Electron Optics

Abstract

The final electron beam diameter, often called the spot size or probe size, limits the best possible image resolution in the SEM and the EPMA. The amount of current in the final probe determines the intensity of emitted signals such as secondary electrons, backscattered electrons, or x rays. Unfortunately, the smaller the electron probe, the smaller the available probe current. Therefore, the operator must adjust the microscope controls to produce the desired result in each microscopy mode: high resolution, high depth-of-field, and micro-analysis. Intelligent use of these instruments requires an understanding of how the components of the electron column control probe size and probe current. In this chapter, we will describe the electron optical column and develop the relationship between electron-probe current and electron-probe diameter (spot size).