High-Q capacitive-piezoelectric AlN Lamb wave resonators

The use of capacitive-piezoelectric transducers, formed by separating a piezoelectric structure from its electrodes by sub-micron gaps, has raised the measured quality factor of aluminum nitride (AlN) Lamb wave resonators (LWR) from the ~1,000 of typical square-edged conventional devices (with contacting electrodes) to over 5,000 at 940 MHz, posting the highest reported Q for non-overmoded pure AlN resonators using d₃₁ (e₃₁) transduction at this frequency range. The Q · f product achieved here is significantly higher than that of a previous 1.2-GHz capacitive-piezoelectric contour-mode ring, mainly due to the use of Lamb wave modes that allow better support isolation to prevent energy loss to the substrate. In addition, the use of interdigital transducer (IDT) electrodes successfully decouples the resonance frequency from overall device dimensions, offering a CAD-definable design parameter for fine-frequency control. The effective coupling coefficient of k_eff² = 0.3% achieved by this device is lower than the 1.6% typically observed for conventional AlN Lamb wave resonators, but still sufficient to avoid pass-band distortion in the 0.1% bandwidth filters needed for next-generation RF channel-selecting communication front-ends.