Nanotube bonding could cut heat in ICs

Heat extraction from ICs can be improved by bonding carbon nanotubes to the thermal interfaces.

From the left: researchers Brett Helms, Frank Ogletree and Sumanjeet Kaur

It is already known that carbon nanotubes have high lengthwise thermal conductivity, and that a ‘brush’ made of nanotubes pressed against a metal surface can make a good thermal interface.

Researchers at the US Lawrence Berkeley National Laboratory claim to have developed a technique to increase six-fold the flow of heat from metal to carbon nanotubes.

“The thermal conductivity of carbon nanotubes exceeds that of diamond or any other natural material but because carbon nanotubes are so chemically stable, their chemical interactions with most other materials are relatively weak, which makes for high thermal interface resistance,” said Berkeley Lab physicist Frank Ogletree. “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.”

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.

The technique is done through gas vapour or liquid chemistry at low temperatures, making it suitable for the manufacturing of computer chips, said the lab.

The reactive reactive molecules used to bridge the nanotube-metal interface are aminopropyl-trialkoxy-silane for oxide-forming metals, and cysteamine for noble metals.

In the research, originally prompted by a request from Intel, vertically aligned carbon nanotube arrays were grown on silicon wafers.

Separately, thin films of aluminium or gold were evaporated on glass, then ‘functionalised’, and allowed to bond with the nanotube arrays.

The majority of the nanotubes within the array may still fail to connect with the metal, said the Berkeley team, which is developing a way to improve the density of carbon nanotube-metal contacts.

It said the technique should also be applicable to single and multi-layer graphene.

The work is described in a Nature Communications paper: “Enhanced thermal transport at covalently functionalized carbon nanotube array interfaces.”

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