Relative humidity sensors based on free nanostructured Al2O3 membranes with open-ended pores

Abstract

Volumetric-surface variants of vertical direction capacitive MDM (metal-dielectric-metal) structures based on high-ordered matrices of free anodic porous alumina membranes for applications in relative humidity sensing elements were designed. The improved humidity sensitivity (4 pF/%), reduced response time and recovery time over a wide humidity range were obtained due to preparing of 30-70 µm thickness nanostructured alumina membranes with open-ended and widened pores (up to 90 nm) without the barrier layer. A special combined method composed of the smooth slow voltage drop at the final stage of the two-stage anodization with the cathode polarization and with the alumina chemical etching was developed to thin and remove the barrier layer. Such the technological approach allowed us to eliminate the effect of electrolyte anions embedded in pore walls of Al2O3 membranes for humidity sensing elements during the adsorption and desorption processes.