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.
