Bell Labs has begun a project to use its 'nanograss' nanostructured surfaces in a liquid cooling system for chips, which has the potential to dissipate powers of greater than 100W/cm sq.

Although currently using multiple cores is seen as the most persuasive strategy for reducing the heat output, Bell Labs expects that eventually liquid cooling will be a requirement.

A droplet of fluid resting on a 'nanograss' surface doesn't seep down between the pillars, reducing the contact area by up to a factor of 50 compared with planar silicon.

Viscous drag is thus dramatically reduced, and coolants should be able to flow much more easily through nanograss-lined channels. "One of the problems with liquid cooling on chips is you very quickly need small channels through the wafer," explained Dr Dave Bishop, vice-president of Nanotechnology Research at Bell Labs.

"Conventional coolants have a viscosity such that when you try to squeeze them through small channels... it's like trying to put cold molasses through a hypodermic needle," continued Bishop. "The advantage of nanograss is it essentially breaks that log jam by lowering the viscous friction. We think it may make liquid cooling practical."

Currently Bell Labs uses conventional lithography to etch the pattern, which could be tricky to adapt to patterning the inside of a channel. However, Bishop said there are a number of alternatives, including using polymers that react to a heating cycle by adopting a nanograss structure. Imprinting techniques could also be used.

"We have thought our way through at the least the basic physics," said Bishop. "[We think] research prototypes in a matter of months, manufacturable prototypes in a matter of years."

The work is based at Bell Labs's Dublin site, a €69m venture in conjunction with IDA Ireland and Science Foundation Ireland which opened last year.

Liquid cooling has been applied before. NEC, Hitachi and Toshiba have all produced liquid-cooled laptops, and in 2002 HP revealed a system based on its inkjet heads that sprayed coolant onto chips. The nanograss surface itself is also being used in a reserve battery application by mPhase Technologies.

www.bell-labs.com