Optical properties restoration apparatus, the restoration method, and an optical system used in the apparatus

摘要

An optical system has fluoride compounds provided in an environment exposed by vacuum ultraviolet light or plasma light, which has higher photon energy than an absorption wavelength of a base stock of the optical system. 1-layer of a protective film of SiO₂ or metal oxides having a film thickness of 2-20 nm is formed at least on the light irradiation side (inner side) of the optical system to prevent the stripping of the fluorine atoms from the surface of the optical system. In addition, the protective film is a 1-layer film selected from one of SiO₂, MgO, TiO₂, or ZrO₂.