The latest generation of gigahertz-clock-rate RISC and CISC CPUs is becoming more challenging to fit into designs. These chips are squeezing into tighter and tighter spaces with no place for heat to escape. Total power-dissipation levels now reside at about 110 W, and peak power densities are reaching 400 to 500 W/mm2 and are constantly climbing. As a result, higher performance and greater reliability levels are extremely tough to attain.
Until now, the solution was to use active heatsinks and passive fluid heat pipes. These types of structures are costly and bulky, and they take up too much real estate on the pc board. The heatsink can be some 3000 times larger than the CPU it's cooling. A standard 120-W Intel Pentium 4 microprocessor generally uses a heatsink that dwarfs the CPU hotspot that it's trying to cool.
Microelectromechanical-systems (MEMS) heat exchangers have found some success here, but the key missing element has been a practical means of removing the heat. The Pentium 4 currently consumes 82 W. Meanwhile, the Prescott is projected to consume 100 to 110 W when it's launched. And, the Itanium will consume about 130 W. These figures will climb with time.
Cooligy Inc. believes it has found the answer thanks to a deep knowledge of cooling microstructures and a novel electrokinetic pump that makes the solution practical and inexpensive (Fig. 1). The company is now ready to introduce to the market the first commercially available self-contained heat-exchange system based on a MEMS heat exchanger and an electrokinetic pump. Andy Keane, Cooligy's vice president of marketing, sees this device as an opportunity for his company to introduce a "disruptive" technology, much like MEMS itself was considered back in the 1980s when it was first commercialized.
This microfluidic system will initially target high-performance CPUs that are used in very restricted spaces in workstations, 1U servers, and small form-factor PCs. Later versions are being planned for other types of ICs, including graphics processors, FPGAs, DSPs, and other dense ICs.
BACK TO BASICS
As professors at Stanford University's Mechanical Engineering Department, Cooligy founders Ken Goodson, Tom Kenny, and Juan Santiago work extensively on MEMS heat exchangers and have become quite familiar with the problems of cooling hot structures and removing the heat. They figure that the best way to remove heat from a CPU is to minimize the distance heat must travel from a CPU's hotspot to the heat collector before it's carried away. In their new microchannel structure, that distance is a mere 1.5 mm, where heat travels to the microchannel that contains the pumped fluid (Fig. 2).
Made by reactive ion etching, the MEMS structure is so efficient that if it were unfolded and laid out flat, its cooling area would be 20 times larger than the actual structure and the die it cools. Its active cooling area is 10 to 20 times the area of the die, yet it fits directly over the die with very little extra space used. Other standout dimensions include a 3:1 aspect ratio (channel height to width) and channel widths of about 50 to 150 µm (about the width of a hair).
Simply having an efficient heat exchanger is only half the story, though. A practical means must carry that heat away. Goodson, Kenny, and Santiago hit upon a simple idea: an electrokinetic pump that can be made from conventional components—and thus at low cost in high volumes. Using a pump the trio developed at Stanford University as a model, they set about to optimize it for CPU heat removal and convection.