Laterally coupled BAW filter using two acoustic modes

We study the use of the second-order thickness shear (TS₂) plate wave mode in a solidly-mounted, laterally acoustically coupled bulk acoustic wave filter to form a wider passband. Typically, the first-order thickness-extensional (TE1) mode is used to create the coupled even and odd mode resonances that build up the filter passband. An unwanted response can arise due to the TS₂ mode. By designing the dispersion of the thin-film reflector stack appropriately, the TS₂ mode can be brought close to TE1 so that both plate wave modes contribute to the passband. Measurement results for filter with varied structure parameters (electrode width, gap width, number of electrodes) are presented. For a 31-electrode filter with electrode width 9 μm, gap width 1.5 μm, and electrode length 200 μm, a total of 6 standing-wave resonances can be identified in the passband. The resonances span a frequency range of 230 MHz at 2 GHz (11.5%).