=================================================================================

In bulk materials, SEs are emitted in the backward direction as shown in Figure 4818a, while SEs from TEM (transmission electron microscopes) thin films are emitted in both backward and forward directions in wide spatial directions and wide energy bands.

Figure 4818a. Secondary electron emission: (a) backward-SEs, (b) forward-SEs

In principle, the SE detectors in principle can be installed above or below TEM thin film as indicated in Figure 4818a (b). However, SEM detectors are conventionally installed above the sample stage due to the following reasons:
          i) SEM systems are mostly used for observing bulk materials.
          ii) More SEs are generated from the top surface than bottom surface of the TEM sample. (see page999)

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) from bulk materials 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.