COMPENSATION OF BIREFRINGENCE IN A LENS COMPOSED OF CRYSTAL LENSES

摘要

The invention relates to a lens (1), in particular a projection lens for a micro-lithography projection exposure system, composed of at least one first group (3) of lenses (7) or lens sections consisting of a first crystal material and of at least one second group (5) of lenses (11) or lens sections consisting of a second crystal material. An outer aperture beam (15) is subjected to a first optical path difference for two mutually perpendicular linear polarisation states in the first group (3) and a second optical path difference in the second group (5). The two optical path differences approximately compensate one another, as a result of the different crystal materials. This can be achieved in particular with calcium fluoride as the first crystal material and barium fluoride as the second crystal material.