at Rice University have brought graphite’s potential as a mass data storage
medium a step closer to reality.
Figure 1. The researchers have used industry-standard
lithographic techniques to deposit 10-nanometer stripes of amorphous graphite
Advances by the Rice University lab of James
Tour have brought graphite’s potential as a mass data storage medium a step
closer to reality and created the potential for reprogrammable gate arrays that
could bring about a revolution in integrated circuit logic design. In a paper
published in the online journal ACS Nano
, Tour and postdoctoral associate Alexander
Sinitskii show how they’ve used industry-standard lithographic techniques to
deposit 10-nanometer stripes of amorphous graphite, the carbon-based
semiconducting material commonly found in pencils, onto silicon (see Figure 1).
This facilitates the creation of potentially very dense, very stable
nonvolatile memory for all kinds of digital devices.
With backing from a major manufacturer of memory chips, Tour and his team have
pushed the technology forward in several ways since a paper that appeared last
November first described two-terminal graphitic memory. While noting advances
in other molecular computing techniques that involve nanotubes or quantum dots,
he said none of those have yet proved practical in terms of fabrication.
Not so with this simple-to-deposit graphite. “We’re using chemical vapor
deposition and lithography-techniques the industry understands,” said Tour,
Rice’s Chao Professor of Chemistry and a professor of mechanical engineering
and materials science and of computer science. “That makes this a good
alternative to our previous carbon-coated nanocable devices, which perform well
but are very difficult to manufacture.”
Graphite makes a good, reliable memory “bit”
for reasons that aren’t yet fully understood. The lab found that running a
current through a 10-atom-thick layer of graphite creates a complete break in
the circuit-literally, a gap in the strip a couple of nanometers wide. Another
jolt repairs the break. The process appears to be infinitely repeatable, which
provides addressable ones and zeroes, just like today’s flash memory devices
but at a much denser scale.
Graphite’s other advantages were detailed in Tour’s earlier work: the ability
to operate with as little as three volts; an astoundingly high on/off ratio
(the amount of juice a circuit holds when it’s on, as opposed to off); and the
need for only two terminals instead of three, which eliminates a lot of
circuitry. Graphite is also impervious to a wide temperature range and
radiation, which makes it suitable for deployment in space and for military
uses where exposure to temperature extremes and radiation is a concern.
2. When current is applied to a graphite-filled via, the graphite alternately
splits and repairs itself.
Tour’s graphite-forming technique is
well-suited for other applications in the semiconductor industry. One result of
the previous paper is a partnership between the Tour group and NuPGA (for “new
programmable gate arrays”), a California
company formed around the research to create a new breed of reprogrammable gate
arrays that could make the design of all kinds of computer chips easier and
The Tour lab and NuPGA, led by industry veteran Zvi Or-Bach (founder of
eASIC and Chip Express), have applied for a patent based on vertical arrays of
graphite embedded in vias, the holes in integrated circuits connecting the
different layers of circuitry (see Figure 2). When current is applied to a
graphite-filled via, the graphite alternately splits and repairs itself (a
process also described in the latest paper), just like it does in strip form.
Essentially, it becomes an antifuse, the basic element of one type of field
programmable gate array (FPGA), best described as a blank computer chip that
uses software to rewire the hardware.
Currently, antifuse FPGAs can be programmed once, but this graphite
approach could allow for the creation of FPGAs that can be reprogrammed at
will. Or-Bach said graphite-based FPGAs would start out as blanks, with the
graphite elements split. Programmers could “heal” the antifuses at will by
applying a voltage, and split them with an even higher voltage.
Such a device would be useful to computer chip designers, who now spend
many millions to create the photolithography mask sets used in chip
fabrication. If the design fails, it’s back to square one. “As a result of
that, people are only hesitantly investing in new chip designs,” said Tour.
“They stick with the old chip designs and make modifications. FPGAs are chips
that have no specific ability, but you use software to program them by
interconnecting the circuitry in different ways.” That way, he said,
fabricators don’t need expensive mask sets to try new designs.
“The number-one problem in the industry, and one that gives an opportunity for
a company like ours, is that the cost of masks keeps moving up as people push
semiconductors into future generators,” said Or-Bach. “Over the last 10 years,
the cost of a mask set has multiplied almost 10 times. If we can really make
something that will be an order of magnitude better, the markets will be happy
to make use of it. That’s our challenge, and I believe the technology makes it
possible for us to do that.”
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