2011 Annual Report - Center for Integrated Nanotechnologies - Los ...

Theory & Simulation of Nanoscale Phenomena Thrust
Potential New Nanolithography Tool from
Directed Assembly of Mixed Polymer Brushes
Scientific Accomplishments: Top-down lithography is a
well-established industrial technology that allows for the patterning
of geometrical designs with nanometer-level precision
and accuracy. Though conventional lithography has achieved
the continual shrinkage of surface features, it is widely believed
that fundamental limitations will soon be reached. These limitations
could be overcome by combining molecular self-assembly
with lithographic techniques in a process known as templated
self-assembly. Most work in this area to date has focused on
the use of self-assembling block copolymers. An alternative
polymer system is a mixture of two different homopolymers
that are covalently bonded to a substrate to produce a polymer
“brush”. The two different polymers will prefer to phase separate
from each other, but because of their covalent bond to the
substrate, only micro-phase separation is possible, resulting
in nanoscale self-assembled patterns. Mixed brushes have
various advantages over diblock copolymers; in particular they
could be grown on three-dimensional substrates and on a wider
variety of substrates.
Our idea was to use boundary interactions to orient these nanoscale
assemblies into long range ordered patterns. Through
a two-step polymerization scheme, micron-scale mixed polymer
brush regions bordered by a region of pure polymer could be
produced. We anticipate that the interaction with the pure polymer
would direct the phase separation parallel to this interface
by drawing the compatible polymer chains to the interface, forming
the first nanoscale domain in the phase separation pattern.
To demonstrate that this ordering mechanism in mixed polymer
brushes is possible, we carried out theoretical simulations with
CINT Users Su-Mi Hur and Glenn Fredrickson at the University
of California, Santa Barbara. We calculated the morphology of
a mixed polymer brush consisting of two polymers (A and B) of
the same length, both grafted to a hard substrate, using selfconsistent
field theory. When the brush is not confined laterally,
a disordered pattern forms, consisting of regions rich in either
polymer A or B. However, when a pure brush region of polymer
A is introduced, it templates the assembly of domains of
polymer A next to the pure brush region. Slowly annealing the
system leads to a regular pattern of nanoscale stripes. The calculations
also show that the pattern is robust to perturbations,
such as roughness in the pure brush region.
Density of polymer A in a mixed brush near a rough region of pure
polymer A (right in both images). Well-ordered stripes persist even with
line edge roughness.
Top view of the density A polymer in a symmetric mixed polymer
brush. Top image: bulk polymer brush; middle image: polymer brush
constrained by a pure brush region, that is quenched from high to low
temperature quickly; bottom image: a confined polymer brush that has
been slowly annealed.
Significance: These calculations demonstrate a proof-of principle,
that templating a mixed polymer brush can lead to wellordered
domains that display long range order. In principle one
could make the patterns in any desirable shape or geometry.
The size of the pattern features is determined by the polymer
molecular weight, which is tunable by synthesis. In general, the
synthesis of mixed polymer brushes is simple and uses inexpensive
reagents. Once demonstrated experimentally, this new
technique for nanolithography could have profound implications
on a number of technologies that utilize high-density patterned
surfaces.
Reference:
“Self-consistent field simulations of self- and directed-assembly
in a mixed polymer brush”, S.M. Hur, A. L. Frischknecht, D. L.
Huber, and G. H. Fredrickson. Soft Matter 2011, 7, 8776-8788.
CINT Contacts: Amalie Frischknecht and Dale Huber.
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40The Center for Integrated Nanotechnologies | 2011 Annual Report