Antireflective bilayer coatings based on Al₂O₃ film for UV region

Bilayer antireflective coatings consisting of aluminium oxide Al₂O₃/MgF₂ and Al₂O₃/SiO₂ are presented in this paper. Oxide films were deposited by means of e-gun evaporation in vacuum of 5 × 10^-3 Pa in the presence of oxygen, and magnesium fluoride was prepared by thermal evaporation on heated optical lenses made from quartz glass (Corning HPFS). Substrate temperature was maintained at 250 _C during the deposition. Thickness and deposition rate were controlled with a thickness measuring system Inficon XTC/2. The experimental results of the optical measurements carried out during and after the deposition process have been presented. Physical thickness measurements were made during the deposition process and resulted in 44 nm/52 nm for Al₂O₃/MgF₂ and 44 nm/50 nm for Al₂O₃/SiO₂ system. Optimization was carried out for ultraviolet region with minimum of reflectance at 300 nm. The influence of post deposition annealing on the crystal structure was determined by X-ray measurements. In the range from ultraviolet to the beginning of visible region, the reflectance of both systems decreased and reached minimum at 290 nm. The value of reflectance at this point, for the coating Al₂O₃/MgF₂ was equal to R_290nm = 0.6 % and for Al₂O₃/SiO₂R_290nm = 1.1 %. Despite the difference between these values both are sufficient for applications in the UV optical systems for medicine and UV laser technology.