Ion mobility is improved by both sufficiently heating the source material, and by properly heating the glass substrate. Hard materials with high melting points, such as dielectric coatings, will require much more evaporative energy. Preheating the glass substrate is also mandatory. Good ion mobility is of supreme importance for hard dielectric coatings. For similar reasons, the coating thickness plays a part in the film roughness.
Conglomerate growth is almost a direct function of film thickness.
The simplest, and oldest, process for applying thin film coatings is with thermal evaporation. These chambers use a small container, called a boat , to hold the source material. The boat is usually made of tungsten metal. An electric current is passed through the boat, much like a light bulb filament, and it gets hot enough to heat the source to the point of ion evaporation into the chamber. Thermal evaporation is still an excellent way to apply metal coatings, and other low melting point materials.