A Multifunctional Humidity-Gas Sensor Fabricated on a Silicon Substrate

Abstract

We have developed a multifunctional humidity-gas sensor on a silicon substrate by using thin-film technology and IC fabrication process. The size of the sensing film is 400μm×400μm, which is defined by photolithography. The sensing film is composed of Fe₂O₃(91mol%), TiO₂(5mol%) and MgO(4mol%). It is deposited by rf sputtering on a porous Al₂0₃ film, which is made from Al by anodic oxidation. A Ta film is used as the bottom electrode while a Pt film is used as the top electrode. A part of the Ta film is oxidized into Ta₂O₅ just below the porous Al₂O₃ film. The impedance between the two electrodes is measured as a function of relative humidity or gas concentration.
This sensor is sensitive to humidity when it is kept at room temperature. It is also sensitive to hydrogen gas when it is heated to 400°C. The porous structure increases the adsorption of water and gas molecules. The sensor is entirely made of stable materials with high melting points. So its sensitivity is recovered to its initial value by heat cleaning. The response times of the sensor to humidity change and concentration change of hydrogen gas are short.