Patent ID: 7453630

Claim:
A method to use an optical apparatus, comprising steps of: applying in advance a protective film of a metal oxide selected from the group consisting of Al 2 O 3 , MgO, TiO 2 , and ZrO 2 to an optical system, wherein a total thickness of all film applied to the optical system is 2 nm-10 nm, and said protective film suppresses the stripping off of a structural element from the surface of a base stock of said optical system or the oxidation of the surface of the base stock of said optical system, by the irradiation of vacuum ultraviolet light over time or the plasma exposure to the base stock, incorporating said optical system into a desired device that has vacuum ultraviolet light sources or plasma light sources which has higher photon energy than an absorption wavelength of the base stock of said optical system, and forming the protective film on the optical system, wherein the optical system comprises of mono-crystal fluoride material having the crystal axis (the c axis) along the direction of the light irradiation, and a perpendicular surface of said protective film is coated by Si02 or metal oxides.