Electrode Structure and Heat Treatment Effects on Stabilization of Capacitive Humidity Sensor with Plasma-Polymerized Thin Film

Abstract

Organic as-grown films obtained by rf plasma excitation have, in general, unstable properties that appear with time, which induce deviations from accurate values of the relative humidity (RH). In this paper, the improvement of the humidity properties of the as-grown films by heat treatment, and the extension of lifetime through the arrangement of electrodes of the sensors are described. Plasma-polymerized polycresol (PC) films, used as a material to sense water vapor, are subjected to two different heat treatments, i.e., the continuous heat cycle and the alternative trapezoidal form of the temperature cycle in vacuum. These heat treatments led to the evaporation of small unreacted molecules, and at the same time, dissociation, repolymerization and cross-linking with radicals of the polymer chain. Thus, these treatments cause a decrease in the concentration of hydrophilic groups (OH) and in free volume, which limits the amount of adsorbable water molecules in the film. Consequently, improved and stable RH sensors were obtained.