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

148 Chapter 5
Section 4-10. Films of thickness 10 � 20 nm conduct sufficiently to prevent
charging of most specimens. Because this thickness is greater than the SE
escape depth, the SE signal comes from the coating rather than from the
specimen material. However, the external contours of a very thin film
closely follow those of the specimen, providing the possibility of a faithful
topographical image. Gold and chromium are common coating materials.
Evaporated carbon is also used; it has a low SE yield but an extremely small
grain size so that granularity of the coating does not appear (as an artifact) in
a high-magnification SE image, masking real specimen features.
Where coating is undesirable or difficult (for example, a specimen with
very rough surfaces), specimen charging can often be avoided by carefully
choosing the SEM accelerating voltage. This option arises because the
backscattering coefficient � and secondary-electron yield � depend on the
primary-electron energy E0. At high E0 , the penetration depth is large and
only a small fraction of the secondary electrons generated in the specimen
can escape into the vacuum. In addition, many of the backscattered electrons
are generated deep within the specimen and do not have enough energy to
escape, so � will be low. A low total yield (� + �) means that the specimen
charges negatively; according to Eq. (5.4). As E0 is reduced, � increases and
the specimen current Is required to maintain charge neutrality eventually falls
to zero at some incident energy E2 corresponding to (� + �) = 1. Further
reduction in E0 could result in a positive charge but this would attract
secondaries back to the specimen, neutralizing the charge. So in practice,
positive charging is less of a problem.
For an incident energy below some value E1, the total yield falls below
one because now the primary electrons do not have enough energy to create
secondaries. Because by definition � < 1, Eq. (5.4) indicates that negative
charging will again occur. But for E1< E0 < E2, negative charging is absent
even for an insulating specimen, as shown in Fig. 5-20.
1
total yield (���)
E 1
zerocharging
region
E 2
Figure 5-20. Total electron yield (� + �) as a function of primary energy, showing the range
(E1 to E2) over which electrostatic charging of an insulating specimen is not a problem.
E 0