Patent Number: 
Section: claims

1. An illumination system configured to condition a radiation beam, comprisinga mirror configured for grazing or normal incidence reflection comprising a stack of a plurality of alternating layers including a layer made of metal non-metal compound adjacent a reflection surface of the mirror with a non-metal layer between a substantially pure metal layer and the layer made of metal non-metal compound anda hydrogen radical source configured to supply gas containing hydrogen or hydrogen radicals to the reflection surface,wherein the layer made of metal non-metal compound to protect the substantially pure metal layer from the hydrogen or hydrogen radicals. 2. An illumination system according to claim 1, wherein the metal non-metal compound consists of a transition metal carbide, nitride, boride or silicide compound or mixture thereof. 3. An illumination system according to claim 1, wherein the metal non-metal compound is a metal silicon compound. 4. An illumination system according to claim 3, wherein the metal and silicon are present in the metal non-metal compound in a stoichiometric ratio. 5. An illumination system according to claim 1, wherein the metal in the metal non-metal compound is selected from the group of Mo and Ru. 6. An illumination system according to claim 5, wherein the metal non-metal compound is selected from the group consisting of Mo and Si in a ratio of two-to-one and Ru and Si in a ratio of two-to-three. 7. An illumination system according to claim 1, wherein the radiation beam is an EUV beam. 8. An illumination system according to claim 1, wherein the layer made of the metal non-metal compound being provided between said stack and the reflection surface. 9. An illumination system according to claim 8, wherein the stack comprises a plurality of substantially pure metal layers, alternating with non-metal layers. 10. An illumination system according to claim 8, wherein the stack of layers comprises only one layer of the metal compound. 11. An illumination system according to claim 1, wherein the layer made of the metal non-metal compound has a thickness sufficient to provide substantially all reflectivity of the mirror at an incidence angle at which the radiation beam is incident on the reflection surface. 12. An illumination system according to claim 1, wherein the layer made of the metal non-metal compound is provided between the reflection surface and a layer made of metal only and having a thickness sufficient to provide all substantially reflectivity of the mirror at an incidence angle at which the radiation beam is incident on the reflection surface. 13. A photolithographic apparatus comprisingan illumination system configured to condition a radiation beam and a hydrogen radical source configured to supply gas containing hydrogen or hydrogen radicals into the illumination system, the illumination system comprisinga mirror comprising one or more layers made of metal non-metal compound adjacent a reflection surface of the mirror with a non-metal layer between a substantially pure metal layer and the layer of metal non-metal compound, wherein the mirror includes one or more substantially pure metal layers such that a ratio between a number of the layers made of metal non-metal compound and a number of the metal layers is less than about 50%. 14. A photolithographic apparatus according to claim 13, wherein the metal non-metal compound consists of a transition metal carbide, nitride, boride or silicide compound or mixture thereof. 15. A photolithographic apparatus according to claim 13, wherein the metal non-metal compound is a metal silicon compound. 16. A photolithographic apparatus according to claim 15, wherein the metal and silicon are present in the metal non-metal compound in a stoichiometric ratio. 17. A photolithographic apparatus according to claim 13, wherein the metal in the metal non-metal compound is selected from the group of Mo and Ru. 18. A photolithographic apparatus according to claim 17, wherein the metal non-metal compound is selected from the group consisting of Mo and Si in a ratio of two to one and Ru and Si in a ratio of two to three. 19. A photolithographic apparatus according to claim 13, wherein the mirror comprises a stack of a plurality of alternating layers adjacent the reflection surface of the mirror, the one or more layers made of the metal non-metal compound being provided between said stack and the reflection surface. 20. A photolithographic apparatus according to claim 19, wherein the stack comprises a plurality of substantially pure metal layers, alternating with non-metal layers. 21. A photolithographic apparatus according to claim 19, wherein the stack of layers comprises only one layer of the metal compound. 22. A photolithographic apparatus according to claim 13, wherein a layer made of the metal non-metal compound of the one or more layers made of the metal non-metal compound has a thickness sufficient to provide substantially all reflectivity of the mirror at an incidence angle at which the radiation beam is incident on the reflection surface. 23. A photolithographic apparatus according to claim 13, wherein a layer made of the metal non-metal compound of the one or more layers made of the metal non-metal compound is provided between the reflection surface and a layer made of metal only and having a thickness sufficient to provide substantially all reflectivity of the mirror at an incidence angle at which the radiation beam is incident on the reflection surface. 24. A photolithographic apparatus according to claim 13, comprising a radiation source configured to emit radiation to the mirror. 25. A method of removing contamination from a mirror with a reflecting metal containing layer, the method comprisingsupplying hydrogen radicals to a reflection surface of the mirror andprotecting the mirror against damage due to the supply of the hydrogen radicals by using one or more layers made of a metal non-metal compound in the mirror adjacent the reflection surface,wherein the mirror includes one or more substantially pure metal layers with a non-metal layer between a substantially pure metal layer and the layer of metal non-metal compound, such that a ratio between a number of the layers made of metal non-metal compound and a number of the metal layers is less than about 50%. 26. A device manufacturing method comprisingprojecting a patterned beam of radiation onto a substrate, wherein the radiation that goes into the beam is reflected by a mirror, the method comprising a step ofremoving contamination from a mirror using a supply of a gas containing hydrogen radicals to a reflection surface of the mirror,wherein the mirror comprises one or more layers made of a metal non-metal compound adjacent the reflection surface,wherein the mirror includes one ore more substantially pure metal layers with a non-metal layer between a substantially pure metal layer and the layer of metal non-metal compound, such that a ratio between a number of the layers made of metal non-metal compound and a number of the metal layers is less than about 50%.