Homogeneous TiO₂-SiO₂ ultralow-expansion glass for extreme ultraviolet lithography evaluated by the line-focus-beam ultrasonic material characterization system

A TiO₂-SiO₂ glass ingot was fabricated by the soot method and homogenized by the zone-melting method. Elastic homogeneities of specimens prepared at different production stages were evaluated by measuring leaky surface acoustic wave (LSAW) velocity V_LSAW using the line-focus-beam ultrasonic material-characterization system at 225MHz. The homogeneous VLSAw distribution was within ±1.13m/s, corresponding to a variation of the coefficient of thermal expansion (CTE) of ±5 ppb/K, and no striae were observed. V_LSAW changes associated with residual stress inside the ingot could be detected. The relationships among V _LSAW, TiO₂ concentration, density, CTE characteristics, and fictive temperature were investigated. We found the possibility for extremely homogeneous TiO₂-SiO₂ glass with zero-crossing in CTE at a desired room temperature.