IEEE Spectrum August, 2015 - 18

bAmboo-like crystAls
helP tiny chiP Wires
keeP shrinking
segmented structure could help electrons flow
in future chips

when it comes to talk of the
end of Moore's Law, transistors
attract much of the attention. But the
kilo meters of copper wires that connect these devices on each chip have
miniaturization problems of their own.
Now it seems the issue might not be as
bad as engineers once thought. A team
based at IBM and GlobalFoundries
examined wires that will be needed for
chips after the arrival of the 7-nanometer
node, a manufacturing stage expected
in three or so years. They found indications of a crystal structure that might
actually help speed signals and reduce
energy consumption.
The boost comes in resistivity, a measure of how strongly a material opposes
the flow of current. Copper has a very
low intrinsic resistivity. But this "bulk
property" breaks down in small wires.
Electrons bump up against side walls
and scatter off grain boundaries, the
planar surfaces inside the wire where
a copper crystal changes its orientation.
The problem is only expected to get
worse as copper wiring shrinks further.
Adam Pyzyna of IBM's Thomas J. Watson
Research Center, in Yorktown Heights,
N.Y., and his colleagues tested a variety
of wire sizes and shapes. Intriguingly, for
certain geometries, they found it might
18

aUG 2015

nORTh aMERICan

Copper Crystals Contained: The
banded appearance of this cross section of a
copper interconnect reveals a crystal structure
that could carry current better in future chips.

be possible to create wires with crystalline grain boundaries that naturally run
perpendicular to the length of the wire.
Electrons are more likely to pass straight
through such structures, which would
lower the resistivity of the wires.
The effect, which was reported in June
at the Symposia on VLSI Technology and
Circuits, in Kyoto, is modest. The team
reported that wires with this bamboo-like
structure exhibited resistivity about 20 to
30 percent lower than what's been forecast
by the International Technology Roadmap
for Semiconductors, an industry-wide
plan for future chip technology.
"This says it's not any worse than you
think it's going to be and there's potential
to make it better," Pyzyna says. The trick
will be figuring out how to bring the structure into production, he says. A wire's
resistance depends not only on how low
the resistivity of the material is but also on
how large the wire's cross section is. Today
chip wires look like tall rectangles in cross
section; they're shrunk in width so they
can be packed together effectively, while
they're made as tall as possible to maximize the overall area and keep resistance

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low. But Pyzyna and his colleagues saw
the bamboo-like effect only in wires with
squarer cross sections. He says it will take
additional effort to determine whether it's
possible to grow the bamboo structures
taller to keep the resistance down.
If these structures, which were once
common in larger chip wires, can be
remade at smaller scales, it could also
help with another problem facing
interconnect: the diffusion or "electromigration" of copper ions out of the wire.
Current can dislodge these atoms and propel them out of the wire and into material
surrounding it, leaving behind voids that
stop the flow of current. This process generally happens along grain boundaries,
says Suman Datta of Pennsylvania State
University. With a bamboo structure, this
is less likely to occur, because the current
flows perpendicular to the grain boundary and so can't as easily carry atoms to
the perimeter of the wire.
Electromigration is one of the main
reasons that copper wires must still
shrink aggressively. Today, lining materials such as tantalum and tantalum nitride
are used to prevent copper atoms from
migrating out of their channels. The
thickness of these liners can't be reduced,
so it's the copper that must shrink.
But there might be ways to give copper
breathing room. At the same conference,
a team led by H.-S. Philip Wong of Stanford
University discussed how graphene could
be used to line the interconnect instead of
tantalum. The savings for space could be
drastic; a nanometers-thick layer of tantalum nitride could potentially be replaced
with a liner of graphene just 0.3 nanometer
thick, Wong says. That would free up space
for more conductive copper, easing the
miniaturization requirements.
The success of this strategy will as
always come down to manufacturability. "[Graphene] has good barrier properties but it's usually quite defective,"
Datta says. On the other hand, he adds,
"It's always a question, what graphene
is useful for. Maybe this is it."
-r achel courtl a nd

lynne GiGnAC/ibm

nEwS

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Table of Contents for the Digital Edition of IEEE Spectrum August, 2015