Free standing and solidly mounted Lamb wave resonators based on Al0.85Sc0.15N thin film

Lamb wave microresonators with wavelengths $λ$ of 5–8 μm, vibrating in the S0 mode, and having 75 electrode pairs were fabricated and characterized. The results were compared to theoretical predictions obtained by finite element simulation. The active material was a 1 μm-thick Al_0.85Sc_0.15N thin film. Two types of acoustic isolation solutions were implemented: the first one with freestanding plates fixed by two bridges to a device frame [freestanding Lamb wave resonator (FS-LWR)] and the second one containing an acoustic W/SiO₂ 5-layer reflector [solidly mounted Lamb wave resonator (SM-LWR)]. All devices showed excellent agreement with FEM predictions, regarding resonance frequency and piezoelectric coupling. The quality factors of the SM-LWR devices were 5–6 times larger than the ones of the freestanding structures fabricated by the same Al_0.85Sc_0.15N deposition process: we achieved a figure of merit of 12–18 (Q_p = 771, Q_s = 507, k² = 2.29%) at an operation frequency of 1430 MHz, which is so far the best performance realized with a MEMS Lamb wave resonator having a large number of electrode fingers. This performance opens up perspectives for filter applications.