Model of Stress Generation in Anodic Aluminum Oxide Films: Part II. Surface Stress Accumulation Preceding Formation of Self-Organized Pore Arrays

Abstract

Growth of self-organized porous anodic alumina films initiates after growth of the initial compact barrier oxide becomes unstable. The process of self-ordering is characterized by evolution of an irregular distribution of incipient pores into the characteristic steady-state pore array. We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide. To account for oxide topography evolution, the model domain is chosen to be a spherical shell of oxide representative of the film during the transition to the steady-state porous layer. The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of Al+3 and O−2 ions. The calculation results successfully depict the buildup of compressive stress within a layer of a few nanometers thickness at the oxide surface, as inferred previously from in situ stress measurements. Additionally, the model predictions are consistent with observations of tensile bulk stress in the oxide accompanying inward oxide flow during the transition to the steady-state pore array. This transition is due to competitive growth of the initially nonuniform distribution of incipient pores until a monodisperse steady-state distribution is attained.