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Intrinsic Birefringence in Crystalline Optical Materials for 193 nm and 157 nm Lithography
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JOHN H. BURNETT, ZACHARY H. LEVINE & ERIC L. SHIRLEY,
National Institute of Standards and Technology, Gaithersburg, MD, USA
ABSTRACT
Calcium fluoride and other crystalline fluoride materials are being exploited for latestgeneration lithography optics, making up a significant component of the optics of 193 nm lithography systems and being potentially the exclusive optical materials for 157 nm systems. Improvements in the crystalline material quality seemed to have brought the most troubling material property, stress-induced birefringence, into specification. However, we have recently shown that these materials also have a completely overlooked intrinsic birefringence far over specifications. This has significant implications for lithography system design and performance, especially for 157 nm lithography. We discuss the effect, its impact on lithography optics and how system designers are attempting to compensate for the effect.
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