IEEE Spectrum February, 2012 - 16

years since the iconic Hp-35, the first pocket scientific calculator,
was introduced. It marked the beginning of the end of the slide rule.

a mixture of block
copolymer, having
two components, is
spread out across a
templated surface.

Directed self-assembly,
which harnesses the ability of
some molecules to rearrange
themselves into ordered,
nanometer-scale structures,
has long been eyed as a way
to make smaller features. But
the process doesn't naturally
lend itself to chip designs,
in which transistors and
contacts are placed at varying
distances. Molecules that
self-assemble tend to form
highly regular and extended
arrays of alternating lines
or tiled configurations
of close-packed circles.
A group led by H.-S.
Philip Wong, an electrical
engineering professor at
Stanford University, has
found a way around this
limitation by using ordinary
lithographic methods to carve
indentations that can act as
guiding templates for smaller
self-assembled features.
The team coated the entire
surface of the silicon with a
copolymer, each molecule of
which consisted of a string of
polystyrene linked to a string
of poly(methyl methacrylate),
or PMMA. When heat was
applied, the PMMA side of
16

each chain was drawn to
other PMMA chains, balling
together to form vertical
cylinders within a matrix of
polystyrene. Dissolving the
PMMA left a polystyrene
surface with holes that could
be filled with conductive
material to form electrodes.
Successive etching steps
left holes only where the
indentations had been.
At the December IEEE
International Electron Devices
Meeting, in Washington, D.C.,
Xin-Yu Bao, formerly a post-
doctoral researcher in Wong's
lab, showed that these small
indentations could be used
to reproduce a design for
a memory circuit, an IBM
22-nm static RAM. Wong
says the demonstration puts
directed self-assembly in
the running to compete with
long-delayed, next-generation
lithography tools, which
use extreme ultraviolet light
(EUV) to create patterns.
Directed self-assembly "can
do what EUV can do, with-
out the [US] $100 million
investment," says Wong.
At the SPIE Advanced
Lithography symposium

Na * Ieee Spectrum * february 2012

Heat causes the
polymer to orient so
that one component
faces inward.

in San Jose, Calif., this
month, Wong's team will
show that their template
technique can pattern holes
for a broad range of logic
circuits, including adders
and flip-flops, using a
standard library of circuit
components. These designs
require simple indentations
capable of holding one or
two holes. But by altering
the shape and size of the
guiding templates, Wong
says he expects to be able to
create close-packed holes
in squares, L's, T's, and
other configurations. He
believes that this alphabet
of shapes will enable
chipmakers to drive holes
even closer together than
can be accomplished with
conventional lithography.
A number of other
research groups are work-
ing on different aspects
of directed self-assembly.
"What Stanford has done is
to take all those ingredients
and bake a pie," says Chris
Bencher, who is a member of
the technical staff at Applied
Materials and has collabo-
rated with Wong's group.

the inward-facing
component can
be etched away,
leaving a matrix
of nanometerscale holes.

But Bencher notes there
are two ways to make arbi-
trary patterns with directed
self-assembly. One is to pat-
tern small troughs, as Wong's
group has done. The other is
to create a regular array of
self-assembled structures and
then eliminate the ones that
aren't needed. He suspects
that Wong's technique, which
will require fewer steps,
might be the first to make
its way into industrial semi-
conductor manufacturing
but that the latter method
may ultimately be better
suited to driving down the
distance between holes.
Whichever strategy is
adopted, researchers must
still work to reduce the rate
of defects from factors like
impurities in the polymer.
Bencher and his colleagues
have found that roughly one
out of every 25 million holes
is missing. That's a low rate,
but it's still too high to meet
the exacting standards of the
chip industry, whose ICs have
upward of a billion contacts.
But, says Bencher, "we are
definitely getting close."
-Rachel Courtland
spectrum.ieee.org

EMILy CoopER

40
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Table of Contents for the Digital Edition of IEEE Spectrum February, 2012
