1. Nanopatterning via Pressure-Induced Instabilities in
Thin Polymer Films
By Ximin He, Jurjen Winkel, and Wilhelm T. S. Huck*
Polymer thin films are the bedrock of modern electronic devices,
playing key roles in their fabrication (photoresists) and operation
(dielectrics, light emitting layers, sensor films, etc.). Conse-
quently, enormous efforts have been devoted to understanding
their fundamental properties and to pattern thin polymer films,
especially now that the thickness of the films and the feature sizes
are reaching molecular dimensions. The vast bulk of these
studies has employed films at thermodynamical equilibrium, but
as we will demonstrate here, non-annealed films containing
residual stresses from the preparation of the films by spincasting
can be harnessed in new patterning strategies. Nanoimprint
lithography (NIL) is an extremely powerful technique for rapid
large area nanopatterning.[1,2]
The basic principle involves placing
a master with nanoscale topographic features on top of a thin
polymer film, and subsequently heating the polymer film above
the glass transitions temperature (Tg) while applying pressure, so
that the polymer flows to fill the relief pattern of the master. A thin
residual layer usually remains beneath the master protrusions,
which needs to be removed by a subsequent etching process
before the pattern can be transferred into the substrate.
Solvent-assisted NIL[3]
unexpectedly resulted in pattern replica-
tion without any residual film between the features, due to
dewetting of the plasticized film. Alternatively, films on
completely non-wettable fluorinated surfaces break up during
imprinting and heating, leading to individual polymer nano- and
microstructures.[4]
Scanning-probe lithography has been used
extensively to produce nanostructures in self-assembled mono-
layers on gold by ‘shaving’ the chemisorbed alkanethiols.[5]
Interestingly, residue-free nano- to micrometer-scale trenches
were generated by scribing poly(3-hexylthiophene-2,5-diyl)
(rr-P3HT) thin films with an AFM tip, presumably as a result
of the release of residual strain in the films.[6]
These examples
open up interesting opportunities in developing a general strategy
based on film rupturing phenomena for creating features without
residual films using NIL. Dewetting of thin polymer films at
temperatures above the Tg is a well-understood, although often
undesirable phenomenon.[7–10]
The stability of ultrathin polymer
films (< 100 nm) is governed by weak surface interactions, and
forces acting on the thin film can easily destabilize the continuous
film leading to dewetting phenomena (film rupture).[7]
Depend-
ing on the combined effect of the weak forces (van der Waals,
hydrogen bonding) films may rupture either by spontaneous
dewetting (unstable films) or by nucleation and growth of holes
(metastable films). Numerous studies have investigated the role
of film thickness, substrate surface energy on rupture of thin
wetting films and nanoscale topography.[8–13]
Nanoscale surface
energy patterns lead to local variations of the effective interface
potential and concomitant variations in dewetting kinetics, which
results in pattern replication in the dewetted films.[14]
It should be
noted that all dewetting studies are carried out using polymer
films that are heated above the Tg, below which temperature the
polymer chains are in the glassy state and immobile, hence
resisting flow. Here, we present a non-conventional lithographic
technique to fabricate nanometer-scale periodic patterns based on
pressure-induced local rupturing at temperatures significantly
below the Tg. Our approach is based on the presence of residual
stresses in non-annealed spincoated polymer thin films. The fast
evaporation of solvent during spin-coating leads to frozen-in
non-equilibrated chain conformations of the polymer, thus
generating residual stresses within the film, which can be an
important reason for thin film instability and a driving force for
hole-nucleated dewetting.[15,16]
Any internal or external distur-
bances, e.g., heating, dust particles, film defects, or, as we show in
this communication, local mechanical forces acting on the film
surface, provide a route for the residual stress to be relieved.
The patterning procedure we have developed here is shown
schematically in Figure 1. Firstly, fluorinated sharp (< 10 nm tip
radius) stamps, containing V-shaped lines (Fig. 1c) or cones (Fig.
1d), are placed on top of a spincoated polymer film (10–60 nm
thick) on hexamethyldisilazane (HMDS)-treated Si wafers (to
avoid differences in thickness, the edges of the wafers were cut off
after spincoating and only the center area was used for
imprinting). Subsequently, the stamps are brought into contact
with the polymer film surface using a pressure of 3–5 bar. After
10–50 s, the stamps were lifted away from the film, at which time
periodic patterns had formed triggered by the local stimulus from
the tips. It should be noted that films on HMDS-treated Si are
easily patterned at room temperature, whereas films on untreated
Si, require moderate heating, albeit well below the Tg. Below, we
will describe how the pattern size can be tuned by controlling the
imprinting temperature and film thickness, and the effect of
surface energy and polymer molecular weight.
Figure 2a shows the scanning electron microscopy (SEM)
image of the line array formed in a 44-nm thick polystyrene (PS,
Mw 50k) film on HMDS-Si by using V-shaped stamps as shown in
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[*] Prof. W. T. S. Huck, X. He
Melville Laboratory for Polymer Synthesis
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1 EW (UK)
E-mail: wtsh2@cam.ac.uk
Prof. W. T. S. Huck, X. He
The Nanoscience Centre, University of Cambridge
11 J.J. Thomson Avenue
Cambridge CB3 0FF (UK)
Dr. J. Winkel
Kodak European Research
332 Science Park, Milton Road
Cambridge, CB4 0WN (UK)
DOI: 10.1002/adma.200803547
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Figure 1a. The pattern was very uniform and complete over the
whole of the imprinted area (3 mm  3 mm). Clear narrow
trenches can be seen in the insert close-up image. Rims at the
edges of the trenches are observed, which are typical for polymer
film dewetting processes.[8]
Similarly, a hole
array was formed in PS films on HMDS-Si,
using the stamp containing 2D conical-shaped
sharp tips, as shown in the SEM image in
Figure 2b. We can observe in the high-
magnification insert image that each dot has
a rim surrounding a central pore. In all
experiments, both the trenches and the holes
are larger than the size of the tip and hence the
patterns are not exact negative replicas of the
master. This is a strong indication that these
patterns are not the result of a standard NIL
imprinting process. To examine the patterns in
more detail, a thin film of platinum (Pt) was
sputtered onto the imprinted films followed by
a PS lift-off procedure by soaking and sonicat-
ing the samples in toluene. SEM images
(Fig. 2c and d) clearly show 52 nm wide Pt lines
and Pt dots of 36 nm diameter, which indicates
that the patterns formed in the imprinting
process lead to completely dewetted, residue-
free features. In contrast, much thicker films
(100 nm) imprinted with the same stamp do
not show these completely dewetted patterns;
instead, typical imprinted features that repli-
cate the features but leave a thin residual film
are observed (in other work, Stutzmann et al.
have used similar stamps in the solid state
embossing, or ‘microcutting’, of semiconduct-
ing polymers[17]
). We therefore propose an
alternative route to pattern formation in these
films, based on a rupturing instability triggered
by a local mechanical stimulus, which nucle-
ates a hole in the film. The ‘holes’ then coarsen
by releasing the internal tensile stress of the film. The key to this
mechanism are residual stresses within the polymer films
resulting from spincoating coupled with fast evaporation of the
solvent.[15]
Indeed, freshly prepared (non-annealed) thin film that
had been stored in an ambient environment for a maximum of
2 days tended to form the patterns much more readily in
comparison with films that had been vacuum-annealed at 80 8C
for 48 h (which showed strong sample to sample variation). The
increased chain mobility and lowering of the Tg due to some
remaining solvent in thin films and the presences of residual
stresses after spincoating, as well as the effect on rupturing or
dewetting phenomena, has been well documented.[8,15,18]
To study the effect of patterning temperature on the pattern
size, we conducted a series of experiments at different
temperatures. Figure 3 shows the clear increase in pattern size
with increasing imprinting temperature for both 44 and 60 nm
thick films. The line patterns are plotted as increase in width
versus patterning temperature, where the surface area versus
temperature is plotted for the hole patterns, as the sharp conical
tips allow the residual stress to be released in two dimensions. By
controlling the temperature from 25 to 115 8C, the average width
of trenches in 40 (60) nm-thick PS films can be changed from
59 nm (71nm) to 341 nm (392 nm). Similarly, the average hole
areas changed from 804 nm2
(1017 nm2
) to 13463 nm2
(14950 nm2
). The feature sizes increased more pronouncedly
Figure 1. a) and b) Schematic procedure of the patterning process. A 10–60 nm thick polymer
film, spun cast on a HMDS treated Si/SiO2 substrate, is approached by a fluorinated Si stamps
containing sharp tip arrays, with tip radius of < 10 nm, tip length of 0.5–1.5 mm, periodicity of
3 mm. a) 1D V-shaped stamp produce lines; b) 2D conical-shaped tips produce holes. Step 1.
Stamp gently touches the film surface by applying a pressure of 3–5 bar. Step 2. Pattern formation
occurs under areas in contact with stamp. After 10–50 s, stamp is lifted up from the film. c) and d)
SEM images of stamps used: top-view of c) 1D V-shaped lines and d) 2D conical-shaped dots
stamps. Insert: side-view of stamp.
Figure 2. Scanning Electron Microscope (SEM) images of a)1D line and b)
2D hole array patterns in 44-nm thick PS (Mw 50K) film on HMDS-Si and
sputter-deposited Pt after lifting off PS (c: 1D lines, d: 2D dots). % 10 nm
thick Pt was sputter deposited on patterned PS film for SEM measurement,
and subsequently PS was lifted off to verify that the patterns reached the
SiO2 surface. Insert: high-magnification images. Scale bar: 2 mm.
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at patterning temperatures above 75 8C which is close to the Tg for
a 40–60 nm thin film of PS (70–90 8C as determined by
ellipsometry measurements).[18]
The AFM images in Figure 3
show the progression of the feature size increases. From the
cross-sectional profile of the AFM, we can observe clearly the rims
around the imprinted features and rim heights increase with the
increase of the feature sizes. Close agreement between the
reorganized rim volume and the volume of the trench or hole
implies that the transformation involves mostly mass transport
without removal of material. The rims also support a mechanism
based on dewetting phenomena rather than a cavitation of the
film.[19]
It is known that the size of features formed during dewetting
depends on the film thickness.[7,11]
Figure 4 shows the increase in
feature sizes with increasing film thicknesses.
It should be noted that holes and trenches were
only formed when using sharp tips and films of
<60 nm thickness. We found that flat-topped
molds containing 20 nm lines on a 100 nm
pitch did not produce any pattern under the
conditions used to produce the patterns shown
above (tested for 12–100 nm thick PS films,
room temperature, 5 bar). It was also observed
that the pattern produced by sharp tips did not
reach the substrate when 100 nm thick PS films
were used. In those experiments, we found
trenches or holes with feature sizes more
commensurate with the tip dimensions
(% 12–20 nm) and these patterns almost dis-
appeared when the films were annealed at
120 8C as a result of viscous flow and relaxation
process in the mobile polymer film.[20–22]
Finally, a number of other control experi-
ments were performed to investigate the limits
of the pressure-induced instability patterning
procedure. In thin-film studies, the surface
wettability and film-substrate interactions are
of crucial importance. We found that we could
not pattern thin PS films on oxygen-
plasma-treated Si substrates using room tem-
perature imprinting conditions, reflecting the increased stability
of PS films on SiO2. We measured contact angles of water and PS
toluene solution on oxygen-plasma treated Si wafers (14.58 Æ 48
and 3.18 Æ 18, respectively), which are much lower than the
corresponding values for HMDS-Si (110.88 Æ 58 and 45.38 Æ 48,
respectively). These results indicate that the pattern size could be
tuned further by altering the surface chemistry using functional
silanes.[14]
Furthermore, the results discussed above are all
obtained using PS with Mw %50k. To investigate the effect of
polymer Mw, %30 nm thick PS films of Mw of 10.2k, 113.5k, and
990.5k were patterned by this approach. We found that the
PS-10.2k film gave clear patterns under the same patterning
conditions, while PS-990.5k barely showed indentation patterns
at room temperature due to its good film stability and high
viscosity. These results are in line with other published results
that low Mw polymer films are more easily destabilized.[8]
Additionally, the spincoating solvent also has effect on the
dewetting patterning process, as high boiling point solvents dry
slower, allowing more strain relaxation, resulting in finer
dewetting features (we did not investigate an alternative
explanation: the presence of higher levels of remaining solvents).
The patterns in the film were stable at room temperature for at
least 2 months, without signifant increases in size. When
patterned films were annealed films at the polymer Tg for 5 min,
AFM measurements and SEM images showed that the pattern
sizes increased considerably (several micrometers), and the rim
heights decreased while rim widths increased. These results are
in line with ‘standard’ dewetting results.[7–11]
Since instabilities are a universal phenomenon of polymer thin
films, our technique is applicable to many polymers. Here we
demonstrate the line and hole patterns in a conjugated polymer,
poly(3-hexylthiophene) (P3HT, Mw 87k), and poly(methyl metha-
crylate) (PMMA, Mw 120k) (Fig. 5). Very uniform, 98 nm wide
Figure 3. Trench widths (left axis) in 44 nm-(&) and 60 nm-(*) PS films (Mw 50k) and hole
areas (right axis) in 40 nm-(&) and 60 nm-(*) PS films versus patterning temperature. Inserts:
AFM topography images and cross-section profiles of the patterns corresponding to indicated
data points.
Figure 4. Plots of line width (&) and hole area (*) as the function of film
thickness. Patterning was conducted with PS films of different thickness
spun cast on HMDS-treated Si at 25 8C.
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trenches were formed in 38 nm thick P3HT films at 100 8C at a
pressure of 5 bar for 50 s. Likewise, regular 28 nm wide holes
(measured at the hole bottom) were produced in 60 nm thick
PMMA films at 25 8C at a pressure of 5 bar for 50 s.
In summary, we have demonstrated how the residual stresses
in spincoated films, which so easily leads to undesirable
rupturing of thin polymer films at elevated temperatures, can
be exploited to produce highly controlled nanoscale patterns via
pressure-induced local rupturing. Our technique not only allows a
systematic study of the properties of non-annealed films, but also
serves as an important lithographic tool in that no residue is left in
the patterned areas. The sizes of the holes and trenches formed
here can be tuned from the nano- to the micrometer-scale by
controlling the film thickness, substrate wettability, patterning
temperature and post annealing time (the latter not shown above,
but annealing of the films above the Tg leads to a further retraction
of the films and coarsening of the holes).
The rupturing induced by pressure through a sharp tip array is
in contrast to ‘classical’ dewetting or cavitation studies, which take
place at temperatures above the Tg. Likewise, NIL is usually
carried out at high temperatures and high pressure (10–40 bar).
The very sharp tips used here lead to very rapid pattern formation
(seconds rather than minutes) in films at room temperature (or
slightly above) and at low pressures (3–5 bar) (although the local
pressure under the sharp tips will be much higher). In further
contrast to standard NIL is the absence of a residual ‘skim’ layer,
which obviates the need for an additional dry-etching step before
metallization or other evaporation processes that are part of
device fabrication procedures. Apart from the patterning results,
the experiments should also be of relevance for polymer
electronics studies, where spincoating is used as a standard
fabrication tool, but where the residual stresses might well impact
film morphology and hence device performance.
Experimental
Si substrates were cleaned in DI water, acetone, iso-propanol sonication
bath for 15 min each before use. Following a 30-second oxygen plasma
(power 100 Watt), Si substrates were coated with a HMDS by vapor phase
deposition at 175 8C for 15 min. Contact angles were measure using a
home built contact angle rig fitted with a microscope and camera. The
roughness of HMDS layers and of the PS films, as determined by atomic
force microscopy (AFM) was below 1 nm.
Polystyrene (PS, Mw 50k, PolyScience Inc.) films of certain thickness
were spun cast from toluene solution of various concentration onto fresh
HMDS-treated, oxygen-plasma-treated and untreated Si substrates with
different spin speeds. Cleaning and coating were performed in a class
100 clean room. Patterning was performed on freshly prepared PS films
stored in ambient atmosphere and, for control experiment, dried PS films
by vacuum annealing at 80 8C for 48 h. Thickness was checked by Alpha-SE
Spectroscopic Ellipsometer and Dektak Profilometer. 12, 20, 44, 60, and
100 nm thick PS films (Mw 50k), and also 20–30 nm thick film made from
polystyrene of Mw of 10.2k, 113.5k, and 990.5k were studied in this work.
Polymethyl methacrylate (PMMA, Mw ¼ 120k, Aldrich) films and poly-3-
hexythiophene (P3HT, Mw ¼ 87k, Aldrich) films were prepared and
patterned in the same way as PS films. The glass transition temperatures
of the polymer thin films that were used in our study were measured using
an Alpha-SE Spectroscopic Ellipsometer.
The 1D V-shaped tip array, periodicity 3 mm, tip height 1.5 mm, and 2D
tip array, periodicity 2.12 mm, tip height 0.3–0.5 mm, tip radius < 10 nm for
both arrays, are commercially available (NT-MDT). Patterning was
conducted using a nanoimprinter (Obducat) at room temperature
(constant at 25 8C in clean room) or various elevated temperatures with
applied force of 3–5 bar for 10–50 s. The sample plate of the nanoimprinter
was pre-heated to the temperature that would be used for imprinting and
the instability patterning took place right after the sample (Si tip master
covered PS film on Si substrate) was placed onto pre-heated plate. The
pattern morphologies were checked by SEM (Leo Variable pressure SEM)
and AFM (Dimension 3100 AFM). All the pattern widths and diameters
were measured at the trench and hole bottoms and average values were
taken from at least ten measurements.
Acknowledgements
This work was supported by Kodak European Research, Cambridge and the
Cambridge Gates Trust (XH).
Received: December 1, 2008
Revised: December 19, 2008
Published online:
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