Scanning electr on microscopy (SEM) and x-ray microanalysis can
produce magnified images and in situ chemical information from
virtually any type of specimen. The two instruments generally
operate in a high vacuum and a very dry environment in order to
produce the high energy beam of electrons needed for imaging and
analysis. With a few notable exceptions, most specimens destined
for study in the SEM are poor conductors and composed of beam
sensitive light elements containing variable amounts of water. In
the SEM, the imaging system depends on the specimen being
sufficiently electrically conductive to ensure that the bulk of the
incoming electrons go to ground. The formation of the image depends
on collecting the different signals that are scattered as a
consequence of the high energy beam interacting with the sample.
Backscattered electrons and secondary electrons are generated
within the primary beam-sample interactive volume and are the two
principal signals used to form images. The backscattered electron
coefficient ( ? ) increases with increasing atomic number of the
specimen, whereas the secondary electron coefficient ( ? ) is
relatively insensitive to atomic number. This fundamental diff-
ence in the two signals can have an important effect on the way
samples may need to be prepared. The analytical system depends on
collecting the x-ray photons that are generated within the sample
as a consequence of interaction with the same high energy beam of
primary electrons used to produce images.
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