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

We proposed a new coefficient-of-thermal-expansion (CTE) evaluation method for ultra-low expansion glasses using the line-focus-beam ultrasonic material characterization system. In this paper, we investigated evaluation procedures for photomasks and optical mirrors with practical size used as reflective optics in extreme ultraviolet lithography (EUVL) systems. Two specimens were prepared with their surfaces parallel to the striae plane from commercial TiO2-SiO2 ultralow- expansion glass ingots. Homogeneities/inhomogeneities of specimens were evaluated at 225 MHz. Evaluation procedures with sufficient accuracy were established for analysis of striae parameters such as striae periodicity and variations. Our ultrasonic method should be standardized as a new evaluation method not only for development of the EUVL-grade glass and evaluation of the production processes, but also for quality control and selection of the production lots.