When the electron beam hits a sample it interacts with the atoms in that sample. There are a number of outcomes. 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.
Generally heat is how most of the energy is dispersed. We collect the electrons coming out of the material in order to produce the traditional SEM images (called micrographs).
The volumes involved in the production of secondary electron (SE), backscattered electron (BSE) and X-rays, form into a shape that ranges from a tear-drop to a semi circle within the specimen. This shape is called an interaction volume and its depth and diameter depends on the kV as well as the density of the specimen. Approximately the top 15nm of the volume comprises the zone from which SE can be collected, the top 40% is the region from which BSE can be collected and X rays can be collected from the entire region.
Show the interaction volumes of:
Electron-matter interactions can be divided into two classes:
- Elastic scattering – the electron trajectory within the specimen changes, but its kinetic energy and velocity remains essentially constant. The result is generation of backscattered electrons (BSE).
- Inelastic scattering– the incident electron trajectory is only slightly perturbed, but energy is lost through the transfer of energy to the specimen. The result is the generation of:
- phonon excitation (heating);
- cathodoluminescence (visible light fluorescence);
- continuum radiation (bremsstrahlung);
- characteristic x-ray radiation;
- plasmon production (secondary electrons);
- auger electrons (ejection of outer shell electrons).
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