Two Surface acoustic wave (SAW) resonators were fabricated on langasite substrates with Euler angle of (0°, 138.5°, 117°) and (0°, 138.5°, 27°). A dipole antenna was bonded to the prepared SAW resonator to form a wireless sensor. The characteristics of the SAW sensors were measured by wireless frequency domain interrogation methods from 20 °C to 600 °C. Different temperature behaviors of the sensors were observed. Strain sensing was achieved using a cantilever configuration. The sensors were measured under applied strain from 20 °C to 500 °C. The shift of the resonance frequency contributed merely by strain is extracted from the combined effects of temperature and strain. Both the strain factors of the two SAW sensors increase with rising ambient temperature, and the SAW sensor deposited on (0°, 138.5°, 117°) cut is more sensitive to applied strain. The measurement errors of the two sensors are also discussed. The relative errors of the two sensors are between 0.63% and 2.09%. Even at 500 °C, the hysteresis errors of the two sensors are less than 5%.

We report in this paper on the study of surface acoustic wave (SAW) resonators based on an AlN/titanium alloy (TC4) structure. The AlN/TC4 structure with different thicknesses of AlN films was simulated, and the acoustic propagating modes were discussed. Based on the simulation results, interdigital transducers with a periodic length of 24 μm were patterned by lift-off photolithography techniques on the AlN films/TC4 structure, while the AlN film thickness was in the range 1.5–3.5 μm. The device performances in terms of quality factor (Q-factor) and electromechanical coupling coefficient (k²) were determined from the measure S₁₁ parameters. The Q-factor and k² were strongly dependent not only on the normalized AlN film thickness but also on the full-width at half-maximum (FWHM) of AlN (002) peak. The dispersion curve of the SAW phase velocity was analyzed, and the experimental results showed a good agreement with simulations. The temperature behaviors of the devices were also presented and discussed. The prepared SAW resonators based on AlN/TC4 structure have potential applications in integrated micromechanical sensing systems.

High temperature characteristics of langasite surface acoustic wave (SAW) devices coated with an AlN thin film have been investigated in this work. The AlN films were deposited on the prepared SAW devices by mid-frequency magnetron sputtering. The SAW devices coated with AlN films were measured from room temperature to 600 °C. The results show that the SAW devices can work up to 600 °C. The AlN coating layer can protect and improve the performance of the SAW devices at high temperature. The SAW velocity increases with increasing AlN coating layer thickness. The temperature coefficients of frequency (TCF) of the prepared SAW devices decrease with increasing thickness of AlN coating layers, while the electromechanical coupling coefficient (K²) of the SAW devices increases with increasing AlN film thickness. The K² of the SAW devices increases by about 20% from room temperature to 600 °C. The results suggest that AlN coating layer can not only protect the SAW devices from environmental contamination, but also improve the K² of the SAW devices.

The strain sensitivities of the langasite surface acoustic wave resonators are strongly dependent on the temperature, which causes measurement strain to shift greatly at high temperature. To cancel the effect of temperature, a surface acoustic wave strain sensor which consisted of three langasite surface acoustic wave resonators was presented in this work. The surface acoustic wave resonators are configured to ensure that the propagation directions of the surface acoustic wave in the three surface acoustic wave resonators are parallel, perpendicular and at an orientation of 30° to the applied strain. The strain sensitivities of the three surface acoustic wave resonators from room temperature to 250 °C were investigated. Functional relationships of the resonance frequency shifts of the three surface acoustic wave resonators due to both the temperature and applied strain were established. It was deduced that the strain was linearly represented by the resonance frequency shifts of the three surface acoustic wave resonators. Experimental results demonstrated that the langasite surface acoustic wave strain sensor was well worked within the temperature range from room temperature to 250 °C. The average values of the calculated strains were in agreement with the practical applied strains. There was a little fluctuation between the calculated and preset strains. The measurement error was about 1%. The maximal standard deviation was about 18.5 με.

This work presents the sensitivity enhancement of AlN based surface acoustic wave (SAW) pressure sensor fabricated on the Si (100) wafer. Based on the theoretical analysis, a novel concept is proposed to improve pressure sensitivity with crystalline direction. The effect of wave propagation along < 100> and < 110> crystalline direction on pressure sensitivity of the SAW sensor has been studied by the finite element method. The simulation results indicate that the Si (100) based SAW sensor oriented in < 110> direction shown to possess high sensitivity compared to < 100> direction. The increase in sensitivity along the < 110> direction is due to a larger $\gamma $ (velocity sensitivity to strain coefficient) value that results in increased velocity and large frequency shift. Two SAW pressure sensors, PS $_{< 100>}$ and PS $_{< 110>}$ with wave propagation along < 100> and < 110> directions, respectively, have been fabricated on Si (100) wafer to verify the sensitivity enhancement concept. Both sensors have been tested for the pressure range from 0–2 MPa. The measured sensitivity for the PS $_{< 100>}$ and PS $_{< 110>}$ are obtained as 112.68 ppm/MPa and 203.83 ppm/MPa, respectively, Which is significantly improved by 80%. Additionally, the repeatability and linearity of PS $_{< 110>}$ have been measured and the deviation of the measured pressure sensitivity curve from the best linear fit is very small. The obtained results show that the developed sensor design has sufficiently high sensitivity with linear response and can be used for high pressure sensing applications.