High-Frequency and Spectrum-Clean Shear-Horizontal Acoustic Wave Resonators with AlN Overlay.

Abstract

By bonding the sub-wavelength-thick lithium niobate (LiNbO3) layer to high-phase-velocity (vp) substrates, such as Si, the shear-horizontal (SH) modes no longer couple with the bulk modes leaking into substrates. As the propagation loss is no longer the major concern for these types of nonleaky SH wave devices, the YX-LiNbO3 with a low rotation angle providing ultra-large coupling coefficient (keff2) can be used. In addition, by overlaying a high-velocity layer such as AlN on top of LiNbO3/Si, the vp of the SH wave can be significantly enhanced at a small cost of keff2. By a careful design of the stack, both the wide-band spurious (Lamb wave) and near-band spurious (Rayleigh wave) are suppressed successfully. This paper focuses on the design of layered substrate not only to optimize its resonance characteristics—series frequency (fs), quality factor (Q), keff2, and temperature coefficient of frequency (TCF)—but also for eliminating the out-of-band spurious responses. The optimized substrate design demonstrates the minimal propagation loss, high fs of 3 GHz, large keff2 of 14.4% and a spurious-free response at 0–6 GHz. These novel nonleaky SH wave devices can potentially enable the low loss and wideband processing functions, which is promising for the 5G/6G radio frequency (RF) communication systems.