Determining the Linear Response of BAW Resonators within a Filter

The continued evolution of Bulk Acoustic Wave (BAW) resonator technology employed in RF filter fabrication demands ever more precise characterization and modeling techniques. To this end, BAW technologists fabricate test vehicle wafers comprising an array of resonators with varied areas, shapes, and other parameters. Resonator models to be employed in filter designs are constructed from the S11 parameter measurements of these resonator arrays. But because the test vehicle resonator arrays and the filter designs are fabricated on two different sets of wafers which employ subtly different fabrication processes, the resonators that are used to model the filter and those resonators comprising the filter will undoubtedly have slightly different resonant frequencies and effective coupling constants. To characterize the resonators in a fabricated filter more precisely, an in-situ characterization scheme is proposed. To this end, a Laser Doppler Vibrometer (LDV) system is used to measure/record the surface velocity of each of the resonators in a filter while set filter is being stimulated with RF power. The LDV system captures both the magnitude and phase of this surface velocity across the surface of each resonator, and this data is used in conjunction with a two-port measurement of the filter to extract the S11 parameter responses of each resonator in the filter. This characterization scheme is a powerful tool to debug and fine tune any arbitrary filter.