L-band Lithium Niobate-On-Insulator SH₀ Resonators Avoiding Transverse Spurious Modes through Electrode Apodization

This work discusses how apodization of the interdigital transducers (IDT) of shear horizontal (SH0) acoustic wave resonators allows to avoid the presence of spurious modes between resonances and enhance the achieved electromechanical coupling and quality factor. Achieving spurious-free resonators is fundamental to face the design of ladder filters with flat passbands. The proposal is demonstrated with manufactured resonators on a 30 degrees YX-cut lithium niobate-on-insulator (LNOI) structure comparing the performance of apodized and non-apodized devices. Clean resonators without transverse spurious modes have been obtained showing an improvement in quality factor and electromechanical coupling coefficient, attaining k(t)(2) = 17%, Q(max) similar to 800 and impedance ratios of 69 dB.