Signals in Scanning Electron Microscopy

When an electron beam hits matter, it begins to broaden because of strong elastic scattering effects. Simultaneously, inelastic interactions cause an energy loss of the electrons. If the sample is very thick, the energy will completely be transferred to the sample. The overall result is a pear-shaped interaction volume. The penetration depth t depends on the electron energy (t ∼ V) and on the material (t ∼ 1/atomic mass).

Mainly, three signal are measured in the SEM providing different information about the sample:

Since electrons in the conduction or valence band need a small amount of energy (work function) only to be transferred into vacuum, the energy of secondary electrons (SE) is low (>50 eV). Because of their low energy, SEs can only escape from the sample if they are generated close to the surface. Therefore, SE images are a means to get topographic images.

Examples

The collision of an electron from the beam with a nucleus leads to the deflection of its path as a result of Coulomb forces (Rutherford elastic scattering). Sometimes, the electrons are completely scattered back and leave the surface of the sample. Since heavy atoms with a high atomic number are much stronger scatterers of electrons than light ones, they cause a higher signal. Therefore, images with back-scattered electron contain compositional information (cf., STEM).

Examples

A useful introduction into SEM is provided by B. Hafner.