Flexural Plate Wave Sensors
What is a Flexural Plate Wave Sensors ?
In thin plates, i.e. diaphragms with thickness smaller than the wavelength, a series of symmetric and antisymmetric plate modes can be generated. These so-called Lamb waves have a particle displacement similar to Rayleigh waves, i.e. particle motions describe a retrograde ellipsis with the major and minor axes normal and parallel to the surface, respectively. The wave velocity depends on the plate material and the plate thickness. The advantage of the lowest antisymmetric mode, the so-called flexural plate wave (FPW) mode, is a wave velocity smaller than that of SAW devices. It decreases with decreasing plate thickness and becomes lower than the wave velocity of liquids.
This determines a couple of unique features that makes FPW sensors very attractive. The first is that, for a given wavelength, the corresponding frequency is comparatively low, in the range of 5-20 MHz, which alleviates the requirements on the associated electronics.
The second is that FPW sensors are best suited to the measurement of fluid properties, such as liquid viscosity, and gravimetric (bio)chemical analysis in solutions. In this latter application, the plate being very thin and significantly affected by surface perturbations, the achievable mass sensitivity can be extremely high. Typically, the plate is a few-micron thick rectangular silicon-nitride diaphragm with a piezoelectric overlayer, such as zinc oxide, in which the waves are generated by means of IDTs. Unfortunately, those FPW sensors are still fragile and the fabrication process must be further optimized. Another version of excitation involves a magnetic field.