Ellipsometric Studies of Anodic Oxide Films Formed on Tantalum in Dilute Phosphoric Acid

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Abstract
Further ellipsometric data which confirm the two‐layer nature of the anodic oxide films grown on tantalum in dilute phosphoric acid are reported. The two layers are believed to arise, respectively, from growth at the oxide/solution interface due to metal ion transport and from growth at the metal/oxide interface due to oxygen ion transport. The outer layer has a lower refractive index and dielectric constant due to phosphate incorporation. The effects have been observed in dilute phosphoric acid but not in the previous work in dilute sulfuric acid and citric acid of comparable concentration since phosphate is incorporated relatively strongly during the growth process. A model with a linear gradient of index is shown not to give an adequate representation of the optical results.

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