Key components are required for a scanning electron microscope to operate, including:
Electrons are produced at the source by thermionic heating. These electrons are then accelerated to a voltage between 1-40 kV and condensed into a narrow beam which is used for imaging and analysis.
There are 3 commonly used types of electrons sources:
- Tungsten filament
- Solid state crystal (CeB6 or LaB6)
- Field emission gun (FEG)
Tungsten (W) electron filament
This consists of an inverted V-shaped wire of tungsten, about 100 µm long, which is heated resistively to produce electrons. This is the most basic type of electron source.
SEM image of a typical tungsten filament
Lanthanum hexaboride (LaB6) or Cerium hexaboride (CeB6)
This is a thermionic emission gun. It is the most common high-brightness source. This solid state crystal source offers about 5-10 times the brightness and a much longer lifetime than tungsten.
SEM image of a typical solid state crystal electron source
Note: The Phenom scanning electron microscope uses a CeB6 (trademarked as CeBix™) electron source. It is the only SEM in its price range that does not use a tungsten filament.
Learn more and see image comparisons on the Phenom World blog: Tungsten vs. CeB6 electron source
Field emission gun (FEG)
This is a wire of tungsten with a very sharp tip, less than 100 nm, that uses field electron emission to produce the electron beam. The small tip radius improves emission and focusing ability.
SEM image of a field emission gun electron source
A series of condenser lenses focus the electron beam as it moves from the source down the column. The narrower the beam the smaller the spot it will have when contacting the surface, thus the term ‘spot size’.
After the beam is focused, scanning coils are used to deflect the beam in the X and Y axes so that it scans in a raster fashion over the surface of the sample.
Samples are mounted and placed into a chamber that is evacuated. The sample chamber can include a translation stage, tilt and rotation devices, feed-throughs to the outside, temperature stages, optical cameras, and a variety of other devices to assist in imaging the sample.
The detectors for SEM collect the electrons coming off of the sample. Two types of electrons are typically used for imaging: secondary electrons (SE) and backscattered electrons (BSE).
Secondary electrons are low energy electrons produced when electrons are ejected from the k-orbitals of the sample atoms by the imaging beam. The most popular detector in SEMs is the Everhart-Thornley detector. It consists of a Faraday cage which accelerates the electrons towards a scintillator. This in turn produces a current which is directed towards a photomultiplier and the amplified signal is read on the monitor.
Backscattered electrons are higher energy electrons that are elastically backscattered by the atoms of the sample. Atoms with higher atomic numbers backscatter more efficiently and therefore this detector can give compositional information about the sample. These detectors can either be scintillators or semiconductors. An advantage of having a semiconductor detector is that it can be split into sections can be switched on or off to control the contrast and directionality, which gives valuable topographical information about the sample as well.