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INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 16 (2005) 2936–2940 doi:10.1088/0957-4484/16/12/036 Nanolithography based contacting method for electrical measurements on single template synthesized nanowires S Fusil1 , L Piraux2,4 , S Mátéfi-Tempfli2 , M Mátéfi-Tempfli2 , S Michotte2 , C K Saul3,5 , L G Pereira3 , K Bouzehouane3 , V Cros3 , C Deranlot3 and J-M George3 1 LMN, Université d’Evry Val d’Essonne, Boulevard F Mitterrand, 91025 Evry cedex, France Unité de Physico-Chimie et de Physique des Matériaux, Place Croix du Sud, 1, 1348-Louvain-la-Neuve, Belgium 3 Unité Mixte de Physique CNRS/Thales and Université Paris Sud XI, Route départementale 128, F-91767 PALAISEAU Cedex, France 2 E-mail: [email protected] Received 1 August 2005, in final form 7 September 2005 Published 25 October 2005 Online at stacks.iop.org/Nano/16/2936 Abstract A reliable method enabling electrical measurements on single nanowires prepared by electrodeposition in an alumina template is described. This technique is based on electrically controlled nanoindentation of a thin insulating resist deposited on the top face of the template filled by the nanowires. We show that this method is very flexible, allowing us to electrically address single nanowires of controlled length down to 100 nm and of desired composition. Using this approach, current densities as large as 109 A cm−2 were successfully injected through a point contact on a single magnetic multilayered nanowire. This demonstrates that the technique is very promising for the exploration of electrical spin injection in magnetic nanostructures. 1. Introduction Over the last decade, many groups have been exploring the concept of using the pores in nanoporous media as templates to prepare arrays of nanowires or nanotubes by various synthesis techniques. This so-called ‘template method’ makes it possible to fabricate various nanowires that are difficult to form using a conventional nanolithographic process. Though there now exists a huge range of hosts, most studies in this area have entailed the use of two types of templates: track-etch polymer membranes and nanoporous aluminas. Under optimized fabrication conditions, these two types of templates exhibit well defined nanoporosity with geometrical parameters controllable to a large extent [1, 2]. Also, depending on the thickness of the nanoporous media, above or below a few micrometres, these templates are either 4 Author to whom any correspondence should be addressed. 5 Present address: Departamento de Fisica, Universidade Federal do Parana, CP 19044, 81531-990, Curitiba, PR, Brazil. 0957-4484/05/122936+05$30.00 © 2005 IOP Publishing Ltd self-supported or supported on a substrate [3, 4]. This templating method has proven to be reliable for the synthesis of a variety of nanowired materials including noble metals, ferromagnets, superconductors, semimetals, alloys, oxides, carbon nanotubes, conducting polymers and different types of multilayers. The resulting cylinders have diameters ranging between a few nanometres and a few microns and the aspect ratio (length to diameter) can be controlled to a large extent up to 103 . Using different approaches leading to measurements on both single wires and/or arrays, a variety of novel physical properties and potential applications have been identified in relation to the nanowire composition and to their nanoscopic dimensions [5]. Fundamental transport properties such as localization and electron–electron interaction effects have been studied in ultrathin electrodeposited Au nanowires [6]. Nanowires of pure Bi and of n-type-doped Bi were fabricated either by pressure injecting liquid melt into the nanochannels of anodic alumina templates or by electrodeposition. These Bi Printed in the UK 2936 Nanolithography based contacting method on single nanowire nanowires show finite size effects, promising thermoelectric properties and semimetal–semiconductor transition when the diameter is reduced [7, 8]. Other interesting results were recently obtained in 1D superconducting nanowires such as the destruction of superconductivity by the formation of phase slip centres [9, 10]. Nanotubules of conducting polymers have also been prepared either electrochemically or by chemical synthesis. These nanotubes show enhanced conductivity compared to bulk materials and controllable dimensionality of conduction [11, 12]. For electrodeposited magnetic nanowires, magneto-transport measurements have enabled us to investigate field induced magnetization reversal mechanisms [13, 14] and electronic transport through domain walls [15]. In addition, the transport properties of nanowires have attracted a great deal of research interest because the current is flowing naturally along the axis of segmented nanowires, i.e. in the so-called CPPGMR geometry (current perpendicular to the plane–giant magnetoresistance). Various magnetic multilayered nanowires have been successfully fabricated using a single bath and a pulse-plating method [16]. CPP-GMR studies on a large set of nanowires with different materials and structures have allowed us to extract a large bunch of important parameters for spintronics such as spin diffusion length, spin asymmetry coefficients in the bulk and at the interface, and interface resistance [17]. Current induced magnetization has been reported in Co/Cu multilayered wires [18] as well as quantized spin transport in nanoconstrictions based on electrodeposition in nanopores [19]. In most cases, the electrical measurements are performed by attaching leads on either side of the nanoporous media, so that the number of wires contributing to the conduction measurements is unknown. However, it is often highly desired to perform electrical transport measurements on single nanowire. Till now, two different experimental approaches have been developed to address electrically one single nanowire. In the former the wires are kept embedded in the nanoporous media, while in the latter the wires are removed from the host nanoporous material and electrodes are next attached to some of them. Considering the first approach, a self-contacting technique has been successfully employed by using polymer templates with relatively low pore density (typically less than In this method, prior to 109 pores cm−2 ) [13, 14]. electrodeposition, a metallic film is deposited on the top face of the membrane. This film is thin enough to ensure that the pores are not closed, so they can thereafter be filled by the solution. Since the wires in the array do not grow exactly at the same rate, the first emerging wire (detected by a sharp increase of the plating current) stops the growth of the others. Using this procedure, a single nanowire is contacted reliably during the growth process and electrical contacts are therefore established on the two extremities of a 6–20 µm long nanowires, allowing two-probe measurements. Due to the extremely large aspect ratio of the nanowire, the contact resistance is usually small compared to the sample resistance. However, this method is not suitable using anodic porous alumina as template because of the much higher pore density (up to 1011 pores cm−2 ), that makes it difficult to contact a single wire. This drawback of the porous alumina template was partially solved [20] using a pair of crossed narrow electrodes patterned on the opposite faces of a 60 µm thick alumina membrane, allowing us to contact more easily a single or a few nanowires. A different way was followed by using a singlepore membrane [21], in which a Bi nanowire was grown electrochemically and contacted using two macroscopic electrodes on either side of the membrane. The second approach is based on the use of optical or e-beam lithography technique to connect a single nanowire [22–24]. The process requires several steps prior to the lithography process, which have been carefully optimized, including the dissolution of the membrane, the dispersion of an appropriate density of wires on a substrate and the cleaning of the wires to remove all residues from the surface. The advantage of the lithography technique is that four-point electrical measurements can be performed on an isolated nanowire and that segments as small as 500 nm can be probed [22]. However, the extremely high surface to volume ratio of the nanowires makes them more sensitive to oxidation when removed from the template. Therefore, it is often difficult to prevent nanowire burn-out by static discharge due to high resistance electrical contacts between the nanowires and the finger-like electrodes (mainly caused by oxidation and imperfect cleaning of the nanowires). In this paper, we propose a new reliable method that allows the electrical connection of one single nanowire in a large assembly of wires embedded in a nanoporous template with a high pore density (such as anodic porous alumina template). Nanocontacts on single wires are performed by indentation of an ultrathin insulating photoresist layer deposited on the top face of a thin porous alumina layer (1 µm) supported on a silicon substrate in which the lengths of the electrodeposited nanowires are uniform and equal to the thickness of the template. A modified atomic force microscope (AFM) designed for local resistance measurements [25] is used as a nanoindenter, yielding an access point onto individual nanowires at the surface of the template. The indented holes are subsequently filled by metal to establish contacts on single nanowires. 2. Experimental details Figure 1 illustrates the fabrication process used to create a nanocontact onto a single wire embedded in a thin alumina template. The process begins with the anodization of a ∼0.5 µm thick Al layer evaporated onto the Ti (5 nm)/Au (100 nm)/Nb (20 nm)/Si substrate. While the Ti and the Nb layers serve as buffer and adhesion layers, the gold layer was used as electrode for the subsequent electrochemical and measurement processes. The nanopores in the template are formed by complete anodic oxidation of the Al layer, performed in 0.3 M oxalic acid solution at 2 ◦ C under a constant voltage of 60 V. At the end of the anodization process, residual alumina was eliminated completely from the bottom of all the nanopores, by etching with a solution of 5 wt% H3 PO4 at 30 ◦ C, in order to fill conducting material into the nanopores electrochemically using the gold layer as a working electrode on which electrodeposition takes place. The interpore distance and pore length increase in proportion to the applied anodization voltage and time, respectively. The use 2937 S Fusil et al Figure 1. Schematic illustration of the single-wire contacting process on an array of nanowires electrodeposited in a supported nanoporous alumina template. of supported alumina templates prevents one from breaking the fragile template during the handling and nanocontact fabrication process. Such a type of templates also shows improved thermal stability (up to 1000 ◦ C) compared to thick self-supported alumina membranes. The final thickness of the alumina template was about 0.75 µm and the pore density was close to 1010 cm−2 . The pore diameter used in this study was 85 nm and the interpore distance was about 140 nm. Electrodeposition into the pores was carried out in a conventional three-electrode cell with a platinum counterelectrode and a Ag/AgCl reference electrode. Arrays of Co/Cu multilayered nanowires were fabricated using a pulseplating method in which the two metals are deposited from a single sulfate bath (with copper kept in dilute concentration) by switching between the deposition potentials of the two constituents [16]. Co (14 nm)/Cu (7 nm) nanowires were electrodeposited within nanopores and the deposition process was stopped as soon as the first nanowires emerged at the surface, as detected by a rapid increase of the electrochemical current. This corresponds to an increase of the effective electrode area caused by the formation of caps on the template surface. The surface of the filled alumina template at this stage was examined by conductive tip atomic force microscopy (CTAFM): when a tip–sample bias is applied, both topographical and local resistivity data are acquired simultaneously. As the growth rate is inhomogeneous in different pores, only a few pores show a complete filling followed by hemispherical metallic cap growth at the end of the deposition process; see figure 2(a). Prior to the nanolithography step, the template was thinned by mechanical polishing using colloidal silica 2938 (Syton® ) in such a way that a large proportion of the nanowires end at the template surface, figure 2(b). The thinning procedure of the alumina/nanowire layer is controlled with a resolution of few tens of nanometres. This makes it possible to adjust the length of the nanowire over a wide range (from a few micrometres down to ≈100 nm). In figures 2(a) and (b), we show the top view (topography and resistance) images of the alumina pore structure filled with Co/Cu nanowires before (a) and after (b) polishing. The conductive surface ratio increases from 2% for the as-grown sample up to 37% after the second polishing step. At this stage, the measured wire length is about 350 nm, corresponding to an aspect ratio of 4, and about 50% of the pores are filled with metal. During the mechanical polishing, an intrinsic roughness of 20 nm corresponding to the abrasive particle size is created. As a consequence, the nanowires emerge from about 20 nm above the alumina top surface (figure 2(c)). In the fabrication process of the nanocontact, we first deposit by spin coating a thin (60 nm) insulating polymer resist. Then a second thick (1.2 µm) photoresist is spin coated and designed by a classical optical lithographic process: a 30 × 30 µm2 aperture is opened on the thin base resist. The electrical insulation realized by the thin resist inside the aperture is checked by CT-AFM. The CT-AFM is then used to create a hole by indentation of the 60 nm resist until a good electrical contact between the tip and the underlying nanowire is obtained. More details about this nanoindentation step can be found in [26]. The bottom of the hole is enlarged and cleaned from resist residues by a soft O2 –Ar plasma etching. Then a 100 nm Au layer is sputtered to fill this access point to the nanowire and create the nanocontact. The overall shape of this nanocontact can be represented by the inverted 3D image of the indented hole obtained by tapping mode AFM using ultrasharp tips, figure 2(d). This inverted 3D image is just a trick to visualize the shape and size of the nanocontact to be deposited since it is not possible to see the bottom of the hole in the raw image (image inversion option of the AFM data processing software: the bottom of the indented hole, i.e. the future contact surface, is transformed to the top of the hill). The interpore distance (140 nm, figure 2(c)) is significantly larger than the size of the hole (≈50 nm, figure 2(d)) and thus the probability to connect a single wire is very high. Finally, the top electrical contact is designed in the Au layer by subsequent technological steps such as optical lithography and Ar ion milling. The area of each alumina template is about 1 cm 2 , on which are fabricated about 15 nanocontacts. The whole process allows us to connect separately individual nanowires at different locations on the same sample and provides an easy way to perform single-wire magneto-transport measurements. In the inset of figure 3, we show an I (V ) curve obtained at room temperature on a single Co/Cu nanowire. An Ohmic behaviour is found for dc current up to 12 mA (otherwise, all electrical measurements were performed using dc signals), which corresponds to a current density of 2.5 × 108 A cm −2 . This proves the very good quality of the electrical contact on both extremities of the wire. The wire resistance (∼10 ) is consistent with the estimated mean resistivity of the multilayer (∼15 µ cm) and geometrical parameter (S/L ∼ 1.5 × 10−6 cm). The GMR curve obtained with the field perpendicular to the wire axis is reported in figure 3. A GMR Nanolithography based contacting method on single nanowire Figure 2. CT-AFM 10 µm × 10 µm images (left, topography; right, local resistivity) of as-grown sample with a few emerging nanowires with hemispherical conducting heads (a) and polished sample with 37% of conductive surface ratio (b); tapping mode AFM 1 µm × 1 µm image of magnetic nanowires embedded in a nanoporous alumina template after polishing (c); tapping mode AFM inverted image of a typical nanoindented hole after plasma etch, x y scale is 1 µm, z scale is 100 nm (d). Figure 3. Variation of the resistance as a function of the magnetic field at 300 K measured on a single Co/Cu multilayered nanowire. The magnetic field is applied perpendicular to the wire axis. Dark squares correspond to an applied current of 100 µA and open circles to an applied current of 15 mA. The inset shows the I (V ) curve. ratio of 3.1% has been measured under an excitation current of 100 µA. This GMR value is in good agreement with that expected from previous studies on the Co/Cu nanowires system [17, 27]. To demonstrate the stability of the system when high current density is reached, we also add in figure 3 the magnetoresistance curve recorded with a current of 15 mA (corresponding to a current density of 3 × 108 A cm−2 ). A small increase of the wire resistance leading to a small decrease of the GMR ratio (2.95%) was observed. This can be attributed to an increase of the nanowire temperature due 2939 S Fusil et al to Joule heating. The first effect of degradation, evidenced by irreversible change of the wire resistance, appears for a current density of 6 × 108 A cm −2 . Finally, burn-out of the nanowire was observed at 1.8×109 A cm −2 . A possible reason for the very good resistance of the nanowires to high current densities is that heat is evacuated by conduction through the alumina matrix surrounding the nanowire. Thanks to the very large wire density in alumina templates, heat transfer is also favoured by the neighbouring wires that increase the thermal conductivity of the Al2 O3 /metal nanocomposite system. 3. Conclusions As a conclusion, we demonstrate a new reliable method that allows the electrical connection of one single nanowire in a large assembly of wires embedded in a nanoporous template with a high pore density (such as an anodic porous alumina template). Nanocontacts on single wires are performed by indentation of an ultrathin insulating photoresist layer by using a modified atomic force microscope (AFM) designed for local resistance measurements. The good quality of the contact was evidenced by magnetoresistance measurement. CPP-GMR was clearly visible and we have shown that current densities as large as 6 × 108 cm −2 can pass through the nanowire without deterioration. Our approach is very flexible and promising to investigate the influence of high electric current density on the magnetic state of a ferromagnet, i.e. current induced magnetization reversal, domain wall motion and magnon generation. Acknowledgment The work was partially supported by the Belgian Science Policy through the Interuniversity Attraction Pole Program PAI (P5/1/1). References [1] See Ferain E and Legras R 2001 Nucl. Instrum. Methods B 174 116 and references therein [2] Furneaux R C, Rigby W R and Davidson A P 1989 Nature 337 147 Masuda H and Fukuda K 1995 Science 268 1466 [3] Rabin O, Herz P R, Lin Y M, Akinwande A I, Cronin S B and Dresselhaus M S 2003 Adv. Funct. Mater. 13 631 2940 [4] Dauginet-De Pra L, Ferain E, Legras R and Demoustier S 2002 Nucl. Instrum. 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