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Cooling Chips with an Ion Breeze

By using an electric charge to put molecules in motion, a new device can make a computer’s fans more efficient.
August 22, 2007

Electrodes that send a flow of ionized air over the surface of a silicon chip could make the cooling fans in computers and laptops much more effective. Researchers at Purdue University and Intel found that a device that generates an ionic breeze keeps computer chips 25 ºC cooler than fans alone. By enabling the use of smaller fans, the device could lead to more-compact laptops.

Cool breeze: Infrared images of a microchip’s surface show one of the two electrodes that generate a cooling breeze of ionized air in a new device developed at Purdue University. In the top image (red), fans keep the chip’s surface temperature at 60 ºC. In the bottom image (blue), activating the electrodes brings the temperature down to 35 ºC.

As microchips get crowded with more and more components, today’s cooling methods will no longer be adequate. Currently, heat is drawn away from chips by metal heat sinks–panels attached to arrays of fins or prongs that maximize heat-dissipating surface area. The fans in a computer cool the heat sinks and blow out the hot air. But air cooling “has been stretched to the limit in its capacity for heat removal,” says Suresh Garimella, a mechanical-engineering professor at Purdue. And besides, fans can be bulky and noisy.

The new device is small and can be integrated directly into a computer chip. By placing it at specific “hot spots” on a chip, engineers could enhance the cooling fan’s effectiveness in those areas. This could lead to smaller fans that work just as well as current fans, says Garimella, and thus to thinner, smaller laptops. The eventual goal is to develop cooling technologies for small notebooks and handheld computers, says Rajiv Mongia, an Intel research engineer who worked with the Purdue researchers on the new device.

Garimella and his colleagues built their experimental cooling system on a mock computer chip. The system consists of two electrodes–a stainless-steel wire that acts as the positively charged anode, and a copper tape that serves as the cathode–that are separated by a few millimeters.

Applying a voltage across the electrodes makes electrons in the air collide with oxygen and nitrogen molecules, stripping them of electrons and creating positively charged ions. The ions move toward the negatively charged cathode, dragging surrounding air molecules with them and creating a breeze. The researchers found that while a fan blowing over a heat sink cooled the surface of their chip to about 60 ºC, adding the ion breeze cooled it down to 35 ºC.

Garimella says that because the device uses strips of metal as electrodes, as opposed to sharper tips, the ion breeze sweeps a larger portion of the chip–although it does not generate enough air pressure to cool the chip without the aid of a fan.

The ion breeze faces stiff competition from other experimental chip-cooling techniques. Computer makers have recently started to explore liquid cooling, in which a pump pushes water or another liquid through pipes. (Apple’s Mac Pro computers use this system.) But most liquid-cooling systems are complicated and increase manufacturing costs; the Purdue device could provide a cheaper alternative. “Our invention allows us to extend the performance of air cooling without having to switch to more aggressive and expensive methods such as liquid cooling,” Garimella says. “At the same time, we do not add any extra volume.”

A lower-volume approach to liquid cooling, however, may be forthcoming from Cooligy, based in Mountain View, CA. The company is developing a microchannel-based cooling technology licensed from Stanford University. The technology is a smaller, on-chip version of the pump-and-pipe method of circulating liquids. In Cooligy’s device, cooling liquid circulates through tiny channels carved into a silicon layer that sits on top of a computer chip.

Girish Upadhya, director of applications engineering at Cooligy, has cautious praise for ion-breeze cooling, which he calls “a unique approach which may have specific applications in spot cooling.” But he suspects that the Purdue device could prove difficult to incorporate into computer chips. “The hard part is to come up with a specific product using such an approach,” Upadhya says.

Intel, which collaborated with the Purdue researchers, is keeping its options open. The company has also worked on a similar ion-pump approach with researchers at the University of Washington, in Seattle. (See “Tiny Pump Cools Chips.”)

But Garimella is confident that the Purdue device will yield practical applications. First, however, the researchers will have to make it smaller and more rugged. “The device is at the millimeter scale, and we are working on reducing it to the scale of tens of micrometers,” Garimella says. A smaller device, he says, can achieve the same cooling effect with lower voltages. And that, he adds, “would make the technology commercially viable.”

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