Low-Cost Parylene Based Micro Humidity Sensor for Integrated Human Thermal Comfort Sensing

In this paper, we report a CMOS-MEMS compatible Parylene C based Humidity Sensor (PHS) to be used for integrated human thermal comfort sensing for smart buildings. Interdigitated platinum (Pt) electrodes are deposited on a silicon substrate. A parylene C thin film as hygroscopic layer is coated on the electrodes using room-temperature chemical vapor deposition (CVD) technique. Three sensors with various dimensions (1.2 mm
²
, 4.8 mm
²
, and 7.5 mm
²
) are fabricated to study the size effect of the sensor on the sensitivity. The impedance, phase and capacitive response of the sensor at different frequencies of the operating voltage under various relative humidity (RH) levels are investigated. The overall impedance and capacitance changed from 23.02 to 3.744 MŸ and 64.165 to 194.14 pF respectively at 100 Hz operating frequency for the 4.8 mm
²
sensor when RH is increased from 0.1 to 92%. The measured PHS's sensitivity at the frequencies of 1~100 kHz shows highest (1.428 pF/%RH) at low frequency (100 Hz). The PHS with large sensing area showed higher sensitivity (0.11 ~ 0.53 pF/%RH) compared to medium and small sensors. Moreover, the PHS is tested for 3 days depicting good stability with respect to time.