Objective with crystal lenses

摘要

Objective, in particular a projection objective for a microlithography projection-exposure installation, with at least one fluoride crystal lens. A reduction in the detrimental influence of birefringence is achieved if this lens is a (100)-lens with a lens axis which is approximately perpendicular to the {100} crystallographic planes or to the crystallographic planes equivalent thereto of the fluoride crystal. In the case of objectives with at least two fluoride crystal lenses, it is favorable if the fluoride crystal lenses are arranged such that they are rotated with respect to one another. The lens axes of the fluoride crystal lenses may in this case point not only in the <100> crystallographic direction but also in the <111> crystallographic direction or in the <110> crystallographic direction. A further reduction in the detrimental influence of birefringence is achieved by the simultaneous use of groups with (100)-lenses rotated with respect to one another and groups with (111)-lenses or (110)-lenses rotated with respect to one another. A further reduction in the detrimental influence of birefringence is obtained by covering an optical element with a compensation coating.