Radial extensional mode AlN-film resonator with high coupling factor

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There is strong demand for MHz-band resonators integrated in IC chips. Although AlN-film resonators, such as FBARs, have good compatibility with CMOS fabrication, it is difficult to produce low-frequency devices because the vibration mode is thickness extension. It has recently been reported that a disc-type AIN-film resonator can excite radial extensional (RE) modes strongly [6]. In this study, the electrical characteristics of the fundamental RE mode were focused on, and the relation between the Q value and the structure of the resonator was investigated. Because the RE resonator, such as a disc-type resonator, had a null point at the center of the resonant part, it was difficult to connect an electric lead line at the null point using conventional AIN-film fabrication processes. We adopted a tuning-fork-type structure, which consisted of two RE resonant parts, a connecting part, and two supporting beams. Because the tuning-fork-type resonator had a null point at the center of the connecting part between the RE resonant parts, it was easy to connect the supporting beams, which acted as the electric lead lines too, at the null point. A fabricated RE resonator exhibited a high coupling factor of 2.8% and a high Q value of 3000 at the series resonant frequency and 4000 at the parallel resonant frequency. The figure of merit was 71 at the series resonant frequency and 94 at the parallel resonant frequency. These are the highest values for contour-mode AIN resonators so far reported and indicate that the RE resonator integrated in an IC chip is suitable for oscillator and filter applications.