Ultra-long metal nanowire arrays on solid substrate with strong bonding
© Xu et al; licensee Springer. 2011
Received: 7 June 2011
Accepted: 9 September 2011
Published: 9 September 2011
Ultra-long metal nanowire arrays with large circular area up to 25 mm in diameter were obtained by direct electrodeposition on metalized Si and glass substrates via a template-based method. Nanowires with uniform length up to 30 μm were obtained. Combining this deposition process with lithography technology, micrometre-sized patterned metal nanowire array pads were successfully fabricated on a glass substrate. Good adhesion between the patterned nanowire array pads and the substrate was confirmed using scanning acoustic microscopy characterization. A pull-off tensile test showed strong bonding between the nanowires and the substrate. Conducting atomic force microscopy (C-AFM) measurements showed that approximately 95% of the nanowires were electrically connected with the substrate, demonstrating its viability to use as high-density interconnect.
Recently, template-based methods have been successfully adopted for fabrication of metal, semiconductor and polymer nanowire arrays by electrodeposition. These methods have used heavy ion-irradiated polycarbonate (PC) film or patterned porous anodic alumina membranes (AAM) as the template . AAM has the advantages of high-density, highly ordered pore channels and adjustable pore size compared to the porous PC film. The nanowire arrays obtained can be used for interconnects , ultrahigh density capacitors , batteries , memories , biosensors [6–8], etc. For all these applications, length and aspect ratio of the nanowires are the most important parameters to consider. Increasing the length of nanowires will not only enhance the surface area dramatically but also increase the amount of nano-devices grown in one single nanowire . High-aspect ratio metal nanowire arrays with length above 10 μm are usually obtained by using a sputtered metal seed layer on the back of AAM template with follow-on electrodeposition . However, the main disadvantage when using these metal substrates is the lack of possibility for readily integrating the nanostructure array with the current microsystem technology. Also, the metal thin film deposited on the back of the AAM is thin and soft and is not a strong physical support . This is a hindrance to achieving uniform length nanowire arrays with large area coverage because mechanical polishing is required to level the unevenness of the deposited nanowire tops . To address these difficulties, there have been many studies where AAMs were fabricated by anodization of evaporated or sputtered aluminium films on solid substrates, such as Si or glass [12, 13]. In order to achieve long nanowires, a thick AAM is required, and this requires the use of a thick Al layer and a long anodization time . The deposition of a thick Al film (above 10 μm) on a solid substrate is neither easy nor cost-effective, especially when considering the quality of the evaporated Al  material. However, by using the electron-beam evaporation, an Al film up to 50 μm thick  and a length of about 9 μm nanowire has recently been demonstrated , most of the publications about nanowire arrays on Si substrates show values below 3 μm [1, 9, 13, 16]. This is due to the high cost of evaporation and thermal-stress which causes detachment during the anodization of the thick Al film. A further disadvantage of using this AAM-Si template created from an evaporated Al film is that the pores of the AAM tend not to be open at the bottom of the channels due to the insulator barrier layer between AAM and Si substrate, this results in a further step to remove the barrier layer, weak bonding between AAM and Si or even the detachment of the AAM from the Si surface have been observed if the barrier were not properly removed . It would also be very difficult to achieve large area uniform nanowire arrays on a solid substrate using the AAM template prepared by the above techniques, due to the difficulty in achieving uniform Al thickness over a large area, and of maintaining appropriate homogeneity of the template's pore channels .
Recently, Taberna  attached a commercial AAM to a smooth copper foil as an electrode without using any conductive glue between AAM template and the copper foil. In this work, another thicker copper foil was put on the top of AAM and worked as an anode with a cellulose paper separator between the AAM and anode copper foil. Copper nanowire arrays, directly attached to the smooth copper foil were achieved. Theoretically, this method can also achieve ultra-long nanowires on other solid substrates, but this setup requires a flat anode which is not process friendly when mesh type anodes are required. Furthermore, it is not easy to get homogeneous nanowire arrays with a large area due to the pore blocking effect of the cellulose paper, specifically when the template's pore diameters are comparable to the width of the cellulose fibres. Nano-pillar gold arrays on a gold-coated glass substrate have also been achieved using a similar procedure . However, the length of these nano-pillars was less than 1 μm. So far, no nanowire arrays with length above 9 μm on Si or glass substrate have been achieved by this template method. In this paper, we report an effective procedure using a new setup for the fabrication of ultra-long metal nanowire arrays directly onto Si and glass substrates. Both copper and silver nanowires were fabricated using this setup. The length of these nanowire arrays can be up to 30 μm with length uniformity in an area covering up to 25 mm in diameter. This technique when combined with lithography technology enabled micrometre-size-patterned nanowire array pads on a glass substrate to be fabricated. The strong bonding between nanowires and the bottom substrate was demonstrated by a tensile stress test, performed on the large area nanowires and the subsequent scanning acoustic microscopy (SAM) analysis on patterned nanowires. Furthermore, conductive atomic force microscopy analysis on the nanowire-AAM composite was performed to show the uniform conducting properties of the composite and to prove that the each wire was electrically connected to the substrate.
Electrodeposition of copper nanowires was carried out at room temperature at a constant current with a density of 1.0 mA/cm2. Electrodeposition usually takes about 5 to 12 h from an electrolytic bath where a typical copper bath consists of 200 g l-1 of CuSO4.5H2O and 20 g l-1 of H2SO4 and a silver bath consists of 16 g l-1 of Ag2SO4, and 224 g l-1 of potassium thiocyanate. A two electrode cell was used for deposition and copper or Ag foil was used as counter electrode. The deposition was performed at a stirring speed of 500 rpm at room temperature. The commercial AAM used in this paper are Anodisc™ membrane filter with a 200-nm nominal pore diameter from Whatman plc (Maidstone, Kent, UK). To obtain nanowire arrays on substrate, the template was removed in 6.0 M KOH, washed with plenty of de-ionised water and dried in air.
Pull-off adhesion tensile test
The adhesion testing was carried out using an Elcometer 110 P.A.T.T.I. pneumatic tester according to the ASTM D4541-95e1 standard. A Ag nanowire-AAM on Si sample was selected to avoid the effect of oxidation of the copper nanowire tips. The sample size was 11 mm in diameter. The diameter of the test stud was 8.16 mm which is related to 0.52-cm2 test area. To run the adhesion test, the stud was attached to the Ag nanowire-AAM surface with Araldite epoxy adhesive as shown in Figure S1a (see Additional file 1). The Si wafer was attached on a flat metal base plate as shown in Figure S1b (see Additional file 1). For execution of a pull-off test, the stud of the test equipment has to be firmly attached to the nanowire-AAM surface. To ensure this, the surfaces of the stud and the nanowire-AAM composite were polished using a coarse sand paper and subsequently cleaned in acetone before applying the adhesive epoxy. After the sample was mounted, the specimen under test was pulled perpendicular to the wafer substrate until some part of the structure under ruptures.
SAM characterization was carried out by SONIX HSl000 with software IC5.98d and transducers with nominal frequencies of 75 MHz. The transducer tip and sample were immersed in distilled water at room temperature. The test was performed with the Si substrate side up, in order to avoid signal scattering due to the rough surfaces of the nanowires. Signals from the sample were transferred to a personal computer for advanced numerical data analysis.
Conductive AFM characterization
Conductive AFM characterization was performed using a Dimension D3100 scanning probe microscope. The tip used in this study was electrically conductivity probes made from 0.01 to 0.025 Ωcm antimony (n)-doped Si. The nominal tip radius is 20 nm and the front and backside of the tip was coated with 20 nm Pt/Ir on top of 3 nm Cr. (see Figure S2 in Additional file 1) The scanning parameters (scan rate = 1 Hz; integral gain = 2; proportional gain = 3 to 4; deflection set point = 1 V; DC sample bias = 1 to 2 V) were set to these values for all experiments.
Results and discussion
A useful method has been developed to fabricate ultra-long metal nanowire arrays, from micrometre-size pattern to large area (of 25 mm diameter), on a solid substrate by using a ready-made AAM as template. By mechanically polishing the surface after nanowire fabrication, nanowire arrays with uniform length up to 30 μm were achieved. The nanowires and substrate were bonded via intermediate metal film layer, which showed a very good adhesion. Conducting AFM analysis showed that nearly 95% of the nanowires had good electrical connection with the substrate. While only metal nanowires fabricated on Si and glass substrate are shown in this letter, the approach described here can be adjusted to deposit any conductive materials, oxides, conductive polymers and semiconductors, on any solid substrate as well as flexible polymer substrate. In addition, by slightly changing the fabrication procedures, individually addressable nanowire pads/electrodes with micrometre size can also be fabricated (see Figure S3 in Additional file 1). It is also possible to achieve longer (higher aspect ratio) and larger area coverage of these nanowire arrays with uniform length for nearly 100% pore filling, if in-house fabricated AAM with homogenous pore distribution and larger size template is used.
This work is financially supported by Enterprise Ireland (EI) under the commercialization fund CFTD/2008/322 and by "Irish Research Council for Science, Engineering and Technology (IRCSET)", co-funded by Marie Curie Actions under FP7". The authors acknowledge the assistance from Pio Jesudoss, Paul Tassie and Tony Compagno for their help with the characterization equipments.
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