Frequency tuning of film bulk acoustic resonators

This paper suggests a design for a Film Bulk Acoustic Resonator (FBAR) which utilizes a secondary piezoelectric layer for purposes of tuning the FBAR's resonant frequency. Currently, many ceramic resonators have difficulties in on-chip integration, power handling and electrode fabrication. FBARs are not only simple to fabricate and capable of full integration with CMOS/RF IC circuitry, but are also compact and can achieve high frequencies (GHz) with high quality factors. It is widely accepted that piezoelectric actuators encounter a significant change in mechanical stiffness between their open-circuit and closed-circuit states. In addition, it has been previously shown that the resonant frequency of a multi-layer FBAR is a function of the acoustic impedances and, correspondingly, the acoustic velocities, of its respective layers. Since the effective modulus term of the acoustic velocity of an FBAR layer is dependent on both the mechanical properties and electromechanical coupling of its piezoelectric element, and since electromechanical coupling can be altered by means a previously investigated shunt capacitor tuning concept, the stiffness of the piezoelectric tuning layer can be adjusted to vary the resonant frequency of the FBAR. Since difficulties have existed in matching FBAR resonant frequencies to specified values or making the frequencies stable during temperature variations, an active tuning capability for FBARs could offer many possible improvements. This work describes the application of the shunt capacitor tuning to a FBAR resonator and looks at the effects that varying different FBAR parameters have on the frequency range and degree of tunability of the device.