Scanning Electron Microscopy and X-Ray Microanalysis

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correcting the astigmatism, although it generally produces

an image that is far inferior to the in-focus images obtainable

with a properly stigmated beam. If the focus is further

adjusted, past this point of symmetry, the image will again

exhibit large amounts of shearing and stretching of the fine

details, but in a different direction. This sequence of effects

can be seen in. Fig. 5.8. In. Fig. 5.8a the sample is shown

when the objective lens is overfocused, with the beam

crossover occurring above the sample surface, corresponding

to the left diagram in. Fig. 5.7. In panel. Fig. 5.8b, the same

sample with the same astigmatic beam is shown in underfo-

cus, the right side diagram in. Fig. 5.6.. Figure 5.8c shows

the best achievable focus; here, the shearing and stretching is

minimized (or at least balanced), suggesting the cross section

a

b

c

. Fig. 5.8 Three SEM micrographs showing strong astigmatism in the X
direction, when the sample is a overfocused, b underfocused, and c near
focus. Note that the shearing or “tearing” appearance of fine detail in a
appears to be in a direction perpendicular to the effect in b

b

a

c

. Fig. 5.9 Three additional SEM micrographs from the same field of
view shown in Fig. 5.8 above. Here the beam shows strong astigmatism
in the Y-direction, when the sample is a overfocused, b underfocused,
and c near focus

Chapter 5 · Scanning Electron Microscope (SEM) Instrumentation