LiNbO3 ultrasonic transducers with an inverted-domain layer for radiation to a solid medium

Heat treatment of lithium niobate (LiNbO₃) plates induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, the ultrasonic transducer using such a piezoelectric plate is expected to operate over a wide frequency range. In this paper, it is shown both theoretically and experimentally that broadband ultrasonic transducers for radiation to solid media can be obtained when the inversion layer is on the side of the acoustic load medium and the inversion layer-to-plate thickness ratio is around 0.3. A 50-110 MHz ultrasonic transducer was fabricated using a 36° rotated Y-cut LiNbO₃ plate with an inversion layer. The conversion loss characteristic was evaluated when it was bonded to an acoustic medium of fused-silica by a low-temperature Au-diffusion bonding technique. The 3-dB specific bandwidth was as large as 74% and the minimum conversion loss was as low as 2 dB.