Download Scanning Electron Microscope Operation

Figure 25. This schematic depicts generation of secondary electrons near the surface from the
primary beam and backscattered electrons.
To the first order, the energy distribution of the secondary electrons is
independent of the energy of the primary beam. Because of this
characteristic, a “universal” curve can be generated for all metals, Figure
26. The curve rises from a zero emission coefficient for zero secondary
electron energy, to a peak between 1.3 and 2.5 eV for most metals, and
then falls to a low value for secondary energies greater than 10 eV. For
insulators, the peak falls at a lower secondary electron energy, because
there is a lower potential barrier at the surface of insulators than there is
with metals, and this allows a larger number of slower secondary electrons
to escape. The SEM video system produces a brighter pixel where the
secondary electron current striking the electron detector is greater.
Figure 26. This graph shows the envelope of probabilities of secondary electron escape from the
surface of metals as a function of the secondary electron energy. The data shows a probability of
escape peak between 1.3 and 2.5 eV. This peak denotes the general energy level of the secondary
electrons.
Title: Scanning Electron Microscope Operation
Author: Roger Robbins
The University of Texas at Dallas
Page 43 of 85
Date: 9/29/2015