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Backscattered electrons are those which leave the spec-
imens with energies between about 50 eV and the inci-
dent beam energy. These electrons emerge from depths of
up to about 0.3 to the electron range R, where R can be
approximated as
R = 0.0276AE1.67/(Z0.889ρ)µm, (8)
where A is the atomic weight of the sample (g/mol), Z
is the atomic number, ρ is the density (g/cm3), and E
is the beam energy in keV. For medium atomic weight
elements, and beam energies in the range 10 to 20 keV,
the range is of the order of a few microns. The backscat-
tered image therefore contains information from regions
beneath the sample surface, but the high energies also en-
able these electrons to be conventionally collected in the
LPSEM.
The yield η of backscattered electrons is independent
of the beam energy (over the range 1 to 100 keV) but
is a monotonic function of the atomic number Z of the
specimen. η can be approximated by the function
η = −0.0254 + 0.016Z − 0.000186Z2
. (9)
For compounds and alloys Z can be replaced by the arith-
metic mean value of Z derived from the chemical compo-
sition. η varies between 0.05 for carbon (Z = 6) to about
0.6 for gold (Z = 79), thus at typical SEM beam energies
the backscattered signal is larger than the secondary elec-
tron signal. However, because the backscattered electrons
are relatively high in energy they are not easily deflected
toward a detector. Thus efficient collection of this signal
requires a detector which subtends a large solid angle at
the specimen. Typically this is achieved by using an an-
nular p–n junction or Schottky barrier solid state device
directly above the sample, and concentric with the beam.
With such an arrangement 50% or more of the backscat-
tered signal can be collected.
1. Atomic Number Contrast
The variation of η with Z produces image contrast which
is directly related to the mean atomic number of the irra-
FIGURE 10 Atomic number contrast from aluminum–copper
alloy.
diated volume of the specimen. Thus in materials such as
multiphase alloys, regions of the specimen with different
atomic numbers will display contrast, the magnitude of
which will depend on the relative change in Z (Fig. 10).
Under normal imaging conditions regions with an effec-
tive difference of only about 0.5 units in Z can be dis-
tinguished. The fact that the variation of η with Z is not
only monotonic but almost linear also makes it possible
to perform a simple form of chemical microanalysis on
a sample by measuring the variation in the backscattered
signal and comparing this with the signal produced under
identical conditions from suitable pure element standards.
The atomic number contrast effect is of importance in
many biological applications, since heavy metal reagents
having affinities for specific groups can be used as stains.
In the backscattered image these stained regions then ap-
pear bright against the predominantly low atomic num-
ber carbon matrix. Because the backscattered electrons
are collected from depths up to about 0.3 of the electron
range [as determined from Eq. (8)] the labeled regions can
be observed at significant depths beneath the surface of a
specimen. For example, at 15 keV a penetration in excess
of 1 µm is possible, although the lateral spatial resolution
will, correspondingly, be poor.
The atomic number contrast is superimposed on any to-
pographic contrast present from the sample, since changes
in surface orientation also lead to changes in the backscat-
ter yield. In addition, the fact that the backscatter elec-
trons travel in straight trajectories from the sample to the
detector produces shadowing of any surface relief. The to-
pographic and chemical components of the signal can be
separated, at least partially, by using multiple detectors.
An annular detector divided into two halves will mini-
mize topographic contrast and maximize atomic number
contrast when the signals are added, since the shadowing
seen by one segment of the detector will in general not be
present in the signal from the other segment, whereas the
situation will be reversed if the two signals are subtracted
since both segments will see the change in signal due to
the change in atomic number in the same way.