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

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 +/- 2 sigma (sigma: 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.