Control of Pitting Sites on Al for Electrolytic Capacitors Using Patterned Masking Film

Abstract

Pitting sites on an Al foil for electrolytic capacitors were controlled by printing a hole-array mask on the foil. This process included the formation of a thin rubber film on a hole-array stamp, and the transfer of this film onto Al by microcontact printing. A thin rubber film with a hole-array configuration on Al functioned as an insulating mask during Al electrochemical etching. Consequently, highly ordered square pits with an arrangement identical to that of the holes of the rubber film were successfully obtained.Anodization of Al in an electrolytic solution has attracted much interest due to its applicability of several types of functional devices. 1-6 The etching of high-purity Al has been widely used to produce a large surface area on an Al electrode for electrolytic capacitors. For their application to high-voltage electrolytic capacitors, Al foils predominantly composed of the ͑100͒ plane are etched under a dc condition in a chloride-containing electrolyte. This process produces fine tunnel pits oriented along the ͗100͘ direction of Al, 7-13 which significantly increases the surface area of the Al foil. According to the theoretical calculation of capacitance, the arrangement of uniform tunnel pits at regular intervals is essential for maximizing the surface area of Al electrodes. 14 However, tunnel pits obtained by conventional anodic etching are not regularly distributed on an Al foil. We previously reported the precise site control of tunnel etching by the pretexturing of Al using an imprinting mold. 15 In this study, a pretextured array of concaves on Al functioned as an initiation site for tunnel pits, and ideally ordered tunnel pits were successfully obtained. If an Al surface is covered by a patterned masking film that shows resistance against electrochemical etching, the anodic dissolution of Al would occur only at regions exposed to the electrolyte. Here we describe the precise control of the initiation sites of tunnel pits on Al using a patterned masking film based on the microcontact printing, in which a thin mask was formed on Al using a soft mold of poly͑dimethylsiloxane͒ ͑PDMS͒. When we compared the two processes, the masking process by the soft mold is simpler than imprinting using a hard mold and has an advantage in accomplishment of repeated use for forming a highly ordered array of pits on Al, because it does not include the mechanical pressing of an imprinting mold on Al, which leads to the wearing out of the mold during repeated use. Although the pattern resolution by the soft molds is lower than that by the hard molds, the resolution of the soft molds is enough for the site control of the tunnel pitting on Al foils for high-voltage electrolytic capacitors, which have been attracting increasing interest in the field of electrolytic capacitors.The method for printing using the elastomeric stamp was proposed by other researchers, where the self-assembled monolayer of alkanethiol was used as a masking material. [16][17][18][19][20] In the present study, we used poly͑chloroprene͒ rubber ͑C...

Highly Ordered Anodic Porous Alumina Formation by Self‐Organized Anodizing

“…In the direct patterning of aluminum, each sample can be indented individually, although this makes the techniques time-consuming and also limits their applications to the laboratory scale. Therefore, the imprinting lithography with a master stamp (mold) is the most widespread method used for prepatterning of aluminum [420][421][422][423][424][425][426][427][428][429][430][431][432][433][434][435][436]. Stamps with an arranged array of convexes are usually prepared lithographically, and can be used several times for the pretexturing of aluminum.…”

“…Stamps with an arranged array of convexes are usually prepared lithographically, and can be used several times for the pretexturing of aluminum. The mold used for nanoindentation of aluminum can be made from SiC [420][421][422][423][424][425][426][427][428][429], Si 3 N 4 [430][431][432], Ni [433][434][435] and poly(dimethylsiloxane) (PDMS) [436]. The imprinting of aluminum using a master stamp and fabrication of anodic porous alumina from prepatterned aluminum is illustrated in Figure 1.21.…”

“…The different shape and arrangement of convexes in the master mold leads to various nanopore arrays in anodic porous alumina. The indentation of triangular, square, and graphite structure lattices results in hexagonal, square, and triangular cells, respectively (Figure 1.22) [424][425][426][427]430,432,434,436]. The porous alumina array with a Moiré pattern was also fabricated using indentation with the mold [431].…”

Structural features of self-organized nanopore arrays formed by anodization of aluminum in oxalic acid at relatively high temperatures

“…refs. [19,20], Si 3 N 4 , [21], Ni [22] or poly(dimethylsiloxane) (PDMS) [23]. Recently, an optical diffraction grating (5 mm × 5 mm, 1200 lines/mm 2 ) [24] and nanosphere lithography (NSL) employing 2D hexagonal close-packed arrays of polystyrene [25] and Fe 2 O 3 spheres [26] have been successfully used for pre-patterning of aluminum substrate before anodizing.…”

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

“…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.…”