Measuring and tailoring CTE within ULE glass

Corning Incorporated is improving material and metrology in order to meet the requirements for both EUVL optics and photomask substrate applications. The EUV optics requirements present a unique challenge to the lens designer. The temperature of each optic in the system can experience a different thermal profile based upon the geometry of the element and the intensity of the beam at each element location. This places a need on the optical material for small variation in the coefficient of thermal expansion (CTE) uniformity and the ability to achieve targeted optimum zero CTE cross-over temperatures. This paper addresses Corning’s ability to target specified CTE values as well as discusses a new metrology tool for measuring CTE variations within the glass. Past data suggested that index variation within the material were related to CTE variations. This correlation was investigated with the results presented here. This preliminary work suggests a new metrology tool with the capability of non-destructively measuring peak to valley (P-V) CTE variations to within 70 parts per trillion per degree Kelvin (ppt/K) at possible spatial frequencies in the micron range on thick or thin samples. This technique is vital for certifying photomasks and will be the foundation needed to reduce CTE variations in photomasks and optics to targeted values of less than 1 ppb/K for future EUVL needs.