In bulk materials, SEs are emitted in the backward direction as shown in Figure 4818a, while SEs are emitted in both backward and forward directions. One of the reasons why the SEM detectors are inserted above the sample stage is because SEM systems are mostly used for observing bulk materials. However, the SE detectors in principle can be installed above or below TEM thin film as indicated in Figure 4818a (b).
Figure 4818a. Secondary electron emission: (a) backward-SEs, (b) forward-SEs
It should be noted that more SEs are generated at a higher sample tilt angle in TEM because of the bigger interaction volume. Furthermore, it is different from emission of secondary electrons (SEs) in SEM, not all the incident electron can produce SEs because the TEM specimen is very thin.
The schematics in Figure 4818b shows the electron optical column in a modern analytical electron microscope operated in STEM mode.
Figure 4818b. Schematics of the electron optical column in a modern
analytical electron microscope operated in STEM mode.