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

74 Chapter 3
Figure 3-9 summarizes these conclusions in terms of the current-density
profile at the specimen plane, which is directly observable (with the radial
distance magnified) as a variation in image intensity on the TEM screen.
Condenser aperture
The condenser aperture is the small hole in a metal diaphragm located just
below the polepieces of the C2 lens. In order to center the aperture on the
optic axis, the diaphragm is mounted at the end of a support rod that can be
moved precisely in the horizontal plane (x and y directions) by turning knobs
located outside the microscope column, or by an electric-motor drive. The
aperture is correctly aligned (on the optic axis) when variation of the C2
current changes the illumination diameter but does not cause the magnified
disk of illumination to move across the viewing screen. In practice, there are
three or four apertures of different diameter (D � 20 � 200 �m), arranged
along the length of the support rod, so that moving the rod in or out by
several mm places a different aperture on the optic axis. Choosing a larger
size increases the convergence angle � of the illumination but allows more
electrons to reach the specimen, giving higher intensity in the TEM image.
Condenser stigmator
The condenser-lens system also contains a stigmator to correct for residual
astigmatism of the C1 and C2 lenses. When such astigmatism is present and
the amplitude control of the stigmator is set to zero, the illumination (viewed
on the TEM screen, with or without a TEM specimen) expands into an
ellipse (rather than a circle) when the C2 lens excitation is increased or
decreased from the focused-illumination setting; see Fig. 3-10. To correctly
adjust the stigmator, its amplitude control is first set to maximum and the
orientation control adjusted so that the major axis of the ellipse lies
perpendicular to the zero-amplitude direction. The orientation is then correct
but the lens astigmatism has been overcompensated. To complete the
process, the amplitude setting is reduced until the illumination ellipse
becomes a circle, now of smaller diameter than would be possible without
astigmatism correction (Fig. 3-10e). In other words, the illumination can be
focused more tightly after adjusting the stigmator. To check that the setting
is optimum, the C2 current can be varied around the fully-focused condition;
the illumination should contract or expand but always remain circular.
Condenser-lens astigmatism does not directly affect the resolution of a
TEM-specimen image. However, it does reduce the maximum intensity
(assuming focused illumination) of such an image on the TEM screen and