Accurate calibration line for super-precise coefficient of thermal expansion evaluation technology of TiO₂-doped SiO₂ ultra-low-expansion glass using the line-focus-beam ultrasonic material characterization system

We obtained an accurate relationship between the leaky surface acoustic wave (LSAW) velocities and TiO₂ concentrations. Such a relationship is needed for the line-focus-beam ultrasonic material characterization (LFB-UMC) system as a new technology for evaluating the coefficient of thermal expansion (CTE) of ultra-low-expansion glasses with extremely high precision. Using commercially available TiO₂-SiO₂ glass with periodic striae, averaged velocities for LSAW propagation parallel to the striae plane at 225 MHz and averaged TiO₂ concentrations by X-ray fluorescence analysis were measured on the surface of seven specimen substrates prepared perpendicular to the striae plane. The obtained sensitivity of LSAW velocity to TiO₂ concentration was -0.0601 wt%/(m/s), and the measurement resolution was estimated to be ±0.0084wt% for ±2σ (σ: standard deviation) from the LSAW velocity measurement resolution of ±0.14 m/s. The TiO₂ concentration for the standard TiO ₂-SiO₂ glass specimen with a LSAW velocity of 3308.18 m/s for system calibration was determined to be 7.09 wt %. This ultrasonic technology will contribute greatly to the development of the ultra-low-expansion glasses needed for extreme ultraviolet lithography (EUVL) systems.