Surface acoustic wave sensor

The structure of the basic surface acoustic wave sensor allows for the phenomena of pressure, strain, torque, temperature, and mass to be sensed. The mechanisms for this are discussed below:

The phenomena of pressure, strain, torque, temperature, and mass can be sensed by the basic device, consisting of two IDTs separated by some distance on the surface of a piezoelectric substrate. These phenomena can all cause a change in length along the surface of the device. A change in length will affect both the spacing between the interdigitated electrodes---altering the pitch---and the spacing between IDTs---altering the delay. This can be sensed as a phase-shift, frequency-shift, or time-delay in the output electrical signal.

When a diaphragm is placed between the environment at a variable pressure and a reference cavity at a fixed pressure, the diaphragm will bend in response to a pressure differential. As the diaphragm bends, the distance along the surface in compression will increase. A surface acoustic wave pressure sensor simply replaces the diaphragm with a piezoelectric substrate patterned with interdigitated electrodes. Strain and torque work in a similar manner, as application to the sensor will cause a deformation of the piezoelectric substrate. A surface acoustic wave temperature sensor can be fashioned from a piezoelectric substrate with a relatively high coefficient of thermal expansion in the direction of the length of the device.

The accumulation of mass on the surface of an acoustic wave sensor will affect the surface acoustic wave as it travels across the delay line. The velocity v of a wave traveling through a solid is proportional to the square root of product of the Young's modulus E and the density ${\displaystyle \scriptstyle \rho }$ of the material.

${\displaystyle v\propto {\sqrt {E/\rho }}}$

Therefore, the wave velocity will decrease with added mass. This change can be measured by a change in time-delay or phase-shift between input and output signals. Signal attenuation could be measured as well, as the coupling with the additional surface mass will reduce the wave energy. In the case of mass-sensing, as the change in the signal will always be due to an increase in mass from a reference signal of zero additional mass, signal attenuation can be effectively used.

The inherent functionality of a surface acoustic wave sensor can be extended by the deposition of a thin film of material across the delay line which is sensitive to the physical phenomena of interest. If a physical phenomenon causes a change in length or mass in the deposited thin film, the surface acoustic wave will be affected by the mechanisms mentioned above. Some extended functionality examples are listed below:

Chemical vapor sensors use the application of a thin film polymer across the 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 with resolution down to parts per trillion, allowing for the creation of a sensitive "lab on a chip."

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.

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.

Surface acoustic wave devices are made sensitive to optical wavelengths through the phenomenon known as acoustic charge transport (ACT), which involves the interaction between a surface acoustic wave and photogenerated charge carriers from a photoconducting layer. Ultraviolet radiation sensors use a thin layer of zinc oxide across the 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.[4] This interaction produces measurable decreases in both the velocity and amplitude of the acoustic wave signal.

Ferromagnetic materials (such as iron, nickel, and cobalt) change their physical dimensions in the presence of an applied magnetic field, a property called magnetostriction. The Young's modulus of the material is dependent on ambient magnetic field strength. If a film of magnetostrictive material is deposited in the delay line of a surface acoustic wave sensor, the change in length of the deposited film in response to a change in the magnetic field will stress the underlying substrate. The resulting strain (i.e., the deformation of the surface of the substrate) produces measurable changes in the phase velocity, phase-shift, and time-delay of the acoustic wave signal, providing information about the magnetic field.

Surface acoustic wave devices can be used to measure changes in viscosity of a liquid placed upon it. As the liquid becomes more viscous the resonant frequency of the device will change in correspondence. A network analyser is needed to view the resonant frequency.