Patterning of nanoporous anodic aluminum oxide arrays by using sol-gel processing, photolithography, and plasma etching

Patterned anodic aluminum oxide (AAO) arrays were fabricated by using sol-gel processing, photolithography, plasma etching, and two-step anodization. The fabrication process included the following general steps. First, a layer of aluminum was evaporated onto silicon wafers. Second, a silica layer was deposited by spin-coating a sol. Next, a patterned layer of a photoresist was deposited by standard photolithographic procedures. Subsequently, the pattern of photoresist was transferred to the underlying layer of silica by fluorocarbon plasma etching. Finally, AAO arrays were formed by the two-step anodization of exposed aluminum (e.g., not covered by silica). Deposition and etching of the silica layer were monitored by spectroscopic ellipsometry. Electrical measurements were performed to investigate the ability of the silica layer to act as a barrier that prevents anodization of aluminum. The structure of patterned AAO arrays was studied by scanning electron microscopy. A clear boundary was observed between two regions. The first region, nanoporous AAO, had pores arranged in an approximately hexagonal order, whereas the second region, aluminum covered with silica, contained no pores. The patterned AAO arrays have two key advantages over other AAO arrays. These arrays exhibit enhanced mechanical strength and possess electrical conductivity, due to the incorporation of regions of aluminum. In addition, the reported fabrication method allows formation of the nanoporous alumina patterns on the microscopic length scale. All the fabrication steps can be integrated into a suitable fabrication line, because they are carried out using standard microfabrication tools. These properties and advantages will facilitate the incorporation of patterned AAO arrays into micro- and nanodevices.