Page 175 - Science at the nanoscale

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June 5, 2009
8.2. Electron Microscopy
Electron Gun
Magnetic Lenses
Scanning Coil
Secondary Electron
Detector
SEM
Image
Specimen
Figure 8.5.
(a) Schematic drawing of the main components of a SEM. (b)
Photo of a SEM unit.
Pointed Cathode
-
Cathode (-)
Anode (+)
Electron Beam
(a)
(b)
Figure 8.6.
Schematic of operation of (a) a thermionic electron gun and
(b) a field emission electron gun. Electron Beam + High Voltage 165 ch08
chamber. Figure 8.5 shows a schematic of a typical SEM and a
photograph of a commercial SEM.
There are two types of electron source (or electron gun):
thermionic or field emission guns. Figure 8.6 illustrates the
operations of these two types of electron gun. A thermionic
electron gun relies of electrons emitted from a heated wire or
a filament. This filament is usually a bent tungsten wire that
functions as the cathode. The bent portion becomes heated up
when an electric current passes through the filament. The outer
orbital electrons of the tungsten atoms are emitted when they