Electron sources thoriated tungsten

Thoriated tungsten, whose surface has been carburized at high temperatures, have lower work functions than pure tungsten and emit equivalent electron beam currents at lower temperatures thereby extending the life and stability when compared to regular tungsten sources. These sources require a stable, high vacuum (10 to 10 torr) and are more difficult to fabricate. 

Electron sources used in EBL exposure tools are similar to those used in conventional electron microscopes. They can be divided into two main groups— thermionic or field emission—depending on the way in which they emit electrons. The sources that rely on the emission of electrons from a material that is heated to a temperature at which electrons are emitted from the surface are referred to as thermionic sources. These sources are fabricated from materials such as tungsten, thoriated tungsten, or lanthanum hexaboride (LaBe). ... 

When heated, some surfaces emit copious amounts of electrons (thermoelectron emission). Tungsten and thoriated tungsten are common examples but lanthanum hexaboride (LaB ) is an interesting material in that, at a temperature of 1700 C, it has an electron emission of >20 A/cm, which is much higher than that of tungsten at the same temperature. Hot surfaces of these materials are used as electron sources in some ion and plasma sources. 

---