Direct measurement of spurious mode properties in thin film BAW resonator

The acoustic wave fields excited in a 1.8 GHz AlN thin film bulk acoustic wave resonator are measured in detail with a heterodyne laser interferometer. The electrical response of the resonator exhibits a strong thickness resonance onto which spurious modes, caused by lateral standing plate waves, are superposed. The acoustic wave fields are measured with a heterodyne interferometer and are used to calculate dispersion diagram of the laterally propagating waves responsible for the spurious electrical responses. The dispersion diagram features a discrete eigenmode spectrum due to the finite lateral dimensions of the resonator. An equivalent circuit model for a multi-mode resonator is fitted to the mechanical resonator response extracted along a single curve in the dispersion diagram, and is used to determine properties, such as Q-values, of the individual lateral eigenmodes. Measured wave field images, extracted dispersion curves, and the eigenmode spectrum with the model fitting results are presented.