Evaluation of TiO 2 -SiO 2 ultra-low-expansion glass fabricated by the soot method using the line-focus-beam ultrasonic material characterization system

In this paper, we tried to fabricate a TiO₂-doped SiO₂ (TiO₂-SiO₂) glass ingot by the soot method, and homogenized the glass ingot by the zone-melting method. Homogeneities of the specimens were evaluated by measuring leaky surface acoustic wave (LSAW) velocity using the line-focus-beam ultrasonic material characterization system at 225 MHz. Two-dimensional LSAW velocity distributions having an average velocity of 3304.08 m/s with a maximum velocity difference of 3.85 m/s were measured for a homogenized specimen. Striae were not observed for the specimen. The velocity difference corresponds to 17.0 ppb/K from the sensitivity of the LSAW velocity to the CTE {4.41 (ppb/K)/(m/s)}. However, the velocity distributions excluding the edge parts were within ±1.13 m/s, corresponding to the CTE specification of ±5 ppb/K required for EUVL-grade glass. We also discussed the relationship between LSAW velocities and fictive temperatures by heat-treating a part of the homogenized specimen.