Microcontact Printing of Octadecylsiloxane on the Surface of Silicon Dioxide and Its Application in Microfabrication

9576 J. Am. Chem. Soc. 1995, 117, 9576-9577

Microcontact Printing of Octadecylsiloxane on the PDMS Surface of Silicon Dioxide and Its Application in r solution of Microfabrication CH3{CH2)17SiCI3 in hexane Younan Xia, Milan Mrksich. Enoch Kim, and SiO? George M. Whitesides*

Department of Chemistry, Harvard University microcontact print Cambridge, Massachusetts 02138 SAM Received January 13, 1995 JUL J! This paper describes the use of microcontact printing (//CP)1 for patterning self-assembled monolayers (SAMs)2 of alkylsiloxanes on the surface of silicon dioxide. is a convenient //CP dip in solution of CI3SiR technique for generating patterned SAMs of alkanethiolates on in hexane or toluene gold,19 but it has not been applied to Si/SiOi and glass. The SAMs of alkyltrichlorosilanes on the hydroxyl-terminated surfaces are less ordered than those of alkanethiolates on gold, and they form more slowly.5 Here we demonstrate that //CP can, nonetheless, be used to produce patterned SAMs of alkylsiloxanes on Si/SiOj. Although these patterned SAMs R = -(Ch2)3NH2, (CH2)3Br, -(CHj)3NCO, -(CH2)3SH. provide only partial selectivity in the etching of SiCF in aqueous and HF/NH^F solution, they are useful in other ways. In particular, - in hexane (~0.4% w/w) for ~5 min to derivatize the of with siloxanes terminated in remaining regions Si/Si02 H,0/-COOH SAM(-CH3) 10 Jim a different functional group. Subsequent treatment of a pat- b : i____*_ terned SAM containing vinyl-terminated regions with an aque- 000000 ous solution of KMnCh and KIO4 converted the olefins to OOOOO carboxylic acids.7 We characterized these patterned surfaces 00009 by XPS, SEM, SIMS, and optical microscopy of condensation #00009 figures (CFs)8 (Figure 2). SEM suggests that the edge resolution of these lines is lower than that observed for alkanethiolates on gold (~200 vs ~50 nm)9 Comparison of the properties of nonpattemed SAMs of octadecylsiloxane that were generated by //CP with a flat PDMS

Downloaded via NORTHWESTERN UNIV on June 9, 2020 at 19:38:24 (UTC). (1) Wilbur. J. L.; Kumar. A.; Kim. E.; Whitesides. G. M. Adv. Mater. 1994. 6. 600-604. (2) Dubois, L. H.; Nuzzo. R. G. Annu. Rev. Phxs. Chem. 1992.43,437- 463. Kumar. See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. (3) A.; Biebuyck. H.; Whitesides, G. M. Langmuir 1994, 10. 1498-1511. (4) Xia, Y.; Whitesides. G. M. J. Am. Chem. Soc. 1995. 117, 3274- 3275. (5) For recent studies, see: McGoven, M. E.; Kallury, K. M.; Thompson. M. 1994. 10. 3607 —3614. Parikh, A. N.; Allara, D. L.; Azouz, Langmuir 2. SE of SAMs of I B.: Rondelez, F. J. Phxs. Chem. 1994, 98, 7577-7590. Figure (a) micrographs patterned octadecylsiloxane (6) Substrates were cleaned by heating at 70 °C for ~30 min in Piranha on the surface of Si/SiO;. (b) Condensation figures obtained by solution (a mixture 7:3 (v/v) of 98% HiSOj and 30% H2O2), thoroughly condensing water vapor on SAMs that have complementary patterns rinsed with deionized water, and used Caution: Piranha immediately. of methyl- and carboxylic acid-terminated regions. The rough edges solution is an extremely strong oxidant and should be handled with care! are artifacts from surface tension in the not defects in the pattern, Samples used for XPS. SEM, and SIMS were prepared on silicon wafers liquid, covered with native silicon dioxide. The "ink" used in //CP was a ~0.2% (c) SIMS ion maps of surfaces patterned with SAMs terminated in (w/w) solution of octadecyltrichlorosilane (OTS) in hexane; similar results different functional groups. In all cases, the surface was printed with were obtained toluene as the solvent. The was left in using stamp contact lines of octadecylsiloxane and then filled in with a second siloxane. with the Si/SiOs for —5 s in printing. The was rinsed with hexane stamp The elements are (clockwise, from left) fluorine, and ethanol after each printing. When washed carefully between uses, one imaged upper nitrogen, stamp has been used for more than 100 impressions without loss in the sulfur, and nitrogen. The scale applies to all images. quality of the patterns. 16-Hepladecenyltriethoxysilane was synthesized according to the published procedure. Other alkylsilanes were purchased and immersion in solution (Table 1) that from Aldrich or Hills and were used without purification. stamp by suggested (7) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, the former SAMs were more uniform, although the thicknesses 5, 1074-1078. of the SAMs prepared by these two methods were comparable. (8) Lopez. G. P.; Biebuyck, H. A.; Friesbie, D. C.; Whitesides, G. M. Science 1993, 260, 647 —649. Although the patterned SAM of octadecylsiloxane on Si/SiOz (9) Biebuyck. H,; Whitesides, G. M. Langmuir 1994. 10, 4581—4587. did not protect the surface against etching in aqueous HF/NH4F

Table 1. between SAMs Formed Immersion and Comparison by hydrophilic hydrophobic Printing (-OH.bare Si02) (-CH3, SAM of OTS) contact angle (deg) { ^ SAM _ (~2 nm) H:0 J-91 prepolymer thickness" -— Si02 (~ 200 nm) T, flT ~~o* eT~ (A) native wet wet 18 10 SiO: 1) Self-assemble PMMA prepolymer thermal SiO: <20 <20 28 15 2) Cure under UV for 20 min SAM bv dipping" 93 50 47 22 20.5 light SAM by /

solution, PMMA or PU that was assembled selectively on the hydrophilic regions of the patterned surface provided good protection (Figure 3).10-12 The unprotected areas of SiC>2 were Etch in KOH//-PrOH etched in 4 min. The of formed in completely pattern SiO; (70 °C, 20 min) this step served as a mask for the anisotropic etching of the underlying silicon in KOH/t-PrOH.10-1-1 In these experiments, the liquid prepolymers did not completely wet the surface of thermally-formed Si02 (Table 1), and the lines of polymeric Si liquids that were left on the hydrophilic regions from the Figure 3. Procedure used to fabricate microstructures in silicon. A dewetting minimized their free energy by retracting. As a result, prepolymer of PMMA or PU was allowed to self-assemble on the the final width of these lines was smaller than the width of the hydrophilic regions (that is, bare SiO:) and cured under a UV light lines of bare Si/SiO:.14 source. The polymeric structures were effective resists for etching of Patterned SAMs on and comprising alkyisiloxanes Si/SiO; the exposed regions of SiO: in an aqueous solution of HF (buffered by are to those on glass15 complementary of alkanethiolates gold, NH4F). The resulting pattern of SiO: (PMMA or PU was soluble in silver, and copper. In the past, patterning siloxane monolayers the strongly alkaline solution used for silicon etching) was used as a on the surface of Si/SiO; required UV photolithography.16 mask for the anisotropic etching of the underlying Si in KOH/t-PrOH. While this technique generates patterns effectively, it provides The samples were tilted before the SEM photographs were taken. The SiO: mask remained on the surface after silicon etching, (10) Kim. E.; Kumar, A.; Whitesides, G. M. J. Electrochem. Soc. 1995, 142, 628-633. iittle control over the surface chemistry of the UV-damaged (11) Biebuyck. H.: Whitesides. G. M. Langmuir 1994, 10, 2790—2793. regions and cannot, in general, be used on nonplanar surfaces. (12) Silicon wafers orientation) covered with 0.2-um-thick thermal ((100) Microcontact can be on both and silicon dioxide (MEMC) were cleaned and used for experiments involving printing performed planar etching. After the surface was patterned with octadecylsiloxane. a drop of nonplanar surfaces and does not require routine access to a prepolymer of PMMA (SK-9, Edmund Scientific) or PU (J-91, Edmund photolithography facility for each pattern formation.17 We Scientific; NOA8I, Norland Products Inc.) was on the placed patterned believe this technique will prove valuable for many other area. The assembled prepolymer was cured under a mercury lump (medium — and cell pressure; the distance between the sample and the lamp was l cm) for applications—for example, studies of protein absorption —20 min. The bare regions of silicon dioxide uncovered by polymers were attachment—where high edge resolution (<50 nm) is not critical. etched in a NHjF-buffered aqueous HF solution (250 mL of HaO, 165.5 g ofNHaF, and 40 mL of 48% HF). Similar results were also observed when Acknowledgment. This work was supported in part by ONR and hexadeeane was used instead of of PMMA or PU. liquid prepolymer ARPA. This work made use of MRSEC Shared Facilities (13) Peterson, K. E. Proc. IEEE 1982. 70. 400-424. supported We thank (14) The reduction in width of the assembled polymer was observed in by the National Science Foundation under DMR-9400396. situ by optical microscopy. When silicon wafers with native oxide were Dr. Yuanchang Lu and Dr. Stephen Shepard for their assistance in using used, the final width of the cured polymeric lines was approximately equal the SEM and photolithography facility. The SIMS analysis was carried to the width of the lines of bare because the native oxide is more SiO:, out by Judy Baker at the Center for Microanalysis of Materials. hydrophilic than the thermally-formed oxide (Table 1). We also used gold University of Illinois at Urbana—Champaign, which is supported by surfaces that were covered by hydroxyl-terminated alkanethiolate, and the the U.S. of under Grant DEFG02-91-ER45439. width of the cured polymer lines was approximately equal to the width of Department Energy the lines of hydroxyl-terminated SAM. M.M. is grateful to the American Cancer Society for a postdoctoral (15) We have also observed condensation figures (as well as the fellowship. diffraction patterns of these CFs) on patterned SAMs comprising methyl- and carboxylic acid-terminated siloxanes on glass slides. JA950II5G (16) Calvert. J. M.; Dulcy, C. S.; Pecker.tr. M. C.; Schnur. J. M.; Gcorger, J. H„ Jr.: Calabrese, G. S.; Sricharoenchaikit, P. Solid Slate Technol. 1991. (17) Jackman, R.; Wilbur, J. L.; Whitesides, G. M. Science 1995, 269, October, 77-82. 664 -666.