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

The Transmission Electron Microscope 83
of amplitude and orientation, as discussed in Section 2.6. One way of setting
these controls is to insert an amorphous specimen, whose image contains
small point-like features. With astigmatism present, there is a “streaking
effect” (preferred direction) visible in the image, which changes in direction
through 90 degrees when the objective is adjusted from underfocus to
overfocus of the specimen image (similar to Fig. 5.17). The stigmator
controls are adjusted to minimize this streaking effect.
Selected-area aperture
As indicated in Fig. 3-12c and Fig. 3-14, a diaphragm can be inserted in the
plane that contains the first magnified (real) image of the specimen, the
image plane of the objective lens. This selected-area diffraction (SAD)
diaphragm is used to limit the region of specimen from which an electron
diffraction pattern is recorded. Electrons are transmitted through the aperture
only if they fall within its diameter D, which corresponds to a diameter of
D/M at the specimen plane. In this way, diffraction information can be
obtained from specimen regions whose diameter can be as small as 0.2 �m
(taking D � 20 �m and an objective-lens magnification M � 100).
As seen from Fig. 3-12c, sharp diffraction spots are formed at the
objective back-focal plane (for electrons scattered at a particular angle) only
if the electron beam incident on the specimen is almost parallel. This
condition is usually achieved by defocusing the second condenser lens,
giving a low convergence angle but a large irradiation area at the specimen
(Fig. 3-9). The purpose of the SAD aperture is therefore to provide
diffraction information with good angular resolution, combined with good
spatial resolution.
Intermediate lens
A modern TEM contains several lenses between the objective and the final
(projector) lens. At least one of these lenses is referred to as the intermediate,
and the combined function of all of them can be described in terms of the
action of a single intermediate lens, as shown in Fig. 3-14. The intermediate
serves two purposes. First of all, by changing its focal length in small steps,
its image magnification can be changed, allowing the overall magnification
of the TEM to be varied over a large range, typically 10 3 to 10 6 .
Second, by making a larger change to the intermediate lens excitation, an
electron diffraction pattern can be produced on the TEM viewing screen. As
depicted in Fig. 3-14, this is achieved by reducing the current in the
intermediate so that this lens produces, at the projector object plane, a