3. Instrument

A scanning electron microscope consists of the following components (figure 1 ):

• an electron column, comprising an electron gun, a number of electromagnetic lenses ("condensers"), a number of electrical alignment and adjustment coils, and an electron beam scanning device. This column is maintained under vacuum at a minimum level of 10 ^–3 Pa. In current microscopes, the necessary vacuum is obtained by a primary vane pump coupled with a secondary pumping system consisting of either an oil diffusion pump (or sometimes two in cascade), or a turbomolecular pump. Some higher-performance systems require ion pumping and/or ultra-high-vacuum...