CAPACITIVE-PIEZO TRANSDUCERS FOR HIGHER Q CONTOUR-MODE ALN RESONATORS AT 1.2 GHZ

A “capacitive-piezo” transducer that combines the strengths of capacitive and piezoelectric mechanisms to achieve an impedance and Q simultaneously lower and higher, respectively, than otherwise attainable by either mechanism separately, has allowed demonstration of a 1.2-GHz contour-mode AlN ring resonator with a motional resistance of 889 Ω and Q=3,073 higher than so far measured for any other d31-transduced piezoelectric resonator at this frequency. Here, the key innovation is to separate the piezoelectric resonator from its metal electrodes by tiny gaps to eliminate metal material and metal-to-piezoelectric interface losses thought to limit thin-film piezoelectric resonator Q’s, while also maintaining high electric field strength to preserve a strong piezoelectric effect. In addition, this capacitive-piezo transducer concept does not require dc-bias voltages and allows for much thicker electrodes that then lower series resistance without mass loading the resonant structure. The latter is especially important as resonators and their supports continue to scale towards even higher frequencies.