CF4 Plasma Treatment of Poly(dimethylsiloxane): Effect of Fillers and

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Langmuir 2005, 21, 8905-8912

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CF4 Plasma Treatment of Poly(dimethylsiloxane): Effect of Fillers and Its Application to High-Aspect-Ratio UV Embossing Y. H. Yan,†,‡ M. B. Chan-Park,*,†,‡ and C. Y. Yue‡ School of Chemical and Biomedical Engineering and The Singapore-MIT Alliance, Innovation in Manufacturing Systems and Technology Program, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received June 13, 2005 Surface modification of poly(dimethylsiloxane) (PDMS) was carried out via CF4 plasma treatment. The test PDMS used contains significant amounts of quartz and silica fillers, while the control material is the same PDMS with quartz removed by centrifugation. Fluorination accompanied with roughening was produced on both PDMS surfaces. With short plasma times (15 min or less), a macromolecular fluorocarbon layer was formed on the PDMS surfaces because of the dominant fluorination, leading to significant increase in F concentration, decrease of surface energy, and some roughening. With intermediate plasma times (15-30 min), dynamic balance between fluorination and ablation was achieved, leading to a plateau of the surface roughness, fluorine content, and [F-Si]/[F-C] ratio. At our longest investigated plasma time of 45 min, the plasma ablated the fluorinated covering layer on the PDMS surfaces, leading to significant increase in roughness and [F-Si]/[F-C] ratio and decrease of surface F concentration. The effect of additional quartz in the test PDMS on surface F concentration, [F-Si]/[F-C] ratio, and roughness was dramatic only when ablation was significant (i.e., 45 min). The obtained Teflon-like surface displays long-term stability as opposed to hydrophobic recovery of other plasma-treated PDMS surfaces to increase hydrophilicity. On the basis of the optimized plasma treatment time of 15 min, a microstructured PDMS mold was plasma treated and successfully used for multiple high-aspect-ratio (about 8) UV embossing of nonpolar polypropylene glycol diacrylate (PPGDA) resin.

1. Introduction It has long been recognized that surface modification of polymeric materials is often essential in order to impart desirable surface characteristics while preserving bulk properties. Many reports1-6 describe the employment of CF4 plasma treatments to make fluorinated polymer surfaces exhibiting desirable properties of low surface energy, chemical inertness, and low coefficient of friction. However, CF4 plasma treatment of poly(dimethylsiloxane) (PDMS) to lower its surface energy through fluorination has not previously been reported, although Garra et al.7 described the etching effects of O2 and CF4 mixture plasma on PDMS. PDMS-based elastomers have recently been widely used in micro- and nanotechnologies.8-12 For example, micro* Corresponding author. Email: [email protected]. Telephone: (65) 6790 6064. Fax: (65) 6792 4062. † School of Chemical and Biomedical Engineering, Nanyang Technological University. ‡ The Singapore-MIT Alliance, Innovation in Manufacturing Systems and Technology Program, School of Mechanical and Aerospace Engineering, Nanyang Technological University. (1) Ryan, M. E.; Badyal, J. P. S. Macromolecules 1995, 28, 1377. (2) Hopkins, J.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 4261. (3) Hopkins, J.; Badyal, J. P. S. Langmuir 1996, 12, 3666. (4) Godfrey, S. P.; Kinmond, E. J.; Badyal, J. P. S. Chem. Mater. 2001, 13, 513. (5) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (6) Riekerink, M. B. O.; Terlingen, J. G. A.; Engbers, G. H. M.; Feijen, J. Langmuir 1999, 15, 4847. (7) Garra, J.; Long, T.; Currie, J.; Schneider, T.; White, R.; Paranjape, M. J. Vac. Sci. Technol. A 2002, 20, 975. (8) Owen, M. J. In Siloxane Polymers; Clarkson, S., Semlyen, J. A., Eds.; Prentice Hall: Engelwood Cliffs, New Jersey, 1993. (9) Xia, Y. N.; Whitesides, G. M. Angew. Chem. Int. Ed. 1998, 37, 550. (10) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, 159.

structures formed from PDMS have been used in microcontact printing,10 microfluidics,11 and UV embossing.12 For most applications, including some UV embossing,12 PDMS was used “as obtained” because of its elasticity, chemical inertness, and low interfacial free energy. In other cases, however, the PDMS surfaces had to be modified to tune their surface properties to meet the demands of specific applications. Typically, the surface modifications focus on increasing rather than decreasing the hydrophilicity by O2 plasma treatment13 or UV or UV/ O3 exposure.14,15 In our laboratory, PDMS has been investigated as a mold material for high-aspect-ratio (5 and above) UV embossing. The antiadhesion surface properties created by CF4 plasma treatment are desirable to promote clean demolding and improve PDMS mold life for high-aspectratio UV embossing, especially of relatively nonpolar polymers. Further, CF4 plasma treatment also results in surface roughening, which greatly aids the UV curable liquid resin filling of the high-aspect-ratio microstructures in a PDMS mold because chemical fluorination itself, without the surface roughening, results in increased hydrophobicity that resists resin filling. Both resin filling and easy demolding are critical for good replication of the microstructured PDMS mold by the UV-curable resin employed in UV embossing. Though many other methods such as direct exposure to fluorine gas,16 ion sputtering,17 (11) Chiu, D. T.; Pezzoli, E.; Wu, H. K.; Stroock, A. D.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 2961. (12) Chan-Park, M. B.; Yan, Y. H.; Neo, W. K.; Zhou, W. X.; Zhan, J.; Yue, C. Y. Langmuir 2003, 19, 4371. (13) Ferguson, G. S.; Chaushury, M. K.; Biebuyck, H.; Whitesides, G. M. Macromolecules 1993, 26, 5870. (14) Huck, T. S. W.; Bowden, N.; Onck, P.; Pardoen, T.; Hutchinson, J. W.; Whitesides, G. M. Langmuir 2000, 16, 3497. (15) Ouyang, M.; Yuan, C.; Muisener, R. J.; Boulares, A.; Koberstein, J. T. Chem. Mater. 2000, 12, 1591.

10.1021/la051580m CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005

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plasma polymerization,18-20 vapor deposition,21 and use of fluorine-containing release agents in the polymer formulation22 have also been employed to produce antiadhesion Teflon-like surfaces, to our knowledge, there is no reported one-step method that can easily achieve both controllable hydrophobicity and increased surface roughness in PDMS. The surface fluorination and roughness increases depend also on the filler in the PDMS. Quartz and silica fillers significantly improve the mechanical properties of the cured PDMS.8 Sufficiently high tear strength is necessary for PDMS to be used in high-aspect-ratio microstructural molds. Silastic J RTV, filled with quartz and silica particles, has high tear strength (15 kN/m) and hardness (Shore A, 56).23 Sylgard-184, another similar PDMS also from Dow Corning, filled only with silica,23 has lower tear strength and hardness (50 Shore A hardness and 2.6 kN/m tear strength).23 This paper explores the simple but effective CF4 plasma treatment of PDMS to increase not just surface hydrophobicity but also roughness. A commercial PDMS kit employing quartz and silica fillers, specifically Silastic J RTV from Dow Corning, was selected as the PDMS material, and the same PDMS kit with the quartz filler removed by high-speed centrifugation was used as the control material. Under a constant power supply, the surface modification of PDMS as a function of CF4-plasmatreatment time was investigated. Detailed surface analysis using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements were performed. The effect of quartz filler on the surface modification and air aging was also studied. The optimum plasma treatment condition was subsequently applied to UV microembossing. The embossed microstructures have microwalls measuring 1 mm long, 11 µm wide, and 83 µm high (i.e., aspect ratio of 8), separated by 78-µm-wide microchannels. All tests and characterizations used flat PDMS films, except for microembossing, which used microstructured PDMS molds. 2. Experimental Section Materials. Silastic J RTV from Dow Corning was selected as the PDMS material and used as received. It was supplied in two parts: Silastic J RTV base and Silastic J RTV curing agent. These comprised vinyl-terminated PDMS as base, methylhydrogen siloxane as curing agent, and chromic oxide as catalyst.23 As obtained, both base and curing agent contained 15-40 wt % dimethylvinylated and trimethylated silica fillers; additionally, the base contained 30-60 wt % quartz. An identical PDMS kit with the quartz filler removed was used as the control material. To remove the quartz filler, the needed quantity of base was diluted in heptane to form an approximately 30 wt % solution, then the solution was centrifuged (Jouan KR25i, Thermo Electron Corp) at 18 000 rpm for 1 h, and the supernatant was filtered through a 0.22-µm Millex-GP polyethersulfone filter (Millipore Corp). After volatilizing most of the solvent in a dust-free hood at room temperature for 2 h and in a vacuum oven with reduced (16) Clark, D. T.; Feast, W. J.; Musgrove, W. K. R.; Ritchie, I. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 857. (17) Ryan, M. E.; Fonseca, J. L. C.; Tasker, S.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 7060. (18) Jaszewski, R. W.; Schift, H.; Schnyder, B.; Schneuwly, A.; Gro¨ning, P. Appl. Surf. Sci. 1999, 143, 301. (19) Sandrin, L.; Silverstein, M. S.; Sacher, E. Polymer 2001, 42, 3761. (20) Lau, K. K. S.; Gleason, K. K. J. Phys. Chem. B 1998, 102, 5977. (21) Beck, M.; Graczyk, M.; Maximov, I.; Sarwe, E.-L.; Ling, T. G. I.; Keil, M.; Montelius, L. Microelectron. Eng. 2002, 61-62, 441. (22) Johnston, E.; Bullock, S.; Uilk, J.; Gatenholm, P.; Wynne, K. J. Macromolecules 1999, 32, 8173. (23) Product information on Silastic J RTV and Sylgard-184 obtained from Dow Corning can be retrieved from the website at http:// www.dowcorning.com.

Yan et al. Scheme 1. Schematic of a Cavity Formed by a Flat Glass Sheet or SU-8 Master (for microstructured mold only) and Teflon Picture Frame

pressure (30 mTorr) at 65 °C for 4 h, the residue was collected and used as the base of the control PDMS kit. Polypropylene glycol diacrylate (PPGDA) with average molecular weight 900 was purchased from Sigma-Aldrich. Trimethylolpropane triacrylate (TMPTA) was obtained from Sartomer Chemicals and used as a high-functionality cross-linker of PPGDA. A reactive silicone hexaacrylate supplied as EB1360 by UCB Chemicals was used as a release agent. The UV curing of embossings was initiated with the photoinitiator of 2,2-dimethoxy2-phenylacetophenone, supplied as Irgacure 651 by Ciba Chemicals. PDMS Network Preparation. The Silastic J RTV base and its curing agent were thoroughly mixed in a 10:1 weight ratio. Air bubbles trapped in the prepolymer mixture were removed by degassing at 30 mTorr for 30 min. For preparing flat films, a horizontal spacer mold with a cavity formed from a flat glass bottom and a Teflon picture frame cut-out 1-mm thick as the peripheral spacer (Scheme 1) was used. For fabricating microstructured child molds for UV embossing, a SU-8 master mold (an epoxy photoresist supplied by MicroChem., MA, USA) was placed on the glass bottom (Scheme 1). The degassed prepolymer mixture was poured into the cavity and further degassed at 30 mTorr for another 30 min. Another piece of glass sheet was carefully lowered onto the prepolymer mixture from one edge to realize continuous contact and prevent formation of air bubbles at the interface. Prior to their use to form the casting chamber, the glass sheets and Teflon spacer were cleaned in ultrasonic baths of acetone for 15 min, ethanol for 15 min, and deionized (DI) water for 30 min. The prepolymer was cured at 65 °C for 16 h to produce a PDMS flat sheet or mold. Before use, the cured PDMS films and molds were cleaned in successive ultrasonic baths containing acetone, ethanol, and DI water for 15 , 15, and 30 min, respectively, and then dried under vacuum of 30 mTorr at 60 °C for 24 h. CF4 Plasma Treatment. As is well-known, plasma treatment using Ar (high atomic number) can effectively create stable free radicals on a polymer surface.24 When subsequently exposed to CF4 plasma, the surface free radicals can rapidly react with the F-containing plasma radicals, accelerating the fluorination. Therefore, in this study, a 5-min Ar-plasma pretreatment was carried out before the CF4 plasma treatment. Plasma treatment of PDMS films was carried out at 13.56 MHz in a Technics 800-II Series reactive ion etching system. Prior to plasma treatment, the chamber was scrubbed with 2-propanol, dried, and further cleaned using a 21.3 sccm (standard cubic centimeter per minute) Ar plasma at 350 W for 30 min. The sample was then placed into the chamber, followed by evacuation to a base pressure of 30 mTorr. Ar gas was introduced at a flow rate of 21.3 sccm, and the glow discharge was ignited at 200 W. After a 5-min Ar-plasma pretreatment, the power supply was switched off, and the system was evacuated to base pressure again. CF4 was then introduced at a flow rate of 13.4 sccm, and the plasma was ignited at 70 W. Upon completion of CF4 plasma treatment, the power supply was switched off while CF4 was allowed to flow through the chamber for an additional 5 min. Finally, to reduce the physical adsorption of CF4 on the surface, each sample was purged with N2 for 30 min under vacuum of 30 mTorr. UV Embossing. A PPGDA-based resin consisting of 69/29/ 2/0.2 w/w PPGDA, TMPTA, EB1360, and Irgacure 651 was (24) Hudis, M. In Techniques and Applications of Plasma Chemistry; Hollahan, J. R., Bell, A. T., Eds.; John Wiley & Sons: New York, 1974.

CF4 Plasma Treatment of Poly(dimethylsiloxane) dispensed on a PDMS mold, covered with a 300-gauge Melinex 454 polyester (PET) film, and irradiated at an area-averaged UV intensity of 16 mW/cm2 at 365 nm for 96 s.12 An untreated and a 15-min CF4-plasma-treated PDMS molds were used. After curing, the embossed film was slowly peeled from the PDMS mold with a small peeling angle of a few degrees. Characterization. The particle size of the quartz filler removed from Silastic J RTV base was characterized using scanning electron microscopy (SEM, the JEOL JSM-5600). Samples for SEM were obtained by drying several droplets from a quartz/acetone suspension onto a precleaned silicon wafer. The particle size was analyzed using Smile View software supplied with the SEM instrument. Over 55 measurements in 7 SEM images taken from different locations on the sample surface were made. The silica filler particle size was characterized with laser light scattering (LLS) after filtration of the control base to remove larger particles; some base of the control PDMS kit was diluted in heptane to form 50 wt % solution and was filtered through a 0.22-µm Millex-GP polyethersulfone filter to remove dust. A Brookhaven ZataPlus particle sizer was used to perform a dynamic LLS experiment and obtain the particle size. The light source was a He-Ne laser with a wavelength of 671 nm. For surface chemistry analysis, a Kratos Ultra XPS (X-ray photoelectron spectroscopy) system was used with a monochromatic Al KR1,2 X-ray source operating at 15 kV and 10 mA. The sampling area on the specimen was typically about 700 µm by 400 µm. A step-scan interval of 1 eV was used for wide scans, and 0.1 eV for high-resolution scans; acquisition times were 60 s at both resolutions. The core-level spectra were obtained at a photoelectron takeoff angle of 90°, measured with respect to the sample surface. All binding energies (BEs) were referenced to the C 1s neutral carbon peak at 285.0 eV to compensate for surface-charging effects. Elemental stoichiometries were determined from the high-resolution peak areas using Shirley background subtraction. AFM (atomic force microscopy) measurements were carried out with a Nanoscope IIIa atomic force microscope. All scans were performed in air using the tapping mode. The surface roughness measure Ra, defined as the arithmetic average of the absolute values of the surface height deviations measured from the mean plane, was obtained by evaluation of 50 µm × 50 µm scans according to

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Figure 1. Typical SEM image of quartz filler particles on a silicon wafer substrate.

3.1. Argon Plasma Pretreatment. PDMS with the quartz filler is used here. In this work, the plasma fluorination was typically done with a 5-min Ar-plasma treatment followed by a CF4 plasma treatment. In another set of experiments, the plasma fluorination was carried out with the absence of Ar plasma treatment. XPS and AFM results indicate that, under the same condition of CF4 plasma treatment but without Ar plasma treatment, the rate of fluorination is much slower and the increase in surface roughness is less significant. Without the Ar

plasma treatment, the fluorine content on the PDMS surface increased from 0 to 4.2%, 37.7%, 53.5%, 58.1%, and 15.2% with CF4-plasma-treatment times of 5, 10, 15, 30, and 45 min, respectively. With Ar plasma treatment and similar treatment times, the fluorine content was 42.8%, 54.0%, 65.6%, 62.5%, and 16.1%, respectively. Without Ar plasma, the maximum F content was 58.1% with 30 min CF4 plasma treatment, while with a 5-min Ar-plasma treatment, the fluorine content reached a maximum of 64.6% with 15-min CF4-plasma treatment. This suggests that the Ar plasma treatment introduces some free radicals onto the PDMS surface27,28 and facilitates the following CF4 plasma fluorination. Additionally, the average surface roughness of PDMS increased 1.5-, 1.7-, 1.8-, 2.2-, and 3.2-fold with 5, 10, 15, 30, and 45 min, respectively, of CF4 plasma treatment without the Ar plasma treatment, but 2.2-, 2.5-, 2.9-, 3.1-, and 6.0-fold with Ar plasma treatment, respectively. More importantly, for untreated and only CF4-plasma-treated (i.e., without the Ar plasma treatment) PDMS molds, SEM results reveal that the UV embossings do not have good replication fidelity because of poor resin filling in the microstructures. Therefore, unless otherwise specified, subsequent plasma fluorination refers to CF4 plasma treatment, which was preceded by a 5-min Ar-plasma treatment. 3.2. Fillers. Two kinds of fillers, quartz and silica, are ingredients in the as-supplied Silastic J RTV PDMS. Figure 1 shows a typical SEM image of quartz filler particles, which were separated from Silastic J RTV base, on a silicon substrate. The typical quartz particle is almost spherical, and the average particle size is 1.2 ( 0.3 µm. Because of the excellent solubility of the small dimethylvinylated and trimethylated silica in heptane, the silica filler was not removed from the PDMS formulation during the centrifugation. To confirm this point, a control experiment was carried out using the same centrifugation parameters with 30 wt % solution of Sylgard-184 base (which has the same silica filler as Silastic J RTV, but no quartz)23 in heptane. No silica deposits were found, even after prolongation of the centrifugation time by 2 h. Therefore, the silica filler is present in both as-received and de-quartz-filler control Silastic J RTV materials. The particle size of the silica filler was measured to be 24 ( 6 nm using dynamic LLS.

(25) Ulman, A. An Introduction to Ultrathin Film: From LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (26) Yan, Y. H.; Chan-Park, M. B.; Chew, C. P.; Yue, C. Y. J. Electrochem. Soc. 2004, 151, C685.

(27) Lee, S. D.; Hsiue, G. H.; Chang, P. C. T.; Kao, C. Y. Biomaterials 1996, 17, 1599. (28) Volcker, N.; Klee, D.; Hocker, H.; Langefeld, S. J. Mater. Sci.: Mater. Med. 2001, 12, 111.

Ra )

1

N

∑|Z | N j

j)1

where N is the number of points, and Zj is the height of the jth point with respect to the mean plane. Every reported Ra value is an average of at least three images scanned at different locations on the sample surface. The surface energies of PDMS films were calculated using the Owens-Wendt geometric mean equation25 and static contact angles of deionized (DI) water and diiodomethane, which were measured using the method reported previously.26 The surfaces of the embossed microstructure and mold were characterized using a SEM (JEOL JSM-5600) and an optical profilometer (Wkyo NT 2000).

3. Results and Discussion

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Figure 2. Typical (a) C 1s and (b) F 1s spectra of CF4-plasmatreated PDMS control sample (15-min CF4-plasma-treated PDMS without quartz filler).

3.3. XPS Analysis. XPS analysis gives the elemental C/Si/O ratios of 2.2:1.0:1.1 and 2.0:1.0:1.0 for untreated PDMS samples with and without quartz filler, respectively. The values are in good agreement with the theoretical C/Si/O ratio of PDMS of 2.0:1.0:1.0. In the C 1s spectrum, only one peak component is present at 285.0 eV, corresponding to the C-H/C-Si species. CF4 plasma treatment of the PDMS materials resulted in a substantial amount of F incorporation onto the surface. This was accompanied by a dramatic change in the C 1s spectrum (Figure 2a) due to the formation of C-CFn (286.5 eV), C-F (287.8 eV), CF-CFn (289.8 eV), C-F2 (291.8 eV), and C-F3 (293.9 eV) species1-5 at the expense of the C-H/C-Si functionalities. As indicated in Figure 2b, the incorporated F bonds to Si (F-Si species at 685.6 eV)29,30 as well as C (F-C species at 688.2 eV). Fluorosilyl species existing on silicon surfaces after exposure to F-containing plasmas have been proposed by many researchers.31-33 The [F-Si]/[F-C] ratio and the F content as a function of CF4-plasma-treatment time are shown in Figure 3 parts a and b, respectively. No significant effects of quartz filler on the evolution of the [F-Si]/[F-C] ratio and the F content were observed up to 30-min CF4-plasma-treatment time; in this range of treatment times, F-C bonding predominates over F-Si. At 45-min plasma-treatment time, however, there is a dramatic change in the bond ratio (29) Plank, M. O. V.; Jiang, L. D.; Cheung, R. Appl. Phys. Lett. 2003, 83, 2426. (30) Moulder, J. F. Handbook of X-ray Photoelectron Spectropy: A Reference Book of Standard Spectra for Indentification & Interpretation of XPS; Physical Electronics: Minnesota, 1995. (31) Vasile, M. J.; Stevie, F. A. J. Appl. Phys. 1982, 53, 3799. (32) Houle, F. A. Chem. Phys. Lett. 1983, 95, 5. (33) McFeely, F. R.; Morar, J. F.; Shinn, N. D.; Landgren, G.; Himpsel, F. J. Phys. Rev. B 1984, 30, 764.

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Figure 3. (a) The [F-Si]/[F-C] ratio and (b) surface fluorine content of PDMS samples as a function of CF4-plasma-treatment time.

trend for the quartz-filled sample, but essentially no change for the sample without quartz filler. At 45 min, there is also a dramatic decrease in the F content of the quartz-filled sample from a maximum of 65% to around 15%, but a much smaller decrease for the sample without quartz filler. As reported in previous studies,2,34 the reactive species in CF4 plasma are primarily fluorine atoms with a small concentration of complex fluorocarbon species. Fluorine atoms and F-substituted methyl species (-CFxH(3-x), 0 e x e 3) can graft onto a polymeric surface via hydrogen replacement and the opening of unsaturated bonds to form CF, CF2, and CF3 functionalities,2 whereas high-molecularweight fluorocarbon species (-CmFn, m > 1) can form a thin film of fluorocarbon macromolecules on the substrate surface.34 In the case of PDMS, however, -CH3 replacement is highly probable, considering the lower bond energy of C-Si than that of C-H. In addition to the direct attachment of a F and F-substituted methyl group to Si, resulting in C-F3, C-F2, and C-F species in Figure 2b, fluorocarbon macromolecules are also attached to Si, as indicated by the additional presence of C-CFn and CFCFn species in the C 1s spectra (Figure 2a). With increasing CF4-plasma-treatment time, more macromolecular fluorocarbons arising from CmFn (m > 1) species are attached to Si, leading to the observed decrease of the [F-Si]/[FC] ratio until a CF4-plasma-treatment time of 30 min (Figure 3a). Besides CF4 plasma fluorination, ablation or etching also occurs on polymeric surfaces.35 The two reactions seem to be parallel and competitive, and the relative rates of fluorination and ablation are dependent on treatment time under constant power supply for a fixed polymer. At first, fluorination predominates over ablation (34) Schabel, M. J.; Peterson, T. W.; Muscat, A. J. J. Appl. Phys. 2003, 93, 1389. (35) Anand, M.; Cohen, R. E.; Baddour, R. F. Polymer 1981, 22, 361.

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Figure 4. AFM images of PDMS samples (A) with and (B) without quartz filler as a function of CF4-plasma-treatment time: a, 0; b, 15; and c, 45 min. Table 1. Surface Elemental Contents of 15-min CF4-Plasma-Treated Samples by XPS Analysis as a Function of Duration of Exposure to Air exposure time fresh after 5 days after 15 days after 35 days

PDMS with quartz filler

PDMS without quartz filler

C 1s Si 2p O 1s F 1s

C 1s

Si 2p

O 1s

F 1s

18.7 19.7 19.3 20.9

18.9 21.2 22.4 23.7

4.8 5.2 5.3 5.3

18.8 19.5 19.6 20.1

57.5 54.2 52.7 50.9

5.8 5.9 6.5 6.4

10.8 11.2 12.0 12.4

64.7 63.2 62.2 60.3

so that the fluorine content increases rapidly (Figure 3b). Beyond a critical treatment time, the two reactions reach a dynamic near-equilibrium with slowly varying F content on the surfaces (around 15-30 min for our case, Figure

3b).5,36 For PDMS materials with fillers, the plasmas gradually ablate the fluorinated covering layer and unveil the embedded silica and/or quartz. The increased exposure of silica particles in the control sample without quartz accounts for a substantial increase of [F-Si]/[F-C] ratio at a 45 min treatment time (Figure 3a) and a corresponding slight decrease in F content (Figure 3b). The corresponding trends in the quartz-filled sample are very dramatic at 45 min. That is because the total content of silica and quartz in Silastic J RTV is much larger than the content of silica in the control sample. (36) Park, S. H.; Kim, S. D. Polym. Bull. 1998, 41, 479.

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Figure 5. Surface roughness of PDMS samples as a function of CF4-plasma-treatment time. (The parenthesized numbers are the Ra increase ratios, which are defined as the ratios of the Ra of CF4-plasma-treated PDMS samples to that of untreated PDMS samples.)

Figure 6. Surface energy of PDMS samples as a function of CF4-plasma-treatment time. Solid line: apparent surface energy; dashed line: true surface energy.

The effect of air aging on the surface elemental analysis of 15-min CF4-plasma-treated PDMS samples as a function of air aging time is summarized in Table 1. With prolonged exposure to air, the C and O contents generally increase, while the F content gradually decreases, and the Si content remains nearly constant. After exposure to air for 5 weeks, the F contents are 60.3% (93.2% remains of the initial F content) and 50.9% (88.5% remains of the initial F content), respectively, for the samples with and without quartz filler. Compared with previous reports37-40 in which Ar, H2, N2, or O2 plasma treatment of PDMS samples greatly increases their surface hydrophilicity, but the effect is shortlived and complete hydrophobic recovery was observed in several days, the data reported here indicate that the Teflon-like surface produced by CF4 plasma treatment has long-term stability. Hydrophobic recovery has been attributed to the rotation of surface polar groups attached to the siloxane backbone into the polymer bulk, exposing methyl groups instead due to surface energy minimization40,41 and surface condensation of silanols.42 Additionally, the migration of hydrophobic and uncured lowmolecular-weight silicone oligomers to the surface through (37) Owen, M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 1063. (38) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991. (39) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H. K.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27. (40) Elidiane, C. R.; Giovana, Z. G.; Nilson, C. C. Plasmas Polym. 2004, 9, 35. (41) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285. (42) Yektafard, M.; Ponter, A. B. J. Adhes. Sci. Technol. 1992, 6, 253.

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cracks in a silica-like layer produced by oxidation of the PDMS surface also plays an important role.37,38 With CF4 plasma, the surface energy of -CmFn (m > 1) or -CFxH(3-x) (0 e x e 3) is lower than that of -CH3, and there is no thermodynamic driving force for hydrophilic recovery. Considering our 15-min CF4-plasma-treated samples, the attachment of bulky sterically hindered macromolecular fluorocarbon (-CmFn, m >1) rather than the smaller F-substituted methyl group (-CFxH(3-x), 0 e x e 3) further reduces the possibility of the rotation of fluorocarbon groups into the PDMS bulk. The small decrease of F concentration with aging is likely due to hydrocarbon contamination. For fluorosilyl groups on the exposed silica and/or quartz fillers, it is nearly impossible for them to rotate into the PDMS bulk due to the anchoring of the Si atoms in the bulk silica or quartz. This is suggested by the experimental fact that the sample with quartz filler, which has more fluorine than the control, remains heavily fluorinated after five weeks exposure in the air. Moreover, the specimen cleaning employed in this study, i.e., successive ultrasonic cleaning in acetone, ethanol, and DI water baths, is believed to get rid of most uncured silicone oligomers. These considerations account for the observed long-term stability of the Teflon-like surface produced by CF4 plasma treatment of our PDMS samples. 3.4. AFM Analysis. The effects of CF4-plasma-treatment time on surface topography of the PDMS samples are selectively presented in Figure 4. The smaller Z scale for the samples without quartz filler (Figure 4B) should be noted. In Figure 5, the roughness data are plotted as a function of CF4-plasma-treatment time. As indicated by Figures 4 and 5, the untreated PDMS sample with quartz filler exhibits a rougher surface (Figure 4A-a) with a Ra value of 14.2 ( 0.7 nm compared with 5.9 ( 1.0 nm for the untreated sample without quartz filler (Figure 4B-a). With an increasing treatment time, a gradual roughening of both the quartz-filled and control PDMS surfaces was observed. A similar surface roughening effect has also been observed on some carbon-based polymers.5,6 With short treatment times (e15 min), the surface roughness of both quartz-filled and control samples increases gradually. Between 15 and 30 min of treatment, the surface roughness does not change much. This state continues to 45 min of treatment for the control sample. For the quartzfilled sample, however, the roughness noticeably increases to a Ra value of 85.4 ( 4.3 nm at 45 min from 44.4 ( 4.2 nm at 30 min. Though the samples with quartz filler show higher Ra values than the control samples, both sets of samples have approximate Ra increase ratios (defined as the ratio of the Ra of the CF4-plasma-treated PDMS sample to that of the untreated PDMS sample) at the same treatment time until 30 min; at 45 min, a larger difference of the Ra increase ratio was noticed, with the value of 6.0 for the quartz-filled sample and 4.1 for the control. With short plasma times (e15 min), the increased roughening is attributed to pronounced fluorination. With intermediate times (0 e 15 min < 45 min), it appears that the rates of fluorination and ablation are equal, leading to almost constant roughness. With long plasma times (g45 min), the roughening effect can be attributed to the exposure of silica and/or quartz fillers, resulting from the progressive ablation of the fluorinated covering layer, leading to increased exposure of the fillers with increasing treatment time; the filler exposure is also evident from the increase of [F-Si]/[F-C] ratio (Figure 3a) and F content (Figure 3b) with CF4-plasma-treatment time. The filler size determines the absolute roughness values, but not the roughening ratio increase for the shorter plasma durations.

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Figure 7. (A) PDMS mold, (B) cured PPGDA embossing adhered to untreated PDMS mold, and (C) PPGDA microstructures replicated from 15-min CF4-plasma-treated PDMS mold (with 5-min Ar-plasma pretreatment). (a) SEM images and (b) optical profilometric measurements of height across white line.

Further investigation suggests that the characteristic scale of the height variations (peak-to-valley) in the quartzfilled highly ablated 45-min CF4-plasma-treated PDMS sample (Figure 4A-c) is about 600 nm, which is about half the particle diameter (1.2 ( 0.3 µm). It is also apparent from the image that the X-Y dimensions of these peaks are 1-2 µm. This is strong evidence that Figure 4A-c is an image of a surface that is composed substantially of the exposed quartz filler particles. For the control surface, the visible topology is dominated at all treatment times by a small number of spikes. These spikes are of height about 200 nm at the longest treatment time of 45 min. The number of 200 nm is very interesting because it is just smaller than the pore size (0.22 µm) of the filter that was used to exclude the quartz particles. This suggests that, although the LLS measurements found the silica particles to be 24 ( 6 nm in size, there is a population of

much larger particles, perhaps silica aggregates or small quartz fragments, still present in the control sample. It is also worth noting that the peaks in Figure 4B-c are very narrow compared with those in Figure 4A-c. This is to be expected because the particles that dominate the topology of the control sample are silica. 3.5. Surface Energy Analysis. Apparent surface energies of untreated PDMS samples with and without quartz filler were calculated using the measured contact angles to be 25.4 and 24.6 mJ/m2 (Figure 6), respectively (similar to the reported value of 21.6 mJ/m2 for pure PDMS with no fillers). 8 The higher values can be attributed to the inclusion of fillers. As expected, CF4 plasma treatment decreased the surface energies of PDMS (Figure 6). For both sets of PDMS samples, apparent surface energies decrease significantly until a treatment time of 15 min. For treatment times longer than 15 min, the curves of

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apparent surface energies versus CF4-plasma-treatment time plateau off. These are consistent with the dependence of the F content on CF4-plasma-treatment time (Figure 3b). This trend extends to 45 min for the sample without quartz filler. For the sample with quartz filler, however, an obvious decrease of apparent surface energy is observed at 45 min, probably because of a sudden increase in F-Si bonds (Figure 3a). Because of their inherent chemical inertness, the fluorocarbon groups on PDMS surface lower the surface energy. However, according to Wenzel’s Law,43 the roughening of an already hydrophobic surface further decreases its wettability. Therefore, in principle, ablation-induced surface roughening can also reduce the surface energy. The true surface energy, due to chemical modification alone, can be calculated from the true contact angle (θt), which can be determined by the equation cos θ ) r cos θt,43 where r is a roughness factor defined as the ratio of actual area to the projected area of the surface, and θ is the measured contact angle. As shown in Figure 6 (dashed curves), the true surface energy monotonically decreases with increased plasma treatment time and is similar to the apparent surface energy trend. The sudden decrease of F content with a 45-min treatment time for PMDS with quartz filler (Figure 3b) does not affect the decrease of surface energy because the macromolecular fluorocarbon layer is ablated but F continues to be attached to the exposed surface Si; the F-Si concentration increases. The contribution of the surface roughening to apparent surface energy can be estimated using ∆E ) |Et - E|, where E is apparent surface energy, and Et is true surface energy. The trend of ∆E versus plasma treatment time (data not shown) mirrors the surface roughness trend (Figure 5), with values ranging from almost 0 to 2.6 and 0.4mJ/cm2 for the samples with and without quartz filler, respectively. 3.6. Application of optimum CF4 Plasma Conditions to UV Embossing. The optimum plasma treatment condition determined on a flat PDMS sample, i.e., 15-min CF4-plasma treatment that results in maximum fluorine content (65%) and modest surface roughness (40.7(4.3 nm), was extended to UV embossing of PPGDA using assupplied PDMS rubber. Within the measurement errors of optical profilometry, there is no distinct difference in dimensions between untreated and CF4-plasma-treated PDMS molds. Parts a and b of Figure 7A show the SEM image and optical profile of a representative PDMS mold. The mold had raised, rectangular, 1-mm-long microwalls measuring approximately 75.6 µm wide at the top and 79.6 µm at the bottom. The mold channels measured approximately 83.9 µm deep, 13.3 µm wide at the top, and 9.0 µm at the bottom. As reported previously,12 it is not easy to demold cured PPGDA microstructures from untreated PDMS molds. PET film usually delaminated from the cured embossing, which remained adhered to the PDMS mold (Figure 7B), spoiling the mold. When a 15-min CF4-plasma-treated PDMS mold was employed, however, easy and successful demolding and good replication were achieved. As shown in Figure 7C, the embossed PPGDA microstructures had microwalls measuring 83.4 µm deep, 8.2 µm wide at the top and 13.2 µm at the bottom, separated by cavities (43) Kinloch, A. J. Adhesion and Adhesives: Science and Technology; Chapman & Hall: London, 1987.

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measuring 80.9 µm wide at the top and 75.5 µm at the bottom. The aspect ratio of the embossed microwalls was calculated to be approximately 8. Each dimensional datum reported here was the average of at least five measurements. Because plasma fluorination of PDMS reduces the adhesion of the embossing to the mold, a clean demolding process was achieved. In our experiments, a 15-min CF4plasma-treated PDMS mold has been reused for UV embossing at least 10 times without further cleaning. The width of the microwalls of the tenth embossing decreased an average of 0.5 µm (6%) compared with the first, while the height decreased by 0.9 µm (1%). This demonstrates that multiple high-aspect-ratio UV embossings with fairly good fidelity can be replicated using a single CF4-plasmatreated PDMS mold. 4. Conclusions CF4 plasma treatment of PDMS surfaces gives rise to extensive surface fluorination together with surface roughening. Surface fluorination is proposed to be due to the F-containing reactive species grafting onto the PDMS surface, leading to increased F content and decreased surface energy. Ablation results from removal of the organic fluorocarbon layer, exposing the silica or quartz filler. With short plasma times (15 min or less), a macromolecular fluorocarbon layer was formed on the PDMS surfaces because of the dominant fluorination, leading to a significant increase in F concentration, decrease of surface energy, and some roughening. With intermediate plasma times (15-30 min), dynamic balance between fluorination and ablation was achieved, leading to a plateau of the surface roughness, fluorine content, and [F-Si]/[F-C] ratio. At our longest investigated plasma time of 45 min, the plasma ablated the fluorinated covering layer on the PDMS surfaces, leading to a significant increase in roughness and [F-Si]/[F-C] ratio and a decrease of surface F concentration. The observed progressive decrease of surface energy with plasma treatment was mainly attributed to the surface fluorination with attachment of F or fluorocarbon to Si. The effect of additional quartz in the test PDMS on surface F concentration, [F-Si]/[F-C] ratio, and roughness is dramatic only when ablation was significant (i.e., 45 min). The obtained Teflon-like surface was relatively stable longterm. PDMS molds with high-aspect-ratio (8) microchannels were used “as is” and with the optimal plasma treatment for UV embossing of PPGDA; the PDMS contains the quartz and silica fillers as supplied. Cured PPGDA replicas could not be removed from the untreated mold. However, replicas were successfully, faithfully, and repeatedly formed and demolded from the treated PDMS mold using the optimal treatment time of 15 min, which resulted in maximum fluorine content (65%) and modest surface roughness (40.7 ( 4.3 nm). CF4 plasma treatment is thus demonstrated to be a highly effective means of antiadhesive surface treatment for multiple-use microstructured PDMS molds for high-aspect-ratio UV embossing. Acknowledgment. This research was supported by an A-STAR (Singapore) grant (project no. 022 107 0004). Y. H. Yan acknowledges the financial support of the Singapore-MIT Alliance Program (IMST) through a postdoctoral fellowship. LA051580M