SAN MATEO, Calif. " Borrowing techniques used to make semiconductors and pharmaceuticals, startup Cooligy Inc. says it has developed a way to cool a CPU by etching hundreds of channels on a piece of silicon that sits on top and circulating water through them. The water absorbs heat and transfers it out to a radiator and fan, where it is cooled before being pumped back to the CPU.
Cooligy (Mountain View, Calif.) had two principles in mind when it designed the microchannels. One was to make the total area of the channels large in order to maximize the amount of heat they can take in. If the channels could be unfolded, their area would be about 20 times larger than their side-to-side physical dimensions, according to the company.
Second, Cooligy wanted to keep the channels close to the surface of the CPU, at a distance of about 1 mm, to rapidly transfer the heat. Water was chosen as the cooling medium because of its high boiling point, although it must stay thin and move quickly so it can be replaced by cooler fluid. The company claims it can handle 1,000 watts per square centimeter; today's best passive cooling systems are specified for 250 W/cm2, said vice president of marketing Andy Keane.
Cooligy has been talking to some of the best-known CPU vendors about ways to remove heat from their chips, Keane said, but "nobody wants to talk about it because it speaks to speed and reliability." The only data he's been able to disclose publicly is a seven-year-old chart showing the heat distribution of a PA-RISC 8700 from Hewlett-Packard Co. The 3-D graph shows heat rising like steam from a tea kettle on the corner of the die where the chip's floating-point and integer units are congregated.
It's these hot spots, not just the overall thermal budget, that pose one of the biggest challenges to CPU vendors and system designers. But today's heat sinks and passive fluid heat pipes aren't up to the task, Keane said. And it's only going to get worse as ever-finer process technologies make the hot spots smaller and more numerous.
"It's a heat density problem arising from those parts of the chip that run the fastest," Keane said. "At 90 and 65 nanometers, the hot spots get too small for passive cooling technologies." The fastest chips can go as high as 75 degrees C to 95 degrees C.
The Cooligy system uses an electrokinetic pump to circulate water from the chip to the radiator. Water flows through a porous glass disk that is negatively charged. When a positive electric field is applied, hydrogen ions push water through the openings. It's a well-understood effect, and is commonly used as a filter in the biological sciences, Keane said.
Founded last year by three Stanford University mechanical-engineering professors, Cooligy said it has already developed a prototype system with the help of companies like Intel, Apple Computer and Advanced Micro Devices. Cooligy expects to field the first samples of its Active Micro-Channel Cooling devices next quarter to makers of high-end systems like workstations and 1U servers. Keane said they will be priced in the $25 to low-$30s range.