Dell JSM-6060LV - Manual (Page 5)

J E O L J S M - 6 0 6 0 L V S C A N N I N G E L E C T R O N M I C R O S C O P E
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Figure 2. Schematic illustration of SEM electron
The basic components of a SEM are the vacuum system, electron gun, lens system, electron
detector, imaging system, and the electronics associated with these components. The vacuum
system is necessary to minimize the interference of air particles with the electron beam and to
prevent rapid oxidation of the tungsten filament. The vacuum system consists of roughing
pumps, an oil diffusion pump, and various vacuum fittings, valves and seals that provide a
working pressure in the SEM of 10-6 to 10-4 Torr (10-4 to 10-2 Pa). Our SEM has a vacuum
system that may be switched to low vacuum mode (close to atmospheric pressure). The electron
gun in our JEOL SEM contains a tungsten filament that is heated with a filament power supply
and maintained at a high negative voltage (typically 10-20 kV) during operation. When the
tungsten filament is heated, electrons are emitted from the tip and accelerated to ground by the
10-20 kV potential between the filament
and the anode. Figure 2 shows the typical
configuration of an electron gun in a
SEM. After electrons are emitted from
the gun and accelerated down the SEM
column in an electron beam, they are
controlled and directed to the specimen
by a series of electromagnetic lenses and
apertures.
When the electrons in the electron beam
hit the specimen, a number of electron-
specimen interactions may occur. Some
of these interactions include elastic
scattering of electrons, secondary electron
emission (emission of loosely bound
electrons of the conduction band),
ionization of inner shell electrons
(produces x-rays and Auger electrons),
and excitation of phonons (causes heating of the specimen). If a sample is thin enough, some
electrons will be transmitted all the way through the sample. Different materials and sample
geometries will produce different amounts or different types of secondary electrons, backscattered
electrons, Auger electrons, transmitted electrons, and x-rays, and all of these interactions may be
used for imaging or analysis of the sample. The most common type of imaging in a SEM is
secondary electron imaging (SEI), which involves the use of a secondary electron detector. The
secondary electron detector collects secondary electrons and some backscattered electrons that are
emitted from the specimen surface, amplifies the detected signal, and converts the electron signal
into a video signal that is sent to the monitor.
References
• JEOL JSM-35CF Scanning Microscope Instruction Manual
• J. I. Goldstein, et al., Scanning Electron Microscopy and X-Ray Microanalysis, Plenum Press,
New York (1981).