Study of the performance degradation of thin film aluminum oxide sensor at high humidity

Humidity sensors based on anodised aluminum oxide (Al2O3) thin films are investigated to analyse the mechanism of performance degradation at high humidity. It is observed that at high humidity, the sensor shows significant degradation in the sensitivity and drift in the capacitance characteristics, when exposed for a longer duration. Further, it is found that at high humidity, the capacitance variation becomes almost independent of the Al2O3 film thickness. The effect of temperature on capacitance at high humidity is also studied. The sensor behaviour is discussed in terms of film microstructure, moisture adsorption isotherm and pore surface conductivity. A virtual pore-widening model is proposed incorporating the effect of lateral moisture diffusion inside the pore wall. It is shown that with increasing relative humidity, the moisture seepage in the cell structure surrounding the pores increases, resulting in a change in the pore shape parameter. The effect of pore widening is correlated to the change in sensor capacitance. The results are explained in terms of degradation of dielectric isolation between the pores, leading to the degradation of the sensor characteristics. (C) 2000 Elsevier Science S.A. All rights reserved.