Graphene just
keeps on giving. Sooner or later it will
be integrated into all our electronics and yes it will be an almost complete
reinvention of what we do. Yet it will
take us the rest of the way to the holodec.
What I do find
interesting is the proclivity to alter adjacent copper topology and this has to
be important. Sooner or later we are
wanting topologically shaped layers of metallic that are micron thick at most. This is our first hint of possibility.
Pulling all this
together in a manufacturing program continues to be daunting.
Graphene Helps
Copper Wires Keep Their Cool
An exotic form of carbon could help relieve a
growing problem with the copper used in computer processors.
When people in the chip industry talk about the
thermal problems in computer processors, they get dramatic. In 2001, Pat
Gelsinger, then vice president of Intel, noted that if the temperatures
produced by the latest chips kept rising on their current path, they would
exceed the heat of a nuclear reactor by 2005, and the surface of the sun by
2015. Fortunately, such thermal disaster was averted by slowing down the
switching speeds in microprocessors, and by adopting multicore chip designs in
which several processors run in parallel.
Now the semiconductor industry has another thermal
problem to sort out. As chip components shrink, the copper wiring that connects
them must shrink, too. And as these wires get thinner, they heat up
tremendously.
A potential solution to this interconnect fever has
been found in the form of graphene, an exotic material made from
single-atom-thick sheets of carbon that is a superlative conductor of both
electrons and heat.
Materials scientists already use copper as a
catalyst to grow graphene for other uses. So Alexander
Balandin of the
University of California, Riverside, and Kostya Novoselov, a physicist at University of Manchester,
U.K., who won the 2010 Nobel Prize in Physics for his foundational work with graphene (see “Graphene Wins Nobel Prize”), decided to leave the graphene on the copper
to see how it affected the metal’s thermal properties. In a paper published
in the journal Nano Letters, they report that a sandwich made of
graphene on both sides of a sheet of copper improves the copper’s ability to
dissipate heat by 25 percent—a significant figure for chip designers.
Balandin says that the graphene itself doesn’t seem
to conduct the heat away. Rather, it alters the structure of the copper,
improving the metal’s conductive properties. Heat moving through copper is
usually slowed by the crystalline structure of the metal. Graphene changes
this structure, causing those walls to move farther apart, and allowing heat to
flow more readily, says Balandin.
Studies were done with relatively thick sheets of
copper—much larger than the copper wires found in computer chips—but Balandin
expects that the heat-conducting effect will be seen in thinner copper wires,
too. He’s now working on copper-graphene wires as small as those used in
commercial computer chips.
The problem is an urgent one. This year, Intel is
expected to announce products containing 14-nanometer transistors, with copper
interconnects about on this scale or even smaller. Copper wires will not work
below 10 nanometers, and it’s not clear what will. “We haven’t yet found an
interconnect material that can work beyond 10 nanometers,” partly due to
overheating, says Saroj Nayak, a physicist at the Center for Integrated
Electronics at the Rensselaer Polytechnic Institute in Troy, New York.
Majeed Foad, an electrical engineer at Applied
Materials, a
semiconductor-equipment maker headquartered in Santa Clara, California, who
helps the company track research on new materials, says graphene’s properties
are exciting, but adds that as chip components are miniaturized, they become
more sensitive to high temperatures. It takes a lot of heat to make good
quality graphene—Balandin and Novoselov heated their wires to over 1,000 °C.
Foad says such temperatures would degrade transistors and other chip
components. Balandin, however, points to lab experiments that demonstrate
that graphene can be grown at lower temperatures, at least in the research
setting.
Regardless, Foad says, chip makers won’t be in any
rush to embrace graphene. “Changing materials is very painful, so we will
squeeze every last drop of performance out of what we have,” he says.
It’s clear that simply cramming more transistors
into processors and putting more processors in chips is not going to be tenable
much longer. High-end chips already contain about 50 to 60 kilometers of copper
wiring and multiple cores.
Jonathan Candelaria, director of interconnect research at the
Semiconductor Research Corporation, an industry consortium in Durham, North
Carolina, says that adding more transistors doesn’t improve performance the way
it used to. The solution may again turn out to be adopting fundamentally
different architectures. New ways of designing and packaging chips could help
solve the heat problem, says Candelaria, and this will give the industry time
to work out problems with new materials, perhaps including the new
graphene-copper hybrids.
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