Capacitive humidity sensors based on a newly designed interdigitated electrode structure

This paper presents a study of capacitive humidity sensors constructed based on a newly designed interdigitated electrode (IDE) structure with a polyimide (PI) sensing layer fabricated using micro-electro-mechanical system (MEMS) technology. The humidity sensors use an IDE with increased height, formed by the surface micromachining of a silicon substrate. The fabricated sensors showed higher sensitivity in variable ambient relative humidity (RH) when compared with the humidity sensors of a conventional IDE since the horizontal electric field lines, generated between the thick electrodes, are confined to the PI sensing layer. The effect of the thickness of the PI layer was also investigated. The fabricated sensors with a 4-mu m-thick PI layer showed a higher sensitivity of 37.1 fF/%RH when compared with those of a 2- and 3-mu m-thick PI layers, indicating that the properties of the sensors depend on the thickness of the PI layer. This is because the amount of the fringing electric field lines passing through the PI is determined by the PI thickness. In addition, the sensor with the 4-mu m-thick PI showed very slight hysteresis with a maximum of 2.87%RH and displayed high stability with a variation range of 0.06 pF.