Formation of Porous Aluminum Films with Isolated Columnar Structure Using Physical Vapor Deposition for Medium-Voltage and High-voltage Capacitors

Abstract

An attempt was made to tailor porous aluminum films with an isolated columnar structure by magnetron sputtering for potential application to medium-voltage and high-voltage electrolytic capacitors. The aluminum film was deposited at a relatively high Ar pressure of 3.3 Pa on rough aluminum substrate with a cellular texture. Shadowing effects, which were enhanced by the random incident angle of aluminum atoms at the high Ar pressure and the use of a rough substrate, produced the isolated columnar structure of the deposited film. However, gaps separating neighboring columns were not high enough to maintain high surface roughness after anodic oxide formation, due to the large Pilling-Bedworth ratio for the Al/Al₂O₃ system. The gaps became filled with anodic oxide during anodic film formation. Slight alkaline etching of the deposited film the increased the gaps such that high surface area was maintained to high formation voltages.