Reflective projection system comprising four spherical mirrors

Claim 1

A reflective projection system having a given optical axis for either projecting, into a segment of an annular ring surrounding said given optical axis and lying in a first plane, a demagnified radiation image of an object pattern situated in a second plane or, alternatively, projecting, into said second plane, a magnified radiation image of an object pattern situated in said segment of an annular ring; said reflective projection system comprising: a first spherical-mirror segment of a certain width having a concave surface radius of curvature equal in value to R 1 , said first spherical-mirror segment being spaced a first given axial distance T 0 from said first given plane; a second spherical-mirror segment of a certain width having a convex surface radius of curvature equal in value to R 2 , said second spherical-mirror segment being spaced a second given axial distance T 1 from said first spherical-mirror segment; a third spherical-mirror segment of a certain width having a concave surface radius of curvature equal in value to R 3 , said third spherical-mirror segment being spaced a third given axial distance T 2 from said second spherical-mirror segment; a fourth spherical-mirror segment of a certain width having a convex surface radius of curvature equal in value to R 4 , said fourth spherical-mirror segment being spaced a fourth given axial distance T 3 from said third spherical-mirror segment; the relative values of said respective radii of curvatures R 1 , R 2 , R 3 and R 4 being such that R 2 ≦R 1 ≦R 3 ≦R 4 and 0.95≦(1/R 1 +1/R 3 )/(1/R 2 +1/R 4 )≦1.05; said annular ring having a radius no greater in value than 0.2; and the respective lengths of axial distances T 0 , T 1 , T 2 and T 3 , the respective spatial positions of said first, second, third and fourth spherical-mirror segments, and the respective certain widths of said first, second, third and fourth spherical-mirror segments being such that radiation defining an object pattern situated in said second plane is successively reflected, in order, from said fourth, third, second and first spherical-mirror segments defines an optimized image of said object pattern with a numerical aperture no greater than 0.12 in said first plane with a Strehl ratio of total aberrations at least equal in value to 0.95 which is demagnified by a specified demagnification power value between 3 and infinity, with said second and first spherical-mirror segments by themselves providing a demagnification or magnification power value between 0.8 and 1.2, and with substantially no blockage of said reflected radiation traveling over a path between said first and second planes taking place by the presence an intervening one of said spherical-mirror segments in said radiation path.