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Cooling Hot Chips

Chip substrate

No, the image above is not an architect's model for a new sports complex. It's an electron micrograph of a new refrigeration system for computer chips created by engineers at Sandia National Laboratories. The complex structure is a "package" or substrate for semiconductors that may help manufacturers jump an impending hurdle to the next generations of computers and electronic devices--heat. Microscopic radial pipes, filled with an evaporative coolant, conduct heat away from a computer chip mounted at the center.

David Benson
BIG CHILL. Sandia's David Benson observes the heat being drawn from a chip with an infrared thermal imaging microscope.

As chipmakers pack more and more computing power onto tiny slivers of silicon, they are limited not only by the amount of real estate available but by the heat created by electrical resistance in all those circuits. Soon dime-sized chips may be crammed with as many as 10 million transistors; unless they can be kept cool, they will simply self-destruct. Today's solution--using copper interconnects to carry the heat away--will not be able to cope with the dense semiconductors coming down the pipeline. Alternative materials, such as highly conductive diamond, are too costly. "Heat buildup is becoming one of the major limitations to creating tomorrow's devices," says David Benson of Sandia's Advanced Packaging Department.

Benson and his colleagues turned to the same photolithography technique used to print circuits on chips to create a packaging material for chips that contains thousands of microscopic channels filled with a circulating coolant. A repetitive photolithography-electroplating process creates five- to 50-micron-wide ridges on the surfaces of two metal alloy plates that are mirror images of one another. When the plates are put together, a metal sheet, measuring five one-hundredths of an inch thick and containing an interconnected network of micron-scale passageways, is created.

Photomicrograph
MICRO MAZE. Closeup image shows large ridges (10 to 20 microns wide) and smaller ridges (five microns wide). When mirror-image plates are joined, the spaces between the ridges form channels through which coolant can flow.

A small amount of a cooling fluid--barely a few drops--is injected between the plates; then the edges are sealed. The coolant directly underneath the microchip heats up and evaporates. When the vapor reaches cooler areas, it condenses. Capillary pressure then draws the liquid coolant back to hotter regions where the process repeats. The evaporation and condensation cycle distributes the heat evenly though the substrate.

Sandia says it has created a quarter of a million pipe structures within a square inch of the substrate. Micropipe and coolant geometries can be tailored to the design and operating temperature of specific devices. A coolant and micropipe geometry can be selected that best transfers heat given each device's design and operating temperature range.

According to Sandia, the new substrate could be produced in commercial volumes for about 50 cents per square inch, compared to more than $1 per square inch for present high-performance packaging materials. The lab has applied for a patent and is negotiating licenses to the technology.


Image: RANDY MONTOYA, Sandia National Laboratories
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