Influence of surface conductivity on the response of planar capacitive humidity sensors

Various theoretical models for the response of capacitive and resistive thin-film humidity sensors were proposed, relating device geometry, properties of the moisture sensitive material and ambient humidity to sensor impedance. However not all of the involved processes are well understood and more efforts are needed to validate and improve theoretical models. In the case of surface-type capacitive humidity sensors the influence of the moisture-dependent sheet resistance of sensitive materials is known and a model based on distributed RC transmission lines has been proposed. In order to show the influence of surface conductivity on sensor response, typical interdigitated electrode structures (width/distance: 10 mym/10 mym and 20 mym/20 mym resp.; area: 4940 mym x 6300 mym) were fabricated on oxidized silicon wafers and covered with a 200 nm thick, reactively sputtered fabricated on oxidized silicon wafers and covered with a 200 nm thick, reactively sputtered dense Al2O3 film as dielectr ic layer. These devices are inherently humidity insensitive with a change in capacitance between 10% r.H. and 95% r.H. of 0.6% C(10% r.H.) at 40 degrees C suggesting a surface resistance exceeding 10 E12 ohm per square. Subsequently these devices were conered with thin sputtered Au-Layers to increase surface conductivity in a controlled manner. The thickness was in the range between 2.5 nm and 6.0 nm as interpolated from deposition rate. (IFT)