Measurement of Dispersion Properties of Active and Surrounding Regions in BAW Resonator

Acoustic wave fields both within the active electrode area of a solidly mounted 1.8 GHz bulk acoustic wave resonator, and around it, are measured using a heterodyne laser interferometer. Plate wave dispersion diagrams for both regions are extracted from the measurement data. The waves leaking from the resonator area enable the determination of the dispersion properties of the sourceless outside region. The experimental dispersion data reveal the cutoff frequencies of the acoustic vibration modes in the region surrounding the resonator, and therefore, the lateral energy trapping range of the resonator can be directly determined from the measurements. The onset of the acoustic leakage from the resonator is signalled on one hand by propagating wave modes appearing in the dispersion diagram of the surrounding region, and on the other hand by the abruptly diminishing amplitude of curves in the resonator dispersion diagram. This information is important for verifying and further developing the simulation tools used for the design of the resonators. Experimental wave field images, dispersion diagrams for both regions, and the threshold for energy leakage are discussed.