Patent Number: 
Section: claims

1. An optical element, comprising:a substrate;a multilayer film supported on the substrate, the multilayer film reflecting exposure light, the exposure light comprising at least one of extreme ultraviolet radiation and soft X-radiation from source light in a prescribed wavelength region; anda protective layer provided on the multilayer film, the protective layer having a composition preventing oxidation of the multilayer film, the protective layer having an absorption index relative to non-exposure light, other than the exposure light, that is larger than the absorption index of the protective layer relative to the exposure light. 2. The optical element according to claim 1, wherein the non-exposure light comprises at least one of ultraviolet radiation, visible light, and infrared light. 3. The optical element according to claim 1, wherein a material composition of the protective layer changes along a depthwise direction. 4. The optical element according to claim 1, wherein the protective layer has a surface-side layer on its outermost surface side, the surface-side layer being in a state where it is saturated by oxidation, the protective layer also having an interfacial-side layer on its boundary face side adjoining the multilayer film, the boundary face side being in a state where it is unsaturated by oxidation. 5. The optical element according to claim 4, wherein the protective layer has a composition that prevents oxidation of the multilayer film by the surface-side layer. 6. The optical element according to claim 4, wherein the protective layer exhibits relative absorption of the non-exposure light from the source light by the interfacial-side layer. 7. The optical element according to claim 4, wherein the surface-side layer is formed with SiO2, and the interfacial-side layer is formed with SiO. 8. The optical element according to claim 4, wherein the surface-side layer is formed with TiO2, and the interfacial-side layer is formed with TiO. 9. The optical element according to claim 4, wherein the surface-side layer is formed with ZrO2, and the interfacial-side layer is formed with ZrO. 10. The optical element according to claim 4, wherein the protective layer comprises an intermediate layer between the surface-side layer and the interfacial-side layer. 11. The optical element according to claim 10, wherein the intermediate layer comprises at least one of Si, SiO, SiO1.5, and SiO2. 12. The optical element according to claim 1, wherein the multilayer film is formed by alternately laminating on top of the substrate a first layer that is composed of material with a small refractive index difference relative to the refractive index of a vacuum in the extreme ultraviolet ray region, and a second layer that is composed of material with a large refractive index difference. 13. The optical element according to claim 1, wherein the protective layer is situated on top of the first layer that is the topmost layer of the multilayer film. 14. An exposure apparatus, comprising:a light source that generates extreme ultraviolet radiation;an illumination optical system that guides the extreme ultraviolet radiation from the light source to a mask used for pattern transfer; anda projection optical system that forms a pattern image of the mask on a sensitive substrate, whereinat least one of the mask, the illumination optical system, and the projection optical system comprises the optical element according to claim 1. 15. A device manufacturing method comprising a pattern-exposure step performed using the exposure apparatus according to claim 14.