Nanowires and nanostructures fabrication using template methods: a ...

J Mater Sci: Mater Electron (2009) 20:S249–S254 DOI 10.1007/s10854-008-9568-6

Nanowires and nanostructures fabrication using template methods: a step forward to real devices combining electrochemical synthesis with lithographic techniques S. Ma´te´fi-Tempfli Æ M. Ma´te´fi-Tempfli Æ A. Vlad Æ V. Antohe Æ L. Piraux

Received: 17 July 2007 / Accepted: 4 January 2008 / Published online: 20 January 2008 Ó Springer Science+Business Media, LLC 2008

Abstract One of the great challenges of today is to find reliable techniques for the fabrication of nanomaterials and nanostructures. Methods based on template synthesis and on self organization are the most promising due to their easiness and low cost. This paper focuses on the electrochemical synthesis of nanowires and nanostructures using nanoporous host materials such as supported anodic aluminum considering it as a key template for nanowires based devices. New ways are opened for applications by combining such template synthesis methods with nanolithographic techniques.

1 Introduction Nanowires with controlled geometry could be realized by patterning thin films using advanced nanolithographic techniques such as electron and ion beam lithography and nanoimprint methods. Unfortunately, they do not permit the realization of nanowires with compositional modulation along their axis. Electrochemical reduction of various materials directly inside the nanopores of different host materials gives, due to their confinement, nanowires with geometries defined by

S. Ma´te´fi-Tempfli (&) M. Ma´te´fi-Tempfli V. Antohe L. Piraux Unite´ de Physico-Chimie et de Physique des Mate´riaux, Universite´ Catholique de Louvain (UCL), Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium e-mail: [email protected] A. Vlad Unite´ de Dispositifs Inte´gre´s et Circuits Electroniques, UCL, Place de Levant 3, 1348 Louvain-la-Neuve, Belgium

the nanopore dimensions. Such host materials include nanoporous anodic aluminum oxide films [1–5], nuclear track-etched mica [6–8] and polymer membranes [9–11], diblock copolymers [12], nanochannel array glasses [13] and SiO2 nanocapillary arrays [14]. Using these nanoporous materials as templates for the electrochemical synthesis of nanowires allows the fabrication of a large number of nanowires in parallel and, by varying the electrodeposition conditions (i.e. voltage, current, electrolyte), the realization of multilayered (along their growth axis) nanowires. An essential requirement for producing multilayered structures with the individual layers thicknesses in the nanometer range is the use of an atomistic deposition process. Several techniques, such as molecular beam epitaxy, sputter deposition and e-beam evaporation satisfy quite well this requirement. However, these thin film deposition techniques have some drawbacks, such as high cost, low deposition rates and very poor compatibility with template synthesis methods. Alternative atomistic deposition and cost saving electrochemical deposition techniques were developed for producing thin multilayers, namely the single- [15, 16], the dual- [17–21] and the multiple-bath techniques [22]. The most interesting nanoporous materials used as templates for nanowires synthesis are the track-etched polymers, the diblock copolymers with hexagonal ordering and the nanoporous aluminas. They all show a nanochanneled structure with hollow channels parallel to each other. Among them the nanoporous alumina can stand high temperatures, is insoluble in organic solvents and geometrical parameters can be easily tuned by changing the preparation conditions. We focused our attention to this template considering it as the most promising for the integration of nanowires based structures in silicon processes for microelectronic manufacturing.

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Electrochemical oxidation of aluminum is used since almost a century for protective and/or decorative coating of aluminum products. It has been found that certain electrolytes used for the anodization of the aluminum produce compact oxide layers (i.e. boric acid and borates) while others produce porous ones (i.e. sulfuric, oxalic, phosphoric acids). The first characterization of these oxide layers dates back to the early 1950s [1]. A nanoporous structure is formed when a competitive equilibrium is established between electrochemical oxide growth and field enhanced chemical dissolution of the oxide in certain acidic electrolytes [2–4]. Pores growth is accompanied by a phenomenon of self-ordering [5, 23] tending to a perfectly ordered honeycomb nanoporous structure. Systematic studies on these systems have shown that the degree of ordering could be influenced by the preparation conditions, i.e. anodization time, electrolyte type and its pH, applied voltage and temperature (see for example [24]). Perfect regularity could be obtained by prepatterning the aluminum surface before anodization [25] with various techniques like e-beam lithography or nanoindentation. The main futures of the alumina template are: the ordered arrangement of nanopores in a high density structure of typically 109...1012 pores/cm2; interpore distances controlled by the anodization voltages. Minimal pore diameter is also related to the anodization voltage but it can be increased by pore enlargement (etching) of the nanopores. This can range between 5 and 400 nm in function of the preparation conditions. As a first step toward the above mentioned integration we have developed a supported nanoporous alumina template with numerous advantages compared to the state of the art nanoporous alumina templates.

2 Supported nanoporous aluminum oxide Common alumina membranes obtained by anodic oxidation of aluminum foils are not adequate for direct integration in silicon processes. They could be supported on the remaining non oxidized aluminum substrate or selfsupported if they are removed from this substrate either by selective chemical etching of aluminum, or by diminishing stepwise the anodization voltage [4], or by reversing the applied voltages [26]. Despite the high melting point of Al2O3 (*2,050 °C), the highest working temperature for such nanoporous membranes is limited by the melting point of aluminum (*658 °C) when the alumina membrane is supported on aluminum. It is also limited by the recrystallization temperature of the nanoporous alumina ([400 °C) and its phase transformations ([820 °C) [27] which cause mechanical deformations and cracking of the self-supported membranes during thermal treatments. The minimal thickness of the self-supported membrane is

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limited to *20–50 lm due to its brittleness. When the alumina membrane is naturally supported on aluminum, only AC electrodeposition could be used to grow nanowires inside nanopores due to an oxide barrier layer between the nanopore ends and the aluminum substrate. Some attempts for supported alumina fabrication have been reported. One of them permits DC electrochemical synthesis of nanowires inside nanopores by barrier layer removal [28]. Other attempts use conducting underlayers i.e. ITO [29, 30], Pt [31, 32], between an Al layer and a solid substrate like Si or glass. Unfortunately, none of these attempts succeeded to solve all problems and/or produce such supported membranes in a reliable manner and on large areas. We have developed a fabrication procedure of a supported nanoporous aluminum oxide on silicon (or other substrates) that overcomes the above-mentioned major drawbacks. The process starts with physical vapor deposition of a conducting underlayer on a carefully cleaned substrate, typically Si, SiO2, sapphire, MgO, glass, etc. (see Fig. 1). This conducting layer has a double role of serving as an anodization barrier during electrochemical oxidation of the aluminum layer deposited on it, and also as contacting electrode both during anodization and further electrochemical growth of nanowires inside the pores of the alumina. This conducting layer should not oxidize under anodization conditions, typically being a layer of gold or platinum. Intermediate adherence layers are used between the substrate and the conducting layer, and between this conducting layer and the subsequent aluminum layer. Aluminum layer deposition is a critical step in sense that small thicknesses (up to a few hundred nm) are easy to realize but thicker ones need special deposition conditions. The temperature of the substrate and the deposition rate are key factors for compact aluminum layers deposition without hillock and pitting defects [33, 34]. High deposition ˚ /min at room temperature, permit the rates, up to 6,000 A realization of e-beam deposited Al layers with thicknesses up to 10 lm. Depending on the final objectives, thermal treatment at 400–450 °C/(20 min-4 h) in Ar gas and/or electrochemical polishing in perchloric acid (HClO4) containing electrolytes at constant current densities of 100–200 mA/cm2, can follow the deposition steps to increase grain sizes and decrease surface roughness. Anodic oxidation at constant voltages of the supported aluminum layer is performed in similar conditions as for the case of classical aluminum foils anodization using sulfuric, oxalic or phosphoric acid solutions. In order to diminish oxidation reaction speeds and for a better control of thin layers anodization, low working temperatures of about 0 °C are used. By limiting the current at the end of the process to values similar to the mean anodization


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current, when no more metallic aluminum remains on the substrate, we obtain a nearly defect free structure with nanopores down to the conducting underlayer, see Fig. 2a. Continuing applying the anodization conditions after

Fig. 1 Schematic fabrication

representation

of

the

nanoporous

alumina

complete oxidation of the aluminum leads to the thinning of the barrier layer due to the continuation of the field enhanced chemical dissolution of the oxide. A subsequent pore enlargement step completely eliminates the remaining barrier oxide layer. This allows the growth of nanowires inside nanopores by DC electrodeposition on large surface areas (Fig. 2b). Pore widening to desired diameters follows the anodization step using mainly hydrofluoric and phosphoric acid solutions at temperatures ranging between 30 and 40 °C. The template is well suited for the growth of nanowires or nanostructured nanowires inside nanopores by electrochemical or CVD methods. By using adequate substrates and underlayers it resists to high temperatures of more than 1,000 °C allowing thermal treatments [35]. By controlling the preparation conditions, pore diameters can be adjusted between 5 and 350 nm and pore lengths between 150 nm and 10 lm. Due to the possibility for thick Al layers deposition, a two step anodization is feasible especially for

Fig. 2 Scanning electron microscope (SEM) sectional view of a supported nanoporous alumina template (a) and top view of an array of gold nanowires electrodeposited inside the nanopores after the removal of the alumina host (b)

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applications where an ordered arrangement of the nanopores is important [36, 37]. The final thickness (length of the nanopores) of the supported template is mainly controlled by the thickness of the aluminum layer and also by intermediate preparation steps like polishing and/or depths of an eventual first anodization step.

3 Patterning the supported alumina First patterning experiments (using optical lithography and CT-AFM indentation) on arrays of nanostructured nanowires embedded in the supported alumina matrix for the purpose of single wires contacting [38] were realized with success. Measurements revealed that extremely high current densities of *109 A/cm2 can be continuously injected in such single nanowires without affecting their internal fine structure, i.e. multilayers of Co(14 nm)/Cu(7 nm). This could only be explained if a very good electrical contact (small contact resistance), between the contacting electrodes on the top and bottom of the alumina matrix and the nanowires extremities, and also a highly efficient heat evacuation through the supported matrix are simultaneously realized. We should note that this last aspect is of a paramount importance for an efficient heat evacuation of further high density electronic devices that are based on such nanowire arrays. Two different procedures, combining lithographic and electrochemical template synthesis methods were also developed for the localized growth of the nanowires. A simple localization could be obtained by using a supported alumina template realized with the use of an insulating substrate and patterning the conductive underlayer by an advanced lithographic method. In case of a subsequent electrochemical deposition inside nanopores, the nanowires will grow localized only on the conducting electrodes, see Fig. 3. Such vertically aligned nanowire arrays realized on interdigited electrodes are very promising for applications where high active surfaces are needed, i.e. sensors and biosensor. We adopted a completely different approach [39] by masking the top surface of the alumina template with a patterned SiN layer. For a subsequent electrochemical deposition, only certain regions are exposed in this manner to the electrolyte with nanowires growing only in these regions (Fig. 4). After forming the nanopores by anodic oxidation on the supported template, a 100 nm thick SiN film is deposited by plasma enhanced chemical vapor deposition on top of the alumina layer. Then the sample is coated with a 200 nm thick layer of polymethylmethacrylate (PMMA) and patterned using

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Fig. 3 SEM micrographs from localized nanowires on interdigited electrodes. Supported nanoporous alumina was prepared using a conducting underlayer of gold on SiO2/Si substrate patterned by optical lithography then Au nanowires were grown by electrodeposition inside nanopores. SEM images were taken with increasing magnification from (a) to (c), after the removal of the alumina host

e-beam lithography with different geometries. After resist development, the pattern is transferred to the SiN film using reactive ion etching followed by an oxygen plasma step to strip-out the PMMA layer.


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By carefully adjusting the dimension of the openings in the SiN mask we are able to grow arrays of single nanowires, see Fig. 5. The pattern presented in this figure is particularly interesting for crossbar nanowire circuits where each crossing point should be connected by a single nanowire. Work is in progress to obtain such electrically connected single nanowires in a matrix arrangement combining the two patterning methods presented above. Highly dense circuits based on nanowires can be envisaged using this approach. In addition, the present technique allows the realization of contacts without dissolving the alumina host, thus protecting the nanowires from oxidation and allowing an efficient heat evacuation through the alumina matrix [38]. Fig. 4 SEM micrograph from localized nanowires in artistic pattern by masking the supported nanoporous alumina surface using e-beam lithography

4 Conclusions Electrochemical synthesis using nanoporous materials as templates allows the realization of nanostructures that are difficult to obtain using conventional techniques. Among these templates the supported nanoporous alumina is the most promising. Its particular properties i.e. mechanical stability, good thermal conductivity and high temperature resistance, the possibility of using DC-electrochemical deposition inside nanopores, the easily tunable geometrical parameters (pore diameters and interpore distances) and the possibility for a high density ordered arrangement of nanopores, makes it an ideal candidate for further integration into large-scale fabrication of various nanowires based devices. We have shown that combining nanolithographic techniques and electrochemical template synthesis, using supported nanoporous alumina, a powerful method was developed that opens a way for complex devices and circuits fabrication exploiting particular properties of nanowires and nanostructures. Work is now in progress to go further both for realizing specific nanostructures for fundamental studies and for applications, such as spintronics, magnetic recording, sensors and biosensors, etc. Acknowledgements This work was partially supported by the FNRS, the NANOMOL project (Actions de recherches concerte´es— Communaute´ franc¸aise de Belgique), and by the Interuniversity Attraction Pole Program (P6/42)—Belgian State—Belgian Science Policy. We gratefully acknowledge financial support for this study from the Government of the Walloon Region (NANOTIC project).

References Fig. 5 SEM micrographs from localized arrays of single nanowires in a matrix arrangement masking the supported nanoporous alumina surface using e-beam lithography

1. F. Keller, M.S. Hunter, D.L. Robinson, J. Electrochem. Soc. 100, 411 (1953) 2. J.P. O’Sullivan, G.C. Wood, Proc. Roy. Soc. Lond. Ser. A: Math. Phys. Eng. Sci. 317(1531) 511 (1970)

123

S254 3. G.E. Thompson, R.C. Furneaux, J.S. Goode, G.C. Wood, J.A. Richardson, Nature 272, 433 (1978) 4. R.C. Furneaux, W.R. Rigby, A.P. Davidson, Nature 337, 147 (1989) 5. H. Masuda, K. Fukuda, Science 268, 1466 (1995) 6. P.B. Price, R.M. Walker, J. Appl. Phys. 33, 3407 (1962) 7. C.R. Martin, Science 266, 1961 (1994) 8. R. Parthasarathy, C.R. Martin, Nature 369, 298 (1994) 9. R.P. Fleischer, P.B. Price, R.M. Walker, E.L. Hubbard, Phys. Rev. 156, 353 (1967) 10. W. Kautek, S. Reetz, S. Pentzien, Electrochim. Acta 40, 1461 (1995) 11. E. Ferain, R. Legras, Nucl. Instrum. Methods Phys. Res. Sect. B. 174, 116 (2001) 12. T. Thurn-Albrecht, J. Schotter, G.A. Ka¨stle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Science 290, 2126 (2000) 13. R.J. Tonucci, B.L. Justus, A.J. Campillo, C.E. Ford, Science 258, 783 (1992) 14. R.T. Rajendra Kumar, X. Badel, G. Vı´kor, J. Linnros, R. Schuch, Nanotechnology 16, 1697 (2005) 15. J. Yahalom, O. Zadoc, J. Mater. Sci. 22, 499 (1987) 16. D.S. Lashmore, M.P. Dariel, J. Electrochem. Soc. 135, 1218 (1988) 17. S. Menezes, D. Anderson, in Electrochemical Society Extended Abstracts 342, Vol. 88-2, 174th Meeting of the Electrochemical Society, Chicago, Illinois, October 9–14, 1988, p. 503 18. L.M. Goldman, C.A. Ross, W. Ohashi, D. Wu, F. Spaepen, Appl. Phys. Lett. 55, 2182 (1989) 19. G. Barral, S. Maximovitch, Colloque de Physique. C4, 291 (1990) 20. J.P. Celis, A. Haseeb, J.R. Roos, Trans. Inst. Met. Finish. 70, 123 (1992) 21. A.S.M.A. Haseeb, J.P. Celis, J.R. Roos, J. Electrochem. Soc. 141, 230 (1994) 22. S. Ma´te´fi-Tempfli, L. Piraux, Multiple bath electrodeposition EP1256639A1; method, apparatus and system for electrodeposition of a plurality of thin layers on a substrate WO02092883, US2006243597 (2002)

123

J Mater Sci: Mater Electron (2009) 20:S249–S254 23. H. Masuda, F. Hasegwa, S. Ono, J. Electrochem. Soc. 144, L127 (1997) 24. K. Nielsch, J. Choi, K. Schwirn, R.B. Wehrspohn, U. Go¨sele, Nano Lett. 2, 677 (2002) 25. H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, T. Tamamura, Appl. Phys. Lett. 71, 2770 (1997) 26. M. Tian, S. Xu, J. Wang, N. Kumar, E. Wertz, Q. Li, P.M. Campbell, M.H.W. Chan, T.E. Mallouk, Nano Lett. 5, 697 (2005) 27. P.P. Mardilovich, A.N. Govyadinov, N.I. Mukhurov, A.M. Rzhevskii, R. Paterson, J. Membr. Sci. 98, 131 (1995) 28. K. Nielsch, F. Mu¨ller, A.-P. Li, U. Go¨sele, Adv. Mater. 12, 582 (2000) 29. S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, J. Electrochem. Soc. 149, B321 (2002) 30. S.Z. Chu, K. Wada, S. Inoue, S. Todoroki, Electrochim. Acta 48, 3147 (2003) 31. O. Rabin, P.R. Hertz, Y.-M. Lin, A.I. Akinwande, S.B. Cronin, M.S. Dresselhaus, Adv. Funct. Mater. 13, 631 (2003) 32. N. Yasui, A. Imada, T. Den, Appl. Phys. Lett. 83, 3347 (2003) 33. G.J. Hale, Thin Solid Films 63, 95 (1979) 34. L. Mattsson, Y.H. Le Page F. Ericson, Thin Solid Films 198, 149 (1991) 35. J. Mallet, K. Yu-Zhang, S. Ma´te´fi-Tempfli, M. Ma´te´fi-Tempfli, L. Piraux, J. Phys. D: Appl. Phys. 38, 909 (2005) 36. W. Vinckx, J. Vanacken, V.V. Moshchalkov, S. Ma´te´fi-Tempfli, M. Ma´te´fi-Tempfli, S. Michotte, L. Piraux, Eur. Phys. J. B: Condens. Mater. Compl. Syst. 53, 199 (2006) 37. W. Vinckx, J. Vanacken, V.V. Moshchalkov, S. Ma´te´fi-Tempfli, M. Ma´te´fi-Tempfli, S. Michotte, L. Piraux, X. Ye, Physica C: Supercond. 459, 5 (2007) 38. S. Fusil, L. Piraux, S. Ma´te´fi-Tempfli, M. Ma´te´fi-Tempfli, S. Michotte, C.K. Saul, L.G. Pereira, K. Bouzehouane, V. Cros, C. Deranlot, J.-M. George, Nanotechnology 16, 2936 (2005) 39. A. Vlad, M. Ma´te´fi-Tempfli, S. Faniel, V. Bayot, S. Melinte, L. Piraux, S. Ma´te´fi-Tempfli, Nanotechnology 17, 4873 (2006)