APPLICATIONS for buckyballs have been thin on the ground since the
football-shaped, all-carbon atoms were discovered in 1985. Now, two
researchers in California have found a new use for them 鈥� as an important raw
material in the manufacture of semiconductors and tiny electromechanical
devices.
Alex Hamza and Mehdi Balooch, two engineers at the Lawrence Livermore
National Laboratory in California, have used buckyballs to produce intricately
shaped areas of silicon carbide film on silicon wafers. Silicon carbide is
very useful, as it can be used where silicon is unsuitable. For example,
electronic devices made from silicon carbide can function at up to 600 掳C,
while those made from silicon fail above 75 掳C. Similarly, mechanical
devices made from silicon carbide can function at higher temperatures than the
600 掳C at which silicon softens, and are harder and stronger. Silicon
carbide is also very resistant to corrosion. But silicon carbide devices are
difficult to fabricate, since the material is difficult to grow and etch into
the patterns.
Buckyballs鈥� almost spherical shape gives them unusual properties. The
best-known version consists of 60 carbon atoms arranged in a football shape.
But so far there has only been one other proposed practical use for
buckyballs: researchers from Argonne National Laboratory in Illinois used them
to grow diamond films (Technology, 30 July).
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The new technique relies on the interaction between buckyballs and silicon.
If a stream of buckyballs is fired at a silicon wafer heated to 930 掳C,
the carbon bonds break, so that the molecules 鈥渙pen up like tulips鈥�. These are
then highly reactive, and combine with the raw silicon to cover the surface in
patterns of silicon carbide.
More importantly, the buckyballs do not react with silicon dioxide. This
means that the standard lithographic technique of 鈥渕asking鈥� the circuit
pattern on a wafer of raw silicon can be used to produce a device. After the
bombardment with the carbon molecules, producing the same pattern of silicon
carbide, the oxide can be dissolved with hydrofluoric acid to leave the
silicon carbide microstructure.
The size of the silicon carbide areas is only limited by the lithographical
method itself. 鈥淭he smallest structures we have made are 1 micrometre in
diameter, and range in thickness from a few hundred angstroms to greater than
a micrometre,鈥� says Hamza.
The applications for this technique are not limited to semiconductors. 鈥淥ne
of the most promising is micro-electromechanical devices,鈥� says Hamza. These
could include pressure sensors in car engines to control fuel injection,
flame-out detectors in aircraft engines, accelerometers in airbags or tiny
engine parts built on microchips.
