Development of an optical model for steady state porous anodic films on aluminium formed in phosphoric acid

Porous anodic oxide films on aluminium formed in phosphoric acid (PAA) have been characterized nondestructively by spectroscopic ellipsometry. Compared to previous studies on porous films formed in sulfuric acid, the optical behaviour of PAA films reveals new features which have been attributed to film-substrate interface roughness and optical anisotropy effects. On one hand relatively large interface roughness has been simulated by a graded index model. On the other hand, the implementation of uniaxial anisotropy in the optical model of the PAA film enables to interpret spectroscopic ellipsometry data acquired at multiple angles of incidence in terms of the morphology of the films. More specifically, accurate and physically realistic values are found for the porosity and porous film thickness. Although more difficult to interpret from the optical findings, the thickness of the barrier part of the porous film can also be estimated. The ellipsometry characterizations are confirmed by complementary TEM analysis of various films. Finally, the anisotropy exhibited by the PAA films is in line with recent theoretical predictions of the optical behaviour of arrays of parallel cylindrical capillaries in an isotropic medium proposed by other authors. (C) 1998 Elsevier Science S.A. All rights reserved.