Evaluation of mass-produced commercial LiTaO₃ single crystals using the LFB ultrasonic material characterization system

A mass-production line of lithium tantalate (LiTaO₃) crystals with a maximum charge number of 60 for surface acoustic wave (SAW) devices was evaluated with the line-focus-beam (LFB) ultrasonic material characterization system. Some serious problems associated with chemical compositions were observed and resolved by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), V_LSAW, for two groups of LiTaO₃ wafers: 21 36°Y X-LiTaO₃ wafers selected randomly from crystal ingots grown with different charge numbers in different furnaces, and 14 42°Y X-LiTaO₃ wafers obtained at the top, middle, and bottom parts from 5 crystals selected from 39 crystals grown successively in the same furnace and crucible. Using the measured V_LSAW and the predetermined relationship between V_LSAW and Li₂O concentrations, M(Li₂O), we estimated the average M(Li₂O) controlled in the current mass-production line to be about 48.77 mol% with a maximum difference of 0.75 mol%. The composition for each crystal ingot increased linearly about 0.04 mol% from the top to the bottom, and no dependence on the charge number was observed, as the melt composition used for the mass production was controlled through Curie temperature (T_C) measurements. A nearly true congruent composition of 48.49 Li₂O-mol% was obtained through the precise V_LSAW data for the 42°Y X-LiTaO₃ wafers, that was about 0.3 mol% less than the melt composition in the production line. It was also pointed out that the T_C measurement conditions, including room temperatures surrounding the measurement systems, should be re-examined for reliable production control. A guideline for more efficient mass production of the crystals has been established concerning the true congruent composition as the starting material.