A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS–MEMS technique

This study presents the fabrication of a humidity sensor with a micro heater and a ring oscillator circuit using the commercial 0.35 μm complementary metal oxide semiconductor (CMOS) process and a post-process. The micro heater is utilized to provide a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to generate the signal drift. The humidity sensor is a capacitive type sensor. The structure of the humidity sensor consists of interdigital electrodes and a sensing film. The sensing film, which is polyimide, is coated on interdigital electrodes. The humidity sensor changes in capacitance when the sensing film absorbs or desorbs water vapor. The ring oscillator circuit is employed to convert the capacitance of the humidity sensor into the oscillation frequency output. Experimental results show that the sensitivity of the humidity sensor is about 25.5 kHz/% RH at 80 °C.