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Advanced Scanning Electron Microscopy and X-Ray Microanalysis
Advanced Scanning Electron Microscopy and X-Ray Microanalysis
Advanced Scanning Electron Microscopy and X-Ray Microanalysis Dale E. Newbury National Bureau of Standards Gaithersburg, Maryland
DavidC. Joy A T & T Bell Laboratories Murray Hill, New Jersey
Patrick Echlin University of Cambridge Cambridge, England
Charles E. Fiori National Institutes of Health Bethesda, Maryland
and Joseph I. Goldstein Lehigh University Bethlehem, Pennsylvania
Springer Science+Business Media, LLC
Library of Congress Cataloging in Publication Data
Main entry under title:
Advanced scanning electron microscopy and X-ray microanalysis.
Includes bibliographies and index. I. Scanning electron microscope. 2. X-ray microanalysis. I. Newbury, D. E. [DNLM:
I. Electron Probe Microanalysis. 2.Microscopy, Electron, Scanning. QH 212.53 A244) QH212.S3A38 1986 502./8 /25 85-28261
ISBN 978-1-4757-9029-0 ISBN 978-1-4757-9027-6 (eBook) DOI 10.1007/978-1-4757-9027-6
© 1986 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1986. Softcover reprint of the hardcover 1 st edition 1986
All rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher
Preface
This book has its origins in the intensive short courses on scanning electron microscopy and x-ray microanalysis which have been taught annually at Lehigh University since 1972. In order to provide a textbook containing the materials presented in the original course, the lecturers collaborated to write the book Practical Scanning Electron Microscopy (PSEM), which was published by Plenum Press in 1975. The course continued to evolve and expand in the ensuing years, until the volume of material to be covered necessitated the development of separate introductory and advanced courses. In 1981 the lecturers undertook the project of rewriting the original textbook, producing the volume Scanning Electron Microscopy and X-Ray Microanalysis (SEMXM). This volume contained substantial expansions of the treatment of such basic material as electron optics, image formation, energy-dispersive x-ray spectrometry, and qualitative and quantitative analysis. At the same time, a number of chapters, which had been included in the PSEM volume, including those on magnetic contrast and electron channeling contrast, had to be dropped for reasons of space. Moreover, these topics had naturally evolved into the basis of the advanced course. In addition, the evolution of the SEM and microanalysis fields had resulted in the development of new topics, such as digital image processing, which by their nature became topics in the advanced course.
Thus, following this progression of development, the lecturers of the advanced course have collaborated to produce a book containing the principal topics covered in that course. In comparison to the introductory SEMXM volume, the topics covered in this book are undergoing rapid development. Our current efforts represent a snapshot of the field at this point in time. We fully expect these areas to develop quite substantially,
v
vi PREFACE
and we hope that our efforts to assemble these chapters may be of aid to our readers in making their own contributions to a most exciting field of work.
Dale E. Newbury David C. Joy Patrick Echlin Charles E. Fiori Joseph I. Goldstein
Contents
1. Modeling Electron Beam-Specimen Interactions. . . . . . . . . . 3 1.1. Introduction................................... 3 1.2. Cross Sections for Scattering Processes . . . . . . . . . . . . . 3
1.2.1. General Properties ..................... . . 3 1.2.2. Elastic Scattering. . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2.3. Inelastic Scattering . . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Analytic Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.3.1. X-Ray Generation in Thin Foils. . . . . . . . . . . . 16 1.3.2. X-Ray Generation in Thick Targets. . . . . . . . . 17
1.4. Monte Carlo Electron Trajectory Simulation ........ 19 1.4.1. Formulation............................ 19 1.4.2. Applications of the Monte Carlo Method. . . . . 26
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2. SEM Microcharacterization of Semiconductors ........... 45 2.1. Introduction................................... 45 2.2. Voltage Contrast ............................. . . 45 2.3. Voltage Measurements .......................... 52 2.4. Stroboscopic Microscopy ........................ 56 2.5. Charge Collection Microscopy. . . . . . . . . . . . . . . . . . . . 59 2.6. Charge Collection Images of Semiconductor
Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 2.7. Quantitative Measurements in Charge Collection
Microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 2.8. Practical Aspects of Charge Collection Microscopy ... 75 2.9. Cathodoluminescence Studies. . . . . . . . . . . . . . . . . . . . . 77 2.10. Thermal Wave Microscopy. . . . . . . . . . . . . . . . . . . . . . . 82
3. Electron Channeling Contrast in the SEM ............... 87 3.1. Introduction................................... 87 3.2. Origin of Electron Channeling Contrast. . . . . . . . . . . . . 87
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3.3. Characteristics of Electron Channeling Contrast. . . . . . 89 3.4. The Electron Channeling Pattern . . . . . . . . . . . . . . . . . . 92
3.4.1. Coarse Structure . . . . . . . . . . . . . . . . . . . . . . . . . 92 3.4.2. Fine Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
3.5. Electron Optical Conditions to Observe Electron Channeling Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.5.1. Beam Divergence . . . . . . . . . . . . . . . . . . . . . . . . 99 3.5.2. Beam Current. . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.5.3. Choice of Detectors ...................... 101 3.5.4. Signal Processing. . . . . . . . . . . . . . . . . . . . . . . . . 103 3.5.5. Beam Energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3.5.6. Backscattered Electron Energy. . . . . . . . . . . . . . 105 3.5.7. Specimen Tilt. . . . . . . . . . . . . . . . . . . . . . . . . . . 105 3.5.8. Specimen Condition. . . . . . . . . . . . . . . . . . . . . . 106
3.6. Channeling Micrographs. . . . . . . . . . . . . . . . . . . . . . . . . 107 3.6.1. Polycrystalline Samples . . . . . . . . . . . . . . . . . . . 107 3.6.2. Limit of Spatial Resolution in Channeling
Contrast Images ............... . . . . . . . . . . 111 3.6.3. Limit of Misorientation Detection .......... 113
3.7. Selected Area Electron Channeling Patterns .... . . . . . 113 3.8. Orientation Determination with ECPs. . . . . . . . . . . . . . 117
3.8.1. Determining Orientation . . . . . . . . . . . . . . . . . . 118 3.8.2. Channeling Maps . . . . . . . . . . . . . . . . . . . . . . . . 118
3.9. Deformation Effects on Channeling Contrast. . . . . . . . 123 3.9.1. Channeling Patterns. . . . . . . . . . . . . . . . . . . . . . 123 3.9.2. Deformation Effects in Channeling Contrast
Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Appendix 1: Electron Channeling Maps. . . . . . . . . . . . . . . . . . 137 Appendix 2: Worked Example of Orientation
Determination ........................... 142
4. Magnetic Contrast in the SEM ........................ 147 4.1. Introduction................................... 147 4.2. Magnetic Microstructures. . . . . . . . . . . . . . . . . . . . . . . . 147
4.2.1. Magnetic Domains. . . . . . . . . . . . . . . . . . . . . . . 147 4.2.2. Types of Magnetic Materials . . . . . . . . . . . . . . . 149
4.3. Type I Magnetic Contrast: External Deflection of Secondary Electron Trajectories . . . . . . . . . . . . . . . . . . . 150 4.3.1. Physical Origin.......................... 150 4.3.2. Characteristics of Type I Magnetic Contrast .. 151 4.3.3. Quantitative Theory of Type I Magnetic
Contrast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
CONTENTS ix
4.3.4. Strategy for Observing Type I Magnetic Contrast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
4.4. Type II Magnetic Contrast: Internal Deflection of Primary Electron Trajectories. . . . . . . . . . . . . . . . . . . . . 158 4.4.1. Physical Origin . . . . . . . . . . . . . . . . . . . . . . . . . . 158 4.4.2. Characteristics of Type II Magnetic Contrast. . 161 4.4.3. Calculation of Type II Magnetic Contrast .... 168 4.4.4. Strategy for Observing Type II Magnetic
Contrast. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 4.5. Type III Magnetic Contrast: Polarization of Secondary
Electrons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 4.5.1. Physical Origin . . . . . . . . . . . . . . . . . . . . . . . . . . 173 4.5.2. Experimental Techniques. . . . . . . . . . . . . . . . . . 174
4.6. Dynamic Experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . 175
5. Computer-Aided Imaging and Interpretation. . . . . . . . . . . . . . 181 5.1. Introduction................................... 181 5.2. Some Perceptual Limitations and Artifacts Due to
Prejudice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 5.3. Some Perceptual Limitations and Artifacts of the Eye
and Mind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 5.4. The Image Acquisition and Display System . . . . . . . . . 186 5.5. General Considerations for a Computer and Its
Microscope Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 5.5.1. Considerations for the Microscope. . . . . . . . . . 189 5.5.2. Considerations for the Computer. . . . . . . . . . . 190
5.6. Color Display Systems .......................... 192 5.7. The Photography of Color Displays. . . . . . . . . . . . . . . . 195 5.8. Satellite Processors ............................. 196 5.9. Ways to Obtain Quantitative Intensity Information
from Digital Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 5.9.1. LineScans.............................. 197 5.9.2. Image Intensity Histograms. . . . . . . . . . . . . . . . 199 5.9.3. Histogram Stretching. . . . . . . . . . . . . . . . . . . . . 200 5.9.4. Histogram Normalization ............... . . . 200 5.9.5. Density Slicing ... . . . . . . . . . . . . . . . . . . . . . . . 201 5.9.6. Primary Coloring and Pseudocoloring ....... 201
5.10. Transformations for Electron and X-Ray Micrograph Processing: The Contrast Problem . . . . . . . . . . . . . . . . . 202 5.10.1. Some Preliminaries. . . . . . . . . . . . . . . . . . . . . . . 203 5.10.2. Derivative Transformations. . . . . . . . . . . . . . . . 204 5.10.3. The First Derivative. . . . . . . . . . . . . . . . . . . . . . 205
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5.10.4. The Space Gradient . . . . . . . . . . . . . . . . . . . . . . 206 5.10.5. The Space Laplacian. . . . . . . . . . . . . . . . . . . . . . 206 5.10.6. Isotropic, Nonlinear, Differential Operators. . . 206
5.11. Application of the Derivative Transforms to the Contrast Problem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
5.12. Discrete Approximations to Several of the Derivative Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.13. "Smoothing" Kernels. . . . . . . . . . . . . . . . . . . . . . . . . . . 210 5.14. The Acquisition and Display of X-Ray Images in
Scanning Electron Column Instruments ............ 214 5.14.1. Statement of the Problem. . . . . . . . . . . . . . . . . . 215 5.14.2. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
5.15. Methods to Separate the Characteristic and Continuum Radiation: EDS . . . . . . . . . . . . . . . . . . . . . . 218
5.16. Deadtime Correction for Digitally Acquired EDS Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
5.17. Methods to Separate the Characteristic and Continuum Radiation: WDS ..................... 220 5.17.1. Regional Variance in Background-Corrected
Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 5.17.2. Choice of Scale for Background-Corrected
Images. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 5.17.3. Conversion of Background- and Deadtime-
Corrected Intensity Maps to Quantitative Maps.. ... ...... ..... ... . .. . ... . . . ..... 224
5.18. An Example of Computer-Aided Imaging and Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
5.19. Contrast Criteria for Images. . . . . . . . . . . . . . . . . . . . . . 236 5.20. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
6. Alternative Microanalytical Techniques ................. 243 6.1. Introduction................................... 243 6.2. Strengths and Weaknesses ofSEM/XM. . . . . . . . . . . . . 243 6.3. General Observations on Analysis Methods. . . . . . . . . 247
6.3.1. Quantitative Analysis Procedures. . . . . . . . . . . 247 6.3.2. Detection Limits. . . . . . . . . . . . . . . . . . . . . . . . . 254
6.4. High-Spatial-Resolution Microanalysis . . . . . . . . . . . . . 257 6.4.1. Thin Specimens: Analytical Electron
Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 6.4.2. Analysis at Atomic Resolution ............. 262 6.4.3. Analysis of Bulk Samples. . . . . . . . . . . . . . . . . . 263 6.4.4. Surfaces................................ 266
CONTENTS xi
6.5. Trace Microanalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 6.5.1. X-Ray Analysis Techniques. . . . . . . . . . . . . . . . 272 6.5.2. Secondary Ion Mass Spectrometry. . . . . . . . . . 276 6.5.3. Laser Microprobe Mass Analysis. . . . . . . . . . . . 279
6.6. Molecular Microanalysis. . . . . . . . . . . . . . . . . . . . . . . . . 281 6.6.1. Mass Spectrometry Techniques. . . . . . . . . . . . . 281 6.6.2. Photon Beam Microanalysis ............... 284
6.7. Summary..................................... 294
7. Specimen Coating ................................... 295 7.1. Introduction................................... 295 7.2. Diode Sputter Coating. . . . . . . . . . . . . . . . . . . . . . . . . . . 296 7.3. Low-Voltage Sputter Coating. . . . . . . . . . . . . . . . . . . . . 298
7.3.1. Gold/Palladium......................... 299 7.3.2. Gold................................... 300 7.3.3. Platinum............................... 300
7.4. Very-Low-Voltage Sputter Coating. . . . . . . . . . . . . . . . . 301 7.5. Penning Sputtering ............................. 302 7.6. Ion Beam Sputtering. . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 7.7. Thermal Evaporation ........................... 311 7.8. Coating at Low Specimen Temperatures. . . . . . . . . . . . 313 7.9. Coating Thickness Measurements. . . . . . . . . . . . . . . . . 315 7.1 O. Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 7 7.11. Coating Techniques for X-Ray Microanalysis. . . . . . . . 321 7.12. Non-Thin-Film Coating Methods.. . . .. . .. . . . . . . .. . 322 7.13. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
8. Advances in Specimen Preparation for Biological SEM . . . . . 325 8.1. Introduction................................... 325 8.2. General Methods of Specimen Preparation. . . . . . . . . . 325 8.3. Fixation...................................... 327 8.4. Dehydration................................... 329 8.5. Exposing Internal Surfaces . . . . . . . . . . . . . . . . . . . . . . . 332 8.6. Localizing Regions of Biological Activity and
Chemical Specificity ............................ 336 8.6.1. Immunocytochemical Techniques. . . . . . . . . . . 337 8.6.2. Radioactive Labeling Methods ............. 344 8.6.3. Backscattered-Electron Cytochemical Methods 347
8.7. Modifying Specimen Bulk Conductivity. . . . . . . . . . . . 352 8.8. Specimen Mounting Procedures. . . . . . . . . . . . . . . . . . . 358 8.9. High-Resolution Microscopy. . . . . . . . . . . . . . . . . . . . . 359
8.9.1. Isolation of Object ofInterest .. . . . . . . . . . . . . 359 8.9.2. Stabilization and Conductive Staining. . . . . . . 363 8.9.3. Specimen Coating. . . . . . . . . . . . . . . . . . . . . . . . 363
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9. Cryomicroscopy .................................... . 9.1. Introduction .................................. . 9.2. Low-Temperature States of Water ................ . 9.3. Conversion of the Liquid to the Solid State ........ .
9.3.1. Specimen Pretreatment .................. . 9.3.2. Nonchemical Pretreatment .......... .
9.4. Methods of Rapid Cooling ................. . 9.4.1. Plunge Cooling .................... . 9.4.2. Spray Cooling . . . . . . . . . . . . . . . . . ...... . 9.4.3. Jet Cooling ............................ . 9.4.4. Metal Block Cooling ..................... . 9.4.5. High-Pressure Cooling ................... .
9.5. Sample Handling after Rapid Cooling ... . 9.5.1. Sectioning ............................. . 9.5.2. Fracturing ............................. . 9.5.3. Etching and Replication .................. . 9.5.4. Chemical Substitution ................... . 9.5.5. Low-Temperature Embedding ............. . 9.5.6. Freeze-Drying .......................... .
9.6. Microscopy and Analysis ....................... . 9.6.1. Specimen Transfer ...................... . 9.6.2. Specimen Stages . . . . . . . . . . . . ..... . 9.6.3. Specimen Visualization .................. . 9.6.4. Artifacts of Microscopy .................. .
9.7. Analysis............................. . ..... . 9.7.1. Thin Sections .................... . 9.7.2. Bulk Samples .................. .
9.8. Specimen Damage ............................. . 9.8.1. Thermal Damage ................ . 9.8.2. Ionizing Radiation ................ .
9.9. Conclusion ............................... .
REFERENCES .................................. .
INDEX ............................................ .
CONTENTS
365 365 367 371 371 378 379 380 381 381 382 383 384 385 391 395 397 400 403 407 407 408 409 420 425 426 426 427 427 428 433
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Advanced Scanning Electron Microscopy and X-Ray Microanalysis
