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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
112
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
194 194 194 194
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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
219
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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Contents
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
m
51
70
..c: ()
Ul .r-i
o
••
0
.• •
•t
Q
~
50
Anode current, A
o 2 o 4
30
6
6
o
8
0
0
•
6 0
0
0
0 0
6
0
A 0
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
\
/
\
\
\
/
I
0.01 I
I
/
, \
\
\
\
I
/
\
" --.......
o
0
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-!
.. (>
rn s::
(1)
.. (>
1.5
.. ,
~
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0 0
(1)
~ ~
(>
...
(>
0 0
~ {)
ectS
8
() 10
.9(>
>;
'0
6
. ... ..
e
{)
ICC
(>
2 4
D
1.0
(>
~(>
0
.(>
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(1)
(>
~
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0
m
... (> 0
,
-.(> -.(> ""(>
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-
VA lt~ 1.IO'j 1
1.'(.1 ~1'1
R... llf
A~
nno'J 1(', O-k •
/ l-kV 'UI.l''' 1
-,
."
' .. k'
11
..,.
"
18 "I
'0 1
I.
'1.
Figure 8: Electron micrographs of the deposited film clusters under an overhanging edge as a function of cluster acceleration voltage.
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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.
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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 (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.
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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.
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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-
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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;
96
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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Handbook of Ion Beam Processing Technology
(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)
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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
100
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).
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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
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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.
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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
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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
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Handbook of Ion Beam Processing Technology
ionization to surface structure. This will likely include probing the internal states of sputtered molecules such as NO.
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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. (
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Characterization of Atoms Desorbed from Surfaces
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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
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(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+
MnGaGeAlloy Mn~Ga+
Ge+
GaAs Ga+ As+
Cu+
16
1
10-2
101 NiAr+ CuAr+
GeAr+ MnAr+
c:
OJ
''-
:::J
u
.§ 10-3
Cu;
Ge+
G~oGe+ 2
:.i=
MnGe+ Ga;
d
Qj 0::
104 Ni;
10-2 AsAr+ GaAs+
Ni; NiCu+
+-
As;
GeO~GaAr+
GaAr+ Gai As 0+
G02O+
G02 O+
Mn 2 O+
GaO+
4
16
MnGa+
10-5
10-6
10-3
10- 5
Go++
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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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 ?
4
..........
""
3 2
).,',
,f
5 M
o ",,-0, ,,.., 0, ,. \ ,
I
/~----
//
--.............
JA-...... I
/1
a
_----A
----~
---
I
I
[J
-~----
- --- - - - -----0
I
_/
1 Ii-
-
+
......
-A----
_---A---
-----
I~/
otil 1/ A
-1
-2
t---..'---.. . . .
0
50
I 100
.....I I.-_ _-&I 150 200
1 250
......L.
......., 300
En (eV/Met. at.) Figure 5: Lattice distortion Vd/do of Cu and Pd films grown under Ar ion bombardment as a function of the energy delivered to the substrate per incident metal atom ( En ).
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
108f"t
I
9 Ji 0
6-
a
4-
~ "tJ ~
B
Ne
2-
'
0-
....
'!-- __ ',Ar
Ne
......1:=:: ......
--::::::::::=:: ......
"'-==:::-- ...-::: - Xe8 y=o" ... ----.--...--.--t
- 2-+---Ir----,.r----,.r----,.--""I'.--"'T'I--"'T'I--..... .00 .20 .40 .60 .80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 Gas afom moss / target atom moss Figure 6: Lattice distortion (8d/do) for Au and Pd films as a function of gas-to-target mass ratio and substrate position angle.
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
180
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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Handbook of Ion Beam Processing Technology
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
0.5,u
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( a)
0.5,u
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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Handbook of Ion Beam Processing Technology
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
186
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.
The Modification of Films by Ion Bombardment
187
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 I
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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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Figure 10: Oxygen to copper composition ratio for Cu-O films deposited by reactive evaporation of Cu with concurrent oxygen ion beam bombardment at 100 eV (55).
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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).
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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).
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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).
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._.-._._._.-.-._.- _._._._._._._._._._._._~:-:~~_._._._._._._._._.
__.-
..' .......
' ....
• 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
~0) o
2.2
2.0
1.8
~----I._......l.__......._._I....-___I...
0.0
11
00
.c
.2 ~
:::"';~:::::::::l.::::::,..,.::t·.·::: ~ x
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)
•
H
.x .
,,~ - - - - ' - - - - - + -
o a.. {
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 . 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
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 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-
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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 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~> 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
308
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
310
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
A
B
o
(a) /",-.........
/
/I
( I
\'
/'
\ '\ \
1,-
/'
\' I /-, \ I l
/ilt
\ ' - // \ \
,_/
I /'
.........
\\\\ J
/
\
/1
I \ '- /
-/ I ,_/
\'
,
I
'\ \ I I
"
/-''''\
,......-,
( b)
- \ / '\ \
/
I
I
\ \
\
~ I
,_/ / I
'-----/
I
I ,.-, \ I
\
\ \
I
{
I
I
\ ,_/ /
-,
/
I
'- /\\
-
\
I / /-, \ JI I \ \
"
/1
'-- / ,_/
/
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.
312
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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Handbook of Ion Beam Processing Technology
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::
LJ
~
0
-
t:
Cf)
z
W
t-
Variable angle N 1s XPS spectra of the 4 keV Ar+ bombarded PIQ surface: (a) nitroso, (b) imide, and (c) cyano groups (26). Figure 6:
Z
W
~
t-
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.
378
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
382
Handbook of Ion Beam Processing Technology
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
386
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
392
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 < 2.5 UJ
C
~ UJ
>
2.4
~
(.)
«
a: 2.3 u. a:
UJ
•\ \
.,
\
'
.....
..................
EVAPORATED
...................
..... ............
2.2
--...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
>
0 ~
0
0
• 0.4
0.5
0.6
0.7
0.8
~
~
0.9
1.0
5.0
EJ 10
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--~---.--
......
o
~ 300 C
2.2
•
300·c
2.2
Io
~
I
2.1
It
al,,"
~
~
I
w
> 1= u c(
( a)
/
(b)
,
2.0
I
ff LLI
,.,
a:
t.'
1.9
1.810---1o-----"----'-----'-----'-----'-.......1
20
40
eo
60
100
At CURRENT DENSITY ~cm-2)
50
100
150
O2 ION CURRENT DENSITY
200
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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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Figure 19: The influence of oxygen ion bombardment on the optical properties of Ta20s for various ion energies (31); (a) n 400, (b) k4oo .
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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.
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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 ~
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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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