|
Nanotube
Forests Grown On Silicon Chips For Future Computers,
Electronics
Other Topics:
Nanocrystals
Purdue University
October 2, 2007
West Lafayette, IN -- Engineers have shown how to grow forests
of tiny cylinders called carbon nanotubes onto the surfaces of
computer chips to enhance the flow of heat at a critical point
where the chips connect to cooling devices called heat sinks.
The carpetlike growth of nanotubes has been shown to
outperform conventional "thermal interface materials." Like
those materials, the nanotube layer does not require elaborate
clean-room environments, representing a possible low-cost
manufacturing approach to keep future chips from overheating
and reduce the size of cooling systems, said Placidus B. Amama,
a postdoctoral research associate at the Birck Nanotechnology
Center in Purdue's Discovery Park. |
|
Researchers are trying to develop new types of thermal
interface materials that conduct heat more efficiently than
conventional materials, improving overall performance and
helping to meet cooling needs of future chips that will
produce more heat than current microprocessors. The materials,
which are sandwiched between silicon chips and the metal heat
sinks, fill gaps and irregularities between the chip and metal
surfaces to enhance heat flow between the two.
The method developed by the Purdue researchers enables them to
create a nanotube interface that conforms to a heat sink's
uneven surface, conducting heat with less resistance than
comparable interface materials currently in use by industry,
said doctoral student Baratunde A. Cola.
Findings were detailed in a research paper that appeared in
September's issue of the journal Nanotechnology. The paper was
written by Amama; Cola; Timothy D. Sands, director of the
Birck Nanotechnology Center and the Basil S. Turner Professor
of Materials Engineering and Electrical and Computer
Engineering; and Xianfan Xu and Timothy S. Fisher, both
professors of mechanical engineering.
Better thermal interface materials are needed either to test
computer chips in manufacturing or to keep chips cooler during
operation in commercial products.
"In a personal computer, laptop and portable electronics, the
better your thermal interface material, the smaller the heat
sink and overall chip-cooling systems have to be," Cola said.
Heat sinks are structures that usually contain an array of
fins to increase surface contact with the air and improve heat
dissipation, and a fan often also is used to blow air over the
devices to cool chips.
Conventional thermal interface materials include greases,
waxes and a foil made of a metal called indium. All of these
materials, however, have drawbacks. The greases don't last
many cycles of repeatedly testing chips on the assembly line.
The indium foil doesn't make good enough contact for optimum
heat transfer, Fisher said.
The Purdue researchers created templates from branching
molecules called dendrimers, forming these templates on a
silicon surface. Then, metal catalyst particles that are
needed to grow the nanotubes were deposited inside cavities
between the dendrimer branches. Heat was then applied to the
silicon chip, burning away the polymer and leaving behind only
the metal catalyst particles.
The engineers then placed the catalyst particle-laden silicon
inside a chamber and exposed it to methane gas. Microwave
energy was applied to break down the methane, which contains
carbon. The catalyst particles prompted the nanotubes to
assemble from carbon originating in the methane, and the tubes
then grew vertically from the surface of the silicon chip.
"The dendrimer is a vehicle to deliver the cargo of catalyst
particles, making it possible for us to seed the carbon
nanotube growth right on the substrate," Amama said. "We are
able to control the particle size - what ultimately determines
the diameters of the tubes - and we also have control over the
density, or the thickness of this forest of nanotubes. The
density, quality and diameter are key parameters in
controlling the heat-transfer properties."
The catalyst particles are made of "transition metals," such
as iron, cobalt, nickel or palladium. Because the catalyst
particles are about 10 nanometers in diameter, they allow the
formation of tubes of similar diameter.
The branching dendrites are tipped with molecules called
amines, which act as handles to stick to the silicon surface.
"This is important because for heat-transfer applications, you
want the nanotubes to be well-anchored," Amama said.
Researchers usually produce carbon nanotubes separately and
then attach them to the silicon chips or mix them with a
polymer and then apply them as a paste.
"Our direct growth approach, however, addresses the critical
heat-flow path, which is between the chip surface and the
nanotubes themselves," Fisher said. "Without this direct
connection, the thermal performance suffers greatly."
Because the dendrimers have a uniform composition and
structure, the researchers were able to control the
distribution and density of catalyst particles.
The research team also has been able to control the number of
"defect sites" in the lattice of carbon atoms making up the
tubes, creating tubes that are more flexible. This increased
flexibility causes the nanotube forests to conform to the
surface of the heat sink, making for better contact and
improved heat conduction.
"The tubes bend like toothbrush bristles, and they stick into
the gaps and make a lot of real contact," Cola said.
The carbon nanotubes were grown using a technique called
microwave plasma chemical vapor deposition, a relatively
inexpensive method for manufacturing a thermal-interface
material made of carbon nanotubes, Fisher said.
"The plasma deposition approach allows us great flexibility in
controlling the growth environment and has enabled us to grow
carbon nanotube arrays over a broad range of substrate
temperatures," Fisher said.
The research has been funded by NASA through the Institute for
Nanoelectronics and Computing, based at Purdue's Discovery
Park. Cola also received support through a fellowship from
Intel Corp. and Purdue. |
