High performance Humidity Sensor Based on Deliquescent Salt Diffused PI Film

This research presents a novel method to improve the sensing performance of capacitive-type humidity sensors by diffusing a deliquescent salt of magnesium chloride (MgCl₂) into the thin PI sensing layer. PI sensing film is firstly photo patterned and then immersed into an inorganic salt solution of MgCl₂ for salt diffusion process. Since MgCl₂ salt deliquesces at the humidity higher than 33%RH, the water absorbability and the humidity response of the proposed humidity sensors are significantly enhanced due to the existence of trace deliquescent salts in the PI sensing film. The measured sensitivity of the proposed humidity sensor is 15 pF/%RH which is up to 750-fold larger than the pure PI-based humidity sensor for the case of 10-mum thick sensing film. In addition, the measured sensitivity is 81 pF/%RH for the case of 40-mum thick sensing layer which is up to 810-fold greater than the pure PI-based humidity sensor. The method proposed in this study is feasible of fabricating MEMS-based high-performance humidity sensors in a simple, reliable and low-cost way.