Reflow and electrical characteristics of nanoscale solder.

Solders or metallic alloys are used extensively in a wide range of electronic systems, including surface-mount circuit boards and device packages for integrated circuits (ICs), to form conductive joints between electronic components. In existing devices, the size of solder joints range from millimeters in circuit boards to tens of micrometers in IC ballgrid-array packages. In order to extend millimeterand micrometer-scale electronic integration to the sub-micrometer and nanometer length scales, it is necessary to develop strategies to form electrically conductive joints between nanoparticles (isotropic or anisotropic) and between nanoparticles and substrates. At the present time, two strategies, either a) damascene integration or b) molecular contacts, are being utilized to form sub-micrometer scale interconnects. Damascene integration, which involves the photolithographic processes of patterning, etching, and deposition of metal, is used in copper interconnect layers in ICs. However, damascene integration is an inherently two-dimensional, layer-by-layer methodology that requires extensive processing steps. Moreover, it is prohibitively difficult to form electrical contacts between large numbers of nanoparticles using damascene integration. Although it is possible to form molecular electrical contacts between nanoparticles using self-assembly, organic molecules do not conduct electricity as well as metals and cannot survive high-temperature processing. Metallic solder allows components to be attached together in an easy and inexpensive manner. Moreover, solder joints are not sensitive to local roughness of components as solder forms a liquid during reflow. However, it is not clear if conventional solder will remain functional at the sub-micrometer and nanometer length scales, where the ratio of surface area to volume increases significantly. Solder degradation mechanisms, including oxidation, diffusion, and intermetallic formation, that do not significantly impact solder performance at the millimeter-to-10-mm length scales can render solder joints non-conductive at the sub-micrometer length scale. There are very limited experiments investigating the use of metallic solder in sub-micrometer-scale applications. In one study, 100 nm of a single-component solder, indium (In), was evaporated on a substrate, and, upon heating the substrate, solder spheres with sizes as small as 5 nm were observed. Although this experiment demonstrated that reflow might be possible on the nanoscale, the question of the functionality of solder to form an electrically conductive joint at sub-micrometer and nanometer length scales remained unanswered. In another study, a soldering technique for carbon nanotubes was carried out in a scanning electron microscope using amorphous carbon, which graphitizes under the electron-beam irradiation to form an electrical contact. In a previous study, we demonstrated electrical bonding of nanoparticles to microscale solder pads patterned on substrates using solder reflow. In this communication, we demonstrate that nanoscale solder can be electrodeposited on rod-shaped metallic particles (also known as wires) grown in nanoporous membranes and that solder on these wires (diameter range of 30 to 200 nm) can be reflowed upon heating. In our experiments, the wires contained segments that optimized wetting of solder while limiting intermetallic diffusion. The nanoscale solder reflow was carried out in a home-built tube furnace where extreme care was taken to minimize oxidation of the joints. We were also able to fuse nanowires with each other using solder. The resistance of the fused wires was measured, and we observed electrical resistances that were consistent with a low-resistance ohmic solder joint. We used the strategy of Martin and Mallouk to electrodeposit nanowires in porous membranes (templates) with a nominal pore size ranging from 30 to 200 nm (Figure 1A). We experimented with several different multicomponent