Determination of lateral mode dispersion from full-field imaging of film bulk acoustic resonator motion

A full-field view laser ultrasonic imaging method has been developed that measures acoustic motion at a surface without scanning. Images are recorded at normal video frame rates and heterodyne principles are used to allow operation at any frequency from Hz to GHz. Both acoustic amplitude and phase are recorded allowing Fourier transformation of the displacement images to provide a mapping of excited mode wavenumbers at each frequency. Measurements as a function of frequency provide a means for mode identification and a direct measure of the lateral mode dispersion. Results are presented at frequencies near the first longitudinal plate thickness mode (∼876 MHz) demonstrating simultaneous excitation of lateral modes with nonzero wavenumbers in an electrically driven AlN thin film bulk acoustic resonator. Discussion and analysis are presented illustrating identification of specific lateral modes and illustration of the effects of physical dimensions and material elastic properties on the measured dispersion relations.