Ultra-Low Expansion ULE Glass is the material of choice for substrates for masks and projection optics mirrors in EUV lithography. A key parameter is the temperature of zero crossover which is the temperature at which the coefficient of thermal expansion (expansivity, a(T)) is equal to zero. As the temperature of the glass departs from zero crossover, the absolute value of a(T) increases proportionally to the slope of the expansivity curve. As EUVL matures, substrates will experience wider temperature changes, while higher resolution demands tighter constrains in substrate deformation. This paper presents data showing that through modification of the fictive temperature of the glass, Tf, the expansivity slope can be lowered. This is combined with better control of Tzc, which can be tuned to a narrower range to better match the requirements of each substrate. Depending on the application, performance improvements can exceed 30% with respect to traditional ULE glass.
