Line-focus acoustic microscopy measurements of acoustic properties of LiTaO₃ crystal plates with an inversion layer

Inversion layers of 2 μm and 30 μm thickness were formed at the negative dipole-end (-c) face of a Z-cut LiTaO₃ crystal plate by proton exchange followed by heat treatment. The phase velocities of leaky surface acoustic waves (LSAWs) on both the positive (+c) and negative (-c) faces of the plate were measured by line-focus acoustic microscopy versus the angle of propagation, at a frequency of 225 MHz, and the results were compared with theoretical predictions. It is shown that for an inversion layer of 30 μm thickness the phase velocities of LSAWs on the +c and -c faces are the same, and almost equal to the velocities of the virgin plate. This suggests that the material constants, which are decreased due to the proton exchange, are restored to the original values by the heat treatment.