A Promising Evaluation Method of Ultra-Low-Expansion Glasses for the Extreme Ultra-Violet Lithography System by the Line-Focus-Beam Ultrasonic Material Characterization System

A super-precise method of evaluating the coefficient of thermal expansion (CTE) of ultra-low-expansion glasses was developed using the line-focus-beam ultrasonic material characterization (LFB-UMC) system and was demonstrated for TiO2-doped SiO2 glass. The sensitivity and resolution in the velocity measurement of leaky surface acoustic waves (LSAWs) in CTE were estimated to be 4.40 (ppb/K)/(m/s) and ±0.77 ppb/K for ±2σ (σ: standard deviation) at 225 MHz. LSAW velocity differences caused by different TiO2 concentrations and distributions or striae in the specimens were successfully detected and evaluated, providing two-dimensional information. This ultrasonic method is effective for evaluating ultra-low-expansion glasses needed for extreme ultra-violet lithography (EUVL) systems.