The propagation of longitudinally polarized acoustic modes along thin piezoelectric plates has been previously reported [1], aiming at the design of high frequency and low loss electroacoustic devices suitable for working in liquids. The investigation of the acoustic field profile across the plate revealed the presence of higher order quasi-longitudinally polarized Lamb modes, traveling at velocities close to that of the longitudinal bulk acoustic wave propagating in the same direction. The commonly used fundamental anti-symmetric Lamb modes A_o is suitable for liquid sensing only for very small plate thickness-to-wavelength ratio h/l corresponding to a phase velocity lower than that of the liquid to be tested (v<1400 m/s), which results in low sensor operating frequencies and sensitivity. The fundamental symmetric mode S₀ is much faster than A₀ (thus allowing higher operating frequencies) but it is suitable for liquid sensing application only for a limited h/l range (up to 0.1) corresponding to predominant longitudinal polarization. This latter limitation can be overcome by investigating the propagation of higher order quasi-longitudinal modes with close-to-zero transverse particle displacement component at the surface of the plate contacting the liquid environment. The zero-surface transverse component will prevent attenuation of acoustic energy into the liquid. Disperse, McGill and COMSOL Multiphysics simulations were used to study the dispersion curves and the acoustic field profile of the fundamental and higher order modes traveling in SiN/AlN thin suspended membranes, for fixed SiN thickness (200 nm) and variable AlN thickness. Two higher order modes, qS₁ and qS₂, were found that are slowly dispersive and have dominant longitudinal particle displacement component, at h_AlN/l = 0.8 and 1.6 respectively. The acoustic field profile shows that the modes have close to zero out-of-plane particle displacement at the surface of the thin film. The electroacoustic coupling coefficient K² dispersion curves were studied for four different electroacoustic coupling configurations, based on interdigital transducers (IDTs) positioned at the AlN surface, with or without a metal floating electrode at the opposite layer surface. The theoretical sensitivity to the density-viscosity product of a Newtonian liquid was calculated for the two modes. In order to verify the technological feasibility of such device and to test the technological complexity, Cr/Au IDTs were implemented onto the bare Silicon Nitride (Si₃N₄) thin suspended membrane (200 nm thick, and with the area equal to 1.5 mm x 1.5 mm) by electron beam lithography (EBL) technology. Then a c-AlN layer was deposited by rf magnetron sputtering technique onto the suspended membrane. The IDTs width of 0.5 mm (equal to l/4) was achieved with EBL resolution of 500 nm onto the Si₃N₄ suspended membrane (wavelength l = 2 μm). The piezoelectric AlN thin film was deposited onto a reduced area of the surface of the Si₃N₄ suspended membrane by using a shadow mask with an opening area of 1 mm x 1 mm, thus an AlN layer was sputtered having an “island-like” structure, with the thickness of 1.4 μm and 3 μm. The use of a mask is a method that offers the remarkable advantage to reduce the complexity of the device fabrication process based on several technological steps, including the silicon substrate surface or bulk micromachining and the etching of a sacrificial layer to release the membrane.

 [1] Caliendo, Cinzia. "Longitudinal Modes along Thin Piezoelectric Waveguides for Liquid Sensing Applications." Sensors 15.6 (2015): 12841-12856.