Low Voltage Electron Microscopy

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.

• List of Contributors ixPreface xi1 Introduction to the Theory and Advantages of Low Voltage Electron Microscopy 1David C. Bell and Natasha Erdman1.1 Introduction 11.2 Historical Perspective 21.3 Beam Interaction with Specimen—Elastic and Inelastic Scattering 31.3.1 The Scattering Cross Section 61.3.2 Effects of Specimen Damage 101.4 Instrument Configuration 111.4.1 Scanning Electron Microscope 111.4.2 Transmission Electron Microscope 121.4.3 Scanning Transmission Electron Microscope 121.5 Influence of Electron Optics Aberrations at Low Voltages 121.5.1 Spherical Aberration 131.5.2 Effect of Chromatic Aberration 141.5.3 The Diffraction Limit 151.5.4 Optimizing Spot Size for SEM and STEM 151.6 SEM Imaging at Low Voltages 161.6.1 Primary Contrast Signals and their Detection in SEM 181.6.2 Backscattered Electrons 181.6.3 Secondary Electrons 211.6.4 Charge Balance in SEM 231.6.5 SEM Image Contrast 241.6.6 Microanalysis in SEM at Low Voltages 251.7 TEM/STEM Imaging and Analysis at Low Voltages 261.8 Conclusion 27References 282 SEM Instrumentation Developments for Low kV Imaging and Microanalysis 31Natasha Erdman and David C. Bell2.1 Introduction 312.2 The Electron Source 332.3 SEM Column Design Considerations 362.4 Beam Deceleration 412.5 Novel Detector Options and Energy Filters 432.5.1 Secondary Detectors 432.5.2 Backscatter Detectors 452.6 Low Voltage STEM in SEM 482.7 Aberration Correction in SEM 502.8 Conclusions 53References 533 Extreme High-Resolution (XHR) SEM Using a Beam Monochromator 57Richard J. Young, Gerard N.A. van Veen, Alexander Henstra and Lubomir Tuma3.1 Introduction 573.2 Limitations in Low Voltage SEM Performance 583.2.1 Aberration Correction 583.2.2 Electron Source Energy Spread 593.3 Beam Monochromator Design and Implementation 593.4 XHR Systems and Applications 633.4.1 Elstar XHR Electron Column 643.4.2 Beam Deceleration for Extending Low-Voltage Performance 653.4.3 Combination of a Monochromator with Non-Immersion Lens 673.4.4 XHR Applications 683.5 Conclusions 69Acknowledgements 70References 704 The Application of Low-Voltage SEM—From Nanotechnology to Biological Research 73Natasha Erdman and David C. Bell4.1 Introduction 734.2 Specimen Preparation Considerations 744.3 Nanomaterials Applications 764.3.1 Nanoparticles, Nanotubes and Nanowires 764.3.2 Nanoporous Materials 814.3.3 Graphene 834.4 Beam Sensitive Materials 844.5 Semiconductor Materials 854.6 Biological Specimens 874.7 Low-Voltage Microanalysis 914.8 Conclusions 92References 935 Low Voltage High-Resolution Transmission Electron Microscopy 97David C. Bell5.1 Introduction 975.2 So How Low is Low? 995.3 The Effect of Chromatic Aberration and Chromatic Aberration Correction 1005.4 The Electron Monochromator 1035.5 Theoretical Tradeoffs of Low kV Imaging 1055.6 Our Experience at 40 keV LV-HREM 1095.7 Examples of LV-HREM Imaging 1105.8 Conclusions 114References 1166 Gentle STEM of Single Atoms: Low keV Imaging and Analysis at Ultimate Detection Limits 119Ondrej L. Krivanek, Wu Zhou, Matthew F. Chisholm, Juan Carlos Idrobo, Tracy C. Lovejoy, Quentin M. Ramasse and Niklas Dellby6.1 Introduction 1196.2 Optimizing STEM Resolution and Probe Current at Low Primary Energies 1216.3 STEM Image Formation 1286.3.1 Basic Principles 1286.3.2 ADF Imaging 1326.4 Gentle STEM Applications 1356.4.1 Single Atom Imaging 1356.4.2 Single Atom Spectroscopy 1466.4.3 Single Atom Fine Structure EELS 1526.5 Discussion 1546.6 Conclusion 156Acknowledgements 157References 1577 Low Voltage Scanning Transmission Electron Microscopy of Oxide Interfaces 163Robert Klie7.1 Introduction 1637.2 Methods and Instrumentation 1667.3 Low Voltage Imaging and Spectroscopy 1687.3.1 SrTiO3/BiFeO3 Interface 1687.3.2 Si3N4/SiO2 Interfaces 1707.3.3 Ultrathin SrTiO3 films on GaAs 1757.4 Summary 180Acknowledgements 180References 1808 What’s Next? The Future Directions in Low Voltage Electron Microscopy 185David C. Bell and Natasha Erdman8.1 Introduction 1858.2 Unique Low Voltage SEM and TEM Instruments 1868.2.1 Miniature SEM Columns 1868.2.2 Dedicated Low Voltage TEM 1878.2.3 The Helium Ion Microscope as an Alternative to Low Voltage SEM Imaging 1898.3 Cameras, Detectors, and Other Accessories 1928.3.1 The Direct Electron Detector 1928.3.2 Silicon Drift Detectors for Low kV Nanoanalysis 1958.4 Conclusions 198References 199Index 201