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Experimental image of surface acoustic waves on a crystal of tellurium oxide.[1]

A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.



SAWs were first explained in 1885 by Lord Rayleigh, who described the surface acoustic mode of propagation and predicted its properties in his classic paper.[1] Named after their discoverer, Rayleigh waves have a longitudinal and a vertical shear component that can couple with any media in contact with the surface. This coupling strongly affects the amplitude and velocity of the wave, allowing SAW sensors to directly sense mass and mechanical properties.

Application in electronic components

This kind of wave is commonly used in devices called SAW devices in electronic circuits. SAW devices are used as filters, oscillators and transformers, devices that are based on the transduction of acoustic waves. The transduction from electric energy to mechanical energy (in the form of SAWs) is accomplished by the use of piezoelectric materials.

Schematic picture of a typical SAW device design.

Electronic devices employing SAWs normally use one or more interdigital transducers (IDTs) to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of certain materials (quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, etc.).[2] These devices are fabricated by photolithography, the process used in the manufacture of silicon integrated circuits.

SAW filters are now used in mobile telephones, and provide significant advantages in performance, cost, and size over other filter technologies such as quartz crystals (based on bulk waves), LC filters, and waveguide filters.

Much research has been done in the last 20 years in the area of surface acoustic wave sensors.[3] Sensor applications include all areas of sensing (such as chemical, optical, thermal, pressure, acceleration, torque and biological). SAW sensors have seen relatively modest commercial success to date, but are commonly commercially available for some applications such as touchscreen displays.

SAW in geophysics

In seismology surface acoustic waves travelling along the Earth's surface play an important role, since they can be the most destructive type of seismic wave produced by earthquakes.[4]

SAW in microfluidics

In recent years, attention has been drawn to using SAWs to drive microfluidic actuation and a variety of processes. Owing to the mismatch of sound velocities in the SAW substrate and fluid, SAWs can be efficiently transferred into the fluid, to create significant inertial force and fluid velocities. This mechanism can be exploited to drive fluid actions such as pumping, mixing, jetting, as well as others [5].

See also

External links and references



  1. ^ Lord Rayleigh "On Waves Propagated along the Plane Surface of an Elastic Solid", "Proc. London Math. Soc. s1-17 (1885), p. 4-11"
  2. ^ R. Weigel, D. P. Morgan, J. M. Owens, A. Ballato, K. M. Lakin, K. Hashimoto, and C. C. W. Ruppel, IEEE Trans. Microwave Theory Tech. 50, 738 (2002).
  3. ^ E. Benes, M, Gr¨oschl, F. Seifert, "Comparison Between BAW and SAW Sensor Principles", IEEE Trans. Ultrason. Ferro. Freq. Control 45, 1314 (1998).
  4. ^ K. Aki and P. G. Richards, Quantitative Seismology, Freeman and Co., New York, 1980.
  5. ^ Yeo, L.Y. and Friend, J.R. "Ultrafast microfluidics using surface acoustic waves", Biomicrofluidics, 3, 2009
  6. ^ Two-dimensional visualization of SAWs travelling over crystal surfaces: Y. Sugawara, O. B. Wright, O. Matsuda, M. Takigahira, Y. Tanaka, S. Tamura and V. E. Gusev, "Watching ripples on crystals", Phys. Rev. Lett. 88, 185504 (2002)


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