Microscopists have developed an array of sample preparation techniques that
have extended the applicability of SEM. However these preparations impose an
additional burden in time, cost, complexity and difficulty. Equally important,
all preparations may modify the sample in some way, introducing a layer of
complexity and uncertainty to the interpretation and use of the information
they contain.
The primary imaging signals used in SEM are secondary electrons (SE), which are
electrons from sample atoms ejected by interactions with the primary electron
beam, and backscattered electrons (BSE), which are electrons from the primary
beam that have been scattered back out of the sample by the nuclei of sample
atoms. Secondary electrons have very low energy and can escape to be detected
only if they are created very close to the sample surface. The SE signal has
high spatial resolution and is very sensitive to surface topography. BSE have
higher energy and can travel greater distances through the sample. The BSE
signal has lower resolution and carries information about sample composition
(atomic number). Conventional (Everhardt-Thornley) SE detectors cannot function
in low-vacuum conditions so some LVSEMs rely on BSE as the primary imaging
signal. Though the BSE has less resolution, it is also less sensitive to
charging and does permit imaging of less conductive samples without conductive
coatings.