Resorcinol–formaldehyde aerogels for CMOS-MEMS capacitive humidity sensor

This paper reports the design, implementation and characterization of a capacitive humidity sensor based on a new kind of organic polymer, resorcinol–formaldehyde (RF) aerogel. The developed fabrication is compatible with CMOS process. In the design, aerogel solution is filled into a vertical parallel-plate (VPP) structure, prepared by the sol–gel method and supercritical fluid (SCF) drying for capacitive sensing. The low-density RF-aerogels synthesized by this method exhibit high surface areas, high porosities, and mesoporosity, which are beneficial to moisture diffusion and sensing reaction. The present sensor shows a sensitivity of 0.595% capacitance change per percent relative humidity (%RH) with a response time of 12 s. Further measurements exhibit maximum hysteresis of 1.1%RH and a 48-h signal drift of 0.75%RH have been obtained. Moreover, temperature calibration of the proposed humidity sensor was carried out. The monolithic integration of an on-chip resistive temperature detector (RTD) with the proposed humidity sensor is thus implemented for the temperature compensated readout.