Layered SAW Resonators with Near-Zero TCF at Both Resonance and Anti-resonance

A layered SAW configuration consisting of a thin piezoelectric layer bonded onto a support substrate has been drawing attention for its superior performances—a higher quality factor (Q), higher electromechanical coupling factor (k
²
) and better temperature coefficient of frequency (TCF)—than conventional SAW technologies. However, one of the biggest challenges on the layered SAW is a much worse ΔTCF than conventional acoustic resonator technologies such as LiTaO3 (LT) SAW, temperature compensated (TC) SAW with SiO2 over coat and BAW, where ΔTCF is defined as TCF at anti-resonance (TCFp) minus TCF at resonance (TCFs). The worse ΔTCF causes a serious return loss/VSWR degradation as well as a significant impedance shift of the filters with temperature change. First, this paper analyzes the cause of ΔTCF degradation on layered SAW considering linear and nonlinear effects. Based on the analyses, a new cut of quartz substrate allowing a near-zero ΔTCF as well as near-zero TCFs and TCFp is proposed. The bonded wafer with thin LT on the new quartz substrate is fabricated and resonator performances on the new layered SAW substrate are evaluated. Both TCFs and TCFp are measured near 0 ppm/°C. The ΔTCF is only –2 ppm/°C, which is the best ΔTCF of all the conventional SAW/BAW technologies. In addition, the experimental results confirm a very high Q of over 5,000 and high k
²
of 8.8% at 1 GHz. Moreover, a spurious free out-of-band response is achieved.