A deflector for electron-beam deflection in EMs has been applied to align, tilt, shift, scan the electron beam, and so on. The deflector, usually including a pair of deflection coils, is called the double-deflection system.
The principle of beam scanning in SEM and STEM systems is similar to the electron beam scanning of old TV system as shown in Figure 4955a.
Figure 4955a. Electron beam scanning of old TV system.
Figure 4955b shows the schematic diagram of an SEM system. The beam-scan generator and the relevant lenses are indicated as well.
Figure 4955b. Schematic diagram of an SEM system with a thermionic electron gun.