Low Voltage Electron Microscopy: Principles and Applications

Chapter 5

Low Voltage High-Resolution Transmission Electron Microscopy

David C. Bell

School of Engineering and Applied Sciences, Harvard University, USA

5.1 Introduction

Low-Voltage High-Resolution Electron Microscopy (LV-HREM) has several advantages, including increased cross-sections for inelastic and elastic scattering, increased contrast per electron and improved spectroscopy efficiency, decreased delocalization effects and reduced radiation knock-on damage (Bell et al., 2010, 2011; Kaiser et al., 2011; Sasaki et al., 2010). Together, these often improve the contrast to damage ratio obtained on a large class of samples. Third-order aberration correction now allows us to operate the TEM at low energies while retaining atomic resolution, which was previously impossible. At low voltage the major limitation to resolution becomes the chromatic aberration limit.

The history of electron microscope development shows that the trend in electron gun design has been towards brighter emission with less energy spread (Hawkes and Spence, 2007). Thermal emission yields energy spreads on the order of 1.2 eV; the Schottky field emission gun (FEG) yields 0.7 eV, and the cold FEG yields down to the 0.3 eV range. The competing needs for high brightness, low cost and emission stability have meant that cold FEGs are not as commonly available from microscope vendors as thermal or Schottky emission sources. One way to combine the high current and stability of a thermally-assisted field emitter with ...