Physical Principles of Electron Microscopy: An Introduction to TEM ...

144 Chapter 5
the SE2 and SE3 components is substantially worse than for the SE1
component; the SE2 and SE3 electrons contribute a tail or skirt to the imageresolution
function; see Fig. 5-15b. Although this fact complicates the
definition of spatial resolution, the existence of a sharp central peak in the
resolution function ensures that some high-resolution information will be
present in a secondary-electron image.
objective
escape depth
grid of SE
detector
SE3
SE1
SE2
BSE escape
volume interaction
volume
number of
secondaries
SE1 peak
SE2 + SE3
0
distance from center of probe
(a) (b)
Figure 5-15. (a) Generation of SE1 and SE2 electrons in a specimen, by primary electrons and
by backscattered electrons, respectively. SE3 electrons are generated outside the specimen
when a BSE strikes an internal SEM component, in this case the bottom of the objective lens.
(b) Secondary-image resolution function, showing the relative contributions from secondaries
generated at different distances from the center of the electron probe.
Figure 5-16. SE images of tridymite crystals and halite spheres on a gold surface, recorded
with an SEM accelerating voltage of (a) 10 kV and (b) 30 kV. Note the higher transparency at
higher incident energy. From Reimer (1998), courtesy of R. Blaschke and Springer-Verlag.