Fabrication of Alumina Membrane Filters with Framework Structures by Photolithography

Porous alumina membrane filters (PAMFs) with a framework are a promising membrane filtration material. Their structure, where a thin filter layer is supported by a thick framework, enables the realization of uniform pore size, high permeability, and mechanical durability, allowing for the precise and efficient filtering of various particles. In this study, we fabricated a PAMF with a framework using a technique that combines Al substrate anodization and photolithography. To form a framework structure on an anodic porous alumina layer, a resist pattern was formed on the anodic porous alumina layer by photolithography, followed by the dissolution of the alumina layer at the resist opening. Subsequently, the obtained samples were anodized again to form a filter layer. By detaching the anodic oxide layer from the Al substrate, we obtained a PAMF with a framework. In this study, it was demonstrated that controlling the wettability of anodic porous alumina is essential for forming resist patterns by photolithography on its surface. It was also clarified that prefilling the high-aspect-ratio fine pores with an etchant or electrolyte is important for removing the alumina layer at the resist opening by etching and for reanodization to form a filter layer.