Positive Microcontact Printing with Mercaptoalkyloligo(ethylene glycol)s

1016

Langmuir 2006, 22, 1016-1026

Positive Microcontact Printing with Mercaptoalkyloligo(ethylene glycol)s Milan Saalmink,† Cees van der Marel,‡ Henk R. Stapert,† and Dirk Burdinski*,† Philips Research, High Tech Campus EindhoVen, 5656 AE EindhoVen, The Netherlands ReceiVed September 14, 2005. In Final Form: NoVember 2, 2005 The soft lithographic replication of patterns with a low filling ratio by microcontact printing (µCP) is problematic due to the poor mechanical stability of common elastomeric stamps. A recently described strategy to avoid this problem employs a modified patterning method, positive microcontact printing ((+)µCP), in which a stamp with a mechanically more stable inverted relief pattern is used. In contrast to conventional negative µCP ((-)µCP), in the contact areas a self-assembled monolayer (SAM) is printed of a “positive ink”, which provides only minor etch protection, whereas the noncontacted areas are subsequently covered with a different, etch-resistant SAM, prior to development by chemical etching. With the aim to identify novel, highly versatile positive inks, the patterning of gold by (+)µCP with mercaptoalkyloligo(ethylene glycol)s (MAOEGs), the subsequent adsorption of octadecanethiol (ODT), and the final development by wet chemical etching have now been studied. A polydisperse mixture of mercaptoundecylocta(ethylene glycol) derivatives was found to provide the best patterning results. The surface spreading of the positive ink during stamping, the exchange of printed MAOEGs with ODT, and the choice of the right etching bath were identified as key parameters that influence the achievable pattern resolution and contrast. Due to the modular composition of functionalized alkyloligo(ethylene glycol) derivatives, (+)µCP with these positive inks has the potential for easy adaptation to a variety of materials and development conditions.

Introduction Surface modification and patterning are of great importance for biomedical as well as electronic devices. Patterns comprising different self-assembled monolayers (SAMs) with orthogonal surface properties are frequently utilized for the patterned immobilization of biomolecules or molecular probes, or for the deposition and patterning of conductive materials, for instance, in the electronics industry.1-9 The generation of monolayer patterns has become remarkably easy, since soft lithographic patterning techniques, most importantly microcontact printing (µCP), were introduced about a decade ago.10-13 Patterning by µCP down to the submicrometer range has been demonstrated for Au,10,11 Ag,14-16 Ag alloys,17 Cu,18 Pd,19,20 surface * To whom correspondence should be [email protected]. † Department of Bio-Molecular Engineering. ‡ Department of Materials Analysis.

addressed.

E-mail:

(1) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81-96. (2) Rich, R. L.; Myszka, D. G. J. Mol. Recognit. 2002, 16, 351-382. (3) Frederix, F.; Bonroy, K.; Laureyn, W.; Reekmans, G.; Campitelli, A.; Dehaen, W.; Maes, G. Langmuir 2003, 19, 4351-4357. (4) Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X.; Ingber, D. E. Annu. ReV. Biomed. Eng. 2001, 3, 335-373. (5) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697-719. (6) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519-523. (7) Lauer, L.; Ingebrandt, S.; Scholl, M.; Offenha¨usser, A. IEEE Trans. Biomed. Eng. 2001, 48, 838-842. (8) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821-1825. (9) Zhou, D.; Bruckbauer, A.; Ying, L.; Abell, C.; Klenerman, D. Nano Lett. 2003, 3, 1517-1520. (10) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (11) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002-2004. (12) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (13) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823-1848. (14) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070-1079. (15) Xia, Y.; Venkateswaran, N.; Qin, D.; Tien, J.; Whitesides, G. M. Langmuir 1998, 14, 363-371.

oxide-forming metals, including Si21-23 and Al,24,25 and metal oxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO).26 In µCP an elastomeric stamp, commonly made of poly(dimethylsiloxane) (PDMS), is used to transfer ink molecules to the surface of a substrate where they form a SAM, which is congruent with the surface relief pattern of the stamp. The incompressible elastomer PDMS allows for conformal contact between the stamp and the substrate, which enables homogeneous ink transfer from the stamp to the substrate in the contact regions. The flexibility of the stamp material proves to be problematic, however, when the stamp pattern comprises features with a high aspect ratio or extended featureless regions.5,27-29 Stamp features with a high aspect ratio are prone to buckle, whereas stamps with low filling ratio patterns comprising large recessed areas tend to collapse under the applied printing pressure (Figure 1). Even (16) Tate, J.; Rogers, J. A.; Jones, C. D. W.; Vyas, B.; Murphy, D. W.; Li, W.; Bao, Z.; Slusher, R. E.; Dodabalapur, A.; Katz, H. E. Langmuir 2000, 16, 6054-6060. (17) Burdinski, D.; Brans, H. J. A.; Decre´, M. M. J. J. Am. Chem. Soc. 2005, 127, 10786-10787. (18) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (19) Carvalho, A.; Geissler, M.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2406-2412. (20) Wolfe, D. B.; Love, J. C.; Paul, K. E.; Chabinyc, M. L.; Whitesides, G. M. Appl. Phys. Lett. 2002, 80, 2222-2224. (21) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (22) John, P. M. S.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022-1024. (23) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382-3391. (24) Geissler, M.; Wolf, H.; Stutz, R.; Delamarche, E.; Grummt, U.-W.; Michel, B.; Bietsch, A. Langmuir 2003, 19, 6301-6311. (25) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 11821191. (26) Breen, T. L.; Fryer, P. M.; Nunes, R. W.; Rothwell, M. E. Langmuir 2002, 18, 194-197. (27) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318. (28) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394-1407. (29) Sharp, K. G.; Blackman, G. S.; Glassmaker, N. J.; Jagota, A.; Hui, C.-Y. Langmuir 2004, 20, 6430-6438.

10.1021/la052513v CCC: $33.50 © 2006 American Chemical Society Published on Web 01/07/2006

(+)µCP with Mercaptoalkyloligo(ethylene glycol)s

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Figure 2. Printing and etching scheme illustrating (-)µCP (left) and (+)µCP (right).

Figure 1. Illustration of mechanical problems occurring in µCP with elastomeric stamps, such as PDMS: (left) collapse of low filling ratio patterns, (right) squeezing of high aspect ratio features.

without external load, collapse may occur due to the work of adhesion between the stamp and substrate.30-32 Various concepts were proposed to improve the mechanical stability of the stamp, which include the development of new stamp materials33-35 and stamp designs,27,28,36,37 as well as advanced stamping techniques.30,38 Very recently, the concept of using flat, chemically patterned stamps has been introduced, which solves basic stamp stability issues for a number of applications and makes the use of more simple, direct write techniques for stamp manufacturing possible.39 In a different approach, Delamarche et al. avoided the stability problem by devising an inverted, “positive” microcontact printing ((+)µCP) method.40 The method employs a stamp with a relief pattern exhibiting the inverted surface structure of a stamp used in a conventional negative microcontact printing ((-)µCP) process (Figure 2). An ink is used that forms a first SAM in the contact regions of the substrate, which provides less etch protection than a second SAM subsequently deposited in the remaining areas. In a following etching step material is therefore removed selectively from the less protected contact areas of the pattern. Stamps with a high filling ratio and a high mechanical stability can therefore be used to obtain low filling ratio patterns in the final development step.41 The deposition of the etch-protecting (30) Decre´, M. M. J.; Schneider, R.; Burdinski, D.; Schellekens, J.; Saalmink, M.; Dona, R. Mater. Res. Soc. Symp. Proc. 2004, EXS-2, 59-61. (31) Decre´, M. M. J.; Timmermans, P. H. M.; Sluis, O. v. d.; Schroeders, R. Langmuir 2005, 21, 7971-7978. (32) Huang, Y. Y.; Zhou, W.; Hsia, K. J.; Menard, E.; Park, J.-U.; Rogers, J. A.; Alleyne, A. G. Langmuir 2005, 21, 8058-8068. (33) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042-3049. (34) Li, H.-W.; Muir, B. V. O.; Fichet, G.; Huck, W. T. S. Langmuir 2003, 19, 1963-1965. (35) Trimbach, D.; Feldman, K.; Spencer, N. D.; Broer, D. J.; Bastiaansen, C. W. M. Langmuir 2003, 19, 10957-10961. (36) Tormen, M.; Borzenko, T.; Steffen, B.; Schmidt, G.; Molenkamp, L. W. Microelectron. Eng. 2002, 61-62, 469-473. (37) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314-5320. (38) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. Langmuir 2003, 19, 6567-6569. (39) Sharpe, R. B. A.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. J. Am. Chem. Soc. 2005, 127, 10344-10349. (40) Delamarche, E.; Geissler, M.; Wolf, H.; Michel, B. J. Am. Chem. Soc. 2002, 124, 3834-3835.

SAM by adsorption rather than printing is a further potential advantage of the positive printing method, since it provides more flexibility in the SAM formation process and can yield monolayers with a better structural order and a lower defect density.40,42,43 To be useful for (+)µCP, the ink (i) has to be soluble in a suitable solvent and (ii) has to adsorb and form a stable SAM on the substrate surface, with (iii) only small surface spreading tendencies. The printed SAM must furthermore (iv) not provide a strong etch resistance and (v) not significantly exchange with the molecules forming the second SAM in the subsequent monolayer deposition process.40 For gold patterning, these prerequisites are fulfilled by pentaerythritol tetrakis(3-mercaptopropionate) (PTMP), which is considered to form only poorly ordered SAMs during printing, thus providing a sufficiently low etch resistance.40,44,45 There is, nevertheless, a need for alternative types of positive ink that permit systematic variations of the molecular structure for easy adjustment to different patterning conditions, such as a variation of the substrate material or the etching conditions. We consider the group of mercaptalkyloligo(ethylene glycol)s (MAOEGs) a promising candidate for such an ink system. MAOEG SAMs on gold are now widely studied due to their excellent protein and cell repellency.46-55 Their well-known (41) A more complicated (+)µCP scheme has been described by Kim et al. employing phase separation of organic monomers followed by polymerization to obtain a highly etch resisting polymer layer on a SAM pattern, which consists of a printed hydrophobic SAM with low etch resistance and a subsequently solution deposited hydrophilic SAM as the condensation point of the monomer (Kim, E.; et al. J. Electrochem. Soc. 1995, 142, 628-633). (42) Graham, D. J.; Price, D. D.; Ratner, B. D. Langmuir 2002, 18, 15181527. (43) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17, 3307-3316. (44) Delamarche et al. further proposed cysteamine as a positive ink (Delamarche, E.; et al. U.S. Patent US 6,893,966). (45) Trimbach, D. C.; Al-Hussein, M.; Jeu, W. H. d.; Decre´, M.; Broer, D. J.; Bastiaansen, C. W. M. Langmuir 2004, 20, 4738-4742. (46) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (47) Qian, X.; Metallo, S. J.; Choi, I. S.; Wu, H.; Liang, M. N.; Whitesides, G. M. Anal. Chem. 2002, 74, 1805-1810. (48) Herrwerth, S.; Rosendahl, T.; Feng, C.; Fick, J.; Eck, W.; Himmelhaus, M.; Dahint, R.; Grunze, M. Langmuir 2003, 19, 1880-1887. (49) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 1071410721. (50) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426-436. (51) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605-5620. (52) Ostuni, E.; Chapman, R. G.; Liang, M. N.; Meluleni, G.; Pier, G.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 6336-6343. (53) Zhu, B.; Eurell, T.; Gunawan, R.; Leckband, D. J. Biomed. Mater. Res. 2001, 56, 406-416.

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properties and already widespread use make their application also in (+)µCP attractive. Typically, a variety of derivatives, including those bearing different functional headgroups, such as thiol or halosilane derivatives, are available in most dedicated laboratories. In a SAM the oligo(ethylene glycol) (OEG) part of MAOEGs can adopt any of three basic conformations: helical, zigzag, or disordered. Among the factors that determine the confirmation of the OEG part of the molecules at the surface are the exact molecular constitution,56-59 SAM composition and surface coverage,49,60 temperature,61 and solvent used for SAM formation.62 In a liquid medium these monolayers are generally subject to solvation.63 Hydration results in significant water uptake in the OEG part of the SAM and the formation of a defined SAMwater interphase.64-66 The swelling of the SAM causes significant structural changes and suggests the possibility of a diffusional exchange of molecules and ions with the bulk solution.67-70 In particular, hydroxide ions are considered to diffuse into and to be stabilized within the OEG part of these monolayers.71-74 We therefore postulate that MAOEG SAMs on gold, although generally stable and of high surface coverage, would provide only minor resistance against aqueous and in particular alkaline etching solutions. Accordingly, this study investigates a (+)µCP method comprising the printing of an MAOEG as the positive ink, followed by solution adsorption of octadecanethiol (ODT), the SAMs of which are known to provide excellent etch resistance,75,76 to the noncontact areas, and a subsequent etching step. The studied MAOEGs have the general formula HS(CH2)m(OCH2CH2)nOCH3, with m ) 2, 6, 10, 11, or 16 and n ) 3, 6, 8, 15, or 16, in selected monodisperse or polydisperse compositions. The methoxy terminal group was chosen over a hydroxy functionality due to its chemical inertness. This is believed to allow for future easy adaptation of the ink system for (+)µCP (54) Schwendel, D.; Dahint, R.; Herrwerth, S.; Schloerholz, M.; Eck, W.; Grunze, M. Langmuir 2001, 17, 5717-5720. (55) Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359-9366. (56) Vanderah, D. J.; Parr, T.; Silin, V.; Meuse, C. W.; Gates, R. S.; La, H. Langmuir 2004, 20, 1311-1316. (57) Zwahlen, M.; Herrwerth, S.; Eck, W.; Grunze, M.; Ha¨hner, G. Langmuir 2003, 19, 9305-9310. (58) Valiokas, R.; Svedhem, S.; O ¨ stblom, M.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. B 2001, 105, 5459-5469. (59) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390-3394. (60) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 18, 8862-8870. (61) Valiokas, R.; O ¨ stblom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. B 2000, 104, 7565-7569. (62) Vanderah, D. J.; Valincius, G.; Meuse, C. W. Langmuir 2002, 18, 46744680. (63) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5849-5852. (64) Fick, J.; Steitz, R.; Leiner, V.; Tokumitsu, S.; Himmelhaus, M.; Grunze, M. Langmuir 2004, 20, 3848-3853. (65) Kim, H. I.; Kushmerick, J. G.; Houston, J. E.; Bunker, B. C. Langmuir 2003, 19, 9271-9275. (66) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Langmuir 2003, 19, 2284-2293. (67) Vanderah, D. J.; Arsenault, J.; La, H.; Gates, R. S.; Silin, V.; Meuse, C. W. Langmuir 2003, 19, 3752-3756. (68) Pertsin, A. J.; Grunze, M. Langmuir 2000, 16, 8829-8841. (69) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. B 1997, 101, 9767-9773. (70) Dicke, C.; Ha¨hner, G. J. Phys. Chem. B 2002, 106, 4450-4456. (71) Chan, Y.-H. M.; Schweiss, R.; Werner, C.; Grunze, M. Langmuir 2003, 19, 7380-7385. (72) Kreuzer, H. J.; Wang, R. L. C.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 8384-8389. (73) Pertsin, A. J.; Hayashi, T.; Grunze, M. J. Phys. Chem. B 2002, 106, 12274-12281. (74) Dicke, C.; Ha¨hner, G. J. Am. Chem. Soc. 2002, 124, 12619-12625. (75) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252812536. (76) Zhang, H.; Mirkin, C. A. Chem. Mater. 2004, 16, 1480-1484.

Saalmink et al.

on materials that require very reactive ink headgroups, such as activated silyl groups for the patterning of silicon or metal oxides.21-23 Experimental Section Materials. The stamp material, Sylgard-184 PDMS, was obtained from Dow Corning. It was mixed in a 1/10 curing agent/prepolymer ratio, and cured overnight at 65 °C. Silicon wafers were modified with an about 500 nm thick thermal silicon oxide layer, a titanium adhesion layer (5 nm, evaporated) on top, and finally an evaporated gold layer with a thickness of 20 nm. Prior to use, the substrates with a size of about 1 × 2 cm2 were rinsed with, subsequently, ultrapure water (resistivity >18 MΩ cm), ethanol, and heptane. They were thereafter exposed to a TEPLA 300E microwave argon plasma (0.25 mbar of Ar, 300 W, 5 min). ODT (98% purity) was purchased from Sigma-Aldrich and used as received. Ethanol and n-heptane (p.a. grade) were purchased from Merck. MAOEGs 1-8 were synthesized adopting published methods.49,77,78 Synthesis and characterization details are described in the Supporting Information. Thiol solutions (2-20 mM) were freshly prepared before each set of experiments by dissolving the respective thiols in ethanol. Dissolution was supported by immersion in an ultrasound bath for about 10 min. Patterning. PDMS stamps (about 1 × 2 cm2) bearing various electronic test patterns were equilibrated for a minimum of 2 h in the corresponding ink solutions. Where indicated, the inking time was reduced to 10 min. Temporarily surface-hydrophilized PDMS stamps were obtained by treatment of regular PDMS stamps with an oxygen plasma (TEPLA 300E plasma oven, 0.25 mbar of O2, 300 W, 30 s). Oxidized stamps were inked immediately after the plasma treatment by immersing them in the corresponding ink solution for about 10 min. After removal from the ink solutions, all stamps were rinsed with ethanol and dried in a stream of nitrogen for about 30 s immediately prior to use. Stamping was performed manually by using tweezers for stamp handling and by taking advantage of the natural stamp-substrate adhesion. No extra pressure was applied. Stamped substrates were immersed in ODT solutions (2 mM in ethanol) in polyethylene containers, which were open to the ambient. The samples were etched at room temperature in open polyethylene containers filled with one of the following etching baths: (1) a standard thiosulfate-based etching bath containing K2S2O3 (0.1 M), K3Fe(CN)6 (0.01 M), and K4Fe(CN)6 (0.001 M) in aqueous KOH (1.0 M) solution;10,79 (2) a cyanide-based etching bath containing KCN (0.01 M), K3Fe(CN)6 (0.01 M), and K4Fe(CN)6 (0.001 M) in aqueous KOH (1.0 M) solution;80 (3) an aqueous thiourea-based etching bath containing thiourea (0.1 M), Fe2(SO4)3 (0.01 M), and sulfuric acid (0.01 M).79,81 The first two alkaline etching baths were used either with or without added 1-octanol (at half-saturation).18 All etching baths were prepared freshly before use. Characterization. Optical micrographs were taken of etched patterns. Atomic force microscopy (AFM) analysis was performed on a Veeco MultiMode scanning probe microscope with a NanoScope IV controller or on a TopoMetrix Accurex II system. SAM-modified substrates were analyzed in contact (friction) mode using an Ultrasharp µMash NSC21 type A cantilever, and etched substrates were analyzed in noncontact (tapping) mode using an Ultrasharp µMash NSC16 type A cantilever. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Quantum 2000 instrument from Ulvac-PHI. All XPS data were acquired at a nominal photoelectron takeoff angle of 45°, where the takeoff angle is defined as the angle between the surface normal and the axis of the analyzer lens, using Al KR radiation providing an information depth of about 6 nm. Areas of about 1200 × 500 µm2 were scanned with a spot (77) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (78) Wenzl, I.; Yam, C. M.; Barriet, D.; Lee, T. R. Langmuir 2003, 19, 1021710224. (79) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332-2337. (80) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 0, 1498-1511. (81) Groenewald, T. Hydrometallurgy 1976, 1, 277-290.


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Chart 1

of 100 µm diameter. All neutralizing beams (slow electrons, lowenergy ions) were switched off during the analyses to prevent radiation damage. Atomic concentrations and the binding state of chemical species were determined on the basis of accurate narrow-scan measurements. CasaXPS82 was used for the analysis of the XPS spectra (curve fitting and quantification of peak areas). Analyses were usually performed in duplicate, and average values are reported provided that the spreading of individual results was smaller than 10% of the reported average value.

Results Synthesis. Monodisperse triethylene glycol derived thiol 1 (Chart 1) was prepared by reaction of diethylene glycol monomethyl ether with sodium hydride and subsequent treatment of the sodium salt with an excess of bis(chloroethyl) ether to give the chloride compound. Further reaction with potassium thioacetate, followed by acidic hydrolysis of the resulting thioacetate, provided the distillable MAOEG 1. MAOEGs 2 and 3 were prepared similarly by reactions of triethylene glycol monomethyl ether with sodium hydride. The sodium salts were treated with an excess of 1,6-dibromohexane and 1,10-dibromodecane, respectively, to yield bromide intermediates. Those were allowed to react with potassium thioacetate, followed by acidic hydrolysis of the resulting thioacetate, to provide the distillable MAOEGs 2 and 3. 1,16-Dibromohexadecane is commercially not available. Therefore, the synthesis of 4 started with the bromodecylation of (6-bromohexyl)tri(ethylene glycol) monomethyl ether. Subsequent reaction with potassium thioacetate, followed by acidic hydrolysis, yielded crude 4, which was purified by column chromatography and Kugelrohr distillation. Thiols 5 and 6 were prepared starting from commercially available polydisperse poly(ethlyene glycol) monomethyl ethers with average molecular weights of 350 g/mol and 750 g/mol, respectively, which were converted to the corresponding tosylates with p-toluenesulfonyl chloride and triethylamine. These tosylates were allowed to react with potassium thioacetate followed by hydrolysis and chromatographic purification to provide the desired thiols. Thiols 7 and 8 were synthesized starting from 11bromoundecanol. Protection of the alcohol functions with ethyl vinyl ether (EVE), followed by reaction with the sodium alcoholates of oligo(ethylene glycol) methyl ethers with average molecular weights of 350 g/mol and 750 g/mol, respectively, yielded the EVE-protected compounds after chromatographic purification. Deprotection, tosylation, and subsequent reaction with potassium thioacetate, followed by hydrolysis, yielded the desired thiols, which contained some minor amounts of the corresponding disulfides. All MAOEGs were characterized by 1H NMR spectroscopy, elemental analysis, and mass spectrometry. Overall yields were generally higher than 60%. Positive Microcontact Printing. The studied (+)µCP method consists of three steps (Figure 2). In the first step, a patterned SAM of one of the MAOEGs 1-8, the positive ink, was printed (82) CasaXPS version 2.2.52, http://www.casaxps.com.

Figure 3. Optical micrographs of gold substrates patterned via conventional (-)µCP using ODT as the ink (A) and via (+)µCP with MAOEG 7 as the positive ink for stamping (10 mM in ethanol, 10 s contact time) and ODT (2 mM, 10 min) as the solution-mediated second ink (B). The light areas are elevated gold features. These areas were covered with an etch-resistant ODT SAM prior to etching. In the darker depressed areas the gold was etched away down to the Ti layer. Those areas were covered with no SAM (sample A) or a less-etch-resistant MAOEG SAM of thiol 7 prior to etching (sample B). Since in both cases the same stamp pattern was used, the pattern of sample B is inverted with respect to that of reference sample A.

onto the surface of a gold substrate with a PDMS stamp bearing a surface relief. In the second step, the remaining unmodified areas were filled with a second SAM by self-assembly upon either immersion of the substrate in an ethanolic ODT solution or stamping with an unpatterned flat PDMS stamp loaded with the ODT ink. Finally, the pattern was developed by an etching procedure, in which the initially printed MAOEG SAM provided less etch resistance than the ODT SAM, to yield the corresponding gold pattern. The studied MAOEGs can be subdivided into two groups. MAOEGs 1-4 with the general formula HS(CH2)n(OCH2CH2)3OCH3, with n ) 2, 6, 10, or 16, are monodisperse, bearing an identical tri(ethylene glycol) group and alkyl groups of varying length. The remaining MAOEGs 5-8 were synthesized and used as polydisperse compositions. The (average) molecular formulas are shown in Chart 1 and represent the four possible permutations of short and long alkyl fragments combined with short and long oligo(ethylene glycol) groups. In an initial screening protocol, PDMS stamps bearing a pattern of various test structures were inked with a 10 mM ethanol solution of each of the eight MAOEG compositions for 2 h. Gold substrates were then printed with a contact time of 10-15 s. Following immersion in an ethanolic ODT solution (2 mM) for 5 min, rinsing with ethanol, and drying, they were etched in a standard thiosulfate-ferricyanide bath containing 1-octanol at halfsaturation. The minimum time necessary to completely remove the 20 nm thick gold layer from the contacted areas was deduced from optical inspection of the etching process. Under these conditions unmodified gold was removed completely within about 12 min. For gold covered with thiols 1-7 etching times of 2040 min were observed. Experiments with thiol 8 suffered from a very poor reproducibility, which was correlated with the appearance of a white precipitate on the stamp surface. Figure 3 shows optical micrographs of two substrates that were patterned by (-)µCP (sample A) and (+)µCP (sample B), employing in both cases an ODT SAM as the etch-resistant layer. Sample A was printed with an ODT-inked stamp prior to etching. Sample B was printed with an equal stamp that was inked with MAOEG 7, however. Subsequent immersion in an ODT solution followed by etching yielded an inverted pattern with respect to sample A. Features with nominal dimensions down to 1 µm were reproduced with both printing methods.

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Table 1. (+)µCP with MAOEGs 1-7: Etching Times and Pattern Qualitya thiol

tetch/ min

1 2 3 4 5 6 7

30-40 30-40 30-35 35-40 20-40d 20-40 30-35

quality of pattern and contrastb nonoxidizedc oxidizedc 0 0 0 +/0d +/-d ++

0 +/0d +/-d +

a Experimental conditions: [MAOEG] ) 10 mM (ethanol); contact time 10 s; immersion time in 2 mM ODT solution (ethanol), 5 min; etching solution, standard thiosulfate/ferricyanide/octanol. b The quality has been judged by optical inspection relative to the other experiments of this series. Contrast and resolution: ++ ) very good, + ) good, 0 ) poor, - ) very poor, - - ) no pattern. c PDMS stamp treatment prior to inking and printing: oxidized ) O2 plasma treatment for 30 s, nonoxidized ) no special treatment. d A large variance was observed in the results.

Earlier reports on µCP with MAOEGs describe the use of PDMS stamps that had been temporarily hydrophilized by treatment with an oxygen plasma prior to inking.83,84 To study the influence of the stamp surface treatment on the (+)µCP result, the above experiments were repeated employing plasma-oxidized as well as untreated PDMS stamps. Inking times were reduced to 10 min, to prevent hydrophobic recovery of the oxidized stamps.85,86 The results are summarized in Table 1. It can be concluded that the inking time of the untreated PDMS stamps (2 h vs 10 min) had no significant influence on the etching time and pattern quality. Good contrast and pattern resolution were obtained with MAOEG 5,87 and the best results were obtained with compound 7. Inspection of Table 1 further reveals comparable trends in the (+)µCP results obtained with an untreated and an oxidized stamp. Nevertheless, the pattern obtained with a surface-oxidized stamp was of poorer quality in the case of compound 7. The variations in the observed etching times triggered us to study the influence of the type of etching solution on the rate and quality of the development process. In this context gold substrates were patterned employing compound 7 in combination with ODT as described above and subsequently developed with different standard etching baths, including the above-described alkaline thiosulfate bath, an alkaline cyanide bath, and an acidic thiourea bath. Etching times of 35, 1.5, and 15 min were observed, respectively, which did not change upon the addition of 1-octanol at half-saturation to improve the pattern quality. A rather poor contrast was obtained with the thiourea bath. The cyanide and thiosulfate baths etched very homogeneously and provided a comparably good contrast. For initial screening experiments thiosulfate etching was therefore preferred due to the therewith associated easier working procedures (larger etch window, lower toxicity). It was, nevertheless, eventually found that cyanide etching provided a somewhat better pattern quality in the optimized printing protocol. A low exchange rate of the molecules forming the initially printed SAM with the components of the immersion solution is (83) Zhang, S.; Yan, L.; Altman, M.; La¨ssle, M.; Nugent, H.; Frankel, F.; Lauffenburger, D. A.; Whitesides, G. M.; Rich, A. Biomaterials 1999, 20, 12131220. (84) Zhou, Y.; Valiokas, R.; Liedberg, B. Langmuir 2004, 20, 6206-6215. (85) Hillborg, H.; Tomczak, N.; Ola`h, A.; Scho¨nherr, H.; Vancso, G. J. Langmuir 2004, 20, 785-794. (86) Lai, J. Y.; Lin, Y. Y.; Denq, Y. L.; Shyu, S. S.; Chen, J. K. J. Adhes. Sci. Technol. 1996, 10, 231-242. (87) A poor reproducibility was observed in the results obtained with compound 5.

Table 2. (+)µCP with MAOEG 7: Immersion and Etching Timesa timmersion/ s

tetch/ min

contrastb

2 5 30 60

19 25 27 31

-0 + ++

timmersion/ s

tetch/ min

contrastb

300 600 1500

30 35 45

++ ++ +

a Experimental conditions: [7] ) 10 mM (ethanol); contact time 10 s; immersion in 2 mM ODT solution (ethanol) for varying time lengths; etching solution, standard thiosulfate/ferrisulfate/octanol. b The quality has been judged by optical inspection relative to the other experiments of this series. Contrast and resolution: ++ ) very good, + ) good, 0 ) poor, - ) very poor, - - ) no pattern.

Figure 4. Optical micrographs of gold substrates patterned via conventional (-)µCP using ODT as the ink (A) and via (+)µCP with MAOEG 5 as the positive ink for stamping (10 mM in ethanol, 10 s contact time) and ODT (2 mM, 10 min) as the solution-mediated second ink (B). The light areas are elevated gold features. In the darker depressed areas the gold was etched away down to the Ti layer. Since an identical stamp pattern was used, the obtained substrate pattern in sample B is inverted with respect to the reference sample A, in which conventional (-)µCP was used. The nominal size of the small squares filling the pattern is 10 × 10 µm2. The circular features in the ring transistor structures have nominal widths of 2.5, 1.5, and 1 µm (left to right). The large defects in micrograph B result from reproducible particle defects in the stamp used.

crucial for a good contrast. The influence of the immersion time on the pattern quality was therefore studied for MAOEG 7 under otherwise identical conditions as described before. Immersion times were varied between 2 and 1500 s, resulting in etching times increasing from 19 to 45 min for the longest immersion times (Table 2), which is indicative of a slow exchange of MAOEG for ODT molecules. Immersion times between 1 and 10 min yielded developed patterns with the best contrast. It can be expected that the thiol exchange rate strongly depends on the surface coverage and order of the printed SAM. For a first evaluation of this factor, a series of (+)µCP experiments was performed with MAOEGs 1-7 using various ink concentrations (2-16 mM in ethanol, data not shown), from which we conclude an optimum ink concentration of 10 mM. Higher concentrations resulted in a significant size change of individual features, which was presumably caused by enhanced surface spreading of the MAOEG ink during printing.88,89 The achievable resolution was further found to be ink dependent. With MAOEG 1, which has a relatively low molecular weight, lines with a nominal width of less than 2 µm could not be reproduced. MAOEGs 5 and 7, on the other hand, which have a higher molecular weight, allowed the reproduction of gaps and lines with a nominal width down to 1 µm. Figure 4 shows optical micrographs obtained after (+)µCP on gold with compound 5 as the positive ink and ODT immersion. The circular features in the ring transistor structures have nominal widths of 2.5, 1.5, and 1 µm (left to right). A large (88) Sharpe, R. B. A.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. Langmuir 2004, 20, 8646-8651. (89) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334.


Figure 5. Tapping mode AFM micrograph (A) of the patterned surface of a PDMS stamp bearing square features with a size of 10 × 10 µm2 and a feature height of 2 µm. Friction mode AFM micrograph (B) of a gold substrate which was contacted with the above PDMS stamp for 15 s. The stamp had been inked with a 10 mM solution of 7 in ethanol for 30 min prior to printing. Friction mode AFM micrographs (C-F) of gold substrates that were printed as in (B) with various contact times and subsequently immersed in an ethanol solution of ODT (2 mM) for 15 s. Contact times were 5 s (C), 15 s (D), 30 s (E), and 60 s (F).

variance was observed in the results obtained with compound 5. We have not yet been able to identify the critical parameters for this particular system. On the basis of these initial screening experiments, 7 was concluded to be the most suitable MAOEG derivative for further, more detailed studies. MAOEGs 1 and 4 were used as reference compounds where indicated. Atomic Force Microscopy. Printed gold substrates were analyzed by AFM prior to and after etching. Figure 5A shows a tapping mode AFM picture of the patterned surface of a PDMS stamp bearing protruding square features with a size of 10.0 ×10.0 µm2 and a feature height of 2 µm. Figure 5B displays the result of a friction mode AFM analysis of a gold substrate printed for 15 s with the above stamp inked with thiol 7. The dark square feature corresponds to the contact area. It has a width of 14.0 µm, which translates to a spreading of thiol 7 of about 2 µm in each direction during the printing process. To assess the spreading and exchange characteristics of the MAOEG pattern with an ODT solution, a series of substrates was printed with contact times between 5 and 60 s and subsequently immersed in an ethanolic solution of ODT (2 mM) for 15 s (AFM micrographs C-F in Figure 5). The size of the square features increased significantly with increasing contact

Langmuir, Vol. 22, No. 3, 2006 1021

Figure 6. Friction and tapping mode AFM analysis of patterned gold substrates printed with MAOEG 7 and subsequently immersed (dipped) in an ethanolic solution of ODT (2 mM). From the friction contrast between the areas covered with either MAOEGs or ODT, the image size of the square stamp features with a nominal size of 10 × 10 µm2 after stamping and ODT adsorption was determined. (A) The contact time (tstamp) of the PDMS stamps with the substrate was varied between 5 and 60 s. The dipping time of the printed substrates in the ODT solution was either 15 s (circles) or 5 min (squares). Samples were analyzed with respectively friction or tapping mode AFM before (closed data points) and after etching (open data points) in a standard cyanide etching bath. (B) Friction mode AFM analysis of nonetched substrates. The contact time (tstamp) was constant (15 s), and the dipping time (tdip) in the ODT solution was varied between 15 s and 25 min (closed squares). In one case the printed substrate was contacted with an unpatterned flat stamp (inked with ODT) for 15 s instead of dipping (open circle).

time. Figure 6A shows the average width L of these squares as a function of the stamping time (closed circles). The width increased linearly with stamping time and almost doubled with respect to the nominal value of 10 µm within 60 s. When the dipping time in the ODT solution was increased to 5 min, L became smaller compared to that of the shorter dipped substrates (closed squares). This resulted in a lower increase of the square size with the stamping time, which is indicative of a preferred exchange of the outermost MAOEG molecules of the pattern with ODT molecules from solution. A longer dipping time can therefore compensate for the spreading of the MAOEG during stamping. Although L did not change significantly after 15 s of stamp contact prior to and after immersion in ODT solution for 15 s (Figure 5B,D), an increase of the dipping time to 5 min resulted in a significant decrease of L by about 3 µm (Figure 6B). For very long dipping times (25 min) the square size approached the nominal value of 10 µm. This is indicative of different exchange characteristics of the MAOEG SAMs inside and outside the stamp contact areas. The open circle in Figure 6B corresponds to a sample that was contacted with an ODT-inked unstructured stamp for 15 s, instead

1022 Langmuir, Vol. 22, No. 3, 2006

Figure 7. Tapping mode AFM micrographs (left) and corresponding height profiles (right) of gold substrates patterned via (+)µCP with MAOEG 7 as the positive ink for stamping (10 mM in ethanol) and ODT (2 mM, 5 min dipping time) as the etch-resistant second ink. The stamp-substrate contact time was 15 s (A) or 30 s (B). The light areas are ODT-covered elevated gold features. In the dark depressed areas the gold has been etched away with a cyanide etching bath down to the titanium adhesion layer. The square depressed areas have a nominal size of 10 × 10 µm2 and a nominal distance of 10 µm according to the stamp pattern. The measured average widths are 10.6 ( 0.3 µm (A) and 11.8 ( 0.4 µm (B).

of dipping it in an ODT solution after printing with 7. Considering the smaller L value, the feature shrinkage resulting from the exchange with ODT along the edges appears to have been faster upon stamping than upon dipping in an ODT solution, which makes the process more difficult to control. It was further observed that the contrast obtained after etching of such substrates was poorer, which is indicative of an increased exchange reaction also in the contact areas. Therefore, solution immersion rather than stamping was chosen as the ODT application method in the subsequent experiments. The dipping time was furthermore found to be a determining factor for the quality of the developed pattern. The open circles in Figure 6A result from the AFM analysis of developed samples that were stamped with 7 and dipped in ODT solution for 15 s prior to etching in a standard cyanide bath. In direct comparison with the corresponding data of nonetched substrates (closed circles), similar trends are observed. The depressed square features were found to be slightly smaller than expected on the basis of the SAM patterns, particularly in the case of long immersion times in the ODT solution (open squares). For samples that were stamped for more than 15 s with 7 and dipped for only 15 s in ODT, the selectivity of the etching process was too poor to obtain a clear pattern, which indicates an insufficient etch stability of the adsorbed ODT SAM under these conditions. The best pattern quality was obtained with a stampsubstrate contact time of 15-30 s and a dipping time of 5 min (Figure 7). X-ray Photoelectron Spectroscopy. XPS analysis was performed to correlate the printing conditions and results with the surface coverage and composition of the deposited SAMs. Monolayers of MAOEGs were deposited on unpatterned gold samples either by printing with an unpatterned flat stamp or by immersion of the samples in an ethanolic MAOEG solution (10 mM) for 15, 60, or 1800 s immediately prior to XPS analysis.

Saalmink et al.

Figure 8. Typical S2p XPS spectrum of MAOEG-covered gold. After background subtraction the experimental spectrum has been fitted with three doublets corresponding to three chemically different sulfur species (the S2p peak is a doublet with a peak distance of 1.18 eV and a ratio of 2/1). The signals are assigned to gold thiolates (162.0 eV), unbound thiols (163.5 eV), and sulfoxo species (169 eV) (see the text).

As expected, XPS signals were observed only for the elements Au, C, O, and S. In only a few cases traces of Si were detected in negligible concentrations. The maximum of the Au4f7 peak was shifted to 84.0 eV to adjust for sample charging. The S2p spectra could be fitted assuming the presence of three chemically different classes of sulfur species (Figure 8),90 adopting a doublet structure with a doublet distance of 1.18 eV and a ratio of 2/1. Peaks at 162.0 ( 0.1 eV are assigned to sulfur in gold-bound thiolates, and signals found at 163.5 ( 0.2 eV are assigned to unbound, undissociated thiol groups.90 As a third component, oxidized sulfur species are represented by peaks between 168.0 and 169.5 eV.91,92 The appearance of oxidized species is not unexpected given the oxidation sensitivity of alkanethiols and the fact that all SAM formation experiments were performed in an ambient atmosphere with no special precautions to exclude air oxygen.93,94 Curve fitting and quantification of the XPS spectra provided the raw concentrations at the surface of the samples. The raw concentrations were converted to more meaningful quantities (real atomic concentrations in the SAM layer, layer thickness, and surface coverage with sulfur) by means of a calculation method described elsewhere.95 Values for the inelastic mean free path were taken from P. J. Cumpson et al.96 Table 3 summarizes the results obtained with MAOEGs 1, 4, and 7. The surface coverage with sulfur atoms [S] was utilized to determine the SAM surface coverage, which is [S]max ) 4.67 × 1014 atoms cm-2 for a full layer of sulfur atoms on a [111]oriented gold surface.50,75,97 For all three studied MAOEGs the surface coverage increased by a factor of about 2 or more with an increasing stamp contact time from 15 to 1800 s. The increase in surface coverage was less pronounced for SAMs formed by (90) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 50835086. (91) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657-6662. (92) Pavlovic, E.; Quist, A. P.; Gelius, U.; Nyholm, L.; Oscarsson, S. Langmuir 2003, 19, 4217-4221. (93) Lee, M.-T.; Hsueh, C.-C.; Freund, M. S.; Ferguson, G. S. Langmuir 1998, 14, 6419-6423. (94) Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639. (95) van der Marel, C.; Yildirim, M.; Stapert, H. R. J. Vac. Sci. Technol., A 2005, 23, 1456-1470. (96) Cumpson, P. J. Surf. Interface Anal. 2001, 31, 23-34. (97) XRD experiments indicate that the used sputtered gold surfaces exhibit a [111] orientation. A surface density of [S]max ) 4.67 × 1014 atoms cm-2 corresponds to an area coverage of 0.214 nm2/thiolate on an ideal smooth surface.


Langmuir, Vol. 22, No. 3, 2006 1023

Table 3. Results from XPS Experiments Performed on Gold Substrates Homogeneously Modified with Printed or Adsorbed SAMs of MAOEG 1, 4, or 7a MAOEG 1 4 7

tprint/ s

tdip/ s

[S]/ 1014 atoms cm-2

coverage/ %

f(S-Au)/ %

f(S-H)/ %

f(S-O)/ %

O1s/C1s(exptl)

O1s /C1s(theor)

24 36 62 43 62 81 47 60 98 75 90 90

78 70 76 61 71 76 100 100 97 86 90 89

6 12 16 7 18 22 0 0 3 14 10 11

16 18 8 32 11 2 0 0 0 0 0 0

0.56 0.34 0.79 0.25 0.25 0.36 0.27 0.26 0.26 0.24 0.23 0.18

0.44

15 60 1800

1.1 1.7 2.9 2.0 2.9 3.8 2.2 2.8 4.6 3.5 4.2 4.2

15 60 1800 15 60 1800 15 60 1800

7

0.17 0.32 0.32

a [MAOEG] ) 10 mM (ethanol); the same solution has been used for inking the stamp (for 2 h) and immersion of the substrate. [S] denotes the coverage with sulfur in Au-thiolate bonds; the relative amounts of the three sulfur species present are denoted by f(S-Au), f(S-H) and f(S-O). In the two right-hand columns the concentration ratios O/C are given (“exptl” as determined from the XPS results and “theor” based upon the theoretical composition (Chart 1)).

Table 4. Theoretical O/C Ratios for MAOEGs of the General Formula HS(CH2)11(OCH2CH2)nOCH3 O/C

n)1

n)2

n)3

n)4

n)5

n)6

n)7

n)8

0.14

0.19

0.22

0.25

0.27

0.29

0.31

0.32

solution adsorption of thiol 7. In this case about 75% of the maximum surface coverage on gold was reached already after an immersion time of just 15 s, which is significantly higher than that obtained after the same printing time. SAMs formed from 7 reach a higher surface coverage in a shorter time than those formed from the other two MAOEGs. Comparison of the fraction of oxidized and nonbound sulfur species reveals striking differences between the different MAOEGs. For thiol 7 no oxidized species were detected at the gold surface, whereas for thiols 1 and 4 a significant amount of oxidation products was observed, although starting materials were found to be free of oxidized sulfur species by NMR analysis. The relative concentration of these sulfoxo species decreased with an increasing contact time, which may be caused by one of the following mechanisms. Oxidation can occur at the stamp surface, possibly causing a higher sulfoxo concentration at the surface than in the bulk, which may result in a preferred initial transfer of these species upon contact with the substrate, followed by slow exchange with nonoxidized thiols originating from the bulk of the stamp. Alternatively, a higher sulfoxo concentration at the stamp surface could result from oxidation processes occurring already in the inking solution considering that the more polar oxidation products have a lower affinity for the hydrophobic bulk PDMS.98 Oxidation may also occur at the substrate surface after stamping. In this scenario the decrease of the fraction of sulfoxo species correlates with the increase of the sulfur surface coverage and thus a lower accessibility of the gold-bound sulfur headgroups for atmospheric oxygen.42,99 Currently, we cannot distinguish between these three mechanisms. Nevertheless, printed SAMs of thiol 7 showed no detectable sulfoxo content, which again may either allow for or be a cause of the high SAM completeness. Surprisingly, the content of unbound thiols was found to be smaller for printed than for adsorbed SAMs of 7. We ascribe the generally too high O/C ratio found for thiols 1 and 4 to a high concentration of adsorbed water at these less dense monolayers. Interestingly, for printed as well as for adsorbed SAMs of thiols 7 the observed O/C ratio is smaller than the calculated value. Although this difference is small and independent of the contact time for printed samples, a further decrease was observed for SAMs adsorbed from solution for a longer

immersion time. This observation may be explained by taking into account the polydispersity of thiol 7. It had been shown that the substitution of mercaptododecane with a hydroxo-terminated tetra(ethylene glycol) group results in an increased initial adsorption rate from ethanol to gold when compared to that of the respective unsubstituted alkanethiols.100 This effect reverses, however, already at less than semicomplete surface coverage, probably due to slow reorganization processes, which are necessary for a high packing density of the OEG groups. Consequently, a preferred adsorption of short-chain MAOEG fragments would be expected at a higher surface coverage, as observed here. The further decrease of the O/C ratio with increasing dipping time at a constant high surface coverage suggests that this process is dynamic and allows for an exchange of already adsorbed thiols with the solution.84 Table 4 correlates calculated O/C values for polydisperse 7 with the number of ethylene glycol groups per thiol. It can be deduced that for the longest immersion time thiols with an average of about three glycol oxygen atoms are adsorbed at the gold surface. The absence of a solvent in SAM printing, on the other hand, may prevent this mechanism from becoming effective, which may explain the contact-time-independent higher O/C ratio in this case. In addition to this, we assume a higher driving force for the transfer of long OEG thiols during stamping, since these hydrophilic molecules are less compatible with the hydrophobic PDMS stamp matrix. A low exchange rate of printed MAOEGs with etch-protecting ODT molecules is essential for a good contrast in the heredescribed (+)µCP method. The change of the composition of MAOEG SAMs of compounds 1, 4, and 7 after different exposure times to an ODT solution was therefore studied by XPS. SAMs of thiols 1 and 4 were formed via µCP with an unpatterend flat stamp and a contact time of 15 s. SAMs of 7 were prepared analogously by printing or by adsorption from solution as described above. The MAOEG-modified gold samples were subsequently immersed in a 2 mM solution of ODT in ethanol (98) Delamarche, 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, 8749-8758. (99) Wolf, K. V.; Cole, D. A.; Bernasek, S. L. Anal. Chem. 2002, 74, 50095016. (100) Jung, L. S.; Campbell, C. T. J. Phys. Chem. B 2000, 104, 11168-11178.

1024 Langmuir, Vol. 22, No. 3, 2006

Saalmink et al.

Table 5. Results from XPS Experiments Performed on Gold Substrates, Which Were Homogeneously Covered with Printed or Adsorbed SAMs of MAOEG 1, 4, or 7, and Subsequently Dipped in a Solution of ODT or Printed with an ODT-Inked Stamp for Different Time Periodsa MAOEG no

1b

4b

7b

7c

tdip/ min

[S]/ 1014 atoms cm-2

coverage/ %

f(S-Au)/ %

f(S-H)/ %

f(S-O)/ %

O1s/C1s(exptl)

O1s/C1s(theor)

0.25 1 5 30 0 1 5 30 0 1 5 30 0 1 5 30 0 1 5 30

3.4 3.3 3.5 3.6 1.7 3.7 3.5 4.0 2.0 2.9 3.1 3.5 2.5 3.2 3.6 3.3 3.5 3.1 4.0 3.9

73 71 75 77 36 79 75 86 43 62 66 77 54 69 77 71 75 66 86 84

96 93 95 94 78 67 82 70 61 64 84 85 94 90 97 93 86 88 88 89

4 7 5 6 6 12 9 13 7 36 16 15 6 10 3 7 14 12 12 11

0 0 0 0 16 21 9 17 32 0 0 0 0 0 0 0 0 0 0 0

0.0 0.0 0.0 0.0 0.56 0.02 0.01 0.01 0.25 0.13 0.13 0.15 0.29 0.20 0.18 0.15 0.24 0.14 0.10 0.07

0.0

0.44

0.17

0.32

0.32

a Experimental conditions: (step 1) [MAOEG] ) 10 mM (ethanol), dipping or contact time 15 s; (step 2) immersion in 2 mM ODT solution (ethanol) for 0-30 min. b SAM formed via µCP (contact time 15 s). c SAM formed via immersion in ODT solution (dipping time 15 s).

Figure 9. Change of the O/C ratio as determined by XPS for a gold sample bearing a SAM of MAOEG 1 (green squares), 4 (red circles), or 7 (blue triangles) upon exposure to an ODT solution. The MAOEG SAM was formed either by printing with a flat PDMS stamp (closed data points) or by dipping in a respective ethanol solution (light blue open triangles, [MAOEG] ) 10 mM) for a constant printing/dipping time t1 ) 15 s. The substrates were subsequently dipped in an ethanolic ODT solution (2 mM) for different times t2. The plots are on linear (left) and logarithmic (right) time scales.

for different times between 0 and 30 min, rinsed with ethanol, and dried prior to XPS analysis. The results obtained by means of calculations as described before, starting from the raw concentrations, are summarized in Table 5. For comparison also given are XPS analysis results of ODT SAM formation via adsorption from solution on unmodified gold surfaces (no MAOEG pretreatment). An ODT coverage of larger than 70% was achieved within 15 s. All printed MAOEG samples showed an increase in surface coverage with increasing exposure time, whereby the highest surface coverage increase was observed already within the first minute. For thiol 1 this was accompanied by fast exchange of MAOEG against ODT molecules, which was almost complete within 1 min as deduced from the dramatic reduction of the O/C ratio. In fact, after 30 min the etch resistance of such layers against a standard thiolsulfate bath was indistinguishable from that of an ODT SAM grown on a clean gold substrate under identical conditions. The exchange of 4 against ODT is far less pronounced and ceased already after 1 min. This exchange was strikingly accompanied by a complete substitution of all initially present sulfoxo species, which suggests that the latter exchange with a higher rate than nonoxidized MAOEGs.94,101 After 1 min

the O/C ratio remained virtually constant, indicating no further replacement of MAOEG 4 molecules at the gold surface. Figure 9 summarizes the change of the O/C ratio for MAOEGs 1 (squares), 4 (circles), and 7 (triangles) upon dipping in ODT solution for different times t2. Printed (closed triangles) as well as adsorbed (open triangles) SAMs of 7 showed a steady decrease in their O/C ratio with immersion time in the ODT solution. This decrease was less pronounced for the printed SAM and was paralleled by an overall increase in SAM surface coverage, whereas for SAMs adsorbed from solution a higher decrease of the O/C ratio was found on a virtually constant surface coverage level. This is indicative of an exchange reaction with ODT molecules, which on the basis of these data is significantly slower for the printed than for the adsorbed SAM.

Discussion This study of a (+)µCP method with PDMS stamps is intended to provide a new type of positive ink. The chosen MAOEGs have been shown to form stable SAMs on gold, when adsorbed (101) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032.


from dilute solution.13,25,56,67 The properties of their SAMs formed by µCP are less well-known.83,84 Hence, MAOEGs 1-8 have been evaluated in more detail for their potential use in a (+)µCP method, which comprises printing an MAOEG SAM on a gold surface, filling the remaining unmodified areas with an ODT SAM via adsorption from solution, and finally developing the pattern by etching selectively through the MAOEG SAM. Dramatic differences were found among the eight MAOEGs with respect to the etching times required, the quality of the final gold pattern, the achievable resolution, and the reproducibility of the result. Patterning experiments employing the polydisperse MAOEGs 5, 6, and 8 as the positive ink suffered from a poor reproducibility of the results. These molecules bear an OEG chain, which is at least 4 times as long as the oligomethylene part. The resulting high hydrophilicity makes them poorly compatible with the hydrophobic PDMS stamp material. Stamps inked with thiol 8 (Mav ≈ 987) prior to printing were observed to show white solid residues probably resulting from clustered precipitation of 8 at their surface. However, an oxidative treatment intended to provide the stamp surface temporarily with a more hydrophilic character did not result in improved patterning characteristics with any of the here-studied MAOEGs.86,98 (+)µCP with the monodisperse MAOEGs 1-4 of the general formula HS(CH2)m(OCH2CH2)3OCH3, with m ) 2, 6, 10, or 16, was found to provide after etching only a poor contrast between the printed and the ODT-adsorbed areas of the developed pattern. Especially for the shorter molecules, pattern shrinkage is a problem. This can be ascribed to the tendency of low molecular weight inks to spread over the surface, which allows the molecules to cover substrate areas outside the contact regions. The inverted nature of the (+)µCP method causes this effect to result in shrinkage instead of growth of the final pattern as it is observed in regular (-)µCP techniques. For the printing of alkanethiols on coinage metal surfaces Delamarche et al. found a dependence of the extent of spreading on the molecular weight of the ink as well as on the ink concentration and the contact time during printing.89 A similar concentration dependence was observed for mercaptohexadecanoic acid (MHDA), whereas for the occurrence of a contact time dependence a minimum ink concentration was established.88 Although we found an MAOEG ink concentration of not smaller than about 10 mM to be essential for achieving a good patterning contrast, a further increase of the ink concentration was found to cause dramatic pattern shrinkage. Optimized patterning conditions comprise stamping with an MAOEG-inked stamp (10 mM, ethanol) for 15-30 s, subsequent dipping in an ethanolic solution of ODT (2 mM), and final development in an octanol-enriched cyanide-ferricyanide etch bath. MAOEG 7 provided the best patterning results. In this case an edge shift (shrinkage) of about 0.5 µm compared to the stamp design was observed after printing, ODT adsorption, and etching under the described conditions. The selective penetration of the etching solution through the positive ink pattern is an essential aspect of the (+)µCP process. From the etch results obtained with three standard etching baths, which were based on cyanide, thiosulfate, or thiourea, we conclude a superior etch behavior of the cyanide bath containing ferricyanide as the oxidizing agent. A significantly better patterning result was obtained with an octanol-enriched bath when compared to the same cyanide-ferricyanide bath containing no further additive. Octanol has proven to have a defect healing and thus monolayer reinforcing effect on alkanethiol SAMs on gold, when added to respective etching solutions. The effect has been attributed to the ability of the lipophilic part of the molecule to insert itself into or cover structural defects in the hydrophobic

Langmuir, Vol. 22, No. 3, 2006 1025

SAM, which makes it more difficult for the ligand and the oxidant of the etching solution to reach the metal surface and initiate metal dissolution in the SAM-covered areas.18 The observation that the contrast enhancement in the presence of octanol was not accompanied by an increased etching time for the mixed MAOEG/ ODT SAM-covered substrates suggests that defect healing occurs selectively in the ODT layer. Most likely, this is a consequence of the incompatibility of the hydrophilic MAOG SAM with the rather hydrophobic octanol additive. The superior etch behavior of the cyanide bath may be rationalized on the basis of similar arguments. Strongly hydrated charged species, such as hydroxide, cyanide, and thiosulfate, are more likely than neutral thiourea to penetrate the hydrated upper part of the MAOEG SAM and accumulate at the interface to the underlying oligomethylene sublayer.70-72,102-104 Due to its smaller size, the cyanide ligand can furthermore be expected to have a higher MAOEG SAM penetration probability than the larger thiosulfate ion, which may explain the observed high etch selectivity of the cyanide bath. Similarly, gold covered with a hexadecanethiol SAM was reported to exhibit a higher pinhole density when etched with a cyanide instead of a thiosulfate solution.79 Delamarche et al. found a sufficiently low etch resistance against a cyanide-based etching solution for PTMP, when compared to an adjacent eicosanethiol SAM on gold, which was ascribed to the formation of only poorly ordered PTMP SAMs.40 This low SAM order can be attributed to the multivalence of PTMP in combination with its spherical structure. The etch resistance of PTMP SAMs furthermore depends on the printing conditions, which were ascribed to the different SAM densities obtained.45 An improved (+)µCP result for MAOEG 7 printed substrates could nevertheless not be observed when oxidized instead of regular PDMS stamps were employed. It is therefore believed that an incomplete and imperfectly ordered MAOEG SAM may in fact be beneficial for (+)µCP. A lower SAM completeness allows for more structural flexibility and a higher water content in the upper OEG sublayer of the SAM.56,57,67,68 As a consequence, the penetration of the SAM by the aqueous etching solution can be facilitated, which can yield an improved development contrast.73 Consistent with this hypothesis, the exchange of layers of MAOEG 1, which comprises a short alkyl chain and a very short OEG part, with ODT molecules was virtually complete within 1 min. MAOEG 4 is composed of an equally short OEG part, but a longer alkyl chain. In this case the exchange reaction was mainly restricted to less strongly bound oxidized or undissociated thiols. Although in this respect 4 may be considered a good candidate for a positive ink, due to its long alkyl chain, which stabilizes the SAM against solution exchange, it provides almost as good an etch resistance as ODT. Hence, with MAOEG 4 only a very poor contrast was obtained. MAOEG 7 is a suitable compromise. The relatively long alkyl chain provides sufficient SAM stability for a slow exchange reaction, and the long OEG part makes it sufficiently susceptible to the aqueous etching solution. The exchange of the MAOEG layer of 7 with ODT molecules in the periphery of the stamp contact areas was, nevertheless, found to be comparably fast. Exchange started at the outermost demarcation of the MAOEG SAM and progressed inward to the (102) Zangi, R.; Engberts, J. B. F. N. J. Am. Chem. Soc. 2005, 127, 22722276. (103) Schweiss, R.; Welzel, P. B.; Werner, C.; Knoll, W. Langmuir 2001, 17, 4304-4311. (104) Schweiss, R.; Welzel, P. B.; Werner, C.; Knoll, W. Colloids Surf., A 2001, 195, 97-102.

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borders of the stamp contact areas upon longer exposure to the ODT solution, as deduced from the friction mode AFM analysis. Although the exchange reaction is expected to initiate at exposed sites, such as the edges of the MAOEG layer, we interpret this strong directional effect as an indication for a gradual decrease of the SAM stability from the borders of the contact areas toward the outer demarcation of the peripheral MAOEG-covered areas. One possible reason is a surface coverage decrease of the SAM with increasing spread distance. Alternatively, the different components of polydisperse 7 may have a different spread rate, and a composition gradient may build up between the contact area and the peripheral demarcation. This hypothesis is supported by the observation of a preferred transfer of long, more hydrophilic OEG derivatives of 7 during stamping compared to solution adsorption. A SAM gradient of increasing OEG chain lengths and thus less structural order toward the outer demarcation of the noncontacted areas may explain the faster exchange in the outermost regions. The importance of the ink-stamp compatibility and its influence on ink transfer and surface spreading processes have recently also been recognized in a model describing the edge dominance phenomenon.88 Irrespective of the actual mechanism, a size difference was observed between the larger MAOEG-covered areas in the initial MAOEG/ODT pattern and the smaller depressed areas in the final developed pattern. This is indicative of a reinforcement of these outermost areas of the MAOEG layer by implemented ODT molecules that is sufficient to provide strong etch resistance, even before the reinforcement could be detected in the AFM friction image of the MAOEG/ODT pattern.

Conclusion As part of our program to develop novel soft lithographic patterning methods for the manufacturing of electronic devices, we have studied new inks for (+)µCP, which is particularly useful for the reproduction of structured conductive layers with low filling ratio patterns. Selected MAOEGs were found to be suitable “positive inks” in combination with self-assembled monolayers of ODT as the etch resist for the patterning of thin gold layers. A slow exchange rate of the MAOEG SAM with the subsequently adsorbed ODT molecules, which requires a

Saalmink et al.

certain length of the oligomethylene part of the molecules, and a high affinity for the aqueous etching solution, which requires a minimum chain length of their oligo(ethylene glycol) part, were identified as crucial factors in this system. The selectivity of the etch development process decreased in the order of a cyanide-, a thiosulfate-, and a thiourea-based etching bath. Spreading of the MAOEG molecules during printing was found to increase with increasing stamp-substrate contact time. A preferred exchange of MAOEG 7 with ODT molecules at the periphery of the MAOEG SAM could partially compensate for this spreading effect in the case of the most suitable polydisperse MAOEG with the average molecular formula HS(CH2)11(OCH2CH2)8OCH3. Under optimized patterning conditions a shrinkage of the elevated features of the developed pattern with respect to the original stamp pattern of about 1/2 µm was reproducibly observed, which is somewhat larger than the spreading observed for (-)µCP with ODT or (+)µCP with the PTMP/ODT system, but acceptable for a number of applications, including the production of large-area displays or RFID tags with minimum feature sizes of a few micrometers. The use of the chemically inert methoxy group at the tail end allows for an application of these types of positive ink molecules also for materials different from gold, which require more reactive functional ink headgroups, such as silyl chlorides, for bond formation with the substrate surface. Research toward the use of tunable MAOEG-based positive inks for the patterning of silicon, aluminum, and related materials is currently in progress. Acknowledgment. This work was supported in part by the European Commission, Contract No. NMP4-CT-2004-500355 (NAIMO). We are grateful to our colleague Harry A. G. Nulens for the AFM measurements. We thank our colleagues Michel M. J. Decre´, Ruben B. A. Sharpe, Emiel Peeters, and Wendy Dittmer for many fruitful discussions and their helpful comments. Supporting Information Available: Details of the synthesis and characterization of the mercaptoalkyloligo(ethylene glycol) methoxy ethers. This material is available free of charge via the Internet at http://pubs.acs.org. LA052513V

Positive Microcontact Printing with Mercaptoalkyloligo(ethylene glycol)s

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