Xu et al. Nanoscale Research Letters 2011, 6:525 http://www.nanoscalereslett.com/content/6/1/525 Page 4 of 7 Figure 3 Images of Cu nanowire arrays on Si substrate. (a) Optical image of non polished sample; (b) top view SEM image of non polished sample showing approximately 30% pore filling; (c) cross sectional view SEM image of non polished sample showing the length of nanowires is about 40 to 50 μm; (d) optical image of polished sample shows mirror surface with a large circular area in diameter of 25 mm; (e) top view SEM image of polished sample with 20-μm wires down from the top surface, showing 98% pore filling; (f) cross sectional SEM image of polished sample showing the nanowire arrays with uniform length of approximately 30 μm after the AAM was dissolved. the top of the template first, grow laterally and thereby block the unfilled pores. This will eventually stop the solution to transport into the unfilled pores and prohibit the nanowire growth inside them [11]. The diameter of the nanowires was about 209 nm which is related to the pore diameters of the AAM template [22] used. Smaller diameters of nanowires should be achievable if smaller pore sizes of template are used. The cross section image in Figure 3c shows that the length of these nanowires is about 40 to approximately 50 μm. This is associated to an aspect ratio of 200 to approximately 250 based on the wire diameter. A thin layer of metal film of approxi- mately 2 μm thickness was found between the AAM and substrate due to the physical gap between them. The adhesion between nanowires and Si substrate is strong enough to sustain the force of mechanical polish- ing using a Logitech precision lapping machine. Figure 3d shows a plan view of a Cu nanowire array within an originally 50-μm-thick template after mechanically pol- ishing down to 20 μm from the top surface. The mir- ror-polished large area was achieved, suggesting uniform length of nanowires were obtained. The high-resolution SEM image in Figure 3e shows that nearly 98% pore fill- ing ratio on the top surface of template was achieved at this location in the template. It is worth noting that the filling ratio is highly dependent on the depth of surface polishing due to the unevenness of nanowire growth along the template channels from the bottom (nearly 100% pores were filled on the bottom of channels according to Figure 3c) to top. The cross sectional view of a polished sample in Figure 3f shows that nanowires with a uniform length of approximately 30 μm were achieved. The ultra-long and uniform length of these nanowires are very useful for some applications, such as Z-axis interconnects, because uniform lengths will allow for large surface area contact in an electronic device, lead- ing to maximum electrical and thermal device perfor- mance. Also, this long wire length will provide enough gaps between top and bottom substrates for filling with underfill [23] for mechanical stability [24]. For further the usefulness of nanowire arrays as a Z-axis intercon- nect, the wires need to be bonded on the pre-patterned pads of the wafer/devices. Recently, micrometre-sized Ni pillars (low aspect ratio) were fabricated on top of the pads using lithography processes [25], which is time consuming and expensive to ultra-long nanowires. Alternative to this, we are proposing direct fabrication of nanowires on top of the pads based on this template attachment approach. Micrometre-sized nanowire