For thermal FEGs (field emission guns), electrons are emitted through a reduced potential barrier (reduced by an applied electric field) even when heating the emitter to a lower temperature than the thermal electron emission temperature of 1600 - 1800 K. This phenomenon is called the Schottky effect. Comparing to cold FEGs, thermal FEGs have disadvantage of large energy spread (0.6 -0.8 eV) because of heating the emitter. However, they have smaller emission noise and produce a stable emission current without gun flashing because there is no adsorption of contamination on the emitters.
Figure 4587a shows the schematic illustration of the electric circuit of thermal FEGs (field electron guns). The main part which is different from thermionic electron emission guns is dash-green-boxed. An extracting
electrode and an electrostatic lens are employed in the thermal EFG instead of the Wehnelt electrode that is in thermionic electron emission guns .
Figure 4587a. Schematic illustration of the electric circuit of thermal FEGs (field electron guns).
Many SEMs are
composed of a thermo-ionic cathode from which electrons
are accelerated by a voltage of
15 to 30 keV. Figure 4587b shows the differences of operating electrical field and temperature to emit electrons in various electron guns.
Figure 4587b. The differences of operating electrical field and temperature to emit electrons in various electron guns.
Before the application of field emission electron guns in TEM, the energy resolution of EELS systems had been ~1–2 eV, mainly limited by the energy spreading of thermionic electron source (tungsten filament or LaB6). In the late 1990s, Schottky emission sources became available and have been providing energy resolution greater than 0.5 eV.