Researchers cool CPUs with nano-size fridges

Intel, RTI and ASU are developing a micro-refrigerator that can be mounted on chips

Cramming ever-more transistors into CPUs has not troubled chip makers Intel Corp. and AMD Inc. The problem, rather, has been how to handle the extreme heat generated by the movement of so many electrons in such a tiny space.

With heat sinks and fans not up to the task of cooling the 100-degree-Celsius and greater chips, makers stopped trying to raise processor speeds several years ago and moved to building multicore CPUs instead.

Gamers and performance fanatics responded by using water, liquid nitrogen and other exotic liquids to cool their overclocked CPUs.

And it's not just overclockers pushing the envelope. AMD has gotten into the act, this month enabling its new Phenom II desktop processor to run at a record 6.5 GHz by cooling it with liquid nitrogen. And liquid cooling is even gaining interest from the far more conservative server and data center crowd.

However, despite containment efforts, the danger of mixing liquids with electronics remains.

Chip cooling's next generation

Now, a more elegant solution is on the horizon. Researchers at Intel, RTI International and Arizona State University have developed a micro-refrigerator that can be easily mounted on chips to draw heat from hot spots with surgical precision.

That would allow these nano-scale (10 micron) systems to be smaller and use less electricity than conventional heat sinks, fans or liquid setups, said Dr. Rama Venkatasubramanian, a senior researcher at RTI and a co-author of a paper published this month in the scientific journal Nature Nanotechnology.

The refrigerator in your kitchen uses a mechanical heat pump to compress and circulate a coolant liquid that absorbs heat inside the fridge and dissipates it outside.

The micro-refrigerator is quite different. It is a super-thin film made from thermoelectric molecules such as Bismuth telluride and Antimony telluride.

Thermoelectric materials such as these convert heat into electricity. In other words, "you're using electrons to pump heat away," Venkatasubramanian said.

Venkatasubramanian and the other researchers have been able to reduce heat on a simulated CPU by 15 degrees Celsius. But he is optimistic that by using more thermally conductive materials on the silicon chip, such as improved solder or even carbon nanotubes, the micro-refrigerator could help cool a chip by as much as 40 degrees Celsius.

Such a drastic reduction would allow a chip to run faster or a chip maker to cram even more transistors onto single or multiple cores.

Another advantage of a micro-refrigerator would be its efficiency. Each cooler would be targeted at hot spots on the rear of the chip and use only between 2 and 3 watts when active.

"This is cooling on demand," Venkatasubramanian said.

The closest analogy is to the ice packs that marathon runners wrap around their necks during races, he said. "Because of the large amount of blood supplied to your head and neck area, you can bring your core body temperature down quickly and continue racing."

Cooler CPUs on the horizon

Funded via DARPA grants, the technology is being brought to market by an RTI spin-off, Nextreme Thermal Solutions Inc., where Venkatasubramanian served as CTO for two years.

It might already be used by Intel or AMD if they hadn't gone the multicore route, he said. Venkatasubramanian predicts that within three to four years, chip makers won't be able to continue boosting CPU performance without seeking alternative solutions such as with micro-refrigeration.

Graphic chip makers have also "shown interest in our hot-spot cooling technology as well, as they do get hot in some selective areas when they are running at blistering speed," Venkatasubramanian said.

For all their promise, he said, thermoelectric solutions like this aren't intended to replace heat sinks or fans, but complement them.

As the micro-refrigerator draws heat, it also generates electricity. But Venkatasubramanian said the technology would not work like a hybrid car, such as Toyota's Prius. The devices are hampered because with the more heat they draw, the less electricity they are able to generate. "You can't get something from nothing. You can't violate the second law of thermodynamics," he said.

Copyright © 2009 IDG Communications, Inc.

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