Ellipsometer Study of Anodic Oxides Formed on Sputtered Tantalum and Tantalum–Aluminum Alloy Films

During an ellipsometer study of anodized films of tantalum and tantalum–aluminum, the optical properties of both the metal and anodic oxide films were determined. The complex refractive index of normal density (16.0 g/cm3) tantalum films range from n = 3.43–3.66i  (at  λ = 5461  A) for tetragonal (beta) tantalum to n = 2.90 – 2.41i for bcc tantalum. For porous tantalum films, the absorption coefficient decreases from a value of 3.66 for normal density to 1.70 for a density of 11.7 g/cm3. The refractive index of anodic Ta2O5 is 2.22 and the dielectric constant is 26.7. For tantalum-aluminum alloy films sputtered from a Ta cathode with 23% aluminum area coverage, n = 3.2 – 2.98i. The anodic oxides of these alloy films act as a mixture of anodic Ta2O5 and Al2O3, having a refractive index of 2.12 and a dielectric constant of 22.1.