Anodic film growth on tantalum in dilute phosphoric acid solution at 20 and 85°C

The effects of current density and temperature on the anodic films formed on tantalum in dilute H3PO4 (0.06%wt) solution have been studied by transmission electron microscopy, using ultramicrotomed sections, and Rutherford backscattering spectroscopy. Two-layered films have been identified, comprising an inner relatively pure Ta2O5 layer, adjacent to the metal/film interface, and an outer layer containing incorporated pO(4)(3-) anions. The total amount and depth of incorporated phosphorus species increase with 4 increasing current density and decreasing temperature, in correspondence with the enhancement of the electric field. The formation conditions for the films include those relevant to the commercial anodising of tantalum for capacitors for which the extent of phosphorus incorporation is shown to be comparatively low. (C) 2002 Elsevier Science Ltd. All rights reserved.