High overtone mode bulk acoustic wave resonators with polarization inverted ScAlN or SiAlN/AlN films for next-generation high frequency communication systems

For frequency filter applications in next-generation mobile communications, bulk acoustic wave (BAW) resonators are required to have high frequency operation, high k_eff², high Q, and high power durability. High-overtone mode BAW resonators with multilayer polarization-inverted films can operate at higher frequencies than standard single-layer BAW resonators, while maintaining the device volume and power durability. In this paper, the frequency characteristics and BAW behavior of polarization-inverted AlN film SMRs were investigated using the finite element method and Mason’s equivalent circuit models. Polarization-inverted ScAlN film bulk acoustic wave resonators and SiAlN/AlN solidly mounted resonators (SMRs) operating in the GHz range were experimentally fabricated and evaluated. We theoretically demonstrated that the k_eff² of the AlN film SMRs improves with increasing the number of polarization-inverted layers due to the improvement of the BAW energy trapping into the AlN films. We also experimentally found that the polarization-inverted SiAlN/AlN SMRs operating in high-overtone mode had higher Q and k_eff² than the single-layer AlN SMR.