Patent Number: 
Section: claims

1. A multilayer mirror comprising:a layer of a first material;a layer of silicon;the layer of the first material and the layer of silicon forming a stack of layers;wherein only an exposed region of the layer of silicon comprises a modification that is arranged to improve robustness of the exposed region of the layer of the silicon. 2. The multilayer mirror of claim 1, wherein the modification is arranged to improve the robustness of the exposed region of the layer of the silicon by at least one of:reducing a reactivity of the exposed region of the layer of silicon with hydrogen or atomic hydrogen; andimproving a sputtering resistance of the exposed region of the layer of silicon. 3. The multilayer mirror of claim 2, wherein the reactivity of the exposed region of the layer of silicon with hydrogen or atomic hydrogen is reduced by at least one of:one or more implanted materials provided on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; anda passivation layer covering, or forming part of, the exposed region of the layer of silicon. 4. The multilayer mirror of claim 2, wherein the sputtering resistance of the exposed region of the layer of silicon is improved by at least one of:one or more implanted materials provided on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; anda passivation layer covering, or forming part of, the exposed region of the layer of silicon. 5. The multilayer mirror of claim 1, wherein the modification comprises at least one of:one or more implanted materials comprising at least one of boron, nitrogen and nitride, provided on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; anda passivation layer covering, or forming part of the exposed region of the layer of silicon. 6. The multilayer mirror of claim 5, wherein the passivation layer comprises one or more of a nitride layer, a silicon nitride layer, a boron glass layer, or a silicon oxynitride layer. 7. The multilayer mirror of claim 1, wherein the exposed region of the layer of silicon is a peripheral region of the layer of silicon or a sidewall of the layer of silicon. 8. The multilayer mirror of claim 1, wherein:the multilayer mirror comprises at least one of a plurality of layers of the first material and a plurality of layers of silicon, such that:the plurality of layers of silicon are separated by a layer of the first material, orthe plurality of layers of the first material are separated by a layer of silicon. 9. The multilayer mirror of claim 1, wherein a plurality of exposed regions of the layer or layers of silicon comprises the modification. 10. The multilayer mirror of claim 1, wherein a majority or substantially all of the exposed region or regions of the layer or layers of silicon comprise the modification. 11. A lithographic apparatus comprising:an illumination system configured to generated a beam of radiation;a patterning device configured to modulate the beam;a projection system configured to direct the modulated beam onto a target portion of a substrate; andmultilayer mirror within one of the illumination or projection system, the multilayer mirror comprising:a layer of a first material;a layer of silicon;the layer of the first material and the layer of silicon forming a stack of layers;wherein only an exposed region of the layer of silicon comprises a modification that is arranged to improve robustness of the exposed region of the layer of the silicon. 12. A method comprising:forming a stack of layers including a layer of a first material and a layer of silicon; andmodifying only an exposed region of the layer of silicon to improve robustness of the exposed region of silicon. 13. The method of claim 12, wherein the modifying comprises:reducing a reactivity of the exposed region of the layer of silicon with hydrogen or atomic hydrogen; or improving a sputtering resistance of the exposed region of the layer of silicon. 14. The method of claim 13, wherein the reactivity of the exposed region of the layer of silicon with hydrogen or atomic hydrogen is reduced by at least one of:implanting one or more material on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; andproviding a passivation layer that covers, or forms part of, the exposed region of the layer of silicon. 15. The multilayer mirror of claim 13, wherein the sputtering resistance of the exposed region of the layer of silicon is improved by at least one of:implanting one or more material on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; andproviding a passivation layer that covers, or forms part of, the exposed region of the layer of silicon. 16. The method of claim 12, wherein modifying comprises at least one of:implanting one or more material on a surface of the exposed region of the layer of silicon, or within the exposed region of the layer of silicon; andproviding a passivation layer that covers, or forms part of, the exposed region of the layer of silicon.