DEVELOPMENT OF AN OPTICAL-MODEL FOR STEADY-STATE POROUS ANODIC FILMS ON ALUMINUM FORMED IN PHOSPHORIC-ACID

Porous anodic oxide films on aluminium formed in phosphoric acid (PAA) have been characterized nondestructively by spectroscopic ellipsometr y. Compared to previous studies on porous films formed in sulfuric aci d, the optical behaviour of PAA films reveals new features which have been attributed to film-substrate interface roughness and optical anis otropy effects. On one hand relatively large interface roughness has b een simulated by a graded index model. On the other hand, the implemen tation of uniaxial anisotropy in the optical model of the PAA film ena bles to interpret spectroscopic ellipsometry data acquired at multiple angles of incidence in terms of the morphology of the films. More spe cifically, accurate and physically realistic values are found for the porosity and porous film thickness. Although more difficult to interpr et 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 the oretical predictions of the optical behaviour of arrays of parallel cy lindrical capillaries in an isotropic medium proposed by other authors . (C) 1998 Elsevier Science S.A. All rights reserved.