Surface acoustic wave

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A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate. This kind of wave is commonly used in piezoelectric devices called SAW devices in electronics circuits. SAW devices are employed as filters, oscillators and transformers based on the transduction of acoustic waves.

Discovered in 1887 by Lord Rayleigh who reported the surface acoustic wave mode of propagation and in his classic paper predicted the properties of these waves. Named for their discoverer, Rayleigh waves have a longitudinal and a vertical shear component that can couple with a medium in contact with the device’s surface. This coupling strongly affects the amplitude and velocity of the wave allowing SAW sensors to directly sense mass and mechanical properties.

Schematic picture of a typical SAW device design.

Electronic devices employing the SAW normally utilize one or more interdigital transducers (IDTs) to convert acoustic wave to electrical signal and vice versa utilizing the piezoelectric effect of certain materials (quartz, lithium niobate, lithium tantalate, LGS etc). These devices are fabricated utilizing common processes used in the manufacture of silicon integrated circuits.

SAW filters have enjoyed successful application in the booming cellular telephone market and provide significant advantages in performance, cost, and size over other filter technologies (digital signal processors, quartz crystals (bulk wave) , LC filters, and waveguide filters).

Significant research has been done in the last 20 years in the area of surface acoustic wave sensors. Sensor applications include all areas of sensing (such as chemical, optical, thermal, pressure, acceleration, torque and biological). SAW sensors have seen limited commercial success to date but are commonly commercially available for some applications such as touchscreen displays.