Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al

Thin films of MgF₂ deposited by evaporation is a material widely used for its transparency in the far ultraviolet (FUV) down to ∼115 nm. In this paper the optical properties of ion-beam sputtered (IBS) MgF₂ in the FUV are investigated and compared with MgF₂ films deposited by evaporation. A slightly higher transparency at the 121.6-nm, H Lyman α line was obtained for IBS MgF₂ films compared to films deposited by evaporation, which makes IBS MgF₂ a promising protective material for Al reflective coatings. Experimental work on Al films that were protected with a thin evaporated MgF₂ film followed by an IBS MgF₂ film to produce a protective coating with optimum thickness showed a reflectance at 121.6 nm that was higher by about 3% compared to an Al film protected with an all-evaporated MgF₂ film.