I've read a few studies on comparisons between SE and BSE mode SEM imaging which show BSE imaging displays less charging. I am also working on block face imaging and the instrument vendors (FEI, Zeiss) have also developed specialised BSE detectors for this task, claiming the reduction of charging. But I don't understand the mechanism behind it. Can anyone please explain? Thank you!
secondary electrons are have less energy less than 50ev and BSE has high energy they have enough energy to ionize, so i think roll of energy is important.
Hi Boyin, I think Manish is right.
Although backscatterd electrons are also belonging to the class of secondary electrons ( they are first order scattered electrons) the difference in the kinetic energy of the electrons makes the difference.
Dear Boyin,
I agree with the answers provided above. Even though both BSE and SE are affected by charging and applied electric fields in the vicinity. The magnitude varies largely due to the sheer difference in energy. BSE electrons are scattered close to the surface and elastically hence possess high energy and move relatively faster. Even when high voltages are applied their trajectories change very little. SE come from deep within surface and have lost most of their energies due to several inelastic collisions, as such can be used to reveal sub-surface features and chemical composition. However, owing to poor energy they travel relatively slow and are easily affected by even minor voltages. It is the same principle that Everhart-Thornley detector is based on.
I hope it helps.
https://en.wikipedia.org/wiki/Everhart-Thornley_detector
Of course, the charge on the sample is not depending of what detector is used to make the image but a SE detector is better than a BE detector to SEE the charge. It is because the charge affect mainly the secondary emission (but finally also the backscatered emission when the charge is too strong). Because only low energy secondary electrons contributes to image a charge then you can filter this component of the image if you use the backscatered detector because it can not receive those low energy electrons. That's one of the paradoxe of the in-lens secondary detector which is able to detect very low energy secondary electrons because it is also very sensitive to charging effects and, for that, sometime totally unusable. The BE detector is very useful for such situation but the in-chamber secondary detector can also act as a filter sometime because it is far from the sample and cannot receive the lowest energy secondary electrons.
- Badges
- Science method