Imaging system, in particular a projection objective of a microlithographic projection exposure apparatus

1. An imaging system, having an optical axis and being capable of producing an image field which is extra-axial relative to the optical axis, the imaging system comprising:

a first optical element which, during use of the imaging system, causes a first distribution of retardation in a plane that lies perpendicular to the optical axis; and

at least one second optical element which, during use of the imaging system, causes a second distribution of retardation in a plane that lies perpendicular to the optical axis,

wherein the second distribution of retardation at least partially compensates the first distribution of retardation, and the first and the second optical elements are not designed with rotational symmetry relative to the optical axis.

2. The imaging system according to claim 1 , wherein the first optical element and the second optical element each comprise a cubic crystal material.

3. The imaging system according to claim 1 , wherein the first optical element and the second optical element each comprise an optically uniaxial crystal material.

4. The imaging system according to claim 1 , wherein, during use of the imaging system, the retardation caused by the second optical element is of the opposite sign relative to the retardation caused by the first optical element.

5. The imaging system according to claim 1 , wherein the first optical element and the second optical element have the same crystallographic cut and are rotated relative to each other about the optical axis.

6. The imaging system according to claim 1 , wherein the extra-axial image field is mirror-symmetric relative to a plane of symmetry, and at least one of the first second optical elements is mirror-symmetric relative to the same plane of symmetry.

7. The imaging system according to claim 6 , wherein at least one of the first and second optical elements has as its only symmetry a mirror-symmetry relative to the plane of symmetry.

8. The imaging system according to claim 1 , wherein, during use of the imaging system, for at least one ray falling on the center of the image field, the condition n 1 ×d 1 ≈n 2 ×d 2 is met, wherein n i (i=1, 2) stands for the respective refractive index and d i (i=1, 2) indicates the geometrical path lengths covered by this ray in the first and the second optical element, respectively.

9. The imaging system according to claim 8 , wherein, during use of the imaging system, the condition n i ×d i ≈n 2 ×d 2 is met for at least two rays falling on the center of the image field, wherein the angle between the rays is at least 40°.

10. The imaging system according to claim 5 , wherein the first optical element and the second optical element have a crystallographic (111)-cut arranged with a rotation of 60°+k×120° relative to each other about the element axis, and k is an integer having a value of at least zero.

11. The imaging system according to claim 5 , further comprising third and fourth optical elements, wherein the third and fourth optical elements have a crystallographic (100)-cut arranged with a rotation of 45°+l×90° relative to each other about the element axis, and l is an integer having a value of at least zero.

12. The imaging system according to claim 11 , wherein neither of the third and fourth optical elements is designed with rotational symmetry relative to the optical axis.

13. The imaging system according to claim 12 , wherein, during use of the imaging system, for at least one ray falling on the center of the image field, the condition n 3 ×d 3 ≈n 4 ×d 4 is met, wherein n i (i=3, 4) stands for the respective refractive index and d i (i=3, 4) indicates the geometrical path lengths covered by this ray in the third optical element and in the fourth optical element, respectively.

14. The imaging system according to claim 1 , wherein at least two of the optical elements are joined to each other by wringing in such a way that they form a lens together.

15. The imaging system according to claim 14 , wherein the lens is shaped with rotational symmetry relative to the optical axis.

16. The imaging system according to claim 15 , wherein the lens is a planar-convex lens.

17. The imaging system according to claim 14 , wherein the lens is a lens of the imaging system that is arranged in last position on the image side.

18. The imaging system according to claim 16 , wherein at least one of the optical elements has the shape of a shell that is convex-curved towards the object side.

19. The imaging system according to claim 1 , wherein a liquid is arranged between at least two of the optical elements, and the refractive index of the liquid is substantially equal to the refractive index of the two optical elements.

20. The imaging system according to claim 19 , wherein the liquid is perhydro fluorene.

21. The imaging system according to claim 1 , wherein the optical elements are made of the same crystal material.

22. The imaging system according to claim 2 , wherein the cubic crystal material of at least one of the optical elements is selected from the group that contains magnesium spinel (MgAl 2 O 4 ), yttrium aluminum garnet (Y 3 Al 5 O 12 ), magnesium oxide (MgO), and scandium aluminum garnet (Sc 3 Al 5 O 12 ).

23. The imaging system according to claim 2 , wherein the imaging system has an image-side numerical aperture, during use the imaging system has a working wavelength, and the cubic crystal material at the working wavelength has a refractive index of such a magnitude that the difference between the refractive index n and the numerical aperture of the imaging system does not exceed 0.2.

24. The imaging system according to claim 23 , wherein the difference between the refractive index and the numerical aperture of the imaging system lies in the range from 0.05 to 0.20.

25. The imaging system according to claim 2 , wherein the cubic crystal material of at least one of the optical elements comprises an oxide.

26. The imaging system according to claim 25 , wherein the cubic crystal material of at least one of the optical elements comprises sapphire (Al 2 O 3 ) and a potassium- or calcium oxide among its components.

27. The imaging system according to claim 25 , wherein that the cubic crystal material of at least one of the optical elements comprises at least one material selected from the group that includes 7Al 2 O 3 .12CaO, Al 2 O 3 .K 2 O, Al 2 O 3 .3CaO, Al 2 O 3 .SiO 2 KO, Al 2 O 3 .SiO 2 .2K, and Al 2 O 3 .3CaO 6 H 2 O.

28. The imaging system according to claim 2 , wherein the cubic crystal material of at least one of the optical elements comprises calcium, sodium and silicon oxide.

29. The imaging system according to claim 2 , wherein the cubic crystal material of at least one of the optical elements comprises at least one material selected from the group that includes CaNa 2 SiO 4 and CaNa 4 Si 3 O 9 .

30. The imaging system according to claim 2 , wherein the cubic crystal material of at least one of the optical elements comprises at least one material selected from the group that includes Sr(NO 3 ) 2 , MgONa 2 O—SiO 2 and Ca(NO 3 ) 2 .

31. The imaging system according to claim 16 , further comprising an optical element, which is substantially planar-parallel, placed on the light-exit surface of the planar-convex lens.

32. The imaging system according to claim 31 , wherein the planar-parallel optical element has an element axis and at least two partial elements which have the same crystallographic cut and are arranged in rotated positions relative to each other about the element axis.

33. The imaging system according to claim 32 , wherein the first partial element and the second partial element each have a crystallographic (111)-cut arranged with a rotation relative to each other of 60°+k×120° about the element axis, and k is an integer having a value of at least zero.

34. The imaging system according to claim 32 , wherein the first partial element and the second partial element each have a crystallographic (100)-cut arranged with a rotation relative to each other of 45°+l×90° about the element axis, and l is an integer having a value of at least zero.

35. The imaging system according to claim 31 , wherein the planar-parallel optical element has an element axis and at least four partial elements, first and second partial elements each have a crystallographic (111)-cut arranged with a rotation relative to each other of 60°+k×120° about the element axis, a third partial element and a fourth partial element each have a crystallographic (100)-cut arranged with a rotation relative to each other of 45°+l×90° about the element axis, k is an integer having a value of at least zero, and l is an integer having a value of at least zero.

36. The imaging system according to claim 1 , wherein, during use the imaging system has a working wavelength less than 250 nm.

37. The imaging system according to claim 1 , wherein the imaging system is a catadioptric projection objective with at least two concave mirrors which produces at least two intermediate images.

38. A microlithographic projection exposure apparatus having a projection objective that is an imaging system according to claim 1 .

39. A method for the microlithographic production of micro-structured components, the method comprising:

preparing a substrate on which at least one coating of a light-sensitive material is deposited;

preparing a mask which has structures of which images are to be formed;

preparing a microlithographic projection exposure apparatus according to claim 38 ; and

projecting at least a part of the mask onto an area of the coating by means of the projection exposure apparatus.

40. A micro-structured component which is produced according to the method of claim 39 .