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The material of the device's piezoelectric substrate determines the velocity of the acoustic wave, which is in the range of 1500-4800 m/s. This is 105 times slower than the electromagnetic wave velocity, allowing for a long delay along a relatively short area of propagation. The most common piezoelectric substrate materials are quartz, lithium niobate, and lithium tantalate.
Interdigital transducers (IDTs)
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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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While the magnitude of the delay line transfer function depends on the characteristics of the individual IDTs, the phase of the response is only dependent on the delay and signal and synchronous frequencies:
Some sample frequency responses of various SAW sensors are given in the figures below:
Figure 6: This SAW Hg sensor responds to Mercury concentrations with varying frequency response slopes. Plotting the initial frequency slopes results in a fairly linear response that can be used to accurately determine Mercury concentration.1
Figure 7: This SAW NO sensor responds to Nitric Oxide concentraions with varying frequency response magnitudes. As with the example above, the sensor's response can be approximated linearly to an extent1.
Sensitivity
SAW sensor sensitivities are also dependant on their wavetype, configuration, components, materials, and applications. Some typical sensitivities are listed below:
| Physical quantity | Linear coefficient |
|---|---|
| Temperature | up to 100 ppm*/K |
| Pressure, stress | 2 ppm/kPa |
| Force | 10 ppm/kN |
| Mass loading | 30 ppm/μg·cm2 |
| Voltage | 1 ppm/V |
| Electric field | 30 ppm/V·μm−1 |
Figure
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8: 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.
SAW sensors are often valued for their high degree of sensitivity due to the concentration of energy at the device’s surface, where the external environment can have a greater effect. However, this is oftentimes a design challenge. For example, whereas surface acoustic waves with shear vertical components are very sensitive to changes in gaseous environments, they can undergo severe damping in liquid environments. Furthermore, in environments with large temperature fluctuations, a SAW sensor’s piezoelectric substrate can be affected by these fluctuations, often necessitating an additional “reference” configuration to control for such effects.
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- Electromagnetic feedthrough between IDTs, causing amplitude and phase ripple
- “Triple-transit interference” associated with SAW reflections, causing ripple effects
- Mass-loading by IDT digits, causing SAW velocity changes
- Unwanted bulk wave emissions accompanying SAW emissions, causing passband distortion
- Finite source and load impedances, causing frequency-dependent voltages across IDTs
- IDT diffraction similar to optical systems, causing changes in transition band and shape
- Harmonic frequencies generated by the input IDT (may be desirable or undesirable, depending on application)
Manufacturing materials and processes
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.
| Substrate material | Crystal cut | Linear TK* |
|---|---|---|
| Lithiumniobate LiNbO3 | rotated 128 Y/X cut Y/Z standard cut | 72 ppm**/K 92 ppm/K |
| Lithiumtantalate LiTaO3 | X/112Y 36 Y/X rotated cut | 18 ppm/K 30 ppm/K |
| Quartz (SiO2) | ST-X cut | 0 ppm/K |
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:
| Metal | Substrate adherence | Electrical resistivity (μΩ-cm) | Boiling point (K) | Cost |
|---|---|---|---|---|
| Copper | Good | 1.7 | 3200 | Low |
| Aluminium | Good | 2.65 | 2792 | Low |
| Gold | Poor | 2.2 | 3129 | High |
| Tungsten | Average | 5.0 | 5828 | Mid |
| Titanium | Good | 50 | 3560 | Mid |
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.
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.