Patent Number: 
Section: claims

1. A reflective optical element configured for an operating wavelength in at least one of the soft x-ray and extreme ultraviolet wavelength range, comprising:a multilayer system comprising:at least two alternating layers of respective materials having differing real parts of the index of refraction at the operating wavelength, andat least two additional layers of further material, the at least two additional layers adjoining each other, and situated adjoining at least one of the two alternating layers,wherein the two additional layers differ respectively at a transition from the layer having a lower real part of the index of refraction to the layer having a higher real part of the index of refraction and at a transition from the layer having a higher real part of the index of refraction to the layer having a lower real part of the index of refraction. 2. The reflective optical element according to claim 1, wherein a material of at least one of the two additional layers at a transition from the layer having the lower real part of the index of refraction to the layer having the higher real part of the index of refraction differs from a material of at least another of the two additional layers at a transition from the layer having the higher real part of the index of refraction to the layer having the lower real part of the index of refraction. 3. The reflective optical element according to claim 1, further comprising a protective coating being applied to the multilayer system, wherein a first additional layer of an additional material is situated at a transition between the multilayer system and the protective coating, which results in an increase of the maximum reflectivity of the transition between the multilayer system and the protective coating at the operating wavelength in comparison to the reflectivity without the additional layer, and a second additional layer of another additional material is situated at the transition between the multilayer system and the protective coating, which acts as a barrier between the adjoining layers. 4. The reflective optical element according to claim 1, wherein a thickness of the additional layers is less than a thickness of the alternating layers. 5. The reflective optical element according to claim 1, wherein a thickness of the additional layers is less than one fourth of the operating wavelength. 6. The reflective optical element according to claim 1, wherein a thickness of the additional layers is less than one eighth of the operating wavelength. 7. The reflective optical element according to claim 1, wherein the material having the lower real part of the index of refraction at the operating wavelength is molybdenum and the material having the higher real part of the index of refraction at the operating wavelength is silicon. 8. The reflective optical element according to claim 1, wherein the first additional layer comprises a material selected from the group consisting of ruthenium, rhodium, yttrium, and niobium. 9. The reflective optical element according to claim 1, wherein the second additional layer comprises boron carbide. 10. A projection system having at least one reflective optical element according to claim 1. 11. An illumination system having at least one reflective optical element according to claim 1. 12. An EUV lithography device having at least one reflective optical element according to claim 1. 13. A reflective optical element configured for an operating wavelength in at least one of the soft x-ray and extreme ultraviolet wavelength range, comprising:a multilayer system comprising:at least two alternating layers of respective materials having differing real parts of the index of refraction at the operating wavelength, andat least two additional layers of further material situated adjoining at least one of the two alternating layers,wherein the two additional layers differ respectively at a transition from the layer having a lower real part of the index of refraction to the layer having a higher real part of the index of refraction and at a transition from the layer having a higher real part of the index of refraction to the layer having a lower real part of the index of refraction; andwherein a sequence of the two additional layers at a transition from the layer having the lower real part of the index of refraction to the layer having the higher real part of the index of refraction differs from a sequence of the two additional layers at a transition from the layer having the higher real part of the index of refraction to the layer having the lower real part of the index of refraction. 14. A reflective optical element configured for an operating wavelength in at least one of the soft x-ray and extreme ultraviolet wavelength range, comprising:a multilayer system comprising:at least two alternating layers of respective materials having differing real parts of the index of refraction at the operating wavelength, andat least two additional layers of further material situated adjoining at least one of the two alternating layers,wherein the two additional layers differ respectively at a transition from the layer having a lower real part of the index of refraction to the layer having a higher real part of the index of refraction and at a transition from the layer having a higher real part of the index of refraction to the layer having a lower real part of the index of refraction; andwherein a first of the at least two additional layers of a further material is situated on at least one transition between the two alternating layers, which results in an increase of the maximum reflectivity of the multilayer system at the operating wavelength in comparison to the reflectivity without the additional layer, and a second of the at least two additional layers of another further material is situated on the at least one transition, which acts as a barrier between the adjoining layers. 15. A reflective optical element configured for an operating wavelength in at least one of the soft x-ray and extreme ultraviolet wavelength range, comprising:a multilayer system comprising:at least two alternating layers of respective materials having differing real parts of the index of refraction at the operating wavelength, andat least two additional layers of further respective materials situated adjoining at least one of the two alternating materials,wherein a first of the additional layers of a first of the further materials is situated on at least one transition between the two alternating layers, which results in an increase of the maximum reflectivity of the multilayer system at the operating wavelength in comparison to the reflectivity without the additional layer, and a second of the additional layers of a second of the further materials is situated on the at least one transition, which acts as a barrier between the adjoining layers. 16. The reflective optical element according to claim 15, wherein a composition of the first of the further materials of the first additional layer at a transition from the layer having the lower real part of the index of refraction to the layer having the higher real part of the index of refraction differs from a composition of the first of the further materials of the first additional layer at a transition from the layer having the higher real part of the index of refraction to the layer having the lower real part of the index of refraction. 17. The reflective optical element according to claim 15, further comprising a protective coating being applied to the multilayer system, wherein a first of the at least two additional layers of an additional material is situated at a transition between the multilayer system and the protective coating, which results in an increase of the maximum reflectivity of the multilayer system at the operating wavelength in comparison to the reflectivity without the additional layer, and a second of the at least two additional layer of another additional material is situated at the transition between the multilayer system and the protective coating, which acts as a barrier between the adjoining layers. 18. The reflective optical element according to claim 15, wherein the material of the second additional layer also results in a higher maximum reflectivity at the operating wavelength in comparison to the reflectivity without the additional layer. 19. The reflective optical element according to claim 15, wherein at least the material of the first additional layer has an index of refraction whose value of the real part of the index of refraction at the operating wavelength is either greater than the corresponding value of the alternating layer having the higher real part of the index of refraction or is lower than the corresponding value of the alternating layer having the lower real part of the index of refraction. 20. The reflective optical element according to claim 15, wherein at least the material of the first additional layer has an index of refraction whose value of the real part of the index of refraction at the operating wavelength is between the corresponding values of the alternating layers. 21. The reflective optical element according to claim 15, wherein a thickness of the additional layers is less than a thickness of the layers made of the alternating layers.