Trapezoidal Mode: High Q Guided SAW with Full Transverse Mode Suppression

Achieving transverse mode suppression (TMS) while keeping a high quality factor (Q) and electro-mechanical coupling factor (k²) is key to realize low-loss and wideband filters. Piston mode configurations are promising for TMS, however, they require a large velocity difference (Δv) between the main and slow regions which often needs an additional process such as mass loading. In addition, for shear horizontal (SH) mode devices such as I.H.P / guided SAW (e.g. LT/SiO2/Si), these configurations potentially suffer from large scattering losses on the edge of electrodes. To combat these challenges, an optimized apodization has been proposed and achieved high Q, k² and good TMS. However, TMS especially for lower order modes still needs improvement. This paper proposes a novel TMS scheme that demonstrates a better TMS than piston mode or optimized apodization without any additional process maintaining high Q and k². A better TMS with the new scheme is realized by adding slow regions in optimized locations in apodized transducers. This approach manipulates TMs profiles in order to minimize their transduction efficiency. First, a scalar potential theory is used to analyze TM transductions in conventional configurations. This leads to the discovery of the "trapezoidal mode" approach. This new approach minimizing all TMs is characterized by a trapezoidal-shaped main mode. Finally, guided SAW resonators and filters with the trapezoidal mode as well as optimized apodization and piston mode configurations are fabricated on LT/SiO2/Si without an additional process to increase Δv. The resonator measurements demonstrate that the proposed configuration has the best TMS (full TMS) with similar k² and capacitance to the piston mode. The Bode Q for the new device is higher than for the piston mode resonator. This is attributed to lower scattering losses. The filter measurements confirm that the trapezoidal mode device achieves a ripple free passband with a full TMS and a lower insertion loss than the optimized apodization.