Surface Acoustic Wave Humidity Sensor Based on Hydrophobic Polymer Film

This work presents the experimental results for a surface acoustic wave (SAW) humidity sensor based on fluorinated polyimide (PI) as the sensing layer. The SAW sensor was configured as one port resonator onto an aluminium nitride (AlN)/silicon substrate. The interdigital transducers (IDTs) and reflectors were fully coated with PI film. With the hydrophobic sensing film, the total frequency shift was obtained as 332 kHz in the 10–90% relative humidity (RH) range, and the corresponding sensitivity was calculated as 4.15 kHz/%RH. In contrast to existing SAW humidity sensors, the fluorinated PI-based sensor showed a positive frequency shift in the whole detection range. Based on perturbation theory, the positive shift can be observed due to the stiffening effect during the sorption of vapor molecules occupying polymer-free volume. Furthermore, the sensor was tested in both very low (< 10%RH) and very high (> 90%RH) humidity ranges to verify the reverse trend of increasing frequency. The hydrophobic film was observed to be sensitive in both ranges and the frequency shift was found large in low ranges compared to high ranges, mainly due to the stiffening effect. At high RH levels, two contrary effects were contributing, mass loading, which shifts frequency negatively, and stiffening, which shifts positively. The cancellation between these effects results in low-frequency change at high ranges. Moreover, the sensor exhibits a fast response time, low hysteresis and good repeatability. These findings demonstrate that the SAW sensor with hydrophobic PI sensing film is a good choice for humidity sensing applications.