A micromachined resistive-type humidity sensor with a composite material as sensitive film

A novel resistive-type humidity microsensor was fabricated by a microelectromechanical system (MEMS) technology and thick-film technology. The microsensor was composed of a suspended planar membrane bridged to the base silicon substrate with two beams. Metal electrodes were deposited on the surface of the membrane, and a composite film of poly-[3-(methacryloylamino)propyl] trimethyl ammonium chloride and SiO2 (PMAPTAC/SiO2) was coated as a humidity-sensing material on the top of the electrodes. The humidity-sensing principle of the sensor was based on the conductivity change of coated composite film upon adsorption/desorption of water vapor. The logarithmic impedance of the microsensor measured at 1 kHz decreased linearly with increasing the humidity in the range of 30–90%RH. The applied voltage above 1 V did not influence the humidity response of the microsensor. The maximum difference between humidification and desiccation process was 5.1%RH at 30%RH. The influence of temperature was −0.72%RH/°C on average in the temperature range 15–25 °C and the humidity range of 30–90%RH.