Fluid loading of a Lamb‐wave sensor

We describe the fluid loading of Lamb waves propagating in a 3 mm×9 mm composite membrane consisting of low‐stress silicon nitride deposited by low‐pressure chemical vapor deposition (2.0 μm thick) supporting an aluminum ground plane (0.3 μm), a piezoelectric zinc oxide layer (0.7 μm), and a pair of 100‐μm‐period interdigital Al transducers (0.5 μm). When this device is operated in air, the phase velocity of the lowest antisymmetric mode is 474 m/s; it changes to 304 and 322 m/s, respectively, when the membrane is loaded on one side with de‐ionized water and with methanol. The large fluid‐loading effect, in excellent agreement with theory, and the stability of an oscillator made with this membrane (stability better than 1 part in 106) suggest the utility of the device as a sensitive densitometer and pressure sensor.