Striae evaluation of TiO 2 -SiO 2 ultra-low expansion glasses using the line-focus-beam ultrasonic material characterization system

Abstract

Super accurate evaluation method for TiO 2 -doped SiO 2 ultra-low-expansion glass having periodic striae associated with its fabrication process was investigated using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. To obtain absolute values of leaky surface acoustic wave (LSAW) velocities measured with the LFB-UMC system, proper standard specimens of the glass for system calibration was examined. Using a specimen with a surface perpendicular to the striae plane as the standard specimen, a reliable standard LSAW velocity of 3308.18 m/s within ±0.35 m/s for the calibration was obtained regardless of influence of velocity variations due to the periodic striae. Also, we determined the accurate relationship between the TiO 2 concentrations and LSAW velocities (sensitivity: -0.0601 wt%/(m/s)) so that the TiO 2 concentration of the standard specimen with a LSAW velocity of 3308.18 m/s was determined to be 7.09 wt%. Furthermore, to evaluate more reliably the more homogeneous ultra-low-expansion glasses in the near future, the measurement accuracy of the LSAW velocity was improved with a method using the LFB device with a larger curvature radius R operating at lower frequency from ±0.0053% for R=1.0 mm at 225 MHz to ±0.0020% for R=2.0 mm at 75 MHz.

Evaluation and selection of EUVL-grade TiO2-SiO2 ultra-low-expansion glasses using the line-focus-beam ultrasonic material characterization system

“…And, the velocity differences correspond to 57.2 ppb/K and 33.8 ppb/K for specimens A and B from the sensitivity of the LSAW velocity to the CTE {4.41 (ppb/K)/(m/s)}. 17 The CTE variations were six and two times larger than the specifications (10 ppb/K and 15 ppb/K, respectively). This is considered to be due to the fact that the manufacturer evaluates the CTE of the glass ingots by measuring the velocities of longitudinal waves propagating in the thickness direction of the ingots and uses the two-dimensional velocities of the averaged velocities.…”

“…11 The reproducibility of the LSAW velocities was estimated within ±0.17 m/s for ±2 at 225 MHz, and the resolution for CTE was ±0.74 ppb/K. 17 Apparent velocities averaged on the LSAW propagation region interacting with the periodic striae are obtaned, when the measurement region W D is larger than the period of striae. 11 W is the LSAW propagation distance on the specimen surface in the focused direction.…”

“…So far, we obtained an extremely high resolution of CTE of ±0.74 ppb/K for ±2 for TiO 2 -SiO 2 glasses at 225 MHz. 9,10,17 And, we detected periodic striae caused by the periodic distributions of TiO 2 concentrations as velocity variations. 9, 10 We also investigated methods to evaluate specimens with periodic striae and to prepare appropriate standard specimens with periodic striae.…”