Fabrication of Site-Controlled Tunnel Pits with High Aspect Ratios by Electrochemical Etching of Al Using Masking Film

Abstract

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...

Enlargement of surface area of Al by electrochemical insertion and deinsertion of Li

“…115 μm in a Cl − -containing electrolyte. The electrochemical etching of (100)-oriented Al foil in an electrolyte containing Cl − using direct current (DC) generates tunnel pits in the b 100N direction over the surface [2][3][4]. On the other hand, in the case of the Al foil used in capacitors driven at relatively low voltages, electrochemical etching using alternating current (AC) has been adopted [5][6][7].…”

Inkjet Printing for Site-Controlled Tunnel Pitting with High Aspect Ratio in Al

“…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.…”

“…The load of a small amount of Cu is thought to induce the uniform development of the tunnel pits due to the localized galvanic corrosion. 7 The electrochemical etching of Al was carried out in 7 M HCl under a constant current of 500 mA cm −2 at 55°C. The morphology of the tunnel pits in the Al foil was observed by scanning electron microscopy ͑SEM, JEOL, JSM-6100͒.…”

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

“…This thin Cu layer contributed to the uniform initial etching of Al at the start of the anode etching. 15 The anode etching of Al was carried out in the 7 M HCl electrolyte under a constant-current condition. The obtained samples were observed by scanning electron microscope ͑SEM, JEOL JSM-6100͒.…”

“…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.…”