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Development of a rapid curepolydimethylsiloxane
replicationprocess with near-zero shrinkage
Mohsin Ali BadshahHyungjun JangYoung Kyu KimTae-Hyoung
KimSeok-min Kim
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Development of a rapid cure polydimethylsiloxanereplication
process with near-zero shrinkage
Mohsin Ali Badshah, Hyungjun Jang, Young Kyu Kim, Tae-Hyoung
Kim,* and Seok-min Kim*Chung-Ang University, School of Mechanical
Engineering, 84 Heukseok-Ro, Dongjak-Gu, Seoul 156-756, Republic of
Korea
Abstract. Replicated polydimethylsiloxane (PDMS)
micro/nanostructures are widely used in various researchfields due
to their inexpensiveness, flexibility, low surface energy, good
optical properties, biocompatibility,chemical inertness, high
durability, and easy fabrication process. However, the application
of PDMSmicro/nano-structures is limited when an accurate pattern
shape or position is required because of the shrinkage that
occursduring the PDMS curing process. In this study, we analyzed
the effects of processing parameters in the PDMSreplication process
on the shrinkage of the final structure. Although the shrinkage can
be decreased by decreas-ing the curing temperature, this reduction
also increases the unnecessary curing time. To minimize the
inherentshrinkage in the PDMS replica without an accompanying
curing time increase, we propose a PDMS replicationprocess on a
high modulus substrate (glass and polymer films) with compression
pressure, in which the adhe-sion force between the substrate and
the PDMS, and the compression pressure prevent shrinkage during
thecuring process. Using the proposed method, a PDMS replica with
less than 0.1% in-plane and vertical shrinkagewas obtained at a
curing temperature of 150C and a curing time of 10 min. 2014
Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI:
10.1117/1.JMM.13.3.033006]
Keywords: polydimethylsiloxane; shrinkage; polydimethylsiloxane
on glass substrate; polydimethylsiloxane on polyethylene
tereph-thalate film; shrinkage compensation.
Paper 14024 received Mar. 12, 2014; revised manuscript received
Jun. 18, 2014; accepted for publication Jul. 14, 2014;
publishedonline Aug. 5, 2014.
1 IntroductionDuring the last decade, optical, biomedical, and
chemicaldevices have been revolutionized by the many
well-knownsilicon and glass-based micro/nanofabrication
technologies.However, the use of silicon and glass-based
micro/nanofab-rication techniques is limited in some applications
due totheir complexity and high process cost.13 By
comparison,polymer micro/nanoreplication techniques have
manyadvantages, such as a wide range of available materials, easeof
processing, biomedical compatibility, and low cost. Due tothese
advantages, polymer micro/nanoreplication processesare receiving
more attention in many different fields, suchas photonic devices,
high density data storage devices, elec-tronics, plasmonics, and
chemical and biological sensors.48
Since the polymer micro/nanoreplication techniques
wereintroduced by Whitesides in 1998 as soft lithography,
poly-dimethylsiloxane (PDMS) began to play an important role
inmicro/nanopolymer replication technologies.9,10 In
polymerreplication processes, PDMS is widely used as a moldmaterial
because of its superior properties when comparedwith metal or
silicon mold, such as low surface energy(22 to 24 dyncm), high
optical transparency (85%), inex-pensiveness, flexibility, chemical
inertness, high durability,and an easy fabrication process.1113 The
excellent opticaltransparency and biocompatibility of PDMS make it
attrac-tive not only as a mold material but also as a
componentmaterial in various fields.14,15 PDMS
micro/nanostructuresare fabricated by polymerization of monomer
with initiatorand shrinkage typically occurs during the curing
process.Because of the shrinkage during the fabrication
process,
the application of PDMS molds is limited; micro/nanocon-tact
printing or imprinting processes require very precisealignment.
Also, PDMS micro/nanostructures cannot beapplied to devices that
require either high dimensional accu-racy or further assembly
processes with other devices.16,17
The shrinkage of PDMS during its fabrication process
isinfluenced by the processing conditions, and severalapproaches
have been employed to minimize the shrinkageof PDMS
micro/nanostructures. One well-known method tominimize the
shrinkage is to use a low curing temperature.However, a low curing
temperature requires long curingtimes because the polymerization
time of PDMS is inverselyproportional to the curing temperature
(more than 24 h isrequired to polymerize PDMS at room
temperature).18
Furthermore, the use of a low-temperature curing processalso
decreases the mechanical properties of the final-polym-erized
PDMS,19,20 which negatively affects the durability ofPDMS molds and
devices. A simple and straightforwardshrinkage compensation method,
in which the initial tem-plate pattern was enlarged considering the
shrinkage ratioof PDMS, was widely used.21 However, a strict PDMS
cur-ing condition should be maintained to obtain accurate
patterndimensions after shrinkage because the shrinkage ratio
ofPDMS is sensitive to the curing conditions. Since thePDMS curing
is manually conducted in most laboratoryexperiments, it is almost
impossible to perfectly compensatethe shrinkage using the
redesigning method of template pat-tern. Therefore, a method to
eliminate the shrinkage ofPDMS is required. In this research, a
method to fabricaterapid-cure PDMS microstructures with near-zero
in-planeshrinkage was proposed using a PDMS replication process
*Address all correspondence to: Tae-Hyoung Kim, E-mail:
[email protected];Seok-min Kim, E-mail: [email protected]
0091-3286/2014/$25.00 2014 SPIE
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13(3)
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on a high modulus substrate. The basic shrinkage
character-istics of the PDMS replica at various curing conditions
wereinvestigated using the UV replicated template; the
feasibilityof the proposed rapid-cure PDMS replication method
withnear-zero shrinkage was examined by comparing the shrink-age
amounts of replicated PDMS microstructures on a poly-ethylene
terephthalate (PET) film and glass substrates tothose of the
conventional PDMS replica.
2 Experimental Method to Examine the Shrinkageof a PDMS
Replica
To examine the effects of processing conditions on theinduced
shrinkage, the experimental conditions wereselected to minimize the
influences of other processingparameters. Various materials and
fabrication processescan be used to prepare the template for the
PDMS replicationprocess, and a single template can be reused many
times. Inthis study, UV-replicated polymer templates using a
UV-curable polyurethane-acrylate (UP088, SKC, Ulsan,Republic of
Korea) from a single micro-patterned siliconmaster were used for
experiments.22,23 The polymer templatewas used only once in the
PDMS replication process to avoidthe influences of shape changes in
the template during therepeated PDMS replication processes. Figure
1 shows aschematic of the fabrication process of the polymer
templateand the PDMS replica. A silicon master pattern was
preparedby conventional photolithography and a reactive ion
etchingprocess, and a self-assembled monolayer was coated onto
thesilicon master as an anti-adhesion layer by dipping the sil-icon
master in 2% dimethyldichlorosilane dissolved in
octa-methylcyclooctasilane.22,23 The replicated PDMSmaster
wasobtained from the silicon master, and the cloned
polymertemplates were replicated from the PDMS master using a
UV replication process. Although the
photolithographedphotoresist patterns and the etched silicon
patterns from asingle mask showed good reproducibility and can be
usedas templates for PDMS replicas, the fabrication cost of a
sil-icon-based template is relatively expensive for the
one-time-use template in this study. Furthermore, a slight
difference inpattern height between samples could be caused by
unstablespin coating or reactive etching processes, respectively.
TheUV replication process from a single PDMSmaster pattern isan
appropriate fabrication method for the one-time-use tem-plate in
this study, because it showed high reproducibilityand also provided
a fast and low-cost fabrication methodfor multiple templates. After
preparation of the template, amixture of Sylgard 184 A and B (Dow
Corning, Midland,Michigan) with a 101weight ratio was poured onto
the tem-plate and cured in a convection oven. Finally, the
replicatedPDMS was released from the template and the shrinkage
wasmeasured using a microscope equipped with a motorizedstage
synchronized with a microscope image (STM 6,Olympus, Tokyo,
Japan).
Figure 2(a) shows the fabricated 4-inch silicon master pat-tern
with a 50 50 mm2 micro-pattern area. The patternedarea was divided
into 25 10 10 mm2 unit cells with variousmicrostructures including
lines, rectangular arrays, and dotarrays with pattern widths of 30
to 200 m, as shown inFig. 2(b). To determine the shrinkage of the
PDMS replica,a long distance measurement method was used instead
ofpattern width or pattern pitch measurement using a singlescanning
electron microscope (SEM) or a microscope image,because the long
distance measurement method can providebetter accuracy in terms of
a measurement error ratio to themeasured value. In the short
distance measurement processusing single SEM image, the pixel
selection error due to the
Fig. 1 A schematic diagram of the polydimethylsiloxane (PDMS)
replication process used in this study.
J. Micro/Nanolith. MEMS MOEMS 033006-2 JulSep 2014 Vol.
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Badshah et al.: Development of a rapid cure polydimethylsiloxane
replication process with near-zero shrinkage
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unsharp edge image of pattern is one of the major measure-ment
errors. The pixel selection error shows a great impacton the
shrinkage ratio for short distance measurement, but itsimpact
minimizes in long distance measurement to calculatethe shrinkage
ratio. In this study, the distances between thespecific
microstructures with lengths of 14, 42, and70 mm across the full
4-inch polymer template and repli-cated PDMS were measured using an
optical microscopewith a synchronized stage. Comparing 12 diagonal
distancesbetween the polymer template and the PDMS replica,
12shrinkage values were calculated for a single-PDMS replica.To
allow analysis of variance (ANOVA), more than threesamples were
prepared at each processing condition.
3 Effects of Processing Parameters on the InducedShrinkage of
the PDMS Replica
Among the various processing parameters affecting theshrinkage
of a PDMS replica, the curing temperature andthe PDMS replica
thickness were selected as the designfactors in this research. To
examine the effects of curingtemperature and thickness of the PDMS
replica on theshrinkage, the polymer template was attached to the
bottomof a Petri dish and covered with different amounts ofuncured
PDMS mixtures. The PDMS samples of differentthicknesses on the
templates were then polymerized in aconvection oven at various
temperatures. Figures 3(a)and 3(b) show the changes in PDMS
shrinkage by curingtemperature and thickness. Since the required
curing timefor PDMS is inversely proportional to the curing
tempera-ture, we allowed sufficient curing time for each
experimentaccording to the PDMS manufacturers guide. According
tothe ANOVA results, the curing temperature was the dom-inant
factor affecting the PDMS shrinkage; the effect ofPDMS thickness
was negligible. It was also found thatthe amount of shrinkage was
proportional to the curing tem-perature as shown in Fig. 3(b).
Approximately 2.52%shrinkage (mean value) was obtained from the
PDMS rep-lica cured at a temperature of 150C for 20 min, and
theshrinkage was dramatically decreased to 0.37% at a tem-perature
of 25C for 48 h.
4 Development of a Rapid Cure and Near-ZeroShrinkage PDMS
Replication Process
Although
-
Fig. 4 Schematic of the PDMS molding process with high modulus
materials.
75 100 125 150-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Shri
nkag
e %
Temperature C
PDMS replica without substrate PDMS replica with PET film PDMS
replica with glass
(c)(b)
(d) (e)
(a)
Fig. 5 (a) Comparison of shrinkage ratio between PDMS replicas
without substrate, with glass and withpolyethylene terephthalate
(PET) film at various curing temperatures, and SEM images of (b)
polymermaster obtained from a PDMS master using the UV-replication
method, (c) PDMS replica without sub-strate cured at 150C, (d) PDMS
replica with a PET film cured at 150C, and (e) PDMS replica with
glasssubstrate cured at 150C.
J. Micro/Nanolith. MEMS MOEMS 033006-4 JulSep 2014 Vol.
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Badshah et al.: Development of a rapid cure polydimethylsiloxane
replication process with near-zero shrinkage
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substrate was polymerized in a convection oven with com-pression
pressure of 0.12 kPa. The compression pressure wasapplied to
minimize the thickness of the PDMS layer, whichcan help shrinkage
compensation effects due to the substrate.Furthermore, the applied
compression pressure can alsoimprove the replication quality of a
PDMS replica.20 Asthe high modulus substrates, a soda-lime glass
(Tensilestrength: 19 to 77 Kgmm2), with a thickness of 1 mm,and a
PET film (SH34, SKC, Korea, Tensile strength: 18to 21 Kgmm2), with
a thickness of 188 m, were used.To increase the adhesion properties
between the substrateand PDMS, an O2 plasma treatment was applied
to theglass substrate, and a primer-coated PET film was used.The
shrinkage characteristics of PDMS on the different sub-strates were
compared with the PDMS replica prepared bythe conventional method,
as shown in Fig. 5(a). As shownin Sec. 3, the shrinkage of the PDMS
replica prepared bythe conventional method was increased by
increasing thecuring temperature. However, the PDMS replicas on a
sub-strate showed less than 0.1% shrinkage throughout the
entirecuring temperature range. The slight increase of shrinkage
inPDMS on a substrate cured at a high temperature could be
explained by the thermal expansion of the substrate at
highcuring temperatures; the relatively greater shrinkage ofPDMS on
PET substrates compared with those on glasssubstrates could also be
explained by the difference in thethermal expansion coefficient of
the substrates. The meanmeasured shrinkages of PDMS cured at 150C
were2.52%, 0.09%, and 0.04% for materials without sub-strate, on
PET substrate and on glass substrate, respectively.Figures 5(b)5(e)
show a comparison of the SEM images ofthe line pattern
microstructures on (b) UV replicated polymertemplate, (c) PDMS
without substrate cured at 150C,(d) PDMS on PET substrate cured at
150C, and (e) PDMSon glass substrate cured at 150C. The designed
line width ofthe micro line array was 30 m and the pitch was 60
m.The measured eight-pitch distances were (b) 478.3 m,(c) 466.2 m
(2.52% shrinkage), (d) 477.8 m (0.105%shrinkage), and (e) 478.1 m
(0.042% shrinkage), respec-tively, which are comparable to the
shrinkage valuesmeasured using the long distance measurement
method.The main reason for this large difference between thePDMS
replica with and without substrate may be the adhe-sion force
between the PDMS and the availability of the
Fig. 6 Cross-sectional surface profile of (a) polymer template,
and PDMS replicas (b) without substrate,(c) on a PET film, and (d)
on a glass substrate cured at 150C.
J. Micro/Nanolith. MEMS MOEMS 033006-5 JulSep 2014 Vol.
13(3)
Badshah et al.: Development of a rapid cure polydimethylsiloxane
replication process with near-zero shrinkage
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applied pressure to the high modulus substrate, which
func-tioned to create a permanent bond and prevented the
polym-erization shrinkage of PDMS that typically occurs during
theprocess. The slight difference between the design and mea-sured
pattern shapes of the UV-replicated templates wascaused by the
conventionally cured PDMS master whichwas used as a master in the
UV-replication process.
To examine the vertical shrinkage characteristics of
thereplicated PDMS microstructures, we measured the
three-dimensional (3-D) surface profiles using the confocal
micro-scope (LEXT OLS4100, Olympus, Tokyo, Japan). Figure 6shows
the measured cross-sectional surface profiles of(a) UV replicated
polymer template, and replicated PDMS(b) without substrate, (c) on
PET substrate, and (d) onglass substrate cured at 150C. The
measured pattern heights(mean value) were (a) 3.444 m, (b) 3.350 m
(2.73% ver-tical shrinkage), and (c) 3.411 m (0.09% vertical
shrink-age), and (d) 3.417 m (0.07% vertical
shrinkage),respectively. It was noted that the measured vertical
shrink-age of PDMS cured by conventional method (2.73%) wasalmost
the same as the lateral shrinkage value (2.52%); thevertical
shrinkage of PDMS could also be suppressed by aproposed rapid-cure
PDMS replication method. Althoughthe adhesion force between PDMS
and the substrates cannotsuppress the vertical shrinkage of PDMS,
the compressionforce, which is applied to minimize the thickness of
aPDMS layer on the substrates, can minimize the verticalshrinkage
of PDMS.24 This result clearly shows that the pro-posed rapid cure
PDMS replication approach can minimizenot only the lateral
shrinkage but also the vertical shrinkage.
5 ConclusionA precisely controlled PDMS replication process was
devel-oped to analyze the effects of curing temperature and
thethickness of PDMS on the shrinkage of the final mold.We
confirmed that the curing temperature was the dominantfactor for
the shrinkage in the PDMS replication process, andthe shrinkage
increased proportionally to the curing temper-ature. To realize the
near-zero shrinkage PDMS replica with-out increasing curing time, a
PDMS replication process on asubstrate with a compression pressure
was proposed, and0.04% and 0.1% in plain shrinkage values and
0.07%and 0.09% in vertical shrinkage values were obtained from
thePDMS replicas on glass and PET substrates, respectively.Although
the higher dimensional accuracy was obtainedfrom the replicated
PDMSmicrostructure on a glass substrate,PDMS on glass substrates
loses the flexibility, which is one ofthe advantages of PDMS
microstructures. The PDMS on PETfilm substrate, however, still
provided this flexibility. Based onthese advantages and
disadvantages of the two proposed rapidcure near-zero shrinkage
PDMS replication methods, PDMSon glass substrates for applications
requiring precise align-ment and PDMS on PET film substrates for
applicationsrequiring flexibility seem to be the best options.
AcknowledgmentsThis work was supported by the Human
ResourcesProgram in Energy Technology of the Korea Institute
ofEnergy Technology Evaluation and Planning (KETEP)and was granted
financial resources from the Ministryof Trade, Industry and Energy,
Republic of Korea
(No. 20134030200350) and the Chung-Ang UniversityResearch
Scholarship Grants in 2012.
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Mohsin Ali Badshah received his BSc mechanical engineering
fromthe University of Engineering and Technology (UET)
Lahore,Pakistan, in 2010. He is now pursuing his MS degree from
Chung-Ang University (CAU), Republic of Korea, and is a member of
theNano Manufacturing Technology Laboratory, Chung-AngUniversity.
His research interests include fabrication of nanostructuredevices,
nanotechnology, and biosensors.
Hyungjun Jang is pursuing his BSc in mechanical engineering
fromChung-Ang University (CAU), Republic of Korea, and he is a
memberof the Nano Manufacturing Technology Laboratory,
Chung-AngUniversity. His research interests include fabrication of
micro-
J. Micro/Nanolith. MEMS MOEMS 033006-6 JulSep 2014 Vol.
13(3)
Badshah et al.: Development of a rapid cure polydimethylsiloxane
replication process with near-zero shrinkage
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nanostructure devices, glass microfluidics channel using
vitreous car-bon mold and optical devices.
Young Kyu Kim received his BS degree in mechanical
engineeringfrom Chung-Ang University in 2013. He is now pursuing
his MS andPhD degrees in Chung-Ang University and is a member of
the NanoManufacturing Technology Laboratory, Chung-Ang University.
Hisresearch is fabrication of all-glass micro Fresnel lens using
vitreouscarbon mold for optical lens of concentrator
photovoltaic.
Tae-Hyoung Kim received his PhD degree in informatics from
KyotoUniversity, Japan, in 2006. He is currently an associate
professor atthe School of Mechanical Engineering, Chung-Ang
University. His
current research interests include robust control, multiagent
system,particle swarm optimization, system identification, model
predictivecontrol, iterative learning control, and systems
biology.
Seok-min Kim received his PhD degree from the School of
Mechani-cal Engineering at Yonsei University, Seoul, Republic of
Korea. He iscurrently an associate professor in the School of
Mechanical Engi-neering at Chung-Ang University, Seoul. His current
research inter-ests include design and fabrication of
micro/nanostructures foroptical biosensors, micro fluidic chips,
concentrator photovoltaic sys-tem, digital display, LED lighting,
and enhanced boiling heat transfersurface.
J. Micro/Nanolith. MEMS MOEMS 033006-7 JulSep 2014 Vol.
13(3)
Badshah et al.: Development of a rapid cure polydimethylsiloxane
replication process with near-zero shrinkage
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