Projection lens for a microlithographic projection exposure apparatus
View Patent ↗A projection lens ( 10 ) for a microlithographic projection exposure apparatus has a first optical element, for example a birefringent lens (L 2 ), that has polarization dependent properties causing intensity fluctuations in an image plane of the projection lens. These fluctuation may be produced by a second optical element ( 24 ), for example a polarization selective beam splitting layer ( 28 ), that is arranged downstream of the first optical element. A gray filter ( 32; 132; 232 ) disposed in the beam path reduces the intensity fluctuations.
1. A projection lens for a microlithographic projection exposure apparatus, comprising:
a) a first optical element that has polarization dependent properties causing intensity fluctuations in an image plane of the projection lens, and
b) a gray filter that reduces the intensity fluctuations.
2. The projection lens of claim 1 , in which the first optical element disturbs the polarization state of at least one ray of projection light impinging on the first optical element.
3. The projection lens of claim 2 , comprising a second optical element that is arranged downstream of the first optical element in the propagation direction of the projection light and has polarization dependent properties such that the intensity of projection light emerging from the second optical element depends on the polarization state of projection light impinging thereon.
4. The projection lens of claim 3 , in which the second optical element attenuates the intensity of light rays, whose polarization state is disturbed by the first optical element, to a larger amount than the intensity of light rays, whose polarization state is less disturbed by the first optical element.
5. The projection lens of claim 4 , in which the gray filter attenuates the intensity of light rays, whose polarization state is disturbed by the first optical element, to a smaller degree than the intensity of light rays, whose polarization state is less disturbed by the first optical element.
6. The projection lens of claim 1 , in which the gray filter is a transmissive optical element having a locally varying transmittance.
7. The projection lens of claim 1 , in which the gray filter is a reflective optical element having a locally varying reflectance.
8. The projection lens of claim 1 , in which the gray filter does not modify the phase distribution of the projection light passing through.
9. The projection lens of claim 1 , in which the gray filter is disposed on the first optical element.
10. The projection lens of claim 3 , in which the gray filter is disposed in the beam path between the first optical element and the second optical element.
11. The projection lens of claim 3 , further comprising a catadioptric section in which the first optical element, the second optical element and the gray filter are disposed.
12. The projection lens of claim 1 , in which the gray filter is disposed interchangeably in a filter holder.