Humidity sensor structures with thin film porous alumina for on-chip integration

Our aim was to produce a cheap, reliable, low-power and CMOS-MEMS process compatible relative humidity (RH) capacitive sensor that can be incorporated into a state-of-the art wireless sensor network. Porous alumina, produced by electrochemical oxidation of aluminum thin film under anodic bias (AAO, Anodic Aluminum Oxide) was used for the purpose of the sensing layer. We prepared two different capacitive sensor structures on silicon substrate using semiconductor processing steps and anodic oxidation in addition. The first sensor has an ultra-thin, vapor-permeable palladium upper electrode, while in the second case, an electroplated gold-grid is used for the same purpose. The highest achieved average sensitivity is approx. 15 pF/RH%, which is much higher than the values found in product catalogues of discrete, off-the-shelf capacitive humidity sensors (0.2–0.5 pF/RH%).