Photocatalytic activation of TiO2-functionalized anodic aluminium oxide for electroless copper deposition

Abstract

We demonstrate conductive copper layer patterning of TiO₂-functionalized anodic aluminium oxide (AAO) by means of Cu electroless deposition (Cu-ELD). 4 μm thick AAO layers were obtained by anodization of Al foil at 140 V in 5% H₃PO₄ and functionalization with TiO₂ by dip-coating in a salicylic-acid-based Ti sol. We show that TiO₂ functionalization improves the AAO chemical stability at pH 12.8, allowing its metallization in an alkaline, HCHO-based Cu-ELD bath. In lieu of a noble metal catalyst, surface activation of TiO₂/AAO was accomplished by photocatalytic reduction of a Cu[EDTA] complex under ultraviolet (UV) illumination through a photomask. The effects of UV dose on the photocatalytic activation of TiO₂/AAO were studied in the range 1.25–20 J cm⁻². Uniform surface coverage of reduction products were obtained at doses ≤7.5 J cm⁻², followed by the formation of 100–500 nm particles upon prolonged UV exposure. XPS analysis revealed a mixed phase Cu⁺ : Cu²⁺ composition in a 3 : 1 ratio of the Cu[EDTA] reduction products, which were found to be active as a Cu-ELD catalyst. Conductive Cu patterns with sheet resistance as low as 0.54 Ω sq⁻¹ were obtained within 15 min of Cu-ELD.