The thermal evaporation of materials in vacuum is a versatile and popular method to provide a thin film coating on a substrate. In evaporation a material is heated in vacuum until it boils, the resulting vapour then condenses on the substrate to form a thin film. This film can be from a few atoms thick (less than 1nm) to hundreds or thousands of nm thick. Thermal evaporation is carried out at high vacuum, typically better than 1x10-6 mbar.
For materials with higher melting points which cannot be easily evaporated by resistive heating the e-beam evaporation processes are used. Resistance evaporation systems pass a high electrical current through a filament or a ‘boat’ source in which the material is placed, the heating effect of which is sufficient to evaporate many materials. Electron beam sources use a dedicated power supply to provide a high-power electron beam to heat the source material, and are capable of evaporating higher melting point or reactive materials.
Further specialized sources with closed-loop temperature control are available for applications such as the evaporation of organic materials for OLED displays.