Fabrication of ordered nanoporous anodic alumina prepatterned by mold-assisted chemical etching
© Lai et al; licensee Springer. 2011
Received: 2 October 2010
Accepted: 21 February 2011
Published: 21 February 2011
In this article, a simple and cost-effective method to create patterned nanoindentations on Al surface via mold-assisted chemical etching process is demonstrated. This report shows the reaction-diffusion method which formed nanoscale shallow etch pits by the absorption/liberation behaviors of chemical etchant in poly(dimethylsiloxane) stamp. During subsequent anodization, it was possible to obtain the ordered nanopore arrays with 277 nm pitch that were guided by the prepatterned etch pits. The prepatterned etch pits obtained can guide the growth of AAO nanopores during anodization and facilitate the preparation of ordered nanopore arrays.
In recent years, nanoporous anodic aluminum oxide (AAO) has become a popular template system for the synthesis of various functional nanostructures which have extensive applications in scientific and commercial fields [1–4]. In the syntheis of template-based materials, the template with long-range-ordered nanostructure is attractive, in order that structurally well-defined materials can be consequently produced. In general, Al anodization processes, highly regular arrangement of pores, however, occurs only within a small process window, and the domain size (ordering length) is usually limited to a micrometer scale on Al foils [5, 6]. In order to achieve an ordered pore arrangement over a larger area, Masuda et al. [5, 7] developed a pretexturing process of Al using nanoimprinting with a SiC mold. Shallow indentations on an Al substrate initiate pore nucleation during anodization and lead to a long-range-ordered pore arrangement within the stamped area.
Self-ordered and prepatterned guided growths are two kinds of anodization technology, which are competing in the aspects of product quality and production cost. For prepatterned guided anodization, imprinting methods have been used by several author groups to prepare ordered AAO, wherein nanoindentations are created by transferring patterns from hard master stamp onto the Al surface under a high pressure (5-25 kN cm-2) before anodization [8–10]. Despite the ideally ordered patterns obtained, this method is limited by the pattern transfer protocol, and pattern transferred by imprint lithography directly onto metallic substrates such as Al foils or Al films requires 50-2000 times higher pressures in comparison with imprint lithography on polymer layers . The applied pressure for pattern transfer tends to crack the substrates underneath the Al films, such as silicon and glass with brittle property, and leads to substrate fracture. Otherwise, damage to the imprint stamp often occurs after several runs of imprinting because of the high mechanical stresses.
In the reported literatures, some outstanding methods, such as focused ion beams , optical diffraction gratings , colloidal lithography , block-copolymer self-assembly , and metal mask  were also used to achieve prepatterning of Al substrates, thus avoiding fabrication of the expensive hard imprint stamp. However most of them have limitations in scalability or size of ordered domains. Consequently, a simple and economic method for realization of a long-range-ordered AAO over very large areas (cm2 to wafer size) still faces challenges. Recently, some methods, such as guided electric field method , and step and flash imprint lithography , have been developed to fabricate wafer-scale-ordered AAO.
Ideally, a simple and cost-effective process for preparing ordered AAO should combine with a high-throughput method to create patterned nanoindentations on Al surface. It should also be substrate-friendly to avoid damaging the substrate such as thin Al film-deposited Si.
The reaction-diffusion wet stamping (RD-WETS) method uses a nanopatterned agarose stamp such as poly(dimethylsiloxane) (PDMS) in soft lithography. An agarose stamp soaked with an appropriate chemical reactant can etch/dissolve the desired hard material by simply contacting with the substrate (e.g., HF for SiO2 or HCl/FeCl3 for Cu) [18–20]. Localized etching is mediated by a mold-assisted chemical etching initiated from the stamp microfeatures, and excellent uniformity over areas of several square centimeters can be achieved.
In this study, a simple and reliable method for substrate prepatterning by soft imprinting, using a diffusion-reaction-controlled wet chemical etching method, is developed thus avoiding the use of sophisticated device fabrication procedures. In addition, the highly ordered porous alumina on Al foils with the help of prepatterned indentations by the above-mentioned wet stamping were fabricated.
The master molds for PDMS stamp fabrication were sub-micromter gratings (for 1D pattern) and Si wafers with regular pit arrays (for 2D pattern). The membrane stamp was made by pouring a mixture of PDMS prepolymer (Dow Corning Sylgard 184) and its curing agent (10:1 by weight) into the masters, which was cured for 1 h at room temperature and then for 4 h at 60°C in an oven. The PDMS stamps about 2 mm in thickness were replicated from straight line diffraction grating surface (Thorlabs, Inc. 3600 and 1800 lines/mm), and Si mold with regular pit arrays of 277-nm pitch. The flexible agarose membrane has a better attachment to solid surface. Al samples with a total surface area of 2 × 2 cm2 were cut from an aluminum sheet (99.99%, Alfa Aesar), degreased in acetone and dried.
The Al sheet was electropolished at a constant voltage in perchloric acid/ethanol (1:4 V/V ratio) at 4°C for 30 s, to diminish the roughness of Al foil surface. Patterns on Al substrate were etched using a mold previously soaked in a diluted solution of mixed acid (2%) in alcohol (mixed acid composition: 0.15 M HNO3, 0.6 M H3PO4, and 0.2 M CH3COOH). The nitric acid consumes some of the aluminum material to form an aluminum oxide layer. This oxide layer is then dissolved by the phosphoric acid, and more Al2O3 is formed to keep the oxidation/dissolution cycle going. The diluted etchants moderated the condition of etching reaction and contributed to the formation of nanopatterns. The PDMS stamp was soaked in etching solution for 10 min and absorbed in the latter, and the time period for etching process was within 5 min. After nanoindentation by the RD-WETS process with PDMS membrane stamps, anodization was conducted under a constant voltage in phosphoric acid solution. The ordered AAO structures were examined by scanning electron microscopy (SEM, Hitachi S3000) and atomic-force microscopy (AFM, Digital Instrument Nanoscope LFM-3).
Results and discussions
In conclusion, a novel method for fabricating prepatterned Al foil was developed, which used the reaction-diffusion process mediated by a PDMS template. By means of using the diluted (2%) mixed acid solution as a chemical etchant, the wet soft stamp can indent nanoscale shallow concaves on aluminum without the need of excessive loading. Furthermore, based on the phenomenon of multiple RD-WETS imprinting, 2D prepattern by multiple etching could be made using simple stripe-patterned stamps with selected orientation. After anodization, a uniform, ordered AAO array with 277-nm interpore distance guided by the prepattern was obtained. Combining mold-assisted chemical etching and anodization reaction, this process provides a simple and efficient route to obtain ordered nanostructures for further nanodevice applications.
anodic aluminum oxide
reaction-diffusion wet stamping.
The financial support of this study from the National Science Council, Taiwan ROC (NSC 97-2628-E-006-122 and NSC 99-2221-E-024-004) is gratefully appreciated.
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