Fabrication of Ordered Trench Structures with High Aspect Ratios by Anisotropic Anode Etching of Al(100)

Abstract

Ordered trench structures with high aspect ratios were formed by the anisotropic anode etching of ͑100͒-oriented Al foils. A thin polymer mask with a periodic line and space pattern enabled the selective anode etching of Al in an electrolyte containing Cl − ions. The shape and depth of the obtained trenches were strongly dependent on the temperature of the electrolyte used for anode etching. On the basis of this property, trenches with controlled geometrical structures could be obtained through this process.The development of high throughput processes for the fabrication of microstructures of metal or semiconductor substrates with high aspect ratios has attracted increasing interest in various application fields typified by microelectromechanical systems. Among the various types of microfabrication processes, electrochemical anisotropic anode etching is particularly promising because of its capability for high throughput processing of fine structures with high aspect ratios. In this process, differences in the etching rate depending on the crystal orientation of the substrate allow the anisotropic etching of substrates yielding fine structures with high aspect ratios. Although anisotropic anode etching has been widely applied to the processing of semiconductor substrates such as Si, 1,2 InP, 3 and GaAs, 4 its application to metal substrates has been limited to very few examples. This is mainly due to the poor controllability of the anisotropic etching of metal substrates. For example, the contribution of the space charge layer existing in semiconductor substrates, which contribute to the controlled anisotropic etching of substrates, cannot be expected in metal substrates. Among the very few examples of the anisotropic anode etching of metal substrates, the anode etching of ͑100͒-oriented Al foil has been widely explored. [5][6][7][8][9][10][11] In this process, tunnel pits with high aspect ratios can be formed perpendicularly to the Al͑100͒ surface by anode etching in a Cl − -containing electrolyte. This process has been used for the optimization of the surface area of Al foil, which is used in electrolytic capacitors. In our previous articles, we proposed a process for controlling the sites of tunnel pitting based on the mechanical texturing of Al 12,13 or on masking processes. 14,15 These processes can control the sites of development of the tunnel pits and enable site-controlled tunnel pitting in Al foils. In the present article, we describe the fabrication of ordered periodic trenches with high aspect ratios by the anisotropic anode etching of Al͑100͒ using an etching mask. Here, the term of anisotropic anode etching means a comprehensive one, which includes the various etching processes, e.g., trench formation, on Al͑100͒ in addition to tunnel etching for tunnel pits. Although many studies have been reported on the formation of tunnel pits in Al͑100͒, the fabrication of microstructures other than tunnel pits has not been reported so far. From the viewpoint of microfabrication, the most characteristic feature...