Thermal expansion behavior of proposed EUVL substrate materials

Abstract

The enhanced demands for substrate materials for next- generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material. The dependence of thermal expansion of Zerodur on process parameters is illustrated herein as well as its utility for EUV substrate material demands. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5 degrees C. Laboratory samples of Zerodur exhibit a CTE corresponding to the lowest expansion class of the SEMI standard P37. By further variation of process parameters, the position of zero crossing, e.g. at 30 degrees C, can be varied, revealing an attractive attribute feature of Zerodur. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm- scale: No CTE variation was observed within the error of measurements for a block exhibiting +/- 2 * 10-6 variation in refractive index. A new dilatometer type is in the course of development. First operational results are expected in Summer 2002 with an increased accuracy < ppb/K in the temperature range of 17 to 30 degrees C.