Replicare PDMS_2014

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

J. Micro/Nanolith. MEMS MOEMS 033006-1 JulSep 2014 Vol. 13(3)

J. Micro/Nanolith. MEMS MOEMS 13(3), 033006 (JulSep 2014)


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.


Badshah et al.: Development of a rapid cure polydimethylsiloxane replication process with near-zero shrinkage


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

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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.




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.




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-




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.




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