Working distance is a distance between the specimen and the lower pole piece in SEM system. The final lens, focusing an electron beam on specimen, is operated with a relatively long working distance so that secondary electrons (SE) are easily collected with a lateral detector (for Thornley-Everhardt (ET) detector).
Spherical aberration of the probe-forming lens is increased with the increase of working distance, resulting in a larger electron-probe size. Therefore, the need to have better spatial resolution of SEM leads to shorten the working distance and consequently to change the SEM detector position.
Furthermore, a low take-off angle (α) is more sensitive to topographic effects. For this reason, secondary electron detectors are located at low angles to enhance the topographic resolution of secondary electron images.