The source
At the top of our SEM is the electron source. This is where the beam of electrons is first generated before being directed down the electron column towards the sample. Many SEM instruments you encounter will be designated by the type of source that they have. This is because the source determines both the ultimate performance of an SEM, and also the requirements of that SEM. No matter the type of source though, the electron beam is generated in largely the same way.
There are two groups of electron sources in SEM: thermionic emitters, and field emitters. Thermionic emitters rely on heating a source up to a high temperature, which promotes the spontaneous emission of electrons from the source material. These will commonly be tungsten hairpin filament sources, or hexaboride crystal emitters. Field emitters use a very intense electric field to pull the electrons directly from the source material.
Types of electron sources
The tungsten filament is a thermionic emitter. That is, electron emission is stimulated by heating the filament with electric current. It is the simplest of the electron sources. It is affordable, robust, easy to exchange and align, and amenable to working in variable pressure environments.
However it has the lowest brightness of all the sources, does not achieve as high an ultimate resolution, and has a relatively short lifetime.
Hexaboride crystals, such as LaB6 and CeB6, are also thermionic emitters, like tungsten. But these sources are brighter, meaning the electron beam current is greater than tungsten, and they can achieve better resolution as the region where the electrons are emitted from is smaller.
But increased performance comes with increased requirements, and these sources need better vacuum conditions maintained around the source region. It means they also have longer lifetimes, but at higher cost, so tend to be found on the more expensive instruments.
The field-emission source is the highest performance source for an SEM. Instruments that utilise these are often specifically marketed as FE-SEM to designate this performance. They are the brightest source, maintain longest lifetime, and give the best resolution.
But they require the greatest vacuum conditions, often high-vacuum in the source at all times, and are generally found only on high-end SEM instruments.
Our electron source sits inside a cylinder called a Wehnelt cap, and this sits above the anode. The Wehnelt is held at a negative voltage, while the anode is held at a positive voltage. These two components work together to direct the electron beam emitted from the source down into the optics of the electron column. It is the strength of the voltage applied to the anode that determines your electron beam energy.
On the FlexSEM this is presented as Vacc, or accelerating voltage, and the energy of your beam will be in electron-volts, or eV. For example, a 15 kV accelerating voltage gives you a 15 keV energy electron beam. This becomes important later when we discuss electron-surface interactions.