Patent Number: 
Section: claims

1. A reflective optical element for the extreme ultraviolet and soft X-ray wavelength regions, comprising:a multilayer system,a protective layer system; anda contamination layer that changes in thickness over time during use of the reflective optical element by 2 nm or more,wherein the thickness over time of the contamination layer produces fluctuations in reflectivity of the reflective optical element within a tolerance range of no more than 1%. 2. The reflective optical element according to claim 1, wherein the given contamination layer thickness is approximately equal to or less than the thickness of the protective layer system. 3. The reflective optical element according to claim 1, wherein the protective layer system consists of two layers. 4. The reflective optical element according to claim 1, wherein the protective layer system consists of three layers. 5. The reflective optical element according to claim 1, wherein the protective layer system comprises one or more materials selected from the group consisting of Ce, Be, SiO, SiC, SiO2, Si3N4, C, Y, MoSi2, B, Y2O3, MoS2, B4C, BN, RuxSiy, Zr, Nb, MoC, ZrO2, RuxMoy, RhxMoy, and RhxSiy. 6. The reflective optical element according to claim 1, wherein a side of the multilayer system facing the protective layer system terminates in an absorber layer. 7. The reflective optical element according to claim 1, wherein the multilayer system is a system that consists of molybdenum and silicon layers, ending with a molybdenum layer on a side facing the protective layer system. 8. The reflective optical element according to claim 1, wherein the protective layer system terminates toward a vacuum with a layer of a material for which a build-up of carbon is suppressed. 9. The reflective optical element according to claim 1, wherein the protective layer system terminates toward a vacuum with a layer of a material that is inert to energy deposition. 10. The reflective optical element according to claim 1, wherein the thickness d1 of the protective layer system is such that a standing wave formed by reflection at an operating wavelength λB has a minimum at a distance from a surface of the reflective optical element of 0.1 λB or less. 11. The reflective optical element according to claim 10, wherein the minimum lies in a vacuum. 12. An extreme ultraviolet lithography appliance with at least one reflective optical element according to claim 1. 13. A reflective optical element for the extreme ultraviolet and soft X-ray wavelength regions, the reflective optical element comprisinga multilayer system, anda protective layer system, wherein:the protective layer system comprises at least two layers, one of the two layers terminating toward a vacuum; and a thickness of the protective layer system is such that a standing wave formed by reflection at an operating wavelength λB has a minimum at a distance from the surface of the reflective optical element of 0.1 λB or less, wherein the minimum lies in the vacuum. 14. The reflective optical element according to claim 13, wherein the protective layer system terminates at the vacuum side in one layer of a material with a refractive index whose real part at an operating wavelength λB between 12.5 nm and 15 nm is between 0.90 and 1.03, and whose imaginary part at an operating wavelength λB between 12.5 nm and 15 nm is between 0 and 0.025. 15. The reflective optical element according to claim 14, wherein the imaginary part is between 0 and 0.015 and the real part is between 0.95 and 1.02. 16. The reflective optical element according to claim 13, wherein a reflectivity of the reflective optical element as a function of a thickness of the layer terminating the protective system at the vacuum side drops until a first thickness value d1 is reached, the reflectivity remains essentially constant between the first thickness value d1 and a second thickness value d2, where d2>d1, and the reflectivity drops at thicknesses greater than d2. 17. The reflective optical element according to claim 16, wherein the essentially constant reflectivity comprises a reflectivity decrease of 1% or less of a maximum reflectivity at thicknesses between d1 and d2. 18. The reflective optical element according to claim 16, wherein the reflectivity as a function of the thickness of the protective layer system comprises an inflection point at a thickness dw between d1 and d2, the thickness of the protective layer system is less than dw. 19. The reflective optical element according to claim 16, wherein the thickness of the protective layer system is smaller than d2. 20. The reflective optical element according to claim 13, wherein the protective layer system comprises one or more materials selected from the group consisting of Ce, Be, SiO, SiC, SiO2, Si3N4, C, Y, MoSi2, B, Y2O3, MoS2, B4C, BN, RuxSiy, Zr, Nb, MoC, ZrO2, RuxMoy, RhxMoy, and RhxSiy. 21. The reflective optical element according to claim 13, wherein a side of the multilayer system facing the protective layer system terminates in an absorber layer. 22. The reflective optical element according to claim 13, wherein the multilayer system is a system that consists of molybdenum and silicon layers, ending with a molybdenum layer on a side facing the protective layer system. 23. The reflective optical element according to claim 13, wherein the protective layer system terminates toward a vacuum with a layer of a material for which a build-up of carbon is suppressed. 24. The reflective optical element according to claim 13, wherein the protective layer system terminates toward a vacuum with a layer of a material that is inert to energy deposition. 25. An extreme ultraviolet lithography appliance with at least one reflective optical element according to claim 13.