Interaction between the Rayleigh-type SAW and the SH-wave in a periodic grating on a 128°-LN substrate

Ladder-type filters employing SAW resonators demand high Q-factors at both the resonance and anti-resonance frequencies. Also, spurious-free, low-loss response is especially important for novel filters having multiple passband modes in which one or more of the passbands is far from the resonant frequency of a SAW device. 128° LiNbO₃ is traditionally considered as a substrate in which SH-SAW modes are very weakly coupled to transducer electric fields and thus it may provide a good avenue for improving SAW resonator loss performance. We have experimentally studied a large number of synchronous SAW resonators on 128° LiNbO₃ substrate with different aluminum relative thickness (h/ λ ) and resonant frequencies near 900MHz. All theoretical modeling of the resonators was performed using the FEM/BEM simulation tool FEMSAW. In almost all measured devices and FEMSAW simulations we observed an interaction of the SAW with another acoustic mode. However, the very weak coupling of the IDT to the SH-SAW mode does not explain the data. The observed interaction is likely to be due to mechanical scattering of the strongly excited Rayleigh-type SAW into a shear SAW.