The Growing demands for humidity detection in commercial and industrial applications led to the rapid development of humidity sensors based on different techniques. Surface acoustic wave (SAW) technology is one of these methods that has been found to provide a powerful platform for humidity sensing owing to its intrinsic features, including small size, high sensitivity, and simple operational mechanism. Similar to other techniques, the principle of humidity sensing in SAW devices is also realized by an overlaid sensitive film, which serves as the core element whose interaction with water molecules is responsible for overall performance. Therefore, most researchers are focused on exploring different sensing materials to achieve optimum performance characteristics. This article reviews sensing materials used to develop SAW humidity sensors and their responses based on theoretical aspects and experimental outcomes. Herein the influence of overlaid sensing film on the performance parameters of the SAW device, such as quality factor, signal amplitude, insertion loss, etc., is also highlighted. Lastly, a recommendation to minimize the significant change in device characteristics is presented, which we believe will be a good step for the future development of SAW humidity sensors.

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.

In this paper we demonstrate a novel acoustic wave pressure sensor, based on an aluminum nitride (AlN) piezoelectric thin film. It contains an integrated vacuum cavity, which is micro-fabricated using a cavity silicon-on-insulator (SOI) wafer. This sensor can directly measure the absolute pressure without the help of an external package, and the vacuum cavity gives the sensor a very accurate reference pressure. Meanwhile, the presented pressure sensor is superior to previously reported acoustic wave pressure sensors in terms of the temperature drift. With the carefully designed dual temperature compensation structure, a very low temperature coefficient of frequency (TCF) is achieved. Experimental results show the sensor can measure the absolute pressure in the range of 0 to 0.4 MPa, while the temperature range is from 20 °C to 220 °C with a TCF of −14.4 ppm/°C. Such a TCF is only about half of that of previously reported works.

This work illustrates the temperature stability of an AlN-based surface acoustic wave (SAW) pressure sensor working at different pressures. At atmospheric pressure, the temperature coefficient of frequency (TCF) in the temperature range of 18°C to 460°C is tested to be -35.68ppm/°C. With increasing pressure from 0MPa to 2MPa, the |TCF| is found as decreased by 0.43ppm/°C. A finite element method (FEM) has been used to investigate the phenomena of decreasing |TCF| and it was found that elastic modulus and thermal mismatch are two main factors contributing to the decrease in |TCF|. At atmospheric pressure, the TCF has been simulated as -17.27ppm/°C, when only the elastic modulus effect was considered. With the increase in pressure from 0MPa to 2MPa, the |TCF| is reduced only by 0.003 ppm/°C, which is very small than experimental findings as strain increases only with the pressure. After including the thermal mismatch between the films, the TCF of the sensor at atmospheric pressure is simulated as -26.50ppm/°C. The |TCF| is decreased by 0.49ppm/°C with increasing pressure from 0MPa to 2MPa, as the strain increases with both pressure and temperature. The sensor’s |TCF| decreases with pressure because the frequency decreases with temperature but increases with pressure, which means that pressure can cancel part of the frequency drop caused by the same temperature range, so the |TCF| at high pressure is smaller.