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SAW Physics

History

  • Piezo-electricity was discovered by the Curie brothers in 1880, named by Hankel in 1881 and used by Cady in 1921 in the form of a quartz resonator to stabilise electronic oscillators
  • In 1887, Lord Rayleigh discovered the SAW propagation mode and in a classic paper predicted the wave properties

SAW Sensors

  • Piezo-electric SAW sensors utilise an oscillatory electric field to generate an acoustic wave which propagates on the substrate surface, then transforms back to an electric field for measurement
  • Mechanical strain affects both the propagation path length and the surface wave velocity
  • Changes in frequency and/or phase correlate with surface strain
  • Rayleigh waves have a velocity typically 5 orders of magnitude slower than the corresponding electro-magnetic wave - the slowest to propagate in solids
  • Wave amplitudes are typically 1 nm and wavelengths 1 - 100µm
  • Most acoustic energy is confined within 1 wavelength of the surface so that SAW sensors have the highest sensitivity of the various acoustic wave sensor types
  • SAW sensors typically operate between 100MHz and 1GHz

Piezo-Electric Substrate Materials and SAW Fabrication

  • The most common piezo-electric material for SAW sensors is single crystal quartz
  • First order temperature effects are minimised by selection of cut angle and propagation direction
  • Other piezo-electric materials include zinc oxide (ZnO), langasite (La3Ga5SiO14), lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and lead zirconium titanate (PZT)
  • A photo-lithographic process generates an aluminium inter-digital transducer (IDT) on the surface
  • SAW sensor performance is optimised by adjusting the physical dimensions of the IDT