Optical element and lithographic apparatus

1. An optical element, comprising:

a surface comprising a tilted profile having height differences, thereby providing cavities and elevations having a predetermined maximum height difference; and

a transmissive layer that covers the cavities and the elevations of the optical element, wherein a first height of the transmissive layer in the cavities is substantially equal or larger than the predetermined maximum height difference and the transmissive layer has a second height on the elevations and the second height is about 10-500 nm,

wherein the transmissive layer is enabled to optically filter incident radiation, and

wherein the optical element is a grating.

2. The optical element according to claim 1 , wherein the transmissive layer is substantially planar.

3. The optical element according to claim 1 , wherein the transmissive layer has a roughness of 0-50 nm.

4. The optical element according to claim 1 , wherein the transmissive layer comprises a material selected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pr, Pa and U and combinations thereof.

5. The optical element according to claim 1 , wherein the grating comprises a mirror with a mirror surface and a multilayer stack which is tilted with respect to the mirror surface.

6. The optical element according to claim 1 , wherein the transmissive layer comprises a material with a complex index of refraction.

7. The optical element according to claim 1 , wherein the optical element is an EUV optical element and the transmissive layer is an EUV transmissive layer.

8. The optical element according to claim 1 , further comprising:

a capping layer formed on top of the top surface of the transmissive layer, wherein the capping layer is configured to provide protection to the optical element or provide optical filtering, or both.

9. The optical element according to claim 8 , wherein the capping layer includes ruthenium.

10. A lithographic apparatus, comprising:

an illumination system configured to provide a beam of radiation;

a support configured to support a patterning device, the patterning device configured to impart the beam of radiation with a pattern in its cross-section;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto a target portion of the substrate; and

an optical element comprising a tilted multilayer stack comprising a profile having height differences, thereby providing cavities and elevations having a predetermined maximum height difference; and a transmissive layer in at least the cavities of the optical element, wherein a first height of the transmissive layer in the cavities is substantially equal or larger than the predetermined maximum height difference and the transmissive layer has a second height on the elevations and the second height is about 10-500 nm,

wherein the transmissive layer is enabled to optically filter radiation, and

wherein the optical element is a grating.

11. The apparatus according to claim 10 , wherein the transmissive layer is substantially planar.

12. The apparatus according to claim 10 , wherein the transmissive layer has a roughness of 0-50 nm.

13. The apparatus according to claim 10 , wherein the transmissive layer comprises a material selected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pr, Pa and U and combinations thereof.

14. The apparatus according to claim 10 , wherein the grating comprises a mirror with a mirror surface and the multilayer stack is tilted with respect to the mirror surface.

15. The apparatus according to claim 10 , wherein the transmissive layer comprises a material with a complex index of refraction.

16. The apparatus according to claim 10 , wherein the optical element is an EUV optical element and the transmissive layer is an EUV transmissive layer.