Like glass lenses in optical microscopes (EMs), all electromagnetic lenses suffer from defects such as astigmatism, chromatic and spherical aberrations, coma, and barrel or pincushion distortion. The first three are related to the objective lens. On the other hand, from the operator's standpoint those three are the most important because of their influence on the resolution of the electron microscopes (EMs).
There are mainly three types of SEM objective lenses: i) Asymmetric pinhole lens; ii) Symmetric immersion lens; and iii) Snorkel lens.
Astigmatism in condenser lens is important because it reduces the coherence of the electron beam, while astigmatism in objective lens is important because it induces a serious degradation of spatial resolution.
Figure 3596 shows the schematic diagram of an SEM system with a thermionic electron gun. The purpose of the condenser lens system in the SEM is to
deliver electrons from the first crossover to the specimen
plane in a small-diameter, high-intensity beam. SEMs with
thermionic electron guns have two condenser lenses. The positions of BSE (backscattered electron) and SE (secondary electron) detectors can be different depending on different applications. Refer to BSE and SE detectors.
Figure 3596. Schematic diagram of an SEM system with a thermionic electron gun.