Patent Number: 
Section: claims

1. A mirror, having an optically effective surface and configured to have a reflectivity of at least 0.5 for electromagnetic radiation which has a prescribed working wavelength and impinges on the optically effective surface at an angle of incidence of at least 65° with respect to respective surface normals of the optically effective surface, said mirror comprising:a protective layer composed of a protective layer material and arranged as a topmost layer forming the optically effective surface, wherein the protective layer comprises silicon nitride (Si3N4), or a compound of an element selected from the second period and an element of the 4d transition group,wherein the protective layer has a thickness of not more than 5 nm; andat least one further layer arranged underneath the protective layer with respect to the optically effective surface and which comprises a compound of an element selected from the second period and an element of the 4d transition group. 2. The mirror according to claim 1, wherein the element of the 4d transition group is selected from the group consisting of molybdenum (Mo), niobium (Nb) and zirconium (Zr). 3. The mirror according to claim 1, wherein the element of the second period is selected from the group consisting of beryllium (Be), boron (B), carbon (C), nitrogen (N) and oxygen (O). 4. The mirror according to claim 1, further comprising at least one barrier layer. 5. The mirror according to claim 1, wherein the working wavelength is less than 30 nm. 6. An optical system of a microlithographic projection exposure apparatus, comprising at least one mirror according to claim 1. 7. A microlithographic projection exposure apparatus comprising an illumination device and a projection lens,wherein the illumination device illuminates a mask present in an object plane of the projection lens during operation of the projection exposure apparatus and the projection lens projects structures on the mask onto a light-sensitive layer arranged in an image plane of the projection lens, andwherein the projection exposure apparatus comprises at least one mirror according to claim 1. 8. The mirror according to claim 1, wherein the further layer is configured to at least partly compensate a decrease in reflectivity brought about by the protective layer. 9. A mirror, having an optically effective surface and configured to have a reflectivity of at least 0.5 for electromagnetic radiation which has a prescribed working wavelength and impinges on the optically effective surface at an angle of incidence of at least 65° with respect to respective surface normals of the optically effective surface, said mirror consisting essentially of:a layer which comprises a compound of an element selected from the second period and an element selected from the 4d transition group; andno more than a single further layer, the further layer forming a substrate. 10. The mirror according to claim 9, wherein the element of the 4d transition group is selected from the group consisting of molybdenum (Mo), niobium (Nb) and zirconium (Zr). 11. The mirror according to claim 9, wherein the element of the second period is selected from the group consisting of beryllium (Be), boron (B), carbon (C), nitrogen (N) and oxygen (O).