The Electron Gun (under construction)

The electron microscope generates images by bombarding a sample with electrons and then transforming the results of that interaction into a viewable image. For the image to form, the electrons that are generated need to be tightly controlled.

Types of electron guns

Figure 1. A LaB6 filament. (Larger view)
Heated tungsten
A heated filament made from the metal tungsten. Much in the way that an incandescent lightbulb works, the high voltage that is fed through the filament causes electrons to be kicked off the filament. The amount of energy required is known as the work function.
Lanthanum hexaboride (LaB6)
The LaB6 filament is also a thermal filament. However, its work function is lower than for a tungsten filament, so it is more efficient.
Tungsten field emission gun (FEG)
The FEG gun is not a thermal filament. Instead, electrons are expelled by applying a very powerful electric field very close to the filament tip. The size and proximity of the electric field to the electron reservoir in the filament causes the electrons to tunnel out of the reservoir.


Temporal coherence

Because the emitted electrons in the various types of guns are heated, their energy distribution is not a sharp peak. Instead, they have a Boltzmann distribution that can vary widely depending on the type of filament. For a good microscope, you want ∆E/E to be as small as possible, that is, the energy distribution to be a small fraction of the average energy of the electrons. Following is a list of the average energy distributions:

Distribution (∆E)
Heated tungsten2.5 eV
LaB61.5 eV
Warm FEG1.0 eV
Cold FEG0.25 eV

It is clear that the FEG based microscopes have an advantage over the other types. For this reason, they are most often used for high resolution imaging. Ideally, cold FEGs are the best. However, they suffer from rapid degradation and are not economical.

Spatial coherence

Spatial coherence basically means beam brightness. If the electrons are all emmitted as close to parallel as possible, then the beam will remain together over a long distance. This effect translates to greater brightness. The phase of the electrons also affects coherence. Ideally, the emitted electrons should all be in phase. If they are in phase, the electron waves interact constructively. If the phases are not aligned, then destructive interference will occur, and the beam will be less bright.


Principles of Image Formation