Sensing Mechanisms Mass Loading Film Elasticity Changes Film Conductivity Changes Stress or Strain Temperature Applications Temperature Sensing Pressure Sensing Torque Sensing Chemical Gas Detection Humidity Sensing

Starting with the Rayleigh surface acoustic wave (SAW) delay line, we can see that propagating wave is confined to the top surface of the substrate. For a particle on the surface of the substrate, the propagation of the Rayleigh wave will cause the particle to experience a vertically aligned elliptical motion. Because of this, the SAW is a very sensitive probe for measuring mechanical and electrical properties on its surface. We also note that since there is a vertically polarized displacement, the Rayleigh SAW can only be used for gas sensing or physical sensing applications. Putting the SAW in an aqueous environment will result in the SAW being completely damped out due to energy loss into the liquid.

The Rayleigh SAW is sensitive to mechanical and electrical properties occurring on its surface. For mechanical properties, they are sensitive to mass loading and visco-elastic changes like stiffening and softening. For electrical properties, the devices can be sensitive to any property that interacts with the electrical field that is coupled to the propagating acoustic wave. This effect has been given the term electro-acoustic interactions. The Rayleigh SAW is also sensitive to stress or strain coupled into the SAW substrate through the packaging. Because of this, Rayleigh SAW devices make great platforms for torque and pressure sensing applications. Rayleigh SAW devices can also be tailored with special cuts of piezoelectric substrate to create a very linear SAW frequency versus temperature dependence. The result is a very high resolution temperature sensor.