SWCNTs May Offer Cooler Interconnects
University at Buffalo engineers recently proved with quantum mechanics that single-walled carbon nanotubes (SWCNTs) offer "cooling" properties far superior to those provided by metals in electronics. Nanotubes could replace many of the metals used in ICs now, argues Cemal Basaran, director of the university's Electronic Packaging Laboratory.
Sally Cole Johnson, Contributing Editor -- Semiconductor International, 4/6/2009
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High current densities can induce electromigration and thermomigration, phenomena that damage metal conductors and produce heat — often causing the premature failure of electronics devices.
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| Cemal Basaran, professor, University at Buffalo |
Researchers led by Cemal Basaran, director of the Electronic Packaging Laboratory at the University at Buffalo (Buffalo, N.Y.), spent the past four years performing quantum mechanics calculations to prove that in single-walled carbon nanotubes (SWCNTs) high current densities do not lead to electromigration and thermomigration problems because they generate only 1% of the heat produced by traditional metals like copper.
CNTs are extremely thin, hollow cylinders no thicker than a single atom, thousands of times stronger than metals. Many researchers believe they will eventually replace metals in millions of electronics applications. However, some had assumed that electrical heating process in SWCNTs would be governed by Joule’s Law, where resistance in a circuit converts electrical energy into heat. Basaran and his team are the first to show that mathematically, using quantum mechanics, SWCNTs do not follow Joule’s Law. The difference between metals and SWCNTs lie in the way they conduct electricity, he said.
Even though SWCNTs are conductive, they don’t have metallic bonds, Basaran said, so they don’t conduct electricity the way traditional metals do. In conventional metals, conduction causes a scattering of electrons within the lattice of the material so that when electrons move during conduction they bump into atoms. This creates friction and generates heat.
In SWCNTs, electric conduction happens in a very different, one-dimensional ballistic way. The electrons are fired straight through the material so that the electrons have little interference with the atoms. The minimal amount of friction gives SWCNTs a tremendous advantage over conventional metals, Basaran said. The unique properties of SWCNTs will allow engineers to realize a host of smaller, faster and more powerful new devices that cannot exist right now because of limitations of conventional metals and alloys.
“From an advanced packaging point of view, so far everything in our packages is metal-based,” Basaran said. “Our interconnects are either copper or aluminum, and solder joints are usually a tin-based alloy. Diffusion barriers tend to be nickel and corrosion barriers are gold. However, we’ve reached the point where metals just don’t cut it in very small-scale electronics with high current density. The current densities and temperature gradients are so large that we’re unable to stop the diffusion forces. In SWCNTs, we discovered something very important — they don’t have the diffusion barrier problem.”
Although it won’t be possible to get rid of PCBs and solder joints right away, Basaran said he expects that the interconnects in a first-level package will be replaced by SWCNTs. “This will provide significantly higher density,” he added. “We’ll probably see a system-on-a-chip (SoC) package use this technology first, with less reliance on a PCB and solder joints.”
One of the greatest advantages of SWCNTs is that they can operate at high temperatures. “A big problem for the U.S. Navy and Air Force is high-temperature electronics,” Basaran said. “Systems we have now cannot operate at the temperature levels they want. If, however, you have a silicon-carbide-based semiconductor substrate with SWCNT interconnects, it can be used at high temperature levels. A 600°C package, if it existed on the market, wouldn’t be a problem with SWCNTs.”
SWCNTs can be grown on a substrate at extremely low costs, though with associated difficulties: handling them and controlling the chirality. “When you make the material, you have to control the chirality, because how you fold it makes it either a semiconductor or a metallic. Making it without defects is another trick,” Basaran said. “If you take a single sheet and fold it, it’s an atom thick. If a single atom is missing, it results in a big defect. From what we understand, Japanese researchers have perfected this process. They are able to make high-quality, defect-free CNTs. The manufacturing process is a science.”
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| Single-wall carbon nanotubes can sustain current densities 1000× higher than metals. This image shows a molecular dynamics simulation for the failure of CNTs under uniaxial tension. (Source: University at Buffalo) |
Basaran said he expects to see a SWCNT-based package emerge from Japan within the next five years. And he cautions that if the United States doesn’t invest in the technology now, it may be difficult to catch up.
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