Intrinsic Birefringence in Crystalline Optical Materials: A New Concern for Lithography
John H. Burnett, Zachary H. Levine, Eric L. Shirley
Crystalline optical materials, such as CaF2, expected to be used in193nm and 157nm lithography systems, turn out to have large, intrinsicbirefringences which must be carefully accounted for in the lithographyoptics design.We recently reported intrinsic birefringences in CaF2 and BaF2 thatresult in substantial directional and polarization dependences to theindices of refraction. The magnitudes of the effect in these materials aresignificantly larger than the birefringence specification for 157nmlithography, and put significant optical design constraints on 157nmsystems. Our model for the effect completely describes itscharacteristics, including the angle dependence in the crystal. Approachesto ameliorating the birefringence have been analyzed and are expected towork, though complicating the designs.