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

The Scanning Electron Microscope 147
The large SEM depth of field is seen to be a direct result of the relatively
small convergence angle � of the electron probe, which in turn is dictated by
the need to limit probe broadening due to spherical and chromatic aberration.
As shown in Fig. 5-19a,
��D/(2v) (5.6)
According to Eqs. (5.5) and (5.6), the depth of field �v can be increased by
increasing v or reducing D , although in either case there may be some loss
of resolution because of increased diffraction effects (see page 4). Most
SEMs allow the working distance to be changed, by height adjustment of the
specimen relative to the lens column. Some microscopes also provide a
choice of objective diaphragm, with apertures of more than one diameter.
Because of the large depth of field, the SEM specimen can be tilted
away from the horizontal and toward the SE detector (to increase SE yield)
without too much loss of resolution away from the center of the image. Even
so, this effect can be reduced to zero (in principle) by a technique called
dynamic focusing. It involves applying the x- and/or y-scan signal (with an
appropriate amplitude, which depends on the angle of tilt) to the objectivecurrent
power supply, in order to ensure that the specimen surface remains in
focus at all times during the scan. This procedure can be regarded as an
adaptation of Maxwell’s third rule of focusing (see Chapter 2) to deal with a
non-perpendicular image plane. No similar option is available in the TEM,
where the post-specimen lenses image all object points simultaneously.
5.7 SEM Specimen Preparation
One major advantage of the SEM (in comparison to a TEM) is the ease of
specimen preparation, a result of the fact that the specimen does not have to
be made thin. In fact, many conducting specimens require no special
preparation before examination in the SEM. On the other hand, specimens of
insulating materials do not provide a path to ground for the specimen current
Is and may undergo electrostatic charging when exposed to the electron
probe. As is evident from Eq. (5.4), this current can be of either sign,
depending on the values of the backscattering coefficient � and secondaryelectron
yield �. Therefore, the local charge on the specimen can be positive
or negative. Negative charge presents a more serious problem, as it repels
the incident electrons and deflects the scanning probe, resulting in image
distortion or fluctuations in image intensity.
One solution to the charging problem is to coat the surface of the SEM
specimen with a thin film of metal or conducting carbon. This is done in
vacuum, using the evaporation or sublimation technique already discussed in