Correction of birefringence in cubic crystalline optical systems

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 calcium fluoride optical element, optically transmissive of said beam of radiation, imparting a retardance to said beam, the calcium fluoride optical element being aligned along an optical path for propagation of the beam of radiation therethrough, the calcium fluoride optical element 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 plurality of fused silica lens elements, optically transmissive of said beam of radiation, being aligned along the optical path for propagation of the beam of radiation therethrough, wherein the calcium fluoride optical element or at least one fused silica lens element of said plurality of fused silica lens elements has an aspheric surface and the optical system has less than or about 0.015 waves RMS of wavefront aberration.