In situ electron microscopy applications in physics chemistry and materials science

Edited by
Gerhard Dehm, James M. Howe,
and Josef Zweck
In-situ Electron Microscopy


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

In-situ Electron Microscopy
Applications in Physics, Chemistry and Materials Science


The Editors
Prof. Dr. Gerhard Dehm
Montanuniversität Leoben
Dept. Materialphysik
Jahnstr. 12
8700 Leoben
Austria
Prof. Dr. James M. Howe
University of Virginia
Dept. of Mat. Science & Engin.
116 Engineer's Way
Charlottesville, VA 22904-4745
USA

Prof. Dr. Josef Zweck
Universität Regensburg
Fak. für Physik
93040 Regensburg
Germany

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V

Contents
List of Contributors
Preface XVII

XIII

Part I

Basics and Methods

1

1

Introduction to Scanning Electron Microscopy 3
Christina Scheu and Wayne D. Kaplan
Components of the Scanning Electron Microscope 4
Electron Guns 6
Electromagnetic Lenses 9
Deflection System 13
Electron Detectors 13
Everhart–Thornley Detector 13
Scintillator Detector 15
Solid-State Detector 16
In-Lens or Through-the-Lens Detectors 16
Electron–Matter Interaction 16
Backscattered Electrons (BSEs) 20
Secondary Electrons (SEs) 22
Auger Electrons (AEs) 25
Emission of Photons 25
Emission of X-Rays 25
Emission of Visible Light 26
Interaction Volume and Resolution 26
Secondary Electrons 27
Backscattered Electrons 27
X-Rays 27
Contrast Mechanisms 28
Topographic Contrast 28
Composition Contrast 31
Channeling Contrast 31

1.1
1.1.1
1.1.2
1.1.3
1.1.4
1.1.4.1
1.1.4.2
1.1.4.3
1.1.4.4
1.2
1.2.1
1.2.2
1.2.3
1.2.4
1.2.4.1
1.2.4.2
1.2.5
1.2.5.1
1.2.5.2
1.2.5.3
1.3
1.3.1
1.3.2
1.3.3


VI

Contents

1.4
1.5
1.6
1.7

Electron Backscattered Diffraction (EBSD) 31
Dispersive X-Ray Spectroscopy 34
Other Signals 36
Summary 36
References 37

2

Conventional and Advanced Electron Transmission Microscopy 39
Christoph Koch
Introduction 39
Introductory Remarks 39
Instrumentation and Basic Electron Optics 40
Theory of Electron–Specimen Interaction 42
High-Resolution Transmission Electron Microscopy 48
Conventional TEM of Defects in Crystals 54
Lorentz Microscopy 55
Off-Axis and Inline Electron Holography 57
Electron Diffraction Techniques 59
Fundamentals of Electron Diffraction 59
Convergent Beam Electron Diffraction 61
Large-Angle Convergent Beam Electron Diffraction 63
Characterization of Amorphous Structures by Diffraction 63
Scanning Transmission Electron Microscopy and Z-Contrast 63

Analytical TEM 66
References 67

2.1
2.1.1
2.1.2
2.1.3
2.2
2.3
2.4
2.5
2.6
2.6.1
2.7
2.7.1
2.7.2
2.8
2.9

3

3.1
3.2
3.2.1
3.2.2
3.2.3
3.2.4
3.3
3.3.1
3.3.2

3.4
3.5
3.5.1
3.5.2
3.6
3.6.1
3.6.2
3.6.3

Dynamic Transmission Electron Microscopy 71
Thomas LaGrange, Bryan W. Reed, Wayne E. King,
Judy S. Kim, and Geoffrey H. Campbell
Introduction 71
How Does Single-Shot DTEM Work? 72
Current Performance 74
Electron Sources and Optics 75
Arbitrary Waveform Generation Laser System 80
Acquiring High Time Resolution Movies 81
Experimental Applications of DTEM 82
Diffusionless First-Order Phase Transformations 82
Observing Transient Phenomena in Reactive Multilayer Foils 85
Crystallization Under Far-from-Equilibrium Conditions 88
Space Charge Effects in Single-Shot DTEM 90
Global Space Charge 90
Stochastic Blurring 91
Next-Generation DTEM 91
Novel Electron Sources 91
Relativistic Beams 92
Pulse Compression 93

Contents

3.6.4
3.7

Aberration Correction
Conclusions 94
References 95

4

Formation of Surface Patterns Observed with Reflection
Electron Microscopy 99
Alexander V. Latyshev
Introduction 99
Reflection Electron Microscopy 102
Silicon Substrate Preparation 107
Monatomic Steps 109
Step Bunching 111
Surface Reconstructions 114
Epitaxial Growth 115
Thermal Oxygen Etching 116
Conclusions 119
References 119

4.1
4.2
4.3
4.4

4.5
4.6
4.7
4.8
4.9

93

123

Part II

Growth and Interactions

5

Electron and Ion Irradiation 125
Florian Banhart
Introduction 125
The Physics of Irradiation 126
Scattering of Energetic Particles in Solids 126
Scattering of Electrons 128
Scattering of Ions 129
Radiation Defects in Solids 129
The Formation of Defects 129
The Migration of Defects 130
The Setup in the Electron Microscope 131
Electron Irradiation 131
Ion Irradiation 132
Experiments 132

Electron Irradiation 133
Ion Irradiation 140
Outlook 141
References 142

5.1
5.2
5.2.1
5.2.2
5.2.3
5.3
5.3.1
5.3.2
5.4
5.4.1
5.4.2
5.5
5.5.1
5.5.2
5.6

6

6.1
6.2
6.3

Observing Chemical Reactions Using Transmission Electron
Microscopy 145
Renu Sharma

Introduction 145
Instrumentation 146
Types of Chemical Reaction Suitable for TEM Observation 150

VII


VIII

Contents

6.3.1
6.3.2
6.3.3
6.3.4
6.3.5
6.3.6
6.4
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
6.4.6
6.5
6.5.1
6.5.1.1
6.5.1.2
6.5.2
6.5.3

6.6

7
7.1
7.2
7.2.1
7.2.2
7.2.3
7.3
7.3.1
7.3.2
7.3.3
7.4
7.4.1
7.4.2
7.5

8
8.1
8.2

Oxidation and Reduction (Redox) Reactions 150
Phase Transformations 151
Polymerization 151
Nitridation 152
Hydroxylation and Dehydroxylation 152
Nucleation and Growth of Nanostructures 153
Experimental Setup 154
Reaction of Ambient Environment with Various TEM Components 154
Reaction of Grid/Support Materials with the Sample or with

Each Other 154
Temperature and Pressure Considerations 155
Selecting Appropriate Characterization Technique(s) 156
Recording Media 156
Independent Verification of the Results, and the Effects of the
Electron Beam 157
Available Information Under Reaction Conditions 157
Structural Modification 158
Electron Diffraction 158
High-Resolution Imaging 158
Chemical Changes 161
Reaction Rates (Kinetics) 164
Limitations and Future Developments 164
References 165
In-Situ TEM Studies of Vapor- and Liquid-Phase Crystal Growth 171
Frances M. Ross
Introduction 171
Experimental Considerations 172
What Crystal Growth Experiments are Possible? 172
How Can These Experiments be Made Quantitative? 173
How Relevant Can These Experiments Be? 175
Vapor-Phase Growth Processes 175
Quantum Dot Growth Kinetics 176
Vapor–Liquid–Solid Growth of Nanowires 177
Nucleation Kinetics in Nanostructures 180
Liquid-Phase Growth Processes 183
Observing Liquid Samples Using TEM 183
Electrochemical Nucleation and Growth in the TEM System 184
Summary 187
References 188

In-Situ TEM Studies of Oxidation 191
Guangwen Zhou and Judith C. Yang
Introduction 191
Experimental Approach 192


Contents

8.2.1
8.2.2
8.2.3
8.2.4
8.3
8.3.1
8.3.2
8.3.3
8.3.4
8.3.5
8.3.6
8.4
8.5

Environmental Cells 192
Surface and Environmental Conditions 193
Gas-Handling System 194
Limitations 195
Oxidation Phenomena 196
Surface Reconstruction 196
Nucleation and Initial Oxide Growth 197
Role of Surface Defects on Surface Oxidation 198

Shape Transition During Oxide Growth in Alloy Oxidation 199
Effect of Oxygen Pressure on the Orientations of Oxide Nuclei 202
Oxidation Pathways Revealed by High-Resolution TEM Studies
of Oxidation 203
Future Developments 205
Summary 206
References 206
209

Part III

Mechanical Properties

9

Mechanical Testing with the Scanning Electron Microscope 211
Christian Motz
Introduction 211
Technical Requirements and Specimen Preparation 212
In-Situ Loading of Macroscopic Samples 214
Static Loading in Tension, Compression, and Bending 214
Dynamic Loading in Tension, Compression, and Bending 216
Applications of In-Situ Testing 216
In-Situ Loading of Micron-Sized Samples 217
Static Loading of Micron-Sized Samples in Tension, Compression,
and Bending 218
Applications of In-Situ Testing of Small-Scale Samples 220
In-Situ Microindentation and Nanoindentation 222
Summary and Outlook 223
References 223

9.1
9.2
9.3
9.3.1
9.3.2
9.3.3
9.4
9.4.1
9.4.2
9.4.3
9.5

10

10.1
10.2
10.2.1
10.2.2
10.2.3
10.3
10.3.1
10.3.2

In-Situ TEM Straining Experiments: Recent Progress in Stages
and Small-Scale Mechanics 227
Gerhard Dehm, Marc Legros, and Daniel Kiener
Introduction 227
Available Straining Techniques 228
Thermal Straining 228

Mechanical Straining 229
Instrumented Stages and MEMS/NEMS Devices 230
Dislocation Mechanisms in Thermally Strained Metallic Films 233
Basic Concepts 233
Dislocation Motion in Single Crystalline Films and Near Interfaces 235

IX


X

Contents

10.3.3
10.3.4
10.4
10.4.1
10.4.2
10.4.3
10.4.4
10.5

11
11.1
11.1.1
11.1.2
11.2
11.3
11.3.1
11.3.2

11.4

Dislocation Nucleation and Multiplication in Thin Films 236
Diffusion-Induced Dislocation Plasticity in Polycrystalline
Cu Films 239
Size-Dependent Dislocation Plasticity in Metals 239
Plasticity in Geometrically Confined Single Crystal
fcc Metals 241
Size-Dependent Transitions in Dislocation Plasticity 243
Plasticity by Motion of Grain Boundaries 244
Influence of Grain Size Heterogeneities 245
Conclusions and Future Directions 247
References 248
In-Situ Nanoindentation in the Transmission Electron Microscope
Andrew M. Minor
Introduction 255
The Evolution of In-Situ Mechanical Probing in a TEM 255
Introduction to Nanoindentation 256
Experimental Methodology 260
Example Studies 263
In-Situ TEM Nanoindentation of Silicon 263
In-Situ TEM Nanoindentation of Al Thin Films 269
Conclusions 272
References 274
279

Part IV

Physical Properties

12

Current-Induced Transport: Electromigration 281
Ralph Spolenak
Principles 281
Transmission Electron Microscopy 283
Imaging 283
Diffraction 288
Convergent Beam Electron Diffraction (CBED):
Measurements of Elastic Strain 288
Secondary Electron Microscopy 289
Imaging 289
Elemental Analysis 291
Electron Backscatter Diffraction (EBSD) 292
X-Radiography Studies 292
Microscopy and Tomography 292
Spectroscopy 293
Topography 294
Microdiffraction 294
Specialized Techniques 295
Focused Ion Beams 295

12.1
12.2
12.2.1
12.2.2
12.2.3
12.3
12.3.1
12.3.2

12.3.3
12.4
12.4.1
12.4.2
12.4.3
12.4.4
12.5
12.5.1

255


Contents

12.5.2
12.5.3
12.6

Reflective High-Energy Electron Diffraction (RHEED) 296
Scanning Probe Methods 296
Comparison of In-Situ Methods 297
References 299

13

Cathodoluminescence in Scanning and Transmission
Electron Microscopies 303
Yutaka Ohno and Seiji Takeda
Introduction 303
Principles of Cathodoluminsecence 304

The Generation and Recombination of Electron-Hole Pairs 304
Characteristic of CL Spectroscopy 305
CL Imaging and Contrast Analysis 306
Spatial Resolution of CL Imaging and Spectroscopy 306
CL Detection Systems 307
Applications of CL in Scanning and Transmission Electron
Microscopies 307
Assessments of Group III–V Compounds 308
Nitrides 308
III–V Compounds Except Nitrides 309
Group II–VI Compounds and Related Materials 310
Oxides 310
Group II–VI Compounds, Except Oxides 312
Group IV and Related Materials 313
Concluding Remarks 313
References 313

13.1
13.2
13.2.1
13.2.2
13.2.3
13.2.4
13.2.5
13.3
13.3.1
13.3.1.1
13.3.1.2
13.3.2
13.3.2.1

13.3.2.2
13.3.3
13.4

14
14.1
14.2
14.3
14.4
14.5
14.6
14.7

15
15.1
15.2
15.2.1
15.2.2
15.2.3

In-Situ TEM with Electrical Bias on Ferroelectric Oxides 321
Xiaoli Tan
Introduction 321
Experimental Details 323
Domain Polarization Switching 324
Grain Boundary Cavitation 326
Domain Wall Fracture 331
Antiferroelectric-to-Ferroelectric Phase Transition 335
Relaxor-to-Ferroelectric Phase Transition 341
References 345

Lorentz Microscopy 347
Josef Zweck
Introduction 347
The In-Situ Creation of Magnetic Fields 350
Combining the Objective Lens Field with Specimen Tilt 351
Magnetizing Stages Using Coils and Pole-Pieces 352
Magnetizing Stages Without Coils 356

XI


XII

Contents

15.2.3.1
15.2.3.2
15.2.3.3
15.3
15.3.1
15.3.2
15.3.3
15.4
15.5

Index

Oersted Fields 356
Spin Torque Applications 358
Self-Driven Devices 361

Examples 362
Demagnetization and Magnetization of Ring Structures
Determination of Wall Velocities 364
Determination of Stray Fields 365
Problems 366
Conclusions 367
References 367
371

362


XIII

List of Contributors
Florian Banhart
Université de Strasbourg
Institut de Physique et Chimie des
Matériaux, UMR 7504
23 rue du Loess
67034 Strasbourg
France
Nigel D. Browning
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue
Livermore
California 94550
USA

Geoffrey H. Campbell
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue
Livermore
California 94550
USA
Gerhard Dehm
Austrian Academy of Sciences
Erich Schmid Institute of Materials
Science
Jahnstr. 12
8700 Leoben
Austria

and
Montanuniversität Leoben
Department Materials Physics
Franz-Josef-Str. 18
8700 Leoben
Austria
Wayne D. Kaplan
Technion - Israel Institute of Technology
Department of Materials Engineering
Haifa 32000
Israel
Daniel Kiener
Montanuniversität Leoben
Department Materials Physics

Franz-Josef-Str. 18
8700 Leoben
Austria
Judy S. Kim
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue
Livermore
California 94550
USA


XIV

List of Contributors

and
University of California
Department of Chemical Engineering
and Materials Science
One Shields Avenue
Davis
California 95616
USA
Wayne E. King
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue

Livermore
California 94550
USA
Christoph Koch
Max-Planck-Institut für
Metallforschung
Heisenbergstr. 3
70569 Stuttgart
Germany
Thomas LaGrange
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue
Livermore
California 94550
USA
Alexander V. Latyshev
Siberian Branch of Russian Academy of
Sciences
Institute of Semiconductor Physics
Prospect Lavrent’eva 13
630090 Novosibirsk
Russia

Marc Legros
CEMES-CNRS
29 Rue Jeanne Marvig
31055 Toulouse
France

Andrew M. Minor
University of California, Berkeley and
National Center for Electron Microscopy
Department of Materials Science and
Engineering, Lawrence Berkeley
National Laboratory
One Cyclotron Road, MS 72
Berkeley
CA 94720
USA
Christian Motz
Österreichische Akademie der
Wissenschaften
Erich Schmid Institut für
Materialwissenschaft
Jahnstr. 12
8700 Leoben
Austria
Yutaka Ohno
Tohoku University
Institute for Materials Research
Katahira 2-1-1
Aoba-ku
Sendai 980-8577
Japan
Bryan W. Reed
Lawrence Livermore National
Laboratory
Physical and Life Sciences Directorate
7000 East Avenue

Livermore
California 94550
USA


List of Contributors

Frances M. Ross
IBM T. J. Watson Research Center
1101 Kitchawan Road
Yorktown Heights
NY 10598
USA
Christina Scheu
1Ludwig-Maximilians-Universität
München
Department Chemie & Center for
NanoScience (CeNS)
Butenandstr. 5-13, Gerhard-ErtlGebäude (Haus E)
81377 München
Germany
Renu Sharma
National Institute of Science and
Technology
Center for Nanoscale Science and
Technology
100 Bureau Drive
Gaithersburg
MD 20899-6201
USA

Ralph Spolenak
ETH Zurich
Laboratory of Nanometallurgy,
Department of Material
Wolfgang-Pauli-Str. 10
8093 Zurich
Switzerland

Seiji Takeda
Osaka University
The Institute of Scientific and Industrial
Research
Mihogaoka 8-1
Ibaraki
Osaka 567-0047
Japan
Xiaoli Tan
Iowa State University
Department of Materials Science and
Engineering
2220 Hoover Hall
Ames
IA 50011
USA
Judith C. Yang
University of Pittsburgh
Department of Chemical and Petroleum
Engineering
1249 Benedum Hall
Pittsburgh

PA 15261
USA
Guangwen Zhou
P. O. Box 6000
85 Murray Hill Road
Binghampton
NY 13902
USA
Josef Zweck
University of Regensburg
Physics Faculty
Physics Building Office Phy 7.3.05
93040 Regensburg
Germany

XV


XVII

Preface
Today, transmission electron microscopy (TEM) represents one of the most important tools used to characterize materials. Electron diffraction provides information
on the crystallographic structure of materials, conventional TEM with bright-field
and dark-field imaging on their microstructure, high-resolution TEM on their
atomic structure, scanning TEM on their elemental distributions, and analytical
TEM on their chemical composition and bonding mechanisms. Each of these
techniques is explained in detail in various textbooks on TEM techniques, including
Transmission Electron Microscopy: A Textbook for Materials Science (D.B. Williams and
C.B. Carter, Plenum Press, New York, 1996), and Transmission Electron Microscopy
and Diffractometry of Materials (3rd edition, B. Fultz and J. M. Howe, Springer-Verlag,

Berlin, Heidelberg, 2008).
Most interestingly, however, TEM also enables dynamical processes in materials
to be studied through dedicated in-situ experiments. To watch changes occurring in a
material of interest allows not only the development but also the refinement of
models, so as to explain the underlying physics and chemistry of materials processes. The possibilities for in-situ experiments span from thermodynamics and
kinetics (including chemical reactions, oxidation, and phase transformations) to
mechanical, electrical, ferroelectric, and magnetic material properties, as well as
materials synthesis.
The present book is focused on the state-of-the-art possibilities for performing
dynamic experiments inside the electron microscope, with attention centered on
TEM but including scanning electron microscopy (SEM). Whilst seeing is believing is
one aspect of in-situ experiments in electron microscopy, the possibility to obtain
quantitative data is of almost equal importance when accessing critical data in
relation to physics, chemistry, and the materials sciences. The equipment needed
to obtain quantitative data on various stimuli – such as temperature and gas flow for
materials synthesis, load and displacement for mechanical properties, and electrical
current and voltage for electrical properties, to name but a few examples – are
described in the individual sections that relate to Growth and Interactions (Part Two),
Mechanical Properties (Part Three), and Physical Properties (Part Four).


XVIII

Preface

During the past decade, interest in in-situ electron microscopy experiments has
grown considerably, due mainly to new developments in quantitative stages and
micro-/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 transmission
electron microscope. In addition, the advent of imaging correctors that compensate

for the spherical and, more recently, the chromatic aberration of electromagnetic
lenses has not only increased the resolution of TEM but has also permitted the use of
larger pole-piece gaps (and thus more space for stages inside the microscope), even
when designed for imaging at atomic resolution. Another driving force of in-situ
experimentation using electron probes has been the small length-scales that are
accessible with focused ion beam/SEM platforms and TEM instruments. These are
of direct relevance for nanocrystalline materials and thin-film structures with
micrometer and nanometer dimensions, as well as for structural defects such as
interfaces 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 fields
of chemistry, physics, and the materials sciences. Although experience in electron
microscopy techniques is not a prerequisite for readers, as the basic information on
these techniques is summarized in the first two chapters of Part One, Basics and
Methods, some basic knowledge would help to use the book to its full extent. Details
of specialized in-situ methods, such as Dynamic TEM and Reflection Electron Microscopy are also included in Part One, to highlight the science which emanates from
these fields.
Gerhard Dehm, Leoben, Austria
James M. Howe, Charlottesville, USA
Josef Zweck, Regensburg, Germany
January 2012


j1

Part I
Basics 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.

Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.


j3

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

The scanning electron microscope is without doubt one of the most widely used
characterization tools available to materials scientists and materials engineers. Today,
modern instruments achieve amazing levels of resolution, and can be equipped with
various 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 wide
number of applications. A schematic overview of the signals that are generated when
an electron beam interacts with a solid sample, and which are used in the scanning
electron microscope for microstructural characterization, is shown in Figure 1.1. The
most frequently detected signals are high-energy backscattered electrons, low-energy
secondary electrons and X-rays, while less common signals include Auger electrons,
cathodoluminescence, and measurements of beam-induced current. The origin of
these signals will be discussed in detail later in the chapter.
Due to the mechanisms by which the image is formed in the scanning electron
microscope, the micrographs acquired often appear to be directly interpretable; that
is, the contrast in the image is often directly associated with the microstructural
features of the sample. Unfortunately, however, this may often lead to gross errors in
the measurement of microstructural features, and in the interpretation of the
microstructure of a material. At the same time, the fundamental mechanisms by
which the images are formed in the scanning electron microscope are reasonably
straightforward, and a little effort from the materials scientist or engineer in

correlating the microstructural features detected by the imaging mechanisms makes
the technique of scanning electron microscopy (SEM) being extremely powerful.
Unlike conventional optical microscopy or conventional transmission electron
microscopy (TEM), in SEM a focused beam of electrons is rastered across the
specimen, and the signals emitted from the specimen are collected as a function
of position of the incident focused electron beam. As such, the final image is collected
in a sequential manner across the surface of the sample. As the image in SEM is
formed from signals emitted due to the interaction of a focused incident electron
probe with the sample, two critical issues are involved in understanding SEM images,
as well as in the correlated analytical techniques: (i) the nature of the incident electron
probe; and (ii) the manner by which incident electrons interact with matter.
In-situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science, First Edition.
Edited by Gerhard Dehm, James M. Howe, and Josef Zweck.
Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.


4

j 1 Introduction to Scanning Electron Microscopy

Figure 1.1 Schematic drawing of possible signals created when an incident electron beam interacts
with a solid sample. Reproduced with permission from Ref. [4]; Ó 2008, John Wiley & Sons.

The electron–optical system in a scanning electron microscope is actually designed
to demagnify rather than to magnify, in order to form the small incident electron
probe which is then rastered across the specimen. As such, the size of the incident
probe depends on the electron source (or gun), and the electromagnetic lens system
which focuses the emitted electrons into a fine beam that then interacts with the
sample. The probe size is the first parameter involved in defining the spatial resolution
of the image, or of the analytical measurements. However, the signals (e.g., secondary

electrons, backscattered electrons, X-rays) that are used to form the image emanate
from regions in the sample that may be significantly larger than the diameter of the
incident electron beam. Thus, electron–matter interaction must be understood,
together with the diameter of the incident electron probe, to understand both the
resolution and the contrast in the acquired image.
The aim of this chapter is to provide a fundamental introduction to SEM and its
associated analytical techniques (further details are available in Refs [1–5]).

1.1
Components of the Scanning Electron Microscope

It is convenient to consider the major components of a scanning electron microscope
as divided into four major sections (see Figure 1.2):
.

The electron source (or electron gun).


1.1 Components of the Scanning Electron Microscope
.
.
.

The electromagnetic lenses, which are used to focus the electron beam and
demagnify 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 mentioned
regarding the vacuum system. Within the microscope, different levels of vacuum are

required for three main reasons. First, the electron source must be protected against

Figure 1.2 Schematic drawing of the major
components of a scanning electron
microscope. The electron lenses and apertures
are used to demagnify the electron beam that is
emitted from the electron source into a small

probe, and to control the beam current density.
The demagnified beam is than scanned across
the sample. Various detectors are used to
register the signals arising from various
electron–matter interactions.

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j 1 Introduction to Scanning Electron Microscopy
oxidation, which would limit the lifetime of the gun and may cause instabilities in the
intensity of the emitted electrons. Second, a high level of vacuum is required to
prevent the scattering of electrons as they traverse the column from the gun to the
specimen. Third, it is important to reduce the partial pressure of water and carbon in
the vicinity of the sample, as any interaction of the incident electron beam with such
molecules on the surface of the sample may lead to the formation of what is
commonly termed a “carbonaceous” (or contamination) layer, which can obscure
the sample itself. The prevention of carbonaceous layer formation depends both on
the partial pressure of water and carbon in the vacuum near the sample, and the
amount of carbon and water molecules that are adsorbed onto the surface of the

sample prior to its introduction into the microscope. Thus, while a minimum level of
vacuum is always required to prevent the scattering of electrons by molecules (the
concentration of which in the vacuum is determined from a measure of partial
pressure), it is the partial pressure of oxygen in the region of the electron gun, and the
partial pressure of carbon and water in the region of the specimen, that are in fact
critical to operation of the microscope. Unfortunately, most scanning electron
microscopes do not provide such measures of partial pressure, but rather maintain
different levels of vacuum in the different regions of the instrument. Normally, the
highest 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 <10À8 Pa) may be attained. The nominal pressure in the vicinity of the
specimen is normally in the range of 10À3 Pa. Some scanning electron microscopes
that have been designed for the characterization of low-vapor pressure liquids,
“moist” biological specimens or nonconducting materials, have differential
apertures between the regions of the microscope. This allows a base vacuum as
high as approximately 0.3 Pa close to the sample. These instruments, which
are often referred to as “environmental” scanning electron microscopes, offer
unique possibilities, but their detailed description is beyond the scope of the present
chapter.
1.1.1
Electron Guns

The role of the electron gun is to produce a high-intensity source of electrons which
can be focused into a fine electron beam. In principle, free electrons can be generated
by thermal emission or field emission from a metal surface (Figure 1.3). In thermal
emission, the energy necessary to overcome the work function is supplied by heating
the tip. In order to reduce the work function an electric field is applied (“Schottky
effect”). If the electric field is of the order of 10 V nmÀ1, the height and width of the
potential barrier is strongly reduced, such that the electrons may leave the metal via
field emission.

Although several different electron sources have been developed, their basic
design is rather similar (see Figure 1.4). In a thermionic source, the electrons are
extracted from a heated filament at a low bias voltage that is applied between the
source and a cylindrical cap (the Wehnelt cylinder). This beam of thermionic


1.1 Components of the Scanning Electron Microscope

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

The work function can be lowered by applying an
electric field (Schottky effect). If the field is very
high, the electrons can tunnel through the
potential barrier. Redrawn from Ref. [1].

electrons is brought to a focus by the electrostatic field and then accelerated by an
anode beneath the Wehnelt cylinder.
The beam that enters the microscope column is characterized by the effective
source 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 the
current DI passing through an area DS into a solid angle DV ¼ pa2, where a is the
angular spread of the electrons. With j ¼ DI/DS being the current density in A cmÀ2,
the following is obtained:
b¼

DI

j
¼
¼ const:
DSDV pa2

ð1:1Þ

The brightness is a conserved quantity, which means that its value is the same for
all points along the optical axis, independent of which apertures are inserted, or how
many lenses are present.
Currently, three different types of electron sources are in common use (Figure 1.4);
the characteristics of these are summarized in Table 1.1. A heated tungsten filament
is capable of generating a brightness of the order of 104 A cmÀ2 srÀ1, from an effective
source size, defined by the first cross-over of the electron beam, approximately 15 mm
across. The thermionic emission temperatures are high, which explains the selection
of tungsten as the filament material. A lanthanum hexaboride LaB6 crystal can
generate a brightness of about 105 A cmÀ2 srÀ1, but this requires a significantly
higher vacuum level in the vicinity of the source, and is now infrequently used in SEM
instruments. The limited effective source size of thermionic electron guns, which
must be demagnified by the electromagnetic lens system before impinging on the
sample, leads to microscopes equipped with thermionic sources being defined as
conventional scanning electron microscopes.

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j 1 Introduction to Scanning Electron Microscopy

Figure 1.4 Schematic drawings of (a) a
tungsten filament and (b) a LaB6 tip for
thermionic electron sources. (c) For a fieldemission gun (FEG) source, a sharp tungsten
tip is used. (d) In thermionic sources the
filament or tip is heated to eject electrons, which
are then focused with an electrostatic lens (the

Wehnelt cylinder). (e) In FEGs, the electrons are
extracted by a high electric field applied to the
sharp tip by a counterelectrode aperture, and
then focused by an anode to image the
source. Reproduced with permission from
Ref. [4]; Ó 2008, John Wiley & Sons.

The effective source size can be significantly reduced (leading to the term highresolution SEM) by using a “cold” field emission gun (FEG), in which the electrons
“tunnel” out of a sharp tip under the influence of a high electric field (Figures 1.3
and 1.4). Cold FEG sources can generate a brightness of the order of 107 A cmÀ2 srÀ1,
and the sharp tip of the tungsten needle that emits the electrons is of the order of
0.2 mm in diameter; hence, the effective source size is less than 5 nm. More often, a
“hot” source replaces the “cold” source, in which case a sharp tungsten needle is
heated to enhance the emission (this is termed a “thermal” field emitter, or TFE). The
heating of the tip leads to a self-cleaning process; this has proved to be another benefit
of TFEs in that they can be operated at a lower vacuum level (higher pressures). In the


1.1 Components of the Scanning Electron Microscope
Table 1.1 A comparison of the properties of different electron sources.

Source type

Thermionic

Thermionic

Schottky

Cold FEG

Cathode material
Work function [eV]
Tip radius [mm]
Operating temperature [K]
Emission current density [A cmÀ2]
Total emission current [mA]
Maximum probe current [nA]
Normalized brightness [A cmÀ2 srÀ1]
Energy spread at gun exit [eV]

W
4.5
50100
2800
13
200
1000
104
1.52.5

LaB6
2.7

1020
1900
2050
80
1000
105
1.32.5

W(100) ỵ ZrO
2.7
0.51
1800
5005000
200
>20
107
0.40.7

W(310)
4.5
<0.1
300
104106
5
0.2
2 107
0.30.7

so-called Schottky emitters,” the electrostatic field is mainly used to reduce the work
function, such that electrons leave the tip via thermal emission (see Figure 1.3). A

zirconium-coated tip is often used to reduce the work function even further.
Although Schottky emitters have a slightly larger effective source size than cold
field emission sources, they are more stable and require less stringent vacuum
requirements than cold FEG sources. Equally important, the probe current at
the specimen is significantly larger than for cold FEG sources; this is important
for other analytical techniques used with SEM, such as energy dispersive X-ray
spectroscopy (EDS).
1.1.2
Electromagnetic Lenses

Within the scanning electron microscope, the role of the general lens system is to
demagnify an image of the initial crossover of the electron probe to the final size of
the electron probe on the sample surface (1–50 nm), and to raster the probe across
the surface of the specimen. As a rule, this system provides demagnifications in the
range of 1000- to 10 000-fold. Since one is dealing with electrons rather than photons
the lenses may be either electrostatic or electromagnetic. The simplest example of
these is the electrostatic lens that is used in the electron gun.
Electromagnetic lenses are more commonly encountered, and consist of a large
number of turns of a copper wire wound around an iron core (the pole-piece). A small
gap located at the center of the core separates the upper and lower pole-pieces. The
magnetic flux of the lens is concentrated within a small volume by the pole-pieces,
and the stray field at the gap forms the magnetic field. The magnetic field distribution
is inhomogeneous in order to focus electrons traveling parallel to the optical axis onto
a point on the optical axis; otherwise, they would be unaffected. Thereby, the radial
component of the field will force these electrons to change their direction in such a
way that they possess a velocity component normal to the optical axis; the longitudinal
component 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

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