Sensing humidity using nanostructured SiO posts: Mechanism and optimization

Abstract

Recently it was demonstrated that relative humidity (RH) could be sensed using SiO films fabricated by a newly developed glancing angle deposition (GLAD) technique that enables three-dimensional control of the film microstructure on a 10 nm scale. The performance of the SiO sensors depends critically on their detailed microstructures. A surprising dynamic response range over five orders of magnitude was detected for a SiO sensor with a microstructure of posts and a porosity of 25%. This paper presents a detailed model for the new SiO sensors and reports the optimization of their sensing characteristics. The model is based on a modification of Sillars' medium theory incorporating the effects of the net heat of adsorption of water molecules, changes in conductivity at the interface between SiO and water molecules, water condensation inside the pores due to the capillary effect, changes in conductivity of the condensed water with RH, and pore size distribution on the performance of the sensors in the model. The model predicts that the sensing characteristics of a porous RH sensor are determined mainly by the microstructure of the medium and the complicated properties at and near the interfaces between the medium and the adsorbed or condensed water molecules. Although the model is developed to explain the sensing characteristics of our sensors, it is quite general and can be used to predict and explain the sensing characteristics for any other capacitive humidity sensor with a porous medium as the sensing material. The results of optimized sensors based on the guidance provided by the theoretical model are presented.