SEM

FE-SEM

An electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode or LaB₆.
Thermionic emitters use electrical current to heat up a filament. When the heat is enough to overcome the work function of the filament, the emission of electrons takes place.

A Field Emission Microscope consists of a metallic sample (tungsten) in the form of a sharp tip with diameter of about 100 nm.
The sample is held at a large negative potential (1-10 kV), which gives the electric field near the tip apex to be the order of 10¹⁰ V/m. This is high enough for field emission of electrons to take place. This is also called as a cold cathode field emitter.

Thermionic sources have the following issues: (1) relatively low brightness, (2) evaporation of cathode material due to heat, and (3) thermal drift during operation.

As a field emission gun emits the electrons from a much smaller area then a thermionic gun, field-emission cathode provides narrower probing beams at low as well as high electron energy.

The coherency is much higher and the energy spread is smaller. This further allows the perfectly focused beam.