Practical Electron Microscopy and Database

There are two main types of BE detectors. The Everhart-Thornley (ET) detector for secondary electron (SE) imaging can be used in the detection of backscattered electrons (BSEs) in SEM systems either by turning off the Faraday cage or by applying a negative voltage to the Faraday cage. This is an inefficient way to collect BE. The geometric collection efficiency of BE in an ET detector is only about 1 to 10% [1]. The efficiency is low because most BEs travel away from the sample in the beam direction, but the ET detectors are usually mounted away from the beam axis. However, better BSE images come from dedicated BSE detectors rather than from the ET detector as a BSE detector. In this case, the collection efficiency is improved if a disk-shaped detector is placed at the bottom of the objective lens, with a hole in the middle for the electron beam. Both solid-state and scintillation-type detectors are used in the two types of detectors.

Figure 4580 shows an example of positions of detectors, including two BSE detectors, in a SEM system.

[1] Goldstein, J. L, Scanning Electron Microscopy, 2nd ed., Plenum Press, New York, 1992.
[2] www.nanoimages.com.