Advances in
IMAGING AND ELECTRON PHYSICS VOLUME
159 Cold Field Emission and the Scanning Transmission Electron Microscope
EDITOR-IN-CHIEF
PETER W. HAWKES CEMES-CNRS Toulouse, France
Advances in
IMAGING AND ELECTRON PHYSICS VOLUME
159 Cold Field Emission and the Scanning Transmission Electron Microscope Edited by
PETER W. HAWKES
CEMES-CNRS, Toulouse, France
AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier
Academic Press is an imprint of Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands 32 Jamestown Road, London NW1 7BY, UK 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA First edition 2009 Copyright # 2009, Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email:
[email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://www.elsevier.com/ locate/permissions, and selecting Obtaining permission to use Elsevier material. Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-374986-4 ISSN: 1076-5670 For information on all Academic Press publications visit our Web site at elsevierdirect.com Printed in the United States of America 09 10 11 12 10 9 8 7 6 5 4 3 2 1
Contents
Preface Contributors Future Contributions
1. The Work of Albert Victor Crewe on the Scanning Transmission Electron Microscope and Related Topics
xi xv xvii
1
A. V. Crewe 1. Introduction 2. Some Chicago Aberrations: A Personal Collection Acknowledgments 3. Electron Microscope Studies: Achievements of the Crewe Lab Introduction Construction (Reference Group A) Source Development (Reference Group B) STEM Development and Atomic Images (Reference Group C) The Field Emission SEM (Reference Group D) Energy Loss and Radiation Damage (Reference Group E) Secondary Electron Production (Reference Group F) DNA Labeling (Reference Group G) Nucleosomes (Reference Group H) Attempts at Aberration Correction (Reference Group I) Theoretical Electron Optics (Reference Group J) Optimization and the Super-High-Resolution SEM (Reference Group K) Image Processing (Reference Group L) Three-Dimensional Reconstruction (Reference Group M) Hemoglobin Work (Reference Group N) References (of Chicago Aberrations) References (of DOE Report)
2. A Review of the Cold-Field Electron Cathode
2 6 13 13 14 16 17 18 19 20 21 21 22 23 27 28 30 31 32 37 38
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L. W. Swanson and G. A. Schwind 1. Introduction 2. Work Function
63 65
v
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3. Energy Distribution 3.1. Theoretical Background 3.2. Analytical versus Numerical Results 3.3. Measured Values of the FWHM for the Tungsten Cold-Field Electron 4. Source Optics 5. Column Optics Using the Cold-Field Electron Source 6. Current Stability 6.1. High-Frequency Current Fluctuations 6.2. Long-Term Current Drift 6.3. Cold-Field Electron End-of-Life Mechanisms 7. Summary Acknowledgments References
3. History of the STEM at Brookhaven National Laboratory
67 67 68 72 79 82 86 87 88 94 97 98 98
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Joseph S. Wall, Martha N. Simon, and James F. Hainfeld 1. Introduction 2. Instrument Design Parameters 3. Heavy Metal Cluster Labeling 4. Early User/Collaborator Projects 5. Recent Work 6. Conclusion Acknowledgments References
4. Hitachi’s Development of Cold-Field Emission Scanning Transmission Electron Microscopes
101 102 106 108 115 115 115 116
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Hiromi Inada, Hiroshi Kakibayashi, Shigeto Isakozawa, Takahito Hashimoto, Toshie Yaguchi, and Kuniyasu Nakamura 1. Introduction 2. The Dawn of Hitachi Electron Microscopes (by Hiromi Inada) 2.1. Crewe STEM Shock and Field Emission Development 3. Cold FE-SEM Studies and Expansion to Different Fields 3.1. CFE-STEM Development at HCRL 3.2. CFE-SEM Development at Naka Works 3.3. Studies of Field Emission Stability 4. Expansion of High-Voltage TEMs and STEMs 4.1. Holography Studies by Tonomura with FE-TEMs 4.2. Multistage Acceleration CFEG for TEM Applications 4.3. Commercialized Analytical CFE-TEM/STEM 5. Development of 50-kV STEM in 1970s (by Shigeto Isakozawa) 5.1. Development of Hitachi’s 50kV Prototype CFE-STEM
124 125 125 128 128 131 132 137 137 138 139 141 141
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5.2. Bright-Field Images Obtained with 50-kV CFE-STEM 5.3. Single-Atom Observation 5.4. Development of the Electron Energy-Loss Spectrometer 5.5. Further Development of 50-kV CFE-STEM 6. Hitachi’s First Commercialized Dedicated STEM (by Takahito Hashimoto) 6.1. 200-kV Analytical CFE-TEM, HF-2000 6.2. ‘‘Gate Viewer,’’ Trigger for Dedicated STEM 6.3. Novel Functions for HD Series 7. Cutting-Edge Applications with Customized HD Models (by Toshie Yaguchi) 7.1. 120-kV FE UHV for Nanotubes 7.2. 200-kV FE Ultrahigh-Resolution STEM 7.3. 3D Structural and Elemental Analysis 8. Aberration-Corrected CFE-STEM (by Kuniyasu Nakamura) 8.1. Atomic-Level Characterization Instrument 8.2. Theoretical Consideration of Advantages of Aberration-Corrected CFE-STEM 8.3. Evaluation of Aberration-Corrected CFE-STEM 8.4. Advanced Application Results with HD-2700C 9. Conclusion and Future Prospective (by Hiroshi Kakibayashi) Acknowledgments References
5. Two Commercial STEMs: The Siemens ST100F and the AEI STEM-1
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142 144 147 149 150 150 154 161 165 165 165 169 170 170 172 173 176 181 182 182
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P. W. Hawkes 1. Introduction 2. The AEI STEM-1 3. The Siemens ST100F 3.1. The New ELMISKOP ST100F Scanning Transmission Electron Microscope Introduction Construction of the ELMISKOP ST100F Advantages of the Scanning Transmission Electron Microscope Imaging with Low Radiation Damage Conclusion 3.2. Image Forming Systems General Remarks The Imaging Process Image Forming Properties of Magnetic Lenses Strong Lenses Lens Systems for CTEM and STEM: Similarities and Differences Requirements for Analytical Microscopy Acknowledgments References
188 188 191 195 195 195 198 198 201 203 203 204 205 208 210 215 217 217
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Contents
6. A History of Vacuum Generators’ 100-kV Scanning Transmission Electron Microscope
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Ian R. M. Wardell and Peter E. Bovey 1. The Early Days 2. Design Considerations for a Commercial STEM 2.1. The Objective Lens 2.2. The Specimen Stage 2.3. The Electron Gun 2.4. Location of Analytical Facilities 2.5. The Resultant Assembly 3. The First STEMs and HB5 Development 3.1. The Optical Column 3.2. Stage Development 3.3. Early Airlock and Manipulator 3.4. The Dark-Field Detector 3.5. Secondary Electron Detector 3.6. Diffraction Facilities 3.7. The Energy Analyzer Mk1 3.8. Electronics 4. Improved Resolution 4.1. The MIT HB5 4.2. The University of Illinois HB5 5. Other Developments 5.1. Beam-Blanking Plates 5.2. Detectors and the Virtual Objective Aperture 5.3. Stage Motor Drives 5.4. New Airlock 5.5. Energy Analyzer Mk2 5.6. Gun Lens 5.7. High-Excitation Objective Lens 6. Stages and Cartridges 6.1. Basic Cartridges 6.2. Beryllium Cartridges 6.3. Cold Stage 6.4. Cryo-Transfer System 7. The HB501 7.1. General Development 7.2. HB501UX and High-Resolution Imaging 8. Special and Variant Instruments 8.1. University of Glasgow’s HB5 8.2. The HB501A 9. The HB601
222 224 225 225 226 226 227 227 232 238 240 241 245 246 249 251 254 254 255 256 256 257 261 262 264 266 266 268 268 269 269 270 271 271 273 273 275 276 278
Contents
10. Postscript Acknowledgments References
7. Development of the 300-kV Vacuum Generator STEM (1985–1996)
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H. S. von Harrach 1. Prelude in Oxford (1973–1975) 2. The MIDAS Project (1985–1988) 2.1. System 2.2. Gun Lens 2.3. Objective Lens 2.4. Side-Entry Stage 2.5. MIDAS Performance 3. The HB603 300-kV STEM instruments 3.1. The Prototype Design Phase 4. MIT and ANL Designs 5. Oak Ridge Design 6. Lehigh Design 7. Testing, Testing 8. Record-Breaking Results 8.1. Source Brightness 8.2. Energy Spread 8.3. ADF Resolution 8.4. X-Ray Microanalysis 9. Conclusions Acknowledgments References
8. On the High-Voltage STEM Project in Toulouse (MEBATH)
288 288 289 289 290 291 291 292 293 305 307 310 311 316 316 316 316 317 320 321 322
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Bernard Jouffrey 1. 2. 3. 4.
Introduction The Building Generator The Column 4.1. The Source 4.2. Accelerating Tube 4.3. Lenses 4.4. Spectrometers 5. Suspension of the Platform and the Microscope 6. Recording of The Signal 7. Conclusions Acknowledgments References
325 328 333 336 336 342 346 348 349 351 352 352 353
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9. Scanning Transmission Electron Microscopy: Biological Applications
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Andreas Engel 1. Introduction 2. Image Formation 2.1. Electron-Sample Interactions 2.2. The Optical System 2.3. Detectors 2.4. Single-Electron Counting 2.5. Imaging Modes 3. Imaging Thin Sections 4. Imaging Negatively Stained Samples 5. Mass Measurements Using the Basel STEM 5.1. Principle 5.2. Estimate of the Statistical Error 5.3. Mass Determination of Biological Samples 6. Specific Examples of STEM Imaging and Mass Measurements 7. High-Throughput Visual Proteomics 8. Conclusions and Perspectives Acknowledgments References
10. STEM at Cambridge University: Reminiscences and Reflections from the 1950s and 1960s
358 359 359 360 360 361 363 363 365 366 366 369 371 373 378 380 382 382
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K. C. A. Smith 1. STEM as a Diagnostic Tool for SEM 2. The Observation of Specimens in Water Vapor by Means of STEM 3. High-Voltage STEM Using a Single-Field Condenser—Objective Lens References
Contents of Volumes 151–158 Index
387 393 400 405
407 411
Preface
Tom Mulvey (right) with Elmar Zeitler, Berlin 1990* I have the sad duty of recording the death of our other Honorary Associate Editor, Tom Mulvey, who died on 16 July 2009, a few days before his 88th birthday. He was born on 26 July 1921 in Manchester; during the wartime years he served in the Navy and was sent to the Far East where he was responsible for anti-submarine operations. After being demobbed, he was awarded the MSc degree by Manchester University for a substantial study of electrostatic lenses and then spent several years in the Metropolitan–Vickers research establishment at Aldermaston Court. It was during this period that he met Rita, his future wife, for whom he cared devotedly during the last years of her life. In 1965, he joined the Birmingham College of Advanced Technology, later the University of Aston in Birmingham (today Aston University) and was made Emeritus Professor on his retirement in 1986. The last years of his life were spent near his son Nicholas, close to Oxford. In addition to his own research, he took over from V. E. Cosslett as editor of Advances in Optical & Electron Microscopy and later became an Associate Editor of Advances in Imaging & Electron Physics, to which he contributed several biographical articles on the pioneers of electron microscopy: Ernst Ruska, Dennis Gabor and Jan le Poole; he also wrote the Royal Society memoir on V. E. Cosslett and * Courtesy of Prof. Dr B. Lencova´
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translated Ruska’s book on the early history of the electron microscope. The history of electron microscopy is richer by the presence of his many articles, of which the first (Brit. J. Appl. Phys. 13, 1962, 197–207) is truly remarkable, written before most of the fundamental documents for such a study had been published; a more extended account appeared in Proc. RMS 2 (1967) 201–227. The volume of these Advances that he guest-edited on ‘‘The Growth of Electron Microscopy’’ was a splendid achievement. His research interests were wide-ranging and he introduced many unconventional lens designs. Among the most memorable were boiling-water lenses, pancake lenses, single-polepiece lenses and snorkel lenses, the dimensions of which were often indicated in units of ‘‘England’s Glory’’ matchboxes. For his 80th birthday (but rather belatedly), the Royal Microscopical Society (of which he was an honorary Fellow) brought together many of his friends for a celebration in Birmingham, which is commemorated in a long section of the Proceedings of the society (39, 2004, 206–233). A much fuller biography is to be found there, as well as tributes from many of those present; a bibliography of his publications was published for an earlier birthday and occupies four pages of the Journal of Microscopy (179, 1995, 101–104). One aspect that emerges from the tributes is just how entertaining he could be – he had an unlimited stock of anecdotes about present and past colleagues and jokes ranging from the politically highly incorrect to the purely frivolous. He was also a Distinguished Scientist and Fellow of the Microscopy Society of America and was awarded the gold Kr i z ´ık medal of the Czech Academy of Sciences. He will be greatly missed and we extend our sympathy to all his family. The scanning transmission electron microscope (STEM) has a long history, going back to the 1960s. In the present volume, its development is traced from its inception in the Argonne National Laboratory and the University of Chicago, where A. V. Crewe built the first simple instruments, the success of which owed much to the quality of the field-emission gun. Albert Crewe was unfortunately not able to prepare a new account for this volume and, with his approval, two older documents are reproduced here: ‘‘Some Chicago aberrations’’ and a long report submitted to the US Department of Energy in 1992, which includes a very full list of publications. The field-emission source is the vital element of the STEM, for the brightness of thermionic guns is insufficient for a high-resolution instrument. Chapter 2 offers a review of cold field-emission source properties by L. W. Swanson and G. A. Schwind, both major contributors to our understanding of these guns. They cover both early developments and current research preoccupations. The following group of chapters describes specific instruments and commercial developments. J. S. Wall was a member of Crewe’s team
Preface
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when the first STEMs were being constructed and, as he explains, he was later recruited by Brookhaven National Laboratory to advance the construction of their STEM. Together with M. N. Simon and J. F. Hainfeld, he describes the design of this early home-made instrument and subsequent developments. The spectacular images obtained by Crewe awakened interest worldwide, not least in Japan. Hitachi in particular was stimulated to enter the STEM field and in Chapter 4, H. Inada, H. Kakibayashi, S. Isakozawa, T. Hashimoto, T. Yaguchi and K. Nakamura describe the early work that made STEM development possible and such topics as holography that also benefited from highly coherent sources. The various models are presented, together with some of the most impressive results. The closing paragraphs bring the story up to the aberration-corrected era. This does not exhaust the STEM projects in Japan. For example, a high-voltage instrument constructed at Nagoya University is described by M. Hibino, H. Shimoyaya and S. Maruse ( J. Electron Microsc. Tech. 12, 1989, 296–304) and an aberration-corrected STEM is also available from JEOL. In Europe, Siemens, AEI and Vacuum Generators all embarked on STEM projects. The VG instruments are described at length by I. R. M. Wardell, P. E. Bovey and S. von Harrach in Chapters 6 and 7. But for Siemens and AEI, I was unable to find authors and have therefore written an account of their STEMs myself, based on published information and personal communication from several of those involved (Chapter 5). It was natural that the Laboratoire d’Optique Electronique in Toulouse, already home to two high-voltage transmission electron microscopes, should embark on the design and construction of a high-voltage STEM. In Chapter 8, B. Jouffrey, who launched and piloted this project, describes the principal stages in the preparations for this instrument, which alas was never completed. The foregoing chapters contain much description of instrumentation while Chapter 9 is a reminder that the STEM has proved an invaluable tool for microscopists in many fields of application. Here, A. Engel shows how fruitful it has been in biology. We conclude with a chapter by K. C. A. Smith, who was not only a pioneer of the scanning electron microscope in the Cambridge University Engineering Department but was also responsible for the design and construction of the first Cambridge high-voltage electron microscope, built in the Cavendish Laboratory in the 1960s. It was at that time that the idea of operating the microscope in the STEM mode came to him and it is this little-known early effort that is recorded here. It is worth emphasizing that it was first mentioned in print in 1968, the same year as Crewe’s key paper in the Journal of Applied Physics. This volume should also have contained two more chapters, by M. S. Isaacson and O. L. Krivanek. Circumstances have prevented them
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from completing their chapters in time and their contributions will therefore appear in later volumes of the Advances. I greatly regret that they do not appear here, for M. S Isaacson was one of the pioneers of the STEM while O. L. Krivanek not only produced the first successful corrector of spherical aberration for the STEM but also suggested to me that a thematic volume on STEM would be timely. As always, I am most grateful to all the contributors for writing such readable and informative accounts. Peter W. Hawkes
Contributors
A. V. Crewe Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, USA L. W. Swanson and G. A. Schwind FEI Co. Hillsboro, Oregon, USA Joseph S. Wall, Martha N. Simon, and James F. Hainfeld Biology Department, Brookhaven National Laboratory, Upton, New York 11073, USA Hiromi Inada, Hiroshi Kakibayashi, Shigeto Isakozawa, Takahito Hashimoto, Toshie Yaguchi, and Kuniyasu Nakamura Hitachi High-Technologies Corp., Tokyo, Japan
1
63 101
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P. W. Hawkes CEMES-CNRS, Toulouse, France
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Ian R. M. Wardell Department of Physics and Astronomy, University of Sussex, Brighton, United Kingdom
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Peter E. Bovey Lindfield, West Sussex, United Kingdom
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H. S. von Harrach FEI Electron Optics, Eindhoven, The Netherlands
287
Bernard Jouffrey Laboratoire de Structures, Sols et Mate´riaux (LMSS-Mat), E´cole Centrale Paris, Unite´ Mixte de Recherche (UMR), National Center for Scientific Research (CNRS) 8579, Chaˆtenay-Malabry, France
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Andreas Engel M. E. Mu¨ller Institute for Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, Basel, CH-4056 Switzerland, and Department of Pharmacology, Case Western Reserve University, 10900 Euclid Avenue, Wood Bldg 321D, Cleveland, Ohio 44106, USA
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K. C. A. Smith Fitzwilliam College, University of Cambridge, Cambridge CB3 0DG, United Kingdom
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Future Contributions
S. Ando Gradient operators and edge and corner detection K. Asakura Energy-filtering X-ray PEEM W. Bacsa Optical interference near surfaces, sub-wavelength microscopy and spectroscopic sensors Baranczuk, J. Giesen, Z. K. Simon and P. Zolliker (vol. 160) Gamut mapping C. Beeli Structure and microscopy of quasicrystals C. Bobisch and R. Mo¨ller Ballistic electron microscopy G. Borgefors Distance transforms Z. Bouchal Non-diffracting optical beams A. Buchau Boundary element or integral equation methods for static and time-dependent problems B. Buchberger Gro¨bner bases E. Cosgriff, P. D. Nellist, L. J. Allen, A. J. d’Alfonso, S. D. Findlay, and A. I. Kirkland Three-dimensional imaging using aberration-corrected scanning confocal electron microscopy T. Cremer Neutron microscopy C. J. Edgcombe New dimensions for field emission: effects of structure in the emitting surface
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Future Contributions
A. N. Evans (vol. 160) Area morphology scale-spaces for colour images A. X. Falca˜o The image foresting transform R. H. A. Farias and E. Recami Introduction of a quantum of time (‘‘chronon’’) and its consequences for the electron in quantum and classical physics R. G. Forbes Liquid metal ion sources C. Fredembach Eigenregions for image classification ¨ lzha¨user A. Go Recent advances in electron holography with point sources M. Haschke Micro-XRF excitation in the scanning electron microscope L. Hermi, M. A. Khabou and M. B. H. Rhouma (vol. 162) Shape recognition based on eigenvalues of the Laplacian M. I. Herrera The development of electron microscopy in Spain M. S. Isaacson Early STEM development J. Isenberg Imaging IR-techniques for the characterization of solar cells K. Ishizuka Contrast transfer and crystal images A. Jacobo Intracavity type II second-harmonic generation for image processing L. Kipp Photon sieves G. Ko¨gel Positron microscopy T. Kohashi Spin-polarized scanning electron microscopy O. L. Krivanek Aberration-corrected STEM R. Leitgeb Fourier domain and time domain optical coherence tomography B. Lencova´ Modern developments in electron optical calculations
Future Contributions
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H. Lichte New developments in electron holography M. Mankos, V. Spasov and E. Munro (vol. 161) Principles of dual-beam low energy electron microscopy M. Marrocco Discrete diffraction M. Matsuya Calculation of aberration coefficients using Lie algebra S. McVitie Microscopy of magnetic specimens J. D. Mendiola-Santiban˜ez, I. R. Terol-Villalobos and I. M. Santilla´n-Me´ndez (vol. 161) Determination of adequate parameters for connected morphological contrast mappings through morphological contrast measures I. Moreno Soriano and C. Ferreira (vol. 161) Fractional Fourier transforms and geometrical optics M. A. O’Keefe Electron image simulation D. Oulton and H. Owens Colorimetric imaging D. Paganin and T. Gureyev Intensity-linear methods in inverse imaging N. Papamarkos and A. Kesidis The inverse Hough transform K. S. Pedersen, A. Lee and M. Nielsen The scale-space properties of natural images Y. Pu (vol. 160) Harmonic holography G. X. Ritter and G. Urcid (vol. 160) Lattice algebra approach to endmember determination in hyperspectral imagery R. Ru¨denberg (vol. 160) Origin and background of the invention of the electron microscope, Memoir H. G. and P. Rudenberg (vol. 160) Origin and background of the invention of the electron microscope, Commentary R. Shimizu, T. Ikuta and Y. Takai Defocus image modulation processing in real time S. Shirai CRT gun design methods
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A. S. Skapin The use of optical and scanning electron microscopy in the study of ancient pigments T. Soma Focus-deflection systems and their applications P. Sussner and M. E. Valle Fuzzy morphological associative memories I. Talmon Study of complex fluids by transmission electron microscopy M. E. Testorf and M. Fiddy Imaging from scattered electromagnetic fields, investigations into an unsolved problem N. M. Towghi Ip norm optimal filters E. Twerdowski Defocused acoustic transmission microscopy Y. Uchikawa Electron gun optics K. Vaeth and G. Rajeswaran Organic light-emitting arrays V. Velisavljevic and M. Vetterli (vol. 161) Space-frequence quantization using directionlets M. H. F. Wilkinson and G. Ouzounis (vol. 161) Second generation connectivity and attribute filters E. Wolf (vol. 162) History and a recent development in the theory of reconstruction of crystalline solids from X-ray diffraction experiments
Chapter
4 Hitachi’s Development of Cold-Field Emission Scanning Transmission Electron Microscopes Hiromi Inada, Hiroshi Kakibayashi, Shigeto Isakozawa, Takahito Hashimoto, Toshie Yaguchi, and Kuniyasu Nakamura
Contents
1. Introduction 2. The Dawn of Hitachi Electron Microscopes (by Hiromi Inada) 2.1. Crewe STEM Shock and Field Emission Development 3. Cold FE-SEM Studies and Expansion to Different Fields 3.1. CFE-STEM Development at HCRL 3.2. CFE-SEM Development at Naka Works 3.3. Studies of Field Emission Stability 4. Expansion of High-Voltage TEMs and STEMs 4.1. Holography Studies by Tonomura with FE-TEMs 4.2. Multistage Acceleration CFEG for TEM Applications 4.3. Commercialized Analytical CFE-TEM/STEM 5. Development of 50-kV STEM in 1970s (by Shigeto Isakozawa) 5.1. Development of Hitachi’s 50kV Prototype CFE-STEM
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Hitachi High-Technologies Corp., Tokyo, Japan Advances in Imaging and Electron Physics, Volume 159, ISSN 1076-5670, DOI: 10.1016/S1076-5670(09)59004-0. Copyright # 2009 Elsevier Inc. All rights reserved.
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5.2. Bright-Field Images Obtained with 50-kV CFE-STEM 5.3. Single-Atom Observation 5.4. Development of the Electron Energy-Loss Spectrometer 5.5. Further Development of 50-kV CFE-STEM 6. Hitachi’s First Commercialized Dedicated STEM (by Takahito Hashimoto) 6.1. 200-kV Analytical CFE-TEM, HF-2000 6.2. ‘‘Gate Viewer,’’ Trigger for Dedicated STEM 6.3. Novel Functions for HD Series 7. Cutting-Edge Applications with Customized HD Models (by Toshie Yaguchi) 7.1. 120-kV FE UHV for Nanotubes 7.2. 200-kV FE Ultrahigh-Resolution STEM 7.3. 3D Structural and Elemental Analysis 8. Aberration-Corrected CFE-STEM (by Kuniyasu Nakamura) 8.1. Atomic-Level Characterization Instrument 8.2. Theoretical Consideration of Advantages of Aberration-Corrected CFE-STEM 8.3. Evaluation of Aberration-Corrected CFE-STEM 8.4. Advanced Application Results with HD-2700C 9. Conclusion and Future Prospective (by Hiroshi Kakibayashi) Acknowledgments References
142 144 147 149 150 150 154 161 165 165 165 169 170 170 172 173 176 181 182 182
1. INTRODUCTION Hitachi started researching and developing electron microscopes in 1940 and has since developed and manufactured many electron microscopes. Cold-field emission (CFE) technology developed in a number of different directions. Crewe invented the field emission scanning transmission electron microscope (FE-STEM) and used an early version to observe individual atoms. Stimulated by his reports, Hitachi began developing a CFE-STEM in the mid-1960s and invited Crewe to serve as a consultant to Hitachi. He introduced his CFE technology and helped Hitachi develop and commercialize STEMs and scanning electron microscopes (SEMs). Hitachi’s prototype CFE-STEM opened a new world of analytical electron microscopes equipped with an X-ray analyzer and energy spectrometer. The HFS-2 FE-SEM, which was built at Hitachi’s Naka Works in 1972, was the first step in Hitachi’s development of FE-SEMs. A 50-kV CFE-TEM
Hitachi’s Development of Cold-Field Emission Scanning Transmission Electron Microscopes
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was developed at Hitachi’s Central Research Laboratory (HCRL) by Tonomura and his group in 1973 for application to electron holography, which enabled higher image resolution. To incorporate a sophisticated FE electron gun into microscopes, it was necessary to create an ultrahigh vacuum (UHV) (of the order of 108 Pa) to fundamentally stabilize the FE current. This chapter describes Hitachi’s efforts over the past 40-plus years to develop CFE technology and STEMs. We also introduce cutting-edge application data obtained with the latest CFE-STEMs (with and without an aberration corrector), highlight Hitachi’s contributions to FE technology development, and show how the knowledge gained has been passed from generation to generation at Hitachi. Additionally, we show how CFE SEM/ STEM technologies were established.
2. THE DAWN OF HITACHI ELECTRON MICROSCOPES (BY HIROMI INADA) K. Kasai, one of the founders of the 37th Subcommittee of the Japanese Society for the Promotion of Science, with regard to electron microscopes, moved to Hitachi from one of the Japanese National Laboratories, Electrotechnical Laboratory in 1939 and began to develop electron microscopes by using the experience he gained working on cathode ray oscillography in Germany. His guiding principle was that one should work in a manufacturing company to be able to do innovative practical development. Kasai joined forces with B. Tadano at Hitachi and, in 1940, they developed the first Hitachi TEM (model HU-1) (Komoda 1996a). A second TEM with improved resolution (the HU-2) was developed and installed at Nagoya Imperial University (now Nagoya University) in 1943. Hitachi established a central research laboratory (HCRL) in Kokubunji, a suburb of Tokyo, in 1942 where it continued developing electron microscopes despite the wartime conditions.
2.1. Crewe STEM Shock and Field Emission Development 2.1.1. Trigger for Field Emission Microscopes A. V. Crewe reported the successful development of an SEM equipped with a CFE gun (CFEG) in 1964 (Crewe, 1964; Crewe et al., 1970a). This report greatly affected Hitachi researchers, who were seriously fighting to obtain the highest TEM resolution in the mid-1960s (Komoda 1996b). Crewe and his group were provident in terms of advantages of CFE and the progress of UHV technology, and they began pioneering development of an STEM equipped with a CFEG in the early 1960s. They
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achieved a resolution of 0.5 nm and successfully observed individual atoms of uranium and thorium, even though their microscope had a simple design and contained a single-electron magnetic lens with a CFEG. This breakthrough observation overturned the conventional wisdom about electron microscopy at that time and revealed the power of FE technology. Moreover, they directly observed individual atoms with a STEM, not a TEM, whereas most scientists believed that only a TEM could achieve such high-resolution imaging (Crewe et al., 1968a,b; Crewe, Wall, and Langmore, 1970b). Hitachi started developing SEMs in the early 1960s and commercialized the first thermionic gun SEM (model HSM-2) in 1969. Conventional SEMs with a thermionic gun at that time had a resolution of 10 nm, which was much worse than that of TEMs (0.3 nm); hence, nobody compared performance between TEMs and SEMs. However, Crewe’s development of an SEM in 1964 made it possible to compare SEMs with TEMs using the same criteria. An SEM can be used to acquire time series of various types of signals (secondary, transmission electron, X-ray, and energy loss) from a specimen simultaneously by focusing on an electronically specific point and/ or area by imaging it with a narrow scanning electron beam. The development of the CFE-SEM meant the birth of a new scientific instrument capable of simultaneous nanometer-order analysis and imaging. Although some scientists (Eggenberger, Hart, and Libal, 1968; Siegel et al., 1968; Wiesner and Everhart, 1969) had started early FEG electron microscopes, none of the electron microscope vendors had succeeded in building a CFE-SEM. Accordingly, Hitachi was driven to quickly develop a CFE-SEM due to its importance. R. Ueda (Nagoya University) said in the preface of the first issue revival of the Journal of Electron Microscopy in 1975, ‘‘Crewe STEM was excellent and nobody had imagined before such achievement and his outstanding concept’’ (Ueda, 1975). This statement reflects the sensational effect the FE-SEM had at that time.
2.1.2. Field Emission Gun Development at Hitachi Hitachi launched a major project in 1969 to produce ultrahigh-resolution FE-SEMs and FE-TEMs. As a microscope manufacturer, Hitachi focused on ease of use and ease of production, as well as on imaging capability. In 1970, Hitachi invited Crewe to serve as a technical consultant for its development of a CFE-SEM. Field emission was discovered by R. W. Wood in the nineteenth century (Wood, 1897). Millikan and Lauritsen (1928) and E. W. Mu¨ller (1937) studied the characteristics of this type of emission. FE patterns
Hitachi’s Development of Cold-Field Emission Scanning Transmission Electron Microscopes
127
were investigated (Gomer, 1961), and study of FE as a source of electrons began in 1955 (Butler, 1966; Dyke et al., 1953; Everhart, 1967; Martin, Trolan, and Dyke, 1960). FE electrons can be obtained by applying several thousand volts to the tip of a metal needle (FE tip) with a radius of less than 100 nm. The energy spread of the emitted electrons is narrow (0.2–0.3 eV) because the emitter runs at room temperature. FE electrons are emitted through the potential barrier of the surface from near the Fermi level because the barrier is thinned by the application of a negative electronic field with a strength of 107 V/cm to the metal surface, which is the tunnel effect. The current density J(A/cm2) is obtained using equation 1 from Fowler and Nordheim (Fowler and Nordheim, 1928), and the density strongly depends on the electric field applied to the FE tip and on the work function of the metal. ( ) F2 6:83 105 f3=2 vðyÞ 2 exp J ¼ 1:54 10 ; (4.1) ft2 ðyÞ F where t2(y) and v(y) are elliptic functions nearly equal to 1, F is the electric field strength, and f is the cathode work function. The thermionic electron current density is obtained using the Richardson–Dushman equation. The CFE current density (104–106 A/cm2) is three orders of magnitude larger than that of the thermionic electrons (1–10 A/cm2). The FE source is ideally a point source, and the diameter of the virtual source ranges from 5 to 10 nm because of the small FE tip, which is 1/1000 the source size of the thermionic emission (1–10 m). Thus, changing the FE source can reduce the beam spot size; that is, it can improve SEM image resolution. An electron gun should be designed to have less spherical aberration and mechanical vibration. Crewe used the Butler electrostatic lens, which has less spherical aberration than his prototype design, for his CFE-STEM and thereby obtained high resolution. An FE electron gun with a Butler lens consists of an FE tip and a diode region for the extraction and acceleration electrodes (Figure 1). A. Tonomura at HCRL used a microcomputer to optimize the Butler lens shape and minimize the aberration (Shimoyama, Ichihashi, and Tamura, 1989; Tonomura and Komoda, 1973). Table 1 shows a comparison of the characteristics of emitters with FE and thermionic electron guns. Although applying the FE gun to electron microscopes was an obvious next step, it took many years to develop such a microscope that was practical. This was due to the instability of the FE current and the difficulty of achieving an UHV of the order of 108 Pa.
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FE tip V1 1st anode
V0
2nd anode
FIGURE 1
Schematic diagram of FE gun with Butler electrode lens.
TABLE 1 Characteristics of Emitters
Source brightness Source size Energy spread Temperature Vacuum pressure Lifetime
Field emission W
Thermionic emission (Tungsten filament)
109 A/cm2 sr 5–10 nm 0.3 (eV) 300 K (R.T.) zc
(2.9)
as depicted in Figure 2. The image potential term in Eq. (2.9) is artificially set at 0 at zc. The wave function for the starting condition for the numerical integration for large positive z to the right of the potential barrier is given by c ¼ Cexp(ikz). For negative z to the left of the potential barrier, the wave function separates into an incident and reflected component as c ¼ Aexp(ikz) þ Bexp(–ikz). The tunneling coefficient follows as 2 C (2.10) DðWÞ ¼ A The complete analytical expression for the current density JTF for an emitter that includes the effect of a finite temperature can be obtained by integrating Eq. (2.1) as follows: ð1 pp ; (2.11) JðeÞde ¼ JFN JTFE ¼ sinðppÞ 0 where the dimensionless parameter p < 0.7. To obtain the analytical expressions J(e) and JTFE, a variety of approximations must be made that include the well-known Wentzel-Kramers-Brillouin (WKB) approximation in the derivation of JFN. The degree to which these approximations affect the validity of Eqs. (2.1) and (2.7) was discussed previously (Bahm,
70
L. W. Swanson and G. A. Schwind
Schwind, and Swanson, 2008). They compared the above analytical expressions (A) with an accurate numerical (N) calculation of JTFE J (A) and J(e) throughout the CFE, TFE, and SE regimes. Figures 3 and 4 show the ratios of the FWHM(A)/FWHM(N) and J(A)/J(N) obtained from the numerical and analytical (i.e., Eqs. (2.1) and (2.11)) as a function of T for various work function and J(N) values in the low-temperature (i.e., p < 0.7) range. As observed earlier (Bahm, Schwind, and Swanson, 2008), the analytical and numerically calculated values for the FWHM agree within a few percent over the range of ’, J, and T normally encountered for the low-temperature CFE and the higher-temperature TFE emission regimes. In contrast, J(A)/J(N) in Figure 3 shows a significant variation (up to 30%) from unity over the temperature and work function range that Eq. (2.11) is expected to be valid. For the numerical calculations a value of Ef ¼12 eV was used for tungsten (Mattheiss, 1965). Interestingly, the Eq. (2.11) analytical expression for J(A) does not depend on Ef, whereas J(N) exhibits a strong dependence on Ef as shown in Figure 5. In contrast, the FWHM(N) values are relatively independent of over the range Ef ¼ 4 to 18 eV. Since the discrepancy occurs only between J(N) and J(A), we suspect the problem lies with the approximations associated with D(W) in formulating the analytical expression for JFN. Further studies are necessary to determine the exact cause of the discrepancy between J(N) and J(A). 1.50 j = 4.0 eV j = 4.5 eV j = 5.0 eV
1.45 1.40 1.35
J(A)/J(N)
Ratios
1.30 1.25 1.20 1.15 1.10
FWHM(A)/FWHM(N)
1.05 1.00 0.95 0.90 0
200
400
600 T (K)
800
1000
1200
FIGURE 3 Variation of analytical and numerically calculated current density and FWHM ratios versus T for J(N) ¼ 1 108 A/m2 and the indicated work function values.
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A Review of the Cold-Field Electron Cathode
1.30 1.25 1.20
J(A)/J(N)
1.15
Ratio
1.10 1.05 1.00 FWHM(A)/FWHM(N) 0.95
J(N) = 107A/m2 J(N) = 108A/m2 J(N) = 109A/m2
0.90 0.85 0.80 0
200
400
600 T (K)
800
1000
1200
FIGURE 4 Variation of analytical and numerically calculated current density and FWHM ratios versus T for ’ ¼ 4.5 eV and the indicated J(N) values.
1.5 J(A)/J(N) FWHM(A)/FWHM(N)
1.4 1.3
Ratio
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0
2
4
6
8
10 Ef (eV)
12
14
16
18
20
FIGURE 5 Variation of analytical and numerically calculated current density and FWHM ratios versus Ef for ’ ¼ 4.5 eV and F ¼ 4.5 109 V/m. J(A) and FWHM values are 8.12 108 A/m2 and 0.238 eV, respectively, and independent of Ef.
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L. W. Swanson and G. A. Schwind
The calculated values of FWHM(N) in Figures 6 and 7 versus T for various values of ’ and J(N) show that the sensitivity of FWHM(N) to J(N) and ’ diminishes with increasing T. The results given in Table 2 for a CFE source operating at T ¼ 300 K show that the dependence of the FWHM(N) values on both work function and current density is minimal. Decreasing values of both ’ and/or J(N) result in a slightly lower value for the FWHM(N).
3.3. Measured Values of the FWHM for the Tungsten Cold-Field Electron Other than the fact that, according to Table 1, the (310) plane of clean tungsten possesses the lowest work function, it is not clear why historically it has persisted as the most favored orientation for most commercial applications. From the standpoint of crystallographic symmetry and the amplitude of intrinsic current fluctuations at low temperatures (to be discussed in section 6.1), a case can be made that the (111) plane might be the preferred plane for electron optical applications. The slight decrease in FWHM(N) with decreasing ’ at a fixed value of J(N), according to Table 2, will provide a minimal advantage for a low ’ CFE source. In addition, the vacuum environment encountered in most commercial 0.50 4.00 eV 4.50 eV 5.00 eV
0.45
FWHM(N) (eV)
0.40 0.35 0.30 0.25 0.20 0.15 0
200
400
600
800
1000
1200
T (K)
FIGURE 6 Variation of the FWHM(N) versus T for the indicated values of work function for J(N) ¼ 1 108 A/m2.
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A Review of the Cold-Field Electron Cathode
0.50 J(N) = 107 A/m2 J(N) = 108 A/m2 J(N) = 109 A/m2
0.45
FWHM (N) (eV)
0.40 0.35 0.30 0.25 0.20 0.15 0
FIGURE 7
200
400
600 T (K)
800
1000
1200
Variation of the FWHM(N) versus T for the indicated values of J(N) for ’ ¼ 4.50 eV.
TABLE 2 Calculated values of the FWHM(N) and F for the indicate values of J and w at T ¼ 300 K J (A/m2)
’ (eV)
F(V/nm)
FWHM(N) (eV)
1 10 1 108 1 108 1 107 1 108 1 109
4.0 4.5 5.0 4.5 4.5 4.5
3.368 4.004 4.680 3.532 4.004 4.616
0.207 0.222 0.239 0.206 0.222 0.244
8
applications is such that the CFE source very quickly receives an adsorbed layer of residual gases that alters the work function from the atomically clean surface. Measurement of the energy distribution for the W(310) and W(111) planes of the CFE source has little value for electron optical analysis without the simultaneous measurement of the cathode current density or the more easily measured current per solid angle, referred to as the angular current density (I0 ). As shown by Wiesner and Everhart (1974), for an FE source with apex radius r the following relationship between I0 and J(N) exists:
74
L. W. Swanson and G. A. Schwind
I 0 ¼ JðNÞ
r 2
(2.12)
m
where the angular magnification of the emitted electron m ¼ ao/y is defined as the ratio of the initial launch angle y to the final angle ao with respect to the emitter axis. Implicit in Eq. (2.12) is the assumption that J and I0 are uniform over the solid angle defined by the beam acceptance aperture, as is usually the case for electron optical applications and TED measurements. Using a sphere-on-orthogonal-cone structure (Dyke et al., 1953a) to model the emitter shape and equipotential surfaces between the cathode and anode, Wiesner and Everhart (1973) calculated a variety of emitter parameters as a function of emitter shape. They found that 0.39 <m < 0.64 depending on details of the emitter shape. For source/extractor electrode geometries other than the sphere-on-orthogonal-cone shape modeled by Wiesner and Everhart (1973), trajectory ray-tracing simulation is required to compute the exact value of m. Results of the limited TED measurements will be reviewed where the experimental (FWHM(E)) and I0 values are both available or can be determined from the data. Table 3 summarizes measurements of the key emitter parameters obtained from TED data and FN plots measurement by Swanson and Crouser (1967) for the low work function (111) and (310) planes of clean tungsten. For these relatively low values of I0 , the FWHM (E) values for the (111) and (310) planes are 0.25 and 0.23 eV, respectively, and are in close agreement with the numerically calculated values. However, it will be shown that for larger values of I0 , typically of interest for meaningful electron optical applications, the FWHM(E) values are substantially larger than the calculated values. Experimental values of the TED for an FE source are usually measured using a differential (Braun et al., 1978) or a retarding potential analyzer (Simpson, 1961). The former analyzer type measures the differential curve of Eq. (1), whereas the latter measures the integral of Eq. (1) with e ¼ VR ’c ;
TABLE 3
Plane
W(111) W(310)
(2.13)
Summary of emission parameters for W(111) and W(310) planes at T ¼ 77 K * V b J(N) I0 (A/sr) ’ (eV) (volts) d (eV) (m–1) (A/m2)
0.61 0.54
4.47 4.34
1306 0.154 1230 0.149
2.68 1.02x107 2.70 8.57x106
FWHM(E) (eV)
FWHM (N) (eV)
0.25 0.23
0.21 0.21
* From Swanson and Crouser (1967). The I0 values were derived from the given data. FWHM(E) and FWHM (N) are the experimental and calculated values, respectively.
A Review of the Cold-Field Electron Cathode
75
where VR is the emitter to collector electrode bias potential and ’c is the effective work function of the collector electrode. An important advantage of the integral analyzer is the relative ease with which the TED and I0 can be simultaneously measured. Figure 8 shows a typical output from an integral analyzer where the experimental curve has been fit to the theoretical curve with d, T, and ’c as the adjustable parameters. The Eq. (1) theoretical curve is also shown in Figure 8 using the indicated values for the adjustable parameters. From the value for d and the slope mFN of the FN plot, the work function ’ and b factor can be determined from Eqs. (5) and (6) for the particular crystal plane seen by the opening angle of the analyzer. Fitting the theoretical expression Eq. (1) to the experimental data in Figure 8 requires a temperature of 530 K, even though the measurement was carried out at 300 K. This anomaly is due to the well-known and well-studied effect (Boersch, 1954) caused by statistical mutual Coulomb interactions between electrons in the high current density region near the cathode surface and, to a lesser extent, by the finite resolution of the analyzer. This energy-broadening effect, which has been studied extensively both experimentally (Bell and Swanson, 1979;
1.0
I' = 10mA/sr T = 530 K d = 0.19 eV jc = 3.47 eV
Normalized I or J(E)
0.8
0.6 Differential curve theory Integral curve theory
0.4
Integral curve experimental
0.2
0.0 3.0
3.2
3.4
3.6
3.8 4.0 Voltage
4.2
4.4
4.6
4.8
FIGURE 8 The normalized, experimental retarding potential current obtained from a W-oriented emitter at 300 K and I0 ¼ 10 A/sr is shown along with the best fit to the calculated curve using T, d, and ’c as adjustable parameters. The differential curve calculated from the experimentally derived ’ and F is also shown.
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L. W. Swanson and G. A. Schwind
0.8 CFE b (a) W (b) W (c) W (d) W
0.7
FWHM (eV)
0.6
(mm−1) 1.30 1.90 1.02 1.14
0.5
(a) (b) (c)
0.4 (d)
0.3 0.2 0.1 0 0
20
40
60
80 100 120 140 Angular intensity (mA/sr)
160
180
200
FIGURE 9 The experimental FWHM(E) values for the TED versus I0 obtained from the indicated planes of the tungsten CFE source at 300 K. Values for ’ and b are given in Table 4.
Fransen et al., 1998; Kim et al., 1997; Troyon, 1988) and theoretically ( Jansen, 1990), will be discussed as it relates to W and W CFE sources. Figure 9 compares experimental data obtained in this laboratory for clean W and W oriented emitters with similar data obtained by Troyon (1988). Table 4 provides the respective values of ’, b, r, and m obtained from the TED and I(V) data. The m values, obtained from Eq. (12) using measure r from Figure 10, are in the range expected from trajectory simulations (Wiesner and Everhart, 1973) for a spherical apex emitter. As observed in previous investigations (Schwind, Magera, and Swanson, 2006; Swanson and Schwind, 2009), the FWHM(E) values increase beyond the respective intrinsic (i.e., calculated) values with increasing I0 . Although the data are limited, the contribution of the energy broadening to the FWHM(E) values within Troyon’s and our datasets appears to diminish with increasing b. Because of differing source/anode geometries, comparison of the influence of b on the FWHM values between the two data sets is not possible. Nevertheless, the variation of the energy broadening with b within the two datasets is counter to that observed for the larger-radius SE emitter (Schwind, Magera, and Swanson, 2006). Figure 11 provides a more detailed analysis of the energy broadening for the W and W CFE sources at 300 K where a convolution program (Bronsgeest et al., 2007) has been used to extract the energy-
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A Review of the Cold-Field Electron Cathode
(a)
Acc.V Magn 500 nm 10.0 kV 50 000 x 11/29/05 dE W310
(b)
det Landing E TLD 5.00 keV
mag 70 000 x
HFW 2.15 mm
WD 3.6 mm
500 nm FEI NanoSEM 630
FIGURE 10 SEM photos of the W (a) and W (b) CFE sources used to obtain the data for Figures 8–10.
TABLE 4 Emission characteristics obtained from the indicated CFE sources at T¼300 K* Plane
r (nm) {
W(310) W(310){ W(310) W(111)
— — 160 120
’e (eV)
b (m–1)
4.40 4.40 4.32 4.40
1.30 1.90 1.02 1.14
* Values of the apex radius r were determined from the Figure 11 SEM photos. { Obtained from Troyon (1988) where the value of ’ was derived from the given I(V) data.
m
— — 0.66 0.62
78
(a)
L. W. Swanson and G. A. Schwind
0.35 (a) Exp. (b) Intrinsic (c) Boersch
0.30
(a)
FWHM (eV)
0.25
(b)
0.20 0.15
(c)
0.10 0.05 0.00 0 (b)
2
4
6 8 10 Angular intensity (mA/sr)
12
14
16
0.6 (a) Exp. (b) Boersch (c) Intrinsic
0.5
(a)
FWHM (eV)
0.4
0.3
(b) (c)
0.2
0.1
0.0 0
20
40
60 80 Angular intensity (mA/sr)
100
120
140
FIGURE 11 Relative contributions of the experimental, intrinsic, and Boersch FWHM values versus I0 for the W (a) and W (b) CFE sources at 300 K.
broadening Boersch effect from the experimental TED. The Boersch contribution is described by a two-parameter bell-shaped distribution. One of the fitting parameters is the FWHM(B) of the bell-shaped distribution. By fitting the convolution of the intrinsic energy distribution and the bell-
A Review of the Cold-Field Electron Cathode
79
shaped distribution with its two fitting parameters to the experimental TED, the Boersch contribution could be adequately determined. The results are presented in Figure 10 in terms of the experimental FWHM(E), intrinsic FWHM(N) (i.e., calculated), and the Boersch FWHM(B) contributions. A comparison between the W and W results shows that the energy broadening as determined by the FWHM(B) values not only increases with I0 , but is substantially larger for the W(310) source, which also possesses the smaller value of b. It has been shown for SE sources thatFWHMðBÞ / b j I 0n (Schwind, Magera, and Swanson, 2006; Swanson and Schwind, 2009). However, the limited data reported here for the W and W CFE sources suggest that FWHMðBÞ / bj I 0n . Since all formulations for the variation of b with the cathode/anode geometry give b / 1=rp (Swanson and Schwind, 2009), we conclude from Troyon’s (1988) results (given in Figure 9) and the more recent results for the W and W CFE sources (given in Figure 10) that the Boersch effect diminishes with decreasing source apex radius. Most theoretical and empirical formulations of the energy broadening for high current density FE sources (Knauer, 1981) or for beam crossovers ( Jansen, 1990) are of the form FWHMðBÞ /
I 0l : rk Vos
(2.14)
It therefore appears that the exponent k in Eq. (2.14) may be negative for the CFE source. Further experimental and theoretical investigation will be required to complete our understanding of the role of the source radius in the Boersch effect for the CFE source.
4. SOURCE OPTICS In addition to knowledge of the source energy spread and beam angular intensity as described previously, it is also important to simultaneously know the source reduced brightness Br to fully describe the source optical properties. Unfortunately, Br is not easily measured under the same experimental conditions that the FWHM(E) and I0 values are obtained. A practical definition of Br requires a proper definition of the virtual source size dv as seen from the extractor electrode at voltage Vo. If I is uniform over the acceptance aperture subtending the source, Br is defined by the following relationship: Br ¼
4I 0 pd2v Vo
(2.15)
Bronsgeest et al. (2008) pointed out that a proper definition of dv must take into account the current density distribution in the source image
80
L. W. Swanson and G. A. Schwind
plane. A useful definition for dv that avoids assumptions regarding the intensity profile is the diameter containing a certain fraction FC of the current. For FC ¼ 0.5, various intensity profiles yield a common value for dv measured by scanning the beam across the knife edge and defining dv by the 25% to 75% edge resolution. In addition, the total probe size obtained by the addition of various aberrations, using the FC ¼ 0.5 definition as described by Barth and Kruit (1996), is free of assumptions regarding the spatial distributions of the various aberrations. The value of dv obtained for FC ¼ 0.5 is denoted as (Bronsgeest et al., 2008) r ET 1=2 ; (2.16) dv 50 ¼ 1:67 m Vo where ET is the average initial transverse energy of the emitted electrons. For the CFE emission regime, ET ¼ d; thus, by combining Eqs. (2.15) and (2.16) the following expression for the practical brightness Br50 is obtained: I 0 m 2 Br 50 ¼ 1:44 : (2.17) d r Eliminating m/r from Eq. (2.17) by use of Eq. (2.12) leads to Br 50 ¼ 1:44
JðNÞ ¼ 1:44Br ; pd
(2.18)
where Br ¼ J(N)/pd is the familiar expression for the differential or axial brightness (Worster, 1969). As observed previously (Bronsgeest et al., 2008), the practical source brightness exceeds the axial brightness by 44%. For SEM and TEM applications, one is interested in a FE source that simultaneously provides a large value of Br50 and a small value of FWHM(E). If ’ and bVo are known, Br50 can be calculated using Eq. (2.18). The latter is plotted versus FWHM(E) in Figure 12 for the W and W sources. Since the W curve lies to the left in Figure 12, it is favored over the W source based on the above criteria. The relatively large calculated values of Br50 in Figure 12 are primarily due to the small values of dv50, or, equivalently, the large values of J(N) for these CFE sources. Values of dv50 obtained from Eq. (2.16) and J (N) (obtained knowing b, Vo, and ’) are given in Tables 5 and 6. The average dv50 values are 2.90 and 2.29 nm for the W and W sources, respectively. The latter values of dv50 are an order of magnitude smaller than the typical dv50 values for the SE source (Swanson and Schwind, 2009). The mutual Coulomb interactions that cause the energy broadening also cause a random, non-focusable modification of the electron trajectories. This leads to a radial broadening dblur of the virtual source size and
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A Review of the Cold-Field Electron Cathode
5.E + 09 5.E + 09 4.E + 09
Br (A/m2srV)
4.E + 09
CFE
b (mm−1)
(a) W (b) W
1.14 1.01
(b)
3.E + 09 3.E + 09 2.E + 09 2.E + 09 (a) 1.E + 09 5.E + 08 0.E + 00 0.20
0.25
0.30
0.35 FWHM (eV)
0.40
0.45
0.50
FIGURE 12 Graph shows the source reduced brightness Br versus the corresponding FWHM(E) values for the two CFE sources. TABLE 5
Values of various emission parameters for the W CFE at 300 K*
I0 (A/sr)
7.5 19 44 62 116
F (V/nm)
J(N) (A/m2)
d (eV)
dv50 (nm)
FWHM(E) (eV)
FWHM(B) (eV)
3.840 4.040 4.242 4.340 4.545
1.38 108 3.22 108 6.93 108 9.94 108 19.5 108
0.190 0.201 0.211 0.216 0.227
2.76 2.90 2.98 2.98 2.89
0.357 0.380 0.427 0.412 0.507
0.199 0.247 0.271 0.236 0.369
* The relevant values of ’, b, r, and m are given in Table 4.
TABLE 6
Values of various emission parameters for the W CFE at 300 K*
I0 (A/sr)
2 5 10 15
F (V/nm)
J(N) (A/m2)
d (eV)
dv50 (nm)
FWHM(E) (eV)
FWHM(B) (eV)
3.735 3.932 4.104 4.206
5.42 107 1.32 108 2.71 108 4.02 108
0.165 0.174 0181 0.186
2.78 2.31 2.28 2.29
0.237 0.273 0.301 0.319
0.039 0.062 0.080 0.093
* The relevant values of ’, b, r, and m are given in Table 4.
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L. W. Swanson and G. A. Schwind
a concomitant decrease in Br50. Although dblur occurring at a beam crossover in an optical column has been extensively studied (Jansen, 1990; Kruit and Jansen 2009), the nonhomogeneous field accelerating the particles in the region near the emitter surface of the FE source renders it impossible to treat the radial broadening analytically. Nevertheless, it is expected that the calculated values of Br50 above 1 109 A/m2srV in Figure 12 will be substantially lowered due to the radial broadening that increases dv50 beyond the value calculated via Eq. (2.16).
5. COLUMN OPTICS USING THE COLD-FIELD ELECTRON SOURCE In a focusing column the conservation of the source brightness Br down to the target leads to the following useful relations between the probe current Ip, and the convergence half angle ai, the virtual source size dp (dp ¼ Mdv50) and image (Vi), and the object (Vo) side voltages: Ip ¼
p2 Vi Vi Br ðai dp Þ2 ¼ pI 0 ðMai Þ2 4 Vo
(2.19)
where Br is defined by (E. 15). The total beam size in the image plane is usually limited by a combination of lens chromatic aberration dc, spherical aberration ds, diffraction dl, coulomb interactions dblur, and virtual source size dv. The contributions of these terms to the total beam size in the image plane have been given by Barth and Kruit (1996) in terms of the beam size dt50 that contains 50% of the beam current as follows: dl ¼
663:6
ðnmÞ
(2.20)
dV ai ðnmÞ Vi
(2.21)
1=2
Vi ai
dc ¼ 600Cc
ds ¼ 1:8x104 Cs a3i ðnmÞ dt 50 ¼
h
d4l
þ
1:3=4 d4s
þ M dv 50 þ 2
2
1:3=2 i2=1:3 M2 d2blur
(2.22) 1=2 þ
d2c
;
(2.23)
where M is the overall column magnification. The image-side voltage Vi is in volts, the spherical and chromatic aberration coefficients (Cs and Cc, respectively) are in millimeters, and dV is the full width containing 50% of the particles (FW50) value in electron volts of the energy spread. The above formulation of the aberration contributions gives results that are comparable to the more rigorous wave optical approach.
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83
The total chromatic Cct and spherical Cst aberration coefficients of a two-lens optical system are given by (Orloff, 1983) 3=2 Vi (2.24) Cct ¼ Cci þ Cco M2 V0 3=2 Vi ; (2.25) Cst ¼ Csi þ Cso M4 V0 where Cci and Csi are the image-side aberration coefficients of the objective lens and Cco and Cso are the object-side aberration coefficients of the gun lens. Sources with a large value of dv50 (e.g., an SE source with dv50 30 to 60 nm) require the column to be operated in a strongly demagnified mode. This, combined with a high Vo (e.g., 5 kV), results in such a small gun lens aberration contribution that it can typically be ignored or at most considered a minor contribution to the total column aberrations. To illustrate the latter and other contrasts between the CFE and SE sources, a comparison between the commonly used WZrO SE and the W CFE sources will be made using aberration coefficients of a typical focusing column. Tables 7 and 8 give source parameters and the focusing column aberration coefficients used to calculate dt50 versus Ip for a high (30-kV) and low (1-kV) beam voltage. A computer program was used to calculate the optimum values of M for a range of ai values to minimize dt at each value of ai using Eqs. (2.19) to (2.25). The results of the latter calculations are TABLE 7
Working distance and column aberrations used for the figure 13 analysis
Cso (mm)
20
TABLE 8
0
Cso (mm)
Cci (mm)
Csi (mm)
Working Distance (mm)
75
0.58
0.49
1.0
Source parameters used for the figure 13 analysis
I (A/sr) Vo (V) r (nm) m FW50 (eV) dv50 (nm) ’ (eV) T ( K)
W(111)
W(100)ZrO
15 3693 120 0.62 0.214 2.29 4.40 300
200 4032 370 0.21 0.37 16.8 2.92 1800
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L. W. Swanson and G. A. Schwind
(a) 100 (b) (a) WZrO SE (b) W(111) CFE
dt50 (nm)
(a)
10
(a) (b) 1 0.01
0.1
1
10
100
1000
10,000
100,000 1000,000
Ip (pA)
(b) 100
(b) (a) WZrO SE (b) W(111) CFE
(a)
dt50 (nm)
10
1
(a)
(b)
0 0.01
0.1
1
10
100 Ip (pA)
1000
10,000
100,000 1000,000
FIGURE 13 Plots of the beam size dt50 versus beam current Ip for a typical SEM column for a beam voltage of 1 kV (a) and 30 kV (b). The W CFE source is compared with a W(100)ZrO SE source. Source and column parameters are given in Tables 7 and 8. The dashed lines show the effect of eliminating the gun lens aberrations.
given in Figure 13. Not surprisingly, for Ip < 1000 pA the CFE source provides a lower value of dt than the SE source, although less so for the 30-kV beam voltage. The individual contributions to dt are detailed in
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A Review of the Cold-Field Electron Cathode
(a)
100
10
(a) Total (b) Diffraction (c) Chromatic (d) Spherical (e) Virtual source
(c)
d (nm)
(a) 1 (d) (b) 0.1 (e)
0.01 0.1
1
10
100
1000
10,000
1000
10,000
100,000
Ip (nA) (b)
100
d (nm)
10
(a) Total (b) Diffraction (c) Spherical (d) Chromatic (e) Virtual source
1 (a) (b) (c) 0.1
(e) (d)
0.01 0.1
1
10
100
100,000
Ip (nA)
FIGURE 14 Aberration contributions for the Figure 13 (a) dt versus Ip results for a beam voltage of 1 kV (a) and 30 kV (b).
Figure 14 for the CFE source. For Ip < 1000 pA diffraction, chromatic and spherical aberrations are the primary contributors to dt at Vi ¼ 1 kV, whereas at Vi ¼ 30 kV the primary contributions come from the spherical and
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L. W. Swanson and G. A. Schwind
diffraction terms. At Ip ¼ 1000 pA, all aberrations contribute more or less equally. For different values of Cst and Cct, the relative contributions will be altered. The improvement in performance for Ip < 1000 pA using the CFE source comes with the requirement for a larger value of M (Figure 15), which in turn leads to a larger contribution from the gun lens (see dashed curves in Figure 13) to the overall aberration load in accordance with Eqs. (2.24) and (2.25). As noted previously (Kruit, Bezuijen, and Barth, 2006), this is a consequence for any CFE that achieves a large value of Br by virtue of a small value of dv as opposed to a large value of I0 (see Eq. (2.15)), which is the case for the SE source. The larger required value of M for the CFE source implies more mechanical stability and magnetic shielding requirements for the column. Not withstanding beam current stability issues, the CFE source is a superior source for applications requiring a high resolution or a highly coherent focused beam.
6. CURRENT STABILITY For electron optical applications, current stability considerations are confined to a relatively small fraction of the total current contained in a 1 to 10 mr semi-angle centered on the optical axis of the source. Since the basic 10 (a) W(111) CFE (b) WZrO SE
M
1
0.1
0.01
(a) (b)
0.001 0.1
1
10
100 Ip (pA)
1000
10,000
100,000
FIGURE 15 Graph shows the optimum column magnification M corresponding to the Figure 13 (a) dt versus Ip results.
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mechanisms associated with long current drift and short-term current fluctuations differ, it is instructive to consider the current stability issues in terms of high-frequency fluctuations about a mean current and longterm current drift usually in the direction of reduced current. According to the FN Eq. (2.2), current fluctuations and drift are due to fluctuations or drift in ’ and/or b. Ignoring the pre-exponential term in Eq. (2.2), it can be shown that small current fluctuations dI are given by dI b’3=2 3b’1=2 d’; ¼ 2 dF ¼ F 2F I
(2.26)
where b ¼ 6.83 109 (V/m(eV)–3/2), dF ¼ Vdb. The workfunction fluctuations d’ originate from fluctuations in the adsorbed molecules or atoms dn in the emitting area Ap subtended by the acceptance aperture as follows: d’ ¼ cdn;
(2.27)
where c ¼ 4pm/Ap, where m is the dipole particle per adsorbed particle. Thus, a change in ’ can come about only from a change in the adatom coverage n/Ap or m. To change I due to a change in b requires either a microscopic geometric change (atomic size within Ap) or a macroscopic geometric change in the emitter shape.
6.1. High-Frequency Current Fluctuations The following mechanisms can contribute to varying degrees to fluctuations in I: 1. With a specific pressure of residual gases, fluctuations in n can be caused by adsorption/desorption processes. 2. Fluctuations in m can occur due to thermal induced transitions between two bonding states of adsorbed molecule (Kleint, 1971). 3. Fluctuations in n will occur for mobile adsorbed molecules or atoms due to surface diffusion in and out of Ap (Gomer, 1973). It has been shown that mechanism 3 in the list above can occur even for an atomically clean surface due to mobile, self-adsorbed atoms on a particular crystal plane (Swanson, 1978). These intrinsic current fluctuations vary significantly with crystal plane as shown in Table 1 where the value of dI/I values are approximately inversely proportional to the surface atom density of the respective crystal plane. As an illustration of the highly anisotropic nature of this current fluctuation mechanism, dI/I (310)/dI/I(100) ¼ 7.5 at 500 K (see Table 1) and increases to 67 at 900 K (Swanson, 1978). The temperature threshold where these fluctuations exceed the statistical shot-noise level is as low as 300 K for the (310)
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L. W. Swanson and G. A. Schwind
plane for a frequency bandwidth of 10 kHz (Swanson, 1978). Interestingly, the Table 1 summary of current fluctuations suggests a lower value for the W(111) versus the W(310) plane. To illustrate the extreme sensitivity of a CFE source to a small change in ad atomdensity (n/Ap); Ap ¼ 19.6 nm2 for a typical optical column application with ao ¼ 10 mr, r ¼ 150 nm and m ¼ 0.6. For the W(111) plane, the latter value of Ap contains 114 substrate atoms; thus, one atom diffusing in or out of Ap represents a 1% change in n/Ap. Besocke and Wagner (1973, 1975) observed that the workfunction of the W(110) plane decreased monotonically with increasing concentration of adsorbed tungsten atoms on the W(110) terrace. According to Eq. (2.26), a 1% variation in ’ would cause a 28% variation in I for a CFE source with a workfunction of 4.5 eV and a typical applied field of 3.5 109 V/m. Similarly, a 1% variation in b would result in an 18% variation in I. Clearly, both ’ and b must be controlled to stabilize current from a CFE source. Investigations by Saitou (1977) and Todokoro Saitou, and Yamamoto, (1982) suggest that current fluctuation due to ion bombardment dominates when PxIt > 7 10–12 PaA, whereas current fluctuations due to mobility of adsorbed gas molecules dominates at lower values of PxIt. As shown elsewhere (Swanson and Schwind, 2009), the magnitude of the current fluctuations for an SE source increases with decreasing emitter radius and acceptance aperture solid angle. This result is not unexpected since the rms amplitude of concentration fluctuations for a mobile adsorbed layer is proportional to Ap–1/2 (Swanson, 1978). However, as the radius approaches that of a single atom, as is the case for some versions of room temperature nano tips (Kuo et al., 2006), the lifetime with respect to atomic displacement appears to be exceedingly large. All fluctuations due to atomic motion eventually cease, and only shot noise from the statistics of the emission process remains as the CFE source temperature is reduced below room temperature.
6.2. Long-Term Current Drift If adsorption of foreign molecules from the gas phase or from electronstimulated desorption (ESD) of neutrals and ions (Madey and Yates, 1971) from electrode surfaces were eliminated, then all adsorbate-induced noise and long-term drift related to workfunction change would be minimal. Such a result was approached by the Dyke group in 1960 (Martin, Trolan, and Dyke, 1960) where stable emission current with limited fluctuation and drift was obtained for more than 2500 hours for total currents in excess of 100 A. This result was obtained with a tungsten CFE source in a sealed-off tube with excellent vacuum (
