Monday, January 26, 2009

Cooling Chips with Thermoelectrics

Researchers have made ultrathin refrigerators for microprocessors.

By Kate Greene

Chip chilling: The thermoelectric cooler shown above (center gold square) is attached to a copper plate that is used to spread heat away from hot spots on chips. Credit: Intel

If you could remove the layers of circuitry in your computer and touch the main processor while it's running a video, you would feel its blistering heat, which can exceed 100 °C. Such heat, a natural by-product of shuttling electrons through transistors, can impede performance and even damage the processor in the long run. Traditionally, engineers have used simple copper plates to pull away the heat, and fans or liquid-based cooling systems. But these systems are bulky and can sap energy.

Now researchers at Intel, RTI International of North Carolina, and Arizona State University have shown that it's possible to build an efficient microrefrigerator that can target hot spots on chips, saving power and space, and more effectively cooling the entire system. Their work also demonstrates, for the first time, that it is possible to integrate thermoelectric material into chip packaging, making the technology more practical than ever before. A paper detailing the research was just published in Nature Nanotechnology.

The fundamental technology used to chill the chip, a thermoelectric cooler, isn't new, explains Rama Venkatasubramanian, senior research director at the Center for Solid State Energetics at RTI International. In a Nature paper from 2001, he and his team showed that a material called a nanostructured thin-film superlattice has superior thermal properties to other types of thin thermoelectric materials: the superlattice conducts electricity well but impedes the flow of heat. When an electric current zips through the material, its temperature can drop to about 55 °C.

"People have been talking about using high-efficiency thermoelectric materials for cooling hot spots on chips for years," says Intel manager Ravi Prasher. He says that part of the reason he and his colleagues were able to succeed is because they used a material that has shown exceptional thermal properties, and they relied on Intel's knowledge of chip packaging to build an integrated thermoelectric system that was engineered to fit within the confines of a chip's housing.

To put the microrefrigerator in the chip package, the engineers integrated the cooler onto a square of copper, just like the type that's already used in chip packaging to disperse heat. Usually this piece of copper is in close contact with the chip, but the researchers put the 0.4-millimeter-square cooler in between the chip and the copper. When the microrefrigerator was turned on, it cooled a localized region on the chip by about 15 °C. This is significant, says Venkatasubramanian, because generally speaking, for each five-degree increase in chip temperature, there is a marked decrease in reliability and performance of a chip. In the demonstration, the researchers only used one microrefrigerating unit but foresee using three or four per chip, to cover the hottest areas.