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

The Scanning Electron Microscope 145
Figure 5-17. (a – c) Astigmatic SE image showing the appearance of small particles as the
objective current is changed slightly; in (b) the focus is correct, but the resolution is less than
optimum. (d) Correctly focused image after astigmatism correction.
Because the incident beam spreads out very little within the SE1 escape
depth, the width of this central peak (Fig. 5-15b) is approximately equal to
the diameter d of the electron probe, which depends on the electron optics of
the SEM column. To achieve high demagnification of the electron source,
the objective lens is strongly excited, with a focal length below 2 cm. This in
turn implies small Cs and Cc (see Chapter 2), which reduces broadening of
the probe by spherical and chromatic aberration. Because of the high
demagnification, the image distance of the objective is approximately equal
to its focal length (see Section 3.5); in other words, the working distance
needs to be small to achieve the best SEM resolution. The smallest available
values of d (below 1 nm) are achieved by employing a field-emission source,
which provides an effective source diameter of only 10 nm (see Table 3-1),
and a working distance of only a few millimeters.
Of course, good image resolution is obtained only if the SEM is properly
focused, which is done by carefully adjusting the objective-lens current.
Small particles on the specimen offer a convenient feature for focusing; the
objective lens current is adjusted until their image is as small and sharp as
possible. Astigmatism of the SEM lenses can be corrected at the same time;
the two stigmator controls are adjusted so that there is no streaking of image
features as the image goes through focus (see Fig. 5-17), similar to the
commonly-used TEM procedure. In the SEM, zero astigmatism corresponds
to a round (rather than elliptical) electron probe, equivalent to an axiallysymmetric
resolution function.
As shown in Fig. 5-18, the SEM not only has better resolution than a
light microscope but also a greater depth of field. The latter can be defined
as the change �v in specimen height (or working distance) that produces a