It is due to charge up of specimen surface and this can be handled by reducing probe current. have a look at the following document, hope it will be helpful.

Dear Abdul Samad,

Thank you for the answer. If you could clarify, why is that the charge accumulation is more for thin regions and not much for the thicker regions. How come a thicker region of the same sample is able to provide a conductive path to the electrons.

The sample i am referring to is not coated by a conductive material.

Thanks

Dear Bilal,

Charge can buildup in any area of non-conductive sample,

therefore, it is preferred that sample should be of even thickness and observed in BSED and low vacuum.

Moreover, it is advisable that non-conductive samples should be coated with metalic coating for best results.

Dear Abdul Samad,

I think we are getting a bit off track here. I will try to elaborate the issue so that you may help me better understand it.

Actually, I have prepared a thin TEM lamella using Focused Ion beam in SEM. Now, at low KV's the thin section (less than 100nm) appears more bright as compared to the thicker region (greater than 1.5 micron).

My assumption is that it is sort of an edge effect that is causing more SE electron from the thinner region.

I would appreciate if you may provide me some help in understanding the phenomenon.

Thanks..

Dear Bilal,

Have a look at the below links, it will give you the better idea to understand the problem you mentioned.

http://www.ammrf.org.au/myscope/sem/practice/principles/troubleshooting.php

http://iitkgp.vlab.co.in/?sub=40&brch=263&sim=1596&cnt=3340