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However, transfer functions for the two-port delay line configuration may be generalized. One such transfer function used for rapid simulation tools is as follows¹¹:

where where f is the frequency, k is the piezoelectric coupling coefficient, τ is the delay between IDTs in wavelengths, CS is the capacitance for an IDT digit pair per unit length, NP is the number of IDT digit pairs, and X is defined as:

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Figure 68: Linear coefficients for physical effects on SAW sensors⁶. *ppm = parts per million. For example, when measuring pressure, a change in SAW frequency of 2x10^-6 corresponds to a change in pressure of 1 kPa.

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Manufacturing materials and processes

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Piezoelectric substrate materials

Piezoelectric substrates are ansiotropic (i.e. directionally dependent) crystalline structures, where each individual crystal inside of a substrate has its own polarity. In a polycrystalline material the different polarities of the individual crystallites may cancel each other out, but by applying a ferroelectric polarization process (heating the material while exposing it to a strong electric field), the material's individual polarities can be aligned, and the material as a whole will exhibit the piezoelectric effect just as its individual crystallites do.

While all SAW sensors require a piezoelectric crystalline material, the exact choice of material is dependent on the sensor's application. If the device is meant to measure temperature, a material with a high temperature coefficient is desirable to increase sensitivity to temperature changes. In virtually all other applications, a material with a low temperature coefficient is desirable to minimize unwanted effects due to temperature changes. As Figure 9 below shows, a substrate's temperature coefficient is dependent not only on the material used, but also the material's crystal orientation, or cut. A substrate's coupling factor, which measures efficiency of energy transduction between mechanical and electromagnetic forms, is also dependent both on cut and material.

Figure 9: Temperature coefficients at room temperature for SAW piezoelectric substrate materials. *TK = temperature coefficient. **See note on figure 8 above for explanation of ppm.

Intedigital transducer materials

The choice of metal used for IDTs also tends to be application-specific, although generally a low resistance is desirable as this typically makes the transduction process more efficient. The strength of the metal's adhesion to its substrate, and the boiling point of the metal (which determines the types of depositing processes available) are also important factors, as is cost. Figure 10 below compares these properties of common IDT materials:

Figure 10: Properties of common IDT materials³

Manufacturing process

Manufacturing a simple SAW sensor with a two-port delay line configuration requires little more than the application of the interdigital tranducers onto the piezoelectric substrate. Two alternative processes for this application are shown in Figure 11 below. A photoresist mask is used to aid in both processes. Additional processing may be required, depending on the sensor's application and configuration.

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Figure 11: Process diagram for etching and lift-off processes for manufacturing a SAW sensor³

Applications

Acoustic wave sensors are very versatile in that they may be used alone or as part of a filtered sensor to measure many phenomena, including:

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¹⁰C.K. Campbell (1989), “Applications of Surface Acoustic and Shallow Bulk Acoustic Wave Devices,” Proceedings of the IEEE, Vol. 77, Issue 10, Oct 1989.

11W¹¹W.C. Wilson, G.M. Atkinson (2007), “Rapid SAW Sensor Development Tools,” NASA Langley Research Center.