Vacuum deposition processes are those most commonly used for the construction of optical coatings. Although the films that are produced are solid, there are many significant differences between them and bulk materials and especially in the microstructure of the films. Thermal evaporation, the most widely used process, produces an intensely columnar microstructure that includes a large proportion of pore-shaped voids that have far reaching effects on the properties of the films, optical, thermal, mechanical and chemical. The energetic processes that involve an element of ion bombardment during film growth have been shown to produce a much more compact microstructure, eliminating many of the problems associated with the columnar structure. However, the microstructure still differs in important respects from that of similar bulk material and still exerts an influence on thin film properties.
