Patent ID: 7331676

Claim:
Apparatus for illuminating a mask with a beam of radiation and projecting an image of the mask on a substrate, the apparatus having an optic-axis, a mask location and an image location, the apparatus comprising: an objective for forming the image, said objective including a convex first mirror and concave second mirror spaced apart and facing toward each other, said second mirror being larger than said first mirror and having an aperture therein providing optical access to said first mirror; a prism located in the path of the beam of radiation prior to the mask location, said prism having a plurality of facets, said facets being configured and arranged such that the beam is divided thereby into a plurality of beam-portions, said beam portions being mutually convergent on leaving said prism; and wherein said mutually convergent beam-portions intersect in the mask location then mutually diverge through said aperture in said second mirror such that substantially all of the radiation in said beam portions is incident in an annular zone on said first mirror, said beam portions incident in said annular zone being reflected onto said second mirror and being reflected by said second mirror to image points in the substrate location.