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

The Transmission Electron Microscope 67
Boersch effect) because of the electrostatic interaction between electrons at
“crossovers” where the beam has small diameter and the electron separation
is relatively small. Larger �E leads to increased chromatic aberration
(Section 2.6) and a loss of image resolution in both the TEM and SEM.
3.2 Electron Acceleration
After emission from the cathode, electrons are accelerated to their final
kinetic energy E0 by means of an electric field parallel to the optic axis. This
field is generated by applying a potential difference V0 between the cathode
and an anode, a round metal plate containing a central hole (vertically below
the cathode) through which the beam of accelerated electrons emerges.
Many of the accelerated electrons are absorbed in the anode plate and only
around 1% pass through the hole, so the beam current in a TEM is typically
1%
of the emission current from the cathode.
To produce electron acceleration, it is only necessary that the anode be
positive relative to the cathode. This situation is most conveniently arranged
by having the anode (and the rest of the microscope column) at ground
potential and the electron source at a high negative potential (�V0). Therefore
the cathode and its control electrode are mounted below a high-voltage
insulator (see Fig. 3-1) made of a ceramic (metal-oxide) material, with a
smooth surface (to deter electrical breakdown) and long enough to withstand
the
applied high voltage, which is usually at least 100 kV.
Because the thermal energy kT is small (