Efficient surface neutralization and enhanced substrate adhesion through ketene mediated crosslinking and functionalization

Article abstract of DOI:10.1002/adfm.201201352

Balancing the interfacial interactions between a polymer and substrate is one of the most commonly employed methods to ensure the vertical orientation of nanodomains in block copolymer lithography. Although a number of technologies have been developed to meet this challenge, there remains a need for a universal solution for surface neutralization that combines simple synthesis, fast processing times, generality toward substrate, low density of film defects, and good surface adhesion. The chemistry of ketenes, which combines highly efficient polymer crosslinking through dimerization and surface adhesion through reaction with the substrate, is shown to be well suited to the challenge. The versatile chemistry of ketenes are accessed through the post-polymerization of Meldrum's acid, which can be easily incorporated into copolymers through controlled radical polymerization processes. Further, the Meldrum's acid monomer is synthesized on a large scale in one step without the need for chromatography. Processing times of seconds, low defect density, simple synthetic procedures, and good substrate adhesion make these materials attractive as robust block copolymer neutralization layers.

Full text of DOI:10.1002/adfm.201201352

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Efficient Surface Neutralization and Enhanced Substrate
Adhesion through Ketene Mediated Crosslinking and
Functionalization
Hyunjung Jung, Frank A. Leibfarth, Sanghoon Woo, Sumi Lee, Minhyuk Kang,
Bongjin Moon, Craig J. Hawker,* and Joona Bang*
equipment and processing, and material
challenges as the feature size decreases.
To overcome the inherent limitations of
conventional lithography, “bottom up”
approaches have garnered significant
attention in both academic and indus-
trial settings. Among these approaches,
self-assembly of block copolymers has
emerged as an attractive strategy for
accessing feature sizes below 50 nm.[1,2]
Termed block copolymer lithography, this
approach has several advantages including
the ability to cost-effectively produce uni-
form sphere, cylinder and lamella patterns
on the nanometer scale.[3–17]
One of the grand challenges in the
block copolymer lithography field has been
controlling the orientation of the self-
assembled nanodomains, which ideally
should order perpendicular to the bottom
substrate and maintain high uniformity
over large length scales. In order to achieve
Balancing the interfacial interactions between a polymer and substrate is
one of the most commonly employed methods to ensure the vertical orienta-
tion of nanodomains in block copolymer lithography. Although a number of
technologies have been developed to meet this challenge, there remains a
need for a universal solution for surface neutralization that combines simple
synthesis, fast processing times, generality toward substrate, low density of
film defects, and good surface adhesion. The chemistry of ketenes, which
combines highly efficient polymer crosslinking through dimerization and sur-
face adhesion through reaction with the substrate, is shown to be well suited
to the challenge. The versatile chemistry of ketenes are accessed through the
post-polymerization of Meldrum’s acid, which can be easily incorporated into
copolymers through controlled radical polymerization processes. Further, the
Meldrum’s acid monomer is synthesized on a large scale in one step without
the need for chromatography. Processing times of seconds, low defect den-
sity, simple synthetic procedures, and good substrate adhesion make these
materials attractive as robust block copolymer neutralization layers.
1. Introduction
this vertical orientation of polymer domains, it is imperative to
balance the interfacial interactions between the polymer and
substrate.[8,11,18–21] In the case of poly(styrene-b-methyl methacr-
ylate) (PS-b-PMMA), the PMMA blocks preferentially interact
with silicon substrates and results in a disadvantageous par-
allel orientation of the microstructure.[22–24] To achieve vertical
orientation, one of the most successful strategies for large-area
applications has been to neutralize the substrate with a random
copolymer of poly(styrene-methyl methacrylate) [P(S-r-MMA)].
The conventional and current commercial approach to surface
neutralization is the anchoring of a hydroxyl end-functional
random copolymer P(S-r-MMA) to a native silicon oxide sub-
strate through condensation-type chemistry.[18] By adjusting
the composition of PS and PMMA in the polymer backbone
and covalently grafting the polymers to the substrate, the sur-
face is effectively neutralized and one can achieve the desired
perpendicularly oriented nanostructures over large areas. The
disadvantages of this process include long processing times
of hours to days and incompatibilities with substrates other
than silicon. To overcome these issues, random copolymers
containing cross-linkable units have been developed. The first
polymer of this type contained a thermally reactive benzocy-
clobutene (BCB) comonomer, which provided crosslinked layers
upon dimerization above 200 °C.[25,26] These BCB containing
Conventional “top-down” lithography processes present
number of fundamental limitations for next-generation micro-
electronics including prohibitive cost, highly-specialized
a
H. Jung, S. Woo, Prof. J. Bang
Department of Chemical and Biological Engineering
Korea University
Seoul 136-713, Republic of Korea
E-mail: joona@korea.ac.kr
F. A. Leibfarth, Prof. C. J. Hawker
Departments of Materials
Chemistry and Biochemistry and Materials Research Laboratory
University of California
Santa Barbara, CA 93106, USA
E-mail: hawker@mrl.ucsb.edu
Dr. S. Lee, Dr. M. Kang
LCD R & D Center
Samsung Display, San#24, Nongseo-dong,
Giheung-gu, Yongin-city, Gyeonggi-do 446-711, Republic of Korea
Prof. B. Moon
Department of Chemistry
Sogang University
Seoul 121-742, Republic of Korea
DOI: 10.1002/adfm.201201352
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copolymers provided many advantages, including
greatly reduced processing times and applicability
to a variety of substrates, including Al, SiO2, Si3N4,
Au, etc. The BCB monomer, however, is not com-
mercially available, difficult to synthesize and
provides little, if any, covalent bonding to the
underlying substrate. The other widely employed
surface neutralization material relies on nitrene
formation from photochemical or thermal treat-
ment of an azide for crosslinking.[27] This method
has been readily adopted as a result of the lack of
availability of the BCB monomer, but the approach
suffers from multiple processing steps to intro-
duce the azide post-polymerization, a high surface
defect density at moderate concentrations of azide
and/or long irradiation times, undefined chemistry
of the highly-reactive nitrene, and limited covalent
attachment to the underlying substrate.
In response to the challenge of surface neutrali-
zation, we sought to develop a user-friendly meth-
odology that combines the advantages of simple
synthesis, fast processing times, highly-efficient
crosslinking, low defect density, and covalent
adhesion to the underlying substrate. Herein, we
describe the synthesis and application of Meldrum’s
acid containing polymers for surface neutralization
of block copolymer (BCP) lithography substrates.
Recently, Meldrum’s acid[28] has been employed as
a monomer building block for thermal access to
the valuable ketene functional group.[29–34] The ver-
satile chemistry of ketenes provide for crosslinking
via dimerization while also allowing for facile func-
tionalization via nucleophilic addition. Significant
potential for this chemistry therefore exists in
the BCP lithography field, as ketenes provide an
opportunity to both crosslink the polymer film and
covalently attach it to the substrate in one thermal
treatment step.[28–31,33,34]
Scheme 1. a) One-step synthesis of the Meldrum’s acid containing comonomer, 1.
b) Synthesis of P(S-r-1-r-MMA) random copolymer and its corresponding chemistry,
which provides crosslinking via ketene dimerization and adhesion via nucleophilic addi-
tion to surface-active silonols.
2. Results and Discussion
A distinct advantage of these Meldrum’s acid
based materials is their simple and user-friendly
synthesis. The functional comonomer (1) is syn-
thesized by a one-step process from two commer-
cially available starting materials in high yields.
Specifically, 2,2,5-trimethyl-1,3-dioxane-4,6-dione
(5-methyl Meldrum’s acid) and 4-vinylbenzylchlo-
ride are reacted under mildly basic conditions
and the crude product recrystallized to afford the
desired monomer as an analytically pure crystalline
solid (see Scheme 1a).
The ready availability of 1 provided the oppor-
tunity to thoroughly investigate the consequence
of ketene crosslinking on surface neutralization
properties. Specifically, by varying factors such
as the concentration of 1 in a random copolymer
and the crosslinking temperature and time, many
of the processing parameters could be tuned to
Figure 1. a) Crosslinked thickness of P(S-r-1-r-MMA) thin films as a function of
crosslinking time at 200 °C. The representative SFM phase images of lamellar forming
PS-b-PMMA diblock copolymer prepared on the crosslinked P(S-r-1-r-MMA) layers, in
which b) 3.0 mol% of 1 in P(S-r-1-r-MMA) was crosslinked for 10 s, and c) 10.0 mol% of
1 in P(S-r-1-r-MMA) was crosslinked for 1 h.
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of one hour. Gel content, or the amount of
remaining polymer after crosslinking and
subsequent solvent extraction, was measured
by the ratio between the initial film thickness
and the film thickness after thermal treat-
ment and extensive washing with toluene.
These crosslinked and washed samples were
then used as substrates for the self-assembly
of a lamellar forming diblock copolymer of
(PS-b-PMMA) and their ability to induce ver-
tical orientation was investigated by scanning
force microscopy (SFM).
To understand the influence of ketene
crosslinking on film neutralization proper-
ties, thin films of the random copolymers
were heated at 200 °C and the change in film
thickness was monitored as a function of
time. As previous work demonstrated, 200 °C
is the lowest temperature at which ketene
generation was observed in polystyrene
systems.[29] As seen in Figure 1, final film
thickness increased with the time of heat
treatment, requiring approximately 15 min at
200 °C for the 5% and 10% Meldrum’s acid
containing polymers to reach approximately
100% gel content, i.e., full retention of film
thickness. Each experiment was repeated
numerous times to ensure consistent results,
as denoted by the error bars in Figure 1a. As
expected for partial neutralization layers such
as those crosslinked for 10 s at 200 °C, the
self-assembly of (PS-b-PMMA) showed a mix
of parallel and perpendicular orientation of
domains on the surface (Figure 1b). When
Figure 2. a) Crosslinked thickness of P(S-r-1-r-MMA) thin films as a function of crosslinking
time at 230 °C. The representative SFM phase images of lamellar forming PS-b-PMMA diblock
copolymer prepared on the crosslinked P(S-r-1-r-MMA) layers, in which b) 3.0 mol% of 1 in P(S-
r-1-r-MMA) was crosslinked for 10 s, and c) 10.0 mol% of 1 in P(S-r-1-r-MMA) was crosslinked
for 1 h.
optimize the performance of the polymer neutralization layer.
The synthetic versatility of 1 allows the synthesis of random
copolymers [P(S-r-1-r-MMA)] by reversible addition fragmenta-
tion chain transfer (RAFT) polymerization[35,36] from styrene,
MMA, and 1 without any post-polymerization requirements
(Scheme 1b). This is in contrast to the widely used azide con-
taining crosslinkable copolymer, which requires a two-step,
post-polymerization removal of the dithioester and azide
displacement to obtain the desired copolymer. In order to deter-
mine the effect of crosslink density on material properties,
P(S-r-1-r-MMA) copolymers containing 3, 5, and 10 mol% of
1 were synthesized by adjusting the stoichiometry of the poly-
merization. Specifically, as 1 is similar to styrene, the propor-
tion of styrene and 1 was held constant at a combined 55 mol%
of the final random copolymer and the methacrylate was held
constant in all polymers at 45 mol% (Figure S1, Supporting
Information). The total molecular weight of all polymers was
controlled between 20 000 and 30 000 g/mol with polydispersi-
ties below 1.2.
the crosslinking time was increased, the neutralization was par-
tially improved as seen in Figure 1c. Although the crosslinked
film thickness reached over 80% gel content after 10 min
heating time, a significantly high defect density was observed
over large areas of the thin-film, proving that copolymer layers
crosslinked at 200 °C do not act as effective neutral substrates.
In order to improve the efficiency of crosslinking and, thus,
the neutralization properties of the films, the thermal treat-
ment temperature was increased to 230 °C. Notably, a heating
time of one minute at this temperature produced crosslinked
layers that had a gel content of greater than 50% regardless
of incorporation of 1, which is an appropriate thickness for a
neutral surface (Figure 2a). To probe the neutralization effect
of the crosslinked layers at this temperature, the same lamellar
forming PS-b-PMMA diblock copolymer was prepared on the
crosslinked and washed layers. In this case, we found excellent
perpendicular orientation of the BCP nanodomains regardless
of concentration or crosslinking time (Figure 2b,c, and Figure S2
in the Supporting Information). At crosslinking times of one
minute, our results suggest that the surface defect density is
comparable to neutralization layers fabricated using BCB or
azide containing copolymers. Furthermore, it should be noted
that long range perpendicular orientation was observed with
neutralization layers crosslinked for less than one minute
(Figure 2b), although the film thickness is less than 6 nm.
Polymer neutralization layers were subsequently prepared
by spin casting a toluene solution containing the P(S-r-1-r-
MMA) random copolymer (0.4 wt%, 3000 rpm) on a silicon
wafer, providing uniform films with thicknesses of 11.5 to
13 nm. Thin-films containing different concentrations of 1 were
subsequently crosslinked at various temperatures over a period
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on the surface of silicon wafers.[37,38] To probe
the strength of this binding on neutralization
properties, ultrathin crosslinked films were
prepared by thermal crosslinking at 230 °C
for 60 min and then exposure to mechanical
stress. The water contact angle, film thick-
ness, and neutralization properties were sub-
sequently examined to provide a qualitative
measure of the mechanical integrity of the
neutralization layers. The ketene crosslinked
films were tested against the widely employed
photo-crosslinked azide random copolymer
neutralization layers, which should not
undergo appreciable reaction with silicon
substrates.
In order to model mechanical stress, the
neutralization layers were sonicated in ben-
zene for varying amounts of time and the
film thickness and water contact angle was
subsequently measured. The superior surface
adhesion of the ketene films is immediately
apparent, as the ketene based films remain
adhered to the substrate and do not change
thickness even after sonication for an hour
in a good solvent. This is in direct contrast
to the azide-based films, which lose thick-
ness rapidly even after short sonication times
(Figure 4a). This behavior is further sup-
ported by water contact angle measurements,
which show that the ketene based films show
no change in surface properties after sonica-
tion, whereas the azide based films undergo a
marked decrease in contact angle (Figure 4b).
Figure 3. a) Crosslinked thickness of P(S-r-1-r-MMA) thin films as a function of crosslinking time at
250 °C. The representative SFM phase images of lamellar forming PS-b-PMMA diblock copolymer
prepared on the crosslinked P(S-r-1-r-MMA) layers, in which b) 3.0 mol% of 1 in P(S-r-1-r-MMA)
was crosslinked for 10 s, and c) 10.0 mol% of 1 in P(S-r-1-r-MMA) was crosslinked for 1 h.
This suggests that the ketene crosslinking reaction is much
more efficient at 230 °C. Finally, by increasing the thermal
crosslinking temperature to 250 °C, the kinetics of the Mel-
drum’s acid to ketene reaction speeds up considerably and fully
crosslinked neutralization layers could be obtained with only
10 s of thermal treatment (Figure 3a). This allows perpendic-
ularly orientated BCP domains to be obtained regardless of
heating time at 250 °C, providing evidence of a large processing
window for these materials (Figure 3b,c, and Figure S2 in the
Supporting Information). The orientation of BCP domains on
these surfaces was also examined by grazing-incidence small-
angle X-ray scattering (GISAXS), which was carried out at
the 9A beamline at the Pohang Accelerator Laboratory (PAL),
Korea. For the BCP patterns prepared on the neutralization
layers crosslinked at 230 and 250 °C, sharp bragg peaks were
clearly observed, indicating that the perpendicular orientation
of PS-b-PMMA lamellae persist throughout the film thickness
(Figure S3, Supporting Information).
As these results demonstrate, employing thermally gener-
ated ketenes as a crosslinking group allows the gel content and
surface properties of BCP neutralization layers to be tuned by
varying parameters such as crosslinker concentration, reac-
tion temperature and processing time. The power of this
approach, however, derives from the ability of ketenes to not
only dimerize, but also provide covalent attachment to the sub-
strate via nucleophilic addition of ketenes with silanol groups
Lastly, as a true test of the viability of these films to act as neutral-
ization layers even after significant mechanical stress, a lamellar
forming (PS-b-PMMA) diblock copolymer was assembled on the
surfaces after 60 min of sonication. As shown in Figure 4c, the
films assembled on the azide crosslinked neutralization layer
show a mix of parallel and perpendicular domains, signifying
a significant reduction in the ability to neutralize the surface.
All films crosslinked by ketenes, however, show long-range
perpendicular ordering of the block copolymers (Figure 4d)
and clearly indicate the robust nature and increased perform-
ance of a ketene based crosslinking strategy for surface neu-
tralization in block copolymer lithography. Furthermore, the
same adhesion test was also performed on a different substrate,
thermally deposited Al2O3 layer. As with the results on the sil-
icon wafer in Figure 4, the ketene films on Al2O3 layer exhib-
ited superior surface adhesion and gave rise to perpendicular
ordering of PS-b-PMMA block copolymers (Figure S4, Sup-
porting Information), demonstrating the excellent versatility
and adaptability of this strategy.
3. Conclusions
This study demonstrates that a ketene-based crosslinking
strategy for the fabrication of neutralization layers in BCP lithog-
raphy offers enhanced performance and substrate adhesion
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Figure 4. a) Crosslinked thickness of P(S-r-1-r-MMA) thin films compared to azide containing materials as a function of sonication time after crosslinking
at 230 °C for 1 h. b) The water contact angle of P(S-r-1-r-MMA) surface compared to azide containing materials as a function of sonication time after
crosslinking at 230 °C for 1 h. c) The representative SFM phase image of lamellar forming PS-b-PMMA diblock copolymer prepared on the crosslinked
azide random copolymer after 1 h of sonication. d) The representative SFM phase image of lamellar forming PS-b-PMMA diblock copolymer prepared
on the crosslinked P(S-r-1-r-MMA) layers, in which 10.0 mol% of 1 in P(S-r-1-r-MMA) was crosslinked for 1 h at 230 °C followed by 1 h of sonication.
standards. Mass spectral data were collected on a Micromass QTOF2
Quadrupole/Time-of Flight Tandem mass spectrometer (ESI-MS). SFM
images were obtained on a Digital Instruments Multimode scanning
force microscope in tapping mode. GISAXS experiments were carried out
at the Pohang Accelerator Laboratory (PAL) 4C2 beamlines, Korea. A CCD
detector was used to record 2D GISAXS patterns, which was positioned
at the end of a vacuum guide tube where the X-ray (wavelength: 1.38 Å)
passed through the sample.
Synthesis of P(S-r-1-r-MMA) Random Copolymers: Thermally
crosslinkable P(S-r-1-r-MMA) random copolymer was synthesized via
RAFT polymerization. The initial stoichiometry of 1 compared to styrene
was varied to provide polymers with 3, 5, and 10% of repeat units based
on 1. A representative procedure for the 5% random copolymer follows:
Styrene (1.39 g, 13.3 mmol), methylmethacrylate (1.2 g, 12.0 mmol), 1
(0.365 g, 1.33 mmol), 2,2′-azobis(2-methylpropionitrile) (AIBN) (0.7 mg,
0.0042 mmol), and RAFT agent (12.7 mg, 0.042 mmol) were added to
a Schlenk flask and degassed through three freeze-pump-thaw cycles.
The reaction mixture was allowed to stir at 80 °C for 60 h. The reaction
was quenched by exposing to air and precipitated into 150 mL methanol.
The final product was collected by filtration as a pink powder. Physical
properties were as follows: 3 mol% P(S-r-1-r-MMA): Mn = 24 000 g/mol,
PDI = 1.13; 5 mol% P(S-r-1-r-MMA): Mn = 21 000 g/mol, PDI = 1.15;
10 mol% P(S-r-1-r-MMA): Mn = 27 000 g/mol, PDI = 1.22. 3 mol% of
azide containing copolymer: Mn = 21 000 g/mol, PDI = 1.12.
compared to previous methods. These materials provide effi-
cient surface neutralization for (PS-b-PMMA) self-assembly via
thermal crosslinking of P(S-r-1-r-MMA) with short processing
times and tunable surface properties. This is in contrast to pre-
vious methods, which either provide surface adhesion at the
expense of long processing times and harsh conditions (>2 days
at 140 °C) or provide short processing times at the expense of
surface adhesion. Additional benefits of this strategy include
synthetic utility, as the Meldrum’s acid monomer is simple
to prepare, and tunable surface properties through varying
crosslinking temperature and time. Significantly, conditions
were found that provided large area substrate neutralization
and perpendicular orientation of the (PS-b-PMMA) domains
with excellent substrate adhesion. The robust nature of these
materials offer significant promise as a platform for the further
development of block copolymer lithography.
4. Experimental Section
General Methods: All commercially obtained solvents and reagents
were used without further purification. 1H and 13C solution-state NMR
were recorded on 600 MHz NMR and 500 MHz NMR spectrometers.
Chemical shifts are reported relative to residual solvent peaks (δ 7.26
for CDCl3 in 1H NMR and δ 77.2 for CDCl3 in 13C NMR). IR spectra were
obtained using a Thermo-Nicholet Avatar-330 IR spectrometer with
single-bounce attenuated total reflection (ATR) (Ge crystal) accessory
(Smart MIRacle). Gel permeation chromatography (GPC) was performed
in THF on a Waters equipped with a refractive index detector. Molecular
weights of polymers were calculated relative to linear polystyrene
Preparation of Random Copolymer and Crosslinking Test: A 0.4 wt%
solution of P(S-r-1-r-MMA) in toluene was spin coated on a silicon
substrate (3000 rpm, 40 s). The initial thickness of the random layer,
which was measured by ellipsometry, was 12 nm ( 1 nm). Thin films
were crosslinked at 200, 230, 250 °C for 10 s to 1 h under nitrogen
on a hot plate. To test the reproducibility of this process, at least
10 samples were prepared under each of the conditions. After thermal
crosslinking, all samples were rinsed in a toluene solution, dried in a
vacuum oven, and the residual thickness was measured. To confirm
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the neutral properties of the surface, 40 nm of lamella forming PS-b-
PMMA (74 kg/mol total, PS 38 kg/mol, PMMA 36 kg/mol, purchased
from polymer source) was spin coated on the residual random layer
(1.2 wt% of toluene solution, 3000 rpm spin rate, 60 s). After 72 h at
190 °C of thermal annealing under vacuum, the lamella morphology was
confirmed by SFM.
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Acknowledgements
This work was supported by National Research Foundation of Korea
(NRF) grant funded by the Korean Government (MEST) (No.2012-
014473), and also by the Human Resources Development Program of
KETEP grant (No. 20114010203050) funded by the Korea government
Ministry of Knowledge Economy. F.A.L. and C.J.H. would like to thank
the Nanoelectronics Research Initiative under contract RID#1549 (SRC/
NRI), the Microelectronics Advanced Research Corporation (MARCO)
and its Focus Center Research Program (FCRP)−Center for Functional
Engineered NanoArchitectonics (FENA), the National Science Foundation
(MRSEC Program - DMR-1121053, Graduate Research Fellowship) and
the DOD (NDSEG Fellowship) for financial support.
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Received: May 18, 2012
Revised: August 15, 2012
Published online: October 22, 2012
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