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Hydrophobically Recovered and Contact Printed Siloxane Oligomers for General-Purpose Surface Patterning Ju-Han Kim, Hyun-Sik Hwang, Si-Woo Hahm, and Dahl-Young Khang* Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea Received May 11, 2010. Revised Manuscript Received June 19, 2010 Hydrophobic recovery of elastomeric polydimethylsiloxane (PDMS) has been well-known in various fields, such as microcontact printing (μCP), microfluidics, and electric insulation, etc., which has been believed to be due to the transfer of out-diffused siloxane oligomers in PDMS. The recovery phenomenon has been used to control surface energy of a substrate, due partly to its nanoscale thickness. In this work, we extend the use of recovered oligomers to a generalpurpose surface patterning process, in combination with both dry and wet pattern transfer processes. The out-diffused and transfer-printed oligomers play exactly the same role of “ink” in the conventional μCP; thus, the present method can be termed as “inkless” microcontact printing (IμCP). Also, the detailed nature of recovered oligomers has been investigated, and they are found to have a molecular weight ∼10 times larger than that of pristine, uncured PDMS oligomers. And the molecular weight distribution is very broad with a polydispersity index of ∼15. Then, we present and discuss various aspects of the IμCP process, such as pattern transfer onto substrate via wet or dry etching, effect of process variables on printing results, minimum feature size achieved by the technique, repeated printing with the same stamp, and the generation of more complex patterns from simpler ones by applying multiple IμCP.
Introduction The hydrophobic recovery of elastomeric PDMS has long been studied,1-7 especially in the field of high-voltage electric insulation. During the outdoor services, the hydrophobic nature of PDMS surface can be lost temporarily by electric discharge or contamination. This “lost” hydrophobicity is usually regained, which is desirable in electrical insulation, in relatively short time (from hours to days) after the events. The out-diffusion of low molecular weight (LMW) PDMS molecules is believed to be the cause of this hydrophobic recovery behavior. On the other hand, the PDMS has been the material of choice for various soft lithographic patterning methods,8-12 where it is used as a stamp or mask. Among them, the microcontact printing (μCP) is a simple, fast, and low-cost process and has been widely used for surface patterning, even on curved surfaces. During μCP, the patterned PDMS stamp that is soaked with an ink is brought into contact with a solid substrate and then removed, leaving the pattern of “ink” on the areas where the stamp was in contact with the substrate. It is now well-known that, after the μCP, there *To whom correspondence should be addressed: e-mail dykhang@ yonsei.ac.kr; Tel þ82 2 2123 5835; Fax þ82 2 312 5375.
(1) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991. (2) Hillborg, H.; Gedde, U. W. IEEE Trans. Dielectr. Electr. Insul. 1999, 6, 703. (3) Kim, J.; Chaudry, M. K.; Owen, M. J.; Orbeck, T. J. Colloid Interface Sci. 2001, 244, 200. (4) Olah, A.; Hillborg, H.; Vansco, G. J. Appl. Surf. Sci. 2005, 239, 410. (5) Lawton, R. A.; Price, C. R.; Runge, A. F.; Doherty, H. J., III; Saavedra, S. S. Colloids Surf., A 2005, 253, 213. (6) Hillborg, H.; Tomczak, N.; Olah, A.; Schonherr, H.; Vansco, G. J. Langmuir 2004, 20, 785. (7) Jia, Z.; Gao, H.; Guan, Z.; Wang, L.; Yang, J. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 1317. (8) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (9) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (10) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (11) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (12) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171. (13) Bohm, I.; Lampert, A.; Buck, M.; Eisert, F.; Grunze, M. Appl. Surf. Sci. 1999, 141, 237.
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remains unexpected “contaminant” PDMS material on the substrate,13-17 which is in general unfavorable, as well as ink material. This contaminant PDMS material is believed to be the low molecular weight (or, small), uncured PDMS oligomers migrated onto the stamp surface. Further, the hydrophobic PDMS surface should be made hydrophilic, in applications such as micro/nanofluidics and bioapplications, so that the waterbased hydrophilic materials can be manipulated with PDMS. For this, short exposure of PDMS to oxygen plasma, discharge, or UV/ozone can be used, but all these modifications result in the hydrophobic state in the end, due to the recovery phenomenon, unless some other steps are taken.18-22 Thus, small uncured PDMS molecules have been believed to be responsible for both the recovery of hydrophobicity and the contaminant left after μCP. In this work, we characterize the size (or molecular weight) of those recovered materials directly and show that the recovered oligomers are much larger than the pristine ones. Also, the recovery of siloxane oligomers is shown to be advantageously utilized as a general-purpose surface patterning technique, especially as an etch mask during pattern transfer steps such as wet chemical etching and even plasma-based dry etching processes (Scheme 1). The patterning technique, termed inkless microcontact printing (IμCP), is very simple and does not involve any specific surface chemistry depending on the substrates (14) Graham, D. J.; Price, D. D.; Ratner, B. D. Langmuir 2002, 18, 1518. (15) Glasmastar, K.; Gold, J.; Andersson, A.-S.; Sutherland, D. S.; Kasemo, B. Langmuir 2003, 19, 5475. (16) Sharpe, R. B. A.; Burdinski, D.; Marel, C.v.d.; Jansen, J. A. J.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. Langmuir 2006, 22, 5945. (17) Thibault, C.; Severac, C.; Mingotaud, A.-F.; Vieu, C.; Mauzac, M. Langmuir 2007, 23, 10706. (18) Makamba, H.; Hsieh, Y.-Y.; Sung, W.-C.; Chen, S.-H. Anal. Chem. 2005, 77, 3971. (19) Vickers, J. A.; Camlum, M. M.; Henry, C. S. Anal. Chem. 2006, 78, 7446. (20) Kim, J.; Chaudry, M. K.; Owen, M. J. J. Colloid Interface Sci. 2000, 226, 231. (21) Delamarch, E.; Donzel, C.; Kamounah, F. S.; Wolf, H.; Geissler, M.; Stutz, R.; Schmidt-Winkel, P.; Michel, B.; Mathieu, H. J.; Schaumburg, K. Langmuir 2003, 19, 8749. (22) Langowski, B. A.; Uhrich, K. E. Langmuir 2005, 21, 6366.
Published on Web 07/01/2010
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Kim et al. Scheme 1. Inkless Microcontact Processa
a The patterned PDMS stamp, replicated form a master, is simply made conformal contact onto substrates, such as Si, SiO2, polymers, photoresists, metals, etc. Then the stamp is peeled off the substrate, leaving behind the recovered PDMS materials on areas where the stamp-substrate were in intimate contact. The patterned substrate can be processed further to transfer pattern into the substrate.
to be patterned, thus expanding its applicability to virtually any kinds of substrates. Also, investigations on the various aspects of the IμCP process, such as stamp preparation conditions, minimum feature size attainable, repeated use of the same stamp, and multiple IμCP on the same substrate, are presented.
Experimental Section PDMS Stamp Preparation and Inkless Microcontact Printing. The elastomeric PDMS stamps (Sylgard 184, Dow) were replicated from patterned photoresist/Si substrates, which were prepared by photolithography. The 10:1 (or 5:1) mixture of base resin and curing agent was poured onto PR/Si master substrate, degassed for ∼20 min, and thermally cured at 70 °C or at room temperature. This patterned PDMS stamp was kept in contact with a cleaned substrate (Si, metals, polymers, etc.) for specific duration. Pattern Transfer. For the transfer of printed patterns of recovered PDMS onto a substrate, both the wet chemical etching (for metals such as Al, Mo, and ITO) and the reactive ion etching (for Si and polymers) were used. For the wet chemical etching, UV/ozone treatment of PDMS-printed substrate was applied for 20 min before dipping the samples into wet chemical etchant bath. The wet chemical etchants for metals were kindly supplied by Dongwoo Fine Chem., Ltd. (MA-SO2 for ITO, MA-SO1B for Al, and MA-TO1 for Mo). The plasma-based dry etching was done by home-built RIE system. Characterization. For the gel-permeation chromatography (GPC) measurements, the PDMS contacted Si substrate was washed in tetrahydrofuran (THF), and this solution was directly injected into GPC. Several pieces of the IμCP-ed Si substrate were washed in THF to make the solution suitable for GPC measurements. As a control, a small amount of base resin (uncured, pristine PDMS; part A in Sylgard 184 kit from Dow) was also dissolved in THF, and the solution was analyzed by GPC. Optical microscopy (Olympus BX51) and atomic force microscopy (DI 3100, Veeco) were used to image the patterned surface.
Results and Discussion Molecular Weight of Recovered Siloxane Oligomers. The low molecular weight PDMS molecules have been believed to be responsible for both the recovery of hydrophobicity and the contaminant left after μCP. Aside from the use of unscientific term such as “small” or “low”, however, there remains a basic question 13016 DOI: 10.1021/la1018746
Figure 1. Molecular weight and its distribution of pristine and recovered siloxane oligomers by gel-permeation chromatography.
on how small these recovered molecules are. The recovered molecules might simply be viewed as the unreacted oligomers present inside the cured 3-dimensionl (3D) network structure of PDMS. From the viewpoint of polymerization or cross-linking reaction kinetics, however, these recovered molecules might have a larger molecular weight (MW) than the pristine oligomers. The curing of PDMS is known as an addition polymerization. As the cross-linking progresses, the viscosity increases and the probability decreases for these reactants to encounter others. At the early stage of the curing reaction, however, oligomers are mobile enough to get together, leading to an increase in the molecular weight. Once the 3D network structure starts to form, the growing chains are prone to be trapped inside, and it becomes difficult for them to participate in the curing reaction anymore. These trapped PDMS chains might be responsible for the recovery behavior. Thus, the recovered molecules might have a MW larger than that of the starting oligomers. To directly measure the MW of the recovered material on PDMS stamp surface, a gel-permeation chromatography (GPC) analysis was carried out. The cured PDMS stamps were kept in contact with a cleaned silicon substrate overnight at room temperature. Figure 1 shows the GPC characterization results. The pristine siloxane oligomers has Mn ∼ 4500 and Mw ∼ 19 000 (PI = 4.24), while the recovered ones have Mn ∼ 58 000 and Mw ∼ 912 000 (PI = 15.6). Here, Mn and Mw denote the number-averaged and weightaveraged molecular weight, respectively, and PI the polydispersity Langmuir 2010, 26(15), 13015–13019
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Figure 2. Patterning results with “inkless” microcontact printing (IμCP). (a) Large area (80 μm 80 μm) AFM image of IμCP on Si for 4 h at room temperature. (b) Close-up view (10 μm 10 μm) and section profile of printed PDMS features. (c) AFM image and linecut profile of Si substrate after plasma etching with the printed PDMS as an etch mask. Note the ∼10 times difference in feature heights, ∼12 nm on IμCP-ed Si, while ∼120 nm on etched substrate.
index. The measured MW of pristine oligomers, ∼4500, agrees well with data supplied from the manufacturer and the literature result.23 It is noted that the MW of recovered ones is far larger than that of from other measurements such as mass spectrometry (MS) or matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS),23,24 which might be due to limited mass range surveyed and/or preferential ionization of low mass species. The polydisperse nature of material may further complicate the situation. Considering these limitations or uncertainties in those ionization-based techniques, the GPC analysis is better suited for our purpose. On the basis of GPC results, it is clear that the recovered PDMS molecules are not “small”, in comparison with the pristine, uncured siloxane oligomers. Also, the recovered ones have a very broad molecular weight distribution (MWD), the distribution ranging from ∼10K to ∼1000K. Although there is a range of overlap of molecular weight in Figure 1, the main portion of the recovered molecules has a much higher MW than the pristine oligomers. Therefore, we can conclude that the recovered PDMS material is not small, unreacted PDMS oligomers. Rather, they are partially polymerized and are consisted of more than 10 pristine siloxane oligomers. Inkless Microcontact Printing and Pattern Transfer. The hydrophobic recovery phenomenon can be exploited as a surface patterning method. Contrary to the conventional μCP that uses specialized ink materials such as self-assembled monolayers, proteins, etc., the recovered PDMS molecules are used as an ink layer, thus termed “inkless” microcontact printing (IμCP). In this method, all one has to do is to simply place a patterned PDMS stamp in contact with a substrate for a period of time, as shown in Scheme 1. The atomic force microscopy (AFM) image of such a patterned substrate is shown in Figure 2a for large (80 μm 80 μm) area. Shown in Figure 2b is the close-up view (10 μm 10 μm) with sectional profile. Here, the substrate was cleaned silicon and the PDMS stamp having an array of ∼1 μm sized square pillars was kept in contact with the Si substrate for 4 h at room temperature. It can clearly be seen that the recovered PDMS molecules are left only on the contacted area of the substrate. The thickness of this “ink” PDMS molecules was typically 5-10 nm. The patterned Si substrate was further treated with SF6 plasma (100 W, 30 s), the recovered PDMS “ink” playing the role (23) Briseno, A. L.; Roberts, M.; Ling, M.-M.; Moon, H.; Nemanick, E. J.; Bao, Z. J. Am. Chem. Soc. 2006, 128, 3880. (24) Hunt, S.; Cash, G.; Liu, H.; George, G.; Birtwistle, D. J. Macromol. Sci., Part A 2002, 39, 1007.
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Figure 3. Inkless microcontact printing and pattern transfer. AFM images of IμCP on (a) photoresist/Si substrate for 65 h at room temperature. The PDMS-printed substrate was treated by oxygen reactive ion etching for 2 min (20 sccm, 20 mTorr, 100 W). (b, c) Optical microscope images of wet chemical etched substrates after IμCP on (b) ITO(100 nm)/glass and (c) Al(20 nm)/Si substrates. For IμCP, patterned PDMS with 5 μm line and 10 μm space was contacted onto the substrates for 10 h. Then the PDMSprinted substrates were dipped into the etchant bath for 10 min (for ITO) and 3 min (for Al), after UV/ozone treatment for 20 min.
of an etch resist during this plasma etching. The etched Si surface has a depth of ∼120 nm, shown in Figure 2c. The PDMS “ink” is shown here to be a very good etch resist, yielding an etching depth of 120 nm with a 5-10 nm thick etch resist. The noncovalent printing nature of the IμCP does not limit the choice of substrate to be patterned. Note that one of the limitations of typical μCP is the proper covalent bonding chemistry between the ink material and the substrate. The recovered PDMS, on the other hand, can be printed on various substrates, such as Si, SiO2, metals, polymers, etc. Figure 3a shows the patterning result on polymer/Si surface by IμCP. The PDMS printed photoresist (PR; AZ5214)/Si substrate was treated by O2 reactive ion etching (RIE) (20 sccm, 20 mTorr, 100 W, 2 min). Thin (∼10 nm) printed PDMS was found to be a very good etch resist to oxygen RIE (Figure 3a). In fact, it is well-known that oxygen plasma or uv/ ozone treatment of PDMS convert its surface to silica-like (SiOx) state, which is a good etch barrier for oxygen plasma. The conversion of PDMS into SiOx layer expands the applicability of the IμCP technique. In μCP using SAM materials, it is hard to apply dry etching techniques to transfer the printed SAM pattern onto the substrate because of the extreme thinness and organic nature of the SAM layer used. This is the reason that the wet chemical etching has been the main method for pattern transfer onto a substrate after the conventional μCP process. The IμCP, on the other hand, can be used for both wet and dry etching processes for the pattern transfer. Figure 3b,c shows optical microscope (OM) images of ITO(100 nm)/glass and Al(20 nm)/ Si substrates undergone wet chemical etching after the IμCP. It can be seen that the printed PDMS pattern plays the role of etch barrier quite well during wet chemical etching processes. Effect of Mixing Ratio and Curing Temperature. The amount of unreacted oligomer in the cured PDMS stamp can be varied by changing the stamp preparation conditions, such as mixing ratio between base resin and curing agent and curing temperature.25 Figure 4 shows a series of images after IμCP on Si substrate using different PDMS stamp. For 5:1 (in weight) mixing ratio, the excess amount of curing agent (cf. 10:1 ratio is recommended by the vendor) leads to less oligomers available for the printing, as shown in Figure 4a. For 10:1 mixing ratio, the curing temperature (70 °C or RT) affects the quality of contact printing (25) Cortese, B.; Piliego, C.; Viola, I.; D’Amone, S.; Cingolani, R.; Gigli, G. Langmuir 2009, 25, 7025.
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Figure 4. Effect of PDMS stamp with different mixing ratio and curing temperature on IμCP. AFM images of IμCP on Si substrate: (a) 5:1 mixing ratio, curing at 70 °C for 10 h, and IμCP for 120 h; (b) 10:1 mixing ratio, curing at 70 °C for 4 h, and IμCP for 4 h; (c) 10:1 mixing ratio, curing at RT for 4 h, and IμCP for 4 h.
Figure 5. Minimum feature size achieved by IμCP. AFM images of (a) the original PDMS stamp with 300 nm wide lines and 200 nm tall pattern features and (b) the patterned substrate after IμCP followed by O2 reactive ion ething. The photoresist/Si substrate was printed with the stamp at room temperature for 4 h. After removal of the PDMS stamp, PR/Si was etched by oxygen plasma.
for the same contact time, as shown in Figure 4b,c; the higher the curing temperature, the more cross-linking reaction proceeds, leading to less amount of oligomers that can be recovered and printed. Overall, both the large amount of curing agent and the high curing temperature leads to incomplete pattern printing. One interesting observation in those incomplete printing is that the printing seems to start from edge to center in each pattern, leading to fencelike printed features on substrate. One possible explanation is that the larger surface area around the perimeter of patterns, due to pattern side walls and ceilings, may be responsible for this behavior. Minimum Feature Size Achievable. As the IμCP uses a patterned PDMS stamp and its recovery phenomenon, the patterning resolution would be directly dependent on the properties of PDMS stamp. It is well-known that the pattern features on PDMS stamp may deform due to pairing and collapse, depending mainly on the aspect ratio of these features and the mechanical strength of PDMS material.26,27 Figure 5 shows the AFM images of both the PDMS stamp (Figure 5a) and the patterned substrate (Figure 5b), including sectional profiles at the bottom of each image. The features on the PDMS stamp are equally spaced ∼300 nm wide lines and ∼200 nm tall, leading to aspect ratio of less than 1 (cf. spots on the PDMS stamp were found to have defects due to pairing). This patterned PDMS stamp was kept in contact with a photoresist (AZ 5214)/Si substrate for ∼4 h. After removing the stamp, the substrate was etched by O2 plasma. During the oxygen plasma etching, the recovered thin layer of (26) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042. (27) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. Adv. Mater. 1997, 9, 741.
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Figure 6. Repeated IμCP with the same PDMS stamp. AFM images of printed patterns on Si substrate (a) at 10th and (b) 17th IμCP process.
Figure 7. Multiple IμCP. Square dot arrays and connecting lines are formed after printing twice on the same (Si) substrate using PDMS stamp with 2 μm line width and 2 μm space. The first printing and second printing time were 24 h at room temperature. AFM images of (a) large area (50 μm 50 μm), (b) close-up view (20 μm 20 μm), and (c) perspective view of multiple IμCP-ed surface. The feature height is ∼30 nm from the section profile at the bottom of (b).
PDMS acts as an etch-resist layer. The result shows that submicrometer scale resolution, less than 500 nm in this work, can easily be achieved by the IμCP. Marathon Test. The repeated use of stamp/mold for the patterning is one of the important issues in various patterning techniques because it directly affects the process economics.10-12 One of the very interesting soft lithographic patterning techniques suggested in the literature is the decal transfer lithography (DTL)28,29 and modified versions of it,30-32 all of which involve either the permanent damage to the PDMS stamp or some other treatment steps or specific equipments, which may limit the repeated use of the stamp. In general, it is thought that the unreacted siloxane oligomers exist throughout the cured PDMS stamp;23 that is, once the oligomers are transfer-printed onto a substrate, then more oligomers in the bulk of stamp will diffuse out to replenish the surface. The diffusivity (D) of siloxane chains in cured PDMS network is ∼10-8 cm2/s.33 Then, the diffusion length, L = (Dt)1/2, is estimated to be ∼1 μm for 1 s, which means very fast diffusion from the bulk to the surface. In this regard, the PDMS stamp for IμCP is expected to be used repeatedly. Figure 6 shows the AFM images of printed features at 10th (Figure 6a) and 17th (Figure 6b) IμCP, respectively, using the same PDMS stamp, which confirms (28) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 13583. (29) Childs, W. R.; Nuzzo, R. G. Adv. Mater. 2004, 16, 1323. (30) Zheng, Z.; Azzaroni, O.; Zhou, F.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 7730. (31) Park, K. S.; Seo, E. K.; Do, Y. R.; Kim, K.; Sung, M. M. J. Am. Chem. Soc. 2006, 128, 858. (32) Kim, M.; Kim, Y. S. J. Am. Chem. Soc. 2007, 129, 11304. (33) Mazan, J.; Leclerc, B.; Galandrin, N.; Couarraze, G. Eur. Polym. J. 1995, 31, 803. (34) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (35) Asberg, P.; Nilsson, P. R.; Inganas, O. Langmuir 2006, 22, 2205. (36) Wang, X.; Ostblom, M.; Johansson, T.; Inganas, O. Thin Solid Films 2004, 449, 125.
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that the PDMS stamp can be used many times for IμCP. We note, however, that the curing or cross-linking reaction of PDMS proceeds even at room temperature. Therefore, the repeated use of PDMS stamp may be limited by the gradual curing at room temperature, rather than the shortage of unreacted oligomers inside the stamp. Multiple Inkless Microcontact Printing. As the IμCP process does not involve any surface treatments or other preparation steps for both the substrate and the stamp, it is natural to expect that the multiple IμCP should be possible.25 As shown in Figure 7, the IμCP was done twice on the same substrate, with a 2 μm/2 μm line/space patterned PDMS stamp rotated in the second printing relative to the first one. Instead of checkerboard-like patterns that would be expected from the simple line/space patterned stamp, square dot arrays are formed after the printing twice; the square dots are formed at the crossing locations of the two printing processes, while there are transferred line features only along the edges of the patterns on other printed areas. A consecutive edge-only transfers16,25 might be responsible for this result. In all, the multiple applications of IμCP can lead to the generation of the more complex and intriguing patterns from the simpler patterns.
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Conclusions We have systematically investigated the hydrophobic recovery phenomenon of PDMS in this work. First of all, the recovered oligomers were found to have >10 larger molecular weight compared to that of pristine ones. The recovery of PDMS oligomers were shown to be used to generate desired surface topography. It is remarkable that the ultrathin (∼10 nm or below) recovered layer shows good enough etch barrier performance even in plasma-based dry etching processes, let alone in wet chemical etchings. The IμCP technique was shown to achieve resolution down to ∼300 nm. The stamp can be used repeatedly, only limited by the gradual curing of the stamp at room temperature. The absence of specific chemistry or surface preparation steps in the process allowed one to apply IμCP multiply on the same substrate, enabling the generation of the more complex and intriguing patterns from the simpler ones. Acknowledgment. This work was supported by the National Research Foundation (KRF) grant funded by the Korea government, WCU (World Class University) program (R32-20031).
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