Low Voltage Electron Microscopy Principles and Applications

David C. Bell received his PhD in physics from the University of Melbourne, Australia in 1997 and completed his postdoctoral studies at MIT in 1999. He was research faculty and principal investigator at the University of Minnesota from 2000 to 2002. In 2003, he joined the Center for Nanoscale Systems at Harvard University as a principal scientist and became the Manager for Imaging and Analysis in 2007. He has been a lecturer in applied physics at Harvard since 2003 and is a teaching professor at the Harvard Extension School. In 2007, he was a visiting scientist at the Department of Materials, Oxford University, UK. Dr Bell is one of the renowned experts in the field of elemental analysis using electron microscopy (TEM and STEM) and has co-authored a book on this subject. He has authored more than 70 research papers on the subjects of microscopy, materials science and biology and holds several patents. He is an elected Fellow of the Royal Microscopical Society, UK.

Natasha Erdman received her Ph.D. in Materials Science and Engineering from Northwestern University (Chicago, IL) in 2002. After completing her Ph.D. she worked as a Senior Research Chemist at UOP LLC (currently Honeywell) in Des Plaines, IL focusing on investigation of structure-properties relationship in various catalysts using electron microscopy techniques. In 2004 Dr. Erdman as joined JEOL USA Inc., and currently serves as an SEM and Ion-Beam Product Manager. She has authored over 30 peer-reviewed papers on the subjects of microscopy, materials science, chemistry and biology and is a renowned expert on ion-beam based sample preparation techniques for electron microscopy.

Preface xi

1 Introduction to the Theory and Advantages of Low Voltage

Electron Microscopy 1

David C. Bell and Natasha Erdman

1.1 Introduction 1

1.2 Historical Perspective 2

1.3 Beam Interaction with Specimen—Elastic and Inelastic Scattering 3

1.3.1 The Scattering Cross Section 6

1.3.2 Effects of Specimen Damage 10

1.4 Instrument Configuration 11

1.4.1 Scanning Electron Microscope 11

1.4.2 Transmission Electron Microscope 12

1.4.3 Scanning Transmission Electron Microscope 12

1.5 Influence of Electron Optics Aberrations at Low Voltages 12

1.5.1 Spherical Aberration 13

1.5.2 Effect of Chromatic Aberration 14

1.5.3 The Diffraction Limit 15

1.5.4 Optimizing Spot Size for SEM and STEM 15

1.6 SEM Imaging at Low Voltages 16

1.6.1 Primary Contrast Signals and their Detection in SEM 18

1.6.2 Backscattered Electrons 18

1.6.3 Secondary Electrons 21

1.6.4 Charge Balance in SEM 23

1.6.5 SEM Image Contrast 24

1.6.6 Microanalysis in SEM at Low Voltages 25

1.7 TEM/STEM Imaging and Analysis at Low Voltages 26

1.8 Conclusion 27

References 28

2 SEM Instrumentation Developments for Low kV Imaging and Microanalysis 31

Natasha Erdman and David C. Bell

2.1 Introduction 31

2.2 The Electron Source 33

2.3 SEM Column Design Considerations 36

2.4 Beam Deceleration 41

2.5 Novel Detector Options and Energy Filters 43

2.5.1 Secondary Detectors 43

2.5.2 Backscatter Detectors 45

2.6 Low Voltage STEM in SEM 48

2.7 Aberration Correction in SEM 50

2.8 Conclusions 53

References 53

3 Extreme High-Resolution (XHR) SEM Using a Beam Monochromator 57

Richard J. Young, Gerard N.A. van Veen, Alexander Henstra and Lubomir Tuma

3.1 Introduction 57

3.2 Limitations in Low Voltage SEM Performance 58

3.2.1 Aberration Correction 58

3.2.2 Electron Source Energy Spread 59

3.3 Beam Monochromator Design and Implementation 59

3.4 XHR Systems and Applications 63

3.4.1 Elstar XHR Electron Column 64

3.4.2 Beam Deceleration for Extending Low-Voltage Performance 65

3.4.3 Combination of a Monochromator with Non-Immersion Lens 67

3.4.4 XHR Applications 68

3.5 Conclusions 69

Acknowledgements 70

References 70

4 The Application of Low-Voltage SEM—From Nanotechnology to Biological Research 73

Natasha Erdman and David C. Bell

4.1 Introduction 73

4.2 Specimen Preparation Considerations 74

4.3 Nanomaterials Applications 76

4.3.1 Nanoparticles, Nanotubes and Nanowires 76

4.3.2 Nanoporous Materials 81

4.3.3 Graphene 83

4.4 Beam Sensitive Materials 84

4.5 Semiconductor Materials 85

4.6 Biological Specimens 87

4.7 Low-Voltage Microanalysis 91

4.8 Conclusions 92

References 93

5 Low Voltage High-Resolution Transmission Electron Microscopy 97

David C. Bell

5.1 Introduction 97

5.2 So How Low is Low? 99

5.3 The Effect of Chromatic Aberration and Chromatic Aberration Correction 100

5.4 The Electron Monochromator 103

5.5 Theoretical Tradeoffs of Low kV Imaging 105

5.6 Our Experience at 40 keV LV-HREM 109

5.7 Examples of LV-HREM Imaging 110

5.8 Conclusions 114

References 116

6 Gentle STEM of Single Atoms: Low keV Imaging and Analysis at Ultimate Detection Limits 119

Ondrej L. Krivanek, Wu Zhou, Matthew F. Chisholm, Niklas

Dellby, Tracy C. Lovejoy, Quentin M. Ramasse and Juan

Carlos Idrobo

6.1 Introduction 119

6.2 Optimizing STEM Resolution and Probe Current at Low

Primary Energies 121

6.3 STEM Image Formation 128

6.3.1 Basic Principles 128

6.3.2 ADF Imaging 132

6.4 Gentle STEM Applications 135

6.4.1 Single Atom Imaging 135

6.4.2 Single Atom Spectroscopy 146

6.4.3 Single Atom Fine Structure EELS 152

6.5 Discussion 154

6.6 Conclusion 156

Acknowledgements 157

References 157

7 Low Voltage Scanning Transmission Electron Microscopy of Oxide Interfaces 163

Robert Klie

7.1 Introduction 163

7.2 Methods and Instrumentation 166

7.3 Low Voltage Imaging and Spectroscopy 168

7.3.1 SrTiO3/BiFeO3 Interface 168

7.3.2 Si3N4/SiO2 Interfaces 170

7.3.3 Ultrathin SrTiO3 films on GaAs 175

7.4 Summary 180

Acknowledgements 180

References 180

8 What’s Next? The Future Directions in Low Voltage Electron Microscopy 185

David C. Bell and Natasha Erdman

8.1 Introduction 185

8.2 Unique Low Voltage SEM and TEM Instruments 186

8.2.1 Miniature SEM Columns 186

8.2.2 Dedicated Low Voltage TEM 187

8.2.3 The Helium Ion Microscope as an Alternative to Low Voltage SEM Imaging 189

8.3 Cameras, Detectors, and Other Accessories 192

8.3.1 The Direct Electron Detector 192

8.3.2 Silicon Drift Detectors for Low kV Nanoanalysis 195

8.4 Conclusions 198

References 199

Index 201

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