SenGenuity for Viscosity, Gas and Physical Sensors : SAW Technology : Rayleigh Surface Acoustic Wave Delay Line

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.

Viscosity, Gas and Physical Sensors Application of the Rayleigh SAW as a gas sensor is achieved by placing a gas specific sensing film on the surface of the device. When the sensing film gets exposed to the target gas, mechanical and electrical perturbations in the sensing film will cause a corresponding change in the resonant frequency of a SAW resonator or a change in the delay of a SAW delay line. When the device is combined with an oscillator circuit, the result for both is a change in the oscillation frequency.

Mechanical examples:

Mass loading as a concentration of gas adsorbs (sticks onto) onto the surface of the sensing film will result in a decrease in oscillation frequency.
Changes in a sensing film as a concentration of gas diffuses into the bulk of the sensing film can result in elastic stiffening or softening. Elastic stiffening will result in an increase in the oscillation frequency, while elastic softening or swelling of the sensing film will result in a decrease in the oscillation frequency.
Stress or Strain coupled into the substrate through the sensor packaging.
Pressure exerted on a diaphragm on which the SAW transducer is fabricated.

Electrical examples:

Conductivity changes in the sensing film as it gets exposed to a concentration of gas can result in either an increase or decrease in oscillation frequency depending on whether the gas causes the conductivity of the sensing film to increase or decrease.
Note that in some cases where the sensing film is metal, the device electrodes themselves can become the sensing material. Also note that when you have a metal sensing film in the delay path, you will not be able to observe any electrical properties because the metal film will short out any electric field that is coupled to the propagating acoustic wave.