High-performance capacitive humidity sensor with novel electrode and polyimide layer based on MEMS technology

A high-performance capacitive humidity sensor based on a newly designed electrode and a polyimide (PI) layer is presented in this paper. The humidity sensor consists of a substrate with a cavity, a bottom electrode, a PI sensing layer, and a comb-shaped top electrode with branches. The cavity structure of the substrate was formed to protect the top electrode. In order to enhance the performance of the sensor, the coated PI layer was etched by using an O ₂ plasma asher in accordance with the top electrode passivation. After the PI etching, the humidity sensor showed a high sensitivity of 506 fF/% RH and a fast response time of less than 6 s, which is attributed to the increased contact area between the PI layer and moisture, and shortened moisture absorption path into the PI layer. Further characterizations were carried out to measure the effect of temperature, hysteresis, and stability. The humidity sensor showed a hysteresis of 2.05% RH, little temperature dependence, and stable capacitance value with maximum 0.28% error rate for 24 h.