Chapter One - Toward Quantitative Scanning Electron Microscopy

Abstract

Abstract The scanning electron microscope (SEM) has been a fundamental tool that has underpinned much advancement in research and engineering in various disciplines over many decades. However, it cannot be regarded yet as a true quantitative instrument, particularly with regard to the resulting image contrast and when imaging is carried out at the nanoscale level. Such a limitation is not applied to the SEM’s use as a measuring instrument, in which it performs exceptionally well as a critical dimension tool (CD-SEM). This lack of material quantification manifests itself further when the instrument is operated with low-energy electrons, in what is referred to as low-voltage SEM (LVSEM). This is due to the presence of carbonaceous deposits at the surface and a poor understanding of the emission of secondary electrons from the materials. In this chapter, a short review is given of some of the progress made in the efforts to improve the quantification of the SEM, with the emphasis on research carried out at York. Our results strongly suggest that the currently accepted theory, which explains why there is a correlation between the secondary electron yield and the work function of a metal, is incorrect. In addition, we show that the backscattering coefficient from materials can be strongly influenced by surface layers at low primary electron energy, and that a secondary electron contribution to the backscattering coefficient occurs at low primary beam energy. Finally, we present Auger electron spectra that have been in situ acquired from clean surfaces at high speeds in a high vacuum (10 -7 mbar), and thus represent a new way to determine the composition of nanostructures in an SEM.