A high-speed capacitive humidity sensor with on-chip thermal reset

This dissertation reports a high-speed capacitive humidity sensor integrated on a polysilicon heater. The device has a response time of 1.0sec, a sensitivity of 30fF/%RH, and is capable of thermally resetting the sensing film. High speed is achieved using multiple polyimide columns having diameters of a few microns and allowing moisture to diffuse into them circumferentially. Analytical results show that the high-speed structure introduced in this work has a response time 10 times faster than the conventional thin-film structure.; In order to improve the stability of the device against contamination, two other structures achieving the same fast response but eliminating parasitic air-capacitance around the sensing columns have also been developed. These structures have orthogonal or multi-layered electrodes to eliminate the parasitics, respectively. In the orthogonal structure, the air-gap capacitance of the sensor is reduced by 94% as compared to the original structure. In the multi-layered structure, the sensor output drifts by only 1% when the relative dielectric constant in the air region changes from 1 to 10, representing a very high degree of contamination.; By combining the humidity sensor with an integrated heater, high relative humidity levels >80%RH have been measured while preventing the risk of water condensation on the sensor surface. An accuracy of +/-3%RH has been obtained using this method with measurement errors of +/-0.5°C and +/-2%RH in temperature and relative humidity, respectively. The heater also reduces the recovery time after wetting, allows device self-test in the field, and enables the sensor to recover from contamination, aging, and hysteresis.; The fabricated device has been interfaced through a programmable interface chip which implements thermal reset of the sensing film and allows the measurement of relative humidity at different temperatures, circuit gains, and clock frequencies. An on-chip voltage-controlled current source and a voltage-controlled charge pump have been used to power the integrated heater. Effect of humidity on switched-capacitor charge integrator operation has also been explored.