HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY Principles, Deposition, Film Modification and Synthesis Jerome J. Cuomo Stephen M. Rossnagel Harold R. Kaufman
William Andrew Inc.
HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY
MATERIALS SCIENCE AND PROCESS TECHNOLOGY SERIES Editors Rointan F. Bunshah, University of California, Los Angeles (Materials Science and Technology) Gary E. McGuire, Microelectronics Center of North Carolina (Electronic Materials and Processing) DEPOSITION TECHNOLOGIES FOR FILMS AND COATINGS: by Rointan F. Bunshah et al CHEMICAL VAPOR DEPOSITION IN MICROELECTRONICS: by Arthur Sherman SEMICONDUCTOR MATERIALS AND PROCESS TECHNOLOGY HANDBOOK: edited by Gary E. McGuire SOL-GEL TECHNOLOGY FOR THIN FILMS, FIBERS, PREFORMS, ELECTRONICS AND SPECIALTY SHAPES: edited by Lisa A. Klein HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK: by James J. Licari and Leonard R. Enlow HANDBOOK OF THIN FILM DEPOSITION PROCESSES AND TECHNIQUES: edited by Klaus K. Schuegraf IONIZED-CLUSTER BEAM DEPOSITION AND EPITAXY: by Toshinori Takagi DIFFUSION PHENOMENA IN THIN FILMS AND MICROELECTRONIC MATERIALS: edited by Devendra Gupta and Paul S. Ho SHOCK WAVES FOR INDUSTRIAL APPLICATIONS: edited by Lawrence E. Murr HANDBOOK OF CONTAMINATION edited by Donald L. Tolliver
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MICROELECTRONICS:
HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY: edited by Jerome J. Cuomo, Stephen M. Rossnagel, and Harold R. Kaufman FRICTION AND WEAR TRANSITIONS OF MATERIALS: by PeterJ. Blau CHARACTERIZATION OF SEMICONDUCTOR MATERIALS-Volume 1: edited by Gary E. McGuire SPECIAL MELTING AND PROCESSING TECHNOLOGIES: edited by G.K. Bhat
Related Titles ADHESIVES TECHNOLOGY HANDBOOK: by Arthur H. Landrock HANDBOOK OF THERMOSET PLASTICS: edited by Sidney H. Goodman SURFACE PREPARATION TECHNIQUES FOR ADHESIVE BONDING: by Raymond F. Wegman
HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY Principles, Deposition, Film Modification and Synthesis
Reprint Edition
Edited by
Jerome J. Cuomo and Stephen M. Rossnagel IBM Thomas J. Watson Research Center Yorktown Heights, New York
Harold R. Kaufman Front Range Research Fort Collins, Colorado and Commonwealth Scientific Corporation Alexandria, Virginia
NOYES PUBLICATIONS Westwood, New Jersey, U.S.A.
Copyright © 1989 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without perm ission in writing from the Publisher. Library of Congress Catalog Card Number: 88-38244 ISBN: 0-8155-1199-X Printed in the United States
Published in the United States of America by Noyes Publications Fairview Avenue, Westwood, New Jersey 07675 109876543
Library of Congress Cataloging-in-Publication Data Handbook of ion beam processing technology : principles, deposition, film modification, and synthesis / edited by Jerome J. Cuomo and Stephen M. Rossnagel, Harold R. Kaufman. p. cm. Includes bibliographies and index. ISBN 0-8155-1199-X : 1. Ion implantation. 2. Ion bombardment--I ndustrial applications. I. Cuomo, J.J. II. Rossnagel, Stephen M. III. Kaufman, Harold R. aC702.7.155H36 1989 621.381'7--dc19 88-38244 CIP
About the Editors
Jerome J. Cuomo is presently Manager of the Materials Processing Laboratory at the IBM T.J. Watson Research Center, Yorktown Heights, New York. He is particularly involved in the study of sputtering, ion beam and plasma processing, and is the author or co-author of 55 patents. He has made important contributions to the development of LaB 6 electron emitters and Si 3 N4 as dielectric layers, and also pioneered work in chemical vapor deposition, dendritic solar thermal absorbers, sputtered amorphous silicon, amorphous magnetic bubble domain materials, ion beam modification and synthesis of materials, enhanced plasma processes, and high Tc superconductors. Dr. Cuomo has been active in various capacities in the American Vacuum Society, the American Chemical Society, the Materials Research Society, North Carolina State University, and Tanury Industries. He has also published 85 research papers, chapters in several books, and has edited two books. He is distinguished by having the highest patent level in the IBM Corporation. Stephen M. Rossnagel is presently a research staff member at the IBM T.J. Watson Research Center, Yorktown Heights, New York. His current research is in plasma-based processing, particularly in ion beam and magnetron areas. He received his doctorate in physics from Colorado State University, and has held positions at Princeton University and at the Max Planck Institute in Garching, West Germany. Dr. Rossnagel has published extensively in areas of surface modification by sputtering and also film modification by ion bombardment. He has published over 55 research papers and two books, is the author of 6 patents, and is chairman of the Plasma Science and Technology Division of the American Vacuum Society. Harold R. Kaufman is Professor Emeritus, Colorado State University and is presently involved in research and development of ion and electron sources at Front Range Research, Fort ,Collins, Colorado and Commonwealth Scientific Corp., Alexandria, Virginia. He was active in aerospace propulsion research at NASA Lewis Research Center, was professor of both physics and mechanical engineering at Colorado State University, and from 1979-1984 he was chairman of the Physics Department. Dr. Kaufman is the recipient of the James H. Wyld Propulsion Award of AlAA, and the NASA Medal for Exceptional Scientific Achievement. He is an Associate Fellow of the AIAA, and a member of the American Physical Society
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About the Editors
and the American Vacuum Society. He has also authored over 100 scientific publications. More than half of the broad-beam ion sources presently used in the U.S. industry were designed by Dr. Kaufman.
Contributors
John Baglin IBM, Almaden Research Center San Jose, CA
Dieter M. Gruen Argonne National Laboratory Argonne,IL
Bruce A. Banks NASA Lewis Research Center Cleveland, OH
Paul S. Ho IBM, Thomas J. Watson Research Center Yorktown Heights, NY
R. Mark Bradley Colorado State University Fort Collins, CO Wallis F. Calaway Argonne National Laboratory Argonne,IL Jerome J. Cuomo IBM, Thomas J. Watson Research Center Yorktown Heights, NY Nicholas E. Efremow Lincoln Laboratories, MIT Lexington, MA Michael Geis Lincoln Laboratories, MIT Lexington, MA Willianl D. Goodhue Lincoln Laboratories, MIT Lexington, MA
William M. Holber IBM, Thomas J. Watson Research Center Yorktown Heights, NY Gerald D. Johnson Lincoln Laboratories, MIT Lexington, MA Harold R. Kaufman Front Range Research Fort Collins, CO Eric Kay IBM, Almaden Research Center San Jose, CA Fred Kimock Air Products and Chemicals Allentown, PA Makoto Kitabatake Matsushita Electrical Industrial Co., Ltd. Moriguchi, Osaka, Japan vii
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Contributors
George A. Lincoln Lincoln Laboratories, MIT Lexington, MA Phil J. Martin CSIRO Lindfield, NSW Australia Karl-Heinz Muller CSIRO Lindfield, NSW Australia Roger P. Netterfield CSIRO Lindfield, NSW Australia Hans Oechsner Universitat Kaiserslautern Kaiserslautern, Germany Stella W. Pang Lincoln Laboratories, MIT Lexington, MA David L. Pappas IBM, Thomas J. Watson Research Center Yorktown Heights, NY Michael J. Pellin Argonne National Laboratory Argonne, IL Raymond S. Robinson Colorado State University Fort Collins, CO Stephen M. Rossnagel IBM, Thomas J. Watson Research Center Yorktown Heights, NY
Ronnen A. Roy IBM, Thomas J. Watson Research Center Yorktown Heights, NY Toshinori Takagi Kyoto University Sakyo, Kyoto, Japan Kiyotaka Wasa Matsushita Electric Industrial Co., Ltd. Moriguchi, Osaka, Japan Robert C. White Columbia University New York, NY Nicholas Winograd Penn State University University Park, PA Isao Yamada Kyoto University Sakyo, Kyoto, Japan Dennis S. Yee IBM, Thomas J. Watson Research Center Yorktown Heights, NY Charles E. Young Argonne National Laboratory Argonne, IL Peer C. Zalm Philips Research Laboratories Eindhoven, The Netherlands
NOTICE To the best of the Publisher's knowledge the information contained in this book is accurate; however, the Publisher assumes no responsibility nor liability for errors or any consequences arising from the use of the information contained herein. Final determination of the suitability of any information, procedure, or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. The book is intended for informational purposes only. Expert advice should be obtained at all times before implementation of any procedure described or implied in the book, and caution should be exercised in the use of any materials or procedures for ion beam processing which could be potentially hazardous.
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Contents
1. PERSPECTIVE ON PAST, PRESENT AND FUTURE USES OF ION BEAM TECHNOLOGY Jerome J. Cuomo, Stephen M. Rossnagel and Harold R. Kaufman 1.1 Introduction 1.2 Past Technology 1.3 Present Capabilities 1.3.1 Ion Beam Technology 1.3.2 Sputtering Phenomena 1.3.3 Film Deposition, Modification and Synthesis 1.4 Future Trends 1.5 References
1 1 2 2 2 3 3 4 5
PART I ION BEAM TECHNOLOGY 2. GRIDDED BROAD-BEAM ION SOURCES Harold R. Kaufman and Raymond S. Robinson 2.1 Introduction 2.2 General Description 2.3 Discharge Chamber 2.4 Ion Optics 2.5 Production Applications 2.6 Target Contamination 2.7 Concluding Remarks 2.8 References 3. ELECTRON CYCLOTRON RESONANCE (ECR) ION SOURCES William M. Holber 3.1 Introduction 3.2 Theory of Operation 3.3 Types of Sources and Characteristics 3.4 Etching xi
8 8 9 11 13 16 16 19 20 21 21 22 26 30
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Contents 3.5 Deposition 3.6 References
33 36
4. HALL EFFECT ION SOURCES Raymond S. Robinson and Harold R. Kaufman 4.1 Introduction 4.2 End-Hall Ion Source 4.2.1 Operation 4.2.2 Ion Acceleration 4.2.3 Beam Energy Distribution 4.2.4 Beam Current Density Profile 4.3 Closed Drift Ion Source 4.3.1 Operation 4.3.2 Ion Acceleration 4.3.3 Beam Energy Distribution 4.3.4 Beam Current Density Profile 4.4 Concluding Remarks 4.5 References 5. IONIZED CLUSTER BEAM (ICB) DEPOSITION AND EPITAXY Isao Yamada and Toshinori Takagi 5.1 Introduction 5.2 Experiment 5.2.1 Principles of ICB Operation 5.3 Aspects of Film Deposition with ICB 5.3.1 Kinetic Energy Range of ICB and Effects of the Kinetic Energy 5.3.2 Effects of the Ionic Charge 5.3.3 Film Deposition by Reactive ICB Techniques 5.3.4 Film Deposition by Simultaneous Use of ICB and Microwave Ion Sources 5.4 Summary 5.5 References
39 39 40 40 42 43 46 48 49 50 51 53 53 54 58 58 59 59 64 67 70 70 72 74 75
PART II SPUTTERING PHENOMENA 6. QUANTITATIVE SPUTTERING Peer C. Zalm 6.1 Introduction 6.2 Total Sputter Yield Considerations 6.2.1 Polycrystalline and Amorphous Elemental Targets 6.2.2 Predictions from Linear Cascade Theory 6.2.3 Exceptions to Predictions from Linear Cascade Theory 6.2.4 Ion Effects: The Direct Knock-On Regime 6.2.5 Ion Effects: Due to I-Iigh Fluence 6.2.6 Ion Effects: Reactive and Molecular Ions 6.2.7 Target Effects: Temperature 6.2.8 Target Effects: Single Crystal Targets 6.2.9 Target Effects: Multicomponent Materials
78 78 79 79 81 82 83 84 84 85 86 87
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6.3 Differential Sputter Yield Considerations 6.3.1 Angular Distributions of Sputtered Particles 6.3.2 Energy Distributions of Sputtered Particles 6.4 Experimental Considerations for Sputter Yield Measurements 6.4.1 Ion Beam 6.4.2 Sputtering Target 6.4.3 Measurement Techniques 6.5 Total Sputter Yield Measurements 6.5.1 Mass Loss Techniques 6.5.2 Probe Techniques 6.5.3 Thickness Change Techniques 6.5.3.1 Masking Techniques 6.5.3.2 Optical Methods 6.5.3.3 Thin Film Interface Techniques 6.5.3.4 Other Techniques 6.6 Differential Yield Measurements: Angular and Energy Distributions 6.6.1 Angular Distributions of Ejected Particles 6.6.2 Energy Distributions of Ejected Particles 6.6.3 Combined Angular- and Energy-Resolved Measurements 6.7 Concluding Remarks 6.8 References 7. LASER-INDUCED FLUORESCENCE AS A TOOL FOR THE STUDY OF ION BEAM SPUTTERING Wallis F. Calaway, Charles E. Young, Michael J. Pellin, and Dieter M. Gruen 7.1 Introduction 7.2 Experimental Technique 7.3 Summary of Data 7.3.1 Sputtering Yields 7.3.2 Velocity Distributions 7.3.3 Oxide Coverage and Adsorbates 7.3.4 Sputtering of Alloys and Nonmetallic Compounds 7.4 Conclusion 7.5 References 8. CHARACTERIZATION OF ATOMS DESORBED FROM SURFACES BY ION BOMBARDMENT USING MULTIPHOTON IONIZATION DETECTION David L. Pappas, Nicholas Winograd and Fred M. Kimock 8.1 Introduction 8.2 Analytical Applications 8.3 Energy and Angle Measurements 8.4 Nonresonant Multiphoton Ionization 8.5 Conclusion 8.6 References 9. THE APPLICATION OF POSTIONIZATION FOR SPUTTERING STUDIES AND SURFACE OR THIN FILM ANALYSIS Hans Oechsner
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87 87 89 93 93 95 95 96 96 97 98 98 100 100 100 101 101 102 104 105 106
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112 113 116 116 118 121 123 124 125
128 128 129 134 138 140 142
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9.1 Introduction 9.2 Postionization Techniques Using Penning Processes 9.3 Electron Gas Postionization in Low Pressure Plasmas 9.3.1 Investigations of the Sputtering Process by Plasma Postionization 9.3.2 Electron Gas Postionization for Secondary Neutral Mass Spectrometry SNMS 9.4 Summary 9.5 References
145 146 148 149 156 164 165
PART III FILM MODIFICATION AND SYNTHESIS 10. THE MODIFICATION OF FILMS BY ION BOMBARDMENT Eric Kay and Stephen M. Rossnagel 10.1 Introduction 10.2 Experimental Concerns for Bombardment-Modification of Films 10.3 Effects on Film Properties by Energetic Bombardment 10.3.1 Physical Effects 10.3.1.1 Grain Size 10.3.1.2 Orientation 10.3.1.3 Nucleation Density 10.3.1.4 Defects 10.3.1.5 Lattice Distortion 10.3.1.6 Surface Diffusion 10.3.1.7 Density 10.3.1.8 Epitaxial Temperature 10.3.1.9 Film Stress 10.3.1.10 Surface Topography 10.3.1.11 Implantation of Gas Atoms 10.3.1.12 Optical Properties 10.3.1.13 Resistivity 10.3.2 Chemical Effects 10.3.2.1 Stoichiometry 10.4 Reactive Film Deposition 10.4.1 Reactive Ion Beam Deposition 10.4.2 Reactive Deposition by Dual Ion Beam Synthesis: AIN 10.4.3 Reactive Ion Beam Assisted Evaporation: Cu-O Compounds 10.4.4 Optical Films by Ion Beam Assisted Deposition 10.5 Summary 10.6 References 11. CONTROL OF FILM PROPERTIES BY ION-ASSISTED DEPOSITION USING BROAD BEAM SOURCES Ronnen A. Roy and Dennis S. Vee 11.1 Introduction 11.2 Property Changes 11.2.1 Ion Energy Effects
170 170 171 175 175 175 175 176 176 178 179 180 181 181 182 184 184 184 185 185 187 187 187 188 190 190 190
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11.2.2 Temperature Effects 11.3 Film Structure Modification 11.3.1 Ion Energy Effects 11.3.2 Temperature Effects 11.3.3 Structure-Property Relations 11.4 General Discussion of Ion Bombardment Mechanisms 11.4.1 Materials and Temperature Effects 11.4.2 Property Optimization 11.5 References
199 201 201 202 205 210 213 216 217
12. ETCHING WITH DIRECTED BEAMS Michael Geis, Stella W. Pang, Nicholas E. Efremow, George A. Lincoln, Gerald D. Johnson and William D. Goodhue 12.1 Introduction 12.2 Ion Beam Assisted Etching 12.3 Etching GaAs 12.4 Etching Diamond 12.5 Hot Jet Etching 12.6 Etching Damag" 12.7 Summary 12.8 References
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13. FILM GROWTH MODIFICATION BY CONCURRENT ION BOMBARDMENT: THEORY AND SIMULATION Karl-Heinz Muller 13.1 Introduction 13.2 Film Microstructure, the Role of Impact Mobility and Substrate Temperature 13.2.1 Classification of Film Structure in Terms of Zones 13.2.2 The Henderson Model and Zone-1 Structure 13.2.3 Thermal Mobility and the Zone-1-Zone-2 Transition 13.2.4 Origin of the Zone-2 Structure 13.3 Ion Bombardment Induced Structural Modifications During Film Growth 13.3.1 The Thermal-Spike Approach 13.3.2 The Collision-Cascade Approach 13.3.2.1 Redeposition Mechanism 13.3.2.2 Densification Mechanism 13.3.2.3 Critical and Optimum Ion-to-Atom Arrival Rate Ratios 13.3.2.4 Film Orientation 13.3.3 The Molecular-Dynamics Approach 13.3.3.1 Vapor Phase Growth 13.3.3.2 Vapor and Sputter Deposition 13.3.3.3 Ion-Assisted Deposition 13.3.3.4 Intrinsic Stress Modification 13.3.3.5 Ion-Beam Deposition 13.3.3.6 Ionized-Cluster-Beam Deposition 13.6 Conclusions 13.7 References
219 219 221 230 231 236 237 238
241 241 242 242 242 244 245 247 247 249 249 249 257 259 260 260 262 262 267 270 271 274 274
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14. INTERFACE STRUCTURE AND THIN FILM ADHESION John Baglin 14.1 Introduction 14.2 Factors Affecting Adhesion 14.3 Ion Beam Techniques 14.4 Interface Stitching 14.4.1 Adhesion Enhancement 14.4.2 Examples of Stitching 14.4.3 Stitching Mechanisms 14.4.4 Contaminant Dispersion 14.4.5 Applicability of Stitching 14.5 Low Energy Ion Sputtering 14.5.1 Adhesion Enhancement 14.5.2 Adhesion Mechanism 14.6 Implantation and Adsorption 14.7 Ion Assisted Deposition 14.8 Summary 14.9 References 15. MODIFICATION OF THIN FILMS BY OFF-NORMAL INCIDENCE ION BOMBARDMENT R. Mark Bradley 15.1 Introduction 15.2 Modification of Crystal Structure by Off-Normal Incidence Ion Bombardment 15.2.1 Effect of Bombardment After Deposition 15.2.2 Effect of Bombardment During Deposition 15.3 Topography Changes Induced by Off-Normal Incidence Ion Bombardment 15.3.1 Overview 15.3.2 Ripple Topography Induced by Off-Normal Incidence Ion Bombardment 15.4 Summary 15.5 References 16. ION BEAM INTERACTIONS WITH POLYMER SURFACES Robert C. White and Paul S. Ho 16.1 Introduction 16.2 High and Medium Energy Ions 16.3 SIMS Studies of Polymers 16.4 XPS Studies 16.5 Summary 16.6 References 17. TOPOGRAPHY: TEXTURING EFFECTS Bruce A. Banks 17.1 Introduction 17.2 Ion Beam Sputter Texturing Processes and Effects 17.2.1 Natural Texturing 17.2.1.1 Chemically Pure Materials
279 279 279 281 283 283 287 288 289 291 291 292 292 295 296 296 297
300 300 300 300 301 307 307 307 312 313 315 315 317 320 326 336 336 338 338 338 339 339
Contents
17.2.2 Seed Texturing 17.2.2.1 Seed Materials 17.2.2.2 Diffusion Effects 17.2.2.3 Resulting Topographies 17.2.3 Shadow Masking 17.3 Textured Surface Properties 17.3.1 Mechanical 17.3.2 Electrical 17.3.3 Chemical 17.3.4 Optical 17.4 References 18. METHODS AND TECHNIQUES OF ION BEAM PROCESSES Stephen M. Rossnagel 18.1 Introduction 18.2 Ion Beam Sputtering (IBS) 18.2.1 Comparison to RF Sputtering 18.3 Ion Beam Sputter Deposition 18.4 Ion Beam Assisted Deposition (IBAD) 18.5 Dual Ion Beam Sputtering (DIBS) 18.6 Ion Assisted Bombardment: Other Techniques 18.6.1 Ionized Cluster Beam 18.6.2 Hollow Cathode Magnetron Techniques 18.7 Summary 18.8 References 19. ION-ASSISTED DIELECTRIC AND OPTICAL COATINGS Phil J. Martin and Roger P. Netterfield 19.1 Introduction 19.2 Microstructure of Thin Films 19.2.1 Microstructure and Optical Properties 19.3 Effects of Ion Bombardment on Film Properties 19.3.1 Microstructure 19.3.2 Adhesion and Stress 19.3.3 Compound Synthesis 19.3.4 Crystal Structure and Stoichiometry 19.3.5 Scattering 19.3.6 Optimum Parameters for Ion-Assisted Film Deposition 19.3.7 Summary 19.4 Ion-Assisted Techniques 19.4.1 Ion-Assisted Deposition 19.4.2 Ion Plating 19.4.3 Sputtering 19.4.3.1 Ion Beam Sputtering (IBS) 19.4.3.2 Magnetron Sputtering 19.4.4 Ionized Cluster Beam Deposition (ICB) 19.5 Optical Properties of Ion-Assisted Films 19.5.1 Oxides 19.5.1.1 Silicon Dioxide 19.5.1.2 Aluminum Oxide
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346 346 348 350 353 355 355 357 357 358 359 362 362 362 365 366 368 370 371 371 371 371 372 373 373 373 376 378 378 381 382 382 383 384 387 387 387 389 390 390 390 391 392 393 393 393
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Contents
19.5.1.3 Titanium Dioxide 19.5.1.4 Zirconium Dioxide 19.5.1.5 Cerium Dioxide 19.5.1.6 Tantalum Pentoxide 19.5.1.7 Vanadium Dioxide 19.5.2 Fluorides 19.5.3 Conducting Transparent Films 19.5.4 Nitrides 19.6 Conclusion 19.7 References 20. DIAMOND AND DIAMOND-LIKE THIN FILMS BY ION BEAM TECHNIQUES Makoto Kitabatake and Kiyotaka Wasa 20.1 Introduction 20.2 Principle of Diamond Synthesis 20.2.1 Conventional Synthesis 20.2.2 Synthesis from the Gas Phase 20.3 Experimental Techniques 20.4 Diamond-Like Films 20.4.1 Characterization 20.4.2 Discussion 20.4.3 Applications 20.5 Diamond Particles 20.5.1 Characterization 20.5.2 Discussion 20.6 Conclusion 20.7 References INDEX
395 397 400 401 402 404 404 405 407 407
415 415 416 416 419 420 422 422 425 427 429 429 432 433 433 435
1 Perspective on Past, Present and FutureUses of Ion Bealn Technology
Jerome J. Cuomo, Stephen M. Rossnagel and Harold R. Kaufman
1.1 INTRODUCTION
The work presented in this book deals with ion beam processing: for basic sputter etching of samples, for sputter deposition of thin films, for the synthesis of material in thin film form, and for the modification of the properties of thin filnls. The ion energy range we are concerned with is from a few tens of eV to about 10,000 eV, with primary interest in the range of about 20 to 1-2 keV, where implantation of the incident ion is a minor effect. Of the wealth of types of ion sources and devices available, this book will tend to examine principally broad beam ion sources, characterized by high fluxes and large work areas. These sources include the ECR ion source, the Kaufman-type single- and multiple-grid sources, gridless sources such as the Hall effect or closed-drift source, and hybrid sources such as the ionized cluster beanl systenl. The types of ion sources typically used for surface analysis experiments (for example, depth profiling), high energy ion implantation, or fusion-plasma heating will not be discussed, even though many of the phenomena described in this book have parallels in those areas. The use of ion beams for processing, as opposed to directly extracting ions from a plasma to bombard a sample, has nunlerous advantages for the controlled processing of materials with ion bombardment. The parameters of the ion beam: the flux, the energy, the species and charge state and the direction (and divergence) are all easily quantified and controlled. Ion beanls of the types of interest in this book operate in the pressure range of 1x10- s to 1x10- 3 Torr, which makes them compatible with a number of other physical and chemical processes used in thin film materials processing. This is typically not possible in plasnla-based systems. One other significant advantage to operation in this relatively low pressure region is that the mean free paths both of the incident ions and also of the sputtered atoms are long. There is little scattering due to gas phase collisions, and as such, the complication of charge-exchange modification of the ion flux is minor, as is the thermalization of the sputtered atoms.
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Handbook of Ion Beam Processing Technology
1.2 PAST TECHNOLOGY
The evolution of ion-beam processing has been rapid and impressive. The evolution of this technology can be traced in outline with a few publications. The use of only a few publications is, of course, not fair to the many capable workers in the various supporting or related fields. It does, however, pernut trends to be described that might otherwise be lost in the total volume of publications generated. Broad beam ion sources, as they are currently configured, evolved out of the US space program on electric propulsion. The first broad beam sources of this type were developed in the late 1950's and early 1960's and were tested as propulsion systems in several space-based experiments (1,2). Comnlercial versions of broad beam Kaufnlan-type sources became available first in France, then in the early 1970's in the United States. Significant numbers of publications on the industrial use of ion beams started about 1970, with the early applications emphasizing the simple removal of material (etching) and deposition using nonreactive ion beams (3,4). By the early 1980's, ion-beam processing had progressed to the point where few publications were concerned with etching and deposition using nonreactive ion beams. Instead, the bulk of the publications were about reactive processes, where chemical reactions with, or activated by, beam ions are involved; or with property modification, where the use of the ion beam permitted a property to be modified or enhanced beyond what nlight be possible without the use of an ion beam (5,6). A corresponding, rapid development has taken place in ion sources. A simple, allpurpose ion source typically was used for any and all applications in the early 1970's. By 1982, a wide range of source configurations had been developed, to more efficiently meet the wide range of application needs (7). In the late 1970's, the Ionized Cluster Beam device was developed in Japan, which combined aspects of evaporation with the broad beam ion deposition system. In the early 1980's, the Electron Cyclotron Resonance (ECR) ion source was beginning to be developed, particularly in Japan, although little activity was occurring elsewhere. The driving force in the past technology evolution has been the degree of control possible with ion beam processes, as compared with conlpeting processes. That is, the ion direction, energy, flux, and the background pressure can be both known and independently controlled. It was therefore recognized that ion beam processes could be more directly linked to the fundamental sputter yield and matrix effects than plasma-based processes. 1.3 PRESENT CAPABILITIES
Probably the most obvious indication of present capability is the broad scope of present publications. In 1982 it was possible to give fairly complete surveys of ion source technology and the applications of these sources in article-length publications (5,7). This book is anlple proof that such conlpact publications are no longer possible. 1.3.1 Ion Beam Technology
The fairly wide range of ion sources available in 1982 (7) has further evolved into the even wider range presented in Chaps. 2-5. The more conventional gridded, dc source technology is still used and still important (Chap. 2), with recent advances more in the
Perspective on Past, Present and Future Uses of Ion Beam Technology
3
areas of ease of use and large processing capability. For example, ion sources ranging from 1 cm to 50 or more cm diameter, with planar or dished focusing grids, are available from roughly a dozen commercial sources. The corresponding ion current capabilities range from a few mA to 4-5 A. The emergence of other ion source technologies, such as rf and ECR generation of ions, has broadened the range of ion source applications significantly. ECR ion sources, in which microwave energy is coupled to the ion generating discharge through ion cyclotron resonance, are described in Chap. 3. ECR and rf ion sources are particularly promising in reactive processing, where the cathode lifetime of more conventional (Kaufman-type) ion sources can be a limitation. The development of ECR sources has rapidly increased in the last few years, with perhaps 10-15 companies along with dozens of universities active in the development of these sources. Gridless ion sources, in which electrostatic acceleration of ions is achieved by the interaction of a substantial electron current with a magnetic field, are described in Chap. 4. The technology of Chaps. 3 and 4 is particularly important for the many recent high-flux/lowenergy processing techniques. In the final chapter on ion source technology (Chap. 5), the ions are generated by charging clusters of atoms, rather than isolated atonlS or molecules. This approach also permits a high flux of low energy particles. 1.3.2 Sputtering Phenomena
The fundamental information upon which ion beam applications are based has also expanded, and is covered in Chaps. 6-9. The general quantitative description of sputtering is presented in Chap. 6, along with some of the techniques used to measure sputtering effects. Many recent investigations into the energy and angular distribution of sputtered atoms, as well as related surface phenomena, involve the use of sophisticated instrumentation that permits more detailed descriptions than possible only a few years ago. This instrumentation and results are described in Chaps. 7-9. For exanlple, the bonding states of surface atoms and adsorbed layers can be determined; many collision processes that result in sputtering can be followed in detail; and the resultant velocity-flux distributions can be determined for the individual species (atoms, dimers, trimers, etc.). The sum total of these advances in measuring ability and detailed knowledge is impressive. 1.3.3 Film Deposition, Modification and Synthesis
The present impact of ion beam processing depends directly on the description and understanding of a wide range of industrial applications. These applications are described in Chaps. 10-20. Ion beam deposition processes are characterized by a high average energy (for the sputtered atoms), compared to plasma-based film deposition. This high energy results in improved films properties in many cases, as well as increased film-substrate adhesion. The low pressure operation of these sources results in a line-of-sight film deposition, due to low levels of gas scattering. The charge neutralization of the Kaufmantype ion source permits the sputtering of insulating or electrically isolated targets without charging. In addition, the problem of negative ion formation encountered in plasmabased sputter deposition of some alloys and compounds is not encountered, due to the lack of a significant electric field at the target surface. Simultaneous ion bombardment and film deposition were known in 1982 to give inlproved film properties (similar to the effect of high sputtered particle energy described above) and were felt to be related to the total ion energy in many cases. This relation to energy was described further in 1984 (6). Now we have detailed theoretical and exper-
4
Handbook of Ion Beam Processing Technology
imental descriptions of a number of modification processes. In many cases the energy of the individual ions is relatively unimportant, as long as the energy is below 200-300 eV, and the total ion beam energy per atom (eV/ atom) is the critical parameter. In most of these cases, higher ion energy gives similar results, but with deeper damage that is not "annealed" out by additional bombardment and deposition. That is, there is a severalatomic-layer depth over which ion collisions can "anneal" the structure, and an ion with greater energy disrupts the structure to greater depths than this. Further, there are fairly simple and direct trade-offs that can be made between ion bombardment and substrate temperature. That is, a property modification can be accomplished with ion bombardment that might otherwise require excessive and damaging substrate temperatures. And there are also some processes that are not dependent on total ion energy, as well as some processes that require high energy ions. Several extensive efforts have examined from a theoretical point-of-view the phenomena occurring during ion bombardment of a growing film. The molecular-dynamics computer simulations (Chap. 13) have been particularly successful in modeling some of the changes in physical properties of the films due to the concurrent ion bombardment, as well as effects of substrate temperature and orientation. Other analytical studies (Chap. 15) have exan1ined the formation of topography and preferred orientation in similar circumstances. Perhaps the most successful application of ion beam-assisted deposition techniques has been in the area of dielectric film deposition, where the film's optical properties are of critical interest (Chap. 19). In this area it is clearly possible to tailor the properties of the film through carefully controlled ion bombardment. Additional studies have examined the effects of ion bombardment on the formation of surface structure (Chap. 17), particularly with low levels of impurities. The effects of incident ion bombardment on the properties of polymer surfaces has also been studied (Chap. 16). Finally, ion beams have been used to synthesize structures not readily made by other techniques. Often these structures or films are metastable, in that they would not form under the thermodynamic equilibrium of conventional processes. Examples are the formation of certain Cu and Cr oxides (Chap. 10,11) and the forn1ation of diamond particles and diamond-like thin films (Chap. 20). To summarize the advances in film modification and synthesis presented herein, we are seeing the art of ion beam processing becoming the science of ion beam processing. 1.4 FUTURE TRENDS
The nearly explosive evolution that we have seen in ion beam processing will certainly continue for some time. This can be expected from the fact that publications rates have increased in the last several years. The detailed understanding of ion beam processes should also continue to improve. Any attempt to stand back and view the progress in understanding in terms of years rather than n10nths can only serve to heighten the feeling of progress. We are clearly seeing the creation and refinement of several related scientific disciplines. A broad range of new areas are being exploited with broad beam ion source technology. In addition to the controlled densification and reactive deposition, such areas as modulated doping control, layered structures, 3-dimensional structures, tailored materials,
Perspective on Past, Present and Future Uses of Ion Beam Technology
5
metastable materials, selective deposition, control of sticking probabilities and other areas are developing rapidly. Ion sources are increasing in size, as well as current capability, reliability and control. New types of ion sources, utilizing direct deposition of elemental and compound species, allow a new degree of control over film properties. In addition, there is a clear trend toward the mixing of different types of low temperature deposition and film modification processes. In recent years, techniques such as low pressure CVD, enhanced magnetron sputtering, laser ablation and other optically-enhanced techniques, direct low energy ion beam deposition, and a host of others have been rapidly developed. The combination of these technologies with the emerging low energy, high flux ion beam sources will lead to a new generation of process technologies and material deposition capabilities. The past driving force for ion beam processing was described above as the degree of control possible in such processing. The improvement in process understanding presented in this book only increases the value of control in industrial processes. In short, we can only expect wider use of ion beam processing to result from the inlproved understanding, with this processing used increasingly in the more sophisticated and difficult thin film processes.
1.5 REFERENCES
1.
R.J. Cybulski, D.M. Shellhammer, R.R. Lovell, E.J. Domino and J.T. Kotnik, Results from SERT I ion rocket flight test. NASA TN D-2718 (1965).
2.
W.R. Kerslake, R.G. Goldman and W.C. Nieberding, SERT II: mission, thruster performance and in-flight thrust measurements. J. Spacecraft and Rockets 8: pp. 213-224 (1971).
3.
D.T. Hawkins, Ion milling (ion beam etching), 1954-1975: A Bibliography, J. Vac. Sci. Technol. 12: 1389-1398 (1975).
4.
D.T. Hawkins, Ion milling (ion beam etching), 1975-1978: A Bibliography. J. Vac. Sci. Technol. 16: 1051-1071 (1979).
5.
J.M.E. Harper, J.J. Cuomo and H.R. Kaufman, Technology and applications of broad beam ion sources used in sputtering, part II, applications. J. Vac. Sci. Technol. 21: 737-756 (1982).
6.
J.M.E. Harper, J.J. Cuomo, R.J. Gambino and H.R. Kaufman, Modification of thin film properties by ion bombardment during deposition, in Ion Bombardment Modification of Surfaces: Fundamentals and Applications (0. Auciello and R. Kelly, eds.) Elsevier Science Publishers, Amsterdam, The Netherlands (1984).
7.
H.R. Kaufman, J.J. Cuomo and J.M.E. Harper, Technology and applications of broad beam ion sources used in sputtering, part I, ion source technology. J. Vac. Sci. Technol. 21: 725-736 (1982).
Part I
Ion Beam Technology
7
2 Gridded Broad-Bealn Ion Sources
Harold R. Kaufman and Raymond
s. Robinson
2.1 INTRODUCTION
Broad-beanl ion sources employing grids for the electrostatic acceleration of ions originated in the program for electric space propulsion. The early work in this program, starting from about 1960, included the study of a broad range of concepts,(l) and serves as the foundation for the present ion source technology used in thin film fabrication and processing (2,3). There have been many developments since this early work, but ignorance of this early work has also resulted in repetition of it. The significant use of gridded, broad-beam sources in thin film applications started about 1970, and increased rapidly thereafter (4). This rapid growth resulted from the advantages of these ion sources compared to competitive processes. These advantages include ions that are accelerated into a beam with a well-defined and controlled direction, density, and energy. Both the control and the process definition are more difficult with competitive plasma processes. The thin-film applications of these ion sources have been mostly in research. The early applications were further limited to etching and deposition. In more recent applications the objective can often be described as property modification or enhancement, rather than simple etching and deposition. At present, gridded, broad-beam ion sources are readily available in beanl diameters at the ion source ranging upward from 1 cm to ten's of cm. The ion-beam currents range from a few milliamperes to several Amperes. In the largest beam sizes, ion-beam current is a better measure of capability than size alone. The nlultiAnlpere beanl-current capability of a commercial 38-cm ion source (5) is probably the largest available at ion etching and deposition energies at present.
8
Gridded Broad-Beam Ion Sources
9
The most common working gas is argon. Reactive gases such as nitrogen and oxygen are frequently used, and even more reactive gases incorporating chlorine or fluorine are sometimes used. Until recently, the few successful production applications have usually involved products of very high unit cost, so that the use of highly skilled operators could be justified. (4) More recent technology developments, however, have resulted in ion sources that are much more suited to conventional production applications. The review of technology presented herein will emphasize these recent developments. 2.2 GENERAL DESCRIPTION
The schematic diagram of a gridded broad-beam ion source and its controller (power supplies) is shown in Fig. 1. The working gas is introduced into the discharge chamber, where energetic electrons from the cathode strike and ionize atoms or molecules of the working gas. The ions that approach the ion optics (the screen and accelerator grids) are extracted from the discharge chamber and accelerated into the ion beam. The apertures in the grids are aligned so that the screen grid protects the accelerator grid from direct impingement during nornlal operation. Electrons fronl the neutralizer both charge and current neutralize the ion beam. The actual recombination of these electrons with ions is normally a negligible process. The cathode and neutralizer in Fig. 1 are of the hot-filament type. The electron emission for either of these functions can be supplied instead by a hollow cathode,( 1) which requires a separate gas flow. The gases used for hollow cathodes in industrial applications have been either argon or xenon. The discharge chamber and the ion optics are two major components of the ion source that have been involved in recent technology developments. The function of the discharge chamber is to generate ions efficiently and with little need for maintenance. A variety of discharge chamber configurations have been used, and all use a magnetic field to contain the energetic ions emitted from the cathode and thereby improve the efficiency. Both permanent magnets and electromagnets are used to provide the magnetic field. If an electromagnet is used, an additional power supply is required to energize the electromagnet. The screen grid and the discharge chamber wall are often connected to cathode center-tap potential. If these surfaces are electrically isolated, they will be driven to close to this potential by energetic electrons from the cathode. Because these surfaces are at close to cathode potential, the ions generated in the discharge bOlnbard them nlore energetically than if they were at anode potential. Because of this bombardment, sputtered material from the discharge chamber wall can cause significant contamination (Sec. 2.6). The material sputtered from the screen grid is not as important for contamination because most of it is directed back into the discharge chamber. The recent improvements in discharge chamber configurations have tended to be in the direction of reducing contamination and maintenance requirements.
10
Handbook of Ion Beam Processing Technology
Gas
-~
~
Discharge chamber
II
uijij 1.- Accelerator grid Ion beam
Screen grid -~
~
-~
+ Neutralizer
tcathode
!
Anode
====::dl~
Ion source
Controller
+
ac ,,-...
:>
0
t;
Q) '0:>"
O..-l ,.c:P. ..., P. rtI
::s
u en
ct
+
~ :>
U
~
::s en
.0
1-1 ';cs 0 ...,H
:>..
rtI
-r-l ~
"-"'rtI
~
OJH 00-
H :>.. ,.c:..-l u P. en P.
ac
"'1 :>
~
Q)
~
..-l
g;
::s en
:> rtI-
,,-...
c:: :>
1-1 OJ ~ NH
-rot-
..-l..-l OJ P. u P. u ::s
< en
..-l rtI ~ H..-l ..., P. ::s P. OJ ::s Z en
+
ct
1-1 Q)
~
Figure 1: Schematic diagram of gridded, broad-beam ion source and controller (power
supplies). The improvements in the ion optics cannot be described in such a simple manner. The discharge chamber plasnla within which the ions are created is at a potential close to that of the anode. In being accelerated into the ion beam, the ions gain an energy corresponding to the beam supply voltage, Vb. (For singly charged ions, the energy in e V equals the beanl supply voltage in V.) The ion current that is accelerated equals, in normal operation, the beam supply current, lb. The accelerator voltage is required to provide a potential barrier against neutralizing electrons in the ion beam. Without this barrier the electrons would flow backwards, or backstream, through the ion optics, and give a false indication of ion beam current. Contamination from the accelerator grid often limits the accelerator voltage to values close to the minimum required to prevent backstreanling (Sec. 2.4). The maximum ion beam current, Ib , that can be accelerated is given approximately by (1)
where eo is the permittivity of space, A b is the beam area, elm is the charge-to-mass ratio of the accelerated ions, V t is the total voltage (Vb + Va) , and 19 is the gap between the
Gridded Broad-Beam Ion Sources
11
screen and accelerator grids. This equation is derived from Child's law, (6) but is only approximate because the effective area for ion extraction is less than the total beam area and the effective acceleration distance is greater than the gap between the grids. The actual beam current is usually only 20-50 oib of the approximate value given by Eq. (1). Because the ion-beam current varies as Vi/ 2 , the maximum beam current that can be extracted without direct impingement of energetic ions on the accelerator grid depends strongly on the beam voltage, Vb. Many of the developments in gridded broad-beam ion source have been associated with obtaining high beam currents at moderate beam voltages. Improved reliability and ease of maintenance have also been objectives in recent developments. 2.3 DISCHARGE CHAMBER
The axial-field configuration was the first discharge chamber used (and still being used in many ion sources) for a gridded broad-beam ion source. This configuration, Fig. 2, has a central cathode, a cylindrical anode, and a magnetic field approximately parallel to the axis of the cylinder, with the magnetic field usually generated by an electromagnet (not shown in Fig. 2) (7). The efficiency of ion production is improved if the field strength decreases toward the ion optics, as indicated in Fig. 2.
r
Anode
Figure 2: Axial-field discharge chamber.
A multipole configuration (Fig. 3) was developed later and gives a more uniform ion density at the ion optics. (Note that the uniformity at the ion optics is only one factor in the uniformity at the target.) The initial version of this discharge chanlber used electromagnets for research purposes, (8) but later versions have all used permanent magnets. This discharge chamber presents maintenance problems when used in industrial applications. Specifically, all the recesses and hidden surfaces of this design, result in the
12
Handbook of Ion Beam Processing Technology
requirement for complete disassembly for any thorough cleaning. Removal of the permanent magnets in this design involves a risk of damage to the magnets, so that an ion source with this type of discharge chatnber is nornlally returned to the manufacturer for such cleaning. Magnets
Pole pieces
=3
Cathode
Figure 3: Multipole discharge chamber.
A more recent discharge chamber (Fig. 4) resembles the multipole design, except that the inside surface of the chamber is a snlooth and continuous anode (9). Because the entire inside surface (except for cathode and cathode supports) is at anode potential, this type of discharge chamber has a reduced sputter contamination of the target from the discharge chamber. Further, this inside surface protects the magnet and pole-piece structure from deposits, and is easily removed for any cleaning that it may require. All three of these discharge-chamber configurations are presently being used on different commercial ion sources. Within the limits described above for uniformity of ion density, sputter contamination, and ease of maintenance, all can be used for a variety of applications. The ratio of discharge current, I d to ion-beam current, I b , is typically in the range of 10-20 for these discharge chambers. The discharge voltage should be at or below the sum of the first and second ionization potential for the gas being used in order to minimize the production of doubly charged ions. (For argon, the first and second ionization potentials are 15.8 and 27.6 eV. Their sum is 43.4 eV. The discharge voltage with argon should therefore be less than 43.4 V. To offset some secondary effects, the discharge voltage should actually be 40 V, or even 35 V.) The effect of doubly charged ions is discussed further in Sec. 2.4.
Gridded Broad-Beam Ion Sources
13
pieces
3
Cathode
Tl
Magnetic field Anode
~~~~ Figure 4: Modified multipole discharge chamber. 2.4 ION OPTICS
Many ion-optics configurations have been used. The most frequently used configurations have been: (1) one-grid ion optics for low beam voltages «100-200 V), (2) flat two-grid ion optics for snlall and medium sized ion sources up to 15-20 cm, and (3) dished two-grid ion optics for large ion sources (greater than about 20 cm) and applications that require a large amount of beam focusing or defocusing. One-grid ion optics, (10) Fig. 5, draw ions directly from the discharge plasnla, so that the acceleration distance (lg in Eq. (1)) is the thickness of the plasma sheath. Because this distance can be less than the mechanical spacing between two grids, ion current densities of 1-2 mAlcm2 can be extracted at low voltages - typically less than 100-200 V. Without the protection of the screen grid, the accelerated ions impinge directly on the accelerator grid. This direct impingement is a major shortcoming of one-grid ion optics, and results in both a rapid wear of the grid and substantial contamination of the target with grid material. If a metal grid is used with oxygen, the oxide formed can slow the erosion rate. Fine-mesh (> 40 wires/cm or > 100 wires/inch) stainless-steel screening is readily available and is often used as the grid material for one-grid ion optics. Flat two-grid ion optics (Fig. 1 or 2), were the type originally used on gridded, broad-beam ion sources. (7) These ion optics are widely used in industrial applications, and are at present almost always fabricated from graphite - usually pyrolytic graphite. The very low thermal expansion and sputter yield of graphite makes it a useful material for ion optics. Graphite, however has a small modulus of elasticity (Young's modulus),
14
Handbook of Ion Beam Processing Technology
so that deflections are excessive under electrostatic and gravitational forces when large grids are fabricated from graphite (11). The ion current densities that are obtainable depend on the grid spacing Og in Eq. (1)) and the voltages used. For a typical 1 mm spacing and beam-supply voltages of 500-1000 V (500-1000 eV), the current densities at the ion optics typically range from 1-4 mA/cm2 .
~ I I I I
Accelerator grid
--_~I
Figure 5: An ion source with one-grid ion optics.
Note that the ion-beam current is very sensitive to total voltage. An ion-beam current or current density therefore has little meaning without the corresponding ion energy. For example, higher beam currents can always be obtained at high beam voltages, Vb' of 1500-2000 V. Such high voltages and ion energies are, however, relatively inefficient for sputtering in deposition applications and can cause excessive damage to substrates and photoresist in etching applications. For small ion sources with beam diameters less than about 10 cm, the grid spacing can be reduced to well under 1 mm, resulting in higher ion current densities. To fully utilize a small grid spacing, though, the diameter of a grid hole should not be more than several times the grid spacing, and the grid thickness should be only a fraction of the hole diameter. As the grid spacing is reduced, then, the reduced hole diameter and reduced grid thickness result in an increasingly fragile grid structure. The limit is not a clearcut one, but the increasing difficulty in handling and maintaining fragile grids does result in a practical limit on the minimum grid spacing. A small amount of focusing or defocusing can be obtained with two-grid ion optics by offsetting the apertures in the two grids. The deflection of a beamlet (the ions from a single aperture) with this technique is usually limited to about 4-8 degrees.
Gridded Broad-Beam Ion Sources
15
Almost all dished two-grid ion optics, Fig. 6, are fabricated from molybdenum. Molybdenum has a low thermal expansion and a moderate sputter yield. The modulus of elasticity, however, is more than a factor of ten higher than that of graphite, which results in much more rigid grids. The dished shape greatly reduces the grid deflections from thermal gradients within the grids (12). Dished grids have been used to maintain a grid gap of approximately 1 mm over a 38-cm beam dianleter in a conlmercial ion source (5).
~~
\\
\\
,
\ \
\ \' \ \ \ \
,, \ \
It
I'
II II • I II
.
II
I I
,J' ,, ,,
.,
I,
I,
I I
I I
l!::==========iJ([' Figure 6: An ion source with dished two-grid ion optics.
Dished grids can be used for a large amount of focusing or defocusing, and have frequently been used for such purposes on medium sized ion sources (13). (The grids are dished as indicated in Fig. 6 for defocusing, and in the reversed direction for focusing.) Ion optics configurations other than dished molybdenum grids have been used on ion sources that are physically large. It is necessary, though, to distinguish between an ion source that is physically large and one that has a large beam current, hence a large processing capability. If the entire circular beam area is utilized, the ion-beam current can be shown to be proportional to the square of the ratio of beam diameter to grid gap, (d b/l g )2. (To show this, substitute '1Td£/4 for A b in Eq. (1).) Assuming the same voltages are used, then, if the ion optics of a large ion source are to have a larger ion-current capacity than those of an ion source that is smaller, but otherwise similar, the ratio db/l g must be larger for the large ion source. If this ratio is not larger, the beam current of the large source will be no greater than the small one at the same voltages, regardless of the difference in physical size. Present ion sources that are large and also have correspondingly large beam currents all use dished molybdenum grids to achieve a large value of db/l g •
16
Handbook of Ion Beam Processing Technology
The preceding ion-optics configurations account for almost all industrial applications. There are a number of other configurations that are occasionally used, most of which are described in an earlier publication. (3) 2.5 PRODUCTION APPLICATIONS
As mentioned in the Introduction, the use of gridded, broad-beanl ion sources has been limited mostly to research applications. The few production applications have been limited to products of very high unit cost. The ion-source requirements for a production environment have been given in an earlier paper. (4) These requirements emphasized ease of maintenance and reliable operation. Several ion sources are available that meet these requirenlents. The 38-cm ion source not only meets these requirements, but also has a large processing capability. (5) For example, ion-beam currents of 4-5 A are possible - up to 4 A without exceeding 1000 eV (a beam voltage, Vb' of 1000 V). A cutaway sketch of the 38-cm ion source is shown in Fig. 7. The discharge chanlber is of the type shown in Fig. 4, with an anode that covers and protects the magnet and polepiece structure and is also readily removable for cleaning. The ion optics are dished nl0lybdenunl. As described previously, (4) alignment of the ion optics has been a nlajor problem in both maintenance and reliability. A large number of ion optics that require an alignment step have been used on ion sources in an industrial environment. The serious nature of the alignnlent problem is indicated by the fact that most of these ion optics have accelerator-grid holes that have been worn into noncircular shapes by prolonged operation in a misaligned condition. The ion optics of the 38-cm ion source are specifically designed to obtain a precise alignment from a straightforward assembly procedure (14). That is, a separate alignment step is not required in the 38-cnl ion optics. Such an alignment step depends on the hand-eye coordination of a technician, hence is not easily reproducible. When required, this step can greatly decrease the reproducibility of operation, hence the in-process reliability. Ion sources with the reliability, ease of nlaintenance, and large processing capability of the 38-cm design should find greatly increased use in production applications. 2.6 TARGET CONTAMINATION
The importance of contamination of the target by the ion source depends on the particular application. Most etching processes are relatively insensitive to such contamination, while contamination can be much more critical in the deposition of filnls.
Gridded Broad-Beam Ion Sources
17
o
Figure 7: Cutaway sketch of 38-cm ion source. (From Ref. 5)
The relative magnitudes of contamination from different ion-source components are important in the assessment of such contamination. These relative magnitudes have been calculated from sputter yields and geometrical considerations, and are indicated in Table 1 for a typical ion source. This ion source used a O.4-mm tungsten-wire cathode, a O.4-mm tungsten-wire neutralizer, and flat graphite grids with a beam diameter of 15-16 em. The working gas was assumed to be argon. The vacuum-chamber pressure around the ion source was assumed to be about 2xlO- 2 Pa (1.5xlO- 4 Torr, or 2x10- 4 Torr using an ion gauge calibrated for nitrogen or air). This pressure resulted in an accelerator-tobeam current ratio of about 0.08. The contamination magnitudes in Table 1 are given as ratios of the arrival rates of contamination atoms to the arrival rate of beam ions at the target, which is assumed to be 30 em from the ion source. The results are approximately correct for beam voltages, Vb' from 500-1000 V. Several points can be made from the contamination ratios presented in Table 1. One point is the order of importance of different components for contamination: the accelerator grid is most important, the neutralizer next, and and cathode least. The contamination from the cathode is much smaller than that from the neutralizer because it is bombarded with less energetic ions, it is farther from the target, and the ion optics partially block the material sputtered from the cathode.
18
Handbook of Ion Beam Processing Technology
TABLE 1. Target contamination from a gridded broad-beam ion source, in atom-to-ion
ratios.
Component
Va' 100 V
Va' 200 V
Cathode Neutralizer Accelerator grid
0.lx10- 4 1x10- 4 2x10- 4
0.lx10- s 1x10- 4 6x10- 4
It should be noted, however, that nluch of the material sputtered from the accelerator grid may be resputtered target material. If this is the case, the contamination from the accelerator grid can be substantially reduced from that shown in Table 1. Another point is the importance of accelerator voltage. An accelerator voltage of 100 V is typically required to prevent electron backstreaming at a beam voltage of 500 V, while an accelerator voltage of 200 V is typically required at 1000 V. In this 100-200 V range of accelerator voltage, the sputter yield from the accelerator grid increases drastically with voltage. Operating at an accelerator voltage that is larger (more negative) than necessary can be a major cause of contamination. For example, the use of an accelerator voltage of 200 V, or more, to give a large beam divergence at a beam voltage of 500 V, or less, is questionable from the contamination viewpoint. If reduced contamination is important, the accelerator voltage should be near the minimum necessary to prevent the backstreaming of electrons from the ion beam. The contamination from the accelerator grid can be further reduced by reducing the background pressure in the surrounding vacuum chamber. This is because the ions that bombard the accelerator are generated by charge exchange, and the production of these ions is reduced at a lower pressure. The contamination from the cathode and neutralizer can be reduced by using smaller wire diameters, but the lifetimes will also be reduced, roughly in proportion to the wire dianleter. (There is an effect of ion energy on the contamination ratio fronl the neutralizer, but the magnitude of this effect is small in the 500-1000 eV energy range compared to other uncertainties.) The use of a hollow cathode neutralizer in place of the tungsten-wire neutralizer will further reduce the contamination. The use of a hollow cathode in place of the tungstenwire cathode is much more questionable. Not only is the cathode a relatively minor source of contamination, but the hollow cathode and its keeper can be a source of contamination in the discharge-chanlber plasnla. In conlparison, a hollow-cathode neutralizer is located in a low-density plasma outside of the ion beam and contributes very little to target contamination when correctly oriented.
Gridded Broad-Beam Ion Sources
19
There is another source of target contamination from an ion source that can be important and is not listed in Table 1. This is the sputtered material from cathode-potential surfaces in the discharge chamber - other than the cathode itself. The area of these surfaces varies widely, so that a single typical value cannot be given. However, an example can be given for an ion source in which the back surface of the discharge chamber is mostly at cathode potential (either Fig. 2 or Fig. 3). For such a configuration, the contamination ratio at the target would be roughly 8xl0- 4 for a normal discharge voltage, V d , of 40 V. For a discharge voltage of 35 V, the contamination ratio would drop to roughly 4xl0- 4 • The contamination ratio increases sharply at higher discharge voltages for two reasons. First, a higher discharge voltage increases the voltage through which the ions fall when they strike cathode potential surfaces. Second, because the higher discharge voltage results in a substantial production of doubly charged ions, some of the colliding ions have twice as much energy due to being doubly charged. At a discharge voltage of 60 V, for example, the contamination ratio would be roughly 40x10-4 • Because there is no simple and direct indication of the production of doubly charged ions, many ion-source operators have greatly increased target contamination by operating at excessively high discharge voltages. If target contamination is a problem, the discharge voltage with argon should be decreased from 40 to 35 V or, if the source will operate there, at an even lower discharge voltage. Operation with a design that minimizes the area of the cathode-potential surfaces, such as Fig. 4, should also be considered. As mentioned, the contamination ratios given above are for argon as the working gas. The use of reactive gases can give drastically different results. For example, oxygen will greatly increase the contamination from the neutralizer, but decrease the contamination from the cathode potential surfaces in the discharge chamber. (The oxide apparently vaporizes at the neutralizer temperature, but serves as a protective coating at a lower temperature.) The values given should not, therefore, be considered as typical of operation with reactive gases. 2.7 CONCLUDING REMARKS
A gridded, broad-beam ion source generates an ion beam with a well controlled direction, density, and energy. This improved control constitutes the major advantage of such an ion source when it is compared with most competitive processes. The advantages of these ion sources have been well recognized in research applications. The absence of a correspondingly wide use in production applications is felt to be due to a lack of both designs and processing capability suitable for production. Ion sources presently available should find increasing applications in conventional production environments. The information included herein on contamination should be useful in selecting configurations and operating conditions that will give low target contamination.
20
Handbook of Ion Beam Processing Technology
2.8 REFERENCES
1. H. R. Kaufman, Technology of Electron-Bombardment Thrusters, in Advances in Electronics and Electron Physics, Vo1.36, (L. Marton, ed.), pp. 265-373, Academic Press, New York (1974). 2. H. R. Kaufman and R. S. Robinson, Ion Source Design for Industrial Applications. AIAA J. 20: 745-760 (1982). 3. H. R. Kaufman, J. J. Cuonlo, and J. M. E. Harper, Technology and Applications of Broad-Beam Ion Sources Used in Sputtering. Part I. Ion Source Technology. J. Vacuum Science and Technology 21: 725-736 (1982). 4. H. R. Kaufman, Broad-Beam Ion Sources: Present Status and Future Directions. Vacuum Science and Technology A4: 764-771 (1986).
L.
5. H. R. Kaufman, W. E. Hughes, R. S. Robinson, and G. R.Thompson, Thirty-Eight Centimeter Ion Source, presented at the 7th International Conference on Ion Implantation Technology, June 7-10, 1988, Kyoto, Japan. 6. C. D. Child, Discharge from Hot CaD. Physical Review 32: 492-511 (1911). 7. H. R. Kaufnlan, An Ion Rocket with an Electron-Bombardment Source. Technical Note TN D-585: Jan. 1961.
NASA
8. H. R. Kaufman, Experimental Investigations of Argon and Xenon Ion Sources, NASA Contr. Report CR-143845, June 1975. 9. H. R. Kaufman, R. S. Robinson, and W. E. Hughes, U. S. Patent No. 4,481,062, Nov. 1984. 10. P. LeVaguerese and D. Pigache, Etude d'une source d'ions de basse energie et a'forte densite de courant. Revue de Physique Appiquee 6: 325-327 (1971). 11. R. S. Robinson and H. R. Kaufman, Ion Thruster Technology Applied to a 30-cm Multipole Sputtering Ion Source. AIAA J. 15: 702-706 (1977). 12. V. K. Rawlin, B. A. Banks, and D. C. Byers, Dished Accelerator Grids on a 30-cm Ion Thruster. J. Spacecraft and Rockets 10: 29-35 (1973). 13. H. R. Kaufman, J. M. E. Harper, and J. J. Cuomo, Focused Ion Beam Designs for Sputter Deposition. J. Vacuunl Science and Technology 16: 899-905 (1979). 14. H. R. Kaufman and R. S. Robinson, patent pending.
3 ECR Ion Sources
William M. Holber
3.1 INTRODUCTION
In plasma processing, there are contributions to an etch or deposition from both reactive neutral species and from ions - both of which are usually created in the same discharge. The roles of the ions and neutrals have been explored extensively - however, there are still many unknowns, especially in the low ion-energy reginle (under 100 eV). A process dominated by reactive neutrals tends to be relatively free of physical damage (although not necessarily free of chemical damage), isotropic in its directionality, and may be chemically highly selective. An ion-dominated process may be more spatially directed, but, especially at higher energies, may cause more physical damage and may be less selective. A knowledge of the relative contributions of ions and neutrals to various processes helps to explain the trends which have emerged in recent years in plasma processing for semiconductor applications. The driving force behind these trends is the nl0vement towards smaller, faster, more densely packed semiconductor devices. This requires processing which is more accurate. For example, in etching, the directionality of the etch must be nlore tightly controlled. Thinner, more delicate structures require processing which causes less damage and is more selective. Depositions have to be carried out at lower temperatures and still yield high quality films. However, rates must be kept high enough to satisfy manufacturing needs. The first plasma tools used were higher-pressure devices - up to the Torr region. Etching tended to be isotropic - or if directional, relying to a large degree on sidewall passivation to achieve directionality. Reactive ion etching, currently in wide use, operates at lower pressures, ranging from tens to hundreds of millitorr. Plasma densities in these tools are typically on the order of 10 10 cm- 3 , so that the ion to neutral ratio is about 10- 6 -10- 4 . The energy of ions impinging onto the substrate is dependent upon the operating pressure, excitation frequency, excitation voltage, and gas species, but can achieve an appreciable fraction of the peak rf voltage. Ion energies of several hundred eV are not uncommon.
21
22
Handbook of Ion Beam Processing Technology
More recently, magnetically-active plasmas have received much interest. For example magnetron systems can operate at pressures down to a few millitorr, with plasma densities as high as 1011 cm- 3 - corresponding to an ion-to-neutral ratio of about 10-4 -10- 2 . Ion energies tend to be lower than for RIE systems; typically 100 eV. Electron-cyclotron-resonance (ECR) plasmas are receiving an increasing anlount of attention as one possible means of meeting more stringent processing requirements. ECR plasmas continue the trend from high-pressure rf plasmas, to lower pressure RIE plasmas, to magnetron-type plasmas. They have the capability of operating at lower pressures and higher plasma densities, with a corresponding greater ion-to-neutral ratio (greater than 10% in some cases.) Ion energies can be as low as a few tens of eV. This chapter will begin with the basic theory behind ECR plasmas. A discussion will then be made of various operational considerations and the types of ECR sources currently under investigation. Finally, specific knowledge gained from both etching and deposition experiments carried out using ECR plasmas will be presented. Much of the original work in ECR plasmas was done for plasma fusion applications, where it is an attractive source for both plasma generation and heating (1). This work began in the early 1960's, with applications in snlall plasma mirror machines. With the development of higher-frequency, higher-power microwave sources, which is necessary for the generation of higher density, more energetic plasmas, electron-cyclotron resonance heating has received increased attention for use in larger-scale plasma confinement devices, such as tokamaks. Microwave sources with frequency greater than 100 GHz and peak powers at megawatt levels are now in use. The initial work in applying ECR plasma generation towards materials processing work for semiconductor applications was carried out primarily in Japan starting in the mid-1970's (see, for example, 2,3,4.) This work was aimed at both the development of high-current sources for ion-implantation, where hot-cathode sources have a limited lifetime and can be a source of contamination, and for use in plasma etching. Promising results have since been obtained in both etching and in deposition of various materials. The predominance of the research work has continued to be carried out by a number of groups in Japan, although activity elsewhere is now increasing. The first commercial ECR tools became available several years ago, prinlarily for R + D use, and manufacturing-scale machines are now available also. 3.2 THEORY OF OPERATION
The basic theory behind ECR plasma generation will be presented here. This consists of a discussion of the basic resonance condition, the importance of the magnetic field profile in creating, containing, and extracting the plasma, and the launch of the microwave into the plasma. An electron in motion in a uniform magnetic field will undergo circular motion transverse to the magnetic field direction, with frequency (the cyclotron frequency) We
= e
B/ m
(1)
When an electromagnetic field is applied, energy can be transferred from the field to the electrons. A resonance condition exists for the energy transfer when the electron under-
ECR Ion Sources
23
goes precisely one circular orbit in one period of the applied field. Several considerations have made 2.45 GHz the frequency utilized in all of the ECR materials processing work reported to date. The magnetic field required to obtain the resonance condition at this frequency, 875 Gauss, is reasonably simple and inexpensive to achieve with ordinary water-cooled solenoidal electromagnets. This frequency is commonly used for industrial heating applications (consumer microwave ovens, for example), so that hardware and power supplies are readily available. Finally, although achievable plasma densities generally increase with higher excitation frequency, the densities obtained using 2.45 GHz are high enough to be useful for most current materials processing applications. The radius of motion of the electron in the magnetic field is given by (2)
where V.L is the velocity component of the electron perpendicular to the direction of the magnetic field. The energy distribution of the electrons in the ECR plasma is dependent on parameters such as gas pressure and microwave power density. The basic trend is that electron energy increases as pressure is decreased, since the electrons can undergo more revolutions between collisions, with each revolution resulting in an increase in the electron energy. Under conditions commonly employed, the average electron energy is typically about 5-10 eVe For a transverse electron energy of 5 eV and microwave frequency of 2.45 GHz, the calculated electron radius in the source is approximately 0.01 cm, which is much smaller than the dimensions of the vacuum system. An electromagnetic wave which is right-hand circularly polarized relative to the magnetic field direction can transfer energy to the electrons. When the frequency of the wave nlatches the cyclotron frequency of the electron in the magnetic field, the systenl is in resonance and energy can be very efficiently transferred from the wave to the electrons. The electrons in turn can collisionally transfer energy to both ions and neutrals. This situation is pictured in Figure 1. In general, a wave may not have the appropriate polarization to allow for efficient first-pass absorption through the plasma. In such cases, the portion of the wave having the correct polarization will be absorbed. The rest of the wave may be absorbed on successive passes through the plasma, as the wave is scattered inside a vacuum chamber or microwave cavity. This may not be an efficient method of generating a plasma, since chamber surfaces are generally fairly lossy at microwave frequencies, and because the densest plasmas may not be attainable in such a manner. For a simple, unmagnetized plasma, there is a simple dispersion relation for electromagnetic waves propagating in the plasnla (6). Fronl this dispersion relation, one can derive a critical density for the plasma, given by (3)
where w is the frequency of the wave. For N c too large or w too small, the electromagnetic wave cannot penetrate the plasma. Thus, at a given microwave frequency, the density achievable in the bulk of the unmagnetized plasma is limited to the critical density. For a microwave frequency of 2.45 GHz, the critical density is 7 x 1010 cm 3.
24
Handbook of Ion Beam Processing Technology
In order to obtain a dense plasma, it is necessary to carefully consider how the microwave power is launched into the plasma, with respect to the magnetic field. The transmission and absorption properties of a magnetized plasma are quite complicated, but can be understood at least qualitatively by referring to Figure 2. Here it can be seen that there are regions of propagation and non-propagation for left and right circularly polarized waves along a magnetic field. For right-hand circularly polarized waves (with respect to the magnetic field), the wave will propagate along the magnetic field lines, as long as the magnetic field strength remains above the resonance value (Wb / W = 1). The wave can therefore propagate through a plasma having a density above the critical value. Other relationships exist for wave propagation across magnetic field lines. When the microwave propagation relative to the magnetic field profile is carefully controlled, plasma densities can exceed the critical plasma density by a factor of 10 to 100 (12,35). Since the flux out of a plasma region is directly proportional to the density of the plasma, a dense plasma is important in order to obtain a high flux of ions onto the substrate to be processed. For example, an argon plasma with a density of 3 x 1011 cm- 3 and an average electron energy of 10 eV will have an ion saturation current of about 11 rnA cm-2 • Greater plasma density can directly translate to higher ion flux. Consequences of not launching the microwave properly include radially inhomogeneous plasmas and low plasn1a densities.
...80 (DECREASING)
...E
RESONANCE REGION
I ELECTRON GYRATION ALONG MAGNETIC FIELD LINE RIGHT HAND CIRCULARLY POLARIZED WAVE Figure 1: Circularly polarized electromagnetic wave propagating along magnetic field
lines.
The magnetic field can also aid in the confinement and extraction of the plasma created in the ECR region. For a solenoidal magnetic field, radial losses will be reduced, although not entirely eliminated, since the electrons and ions will be inhibited from crossing magnetic field lines. The transport of the plasma will therefore be primarily along the direction of the magnetic field lines. The gradient along the magnetic field lines will affect the motion of the electrons and the transport of the plasma. Each circulating
ECR Ion Sources
25
electron can be considered as constituting a magnetic dipole, which will be attracted towards regions of weakening magnetic field by a force given by
(4)
F = ( -jl'V)B
where jl is the magnetic dipole of the circulating electron. For cases in which the magnetic field is in a nlirror configuration, the electrons will bounce between the regions of high magnetic field. For a situation in which the magnetic field monotonically decreases in one direction, the plasma will be preferentially extracted along that direction, with the electron motion changing from primarily circular in the source region to more axial as the magnetic field decreases. As will be discussed in the next section, the magnetic field gradient affects the energy of the plasma as well as its extraction. In this simple picture, the role of the ions has been ignored. Although significant energy will not be transferred directly from the microwave excitation to the ions, due to their low nlobility, the ions can still have an effect on the plasnla dynamics. As the electrons are extracted from the source region, primarily along the magnetic field lines, an electrostatic potential is created which will tend to pull the positive ions along the same direction. The ions can also undergo gyration about the magnetic field lines. However, the greater mass of the ions causes them to have an orbital radius much larger than that of the electrons. For an argon ion having kinetic energy of 1 eV transverse to a magnetic field of 300 Gauss, the radius of the ion gyration will be about 2 cnl. In sonle systems, this can be large enough to cause significant loss of ion current to the vacuum chamber walls. Also, this can have an impact on the angle at which the ions will impinge upon the substrate.
IJ.l
IJ.Z
<1
JJ,,.
<
<
1
IJ.,.
>1
1
O-------------l... o 1
2
Electron density (Wp/w)2 z nIne Figure 2: Propagating regions of left and right circularly polarized waves along magnetic field lines. Waves are cut off in the shaded regions. (From Ref. 5)
26
Handbook of Ion Beam Processing Technology
In summary, the action of the magnetic field and microwave excitation source in resonance, has several effects. Energy will be efficiently transferred fronl the microwave electromagnetic field to the electrons, resulting in a discharge that can operate at low pressures and high ionization efficiencies. If the microwave polarization and magnetic field orientation are carefully controlled, very high plasnla densities - over 10 12 cm- 3 - can be obtained. Control of the magnetic field profile can affect both the spatial distribution of the plasma and the energy distribution of the ions and electrons in the plasma.
3.3 TYPES OF SOURCES AND CHARACTERISTICS
There are several configurations which have been used for ECR sources in materials processing. A system typical of the divergent magnetic field type is shown in Figure 3. Microwave power is introduced into the vacuum vessel through a dielectric window; either quartz or ceramic are typically used, since they have low losses at 2.45 GHz. Typically two magnets surround the ECR plasma region. This allows some independence in setting the field strength to the resonance value, and tailoring the field lines and gradient for optimum plasma generation and extraction. Gas may be introduced either into the source directly, downstream near the specimen, or both. For many applications, the microwave applicator, source chamber, and possibly the process chamber must be water-cooled, since microwave power, plasma species, and excited neutrals can all cause considerable heating. A temperature-controlled substrate holder may also be necessary, due to either the high heat load from the plasma or a need to heat the specimen during processing.
r\:7I
~ SOURCE
~MAGNET
COILS
GAS INLET AUXILIARY
~ETCO'L
I
SUBSTRATE
TO VACUUM PUMPS
Figure 3: Example of divergent magnetic-field ECR source.
In some cases, a third solenoid is added to the system, in the vicinity of the substrate. This gives additional flexibility in tailoring the magnetic field lines to a desired profile and can have a significant effect on the energies and flux of ions impinging onto the substrate.
ECR Ion Sources
27
Various schemes have been employed in launching the microwave in the desired manner. In some cases, a single-mode or controlled multimode microwave cavity is constructed. In others, the microwave is treated as a traveling wave launched into the highly absorbing plasma region. In either case, analytical treatment becomes extremely difficult when one considers the effect of both the highly absorbing plasma and the nlagnetic field on the propagation of the microwave. It has been generally found that the gas pressure has a distinct effect on the plasma parameters - electron temperature, plasma density, and plasma potential. These are typically measured with Langmuir probes. It has been consistently found (2,7,8) that the plasnla density, which can be calculated from the measured ion saturation current density and the electron temperature, peaks in value at a pressure of about 1 mTorr. The exact relationship is dependent on the gas used and the magnitude of the absorbed microwave power. Peak values reported in these references exceed 5 x 1011 cm- 3 • It is difficult to compare the results of different reported experiments, since they are often carried out at a stated microwave power level, without a discussion of either the actual plasma source volume, or the total flux of plasma out of the source (in addition to flux density from the source). However, it appears as if the electron temperature falls off more rapidly than the plasma density at higher pressures. Electron temperature reported are typically 5-1 0 eV. The behavior of the plasma as a function of absorbed microwave power is also an important factor. Neither the details of dissociation processes nor the charged species present from various feed gases have been explored in depth for ECR sources. It is to be expected, though, that due to the low pressure and high plasma density relative to RIE type plasmas, there may be substantial differences. For example, the ion output from a very high density ECR oxygen source utilizing extractor grids was found to be over 80% 0+ (12), as opposed to a much smaller fraction for conventional rf plasmas. The plasma density has been measured as a function of applied microwave power. Figure 4 shows the ion saturation current density measured approximately 30 em downstream from the source region, in a divergent magnetic field system. For both the argon and oxygen cases, the pressure in the source and process regions was 3 x 10-4 Torr. The applied microwave power is over 90% absorbed. There is roughly a linear dependence of ion saturation current on microwave power, for both gases studied. The conversion efficiency of microwave power into extracted ion flux is quite good - better than 400 Watts per Ampere - even though the measurement is downstream fronl the source. This demonstrates the high efficiency of the magnetic field in confining and guiding the plasma. The magnetic field profile in this case constituted a weak mirror, with the ion current measured just inside the mirror. Matsuoka and Ono (9,10) have studied the effects of the magnetic field profile on ions extracted from a divergent magnetic field ECR source. They used a third magnetic coil downstream from the source region, to have greater flexibility in tailoring the magnetic field gradient, sonlewhat independently of the source conditions. They could vary the current in the third coil such that the magnetic field varied between a mirror field and a cusp field. It was found that application of a mirror field configuration results in a much denser plasnla in the source region (within the mirror), while the energy and energy dispersion of ions extracted out of the mirror are reduced. Generally, with a mirror field the ion saturation current outside of the mirror region is greater than that for a cu~p field.
28
Handbook of Ion Beam Processing Technology
The pumping system design is critical for ECR systems. The typical operating pressure - 10- 4-10- 2 Torr - is in the region where neither turbomolecular pumps nor mechanical pumps operate at their highest efficiency. High volumetric pump rates are required at the lower end of this pressure range, if high throughput is to be achieved. A simple example will make this clear. In order to deposit Si02 at a rate of 2500 A min- t , onto a five-inch diameter substrate, a reactant flow rate of at least 100 sccm will be required (assuming 10% utilization efficiency). In order to to maintain a pressure of 10- 3 Torr at a flow rate of 100 sccm, an effective volumetric pump rate of 1300 liters/sec is needed. 30
c: ~ :::1
r----.,..--I--TI--~I--__,.I-----,I-----.
o
= Oxygen
o
= Argon
15-
U
c:
o
~
.ao
10
o
~
-
o
(/)
c: ..Q
o
5
~
-
o
o'"-__0I0-1_ _.I_ _.-.._ _
......Lo_ _---I.1_ _- o I
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Microwave Power (kW)
Figure 4: Ion saturation current measured 30 cm downstream from a divergent magneticfield ECR source. The pressure in each case was 3 x 10-4 Torr. (From Ref. 34)
The same basic system can be used for either deposition or etching, with appropriate provision for additional gas inlets and a heated substrate holder. The position of the substrate relative to the source varies from design to design, and usually has some adjustability. In some cases, the substrate is actually immersed in the plasma generation region, while in other cases, it is 20 cm or more remote from the source region. For some applications, it is necessary to apply an external electrical bias to the sample, in order to increase the ion energy above that due to energy gained in both the plasma stream and in the sheath formed at the sample boundary (typically about 20 eV together.) Unlike the electrons, the ions can have a substantial gyroradius, which can cause them to impinge upon the substrate at non-normal incidence. The magnetic field lines may not be normal to the substrate, which can also cause non-normal ion incidence; this may have a radial dependence as well. Application of a modest bias voltage can act to counteract the few eV of transverse energy which an ion may have. Also, some processes may require higher ion energies - for example, if a component of physical sputtering is desired. For substrates which have a dc conductivity, a simple dc bias may be used. For insulating substrates,
ECR Ion Sources
29
such as Si02 , rf bias can be utilized to gain the same effect. Suzuki et al. (11) have reviewed the theoretical foundations for rf-biasing of insulating substrates, and have applied the results to ECR etching of Si0 2 They investigated the etch rate dependence on rf bias, for a number of gases (CF 4 , C 2 F 6 ,C 3 F g , C 4F 10 ' and C 4 C g). As the ratio of carbon to fluorine increases, higher ion energy is required for reasonably rapid etching. The rf peak-to-peak bias needed was found to be typically 100 to 150 volts in this experiment, to yield etch rates of roughly 300 A min- 1 . For some applications, it is desirable to use electrostatic grids to aid in plasma extraction from the source. Grids may improve uniformity of the extracted beam, allow sonle degree of downstream collinlation, and and pernlit the use of just ions, instead of a space-charge neutral plasma. For applications such as a high-density ion source for ion implantation (12), grids may be a necessity. The ion extraction voltage is in some cases ( 13) applied to the entire discharge chamber, relative to a grounded grid. An additional advantage gained by using grids is that potential leakage of microwaves out of the source into the process chamber, which can occur with tenuous plasmas, or if the plasma extinguishes for some reason (e.g. magnet or gas failure), is eliminated. However, for broad beam applications such as etching and deposition, grids are usually not used, since they are a potential source of sputtered contaminants, they reduce the current density which can be extracted, and they tend to be delicate and easily damaged. Another type of ECR plasma source combines an ECR plasma generation region similar to that in the divergent magnetic field case, but without magnetic field extraction and guidance - with a multipole magnetic confinement scheme, achieved with permanent magnets (14,15.) This type of system is shown in Figure 5. The potential advantage lies in that the plasma generation and multipole regions are quite distinct - the source plasma is not mapped into the the multipole region by strong magnetic field lines. The multipole structure limits radial plasma loss to the chamber wall, while the interior region is magnetic-field free. A small-area source can then be expanded into a larger-area multipole region, where the plasma can mix and become spatially more uniform. One disadvantage is that the plasma density in the multipole region may be substantially less than that in the source region, depending on the extraction area and relative sizes of the two volumes. Petit and Pelletier (14) have studied the plasma obtained in such a system, and the results obtained in etching silicon in SF 6' Finally, ECR processing sources have been constructed using permanent magnets only, without electromagnets (15,16.) The permanent magnets are arranged in a multipole array and may be placed inside or outside the chamber. The resultant magnetic field both supplies the 875 Gauss ECR resonance region and also reduces radial plasma loss, while creating a nlagnetic field-free interior region to allow the plasma to mix more uniformly. The microwave has been introduced through a tuned cavity in one case (16), and through an internal multipole antenna array in another (15). Permanent magnet multipole sources are quite compact and have the potential to scale to large diameters. However, the power conversion efficiency (microwave power to extracted plasma flux) appears to be much lower than for electromagnetic coil sources. This is probably because the volume of the source in which the magnetic field is at or near the resonance value is much less.
30
Handbook of Ion Beam Processing Technology
Gas Inlet
Permanent Magnets
Cylindrical Langmuir Probe
Figure 5: Hybrid source consisting of an ECR source region and a permanent magnet multipole region. (From Ref. 14)
Water-Cooled Sample-Holder
3.4 ETCHING
ECR processing offers several potential advantages in etching of materials for electronic applications. These include lower levels of damage and contamination, better profile control, and greater selectivity in materials etched. At the pressures and microwave power levels typically used, the ECR plasma consists of electrons having average energies on the order of 5-10 eV, and electrons and ions bombarding an unbiased substrate with energies of about 10-20 eV. RIE plasmas have similar average electron energies. However, the electrons may impinge upon the substrate with energies nearly as high as the rf driving voltage (over 1000 volts in some cases), and ions may impinge with energies of several hundred eV or more. The energetic electrons may cause damage to the sample directly, or through the creation of x-rays via collisions with the chamber walls. The ions can both act to sputter contaminants from the chamber walls, and cause direct damage to the sample. Since the impinging ions and electrons are far less energetic in the ECR case, it would appear that both contamination from sputtered material and damage fronl energetic charged species should be reduced. Unfortunately, very little work in this area has been done to date. It should be noted that even when rf or dc bias is applied to the substrate to increase the energy of impinging ions, energies of charges species elsewhere in the ECR system will renlain low. Since the inlpinging ions are of lower energy than for RIE plasnlas, the selectivity in etching one material relative to another might be increased. The physical sputter yield of 20-50 eV ions is very low, so that etching with ions at this energy must be a predominately chemical or ion-assisted chemical process. In RIE systems it is often necessary to resort to complex chemistry to obtain high selectivity. For example, changing the relative concentration of carbon and fluorine in the gas by changing feedstock and through the addition of oxygen or hydrogen can change the relative etch rates of silicon and silicon dioxide. This, however, has the disadvantage of being difficult to optimize, pattern and process dependent, and possibly leaving a fluorcarbon film residue.
ECR Ion Sources
31
Some work has been done in studying selectivity in ECR etching. Miyamura et al. (18) studied the relative etch rates of Si and Si02 using C4F 8 and C 3F 8 gases in a RIBE-type ECR system. As shown in Figure 6, with ion energies of 1000 eV and ion flux of 0.3 mA cm-2 , they were able to achieve a relative Si02 to Si etch rate of 30. Theyattribute at least part of the selectivity to the formation of a carbon passivation layer on the silicon surface. Suzuki et al. (11) also used C 3F 8 to study the relative etch rates of Si and Si0 2 • With an rf bias of 200 V peak-to-peak they measured a relative Si02 /Si etch rate of about 4.0, with an absolute Si02 etch rate of 300 A/min. Matsuo and co-workers (13) used an ECR source with a 2-grid extraction system to etch Si02 and Si with C4F 8. They found that the Si0 2 /Si relative etch rate increases with increasing ion flux. The ratio is about 20 at 1 mA cm-2 , with an absolute Si02 etch rate of approximately 1600 A min- 1 • 800 /
Ei = IKeV Ii= 300fLA/cm 2 600 c
·e"'-
4 8
0 C 3 Fa
oct
l1J
ti0:
400
Si02
AC4Fa} / ",,,0 a ./"e /,," ,,-' A C F:
Si
3
:I: 0
t-
l1J
• CF}
e/e
".
~-"
200
....0"'-'
-A-.
().tIl'
~-..
...
.8
1.0
--_A
0 .4
.6
PRESSURE
----
1.4
2.0
( x 10-4 Torr)
Figure 6: Si02 and Si etch rates as a function of gas pressure, for several gases. (From
Ref. 18) Much more work has been done in studying the profiles of etching carried out in ECR systems.(2,8,14,16,17,19) The results obtained have been found to be dependent on the etchant gas, pressure, microwave power, magnetic field confinement, and substrate bias. In a study of silicon etching using SF 6 in a hybrid ECR-multipole apparatus, Petit et al. (14) have made the following observations. At low enough pressures, the etch rate is independent of ion current density and ion energy, and is dependent only on the partial pressure of fluorine atoms. However, the degree of anisotropy and the pressure at which the transition to anisotropy occurs is controlled by the ion current density. Some minimum substrate bias is needed to allow the ion-assisted etching to occur - above this value (-SOV), but below energies at which substantial physical sputtering would occur, the bias seems to have little effect. Figure 7 shows some of their results. Even at an ion current density of only 0.5 mA cm-2 , the etching becomes totally anisotropic at a pressure of 3 x 10- 5 Torr. One conclusion drawn from this work is that spontaneous (non-ion-
32
Handbook of Ion Beam Processing Technology
assisted) etching occurs only above a critical fluorine coverage on the silicon surface. With less coverage, etching will occur only with the assistance of ion bombardment.
1.0
< ~
0 L
~
I
-+-+-+\
0.9
+
+'+
-+--
0.,.
'c0
O.B
~
A=1_~ Vv
I
r
500
-
~oo
N
E 300
'"
.......
<
2; 200
Figure 7: Vertical etch rate V v' anisotropy A and ion current density collected j as a function of SF 6 pressure for a microwave power of 600 W. The resistivity of the ptype silicon wafer is 2-3 Q-cm. (From Ref. 14)
100
o L....-::------L_................ 5
~......I~_---- 0
-I........
1cr
pr~s.ure
~-"
(Torr)
Suzuki et al. (2,8) have similarly found a strong dependence on neutral radical pressure in CF4/02 etching of silicon. They found that with a gas pressure less than 10-3 Torr, spontaneous etching does not occur. Ions are necessary for the reaction to proceed, and act to increase in some nlanner the reaction rate between the surface and radicals. At pressures greater than 10-2 Torr, the neutral radicals alone can etch. In the intermediate pressure regime, both ions and neutral radicals contribute to the etching. For most of this work (8) the ion saturation current from the plasma was approximately 2 mA cm-2 . The etch rates obtained at 10-3 Torr were fairly low under the conditions nlaintained (180 W microwave power, 20% 02' 1 sccm flow rate) - about 500 Amin-to In a study of photoresist etching using 02 in a divergent-magnetic field type ECR apparatus, Tobinaga and co-workers (17) found that the degree of anisotropy of the etching is dependent upon both the energy of the ions in the plasma stream, and the refraction of the ions in the sheath. They identified two pressure regimes. Below about 2 x 10-4 Torr, the ions undergo relatively few collisions with neutrals, the neutral radical species do not contribute much to the etching, and the etching is quite directional, although slow. Above 2 x 10- 4 Torr, the etch rate is higher, but also less directional due to ion scattering and contributions from neutral radicals. Tachi et al. (19) have examined a different approach towards achieving highly anisotropic etching. In both ECR and more conventional RIE systems, they cooled the sample in order to inhibit undercutting from reactive neutral species. The etchant gas used was SF 6 and the pressure was 6.5 x 10-2 Torr, quite high compared to most ECR processing pressures. Despite the relatively high pressure, they found that at substrate temperatures between -130°C and -100 °c, the silicon etching was rapid (greater than 1 /Lm/min),
ECR Ion Sources
33
highly anisotropic (no measurable sidewall etching), and highly selective relative to the photoresist mask (selectivity greater than 30). Using a permanent magnet ECR source, Hopwood et aL (16) have studied etching of masked silicon samples utilizing a CF4/°2 gas mixture at pressures between 3 x 10-4 and 2 x 10- 3 Torr. They found that the anisotropy increases with decreasing pressure, as long as the wafer is biased to a value more negative than the floating potential of the plasma (about -15 V). At 3 X 10-4 Torr pressure, 200 W microwave power, and -50 V wafer bias, the ion flux onto the wafer was about 3 mA cm-2, the anisotropy was about 10, and the etch rate was 400 A/nlin (with a 3.3 cm2 sample.) The etching of semiconductor materials other than silicon may benefit from ECR processing as welL In GaAs etching, for example, the issues of damage, profile control, and UHV compatibility may be even more stringent than in the silicon case. It has been demonstrated (20,21) that etching of III-V compounds using Clz and BCl3 gases in an ECR system can be carried out at lower temperatures and with less danlage and contamination than would be the case in RIE systems. In summary, the work that has been carried out to date in the area of ECR etching has concentrated mainly on rates and profiles, as a function of parameters such as pressure, microwave power, voltage bias on the sample, and ion flux. The general trend that has been found is that a fairly sharp transition towards more anisotropic etching occurs at low pressures; the exact pressure depends on the gas mixture, the voltage bias on the sample, and the ion flux onto the sample. The aspect ratios which can be achieved with little or no sidewall passivation appear to be greater than those attainable with RIE systems. Further work needs to be done in learning how to maintain high rates and good uniformity under these conditions. While some studies have been made on etch selectivity, much work remains to be done. The effect of ECR plasma etching on contamination and damage remains to be studied in depth, as do applications towards materials other than those used in silicon technology.
3.5 DEPOSITION
ECR plasmas have been used to deposit various types of thin films. These include Si02 (22,23,24,25), Si 3N 4 and SiN (22,23,26), Si (27,28,29,30,31), and BN (32,33). There are several possible advantages in using ECR plasmas for deposition, instead of using CVD or more conventional rf-enhanced CVD methods. In some cases, high-quality films can be grown at or near room temperature. This is generally thought to be due to the high density of the plasma - and the resultant high flux of ions onto the sample surface. Directional deposition has been found to be possible, which may aid coverage of nonplanar surfaces. Gas feedstock utilization can be quite high, allowing reasonable rates at low pressures. Films can be plasma doped and annealed at relatively low temperatures. Several researchers have successfully grown thin films of Si0 2 at low temperatures. Using a divergent magnetic field system, Matsuo and Kiuchi (22,23) have deposited high-quality Si0 2 onto Si substrates. The substrates were not externally biased or heated. The ion bombardment energy was about 30 eV, and heating of the substrate due to plasma bombardment raised the sample temperature to between 50 0 C and 150 0 C. 02 was fed directly into the source, while SiH4 was fed into the system downstream, near the
34
Handbook of Ion Beam Processing Technology
substrate. At low flow and deposition rates (400 A min- t ) the resultant films compared favorably with thermally grown oxide, as nleasured by buffered HF etch rate and index of refraction. As can be seen in Figure 8, the deposition rate saturates with microwave power, while the index of refraction improves slightly. When the flow rates of both reactants are raised by a factor of three (23), the deposition rate at the same microwave power nearly triples also.
SiH4 10 cc/min 10 c.c/min
·e.....c:
ec(
"'""
°2
400
~/.
/e
300
II
C
f%
c:
200
g
/
.u; 0
Q.
"---.-
/ •
•
5i02
., )(
"0
.s Q.I
100
II
0
00
_x- x 100
X
x-.-rx200
300
fS ..., 1.5 ~ u
1.4
1.3
C
~
a:
Microwave Power (W) Figure 8: Si02 deposition characteristics. Gas pressure: 2 x 10- 4 Torr. (From Ref. 22)
In another study (24), an investigation has been made of using ECR deposited Si0 2 films to planarize submicron interconnections. Here again, the ECR source was of the divergent magnetic field type. The substrate was heated by the plasma only, to a temperature between 50 0 C and 150 0 C, and gases used were SiH4 , 02' and Ar. RF bias could be applied to the substrate, to increase the energy and directionality of the incident ions. By changing the gas composition and rf and microwave power, the relative amount of etching and deposition on the sidewalls and horizontal surfaces can be varied. Even without rf bias, the ECR-deposited films can fill high aspect ratio features. This is shown in Figure 9, where a comparison is made between experimentally measured ECR films and calculated sputtering results. The ECR films appear to be superior to sputtered films in filling high aspect ratio trenches. Si02 has also been deposited onto Si substrates by simultaneously exposing the substrate to Si atom flux, derived fronl an e-beam evaporated source, and oxygen atom and ion flux, from an ECR source (25). Neither external heating nor rf bias was applied to the substrate. The films obtained were of similar quality to those obtained in rf-plasma CVD systems, although not as good as thermally grown films. It appears as if direct exposure to the plasma causes little damage to the film, and improves its quality through densification. In the same apparatus as used for their Si0 2 deposition, Matsuo and co-workers (22,23) have also studied the deposition of Si3N 4 . They found that under appropriate deposition conditions, the buffered HF etch rate and refractive index of the films were
ECR Ion Sources
35
as good as for thernlal CVD deposited films. In addition, IR absorption spectra show little hydrogen incorporation into the films. A deposition rate of 700 A min- 1 was obtained with 300 W microwave power and flow rates of 20 sccm SiH4 and 30 sccm N 2 • A mininlum of about 150 W microwave power was required to obtain good film properties.
-- 1.0 o
\~ECR(ME~)
~
......
0\
o
S 0:: (!)
\
0.5
\~ SPUTTERING \ (SIMULATED) \
Z
\
::;
--' r::
\
O.O~_--'-""
o
~ ts1
Ha l.5pm
<\
_ _~---J
2.0 ASPECT RATIO (HIS)
1.0
Figure 9: Aspect ratio dependencies of filling ratio for ECR plasma deposition and sputtering deposition. (From Ref. 24)
Manabe et al. (26) have studied the deposition of SiN. The reactants were SiH4 and N 2 and the sample, heated only through plasma borribardment, did not exceed a tenlperature of 60 0 C during depositon. The films were evaluated through refractive index, buffered HF etch rate, and I-V and C-V characteristics. The measured physical properties were close in quality to that for thermal CVD films. The electrical properties were at least as good as for rf-plasma deposited films, with resistivity and breakdown voltage greater than that for thermal CVD films. The deposition rate for these films was quite low - approxinlately 100 A min- 1 - but could be increased to 1000 A min- 1 through changing the gas flow rates. Amorphous silicon filnls have been deposited by a number of researchers (27,28,29,30,31). In one experiment (27), the magnetic field is oriented parallel to the sample surface, to minimize ion bombardment of the sample surface. It was found that the magnetic field strength greatly affects the minimum microwave power necessary to maintain a stable plasma, the deposition rate, and the film structure. Under resonant magnetic field conditions, the deposition rates were lower, the film structure was amorphous, and the mininlum microwave power needed was less than in the non-resonant case. Kobayashi et al. (31) have carried out an extensive study of hydrogenated anlorphous silicon films deposited with an ECR system. The system is of the divergent magnetic field type; argon gas is fed into the source region and SiH4 is fed into the specimen chamber. Electrodes are applied to the deposited a-Si:H films to allow nleasurement of photoconductivity and dark conductivity. IR absorption spectra are used to measure hydrogen bonding in the films. ESR spin density measurements give information on bonding. Without external substrate heating (only heating from the plasnla), deposition rates of about 1400 A min- 1 are obtained, with film quality comparable to that obtained for conventional rf-plasma CVD films, for which substrate heating is required. Substrate
36
Handbook of Ion Beam Processing Technology
heating improves the electrical properties somewhat. The electrical and physical properties of the films appear to be best at the highest microwave power used - 500 W. Kitagawa et al. (29) have deposited phosphorus and boron-doped hydrogenated amorphous silicon films in an ECR system. The feed gases were SiH4 , PH 3 , and B2 H 6 • The films were deposited without external substrate heating - the estimated temperature rise due to plasma bombardment was less than 60 aC. Without annealing, the measured conductivity was very low compared to that for doped a-Si:H films prepared by conventional rf-plasma CVD operated at 250 aC .. Annealing of the films at 200 aC and 300 aC resulted in optical gap and conductivity comparable to that obtained for conventional plasnla deposited films. Yamada and Torii (28) used an ECR source to deposit homoepitaxial films on silicon. The source gas was SiH4 and the ion energy, controlled by biasing the entire plasma source with respect to the substrate, could be raised to as high as 1500 volts. At a temperature greater than 600 aC, homoepitaxial films could be grown for ion energies of 50 - 650 eVe At 300 - 350 eV ion energy, homoepitaxial growth could be obtained at 400 aC. Finally, boron nitride films have also been deposited using ECR plasmas (32,33). Reactants in both cases were N 2 and B2 H 6 • The films formed are generally of good quality. Cubic boron nitride, having a zinc-blende type structure, could be synthesized with a substrate self-bias of greater than 40 V (33), compared to a threshold of over 200 V for deposition carried out in the same system using only the rf bias on the substrate to drive the discharge. In summary, various films have been deposited using ECR systems. The general trend is that films can be deposited at lower temperatures than for CVD or conventional plasma-CVD processing, with conlparable or better film quality. This is thought to be due to the high degree of dissociation and high ion flux from the ECR plasma. Some degree of directional deposition has been demonstrated as well. Although rates are generally low, progress has been nlade in scaling these upward through higher flow rates and greater microwave power.
3.6 REFERENCES
1. A.C. England, IEEE Trans. on Plasma Science, PS-12: p. 124 (1984). 2. Keizo Suzuki, Sadayuki Okudaira, Noriyuki Sakudo, and Ichiro Kanamoto, Jpn. J. Apol. Phys. 16: p. 1979 (1977). 3. Noriyuki Sakudo, Katsumi Tokiguchi, Hidemi Koike, and Ichiro Kanomata, Rev. Sci. Instrum., 48: p. 762 (1977). 4. Noriyuki Sakudo, Katsumi Tokiguchi, Hidemi Koike, and Ichiro Kanomata, Rev. Sci. Instrum., 49: p. 940 (1978).
ECR Ion Sources
37
5. M.A. Heald and C.B. Wharton, Plasma Diagnostics With Microwaves (Robert E. Krieger Publishing Company, Melbourne, Fla. 1978). 6. Francis F ~ Chen, Introduction to Plasma Physics and Controlled Fusion (Plenum Press, New York, 1984). 7. Chen Keqiang, Zhang Erli, Wu Jinfa, Zhen Hansheng, Guan Zuoyao, and Zhou Bangwei, J. Vac. Sci. Technol. A4 p. 828 (1986). 8. Keizo Suzuki, Sadayuki Okudaira, and Ichiro Kanomato, J. Electrochem. Soc. 126: p. 1024 (1979). 9. Morito Matsuoka and Ken'ichi Ono, ADD!. Phys. Lett. 50: p. 1864 (1987). 10. Morito Matsuoko and Ken'ichi Ono, J. Vac. Sci. Technol. A6: p. 25 (1988). 11. Keizo Suzuki, Ken Ninomiya, Shigeru Nishimatsu, and Sadayuki Okudaira, J. Vac. Sci. Technol. B3: p. 1025 (1985). 12. Yasuhiro Torii, Masaru Shimada, and Iwao Watanabe, Nucl. Instr. Meth. Phys. Res. B21: p. 178 (1987). 13. Seitaro Matsuo and Yoshio Adachi, Jpn. J. Apol. Phys. 21: .p L4 (1982). 14. B. Petit and J. Pelletier, Jpn. J. Apol. Phys. 26: p. 825 (1987). 15. Rudolf R. Burke and Claude Pomot, Solid State Technology, p. 67 (Feb. 1988). 16. J. Hopwood, M. Dahimene, D.K. Reinhard, and J. Asmussen, J. Vac. Sci. Technol. B6: p. 268 (1988). 17. Y. Tobinaga, N. Hayashi, H. Araki, and S. Nakayama, J. Vac. Sci. Technol. B6: p. 272 (1988). 18. M. Miyamura, O. Tsukakoshi, and S. Komiya, J. Vac. Sci. Technol. (1982).
20: p. 986
19. Shinichi Tachi, Kazunori Tsujimoto, and Sadayuki Okudaira, Apol. Phys. Lett. 52: p. 616 (1988). 20. Sumio Sugata and Kiyoshi Asakawa, J. Vac. Sci. Technol. B5: p. 894 (1987). 21. Yuhki Imai and Kuniki Ohwada, J. Vac. Sci. Technol. B5: p. 869 (1987). 22. Seitaro Matsuo and Mikiho Kiuchi, Jpn. J. Appl. Phys. 22: p. L210 (1983). 23. Seitaro Matsuo, Extended Abstracts of the 16th (1984 Int.) Cont. on Solid State Devices and Materials, Kobe, pp. 459-462 (1984). 24. Katsuyuki Machida and Hideo Oikawa, J. Vac. Sci. Technol. B4: p. 818 (1986).
38
Handbook of Ion Beam Processing Technology
25. Eiichi Murakami, Shin-ichiro Kimura, Terunori Warabisako, Kiyoshi Miyake, and Hideo Sunami, Extended Abstracts of the 17th Cont. on Solid State Devices and Materi~ Tokyo, pp. 271-274 (1985). 26. Yoshio Manabe, Tsuneo Mitsuyu, and Osamu Yamazaki, Extended Abstracts of ISPC-8, Tokyo, p. 2428 (1987). 27. S.R. Mejia, R.D. McLeod, K.C. Kao, and H.C. Card, Rev. Sci. Instrum. 57: p. 493 (1986). 28. Hiroshi Yamada and Yasuhiro Torii, ADD!. Phys. Lett. 50: p. 386 (1987). 29. M. Kitagawa, K. Setsune, Y. Manabe, and T. Hirao, J. App!. Phys. 61: p. 2084 (1987). 30. Hiroharu Fujita, Hiroshi Handa, Masamitsu Nagano, and Hisao Matsuo, Jpn. J. App!. Phys. 26: p. 1112 (1987). 31. Kazuhiro Kobayashi, Masahiro Hayama, Satoru Kawamoto, and Hidejiro Miki, Jpn. J. ADD!. Phys. 26: p. 202 (1987). 32. T. Maeda, H. Nakae, and T. Hirai, Extended Abstracts of ISPC-8, Tokyo, p. 2434 ( 1987). 33. Akiyoshi Chayahara, Haruki Yokoyama, Takeshi Imura, Yukio Osaka, and Masami Fujisawa, Extended Abstracts of ISPC-8, Tokyo, p. 2440 (1987). 34. J. Forster and W. M. Holber, submitted to J. Vac. Sci Techno!.
4 Hall Effect Ion Sources
Raymond S. Robinson and Harold R. Kaufman
4.1 INTRODUCTION
Hall-effect ion sources have been used in thin film and surface processing applications in the U.S. for a comparatively short time (1). The high-current, low-energy capabilities of these sources make them particularly suited to process improvement during thin-film deposition or thin-film property enhancement such as increasing hardness, passivating surfaces, producing a preferred crystal orientation, activating surface chemical reactions or improving step coverage. They are also suited to a production environment because they are simple to operate, mechanically rugged, and reliable. Hall-effect ion sources were developed to offset the current-density limitations of gridded ion sources. In Hall-effect sources, the accelerating potential difference for the ions is generated with a magnetic field in conjunction with a substantial electron current. This generating process results in a circulating or Hall current in the acceleration region. The ion beam current densities possible with this acceleration process are much greater than those possible with gridded sources, particularly at low ion energies. There are several types of Hall-effect ion sources. Two basic geometries are described herein. The end-Hall type, (2) named because the beam exits the acceleration region at the end (axis) of the magnetic field, and the closed-drift type (3) in which the ion-acceleration channel is annular, rather than circular as it is in the end-Hall. There can be a number of variations within these two general categories that emphasize particular operating or performance characteristics. Hall-effect ion sources usually employ axially symmetric electrodes and magnetic pole pieces although designs without axial symmetry have also been investigated. The development of both gridded and Hall-effect ion sources originated in research for electric space propulsion. The present technology of Hall-effect ion sources is closely related to work done in the Soviet Union, nluch of which was reported in their All-Union Conferences (4-5). This work is acknowledged in the use of the names "closed drift" and "end Hall" as types of Hall-effect ion sources. These names are direct translations of frequently used nanles in Soviet literature.(2)
39
40
Handbook of Ion Beam Processing Technology
A Hall-effect ion source can be defined as a plasma device operating in approximately the glow-discharge regime. The ions are electrostatically accelerated into a beam with the accelerating electric field established by an electron current of comparable magnitude to the beam current, interacting with a magnetic field. One component of the electron motion is counter to the ion flow. Another component is normal to that direction. The current associated with this nornlal component is called the Hall current. In Hall-effect ion sources there is a complete, or closed, path for the Hall current. In addition, for the ions to be accelerated into a beam, rather than much more diffusely, the ion cyclotron radius must be much larger than the total acceleration length.
4.2 END-HALL ION SOURCE
The cross section of a typical end-Hall ion source is shown in Fig. la and a schematic electrical diagram is shown in Fig. 1b. The electron emission from the cathode is controlled with the cathode supply. The anode potential is determined by the anode current, the strength of the magnetic field, and the gas flow. An electromagnet is shown in Fig. 1a, but a permanent magnet can also be used, thereby eliminating one of the power supplies shown in Fig. 1b. A gas flowmeter is included in Fig. 1b because the voltage of the anode supply is normally controlled with the gas flow at a particular beam current. The cathode supply is customarily an ac supply when filament cathodes are used; however, hollow cathodes are also used as electron sources. End-Hall ion sources have been tested at ion-beam currents from a fraction of an ampere to well over an ampere. 4.2. 1 Operation
Various processes that occur in an end-Hall ion source are indicated in Fig. 2. The numbers in Fig. 2 are keyed to the following discussion. The neutral atoms or molecules (1) of the working gas are in the free-molecular flow regime and are introduced to the ion source through a port. Electrons (2) from the cathode approxinlately follow magnetic field lines (3) back to the discharge region enclosed by the anode and strike atoms or molecules (4) therein. Some of these collisions produce ions (5). The mixture of electrons and ions in the discharge region forms a plasma. Because the density of the neutral atoms or molecules decreases rapidly downstream of the anode (toward the cathode) most of the ionizing collisions with neutrals occur in the region surrounded by the anode. The ions that are formed are accelerated both toward the axis of the ion source and in the axial direction toward the cathode. The component of velocity acquired toward the axis can result in the ion crossing the axis, perhaps to be reflected by the positive potentials on the opposite side of the axis. Depending on where an ion is formed, it may cross the axis nlore than once before leaving the ion source. The ions (6) that leave the source thus follow a variety of trajectories and form a divergent beam. The positive space charge and current due to the ions in this beam are neutralized by some of the electrons (7) that leave the cathode. Most of the electrons from the cathode flow back toward the anode and both generate ions and establish the potential difference that accelerates these ions. Because of the radial and axial components of the electric field, most of the ions that are generated in this source leave in the downstream direction. The current to the anode is almost entirely conlposed of electrons--both the original
Hall Effect Ion Sources
Cathode
(a)
Anode
( b)
'0
>
>
E
ac
+
+ E
~
H
(1)H
+J>t (1)r-i
~o.
m>t .cr-i uo.
:;E:[/)
.r-! ~ (:\[/)
tJlo. m~
tJI-
H
[/)0.
u
>
..
U
H
(1) 'O>t Qr-i .co. +J~ m ~ U[/)
ct
Figure 1: End-Hall ion source (Ref. 1).
41
42
Handbook of Ion Beam Processing Technology
electrons from the cathode and the secondary electrons that result from the ionization of neutrals. The excess electron emission from the cathode is approximately sufficient to current-neutralize the ion beam with the electron enlission fronl the cathode when the electron emission equals the anode current. The cathode often is operated in excess of this current. In normal operation, the gas flow is adjusted at constant anode current until the anode voltage is also at the desired value.
; - Cathode
Figure 2: Processes in an end-Hall ion source (Ref. 7). 4.2.2 Ion Acceleration
The approximate shape of the magnetic field in the discharge region is indicated in Fig. 2. There are two nlajor mechanisms by which a potential difference that accelerates the ions is generated in this magnetic field. The first of these mechanisms is the reduced plasma conductivity across magnetic-field lines. The strong-field approximation is appropriate for the field strengths used in the end- Hall source. The ratio of conductivities parallel and transverse to the magnetic field is then
(1)
where w is the electron-cyclotron (angular) frequency and v is the electron collision frequency. The electron collision frequency is determined by the plasnla fluctuations asso-
Hall Effect Ion Sources
43
ciated with anonlalous diffusion when conduction is across a strong magnetic field. Using Bohm diffusion to estimate this frequency,
°11/OJ..=256
(2)
Because Bohm diffusion is typically accurate only within a factor of several, the ratio of Eq. (2) should be treated as correct within an order of magnitude. It should still be expected that (3)
From this difference in conductivity parallel and normal to the magnetic field, it would be expected that the magnetic field lines in Fig. 2 would approximately determine the equipotential contours in the plasma. Further, the field lines closer to the anode would be more positive in potential. Radial surveys of plasma potential made using a Langmuir probe show some potential increase when moving from the axis to a magnetic field line close to the anode, but the increase is only a fraction of the total anode-cathode potential difference. The bulk of this difference appears in the axial direction. That is, parallel to the magnetic field, where, from Eq. (3), the potential difference would be expected to be small. The time-averaged force of a nonuniform magnetic field on an electron moving in a circular orbit can be calculated. For a variation of field strength in only the direction of the magnetic field, this force is parallel to the magnetic field and toward decreased field strength (opposite the field gradient). Assuming an isotropic distribution of electron velocities, two-thirds of the electron energy is associated with motion normal to the magnetic field, and therefore interacts with this field. For a uniform plasma density, the potential difference in the plasma can then be obtained by integrating the electric field required to balance the magnetic-field force on the electron, which gives (4)
where k is the Boltzmann constant, T e is the electron temperature in K, e is the electronic charge, and Band Bo are the magnetic-field strengths in two locations. With B > Bo , the plasma potential at B is greater than that at Bo . Axial surveys of plasma potential in the end-Hall source are found to be in approximate agreement with Eq. (4) (1). There is an additional effect of plasma density on potential, and a more complete description of the variation of plasma potential with nlagnetic-field strength must also include this effect. 4.2.3 Beam Energy Distribution
Ions are generated in the discharge plasma and accelerated into the ion beam. The potential of the discharge plasma varies substantially with position. As a result, the ions have a corresponding range of kinetic energies after being accelerated into the beam, depending upon where the ions were formed. The distribution of ion energy on the axis of
44
Handbook of Ion Beam Processing Technology
the ion beam can be measured with a retarding potential probe. Some curves from a retarding potential probe analysis of an ion beam from a gridless source are shown in Fig. 3(a). (Some of the following discussion has also been reproduced from Ref. 7 with permission.) For convenience, the maximum probe currents in Fig. 3(a) are normalized to unity. Curve A is typical of operation with a low anode voltage, with most of the ions that are generated of the singly charged type. Operation at higher anode voltages often results in a current-voltage distribution more like that of Curve B, with a substantial probe current often measured at voltages well above the anode voltage. This curve shape is believed due to the generation of multiply charged ions in the discharge region. For example, if a doubly charged ion becomes singly charged as the result of charge exchange with the background gas in the vacuum chamber, then the voltage required to stop the ion can be up to twice the voltage available for the acceleration.
p.
1.0
0
~
H
A B
(Va=71V) (Va=156V)
~ r:: OJ
)..4 )..4
:;j
u
( a)
OJ
.Q
0
0.5
f..f
04
'U OJ
N .~
~
ttl
ef..f 0
Z
0
0.04
~ 0.03 ..........
04
H
( b)
0.02
al ~
a
rl
Ul
0.01
0 0
50
100
150
200
Probe potential, Vp, V. Figure 3: Retarding potential probe analysis of the ion beam from an end-Hall source.
(Ref. 7).
Hall Effect Ion Sources
45
Because it is difficult to separate singly from multiply charged ions when there is a large energy spread in all the ions generated, the analysis of the ion beam from a Halleffect ion source is usually carried out as if the ions were all singly charged. With singly charged ions, the energy distributions of the ions are obtained directly from the magnitudes of the slopes of the retarding potential curves. These magnitudes are plotted in Fig. 3(b). The energy of most general interest is the mean energy. The major significance of the rnlS deviation fronl the mean is that the ion beam fronl a Hall-effect ion source departs considerably from a monoenergetic beam. Mean energies and rms deviations are plotted in Fig. 4 against anode potential for a wide range of operating conditions. The mean energy typically corresponds to about 60% of the anode potential, while the rnlS deviation from the mean energy corresponds to about 30%.
150
0 0
Ar Kr
~
°2
100 (a)
eV 50
o
100
(b)
eV
50
o
a
50 100 150 Anode potential, Val V
200
Figure 4: Ion-beam energy as a function of anode potential: (a) nlean energy and (b) rms deviation from the mean (Ref. 1).
Ideally, the mean ion energy should be the same throughout an ion beam. In practice, there is usually some decrease in mean ion energy away from the beam axis. Some of this decrease is due to scattering interactions of energetic ions with the background gas in the
46
Handbook of Ion Beam Processing Technology
vacuum chamber. That is, ions that have been scattered further from the axis have, on the average, lost more energy due to these scattering collisions. Some of this decrease is also due to the production of low-energy ions downstream (in the beam acceleration direction) of the main ion-production and acceleration region. Such low-energy ions, produced both by charge exchange and energetic electron collisions, will tend to be deflected away from the beam axis. This is because the dense central portion of the ion beam is more positive than the rest of the ion beam, and low-energy ions will tend to be deflected more by the snlall potential differences involved than the more energetic ions. It then follows that the fraction of low-energy ions present at large angles from the beam axis will be increased at high background pressures. The mean energy on the ion-beam axis is herein used as the mean energy of an ion beam from an end-Hall source. Ion beam profiles are calculated on the basis of this energy. That is, the off-axis current density is reduced from the measured value in proportion to the reduction in mean energy. For example, if the mean ion energy on the beam axis is 100 eV, and the mean ion energy at 45 0 from the axis is 90 eV, then the current density at 45 0 is treated as if it were only 90% of the measured value. There are some processes that depend on the arrival rate of ions more than on the energy of these ions. Such processes could clearly benefit from a surplus of ions at lower energies. As indicated above, however, most applications are concerned with total energy. The correction to the on-axis mean energy, as described above, is thus a straightforward approach to characterizing the ion beam of an end-Hall source for most applications, and conservative for those applications in which total ion arrival rate is more important then beam energy. As a final energy consideration, doubly charged ions should be discussed. Many operating conditions for a Hall-effect ion source result in a significant production of doubly charged ions. This production can usually be reduced by operating at lower anode voltages. In a gridded ion source, significant beam currents at low energies are difficult to obtain. Even a small fraction of doubly charged ions can cause the same damage as if the entire beam were all singly charged ions, but accelerated at twice the energy. In a gridded ion source, then, a small fraction of doubly charged ions can severely restrict the utility of that source for low-damage processes. Much larger ion-beanl currents are possible at low ion energies with a Hall-effect ion source. It is therefore usually possible to reduce the effect of doubly charged ions by simply reducing the anode voltage, and hence the mean ion energy, without sacrificing beam current. As a result, the Hall-effect source may have to be operated at a lower mean ion energy than a gridded source in the same application. However, with the large current capacity of the Hall-effect source at low ion energies, this lower energy will nornlally not result in a restriction of the beam current. 4.2.4 Beam Current Density Profile
It is preferable to use a screened probe to measure beam current that is pivoted about the center of the exit plane, rather than one that is translated across the beanl axis at a
Hall Effect Ion Sources
47
right angle. This is because the angular spread of a beam fronl the end-Hall source can be quite large, and simple translation of a probe can result in a large oblique angle of the probe to local ion velocity over much of a survey. If a screened probe, such as is used for a retarding potential energy analysis, is oriented at an oblique angle to the local ion velocity, there can be a variable shadowing (vignetting) effect of the probe screen on the measured ion flux. Due to the negative grid potential employed to repel electrons, and the range of ion energies obtained, the magnitude of this shadowing effect is difficult to calculate because it is not simply a geometrical effect. Assuming a pivoted probe at a fixed distance from a gridless ion source, the ion current density can usually be approximated by (1) (5)
where jo is the ion current density on axis, R is the angle from the axis, and n is a beamshape parameter that is evaluated from the experimental profile. Experimentally, values of n usually fall in the range of 1-5 for the type of Hall-effect source described. The angular spread of these profiles is much greater than that found for most gridded sources. Because ions follow nearly straight-line trajectories, the angular variation would be similar at larger distances, but the intensity would vary inversely as the square of the distance. It is often important to know the beam profile at a flat target surface. Assuming the same target distance at the beam axis, a further factor of cos 3 E) would be required, where E) is the angle from the beam axis. The profile along with this modification is indicated in Fig. 5 with a dashed line. Half profiles are shown in Fig. 5 because only minor asymmetries are found in beam profiles.
o
4
Measured current
Dt.. Corrected for energy Corrected to a flat target surface 3
'\
\
\
\ rnA
cm2
Figure 5: Beam current density and correction for off-axis variation in ion energy. The dashed line shows the additional modification for a flat target (Ref. 1).
\ 2
\
\ \ \
\ 1
\ \
\
, '\
o
,
O~--~2~O~---4--'O~---6-l0
Angle from beam axis, deg
48
Handbook of Ion Beam Processing Technology
The variation of both on-axis ion current density and the integrated ion-beam current with mean ion energy are indicated in Fig. 6 for a discharge current of 5 A. The performance shown is for oxygen, but generally similar performance is found for other gases. 4
( a)
rnA
Cffi2 2
a 2
(b)
A
1
a o
50
100
150
Mean ion energy, eV Figure 6: Ion-beam parameters at a discharge current of 5 A. The ion current density was measured at a target distance of 15 cm. (a) Ion current density on axis and (b) ionbeam current (Ref. 1). 4.3 CLOSED DRIFT ION SOURCE
The cross section of a closed-drift ion source is shown in Fig. 7 along with a schematic electrical diagram. Except for certain geometrical considerations the operation of this source can be described in the same way as the end-Hall ion source. Significant differences should be enlphasized however. The magnetic field direction is generally radial and transverse to the ion flow direction, instead of generally along the acceleration direction as it is in the end-Hall. Most closed-drift ion sources have had dielectric channel walls and a channel length at least equal to the channel width, as indicated in Fig. 7 (8-14). In addition, the electron cyclotron orbit is small compared to the acceleration length.
Hall Effect Ion Sources
49
3
---. ---.---------, 5
+
6
Figure 7: Closed-drift extended-acceleration (CDEA), thruster (single stage). (1) Propellant feed; (2) anode distributor; (3) magnetic circuit, pole pieces; (4) magnetic winding; (5) cathode neutralizer; (6) discharge power supply; (7) insulator (Ref. 3). 4.3. 1 Operation
A closed-drift ion source with a variable magnetic field and a variable accelerationchannel length was used to obtain operating parameters. The source is shown in Fig. 8. The closed-drift ion source employs axially synlnletric electrodes and pole pieces, with the magnetic field primarily in the radial direction and the electric field primarily in the axial direction. The downstream magnetic pole pieces are located at y = 0 position. Typical discharge characteristics as a function of gas flow are shown in Fig. 9. Background pressures were in the low 10- 5 Torr range. Characteristic of this device is the minimum flow required for operation, exhibited in Fig. 8 as a rapid increase in voltage as flow is reduced. It is possible to operate over a wide range of currents at a fixed voltage and gas flow, particularly with a longer acceleration channel. The longer channel has a
50
Handbook of Ion Beam Processing Technology
greater conductivity than the short channel because of the increased ionization efficiency of the larger ion production volun1e.
Axis for Langmuir probing Center pole piece
~ I
I
Acceleration channel
I
I
I I 1/F====:::::;-"1
I I
Gas inlet Cutaway side view
Anode electrical connection
1._---- Outer pole piece
Magnet coil
Figure 8: Cross-section of a variable length, adjustable field closed-drift ion source (Ref.
15). 4.3.2 Ion Acceleration
An analysis of the closed-drift acceleration process shows that two distinctly different acceleration processes could take place (16). In one case the electrons in the acceleration region were assumed to be at a negligible temperature (zero). The potential variation throughout the acceleration region then was found to be smooth and continuous. As a result of the continuous and extended acceleration process, this type has been called a closed-drift extended-acceleration (CDEA) source (17-24). If, however, the electrons were assumed to heat up as they flowed from the ion exit to the ion formation region, then a near-discontinuous potential jump occurred at the positive end of the acceleration channel. The remainder of the acceleration was assumed to take place in an axial length of the order of the local electron-cyclotron orbit.
Hall Effect Ion Sources 130
0
~
110
..•
90
0 6
o
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0 6
•
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6
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g 0
fJ
,•
t"
10
o
20
40
60
80
Argon flow, seem.
Figure 9: Discharge voltage as a function of gas flow for variable acceleration channel closed drift ion source. Magnet current, 0.2 A; open symbols, short-channel configuration; solid symbols, long-channel configuration (Ref. 15).
With this acceleration model to serve as a guide, experimental verification of this anode-layer acceleration process was found in studies of the Penning discharge (25-30). Subsequent studies of the Penning discharge give additional verification and information (31-39). These and other studies made clear sonle of the distinctions between the two types of closed-drift sources. 4.3.3 Beam Energy Distribution
Analysis of the beam energy distribution on the ion source axis yields distributions of the shape shown in Fig. 10 that are primarily Gaussian with a substantial spread in energy superimposed on an alnlost uniform distribution. Each of the curves in Fig. 10 was obtained using an 80 V, 5 A discharge, 6.25 A electron emission and identical magnetic
52
Handbook of Ion Beam Processing Technology
fields. The electron temperature for the insulating channel in the ion production region is almost double the electron temperature for the conducting channel, reaching more than 12 eV. The plasma potential can exceed the discharge voltage by twice the electron temperature, resulting in sonle ion energies substantially higher than might appear justified from the discharge voltage.
0.04 - - Insulating channel Mean energy = 64.5 eVe Sigma = 17.5 eVe
Conducting channel Mean energy 44.5 eVe Sigma = 16 eVe
=
0.03
/-
/ I
0.02
\
I
\
I
\
I
I
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/
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10
20
30
40
50
60
70
80
90
100 110 120
Beam ion energy, eVe
Figure 10: Beam energy analysis for conducting and insulating channels (Ref. 15).
Table 1 shows some representative parameters for the ion beam energy distribution over a range of currents.
Table 1: Ion Beam Energy Distribution
Discharge Current
E mean
a
(A)
(eV)
(eV)
2.0 4.0 6.0 8.0 10.0
61.8 64.2 78.9 94.3 101.5
13.2 14.9 16.9 17.0 14.8
Emean/Vdis
4.68 4.31 4.65 5.54 6.87
0.70 0.71 0.77 0.79 0.81
Hall Effect Ion Sources
53
4.3.4 Beam Current Density Profile
Figure 11 presents beanl current density profiles for 2, 4, 6, 8 and 10 A discharge currents at a 1.0 A magnet current for the short channel configuration with 60 sccm Argon flow. No asymmetries in the beam profiles with respect to the centerline of the source were observed. The Faraday probe systenl was positioned 18.6 cm fronl the downstream pole piece of the ion source. As seen in Fig. 11, the shape of the profile changes with increasing discharge current. The increase in current density is greater near the center of the beam than it is at larger radii when compared to current densities at lower discharge currents. The ion generation process in the channel appears to become more effective at higher power levels relative to generation further downstream. Integrated beam current data and operating conditions for the beam current density profiles of Fig. 11 are presented in Table 2.
Table 2: Argon Integrated Beam Current Data.
Discharge Current (A)
Cathode Emission Current (A)
Discharge Voltage (V)
Integrated Beam Current (mA)
2.0 4.0 6.0 8.0 10.0
2.5 5.0 7.5 10.0 12.5
88 90 103 120 125
213 366 620 688 804
In Table 2, E mean is the mean energy of the Gaussian, a is the standard deviation of the Gaussian, and E m/ a and Em/Vdis are the ratios of the mean energy of the Gaussian to the standard deviation and the discharge voltage respectively. The behavior of the beam energy distribution with increasing power exhibits a fairly regular progression in all parameters. The mean energy of the beanl represents an increasing fraction of the discharge voltage for higher discharge voltages probably partly as the result of a decreased charge exchange cross section at higher energies. A number of random processes probably contribute to the generation of a Gaussian profile for the beam energy distribution. The plasma potential at which ions are created will determine their maximum total energy, but collisional processes and charge exchange can alter the makeup of the energy distribution after the ions begin to accelerate along the channel. 4.4 CONCLUDING REMARKS
Hall-effect ion sources generate ion beams with fairly well-controlled direction, a controllable energy range and current density. The major advantage of these sources is the ability to generate large ion currents at low energy. These sources should find in-
54
Handbook of Ion Beam Processing Technology
creased application in thin film and surface processing, especially in production, where a simple, reliable source of large ion currents can have a significant impact.
3.0 (>
Anode current, A
(> (>
2.5
0
•
N
..........
e
2.0
(>
~
..
•r-!
.. (>
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(1)
.. (>
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(1)
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(>
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ectS
8
() 10
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. ... ..
e
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~(>
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.(>
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(>
~
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,
-.(> -.(> ""(>
0
0
0
0.5
""() 0
-(> 0 0
'0
a:,~
o o
5
10
15
20
25
30
35
40
Radial distance from centerline, cm. Figure 11: Beam current density profiles for representative discharge currents (Ref. 15).
4.5 REFERENCES
1. Kaufman, H. R., R. S. Robinson, and R. I. Seddon, End-Hall Ion Sources. J. Vac. Sci. Technol. A5: pp. 2081-2084 (1987).
2. Morosov, A. I., Physical Principles of Cosmic Electro-Jet Engines. Vol. 1, pp. 13-15 (Atomizdat, Moscow, 1978). 3. Kauftnan, H. R., Technology of Closed Drift Thrusters. AIAA Journal 23: pp. 78-87 (1985). 4. Proceedings of the II All-Union Conference on Plasma Accelerators. Academy of Science, U.S.S.R., Minsk (1973).
Hall Effect Ion Sources
55
5. Proceedings of the III All-Union Conference on Plasma Accelerators, Academy of Science, U.S.S.R., Minsk (1976). 6. Proceedings of the IV All-Union Conference of Plasma Accelerators and Ion Injectors, Academy of Science, U.S.S.R., Moscow (1978).
7. Kaufman, H. R. and R. S. Robinson, Operation of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria p. 57 (1987). 8. Seikel, G. R. and Reshotko, E., Hall Current Ion Accelerator. Bull. Am. Phys. Soc. Sere II 7: p. 414 (1962). 9. Lary, E. C., Meyerand, R. C. Jr., and Salz, F., Ion Acceleration in Gyro-Dominated Neutral Plasma-Theory. Bull. Am. Phys. Soc. Sere 117: p. 441 (1962). 10. Salz, F., Meyerand, R. G. Jr., and Lary, E. C., Ion Acceleration in a Gyro-Dominated Neutral Plasma-Experiment. Bull. Am. Phys. Soc. Sere II 7: p. 441 (1962). 11. Seikel, G. R., Generation of Thrust-Electromagnetic Thrusters. Proceedings of the NASA-University Conference on the Science and Technology of Space Exploration, 2: pp. 171-176 (1962). 12. Ellis, M. C. Jr., Survey of Plasma Acceleration Research. Proceedings of the NASA University Conference on the Science and Technology of Space Exploration, 2: pp. 361-381 (1962). 13. Pinsley, E. A., Brown, C. 0., and Banas, C. M., Hall-Current Accelerator Utilizing Surface Contact Ionization. J. Spacecraft and Rockets 1: pp. 525-531 (1964). 14. Brown, C. O. and Pinsley, E. A., Further Experinlental Investigations of a Cesiunl Hall-Current Accelerator. AIAA J. 3: pp. 853-859 (1965). 15. Patterson, M. J., R. S. Robinson, T. D. Schemmel, and D. R. Burgess, Experimental Investigation of a Closed-Drift Thruster. AIAA Paper No. 16. Zharinov, A. V. and Popov, Yu. S., Acceleration of Plasma by a Closed Hall Current. Sov. Phys. Tech. Phys. 12: pp. 208-211 (1967). 17. Morozov, A. I., Esipchuk, Yu. V., Tilinin, G. N., Trofimov, A. V., Sharov, Yu. A., and Shchepkin, G. Ya., Plasma Accelerator with Closed Electron Drift and Extended Acceleration Zone. Sov. Phys. Tech. Phys. 17: pp. 38-45 (1972). 18. Morozov, A. I., Esipchuk, Yu. V., Kapulkin, A. M., Nerovskii, V. A., and Smirnov, V. A., Effect of the Magnetic Field on a Closed-Drift Accelerator. Sov. Phys. Tech. Phys. 17: pp. 482-487 (1972). 19. Epischuk, Yu. V., Morozov, A. I., Tilinin, G. N., and Trofimov, A. V., Plasma Oscillations in Closed-Drift Accelerators with an Extended Acceleration Zone. Sov. Phys. Tech. Phys. 18: pp. 928-932 (1974).
56
Handbook of Ion Beam Processing Technology
20. Melikov, I. V., Experimental Investigation of Anode Processes in a Closed Electron-Drift Accelerator. SOy. Phys. Tech. Phys. 19: pp. 35-37 (1974). 21. Antipov, A. T., Grishkevich, A. D., Ignatenko, V. V., Kapulkin, A. M., Prisnyakov, V. F., and Statsenko, V. V., Double-Stage Closed Electron Drift Accelerator. Abstracts for IV All-Union Conference on Plasma Accelerators and Ion Injectors, Moscow, pp. 66-67 (1978). 22. Bardadymov, N. A., Ivashkin, A. B., Leskov, L. V., and Trofimov, A. V., Hybrid Closed Electron Drift Accelerator. Abstracts for IV All-Union Conference on Plasma Accelerators and Ion Injectors, Moscow, pp. 68-69 (1978). 23. Morozov, A. I., Physical Principles of Cosmic Electro-Jet Engines, 1: pp. 8-16, (Atomizdat, Moscow, 1978). 24. Shadov, V. P., Porotnikov, A. A., Rilov, U. P., and Kim, V. P., Plasma Propulsion Systems: Present State and Development. 30th International Astronautical Congress (Sept. 1979). 25. Knauer, W., Mechanics of the Penning Discharge at Low Pressures. J. App!. Phys. 33: pp. 2093-2099 (1962). 26. Knauer, W. and Lutz, M. A., Measurement of the Radial Field Distribution in a Penning Discharge by Means of the Stark Effect. Apo!. Phys. Lett. 2: pp. 109-111 (1963). 27. Dow, D. G., Electron-Beam Probing of a Penning Discharge. J. ADO!. Phys. 34: pp. 2395-2400 (1963). 28. Knauer, W., Fafarman, A., and Poeschel, R. L., Instability of Plasma Sheath Rotation and Associated Microwave Generation in a Penning Discharge. ADD!. Phys. Lett. 3: pp. 111-112 (1963). 29. Kervalishvili, N. A. and Zharinov, A. V., Characteristics of a Low-pressure Discharge in a Transverse Magnetic Field. SOY. Phys. Tech. Phys. 10: pp. 1682-1687 (1966). 30. Popov, Yu. S., Low-Pressure Cold-Cathode Penning Discharge. SOY. Phys. Tech. Phys. 12: pp. 81-86 (1967). 31. Kervalishvili, N. A., Effect of Anode Orientation on the Characteristics of a LowPressure Discharge in a Transverse Magnetic Field. SOY. Phys. Tech. Phys. 13: pp. 476-482 (1968). 32. Kervalishvili, N. A., Instabilities of a Low-Pressure Discharge in a Transverse Magnetic Field. Sov. Phys. Tech. Phys. 13: pp. 580-582 (1968). 33. Smirnitskaya, G. V. and Nguen, K. T., The Center Potential and Electron Density in a Penning Discharge. Sov. Phys. Tech. Phys. 14: pp. 783-788 (1969).
Hall Effect Ion Sources
57
34. Reikhrudel, E. M., Smirnitskaya, G. V., and Nguen, K. T., Dependence of Current on Parameters in a Penning Discharge. Sov. Phys. Tech. Phys. 14: pp. 789-795 (1969). 35. Popov, Yu. S., Anode Sheath in a Strong Transverse Magnetic Field. Sov. Phys. Tech. Phys. 15: pp. 1311-1315 (1971). 36. Erofeev, V. S. and Sanochkin, Yu. V., Ionization Instability of a Self-Sustaining Low-Pressure Discharge in a Strong Transverse Magnetic Field. Sov. Phys. Tech. Phys. 15: pp. 1413-1417 (1971). 37. Smirnitskaya, G. V. and Nosyreva, I. A., Oscillations in a Low- Pressure Penning Discharge. Sov. Phys. Tech. Phys. 15: pp. 1832-1838 (1971). 38. Barkhudarov, E. M., Kervalishvili, N. A., and Kortkhondzhiya, V. P., Anode Sheath Instability and High-Energy Electrons in a Low-Pressure Discharge in a Transverse Magnetic Field. Sov. Phys. Tech. Phys. 17: pp. 1526-1529 (1973). 39. Mukhamedov, R. R., Similitude Criteria in the Penning Discharge. Sov. Phys. Tech. Phys. 20: pp. 1254-1256 (1975).
5 Ionized Cluster Bea." (ICB) Deposition and Epitaxy
Isao Yamada and Toshinori Takagi
5.1 INTRODUCTION
Thin film deposition techniques are generally characterized by fluxes of single atoms, molecules or ions arriving at a surface, with some probability of either sticking and eventually becoming absorbed in the growing film, or else reacting chemically with other species at the film surface to produce a nonvolatile product. There are two basic techniques for producing fluxes of condensable atoms, molecules or ions: evaporation and sputtering. Evaporation consists of heating a source to a sufficiently high temperature such that atoms evaporate from the surface of the (usually) molten source and condense on the relatively cold sample surface. The atom flux is this case is usually monoatomic and has a kinetic energy on the order of the source tenlperature. Sputtering is the result of energetic ion impact to a cathode or target surface. Atoms are ejected from the target, usually with several eV of kinetic energy. Depending on the background gas pressure and system geometry, these sputtered atoms nlay lose some or all of their kinetic energy before landing on the sample surface. A third deposition process, that of depositing films from fluxes of large aggregates or clusters of atoms is the basis of this chapter. Clusters of atoms can have unique physical and chemical properties, quite unlike the atomic fluxes and unlike the liquid or bulk states of the film. As a result of the unique properties of small clusters, numerous new applications in plasma physics, atomic and molecular physics, surface science, and thin filnl formation become available. The clusters used in this work are aggregates of only a few hundred to a few thousand atoms. In a cluster this size, a large percentage of the atoms are located at or within a few layers of the cluster surface. For example, a cluster of 500 atoms has a radius on the order of 15 Angstroms. Approximately 500/0 of the atoms are on the surface layer and another 28% are on the next layer in. Therefore, the overall structure of the cluster is dominated by the surface atoms, and we should consequently
58
Ionized Cluster Beam (ICB) Deposition and Epitaxy
59
expect that the physical and chemical properties of the cluster are n1uch different from those of bulk and liquid (1). The ICB deposition technique has several features which can be attributed both to the unique properties of small clusters and to aspects of the cluster acceleration process (2,3). One of the n10st significant properties of the ICB deposition technique is an apparent enhancement of the surface adatom migration or diffusion in the depositing film. The ICB deposition process also allows the gradual increase in cluster (or atom) energy without the space charge problems usually associated with low energy ion beams. This is due, again, to the cluster technique where a single charge (on the cluster) is used to accelerate many hundreds of atoms. Thus, the effective kinetic energy for each depositing atom can be increased easily from thermal energies up into a range similar to sputtering. This great sensitivity will be quite important to modifying or tailoring the properties of thin films. The importance of low energy ion beams for film formation can be easily understood when we recognize that the binding energies of the atoms in a solid are in the range of a few eV per atom. For atoms evaporated from thermal sources, the kinetic energies correspond roughly to the temperature of the source and are approximately 0.01-0.1 eV, or much less than binding energies of the film atoms. A strong effect can be expected, however, as the result of bombarding by accelerated ion or neutral atom beams, even at energies of only a few eV which correspond to binding energies. The clusters in the ICB technique initially have thermal energies on the order of 0.1 eV per atom. For a cluster of a few hundred to a thousand atoms, this corresponds to less than 100 eV per cluster. If the cluster is ionized and accelerated by a few hundred to n1any thousands of volts, the average energy for each atom can be carefully increased from the initial thermal energies up to the binding energy of the film atoms and beyond. By working with these high acceleration potentials, space charge problems are strongly reduced, and high fluxes can be achieved. 5.2 EXPERIMENT
The differences in films deposited by the ICB technique, compared to evaporation, will depend critically on the properties of the clusters. The clusters are formed during an adiabatic expansion in a nozzle and then travel relatively unhindered (except for possible ionization and acceleration) to the sample surface. Thus the nozzle region and the dynan1ics of cluster formation will be quite important to the final film. Extensive research has been undertaken on the topic of the dynamics of the vapor expansion and cluster formation, as well as the subsequent properties of the clusters themselves. This chapter examines the basic physics of nucleation during expansion, as well as the kinetic and structural aspects of the clusters after formation. 5.2.1 Principles of ICB Operation
In the Ionized Cluster Beam technique, small clusters of a few hundred atoms each are formed in a source, using techniques somewhat similar to evaporation. As the clusters leave the source, they drift through the vacuum chamber under conditions of pressure low enough that there are no collisions with gas atoms or other clusters. Upon reaching a surface, the clusters condense to form a film. Often the clusters are intentionally ionized in the drift region and accelerated by electric fields to the sample. This acceleration in-
60
Handbook of Ion Beam Processing Technology
creases the net kinetic energy of the cluster, and can have an effect on the properties of the depositing film. The design of an ICB system is broken up into four regions. These are the source region, where the clusters are formed; the ionization and acceleration region; a drift region; and finally the substrate. A typical schematic of the ICB system is shown in Fig. 1. Not shown in the figure is the vacuum chamber and pumping system, as well as any gas supplies. These systenls operate typically in the 10- 5 to 10- 7 Torr region (10- 3 to 10- 5 Pascals). Of these four regions in the system, the drift region is perhaps the least critical. It is in this region that a shutter of some kind is used to control the deposition time or thickness. _ _THERMO OOUFtE SUBSTRATE HOLDER
SHUTTER IONIZED AND NEUTRAL CLUST ERS ELECTRON EM IlTER FOR IONIZATION ELECTRON EMITTER FOR HEATING CRUCIBLE COOLING 1WATER INLET
l
CRUCIBLE
Figure 1: A typical Ionized Cluster Beam (ICB) system. The vacuum system and chamber, as well as the power supplies are omitted for clarity.
In the source region, the clusters are formed by an adiabatic expansion and condensation process (4,5). The nozzle diameter D of the crucible has to be larger than the mean free path A between vapor atoms in the crucible. This causes a viscous flow in the nozzle region. In the case where the nozzle diameter is smaller than the mean free path of the vapor atoms (molecular flow), there are few, if any, collisions between atoms in the nozzle region and agglomeration or clustering of the vapor atonlS will not occur. The ratio of the vapor pressure Po in the crucible to the vapor pressure P outside of the crucible (in the chamber) must be larger than 102 - 105 • Therefore, if film deposition in the 10-7 to 10- 5 Torr range is desired, it is necessary to operate the inner pressure in the crucible in the range of 10-2 to 1 Torr. To cause a sufficient number of collisions in the nozzle to form clusters, it is necessary to make the nozzle thickness-to-diameter ratio (LID) in the range of 0.5 to 2.0. This serves to keep the ratio of the chamber pressure P to the crucible pressure Po high, allowing for low pressure depositions. A simple nozzle shape is cylindrical, with a diameter D and length of 1-2 mm, which is sufficient to form a beam of clusters with a high drift velocity. A multiple nozzle source, where each nozzle satisfies the dimensions mentioned above, can be used for uniform film deposition over a large substrate area (Fig. 2) (6). In this particular system, the auxiliary heating electrode
Ionized Cluster Beam (ICB) Deposition and Epitaxy
61
used for heating the bottom of the crucible is feedback-controlled by the deposition rate monitor signal in order to keep a constant deposition rate. The multiple nozzles on this curtain beanl source place additional requirements on the vacuunl system to sustain a low chamber pressure. The range of the source temperature is determined in order to produce the vapor pressure Po of the order of 10-2 to a few Torr. The crucible can be heated by either resistive heating, electron bombardment heating or by hybrid methods according to the application purposes.
DEPOSITION RATE METER
LU
t-
<{
ex:
~
DIRECT HEATING POWER SUPPLV
~ \
rl J
\
/
I
J..
.1..
ACCELERAT ION VOLTAGE CONl'HOL FOR ELECTRON
lO1
}
~
AUXI L1ARY HEATIN G ELECT RODE EM IllER
Figure 2: A single source, multiple aperture ICB system. This is also known as a "curtain beam". In this system the heating rate of the crucible is feedback-controlled by the deposition rate monitor to provide a steady deposition rate.
The clusters are ionized by an electron impact in an ionization electrode region located just above the expansion nozzle. The ratio of number of ionized clusters to the total number of clusters can be adjusted by changing the electron emission current Ie in the ionization region. In general, the clusters only become singly ionized by this technique due to the low currents. The degree of ionization is defined as the percent of the total cluster flux which is ionized. In a typical system, (7) the degree of ionization obtained for a single nozzle is 5-7% at Ie = 100mA, 7-15% at Ie = 150mA and and 30-35% at Ie = 300mA. The ionized clusters are accelerated by the electric field caused by the potential Va on the accelerating electrode located just outside the ionization region. Typically this electrode would operate at a potential of a few hundred to several thousand volts negative of the system ground. The accelerated ionized clusters bombard the substrate together with neutral clusters which are not ionized in the ionization electrode system. The ionized clusters have a kinetic energy corresponding to the acceleration voltage, whereas the neutral clusters have a kinetic energy corresponding to the ejection velocity. The trajectory of the ionized clusters is controlled to obtain a wide and uniform bombardment on the substrate by optimizing the aspect ratio (LID) of the nozzle or by adjusting the acceleration voltage applied to the acceleration electrode. The uniform radiation by cluster ions of the depositing film is critical in cases where the film properties are dependent on the bombardment by the ions.
62
Handbook of Ion Beam Processing Technology
The shape and spacing of the acceleration electrodes can be optimized by modeling the ion trajectories through the acceleration region to give the best beam uniformity. One acceleration design is shown in Fig. 3, consisting of a grid, an ionization electrode (part of the electron source for ionization of the clusters), an intermediate electrode and the acceleration electrode. It should be noted that the optimum voltage on each electrode is interrelated. The best or optimum focussing conditions result in a broad, uniform beam at the sample. An example of the interrelation between electrodes is shown in Fig. 4 for the case of a grounded grid electrode (V g = 0) and an ionization electrode voltage, V e , equal to negative SOOV. The optimized beam signal is then strongly related to the intermediate electrode and the acceleration voltage.
Figure 3:. Trajectories for Ag clusters in a multipleaperture acceleration system. In this case, the optimum voltage is applied to the internlediate electrode.
o
5
10
15
20
25
Z-AXIS (em)
A shutter is often located in the drift region between the acceleration electrodes and the sample. This shutter is closed during the start-up process. The degree of ionization as well as the beam current and uniformity can also be monitored by placing a Faraday cup in this same region. The sample region generally consists of a heated substrate holder. Typically these substrate mounts are radiation-heated, and contain a thermocouple for the measurement of sample temperature. It is also usually desirable to measure the current to the sample as a means of monitoring the deposition process. There can also be rate monitors of various designs in the sample region. Similar to reactive evaporation or sputter deposition, the ICB technique can be used for the deposition of compound thin films. This allows the deposition of oxide, nitride, hydride or carbide films, for example, while still using relatively simple elemental sources. During a reactive ICB deposition (RICB), an appropriate reactive gas is introduced into
Ionized Cluster Beam (ICB) Deposition and Epitaxy
63
the chamber at a pressure of 10-5 to 10-4 Torr (8). In this low pressure region, no plasma is produced in the acceleration or drift regions, as is the case with Activated Reactive Evaporation (ARE) (9). or ion plating techniques. At higher pressures where a plasnla is formed, the clusters are rapidly destroyed by collisions with energetic particles. This eliminates the advantages of using the cluster technology. The chemical reactions at these pressures all take place at the depositing film surface on the sample. The deposition rate of clusters must be carefully controlled along with the partial pressure of the reactive gas to control the film stoichiometry.
Vg Ve
0-0
=OV =-0.5
kV
o 0--
OL....-----'----L-....-----JL..-..---.&...-....I.....---.&-----L....----------------.--
o
-5 -1 0 ACCELERATION VOLTAGE (kV)
Figure 4: The interrelation between intermediate grid voltage and the cluster acceleration voltage for the conditions of highest beam uniformity for the electrode system shown in Figure 3.
Due to the low pressure of operation of the leB technique, it is possible to operate more than one source simultaneously in a chamber (Fig. 5) (10). This feature is critical for the deposition of alloys, and in particular those alloys whose components have radically different melting points and vapor pressures. A good example of a relevant compound is GaAs. In a multiple-source system, the sources operate completely independently, and can be used simultaneously to form compounds, or in sequence to form nlultilayer or superlattice films.
64
Handbook of Ion Beam Processing Technology
RESISTIVE HEATING WALL Figure 5: A typical arrangement for a ICB deposition with two sources. Each of the sources is completely independent during operation. 5.3 ASPECTS OF FILM DEPOSITION WITH ICB
It has generally been observed that the properties of a deposited thin film are critically dependant on the deposition process, the nature of the vacuum system and the substrate conditions during the deposition. Films deposited by conventional evaporation often differ radically from films of the identical material deposited by physical sputtering, even though in both cases the depositing atoms are arriving at the surface singly or in very small groups. It is therefore not too surprising that films deposited by the ICB technique can have very different qualities than either evaporated or sputtered films. In addition, it should be obvious that by changing the kinetic energy of the cluster from effectively a hundred eV or so to many thousands of eV, the physical properties of the deposited films can also be drastically altered. Due to the parallels between the ICB technique and conventional evaporation techniques, it is quite relevant to examine the underlying phenomena which affect film growth and properties in each case. In many cases the basic phenomena are quite similar, but the ICB technique will vary from conventional evaporation in the magnitude of the effect. The basic phenomena of interest at the substrate surface during the deposition of a thin film are the surface binding energy of the adsorbed atom, the nucleation density or critical size for surface nucleation and the level of surface mobility of the adsorbed atom. The sticking coefficient is a term describing the effective probability of the arriving, con-
Ionized Cluster Beam (ICB) Deposition and Epitaxy
65
densing atom to be adsorbed on the substrate surface. During an actual deposition, these quantities can be considerably influenced by such conditions as: •
The arrival rate of the deposition vapor
•
The substrate temperature
•
The ambient gas pressure
•
The presence of impurities at the surface or in the gas
•
The crystal structure of the surface
•
Structural defects on the surface
•
The presence of electric charge on the vapor and/or the substrate
•
Bombardment by energetic particles
The last term, energetic particle bombardment, usually takes the form of ion bombardment in many practical systems. The effects of ion bombardment during conventional evaporative film growth have been documented at some length (see Chapters 10, 11 and 13). However, the basic, fundamental understanding of the exact mechanisms of ion bombardment-induced modification of film properties is still lacking. In the case of ICB, a number of features are quite different from conventional evaporation, and these features must be included in discussions of the basic film deposition and modification mechanisms. Clearly there is a significant difference compared to evaporation in the arrival of atoms in the form of large clusters to the film surface. In addition, the additional kinetic energy of the clusters, when ionized and accelerated, must be considered, as must the possible effects of the electronic charge those clusters carry. Some of the effects that can be attributed to the ICB technique include: •
Creation of activated centers for nuclear formation
•
Sputtering or in-situ cleaning
•
Substrate surface heating (higher effective surface temperature)
•
Very shallow ion implantation
•
Enhancement of adatom migration
In film formation from the vapor phase, the following processes must be considered: atom arrival, migration, reevaporation, collision or combination of atoms, nucleation, growth, coalescence and continuous film fornlation (11). Even in the cluster beanl deposition, these fundamental processes of film formation will be preserved. Therefore, we can suggest the following concept of the film formation process. When the clusters bombard the substrate surface, both ionized and neutral clusters are broken up into atonlS which are then scattered over the surface with high surface diffusion energy (compared to the single atom condensing from the vapor phase). A scattered atom is physically attracted to the substrate surface but it may diffuse across the surface by junlping fronl point to point (adatom migration) because of high kinetic energy parallel to the surface caused by conversion from the incident kinetic energy. The adatom interacts with other adatoms and/or substrate atoms to form a stable nucleus and finally to cease its move-
66
Handbook of Ion Beam Processing Technology
ment and beconle a chenlically absorbed atom. It should be noted that at least in some cases the clusters are not broken up totally into individual atoms. The use of ionized clusters in the ICB technique has certain advantages over both conventional evaporation and sputter deposition as well as low energy ion beam deposition. One fundamental difference is the low effective charge-to-mass ratio. Generally only one atom in a cluster of many hundred atoms is ionized. Therefore, space charge effectbased problems that can occur with low energy ion beam deposition techniques are significantly reduced. In addition, the charging problems that can occur with highly ionized ion beams can be reduced or eliminated. The other basic phenomena of critical value to the ICB technique is the reduced internal binding of the cluster atoms. Due to this lower level of interatomic binding, upon collision of the cluster with the sample the cluster atoms are more easily dislodged from the cluster for the purpose of surface diffusion. One result of this effect is the ability to deposit epitaxial thin films at substrate temperatures significantly below those of conventional evaporation techniques. The initial kinetic energy of the ionized cluster upon impact with the surface can result in several phenomena, including: a. b. c. d. e.
an increase in the local temperature at the point of impact, a possible implantation of cluster atoms into the bulk of the film, at high enough energies, physical sputtering or desorption from the surface, increased surface diffusion of the surface and cluster atoms, and the creation of activated sites or defects to be nucleation points for film growth.
The presence of ions in the arriving particle flux to the surface can have a great influence on: a. critical parameters in the condensation process, such as coalescence and nucleation, and b. chemical reactions of the condensing atoms with bulk or gas phase atoms. Moreover, these magnitude of these effects can be nlodified by adjusting the acceleration voltage and the content of the ionized clusters in the total flux. An optimum value of the kinetic energy of ionized and accelerated particles in film formation is estimated to be in the range of a few to a hundred eV/ atonl under the good quality film deposition conditions. However, some amount of defects and displacements of atoms are often effective at the initial stage of film formation. Therefore energies above the damage threshold can often be of great value to film formation. In addition, the optimum energy may vary according to the required characteristics characteristics of the film, such as mechanical properties, optical properties, or morphology and the combination of deposit and substrate materials. As an example, low energy implantation was used to fabricate a thin film electroluminescent cell. By combining the implantation of the activator element and the deposition process for the substrate material, both the dose distribution of the activator elements and the selection of the elements can be controlled externally by adjusting the ion beam current and the cluster acceleration voltage. This idea was first demonstrated in 1973 by using Mn implantation during the deposition of zinc sulfide (12) The cell could be operated in either an ac or dc mode, and strong emission was observed.
Ionized Cluster Beam (ICB) Deposition and Epitaxy
67
As an alternative to the topic of ion beam deposition, in the case of ion-assisted deposition, the ions are not the principal depositing species. The concurrent ion bombardnlent during deposition can occur over an even broader range of ion energies than that of primary ion beam deposition. In addition, the effective energy deposited per depositing atom can be carefully controlled. 5.3.1
Kinetic Energy Range of ICB and Effects of the Kinetic Energy
The properties of the vaporized-metal cluster beam have important correlations with the film formation process in the ICB deposition process. Since the cluster has an amorphous structure, the constituent atoms are considered to be loosely coupled to one another compared to those in the crystalline state. An ionized cluster accelerated to an appropriate energy will break up upon striking the sample surface with the kinetic energy distributed for the most part evenly to the individual atoms. The migration of the atoms on the substrate surface has an important role in the film formation kinetics. Moreover, the ICB deposition process allows the production of equivalently low energy and high intensity ion beams without space charge problems. An estimation of the surface disorder caused by the ICB bombardment also supports the beneficial aspects of cluster formation. The surface crystallinity of Si substrates bombarded by Al clusters was measured by ion channeling after the Al film was removed by chemical etching. A Si substrate bombarded by a 500 eV Ar ion beam to a dose of 2x10 15 ions/cm2 was also examined for comparison. Fig. 6 compares the number of displaced atoms for the same cases (13). The results show that the displacement of surface atonlS induced by the Al ICB bombardment is much smaller than that caused by the Ar ion bombardment. Actually, the disorder caused by ICB bombardment is much smaller than that produced by the naturally occurring oxidized layer. It can be concluded that the kinetic energy of the individual atoms in an accelerated cluster as it impacts the surface appear no larger than the order of a few 10's of eV per atom.
e ::
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, I
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~ -Ica -~\\\1 , :(\.\Y~S\\\-S5~\_ ___ ~_ If ACCELERATION VOLTACE (kV)
Figure 6: Comparison of the number of displaced Si surface atoms for Si surfaces bombarded by Ar ions at 500 eV and Al ICB clusters.
68
Handbook of Ion Beam Processing Technology
For an actual film deposition by using the ICB technique, the kinetic energy of the accelerated clusters produces effects such as the following: (1) formation of preferential nucleation sites, (2) surface cleaning by desorption or sputtering, (3) very shallow ion implantation, (4) surface heating at equivalently high temperature, and (5) adatom migration. These effects have been confirmed experimentally. The spatial density of the nuclei at early stages of film growth has been observed to be dependant on the kinetic energy of the incoming clusters with ICB. By increasing the kinetic energy of the ionized clusters, the density of nuclei is increased monotonically at first and then becomes constant. Fig. 7 shows a plot of the nUluber of nuclei as a function of the acceleration voltage on the cluster. The increase in the density of nuclei at higher acceleration voltages is attributed to the effect of ion bombardment. At a high deceleration voltage (on the sanlple) greater than 300 V, the increase of nuclei density is considered to be due to electron bombardment. The spatial density of nuclei on the surface is considered to be determined mainly by such factors as the deposition rate, the presence of charges, structural defects, ion bonlbardment, and statistical fluctuations in the supersaturation conditions. In these experiments the deposition rate and the ionization ratio were kept constant. Therefore it is reasonably assumed that ion bombardment and the resulting creation of surface defects produced the increased number of nuclei (14) .
-
.
~
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:i
~20
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NUMBER OF NUCLE I BY NEUTRAL CLUSTERS ONLY
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~
o........10--4...0-0-10--0.----_-40-0-.......-_-8...0-0----110-.._-12....0 0-..... DECELERATION ACCELERATION APPLI D VOLTA E (V
o
400 800 1200 IONIZED CLUSTER ENERGY (eV)
Figure 7: A plot of the number of nuclei (per unit area) as a function of acceleration voltage for the clusters. The initial energy of the clusters is approximately 100 eVe
Surface cleaning or sputtering effects can also be clearly seen by comparing the Si epitaxial growth in an ultra high vacuum (DRV) and a high vacuum chambers. In the epitaxial growth of Si on an atomically clean and well-ordered silicon surface in the DRV chamber, only a 200 V acceleration voltage was sufficient to deposit epitaxial films at a substrate temperature (T s ) of 500°C. By increasing the acceleration voltage, an improvement of the crystalline quality could be obtained. For the identical deposition case in the high vacuum chamber, the results were quite different. The chamber was evacuated by an oil diffusion pump to a base pressure of 10- 7 - 10- 6 Torr. In this system, an acceleration voltage of at least 6 kV was necessary before the deposited films attained a degree of epitaxy. An amorphous or polycrystalline structure is formed in a range of 0 -
Ionized Cluster Beam (ICB) Deposition and Epitaxy
69
4 kV. In this deposition, no special cleaning process except for chemical cleaning was used prior to the deposition. In this case, higher acceleration voltage is required in order to sputter the native oxide and to remove adsorbed residual gas atoms on the substrate surface during the deposition. From the Rutherford backscattering spectra using 185 keY H+, it was found that the film prepared with clusters accelerated to 6 keY has no oxygen at the interface between the film and the substrate (15). During a relatively high energy ICB deposition, there is a small possibility that atoms from the cluster can be implanted to a shallow depth into the substrate. A measurement of this effect has been made for the case of energetic Cu clusters onto a polyimide surface. This system is of critical importance in the area of thin film electronics and packaging applications. The Cu film in this case was chemically etched away from the polyimide substrate following the deposition. The residual, apparently implanted Cu atoms were measured by a neutron activation analysis technique. The amount of Cu remaining in the polyimide film was found to increase with increasing cluster acceleration voltage. To measure the enhancement of the surface mobility for a condensing atom, the very early stages of film deposition were examined. In this experiment, Au was deposited onto a silicon oxide (SiO) film which had been deposited on an NaCI substrate. The substrate surface was partially covered with a cleaved NaCl plate to study shadowing at the cleaved edge. The average spacing between the SiO and the NaCl cover was about 80ltm. The gold was deposited both by the ionized-cluster beam technique and by conventional vacuum deposition. Fig. 8 shows electron micrographs of the deposited Au film near the edge of the penumbra. In the case of ionized-cluster beam deposition, the deposited gold particles were observed to have migrated under the cleaved NaCl cover. I
'Hlrn
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Figure 8: Electron micrographs of the deposited film clusters under an overhanging edge as a function of cluster acceleration voltage.
70
Handbook of Ion Beam Processing Technology
Even when the acceleration voltage was zero, the migration distance of the deposited particles was greater than in conventional evaporation technique. The increased migration distance could be the result of the breaking up of deposited clusters into atoms upon impact with the film surface. These and related results strongly suggest that the acceleration of the clusters during ICB influences the dynamic processes in the film formation. These dynamic processes include the breaking of clusters into atoms upon bombarding the substrate surface, sputtering of inlpurities from the substrate surface, formation of activation centers for nuclear formation, adatom migration, and shallow implantation. In ICB deposition these processes can be controlled by changing the acceleration of ionized clusters and the content of ionized clusters in the total flux, and consequently the physical properties of the deposited films can be controlled. 5.3.2 Effects of the Ionic Charge
Bombardment by ions at a very low energy during film deposition can enhance film formation activity and chenlical reaction activity, even though the ion content may be only a few percent of the total flux. The effect of the presence of charged particles can be seen in the change in the critical condensation parameters of the depositing materials and subsequently in the growth nlechanism of the nuclei. Ion enhanced chemical reactivities have useful application to the formation of many films, particularly those of compound materials. The presence of ions during deposition has also been fount to influence the preferential orientation of the film. A film having the wurtzite structure such as ZnO or BeO when deposited with ICB techniques shows preferential orientation along the c-axis when a small fraction of the clusters are ionized, even without applying an acceleration voltage to the clusters. An additional example of the ionic charge effect was seen in the formation of a preferentially oriented ZnO films. The ICB deposition was also made without applying the acceleration voltage. (16,17) The crystallinity of the film improved as the current was increased. However, even in the case of the highest ionization electron emission current (Ie = 300mA), the relative flux of ions to neutrals to the surface is very low. Assuming a cluster size of 1000 atoms and a degree of ionization of the clusters of 30-35 °lb, the relative ion-to-neutral atom arrival rate ratio is 0.003 assuming that a cluster contains 1000 atoms. This result demonstrates that the effect of the ionic charge is remarkable even when only a small amount of ions are included in the total arriving flux. 5.3.3 Film Deposition by Reactive ICB Techniques
Conlpound films, such as oxides, nitrides or hydrides, can be deposited by introducing the appropriate reactive gas species into the vacuum chamber during the ICB deposition process. The partial pressure of the reactive gas is typically on the order of 10- 5 -10- 4 Torr. A fraction of the reactive gas introduced into the chamber is ionized and dissociated in the ionization region of the ICB source. These species can become active and may contribute to the reaction at the film surface. Reactive ICB (RICB) deposition mechanisms have been studied (18) by examining the deposition of amorphous, hydrogenated silicon (a-Si:H). In this case, silicon clusters were deposited in a hydrogen ambient at 10- 5 Torr. At this pressure range, there are few gas phase collisions of the hydrogen molecules with the Si clusters, and the reactions take place predominantly at the film surface. The reaction rate appeared to increase with the
Ionized Cluster Beam (ICB) Deposition and Epitaxy
71
acceleration voltage on the clusters. Fig. 9 shows the relative numbers of particles impinging on the substrate surface. Since the background gas pressure before introducing the hydrogen gas was 5x10- 7 Torr, the main particles impinging on the substrate surface are ionized and neutral silicon clusters from the ion source, and the mixed hydrogen gas and doping gases that are introduced into the chamber through the leak valves. Some fraction of the hydrogen molecules are ionized and dissociated in the ionization section of the cluster beam. Therefore, the flux of hydrogen to the sample surface consists of a range of atoms, molecules and ions. Under typical deposition conditions, the arrival rate of Si atoms (within the clusters) was on the order of 1015 - 1016 atoms cm- 2 sec- 1 , as calculated from the nleasured silicon ion-current to the substrate. The ratio of the hydrogen atoms to the hydrogen molecules was estimated from the change of Hand H 2 peaks in a mass spectrum when the electrical input power into the source was varied. From these measurements, the bonlbardment rate of H 2 molecules to the sample was approximately 1016 molecules cm- 2 sec- 1 and the bombardment rate of dissociated hydrogen atoms was estimated to be 1015 atoms cm2 sec- 1 . The rate of impinging hydrogen ions is three orders of magnitude smaller than that of molecular hydrogen. It is not clear yet which state of hydrogen is dominantly involved in the hydrogenation process, but it seems reasonable to consider that hydrogen atoms could have a considerable influence in providing uniform hydrogenation. For doped film formation, the hydrogen gas was mixed with phosphine or diborane on the order of 5000 ppm sequentially in the same chamber. No problems arose as a result of the residual reactive gas used in previous processes. Doped films of either p or n type could be reproducibly deposited at practical deposition rates. Subsequent structural analysis showed that the films mainly consisted of monohydrides. The density of monohydrides can be increased by accelerating the Si clusters to a higher acceleration voltage. For both this case and the parallel case of oxide and nitride RICB deposition, operation at a gas pressure of less than 10- 5 - 10- 4 Torr was sufficient to cause sufficient surface chemical reactions to form the compound films without forming a plasma within the chamber.
Estimated (rom Deposition Rate
Estimated (rom Ion Current
l.---J
l--J
Reevaporation
10 16 10 17 (em -2 sec -1) NUMBER OF IMPINGING PARTICLES
10 13
10 14
10 15
Figure 9: The relative fluxes of each of the arriving particle species for the Reactive ICB deposition of a-Si:H doped films.
72
Handbook of Ion Beam Processing Technology
A unique aspect of the RICB deposition process can be demonstrated by the case of Ti ICB deposition in oxygen to form Ti0 2 • In this case, the crystal phase of the films could be controlled by the content of ionized clusters and the acceleration voltage (19). The oxygen was introduced into the chamber in a range of 10- 5 - 10- 4 Torr and Ti was used as the source material. Fig. 10 shows the change of crystalline structure observed by the X-ray diffractometer. The phase transition from anatase structure to rutile structure can be induced by increasing the cluster acceleration voltage. The change of the structure was also observed to be dependant on the cluster ionization current. These unique results have not been seen with other types of evaporative or sputter-based deposition processes.
Thickness
=2700-3900 A
-4
Po2=2 x l0 Torr Ts=350·C Va = 3 kV
Ti02 Anatase(112)
/
26
30
34 28 (deg)
38
42
Figure 10: X-ray diffractometer traces if titanium oxide films deposited at different ionization currents. 5.3.4 Film Deposition by Simultaneous Use of ICB and Microwave Ion Sources
The simultaneous use of an ion source along with a deposition process was proposed in 1973 (20). Along these lines, the simultaneous use of a microwave ion source and an ICB source has been developed. This technique is attractive because the reactive gas ion energy and the current can be controlled independently from the ICB source operation. Therefore, the reactivities of the gases can potentially be enhanced by this method. Fig. 11 shows a schematic diagram of the simultaneous system the microwave ion source and the ICB source. The details of the microwave ion source are not important to this discussion and have been described elsewhere (21). The microwave source requires permanent magnet around the discharge chamber. The source operation can be set to the Electron Cyclotron Resonance (ECR) condition which results in a very high density plasma. The gas ions are extracted by the extraction electrode applied at Vex = 3 - 15
Ionized Cluster Beam (ICB) Deposition and Epitaxy
73
kV and the extracted ions are then subsequently decelerated down to 500 eV by the retarding filed produced between the source and the substrate holder. This system was used for the deposition of AIN films. High purity Al metal and N 2 gas were used as source materials. Sapphire (0001) and p-type Si (111) were used as substrates. The substrate temperature (T.) was 100°C. The films deposited with neutral N 2 (rather than accelerated ions) were opaque and not characteristic of reacted AIN. Films deposited with concurrent Nt were clear and had high optical transmittance. Measurements by Rutherford Backscattering Spectroscopy (RBS). suggested a composition ratio in these cases of AIN. The obtained film was amorphous and chemically stable up to 1000°C. As an example of oxide film formation using this same technique, AI2 0 3 films have been deposited. For the case of neutral AI-clusters and O2 gas, the transmittance of the film is low particularly at small wavelengths.
IONIZED CLUSTERS AHD NEUtRAL CLUSTERS ACCELERATING ELECTRODE GRID --.;:-.;::0,,1
I ICB SYSTEM
Figure 11: Schematic of the ICB deposition system with simultaneous ion bombardment from a microwave ion source.
On the other hand, the films deposited with neutral Al clusters and O 2 ions had significantly higher transmittances. Also, in the case of the film deposited with both O 2 ions and ionized AI-clusters the film was transparent and its transmittance approached that of the sapphire substrate. It was found from RBS measurements that the composition ratio of oxygen to Al in these last films was 0.67, and that stoichiometric Al2 0 3 films were formed. The film prepared at an incident energy of 500 eV for O 2 ions and an acceleration voltage of 0.5 kV for Al clusters was found to be thermally stable even after annealing at 1000°C. The refractive index (n) is found to increase with increasing ion energy. In addition, the same increase in refractive index for ionized, accelerated clusters compared to neutral clusters was found in this case as was found in the case of AlN (above). The etching rate of these films in 5% HF solutions is found to decrease as a function of increased cluster acceleration voltage. In particular, in the case of using both O 2 ions and ionized AI-clusters, the film prepared at an incident energy of 500 eV was
74
Handbook of Ion Beam Processing Technology
not etched at all in the 5 % HF solution. This indicates that the higher incident energy such as 500 eV may increase the packing density of the film. 5.4 SUMMARY
The deposition of thin films by means of beams of large clusters of atoms rather than individual atoms has been shown to have numerous advantages over other deposition techniques. The clusters are generally formed by condensation during the expansion of a vapor through an aperture into high vacuum. Ionization of the clusters in flight and subsequent acceleration of the clusters to the film surface has also been found to be a sensitive technique for the modification of the properties of the deposited film. These techniques are equally applicable to reactive deposition of compound materials, in which clusters of one species are deposited in the presence of background gas atoms and ions of a reactive species. The critical features of the ICB technique are the control of the cluster kinetic energy through ionization and acceleration, and the subtle characteristics of the clusters themselves. The clusters are characterized by lower levels of inter-atomic bonding than the solid phase. This reduced bonding apparently allows increased surface mobility of the atoms upon arrival at the filnl surface, conlpared to conventionally evaporated films. One result of these effects is a greatly lowered temperature for the deposition of epitaxial films, compared to evaporative of MBE techniques. The control of the cluster kinetic energy, through partial ionization of the clusters and subsequent acceleration by an electric field, results in a broad degree of control in the effective kinetic energy of each of the atoms that arrives at the film surface. In addition, due to the high mass-to-charge ratio of the clusters, such aspects as space charge limited current flow are avoided in nlost cases and charging effects are reduced significantly. The broad range of energy control is not possible in other techniques such as evaporation or sputtering. Deposition with ICB and RICB techniques has been shown to be applicable in a broad number of experimental conditions with many different types of materials. Thus, complex alloys and internletallics can be routinely deposited, as well as conlpounds of new or metastable compositions. Another characteristic of ICB deposition techniques which has no real comparison to other deposition techniques is the ionic nature of the deposition of many compounds. It is possible, by ionization of the clusters even in the absence of cluster acceleration to influence film growth of several ionic compounds. Thus, the presence of even a small number of ions at the film surface can have a drastic effect on film properties. This effect has not been observed with other sputtering based techniques. The ICB technique has been used successfully to deposit organic films of several compositions. This is quite impossible by means of sputtering, as the molecules are generally broken apart by the incident bombarding ion. Evaporation of organic materials has only been marginally successful, as there is little control on the energy of deposition or the degree of decomposition of the polymer.
Ionized Cluster Beam (ICB) Deposition and Epitaxy
75
There are still many unanswered questions regarding the fundamental phenomena underlying the ICB technique. For example, effects that were apparently indicative of physical sputtering have been observed. Oxide contamination on substrate surfaces was effectively removed at high cluster acceleration energy. However, the velocity of the cluster even at these high energies is still below the apparent threshold for physical sputtering observed with single ions. A second area for further study is the increase in surface adatom mobility of the accelerated clusters upon impacting a surface. The average kinetic energy is often less than even the apparently lower bonding present in the cluster. Yet, increased surface diffusion is observed in comparison to evaporation under the same circumstances. This is compounded by recent molecular dynamics calculations, which show a high degree of epitaxy but little lateral motion of the cluster atoms (22). These and other phenomena suggest that there are many careful experiments left to do with ICB techniques before it can be conlpletely understood. The Ionized Cluster Beam techniques have been shown to be valuable additions to the realm of thin film deposition techniques. The processes are well characterized and reliable equipment is available from a number of sources. The films deposited by these techniques are often superior to those deposited by either evaporation or sputtering, and the range of control of the process exceeds other techniques by a great margin. It is hoped that the technique will find greater acceptance and recognition in the future as its features become even more advanced and more and more of the thin film community becomes familiar with the technology.
5.5 REFERENCES
1. J. Borel and J. Buttet (ed.), Small Particles and Inorganic Clusters. Surf. Sci. 106: (1981). 2. P.P. Kulik, G.E. Norman and L.S. Polak, Khimiya Vysokikh Energii 10: p. 203 (1976). 3. T. Takagi, Thin Solid Films 92: p. 1 (1982). 4. T. Takagi, I. Yamada, M. Kumnori and S. Kobiyama. Proc. 2nd Int. Conf. Ion Sources, Vienna (Osterreichiche Studiengeselshaft fur Atomenrgie, Vienna, 1972») p. 790. 5. T. Takagi, I. Yamada and A. Sasaki, J. Vac. Sci. Techno!. 12: p. 1128 (1975). 6. Technical Data, Sumitomo Bakelite Co., Totsuka, Yokohama, Japan. 7. T. Takagi, I. Yamada and A. Sasaki, Inst. Phys. Conf. Ser. 38: p. 142 (1978). 8. T. Takagi, I. Yamada, K. Matsubara and H. Takaoka, J. Cryst. Growth 45: p. 326 (1978). 9. R.F. Bunshah in Deposition Technologies for Films and Coatings ed. by R.F. Bunshah (Noyes, N.J. 1982) 5. 10. T. Takagi, K. Matsubara, N. Kondo, K. Fujii and H. Tokaoka, Jpn. J. App!. Phys. 19: Supple. 19-1, p.507 (1980).
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Handbook of Ion Beam Processing Technology
11. T. Takagi, J. Vac. Sci. Technol. A2: p. 382 (1984). 12. T. Takagi, I. Yamada and A. Sasaki, IEEE Trans. ED-20: p. 1110 (1973). 13. I. Yamada and T. Takagi, Nucl. Instrum. Methods Phys. Res. B21: p. 120 (1987). 14. I. Yamada, H. Takaoka, H. Inokawa, H. Usui, S.c. Cheng and T. Takagi, Thin Solid Films 92: p. 137 (1982). 15. I. Yanlada, F.W. Saris, T. Takagi, K. Matsubara, H. Takaoka and S. Ishiyama, Jpn. J. Appl. Phys. 19: p. 181 (1980). 16. K. Matsubara, I. Yanlada, N. Nagao, K. Tominaga and T. Takagi, Surf. Sci. 86: p. 290 (1979). 17. K. Matsubara, Y. Fukumoto and T. Takagi, Thin Solid Films 92: p. 65 (1982). 18. I. Yamada, I. Nagai, H. Horie and T. Takagi, J. ApDl. Phys. 54: p. 1583 (1983). 19. K. Fukushima, I. Yamada and T. Takagi, J. ADDl. Phys. 58: p. 4146 (1985). 20. K. Fujime, T. Ueda, H. Takaoka, J. Ishikawa and T. Takagi, Proc. Int. Workshop on Ionized Cluster Beam Technique, Tokyo and Kyoto, Japan, p. 195 (1986). 21. J. Ishikawa, Y. Takeiri and T. Takagi, Rev. Sci. Instrum. 55: p. 449 (1984). 22. Karl-Heinz Muller, J. ADDl. Phys. 61: p. 2516 (1987).
Part II Sputtering Phenomena
77
6 Quantitative Sputtering
Peer C. Zalm
6.1 INTRODUCTION
Sputtering is described as the removal of atoms from a solid surface due to energetic particle bombardment. This phenomena was first reported by Grove (1) in 1852, but it was not until the early 1900's that the effect was identified as due to positive ions (2,3). The first descriptions of sputtering on the atomic scale were in terms of evaporation from a hot spot. Later theories were based on a binary collision sequence (3), inducing momentum reversal and ejection of target or cathode atoms. The field of sputtering has matured rapidly in the past three decades (4) due to both improved experimental methods as well as a well developed theory of collision cascades, which could be treated analytically or by computer. Sputtering phenonlena are important from both a fundamental as well as a practical point of view. The study of sputtering can provide basic information about the interactions of ions with matter. Sputtering has also found broad usage in surface analytical techniques, where it can be used as a tool for depth profiling. (5) Perhaps the highest percentage of active users of sputtering are in the thin film and semiconductor fabrication areas. Here, sputtering is used routinely for the deposition of films as well as the etching of patterns and features important to the production of integrated circuit devices as well as device packaging. The rapid increase in the theoretical understanding of sputtering, along with the broad usage in laboratories and manufacturing sites world-wide has resulted in an increased need for accurate experimental data on almost every facet of the phenomena. This chapter will describe and discuss a number of the most promising and widely used techniques in studying the phenomena of sputtering. A few selected exanlples will help to illustrate attainable results. The first topic will be to present well-established trends in absolute yields of monatomic and multicomponent targets, as well as the angle- and energy-distribution of ejected particles, as a function of ion mass, energy, angle-of-incidence, fluence and target preparation. This rather lengthy treatnlent, which is accompanied by a discussion of some theoretical predictions, serves to outline the boundary conditions for any measuring
78
Quantitative Sputtering
79
technique claiming absolute reliability. Also it may serve as an aid in feasibility studies or be used as a set of technique selection criteria. The remainder will be devoted to specific methods not discussed elsewhere in this book. To alleviate later confusion, it is inlportant to give brief definitions of the ternlinology used in this chapter. In general, the experiments used to try to quantify sputtering involve an energetic ion bombarding a fixed target. Each of the parameters relating to the incident ion will have a subscript "i", and each parameter relating to the target, a subscript "t". Some parameters of interest are the incoming ion energy, (Ei ) , angle of incidence with respect to the surface normal (OJ, flux (yJ and total fluence (<1>J, as well as the various masses (AI;, M r), and atomic numbers (Zi' Zr) of the ion and target species. There are several types of measurements that may be done. A static measurement requires the removal of at least ten or more monolayers of material, and is generally taken long after the sputtering event. A dynamic measurement generally occurs in-situ, during sputtering and thus deals with significantly smaller levels of erosion. The" yield, " Y, describes how many atoms are ejected during a sputtering event. The total yield is defined as the average number of atoms ejected per incident ion. In cases where the target is composed of more than one species, the a yield describes the average number atoms of each particular species ejected for each incident ion. An absolute yield measurement is quantitative: a relative yield measurement is less quantitative, but may be very accurate in comparison to a standard. If the yield is measured in ternlS of a specific energy or angular interval, the yield is described as a differential yield. Finally, preferential sputtering describes the case where the composition of the sputtered particles differs from that of the outermost layer or layers of a multicomponent target.
6.2 TOTAL SPUTTER YIELD CONSIDERATIONS 6.2.1 Polycrystalline and Amorphous Elemental Targets
The largest body of experimental data on sputtering is for polycrystalline and amorphous elemental targets. Most measurements concern total yield determinations. Andersen and Bay have compiled the available data and extracted and discussed general trends derived from them in a superb review (6). In general, the total sputtering yield (Y) varies rather smoothly with incident ion energy (Ei ), first increasing to a broad maximum and gradually dropping to zero again for very high energies (MeV). This behavior is found for almost all projectile/target combinations. An example for the case of Si is shown in Fig. 1. The 100-1000 eV energy range, which is most important for thin film processing techniques, will be discussed later. The variation of Y with the angle of incidence (Oi ) of the ion beam can be considerable and depends on ion and target species. The variation is most prominent for light ions (see Fig. 2), but is generally observed for many materials of interest to thin film and semiconductor areas.
80
Handbook of Ion Beam Processing Technology
4 c:
o
~
E o
3
~
2 Figure 1: The observed energy dependence of the sputtering yield of Si for normal incidence ion bombardment of Ne, Ar, Kr and Xe.(7,8) The solid lines are predictions from linear cascade theory (eqs 1-5) with V o = 7.8 eV.
7 • 1 keV H+-Ni )( 50 ke V Art-Au + 1 keV Art-Ag 6 0 1 keV Ar~
- - - - cos- 1 ~ -._-_. cos- 2 ~
5 ...
-
>-
2-
Figure 2: Angle of incidence dependence of the sputter yield relative to the rate observed at normal incidence. The data are compiled from refs. 9-12. The dashed line represents an inverse cosine relationship and the dashed-dotted line an inverse cosine squared dependence.
Quantitative Sputtering
81
The sputter yield has been observed to be strongly dependent on the target species for a given ion species, angle and energy. atomic number Zt as shown in Fig. 3. Qualitative and, if possible, quantitative understanding of these common trends and the more regular exceptions to them will be our concern in the next few paragraphs. 6.2.2. Predictions From Linear Cascade Theory
A linear cascade theory has been developed by Sigmund (14) and others to describe the sputtering event. Many observed regularities in the sputtering behavior of amorphous or polycrystalline elemental targets (6) bombarded with atomic ions can satisfactorily be accounted for by this theory. In this nl0del the incident ion or neutral shares its kinetic energy with target atoms initially at rest in a series of binary collisions, a process in which fast recoils are created. These, in turn, set other target atoms in motion and a continuously increasing number of progressively slower atoms participate in what is an ultimately isotropic cascade. About 1 - 5 x 10 13 sec after impact, the recoil energies at the edges of the cascade have become less than the threshold energy to displace an atom, which is of the order of some 10 eV. The cascade is damped (cooled) by energy dissipation through, e.g., phonon-assisted processes setting in at typically 10 11 to 10 9 sec.
8
Ni
•
eFeGe
Ti
o
20
.Mo Zr"Nb
Ta"W 60
80
Figure 3: The total sputter yield for 1 keV Ar bombardment as a function of target species. The solid points are experimental values (6), and the open points are calculated from Eqs 1-5, using U o from ref. 13.
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Handbook of Ion Beam Processing Technology
Using this approach, the angle and energy dependent sputter yield can be described by (14)
(1)
where U o is the surface binding energy (in eV), usually taken as the sublimation energy, Sn (Ei/E it ) is the reduced nuclear stopping cross section, (e = Ei/Eit = reduced energy), and E it and Kit are scaling constants dependent on the initial target and projectile species. These constants are given by:
(1/32.5) (1
M·
+_ 1 ) Z. Z (Z~/3 + Z2/3)1/2 M 1 tit
{keY}
(2)
t
and an approximate expression (8) 1 3
(3)
which is valid for values of Zt/Zi greater than approximately 1I 16 and less than approximately 5. The reduced nuclear stopping cross section has been estimated as (15)
In(1 + e)
0.5 [e
+ (e/383)3/8
(4)
The angle-of-incidence function, fee), has been found as (14)
n
~
5
±2 3
(5)
for angles that are not too close to grazing incidence. There are several mild complications (16) in the strict usage of these relations which limits the absolute accuracy to about a factor of 2. However, the relative accuracy of these relations will be quite high. 6.2.3. Exceptions To Predictions From Linear Cascade Theory
There are several experimental regimes where this linear collision cascade theory is less accurate or appropriate. Unfortunately, these regimes are also often overlap with types of experiments used in thin film deposition and processing areas. These exceptions can be described in terms of those induced by the ion, and those due to the type of target.
Quantitative Sputtering
83
6.2.4 Ion Effects: The Direct Knock-on Regime
At energies in the 50 to 1000 eV range, there can be a considerable contribution to the yield by means of primary recoils. Some examples of the specific ejection kinematics are shown in Fig. 4 (17). The minimum E i required to initiate target atom ejection is known as the threshold energy and will strongly depend on the particular collision sequence involved and and the angle of incidence 8i. Also, in the near threshold regime ( E i < ~5Eth)' atonl ejection into a preferential angle must be anticipated (17) (see Fig. 8A, below). Based on Eqs. (1) - (4), a fairly accurate estimate (± 25%) can be obtained for the sputtering yield at perpendicular incidence in the near threshold regime for iontarget combinations that do not differ dramatically in mass (1/5 < Zt/Zi < 5) as (17a)
(6)
Here, as before, Va must be inserted in {eV} and E i in {keV} to obtain Y in atoms/ion. This implies that at low energies sputtering yields are largely independent on incident species, a fact confirmed by Fig. 1 and by Anderson and Bay (6). Furthermore, a practical result of this is that for 100eV < E i < 1000eV, the yield is approximately linear with the ion energy. Thus, in many sputter-deposition or etching techniques, the amount of sputtered material scales roughly with the product of current and energy (Le. the power) rather than just the current.
vacuum
~
l.-.r
~
solid
1000 r - - - - - r - r - - - - - - r - - -___
M'
Mt
Ar+~Cu
777797
CD
roo 0
(a)
(b)
®
::>
-....
.c
ur
10
o
®
2 1 0°
30°
60°
90°
-\1i Figure 4: (a) Sonle possible emission mechanisms at low ion energy; in (1) and (2), a primary recoil is produced in the first collision which is ejected directly, or after further collisions ejects a secondary; in (3) the projectile itself undergoes multiple collisions. (b) Predicted threshold energies for the three nlechanisnls shown. (Adapted from Ref. 17).
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Handbook of Ion Beam Processing Technology
6.2.5 Ion Effects: Due To High Fluence
It has been shown in several, well controlled sputtering experiments that the observed yield may increase considerably with ion fluence (6,7,9,18). (Fig 5) The steady state yield is reached only after the target has been eroded to a depth of the order of the projected range, R p , of the incident ion. An obvious explanation is that the (sub-)surface is modified due to ion implantation. Thus the solid is "altered" and after removal of a layer of thickness R a "new" stable target situation, with (slightly) different sputtering conditions, is attained. Another explanation is that the yield increase may be due to a mechanism related to trapped gas release (19). In a recent experiment, the kinetic energy distribution of reenlitted, previously implanted, argon atoms was measured during steady-state Ar ion bombardment of Si (20). A contribution was found which could only be attributed to the explosive expansion of microscopic (~ 10 A) gas bubbles, formed and excavated in the course of ion bombardment and sputtering. Such an event is likely to provide an additional mechanism for target atom ejection. There are additional effects on the sputter yield due to the presence or formation of surface topography. These will be discussed in more detail in a later section.
3
c
O ""'(j)
2 1 keV
E
.....o
eu 0.5 keV
'U
.~
~
1
O"-_.l...-_~_.....J.-_----'--_----L._---IL.-_..L..-...J
o
2 4 6 eroded depth [nm 1~
Figure 5: Dynamical sputter yield measurements as a function of eroded target thickness for Ar bombardment at 50° angle of incidence for amorphous Si (18). Black dots indicate the estimated projected ranges of the ions. 6.2.6 Ion Effects: Reactive and Molecular Ions
A further complication arises when the incoming ion can react chemically with the target species. The formation of a volatile compound product contributes to target atom
Quantitative Sputtering
85
removal and hence the yield (3). An example of this process has been observed (21) during bombardment of Si with F or CI ions, compared to similar mass noble gas ions Ne or Ar ,respectively. Conversely, when an involatile conlpound forms - as for 0 on Si leading to SiO - the sputter yield is generally reduced. As a coarse rule -of-thumb, the magnitude of the target atom yield enhancement or reduction in the steady state, ~ Yreact' for reactive ion bonlbardment can be estimated (22) as: i) ~ Yreact ~ alb, if the projectile P and the target T atoms can form a volatile conlpound TaP b. ii) ~ Yreact ~
volatile. The
~
- bl (a + b) Y phys' if the product is of the form TaPb and is insign implies that only a rough approximation can be given.
This change in the sputter yield can then be added or subtracted to the original, physical sputter yield, Y phys • For polyatomic or molecular ions the observed total yields, at energies above a few hundred eV, are usually higher than those for comparable mass noble gas ion sputtering, even in the absence of a possible chemical reaction with the target atoms. The reason for this lies in the fact that the molecule fragments upon impact. Consequently, the observed Y reflects the sum of the contributions of the individual constituents. As a fair approximation one may assume that the penetrating atoms have the same velocity as the incident molecular ion, and transfer their energy independently to the surface. Then Y can be estimated from the experimental atomic ion yield data, when available, or from eqs. (1-5). In the latter case one finds for perpendicular incidence (22)
(7)
where M stands for molecule. The scaling constants are given by
(K,E)Mt
= ~j
(K,E)iot J
(8)
where the summation runs over all constituents atoms (i). For example, for a cluster of n identical atoms eqs. (7,8) reduce to Yn(Ei ) = nY 1 (E i /n) where Y 1 is the yield of a monatomic ion. There are, of course, additional considerations in this complex topic which can limit the applicability of these relations (16). 6.2.7 Target Effects: Temperature
The influence of target temperature on sputter yield was initially though to be significant (23,24). More recent work by Hofer et al (25) did not observe significant yield enhancements with temperature. It is generally thought that neither the temperature nor the phase of the target, apart from small changes in the sublimation energy, influence sputtering behavior significantly. A possible exception to this rule is the case of crystalline semiconductors, which may become amorphous under ion bombardment (16).
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Handbook of Ion Beam Processing Technology
6.2.8 Target Effects: Single Crystal Targets
As a function of ion energy and crystalline orientation, the sputter yield of single crystal targets can differ significantly from an amorphous or polycrystalline target, as shown in Fig. 6 for Ar on Cu. Although no comprehensive theory on the sputtering of single crystals has been formulated to date a satisfactory explanation of some important aspects follows from the concepts of transparency and channeling. The ordering in the lattice effectively shields subsurface layers into the shadow of surface atoms when viewed in what are known as low index directions. For ion incidence along these more transparent directions, the collision probability with subsurface atoms is reduced and the ion is said to "channel". As ion-atom collisions at shallow depth govern the sputtering process, channeled particles contribute to an effective reduction in the yield. The channeled fraction of the ion beam depends on the "width" and "acceptance angle" of a channel, which is related to the density of the atomic rows in a plane perpendicular, the atom and ion species and the ion energy. Channeling is generally less important at ion energies less than a few keY. However, related effects can be significant during the deposition of films during ion bombardment.
27 keVAr+-Cu( 100)
§10
30
'= en
E 8
....-.,.
o
.
+J
«S
-. 6
, ,.
Q) .~
4
Q')
c
'L: Q) +J +J
20
"- ·~.OIY
'U
2
10
:J
g.
0 L....-....L....-----L.-----I.-..L.-----J...-----L-.L...----l....-----1._L.----l.---+---l...........L...-.....L-...l...-..L..-L---l.----L.--...J 0 10- 1
1 2
5 10 102 Ei[keV]-
(a)
103 0 0 20 0 40 0 60 0 80 0
-\7i
~
(b)
Figure 6: (a) Observed energy dependence of the total sputtering yield for Ar bombardment at 90° incidence on different crystal faces of monocrystalline and polycrystalline Cu. The curves are smooth fits from the data of Refs. 6 and 26-28. (b) The angle of incidence dependance for the yield for 27 keY Ar bombardment of Cu (100) rotated about the (011) axis as compared to polycrystalline Cu (29).
Quantitative Sputtering
87
6.2.9 Target Effects: Multicomponent Materials
The major difference between elemental and multicomponent sputtering is a consequence of the non-stoichiometric removal of surface atoms leading to a change in the surface composition. This field has been recently reviewed by Betz and Wehner (30). This effect depends on ion energy, angle of incidence, fluence, target temperature and composition. In a plasma-sputtering experiment, there may also be contributions from redeposition of scattered, sputtered material. After prolonged bonlbardment eventually equilibrium (i.e. the partial sputtering yields reflect bulk stoichiometry) must be reached for cases without gas scattering. However, this may require removal of a considerable amount of material of the order of 1000A or more. Several mechanisms may result in an enrichment or depletion of the surface in one of the conlponents. According to the collision cascade nlodel the nlonlentunl and energy distribution will depend on the masses of the atoms participating in the cascade, resulting in different ejection probabilities for the individual constituents. This will in general cause preferential sputtering of the lighter component and therefore surface enrichment of the heavier one, as may also be expected on the basis of recoil implantation (31). The effect is, however, fairly weak and probably only dominant for low E i and/or M i . More important will be differences in surface binding energies for the individual components, which depend on composition, resulting in preferential sputtering of the more weakly bound atoms (32). 6.3 DIFFERENTIAL SPUTTER YIELD CONSIDERATIONS
Of interest in both a fundamental and practical sense, the spatial and energy distributions of the sputtered atoms have been studied. Following this chapter, three additional chapters deal with these topics in great detail. 6.3.1 Angular Distributions Of Sputtered Particles
Both the analytical linear collision cascade theory and the thermal spike nlodel of sputtering predict a cosine distribution for the sputtered atom flux (32). The first experimental angular measurements confirmed this and were erroneously taken as evidence for the evaporation-from-a-hot-spot model (3,23). In general, the deviations often take the form of an "under-cosine" distribution, which is reduced normal to the surface, or an "over cosine" distribution, which is more peaked in the direction of the surface normal. In the direct knock-on regime described above (Fig. 4), where specific recoil collisions determine ejection, the angular distribution is under-cosine for perpendicular incidence because emission takes place at large polar angles. For grazing incidence the emission is mainly in the opposite, specular, direction. At intermediate energies (keV to 10's of keV) and for medium to heavy ions, the angular distribution is usually cosine-like, whereas at high incident energy the distribution is (strongly) over-cosine, i.e. peaked in the direction of the surface normal. Some examples are given in Fig. 7.
88
Handbook of Ion Beam Processing Technology
)( 5keV Ar+-Ag
! -60 0 -1keVH+-Ni {)-i =-80°
"'150keV Ar+-Cu {)-i =85 0
Figure 7: Polar plot of experimentally observed angular distributions of the sputtered flux. The data for glancing incidence Ni+ (10) exhibit the behavior typical for the direct knock-on regime. The Ar+ on Ag data (34) show a pure cosine distribution. The Ar+ on eu (33) show energetic mediunl-to-heavy ion bombardment effects.
Other considerations may alter the angular distribution of sputtered atoms. As has been shown convincingly (35,36), surface contamination, either deliberate or resulting from poor vacuum conditions, affects the angular distribution. In addition, surface topography or bOlnbardment-induced texturing can also be significant. In contrast to amorphous materials, single crystal targets show a strongly anisotropic emission as was discovered by Wehner (37). He observed enhanced emission along close-packed lattice directions. Many authors (38-40) confirmed his findings under a variety of conditions (some examples are given in Fig. 8). It is one of the most characteristic features of single crystal sputtering. As an explanation for the so-called Wehner spots, which manifest themselves as a perturbation on the random cosine-like background, momentum focusing collision sequences along atomic rows was proposed (41). Later theoretical work (42,43) indicated that anisotropic enlission need not necessarily be associated with "focusons", but that it might be the consequence of a selective influence of the surface binding energy on the low-energy part of the recoil spectrum. Schematic representations of both nlechanisnls are depicted in Fig. 8. When multiconlponent materials are sputtered the angular distribution can be different for each individual component. magnitude of the effect depends sensitively on Zi , E i , (Ji , and target temperature. Some exanlples are shown in Fig. 9. The available experimental evidence is scarce and sometimes contradictory (30), so no general systematic trend can yet be deduced.
Quantitative Sputteri ng
( c)
(b)
( a)
89
50 eVAr+ -Au(100) 5keVAr+- Cu(111) :J
cd
t
-0 Q)
t
">.
-5'pref
0
~prel
oUo
006 000
~ .....
I~IO
c: Q) ..... Q)
'to'to-
~
/ L....-L......oIlI~.L....-L--.L.....L--L--......"foA--A--"----~------=--::-~
0° 20° 40° 60° 80 0° 20° 40° 60° 80°
polar ejection angle
~
Figure 8: (a) Angular distribution of sputtered particles from Ag (100) by 50 keY Ar , indicating that anisotropic emission (here in the (110) direction) persists to the nearthreshold regime (39). (b). Angular distribution of sputtered neutral particles from eu (111) rotated about the «(10) axis, showing that preferential ejection holds for polyatomic clusters (40). (c) Schematic representation for momentum focussing collision sequence along a close-packed lattice direction (after ref 41) and of a potential minimum deduced by the periodic arrangement of the surface (42). Both mechanisms lead to preferential ejection. 6.3.2 Energy Distributions Of Sputtered Particles
In its simplest form, linear collision cascade theory predicts the kinetic energy (E) distribution of atoms ejected from the target in the direction of the surface normal to be (45,32)
dY/ dE
ex:
E/ (E + D o )3-2m
(9)
where m ~ 0 for 0 < E ~ Do and approaches m ~ 0.25 for E ~ 1keV , and Do is again the (planar) surface escape barrier energy. Refinements to eq. (9) become necessary for since kinetic energies approaching the nlaximum transferable energy T m E i 4M i M t / (M i + M t )2 must always hold, thus in particular for low E i E < Tm and/or M i ions. Several theoretical papers have dealt with this situation (46,47), but unfortunately did not result in an analytical expression. For this reason, a modification of the Thompson formula (eq.9) of the form
90
Handbook of Ion Beam Processing Technology
dY/dE
[1 -/(E + Uo)/T m ]
E
ex:
(10)
(E + Uo)l+l
with I an adjustable parameter, has been adopted frequently although there is no physical justification for such an approach.
( a)
(b)
1.25 _3 ke V Ar +-CoO.41NiO.59
I 0.8
)(
Ar + -Pt O. 5 CU O. 5 )(
•
•
1
)( )(
z
0-0- _. 0 0
Z
---
Z
o300K .575K
0.4 0°
1_-_
0
•
""""80.6
+ +
0
0
•
0
-
+ + )(....!-
::J
0
1- •+ -- + - -
1.0~
--
)(
....
a.
+
+ 2.5 keV
•
•
10keV -x 320keV
)(
+ )(
Z
-
+ )(
20° 40° 60° 80°0° 20° 40° 60° 80° polar ejection angle •
0.75
Figure 9: Variation of the composition of the sputtered particle flux as a function of ejection angle for nletal alloys bombarded with Ar+ (a) the influence of target temperature (76). (b) The influence of incident ion energy (77).
According to the (thermal) spike theory (hot spot model) of sputtering, the expected energy distribution should be of the Maxwell-Boltzmann type (48) dY/dE
ex:
E exp( -E/kT)
(11 )
where T = T sp (~ 103 - 104 OK) is the "temperature" in the spike. A similar distribution, be it at the usually much lower target temperature (eq.(II), with T = T t ), is anticipated when ion induced decomposition followed by outdiffusion and desorption/evaporation occurs (like for the metal component in alkali halides). The former regime is also called prompt thermal sputtering, the latter slow thermal sputtering (for a detailed discussion see ref. 48). Only in the last two decades measurements of kinetic energy distributions of sputtered particles have been reported (see e.g. refs. 49-62). (Some examples are given in Fig. 10.) Often, the data follow the linear collision cascade prediction (eq.(9) with m=O) except in the direct knock-on regime where the empirical eq.(10) is found to work well.
Quantitative Sputtering
(a)
(b)
o ~ .....
91
o 100 1keVAr+-Ti
fj"lr
H+-Fe
=6~ 1 ~\\ ;g 0.8 A +\ \ o
• 2.5 keY +0.5 keY )(0.1 keY
xr\,."
o
O. 6 ~·L x +~ \ > ~)(. \ \ .". Q)
"'0
.~ 0.4 ~ 0.2
g~
•
0
o
XJc
) \ \••
l
'\.
!
\ex I +'t ~ xt ~ ~ ~
4
8
~.
12
16
0
5
ejection velocity [km/s
10 15 20 25 J--~
Figure 10: Velocity distributions of sputtered particles. (a) Ground state Fe atonlS ejected upon low energy H + bombardment, measured perpendicular to the surface. The maximum transferable energy is indicated by the arrow for 0.1 and 0.5 keY bombardment. The curves are fitted, modified Thompson distributions (eq 9) with 1 = 2, 2.5 and 4.5 increasing with decreasing incident ion energy (data from ref. 61). (b) Ground state (gs, a 3F 2 ), metastable (ms, a 1D ) and ionized (Ti+,a4 F 3 / 2 ) Ti atoms ejected upon 1 keV Ar+ bombardment measured perpendicular to the surface. The curves are standard Thompson distributions (Eq 8) with m = 0 and UO,gS = 4.6 eV, Uo,ms = 25 eV and Uo,Ti+ = 9 eV. (59).
Definite contributions from direct knock-on ejection mechanisms have been observed in light ion sputtering of Zr at oblique incidence and large take-off angles (63). A (pronounced) contribution from thermal spikes has occasionally been observed in energetic heavy ion sputtering of polycrystalline metal targets (49) and also in sputtering of alkali halides (52,53) . For alloy targets, eq.(8) seems to apply reasonable well, be it with different values of U o for the individual components, which in addition depend on composition (55). The same behavior was observed for GaAs (54). The general topic of the angular and energy distributions of the sputtered atoms will also be described in the following three chapters. Strong deviations fronl the nornlal cascade behavior, nlainly at the low ejection energy side of the spectrum, have been found (59,60) when targets are bombarded under simultaneous exposure sure to a reactive gas (mostly oxygen), or when the ion itself is chemically active. This is accompanied by a strong decrease (up to 90 %) in neutral ground-state atom emission and an increase in the ejection of particles in an excited or (ionized) state invoked by electronic transitions upon leaving the surface. The probability for excitation decreases rapidly with increasing excitation energy Ex. For a particular target, often the relative population of the excited states is well described by an exp (-Ex/kTeff ) behavior, which indicates that ionization is a rare event compared to excitation. Here T eff is an effective temperature which has no physical meaning, al-
92
Handbook of Ion Beam Processing Technology
though in the past such an exponential dependence has been taken (63) as evidence for a hot spot or local thermal equilibrium model of sputtering. Short-lived excited particles may deexcite through radiative decay, which can be studied spectroscopically and yields information on the surface composition (64). Deexcitation is greatly influenced by the proximity of the surface and hence with the dwell time in the near surface region ( ~ 10 A). The survival probability therefore increases with the velocity normal to the surface, V.l = V2E/M cos 8 according to
exp [
- C(E x ) v~
]
(12)
where C(Ex ) is a constant depending, approxinlately linearly, on the excitation energy. In principle, the survival probability need not depend as smoothly on the particles' kinetic energy as is suggested by eq.(II), since multiple excitation/deexcitation sequences may occur in the egress from the surface. This is not expected to be common, but a related effect, namely oscillatory behavior, with incident energy, of the backscattered ion yield (predominantly with He+ ) from selected metal surfaces has been observed (65). Here it suffices to note that their kinetic energy spectra in general differ from those of neutral ground state atoms. The present discussion only gives arguments in favor of a deficiency at the low energy end and as such it does not explain the observations completely (see e.g. Fig.l0). Let us now briefly turn to cluster emission, which is by no means a rare event, in particular for cluster ion emission (66). Although no completely analytical formulation for the prediction of the kinetic energy distribution of polyatomic clusters on the basis of the linear collision cascade model is available, several approximate solutions exist (67,68). The major differences largely derive from the mechanism adopted for the formation mechanism The n-atomic cluster is assumed to receive momentum as an entity, and subsequently diffracted trough the surface escape barrier, or near-neighbor target atoms are individually sputtered in a single cascade and recombine above the surface because they remain in each others (attractive) potential. In the former case the low energy part of the spectrum is proportional to dY/ dE oc E in the latter to dY/ dE oc En. Both predict a high-energy roll-off of the form dY/ dE oc E(1-5n)/2 well above the dissociation energy of the cluster. Hence experimental evidence will hardly discriminate between both models, which must be considered two extremes in an over simplified description based on kinetic arguments only. In more elaborate treatments it has been tried to account for the internal degrees of freedom in the cluster as well (Le. the rotational and vibrational energy distributions) (69). In passing we note that there is ample experimental evidence that electronically excited molecule emission is a relatively rare event, despite the fact that cluster formation (but apparently in the ground state) or excited/ionized atom ejection is a frequent process. Before closing this section, one comment seems appropriate. The rigorous decoupling of ejection energy and angular distributions in the present treatment is an oversimplification. Theoretical (70) and experimental (58) evidence against such a separation was mentioned in passing, although the bulk of the experimental data cited apparently did not warrant a more elaborate treatment. Very recently, however, a highly sophisticated experimental set-up has been reported (62) which enables dynamical combined energy and angular resolved measurements (Chapter 8). The first results obtained with this novel
Quantitative Sputtering
93
technique clearly show a strong interdependence of energy and angular distributions (i.e. more overcosine for higher ejection energies). This topic and technique will be the subject of a following chapter (Chapter 8). 6.4 EXPERIMENTAL CONSIDERATIONS FOR SPUTTER YIELD MEASUREMENTS
Most quantitative measurements of the total sputter yield have taken place in well characterized, DRV ion beam systems. These devices differ from the broad-beam ion sources described elsewhere in this book in that they are usually small beams (~mm2) of relatively low current, but carefully controlled mass and ion energy. Ion sources of this type are typically operated at much higher ion energy than is found in broad beam devices. It is necessary, however, to describe aspects of these beam-line experinlents, as the quality of these measurements has a direct influence on the results of measurements with broad beam sources. 6.4.1 Ion Beam
The energy spread of the incident ion beam must not influence the yield determination. As in most cases (cf. Figs. 1,6) the total yield exhibits a sublinear dependence on the incident energy, a (symmetrical) distribution around the mean with a half width of 100/0 is usually sufficient. Strongly skewed E distributions do affect the reliability of the measurement and must consequently be avoided. Of particular interest for the deposition and etching of thin films is the energy range of 0.1 to 1 keY. For these studies, particular care must be taken because of the strong dependence of both the yield and the sputtering threshold on ion energy (71). Low energy ion beams are difficult to handle in many cases, due to space-charge blow-up. Three solutions exist to this problem: i) neutralizing the beam by thermal electrons from a hot filament; ii) placing the ion source very close to the target; iii) decelerating energetic ions in front of the target. The first precludes the use of electromagnetic focussing optics and current measurements on the target. (Effectively, the beam is a plasma and is self-shielding to electric fields.) The second bears the risk of target contamination from the source - (and reverse : sputtered target atoms may end up in the source) - and may hinder the use of in situ diagnostic techniques. The third is highly preferable, be it that a very good final deceleration lens is needed (73) (for a treatise on transport and lens design for ion beams in general see ref. (72) ). A single mass ion beam in a well-defined charge-state is required for reliable and reproducible yield determination. Therefore inclusion of a mass (and energy) separation stage in the experimental beam line is necessary. Preferably, such a system should also bend the trajectory to prevent energetic multiply-charged ions or clusters, which are frequently observed when employing liquid metal ion sources, from reaching the target (74). The ion flux delivered to the target is an important factor in sputtering yield determinations, since it affects the outcome (and reliability) of an experiment in several ways. The adsorption of background gases will also affect the yield and must consequently be
94
Handbook of Ion Beam Processing Technology
avoided or, at least, minimized. This imposes restrictions on the vacuum system and/or the ion flux. Taking sticking probability of the gas on the target surface (y g) and resputtering of the adsorbate (with yield Y g) by the incident ion flux (epi) into account, Andersen and Bay stated that a reasonable demand is (6)
~
Here
r
g
10 Yg
(13)
is the arrival rate of the gas given by (14)
with P g , T g ,Mg the pressure (in Torr), temperature and molecular mass of the gas. Unfortunately, Yg and Y g are seldonl known and unless an in situ surface sensitive diagnostic tool is available to actually monitor the contamination of the target level safe upper limits must be assumed (Le. epi ~ 100 r g ). Further complications, associated with ion beam induced cascade nlixing or recoil inlplantation of the adsorbate into the topnlost atomic layers of the target, affect both the steady-state behavior and the time scale on which equilibrium is reached. Experimental methods to deal with this problem, and determination of Y g derive from the work of Morita (75). The incident ion beam may heat the target and induce undesirable artifacts (like recrystallization and evaporation). The local temperature rise, !:l T , during bombardment can be monitored externally by an (infrared) optical pyrometer or in situ by a thermocouple attached to the target. A rough estimate of !:l T , assuming a senli infinite solid, homogeneous deposition of the beam energy over the project range, R p , in the target, no radiative losses and a beam diameter d large compared to R p is Q Cd
(15)
based on a mathematical derivation analogous to the one used in ref. (76). Here Q is the input power, i.e. current times acceleration voltage on the target, and C the thermal conductivity (in Watt °K-i m- i ) and d is the beam diameter. As mostly samples are fairly thin and heat conduction to the target holder is not perfect, eq. (15) nlust be considered a rather conservative estimate - (that is : if eq. ( 15) predicts a temperature increase of the order of !:IT ~ lOOK or more, special care should be taken). Precise knowledge of the total irradiation dose, Le. the spatially integrated fluence (f cI>i) is necessary for an absolute sputtering yield determination. Simple current integration is usually insufficient because the emission of a considerable fraction of charged secondaries gives rise to large errors. The most favorable solution is to make the target part of a Faraday cup or, if there is no need to collect the sputtered material for later investigation, a Faraday cage (6). If this is not possible a retractable Faraday cup may be inserted periodically in the beam. This method is also recommended with insulating targets, which often need flooding with thermal electrons supplied by a hot filament to prevent local charge-up of the sample - (note that the design nlust be such that con-
Quantitative Sputtering
95
tamination by evaporation from the filament must be avoided). The Faraday cup aperture facing the beam can be coated with a phosphor like wurtzite to allow beam positioning and to give an indication of focusing quality. 6.4.2 Sputtering Target
The preparation and cleanliness of the sputtering target are critical to the accurate determination of the sputter yield. It is desirable to fabricate the target in-situ under ultra-high vacuum conditions. Methods for this include fracture, vapor or sputter deposition (76) or direct deposition of a low energy ion beam (73). In many other cases, however, targets must be cleaned through a variety of chemical, heating, or in-situ sputtering techniques. In addition, in-situ surface-analytical techniques are highly desired, and considered indispensable for measurements on multicomponent targets. Surface topography may have a (strong) influence on the average sputtering yield, as becomes evident through eq. (5) and Fig. 2. In addition, redeposition of sputtered material may be a problem. The effect of surface topography can be reduced somewhat by rotating the target, although surface topography development will still occur in the range 30° ~ (Ji ~ 75° if initial surface undulations with lateral dimensions larger than individual cascade sizes (Le. ~ 10 - 100 nm ) are present on the target. The crystalline state of the target may also influence the measurement. The particular characteristics of mono-crystal sputtering have been treated previously and need not be repeated here. It is generally very difficult to obtain nearly amorphous nletal targets. A suitable alternative may be (fine grain) polycrystalline material. However, in order to avoid the specific aspects of crystal sputtering the material must not be textured, that is : the individual crystallites in the target must be randolnly oriented. (Textures may occur in rolled, evaporated or even sputter deposited material). In addition, as will be described in a later chapter (Chapter 15), the (prolonged) ion-bombardment itself may transform a non-textured surface in a textured one (77). 6.4.3 Measurement Techniques
There is a clear lack of reproducibility among sputtering yield data collected prior to about 1965. This may largely be attributed to the generally inferior vacuum (> 10-7 Torr) conditions in those measurements (78). Also, many data were obtained in plasnla discharges, which enable large current densities at low E i (~ 0.1 - 1 keV) thereby keeping in principle the target surface dynamically clean. Unfortunately all other irradiation conditions, like beam purity, charge state and hence bombardment energy and dose determination, are largely undefined in a plasma. Nevertheless, many systematics in sputtering phenomena were uncovered in these older experiments and many of the proposed measurement methods are in use today as only some of the tools must be considered outdated. The sputtering yield determination methods can be grouped in four categories (79): i) decrease of target mass (or areal density); ii) decrease of target thickness; iii) collection of the sputtered material; iv) detection of sputtered particles in flight;
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Handbook of Ion Beam Processing Technology
It is obvious that i) and ii) can exclusively be used for total yield determinations, whereas the other two will predominantly be concerned with differential yields. Each individual technique will have its own advantages and disadvantages. 6.5 TOTAL SPUnER YIELD MEASUREMENTS
Measurements of the total sputter yield are generally performed by a quantitative measurement of the target nlass or thickness following or even during the bonlbardment of a known flux of carefully controlled ions. 6.5.1 Mass Loss Techniques
The weight-loss of the target can be determined by microbalence techniques outside of the vacuum chamber. However, there are obvious disadvantages of this technique, including poor sensitivity (or very high doses) and the effect of exposure to air and water vapor during the measurement. In-vacuum gravimetric, microbalence techniques eliminate air-contamination effects, but are very delicate and difficult measurements. A highly sensitive technique for in-vacuum measurements of the mass of the collected, sputtered particles is the Quartz Crystal Oscillator Microbalence (QCOM) technique. The resonant frequency of an oscillating mechanical system, such as a piezoelectric quartz crystal, is determined by the mass and restoring force, or elastic constant, of the device. Mass changes of the system affect its resonant frequency. Conversely, a frequency change, df, can be used as a very sensitive monitor of mass loss (or gain), dm. For a quartz oscillator the basic relation of both quantities is a linear one dm/mQ = - k df/ f R , where mQ is the total mass of the quartz, f R its resonant frequency and k is related to the elastic constant, provided df ~ f R/ 50 . The sensitivity of a quartz resonator is enormous. For a typical resonator operating at 6 MHz (AT cut) the mass sensitivity is dm ~ -1 x 10- 8 df (Hz g cm- 2 ) while frequency changes of df = 1 Hz are easily detected. This makes the QCOM the fastest and most sensitive DHV-compatible technique allowing for in situ dynamic absolute yield determination with sub-monolayer resolution (79). Moreover, it is possible to design crystal holders such that concurrent ion current measurement is possible (7,9,25,79,80) (see Fig. 11) and to enable combination with surface analytical techniques, for example, thin film interface detection. Targets can be deposited on the quartz crystal in situ or elsewhere. Irradiation of the deposited layer will stress the film which in turn gives rise to frequency changes (81). This can be renledied by applying a rather thick (~5 p.m) nletal (AI or Ag work well) film between the sputtered target material and the crystal. Often as-delivered crystals have very rough surfaces, with undulations of the order of 0.5 p.m Then, such metal buffer layers, when carefully polished, are also advantageous in preventing initial surface topography of the deposited targets .
Quantitative Sputtering
6
MHz~
resonance tre q uenc1{t---_-L--I
~ 1+
-~~~q
ion current
97
-50 '" 100 V
T
Figure 11: Schematic drawing of a holder designed for a quartz crystal oscillator (Q), with a target (T) deposited on to it, with which continuous ion current measurements can be made during sputtering. A beam defining aperture biased negatively suppresses secondary electron emission and helps define the ion beam. (79)
There are two other problems with the use of QCOMs. First, beam heating causes frequency changes. Thus either the input power must be kept low or the crystal temperature must be stabilized in a special set-up (25). The second relates to the (radial) position dependent sensitivity of the QCOM, which may lead to severe errors unless the eroded area is either located very precisely and reproducably or exceeds the "active" area of the QCOM (7). By ensuring homogeneous irradiation this problem can be circunlvented. The fact that QCOMs are readily commercially available and can be used conveniently and with confidence has lead to wide spread application in sputtering yield studies. 6.5.2 Probe Techniques
Several techniques exist to measure on an atomic scale the change in the thickness or composition of targets following sputtering. One technique routinely used in many laboratories is Rutherford Backscattering Spectroscopy (RBS). A second general technique is the use of probe-beam-excited x-ray emission from the target. In Rutherford Backscattering Spectroscopy (RBS), light, high energy ions (e.g. He 2 at 2 MeV) incident on a solid will occasionally undergo an elastic collision with one of the target atoms. By collecting and measuring the flux and energy of these backscattered ions, the composition as a function of depth below the surface can be calculated. With RBS in situ absolute and dynamic sputtering yield determinations are possible with an a priori accuracy of some 10 % • This may be improved further when the areal density of the deposited target film is precisely known, thus allowing for additional calibration of the technique. Thick targets can also be used provided a marker layer is applied
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(e.g. by implantation of heavy ions at high energy (~MeV) to a dose of about 1016 / cm 2 extracted from the same accelerator as used for the He probe beam). Special care must be taken to avoid sputter-beam-induced marker diffusion. RBS is essentially non-destructive, because the sputtering yield of MeV He ions is negligible. It requires, however, highly polished surfaces and interfaces and uniform irradiation by the sputtering beam. The bombardment of materials by high energy protons (100-200 keY) or energetic electrons (~1 OkeV) can cause x-ray enlission characteristic of the constituent elenlents, which can be detected to determine the concentration. For thin (see below) targets the X-ray intensity is proportional to the areal density of the film. This type of measurement can be made absolute by calibrating the X-ray intensity against films whose areal density is precisely known or determined previously (by e.g. RBS). Then it is possible to measure continuously, Le. dynamically, the sputtering yield in much the same way as with RBS. Both electron (18,83) and proton (84) probe beams have been used successfully in in-situ dynamic and absolute (after calibration) yield determinations. The absolute accuracy is estimated at 15 0/0, but relative results as a function of sputter-ion energy or fluence are much better, provided that substrates are selected carefully to avoid X-ray line interference. Also flat surfaces and homogeneous irradiation are a prerequisite. This requirement may be relaxed somewhat when electrons are used as a probe, since the beam can easily be scanned over a large part of the ion irradiated target area while integrating or averaging the X-ray yield. A further advantage of electron bombardment is that also Auger electron enlission takes place so that simultaneously the areal density data and Auger depth profile information at the same point of analysis can be obtained if an electron energy analyzer is available (18,83). On the other hand, the fact that there is very little bremsstrahlung radiation with PIXE (in contrast to electron-excited X-ray emission) to interfere with the detection of elements in very low concentration, favors the use of a H probe beam when submonolayer amounts of (contaminant) material need to be analyzed. 6.5.3 Thickness Change Techniques
6.5.3.1 Masking Techniques. By masking one area of a sputtering target, the total sputter yield can be determined by subsequent examination of the resulting step after bombardments (12). Care must be taken to avoid contamination by sputter deposition of the mask material onto the target. SEM techniques are limited to the range of 0.1 to 10 ,urn due to the resolution and focal length of the SEM. Mask and target thicknesses in the range 0.1-10 ,urn can also easily be measured with stylus instruments (see later) and (up to about l,u m) with ellipsometry. Selective wet chemical etching of the individual masking layers enables step height deternlination. One must be aware of the fact that the etch selectivity may be influenced by ion bombardment Smooth surfaces are mandatory, but for relative yield determinations only locally uniform irradiation is required. An implementation by the author (85) is shown in Fig. 12.
Quantitative Sputtering
99
(b)
(a)
CD
11
mm
~ ~ ~ ~ ~ ~ ~ ®
~.
~~
® @)
Figure 12: The multiple-masking yield determination method. (a) Top view of a threelayer target; the arrow denotes an easy cleavage direction; the encircled areas are so small that local ion beam inhomogenity may be assumed. (b) Cross section of the target (along the arrow); (1). the initial thickness of the layers are measured prior to irradiation; (2). after irradiation the sample can be immediately inspected by SEM (after fracture) or ellipsonletry and the thickness decrease can be established; or (3) and (4). layer-by-Iayer is selectively etched away and step heights are measure in-between (by a stylus device, for example). The yields of the individual layers can be determined from from the step heights and the densities of the layers (85).
The mechanical (vertical) displacement of a very small radius stylus (~O.l,um ) as it is moved over a surface can also be used to probe minute changes in surface topography. Conversion of the displacement normal to the surface into an electric signal enables detection of height changes of the order of 1 nm, when properly processed, and provided stylus dimensions do not interfere with the detection of the feature's full height. Instruments with this capability are commercially available under the name Talysurf or Alphastep. The application of this technique to sputtering is extremely straight-forward. The stylus technique is a non-vacuum, hence static, yield determination method. It is very
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Handbook of Ion Beam Processing Technology
easy, but time-consuming. Indentation of the target surface by the stylus, along with irradiation-induced swelling or densification limit the accuracy of the depth determination to the order of 10 nm, Le. far beyond the potential limits of the instrument itself. This necessitates ion erosion depths in excess of ~ 0.1 JLm in order to obtain reliable data, representing thus steady-state conditions. Then absolute accuracies of 10 % , relative accuracies of 5 % and reproducibility within 2 % are attainable. 6.5.3.2 Optical Methods. Conventional optical interferometry for length difference determination has been applied to sputter crater depth measurements (86). This technique measures the phase difference'!' between two laser beanls reflected off the sputtered and unsputtered target surface, which is related to the sputtered depth 8 and the wavelength of the laser light A through 4 'TT 8 = A '!'. For transparent materials the sputtering yield can only be extracted in an indirect way, viz. by conlparing measured phase and reflectance data with a theoretical relationship calculated under certain nlodel assumptions. This procedure is cumbersome, but yield averaged over a sputtered depth A/4n (n = refractive index) may be obtained in a simple way from the ion fluence needed in between successive extrema of the reflectance curve (87). The optical system requires only one vacuum window (plus mechanical rigidity). This method is applicable both to bulk and thin filnl materials, and enables simply and direct in-situ dynamic absolute yield determinations. The overall accuracy, however is relatively poor due to constraints such as the requirement for optically flat surfaces, and possible bombardment induced changes in the optical constants. 6.5.3.3 Thin Film Interface Techniques. This type of technique makes use of a thin film, preferably of well-known areal density, of target material A deposited onto a flat substrate B. During sputtering the composition of the target surface or the ejected particle flux is monitored continuously or intermittently in situ by some analytical technique. As soon as the detector signal representative for A starts to decrease and one typical for B starts to come up it is assumed that the interface AB is reached. From the fluence needed and the film thickness, the yield can be extracted. The basic situation is shown in Fig. 13.
The technique is limited by beam uniformity and redeposition from the walls of the crater. In addition, interface broadening by ion bombardment-enhanced diffusion, segregation and cascade mixing necessitate fHnl thicknesses of at least 10 times the projected range, or more. This, by definition, rendered the technique suitable for only steady-state bulk sputtering yield measurements, and may lead to the unfortunate side effect of topography development. The accuracy will be limited to about 15 % . Various surface analytical tools have been used in sputtering yield determinations by thin film interface detection, viz. low energy ion scattering (with its one monolayer probe depth), Auger electron spectroscopy (88,89) (combining excellent elemental resolution with a probe depth of some 10 A) and secondary ion (90) (high sensitivity) or neutral (91) (which does not suffer from matrix effects) mass spectrometry. No clear preference can be given, however, because the particular advantages of an individual analysis method are largely lost in the course of the thinning process. A following chapter by H. Oechsner (Chapter 9) will discuss the secondary neutral measurements in more detail. 6.5.3.4 Other Techni_lues. Quite a few individual experinlents have been developed that uniquely determine the sputter yield of a particular system. These techniques are based on such phenomena as interference changes in dielectric films (92), changes in
Quantitative Sputtering
101
electrical resistivity, breakthrough in thin self-supporting films (93), and other more specialized techniques (16).
A
B
<
Z
dose q>AB
"ideal"
"real"
Figure 1 3: Schematic representation of the thin-film interface detection measurement. 6.6 DIFFERENTIAL YIELD MEASUREMENTS: ANGULAR AND ENERGY DISTRIBUTIONS 6.6.1 Angular Distributions Of Eiected Particles
The standard method for the measurement of the angular distribution of sputtered particles has been the accumulation of those particles on a collector of some kind. The total yield is then usually found by integration. The only collection technique that allows for accumulation and detection of ejected volatile or chemically highly reactive target constituents is matrix isolation trapping (94). Here the sputtered particles are co-condensed with an inert gas at high dilution ratios (~ 104 ) onto a cold (~ lOOK) substrate. The resulting solid, inert-gas matrix, isolates the species of interest and their concentration is determined, with an accuracy of some 10 % , by optical absorption spectroscopy from known oscillator strengths. Thus an in situ, potentially dynamic, yield determination with high sensitivity - (changes of the order of f:J.y ~ 0.01 are detectable) - becomes possible. Difficulties with the calibration of the trapped atom fraction enables relative measurements only and the method may be restricted to low yield or low dose experiments, because the necessary inert gas matrix otherwise might become optically opaque. Several other dedicated techniques have been applied to accumulation-based angular distribution studies. For metallic targets, measurement of the electrical resistivity have been used (95) to determine the (local) thickness of the deposited layers. Film oxidation and especially electrical size effects (93) may give rise to systematic errors.
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A number of more generally applicable options are open and preferable when the sputtered species are involatile. A single quartz crystal oscillator microbalance (25), rotatable around the target, or an array of QCOMs (96) has successfully been used as a collector in angular distribution studies and allows for sensitive and direct differential yield determinations. In principle any surface analytical technique can be used to measure the thickness of the layer deposited on the collector, be it that the lateral resolution must meet the requirement 8ep ~ 50. The most widely used method was the determination of the absorption of light in the layer collected on a glass plate. Absorption roughly follows an exponential law, but for thicker films problems may arise because of multiple reflection resonances within the film. The absorption measurenlents are based on the approximation that reflection coefficients do not change as a function of film thickness, but unfortunately they usually do. Also impurity incorporation in the deposited layer and oxidation of the collected nlaterial strongly affect the optical properties, although the latter problem is absent in in-situ measurements (97). Good, quantitative and potentially but not necessarily in situ techniques for analyzing the collected layers, which have been used in angle resolved sputtering experiment are RBS (98), PIXE (98), electron-excited X-ray enlission (99) and Auger electron spectroscopy (AES) (100). These methods are very sensitive and combine a good absolute accuracy ( ~ 15 0/0) with an excellent relative, Le. angle-dependent, determination. AES is particularly suited for (sub)monolayer thicknesses of the deposit. 6.6.2 Energy Distributions Of Eiected Particles
The energy distribution of the sputtered atoms may be relevant to properties of films deposited of those atonlS. For this reason, the following three chapters will discuss specific techniques used to measure these quantities. For completeness, there are several other techniques which have been used to measure sputtered atom energy distributions. A critical review of the measurement techniques for determination of the energy distributions of sputtered particles has been published by Thompson (101). The oldest technique for this measurement has been calorimetry (95,102,103). However, this technique has no energy resolution and only measures the average energy of all of the deposited atoms. This nlay, however, be useful infornlation for film deposition processes. Two later methods are based on time-of-flight measurements, and they will be discussed below. A third technique measures the energy distribution in-flight, and will be discussed in more detail in the following chapter. Thompson and co-workers have implemented a pioneering, mechanical, solution to allow for time-of-flight studies. (Fig. 14) (49). The sputtered particles are collected on a high-speed spinning rotor which is optically coupled to a deflection system. Thus a pulsed ion beam impinges on the target and a fraction of the sputtered particle flux impinges on the rotating collector after passing a diaphragm. The angular distance from the "beam-on" point is determined by the flight time of the ejected particles, hence their velocity, and the speed of rotation (ignoring possible delay time corrections). By selecting the latter, the optimal operating regime for the velocity or energy of the emitted atoms can be chosen. After completion of the experiment, at one specific setting, the collector is renl0ved and analyzed. The amount of deposited material is so low that auto-radiography or other tracer methods must be used, to avoid the necessity of huge irradiation doses. This restricts applicability of the technique to a limited number of elements. Although the system has a few clear disadvantages - it does not distinguish between atom and cluster
Quantitative Sputtering
103
emission, the duty cycle is low and for every change in experimental conditions the vacuum must be broken - it is still in use today (104). Another time-of-flight apparatus has been built by de Vries and collaborators (105) (Fig. 15). A mass- and energy-analyzed ion beam impinges on the target. Neutral particles ejected from the target in a small solid angle ( "" 10- 4 sterad.), defined by a diaphragm which is charged to repel ionic species, are post ionized and accelerated just in front of the entrance of a quadrupole mass spectrometer. Here the freshly formed ions are massanalyzed and detected by an electron multiplier. The detector stage must preferably be misaligned slightly to prevent multiplier exposure to the fast (neutralized) reflected sputtering projectiles. The incident ion beam is modulated by a hardware generated pseudorandom binary sequence and the time-of-flight distribution is extracted from the measured signal by deconvolution. This correlation technique ensures that only those signals, sufficiently above background, are accepted which stem directly from neutrals ejected during ion bombardment. Of course corrections must be applied to the raw data for a velocity dependent ionization efficiency and for delay times caused by the transit times of the incident ions from modulator to target and the detected particles from ionizer to multiplier.
light beam ---..... mirror ---.---.\Q
photocell
_-----~
rotor collecting dispersed deposit
r-t!collimator
,~
--+ deflection plates
-- ~,,1t)\ ~
ejected particles
target
~
~
deflected ions
Figure 14: Mechanical time-of-flight spectrometer with rotating collimator constructed by Thompson et al (49). Details are discussed in the text.
Advantages of this technique are the improved duty cycle due to the pseudorandom beam chopping and the excellent, mass-resolved, sensitivity of the spectrometer. Differentiation between ejected neutral atoms and a possible contribution from clu#rs fragmented in the post-ionizer is tiresome. It can be achieved by comparing their time-of-flight spectra, by varying the incident ion energy and studying the relative contributions of all emitted species to the non-energy-analyzed mass spectrum, or by varying
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Handbook of Ion Beam Processing Technology
the ionization efficiency e.g. by changing the electron energy when an electron impact ionizer is used, and examining the signal response. A highly sophisticated technique for the in-flight investigation of the sputtered particle flux is laser induced fluorescence spectroscopy (LIF). In this nlethod, atonlic (or molecular) transitions in the ejected particles can be induced by irradiation with tunable, cwdye, lasers. Such transitions from a given lower level to an upper level are followed by spontaneous (or stimulated) radiative decay to the same (or another) lower level. The frequency and intensity of the emitted fluorescent radiation can then be used to identify the ejected particles as well as to determine their concentration (or the population distribution over ground state and metastable states) in the sputtered flux. Moreover, by exploiting the Doppler broadening of the absorption line the velocity distribution of the sputtered atoms (in a specific state) can be obtained in situ. The use of this technique will be discussed in more detail in Chapter 7.
ion source &optics
~
pseudorandomly chopped -r focused ion beam
.. quadrupole mass spectrometer
~~~~~i~' i.
~f ~
Ii
C
II
diaphragm post&ionizer accelerator
II •• I, ~
\
~target
-U
ejected species
~ flight length Figure 15: Electrical time-of-flight spectrometer with quadrapole mass analyzer designed by de Vries and co-workers (105). Details are described in the text. 6.6.3 Combined Angular- And Energy-Resolved Measurements
In principle, each of the methods discussed in the previous subsection can be used for combined energy- and angular-resolved sputtering investigations. In our laboratory a set-up akin to the one shown in Fig. 15 has recently been completed (106), which allows automated in-situ rotation of the complete detector stage over a significant angle ( ~3000) around the target. The incident ion beam direction is fixed, but since the target holder is equipped with a double tilting facility still most incident- and ejection-angle combinations can be obtained. It is clear, however, that complete mapping of the angularand energy-distributions will be very time-consuming.
Quantitative Sputtering
105
Another technique, employing mulit-phonon resonance ionization of the sputtered atoms, has been developed by Winograd and co-workers (107). This technique has been successful at measuring the angular- and energy-dependance simultaneously, and will be discussed in detail in a later chapter (Chapter 8). 6.7 CONCLUDING REMARKS
Our knowledge of the phenomena accompanying sputtering has tremendously advanced in the last three decades. For noble gas ion bOITlbardnlent of elenlental targets, the experimental and theoretical situation is now such that the role of many parameters in the sputtering process (e.g. Zi' E i , (Ji'
i' Zt, T t) is exposed and understood, at least qualitatively, but to a large extent also quantitatively. In this context one should bear in mind, however, that at best half of the experimental results published to date may be considered reliable according to the standards laid down in this paper. Nevertheless, experimental techniques have matured or become available with sufficient sophistication to generate the accurate and detailed information needed to provide the answers to most practical and fundamental questions. Our understanding will definitely refine and expand in the next decades, but whether or not completely new, hitherto hidden, phenomena will be uncovered remains to be seen. Perhaps the greatest challenge in the future lies in the exploration of the technologically important low-energy, near threshold, regime. In sharp contrast, our knowledge of compound target sputtering is still very limited, in spite of many useful contributions that deserve to be acknowledged. Yet, from a practical point of view, compounds are the most ubiquitous and important materials. Furthermore, only if we can handle compound sputtering properly we may hope to fully understand sputtering with reactive ions or in the presence of background gases. Unfortunately, to date there is no comprehensive theory to elucidate the relative importance of the various parameters that determine the sputtering behavior. It must be acknowledged that the task is formidable, to encompass in one model simultaneously such processes as ion enhanced segregation and diffusion, ion beam mixing, deconlposition and preferential sputtering etc. On the other hand, there is little incentive to embark on such a program as long as reliable systematic experimental information is virtually non-existent. For example, not even for one binary compound absolute yield data over three decades of E i (O.1-100keV) for more than one Zi (e.g. Ne+, Ar+, Kr+, Xe+) are available to base general trends and scaling laws on. And it is not clear whether or not binary metal alloys, compound semiconductors (e.g. 111-Vs) and insulators (e.g. oxides and nitrides) may be treated on the same footing or must be regarded as distinct classes. As discussed in this paper, the experimental tools to tackle these problems are there. For, potentially dynamic, total yield determinations several measurement techniques that are widely applicable, readily accessible and relatively convenient can be recommended: i) the quartz crystal oscillator microbalance; ii) Rutherford backscattering spectroscopy; and iii) probe beam induced X-ray emission. As for the investigation of compound target sputtering an in situ surface analysis facility with elemental sensitivity is deemed necessary, in particular electron-excited X-ray emission combined with Auger electron spectroscopy must be considered most promising. Other techniques and combinations should not a priori be disregarded, however. In dif-
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Handbook of Ion Beam Processing Technology
ferential yield studies, the collector method is to be preferred for measurements on the angular distributions of ejected particles. For kinetic energy distributions of sputtered particles two techniques are recommended viz; i) mass-spectrometric time-of-flight; and ii) laser induced fluorescence. The former is restricted to the investigation of the steady-state sputtering behavior. The latter, with its ability to discriminate against the atomic state of the emitted particle, holds nlost the promise although the applicability is presently rather limited. All the above-cited nlethods have ideally the capability to generate data with a reproducibility of ~ 2 0/0, i.e. for one specific (Zi , E i , (Ji , Zt) combination on one and the same apparatus, a relative accuracy of ~ 5 % (Le. for variation of one variable), and absolute yields to approx 10 % (Le. for the total number of ejected atoms per incident ion). Stronger claims occasionally found in the literature seem totally unwarranted. Under most practical circumstances these figures need not deteriorate by more than a factor of two provided maximum experimental care is taken -Le. DHV, well-defined ion beam and target conditions, and constant awareness of the variety of phenomena that may interfere with the measurement.
NOTE: An expanded version of this work has been published recently. Please see Ref. 16.
6.8 REFERENCES
1. W.R. Grove, Trans. Roy. Soc. London 142: p.87 (1982), Philos. Mag. 5: p. 203 (1853).
2.
J. Stark, Die ElectriziHit in Gasen (Barth, Leipzig, 1902).
3.
J. Stark, Zeitsch. f. Elektrochem. 14: p. 752 (1908); 15: p. 509 (1909).
4. A more truthful and complete historical survey was given by G.K. Wehner, 2nd Symp. on Sputtering (Spitz a.d. Donau, 1986) 5.
H. Oechsner (ed.), Thin Film and Depth Profile Analysis, Springer, Berlin (1984).
6. H.H. Andersen and H.L. Bay, in: Sputtering by Particle Bombardment Vol. I, ed. by R. Behrisch, Chapt. 4, pp. 145-218, Springer, Berlin (1981). 7.
P. Blank and K. Wittmaack, J. Appl. Phys. 50: p. 1519 (1979).
8.
P.C. Zalm, J. Appl. Phys. 54: p. 2660 (1983).
9.
H.L. Bay, J. Bohdansky, W.O. Hofer and J. Roth, Appl. Phys. 21: p. 327 (1980).
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10. T.J. Whetten, A.A. Armstead, T.A. Grzybowski and A.L. Ruoff, J. Vac. Sci. Technol. A2: p. 477 (1984). 11. H. Oechsner, Z. Physik 261: p. 37 (1973). 12. I.H. Wilson, S. Chereckdjian and R.P. Webb, Nucl. Instr. & Meth. B7/8: p. 735 (1985). 13. K.A. Gschneider, Solid State Phys. 16: p. 275 (1964). 14. P. Sigmund, Phys. Rev. 184: p. 383 (1969); 187: p. 768 (1969). 15. W.D. Wilson, L.G. Haggmark and J.P. Biersack, Phys. Rev. B15: p. 2458 (1977). 16. P.C. Zalm, Surface and Interface Analysis 11: p. 1 (1988). 17. Y. Yamamura and J. Bohdansky, Vacuum 35: p. 561 (1985). 17a. P.C. Zalm and L.J. Beckers, J. Vac. Sci. Technol. B2: p. 151 (1984). 18. J. Kirschner and H.W. Etzkorn, ADDI. Phys. A29: p. 133 (1982). 19. K. Wittnlaack, Nucl. Instr. & Meth. B2: p. 569 (1984). 20. G.N.A. van Veen, F.H.M. Sanders, J. Dieleman, A. van Veen, D.J. Oostra and A.E. de Vries, Phys. Rev. Lett. 57: p. 739 (1986). 21. S. Tachi and S. Okudaira, J. Vac. Sci. Technol. B4: p. 459 (1986). 22. P.C. Zalnl, Vacuum 36: p. 787 (1986). 23. A. von Hippel, Ann. d. Phys. 80: p. 672 (1926); 81: p. 1043 (1926); 86, 1006 (1928). 24. R.S. Nelson, Phil. Mag. 11: p. 291 (1965). 25. K. Besocke, S. Berger, W.O. Hofer and U. Littmark, Rad. Eff. 66: p. 35 (1982); W.O. Hofer, K. Besocke and B. Stritzker, ADDI. Phys. A30: p. 83 (1983). 26. A.L. Southern, W.R. Willis and M.T. Robinson, J. ADDI. Phys. 34: p. 1 & p. 153 (1963). 27. G.D. Magnuson and C.E. Carlston, J. ADDI. Phys. 34: p. 11 & p. 3267 (1963). 28. T.W. Snouse and L.C. Haughney, J. ADDI. Phys. 37: p. 2 & p. 700 (1966). 29. V.A. Molchanov, V.G. Tel'kovskii and V.M. Chickerov, Sov. Phys.-Doklady 6: p. 3 & p. 222 (1961).
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30. G. Betz and G.K. Wehner, in: Sputtering by Particle Bombardment Vol. II, ed. by R. Behrisch, Chapt. 2, pp. 11-90. Springer Berlin (1983). 31. G.S. Anderson, J. Appl. Phys. 40: p. 2884 (1969). 32. P. Sigmund, in: Sputtering by Particle Bombardment Vol. I, ed. by R. Behrisch, Chapt. 2, pp. 9-71. Springer, Berlin (1981). 33. G. Brauer, D. Hasselkamp, W. Kruger and A. Scharmann, Nucl. InstL & Meth. B12: p. 458 (1985). 34. M. Szymonski, W. Huang and J. Onsgaard, Nucl. InstL & Meth. B16: p. 263 (1986). 35. M. Saidoh, H.L. Bay, J. Bohdansky and J. Roth, Nucl. Instr. & Meth. B13: p. 403 (1986). 36. G. Carter, B. Navinsek and J.L. Whitton, in Sputtering by Particle Bombardment II ed. by R.Behrisch, Chap. 6 p. 231, Springer, Berlin, 1983. 37. G.K. Wehner, J. Appl. Phys. 26: p. 1056 (1955); Phys. Rev. 102: p. 690 (1956). 38. J. Linders, H. Niedrig and M. Sternberg, Nucl. InstL & Meth. B2: p. 649 (1984); B13: p. 353 (1986). 39. A. van Veen and J.M. Fluit, Nucl. InstL & Meth. 170: p. 341 (1980). 40. H. Gnaser and W.O. Hofer, presented at 2nd Symp. on Sputtering, Spitz a.d. Donau, 1986. 41. R.H. Silsbee, J. Appl. Phys. 28: p. 1246 (1957). 42. C. Lehmann and P. Sigmund, Phys. Stat. Sol. 16: p. 507 (1966). 43. D.E. Harrison, J.P. Johnson and N.S. Levy, ADDI. Phys. Lett. 8: p. 33 (1966); ADPI. Phys. 39: p. 3742 (1968).
L
44. H.H. Andersen, B. Stenum, T. Sorensen and H.J. Whitlow, Nucl. Instr. & Meth. 209/210: p. 487 (1983). 45. M.W. Thompson, Phil. Mag. 18: p. 377 (1969). 46. H.M. Urbassek, Nucl. Instr. & Meth. B4: p. 356 (1984). 47. G. Falcone and A. Oliva, ADDI. Phys. A32: p. 201 (1983). 48. R. Kelly, Surf. Sci. 90: p. 280 (1979); Rad. Eff. 80: p. 273 (1984). 49. M.W. Thompson, B.W. Farmery and P.A. Newson, Phil. Mag. 18: p.361 (1968). 50. H. Oechsner,
z.
Physik 238: p. 433 (1970).
Quantitative Sputtering
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51. R.A. Weller and T.A. Tombrello, Rad. Eff. 37: p. 83 (1978). 52. H. Overeinder, M. Szymonski, A. Haring and A.E. de Vries, Rad. Eff. 36, p. 63 & p. 189 (1978); 37: p. 205 (1978); 38: p. 21 (1978). 53. W. Husinsky and R. Bruckmiiller, Surf. Sci. 80: p. 637 (1979). 54. M. Szymonski and R.S. Bhattacharya, Aool. Phys. 20: p. 207 (1979). 55. M. Szymonski, Acta Phys. Pol. A56: p. 289 (1979); Aool. Phys. 23: p. 89 (1980). 56. B. Schweer and H.L. Bay, Aool. Phys. A29: p. 53 (1982). 57. D. Grischkowsky, M.L. Yu and A.C. Balant, Surf. Sci. 127: p. 315 (1983). 58. W. Berres and H.L. Bay, Aool. Phys. A33, 235 (1984). 59. E. Dullni, Nucl. Instr. & Meth. B2: p. 610 (1984); Aool. Phys. A38: p. 131 (1985). 60. W. Husinsky, G. Betz and 1. Gigris, Phys. Rev. Lett. 50: p. 1689 (1983); J. Vac. Sci. Technol. A2: p. 698 (1984). 61. H.L. Bay and B. Schweer, in: Proc. Syn1o. Surf. Sci. (Obertrauen, 1985) p. 147. 62. J.P. Baxter, G.A. Schick, J. Singh, P.H. Kobrin and N. Winograd, J. Vac. Sci. Technol. A4; p. 1218 (1986). 63. C.A. Andersen and J.R. Hinthorne, Anal. Chern. 45: p. 1421 (1973). 64. G.E. Thonlas, Surf. Sci. 90: p. 381 (1979). 65. T.W. Rusch and R.L. Erickson, in: Inelastic Ion-Surface Collisions, ed. by N.H. Tolk, J.C. Tully, W. Heiland and C.W. White, pp. 73-104. Academic Press, New York (1977). 66. A. Benninghoven, F.G. Rudenauer and H.W. Werner, SIMS. Wiley, New York (1986). 67. R.A. Haring, H.E. Roosendaal and P.C. Zalm, Nucl. Instr. & Meth. B28: p. 205 (1987). 68. G.P. Konnen, A. Tip and A.E. de Vries, Rad. Eff. 26: p. 23 (1975). 69. J. Weng and E. Veje, Phys. Rev. B31: p. 1600 (1985). 70. M.H. Shapiro, P.K. Haff, T.A. Tombrello, D.E. Harrison and R.P. Webb, Rad. Eff. 89: p. 234 (1985).
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71. P.C. Zalm, Rad. Eff. Lett. 86: p. 29 (1983). 72. E.H.A. Granneman and M.J. Van der Wiel, in: Handbook on Synchrotron Radiation Vol. I, ed by E.E. Koch, Chapt. 6, pp. 367-462. North Holland, Amsterdam (1983). 73. G.E. Thomas, L.J. Beckers, J.J. Vrakking and B.R. de Koning, J. Cryst. Growth 56: p. 557 (1982). 74. T. Tokuyama, K. Yagi, K. Miyake, M. Tamura, N. Natsuaki and S. Tachi, Nuc!. Instr. & Meth. 182/183: p. 241 (1981). 75. K. Morita, M. Takami and H. Ohno, Surf. Sci. 148: p. L667 (1984); 157: p. L361 (1985). 76. L.G. Pittaway, Brit. J. App!. Phys. 15: p. 967 (1964). 76a. J.E. Greene, Crit. Rev. Solid St. & Mat. Sci. 11: p. 47 & p. 189 (1983). 77. G.N. van Wyk and H.J. Smith, Rad. Eff. 38: p. 245 (1978). 78. R. Behrisch, Ergeb. Exakte Naturwiss. 35: p. 295 (1964). 79. J. Fine, in: Physics of Ionized Gases 1980, Invited lectures, ed. by M. Matic, pp. 379-420. Boris Kidric Institute, Beograd (1980). 80. R.J. MacDonald and D. Haneman, J. App!. Phys. 37: p. 1609 (1966). 81. E.P. EerNisse, J. App!. Phys. 42: p. 480 (1971); J. Vac. Sci. Techno!. 11: p. 408 (1974); 12: p. 564 (1975). 82. L. Reimer and G. Pfefferkorn, Rasterelektronenmikroskopie. (1977).
Springer, Berlin
83. J. Kirschner and H.W. Etzkorn, App!. Surf. Sci. 3: p. 251 (1979). 84. B.D. Sartwell, J. App!. Phys. 50: p. 7887 (1979). 85. P.C. Zalm, L.J. Beckers and F.H.M. Sanders, Nuc!. Instr. & Meth. 209/210: p. 561 (1983). 86. S.T. Kang, R. Shimizu and T. Okutani, Jpn. J. Appl. Phys. 18: p. 1717 (1979). 87. J.E. Kempf, in: Secondary Ion Mass Spectrometry II. ed. by A. Benninghoven, C.A. Evans, R.A. Powell, R. Shimizu and H.A. Storms, p. 97. Springer, Berlin (1980). 88. M.L. Tarng and G.K. Wehner, J. Vac. Sci. Technol. 8: p. 23 (1971); J. Appl. Phys. 42: p. 2449 (1971); 43: p. 2268 (1972). 89. J.N. Smith, C.H. Meyer, J.K. Layton, J. App!. Phys. 46: p. 4291 (1975).
Quantitative Sputtering
111
90. W.O. Hofer and P.J. Martin, Aool. Phys. 16: p. 271 (1978). 91. H. Oechsner, H. Schoof and E. Stumpe, Surf. Sci. 76: p. 343 (1978). 92. R. Kelly, J. Appl. Phys. 39: p. 5298 (1986); L.Q. Nghi and R. Kelly, Can. J. Phys. 48: p. 137 (1970). 93. E.H. Sondheimer, Adv. Phys. 1: p. 1 (1952). 94. C. Steinbruchel, D.M. Gruen and J. Dawson, J. Vac. Sci. Technol. 16: p. 25 (1979); Surf. Sci. 93: p. 299 (1980). 95. C.H. Weijsenfeld, Philips Res. Rep. Suppl. No.2 (1967). 96. E. Taglauer and J. Onsgaard, Appl. Phys. Lett. 48: p. 575 (1986). 97. K. Rodelsperger, W. Kruger and A. Scharmann, Z. Physik 269: p. 83 (1974). 98. R.G. AlIas, A.R. Knudson, J.M. Lambert, P.A. Treado and G.W. Reynolds, Nucl. Instr. & Meth. 194: p. 615 (1982); B2: p. 679 (1984).
99. W.O. Hofer, Rad. Eff. 19: p. 263 (1973); Mikrochim. Acta Suppl. 7: p. 185 (1977). 100. M. Kaminsky and S.K. Das, J. Nucl. Mat. 53: p. 162 (1974); 60: p. 111 (1976). 101. M.W. Thompson, Nucl. Instr. Meth. B18: p. 411 (1987). 102. H.H. Anderson, Rad. Eff. 3: p. 51 (1970); 7: p. 179 (1971). 103. J.A. Thornton, Thin Solid Films 119: p. 87 (1984). 104. F. Lama, J.A. Strain and P.D. Townsend, Rad. Eff. 99: p. 301 (1986). 105. R.A. Haring, R. Pedrys, D.J. Oostra, A. Haring and A.E. de Vries, Nucl. Instr. & Meth. B4: p. 40 (1984); B5: p. 476 (1984). 106. A.W. Kolfschoten and J. van Laar, to be published. 107. P.H. Kobrin, G.A. Schick, J.P. Baxter and N. Winograd, Rev. Sci. Instrum. 57: p. 1354 (1986).
7 Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
Wallis F. Calaway, Charles E. Young, Michael J. Pellin, and Dieter M. Gruen
7.1 INTRODUCTION
In this section, an attempt will be made to familiarize the reader with laser-induced fluorescence spectroscopy (LFS) and how it can be utilized to examine the sputtering process. As discussed earlier, when energetic ions strike a solid, the kinetic energy is given up to the matrix. A small portion of that energy is in turn imparted to particles in the solid which are ejected, or sputtered from the surface. One of the more fruitful avenues for achieving an understanding of the sputtering mechanism has been the examination of the number, energy, and direction of the particles ejected from the surface due to ion bombardment (1). During sputtering both neutral and charged species are produced in a variety of electronic states (2). In the most general case, the majority of sputtered particles are ground state neutral atoms. These particles, being neutral, are nearly impossible to manipulate and are more difficult to detect than secondary ions. Before LFS, various techniques were enlployed in the study of sputtered neutral particles and some success was achieved (1,3,4). Generally speaking, however, these experiments suffer from poor sensitivity and it is not possible to distinguish ground from excited state atoms. Sputtering can be thought of as generating a local concentration of neutral gas phase atoms in front of a target. This perspective is particularly useful for one concludes from this that all spectroscopic techniques applicable to gas phase atoms can be used in the investigation of sputtering. The advent of narrow-band lasers capable of being frequency tuned to resonant transitions of atoms brought a new era to atomic spectroscopy (5). An experimental method which proved to be particularly useful was LFS. In this technique, the frequency of a laser is scanned through an atomic (usually a resonance) transition causing excitation due to photon absorption. In the absence of collisions, the atoms thus excited give up that energy by fluorescing at the excitation or various other specific frequencies depending on the element. Because atomic transitions are very narrow and most often well separated in frequency fronl transitions of other elenlents, resonance excitation by a laser results in an element specific process. Therefore, detection of the laser-induced fluorescence need not be frequency specific in order for the LFS technique to be element
112
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
113
specific. The need for high-resolution spectrometers is therefore eliminated. Furthermore, LFS is very sensitive conlpared to absorption spectroscopy, where a small change in a large signal is the usual method of detection. The fluorescence signal is proportional to the intensity of the transition as well as the efficiency of collecting and detecting the emitted photons. The development of narrow-band width lasers has allowed the line shape of the atomic transitions to be measured. From the line shape, it is possible to calculate the velocity distribution of the atoms by determining the Doppler frequency shift observed for the atomic transition (5). Thus, from LFS nleasurenlents both the yield and kinetic energy of sputtered species can be obtained. In this chapter, the LFS technique, as it applies to the study of the dynamics of ion sputtered neutrals, will be described. It is not the intent of the authors to give an overall review of LFS or the application of LFS to related topics such as neutral atomic beams or neutrals in plasma. Further, due to space constraints, the depth of this review will be limited. The authors' intent is to convey an understanding of LFS of sputtered species in sufficient detail so that a researcher unfamiliar with the techniques can gain general knowledge of the field. For a nlore detailed discussion, the reader is referred to other recent reviews on this topic (6-8). 7.2 EXPERIMENTAL TECHNIQUE
The experimental arrangenlent for a typical LFS systenl used to study sputtered particles is shown in Fig. 1 (9). The apparatus can be divided conceptually into three main parts - a fluorescence excitation source (laser), an apparatus for generating the sputtered atonls, and equipment for collecting and detecting the fluorescence. There are criteria for selection and design of each component and each will be discussed in turn. The type of laser employed for LFS studies is in general a tunable dye laser. By selecting an appropriate dye, the laser can be made to operate in any part of the visible spectrum. In many of the first LFS experiments where sputtered atoms were observed, the elements that were examined were selected because they had resonance transitions in the visible region of the spectrum, and thus, are easily reached with the dye laser (10-11). Often in sputtering experiments, the element of interest is a transition metal. Unfortunately, many of these elements have their strongest transitions in the near ultraviolet (UV) region (250 to 350 nm). To probe these atoms requires generating UV light by frequency doubling the visible output from a dye laser. This is most easily accomplished by enlploying a pulsed dye laser because its high peak power allows efficient frequency doubling. For sputtered atoms, this was first accomplished by Elbern, Hintz, and Schweer (12). Light atoms are more difficult to detect by fluorescence since photons at wavelengths shorter than 250 nm must be employed. Only recently, have results on such systenls been reported. For instance, the velocity distribution of carbon atoms sputtered from graphite and TiC was measured by Raman shifting the output of a dye laser into the vacuum ultraviolet (166 nm) (13). Also, the velocity distribution of Zn atoms sputtered from Zn and ZnS targets has been determined using a two-photon transition at 160 nm (14). The development of a new frequency doubling crystal, B -BaB0 4 , which can efficiently operate down to 198 nnl (15), has greatly enhanced the prospects for expanding the capabilities of the LFS technique to shorter wavelengths.
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Handbook of Ion Beam Processing Technology
TO LIGHT BAFFLES AND BREWSTER WINDOW
Figure 1: Typical experimental arrangement used to study sputtered particles using LFS (9).
SAMPLE LENS
TO ION PUMP, LIGHT BAFFLES, AND BREWSTER WINDOW
Historically, flash lamp (12) or YAG (16) pumped dye lasers have been employed to generate tunable UV light. These systems, while generating sufficient energy to allow efficient frequency doubling, operate at low repetition rates which limits the speed and sensitivity of the LFS technique. With the application of excimer-pumped dye lasers, this constraint has been greatly reduced (17). The newer excimer lasers can operate at repetition rates above 100 Hz, improving the duty factor for LFS experiments by an order of magnitude over previous systems. Commercial excimer-pumped dye lasers typically have bandwidths greater than 1 GHz corresponding to a velocity resolution of approximately 600 m/s. This may not be sufficient resolution for some sputtered velocity distributions. It is worth noting that very recently a new excimer pumped dye laser has become available which has a bandwidth of 500 MHz (18). There are continuous wave (cw) dye lasers operating as single- mode lasers, and these have bandwidths at or below 20 MHz. Such resolution far exceeds the requirements of sputtering experiments, and frequency doubling the output of such a laser directly is difficult due to its low power. A different approach which has proven quite successful in our laboratory is to use a single-mode cw laser to generate the desired frequency and then to amplify its output using dye cells pumped by a pulsed laser [19]. In this configuration, an Ar+ ion pumped ring dye laser is used to generate several hundreds milliwatts of cw visible light. This very narrow-band laser light is amplified by a series of excimer-pumped dye cells. Typical gain through the set of amplifiers is 106. In this manner, sufficient peak laser intensity is generated so that efficient frequency doubling of the visible beam is possible using a doubling crystal. This laser system typically produces tunable ultraviolet light of 1 mJ/pulse. Due to the fact that the amplifiers are pumped by a pulsed laser, the
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
115
bandwidth of the output beam is broadened to 120 MHz (near the Fourier transform limit of the pulse width). Even with this broadening, the laser described has substantially better resolution when compared to other pulsed laser systems without a significant sacrifice in laser intensity. This narrower bandwidth can be important in some applications where the mean kinetic energy of the sputtered particles is low due to a low binding energy or where small shifts in velocity distributions are under investigation. The requirements of an ion beam for LFS experiments are not particularly stringent. Currents in the microampere regime at energies of a few keV can produce densities of 108 atomsl cm3 for typical experimental arrangements. The main criteria of importance is the ion beam spot size which should be kept to millimeter size in order to nlaintain a reasonable spatial and angular resolution (6,8). Other refinements such as differentially pumping of the ion beam or mass filtering of the ion beam can be important in specific experinlents where chemical contamination is a major consideration. There are some advantages for pulsing the ion beam. By matching the duty factor of the ion beam with that of the laser, damage to the sample during an experiment can be minimized. While such danlage can be inconsequential for pure polycrystalline samples, experiments on single crystals or surface layers must be performed with low ion doses and pulsed ion beams. Particular attention must be paid to the ion pulse width to assure that all velocities have reached steady state concentration in the volume being probed. For example, for an ion pulse of 100 JJ.S all velocities above 100 ml s can be found in a probe volume 1 cm away from the target. For a typical pulsed experiment (100JJ.s by 10 JJ.A ion pulse; 100 m/s resolution with 100 pulses averaged together), an entire velocity distribution can be recorded with removal of less than one monolayer (9, 20). In Doppler-shifted experiments, the frequency, v, at which a particular atom absorbs radiation, depends on the velocity, v, of that atom as given by the equation: v =
vo( 1
+ v I c cos 8)
(1)
where v0 is the frequency of the atom at rest, c is the speed of light, and 8 is the angle between the direction of propagation of the laser and the velocity vector of the sputtered atom being probed. As a consequence, in order to measure the Doppler shift the laser beam must not be parallel to the sample surface (Doppler-free spectra are obtained by directing the laser parallel to the surface). While any angle, other than 90° from surface nornlal, will have a projection in the direction of the laser beam, velocity resolution is lost as one approaches 90° For this reason, it is desirable to probe the sputtered flux close to the surface normal. This can be accomplished by passing the laser beam alongside the sample as shown in Fig. 1, or by cutting a hole in the sample for the laser beam to pass through (16, 20). Keeping in mind that in a pulsed experiment the fluorescence intensity is proportional to the nunlber density of the elenlent in the specific atomic level being nleasured, there are two philosophies as how best to probe the sputtered particles. If the probe volume is far enough away from the sample, it becomes a differential element of the density. Unfortunately, the nunlber density and, thus, the signal are low the further the probe volume is away from the target, dropping off as the square of the distance. On the other hand, in the region near the sample surface, the particle density is high and the fluorescence is much more intense. However, as the size of the fluorescing volume becomes comparable
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Handbook of Ion Beam Processing Technology
to either the distance to the sample or the size of the ion beam, the fluorescence becomes proportional to the integral of the density over the geometry being probed, and this can be difficult to deconvolute. In general, as laser technology has progressed and the sensitivity of the LFS technique has been improved, it has become more common to measure a differential element of the sputtered density by detecting the fluorescence further away from the sample. A second factor in design consideration is how best to restrict the distance and angle which are interrogated. One approach is to aperture the sputtering so that the laser intercepts only a specific angle at a specific distance (8). In such a system, all fluorescence generated is collected, usually by two hemispherical collecting mirrors (21). In this design, angular distributions are determined by rotation of the sample about the fixed aperture (22); thus, the incident ion beam angle continually changes as the sample is rotated. An alternative experimental arrangement is shown in Fig. 1. Here, no effort is made to restrict the intersection of the sputtered atonlS and the laser. Instead, the fluorescence is imaged by light collection optics, and thus, the volume probed is controlled by slits in front of the detector. In this manner, translation of the sample with respect to the probe volume is used to alter the angular distribution (9). In all LFS experiments, the fluorescence is imaged onto a photomultiplier for detection. As noted above, it is not necessary to have high resolution discrimination of the light which is collected since the laser is exciting only the atoms of interest. It is necessary to suppress stray light, particularly scattered probe laser light. This can easily be accomplished by a low resolution spectrometer or by an interference filter which transmits the fluorescing wavelength. It is also desirable to gate the detector in order to discriminate against noise. Photon counting techniques are generally employed although sonletimes gated charge digitizing or transient digitizing have been employed. 7.3 SUMMARY OF DATA
There are a number of distinct types of information that can be derived from LFS experinlents of sputtered species nlaking such measurements extremely useful for a variety of surface studies including depth of origin of sputtered species, surface chemistry, and electronic excitation of sputtered species. Using LFS, one can determine the number of sputtered atoms (sputtering yield) and the velocity distribution of the sputtered species. The development of the Sigmund-Thompson model (23-24) to explain the yield and kinetic energy distribution of atoms ejected from an ion-bombarded surface has allowed LFS measurements to be related to the fundamental mechanism involved in ion solid interactions. Because LFS measurements are state specific, these experinlents enable one to probe sputtering yields for various atomic levels, to identify the dominant as well as minor species ejected during sputtering, and to observe molecules which are sputtered. From sputtering yields, modification of surfaces composition can be monitored since sputtering is particularly sensitive to the top most layer of a sample (25-26). Also, measurements of velocity distributions can be related to surface chemistry through the binding energy of the surface. 7.3.1
Sputtering Yields
The substantial contribution the LFS technique has had to the understanding of the sputtering process is best exemplified by measurements of the sputtering yield. Because
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
117
they are state specific, LFS experiments demonstrate that the bulk of the sputtered flux from clean metal surfaces consists of neutral ground state atoms. While this is not a particularly surprising result, prior to the application of LFS to the sputtering process, no method for discriminating neutral ground state atoms from the balance of the neutral flux had been devised. The state specificity of LFS has allowed researchers to determine the relative population of low lying (0-1 eV) excited electronic states of atoms that are produced during sputtering. Light emission from sputtered atoms had demonstrated that metastable states (1-4 eV) were populated to some extent during sputtering. However, only with LFS has it been possible to determine in a systematic way populations in low lying states (2). A prime example is Fe where the relative population of the five-fine-structure levels of the ground state and the low-lying a 5 F state has been determined (20,27-28). These levels span an energy range up to 1 eV above the ground state and have been found to have significant populations which decrease with increasing energy. A sunlmary of available data for Fe is given in Table 1. Various authors have reported data on other elements (11,16,29-30) all of which is plotted in Fig. 2. The relative populations can be roughly characterized by Boltzmann distributions with a range of 350 - 1000 0 K for finestructure components of the ground state and up to 2000 0 K for other low lying excited states depending upon the particular element. The surface of the targets is near room temperature for all of these experiments, and thus, the "temperatures" quoted above do not actually reflect thermal equilibrium. Various explanations for the observed distributions in populations have been proposed, none being wholly satisfying (31- 32). Table 1: Relative population of the a 5D and a 5F multiplet levels in sputtered Fe and the
energy separation of these levels from the ground state.
Multiplet
Energy
Level
(eV)
a 5D
4
0.000
a 5D
3
a 5D a 5D
Fractional Population (ref.25)
(ref.20)
(ref.26)
0.56
0.68
0.61
0.052
0.24
0.20
0.22
2
0.087
0.12
0.062
0.090
t
0.11
0.054
0.028 0.016
a 5D o
0.12
0.016
a 5F 5
0.86
0.0060
a 5F 4
0.91
0.0037
a 5F 3
0.95
0.0023
a 5F 2
0.99
0.0013
a 5F
1.01
0.0007
t
118
Handbook of Ion Beam Processing Technology
•
Ti [27}
•
Ti [28]
• •
Zr[16]
o
a
Fe [25]
Q)
0
Fe [20]
•
Fe [26]
c::
.2 ~
10. 1
U[11]
~
C-
0.~
~ Q)
a: "C Q)
10. 2
.E
C)
·cu
;:
• • 10. 3 0.0
2000 K
• 0.5
1.0
1.5
Energy (eV) Figure 2: Plot of the relative populations of excited state atoms as a function of their energy above the ground state.
The task of determining absolute yields was addressed early in the development of LFS of sputtered species (12). This was accomplished by combining relative yields for all populated electronic levels of an element with an absolute value for the total sputtering yield. A detection limit for the LFS experiments could then be calculated based on the dimensions of the volume being probed and the collection efficiency of the detector system. In this manner, detection limits of 102 atoms/cm3 and 104 atoms/cm3 have been reported for visible cw (33) and pulsed UV (34) experiments. An experimentally simpler nlethod which has proven useful is to calibrate the detection efficiency of the LFS apparatus by measuring Raleigh scattering when a rare gas is introduced into the vacuum chamber and then to use known spectroscopic data to calculate branching ratios for the fluorescing level (35). This technique has proven to be just as accurate and does not require knowing or measuring the absolute sputtering yield. 7.3.2 Velocity Distributions
The application of Doppler-shifted LFS techniques to sputtered species allowed for the first time precise determination of the kinetic energy distribution of individual atomic states (10,12). Since those first experiments, velocity distributions of more than a dozen
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
119
elements have been measured including C (13), Li (36), Na (10,37-39), Al (40), Ca (41), Ti (29,42), Cr (41,43), Fe (12,19-20,44-45), Zn (14), Zr (16), Ba (46), Sm (38), and U (11). Again Fe will be used as an example of a typical system. Fig. 3 shows the velocity distributions of sputtered Fe for all five ground state fine structure levels. This data was obtained by bombarding a pure Fe target at normal incidence with 3 keY Ar+, while probing the sputtered density normal to the surface. Each data point is an average of 100 laser shots. Since the repetition rate of the experiment is 40 Hz, the time required to collect the data for each distribution in Fig. 3 is only about 5 minutes. The resolution of the laser is estimated to be 250 MHz (UV) which yields a velocity resolution of ~ 70 m/s.
z
o ~
.....J
:::>
a.. o a..
Figure 3: Velocity distribution of all five fine structure levels of the Fe ground state sputtered from a pure Fe target nornlal to the surface by 3 keV Ar+ as determined by Doppler-shifted LFS (20).
00
cPCO o
w
>
~
.....J UJ ~
-4
o
4
8
12
16
VELOCITY/kms -1 The solid line in Fig. 3 is a least squares fit to the data using the Sigmund-Thompson model for particles ejected normal to the surface, (2)
120
Handbook of Ion Beam Processing Technology
where g(u) is the number density distribution and the reduced velocity is u = V/Vbinding. In the fit, the binding velocity is calculated from the sublimation energy (4.315 eV), and the exponential factor, n, is assunled to be 2. Thus, only the signal height and base line were adjusted for the fit. The excellent agreement between the data and the fit is indicative of the model's validity. As can be seen in Fig. 3, all five levels appear to have the sanle energy distribution. This appears to be the trend for all clean metal targets. Besides Fe (20,47), velocity distributions of excited state neutral species of Ti (30,40), Ba (46), Ca (48), and Zr (16) have also been measured. Results from these also agree with the linear cascade model of Sigmund and Thonlpson. More recently, experiments to explore the angular variation of the velocity distribution have been undertaken. As yet, very few systems have been studied by LFS. It is worth noting that instruments employing resonant ionization instead of LFS have a distinct advantage in measuring angular distributions since all angles can be sampled at one time (49). The quality of data from LFS studies is high and can be put to good use, if the time and patience is available to map out the distributions one angle at a time. Angular distribution measurements deviate from the Sigmund-Thompson model and so this is a likely area where future effort will be directed. The variation of particle density with angle is predicted to be cosine. The measured angular distribution for Fe has recently been nleasured and found to deviate considerably from a cosine distribution at higher incident particle energies (28,50) as shown in Fig. 4. The observed "overcosine" result appears to be in agreement with recent refinements to the original linear cascade model (51).
•
3 keV
•
Figure 4: The relative yield of sputtered ground state Fe atoms as a function of ejection angle for normal incidence Ar+ at 1 and 3 keY (28).
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
121
Similar phenomenon come into play when the velocity distribution as a function of angle is measured. An example of such data is given in Fig. 5 where the velocity distribution of Fe atoms sputtered fronl a Fe (110) single crystal are plotted for two of the ground state spin orbit split components (J =4 and J =0) at two ejection angles (30 0 and 83 0 ) (9). In Fig. 5 the solid lines are fits to the data using the Sigmund-Thompson model, taking into account broadening due to the laser bandwidth and laser power. As can be seen from the data, at angles that are nearly parallel to the surface, the velocity distributions have a "hot" tail compared to the model. Recently, this result for Fe in the J =4 ground state was reproduced by a second research team, and an attempt to explain the tail based on an insufficiently developed collisional cascade was proposed (50).
1.0
1.0 5
Fe a D..
?-
e = 83°
0.8
oE
~
toE ~
0.8
0
0.6
.-Vi~
0.6
W
0.4
~
0."
.-Vi~ Z .-6
Z
~ 0.2
0.2
-..
0.0 -3
2
VELOCITY/kms
0
Z
.I.U
12
15
18
12
15
18
aSD o
e = 83°
0.8
.E
~
9
1.0
Fe
.-Vi~
6
VELOCITY/kms -1
1.0
t-
3
-I
t~
0.6
0.8
.E 0
~
0.6
t:: V) Z
W
0.<4
~
~
:-.,
....
-. ....
0.2
., ..
"
0.4
0.2
-11--..-.
,.~
, ....................
0.0
0.0
-2
-3
2
VELOCITY/kms
-I
3
6
9
VELOCITY/kms -1
Figure 5: Velocity distribution of the J =0 and J =4 spin orbit split components of the ground state Fe atoms (a5D J ) ejected at 30 0 and 83 0 from a pure Fe target during 3 keY Ar bombardnlent (9). 7.3.3 Oxide Coverage and Adsorbates
One of the most interesting and useful phenomena associated with secondary ion mass spectrometry (SIMS) is the increase in ion yields observed when a metal surface is covered with oxygen. Absolute yields of secondary ions increase significantly with oxygen
122
Handbook of Ion Beam Processing Technology
partial pressure and sometimes can become a substantial fraction of the total yield. Clearly, when this happens significant reductions in the neutral yields nlust result. This effect has been studied using LFS by directly probing the neutral yields during oxide formation. The systems that have been studied include Na (38), Al (40), Ca (48, 52), Ti (29-30,40,53), Cr (48, 52, 54-55), Fe (56), Zr (52), and Ba (46). Both yields of neutrals (29-30, 40, 46, 48, 52-56) and velocity distribution (30, 38, 40, 46, 48, 54) as a function of oxygen partial pressure have been determined. For oxygen covered surfaces, the ground state neutral yield is observed to decrease while the mean kinetic energy of the sputtered metal atoms has been found to increase. These results have been attributed to a change in the binding energy, caused by the formation of the oxides. A difficulty associated with measurements of oxidized surfaces is determining the extent of the coverage as opposed to simply correlating changes in ejected particle density with oxygen dose. It has been shown that Auger electron spectroscopy (AES) can be combined with LFS of sputtered particles in order to obtain such results (29, 53). For the nleasurenlents to be nleaningful, it is necessary to perfornl the LFS experiments in the static mode. That is, the ion dose is maintained sufficiently low so that the probability of individual ions striking any part of the surface damage by previous ions is nearly zero. Generally speaking, doses that remove less than 1 % of a monolayer are small enough to assure meaningful static mode data. While velocity distributions have yet to be obtained when operating in the static mode, sputtering yields have (29, 53). An example of the type of results which can be obtained is given in Fig. 6.
150
o to
1
Static LFS
MONOLAYER
~
fj.
aJF 2
X
aJF
[J
aJF 4
(/)
~
-+-
·c
100
fu~
:J
1 to 2 MONOLAYERS
-ci L a "'--/ u ~
>-
J
~xx
50
0
QJ
I-
o
0
3keV Ar +
a 0
0.5
1
1.5
O/Ti (Auger Signal Ratio) Figure 6: Relative yields for the three ground state fine-structure levels of Ti as a function of O/Ti AES signal (29).
Recently, the effect that coverages of small molecules other than oxygen have on the sputtering process have been studied. Nitrogen on Al and Ti (40), and nitrogen, carbon, CH 4 , NH 3 , and SF 6 on Cr (54-55) have been investigated. Similar to oxygen, all of these
Laser-I nduced Fluorescence as a Tool for the Study of Ion Beam Sputtering
123
species suppress sputtering and also shift the velocity distribution to higher energies. Specific chemical interactions and/or compound formation appear to correlate with the magnitude of the shifts and suppression of sputtering yields. In the future, it is likely that these types of studies will be expanded to include precise knowledge of the extent of surface coverage, the surface composition, and the surface structure in order to better understand the details of the sputtering process in these complex systems. 7.3.4 Sputtering of Alloys and Nonmetallic Compounds
In addition to adsorbed species, the effect of substrate composition on sputtering yield and kinetic energy of ejected particles has been investigated. Early on, LFS investigations revealed that Fe atoms sputtered from type 316 stainless steel were ejected with a higher mean kinetic energy than Fe from a pure Fe target [12]. More recently, sputtering of Cr from alloys has been compared to LFS results for a pure Cr target (54). The velocity distributions all show a shift in the mean kinetic energy which has been attributed to changes in the binding energy in the alloys. Yields also show changes. The yield for Fe sputtered from type 316 stainless steel and from inconel (44) and the yield for Cr from A4 stainless steels (54) have been determined. Results indicate that the yield is proportional to the concentration of the element in the alloy. It is important to recognize that, since the so-called "matrix effect" of SIMS is not very important for neutral yields, changes in the sputtering yield in LFS studies tend to reflect variations in surface concentration of that species. Bulk concentrations are reached only after bombardment with much larger ion doses. The significance of these findings has recently become more apparent with the reporting of experiments which have demonstrated that approximately 70% of all sputtered atoms originate from the topmost layer of the surface (25-26). Thus, LFS nleasurements of alloys can determine surface concentrations with monolayer resolution when the experiment is conducted in the static mode regime. An area which will certainly receive much attention in the future is LFS of sputtered particles from nonmetallic targets. Some of the first LFS experiments applied to the study of sputtering used sodiunl salts as the target material. In these experiments both NaCI (37,39) and Nal (10,21) were employed. The importance of extending LFS to the study of sputtering of crystalline material is in exploring variations of sputtering yields and binding energies. One of the comprehensive studies in this area was performed by Husinsky et al. (54-55). In a series of experiments, the velocity distribution and sputtering yield for Cr from Cr, Cr203 and Cr3C 2 has been examined. These experiments were conducted with and without oxygen and carbon coverages. Since binding energy can be determined from the sputtering distribution, it is possible to probe the chemical nature of the surface in a very precise manner. The newest area of LFS sputtering research is the extension of the method to molecules. To the authors' knowledge, only one system has been studied to date. In a series of experiments, the rotational and vibrational energy distribution of S2 molecules sputtered from a sulfur and frozen CS2 targets during Ar+ bombardment has been measured by LFS and time-of-flight techniques (57). Fig. 7 shows an exanlple of the type of data which has been obtained. With both rotational and vibrational degrees of freedom available for energy transfer during the ejection process, mapping the energy flow during sputtering is much more complex. However, the increased amount of information leads
124
Handbook of Ion Beam Processing Technology
to a much more detailed understanding of the sputtering process. For example, based on the LFS data for S2 , it has been concluded that the ejection mechanism for S2 is a double collision at the surface. 1.0 c: .Q
c;
"5
0.8
co
a-
lii
o o
380-750 meV 47-750 meV
A
9-21 meV
-1500K
0.6
c: .Q
(a)
c;
.0
0.4
:> G)
.~
c;
0.2
CD
a:
0.0
0
1
234
Vibrational Quantum Number
1600
g
1400
Q)
1200
:; c; (b)
Qi
a. E
1000 800
Q)
I-
lii
c: .Q
1U 0 a:
600 400
----+--
----------.-
200 0
0
1
2
380-750 meV 47-750 meV 9-21 meV
3
4
Vibrational Quantum Number Figure 7: Vibrational population and rotational temperature of S2 sputtered during Ar+ bornbardnlent as a function of the vibrational level (57). The data is given for three different velocity intervals selected by time-of-flight. The solid line in (a) is a Boltzmann distribution of 1500 o K. 7.4 CONCLUSION
In this review, the authors have tried to give the reader a taste for the types of information that can be deduced from LFS sputtering experiments and the directions future work might take. In addition, detailed information on the experimental technique has been supplied with the goal that a researcher could use this text to help design a LFS apparatus for studying the sputtering process.
Laser-Induced Fluorescence as a Tool for the Study of Ion Beam Sputtering
125
Work supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-Eng-38.
7.5 REFERENCES
1.
M. W. Thompson, Nucl. Instrum. Methods B18: p. 411 (1987).
2.
G. Betz, Nucl. Instrum. Methods B27: p. 1 (1987).
3.
H. Oechsner and W. Gerhard, Surf. Sci. 44: p. 480 (1974).
4.
R. V. Stuart, G. K. Wehner, and G. S. Anderson, J. Appl. Phys. 40: p. 803 (1969).
5.
Laser Spectroscopy, W. Demtroder, Springer Series in Chern. Phys. 5, SpringerVerlag, Berlin, 1982.
6.
H. Bay, Nucl. Instrum. Methods B18: p. 430 (1987).
7.
D. M. Gruen, M. J. Pellin, C. E. Young, and W. F. Calaway, J. Vac. Sci. Technol. A4: p. 1779 (1986).
8.
W. Husinsky, J. Vac. Sci. Technol. B3: p. 1546 (1985).
9.
C. Young, M. J. Pellin, W. F. Calaway, and D. M. Gruen, Conf. Dynamics of Molecular Collisions, Snowbird, UT, July 14-19, 1985.
10.
W. Husinsky, R. Bruckmuller, P. Blum, F. Viehbock, D Hammer, and E. Benes, Appl. Phys. 48: p. 4734 (1977).
11.
R. B Wright, M. J. Pellin, D. M. Gruen, and C. E. Young, Nucl. Instrum. Methods 170: p. 295 (1980).
L.
12. A. Elbern, E. Hintz, and B. Schweer, J. Nucl. Mater. 76&77: p. 143 (1978). 13. P. Bogen, H. F. Dobele, and Ph. Mertens, J. Nucl. Mater. 145-147: p. 434 (1987), 14. H. F. Arlinghaus, W. F. Calaway, C. E. Young, M. J. Pellin, D. M. Gruen, and L. L. Chase, Proc. 19th Symp. Optical Materials for High Power Lasers. Oct. 26-28, 1987, Boulder, CO. 15. W. L. Glab and J. P. Hessler, Appl Optics 26: p. 3181 (1987). 16. M. J. Pellin, R. B. Wright, and D. M. Gruen, J. Chern. Phys. 74: p. 6448 (1981). 17. V. M. Bermudez, J. W. Hudgens, and M. A. Hoffbauer, Appl. Optics, 23: p. 3638 (1983). 18. M. Littman and J. Montgomery, Laser Focus/Electro-Optics 24, 70 (1988). 19. C. E. Young, D. M. Gruen, M. J. Pellin, and W. F. Calaway, Fusion Technology 6: p. 434 (1984). 20. C. E. Young, W. F. Calaway, M. J. Pellin, and D. M. Gruen, J. Vac. Sci. Technol. A2: p. 693 (1984). 21. R. Bruckmuller, W. Husinsky, and P. Blum, Proc. 7th Inter. Vac Congr. & 3rd Intern. Conf. Solid Surfaces, Vienna, p. 1469 (1977). 22. W. Berres and H. L. Bay, Appl. Phys. A33: p. 235 (1984). 23. P. Sigmund, Nucl. Instrum. Meth. B27: p. 1 (1987). 24. M. W. Thompson, Philos. Mag. 18: p. 377 (1968).
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Handbook of Ion Beam Processing Technology
25. M. F. Dumke, T. A. Tombrello, R. A. Weller, R. M. Housley, and E. H Cirlin, Surf. Sci. 124: p. 407 (1983). 26. J. W. Burnett, J. P. Biersack, D. M. Gruen, B. Jorgensen, A. R. Krauss, M. J. Pellin, E. L. Schweitzer, J. T. Yates, and C. E. Young, J. Vac. Sci. Technol. 6: p. 2064 (1988). 27. B. Schweer and H. L. Bay, Proc. 4th Intern. Conf Solid Surfaces, Cannes, Vol. II, p. 1349 (1980). 28. Y. Matsuda, Y. Yamamura, Y. Ueda, K. Uchino, K. Muraoka, M. Maeda, and M. Akazaki, Jap. J. Appl. Phys. 25: p. 8 (1986). 29. M. J. Pellin, C. E. Young, M. H. Mendelsohnl, D. M. Gruen, R. B. Wright, and A. B. Dewald, J. Nucl. Mater. 111&112: p. 738 (1982). 30. E. Dullni, Appl. Phys. A38: p. 131 (1985). 31. D. M. Gruen, M. J. Pellin, C. E. Young, and M. H. Mendelsohn, Physica Scripta. T6: p. 42 (1983). 32. R. Kelly, Nucl. Instrum. Meth. 209/210: p. 509 (1983). 33. W. Husinsky, R. Bruckmuller, and P. Blum, Nucl. Instrum. Methods 170: p. 287 (1980). 34. B. Schweer, D. Rusbuldt, E. Hintz, J. B. Roberto, and W. R. Husinsky, J. Nucl. Mater. 93&94: p. 357 (1980). 35. C. E. Young, M. J. Pellin, D. M. Gruen, and J. H. Norem, J. Appl. Phys. 53: p. 4726 (1982). 36. R. P. Schorn, H. L. Bay, E. Hintz, and B. Schweer, Appl. Phys. A43: p. 147 (1987). 37. M. L. Yu, D. Grischkowsky, and A. C. Balant, Appl. Phys. Lett. 39: p. 703 (1981). 38. R. Bruckmuller, W. Husinsky, and P. Blum, Rad. Effects 45: p. 199 (1980). 39. W. Husinsky, R. Bruckmuller, Surf. Sci. 80: p. 637 (1979). 40. E. Dullni, Nucl. Instrum. Meth. B2: p. 610 (1984). 41. W. Husinsky, I Girgis, and G. Betz, J. Vac. Sci. Technol. B3: p. 1543 (1985). 42. H. L. Bay, B. Schweer, P. Bogen, E. Hintz, J. Nucl. Mater. 111&112: p. 732 (1982). 43. W. Husinsky, G. Betz, I Girgis, and F. Vjiehbock, J.Nucl. Mater. 128&129: p. 577 (1984). 44. E. Hintz, D. Rusbuldt, B. Schweer, J. Bohdansky, J. Roth and A. P. Martinelli, Nucl. Mater. 93&94: p. 656 (1980).
L.
45. C. Honda, M. Maeda, K. Nishimura, K. Muraoka, and M Akazaki, Appl. Phys. B33: p.1 (1984). 46. D. Grischkowsky, M. L. Yu, and A. C. Balant, Surf. Sci. 127: p. 315 (1983). 47. B. Schweer and H. L. Bay, Appl. Phys. A29: p. 53 (1982). 48. W. Husinsky, G. Betz, and I. Girgis, J. Vac. Sci. Technol. A2: p. 698 (1984). 49. J. P. Baxter, G. A. Schick, J. Singh, P. H. Kobrin, and N. Winograd, J. Vac. Sci. Technol. A4: p. 1218 (1986). 50. Y. Matsuda, S. Matsubaguchi, C. Honda, M Maeda, T. Okada, Y. Yamamura, K. Muraoka, and M Akazaki, J. Nucl. Mater. 145-147: p. 421 (1987).
Laser-I nduced Fluorescence as a Tool for the Study of Ion Beam Sputtering
127
51. K. T. Waldeer, and H. M. Urgassek, Nucl. Instrum. Meth. B18: p. 518 (1987). 52. G. Betz and W. Husinsky, Nu£LlD.s.trum.
M~.tb.~
B13: p. 343 (1986).
53. M. J. Pellin, C. E. Young, D. M. Gruen, Y. Aratono, and A. B. Dewald, Surf. Sci. 151: p. 477 (1985). 54. W. Husinsky, P. Wurz, B. Strehl, and G. Betz, Nucl. Instrum. Meth. B18: p. 452 (1987). 55. W. Husinsky and G. Betz, Nucl. Instrum. Meth. B15: p. 165 (1986). 56. R. Behrisch, J. Roth, J. Bohdansky, A. P. Martinelli, B. Schweer, D. Rusbuldt, and E. Hintz, J. Nucl. Mater. 93&94: p. 645 (1980). 57. R. De Jonge, K. W. Benoist, J. W. F. Majoor, A. E. De Vries, and K. J. Snowdon, Nucl. Instrum. Meth. B28: p. 214 (1987).
8 Characterization of Atoms Desorbed from Surfaces by Ion Bombardment Using Multiphoton Ionization Detection
David L. Pappas, Nicholas Winograd and Fred M. Kimock
8.1 INTRODUCTION
Ion beams are now extensively utilized for probing the structure and composition of solid surfaces and interfaces. For example, with secondary ion mass spectrometry (SIMS), the charged component of the material desorbed from the surface by an ion beam is extracted into a mass spectrometer and analyzed. Recently, it has beconle evident that there are a number of advantages to be gained by measuring the sputtered neutral flux. First, the sensitivity of the measurement is potentially increased since the fraction of ejected neutrals is usually quite large conlpared to the corresponding ion fraction. Second, the ion yield has been shown to be quite variable and highly influenced by the sample matrix. The yield of neutral particles, however is predicted to be far less susceptible to these effects, making quantification more straightforward. Finally, many theories of particle/surface interactions require information about neutral species. Careful examination of such species may lead to a better understanding of the basic features of the desorption phenomenon. In this chapter, we exanline the idea of sampling the desorbed neutral particles using multiphonon resonance ionization (MPRI) (1). In particular, we shall consider how this method preserves the inherent advantages of sputtered neutral particle analysis. For exanlple, the ultinlate sensitivity is a direct function of the efficiency of MPRI in collecting and counting the abundant neutral atoms. Additionally, by decoupling particle desorption from ionization, quantitative assessments are more easily made. These features are illustrated by the results of a variety of experiments which demonstrate the potential and applicability of MPRI. To detect these neutral particles, a laser beam is passed through the cloud of ejecting material as shown in Fig. 1. The laser light is tuned to induce a transition between two bound electronic states of the species of interest. The excited target atom is subsequently
128
Characterization of Atoms Desorbed from Surfaces
129
ionized by an additional photon and then collected. Using conlmercially available pulsed laser systems, MPRI can be applied to every element in the periodic table, except He and Ne (2).
Nd: YAG
TO
TOFMS
POL
POL
UHV WEX
Figure 1: Schematic of the basic experiment for analysis of desorbed neutral particles using multiphoton resonance ionization (MPRI). PDL: pulsed dye laser; HG: harmonic generator; WEX: wavelength extension system; Be: beanl conlbiner. While improvenlents in quantification are also available to other methods of neutral post-ionization, the ionization efficiency of MPRI is much greater. In fact, with sufficient photon intensity, every atom which enters the laser beam can be converted into an ion. Furthermore, since the ionization is selective to one species, mass spectral interferences are removed, allowing for the employment of mass spectrometers which emphasize efficiency, rather than resolution. An alternative approach has been developed however, in which high-powered laser light is used to non-selectively ionize the desorbing particles. A discussion of this technique, simply referred to as multiphoton ionization (MPI), is also included in this chapter.
It should also be noted that although the lack of detailed knowledge of molecular spectroscopic transitions makes MPRI analyses of molecules more complex, it has been successfully demonstrated (3) and should find many practical applications. 8.2 ANALYTICAL APPLICATIONS
To date, a substantial research effort has focused on developing MPRI of sputtered neutrals as a quantitative surface analysis tool. This has been stimulated by the potential
130
Handbook of Ion Beam Processing Technology
gains in sensitivity and quantification over other analytical techniques. A schematic of an MPRI apparatus designed to exploit this potential is shown in Fig. 2 (4). Briefly, a high current (60 }LA, 10 KeV) ion pulse (2-5 }Ls) is directed onto a target, causing removal of a fraction of the surface material. A few hundred nanoseconds later, a pulsed laser is fired so that the light intersects the largest possible fraction of the ejecting particles. The photoions are subsequently extracted into a tinle-of-flight (TOF) mass spectrometer equipped with an ion reflector (5). The pulsed nature of the experiment facilitates its combination with the high transmission TOF device, thus maximizing efficiency throughout the systenl. In addition, the low duty cycle keeps the total amount of material removed in a typical experiment in the monolayer regime, thus minimizing perturbation of the substrate by the analysis. Due to the photoionization selectivity, low resolution mass spectrometers are often sufficient, however the use of the reflector makes isotopic analyses possible. sample transfer system pulse/steering plates
DPIO-OI duoplasmatron ion source
r
Ar gas inlet
quadrupole mass spectrometer
"-- ......"
ion reflector ----..:--_ _
\
"' ... _... .-''
I
,
\ I
I
de'flectors
Figure 2: Schematic of the apparatus enlployed by Winograd (fronl ref. 4) for MPRI. This instrument features a high current ion source (5U60 }LA Ar+, 10 keY) and ion reflecting time-of-flight mass spectrometer.
Using this apparatus, a number of experiments have been performed to elucidate the capabilities of MPRI. The measured signal is dependent upon six factors: ion current, duty cycle, sputter yield, concentration, useful fraction and detection efficiency (6). Of particular interest is the useful fraction, which is defined as the number of countable particles relative to the total quantity of sputtered material. In SIMS, this is simply the ion fraction, but for MPRI it is the fraction of atoms desorbed in the particular electronic state
Characterization of Atoms Desorbed from Surfaces
131
of interest. Sputtering of atoms in a variety of metastable energy states could provide a mechanism for signal loss. Fortunately, it has been found that the ground state usually contains the largest fractional population of ejected neutral species. The electronic partitioning of sputtered atoms has been the subject of several studies (3,7,8). In Table I, the results of measurements on clean and air-exposed Fe foil substrates are presented. Note that the data are for ground state neutral atoms, monomeric ions and the sum of the excited atoms in the ground state multiplet. It is clear that for both the clean and airexposed solids, most of the surface atoms are ejected as ground state neutrals thus providing some indication of the relative sensitivity factors of SIMS and MPRI. For many elements, the neutral fractions are substantially greater than those observed for Fe. It should also be noted that rarely is there an appreciable excited neutral population found outside the ground state manifold (9). These results provide a valuable comparison of the effects of the sample matrix on sputtered particle analysis. Such matrix effects are commonly encountered in SIMS, manifested by secondary ion intensities which vary strongly with the san1ple composition, thereby limiting efforts at quantification. This can also be seen in the data of Table 1. Note that upon cleaning, the Fe+ intensity actually decreases despite the fact that the ion beam is sampling a greater number of Fe aton1S. Since the exact surface stoichiometry was not known, it is difficult to judge the absolute accuracy of the data, however the trends for the Fe O yields seem to be at least qualitatively correct.
Table 1: Fractional Monomer Population of Fe Sputtered from Clean and Air-Exposed
Fe Foil.
species
air-exposed intensity
fraction
clean intensity
fraction
Fe
2.36
0.337
18.4
0.512
Fe tot
2.65
0.378
16.2
0.451
Fe+
2.00
0.285
1.30
3.62 x 10- 2
We also report here new data on sputtered useful fractions obtained from a nonmetallic substrate. The method of data collection is identical to that which has been previously described (3). The yields obtained for Gao, Ga* and Ga+ desorbed from GaAs (111) are sumn1arized in Table 2. Once again, the sputtered neutral fractions for both matrices are larger than the corresponding ion fractions. Note also that all signals drop to some extent upon cleaning, which is not predicted. The reduction in the Gao intensity is small and may be attributed to the effects of differential sputtering. The large decrease for the Ga+ ions however, can only be the result of a significant change in ionization probability.
132
Handbook of Ion Beam Processing Technology
Table 2. Fractional Monomer Population of Ga Sputtered from Clean and Air-Exposed
GaAs (111).
species
Ga+
air-exposed
clean
intensity
fraction
intensity
fraction
81.6
0.451
77.7
0.550
81.0
0.447
61.9
0.438
18.4
0.102
1.71
1.21x10-2
Another useful comparison between SIMS and MPRI can be made from the data displayed in Fig. 3. For the clean surface, the ground state InO intensity is several orders of nlagnitude greater than the In+ ion signal. When the surface is carefully dosed with oxygen, the signals converge to a steady state condition in which the sample composition corresponds roughly to In203. At this point, the summed neutral yield has dropped to 40% of its original value, while the In+ intensity has increased by a factor of '" 200. Although the ground state neutral yield does not exactly correspond to In .... In203' it does closely parallel the true surface chemistry, while the secondary ion yield responds in a counter-intuitive nlanner (10). Real analyses via MPRI have been highly successful for many materials. For example, Parks et al. have made several determinations for semiconductor dopant concentrations. Using data obtained from an analysis of Ga in Si, they report a detection limit of 2 partsper-billion (11). The linearity of MPRI is demonstrated in Fig. 4, in which the results of nleasurenlents of V in NBS steel samples are presented (11). While quantitative SIMS usually requires a set of standards which closely parallel the compositions of the matrices containing the sample, the requirements for MPRI are less stringent. Demonstrating this, trace concentrations of B in two different substrates were determined with no appreciable matrix effect observed (12). The apparatus for these experiments now features a O.lmm diameter ion beam which is rasterable over a 1mm x 2mm domain, thus allowing the measurements of depth distributions (13). This MPRI instrument has subsequently been used to measure the diffusion coefficient of Ti in the electro-optic material LiNb0 3, via the analysis of depth profile data (14).
Characterization of Atoms Desorbed from Su rfaces
133
~
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~ 0.75
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Figure 3: Variation of MPRI and static SIMS intensities with oxygen exposure. (0) In+ secondary ion intensity; (x) intensity of ground state In atoms, and (0) intensity of lowest In excited state neutral atonlS as deternlined by MPRI (from ref 10).
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Figure 4: Results of analyses of several NBS steel samples. Vanadiunl was detected using MPRI with the apparatus employed by Parks (from ref. 11).
The present measured detection limit for resonant post-ionization is reported at 500 parts-per-trillion for 54Fe in Si (Fig. 5), obtained using an apparatus which features a high transmission time-of-flight system, equipped with two energy analyzers for background
134
Handbook of Ion Beam Processing Technology
reduction (15,16). Eliminating the background appears to be the prime consideration and is probably the key to the present quoted limits. The largest contributor to the background is spurious arrival of secondary ions. These can often be removed on the basis of their excess energy over the photoions, as was the case for the apparatus which yielded the data in Fig. 5 and the ion reflector shown in Fig. 2. The use of accelerating pulses to give these secondary ions an even larger dose of energy has also been successfully employed (16,4). In addition, it should be noted that low-level nonresonant ionization can occur concomitantly with MPRI. This may become significant in the case of trace analysis, providing an additional requirement for mass resolution. A careful choice of laser scheme, allowing for low energy but high power in the ionization step, can minimize this problem. With the background at a minimum, calculations indicate that sensitivities to even lower concentrations are well within reach.
800~--------------------.
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60 keY Fe+ dose lOll atoms/cm 2
100 .J:J
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---------
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_ 54 Fe detection limit
----
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_
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o
200
400 DEPTH
600
(nm)
Figure 5: MPRI depth profile of 56Fe implanted into Si(lll). An implantation dose of
1011 atoms/cm2 (60 keY) was used (from ref. 15). 8.3 ENERGY AND ANGLE MEASUREMENTS
Fundamental studies of the ion/solid interaction using MPRI with single crystal substrates have proven to be quite valuable for understanding the collisions of low to medium energy particles with surfaces. The scientific gains of such experiments are urgently needed in a number of disciplines ranging from the elucidation of plasma processes and catalytic mechanisms to ion etching and the modification of electronic materials. Due to the surface specificity of ion induced desorption, SIMS has been directed toward structural considerations for a number of years. Specifically, the energy and angular characteristics of the sputtered flux have been of prime interest due to the predictions nlade by the numerous theories of particle ejection. It has long been known that the angular distribution of desorbing atoms from an ordered substrate is dependent upon the
Characterization of Atoms Desorbed from Surfaces
135
symmetry of the surface (Fig. 6) (17). Using the angle-resolved SIMS technique, these angle dependent yields have been observed for particle ejection from single crystal surfaces. Such experiments, however, require a theoretical comparison which not only models the mechanics of the desorption but also the interactions of the image charge, which alters the trajectory of the departing ion. Thus far, the latter has proven to be difficult.
Figure 6: Illustration of preferred ejection directions of atoms desorbed from a (100) crystal surface (from ref. 17).
To combat these problems, an apparatus has been built which is capable of simultaneous energy-and angle-resolved neutral (EARN) desorbed atom nleasurements ( 18,19). The detection scheme is depicted in Fig. 7. An ion beam pulse is used to remove a small fraction of the surface material. The ejecting neutral species pass through the extraction grid while the secondary ions are repelled. A short time later, a ribbon shaped laser pulse is fired which intersects a slice of this desorbing particle cloud and ionizes the neutral atoms via MPRI. The energies are scanned by systematically varying the time interval between the primary ion and laser pulses. The photoions are then collected onto a spatially resolved detector where they are imaged and counted. From the coordinates of the detection point, the angular trajectory away from the solid can be determined. Note that due to the unique geometry of the EARN experimental configuration, it would be extremely difficult to make such measurements without selective laser ionization. The EARN apparatus has been successfully employed for a number of studies. A three-dimensional intensity map obtained from a clean Rh(lll) surface is shown in Fig. 8 (20). The laser has sanlpled the +30 0 and -30 0 azinluths and a polar angle range of o- 90 0 • The map indicates that the angle and energy distributions are dependent upon one another. Similar results have also been found for sputtering from polycrystalline materials. This is the first observation of such behavior and it is not predicted by the heretofore popular transport theories of sputtering. A more recent theoretical approach has emerged, referred to as classical dynamics, which follows the motion of the individual atonlS within the solid as described by Hamilton's equations. It is notable that the obser-
136
Handbook of Ion Beam Processing Technology
vations of Fig. 8 have been accurately predicted by this treatment (21) (22). A detailed example of the classical dynamics procedure has been presented (17) (22).
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neutral (EARN) desorbed atom distributions (from ref. 19). It has long been of interest to determine the precise location of adsorbed species on single-crystal surfaces. This would be valuable in the field of heterogeneous catalysis. The experimental and theoretical angular distributions of Rh sputtered from Rh( 111) and p(2x2)O/Rh(111) are represented in Figs. 9 and 10, respectively (21-24). Note that in the case of the clean surface, the intensity for the _30 0 azimuth is greater than that for the +30 0 azimuth. This is not consistent with the symmetry of the topmost Rh(111) layer, indicating a role played by the second layer atoms. This has also been confirmed by the classical dynamics treatment. By following the trajectories of individual ejecting atoms, it is found that the peaks along these two directions can be ascribed to channeling of a surface atom between two other surface Rh atoms and that the _30 0 peak is greater in intensity because of a collision from a second layer Rh atom (23).
Characterization of Atoms Desorbed from Su rfaces
137
o 40'
_______
-90...------ POLAR EJECTION ANGLE tdeg)
-300 Azimuth Figure 8: Kinetic energy and polar ejection angle distributions of neutral Rh atoms ejected in the ± 30 0 azinluthal directions. These directions are defined in the inset, in which the open circles represent surface Rh atoms, the filled circles denote second layer Rh atoms and the X marks indicate expected surface adsorption sites (from ref. 20).
Upon exposure to oxygen, both peaks shift toward the normal due to the blocking effect of the oxygen overlayer, however the -30 0 azimuth is more strongly affected. In order to explain this observation, theoretical distributions were generated for oxygen adsorbed in each of three distinct sites. It was found that placing the oxygen in the C-site (directly over a third layer atom) yields results which more closely parallel those of the experinlent than the B-site (directly over a second layer atom) or atop geometries (24). The C-site is the location a Rh atom would occupy in the next surface layer of the solid, if it existed. This is also the adsorbate location predicted by dynamical LEED calculations. However, when the clean Rh(lll) surface is exposed to ethylene p(2x2), a different behavior is observed. It appears that the adsorbed ethylidyne species, C 2H 3 occupies the B-site, but stands tall enough to influence the particle trajectories in both the +30 0 and -30 0 directions (25). These experiments demonstrate the effectiveness of EARN in acquiring an understanding of particle bombardment effects and their relation to surface structure. Other results of this combined experimental and theoretical approach are that oxygen not only serves as a blocking agent, but that it also alters the surface binding energy. In addition, it is indicated that the most probable energy for a sputtered atom is simply the energy cost to remove the atom from the surface, rather than one-half the bulk heat of sublimation, as was previously thought (26). Future work will focus on some of the fundamental physics of sputtering through the study of the relationships of atomic excitation ad
138
Handbook of Ion Beam Processing Technology
ionization to surface structure. This will likely include probing the internal states of sputtered molecules such as NO.
Rh {Ill}
EAM-A
EXPERIMENT
PAIR 5-IOeV
Figure 9: Experimental (center column) and theoretical polar angle distributions of Rh atoms sputtered from the Rh( 111) surface under Ar+ ion bombardment. The theoretical results were generated with the molecular dynamics treatment using pair-wise additive atomic interaction potentials (right column) and a many-body embedded atom potential (left column) (from ref. 22). 8.4 NONRESONANT MULTIPHOTON IONIZATION
Although the discussion to this point has centered on resonant ionization processes, sputtered neutral analysis can also be carried out by nonresonant multiphoton ionization (MPI) (27) (28). The basic geometry of such an approach is quite similar to that which has been discussed and, in fact the same instrument can be used for MPRI as well as MPI. The main difference is that high intensity, nontunable UV light is focused into a small spot 10-3cm2 ) over the sample. The resultant high power densities will induce nonselective ionization of all moieties which enter the beam. (
rv
Characterization of Atoms Desorbed from Surfaces
139
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8 The mass spectra obtained often look very much like SIMS spectra (Fig. 11) (29). The chief advantage of MPI is its generality. No prior knowledge of the sample composition is required as is often the case with MPRI. This is particularly useful when multielement analysis of complex materials is desired. In addition, since this technique also samples the sputtered neutral flux, most of the disadvantages associated with secondary ion analysis are again avoided. The main drawback is that the sensitivity is limited by the small ionization volume. Furthermore, conditions of 100 % ionization are more difficult to achieve than with the resonant approach and variations in the ionization volume are more significant than for MPRI. These effects may make quantification more difficult.
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Handbook of Ion Beam Processing Technology
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CHANNEL NUMBER (10 ns/channel) Figure 11: Portion of a time-of-flight mass spectrum obtained using nonresonant multiphoton ionization (MPI) of atoms desorbed from an NBS copper target. NBS quoted conlposition is 197Au (11 ppm), 209Bi (6ppm), and 206Pb (4 ppm). Peaks at mass 204, 207 and 208 are Pb isotopes while the intensities at mass 194, 195, 196 and 198 are due to Pt contamination (from ref. 29).
Despite these limitations, the useful yields of MPI (number of atoms counted per incident ion) are roughly equivalent to those of SIMS (3). This is demonstrated in Fig. 11, in which the sputtered neutral mass spectrunl obtained from an NBS copper sample is shown (29). The sensitivity is more than sufficient for measuring impurity components in the parts-per-million regime. The MPI method has also been applied to GaAs substrates, yielding Ga and As signals which are on the same order of magnitude, contrary to what is found in SIMS (27). A depth profile, obtained using MPI, of an Al sample implanted with Ti is presented in Fig. 12. This demonstrates one of the many applications of this technique, although it is notable that the ultimate sensitivity of the measurement was reported to be limited by a hydrocarbon isobaric interference (30). 8.5 CONCLUSION
In summary, we have considered the value of studying ion-induced desorbed neutral species. Although there are now several methods available for interrogating these particles, resonant laser ionization has denlonstrated the greatest sensitivity, selectivity and efficiency. This can be critically important for measurements of trace-level impurities present in only the topmost layers of the solid. MPRI has also shown to be an effective method for investigating the basic properties of these desorbing species.
Characterization of Atoms Desorbed from Surfaces
..
141
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SPUTTER TIME (min) Figure 1 2: Depth profile of 48Ti inlplanted into AI, determined by the MPI approach
(from ref. 30).
The approach has led to a more accurate characterization of surface structure and a better understanding of the processes which influence the ejection of particles from ionbombarded surfaces, evidenced by the interesting results obtained using the EARN apparatus. Finally, in cases where selectivity is not a requirenlent or the sample composition is unknown, nonresonant MPI can be used to ionize all species which enter the beam. It is interesting to note that for molecular analysis, ionization usually occurs through bound electronic states and the MPI and MPRI approaches are formally identical. Perhaps an effective approach for some determinations might be to use the focused ultraviolet laser output for simultaneous identification of substrate components and molecular analysis followed by ultra-sensitive MPRI for quantitative and/or trace measurements of a particular species. Acknowledgements
The authors are grateful for the financial support of the National Science Foundation, the Office of Naval Research and the IBM Corporation. We would also like to thank David M. Hrubowchak, Curt T. Reimann and Matthew H. Ervin for their assistance in the laboratory in the preparation of this manuscript.
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Handbook of Ion Beam Processing Technology
8.6 REFERENCES
1. Winograd, N., Baxter, J. P., Kimock, F. M., Multiphoton resonance ionization of sputtered neutrals: a novel approach to materials characterization. Chern. Phys. Lett. 88: pp. 581-584 (1982). 2. Hurst, G. J., Payne, M. G., Kramer, S. D., Young, J. P., Resonance ionization spectroscopy and one atom detection. Rev. Mod. Phys. 51: pp. 767-819 (1983). 3. Pappas, D. L., Hrubowchak, D. M., Ervin, M. H., Winograd, N., Quantitative aspects of surface analysis using multiphoton resonance ionization, submitted. 4. Pappas, D. L., Hrubowchak, D. M., Ervin, M. H., Winograd, N., in preparation. 5. Mamyrin, B. A., Karataev, V. I., Schmikk, D. V., Zagulin, V. A. The mass reflectron, a new nonmagnetic time-of-flight mass spectrometer with high resolution. Sov. Phys. JETP 37: pp. 45-48 (1973). 6. Kimock, F. M., Baxter, J. P., Pappas, D. L., Kobrin, P. H., Winograd, N., Solids analysis using energetic ion bombardment with multiphoton resonance ionization with timeof-flight detection. Anal. Chern. 56: pp. 2782-2791 (1984). 7. Kimock, F. M., Pappas, D. L., Winograd, N., Matrix effects on the electronic partitioning of iron atoms d~sorbed from surfaces by energetic ion bombardment. Anal. Chern. 57: pp. 2669-2674 (1985). 8. Wright, R. B., Pellin, M. J., Gruen, D. M., Young, C. E., Laser fluorescence spectroscopy of sputtered uranium atoms. Nucl. Inst. Meth. 170: pp. 295-302 (1980). 9. Pellin, M. J., Gruen, D. M., Young, C. E., Wiggins, M. D., Electronic excitation of Ti atoms sputtered by energetic Ar+ and He+ from clean and monolayer oxygen covered surfaces. Nucl Inst. Meth. Phys. Res. B18: pp. 771-776 (1987). 10. Kimock, F.M., Baxter, J. P., Winograd, N., Ion and neutral yields from ion bombarded metal surfaces during chemisorption using low dose SIMS and multiphoton resonance ionization. Surf. Sci. 124: pp. L41-L48 (1983). 11. Parks, J. E., Schmitt, H. W., Hurst, G. S., Fairbank, W. M., Jr., in: B.~sonanc~ Ionization Spectroscopy 1984 (G. S. Hurst and M. G. Payne, eds.), pp. 167-174, The Institute of Physics, Boston (1984). 12. Parks, J. E., Beekman, D. W., Schmitt, H. W., Taylor, E. H., Materials analysis using sputter initiated resonance ionization spectroscopy. Nucl. Inst. Meth. Phys. Res. B10/11: pp. 280-284 (1980). 13. Parks, J. E., private communication. 14. Parks, J.E., Spaar, M. T., Cressman, P. J., in: Secondary Ion Mass .fu2ectroscopy YJ, in press.
Characterization of Atoms Desorbed from Surfaces
143
15. Young, C. E., Pelling, M. J., Calaway, W. F., Jorgensen, B., Schweitzer, E. L., Gruen, D. M., Laser-based secondary neutral mass spectroscopy: useful yield and sensitivity. Nucl. Inst. Meth. Phys. Res. B17: pp. 119-129 (1986). 16. Pellin, M. J., Young C. E. Calaway, W. F., Burnett, J. W., Jorgensen, B., Schweitzer, E. L., Gruen, D. M., Sensitive low damage surface analysis using resonance ionization of sputtered atoms. Nucl. Inst. Meth. Phys. Res. B18: pp. 446-451 (1987). 17. Winograd, N. in: Progress in Solid State Chemistry (C. M. Rosenblatt and W. L. Worrell eds), Vol. 13, pp. 285-375, Pergamon Press, Oxford (1982). 18. Kobrin, P. H., Schick, G. A., Baxter, J. P., Winograd, N., Detector for measuring energy- and angle-resolved neutral-particle (EARN) distributions for material desorbed from bombarded surfaces. Rev. Sci. Instrum. 57: pp. 1354-1362 (1986). 19. Baxter, J. P., Schick, G. A., Singh, J., Kobrin, P. H., Winograd, N., Angular distributions of sputtered particles. J. Vac. Sci. Technol. A4: pp. 1218-1221 (1986). 20. Singh, J., Reimann, C. T., Baxtr, J. P. Schick, G. A., Kobrin, P. H., Garrison, B. J., Winograd, N., Detection of neutral atoms sputtered from ion-bombarded single-crystal surfaces Rh(lll) and p(2x2)O/Rh(III): Ejection mechanism and surface structure determinations from energy- and angle-resolved measurements. J. Vac. Sci. Technol. A5: pp. 1191-1193 (1987). 21. Garrison, B. J., Reimann, C.T., Winograd, N., Harrison, D. E., Jf., Energy and angular distributions of Rh atoms ejected due to ion bombardment from Rh(III): A theoretical study. Phys. Rev. B36: pp. 3516-3521 (1987). 22. Garrison, B. J., Winograd, N., Deaven, D. M., Reimann, C. T., Lo, D. Y., Tombrello, T. A., Harrison, D. E., Jr., Shapiro, M. H., Many-body embedded atom potential for describing the energy and angular distributions of Rh atoms desorbed from ion-bombarded Rh(III). Phys. Rev. B37: in press. 23. Winograd, N., Kobrin, P. H., Schick, G. A., Singh, J., Baxter, J. P., Garrison, B. J., Energy- and angle-resolved detection of neutral atoms desorbed from ion bombarded single crystals. Rh(lll) and p(2x2)O/Rh(III). Surf. Sci. 176: pp. L817-L824 (1986). 24. Reimann, C. T., Walzl, K. N., EI-Maazawi, M. S., Deaven, D. M., Single, J., Garrison, B. J., Winograd, N., in preparation. 25. Reimann, C. T., Walzl, K., EI-Maazawi, M., Garrison, B. J., Winograd, N., in: Secondary Ion Mass Spectrometry VI, in press. 26. Garrison, B. J., Winograd, N., Lo, D., Tombrello, T. A., Shapiro, M. H., Harrison, D. E., Jr., Energy cost to sputter an atom from a surface in keY ion bombardment processes. Surf. Sci. 180: pp. L129-L133 (1987). 27. Becker, C. H., Gillen, K. T., Surface analysis of contaminated GaAs: comparison of new laser-based techniques with SIMS. J. Vac. Sci. Technol. A3: pp. 1347-1349 (1985).
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28. Becker, C. H., Gillen, K. T., Nonresonant rnultiphoton ionization as a sensitive detector of surface concentrations and evaporation rates. Appl. Phys. Lett. 45: pp. 1063-1065 (1984). 29. Becker, C. H., Gillen, K. T., Surface analysis by nonresonant rnultiphoton ionization of desorbed or sputtered species. Anal. Chern. 56: pp. 1671-1674 (1984). 30. Becker, C. H., On the use of nonresonant nlultiphoton ionization of desorbed species for surface analysis. J. Vac. Sci. Technol. A5: pp. 1181-1185 (1987).
9 The Application of Postionization for Sputtering Studies and Surface or Thin Fill11 Analysis Hans Oechsner
9.1 INTRODUCTION
The knowledge of the composition and the kinetic properties of the neutral particle flux leaving an ion bombarded solid is of practical importance in modern surface and thin film technology for mainly two reasons: -Corresponding data are necessary input parameters for the control and the optimization of thin film deposition processes by sputtering, -Mass analysis of the ejected particles supplies with direct information of the surface composition and - when combined with controlled sputter removal - of concentration depth profiles in the surface near region or of thin film structures. Since in the majority of all cases the sputtered particle flux consists aln10st exclusively of neutral particles, and the small fraction of secondary ions is subjected to the well known "matrix effects" in a difficult to understand manner, mass and energy analysis of the neutral atoms and molecules removed from a solid surface by ion or neutral particle bombardment promises more quantitative inforn1ation than the analysis of the secondary ions. The most obvious technique for the necessary postionization of sputtered neutrals would be to use an electron beam arrangement as in an residual gas analyzer. Corresponding early investigations succeeded in getting mass spectrometric signals mainly of the neutral atoms sputtered from elemental metal targets (1-3). Recent work on electron beam postionization improved the detection sensitivity down to the 10 ppm range for intense sputter removal and optimization of the geometrical and the ion optical conditions for the transfer of postionized neutrals into a quadrapole mass spectrometer (4). Nevertheless, the postionization probabilities aO also in recent electron beam arrangements (4-5) are estimated to approach at best values around 10 4. This is due in essence to the relatively high kinetic energies of sputtered neutral atoms with average values in the order of 10-20 eV, e.g. the short dwelling time of such particles in the electron beam volume (see sect. 9.3.1).
145
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Handbook of Ion Beam Processing Technology
The application of electrical gas discharges for the postionization of sputtered neutrals has also started more than 20 years ago. W.E. Cooper and coworkers employed a magnetically sustained glow discharge and were already able to detect sputter created neutral dimers from an elemental Cu target (6-7). While electron impact and Penning ionization by metastable noble gas ions could have contributed in that case, Oechsner et al. used the electron component of a resonantly excited low pressure high frequency discharge forming a spatially expanded dense Maxwellian electron gas for postionization by electron impact (8-10). Penning ionization involving heavy interactions between sputtered neutrals and nleta-stable rare gas ions is applied in glow discharge mass spectrometry GDMS as introduced by Coburn and Kay (11). The present chapter starts with a short description of the principle and the application of plasma postionization by Penning processes for mass spectroscopy of sputtered neutrals. The main part is devoted to postionization of neutrals atoms and molecules originating from an ion bOlnbarded solid surface by the interaction with a dense electron gas achieved in special low pressure hf-plasma (12). In this context energy distribution measurements of sputtered atoms and molecules, and surface or depth profile analysis by Secondary Neutral Mass Spectronletry SNMS, are presented and discussed in some detail. 9.2 POSTIONIZATION TECHNIQUES USING PENNING PROCESSES
Penning ionization of a sputtered species is described by
x +
A* ... X+ + A + e- + ~E
(1)
were A * denotes a particle excited into a high energy metastable state. Examples of nletastables with sufficient internal energy to ionize a sputtered species with an ionization energy of a few eV are Ne* and Ar* in their 3P2 ,O states with a stored electronic energy of about 11.6 eV and 16.6 eV, respectively. Such particles occur with sufficient density in hf or dc plasmas of the noble gases Ne and Ar. The essential condition for effective Penning ionization is a sufficiently high probability for heavy particle collisions between X and A *, Le. short mean free paths in the postionizing plasma. Therefore, Penning postionization involves relatively high working pressures which vary from about 0.1 mbar up to atmospheric pressure. The residual energy ~E being not consumed in the ionization process itself can appear as kinetic energy, predominantly of the generated electron, or as an additional photon. It can, however, be also stored in a new molecular particle containing e.g. a metal and a noble gas atom. Depending on the operation conditions, such particles are well known to be superimposed, for example, as positively charged "Argides" to the postionized particle flux (13). Corresponding examples are shown in Fig. 1. Such species and other molecular particles created by atomic collisions in a high pressure postionizing plasma conlplicate the corresponding mass spectra. Since the initial kinetic properties of the sputtered species are destroyed by the atomic collisions involved, Penning postionization obviously cannot be used for energy distribution measurements in sputtering. The thernlalization of the sputtered postionized particles, however, prevents also a separation between the originally more energetic particles from the sputtered surface and low energy plasma particles by a a potential step in the ion extracting system. Hence, mostly high resolution double focus-
The Application of Postionization
147
sing mass spectrometers are employed for GDMS using working pressures in the nlbar regime or at even higher values (14). NiCu Alloy Ni+
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Figure 1 Relative GDMS signals from different samples obtained with an rf discharge
(13.56 MHz, rf power 100 W) in Ar at 8x10- 2 mbar. According to Coburn et al.(13). The quantification of GDMS involves calibration by standard samples of well known composition obtained under constant and reproducible discharge conditions. For Ar a pressure around 6.2-6.5 mbar has found to be of particular advantage, since there the variations of the GDMS signals with the discharge pressure pass through a minimum (15). The composition of the working gas has to be controlled precisely in order to distinguish between impurity particles from the sample and from the working gas. Nevertheless, GDMS at high operation pressures has its merits as a technique for bulk analysis with extremely high detection sensitivity (14-15). In corresponding systems the sample material has often to be machined into a rod-like shape, and then is used as an active part of the electrode system for the excitation of the GDMS plasma. When the interpretation difficulties with respect to the origin of detected species can be solved, sample constituents have been shown to be detectable down to the ppb range by sufficiently long particle collection times.
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Handbook of Ion Beam Processing Technology
9.3 ELECTRON GAS POSTIONIZATION IN LOW PRESSURE PLASMAS
The postionization efficiency of sputtered neutrals can be increased over that with electron beams, when the postionizing volume filled with energetic electrons of sufficiently high density is enlarged. Such conditions can be met by the electron component of a dense low pressure plasma, where the electron density can be increased over that in electron beams due to the space charge compensation by the background of positive plasnla ions. Since the quasineutrality of a plasma prevents any potential wells from being introduced by the space charge of an electron beam, the trapping of low energy postionized species is avoided. Hence, not only mass spectrometric analysis of postionized sputtered neutrals becomes possible. Moreover, also the energy distributions of the sputtered particles can be detected in a reliable manner, when the working pressure is reduced to such a value that interfering influences by heavy particle collisions become negligible. Such conditions are well achieved when employing the so called electron cyclotron wave resonance (ECWR) for plasma excitation (12). Then, at working pressures which reach from a few 10-4 mbar for Ar down to a few 10- 6 ITlbar for Xe as working gases, plasma or electron densities n e around 109 to 1010cm- 3 are produced. For comparison, an electron beam of 1 mA/cm2 at 100 eV contains an n e of about 107 cm- 3 • With ECWR the plasnla is generated by inductive coupling, Le. without any internal plasma excitation electrodes, in a simply shaped volume which forms, e.g., an internal chamber in an ultrahigh vacuum system. A small tunable dc magnetic field around 10-15 Gauss has to be superinlposed to nleet the conditions for ECWR (12). As proved by dc probe measurements the electron component of such a plasma forms a Maxwellian electron gas with temperatures T e corresponding to 10 - 20 eV. The ionization probability a~ of a species X entering the electron gas volume is given by a convolution of the ionization function Qxi(ve ) for electron impact, the (Maxwellian) velocity distribution f(v e ) of the plasma electrons and, via the dwelling time, of the velocity distribution Nx(v x) of the sputtered species X, Le. by (16)
(2)
Corresponding aO values have been determined experimentally for different atoms sputtered from metal samples to be around 2-3 x 10-2 (17). As T e and n e vary oppositely when the working pressure is changed (e.g., for Ar between 10- 4 and a few 10- 3 nlbar), aO x for a species X is relatively well constant within the range of the operation conditions employed in ECWR. As an important consequence, the postionization probability aO x for a certain sputtered species X is an apparatus constant when the ECWR-plasma is operated under sufficiently constant experinlental conditions. Sputtered molecules are, of course, also subjected to electron dissociation processes when traversing the postionization volume (18). Then, an effective aO x has to be determined with which a molecular species X entering the postionizing plasma leaves it as the corresponding ion X+.
The Application of Postionization
149
Apart fronl the high values of electron density and temperature n e and T e yielding high postionization probabilities aO x' and the low working pressure, the application of an ECWR plasma for postionization purposes displays several other advantages. Such are -constant T e throughout the plasma chamber and smooth symmetrical distributions of plasma density and potential being well described by analytical functions (19), -high purity of the plasma atnlosphere due to desorption of inlpurities fronl the chamber walls by continuous low energy ion bombardment at about 20 eV and continuous bake-out due to the dielectric losses in the wall material (glass or ceramics), -no introduction of impurities from hot filaments or other plasma exciting electrodes (impurity particles are only introduced by not sufficiently clean working gases, and eventually from ion beam sources involved in the measurements), -positive plasma ions forming the background for electron charge compensation can be extracted and employed for the sample bombardment (" direct bombardment mode"), -the plasma electrons form a very appropriate electron reservoir for charge compensation during the investigation of insulators by ion bombardment. 9.3.1 Investigations of the Sputtering Process by Plasma Postionization
Postionization by electron impact in a low pressure noble gas plasma excited by electron cyclotron wave resonance ECWR was first developed about 20 years ago for the determination of energy distributions of sputtered neutral particles, which were mostly unknown at that time (8,9,19). In such early arrangements a planar elemental sputtering target was bombarded with ions of the ECWR plasma at normal incidence for well controlled bOlTlbarding energies around 1 keY. Energy analysis of the postionized sputtered species was performed by a retarding field arrangement similar to that of a LEED detector. After deconvolution with respect to the electron density and potential distribution along the traveling path through the postionizing plasma, the energy distributions of the sputtered neutral particles ejected normal to the surface from elemental polycrystalline targets were found to depend on the target material and to peak at an energy around a few eV (9,19). When calculating the measured outside energy distributions back to those inside the sputtering target by assuming a planar surface potential well of the height U o of the surface binding energy (or the heat of sublimation), a uniform E i -2 -behavior was found for the inside distribution (20). From this the relation
N(E)
27/4
E
(1
+ E)3
(3)
is derived for the "outside" distributions being normalized to their maximum value. In Eq. 3 a reduced energy E = E/Uo is used (19). The measured energy distributions for particles ejected parallel to the surface normal have been found to be well described by Eq. 3 which predicts the distributions to peak at E = U o /2 and to approach N(E)~E-2 at higher ejection energies E.
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Handbook of Ion Beam Processing Technology
Such a behavior was coincidentally derived theoretically by M.W. Thompson (21), assuming the formation of isotropic sputtering cascades in the solid target, and later in an expanded sputtering theory (22). Experimentally, the E-2 -dependence at the backward slope of N(E) has been independently found at bombarding energies in the 40 keY regime with a different experimental approach involving radioactive tracers (21). The behavior according to the formula in Eq. 3 has been more recently confirmed by other techniques as e.g. Doppler shift measurements in laser fluorescence spectroscopy (23) where the generation of isotropic collision cascades in the target can be assumed. When the retarding field analyzer is replaced by a quadrupole mass spectrometer the composition of the sputtered particle flux can be determined. When combining energy and mass analysis with electron gas postionization in an ECWR plasma, the energy distributions of the different neutral atomic and molecular species sputtered from elemental and nonelemental targets can be determined separately (24-26). A corresponding arrangement is schematically shown in Fig. 2 (25). The target can be bombarded under a well controlled angle of incidence by a noble gas ion beam being extracted from the postionizing plasma by nleans of an ion optical immersion lens. The sputtered neutrals enter the postionizing plasma through an electrical diaphragm. This second ion optical system prevents charged particles of any sign to penetrate in both directions, but can be opened for charged species of one sign in one direction (see also section 9.3.2).
ION GUN
R/ I I
TWIN PARALLEL
L1-l I Hf-PLASMA I
QUADRUPOLE
PLATE ANALYSERl
\MASS SPECTROMETER
~III
;//
11 - 1
III~II
!!I~ lilT ELECTRICAL DIAPHRAGM
Figure 2: Scheme of an apparatus for combined and angle resolved energy and mass analysis of sputtered neutral particles (25-26). The Maxwellian electron component of a hf plasma excited by electron cyclotron wave resonance (12) is used for electron impact postionization. For conlparative secondary ion measurements the plasma is switched of and the external ion gun is used.
The Application of Postionization
151
Angle resolved energy distribution measurements of sputtered neutrals are shown in Figs. 3 and 4. A comparison between the energy distribution of atoms and homonuclear dimers sputtered from polycrystalline Mo is presented in Fig. 3 for experimental conditions under which an isotropic collision cascade is expected to develop within the target (27). Hence, the atom distribution agrees almost completely with the formula of Eq.3. The much narrower energy distribution for the sputtered dimers can give valuable information on the formation mechanism of such particles which will be discussed later in this section. The energy distribution of sputtered trimers which have been measured for the first time with the arrangement in Fig. 2 (27) are still narrower than those of the dimers.
2000 eV Ar+ -.- M0
1,0
1 w
05 '
z
o
10
30 E in eV ---
50
70
Figure 3: Normalized energy distributions N(E) of neutral Mo atoms and M0 2 dimers ejected parallel to the surface normal from a polycrystalline Mo target under normal bombardment with Ar+ ions of 2000 eVe The arrangement in Fig. 2 was modified accordingly (Measurenlents by K. Franzreb (27)).
For bombarding energies on the order of only a few 100 eV or oblique ejection, the energy distributions of sputtered neutral atoms are found to deviate clearly from a behavior according to Eq. 3 (25,26). This gives strong evidence that an isotropic collision cascade has not been fully developed which is obviously expected for low bombarding energies and oblique bombarding and/or escape angles, i.e when only a few near-surface collisions lead to particle ejection. The variation of the shape of the energy distributions with the bombarding energy Eo in the low Eo regime coincides surprisingly well with the predictions of a nl0re elaborated theoretical description of the sputter cascade given by M. Urbassek which includes anisotropy effects by a more general solution of the corresponding transport equations (28). Most interestingly, energy distributions as those shown in Fig. 4 for different bombarding and ejection angles can be almost quantitatively described when the contributions from the subsequent generations in the developing collision cascade are superimposed (26). Therefore, the evolution of bombardment induced atomic collision cascades in the surface near region of solids can be differentially probed
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Handbook of Ion Beam Processing Technology
when the atomic energy distributions at different take-off angles are measured with sufficient precision as can be done by means of an experimental setup as that in Fig. 2.
2000 eV Ar+ ~ Ni
1,0
los w'
z
o
10
30 E in eV
50
70
~
Figure 4: Nornlalized energy distributions N(E) of neutral Ni atoms sputtered from polycrystalline Ni for different ratios of the bombarding and the ejection angle, respectively. Bombardment with Ar+ ions of 2000 eV (27).
As another attractive possibility, secondary neutral and secondary ion energy distributions can be measured alternatively in-situ when the arrangement for the sample bOITlbardment by plasma ions is replaced by a separate conventional ion gun as indicated in Fig. 2. In this "External Bombardment Mode" (EBM) (29) the electrical diaphragm between the sample and the postionizing plasma again is closed for charged particles when studying sputtered neutrals. For in-situ secondary ion measurements the plasma is switched off and the diaphragm is opened for positive (or negative) secondary ions from the sample. Such comparative measurements have been performed for different elemental metal targets on which the surface oxygen concentration has been varied in a controlled manner (25). Corresponding results for a polycrystalline Ta target are presented in Fig. 5. From such measurements the variation of the ionization probability in the secondary ion formation has been quantitatively determined as a function of the particle ejection velocity for the first time (25). Via the variation of the oxygen coverage, such combined secondary ion and secondary neutral measurements give for the investigated systems a direct differential insight into the "matrix effects" in secondary ion formation.
The Application of Postionization
( a)
153
78.8 70.0
78.8 70.0
69.6
69.6
62.7
62.7
50.0
50.0
39.2
39.2
22.4
22.4
9.2
9.2
2.6
2.6
0.6
o
75 E/eV
150
0.4
o
150
75 E/eV
C~/%
c~/%
78.8
------1 78.8
70.0
---~70.0
69.8
(b)
----t69.8
69.6
-----169.6
67.2
--_---l 67.2
62.7
----162.7
39.2
39.2
27.4
a
0.6 75 E/eV
150
27.4
o
0.6
75
150
E/eV
Figure 5 Normalized energy distributions of (a) neutral Ta atoms and TaO molecules and (b) the corresponding positive secondary ions ejected under 45° from a polycrystalline Ta surface for different values of the oxygen surface concentrations c~. Bombardment with Ar+ ions of 2000 eV under 45° (25).
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Handbook of Ion Beam Processing Technology
The variation of the integrated mass spectrometric signals of the postionized neutral atoms and molecules sputtered from targets of different surface and bulk composition is an important key for the understanding of molecule formation in sputtering. As an example the variation of the neutral TaO and Ta-signals during oxygen removal again from a polycrystalline Ta surface is shown in Fig. 6 (30). The nonmonotonic behavior of the TaO-signal is understood from the so called Direct Emission Model for the formations of sputtered molecules which applies to systems with strong (ionic) atomic bonds and sufficiently large differences between the atomic masses of the surface constituents (31). According to this nlodel a light surface atonl like 0 with a sufficiently strong bond to an adjacent heavy surface atom like Ta is co-ejected with the heavier particle, when the latter gets sufficiently high outward directed momentum in a binary atomic impact from the bonlbardnlent induced collision cascade. The direct enlission nlodel predicts, e.g., maximum metal-oxide (MeO) formation for an oxygen surface concentration of 500/0 (31-32). Hence, a variation of the MeO signal as that shown in Fig. 6 enables via the Direct Emission Model the determination of the surface concentration of oxygen or other strongly bonded components without any external standards.
0.7
0.6
Figure 6: Integral signals of neutral Ta and TaO particles during sputter removal of a thin oxide layer (~2 monolayers) from a polycrystalline Ta surface by 4 keY Ar+ ions under 45° incidence. For comparison the simultaneously measured AES signal of the 510 eV oxygen peak is included. An equivalent of about 5 monolayers is removed along the entire bombarding time axis (30).
0.5
O.L
0.3 ~
.iii c
0.2
ell
]
" .'-I...°
5 , 0 tV ( A ES J
arbitrary units
0.1
''--'--
0
100 bombarding
150 tim~
'-'-
200 250 Is_
300
The results in Fig. 7 refer to an NiW-alloy with different W bulk concentrations (33). Neutral sputter generated molecules up to tetramers are detected showing a characteristic variation of the molecule signals with the bulk composition. Similar results have been obtained for other binary alloy systems, and led to the so-called Atomic Combination Model for the formation of sputtered molecules (33-35). This model applies to the formation of molecules with low atomic bond strengths and comparable masses of the atomic constit-
The Application of Postionization
155
uents. It predicts that atoms ejected from one single sputtering cascade can combine to a molecule when leaving the surface, if their momentum is properly correlated, Le. when their relative kinetic energy is smaller than the attractive part of the interacting potential at the individual distance of the ejected particles (33). Consequently, molecular contributions from such samples to which the atomic combination model applies are always by orders of magnitude below the molecular signals referring to the Direct Emission Model. The Atomic Combination Model has been well confirmed from the variation of molecular signals with the concentrations of the sample constituents which determine under stationary conditions the composition of the sputtered particle flux (33).
100
95
--at%Ni 90 85
80
Ar+,1.2 keY Ni -W alloys
Ni
_o---~-.r- NiW
W2
~Nj2W ?<~~2 ~~ -0-
10-5
~
o
_ _...&.-_ _---a...
=0-
NiW3
Ni 2 W2
" ' -_ _-'---_.....J
10 15 at%W--
20
Figure 7: Normalized integral SNMS-signals of atomic and molecular neutral species sputtered from polycrystalline NiW for 3 different bulk compositions. The measurements refer to sputter equilibrium under normal bombardment with 1.2 keY Ar+ ions (33).
Since the postionization probability, a~, for a sputter created atomic or molecular species X is a constant of the postionization equipment. the height of the mass spectrometric signal is directly proportional to the partial sputtering yield Y x of the individual species x. Taking into account that the total sputtering yield Y tot is the sum of all partial yields Y x' and that a solid is stoichiometrically sputter removed under equilibrium conditions, partial sputtering yields can be determined quantitatively as a function of the bombarding parameters as, e.g. the bombarding energy Eo (36). Corresponding results for a Nb2 0 s target are presented in Fig. 8 (37).
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Handbook of Ion Beam Processing Technology
+
0.6
Ar --- Nb2 0S normal incidence
c
0.5 ell
0.4
~
~ 50.3 L. d
a. c
0.2
>-
0.1 100
200
300
400
500
600
EoineVFigure 8: Partial sputtering yields Y x in particles/ion for stationary sputter removal of Nb2 0 5 under normal bombardment with Ar+ ions as a function of the bombarding energy Eo (37).
If a tetramer, as being for instance detected in Fig. 7 with a relative intensity of about 10- 5 would have been an impurity particle at the target surface, this had also been detected in the neutral particle flux leaving the surface. Such considerations led to the development of Secondary Neutral Mass Spectrometry SNMS employing postionization of neutral sputter removed surface particles by the electron component of an ECWR plasma. The potentialities and some characteristic applications of SNMS as a novel quantitative technique for surface and depth profile analysis (10,32) will be described in the next section. 9.3.2 Electron Gas Postionization for Secondary Neutral Mass Spectrometry SNMS
Mass spectrometric identification of atoms and molecules removed from a solid surface is one of the most direct nlethods for compositional surface analysis. When such a procedure is combined with a controlled laterally homogeneous sputter removal of the sample, and all of a representative fraction of the removed particles are continuously nl0nitored, concentration depth profiles are obtained. Since positive or negative secondary ions can immediately be analyzed, Secondary Ion Mass Spectrometry SIMS is widely employed as an analytical technique with excellent detection sensitivity, and numerous highly sophisticated SIMS instruments have been developed. The strong and mostly hard to understand influence of surface chemistry on the ionization probability in secondary ion formation is, however, a strong drawback of SIMS, and makes the quantification of this technique often extremely difficult, if not impossible. Such "matrix effects" are cir-
The Application of Postionization
157
cumvented by Secondary Neutral Mass Spectrometry where - as the main difference from SIMS - the ejection and the ionization process of the analyzed particles are experimentally separated. For constant operation conditions of the postionizing system involved, an ejected species X is postionized and consequently detected with a constant probability which then is a particle specific apparatus constant of the respective SNMS systenl. Thus, an SNMS signal I of a postionized species XO is proportional to the ejection probability of X or the respective partial sputtering yield Yx' Le.
(4)
where I p is the bombarding ion current and TJx a geometry, transmission and amplification factor of the SNMS arrangement (32). The factor (1 - ai) refers to the secondary ion fraction in the sputtered particle flux with ai being the ionization probabilities in the formation of positive or negative secondary ions of X. In almost all cases, however, ai is very small compared to unity, and the last factor in Eq. 4 can be disregarded. While Eq. 4 holds for any sputtered neutrals, the situation becomes very simple when the sample is atomically sputtered, Le. when the nlolecule contribution to the sputtered flux becomes negligibly small. Then the partial sputter yields under sputter equilibrium are given by Y x = cxYtot , and we obtain (5)
which means that the SNMS signal is directly proportional to the concentration Cx of the constituent X in the sample. D x = TJxa~ is the detection factor for X. For the quantification of SNMS only relative detection factors D~el = Dx/D y are necessary, which consecutively can be all related to one reference element R. Taking additionally into account that LC x = 1 , the concentration Cx of a conlponent X in an atomically sputtered sample is obtained from the signals I(XP in the corresponding SNMS spectrum by
(6)
with Dfel = Di/D r . Quite obviously, the bombarding current I p and Y tot cancel when all I(XP) are taken from the same SNMS spectrum. Examples for relative sensitivity factors with Fe as the reference element are given in Table 1. Such values can be easily determined from standard samples of well known composition, and change only little when the operation conditions of the SNMS plasma are not maintained precisely in the daily laboratory work.
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Handbook of Ion Beam Processing Technology
Relative SNMS detection factors with Fe as reference element of different sputtered atoms X for a certain SNMS apparatus (from Ref. 16).
Table 1:
Element X 0.15 1.02 0.52 0.69 0.95
C
Si p S
Cr Fe Co Ni Cu Mo
1
0.79 0.56 0.53 1.10
The factor YJx and hence D x may vary for samples of different composition in a different way, when the bombarding energy Eo is changed, in particular, in the low energy regime of few 100 eV. Such residual influences, which should not be mixed up with the matrix effects in SIMS, originate mainly from variations of the angular distribution of sputtered neutrals (38). They can be neglected for Eo around 1 keY where the angular distributions show in general only small deviations from a cosine-like behavior.
~
MUltiPlier
~~~
~~J
~\
Ion Gun-\
---Quadru pole Mass Spectrometer
)
v
.....Hf-Plasma
Sample
Figure 9: Schematic diagram of the SNMS arrangenlent for the direct bombardment mode DBM and the separate bombardment nlode SBM.
The Appl ication of Postionization
159
The scheme of an SNMS arrangement is show in Fig. 9 (16). Such a system can be operated in three different modes: a) In the direct bombardment mode DBM the sample is perpendicularly bombarded with plasma ions being extracted by an ion optical systenl in front of the sample (39). The bombarding energy Eo can be varied from a few keY down to a few 10 eV, the latter values being important for minimizing collisional mixing and implantation effects at the sanlple surface. For appropriate operation conditions extremely high homogeneity of the bombarding ion current of 1-2 mA/cm2 with deviations of less than 10- 5 across a diameter of 8-10 mm is achieved. b) In the separate bombardment mode SBM the investigated surface is hit by a rastered ion beam, which originates from a conventional ion gun mounted oppositely to the target and penetrates the postionizing plasma. Surface charging at electrically insulating samples is automatically and precisely compensated by a retardation current of plasma electrons onto the floating sample (40). c) Finally, when the plasma is switched off comparative in-situ SIMS measurements of conducting samples become possible. The external bombardment mode EBM, where a sample mounted outside the SNMS plasma is bombarded with an ion gun and the ejected neutral surface particles enter the postionizing chamber through an electrical diaphragm (29) has already been introduced in sect. 9.3.1 (see Fig. 2). The EBM can be also employed for comparative in situ SNMS-SIMS investigations (29,25). The different bombardment modes of SNMS can always be operated under ultrahigh vacuum conditions, and the postionizing plasma volume can be laid out as an internal chamber in an DRV system. The DBM-SBM configuration in Fig. 9 is similar to a commercially available SNMS systema . The utility of comparative SNMS and SIMS measurements and, simultaneously, the advantage of SNMS is demonstrated in Fig. 10 showing the main secondary neutral and secondary ion signals from an GaAs sample under Ar+ bombardment (32). While the ratio of the positive secondary ions for Ga and As is around 102 , the SNMS signals are almost of equal height, revealing clearly the one-to-one sample composition without any further evaluation. The As+ signal is still snlaller than an GaO+ signal, indicating an oxygen impurity which may be mainly responsible for the secondary ion formation. As an example for the quantitative analysis of an unknown sample by SNMS, the mass spectrum of postionized neutrals from an Fe sample containing a large number of low concentration constituents is shown in Fig. 11 (16, 41). Since the signal for the main Fe isotope is around 107 cps and the background amounts only to 1-2 cps, the detection sensitivity of SNMS reaches down to the sub-ppm range (16,42). As the sensitivity factors of SNMS are in general of the same order (see Table 1) the sample composition can be taken within a factor of 2 directly from the height of the SNMS signals without any further evaluation (16). This is shown in Fig. 12 (41).
a
Commercial system: INA 3, from Leybold AG, Kaln.
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Handbook of Ion Beam Processing Technology
SIMS
SNMS
I p =9.IO-'A
01 Tp= 1.2l( 10"A bl ~ 3.3. lO,sA
5"000·
Ar~O.65 keV Az O.5cm z
As
Ar·, 3keV A zlO·lcm 1
59 0 • 0
5'Ga
€)
'Gaol@ Goa·
~
c:
Sx 10eps
x
I
I
~,fr"p,
65 7S 85 95
Go
c
t2S x lO~eps
I
x
6570 7'i 00
m/e
m/e
Figure 10: Parts of linear SNMS and SIMS spectra for GaAs (32).
Q 1/1 0. U (]I
Mn
o
Ar Si
Cr
Ni.Co
FeC MO,Nb
FeO
AI
o
Ti V
pS
FeSi
As
Zr Ar Sf>
~
~~ ~2~ a
t
~
"
1
m/z
66
88
Q. F'2 1/1
Q. U
FeNi ...
To FeCr
100
~
Figure 11: Logarithmic SNMS spectrum of an Fe sample containing a large number of low concentration components. (NBS standard 1261 a). SNMS analysis with the direct bombardment mode DEM under normal bombardment with Ar+ ions of 2 keY. (a) mass range 0-110 amu, (b) mass range 100-190 amu (From ref. 41)
The Appl ication of Postion ization
161
Since the molecular contributions in the SNMS spectrum of Fig. 11 remain small, the quantification procedure described in Eqs. 4-6 becomes applicable. Using the corresponding relative sensitivity factors, the concentration of different isotopes in the present sample determined by SNMS are given in Table 2 and compared with the corresponding certified values being available in this case (16).
Fe
10 7 cps
SNMS-Signal
lOb
Mn Si. Cr"'
10 5
Ni
c
•
AI 10t.
V Nb
-I _
Zr e.
60;-e
10 3
Ti e
• CO
~4H:lJ
s
S
B
• ~b
Ce
10 2 La
•
10 1 Concentration
10 0 10-1
lOa
10 1
10 2
103
105
lOt.
10 6
ppm
Figure 1 2: Correlation plot of the uncorrected absolute SNMS signals from Fig. 11 versus the concentrations in the analyzed standard sample. (From ref. 41)
When employing plasma electrons for a controlled compensation of the positive charge deposited by the bombarding ion current, SNMS is well applicable to the analysis of electrically insulating specimens (43). An example is given in Fig. 13 referring to an highly insulating Ce0 2 layer (44). The high detection sensitivity is maintained also in this case. Thus a number of impurities with concentrations in the 10 ppnl range can be detected. In particular, the W concentration in the order of 100 ppm found in the sputter deposited layer stems presumably from hot tungsten filaments in the deposition system. For the evaluation of the molecular signals appearing in the SNMS spectra from such samples, the relation between the molecule formation probability and the surface concentration as given by the direct emission model (see section 9.2 and refs. 31-33 and 45) becomes important.
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Handbook of Ion Beam Processing Technology
Table 2: Quantitative SNMS analysis of isotope concentrations in a homogeneous multi-element sample (NBS Standard 1261a). From Ref. 16.
Mass CeTtified
C12
1.77x10-2 4.15xlO- 3 2.8x10- 4 2.47x10- 4 6.11x10- 3 2.06x10- 2 3.0x10- 4 5.0x10- 3 2.49x10- 4 2.61x10- 4
Sp8 p31 S32
Cr52 Fe 57 C 0 59 Ni60 CU63
M 0 98
Cu
Isotope concentration bySNMS
1.71x10-2 4.15x10- 3 2.35x10- 4 2.37x10- 4 6.05x10- 3 2.06x10- 2 5.2x10- 4 7.3x10- 3 3.13x10- 4 3.75x10- 4
1000 eV Ar~ -
(eOl
2.2mAlcm 'l
10'
m
e
Figure 13: SNMS-spectrum of an insulating Ce0 2 layer on Si obtained under normal bombardment with Ar+ ions of 1000 eV (see ref. 44)
In addition to the highly quantitative nature of the technique, the high depth resolution obtained with the direct bOITlbardnlent mode DBM is the most attractive property of SNMS. This is due to the very low bombarding energies and the high lateral homogeneity of the bombarding ion current, which can be established with this technique (39).
The Application of Postionization
163
The SNMS depth profile of a sputter deposited multilayer systenl with a total thickness of 2000A, consisting of Ta-Si double layers of only 200 A is shown in Fig. 14 (16). The high depth resolution obtained for normal bombardment with Ar+ ions of 200 eV is denlonstrated by the large variations of the Si-signals of the individual Si sublayers and the detection of an additional Si cover on top of the outermost Ta-Iayer. Even structural details, as a small shoulder at the first Si-Ta interface above the substrate (see arrow in Fig.14) are highly reproducible by low energy SNMS depth profiles at different positions across the backing Si-wafer. Two clearly different experimental interface widths are revealed by the experimental profile in Fig. 14, namely a broader Ta-Si transition and a much sharper Si-Ta interface when proceeding from the surface of the layer system down to the substrate. This behavior is due to the variations of the sputtering yields, Le. the removal velocity, when crossing both types of interfaces: While the heavy Ta particles are less effectively sputter removed from a low mass Si underlayer, the light Si is expected to be rapidly sputter removed from the high mass Ta-backings. Such influences can be taken into account in a quantitative manner since in SNMS both species are recorded with constant sensitivity, and hence the variations of the total sputtering yield during profiling across the layer system can be determined (46).
~
u ~
Si substrate
o
X
o
~10
QI
>
o
50
100
150
200
.250..
sputt ermg time In s
Figure 14: SNMS sputter depth profile of Si in a Ta-Si multilayer structure produced by sequential sputter deposition of Ta and Si on a (100)Si-wafer. Nonlinal thickness of the individual Ta-Si double layers was 200 A. Sputter profiling has been performed with normally incident Ar+ ions of 200 eV at a bombarding current density of 1.8 ma/cm2 (Sample by courtesy of R.v. Criegern and H. Rehnle, Munich.) From Ref. 16
The 84-16% interface widths of the sharper Si-Ta interfaces are found to be 14 ± 2 A across the entire thickness of the multilayer system. This demonstrates the ex-
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Handbook of Ion Beam Processing Technology
tremely high depth resolution obtained with low energy SNMS. Such a small interface width agrees well with the depth of an atomic microroughness which is induced even at ideally plane, defect free surfaces due to the statistical character of the sputtering process itself (47). Taking into account additionally, that the information depth of SNMS at such low bombarding energies as used for the results in Fig. 14 is in the order of 1-2 atomic distances, the very low experinlental transition widths for the Si-Ta transitions refer to sputter depth profiling across atomically sharp interfaces. The experimental possibilities of SNMS enable also high resolution depth profiles on highly insulating thin film structures. An example is given in Fig. 15 (41) for an 70 nm Si3N 4 - and 30 nm Si02 -layer, both being strong electrical insulators, on a Si substrate. Making use of the quantification possibilities for the SNMS signals, the layer stoichiometry is almost ideally obtained from the corresponding SNMS data.
100
80 -;;e. .~
c: a
60 E C Q.I
U
c: a
u
C OJ 40 E Q.I
W
20
20
40
60
80
100
Sputter time in s
Figure 1 5: SNMS depth profile of a highly insulating Si3N 4 (70 nm) / Si02 (30 nm) layer structure on Si under normal bombardment with Ar+ ions of 600 eV (From ref. 41).
9.4 SUMMARY
In conclusion, the postionization of sputtered neutrals by the Maxwellian electron component of an ECWR plasma is not only a very useful means for the investigation of the sputtering process itself. The SNMS technique based on this postionization method offers, in addition, new and very attractive possibilities for the quantitative analysis of arbitrary sanlples and thin filnl structures with extremely high depth resolution.
The Application of Postionization
165
9.5 REFERENCES
L....A1m1.
1.
Honig, R.E., Sputtering of surfaces by positive ion beams of low energy Phys. 29: p. 549 (1958).
2.
Smith, A.J., Camby, L.A., and Marshal, D.J. Mass analysis of sputtered particles LAppl. Phys. 34: p. 2489 (1963).
3.
Kaminsky, M. Mass spectrometric studies of the species of particles leaving a monocrystalline target in a charged or uncharged state under high energy ion bombardment, Advanc. Mass. Spectre 3: p. 69 (1966).
4.
Lipinsky, D., Jede, R., Ganshow, D., and Benninghoven, A. Performance of a new ion optics for quasi-simultaneous secondary ion, secondary neutral and residual gas mass spectroscopy, J. Vac. Sci. Technol. A3: p. 2007 (1985).
5.
Gnaser, H., Fleischhauer, J. and Hofer, W.O. Analysis of Solids by secondary ion and sputtered neutral mass spectrometry. Appl. Phys. A37: p. 211 (1985).
6.
Woodyard, J.R. and Cooper, C.B., Mass spectrometric study of neutral particles sputtered from Cu by 0 to 100 eV Ar ions. J. Appl. Phys. 35: p. 1107 (1964).
7.
Comas, J. and Cooper, C.B. Mass spectrometric study of sputtering of single crystals of GaAs by low energy Ar ions. J. Appl. Phys. 38: p. 2956 (1967).
8.
Oechsner, H., and Reichert, L., Energies of neutral sputtered particles. Phys. Lett. 23 p. 90 (1966).
9.
Oechsner, H., Energy distributions of neutral particles sputtered by low energy ion bombardment. Proc. 8th Int. Cont Phen. loniz. Gas. (Springer Verlag, Vienna 1967)31.
10. Oechsner, H., and Gerard, W., A method of surface analysis by sputtered neutrals. Phys. Lett. 4)A: p. 211 (1972). 11. Coburn, J.W., and Kay, E., A new technique for the elemental analysis of thin surface layers of solids. Appl. Phys. Lett. 19 p. 350 (1971). 12. Oechsner, H., Electron cyclotron wave resonances and power adsorption effects in electodeless low pressure H.F. plasmas with a superimposed static magnetic field. Plasma Physics 16: p. 835 (1974). 13. Coburn, J.W., Eckstein, E.W. and Kay, E., A mass spectrometric study of neutralsputtered species in an rf glow discharge sputtering system. J. Vac. Sci. Technol. 12: p. 151 (1975). 14. Sanderson, N.E., Hall, E., Clark, J., Charalambous, P. and Hall, D., Glow Discharge Mass Spectrometry - A powerful technique for elemental analysis of solids. Mikrochim. Acta (Wien) I: p. 275 (1987). 15. Jakubowski, N., Stuewer, D., and Vieth, W., Performance of a Glow Discharge Mass Spectrometer for simultaneous multi-element analysis of steel. Anal. Chern. 59: p. 1825 (1987). 16. Oechsner, H., Recent applications of Secondary Neutral Mass Spectrometry SNMS for quantitative analysis of homogenous and structured sanlples, Nucl. Instr. Meth. Phys. Res. B, (in press, 1988).
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17. Halden, T. , Untersuchungen zur lonisierung zerstaubter Neutralteilchen in Edelgas-Niederdruckplasmen. Thesis, University of Kaiserlautern (1984). 18. Gerhard, W., and Oechsner, H., Mass spectrometry of neutral molecules sputtered from polycrystalline metals by Ar+ ions of 100-1000 eVe Z. Physik B22: p. 41 (1975). 19. Oechsner, H., Energieverteilugen bei der lonenbeschuss. Z. Physik 238: p. 433 (1970).
Festkorperzerstaubing
durch
20. Oechsner, H., Energy distribution in sputtering process. Phys. Rev. Lett. 24: p. 583 (1970). 21. Thompson, M.W., On the energy spectrum of ejected atoms during the high energy sputtering of gold. Phil. Mag. 18: p. 337 (1968). 22. Sigmund, P. Collision theory of displacement damage, ion ranges and sputtering. Rev. Roum. Phys. 17: p. 823 (1972). 23. Husinsky, W., Betz, G., and Girgis, I., Ground state and excited state sputtering Doppler shift laser-fluorescence studies of Cr and Ca targets. J. Vac. Sci. Technol. A2: p. 698 (1984). 24. Bernhardt, F., Oechsner, H., and Stumpe, E., Energy distribution of neutral atoms and molecules sputtered from polycrystalline silver. Nucl. Instr. Meth. 132: p. 329 (1976). 25. Wucher, A., and Oechsner, H., Emission energy dependance of ionization probabilities in secondary ion emission from oxygen covered Ta, Nb and Cu surfaces. Surface Sci. 199: p. 567 (1988). 26. Dembowski, J., Oechsner, H., Yanlamura, Y., and Urbassek, M., Energy distributions of neutral atoms sputtered from Cu, V, and Nb under different bombardment and ejection angles. Nucl. Instr. and Meth. Phys. Res. B18: p. 464 (1987). 27. Franzreb, K., Energie- and winkeldifferentielle Ausbeuten neutraler Atome and Molekule bei der Zerstaubung von Cu, Ni, Mo, Wand deren Legierungen mit Ar+-Ionen von 200 eV - 3050 eVe Diploma thesis, University of Kaiserlautern ( 1987). 28. Urbassek, M., The energy distribution of sputtered particles at low bombarding energies. Nucl. Instr. and Meth. Phys. Res. B4: p. 356 (1984). 29. Oechsner, H., Ruhe, W., Stumpe, E., Comparative SNMS and SIMS studies of oxidized Ce and Gd. Surface Sci. 85: p. 289 (1979). 30. Oechsner, H., and Stumpe, E., Emission of neutral particles and secondary ions from ion bombarded Ta covered with oxygen and nitrogen. Proc. IV. Int. Conf. Solid. Surf. and 3rd ECoss (D.A. Degras, M. Costa, eds.) Le Vide, les Couches Minces, Suppl. 201 (1980) p. 1234. 31. Oechsner, H., Molecule formation in oxide sputtering. in ; Secondary Ion Mass Spectrometry SIMS III (A. Benninghoven, J. Giber, J. Lazlo, M. Riedel and H.W. Werner, eds.), Springer Sere Chern. Phys. 19: pp. 106, Springer Verlag, Berlin, Heidelberg, New York (1982). 32. Oechsner, H., Secondary Neutral Mass Spectronletry (SNMS) and its application to depth profile and interface analysis. in: Thin Film and Depth Profile Analysis (H. Oechsner, ed.), Topics in Current Physics, 37: p. 63, Springer Verlag, Berlin, Heidelberg, New York, Tokyo (1984).
The Application of Postionization
167
33. Oechsner, H., Formation of sputtered molecules. in: The Physics of Ionized Gases (SPIG 84), (M.M. Popovic, P. Kristic eds.) p. 571, World Scientific Publ, Singapore, Philadelphia (1985). 34. Oechsner, H., and Gerhard, W., Mass spectroscopy of sputtered neutrals and its applications for surface analysis. Surface Sci. 44: p. 480 (1974). 35. Gerhard, W., A model calculation of the neutral molecule emission by sputtering processes. Z. Physik B22: p. 31 (1975). 36. Oechsner, H., Schoof, H., and Stumpe, E., Sputtering of Ta20s by Ar+ ions at energies below 1 keV. Surface Sci. 76: p. 343 (1978). 37. Oechsner, H., Schoof, H., and Stumpe, E., Total and differential sputtering yields of Ta- and Nb-oxide for 100-600 eV Ar+ measures with sputtered neutral mass spectroscopy. Proc. 7th In1. Vac. Congr. and 3rd In1. Conf Sol. Surf. (R. Dobrozemsky, F.G. Rudenauer, E.P. Viehbock, A. Breth, eds.) p1497, Berger, Vienna (1977). 38. Wucher, A., Novak, F., and Reuter, W. Relative elemental sensitivity factors in secondary neutral mass spectroscopy. J. Vac. Sci. Technol. A6: p. 2265 (1988). 39. Stumpe, E., Oechsner, H., and Schoof, E., High resolution sputter depth profiling with a low pressure hf plasma. Aool. Phys. 20: p. 55 (1979). 40. Muller, K-H., Siefert, K. and Wilmers, M., Quantitative chemical surface, in-depth, and bulk analysis by secondary neutral mass spectroscopy (SNMS). J. Vac. Sci. Technol. A3: p. 1367 (1985). 41. Jede, R., and Peters, H., Quantitative Oberflachen- and Tiefenprofilanalyse mit der Massenspektrometrie zerstaubter Neutralteilchen. Techn. Messen 54: p. 343 (1987). 42. Muller, K.-H., and Oechsner, H., Quantitative Secondary Neutral Mass Spectrometry analysis of alloys and interfaces. Mikrochim. Acta (Wein) Suppl. 10: p. 51 (1983). 43. Geiger, J.F., Kopnarski, M., Oechnser, H., and Paulus, H., SNMS analysis of insulators. Mikrochinl. Acta (Wein) I: p. 497 (1987). 44. Stewart, A.F., Guenther, A.H., Raj, T., and Oechsner, H., Quantitative chemical analysis of optical coatings by Secondary Neutral Mass Spectroscopy. Surf. Coatings and Technol. (in press, 1988). 45. Oechsner, H., and Wucher, A., Quantitative analysis of thin oxide layers on tantalum by sputtered neutral mass spectrometry (SNMS). Aool. Surf. Sci. 10: p. 342 (1982). 46. Wucher, A., and Oechsner, H., Depth scale calibration during sputter renloval of multilayer systems by SNMS. Fresenius Z. Anal. Chern. (in press, 1988). 47. Oechsner, H., Ion beam induced effects in thin film analysis. Fresenius Z. Anal. Chern. 314 p. 211 (1983).
Part III Film Modificaton and Synthesis
169
10 The Modification of Fill11s by Ion BOl11bardl11ent
Eric Kay and Stephen M. Rossnagel
10.1 INTRODUCTION
Energetic particle bOlnbardment of the depositing film is generally observed in most types of plasma-based or ion-beam-based film deposition techniques. The energetic bombardment may originate from a variety of sources, including reflected, neutralized primary ions from the target surface, energetic ions and electrons from the plasma, negative ions created at the target surface during sputtering, charge exchange neutrals, and energetic sputtered atoms from the target. The energetic species may be the condensing filnl atoms, gas atoms from the working gas, or impurity atoms from the charrlber walls or the background gas. The effects of the energetic bombardment on the film properties are quite numerous, and are described in some detail in the following chapters. As a general classification, the energetic bombardment of the growing film during deposition may result in either physical, structural changes to the film or changes in the chemical nature of the film. The physical changes can take the form of reduction in grain size, preferred crystalline orientation, increased (or decreased) packing density, lattice expansion and contractions, surface topographical effects, enhanced surface or bulk diffusion, changes in the nucleation density in the early stages of film growth and other related effects. Many physical properties which are sensitive to microstructure can be expected to reflect these bombardment-induced changes. As examples so far, the stress and adhesion of the film can be altered as well as the stability of the film in air. Various optical properties such as the index of refraction, the amount of scattering can be controlled. Many examples of bombardment-induced effects on magnetic and electrical properties can be cited. When energetic particle bombardment during film growth changes the resultant chemical composition of the film, many of the above mentioned physical properties will reflect these changes. For example, very small variations in stoichiometry can cause dramatic changes in the electrical properties. Ion bombardment has also been shown to greatly influence chemical kinetics during film growth in chemically reactive plasmas. Greatly enhanced rates of product formation have been reported, for example, in the
170
The Modification of Films by Ion Bombardment
171
oxidation of metals at room temperature. Likewise, in the synthesis of plasma polymerized thin films, both the rate of polymerization as well as the resultant chemical composition and structure of the polymer film can dramatically be influenced by controlling the energy and flux of ions bombarding the growing film.
10.2 EXPERIMENTAL CONCERNS FOR BOMBARDMENT-MODIFICATION OF FILMS
As mentioned briefly above, energetic bombardment is usually the rule, rather than the exception, during most filnl depositions by sputtering or in a plasma. While it is generally understood that film deposition in a plasma environment is accompanied by some level of energetic bombardment, the same phenomenon is often overlooked during ion beam sputter-deposition of fi1nlS. Alternative techniques based on evaporation also allow significant levels of energetic bombardment during deposition. In one class of experiments, a separate ion beam can be directed at the growing film surface during the film deposition process. In a second class, ions can be created by ionizing the background gas as well as some of the evaporated atoms which are then accelerated to the sample surface electrically. A discussion of each of these techniques is included in Chapter 18. In a plasma-based deposition, the substrate (that the film is being deposited on) is immersed in the plasma. This is true not only for conventional dc or rf diode plasmas, but also for nlagnetron sputter deposition. The plasma in this latter device is highly concentrated near the cathode, but also extends outward, filling the entire chamber. Plasmabased deposition systems are characterized by a plasma potential, which is always positive of the most positive surface in the chamber. In a dc configuration this plasma potential is usually of the order of 3-5 eV. While this may seem insignificant, ions being accelerated across the sheath toward a grounded substrate will have energies an order of magnitude higher than the thermal species arriving at the film surface, and can definitely cause changes in the properties of the film. In rf capacitively voltage-divided diode systems, the plasma potential can reach several hundreds of volts, depending on various parameters, such as the surface area of the powered electrode as compared to the total area of grounded surfaces in contact with the plasma. Plasma potentials of 30-40 V are common (1,2). In such configurations therefore the growing film, though grounded, is inevitably subject to pronounced ion bombardment during film growth. Likewise in supported (triode) discharges, where electrons are injected into the plasma fronl an external source, such as from a thermionic emission filament or hollow cathode, plasma potentials of several tens of volts can be obtained depending on the relative polarity of the filament and electron accelerating electrode with respect to ground (3). Often in conventional plasma processing a negative dc voltage is deliberately applied (or induced by an rf power supply) to the sample, causing ion bombardment from the plasma to become even more pronounced. This technique is known as bias sputtering and affords a very inlportant additional degree of freedom, in that the degree of ion bombardment of the growing film can be systematically altered in an otherwise fixed plasma configuration. This technique can and is commonly being used to great advantage to control filnl microstructure and properties. The kinetic energy of the neutral sputtered species themselves can also be relevant to the deposition of films in various plasma modes. As described in earlier chapters, sputtered neutrals leave the sputtering target with an average 3-10 eV translational energy,
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Handbook of Ion Beam Processing Technology
independent (to first order) of the energy of the primary ions hitting the sputtering target. These energetic sputtered particles then condense on the film surface. In addition, a significant fraction of the primary high energy gas ions hitting the sputtering target (in either an ion beanl or a plasnla system) are Auger neutralized and reflected as energetic neutrals, maintaining a large fraction of their primary energy. These neutrals are, however, directed towards the growing film at the substrate. As neutrals they are now unaffected by the voltage drop across the sheath at either the target or substrate surface. The reflection probability scales with the relative masses of the incident ion and the target species. Calculations of the reflection probabilities for heavy (Z > 4) ion species at low energies have been published by Eckstein and Biersack (4). In general, the reflection coefficient falls off sharply for cases where the ion mass exceeds the target atom mass. The degree to which these two types of energetic neutrals (sputtered atoms and target-reflected particles) will maintain their kinetic energy depends primarily on the nUITlber of energy dissipating collisions they suffer in traversing the plasma toward the substrate. The mean free path (related to the discharge pressure) as well as the distance from the target to the substrate will dictate these collision processes. Calculations (5) and experinlental evidence indicated that in commonly used sputter deposition chamber configurations operated at pressures of several 10's of mTorr, these energetic particles have lost a large fraction of their kinetic energy by the time they reach the substrate. This is definitely not the case in nlany of the discharges operated at 3 nlTorr or less, such as various magnetron and triode configurations. Here only very few collisions are suffered in transit across the plasma and therefor the film surface is inevitably subject to bombardnlent by particles at energies much greater than thermal energies. This situation also prevails in single and dual ion beam sputtering experiments which are usually performed in long mean free path environments. Again, neither the energetic sputtered neutrals not the reflected primaries will suffer a significant number of collisions in transit from target to film surface in such an environment. In addition to these energetic neutral bOITlbardment effects a very pronounced pressure dependance must be recognized when considering ion bombardment of the growing film in various plasma configurations. We have already seen that in most plasma configurations the substrate surface is at a negative potential with respect to the plasnla potential, the magnitude of this potential in a given configuration depending on whether the substrate is grounded or deliberately biased from an external power source. The subsequent positive ion bombardment of the growing film surface will be significantly different in the low (~ 1 mTorr ) to high pressure (~ 10 mTorr pressure discharge modes. In the low pressure nlodes (e.g. nlagnetrons and triodes) the background gas ions will bombard the substrate surface with full substrate fall potential with minimal energy spread. This is because, to first order, the ions are accelerated across the substrate sheath without suffering any energy dissipating collisions, in such long mean free plasmas. This situation comes very close to monoenergetic ion bombardment of the entire film surface and results in film growth conditions quite similar to those achieved in long mean free path dual and single ion beam experinlents (see Fig. 1). In cases where the plasma potential is known or can be estimated, absolute values can be attached to the ion energies as in the case of beam experiments (3). The major difference between the two approaches lies in the fact that the angle of incidence at the substrate can be varied in a dual ion beam
The Modification of Films by Ion Bombardment
173
configuration whereas in a plasma the ions inevitably traverse the substrate sheath orthogonally. Important ion angle of incidence effects on the microstructure of the film have been reported (see, for example, Chapter 15). Furthermore, in an ion beam approach, the experiment does have independent control over the energy and flux of ions bombarding the growing films surface. In all plasma approaches, however, the ion energy and flux are strongly coupled and it is not possible to independently change the plasma/substrate interface conditions without also changing the plasma/target interface conditions. Clearly, for quantitative studies in which changes in the flux of depositing particles as compared to the flux of monoenergetic ions arriving at the film growing surface need to be explored, the ion beam experiments are far more flexible and convenient.
Kr 84 p=2mTorr Supported Triode Discharge
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In high pressure (several tens or hundreds of mTorr) plasmas the energy and type of ions hitting the growing film is not as straightforward to determine as in the low pressure case. Background gas ions will suffer resonance charge transfer collisions in crossing the substrate sheath and only a small fraction will reach the full sheath potential. Care must be taken in conlparing ion energy dependent effects in high pressure plasnlas with those in low pressure and ion beam experiments. In the high pressure plasmas (Le. > 10 mTorr) the bias voltage at the substrate only represents the maximum energy that only a small fraction of the background gas ions reach. Most of the ions never exceed energies above approximately 20-300/0 of that indicated by the bias voltage. This is important since most ion bombardment induced microstructural changes such as lattice distortions, crystallographical orientation effects, resputtering or gas trapping, etc. are very much dependent on the absolute kinetic energy of the incident particles.
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Handbook of Ion Beam Processing Technology
In addition, as can be seen from the exanlple in Fig. 2, the cross section for ionization in the plasma of the sputtered species leaving the target as neutrals can be very significant. Penning ionization collisions with electronically excited metastables of the background gas has been shown to be very significant. Ten percent of the sputtered neutrals can be readily ionized in transit across the plasma and therefore will be accelerated across the substrate sheath. Since they will not be subject to resonance charge transfer collisions while crossing the sheath these sputtered atoms will reach full sheath potential. That is, these condensing particles on the film surface will arrive with large kinetic energies, especially if the substrate is highly biased with respect to the plasma potential. This latter condition is a very important aspect of so-called 'ion plating', where the film growing particles are partially ionized in the plasma and then accelerated towards the growing film surface. In contrast to conventional bias-sputtering, in the ion plating process, the film is grown on the high powered electrode (cathode) and the metal particles are injected into the plasma fronl an external source, e.g., by electron beam evaporation. Also, the ratio of condensible metal species to background gas ions hitting the growing film surface is usually higher than in bias sputtering. 100,000 .....-
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It is reported that the dense microstructures obtained by this procedure very much resemble recrystallized high temperature film structures and are likewise very much less subject to failure at intergranular interfaces. One very important consequence of this is their excellent substrate adhesion characteristics in addition to the fact that they can be deposited with the bulk temperature of the substrate being essentially at ambient temperatures.
The Modification of Films by Ion Bombardment
175
We have seen that pressure in a plasma system very much controls the energetics of the particles arriving at the substrate. By changing the pressure, energetics of the bombardments species can be manipulated with inlportant consequences on the resultant filnl microstructure. Many of the experiments undertaken to quantify the effects of energetic bombardment on the deposition of thin films have taken place in lJHV systems. This is similar to the case of the sputtering nleasurements described in Chapter 6. Many other studies of energetic bombardment-modification have been done in more conventional HV systems. Much of the work described in the next chapter uses broad beam gridded sources operating in the 10 5 Torr range. First, however, it is appropriate to discuss in more detail some of the types of changes in film properties that accompany energetic particle bombardment during film deposition.
10.3 EFFECTS ON FILM PROPERTIES BY ENERGETIC BOMBARDMENT
We have taken the somewhat arbitrary step of grouping the types of effects that have been attributed to energetic particle bombardnlent during film deposition into two basic groups: physical and chemical. While many of the observable features that may be used to characterize thin films, such as the resistivity or the index of refraction, may clearly be the result of a combination of several effects, both physical and chemical, many other features, such as grain size and orientation are more indicative of structural effects. This general delineation will also allow us later to describe as a separate topic the r\:iactive deposition of compound filnlS. 10.3.1. Physical Effects
10.3.1.1 Grain size. Several groups have consistently reported effects of energetic ion bombardment on the grain size of the resulting film. In most of the cases, the result of energetic bombardment was a reduction in the average grain size. Measurements by Huang, et al (6) show this for the case of Ar bombardment of Ag films in a DHV dual ion beam system. (Fig. 3). Interestingly, the average grain size was not further reduced after the energy delivered to the growing film surface by the energetic ions per arriving Ag atom exceeded approximately 40 eV/atom. Related work by Roy, et al (7) which will be described in more detail in the next chapter shows a similar effect for concurrent Ar bombardment during the evaporative deposition of Cu. That work also showed a dependance not only on the average energy deposited per Cu atom, but also on the absolute ion energy. 10.3.1.2 Orientation. One effect of ion bombardment during deposition can be the production of a film with a preferred orientation. In one set of experiments by Kay and coworkers on f.c.c. metal films grown on amorphous substrates it was clearly demonstrated that energetic ion bombardment at normal incidence leads to film growth with a large fraction of the (111) lattice planes parallel to the surface (8). It was further shown that the degree of this (111) orientation depended very much on En' the energy delivered to the film surface per arriving metal atom. Both the En threshold for orientation as well as the maximum were clearly indicated (8). The bulk substrate temperature in these experiments was room tenlperature. More detailed studies (6) of the ion bonlbardment ef-
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Handbook of Ion Beam Processing Technology
fects of much thinner Lc.c. films (50 nm) indicated that both the dominance of the (111) orientation and the significant changes in the (100) plane are also thickness dependent, which can be explained by surface and strain energy consideration; where, in very thin films the surface energy effects are most important and, as the filn1s becon1e thicker, volume recovery processes become increasingly important so that the minimum strain energy for the (hkl = 200) favors the tendency of more (100) oriented grain growth.
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A further series on glancing-angle ion bombardment of grains has been reported (9-11). A later chapter by Bradley (Chapter 15) discusses these experiments in more detail, arriving at a model consistent with the key experimental observations. 10.3.1.3 Nucleation density. Energetic ion bombardment during the very early stages of thin film coalescence and growth often results in an increase in the spatial density of nuclei on the surface. The net number of these sites on the surface, though, will be also a function of the loss rate due to annealing, which increases with increasing substrate temperature. One group has shown that the ion bombardment during the early stages of film growth of Ge films can either increase or decrease the number of nucleation sites, depending on the choice of substrate material and temperature. (12) Ion bombardment is also likely to lead to enhanced levels of surface diffusion (discussed below) which n1ay overshadow this effect. The increased nucleation density may in some cases be related to the reduced grain size observed above. 10.3.1.4 Defects. Ion bombardment during film deposition has been found to increase the density of dislocations in the resulting filn1s. Recent work would indicate that the
The Modification of Films by Ion Bombardment
177
energy of the incident ions influences the type and migration of defects introduced during film growth and that the concomitant rate of deposition of condensible atoms influences to what degree deep lying defects can migrate to the film surface during film growth or be trapped. In addition, the temperature at which the film is grown influences the degree of order in the growing film crystals and greatly affects the migration and elimination of different classes of defects. So, for example, Kay and coworkers found the dislocations density in thin Ag films deposited at room temperature to rise sharply up to En ~ 45 eV / Ag atom and then tended to level (see Fig. 4). On the other hand the twin fault probability decreased sharply across the same energy regime.In a different study by Greene et al (13) the density of defects was found to decrease at higher deposition temperature and lower ion energies.
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More recent studies by Hultman et al serve as an excellent example of the cOlnbined effects of deposition temperature and the ion energy delivered per condensible particle on the defect type and density during epitaxial growth (14). At a given ion bombardment energy the dislocation number density n d in epitaxial layers was found to decrease with increasing substrate temperature, due to higher adatom mobilities. However, at a fixed deposition temperature, n d decreased much more rapidly with increasing ion energy until a minimunl defect density was obtained at a critical ion bombardment energy. At greater than this critical energy, nd increased rapidly as the films became polycrystalline. Ion irradiation apparently played at least two major roles. At the lower ion energies, the primary effect was to enhance adatonl mobilities thereby accelerating the rate at which defects were annealed out during deposition. At energies above the critical energy, the increased projected range of the impinging ions resulted in a larger fraction of the
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Handbook of Ion Beam Processing Technology
irradiation induced defects being trapped in the growing film. Eventually, n d became high enough that renucleation occurred during film growth and polycrystalline films were obtained. 10.3.1.5 Lattice distortion. Ion bombardment during film growth not only induces high degrees of preferred orientation but can also readily distort the unit cell dimension relative to the equilibrium bulk value. Distortions from cubic to tetragonal are frequently observed. So, for example, lattice dilations can be systematically induced by changing the incident ion energy per depositing particle. Figure 5 shows such dilations for the (111) lattice spacings for different metals deposited at room temperature as a function of En . The Pd and Cu systems go through a maximum which, as in the case of defect formation, may reflect an "annealing" effect at the higher En values. This same system was studied earlier in a biased d.c. triode plasma system giving very similar results including showing a distinct maximum in the lattice distortion as a function of En. Similar results have also been recently observed by Roy et al (7). Copper and Au behave qualitatively quite differently which demonstrates that energetic particle bombardment during film growth affects the crystal structure of different metals to different degrees, probably due to their different intrinsic mechanical properties. Energetic neutral particle bOlnbardnlent can also be expected to give rise to similar lattice distortions. Unit lattice parameter changes due to energetic neutrals greater than 1 % have been reported (15). In general, however, the topic of energetic neutral bonlbardment during plasnla or ion beam sputtering is often ignored or forgotten.
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The Modification of Films by Ion Bombardment
179
A series of ion beam sputter deposition experiments by Kay et al has examined such changes in lattice spacing which can be clearly attributed to energetic neutral bombardment during deposition ( 15). In these experiments, an ion beam from a Kaufman-type ion source was incident on a sputtering target at 50 degrees fron1 the target normal. Substrates were arrayed such that a range of angles from the target to the sample were surveyed. Three general results were obtained. First, the films were highly oriented, with the (111) planes parallel to the surface. Second, the (111) lattice spacing was observed to increase with increasing deposition angle in most cases. Finally, there was a clear correlation between the magnitude of the lattice expansion and the ion-to-target mass ratio, the largest expansions being for the smallest ratio. These lattice distortion effects can be attributed to the reflection and Auger neutralization of the energetic ions sputtering the target. This energetic neutral bombardment effect on the growing film would be expected to be largest for deposition angles close to the incidence angle (50°) and targets for the cases of high reflection (low ion-to-target mass ratio). (See Fig. 6.) 12
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10.3.1.6 Surface diffusion. Quite a few authors have observed what appears to be enhanced surface diffusion of surface adatoms in cases of ion bombardment during deposition. Perhaps the only, fully quantitative work in this area are studies of individual atoms and groups of atoms on field emission tips during very low level ion bombardn1ent (16,17). While these studies are indeed important, it is not clear how the results compare to a realistic case of ion bombardment during deposition. In a thin film deposition mode, perhaps the classic example of bombardment-enhanced changes in surface diffusion is the much-discussed work of Marinov and co-workers (18). In this work, energetic ions incident onto a surface in the early stages of film growth lead to much larger cluster sizes and increased inter-cluster distances. The increased cluster sizes are thought to be due to both
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Handbook of Ion Beam Processing Technology
enhanced surface adatom mobility as well as the destruction or break-up of smaller clusters due to the ion bombardment. A model describing this latter effect has been proposed by Robinson (19). Clusters of some sub-critical size will be dissociated by the energy of the incident ion into free adatoms. Increased surface diffusion may be caused by the excitation of surface phonons by the ion impact or else the formation of very shallow collision cascades. The effect of ion bombardment on the surface may also be to alter the surface binding energies for adatoms. Barnett, et al have used a thermally-stimulated desorption experiment to measure both increases and decreases in surface binding energy induced by ion bombardment (20). One technique of estimating the magnitude of the enhancement of surface diffusion due to ion bombardment has been to use measurements of the resulting films and structures afterwards. An example of the significant changes in film morphology which can be traced to surface diffusion is the case of impurity-induced sputter cones (21,22). In this case, the arriving flux of energetic ions greatly exceeds the arrival rate of condensing, film atoms. The condensing adatoms diffuse across the surface and participate in the formation of clusters or islands, which would be the first stage of film growth in a conventional deposition process. Due to the high ion fluxes, the surface is sputtered at a significant rate. The clusters, or islands, however, may be stable under this bombardment because the arrival rate of adatoms due to surface diffusion exceeds the removal rate by sputtering. As the net sputtering rate of these clusters is low compared to the areas between clusters, the surface topography changes with increased sputtering time and sputter cones are formed. The spatial density of these cones is a measure of the surface diffusion. Measurements of surface diffusion with this techniques have shown a strong influence of the incident arrival rate of ions on the magnitude of the surface diffusion (Chap. 17). 10.3.1.7 Density. Films fornled by evaporation are often characterized by an open columnar structure with extended void structures. Sputtered films, depending on the deposition temperature and sputtering conditions, may have a variety of crystalline forms. Previous work by Movchan and Demichisin (23) and also by Thornton has described these effects for sputtered films in a classic drawing of film structures (24), which is shown as Figure 1 in Chapter 19.
Concurrent ion bombardment during an evaporative deposition has been shown to modify the columnar structure of the film, resulting in smaller grain sizes and increased density. This result has been also modeled by means of molecular dynamics calculations, and this work is described in detail in Chapter 13. One result of the reduction in voids and the elimination of the columnar structure is that the films are less porous, and as such, less susceptible to environmental change over time (25). This is critical for optical films, and this feature will also be discussed in more detail in Chapter 19. Another result of the reduction in voids and the increase in film density to near bulk values is an increase in the optical index of refraction. A recent detailed study (26) of the effect of ion bombardment during film growth on optical properties of thin Cu filnls by Parmigiani et al identified the structural origins of the observed non-bulk-like optical density as being associated with voids and grain size. It was shown that appropriately modifying the bulk dielectric function to account for the bOlnbardment induced, observed changes in voids and grain size, allowed accurate mod-
The Modification of Films by Ion Bombardment
181
eling of the observed optical density. In contrast to other reported findings in these relatively thin films (520 A ) prepared with normalized ion energies. En of 41-96 eV, the specific density decreased, from 7.587 to 6.867 g/cm3, as En increased. The absolute ion energy bombarding these films during film growth was much higher (500 eV) than that used in the nlolecular dynamics modelling used by Muller in Chapter 13. These results indicate that absolute ion energy as well as the ion to atom ratio are critical and no universal statement about effects on film density are valid unless all three are clearly defined. 10.3.1.8 Epitaxial temperature. Enhancements in epitaxy and the lowering of the minimum temperature required for epitaxial growth have been observed as a result of concurrent bombardment of film surfaces during growth (27,28). In a related nlode, similar effects have been observed for the direct deposition of low energy ion beams of metallic and semiconductor species. The latter includes mass-filtered low energy beams of Ag and Si at energies of 25-100 eV (29,30) as well as Ion Cluster Beam (ICB) experiments in which a fraction of the vapor stream in an evaporation mode is ionized and accelerated to the substrate. The earlier chapter on ICB (Chapter 4) describes some of these experiments. Care nlust be taken that this lowering of the epitaxial tenlperature by ion bombardment be viewed in parallel with defect formation during epitaxy as a function of ion energy as mentioned in the earlier section of this chapter. Muller has modeled the process of low energy bombardment during deposition and has found that there is a local atomic rearrangement which may result in a relaxation of atoms into lower energy sites. (31,32). This topic is described in more detail in Chapter 13 10.3.1.9 Film stress. Numerous experiments have reported significant changes in the resultant film stress attributable to energetic bombardment during deposition. The development of stress in films under ion bonlbardment has been attributed to several factors, including recoil implantation, implantation of inert gas species, the formation of local thermal spikes which result in an annealing-like effect, changes in the impurity level of the fHnl (33,34), enhanced surface mobility, as well as other features. Thornton and Hoffman (35-37) and others have generated a large body of work over the past 15 years dealing with filnl stress-related issues as encountered in plasmas in which effectively the energy and flux of particles bombarding the growing film have been systematically changed. These changes were induced by changing the bias on the sample as well as the chamber pressure in various discharge configurations. Clearly, in addition, thermal expansion mismatch with the substrate can cause severe stress-induced interfacial problems, often resulting in film peeling. Hirsch and Varga have noted that Ge films deposited with concurrent ion bonlbardment were less likely to peel off the substrate, presumably due to lower intrinsic stresses in the film (38). They observed a critical ion-to-atom arrival ratio for a reduction in stress sufficient to eliminate peeling. Systematically changing En' the energy delivered per arriving condensible atonl, has been shown in well defined beam experiments to change stresses from tensile to compressive (6,39) which suggests that film stress can be tailored at will, provided the other ion bombardment induced nlicrostructureal changes are compatible with particular applications. In the case of ion beam sputter deposited films, it is quite possible, depending on the particular geometry of the target and the sample and the relative masses of the target and gas atoms, that the films will receive a significant flux of reflected, energetic neutrals
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during the film deposition. Huang et al (6,40) have observed tensile-to-compressive stress changes, which correlate with measured lattice expansion and smaller grain sizes. The energetic bombardment of the films in these studies was strictly due to the reflected, energetic neutrals. Sun (41) in a similar study of ion beam sputtered Mo films has reported a similar result. That work found that the film stress becomes more compressive with increasing incident ion energy, or effectively increasing energy in the reflected neutral atoms. 10.3.1.10 Surface Topography. The surface nlorphology of a film often critically depends on the flux and type of energetic species arriving at the surface during the deposition. One aspect of the surface topography is related to grain size and orientation, which have been briefly described above. Another aspect of surface topography may be related to surface diffusion, which may be enhanced by energetic particle bombardment. A third aspect of the surface topography is a consequence of physical sputtering (often known as resputtering) which is an inevitable consequence of energetic bonlbardnlent above the threshold for sputtering (typically a few tens of e V). The sputter yield has been found to be strongly dependent on the angle on incidence for the ion or energetic neutral. The result of energetic bombardment during deposition is that topographical features which protrude up from the rest of the surface plane are more rapidly etched than the flat surfaces. Thus, the result of the resputtering is a smoother, more featureless film. Contributing to this result is the inordinately high yield for such topographical features as over-hangs, which can be forward sputtered down onto the underlying surface.
These effects in cOlnbination have a practical application in the deposition of films, for example, on electronic devices and packaging structures. Bombardment during deposition results in increased coverage of the depositing film over steps or lines that might be present in a complex device structure. This results in better electrical properties (such as lowered via resistance) and longer lifetimes due to less crevice or crack formation. On the negative side, however, bombardment during deposition adversely affects photoresist structures that might be used for lift-off depositions. In addition to the energetic damage to the resist, the resputtering and enhanced surface diffusion results in increased coating of the undersides of the resist structures (better step coverage), which inhibits lift-off of the film. Morphological features (surface roughness) of thin films can greatly influence the magnetic properties such as the coercivity (threshold energy for domain motion) which greatly impacts the magnetization reversal process in all magnetic recording devices. For example, comparison of Fig. 7(a) and (b) shows the effect of energetic ion bombardment during filnl growth on the morphology of a Ni filnl. Figure 7 (c) shows that energetic neutral bombardment, as described in the earlier section, gives rise to similar smoothing of film morphology. In fact, in long mean free path experiments (ion beams and low pressure plasmas) where the sputtered particles (1-10 eV) retain their kinetic energy until they deposit on the substrate, much smoother film morphologies are observed at similar thicknesses and deposition rates than is the case for thermally evaporated films (15), (Fig. 7 (e),(f)).
The Modification of Films by Ion Bombardment
0.5,u
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(d )
( a)
0.5,u
0.5,u
(e )
( b)
0.5,u
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183
(f )
Figure 7: Film morphology changes in Ni films grown under different bombardment conditions: (a) on a grounded substrate at 9.2 Pa Ar pressure, (b) in the presence of ion bombardment (biased substrate) in a 9.2 Pa Ar discharge, (c) on a grounded substrate in a 0.13 Pa Ar discharge, (d) in the presence of ion bombardment (biased substrate) in a 0.13 Pa Ar discharge, (e) evaporated Ni film, and (f) sputtered Ni film produced in an Xe ion beam system with secondary ion beam off. All films were approximately of similar thickness.
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10.3.1.11 Implantation of gas atoms. Energetic gas particles, ego inert gas atoms, bombarding the growing film surface can be expected to be trapped during the film growth depending primarily on the energy of the incident particles and the temperature of the substrate and the degree of crystallographic order of the growing film. Early work by Winters, et al (42) demonstrated that at room temperature several atomic percent of the inert gas can be readily trapped in a growing polycrystalline film. The amount of gas trapped can be systematically altered by control of the incident energetic ion flux and the substrate temperature as well as the relative flux of energetic gas particles to condensible metal atoms. In polycrystalline nletal films the trapping probability of energetic inert gas particles dropped off dramatically at deposition temperatures above approximately 350 o C, presumably due to enhanced diffusion of inert gas to the surface along grain boundaries and a lower sticking probability. The resputtering of previously embedded inert gas also showed a temperature and "coverage" dependance. Quite reasonable predictions of inert gas trapping near room temperature can be made from simple sticking probability and resputtering coefficients readily available in the literature. So, for example, the prediction that inert gas content must go through a maximum as the energy of the inert energetic gas particles per arriving metal condensible increases was experimentally verified by Zieman et al (43). The question of where primarily the inert gas is trapped within a polycrystalline filnl is less clear. Recent structural studies by Parmigiani et al (44) on very thin metal films showed that whereas the observed lattice deformation, grain size, stress and the measured quantity of voids were strongly dependent on the energy delivered to the growing film during film growth. On the other hand, the amount of gas trapped did not correlate with the observed crystollographic anomolies. Evidence is presented that most of the gas in these thin polycrystalline films is trapped in voids within or between crystallites, quite in contrast to gas trapped in epitaxially grown films (14) and films bombarded with energetic ions after deposition. Early work by Cuomo et al (45) showed much higher levels of inert gas trapping can be accomplished in anlorphous films, for exanlple in transition metal-rare earth alloy deposited at room temperature. Several examples have been reported (42) where inert gas has been trapped in both polycrystalline and epitaxial films in which diffraction data shows the gas to be in the solid, crystalline state. Recent work by Cuomo et al has shown that very high levels of inert gas can be trapped in various void structures within the film (46). In these cases, depending on the type of void and the gas atom size, very low stress films have been produced with gas incorporation levels as high as 25 % • 10.3.1.12 Optical properties. The optical properties of thin films can be significantly altered by concurrent ion bombardment, particularly during evaporative deposition. Perhaps the most significant effect is the above mentioned change in the density of the film. Ion bombardment during deposition, at least at reasonably low levels, results in increases in film density and an increase in the index of refraction to near bulk values. A result of these changes is to reduce changes in the index of refraction upon exposure to air and water vapor. Another significant effect of ion bombardment is to alter the surface topography of the films. The general result is that lAD films are smoother with reduced optical scatter as compared to evaporative films. The general topic of ion bombardment nlodification of optical and dielectric filnls will be discussed is great detail following chapters. 10.3.1.13 Resistivity. The electrical resistivity of a thin film can be modified by both structural and chemical changes in the film. The structural effect on the resistivity results
The Modification of Films by Ion Bombardment
185
from the general decrease in grain size for bombardment-modified films. This generally increases the resistivity due to increased scattering at grain boundaries, as observed by Huang et al. (6) However, the role of ion bombardment on the degree of impurity incorporation can also influence the electrical resistivity of the deposited film (39) and will be discussed in more detail in the following chapter. 10.3.2 Chemical Effects
10.3.2.1 Stoichiometry. Energetic particle bombardment during film deposition can have a significant effect on the chemical composition of the resulting film. One obvious case is that of reactive deposition or etching, where the incident ion or neutral chemically reacts with the film atoms on the surface, forming a compound. If the compound is desorbed, this process is known as reactive etching. This topic will be discussed in detail in Chapter 12. If the product has a low vapor pressure at the temperature of the sample, then a compound film may be formed. This subject, reactive deposition, will be discussed below and also in later chapters.
Energetic particle bombardment during deposition may also contribute to more subtle changes in film composition. For example, ion beam cleaning is routinely used to sputter clean surfaces of contaminants prior to deposition. In addition, low level ion beam bombardment during the (ilm deposition process has been found to reduce contamination from background gas species, resulting in higher purity films (39). In general, from detailed studies from Winters et al (47) it can be seen that low Z number chemisorbed impurities (eg. N, 0, C), etc.) will be resputtered with greater probability from a growing higher Z number metal film than the metal atoms, thereby contributing to a lower impurity trapping of these common background constituents. Ion bombardment during the deposition of a compound film may alter the relative composition of the film, due to preferential sputtering of the higher-yield species from the film. This is quite similar in concept to the formation of an altered layer on an alloy target during sputtering of the target. A clear example of this effect is from the earlier work (48) in which alloys of Gd and Co were ion beam sputter deposited in the presence of a Ar ion beam directed at the film (Fig. 8). More recent work with the 4 and 5 component alloys used for high temperature superconducting films has demonstrated similar effects (48). A more subtle chemical effect is the contamination by sputter deposition from other surfaces. In an ion beam experiment, the reflected neutrals from the ion bombardment of the target often have sufficient energy to cause sputtering. This has been described above in terms of changes in the film deposition rate and physical properties. In addition, these energetic particles often sputter other surfaces within the vacuum chamber, such as the chamber walls or other fixtures. This sputtering results in the sputter deposition of impurities onto the film. Unfortunately, the seriousness of this effect often depends on what was coated onto the walls in previous sputtering runs. One solution to this problem is the coating of all interior chamber surfaces with the desired target material. This aspect of chamber conditioning is often overlooked in ion beam experiments. A very recent paper by Winters et al (49) on nlulticomponent sputtering demonstrates that the incident ion energy is critical in deciding if, and to what degree, preferential sputtering will result from targets containing highly dissimilar mass atoms. It is shown that the direct collision sputter processes near the service (as opposed to the collision cascade
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Handbook of Ion Beam Processing Technology
processes) play a more or less dominant role in various energy regimes leading to quite different compositional changes.
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In all plasma deposition systems where the plasma potential is above the sputtering threshold, i.e., several tens of volts (see earlier section) this contamination source involving all grounded surfaces in contact with the plasma (eg. fixtures, chamber walls, shutters, etc) can cause serious problems depending on the level and type of impurities that can be tolerated in the film. Chamber contaminants at the several thousand parts-per-million level are very difficult to avoid. Reference (2) demonstrates these points rather convincingly in the sputtering of a noble metal in a supported discharge V.H.V. system. where stainless steel from the grounded charrlber walls ended up in the film at the several 1000 ppm level depending on the plasma potential. The alteration of the chenlical stoichiometry of a film due to concurrent ion bombardment has been found to cause changes in other aspects of the film properties. As mentioned above, the electrical resistivity is often related to either the impurity level in a film or else the composition ratios in an alloy. The stress in a film has been correlated to the presence of impurities for the case of Nb (39). The optical properties of the film, in particular the absorption coefficient and to some degree the index of refraction are also sensitive to ion-bombardment induced chemical changes. Energetic bombardment in the case of a dielectric oxide film may result in the formation of other oxidation states, often described as sub-oxides. These materials often have increased absorption levels over the desired oxide material. Perhaps the worst case is Ti0 2 , which readily forms sub-oxides due to ion bombardment (50). Nevertheless, energetic bombardment during the reactive deposition of optical films has demonstrated clear advantages over other techniques. This general topic will the the basis of later Chapters.
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10.4. REACTIVE FILM DEPOSITION 10.4.1 Reactive Ion Beam Deposition
Typically, compound thin films are deposited using such techniques as reactive sputtering and reactive evaporation, in which a metal is deposited in the presence of a background reactive gas or plasma. These techniques lack control over the fundamental deposition paranleters, since only external variables such as gas pressure, flow ratios, rf power, and electrode voltages are controlled. In the ion beam assisted deposition techniques described here, direct measurement and control are often available for the fundamental deposition parameters of metal atom arrival rate, reactive species arrival rate (as ions), energy of arrival of the reactive species, and direction of arrival of both metal and reactive species. The background gas pressure is low (10- 5 Torr) and mayor may not participate in compound formation. A good example of a dual ion beam reactive deposition process is the synthesis of AIN, where control and quantitative analysis of the process is demonstrated (51). This will be discussed below. This technique has been used to synthesize and study other compound systems such as TiN, ZrN, HfN, as well as the higher nitrides Tix N y ,Zr3N 4 and Hf3N 4 (52). The dual ion beam deposition technique has been extended to include in situ monitoring of particle fluxes to allow a complete analysis of incorporation probabilities and sputtering yields over a wide range of film composition obtained in each deposition run. Together, these features represent an approach to compound film formation giving substantial quantitative information on which to base an analysis of film properties. As a comparative example, we will discuss a study of Cu-O compound formation, using ion beam assisted evaporative deposition. In this case, a variety of compounds could controllably be formed by systematically varying the oxygen ion energy and ion-to-Cu atom arrival rate ratio. The ion energies in this case ranged from 70 to 200 eV per singly ionized oxygen ion. It appears that a new metastable phase of Cu-oxide has been produced in this work. A third example which will be discussed in later chapters is the area of ion assisted deposition of films for optical applications by both dual ion beam and ion beam assisted evaporation techniques. The films produced are clearly superior optically and structurally over comparative films produced conventionally with no concurrent ion bombardment. 10.4.2 Reactive Deposition by Dual Ion Beam Synthesis: AIN
Aluminum nitride (AIN) is an inert material of interest for a capping or diffusion boundary layer for GaAs devices. In this experimental work (51), an Al target was sputtered with an Ar ion beanl while a second ion source was directed at the growing film surface. The second beam was generated from N 2 at energies of 100-500 eV. The substrate location was oriented such that there was a gradient in both the Al deposition rate and the nitrogen ion bOITlbardment rate across the sample plane. The rate of incorporation of nitrogen into the sample film is shown in Fig. 9 (51). It was observed that very little nitrogen was incorporated in the film in the absence of directed ion bombardment, and also that it was not possible to exceed a saturation value of N / Al = 1.0 even under excess nitrogen ion fluxes. The Ar incorporation rate was 1.5% or lower, and no oxygen contamination was measured. The visual appearance of these films changes with increasing N content: shiny metallic in regions of low N content (N/Al <0.54); gray and dull (0.54 < N/Al < 0.82); shiny gray (0.82 < N/Al < 1.0); transparent (N/Al = 1.0, stoichiometric AIN). The resistivity increases from a value of 20 JLQcm for N/Al = 0.1 to insulating for AlN.
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Handbook of Ion Beam Processing Technology
The mononitrides TiN, ZrN, and HfN also have been synthesized and studied by dual ion beam deposition (52). They are also very hard and useful as wear resistant surfaces and make excellent contact materials and diffusion barriers, for example, for aluminum. In this work, the nitride composition was altered by adjusting the relative concentration of nitrogen and argon in the second ion beam. When 100% nitrogen gas is used, the film composition shifts to a higher nitrogen concentration producing the insulating phase Zr3N4 and Hf3N 4. The TixNy that is produced is a bluish in color and conductive. The structure studies for Zr3N4 and Hf3N 4 indicate that they are an ordered defect structure with about 25% vacancies. This is confirmed by bandstructure and density of state calculations as well as by the presence of an inert gas concentration about 19 % when the structure is produced under ion bombardnlent (53). The inert gas is thought to occupy the vacancy site in the structure and, if saturated, would be about 25 at. % . The gas incorporation varies with the size of the inert gas ion as was also found for sputtered amorphous metal films (54).
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The use of low energy, reactive ion beams during filnl deposition has been growing rapidly in recent years as more reliable sources become available. Many of these applications require the use of oxygen in the ion beam at low energy. Such modifications as single grids, thoriated filaments and the coating of the anodes with conductive oxides (iridium oxide, for example) have alleviated some of the problems associated with oxygen and hot filaments. The Hall-Effect or Closed-drift ion source (Chap. 4) has been developed to overcome some of these problems as have nlicrowave-based sources (Chap. 3). In recent work by Guarnieri, et aI., a systematic study of Cu-O compound formation
The Modification of Films by Ion Bombardment
189
prepared by low energy « 200 eV) oxygen ion bombardment of the growing film has been reported (55). In this study the copper was deposited by resistive heating evaporation at rates ranging from 0.03 to .3 nnl/sec. A single grid was used on the ion source to obtain 0t ion current densities of .01 to 0.2mA/cm2 at the substrate. The O/Cu relative arrival rate at the substrate was varied from 0.1 to 3.1. During deposition, the background O 2 was varied from 5x10- s to 7x10- 4 Torr. In the absence of an ion beam, less than 2% oxygen was incorporated into the films at these high background pressures. After deposition the composition of the films was measured by in situ Rutherford backscattering using 2.3 MeV He ions. The structure or presence of compounds was determined by DebyeScherrer-Hull x-ray analysis. The measured o/eu ratio in the film is plotted versus the 0/Cu ratio arriving at the substrate in Fig. 10. The dotted line with a slope of one corresponds to an 0/Cu ratio in the film equaling the arrival ratio where all of the oxygen is supplied by the ion beam. For 100 eV ions, the constant composition of the curves around the stoichiometric compositions in Fig. 10 strongly suggests the formation of compounds in itself. Subsequent x-ray analysis confirmed the existence of polycrystalline CU20 and CuO. The material whose composition is labeled CUsO 4 was also determined to be polycrystalline. It was concluded that this deposition process has produced a new metastable compound: CUS0 4 •
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Handbook of Ion Beam Processing Technology
10.4.4 Optical Films by Ion Beam Assisted Deposition
The deposition of high quality optical thin films is an impressive example of ion beam assisted deposition. This topic will be covered in detail in a later chapter by Martin and Netterfield. 10.5 SUMMARY
Energetic particle bombardment can have a significant effect on the properties of evaporated and sputter deposited films. In nlany cases, the bonlbardnlent is intentionally added by the addition of ion beams, ionization and acceleration of evaporated flux or substrate bias. However, in many experiments, particularly those using ion beam sputtering, the magnitude of the energetic bombardment by neutrals is ignored or underestimated. The effects of energetic bombardment include significant physical changes in the crystal sizes and orientations, defect densities, electrical and optical properties, chemical stoichiometry and surface morphologies. While there exists no general model for the effect of energetic bOlnbardment on growing film properties, a range of experiments have started to systematically explore the interrelations of the various physical and chemical effects. These results will be helpful in elucidating what phenomena are relevant in any particular experinlental system.
10.6 REFERENCES
1. K. Kohler, J.W. Coburn, D.E. Horne, E. Kay and J.H. Keller, J. ADO!. Phys. 57 (1): p. 59 (1984). 2. P. Ziemann, K. Kohler, J.W. Coburn and E. Kay, J. Vac. Sci. & Techno!. Bl(I): p. 31 (1983). 3. P. Ziemann and E. Kay, J. Vac. Sci. & Techno!. 21(3): p. 828 (1982). 4. W. Eckstein and J.P. Biersack, Reflection of Heavy Ions. Z. Phys. B 63: pp. 471-478 ( 1986). 5. R.E. Somekh, Vacuum 34: p. 987 (1984). 6. T.C. Huang, G. Lim, F. Parmigiani and E. Kay, J. Vac. Sci. & Techno!. A3: p. 2161 (1985). 7. R.A. Roy, D.Yee, J.J. Cuomo, Control of microstructure and properties of copper fHnls using ion assisted deposition. J. Vac. Sci. & Technol A6: pp. 1621-1626 (1988). 8. P. Ziemann and E. Kay, J. Vac. Sci. & Techno!. AI: p. 512 (1983). 9. LockSee Yu, J.M.E. Harper, J.J. Cuomo and D.A. Smith, Alignment of Thin Films by Glancing Angle Ion Bonlbardment During Deposition. ADO!. Phys. Lett. 47: pp. 932-933 (1985).
The Modification of Films by Ion Bornbardment
191
10. Lock See Yu, J.M.E. Harper, J.J. Cuonlo and D.A. Smith, Control of Thin Film Orientation by Glancing Angle Ion Bombardment during Growth. J. Vac. Sci. & Technol. A4: p. 443 (1986). 11. R.M. Bradley, J.M.E. Harper and D.A. Smith, Theory of Thin Film Orientation by Ion Bombardment During Deposition. J. Appl. Phys. 60: pp. 4160-4164 (1986). 12. E. Krikorian and R.J. Sneed, Astrophys Space Sci. 65: p. 129 (1979). 13. J .E. Greene, Low Energy Ion BOITlbardnlent during FHnl Deposition from the Vapor Phase: Effects on Microstructure and Microchemistry. Sol. St. Tech. 30: p. 115 (1987). 14. L. Hultman, Dissertation 186, p163-177, Linkoping University, Sweden, 1988. 15. E. Kay, F. Parmigiani and W. Parrish, Effect of Energetic Neutralized Noble Gas Ions on the Structure of Ion Beam Sputtered Metal Films. J. Vac. Sci. & Technol. A5: p. 44 (1987). 16. M. Drechsler, M. Junack and R. Meclewski, Surf. Sci. 97: p. 111 (1980). 17. Zh. I. Dranova amd I.M. Mikhailovskii, Sov. Phys. Sol. St. 12: p. 104 (1970). 18. M. Marinov, Thin Solid Films, 46: p. 267 (1977). 19. H.R. Kaufman and R.S. Robinson, J. Vac. Sci. & Technol., 16: p. 179 (1979). 20. S.A. Barnett, H.F. Winters and J.E. Greene, Surf. Sci. (in press, 1987). 21. S.M. Rossnagel, R.S. Robinson and H.R. Kaufman, Surf. Sci. 123: p. 89 (1982). 22. R.S. Robinson and S.M. Rossnagel, Diffusion Processes in Ion BOITlbardment Induced Surface Topography, in Ion Bombardment Modification of Surfaces. Ed. by O. Auciello and R. Kelly (Elsevier, NY 1984) 299. 23. B.A. Movchan and A.V. Demchisin, Investigation of the Structure and Properties of Thick Vacuum-deposited films of Ni, Ti, W, Alumina and Zr02 • Fiz. Met. Metalloved 28: pp. 653-660 (1969). 24. J.A. Thornton, Influence of Apparatus Geometry and Deposition Conditions on the Structure and Topography of Thick Sputtered Coatings. J. Vac. Sci. & Technol. 11: pp. 666-670 (1974). 25. P.J. Martin, H.A. Macleod, R.P. Netterfield and C.G. Sainty, Appl. Opt. 22: p. 178 (1983). 26. F. Parmigiani, E. Kay, T.C. Huang and J.D. Swalen, Appl. Opt. 24: p. 3335 (1985). 27. C. Schwebel, F. Meyer, G. Gautherin and C. Pellet, J. Vac. Sci. & Technol. B4: p. 1153 (1986).
192
Handbook of Ion Beam Processing Technology
28. S.V. Krisnaswamy, J.H. Rieger and M.H. Francombe, J. Vac. Sci. & Techno!. to be published. 29. G.E. Thomas, L.J. Beckers, J.J. Vrakking and B.R. de Koning, J. Cryst. Growth 56: p. 257 (1982). 30. P.C. Zalm and L.J. Beckers, ADD!. Phys Lett. 41: p. 167 (1982). 31. K-H. Muller, J. ADD!. Phys. 58: p. 2803 (1986). 32. K-H. Muller, ADD!. Phys. A40: p. 209 (1986). 33. E.H. Hisrch and I.K. Varga, Thin Solid Films 69: p. 99 (1980). 34. D.R. Brighton and G.K. Hubler, Binary Collision Cascade Prediction of Critical Ion-to-Atom Arrival Ratio in the Production of Thin FIlms with Reduced Intrinsic Stress. Nuc!. Instr. & Meth. in Phys. Res B28: pp. 527-533 (1987). 35. D.W. Hoffman and J.A. Thornton, Thin Solid Films, 40: p. 355 (1977). 36. J.A. Thornton and D.W. Hoffman, J. Vac. Sci. & Techno!. 18: p. 203 (1981). 37. J.A. Thornton and D.W. Hoffman, J. Vac. Sci. & Techno!. A3: p. 576 (1985). 38. D.W. Hoffman and M.R. Gaerttner, J. Vac. Sci. & Technol.17:425 (1980). 39. J.J. Cuonlo, J.M.E. Harper, C.R. Guarnieri, D.S. Vee, L.J. Attanasio, J. Angilello, C.T. Wu and R.H. Hammond, J. Vac. Sci. & Techno!. 20: p. 349 (1982). 40. E. Kay, Examples of Ion Bombardment Effects on Film Growth and Erosion Processes - Plasma and Beam Experiments in Erosion and Growth of Solids Stimulated by Atom and Ion Beams. Ed. by G. Kiriakidis, G. Carter and J.L. Whitton, (NATO-ASI series, 112 (1986). 41. S.S. Sun, J. Vac. Sci. & Techno!. A4: p. 572 (1986). 42. H.F. Winters and E. Kay, J. ADD!. Phys. 38: p. 3928 (1967). 43. P. Ziemann and E. Kay, J. Vac. Sci. & Techno!. AI: p. 512 (1983). 44. F. Parmigiani, E. Kay, T.C. Huang, J. Perrin, M. Jurich and J.D. Swalen. Phys. Rev. 33: p. 879 (1986).
~
45. J.J. Cuomo and R.J. Gambino, J. Vac. Sci. & Techno!. 14: p. 152 (1977). 46. J.J. Cuomo, unpublished. 47. H.F. Winters and P. Sigmund, J. ADD!. Phys. 45: p. 4760 (1974). 48. J.M.E. Harper and R.J. Ganlbino, J. Vac. Sci. & Techno!. 16: p. 1901 (1979).
The Modification of Films by Ion Bombardment
193
49. H.F. Winters and E. Taglauer, Phys. Rev. B 35(5): p. 2174 (1987). 50. S.M. Rossnagel and J.R. Sites, XPS of Ion Beam Sputter-Deposited Si02, Ti0 2 and Ta20s. J. Vac. Sci. & Techno!. A2: p. 376 (1984). 51. J.M.E. Harper, J.J. Cuomo and H.T.G. Hentzell, A oo!. Phys. Lett. (1983).
43: p. 547
52. D.S. Yee, J.J. Cuomo, M.A. Frisch and D.P.E. Smith, J.Vac. Sci. & Techno!. A4: p. 381 (1986). 53. Karl-Heinz Schwarz, private communication, 1985. 54. F.T.J. Smith, J. ADO!. Phys. 41: p. 4227 (1970). 55. C.R. Guarnieri, S.D. Offsey and J.J. Cuomo, J. Vac. Sci. & Techno!. (to be published).
11
Control of Fill11 Properties by lon-Assisted Deposition Using Broad Beal11 Sources Ronnen A. Roy and Dennis S. Vee
11.1 INTRODUCTION
Ion bombardment of films during growth has long been recognized as an important tool in modifying resultant film properties. Beginning with bias sputtering and techniques such as ion plating, researchers have sought to control film properties by varying the amount of bombardment (1-4). While a large body of literature exists detailing the qualitative relation between increased ion bombardment and film property changes, the understanding is often not sufficient to reproduce the sanle results in different deposition systems. With the advent of broad beam ion sources the ability to modify and reproducibly control film properties (5-7) has improved due to several factors. A major advantage is that ion energy and ion flux are decoupled, allowing for independent variation of either parameter. Another advantage is that the plasma is contained in the ion source, providing a much simpler deposition environment near the substrate. Thus, one can better quantify the anlount and energy of the various species incident on the film/substrate during growth, making the interpretation of data much simpler. In the present chapter we focus on the application of ion-assisted deposition (lAD) for property modification of various metal films (6-10). Based on the results of niobium, chromium, copper, and tungsten, the importance of parameter selection, such as ion energy, in controlling property changes is highlighted. Furthermore, the modifying effects of substrate temperature (Ts )' substrate type, and type of material, on film properties is also highlighted. A review of filnl physical property changes, microstructure changes, and their interrelation is given. In the last part of the chapter operative ion bombardment mechanisms and their relation to deposition parameters is discussed. 11.2 PROPERTY CHANGES 11.2.1 Ion Energy Effects
Data showing the effect of ion bombardment on film properties is either expressed simply as ion flux (assunling constant atom flux), as a relative Ar ion/nletal atonl flux at the substrate, or as energy in eV/ atom, which is simply the product of the relative ion/atom flux and the average ion energy.
194
Control of Film Properties by lon-Assisted Deposition
195
The range of ion energies used in studies cited in the present chapter is 60-800 eV. This covers a large portion of the range typically used in lAD and in plasma deposition systems. As will be seen, changing the ion energy has profound effects on behavior of certain properties under increased ion flux, while other properties show less drastic change. For the case of copper, the effect on various properties was examined using concurrent Ar ion bombardment dUrin~ evaporative deposition of Cu. The films were typically 5 JLm thick, deposited at 5-20 A sec-to Small Kaufman ion sources were used to generate ion beams at energies between 62 and 600 eV. A single grid configuration was used at 62 and 125 eV, while dual grid was used at higher energies. The stress was found to be modified by ion bombardment in the resulting films. The stress modification was found to be qualitatively the same at 600 and 62 eV (Fig. 1), showing a decrease in tensile stress. This general phenomenon has been observed in other studies(6,7,11).
5
m-
e
4
600 eV, 62 C • Si(100)
x
E
-...u
3
(/)
Q)
(a)
c >-
~
"
I
L
;
:9-
\ \
(/) (/)
\,
~
en
u '00 c
62 eV, 107 C o Si (100) A Si(111)
\
\
0
\
'S -1
\~
/JIg,.';
..,-
-_._._.-...-'
A.;'"''
0
E
-2 0
10
5
15
20
30
25
35
eV /atom
5
m-
e
4
600 eV, 62 C • Si(100)
x
E
-...u
3
62 eV, 107 C o Si (100) A Si(111)
(/)
Q)
(b)
c >-
2
:9(/) (/)
~
en u c
0
'00
.~-_._.:_.....-._.~/'!-~....-._._._._._._._._._._._._.~
'S -1
0
.E:
-2 0.0
0.1
0.2
0.3
0.4
0.5
Ion / atom ratio
Figure 1: Intrinsic Stress in 5 JLm copper films, shown as a function of (a) eV/atom and
(b) Ar/Cu arrival rate ratio, for films subjected to 62 and 600 eV ion bonlbardment (8).
196
Handbook of Ion Beam Processing Technology
Figure 2 shows the stress as a function of ion flux fronl studies of chronlium (7) and tungsten (11). Within each material the stress behavior appears qualitatively the same over a broad range of ion energies. However, these studies showed that the changes in stress were related to the ion energy, and as ion energy is lowered, the flux at which compressive stress is reached becomes higher, a result consistent with previous theories of stress modification (2).
• 100 eV o 200 eV A 300 eV x 400 eV • 800 eV
(a) tJ) tJ)
~
+-'
en
Q)
C>
co
~ -2
<X:
-4 0.0
0.02
0.01
0.03
0.04
0.05
0.06
20 N-
E
(J
15
......... tJ)
Q)
c >-
"0
C(
(b)
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0
~en
-5
co
A
~"''''
-10
~
!
.
...... ......
....,"" ...
5
~
Q) 0)
. ."- ,,< ......
0
en en
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I
._.-._._._.-.-._.- _._._._._._._._._._._._~:-:~~_._._._._._._._._.
__.-
..' .......
' ....
• 200 eV A 400 eV o 600 eV
"'~
m -15 >
<X:
-20 0.0
0.02
0.04
0.06
0.08
0.10
0.12
2 ion flux (mA/ cm )
Figure 2: Film stress in (a) Cr (7) and (b) W (10) films deposited by evaporation with concurrent Ar ion bombardment of varying energies.
Two other film properties, the microhardness and electrical resistivity, may also be modified by concurrent bombardment during deposition. In the case of Cu, the resistivity and microhardness behavior were found to have strong energy dependence (8). At 600 eV ion energy, the resistivity of films rises steeply with ion flux, surpassing 10J-tn-cm at high flux levels (Fig. 3 (a), top graph). On the other hand films deposited with concurrent 62 and 125 eV Ar bombardment (Fig. 3 (a), middle and bottom graphs) show only a small
Control of Film Properties by lon-Assisted Deposition
197
increase in resistivity from about 2 p,Q-cm at low levels to 2.6 p,Q-cm at high levels. Qualitative differences were also seen in microhardness changes under ion bombardment as the primary ion energy is lowered (Fig. 3(b)). At high energy little substrate dependence was seen (Fig. 3 (b), top and middle graph), while at low energy a strong dependence on substrate type occurs, although the absolute changes in microhardness were smaller. ( a)
(b)
14 .......----.--__._--.----r-----.----.---_----.
400 350
...
300
600 eV, Si(100} 0103 C A 77 C x 62 C
250 200
600 eV, Si/Si~ ... 230C .103 C .. 77 C +62 C
4
150
600 eV, Si I Si~
100
• Ts=230 C o Ts=103 C .. Ts=62 C
2
50
0
0
3.0
350
2.8
300
2.6
250
2.4
200 125 eV, 80 C .Si(loo} "Si(lll) • Si/Si~
2.2 2.0
.~
I
100
• Si(100} .. Si(111} • Si/Si~
01....----1...-..........-...1....-----11....----"'--..........-...1....-----1 250
2.4
N
E
r-----r'-.....,._-~---,r----,--.....,._-~---,
200
..···x
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1.8
~----I._......l.__......._._I....-___I...
0.0
11
00
.c
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50
0
.
62 eV, 107 C o Si(100) x Si(111} •
....L__----I
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 lonl Atom Arrival Ratio
..0"
150
o
oSi(loo) x Si(111} • Si/Si~
Q.)
1.6
~
62 eV, 107 C
.iii 0::
..
125 eV, 82 C
1.6
?
..
50
2.6
3C .:;
....
150
1.8
Eu E .c
....----;---;--..... ----------------;_.._. _-
Si/Si~
OL...------I.-......&...-..........- - - - J L . - - - - I . . . - - - - - . . . . L - - - - - - I
0.0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 lonl Atom Arrival Ratio
Figure 3: Resistivity (a) and microhardness, (b) for copper films exposed to 600, 125 v, and 62 eV Ar ion bombardment during growth.
Figure 4 shows resistivity vs ion flux from studies of niobium (6), chromium (7) and tungsten (11) films evaporated at about 400°C, with ion energies from 100 to 800 eV.
198
Handbook of Ion Beam Processing Technology
As the ion energy increases the resistivity behavior changes. Below about 300 eV, increased ion flux causes resistivity decreases, while at higher energies the resistivity increases. 40
I
I
I
Nb 400°C
35
• 100 eV x 400 eV
30 ... •••••• x
(a)
•••• x ••
25 ...
15 ... 101...---..L..----I-.-----L.--.........L---.J 0.02 0.04 0.05 0.0 0.01 0.03 60
r----~--_r_--....._..........- - _ _ . _ _ - _
Cr 360°C 50
(b)
40~
x
"
30
Figure 4: Resistivity in
.••.x·
~~~'"'S~:::::~:~:_~._._._.. "
20
: ~gg ~~
10
• 300 eV x 400 eV a 800 eV ......._ _..a...._ _L....-_---L._ _...L__
"'-0
0
~
--------.o.
~ _
0.0
0.01
0.02
0.03
(a) Nb (6), (b) Cr (7),
0.04
_.J
0.05
0.06
100 r-----.---~-__r--"""T"'"--......---
W 450°C
90 80
( c)
70
x
)(
200 eV x 400 eV • 600 eV
o
············x··.. ·········.. ·x···········.. ····)(,····~····
x
~
60 50 40
o
--------..---.----------------------v---v---------D-
30 20 ........- .......- - . . a . . . . - - L . . . . - - - - L - - . . . L - - - - - - J 0.0 0.02 0.04 0.06 0.08 0.10 0.12 2 current density (mA/cm )
and (c) W (10) evaporated films, shown as a function of ion flux for different concurrent bombardment-ion energies.
Control of Film Properties by lon-Assisted Deposition
199
11.2.2 Temperature Effects
In studies of both Nb (6) and W (10) increasing the substrate temperature was seen to have an effect on stress modification. Figure 5 shows the stress and resistivity behavior as a function of ion flux that was observed for niobium films evaporated at various temperatures under 100 eV bombardment. The maximum in tensile stress shifts to higher flux at lower temperatures, reaching approximately the same value before decreasing, analogous to the effect of lower ion energy seen in tungsten films. Furthernlore, the flux at which the maximum stress is reached corresponds to the point at which a minimum resistivity is obtained, above which flux level little resistivity change occurs. Similar stress behavior is also seen in the case of tungsten deposited under 400 eV ion bombardment, as shown in Fig. 6. In contrast to the 100 eV Nb films, however, the W resistivity behavior was found to be qualitatively different at different temperatures, increasing at high temperatures and decreasing at low temperatures (Fig. 7).
RELATIONSHIP OF CT mol WITH PURITY Ar+ ENERGY: 100 eV
~mOl CTmoll CT mOl 8!400'C 200·1'50.C
ICTma1 ........ IOO·C
N
E
u .......
( a)
c
~
"'0 (1)
g
~
G
b
en en w 0: ren
8 -2~
~~
:::[
i= ~
eo.
:;g (b)
•
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.x .
,,~ - - - - ' - - - - - + -
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A.~
F
[
F
~~ 60~.
40~1".
,,--H
i~I----.---
CRITICAL
BUL~~~~~·~...--I:::=:I""" . L~et'"2' ~ I I I o
0.01
0.02
0.03
ION FLUX j (mA/cm 2 )
Figure 5: Stress (a) and resistivity (b) as a function of ion flux in Nb films evaporated with concurrent 100 eV ion bombardment, shown as a function of substrate temperature.
200
Handbook of Ion Beam Processing Technology
o
N
o I
Thickness
oN L-
L.-
.L..-.
10
L.-
L...-
5000
A
---I"--
---I
0.12
0.08
0.04
Ion Flux (mA/cm 2)
Figure 6: Stress as a function of ion flux for evaporated W films shown at different substrate temperatures, displaying behavior similar to Nb films in Fig. 5.
+
0 0
E
\
N
U I
E
..c 0 <.0
0
'12
I
0
r-
'-'
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> -+oJ
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-
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o
Ul
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Q)
~
a
0
°el O
to
O'--_---I.._ _.....L.-_ _.L--_........I._ _.....L..._ _..L.....-_---L._ _.....L..._ _. . . I . - - _ - 1
o
0.02
0.04
0.06
0.08
0.10
Ion Flux CmA/cm 2 ) Figure 7: Resistivity as a function of ion flux for tungsten filnlS evaporated at various substrate temperatures with simultaneous 400 eV ion bombardment.
Control of Film Properties by lon-Assisted Deposition
201
11.3 FILM STRUCTURE MODIFICATION 11.3. 1 Ion Energy Effects
Such property changes as described above may be related to changes in film microstructure. Results of x-ray diffraction studies of ion energy effects on film microstructure are shown for Cu (Figs. 8,10) and Cr (Fig. 9). Figure 8 shows crystallite size versus bombardment for Cu films subjected to ion bombardment at different energies. There is a clear dependence of the crystallite size on ion energy in the Cu films, even for the same deposited energy-per-atom (eV / atom). At 600 eV the size decreases to about 300 A at high ion flux while at 62 eV it remains above 1000 A.
1600
.----,.....---...----....---~--_r__--_r__--.......__-____.
600 eV • T s =230 C • 103
1400
• •
1200
125 eV,82 C
62 eV, 107 C
I) --------
A···········
77 62
1000
800
600
• 400
•• • •
200
OL---...J....---....L..---..........
--.....L-------L...-----L.-----L------I
o
5
10
15
20
25
30
35
40
eV/atom Figure 8: Crystallite size measured by x-ray diffraction for evaporated copper films as a function of ion flux, for 600, 125, and 62 eV ion energy.
Films deposited with no bombardment have grain sizes between 1000 and 1400A , depending on substrate temperature. The input of as little as 5 eV/ atom reduced grain size fronl 1500 to 600A at 230°C. At higher flux levels the grain size reaches fairly constant levels. On the other hand, at 62 eV little change in size with increased ion flux was seen, with grain size at high ion flux levels similar to unbombarded films.
202
Handbook of Ion Beam Processing Technology
Figure 9 shows grain size changes in Cr films deposited at 360° C. When subject to 400 eV ion bombardment, high levels of ion flux are seen to decrease grain size, while 100 eV ion bonlbardnlent causes a monotonic increase in grain size over the flux range studied. 260 _ - -__---,.---~--___r"--___, 240
o~ 220 CD N (J5
.~
200
co +-'
en
5'
/:
______e
:/
180
e 100 eV 400 eV
160
.&
140 L..-_ _ 0.01 0.0
.....L..._ _----L.
0.02
..I....-_ _.........._ _- - - - '
0.03
0.04
0.05
2
Ion flux (mA/ cm ) Figure 9: Crystallite size as a function of ion flux for evaporated Chromium films deposited at 360°C.
The texture, or orientation of the evaporated copper films was found to show a strong dependence on ion energy as shown in Fig. 10. Figure 10 (a) shows that for copper films deposited with 600 eV ion bonlbardment the data show a peak in (111) orientation at very low flux, followed by a decrease in orientation at high ion flux, consistent with results observed by Huang et al (12) for thin silver films. However, at 125 eV ion energy the degree of (111) orientation was found to increase monotonically over the 0 to 40 eV/ atom range studied, sharply between 0 and 2 eV, and gradually thereafter; showing very strong (111) texture at high flux, slightly more so on Si substrates than on Si02 • At 62 eV the orientation effects showed a strong substrate dependence, with a monotonic increase in (111) texture in films deposited on Si, while films deposited on the oxide substrates show a slight (200) texture, showing little change under increased ion bombardment. 11.3.2 Temperature effects
Figure 11 shows the crystallite size as a function of ion flux for tungsten films deposited under 400 eV ion energy at different temperatures (10). Only at high temperature, where the grain size is fairly large in the absence of ion bombardment, does a significant decrease occur as a function of increasing ion flux. At high flux no systematic temperature differences in grain size were seen. This behavior appears similar to that of copper films exposed to 600 eV ions, where the temperature dependence of grain size was nearly eliminated at high ion flux (Fig. 8, 600 eV curves).
Control of Film Properties by lon-Assisted Deposition
203
102 .---r---~-~-_r__-~-__r_-.....,....-....... 'il230 C .103 C
062 C \
\
( a)
10 :'. ~
\ \ \
~--Q
600 eV, Si02
1'-'-..a...--....&....-.....L--.........- - - I - -........- - - - - L - - - - - I 104 r---.......--~-_-
-
-_...
-_
(b)
125 eV, 82 C
10
........- - -.........- .........
1--~-
-----&.-----I-----I
103 .---r---~-~-_r__-~-__r_-.....,....-....... 0
".;:;
co
c:
•
• Si(100) oSi/Si02
~
'en c
Q,)
C ( c)
~
co
10
Q,)
0-
0 0
~
62 eV, 107 C
............
""'"" ""'"" ....-
",
•
~-----------------~-------------
........._-10- 1 --~- - - -.........- .........- - - - - & . - - - " ' - - - - - I 0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 lon/Atom Ratio
Figure 10:
Crystallographic texture of evaporated copper films vs ion flux for (a) 600, (b) 125, and (c) 62 eV ion energy. Shown is the (111)/(200) ratio obtained using BraggBrentano diffractometer geometry.
204
Handbook of Ion Beam Processing Technology
400
350 • 750 C ... 450C o 300 C
300
~ OJ
N
U5
~ co
>. en
U
250
200
•••••_ •••••••••- •••••••••- - _ . - . _ . _ - 0
150
100 0.0
0.02
0.04
0.06
0.08
0.12
0.10
current density (mA/cm 2)
Figure 11: Crystallite size as a function of ion flux for tungsten films subjected to 400 eV Ar bombardment at different substrate temperatures.
Nb, 100 eV
250 r - - -
.......-...,
-....---r---~--r---r--
• 100 ~C
• 200 0 C o 400 C 50 L-_J....-_.L-_....L...-_...&..-_......I...._........._ ........._ 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 2 Ion flux (mAl cm )
x 10-3
Figure 1 2: Grain size as a function of ion flux for Niobium evaporated at different temperatures under 100 eV concurrent ion bombardment.
Figure 12 shows grain size as a function of ion flux for niobium films deposited under 100 eV ion bombardment at various temperatures. In contrast to 400 eV tungsten films,
Control of Film Properties by lon-Assisted Deposition
205
an increase is seen at all temperatures. Also, at all flux levels increased temperature acts in concert with ion bonlbardnlent to increase the grain size. 11.3.3 Structure-Property Relations
12
E
80
....
(a)
10
70
:
60
...
.:§.
8
:~
6.
"'1.
C
•• ...
.. •
4 Bulk C 2
\
__
i~:T6~ g
Ts=62 C 125 eV 0 Si(100) ¢ Si0 2
u
.*&
Cu 600 eV
:!
:
50
400
600
800
A
40
62 eV + Si(100) x Si02
•
30
~-
50
100
80 0
I
o
...
200
~
_ _--'
250
300
r----r--,--~--r--r---_-_--.
(d)
• Copper A
Tungsten
o Chromium
...
• Niobium
•
40 20
0
Crystallite Size (A)
20
60
150
W 400 eV, Si02 (b) • 750 C ... 450 C o 300 C
0
0
.....
-.L.._ _
1000 1201) 1400 1600
120 100
.....
A ~
------------~----------------------10 o '--_----"'__--'-__
Crystallite Size (A)
0
0
Bulk Nb,Cr
OL...-.-.l....--.L.--....L..---'------~......
200
Cr, 360 C o 100 eV A 400 eV Nb, .00 eV • 100 C • 200 C ... 400 C
(e)
Bulk W
-----------~----------------------
Io--_....L...._----L_ _-'--_--L..._ _.l....-_..J
100
'50
200
250
300
Crystallite Size (A)
350
400
5
_~~~:~_~
__:
O-...........--.L..-.....&---'---~____L
o
~~
_
___L.._..J
~~1~1~1~1~
Crystallite Size (A)
Resistivity vs Crystallite Size for Metal Films Evaporated Under Ion Bombardment
Figure 13: Resistivity as a function of crystallite size is shown in (a) for copper,( b) for tungsten, (c) for Nb and Cr, and in (d) all materials with resistivity normalized to the respective bulk material values.
The studies cited above reveal a clear change in the nature of film structure and property behavior under ion bombardment as ion energy is lowered. At 600 eVa large increase in resistance and hardness occurs, which can be directly related to tbe decrease that ion in grain size under increased ion flux (8,13,14). At lower energy the f bombardment does not strongly decrease the grain size is reflected in the lo\) resistivity
206
Handbook of Ion Beam Processing Technology
and microhardness. In the case of tungsten, niobium, chromium, and copper, Fig. 13 shows the inverse relation between resistivity and grain size, which is consistent with work by Parmigiani et al (15), who found that crystallite size effects dominate other microstructural features in determining resistivity in ion bombarded silver films. The data are plotted as normalized resistance R/R bu1k (Fig. 13 (d)), and fall on roughly two curves, one for Cu and W, and one for Nb and Cr. Interestingly, the Cr and Nb films approach their bulk resistivity values at relatively small grain size in comparison with copper and tungsten. Nonetheless, the general trend shows that the resistance behavior in films subjected to high energy ion bombardment is often correlated to changes in grain size, showing large increases where grain size decreases markedly.
50
E
45
(J
9
40
::l..
C
35
':;
(a)
't; 30 'en ~ 25
ro
u 'j; u a> W
20 15 10
L . . . . - _.........._ _L . . . . - _.........._ _L....-_.......L..._--I
0.06 r - - - - , - - - - , r - - - - - r - - - - , r - - - - r - - - - - ,
c
• 100 eV 200 eV & 300 eV
0.05
o
a>
0>
x>- 0.04
(b)
0
c
a
'';::;
u
~
0.03
'. \ \
,~
E 0.02
a ~
'~~". \\ ~.-., \
'0 ....
.
..........
\·-,~~~~:':.._.,.-~=..-::2:::::::=.=::,2=:--=:-..::7..::7.:2
0.01 0.0 2 Ion flux (mA/ cm )
Figure 14: Resistivity (a) and oxygen content (b) in evaporated Cr films subjected to
100,200, and 300 eV ion bombardment. At 200 and 300 eVa minimum in resistivity and oxygen content is reached, while at 100 eVa the bombardment at the highest flux examined is not yet sufficient to reach the minimum oxygen level. At lower ion energies the resistance decrease seen in Nb (Fig. 5) may be attributed to selective oxygen removal. This is also suggested for Cr films (Fig. 14 (7)). Both resistivity and oxygen content decrease initially, levelling off at a flux level determined by the ion energy. Although oxygen removal also takes place at higher ion energies, it is
Control of Film Properties by lon-Assisted Deposition
207
likely that grain size decreases (Fig. 9, 400 eV) contribute to the resistivity increases seen at 400 and 800 eV (Fig. 4(b». In deposition regimes where grain size changes are smaller (low ion energy in the case of copper, low substrate temperature in the case of tungsten) the effect of crystallographic texture changes on film properties becomes apparent. Properties sensitive to texture, such as hardness and resistivity, reflect the changes in texture caused by ion bombardment. For instance, in copper films deposited by evaporation with concurrent ion bombardment at 62 or 125 eV energy, the resistance and hardness trends nearly parallel the (111)/(200) ratio when plotted as a function of ion flux (compare Figs. 3(a) and 3(b), bottom two graphs with Figs. 10(b) and 10(c». In tungsten films evaporated at 450°C substrate temperature grain size did not vary strongly with ion energy or flux, but texture changed strongly. Figure 15 shows texture as a function of resistivity for 185 A grain size films produced by various combinations of ion energy and flux, revealing a large resistance increase for (110) orientation, similar to the resistance increase seen with increasing (111) orientations in Cu films deposited under 62 eV ion bombardment (see Fig. 3(a) bottom and Fig. 10(c». 100
Tungsten 90
c--r
E u
...........
185
80
70
°
E. ~
'>
A Crystallite Size
60
'';::; (f)
'eQ)n
.450 °e, 200 eV
50
a::
..
40
°e, 400eV °e, 600 eV 300 °e, 400 eV
0450 x 450
•
o 750°C, 400 eV
30
20 0
10
15
20
25
30
(1 10/2 1 1) ratio Figure 1 5: Resistivity as a function of film orientation for evaporated tungsten deposited under various conditions. All data points are taken from films having 185 A crystallite size, so that resistance changes can be attributed to other factors.
Figure 16 (a) through (c) shows the (211)/(110) intensity ratios and the planar stress in tungsten films (10) as a function of ion flux at 750, 450 and 300°C substrate temperatures, revealing a correlation between the maximum in tensile stress and the maximum in the (211)/(110) ratio. In Fig. 16(d) stress is plotted as a function of (211)/(110) ratio, showing more explicitly this correlation. Figure 17 shows the (200)/(110) ratio and planar stress in Cr films deposited at 360°C. Similar to the case for tungsten, the degree of tensile stress is associated with the degree of (200) crytsalline orientation. Thus, especially when grain size changes are not large, film texture changes induced by ion
208
Handbook of Ion Beam Processing Technology
bombardment can produce large changes in properties sensitive to crystalline orientation, including resistivity, hardness, and stress.
I
I
15
I
-
10 -
-
5
N~ U
...........
(a)
o
~
z
r o c.n c.n w
0::
I-
- -5 c.n
-10 I
I
15
I
-
10
-
5
-
0
450 C
(b J • (211) I (110) Ratio
2
o Stress (dynes I em ) - -5 10-3
>I-
en Z
W
I
P'
,/
10- 1 ~ / ~....
I-
( c) z
-.J
I
I
I
I
I
..
750 C
\
\ ~~ \'
\'e-__ \
-------- ........... ...-e
-10 15
-
10
-
5
-
0
-
-5
\ \
LJ.j
~
I
,'1
>
I
\
0
10-2 -
LJ.j
"" "" ""
"" , "
0::
10-3 0.0
"... , ...
0-----0
I
I
I
0.02
0.04
0.06
I
0.08
current density (mAl em 2)
0.10
-10 0.12
Figure 16: Crystalline orientation and stress in evaporated tungsten films. Shown in (a) through (c) are stress and (211/110) ratio as a function of ion flux at different temperatures. In (d) the stress is plotted as a function of (211/110) ratio.
Control of Film Properties by lon-Assisted Deposition
15 , . - - - - - - , , - - - - - r - - - - - , . - - - r - - - - r - - - - - ,
10
•
x
N
(d )
A
E u
I
.........
en
tensile
Q)
c
Q
0
en en ~
450.:omIPreSSive
en
400eV
-5
• 300 C
.200 eV
450C • 750 C
-600 eV
A
-10
L....-_--.JL..--_---L_ _- - l ._ _--I-_ _.....r...._ _......
0.0
0.05
0.10
0.15
0.20
0.30
0.25
(211 )/( 11 0) Ratio
Figure 16: continued.
10
10 100eV
8
0)
0
:~
0
'.;::i
co
0::
0"
x
N
E
6
~ l/)
Q)
c
-......
.:s>-
4
0 0
• (200) 1(110) Ratio
~
o
l/) l/)
Stress
e
U5
2
10-1
0 10
~
0 ';:; co
400 eV
8 0)
• (200) I (110) Ratio t:. Stress
0::
0"
0
6 N
x E
4
~
2
c >-
l/)
Q)
-......
0" 0
~
10-1
~
0
.:s l/) l/)
e
U5 -2 -4
0.0
0.01
0.02
0.03
0.04
0.05
2 Ion Flux (mAl cm )
Figure 17: Crystallite orientation and stress in Cr filnls deposited at 360°C.
209
210
Handbook of Ion Beam Processing Technology
11.4 GENERAL DISCUSSION OF ION BOMBARDMENT MECHANISMS
The studies of ion assisted deposition of Cu, Nb, Cr, and W evaporatively deposited films suggest that different mechanisms dominate in different regimes in parameter space. Figure 18(a) shows a schematic of various regimes in which different mechanisms occur, adapted from previous work of Harper et al (5). The regimes are plotted as a function of ion energy and ion/atom flux, and their delineation is based in part on earlier studies and on arguments developed below. The re-sputtering regime is based on data from Ar bombardment of Cu surfaces (16), while the implantation regime is based on Ar implantation in W (17). Using different ions or nletals will cause the boundaries of these regimes will shift slightly. The 're-sputtering' regime exists at the highest combinations of ion energy and flux, and in the upper limit, leads to no film deposition at all. In the range where significant fractions of incident metal flux are resputtered (> 10 % ), other studies have shown strong effects on film surface topography (18,19), crystallographic texture (20), and planarization (21,22). The 'implantation' regime corresponds to high ion energy and high relative flux, whose effects on film structure are reviewed below. One observation that can be made concerning results of ion bombardment of metal filnls concerns the large changes in property modification that take place when ion energy is lowered in the range of 800 to 60 eV. It is clear that bombardment at high energy causes changes in internal microstructure, influencing a variety of properties, including resistivity, hardness, and stress in the case of copper (8,9). Similar effects were seen in the tungsten study at 750°C, 400 eV (10), where a large decrease in grain size is correlated with an increase in resistivity at high flux; and in the increase in resistivity in Cr and Nb films deposited under ion bombardment at energies above 300 eV, where grain size also decreased. While no single mechanism has been confirmed to explain the grain size decrease with increased ion bombardment, work on thin films (6-8,23) and bulk metals (17) has shown a rapid increase in Ar implantation at energies above 100 eV. In copper films a reciprocal relation between Ar content and grain size for films deposited under high energy bombardment has been seen (9). It was argued that as Ar content increases the Ar incorporated in grain boundaries may reach levels sufficient to limit grain growth. As an example at high energy and high flux, to accommodate 1 % Ar in grain boundaries, the grain size must be 300A or less (Fig. 19). In this manner, the resistance increases seen in films subjected to high energy ion bombardment were not considered to be caused by the high Ar content itself, but by the decreased grain size caused by copious defect creation at high Ar incorporation levels and the subsequent prevention of grain growth during deposition by the Ar incorporated in the grain boundaries. In addition to grain size decreases, other studies of ion assisted deposition of metals (4) and ion bombardment of single crystal semiconductors (24) show that a high density of dislocations and interstitials are created under ion bombardment of several hundred eV. Thus, a variety of damage to filnl structure can take place under high energy bombardment. Figure 18 (b) shows the ion flux and ion energy ranges used in various studies cited in this chapter. It is clear that the data in the 400-800 eV range lies substantially within the implantation regime (arbitrarily defined as >0.1 % Ar). This region thus represents a regime in which substantial implantation and film damage occur.
Control of Film Properties by lon-Assisted Deposition
211
2 10
10
0
.~
(a)
a:: E 10-1 0
~c .2
10-2
10-3
10-4
Figure 18: Schematic of various ion bombardment processes shown as a function of relative ion flux and ion energy. In (a) various regimes are defined and quantitative estimates are given as to their effect at different points. In (b) and (c) these regimes are compared with experimental data for different materials.
2
10
10
s
(b)
-Cu(8) ___ Cr(7)
----w (10) ····.. ·Nb (6) No Effect
10-4 1 - -_ _........_ _----L.
~.L--..JIt,.,...I_L___ ___1
102 r----~___r-'"T'"'I""-"""I"":""-.....---........- - - - .
10
0
( C)
.~
a:: E 10-1 0
S R
~c .2
10-2
10-3
10-4 10-1
• Ni (31) o Cu (8) • Nb (6) to Cr (7) o Cr (2) • Ge (29) )( Ge (28)
No Effect
10
2 10
Ion Energy (eV)
3 10
4 10
212
Handbook of Ion Beam Processing Technology
Also apparent in Fig. 18 (b), is the fact that data obtained at lower energies were mostly produced under conditions outside the implantation regime. The behavior of properties in these films is quite different and suggests that other mechanisms are responsible for property modification. For copper at 125 eV and below (8), niobium at 100 eV (6), and W (10) and Cr (7) at 200 eV and below, the resistivity remains low up to high levels of ion flux, in part because of less film damage. This is also reminiscent of early work on bias sputter deposition where resistivity minima were seen at around 100 V bias (25). These results are in keeping with the low Ar implantation probability (17,23), short ion range, and previous observations that subsurface interstitial or dislocation damage is not seen in studies of low energy «50 eV) borrlbardnlent in single crystal senliconductor surfaces (24). Although overt signs of film damage are absent, other features of microstructure and properties vary significantly in these films produced in the low-to-moderate ion energyflux regime. This regime is designated by the term 'densification' in Fig. 18(a) because of observed changes in film properties that may be related to density (see ref 26). In Fig. 18 (c) data marking the transition from tensile to compressive stress from various studies are shown. This transition represents the arrival at near-maximum film density, since further ion bombardment produces only moderate compressive stress (6-10), a result attributed to plastic flow (27). Also shown in Fig. 18(c) is data from ion bombardment of semiconductors. In the case of amorphous germanium deposited by evaporation, Yehoda et al(28) found that the void fraction was inversely proportional to E1 or eV/atom for ion energy in the range of 15-1 00 eV. Shown in Fig. 18 (c) is the energy necessary to reach a maximum density, about 4 eV/atom, approximately equal to the bond strength. Hirsch and Varga, on the other hand (29), have observed an E3/2 dependence for stress relief in Ge films for ion energy between 65 and 300 eV, and Brighton and Hubler (30) have shown this to be related to the range of damage predicted by simple binary collision simulations. Also shown is data from Hoffmann and Gaerttner (2), suggesting an E1/2 dependence of stress relief in Cr films. In addition to the stress minimization and direct density measurements, other features of film structure may reflect densification effects. In evaporated copper films it was proposed that the increased texture seen at low energies was related to forward-sputter densification of the growing film, among other things (8). The fact that the (111) orientation in Cu films was found to be strongest at 125 eV is in keeping with two-dimensional lattice-dynamics simulations of Ni film growth that show the degree of epitaxy and film density saturate at about 100 eV (31) for moderate ion/atom ratios of about 0.2. Since the (111) plane in fcc materials is that of highest density, it is reasonable that this plane becomes preferentially favored at high ion flux. This is also the case in Wand Cr subject to 400 eV ion bombardment, where above 0.1 ratio the (110) plane (close packed in bcc structure) becomes the favored orientation (Figs. 16 and 17). Figure 18 suggests that at high ion energy and moderate flux the implantation and densification regimes overlap. In this area, these mechanisms may produce countering effects on film microstructure. As a comparison, for copper films produced at 125 eV, Fig. 18 (b) shows that the parameter space explored extends primarily in the densification regime. This is reflected in a monotonic increase in film orientation (Fig. 10(b)), reaching a maximum (111) orientation about 10,000 times normal. In contrast, in Cr and W films subjected to 400 eV ion bombardment, where both densification and implantation occur, a weaker preferred orientation, about ten times random, is seen. Similarly, at high flux
Control of Film Properties by lon-Assisted Deposition
213
and moderate-to-high energy, Fig. 18(b) shows that the implantation and re-sputtering regimes overlap. Yu et al (20) observed the combination of these two mechanisms to produce preferred orientation in the direction of the ion beam. Crystallites with open channels in which Ar could be implanted tended to grow in the beam direction, whereas densely-packed planes were re-sputterred and tended not to grow. At very low ion energy or flux Fig. 18 indicates that ion bombardment predominately influences processes taking place at the film surface. In this regime desorption of weakly bound species as well as displacement of surface adatoms may take place. These effects are reflected in the selective renloval of oxygen in Nb and Cr (6,7) and the increased grain size in Cr and Nb films subjected to 100 eV bombardment and Cu films subjected to 62 eV bombardment.
"~
~ en
10
/ , 1 ) Cu G.B. atoms/total Cu atoms
600 eV, Si(100)
>!Ii
.T,=103 C
"0
§ o co
c "ffi
~ '(f<
• T,=62 C
• •
• (2) Ar monolayer in G.B./total Cu atoms
10-1 ~_--..J._--l.----l....---I--...L-...L......L-.L~_ _...L---.IlI----'---I.--'-"""""'''''''' 4 3 2 10 10 10 Crystallite Size (A)
Figure 19: Argon content as a function of crystallite size for evaporated Cu films. Shown are data for films deposited under 600 eV concurrent bon1bardnlent. The solid lines represent (1) the percent of of copper atoms residing in grain boundaries as a function of grain size, and (2) the Ar content (in percent) that can be accommodated in a film assuming that 100 % of Ar atoms reside in grain boundaries, and form a monolayer coverage in each boundary.
Figure 20 shows the succession of regimes encountered as ion energy is increased at three different relative flux values. The relative importance of various mechanisms is observed to increase and then saturate as ion energy is increased. Also shown is the fact that the succession of mechanisms encountered is different at different ion/atom ratios. At 0.1 ratio all the different mechanisms are encountered as ion energy increases, while at 10 and 0.001 ratios the implantation regime is not encountered. In the former case this is because the ion flux is so high that film re-sputtering occurs before significant implantation can take place. In the latter case, the relative ion flux is so low that significant incorporation can never take place. 11.4.1 Materials and Temperature Effects
214
Handbook of Ion Beam Processing Technology
Although the mechanisms discussed above may apply to a wide range of materials, their effects on film structure vary among different materials type, as illustrated by the studies of refractory metals. At low substrate temperature «450°C) in tungsten, for example, the grain size is relatively small in the absence of ion bombardment, due to low adatom mobility (32-35). Even 400 eV bombardment, which would implant significant amounts of argon (see Fig. 18(b», produced little decrease in grain size (Fig. 10). In part this may be due to the fact that such small grains can incorporate significant amounts of gas in grain boundaries as indicated in Fig. 19. Thus, properties such as resistivity were found to be dominated by oxygen removal, which caused resistivity decreases. On the other hand,in the case of W deposited at 750°C, grain size without ion bombardment is significantly larger and 400 eV ion bombardment causes significant decreases, resulting in large resistivity increases. Therefore, ion bombardment did not produce significantly lower resistivity. In the case of copper, the resistivity increases seen under ion bombardment highlight the material differences between copper and the more refractory metals. One difference is that even at 100° C , due to the low melting temperature, the grain size is relatively large without ion bombardment, making films more susceptible to grain size decreases caused by ion bombardment. A second difference is the lack of oxygen in unbombarded Cu films, such that selective oxygen removal does not appear as prominent a mechanism as in the refractory metal films. These results indicate that the the position of the bombardment regimes of Fig. 18 in flux-energy parameter space is shifted by changes of substrate temperature. In the bcc metals at low temperature, the desorption of oxygen is important up to moderate ion flux levels; while at high temperatures, where oxygen content is less in the absence of bombardment, very low ion flux already begins to densify the films (see Fig. 5). This suggests that the onset of the structural densification regime shifts toward lower flux with increased substrate temperature. At very high temperatures, earlier studies of plasma deposition of metals has suggested that at TIT m in the range of 0.5-0.7 the effect on film structure of changing the plasnla environment is minimized (36). Thus many of the mechanisms present at lower temperatures may be suppressed because of high bulk atom diffusion. Other recent work using high energy (> 1OKeV) ion bOlTlbardment of non-growing films has shown actual grain size increases at high T/T m (>0.5), where damage is annealed over time (37). Thus, at very high temperatures one might expect high energy ion bombardment to enhance grain size in ion assisted deposition at very high temperatures. However, the studies cited in the present chapter involve relatively low TIT m,<0.37 for Cu, <0.30 for Cr, <0.28 for W, and <0.24 for Nb. Under high energy bombardnlent (>400 eV) for Wand Cu, at the highest temperature examined the grain size decreases were actually the greatest (8,10). Thus, at high energy and low temperature (0.33T m) ion bOlTlbardment creates defects that are relatively stable. It should also be emphasized that the ion energy range in which defect and damage creation occurs is highly dependent on the material type. While elemental metals reviewed in this chapter show little sign of damage, both in terms of microstructure and properties, at energies below 100 eV, early studies of sputtered Nb 3 Ge suggest that energies as low as 1-10 eV are sufficient to damage film properties (38,39). The reasons for this lie in the metastable nature of the Nb-alloy superconductors and the extreme sensitivity to slight changes in composition. Changing the type of ion also clearly modifies the picture described above, especially in the case of non-metals. In cerium oxide films subjected to concurrent oxygen ion
Control of Film Properties by lon-Assisted Deposition
215
bombardment, Martin et al. (40) have observed for an ion/atom ratio of 1 that density does not saturate until about 150 eV, well above limits outlined in Fig. 18. Using partially-ionized vapor at an ion/atom ratio of 0.5 Hasan et al (41) have observed nucleii size to grow monotonically up to energies of at least 300 eV in indium films. This is a regime in which severe film damage and implantation of gas would occur in continuous filnls if inert ions were used to bOlnbard the growing film. Such results show that extrapolation of the scheme presented in Fig. 18 to other materials, ions, or stages of film growth, must be done with care.
Surface Desorption Re-sputtering tion
(a)
Surface Desorption
Implantation
Figure 20: Ion bombardment mechanisms as a function of ion energy based on Fig. 18. Ion/atom ratios are (a) 10, (b) 0.1, and (c) 0.001.
(b)
Surface Desorptio
( c)
0.1
10
100
Ion Energy (eV)
1000
10000
216
Handbook of Ion Beam Processing Technology
11.4.2 Property Optimization
Knowledge of mechanisms mentioned above may be important for those concerned with optimizing specific film properties and faced with various experimental constraints typically encountered in more complex deposition environments. For instance, the results of the Nb and W work show that various combinations of temperature, ion flux and ion energy can be used to produce the same property, namely zero-stress. Faced with the constraints on one of these parameters, the other two can be appropriately adjusted to compensate for this constraint. Other operator-controlled parameters, which indirectly influence film properties can also be varied. In the case of stress changes in refractory metals these include the metal evaporation rate and the background gas pressure, which effectively determine the metal atom/impurity atom arrival ratio, thus the film oxygen content, and thereby the stress (7). The eu study illustrates that combinations of properties can be optimized by careful selection of parameter space. As ion flux increases, certain properties show qualitatively similar behavior at different energies, while others change behavior as the energy is lowered. Figure 21 shows the qualitative difference in hardness, resistivity and stress at 62 as opposed to 600 eV. At 62 eV the decrease in tensile stress seen at all energies is no longer coupled to the sharp increases in resistance and hardness. Similarly, in the case of W films, zero stress could be produced with a minimum resistivity by increasing substrate temperature or decreasing ion energy.
62 eV
600 eV
/,/."MiCr~-h-~-;d~-~~~.~-----_·
ensile Stress ~
~
-f-I
-f-J
I-
(a)
I-
ID 0...
~~
o I-
Microhardness,,'," ...··
....<::.::< .
CL
'~ ..,
.......~~.~:~.:::.:~.:
ID
(b) 0..
o ln..
.,
// R~~;:tivitY
" . :~.~:······Resistivity .
Ion Flux
Ion Flux
Figure 21: Qualitative trends in stress, microhardness, and resistivity for evaporated copper films, shown at (a) 62 eV and (b) 600 eV ion energy.
In summary, recent studies of ion assisted metal deposition show that the importance of various bombardment mechanisms is determined by the type of material and the region
Control of Film Properties by lon-Assisted Deposition
217
in deposition parameter space, defined by ion energy, flux, and tenlperature. No simple behavior is seen, but knowledge of important structure-determining mechanisms allows for flexibility in obtaining desired properties.
11.5 REFERENCES
1.
D. W. Hoffman and J. A. Thornton, J.Vac. Sci. Technol. 20(3): p. 35 (1982).
2.
D.W. Hoffman and M. R. Gaerttner, J. Vac. Sci. Technol. 17(1): p. 425 (1980).
3.
R. S. Berg and G. J. Kominiak, J. Vac. Sci. Technol. 13(1): p. 403 (1976).
4.
P. Ziemann and E. Kay, J. Vac. Sci. Technol. 21(3): p. 828 (1982).
5.
J. M. E. Harper, J. J. Cuomo, R. J. Gambino, and H. R. Kaufman, in Ion Bombardment Modification of Surfaces: Fundamentals and Applications. O. Auciello and R. Kelley, eds., Elsevier Science Publishers B. V., Amsterdam, 1984
6.
J. J. Cuomo, J. M. E. Harper, C. R. Guarnieri, D. S. Yee, L. J. Attanasio, J. Angilello, C. T. Wu, and R. H. Hanlmond, J. Vac. Sci. Technol. 20(3): p. 349 (1982).
7.
D. S. Yee, J. Floro, D. J. Mikalsen, J. J. Cuomo, K. Y. Ahn, and D. A. Smith, J. Vac. Sci. Technol. A3(6): p. 2121 (1985).
8.
R. A. Roy, J. J. Cuomo, and D. S. Yee, J. Vac. Sci. Technol. A6: p. 1621 (1988).
9.
R. A. Roy, D. S. Yee, and J. J. Cuomo, (to be published)
10. D. S. Yee and R. Petkie (to be published) 11. J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol. 18(2): p. 203 (1981). 12. T. C. Huang, G. Lim, F. Parmigiani, and E. Kay, J. Vac. Sci. Technol. A3(6): p. 2161 (1985). 13. E. O. Hall, Proc. Phys. Soc. London B64: p. 747 (1951). 14. N. J. Petch, J. Iron Steel Inst. 174: p. 25 (1953). 15. F. Parmigiani, E. Kay, T. C. Huang, J. Perrin, M. Jurich, and J. D. Swalen, Phys. Rev. B 33(2): p. 879 (1986). 16. B. N. Chapman, Glow Discharge Processes. John Wiley and Sons, New York p.394 (1985) 17. E. V. Kornelsen, Can. J. Phys. 42: p. 364 (1964). 18. R.S. Robinson and S.M. Rossnagel, J. Vac. Sci. and Technol. 21: p. 790 (1982). 19. S.M. Rossnagel, Effects of Surface Impurities and Diffusion on Ion Bombardment Induced Topography Formation in Erosion and Growth of Solids Stimulated by Atom and Ion Beams. Ed. by G. Kiriakidis, G. Carter and J.L. Whitton, (NATO-ASI series, 112 (1986). 20. L. S. Yu, J. M. E. Harper, J. J. Cuomo, and D. A. Smith, Appl. Phys. Lett. 47: p. 932 (1985). 21. H. P. Bader and M. A. Lardon, J. Vac. Sci. Technol. A3: p. 2167 (1985). 22. H. P. Bader and M. A. Lardon, J. Vac. Sci. Technol. B4(4): p. 833 (1986). 23. H. F. Winters and E. Kay, J. Appl. Phys. 38: p. 3928 (1967).
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Handbook of Ion Beam Processing Technology
24. B. R. Appleton, Mat. Res. Soc. Symp. Proc., Beam Solid Interactions and Transient Processes, (1987) 25. A. G. Blachman, Met. Trans. 2: p. 699 (1971). 26. R. D. Bland, G. J. Kominiak, and D. M. Mattox, J. Vac. Sci. Technol. 11(4): p. 671 (1974). 27. B. Window, F. Sharples, and N. Savvides, J. Vac. Sci. Technol. (1988) A6: p. 1935 ( 1988). 28. J. E. Yehoda, B. Yang, K. Vedam, and R. Messier, J. Vac. Sci. Technol. A6: p. 1631 (1988). 29. E. H. Hirsch and I. K. Varga, Thin Sol. Films 69: p. 99 (1980). 30. D. R. Brighton and G. K. Hubler, Nucl. Inst. Meth. Phys. Res. B28: p. 527 (1987). 31. K.-H. Muller, Phys. Rev. B, 35(15): p. 7906 (1987). 32. L. S. Palatnik and Y. F. Komnik, SOy. Phys. Doklady 4: p. 196 (1959). 33. L. S. Palatnik and Y. F. Komnik, SOy. Phys.
Dokla~"y
5: p. 1072 (1960).
34. L. S. Palatnik and N. T. Gladkikh, SOy. Phys. Sol. St. 4: p. 307 (1962). 35. B. A. Movchan and A. V. Demchishin, Phys. Met. Metalogr. 28: p. 83 (1969). 36. J. A. Thornton, Ann. Rev. Mat. Sci. 7: p. 239 (1977), and refs. therein 37. H. A. Atwater, Mat. Res. Soc. Proc., Fundamentals of Beanl-Solid Interactions and Transient Processing, (1988) in press 38. N. Chencinski and F.J. Cadieu, J. Low Temp. Phys. 16: p. 507 (1974). 39. F. J. Cadieu and N. Chencinski, IEEE Trans. Mag., Mag. 11: p. 227 (1975). 40. P. J. Martin, R. P. Netterfield, and W. G. Sainty, J. Appl. Phys. 55: p. 235 (1984). 41. M. A. Hasan, J. Knall, S. A. Barnett, J.-E. Sundgren, and J. E. Green, unpublished. Also see J. E. Green, Mat. Res. Soc. Symp. Proc. 75: p. 39 (1987).
12
Etching v#ith Directed Beal11s Michael Geis, Stella W. Pang, Nicholas E. Efremow George A. Lincoln, Gerald D. Johnson and William D. Goodhue
12.1 INTRODUCTION
Anisotropic dry etching of a substrate is usually accomplished with two etching components. The first is a flux of chemically reactive species, that can react with the substrate to form volatile reaction products. The second component consists of a directed flux of energetic (100 to 1000 eV) ions. The ions cause the etching of the substrate both by sputtering and by catalyzing chemical reactions between the chemical flux and the substrate. Control of the chemical flux and the energetic ions and their effects on etching are the subjects of this chapter. Several characteristics of a dry etching technique are desirable to control. These include the etching selectivity between various nlaterials, the etching rate, and the degree of anisotropy. By controlling the components, chemical and ion fluxes, the etching characteristics can be determined. Two etching techniques that control these components, ion beam assisted etching (IBAE) and hot jet etching will be discussed. IBAE has one of the highest etching anisotropy ratios (ratio of the vertical etch depth to the etched distance under the etching mask) and hot jet etching has one of the highest etching rates and highest etching selectivity, approaching that obtained by wet etching. 12.2 ION BEAM ASSISTED ETCHING
Reactive ion etching is the most commonly used anisotropic dry etching technique and etches by impinging fluxes of energetic ions and chemically reactive atoms and molecules on a substrate. The ions and the reactive flux are produced in the same plasma which makes independent control of the ions and the reactive flux very difficult. Several investigators have attempted to obtain independent control of the ions and the reactive flux by using reactive gas in an ion source. The ion source produces both the ions and the reactive flux and allows the energy of the ions to be controlled independently of the reactive flux (1,2). However, since most ion sources can only operate at a fraction of mTorr or less, the chemical etching component is usually limited to a few tens of nanometers per
219
220
Handbook of Ion Beam Processing Technology
minute as shown in Fig 1. This limitation can be overcome by supplying the reactive flux from an effusive source close to the substrate and using an inert gas (Ar or Xe) in the ion source. This approach which is based upon the work of Coburn et al (3) is called either ion beam assisted etching (IBAE) (4) or chemically assisted ion beam etching (CABlE) (5).
~
'e
·e
'0
E
~
...w c(
a:
Cl
z
i
...CJ w
'.0
I
-.....-.~--#-
0.'
L..-'-J-----L.....L.-..L..L.I.............-
'0. 4
RE------1
IBAE AND HOT JET ETCHING
---1
,-0.1
-...L.-...L....L.-,L,.jL.I..I...LL.-- - ' - - . L . . . - . I o - . l - - l............
'0· 3
'0' 2
FLUX EQUIVALENT PRESSURE (Torr)
Figure 1: Etching rate of GaAs, Al and W as a function of Cl2 pressure. The etching rate was calculated assuming that every Cl2 molecule striking the substrate reacts to form volatile products. Since usually less than 10 % of the molecules react with the substrate, the actual etching rate is about one tenth of that shown on this graph. The approximate operating pressure ranges (on the horizontal axis) are shown for four anisotropic dry etching techniques; reactive ion beanl etching (RIBE), ion beam assisted etching (IBAE), hot jet etching, and reactive ion etching (RIE).
An IBAE system, (6) schematically shown in Fig. 2, consists of a Kaufman ion source which is used to supply ions from an inert gas at a controlled energy and flux. The reactive flux is supplied from several effusive sources that direct the reactive flux on the substrate. A liquid nitrogen cold trap near the substrate is used to cryopump etching products and unused reactive flux. With the additional pumping from the cold trap, fluxes of reactive gas equivalent to 20 mTorr can be obtained on the substrate while still maintaining a sufficiently low pressure «0.03 mTorr) to operate the ion source and transport an ion beam to the substrate. As shown in Fig. 1, this could result in a etching rate on the order of tens of micrometers per minute. This approach has the additional advantage that the reactive flux is both directed and in a controlled chemical state. Unlike, RIE where the chemical flux is isotropic and in a variety of chemical and electronic states. The controlled nature of IBAE results in higher etching selectivity and higher etching anisotropy than are usually obtainable with RIE. Generally, for device processing RIE is an acceptable technique. However, when unusual etched structures are required or special materials need to be etched, the control allowed with IBAE becomes important. The
Etching with Directed Beams
221
remaining discussion of IBAE will be concerned with forming unusual structures in GaAs and etching diamond, an unusually difficult material to etch.
,
Argon Gas
Reactive
----...:---1-----_ Gas
II I
I
1 II I 1
\\~\I I \\\~I
1 I I I
I,
A rg on
I ~, Ions 1
.......--...,\'.!\llil r--..-"'-"'----I \ \1 ~\~ L N2 \ ~ ~ 1\ I I ' - - - -.... \1\1'1 ~ I \ ~ ~ I' I Reactiv~1 ~ I'J '\
1
l l :I ~
Cold Trap
Flux
1 \1
\ I 1
I,
t
1
L N2
I
I
Va c u u m •
Pump
J
,
1'1 ,I ~ I
Substrate
Figure 2: Diagram of the IBAE system that consists of an ion source, one or more reactive gas sources (only one shown), and a cold trap to pump the unused reactive gas and the reaction products. The vacuum chamber is made of stainless steal and these systems are usually pumped with either cryopumps or diffusion pumps. The gas sources are usually 3 to 10 cm fronl the substrate.
12.3 ETCHING GaAs
GaAs will react with Cl2 to form volatile products by the chenlical reaction: GaAs(s)
+ 3CI2 (g)
....
GaCI3(g)
+ AsCI3 (g)
This reaction does not occur spontaneously, but requires some activation energy, which can be supplied by an ion flux. Therefore, this reaction is an excellent candidate for both RIE and IBAE, since the reaction can be controlled by the ion flux. However, unlike IBAE, the plasma used to produce ions with RIE also forms CI atoms which will etch GaAs spontaneously and results in an isotropic etching component. (7) The isotropic etching degrades etching control and limits the anisotropic ratio obtainable with RIE. To obtain controlled etching, several experiments were performed to determine the important parameters with IBAE. Figure 3 shows the etching of GaAs as a function of the reactive flux equivalent pressure. The etching rate increased by a factor of 20 over the sputtering rate with the addition of 10 m Torr of C12 . For reactive pressures less then 1 mTorr the etching rate is a strong function of the reactive flux and small variations in the flux can cause considerable variation in the etching rate of the substrate surface. To avoid this difficulty, the etching is usually performed at reactive pressures in excess of 2
222
Handbook of Ion Beam Processing Technology
mTorr. The additional punlping speed of the cold trap allows for uniform coverage of the sample with reactive flux at equivalent pressures in excess 2 mTorr, making it possible to etch 5 cm diameter wafers and obtain ± 5 % uniformity over 4 cm2 area. This uniformity reflects variation of both the ion beam and the reactive flux. It is interesting to note that the etching rate does not saturate, even at high reactive pressures of 10 mTorr, as shown in Fig. 3. This pressure represents a reactive molecular flux 2 x 104 times larger than the ion flux of 20 ,uAcm- 2 . The etching rate at this pressure, 85 nnl min- 1 represents the removal of approximately 50 substrate atoms for every incident Ar+ (8). When the ion beam energy is increased from 10mWcm-2 (equivalent to 20,uAcm- 2 of 500 eV ions) to 1-2 Wcm- 2 (lmAcm- 2 of 1-2 keVions ) the etching rate increases to 5-10 ,urn min- 1 This etching rate is about one-quarter to one-half of the maximunl etching rate based on the available Cl2 assuming that the reaction products are GaCl3 and AsCI3 • At these high rates the anisotropic character of the etchings degraded. This is due to the heating of the substrate by the ion beam which causes the Cl2 to etch the GaAs spontaneously. High anisotropic etching rates can be obtained with a directed flux of CI atoms on GaAs, as described later in this chapter.
80 70
c
'E
"".=.E
...w
60 50
~
a:
40
C)
!:
J:
...w (J
30 20 10
0
2
3
4
5
6
7
8
9
10
CI 2 PRESSURE (mTorr)
Figure 3: Etching rate of GaAs as a function of the reactive flux equivalent pressure. A 20,uAcm- 2 beam of 500 eV Ar+ was used to obtain this data.
For GaAs samples held at room temperature during etching, the etched profile is dependent upon the collimation (penumbra) of the ion flux. Figure 4 illustrates the effect of penumbra on the etched structure. By increasing the ion source to substrate distance, the collimation is improved and the undercutting is reduced. The collimation was found to depend to first order on the ratio of the ion beam diameter and the substrate to ion source distance. Figure 5 shows the actual structures obtained in GaAs for two different substrate to ion sources distances. For the etching conditions used and collimation less than ± 1.5 0 , the undercutting of the etched structures only weakly depends upon the ion beam collimation and other effects like mask erosion become important.
Etching with Directed Beams
223
Figure 4: Diagram showing the effect of the ion source to substrate distance on the profile of the etched structure. RESULTING PROFILES
a.
b.
Figure 5: Scanning electron micrograph of GaAs samples etched with (a) a substrate-to-source distance of 19 em, and (b) substrate-to-source distance of 45.5 em. A 20 jLAcm-2 beam of 500 eV Ar+ and an equivalent reactive flux of 2 mTorr was used to obtain these profiles.
224
Handbook of Ion Beam Processing Technology
For comparison, Fig. 6 shows a profile obtained in GaAs with an aspect ratio in excess of 10:1. The ion beam divergence ranged from ± 0.7° at 2 mTorr equivalent chamber pressure to ± 1.2° at 20 mTorr (9).
Figure 6: Scanning electron micrograph of a GaAs sample etched under the same conditions as the sample shown in Fig. 4(b). The grating is etched to 1.5 p.m deep.
Extremely high aspect ratio structures (40 to 1) have been etched in this system as shown in Fig. 7. This structure was obtainable because both the ion beam penumbra was maintained below ± 1.5° and the etching selectivity between the nickel mask and the GaAs is in excess of 1:100 (4). These structures contain defects that are not commonly seen in etched structures with smaller aspect ratios. The interior of the gratings is easily contaminated due to trapped etching products and to the large surface area. Such contamination can be seen in the grating in Fig. 7. When attempts are made to remove the contamination with solvents, the structure becomes deformed, as in Fig. 8. Once the facets of the grating touch, they will not separate even when the solvent has completely evaporated. A second type of defect appears as several dark patches on the sidewalls of the grating, as shown in Figs. 7 and 9. This defect appears to be small holes in the wall of the gratings and may be caused by the ion beam penumbra.
Etching with Directed Beams
225
Figure 7: Scanning electron micrograph of a GaAs sample etched under the same con-
ditions as the sample shown in Fig. 4(b). The grating is etched 4.5 p.m deep. A combination of 35 nm of nickel and 100 nm of Si0 2 were used as a mask.
Figure 8: Scanning electron micrograph of the same GaAs sample as shown in Fig. 7, after the sample was cleaned in acetone and air dried.
Figure 9: Scanning electron micrograph of the same sample shown in Fig. 7. The dark patches have been magnified and appear to be cavities in the sidewalls of the gratings.
226
Handbook of Ion Beam Processing Technology
Independent control of the ion flux and the reactive fluxes require techniques to measure the relevant parameters. The ion energy and ion species are determined by the gas feed and the power supply settings used with the ion source. The ion current is measured with a Faraday cup and the ion flux penumbra is determined with a pin-hole camera. Figure 10(a) shows a schematic drawing of the camera with a pin hole, usually 20-200 /Lm in diameter, and a self-developing ion beam resist (10) to record the intensity of the ion beam. Figure 10(b) shows a optical micrograph of the exposed resist and the penumbra was calculated from standard geometric considerations. The reactive flux is determined using an opening in the etching table at the same position where the substrate is etched. The opening is connected to a manometer. When some of the molecules of the gas flux enter the opening, a pressure increase will occur at the manometer. Figure 11 shows an illustration of the basic system. The molecular flux on the sample is given by:
(1) where Z is the reactive flux in molecules s- l cm- 2 , P is the pressure measured at the manometer in Torr, m is the mass of the molecules in atomic units, and T is the temperature of the gas in the manometer in K.
ION BEAM
V PIN HOLE
(20-200 Mm
=== SELF DEVELOPING
luu"m""",~;ON BEAM
RESIST
I 1 mm I (a)
(b)
Figure 10: (a) Diagram of a pin-hole camera used to determine the ion beam penumbra, and (b) the image obtained in the self-developing ion beam resist using a 100 /Lm pin hole and a hole-to-resist distance of 2 em. The image is a replica of the carbon grid used to extract the ions in the ion source.
Etching with Directed Beams
227
REACTIVE,-\\ GAS JET
rnm\\ ," ", ," II I ,
\ I ",
,
, I I
,
'
,
I
,
"~,--"REACTIVE FLUX
',
\
,
\
\
,
I
,
\
I
c~PAhTANCE
;'MANOM~;E~
Figure 11: Diagram of the technique used to measure the equivalent reactive flux pressure. The pressure determined by the manometer is directly related to the reactive flux provided the diameter of the hole in the etching table is small compared to the mean free path at the measured pressure.
Jel
2
JET
§r-----~l: ~
o
STEPPING MOTOR
(J
SAMPLE WITH MASK
ION GUN
CI 2 JET
INTERFACE 1------1 COMPUTER UNIT Figure 12: Schematic drawing of the sample-to-beam geometry and computer-controlled sample stage used in chlorine IBAE.
228
Handbook of Ion Beam Processing Technology
The high anisotropy and high etching selectivity of IBAE makes possible the fabrication of unique etched structures. A modified system used to fabricate these structures is shown in Fig. 12, which is a schematic of the etching geometry and computer-controlled sample stage for angular etching (11,12). Two examples of such structures etched with this system will be discussed here. Figure 13 shows SEM micrographs of etched walls in (100) GaAs with the edge alignment along the (011) cleaved plane. The first micrograph shows a sidewall etched at four different angles. By increasing the number of steps to 800 etching angles, a smoothly shaped etched curve can be obtained as shown in Fig. 13(b). This structure has been used to obtain surface emitting diode lasers. Figure 14 shows the basic structure with the diode lasers formed in the GaAs/AIGaAs semiconductor with the IBAE laser facet and reflector (13).
(a)
(b)
4 DISCRETE ANGLES USED
800 DISCRETE ANGLES USED
Figure 13: SEM micrographs of etched sidewalls in GaAs. (a) Sidewall contour generated by four discrete angle runs. (b) Sidewall contour generated by 800 discrete angle runs made under computer controlled.
LASER CONTACTS IN OPENINGS IN Si 3 N 4
\
NilGe/Au BACK CONTACT
Figure 14: Schematic diagram of a monolithic two-dimensional surface-emitting GaAs/AIGaAs diode laser array. The mirrors were etched using IBAE and then metallized.
Etching with Directed Beams
229
The second example of fabrication is schematically shown in Fig. 15. A grating was etched into GaAs using IBAE. After the etching the Ni mask was removed and a second etch, 20° from the substrate normal, was performed. By varying the time of the second etch, the width of the grating lines could be modified. Figure 16 shows four structures etched using this approach. This technique has since been used to fabricate ultrasmall GaAs/AlGaAs quantum well structures having quantized energy states in two dimensions.
DIRECTION OF Ar + Cl z BEAMS FOR
FIRST ETCH
E~C:8~~1 ~'ORMS CO\-~~:~NgO~~~N
SEltCOND
I
I I
I 200inml
15 nm
I
-
II
I I
I I
'
I I
I
II
I _..I
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Figure 15: Schematic drawing showing the geometry employed to fabricate thin GaAs columns. After the initial vertical chlorine IBAE and mask removal steps an angled chlorine IBAE is used to thin the columns.
1--240nm
(8)
(el
(bl
(d)
I--l!'m----j SEM micrographs (b), (c), (d) show the results of angled etching the 80-nm-wide columns shown in micrograph (a). The column micrographs (b), (c), and (d) were etched for 1.0, 1.5, and 2.0 min, respectively, and have respective widths of 40, 22, and 9 nm. Figure 16:
230
Handbook of Ion Beam Processing Technology
12.4
ETCHING DIAMOND
Although it is well known that graphite can be etched using oxygen in RIE system, diamond etches very slowly under the same conditions (14,15). The inert nature of the diamond surface is believed responsible for the slow etching. Oxygen does not absorb on the surface of diamond as it does with graphite (16). Therefore, energetic ions from the plasma can only sputter diamond, while with graphite the ion causes a local chemical reaction with O 2 , renloving substantial amounts of carbon. The ability to control the chemical composition of the reactive flux with IBAE makes it possible to overcome the difficulty. Using nitrogen dioxide, N02 , as a reactive flux and an ion flux of Xe ,etching rates of 200 to 300 nnl min 1 and differential etching ratio of diamond to Al of 100 to 1 can be easily obtained (15). The absorption of N02 on the dianlond surface is believed to be responsible of the enhanced etching rate. This is indicated in Fig. 17 which shows the etching rate of diamond as a function of substrate temperature. As the diamond temperature is decreased and the stability of the absorbed N02 filnl on the diamond is increased, the etching rate of the diamond increases. Below - 25 0 C a visible film of N02 is formed on the diamond and the decrease of the etching rate with temperature below 0 0 C is believed due to the inability of the ions to penetrate the N02 film to the diamond- N02 interface and catalyze the etching. When O 2 was used instead of N02 the etching rate of diamond was comparable to that obtained with sputtering.
c 'E
2000
"-
~ w
...«
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Z
1000
J: U
...w 500
0'------~---.L....---....L-----.1.----L..------l..------J
o
25
50
75
100
TEMPERATURE (Oe)
Figure 1 7: Etching rate of diamond as a function of substrate temperature.
IBAE of diamond has been used to fabricate the first practical dianlond device (17). Figure 18(a) shows a schematic drawing of a permeable base transistor, PBT. The device was fabricated by etching a grating in semiconducting diamond and evaporating Al on the horizontal surfaces of the grating. The aluminum on the top of the grating formed the
Etching with Directed Beams
231
collector contact and the aluminum on the bottom formed the base contact. Chemical vapor deposited Si0 2 insulated the base contact from the diamond. Figure 18(b) shows the collector current as a function of emitter-collector voltage for several base voltages. The device performance was limited by the high resistance of the diamond substrate, but future devices with higher quality diamond substrates may overcome this limitation.
COLLECTOR
~
.=,
80 -
I-
Z ~ 60-
a:
BASE
::J (,) 40-
'.
.
.
'.
BORON-DOPED DIAMOND (lib)"
a:
oI(,)
....
AI EMITTER
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w
OHMIC CONTACT
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o
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10
I
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.
I
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40
I
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COLLECTOR-EMITTER VOLTAGE (Vj
Figure 18: (a) Schematic diagram of a diamond permeable base transistor. To fabricate this device, lBAE was used to produce the grating in the diamond. (b) Collector current as a function of collector-emitter voltage for base-emitter voltage steps of 2V.
lBAE has been used to etch other materials. Some of the highest reported etching rates (10 J.tm min-I) and anisotropic ratios 20 to 1 for Al films have been obtained using this technique (18). lBAE has also been used to etch InP for optical wave guides (19). 12.5 HOT JET ETCHING
The commonly used dry etching techniques, such as RIE, obtain anisotropic etching with a directed flux of ions in the presence of an isotropic flux of chemically reactive species. Hot jet etching eliminates the ions and obtains directed etching with a directed flux of chemically reactive species (20,21). This flux is composed of radicals formed by thermal decomposition of comparatively unreactive gas. From 10 to 100% of the radicals react spontaneously with the substrate depending on etching conditions. Hot jet etching has demonstrated significant improvements over RIE with increased etching rates and higher selectivity between the substrate and the mask. Figure 19 shows a schematic drawing of the system which is similar to the IBAE system. The difference is that an ion beam is used only to sputter clean the surface of the substrate prior to etching and the reactive flux is in the form of radicals. The radicals are obtained from the thermal decomposition of a comparatively unreactive gas. The hot jet used to heat the gas is shown in Fig. 20. It consists of a tungsten tube resistively heated
232
Handbook of Ion Beam Processing Technology
to temperature between 1500 and 3200°C. The ends of the tube are held in position by springs made of 1 mm tungsten wire and the gas is fed in one end through a carbon or alumina tube. Other metals, like Re or Ir, have been used successfully with the hot jet. UNREACTIVE GAS I
•
.......----HOT JET
COLD TRAP
Figure 19: Schematic diagram of a hot jet etching system.
'-CLAMP
TUBE (900-3000 C)
/1----
REACTIVE ~ RADICALS
r
TUBE
~ SOURCE
~~~':":"":"':'~~~ Y--J
GAS
1!"C~------'IJiiIi"'iIiIiIiIIr~~o.:..:..:.:.;~~
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.,.,..----W WIRE SPRINGS
1
50-150 A
1 em
Figure 20: Scale drawing of the hot jet. The hot jet dimensions were found not to be
critical.
Etching with Directed Beams
233
The temperatures necessary to obtain substantial radical formation are on the order of a few thousand degrees centigrade. This may seem surprising since the bonding energy of most compounds is on the order of a few electron volts (equivalent to a few 104 °C). But the low pressure these jets operate at (20-100 Torr in the jet) and the significantly higher entropy for the radical states than the room temperature states, allow for this lower temperature operation « 3300°C). Figure 21 shows the equilibrium percentage of radical fornlation for several compounds as a function of temperature. These calculations assumed an estimated pressure in the jet of 20 Torr. Since the mass flow of gas is held constant in these experiments, the gas pressure in the hot jet is assumed to increase with the square root of temperature and the percentage of gas disassociation. Gases like F 2 , C12 , CF 3 CF 3 and CH3 CH 3 can be easily decomposed into free radicals at temperatures less than 1800°C However, gases like H 2 N 2 , and CF4 require higher temperatures or lower gas pressures to obtain thermal decomposition. 100 90
80
0
z t= c(
70 .
(3 0
60
en en c( en
2i to-
Z
50 40
UJ
U
a: w ~
30 20 10 ; 0 800
1000
1200
1400 1600 TEMPERATURE (K)
1800
2000
2200
Figure 21: Percentage of radicals formed by thermal dissociation.
Using the hot jet technique a variety of materials have been etched, as shown in Table 1. The jet-to-substrate distance in these experiments is approximately 10 cnl with the exception of the etching rate measurements made with GaAs, for which the jet-tosubstrate distance was approximately 4 cm. (100) cut, n-type GaAs and Si substrates were used to determine the etching rates. Si02 films were obtained by thermal oxidation of Si wafers and PH is a novolac-resin photoresist. The discussion here, however, will concentrate on GaAs and Si. Figure 22 shows a scanning electron micrograph of a GaAs sanlple etched with a flux of Cl. The masking material is 0.5 /lm of photoresist and etched depth is approximately 50 p,m with an aspect ratio of 5. The aspect ratio of the etched structure can be varied by the jet tenlperature and the gas pressure. At high jet temperatures (> 2500°C) and low jet pressures « 1 mTorr) most of the Cl2 molecules decompose into CI and approximately 50% of the available chlorine flux reacts with the substrate. Under these condi-
234
Handbook of Ion Beam Processing Technology
tions an anisotropy ratio of 10 is easily obtained. If the temperature is lowered or the gas pressure is increased, fewer of the atoms are formed and the anisotropy ratio is reduced. Figure 23 shows the etching rate of GaAs at two different jet heater currents as a function of flux equivalent pressure. At low flux equivalent pressures, < 1 mTorr, the etching rate is proportional to the flux and a high anisotropic ratio is obtained. At higher pressures (> 1 mTorr) and 85 A heater current (estimated jet temperature of 2300°C) or >4 mTorr and 95 A heater current (estimated jet temperature of 2500°C), the etching rate does not increase as fast with pressure and the anisotropic ratio decreases. This ratio is about 2 for a heater current of 85 A and a flux equivalent pressure of 7.5 mTorr.
Table 1: Hot-jet etching rates (nm min-I) of variety of commonly used materials for se-
veral gases.
Source gas
GaAs
<5
Si
SiOz
PH
Au
10 2x10 3
< 0.1
< 0.1
<1
<2
<2 <2
<2
Pb
600
Figure 22: Scanning electron micrograph of a GaAs sample etched with a flux equivalent pressure of 6 mTorr. The 0.5 JLm thick photoresist mask is visible on top of the etched structure.
Etching with Directed Beams
235
3r------r----r------,-----r-------.----~-----.--------,
95 A
2
ETCH RATE (J.Lm min- 1 )
.",......---
o
,,-
-- - - - - 0 - - - - - - ' 0
85 A
,,0. ----
/ ~
/ /
,6 o
2
3
4
5
6
7
8
FLUX EQUIVALENT PRESSURE (mTorr) Figure 23: Comparison of the etching of GaAs for two different heating currents as a function of Cl2 flux-equivalent pressure. The two sketches show the profiles obtained at low and high flux equivalent pressures.
Silicon is usually etched with F obtained from the thermal decomposition of SF 6 • Since F is extremely reactive, a great deal of care must be used in the jet design. At low temperatures « 3000K), F reacts with the tungsten jet to from a variety of tungsten salts and the jet will be etched away. At higher temperatures (> 3000K), these salts are less stable and tend to decompose back into Wand F. However, at still higher temperatures (> 3500K), the W will evaporate and the jet will fail in a short time. Figure 24 shows the gasification rate of W, Pt, and Ir in an atmosphere of 1 Torr of F as a function of temperature. The evaporation and reaction rates for these metals were obtained from the literature (21). The optimum operating temperature under our conditions is between 3000 and 3500K for W. However, even at these tenlperatures there is substantial loss of material and we have only been able to operate for as long as 40 min before the tungsten jet fails. Pt is more inert than W, but its higher evaporation rate makes it unsuitable for a jet, since the evaporated Pt masks the substrate and stops the etching. Ir jets have operated for several hours without any sign of deterioration provided they are operated between 1750 and 2500 K. However, Ir jets do not provide etching rates as high as similar jets formed of W. Hot jet etching is unique, since it requires no ion beam or plasma and still etches anisotropically. The etching rates for GaAs and Si are some of the highest reported anisotropic etching rates for any dry etching technique. The etching selectivity between Si and Si02 of 1000 to 1 and between GaAs and photoresist of 105 to 1 are exceptionally
236
Handbook of Ion Beam Processing Technology
high. The high selectivity makes etching through stencil masks possible. Since the mask can be made of some inert material, like Ni, the mask is not etched by the hot jet technique and can be repeatedly used. Resolution below 100 nm has been demonstrated using hot jet etching in combination with stencil masks (22).
TEMPERATURE (K) 4000 3500 3000
2500
2000
1500
5
2 10. 4
GASIFICATION 2 10. 5 RATE 2 9 cm- s·1 2 10. 6
2 10.7
F I SF41 SFs SFzI SF I S I
2 10- 8 L....-..J.......J..--l.-..L-_...J.....-----L._-..L-_...J.....-----L_..........- - - ' 6.5 7 2.5 3 3.5 4 4.5 5 5.5 104 /T (K- 1 )
Figure 24: The gasification rates of three metals in an atmosphere of 1 Torr of F as a function of temperature. 12.6 ETCHING DAMAGE
Damage produced in GaAs and Si with RIE and IBAE has been characterized (23-25). The danlage originates from two sources, crystal defects and inlpurity deposition in the semiconductor. The energetic ion flux is responsible for the crystal defects. This damage can be minimized by using a flux of chemical species that absorb on the substrate, by using low energy ions « 500 eV) , and by using ions of high atomic weight, like Xe+. The effect of chemical species on a GaAs substrate is shown in Fig. 25, which compares the electrical characteristics of Schottky diodes formed on GaAs etched surfaces. Schottky diodes formed on GaAs etched by ion beam sputtering alone, became very leaky with a breakdown voltage below 0.05 V. Diodes that were etched with the same ion energy and current, but with the addition of 0.5 mTorr flux-equivalent pressure of Cl2 had the same breakdown voltage and ideality factor as the control diodes formed on unetched GaAs. If the Cl2 is replaced by N0 2 which will not form volatile products with GaAs, then the etching rate will decrease to a value below that of sputtering alone. However, the Schottky diode characteristics obtained from substrates etched with N0 2 show minimal damage and are similar to those of the control. This demonstrates that the flux of reactive species reduces damage not just by etching, but perhaps also by forming a protective layer on the substrate surface. Ion energy is easily controlled for both RIE and IBAE, and with IBAE the ion mass can be controlled. The damage effects of ion energy and mass are
Etching with Directed Beams
237
discussed elsewhere (25), but their contribution to substrate damage is substantially reduced by the reactive flux. 102
,.......------r-----"r-'-----T------..----_
10
........
............. 10- 1
..,
10- 3
. ..
..
10-4
CONTROL Ar 500 V; CI 2 5 X 10-4 Torr
10-5 10-6
Ar 500 V
-------'-----.Jo
0.2
---L.
0.4
0.6
.....L...
--J
0.8
BIAS (V)
Figure 25: Semilog plot of forward current density as a function of voltage for a control sample, sample etched by Ar+ at 500 eV, and sample etched by IBAE with Ar+ at 500 eV and Cl2 flux equivalent pressure of 0.5 mTorr.
Impurities introduced by RIE and IBAE include metals sputtered from the etching chamber (Fe, Ni an Cr) and deposits from the reactive gases (polymers). With appropriate chamber design, gas mixtures and clean-up techniques, these impurities can be nlinimized. Both RIE and IBAE can be used to produce danlage free etched GaAs by controlling the reactive flux, by controlling the ion energy, and by minimizing sources of contamination (26). 1 2.7
SUMMARY
A comparison of the environments for RIE, IBAE and hot jet etching is shown in Fig. 26. The RIE environment consists of directed flux of a few hundred eV ions, hot electrons, and an isotropic flux of chenlically reactive species. These reactive species have a room temperature velocity distribution, but the radicals and metastables in this flux represent a chemical potential energy of several thousand degrees Kelvin. IBAE is similar to RIE except there are no hot electrons, radicals, or nletastables in the reactive flux and the chemical potential energy is near room temperature. This reduces the isotropic etching component below that obtain with RIE. Hot jet etching has no ions or electrons and the anisotropic nature of the etching is due to the directed flux of radicals which have a chemical potential energy and velocity with respect to the substrate of several thousand degrees Kelvin (27).
238
Handbook of Ion Beam Processing Technology
kT in eV KELVIN
~ONS
IONS
I
ELECTRONS
w
CC
10
::::> t
I
105
CC
w ~
~
w t-
0.01
100
......
lO-3~
HOT JET ETCHING
-..I.o
_
REACTIVE ION ION BEAM ETCHING ASSISTED ETCHING
Figure 26: A comparison of the etching components for hot jet etching, RIE and IBAE. The vertical axis represents either the energy or the equivalent temperature of the etching components. Since the actual energy or temperature varies considerably between etching systems and etching conditions for the same technique, the position of each component in the figure indicates only the order of magnitude of the actual condition.
The commonly used dry etching technique (RIE) obtains anisotropic etching with a directed flux of ions in the presence of an isotropic flux of chemically reactive species. By controlling the chemical potential, as in IBAE, of by making the flux of chemically reactive species directions, as in hot jet etching, significant improvenlents over RIE have been obtained. These improvements include increased etching anisotropy, high selectivity between the substrate and the mask, and high etching rates. ACKNOWLEDGEMENTS
This work was sponsored by the Department of the Air Force and NASA. 12.8 REFERENCES
1.
P. D. DeGraff and D. C. Flanders, J. Vac. Sci. Techno!. 16: 1906 (1979).
Etching with Directed Beams
239
2.
M. A. Bosch, L. A. Coldren, and E. Good, App!. Phys. Lett. 38: 264 (1981).
3.
J. W. Coburn, and H. Winters, J. App!. Phys. 50: 3189 (1979).
4.
M. W. Geis, G. A. Lincoln, N. N. Efremow, and W. J. Piacentini, J. Vac. Sci. Techno!' 19: 1390 (1981).
5.
J. Chinn, A. Fernandez, I. Adesida, and E. D. Wolf, J. Vac. Sci. Techno!. AI: 701 (1983); J. D. Chin, I. Adesida, and E. D. Wolf, J. Vac. Sci. Techno!. B1: 1028 (1983).
6.
G. A. Lincoln, M. W. Geis, S. Pang, and N. N. Efremow, J. Vac. Sci. Techno!. B1: 1043 (1983).
7.
E .L. Hu and R. E. Howard, ADD!. Phys. Lett. 37: 1022 (1980).
8.
Note ref. 6 contains an error. On page 1045 is should read "approxin1ately 50 substrate atoms.. " not "approximately 500 substrate atoms."
9.
S. W. Pang, J. N. Randall, and M. W. Geis, J. Vac. Sci. Techno!. B1: 341 (1986).
10.
M. W. Geis, J. N. Randall, T. F. Deutsch, P. D. DeGraff, K. E. Krohn, and L. A. Stern, App!. Phys. Lett. 43: 74 (1983); M.W. Geis, J. N. Randall, T. F. Deutsch, N. N. Efremow, J. P. Donnelly, and J. D. Woodhouse, J. Vac. Sci. Techno!. B1: 1178 (1983).
11. W. D. Goodhue, S. W. Pang, G. D. Johnson, D. K. Astolfi, and D. J. Ehrlich, App!. Phys. Lett. 51: 1726 (1987). 12. W. D. Goodhue, G. D. Johnson, and T. H. Windhorn, Inst. Phys. Conf. Ser. 83: 349 (1987). 13. J. P. Donnelly, W. D. Goodhue, T. H. Windhorn, R. J. Baily, and S. A. Lambert, Apo!. Phys. Lett. 51: 1138 (1987). 14. T. J. Whetten, A. A. Armstead, T. A. Grzybowski, and A. L. Ruoff, J. Vac. Sci. Techno!' A2: 477 (1984). 15. N. N. Efremow, M. W. Geis, D. C. Flanders, G. A. Lincoln, and N. P. Economou, J. Vac. Sci. Techno!. B3: 416 (1985). 16.
J. M. Thomas, The Properties of Diamond, edited by J.E. Field (Academic Press, London, NY 1979) p212.
17. M. W. Geis, N. N. Efremow, and D. D. Rathman, J. Vac. Sci. Techno!. B6: 1953 (1988). 18. N. N. Efremow, M. W. Geis, R. W. Mountain, G. A. Lincoln, J. N. Randall, and N. P. Economou, J. Vac. Sci. Techno!. B4: 337 (1986). 19. N. L. DeMeo, J. P. Donnelly, F. J. O'Donnell, M. W. Geis, and K. J. O'Connor, Nuclear Instr. Methods Phys. Res. B7/8: 814 (1985). 20. M. W. Geis, N. N. Efremow, and G. A. Lincoln, J. Vac. Sci. Techno!. B4: 315 (1986). 21. M. W. Geis, N. N. Efremow, S. W. Pang, and A. C. Anderson, J. Vac. Sci. Techno!. B5: 363 (1987). 22. S. W. Pang, M. W. Geis, W. D. Goodhue, N. N. Efremow, D. J. Ehrlich, R. B. Goodman, and J. N. Randall, J. Vac. Sci. Techno!' B6: 249 (1988).
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Handbook of Ion Beam Processing Technology
23. S. W. Pang, G. A. Lincoln, R. W. McClelland, P. D. DeGraff, M. W. Geis, and W. J. Piacentini, J. Vac. Sci. Techno!. B1: 1334 (1983). 24. S. W. Pang, M. W. Geis, N. N. Efremow, and G. A. Lincoln, J. Vac. Sci. Techno!. B3: 398 (1985). 25. S. W. Pang, Microelectronic Engineering 5: 351 (1986). 26. S. W. Pang, W. D. Goodhue, T. M. Lyszczarz, D. J. Ehrlich, R. B. Goodman, and G. D. Johnson, subnlitted to J. Vac. Sci. Techno!. 27. M. W. Geis, N. N. Efremow, and G. A. Lincoln, J. Vac. Sci. Techno!. A 5: 1928 (1987).
13 Film Grov#th Modification by Concurrent Ion BOlTlbardlTlent: Theory and SilTlulation
Karl-Heinz Muller
13.1 INTRODUCTION
Thin films play an important part in various technologies, most notably in optics and microelectronics. The physical properties of thin films are known to be quite different from those of the parent bulk materials. This difference can be attributed to the microstructural aggregation during the free-atom-to-solid phase transformation. In most vacuum deposition techniques where films are layed down on substrates held at temperatures of less than half the melting temperature of the bulk film material, growth occurs under highly nonequilibrium conditions. Here, the condensation rate is many orders of magnitude larger than the re-evaporation rate which is unlike the conditions characterizing the solidification of a liquid. The nonequilibrium conditions during growth result in the formation of amorphous adsorbates and metastable phases containing lattice vacancies and pores. These defects often correlate with substrate temperature, and their presence can be minimized by choosing an optimum deposition temperature. This temperature should be high enough to permit spontaneous annealing of defects formed during particle adsorption, yet not so high as to induce new defects thermally. Unfortunately, only a narrow temperature-window exists for most materials and to compound this limitation, substrate-film material interdiffusion may occur in this temperature range. A nonequilibriunl approach to reduce defect formation during deposition is to supply any additional excitation required to grow improved material through some specific interaction with the film surface, while at the same time keeping the bulk of the film at low temperature. To achieve this one can either give sufficient kinetic energy to the arriving species which induce surface vibrations and displacements, as in ion-beam deposition and ionizedcluster-beam deposition, or assist the growing film with an energetic ion beam as in ion-assisted deposition. Deposition techniques of this kind together with the gen~. al experimental aspects of ion bombardment effects have been subjects of several comprehensive reviews (1)-(5).
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Handbook of Ion Beam Processing Technology
In this chapter we review current theories of the effect of ion bombardment on the mechanisms and dynamics underlying film growth. In the following section we discuss models that offer an explanation of the origin of defect formation in the adsorbate and the effect of substrate temperature. This expository section will be followed by an overview of theoretical models which describe ion-bombardment-induced structural modifications in thin films. These models are based on a thernlal-spike or collision-cascade description, or on a molecular dynamics approach.
13.2 FILM MICROSTRUCTURE, THE ROLE OF IMPACT MOBILITY AND SUBSTRATE TEMPERATURE 13.2.1 Classification of Film Structure in Terms of Zones
A useful overview of the relationship between the microstructure of vacuumdeposited coatings and the most prominent deposition parameters is provided by the socalled empirical structure-zone models. The first such model was proposed by Movchan and Demchishin (6). They found that the microstructure of thick evaporated coatings of Ti, Ni, W, ZrOz and Alz0 3 could be represented as a function of TIT m by three zones where T is the substrate temperature and T m is the melting temperature. Each zone had its own characteristic structure and physical properties. The low-temperature zone-l ( TIT m < 0.3) microstructure was columnar and consisted of tapered units defined by voided growth boundaries. The intermediate tenlperature zone-2 structure (0.3 < TIT m < 0.5) consisted of columnar grains, which were defined by grain boundaries and increased in width with TIT m according to activation energies typical of surface diffusion. The high temperature zone-3 (T IT m > 0.5) structure was composed of equiaxed grains, which increased in size across the zone according to processes having activation energies typical of bulk diffusion. The latter zone model has been extended by Thornton (7) to include magnetron sputtered metal films by incorporating another parameter-axis to account for the effect of Ar working gas pressure. A transition zone between zones 1 and 2 has been identified, consisting of a dense array of poorly defined fibrous grains. A great number of theoretical investigations have been undertaken in order to better understand the origin of structures in these different zones. 13.2.2 The Henderson Model and Zone-1 Structure
The cause of the porous columnar microstructure of zone-l vapor-deposited films was first elucidated by Henderson, Brodsky and Chaudhari (8). They based their theoretical investigation on the elementary notion that condensing atoms or molecules have a limited mobility and that the shadowing of uncoated portions of the substrate can occur by atoms or molecules that have previously been condensed. Figure 1 shows the results of a computer simulation (8) in which hard spheres, representing atoms, were launched sequentially and travelled in straight lines, intersecting the substrate at an angle of 45 0 • Initial positions in the x-y plane were randomly selected. At its point of impact on the growing film, an incident sphere was assumed to stick and permitted to relax only to the nearest "pocket" where it could make contact with previously deposited spheres. To reduce finite size effects, periodic boundary conditions were applied. Figure 1 shows a slice of film cut parallel to the x-z plane which has a depth of five sphere-diameters. Even at normal incidence, the simulation yielded a density of only 46 % of the close-packed crystalline density. This density is considerably less than the 90% actually obtained for silicon de-
Fi 1m Growth Modification by Concurrent Ion Bombardment
243
posits (9). Similar simulations prepared by Dirks and Leamy(10) showed that hard spheres tend to deposit into columns which lean in the direction of the incident flux and which are interspersed with microvoids. This columnar character was found to be more pronounced if the angle of incidence is oblique. Kim et al. (11) performed simulations in which impinging particles were allowed to bounce from the surface of microcolumns. The bounce probability en at the nth collision was assumed to be
en =
a[1
+
(n - 1)b]-3/2
(1)
where 0 < a < 1 and b > 0 are given material constants. These modified simulations yielded denser coatings (670/0 crystalline bulk density) without significant alteration of the columnar orientation, though it was more difficult to identify the columnar growth visually.
Figure 1: Microstructure for deposition under 45° incidence angle from Ref. (8). The slice is parallel to the x-z plane and five sphere (atom) diameters thick.
A somewhat similar idea, though based on a more tenuous physical picture, was used by Bangjun and Macleod (12) who introduced an increased inlpact mobility simply by arranging that only a certain fraction R 1 of the arriving particles remained at the nearest stable position, while another fraction R 2 continued to move to the next nearest, R 3 to the next, and so on. The sum, ~Ri is unity and the average distance, A, moved by a particle after impingement was approximated by
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Handbook of Ion Beam Processing Technology
A = (0.6R 1
+
1.6R2
+ 2.6R3 + ... )ao
(2)
where ao is the hard sphere diameter. This procedure also resulted in denser films. Bangjun and Macleod (12) have also investigated the cause of nodular growth. At first sight there appears to be a problenl of scale associated with the Henderson model. The dendritic features of the model are an order of magnitude smaller than columns commonly seen in micrographs of films of appropriate thickness. It has been shown ( 13), however, that the real structure of zone-l coatings is a heirarchial one in which there is a continual clustering of smaller units into larger ones as film thickness increases, and the basic fundamental units are of a size comparable to those predicted by the Henderson model. The Henderson model has been particularly successful in reproducing an empirical law in vapor-deposition relating the angle of inclination of the columns, {3, to the angle of incidence of the depositing vapor, a, by the simple expression 2 tan {3 = tan a ,known as the tangent rule. The model reproduces the tangent rule up to angles of incident, a , of 70°. 13.2.3 Thermal Mobility and the Zone-1 - Zone-2 Transition
Using a simple lattice-gas model, Muller (14) extended the two-dimensional (hard disk) Henderson model to film deposition at elevated temperatures T. Film atoms were sited in a triangular lattice, where the film extended in the x-direction and grew in the z-direction. In this model, if the thermal energy, E T , gained by an atom at site i as a result of local thermal fluctuations (during a vibrational period), was found to be larger than the local activation barrier, it was required to jump to a nearby randornly selected empty site j. The thermal energy E T is defined in terms of Boltzmann statistics (15) by ET
= - kT In( 1 -
R)
(3)
where k is the Boltzmann constant and R a uniformly distributed random number in the interval (0,1). The local activation barrier was chosen in a simplified fashion, taking only the nearest neighbor interaction ( > 0 ) into account, and using a constant-valued and side-configuration independent saddle point energy, Q. For an atomic Jump from site i to site j, with respective nearest-neighbor numbers N i and N j ,the local energy barrier, ~E , was evaluated according to Q (4)
; otherwise For an atom sitting at a step site i, the energy barrier to be surmounted was approximated by ~E
= (Ni
-
1)<1>
(5)
As only substrate temperatures less than 0.4 T m were considered, the desorption of atoms could be neglected.
Film Growth Modification by Concurrent Ion Bombardment
245
The sites i, which atoms might leave by hopping, were chosen at random. The distance a newly condensed particle migrates before it becomes buried by new depositing material depends on the varying ratio LlE/kT along its hopping path, on the attempt frequency for hopping (1013 S-l) and on the flux of the arriving species. As hopping probability drops dramatically with increasing value of (Ni - N j ), where N i > N j migrating atoms become trapped at sites with a large number of occupied neighboring sites (energetically favorable sites).
Figure 2:
(e)
kT=O.04Q
Film microstructure of a vapor-deposited film at different substrate temperatures, (a) kT = 0.03 0, (b) kT = 0.037 0 and (c) kT = 0.04 O. The deposition rate is 4.5 (dense) layers per second and the vapor impingement angle is 45°
Figure 2 shows film microstructure corresponding to different substrate temperatures, assuming
The zone-2 structure which is characterized by columnar grains separated by metallurgical grain boundaries, has been studied theoretically by Srolovitz (16) who developed a statistical model for the evolution of grain structure during deposition. As in zone-2 the thermal atomic mobility on the surface greatly exceeds that in the bulk, microstructural evolution is regarded as being totally controlled by the film surface, with the bulk microstructure remaining static behind the advancing surface. This led Strolovitz
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Handbook of Ion Beam Processing Technology
(16) to a two-dimensional model of microstructural evolution in which the linear relationship between time and depth in a film was used. A Monte Carlo computer simulation was enlployed, which considered curvature-driven grain growth (17) (18) and secondary grain growth (19). The latter is the process by which grains having low surface energy grow at the expense of those with higher surface energy. Srolovitz (16) nlapped the surface microstructure onto a discrete lattice in which each lattice site i represented a small surface element of a grain. Each lattice point was assigned a number between 1 and M, corresponding to the orientation of the grain in which it was embedded. A site i was assigned an energy defined by (6)
where Si is one of the M orientations on site i, J is the grain boundary energy constant and 8ab the Kronecker delta. The sun1 is taken over all nearest-neighbor sites. The last term in (6) is a site energy, representing variations of surface energy with grain orientation. The probability P for a transition between old (1) and new (2) orientations is taken as ; if E(I)
> E(2)
p=
(7)
exp{ (E(2) -E( 1)) /kt}
; otherwise
The evolution of film microstructure under the influence of curvature-driven grain growth is shown in Fig. 3. Most of the grains present at the substrate do not extend to the film surface. Their disappearance is the result of growth competition between adjacent grains.
Figure 3: Cross-section of a filn1 perpendicular to the surface from Ref. (16).
Film Growth Modification by Concurrent Ion Bombardment
247
The growth of zone-3 films can be characterized as high temperature crystal growth. The pioneering theory of crystal growth and the equilibrium structure of crystal surfaces is attributed to Burton, Cabrera and Frank (20). A number of Monte Carlo conlputer simulations based on the solid-on-solid model and the Ising model have been performed by Gilmer et al. (21) (22) and the surface roughening transition which occurs at temperatures below the melting point of a crystal has been investigated (23). 13.3 ION BOMBARDMENT INDUCED STRUCTURAL MODIFICATIONS DURING FILM GROWTH
It is usually profitable to optimize deposition temperature since defects in a growing film strongly correlate with substrate temperature. Such optimization involves the balancing of competing processes. If the temperature is too low, defects associated with unannealed configurations will remain, whereas if the temperature is too high, thermally induced defects will appear. At very high temperatures diffusional mixing between the substrate and film material can occur. Unfortunately, experimentally useful temperature windows do not exist for all systems. One nonequilibrium approach designed to reducing defect formation during the growth process, is to supply excess excitation to the surface through some specific surface interaction, while keeping the bulk of the adsorbate cool. One experimentally expedient way of supplying additional kinetic energy to the surface is to bombard the film during growth with ions. Low-energy ion bombardment of a film during its deposition leads to extensive property modification (1)-(5). Among those physical properties that are observed to change are the grain size, the degree or direction of orientation, film density and the number of voids, film stress, other related properties such as the electrical resistivity, complex permittivity, and film stability. Chemical and compositional changes can also occur, and some of these are the formation of compounds or the incorporation of gas into the film. Ion bombardment during deposition occurs to some extent in just about any plasma-based technique, for example in rf and magnetron sputter deposition. Broad-beam ion sources such as Kaufman sources are the principal tools used for facilitating energetic ion bombardment of depositing film surfaces. Ionbeam-based techniques are characterized by a high degree of control over the bombardment process in terms of ion energy and bombarding flux and the ability to operate at low pressures. The process in which a broad-beam ion source is operated in conjunction with an evaporation source is appropriately referred to as "ion-assisted deposition". Owing to the great importance of the ion bombardment process a number of theoretical models have been developed recently which were aimed to reveal some of the fundamental mechanism involved. These theoretical investigations have led to significant advances being made in the understanding of the role of ion energy, relative ion-to-atom arrival rate ratio, the ion mass and the chemical nature of the ions used. 13.3.1 The Thermal-Spike Approach
The effect of energetic ion bombardment on zone-1 film microstructure was first modelled by Muller (24). It was assumed that temperature spikes, created by penetrating ions slightly below the film surface, induce atomic reordering in the porous dendritic network of the growing film. The temporal and spatial evolution of a temperature spike was determined by solving the classical equation of heat conductivity. In a homogenious nledium, a spherically-symmetric temperature pulse at time t after creation takes the form
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Handbook of Ion Beam Processing Technology
c 2 t- 6 / 7
T(r,t)
196f32
where
f3 = X(T) T-1/2
and
2
r _ [1 _ _ ct4 / 7
c =
[ 1715 4'17
]2
(8)
f32E _ _ ]2/7. pk
Here, r is the distance from the spike origin and X(T) is the temperature-dependent thermal diffusivity inside of the spike for which a standard expression of the kinetic gas theory was used (25). The quantity p denotes the atomic number density of the film and E is the energy of the incoming ion which is converted into vibrational energy. The effect of inhomogeneities of a porous film structure on the evolution of the temperature pulse was not considered. The initial spike-temperature distribution was chosen to have a width equal to the distribution of damage determined from three-dimensional collision cascade calculations (26) and the position of the spike center below the film surface was equated with the average damage depth. In the vicinity of the spike center, reordering of atoms during the rapid pulse expansion (~1 0-11 S) was approximated by allowing the atoms to hop between discrete lattice sites with a temperature-dependent (and therefore timedependent) rate v , given by v =
(8k t /
'17
m ao2) 1/2 e -LiE/kT
(9)
Here, m is the mass of film species and a o the lattice spacing. The local activation energy barrier ~E was chosen as in Eqs. (4) and (5). Figure 4 shows Monte-Carlo simulation results for the microstructure of a two-dimensional Ar+ ion-assisted Ni-film. Since the chosen Ar+ bombardment energy was small, Ar incorporation was assumed to be insignificant and was neglected.
(a)
Figure 4: Columnar microstructure for a vapor impingement angle of 45° (a) no ion bombardment, (b) 150 e V Ar ion bombardment at normal incidence during growth and an ionto-atom arrival rate ratio of 1.
Film Growth Modification by Concurrent Ion Bombardment
249
As can be seen from Fig. 4., at an ion-to-atom arrival rate ratio of one, the porous microcolumnar growth is disrupted and bridging develops, causing the closure of open voids. This is in accordance with experimental observations of modifications to some zone-1 films (27), in which ion-bombardment during deposition resulted in films which absorbed less moisture when exposed to air. The model did not, however, predict the film densification observed in ion-assisted zone-1 films (27). The reason for this is that the model considers only isotropic atomic rearrangements during the expansion of the spike and therefore any downward motion of atoms due to multiple collision momentum transfer from the impinging ions is not accounted for. 13.3.2 The Collision-Cascade Approach
13.3.2.1 Redeposition mechanism. Using computer modelling, Brett (28) has recently studied the structural modifications caused by ion bombardment during growth. He used a two-dimensional Henderson model (8) in which hard disks (atoms) aggregate to simulate vapor deposition. A simple process was used to simulate the effect of ion bombardment. The bombardment event sequence entailed the calculation of the straight trajectory of an ion to determine the struck target film atonl, the determination of the target atom sputter probability, the ejection of the sputtered atom, the possible redistribution of the sputtered atom, and the relaxation of the film region surrounding an ejected atonl site. Relaxation to the nearest cradle between two neighboring atoms was assumed for atoms left with one nearest-neighbor bond. Ions were not incorporated into the film. The sputtering yield was chosen as Y = 1/n where n is the number of bonds a target atom forms with other atoms. The results showed that for an ion-to-atom arrival rate ratio of one, the packing density increased slightly from 0.61, without bombardment, to 0.66 with bombardment, and that microcolumns were less well defined. Branching and bridging suppressed the formation of continuous voids. Although the nl0del was simple, and did not describe accurately atomic dynamics and collision cascades, it included one of the essential mechanisms which contributes to densification, namely, the redeposition of sputtered atoms. 13.3.2.2 Densification mechanism. Muller (29) has elucidated the essential mechanisms associated with ion-induced densification of zone-1 films. When ions with energies of a few hundred eV strike the surface of a porous film they penetrate the material to an average depth no grater than a few interatomic spacings. The interacting ions produce phonons, vacancies, knock-on atoms and electronic excitations. The ions are either backscattered or incorporated, while knock-on atoms may leave the surface as sputtered atoms or penetrate deeper into the film where they become trapped as interstitials, preferentially in quenched-in vacancy sites. Vacancies near the surface which are produced by bombarding ions are partially refilled by newly arriving vapor atoms. For a sufficiently large ion-to-vapor arrival rate ratio the latter mechanism will result in downward packing of material such that filnls no longer grow in a porous colunlnar network, but instead grow in a densely packed structure. The densification mechanism is illustrated in Fig. 5, where ion-bombardment and vapor condensation are viewed as alternating processes. Muller (29) considered a vapor deposited Zr02 film assisted by 0+ ions. The time evolution of the atomic number densities Nn(z,t) of species n (Zr, 0 or 0+ ion) of a realistic three-dimensional film, where Z is the film depth, were determined by solving a systenl of coupled differential equations:
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Handbook of Ion Beam Processing Technology
h(k) [Mn(z,t) + 8km Ik(z,t)] +
L 8nl jA (1) A1(z,t)
(10)
1
Here, jI(k) is the ion flux of ion species k and jA (1) the partial vapor flux of atomic species 1. The term Mn(z,t) is the matter transport distribution function, defined as the difference between normalized interstitial and vacancy distributions. The term Ik(z,t) is the ion incorporation distribution function. Both functions M n and I k depend also on the number densities N n of all species involved, which makes Eq. (10) a system of coupled differential equations. The term A 1(z,t) is the vapor adsorption distribution function. The 8-symbol in Eq. (10) is the Kronecker delta. As recoiled surface atoms and implanted bonlbarding species were assunled to be strongly bound at void sites, no diffusion term appears in Eq. (10). Equation (10) was solved numerically where, after each small time step, ~t, the functions M n and I k were calculated using a modified version of the three-dinlensional Monte Carlo collision cascade code TRIM (26) in which the gradual change of film composition with depth was included.
(a)
NO ION BOMBARDMENT Figure 5:
t=O SURFACE DEPLETION
I
(b)
>-
rOO z LU
o
t =6t
REFILLING
r---,
(e)
I
I
I I I I I
I I
t =6t
I
DEPTH Z
(a) Film density distribution without ion bombardment, assuming an abrupt density decrease at the surface. (b) Density distribution after ion irradiation which lasts t. Material is redistributed due to sputtering, recoil implantation and ion implantation. (c) Refilling of the depleted surface region by condensing vapor during t. The refilled density is that attained without ion bombardment.
Film Growth Modification by Concurrent Ion Bombardment
251
The cascade calculations used the Biersack-Ziegler universal potential for the atomatom interaction and the electronic loss was described by an improved version of the Lindhard-Scharff function (30). The parameters of the collision cascade model, surface binding energy E s ' site binding energy E b and cut-off energy E e , were chosen as E s = E b = E e = dHa /2, where dHa is the heat of atomization of the deposited material. The porous atomic network of a zone 1 film was approximated by an amorphous film of corresponding density. For the vapor adsorption probability distribution A 1(z,t) (see Fig. 5(c» it was assumed that the immobile condensing vapor refilled the surface vacancies produced by bombarding ions down to a depth of about one or two interatomic distances. The refilling of deeper-lying vacancies appeared to be improbable under the assumption of linlited adatom mobility. The resulting mass density distributions at different times during growth are shown in Fig. 6. In the absence of ion bombardment, the Zr02 film was assumed to grow with a packing density of 0.77, corresponding to a mass density of 4.41 gcm- 3 as found experimentally (27). From time t=O the growing film was assisted by 0+ ions where the ion-to-vapor (atom) flux ratio was 0.22. As seen in Fig. 6, redensification takes place up to a depth of about 3 nnl.
0+
.. Zr02 (growing)
600eV
5.6
lX I
J1/J A
= 30°
= 0.22
J I = 200 jlA/cm 2
"""'
M
E 4.8
u
......... 0)
'-'
Q
4.0
t
3.2
t
-4.8
t
=° s
-2.2
o
= 4.6 2.4
9.6 s
s
4.8
7.2
DEPTH Z (nm) Figure 6: Calculated mass density distribution, p , of a 600 eV 0+ -assisted Zr02 film as
a function of film depth x for different times from Ref. (29). For time t ~ 0, the film growth is without ion bombardment. The ion incidence angle is 30°, the ion current density 200 ,uAcm- 2 and the ion-to-atom flux ratio is jI/jA = 0.22.
The average mass density versus the ion-to-vapor flux ratio is shown in Fig. 7. The refilling depth was about 1.5 interatomic distances. The leveling-off of the experimental data presumably indicates that the atomic saturation density was attained and the system resisted a further densification due to repulsive forces between film atoms - a mechanism
252
Handbook of Ion Beam Processing Technology
which was not included in the model. Interestingly, the results were found to be insensitive to the choice of the parameters E b and E s of the collision cascade model, as long as Eb
+ E s = ~Ha(31)
5.2
•
EXPERIMENT
0
THEORY
5.0
('I')
E u
"'0)"
4.8
'-"
Q.
600eV
4.6
~ Zr02 (growing)
0+
aI= 30
4.4 0.2
0.1
0.3
0.4
0.5
J 1 /J A Figure 7: Average mass density for 600 eV 0+ -assisted Zr02 films as a function of the ion-to-atom arrival rate ratio, h/jA The experimental data was taken from Ref. (27).
The approximate steady state solution (32) of Eq. (10) was obtained as
L 81njA(1) /jA + [8nk '1J 1
Y(n)] h(k)/jA
No - - - - - - - - - - - - - - - 1 - Y jI(k)/jA
(11)
Here, No is the total average atomic density attained without ion bombardment and jA the total flux of all vapor species (jA = LjA(1). The term '1J is the ion incorporation probability and Y(n) the sputtering yield of species n. The quantity y in the denominator denotes the average number of empty sites a single incoming ion creates between the surface the the refilling depth d. The average number of empty surface sites, y , is given by
d y
= -
f
o
d Ik(z)dz -
f 0
L Mn(z)dz n
(12)
Film Growth Modification by Concurrent Ion Bombardment
253
Equations (11) and (12) can be evaluated much more easily than the computer-timeintensive set of differential equations given by Eq. (10). Calculations using Eqs. (11) and (12) were performed for a vapor deposited Ce02 film assisted by 0+ ions. As it had been found experimentally that the ratio 0/Ce approaches a value of about 3 at an ion-to-atom flux ratio of 0.8, and stays constant for larger ratios (33), the calculation was performed such that loss of excess oxygen was allowed if the 0/Ce ratio in the film exceeded 3. Figure 8 compares the experimental results with the calculation. The experimental finding that densification is optimal at an intermediate energy was reproduced by the model (34). Densification decreases at high bombarding energies because the cross-section of surface vacancy production decreases in this energy regime with increasing bombarding energy.
jI /jA= 1
~
M
6
IE () 0)
"'-'
Q
>-
I(j)
z
5
UJ 0 O+~Ce02
~ ......J I..L.
(GROWING) • EXPERIMENT cTHEORY
4
200
400
600
ION ENERGY E (eV) Figure 8: Calculated mass density for 0+ -assisted Ce02 film versus the ion bombardment energy E at the fixed ratio of h/jA = 1 from Ref. (32). The experimental data is from Ref. (33).
In the limit of zero vapor flux, which describes etching, recoil implantation and ion incorporation cause the density near the surface to increase beyond its saturation value. To account for the relaxation processes which take place during etching, Eq. (10) was modified and diffusion of atoms from overdense and nonstoichiometric regions into less dense areas was included in terms of random walk theory (35). A comparison of the time dependence of the measured and calculated densified layer thickness for an 0+ ion beanl etched vapor deposited Zr02 film is shown in Fig. 9.
254
Handbook of Ion Beam Processing Technology
12 - - llH a =8 eV
10
Ec:
-en
p=0.53 8
en
w z ~ u 6
:c ~
p=1.0
a: UJ
>
< 4 ...J
2
I o
1
10
100
1000
TIME (5) Figure 9: Conlparison between calculated and measured (vertical bars) time dependence of the thickness of the densified surface layer for etching of a vapor deposited Zr02 film by 1.2 keY 0t ions of current density 0.2Am- 2 (35). The predicted thickness varies with the probability p for atomic jumps towards the surface.
The layer thickness was determined experimentally by ellipsometric measurements where it was assumed that during etching the film consisted of an unperturbed base layer of refractive index 1.77 on the substrate, and a densified homogenous layer of index 2.0 near the surface. The theoretical predictions agreed well with experinlent if the junlp probability, p, for diffusion towards the surface was chosen slightly greater than 0.5. This was found to be reasonable as a driving force for diffusion will exist because the damage and vacancy concentrations decrease with depth below the surface, resulting in a preferential particle migration towards the surface. The calculated surface recession rate was also found to agree well with experiment. The calculations demonstrated that the large surface recession rate at commencement of etching was caused by the rapid depletion of porous material from the surface which was related to sputtering and recoil implantation
Film Growth Modification by Concurrent Ion Bombardment
255
processes. At a later stage, atoms from overdensified and nonstoichiometric regions diffused to the surface where they partially refilled the ion-induced depleted area. This then reduced the surface recession rate. Sinlilar calculations were perfornled for N+ ion beanl etching of an Al film (36). Ferron et al. (37) have recently investigated sputter deposition (glow-discharge) of Si, using a Monte Carlo technique. They took into account rearrangement effects due to argon and silicon bombardment. The deposition simulation process was confined to two dimensions (the film extended in the x-direction and grew in the z-direction). Approaching silicon atoms (or argon atoms) progressed along a randomly selected column in a 100 x 100 square grid. An incoming atom was assumed to stop either where it arrived on top of another atom or when it reached a preferential site with at least one occupied neighboring site. While the probability for stopping on top of an atom was taken to be one, the probability for stopping at a preferential site was assumed to be proportional to exp (- EI (NEb)) where E is the kinetic energy of the atom, N the number of neighbors around the preferential site, and E b the binding energy. The kinetic energy distribution of sputtered silicon atoms, SeE), was taken as
(13)
The collision-cascade-induced rearrangements were treated in an approximate way (38) where the energy loss of the impinging species was calculated assuming a continuous slowing down as dEldz ~E1/3. After each travelled interatomic distance, the dissipated energy was distributed in a hemispherical zone whose volume was proportional to that energy. Film atoms located in each hemispherical zone gained an amount of energy which was assumed to depend on the distance from the collision point. Furthermore, simple assumptions were made for the jump probability and jump directions of knock-on atoms. Argon trapping was neglected. These Monte Carlo simulations predicted a silicon film packing density of 0.5 if incoming silicon had zero kinetic energy. The density increased for the energy distribution of Eq. (13). The distribution of Eq. (13) is valid for high vacuum conditions and sets an upper energy limit for the sputtered silicon atoms. Figure 10(a) shows the porous structure formed from silicon atoms arriving with an energy distribution given by Eq. (13). Figure 10(b) displays the dense structure obtained under 50 eV Ar+ bombardment in which the ion-to-atonl arrival rate ratio is 0.5. Figure 11 shows the resulting silicon film density versus argon energy at different Ar-to-Si arrival rate ratios. Due to the bombardment-induced recoil implantation, the density increases with both argon ion energy and the arrival rate ratio. Ferron et al. (37) also investigated the variation of surface roughness with argon ion energy and arrival rate ratio.
256
Handbook of Ion Beam Processing Technology
(a)
::./····H'jL",·,,·""·"·ir"
'Ill,
(b)
I
..,"'"
':C"~i~rF;"~I'I;111 ,.""iii,
illl'li
I
11/111'111
I! I ,I I
I
II : Figure 10: Si-film microstructure from Ref (37), (a) without argon ion bombardment where the energy distribution of the Si atonlS is that of Eq. (13); (b) with 50 eV argon ion bombardment and an atom-to-ion ratio of 0.5.
Film Growth Modification by Concurrent Ion Bombardment
257
1.0
0.9
Q... 0.8 ~
+(J)
c
C1J
Q
0.7
0.6
0.5
o 20 40 60 80 100 Argon Energy EAr (eV) Figure 11: Packing density versus argon ion energy EAr for different ion-to-atom arrival rate ratios h/jA from Ref. (37). -0, j1/jA = 0.3; e, j1/jA = 0.5 ; C, h/jA = 0.7, • h/jA = 0.9
13.3.2.3 Critical and optimum ion-to-atom arrival rate ratios. Brighton and Hubler (39) used the collision cascade model to predict a critical ion-to-atom arrival ratio above which stress annealing was observed in vapor deposited Ge films with the assistance of an Ar+ ion beam. The model was based on the simple assumption that the necessary condition for stress annealing was that each atom deposited in the film be affected by a cascade, Le. be within the cascade volume. The critical ion-to-atom flux ratio, (hi jA)c , was expressed as (14)
where N is the film atomic number density and V the average volume affected by a cascade. The average volume, V, was estimated under several assumptions from range straggling values, obtained by employing the collision cascade code MARLOWE (40) for Ar+ incident on amorphous Ge.
258
Handbook of Ion Beam Processing Technology
Brighton et al. fitted their results to simple power laws and found that 150 E-1.59
; 0.2 keY
< E < 2 keY > (15)
4760 E-2.04
; 2 keV
< E < 5 keV
where the result was sensitive to variations in the ion range. The critical ion-to-atonl flux ratio of Eq. (15) versus the ion energy, along with the data of Hirsch and Varga (41), are plotted in Fig. 12. Hirsch and Varga (41) suggested a thernlal spike model to explain their data and postulated that the critical condition for stress annealing was that the thermal spike induced rearrangement rate be equal to the atom arrival rate. Using a formula from Seitz and Koehler (42), they found (h/jA)c~E-1.67which approximates the experimental energy dependence. But, their model was unable to predict the absolute magnitude of the critical ion-to-atom ratio because of uncertainties in the activation energy for thermal hopping and other material-dependent parameters. Brighton and Hubler (39) concluded that mechanisms based on such surface effects, as enhanced surface adatom diffusion or thermal spike phenomena do not appear to be consistent with the experimental data. A somewhat similar concept to the above was recently proposed by Grigorov et al. (43). They postulated that an optinlum ion bOITlbardnlent flux (h)opt exists, where a film grows with highest degree of order and maximum packing density, if one atomic displacement per condensing particle during the ion-assisted growth process takes place. Therefore, the optimum ion flux is
( h. ) opt = NI .JA
(16)
D
Where Nn is the average number of displacements generated by one ion and jA is the fraction of the flux of condensing particles which are not sputtered. The average number of displacements, N n , was determined from analytical expressions where the differential atom-atom scattering cross-section was based on the Thomas-Fermi potential and the electronic stopping cross-section on the Lindhard function. Threshold displacement energies were taken from the literature. Grigorov et al. (43) showed that there is a correlation between the optimum ion flux deduced from published data of several compounds (TiN, Ti02, Zr02, Si02 and MgF2) and the corresponding calculated values of (h)opt which supported the basic idea expressed by Eq. ( 16). Carter et al. (44) conducted some general theoretical investigations of the ion-assisted growth process, suggesting optimized experimental conditions for the most efficient use of incident species. Not surprisingly, they found that for unity sticking coefficient and no surface segregation, the condition for the most efficient use of deposition material was jA/jI >> Y where Y is the sputtering yield of the film species. This condition is assured with low (~ 1keV) or high (~ 1OOkeV) energy ion irradiation because for these energies the sputtering yields are low enough for all incident and substrate species to ensure rapid film growth.
Film Growth Modification by Concurrent Ion Bombardment
o
x
I-
~
0.01
~
o
....ed:
259
i
o
I-
~
0.001
~1T
X HIRSCH & VARGA DATA o BINARY CASCADE MODEL
1.00 E-4 '"'----......I....-..L.......L....I.o..I...I...I.~......I...._ 0.01 0.1
~----I.-----I......Io._.............
..............................
..........
1
10
ION ENERGY [keY] Figure 1 2: Critical ion-to-atom arrival rate ratio, (hi jA)c , for stress anneal of a Ge film as a function of the Ar ion bombardment energy fronl Ref. (39).
In order to secure optimal atomic mixing and homogenization, Carter et al. (44) found that high ion energies should be used simply because the number X of redistributed atonlS per ion increases with ion bombarding energy. Therefore, a necessary condition for growth-efficient use of deposit material and optimal atomic homogenization should be X > jAI jI >>Y. It was argued that due to the growth of the filnl during bonlbardment, the mixing near the film-substrate interface and the film surface is incomplete over the mixing range which is in the order of several times the ion penetration depth. To assure that a large fraction of the film is completely homogenized at high ion energies, an initially increasing, then constant and finally decreasing ion energy for the initial and final phases of deposition was suggested. 13.3.2.4 Film orientation. Bradley et al. (45) studied theoretically the development of orientational order in thin films grown with off-normal incidence ion bombardnlent during deposition and compared their results with experimental work of Yu et al. (46). It was assumed that the dominant selection mechanism for grain orientation is the difference in sputtering yields between grains which are oriented, thus channeling the ion beam, and those which are not. In their model, the degree of orientational order at the surface was found to increase with increasing ion flux and then saturate at a maximum value. The detailed predictions depended on several nlaterial parameters, such as the extent to which
260
Handbook of Ion Beam Processing Technology
the sputtering yields differ from aligned to misaligned grains, the degree of epitaxy without ion bombardment, the total acceptance angle of the channeling directions and the ion-to-atom arrival rate ratio at which deposition and resputtering rates are equal. (See Chapter 15 for a more complete discussion of this topic.) 1 3.3.3 The Molecular-Dynamics Approach
In molecular-dynamics (MD) a system of coupled classical equations of motion is solved numerically. The result describes the evolution of a representative sample of a many-body atomic system in time and space by fully allowing for mutual atomic interactions and collisions.
2
- V[L D ) j
+ L D~]
(17)
ij
Here, ri is the position of particle i at time t and mi its mass. A three (3) body potential f?~ is necessary if the dynamics of a covalently bonded material has to be investigated. In the case of metallic bonds a two body (2) potential ff) is usually sufficient. 13.3.3.1 Vapor phase growth. Leamy, Gilmer and Dirks (47) first used the MD approach with a simple two-body interaction of Lennard-Jones form to study the microThese were more realistic growth structure evolution of vapor deposited films. simulations than those performed earlier by Henderson et al. (8). In the work of Leamy et al. a formulation (48) of the Langevin formalism was employed to treat the interaction of energetic incident atoms with those atoms within the deposit that were located beyond the potential interaction ranges. This stratagem extensively reduced the amount of computation and allowed the simulation of the deposition of up to 104 particles. Atoms were deposited serially starting from random x and y positions onto a perfect 20 x 20 array of substrate atoms. Once deposited, atoms within the deposit were not allowed the small displacements which are characteristic of structural relaxation. This affected the deposition process negligibly but lead to an unrealistic strained short range order. Leamy, Gilmer and Dirks (47) simulated deposits at vapor beam incidence angles of a = 0° and a = 60° relative to the normal for vapor beam temperatures of 0.2 elk and 2 elk where e is the depth of the Lennard-Jones potential and k the Boltzmann constant. At 0.2 elk, which is 2/7th of the melting point of a Lennard-Jones crystal, the impact mobility of deposited atoms was found to be quite low and on average an impinging atom only traveled one atomic diameter following its impact. Growth was therefore similar to that in the Henderson model, though the attractive force between atoms bent the trajectories of incident atoms towards already existing aggregates and thicker microcolumns developed. At the very high vapor beam tenlperature of 2 elk a few voids were present at a = 0° but no columnar formation was evident. The columnar structure was still well defined at a = 60° and the column diameter was larger than at low vapor temperature. The mean displacement of an impinging atom after impact was about four atomic diameters. Schneider et al. (49) have performed full MD studies in order to investigate the role of relaxation in epitaxial vapor-phase growth of Lennard-Jones particles. In their simu-
Film Growth Modification by Concurrent Ion Bombardment
261
lations the motion of the whole system was followed without any further approximation and relaxation was fully considered. They deposited at normal incidence up to about 2000 atoms onto a perfect array of two close-packed layers of 14 x 16 = 224 substrate atoms such that films of a thickness of more than 10 atomic layers could be studied. The vapor beam temperature was chosen about 1.3 times higher than the melting temperature of a Lennard-Jones crystal. The substrate temperature was varied by periodically resetting the velocities of atoms in the upper substrate layer to the corresponding Maxwellian distribution. Figure 13 shows the structure which developed at zero substrate temperature. The atoms were found to be well arranged in hexagonal patterns with no evidence of in-plane disorder. The number of particles in the various layers was always less than 224 and decreased with increasing height z. At an elevated substrate temperature of about half the melting point temperature, the growth was into fully completed layers. As molecular dynamics can only describe the dynamics of a system over an extremely short time interval of about 1 nanosecond, thermally activated atomic hopping, which occurs on a much larger time scale, was not included. Therefore, the density enhancement at elevated film temperatures was mainly caused by the increased distance an impinging atom could travel on the hot film surface.
10
Y
Figure 13:
5
0 0
5
X
15
10
15 (b)
10
y
O~W
5
(a) Arrangements of vapor deposited atoms in the first deposited layer on top of the movable substrate layer. (b) Arrangement of atoms in the fifth deposited layer on top of the fourth layer. The close-packed structures contain grain boundaries and voids. (From Ref. (49».
15
X Schneider et al. (50) also studied the vapor-phase growth of a mixture of two differently sized Lennard-Jones particles as a function of relative atomic size and substrate temperature. They found that for equal numbers of both species deposited, there was an abrupt change from a layered-crystalline to a nonlayered-disordered ("amorphous-like") structure as a function of the ratio of atomic radii. This transition occurred at a radii ratio
262
Handbook of Ion Beam Processing Technology
of 0.9 and the system was disordered for lower ratios. Substrate temperature or size of substrate atoms did not significantly affect the transition. Schneider et al. (51) also studied the epitaxial growth of silicon from the vapor phase onto a Si (111) substrate. The silicon atoms interacted via the Stillinger and Weber potential (52). This potential comprises both two-body and three-body contributions (see Eq. 17) in order to describe the tetrahedral bonding. They found that at low substrate temperature the growth was not well ordered, opposite to what was found for a spherically symmetric potential that was used to describe growth of metallic films (49). At higher substrate temperatures the growth tended towards more properly stacked, crystalline Si layers which has been observed experimentally. 13.3.3.2 Vapor and sputter deposition. Muller (53) has performed two-dimensional MD simulation for film growth where the film extended in the x-direction and grew in the z-direction. Relaxation was fully considered. Though the two-dimensional approach neglected certain degrees of freedom of the interacting particles and over-emphasized disorder because of missing stabilizing bonds in the y-direction, it was computationally much faster than a three-dimensional simulation and still reflected many of the essential features of the full description. Muller (53) investigated the density of films formed from about 600 atoms as a function of the incidence kinetic energy of impinging atoms. The arriving species were assumed to be monoenergetic and to interact via a Lennard-Jones potential. The energy was varied between 0.02e and 2 e . The substrate temperature was kept at zero by periodically resetting the velocities of atoms in the second layer (seen from the bottom) of a perfect five-layer substrate (40 atoms wide) to zero. In such a way it was guaranteed that the evolving structure was solely related to the impact mobility and not the thermal mobility. The microstructure was found to be porous and microcolumnar at low energies while voids and grain boundaries became less frequent at higher energies. Figure 14 shows the packing density, defined as the fraction of atoms occupying the first 10 layers above the substrate, as a function of the incident kinetic energy in units of e at incidence angles of a = 0 0 and a = 45 0 • As void and colunln formation were more pronounced at oblique incidence, the density was found to be largest at normal incidence. Figure 14 shows clearly that vapor-deposited zone-1 films are of higher microporosity than sputter-deposited films. The incident kinetic energy of most impinging atoms in sputtering, depending on the residual gas pressure, can be assumed to be greater than 0.5 e. The distance which impinging atoms moved after impact (the impact mobility distribution) was also investigated and was found to increase monotonically with incident kinetic energy. 13.3.3.3 Ion-assisted deposition. Muller (54) has performed two-dimensional MD simulations to elucidate ion-surface interaction processes which lead to microstructure modifications of a zone-1 ultra-thin Ni film irradiated during growth by Ar ions. In these simulations the Ni-Ni interaction was of a Lennard-Jones fornl while the Ar-Ni interaction was described by a Moliere potential (55). The rectangular simulation cell which contains the growing film was open along the positive z axis and the substrate consisted of several close-packed rows (40 atonlS each) of Ni-atoms. The filnl atoms in the bottom layer were fixed at their ideal lattice sites while atoms in the second layer were permitted to move. Whenever an atom of the second layer moved downwards and crossed the second layer position, its velocity was reset to zero which meant that the substrate behaved as an ideal heat sink of zero temperature simulating a much larger cold substrate. Periodic boundary conditions were applied in the x-direction.
Film Growth Modification by Concurrent Ion Bombardment
263
1.0
>- 0.9
t-
Ci5 Z
W
C
(!)
z
~
0.8
()
« a..
0.7
0.6
o
0.5
1.0
1.5
2.0
KINETIC ENERGY E (e) Figure 14: Packing density as a function of the incident kinetic energy of adatoms for different angles of incidence, a , from Ref. (53). The vertically aligned pairs of points show results for different adatom introduction sequences.
Vapor atoms and ions were introduced sequentially from random positions x and inlpinged under fixed angles. It was necessary to choose the vapor introduction rate low enough to avoid mutual interactions between approaching vapor atoms and to give sufficient time to the system for atomic vibrations to relax and obtain zero surface temperature before a new atom condensed. The elapsed time for relaxation, after an ion struck the surface, was chosen long enough to guarantee that all collisional displacements had terminated, sputtered atoms had left the top of the simulation cell and the surface temperature had dropped close to zero. This insured that the next arriving vapor atonl condensed onto a cold substrate. The removal of kinetic energy from a two-dimensional MD cell is known to be slower than that for a three-dimensional cell and some of the sputtering and atomic rearrangenlent occurring in the late stage of an ion impact event may be partially thermal in origin. The great advantages of two-dimensional MD calculations are that it is computationally far less expensive and facilitates the tracking of particle trajectories but still reflects many essential features of a full three-dimensional simulation. Careful studies of atomic trajectories during ion impact onto a porous microcolumnar surface structure clarified the essential mechanisms which contribute to an improvement in layer growth. Muller (54) found: (i) ion borrlbardment during growth renloves overhanging atoms and causes void regions to remain open until filled by new depositing atoms; (ii) sputtered atoms are redeposited mainly in voids; (iii) ions induce surface diffusion (diffusion dis-
264
Handbook of Ion Beam Processing Technology
tance is a few interatomic spacings), local heating, collapse of voids and recrystallisation. Figure 15(a) and 15(b) show snapshots during a typical collisional event where a 100 eV Ar ion hit the center of a nlicrocolumn. (Here, € = 1.3 eV which reproduced the cohesive energy of Ni with a two-dimensional close-packed lattice.) The atomic displacements are indicated by straight lines (not trajectories) attached to the corresponding atoms with their origin at the atoms' t=O positions. Displacements less than half the lattice spacing are not indicated. The initial violent ion-surface interaction and the formation of knock-on atoms require only about 0.2 ps while the succeeding freezing process, required to establish the final atomic configuration, takes about lOps. It can be seen from the figures that the microcolunln (or void) collapses after the ion inlpact (densification process), leading to an increased packing.
t = 0.21 ps
( a) Figure 15: Structural evolution at the surface of a Ni film during the impact of a 100 eV Ar ion (Ref. (54)).
t=13.06PS~ (b)
Film Growth Modification by Concurrent Ion Bombardment
265
The microstructures which evolve when about 500 Ni atoms arrive at the substrate while 50 eV Ar ions impinge with different fluxes, are displayed in Fig. 16. The vapor impingement angle was chosen to be 0° while the ion bombardment angle was 30° - a geometrical configuration which is used frequently in ion-assisted deposition. While the ion-to-atonl flux ratio, hiiA , was varied, the sequence of initial atom positions x at introduction stayed unchanged. Instead of determining the kinetic energies of the vapor atoms from a Maxwellian distribution in a random fashion, a constant energy of 0.1 e was assumed for simplicity. As can be seen from Fig. 16, concurrent ion bombardment disrupts microcolumnar growth and improves the crystal order and homoepitaxy. A low illiA ratio causes the closure of open, long voids while a large ion-to-atom flux ratio causes the total disappearance of microporosity. The sputtering yield was found to be 0.35. In none of the examined cases was any Ar found entrapped in these films. The reason for this was the purely repulsive Ar-Ni Moliere interaction. The packing density, defined as the fraction of atoms occupying the first nine layers above the substrate, is shown in Fig. 17 for the two ion energies E= 10 and 50 eVe The density increases linearly
( a) Figure 16: Typical microstructure obtained for condensing Ni vapor atoms arriving under normal incidence (a) without ion bonlbardment, (b) with Ar ion bombardment of E = 50 eV, 0 a = 30 x and h/iA = 0.04, (c) with Ar ion bOITlbardnlent of E = 50 eV, a = 30° and h/iA = 0.16 (Ref. (54)).
(b)
vapour
~
+ ~
( (
( c)
D
with ion-to-atom flux ratio. Because of the relatively snlall number of atoms involved in these simulations, limited by the otherwise unacceptable time of computation, the data exhibited considerable scattering but the trends are discernible. For illiA > 0.3 at E = 10 eV and hiiA > 0.2 at E= 50 eV the curves in Fig. 17 are expected to reach the maximum packing density and then stay constant. Such a linear increase in density and then
266
Handbook of Ion Beam Processing Technology
levelling-off at maximum packing was recently found experimentally by Yehoda et al. (56) for 15-110 eV Ar ion bombardment of a growing Ge film. Linear increases in packing density were also found experimentally for ion-assisted vapor deposited Zr02 and Ce02 films (27) (35) as well as for A120 3 , Ta20s and Ti02 films (57,58). Figure 18 displays the density versus ion energy for a fixed value of JII jA = 0.16. The density increases rapidly at low ion energies because a weakly bonded porous structure is easier to reorder and densify than a more closely packed one. A similar energy-dependence was found experimentally by Yehoda et al (56). The interesting regime of ion energies larger than 100 eV was not considered theoretically as the required larger relaxational time, which followed each ion-induced collision sequence, made calculations extremely computer-time intensive.
1.0
Ar +
-
Ni (growing)
~ 0.9 CiS
z w o
C!'
z
~
()
~
0.8
0.7
Figure 17: Packing density as a function of the ion-to-atom flux ratio, energies of E = 10 eV and 50 eV (Ref. (54)).
hi jA , for Ar ion
Figure 19(a) and (b) show the atomic number density versus the height of the film corresponding to the microstructures of Figs. 16(a) and (c). The substrate layers are included. Figure 19(a) indicates disorder in the adsorbate which is overemphasized because of nlissing bonds in the y-direction. Figure 19(b) exhibits the ion-beam-induced improvement of structural order - a higher degree of crystallinity and homoepitaxy. The order was found to improve with increasing ion-to-atom flux ratio and ion bombardment energy. The effect of ions other than Ar has been investigated for the case of Ti arcevaporation, where Ti ions of 50 eV bombarded a growing film of Ti atoms, arriving with about 0.1 eV kinetic energy (58). Ion-to-atom flux ratios up to 0.3 were studied. Almost all Ti ions were found to become entrapped in the growing film and the sputtering yield was almost zero. The simulations predicted the density to increase linearly with the ionto-atom ratio up to its maximum value at h/jA~0.2 and to stay constant at the maximum density for larger flux ratios.
Film Growth Modification by Concurrent Ion Bombardment
267
1.0
>0.9 ~ U5 z
u.J 0
~ S2
()
« 0- 0.8
Ar+ - Ni j I/jA =0.16
0.7
o
50 E (eV)
100
Figure 18: Packing density versus Ar ion energy, E, for a fixed ion-to-atom flux ratio of h/jA = 0.16 from Ref. (54).
13.3.3.4 Intrinsic stress modification. Experimentally it has been found that in general, filnlS deposited by evaporation are in tensile stress and concurrent ion bombardment can reduce this stress towards zero, and often results in a film in compressive stress (59).
Muller (60) has calculated the dependence of the intrinsic mechanical film stress on ion energy for an Ar+-assisted vapor deposited Ni film using the two-dimensional MD approach. The average film stress (61) is related to the xx component of the surface stress tensor (62) (63) fxx by (J = fxxlh where h is the film thickness and
(18)
Here, ~ is the film atom-atom interaction potential, Xh Zi and Xj' Zj are the coordinates of film particles i and j and rij the distance between film atoms i and j. The quantity L denotes the length of the simulation cell. Figure 20 shows the calculated intrinsic stress of a Ni film versus the Ar ion bonlbarding energy for an ion-to-atom flux ratio of JII jA = 0.16. The atoms arrived with a kinetic energy of 0.1 e. The tensile stress passes over a maximum at about 20 eV and decreases further with increasing bombarding energy.
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Handbook of Ion Beam Processing Technology
40 .................- . - - - - - - - - - - - - - - - - - - ,
jl,j A= 0
30
( a)
> ..... ~ 20 w
0
10
10
5
15
30
25
20
. HEIGHT Z
40
.-..-.-P'-+"......-~-----------..,
E=50 eV jl/jA = 0.16
30
>I-
(b)
~ 20 UJ
o
10
o
...........................J~~UUUh
~
o
It
,\hAf
+_L6_.....~--J,~~ ....................- - - l
5
10
HEIGHT Z (VS/2
15 SO
20
25
units)
Figure 19: Atomic density versus height z (a) without ions, (b) with 50 eV Ar ion bombardment and h/jA = 0.16 (Ref. (54)).
Film Growth Modification by Concurrent Ion Bombardment
269
The MD simulations revealed that at zero ion energy the film structure is rich in defects - large micropores and open voids exist, which are wider than the atom-atonl interaction range. With increasing ion energy the pores become smaller and the tensile stress reaches a maximum when the short range attractive interatomic forces can act most effectively across voids. At larger ion energies, the defects gradually disappeared and a well-layered crystal structure evolved, resulting in almost zero stress. When a higher incident kinetic energy of 0.5 e was used for the adatoms, the film stress at zero ion energy was larger and the tensile stress decreased monotonically over the whole ion energy range. With the Lennard-Jones interaction strength of e = 1.3eV for Ni, the maximum tensile stress value was similar to that found for vapor deposited refractory metals. There was no investigation into the effect of Ar ion bombardment at energies larger than 100 eV as the twodimensional simulation cell was believed to be too small for a realistic consideration of interstitial formation which would result in compressive film stress.
0.02
~
0<
Co) ~
~
en
0.01
UJ
a::
Ar+-- Ni (growing)
.....
en
jAr /j A=O.16 c:
o
'c;; c: ~
o
20
40
60
80
BOMBARDING ENERGY E [eV]
Figure 20: Intrinsic mechanical stress of an ion-assisted, vapor-deposited Ni film as a function of the Ar ion bombarding energy, where h/jA = 0.16 (Ref. (60)).
Experimental data for magnetron sputter deposition (64) (65) show that for refractory metals the intrinsic stress passes over a maximum tensile value and decreases as a function of decreasing working gas pressure and finally becomes compressive. The arrival kinetic energy of neutralized fast primary ions reflected from the target and the arrival energy of sputtered atoms increase with decreasing pressure due to the reduction of collisions with gas atoms. Muller (60) has revealed by MD studies that the variation in tensile stress in magnetron sputter deposition is not only related to an increase in the bombarding energy of fast reflected neutrals but also to an about equal proportion to an increase in incident kinetic energy of sputtered atoms. This is understandable as it has been shown that microporosity of zone-1 films decreases with increasing incident kinetic energy of adatoms (53).
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Handbook of Ion Beam Processing Technology
13.3.3.5 Ion-beam deposition. Dodson et al (66) (67) investigated the mechanisms controlling low energy (10-100 eV) Si-beam deposition at different incident angles onto a relaxed but unreconstructed (Ill) silicon substrate using a MD technique. To describe the covalent Si-Si interaction they employed the Dodson potential (68). Sets of trajectories of 30 successively impinging Si atoms were analyzed to determine trends in the deposition of atoms. It was found that at normal or near-normal incidence of 10 eV Si atoms, 30% of the incident atoms came to rest on top of the Si( 111) surface, whereas 70% penetrated into the top double layer and stopped in interstitial sites. At an incidence angle of 30° relative to the film normal, 50-60% of impinging atoms stopped in interstitial sites, while the remainder formed a new layer on the surface. Thermal diffusion was thought to bring eventually the interstitials to the surface, a process which cannot be described by MD because of the long tinle required. The detailed adsorption dynamics was found to be sensitive to the beam orientation, especially if it was along a bulk channeling direction. Higher energies than 10 eV at normal incidence were not considered because of uncertainties in the hard core repulsive Si-Si interaction. In the case of grazing incidence, surface channeling was predicted where the long-range attractive potential and the short-range repulsive potential generated by the substrate surface atoms produced a potential-well which guided an incoming atom along the surface. For given beam energy a critical angle was found below which surface channeling occurred and above which either the atom bounced off or rapidly adsorbed onto the surface. For a surface channeling trajectory on an idealized surface, the loss of kinetic energy by surface phonon excitations was extremely slow and ranges of thousands of angstroms were possible. Figure 21 shows an incidence angle versus beam energy phase-diagram, summarizing the different ion-surface interaction processes which are rapid adsorption, scattering (bounce-off) and surface channeling.
60 0eI)
RAPID ADSORPTION
"0
W -I <.:J
RA
z < 75 w
u
Z W C
.....
SURFACE CHANNELING
U
Z ~
90 0
50 BEAM ENERGY (eV)
100
Figure 21: Incidence angle versus Si-beam energy phase diagram from Ref. (66), identifying regions of rapid adsorption (RA), scattering and surface channeling.
Film Growth Modification by Concurrent Ion Bombardment
271
Dodson (66) also investigated how the excess kinetic energy and momentum of the atomic beam was carried away by substrate lattice excitations. He speculated that this excess energy, which provides a local and short-lived region of vibrational excitation, may serve to selectively anneal nearby metastable configurations which arise during the growth process. 13.3.3.6 Ionized-cluster-beam deposition. In ionized-cluster-beam deposition (ICBD) clusters are formed when supersaturated vapor expands adiabatically during the ejection of atoms through a nozzle into a high vacuum region. (Please see Chapter 5 for a more complete discussion.) A certain percentage of these neutral clusters are then ionized by electron bombardment and accelerated along an electric field. The clusters are singlecharged and the cluster size distribution is rather broad with cluster sizes ranging up to 2000 atoms I cluster (69). Several theoretical studies were undertaken to understand the cluster formation process (70) (71). Because ICBD offers control over such deposition parameters as cluster kinetic energy and the percentage of ionized clusters, it is becoming an important film growth technique. For different film substrate combinations it was demonstrated that nucleation density, packing density, sticking coefficient, and epitaxial growth (72) strongly depend on the acceleration voltage. Muller (73) recently investigated the growth dynamics of ionized-cluster-beam deposition by employing a twodimensional MD simulation where atoms interact via a Lennard-Jones potential. For simplicity single-sized clusters of hexagonal shape were chosen which corresponded to a representative cluster size of about 700 atoms in three dimensions. The randomly oriented clusters approached the substrate at normal incidence from random positions above the substrate. The substrate consisted of a nunlber of perfect close-packed rows each containing 50 atoms of cluster atom type. The substrate temperature was kept at zero degrees during deposition by periodically resetting the velocities of atoms in the second lowest substrate row to zero. When a cluster hit the substrate with the low initial kinetic energy per cluster atom of E/N = O.le, which represents approximately nozzle ejection kinetic energy, the cluster stayed intact because its impact energy was too small to cause deformation or break-up. At the higher kinetic energy of E/N = 2 e (equivalent to about 1.5 keY acceleration voltage which is typically used in ICBD) clusters deformed and melted upon impact as shown in Fig. 22. Significant phonon excitation occurred in the nearby surface region but still no clusters of material were fast enough to escape the attractive interaction as the incident kinetic energy quickly dissipated into the substrate. The cluster rapidly cooled (~ 5 ps) and recrystallized, adopting the substrate crystal structure. At higher incident kinetic energies, some of the impact-induced surface damage was found not to anneal. No significant enhancenlent of surface mobility in ternlS of diffusion of atoms along the surface was observed in any of the cases studied, which is in disagreement with common belief that neutral and ionized clusters break-up into atoms which then scatter over large distances along the surface (74). The evolving film microstructure, resulting when a nunlber of clusters successively approached the surface, was investigated. Figure 23(a) shows the structure for a cluster energy per atom of E/N = 0.1 e. To obtain an idea of the fate of individual clusters, lines were drawn around the clusters to nlake thenl distinct. Because of their low kinetic energy they stayed almost intact and, due to their random orientation at arrival, the film consisted
272
Handbook of Ion Beam Processing Technology
of snlall crystallites of cluster size. Polycrystalline growth was experimentally observed for neutral cluster beam deposition (75).
E/N=2E
( a)
1=0
•
t (a)-(c) Cluster impact with a perfect substrate at an incident kinetic energy per atom of E/N=2e from Ref. (73). Figure 22:
1=1.1 ps (b)
"v
w
1=7 ps ( c)
At the higher cluster energy of E/N = 1.5 e (Fig. 23 (b» most of the voids disappeared and due to the more pronounced atomic rearrangements and local heating, a high degree of structural order was attained. It is of interest that at this energy no significant atomic mixing at cluster-substrate or cluster-cluster interfaces took place. The degree of homoepitaxial growth, H, was defined as the portion of deposited atoms located in narrow bands ± 0.1(/3/2) a o above and below the positions of ideal layers at z = n(/3 /2) ao , (n = 1,2,3 ... ). A value of H = 0.2 indicated no homoepitaxy because for randomly distributed atoms a fraction of 0.2 would be positioned in these bands; H = 1 meant perfect homoepitaxy. Figure 24 shows the increase of homoepitaxy with cluster energy. Figure 24 also displays the increase in density with increasing cluster energy. A similar density increase was found experimentally in the case of Au films produced by ICBD (76). The mechanism for densification is simply that cluster kinetic energy is gradually transferred into cluster and surface heating, and the downward cluster motion causes the filling of voids.
Film Growth Modification by Concurrent Ion Bombardment
273
( a)
Figure 23: Resulting film microstructure when several clusters successively approach the substrate (Ref. (73». The cluster kinetic energy per atom is (a) E/N = 0.1 E and (b) E/N = 1.5 E.
1.0
1.0
>-
I-
Ci5 :t:
Z
0.6
UJ
c
>X
0.9 C'
0:: w
()
« I-
o ::E o
z ~
« Cl.
0.2
J:
0.8
o
0.5 1.0 CLUSTER ENERGY E/N (E)
1.5
Figure 24: Homoepitaxy, H, and packing density versus ion cluster kinetic energy per atonl, E/N, from Ref. (73).
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Handbook of Ion Beam Processing Technology
13.6
CONCLUSIONS
Up to now a number of ion bombardment induced effects on film growth have been studied theoretically. The molecular dynamics and collision cascade approaches have been used to solve theoretically the complex many-body problenl of nonequilibrium growth associated with ion bombardment. The former approach yields the most realistic description of film growth modifications at the atomic level and details the ballistic and relaxational effects without further approximation. The collision cascade description however, though computationally more economical, can treat relaxation processes only in an approximate manner. The calculations carried out have clearly demonstrated how the two crucial quantities, ion energy and ion-to-atom arrival rate ratio, control microstructure and chemical composition in the near surface region of a growing film. The modifications of film growth induced by ion bombardment that have been modelled so far include: disruption to porous microcolumnar growth, increases in density and crystallinity, changes in film-grain orientation and nlodification of intrinsic stress. There remain however, several important areas of study that have not yet received sufficient attention. One may count among these: modifications caused by concurrent ion bombardment during the nucleation stage of filnl growth, alterations to grain size distributions, and changes in film-substrate adhesion.
13.7 REFERENCES
1.
Takagi, T., Role of ions in ion-based film formation. Thin Solid Films 92: 1-17 (1982)
2.
Greene, J.E. and Barnett, S.A., Ion-surface interactions during vapor phase crystal growth by sputtering, MBE, and plasma-enhanced CVD: Applications to semiconductors. J. Vac. Sci. TechnoL 21: 285-302 (1982)
3.
Harper, J.M.E., Cuomo, J.J., Gambino, J.R. and Kaufman, H.R., in: Ion Bombardment Modifications of Surfaces, (0. Auciello and R. Kelly, eds.), pp 127-162, Elsevier, New York (1984)
4.
Martin, P.J., Ion-based methods for optical thin film deposition. J. Mater. Sci. 21: 1-25 (1986)
5.
Cuomo, J.J., and Rossnagel, S.M., Property modification and synthesis by low energy particle bombardment concurrent with film growth. NucL Instr. Meth. in Phys. Research. B 19-20: 963-974 (1987)
6.
Movchan, B.A. and Demchishin, A.V., Investigation of the structure and properties of thick vacuum-deposited films of nickel, titanium, tungsten, alumina and zirconium dioxide. Fiz. Met. Metalloved. 28: 653-660 (1969)
7.
Thornton, J.A., Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings. J. Vac. Sci. TechnoL 11 : 666-670 (1974)
8.
Henderson, D.J., Brodsky, M.H., and Chaudhari, P., Simulation of structural anisotropy and void formation in amorphous thin films. ApoL Phys. Lett. 25: 641-643 (1974)
Film Growth Modification by Concurrent Ion Bombardment
275
9.
Fritzche, H., Tanielian, M., Tsai, C.C., and Gaczi, P.J., Hydrogen content and denJ. ADDI. Phys. 50: sity of plasma-deposited amorphous silicon-hydrogen. 3366-3369 (1979)
10.
Dirks, A.G. and Leamy, H.J., Columnar microstructure in vapor-deposited thin films. Thin Solid Films 47: 219-233 (1977)
11.
Kim, S., Henderson, D.J., Chaudhari, P., Computer simulation of anl0rphous thin films of hard spheres. Thin Solid Films 47: 155-158 (1977)
12.
Bangjun, L., and Macleod, H.A., Thin-film microstructure modelling. Proc. SPIE. Vol. 540: 150-155 (1985)
13.
Messier, R., Giri, A.P., and Roy, R.A., Revised structure zone model for thin film physical structure. J. Vac. Sci. Technol. A2: 500-503 (1984)
14. Muller, K.-H., Dependence of thin-film microstructure on deposition rate by means of a computer simulation. J. ADDI. Phys. 58: 2573-2576 (1985) 15.
Outlaw, R.A., and Heinbockel, J.H., A potential energy scaling Monte Carlo sinlulation of thin film nucleation and growth. Thin Solid Films 108: 79-86 (1983)
16.
Srolovitz, D.J., Grain growth phenomena in films: A Monte Carlo approach. J. Vac. Sci. Technol. A4: 2925-2931 (1986)
17. Anderson, M.P., Srolovitz, D.J., Grest, G.S., and Sahni, P.S., Computer simulation of grain growth - I. Kinetics. Acta Metall. 32: 783-791 (1984) 18. Srolovitz, D.J., Anderson, M.P., Sahni, P.S., and Grest, G.S., Computer simulation of grain growth - II. Grain size distribution, topology, and local dynamics. Acta Metall. 32: 793-802 (1984) 19. Srolovitz, D.J., Grest, G.S., and Anderson, M.P., Computer simulation of grain growth - V. Abnormal grain growth. Acta Metall. 33: 2233-2247 (1985) 20. Burton, W.K., Cabrera, N., and Frank, F.C., The growth of crystals and the equilibrium structure of their surfaces. Trans. Roy. Soc. London 243A: 299-358 (1951) 21. Gilmer, G.H., and Jackson, K.A., in: Crystal growth and materials, (E. Kaldis and H.J. Scheel, eds.), pp 80-113, North-Holland Publishing Company (1977) 22. Gilmer, G.H., Computer models of crystal growth. Science 208: 355-63 (1980) 23. Weeks, J.D., and Gilnler, G.H., Dynamics of crystal growth. Advances in Chem. Phys. 40: 157-228 (1979) 24. Muller, K.-H., Monte Carlo calculation for structural modifications in ion-assisted thin film deposition due to thermal spikes. J. Vac. Sci. Technol. A4: 184-188 ( 1986) 25. Sigmund, P., Energy density and time constant of heavy-ion-induced elastic-collision spike in solids. ADOL Phys. Lett. 25: 196-171 (1974) 26. Biersack, J.P., and Haggmark, L.G., A Monte Carlo computer program for the transport of energetic ions in amorphous targets. Nucl. Instr. and Methods 174: 257-269 (1980) 27. Martin, P.J., Netterfield, R.P., and Sainty, W.G., Modification of the optical structural properties of dielectric Zr02 filnlS by ion-assisted deposition. J. Aool. Phys. 55: 235-241 (1984)
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Handbook of Ion Beam Processing Technology
28. Brett, M.J., Structural transitions in ballistic aggregation simulation of thin film growth. J. Vac. Sci. Techno!. (in press) 29. Milller, K.-H., Model for ion-assisted thin-film densification. J. App!. Phys. 59: 2803-2807 (1986) 30. Biersack, J.P., and Ziegler, J.F., in: Ion Implantation Techniques (H. Rilssel and H. Glawischnig, eds.), p. 122, Springer, New York (1982) 31. Biersack, J.P., and Eckstein, W., Sputtering studies with the Monte Carlo program TRIM.SP. App!. Phys. A 34: 73-94 (1984) 32. Milller, K.-H., Modelling ion-assisted deposition of Ce02 films. App!.Phys. A 40: 209-213 (1986) 33. Netterfield, R.P., Sainty, W.G., Martin, P.J. and Sie, S.H., Properties of Ce02 thin films prepared by oxygen-ion-assisted deposition. App!.Opt. 24: 2267-2272 (1985) 34. Milller, K.-H., Sumnlary abstract: Molecular dynamics and collision cascade studies of ion-assisted thin film deposition. J. Vac. Sci. Techno!' A5: 2161-2162 (1987) 35. Milller, K.-H., Netterfield, R.P., and Martin, P.J., Dynamics of zirconiunl oxide thin-film growth and ion-beam etching. Phys. Rev. B 35: 2934-2941 (1987) 36. Netterfield, R.P., Martin, P.J., Milller, K.-H., Sainty, W.G., Pacey, C.G., and Filipczuk, S.W., Growth and dynamics of aluminum nitride and aluminum oxide thin-films synthesized by ion-assisted deposition. J. App!. Phys. 63: 760-769 ( 1987) 37. Ferron, J., Koropecki, R.R., and Arce, R., a-Si thin-film growth by sputtering: A Monte Carlo study. Phys. Rev. B 35: 7611-7617 (1986) 38. Schwarz, S.A., and Helms, C.R., A statistical model of sputtering. J. App!. Phys. 50: 5492-5499 (1979) 39. Brighton, D.R., and Hubler, G.K., Binary collision cascade prediction of critical ion-to-atom arrival ratio in the production of thin films with reduced intrinsic stress. Nuc!. Instr. Meth. in Phys. Research B 28: 527-533 (1987) 40. Robinson, M.T., and Torrens, LM., Computer simulation of atomic-displacement cascades in solids in the binary-collision approxinlation. Phys. Rev. B 9: 5008-5024 (1974) 41. Hirsch, E.H., and Varga, LK., The effect of ion irradiation on the adherence of germanium films. Thin Solid Films 52: 445-452 (1978) 42. Seitz, F., and Koehler, J.S., Displacement of atoms during irradiation. Solid State Phys. 2: 305-448 (1956) 43. Grigorov, G.L, Martev, LN., Langeron, J.-P., and Vignes, J.-L., A choice of optimum density of ion bombardment by ion-assisted PVD films. Thin Solid Films (in press) 44. Carter, G., and Armour, D.G., Parameter optinlization for film honlogenization during ion-assisted deposition. Vacuum 36: 337-340 (1986) 45. Bradley, R.M., Harper, J.M.E., and Smith, D.A., Theory of thin-film orientation by ion bombardment during deposition. J. App!. Phys. 60: 4160-4164 (1986) 46. Yu, L.S., Harper, J.M.E., Cuomo, J.J., and Smith, D.A., Control of thin film orientation by glancing angle ion bombardment during growth. J. Vac. Sci. Techno!. A4: 443-447 (1986)
Film Growth Modification by Concurrent Ion Bombardment
277
47. Leamy, H.J., Gilmer, G.H., and Dirks, A.G., in: Current Topics in Materials Science (E. Kaldis, ed.), Vol. 6, pp. 309-344, North Holland Publ., Amsterdam (1980) 48. Shugard, M., Tully, J.C., and Nitzen, A., Dynamics of gas-solid interaction: Calculations of energy transfer and sticking. J. Chem. Phys. 66: 2535-2544 (1977) 49. Schneider, M., Rahman, A., and Schuller, LK., Role of relaxation in epitaxial growth: A molecular-dynamics study. Phys. Rev. Lett. 55: 604-606 (1985) 50. Scheider, M., Rahman, A., and Schuller, LK., Vapor-phase growth of amorphous nlaterials: A molecular dynamics study. Phys. Rev. B 34: 1802-1805 (1986) 51. Schneider, M., Schuller, LK., and Rahman, A., Epitaxial growth of silicon: A molecular-dynamics simulation. Phys. Rev. B 36: 1340-1344 (1987) 52. Stillinger, F.H., and Weber, T.A., Computer simulation of local order in condensed phases of silicon. Phys. Rev. B 31: 5262-5271 (1985) 53. MUller, K.-H., Role of incident kinetic energy of adatoms in thin film growth. Surf. Sci. 184: L375-L382 (1987) 54. Muller, K.-H., Ion-beam-induced epitaxial vapor-phase growth: A moleculardynamics study. Phys. Rev. B 35: 7906-7912 (1987) 55. Wilson, W.D., Haggmark, L.G., and Biersack, J.P., Calculations of nuclear stopping, ranges, and straggling in the low-energy region. Phys. Rev. B 15: 2458-2468 (1977) 56. Yehoda, J.E., Yang, B., Vedam, K., and Messier, R., Investigation of the void structure in amorphous germanium thin films as a function of low energy ion bombardment. J. Vac. Sci. Technol. A6: 1631 (1988). 57. Williams, F.L., Jacobson, R.D., Exarhos, G.J., McNally, J.J., and McNeil, J.R., Optical characteristics of thin film deposition at low tenlperature using ion-assisted deposition. J. Vac. Sci. Technol. A6: 2020 (1988). 58. Martin, P.J., McKenzie, D.R., Netterfield, R.P., Swift, P., Filipczuk, S.W., Muller, K.-H., Pacey, C.G., and James, B., Characteristics of titanium arc evaporation processes. Thin Solid Films 153: 91-102 (1987) 59. Cuomo, J.J., Harper, J.M.E., Guarnieri, C.R., Yee, D.S., Attanasio, L.J., Angilello, J., Wu, C.R., and Hammond, R.H., Modifications of niobium film stress by lowenergy ion bombardment during deposition. J. Vac. Sci. Technol. 20: 349-354 ( 1982) 60. Muller, K.-H., Stress and microstructure of sputter-deposited thin films: Molecular dynamics investigations. J. Apol. Phys. 62: 1796-1799 (1987) 61. Segmuller, A., Angilello, J., and La Placa, S.J., Automatic x-ray diffraction measurement of the lattice curvature of substrate wafers for the deternlination of linear strain patterns. J. Aool. Phys. 51: 6225-6230 (1980) 62. MacLellan, A.G., A statistical-mechanical theory of surface tension. Proc. R. Soc. London Ser. A 213: 274-289 (1952) 63. Harasima, A., Statistical mechanics of surface tension. J. Phys. Soc. Jon. 8: 343-346 (1953) 64. Hoffmann, D.W., and Thornton, J.A., Internal stresses in Cr, Mo, Ta and Pt films deposited by sputtering from a planar magnetron source. J. Vac. Sci. Technol. 20: 355-358 (1982)
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65. Wu, C.T., Intrinsic stress of magnetron-sputtered niobium films. Thin Solid Films 64: 103-110 (1979) 66. Dodson, B.W., Atomistic simulation of silicon beam deposition. Phys. Rev. B 36: 1068-1074 (1987) 67. Dodson, B.W., and Taylor, P.A., Interaction of a 10 eV silicon beam with the Si(lll) surface: A molecular dynamics study. J. Mater. Res. 2: 805-808 (1987) 68. Dodson, B.W., Development of a many-body Tersoff-type potential for silicon. Phys. Rev. B 35: 2795-2798 (1987) 69. T. Takagi, Ionized cluster beam technique. Vacuum 36: 27-31 (1986) 70. Stein, G.D., Design and use of metal cluster beam sources: Implications for thin film devices. in: Proceedings of the International Ion Engineering Congress - ISIAT (Ion Source and Ion Assisted Technology) 1983 and IPAT (Ion Plating and Allied Technology) 1983, Kyoto, (T. Takagi, ed.), pp. 1165-1176B 71. Yang, S.N., and Lu, T.M., Condensation of metal and semiconductor vapor during nozzle expansion. J. ADDt Phys. 58: 541-544 (1985) 72. Yamada, I., Inokawa, H., and Takagi, T., Epitaxial growth of Al on Si(lll) and Si(100) by ionized-cluster beam. J. ADDt Phys. 56: 2746-2750 (1984) 73. Milller, K.-H., Cluster-beam deposition of thin films: A molecular-dynamics simulation. J. ADDt Phys. 61: 2516-2521 (1987) 74. Takagi, T., Yamada, I., and Sasaki, A., An evaluation of metal and semiconductor films formed by ionized-cluster beam deposition. Thin Solid Films 39: 207-217 (1976) 75. Kuiper, A.E.T., Thomas, G.E., and Schouten, W.J., Ion cluster beam deposition of silver and germanium on silicon. J. Cryst. Growth 51: 17-40 (1981) 76. Yamada, I., Matsubara, K., Kodama, M., Ozawa, M., Takagi, T., Characteristics of thin films formed by the ionized-cluster bean1 technique. J. Cryst. Growth 45: 326-331 (1978)
14 Interface Structure and Thin Film Adhesion
John Baglin
14.1 INTRODUCTION
Strong, stable adhesion of thin film coatings is of vital technical significance in a great diversity of applications, including metallization and packaging of semiconductor devices; production of coatings to protect surfaces from abrasion, wear and corrosion; and the development of mirror coatings for high-power laser applications. Systems of prinlary concern generally involve joining materials which display little or no bulk chemical affinity or solubility, such as metals and ceramics. It therefore seemed all the more miraculous when, in recent years, experimental reports seemed to show that energetic ion beam irradiation of such interfaces was normally beneficial to adhesion (1,2,3). Today, it is evident that the effect is related to equilibrium chemical bonding and that different approaches using low energy ions to tailor the interface show even greater promise in effectiveness and practicality. Moreover, it is now becoming clear that not only chemical bonding but also the elastic properties of the interface materials play an important part in altering adhesion performance. Ion beams can often be applied to assist adhesion by modifying interface roughness, overcoming contaminant effects, and improving fracture toughness of the interface region. 14.2 FACTORS AFFECTING ADHESION
The interface energy 'Yh which describes the state of electronic bonding of the interface atoms, must always be an important quantity governing interface adhesion. It is related to the thermodynamic energy of adhesion E ad by the expression
where 'Ys and 'Yf are surface free energies for the substrate and film respectively. However, interface chemical bonding is by no means the only factor required to attain good adhesion performance.
279
280
Handbook of Ion Beam Processing Technology
Some of the factors of primary concern for the production of good adhesion are identified schematically in Fig. 1. "Good adhesion" is a term whose definition must always be made in the context of the perfornlance required of the fHnl, which might for example mean withstanding peeling, shear, normal pulling, scratching or other abrasion, or stress delanlination. In each type of test, the chosen figure of merit (e.g. peel force per mm) will be found to be related in a unique way to 'Yi and also to the elastic properties of the materials at and near the interface. A comprehensive example of such a relationship was derived (4) by K.S. Kim (Fig. 2) for the case of the peel test. In this analysis for the Cu-Si and Cu-polyinlide systems, the peel force P is shown to depend heavily on the interface energy 'Yi and on the yield stress (a y), Young's modulus (E) and thickness (t) of the film. Serious attention must be paid to the selection or tailoring of those material properties, in addition to the interface chemistry, in order to tailor the best adhesive bond.
Interface chemistry
differential thermal expansion
plastic deformation
Figure 1: A schematic illustration of some major factors affecting the adhesion performance of a thin film coating on a substrate.
Figure 1 is drawn for the case of a film being peeled at the interface, because it represents the basic release phenomenon prevailing in all cases, namely the initiation and propagation of an interface fracture. The ultinlate goal for good adhesion performance will be to make the initiation and propagation of that crack as energetically costly as possible. The diagram simply identifies some of the factors which influence that cost. They include, of course, interface electronic bonding (ionic, covalent or metallic). (Van der Waals forces alone can not offer sufficient adhesion strengths for the kind of technological applications in consideration here.) Contaminants may inhibit (or may assist) direct bonding between film and substrate. Roughness of the interface may serve to increase contact area geometrically; it may also serve to distribute interface stress over a
Interface Structure and Thin Film Adhesion
281
larger volume of material, and to pin cracks started at the interface, thereby increasing the fracture toughness of the interface.
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/
6El'
Figure 2: Peel test parameters. The curves refer to peel test removal of a copper film from silicon or polyimide, and they relate the peel force P, the interface free energy y, yield stress a y of the film, t the film thickness and E the Young's modulus. The computation was made by K.S. Kim 4 • It serves to illustrate the importance of material properties other than y in determining adhesion performance.
In the Sections to follow, we shall outline some of the ways in which ion beam techniques have been found useful in tailoring such factors for enhanced adhesion performance. 14.3 ION BEAM TECHNIQUES
We shall discuss four completely different ways of employing ion beams for adhesion enhancement. They are illustrated in Fig. 3, and Table 1 lists their features and anticipated effects on the interface adhesion.
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Handbook of Ion Beam Processing Technology
Table 1: Ion beam techniques for enhancing thin film adhesion.
Interface "stitching"
Substrate pre-sputtering
Ions (or electrons) penetrate beyond interface. Typical keY.
energy
> 100
Interface structure disturbed by both electronic and ballistic energy loss of ions in transit.
Substrate sputter etched in vacuum directly before film deposition. Energy of ions chosen to enhance preferential sputtering of compound substrate. Typically 500 eV Ar+.
No "ion beam mixing" beyond interface without bulk chemical reactivity of film and substrate.
Substrate surface bonding structure disordered; dangling bonds; polymer scission/cross- tin king.
Substrate surface bonding configuration disru pted; disorder offers new film/substrate bond configurations.
Possible to alter elemental conlposition of surface before deposition.
Contaminant layers disturbed. Post-an neal im proves adhesion (re-ordering). Rad ia tion dam age substrate likely.
to
Stable binary/ternary bonding to be established when film deposited. Contaminant moved. Substrate roughened, interface.
layers
may giving
re-
be tough
Post-anneal improves adhesion.
Interface implantation
Ion assisted deposition
Add reactive species selecin tively interface region to assist bonding chemistry.
Ion bombard sample surface during deposition of film.
Ilnplanting at elevated temp. may form interface precipitates, fracture toughening interface. (Delivers all of benefits stitching.)
Control of intrinsic stress in deposited film. Interface sputtercleaning.
Interface Structure and Thin Film Adhesion
283
IONS E
~
100 keV
a) Ion beam •stitching'
b) Substrate pre-sputtering In vacuum
~
,..,.deposlled species
c) Ions Implanted at Interface
•
SUBSTRATE;
d) Ion assisted deposition
Figure 3: Ion beam techniques for assisting thin film adhesion: (a) Ion beam "stitching"
(b) Substrate pre-sputtering in situ prior to film deposition (c) Implantation of active ion species in the interface region (d) Ion assisted deposition. 14.4
INTERFACE STITCHING
14.4.1 Adhesion Enhancement
Beginning with the work of Collins et al (5) an impressive record of experimental observation has accumulated, in which interface irradiation with almost any species of ion (or even photons) can apparently improve the adhesion of films which normally have very weak attachment to a substrate. Until very recently, most observations were qualitative, depending on weak threshold estimates of adhesion such as the scotch tape test, and few were supported by further characterization of the altered interface. A detailed list of reported results has been assembled by Baglin (1). Table 2 represents a new selective list which is intended simply to illustrate the diversity of the method. It is interesting to note that some enhanced adhesion has been achieved with electrons or UV light, in addition to that produced by irradiation with MeV ions whose primary mode of energy transfer in the interface region was electronic. However, in some cases the ions have been used at an energy where collisional processes dominate at the inter-
284
Handbook of Ion Beam Processing Technology
Table 2: Examples of ion beam stitching for adhesion enhancement. Film
1\1
Substrate
Beam
soda glass -organic wash -water wash
Ar 120keV
Au
glass
S 12MeV
Cu
Al with native oxide Teflon
a 12MeV
I\u I\u
Si Sia 2 InP ferrite ferrite W CaF 2 Al 20 3 GaAs Teflon
I\g
Si
Pd Cu
Sia 2 InP A1 20 3 AI 20 3
Cu
alumina
Pt
!\u
Collins et aI. s
IEl2 to lEIS lEl4 to lEIS IEl2
Scotch Tape Scratch Scotch Tape Scratch Scotch Tape
Jacobson et al. 6
S 12MeV
lEIS 2El5 SEI4 3El3 2EIS 2El4 2El4 5EIS SEI4 IEl3 IEI4 5EIS lE16 2EIS SE14 2El5 3EIS
Scotch Tape
Werner et at. 7 and Griffith et a1. 8
Peel
Baglin et al. 9• IO
Crwith thin oxide
P IMeV Ne 2S0keV lie 200keV e 7keV
<2EIS < SEIS -2E16 SE17
Scotch Tape Peel
Bottiger et a1. II
alumina glass glass Si Si Si
lIe 2MeV lIe 2MeV e lOkeV He 2MeV e S-30keV lIe 2MeV e S-30keV lIe 2MeV e 5-30keV e 5-30keV lIe 2MeV e S·30keV
lEI4 4EI6 3El8 lEIS -6E16 >2EIS SEl7 >6E16 SEI? 7EI7 lEIS 4El7
Q-tip
Mitchell ct a1. 12,13
GaAs GaAs
lIe Ne lIe Ne lie Ne
Comment
Measured adhesion vs. dose Increase x(50-100) max.
Cu-At adhesion strong as deposited if no AI oxide
to 6E16
glass
Sn
Scratch Pull
-6E13 -lEIS
C120MeV Cl20MeV Ct20MeV CI20MeV F SMeV Cl20MeV CI20MeV CI20MeV CI20MeV He l.SMeV II IMeV CI20MeV F lOMeV CI20MeV CI20MeV C120MeV CI20 MeV
Reference
200keV 280keV 200keV 280keV 200keV 280keV
Si0 2 (suprasil) Teflon Cu
Threshold Dose Adhesion or Test Range of Doses (em-I)
Pcel Pcel
Adhesion rises at -SE1S cm- 2; saturates at ......2E16 cm- 2; Poor adhesion; heating gives x3 Substrate damage by SE I5 leads to detachment Strong bond Poor bond Weak bond
Interface Structure and Thin Film Adhesion
Table 2: Continued. Film
Au
Substrate
Teflon
20A interface zone; XPS shows ternary complex
No extensive interface mixing
lEl6
h" lOeV,21ev h" 2leV
< IEI6 < IEl6
Q-tip
Mitchell et aI. 16
Si
N 6.5MeV N 3.4MeV C 3.3MeV F 3.0MeV P 3.4MeV N 6.5MeV N 3.4MeV C 3.3MeV F 3.0MeV P 3.4MeV
9El5 2El5 < 1.31216 < 5EI5 2EIS > 1.5El6 > lEl7 >4E16 > 2.5E16 2.5E16
Q-tip/ Scotch Tape
Berkowitz et aI. l ?
lie 2MeV e lOkeV lIe 2MeV e lOkeV lie 2MeV e lOkeV He 2 MeV
....,IEI4 9EI6 >2E17 8EI? >4E16 IEl8 ...., IEl4
Q-tip
Ne 280keV
to 2EI6
Peel
Interface contaminant effects;
Ne 280keV
to 2EI6
Peel
Interface contaminant effects
h" 3.5-6eV 2leV h" 3.5-6eV 21eV h" 3.SeV h" 6-21eV
not known
soda-lime glass
II lOOkeV
fixed test dose IEI6
Si0 2 Si0 2 on Si glass
II lOOkeV
;\1
glass
Au
glass glass Al 20 3 AI 20 3 -clean -water wash -ethanol wash A1 20 3 -clean -water wash -ethanol wash Si GaAs glass Si0 2 on Si
AI
Sofield et al. 14
Si Si
Si0 2 (vitreous)
I"c
Pull
Pronko et a1. 1S
Au
;\1
Comment
Scotch Tape
Au I't
Au
5E6 rads
Reference
> IEl5 >4E15 >2E15 >2E15
Mowith native oxide
eu
y O.2-3.0MeV (lMeV mean e 240eV to 3.5keV
Threshold Dose Adhe~<;ion Test or Range of Doses (em- 2)
Ni IMeV
Au Ag Cu AI
I't
Beam
No No No No
Scotch Tape
adhesion adhesion adhesion adhesion
found found found found
Sood et al. IR ,I9 No enhancement found No enhancement found
Baglin 20
Q-tip
Kellock ct aPI Gazecki et a1. 22
Adhesion enhanced Adhesion enhanced Film detached Adhesion enhanced
Scratch
Battaglin et al.23.24.2s
Enhancement x4; diminished Pe-C bonding Enhancement x3 Enhancement x3 Enhancement x5
285
286
Handbook of Ion Beam Processing Technology
Table 2: Continued. Film
CU
Au Au
Substrate
Beam
Si0 2 glass SiO sub 2 glass Sia 2 Ta with native oxide
Si with native oxide
Sia 2
Threshold Dose Adhesion Test or Range of Doses (em- 2)
Reference
IEl6
2.85MeV/aml for C a Si Cl Ni 2.5MeV/amu for Si Cl Ni Ni
Comment
Enhancement x2 Negligible eITect Negligible eITect No enhancement No enhancement Scotch Tape
Stokstad et aP6
find D lh = 1017(dE/dx)-3.ocm -2
5El6 lEl6 1.5El5 3.3E14 6.4E13 D th
= 6xlO I8 (dE/dx)··4.1 cm -2
9.7E15 2.5E15 2.5E14 2.3EI3
eu
Mo
a 2MeV Si 3MeV Ni 3MeV Au 5MeV
2El6 5EI5 IEI5 5El4
AI
Si
hv 4-6eV in air
lE14-1E16
ZnS
Si0 2
Scotch Tape and pin pull
Ingram and Pronk0 27
Q-tip
Kellock et aP8
e 1-3keV
to IEl7 IE13-5E14
Scotch Tape Pull Scotch Tape
Sofield and Trehan 29
C130MeV
Adhesion enhanced Correlated with interface chemistry (X PS)
Au
Si Al 20 3 fused silicon Si0 2 grown
Cl20MeV
2EI3 2EI4 IEI5 5El4
Q-tip Scotch Tape Scotch Tape Scotch Tape
Ingemarsson and Tombrcllo 30
I;c
Teflon
S 16MeV
> IEI2
Q-tip, Scotch Tape Q-tip
Ingemarsson et aPI
Adhesion enhanced. CEMS shows chemical bonding Adhesion decreased with dose. Radiation damage?
Scotch Tape Pull
nardin et al.32.33
Adhesion improved only when initial surface contaminated. Otherwise adhesion good as deposited.
Musket et aJ.34
Adhesion enhanced.
PVC Au
Si0 2 "mall particles
5EI2
°
(;aAs
lOOkeV 180keV
GaAs -water wash -propanol
160/200keV
rused
He 200keV
silica
a
2E13-2E 15
5EI5
Pull
IEl4
Scotch Tape Q-tip
Interface Structure and Thin Film Adhesion
287
face, also with excellent effect. At present, there seems to be no evidence that either mechanism alone is uniquely responsible for adhesion enhancement. In fact, it seems that any process capable of disrupting interface electronic bonding will be helpful to sonle degree. It will be noted that not all systems have produced good adhesion. That fact should be no surprise if we concede that the specific chemistry available for the interface atom species will determine whether or not stable complex phases can form. Much of the work reported in Table 2 was probably done on substrates carrying some contaminants residual from wet cleaning. It nlay be that many of the early successes of ion stitching stemnled from contaminant layer dispersion rather than direct bonding of the simple system stated. 14.4.2 Examples of Stitching
We shall consider in detail two representative systems for which substantial quantitative characterization has been reported, namely Cu-Teflon (9) and Cu-AI2 0 3 (10). Figure 4 shows the adhesion enhancement produced for each of these systems by stitching with 250 keV Ne+ ions at various doses. Typically, in the CuiAl2 0 3 case, adhesion increases with radiation dose, the most significant rise occurring between 1015 and 10 16 ions/cm2 . (This would probably be true for Cu/Teflon too, were it not for the physical radiation damage to the polymer at 1015 ion/cm2 , above which dose the Teflon substrate discolors and becomes weakened.) The resulting adhesion in each case is quite strong, even though in neither case can the deposited copper chemically break up Al2 0 3 in bulk form. The adhesion of the as-deposited Cu filnls was too little to measure; some films detached spontaneously. However, ion irradiation produced a bond of reasonable strength, which was substantially increased after the Al2 0 3 sample had been annealed in a helium furnace for 1 hour at 450°C. It is interesting to note that, initially, there were reasonable expectations that irradiation must create, at best, a transient interface condition which would relax by segregation to the as-deposited state. Improvement with heating suggests that the bonded interface was indeed in a thermally stable condition, a feature common to other stitched systems. In the bonded CuiAl2 0 3 system, conlpositional analysis of the newly peeled surfaces showed no evidence of extended intermixing of film and substrate. On the contrary, no evidence of residual Al2 0 3 on the peeled Cu was found, while less than half a monolayer of Cu remained at the surface of the exposed substrate. This clearly demonstrated that the "stitched" interface layer responsible for adhesion was itself no thicker than the contact planes of atoms. The existence of a chemically bonded joining layer or interface phase is inferred, and the concept was supported in principle by the work of Ogale el al. (35) on ion stitching in the Fe/Al2 0 3 system (whose behavior should be not unlike that of CuiAI2 0 3). These authors used Conversion Electron Mossbauer Spectrometry (CEMS) to study the electronic bonding state of interface Fe deposited on A130 3, before and after irradiation. The change of signal from that of metallic a-Fe to that of Fe 3+, produced by irradiation and heat treatment, must be attributed to chemical (probably ternary) bond formation which would produce enhanced interface adhesion. Similar CEMS studies of Fe on Teflon and on PVC following irradiation by 16 MeV S3+ ions were reported by Ingemarsson et al (31). For Fe on Teflon, bonding of Fe with F and with C was identified, while for Fe on PVC, FeCl2 bonding was inferred.
288
Handbook of Ion Beam Processing Technology 30r----,---.-----,------;r-----,
20 ::r:
(a)
l - __
I(J
Z
substrate damaged
I.LJ
a:::
I-
V>
10
...J I.LJ I.LJ
a.
O=-__ o
~--~--~-----JL....------::! 1
4
5
xl 0 15 )
DOSE
12.-----.------.-----r------, ......, 10
ions
E
Cu on sapphire
Ol '--'
8 (b)
::r:
I(J
6
Z I.LJ
a:::
l-
4
...J I.LJ I.LJ
2
V>
a.
He+(200keV)
--------------------0':-
-:-'-::-
o
--=-'-=--__---::":-
----' 40
Figure 4: Adhesion enhancement produced for (a) Cu on Teflon and (b) Cu on Al 30
3
(sapphire) by ion beam stitching using Ne (250 keY) or He (200 keY). (10). 14.4.3 Stitching Mechanisms
In view of experimental evidence discussed above, there is strong reason to propose that the chemical bonding at the interface which is required to account for the strong adhesion observed, must reside within islands of stable configurations of film and substrate atoms, constituting part or all of the interface layers of atoms. Electronic bonding continuity from film to substrate would then be established. It would be appropriate to describe such a region as a (planar) interface phase, whose composition and structure need not be those of any 3-dimensional conventional solid compound phase.
Interface Structure and Thin Film Adhesion
289
As a reminder for those accustomed to seeing the effects of "ion beam mixing" in the growth of thick layers of compound phase material at a reactive interface, we emphasize that, for stitching, we refer to a different kind of systenl, in which there is no bulk chemical affinity, no negative heat of compound formation, and therefore no thermodynamic driving force for radiation assisted diffusion across the interface (36). Accordingly, the only net intermixing produced should be that of a purely ballistic nature, extending sparsely one or two monolayers deep, depending on the momentum of the mixing ions and their collisional energy loss. Of course, the existence of a compound interface phase whose stability exists only for a 2-dimensional interface configuration, would provide a strong driving force for migration and rearrangement of atoms of both film and substrate at the interface plane, given the transient dissolution of interface electronic bonds by the ion or electron beam. Ballistic mixing would not be a necessity for this process of interface transport, although collisional displacenlents of interface atoms could only serve to help. It is therefore suggested that irradiation assists the formation of a compound interface phase, wherever the systenl chenlistry favors the existence of such a phase. Competition among random atomic configurations and destruction of the ordered phase by subsequent irradiation will, however, usually cause the interface phase formed in this way to be inconlplete and discontinuous, thereby lacking the ability to create optimal adhesion. Alternative processes using substrate pre-sputtering can overcome this limitation (see Section 5). 14.4.4 Contaminant Dispersion
How will the irradiation process affect an interface in which a thin layer of contaminant species has been interposed between film and bulk substrate? Such a layer could be, for exanlple, a substrate surface oxide. In most cases, this layer could prevent the formation of bonds between film and substrate, and thereby spoil adhesion. The irradiation process can be understood in terms of the thermodynamic argument of Section 14.4.3. The contaminant species can be regarded as a new contributor to the range of possible interface bonding configurations. Ion mixing will succeed in producing random interface displacements. During and after the ion cascade, diffusion at the interface will occur in the thermodynamically driven directions such as forming ternary chemical bonding configurations, or developing precipitate clusters. It will be possible for a reactive "contaminant" to participate in new electronically bonded configurations and assist adhesion; more often, clusters or islands of a non-reacting layer nlay form, allowing film-substrate bonds to form independently. The case of Pt deposited on to oxidized Ni surfaces was examined by Sood and Baglin (37) with results shown in Table 3. Although adhesion between these metals is strong in the absence of oxide, a native nickel oxide layer is sufficient to destroy adhesion. Ion beam stitching readily improved the bonding, supposedly by dispersal of the oxide; however, no data were taken to prove the mechanism. In the same Table, similar results are displayed for Cu deposited on oxidized Cr (11). In that case, a thick oxide (80A), grown before film deposition, could not be overcome by the stitching ion beam even at high doses. However, good adhesion resulted from
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Handbook of Ion Beam Processing Technology
stitching through a thin native oxide. It is possible that stitching induces the creation of a chemically bonded ternary Cu-Cr-O interface layer whose stability depends on an interface configuration, whereas for thicker oxides the depth of ion beam mixing would not be sufficient to include both the Cu and the substrate Cr.
Table 3: Adhesion enhancement by dispersion of interface contaminant layers using ion
beanl stitching. Peel test data are shown for Pt deposited on Ni with and without a native oxide. Similar results are listed for Cu on Cr carrying a surface oxide (13).
Film
Substrate
Ion Beam
Pt(700A)
Ni, no oxide
None
Ni + native oxide
None He+(200 keY) Ne+(250 ke V)
Cu(500A)
Cr, native oxide (20A)
Cr, grown oxide (80A)
None He+(200 ke V)
Dose ion/cm 2
Peel Strength gm
> 200.0
2 x 10 16 2 x 10 16
-
Ne+(250 keV)
5 x 10 15 6 x 10 16 5 x 10 15
None Ne+(200 ke V) Ne+(250 keY)
6 x 10 16 6 x 10 16
-
<0.1 30.0 > 200.0 < 0.1 0.4 2.0 > 25.0 <0.1 0.2 0.5
Substrate surfaces can be contaminated not only by native oxides but also by residues adsorbed after exposure to solvents, moisture, etc. The surface of the Al20 3 for example, readily adsorbs OH upon exposure to moisture. Such layers can greatly affect subsequent adhesion, as shown in Table 4, which displays ion stitching results for samples of Cu on substrates of sapphire which had been (a) sputter/heat cleaned or (b) water washed or (c) organic solvent washed. In examples (a) and (b), ion stitching and subsequent heat treatnlent achieved adhesion improvenlent to some degree; however, residue from a single ethanol wash totally inhibited adhesion, defeating attempts by ion stitching to establish a bond. Ogale et al (38) have reported using CEMS to examine film-substrate bonding conditions following the same series of treatments for Fe/Al2 0 3 samples. Their spectra identify the change of state from a-Fe (as-deposited) to Fe 3+ (bonded) after irradiation in all cases.
Interface Structure and Thin Film Adhesion
291
Evidently, ion beam stitching can often overcome the effects of minor contaminant layers. However, good adhesion still depends on the thermodynamic constraints governing ion beam nlixing and interface chenlistry.
Table 4: Effects of ion stitching on Cui Al2 0
3 adhesion when contaminants have been introduced by substrate washing in pure water or ethanol (from ref. 10) Ne+ dose was 2 x 10 16 ions I cm2 at 280 keY.
Substrate
Surface
surface
layer
preparation
Peel strength (gm) 450°C, I hr.
Not heated As deposited
Ne+
As deposited
Ne+
Sputter clean; O 2 exposure
Normal (Oxygen)
3.5
6.4
13.5
13.0
Water wash
Hydrated (OH)
0.9
4.1
5.3
20.0
Ethanol wash
Hydrocarbon (HxC y)
0.0
0.0
2.2
4.2
14.4.5 Applicability of Stitching
The most obvious difficulty with ion beam stitching as a process technique is the requirement for substantial doses of high energy ions to be delivered from an ion implanter or high energy accelerator. This is an inconvenient and costly technology except for highly specialized applications. Possibly some of the same purposes would be served more readily by interface irradiation with UV light or electron beams, which can also enhance adhesion (16,21,39). However, long exposures are often required, and the quantitative effect of these beams has yet to be tested. A further concern is for the radiation damage in both filnl and substrate that can be sustained during stitching at high doses, and the possibility of problems with bubbles or precipitates of the implanted species. 14.5
LOW ENERGY ION SPUTTERING
Although stitching has shown qualitative success in a huge variety of systems, it is a process basically dependent on making improved use of whatever atomic species already exist at the interface, by creating disorder to promote new bonding arrangenlents. It would seem to be preferable, in general, to control the elemental abundances and contaminants at the interface during the film deposition process. This can readily be at-
292
Handbook of Ion Beam Processing Technology
tempted using a low energy ion beam to pre-sputter the substrate within the vacuum immediately prior to the deposition of the film. The process serves to remove contaminants, to permit the controlled introduction of active adsorbates if required, and in the case of conlpound substrates, to give scope for preferential sputtering in order to prepare a substrate surface of chosen composition. 14.5.1 Adhesion Enhancement
The method of pre-sputtering has been tried, with spectacular success, in a few filmsubstrate systems so far. Figure 5(a) shows the dramatic increase in peel strength obtained for Cu on Teflon (40) by sputter etching the Teflon surface briefly in situ with 500 eV Ar ions (50p.A/cm2 ) from a Kaufnlan source, inlnlediately before depositing the Cu film from a thermal source. An exposure of only 30 seconds led to a strong bond with a peel strength of 80 gm, four times greater than that obtained by ion stitching. Similar success was achieved for the Cui Al2 0 3 case, where pre-exposure of sapphire substrates to the low energy argon ion beam had a dramatic effect upon adhesion of promptly deposited copper (41). Figure 5(b) displays the resulting peel strength as a function of exposure time, for as-deposited samples, and for samples annealed in flowing helium for 1 hr at 450°C. The peel strength obtained (120 gm) was six times better than that produced by stitching, constituting a very strong bond that was both reproducible and reliable at temperatures at least up to 650°C. By contrast, the technically very useful system of copper deposited on Si02 or glass has responded poorly to the same treatment, displaying marginally enhanced adhesion peel strength of the order of a few grams at best. (Its performance after ion stitching was also poor.) 14.5.2 Adhesion Mechanism
By using the pre-sputtering technique, the primary objectives sought were the removal of contaminants from the substrate surface and the alteration of the substrate surface composition in order to enhance bonding with the deposited film. Accordingly, evaluations were made of the chemical bonding changes at the interface coinciding with improved adhesion. For Cu-AI2 0 3 , preferential sputtering is expected to enrich the Al2 0 3 surface in Al (42,43). (Initially, the surface termination layer would consist of oxygen atoms.) After the optinlunl sputtering time, corresponding to the peak of the curves in Fig. 5 (b), the surface composition is presumably ideal for the added Cu to enter into stable ternary bonding configurations, or interface phases. In order to investigate the nature of electronic bonding in interfaces in this way, Schrott et al (41) reproduced the preparation process under UHV conditions, depositing only a very thin layer of Cu to enable the use of X-ray Photoelectron Spectroscopy (XPS) with best sensitivity at the interface. Figure 6 shows the Cu(L3M 4 ,sM4 ,s) Auger line profile both in the absence of pre-sputtering (dashed line) and after preparation using optimum exposure (solid line). The latter sample produced a new line, shifted from the metallic Cu line to a position not identifiable with the known binary compounds. It was believed to indicate a ternary chemical bonding environment for the attached Cu at the interface (which need not correspond to the known bulk compound phases of Cu-AI-O).
Interface Structure and Thin Film Adhesion
.r-e~
~
E
Ol
:r: (a)
t-
t:)
z
I
50
W
0:::
•
t-
V)
-J
w w
•
I·· •
75
-....;
25
293
Cu on Teflon
a..
0
1
0
2
3
E 120
•
3 ~
t:)
(b)
I I
80
W
0::: V)
40
.. /
W
/
W
a..
"./
o --- ... o
450·C 1 hr.
,
I
t-
\ as
\
,,-- -"d.:posited
.,,' 5
Figure 5: Adhesion produced for (a) Cu on Teflon and (b) Cu on Al20 3 by means of presputtering the substrate for the time shown, in vacuum, directly before deposition of Cu. Sputtering was done with Ar+ ions (500 eV, 50f.1A/cm2). (From Refs. 40 and 41.)
(min)
Cu on sapphire
\ .\ ,.
I
Z
--J
"
/
5
4
ION BEAM EXPOSURE
\
.', .... ~" ... ......
10
15
ION BEAM EXPOSURE
20
25
(min)
Similar XPS studies made for Cu-Teflon adhesion were reported by Chang et al (40). A new peak in the CU(2P3/2) spectrum indicated the existence of a new bonding environment for Cu when deposited on pre-sputtered Teflon, believed to involve Cu-F bonds.
294
Handbook of Ion Beam Processing Technology
10
CuA'x Cu CUzO
Cu on c-axis SAPPHIRE
11('
~
(I)
8
----- NOT SPUTTERED - - SPUTTERED (500eV Ar+) before Cu deposition;
6
heated 500°C 1 hr. t following Cu deposition
==C :J
.a L-
a
'--"
>t::
4
(/)
z
w rz
2
_ _i--~~------
oL-_~~02....:..L..J-_-'-_-L-----L._-.J.---I
930
920
910
900
KINETIC ENERGY
890
880
870
(eV)
Figure 6: Electron spectra corresponding to Cu (L 3M 4 ,sM4 ,s) Auger lines from Cu de-
posited thinly on Al2 0 3 (c-axis sapphire) substrates as observed. The dashed line was obtained fronl the sample deposited directly on clean Al2 0 3 and heated to 450°C for 1 hour (poor adhesion); the solid line originated from a sample prepared by substrate sputtering for optimum adhesion (500 eV Ar+), followed by the same annealing cycle. The broad peak of the solid line is believed to represent a well defined ternary Cu-Al-O bonding environment (41). It is interesting to note that Cui Al2 0 3 sanlples prepared using pre-sputtering showed no further systematic increase in adhesion to be available by subsequent ion stitching efforts with Ne+ or He+ at 250 keY. This observation implies that presputtering already achieves (with better efficiency) the interface changes which stitching might otherwise provide. This will clearly be true with respect to overcoming interface contaminant layers. It also supports the proposition that interface chemical bonding reconfiguration has a nlajor effect in both approaches. This is handled in a controlled way by pre-sputtering, whereas stitching can at best offer disordering from which some new bonding may result. From the evidence of these experiments, it seems reasonable to attribute increased adhesion primarily to the creation of new interface chemical bonding. However, a further consideration arose from the work on Teflon. Sputtering roughened the substrate surface, as Chang et al observed by SEM, producing irregularities of typical dimension 1000A after 20 seconds exposure, and eventually, after 5 min exposure, tall dendritic features
Interface Structure and Thin Film Adhesion
295
of dimension about l,um. Such a surface must assist adhesion of a deposited film, independently of changing interface chenlistry, for a variety of reasons. It presents an increased contact area; it offers mechanically interlocking structure; and the interface ultimately formed will be toughened against delamination fracture by the ability of such irregularities to pin interface cracks should they develop. How much of the observed adhesion success should be attributed to this toughening of the entire interface region is not known. An apparent inconsistency is presented by the fact that all the increase in adhesion strength shown in Figure 5(a) was achieved in the first 30 seconds of sputtering, whereas the scale of surface roughness continued to expand greatly as sputtering continued for 6 minutes, without any apparent increase in subsequent adhesion. It is possible that the intrinsic physical weakness of the thin dendritic structure would improve the strength of adhesion, while providing a different mechanism of failure - the breaking of the substrate structure itself rather than the atomic-scale interface bonds. In this case, the longer the sputtering time the weaker would residual dendrites become. Competition between this and the interface chemistry enhancement may account for the apparent independence of net adhesion on sputtering time after the initial 30 second time period. Considerations such as these will probably apply in many systems where ion beam treatment will readily roughen surfaces, this being especially likely for polymer substrates. Control of interface toughness in this way can constitute a powerful tool for improving adhesion performance in general. 14.6 IMPLANTATION AND ADSORPTION
Although we have cited examples where improved adhesion can be attributed to reconstructed chemical bonding among interface atoms, it may not be assumed that all systems can generate such interface phases. In view of the lack of success with either stitching or pre-sputtering to make Cu adhere strongly on Si0 2, for example, an alternative approach seems desirable, namely, to add atoms of a reactive species selectively at the interface in order to link the materials of film and substrate. Such additions could be made by adsorption of selected species on a pre-sputtered substrate prior to deposition. Indirectly, this was the effect of experiments by Pepper (44), who introduced adsorbate gases in vacuum prior to measuring the friction coefficient between various metallic surfaces and sapphire. Small exposures to 02' N 2 and C12 dramatically changed the interface friction, and they would presumably also have significant effects on adhesion. The scope for such interface tailoring is considerable, and more experimental study is needed. A different means of introducing reactive species in the interface region is ion implantation. The method offers precise control over the quantity of implanted materials, and no doubt also offers most of the interface atom mobility available from ion stitching, thereby enabling the active species to form new bonding configurations with both film and substrate. Ion range straggling will cause a broad dispersion of the implanted species for some distance on either side of the interface, which may create problems of contamination or damage. However, in some materials, e.g. ceramics, implantation can increase the fracture toughness of the bulk solid near the interface (45) a desirable benefit. Table 5 sumnlarizes the adhesion enhancement achieved by Madakson and Baglin (46) by implanting Ti+ or Cr+ at the interfaces of Cu with sapphire or Si0 2 substrates.
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Handbook of Ion Beam Processing Technology
Elevated temperatures were used as shown, either during or after the implantation. Ti+ was clearly very effective for both the Alz0 3 and SiO z cases. Cr+ was less helpful in Alz0 3 and had little effect in SiO z. The interface regions of Cui Alz0 3 were found by TEM to contain precipitates of size about 100A, whose composition was not verified but which may well have contained compounds of AI, Cu, and Ti (or Cr). Such precipitates would be expected to contribute greatly to interface toughening, while assisting interface chemical bonding linkage. The relative contribution made by toughening to the observed adhesion was not established in this case. Nevertheless, the effect seems certain to be beneficial in general.
°
Table 5: Adhesion (peel strength) enhancement produced for Cu on SiO z and Alz0
3 by means of ion implantation of a reactive metal species (Ti+ or Cr+ ) to lodge at the interface. Annealing took place either during implantation or afterwards, as shown in the right-hand column.
SUbstrate
Implanted species
Peel strength (gm)
Dose (ionsjcm
2 )
-
Fused quartz
none Cr+ Ti+
5 x 10 16 5 x 10 16
Sapphire
none Cr+ Ti+ Ti+
5 x 10 16 5 x 10 15 5 x 10 16
-
Not heated
heated
<0.1 0.2 19.0
0.6 66.0
0.4 1.5 >210.0 >210.0
0.6 88.0 150.0 >210.0
-
Heat cycle
450°C implant 350°C anneal
350°C 450°C 450°C 450°C
anneal implant implant implant
14.7 ION ASSISTED DEPOSITION
Stress energy stored within a deposited fHnl can serve to assist (or initiate) delamination from the substrate, and it represents the equivalent of weakening adhesion performance. Ion beam assisted deposition, in which a metal film is deposited while the growing surface is subject to bombardment with low energy inert ions, has been shown (47,48) to produce reductions in the stress of deposited films. The process may easily be adopted as an extension to exposure of a substrate to pre-sputtering treatnlent described above. There is evidently much to recommend the use of this technique whenever film stress constitutes an adhesion problem. 14.8 SUMMARY
Ion beam processing clearly has much to offer in the enhancement of thin film adhesion, due to a variety of useful effects. We have seen the importance of fracture inhibition by means of interface toughening, substrate roughening and reduction of internal stress. It should also be pointed out that the elasto-plastic properties of film and substrate might sometimes be tailored with benefit for adhesion performance. The toughening of the bulk substrate and film in the neighborhood of the interface must also be important, since superb interface adhesion alone will only invite failure in the adjoining
Interface Structure and Thin Film Adhesion
297
solid when large stresses are applied. A critical case in point is the bonding of metals on ceranlics where thermal cycling will produce large shear stresses due to differential thermal expansion, which can easily produce fracture in the bulk conlponents. The critical electronic bond linkage between materials having no bulk chemical affinity can evidently be established in many situations by ion stitching or by pre-sputtering of compound substrates. However, it can be anticipated that in many materials systems, additional reactive species will need to be added either by implantation or by adsorption on the substrate, in order to create a stable interface phase. It can not be assumed that such interface phases will be described by bulk phase diagrams. Therefore, in order to predict the requirements for complex chemical bonding in the interface plane, detailed modelling of such junction phases is needed. Provided that such guidelines can be developed, the interface tailoring made practicable and convenient by ion beam techniques can be implemented to the best advantage in a variety of critical technologies.
14.9 REFERENCES
1.
J.E.E. Baglin, "Ion Beam Effects on Thin Film Adhesion", Chapter 15 in Ion Beam Modification of Insulators eds. P. Mazzoldi and G.W. Arnold, Elsevier, Amsterdanl ( 1987).
2.
J.E.E. Baglin in Surface and Colloid Science in Computer Technology ed. K.L. Mittal, Plenum Press, New York (1986), p. 211.
3.
J.E. Griffith, Y. Qiu and T.A. Tombrello, Nucl. Instr. and Methods 198: 607 (1982).
4.
K.S. Kim, Elasto-Plastic Analysis of the Peel Test. University of Illinois Report No. UILU-ENG 85-6003, March 1985. Also: J. Eng. Mat. and Technol. 86-WA/EEP-3: 10 (1986).
5.
L.E. Collins, J.G. Perkins and P.T. Stroud, Thin Solid Films 4: 41 (1969).
6.
S. Jacobson, B. Jonsson and B. Sundqvist, Thin Solid Filnls 107: 89 (1983).
7.
B.T. Werner, T. Vreeland, M.H. Mendenhall, Y. Qiu and T.A. Tombrello, Thin Solid Films 104: 163 (1983).
8.
J.E. Griffith, Y. Qiu and T.A. Tombrello, Nucl. Instr. and Meth. 198: 607 (1982).
9.
J.E.E. Baglin, G.J. Clark, J.
B~ttiger,
Mat. Res. Soc. Symp. Proc. 25: 179 (1984).
10. J.E.E. Baglin and G.J. Clark, Nucl. Instr. and Methods B7/8: 881 (1985). 11. J. B~ttiger, J.E.E. Baglin, V. Brusic, G.J. Clark and D. Anfiteatro, Mat. Res. Soc. Symp. Proc. 25: 203 (1984). 12. LV. Mitchell, J.S. Williams, D.K. Sood, K.T. Short, S. Johnson and R.G. Elliman, Mat. Res. Soc. Symp. Proc. 25: 189 (1984). 13. LV. Mitchell, J.S. Williams, P. Smith and R.G. Eillman, Appl. Phys. Lett. 44: 193 (1984). 14. C.J. Sofield, C.J. Woods, C. Wild, J.C. Riviere and L.S. Welch, Mat. Res. Soc. Symp. Proc. 25: 197 (1984). 15. P.P. Pronko, A.W. McCormick, D.C. Ingram, A.K. Rai, J.A. Woolam, B.R. Appleton and D.B. Poker, Mat. Res. Soc. Symp. Proc. 27: 559 (1984).
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Handbook of Ion Beam Processing Technology
16. LV. Mitchell, G. Nyberg and R.G. Elliman, App!. Phys. Lett. 45: 137 (1984). 17. A.E. Berkowitz, R.E. Benenson, R.L. Fleischer, L. Wielunski and W.A. Lanford, Nuc!. Instr. and Meth. B7/8: 877 (1985). 18. D.K. Sood, W.M. Skinner and J.S. Williams, Nuc!. Instr. and Meth. B7/8: 893 ( 1985). 19. D.K. Sood, P.D. Bond and S.P.S. Badwal, Mat. Res. Soc. Symp. Proc. 27: 565 (1984). 20. J.E.E. Baglin, Mat. Res. Soc. Symp. Proc. 47: 3 (1985). 21. A.J. Kellock, G.L. Nyberg and J.S. Williams, Vacuum 35: 625 (1985). 22. J. Gazecki, G.A. Sai-Halasz, R.G. Elliman, A. Kellock, G.L. Nyberg and J.S. Williams, Applic. Surf. Sci. 22/23: 1034 (1985). 23. G. Battaglin, M. Carbucicchio, R. Dal Maschio, F. Marchetti, P. Mazzoldi and A. Valenti, XIV Internat. Congress on Glass, New Delhi, India, March 1986. 24. G. Battaglin, P. Mazzoldi and R. Dal Maschio, in Induced Defects in Insulators. ed. P. Mazzoldi, Les Editions de Physique, Le Vlis Cedex (France) (1984) p. 235. 25. M. Carbucicchio, A. Valenti, G. Battaglin, P. Mazzoldi and R. Dal Maschio, Rad. Eff. 98: 21 (1986). 26. R.G. Stokstad, P.M. Jacobs, 1. Tserruya, L. Sapir and G. Mamane, Nuc!. Instr. and Meth. B16: 465 (1986). 27. D.C. Ingram and P.P. Pronko, Nuc!. InstL and Meth. B13: 462 (1986). 28. A.J. Kellock, J. Liesegang, G.L. Nyberg and J.S. Williams, Mat. Res. Soc. Symp. Proc. 75: 179 (1987). 29. C.J. Sofield and P.N. Trehan, Rad. Eff. 98: 35 (1986). 30. P.A. Ingemarsson and T.A. Tombrello, Mat. Res. Soc. Symp. Proc. 119: (1988) (to be published). 31. P.A. Ingemarsson, T. Ericsson, A. Gustavsson-Seidel, G. Possnert, B.V.R. Sundqvist and R. Wappling, Proc. of the 12th International Conference of Hosei University on "Application of Ion Beams in Material Science". Tokyo, September 1987. 32. T.T. Bardin, J.G. Pronko and D.K. Kinell, Mat. Res. Soc. Symp. Proc. 77: 731 ( 1987). 33. T.T. Bardin, J.G. Pronko, L. Senbetu and D.A. Kozak, Mat. Res. Soc. Symp. Proc. 119: (1988) (to be published). 34. R.G. Musket, LM. Thomas and J.G. Wilder, App!. Phys. Lett. 52: 410 (1988). 35. S.B. Ogale, D.M. Phase, S.M. Chaudhari, S.V. Ghaisas, S.M. Kanetkar, P.P. Patel, V.G. Bhide and S.K. Date, Phys. Rev. B35: 1593 (1987). 36. W.L. Johnson, Y.T. Cheng, M. Van Rossum and M-A. Nicolet, Nuc!. Instr. and Meth. B7/8 657 (1985). 37. D.K. Sood and J.E.E. Baglin, Unpublished work (1984). 38. S.B. Ogale, Physics Department, University of Poona, Pune 411007, India, 1987 (Unpublished results).
Interface Structure and Thin Film Adhesion
299
39. A.J. Kellock, J.S. Williams, G.L. Nyberg and J. Liesegang, Mat. Res. Soc. Symp. Proc. 119: (1988) (to be published). 40. Chin-An Chang, J.E.E. Baglin, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51: 103 (1987). 41. J.E.E. Baglin, A.G. Schrott, R.D. Thompson, K.N. Tu and A. Segmiiller, Nucl. Instr. and Meth. B19/20: 782 (1987). 42. R. Kelly, Chapter 7 in Physics and Chemistry of Solid Surfaces. Vol 5. eds. R. Vanselow and R. Howe, Springer-Verlag, Heidelberg (1984) p. 159. 43. P. Varga and E. Taglauer, J. Nucl. Mat. 111/112: 726 (1982). 44. S.V. Pepper, J. Apol. Phys. 47: 2579 (1976). 45. C.J. McHargue, Chapter 6 in Ion Beam Modification of Insulators. eds. P. Mazzoldi and G.W. Arnold, Elsevier, Amsterdam (1987). 46. P.B. Madakson and J.E.E. Baglin, Mat. Res. Soc. Syn1o. Proc. 93: 41 (1987). 47. J.M.E. Harper, J.J Cuomo and H.R. Kaufman, J. Vac. Sci. Technol. 21: 737 (1982). 48. J.M.E. Harper, J.J. Cuomo, R.J. Gambino and H.R. Kaufman, Chapter 4 in Ion Bombardment Modification of Surfaces: Fundamentals and Applications. eds. O. Auciello and R. Kelly, Elsevier, Amsterdam (1984).
15 Modification of Thin Fil1115 by Off-Norl11al Incidence Ion BornbardlTlent
R. Mark Bradley
15.1 INTRODUCTION
Ion bombardment can significantly alter many properties of thin films. The effects of ion bombardment include gas incorporation, compound formation, reduction of intrinsic stress, increased density, changes in preferred orientation, and modification of film topography (1). Ion bombardment is present in many thin film deposition environments, so it is important to understand its effects. In addition, there is currently a movement toward systematically employing ion bombardment in materials nlodification, particularly when low-temperature processing is needed (2). The vast majority of work carried out so far has been concerned with the effects of normally-incident ion beams. The role played by the energy, flux and species of the incident ion beam has been studied extensively. The effect of varying the angle of incidence of the ion beanl has received much less attention, however. Of course, most phenomena which occur for normal incidence bombardment also occur when the angle of attack is changed. In this review we will concentrate on the effects of off-nornlal incidence beams which are not found when a nornlally incident beam is employed. As we shall see, there are several interesting orientation effects which are peculiar to off-normal incidence bombardment, and these may prove useful in applications. Both theoretical and experimental work will be reviewed when available. 15.2 MODIFICATION OF CRYSTAL STRUCTURE BY OFF-NORMAL INCIDENCE ION BOMBARDMENT 15.2.1 Effect of Bombardment After Deposition
Normal-incidence ion bonlbardment of a thin film can lead to substantial changes in crystal structure. Typically, the random slowing-down of the incident ions damages the
300
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
301
lattice, leading to a decrease in the amount of crystal order. This occurs in a wide variety of crystalline metals, semiconductors and insulators. On the other hand, in certain polycrystalline metals ion bombardment at keV energies leads to a preferred orientation for one of the crystal axes of the crystallites, thereby increasing the degree of crystalline order. Bombardment of evaporated copper specimens by 40 keY eu ions (3) has been found to produce a (110) fiber texture in which each crystallite tends to have a (110) axis normal to the surface, for example. Van Wyk (4) has shown that if a copper film is bombarded by an off-normal incidence beam of 40 keV Cu ions, a preferred orientation still results. When normal incidence bombardment is employed, a (110) fiber texture is produced. After off-normal incidence bombardment, the crystallites tend to have a (110) axis aligned with the beam rather than the normal to the film surface. Whether normal incidence or off-normal incidence bombardment is employed, the crystallites tend to be aligned so that they channel the ion beam. This can be understood as follows (3)-(5). In crystallites which do not channel the beam, thermal spikes (6) are generated by the stopping of the incident ions, resulting in the formation of hot, highly damaged regions. Conversely, in crystallites which do channel the beam, electronic stopping is predominant and thermal spikes are rare. These crystallites suffer much less damage and act as centers for the recrystallization of the highly damaged regions, leading to a preponderance of crystallites which channel the beam. In fcc materials, channeling is most nlarked along the (110) direction, followed by the (100) and (111) directions. The tendency for (110) planes to be aligned with the beam in ion-irradiated copper is therefore to be expected. 15.2.2 Effect of Bombardment During Deposition
The effect of normal-incidence ion bombardment during deposition has been studied by Dobrev (7), who condensed silver on an amorphous carbon substrate during bombardment with 10 keY Ar ions. When the ion flux was low enough that deposition prevailed over sputtering, islands of silver formed which tended to have a (110) axis normal to the substrate. In contrast, the islands were randomly oriented in the absence of ion bombardment. Thicker films deposited with concurrent ion bombardment had a distinct (110) fiber texture, while unbolnbarded films displayed a slight (111) texture. These results can be understood in much the same way as the results obtained for postdeposition normal- incidence ion bombardment. The effect of off-normal incidence bombardment applied during deposition has also received some attention. Hoffnlan (8) has observed anisotropic stresses in films deposited in a magnetron sputtering system, and attributed this to the off-normally incident particle flux reaching the film surface. The compressive film stress was found to be smaller in the direction parallel to the cathode axis for all deposition rates studied. A similar effect has been observed in sputtered AIN films which were bombarded during growth with a 200 eV non-normal nitrogen beam (9). In the absence of ion bombardment, the AIN films had a high intrinsic compressive stress, and the buckling patterns fornled showed no evidence of anisotropic stresses. However, when films were deposited with concurrent glancing angle ion bombardment, the direction of the corrugations was parallel to the inplane conlponent of the ion beanl direction. This shows that the compressive stress was smaller in the direction parallel to the beam, in agreement with the observations of
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Handbook of Ion Beam Processing Technology
Hoffman. So far no explanation of these results has been advanced, but it is clear that off-normal incidence ion bonlbardnlent nlust lead to the formation of an anisotropic crystal structure. The effect that low energy off-normal incidence ion bombardment has on crystal structure when applied during film deposition has been studied in detail by Yu et al. (10)-(12). Niobiunl was sputter-deposited onto a fused silica substrate, and a 200 eV Ar ion beam at 20 0 from glancing angle was simultaneously directed upon the growing film. In the absence of ion bombardment, a pronounced (110) fiber texture was formed, i.e., most of the film grains shared a (110) crystallographic axis oriented perpendicular to the plane of the substrate. A Schulz X-ray pole figure of one such film (Fig. 1) shows a central spot corresponding to diffraction from (110) planes parallel to the substrate surface. The circle around the central spot represents diffraction from (110) planes at 60 0 from parallel to the substrate. The uniform intensity of the circle indicates that on an amorphous silica substrate, the crystallites had random azimuthal orientation. Such a (110) fiber texture is commonly found in evaporated or sputtered thin films of bcc metals, while fcc metals usually have a (111) texture.
,
/
\.,
'.
,i \
•.•..
...•.....•/ ."
.... ,
Figure 1: Pole figure of Nb film deposited without ion bombardment, showing (110) fiber texture with no azimuthal ordering (from Ref. 10, with permission).
A typical pole figure of a filnl prepared with off-normal incidence bombardment during growth is shown in Fig. 2. There is a strong central peak, indicating that the fiber texture is still present. The intensity of the circle is no longer uniform, however, so the azinluthal distribution of the film grains has been modified. In fact, the grains tend to be oriented so that the incident ions see a planar channeling direction between (110) planes. TEM diffraction patterns obtained from thinner films also display nonuniform rings, and thus confirm that there is a nonuniform azimuthal distribution of crystallite orientations (12).
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
I
303
..
Figure 2: Pole figure of Nb film deposited with concurrent 200 eV Ar+ ion borrlbardment at 20° from glancing angle and with an ion/atom arrival rate ratio of 1.0. The pole figure shows a (110) fiber texture with a restricted set of azimuthal orientations. The direction of ion bonlbardment was from the top to the bottom of the figure (fronl Ref. 10, with permission) .
The fraction of nlaterial which channels the ion beanl can be conlputed by taking the ratio of the intensity of the X-ray diffraction maxima on the 60° circle to the intensity of the central spot. The results (Fig. 3) show that the degree of orientation increases with the incident ion flux. About half of the grains are aligned to within 5 ° of a channeling orientation when the ion/atom arrival rate ratio is 1.3. At this rate of ion bombardment, about three-quarters of the arriving niobium atoms are resputtered. For the low ion energies employed in these experiments, the thermal spike picture (6) is invalid because the ion penetration depth is only several angstroms. A different approach to explaining these results is therefore needed. Bradley et al (13) have developed a theory of thin film orientation by low energy off-normal incidence ion borrlbardnlent applied during deposition which does not use the" thermal spike" concept. In this theory, the selection mechanism for grain orientation is taken to be the difference in sputtering yields between grains which are oriented so that they channel the ion beam and those which are not. This difference, which is as high as a factor of 5 in some materials (14), leads to a larger growth rate for aligned grains than for misaligned grains, and hence to overall orientational order. As the film grows, it can be described by the fraction of material at the surface that channels the ion beanl, x(t). Bradley et al found that the tinle development of the orientational order at the surface is governed by the differential equation
dx dt
Y [_yx 2 de
+ (y -
l)x
+d
]
(1)
where y = (de / db)r. Here y is the rate that the film thickness grows in the absence of ion bombardment, r is the ion/atom arrival rate ratio, and d is the fraction of azinluthal
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Handbook of Ion Beam Processing Technology
orientations which channel the ion beam. The parameters de and db are materialdependent and must be taken from experiment. Roughly speaking, de is the thickness of the fHnl which nlust be deposited for the steady state to be approached in the absence of ion bombardment, and characterizes the extent to which newly-deposited crystallites are aligned with the grains beneath them. The ratio d b/ y is the time needed for the steady state to be approached in the linlit of perfect epitaxy (de = 00) for r = 1. A small value of db results from a substantial difference between the sputtering yields from crystallites which channel the ion beam and those which do not.
1.0
z o ~
0.8
.Z
lLJ
a:: o lLJ lLJ lLJ
o
.•:.• ..•• I
IJ..
o 0.4
a:: (.!)
...• •
0.6
0.210.0 0.0
• 0.1
0.2
0.3
0.4
0.5
CURRENT DENSITY AT SUBSTRATE (mA/cm 2 )
0.0
0.5 1.0 1.5 ION/ATOM ARRIVAL RATE RATIO
Figure 3: Degree of orientation of Nb films vs. the arrival rate ratio of ions/atoms during deposition (from Ref. 10, with permission).
On physical grounds, one would expect the degree of orientational order far from the substrate in a thick film to be spatially unvarying and independent of the initial degree of order in the film. This is indeed the case: Eq. (1) shows that x(t) converges to the limit [y-1
+V(Y-1)2+4~y 2y
(2)
for all x(O) in the interval [0, 1]. We expect ~ to be small in most systems. For ~ >> 1, Fig. 4 shows that as the ion/atom flux ratio r is increased from zero, the ion beam is unable to induce appreciable
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
305
orientational order until r approaches the critical value db/de. As r is increased further, the asymptotic order X oo increases rapidly until it saturates at 1 for r > > db/de. Physically, this means that the ordering influence of the ion beam has little effect until it becomes strong enough to prevail over the disorder due to imperfect epitaxy. It is important to note, however, that for r greater than some critical value r*, sputtering will remove more material than is being deposited. The fraction of material which channels the ion beam in a thick film therefore cannot be made arbitrarily close to 1 simply by increasing r. The observations of Yu et al (shown in Fig. 3) are in qualitative agreement with these predictions. In particular, their data for X oo rise slowly as r is increased from zero until, as a critical value of r is approached, X oo begins to grow more rapidly.
1.0
~=I.O
0.8
te
0.6
)(
0.4
0.2
o
3
2
Figure 4: Plot of the asymptotic degree of orientational order several values of ~ (from Ref. 13).
X oo
vs. y
= (dd/de)
r for
The theory also gives the time dependence of the degree of orientational order. For large enough times t, the degree of order at the film surface x(t) converges exponentially towards X oo , Le., x(t) ~
X
oo
+ Ae -tiT
(3)
The relaxation time
(4)
is a nleasure of how long the film must be grown before the asynlptotic ordering is approached. Like xoo ' 'T is independent of x(O). The coefficient A does depend on x(O), however.
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Handbook of Ion Beam Processing Technology
Perhaps the most surprising aspect of the expression for the relaxation time is that for Ll < 1/2 there is a peak in T as a function of r (Fig. 5). This peak occurs at y = 1 - 2Ll where T achieves the value
In real systems Ll should be rather small, so the peak will be quite pronounced. This peak is the result of the presence of two competing effects: As r is increased from zero, the asymptotic degree of order X oo increases, and so does the time needed to reach it. As r is increased, however, the ion beam becomes more effective at modifying the film's structure. When the ordering influence of the ion beam becomes stronger than the disorder caused by imperfect epitaxy, the latter effect prevails and T decreases as r is increased. Of course, the peak will only be found in systems in which r* > (1 - 2Ll) db/de. It would be very interesting to observe this peak experimentally.
4
:3
2
o Figure 5: Plot of the relaxation tinle
2
T
vs. y
=
:3
(de/db) r for several values of
~
(from
Ref. 13). The theory has a nunlber of implications for the efficacy of thin film orientation by ion bombardment. If de/db is greater than r* and Ll is small, ion bombardment cannot induce appreciable orientational order. To ensure that db/de is much smaller than r*, one should look for circumstances in which the sputtering yields from channeling and nonchanneling orientations differ markedly, and in which epitaxy is good. If db/de < < r, the increase in X oo obtained by an increase in r becomes smaller and smaller once r exceeds db/de . Moreover, this increase in order is gained only at the expense of slower and slower growth of the film. A value of r several times larger than db/de will yield a well-ordered film and still give a reasonable deposition rate. Finally, values of r in the immediate vicinity of d b/ de should be avoided since the convergence time T may be excessively long.
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
307
Several variables can be adjusted in an experiment to optimize orientational order. The length de can be modified by changing the substrate temperature. In addition, the parameters D. , db and r* can be changed by varying the angle B between the beanl and the surface normal, the ion species, and the ion energy. Consider the effect of changing the ion species, for example. An ion which is too large will not channel well, so D. will be small. There will also be little difference between the sputtering yields from aligned and misaligned material, so db will be large. To maximize xoo ' therefore, the smallest ions which do not react chemically with the substrate should be employed. Finally, it is clear that the value B = 70° chosen by Yu et al. is not optimal for deposition of bcc metals. These authors suggest that a higher degree of orientational order would be obtained using a beanl at B = 45° (10)(11). Axial channeling along a (100) direction would then occur, and as a result, each crystallite would tend to have a (100) axis aligned with the beam, while a (110) axis would tend to lie normal to the substrate surface. 15.3. TOPOGRAPHY CHANGES INDUCED BY OFF-NORMAL INCIDENCE ION BOMBARDMENT 15.3.1
Overview
Normal incidence ion bombardment can lead to the formation of sputter cones or etch pits on solid surfaces (15). Sputter cones project above the ambient level of the solid surface, and in crystalline materials they are actually pyramids which reflect the crystallographic symmetry of the underlying material. The presence of low sputter-yield impurities can lead to the formation of sputter cones since these contaminants temporarily serve to protect the material below from erosion. Whether or not impurities are the only source of sputter cones is the subject of an ongoing debate. Etch pits, on the other hand, are formed in regions where the atomic binding energies are reduced by the presence of dislocations or other defects. They also possess crystal symmetries in most crystalline materials. Silicon is an exception, however, apparently because ion bombardment anlorphizes the surface layer in which the etch pits form. Sputter cones and etch pits are also produced during off-normal incidence ion bombardment. The sputter cones have their axes aligned with the direction of the incident ion beam, regardless of the material being sputtered. This is just what one would expect if contaminants which resist erosion lead to cone formation. On the other hand, the etch pits formed during off-normal incidence ion bombardment begin to overlap as erosion continues, and ultimately a periodic ripple structure results. These ripple topographies are considered in detail in the next section. 15.3.2 Ripple Topography Induced by Off-Normal Incidence Ion Bombardment
Off-normal incidence ion bombardment at keV energies often produces periodic height modulations on solid surfaces (16)-(23). Ripple topographies have been observed on amorphous solids such as glass (16)(17), Araldite (18)(19), fused silica (20) and vitreous carbon (19), and on crystalline solids such as copper (21), iron (22) and sapphire (23). The wavelength of these ripples is typically on the order of 0.1-1 ,um although recently wavelengths as short as 250 A have been observed (23). The ripple orientation displays a simple dependence on the angle of incidence of the ion beam for amorphous materials. For angles of incidence Bless than a critical angle Be from the normal, the wave vector of the modulations is parallel to the component of the ion beam in the surface
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Handbook of Ion Beam Processing Technology
plane. The wave vector is perpendicular to this component when the ion beam direction is close to grazing incidence (Fig. 6). Finally, at normal incidence one sometimes finds an interlocking grid of hillocks and depressions in which several ripple orientations are present (16)(17).
(a)
(b)
Figure 6: Dependence of the ripple orientation on the angle of incidence 8 (a) Orientation for small 8. (b) Orientation for 8 close to '17/2 (31).
The situation for crystalline materials is much more complicated and is presently rather poorly characterized. Elich et al (21) bonlbarded single crystal surfaces of (100) copper and rotated their specimens about an in-plane (100) direction. They observed waves transverse to the ion beam for angles 8 less than the maximum in the sputter yield at 8max . Close to 8max the ripples developed flat (110) facets, while no ripples were found
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
309
for 8 > 8max . Lewis et al (24) bombarded the (11,3,1) surface of single-crystal Cu and found height modulations which were neither parallel nor perpendicular to the ion beam direction. Similar findings were nlade by Vasiliu et al (22) in their studies of ionbombarded polycrystalline iron. Finally, Mazey et al (25) applied normal incidence ion bombardment to polycrystalline copper and studied the resultant ripple patterns. They found that the wave orientation varied from crystallite to crystallite, and that the waves tended to be oriented along lattice directions with low Miller indices. Waves were not formed at all on (100) surfaces. Formation of ripple topographies could be problematic in a variety of applications, e.g., in ion polishing or milling. It is also possible that ripples are produced when offnormal incidence ion bornbardment is applied during deposition; the topography of films made in this way has not yet been studied. On the other hand, off-normal incidence ion bombardment may prove to be an inexpensive and simple way of making diffraction gratings, since the ripples can have wavelengths comparable to visible light. It is therefore of considerable practical interest to understand and control ripple formation. Early discussions of the phenomenon suggested an analogy with the ripple structures formed when air or water flows over a sand bed (16)(17). Although the gas pressures were rather large in the original work of Navez et al (16)(17), subsequent work has shown that the waves persist at pressures so low that any hydrodynamical flow effects can safely be neglected (22)(26). A much better analogy is found in the sandblasting of solids, as pointed out by Carter et al (26). When a solid surface is eroded by a stream of abrasive particles at off-normal incidence, a regular ripple pattern is created with wave vector parallel to the surface component of the incident stream (27). Moreover, the variation of the erosion rate with the angle of incidence has a similar form in sandblasting and ion beam sputtering of solids. This is where the analogy ends, however. The wavelength of the ripples formed by sandblasting is comparable to the distance over which a single particle is in contact with the solid surface. In contrast, the wavelength of the ripples formed by ion sputtering can be two orders of nlagnitude larger than the surface component of the ion range (26)(28). Two other explanations of ripple topographies have been proposed. Carter et al (26) and Hajdu et al (29)(30) have suggested that the height modulations may be due to surface buckling caused by incorporation of the bombarding noble gas ions into the target. This is certainly a plausible explanation for the waves formed by normal incidence ion bombardment. However, this theory fails to account for the observed relationship between the ion beam direction and the wave orientation when off-normal incidence ion bombardment is applied to amorphous nlaterials. In addition, waves were found in the experiments of Vasiliu et al (22), even though the noble gas content of the target was presumably quite small at the high temperatures they studied. Another explanation has been proposed by Mazey et al (25). These workers found that ion bombardment can produce dislocation arrays, and suggested these may lead to the creation of ripple structures. However, they do not explain in detail how regular arrays of dislocations form, nor do they show a definite correspondence between the ripples and the dislocation structures. Recently Bradley and Harper (31) advanced a quantitative theory of the ripple topography induced by ion bombardment of amorphous solids. Their theory is based on Sigmund's approach to sputtering (32), in which the rate that material is sputtered from
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Handbook of Ion Beam Processing Technology
a point on the surface of a solid is proportional to the power deposited there by the random slowing-down of ions. The average energy deposited within the solid by an incident ion is taken to have a Gaussian distribution about the point of maximum energy deposition. In general, the widths of the distribution parallel and perpendicular to the beam direction differ. This Gaussian approximation has been shown to be reasonably accurate in many applications (33). Theoretical studies of surface erosion by ion bombardment almost universally assume that the sputtering yield is independent of the curvature of the surface. This assumption has proven to be quite useful in studying the time evolution of the ion-bombarded surfaces and is a reasonable approximation when the radius of curvature at an arbitrary point on the surface is much larger than the ion range. As shown by Bradley and Harper (31), however, the sputtering yield does depend on the curvature of the surface in Sigmund's theory of sputtering, and this dependence is found explicitly. It is this dependence which leads to the growth of waves as the surface is eroded. Bradley and Harper find that when the ion beam is normally incident on a periodic surface disturbance, the troughs are eroded more rapidly than the crests. Thus, sputtering increases the amplitude of the perturbation and so leads to an instability. To see heuristically why this is so, consider the effect of a beam norn1ally incident on a trough (Fig. 7a) and a crest (Fig. 7b). The energy deposited at the point 0 by ions striking the surface at 0 is the same as that deposited at 0' by ions striking the surface there. However, the average energy deposited at 0 by an ion which hits the surface at A is greater than that deposited at 0' by an ion incident at A', and similarly for Band B'. Thus the rate of erosion at 0 is greater than that at 0', and hence the amplitude of a wave is increased by ion bOlnbardment. Typically, before ion bombardment has begun, a very broad range of wavelengths are present in a Fourier decomposition of the surface height. Wavelengths from atomic dimensions to the size of the sample are represented, although the amplitude of each component is small if the surface is initially quite flat. Moreover, when the Fourier amplitudes are sn1all, to a good approximation they evolve independently of each other. If no processes tending to counter the instability due to sputtering are taken into account, all sinusoidal perturbations of the surface are unstable, and those with the shortest wavelength grow fastest. The wavelength A. of the ripple structure would then be comparable to the microscopic cutoff length for the theory, the mean energy deposition range a. However, experiments show that A. can be two orders of magnitude larger than a (26)(28). To bring the theory into agreement with experiment, the effect of surface self-diffusion is incorporated into the theory. Surface self-diffusion slows the growth of short wavelength disturbances more than it retards the growth of long wavelength perturbations. The observed wavelength is the one which grows fastest, and this represents a compromise between the instability induced by sputtering, which is most effective at short wavelengths, and the retarding effect of surface diffusion, which favors the growth of long wavelength disturbances.
311
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
1 J
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o
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/
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Figure 7: A normally-incident ion beam striking a trough (a) and a crest (b). The arrows indicate the beam direction. Contours of equal energy deposition (dotted lines) are shown for ions striking the surface at 0,0', A,A', and B,B'. For clarity, the radius of curvature has been taken to be quite small (31).
At sufficiently high temperatures T and low fluxes f, thermally-activated surface selfdiffusion dominates ion bombardment induced diffusion. The theory predicts that in this regime the selected wavelength A varies as A ~ (fT)-1/2 exp( - VE/2kB T)
(5)
where VE is the activation energy for surface diffusion and k B is the Boltzmann constant. The magnitude of the wavelength given by the theory is in reasonable accord with the high-temperature experiments of Vasiliu et al. (22). In the opposite limit of low temperature and high ion fluxes, surface self-diffusion induced by ion bombardment is predominant. More careful measurements of the high-flux diffusion constant are needed before the theory can be tested in this regime. The theory also predicts the dependence of the ripple orientation on the angle of incidence O. For small angles 0, the wave vector of the ripples is parallel to the surface component of the beam direction. On the other hand, for angles close to grazing incidence, the wave vector of the ripples is perpendicular to the beam direction. Finally, for normal incidence bombardment, waves with several different orientations may be present.
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Handbook of Ion Beam Processing Technology
These orientations are selected by the influence of surface imperfections, impurities and the sample boundary. These predictions are in excellent agreement with experiment. Although the theory of Bradley and Harper seems to work well for amorphous solids, it will have to be nlodified for crystals. In particular, ion channeling occurs for certain incidence angles, and this must be incorporated into the theory. Moreover, facets appear only when the wave amplitude is comparable to the wavelength, a regime in which the linear stability analysis employed by Bradley and Harper fails. Nonlinear interactions of the Fourier components of the surface height must be taken into account before the theory will give facetting. Finally, a fully developed theory of ripple topography on crystal surfaces must predict the rather complex dependence of the wave orientation on the angle of the ion beam incidence. 1 5.4 SUMMARY
Off-normal incidence ion bombardment can have an orienting effect on both the bulk crystal structure and the topography of solids. When an off-normal incidence keV beam is directed upon a polycrystalline filnl, a preferred orientation develops in which the crystallites tend to have a particular crystal axis aligned with the ion beam. Similarly, when a low energy beam is applied at off-normal incidence during deposition, crystalline ordering is increased beyond what would occur without ion bonlbardment. In the absence of ion bombardment, thin-film deposition processes often produce polycrystalline films with pronounced fiber textures in which most grains have a particular crystal axis perpendicular to the substrate surface. The grains typically have a random distribution of azimuthal orientations on amorphous substrates. Experiments by Yu et al (10)-(12) on niobium films have demonstrated that significant azimuthal order can be induced by offnormal incidence ion bombardment applied during growth. A detailed kinetic theory of this process has been proposed (13) which is in accord with the experiments performed to date. More detailed studies are needed to fully test the theory, however. Ripple topographies have been widely observed on both crystalline and amorphous solids which have been bombarded with an off-normal incidence ion beam at keV energies. On amorphous solids, the ripple orientation is fixed by the direction of the ion beam; the ripples are perpendicular to the direction of a near-normally incident beam, while they are parallel to the beam when the angle of incidence is close to glancing angle. Bradley and Harper (31) have advanced a theory which explains the origin of the oriented ripples fornled on amorphous solids. The theory predicts both the wavelength and orientation of the height modulations as a function of the angle of beam incidence, and is in reasonable agreement with experiment. Further theoretical and experimental work is needed to clarify the role played by crystal structure in the formation of wave-like topographies on crystalline solids. ACKNOWLEDGEMENTS I would like to thank Jim Harper and David Smith for their collaboration on much of the work described here, and for allowing their experimental data to be reprinted. I am also grateful to Phil Strenski for many helpful discussions.
Modification of Thin Films by Off-Normal Incidence Ion Bombardment
313
15.5 REFERENCES
1.
Harper, J. M. E., Cuomo, J. J., Gambino, R. J., and Kaufman, H. R., in: Ion Bombardment Modification of Surfaces: Fundamentals and Applications (0. Auciello and R. Kelly, eds.), pp. 127-162, Elsevier, Amsterdam (1984).
2.
Harper, J. M. E., Ion beam techniques in thin film deposition. Sol. St. Technol. 30: 129-134 (1987).
3.
Van Wyk, G. N., and Smith, H. J., Crystalline reorientation due to ion bombardment. Nucl. Instrum. Meth. 170: 433-9 (1980).
4.
Van Wyk, G. N., The dependence of ion bombardment induced preferential orientation on the direction of the ion beam. Rad. Eff. Lett. 57: 45-50 (1980).
5.
Marinov, M., and Dobrev, D., The change in the structure of vacuunl- condensed hexagonal close-packed metal films on ion bombardment. Thin Solid Films 42: 265-8 (1977).
6.
Brinkman, J. A., On the nature of radiation damage in metals. J. Appl. Phys. 25: 961-970 (1954).
7.
Dobrev, D., Ion-beanl-induced texture formation in vacuum-condensed thin metal films. Thin Solid Films 92: 41-53 (1982).
8.
Hoffman, D. W., Stress and property control in sputtered metal films without substrate bias. Thin Solid Films 107: 353-8 (1983).
9.
Yu, L. S., unpublished.
10. Yu, L. S., Harper, J. M. E., Cuomo, J. J., and Smith, D. A., Alignment of thin filnls by glancing angle ion bombardment during deposition. Appl. Phys. Lett. 47: 932-3 (1985). 11. Yu, L. S., Harper, J. M. E., Cuomo, J. J., and Smith, D. A., Control of thin film orientation by glancing angle ion bombardment during growth. J. Vac. Sci. Technol. A4: 443-7 (1986). 12. Harper, J. M. E., Smith, D. A., Yu, L. S., and Cuomo, J. J., Microstructure of niobium films oriented by non-normal incidence ion bombardment during growth. Mat. Res. Soc. Symp. Proc. 51: 343-8 (1985). 13. Bradley, R. M., Harper, J. M. E., and Smith, D. A., Theory of thin- film orientation by ion bombardment during deposition. J. Appl. Phys. 60: 4160-4 (1986). 14. Roosendaal, H. E., in: Sputtering by Particle Bombardment I (R. Behrisch, ed.), Vol. 47 of Topics in Applied Physics, Chap. 5, Springer, Berlin (1981). 15. Carter, G., Navinsek, B., and Whitton, J. L., in: Sputtering by Particle Bombardment II (R. Behrisch, ed.), Vol. 52 of Topics in Applied Physics, Chap. 6, Springer, Berlin (1983). 16. Navez, M., Sella, C., and Chaperot, D., Etude de l'attaque du verre par bombardment ionique. C. R. Acad. Sci. 254: 240-2 (1962). 17. Navez, M., Sella, C., and Chaperot, D., in: Ionic Bombardment, Theory and Applications (J. J. Trillat, ed.), pp. 339-55, Gordon and Breach, New York (1964). 18. Dhariwal, R. S., and Fitch, R. K., In situ ion etching in a scanning electron microscope. J. Mat. Sci. 12: 1225-32 (1977).
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19. Lewis, G. W., Nobes, M. J., Carter, G., and Whitton, J. L., The mechanisms of etch pit and ripple structure formation on ion bombarded Si and other amorphous solids. Nucl. Instrum. Meth. 170: 363-9 (1980). 20. Motohiro, T., and Taga, Y., Characteristic erosion of silica by oblique argon ion beam bombardment. Thin Solid Films 147: 153-165 (1987). 21. Elich, J. J. P., Roosendaal, H. E., Kersten, H. H., Onderdelinden, D., Kistemaker, J., and Elen, J. D., Relation between surface structures and sputtering ratios of copper single crystals. Rad. Eff. 8: 1-11 (1971). 22. Vasiliu, F., Teodorescu, I. A., and Glodeanu, F., SEM investigations of iron surface ion erosion as a function of specimen temperature and incidence angle. J. Mat. Sci. 10: 399-405 (1975). 23. Park, S. I., Marshall, A., Hammond, R. H., Geballe, T. H., and Talvacchio, J., The role of ion-beam cleaning in the growth of strained layer epitaxial thin transition nletal filnls. J. Mat. Res. 2: 446-455 (1987). 24. Lewis, G. W., Carter, G., Nobes, M. J., Cruz, S. A., The development of tailed-cones on non-normal incidence ion bombarded solids. Rad. Eff. Lett. 58: 119-124 (1981). 25. Mazey, D. J., Nelson, R. S., Thackery, P. A., Electron microscope examination of surface topography of ion-bombarded copper. J. Mat. Sci. 3: 26-32 (1968). 26. Carter, G., Nobes, M. J., Paton, F., Williams, J. S., and Whitton, J. L., Ion bombardment induced ripple topography on amorphous solids.. Rad. Eff. 33: 65-73 (1977). 27. Finnie, I., and Kabil, Y. H., On the formation of surface ripples during erosion. Wear 8: 60-69 (1965). 28. Nelson, R. S., and Mazey, D. J., in: Ion Surface Interactions. Sputtering and Related Phenomena (R. Behrisch, W. Heiland, W. Poschenrieder, P. Staib, and H. Verbeek, eds.), pp. 199-206, Gordon and Breach, London (1973). 29. Hajdu, C., Paszti, F., Fried, M., and Lovas, I., Periodic surface deformations caused by high dose ion bombardment induced lateral stresses. Nucl. Instrum. Meth. B 19/20: 607-610 (1987). 30. Hajdu, C., Paszti, F., Mezey, G., and Lovas, I., Stress model for the formation of wave-like structures on high-dose ion implanted materials. Phys. Stat. Sol. A 94: 351-2 (1986). 31. Bradley, R. M., and Harper, J. M. E., Theory of ripple topography induced by ion bombardment. J. Vac. Sci. Technol. A6: 2390 (1988). 32. Sigmund, P., A nlechanisnl of surface micro-roughening by ion bombardment. L. Mat. Sci. 8: 1545-53 (1973). 33. Sigmund, P., Theory of sputtering. I. Sputtering yield of amorphous and polycrystalline targets. Phys. Rev. 184: 383-416 (1969).
16 Ion Beal11 Interactions v#ith PolYl11er Surfaces
Robert C. White and Paul
s. Ho
16.1 INTRODUCTION
Recently there has been increasing interest in studying ion beam interactions with polymeric and insulating materials. The impetus arises from a wide range of applications of ion beam technology for synthesis and processing of such materials. This can be achieved to a high degree of control and precision, as exemplified by ion beam etching (1,2) and lithography (3,4) of polymer films in the processing of semiconductor devices. When an energetic ion strikes a solid target, it loses energy by two main interaction nlechanisnls. One is through the electrons and the other is through the nuclei of the solid target. These energy loss interactions occur until the ion comes to rest, generally in a neutral state by that time. The neutralization of the ion is a further electronic process which occurs, in addition to the energy loss process. The total energy loss rate can be expressed as the sum of the two independent loss rates as: (dEl dr)T
= (dEl dr)elec + (dEl dr)nuc1
(1)
The energy loss processes induce a large number of atomic displacements (the "nuclear" portion of Eq. (1)) and bond breaking (the "electronic" portion of Eq. (1)) in the solid. The study of the nature of such radiation damage, although long-standing for crystalline solids, has been rather limited for polymers. Compared to crystalline solids, polynlers as a class of materials, have distinct and interesting classes of radiation damage. This arises from the molecular structure and chemistry of the polymers. Upon bombardment by energetic ions, the polymer within the depth of penetration can undergo chain scission or crosslinking to yield different molecular structures on the surface. In addition, the polymer contains several chemical components, (e.g. C,N,0 and H), each of which can interact with the incident particle and become ionized or excited. Activated species will then thernlalize, and recoITlbine or leave
315
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Handbook of Ion Beam Processing Technology
the surface, resulting in further modification of compounds near the polymer surface. The extent to which these radiation effects alter the polymer properties depends on the chemistry and structure of the polymer as well as on the nature of the ion beam, particularly the ion type, energy and dose. The ion species as well as the polymer substrate will determine, to a large extent, the surface compounds and the liberated species. An increase of ion energy up to a few keV generally enhances the radiation effects without changing significantly the final products. As will be discussed, it appears that the higher end of this regime, from ion energies of 500 eV to a few keY, may be similar to the electronic regime above 100 keY ion energies, as far as final near-surface chemical products are concerned. One key aspect of ion beam interaction with polymers, therefore, concerns the chemical and physical modification near the polymer surface. So far, the majority of the ion beam studies on polymers have focused on the technological applications of ion beams, where a high ion energy, from several hundred keV to MeV, is required. In this energy range, there is ample energy transferred from the colliding ions to the substrate to induce various types of electronic excitations in the polymer molecules. This topic has been reviewed recently by Brown (5) and Venkatesan (6). These authors concluded that while these studies are useful for technological applications and the results reveal the interesting nature of the excitations induced by ion bombardment, the energy loss mechanism is complex and little has been understood about the specific chemistry induced by ion bOITlbardment. Recently, systematic studies have been carried out, particularly by Briggs and Hearn (7,8), to establish secondary ion mass spectroscopy (SIMS) as a technique to analyze the changes in the composition and chemistry induced by ion bOITlbardnlent of the polymer surface. These studies usually enlployed inert gas ions or neutrals, e.g. Ar+ and Xe+ ions, with several keY energy. The mass and flux of the ions sputtered off from the surface was then measured to deduce the chemical changes. It was emphasized that a static SIMS condition must be used where mass spectra are acquired with a total ion dose as low as possible to avoid damage caused by the measuring beam. The development of this technique shifts the emphasis of the study to low energy ion beams with low dose rates as well as to the initial stage of damage formation. Photoemission spectroscopies, particularly XPS (in which the photoelectrons are ejected by X-ray bombardment), have also been applied to investigate the chemical nlodification of polymer surfaces by ion bombardment. These techniques have high sensitivities for identifying the composition in the near surface region. With proper analysis of spectral features, the change in the chemistry can also be monitored, although this is not straightforward due to the large number of products that can be induced by ion bombardment. Because the detection distance is confined within the electron escape distance «50A), these techniques are better suited for studying interactions with relatively low energy ion beanls. In this regard, the technique is complimentary to SIMS although the combined use of these two techniques has not been fully explored. It is the goal in this chapter to exanline efforts aimed at understanding the chenlical effects of ion bombardment of polymer surfaces, as well as changes in the chemical reactivity of those surfaces when metals are subsequently deposited on them. This chapter will first review the studies using ion beams of high and nlediunl energies, then follow by a discussion on SIMS studies in the low energy range. Finally, a discussion of XPS studies using low energy ions is included, describing some recent results on polyimides obtained in the IBM laboratory using both inert and reactive ion beams.
Ion Beam Interactions with Polymer Surfaces
317
16.2 HIGH AND MEDIUM ENERGY IONS
To date, the most study of the effects of ion bombardment of polymers has been in the high ion energy ranges. The most recent review articles on the subject by Brown (5) and by Venkatesan (6) focus on the nlajority of work done with ions of implant energies from several hundred keV to MeV. In these energy ranges, the energy transferred from the colliding ion to the substrate is sufficient to induce all possible electronic excitations. Brown (5) reviewed the ion bombardment effects in polymers by concentrating on the comparison with the laser ablation of polymers. He emphasized that the specific chemistry induced by bombardment with energetic ions is much less understood than that of nonionizing photostimulated ablation of organic systems. This is due in large part to the nonspecific nature of the excitation induced by ion bombardment. The excitation can involve many different states including ionized as well as neutral species. As an example, the radiation of polyglycidylmethacrylate (PGMA) with UV light causes chain scission since the radiation is non ionizing, but x-rays or electrons cause the dominant reaction to be cross linking (9). It seems that the ionization produced by x-rays or energetic electrons is responsible for promotion of crosslinking reactions. As far as ion bombardment is concerned, any ion energy above the ionization threshold is capable of causing some ionization in the bonlbarded substrate. However, this effect is more pronounced at higher ion energies (2 MeV Ar+ ions will ionize 10-20 atoms per layer) than at lower energies (100 keV H ions typically ionize only one atom every second or third atomic layer). High energy ions (> 10 keY/amu) deposit a large amount of energy in ionizing the target atoms. This results in significant destruction of bonds in the films and causes the polymer to undergo rapid dissociation. Work performed by Geis et al (10) on nitrocellulose indicates that the degree of crosslinking induced by bOITlbardment is reduced for substrates with volatile products if high mass, low energy ions are used. This is probably the case for the Ar+ bombardment of polyimide at energies between 500 eV and 2keV. It was found that above a 50 eV threshold, when heavy ions are used, the decomposition products are all volatile and the etch rate is proportional to the incident ion energy (11), as shown in Fig. 1. Venkatesan (6) has reviewed the effects of high energy ion beam irradiation in polynler films by focusing on the preferential sputtering of multiconlponent polymers as a method for new material synthesis. The studies reported in this review suggest exciting research in the field of ion-polymer interaction with impact on electronic transport in disordered systems, dynamical radiation chemistry and novel materials synthesis. Using a quadrupole mass spectrometer, the study of transient emission of molecular species produced by an ion pulse was shown to yield information about the diffusion and reaction kinetics of various molecules in the polymers. The fact that polymers undergo dissociation and those atoms which form volatile species are selectively depleted from the film can be utilized to produce useful inorganic composites by ion bombardment of polymers. For exanlple, hard SiC composite filnls have been produced by ion beam irradiation of organo-silicon polymers. After a sufficient ion dose, polymer dissociation leads to a predominantly carbon containing film with increased electrical conductivity. Experiments on ion irradiated pure carbon films indicate that a graphitic fornl of carbon was produced from the polymer films at high irradiation doses. While experiments on disordered conductors have modified highly conducting materials to form metals with poor electrical conductivity (with resistivity approximately 10- 3 -10- 4 Qcnl ), nletals with comparable conductivity can be formed starting with insulating materials.
318
Handbook of Ion Beam Processing Technology
I mA/Cm 2
Ar+
Vi'
"E
~ w
~
0:
:I:
~
w
0.1
Eth & 50 eV
0.01
Etching rate for nitrocellulose films at an Ar+ ion current density of ImA/cm2 at different ion energies. The rate is linear with energy above a 50 eV threshold (after Geis et al (10».
Figure 1:
Among the chemical and physical modifications induced by ion bombardment of polymers, changes in solubility have attracted considerable attention owing to the interest in microlithography. The solubility changes are believed to be caused by nlodification of the molecular weight distribution due to bond breaking and reforming. The molecular weight distribution of implanted polystyrene (PS) has shown considerable change upon bOlnbardnlent (3). These changes were observed following bombardment of nearlymonodispersed PS samples. The use of samples with known molecular weight distribution allowed the application of the gel theory for determining the chemical yields. This method is a direct, relatively simple tool to evaluate the chemical modifications in bombarded polymers by deternlining the ratio of crosslinking to scission reactions but gives little direct information regarding specific chemical changes. The development of ion lithography and ion-implantation technology in microelectronics has brought out the need for studies of ion bombardment of polymer resist films. In nlaking microcircuits with submicron elements, resist masks of high-nlolecular-weight compounds (electron and x-ray resists) can be processed by electron, x-ray, vacuumultraviolet and ion lithography. The changes in the properties of high-molecular-weight organic photoresists subjected to ions with doses up to 1016 ions/cm2 and the possible use of such materials as photoresist masks have been investigated by Valiev et al (4). The aim of their work was to explain the effect of the action of medium-energy ions on positive electron and x-ray resists and the topological characteristics of nlasks of these resists. In the experiments, films of polymethylmethacrylate (PMMA) and and polyhexenesulfone (PHS) were deposited on the surface of either silicon or thermally oxidized silicon. These filnls were bonlbarded by N+ ions of 25 to 200 keY. They found that after sputtering, the
Ion Beam Interactions with Polymer Surfaces
319
film thickness was reduced and the surface was left with a graphite-like coating as judged by mechanical properties and solubility. Emmoth et al (12) have used substrates of Be, Si, Cr, and Mo covered by a 400-nm thin film of the electron lithography resist poly(methylmethacrylate) (PMMA), and irradiated by Ar+ ions. The photon emission from deexciting sputtered particles ejected during the ion bombardment was detected. The spectral scans of observed photon radiation were different for PMMA on different substrates. The average sputter yields for Ar+ bonlbardnlent of PMMA at ion energies 30 and 60 keY were found to be 320 and 375 atoms/ion, respectively. The authors concluded that excitation and ejection processes are related to the collision cascades and possibly also to collective electronic excitations induced by high energy ion bOITlbardnlent. Thin films of photoresist material (PMMA and AZ 1450J) have been irradiated with H + and He+ ions in the low MeV energy region (13). The composition and thickness of the irradiated layers were determined by RBS techniques. Results are shown in Fig. 2. The sputter yields of the polymer materials were also measured and were found to vary between 100-20,000 atoms/inconling ion. This could not be explained by conventional sputtering theories. It was assumed that these high erosion rates and compositional changes were connected with the electronic losses of the bombarding ions, giving rise to bond breaking of the resist molecules, as with the lower energy work of Emmoth et al (12).
0,4
Z 0
0,3
Q .
0,2
---
z
a::
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~
0
<
0,1
0
0
100
filled = proton bombardment
200 300 ION DOSE (jJ(/cm 2 ) open
= He
400
particle bombardment
Figure 2: The compositional change of oxygen and carbon of the PMMA resist as a function of the ion exposure dose (after Braun et al (13)). (Filled symbols = proton bombardment, open symbols = He ion bombardment).
Watanabe and Ohnishi (2) have studied the relationship between etching characteristics and silicon content for organosilicon polynlers under oxygen reactive ion etching conditions (02-RIE). It was confirmed from XPS data that a protective layer, which has high resistance against 02-RIE, is formed on the polymer surface and that most silicon
320
Handbook of Ion Beam Processing Technology
atoms exist in the form of Si02 at the protective layer surface. The bulk etching rate for the polymers in this process is inversely proportional to the silicon content. These results suggest that the rate determining step in the etching process is the sputtering of Si0 2 formed by the polymer oxidation. Furthermore, for a polymer with a lower silicon content than some threshold value, the protective layer is porous, and the underlying polymer is attacked by radical species during 02-RIE. 16.3 SIMS STUDIES OF POLYMERS
The secondary ion mass spectrometry (SIMS) of polymer surfaces is used to investigate the chenlical state of the surface by measuring the sputter products fronl the surface upon bombardment by a low dose, low energy ion beam. In order to elucidate the surface structure by this method it is important to evaluate the effect of the ion beam probe itself. In general, SIMS is the mass spectrometric analysis of elemental and molecular fragments which leave a surface under bOlubardment by energetic (0.5 to 15 keY) particles. These particles are either single atoms or molecules, and may be ionized. Of relevance to this technique is the relationship between the substrate damage and the ejected particles detected. The detected secondary ions are generated as the result of the intersection of the collision cascade within the material and the surface. It is this intersection which results in the sputtering event. So the SIMS experiment is based on only the changes induced right at the surface, and not total damage to underlying layers. We will concentrate here on the effects of ion beam interaction with a sample, instead of the mechanism for emitting the secondary ions, which is normally the important parameter for SIMS. The principal benefit of SIMS is the ability to obtain semiquantitative elemental and in some cases chemical analysis of a surface with high sensitivity. SIMS has been used for analysis of many solid materials, however non-conducting materials are a problem due to sample charging under the influence of an ion beam. The only studies published to date which attempt to take advantage of the low dosage regime are ternled "static" SIMS measurements. Static SIMS generally implies a primary beam with intensity below 10 nA, and energy as low as 2 keV, which corresponds to a low dosage regime of approximately 6xl0 10 ions / cn12 per second. In this nlode, surface integrity is assumed to be disrupted only over a period of time much longer than typical analysis times. Analysis times ranging from 3 minutes to 30 minutes would thus yield total doses of 1 x 10 13 to 1 x 1014 ions / cm2 at 10 nA. It has been found that doses in this range actually do cause damage which has been observed by XPS in the PMDA-ODA polyimide when the ion is Ar+ at approximately 500 eV. These results will be discussed in detail below. Van der Berg (14) has written a recent review of the neutral and ion beam SIMS of non-conducting materials. His focus is on the technique itself, particularly regarding the control of surface potential or charging, and the development of neutral beam sources, since their use greatly alleviates the problems caused by sample charging. His comparison of neutral beam results to conventional ion beam work is of relevance to the understanding of ion beam modification of polymer surfaces. The non-conducting sample tends to charge primarily due to the impact of the ion beam, making a local sample potential which is ill defined and difficult to control. This affects particularly ion beam interactions with a non-conducting polymer. Such local potentials will vary greatly for different polymer samples which have inherently varied conduction mechanisms, both on a surface and through a volume. Chemistry occurring under these conditions is ill defined and difficult to interpret. It was his finding that SIMS of insulating targets under ion, electron or atom
Ion Beam Interactions with Polymer Surfaces
321
bombardment is qualitatively similar in that the same characteristic molecular fragment ions occur. Differences in peak height distribution are at least partially explainable in terms of a less precise control of the surface potential in the case of ion and electron bombardment, and not necessarily a yield problem. On the other hand, essential differences between the two types of bombardment are observed in the damage rate in some polymer materials and the sputter rate in a range of low conductivity materials. Specifically, the result for 2keV Ar atom bombardment of I-mm thick polyethylene terephthalate (PET) at a flux density of 3xl09 atoms/cm2 by Brown et al (15). was essentially the same as that for Ar+ carried out under similar conditions with electron beam charge compensation (16). For the most part, the secondary ion spectrum consisted of simple fragmentation of stable products such as OH, CO, CO2 and COOH along with high molecular weight fragments corresponding to cleavage of ethylene liberating arene fragments. The only real difference in the two was a lower intensity for the higher mass fragments in the ion beanl case, and an overall intensity of about a factor of two higher for the atom beam case as compared to the ion beam case. This is likely to be due only to a variation in surface potential for the ion beam case, since it was found that intensities there can be controlled by biasing the substrate appropriately. On the other hand, XPS studies on damage induced by keY-range Ar+ bombardment of polystyrene (PS) at a dose of 1013 to 1014 ions/cm2 indicated destruction of the aromatic ring which was manifested as a loss of the shake-up structure in the CIs spectrum at this dosage, shown in Fig. 3, and the loss of intensity over time of the mass 91 peak (C 7H 7 +) in the SIMS spectra which is characteristic of the aromatic ring structure (17). Comparison of atom to ion bombardment (18) indicated that loss of this mass 91 peak under ion bombardment occurred at approximately four times the rate observed for atom bombardment. Apparently, in addition to nuclear damage caused by the atom or ion beam, the ion beam caused significant charge induced danlage. This would indicate an enhancenlent of the electronic part of Eq. 1 for 2 keY ions compared to atoms. Van der Berg has postulated ion neutralization at the surface resulting in bond destabilization as the electronic interaction. These experiments, carried out after careful calibration procedures, provide evidence for the existence of a charge state induced damage mechanism and a substantial electronic sputtering contribution in these materials under ion bombardment. This is a surprising result when one considers the relatively low dose and low energy employed, where atomic displacements should be prevalent. There has been a considerable theoretical effort in understanding the ion impact and sputtering process occurring during SIMS. If a metal surface is altered by the presence of electronegative elements like 0 or electropositive elements like Cs, one can obtain as much as three orders of magnitude increase in secondary ion yields. Therefore secondary ion yields are dependant on the surface chemical state, as well as the presence of background contamination. These phenomena have also been been employed to enhance yields and hence increase the sensitivity for SIMS. Much of past work has focussed on the separation of the ionization process from the microscopic details of the ion impact event.
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Handbook of Ion Beam Processing Technology
(a)
(b) ,10
.10
I
SE,eV
291
Figure 3: Damage to PS surface during ion bombardment. Spectrum (a) is the C Is peak and associated shake-up satellite from fresh PS. Spectrum (b) is from the same surface after 4 keY Ar+ bombardnlent to a dose of 1.6xl014ions/cm2 (after Briggs et al (17)).
Yu and Lang (19) have proposed an electronic tunneling model for secondary ion yields from metals, semiconductors and oxides. This model works for some systems and is found to be dependent on the "global electronic state" of the surface. It particularly applies when delocalized metal bonds are involved. Yu and Mann (20) proposed a local chemical bonding model for secondary ion yields which reflects changes in chemical environment. This model applies where a local chemical bond is broken when the sputtered atom leaves the surface. It works well for ionic solids with localized bonding states. Both models of ionization provide a sinlple theoretical framework for understanding ionization of sputtered atoms when valence electronic states are involved. An example of an intermediate case is that of covalently bonded semiconductors such as Si and Ge. Polymer surfaces and metal-on-polymer interfaces provide an interesting internlediate case between the limit of one electron dependence (bond-breaking model) and a continuum of substrate electrons involved (tunneling model). The polymer surface can be described as a substrate with "islands" of delocalized charge centers (Le. aromatic systems) which are essentially localized from one another (the reason for poor conductivity). This can be easily visualized by considering one chain of a polymer such as the PMDA-ODA polyimide, shown in Fig. 4(a). The PMDA part represents a planar aromatic, delocalized electronic system covering approximately 50 A2 This electronic system is isolated from the pi electrons of ODA, which again are delocalized over the ODA part of the molecule. The anlount of electronic isolation is dependent on the actual orientation of the two aromatic structures in the repeat unit, and is the subject of some controversy. Observations by Ishida et al (21) indicate that there is some overlap between the electronic states of the two parts resulting in observable signals in the UV-visable absorption spectrum. The chain is then made up of these repeating delocalized but isolated electron density is-
Ion Beam Interactions with Polymer Surfaces
323
lands. In the solid, there is also a great deal of isolation between chains as well with only weak intermolecular interactions as indicated by UV-visable absorption data (21).
(A)
EXPERIMENTAL C 1s EMISSION
(8) CARBONYL
-290
-288
-286
-284
-282
-280
BINDING ENERGY (eV)
Figure 4: (A) Repeat unit of the polyimide PMDA-ODA with constituent atoms indicated by size. Largest are oxygen, then carbon, nitrogen, with hydrogen the smallest. (B) Experimental C Is spectrum from cured PMDA-ODA with contributions of chemically different carbons within the repeat unit indicated by the approximate binding energy.
A fundamental issue not addressed in the theoretical models is the effects of electronic and structural modifications caused by the sputtering event. There is little known about the properties of cation vacancies created by sputtering. In the bond-breaking model, a cation vacancy (negatively charged center created by the sputtering of a positive ion off the surface) is important. Ground or excited states are possible for this surface state. The tunneling model indicates this to be less inlportant since screening of this state by conduction electrons in a metal surface is rapid/ For a polymer, however, this raises the issue of possible screening mechanisms in the insulator made up of delocalized electronic "is-
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Handbook of Ion Beam Processing Technology
lands" as well as possibly much longer lifetime excited states due to the relative stability of a screened center by confined but delocalized electrons. The cation vacancy question should of course not be confused with the charging problem mentioned above. The charging of the insulating substrate under ion bombardnlent will be on the average positive in sign, due to secondary electron emission as well as the accumulation of positive charge from the ions. Cation vacancies on the other hand will be localized and contribute to local field variations. Briggs and Hearn (7) have recently reviewed their results of static SIMS measurements on polymethyl methacrylate (PMMA). This is a standard photoresist polymer but not an aromatic. Their study covered doses up to 1.2 X 1014 ions/cm2 . Of interest in this review is the understanding of the mechanism by which large molecular fragments are sputtered from the surface of polymers. In general, small fragnlents make up the highest intensity of secondary ions in a positive or negative ion spectrum, with larger fragments of greater than 100 amu appearing far less frequently. Various fragments of the polymer were found of different chemical or structural origins within the polymer were found to vary in absolute intensity as a function of ion dose when a PMMA sample was bombarded by 4 keV Xe+ ions. The observable fragments appear to fall into one of four classes as shown in Fig. 5. Major fragments of the whole polymer (-185 in Fig. 5) appear only at very low doses and are not detected at doses higher than approximately 5 x 10 13 ions/cm2 . Smaller repeat unit fragments (+41 in Fig. 5) make up the majority of secondaries, along with aromatic secondary products. The small fragments steadily decrease in intensity as a function of dose, as the aromatic secondaries (+ 91 in Fig. 5) increase up to approx. 6 x 1013 ions/cm2 and then decrease slowly as the dose is increased. The fourth type are backbone fragments which are weakly observable up to the maximum doses (+ 133 in Fig. 5). Briggs and Hearn have interpreted their results as follows. At low doses the primary impact sites are well separated and ion yields are more or less constant, increasing slightly if emission of an ion is aided by chain scission events. As the nunlber of impact sites grows with increasing dosage, sputtering of major fragments declines and ions derived from damage increase in intensity. Secondary groups are preferentially lost, crosslinking occurs through carbon radicals, and ultimately aromatic structures are formed, resulting in secondary aromatic structures not a part of the original polymer structure. As a function of ion mass and energy, the result was virtually the same except that increasing energy or mass increased the process rate as a function of dose (8). Comparison with other polymers such as polyvinyl chloride (PVC) and (PS) showed a similar effect, with PVC being nl0re sensitive to ion dose and PS appearing far less sensitive. The effect of 1014 ions/cm2 on PS was much less obvious. This seems reasonable as PS has no electronegative elements and is highly aromatic. Generally aromatic components in polymers increase radiation stability. The overall conclusion is that main chain scission is induced by side chain elimination (Le. loss of fragment groups), similar to other forms of radiation damage. The discussion (7) focussed on the production of high mass fragments, leaving out interpretation of the production of atomic and low molecular weight fragments, which are in general at least 3x the intensity of the higher molecular weight secondaries.
Ion Beam Interactions with Polymer Surfaces
325
+91
+ .. 1
+133
o
Dose /10
14
• ions cm- 2
1·2
Figure 5: Variation in absolute intensity of prominent peaks in the positive and negative SIMS of PMMA as a function of 4 keV Xe+ ion dose (after (7)).
Magee (22) attempted to compare the sputtering results of polymers to those of metals, considering momentum transfer processes in the sputtering of organic materials. He split momentum transfer processes into two distinct categories. The first termed direct knock-on sputtering, involves ejection from the surface of a target atom which undergoes only a snlall number of collisions after impact of the prinlary ion. The second, linear cascade regime, involves target atoms which are energetic enough to generate secondary and higher order recoil of other sample atoms such that some of the higher order recoils result in the ejection of fragments which were not a part of the original inlpact event. His point is that the two types of ejection mechanisms will vary greatly in the number of constituent atoms, energy, etc. of the ejected secondary particles for organic samples. It was concluded that large intact organic molecules are ejected primarily from the second mechanism, which results in fragnlents of low kinetic energy ejected at considerable distances from the original point of impact (30-40 A is determined for metals by Monte Carlo methods). Because of the low energy of the multiple collision linear cascade process, he assumes that minimal radiation damage occurs for particles ejected by this process, and therefore larger ejected particles are possible. The direct knock on sputtering produces, on the other hand, small high energy clusters or atomic species. The balance between these two processes is influenced by bombarding particle energy and mass. Low mass particles used for sputtering will favor direct knock-on processes due to the higher efficiency of energy transfer between atoms of similar mass. Low energy would also favor direct knock-on processes due to the energy requirement for sustaining extended collision cascades. Therefore low energy, low mass sputtering of organics should result in a large fraction of low mass fragments, with increasing radiation damage occurring as a function of dose. If one subjects a polymer to a low dose, this corresponds to a low density of impacting particles per unit area, and radiation damage
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Handbook of Ion Beam Processing Technology
would occur only very close to the point of impact. For high energy or high mass ions however, the damage to the polymer substrate is not necessarily the same as that to the fragments. Any process able to sustain a linear cascade, where many energy loss processes are occurring, must by definition be causing considerable amounts of radiation damage to the substrate, even though large intact secondary ions are being emitted from the surface. As a summary, the contribution to understanding the interaction of ion beams with polymer surfaces made by SIMS studies is significant. Unfortunately, the theoretical considerations are too complicated to correctly understand the effects in covalently bonded semiconductor (23) as yet, so a conlplete understanding of polynlers is currently not possible. Experimental observations, however, are quite sophisticated at the present time, and will permit a great deal of understanding to the problem in the future. Static SIMS measurements and theoretical analyses of low energy, low dose ion beam interactions therefore hold great promise for investigations of ion beam/polymer interactions. 16.4 XPS STUDIES
We will focus attention in this section on XPS studies of sputtering of polynler surfaces by a directed ion beam in the <5 keY energy regime. In this energy range for non-polymer targets, one expects predominantly atomic displacements, where the primary particle transfers momentum to the substrate atoms via screened coulombic interactions between the nuclei of primary and impacted particles (Le., one expects emphasis on the nuclear part of Eq. ( 1)). Consistent with this idea is the expectation that chemical bonding of the sample atonlS would be unimportant to energy loss processes of this type (22). The collision cascade within the solid would therefore proceed regardless of the local chemistry, or electronic interactions. As we have discussed in the last section, however, this is likely not the case for polymers, where even at low energies some contribution of the electronic part in Eq. 1 is possible. In addition, the presence of electronegative groups in the polymer may also be important as mentioned above, in the amount of electronic interaction one may expect at lower ion energies. This effect will be further examined by comparing the changes of surface chemistry induced by sputtering with inert and reactive ions. Surface modification of polymers has been used to change surface chemistry and as a result, improve adhesion of metals films (24). This is an important area of study with polyimide because of applications in microelectronics. The mechanism responsible for the increased adhesion after ion bombardment of polyimide films is not well established. It is possible that crosslinking induced by sputtering and increased surface area due to roughened surface topology increase the mechanical strength of the interface. Removal of adsorbed species or contaminants by light dose sputtering of the surface may aid in chemical bonding as well. The underlying problem in such studies is the lack of reliable correlation between adhesion and interface studies. Most of the XPS studies have been carried out on polyimides to determine chemical changes in the polymer after ion bombardment. XPS yields core level intensities as a function of their binding energy within the solid. The electron emission spectrum reflects the different chemical states of the atoms emitting the electrons. An example of this was shown in Fig. 4 for the C Is emission from the polyinlide PMDA-ODA before any ion beam treatment. Origins of major features are discernible and labeled accordingly. As
Ion Beam Interactions with Polymer Surfaces
327
seen in Fig. 4, for the case of this polyimide, carbons from the PMDA part emit electrons which are separated from those of the ODA part by approximately 1 eVe Carbons attached to oxygens through carbonyl bonds on the PMDA are shifted by 4 eV from the main peak. This is a result of the chemical structure of the cured polyimide, and is highly reproducible. The components in the CIs spectrum reflect the different chemical environments of the carbon atom in PMDA-ODA. Molecular orbital calculations for the repeat unit shown in Fig. 4(a) have been used to understand details of the chemical shifts occurring for CIs and for 0 1s spectra, as well as for the interaction of this polymer with deposited Cr and Cu (25). These details are inlportant for analyzing XPS spectra fronl the polymer alone, and this serves to exemplify the complicated structure of the spectra as a result of many different chemical interactions within the polymer. Because of the inherent linewidths observed in these spectra (instrumental resolution for the spectrunl shown in Fig. 4 is approximately 0.8 eV) each chemically different carbon is not discernible. Therefore, detailed analysis based on curve fitting of the XPS data is not clear cut and could lead to false interpretations. By interpreting the data on the basis of molecular orbital calculations, one could understand changes in chemistry and how they would manifest in the XPS spectra. The problem with this approach is to have a reasonable idea of the structure as input for the calculation. This is difficult with the present state of understanding of ion beam interactions with the polymer. Contarini, et al (26) have used angular resolved XPS and time-of-flight direct recoil (TOF-DR) spectrometry to characterize the surface changes in the polyimide isoindroquinazolinedione (PIQ) induced by 4 keV Ar+ bombardment. This study was performed with only one relatively high ion dose of 5xl0 16 ions/cm2 This represents a rather large dose of energetic ions, particularly at what is approaching a high energy range with such heavy ions. A stoichiometry of CH2 .2 as determined by DR intensities indicates the presence of an uppernlost hydrocarbon layer on the initial surface. This was in agreenlent with deconvolution of the XPS data. Upon Ar+ bombardment the H, N, and 0 concentrations decreased and the surface layer carbon concentration increased to > 94 at. 0/0, leaving a H/C ratio of 0.7. This carbonaceous layer exhibited C Is chemical shifts and line shapes that were similar to those of an ion bombarded graphite surface, along with an enhanced electrical conductivity. This result is common for most organic polymers at high ion doses and/or ion energies as previously discussed. The problem with determining any further information regarding the chemical state of surface constituents is that the observable XPS binding energy shifts are not large enough to cleanly resolve different chemical environments. Contarini et al suggested that N 1s peaks indicate the presence of nitroso, imide and cyano groups, as shown in Fig. 6. These identifications were made based on handbook values for XPS chemical shifts. The lowest binding energy peak representing more than half the N 1s intensity corresponds to the cyano groups. The 0 1s presented only a broad peak with no discernible features, as was the case with C Is. The CIs however did exhibit a high binding energy tail, but with only 6 at. °ib Nand 0 left on the surface, this tail had a greater intensity than would be expected if it was only a result of Nand 0 bonded to C. BOITlbardnlent of graphite at the sanle dose left a CIs spectrum virtually identical to that of the bombarded polyimide. We note here that we have found similar results for high dosage on PMDA-ODA in data presented below. They have concluded that the carbonaceous overlayer remaining after bombardment can not be unambiguously identified as graphitic even though it is significantly conductive.
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Handbook of Ion Beam Processing Technology
N Is Ar+ bombarded
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BINDING ENERGY, eV More recently, Bachman and Vasile (27) have presented C Is XPS spectra taken from Ar+ bombarded PMDA-ODA in the dosage range from 2xl0 14_1xl0 15 ions/cm2 • Films of 5 micron thickness were exposed to bombardment by 1, 2, and 3.5 keY ions. They investigated ultrahigh purity graphite and high density polyethylene (HDPET) for comparison of CIs position. They concluded that ion bombardment diminishes carbonyl groups at the surface, and proposed that a light ion dose (2xl0 14 ions/cm2 ) at lkeV probably removes adsorbed gases and therefore may be expected to improve in-situ deposited metal bonding. Unfortunately, there was no parallel measurement of adhesion to metals deposited on these surfaces. Work at IBM initially focused on the higher dosage regime from about 5xl0 14 to lxl0 17 of 1500 eV Ar+ ions on PMDA-ODA (28). Lack of high resolution in the XPS analysis allowed only analysis of total atomic composition at the surface. As can be seen in Fig. 7, it was found that over this range of dosage, the surface carbon content increased by about 400/0 as the 0 component dropped approximately 70 % and N went effectively to zero. This result is consistent with those obtained for most organic polymers, where a carbonaceous overlayer is left on the surface. These filnls were approximately 5 microns before sputtering, and a distinct increase in conductivity of the surface was observed as a shift to lower binding energy of about 3.0 eV for core level peaks. Sputtering with reactive ions of ~ and O 2 at similar energies has given a much different result. There was a depletion of surface C probably as a result of more volatile products being formed such as hydrocarbon compounds or CO and CO2 , This would be expected to alter the surface chemistry of PMDA-ODA particularly as it reacts with deposited metals.
Ion Beam Interactions with Polymer Surfaces
1.4 1500 eV Ar
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So far, most of the ion beam studies have been carried out with an ion dose too high to permit detailed observation of the development of damage formation. For this purpose, we have carried out a series of high resolution XPS studies with low energy ion beams. If PMDA-ODA were lying more or less flat on the surface, the PMDA-ODA surface density would be approxinlately 1.3x1014 repeat units/cn12 , so a dose of 2x10 14 corresponds approximately to 1 ion/repeat unit at the surface. Since the polymer is not ordered in such a fashion, this number represents only an upper limit on the density of repeat units at the surface. Accordingly, our studies on surface chemistry of polyimide employed high resolution XPS for an Ar+ dose ranging from 1x1012 to 1x1017 ions/cm2 . The low end of this range was chosen to allow the observation of dosage effects at well below the level of one ion/repeat unit, and to coincide with the levels of ion dose used for static SIMS where damage to the polymer is expected to be minimal. Additionally, ion energies were maintained below 1 keV to remain in the atomic displacement range of ion interactions. The ion beam energy was maintained between 400 and 500 eV, with a sample bias of an additional 90 V for control of secondary electron emission and therefore sample charging as discussed in the previous section. All bombardment discussed here was performed in a normal incidence geometry for ion impacts of the surface. This will likely result in a higher degree of fragmentation of the polymer substrate, as pointed out by Magee (22), although the mass dependent factor makes this less clear. Lighter mass ions will in general favor direct knock-on sputtering events leading to a high degree of fragmentation, as will low energy ions, since not enough energy to attain extended collision cascades is available. The intermediate mass of Ar+ may tend to enhance collision
330
Handbook of Ion Beam Processing Technology
cascades, but this will be suppressed both by the normal incidence geometry and by the low energy of the impinging ions. Figure 8 shows the result for the overall intensity changes as a function of ion dose for these experiments. For ion doses less than approximately 5xl0 14 there is little change in the total intensity from the three observable atomic constituents of the surface. This is roughly (2xl0 14vs5xl0 14) consistent with the estimate above for an upper limit on surface density of polyimide, and for ion sputtering which should be dominated by nuclear interactions. Above this nominal dose, N 1sand 0 1s intensities begin to drop off as the CIs intensity begins to increase, consistent with the 1500 eV Ar+ bombardment discussed above. 1.4
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Ion Beam Interactions with Polymer Surfaces
331
It is instructive to examine the core level spectra individually in order to ascertain whether or not specific chemical information can be obtained. All of the data reported in this section was taken from polyimide films of 100-200 A thickness, where charging effects have been found to be negligible even in the unsputtered film. The binding energy scale was set by calibration against Au 4f7 / 2 and therefore any shifts due to charging or changes in surface conductivity are reflected in the data as presented. We have found previously that PMDA-ODA films on the order of 100 to 200 A thickness have displayed only small charging shifts, generally less than 0.5 eV (25). Figure 9 shows C Is core level spectra taken over a range of Ar+ ion dosages. The initial untreated surface, cured in vacuum, displays the characteristic 1 eV splitting of the low binding energy side, 2000
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assigned as predominantly ODA carbons at 284 eV and PMDA carbons at 285 eV (5). Intensity from the carbonyl carbon is the high binding energy peak at approximately 288 eVe These assignments have been discussed in detail relative to molecular orbital calculations by White et al (25). Upon an initial dose of approximately 2x10 13 one finds that there is a slight shift to lower binding energy of the entire peak, but otherwise relatively little change, save for the possible reduction of carbonyl intensity. The next dose at 4.5x10 13 shows a drop in the carbonyl intensity, concurrent with a shifting to lower binding energy of the carbonyl peak as well as the PMDA side of the main peak. A dose of 2x10 14 exhibits a loss of about half of the carbonyl intensity and a concurrent filling in of the valley between the carbonyl and the main peak. This behavior is consistent with the stepwise removal of carbonyl oxygen from the polymer, as can be seen from the result of a molecular orbital calculation shown in Fig. 10. In this calculation the renloval of one carbonyl oxygen from the PMDA-ODA repeat unit causes a loss of 1/4 the carbonyl intensity, as well as a shift to lower binding energy of that peak. By comparing the shape of the main low BE peak to the experimental data in Fig. 9, it can be seen that a similar
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Handbook of Ion Beam Processing Technology
asymmetry remains in this main peak, consistent with the removal of carbonyl oxygen from the repeat unit. 12
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-280
BINDING ENERGY (eV) Figure 10: Calculated CIs XPS spectra for PMDA-ODA before and after removal of one carbonyl oxygen from the repeat unit. The spectrum labelled after cleavage is for a repeat unit where one carbonyl oxygen is replaced by two hydrogens (courtesy B.D. Silverman).
Figures 11 and 12 show similar set of spectra for N Is and 0 Is. The change in 0 1s is dramatic, with losses only occurring on the low binding energy side, corresponding to the carbonyl oxygen. Loss of carbonyl oxygen is also consistent with the rate of loss of 0 Is intensity being faster than that of N Is (Fig. 8 (b) and (c)). The changes in N 1s in Fig. 11 indicate a chemical state change in the N as the low binding energy shoulder appears at dosages higher than 5x1014 . This indicates nitrogen remaining on the surface in a configuration allowing restoration of electron density, also consistent with removal of the electronegative carbonyl oxygen. Possible changes to surface chemistry with metals, induced by the ion bOll1bardnlent process, were also investigated. Figure 13 shows the deposition of 1 A of Cu onto a polyimide surface after bombardment by 7 x 10 14 Ar+ / cm2 . There is little indication of chemical reaction occurring between Cu and the bombarded surface. Although Cu is known not to react strongly with polyimide (25), there seems to be even less reaction after bombardment. This is also the case for a range of ion doses followed by Cu deposition.
Ion Beam Interactions with Polymer Surfaces
333
3500 N 1s Grazing Emission 3000 2500
Art Bombarded
2000
~
V1
zw
1500
t~
1000 500 0 -500 -408
_ _. . . l . - _ ~_ _--L-_ _. . L . - _ ~_ _...L....._--1
~_-.&..
-406
-404
-~02
-400
-398
-396
-394
-392
BINDING ENERGY (eV)
Figure 11: N Is XPS spectra from the PMDA-ODA surface following successive doses of 500 eV Ar ions.
5000 015 Grazing Emission 4000
3000
~
(/)
z
w
2000
t-
~
1000
0
-1000 '---_......_ _. . L . - _ - - L -_ _ -540 -538 -536 -534 -532
...L.--_----L._ _. . . I . - _ - - - - L _ - - - l
-530
-528
-526
-524
BINDING ENERGY (eV)
Figure 1 2: 0 1s XPS spectra from the PMDA-ODA surface following successive doses of 500 eV Ar ions
334
Handbook of Ion Beam Processing Technology
In addition, no change was observed in N or 0 Is spectra. For the reaction of the surface with Cr, a more reactive nletal, Fig. 14 shows successive deposition of Cr on a bombarded sample. Although not all doses were followed by Cr deposition, many other levels were observed with little difference in result. It is apparent that Ar+ ion bombardment does not increase chemical reactivity of the polynler surface toward Cr or Cu. The only observed reactivity of these bombarded surfaces is shown in Fig. 15. A rather strong interaction is found between the Nand Cr, forming a lower binding energy peak with successive depositions. This may be due to an increase in N endgroups as chain scission occurs, or perhaps an increase in reactivity of the original imide site as carbonyl oxygens are removed. 2000 r-----.--__r------r----'T---r-----,..--~-__, C 1s Grazing emission
1500 At+ Bombarded 7xl0(14) Ar+/cm2 dashed
1000
+ 1..\ Cu dotted
~
I-
U5
Z w I~
500
o
-500
L.....-_---I...-_----II......-_......L..-_----L_ _-'--_.........L._
-296
-294
-292
-290
-288
-286
-284
_....L...-_---J
-282
-280
BINDING ENERGY (eV)
Figure 1 3: CIs XPS spectra from the PMDA-ODA surface showing first the untreated sample, than Ar ion bombarded surface, and then the same bombarded surface with the equivalent of 1 A Cu deposited after bonlbardment.
The interpretation of XPS results for deducing the surface chemistry after bombardment is not straightforward. The difficulty arises because XPS data alone can not determine unambiguously the reaction product generated by many reaction paths in ion bombardment. For example, one cannot simply deconvolute the spectrum after bombardment based on the components of the spectrum of the original surface. To do so, it would require complementary studies by SIMS and molecular orbital calculations. Such a combined study has not been carried out so far. In spite of the problem with interpretation, the high resolution XPS data do show, however, that Ar ion energy of 1500 eV and 500 eV appear to be similar in the overall intensity changes of each of the constituents and that use of 500 eV ions for chemistry studies is possible for low doses. We also see from this result that it is difficult to remove surface contaminants by ion sputtering without affecting the polymer surface chemistry, even at low dosage. We have also found that there is no evidence of implanted Ar in the polymer, which has been investigated in a surface sensitive mode with grazing emission XPS, as well as with normal
Ion Beam Interactions with Polymer Surfaces
335
7000 C 1s Grazing emission
6000
5000
Cured PMDA-ODA
Att Bombarded
.~ .a
4000
>-
3000
oS
I(j) Z
~
~
2000
1000
0
.,.:_.::~= ..
-1000 '--_....I----L.--J-..--J---..1.--..I.--~--.I -296 -294 -292 -290 -288 -286 -284 -282 -280 BINDING ENERGY (eV)
Figure 14: C Is XPS spectra from the PMDA-ODA surface showing first the untreated sample surface, then the surface following Ar ion bombardment, and then the same surface with the equivalent of 1 A Cr and the 2 A Cr deposited after bombardment.
3500
~--,.----r----r----r---r---'
N 1s Grazing Emission
3000 Cured PMOA-ODA"
2500
2000
At" Bombarded
>I-
~
~
1500
~
1000
500
-500 '--_ _. . . L - - - . . J - - - - - - L - - - - J - - - - - l - - - - " -394 -396 -398 -400 -402 -404 -406 BINDING ENERGY (eV)
Figure 15: N Is XPS spectra from the PMDA-ODA surface showing first the untreated sample surface, then the surface following Ar ion bombardment, and then the same surface with the equivalent of 1 A Cr and then 2 A Cr deposited after bombardment.
enlission XPS which gives the greatest depth of sampling. Interestingly, there is no development of a 282 eV peak upon deposition of Cr after ion bombardment, as is seen at
336
Handbook of Ion Beam Processing Technology
Cr coverages of greater than a few monolayers on the untreated polyimide surface. This is additional evidence of the reduced chemical reactivity of the ion beam treated polymer surface. 16.5 SUMMARY
In the past, studies of ion beam interactions with polymer surfaces have been largely motivated by technological applications, therefore the emphasis has been centered on high energy and high dose reginles. Although such practical conditions are essential in some applications, many basic questions regarding the nature of the damage formation process and the accompanying chemical and structural changes of the polymer have not been understood. This is partly due to the complexity of the energy loss mechanism in polymers for high energy ions, and partly due to the multi-component chemistry and the chain structure of the polymers. In spite of these difficulties, considerable advances have been made in developing analytical techniques, in particular SIMS and photoemission spectroscopy, for investigating the nature of ion beam interactions with polymers. It soon became apparent that the complexity of the beam-polymer interaction can be reduced by focusing on the low ion energy regime, i.e. less than a few keY, where nuclear (atomic displacement) stopping of the ions dominates the energy loss mechanism. The SIMS study has been advanced by the development of static SIMS measurements where the analysis time is minimized to reduce the effect of the analysis beam. With this technique, the rate of damage formation and the composition of the molecules sputtered off the surface has been measured for a number of polymers. Photoemission spectroscopy can measure the changes in composition as well as in chemistry of the bombarded surface. With the high surface sensitivity, this technique has revealed chemical and compositional changes of polyimide surfaces by ion doses in the 10 12-10 14 ions/cm2 range. This corresponds to an average of less than one ion impinging on one polymer repeat unit, but within the range used for static SIMS measurements. The interpretation of the XPS data for deducing the chemical change is not straightforward due to the difficulty of determining the reaction products on the polymer surface. In this regard, SIMS and XPS are complimentary although the potential of such combined studies has not been explored. Overall, the current status of the field is in an early stage of development with much more work needed in order to draw broad conclusions useful in application and technology of the future. Indeed ion implantation and sputtering processes are already a major part of current manufacturing technology, particularly for the semiconductor industry. We expect increasing efforts in the future to enhance the basic understanding of ion beam interactions with polymer surfaces. 16.6 REFERENCES
1.
A.J. Steckl, S. Balakrishnan, H.S. Jin, and J.C. Corelli, Microelectron. Eng. 5: 461 ( 1986).
2.
F. Watanabe, Y. Ohnishi, J. Vac. Sci. Technol. B4: 422 (1986).
3.
O. Puglisi, A. Licciardello, L. Calcagno, G. Foti, Nucl. Instrum. & Meth. Phys. Res. B19-20: pt 2,865 (1987).
Ion Beam Interactions with Polymer Surfaces
337
4.
K.A. Valiev, V.A. Danilov, S.V. Peshekhonov, A.V. Rakov, and A.G. Shchuchkin, Sov. Microelectron. 12: 101 (1983).
5.
W.L. Brown, Rad. Eff. 98: 115 (1986).
6.
T. Venkatesan, Nucl. Instrum. & Methods Phys. Res. B7-8: 461 (1985).
7.
D. Briggs and M.J. Hearn, Vacuum 36: 1005 (1986).
8.
D. Briggs and M.J. Hearn, Int. J. Mass Spect. Ion Proc. 67: 47 (1985).
9.
Y. Yamashita, K. Ogura, M. Kunishi, R. Kawazu, S. Ohne, and Y. Mizokami, J. Vac. Sci. Technol. 16: 2026 (1979).
10. M.W. Geis, J.N. Randall, T.F. Deutsch, P.D. DeGraff, K.E. Krohn, and L.A. Stern, Appl. Phys. Lett. 43: 1 (1983). 11. M.W. Geis, J.N. Randall, T.F. Deutsch, N.N. Efremow, J. P. Donnelly, and J.D. Woodhouse, J. Vac. Sci. Technol. B1: 4 (1983); M.W. Geis, J.N. Randall, R.W. Mountain, J.D. Woodhouse, E.I. Bromley, D.K. Astolfi, and N.P. Economou, L. Vac. Sci. Technol. B3: 1 (1985). 12. B. Emmoth, G.M. Mladenov, J. Appl. Phys. 54: 7119 (1983). 13. M. Braun, B. Emmoth, G.M. Mladenov, H.E. Satherblom, J. Vac. Sci. Technol. AI: 1383 (1983). 14. J.A. Van-den-Berg, Vacuum 36: 981 (1986). 15. A Brown and J.C. Vickerman, Surf. Interface Anal. 8: 75 (1986). 16. D. Briggs, Surf. Interface Anal. 4: 151 (1982). 17. D. Briggs and A.B. Wooten, Surf. Interface Anal. 4: 109 (1982). 18. A. Brown, J.A. Van der Berg and J.C. Vickerman, Spectrochim. Acta, 40B: 871 (1985). 19. M.L. Yu and N.D. Lang, Phys. Rev. Lett. 50: 127 (1983). 20. M.L. Yu and K. Mann, Phys. Rev. Lett. 57: 1476 (1986). 21. H. Ishida, S.T. Wellinghoff, E. Baer, J.L. Konig, Macromolecules, 13: 22. C.W. Magee, Inter. J. Mass Spec. and Ion Phys. 49: 211 (1983). 23. M.L. Yu, RC12430 (IBM Internal Report) 1987. 24. I.H. Loh, J.K. Hirvonen, J.R. Martin, P. Revesz, and C. Boyd, in Polymer Surfaces, Interfaces and Adhesion, (MRS Symposia Proceedings 1987) in print, and R.D. Goldblatt, L.J. Matienzo, J.F. Johnson, and S.J. Huang, Journal of Polymer Science, in press. 25. R.C. White, R. Haight, B.D. Silverman, and P.S. Ho, Aool. Phys. Lett. 51: 481 (1987); R. Haight, R.C. White, B.D. Silverman, and P.S. Ho, J. Vac. Sci. Technol. A6: 2188 (1988). 26. S. Contarini, J.A. Schultz, S. Tachi, Y.S. Jo, J.W. Rabalais, Apol. Surf. Sci. 28: 291 (1987). 27. B.J. Bachnlan and M.J. Vasile, Soc. Plas. Eng. Tech. Pap. 34: 1003 (1988). 28. W. Bartha, J. Clabes and P.S. Ho, to be published.
17 Topography: Texturing Effects
Bruce A. Banks
17.1 INTRODUCTION
Although evidence of sputter deposition has been reported as early as 1775 by Joseph Priestly (1) in his "Experiments on Effects of Giving a Metallic Tinge to the Surface of Glass," topography effects associated with sputter etching were not reported until 1942, when Gunterschulze and Tollmien observed microscopic cones on metal glow discharge cathodes (2). Over the past several decades, there has been a growth of interest in altering surface morphology of materials on a microscopic level. Utilization of the unique chemical and physical properties of microscopically textured surfaces will demand not only knowledge of the existence of texturable surfaces, but an understanding of how to control and tailor the developnlent of specific morphologies. The intent of this chapter is to provide a practical guide to issues pertinent to the development and potential application of ion beanl textured surfaces. Topics such as the types of surface morphologies that can be developed on various materials, methods used to produce the morphologies, and properties of these textured surfaces are presented.
17.2 ION BEAM SPUnER TEXTURING PROCESSES AND EFFECTS
The material ejection phenonlena associated with the interaction of ion beams with material surfaces can be divided into two processes; physical sputtering and chemical sputtering. Material presented in this chapter will deal only with physical sputtering associated with Ar, Xe, or Hg ion beams where chemistry does not playa role in the ejection phenomena and resulting morphologies. Sputter texturing is a roughening of a bonlbarded target surface that occurs as a result of spatial variation in the sputter yield of the surface. Generally, texturing is observed on a microscopic level with cones or rills of the order of 20 microns or less. However, long duration ion beam sputtering experiments have been known to produce cones up to several millimeters in length (Fig. 1).
338
Topography: Texturing Effects
339
There is a variety of ways in which spatial variations in the sputter yield of a target may occur. The target material and its properties may lend itself to a natural development of spatial variation in sputter yield which, in turn, causes the formation of a left-standing cone structure (one that remains after removal of material by sputtering). The target material can also be seeded by sputter deposition of a different material with atoms that can nucleate into segregated microscopic sites of sputter resistance. Microscopic or macroscopic shadow masking by fine particles or lithographic techniques can be used to produce sites of sputter resistance or protection on a sputter target which results in the development of a surface texture.
Figure 1: Surface texture (cast epoxy replica) of a frozen mercury sputter target after
7,689 hours of mercury ion beam sputtering. 17.2.1 Natural Texturing
17.2.1.1 ChemicaUy pure materials. Some materials which are chemicaUy pure (essentially a single crystal) with no apparent initial spatial variation in sputter yield, develop a sputter textured surface. Pyrolytic graphite bombarded normal to the graphite lamella evolves into an extremely rough surface composed of very narrow cones or whiskers (Fig. 2). Surface whiskers ranging from 2-50 mm in length and 0.05-0.5 JLm in diameter have been observed (3). Minor impurities within the graphite may contribute to a left-standing structure if the impurities have a lower sputter yield than the graphite or if the protection sites are in continual receipt of surface diffusion transported material. However, as illustrated in Fig. 3 (4), the sputter etch rate of carbon is extremely low already, and identical surface texturing is found to occur in extremely pure synthetic gas (as opposed to natural gas) derived pyrolitic graphite (5). In addition, Floro et al (3) report that seeding the surface with Fe impurity atoms inhibits whisker formation.
340
Handbook of Ion Beam Processing Technology
(b)
(al
Figure 2: Surface textures produced on pyrolytic graphite by ion beam sputter etching:
(a) narrow cones, and (b) whiskers.
C A~o,
TI C,
,
••
To NiC,
-.• ~~ ~
w Al1350 GlIssl'" ClI NO
'0
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--
=-
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:
i ~•
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• ••
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IS
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I
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•~
;,
GIA.
Rh U
Th PI
40
V Mn
-• -. -.
Sm
=--
•
E, G.
45 200
400
600 SPUTIER ETCH RATE, A/min
11m
Figure 3: Range of sputter etch rates observed for various materials bombarded by a normally incident 500 eV argon ion beam at a current density of lmA/cm2
Topography: Texturing Effects
Au Gd Dy Pd Sn Ag
50
Bi12GelOO InSb GaP GaSb CdS
55
r•• ••
•• •
PbTe
•
I
••
Pb Sb Rb Bi
341
•
61
4(XX)
6(XXl
•
10 (XX)
SPUTIER ETCH RATE, Almin
Figure 3: continued.
2.5
r------r----------,r--r-----,
2.0
Figure 4: Variation of sputtering yields with angle of bombarding ion incidence. Credit to H. Oechsner
0.5
o
30
Angle of incidence
60
a (degrees)
90
A perfectly pure homogeneous material may tend to develop an alteration of its initial microscopically rough surface morphology upon ion bombardment simply because sputter yields are a function of the angle of incidence, as shown in Fig. 4 (6). In addition, ions impinging at near grazing incidence upon the sides of cone structures are reflected as fast atoms (Fig. 5) (7).
342
Handbook of Ion Beam Processing Technology
,
INCIDENT fOOl] BEAM [ [010]
J
I
0.9
!I
8 -~-~IIOO]
if
r
\
0.8
0.7
g ::::
\1./
(/)
E
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~
0.5
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I
\ J
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\;
~
o o
0.4
g .~
~
0.3
••.•.•.•••••••• Xe ----Ar
-------Ne
~
Figure 5: Particle reflection coefficient as a function of incidence angle for various 3 keV ions bombarding Cu. Credit to M. Hou and M.T. Robinson.
----N
--·--H
0.1
L---._----L_ _....J,...._ _...L---~----...--- 0
60
70
Angle of incidence,
80
e,
90
degrees
Thus, as the angle of incidence is increased away from normal, the sputter yield increases, which tends to compensate for the effects of reduced projected ion beam current density. However, a maximum yield is reached typically between 60 0 and 80 0 from the surface normal. Beyond this maximum yield angle, reflection begins to occur with an accompanying reduction in the sputter yield. At some angle, 100 % reflection occurs and therefore, negligible sputter etching occurs (Fig. 6) (8). Thus, minor surface anomalies
c:
a:
1: Q) '0
~o o
c:
.2
'0 Q)
~ ~
c:
.2
Figure 6: Typical sputter yield and ion reflection coefficient dependence upon bombarding ion incidence angle. Credit to O. Auciello.
Topography: Texturing Effects
343
subjected to ion bombardment evolve through a series of geometrical changes that are the result of yield and reflection angular dependencies as well as the sputter deposition processes involved. If, during this process, a narrow cone angle surface asperity emerges, then scattered fast atoms may sputter material from the base of the cone at high rates because of overlapping arrival of the normal bombarding ions. The sputter ejected material can further contribute to preservation of narrow cone structures by deposition on their surfaces. The undercutting phenomenon around the base of cones and cone evolution have been investigated by numerous researchers (8-13).
Figure 7: The effects of ion beam sputter etching alumina ceramic caused by the presence of voids and sputter yield angular dependence. (a)
(a.) Surface prior to ion beam sputtering.
(b.) Surface after argon ion beam sputtering with lxl021 ions/cm 2 at 1.75 keY.
(b)
Chemically pure materials with small distributed voids develop a surface texture which is strongly indicative of the sputter yield angular dependence (Fig. 7). Such materials tend to develop concave surface features centered around void sites.
344
Handbook of Ion Beam Processing Technology
Polycrystalline materials, as one would expect, have spatial variations in sputter etch rates caused by the sputter yield dependence upon crystallographic orientation. The sputter-etch rate's crystallographic dependence tends to cause increased visibility of the ion bombarded polycrystalline surfaces due to microscopic crystallite plateaus of varying elevations and grain boundary chamfering. Quasicrystalline materials may have spatial variations in sputter yield because amorphous regions have different yields than crystalline regions. Fluoropolymers such as (PTFE Teflon R ) , fluorinated ethylene propylene polytetrafluoroethylene (REF TeflonR ), and perfluoroalkoxyethylene (PFA TeflonR ) all develop cone shaped surface textures (Fig. 8) as a result of ion beam sputtering (14). Rost, et al (15) determined by x-ray diffraction that the degree of crystallinity of a PTFE surface was increased as a result of the left-standing surface structure formation. Morrison and Robertson (16) also suggest that the etch pattern reflects the crystallinity of the PTFE, and that preferential sputter etching occurs along grain boundaries and/or interstitial amorphous regions. The sputter texturing of fluoropolymers is of importance because of the ease in which large surface structures can be produced, the ability to bond to these surfaces, and the ability to transfer cast these surfaces onto elastomers of industrial or biomedical interest. Fluoropolymers such as PTFE have high sputter yields (15-600 atomslion) which are dependent upon the incident ion beam power density (Fig. 9) (14), allowing textured surfaces to develop in a few seconds with the use of a high power density beam. The cones always point in a parallel direction to the incoming ions.
Figure 8: Textured PTFE surface produced by 30 minutes of exposure to 750 eV argon ions at a current density of 0.6mA/cm2
Topography: Texturing Effects
ARCON
345
Nt:RCUR Y
ION Et£RCY. eV ARCON W£RCURY
" V
1250 1000
o
500
• •
250
•
o
[J
750
"
1000
750
500
250
Figure 9: PTFE etch rate as a function of ion beam power density for argon and mercury
ions. 17.2.1.2 Mixed composition materials. Most engineering materials fall under this category in that they, by design, are alloys, or contain a mixture of chemical ingredients to elicit desired functional properties. Polymers such as segmented polyurethane (BiomerR ) have spatial variations in chemical composition which could contribute to the development of a sputter textured surface. However, the observed surface features typically have much larger dimensions than would be expected from the viewpoint of molecular chemistry (Fig. 10).
Figure 10: Argon ion beam sputter textured segmented polyurethane (BiomerR )
346
Handbook of Ion Beam Processing Technology
Metal alloys will often develop two levels of surface features that consist of large rounded bumps, several microns in diameter, which have smaller submicron rills or cones (Fig. 11). Because of sputter yield differences, the surface of a multicomponent sputter target will become enriched in the low sputter yield species and depleted in the high sputter yield species. Such alteration of surface chemistry and surface texture development contributes to the complexity of interpretation of sputter profiling Auger analysis of these surfaces. Table 1 summarizes the natural textures that result from ion beam sputter etching various materials (5,14,17).
Figure 11: Ion beam sputter textured MP35N (35% Ni; 35% Co, 20% Cr; 10% Mo) produced by 30 minutes exposure to 2,000 eV Xe ions at a current density of 2mA/cm2 •
17.2.2 Seed Texturing
Ion beam sputter etching of a low melting temperature, pure material which is subjected to simultaneous arrival of a high melting temperature material (the seed material) will often result in the development of a left-standing surface texture. This can be accomplished by allowing the sputter ejected material from a seed target to arrive on the target to be sputter textured as shown in Fig. 12. The size, shape, and spacing of the cones or other surface structures developed as a result of seed texturing depend upon ion beam current density, ion energy, substrate material, substrate temperature, seed material, seed target area, seed target orientation with respect to the target to be textured, and duration of ion bombardment. Although seed texturing can often be most easily accomplished by simultaneous sputter etching of two dissimilar target materials, it may also be achieved by the sputter etching of a target while thermally evaporated high melting temperature seed material simultaneously arrives at the surface of the target to be textured. 17.2.2.1 Seed materials. Wehner and Hajicek (18) have shown that in the case of molybdenum seed atoms arriving on a copper surface to be textured, as few as 1 molybdenum atom per 500 sputter etched copper atoms was adequate to produce widely spaced cones. If the arrival flux of molybdenum seed atoms was greater than 1 atom per 20 copper target atoms, a continuous coverage of cones was observed.
Topography: Texturing Effects
Table 1. Bombarding Ion
oS E
r::-
S
~
:)
Q
1 CIl
~
t2
Material
Resulting Surface Morphology
Ref.
20
Concave depress ions. severa 1 mi crans Autll in diameter by a few microns deep
0.5
30
Narrow canes. submicron to microns in length
750
0.5
30
Wide angle canes
1.700
0.5
Alumina (parous ceramic)
Ar
1.750
Ch lorotri fluoroethylene
Ar
750
Fluorinated
Ar
Ar
f\J
10
Auth Auth
f;~~l(~~pp~~~r~~ Gl ass Hanes 25 MP35N (35% Ni i 35% Co; 20% Cr; 10~ Mo)
Xe
2.000
Auth
2.000 Smooth _
Several micron bumps with submi cron ri 11 s on them
30
Several micron bumps with submicron canes or ri 11 s an them
Nylon 30
Auth
Rills
Auth
Narrow canes. submicron to tens of microns in length
Auth
No texture
Auth
Perfluoro a 1koxyn
Ar
750
0.5
Po1yethy1ene
Ar
300500
-
PolyimMle (Kaptoif)
Ar Ar Ar
1.000 1.8 1.000 1.8 1.OGO 1.8
10 30 115
Polyolefin (Hexyit)
Hg
700 0.7
240
Pal yoxymet hy1ene
Ar
Po lyoxymethy1ene
Ar
500 0.6
Po 1yoxymethy 1ene (Oelrin)1!>
At'
300 0.3
Po1ytet ra fl uoroethyle~ (PTFE
At'
750 0.5
30
Narrow cones. submicron to tens of microns in length
14
Po lyurethane
Ar
500
60
rv
1/2 micron diameter hemispherical bumps and ri 11 s
Auth
Po lyurethane
Ar
400
360
Submicron bumps on mounds
Auth
Pyro lyt ic graphite (basal plain)
Ar
S i 1i cane Rubber
Ar
300500
Stainless steel. Series 316
Xe
1.800
Tefze1lii1
Ar
I
(Celcon)~
(Oelrin)~
60
Separate submicron surface pits Connected submicron pits Connected submicron pits with separated narrow canes. severa 1 microns tall
17
2 micron diameter granular surface bumps
Auth
Branched fibrals. tens of microns tall
Auth
Branched fibrals. tens of microns tall
Auth
tV
Autll
2.400 Narrow cones
Teflon~
(Tecof1ex)~
n.J
1-2 micron
Auth
No texture 2
30
N tv
750
0.3
72
30 1nc1uded angle cones and rill s 8 )Am high
No texture
Auth Auth 14
347
348
Handbook of Ion Beam Processing Technology
o
_______ ::IO~ ·-0 00 00
(£) - - -
••
./0
. /0
TEXTURED TARGET
·~W
~DTARGU
$
~~SPunER TARGET
~;
II II
ION SOURCE
I: II
ION BEAM
,I \\ \\
''':
.:i~
TEXTURED ~ SURFACE ~
~~/
TO BE
TEXTURED
.DTARGU
Figure 12: Seed texturing by ion beam sputtering.
Although the seed material elements generally are those elements which have a low sputter yield compared to the target material to be textured, Wehner (19) has shown that seed materials which merely have a higher melting temperature than the target material to be textured successfully produce cones. The collective data of Wehner (19), Hudson (20) and Heil (21) support texturing with a higher sputter yield for the seed material than the target material. When refractory materials are used as seed materials to texture other refractory materials of low melting temperatures, elevated target temperatures are required during ion bombardment to elicit texture formation. Figure 13 (20, 22) illustrates examples of seed and substrate target material combinations which successfully produce textured surfaces. Figure 14 identifies elements in the periodic table which have been successfully textured using tantalum as the seed material (20). 17.2.2.2 Diffusion effects. The concept of seed atoms being able to migrate across the surface and nucleate into clusters of sputter protection of an underlying substrate material is easily understood if the sputter yield of the seed material is lower than that of the substrate material. However, in cases which the surface texture develops in spite of the seed material having a higher sputter yield than the substrate material, the protection mechanism is less obvious. Robinson (23) suggests that sputter protection by higher sputter yield seed clusters can successfully occur provided that the arrival and mobility of the seed material is sufficient to replenish the losses due to sputtering of the seed cluster. The ability of seed atoms to migrate across a substrate surface and nucleate into sites of sputter protection is controlled by the surface diffusion processes. If the surface diffusion of the seed atoms depends upon the substrate temperature and diffusion activation energy, then one would expect that at constant substrate temperatures, the seed clusters and resulting average cone spacing would be dependent upon the inverse square root of the ion beam current density (24-29).
Topography: Texturing Effects
Seed Metal
349
Substrate Metal
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IA
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83
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Figure 14: The periodic chart of the elements. The shaded elements were successfully textured using tantalum as the seed material. The texturing process was unsuccessful for the cross-hatched materials. Untested elements are unshaded (20).
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Handbook of Ion Beam Processing Technology
However, for 1,000 eV argon ions at current densities equal to or greater than 0.7 mA/cm2 , a nearly linear dependence of cone spacing on current density is observed for molybdenum seed textured copper at 300° (28). This dependence is attributable to ion impact enhanced diffusion occurring in addition to the thermal surface diffusion (28, 29). Figure 15 shows the dependence of average cone spacing upon the ion beam current density for molybdenum and tantalunl seed textured copper (28). 15.0
12.5
Figure 1 5: Average cone spacing of seed textured copper as a function of ion beam current density for 1,000 eV Ar ions (28).: (a) Mo seed with substrate at 300°C, and (b) Ta seed.
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17.2.2.3 Resulting topographies. The morphologies that can be achieved as a result of seed texturing can depend upon both the substrate and seed material combinations as well as the sputtering conditions and duration. Rossnagel and Robinson (30) have observed that copper sputter textured with seed materials of carbon, tungsten and tantalum can result in surface features which are ball and stick structures, rills, or cones, respectively. Figure 16 shows several typical forms of surface features which have been observed as a result of ion beam seed texturing of various materials with tantalum as the seed material.
Topography: Texturing Effects
351
Figure 16: Forms of surface features resulting from ion beam sputter texturing with tantalum as a seed material:
(a.) Ball-on-stick structures on copper,
(c.) Random rills on nickel,
(b.) Cones on silicon,
(d.) Parallel rills on platinum, and
(e.) Cones and rills on larger mounds on gadolinium.
352
Handbook of Ion Beam Processing Technology
Table 2 lists the types of morphologies that have been observed for a variety of tantalum seed textured materials. Practical applications of seed texturing require the ability to alter the density and height of the cones.
Table 2: Morphologies resulting from texturing various materials using Ta as a seed ma-
terial and a Xe ion beam of 500 - 2000 eV energy with current densities of 0.2 to 2 mA/cm2 . MATERIAL
TEXTURE GEOMETRY
Alunlinum Antimony Beryllium Bismuth Cadmium Carbon Chromium Cobalt Copper Gadolinium Germanium Gold Graphite Hafnium Iron Lead Magnesium Molybdenum Nickel Niobiunl Platinum Silicon Silver Tantalum Tin Titanium Tungsten Zinc Zirconium
Cones or ball and stick structures Cones Irregular surface cavities Cones Wide angle cones with balloon tops Sparse cones Cones Randonl rills Cones or ball on stick structures Cones or rills on larger mounds Random Rills Cones or random rills Cones Parallel rills Random Rills Cones and random rills, mixed Cones and randonl rills No texture Cones or random rills Very snlall (sub-micron) rills on mounds Cones or parallel rills Cones, or cones and random rills on larger mounds Cones No texture Random rills Random and parallel rills No texture Wide cones with balls on top Random rills
In general, closely spaced cones can be achieved by means of higher seed arrival rates, low current densities, and low substrate temperatures. Similarly, widely spaced cones can be achieved by lower seed arrival rates, higher beam current densities, and high substrate temperatures. Cone height, to a limited extent, is dependent upon duration of ion beam
Topography: Texturing Effects
353
sputtering. However, most material seed combinations appear to eventually develop an equilibrium height structure that does not substantially increase with sputtering duration.
17.2.3 Shadow Masking
Large surface features can be produced by providing sites of sputter protection over an underlying substrate to be macroscopically textured. Although seed texturing produces microscopic sites of sputter protection, the typical surface diffusion processes are inadequate to provide large regions of sputter protection. Several other macroscopic techniques lend themselves to the production of textured surfaces which may have potential for aerospace or industrial application. The most common technique for protection of large areas is the use of a photoresist as a sputter mask. Although this provides large area protection, the depth of the structure that can be etched depends upon the sputter yield of the photoresist and its thickness. A sputter resist can, in fact, be replenished such that it has near infinite life if the sputter resist has a sufficiently low sputter yield relative to the substrate material. Carbon is one such material which can be deposited on most materials and be replenished by simultaneous deposition of carbon, as shown in Figure 17.
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354
Handbook of Ion Beam Processing Technology
The replenishment in Fig. 17(a) is from the graphite target. The etching is by glancing collisions by ions from the ion source, with rotation of the target to avoid shadowing behind the mask. The location of the beam maximum on the target, together with the graphite comb, assure replenishment on the etched graphite mask. Carbon atoms landing upon a carbon surface will tend to fully compensate for the low rate of sputter loss of the carbon mask. However, carbon atoms arriving on a higher sputter yield material will be dominated by the high material loss associated with the high sputter yield substrate material, thus preventing a protective layer from being built up. As a result, a thin diamondlike carbon sputter mask can be selectively deposited and replenished to maintain a proper balance of the sputter etch and deposition rate (31). By selectively depositing a carbon film on a substrate using photoresist techniques with diamondlike carbon film deposition, plateau-shaped structures of any desired crosssectional shape, spacing, and height may be produced. This technique has been demonstrated on silicon dioxide in which there was a slight actual increase in the sputter mask while the silicon dioxide was being sputter etched in regions of no protection (31). High sputter yield materials such as the fluoropolymers PTFE and FEP Teflon R lend themselves favorably to the development of large surface features. This can be accomplished by simply distributing any average sputter yield material on the surface of the fluoropolymer to act as a sputter shield. For example, powdered sodium chloride, dusted or pressed against the surface of PTFE or FEP TeflonR will provide adequate sputter shielding of the underlying fluoropolymer to allow the development of tall, plateau-shaped structures as shown in Fig. 18 (5). The sodium chloride can them be washed off in water after the sputter texturing is completed.
Figure 18: Sodium chloride dust shadow masked FEP Teflon which has been ion beam sputter-etched.
Rectangular sputter etched pits can be produced in most materials by placing an electroformed metal mesh screen in close proximity of the substrate during ion beam sputtering. Through the use of a fine nickel electroformed mesh, regular surface pits can be sputter etched on a repeat distance of the order of 10-2 mm.
Topography: Texturing Effects
355
17.3 TEXTURED SURFACE PROPERTIES
The ability to alter the surface morphology of materials on a microscopic scale, which is much smaller than is possible by conventional mechanical or chemical means, enables the development of unique surface properties which may have industrial, aerospace, or medical applications. 17.3.1 Mechanical
Textured surfaces of metals (whether natural or seed textured) can be used to mechanically interlock with other metal textured surfaces (5). This is accomplished by placing the textured surfaces in such a position so they are facing each other, and then plastically deforming the surface microstructures of one surface into the other such that the cone structures mechanically interlock, somewhat like Velcro R • Tantalum seed textured copper and aluminum surfaces can be mechanically bonded by a simple hammer blow which causes the surface cones to nest, deform, and thus interlock. Two pieces of textured copper bonded to each other had measured bonds of 228 kPa (33 lbs. per square inch) tensile strength and 572 kPa (83 lbs. per square inch) shear strength. Textured surface cones on PTFE TeflonR and FEP TeflonR are ideally suited for adhesive bonding, even though the smooth surfaces of these materials are typically difficult to bond. The bonding adhesive must be applied as a fluid and have a small enough contact angle with the fluoropolymer to allow the uncured adhesive to flow in and around the surface microstructures. When the adhesive hardens, the surface microstructures become potted in the adhesive, thus forming a predominantly mechanical bond. The tensile and shear strengths of epoxy bonds to ion beam natural textured fluoropolymers have been demonstrated to be superior to conventional sodium/napthalene chemically etched surface treatments (32). Figure 19 compares the tensile and shear strengths of epoxy bonded PTFE Teflon which had been ion beam textured with untreated and conventional sodium/napthalene chemically treated surfaces. Because the bond involves mechanical interlocking around fluoropolymer cones, the bond strength remains high independent of the amount of time between surface treatnlent and epoxy bonding. As can be seen in Fig. 19, the bond strengths of chemically treated surfaces are not only lower, but are greatly reduced if the epoxy bonding is not immediately performed. Textured surfaces can be imparted indirectly to elastomeric materials that are not easily textured by sputtering processes. To accomplish this, a transfer casting technique is used (33). First the desired morphology is sputter etched or textured onto a fluoropolymer surface such as PTFE TeflonR . Then, a thin mold release agent may be applied, if required, and an uncured elastomer is case over the textured surface. Upon curing, the elastomer is then peeled from the sputter-textured surface to yield a transfer case negative of the fluoropolymer surface. Figure 20 depicts such a transfer case of silicone rubber peeled from a PTFE Teflon R surface which had an array of pits produced by ion beanl sputtering through an electroformed nickel screen mesh (5). The small surface features of a natural sputter textured PTFE surface can also be transfer cast through the application of a thinly applied mold release agent. Transfer casting has great potential application for biomaterials used for surgical implant applications where surface morphology and chemistry have active roles with respect to tissue response (34).
356
Handbook of Ion Beam Processing Technology
SURFACE TREATMENT
2500
Untreated Chern etch 1 min. bond 24 hrs later Chern etch 5 min, bond immediately 30 min Ion beam texturing by 750 eV argon Ions 8t 0.5 mA/cm 2 • then bonded 20 days later
TENSILE STRENGTH
1500
15
SHEAR STRENGTH
10
Figure 19: Tensile and shear strengths of epoxy bonded PTFE (bulk tensile strength of PTFE = 3,000 - 4,500 psi).
1(00
500
Figure 20: Transfer casting scanning electron photomicrograph: (a.) Polytetrafluoroethylene (PTFE TeflonR ) substrate after transfer casting showing pits produced by ion beam sputtering through an electroformed nickel mesh mask, (b.) Silicone rubber (SilasticR ) transfer cast pillar morphology resulting from the negative of a pit morphology.
Topography: Texturing Effects
357
Textured surfaces may alter the fatigue properties of materials. An experinlental investigation of the effect of ion beam texturing on the fatigue strength of MP35N (35% Co, 35% Ni, 20% Cr, 10% Mo) has been conducted for both natural textured and square array shadow mask pit structured surfaces (35). The natural texture consisted of mounds 10 - 20 microns in diameter with superimposed rill structures which were 10ths of microns high. The rectangular pit structures with dimensions of approximately 150 microns on the edges and 70 to 100 microns deep, were formed by ion beam sputtering through a nickel electroformed mesh mask. The results of this investigation indicate that for fatigue failure at 5x106 cycles, the natural textured surface and the square hole pit surface were reduced in fatigue strength by 50% and 60%, respectively, from that of an untreated smooth surface finish. Textured surfaces enhance nucleate boiling heat transfer rates over smooth surface materials because of the increased number of bubble nucleation sites. A comparison of nucleate boiling heat transfer, using Freon 113 as the working fluid on tantalum seed textured copper and untreated copper, showed improvement by a factor of between 2 and 4 in the heat transfer properties of the textured surface for the same temperature difference. Such improved heat transfer characteristics, if demonstrated as durable, may significantly reduce the size and cost of industrial reboilers through the reduction of required heat transfer area (36). 17.3.2 Electrical
Ion beam sputter texturing of materials can alter the surface electrical properties of materials with negligible effect on their bulk properties. Ion beam sputter texturing of polyimide (KaptonR ), 8 microns thick, by exposure to 1 keY argon ions at 1.8mA/cm2 for 30 minutes reduces the electrical sheet resistance from greater than 107 ohms/square to 104 ohms/square (37). This reduction in sheet resistance may have applicability to prevent spacecraft charging on thermal blanket materials. Sputter textured surfaces of some metals and pyrolytic graphite have surface microstructure that tend to trap electrons (both primary and secondary) when subjected to electron bombardment. Ion beam textured pyrolytic graphite has been shown to have a lower secondary electron emission and reflected primary yield than carbon soot, which has long been regarded as the ultimate surface for capturing electron beams without significant secondary or reflected primary electron release, as shown in Fig. 21 (5). Textured surfaces have application in depressed collectors, used in travelling wave tubes, which collect the spent electron beam from the microwave amplifier. The reduction of reflected or secondary emitted electrons reduces power losses in these devices. 17.3.3 Chemical
The chemical properties of ion bombarded surfaces are modified predominantly through the breaking of chemical bonds and altering the population of species on the surfaces of materials. Both natural and seed texturing can produce spatial variations in the surface chemistry of materials. Documentation of surface chemistry alteration of mateIials caused by ion beanl sputtering has been published by Banks (5) and Kowalski (38) for polymers and metal alloys. The combined modifications of surface chemistry and morphology caused by ion beam texturing can elicit chemical and physical effects with
358
Handbook of Ion Beam Processing Technology
other fluids in contact with these surfaces. Liquid contact angles to textured and untreated surfaces are also different (5). Textured surfaces on surgical implants cause alterations in both the type and kinetics of cellular response around the inlplant. This may be used advantageously to elicit desired biological responses to surgical implants (34). The extended surface area of textured surfaces, as well as the spatial variation in surface chemistry, have given them potential as catalytic surfaces. Such catalytic surfaces are currently being considered for advanced heat transfer cooling systems for conversion of para hydrogen to ortho hydrogen for hydrogen oxygen rocket propulsion engines. 1.2
o o L:1
SMOOTH PYROlYTIC GRAPHITE SOOT ON PYROlYTlC GRAPHITE DISCHARGE CHAMBER TRIODE NATURAL TEXTURED PYROlYTIC GRAPHITE (lOCO eV ARGON IONS AT 5 mA/cm 2 FOR 6 hr)
( a)
o o L:1
SMOOTH PYROlYTIC GRAPHITE SOOT ON PYROlYTIC GRAPHITE DISCHARGE CHAMBER TRIODE NATURAL TEXTURED PYROlYTlC GRAPHITE (lOCO eV ARGON IONS AT 5 mA/cm 2 FOR 6 hr)
(b)
600
800 lOCO 1200 1400 1600 PR IMARY ELECTRON ENERGY. eV
1800
2OCO
Figure 21: Characteristics of normally incident electron bombardment of various carbon surfaces: (a.) Secondary electron emission ratio, (b.) Reflected primary electron yield
17.3.4 Optical
Ion beam sputter texturing tends to increase the diffuse reflectance and absorbtance for opaque materials and increase the diffuse transmittance for transparent materials. As one would expect, textured cone or rill structures on opaque metal surfaces act as anechoic chambers to light if their spacing is of the order of the wavelength of the incoming light. The absorptances of the standard solar spectrum (air mass 0) of copper, silicon, aluminum, titanium, and 316 stainless steel are increased to .0945, 0.973, 0.972, 0.957, and 0.927, respectively by tantalum seed texturing (39). Figure 22 shows the resulting reflectance as a function of wavelength for ion beam textured copper and silicon (5). Such surfaces are ideal high solar absorbers.
Topography: Texturing Effects
359
.~
~
.3
u
.2
~ 0::
•
:z ~ LoU
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Figure 22: Spectral reflectance of tantalum seed textured metals by a 1,000 eV Xe ion beam at 2 mA/cm2 (Top graph is for Cu, lower graph is for Si).
The thermal emittance of potential radiator materials can be enhanced by sputter texturing if the surface features are sufficiently large to be appropriate for the radiator temperatures involved. Tantalum seed texturing in a triode configuration within the discharge chamber of an electron bombardment (Kaufman) ion source allows high ion fluxes and high substrate temperatures which are conductive to the development of cone structures, several microns tall, that have high thermal emittance for radiators operating at 900 K (40). Using this technique to texture surfaces of Cu, Ti, Ti-16 % Al-2.5 % V, Nb-1 0/0 Zr, and Type 304 stainless steel results in thermal emittances at 900 K of 0.983, 0.80, 0.521, 0.376, and 0.89, respectively (40). Such high temperature radiator surfaces have the advantage of durability for space applications because the emittance is dependent upon the surface morphology rather than chemistry, and no coating is required to remain adherent during the large thermal excursions that may be required for advanced solar dynamic power system or nuclear space power system applications.
17.4 REFERENCES
1.
Priestly, J., The History and Present State of Electricity with Original Experiments, Vol. 1, Third Ed., London: Bathurst and Lounder (1775).
2.
Auciello, 0., Ion Bombardment Modification of Surfaces (0. Auciello and R. Kelly, eds. ), p. 3, Amsterdam: Elsevier Publishing Co. (1984).
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Handbook of Ion Beam Processing Technology
3.
Floro, J.A., Rossnagel, S.M. and Robinson, R.S., lon-bombardment- induced whisker formation on graphite. J. Vac. Sci. Techno!. A1(3): 139 (1983).
4.
Con1monwealth Scientific Corp., Ion Beam Etch Rates, Bulletin No. 137-78.
5.
Banks, B., Ion beam applications research - A 1981 summary of Lewis Research Center Programs, NASA TM No. 81721 (1981).
6.
Oechsner, H. App!. Phys. 8: 185 (1975).
7.
Hou, M., and Robinson, M.T. App!. Phys. 17: 371 (1978).
8.
Auciello, 0., Ion interaction with solids. J. Vac. Sci. Techno!. 19(4): 841 (1981).
9.
Carter, G., and Nobes, M. J., in Ion Bombardment Modification of Surfaces (0. Auciello and R. Kelly, eds.) pp. 163-223, Amsterdam: Elsevier Publishing Co. (1984).
10. Wilson, I., Belson, J., and Auciello, 0., in Ion Bombardment Modification of Surfaces (0. Auciello and R. Kelly, eds.), pp. 225-295, Amsterdam: Elsevier Publishing Co. (1984). 11. Broers, A.N., Unpublished thesis. University of Cambridge, England (Feb. 1965). 12. Rossnagel, S.M., and Robinson, R.S., Monte Carlo n10del of topography development during sputterin~ J. Vac. Sci. Techno!. A1(2): 426 (1983). 13. Sigmund, P., A mechanism of surface micro-roughening by ion bombardment. Mat. Sci. 8: 1,545-1,553 (1973).
L.
14. Banks, B.A., Sovie, J.S., Miller, T.B., and Crandall, K.S., Ion bean1 sputter etching and deposition of fluoropolymers, NASA TM No. 78888 (1978). 15. Rost, M., Erler, H.J., Giegengack, H., Fiedler, 0., and Weissmantel, C., Thin Solid Films, 20: S15 (1974). 16. Morrison, D., and Robertson, T., Thin Solid Films, 15: 87 (1973). 17. Mirtick, M.J., and Sovie, J.S., Optical and electrical properties of ion beam textured Kapton and Teflon, NASA TM No. 73778 (1977). 18. Wehner, G.K., and Hajicek, D.J., Cone formation on metal targets during sputtering. Journal of Applied Physics, 42(3): pp. 1145-1149 (1971). 19. Wehner, G.K., Whiskers, cones, and pyramids created in sputtering by ion bombardment. Prepared for NASA Grant NSG-3041, Report No. CR-159549 (March 1979). 20. Hudson, W.R., Ion beam texturing. J. Vac. Sci. Techno!. 14: pp. 286-289 (1977). 21. Heil, 0., Particle bombardment bonding and welding investigation, U.S. Army, Fort Monmouth, NJ, Contract No. DA 28-043-AMC-00429(E), Second Quarterly Report (Aug. 1965). 22. Wehner, G.K., Cone formation as a result of whisker growth on ion bombarded metal surfaces. J. Vac. Sci. Techno!. A3(4): 1821 (1985). 23. Robinson, R.S., Physical processes in directed ion beam sputtering. Prepared for NASA Grant 3086, Final Report No. CR-159567 (March 1979). 24. Kaufman, H.R., and Robinson, R.S., Ion bean1 texturing of surfaces. J. Vac. Sci. Techno!. 16(2) 179 (1979).
Topography: Texturing Effects
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25. Robinson, R.S., Ion beam microtexturing of surfaces. Prepared for NASA Grant NAG 3-43, Final Report No. CR-165383 (May 1981). 26. Robinson, R.S., Ion beam microtexturing and enhanced surface diffusion. Prepared for NASA Grant NAG 3-43, Final Report No. CR-167948 (Feb. 1982). 27. Rossnagel, S.M., and Robinson, R.S., Surface diffusion activation energy determination using ion beam microtexturing. J. Vac. Sci. Technol. 20(2): 195 (1982). 28. Rossnagel, S.M., Robinson, R.S., and Kaufman, H.R., Impact enhanced surface diffusion during impurity induced sputter cone fornlation. Surf. Sci. 123: pp. 89-98 ( 1982). 29. Robinson, R.S., and Rossnagel, S.M., Ion-beam induced topography and surface diffusion. J. Vac. Sci. Technol. 21(3): pp. 790-797 (1982). 30. Rossnagel, S.M., and Robinson, R.S., Quasi-liquid states observed on ion beanl microtextured surfaces. J. Vac. Sci. Technol. 20(3): pp. 506-509 (1982). 31. Banks, B.A., and Rutledge, S.K., Ion beam sputter deposited diamondlike films, NASA TM No. 82873 (June, 1982). 32. Mirtich, M.J., and Sovie, J.S., Adhesive bonding of ion beam textured metals and fluoropolymers, NASA TM No. 79004 (Dec. 1978). 33. Banks, B.A., Weigand, A., and Sovie, J.S., Texturing polymer surfaces by transfer casting, U.S. Patent No. 4,329,385 (June 11, 1982). 34. Banks, B.A., Ion bombardment modification of surfaces in biomedical applications, in Ion bombardment modification of surfaces in biomedical applications, in Ion bombardment modification of surfaces (0. Auciello and R. Kelly, eds.), pp. 399-434, Amsterdam: Elsevier Publishing Co (1984). 35. Wintucky, E.G., Christopher, M. Bahnuik, E., and Wang, S., Ion beam sputter etching of orthopedic implant alloy MP35N and resulting effects on fatigue properties, NASA TM No. 81747 (April, 1981). 36. Park, E.L., and Tasuda, H.K., Nucleate boiling from ion-beam textured surfaces coated with RF plasma deposited polynlers, Final report from NASA Grant No. NSG-3199, University of Missouri at Rolla (1979). 37. Mirtich, M.J., and Sovie, J.S., Optical and electrical properties of ion-beam-textured Kapton and Teflon. J. Vac. Sci. Technol. (15)2: 697 (1978). 38. Kowalski, Z. W., Review - Ion beanl sputtering and its biomedical applications. Theoretical concepts and practical consequences. Clinical implications and potential use. J. Mat. Sci. 20: pp. 1521-1555 (1985). 39. Hudson, W.R. Weigand, A.J., and Mirtich, M.J., Optical properties of of ion beam textured metals, NASA TM No. X-73598 (Feb. 1977). 40. Mirtich, M. J., and Kussmaul, M. T., Enhanced thermal emittance of space radiators by ion-discharge chamber texturing, NASA TM No. 100137 (March 1987).
18 Methods and Techniques of Ion Bea." Processes Stephen M. Rossnagel
18.1 INTRODUCTION
A number of experimental configurations and techniques have been used with broad beam ion sources. Chapters 2-5 have described the operation of several types of these ion sources in some detail. Other chapters have described several of the applications of ion beam technology for the modification of material properties (Chaps. 10-17) and the production of compound or novel thin films (Chaps. 19,20). The purpose of this chapter is to describe applications of ion sources to film deposition, modification and synthesis from the experimetnal viewpoint. In addition, we will attempt to discuss some of the practical aspects of these techniques, including output levels, deposition and etching rates, etc. A variety of plasma-based sputtering and bombardment-modification techniques are available and have been described in the literature. In these techniques, ion bombardment occurs at the sample surface during the film deposition, inasmuch as the sample is immersed in the plasma. Additional bombardment is incurred by biasing the sample negative of the local plasma potential in either a dc or an rf mode. These techniques will not primarily be discussed in this chapter due to length considerations. The discussion will be constrained to techniques utilizing ion bombardment in the form of an ion beam. 18.2 ION BEAM SPUTTERING (IBS)
Direct ion beam sputtering of a sample is commonly used for moderate-rate etching and cleaning. Ion beams of the gridded, Kaufman type and the non-gridded Hall-effect type are charge neutralized, usually by hot filaments inserted in the beam. This reduces or eliminates the charging problems caused by positive ion bombardment of insulating or floating samples. Typical output levels for a Kaufman, gridded source are in the 0.1 to 5 mAl cm2 range at several hundred to 1500 eV. The etching rates of many materials at 500 eV ion energy and 1 mA/cm2 are shown in Table 1 (1). Most etch rates are in the 150 to 2300 A I min. range. An exception is graphite, which is about 50 AI min. As a result of the non-chenlical nature of inert-gas ion beam sputtering, there is poor selectivity be-
362
Methods and Techniques of Ion Beam Processes
363
tween most materials. When used with some sort of mask structure over the sample, this usually results mask erosion at a rate comparable to that of the sample. Approximate sputtering rates (A/min.) for inert gases at 500 eV and 1mAlcm2 with a 90 0 incidence angle (1, with permission).
Table 1.
C AI Si Ti
V Cr Fe Ni
Cu Zr
Mo
Ag Sn To
W
Pt Au
Xe
Kr
Ar
Ne
44.00 730.00 380.00 380.00 370.00 : 31 0.00 550.00 580.00 480.00 530.00 580.00 660.00 1,000.00 1,100.00 410.00 620.00 350.00 : 540.00 : 1,400.00 2,200.00 1,800.00 230.00 420.00 213.00 380.00 440.00: 880.00: 870.00 1,700.00 I
570.00 : 440.00
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
50.00 57.00 630.00 520.00 400.00 320.00 340.00 290.00 340.00 : 330.00 680.00 710.00 500.00 450.00 570.00 460.00 1,100.00 510.00 520.00 540.00 560.00 : 520.00 2,200.00 2,200.00 I
I
I
I
I
I
I
I
I
630.00 61 0.00 590.00 61 0.00 1,1 00.00 : 1,1 00.00 2,100.00 2,000.00 I
I
I
Etching through masks can result in the problem of a greater etch depth adjacent to the mask edge (known as "trenching"). This effect is due to glancing reflections of ions incident on the near-vertical sides of the mask opening. Another problem can be the deposition of material sputtered in an undesirable location (from the sample, for example on the near vertical side of the etch region.) Both of these problems are often avoided or controlled by moderate off-normal etching. The off-normal etching has been incorporated both throughout many etching steps, and also as a final clean-up step for normal incidence etching. For sanlples with mask openings aligned with several directions, the off-normal etching is often accomplished with a rotating or swash-plate motion. Reactive Ion Beam Etching utilizes some aspect of a chemical reaction along with the energetic ion bombardment. This topic was treated in detail in Chapter 12. The reactive species may be added to the ion beam and accelerated as an ion towards the etching surface. Alternative techniques use a neutral directed stream of the reactive species in combination with inert gas ion bombardment from the ion source.
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The object with reactive ion beam etching is usually: (1) to increase the etch rate by utilizing a chemical reaction that generates a volatile product, or (2) to increase the difference in etch rates between adjacent layers. In the latter case, it is desirable to have a significantly faster etch rate on a sample, as compared to a mask surface. Chlorine and fluorine are frequently used as the reactive gas species. To reduce gas handling hazards, the reactive species are often incorporated into chlorocarbon and fluorocarbon gases. Depending on the type of ion source and the reactive species used, adding the reactive gas to the ion beam (through the ion source) may result in substantial etching and damage to the ion source. A hot filament is typically present inside a Kaufman-type source. This filament may react rapidly with the reactive gas species. An alternative is to use a hollow cathode electron source in place of a filanlent. Hollow cathodes require a separate flow of inert gas (Ar, Kr, or Xe) for operation, which will dilute the reactive gas. A second alternative is to use a filament-less source, which relies on a high voltage dc discharge to cause sufficient ionization within the source (2). These sources may be susceptible to etching within the discharge chamber, potentially leading to contamination of the beam. They may also have a broader energy spread in the resultant beam.
ION SOURCE Figure 1: Configuration for inert or reactive ion beam etching.
----------.=---------=I
I I I I I I
I I I 'V 'V 'V 'V
ION BEAM
SAMPLE
Microwave and rf energy is also used to generate ions, and are alternatives to a dc discahrge. Microwave ECR ion sources are one form of microwave source. (See Chap. 3.) Both microwaves and radio frequency discharges are also being studied as means of avoiding hot filaments (in the Kaufnlan-type source). Alternative ion sources, based on microwave or rf discharges, will probably be available in the near future. The ECR source can be constructed of materials with low etching rates for the particular reactive species chosen. Due to the nature of the ECR source when used without grids, the ion beam generated has low energy and poor directionality. Work is continuing on various modifications to these sources to produce more uniform beams. One obvious modification is the addition of grids to the front of the ECR source. This is appropriate for some reactive gas species, but may result in high levels of grid erosion and sample contamination with other gas choices.
Methods and Techniques of Ion Beam Processes
365
18.2.1 Comparison to RF Sputtering
A large portion of the thin film industry relies on rf-diode sputtering for both substrate cleaning and material removal (etching). Ion beam sputtering, however, has clear advantages over rf techniques. Ion beam sputtering has a background pressure in the 0.05 to 0.5 mT range for most of the sources described in this book. (This pressure results from the required discharge chamber pressure, typically 0.5 mT, and the available pumping.) At those background pressures, atoms that are sputtered from the target or samples in the ion beam have a long mean-free path far gas-phase collisions. This distance is typically ten's of cm, and usually exceeds the physical dimensions of the chamber. The comparable rf-diode etching case, however, operates at chamber pressures of 10 to 200 mTorr. At these pressures, the sputtered atoms are scattered near the sample surface, and are thus quite likely to be redeposited back onto the etched surface. This can be a critical problem for complex device or packaging samples if the sample has large areas of insulating or polymer materials, which can then be redeposited onto nletallic surfaces. This redeposition results in increased contamination at the metal surface which will never be removed by additional sputtering in the rf-diode mode because it is continuailly being added. Ion beam techniques are characterized by independent control of the incident ion energy and flux. This is not possible with rf etching techniques, in which the energy and flux are directly coupled through the space-charge limited current flow at the sheath. In addition, as the chamber pressures are higher, there exists the possibility of collisions between the ions and gas atoms during the ion's transit of the sheath. This results in reduced ion energy as well as reduced anisotrophy of the sputtering. While this is not a particularly controllable process, many manufacturers of sputtering equipment use this effect to "claim" low energy bombardnlent processes. Low energy with broad beam sources is routinely achieved with either the Hall-effect, single grid Kaufman-type or ECR microwave sources without grids. Due to the controlled energy, flux, angle and low pressure of operation, ion beam processes are related more closely to the fundamental sputter yield and matrix effects than rf plasma sputtering processes. Another clear advantage of ion beam processing compared to rf-diode sputtering is the ability to sputter at non-nornlal incidence with ion beams. The acceleration of the ions across the sheath in a plasma-etching configuration can only occur in the direction normal to the sheath, hence normal to the etched surface. Ion beam sputtering has no such constraints, and therefore allows much greater process control. Other areas of comparison are in the plasma containment and shielding areas. A plasma tends to fill the chamber in which it is enclosed. Thus, rf systems experience moderate levels of wall-bombardment and desorption during the sputtering process. This can lead to contamination of the sample, either by wall material or by whatever was deposited or or adsorbed on the wall. Thus, this type of system is sensitive to vacuum openings and what material was last sputtered in the chamber. Due to these problems, rf techniques do not often transfer well from one machine to the next, and extensive trials are often needed to obtain similar etching results in a new nlachine. In addition, rf systems must be carefully shielded to reduce radiation of the rf potential. This means that all leads (heaters, thermocouples, etc) must be shielded or filtered. The gridded and nongridded dc ion sources discussed earlier in this book do not suffer fronl these problems.
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Handbook of Ion Beam Processing Technology
1 8.3 ION BEAM SPUnER DEPOSITION
As shown in Fig. 2, ion beam sputtering is commonly used for the deposition of thin films. While the resultant deposition rates are moderate (from a few up to hundreds of AI min) when compared to a process such as magnetron sputtering, many advantages can be realized by this technique. The usual design approach is to select the deposition configuration assuming a cosine distribution for the sputtered material for all portions of the target bombarded by the ion beam. Additional motion is then incorporated in the design to accommodate small departures from the ideal cosine distribution. If a texture is permitted to develop on the target during the sputtering process, (see, for example, Chap. 17) additional motion may be incorporated for the additional departure from the cosineemission distribution as the texture develops. The additional motion can be a small angular variation of the target relative to the ion source to compensate for the development of a texture. Or it can be a rotational or planetary nlovement of the sample holder to accomplish averaging during deposition. Finally, it can be a rotation (either continuous or stepwise between runs) of the target to reduce the tendency to develop a texture. One clear advantage of ion beam sputter deposition is the ability to sputter new compositions of materials without the fabrication expense of large sputtering targets. Alloy conlpositions can be adjusted sinlply by adding or renloving material from the target surface. (Note that an ion beam target is not electrically stressed, so one piece targets are not required, as in the case of magnetron or rf-diode sputtering.) This ease of making targets is also a clear advantage for the sputter deposition of alloys or compounds which are not stable in bulk form or cannot be easily fabricated into a sputtering target. A second advantage is that the samples are not immersed in a dense plasma, as would be the case in various plasma-based sputtering systems. This absebce of a plasma will reduce damage levels in sensitive devices. A related advantage is the absence of negative ion formation with ion beanl. sputtering. In several cases of sputtering from compound targets, or even from elemental targets in the presence of oxygen, negative ions may be formed at the target surface. In the diode, plasma sputtering case, these ions are accelerated by the target sheath and directed at the sanlple at high energy. The ions become charge neutralized in the plasma, and impact the growing film at high energy. This bombardment can cause significant changes in the net deposition rate, as well as the chemical composition of the film. Due to the very low sheath voltage at an ion beam target, the negative ion problem is effectively elinlinated.
[========J I
I
I
ION SOURCE
I
1111,LE
TARGE~
T~IN FILM
Figure 2: Ion beam sputter deposition.
Methods and Techniques of Ion Beam Processes
367
A fourth advantage is the ease of sputtering magnetic materials. The magnetization of the target is not critical to ion beam sputter deposition. In the case of magnetron sputtering, a target made of a magnetic material will tend to shunt the magnetic field and significantly reduce the plasma density. While techniques are available for magnetron sputtering of magnetic materials, in general the problem remains awkward. The types of ion sources described in this book all tend to operate at sufficiently low pressure (~ 0.5 mTorr) that they can be operated independently of each other. That is, several ion sources can be operated side-by-side in the same chamber with little interaction between sources. This is difficult at best with magnetrons. Multiple ion sources and targets may therrfre be used to construct alloy or multilayer films. For example, several systems have been developed which use four separate Kaufman-type ion sources. Three of the sources are used to independently sputter-deposit material from separate targets, and the fourth source is directed at the sample for cleaning and ion-assisted deposition (see Sec. 18.4). Ion beams are quite useful for the reactive deposition of compound films, such as oxides or nitrides. The general technique (for a single ion source system) is to sputter a metallic target (e.g. Al or Ti) with an inert-gas based ion beam and bleed in a sufficient background pressure of the reactive gas (e.g. O 2) into the chamber to cause the compound to form at the substrate surface. An alternative technique is to sputter from a conlpound target with a lower level of the reactive gas species in the chamber. This is done less frequently than the elemental sputtering case, because the sputter yield for the compound can be as low as 5 % of the yield for the elemental target. Generally the background pressures needed for sufficient oxidation of the film, for example, are in the 10- 5 Torr range. This low pressure results in little contamination of the target and little degradation of the ion source. As a comparison, the deposition of compound oxide films by means of magnetron sputtering usually results in the transition of the cathode from a metallic mode to an oxide mode at sufficient background pressures of the reactive gas to form good films. This transition is accompanied by a very significant reduction in deposition rate (by a factor of 4 to 25). The low operating pressure of broad beanl ion sources results in little gas scattering of the sputtered atoms during the transit from the target to the sample. Thus, the kinetic energy of the deposited atoms is not reduced, and may average 5-20 eV per atom, depending on the ion energy, sputtering geometry, and gas and target species. This additional kinetic energy, when compared to evaporation or high pressure sputtering, will cause significant changes in the properties of the deposited films. Often the grain size is smaller than comparable evaporated films, and there may be some degree of preferred orientation. The films are also generally denser with less of a columnar structure, and other properties, such as the intrinsic stress, the electrical and optical properties, and the adhesion to the substrate nlay all be nlodified. In addition, the low pressure allows a significant fraction of the ions that are elastically reflected from the target surface (and Auger neutralized) to impact the growing film. This may also cause changes in film properties, as well as sputter the deposited filnl, resulting and a modified deposition rate and composition profile (3). One particular process application makes use of the low levels of gas scattering present in ion beam experiments. By using a focused ion source and hence a small sputtering area, it is possible to deposit sputtered films in a mode compatible with lift-off, or
368
Handbook of Ion Beam Processing Technology
photoresist technology (Fig. 3). The low levels of gas scattering, and hence the line-ofsight deposition, allow the use of conventional lift-off (photoresist masking) techniques used routinely for evaporation-based depositions. The superior film qualities described above can be important in such applications. In addition, the ion beam technique allows considerable latitude in the deposition of alloys with good film properties. Evaporation fronl nlore than one source, while routinely accomplished in many laboratories, has proven to be difficult in terms of controlling alloy composition over large areas and on a run-to-run basis in production. Perhaps the improvements in ion source reliability and serviceability discussed in Chap. 2 will make multi-source production applications more practical in the future.
,/
FOCUSED
-_
-
" '/"
....... ............... ........
"---
I I
\ \
I
I
\
I
I
\
I
I \
I
I
\ \ \ \ \
I I I I I
Figure 3: Ion beam sputter deposition using focused ion source for lift-off deposition
-
I
I \
I I I I I I
TARGE~ Bea~
ION SOURCE
focused to 1
\ c~
LIFT-OFF SUBSTRATE
2
area
Practical deposition problems often involve contamination of the deposited film. If the ion source is too close to the target, contamination from the ion source may be deposited on the target, and subsequently sputtered off and deposited along with the film. Poor ion source operation can aggravate this contanlination problem. For example, as described in Chap. 2, excessive discharge and accelerator voltages can greatly increase contamination from the ion source. Contamination from other hardware in the vacuum system can also be a problem. Any surfaces that appear unusually clean may be clean because they are being bombarded by beam ions, and therefore be a source of contamination. If two-grid optics are used (on the ion source) and they require periodic alignment during maintenance, contamination due to nlisaligned grids can be a recurring problem. The solution for misaligned grids is either to go to a two-grid design that provides alignment automatically, or to institute a routine procedure to check alignment (Chap. 2).
1 8.4 ION BEAM ASSISTED DEPOSITION (IBAD)
Due to the low operating pressure of the various broad-beam ion sources, it has been possible to cOlnbine ion beanl bombardnlent and sputtering with evaporative deposition.
Methods and Techniques of Ion Beam Processes
369
Depending somewhat on the configuration, the two techniques can operate independently in the same chamber. A general schematic is shown in Fig. 4. The means of evaporation is not critical, but e-guns have been used most frequently. Ion bombardment in this type of arrangement has three basic functions. First, the ion source can be used to etch or clean the sample surface prior to a deposition. This etching may be necessary to renlove a particular layer on the sample surface, or may simply be used to sputter-clean atmospheric contamination. The second general mode of operation is to use the ion source concurrently with the evaporation to bombard the growing filnl with energetic (inert) ions. This has been found to cause great improvement in certain film properties (see Chapters 10, 11, 13, 19). The third mode of operation is to use the ion bombardment during deposition in a reactive mode. In this case, the ion beam is composed of a species which will react with the evaporated species at the surface of the sample. This is routinely used for the deposition of dielectric films of high optical quality and stability (see next chapter). In this third case, the ion source may be susceptible to accelerated wear due to the reactive species.
SAMPLE '- THIN FILM
/1"-
/f'\.
I I
I I ,
I
,
I , I
, I I
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I
\
\ \ \
\
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Figure 4: Ion Beam Assisted Deposition (IBAD).
The principle advantage of lAD is that the operator has control over the relative arrival rates of each species at the sample surface, as well as control over the ion energy. This is quite unlike a plasma-based reactive deposition, where the flux and energy of the energetic species ar e strongly coupled, and the arrival rate of the reactive species is affected by the deposition rate and gettering effects on the walls.
370
Handbook of Ion Beam Processing Technology
Each of the ion sources described in earlier chapters has ranges of output currents and energies. The 2-gridded Kaufnlan source is nlore suitable for higher energy (hundreds of eV) and high flux. These sources operate best with inert gases. However, such sources are routinely used in oxygen for the deposition and modification of dielectric films (Chap. 19). The single-gridded Kaufnlan source has high output current densities at low energy (20-150 eV). In a reactive environment with O 2 and N 2 , the single grid can last 3-4 times longer than with Ar. This increased durability is due, apparently, to the formation of lower sputter yield compounds (oxides or nitrides) on the grid surface. These compounds are not present when operating the source with inert gases. The gridless, end-Hall ion source is quite useful for high current, large area, low energy bombardment. This is most appropriate for reactive deposition or bonlbardment-during-deposition cases. This source has only a single filament, which is easily accessible on the front of the source, and is thus more compatible with reactive species than the conventional Kaufman-type sources. The microwave ECR sources are the most compatible with reactive gas species. However, without some nleans of acceleration (grids, for example), the ion energy is low and the ion beam somewhat dispersed. There are practical limits as to how close any of these ion sources can be located relative to other devices in the vacuum chamber. If the evaporation source is a simple oven or boat, a line-of-sight shield should be mounted between the vapor source and the ion source to prevent excessive contamination of the ion source. If the vapor source is an e-beam evaporation source, the large magnetic field and currents of the vapor source can adversely affect ion source operation at distances up to 20-30 em. In addition, overly large evaporation sources will provide a large heating flux to the ion source, causing potential damage to cable connections (often spring-loaded) or to permanent magnets. 18.5 DUAL ION BEAM SPUTTERING (DIBS)
This technique replaces the evaporative material source with ion beam sputter deposition from another ion source, as shown in Fig. 5. (4,5) The deposition technique has been described above (Fig. 2 ). The second ion source has typically been of the singleor 2-gridded Kaufman type, although there is no reason why either the Hall effect or ECR
ION SOURCE 1
..-=---------=-
Figure 5: Dual Ion Beam Sputtering.
ION SOURCE 2
o 'J./ ~
TARGET
Methods and Techniques of Ion Beam Processes
371
source could not be used. In nlany cases, the two ion beanls pass through each other in this deposition mode. The fluxes and cross-sections are low enough that there is little if any interaction. The more usual problem is the selection of a poor geometry, such as one where the beam of one source can strike the body of the other source.
18.6 ION ASSISTED BOMBARDMENT: OTHER TECHNIQUES 18.6.1 Ionized Cluster Beam
This technique utilizes a beam of charged and neutral clusters that originate from a thermal source. A more complete discussion of this technique has been given earlier in Chap. 5. Clusters are formed in the nozzle region of an enclosed, heated crucible. These clusters are composed of a few hundred atoms, and have nominally a few tens to 100 or so eV of kinetic energy before acceleration (equivalent to a fraction of an eV per atom). Some fraction of the clusters can be ionized in-flight by bombardment with electrons at several hundred eV. The clusters are usually singly ionized. The ionized clusters can then be accelerated to strike the growing film at the sample surface. The net kinetic energy of the cluster can be increased in this manner to several thousand eV. This is equivalent to increasing the average energy-per-deposited atom up to 20 or so eV. The relatively high voltages used (several keY) and the low currents of charged clusters (micro-to-milliamps) elinlinates space-charge linlitations commonly found with low energy beams. The properties of the films deposited with this technique depend dramatically on the degree of ionization of the clusters and the acceleration energy. Some of these results are described in Chap. 5. A more comprehensive discussion of the deposited film properties has also recently been published (6). 18.6.2 Hollow Cathode Magnetron Techniques
Recently, modifications of magnetron sputtering systems have been published which allow some degree of ion beam bombardment during deposition (7). The technique is based on the use of a triode discharge, in which an auxiliary source of electrons is coupled to a magnetron cathode. Hollow cathode electron sources have been used for their ruggedness in high magnetic fields and high output levels. The hollow cathode is inserted into the fringe field of a planar magnetron, near the front cathode surface (Fig. 6). The hollow cathode is started and biased sufficiently below the local plasma potential (approximately 20-40 V in a de mode) so that several amperes of electrons are emitted into the magnetron cathode region. These electrons cause additional ionization, and can allow operation in the magnetron mode at pressures as low as the high 10 5 Torr range. At these low pressures, the nlagnetron is conlpatible with the broad-beam Kaufman ion source (Fig. 6). The ion source can then be used, much as described in earlier discussions using evaporation or ion beam sputtering, to bombard the growing film in a controllable manner. 18.7 SUMMARY
Ion beam techniques have clear advantages over plasma-based sputtering, sputter deposition and reactive processing. Ion beams are generally characterized by independent control of ion energy and ion flux. In addition, ion beam sources typically operate at pressures well below other plasma processes, resulting in less scattering of the atoms and
372
Handbook of Ion Beam Processing Technology
the ability to operate multiple processes side-by-side in the vacuum chamber. This chapter has provided a brief look at some of the nlodes of operation and the practical processing techniques utilizing broad beam ion sources.
MAGNETRON
ION SOURCE
-r----~ «-
HOllOW CATHODE
,.. . . . . . . .".,...., ,
','
~
~
"'"
/
SAMPLES
Figure 6: Hollow cathode enhanced planar magnetron with conventional Kaufman-type ion source for bOInbardment of the sample during deposition.
18.8 REFERENCES
1.
H.R. Kaufman and R.S. Robinson, Operation of Broad Beam Sources (Commonwealth Scientific, Alexandria, VA, 1987).
2.
Annatech. Ltd., 5510 Vine St. Alexandria, VA, USA.
3.
E. Kay, F. Parmigiani and W. Parrish, J. Vac. Sci. Technol. A5: 44 (1987).
4.
C. Weissmantle, Ion beam deposition of special film structures. J. Vac. Sci. Technol. 18: 179-184 (1981).
5.
See, for example, J.J. Cuomo in Physics Today, May 1980.
6.
T. Takagi, Ionized Cluster Beanl Deposition and Epitaxy (Noyes Publications, Park Ridge, NJ 1988).
7.
J.J. Cuomo and S.M. Rossnagel, Hollow Cathode Enhanced Magnetron Sputtering. J. Vac. Sci. Technol. A4: 393 (1986).
19
lon-Assisted Dielectric and Optical Coatings
Phil J. Martin and Roger P. Netterfield
19.1 INTRODUCTION
Vacuum deposited thin films have been employed to modify the optical properties of precision optical components for nearly half a century. It was soon realized that the limiting performance of an optical device was determined by the quality of the thin film coating, which in turn was ultimately dependent upon the deposition conditions. Thin films rarely attain the desirable bulk material properties of density, composition, crystal structure and optical properties, and the quest to close the gap between film and bulk properties is a source of intense international research. The total world market for thin film products for optical purposes has been estimated to be 500 M$ covering traditional multilayer antireflection coatings, laser mirrors, beanI splitters etc. through to coatings for optical data storage (1). The most significant improvements in optical thin film properties have resulted from the introduction of the ion-assisted deposition process. Film properties are more predictable and the stability of the optical performance is greatly enhanced when optical dielectric materials are deposited by ion-based techniques (2) (3). The main ion-based techniques will be described and the properties of the principal optical materials reviewed. These materials include dielectric oxides, fluorides, transparent conductors, and nitrides. 19.2 MICROSTRUCTURE OF THIN FILMS
Thin filnIs deposited by physical vapor deposition techniques frequently have properties dissimilar to those of bulk materials. Among such properties the most important are composition, stoichiometry, defect-density and grain size. The most distinguishing feature of some films is the occurrence of disordered low-density voided regions interspersed throughout the film thickness. A voided film microstructure is largely responsible for the performance gap in optical properties between bulk and thin film material. Thin film microstructure is influenced by the nlaterial itself and by such deposition parameters as substrate temperature, residual gas pressure and angle of incidence.
373
374
Handbook of Ion Beam Processing Technology
Thin film structures were qualitatively classified by Movchan and Demchishin (4) for thick metal and oxide deposits. The model proposed that film structure can be divided into three structural zones, each of which is determined by the substrate temperature T and the film material melting point T m. Zone I (TIT m < 0.25-0.3) contained tapered columns with domed tops which formed due to low adatom mobility. The second region, Zone II (0.25-0.3 < TIT m < 0.45) comprised of a region of smooth-topped granular structures, while Zone III (TIT m > 0.45) is formed from equiaxial crystallites having a polyhedral structure. This simple zone model has been useful in classifying the nlain structural features of evaporated films, and has been extended to include sputtering conditions by Thornton (5), who added a third axis to accommodate variations in working gas pressures. The model, shown in Fig. 1, shows schematically the interplay between structure, gas pressure and substrate temperature. The general observation is that an open grain boundary microstructure dominates when adatom diffusion is insufficient to overcome shadowing effects, while a closed-type structure results from surface and volume recrystallization. COWNNAR GRAINS
I'
Figure 1: The three dinlensional zone structure model of Thornton (5).
Messier (6) has attenlpted to describe the physical structure of films in terms of an evolutionary growth model. Figure 2 shows film growth as the self-organization of structures resulting from the competition for maximum cone growth. The conical columns evolve through growth-death competition: their density being determined by the size distribution of nucleating clusters on the substrate. The model assumes that the process continues with increasing film thickness. More sophisticated computer models have been used to simulate thin film growth as an atom-by-atom event and the columnar nature of thin film growth is readily observed (see Chap. 13). Unfortunately, the basic simulated columnar unit is generally only some 10 atoms wide, Le. much snlaller than those observed in optical films by electron microscopy. A solution to this problem was suggested by Messier (6) who made careful observations, by several techniques, of the microstructure of a-Ge. The basic structural unit resolved by electron microscopy was found to be a few nanometers across. As the
lon-Assisted Dielectric and Optical Coatings
375
film grows the dendritic columns cluster together into larger groups. The voids within a group are lost and larger voids appear between the clustered columns. As the film grows the process is repeated and a larger voided columnar film evolves. Figure 3 shows the resulting structure observed for various film thicknesses as revealed by field-ionmicroscopy (FIM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
2: Growth-death competition in conical column evolution (6).
Figure
Figure 3: Three levels of microstructure in a-Ge (6).
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Handbook of Ion Beam Processing Technology
Apart from columnar microstructure, thin films are also frequently found to contain nodular defects. Guenther (7) has studied these defects in some detail and defined them as isolated structures protruding from the thin film surface in a domelike manner when observed in high resolution optical microscopy or SEM. The nodules appear as inverted cones which extend throughout the whole single or nlultilayer film stack. Such defects frequently separate from the layer leaving a hole behind and pose a limitation to the performance of the films. The origins of nodular growth can be traced in this case to at least three sources. (1) Defects on the original substrate resulting fronl surface roughness or polishing residue etc; (2) coating spatters from the electron-beam evaporator; and (3) excessive oxygen flux impinging on the growing film.
19.2.1 Microstructure and Optical Properties
A useful thin-film parameter related to the microstructure, and which takes account of the voids, is the packing density p. This is a relative measure of film density expressed in terms of that for the ideal bulk material. p is defined as
p
=
Volume of the solid part of the film (Le. columns) Total volume of film (i.e. columns plus voids)
Values of p for most evaporated films lie in the range 0.7 - 0.95. (8). If a thin film contains voids, then its optical refractive index n will differ from that of the bulk material. There have been several attempts at relating the refractive index nr of dielectric films to packing density. The three dominant theories are those of MaxwellGarnett, Bragg and Pippard and Kinoshita and Nishibori. (9) According to Maxwell-Garnett 2
(1 - p)(ns
+ 2)nv2 + p(nv2 + 2)ns2 2
(1 - p)(ns
+ 2) + p(nv2 + 2)
(1)
where n s is the index of the solid material of the film (columns) and n v the index of the voids in the filnl. The Bragg and Pippard relation is 2
(1 - p)n v (1
+ (1
2 2
- p)n v n s
+ p)nv2 + (1
2
(2)
- p)ns
which assumes the film particles to be cylindrical. The linear relationship given by Kinoshita and Nishibori, although empirical, is the nlost convenient to use and is given by
(3)
lon-Assisted Dielectric and Optical Coatings
377
These models have been investigated in detail by Harris et al (10). Their general conclusion was that the linear model was useful for low index materials but poor for high index films, while the Bragg-Pippard expression is suitable for low to intermediate packing densities. The main difficulty in testing these relations is the limited range of packing densities that can be achieved with optical materials deposited by conventional evaporative techniques. 1.0 Z
0
i=
( a)
u
u.
C)
Z ~
U
a.
50
100
150
200
250
300
250
300
SUBSTRATE TEMP.(e) 2.2
(b)
100
150
200
Td{C)
Figure 4: (a) Packing fraction as a function of substrate temperature (11). Refractive index as a function of substrate temperature (11).
(b)
The importance of the packing density-temperature dependence is illustrated in Fig. 4(a). Here the packing density is plotted as function of substrate temperature for a range of optical materials (11). It can be seen that CaF2 for example, has a very low packing density (only 0.6 at 50°C) which cannot be raised to unity for substrate tenlperatures up to 300° C. The situation is better for other materials, but unity packing density is still not achievable. In terms of the refractive indices, Fig. 4(b), shows that significant variations in index occur, e.g. for Zr02 at 50°C an index of 1.8 is measured and this rises to around 2.15 at 300°C. The different behavior of each material with substrate temperature renders accurate optical multilayer deposition difficult at best. Furthermore, there exists a complex interplay between the deposition parameters and the film properties.
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Handbook of Ion Beam Processing Technology
This is illustrated by Table 1 compiled by Ritter (12). Two dots represent a strong dependency of filnl properties on deposition conditions, one dot an established dependency, and a dot in parenthesis indicates a possible dependence. The difficulties are enhanced when a single process parameter influences more than one film property. The principal ainl in optical thin film deposition is then to reduce the number of process parameters and/or control them to achieve film reproducibility. Table 1: The Influence of Deposition Process Parameters on Film Properties (Ritter
(12)). Film property
Substrate material
Substrate cleaning
•
Refractive index Transmission Scattering
••
Geometric thickness Stress Adherence Hardness Temperature stability
•• • • •
Insolubility
e·)
Resistance to laser radiation
e.)
Defects
Starting material
••
• • ee)
•• • • • •
Glow discharge
•
• •
(.)
(.)
• ••
• • •
Evaporation method
Rate
Pressure
•• •• • •
•• •
•
• • •
• ••
(.) (.)
• • • • • • • •
..
Vapour
••
(.)
• • • •• • ••
(.)
Substrate tempera lure
•• (.)
• • • • • •
•• •• •
•
•• • •
e
•• ••
19.3 EFFECTS OF ION BOMBARDMENT ON FILM PROPERTIES
When an energetic particle is incident upon a solid surface, a number of complex processes occur simultaneously. Energetic incoming ions transfer momentum, charge and energy to the developing film, and this is likely to influence the fundamental processes involved in film formation. Among the dominant ion-surface interaction processes are sputtering, implantation, ionreflection and trapping. Basic ion-surface interaction phenomena have been extensively reviewed by many authors, aspects of which are covered elsewhere in this book. In this section however, we give only a brief summary of the role of ions in film deposition technology and their influence on film properties. 1 9.3. 1 Microstructure
Mattox and Komniak (13) demonstrated that ion bombardnlent from a plasma during the deposition of tantalum films could interrupt columnar growth with the result that film density rises close to that of the bulk material. The crystallite size was also decreased with ion bombardment. The measurements were made during planar dc sputtering, in which the substrate was biased negative to attract ions. A similar experiment, performed by
lon-Assisted Dielectric and Optical Coatings
379
Bunshah and Juntz (14) using electron-beam evaporation, showed that a negative bias on the substrate refined the film grain structure. More controlled experiments are possible with the introduction of a monoenergetic ion source into the vacuum deposition chamber. In the case of metallic film deposition, it was quickly recognized that ion irradiaton induced films with preferred orientation. Dobrev and Marinov (15) published several reports on the effects of 1-10 keY argon-ion bombardment on the growth of silver, gold, cadmium and cobalt films. Ion bombardment was found to enhance the surface mobility of adatoms and clusters, and also to accelerate nucleation. Other studies on the condensation of Zn and Sb under ion bombardment confirmed these earlier observations.
Figure 5: The influence of ion bombardment on the structure of magnetron sputter deposited TiN films (a) No ions, (b) Ion bombardment during deposition, i.e. biased deposition (16).
Modification of columnar growth by ion bombardment during deposition is most strikingly illustrated in Fig. 5 (16). Films of TiN were deposited to a thickness of about 3 nm by magnetron sputtering both with and without substrate bias. In the case of optical thin films direct observation of microstructure modification has proved to be more difficult, and the evidence for densification is indirect. The densification effect in Zr02 prepared by evaporation and ion-assisted deposition (17) was inferred from measurements
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Handbook of Ion Beam Processing Technology
of the spectral transmittance curves of layers immediately after deposition and on venting to a humid atmosphere. The results shown in Fig. 6 indicate a shift in the transmittance curve for the evaporated layer and no detectable change in the oxygen-assisted film. The interpretation is that water vapor from the atmosphere is taken up by capillary action in the evaporated film due to the columnar structure. This results in a modification of the film refractive index n as described previously. In the absence of columnar growth, film properties remain stable upon exposure to the atmosphere and therefore no effect is seen in the oxygen-assisted films. Further evidence of water vapor penetration was provided by nuclear reaction analysis (17). Hydrogen (from adsorbed water vapor) was measured in evaporated and in ion-assisted ZrO z films as a function of depth. The ion-assisted layer showed a dramatic reduction in hydrogen content which was only slightly above the background level at the surface of the filnl.
NO IONS
0.9
w
\
zoct
( a)
\
\
o
\ \
~
~
(J)
z
oct
...a:
0.8
Figure 6: 0.7L....-_.....1--_----L_ _. L . . . - _ - - I . - _ - - - - I o _ - - ' " 400
500
600
700
WAVELENGTH (nm)
1.0r----r------r--,----r-----,---,
w o z oct
(b)
l:
0.9 vacuum/air
~
(J)
z oct a:
...
0.7L.-_...J.....-_----L_ _. L . . . . . - _ - - L - . - - - - - - I . . - - J 700 600 400 500 WAVELENGTH (nm)
Spectral transmittance of ZrOz films deposited, (a) in the absence of ion bombardment, (b) under 0t ion bombardment. Changes in the vacuum and air measurements indicate film porosity (1 7).
lon-Assisted Dielectric and Optical Coatings
381
19.3.2 Adhesion and Stress
Film adhesion to a surface is a critical parameter in optical applications and in general ion bombardment of the substrate prior to deposition and during the early stages of film growth can be expected to result in an improvement in adhesion. Surface contaminants remaining from the cleaning processes and loosely bonded surface adatoms can be sputtered provided the incident energy is high enough. In the case of sputter deposition the average energy of the depositing atoms is between 10 and 20 times that of thermally evaporated atoms such that the adhesion of sputtered films is intrinsically higher, particularly in the case of gold films (18). The adhesion of gold to a substrate can be improved by means of a glow-discharge in oxygen prior to deposition (19). The precise nature of the role played by ion bombardment in enhancing adhesion is not clear but several regimes are evident from the various ion energies used in practice. In the case of low-energy oxygen ion bOlnbardment (0.1 - 1 keY), recent studies show that the greatly increased adhesion of gold to silica can be attributed to an increase in the area of contact between the gold film and its substrate. Film nucleation is increased and coalescence accelerated. Enhanced wetting of the substrate by the film is thought to be related gold oxygen and/or gold-oxygen-silicon bonding at the substrate-film interface. Such bonding is improved by oxygen ion bombardment. Monolayer fornlation of stable gold oxides at the interface has also been considered possible. As the incident ion energy is raised, other regimes of film bonding become dominant. For energies of 10 keY and above atomic mixing of the substrate and film atoms occurs. Diffusion of the film atoms into the substrate is also enhanced due to the creation of lattice defects by the incident ions. The term dynamic recoil mixing (DRM) is generally reserved for the technique in which a depositing flux of sputtered film atoms is simultaneously bombarded by a second ion beam whose energy is 10 keY or more, and the current density of 10A/mz. During bombardnlent, the conditions are adjusted to maintain a dynamic balance between resputtering of the film and deposition. A mixing process then occurs at the substrate-film interface leading to enhanced bonding. DRM has also been shown to induce silicide formation in gold on silicon; 30 keY Ar ion bombardment of a 30-nnl thick gold film resulted in the formation of the metastable amorphous silicide AU76Siz4. Other silicides have been observed with 200-300 keY ion bombardment, the principal phases being AusSiz , AusSi, AUlOSi3 and AU3Si (18). Post-irradiation of deposited films has also been shown to enhance adhesion when the incident ion energies are in the MeV region. The energies studied to date vary from 0.1-21 MeV and the species may be inert or reactive gas ions. The films are effectively "stitched" to the substrate by the ion beam. The mechanism is thought to be due to a high-temperature electron spike forming around the track of each ion as it penetrates the substrate, stitching the film at the interface (20). Ion bombardment also influences film stress. Early experinlents by Hirsch and Varga (21) found that both the adhesion and stress of germanium films were positively influenced during argon-ion assisted deposition. A critical ion density was determined for maximum effect and related to ion-induced thermal spike effects. The stress in Nb films has also been modified from tensile to compressive by ion assisted deposition when the
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substrate temperature was raised to 400 0 C and a sufficiently high argon ion flux directed at the growing film (22). 19.3.3 Compound Synthesis
Although the properties of thin films are strongly dependent upon nlicrostructure, film stoichiometry also plays a crucial role in optical performance. Often, dielectric oxide films are deposited by electron-beam evaporation in a background pressure of oxygen in order to compensate for any oxygen depletion during the evaporation of the bulk oxide material. Pranevicious (23) showed that by evaporating aluminum and silicon monoxide at a constant rate of between 0.5 to 2 nm/s, while bombarding with 5 keV oxygen ions, Al2 0 3 and Si0 2 were formed at doses of 1021 and 1023 ions cm- 3 respectively. The refractive varying the ion dose, and indices of 1.85 and 1.46 were achieved with a zero ion dose and 1023 ions cm- 3 respectively. Aluminum nitride has also been synthesized by several groups using nitrogen ion-assisted deposition. Grigorov and Martev (24) demonstrated that continuous bombardment of a growing film of Ti with reactive or inert gas ions in a reactive-gas atmosphere can stimulate a surface chemical reaction and increase the gettering action of the film. The capture coefficient and sorption ratio of titanium films was increased sevenfold by 1 keV Ar+ irradiation. Titanium nitride films have been successfully synthesized by this process. Oxide, nitride, oxynitride and carbide films have been synthesized by many groups using ion-assisted processes. Variations in results are mainly traced to ion energies and ion fluxes which influence crystal orientation, grain sizes and stoichiometry. The optical properties of such films will be discussed in section 19.5. 19.3.4 Crystal Structure and Stoichiometry
The structural state of a surface is modified by the impact of sufficiently energetic particles. Naguib and Kelly (25) have found a correlation between the ratio Tc/T m' and the behavior of the surface under impact where T c is the crystallization temperature and T the melting point of the material. When Tc/T m is less than 0.3 the surface either renlains or becomes crystalline, and when greater than 0.3 remains or beconles amorphous. The model has been successfully applied to all published results, and Table 2 lists the data for a number of optical materials. Ion-assisted films of Zr0 2 have been examined in some detail. (1 7) Films deposited at room temperature without ion bonlbardment show no X-ray diffraction lines. When deposited onto heated substrates the monoclinic phase is found. However, with ionassisted deposition, the cubic phase of Zr02 is identified only when the ion: atom arrival rate is greater than 1 to 75. The mechanisnl of crystallization is not yet clear but it nlay be the result of temperature or displacement-spike effects. The reduction of compound films under ion impact, though studied extensively by several authors, has yielded conflicting results (26). In the case of Ti02 , however, there is general agreement that ion bombardment causes a reduction to a lower oxide phase.
lon-Assisted Dielectric and Optical Coatings
383
Table 2: Crystal structure and stoichiometry of optical materials following ion bombardment. Amorphous (Am), Crystalline (Cr), Stoichiometric (St), temperature of crystallization (Tc )' melting point (T m ) after Naguib and Kelly (25).
Material
Crystal structure
Tc/T m
Structure following ion impact
Si0 2 Al2 0 3 Al2 0 3 Ti0 2 TiO Ti2 0 3 Zr0 2 Nb 2 0 s Ta2 0 S ZnS ZnSe
Hexagonal Hexagonal Cubic Tetragonal Cubic Hexagonal Cubic Monoclinic Tetragonal Hexagonal Hexagonal
0.57 0.43
Am Am Cr Am,St Cr Cr Cr Am,St Am,St Cr Cr
0.35
0.27 0.42-0.49 0.38-0.46
19.3.5 Scattering
Thin films produced by ion beam techniques have been shown to have reduced optical scattering. The most notable example is in high-reflectance coatings for use in ring-laser gyroscopes where losses of less than 10 ppm have been reported for multilayer films produced by ion-beam (27) and RF magnetron (28) sputtering. Ion-assisted deposition has also been shown to reduce the optical scattering from surfaces. AI-Jumaily et al. (29) exanlined these effects for nletal (Cu and Mo) and dielectric (Si0 2 and Ti02 ) lAD films.
J=OJLA/cm 2 J=15JLA/cm 2
0.2
0.4
0.6
0.8
1.0
1.2
1.4
SPATIAL FREQUENCY,um-l Figure 7: Power spectral density of unbombarded and ion-assisted Ti02 films (29).
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Handbook of Ion Beam Processing Technology
Measurements of the scattering-power spectral density versus spatial frequency show that the scattering from lAD Cu coated Cu substrates is less than for the bare substrate particularly at higher spatial frequencies. Si02 films increase scattering from a Si substrate in contrast to that for lAD Ti02 layers. Figure 7 shows a comparison between the power spectral density versus spatial frequency curves for evaporated and lAD Ti0 2 samples. It was presumed that this difference is the result of a suppression by the ion bombardment of low spatial frequency microstructure. Attempts at correlating these optical scattering measurements with microroughness using a surface profilometer (Talystep) were inconclusive, but this may have been due to film crystallite size of the film being much smaller than the lateral resolution of the Talystep stylus (30). McNally et al. (31) have made similar measurements for Ta20S and A120 3 • As a general rule sputtered films produce less scatter than evaporated films with lAD reducing the effect even further. These results are consistent with increasing adatonl mobility. 19.3.6 Optimum Parameters for lon-Assisted Film Deposition
A major challenge in any theoretical study of ion-assisted film deposition is the prediction of optimum conditions to achieve the desired film properties. Muller (32) has made use of the TRIM.CAS computer code (33) and developed a dynamical Monte Carlo lAD growth model.
r_:-:--'"t._- ..-
0.02
,.~
I
,.1
N
~
o -0.04 ~
-0.08
o
2
4 OEPTH
6
(nm)
Figure 8: The distributions M o , M Zr and 10 as a function of depth for 600 eV 0+ bombardment of a Zr02 film (32).
This code provides all the necessary details regarding the ion-surface interaction processes of sputtering, reflection, trapping, etc. The vacancy and interstitial distributions near the surface of the growing film are calculated and the ion trapping probability conlputed. Figure 8 shows the average atomic rearrangement distributions M zr and M o for Zr and atoms respectively, and the ion trapping probability for a film of Zr02 bombarded with 600 eV 0+ ions. The M o and M zr distributions indicate a reduction at the surface (due to sputtering and forward recoils) and increase below the surface (as a result of recoiled surface atoms). The complete model (discussed in Chap. 13) may be regarded as a sequential process in which (a) a film is deposited with a reduced density, (b) the surface
°
lon-Assisted Dielectric and Optical Coatings
385
is depleted and deeper layers densified by ion-surface interactions, and (c) the depleted surface layer is filled by the incoming vapor (Fig. 9). The process is then repeated for the growth of the film. The agreement with experimental data is good in the case of Zr02 and Ce02 oxygen ion-assisted film growth.
(a)
NO ION BOMBARDMENT
t=0 Figure 9: Surface depletion by ion impact and subsequent refilling by incoming vapor. Sequential lAD model due to Muller (32).
>t-
ooZ
UJ
o
REFILLING (c)
DEPTH X
5.2
(')
•
EXPERIMENT
0
THEORY
Figure 10: Theoretical
and experimental data for the densification of lAD Zr02 films. Density is plotted as a function of the ratio of ion-to-vapor flux II/l v
5.0
E 0
.......
~ Q.
?/
4.8
(32).
600eV
4.6
0+- - . Zr02 (growing) (XI
4.4 0.1
0.2
0.3
=30° 0.4
0.5
J1/J v
Figure 10 shows the experimental and theoretical data for film density as a function of ion-to-vapor arrival rate ratio. Good agreement with experimental data is also seen in
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Handbook of Ion Beam Processing Technology
Fig. 11 for the influence of ion energy on Ce0 2 film density. The film density is low for low energies since the cross section for forward recoils and sputtering is low. An optimum value at about 150 eV is reached above which the ion energy is sufficiently high that vacancies are created below the surface and not accessible to the incoming vapor stream. The refilling depth of the depositing vapor is between one and two atomic diameters. Recent molecular dynamics calculations confirm the basic findings of the Monte Carlo model (34).
7 O+~
C')
Ce O
2
Figure 11: The density of lAD Ce02 film as a function of ion-to-vapor ratio, jI/jv, for different energies compared with theoretical calculations (32). The theoretical calculations are shown by the solid lines.
6
E
0
......
A 25 eV
~
>~ Ci5 zw 5
0 150 0 600
eV eV
0
4
0.2
0.6
1.0
1.4
10N-TO-VAPOUR FLUX RATIO J1/J v
Carter and Armour (35) have developed an approxinlate analysis of the formation of a binary compound by lAD. Their analysis shows that the net rate of deposition of a film comprising of a flux J A of A (atoms) and JBof B (ions) is given by (4)
where Sand yare the condensation (sticking) and trapping probabilities of the two species A and B respectively, nA and nB their surface concentrations and YA and YB their respective sputtering yields per surface atom. The theory predicts that homogenized films of the desired composition from separate sources of atomic deposition and ion irradiation are produced when the ion energy is high. Furthermore, filnl homogenization near the surface of the film is best achieved by lowering the ion energy or by oblique incidence at the start and end of the deposition. The binary collision cascade nlodel MARLOWE has been used by Brighton and Hubler (36) to predict the critical ion-to-atom arrival ratio necessary to reduce the intrinsic stress in films deposited with ion assistance. The computed data were in good
lon-Assisted Dielectric and Optical Coatings
387
agreement with the results of Hirsch and Varga (21) for Ar+ ... Ge over the range 200 eV to 2000 eV. The conclusion reached was that the prinlary influence of the ion beam on the film structure occurs through atomic displacements in the bulk, rather than by surface diffusion of adatoms or thermal spikes. Grigorov et al. (37) have developed a simple physical model describing the mechanism of film densification by lAD. The optimum relative density of ion bombardment is given by (Cion )Opt = (ND) -1 where N D is the number of displacements per incident ion and the optimum ion density is chosen such that the number of displacements created is equal to the number of depositing atoms. The authors have reported a good correlation with data for lAD of TiN, Zr02 , Ti0 2 , Si0 2 and MgF 2 . 19.3.7 Summary
The main benefits from ion-bombardment of growing films may be summarized as: (a) enhancement of surface mobility of adatoms (b) stimulation or acceleration of the nucleation and growth of the nuclei and the coalescence at the initial stage of film formation (c) development of preferred crystal orientation (d) crystallization-amorphization (e) increased adhesion (f) modification of film stress (g) stimulation of sorption and enhanced surface reactivity. 19.4 ION-ASSISTED TECHNIQUES
lon-assisted technology may be classified according to the energy distribution of the neutral species, the percentage of ionization possible and the energy distribution of the assisting ions. 19.4.1 lon-Assisted Deposition
The term ion-assisted deposition (lAD) is generally applied to that technique in which evaporation and sinlultaneous irradiation of a film is performed with a low-energy highflux ion source (Fig. 12). Optical thin films are routinely produced by electron-beam evaporation from a multi-crucible turret-type electron gun which facilitates the sequential deposition of multi-layered optical thin film stacks. Such systenls employ substrate heating, evaporation rate-monitoring, and optical transmittance or reflectance monitoring. The energy range of the depositing atoms and molecules from the evaporation source is typically 10-2 to 1 eV and a small fraction nlay be ionized through interaction with the electron beam. If a reactive gas is introduced during evaporation by backfilling, the process is termed reactive evaporation. Compound formation depends upon (a) an adequate supply of reactant, (b) collisions between reactant species and (c) reaction between colliding species. Reactivity can be enhanced by ionization through an electrical discharge during evaporation. The process is then classified as Activated Reactive Evaporation (ARE)(38). When ion bombardment is performed with an ion gun during deposition a greater control over film properties is possible than in ARE. Early attempts at direct beam-assisted deposition were made by Heitmann (39) and later by Ebert (40) using directed discharge tubes. Although successful to some degree, these devices suffered primarily from an insufficient ion flux to fully influence the growing films.
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Handbook of Ion Beam Processing Technology
The most useful ion source developed to date has been the Kaufman-type ion gun. This device (described in Chap. 2) is a high intensity ion gun which can provide useful ion fluxes over the energy range 30 to 1500 eV with both inert and reactive gases. Ion energy is well-defined, and the width of the energy distribution is less than 10 eV at 500 eV beam energy (41). The ratio Ot:O+ for oxygen operation has been measured by McNeil et al (42) to be approximately 3. High current sources can produce ion beam densities up to 200A m- 2 over small areas. The problem of surface charging on insulating substrates can be overcome by use of an electron emitting filament mounted in the ion beam or near the substrate. A present limitation on the continuous operation of the source, particularly with oxygen, is filament lifetime. Sputtering of tungsten filaments also leads to detectable amounts of tungsten in deposited films (43). This can be reduced by the use of thoriated iridium filaments and the lifetime also extended but at the expense of small iridium and thorium contamination (0.2 and 0.03 atomic percent respectively) (44). A recent innovation has been the development of a filamentless ion source employing a cold cathode electron emitter. These sources have not been widely tested to date (45). A gridless ion source for larger area and lower energy assisted deposition has also been developed (46). (See Chap. 4) Light source ~
detector
light source
Figure 12: Experimental system for the study of lAD showing transmittance and reflectance monitoring, ellipsometric monitoring and ion scattering spectrometer (18).
Some experimental systems employ high energy ion-assisted deposition with ion energies up to 40 keY. These technologies have generated a range of terminologies: ionassisted coating (lAC) (47), ion-vapor deposition (lVD) (48), ion-beam enhanced deposition (IBED) (49) and ion-beam assisted deposition (lBAD) (50). In certain cases the ion energy can be increased to 500 keY (51) and the deposition and irradiation carried
lon-Assisted Dielectric and Optical Coatings
389
out sequentially as an ion implantation process. Most of the high energy processing has concentrated on such tribological coatings as TiN and BN. 19.4.2 Ion Plating
The technique of ion plating is generally attributed to Mattox (51) although patents relating to the process can be traced to Berghaus (1938) (52). The technique refers to a process in which the substrate and/or growing film is exposed to energetic particles with the purpose of improving adhesion and/or other film properties. In this general context, lAD has been defined as high vacuum ion plating by Aisenberg and Chabot (53). Figure 13(a) shows the essential components in an ion plating deposition system. Ions are produced by thermally evaporating material in the region of a 1-5 kV inert gas discharge operating at a pressure of around 10-2 Torr. The ionized atoms are then accelerated by an electric field to the substrate. Multiple collisions with the inert gas result in energy loss and charge transfer. Teer (54) has estimated the average energy of the ions arriving at the cathode to be 300 eV, and the average energy of the neutral particles to be 135 eV. The process has a high throwing power in that gas scattering enables sides of the substrate to be coated, although often some degree of rotation is required.
ION PLATING
(a)
f
(b)
S UBSTRATE BIAS SUPPLY
Figure 13: Thin film deposition techniques, (a) ion plating, (b) ion beam sputtering.
Ion plating is routinely employed in the deposition of TiN wear-resistant coatings. Pulker et al. (55) have reported excellent results for optical films of Ti02, Ta20s, Zr02' Al20 3 and Si02 deposited onto unheated substrates. The films had
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Handbook of Ion Beam Processing Technology
high packing densities (>0.97) and high refractive indices. The films were synthesized by evaporation of metals in an oxygen discharge in which a low-voltage ion beam source was operated to enhance reactivity. The reported deposition rate was 0.4 to 0.6 nml s. 19.4.3 Sputtering
19.4.3.1 Ion Beam Sputtering. (IBS) The high intensity Kaufman ion source is routinely used to sputter elemental, alloy or compound targets and the sputtered material deposited as a thin film (Fig. 13 (b)). The substrate may also be heated during deposition. The IBS technique offers the following advantages over plasma technology: (a) the growing film is not exposed to large fluxes of electrons, (b) the processing may be carried out in high vacuum conditions (typically 10-4 -10- 5 Torr), (c) the ion beam can be focussed or apertured into a defined shape, and (d) the depositing atoms have a mean energy of 3-10 eV (compared to 0.1 eV or so in thermal evaporation) and the resulting film adhesion and structure are greatly improved. Sputter deposition of compound and alloy films is feasible since stoichiometry is preserved in the growing filnl. Deposition rates are determined purely by the sputtering yield of the target and the available ion beam current, and deposition rates of 1 micronlhr are readily obtainable, depending on the ion-target combination and source-substrate distances.
The growing film may also be irradiated with ions from a second ion source. This technique is termed dual ion beam sputtering (DIBS Fig. 13 (c)) and has been used extensively by Weissmantel (56) to both modify the film structure and to synthesize compounds. The second ion source may operate with inert or reactive ions with energies ranging from 20 eV to 10 keV. The higher energy technique is termed dynamic recoil mixing as discussed previously. A hybrid technique with less control involves mounting the substrate very close to the sputter target such that a fraction of the primary sputtering beam impinges on the growing film. The benefits of dual-ion-beam deposition can then be realized with the operation of a single ion source (57). Ion beam sputtering techniques are now widely used in optical thin films for the deposition of Si02, Ti02, A120 3 , Zr02, MgF2, AIN, Si3N 4 and BN. Other related materials such as diamond-like carbon have also been deposited (58). 19.4.3.2 Magnetron Sputtering. Magnetron sputtering is a variant of plasma-based sputter deposition techniques (Fig. 13 (d)). Secondary electrons, created at the target surface by ion bombardment, accelerate and ionize the gas atoms to sustain a discharge. The applied power is dc for conducting targets and rf for insulating targets. Higher efficiency is achieved by confining the primary electrons to paths close to the cathode surface with applied magnetic fields. Ionization efficiency is inlproved and higher sputtering rates result (59). The working gas is usually argon, but reactive gases can be added or substituted in the reactive sputter deposition of oxides, nitrides or carbides.
A recent innovation has been the development of magnetron sources with unbalanced nlagnetic fields (60) which are capable of giving ion fluxes at the substrate greater than the flux of the depositing atoms. Magnetron sputtering is a powerful technique for large area optical thin film coating such as in the coating of architectural glass or roll coating. High quality optical layers of most materials have now been deposited by sputtering (61).
lon-Assisted Dielectric and Optical Coatings
391
Substrate t::::===========-=f Film Ion beam 2 (Inert or \ reactive) ( c)
Ion source
Ion beam (Inert or reactive) Target
,-====:::::::sWater
t t SUBSTRATE
tI
I
I I I
SPUTTERED MATEnlAL
/"
I
ANODE
(d )
Figure 13: (c) dual ion beam sputtering, (d) magnetron sputtering 19.4.4 Ionized Cluster Beam Deposition (ICB)
The final technique considered in optical thin film deposition is the ionized cluster beam method introduced by Takagi (62). The basic system is shown in Fig. 13(e). Vaporized source material at high temperature is ejected through a nozzle in the crucible into a high vacuum chamber. Conditions are such that the emerging vapor undergoes adiabatic expansion, cooling to a supersaturated state. This results in the formation of atomic aggregate clusters. Energetic electrons are used to positively ionize some of the clusters (500-2000 atoms) which may then be subsequently accelerated to the substrate by an applied electric field. The growing film is also bombarded with neutral clusters, atoms and ions. Some researchers report that in the case of Ag the cluster size is only 25 atoms (63). The assumption is that the ionized cluster is broken upon impact with the
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Handbook of Ion Beam Processing Technology
substrate. Each atom in the cluster retains an average energy given by E = eVa/N , where Va is the accelerating voltage and N the number of atoms in the cluster. The average energy of the depositing aton1S can be varied with Va . The typical values of E are 0.1-10eV. Reactive deposition is possible by introducing reactive gases into the system through a nozzle close to the metal-vapor source. ICB offers improved film adhesion and surface heating which leads to improved crystallization, and the benefits of ion-assisted deposition. In addition, less surface disorder is introduced by ICB than in n10st ion bean1 techniques, enabling low temperature epitaxial film growth. The deposition rate is approximately 10 nm/min (Si) with a uniformity of 10% over 0.1 m 2 area. The technique ZnO, has been used on a wide range of materials including BeO, PbO, Ti0 2 , Si02 , ZnS, CaF2 , PbF2 , MgF 2 , a - Si and Cd1_xMnxTe (64). Substrate
Accelerating electrode \ I
Ionized
& neutral clusters
Crucible
Heating
Figure 13: (e) Ion Cluster Beam deposition apparatus.
19.5 OPTICAL PROPERTIES OF ION-ASSISTED FILMS
This section is concerned with a survey of the optical properties of thin films deposited by ion-assisted techniques. The survey is restricted to dielectric materials and some nitride materials.
lon-Assisted Dielectric and Optical Coatings
393
19.5.1 Oxides
19.5.1.1 Silicon Dioxide. Silicon dioxide is readily deposited with good optical properties by all the ion-based deposition technologies. When prepared by conventional evaporation, usually electron-beam evaporation of Si02 , the filnlS may be porous of variable index and sensitive to the substrate temperature. Guenther (7) has shown that the columnar microstructure present in films deposited on room temperature substrates can be reduced at elevated temperatures. Pulker et al. (55) report a dense glass-like structure and packing density >0.97 for ion-plated Si0 2 . The films were stoichiometric and with UV properties equivalent to high quality fused silica. The refractive index at 550 nm ( n 550 ) was however 1.49, Le. higher than that of fused silica. Unity packing density of Si02 has not been observed in evaporated films on substrates heated as high as 250 ° C and values range from 0.95 to 0.98 (8). Allen (65) has investigated the effect of ion species and ion flux on the refractive index of lAD silica prepared by evaporation of Si02 • Bombardment with Ar+ and 0t increased the refractive index relative to that of unbombarded films with the extinction coefficient too low to measure at 550 nm. The absorptance at 1.06 JLm was also low (9xl0- 6 ) increasing to 2.1xl0- 5 at 325°C substrate temperature with a 345 nm thick film. Silica is stable under ion impact and oxide reduction (and hence increased absorption) was not observed as in the case of Ti02 • This is consistent with earlier studies (65a). Allen (66) has also prepared Si02 by IBS and found the films to be in a compressive stress of 5 x 10- 8 N m- 2 . DIBS deposition of Si02 has been reported by Emiliani and Scaglione (67). Films were prepared by sputtering a Si target with 1.2 keY Ar+ (25-40 rnA) and irradiating the growing films with a 300 eV - 900 eV « 10 rnA) mixed 0t and Ar+ beam. Film growth rate was 0.1 nml s. The refractive index and extinction coefficient k decreased with increasing ion beam current. Values of k in the visible region were 2 x 10- 4 rising to 5 x 10-4 under the best conditions: a mid-range refractive index of 1.47 was measured. The best values for Si02 prepared by DIBS are those reported by Kalb et al. (27): n 633 = 1.46 and k = 5 X 10-6 . These high performance films used in ring laser gyroscopes had transmission losses of 20-150 ppm absorptance of 20-40 ppm and scattering losses of < 1 ppm. Figure 14 summarizes the optical refractive index values for Si02 deposited by ion-assisted and sputtering techniques. The best results from each author are plotted. 19.5.1.2 Aluminum Oxide. Aluminium oxide has one of the highest packing densities (0.95) of the dielectric oxide materials when deposited by conventional evaporation, as reported by Reale (11). A constant refractive index of 1.60 up to a substrate temperature of 300°C was observed. Magnetron sputtering has been used to prepare films with an index of n546 = 1.63 (68) and absorptances of less than 0.01 (69). Pawlewicz et al. (70) have reported a higher mid-range n of 1.67 for Al2 0 3 prepared by rf diode sputtering of aluminum in argon-oxygen mixtures. These films were found to have a cubic structure. Ion-assisted deposition (71) has been used to produce high quality films with low extinction coefficients (eg. n 63 3 = 1.65, k = 1.8xl0- 6 ). The dispersion of the refractive index for Al2 0 3 is summarized in Table 3 for recently published data on films prepared by ion-based techniques.
394
Handbook of Ion Beam Processing Technology
0
>< 1.6 w c
0 C)
~
w > i= U 1.5
6
«
~
6-
a:
LL
w
a:
1.4
1.3 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
WAVELENGTH(lJm) Figure 14: Summary of the refractive index as a function of wavelength of Si02 as prepared by ion-based techniques. Data are compiled from references (3) and (55). Key: o lAD, 6 sputtering, S reactive evaporation, e ion plating.
Table 3: Refractive indices of Al2 0 3 produced by ion-based techniques.
Wavelength A(JLm)
0.25 0.30 0.35 0.40 0.45 0.50 0.54 0.55 0.60 0.70 0.80 1.00 1.00 1.06 5.00
Refractive Index
Reference
n
1.75 1.70 1.70 1.70 1.69
1.70 1.63
1.68 1.65 1.67 1.68 1.64 1.67 1.67 1.74
70 70 31 70 55 72 68 55 71 55 70 72 70 70 70
lon-Assisted Dielectric and Optical Coatings
395
19.5.1.3 Titanium Dioxide. This nlaterial has received more attention than other dielectric oxide by virtue of its high refractive index, transparency throughout the visible and near infrared regions, chemical stability and hardness. Titanium dioxide occurs in three main crystalline forms; rutile, anatase and brookite (not seen in thin filnl form). The most desirable phase for optical applications is rutile. The substrate temperature and the degree of oxidation of the film are critical parameters in Ti02 deposition, and the optical properties obtainable vary with the deposition technique. A comprehensive study of reactive evaporation methods by Pulker et al. (73) has highlighted the problems encountered in Ti02 deposition by conventional techniques. The refractive indices are dependent upon substrate temperature, oxygen partial pressure during deposition and deposition rate. Reproducible results were obtained only when Ti or Ti30 s starting nlaterials were reactively evaporated. The variations in index with successive evaporations of other source materials are attributable to composition changes in the crucible during deposition, which in turn results in a varying vapor stream composition.
One major problem encountered with reactive evaporation is the reduced packing density of the deposited nlaterial, a parameter which can only be improved by depositing at elevated substrate temperatures in order to enhance surface mobility of depositing atoms and molecules. Grossklaus and Bunshah (74) have prepared rutile by reactive evaporation, but only at very high oxygen partial pressures and elevated substrate temperatures up to 1100°C . Higher packing densities are achieved by sputtering techniques which promote a greater mobility of surface atoms. Considerable control over the oxide phase and grain size has been achieved by r.f. sputtering. Rutile is obtained over a wide range of temperatures at high oxygen pressures and the corresponding grain size varies fronl 10 to 60 nm. The influence of grain size on the refractive index is difficult to separate from packing density effects but is reported to increase n by approximately 5% (75). In general, sputtering is the preferred method for depositing Ti0 2 with reproducible properties, although good results have also been obtained with reactive rJ. biased ion plating. Using this technique Suzuki and Howson (76) have obtained high quality Ti02 films (n633 = 2.49) on water-cooled glass substrates. However, the refractive index is sensitive to deposition rate and oxygen partial pressure. Ti0 2 was also prepared by d.c. magnetron sputtering and r.f. enhanced d.c. magnetron sputtering. In the latter technique, r.f. bias is applied to the substrate. One advantage in these techniques is that the source material can be titanium which is sputtered at a relatively high rate. Ion beams have been used by Takiguchi et al. (77) to deposit titanium oxides directly by sputtering metal targets in oxygen, but the most successful application has been in ion-assisted deposition (lAD). Heitmann (39) evaporated Ti2 0 3 with oxygen-ion assistance. The refractive index was estimated to be between 2.2 and 2.3 at 550 nm and found to depend slightly on deposition rate. The absorption coefficient at 633 nm was estimated to have an upper limit of 40 cm- i , and at 10.5.um was 103 cm- i . Single crystal rutile attains a comparable absorptance only at 11.6.um . The difference was presumed to derive from a structural effect, since the lAD films were all amorphous. The experiments of Heitmann were later repeated in greater detail with a refined Heitmann ion source (40). Using both TiO and Ti2 0 3 , the effect of neutral oxygen,
396
Handbook of Ion Beam Processing Technology
positive and negative ions, and excited molecules on the absorption and refractive index were investigated in the substrate temperature range 50 to 325°C. Further demonstrations of the success of lAD were made by Allen (79) with negative ions (and electrons) and TiO as the starting material. The absorption coefficients obtained at 1.06JLm are given in Table 4. X-ray diffraction measurements showed these films to be amorphous. Table 4: Absorptance measured at 1.06JLm for Ti0 2 films for increasing ion beam cur-
rents (79). Source Current (mA)
150 250 350
Absorptance €X
1.9x10- 1 1.8x10- 1 9.0x10- 3
Absorptance Coefficient (cm- 1 )
1992 29 160
Ion beam sputtering of a metallic Ti target with Ar in a background of oxygen has been denlonstrated to produce high index films (n633 = 2.52), with a visible optical absorption of 0.3 percent (film thickness 200-400 nm) with an oxygen fraction of 30 percent (78). Allen (79) has studied the influence of 300 and 400 eV argon and oxygen positive ions with a Kaufman ion source. The best results were found for oxygen bombardment where very low values of the extinction coefficient were obtained. Titania (Ti02 ) films with a minimum absorptance and a refractive index of 2.49 were produced using 300 eV oxygen ions at an ion-to-molecule ratio of 0.12. Allen (66) has also prepared Ti02 by ion beam sputtering. A metal target was sputtered with 1.4 keV Ar+ while the growing films were bombarded with 100 eV oxygen ions with current densities ranging from 7 to 54 JLAcm- 2 • The absorptance constant at 1060 nm decreased from 27 to <5 cm- 1 with increasing oxygen ion current density. The refractive index was independent of the oxygen current density. The dispersion of refractive index of lAD, IBS and conventionally deposited titania are shown in Fig. 15. The high index of the IBS film was attributed to the higher film density. The superior results for DIBS titania are confirmed by Kalb et al. (27). The effect of low energy bombardment on optical properties has been studied by McNeil et al. (42). Film absorptance is reduced for the lower energy case although no figures on index or absorptance were given. It was found that the ion bombarded films had considerably less hydrogen and hence lower water vapor penetration than films deposited without ions. All lAD studies of titania have shown that the deposited films are amorphous. These observations are consistent with earlier studies of bOlnbardment-induced structural changes in solids. Furthermore, a Ti02 sample subjected to an intermediate (inert gas) ion dose (10 17 -1020 ions m- 3) will revert to the lower oxide Ti2 0 3 (25). Since film growth is essentially a layer by layer process, with each layer being ion bonlbarded, the situation is equivalent to post-bombardment of bulk oxides.
lon-Assisted Dielectric and Optical Coatings
397
2.7
2.6
:s>< 2.5 UJ
C
~ UJ
>
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(.)
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a: 2.3 u. a:
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.,
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.....
..................
EVAPORATED
...................
..... ............
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--...a..---"'-'-. .675 .. 575 625
2.1L...-_--L--.....- -....... 375 425 475 525
-~~
725
WAVELENGTH (nm) Figure 1 5: Conlparison of the refractive indices of evaporated, lAD and ion beam sputtered (IBS) titania films (65,66).
The refractive index dispersion of Ti0 2 obtained by various techniques is summarized in Fig. 16. It is interesting to note that there is a considerable spread in the data points, with very few points approaching the optical properties of bulk rutile. lAD data points show good reproducibility between various groups, but in general fall short of the rutile values at most wavelengths. All the lAD data was taken from samples deposited on unheated substrates which, for films prepared by conventional evaporation, generally leads to inhonlogeneous coatings. The lAD data is, however, in good agreement with that of Cherepanova and Titova '(80) for Ti02 films deposited by evaporation on substrate maintained at 300 ° C . This indicates that ion-assistance is equivalent to enhancing film atom mobility by substrate heating. ICB deposited titania filnls have been reported to have an exceptionally high refractive index (81). The films were deposited with a high content of rutile structure when the ionization current was raised. With increasing acceleration voltage, refractive index increased while absorptance decreased. 19.5.1.4 Zirconium Dioxide. This nlaterial is hard with a high refractive index that is highly sensitive to deposition conditions (82). The packing density of Zr02 is considerably less than that for other dielectrics when deposited by evaporation. Reale (11) gives a figure of 0.7 at 50°C and 0.95 at 300°C substrate temperature with a corresponding rise in index from 1.80 to 2.15. Perveev et al. (83) report a 12% porosity for evaporated Zr02' With such a poor packing density it is not surprising that wide variations in optical properties have been reported.
398
Handbook of Ion Beam Processing Technology
3.0 2.9 2.8
><
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c
z
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WAVELENGTH (J-Im) Figure 16: Summary of the refractive index as a function of wavelength of Ti02 films prepared by ion-based techniques. Data are compiled from references (3), (55) and (81). Key: 0 lAD, ~ sputtering, 0 reactive evaporation, e ion plating, leB.
*
The growth of Zr02 can be substantially influenced by ion bombardment. A detailed study of the modification of the optical and structural properties of dielectric Zr02 produced by ion-assisted deposition has been made by Martin et al. (17) (84). The effect of ion irradiation on the optical properties is detected by vacuum-air effects in the refractive index as shown earlier in Fig. 6. Stable films which did not adsorb water were produced for a nlolecule-to-ion-arrival-ratio of 3.5 with 1200 eV 0t ions. The result was attributed to a reduction of microvoid density which is otherwise high in conventionally evaporated films. Variations in crystal structure and film packing density have a strong influence on the optical properties of Zr0 2 . Fig. 17 shows the refractive index at 550 nm as a function of argon and oxygen ion current density. With argon, a vacuunl-air variation effect is observed up to an index of 2.138. At high current densities the index decreases due to preferential sputtering of oxygen and simultaneous incorporation of argon into the layer. Films produced by oxygen bombardment have a higher index of 2.19. The highest indices for both ion species are observed only when the substrate is heated. The effect of ion bombardment on film density is detected with a high degree of sensitivity by in-situ ellipsonletric nlonitoring of the deposition (85). Figure 18 shows the ~ - '!' plot for a film deposited without ion assistance over the region A to B. At point B the ion assisted deposition commenced and the modification to the film refractive index is registered as a change in the ~ - '!' plot. The refractive index of the initial layer was
lon-Assisted Dielectric and Optical Coatings
399
n633 = 1.76 and that for the assisted layer was n 633 = 2.08 (ion :molecule ratio 1:2). When the steady state value of the optical properties is matched to the evaporated layer the result indicates that the ion bOll1bardnlent has densified the evaporated layer below the surface to a depth consistent with the expected damage layer. Ion assistance results in an immediate change in the optical properties of subsequently deposited layers as well as the near surface layers of less dense evaporated material. The optical data for Zr02 is presented in Table 5. 2.3,.......,----r--r-----r--~---.--
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100
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Figure 1 7: The influence of ion bombardment on the refractive index of Zr0 2 measured in vacuum and air; (a) argon ion bonlbardment, (b) oxygen ion bombardment.
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400
Handbook of Ion Beam Processing Technology
19.5.1.5 Cerium Dioxide. Cerium dioxide has been prepared by sputtering and also by ion-assisted deposition (87-89). As with other oxides prepared by lAD, vacuum-air changes in the refractive index are observed until the ion current density reaches a certain value. A gradual decay from the maximum value of ns so = 2.4 is observed at high current densities which is thought to be a result of trapping of oxygen in the film and/or preferential sputtering effects. The mass difference between Ce and is of such magnitude that in the absence of chemical effects preferential sputtering is to be anticipated. Ion irradiation at any energy increases the extinction coefficient relative to an unbombarded film. The packing density of films deposited without ions is only about 0.55. As with Zr02, the packing density can be improved with lAD. No crystallographic phase changes are observed although structural studies do indicate some preferred orientation effects, particularly in films heated to 300°C. Optical values for Ce02 are shown in Table 6.
°
Table 5: Refractive indices of Zr02 produced by ion-based techniques.
Wavelength i\(/-tm)
0.25 0.40 0.45 0.50 0.55 0.55 0.70 0.80 1.00 1.06 1.06 1.11
Refractive Index
Reference
n
2.47 2.2 2.27 2.23 2.24 2.15 2.19 2.15 2.15 2.15 2.20 2.10
70 70 55 55 55 70 55 70 70 70 85 86
Table 6: Summary of the refractive indices of Ce02 prepared by ion-based methods.
Wavelength i\(/-tm)
0.55 0.56 0.58 0.76 1.06 5.00 10.00
Refractive Index
Reference
n
2.4 2.5 2.49 2.45 2.45 2.25 2.10
89 86 86 86 86 88 88
lon-Assisted Dielectric and Optical Coatings
401
19.5.1.6 Tantalum Pentoxide. Tantalum oxide is a high refractive index material that is useful as an alternative to Ti02 in some applications and is readily prepared by sputtering from either an oxide target or an elenlental Ta target in an oxygen atmosphere. With oxide targets, a 90: 10 partial pressure mixture of argon and oxygen is usually selected for both diode and ion-beam sputtering (90). The percentage of oxygen is usually increased to at least 25 per cent for Ta targets in order to reduce the optical absorptance of the films. Ta20S has also been deposited using electron-beam evaporation of Ta20S although considerable outgassing of the source material occurs and oxygen backfilling nlust be enlployed to minimize optical absorption. Optical performance is also sensitive to substrate temperature and deposition rate (91). Ion-assisted deposition has been used to obtain some high index material (nsso = 2.1, k 633 = 3 x 10-6 (71» . McNally et al. (31) have studied Ta20S films prepared by 0t ion assisted deposition as a function of ion energy and current density. Their data is summarized in Fig. 19. 2.30
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02+ CURRENT DENSITY (p.A cm- 2 )
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10
20
30
40
50
60
70
02+ CURRENT DENSITY (IlA
80
90
cm- 2 )
Figure 19: The influence of oxygen ion bombardment on the optical properties of Ta20s for various ion energies (31); (a) n 400, (b) k4oo .
402
Handbook of Ion Beam Processing Technology
The results show marked variations in optical properties as a function of beam energy and current, and this was interpreted as bombardment induced reduction in the film stoichion1etry. The highest index at 400 nn1 occurs for 300 eV bombardment, and the lowest extinction coefficient reported was k 400 = 2 x 10-4 for 500 eV bombardment and a 5/LA cm- 2 current density for a deposition rate 0.3-0.4 nm/s. Higher current densities increased the film absorption through preferential sputtering of oxygen. Flory et al. (92) have successfully deposited Ta20s (under 250 eV Ar+ bombardment) with an estimated film packing density of 0.99. Optical filters made when the layers were incorporated in a multilayer stack with Si02, were found to show excellent stability when exposed to a moist atmosphere. The optical data for Ta20s is summarized in Fig. 20.
2.7 2.6 X UJ
c
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2.5
Z
w 2.4
> i=
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u.
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1
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0
~
'XI 2.1 2.0
~
~&
~
jl
~
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• •
0
~
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Figure 20: Summary of the dispersion of the refractive index of Ta20s prepared by varion plating. ious methods; Key: 6 sputtering, 0 lAD, 0 reactive evaporation,
e
19.5.1.7 Vanadium Dioxide. There is considerable interest in V0 2 because it exhibits a monoclinic-to-tetragonal phase transition at about 68 0 C which is accompanied by a semiconductor-to-metallic change in its electronic and optical properties. Its electrical conductivity can change by up to a factor of 104 while the transn1ittance and reflectance changes significantly in the infrared region. The temperature width of the transition can be varied from tens of degrees for films, down to about 0.5 0 C for bulk single crystals. Switching times of 30 ns have been demonstrated (93). The n1aterial has been shown to have potential for many device applications including visualization of microwave and infrared radiation, optical data storage, optical temperature sensing, coherent optical data
lon-Assisted Dielectric and Optical Coatings
403
processing, transmit/receive switching, fast random access laser scanning, and most recently in thermochromic energy-efficient "smart" windows (94-98). Thin filnls of V0 2 have been prepared by many techniques but in recent times there has been considerable interest in their preparation by reactive evaporation (99), ionassisted deposition (100) and in ion-beam sputtering (101). Since vanadium has many stable oxide phases, it is difficult to produce as-deposited films of the correct stoichiometry. For lAD films both mixed Ar+ and 0t beams, as well as pure 0t beams, have been used to bombard the growing film. However, in all reported cases an annealing post-deposition treatment between 500 and 600°C in a reduced oxygen atmosphere has been used to optimize the oxidation state, increase the crystal size, and hence improve the phase transition. The energy of the ions in the assist beam has been shown to influence the crystal orientation. 70 ~
=3.4
Slm
80
50
'$. w
40
z -c ~
30
0
itJ) z -c
...a:
20
10
20
30
40
50
60
70
80
90
TEMPERATURE (·C)
Figure 21: Comparison of transmission as a function of temperature for ion-assisted and standard reactively evaporated V0 2 films; (a) 100% oxygen in ion-beam, (b) 400/0 oxygen and 1.5 x 10- 3 Torr O 2 background, (c) standard film (44). in the films. (A hysterisis loop is shown in each case.)
As-deposited V0 2 films with sharp switching transitions have been prepared by ionbeam sputtering (102). The correct combination of oxygen partial pressure, vanadiunl deposition rate and substrate temperature is required to produce V0 2 films with a high degree of crystallinity. The transition temperature can be varied considerably by doping. Tungsten doping of bulk V0 2 lowers the transition temperature in excess of 20 ° C / at. °ib and Nb 8 ° C / at. %. Films have been successfully doped by both ion-assisted and ionbeam sputtering techniques (44,103). Figure 21 shows the optical transmittance of doped and undoped lAD filnls.
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Handbook of Ion Beam Processing Technology
19.5.2 Fluorides
MgF 2 is widely used wherever a low refractive index layer is required, e.g. antireflectance of silica, and for enhancing the reflectance of aluminum mirrors in the vacuum ultraviolet region (VUV). Under normal evaporation conditions the material is porous with a low packing density (0.72 (8». The microstructure is columnar with column dianleters in the range 13-20 nm (104). Several reports have been nlade of the ionassisted deposition of MgF 2 with a range of ion species including H 2 0, Ar, O 2 and C 2 F 6 (105). In general the films are densified but suffer from increased UV and VUV absorptance due to changes in stoichiometry and/or oxide formation. Although ion energy does not influence the results greatly, lAD films are more absorbing at lower wavelengths than evaporated layers. The adhesion of films is greatly improved and the stress reduced (106) with lAD. The University of Arizona group (107) has studied eight of the lanthanide trifluorides prepared by Ar+ lAD: SmF3 , EuF 3 , GdF 3 , TbF 3 , HoF 3 , ErF 3 , YbF 3 and LuF 3. In the absence of ion bombardment the indices of the trifluorides are around 1.5 in the visible and near-UV regions. Ion assistance was found to increase the index of TbF 3 from 1.53 to 1.57 and that for GdF 3 from 1.56 to 1.58. VUV transparency was preserved provided low ion energies and fluxes were used. Typical operating conditions were 100 eV to 300 eV and 20 JLAcm- 2 for deposition rates of 0.6 nm/s. Other fluoride materials deposited by lAD include ThF 4 and Na3AlF 6 (cryolite) (108) (109). ThF 4 is transparent over the range 0.26JLm -12JLm The refractive index was found to increase from 1.54 to 1.60 with 300 eV Ar+ bombardment at a current density level of 70JLAcm- 2 . The bombarded films were also found to have an enhanced resistance to moisture penetration. Cryolite films are sensitive to low energy Ar+ bombardment and high extinction coefficients have been reported (110). When oxygen assistance is used the film index can be increased from nsso = 1.34 to 1.37 (109). 19.5.3 Conducting Transparent Films
Films that simultaneously exhibit high optical transparency (>80 per cent) in the visible region and high electrical conductivity (> 1030hm- 1cm- 1 ) are useful in numerous applications and devices, including transparent heating elements in aircraft and automobile windshields, antistatic coatings, gas sensors and display devices. The more commonly used materials include tin oxide (TO) doped with antimony (ATO) or fluorine (FTO), indium oxide (10) doped with tin (ITO), zinc oxide (ZO) doped with indium (IZO) and calcium stannate. There are several extensive reviews describing the properties and preparation of these films, but only specific examples will be reviewed here (111-113). In the review of Chopra et al. (111) many examples are given of the preparation of most of the materials by (a) reactive sputtering of metallic targets, (b) sputtering from oxide targets, (c) ion beam sputtering and (d) reactive ion plating. The deposition rates for these techniques is typically 10-50 nm min- 1• There have been few reports of conducting transparent films deposited by lAD. Ebert (40) deposited 10 (indium oxide) under neutral and ionized oxygen bombardment but found that useful films could only be deposited with low absorption in the case of ions. When doped with 30 per cent Sn, a sheet resistance of 800hm / square and a film index of nsso = 2.05 were measured. Martin et al. (114) successfully deposited ITO films onto
lon-Assisted Dielectric and Optical Coatings
405
ambient temperature substrates under 100eY 0t lAD. These films had a refractive index of n550 = 2.13 and sheet resistance of 800 Ohm/square . When the substrate temperature was increased to 400°C these respective values reduced to 2.0 and 25 Ohm/square, respectively. Microscopy studies showed that films deposited on room temperature substrates were amorphous, but became crystalline upon heating to 100° C or greater. 19.5.4 Nitrides
Nitride films find application in optics, electronics and tribology and several surveys have been published reporting the mechanical, structural and electrical properties of these filnls. In this section only recent studies of the optical properties of nitrides prepared by lAD and related techniques will be addressed. Boron nitride films have been synthesized by Ion Beam Sputtering (IBS) (115) (116), (lAD) (117), reactive evaporation (118) (119), rf sputtering (120) and IBAD (121) ( 122). The variation in properties of deposited BN films is largely structurally related in that some groups report an anlorphous structure and others the cubic phase. Bouchier et al. (116) have made detailed studies of BN deposited by reactive IBS by sputtering B with Nt beams of energy 0.5 to 4 keY. The refractive index of stoichiometric films were determined to be n546 = 2.03 which is to be compared with the bulk cubic BN value of 2.11. The film density was however, less than bulk (2.01 compared to 3.45 gcm- 3 ) and deposition rate low (0.5nm min- 1 ). Holmes and Barnett (115) deposited BN by IBS of a pyrolytic BN target, and achieved the higher deposition rates of 4.5 - 9.0nm nlin- 1 . Films deposited at higher rates were found to be more absorbing. Sainty et al. (117) have recently synthesized BN by B evaporation and low-energy
Nt lAD. The films were amorphous when deposited on room temperature substrates, and hexagonal when deposited on substrates heated to 300°. Stoichiometric films had a low extinction coefficient throughout the visible region (10- 2 ) and a refractive index of n633 = 1.9. Several groups have reported the formation of cubic BN when the assisting ion beam energy is raised to higher energies. Satou and Fujimoto (121) report the cubic phase for 40 keY Nt assisted deposition and Bricault et al (122) obtained a high refractive index of n633= 2.100 close to bulk cubic boron nitride for B films implanted with 120 keY Nt. BN is found to be highly transparent, hard and a promising material for optical applications. Silicon nitride thin films find important application in the passivation layers of microelectronic devices, and in recent times have been shown to produce very stable edge filters and antireflection coatings when used in combination with Si0 2 in multilayer optical coatings. The favored deposition technique for optical applications has been sputtering which produces refractive indices as high as 2.1 at 633 nm, and useful transmission in the wavelength range from 250 nm to 9p.m (123). N 2 - O 2 mixtures, used in many techniques to produce silicon oxynitride films (124), have been found to produce predominantly oxide layers. Ion-assisted evaporated layers with extinction coefficients less than 3.5x10- 6 have been reported (125). Holmes and Barnett (115) report deposition rates of 4.5nm/min. for reactive ion beam sputtering, which is much higher than the maximum of 1 nm / min obtained by Bouchier et al. (116) using the same technique. Introduction of NH 3 and N 2 to the proc-
406
Handbook of Ion Beam Processing Technology
ess greatly increases the deposition rate but produces hydrogen-contaminated films. A novel method for producing hydrogen free silicon nitride films reported by Kitabatake and Wasa (57), involves ion-beam sputtering with a nlixture of Ar and N 2 where the substrate is also bombarded at a glancing angle by the sputtering ion-beam. Aluminum nitride films have been produced by many techniques including reactive dual ion-beam sputtering, rf and dc sputtering and ion-assisted deposition. As the energy of the N 2 ions in the assist beanl is increased from 100 eV to 500 eV in IBD (126) (127) the film crystallite orientation changes from c-axis perpendicular to parallel with respect to the film plane. Figure 22 shows the optical properties of a number of films prepared by lAD (128) under different conditions. It can be seen that the extinction coefficients decrease as the energy of the assist beam is reduced. Reductions in absorptance have been observed for reactive ion-beam sputtered films using a mixture of N 2 and 25 % H 2 compared to pure nitrogen (129). Titanium nitride, TiN, has a reflectivity in the visible region similar to that of gold, rendering it suitable for decorative coating applications (130). TiN is also suitable as a selective transparent film in "heat mirrors" due to a high IR reflectance (131). All ionbased techniques are suitable for TiN deposition. lAD of evaporated Ti with Nt or Ar+ in a nitrogen atnlosphere (ion-stimulated sorption) is successful in the formation of TiN. The refractive index at 400 nm is between 2.5 and 3 decreasing with increasing wavelength to around 1.5 at 700 nm (132). The extinction coefficient rises sharply in the near IR. The optical properties of TiN are sensitive to variation in stoichiometry and surface oxidation (132-133).
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Figure 22: Refractive index and extinction coefficient of AIN filnls as a function of wavelength for different ion energy and ion-to-vapor relative arrival rates. B. 75 eV Nt , 1.7 A. 75 eV Nt , 1.05 C. 75 eV Nt ,3.4 D. 500 eV Nt ,2.5
lon-Assisted Dielectric and Optical Coatings
19.6
407
CONCLUSION
Single and multilayer thin film coatings play an essential role in modern optics and are applied to the surfaces of virtually all optical components in commercial instruments. The properties of deposited films have usually been inferior to those of the bulk material thus making coatings the limiting factor in a number of optical systenls. Bombardment of the growing film with ions has been shown to have a dominating influence on film properties where many of the induced modifications are beneficial. Examples include improved adhesion, increased density, production of stable bulk-like films, reduced stress, improved stoichiometry, and compound synthesis. The technique is attractive since it can be easily retrofitted to existing deposition equipment. Ion sources developed earlier were limited by shortcomings in their ion flux density, reliability and filament lifetime. Nevertheless, with new ion sources, ion assistance is now commonplace in a wide number of specialized applications, and as familiarity and equipment reliability improve, its use in routine coating should be ensured.
19.7 REFERENCES
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Handbook of Ion Beam Processing Technology
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Handbook of Ion Beam Processing Technology
49. Fatkin, J., Kohno, A. and Kanekama, N., Characterization of nitrides prepared by ion beam enhanced deposition of aluminium, silicon and titanium. Jape J. Appl. Phys. 26: 856-862 (1987). 50. Wolf, G.K., Zucholl, K., Barth, M. and Ensinger, W., Equipment for ion beam assisted deposition. Nucl. Instrum. Methods B21: 570-573 (1987). 51. Mattox, D.M., Film deposition using accelerated ions. Electrochem. Technol. 2: 295-298 (1964). 52. Berghaus, B., U.K. Patent 510, 993 (1938) 53. Aisenberg, S. and and Chabot, R.W., Physics of ion plating and ion beam deposition. J. Vac. Sci. Technol. 10: 104-7 (1973). 54. Teer, D.G., The energies of ions and neutrals in ion plating. J. Phys. D. 9: L187-189 (1976). 55. Pulker, H.K., Haag, W., Buhler, M. and Moll. E., Optical and mechanical properties of ion-plated oxide films. Proc. 5th Int. Conf. Ion and Plasma Assisted Techniques (H. Oechsner, ed.), pp 299-306, CEP, Edinburgh (1985). 56. Weissmantel, C., Ion beam deposition of special film structures. Technol. 18: 179-184 (1981).
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57. Kitabatake, M. and Wasa, K., Hydrogen-free SiN films deposited by ion beam sputtering. Appl. Phys. Lett. 49: 927-929 (1986). 58. Mirtich, M.J., Swec, D.M. and Angus, J.C., Dual-ion-beam deposition of carbon films with diamond-like properties. Thin Solid Films 131: 245-254 (1985). 59.
Danilen, B.S. and Sirchin, V.K., Magnetron systenls for ion sputtering of materials. Prib. Teck. Eksp. 4: 7-18 (1978).
60. Window, B. and Savvides, N., Unbalanced dc magnetrons as sources of high ion fluxes. J. Vac. Sci. Technol. A4: 453-456 (1986). 61. Pawlewicz, W.T., Martin, P.M., Hays, D.D. and Mann, LB., Recent developments in reactively sputtered optical thin films. Proc. Soc. Photo-Opt. Instrum. Eng. 325: 105-116 (1982). 62. Takagi, T., Ionized cluster beam technique. Vacuum 36: 27-31 (1986). 63. Kuiper, A.E.T., Thomas, G.E. and Schouten, W.J., Ion cluster beam deposition of silver and germanium on silicon. J. Cryst. Growth 51: 17-40 (1981). 64. Takagi, T. Development of new materials by ionized-cluster beam technique. Mat. Res. Soc. Symp. Proc. 27: 501-511 (1984). 65. Allen, T.H., lon-assisted deposition of titania and silica films. Proc. Int. Ion Engineering Congress. Vol. 2. Kyoto (T. Takagi, ed.), pp 1305-1310, lonics Co., Tokyo (1983). 66. Allen, T.H., Reactive ion beam sputtered optical coatings in Proc. 1987 Soc. Vac. Coaters Tech. Conf. Boston (1987). 67. Emiliani, G. and Scaglione, S., Properties of silicon and aluminum oxide thin films deposited by dual ion beam sputtering. J. Vac. Sci. Technol. 96: 265-270 (1982). 68. Nowicki, R.S., Properties of rf-sputtered Alz0 3 films deposited by planar magnetron. J. Vac. Sci. Technol. 14: 127-133 (1977).
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69. Deshpandey, C. and Holland, L., Preparation and properties of Al2 0 3 films by dc and rf magnetron sputtering. Thin Solid Films 96: 265-270 (1982). 70. Pawlewicz, W.T., Hays, D.D. and Martin, P.M., High band gap oxide optical coatings for 0.25 and 1.06 ,um fusion lasers. Thin Solid Films 73: 169-176 (1980). 71. Binh, L.N., Netterfield, R.P. and Martin, P.J., Low loss optical waveguiding in ionbeam-assisted-deposited thin films. ADDI. Surf. Sci. 23: 656-662 (1984). 72. Varasi, M., Misiano, C. and Lasaponara, L., Ion beam deposition of optical thin filnls in Proc. Int. Ion Eng. Congo Vol. 2, Kyoto (T. Takagi, ed.) pp 1041-1048, lonics Co., Tokyo (1983). 73. Pulker, H.K., Paesold, G. and Ritter, E., Refractive indices of Ti02 films produced by reactive evaporation of various titanium-oxygen phases. ADDI. Opt. 15: 2986-2991 (1976). 74. Grossklaus, W. and Bunshah, R.F., Synthesis of various oxides in the Ti-O system by reactive evaporation and activated reactive evaporation techniques. J. Vac. Sci. Technol. 12: 593-597 (1975). 75. Schiller, S., Beister, G., Sieber, W., Schirmer, G. and Hacker, E., Influence of deposition parameters on the optical and structural properties of Ti02 films produced by reactive d.c. plasnlatron sputtering. Thin Solid FilnlS 83: 239-245 (1981). 76. Suzuki, K. and Howson, R.P., Ion plating for optical coating in Proc. Int. Ion Eng. Congo Vol. 2, Kyoto (T. Takagi, ed.) pp 889-899, lonics Co., Tokyo (1983). 77. Takiguchi, K., Ogawa, S. and Takahashi, Y., Preparation and properties of TiD thin films prepared by dual ion beam sputtering. Proc. Int. Ion. Eng. Congo Kyoto Vol. 2 (T. Takagi, ed.) pp 1337-1340, lonics Co. Tokyo (1983). 78. Demiryont, H. and Sites, J.R., Effects of oxygen in ion-beam sputter deposition of titaniunloxides. J. Vac. Sci. Technol. A2: 1457-1460 (1984). 79. Allen, T.H., Properties of ion-assisted deposited silica and titania filnlS. Proc. Soc. Photo-Opt. Instrum. Eng. 325: 93-100 (1982). 80. Cherepanova, M.N. and Titova, N.F., Multilayer vacuum coatings employing layers of titanium dioxide and silicon dioxide. Sov. J. Opt. Technol. 46: 694-696 (1979). 81. Fukushima, K., Yamada, I. and Takagi, T., Characteristics of Ti02 films deposited by a reactive ionized cluster beam. J. ADDI. Phys. 58: 4146-4149 (1985). 82.
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96. Moiseev, V.V., Ogrin, Yu, R., Kutsevich, LV., Potapov, V.T. and Sokolovskii, A.A., Possible use of vanadium oxide films in fiber-optics temperature pickups. Sov. Tech. Phys. Lett. 8: 246-247 (1982). 97. Eden, D.D., Some applications involving the semiconductor-to-metal phase transition in VO z. Proc. Soc. Photo-Opt. Instrum. Eng. 185: 97-102 (1979). 98. Babulanam, S.M., Eriksson, T.S., Niklasson, G.A. and Granqvist, C.G., Thermochromic VO z films for energy-efficient windows. Solar Energy Mater. 16: 347-363 (1987). 99. Case, F.C., Reactive evaporation of anonlalous blue VO z. 1550-1553 (1987).
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105. Gibson, V.J. and Kennemore, C.M., Ambient temperature deposition of MgF2 with noble and chlorofluorocarbon ion assistance. Proc. Soc. Photo-Opt. Instrum. Eng. 678: 130-133 (1986). 106. Jacobs, S.D., Hrycin, A.L., Cerqua, K.A., Kennemore, C.M. and Gibson, V.J., Adhesion enhancements and internal stress in MgF2 films deposited with an ion beam assist. Thin Solid Films 144: 69-76 (1986). 107. Lingg, L.J., Targove, J.D., Lehan, J.P. and Macleod, H.A., lon-assisted deposition of lanthanide trifluorides for VVV applications. Appl. Opt. to be published. 108. AI-Jumaily, G.A., Hazlovitsky, L., Mooney, T. and Smajkiewiez, A., Optical properties of ThF 4 films deposited using ion-assisted deposition. Appl. Opt. (in press). 109. Martin, P.J., lon-assisted thin film deposition and applications. 585-590 (1986).
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110. Targove, J.D., Messerly, M.J., Lehan, J.P., Weng, C.C., Potoff, R.H., Macleod, H.A., McIntyre, L.C. and Leavitt, J.A., lon-assisted deposition of fluorides. Proc. Soc. Photo. Instrum. Eng. 678: 115-122 (1986) 111. Chopra, K.L., Major, S. and Pandya, D.K., Transparent conductors - a status review. Thin Solid Films 102:1-46 (1983). 112. Dawar, A.L. and Joshi, J.C., Semiconducting transparent thin films: their properties and applications. J. Mater. Sci. 19: 1-23 (1984). 113. Halnberg, I. and Granquist, C.G., Evaporated Sn-doped In203 films: basic optical properties and applications. J. Appl. Phys. 60: R123-R159 (1986). 114. Martin, P.J., Netterfield, R.P. and McKenzie, D.R., Properties of indium tin oxide films prepared by ion-assisted deposition. Thin Solid Films 137: 207-214 (1986). 115. Homes, S.J. and Barnett, G.J., Optical properties of ion beam sputtered nitride coatings. Proc. 5th Int. ConL Ion and Plasma Assisted Technique (H. Oechsner, ed.) pp 417-421, CEP, Edinburgh (1985). 116. Bouchier, D., Bosseboeuf, A. and Gautherin, G., Preparation and characterization of ion-beam sputtered deposited boron nitride. Proc. Int. Symp. Trends and New Applications in Thin Films, Strasbourg 1987. To be published. 117. Sainty, W.G., McKenzie, D.R., Cochayne, D.J.H., Dwarte, D.M., Martin, P.J. and Netterfield, R.P., The structure and properties of ion beam synthesized boron nitride films. J. Appl. Phys. (in press). 118. Chopra, K.L., Agarwal, V., Vankar, V.D., Deshpandey, C.V. and Bunshah, R.F., Synthesis of cubic boron nitride films by activated reactive evaporation of H 3B0 3. Thin Solid Filnls 126: 307-312 (1985). 119. Inagawa, K., Watanabe, K., Ohsone, H., Saitoh, K. and Itoh, A., Preparation of cubic boron nitride films by activated reactive evaporation with a gas activation nozzle. J. Vac. Sci. Technol. A(5): 2696-2700 (1987). 120. Seidel, K.H., Reichelt, K., Schaal, W. and Dimigen, H. The preparation of cubic boron nitride films by reactive diode sputtering. Thin Solid Films 151: 243-249 ( 1987). 121. Satou, M. and Fujimoto, F., Formation of cubic boron nitride films by boron evaporation and nitrogen ion beam bombardment. Jap. J. Appl. Phys. 22: L171-L172 (1983).
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122. Bricault, R.J., Sioshansi, P. and Baker, S.N., Deposition of boron nitride thin films by ion beam assisted depositon. Nucl. Instrum. Meth. B21: 586-687 (1987). 123. Martin, P.M. and Exarhos, G.J., Relationship between stress, composition and microstructure in sputtered silicon nitride. J. Vac. Sci. Technol. A(3): P615-616 (1985). 124. Eriksson, T.S. and Granquist, C.G., Infrared optical properties of electron-beam evaporated silicon oxynitride films. Appl. Opt. 22: 3204-3206 (1983). 125. Netterfield, R.P., Martin, P.J. and Sainty, W.G., Synthesis of silicon nitride and silicon oxide films by ion-assisted deposition. ADDl. Opt. 25: 3808-3809 (1986). 126. Erler, H.-J., Reisse, G. and Weissmantel, C., Nitride deposition by reactive ion beam sputtering. Thin Solid Films 65: 233-245 (1980). 127. Harper, J.M.E., Cuomo, J.J. and Heatzell, H.T.G., Quantitative ion beam process for the deposition of compound thin films. ApDl. Phys. Lett. 43: 547-549 (1983). 128. Netterfield, R.P., Muller, K.H., McKenzie, D.R., Goonan, M.J. and Martin, P.J., Growth dynamics of aluminum nitride and aluminum oxide thin films synthesized by ion-assisted deposition. J. Appl. Phys. (in press). 129. Bhat, S., Ashok, S., Fonash, S.J. and Tongson, L., Reactive ion beam deposition of aluminum nitride thin films. J. Electron. Mater. 14: 405-418 (1985). 130. Perry, A.J., The structure and color of some nitride coating. Thin Solid Films 135: 73-85 (1986). 131. Grigorov, G.I., Martev, I.N. and Balabanov, S., Optically selective coatings of Ti-N conlpounds obtained by ion-stinlulated sorption. Thin Solid Filnls 137: 1-5 (1986). 132. Martin, P.J., Netterfield, R.P. and Sainty, W.G., Optical properties of TiN produced by reactive evaporation and reactive ion-beanl sputtering. Vacuum 32: 359-362 (1982). 133. Sundgren, J.E., Structure and properties of TiN coatings. Thin Solid Films 128: 21-44 (1985).
20 Dial110nd and Dial11ond-like Thin Fill11s by Ion Beal11 Techniques
Makoto Kitabatake and Kiyotaka Wasa
20.1 INTRODUCTION
Metastable synthesis and the nl0dification of the properties of nlaterials play important roles in present-day industry and are expected to produce new materials for future industry. Ion beam techniques (ion bombardment modification and ion assisted deposition) are recognized as metastable synthesis and modification processes. Ion collisions generate temperature and displacement spikes on the surface of the materials and also may lead to increased chemical activity at the surface. These spikes create localized and rapidly collapsing conditions of high temperature and high pressure. These techniques are expected to be analogous to the macroscopic quenching processes used in the formation of metastable high-temperature and high-pressure phases of materials. Diamond is the well-known metastable high-temperature and high-pressure phase of carbon crystal (1). Graphite is the stable phase of carbon crystal under normal conditions. In vacuum, diamond can withstand very high temperatures without transforming to graphite. This high stability of diamond is caused by energetical bonding of carbon in the diamond crystal. Large activation energies are required to disrupt the crystalline form of diamond. The activation energy is estimated to exceed 60 kcal/mol. This value coincides with the vaporization energy of graphite. This large activation energy enables us to synthesize the metastable state of diamond, permits its formation with negligible change to graphite. The outstanding mechanical, physical, and chemical properties of diamond are expected to be valuable to future industrial applications (2). The synthesis of diamond in thin film form is recognized as one of the principle objectives for the research in diamond formation. The metastable syntheses of diamond in thin film form under low-temperature and low-pressure conditions is greatly desired to minimize thermal expansion coefficients with the substrate, as well as reduce the high temperature on other portions of the fabricated structure.
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We have reported the growth of diamond crystals under room-temperature and lowpressure conditions using ion beam techniques (3,4). In this chapter, diamond and dianlond-like thin films prepared by ion beam sputter deposition (single ion beam) are discussed.
20.2 PRINCIPLE OF DIAMOND SYNTHESIS
Figure 1 shows the general directions in the the area of diamond synthesis in block diagram form. The high-temperature/high-pressure synthesis under thermodynamic equilibrium conditions evolved to either a low-pressure/high-temperature or highpressure/low-temperature process, and then finally to a low-temperature/low-pressure synthesis. This chapter describes this low-temperature and low-pressure synthesis of diamond. The next section discusses the evolution of this synthesis via the low pressure/high temperature process.
CHEMICAL CVD - - - - - - - - - - - - - - - - - - - - - - Reactivity Cubic Diamond High Temperature .--------~
L-
~
1-------,.
Low Pressure
Low Temperature
ION ASSISTED DEPOSITION
Hi h Pressure Hexagonal Diamond compress along graphite c-axis - - - - - - - - - - - - - - Spikes (atomic scale)
PHYSICAL Figure 1: Evolution of diamond synthesis. 20.2.1 Conventional Synthesis
The pressure-temperature (P-T) phase and reaction diagram for carbon is shown in Fig. 2. The thermodynamic phase boundaries and the P-T region, in which the practical synthesis were undergone, are illustrated. The conventional methods synthesize diamond fronl some form of graphite (G), in general, hexagonal graphite. There are two crystallographic types of diamond: cubic diamond (CD) and hexagonal diamond (HD). The catalyst-solvent method is performed in the P-T range indicated by the crosshatched region A (1500-2500 K, 50-100kb) in Fig. 2.(5) In this method, diamond is synthesized from graphite under the P-T regions in which diamond is stable. This method
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417
results in the formation of cubic diamond and can produce large and perfectly formed diamond crystals. On the other hand, when the crystallized graphite is compressed in the direction in which the c-axes of the crystallites are parallel to each other and to the direction of compression, hexagonal diamond can form in the region indicated by B in Fig. 2. Experimental results suggest that hexagonal diamond can form even at room temperature but it is not stable enough to be retrieved unless a setting temperature exceeding approximately 10000e is applied. That is, the domains of crystallites are so small that they are not stable in depressurization. This result reported by Bundy et al (6) indicates a possibility of low-temperature synthesis of diamond. Hexagonal diamond is synthesized normally in the fine-grained form. Hexagonal diamond mixed with cubic dianl0nd is also produced by shock-quench method (7).
300..----.-------.------.r---...--.....-...
00
2000
4000
Temperature(K) Figure 2: Pressure-temperature (P-T) phase and reaction diagram for carbon.
The diamond crystal structure is oriented such that each carbon atom is surrounded by four other carbon atoms (fourfold symmetry). The difference of the next-nearestneighbor position distinguishes between cubic and hexagonal diamond. Figure 3 shows configurations of fourfold symmetry carbon of hexagonal and cubic diamond. The [llllcD direction of cubic diamond corresponds to the [OOOllHD direction of the hexagonal diamond. The 60 degree rotated bonding of the fourfold coordinated carbons along the [111 lCD direction of cubic diamond coincides with that of hexagonal diamond along the [OOOllHD direction as shown in Fig. 3. The periodicity of hexagonal diamond along [OOOllHD direction is double the length of that of cubic diamond along the [llllcD direction (7).
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The graphite-diamond transformation is considered in the following discussions (7). It was noted that (cubic) diamond produced by shock wave down the c-axis of graphite showed a [112]cD preferred orientation. This result suggests that [OOOI]G direction of graphite becomes [112]cD of cubic diamond. In addition, the bond length between carbon atoms parallel to [110]cD direction of cubic diamond is almost same (2% difference) as that parallel to [1210]G direction of graphite. It is reasonable to say, that [1210]G direction of graphite changes to [11 O]CD of cubic diamond. Hence the structural relationship between the basic parent graphite and the synthesized (cubic and hexagonal) diamond is recognized as shown in Fig. 3.
Cubic diamond
Hexagonal diamond
(00011
[f2fOl
rl~~~J~~E/~~L...-[10fO]
Graphite
Lt
•
Figure 3: Relation between the carbon configurations of cubic diamond, hexagonal diamond, and graphite.
A mechanism of the graphite-diamond (hexagonal) transformation was proposed as shown by arrows in Fig. 3(c) (6-7). The basal planes of graphite are assumed to approach each other during compression. Rows of carbon atoms in the graphite [1210]G direction
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
419
are displaced together at the same time,and the adjacent rows in the basal plane of graphite take alternate displacements in the graphite [OOOl]a direction. When carbon atonlS in the adjacent basal plane of graphite are joined, hexagonal diamond is constructed. The [1210]HD direction of hexagonal diamond is parallel to the [1210]a direction of graphite. The [OOOl]HD direction of hexagonal diamond is normal to the [OOOl]a direction and parallel to the [1010]a direction of graphite. For the graphite-to-cubic diamond transformation, a more complicated rearrangement of the carbon atoms is necessary than for the graphite-to-hexagonal diamond transformation. It is believed that a higher temperature (or equivalent energy) is required for the rearrangement to achieve the cubic crystalline form. Because of the relatively short period of time under the conditions of high temperature and high pressure, the shock-quench method sometimes results in the formation of hexagonal diamond. 20.2.2 Synthesis from the Gas Phase
In this decade, the growth of diamond from gas phase has been progressing rapidly. These deposition techniques are performed under moderate to low vacuums at far from thermodynamic equilibrium. Films have been deposited that show either crystalline dianl0nd or diamond-like amorphous structure. These vapor deposition techniques are subdivided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD is based on the ion beam techniques and utilizes some aspect of energetic particle bombardment (usually ions) during the deposition. The first report of this process was in a paper entitled "Ion-Beam Deposition of Thin Film of Diamond-Like Carbon" was published by Aisemberg and Chabot in 1971 (8). They noted that the diamond-like carbon films were deposited using 40 eV carbon ions (positive) in an ion-beam deposition technique at room temperature. The equivalent thermal energy of these ions exceeds 105 degrees. The deposited filnls were amorphous but were transparent, highly insulating, and hard enough to scratch glass. Several years later, Weissmantel et al. reported that diamond-like films were deposited using dual ion beam sputter deposition techniques (9). Wasa et al reported that a diamond-like filnl was deposited by rf-nlagnetron sputtering using a diamond powder target (10). In this case, the carbon particles were sputtered from the target. The deposited film was bombarded with additional argon ions (from the plasma or the ion beam) to modify its properties. Using a variety of CVD methods(11)-(14) thin, polycrystalline films and micron-scale particles of cubic dianlond have been deposited under high temperature and low-pressure conditions. It is noted that atomic hydrogen plays an important role in diamond crystallization and nucleation in CVD techniques. In one aspect of this CVD process, the atomic hydrogen apparently interacts strongly with graphite but weakly if at all with diamond. Such effects selectively etch graphite, effectively suppressing the graphite deposition, and resulting in net diamond crystal growth. A plasma discharge or a hot filament ( > 2000 ° C) are used to create atomic hydrogen. It should be noted that atomic hydrogen also contributes to the creation of hydrocarbons, which may have introduced a fourfold symmetry (low levels of methane or other hydrocarbons are usually added to the discharge). On the surface of the substrate, the absorbed hydrocarbons are decomposed, release hydrogen, and assist in the construction of the diamond configuration of carbon. In order to decompose the hydrocarbons, high temperature (> 400°C) is necessary. Spontaneous nucleation of diamond crystals has been observed mostly on defects like
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scratches, grain boundaries, dislocations, etc.. Recent reports, however, suggest that after a sufficient incubation time, spontaneous nucleation has been observed in the absence of defects or seed crystals (15). The CVD-synthesized diamond crystal particles take octahedral (constructed with [111] faces), internlediate cubo-octahedral or cubic form (constructed with [100] faces). The CVD methods also create diamond-like amorphous carbon films on the substrate at lower substrate temperature (16-17).
20.3 EXPERIMENTAL TECHNIQUES
The ion-beam sputter deposition system, which has been used for the present experiments, is illustrated in Fig. 4. The graphite disk target (purity 99.999%, 100mm in diameter) was bonded to the water-cooled holder. An electron-bombardment ion source using a hot filament was employed. The ion-beam aperture was 25 mm in diameter. The ion energy and the ion current were 1200 eV and 60 rnA, respectively. The incident angle of the ion beam to the target was about 30° from the normal to the target surface. Argon and hydrogen gas were introduced to the ion source and the deposition chamber through mass flow controllers. The substrate was fixed on the substrate holder and placed near the target as illustrated in Fig. 4. The substrate holder was movable in the deposition chamber, allowing the angle of the substrate to the ion beam to change, while keeping the substrate near room temperature with water cooling.
t:"::
.
···:i·~.n:::~ ~p.f1 Sub strate ... ...... . .............-.. .,
••••••: •••:li,. ~
H2
----
~
.~
--~--a..----f
to Pump
Figure 4: Ion-beam sputter deposition systenl.
The surface of the substrate was fixed almost parallel to the direction of the ion beam. The ion beam sputtered the target and in addition bombarded the surface of the substrate
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
421
at grazing incidence. The ion current densities were about 1 and 0.04 mAlcm2 at the target and the substrate, respectively. The ion beam, which bombarded the substrate at a grazing incidence, perturbs the arrangement of the surface atoms in the deposited carbon film. The energy of each bombarding ions was high, but the momentum component normal to the surface of the substrate was low due to the grazing incidence. The localized atomic-scale activations by these grazing ions were formed in linear regions along the surface. The affected surface area is considerably larger for the ion impact at grazing incidence, as compared to one near normal incidence. Therefore it is considered that the uniformity of the deposited film using the additional bombardment of ions at grazing incidence is improved over that using a similar nurrtber of ions at normal incidence (from a second source, for example). In order to estimate the effects of incident angle of bombarding ions, the angle between the substrate holder and the ion beam was varied between o and 7 degrees. Table 1 summarizes the sputtering conditions. Si(lll) or fused quartz plates 0.2-0.3 mm in thickness were used as the substrates. The substrates were sometimes covered with Ni or Ti thin films. It is believed that these metal films may assist in the nucleation of diamond crystals. The metal thin films are also convenient for later preparation of samples for transmission electron microscopy (TEM). For example, the Ni thin film was dissolved by a mixed solution of nitric acid and acetic acid after the deposition. The the carbon films which were deposited on the Ni or Ti covered substrates were then peeled off and used as samples for TEM. TABLE 1. Sputtering conditions.
Target Target Dimension Ion Source Energy Current Beam Dia. Gas Pressure Target-Ion Source Spacing Growth Rate
Graphite Plate (99.9999 0/0) 100 nlnl diameter 1200 eV 60mA 25 nlm diameter (a) 5 x 10- 5 Torr (b),(c) 2 x 10-4 Torr 250mm 300-400 nnl / hr
In order to study the effect of hydrogen ion bombardment, films were deposited under three different experimental conditions as follows: (a) Pure argon fed through the ion source. The argon ions (Ar+ and/or Ar2 +) bombarded the surface of the substrate at a grazing incidence. The kinetic energy was about 1200eV. (b) Hydrogen was also fed through the vacuum chamber. Hydrogen molecules (H2 ) with low thermal kinetic energy « 10- 1 eV) inlpinged on the surface of the substrate during the deposition. The bombarding ions were argon as in case (a).
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(c) Hydrogen was also fed through the ion source. The grazing ions were mixture of argon and hydrogen (Ht and/or H+). The ion current ratio [Ar/H] was about 5/1. The deposition chamber was evacuated to 10- 7 Torr before the deposition and was maintained at pressure of (a) 5 x 10- 5 Torr argon or (b) and (c) 2 x 10- 4 Torr of argon and hydrogen (l: 3 in pressure) during the deposition. The mean free path under these pressures are estimated at more than 50 cm. The ion beam therefore impacts the surface of the substrate without a significant number of collisions with gas molecules, and almost all of the ion beam energy is supplied to the surface of the substrate. When hydrogen was introduced to the ion source, the discharge became unstable and the ion beam operation was difficult to control. High discharge voltage and high cathode current stabilized the operation. The films, which were deposited under these (a), (b), and (c) conditions, are designated film (a), film (b), and film (c), respectively.
12.4 DIAMOND-LIKE FILMS 20.4.1 Characterization
The TEM image and the electron diffraction analysis (EDA) were obtained. The samples were deposited on the Si substrate covered with a Ni thin film. The TEM image and the EDA pattern of film (a), which is similar to that of film (b) is shown in Fig. 5. The pattern is a halo and indicates that these films consist of an amorphous structure. Careful observation of the TEM image in Figure 5 points out that the amorphous structure of these films include many defects and clusters and faded linear tracks of about lOOP.. wide.
Figure S: TEM image and EDA pattern of diamond-like film (a) deposited using argon
ion bombardment. Figure 6 shows the typical TEM image and the EDA pattern of film (c). Small particles of size 0.1-1 J.lm were observed only in film (c). Diffraction spots were superimposed
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
423
on the halo in the EDA pattern of film (c). The EDA suggests that these particles are cubic diamond crystals (discussed below) surrounded by amorphous structure. This interpretation is consistent with visual observations in which diamond particles were sometimes formed in the amorphous structure. The density of these particles was below 0.1 % per unit area. In the amorphous structure, there are linear tracks of about 100 A wide which is clearer than those of film (a) in Fig. 5. The tracks observed in Figs. 5 and 6 are believed to be the trails which are affected by the grazing argon ions. The clusters and defects in the amorphous structure are barely observed in Fig. 6. The amorphous structure of film (c) is more uniform and rigid than that of film (a) and film (b). The carbon atoms deposited under argon and hydrogen ion bombardment exhibited uniform and rigid amorphous structure which sometimes included diamond crystals.
0.1 11m
. ( 02 I)
_
~1110]
,TIl ~
. (1111 0211
I 1 ) 2 I
·(222)
• ( J J J )
\[111] Figure 6: TEM image (upper right) and EDA (lower left) pattern of diamond-like film
(sample type c) deposited using argon and hydrogen ion bombardment. Vicker's hardness of film (a), film (b), and film (c) deposited on Si substrates and 1.5 in thickness, was more than 3000 kg/mm2 for 10 g of load. This value is higher than that of glass (<::< 1800) but lower than that of diamond ( <::<10,000). These films were hard but brittle. When we measured the hardness of thinner films than these, or for more than 10 g of load, the deposited films cracked and detached from the substrates. Using this type of hardness measurement, it is difficult to quantitatively determine differences between these three types of films. JLm
Figure 7 shows the Raman scattering spectra of the films. The wavelength of the excitation laser beam is 5145 A. The spectra show the broad peak from 11 00 to 1700 cm- I . The peak at 1558 cm- I , which is observed in film (a) and film (b), is attributed
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to the background oxygen molecules in the measuring atmosphere. The intensity ratio of the spectra of film (a) to film (b) to film (c) was 1 : 1.5 : 15. Therefore, it is believed that the atomic structure of film (c) is more uniform and rigid than that of film (a) and film (b). A similar result was also inferred from the TEM measurement as mentioned above. It is reported that the Raman scattering spectrum of diamond shows a peak at 1332 cm -1 that of graphite shows it at 1580 cm -1, and that of amorphous carbon (graphitic structure) shows broad peaks at 1600 and 1360 cm- 1 (18). In addition, some polymers have peaks in the range of 1100 - 1700 cm- 1 It is believed that the diamond-like amorphous carbon structure contains (possibly modified) diamond, graphite (graphitic), and polymer structures.
....
....-.-
c
:::a
.
.c ... CO
-.......-
....>en
c
....C
Q)
c
CO
(e)
E CO a:
x15 1000
1400
1800
Raman Shift (em- 1 ) Figure 7: Raman scattering spectra of diamond-like films.
The refractive indices and the electrical conductivities of film (a), fHnl (b), and film (c) are shown in Table 2. The refractive indices were measured by ellipsometry. Wavelength of the measuring light was 6328A. The refractive indices of these films are higher than the value of diamond (2.4) and the diamond-like films deposited by other nlethods (2.0-2.8). Film (c) shows a lower refractive index than film (a) and film (b). Film (a) exhibits high electrical conductivity. The electrical conductivity decreases from film (a) to film (c). The optical absorption spectra of these films are shown in Fig. 8. The large optical absorption of film (a) and (b) cause the apparently large refractive indices (measured by ellipsometry). The light absorption, the refractive index, and the electrical conductivity all decrease from film (a) to film (c). The hydrogen ion bombardment may be responsible for the sputter deposited carbon films being transparent and insulating. The optical en-
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
425
ergy gap, which is estimated from the optical absorption spectra of film (c), is 1.04 eVe This is narrower than that of diamond (5.6 eV). It is believed that the light absorption and electrical conductivity are caused by the presence of graphitic structure within the amorphous structure. Precipitation of the graphitic structure is suppressed by the hydrogen ion beam.
TABLE 2:
The refractive indicies and electrical conductivities of films (a), (b), and (c)
n a( ohm-cm)-l
(a)
(b)
(c)
3.1 103
3.1 10
2.8 10-2
1600
2000
Wavelength (nm) Figure 8: Optical absorption spectra of diamond-like films. 20.4.2 Discussion
Diamond particles could not be observed in films (a) and (b) which were deposited without hydrogen ion bombardment. The growth of diamond was observed in film (c) under bombardment of the substrate by argon and hydrogen ions. Moreover film (c) is transparent, insulating, uniform, and rigid in comparison with films (a) and (b). Hydrogen molecules with thermal energy (low temperature) have little effect on the structure of the resultant carbon films, because of their low chemical and physical activity. The hydrogen ion bombardment plays an important role in the growth of diamond at room
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temperature and the formation of the uniform and rigid diamond-like amorphous carbon structure. The effects of simultaneous bombardment of the substrate with hydrogen and argon ions are thought to be as follows: Energetic activation and rapid quenching occur at the surface of the film during deposition. Bombardment with argon ions supplies the thermal and displacement spikes to the configurations of the carbon atoms on the substrate. The ~ 100 A-wide trails are shown in the deposited films (Figs. 5 and 6). The spikes made by grazing argon ions may create high-temperature regions with increasing pressure within the ~ 100A -wide line region. The diamond structures are partly formed around these spikes. These diamond structures are restricted in their growth, because they are surrounded by the amorphous regions. The carbon atoms, which compose the non-diamond (mainly graphitic) structures, are easily activated by hydrogen ion bombardment and change a gas phase of hydrocarbons. The non-diamond structures can thus be selectively removed, but this removal is not complete. If this removal was complete, only diamond would have grown on the surface of the substrate. The high refractive index, electrical conductivity, and light absorption of the diamond-like amorphous films are caused by the non-diamond (graphitic) structures which are not removed and remain in the film. The depositing carbon atoms incident on the substrate surface in the present ionbeam-sputter deposition case exhibit relatively high kinetic energy on the order of several eV per depositing atom). When the cooling of the target was inadequate, the deposited films became brownish and electrically conductive. The inadequate cooling caused by the increase of the thermally agitated carbon particles of low energy from the target. It is believed that the low energy particles mainly form non-diamond structures and make the filnl brownish in color and electrically conductive. The reaction of the incident hydrogen ions and the incident carbon atonlS leads to an increase in the partial pressure of hydrocarbons in the chamber. Figure 9 shows mass spectroscopy of the depositing atmosphere of film (c). Various C-H compounds exist (CHn where n = 1-4), as well as a strong signal from C 2 H n • It has been noted above that these hydrocarbon molecules appear to play an inlportant role in CVD-based growth methods for diamond. The diamond structures formed by these deposition mechanisms have many broken bonds. The broken bonds make the configurations highly unstable, and they may collapse rapidly. If the broken bonds are rapidly passivated (or pinned) by hydrogen, the diamond structures become less active so that they may grow to a stable size. The nucleation of the diamond seed crystals must be achieved, however, before diamond crystals can be grown. This nucleation process is discussed in a later section.
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
Ar
H
C 2 Hn
427
Ar
CH n
. o
,I II 10
II
I
I
I
30
Mass Number Figure 9: Mass spectroscopy of the atmosphere during the deposition of diamond-like
film (c). 20.4.3 Applications
The wear and friction behavior of diamond-like carbon films has been studied (19). Polymethylnlethacrylate (PMMA) substrates were covered with the sputtered diamondlike films. The thickness of these films were about 200A. The sliding frictional coefficient was measured using a hard sphere with 10-g load and a 2-cm sliding length. Figure 10 shows the sliding coefficient of friction versus the number of sliding cycles for PMMA substrates covered with the diamond-like films. Figure 11 shows the surface images of the diamond-like films after 600 sliding cycles. The sliding frictional coefficient of PMMA covered with film (a) is moderately low and declines slowly with an increasing number of cycles. Following these tests, the surface showed broken pieces of detached film, as shown in Fig. 11. Filnl (a) is broken along the grain boundaries which is observed in the TEM image shown in Fig. 5. These results suggest that film (a) contains a graphitic structure which is consistent with the observed low frictional values. The content of the graphitic structure was also pointed out in the preceding characterization (sec. 20.4.1). The sliding frictional coefficient of PMMA covered with film (c) is as large as that of non-covered PMMA (Fig. 10). The broken pieces of the detached film are not observed on the surface following these tests, but scratches are evident. The scratch depth is higher than that of the non-covered PMMA. The broken pieces of film (c) do not serve as a lubricant but are hard enough to scratch PMMA. Film (c) is broken sharply along the course of the sliding sphere in comparison with film (a) which as previously, strongly adheres to the substrate.
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O. 9.-----.-----r-,.....-r-.----.-~-,.....
......___,
PMMA _ ••- (e)
0.8
--- (a)
-- -- layered (a)/le)
0.7 ~
ell
0.6
'1:) ell
o
U
c
o
;: u '':
U.
-,;-,"",
,
0.2 .,
\~
.-....... - .......
'................."'., ~---------------------_:~~~~:~~~~
0.1
Number of cycles Figure 10: Sliding frictional coefficient of PMMA substrates covered with diamond-like films versus number of sliding cycles.
.' ';
(a)
(e)
Figure 11: Surface images of PMMA covered with diamond-like films after 600-cycles
sliding.
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429
Layered structures were fabricated of alternating layers of films (a) and (c). The thickness of these layered films was 200A (film(a) = 100A, film(c) = 100A). The sliding frictional coefficient of PMMA covered with these layered films shows low value (0.16) and is stable to many cycles. No scratches or the broken pieces of detached film were observed on the surface of this coated PMMA substrate after more than 1000 sliding cycles. Film (c) is uniform, rigid and strongly adheres to PMMA. Film (a) serves as a lubricant. This layered film acts as a wear-resistance coating. It is also noted that it is adaptable to flexible films (magnetic tape, etc.).
20.5 DIAMOND PARTICLES 20.5.1
Characterization
The EDA pattern shown in Figure 6 can be indexed as cubic diamond whose lattice constant coincides with that of the natural diamond within experimental error. The plane indices corresponding to each diffraction spot are also indicated in Figure 6. The particles shown in Fig. 6 agree with the well-defined morphology of diamond. These crystal particles exhibit the (111) and (100) faces and cubo-octahedral form as shown in Fig. 12. It is noted that the [111] and [110] directions of the diamond crystal are parallel to the surface of the substrate.
Figure 12: Face identification of cubo-octahedral form of the cubic diamond particle shown in Fig. 6.
The angle between the ion beam and the surface of the substrate was changed from almost zero to 7 degrees. This resulted in slightly increased ion bombardment during the deposition process. The other sputtering conditions were kept the same as mentioned above. In this case, Si substrates covered with a thin Ti layer were used. The typical TEM image and the EDA pattern of the particles included in amorphous structure are shown in Fig. 13.
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0.1 11m /[111ICD
+ . (i31)
• ( '2 2 0 )
('jlI)~ (110)cD Main Spots •
Cubic Diamond
Additional Spots ...
HGKagonal Diamond
Figure 13: TEM image and EDA pattern of the particles included in a diamond-like film deposited using ion bombardment at 7°.
These particles do not show a well-defined morphology like those in Fig. 6. Figure 14 shows the EDA pattern of the tilted particles shown in Fig. 13. In Fig. 14, there are main spots which correspond to cubic diamond and additional spots which are observed at the center between the main spots belong the [11l]CD directions. Such additional spots suggest the existence of double periodicity along the [11l]CD direction of cubic diamond. In Sec. 20.2 it was discussed that the periodicity along [OOOI]HD of hexagonal diamond is double as long as that along [11l]CD of cubic diamond and hexagonal diamond is made normally in the fine-grained form (that is, does not take well-defined morphology). It is believed that the particles shown in Fig. 13 are hexagonal diamond. The EDA pattern shown in Fig. 13 can be indexed as also shown in Fig. 13 using the main and additional spots explained above. These hexagonal diamond particles take the same crystalline orientation as cubic diamond in Fig. 6. It has been suggested that the nucleation of diamond crystals occurs on the defects of the surface of the substrate. In order to form such defects, the surface of the Si substrate was scratched by SiC powder. The sputtering conditions were same as that mentioned above. The scratched substrate was then covered with Ni thin film. The shallow scratches had little effect on the nucleation of diamond crystals. The deep scratches (over several hundred nm) worked as a source of nucleation centers. Figure 15 shows the TEM image of the carbon film deposited on the deeply scratched surface of the substrate. The smooth area (indicated by (s) in Fig. 15) of the film is deposited on the smooth area of the substrate surface. The granular area (indicated by (g) in Fig. 15) of the film is deposited on the trails of the scratches. The EDA of the (s) region gave a halo and therefore the (s) area took amorphous structure. Diffraction spots were superimposed on the halo in the EDA of the (g) area. These spots can be indexed by hexagonal
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
431
diamond as shown in Table 3. The hexagonal diamond crystallites are thus selectively grown on the trails of the deep scratches.
/lOO1leD
•
•
Main Spots cubic Diamond
~11]eD
~l110]eD
Additional Spots ..
Hexagonal Diamond
Figure 14: EDA pattern of the tilted particles shown in Fig. 13.
(s)
Figure 15: TEM image of the carbon film deposited on the deeply scratched surface.
432
Handbook of Ion Beam Processing Technology
TABLE 3: Observed and calculated lattice spacing, d.
d obs
deal
(hkl)
3.09 1.92 1.64 1.55 1.35
3.09 1.93 1.64 1.54 1.37
(004) (103) (105) (008) (107)
hexagonal diamond (ASTM card 26-1082)
20.5.2 Discussion
The EDA of cubic and hexagonal diamond particles shown in Figs. 6 and 13 exhibit the same crystalline orientation. The [llllcD and [IIOlcD directions are parallel to the surface of the substrate. Hexagonal and cubic diamond are grown on the surface of the substrate as illustrated in Fig. 3. Such results indicated that the nucleation of cubic and hexagonal diamond is based on the same mechanism. This mechanism may be similar to that of the Bundy's low-temperature and highpressure process (see Sec 20.2). The compression nlade by ion collisions is effective in the direction normal to the surface of the substrate. If there exists a graphite structure, whose c-axis is normal to the surface of the substrate, such compression causes the graphite-diamond (hexagonal) transformation. This relationship between graphite crystalline orientation and the compressional orientation most probably causes the transformation to diamond. The crystalline diamond particles synthesized by above mentioned mechanism exhibit the crystalline orientation whose [llll cD and [IIOlcD directions are parallel to the surface of the substrate. These discussions are directed towards the nucleation of diamond. The nucleation of diamond is attributed to the modulation of the already deposited arrangement of carbon atoms. For the present preparations, the surface of the substrate was covered with Ni or Ti. At this stage, it is difficult to discuss the effect of (catalyst) metals on nucleation. In order to solve this problem, more advanced and detailed studies are required. After nucleation, diamond particles grow under the physically and chemically activated ion-beam-sputter deposition process as described above. Argon and hydrogen ions play physical and chemical roles, respectively, in the activation. The activation affects the crystallographic feature of the deposited diamond particles. If chemical activation is dominant, cubic diamond can be grown by the moderate chemical process. If the ion beam and the surface of the substrate are parallel to each other, the physical effects of ion collisions are suppressed and chemical effects of activated hydrogen become dominant. In this case, cubic diamond particles with a well-defined morphology are grown as shown in Fig. 6. On the other hand, the hexagonal diamond particles without a defined
Diamond and Diamond-Like Thin Films by Ion Beam Techniques
433
morphology are grown when the incident angle of the ion beam at the surface of the substrate is closer to normal as shown in Figs. 13 and 15. (The incident angle at the wall of the deep scratched trails is also high.) The high incident angle enhances the physical activation. The physical activation may cause the growth of hexagonal diamond. These growing conditions of hexagonal diamond are similar in a general sense to the shockquench method.
20.6 CONCLUSION
Diamond-like amorphous carbon films were deposited by ion beam sputtering. Three types of film were prepared under different deposition conditions. Argon and hydrogen ion bombardment at grazing incidence caused the filnls to be harder, more transparent, insulating, more uniform, and rigid. Diamond crystals were grown at room temperature by the present ion beam sputter deposition. The ion bombardment plays an important role in the nucleation and growth of diamond. The nucleation of diamond is attributed to the modulation of the already deposited arrangement of carbon by the argon ion bombardment. Argon and hydrogen ions play physical and chemical roles in the activation, respectively. The deposited diamond particles took two types of crystalline form: cubic diamond and hexagonal dianlond. The physical activation causes the growth of hexagonal diamond. Cubic diamond is grown where the chemical activation is dominant.
20.7 REFERENCES
1.
Bundy F.P., Strong H.M., Wentorf R.H., Jf., in: Chemistry and Physics of Carbon (P.L. Walker, Jr. and P.A. Thrower, ed) Vol. 10 pp. 213-263, Marcel Dekker Inc., New York (1973).
2.
Chrenko R.M., Strong H.M., Physical properties of diamond. GE Technical Infornlation Selies RD-54: 1-45 (1975).
3.
Kitabatake M., Wassa K., Growth of diamond at room temperature by an ion-beam sputter deposition under hydrogen-ion bombardment J. ADDI. Phys. 58: 1693-1695 (1985).
4.
Kitabatake M., Wasa K., Diamond films by ion-assisted deposition at room temperature. J. Vac. Sci. Technol. to be published.
5.
Bundy F.P., Bovenkerk H.P., Strong H.M., Wentorf R.H., Jr., Diamond-graphite equilibrium line from growth and graphitization of diamond. J. Chem. Phys. 35: 383-391 (1961).
6.
Bundy F.P., Kasper J., Hexagonal diamond - a new form of carbon. J. Chem. Phys. 46: 3437-3446 (1967).
7.
Wheeler E.J., Lewis D., The structure of a shock-quenched dianlond. Mat. Res. Bull. 10: 687-693 (1975).
8.
Aisenberg S., Chabot R., Ion-beam deposition of thin films of diamondlike carbon. J. ADDI. Phys. 42: 2953-2958 (1971).
9.
Weissmantel C., Ion beam deposition of special film structures. J. Vac. Sci. Technol. 18: 179-185 (1981).
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10. K. Wasa; Japan Patent 53-10394; July 15, 1976; assigned to Matsushita Electric Industry. 11. W.G. Eversole; U.S. Patent 3,030,187 and 3,030,188; Apr. 17, 1962; assigned to Union Carbide Corporation. 12. Angus J.C., Will H.A., Stanko W.S., Growth of diamond seed crystals by vapor deposition. J. ADD!. Phys. 39: 2915-2922 (1968). 13. Spitsyn B.V., Bouilov L.L., Derjaguin B.V., Vapor growth of diamond on diamond and other surfaces. J. Cryst. Growth 52: 219-226 (1981). 14. Kamo M., Sato Y., Matsumoto S., Setaka N., Diamond synthesis from gas phase in nticrowave plasnla. J. Cryst. Growth 62: 642-644 (1983). 15. Badzian, A.R., Badzain, T., Roy, R., Messier, R. and Spear, K.E., Crystallization of diamond crystals and films by nlicrowave assisted CVD (Part II). Mater. Res. Bull. 23: 531-548 (1988). 16. Holland L., Ojha S.M., The growth of carbon films with random atomic structure from ion impact damage in a hydrocarbon plasma. Thin Solid Films 58: 107-116 (1979). 17. Vola H., Moravec T.J., Structural investigation of thin films of diamondlike carbon. J. ADD!. Phys 52: 6151-6157 (1981). 18. Sato Y., Kamo M., Kanda H., Setaka N., Carbons deposited on diamond surface. (in Japanese) Hyoumenkagaku 1: 60-66 (1980). 19. Hirochi K., Kitabatake M., Wasa K., Wear and friction behavior of diamond-likecarbon films prepared by ion-beam deposition. (in Japanese) Oyobutsuri 56: 256-262 (1987).
Index
Activated reactive evap. - 387 Adhesion - 279, 381 Adhesion enhancement - 283 Adhesion mechanisms - 292 Ag films - 176 Al etching - 220 Alloy sputtering - 123, 154, 186, 345 Al2 0 3 films - 393 Aluminum nitride - 73,187,406 Amorphous Ge films - 375 Amorphous Si - 35, 71 Angular dependence of sputtered atoms - 87,101, 134 Angular dependence of sputter yield - 80, 342 Anisotropy - 32 Ar implantation - 210 Aspect ratio - 224 Axial discharge - 11 Backstreaming - 18 Beam current - 10 Beam profile - 47 B-N deposition - 36, 405 Bombardment after deposition 300 Bombardment modification mechanisms - 210 CaF2 films - 377 Cascade sputtering theory - 81
Cathode - 9 Ce02 films - 253, 386, 400 Chain scission - 315 Channeling - 259 Child's Law - 10 Chlorine etching - 221 Closed-drift ion source - 48 Cluster emission by sputtering - 92 Collimation effects - 222 Collision cascade effects - 249 Columnar structure - 248, 379 Compound synthesis - 382 Conductivity - 42 Contaminant dispersion - 289 Contamination - 18 Cr films - 198 Critical density - 23 Cross-linking - 315 Crystalline orientation - 208, 416 Crystallite size - 201, 204 Crystal structure - 300, 382 Cubic diamond - 416 Cu films - 178, 195 Cu-on-AI 2 0 3 - 287 Cu-on-polyimide - 280 Cu-on-Si - 280 Cu-on-teflon - 293 Cu-oxide - 188 Curvature-driven grain growth - 246 Cu sputtering - 346 CVD - 419 Cyclotron frequency - 22 Cyclotron radius - 23 435
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Defects - 176, 378 Delocalized metal bonds - 322 Densification - 210, 249, 385 Diamond etching - 230 Diamond films - 415 Diamond particles - 426 Diamond synthesis - 416 Diamond transistors - 231 Diode laser array - 228 Direct knock-on sputtering - 83 Discharge chamber - 11 Doppler shift - 115 Doubly-charged ions - 46 Dual grid source - 15 Dual ion beam sputtering - 370 Dynamic recoil mixing - 381 ECR multipole - 30 ECR sources - 21 Electric propulsion - 2 Electron gas postionization - 148 Ellipsometry - 388, 399 End-Hall source - 40 Energetic neutrals - 172 Energy distribution of sputtered atoms - 88, 102, 134, 149 Epitaxial growth - 177 Epitaxial temperature - 181 Epitaxy - 305 Etching damage - 236 Etch profiles - 222 Film composition, impurities - 185 Film density - 190 Film growth - 374 Film microstructure - 373 Film orientation - 259, 302 Film stress - 181 Film topography - 307 Fluorides - 404 Focussed ion beams - 368 Focussing - 14 Friction - 427 GaAs etching - 33, 220 Gas implantation - 184 GdCo films - 186 Ge films - 176, 181, 259 Glow discharge mass spectroscopy 146 Graphite - 416
Grids: ECR - 29 Hard carbon films - 422 Hardness - 378, 423 Henderson structure model - 242 Hexagonal diamond - 416 Hg sputtering - 339 Hollow cathode - 18 Homoepitaxy - 273 Hot jet etching - 231 ICB nozzle - 60 Implantation - 295 Impurity induced topography - 340 Impurity seeding - 346 Interface energy - 279 Interface roughening - 295 Interface roughness - 280 Interface stitching - 282 Interface structure - 283 Intrinsic texturing - 339 Ion acceleration - 50 Ion-assisted deposition - 373, 387 Ion-assisted deposition model - 262 Ion-atom arrival ratios - 257 Ion beam assisted deposition - 369 Ion beam assisted etching - 219 Ion beam deposition model - 270 Ion beam energy distribution - 44, 51 Ion beam focussing - 62 Ion energy effects on films - 194 Ionic charge effects - 70 Ion optics - 13 Ion plating - 389 Ionization cross-section - 174 Ionization probability - 148 Ionized cluster beam - 58, 391 Ionized cluster beam model - 271 Kaufman sources - 8 Langmuir probes - 27 Laser-induced fluorescence - 113 Lattice distortion - 178 Lattice spacing, diamond - 432 Low energy ion beams - 93 Magnetron sputtering - 390 MARLOWE code - 257, 386 Masking techniques - 98
Index
Mass loss sputtering measurements 96 Matrix effects - 131, 152 Mechanical interlock of surfaces 355 Methane - 426 Microhardness - 196 Microstructure - 378 Molecular dynamics - 260 Molecular flux - 226 Multicomponent sputtering - 87, 185 Multilayer sputtering - 163 Multiphoton resonance ionization - 128 Multiple ICB - 61 Multipole discharge - 12 Nb films - 198, 302 Neutralizer - 9 Ni films - 178 Nitrides - 187, 405 Nitrocellulose - 318 Nitrous oxide etching - 236 Non-resonant multiphoton ionization - 138 Nucleation - 68, 430 Nucleation density - 176 Off-normal incidence bombardment - 301 Optical density - 180 Optical interferometry - 100 Optical properties - 184, 358, 376 Orientation - 175 Oxide coverage - 121 Oxygen incorporation - 206 Packing density - 266, 395 Packing fraction - 377 Pd films - 178 Peeling - 280 Peel testing - 281 Penning ionization - 146, 174 PET - 321 PGMA - 317 Photoresist etching - 32 Plasma potential - 171 PMDA-ODA - 322 PMMA - 318, 427
Polyimide - 317 Polymer composit. change - 319 Polymeric materials - 315 Polystyrene - 318 Potential gradient - 43 Property optimization - 216 PTFE sputtering - 344 Pt-Ni - 289 PVC - 324 Quartz-crystal oscillator microbalance - 96 Raman spectra - 424 Reactive deposition - 187,367 Reactive ICB - 70 Reactive sputtering - 84 Redeposition - 249 Refilling - 250 Reflection coefficient - 342 Refractive index - 377, 399, 425 Relaxation time - 305 Resistivity - 184, 196 Resists - 318 Resputtering - 215 Rf-diode - 365 Rills - 351 Ripple topography - 307 Rutherford backscattering (RBS) 97 Sandblasting analogies - 309 Scattering - 378, 383 Secondary neutral mass spectroscopy - 145 Sensitivity factors - 159 Shadow masking - 353 Si etching - 31, 235 Sigmund-Thompson model - 119 Silicides - 381 SIMS of polymers - 320 Single crystal sputtering - 86 Single grid source - 14 Si-nitride deposition - 35 Si0 2 deposition - 33 Si0 2 etching - 31 Si0 2 films - 393 Sputter cones - 339 Sputter deposition - 366 Sputter deposition model - 262 Sputtering of molecules - 123
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Sputter rates - 363 Sputter yield - 79,95,116 Stability in air - 380 Stitching - 381 Stitching mechanisms - 288 Stoichiometry - 185, 382 Stress - 195, 378, 381 Stress modification model - 267 Structure-zone model - 242, 374 Substrate bias - 28 Substrate effects - 203 Substrate temperature - 245 Surface binding energy - 180 Surface damage - 67 Surf9.ce depletion - 385 Surface diffusion - 179, 348 Surface mobility - 69 Surface roughness - 182 Surface topography - 182 Ta20s films - 401 Target temperature - 85, 94 Target topography - 95 Teflon sputtering - 344 Temperature effects - 199 Temperature stability - 378 Textured materials - 352 Texturing - 338 electrical aspects - 357 mechanical aspects - 357 Thermal mobility - 244
Thermal spike - 247 Thermal sputtering - 90 Thin film interface detection 100 Time-of-flight, direct recoil spectrometry - 327 Time-of-flight techniques - 103 TiN films - 379 Ti0 2 - 382, 395 Transmission - 378 Transmittance - 425 Transparent, conducting films - 404 Trenching - 363 TRIM - 250,384 Triode sputtering - 173 Troughs, crests - 311 Vapor phase growth model - 260 Velocity distributions of sputtered atoms - 118 V0 2 fillns - 402 Voids - 376 W etching - 220 W films - 198 Whiskers - 339 XPS analysis - 322, 326 X-ray diffraction - 302
zr0 2 films - 249, 254, 377, 397
