High-order resonances of AlN-film radial extensional mode resonator

There is strong demand for MHz-band resonators integrated in IC chips. Although AIN-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 AlN-fiIm resonator can excite a strong fundamental radial extensional (RE) mode theoretically, and the fabricated tuning-fork-type 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 [1). Generally, resonators which have a high-coupling and high-Q fundamental mode excite a lot of high-order resonant ones. The electrical characteristics of the high-order RE modes were focused on, and the resonant properties at the several-hundred-MHz-band level were investigated. An RE resonator fabricated by MEMS technology showed 3rd, 5th, 7th, 9th, and 11th order resonances between 160 and 530 MHz, and the figure of merit of the 5th order resonance was over 2. It means that a Pierce type oscillator using the RE resonator operates at 5th order resonant frequency. The high-order RE resonators have the potential to operate at a frequency of more than several-hundred MHz by further optimizing the fabrication process.