MIT materials scientists
have developed a new technique for growing carbon nanotubes that could replace
the vertical wires in chips, permitting denser packing of circuits.
Source: "Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes
with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition
of the Carbon Feedstock," Gilbert Nessim, Carl V. Thompson et al, Nano
Letters, Aug. 31, 2009
Results: Researchers in the lab of MIT materials science professor Carl V.
Thompson grew dense forests of crystalline carbon nanotubes on a metal surface
at temperatures close to those characteristic of computer chip manufacturing.
Unlike previous attempts to do the same thing, the researchers' technique relies
entirely on processes already common in the semiconductor industry. The researchers
also showed that the crucial step in their procedure was to preheat the hydrocarbon
gas from which the nanotubes form, before exposing the metal surface to it.
Why it matters: The transistors in computer chips are traditionally connected
by tiny copper wires. But as chip circuitry shrinks and the wires become thinner,
their conductivity suffers and they become more likely to fail. A simple enough
manufacturing process could enable carbon nanotubes to replace the vertical
wires in chips, permitting denser packing of circuits.
How they did it: In a vacuum chamber, the researchers vaporized the metals
tantalum and iron, which settled in layers on a silicon wafer. Then they placed
the coated wafer at one end of a quartz tube, which was inserted into a furnace.
At the wafer's end of the tube, the furnace temperature was 475 degrees C; but
at the opposite end, the temperature varied. The researchers pumped ethylene
gas into the tube from the end opposite the wafer. When the temperature at that
end approached 800 degrees, the ethylene decomposed, and the iron on the wafer
catalyzed the formation of carbon nanotubes.
Next steps: The researchers are trying to determine whether different combinations
of metals and hydrocarbon gases can lower the catalytic temperature even further
and improve the quality of the nanotubes.
Funding: The research was sponsored by the MARCO Interconnect Focus Center
and partially by Intel (Gilbert Nessim, who was a graduate student in Thompson's
lab, was supported by an Intel Fellowship).
Posted September 10th, 2009
