Patent ID: 7116496
Filing Date: 2006-10-03
Classification: G02B,G03F

Abstract:
1. A projection optical system comprising: a concave mirror with an aperture on its optical axis; a variable aperture stop located at the concave mirror that determines a numerical aperture (NA) of the system; a main lens group with positive refracting power arranged adjacent the mirror and spaced apart therefrom, the main lens group comprising in order towards the mirror: first and second prisms each having respective first and second flat surfaces, wherein the second flat surfaces are arranged adjacent the positive subgroup on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent object and image planes, respectively; wherein the optical system is anastigmatic, projects an image of a pattern located in said object plane to said image plane at substantially unit magnification and has an exposure field at the image plane that is variable in size depending on the adjustment of the variable aperture stop.