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).
three signal are measured in the SEM providing different information
about the sample:
in the conduction or valence band need a small amount of energy
(work function) only to be transferred into vacuum, the
of secondary electrons (SE) is low
eV). Because of their low energy, SEs can only escape from the
sample if they are generated close to the surface. Therefore,
are a means to get topographic images.
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
(EDXS) and wavelength-dispersive (WDXS) spectrometers
can be installed at an SEM, providing laterally resolved information
about the sample composition.
A useful introduction into SEM is provided by B. Hafner.