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鈥攔emoving 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鈥攅ach less than half a millimetre
long鈥攚as made by etching shapes into thin silicon sheets. 鈥淵ou have to
think in two dimensions when you design it, but know how it will turn out once
it鈥檚 been folded into position,鈥� says Paul Kladitis of the University of
Colorado at Boulder. 鈥淚t鈥檚 like being some kind of pop-up book artist.鈥�
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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. 鈥淭he solder wets the
gold pads and pulls the wing up into a fan blade position,鈥� he says.
The fan is powered by a so-called 鈥渟cratch drive鈥�, which nudges it around. At
the end of thin silicon plates attached to the hub are silicon 鈥渇eet鈥�. 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.
鈥淲e used nine scratch plates in a circle like a merry-go-round,鈥� says
Kladitis. 鈥淲hen we drive them with a voltage alternating at 2 kilohertz, we get
speeds from 50 to 180 revolutions per minute.鈥� 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. 鈥淵ou 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,鈥� he says.
鈥淚鈥檓 impressed they got such an elaborate structure to rotate so well with
integrated motors,鈥� 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 鈥渋ngenious鈥�, but says the speeds achieved are 鈥渞ather slow鈥�.
鈥淲e鈥檙e striving for in excess of 1 million rpm in our motor,鈥� he says. Spearing
has other concerns too. 鈥淚 am not a big fan of frictional or sliding contacts in
micro electro-mechanical devices,鈥� he says. 鈥淔riction and wear tend to be
potential show-stoppers at these scales.鈥�