Patent Number: 
Section: claims

1. A reflective optical element for an extreme ultraviolet (EUV) wavelength range, comprising:a substrate having a surface, anda multilayer system that extends over the surface of the substrate,wherein the multilayer system comprises at least two alternating layerscomposed of at least two different materials with mutually differing real parts of the refractive index at a wavelength in the EUV wavelength range,wherein a given one layer of one of the at least two materials and a further layer or layers arranged between the given one layer and another layer, which is closest at an increasing distance from the substrate, of a same material as the material of the given one layer form a stack,wherein, in the stack, the material of the layer with the lower real part and/or the material of the layer with the higher real part of the refractive index is a combination of at least two substances,wherein respective proportions of the at least two substances in the combination differ between at least one partial surface of the multilayer system and a remaining surface of the multilayer system. 2. The reflective optical element as claimed in claim 1, wherein the partial surface and the remaining surface form an entire surface of the multilayer system, wherein the reflective optical element is configured for different angles of incidence of EUV radiation over the entire surface of the multilayer system, and wherein the proportions of the at least two substances vary in dependence on the different angles of incidence. 3. The reflective optical element as claimed in claim 1, wherein the proportions of the at least two substances vary continuously. 4. The reflective optical element as claimed in claim 1, wherein in the stack, the material of the layer with the lower or the higher real part of the refractive index is a combination of exactly two substances. 5. The reflective optical element as claimed in claim 1, wherein, in the stack, the layer with the lower or the higher real part of the refractive index is constructed from sub-layers from the at least two substances, and wherein respective thicknesses of the sub-layers over the at least one partial surface are different than are sub-layer thicknesses over the remaining surface of the multilayer system. 6. The reflective optical element as claimed in claim 1, wherein, in the stack, the material of the layer with the lower or the higher real part of the refractive index has a mixture ratio of the at least two substances which is different over the at least one partial surface than is a mixture ratio of the at least two substances over the remaining surface of the multilayer system. 7. The reflective optical element as claimed in claim 1, wherein a total thickness and/or a ratio of a thickness of the layer with the lower real part of the refractive index to a total thickness of the stack over the at least one partial surface differs from a total thickness of the stack over the remaining surface of the multilayer system. 8. The reflective optical element as claimed in claim 1, wherein the material of at least one of the alternating layers varies in density. 9. The reflective optical element as claimed in claim 1, wherein the material with the higher real part of the refractive index is silicon, the material with the lower real part of the refractive index is molybdenum, and the at least two substances are selected from the group consisting of molybdenum, ruthenium, niobium, scandium, titanium, carbon, and carbide. 10. The reflective optical element as claimed in claim 1, wherein the material with the higher real part of the refractive index is silicon, the material with the lower real part of the refractive index is ruthenium, and the at least two substances are selected from the group consisting of silicon, boron carbide, beryllium, boron, and carbon. 11. An optical system for EUV lithography having a reflective optical element as claimed in claim 1. 12. An EUV lithography apparatus having a reflective optical element as claimed in claim 1. 13. The reflective optical element as claimed in claim 1, further comprising further stacks, wherein, in each of the further stacks, a material of a layer with a lower real part and/or a material of a layer with a higher real part of the refractive index is a combination of at least two further substances, andwherein respective proportions of the at least two further substances in the combination of the further substances differ between at least one further partial surface of each of the further stacks and at least one remaining surface of each of the further stacks. 14. The optical system as claimed in claim 13, configured for receiving extreme ultraviolet radiation incident on the reflective optical element with an angle of incidence that varies over a total surface of the multilayer system, wherein the proportions of the at least two substances vary in accordance with the angle of incidence. 15. An EUV lithography apparatus having an optical system as claimed in claim 13.