Chips best served cool

Engineers at Purdue University have developed a tiny "micro-pump" cooling device small enough to fit on a computer chip that circulates coolant through channels etched into the chip.

The microelectromechanical (MEMS) device has been integrated onto a silicon chip that is about 1cm².

Current computer chips are cooled primarily with an assembly containing conventional fans and heat sinks. But because chips a decade from now will likely contain upwards of 100 times more transistors and other devices, they will generate far more heat than chips currently in use, Garimella said.

"Our goal is to develop advanced cooling systems that are self-contained on chips and are capable of handling the more extreme heating in future chips," said Suresh Garimella, director of Purdue's Cooling Technologies Research Center. The centre was formed to help corporations develop miniature cooling technologies for a wide range of applications, from electronics and computers to telecommunications and advanced aircraft.

The prototype chip contains numerous water-filled micro-channels about 100 microns wide. The channels are covered with a series of hundreds of electrodes which receive varying voltage pulses in such a way that a travelling electric field is created in each channel. The travelling field creates ions, which are dragged along by the moving field.

"Say every sixth electrode receives the same voltage, these varying voltages from one electrode to the next produce a travelling electrical field that pulls the ions forward, causing the water to flow and inducing a cooling action," Garimella said. "Essentially, you are pumping fluid forward."

This pumping action is created by electrohydrodynamics, which uses the interactions of ions and electric fields to cause fluid to flow.

"Engineers have been using electrohydrodynamics to move fluids with electric fields for a long time, but it's unusual to be able to do this on the micro-scale as we have demonstrated," Garimella said.

The researchers also have added a feature to boost the force of the pumping action. A thin sheet of piezoelectric material, which expands and contracts in response to an electric current, was glued on top of the cover of the liquid-filled channels.

"This material acts as a diaphragm that deforms up and down when you give it a voltage, causing it to push additional flow through the channels," Garimella said. "We have developed mathematical models that show this piezo action enhances the electrohydrodynamic performance."

The diaphragm has enhanced the pumping action by 13 percent in the current prototype, but the modelling indicates a possible enhancement of 100 percent or greater, he said.

"Although electrohydrodynamics has generally not been considered practical for pumping applications due to the assumption that it requires a large amount of energy and does not produce enough motive force for thrust, the method has been shown to be far more efficient for micro-cooling applications," Garimella said. "We have shown that the power input required is in the microwatts, but you can get milliwatts of cooling. In other words, the cooling effect is more than a thousand times greater than the energy needed to drive the system. That's because all we need to do is create enough of a flow to induce cooling."

However, several major challenges remain.

"One big challenge is further developing mathematical models that are comprehensive and accurate because this is a very complicated, dynamic system," Garimella said. "You've got fluid flow on a micro-scale, you've got electrohydrodynamic effects, electrical fields and a moving diaphragm."

Other challenges include sealing the tiny channels to prevent water leakage and designing the system so that it could be manufactured under the same conditions as semiconductor chips.