Optical system and photolithography tool comprising same

摘要

An optical system, for example a lens for a photolithography tool, includes a group of optical elements (L 1 , L 2 ) that each comprise a birefringent cubic crystal such as CaF₂. The crystal lattices of the crystals have different orientations, e.g. for reducing the overall retardance of the group by mutual compensation. The [110] crystal axis of at least one optical element (L 1 , L 2 ) is tilted with respect to an optical axis ( 34 ) of the system ( 10 ) by a predefined tilting angle (theta₁, theta₂) having an absolute value between 1° and 20°. This reduces the magnitude, but not significantly changes the orientation of intrinsic birefringence. By selecting an appropriate tilting angle it is possible to achieve a better performance of the optical system. For example, the overall retardance of the optical system may be reduced, or the angular retardance distribution may be symmetrized.