LT/Quartz Layered SAW Substrate with Suppressed Transverse Mode Generation

Abstract

The layered SAW with a thin piezoelectric layer on a support substrate is a promising technology for its high quality factor (Q), high electromechanical coupling factor $k^{2}$ and small temperature coefficient of frequency (TCF). A large $\Delta\mathrm{TCF}$ (TCF difference between resonance and anti-resonance) is one of the biggest challenges for this technology. Last year, we proposed a new LT/quartz substrate providing a near-zero ATCF [1]. Another challenge for layered SAW is transverse mode suppression. This paper analyzes the transverse modes on layered SAW and proposes a LT/quartz substrate which drastically suppresses the transverse mode generation. Dispersion/slowness curves for SAW propagating on metal gratings on layered SAW substrates are calculated by FEM to analyze the mechanism of transverse mode generation. It is revealed that most layered SAW substrates such as LT/Si, LT/sapphire or LT/quartz have convex slowness curves which cause transverse mode generation. One way to suppress the transverse modes is to have a vertical slowness curve. Based on careful slowness calculations, a quartz orientation is optimized to allow a vertical slowness curve in LT/quartz near the propagation angle $\theta=0$ . The optimized quartz orientation is 69°Y90°X which is the same orientation as the one we proposed in [1]. The LT/69°Y90°X quartz bonded wafer is fabricated and a transverse mode free response is experimentally confirmed without any apodization or piston mode configuration for 1 port resonators. In addition, both TCF and $\Delta \mathrm{TCF}$ are measured near 0 ppm/°C, and a spurious free out-of-band response is achieved while maintaining a high Q and high $k^{2}$ .