Patent ID: 6813098
Filing Date: 2004-11-02
Classification: G02B,G03F

Abstract:
An optical system comprising along an optical axis:a concave spherical mirror; a variable aperture stop located at the 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: a first subgroup having positive refractive power; a second subgroup having negative refractive power and spaced apart from the first subgroup by an air space; 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; and wherein the optical system has an exposure field size at the image plane that is adjustable by adjusting the variable aperture stop to image two or more 34 mmÃ—26 mm step-and-scan fields.