Effect of Sensing Film Thickness on Drift Phenomenon of Capacitive-Type Humidity Sensors

Abstract

The effect of a sensing film's thickness on the drift phenomenon of capacitive-type humidity sensors in a hot and humid atmosphere was investigated. Poly͑methyl methacrylate͒ ͑PMMA͒ was chosen as the model sensing material for this purpose. The electrical capacitance measured at 0% relative humidity ͑RH, C 0 ) decreased, while that measured in a wet atmosphere (C X ) increased by aging the sensors in a 40°C and 90% RH atmosphere. This aging test showed an increase in the resulting sensitivity (C X /C 0 ). The drift of sensitivity can be closely related to the increase of water sorption ability of the sensing polymer. The magnitude of the drift of the water sorption ability was larger for the thicker film. The large increase of film thickness induced by aging was also observed for the thicker film. These results suggest that an irreversible volumetric change in PMMA film caused by the slight swelling that occurs in a hot and humid atmosphere is one of the important contributive factors associated with the drift phenomenon of the water sorption ability of PMMA film.