Development of an ultrasonic system for super-precise measurement of zero-CTE temperature of EUVL-grade TiO2-SiO2 ultra-low-expansion glasses

This paper presents the development of a practical system for super-precise evaluation of zero-CTE temperatures T(zero- CTE) of TiO 2 -SiO 2 glasses for extreme ultraviolet lithography (EUVL) by measuring leaky surface-acoustic-wave (LSAW) velocity VLSAW with a line-focus-beam ultrasonic material characterization (LFB-UMC) system. This new system can evaluate T(zero-CTE) from 20 to 150°C on the surfaces of glass substrates for photomasks and optical mirrors located at different positions in EUVL systems. This system operates in a stabilized temperature measurement environment (e.g., 23.00°C). It was demonstrated at 225 MHz for homogenized TiO 2 -SiO 2 glass specimens with different annealing temperatures and realized an extremely homogeneous glass ingot with a ΔT(zero-CTE) of 1.6°C around 23.2°C. This ultrasonic system enables both glass manufacturers and users to speedily inspect all glass substrates with reliable data of T(zero-CTE).