Intrinsic Birefringence in Crystalline Optical Materials for 193 nm and 157 nm Lithography
John H. Burnett, Zachary H. Levine, Eric L. Shirley
lcium fluoride (CaF2) and other crystalline fluoride materials are beingexploited for latest-generation lithography optics, making up a significantcomponent of the optics of 193nm lithography systems and potentially theexclusive optical materials for 157nm systems. Improvements in thecrystalline material quality seemed to have brought the most troublingmaterial property, stress-induced birefringence, intospecification. However, we have recently shown that these materials alsohave a completely overlooked intrinsic birefringence far overspecifications. This has significant implications for lithography systemdesign and performance, especially for 157nm lithography. We discuss theeffect, its impact on lithography optics, and how system designers areattempting to compensate for the effect.