared with the expected one and the defect points are localized. A modern scanning electron microscope uses many kinds of interaction between the primary electron beam and matter (Oatley et al., 1965; Reimer, 1985). Among them, three types are used in electron beam testing: secondary emission, current absorption, and electron beam induced conduction (EBIC). The secondary emission from specimens includes the voltage contrast (Oatley and Everhart, 1957);bright at low voltage and dark at high voltage. Its spatial resolution easily attains 0.1 pm, while the temporal resolution can be much improved by a stroboscopic SEM (Plow and Nixon, 1968); stroboscopic micrographs with 1.5 ps were reported by Hosokawa et al. (1978b). If the electron beam has a current of about 1 nA, the loading effect to the

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tested device would be very slight. The electron beam becomes a very excellent probe. On the other hand, if the current of an electron beam is fairly large, the charge deposition to the floating electrode or the current injection to the p-n junction will change the status of the device or the circuit. That is, an electron beam can be used as: (a) a high resolution probe, and/or (b) a contactless current feeder. By using a fine mechanical probe connected with an oscilloscope, one can measure the voltage at the internal lines whose widths are five times larger than the probe tip radius. At present, the probe with a tip radius of 0.25 pm is commercially available. The stray capacitance of a probe is 2 to 4 pF; in the case of a FET probe, about 0.2 pF. Compared with the mechanical probe, the probing with an electron beam has the following advantage: (i) (ii) (iii) (iv) (v)

less loading effect to device operation, higher spatial and temporal resolutions, easy positioning, nondestructive probing under controlled irradiation conditions, and easy display of two-dimensional voltage distribution.

In the next part, we shall review how the electron beam is used and what information can be derived from it. The main topic is the scan-display methods where the electron beam is used as a probe. Furthermore, the contactless current feedings are discussed, although they have not yet been practically used. The voltage contrast is the key technology in the electron beam testing; this item has been reviewed recently by Gopinath (1987) and the third part will be complementary to it. The electron beam testing is not completely nondestructive; the irradiation effects are treated in Part IV. The electron optical column differs in some respects from a usual SEM; the fifth part is concerned with this. At present, the commercial electron beam tester uses the probing action of an electron beam and it can resolve 0.1 pm spatially and subnanosecond in time. Its voltage sensitivity reaches 10 mV. It, however, needs (i) high initial cost and (ii) high skill in testing. Nevertheless, the VLSI age urgently requires the testing with higher spatial and temporal resolutions. The semiconductor industry has evaluated the electron beam testing as very useful in function testing and failure analysis of VLSIs and is beginning to apply it in the design verification and quality control of VLSIs. Table I summarizes the trend and tasks to be solved for practical use of electron beam testing. The main task is to increase its cost performance,

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K. URA A N D H. FUJIOKA TABLE I TRENDOF ELECTRON BEAMTESTING: AIMSAND METHODS

Increase of cost performance Less initial cost : Electrostatic electron optical column High throughput Easier manipulation Fully automated measurement combined with C A D database Higher reliability Various testing environments: Various kinds of specimen stages Improvement of fundamental performances Spatial and temporal resolutions Quantitative voltage measurement

I

because the fundamental performances such as spatial, temporal and voltage resolutions meet the present demand. Of course, higher resolutions would be needed in the near future. The electron optical column of the present electron beam tester is based on the conventional SEM. The accelerating voltage must be low, i.e. below about 3kV, in order to avoid variation of device characteristics. In this case, the electrostatic column has an advantage of low cost, and it also promises easy and fast control with a computer. For the improvement of throughput and manipulation, the fully automated testing is decisively effective. In this case, the electron optical column must be reliable first of all and controllable with a computer; this rather troublesome task has been resolved recently (see Part VI). Furthermore, the system must be combined with various CAD databases and, in some leading systems, this is realized (see Part VII). The linewidth measurement of photoresist and mask inspection are very important in the process control. They are briefly sketched in the last part, because their testing systems are very closely related with the art of electron beam testing. 11. VARIOUS METHODS OF ELECTRON BEAMTESTING

Here, various methods that are closely related with the electron beam itself are discussed. That is, whether an electron beam is a probe or a feeder, pulsive or continuous, line scan or raster scan and so on. Table I1 summarizes eight main methods for probe use; these and some derivatives are discussed in the following. There are some scan-display methods to observe the passivated devices, which are discussed in the next part for the sake of convenience.

TABLE I1

VARIOUSSCAN AND DISPLAY MODE Status of electron beam Pulse Name of mode

Continuous

Real-time waveform Voltage coding Stroboscopic image Stroboscopic waveform (sampling) Logic state mapping

0

Logic state tracing

Phase

Scan Frequency

0

Plane

Line

Display on CRT Spot

s

y

0

t

V(t)

0 0

Fixed Incremental shift Incremental shift Bit pattern modulation

0 0

0

Frequency mapping

Incremental shift

Frequency tracing

sB=f,*hF

0 0

Brightness

2

0

2.

x

I’

W(X.Y))

x cp

Y

Ucp)

x

cp

w,cp)

x

y

Bit pattern

x

/B

w ,.L *

x

Y

Vf,)

Wcp)

hF)

N

W

4

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K. URA AND H. FUJIOKA

A . Real-Time Waveform Mode and Voltage Coding Mode

In the usual SEM, when the raster scan is stopped and the electron beam is fixed on an electrode of a device, the detected secondary electron current varies according to the voltage variation: a high signal level to a low voltage and vice versa. By this real-time waveform mode, a voltage variation of several tens kHz can be measured without any trouble. Its highest frequency is limited, in principle, by the shot noise of the electron beam: the actual bandwidth of a SEM is adjusted taking account of this. If the electron beam current is increased, the higher time resolution can be obtained. Ostrow et al. (1982)reported a time resolution of 50 ns. A 10 pm width line of microprocessor Z-80 was irradiated with an electron beam whose diameter and current are 3 pm and 0.1 pA, respectively. The accelerating voltage was set at 700 volts in order to balance the primary beam current to the secondary emission current. If the electron beam is scanned in a usual way on a device to which the high frequency signal is applied, the secondary electron current will be varied according to voltage variation. One will observe stripes in the display with the TV scan rate. If the driving frequency of a device coincides with a multiple integer of TV scan rate, the display stands still. This voltage coding mode was reported by Lukianoff and Touw (1975). Several MHz can be observed, but limited to rather a simple time sequence. Figure 1 shows a series of micrographs of one part of a 1 Mbit DRAM (dynamic random access memory) by some observing modes: its right part is an address counter. (This device and necessary data were supplied from the Semiconductor Group, Matsushita Electronics Co., Kyoto, Japan.) Figure l(a) shows the micrograph when any voltage is not applied. Figurel(b) shows a voltage coding micrograph of it. If the frequency on the line is high, the vertical stripe becomes fine. The clock signal is seen at the lines in the center region. It is seen that the frequency is counted down from the rightmost line to inner lines. B. Stroboscopic Image Mode and Stroboscopic Waveform Mode (Sampling Mode)

The time resolution can be increased drastically by the stroboscopic image and waveform modes. Stroboscopy has been well known in the optical measurement of rotating speed of a body. It was first applied to the electron optical instrument by Dubinina et al. (1959). In 1968, Plows and Nixon first reported the stroboscopic scanning electron micrographs of an 8 MHz IC with a SEM.

FIG.I . Observation of the address counter of a 1 Mbit DRAM. An accelerating voltage of 1 k V and a beam current of 1 nA. (a) SEM micrograph without voltage application; (b) Voltage coding mode by 0.4 Hz signal application; (c) Stroboscopic image by a beam pulse of 5 MHz and 5 ns; (d) Logic state mapping at a clock of 5 MHz, pulse beam of 1.25 MHz and 10 ns. The scanned region is shown by the black arrows in (a);(e)Frequency tracing at 2.5 MHz, where fs = 2.55 MHz and JF = 50 kHz; (f) Frequency mapping. The scanned region is shown by the white arrows in (a). 239

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K. URA AND H. FUJIOKA

+

Electron gun

-Y-

Deflector 1

Aperture

Lens

SG

Scan coils

1 Phase A

Specimen

T Phase B

Phase C

FIG.2. Principle of a stroboscopic SEM.

The principle of stroboscopic image mode is the same as that of optical one. Figure 2 shows its principle. First of all, the device voltage must be periodic. The same phase appears in every cycle. If the electron pulses irradiate the device at a certain fixed phase of the device voltage, the electron pulses meet the device at the same phase. The stroboscopic micrograph can be taken with the usual secondary detector of a SEM, and those at the other phases can be done by shifting the phase difference between the electron pulse and the device. Figure 3 shows an example by Fujioka et al. (1980a): a sequence of stroboscopic micrographs taken at every 1 ns. While this IC is a rather old type and has a large size, it may be suitable for demonstration. It is an inverter working at 8.6 MHz. The pulse width of the electron beam was 0.2 ns. “IN” at the right bottom corner of each frame means the input terminal and “OUT” at the left one means the output terminal. Because this IC is an inverter, the contrast at “IN” should be inverse to that of “OUT”.At the phase “3 ns”, “ I N ’ becomes dark, that is, it takes a high level. At “9 ns”, “OUT” becomes bright; this means a signal delay of 6 ns. By spot irradiation and by shifting the phase difference sequentially, the stroboscopic waveform can be recorded. This is just the principle of a sampling oscilloscope and is also called the sampling mode.

ELECTRON BEAM TESTING

24 1

FIG.3. Stroboscopic micrographs of an inverter IC. A repeating frequency of 8.6 MHz, an electron beam pulse width of 0.2 ns, an average beam current of 12 PA. (From Fujioka et al., 1980a, copyright 0 1987 IEEE).

The time resolution of the stroboscopic image or waveform mode is determined by the pulse width of an electron beam, the accuracy of synchronization between the electron pulses and the device, and the transit time effect of secondary electrons; it does not depend on the time response of the secondary electron detector and the amplifiers. The stroboscopic image and waveform modes are equipped as standard functions in an electron beam tester.

242

K. URA AND H. FUJIOKA

Figure l(c) shows the stroboscopic micrograph of Fig. l(a) at 5 MHz and a electron beam pulse width of 5 ns. The stroboscopic image and waveform modes have been applied to function testing and failure analysis of LSIs and VLISs as well as of a surface acoustic wave device; the latter is made of LiNbO, and has piezoelectricity which enables observation of its voltage contrast (Feuerbaum et al., 1980). Hosokawa et al. (1978b) reported the stroboscopic micrographs at an electron beam pulse width of 1.5 ps, and Fujioka and Ura (1981) reported the stroboscopic waveforms at an electron pulse width of 5 ps. These are the highest records to date. The stroboscopy can be applied only to periodic phenomena. If the period is long, the recording time will become long. Ikuta and Shimizu (1985) reported the multi-frame line-sampling stroboscopy to observe the wall motions of magnetic domains of Si steel at 60 Hz. One period is divided into several phase points for which multi-frames are displayed on the CRT simultaneously. The electron beam scans a full horizontal line in 1 ms, while it moves only slightly for the usual stroboscopic image mode. In one period, one horizontal line of each frame is displayed on the CRT. This reduces the recording time enormously: in several seconds, rnulti frame recording of a 60 Hz phenomenon is finished. Sano et al. (1986) reported the multi-sampling waveform mode to record the waveforms at two phase points at the same long period. The example was a CMOS microprocessor whose clock cycle and repeating frequency are 6 MHz and 20 kHz, respectively.

C. Logic State Mapping and Tracing Crichton et al. (1980) reported logic state mapping to observe the logic states of connecting lines such as bus lines. These lines are expected to be vertical in the scan mode as shown in Fig. 4(a) schematically. Figure 4(b) shows the applied voltage to line(A) and (B) respectively. In the usual stroboscopic image mode, the phase difference between the electron pulses and the device is fixed during the taking of a micrograph. In the logic state mapping, the phase difference is fixed during a horizontal scan, and it is shifted by an increment for the next horizontal scan. At the device, the electron beam scans along the same horizontal line, while the vertical scan of the display CRT is taken as proportional to the phase shift; the variation of the logic states along the connecting lines are displayed vertically as shown in Fig. 4(c). Figure l(d) shows an example. The rightmost line works a half cycle of the center region. The next left line from it is counted down a half of it and so forth. If the electron beam scans the device vertically synchronized with phase shifting, the phase shift image mode is observed (Crichton et al., 1980). This

243

ELECTRON BEAM TESTING

IPE

.

VA

Ve

w

w ! P

(b)

(0)

(c)

FIG.4. Principle of logic state mapping. (a) schematics of two lines, (b) applied voltage to these, and (c) schematic diagram of logic state mapping.

gives the same information as the logic state mapping in the case of a simple configuration of lines, and it is effective to reduce the contamination and charging of passivation layer (Ura et al., 1982). Brust and Fox (1985, 1986) proposed the logic state tracing. It displays only the connecting lines which have the specified logic state. Let g ( t )represent voltage variation at an arbitrary line in a device, and g,(t) the voltage variation corresponding to the specified signal sequence (bit pattern); the correlation function f ( ~is)then defined as I-

If g coincides with gy,then f takes a large value at z = 0. If the primary electron beam is modulated by the inverse of g,(t), the detected secondary electron current become h(g(t)g,(t)).By averaging the output signal over several periods, one can obtain the correlation. Actually, the variations of materials and the surface topographs may affect the value of the correlation function. Then the electron beam is modulated by g,(t) and its inverse alternatively, and two correlations are calculated. One displays only such lines that the difference between two correlations exceeds over a threshold. Figure 5 shows an example by B r u t and Fox (1 986). D . Frequency Mapping and Tracing The clock cycles inside of an LSI are not always fixed. An example is the clock of the internal voltage generator for the substrate bias in a DRAM. Its

244

K. URA AND H. FUJIOKA

250 prn

good

device

faulty

device

FIG.5. Logic state tracing of amultiplexer HEF 4512 (From Brust and Fox, 1986):(a) usual voltage contrast, (b) for “good” sample, and (c) for “no-good” sample.

frequency depends on the supply voltage and the environmental temperature. The frequency of such a line can be “mapped” (Brust et al., 1984). Furthermore, the lines which have the specified frequency can be “traced” and displayed two-dimensionally (Brust et al., 1984; Brust and Fox, 1984). In the frequency tracing, the primary electron beam is modulated by a frequency fB and scanned in the usual way. When the intermediate frequency amplifier is set after the secondary electron detector and its frequency is fiF,the connecting lines whose frequency is either of fa & fiFare displayed brightly on the CRT. Usually, the output from the IF amplifier is shaped in binary. Figure 1 (e) shows the frequency tracing at 2.5 MHz of Fig. 1(a). In actual cases, both modulated electron beam and device voltage contain many higher harmonics, and some care must be taken to ensure correct interpretation. The modulation ratio of 50% is desirable for the primary electron beam, where the content of higher harmonics is least. As the amount of higher harmonics of a device becomes greater, the measuring time becomes longer. In order to avoid cross talk, fiFshould be as small as possible, but as for flicker noise it should be high. Its bandwidth must be sufficient for scanning and displaying. The frequency mapping resembles the logic state mapping. The primary beam scans the same horizontal line, the beam modulation frequency is shifted gradually, and the vertical scan of the CRT display is proportional to the shifted frequency. Figure l(f) shows an example. The distance from the upper side to the traces corresponds to fB = f, f fiF.Countdown of the frequency is clearly seen. If f , is equal to f i F , a continuous trace is observed independent of fB as seen in this figure. This situation is the same in the frequency tracing.

ELECTRON BEAM TESTING

245

E . Current Feeding As stated in the introduction, the electron beam can be applied to contactless current feeding. If the probing is carried out with another electron beam, the fully contactless testing is realized.

1 . Shortlopen Test Between Floating Electrodes by Charge Deposition The shortlopen test of a circuit with an electron beam has been investigated by Engel and Holmstrom (1970), Sebeson (1973), Engelke (1974), Pfeiffer et al. (1981), Hohn et al. (1982), and Brunner and Lischke (1985). Figure 6 shows a schematic diagram of a three-dimensional conductor network (Pfeiffer et ul., 1981). The openlshort of about ten thousand junction points is tested. Three electron beams are prepared. Two are writing flood beams to charge the electrodes from the upper and lower sides and one to probe voltage contrast on the upper surface. In this case, the testing is fully contactless. If the measured voltage contrast coincides with the expected ones, the connection is correct. Pfeiffer et al. (1986) succeeded in testing 12,000 pads within one minute including installation and removal of a sample to/from the vacuum chamber. The testing is possible by one electron beam which is used in charging and probing as similarly as in the storage tube (Knoll and Kazan, 1952), although it takes a long time (Engel and Holmstrom, 1970; Sebeson, 1973; Engelke, 1974). Brunner and Lischke (1985) proposed the testing by using one electron beam with the same accelerating voltage.

FIG 6 Schematics of a package module (From Pfeifler et al., 1981)

K.URA AND H. FUJIOKA

246

Drake et al. (1986) tested short/open of a liquid crystal display matrix on an insulator substrate, where a conventional SEM was used to charge and probe the device. The short/open test requires no DC supply voltage. Due to rather high impedance of an electron beam, a leakage resistance below several megohms will be taken as short (Engel and Holmstrom, 1970; Sebeson, 1973; Pfeiffer, 1982; Brunner, 1984). 2. EBIC at p-n Junctions Shaver (1981) reported a new test method. The electron beam was irradiated to reverse-biased p-n junctions between an NMOS flip-flop circuit and the substrate as shown in Fig. 7. The multiplied current sets the logic state of a subsystem where the whole system was made on one Si-wafer. The setting time per one point was 280 ns where a 5 kV and 7 nA electron beam was used. In this case, the logic states are held regardless of leakage as long as the DC supply voltage is maintained. When an electron beam is irradiated to an open-state p-n junction, the hole will move to the p-type region and the electron to the n-type; the electron beam induced voltage will appear. If no leakage current flows, its voltage contrast can be easily observed. Ashburn et al. (1979) tested the short/open between the emitter and collector of the bipolar transistor. The same technique was applied to observe the latch-up in the p-well region of a CMOS LSI. Canali et al. (1986) measured the latch-up sensitivity by using an electron beam, followed sequentially the phenomenon by stroboscopy and localized the “firing” point. 3. Mechanical Current Feeder and Voltage Contrast The electron beam is an excellent probe because of its high impedance; this, however, is a disadvantage in the case of the current feeding. A fine

c-

t-

-

DIODES TO SUBSTRATE

---c

ELECTRONS

-

FIG.7. Schematics of electron beam switch latch (ESL) (From Shaver, 1981).

ELECTRON BEAM TESTING

247

FIG.8. Mechanical current feeder on a D-latch circuit observed in a SEM (from Miura and Tamaru, 1986):(a) before contact of the feeder to “ I N point, and (b) after contact.

mechanical current feeder which is fundamentally the same as a mechanical probe has low source impedance. It requires high skill to exactly touch the feeder on the aimed electrode. If this is carried out in a SEM observing the voltage contrast, the difficulty can be removed drastically. This technique has been proved very effective in failure analysis (Wolfgang, 1983). Figure 8 shows an example by Miura and Tamaru (1986).

111. VOLTAGE CONTRAST BY ELECTRON PROBE

Voltage contrast can be isolated from topography and material contrasts by subtraction of I , from I , , where I , and 1, are the detected secondary electron current without and with voltage application, respectively. The sample and hold circuit was used by Oatley (1969), and Gonzales and Powell (1978a). The lock-in amplifier was used by Flemming and Ward (1970), and Balk et al. (1976). Cocito et al. (1980) commented that the former had higher voltage sensitivity than the latter. Obyden et al. (1980) proposed normalization by (1, - Io)/Zo. These are the point-by-point subtraction, that is, the voltage is switched at each point. In the frame-by-frame subtraction, the whole image without voltage application is stored and the one with voltage application is stored similarly. In this case various image processings can be applied after taking images (Touw et al. 1977; Furukawa et al. 1979). By Fujioka et al. (1982), the subtraction yields almost the same quality image as the normalization does; this is not the case for pseudo-color display where the median filter is applied (Fujioka and Ura, 1982). Here we shall describe the fundamentals of voltage contrast briefly, and discuss special topics on the address error of primary electron beam near the electrode, the transit time effect and voltage contrast of passivated devices.

248

K. URA AND H. FUJIOKA

Readers should refer to the latest review article by Gopinath (1987) for the details on S/N, local field effect, linearization and so on. A . Fundamentals of Voltage Contrast

The voltage contrast usually results from variation of collecting efficiency of secondary electrons with a detector, not of secondary emission yield. The exception is the contrast for negatively charged-up insulator surface, which will be discussed in Section 1V.E.1. The electric field (and magnetic field in some cases) constitutes a kind of energy filter. When the potential of an irradiated point is varied, the initial energy of a secondary electron is biased by this amount, and the collecting efficiency will vary. This is one mechanism of voltage contrast. Another is stronger. In the case of a positive potential of an electrode, the secondary electron is emitted to a retarding electric field which acts as a potential barrier. For a two-dimensional line with a width of 2a, the potential function $(x, z) is approximated by $(x,z) = Ez

+ (V,/n)(tan-'((a - x)/z) + tan-'(@ + x)/z))

(2) where E is the extracting field to secondary electrons and the line thickness is assumed as negligible compared with 2a. The potential barrier is represented which are by the saddle point of potential. Its position (0,z,) and potential estimated from Eq. (2), are as follows.

+,,

1

(3)

+ (2V,/n)tan-'(a/z,).

(4)

= (2V,)/(nEa) -

Z,/U

$,,

=

Ez,

Some of the emitted electrons are returned by this potential barrier. In a usual Everhart-Thornley detector (Everhart and Thornley, 1960) of a SEM, the extracting field at the specimen surface is not so strong; the voltage contrast mainly results from this potential barrier. Let the initial energy and ejecting angle be e W and ci, respectively. The condition under which a secondary electron can reach the detector is written as

a,

WL(4 6 )5 W ( 4 5 W"(4 (5) where WL(ci, V,) and Wu(a,V,) can be calculated from trajectory tracing of secondary electrons. Assuming a two-dimensional model, the detected secondary current lDET is given by

j:Z2 jw, wu

IPE6 IDET =

dci

j"' :j da

-n/2

N( W )cos ci d W

N(W)cosadW,

249

ELECTRON BEAM TESTING

where I,, is the primary current, 6 the secondary emission yield, and N ( W )the energy distribution of secondary electrons. The acceptance diagram is obtained if Eq. ( 5 )is plotted on the plane o! and W. It is noted here that Eq. ( 6 )can be applied to the topography contrast in a SEM by taking account of variation of 6 due to the variation of material and injection angle to specimen. Indeed, Miyoshi and Yamazaki (1986) calculated the secondary electron intenstity profile of a photoresist pattern. Figure 9 shows the calculated intensity profile and the measured one by Yamazaki and Miyoshi (1986).The calculation explains the features of the measured profile very well. If the voltage contrast is determined only by the potential barrier, Eq. ( 6 )is well approximated by

4B= G 4 m .

(8) N ( W ) is summarized by Kollath (1956). In the case of metals, it is conveniently approximated by

:1

N ( W )=

A

=

J4/3

and 1

(

N(W)dW = ( 2 / A 4 ) e - ” m 1 + A f l + -A2W + A3W31Z (9) 2 l ) When another earthed line is adjoined by a spacing of 2b, its potential function is approximated as follows. If the lines on a plane of z = 0 have a width of 2 4 a spacing of 2b, and a negligible thickness, and if the potential distribution in the spacing is assumed to be independent of other electrode potentials, then the potential function 4(x, z) is written as

+ + b,

$(x,z) = ( K / 2 ) ( f ( ~ a

Z)

-f ( x -a

-

b, 2))

+ Ez,

z > 0. (10)

f ( x ,z ) is analytically given in the following two cases.

Case A:

xlb,

1x1 < b

-1,

x<-b

(11)

f ( x , z)

=2 [Z-In ( z 2

nb 4

+

(b - x ) ~ ) ( b ; z2 + ( b + x ) ~

- (P)tan-1(:)]

+

X )

‘1

tan-’ ( b X)

250

K. URA AND H. FUJIOKA

TOP EDGE 1.2pm --_

-6

-4

0

-2

4

2

6

8

10

12

DISTANCE X ( p m )

-

0-8

w v)

2.0pm

0' 0

I

500

I

1000

I

1500

I

2000

DISTANCE X FIG.9. Secondary electron intensity profile contrast with a top detector in the case of a metalized photoresist with a height of 1 pm and a width of 2 pm (From Yamazaki and Miyoshi, 1986):(a) cross section and dimensions, (b) calculated, and (c) measured.

25 1

ELECTRON BEAM TESTING

Case B:

s(x, 0) =

1x1 < b

(2/n)sin-'(x/b), 1, -1,

x>b x<-b

(13)

+ z 2 + bZ- J(x' + zz + bz)2- 4x2bZ (14) where the plus sign is taken for x > 0 and vice versa. While the actual potential distribution in the spacing may be given by mixing Eq. (1 1) with Eq. (13), the difference of Eq. (12) from Eq. (14) is rather slight at the point a little far from the spacing. Similarly, Eq. (2) in which a + b is substituted into a, well approximates the potential function 4(x, z ) for Case A and Case B. In a conventional SEM, the voltage contrast appears usually for a positive electrode potential, and it becomes slight for a negative potential, because it results from the potential barrier. By biasing the whole device positively, the observable range of voltage contrast can be extended to the negative side (Furukawa et al., 1979). If one measures the shift of energy distribution of secondary electrons by using an energy analyzer, the electrode potential will be measured regardless of its sign. This was first tried by Wells and Bremer (1968). Flemming and Ward (1970) and later Fentem and Gopinath (1974) included an energy analyzer in a feedback loop to measure the electrode voltage automatically. These are called the voltage linearization. The voltage sensitivity is determined by S/N of the detected secondary electron current (Gopinath, 1977),which can be improved by signal processing such as filtering, integration and averaging. The theory on signal processing was compared with experiments by Fujioka et al. (1985b). The accuracy of voltage measurement, or voltage resolution, is determined not by S/N but the local field effect (LFE). The detected secondary electron current depends on the electrode size, typically on the line width, and on the neighboring electrode potential. The former is called the type I and the latter the type I1 (Nakamae et al., 1981b). The LFE results from focusing, defocusing, or deflection action to a secondary electron by the local field near the tested lines. It is written as VR -

VkO = k 1 K

+ kZVn7

VRO

=

vR(K = 0,

= 0)

(15)

where VR, K, and V , are the retarding grid voltage of an analyzer, the tested electrode voltage, and the next neighboring line voltage, respectively; k , and k 2 correspond to the type I and I1 LFE of the analyzer, respectively. They have been measured by Nakamura and Sat0 (1982) for a parallel plate retarding field energy analyzer in the case of 2a 2 10 pm and 2b 2 10 pm. Now let us consider the impulse of a secondary electron by the local field for a given V , . The intensity of the local field is proportional to (2a)-'. The

252

K. URA AND H. FUJIOKA

transit time of a secondary electron which is ejected perpendicularly is proportional to (2a/E)'I2 in the case of Ea >> W and V,, and to 2a in the case of Ea << W and V,. Therefore, the LFE is expected to be

k l and k, a

i

(E2a)-'I2 in the case of Ea >> W and V,, (E2a)O in the case of Ea << W and V,,

(16) (17)

and also

k , a (2b)-', (18) where e W is the mean initial energy of the secondary electrons. From Eq. (16), the LFE can be reduced by increasing the extracting field but its reduction rate is rather slow. If the energy analyzer is sensitive to a departure angle of a secondary electron from the local field, the LFE cannot be removed, because the local field will focus, defocus, or deflect the secondary electron. It can be reduced by a specially designed energy analyzer (see Part V.C), but not completely. Miyoshi et al. (1983) reported reduction of the type I1 LFE by built-in electrodes which encircled the tested points (Herrmann and Kubalek (1986) calculated this). In principle, the LFE can be corrected by preliminary measurement of k , and k2 of Eq. (1 5) and by probing the adjoined electrode potential. This, however, has been never tried as far as the authors know. B. Address Error of the Primary Electron

When the specimen is negatively charged up or the abnormal voltage is applied to a large-size electrode, the distorted image is observed occasionally. It is important for image interpretation to know the amount of address error due to surface potential distribution. The address error of the primary electron with an accelerating voltage of V , can be easily calculated for a rectangular plane electrode with a size of 2a and 2b and a voltage of I/ (Ura, 1981). It is given by AX

-[47EV ,

= V

+

-(T)ln(: b y (U

- (?)In(

(U

+ + + y)' + + y)' + x ) +~ ( b - y), -x ) + ~ ( b - y), a x)' (b a - x ) ~ (b

{ (:1:)

-(a + x ) tan-'

+

tan-'(%)}

+ (a + x ) { t a n - ' ( S ) + t a n - ' ( s ) } .

(19)

ELECTRON BEAM TESTING

253

where x and y represent the address coordinates for V = 0. A y is obtained by exchanging x and y, a and b, respectively. The address error is at most

C . Transit Time Eflect on Voltage Contrast If the voltage variation becomes so fast that the secondary electron experiences a different phase of device voltage during its transit through the device field, the voltage contrast would be varied. As a result, the rising and falling waveforms may be measured as schematically shown in Fig. 10. This effect was first investigated by Fujioka et al. (1985a) in the case of a two-dimensional line electrode. The field of Eq. (2) is assumed as quasi-static,

t

t

Ib)

FIG. 10. Transit time effect of secondary electrons on waveform measurement. For curve I, the transit time through the local field is much less than the time T (from Fujioka et al., 1985a): (a) for rising edge, and (b) for falling edge.

K.URA AND H. FUJIOKA

254

/ /

0.01' 0.01

' ' """'

0.1

' ' """'

0.1

1

' '

10

'

eE FIG.1 1 . Normalized time advance of stroboscopic waveform versus the transit angle of a secondary electron. OE = ( 4 a / T ) / d w

and the calculation method of Section 1II.A is applied to each starting phase of a secondary electron. The calculated result is shown in Fig. 11. T, z, and z - are defined in Fig. 10. OE is the transit angle of an electron and defined by (4a/T)/(2e/m)E2a)'''. For example, if 2a = 1 pm, T = 50 ps, and E = 500 volts/mm, then 2a/T = 0.02 and OE = 0.095, z + / T 0.034, that is z+ 1.7 ps. If T = 10 ps and the others are the same, z+ 1.4 ps. The phase advance at the rising edge is almost equal to the one at the falling edge and the variation of pulse width is rather small. If the pulse width becomes narrower, the peak value is reduced. Ozaki et al. (1985) estimated dulling of a rising edge as 120 ps in the case of an opened SMA connector. They also noted that the jitter of a pulse generator may become serious for measurement of a fast waveform. Nakamae et af. (1986a) calculated amplitude reduction and phase shift in the case of a kind of coaxial-line. It well agreed with the experiment, where one GHz sinnusoidal voltage was applied and it was calibrated within an accuracy of 5%. Clauberg (1 987) calculated the transit time effect for the electrodes which are different from the Fujioka et al. model.

--

-

D. Voltage Contrast in Passivated Devices

The voltage of buried lines in the passivation layer of a device can be observed by the following: (a) removal of the passivation layer,

ELECTRON BEAM TESTING

255

(b) coating with antistatics,

(c) resistive coupling between the surface and the buried electrode by EBIC, and (d) capacitive coupling between the surface and the buried electrode. The last is called the capacitive coupling voltage contrast (CCVC) (Crosthwait and Ivy, 1974). While the removal of passivation has been used conventionally (Wolfgang, 1983), it takes time and skill; often the device is damaged. Arima et al. (1983) applied the ion beam milling to remove an area of several pm2 by Ga’. Hosoi et al. (1985a) successfully observed stroboscopically a logic VLSI with a rule of 1.7 pm which is covered by antistatics. This mechanism has not yet been clarified. If the primary electron has sufficient energy to penetrate the passivation layer into the buried electrode, the surface potential becomes equal to that of electrode by EBIC (Taylor; 1978, 1981). Nakamae et al. (1981~)measured stroboscopically the waveform of a passivated MOS FET by this method where the electron irradiation was controlled very carefully so as to not exceed the threshold dose (see 1V.C). If the electron beam has such an energy that it cannot penetrate the passivation layer but it is larger than its second cross over energy, the surface will be negatively charged as will be stated in 1V.E. In this case, the voltage contrast cannot be observed stably. If the electron energy is less than the second cross over energy, the surface potential will reach an equilibrium one which is a few volts higher than the neighboring potential. In this case, the CCVC can be observed for a while, a storage time, just after the voltage variation of the electrode, bright for the voltage variation from “High” level to “Low” level and vice versa. Uchikawa and Ikeda (1983)reported that the CCVC for S i 0 2 layer can be observed repeatedly after switching of the electrode potential when the primary electron energy is lower than 1 keV, while it could not be if the normal component energy exceeds 1 keV. 1. Surface Voltage just afer Voltage Variation

If the contribution of EBIC is negligible, the surface potential of a passivated device just after variation of the buried electrode is the same as that in the case of no electron irradiation. Ookubo et al. (1985) calculated this by a relaxation method assuming a periodic boundary condition (Herrmann and Kubalek (1986) also calculated this). The two-dimensional potential is calculated for a simplified model, where a dielectric thin plate with a thickness of d is put on the two-dimensional

256

K. URA AND H. FUJIOKA m

1

0

0

1

2

4

3

X/a FIG. 12. Potential variation along the passivation surface that has a thickness of d and is laid on the two- dimensional line and space electrodes. The specific dielectric constant of 3.5 correspondsto PSG and 8 to SiN.

electrode. In this case, Eq. (12) is reduced to

f

f(x,z>d) =-2 2&* .(-1y [t?)ln( nb E* + 1 ,,=o

+

(F) (-)+ tan-

1

z

b+x 2nd

(z + 2nd)’ + ( b - x)’ ( z 2nd)’ (b x)’

+

+ +

(q)(%)I. + tan

1

z 2nd

(21)

+

where K = (E* 1 ) / ( ~ *- l), E* is the specific dielectric constant of a passivation layer. If E* is one, only the first term remains and agrees with Eq. (12). The alternating series of Eq. (21) can be very rapidly summed up by Euler’s transformation. Figure 12 shows the surface potential of passivation layer.

2. Storage Time of CCVC The CCVC can be observed only for a storage time. It is expected that the storage time depends on the deposited charge on the passivation surface. Figure 13 shows the measured result for a microprocessor 8085A whose passivation layer is 0.8 pm thickness SiN and irradiated by 1 keV electrons (Ura et al., 1982); toN means the storage time when the electrode is switched from “Low” to “High”, and toFFvice versa. The latter is about twice the former. Gorlich et al. (1986) measured the storage time for 0.1 x 0.1 mm’ electrodes passivated with a 0.36 pm thickness SO, layer. The result is almost the same as that by Ura et al. (1982); a storage time of 1 second corresponds to about A/m’. They also measured the dependency on thickness; the storage time is inversely proportional to the thickness, as expected. Watanabe

251

ELECTRON BEAM TESTING

1 10.6

10.5

1 0-b

10-2

Electron Irradiation (C/rnz.secI FIG. 13. Measured storage time of CCVC vs electron irradiation in the case of the microprocessor8085A. (From Ura et al., 1982.)

et a!. (1985) calculated the effective secondary emission yield, while Sugiyama et al. (1986) tried the application of the “internal electric field model” in order to explain the reversible appearance-disappearance of the voltage contrast.

3. Some Observation Methods of the CCVC As stated above, the product of storage time to irradiation current density coulombs. This amount is rather low and one is typically an order of must devise some scan-displaymethods to observe the CCVC practically. The key point is to erase or initialize the surface potential by some effective method as noted by Fujioka et al. (1983). Takashima (1982) reported the alternate phase scan method to observe a stroboscopic image at the phase “A”. After one horizontal line scan, the device phase is switched to another phase “B” but the response at this phase has not been displayed on CRT. After one horizontal line scan, the phase is switched back to “A” and so forth. In the usual stroboscopic waveform measurement, the secondary electron signal at a certain phase is summed or averaged for n times of period in order to obtain a certain value of S/N. In this case, the irradiation time greatly exceeds the storage time. Todokoro et al. (1983) reported the fast phase scan

258

K. URA AND H. FUJIOKA

0

1

2

0

1

2

Time (11s) Time (11s) FIG.14. Comparison of random phase scanning to usual scanning in stroboscopic waveform mode: (a)by usual (serial) scanning, and (b) by random phase scanning. (From Ookubo et al., (b) Random scanning (a) Serial scanning 1986.) FIG.14. Comparison of random phase scanning to usual scanning in stroboscopic waveform mode: (a)by usual (serial) scanning, and (b) by random phase scanning. (From Ookubo et al.,

method. They divided n into k and 1. After each k times of period, the phase is 1986.) shifted by an increment. When the total shift becomes one period, the phase is 1 times. started from beginning. is repeated method. Theythe divided n intoThis k andprocedure 1. After each k times of period, the phase is Ookubo et al. (1986) reported the random phase scan method for the shifted by an increment. When the total shift becomes one period, the phase is stroboscopic waveform measurement. In this case, the phase shift is not by an started from the beginning. This procedure is repeated 1 times. increment random fashion.the The signalsphase at each phase are summed al. a(1986) reported random scan method for the Ookubobutet by each time. Figure 14 shows the result, where they used an energy stroboscopic waveform measurement. In this case, the phase shiftanalyzer is not by for an the voltagebut linearization (It0fashion. et al., 1985a). It shows its effectiveness. increment by a random The signals at clearly each phase are summed al. (1986) deposited a thinwhere metalthey piece which had the same size eachEkuni time. et Figure 14 shows the result, used an energy analyzer for 2 x 2 pm’ as the measured electrode. The CCVC well agreed with the voltof the voltage linearization (It0 et al., 1985a). It shows clearly its effectiveness. age Ekuni variation of (1986) the buried electrode in metal the whole region positive et al. deposited a thin piecevoltage which had theofsame size 2.5 keV. This and negative levels, where the primary electron energy was of 2 x 2 pm' as the measured electrode. The CCVC well agreed with the voltmechanism beenelectrode clarified.in the whole voltage region of positive age variationhas ofnot theyet buried Menzel and Buchanan (1986) proposed an erase operation the stroThis and negative levels, where the primary electron energy was 2.5in keV. boscopic waveform measurement where the spot and raster scan are switched mechanism has not yet been clarified. 50 ms.(1986) proposed an erase operation in the stroalternatively by every Menzel and Buchanan boscopic waveform measurement where the spot and raster scan are switched alternatively by every 50 ms. IRRADIATION EFFECTS IV. ELECTRON

IRRADIATION EFFECTS It is assumed forIV. theELECTRON electron beam testing that the characteristics of the tested device would be never changed. This is not always realized, because the it in some is whether its effects energetic electronfor may It is assumed theaffect electron beam sense. testingThe thatmatter the characteristics of the or not; the same on the concerned characteristics is within a permissible limit tested device would be never changed. This is not always realized, because the A irradiation condition in one butis not in another. in some sense. The case matter whether its effects energetic electron maymay affectbeit permissible slight change of the irradiation condition may often cause a great difference in on the concerned characteristics is within a permissible limit or not; the same the results. Itcondition is most desirable certify the in irradiation under the actual A irradiation may be to permissible one case effects but not in another. condition. slight change of the irradiation condition may often cause a great difference in the results. It is most desirable to certify the irradiation effects under the actual condition.

ELECTRON BEAM TESTING

259

Topics on the electron range, the backscattering range, the secondary emission, and EBIC are referred to in review articles by Birkhoff (1958), Kanaya and Okayama (1972), Seiler (1967,1983), Leamy (1982), Schick (1985), and Reimer (1985). Here the contamination and direct current injection are briefly commented on. The characteristics variation of a MOS FET by nonpenetrating electrons, and charging of a floating electrode and an insulator are discussed. A . Beam-Induced Specimen Contamination

As is well known, an image observed in a TEM or a SEM is degradated in a time lapse, in a SEM, a dark image. Details should be referred to in the review articles, for example by Hren (1979). Here some comments are added. The contamination is explained by surface migration of hydrocarbon molecules to the irradiated region where they are fixed by the electron irradiation. Although the present electron beam testing system uses the oil-free pumps, wiring to the tested device and the device itself contain hydrocarbon compounds. The line scan mode or spot mode is well used in the electron beam testing. This means an undesirable condition for contamination, because the contamination rate increases with an inverse square of beam diameter (Miiller, 1971). It is necessary to avoid the excess focusing of an electron beam. The low accelerating voltage is an important condition for the modern electron beam tester, as discussed in 1V.C. In this case, the contamination would increase especially for an insulator, because the secondary electrons would contribute to the contamination more than the primary electrons do. Contamination may still be an important hindrance to the quantitative measurement in electron beam testing.

B. Direct Contribution to Device Current When the primary beam current I,, is irradiated to metal lines, electrodes and diffusion regions of a transistor, the net current flowing into a device becomes IpE(1 - 6). (22) Actually, the secondary emission yield 6 may vary by the contamination and the emitted electrons will be redistributed by a potential barrier in the case of a positive electrode potential. In an LSI, some parts may be floating, that is, in no-connection from the other parts under some operating conditions. These may have only the capacitance. Let the capacitance be 0.1 pF. A voltage variation of one mV

260

K. URA AND H. FUJIOKA

results from an electric charge of coulombs, for example, a net current of 1 nA and an irradiation time of 0.1 p. If the pulse width is 1 ns, 100 times irradiation will cause this voltage change. When a p-n junction is irradiated, the EBIC current will be added to the junction current. Whether this is permissible or not depends strongly on the device itself and its working conditions. If the irradiated point is within a diffusion length from a p-n junction, the EBIC contributes directly to the device current also (Leamy, 1982). C . Characteristics Variation of MOS FETs by a Non-Penetrating Beam

The irreversible characteristics variation of a passivated bipolar transistor was reviewed by Keery et al. (1976). This was explained by charging-up at the interface between Si and S O z . Figure 15 shows a cross section of a MOS FET. It had been believed that MOS FETs would be insensitive to radiation regardless of X-rays or electrons. Hughes and Giroux (1964) found that this was not the case. Since then, many investigations have been carried out (Churchill et al., 1982). The variation of flatband voltage or threshold voltage is used to represent the irradiation effect. When the total dose is small, this variation is proportional to the dose. The variation of threshold voltage depends on the gate voltage during irradiation. At a constant dose, it takes a minimum value at a negative gate voltage (Barrey & Page, 1966). If the primary electron reaches the gate oxide, it generates electron-hole pairs. The holes will be trapped in the gate oxide, which causes the shift of the threshold voltage. The above results are by the penetrating irradiation to the active gate oxide. If the electron range is less than the distance to the gate oxide, it had

Poly-Si Gate 0.43pm

\

-

P Si

’ Gate SiO2

0.085pm

FIG. 15. Cross section of a MOS FET. (From Nakamae et al., 1981a.)

26 1

ELECTRON BEAM TESTING

been supposed that no appreciable variation could appear (Keery et al., 1976; Menzel and Kubalek, 1981). The characteristics variation of MOS FETs by a non-penetrating electron beam was first reported by Nakamae et a!.(1981a).The variation of threshold voltage is proportional to the irradiated electron charge density for small dosage. Figure 16 shows the irradiated charge density at a threshold voltage

C/cm'

1o

-~

1o-6

1o-8

10-

0

5

15 k V

10

FIG.16. Electron irradiation charge density which causes a variation of the threshold voltage of 10 mV versus the accelerating voltage of primary electrons.

Authors Nakamae et al. (1981) Todokoro el al. (1983) Miyoshi et al. (1982) Arima et al. (1983) Gorlich et al. (1985)

Thickness to gate oxide (pm) 1.83 1.2 2.3

Channel length (pm) 25 2.1 2.2 5

Gate voltage (volts) 4

0 0

262

K. URA AND H. FUJIOKA

variation of 10 mV as a function of the accelerating voltage. From the range theory, the primary electron could not reach the gate oxide below 12 keV. The region from 12 keV to 8 keV is interpreted as the range straggling. The point 5 keV needs another explanation. It is concluded that X-rays will be radiated from the upper part of the MOS FET and absorbed at the gate oxide (Nakamae et al., 1981a). This evidence has been followed by Miyoshi et al. (1982), Todokoro et al. (1983), Arima et al. (1983) and Gorlich and Kubalek (1985). Their results are summarized in Fig. 16. While the measuring conditions are different from each other in Fig. 16, it clearly shows the abnormal energy deposition at the gate oxide by a non-penetrating electron beam. They reported also that the irradiation effect becomes larger as the channel length of FETs becomes shorter than several pm. Miyoshi et al. (1982) and Mitsuhashi et al. (1983) measured the threshold voltage variation at the pulse operation and compared it with that at a dc beam. At the accelerating voltage of 1 kV, acharge density of lo-' to lo-' C/cm2 is permitted. If one scans a 30 pm x 30 pm area by 1 nA beam for 10 seconds, then the charge density becomes lo-' C/cm2. This means that not only the low accelerating voltage but also a very careful control of electron irradiation is needed for electron beam testing. D. Charging4.Jpof a Floating Electrode A floating electrode can be charged up by an electron beam. This application is reviewed in 1I.E. The basic property is the relation between the potential variation and the irradiated charge, and not all of the latter contributes to the former. In some respects, this problem is similar to charging an insulator. The charging characteristics have been investigated by Engel and Holmstrom (1972), Sebeson (1973), Engelke (1974), Pfeiffer et al. (1981), and Brunner and Lischke (1985). The positive charging is more complicated than the negative one. In the former, redistribution of secondary electrons by the potential barrier degradates the charging characteristics; the net charging current is not given by Eq. (22). The role of the potential barrier has been hardly investigated. This is discussed as follows. It can be represented by the saddle point of potential, which can be estimated by Eqs. (3) and (4) in the case of a line electrode. Often a grid with an extracting voltage of V,is put at z = 1 above the specimen in order to prevent the appearance of a potential barrier. The potential function in this case is given as follows. Taking into account the extracting grid, the potential function is calculated from Eq. (10) and by

f(x,z)

[-

= sgn(x)

1

(2) 71 tan-'

{(cosh(nx/l)+

sinh(nx/l) '>tan(+)}.

(23)

263

ELECTRON BEAM TESTING

In the case of a disc plate with a radius of a, the axial potential function is given by m

$(O, z) = E z

+ (V,/2) 1

{ g(2nl+

z) - g(2nI - z )

n=--m

+ g(2nl-

-g(2nI - 21 - z)},

21 + z ) (24)

g(z) = sgn(z) - z/(a2

+ z’)~’~.

(25)

The condition under which the saddle point appears, is written as By using Eqs. (23) and (24), the saddle point condition is calculated. Figure 17 shows V,,,/V, and V,,,/V, versus a/l. Fujioka et al. (1986~)measured the floating gate potential of a MOS FET. The gate electrode was connected with a pad electrode which had a size of 0.2 x 0.2 mm2 and then connected with an external terminal. This pad was irradiated by an electron beam. At first, the drain current of the MOS FET was measured as a function of the gate voltage. In the charging experiment, the 100

f

10

\

4 >

3 \ =cn.

>

+-

1

I

0.1 0.1

1

3

a/[ FIG. 17. Appearance condition of the saddle point in the potential distribution. An extracting grid is set at a distance of 1 above the plane electrode, which is a two-dimensional line with a width of 2a or a disc with a radius a.

264

K. URA A N D H. FUJIOKA

I

100 70

50

30

4-

w

Va = 1 kV

IP= 30 pA

2

10

200

-2

-

s -

Y

9-4 ~

-100 -50

-30

(b) FIG. 18. Charging characteristics of a floating pad electrode by an electron beam with a secondary emission yield higher than one: (a) positive charging for positive V,; (b) negative charging for negative V,. The arrows in (b) mean that the irradiation was stopped there. (From Fujioka et al., 1986c.)

ELECTRON BEAM TESTING

265

gate voltage VG was calibrated from the drain current. A control grid with a voltage of V, was set at 1 mm above the device. The total capacitance including wiring was 185 pF. Figure 18(a) shows the results in the case of 1 keV irradiation. When V, < 10 volts, VG is saturated. This is due to the potential barrier as is stated in the above. It is seen that the positive charge deposited on the floating gate is at most only about 6% of the irradiated electron charge. Figure 18(b) shows the negative charging characteristics at 1 keV and negative V,. In this case, about 10% of secondary electrons are returned to the floating electrode. If V, is negatively biased strongly, say - 1000 volts, almost all the secondary electrons would be returned to the pad electrode. This means that a floating electrode can be charged negatively by applying a strong retarding field, even if the secondary emission yield is larger than one. E. Charging of Passivated Devices by Non- Penetrating Electron Beam Irradiation

The LSI is usually covered by a passivation layer such as SiOt or SiN. As is well known, if an insulator is irradiated by a non-penetrating electron beam where the secondary emission yield is less than one, it is negatively charged-up and the highly bright part in the image is observed. In extreme cases, the image is distorted. The latter means address error of primary electrons due to a large potential difference. Various interpretations have been reported by many authors and reviewed by Pawley (1972), Shaffner and Hearle (1976), and Reimer (1985). In Knoll and Kazan’s textbook (p. 17) on the storage tube (1952), the redistribution of secondary electrons due to the surface potential barrier is illustrated in the case of the secondary emission yield higher than one, and it is also shown that the potential of the irradiated part is higher by a few volts than that of the neighboring area. 1. Negative Charging

The negative charging is often observed in the case of a photoresist whose cross section is shown in Fig. 9(a). In order to explain the bright part, one must find out how the secondary emission yield increases. Shaffner and Hearle (1976) assumed that just the surface would become positive to the inner region of a insulator because the secondary emission takes place at the very near surface region, and that this positive electric field would make the surface potential barrier lower; that is, a kind of Schottky effect would arise. This model disregards the EBIC. The electron-hole pairs which are generated by the primary electron, would cancel the strong electric field. Indeed Nunnes de Olivera and Gross (1975) postulated in their theory on

266

K. URA AND H. FUJIOKA

-I

40

-30 I

-20

- 10

0

lo

VG, ( V l FIG.19. Temporal variation of analyzed energy distribution curve of secondary electrons from SiO, film which has a thickness of 1.4 pm and is irradiated with 3 keV electrons. The parameter is the starting time of measurement.The analyzer is a parallel plate retarding field type. (From Taylor, 1978.)

space-charge conduction in an insulator that the electric field in the penetrating region would be determined by EBIC, and this would be fairly lower than that in the non-penetrating region where conduction was limited by space-charge. This theory has been applied to MOS structures where mobility and life time are measured (for example, Taylor and Mehdi, 1979). Figure 19 (Taylor, 1978)shows the temporal variation of analyzed S-curve of secondary electrons with a parallel plate retarding field energy analyzer, where the insulator layer was steam-grown SiO, on n-Si with a thickness of 1.4 pm. The primary electron had an energy of 3 keV and a current of 1 nA, and an area of 1 x 1 pm2 was irradiated. It is noted that (i) the secondary emission yield increases to 70 sec and after this it cannot be seen there, (ii) the energy distribution becomes broader and broader with the lapse of time, and even two steepest parts of the S-curve appear after 80 sec. In the case of negative charging of a photoresist in Fig. 9(a), the central upper part and edge parts become bright and the upper region between them becomes dark, which is seen for example in Fig. 36. This dark part means that it has the higher potential than that of the central bright part. These two evidences suggest the model shown in Fig. 20. The bright part is negative to the Si substrate and EBIC would occur between the surface and the

ELECTRON BEAM TESTING

267

PE

&

Insulator

Surface potential

t FIG.20. Negative potential region and secondary emission multiplication for negatively charged-up insulator surface.

substrate. On the other hand, the electrons from the bright part would experience the strong accelerating field along the surface. Some of them would strike there with a low angle. It may happen that the electron would turn out another electron and would itself be reflected specularly; if so, the secondary emission yield will exceed one. Furthermore, the emitted secondary electrons will have a wide energy distribution which is seen in Fig. 19. 2. Positive Charging The CCVC disappears after the storage time (see III.D.2). The surface potential may be uniform on the passivation surface and its equilibrium value may be the same regardless of the process of “High” to “Low” or “Low” to “High”. Gorlich et al. (1986) analyzed it based on a one-dimensional model. They approximated the detected secondary electron current by Eq. (7), where $,, = 0 is assumed. Combining it with the charging equation, they calculated charging process and compared it with experimental results. They also succeeded in explaining the difference between t o F F and CON. In the case of spot irradiation for the waveform measurement, the situation is quite different from the above. As seen from Fig. 14(a), the CCVC is decreased after a short time which might be different from the storage time in the case of the raster scan. The waveform seems to take the equilibrium values, which are different from each other depending on the process of “High” to “Low” or “Low” to “High. Fujioka et a/. (1983) measured them with a parallel plate energy analyzer when the primary electron energy was 1 keV and the secondary emission yield was larger than one. Figure 21(a) shows the measured voltage shift for the CCVC in the case of the A1 stub plate of 10 x 10 mm2 coated with a SiO,

-51

-10

FIG.21. Shift of retarding voltage vs. that of specimen electrode buried inside a passivation layer: (a) in the case of a stub electrode; (b) in the case of a 10 pm width line electrode. (From Fujioka et a!., 1983.) 268

ELECTRON BEAM TESTING

269

-

Central region (do)

c _

Core region (dc) SE redlstribution region

FIG.22. Schematics of the surface potential irradiated by an non-penetrating electron beam which has a secondary emission yield higher than one. (From Fujioka et al., 1986d.)

passivation layer of 1 pm thickness. It well agreed with the voltage variation of the buried A1 stub electrode. Figure 21(b) shows the result in the case of the 10 pm width A1 line. It is remarkable that only the slight shift of the surface potential was measured compared to the stub electrode. To explain the surface potential of passivated devices, Fujioka et al. (1986d)proposed the “core” model which is a modified version of Takashima’s model (1982).They named the irradiated region the “core” as shown in Fig. 22. They assumed that the core itself would be retained regardless of voltage variation of the buried electrode and that its equilibrium potential would vary according to the latter. They studied the equilibrium potential of the core by taking account of redistribution of secondary electrons; the binary potential distribution was assumed, the saddle point potential was calculated from Eq. (2), and the amount of redistribution was estimated from Eq. (7). Their important conclusions are as follows. At first, when the buried electrode is a stub plate, the secondary electrons are redistributed to the core in the same manner independent of the voltage variation of the stub electrode. Therefore, the variation of the surface potential is the same as that of the stub plate as shown in Fig. 21(a).Secondly, when the buried electrode is a line electrode with a finite width, a certain leakage path of the electric charge from the irradiated part must be assumed in order to explain Fig. 2l(b). As the leakage path, Fujioka et al. (1986d)assumed only the surface conduction to the core and the exterior region. It seems more reasonable to assume EBIC to the buried electrode instead of leakage to the exterior region (see below). 3. EBIC by a Non-Penetrating Electron Beam

The EBIC in (SiO,) by a non-penetrating electron beam was reported by Taylor and Mehdi (1979).They measured this in the case of a MOS structure.

270

K. URA AND H. FUJIOKA

They observed it as soon as the primary electron reached the SiO, layer by penetrating the A1 thin film which was kept at a negative voltage. Takashima (1982) found that the stroboscopic waveform can be measured without appreciable distortion by applying a negative voltage (- 3 volts) instead of a positive extracting voltage to a grid which was set at a distance of 1 mm above the device. While the mechanism was not accounted for by this author, this evidence may result from the resistive coupling due to EBIC between the surface and the buried electrode, because the surface seemed to be negative in this condition.

v. ELECTRONOPTICAL COLUMN OF ELECTRONBEAMTESTING The electron beam tester is different from a usual SEM in some respects, and these differences are discussed in the following. A. Low Accelerating Voltage Electron Gun

As is stated in the previous sections, the low accelerating voltage is needed in order to avoid the characteristics variation of a device and the negative charging of a passivated device. On the other hand, it may increase contamination. As the accelerating voltage of a conventional SEM is decreased, the beam current decreases faster than the accelerating voltage. This is due to reduction of electric field intensity at the cathode by space-charge. Ohye et al. (1973) derived a formula by which one can estimate the space-charge-limited current density of an emitter from field calculation in the case of no space-charge. This has been certified by an exact numerical calculation of Poisson field (Takaoka and Ura, 1986). In the space-charge-limited case, the current density at the cathode is proportional to a power of 1.5 of the anode voltage. Because the gun brightness is proportional to both anode voltage and cathode current density, it decreases by a power of 2.5 of the anode voltage. In order to reduce the space-charge effect, the following methods have been taken: (a) reduction of the distance between the cathode and the anode, (b) introduction of the additional extracting anode before the final anode, and (c) reduction of the cathode tip radius. By method (b), Yamazaki et al. (1984) increased the brightness by one order and studied the effect of various kinds of electrode shapes on the brightness. It is noted here that an increase of a gun brightness is not always ac-

ELECTRON BEAM TESTING

27 1

companied with an increase of the brightness on the specimen plane. This was pointed out Uchikawa et al. (1983) in the case of a pointed cathode gun, and later by Katsuta et al. (1986)in the case of a magnetic-field immersed field emission gun. The characteristics of an electron gun have been drastically improved by the new emitter material. LaB, has been practically used in the high brightness purpose. A thermionic emission gun with this has usually a higher brightness by one order compared with a conventional tungsten hairpin cathode gun. The disadvantage of a LaB, emitter is its variation in both size and shape in operation. It depends on many factors: emitter temperature, orientation of the crystal, vacuum environment, operation time, tip radius, cone angle of the emitter, and so on. Takigawa et al. (1982) studied a variation of optimum Wehnelt voltage in a time lapse. Hagiwara et al. (1982) investigated the optimum condition for the tip radius and cone angle. Furukawa et al. (1983) recommended a large tip radius for the electron beam lithography. These were carried out under the practical vaccum environment. In such a case, the tip radius becomes less and less independent of crystal orientiation, while the (100) facet is most stable in the ultra-high vacuum (Oshima et al., 1980).Davis et al (1986) reported that the residual oxygen promotes the material evaporation and its speed depends on the crystal surface and that the facet nearest to the original cone shape grows selectively. A usual emitter is (100) orientation, tip.radius of 15 pm, cone angle of 90 degrees. For optimum operation of a LaB, gun, it is necessary to adjust the bias voltage according to the emitter deformation. A field emission gun has much higher brightness and less beam current than a LaB, gun. The former has an advantage of finer beam diameter and less beam current, and loses it for larger beam current. Orloff (1984) commented on an advantage of the field emitter or thermal field emitter over the LaB, emitter for a spot size of 0.1 pm and less, while Broers (1972)mentioned 30nm. The trajectory of a field emission gun can be calculated very rapidly by applying the ordinary paraxial ray and aberration theory. The general trajectory is expressed by a sum of two paraxial rays and aberrations (Ura and Takaoka, 1986). As emitter materials, W and Zr/W are practically used. Zr/W has a lower work function and narrower angular confinement than those of W; it is used also as a thermal field emitter (Swanson and Martin, 1975). Recently, a Ti/W thermal field emitter was developed by Hosoki et al. (1986). It has similar angular confinement and works at a lower temperature of 1200"C, while ZrW works at 1500°C. The emission characteristics of T i c have been reported by Fujii et al. (1985). The most difficult problem in practical use of a field emitter has been its emission stability. As well known, the emission current from a field emitter

P x loo ( Pa-A

1

FIG.23. Emission current fluctuation of field emitter as a function of a product of residual gas pressure and emission current. (From Todokoro et al., 1982.)

fluctuates more or less. It depends on vacuum environment and working conditions (for a recent review, see Adachi (1985)).Todokoro et al. (1982)found out that the current fluctuation depends on a product of residual gas pressure and emission current as shown in Fig. 23. In the region where the fluctuation increases accompanied with the product, fluctuation is due to ion bombardment of an emitter by nearby ionized gas molecules. In the region where it is independent of the product, it is due to molecule adsorption by surface migration. This means that if one wishes to use a field emitter at a large emission current, one must improve vacuum environment accordingly. The thermal field emission is preferred. The microdischarge in vacuum is very fatal to practical use of a field emission gun. In the low accelerating voltage, this becomes less serious. Furthermore, recent improvement of vacuum technology, automated control of working conditions, and quality control at the production line have made a low accelerating voltage field emission gun commercially available. (Todokoro et al. (1985a) reported an electron beam tester that is equipped with it). B. Pulse Gate

The electron beam pulses can be generated by various pulse gating methods: a) beam intensity modulation at the Wehnelt electrode (Lukianov

ELECTRON BEAM TESTING

273

and Spivak, 1966;Szentesi, 1972; Kotorman, 1980);b) space focusing action by a klystron cavity lens (Oldfield, 1976) or an electrostatic einzel-lens (Plies, 1982);c) phase focusing action by a klystron cavity buncher (Hosokawa et al., 1978a); and d) beam deflection across a chopping aperture. The last method, i.e., the deflection gating scheme, has been most widely employed in the electron beam testing instruments because of its simplicity and wide adjustable pulse duration from nanosecond to picosecond regions. In the following, we restrict the discussion mainly to generation of picosecond electron beam pulses by electron beam deflection methods, where the picosecond pulses is a term for pulses with about 10-300 ps durations. The first part is concerned with deflection systems, the second part with electron optics, and the third with measurement of pulse waveforms. 1. Electron Beam Dejection System Electron beam pulses with repetition rates from megahertz to gigahertz regions and durations of nanosecond to picosecond regions can be produced by deflecting the primary electron beam across a chopping aperture as shown in Fig. 24. Here we discuss a) installation of the deflection structure into the SEM column, b) types of deflection structure, c) methods of driving of the deflection structure, and d) some examples of generation of picosecond electron beam pulses in the SEMs. a. Installation of Dejection Structure The items which should be considered in installing the deflection structure into the column are as follows: 1) installation brings as little structural modification in the column as possible, and 2) installation brings as little degradation in the electron optical

Chopping aperture (FA1 1.

FIG.24. Generation of electron beam pulses by a deflection structure and a chopping aperture.

K. URA AND H.FUJIOKA

274

Aperture (FA11 ( 1 )

Aperture (FAI)

(n)

(m)

FIG.25. Three different arrangements of deflection systems.

properties of the original SEM as possible. These two are demands which oppose each other, and to what extent each requirement can be satisfied, determines the arrangement of the deflection system. The arrangement of the deflection system is divided into three types: types I, 11, and I11 as shown in Fig. 25. In the type I arrangement, the deflection structure is installed between the electron gun and the first lens. In usual operating conditions of the SEM, deflected beams are chopped by the objective lens aperture whose demagnified virtual image is produced above the first lens: this image is termed final aperture image (FAI) (Gopinath and Hill, 1977). As all electrons passing through the FA1 aperture reach the specimen, while the others do not, the FA1 acts as an effective chopping aperture. The type I arrangement has been widely used (MacDonald et al., 1969; Feuerbaum and Otto, 1978; Feuerbaum et al., 1978; Hosokawa et al., 1978a; Menzel and Kubalek, 1979; Sadorf and Kratz, 1985) since the first stroboscopic SEM by Plows and Nixon (1968), because it requires least modification in the column. This arrangement has a shortcoming degrading of the electron beam spot size caused by the apparent lateral movement of the gun crossover. The chopping degradation factor, i.e., the ratio of apparent to real crossover size, is typically of the order of 2-5 (Gopinath and Hill, 1977; Menzel and Kubalek, 1979; Fujioka and Ura, 1983; Lischke et al., 1983). This crossover movement, however, would not bring so severe an influence in the final beam spot size at the specimen, so long as the magnification in the electron optical column, M, is much less than unity, as is the case in the usual operating conditions for the instruments with W or the LaB, electron gun. The type I arrangement is not applicable to the column with the field emission (FE) gun where M is around 1. In the type I1 arrangement, the second crossover is made near the deflection pivot point so as to minimize the chopping degradation (Lin and

ELECTRON BEAM TESTING X

t

275

X

'

t

i

-h(a! (b) FIG. 26. Deflection pivot point:(a) the case where the electron transit time effect is negligibly small, and (b) the general case where the transit time effect cannot be neglected.

Beauchamp, 1973; Gopinath and Hill, 1977): the deflection structure is installed between the first lens and the second lens (Fentem and Gopinath, 1976;Gopinath and Gopinathan, 1978),or between an additional lens and the first lens (Schmitt et al., 1986; Ueda et al., 1986; Ozaki et al., 1987). The deflection pivot point P is defined as a virtual point from which all electrons entering along the optical axis are assumed to emerge straight regardless of deflection angle as shown in Fig. 26. There arises no apparent movement of the crossover if the second crossover is made at the deflection pivot point. As is commonly known, the pivot point P coincides with the center of the deflection field, 0 as shown in Fig. 26(a),when the electron transit time effect caused in the deflection process is negligibly small. In a general case where the transit time effect cannot be neglected, however, the pivot point P is not positioned on the axis but formed at the off-axial position (It0 et al., 1985b) as shown in Fig. 26(b).This shift of the deflection pivot brings some image shift at the specimen plane but no degradation in the spatial resolution, as will be discussed in detail later. In the type I11 arrangement (Eidson and Scudder, 1981; Todokoro et al., 1983; Rose and Zach, 1984), the signal at the second deflector is delayed by the electron transit time between the first and second deflectors so as to form the deflection pivot at the center of the deflection structure where the chopping aperture is placed. Then the second crossover is formed at the deflection pivot in the same manner as the type I1 arrangement. b. Types of DeJector There are three types of deflection structures which have been used to generate picosecond electron beam pulses in the SEM: a) reentrant cavity structures (Hosokawa et al., 1978a),b) trough-type travelling wave structures (Fentem and Gopinath, 1976; Feuerbaum and Otto, 1978; Ozaki et al., 1987), and c) plate capacitors (Fujioka et al., 1978; Menzel and Kubalek, 1979; Sadorf and Kratz, 1985; Schmitt et al., 1986; Ueda et al., 1986). The reentrant cavity structure has the highest deflection efficiency (deflection angle divided by the square root of the deflection power) owing to its

276

K.URA AND H.FUJIOKA

resonant structure, but has a disadvantage in that the deflection frequency is fixed to a resonant microwave frequency such as 1 GHz. The trough-type travelling wave structure (meander line with the shielding plate, Yamada and Takagi, 1972) requires the fixed primary electron energy in order to coincide the electron velocity with the velocity of the travelling deflection wave, which permits a long deflector length and therefore a high deflection efficiency. The basic feature of this structure is its wide bandwidth from dc to some GHz. The greatest advantage of the plate capacitor is its simplicity of structure. All the commercial instruments now available use the plate capacitor plus the type I or type I1 arrangement. c. Driving Methods Generation of electron beam pulses can be realized by five different driving methods (Fujioka and Ura, 1983)as shown in Fig. 27:

a) b) c) d) e)

X deflector (sinewave), X deflector (sinewave) + Y deflector (sinewave), X deflector (dc + pulse), X deflector (dc + pulse) + Y deflector (pulse), X deflector (sinewave$) + Y deflector (pulse; 2f/N).

A , , d,, and D,are defined in Fig. 24. t denotes the pulse width generated and I , is the pulse beam current. Applying a sinusoidal voltage with an angular frequency w to one deflector (X deflector) results in a sinusoidal movement of the beam across a centrally placed aperture as shown in Fig. 27(a). The repetition rate of generated beam pulses is twice the deflection frequency f = o/(2n).It should be noted that for stroboscopic studies, the repetition rate of the pulse has to be equal to or 1/N ( N = 2,3,. ..) of the event repetition rate; frequency multiplexer or dividers so as to adjust both repetition rates are required. It is seen from Fig. 27(a) that the following relation holds A , sin(oz/2) = ( d ,

+ D,).

(27)

and thus the pulse width z is given when COT << 1 as z = 2(d,

+ D,)/A,w.

(28)

In the driving method (b), the second deflector (Y deflector) is added perpendicularly to the first one (X deflector) so that a beam would trace a Lissajous figure around the chopping aperture. Shift coils are used to make the Lissajous figure cut across the aperture. The repetition rate of the pulse coincides with the deflection frequency as shown in Fig. 27(b). The pulse width given by Eq. (28) still holds. In the driving method (c), the deflector is dc-biased so that the electron beam is normally off and the beam pulse is generated when the beam drops

ELECTRON BEAM TESTING

277

At

AA

^I Icl

[e) FIG.27. Five different methods of driving deflection structures. (From Fujioka and Ura, 1983, with permission of the Foundation for Advances in Clinical Medicine, Publisher.)

into the chopping aperture on the bottom of an excitation voltage pulse as shown in Fig. 27(c). The pulse width is determined by the width of the deflection pulse itself. The repetition rate of generated pulses coincides with the deflection frequency. In the driving method (d), the X deflector is also dc-biased as in method (c) but the beam pulse is generated when the beam traverses the chopping aperture on the rising edge of an excitation voltage pulse. The flyback during the falling edge of the excitation pulse is blanked by an additional deflector (Y deflector) placed orthogonally to the X deflector (Thomas et al., 1976). In this case, pulse width is given as = (d,

+ D,)t,/A,

(29)

278

K. URA AND H. FUJIOKA

where t, is the time required to deflect the beam by A , at the chopping aperture plane. The pulse repetition rate coincides with the deflection frequency. The last method (e) shown in Fig. 27(e)permits a reduction in the repetition rate of beam pulses generated by method (a) (Morimura and Ura, 1973; Ura et al., 1982). The pulse repetition rate coincides with the frequency of the excitation voltage pulse applied to the Y deflector. The pulse width is determined by the X deflection and thus Eq. (28) still holds.

d. Generation of Picosecond Electron Beam Pulses Examples of generations of picosecond electron beam pulses by realized combination of system arrangements, deflection structures, and driving methods are listed in Table 111; # 1 - # 5 are classified into the type I system arrangement. The deflection structures are installed between the gun and the condenser lens of the original SEM except for #4. In #4, an additional lens is positioned between the deflection structure and the condenser lens, and the FA1 is formed between the gun and the deflection structure. Examples of the type I1 system arrangement are # 6 - # 10. Generation of picosecond electron beam pulses by the type 111 arrangement has not been reported in the literature, though beam pulses of 1 ns duration are generated (Todokoro et al. 1983). It should be noted that Hosokawa et al. (1978a)were able to obtain a beam pulse of 0.2 ps duration using a phase focusing (bunching) technique together with a deflection method. For some other examples the reader may refer to Fujioka and Ura (1983). 2. Electron Optics of Picosecond DeJection System As is seen from Table 111, the driving method (d) is most widely used to generate picosecond electron beam pulses over a wide frequency range of from 1 MHz to 250 MHz. In this section we treat the electron optics when electron beam pulses are generated by using the rising edge of an excitation voltage pulse. This is a special case of the sinusoidal wave driving where a sinusoidal voltage of V, sin o t is applied to the deflector. One can use the results derived by Hosokawa et al. (1977) by letting oV, = k and w + 0 where k is the rising rate of the applied voltage pulse. In the following, however, we will solve the paraxial trajectory equation again. All the deflection structures in rf fields have an off-axial longitudinal component of electric field (Panofsky and Wenzel, 1956) which, in combination with the transit time effect, gives rise to an additional longitudinal velocity spread in beam pulses. This velocity spread causes chromatic aberration which may influence the beam diameter at the specimen plane. Furthermore, it causes debunching of beam pulses which may enlarge the pulse width at the specimen plane (Ura and Morimura 1973).Special attention should be paid to these effects, when the generation of beam pulses with

TABLE I11 EXAMPLES OF GENERATION OF P I ~ S E C O N ELECTRON D BEAM PULSES System arrangement (Fig. 25) Fujioka et. a[. 1978 Hosokawa et al. 1978a Feuerbaum & Otto 1978 # 4 Menzel & Ku balek 1979 # 5 Sadorf & Kratz 1985 # 6 Fentem & Gopinath 1976 # 7 Schmitt et al. 1986 # 8 Gopinath & Gopinathan 1978 # 9 Ueda et al. 1986 # 10 Ozaki et al. 1987

#I #2 #3

I I I I I I1 I1

I1 I1 I1

Deflector type Plate capacitor Reentrant cavity Travelling wave structure Plate capacitor Plate capacitor Travelling wave structure Plate capacitor Travelling wave structure Plate capacitor Travelling wave structure

Driving method (Fig. 27)

Pulse width (PS)

a b

200 7.6 300 11 15 5-8

C

d d a C

d d d

40 100 65 40

Pulse frequency (MH4

Accelerating voltage

8.6 lo00

25 25 2.5 10 3 10 2.2 12.5 1 1

50 250

loo00 10 1

(kV)

280

K. URA AND H. FUJIOKA X

-b-a

-a

0

a a+b

2

(C)

FIG.28. A model for analysis: A parallel plate capacitor is assumed as a deflection structure.

around 10-picosecond duration is the question. In the following, the electron optical treatment taking account of the effects caused by the longitudinal velocity spread will be described.

a. Analysis of the Pulse Gate Figure 28 shows the model for analysis where a parallel plate capacitor is assumed as a deflection structure (see Fig. 28(a)). We solve the paraxial trajectory equation under the following assumptions: 1) field distribution and electron motion are two-dimensional in the x-z plane,

2) transverse electric field distribution on the axis, E X o ,is trapezoidal as shown in Fig. 28(b), 3) quasi-static approximation is valid, 4) deflection voltage pulse is assumed to be rising with a constant rate of k = Vd/tr while the electron is travelling through the deflector as shown in Fig. 28(c), thus f(t) = (VJdt,)t = kt/d and the electron transit time T = 2(a + b)/u, = h/u, is shorter than the rising time t, where V, is the amplitude of the deflection voltage and d is the distance between the deflection plates, and

28 1

ELECTRON BEAM TESTING

5 ) longitudinal velocity variation Au, caused in the deflection field is much smaller than the incident electron velocity uo. For the electron which has the initial condition at z = - ( a

+ 6)

u, = 0 0 , x = xe, x’ = x i , and Ae = ( A U , / U at ~ )the ~ exit of the deflector ( z = a calculated as follows: x’ = xb,

x = xo,

T

+ C(+) + D ] - uF[(t

- 2F2(+)’

+ b) are

+ xb

t0

x: = aF-

Ae = (:)2[

(30)

+

4)~;]: +

(33)

where to denotes the time at which the electron reaches the center of the deflector, and a = - V, - - -h T

Edt,

hkT -dV,

q = -b 2a

(34) (35)

F = -I- l + q 21+2q

+ 2q + 2q2 ( 1 + 2q)2 c = - -1 1 + 3q + 7 q 2 / 2+ 7 q 3 / 5 1

R=-F 12

1

(1

6

D

1 6

=-F2

1

(37)

+

+ 2q + 2q2 (1 + 2q)2

(39)

As can be seen in Eqs. (34)-(39), u, q, F, R , C , Dare all dimensionless quantities. The relativistic correction is neglected in deriving Eqs. ( 3 1)-(33) because the electron accelerating voltage V , often used in the recent electron beam testing is low around 1 kV. The length of the fringing field region can be estimated by using the following relation by Recknagel (1983):

282

K. URA AND H. FUJIOKA

b. Simplijied Formula for Pulse Width With the aid of Eq. (32), the deflection amplitude A. at the chopping aperture is given as

where L,, is the distance from the center of the deflector to the chopping aperture (see, Fig. 24), and the effect of the fringing field is neglected, i.e., q = 0. Substitution of Eq. (41) into Eq. (29) gives

V, d d, 7=2----

+ D, Lda

If the accelerating voltage V,, the deflector dimensions h and d , the rising rate of the deflection voltage pulse k, and three parameters at the chopping aperture (FAI) plane, L d a , d,, and Da are given, then the pulse width z can be readily estimated. For example, when

V , = 1 kV,

h = 30mm,

d = 2mm

k = 7.5 x lo9 V/s (t, = 3 ns) L,, = 42 mm,

d,

=

165 pm,

Da = 5 jtm,

(43)

z is calculated as 72 ps for the measured value of 65 ps (FWHM) (Ueda et al., (1986). It should be confirmed that the assumption of T c t, ( = 3 ns) is fully satisfied, because the transit time T for 1 keV electron is 1.6 ns. More precise treatment which takes the spatial conditions of the electron beams entering the deflector and the longitudinal velocity spread into account will be discussed later in Section V.B.2.d.

c. Dejection Piuot It is well known that in a parallel plate capacitor the pivot point P coincides with the deflector center 0 as shown by the # 1 trajectory in Fig. 29(a), if the deflection field is static. In this situation electrons aiming at the deflector center 0 as in the case of type I1 arrangement are deflected as if they are emerged straight from the point 0 = P. Let us discuss the case where the deflector is driven by a pulse voltage. From Eqs. (31) and (32),the trajectory of the electron ejected from the deflector is expressed as to 1 x = (aF- + x b ) z - ahR + xo + - h x b . (44) T 2 If the deflected trajectory is traced back to the center plane of the deflector, the x coordinate at z = 0 is given by 1 x ( z = 0) = -ahR + x o + -hxb (45) 2

ELECTRON BEAM TESTING

283

-h-

(a) (b) Deflection pivot in type I1 arrangement: (a) the case where the electron transit time effect is zero (static field case) or negligibly small, and (b) the general case where the transit time effect cannot be neglected. Flci. 29.

which contains no time dependent term. Thus the deflection pivot point is given by substitution of x, = xb = 0 in Eq. (45): P(z, X)

= (0, - ahR) = ( O , ~ X ) .

(46)

With the aid of Eq. (34), the shift of the deflection pivot 6x is expressed as

where q = elmo (e and m, are the charge and the mass of an electron) and the fringing field effect is neglected in the last equation. As is seen from the first equation, 6x approaches 0 as long as the rising time of the excitation voltage pulse is much greater than the electron transit time T. It should be noted that in ideally working travelling wave type deflectors, this type of deflection pivot shift will not occur because the deflection field is equivalent to static field for electrons travelling with the same velocity as the deflection wave: T/tr= 0. From the second and the third equations, it is seen that 6x increases in proportion to the rising rate of the pulse voltage and the third power of the electron transit time, and decreases linearly with the distance between deflection plates. In the type I1 arrangement, the second crossover is made at the deflector center 0. The injection condition is expressed as x,

+ -21h x b

= 0.

With substitution of Eq. (48) into Eq. (45), electrons are deflected so as to emerge straight from the point P as shown by the # 2 trajectory in Fig. 29(b). Since the point P does not vary with time, the pivot shift does not bring about the degradation in the spatial resolution due to beam blurring, though the

284

K. URA AND H. FUJIOKA

image shift occurs by MSx at the specimen plane where M is the electron optical magnification. By using other alignment coils different from ones attached to the original SEM, the image shift caused by the pivot shift can be corrected without affected the electron optical axis at the static state. Let us consider the case defined by Eq. (43).By substituting k = 7.5 x lo9 Vs, T = 1.6 ns, and d = 2 mm into the third relation of Eq. (47), we obtain Sx = 0.23 mm which will produce the image shift of 2.3 pm at the specimen plane when M = 0.01. If the accelerating voltage is increased by a factor of 10 to 10 kV, then the image shift is reduced by a factor of about 30 to 0.078 pm. If the pulse width is enlarged by reducing k, then the image shift is reduced accordingly. Thus it is seen from these numerical examples that the image shift caused by the shift of deflection pivot is notable in the case of low keV and picosecond beams. In the type I11 arrangement where deflection structure is composed of a set of two parallel plate capacitors of length h/2, the pivot point is positioned at the center plane of the structure and the shift of the pivot point is given by the same formula as Eq. (47). So far, the deflection voltage pulse is assumed to be rising while the electron is travelling through the deflector. Here we treat the case where the rising time t, = T2 is shorter than the transit time T = TI T2 + T3as shown in Fig. 30. Let the origin of the z-axis be the position of the electron at the center of the rising edge (see Fig. 30), then the coordinates of P(z, x)are calculated as (0,Sx):

+

-Vd

-

tT1-I-

T 2 4 -

T 3 4

(b)

FIG.30. A model for analysis when the rising time of the applied pulse voltage is shorter than the electron transit time.

285

ELECTRON BEAM TESTING

where h , , h,, h , are given in Fig. 30 and

c1,,

are defined as

vd

hl

a1 =

a 2 , and a,

---

dV, vd

h2

a2 = 2-

-

d

h3

a,=--

va vd

dV,

It should be noted that the first term of Eq. (49)vanishes in the travelling wave type deflector, but the second term remains. Let us consider the case where T, = T, = 500 ps, T2 = 600 ps, V , = 1 kV, Vd = 1.25 volts, h = 30 mm and d = 0.8 mm, then 62 = -0.16 mm. This result agrees fairly well with the result of 0.17 mm by numerical ray tracing by computer (Ito et al., 1985b). d. Method of Longitudinal Emittance Diagram From Eqs. (31)-(33), the time t , and the longitudinal velocity variation A, at the exit of the deflector for the electron which has the initial condition (xo,xb) and can pass through the chopping aperture can be obtained. The longitudinal emittance diagram is the set of points (t,,A,). The time t , and the longitudinal velocity variation A, at the specimen plane is calculated by using the following relation in the drift space: t,

=

Les

t , - -Ae

(53)

VO

As = A,

(54)

where L,, is the distance between the deflector exit and the specimen plane. From the longitudinal emittance diagram which is the set of points ( t s ,A,), the pulse width z and the longitudinal velocity spread A at the specimen plane can be obtained. Figure 31 shows an example of calculation of pulse width and velocity spread as a function of k with other conditions as given in Eq. (43): c1 = 2.4 x 10-"k. The open circles in the figure denote the measured pulse width (Ueda et al., 1986) which will be described in Section V.B.3. The region where the pulse width increases with pulse rising rate k is due to an increase in the logitudinal velocity spread introduced in the deflection process. The attainable minimum pulse width is limited to about 10 ps. 3. Measurement of Pulse Waveform

In the various methods to measure the pulse waveforms directly, the Lissajous figure method that transforms a time distribution of the beam pulse

286

K. U R A A N D H. F U J I O K A 10-8

10'~

D

10'"

0.01 lo8

0.1

lo9

10

1

1o'O

10"

10l2

k (Wsec) FIG.31. A n example of calculation of pulse width and longitudinal velocity spread as a function of k and a by the method of longitudinal emittance diagram. The open circles in the figure denote the measured pulse width.

into a spatial intensity distribution (Ernst and VonFoerster, 1954) has the highest time resolution. In the Lissajous figure method, an additional measurement deflector which works in synchronism with the pulsing deflector is installed at the specimen plane as shown in Fig. 32. The waveform of the beam pulse is transformed into the line intensity profile 6 A at the detector plane, since the time distributed electrons are differently deflected depending on arrival time at the deflector. There are two methods (a) and (b) as shown in Fig. 32 to get the pulse waveform from the line intensity profile. In method (a), the line intensity profile 6 A is scanned across the aperture by a scan coil synchronous with the spot of a cathode-ray tube (CRT). Electrons passing through the aperture are collected, amplified, and sent to the CRT as the video signal. The displayed image represents a waveform of the beam pulse (Hosokawa et al., 1978a). In this method, the calibration between the deflection amplitude and the time is required. Hosokawa et al. (1978a) measured a beam pulse of 0.2 ps (FWHM) with a measurement accuracy of 0.08 ps by this method, and close agreement was obtained between the measured pulse width and that calculated with the help of the longitudinal emittance diagram. In method (b), 6 A is scanned across aperture by a continuous variation of

287

ELECTRON BEAM TESTING

I generator

Delay circuit

(a)

(b)

FIG.32. Measurement of pulsed beam waveforms by the Lissajous figure method: A scan coil (a) or a delay circuit (b) is used to scan the line intensity profile 6 A across an aperture.

the delay time between the deflection voltage at the pulsing deflector and that at the measurement deflector (Menzel and Kubalek, 1979). In this method, the delay circuit with a high delaying accuracy is required. Menzel and Kubalek (1979) measured a beam of 11 ps (FWHM) with an accuracy of 3 ps by this scheme. Recently Ueda et al. (1986) have reported a compact retractable measurement unit which evaluates five parameters of electron beam pulses (the dc and pulsed beam currents, the spot sizes of dc and pulsed beams, and pulse width) simultaneously. The pulse waveform of 65 ps (FWHM) was measured with an accuracy of 6 ps by method (b); the delay circuit used allowed the total delay of 1.6 ns in steps of 1 ps within an increment accuracy of 0.1 ps (Fujioka et al., 1986a).The measured pulse width agreed fairly well with the calculation by the method of logitudinal emittance diagram as shown in Fig. 31. C . Secondary Electron Detectors

A conventional SEM uses mostly the Everhart-Thornley type secondary electron detector (Everhart and Thornley, 1960). This has a high sensitivity

288

K. URA AND H. FUJIOKA

----------

Specimen

2 cm FIG.33. Hemispherical retarding field analyzer to reduce LFE. (From Nakamae et al., 1986.)

and stability. The micro-channel plate (MCP), a secondary electron multiplier, has been improved in its stability and some electron beam systems use it. It has such an advantage that the collection of the secondary electrons is axially symmetrical as shown in Fig. 33. The quality of SEM contrast strongly depends on the configuration of objective lens, specimen stage and secondary electron detector, which is reviewed by Reimer and Riepenhausen (1985). The secondary electron detector is designed to collect low energy electrons, say to 50 eV. Actually, it collects not only direct secondary electrons (type I) from the specimen but also secondary electrons (type 11) which are generated at the wall of an objective lens by high energy backscattered electrons. The type I1 electrons degradate SEM contrast; they contribute to the background level. This can be improved by setting the detector at the upper side of the objective lens (Kawamoto et al., 1984).This top detector has another advantage in that the working distance to the stage is small. This makes it possible to increase the beam current. The disadvantage is a narrower field view. For linearization and quantitative measurement of voltage, a secondary electron energy analyzer is needed. From the point of view of S/N, a retarding

ELECTRON BEAM TESTING

289

field energy analyzer, that is, an integration type, is preferable (Gopinath, 1977). Although the high energy resolution for this purpose is desirable (Menzel and Kubalek, 1983), it is not critical. The most important items for energy analyzer for this purpose are: (i) high transparency of secondary electrons, and (ii) slight LFE. Furthermore, it may not degradate the characteristics of the primary electron beam and especially, it may not have any harmful deflection field. For a voltage linearization, the energy analyzer is usually included in a feedback loop. Ishizuka et al. (1984) constructed a software feedback loop in which the retarding grid voltage is controlled iteratively by software. The present authors use the open loop method because of its flexibility (Fujioka and Ura, 1985). Various energy analyzers have been devised; they are reviewed by Menzel and Kubalek (1983).Their characteristics, however, are usually influenced by the LFE which is briefly discussed in 1II.A. Hereafter, the LFE reduction type energy analyzer is discussed. Fentem and Gopinath (1974) reported a hemispherical analyzer. It is expected that it would not be very sensitive to the LFE. Goto et al. (1981) improved it by inserting a buffer grid after the retarding grid and later adding a plane grid to keep the extracting field for secondary electrons uniform (Ito et al., 1985a). Todokoro et al. (1985a) set a hemispherical analyzer on the upper side of the objective lens. Although these analyzers have the improved performances as shown in Table IV, the combination of the analyzing hemispherical field with the extracting field of secondary electrons is not clear-cut. Nakamae et al. (1 985) proposed a hemispherical retarding energy analyzer for secondary electrons, where the design principle is very clear. As is well known, the parallel plate diode has a virtual cathode behind the real one. If the longitudinal velocity is neglected, a spot size on the virtual cathode is the same as that on the cathode. This situation can be applied to the design of the analyzer; the tested electrode of a LSI corresponds to the cathode, the extracting grid to the anode. Actually, the longitudinal velocity cannot be neglected and the local field affects the size at the virtual source point. Even so, it is estimated at about several tens pm. If one can take the radius of hemisphere larger than 10 mm, the LFE would be neglected in principle. Figure 33 shows the cross section of the analyzer which was designed by the above principle (Nakamae et ul., 1986b).By this, the LFE has been reduced by about one order, while theoretically by two orders. Kawamoto (1986) reported a similar improvement by setting a parallel plate retarding field analyzer in the top detector. In this case, it is supposed that the secondary electron would meet the retarding grid perpendicularly. Garth er al. (1985) have applied the adiabatic theorem in the strong magnetic field to secondary electrons. A strong unipole magnetic field is

290

K. URA AND H. FUJIOKA

TABLE IV

REDUCTION OF LFE Authors

Line width (Pm)

Spacing

8 8

12 12 2 5 4

Nakamae et. al. (1981b)' Nakamae et al. (1981b)' Nakamae et a/. (1986) Ito et al. (1985) Todokoro et al. (1985) Kawamoto et al. (1984) Garth et al. (1986) ~

2 5 4 4 2

(Pd

1-k1 (%)

60 4 6 6 3

k*

(%I 3 4 1

2

1

~~

' Conventional SEM detector.

* Parallel plate retarding field analyzer. k , and k,

are defined by Eq. (15).

applied at the specimen plane and it is decayed at the retarding grid of a parallel plate energy analyzer. By this, the secondary electrons which have a large emitting angle meet the retarding grid almost perpendicularly. They succeeded in reduction of the type I1 LFE by an amount similar to that of the above authors, while it seems that the detected current is much depressed due to the weak extracting field in the case of a positive electrode voltage. Table IV summarizes the results by the above authors. Plies and Kolzer (1986) designed a hemispherical analyzer which is set on the upper side of the objective lens and whose center is set at the projected image of the tested electrode by the objective lens. D. Peripheral Equipment for Specimen Chamber This section describes some specimen chamber peripheral equipment which is useful for practical electron beam testing. 1. Temperature Control Unit

To compare the measured result with the simulated one in the design verification of newly developed devices, the experiment should be carried out at the same temperature at which the simulation was done. The device under test (DUT) has to be cooled or heated. Temperature control of the DUT is also required in the failure analysis to inspect the device which works well at room temperature but breaks down at a high or low temperature. To meet these purposes, a simple cooling/heating unit which sprays cool or warm air to the rear side of the chip carrier exposed to the air was reported (Kollensperger et al., 1984).

ELECTRON BEAM TESTING

29 1

Most recently, a more practical temperature control unit is proposed (Todokoro et al., 1986; Yamada et al., 1986). A heat sink which is placed in contact with a heat block is attached to the rear side of the DUT with a ceramic paste. In the heat block temperature-controlled cool or hot dry air is circulated through the outer tube. The temperature which is monitored by a thermocouple connected to the heat sink is controlled over a range of - 70°C to 100°C. The dependence of temperature on propagation delay in 16 kbit bipolar RAM is measured by using this unit in the temperature range of -40°C to 70°C. A fairly good agreement between measured and simulated results are reported. A similar cooling/heating unit is also reported (Hosoi et al., 1986), which utilizes a fluorine-contained inactive liquid as a thermal medium and controls the temperature over a range from room temperature to 120°C within an accuracy of _+ 3°C. 2. Mechanical Microprobe as a Signal Feeder A method to feed an electrical signal directly onto the internal signal line on the integrated circuits by way of a mechanical microprobe positioned with the help of micromanipulators installed in the SEM specimen chamber has been proposed (Wolfgang, 1983). Probe placement is very simple, since the moment when the probe is contacted onto the line is easily monitored by using voltage contrast. A contact impact can be minimized. The mechanical signal feeder with a tip diameter of 2 pm (see Fig. 8) is effectively used in the failure analysis of CMOS gate arrays (Miura and Tamaru, 1986).

VI. AUTOMATIC CONTROL SYSTEM OF ELECTRON OPTICAL COLUMN To obtain accurate and easy operation and a high throughput in various kinds of electron beam instruments such as electron beam testers, electron beam linewidth measurement instruments, and electron beam pattern inspection systems, there is increased demand for automatic computer control of electron optical parameters such as accelerating voltage, beam current and beam diameter. For example, in a test method that utilizes the electron beams as micro current sources for inducing voltage or for activating beam switches built within integrated circuits (Lukianoff and Langner, 1983), an automatic conversion from a high accelerating voltage and large beam current (e.g., 10 kV and 1pA) mode for micro current injection to a low accelerating voltage and small beam current (e.g.. 1 k V and 1 PA) mode for probing is essential.

*-

f

CdUn

* 1I

\D

I

meter

'""oo"o;;"""' HT GA SA CL MG IS SR ST OL

Interface box Reaister

h)

N

I -

I

I

B C

A

1

II

Gun alignment STB alignment

SA Condenser lens CL Maanification

I ,

PCD on/of f

IContrast Manual/Computer PCD control

I

I

AFD control

I

I

I I I I SEM {voltmet erF 1

1 I

I

FIG.34. Block diagram of a fully computer-controlledscanningelectron microscope system. (From Fujioka et al., 1986e.)

ELECTRON BEAM TESTING

293

In the inspection of insulating materials such as photoresist patterns, the use of the accelerating voltage which causes no surface charging is required. To determine the accelerating voltage at which specimen charging starts, the accelerating voltage must be changed continuously without changing other electron beam irradiation conditions. The computer-controlled column is best suited for this purpose. Furthermore, full automation is the general trend for all measuring systems. Many papers have been reported on computer control of the SEM for image recording and processing (Herzog et al., 1974) and automatic focusing and stigmatizing (Tee et al., 1979).A few studies have been presented, however, about computer control of electron optical parameters such as accelerating voltage, beam current and beam diameter. Furukawa et al. (1985) have reported a simple control system which was designed to try to reproduce the state of the electron optical column with the aid of the control table stored in the computer: the table contains the control data for each of the electron optical elements of the column which are measured manually in advance for various operating conditions. This method is not proof against the hysteresis in magnetic lenses that affects the reproducibility of the system in particular when the accelerating voltage is changed over a wide range, for example, from 5 kV to 10 kV and the variation with time in the emission properties of the electron gun. A similar table-controlled system which allows a rather narrow accelerating voltage variation of from 0.5 k V to 1 kV has been reported (Yoshizawa et al., 1986). Fujioka et al. (1986e, 1987) have developed a sophisticated system which makes effective use of lens initialization to minimize the hysteresis effect and feedback control of the beam current to deal with the time variation of the gun emission characteristics. The system allows a high standard of automatic control of electron optical parameters and automatic preparation of the control table with a high reproducibility. In this part, we describe the system of Fujioka et al. and its experimental performance. A . Control System

The system is illustrated in Fig. 34. The interface box with a CPU and a control register connect the stroboscopic SEM and the minicomputer which has an analog input module (AIM), a GP-IB bus and an RS 232C bus as 1 / 0 (input/output) devices. A standard stroboscopic SEM based on JEOLJSM/IC845 is modified so as to communicate with the interface box and the minicomputer. The relation between control elements and controlled variables is illustrated in Table V. The control parameters for each control element and

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K. URA AND H.FUJIOKA TABLE V CONTROLLED VARIABLES AND CONTROL ELEMENTS

Controlled variables

Control elements

Accelerating voltage

Electron gun

Beam axis

Gun alignment coils

STB lens alignment coils

Beam current

Condenser lens

SE signal Brightness

SE detector

--

HT: Gun high voltage 0.5 5 kV (100 V steps) 5 10 kV (1 kV steps) GA: Coil current ( H T , CL-coarse) tilt x, y 12 bit shift x, y 12 bit SA: Coil current (HT, CL-coarse) 12 bit tilt x, y shift x, y 12 bit CL: Excitation current (HT, 1,) coarse code 4 bit fine code 12 bit B: C:

Contrast Image Focus

Control parameters

Objective lens

Stigmatism

Stigmator

Image shift

Image shift coils

Scan rotation

Scan rotator

Amplifier offset (HT, CL-coarse, WD) 12 bit PMT HV (HT, CL-coarse, WD) 12 bit

OL: Excitation current (HT, WD) coarse code 8 bit fine code 12 bit ST: Coil current (HT, W D ) x, Y 12 bit IS: Coil current (HT, WD) x, Y 12 bit SR: Coil current (HT, W D ) 0 12 bit

setting resolutions are also shown in this table. The control parameters are: the gun high voltage ( H T ) ,the driving currents for the gun alignment coils (GA), the STB lens alignment coils (SA),the condenser lens (CL), the image shift coils (IS),the scan rotation controller (SR), the stigmatizor (ST),and the objective lens (OL),the magnification (MG), and the brightness (B) and contrast (C) of the secondary electron (SE) detector. The STB lens is used in the stroboscopic operation to focus the beam at the center of the deflector (type I1 arrangement described in Section V.B.1.a). Also controllable are the following two attachment units: the probe beam current detector (PCD) which is put into the electron optical axis as required to measure the probe beam current I,, and the automatic focusing device

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(AFD). The PCD serves as a beam shutter which prevents the unnecessary irradiation of electron beam to the specimen. The system enables the lens initialization to be made by feeding a maximum rated current to each lens during a prescribed time period so that the effect of the hysteresis in the lens can be initialized. The system has two feedback loops: one for adjusting the alignmeht coil currents GA, SA and CL-fine code so that the beam current measured by the PCD coincides with the specified value within an accuracy of L lo%, and the other for adjusting the offset voltage of the preamplifier and the high voltage of the photomultiplier tube (PMT) so that the current B and C values coincide with the specified values (ordinarily, 0-5 volts) within an accuracy of f O . l volt. B. Procedure for Control

The automatic control of electron optical parameters is feasible by the following two methods in this system, the table control method which utilizes only the control table and the feedback control method which uses both the control table and the feedback control loops. 1. Table Control Method

The key point of the table control is how to keep the effect of hysteresis in magnetic lenses to a minimum in order to obtain the table with a high reproducibility. The ease of the preparation and renewal of the table is also an important point. The beam axis is related to the accelerating voltage (gun high voltage) and also to the excitation currents of the condenser lens. The setting currents for gun and STB lens alignment coils are tabulated as a function of accelerating voltage and coarse code of the condenser lens. This situation is described as coil currents [HT, CL-coarse] in the G A and SA columns of Table V. This notational convention is the same as other control parameters. The brightness and contrast of the SE detector are tabulated as a function of HT, CL-coarse, and working distance ( WD), and the control parameters for image-related elements (OL, ST, IS, S R ) as a function of HT and WD. Before starting the control table construction, we measure the maximum as a function of HT and CL-coarse code: IPmax [HT, CLbeam current IPmax coarse] is tabulated and stored in the computer. Then the control table to get a focused and image-shift corrected image for specified accelerating voltage V, and beam current I , is made according to the following procedure:

1) set the accelerating voltage V , and the working distance WD, 2) initialize the lenses,

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3) select the CL-coarse code which would realize I , with the aid of the IPmax [HT, CL-coarse] table and set it to the condenser lens, 4) keep alignment coil currents under feedback control so that I , approaches IPmax: we can get GA, SA, [HT, CL-coarse], 5) keep CL-fine code under feedback control so that I , coincides with the specified value: we can get CL-fine code [ H T , I,], 6) keep B, C of the SE detector under feedback control so as to coincide with specified values: we can get B, C [HT, CL-coarse, WD], 7) focus the image by the AFD: we can get OL-fine and coarse codes [HT, WD], and finally, 8) adjust the coil currents manually so that stigmatism, image shift, and scan rotation can be minimized: we can get ST, I S , SR, [HT, WO]. Thus tabulation of all the parameters listed in Table V is finished. It is seen from the processes 4) to 7) that the preparation and renewal of the control table, except for the image-related parameters, can be made automatically without any manual operation. The procedure for table control is as follows: 1) set WD and input the required values of V,, I , , and M G from the keyboard, 2) practice the lens initialization, which reset the electron optical column to the state when the control table was created, 3) read out the control data GA, SA, CL, I S , SR, S l : OL, and B, C from the control table and set them to the register in the interface box through a RS 232C bus, and 4) transfer the data in the register to each control element. The table control method is useful especially in the case when the range of the accelerating voltage change is narrow, as will be shown in Section V1.C.

2. Feedback Control Method In the feedback control method, the following three processes follow after the processes 1) to 4) of the table control method: 5) adjust the alignment coil currents G A , SA and CL-fine code so that the measured beam current coincides with the specified value, 6) adjust the offset and high voltage of the SE detector so that the measured B and C values coincide with the specified values, and finally, 7) focus the image by the AFD. By using the feedback control method, we can get a high reproducibility of the system even when the accelerating voltage is changed over a considerably wide range of, for example, from 1 kV to 10 kV as will be shown in Section V1.C.

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C . Experimental Performance

In this section the basic performance of the system is measured by using a standard SEM magnification calibration specimen (by feedback control method) and a LSI photoresist specimen (by table control method). In the experiment, the SEM micrographs are recorded with the computer-controlled image acquisition system (not illustrated in Fig. 34) which is comprised of a digital probing device (DPD) and an image processing device (IP). The specimen is scanned 512 x 512 pixel array by the DPD. The IP allows the simultaneous storage of twelve 512 x 512 pixel images of 8 bit resolution, which can be transferred to the computer for nonvolatile storage on the floppy disk. The photographic record of the image is taken from the display on the CRT of the IP. 1. Feedback Control Method

The experimental specimen consists of square mesh grid of 33 pm mesh pitch and latex microspheres of 1 . 1 pm in diameter as shown in Fig. 35. The experiment was done by allowing the accelerating voltage to change from 1 kV to 10 kV with the beam current maintained constant at 10 PA. The results which demonstrate fairly good controllability of the system are shown in Fig. 35. The image rotation of 1 degree left uncorrected is due to the fact that the computer control of the SR was unavailable when the experiment was done. The total time to get a SEM image was 30 sec: 5 sec for processes 1) to 4), 10 sec for processes 5 ) and 6), and 15 sec for process 7). 2. Table Control Method The table control method was applied to the observation of photoresist charging. The photoresist is patterned on the base material of polycrystalline (Poly-) Si. The 40 pm x 40 pm area of the specimen which contains 3 rectangular (3 pm x 30 pm) and I5 square (3 pm x 3 pm) photoresist patterns was observed as shown in Fig. 36. The micrographs were recorded when the accelerating voltage was changed in sequence from 0.7 k V to 1.5 kV in 100 volt steps with the beam current set to 6 PA. The average error of the beam current from the set value was about - 10% and the image shift was within k0.5pm in both x and y directions. The control time was 5 sec in contrast with 30 sec in the feedback control method. The recording time per micrograph was 9 sec. The total time required to get a series of 9 micrographs from which the charging start voltage of the photoresist was found to be 1.2 kV was about 2 minutes.

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FIG.35. Experimental results of feedback control of the system: the accelerating voltage is changed from 1 kV to 10 kV in steps of 1 kV with the beam current maintained constant at 10 PA. (From Fujioka et al., 1986e.)

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FIG.36. Experimental results of table control of the system: the accelerating voltage is changed from 700 V to 1.5 kV in steps of 100 V with the beam current set to 6 PA. (From Fujioka ef al., 1987.)

D. Prospects The replacement of magnetic structures in the electron optical column with electrostatic ones as far as possible will solve the problems due to the hysteresis and shorten the response time, though a new problem of contamination and some increase in aberration may occur. The automation of measurement and correction of image-related variables such as image shift with the help of the pattern recognition technique will improve the system performance still more. VII. EB TESTER SYSTEM Recently the instruments or systems which are used for electron beam (EB) testing are most often called “EB tester”, from simple stroboscopic SEMs to large-scale systems linked to the computer-aided design (CAD) database

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system. The generation of EB testers may be divided into the following three generations from a viewpoint of the level of automation of testing.

I) the first generation: the period associated with simple systems of which main instrument is a standard SEM or standard stroboscopic SEM (or modified SEM), 11) the second generation: the period associated with practical systems with several additional subsystems each of which are systematically controlled by one or more computers for easy operation and automation of some testing processes, and 111) the third generation: the period associated with automated systems linked to the CAD database system for automation of some or complete processes of various kinds of testing. It should be noted that from a viewpoint of cost performance, an EB tester of the second or third generation is not always more advantageous than that of the first generation: cases often arise in which the EB tester of the first generation is sufficient for some specific purposes. The present application of EB testers is divided into two broad categories: 1) design verification during the development phase by comparing measured data with designed data obtained by simulation on the characteristics of internal waveforms such as voltage amplitudes, propagation delays, rise and fall times; and 2) fault diagnosis (failure analysis) during the development and production phases which is put into practice on and after some faults are found by means of LSI testers or some other methods. Faults diagnosis is subclassified into the following four categories: a) location of faults by comparing the logic data measured by static (dc) voltage contrast between faulty and fault-free reference devices, b) location of faults by comparing the logic data measured by dynamic (stroboscopic) voltage contrast between faulty and reference devices, c) location of faults by comparing the measured logic data of the faulty device with the expected logic data generated with the aid of the CAD system, and d) location and resolution of faults by using various testing modes such as the stroboscopic image and waveform modes and the logic state mapping. Thus the EB tester of the third generation for faults diagnosis of subclass c), for example, is described by the notation [III.2.c]. In the following, the EB tester systems and their application to various testing of LSI circuits are described. A. EB Testers of the First Generation

With EB testers of the first generation, a variety of works have been performed which demonstrate that electron beam testing is a powerful

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alternative to the method using mechanical probes for inspection of modern high-density and fast integrated circuits. The fields of application of the [I,l] testers to design verification extend over a wide range, i.e. functional testing of dynamic and static MOS RAMs (Feuerbaum et al., 1978; Wolfgang et al., 1979; Nakamae et al., 1981~;Lukianoff et al., 1981; Todokoro, et al., 1983), microprocessors (Crichton et al., 1980; Ura et al., 1982), MOS speech synthesizer devices (Bestente et al., 1985), bipolar devices (Gopinath and Gopinathan, 1978; Fujioka et al., 1980a; Todokoro et al., 1985b),and surface acoustic wave devices (Feuerbaum e f al., 1980). Many effective applications to faults diagnosis have also been reported, i.e. location and resolution of faults on dynamic MOS RAMs (Gonzales and Powell, 1978b; Yuasa et al., 1980) and bipolar TTL ICs (Nakamae et al., 1981d)with [I,2.d] systems and location of failures on microprocessors with a [112.a] system (Bergher et al., 1986). The various experimental supports with EB testers of the first generation have encouraged one to develop more practical systems of new generations aiming at automation of testing. B. E B Testers of the Second Generation

In 1982, Miyoshi et al. have developed a practical EB tester system [11,1] which is composed of a standard stroboscopic SEM with a LaB, gun operating at low accelerating voltages of 1-2 kV, a general purpose LSI drive and delay controller unit, and an image acquisition and processing unit each of which is systematically controlled by a microprocessor and a desktop minicomputer to get easy and accurate operation for electron beam positioning, waveform sampling, and stroboscopic image acquisition. Effective feedback from measurements with the EB tester to circuit design that improves the reliability and performance of various kind of VLSI circuits has been reported: CMOS/SOS microcomputer (Koike et al., 1982),64 kbit CMOS static RAM (Uchida et al., 1982), 256 kbit dynamic RAM (Sato and Saito, 1983), 1 Mbit CMOS dynamic RAM (Shimizu et al., 1985). Kollensperger et al. (1984) have developed a similar practical EB tester [II,l] whose special feature is that its electron optical column is rotated by 180" so that the pins of the package are exposed to the air, making an optimal signal feed to the DUT is attainable. An EB tester with a high time resolution of 10-100 ps which is of use for high speed devices, such as GaAs ICs is presented (Goto et al., 1984;Todokoro et al., 1986; Ozaki et al., 1987). A standard IC tester aided EB tester system [II,2.a] has been reported. The IC tester provides the signal for electron beam positioning and the test vector sets to the DUT (Walter et al., 1982). Hosoi et al., (1985b) developed an EB tester system [II,2.d] directly combined with an LSI tester in order to resolve

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the following two practical difficulties in the failure analysis of complex random logic devices by means of stroboscopic voltage contrast: 1)what kind of waveforms should be applied to the faulty device to resolve failures; and 2) how to generate the waveform and how to apply it to the faulty device. The system has been successfully used for resolution of failures on a CMOS microprocessor containing 63,000 transistors, an NMOS microprocessor containing 16,000 transistors, and a bipolar gate array containing 10,000 elements, and a NMOS controller LSI containing 29,000 transistors. Various kinds of test systems which automatically locate the faults by comparing the internal logic data between faulty and fault-free devices have been reported: [11,2.a](Thangamuthu et al., 1982; Furukawa et al., 1985; Oxford and Propst), [II,2.b] (May et al., 1984)and [II,2.b,d] (Shinkawa et al., 1986).May et al.’sdynamic fault imaging system for random logic devices such as microprocessors which consists of a stroboscopic SEM operating at a low accelerating voltage of 0.7 kV, an IC tester and an image array processor is most distinctive. The fault propagation in time and space can be observed on a color monitor by acquiring sequences of dynamic voltage contrast images for both faulty and fault-free devices and then comparing them in the image processor to show how faults nucleate and how they spread out. C . E B Testers of the Third Generation

One of the most time-consuming processes in the design verification and fault diagnosis using EB testers of the first or second generation is the node location process; wiring patterns corresponding to a certain circuit node to be tested have to be located manually on an observed SEM image of the DUT. Since the number of wires in a modern LSI circuit has amounted to several thousands, a manual node location has become extremely troublesome. Several authors point out that the combining of an EB tester with a CAD system is useful in order to resolve this problem (Walter et al., 1982; Thangamuthu et al., 1982; Kollensperger et al., 1984).Kuji et al. (1983, 1986) have developed the first fully automated EB tester system [III,2.c] which has strong links with a CAD database system. The system consists of a standard scanning electron microscope equipped with a motor driven X-Ystage, an LSI driver which generates test patterns, and a host computer in which the LSI CAD system is installed. The CAD system they used (Sudo et al., 1983) includes the following three design data groups which are mutually linked as shown in Fig. 37 where all of the interconnection nodes are identified by a serial net number: 1)the logic circuit data group which holds information on a logic design level, such as logical connections among standard cells; 2) the placement and routing data group which holds information on a layout design

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ELECTRON BEAM TESTING DESIGN DATABASE

LOGIC SIMULATOR

EBT

-PI QEFERENCE

-T

PLACEMENT AND ROUTING DATA GROUP

1-5: NET NUMBERS OBSERVED LOGIC-STATE

MASK PATERN DATA GROUP COMPARISON

DESIGNED LOGIC-STATE MAP FIG.37. An example of the design database where the logic circuit data group is mutually linked with the mask pattern data group via the placement and routing data group. (From Kuji et a/., 1986, copyright 0 1987 IEEE.) U

level, such as the position of logic cells and interconnection patterns; and 3) the mask pattern data group which holds information on a mask design level, such as the size and position of the patterns. Thus, when a net number is given, the corresponding interconnection patterns can be easily identified through the internal links in the design database. The outline of the testing procedure for locating faults with this system after some suspicious circuit region is detected by using an LSI tester is as follows: 1) Input the net numbers which belong to the suspicious circuit region, then the corresponding designed interconnection patterns are displayed on the CRT terminal. 2) Determine the observation zone which contains the net numbers to be checked by moving the cursor on the screen of the CRT. 3) Set the SEM observation area by moving the X-Y stage on which the DUT is mounted. 4) Sample the static voltage contrast image at an accelerating voltage of 1 kV, then binary logic-state image is obtained by thresholding. 5 ) Compare the logic state for each net between the observed and designed logic-state images by matching and superimposing both images, then

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net numbers with incorrect logic states are picked up and displayed on the screen of the CRT. 6) Trace the propagation of the nodes with an incorrect logic state, then faulty cells are finally pinpointed. Processes 1) to 6) are performed automatically with the aid of the host computer. It should be noted that this system is effective for pinpointing faults in an early stage of the development phase, when a fault-free reference device is not present. It will shorten the turnaround time (TAT) from test to redesign of newly developed LSIs. Utilizing the system, a fault diagnostic test of a 40kgate LSI circuit has been performed successfully (Tamama and Kuji, 1985). In succession from the earlier system, Kuji and Tamama (1986) and Tamama and Kuji (1986) have developed a new system to which a generalpurpose LSI tester is directly combined to generate test patterns and apply them to the DUT. A 75k-transistor CMOS digital color-encoding device has been successfully tested by using this system. If the design data groups exist separately in the database system, it is not straightforward to obtain the one-to-one correspondence between the logic circuit node (net-list data) and the interconnection pattern (layout data). A solution is given by Kuji and Tamama (1986). A net-list extractor is used to generate a transistor level net-list from layout data written in the CALM GDS-I1 format and a link is created between the net-list and the layout pattern. On the other hand, the circuit data described in the Hierarchical Specification Language (HSL) is expanded into transistor and gate levels by using the Hierarchical Design Database Manipulator system. Then, the netlists extracted from the layout data and the circuit data are automatically matched in transistor level, thus correlation between the layout pattern and the logic circuit node is formed. This procedure is effectively applied to the failure diagnosis of a 20k-gate CMOS parallel processor manually designed by CALMAGDS-11. An EB tester system [III,2.c] similar to one described above, which seeks high throughput at relatively low cost has been developed and practically used for fault diagnosis in the mass-production phase of the Level Sensitive Scan Design (LSSD) random logic devices (Okada and Asai, 1987). An EB tester system [111, 13 for design verification fully automated by connecting a standard stroboscopic scanning electron microscope and the CAD database system has been presented (Kuji et al., 1985). A similar fully automated system which can deal with the layout data written in the format of Applicon 860, CALMA GDS-11, or CIF and the circuit design data described in the SPICE or SDL format have been reported (Concha et al., 1986). Other attempts to make effective use of CAD database system for design verification have been reported by several authors (Komatsu et al., 1986;

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Vernay et al., 1986; Battu’ et al., 1986; Guiguet et al., 1986). A method which intends to locate the faults on lower metallizations of multilayer devices by using a fault dictionary has also been proposed (Okamoto et al. 1985). The dictionary describes the relation between the internal faults and the combination of the input test pattern and the faults on the uppermost metallization. D. Peripheral Equipment and Techniques

In this section, some peripherals for EB tester system are described. 1 . Synchronizing Circuitry The accuracy of synchronization between the electron beam pulses and the periodic event on the DUT is one of the three factors which affect the time resolution in stroboscopy together with other two factors, i.e., the pulse duration and the transit time effect. A typical constitution of the synchronizing circuitry is shown in Fig. 38. A signal from the synthesized signal generator is divided into two parts. One part, after passing through the delay generator and the pulse generator, is fed to the pulse gate. The other, after passing through the IC or LSI driver, is fed to the DUT. The accuracy of the synchronization is determined by the sum total of the jitter generated in the synchronizing circuitry (Ozaki et al., 1987). To achieve a high synchronization accuracy, the

Pulse gate

generator

Delay generator

: Pulsed beam Synthesized signal generator

FIG. 38. A typical constitution of the synchronizing circuitry of EB testers.

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jitter in each component of the circuitry should be suppressed to as low as possible. The resolution of the time scale is determined by the resolution of the delay generator (circuit), or phase shifter. Fujioka et al. have developed three types of delay circuits which can be controlled by a computer: 1) 200 ps resolution phase locked loop (PLL) delay circuit (Fujioka et al., 1978); 2) 100 ps resolution delay circuit which consists of three units of multiplexer IC/transmission line combinations; a unit consists of an 8-line multiplexer IC and transmission lines terminated to inputs of the multiplexer IC which connects one of eight inputs to the output according to the three-bit code present on the address input of the IC, then the output is delayed in proportion to the length of the transmission line through which the signal propagates (Fujioka et al., 1980b, 1984); and 3) 0.1 ps resolution delay circuit which consists of continuously variable mechanical phase shifter of broad bandwidth and a pulse motor (Fujioka and Ura, 1981; Fujioka et al., 1986a). The second type has become one of the standard delay circuits for scanning electron microscope stroboscopy and has been used in practical EB testers with a high time resolution better than 100 ps (Todokoro et al., 1983, 1986; Ozaki et al., 1987). The third one has been effectively used in the experiment where a high delay resolution of several picoseconds or less is required, for example, in the measurement of voltage waveforms on gigahertz semiconductor devices (Fujioka and Ura 1981), current waveforms of picosecond electron beam pulses (Ueda et al., 1986), and transit time effects on voltage contrast (Nakamae et al., 1986a). Todokoro et al. (1986) has proposed a new timing control method which realizes a total delay of 100 ps in steps of 10 ps. A timing of beam pulse generation is shifted by changing a small offset voltage applied to the deflector plate. A delay greater than 100 ps is performed by using multiplexer and transmission line combinations. Ozaki et al. (1987) developed a multiplexer IC/transmission line type delay circuit with a 10 ps resolution by connecting variable capacitors to the transmission lines in order to correct the delay step. 2. Electron Beam Positioning

In the first stage of the electron beam positioning, a two-dimensional image for searching the target points on the DUT is displayed by means of the “image scan mode” on the screen of the CRT from which the addresses to be tested can be read out. Then, “the point probe” mode allows one to hit the electron beam probe on the target points according to the specified addresses. The positioning error caused by the difference in the driving method of the scanning coils between the image scan mode and the point probe mode is measured. The method to correct it by software has been proposed (Fujioka et al., 1986b) and a positioning accuracy of k0.03 pm was reported.

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3. Processing of Multilayer Devices As the use of multilayer metallization techniques in LSI manufacturing has become widespread, the failure analysis becomes more and more difficult. A solution to solve this difficulty by means of laser irradiation has been reported (Sanada and Nikawa, 1986). In order to obtain the voltage contrast image on the lower level metallization of a double-layer metallization device, a 30 pm diameter open hole is made through the upper level metallization and the isolation layer without destroying any electrical functions by using a pulse driven Na-YAG laser. By this technique, a silicon gate CMOS LSI with double-layer metallization is analyzed.

VIII. MEASUREMENT OF MICROSTRUCTURES As the semiconductor feature size becomes smaller and smaller into the submicron range, even a very small change less than 0.1 pm in the geometrical dimensions would give rise to remarkable changes in electrical specifications of a circuit. Therefore, routine in-process inspection and measurement techniques with sufficient accuracy of microstructures produced in the microfabrication processes have become extremely important. Microstructures to be measured are photoresist patterns and film patterns after etching. Of particular importance is the measurement of the photoresist patterns, since if the quality of the resist patterns is poor or the feature sizes are not within a specified range, the resist may be stripped and the complete photoresist process repeated, and only acceptable wafers go on to be etched (McGillis, 1983). The photoresist patterns have a 1-2 pm thickness and their edges are usually tapered. The cross section of the photoresist pattern is approximately trapezoidal. The resolution and the depth of field by means of optical methods is no longer sufficient for most applications. Even by allowing the depth of field to decrease to less than 1 pm, conventional optical methods will probably not be able to obtain the sub-0.5 pm resolution (Steckl, 1986). Furthermore, the optical methods have many drawbacks due to scattering, diffraction and interference at pattern edge which always cause measurement errors (Nyyssonen, 1979; Yamaji et al., 1985). The method by scanning electron microscopy utilizing low accelerating voltages is more suitable. The resolution better than 15 nm and the depth of field on the order of several micrometers are obtainable using secondary electron signals at a low accelerating voltage of 1 kV. The use of such a low accelerating voltage is preferable to reduce sample damages. By selecting a proper accelerating voltage, between 0.7 to 1 kV, sample charging can be also minimized.

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In the following, we will describe the measurement of microstructural dimensions such as the linewidth, the slope angle, the height and the edge profile by using the secondary electron signals in the scanning electron microscope. An electron beam pattern inspection system will also be described. A . Linewidth Measurements

Special attention should be paid to the distinction between a width and a spacing measurement (Jensen and Swyt, 1980). A cross section of a microstructure is illustrated schematically in the upper part of Fig. 39, and a typical secondary electron intensity profile is shown in the lower part of the figure. So long as the intensity profile is reiterative, it will always provide an exact line spacing by measuring any corresponding two points in the profile. On the other hand, a measurement of the linewidth involves a left edge to right edge measurement. An accurate location of the measured edge in the profile relative to the true edge of the structure is essential. In order to minimize the error in the edge location, it is important to know the correspondence between the topographic intensity profile in the SEM and the real surface topography of the microstructure. 1. Intensity Projile of the Microstructure

To explain a topographic intensity profile from microstructures, George and Robinson (1975) calculated the secondary electron yield using twodimensional Monte Carlo computations. In the actual measurement of

Microstructure

'-Width--: SE intensity! profile/ I Spacing

WidthFIG.39. A cross section of a microstructure (upper part) and a typical secondary electron intensity profile (lower part).

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microstructures, however, some emitted secondary electrons are unable to reach the detector due to collisions with the microstructure itself or the objective-lens pole piece. Thus, an analysis which takes secondary electron collection efficiency into account is required. Miyoshi and Yamazaki (1986) have proposed a new simple method to analyze the intensity profile using the secondary electron emission conditions based upon the modified diffusion model and the acceptance diagram method which traces the secondary electron trajectories from each ejection point to the detector. The procedure for calculation is described in Section 111.1. According to the analysis by Miyoshi and Yamazaki, the intensity profile from the microstructure (photoresist pattern on the substrate in their case) illustrated in Fig. 39 is explained as follows. The flat regions denoted by A and G depend on the secondary electron yields on the substrate and pattern materials, respectively; the region B to C represents the decrease in collected current due to collisions of secondary electrons with the slope region of the pattern; the locations of the minimum point C and the maximum point F shift outside by about one half of the probe size from the positions relative to the true bottom and top edges; the slope region denoted by D is due to the drastic change in secondary electron yield; and the region F to E is due to the gentle change in secondary electron collection efficiency. Thus, predicted intensity profiles show good agreement with the measured intensity profiles as shown in Fig. 9. 2. Methods of Linewidth Measurements In the commonly used threshold method, the intersection of the intensity profile and a properly set threshold level is defined as the edge. The linewidth is a left edge to right edge distance. However, this method produces a certain error due to the difference between the measured intersection and the position corresponding to the true edge (point P in Fig. 39). Another problem in the threshold method is that the linewidths are different for different threshold levels and different slope angles (4 in Fig. 39). To determine the point P corresponding to the true bottom edge of the photoresist pattern, Miyoshi et al. (1986a) proposed a new measurement method called the “linear regression method” in which the intensity profile is approximated by two lines at each pattern edge. One is the average line of the intensity profile on the substrate and another is the slope line corresponding to the slope of the pattern edge. The intersection of two lines is defined as the bottom edge of the pattern. They have evaluated the correlation between measured linewidths by the linear regression method and the true linewidt hs measured from SEM cross-sectional images. The absolute accuracy defined as the mean value of the differences between measured values and true values was 9 nm when the slope angle 4 varies from 72 to 85”.

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B. Three-Dimensional Measurements

Various kinds of three-dimensional measurement methods for microstructural dimensions such as the slope angle and the height have been reported by many authors (Hoover, 1971; Hieke et al., 1977; Kato et al., 1977; Frosien, 1986; Miyoshi et al., 1986b). The most distinct method among them is the method by Miyoshi et al. which measures the height h and the average slope 4 by using the principle of stereoscopy and produces the true edge profile automatically without destroying the sample. Figure 40 shows the measured results of the photoresist pattern; the measured edge profile (a) and the cross-sectional SEM image (b)

image

(a) (b) FIG.40. Measured edge profiles of the photoresist pattern: (a) comparison of the measured edge profile by the stereoscopic method with that from the cross-sectional SEM image, and (b) the cross-sectional SEM image. (From Miyoshi et a!., 1986b.)

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are shown. The measurements were carried out at the specimen tilt angles for stereoscopy of 0 and 5". The measured result for the average slope angle and the height are 77.4" and 1.35 pm which are in good agreement with the true values of 77.0" and 1.40 pm measured from the cross-sectional SEM image. The absolute measurement accuracy is 0.5% for slope angle measurement and 3.5% for height measurement.

C . Pattern Inspection There are three kinds of pattern defects which decrease device production yield (Kato et al., 1986): 1) systematic defects characterized by linewidth variations or edge roughness caused by the process failures such as underexposure or overdeveloping; 2) random defects characterized by pinholes or intrusions caused by various factors in the process; and 3) repeating defects caused by a stepper with a defective reticle. To detect all types of defects, a pattern inspection system that consists of a scanning electron microscope, an image processing unit, and a host computer that is connected to the CAD system has been reported (Kato et al., 1986; Saitoh et al., 1986). The measured pattern data are compared with the design pattern data generated from the CAD system and defects are automatically extracted. This type of pattern inspection system which has a tight link with a CAD system will be a valuable aid for improving the productivity of future devices with submicron patterns.

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A

matrix elements, 51, 53 radial trajectory equations, 50, 52 source collector distance, 54 Cylindrical electrostatic mirror, temporal matrix, 87-88

Aberration coefficient, 108 Aberrations chromatic, 96.98, 105-1 11, 115 spherical, 106-107, 113 AEG. 155 influence on GE, 155 microscope at Naval Res. Lab., 220 Alexander, R. G., 218 American Cyanimid Co. Labs., 171, 196,227 Anderson, P. A,, 139-143 Anderson, T. F., 172,226 Applications of EMB, 174 Asherman, E., 217 Astigmatism, 106

B Bachman, C. H., 155-161 Baker, R. F., 176, 180, 181,228 Banca, M.C., 138, 170, 182,208,215 Barnes, R. B., 172, 182, 215 Beale, R., 163 Benson, I., 160 Boersch, H., 172 Brightness, 114-1 16 Burger, G. F., 183 Burnett, C. E., 177 Burton, C. J., 227 Burton, E. F., 149

c Caledonian quadruplet, 113 Circular ion beam, 84-87 Clark, G. L., 223 first EM, symposium (Chicago 1942). 223 Cohen, A,, 140, 143 Columbe, M. J., 161 Cowdry, E. V., 144 Cyclotrons as sources, 101 Cylindrical electric mirror coordinate system, 49-50 ion trajectory parameters, 53

D d'Alessandro, L., 21 7 Davis, C., 193 Davisson, C. J., 194 Demagnification factor, 95-96 Disk of least confusion, 107 Dornfeld, E. G., 183, 186,201, 204 Duffendack, 0. S., 223

E Eastman Kodak, 150-152 EB testers, 299-307 application of design verification, 300 fault diagnosis, 300 classification of, 299-300 first generation, 301-302 second generation, 301-302 third generation, 302-305 combining of, with CAD database system, 302-303 peripheral equipment and techniques, 305307 delay circuit, 306 electron beam positioning, 306 phase shifter, 306 processing of multilayer devices, 307 synchronizing circuitry, 305- 306 Elastic recoil detection analysis (ERDA), 117118 Electric field circular ion beam, 84-87 homogeneous, ion movement, 75-76 trochoidal ion beam, 87 see also Ion focusing Electric field potential, hexapole, 67 Electric plane mirror, 54 319

320

INDEX

Electric quadrupole, 65-67 temporal matrix, 88-89 Electric toroidal deflectors, 42-43 Electron beam deflection pivot, 275,282-285 deflection structure, 273-278 driving method of, 276-278 installation of, 273-275 types of, 275-276 deflection system, 273-285 electron optics of, 278-285 picosecond pulses, generation of, 278 pulse gate, 272-287 pulse wave form, measurement of, 284-287 tester, see EB tester testing, method of contactless current feeding, 245 contactless testing, 245 frequency mapping, 244 frequency tracing, 244 logic state mapping, 243 phase shift image mode, 242 real-time waveform mode, 238 sampling mode, 240 stroboscopic image mode, 238-242 stroboscopic waveform mode, 240-242 voltage coding mode, 238 Electron gun, low accelerating voltage, 270272 Electron irradiation effects, 258-270 beam-induced specimen contamination, 259 floating electrode, charging-up of, 262-265 MOS FETs, characteristic variation of, 260262 passivated devices, charging of, 265-269 core model, 269 Electron optical column, automatic control system of, 291-299 control system, 293-295 feedback control method, 296-297 table control method, 295-296,297-299 Electrostatic focussing, I 12- 1 13 Electrostatic mirrors cylindrical electric mirror, 49-54 electric plane mirror, 54 energy focusing with, 81-83 Electrostatic potential, electric quadrupole field, 65 Ellis, S. G., 200, 203 Euler-Lagrange equations, 15-16

F

Farrand, C. L., 215 Felheimer, C., 204 Field free space ion beam focusing, 24 temporal matrix, 84 time focusing methods, 80-81 Fitzsimmons, K., 139-143 Flory, L. E., 178 Fullam, E. F., 228

G Gabor, D., 134 G E microscopes, 154- 163 Gutenberg Bible, 127

H Hall, C. E., 138, 149-152 Halma, H., 208 Harvey, G. G., I51 Heat induced damage, 122, 129 Helmholtz-Lagrange law, 3-4 Herzog shields, 43 Hillier, J., 137, 169-180, 189, 198,203,208,225 Holley, R., 187 Hughes, A. L., 146 Hutter, R. G. E., 221

I IONBEAM, 98 Ion beam, characterization, 5 Ion focusing in space, 2 aberration coefficients, 20,22 axial phase subspace representation, 3 axial space transfer matrix, 19,22 beam position with collector current at bottom of valley, 7 , 9 collected current peak shape, 7-8 current density, 7-8 cylindrical electric mirror, 49-54 distance between median plane projection and beam axis, 11 electric and magnetic quadrupoles, 65-67 electric hexapoles, 67-68 electric plane mirror, 54 electric toroidal deflectors, 42-43

INDEX Ion focusing in space (Conrinued) Euler-Lagrange equations, 15-16 extrema of the function used, 12-13 field free space, 24 general laws, 2-4 higher order multipoles, 68 image width and aberrations, 10-12 ion trajectory calculation, 15- 18 cylindric coordinate system, 16- 17 magnetic fields, 55 aberration coefficients, 58 Cartesian coordinate system, 62-63 entry boundary fringing field matrix elements, 64 field free space length, 56-57 homogeneous, 55-59 matrix elements, 55-56,61-62 movement equations, 59 optical parameters, 57 reference planes and trajectory parameters, 60 spectrometers, 58 symmetric homogeneous field analyzer, 56 wedge field, 59-64 movement equations, 14-15 integration, 47 trochoidal ion beam, 38 optical parameters of individual analyzers, 20,22-24 optimum beam parameters, 13-14 optimum conditions, 12-14 plane condenser deflector, 47-49 product of linear and angular magnifications, 4 quality factor, 12 radial phase subspace representation, 2-3 radial space and time transfer matrix, 19-21 recorded current correlation with emitted ion properties, 5-6 rectangular cross, 3-4 relative mass difference, 18 relativistic and unrelativistic relations, 6 resolution, 6- 10 resolution formula, 13 c peaks, 7 source and collector places, analyzers with direct and reversed geometries, 23-24 space transfer matrix, 18-20

32 1

spiral electric deflector, 43-47 superimposed electric and magnetic fields boundary and fringing field distribution, 31-32 coordinate system and boundary curvature radius, 33 crossed fields, circular main path, 25-38 electric potential, 33-34 external beam axis, 35 field integrals, 37 inhomogeneous, 25 ion distribution along constant mass parabola, 40 ion trajectory, 35-36 observation point abscissa and ordinate, 41 parallel homogeneous fields, 39-42 second order matrix elements, 28-29 transfer matrix elements, 27, 38 trochoidal ion beam, 38-39 symbols in derivation of resolution, 7,9 tandem systems crossed field element, 73-74 entry boundary profile, 72 with successive electric and magnetic sectors, 70-73 system matrix and aberrations, 69-70 wide energy spread focusing mass spectrometer, 71-72 lon focusing in time, 74 initial velocity, 76-77 instruments, 89-90 ion movement in homogeneous electric fields, 75-76 ion packet formation, 76-78 length, 77 optimum conditions, 79-80 resolution, 78- 79 temporal matrix circular ion beam, 84-87 cylindrical electrostatic mirror, 87-88 electric quadrupoles, 88-89 elements, 83-84 field free space, 84 magnetic quadrupoles, 89 plane electrostatic mirror and condenser. 88 trochoidal ion beam, 87 time focusing methods

322

INDEX

Ion focusing in time (Continued) energy focusing with electrostatic mirrors, 81-83

field free space, 80-81 impulse field focusing, 81 source-collector flight time, 83 Ion-optical index function, 16 Ion optical refractive index, 15 Ion optics, 1-2 developments and refinements, 90-91 Ion packet, formation, 76-78 Ion trajectory calculation, 15-18 K Kerr, C. H., 138 rationing of microscopes, 139,207 War Production Board, 138, 173 Knoll, M., 136 K X-ray excitation, 101 L Ladd, W. A., 138,149, 153,228 Columbian Carbon Co., 139 Lagrange’s law, 4 Lanthanides, 128 Lens achromatic, 115 electrostatic coaxial, 113 misalignment, 105-106 plasma, 97 space charge, 97 superconducting, 115 superconducting solenoid, 110 Liouville’s theorem, 3, 114 Litwak, A. A., 204 Longitudinal emittance diagram, method, of, 285

Marvin, H. B., 154 Matheson, R. L. A., 223 McMillen, H., 145 Mees, C. E. K., 150 Microprobes collimated, 100, 119 focussed, 101, 121 high energy, 101 Microstructure measurements, 307-31 1 linewidth measurements, 308-309 intensity profile, 308-309 linear regression method, 309 threshold method, 309 pattern inspection, 31 1 three-dimensional measurements, 310-31 1 MIT, 151,222,227 Morgan, J., 176, 182, 185 Morton, G., 181 Movement equations, 14-15 integration, 47 magnetic wedge field, 59 trochoidal ion beam, 38 Mudd, S., 225-226

N Nat. Res. Council Fellowship, 226 Newberry, S. P., 143-149,161-163 Newman, S. B., 228 NIH, 139 Non-stigmatic images, 95 Normalized emittance, 114 Nuclear reaction analysis (NRA), 117 0 OBrien, H. C., 228 Ogilvie, R., 152 OXRAY, 98

M Magnetic field circular ion beam, 84-87 trochoidal ion beam, 87 see also Ion focusing Magnetic quadrupole, 65-67 magnetostatic potential, 65 temporal matrix, 89 Marton, L., 136, 164-166,220-222,225 his fourth microscope, EMA, 164

P Packer, D., 146 Pease, D. C., 228 Performance of early EM, 188 Picard, R. G., 184, 194-197,204,208 Picture element (pixel), 121 Pike, S . W., 201,205,212 PIXE, peak interference in spectrum, 96, 117, 122- 129

Plane condenser deflector, 47-49

323

INDEX Plane electrostatic mirror and condenser, temporal matrix, 88 Porter, K. R., 227 PRAM, 98 Prebus, A,. 137, 149, 152, 170 Printers' ink, 127 Proton induced X-ray emission (PIXE), 96,98, 105-Ill. 115

0 Quadrupole doublet, 95, 105 lens, 98, 105 triplet, 96, 105 Quadrupole lens, achromatic, 96, 109, I12

R Radial trajectory equation, 50 Radiation damage, 122 Ramberg, E. W., 178, 181,199,201 Ramo, S., 155-161 RCA Laboratories, Camden, 169 RCA Laboratories, Princeton, 182 Reeber, H. E., 204 Reisner, J. H.. 184,194-197,200,201-202,204, 208 Rempfer, G . F., 216-220 Rempfer, R.. 216-219 Residual gas scattering, 112, 116 Richards, A. G., Jr., 228 Rochow. T. C . , 227 Rudenberg, R.. 216,218,225 Runge, F., 183, 185, 186 Ruska, E., 134, 136 Russian quadruplet, 95, 101, 105 Rutherford backscattering (RBS), 117

S Sample labelling, 122 Sarnofl, D., 163, 182 Scanning proton microscope, 104 Schmidt, F. O., 149, 151,227 Scott, G . H., 143-149 Secondary electron detectors, 287-290 energy analyzers, 287-290 Secondary electron emission, 104, 118 Siemens Company, 136

Smith, P. C., 183-185, 199,204,208 Smith, T. A,, 163,225 Snell's law, 4 Snyder, R. L., 178 Space focusing, 80- 8 1 Space transfer matrix, 18-20 Specimen chamber peripheral equipment, 290- 29 1 mechanical probe, as a signal feeder, 29 1 temperature control unit, 290-291 Spiral electric deflector, 43-47 electric potential, 46 fringing field components, 46 ion trajectory coordinate system, 43-44 transfer matrix elements, 45-46 Spot size, 101-104 Steever, R., 140, 143 Strong, J., 136 Summers, S. S., 161

T Thermally cooled protons, 116 Time of flight analyzer field free space, 80 resolution definition, 78 Topography contrast, 249 Trace elements in normal and diseased organs, 124,125 TRANSPORT, 98 Trochoidal ion beam, 38-39,87

V Vance, A. W., 166-168,178, 185 Van de Graaff accelerators, 97 Vassos,J., 173, 209 Vees, J., 215 Voltage contrast, 247-258 address error, of primary electron, 252 capacitive coupling, 255 alternate phase scan, 257 random phase scan, 258 storage time of, 256 local field effect, 251 potential barrier, 248 resistive coupling, 255 transit time effect on, 253-254 voltage linearization, 251 voltage resolution, 251 voltage sensitivity, 251

324

INDEX

W Walliston wires, 104 Washington State University, 139-143 Washington University of St. Louis, 143-149 Weber, R. L., 223 Wien filter, 73-74 Williams, R.C., 227 Wilson, A., 173 Wyckoff, W. G., 184,199, 227

Y Yoshii, Z., 143

Z Zollers, S . M., 183-186,201 Zworykin, V. K., 154, 163-179,208,209, 212,225