$\gamma -\hbox {Al}_{2}\hbox {O}_{3}$ -Coated Porous Silicon for Trace Moisture Detection

A simple moisture sensor working at room temperature has been fabricated using porous silicon (PS) coated with porous γ-aluminium oxide ( γ-Al₂O₃). Porous silicon was formed by electrochemical anodization of p-type silicon. Thereafter, a porous layer of the PS was dip coated with γ-Al₂O₃ film. The γ-Al₂O₃ coating solution was made by a simple sol-gel method. The coated structure was sintered at 500°C for 30 m to form a nano porous layer of metal oxide on the porous silicon. Sintering the γ-Al₂O₃ -coated PS sample improves the stability of the sensor. Two different sensors with membrane-type metal contact have been fabricated by varying electrochemical parameters of PS. The sensors were tested to detect low moisture content in the N₂ gaseous atmosphere in the range of 25 to 200 μl. Results show that suitable pore morphology of the hybrid structure will enable one to obtain characteristics of a sensor comparable to the commercial porous alumina moisture sensor. It was further observed that the sensor output is highly reproducible and has negligible hysteresis.