A Super-Precise CTE Evaluation Method for Ultra-Low-Expansion Glasses Using the LFB Ultrasonic Material Characterization System

Abstract

A super-precise method of evaluating the coefficient of thermal expansion (CTE) of ultra-low-expansion glasses for future extreme ultra-violet lithography (EUVL) systems was developed using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. Evaluation was demonstrated for two commercial glasses, TiO2-SiO2 glass (C-7971) and Li2O-Al2O3-SiO2 glass ceramic (Zerodur). For the C-7971 specimens, the sensitivity and resolution in the velocity measurement of leaky surface acoustic waves (LSAWs) for the CTE were estimated to be 4.40 (ppb/K)/(m/s) and ±0.77 ppb/K for ±2σ (σ: standard deviation). LSAW velocity differences caused by different TiO2 concentrations and distributions or striae in each specimen were successfully detected and evaluated. For the Zerodur specimens, LSAW velocity differences associated with the chemical compositions and crystallization conditions were observed among different ingots and specimens. This ultrasonic method is expected to be an extremely useful and effective CTE evaluation technology and to contribute to improving and developing EUVL-grade glass materials.

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

“…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.…”

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

“…The result was ±2 =±0.07 m/s (±0.0020%), which was a significant improvement compared to the ±0.0053% obtained at 225 MHz with a cylindrical lens of R=1.0 mm. 5 The achieved measurement accuracy of LSAW velocity corresponds to ±0.29 ppb/K in CTE. …”

“…We recently proposed this system as a new evaluation technology for CTE of ultra-low-expansion glasses in EUVL systems. 5 Previous investigations involving commercial TiO 2 -SiO 2 glass have demonstrated a CTE sensitivity of 4.40 (ppb/K)/(m/s) and a resolution of ±2 =±0.77 ppb/K ( : standard deviation) at 225 MHz. 5 This resolution is higher than those obtained by conventional CTE evaluation techniques.…”

“…5 Previous investigations involving commercial TiO 2 -SiO 2 glass have demonstrated a CTE sensitivity of 4.40 (ppb/K)/(m/s) and a resolution of ±2 =±0.77 ppb/K ( : standard deviation) at 225 MHz. 5 This resolution is higher than those obtained by conventional CTE evaluation techniques. 1,6,7 Two-dimensional information of the LSAW velocities reflects the substrate surface characteristics which play a very important role for the reflective optics in the EUVL systems.…”

Evaluation of synthetic silica glasses by the ultrasonic microspectroscopy technology

“…We proposed an evaluation method of glass materials using ultrasonic microspectroscopy (UMS) technology [1][2][3]. UMS technology was previously applied to characterization of silica glasses, and it was found that acoustic properties, viz.…”