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The process for the site-controlled formation of tunnel pits with high aspect ratios by anode etching of Al͑100͒ foil is described.Introduction of a small amount of Cu on the Al foil, which has a patterned thin layer of polymers, initiated the uniform development of pits and generated the tunnel pits with sufficiently high aspect ratios. The obtained ordered array of tunnel pits with high aspect ratios will be used for the preparation of several types of functional devices in addition to the improvement of electrolytic capacitors, which use the Al foil with tunnel pits as a surface-enlarged electrode.Formation of tunnel pits on Al by the anisotropic anode etching of Al͑100͒ foils in an electrolyte containing Cl − ions has been widely used for the enlargement of the surface area of electrodes in electrolytic capacitors. 1-10 To maximize the capacitance of the electrolytic capacitors, the size and interval of the tunnel pits must be controlled precisely by considering the operating voltages. 1 In our previous report, we showed the control of the initiation sites of the tunnel pits using a patterned mask composed of a polymer film on Al. 11 In this process, a very thin patterned mask of poly͑chloro-prene͒ rubber ͑CR͒ with the thickness of ϳ200 nm formed on Al can be used to control the initiation of tunnel pits at the initial stage of the anode etching of Al. However, tunnel pits with sufficient depth could not be formed due to the preferential growth of the tunnel pit in the lateral directions. In the present report, we show the process for the formation of tunnel pits with sufficiently high aspect ratios. For the uniform growth of tunnel pits with high aspect ratios, we introduced a small amount of Cu on an Al surface as an initiator for the uniform development of tunnel pits. The uniform development of the pits effectively improved their growth with uniform depth, and yielded the tunnel pits with sufficiently high aspect ratios. The obtained ordered array of tunnel pits on Al prepared by the present process will be used for the fabrication of various types of functional devices in addition to the improvement of the capacitance in electrolytic capacitors. Figure 1 shows the process for the fabrication of highly ordered tunnel pits with high aspect ratios on Al. A patterned mask composed of a polymer film placed on Al with the ͑100͒ plane ͑Ͼ95%͒ was formed using a process reported previously. 11 A thin patterned mask of CR was formed using a poly͑dimethylsiloxane͒ ͑PDMS͒ stamp, which has an ordered hole array. Diameter, depth and interval of holes of the stamp were 1.5, 1.5, and 5.0 m, respectively. Size of the stamp was typically 6 ϫ 6 mm. Before making a patterned mask, an Al foil was electrochemically polished in a mixed solution of perchloric acid and ethanol. Then, a small amount of Cu was formed on the surface of Al by vacuum evaporation or sputtering. The amount of deposited Cu was controlled using a quartz crystal thickness monitor. Electrochemical etching of Al was carried out under a constant-current cond...

High resolution inkjet printing was applied to the site control of the anodic tunnel etching of Al. In this method, a dot array of polymer resin was formed on Al͑100͒ by inkjet printing and was transferred to the oxide layer by anodization. The anodic etching of Al with a mask in a HCl electrolyte resulted in the site-controlled initiation of the tunnel etching of Al and the uniform tunnel pits with high aspect ratio. The interval of the tunnel pits could be successfully controlled by the inkjet patterning on the micrometer scales.Inkjet printing has attracted considerable interest for preparing various types of functional devices owing to its advantages for making desirable patterns on a substrate in noncontact mode. There have been several applications of inkjet printing for the preparation of a wide variety of functional devices, such as electronic or biological devices in addition to the printing of graphical materials. 1-4 One attractive application of inkjet printing is in the fabrication of fine structures with high aspect ratios. For such applications, novel processes combined with inkjet printing and an anisotropic fabrication technique, which allow the formation of structures with high aspect ratio, are essential. In addition, inkjet printing with high spatial resolution is desirable for this purpose. Recent progress in inkjet printing has demonstrated the possibility of patterning extremely high resolutions, e.g., an ϳ3 m interval in dot arrays. 5 In the present article, the fabrication of arrays of fine tunnel pits with high aspect ratios is described using the combined process of inkjet printing and anodic etching of Al. The formation of a fine pattern by high resolution inkjet printing and the subsequent anisotropic anodic etching of Al͑100͒ generated the site-controlled ordered array of tunnel pits with high aspect ratios. The site control of the tunnel pitting in Al was conducted by a method similar to previously reported articles. 6,7 In this process, the polymer mask formed by contact printing generated the selective tunnel pitting of Al during the anodic etching in the electrolyte containing Cl − . This process was originally developed for widening the surface area of electrodes in electric capacitors. [8][9][10][11][12] The introduction of inkjet printing allows the desirable arrangement of tunnel pitting in noncontact mode. The obtained fine structures are the first examples of the microscale hole array structures of metal with high aspect ratios based on inkjet patterning and are applied to various types of functional devices, which require a fine structure with a high aspect ratio besides its use in electric capacitors. Figure 1 shows the schematic of the site-controlled tunnel pitting of Al using the combined process of inkjet printing and anodic etching. An Al foil ͑99.99%, 110 m thick, Toyo Aluminum͒ with ͑100͒ plane ͑Ͼ95%͒ was electrochemically polished in a mixture solution of perchloric acid and ethanol. The fine pattern composed of the dot array of acrylic polymer was formed on the A...

We describe the fabrication of hierarchical structures of Au composed of micrometer-scale quadrangular prisms and a nanometer-scale fine pillar array. Such structures were prepared using a template formed by the site-controlled tunnel etching of Al and its subsequent anodization and the electrodeposition of Au. The hierarchical structure was applied as a substrate for surface-enhanced Raman scattering.

The most appropriate conditions in the site-controlled anode etching of Al using a mask film for the formation of tunnel pits with high aspect ratios over 20 were examined. Anode etching under a reducing current density at an appropriate ratio against time (200mAcm−2normals−1) was effective for the suppression of the undesirable lateral dissolution of the Al surface. Moreover, the addition of a small amount of normalH2SO4 to HCl electrolyte increased the depth of the obtained tunnel pits. By adopting the appropriate anode etching conditions, these improvements could generate site-controlled deep tunnel pits with depths of over 50μm .

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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...

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