The effect of chemical additives in phosphoric acid anodization of aluminum-tantalum thin films

摘要

Biosensor development has relied on the process of anodization of sputtered aluminum-tantalum thin films to develop specific optical properties for interference color generation. The anodization of thin sputtered films of aluminum (140 nm) on tantalum (230 nm) on silicon was completed in 0.4 M phosphoric acid under potentio-static conditions (4 V) and with various additives: 0.1 M and 0.4 M citric acid, 3% (v/v) and 10% (v/v) of a commercial anodizing solution consisting of glycerol and glycolic acid, and 0.1 M and 0.4 M oxalic acid. The microstructural and optical properties of these films were analyzed using scanning electron microscopy, X-ray diffraction, and ellipsometry. The interference colors produced by the anodic aluminum and tantalum oxides on top of the tantalum metal layer were also observed. The addition of 0.1 M citric acid, 3% (v/v) glycerol and glycolic acid-based solution, and 0.1 or 0.4 M oxalic acid all produced changes to the porous microstructure of the anodic alumina, including lower roughness and more homogeneous surfaces by inhibition of the dissolution from phosphoric acid. Oxalic acid acted as the strongest inhibitor and significantly reduced surface roughness and pore size, increased the composite refractive index of the anodized film, and significantly changed the interference colors compared to films where no additive was used.