Fast response Humidity Sensors for a Medical Microsystem

A fast response humidity sensor was fabricated to equip a medical microsystem for diagnosis of pulmonary diseases. Its main characteristics are reported in this paper. The sensor is based on a capacitor made of a divinyl siloxane benzocyclobutene (BCB) thin film in between parallel plate electrodes. It was fabricated with compatible CMOS technology. It exhibits good linearity, good sensitivity, and a short response time. Equilibrium capacitance variations versus humidity can be considered linear with a linearity error less than 2% of the humidity level. The sensitivity is 0.1 pF by per cent of the humidity level. The device displays a typical adsorption time of 650 ms, a minimum adsorption time of 400 ms, and a desorption time of a few seconds at ambient temperature. Its performances were compared to other types of capacitive humidity sensors fabricated for the same medical use. The static behavior and the dynamic behavior of the device are reported. They are interpreted according to physical processes of gas adsorption and diffusion in and through glassy polymers. The suitability of the conventional Dual-Mode model to explain water sorption in BCB is discussed. This investigation is a starting point in a modeling process to improve the design of the sensor interface circuitry.