Surface Acoustic Wave (SAW) Sensors are a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. Surface Acoustic Wave (SAW) Sensor transduces an input electrical signal into a mechanical wave which can be easily influenced by physical phenomena, unlike an electrical signal. The SAW sensor device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.

Surface Acoustic Wave (SAW) Sensor DesignSurface acoustic wave technology takes advantage of the piezoelectric effect in its operation. The basic surface acoustic wave device consists of a piezoelectric substrate, an input interdigitated transducer (IDT) on one side of the surface of the substrate, and an output interdigitated transducer on the other side of the substrate. All surface acoustic wave sensors use an input interdigitated transducer (IDT) to convert an electrical signal into an acoustic wave. The acoustic wave travels across the surface of the device substrate to the other interdigitated transducer, converting the wave back into an electric signal by the piezoelectric effect. Any changes that were made to the mechanical wave will be reflected in the output electric signal. The space between the IDTs, across which the surface acoustic wave will propagate, is known as the delay-line. This region is called the delay line because the signal, which is a mechanical wave at this point, moves much slower than its electromagnetic form, thus causing an appreciable delay.

Applications of Surface Acoustic Wave (SAW) Sensors

  • Pressure, Strain, Torque, Temperature, and Mass Sensors – Pressure, strain, torque, temperature, and mass can be sensed by the basic SAW device, consisting of two IDTs separated by some distance on the surface of a piezoelectric substrate.
  • Chemical Vapor Sensors and Gas Sensors – Chemical vapor sensors use the application of a thin film polymer across the SAW delay line which selectively absorbs the gas or gases of interest. An array of such sensors with different polymeric coatings can be used to sense a large range of gases on a single sensor of Electronic Nose.
  • Bacteria & Virus Biological Sensors – A biologically-active layer can be placed between the interdigitated electrodes which contains immobilized antibodies. If the corresponding antigen is present in a sample, the antigen will bind to the antibodies, causing a mass-loading on the device. These sensors can be used to detect bacteria and viruses in samples, as well as to quantify the presence of certain mRNA and proteins.
  • Humidity Surface Acoustic Wave (SAW) Sensors – Surface acoustic wave humidity sensors require a thermoelectric cooler in addition to a surface acoustic wave device. The thermoelectric cooler is placed below the surface acoustic wave device. Both are housed in a cavity with an inlet and outlet for gases. By cooling the device, water vapor will tend to condense on the surface of the device, causing a mass-loading.
  • Ultraviolet Radiation Sensors – Surface acoustic wave devices can be made sensitive to optical wavelengths through the phenomena known as acoustic charge transport (ACT). Ultraviolet radiation sensors employ the use of a thin film layer of zinc oxide across the SAW sensor delay line. When exposed to ultraviolet radiation, zinc oxide generates charge carriers which interact with the electric fields produced in the piezoelectric substrate by the traveling surface acoustic wave. This interaction decreases the velocity and the amplitude of the signal.
  • Magnetic Field Sensors – Ferromagnetic materials, such as iron, nickel, and cobalt, exhibit a characteristic called magnetostriction, where the Young’s modulus of the material is dependent on magnetic field strength. If such a material is deposited in the SAW sensor delay line of a surface acoustic wave sensor, a change in length of the deposited film will stress the underlying substrate. This stress will result in a strain on the surface of the substrate, affecting the phase velocity, phase-shift, and time-delay of the signal.