High Frequency Strip-Type Solidly-Mounted Shear Mode Bulk Wave Resonator Using X-LT

Abstract

The common materials of bulk acoustic wave (BAW) resonators are AlN and ScAlN. For a wider bandwidth, the Sc to Al ratio in ScAlN is increased up to 40%, but the Q factor often deteriorates and the impedance (Z) ratio does not improve significantly. For better filter performance, this study investigated monocrystalline LiTaO 3 (LT) as a piezoelectric material for a high frequency strip type thickness shear mode (TS) solidly mounted resonator (SMR). We selected the SMR instead of a film BAW resonator (FBAR) with a cavity, because the suspended ultrathin LT plate is fragile. The strip type TS mode SMR is composed of two Al electrodes, a 2.0 µm thick X LT plate, an Al electrode, eight layers of SiO 2 and Ta film (4+4 layers) and a 350 µm thick Si substrate from the top. The fabricated SMR shows a center frequency of 1 GHz, a bandwidth of 6.7% and an $Z$ ratio of 62 dB. The temperature coefficients of frequency (TCF) at series and parallel resonance are as good as - 17 and -34 ppm/°C, respectively.