Estimating the quality of thin aluminum layers through their anodizing behavior

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Porous anodic alumina (PAA) grown on aluminum bulk, sheets, and foils in aqueous acid electrolytes has received numerous studies due to its unique self-organized nature and diverse potential applications. However, the formation of well-self-ordered PAA on thin aluminum layers on substrates remains a challenge due to the thickness-related limitation for pore selfordering behavior. It is anticipated that the morphology and inner structure of a thin Al layer are important factors influencing further preparation of a high-quality PAA film with a desired morphology and pore configuration. Here we demonstrate that the anodizing behavior during formation of barrier-type alumina films on thin Al layers prepared by various PVD techniques can be effectively used for estimating the suitability of the films as precursor layers for more advanced anodic treatments. In this course, a systematic study was performed on anodizing thin Al layers prepared via (1) thermal evaporation, (2) magnetron sputtering, and (3) ion-beam-assisted deposition on SiO2 substrates, striving to reveal and exploit the effect of morphology, structure, and physical properties of thin Al films on their anodizing behavior. A citric acid solution of pH 3.0 and a borate buffer of pH 7.4 were employed as electrolytes due to the substantial difference in the pH values and hence the dissolution ability towards Al and Al2O3. A 100 m thick Al foil (99.999%) was used as a reference material. All thin Al films were of 99.999% purity and 500 nm thickness. The findings revealed that the voltagetime and current-time responses for galvanostatic and potentiodynamic anodizing of the Al films, unlike the case of Al foil, are certainly influenced by the film morphology, structure, grain size, and the features of air-formed surface oxide films. A sequence of characteristic breakdown events occurring in the thin films was proved to be certainly dependent upon the individual film characteristics and anodizing variables, also reflecting a number of systematic structural transitions within the films. Based on the anodizing behavior, SEM, and TEM observations, we advanced the existing models of anodic film growth [1] suggesting a driving mechanism for the breakdown events during anodic oxidation of thin aluminum films.

anodizing; aluminum

KOLÁŘ, J.; MOZALEV, A.

11. 7. 2017

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