Surface Acoustic Wave Sensors
Surface acoustic wave sensors (SAW) sensors are made by a thick plate of piezoelectric material, typically ST-cut quartz, lithium niobate or lithium tantalate, where predominantly Rayleigh waves propagate along the upper surface.
What is a Surface Acoustic Wave Sensors?
Surface wave generation is efficiently accomplished by a particular electrode configuration named interdigital transducer (IDT). An IDT, in its simple version, is formed by two identical comblike structures whose respective fingers are arranged on the surface in an interleaved alternating pattern. The IDT period length d, or pitch, is the spacing between the center of two consecutive fingers of the same comb. When an AC voltage is applied to the IDT, acoustic waves are generated which propagate along the axis perpendicular to the fingers in both directions. The maxi mum wave amplitude is obtained when constructive interference among the fingers occurs. This happens at the characteristic or synchronous frequency f o = v/d, where v is the SAW velocity in the material. Typical SAW characteristic frequencies are 30-500 MHz. Two basic configurations are possible: one-port SAW resonators with a single IDT, and two-port SAW delay lines with two IDTs separated by a distance L.
Similarly to what happens with BAW devices, SAWs can be used as high-frequency reference elements in filters and oscillators, but they can also be made responsive to a variety of quantities to have them work as sensors.
The primary interaction mechanisms are those that affect the frequency by changing the wave velocity, the IDT distance, or both. Temperature, strain, pressure, force, and properties of added surface materials are examples of measurand quantities. In particular, the accumulated surface mass produces a decrease in frequency.
Compared to QCMs, the higher values of the unperturbed frequency and the fact that vibrations are localized near the surface, becoming more affected by surface interactions, determine a higher sensitivity of SAWs in gravimetric applications.
This fact is advantageously exploited in sorption gas and vapor sensors where SAWs coated with chemically-active films can achieve significantly low detection limits. Due to the configuration of the IDT electrodes, SAW sensors are also responsive to the electric properties of the coating film or the surrounding medium by means of the acoustoelectric coupling. The improvement over quartz crystal TSM sensors offered by SAW sensors in air cannot be extended in liquids because of the vibration component normal to the surface involved in Rayleigh waves, which causes acoustic energy radiation into the liquid with a consequent excess of damping.
In principle, IDTs can generate a spectrum of transversal horizontally and vertically polarized waves as well as longitudinal waves, which propagate on the surface or into the volume of the piezoelectric material. Material properties, crystal cut, and sensor geometry are responsible for which modes appear and in what extent. A whole family of SAW-like devices has been developed. The most important ones are further described.
Surface Acoustic Wave Sensors Ilustration
- Interdigital transducer configuration as used in SAW sensors;
- Structure of a surface acoustic wave sensors