Experimental Study of Parylene Based Capacitive Humidity Sensor for Integrated Thermal Comfort Sensing Application

In this letter, we report the temperature effect and noise characterization of the Parylene-based capacitive humidity sensor which can be used for integrated CMOS compatible thermal comfort sensing. The sensor is comprised of a pair of interdigitated Pt electrodes deposited on a silicon substrate and a parylene hygroscopic layer coated on the electrodes using room temperature chemical vapor deposition process. The sensor was characterized at different relative humidity (RH) values and working temperatures in a testing setup. The capacitance of the sensor increases with the RH due to the absorption of water molecules on the surface of the sensing film. The capacitance of the sensor also increases with temperature, which is due to the stronger thermal movement of the molecules that helps increase the polarization at increased temperature levels. Besides, a readout circuit was done on a printed circuit board using capacitance to voltage converter for the sensor. The sensor achieved a maximum sensitivity of 76.2 mV/%RH and response and recovery times of about 31.5 and 29.2 s, respectively. Besides, the sensor shows a noise level of 1.399 mV rms within 25 Hz bandwidth in the output voltage response. The experimental results also indicated a resolution of about 0.072-2% within the ASHRAE standard approved RH range (25-95%) for the fabricated sensor.