Birefringence dispersion in photomask substrates for DUV lithography

To further extend the lifetime of optical lithography, stronger requirements are placed on the optical materials used in stepper systems. For example, photomask requirements from mask makers and users include good transmission uniformity, low induced absorption, low defect density and low thermal expansion. However little emphasis and attention has been placed on the magnitude of birefringence. In this paper we present results on the dispersion of birefringence in fused silica photomask substrates measured from the visible to DUV wavelengths. An examination of the wavelength dependent stress-optic response that produces birefringence is performed and found to increase from 633nm to the DW. Measurements were performed on substrates with levels of birefringence that varied by an order of magnitude. Birefringence in photomasks results in a change in the polarization of an optical beam and for some lithography tools this can influence system performance and degrade image quality. Therefore, we believe an emphasis on low birefringence photomask substrates can possibly improve system performance.