Sensitivity enhancement of capacitive-type photoresistor-based humidity sensors using deliquescent salt diffusion method

This study presents a novel method for enhancing the moisture sensing performance of capacitive-type humidity sensors through the diffusion of a deliquescent salt of magnesium chloride (MgCl2) into a thin polyimide (PI) sensing film. The diffusion process is performed by immersing the photo-patterned PI film in an inorganic salt solution of MgCl2. The results show that the traces of MgCl2 salt in the PI sensing film enhance both the water absorbability and the humidity sensing performance of the proposed sensor. The sensitivity of the humidity sensor is found to be as much as 32.45 pF/%RH for a sensing layer of 10 μm thick. These performance characteristics correspond to an improvement of 1361-folds compared to the sensing performance of a humidity sensor with a conventional pure PI sensing film of the equivalent thickness. A 72 h test shows that the proposed humidity sensors have good stability for long-term measuring. The novel method proposed in this study provides a simple, reliable, and low-cost means of realizing highly sensitive humidity sensors.