Accurate Calibration Line for Super-Precise Coefficient of Thermal Expansion Evaluation Technology of TiO2-Doped SiO2Ultra-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 ±0.0084 wt % for ±2 σ (σ: standard deviation) from the LSAW velocity measurement resolution of ±0.14 m/s. The TiO2 concentration for the standard TiO2–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.