Aluminium pre-patterning for highly ordered nanoporous anodized alumina

Aluminium pre-patterning for highly ordered nanoporous anodized alumina

Photonics and Nanostructures – Fundamentals and Applications 5 (2007) 136–139 www.elsevier.com/locate/photonics Aluminium pre-patterning for highly o...

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Photonics and Nanostructures – Fundamentals and Applications 5 (2007) 136–139 www.elsevier.com/locate/photonics

Aluminium pre-patterning for highly ordered nanoporous anodized alumina V. Stasi a, G. Cattaneo b, S. Franz b, M. Bestetti b, M.C. Ubaldi c,1, D. Piccinin c,*,1, S.M. Pietralunga c,1 a

Fondazione Politecnico di Milano, via Garofalo 39, 20133 Milano, Italy b Politecnico di Milano, p.zza L. da Vinci 32, 20133 Milano, Italy c CoreCom, via G. Colombo 81, 20133 Milano, Italy

Received 31 January 2007; received in revised form 29 June 2007; accepted 22 July 2007 Available online 3 August 2007

Abstract A pre-patterning method of aluminium surface, in order to obtain highly ordered nanoporous anodized alumina on large areas is presented. Aluminium single crystals have been used as a substrate and a 2D hexagonally closed-packed lattice of shallow pits, with diameter of about 200 nm and period of 350 nm, has been successfully achieved by direct writing laser lithography (DWL) and wet etching. Finally, anodic oxidation of the single crystal at high cell voltage in phosphoric solution results in oxide growth with pore ordering superimposed by the pre-patterning procedure. # 2007 Elsevier B.V. All rights reserved. PACS : 81.16. c; 82.45.Yz; 81.65.Cf; 81.05.Rm Keywords: Nanoporous anodic alumina; Direct writing laser lithography; 2D lattice; Aluminium pre-patterning; Aluminium anodization

1. Introduction Self-organized porous anodic alumina (PAA) nanostructures have attracted considerable attention in both scientific and commercial fields related to nanotechnology [1–5]. Porous anodic alumina films are suitable templates for growing one-dimensional nanostructures such as nano-tubes and nano-wires [6], thanks to the possibility of obtaining a 2D self-ordered arrangement of nano-sized pores. Besides, the distance between pores can be controlled by tuning the anodization voltage. Therefore, PAA with parallel nano-pores is also

* Corresponding author. Tel.: +39 0223998900; fax: +39 0223998922. E-mail address: [email protected] (D. Piccinin). 1 www.corecom.it. 1569-4410/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.photonics.2007.07.009

interesting in view of applications as a 2D-photonic crystal [8], or simply for its optical properties as an heterogeneous material (with an equivalent refractive index) [7], if the diameter of pores is much smaller than the optical wavelength. The process of aluminium anodization to porous oxide is usually performed in acid solutions, such as sulphuric, oxalic and phosphoric acids [9]. When the anodization voltage lies within a well defined range, pores self-organize into ordered domains of hexagonally closed-packed (HCP) array. However, the well known honeycomb-like structure of PAA films features short-distance ordering (ranging from several tens of nanometers to a few microns). To achieve a highly ordered pore arrangement over larger areas, several patterning techniques have been studied. Masuda and Fukuda [10] first proposed a two-step anodization process. Subsequently Masuda and co-workers also

V. Stasi et al. / Photonics and Nanostructures – Fundamentals and Applications 5 (2007) 136–139

presented a mechanical pre-texturing process [11]. Recently, some modified methods, such as aluminium pre-patterning by optical diffraction grating [12], holographic lithography [13], atomic force microscope [14], scanning probe lithography [15], focused-ion beam [16] and electron beam lithography [17] have been also attempted. In this work a pre-patterning method based on direct writing laser (DWL) lithography process is presented. Since DWL is a direct writing technique, no photolithographic mask is required; in fact, the exposed resist layer acts itself as a mask. A film of positive photoresist is deposited by spin-coating on the aluminium substrate and a hexagonally closed-packed array layout is then written on it. By tuning the pattern parameters, the 2D lattice impressed on the resist can exactly match the constraints deriving from the anodization conditions, leading to a more regular ordered pattern of pores. In particular, the dimensions of the realized pores can be made compatible with PAA behaving as a PBG structure in the infrared range. By subsequent etching process, shallow pits are produced on aluminium surface. These pits, as experimental results demonstrate, induce the pore initiation during the anodization step and lead to an ideally ordered pore arrangement within the stamped area. 2. Experimental The substrate consists of a polished [1 1 1]-cut aluminium single crystal of purity 99.9999%, thickness 1 mm and 12 mm diameter. A 350 nm layer of UV6 photoresist is spun on the substrate at 6000 rpm for 50 s and softbaked at 130 8C for 60 s. The DWL system

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essentially consists of a stationary focused solid state writing laser beam at 266 nm, and a high-precision translation stage for 6 in. wafers, featuring a 8 nm resolution. HCP arrays with pore diameter of 220 nm and interspace distance of 350 nm have been exposed on an area of 20 mm  50 mm. Fig. 1 shows the 2D pattern of pores in the resist after post exposure bake (90 s at 132.5 8C) and development (45 s in TMAH solution at 2.38%). The array pattern on resist has then been transferred on the Al substrate by wet etching in a chrome etchant (Shipley Chrome Etch18). After the etching process, the resist has been stripped, by dipping the sample into an acetone ultrasonic bath for 10 min. Fig. 2 shows the results of an AFM scanning of the indented Aluminium surface; obtained pits measure about 120 nm in depth, compatible with the indentations of the aluminium surface, as would be spontaneously induced by the anodization process. Anodic oxidation was carried out in 0.5 M phosphoric acid solution (water:ethanol = 4:1) at 10 8C in a thermostated cell, at 195 V of cell voltage for 30 min. Fig. 3 shows a SEM image of the anodized surface, in correspondence to the border between a pre-textured region and a non pre-patterned one. The effect of pretexturing on the ordering of alumina oxide pores is brought into evidence. As expected, after a single anodization step, the anodic oxide has an ordered arrangement of pores in the pre-patterned region, while it is completely disordered in the non pre-patterned region. The average interpore distance measures 350 nm in the ordered zone, as from the design of the photoresist mask.

Fig. 1. (Left) SEM image of the resist mask wrote by DWL technique; (right) detail of the pore layout.

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Fig. 2. AFM images of DWL pre-patterned Al after wet etching.

The fracture of the pre-patterned oxide could be explained as a result of a higher growth rate in comparison to the porous oxide in the external region. Fig. 4 is a SEM image of the large ordering areas (tens of square microns) that can actually be achieved using the DWL pre-patterning process. 3. Conclusions

Fig. 3. SEM image of anodized single crystal, showing a border between ordered pore array in the pre-patterned area and disordered pore array without any pre-patterning.

The experimental results demonstrate the effectiveness of the proposed process on high purity single crystal aluminium substrate, in obtaining nanoporous alumina ordered on large areas. DWL lithography followed by wet etching has been demonstrated to be a simple and versatile process for the aluminium prepatterning. Nanoporous alumina oxide grown by anodic oxidation has an ordered arrangement of pores in the pre-patterned region, while it is completely disordered in the non pre-patterned region. By comparison of the average interpore distances (350 nm inside the prepatterned region and 290 nm outside) it can be inferred that oxide anodic growth, at least for short-time anodization, is driven by the pre-pattering configuration. By tailoring the exposure layout and etching parameters, closed hexagonal patterns can be generated which best match the natural tendency of anodic alumina to self-ordering. Acknowledgments

Fig. 4. Anodized alumina ordered over large areas.

This work was supported by grants from the Fondazione Cariplo (Rif. 2004.1105/11.4988—Bando Ricerca Applicata), under the contract ‘‘Sviluppo di dispositivi ottici basati su materiali nanostrutturati autoaggreganti prodotti per via elettrochimica’’.

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