Electron Optics

The amount of current in a finely focused electron beam impinging on a specimen determines the magnitude of the signals (x-ray, secondary electrons, etc.) emitted, other parameters being equal. In addition, the size of the final probe determines the best possible resolution of the scanning electron microscope (SEM) and electron microprobe (EPMA) for many of the signals that are measured. Therefore the electron optical system in these instruments is designed so that the maximum possible current is obtained in the smallest possible electron probe. In order to use the instruments intelligently it is important to understand how the optical column is designed, how the various components of the optical system function, and how the final current and spot size are controlled. In this chapter we will discuss the various components of the electron optical system, develop the relationship between electron probe current and spot size, and discuss the factors which influence this relationship.