Electron Channeling Patterns

Electron channeling contrast arises directly from the inter-
action of the electron beam with a crystalline sample. For
beams incident at random angles on a crystal there is an
approximately constant probability of the electron being
scattered out of the sample and being collected. However,
if the electron is incident along one of the symmetry direc-
tions of the crystal lattice then the electron may penetrate,
or channel, a significant distance into the specimen before
being scattered. In this case the chance of the electron
emerging and being collected is reduced. The backscat-
tered signal from a crystal recorded as a function of the
angle of incidence of the beam therefore shows a vari-
ation which displays directly the symmetry elements of
the lattice. Figure 11 shows an example recorded from the
(111) face of a silicon crystal. The 3-fold symmetry as-
sociated with the (111) face is immediately evidence, in
the arrangement of bands which cross at the (111) pole.
The angular width of the bands is twice the Bragg angle
θB where
θB = λ/2d. (10)
λ is the electron wavelength and d is the lattice spacing.
For electrons of 20 keV energy λ is 0.087 A, so lattice ˚
spacings of a few angstroms will produce bands with a
width of the order of 0.02 rad, about 1◦. Higher order
reflections produce families of lines parallel to the band
at spacings of θB. By calibrating the angular width of the
display from a known crystal, the symmetry and lattice
spacings of an unknown crystal can rapidly be determined.
The pattern observed will not change when the sample is
moved laterally because the symmetry will not alter, but
tilting or rotating the crystal will change the symmetry
and cause the channeling pattern to change as if rigidly
fixed to the lattice. This fact can be used to build up a
FIGURE 11 Electron channeling pattern from the (111) face of a
silicon crystal.
channeling map of a crystal, showing all the symmetry
elements exhibited by the lattice.
Channeling contrast comes from the top few hundred
angstroms of the crystal surface, the quality of the pattern
is therefore very dependent on the quality of the surface.
Small amounts of surface contamination or mechanical
damage will lead to the degradation, or elimination, of the
pattern. The electron channeling pattern can therefore be
used as a sensitive test of surface condition and crystal
quality, and electron channeling has been widely used in
the study of wear and deformation, and in the investigation
of rapid thermal annealing by laser or electron sources.
Similarly, information can be learned at higher resolution
from a static beam when it is referred to as an electron
backscattered pattern. However, the signal strength is dra-
matically reduced necessitating either extremely long ex-
posures or expensive photosensitive detectors to record
the patterns.