Low Voltage Electron Microscopy: Principles and Applications

Chapter 2

SEM Instrumentation Developments for Low kV Imaging and Microanalysis

Natasha Erdman¹ and David C. Bell²

¹JEOL USA Inc., USA

²School of Engineering and Applied Sciences, Harvard University, USA

2.1 Introduction

Scanning electron microscopes have historically been operated at relatively high voltages, between 15–40 kV; however, recent instrumental developments have now enabled very much the possibility for ultra-low voltage imaging in the order of tens of volts of accelerating voltage. The benefits of low voltage microscopy in SEM for observation of non-conductive specimens and high spatial resolution combined with enhanced surface detail have been known since late 1960s (Thornley, 1960; Thornley and Cartz, 1962), the capability for good SEM resolution at lower voltages (sub 5 kV) was extremely lacking at that time. The rapid growth of the semiconductor industry in the early 1980s dictated the development of new SEM columns with high-resolution across a range of accelerating voltages (in particular the 1 kV range) that could accommodate handling of large wafer semiconductor specimens.

The fundamental limitations to the operation of the scanning electron microscope, in particular in the low voltage regime, have been outlined early on in the development of SEMs (Oatley et al., 1965):

1. Brightness of the electron source. According to Langmuir (1937), the maximum current density for electrons generated by thermionic cathode emission and brought to a focus by an electron-optical ...