Impact of Si₃N₄ layer insertion to hybrid modes in low-velocity surface acoustic wave resonator on LiTaO₃/SiO₂/Si substrate

Abstract This paper discusses the influence of Si 3 N 4 insertion for the hybrid mode suppression in the low-velocity interdigital transducer (IDT)/LiTaO 3 (LT)/SiO 2 /Si surface acoustic wave (SAW) resonator structure. The hybrid mode is caused by the coupling between the main shear-horizontal (SH) SAW and unnecessary Rayleigh SAW, and three-dimensional finite element analysis is necessary for the theoretical analysis. It should be noted that the hybrid mode exists even when the Rayleigh mode excitation is completely suppressed. First, it is shown that the insertion shifts the hybrid mode resonances to the HF side and is weak with scarce influence on the main SH SAW responses. It is also shown that adding acoustic loss in Si 3 N 4 is effective in suppressing the hybrid mode responses selectively. Then, the use of periodically slotted Si 3 N 4 is studied for the hybrid-mode scattering instead of the absorption. The slots are X-shaped and filled with SiO 2 . It is demonstrated that the hybrid mode can be suppressed without badly affecting the main resonance by properly designing the slot shape and width.