Formation and Dielectric Properties of Anodic Films Formed on Ta-W Alloys at Various Formation Voltages

Abstract

Barrier-type anodic films are formed on magnetron sputtered Ta-W alloy films to various formation potentials in 0.1 mol dm −3 ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising an outer thin Ta 2 O 5 layer free from tungsten species and an inner layer containing both tantalum and tungsten species. Slower migration of W 6+ ions with respect to Ta 5+ ions results in the formation of the two-layered films. Because of the absence of more soluble tungsten species in the outer layer, the anodic films grow at high current efficiency. The reciprocal of capacitance of the anodic films changes linearly with formation voltage, as a consequence of linear thickening of the anodic films with the formation potential. The capacitance is enhanced by the addition of tungsten, particularly at low formation potential. The shift of the potential, at which the anodic film growth commences, to the noble direction, contributes to the enhanced capacitance at the low formation voltages.