Langmuir 2002, 18, 3415-3417
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Microcontact Printing Directly on the Silicon Surface Yongseok Jun, Duc Le, and X.-Y. Zhu* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received November 19, 2001 Alkoxyl thin films were deposited directly onto silicon surfaces by microcontact printing. This is based on an efficient reaction between alcohol functional groups and chlorine-terminated silicon surfaces for thin film assembly [Langmuir 2000, 16, 6766]. The features formed by microcontact printing show high resolution and fidelity, with no evidence of edge diffusion or island formation. These represent marked improvements over siloxane-based approaches.
1. Introduction Since the successful demonstration of soft-lithography techniques,1 particularly microcontact printing (µCP) using self-assembled monolayers (SAMs) of thiols on gold surfaces, a number of attempts have been made to extend the technique to silicon and related surfaces.2-5 The importance of patterning silicon surfaces cannot be overstated. Among prominent examples are microelectronics, micromachines, and microfluidics. Past attempts in patterning silicon surfaces via µCP have relied on siloxane SAMs formed from alkyltrichlorinesilane on oxide-terminated silicon. Although siloxane patterns generally are not good resists for the selective aqueous etching of silicon oxide, they can serve as primary patterns onto which secondary patterns are generated. For example, polymers and metals have been successfully deposited on hydrophilic regions within patterns generated by µCP of siloxane SAMs.2-5 Recently, Nuzzo and co-workers also demonstrated the use of long-chain alkylsiloxane (22 carbon) SAMs directly as resists for aqueous etching.6 An increase in alkyl chain length from octadecyl (18 carbon) to decosyl (22 carbon) showed significant improvements in film and pattern quality from µCP. While siloxane SAMs are applicable to a wide variety of surfaces, a problem arises from the complexity of the monolayer formation process due to competitions between intermolecular covalent bond formation and surface attachment or between cross-linking and close packing.7 The deposition process is highly sensitive to experimental variables, such as temperature, alkyl chain length, and the concentration of a trace amount of water. More often than not, this process leads to polymers, microstructures, and other precipitates on the surface, particularly when the organic group deviates from simple long-chain alkyls.8,9 This complexity may be the reason for the low resolution (1) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (2) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576. (3) Jeon, N. L.; Nuzzo, R. G. Langmuir 1995, 11, 3024. (4) Jeon, N. L.; Clem, P. G.; Nuzzo, R. G.; Payne, D. A. J. Mater. Res. 1995, 10, 2996. (5) Jeon, N. L.; Clem, P. G.; Payne, D. A.; Nuzzo, R. G. Langmuir 1996, 12, 5350. (6) Erhardt, M. K.; Nuzzo, R. G. Langmuir 1999, 15, 2188. (7) (a) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (b) Wasserman, S. R.; Tao, Y.; Whitesides, G. M. Langmuir 1989, 5, 1074. (c) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (d) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (e) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (f) Stevens, M. J. Langmuir 1999, 15, 2773. (8) Bunker, B. C.; Carpick, R. W.; Assink, R. A.; Thomas, M. L.; Hankins, M. G.; Voigt, J. A.; Sipola, D.; de Boer, M. P.; Gulley, G. L. Langmuir 2000, 16, 7742.
(∼300 nm) of patterns from µCP of siloxane monolayers.2 Edge diffusion and island formation in noncontact regions have been cited as the source for poor resolution.6 Recently, our laboratory has demonstrated an alternative to siloxane chemistry for monolayer formation on silicon or silicon oxide surfaces.10,11 This process consists of two steps: in the first step, a silicon or silica surface is activated to give Si-Cl functional groups; in the second step, surface Si-Cl reacts with the alcohol functionality for organic attachment and monolayer formation. There is no competition between intermolecular and molecule-surface attachment reactions. Such a two-step approach has been demonstrated for the formation of close-packed alkoxyl monolayers on both silicon and silica surfaces. In this report, we apply the two-step monolayer assembly strategy to microcontact printing on silicon and demonstrate the high resolution of resulting organic patterns on the surface. 2. Experimental Section The silicon samples were 1 × 1 cm2 slices of a polished Si(111) wafer (Wafernet). They were cleaned by rinsing with acetone, dried, and then oxidized in 3:1 concentrated H2SO4/30% H2O2 (piranha) solution for 2 h at 100 °C (caution! piranha solution is a strong oxidant and reacts violently with organic substances), followed by thorough rinsing with 18 MΩ cm H2O. The surface was etched in 40% aqueous NH4F solution for 4-7 min to yield a monohydride silicon surface. After removal from the NH4F solution, excess NH4F solution on the surface was blown off under an Ar stream. Upon drying, the sample was transferred to a glass reaction cell and evacuated to a base pressure of 1 × 10-5 Torr. The Cl-terminated silicon surface was obtained from exposing the H-Si(111) surface to ∼2 Torr of Cl2 gas at a temperature of 80 °C for 15 min. Because the Cl-Si(111) surface was sensitive to moisture, the sample was removed from the glass cell under argon in a glovebag and subsequent stamping experiments were also carried out inside the glovebag. Poly(dimethyl siloxane) (PDMS) stamps were gifts from Prof. Y. Xia (University of Washington, Seattle). Patterns of regular straight lines with 5 µm width, 5 µm spacing, and 1.5 µm depth were used for stamping. Dodecanol (98+%, Aldrich) solution (0.1 M) in isooctane (99%, Aldrich) was applied to the PDMS stamp with a Q-Tip. After the evaporation of isooctane solvent, the dodecanol-covered stamp was brought into contact with the ClSi(111) surface on a hot plate maintained at a constant temperature of 70 °C for a total stamping time of 1-30 min. The silicon sample was subsequently rinsed with CH2Cl2 to remove excess alcohol molecules weakly adsorbed on the surface. Atomic force microscopy (AFM) imaging of the surface was carried out with the NanoScope IIIa from Digital Instruments. An oxide(9) Cabibil, H.; Pham, V.; Lozano, J.; Celio, H.; Winter, R. M.; White, J. M. Langmuir 2000, 16, 10471. (10) Zhu, X.-Y.; Boiadjiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Langmuir 2000, 16, 6766. (11) Major, R. C.; Zhu, X.-Y. Langmuir 2001, 17, 5576.
10.1021/la011692n CCC: $22.00 © 2002 American Chemical Society Published on Web 04/03/2002
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Langmuir, Vol. 18, No. 9, 2002
Figure 1. The dependence of film thickness on stamping time for dodecoxyl monolayer patterns from the µCP of dodecanol on Cl-Si(111). The insets are AFM topographical images for stamping times of (a) 1 min, (b) 10 min, and (3) 30 min. Inset d shows an AFM image of the surface from stamping on H-Si(111) for 10 min. sharpened silicon nitride tip with a nominal force constant of 0.06 N/m and a nominal tip radius of ∼30 nm was employed. Images were taken in the tapping mode.
3. Results and Discussion Figure 1 shows the dependence of film thickness as a function of stamping time for dodecanol on Cl-Si(111). The thickness values are obtained from topographical AFM images, such as those shown in insets. Patterns with sharp contrast are obvious for stamping times from 1 to 30 min (insets a-c). The saturation coverage (thickness 16-18 Å) is achieved for a stamping time of g30 min. This saturation thickness is close to the molecular length in the all-trans configuration and corresponds to a nearly close-packed film. For comparison, inset d shows a background image of stamping for 10 min directly on the H-Si(111) surface without the chlorination step. Although the covalent attachment of alcohol molecules to the H-Si(111) surface has been reported,12 we see no deposition of molecules from µCP under the experimental conditions used. The Cl-terminated Si surface is more reactive toward alcohol molecules than the H-terminated surface, thus making µCP possible within a reasonable time window. Note that surface Si-Cl species in the nonstamped region are reactive toward water. These areas are easily converted to the hydrophilic -OH-terminated surfaces upon exposure to moisture or water. Thus, the pattern obtained from the above stamping process consists of alternating hydrophobic and hydrophilic regions. We now examine in more detail the quality of molecular patterns from the µCP process. Figure 2A shows AFM topographical images of a pattern at submonolayer coverage. The film thickness in the printed region is 6 Å (see cross section in Figure 2C), which corresponds to 1/3 of a monolayer. Figure 2B shows the corresponding phase image, which is positively related to the hardness of the surface. As expected, regions with molecular film (lighter area in A) correspond to softer material (darker area in B). Although the coverage of alkoxyl groups in the printed region is only 1/3 of a monolayer, the morphology of the (12) Boukherroud, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M. Langmuir 2000, 16, 7429.
Letters
Figure 2. AFM images obtained after µCP of dodecanol on Cl-Si(111) for ∼6 min at 80 °C: (A) topographical image; (B) phase image; (C) cross section of the solid line in (A); (D) magnified topographical image. The scale bar in (A) is 5 µm, and that in (D) is 200 nm. Several lines near the solid line in (A) are averaged to reduce scan noise and give the cross section in (C).
film is smooth, with no evidence for islands and pinholes. The covalently attached alkoxyl groups must adopt a highly tilted geometry to form a smooth film. A zoomed-in picture (1 × 1 µm2) of the boundary between printed and nonprinted regions in Figure 2D demonstrates the excellent resolution of the pattern. We estimate from the image a boundary resolution of
