| 摘要 |
PROBLEM TO BE SOLVED: To obtain a thin fluoride film that ensures a large refractive index difference and high optical performance even in the UV region by selectively removing only silicon dioxide from a mixed thin film comprising silicon dioxide and a fluoride formed on a substrate. SOLUTION: The inside of a vacuum deposition vessel 9 is kept under a prescribed pressure or below and silicon dioxide from an electron gun evaporation source 1 and a fluoride from a resistance heating evaporation source 2 are deposited on a heated substrate 6 while controlling the evaporation rates and film thickness with quartz oscillators 3, 4 to form a mixed thin film. The mixing ratio is freely controlled because the evaporation rates are controlled. The fluoride is one or more selected from aluminum fluoride, barium fluoride, etc. The substrate 6 is preferably a fluoride substrate. When a substrate other than the fluoride substrate is used, it is protected, gaseous hydrogen fluoride or gaseous fluorine is allowed to act on the mixed thin film and only the silicon dioxide is selectively leached out. The refractive index of the resulting thin fluoride film can be arbitrarily adjusted by varying the mixing ratio between the silicon dioxide and the fluoride.
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