Physical Principles of Electron Microscopy: An Introduction to TEM ...

62 Chapter 3
electron
emission
current
small R b
large R b
saturation point
cathode temperature or filament heating current
Figure 3-3. Emission current I e as a function of cathode temperature, for thermionic emission
from an autobiased (tungsten or LaB6) cathode.
area according to the feedback mechanism just described, so Ie increases
rapidly with filament temperature T, as expected from Eq. (3.1). As the
filament temperature is further increased, the negative feedback mechanism
starts to operate: the beam current becomes approximately independent of
filament temperature and is said to be saturated. The filament heating
current (which is adjustable by the TEM operator, by turning a knob) should
never be set higher than the value required for current saturation. Higher
values give very little increase in beam current and would result in a
decrease in source lifetime, due to evaporation of W or LaB6 from the
cathode. The change in Ie shown in Fig. 3-1 can be monitored from an
emission-current meter or by observing the brightness of the TEM screen,
allowing the filament current to be set appropriately. If the beam current
needs to be changed, this is done using a bias-control knob that selects a
different value of Rb , as indicated in Fig. 3-3.
Schottky emission
The thermionic emission of electrons can be increased by applying an
electrostatic field to the cathode surface. This field lowers the height of the
potential barrier (which keeps electrons inside the cathode) by an amount ��
(see Fig. 3-4), the so-called Schottky effect. As a result, the emissioncurrent
density Je is increased by a factor exp(��/kT), typically a factor of 10
as demonstrated in the Appendix.
A Schottky source consists of a pointed crystal of tungsten welded to the
end of V-shaped tungsten filament. The tip is coated with zirconium oxide
(ZrO) to provide a low work function (� 2.8 eV) and needs to be heated to