The Formation of Porous Anodic Aluminum Oxide Confined in μm-Size Contact and Trench Patterns

Abstract

A porous anodic aluminum oxide ͑AAO͒ template was fabricated selectively on a patterned substrate from a sputter-deposited Al film. The pattern size was varied from 2 to 500 m with a depth of 1.5 m. The overall surface undulation of Al resulting from the underlying patterns was planarized by chemical mechanical polishing. During the anodization of the confined Al, the anodization rate was significantly retarded at the vertical sidewall of the pattern. This suggests that the stress should be carefully controlled for the successful integration of a porous AAO structure on a patterned substrate.Anodic aluminum oxide ͑AAO͒ is known for its self-assembled periodic pore structure with a pore size of tens of nanometers and is widely used as a template for the formation of various nanostructures and nanodevices. 1-4 Most notably, the development of a twostep anodization process to form a more or less aligned pore structure 5 and the formation of aligned pore nucleation sites using prepatterned nanoimprint indentation 6-8 greatly facilitates the application of this template. However, the use of this material as a template for nanodevices is relatively new, and several critical issues need to be resolved before the template can be applied successfully.The formation of a AAO pore array on selective areas of a wafer is an important step when we are applying a porous AAO as a template incorporated in the process for fabricating integrated circuits ͑IC͒. Indeed, several methods for defining an AAO template on selective areas of a wafer have been reported. 9-11 One method uses the selective opening of a surface dielectric layer, which is deposited on the top of an Al film by patterning, which defines the AAO area. 9 However, this approach has an inherent limitation because the pore propagates underneath the dielectric passivation layer. 9 Another method typically employs the formation of AAO over the whole area of a wafer first followed by the selective etching of the unnecessary area using wet chemicals. 10,11 In this case, it is difficult to define the small feature size of a patterned AAO area because the etching of AAO is commonly carried out by wet-chemical processes. Moreover, further steps are needed to passivate each patterned AAO area after the etching process. ExperimentalThis paper proposes a new approach to forming AAO on selective areas of a wafer, which are defined as contacts and trenches. Figure 1 shows a schematic diagram showing the processing steps. First, a TEOS SiO 2 ͑2 m͒ layer was deposited on a Si wafer by plasma-enhanced chemical vapor deposition ͑PECVD͒ ͑P-5000, Applied Materials͒. Using the standard optical lithography process, patterns with various widths ͑2-500 m͒ were transferred to the SiO 2 layer by reactive ion etching ͑RIE͒ ͑P-5000, Applied Materials͒ to produce a pattern depth of 1.5 m. Ti ͑50 nm͒, TiN ͑50 nm͒, and Al ͑8 m͒ were successively deposited by a direct current ͑dc͒ magnetron sputtering on the patterned SiO 2 . Second, the topology of the Al film due to the underlying patterns was re...

Novel Three‐Dimensional Nanoporous Alumina as a Template for Hierarchical TiO2 Nanotube Arrays

“…An example is shown in Figure 2c as an area marked with a black line. This observation is consistent with the anodization behavior of Al in a confined micrometer‐sized trench,33 and implies that the accumulation of compressive stress during anodization plays an important role in governing the oxidation kinetics. Previously, Lee et al observed long Al nanopillars at the irregular cell junctions of porous alumina formed by malonic acid‐based hard anodization 34.…”

Effect of Electrostriction on the Self-organization of Porous Nanostructures in Anodized Aluminum Oxide

“…The model proposed by Singh et al in [31,32] included the contributions of elastic stress to the surface activation energies, and it was shown that these effects can result in a short-wave instability which can explain the formation of hexagonally ordered pores. The validity of this model was explored experimentally in [33,34,35,36].…”

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

“…[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][11][12][13][14][15][16][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][16][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.…”