Effect of Water Adsorption on Electrical Characteristics of Porous Silica Films

Abstract

The effects of water adsorption on the dielectric constant and leakage current of porous silica thin films were investigated. The films were prepared by the sol–gel method and their pore surfaces became hydrophobic with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) or hexamethyldisilazane (HMDS) vapor phase treatment. Permittivity and leakage current were measured within the relative humidity condition range from 3 to 50%. The dielectric constant increased with increasing relative humidity, and water adsorption properties were calculated from the dielectric constant. Then, the number densities of water adsorption sites were nearly the same in the two films. Leakage current began to increase at a relative humidity of 8%. The slope of leakage current relative to electric field was described well by the Pool–Frenkel leakage current model above an electric field strength of 0.3 MV/cm. To consider the effect of water adsorption on the leakage current model, we used the Pool–Frenkel current equation for ionic conduction. This model revealed that the ionization energy of proton emission from the silanol group decreased with increasing relative humidity; thus, leakage current increased with increasing relative humidity.