Optical Interference During rf-GDOES Depth Profiling of Anodized Aluminum-Tantalum Alloy Films

The optical interference effect observed in radio frequency glow discharge optical emission spectroscopy (rf-GDOES) depth profiles of anodic oxide films formed on several aluminum-tantalum alloys is investigated and then used to determine refractive index, thickness, and composition of the anodized Ta-Al alloy layer. Refractive indices at the tantalum wavelength (1.4-2.4) were found to increase approximately linearly with the Ta₂O₅/Al₂O₃ ratio in the inner layer of the anodic oxide film. The optical interference effect was used to determine the composition of the inner film layer from the intensity/time rf-GDOES depth profile with excellent agreement with RBS-derived values. Good accuracy is obtained for constant sputtering rates and hence good interfacial depth resolution and for intensity signals oscillating uniformly across the film. A procedure is also proposed for calculating the thickness of the transparent layer of the anodic oxide film using the refractive index value derived from calibration plots of refractive index versus Ta₂O₅/Al₂O₃ ratio. The quantified depth profile of anodized Ta-24at.%Al alloy is presented. There is excellent accuracy of the quantified composition. The accuracy of the film depth, however, has a strong correlation with calculated density.