Selective Wet-Chemical Etching of the Barrier Layer during Formation of Porous Anodic Aluminum Oxide Template

Abstract

A porous anodic aluminum oxide ͑AAO͒ template with a TiN bottom electrode was fabricated on a SiO 2 /Si substrate. The anodizing process ended up stably with the formation of TiO 2 pillars penetrating the barrier layer, and the TiO 2 pillars were selectively and uniformly removed using standard clean-1 solution ͑NH 4 OH:H 2 O 2 :H 2 O = 1:1:5͒. Ru nanowires were fabricated in the AAO nanochannels with an aspect ratio of 10:1 by an atomic layer deposition process using Ru͑EtCp͒ 2 as a precursor and O 2 as a reducing agent. The contact between Ru nanowires and a TiN bottom electrode was confirmed by a transmission electron microscope. The current-voltage characteristic of each Ru nanowire through the TiN bottom electrode was resolved by a conductive atomic force microscope, which showed the uniform ohmic behavior of the contact with a resistance of as low as 2.7 Ϯ 0.3 k⍀ per single Ru nanowire. The stable wet-chemical route to the selective and uniform removal of the barrier layer of the AAO template was applied to devices that require a good electrical connection to nanowires with high aspect ratios, such as sensors, batteries, or optoelectronic devices.An array of vertically aligned nanowires has attracted considerable interest for device applications that require either a large surface area or the unique properties of nanowires, such as sensors, batteries, field emission devices, and optoelectronic devices. [1][2][3][4] While there have been various attempts to fabricate these structures, the use of an anodized aluminum oxide ͑AAO͒ as a template is one of the most intensively investigated processes. [5][6][7][8][9] In this respect, it is not surprising that many researchers have attempted to develop a process of directly integrating the AAO template on a substrate, namely, fabricating an AAO template starting from a deposited Al thin film instead of using Al foil. 10-17 However, there are various challenges in accomplishing this task, including the surface roughness of an Al film, 18 the pattern of the template, 18,19 and the stress caused by volume expansion. 17,18 In particular, the removal of a bottom barrier layer formed under the nanochannel is one of the major issues to resolve. [10][11][12][13][14][15] The fact that the thickness of the barrier layer is approximately half that of the surrounding wall 20 highlights the need for the development of a method to remove this bottom barrier layer selectively without destroying the entire template structure.There have been various attempts to selectively remove the bottom barrier layer of AAO including voltage reduction, 21 insertion of a noble metal underlayer, 10 plasma-assisted dry etching, 11,12,22 oxide breakdown by applying overpotentials, 13 or application of a reverse bias. 10,14 However, none of these approaches appears to be successful because each of these processes demonstrates some limitations, such as the continuous diameter reduction in the nanochannels, template delamination caused by extensive gas evolution, and the difficulty in applying to high...

Alumina Templates on Silicon Wafers with Hexagonally or Tetragonally Ordered Nanopore Arrays via Soft Lithography

“…Several researchers have reported uncovering process to open barrier layer. 26,[32][33][34] Among those methods, Kim and coworkers successfully demonstrated a barrier layer-removal method, having conductive electrode at the bottom of nanoporous alumina. In their method, electrical conducting TiN film, placed underneath of the AAO, was partially electrochemically oxidized, and then the metal oxide was chemically etched away.…”

“…In their method, electrical conducting TiN film, placed underneath of the AAO, was partially electrochemically oxidized, and then the metal oxide was chemically etched away. 26 In this study, following Kim's method, TiO 2 was selectively removed by dipping in SC1 solution, as described in the experimental section, and conductive TiN layer was exposed and used for an electrode during Cu electroplating. TiO 2 etching was performed for a short controlled time in order to reduce TiN etching.…”

Gold Microshell Tip for In Situ Electrochemical Raman Spectroscopy

“…A glass micropipet was prepared by a conventional pulling method using a CO 2 laser puller (Supporting Information). A ruthenium (Ru) thin film was deposited on the inside wall of the micropipet by atomic layer deposition (ALD)26, 27 to serve as an electrical contact (Figure 1A) (Supporting Information). ALD allowed successful deposition of a high‐aspect‐ratio structure on the micropipet without clogging, and the Ru thin film as made was superior in conductivity (with estimated resistivity of ca.…”

Nano-Material and Structural Engineering for Thermal Highways

“…Both USPI's and KPI's laboratories have accumulated experiences on preparing the AAO templates, as reflected in many of their articles [7][8][9][10][11][12][13] and these experiences greatly eased this pursuit. As target materials, we used thermally evaporated Bi film and CNT film formed by a membrane infiltration method.…”