Hidden in Plain Sight: Calcium Fluoride's Intrinsic Birefringence

John H. Burnett, Zachary H. Levine and Eric L. Shirley

For years, lithographers have applied progressively shorter exposure wavelengths to reduce the minimum feature size on integrated circuit chips. However, this approach recently encountered an unexpected and little-known phenomenon: the intrinsic birefringence of cubic crystals, which could undermine the drive to shorter-wavelength lithography.

State-of-the-art large-scale chip fabrication uses 248-nm excimer lasers as illumination sources, and pilot lines are testing prototype 193-nm systems. Next in the progression are 157-nm systems, which have been under intense development for several years. The semiconductor industry has scheduled the introduction of production-level 157-nm steppers within the 2005 to 2007 time frame, with the expectation that these tools will deliver critical feature sizes down to 65 nm…