Effect of Ambients and Contact Area on the Asymmetric Conduction of Anodic Tantalum Oxide Films

Abstract

The effect of moisture and metallic contact area has been investigated on the asymmetry properties of anodic tantalum oxide films, approximately 2400Aå thick. Moisture has been reversibly desorbed and adsorbed by vacuum baking followed by exposure at 50–85% RH. The cathodic currents are not affected significantly by this cycle, while the anodic leakage and charging current can be changed by factors of 106 and 102, respectively. The contact area experiment confirm previous results that the asymmetry is due to local inhomogeneities in the oxide film, since nonpolar characteristics, in the absence or presence of moisture, can be obtained with a contact area less than 0.01 mm2. This result indicates that the moisture effect is associated with defects in the oxide film. The oxide film can thus be considered as a normal dielectric with high symmetrical anodic and cathodic breakdown voltages. Distributed on the average 0.1 mm apart throughout the film are heterogeneities of an unknown nature which cause a symmetrical but lower breakdown than the rest of the oxide. The asymmetry results from an increase in the anodic breakdown at these distinct sites due to the presence of water.