Birefringence dispersion in fused silica for DUV lithography

A shift to shorter wavelength radiation sources along with improvements in the quality of lithography grade optical materials has driven the production of integrated circuits with smaller feature sizes. Optical characterization of these materials, in some cases, is performed at visible wavelengths due to the complexity level associated with measuring in the DUV. Birefringence measurements of stepper lens blanks, for example, are typically measured at 633nm using a Helium Neon laser source. However, knowledge of the correlating DUV birefringence values is needed for determining the acceptable magnitude of birefringence in the material and for predicting the magnitude of loss in CD contrast. In this paper we report results on how the birefringence in Coming's HPFS(R) synthetic silica glass changes at the DUV wavelengths used in lithography systems. An examination of the wavelength dependent stress-optic response that produces birefringence was performed and found to increase from 633nm to 193nm. Birefringence in lithography stepper lens elements degrades imaging performance so an understanding of its dispersion is important for system designs.