Effect of Heat-Treatment on Characteristics of Anodized Aluminum Oxide Formed in Ammonium Adipate Solution

Abstract

The effects of heat-treatment ͑before and after anodization͒ on the microstructure and electrochemical characteristics of anodized aluminum oxide films formed in 85°C aqueous ammonium adipate electrolyte were investigated. The morphology and crystal structure of the anodized oxide were examined by transmission electron microscopy. The capacitance, relative dielectric constant, electrochemical impedance spectroscopy, and I-V behavior of the oxide film were also determined. Both pre-and post-heattreatment at 500°C could induce the formation of crystalline ␥Ј-Al 2 O 3 in the outer layer of anodized oxide and consequently increase the relative dielectric constant of the film. The differences in the morphology and crystalline characteristics between the anodized oxide subjected to pre-and/or post-heat-treatments led to variations in the electrochemical properties. The pre-heattreatment could economize the required charge to anodize the oxide and retard the growth of film thickness during anodizing. Thermal-induced phase transformation from amorphous to crystalline oxide of the anodized film due to post-heat-treatment, could leave some defects and substantially decrease the electrical resistance of the oxide layer. The reanodization further developed and extended the crystalline oxide formation, which subsequently increased the relative dielectric constant of the oxide film.Barrier aluminum oxide finds many applications in the integrated circuit process, 1 thin film transistor/liquid crystal display fabrication, 2 metal-insulator-metal cathodes for electron-beam lithography, and electrolytic capacitors. 3 Depending on the anodizing conditions, either amorphous or crystalline barrier aluminum oxide can be formed. Because the crystalline oxide can sustain a higher voltage, 4 has a higher relative dielectric constant, 5,6 and possesses a lower ionic conductivity than that of the amorphous oxide film, work on developing an anodized film with a high degree of crystallinity is of continuous interest to researchers. 7 Crystalline oxide growth may be promoted by the presence of a thin layer of thermal oxide on the surface of aluminum. 5,[8][9][10][11][12] The thermal oxide, which contains ␥-Al 2 O 3 13,14 crystals, becomes incorporated into the growing barrier oxide and acts as nuclei for the amorphous to ␥Ј-Al 2 O 3 transformation. 10,11 But the development, during the anodization, of ␥Ј-Al 2 O 3 within the anodized oxide film formed on aluminum covered with a thin thermal oxide is strongly influenced by the nature of the anion species of the electrolytes. 15 Although the film morphology, crystalline characteristics, and growing mechanism of dielectric oxides anodized in borate and phosphate electrolytes had been extensively reported, 10-12,15-17 those formed in ammonium adipate solution, a common medium to anodize the aluminum for use in lowvoltage electrolytic capacitors, were rarely explored.In a recent study, the dependence of the microstructure of aluminum anodized film on the forming voltage in ammonium adipate solution...

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