Study of the effect of humidity on AlO— x thin-film resistors

Abstract

The effect of humidity on the d.c. resistance of AlO— x thin-film planar resistors on Teflon is reported. The AlO— x films were obtained by immersing Al thin films in distilled water at 58 °C. The porosity variation and the thickness dependence of the porosity of the AlO— x films were obtained by the method of packing fractions using refractive index data. Results indicate that the porosity decreases with thickness from 67% at 120 nm to 48% at 450 nm with a porosity scatter of ±6%, and that films thicker than 220 nm may be more moisture sensitive. A change in the d.c. resistance of three orders of magnitude (10 9 Ω to 10 12 Ω) was observed between 20 and 100% R.H. The AlO— x resistor response (on a log scale) to changes of relative humidity from 20 to 100% is similar to the nature of the adsorption isotherm curve. The observed porosity scatter mainly leads to variations in hysteresis between films and also affects the reproducibility to some extent. Aging in the ambient atmosphere over a five month period resulted in an increased d.c. resistance in the 40 to 90% R.H. range, apparently due to a partial phase transformation to Al 2O 3. The high sensitivity of the d.c. resistance to relative humidity and its fast response to step changes in relative humidity indicate a potential use of these resistors as humidity sensors.