In-situ Electron Microscopy

Edited by Gerhard Dehm, James M. Howe, and Josef Zweck

Applications in Physics, Chemistry and Materials Science

57268File AttachmentCover.jpg

Edited by

Gerhard Dehm, James M. Howe,

and Josef Zweck

In-situ Electron Microscopy

Related Titles

Van Tendeloo, G., Van Dyck, D.,Pennycook, S. J. (Eds.)

Handbook of Nanoscopy

2012

Hardcover

ISBN: 978-3-527-31706-6

Tsukruk, V. V., Singamaneni, S.

Scanning Probe Microscopyof Soft Matter

Fundamentals and Practices

2012

Hardcover

ISBN: 978-3-527-32743-0

Baró, A. M., Reifenberger, R. G. (Eds.)

Atomic Force Microscopy in Liquid

Biological Applications

2012

Hardcover

ISBN: 978-3-527-32758-4

Bowker, M., Davies, P. R. (Eds.)

Scanning Tunneling Microscopyin Surface Science2010

Hardcover

ISBN: 978-3-527-31982-4

García, R.

Amplitude Modulation AtomicForce Microscopy2010

Hardcover

ISBN: 978-3-527-40834-4

Codd, S., Seymour, J. D. (Eds.)

Magnetic Resonance Microscopy

Spatially Resolved NMR Techniques

and Applications

2009

Hardcover

ISBN: 978-3-527-32008-0

Stokes, D.

Principles and Practice ofVariable Pressure

Environmental Scanning Electron

Microscopy (VP-ESEM)

2009

Hardcover

ISBN: 978-0-470-06540-2

Edited byGerhard Dehm, James M. Howe,and Josef Zweck

In-situ Electron Microscopy

Applications in Physics, Chemistry and Materials Science

The Editors

Prof. Dr. Gerhard DehmMontanuniversität LeobenDept. MaterialphysikJahnstr. 128700 LeobenAustria

Prof. Dr. James M. HoweUniversity of VirginiaDept. of Mat. Science & Engin.116 Engineer's WayCharlottesville, VA 22904-4745USA

Prof. Dr. Josef ZweckUniversität RegensburgFak. für Physik93040 RegensburgGermany

All books published by Wiley-VCH are carefullyproduced. Nevertheless, authors, editors, and pub-lisher do not warrant the information contained inthese books, including this book, to be free of errors.Readers are advised to keep in mind that statements,data, illustrations, procedural details or other itemsmay inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from theBritish Library.

Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publica-tion in the Deutsche Nationalbibliografie; detailedbibliographic data are available on the Internet athttp://dnb.d-nb.de.

# 2012 Wiley-VCH Verlag & Co. KGaA,Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translationinto other languages). No part of this book may bereproduced in any form – by photoprinting, micro-film, or any other means – nor transmitted or trans-lated into a machine language without writtenpermission from the publishers. Registered names,trademarks, etc. used in this book, even when notspecifically marked as such, are not to be consideredunprotected by law.

Cover Design Adam-Design, WeinheimTypesetting Thomson Digital, Noida, IndiaPrinting and Binding Strauss GmbH, Mörlenbach

Printed in the Federal Republic of GermanyPrinted on acid-free paper

Print ISBN: 978-3-527-31973-2ePDF ISBN: 978-3-527-65219-8ePub ISBN: 978-3-527-65218-1mobi ISBN: 978-3-527-65217-4oBook ISBN: 978-3-527-65216-7

Contents

List of Contributors XIIIPreface XVII

Part I Basics and Methods 1

1 Introduction to Scanning Electron Microscopy 3Christina Scheu and Wayne D. Kaplan

1.1 Components of the Scanning Electron Microscope 41.1.1 Electron Guns 61.1.2 Electromagnetic Lenses 91.1.3 Deflection System 131.1.4 Electron Detectors 131.1.4.1 Everhart–Thornley Detector 131.1.4.2 Scintillator Detector 151.1.4.3 Solid-State Detector 161.1.4.4 In-Lens or Through-the-Lens Detectors 161.2 Electron–Matter Interaction 161.2.1 Backscattered Electrons (BSEs) 201.2.2 Secondary Electrons (SEs) 221.2.3 Auger Electrons (AEs) 251.2.4 Emission of Photons 251.2.4.1 Emission of X-Rays 251.2.4.2 Emission of Visible Light 261.2.5 Interaction Volume and Resolution 261.2.5.1 Secondary Electrons 271.2.5.2 Backscattered Electrons 271.2.5.3 X-Rays 271.3 Contrast Mechanisms 281.3.1 Topographic Contrast 281.3.2 Composition Contrast 311.3.3 Channeling Contrast 31

V

1.4 Electron Backscattered Diffraction (EBSD) 311.5 Dispersive X-Ray Spectroscopy 341.6 Other Signals 361.7 Summary 36

References 37

2 Conventional and Advanced Electron Transmission Microscopy 39Christoph Koch

2.1 Introduction 392.1.1 Introductory Remarks 392.1.2 Instrumentation and Basic Electron Optics 402.1.3 Theory of Electron–Specimen Interaction 422.2 High-Resolution Transmission Electron Microscopy 482.3 Conventional TEM of Defects in Crystals 542.4 Lorentz Microscopy 552.5 Off-Axis and Inline Electron Holography 572.6 Electron Diffraction Techniques 592.6.1 Fundamentals of Electron Diffraction 592.7 Convergent Beam Electron Diffraction 612.7.1 Large-Angle Convergent Beam Electron Diffraction 632.7.2 Characterization of Amorphous Structures by Diffraction 632.8 Scanning Transmission Electron Microscopy and Z-Contrast 632.9 Analytical TEM 66

References 67

3 Dynamic Transmission Electron Microscopy 71Thomas LaGrange, Bryan W. Reed, Wayne E. King,Judy S. Kim, and Geoffrey H. Campbell

3.1 Introduction 713.2 How Does Single-Shot DTEM Work? 723.2.1 Current Performance 743.2.2 Electron Sources and Optics 753.2.3 Arbitrary Waveform Generation Laser System 803.2.4 Acquiring High Time Resolution Movies 813.3 Experimental Applications of DTEM 823.3.1 Diffusionless First-Order Phase Transformations 823.3.2 Observing Transient Phenomena in Reactive Multilayer Foils 853.4 Crystallization Under Far-from-Equilibrium Conditions 883.5 Space Charge Effects in Single-Shot DTEM 903.5.1 Global Space Charge 903.5.2 Stochastic Blurring 913.6 Next-Generation DTEM 913.6.1 Novel Electron Sources 913.6.2 Relativistic Beams 923.6.3 Pulse Compression 93

VI Contents

3.6.4 Aberration Correction 933.7 Conclusions 94

References 95

4 Formation of Surface Patterns Observed with ReflectionElectron Microscopy 99Alexander V. Latyshev

4.1 Introduction 994.2 Reflection Electron Microscopy 1024.3 Silicon Substrate Preparation 1074.4 Monatomic Steps 1094.5 Step Bunching 1114.6 Surface Reconstructions 1144.7 Epitaxial Growth 1154.8 Thermal Oxygen Etching 1164.9 Conclusions 119

References 119

Part II Growth and Interactions 123

5 Electron and Ion Irradiation 125Florian Banhart

5.1 Introduction 1255.2 The Physics of Irradiation 1265.2.1 Scattering of Energetic Particles in Solids 1265.2.2 Scattering of Electrons 1285.2.3 Scattering of Ions 1295.3 Radiation Defects in Solids 1295.3.1 The Formation of Defects 1295.3.2 The Migration of Defects 1305.4 The Setup in the Electron Microscope 1315.4.1 Electron Irradiation 1315.4.2 Ion Irradiation 1325.5 Experiments 1325.5.1 Electron Irradiation 1335.5.2 Ion Irradiation 1405.6 Outlook 141

References 142

6 Observing Chemical Reactions Using Transmission ElectronMicroscopy 145Renu Sharma

6.1 Introduction 1456.2 Instrumentation 1466.3 Types of Chemical Reaction Suitable for TEM Observation 150

Contents VII

6.3.1 Oxidation and Reduction (Redox) Reactions 1506.3.2 Phase Transformations 1516.3.3 Polymerization 1516.3.4 Nitridation 1526.3.5 Hydroxylation and Dehydroxylation 1526.3.6 Nucleation and Growth of Nanostructures 1536.4 Experimental Setup 1546.4.1 Reaction of Ambient Environment with Various TEMComponents 1546.4.2 Reaction of Grid/Support Materials with the Sample or with

Each Other 1546.4.3 Temperature and Pressure Considerations 1556.4.4 Selecting Appropriate Characterization Technique(s) 1566.4.5 Recording Media 1566.4.6 Independent Verification of the Results, and the Effects of the

Electron Beam 1576.5 Available Information Under Reaction Conditions 1576.5.1 Structural Modification 1586.5.1.1 Electron Diffraction 1586.5.1.2 High-Resolution Imaging 1586.5.2 Chemical Changes 1616.5.3 Reaction Rates (Kinetics) 1646.6 Limitations and Future Developments 164

References 165

7 In-Situ TEM Studies of Vapor- and Liquid-Phase Crystal Growth 171Frances M. Ross

7.1 Introduction 1717.2 Experimental Considerations 1727.2.1 What Crystal Growth Experiments are Possible? 1727.2.2 How Can These Experiments be Made Quantitative? 1737.2.3 How Relevant Can These Experiments Be? 1757.3 Vapor-Phase Growth Processes 1757.3.1 Quantum Dot Growth Kinetics 1767.3.2 Vapor–Liquid–Solid Growth of Nanowires 1777.3.3 Nucleation Kinetics in Nanostructures 1807.4 Liquid-Phase Growth Processes 1837.4.1 Observing Liquid Samples Using TEM 1837.4.2 Electrochemical Nucleation and Growth in the TEM System 1847.5 Summary 187

References 188

8 In-Situ TEM Studies of Oxidation 191Guangwen Zhou and Judith C. Yang

8.1 Introduction 1918.2 Experimental Approach 192

VIII Contents

8.2.1 Environmental Cells 1928.2.2 Surface and Environmental Conditions 1938.2.3 Gas-Handling System 1948.2.4 Limitations 1958.3 Oxidation Phenomena 1968.3.1 Surface Reconstruction 1968.3.2 Nucleation and Initial Oxide Growth 1978.3.3 Role of Surface Defects on Surface Oxidation 1988.3.4 Shape Transition During Oxide Growth in Alloy Oxidation 1998.3.5 Effect of Oxygen Pressure on the Orientations of Oxide Nuclei 2028.3.6 Oxidation Pathways Revealed by High-Resolution TEM Studies

of Oxidation 2038.4 Future Developments 2058.5 Summary 206

References 206

Part III Mechanical Properties 209

9 Mechanical Testing with the Scanning Electron Microscope 211Christian Motz

9.1 Introduction 2119.2 Technical Requirements and Specimen Preparation 2129.3 In-Situ Loading of Macroscopic Samples 2149.3.1 Static Loading in Tension, Compression, and Bending 2149.3.2 Dynamic Loading in Tension, Compression, and Bending 2169.3.3 Applications of In-Situ Testing 2169.4 In-Situ Loading of Micron-Sized Samples 2179.4.1 Static Loading of Micron-Sized Samples in Tension, Compression,

and Bending 2189.4.2 Applications of In-Situ Testing of Small-Scale Samples 2209.4.3 In-Situ Microindentation and Nanoindentation 2229.5 Summary and Outlook 223

References 223

10 In-Situ TEM Straining Experiments: Recent Progress in Stagesand Small-Scale Mechanics 227Gerhard Dehm, Marc Legros, and Daniel Kiener

10.1 Introduction 22710.2 Available Straining Techniques 22810.2.1 Thermal Straining 22810.2.2 Mechanical Straining 22910.2.3 Instrumented Stages and MEMS/NEMS Devices 23010.3 Dislocation Mechanisms in Thermally Strained Metallic Films 23310.3.1 Basic Concepts 23310.3.2 DislocationMotion in Single Crystalline Films and Near Interfaces 235

Contents IX

10.3.3 Dislocation Nucleation and Multiplication in Thin Films 23610.3.4 Diffusion-Induced Dislocation Plasticity in Polycrystalline

Cu Films 23910.4 Size-Dependent Dislocation Plasticity in Metals 23910.4.1 Plasticity in Geometrically Confined Single Crystal

fcc Metals 24110.4.2 Size-Dependent Transitions in Dislocation Plasticity 24310.4.3 Plasticity by Motion of Grain Boundaries 24410.4.4 Influence of Grain Size Heterogeneities 24510.5 Conclusions and Future Directions 247

References 248

11 In-Situ Nanoindentation in the Transmission Electron Microscope 255Andrew M. Minor

11.1 Introduction 25511.1.1 The Evolution of In-Situ Mechanical Probing in a TEM 25511.1.2 Introduction to Nanoindentation 25611.2 Experimental Methodology 26011.3 Example Studies 26311.3.1 In-Situ TEM Nanoindentation of Silicon 26311.3.2 In-Situ TEM Nanoindentation of Al Thin Films 26911.4 Conclusions 272

References 274

Part IV Physical Properties 279

12 Current-Induced Transport: Electromigration 281Ralph Spolenak

12.1 Principles 28112.2 Transmission Electron Microscopy 28312.2.1 Imaging 28312.2.2 Diffraction 28812.2.3 Convergent Beam Electron Diffraction (CBED):

Measurements of Elastic Strain 28812.3 Secondary Electron Microscopy 28912.3.1 Imaging 28912.3.2 Elemental Analysis 29112.3.3 Electron Backscatter Diffraction (EBSD) 29212.4 X-Radiography Studies 29212.4.1 Microscopy and Tomography 29212.4.2 Spectroscopy 29312.4.3 Topography 29412.4.4 Microdiffraction 29412.5 Specialized Techniques 29512.5.1 Focused Ion Beams 295

X Contents

12.5.2 Reflective High-Energy Electron Diffraction (RHEED) 29612.5.3 Scanning Probe Methods 29612.6 Comparison of In-Situ Methods 297

References 299

13 Cathodoluminescence in Scanning and TransmissionElectron Microscopies 303Yutaka Ohno and Seiji Takeda

13.1 Introduction 30313.2 Principles of Cathodoluminsecence 30413.2.1 The Generation and Recombination of Electron-Hole Pairs 30413.2.2 Characteristic of CL Spectroscopy 30513.2.3 CL Imaging and Contrast Analysis 30613.2.4 Spatial Resolution of CL Imaging and Spectroscopy 30613.2.5 CL Detection Systems 30713.3 Applications of CL in Scanning and Transmission Electron

Microscopies 30713.3.1 Assessments of Group III–V Compounds 30813.3.1.1 Nitrides 30813.3.1.2 III–V Compounds Except Nitrides 30913.3.2 Group II–VI Compounds and Related Materials 31013.3.2.1 Oxides 31013.3.2.2 Group II–VI Compounds, Except Oxides 31213.3.3 Group IV and Related Materials 31313.4 Concluding Remarks 313

References 313

14 In-Situ TEM with Electrical Bias on Ferroelectric Oxides 321Xiaoli Tan

14.1 Introduction 32114.2 Experimental Details 32314.3 Domain Polarization Switching 32414.4 Grain Boundary Cavitation 32614.5 Domain Wall Fracture 33114.6 Antiferroelectric-to-Ferroelectric Phase Transition 33514.7 Relaxor-to-Ferroelectric Phase Transition 341

References 345

15 Lorentz Microscopy 347Josef Zweck

15.1 Introduction 34715.2 The In-Situ Creation of Magnetic Fields 35015.2.1 Combining the Objective Lens Field with Specimen Tilt 35115.2.2 Magnetizing Stages Using Coils and Pole-Pieces 35215.2.3 Magnetizing Stages Without Coils 356

Contents XI

15.2.3.1 Oersted Fields 35615.2.3.2 Spin Torque Applications 35815.2.3.3 Self-Driven Devices 36115.3 Examples 36215.3.1 Demagnetization and Magnetization of Ring Structures 36215.3.2 Determination of Wall Velocities 36415.3.3 Determination of Stray Fields 36515.4 Problems 36615.5 Conclusions 367

References 367

Index 371

XII Contents

List of Contributors

XIII

Florian BanhartUniversité de StrasbourgInstitut de Physique et Chimie desMatériaux, UMR 750423 rue du Loess67034 StrasbourgFrance

Nigel D. BrowningLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

Geoffrey H. CampbellLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

Gerhard DehmAustrian Academy of SciencesErich Schmid Institute of MaterialsScienceJahnstr. 128700 LeobenAustria

andMontanuniversität LeobenDepartment Materials PhysicsFranz-Josef-Str. 188700 LeobenAustria

Wayne D. KaplanTechnion - Israel Institute of TechnologyDepartment of Materials EngineeringHaifa 32000Israel

Daniel KienerMontanuniversität LeobenDepartment Materials PhysicsFranz-Josef-Str. 188700 LeobenAustria

Judy S. KimLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

andUniversity of CaliforniaDepartment of Chemical Engineeringand Materials ScienceOne Shields AvenueDavisCalifornia 95616USA

Wayne E. KingLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

Christoph KochMax-Planck-Institut fürMetallforschungHeisenbergstr. 370569 StuttgartGermany

Thomas LaGrangeLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

Alexander V. LatyshevSiberian Branch of Russian Academy ofSciencesInstitute of Semiconductor PhysicsProspect Lavrent’eva 13630090 NovosibirskRussia

Marc LegrosCEMES-CNRS29 Rue Jeanne Marvig31055 ToulouseFrance

Andrew M. MinorUniversity of California, Berkeley andNational Center for ElectronMicroscopyDepartment of Materials Science andEngineering, Lawrence BerkeleyNational LaboratoryOne Cyclotron Road, MS 72BerkeleyCA 94720USA

Christian MotzÖsterreichische Akademie derWissenschaftenErich Schmid Institut fürMaterialwissenschaftJahnstr. 128700 LeobenAustria

Yutaka OhnoTohoku UniversityInstitute for Materials ResearchKatahira 2-1-1Aoba-kuSendai 980-8577Japan

Bryan W. ReedLawrence Livermore NationalLaboratoryPhysical and Life Sciences Directorate7000 East AvenueLivermoreCalifornia 94550USA

XIV List of Contributors

Frances M. RossIBM T. J. Watson Research Center1101 Kitchawan RoadYorktown HeightsNY 10598USA

Christina Scheu1Ludwig-Maximilians-UniversitätMünchenDepartment Chemie & Center forNanoScience (CeNS)Butenandstr. 5-13, Gerhard-Ertl-Gebäude (Haus E)81377 MünchenGermany

Renu SharmaNational Institute of Science andTechnologyCenter for Nanoscale Science andTechnology100 Bureau DriveGaithersburgMD 20899-6201USA

Ralph SpolenakETH ZurichLaboratory of Nanometallurgy,Department of MaterialWolfgang-Pauli-Str. 108093 ZurichSwitzerland

Seiji TakedaOsaka UniversityThe Institute of Scientific and IndustrialResearchMihogaoka 8-1IbarakiOsaka 567-0047Japan

Xiaoli TanIowa State UniversityDepartment of Materials Science andEngineering2220 Hoover HallAmesIA 50011USA

Judith C. YangUniversity of PittsburghDepartment of Chemical and PetroleumEngineering1249 Benedum HallPittsburghPA 15261USA

Guangwen ZhouP. O. Box 600085 Murray Hill RoadBinghamptonNY 13902USA

Josef ZweckUniversity of RegensburgPhysics FacultyPhysics Building Office Phy 7.3.0593040 RegensburgGermany

List of Contributors XV

Preface

Today, transmission electron microscopy (TEM) represents one of the most impor-tant tools used to characterize materials. Electron diffraction provides informationon the crystallographic structure of materials, conventional TEM with bright-fieldand dark-field imaging on their microstructure, high-resolution TEM on theiratomic structure, scanning TEM on their elemental distributions, and analyticalTEM on their chemical composition and bonding mechanisms. Each of thesetechniques is explained in detail in various textbooks on TEM techniques, includingTransmission Electron Microscopy: A Textbook for Materials Science (D.B. Williams andC.B. Carter, Plenum Press, New York, 1996), and Transmission Electron MicroscopyandDiffractometry ofMaterials (3rd edition, B. Fultz and J.M.Howe, Springer-Verlag,Berlin, Heidelberg, 2008).

Most interestingly, however, TEM also enables dynamical processes in materialsto be studied through dedicated in-situ experiments. To watch changes occurring in amaterial of interest allows not only the development but also the refinement ofmodels, so as to explain the underlying physics and chemistry of materials pro-cesses. The possibilities for in-situ experiments span from thermodynamics andkinetics (including chemical reactions, oxidation, and phase transformations) tomechanical, electrical, ferroelectric, and magnetic material properties, as well asmaterials synthesis.

The present book is focused on the state-of-the-art possibilities for performingdynamic experiments inside the electron microscope, with attention centered onTEM but including scanning electron microscopy (SEM). Whilst seeing is believing isone aspect of in-situ experiments in electron microscopy, the possibility to obtainquantitative data is of almost equal importance when accessing critical data inrelation to physics, chemistry, and the materials sciences. The equipment neededto obtain quantitative data on various stimuli – such as temperature and gas flow formaterials synthesis, load and displacement for mechanical properties, and electricalcurrent and voltage for electrical properties, to name but a few examples – aredescribed in the individual sections that relate to Growth and Interactions (Part Two),Mechanical Properties (Part Three), and Physical Properties (Part Four).

XVII

During the past decade, interest in in-situ electron microscopy experiments hasgrown considerably, due mainly to new developments in quantitative stages andmicro-/nano-electromechanical systems (MEMS/NEMS) that provide a ‘‘lab on chip’’platform which can fit inside the narrow space of the pole-pieces in the transmissionelectron microscope. In addition, the advent of imaging correctors that compensatefor the spherical and, more recently, the chromatic aberration of electromagneticlenses has not only increased the resolution of TEMbut has also permitted the use oflarger pole-piece gaps (and thus more space for stages inside the microscope), evenwhen designed for imaging at atomic resolution. Another driving force of in-situexperimentation using electron probes has been the small length-scales that areaccessible with focused ion beam/SEM platforms and TEM instruments. These areof direct relevance for nanocrystalline materials and thin-film structures withmicrometer and nanometer dimensions, as well as for structural defects such asinterfaces in materials.This book provides an overview of dynamic experiments in electron microscopy,

and is especially targeted at students, scientists, and engineers working in the fieldsof chemistry, physics, and the materials sciences. Although experience in electronmicroscopy techniques is not a prerequisite for readers, as the basic information onthese techniques is summarized in the first two chapters of Part One, Basics andMethods, some basic knowledge would help to use the book to its full extent. Detailsof specialized in-situ methods, such as Dynamic TEM and Reflection Electron Micro-scopy are also included in Part One, to highlight the science which emanates fromthese fields.

Gerhard Dehm, Leoben, AustriaJames M. Howe, Charlottesville, USAJosef Zweck, Regensburg, GermanyJanuary 2012

XVIII Preface

Part IBasics and Methods

In-situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science, First Edition.Edited by Gerhard Dehm, James M. Howe, and Josef Zweck.� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

j1

1Introduction to Scanning Electron MicroscopyChristina Scheu and Wayne D. Kaplan

The scanning electron microscope is without doubt one of the most widely usedcharacterization tools available tomaterials scientists andmaterials engineers. Today,modern instruments achieve amazing levels of resolution, and can be equipped withvarious accessories that provide information on local chemistry and crystallography.These data, together with the morphological information derived from the sample,are important when characterizing the microstructure of materials used in a widenumber of applications. A schematic overview of the signals that are generated whenan electron beam interacts with a solid sample, and which are used in the scanningelectronmicroscope formicrostructural characterization, is shown in Figure 1.1. Themost frequently detected signals are high-energy backscattered electrons, low-energysecondary electrons and X-rays, while less common signals include Auger electrons,cathodoluminescence, and measurements of beam-induced current. The origin ofthese signals will be discussed in detail later in the chapter.

Due to the mechanisms by which the image is formed in the scanning electronmicroscope, the micrographs acquired often appear to be directly interpretable; thatis, the contrast in the image is often directly associated with the microstructuralfeatures of the sample. Unfortunately, however, this may often lead to gross errors inthe measurement of microstructural features, and in the interpretation of themicrostructure of a material. At the same time, the fundamental mechanisms bywhich the images are formed in the scanning electron microscope are reasonablystraightforward, and a little effort from the materials scientist or engineer incorrelating themicrostructural features detected by the imagingmechanismsmakesthe technique of scanning electron microscopy (SEM) being extremely powerful.

Unlike conventional optical microscopy or conventional transmission electronmicroscopy (TEM), in SEM a focused beam of electrons is rastered across thespecimen, and the signals emitted from the specimen are collected as a functionof position of the incident focused electron beam.As such, thefinal image is collectedin a sequential manner across the surface of the sample. As the image in SEM isformed from signals emitted due to the interaction of a focused incident electronprobewith the sample, two critical issues are involved in understanding SEM images,aswell as in the correlated analytical techniques: (i) the nature of the incident electronprobe; and (ii) the manner by which incident electrons interact with matter.

j3

In-situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science, First Edition.Edited by Gerhard Dehm, James M. Howe, and Josef Zweck.� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

The electron–optical system in a scanning electronmicroscope is actually designedto demagnify rather than to magnify, in order to form the small incident electronprobe which is then rastered across the specimen. As such, the size of the incidentprobe depends on the electron source (or gun), and the electromagnetic lens systemwhich focuses the emitted electrons into a fine beam that then interacts with thesample.Theprobe size is thefirst parameter involved indefining the spatial resolutionof the image, or of the analyticalmeasurements.However, the signals (e.g., secondaryelectrons, backscattered electrons, X-rays) that are used to form the image emanatefrom regions in the sample that may be significantly larger than the diameter of theincident electron beam. Thus, electron–matter interaction must be understood,together with the diameter of the incident electron probe, to understand both theresolution and the contrast in the acquired image.

The aim of this chapter is to provide a fundamental introduction to SEM and itsassociated analytical techniques (further details are available in Refs [1–5]).

1.1Components of the Scanning Electron Microscope

It is convenient to consider themajor components of a scanning electronmicroscopeas divided into four major sections (see Figure 1.2):

. The electron source (or electron gun).

Figure 1.1 Schematic drawing of possible signals createdwhenan incident electronbeam interactswith a solid sample. Reproduced with permission from Ref. [4]; � 2008, John Wiley & Sons.

4j 1 Introduction to Scanning Electron Microscopy

. The electromagnetic lenses, which are used to focus the electron beam anddemagnify it into a small electron probe.

. The deflection system.

. The detectors, which are used to collect signals emitted from the sample.

Before discussing these major components, a few words should be mentionedregarding the vacuum system.Within themicroscope, different levels of vacuum arerequired for three main reasons. First, the electron source must be protected against

Figure 1.2 Schematic drawing of the majorcomponents of a scanning electronmicroscope. The electron lenses and aperturesare used to demagnify the electron beam that isemitted from the electron source into a small

probe, and to control the beam current density.The demagnified beam is than scanned acrossthe sample. Various detectors are used toregister the signals arising from variouselectron–matter interactions.

1.1 Components of the Scanning Electron Microscope j5

oxidation, whichwould limit the lifetime of the gun andmay cause instabilities in theintensity of the emitted electrons. Second, a high level of vacuum is required toprevent the scattering of electrons as they traverse the column from the gun to thespecimen. Third, it is important to reduce the partial pressure of water and carbon inthe vicinity of the sample, as any interaction of the incident electron beam with suchmolecules on the surface of the sample may lead to the formation of what iscommonly termed a carbonaceous (or contamination) layer, which can obscurethe sample itself. The prevention of carbonaceous layer formation depends both onthe partial pressure of water and carbon in the vacuum near the sample, and theamount of carbon and water molecules that are adsorbed onto the surface of thesample prior to its introduction into themicroscope. Thus, while aminimum level ofvacuum is always required to prevent the scattering of electrons by molecules (theconcentration of which in the vacuum is determined from a measure of partialpressure), it is the partial pressure of oxygen in the region of the electron gun, and thepartial pressure of carbon and water in the region of the specimen, that are in factcritical to operation of the microscope. Unfortunately, most scanning electronmicroscopes do not provide such measures of partial pressure, but rather maintaindifferent levels of vacuum in the different regions of the instrument. Normally, thehighest vacuum (i.e., the lowest pressure) is in the vicinity of the electron gun and,depending on the type of electron source, an ultra-high-vacuum (UHV) level(pressure

electrons is brought to a focus by the electrostatic field and then accelerated by ananode beneath the Wehnelt cylinder.

The beam that enters the microscope column is characterized by the effectivesource size dgun, the divergence angle of the beam a0, the energy of the electrons E0,and the energy spread of the electron beam DE.

An important quantity here is the axial gun brightness (b), which is defined as thecurrent DI passing through an area DS into a solid angle DV¼pa2, where a is theangular spread of the electrons. With j¼DI/DS being the current density in Acm�2,the following is obtained:

b ¼ DIDSDV

¼ jpa2

¼ const: ð1:1Þ

The brightness is a conserved quantity, which means that its value is the same forall points along the optical axis, independent of which apertures are inserted, or howmany lenses are present.

Currently, three different types of electron sources are in commonuse (Figure 1.4);the characteristics of these are summarized in Table 1.1. A heated tungsten filamentis capable of generating a brightness of the order of 104 A cm�2 sr�1, froman effectivesource size, defined by the first cross-over of the electron beam, approximately 15mmacross. The thermionic emission temperatures are high, which explains the selectionof tungsten as the filament material. A lanthanum hexaboride LaB6 crystal cangenerate a brightness of about 105A cm�2 sr�1, but this requires a significantlyhigher vacuum level in the vicinity of the source, and isnow infrequently used in SEMinstruments. The limited effective source size of thermionic electron guns, whichmust be demagnified by the electromagnetic lens system before impinging on thesample, leads to microscopes equipped with thermionic sources being defined asconventional scanning electron microscopes.

Figure 1.3 Schematic drawing of theelectrostatic potential barrier at a metal surface.In order to remove an electron from the metalsurface, the work function must be overcome.

Thework function can be lowered by applying anelectric field (Schottky effect). If the field is veryhigh, the electrons can tunnel through thepotential barrier. Redrawn from Ref. [1].

1.1 Components of the Scanning Electron Microscope j7

The effective source size can be significantly reduced (leading to the term high-resolution SEM) by using a cold field emission gun (FEG), in which the electronstunnel out of a sharp tip under the influence of a high electric field (Figures 1.3and 1.4). Cold FEG sources can generate a brightness of the order of 107 A cm�2 sr�1,and the sharp tip of the tungsten needle that emits the electrons is of the order of0.2mm in diameter; hence, the effective source size is less than 5 nm. More often, ahot source replaces the cold source, in which case a sharp tungsten needle isheated to enhance the emission (this is termed a thermalfield emitter, or TFE). Theheating of the tip leads to a self-cleaning process; this has proved to be another benefitof TFEs in that they can be operated at a lower vacuum level (higher pressures). In the

Figure 1.4 Schematic drawings of (a) atungsten filament and (b) a LaB6 tip forthermionic electron sources. (c) For a field-emission gun (FEG) source, a sharp tungstentip is used. (d) In thermionic sources thefilament or tip is heated to eject electrons, whichare then focused with an electrostatic lens (the

Wehnelt cylinder). (e) In FEGs, the electrons areextracted by a high electric field applied to thesharp tip by a counterelectrode aperture, andthen focused by an anode to image thesource. Reproduced with permission fromRef. [4]; � 2008, John Wiley & Sons.

8j 1 Introduction to Scanning Electron Microscopy

so-called Schottky emitters, the electrostatic field ismainly used to reduce the workfunction, such that electrons leave the tip via thermal emission (see Figure 1.3). Azirconium-coated tip is often used to reduce the work function even further.Although Schottky emitters have a slightly larger effective source size than coldfield emission sources, they are more stable and require less stringent vacuumrequirements than cold FEG sources. Equally important, the probe current atthe specimen is significantly larger than for cold FEG sources; this is importantfor other analytical techniques used with SEM, such as energy dispersive X-rayspectroscopy (EDS).

1.1.2Electromagnetic Lenses

Within the scanning electron microscope, the role of the general lens system is todemagnify an image of the initial crossover of the electron probe to the final size ofthe electron probe on the sample surface (1–50 nm), and to raster the probe acrossthe surface of the specimen. As a rule, this system provides demagnifications in therange of 1000- to 10 000-fold. Since one is dealing with electrons rather than photonsthe lenses may be either electrostatic or electromagnetic. The simplest example ofthese is the electrostatic lens that is used in the electron gun.

Electromagnetic lenses are more commonly encountered, and consist of a largenumber of turns of a copperwirewound around an iron core (the pole-piece). A smallgap located at the center of the core separates the upper and lower pole-pieces. Themagnetic flux of the lens is concentrated within a small volume by the pole-pieces,and the strayfield at the gap forms themagneticfield. Themagneticfield distributionis inhomogeneous in order to focus electrons traveling parallel to the optical axis ontoa point on the optical axis; otherwise, they would be unaffected. Thereby, the radialcomponent of the field will force these electrons to change their direction in such away that they possess a velocity component normal to the optical axis; the longitudinalcomponent of the field would then force them towards the optical axis. Accordingly,the electrons move within the lens along screw trajectories about the optical axis due

Table 1.1 A comparison of the properties of different electron sources.

Source type Thermionic Thermionic Schottky Cold FEG

Cathode material W LaB6 W(100) þ ZrO W(310)Work function [eV] 4.5 2.7 2.7 4.5Tip radius [mm] 50–100 10–20 0.5–1 20 0.2Normalized brightness [A cm�2 sr�1] 104 105 107 2� 107Energy spread at gun exit [eV] 1.5–2.5 1.3–2.5 0.4–0.7 0.3–0.7

1.1 Components of the Scanning Electron Microscope j9

to the Lorentz force associated with the longitudinal and radial magnetic fieldcomponents.

Generally, in order to determine the image position and magnification (demagni-fication) for the given position of the object, it is possible to use the lens formula:

1F¼ 1

Uþ 1

Vð1:2Þ

where F is the focal length of the lens, U is the distance between the object and thelens, and V is the distance between the image and the lens. The magnification(demagnification) of the image – that is, the ratio of the linear image size h to thecorresponding linear size of the object H – is equal to (see Figure 1.5):

M ¼ hH

¼ VU: ð1:3Þ

IfU� F, then for the total demagnification of a three-lens system a spot is obtainedwith a geometric diameter of

d0 ¼ F1F2F3U1U2U3 dgun ¼ Mdgun ð1:4Þ

where dgun is the initial crossover diameter. To obtain d0 � 10 nm for a thermioniccathode, which possesses an initial crossover dgun of�20–50 mm, the total demagni-fication must be �1/5000. A Schottky or field-emission gun can result in dgun �10 nm, such that only one probe-forming (objective) lens is necessary to demagnifythe electron probe to d0 � 1 nm. The distance between the objective lens and thesample surface is termed the working distance of the microscope. From the abovediscussion, it follows that a short working distance will lead to a stronger demagni-fication and thus to a smaller electron probe size.

Figure 1.5 Schematic drawing of the relationship between focal length and magnification for aideal thin lens. Reproduced with permission from Ref. [4]; � 2008, John Wiley & Sons.

10j 1 Introduction to Scanning Electron Microscopy