Patent ID: 7009769

Claim:
A photolithography tool of a scanning-type configured to project a mask pattern onto a substrate using a beam of radiation of a wavelength less than or equal to about 200 nm, comprising an optical system configured to transmit said beam of radiation toward the substrate, the optical system comprising a plurality of calcium fluoride lens elements, optically transmissive of said beam of radiation, each having respective optical axes and imparting a retardance to said beam, said plurality of calcium fluoride lens elements being aligned along an optical path for propagation of the beam of radiation therethrough, each of said calcium fluoride lens elements comprising cubic crystalline calcium fluoride that is aligned with its [111] lattice direction, or a lattice direction optically equivalent to the [111] lattice direction, substantially parallel with its optical axis and a first calcium fluoride lens element of said plurality of calcium fluoride lens elements is rotated about its optical axis with respect to a second calcium fluoride lens element of said plurality of calcium fluoride lens elements, wherein the optical system has less than or about 0.015 waves RMS of wavefront aberration.