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

D Ifezue

doi:10.1007/s11665-013-0516-0

Abstract

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.

Highlights

Typical transmission electron micrographs of anodic oxide films formed on Ta-Al alloys reveal a thin outer alumina layer and an inner layer composed of units of Ta2O5 and Al2O3
Corresponding rf-GDOES depth profiles show oscillation of the tantalum and aluminum intensity signals, which is attributed to double reflection of the light emission incident on the film surface during sputtering
The optical interference effect on anodic oxide films formed on Ta-Al alloys is generally observed for films the thickness of which is a tiny fraction of the wavelengths of Tantalum and Aluminum

Summary

Introduction

Typical transmission electron micrographs of anodic oxide films formed on Ta-Al alloys reveal a thin outer alumina layer and an inner layer composed of units of Ta2O5 and Al2O3. One portion of the light emission is reflected from the specimen surface, while the remaining portion traverses through the sample thickness and is subsequently reflected from the film/alloy interface Both reflected lights interfere, and the changing sample surface results in the observed modulation of the intensity signal with depth. The optical interference effect on anodic oxide films formed on Ta-Al alloys is generally observed for films the thickness of which is a tiny fraction of the wavelengths of Tantalum and Aluminum. These films may be of specific interest in building nanostructures (Ref 4)

Methods

Results

Conclusion