Patent ID: 7679831

Claim:
A projection objective with an optical axis, the projection objective configured to project an image of a mask which can be positioned in an object plane onto a light-sensitive coating which can be positioned in an image plane, the projection objective comprising: at least one lens which has at least one curved lens surface and includes at least four lens elements comprising intrinsically birefringent material which are arranged so that they contact each other in a mutually adjacent relationship along the optical axis of the projection objective; wherein: the at least four lens elements comprise two pairs of lens elements, the lens elements in the one pair of lens elements have a crystallographic [111]-cut and are arranged relative to each other with a rotary offset about the optical axis of the projection objective, the lens elements in the other pair of lens elements have a crystallographic [100]-cut and are arranged relative to each other with a rotary offset about the optical axis of the projection objective, three of the four lens elements have a convex surface which is faced towards a concave surface of an adjacent one of the lens elements, all successive lens elements of the lens elements are separated from each other by a curved boundary, each of the curved boundaries is rotational symmetric relative to the optical axis of the projection objective, and the projection objective is a microlithographic projection exposure apparatus projection objective.