May 28, 2001
Miniature refrigerators built to help computer chips keep their
cool
Tiny refrigerators may soon be deposited directly onto computer chips to cool
their overheated circuits. A team of researchers from several institutions has found
a way to grow minuscule cooling devices on top of chips, placing them in the exact
spots where they are most needed. Heat generated by a chip's electric currents is
one of the main obstacles to making computer components smaller and speedier.
"As computers get faster, they get hotter, to the point where fans don't work
to cool them anymore," said Ali Shakouri, an assistant professor of electrical
engineering at UCSC and technical director of the team that has built the new coolers.
The temperature of a Pentium chip can soar to nearly 200 degrees Fahrenheit when
it is operating, Shakouri said. Densely packed transistors working at maximum capacity
create "hot spots" on chips that can get 30 to 50 degrees hotter than the
rest of the chip.
"The heat can reduce the lifetime, power, and speed of the chip," Shakouri
said.
To make the new "micro-refrigerator" cooling device, the researchers deposited
200 alternating layers of pure silicon and a silicon-germanium-carbon compound onto
a silicon chip. They used a technique called molecular beam epitaxy, which aims a
stream of molecules toward the surface of the chip, where they attach. The total
thickness of the 200-layer deposit is just one-tenth that of a human hair. The molecular
beam epitaxy technique is so precise, the thickness of the deposited material can
be controlled to within one or two layers of atoms, Shakouri said.
When an electric current is run through wires attached to the micro-refrigerator
on one side and the chip on the other, the deposited material only allows high-energy,
or "hot," electrons to pass through it and complete the circuit. Low-energy,
or "cold," electrons stay at the top surface of the deposit. This heat
imbalance cools down the top surface of the chip.
These "thermoelectric" coolers can be fabricated using the same techniques
used to etch circuits onto chips. This will allow engineers to deposit the tiny devices
onto potential hot spots during chip manufacture, Shakouri said. Since the coolers
can be targeted to the circuits that need them, instead of to the whole chip, the
cooling process should be much more energy-efficient than conventional methods.
The new micro-refrigerators cool computer chips by almost 13 degrees Fahrenheit.
To be used in commercial applications, though, they will have to be able to cool
chips by 30 to 50 degrees. This goal should be attainable, Shakouri said.
"We hope to improve efficiency to the point where these will replace conventional
cooling materials," he said.
If the coolers can be made more efficient, their use may not be limited to the Lilliputian
world of computer chips, he added.
"If we could improve efficiency by a factor of three or four, which in principle
is possible, these devices could be used not just for chips but for refrigerators
at home," Shakouri said. Such refrigerators, which would not need compressors,
would be much quieter than refrigerators in use today.
The thermoelectric cooling machine even has a potential application when it is run
in reverse, Shakouri said. Instead of using an electric current to create a temperature
imbalance in a material, the device could use such an imbalance to generate electric
current. The device could then be used, for example, to extract usable power from
the heat that emerges from a car's engine.
"When you run a car, only one-third of the energy consumed is used to run the
car, and the other two-thirds is wasted as heat," Shakouri said. "If we
could make more efficient thermoelectric materials, we could put them around the
engine and use some of that waste heat."
Shakouri and his coworkers reported their findings in the March 12 issue of Applied
Physics Letters. Shakouri's coauthors are Xiaofeng Fan, Gehong Zeng, Chris LaBounty,
and John E. Bowers at UC Santa Barbara; Edward Croke of HRL Laboratories, in Malibu,
California; Channing C. Ahn of the California Institute of Technology in Pasadena;
and Scott Huxtable and Arun Majumdar at UC Berkeley.
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