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

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), VLSAW, for two groups of LiTaO3 wafers: 21 36 degrees Y X-LiTaO3 wafers selected randomly from crystal ingots grown with different charge numbers in different furnaces, and 14 42 degrees 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 VLSAW and the predetermined relationship between VLSAW 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 (TC) measurements. A nearly true congruent composition of 48.49 Li2O-mol% was obtained through the precise VLSAW data for the 42 degrees 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 TC 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.