The electron gun
The electron gun refers to the top region of the SEM that generates a beam of electrons. The simplest and cheapest gun uses a heated tungsten wire to produce electrons. Other more expensive types use crystals (lanthanum hexaboride: LaB6; or tungsten) and are either heated or a large electrical potential is used to pull the electrons out of them, cold. The gun is made up of a number of components.
In the diagram the filament (also called the emitter) is surrounded by the Wehnelt cylinder that closes over the filament assembly and has a small hole in the centre through which electrons exit. The electrode pins run to the filament through an insulator disc, and carry the current flow to the filament. An actual assembly, minus Wehnelt cap, is shown in the photograph. The schematic (a-c) shows three filament types: a) a tungsten (W) wire, b) a lanthanum hexaboride crystal assembly: LaB6, and c) a tungsten crystal (for field emission guns: FEGs). The tip of a tungsten wire hairpin filament is about 10µm in diameter whereas the tungsten crystal is sharpened to a much narrower tip.
Below the cap sits an anode, which, being positive, attracts the electrons away from the filament. If the filament is broken, the beam current will not increase on the SEM because no electrons can be produced (see SEM Operation).
The electron gun is used to provide a large, stable current in a small electron beam.
The electron gun produces a source of electrons (comprised of free electrons i.e. detached from the atom) and accelerates these electrons in an energy range typically 1-40kV. The conventional electron gun (triode) has three components, a hot wire (called the filament or cathode [- ve] or electron emitter), a Wehnelt (grid) cap [-ve], and an anode [+ ve]. In a thermal emission (thermionic) filament the tungsten filament is heated white hot by a filament current. This results in the emitting of thermal electrons. The emitted electrons are those that have overcome the work function energy of the material.
The hole in the anode allows a fraction of the electrons to continue down the column through the lenses to produce a smaller, more cohesive beam. Electrons that strike the anode are returned to the high voltage power supply via ground. The portion of the beam that leaves the anode through the hole is termed the beam current.
Two important parameters for any electron gun are the amount of current produced and the current stability. At the saturation point the beam is most stable.
A constant beam current is required to create a good quality image because all image information is recorded as a function of time. The electron micrograph is a scanned image on intensity values, projected as a function of the position upon the specimen. Since the micrograph is acquired over a period of time of minutes (at the slow scan rate used for photographs of high quality), any changes in the filament emission will affect the image intensity at that point in the scan. This will produce a poor quality image because the brightness will vary across the image. A constant beam current is dependent on saturating the filament properly.