Illumination system of a microlithographic projection exposure apparatus with a birefringent element

1. An illumination system, comprising:

a mirror arrangement comprising a plurality of mirror units displaceable independently of each other to alter an angle distribution of light reflected by the mirror arrangement, each mirror unit comprising at least one mirror element; and

at least one element that is in front of the mirror arrangement in a propagation direction of the light so that the at least one element is configured to produce at least two different states of polarization incident on different mirror units and/or different mirror elements,

wherein:

the at least one element comprises a birefringent element configured to split a partial ray of the light into an ordinary partial ray and an extraordinary partial ray;

an entire portion of the partial ray contributes to the ordinary partial ray;

the entire portion of the partial ray contributes to the extraordinary partial ray;

the ordinary partial ray and the extraordinary partial ray have mutually perpendicular polarization directions;

the ordinary partial ray is deflected by the mirror arrangement in a first direction;

the extraordinary partial ray is deflected by the mirror arrangement in a second direction different from the first direction; and

the illumination system is configured to be used in a microlithographic projection exposure apparatus.

2. The illumination system as set forth in claim 1 , wherein the ordinary partial ray and the extraordinary partial ray are incident on the different mirror units or the different mirror elements.

3. The illumination system as set forth in claim 1 , wherein the ordinary partial ray and the extraordinary partial ray are incident on mutually adjacent mirror units or on mirror elements of the same mirror unit.

4. The illumination system as set forth in claim 1 , wherein the mirror arrangement comprises first mirror units and second mirror units, the first mirror units are optimized for a first polarization direction, and the second mirror units are optimized for a second polarization direction perpendicular to the first polarization direction.

5. The illumination system as set forth in claim 1 , further comprising a microlens arrangement comprising a plurality of microlenses, wherein the birefringent element is between the microlens arrangement and the mirror arrangement.

6. The illumination system as set forth in claim 1 , further comprising a rotator element configured to rotate the polarization state, wherein the rotator element is upstream or downstream of the birefringent element in the light propagation direction.

7. The illumination system as set forth in claim 6 , wherein the rotator element comprises a lambda/2-plate.

8. The illumination system as set forth in claim 1 , wherein the birefringent element comprises an optically uniaxial crystal material.

9. The illumination system as set forth in claim 1 , wherein the birefringent element comprises an optically active material.

10. The illumination system as set forth in claim 1 , wherein the illumination system has an optical axis, and the birefringent element has an optical crystal axis that is not oriented in parallel relationship with the optical axis.

11. The illumination system as set forth in claim 1 , further comprising an arrangement of at least two channels associated with respectively different regions of the mirror arrangement, wherein, for those channels the polarization state of the light emanating from the respective channel and incident on the mirror arrangement is adjustable independently of each other.

12. The illumination system as set forth in claim 11 , wherein the channels each have at least one respective lambda/2-plate, wherein lambda/2-plates of different channels are adjustable independently of each other.

13. The illumination system as set forth in claim 12 , wherein the illumination system has an optical axis, and at least one of the lambda/2-plates is arranged rotatably about the optical axis.

14. The illumination system as set forth in claim 1 , wherein a lambda/4-plate is between the birefringent element and the mirror arrangement.

15. The illumination system as set forth in claim 14 , wherein the illumination system has an optical axis, and the lambda/4-plate is arranged rotatably about the optical axis.

16. The illumination system as set forth in claim 1 , wherein the birefringent element has plane-parallel geometry.

17. The illumination system as set forth in claim 1 , wherein the birefringent element has a varying thickness profile as measured in the light propagation direction.

18. The illumination system as set forth in claim 1 , wherein the birefringent element comprises a prism.

19. The illumination system as set forth in claim 1 , wherein the birefringent element has at least substantially wedge-shaped geometry.

20. The illumination system as set forth in claim 1 , wherein the birefringent element comprises at least one retarder that produces, for light passing therethrough, an effective retardation which is less than one half of an operation wavelength of light for the illumination system.

21. The illumination system as set forth in claim 20 , wherein the position of the retarder is variable in a plane perpendicular to the light propagation direction.

22. The illumination system as set forth in claim 20 , wherein the birefringent element comprises at least two retarders.

23. The illumination system as set forth in claim 22 , wherein the at least two retarders produce, for light passing therethrough, effective retardations of opposite sign.

24. The illumination system as set forth in claim 1 , wherein the birefringent element comprises at least two sub-elements, and the position of the at least two sub-elements with respect to each other is variable.

25. The illumination system as set forth in claim 1 , further comprising a polarizer configured to refresh the polarization state.

26. An apparatus, comprising:

an illumination system according to claim 1 ; and

a projection objective,

wherein the illumination system is configured to illuminate a reticle, and the projection objective is configured to image the reticle onto a substrate.

27. A method of using an apparatus comprising an illumination system and a projection objective, the method comprising:

using the illumination system to illuminate a reticle; and

using the projection objective to image the reticle onto a substrate,

wherein the illumination system is an illumination system according to claim 1 .