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Manufacturing materials and processes
| 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 |
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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.
| 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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