Solidly mounted resonator using shear horizontal mode plate wave in LiNbO₃ plate

A 0-th shear horizontal (SHo) mode plate wave has an electromechanical coupling factor k² larger than 50% in a (0°, 120°, 0°) LiNbOs (LN) plate thinner than 0.1λ (λ is a pitch of interdigital transducer). However, the structure is fragile, because the plate thickness is as thin as 0.5∼0.6 μm for devices working in several hundred MHz range. To address this issue, a solidly mounted resonator (SMR) structure was applied to SH mode plate resonators. The materials and layer thicknesses of a Bragg reflector, the Euler angle and thickness of a LN plate, and the thickness and material of electrodes were investigated to obtain large k² and spurious-free response by finite element method (FEM) simulation. The designed Bragg reflector is composed of 6 layers of SiO2 and AlN. The best Euler angle is (0°, 90°, 0°) and the suitable thickness of a LN plate is 0.3 to 1 λ. Also, heavy and thick electrodes are better for interdigital transducers (IDT). This design is significantly different from that of the cavity type. A SMR type plate wave resonator of several hundred MHz was fabricated using 2 μm thick LN, and a wide bandwidth of 20% was measured.