| 摘要 |
PROBLEM TO BE SOLVED: To prevent the alteration of a spectroscopic property with lapse of time, even when an apparatus is left standing or kept in the air by forming a hydrophobic protective film with a specified film thickness on the uppermost layer of a substrate forming an optical thin film for a specified wavelength region. SOLUTION: An antirefbction film, reflecting film and the like comprising a low refractive index layer and a high refractive index layer are formed on a substrate 3 made of quartz glass or the like, as an optical thin film for light within a wavelength region from 190 nm to 250 nm, and a hydrophobic protective film 1 with 1-10 nm film thickness is further formed on the substrate 1. The protection film 1 has high durability to laser and is made of a low refractive index fluororesin or the like and prevents the penetration of humidity in the atmospheric air due to the hydrophobic property. The absorption loss is lowered by controlling the film thickness of the fluororesin, having light absorptive property in an ultraviolet-ray region of 250 nm or shorter wavelength, to be 1-10 nm. Tetrafluoroethylene resin, trifluorochloromethylene resin, vinyl fluoride resin, tetrafluoroethylene- hexafluoropropylene copolymer, vinylidene fluoride resin, or polyacetal resin is preferably used as the fluororesin. |
