Electrical surface microrelief of anodic oxide films on aluminum

The morphology and electrical microstructure of different anodic oxide films on aluminum were studied using atomic force microscopy and scanning capacitance microscopy. It was shown that the small basic element in the texture of both thin (0.05 mum) and thick (0.8 mum) anodic oxide films represents a disklike element ("grain") similar to200 x 200 x 30 nm in size. For films with a rough surface relief, the capacitance (and, consequently, the surface potential) shows strong fluctuations in the vicinity of coarse (similar to5-8 mum) pores. Because of this, the image of the surface obtained using, atomic force microscopy does not coincide with that obtained by scanning capacitance microscopy (the opposite contrast effect). The manifestation of the opposite contrast correlates with an increase in the surface potential of the anodic oxide films measured by an independent method. A series of experiments under atmospheric conditions at different distances from the end of the cantilever to the surface of anodic oxide films showed that the influence of the surface field is detectable at long distances (up to 0.7 mum). It was shown that, at a test temperature of 120degreesC, the opposite contrast disappears: the images obtained in the semicontact (atomic force microscopy) and noncontact (scanning capacitance microscopy) modes coincide with each other. The results obtained suggest a relationship between the formation of electrostatic nanosized irregularities at the surface of anodic oxide films and the sorption of water molecules under atmospheric conditions.