Zero expansion glass ceramic ZERODUR®roadmap for advanced lithography

The zero expansion glass ceramic ZERODUR^® is a well-established material in microlithography in critical components as wafer- and reticle-stages, mirrors and frames in the stepper positioning and alignment system. The very low coefficient of thermal expansion (CTE) and its extremely high CTE homogeneity are key properties to achieve the tight overlay requirements of advanced lithography processes. SCHOTT is continuously improving critical material properties of ZERODUR^® essential for microlithography applications according to a roadmap driven by the ever tighter material specifications broken down from the customer roadmaps. This paper will present the SCHOTT Roadmap for ZERODUR^® material property development. In the recent years SCHOTT established a physical model based on structural relaxation to describe the coefficient of thermal expansion’s temperature dependence. The model is successfully applied for the new expansion grade ZERODUR^® TAILORED introduced to the market in 2012. ZERODUR^® TAILORED delivers the lowest thermal expansion of ZERODUR^® products at microlithography tool application temperature allowing for higher thermal stability for tighter overlay control in IC production. Data will be reported demonstrating the unique CTE homogeneity of ZERODUR^® and its very high reproducibility, a necessary precondition for serial production for microlithography equipment components. New data on the bending strength of ZERODUR^® proves its capability to withstand much higher mechanical loads than previously reported. Utilizing a three parameter Weibull distribution it is possible to derive minimum strength values for a given ZERODUR^® surface treatment. Consequently the statistical uncertainties of the earlier approach based on a two parameter Weibull distribution have been eliminated. Mechanical fatigue due to stress corrosion was included in a straightforward way. The derived formulae allows calculating life time of ZERODUR^® components for a given stress load or the allowable maximum stress for a minimum required life time.