THE HUM of a computer fan could soon be history. In future, the microchips
inside a PC could be cooled by armies of microscopic fans actually grown on the
surface of the chips—removing the need for a large, noisy cooling
system.
The inventors of the microfan, which is small enough to sit on the head of a
pin, speculate that it could also be used to propel tiny flying machines or pump
chemicals around lab-on-a-chip devices for analysis.
The fan, which has eight blades—each less than half a millimetre
long—was made by etching shapes into thin silicon sheets. “You have to
think in two dimensions when you design it, but know how it will turn out once
it’s been folded into position,” says Paul Kladitis of the University of
Colorado at Boulder. “It’s like being some kind of pop-up book artist.”
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Each fan blade is connected to a central hub by a hinge. To pull the flat
blades up into position, Kladitis deposited gold pads on either side of the
hinge. He then dropped a small blob of solder onto the gold pads. Surface
tension between the solder and gold pads raised the blade. “The solder wets the
gold pads and pulls the wing up into a fan blade position,” he says.
The fan is powered by a so-called “scratch drive”, which nudges it around. At
the end of thin silicon plates attached to the hub are silicon “feet”. These
rest on an insulating layer of silicon nitride that coats a silicon substrate
beneath the fan (see Diagram).
To drive the fan, the scratch plate and the silicon base are connected to an
alternating power supply. The difference in electrical potential between the
scratch plate and the base produces cycling electrostatic forces that rapidly
pull the scratch plate down onto the insulator coating and up again. Each time
the scratch plate bows downward, the foot pushes against the insulator and
nudges the fan around.
“We used nine scratch plates in a circle like a merry-go-round,” says
Kladitis. “When we drive them with a voltage alternating at 2 kilohertz, we get
speeds from 50 to 180 revolutions per minute.” At 3 kilohertz, an
electrostatically induced wobble in the scratch plate makes the foot push in the
opposite direction, driving the fan into reverse at 100 rpm.
Kladitis says the fan could be used to pump chemicals around microchip-based
chemistry labs. “You could also put this fan right next to an electronic
component in a computer to cool it, or even use it as some kind of micro-vehicle
propulsion system,” he says.
“I’m impressed they got such an elaborate structure to rotate so well with
integrated motors,” says Kris Pister, who is working on insect-like
micromachines at the University of California at Berkeley. Mark Spearing,
currently testing microturbines at the Massachusetts Institute of Technology,
calls the fan “ingenious”, but says the speeds achieved are “rather slow”.
“We’re striving for in excess of 1 million rpm in our motor,” he says. Spearing
has other concerns too. “I am not a big fan of frictional or sliding contacts in
micro electro-mechanical devices,” he says. “Friction and wear tend to be
potential show-stoppers at these scales.”