Patent Number: 
Section: claims

1. An optical system, comprising:a plurality of elements arranged to image radiation at a wavelength λ along a radiation path from an object field in an object surface to an image field in an image surface,wherein:the plurality of optical elements comprises mirror elements including first and second mirror elements;the first mirror element has a rotationally asymmetric reflective surface which deviates from a best-fit rotationally symmetric reflective surface by about λ or more at one or more locations;the second mirror element has a reflective surface comprising a reflective coating that is a non-rotationally symmetric, graded coating comprising a multilayer stack of different materials;a layer in the multilayer stack has a geometrical layer thickness which varies according to a first grading function in a first direction and which varies according to a second grading function in a second direction perpendicular to the first direction, the second grading function being different from the first grading function. 2. The optical system according to claim 1, wherein:the multilayer stack comprises a cap layer on a radiation entry side facing away from a substrate of the second mirror element; andthe cap layer has a geometrical layer thickness which varies according to a third grading function in a third direction and which varies according to a fourth grading function in a fourth direction perpendicular to the third direction, the fourth grading function being different from the third grading function. 3. The optical system according to claim 2, wherein the geometrical layer thickness of the cap layer increases from an origin towards an edge of the second mirror element with a first amount of increase in the third direction, and a second amount of increase between the origin and an edge region is significantly larger in the fourth direction. 4. The optical system according to claim 2, wherein the geometrical layer thickness of the cap layer is essentially uniform in a central zone around an origin at least up to radial coordinates corresponding to an outer edge of a region corresponding to a first sub-aperture corresponding to a central field point, and the geometrical layer thickness of the cap layer increases outside the central region slightly in the third direction and stronger in the fourth direction. 5. The optical system according to claim 2, wherein the cap layer comprises a material selected from a group consisting of ruthenium, aluminium oxide, silicon carbide, molybdenum carbide, carbon, titanium nitride, titanium dioxide, and a mixture, an alloy or a compound of ruthenium, aluminium oxide, titanium nitride and titanium dioxide. 6. The optical system according to claim 2, wherein the multilayer stack comprises a plurality of intermediate layers between the cap layer and the substrate, and each of the plurality of intermediate layers has a uniform layer thickness. 7. The optical system according to claim 2, wherein the multilayer stack comprises a plurality of intermediate layers arranged between the cap layer and the substrate, and the material of the cap layer has a specific absorbance for the radiation at wavelength λ which is greater than a specific absorbance of each of the materials of the intermediate layers. 8. The optical system according to claim 2, wherein the material of the cap layer has a specific absorbance for the radiation at wavelength λ which is greater than a specific absorbance of silicon and/or molybdenum. 9. The optical system according to claim 2, wherein the material of the cap layer has a specific absorbance characterized by an extinction coefficient k greater than 0.013 in a wavelength range between about 13 nm and 14 nm. 10. The optical system according to claim 2, further comprising a filter layer disposed on the cap layer on the radiation entry side, wherein the filter layer comprises a material configured to absorb radiation at the wavelength λ, and the filter layer has a geometrical thickness which varies spatially. 11. The optical system according to claim 10, wherein the filter layer comprises a material having a greater specific absorbance at wavelength λ than the material of the cap layer. 12. The optical system according to claim 10, wherein the filter layer comprises a material having a smaller specific absorbance at wavelength λ than the material of the cap layer. 13. The optical system according to claim 10, wherein the filter layer comprises a material selected from a group consisting of ruthenium, aluminium oxide, silicon carbide, molybdenum carbide, carbon, titanium nitride, titanium dioxide, and a mixture, an alloy or a compound of ruthenium, aluminium oxide, titanium nitride and titanium dioxide. 14. The optical system according to claim 1, wherein:the multilayer stack comprises a stack of bilayers;each bilayer comprises a layer of a first material having a first refractive index and a layer of a second material having a second refractive index which is lower than the first refractive index,for each bilayer, the layer of the first material is thicker than the layer of the second material;for each bilayer, a ratio between a geometrical thickness of the first layer and a geometrical thickness of the second layer varies according to third grading function in a third direction; andfor each bilayer, the ratio varies according to a fourth grading function, different from the third grading function, in a fourth direction perpendicular to the third direction. 15. The optical system according to claim 1, wherein the second mirror element is positioned optically remote from a pupil surface of the optical system at a position where condition P(M)<1 is satisfied, andP(M)=D(SA)/(D(SA)+D(CR)),where D(SA) is a diameter of a sub-aperture of a ray bundle originating from a field point in the object surface on a respective surface M, and D(CR) is a maximum distance of chief rays of an effective object field imaged by the optical system measured in a reference plane of the optical system on the surface M. 16. The optical system according to claim 15, wherein:the reference plane is a symmetry plane of the optical system;the condition P(M)<0.99 holds for a position of the second mirror element;the second mirror element is positioned in an intermediate region optically between a pupil surface and a field surface of the optical system at a position where condition 0.99>P(M)>0.95 is satisfied; and/orthe second mirror element is positioned optically near to a field surface where the condition 0<P(M)<0.93 is satisfied. 17. The optical system according to claim 1, wherein the second mirror element is positioned at or optically close to a pupil surface of the optical system where the condition 0.98<P(M)<1 is satisfied. 18. The optical system according to claim 1, wherein a third mirror element comprises a non-rotational symmetric coating which is a one-dimensionally graded coating comprising a multilayer stack of layers of different materials, the layers having a geometrical layer thickness which varies according to a third grading function in the first direction of the coating, and the layers having a geometrical layer thickness which is substantially constant in the second direction perpendicular to the first direction. 19. The optical system according to claim 18, wherein the third mirror is configured so that an average angle of incidence varies strongly according to a substantially linear function in the first direction, n and the average angle of incidence is substantially constant in the second direction perpendicular thereto. 20. The optical system according to claim 1, further comprising a third mirror having a reflective coating that is a non-rotationally symmetric, graded coating comprising a multilayer stack of different materials, whereina layer in the multilayer stack of the third mirror has a geometrical layer thickness which varies according to a third grading function in a third direction and which varies according to a fourth grading function in a fourth direction perpendicular to the third direction, the fourth grading function being different from the third grading function. 21. The optical system according to claim 1, wherein the second mirror element is configured to increase symmetry of a spatial intensity distribution present in an exit pupil of the optical system relative to symmetry of spatial intensity distribution that would be present in the exit pupil of the optical system in the absence of the second mirror element. 22. The optical system according to claim 1, wherein the second mirror element is effective to increase rotational symmetry of an intensity distribution present in an exit pupil of the optical system relative to rotational symmetry of spatial intensity distribution that would be present in the exit pupil of the optical system in the absence of the second mirror element. 23. The optical system according to claim 22, wherein the spatial intensity distribution present in the exit pupil of the optical system is characterized by an apodization parameter APO representing a normalized azimuthal variation of the intensity in an edge region of the exit pupil according to:APO=(IMAX−IMIN)/(IMAX+IMIN),where IMAX is the maximum value of the intensity in an edge region of the exit pupil, and IMIN is the minimum value of the intensity in the edge region of the exit pupil, andwherein the apodization parameter APO is decreased by at least 1% relative to the apodization parameter APO that would for the optical system in the absence of the second mirror element. 24. The optical system according to claim 1, wherein the second mirror element has a rotationally asymmetric reflective surface which deviates from a best-fit rotationally symmetric reflective surface by about λ or more at one or more locations. 25. A projection-exposure system, comprising:an illumination system; anda projection objective,wherein:the illumination system is configured to receive radiation from a primary light source and to illuminate a pattern in an object surface of the projection objective;the projection objective comprises the optical system according to claim 1; andthe projection-exposure system is a microlithography projection-exposure system. 26. A projection-exposure system according to claim 25, wherein the radiation is EUV radiation at a wavelength 13 nm<λ<14 nm. 27. A method, comprising:using a projection-exposure system to fabricate semiconductor devices or other types of micro devices, wherein the projection-exposure system comprises:an illumination system; anda projection objective, andwherein:the illumination system is configured to receive radiation from a primary light source and to illuminate a pattern in an object surface of the projection objective; andthe projection objective comprises the optical system according to claim 1.