An off-axis catadioptric projection optical systems for use in lithography tools for processing modulated light used to form an image on a substrate is provided. In one embodiment the optical system includes an off-axis mirror segment, a fold mirror, a relay, an aperture stop and a refractive lens group....http://www.google.com/patents/US7511798?utm_source=gb-gplus-sharePatent US7511798 - Off-axis catadioptric projection optical system for lithography
