Surface Acoustic Wave Sensor

SAWCAL 1.0

SAW Delay Line Parameter Calculator

Lord Rayleigh

The stress-free boundary imposed by the surface of a crystal gives rise to a unique acoustic mode whose

propagation is confined to the surface and is known as a surface acoustic wave (SAW).

In 1887 Lord Rayleigh discovered a mode of propagation in which acoustic energy is confined very near the surface of an isotropic solid. This mode is known as the Rayleigh wave.

SAW energy is concentrated near the surface in a small region of approximately one wavelength in depth, therefore Piezoelectric thickness must be at least two wavelength thick to ensure correct conditions for Rayleigh wave propagation.

Deformation due to SAW propagation

Distribution of potential energy

SAW devices are used in considerable range of applications in modern electronics. They have application such as chemical sensors and also have potential for the measurement of pressure, temperature, flow, magnetic field and radiation.

Basic SAW Delay line with uniform IDT

R. M. White in the University of California at Berkeley discovered that surface acoustic waves could be excited and detected by interdigital transducer (IDT) on the surface of piezoelectric material. Fig. Below shows a basic SAW delay line.

In SAW devices, the change of electrical conductivity and the mass change of the chemical interface cause the change of phase velocity.

Equivalent circuit for a SAW delay line

According to Smith’s analysis of interdigital transducers, each IDT can be represented by a parallel circuit consisting of a radiation conductance G_a(f), an acoustic susceptance B_a(f) and a transducer capacitance C_t. Figure below shows this equivalent circuit.

IDT and its equivalent circuit

G_a(f), B_a(f) and C_t can be calculated from the following equations:

G_a(f) = G_a(f_o) [Sin (x) / x]²

B_a(f) = G_a(f_o) [(Sin (2x) – 2x) / 2x²]

C_t = NWC_o

Where x = NB(f – f_o)/f_o, G_a(f_o) = 8K²f_oNC_t and C_o is capacitance per electrode pair per unit length. The susceptance B_a(f) is negligible when f is near the center frequency f_o. Using this model, the overall equivalent circuit of the SAW delay line can be sketched as shown in the above Figure.

Fabrication

A piezoelectric thin film (i.e. ZnO) is deposited on a silicon substrate by RF sputtering technique.

Metal layer is deposited on the film (for instance using electron beam evaporation) and then is patterned to form IDTs (lift off).