Scanning Electron Microscopy and X-Ray Microanalysis

75 5

of the electron beam is approximately circular.. Figure 5.9

shows the sample field of view seen in. Fig. 5.8, but imaged

using a beam with pronounced astigmatism in the Y direc-

tion. In general the SEM beam will be astigmatic in both X

and Y, and the operator must correct this beam distortion

along both axes using the X and Y stigmators. When this is

performed correctly, a series of image like those in. Fig. 5.10

can be obtained. In. Fig. 5.10, both overfocused and under-

focused images show loss of fine detail, but no directional

distortion is present. The other significant improvement in

. Fig. 5.10c over. Figs. 5.8c and 5.9c is that the best-focus
image is much sharper when the image is properly stigmated.
While this last benefit is the real reason to master the art of
image stigmation, the characteristic appearance of images
like those in. Figs. 5.8a, b and 5.9a, b are very handy when
adjusting the stigmation controls on the microscope.
Learning how to properly adjust the stigmation coils on an
SEM can be one of the most challenging and frustrating skills
to develop when first learning to use the instrument. However,
as can be seen in the previous figures, being able to quickly and
accurately minimize astigmatism in your electron micrographs
is an essential milestone along the journey to becoming an
expert scanning electron microscopy and X-ray microanalyst.

14 SEM Imaging Checklist

SEMs are very flexible instruments, and the SEM operator has

control over a large number of electron beam, detector, and

stage parameters. Consequently, the number of different imag-

ing conditions that may be employed to analyze any given sam-

ple is nearly infinite, and it is the job of the analyst to choose

these conditions wisely to obtain useful information to meet

the needs of the analysis. Fortunately, in many situations these

choices can be narrowed to using one of the four basic modes of

SEM operation: (1) High Depth-of-Field Mode, (2) Resolution

Mode, (3) High-Current Mode, and (4) Low-Voltage Mode.

Below you will find practical information on how to con-

trol the fundamental electron optical parameters described

earlier in the text and specific guidance for operating the

SEM in the four basic modes just mentioned. Experienced

SEM operators will have mastered these four modes and will

be comfortable moving between them as needed. Choosing

any one of these modes is a compromise, since each of them

sacrifices microscope performance in some areas to achieve

other imaging goals. Appreciating the strengths and weak-

nesses of each mode is essential to understanding when each

mode is warranted. Of course some analyses will demand

imaging conditions that do not fall neatly into one of these

four basic modes, and the expert SEM operator will use the

full flexibility of the instrument when required.

5.3.1 High Depth-of-Field Mode

Anyone familiar with compound light microscopes (LMs)

understands that they have a very limited Depth-of-Field

(DoF), meaning there is a limited range of vertical heights on

the sample surface that will all appear to be in focus simul-

taneously. Parts of the sample that fall outside this range

appear blurry. One of the advantages of the SEM over the

light microscope is that it is capable of a much deeper depth-

of- field than the LM.

a

b

c

. Fig. 5.10 Three additional SEM micrographs from the same field of
view seen in. Figs. 5.8 and 5.9 above. Here the sample is imaged with
a fully corrected beam, so neither the overfocused image a nor the
underfocused image b shows significant anisotropic fine detail. Fur-
ther, the in-focus image in panel c is much sharper then the best-focus
images obtained in panel c of. Figs. 5.8 and 5.9

5.3 · SEM Imaging Modes