Patent Number: 
Section: claims

1. A mirror having an optical effective surface, comprisinga mirror substrate;a reflection layer stack for reflecting electromagnetic radiation impinging on the optical effective surface; anda layer composed of a group III nitride arranged between the mirror substrate and the reflection layer stack, wherein the group III nitride comprises at least one of gallium nitride (GaN), aluminum nitride (AlN) or aluminum gallium nitride (AlGaN);wherein the layer has a thickness of at least 0.1 μm and of at most 100 μm;wherein said mirror is configured to be arranged in a microlithographic projection exposure apparatus; andwherein a surface profile of the layer is configured to at least partly compensate for mechanical strain formed between layers of the reflection layer stack. 2. A mirror having an optical effective surface, comprisinga mirror substrate;a reflection layer stack for reflecting electromagnetic radiation impinging on the optical effective surface; anda layer composed of a group III nitride arranged between the mirror substrate and the reflection layer stack, wherein the group III nitride comprises at least one of gallium nitride (GaN), aluminum nitride (AlN) or aluminum gallium nitride (AlGaN);wherein the layer has a thickness of at least 0.1 μm and of at most 100 μm;wherein said mirror is configured to be arranged in a microlithographic projection exposure apparatus; andwherein the group III nitride is amorphous. 3. The mirror as claimed in claim 1, wherein the layer has a thickness of at least 0.5 μm and of at most 50 μm. 4. The mirror as claimed in claim 1, wherein the layer is arranged directly on the mirror substrate. 5. The mirror as claimed in claim 1, further comprising a polishing layer on which the layer is arranged. 6. The mirror as claimed in claim 1, wherein the mirror substrate is produced from a metallic material. 7. The mirror as claimed in claim 1, wherein the mirror substrate is produced from an amorphous material. 8. The mirror as claimed in claim 1, configured for an operating wavelength of less than 30 nm. 9. An optical system of a microlithographic projection exposure apparatus, comprising at least one mirror as claimed in claim 1. 10. A method for producing a mirror for a microlithographic projection exposure apparatus, comprising:applying a layer system on a mirror substrate, said layer system having a reflection layer stack configured to reflect electromagnetic radiation impinging on the optical effective surface,applying a layer composed of a group III nitride before the reflection layer stack is applied to the mirror substrate, wherein the group III nitride comprises at least one of gallium nitride (GaN), aluminum nitride (AlN) or aluminum gallium nitride (AlGaN), and wherein the layer has a thickness of at least 0.1 μm and of at most 100 μm, andworking a surface profile into the layer, wherein the surface profile at least partly compensates for mechanical strain formed between layers of the reflection layer stack. 11. The method as claimed in claim 10, further comprising polishing the layer before the reflection layer stack is applied. 12. The mirror as claimed in claim 1, further comprising an adhesion promoter layer situated on the mirror substrate and on which the layer is arranged. 13. The mirror as claimed in claim 7, wherein the amorphous material is a titanium dioxide (TiO2)-doped quartz glass. 14. The mirror as claimed in claim 8, configured for an operating wavelength of less than 15 nm. 15. The optical system as claimed in 9, wherein the microlithographic projection exposure apparatus is an illumination device or a projection lens. 16. The mirror as claimed in claim 5, wherein the polishing layer comprises amorphous silicon. 17. The mirror as claimed in claim 2, wherein a surface profile of the layer is configured to at least partly compensate for mechanical strain formed between layers of the reflection layer stack. 18. The mirror as claimed in claim 1, wherein the group III nitride is amorphous. 19. The mirror as claimed in claim 12, wherein the adhesion promoter layer is comprised of a first material, wherein the substrate is comprised of a second material, and wherein the first material is different from the second material. 20. The mirror as claimed in claim 2, wherein the mirror substrate is produced from a metallic material or from titanium dioxide (TiO2)-doped quartz glass.