TechFront Research and Development in Manufacturing and Technology
Nanotube Soldering Process Creates Tiny Electrical Pathways
S
cientists at the University of Illinois (Champaign, IL) have devised a way to heal gaps in tiny wires too small for the smallest soldering iron. A research team, led by electrical and computer engineering professor Joseph Lyding and graduate student Jae Won Do, has published its results in the journal Nano Letters.
Carbon nanotubes, which are similar to graphene but cylindrical, are like tiny hollow wires of carbon just an atom thick. Researchers have been exploring using them as transistors in place of traditional silicon, because carbon nanotubes are easier to transport onto alternate substrates, such as thin sheets of plastic, for low-cost flexible electronics or flat-panel displays.
The carbon nanotubes are high-quality conductors, but creating single tubes suitable to serve as transistors is very dif- ficult. Arrays of nanotubes are easier to make, but the current has to hop through junctions from one nanotube to the next, slowing it down. In standard electrical wires, such junctions would be soldered, but the researchers needed to bridge these gaps on a very small scale.
“It occurred to me that these nanotube junctions will get hot when you pass current through them, kind of like faulty wiring in a home can create hot spots,” said Lyding. “In our case, we use these hot spots to trigger a local chemical reac- tion that deposits metal that nanosolders the junctions.” The nanosoldering process is simple and self-regulating. A carbon nanotube array is placed in a chamber pumped full of metal-containing gas molecules. When a current passes through the transistor, the junctions heat because of resis- tance as electrons flow from one nanotube to the next. The molecules react to the heat, depositing the metal at the hot spots and effectively “soldering” the junctions. Then the resis- tance drops, as well as the temperature, so the reaction stops. The nanosoldering process takes only seconds and improves the device performance by an order of magnitude—almost to the level of devices made from single nanotubes, but much easier to manufacture on a large scale.
Electrical and computer engineering professor Joseph Lyding led the research team that developed a way to heal gaps in wires too small for even the world’s tiniest soldering iron.
“It would be easy to insert the CVD process in existing process flows,” Lyding said. “CVD technology is commercially available off-the-shelf. People can fabricate these transistors with the ability to turn them on so that this process can be done. Then when it’s finished they can finish the wiring and connect them into the circuits. Ultimately it would be a low- cost procedure.” Now, the group is working to refine the process. “We think we can make it even better,” Lyding said. “This is the prelude, we hope, but it’s actually quite significant.”
The National Science Foundation and the Office of Naval Research supported the work. Lyding and Rogers also are
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Lyding’s group teamed with Eric Pop, an adjunct profes- sor of electrical and computer engineering, and John Rogers, Swanlund professor in materials science and engineering, experts on carbon nanotube synthesis and transfer, as well as chemistry professor Greg Girolami, an expert in a process that uses gases to deposit metals on a surface, called chemical vapor deposition (CVD).
Photo courtesy L. Brian Stauffer
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