condenser lens

The condenser lens is a lens that controls the probe current and probe-diameter by a combined use with the objective aperture.
As shown in Figures below, when the lens excitation is strong, the number of electrons passing through the objective aperture is small, whereas when the excitation is weak, the number of electrons passing through the aperture is large. Furthermore, when the lens is strongly excited, the focal length of the lens becomes short, and thus the electron probe size becomes small. Therefore, the probe current and probe diameter are controlled by the condenser-lens excitation. Normally, the condenser lens is strongly excited for high-resolution image observation to obtain a small probe size, and weakly excited for analysis such as EDS to obtain a large probe current.
An SEM equipped with a thermionic-emission gun uses the multi-stage condenser lens system to control the probe diameter in a wide range. Since the crossover (electron source) of this gun is as large as 20 µm, the crossover needs to be demagnified to about 1/1000 with a high reduction rate. An SEM equipped with a Schottky-emission gun has an electron source as small as 15 to 20 nm, and thus the lens with a low reduction rate is allowed. An SEM equipped with a field-emission gun has a sufficiently small source size. Thus, the condenser lens is mainly used to control the probe current with a one-stage condenser lens because the control range is small.