Porous anodic alumina (PAA) films are widely used as templates for functional nanostructures, because of the high regularity and controllability of the pore morphology. However, growth mechanisms have not yet been developed that can explain quantitative relationships between processing conditions and oxide layer geometry. Here, we present a model for steady-state growth of these amorphous films, incorporating the novel feature that metal and oxygen ions are transported by coupled electrical migration and viscous flow. The oxide flow in the model arises near the film-solution interface at the pore bottoms, in response to the constraint of volume conservation. The hypothesis of viscous flow was successfully validated through detailed comparisons to observations of the motion of tungsten tracers in the film. Predictions of localized tensile stress near nanoscale ridges at the metal-film interface were supported by observations of voids at these sites. We suggest that the ordering of PAA may be explained by a mechanism in which metal-film interface motion is regulated by the combination of ionic migration in the oxide and stress-driven interface diffusion of metal atoms.

Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.

High-purity aluminum foils were examined using positron annihilation spectroscopy ͑PAS͒ after dissolution for various times in 1 M NaOH at room temperature. Measurements of the S and W shape parameters of the annihilation photopeak at 511 keV show the presence of voids of at least nanometer dimension located at the metal-oxide film interface. The large S parameter suggests that the metallic surface of the void is free of oxide. Voids are found in as-received foils and are also produced by dissolution in NaOH, evidently by a solid-state interfacial process. Atomic force microscopy ͑AFM͒ images of NaOH-dissolved foils, after stripping the surface oxide film in chromic-phosphoric acid bath, reveal cavities on the order of 100 nm size. The average cavity depth is in quantitative agreement with the PAS-derived thickness of the interfacial void-containing layer, and the dissolution time dependence of the defect layer S parameter closely parallels that of the fractional coverage of the foil surface by cavities; thus, the cavities are believed to be interfacial voids created along with those detected by PAS. The cavity distribution on the surface closely resembles that of corrosion pits formed by anodic etching in 1 M HCl, thereby suggesting that the interfacial voids revealed by AFM serve as sites for pit initiation.

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Chloride ions and polyethylene glycol ͑PEG͒ are used together as additives to copper damascene electroplating baths, in which they suppress deposition. When the Cl − concentration is lower than the order of 1 mM, suppression abruptly breaks down below a critical potential, around which hysteresis between active and inhibited deposition is observed. A mathematical model is presented which successfully predicts the observed Cl − concentration-dependent breakdown of PEG suppression and currentpotential hysteresis. The model assumes that adsorbed Cl − ions are involved in binding of PEG to the Cu surface, and that these ions are incorporated in the deposited film. The expressions for Cl − incorporation and adsorption are consistent with experimental measurements of Cl in deposits. Hysteresis was found to depend on the high sensitivity of polymer surface coverage to the concentration of adsorbed Cl − ions, possibly because each PEG molecule has a small number of binding sites to the surface.

The mechanism controlling passivation of metal surface at tips of corrosion tunnels formed on aluminum by anodic etching in hot aqueous chloride solutions was studied. Theoretical considerations based on transport processes were used to rationalize SEM observations and potential measurements. In the mechanism proposed here, electrolyte diffusion in the tunnels served to maintain the tips of all tunnels at the repassivation potential, which is the critical value below which active aluminum surfaces become passive.

Porous anodic alumina ͑PAA͒ films, formed by anodic oxidation in acidic solutions, contain hexagonal arrays of parallel cylindrical pores, with pore diameter and spacing between ten and several hundred nanometers. Simulations were developed for the electrical potential distribution in the film during steady-state PAA growth, and used to calculate the rates of metal-film and film-solution interface motion. In particular, a model using the assumption of no space charge ͑Laplace's equation͒ and one based on the current continuity equation, in each case coupled with high-field ionic conduction, were evaluated with respect to the requirement that the interface profiles are time invariant. Laplace's equation, on which prior simulations of PAA growth were based, yielded unrealistic behavior with highly nonuniform interface motion, suggesting the presence of significant space charge. In contrast, interface motion predicted by the current continuity equation was uniform, except near convex ridges on the metal-film interface between pores. To fully rationalize the steady-state PAA geometry, phenomena other than conduction should be considered, which are able to provide inhibition of the oxidation rate on these ridges.

A model for transport in amorphous anodic oxide films was developed in which ion migration was driven by gradients of mechanical stress as well as electric potential and which included viscoelastic creep of the oxide. Simulations were presented for the galvanostatic growth of planar barrier-type anodic aluminum oxide films. It is assumed that stress originates at the metal-film interface due to the volume change upon oxidation. The average stress in the film decayed during growth and evolved from compressive to tensile with increasing applied current density. The model was fit to stress-thickness measurements using a viscosity of 1×1012Pas on the same order of magnitude as that of many other amorphous materials displaying viscous creep. The current density increased exponentially with electric field, in agreement with an empirical high field conduction behavior. The metal ion transport number was predicted based on the motion of markers in the film and increased with current density in quantitative agreement with experimental measurements. The model represents a unified quantitative interpretation of ionic conduction, transport numbers, and mechanical stress measurements in anodic films.