Gap reduction based frequency tuning for AlN capacitive-piezoelectric resonators

A voltage controlled resonance frequency tuning mechanism, capable of effecting 1,500 ppm frequency shifts or more, is demonstrated for the first time on an AlN capacitive-piezoelectric resonator. The key enabler here is a compliant top electrode suspension that moves with applied voltage to effectively vary capacitance in series with the device, hence changing its series resonance frequency. Capacitive-piezoelectric AlN micromechanical resonators, i.e., those with electrodes not directly attached to the piezoelectric material, already exhibit high Q-factors compared to attached-electrode counterparts, e.g., 8,800 versus 2,100 at 300 MHz; are on/off switchable; and, as shown in this work, can exhibit electromechanical coupling C_x=C₀ of 1.0%. This new ability to tune frequency without the need for external components now invites the use of on-chip corrective schemes to improve accuracy or reduce temperature-induced frequency drift, making an even more compelling case to employ this technology for frequency control applications.