Patent ID: 7199922

Claim:
A projection lens for imaging a pattern arranged in an object plane onto an image plane employing electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region, the projection lens comprising a plurality of imaging mirrors having reflective coatings and defining an optical axis of the projection lens, wherein the mirrors are arranged between the object plane and the image plane and are aligned for projection of an off-axis field from the object plane to the image plane, wherein the mirrors are configured and arranged with respect to one another such that every mirror is irradiated over a range of angles of incidence that is characteristic of that particular mirror, wherein at least one of the mirrors, having the largest range of angles of incidence, has a centric, graded, reflective coating that has a film-thickness gradient that is rotationally symmetric with respect to a coating axis, and wherein the coating axis is substantially coincident with the optical axis of the projection lens.