neutral chemical to act as a base, then
adding the block copolymer on top,
and heating the whole thing. The polymers would form alternating lines—
think of them as red stripes and blue
stripes—and one set, say the red, would
be washed away. The remaining blue
stripes would act as a pattern for the
same etching process used in current
photolithography to inscribe circuits
into silicon.
Photolithography also relies on
chemicals—photoresists that are patterned through exposure to light, creating a pattern that is used to define which
parts of the wafer to etch. Many modern
designs, however, require multiple
masks, driving up costs. The machines
for focusing the light beams to ever-smaller sizes are also expensive, so DSA
could, the thinking goes, create smaller
features and do it more cheaply.
A few years ago, DSA was seen as the
next big thing, and many chipmakers
were investing heavily in it, says Kurt
Ronse, director of the advanced lithography research program at the Interuniversity Microelectronics Center
(imec) in Leuven, Belgium. The focus
was on a block copolymer consisting of
polystyrene and polymethyl methacrylate, or PS-PMMA, which was targeted
for features with a 28nm pitch, in
which lines and spaces are each 14nm
wide. At the time, the anticipated next
photolithography technology was extreme ultraviolet (EUV), but progress
toward making it commercial seemed
to be stalled.
Too Many Defects
The hype over DSA did not last, says
Ronse. “Very quickly it turned out that
one of the big issues was going to be
defects,” he says. Sometimes the copolymer would leave a gap, or the
stripes would get too close to each
other or even cross, a phenomenon
called “line edge roughness.” The undercoating might have a pinhole or a
slight bump that would throw the
process off.
“It uses thermodynamics to form
the patterns,” says Charles Black, di-
rector of the Center for Functional
Nanomaterials at Brookhaven Nation-
al Laboratory in Upton, NY. “It usually
ends up with a defect here or a defect
there, and microelectronics is really in-
tolerant of that.”
Over time, Ronse says, researchers
managed to reduce the level of de-
fects, while at the same time, EUV was
beginning to look more promising.
The combination made it seem less
likely that DSA would provide the low-
cost alternative the industry sought.
“After a couple of years of develop-
ment, the window of opportunity for
28nm pitch was starting to close,”
Ronse says.
Though the hype has faded, he says,
work continues. Some of the companies imec works with have ended their
DSA research, while others are still going, though they have directed some of
their chemistry work toward developing photoresists for EUV. “They have
not stopped DSA development, but by
putting less effort in it, it definitely
slows down,” Ronse says.
While research into DSA is not as ag-
gressive as it once was, it has not
stopped, says Christopher Ellison, as-
sociate professor of chemical engi-
neering at the University of Minnesota.
“I don’t think it’s going away,” he says.
“It’s not growing at a rapid rate.”
Part of the reason for that slowdown
may be that it is not the optical physics
that the chipmaking industry has al-
ways relied upon. “It makes it challeng-
ing for industry to accept because it’s
so radically different,” Ellison says.
Surface Tension
While PS-PMMA may seem less attrac-
tive than it once did because its resolu-
tion limit is 11 nm, chemists are pursu-
ing other block copolymers that could
go even smaller. Some materials have a
high chi, a measure of how incompati-
ble the two polymers are. High-chi co-
polymers can perform the same self-
assembly as PS-PMMA, but at a smaller
size. Ellison’s group has created struc-
tures just four to five nm wide.
The trouble with high-chi copolymers is that it can be difficult to get
them to stand up straight. To form parallel strips, they need to orient themselves perpendicular to the surface
they are on. Often, however, they will
flop over onto their sides, destroying
the pattern.
Because self-assembly is a thermodynamic process, it is controlled by
differences in surface energy, and the
top “surface” is open air. Some researchers, such as Ellison and his colleague C. Grant Willson, a professor
of chemical engineering at the University of Texas at Austin, have been
working on topcoats, films of chemicals that change the surface energy
on top of the copolymer so that it
aligns itself correctly.
Paul Nealey, a professor of molecular
engineering at the University of Chicago
and one of the pioneers of DSA, says
there are three or four methods to control the orientation of high-chi materials. Nealey and Karen Gleason, a professor of chemical engineering at the
Massachusetts Institute of Technology,
Cambridge, MA, published a paper in
Nature Nanotechnology in March in
which they reported using a technique
called initiated chemical vapor deposition to create a topcoat that forced the
block copolymer to line up the way they
wanted it. The approach allowed them
to create features only 9.3nm wide.
Another approach exposes the copolymer to vapor from a solvent to
change the balance of energy. A technique explored by IBM mixes in an additive to the copolymer that changes
how it responds to surface energy at
the air interface.
However they are controlled, block
copolymers cannot do everything on
their own. Some lithography must be
used to inscribe guiding lines for them
to follow—that is the “directed” part of
DSA. Because the material forms lines
much thinner than those inscribed,
they require less-advanced photolithography processes. At some resolutions,
the guiding pattern could be created by
the current state-of-the-art process,
193nm immersion lithography, which
uses a liquid to focus the light beam to a
Because self-
assembly is a
thermodynamic
process, it is
controlled by
differences in surface
energy, and the top
“surface” is open air.