Ultrasonic flexural Lamb waves in thin films have applications for sensing and actuation. We begin with a review of the Rayleigh-Lamb dispersion relation, which governs the propagation of Lamb waves in free, homogeneous, isotropic plates. We derive the dispersion relation for the lowest-order flexural (A0) mode in terms of in-plant tension, stiffness, mass per unit area and rotary inertia. A radiation-load method is used to treat viscoelastic loading. The resulting dispersion relations accurately predict sensor response -- changes of wave velocity, frequency and attenuation -- to various measurands, including force, pressure, mass changes, fluid density and viscosity, and elastic moduli of gels.
We consider the long-wavelength limit of the lossless and very lossy flexural modes that can exist when an inviscid fluid contacts a Lamb-wave device. Of primary interest for sensing is the lossless (Scholte) mode, whose phase and group velocities depend on fluid density; additionally, this mode becomes lossy if the fluid is viscous.
Different transduction methods are described: piezoelectric, electrostrictive and thermoelastic wave generation; and piezoelectric, capacitive and optical wave detection. Piezoelectric transduction with interdigital transducers (IDTs) is emphasized.
Experiments include: chemical vapor tests with polymer-coated membranes (showing high mass sensitivity -- up to 1000 cm^2/g -- and low detection limits); density measurements with low-viscosity liquids (one sided loading with carbon tetrachloride reduced the center frequency of the IDTs on a 2.9-MHz device by 55%); measurements of the viscosity of glycerol-water solutions (up to 1035 centipoise). Use of ultrasonic surface-wave and flexural-Lamb-wave delay lines for implementing accurate position sensors is also demonstrated, along with optical detection schemes for the A0 mode.
Fabrication of the following thin-film Lamb-wave devices is described: piezoelectricZnO/Al/silicon nitride (1-6 um thick); electrostrictive Al/silicon nitride (0.45-0.60 um thick); piezoelectric ZnO/Al? silicon nitride with resistive poly silicon or nichrome heating elements on the membrane (106 um thick )