Thickness-tunable high-velocity interlayers for coupling and FBW control in dual-mode hybrid SAW/BAW resonator filters

This paper reports a comprehensive and systematic design methodology for controlling the effective electromechanical coupling coefficient ( K e f f 2 ) of solidly mounted dual-mode Laterally Coupled Alternating Thickness (LCAT) and hybrid Surface/Bulk Acoustic Wave (SAW/BAW) resonators. By systematically varying the thickness of a high-acoustic-velocity AlN interlayer between the piezoelectric Sc 0.4 Al 0.6 N layer and the SiO 2 reflector, bidirectional redistribution of K e f f 2 between Rayleigh and Sezawa modes is demonstrated through finite element modeling. Simulation results show K e f f 2 -tunability from 1.97–4.69% and 7.72–2.45% for the Rayleigh and Sezawa modes of LCAT resonators, respectively, and from 9.3–18.5% and 21.5–10.2% for quasi-Rayleigh and quasi-Sezawa modes in hybrid SAW/BAW resonators. Theoretical Integration of these thickness-tunable resonators enables dual-passband filter functionality with independently controllable fractional bandwidths (FBWs) from 3.1%–10%. All filters were synthesized using modified Butterworth-Van Dyke equivalent circuit models with quality factors calibrated against twelve fabricated solidly mounted ScAlN resonators from recent literature (Q = 50–400), demonstrating FBW-tunability across realistic loss scenarios. The design methodology offers a systematic approach for achieving flexible bandwidth allocation in dual-passband filters for advanced high-frequency applications, with experimental implementation currently underway.