ANODIZATION MECHANISM AND PROPERTIES OF BI-LAYER TANTALUM OXIDE FORMED IN PHOSPHORIC ACID

Open Access
Author:
Sloppy, Jennifer D.
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 11, 2009
Committee Members:
  • Sarah Elizabeth Dickey, Dissertation Advisor
  • Elizabeth C Dickey, Committee Chair
  • Digby D Macdonald, Committee Member
  • Clive A Randall, Committee Member
  • Thomas E Mallouk, Committee Member
  • Nikolas J Podraza, Committee Member
Keywords:
  • Anodic Film
  • Tantalum anodization
  • Point Defect Model
  • Electrochemical Impedance Spectroscopy
  • Kinetics of Anodization
  • spectroscopic ellipsometry
  • anodic growth laws
Abstract:
The electrochemical growth of tantalum oxide thin films on tantalum anodes is a longstanding field of scientific inquiry. The process of anodic oxidation is of industrial importance because it is used to form the dielectric material of electrolytic capacitors. From an academic perspective, the study of anodic oxidation is of interest because it examines the ionic conduction of materials under an electric field. This work describes the formation of thin anodic films on tantalum when anodization is performed in dilute phosphoric acid. It has long been known that such materials exhibit a bi-layer structure, but studies on the formation and characteristics of the individual layers are lacking. A quantitative description for the formation of the two individual layers is achieved by determining the kinetic rate constants and thermodynamic transfer coefficients of point defect reactions. The Faradaic impedance due to the formation of oxygen vacancies and tantalum interstitials is determined; the experimental impedance is described using an equivalent electrical analog, and the model is optimized to experimental electrochemical impedance spectra (EIS). Empirical investigations of the growth rates of the individual oxide layers were made using FT-IR reflectance spectroscopy, transmission electron microscopy (TEM), and spectroscopic ellipsometry (SE.) The empirical investigations of oxide growth rates provide verification for the parameters of the reported point defect model (PDM). The optical properties of the individual layers formed under a variety of formation conditions are determined using SE. It is demonstrated that the dielectric functions of the inner layer of pure oxide are significantly different than the dielectric functions of the outer layer, which contains incorporated phosphate. Comparisons of the optical properties are made to oxide materials formed without incorporated species.