Optical System, In Particular Objective Or Illumination System For A Microlithographic Projection Exposure Apparatus

1 . An optical system having an optical axis, the optical system comprising:

at least two lens groups with lenses made of intrinsically birefringent material and being arranged in the optical system in succession and in mutually adjacent relationship along the optical axis,

wherein:

the lens groups respectively comprise a first subgroup with lenses in a (100)-orientation and a second subgroup with lenses in (111)-orientation,

the lenses of each subgroup are arranged rotated relative to each other about their lens axes,

the (100)-lenses and the (111)-lenses of each lens group are arranged in alternate relationship, and

the optical system is a microlithographic optical system.

2 . An optical system according to claim 1 , wherein the lenses of each subgroup are rotated relative to each other about their lens axes in such a way that each subgroup has an azimuthally symmetrical distribution of the retardation for two mutually perpendicular polarisation states.

3 . An optical system according to claim 1 , wherein the lenses of each subgroup are rotated relative to each other about their lens axes in such a way that each subgroup has substantially reduced values of the retardation in comparison with a non-rotated arrangement of the lenses.

4 . An optical system according to claim 1 , wherein the first subgroup has two (100)-lenses which are arranged rotated relative to each other about their lens axes through 45°+k*90° and the second subgroup has two (111)-lenses arranged rotated relative to each other about their lens axes through 60°+1*120°, wherein k and l are integers.

5 . (canceled)

6 . An optical system according to claim 1 , wherein the lenses of one of the subgroups are arranged rotated about their lens axes relative to the lenses of another of the lens groups.

7 . An optical system according to claim 1 , wherein lenses of a subgroup of a lens group are respectively of a maximum thickness D i and are made from a material with an intrinsic birefringence Δni and lenses of a subgroup of another lens group are of a maximum thickness D j and are made from a material with an intrinsic birefringence Δn j so that the condition Δn i *D i =Δn j *D j is fulfilled in pairs for two respective lenses, and wherein i is an integer and j is an integer.

8 . An optical system according to claim 7 , wherein the condition D i , D j ≦30 mm is fulfilled for the maximum thicknesses D i and D j .

9 . An optical system according to claim 1 , wherein the number of lens groups is at least three.

10 . An optical system according to claim 1 , wherein the number of lens groups is at least four.

11 . An optical system according to claim 1 , wherein the intrinsic birefringence of the material of at least one of the lenses is at least Δn=50 nm/cm.

12 . An optical system according to claim 1 , wherein the lenses at least partially comprise a crystal material of a cubic crystal structure.

13 . An optical system according to claim 1 , wherein the optical system comprises at least one lens of a crystal material selected from the group consisting of MgAI 2 O 4 , MgO and garnets.

14 . An optical system according to claim 1 , wherein the optical system comprises at least one lens of a crystal material selected from the group consisting of NaCl, KCl, KI, NaI, RbI and CsI.

15 . An optical system according to claim 1 , wherein the optical system has an image-side numerical aperture (NA) of at least 0.8.

16 . An optical system according to claim 1 , wherein the resulting maximum retardation of a beam with a working wavelength λ is less than λ/10.

17 . An optical system, comprising:

at least one lens of a crystal comprising a material selected from the group consisting of MgAI 2 O 4 , MgO and garnets,

wherein at least two elements of the crystal material have the same crystal cut and are arranged rotated relative to each other about the lens axis, and/or there are two different crystal cuts of the crystal material, and wherein the optical system is a microlithographic optical system.

18 . An optical system, comprising:

at least one lens of a crystal material selected from the group consisting of NaCl, KCl, KI, NaI, RbI and CsI,

wherein at least two elements of the crystal material have the same crystal cut and are arranged rotated relative to each other about the lens axis, and/or there are two different crystal cuts of the crystal material, and wherein the optical system is a microlithographic optical system.

19 . An optical system according to claim 17 wherein the two elements are wringed together so that they jointly form a lens.

20 . An optical system according to claim 17 , wherein the two elements form two separate lenses.

21 . An optical system according to claim 17 , wherein the combination of the two elements affords an azimuthally symmetrical distribution of the retardation for two mutually perpendicular polarisation states.

22 . An optical system according to claim 17 , wherein the combination of the two elements leads to a substantial reduction in the values of the retardation in comparison with a non-rotated arrangement or in comparison with the situation where there are only elements of the crystal material in the same crystal cut.

23 . An optical system according to claim 17 , wherein the maximum beam angle occurring relative to the optical axis in the lens of the crystal material is not less than 25°.

24 - 26 . (canceled)

27 . An optical system according to claim 1 , wherein a working wavelength of the optical system is less than 250 nm.

28 . A microlithographic projection exposure apparatus comprising an objective according to claim 1 .

29 . A microlithographic projection exposure apparatus comprising an illumination system according to claim 1 .