Patent ID: 9625827
Date: 2017-04-18
CPC Classifications: G02B,G03F,G21K

Claim:
1. An imaging optical unit for EUV projection lithography for imaging an object field in an object plane into an image field in an image plane, the imaging optical unit comprising: a plurality of mirrors for guiding imaging light from the object field to the image field; an aperture stop, which is tilted by at least 1° relative to a normal plane which is perpendicular to an optical axis, wherein the aperture stop is configured with a circular stop contour, wherein the aperture stop is arranged in such a way that the following applies to mutually perpendicular planes: a deviation of a numerical aperture NAx measured in one of these planes from a numerical aperture NAy measured in the other one of these two planes is less than 0.003, averaged over the field points of the image field.