Broadband ultrasonic transducers using a LiNbO3 plate with a ferroelectric inversion layer

Some heat treatment of a LiNbO₃ plate 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 that broadband ultrasonic transducers can be obtained at a certain thickness ratio of inversion layer to plate and that the transducer characteristics differ depending on whether the inversion layer is on the front side or on the backside. The broadband characteristics are experimentally demonstrated by fabricating transducers with 9 MHz or 75 MHz center frequency using 36° rotated Y-cut LiNbO₃ plates with a ferroelectric inversion layer.