SEM: Interaction with sample

1. When the electron beam hits a sample surface, it interacts with the atoms in that sample.

2. As a result, some electrons are bounced back out of the sample (backscattered electrons), others knock into atoms and displace electrons that, in turn, come out of the sample (secondary electrons); alternatively X-rays, and light or heat (in the sample) can be the result of these interactions.

3. Generally, the energy is dissipated in the form of heat. We collect the electrons coming out of the material in order to produce the traditional SEM images (called micrographs).

4. The volumes involved in the production of secondary electron, backscattered electron and X-rays, form into a shape (called as Interaction volume) that ranges from a tear-drop to a semi circle within the specimen.

5. The depth and diameter (see next slide) depend on the applied voltage as well as the density of the specimen. The top 15 - 25 nm of the thickness comprises the zone from which secondary electrons can be collected, the top 40% is the region from which backscattered electrons can be collected and X-rays can be collected from the entire region.

Kanaya - Okayama Depth penetration formula

The penetration of incident electrons is determined by the electron stopping power of the specimen, which decreases with increasing atomic number (Z). The penetration depth of incident electrons is given by Kanaya -Okayama Formula [4], as shown in Figure 17.04.

Ref.[4]. K. Kanaya , S. Okayama, J. Phys. D., J. Appl. Phys. 5 (1972) 43.