hello everyone
I just wondered , what is the theory behind the ability of different SEM detector to collect and detect a specific type of electrons .
for example: there is a secondary electron detector (SED) which detect only secondary electrons , and on the other hand there is the backscattered electrons detector (BSE) which detect only BSE .
Different detectors are capable of detecting electrons of different energy levels. For example, SE are low-energy (
Different detectors are capable of detecting electrons of different energy levels. For example, SE are low-energy (
Different energy and directions. BSD usually is coaxial with a beam and SE detector is angularly and shifted. This tilt provides "pseudo-3D" effect for SE pictures.
The directions for electrons are self-evident from geometry.
There are a lot of classical book about SEM technics. For example:http://www.springer.com/us/book/9781461344247
In addition: many BSE detectors are semiconductor detectors. Sensitivity of sc detectors depends on electron energy, therefore low-energy SE make negligible signal corresponding to high-energy BSE.
Most SE detectors are scintillation detectors with photoelectronic multiplier, so they are able to collect each type of electron signal. But their angular size is relatively small, so they collect only small portion of BSE. To increse SE collection ability, there is a collecting grid in front of detector(Everhat-Thornley detector). If you apply 200-500 V on this grid, you can collect all SE emmited by the sample(if working distance is enough), but this voltage doesn't affect high-energy BSE, so you can separate signals again.
There is an effect of BSE to SE conversion: BSE from sample surface hit the chamber walls and generate SE3 signal. SE detector collects this SE3 signal, so some BSE contrast may present in SE images(fpr more information see here: Reimer L., Volber B. // SCANNING, 1979, V. 2, p. 238).
Also there are some types of SE and BSE detectors inside electron column. They separate SE and BSE by lower electron optics.
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