Effect of Electrolyte Species on Dielectric Property of Crystalline Anodic Alumina Film

Abstract

Dielectric properties of crystalline anodic barrier films formed on aluminum by multiple step anodizing were evaluated with focusing on the effects of each processing stage and electrolytes species. An aluminum sheet was first immersed in a boiling water to form a hydrated oxide and subsequently anodized up to 350 V in a mixture of boric acid—sodium borate solution or ammonium adipate solution. After the first anodizing, the leakage current of the film formed in borate was much higher than that of the film formed in adipate. After the subsequent heating in air at 500°C, the leakage current of the film formed in borate showed few change although that of adipate remarkably increased. These results suggest that the degree of crystallization accompanied by defects formation after the first anodizing of the film formed in adipate is lower than that in borate. The leakage current after the second anodizing of both films in the borate was effectively suppressed; however, it is still high in the film formed in borate. TEM observation indicated that the film formed in adipate was thinner than the other; despite their same withstand voltage, suggesting higher permittivity of the film formed in adipate. In accordance to that, the capacitance of the film formed in adipate was higher than that of borate. These differences in dielectric properties between the films would be attributed to the difference in anion incorporation depth into the films, particularly incorporation depth of borate.