PROJECTION LENS AND METHOD FOR SELECTION OF OPTICAL MATERIALS IN SUCH A LENS

The invention relates to a projection lens (10) for a microlithographic projection illumination unit, comprising several groups of serial optical elements, at least one first optical element made from a pure earth alkali metal fluoride crystal and at least one second optical element made from a mixed earth alkali metal fluoride crystal, comprising at least two different earth alkali metals, are arranged in at least one group. All optical elements made from a mixed earth alkali metal fluoride crystal and preferably none of the optical elements made from a pure earth alkali metal fluoride crystal fulfil the condition mi = GPLi * DB (θi) ≥ S. The parameter mi is a lag factor associated with each optical element Li, GPLi the geometrical path length of an aperture beam (28) incident on the optical element Li with a maximum aperture angle, θi is the aperture angle between the aperture beam (28) and the optical axis (26) of the optical element Li, DB(θi) is a measure of the birefringence of the optical element Li, independent of the material and the crystal orientation of the optical element Li and S is a unitary threshold value for all optical elements Li. The projection lens has a low birefringence and is more economical to produce than with all optical elements made from expensive mixed crystals.