A simple humidity sensor with thin film porous alumina and integrated heating

We developed a simple, capacitive, AAO (Anodic Aluminum Oxide) based humidity sensor structure with integrated heating and temperature measuring capabilities. The sensor was prepared by conventional semiconductor processing steps and an additional electrochemical oxidation of a deposited thin film aluminum layer. The achieved average sensitivity (approx. 2.5 pF/RH%) was one magnitude higher than the commercially available discrete capacitive sensors’. With a p-n junction below the capacitive structure we were able to heat the chip to the desired temperature. At the same time the temperature was calculated form the measured forward voltage using calibration data. The integrated heating enabled the study of the sensing mechanism at different sensor temperatures and was also useful for eliminating the drift (“reset the sensor”) caused by measurements at high humidity and several days of storage at room-conditions.