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January 23, 2014

Carbon Nanotubes Aid Microprocessor Cooling

Tiffany Trader
Ogletree-CNT-graphic

The semiconductor industry is facing several hurdles in developing the next generation of microprocessors. As more and more transistors are squeezed onto a chip, overheating threatens to negate performance gains. Countering this is a two-step process that begins with getting the heat out of the chip and onto the circuit board, where it can then be dispersed using fans or other techniques.

Recently researchers have begun assessing the feasibility of using carbon nanotubes to conduct heat away from the microprocessor chips. The approach could be the key to maintaining the performance levels of densely packed, high-speed microprocessors in the decades to come. It is also expected to be compatible with single- and multi-layer graphene devices, which are facing the same cooling issues as silicon.

Through a partnership with Intel Corp, scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a “process friendly” technique using carbon nanotubes in combination with organic molecules that serve as bonding agents. The study took place at Berkeley Lab’s Molecular Foundry, one of five DOE Nanoscale Science Research Centers, under the direction of Frank Ogletree, a physicist with Berkeley Lab’s Materials Sciences Division.

Carbon nanotubes are known for having very high thermal conductivity, desirable for this microprocessor cooling application, but they also demonstrate high thermal interface resistance – which has until now precluded their use as a cooling agent. Working with Intel engineers Nachiket Raravikar and Ravi Prasher, the Berkeley team was able to increase and strengthen the contact between carbon nanotubes and the surfaces of other materials, thereby reducing thermal resistance and substantially boosting heat transport efficiency.

Crucial to the study was the use of organic molecules, which formed strong covalent bonds between carbon nanotubes and metal surfaces. With the improved adhesion, the flow of heat from the metal to the carbon nanotubes increased six-fold, laying the groundwork for next-generation computer chips that are faster and more efficient. And because the technique relies on vapor or liquid chemistry at low temperatures, it is suitable for computer chip manufacturing.

“We’ve developed covalent bond pathways that work for oxide-forming metals, such as aluminum and silicon, and for more noble metals, such as gold and copper,” said Ogletree, who is also a staff engineer for the Imaging Facility at the Molecular Foundry. “In both cases the mechanical adhesion improved so that surface bonds were strong enough to pull a carbon nanotube array off of its growth substrate and significantly improve the transport of heat across the interface.”

While early results are promising, a majority of the nanotubes may still fail to connect with the metal, so the next step for the Berkeley team is to improve the density of the contact material.

A paper detailing this research appears in the journal Nature Communications with the title “Enhanced Thermal Transport at Covalently Functionalized Carbon Nanotube Array Interfaces.”