Monitoring relative humidity using ZNO based film bulk acoustic-wave resonator

Humidity is a dynamic parameter that is essential for various fields of industry as well as human lives. There is a substantial interest in the development of relative humidity (RH) sensors for applications in monitoring moisture level at home, in clean rooms, cryogenic processes, medical and food science, and so on. Humidity sensors based on changes in the capacitance or resistance of the sensing element from absorption of water vapor have been investigated extensively. Alternatively, surface acoustic wave (SAW) resonant sensors have also been developed with polymer films deposited on top of a SAW resonator as the sensing layer. In this chapter, a novel RH sensing device using ZnO based film bulk acoustic-wave resonator (FBAR) was described. The resonant frequency of the FBAR decreased in a two-stage manner as RH increased in the environment. For low RH (RH < 50%), a frequency downshift of 2.2 kHz per 1% RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film, resulting in a frequency drop. For high RH (RH > 50%), a frequency downshift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface. It was demonstrated that the two-stage response of the FBAR can be interpreted using the power law theory for semiconductor gas sensors and the mass loading effect of the resonator. UV light was applied to monitor its effects on the humidity sensing performance of the FBAR. UV can enhance the sensitivity at low RH (frequency downshift increased to 3.4 kHz per 1% RH change), while degrading the sensitivity at high RH (frequency downshift decreased to 5.7 kHz per 1% RH change). The mechanism of the influence of the UV illumination was interpreted.