Ellipsometric Measurements of the Barrier Layer in Composite Aluminum Oxide Films

The composite oxide film (hydrous + anodic) formed on aluminum foil was chemically stripped to remove only the outer hydrous layer. Ellipsometry of the remaining barrier film showed it to be thinner and have a higher refractive index than conventional anodic barrier films grown to the same voltage. Reanodization to improve film stability gave a further increase in refractive index which indicated that the barrier film had contained some voids. The barrier film is almost entirely crystalline γ-Al2O3 and the higher field strength compared with conventional amorphous anodic films on aluminum is believed to be a characteristic of the denser oxide.