Direct Patterning of NiB on Glass Substrates Using Microcontact

Langmuir 2003, 19, 6283-6296

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Direct Patterning of NiB on Glass Substrates Using Microcontact Printing and Electroless Deposition Matthias Geissler, Hannes Kind, Patrick Schmidt-Winkel, Bruno Michel, and Emmanuel Delamarche* IBM Research, Zurich Research Laboratory, Sa¨ umerstrasse 4, 8803 Ru¨ schlikon, Switzerland Received February 24, 2003. In Final Form: April 29, 2003 We investigate a method to direct the electroless deposition (ELD) of NiB on glass by microcontactprinting a reagent from an elastomeric stamp onto a glass substrate. There are three variants of this method depending on the reagent to be printed. ELD of a metal on a glass substrate necessitates the pretreatment of the glass with organic linkers that can bind a catalyst from solution. We use colloidal Pd/Sn as catalyst and immobilize these particles via an amino-functionalized silane such as 3-(2aminoethylamino)-propyltrimethoxysilane (EDA-Si) grafted to the glass substrate. The first variant includes microcontact-printing EDA-Si onto glass to bind the colloidal Pd/Sn catalyst at well-defined locations on the substrate. Here, the stamp is first hydrophilized with an O2-based plasma and then inked using different methods that include wet-inking, inking of the silane through the vapor phase, and contact inking. ELD of NiB initiates in those regions of the substrate that were previously microcontact-printed. This approach entails the problem of inking and printing of an excess of silane, which can be washed away from the printed regions and can thereby induce the ELD of NiB grains adjacent to the desired pattern. In the second approach, the entire glass is uniformly derivatized with EDA-Si, and colloidal Pd/Sn particles are inked onto a stamp and microcontact-printed to activate the substrate where desired. These colloids do not diffuse on the substrate during printing and subsequent steps, allowing the formation of NiB patterns having excellent contrast and accuracy even over areas as large as 4′′ in diameter. Similar to the first method, inking and reusing stamps is inconvenient because the colloids are suspended in a solution of concentrated HCl and the stamp needs to be hydrophilized and covered with a thin layer of a polyelectrolyte to achieve homogeneous inking of the stamp with these colloids. The third and most promising approach is to derivatize the glass substrate homogeneously with both EDA-Si and Pd/Sn colloids and then deactivate the catalyst selectively prior to the ELD step by microcontact-printing eicosanethiol (ECT). In this case, it is possible to employ hydrophobic PDMS stamps, to reuse them, to optimize the inking and printing conditions, and to form high-quality NiB structures with lateral dimensions ranging from several hundred to below 1 µm. This work suggests that microcontact printing (µCP) and ELD can be combined in a variety of manners to provide interesting alternatives to conventional microfabrication methods that otherwise include the deposition of metals in a vacuum process and patterning methods based on photolithography.

1. Introduction The work reported here extends the direct metallization of a substrate using ELD and µCP to the patterning of NiB onto glass via three related strategies. There is general interest in developing novel lithographic techniques that are more economical and have different processing capabilities than the methods currently used. Soft lithographic techniques1-4 and µCP5,6 in particular are promising because they can pattern planar or curved surfaces and place self-assembling films or proteins on a substrate with spatial control at length scales ranging from many centimeters down to 100 nm. When patterning a substrate entails structuring a metallic layer, ELD7,8 constitutes an interesting alternative to the standard deposition of metals in a vacuum process. * Corresponding author. Telephone: +41-1-7248283. Fax: +411-724-8966. E-mail: [email protected]. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551575. (2) Brittain, S.; Paul, K.; Zhao, X.-M.; Whitesides, G. M. Phys. World 1998, May, 31-36. (3) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (4) 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, 697719. (5) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 6, 20022004. (6) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511.

1.1. Electroless Deposition. ELD is a convenient method to obtain thin metallic films on a variety of insulating substrates at low cost.7 This method uses metastable solutions that contain a reducing agent and complexed metal ions as the source of the metal. The presence of complex-forming molecules inhibits the spontaneous reduction of the metal ions in solution until a substrate activated with the proper catalyst is immersed into the ELD bath. ELD initiates at the catalytic sites of the surface and proceeds in an autocatalytic manner. A variety of metals, including Cu, Ag, Au, Co, Ni, and some alloys of these metals, can be electroless-deposited from solution. Some examples exist where ELD of Cu, Ni, or CoP layers has been successfully implemented into the fabrication of fine metallic patterns of microelectronic devices.9 Palladium-based catalysts and Pd/Sn mixed colloids10-14 in particular are the most widely used catalysts to activate (7) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (8) Schlesinger, M. In Modern Electroplating, 4th ed.; Schlesinger, M., Paunovic, M., Eds.; John Wiley & Sons: New York, 2000; pp 667684. (9) O’Sullivan, E. J.; Schrott, A. G.; Paunovic, M.; Sambucetti, C. J.; Marino, J. R.; Baily, P. J.; Kaja, S.; Semkow, K. W. IBM J. Res. Dev. 1998, 42, 607-620. (10) Rantell, A.; Holtzman, A. Plating 1974, 61, 326-331. (11) Rantell, A.; Holtzman, A. Electroplat. Met. Finish. 1974, 27, 15-20. (12) Feldstein, N.; Schlesinger, M.; Hedgecock, N. E.; Chow, S. L. J. Electrochem. Soc. 1974, 121, 738-744.

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nonconductive substrates such as ceramics and plastics. Pd/Sn colloids are obtained by mixing acidic (HCl) solutions of PdCl2 and SnCl2.15 The resulting colloids are negatively charged and have a Pd-rich core and a SnClx shell comprising Sn2+ and Sn4+ species.16 These colloids are stable against oxidation (oxidized Pd is not a catalyst for ELD) and agglomeration when kept in acidic solution. Their immobilization onto a surface can be achieved through electrostatic interaction when exposing a positively charged substrate to a solution containing the colloids. An acceleration step (usually using an aqueous solution of HBF4) can be applied to remove part of the shell of the deposited colloids and to make the catalytic core more accessible to the reactants present in the ELD bath.14,17-20 This ensures a better initiation (strike) of the ELD and a plated film with homogeneous thickness. Patterning an electroless-deposited metal layer can be achieved either by directing the ELD process or by depositing a blanket layer and then etching it selectively. We favor the first method because it does not require an etching step and might consequently be easier and more economical. Directed ELD has been demonstrated by either selectively activating a noncatalytic surface or deactivating a surface that was entirely active for ELD using, for example, conventional resist-based photolithography,9,21-23 patterning catalyst-binding monolayers,24-26 selective laser-assisted pyrolysis of catalytic layers,27 and microcontact printing of catalysts onto substrates.28-30 1.2. Microcontact Printing. Microcontact printing uses a micropatterned elastomeric stamp that is inked, dried, and placed on a substrate to localize a chemical reaction between molecules from the ink and the substrate in the areas of contact.6 Stamps are formed by covering a patterned mold (master) with the liquid prepolymers of poly(dimethylsiloxane) (PDMS), curing the polymer, and releasing it from the master. PDMS provides several advantages: it is (i) commercially available, (ii) elastomeric, forming an intimate (conformal) contact with substrates,31 (iii) resistant to many types of chemicals and (13) Matijevic´, E.; Poskanzer, A. M.; Zuman, P. Plat. Surf. Finish. 1975, 62, 958-965. (14) Osaka, T.; Takematsu, H.; Nihei, K. J. Electrochem. Soc. 1980, 127, 1021-1029. (15) Fujinami, T.; Watanabe, J.; Honma, H. Trans. Inst. Met. Finish. 1996, 74, 193-197. (16) Cohen, R. L.; D’Amico, J. F.; West, K. W. J. Electrochem. Soc. 1971, 118, 2042-2046. (17) Osaka, T.; Nagasaka, H.; Goto, F. J. Electrochem. Soc. 1980, 127, 2343-2346. (18) Kim, J.; Wen, S. H.; Jung, D. Y.; Johnson, R. W. IBM J. Res. Dev. 1984, 28, 697-709. (19) Froment, M.; Queau, E.; Martin, J. R.; Stremsdoerfer, G. J. Electrochem. Soc. 1995, 142, 3373-3377. (20) van der Putten, A. M. T.; de Bakker, J.-W. G.; Fokkink, L. G. J. J. Electrochem. Soc. 1992, 139, 3475-3480. (21) Shacham-Diamand, Y. J. Micromech. Microeng. 1991, 1, 6672. (22) Cho, J. S. H.; Kang, H.-K.; Wong, S. S.; Shacham-Diamand, Y. MRS Bull. 1993, 18, 31-38. (23) Chen, M.-S.; Brandow, S. L.; Dulcey, C. S.; Dressick, W. J.; Taylor, G. N.; Bohland, J. F.; Georger, J. H.; Pavelchek, E. K.; Calvert, J. M. J. Electrochem. Soc. 1999, 146, 1421-1430. (24) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calvert, J. M. Chem. Mater. 1993, 5, 148-150. (25) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210-220. (26) Potochnik, S. J.; Pehrsson, P. E.; Hsu, D. S. Y.; Calvert, J. M. Langmuir 1995, 11, 1841-1845. (27) Shafeev, G. A.; Themlin, J.-M.; Bellard, L.; Marine, W.; Cros, A. J. Vac. Sci. Technol., A 1996, 14, 319-326. (28) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (29) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215. (30) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373.

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pH environments, (iv) transparent, (v) nontoxic, and (vi) surface-modifiable using plasma treatments and grafting of silanes.32-35 Moreover, the same stamp can, in principle, be reused many times. Printing alkanethiols onto Au36-39 is the best-explored variant of µCP. Forming self-assembled monolayers (SAMs) of alkanethiols by printing also works on other substrates such as Ag,40 Cu,41,42 and Pd.43-45 Microcontact printing on Si/SiO2 and Al/Al2O3 is possible using silanes as the ink,46-49 and to some extent on metal oxides such as indium/tin oxide, using alkanephosphonic acids.50,51 Microcontact-printed SAMs can locally protect a substrate from etching, although these layers are usually only 2-3 nm thick.37,52,53 A patterned SAM can also direct the deposition of a material onto a substrate by metallization,54,55 crystal growth,56,57 or chemical amplification,58-61 and it can modify the wetting and adhesion properties of substrates.62-64 (31) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318. (32) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 10131025. (33) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 12301232. (34) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870-5875. (35) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. Adv. Mater. 2001, 13, 1164-1167. (36) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 32743275. (37) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron. Eng. 1996, 32, 255-268. (38) Larsen, N. B.; Biebuyck, H. A.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017-3026. (39) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. A. J. Phys. Chem. B 1998, 102, 3324-3334. (40) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070-1079. (41) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601-603. (42) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (43) Carvalho, A.; Geissler, M.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2406-2412. (44) Love, J. C.; Wolfe, D. B.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 1576-1577. (45) Wolfe, D. B.; Love, J. C.; Paul, K. E.; Chabinyc, M. L.; Whitesides, G. M. Appl. Phys. Lett. 2002, 80, 2222-2224. (46) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382-3391. (47) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (48) St. John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022-1024. (49) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (50) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182-1191. (51) Breen, T. L.; Fryer, P. M.; Nunes, R. W.; Rothwell, M. E. Langmuir 2002, 18, 194-197. (52) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (53) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332-2337. (54) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (55) Moffat, T. P.; Yang, H. J. Electrochem. Soc. 1995, 142, L220L222. (56) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495-498. (57) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. Adv. Mater. 1999, 11, 1433-1437. (58) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179-6180. (59) Tao, Y.-T.; Pandian, K.; Lee, W.-C. Langmuir 1998, 14, 61586166. (60) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208-1214. (61) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862-6867. (62) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (63) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 27902793.

Direct Patterning of NiB on Glass Substrates

1.3. Combining Electroless Deposition and Microcontact Printing. We recently described three ways to combine ELD and µCP in view of fabricating transistor gates of thin-film transistor liquid-crystal displays (TFTLCDs).65-67 Introducing both methods into display fabrication procedures is technologically interesting and, in principle, economical because they provide alternatives to vacuum sputtering of metals and their patterning using a photoengraving process (PEP)stwo steps that make the overall display fabrication procedure costly. Employing ELD and µCP is challenging, however, because the plated and patterned structures must be free of defects, accurate, fabricated on large substrates (>15′′), and have good electrical performance. In the first approach named “plate & PEP”, we established the ELD of a blanket film of a metal such as NiB on 15′′ glass substrates for display applications and used conventional photolithography to pattern the plated metal layer.65 We characterized the plated films and determined the important parameters and the processing steps necessary to achieve high-quality NiB deposits.68 The procedure to pattern the plated metal film includes spin-coating a photoresist onto the plated substrate, exposing the resist to UV light and developing it, and, finally, etching the metal selectively by using the patterned resist as a mask. The second approach combined ELD and µCP by electroplating, on top of the electroless-plated metal film, a metallic layer that was printed and etched selectively to provide a sacrificial masksan approach that we called “plate, iplate & print”.66 It allows a large variety of metals to be deposited and patterned using µCP when the mask is electroplated Cu. The third approach, “plate & print”, combined ELD and µCP in a more direct manner. Specifically, Cu was electroless-plated on glass, printed with alkanethiols, and selectively etched.67 1.4. The “Print & Plate” Strategy. In this article, we investigate and evaluate an even more direct strategy to combine ELD and µCP. As the printing step is done prior to plating, an etch step to pattern the deposited layer is no longer necessary. This strategy, which we call “print & plate”, has several variants, as illustrated in Figure 1. All of these variants were pursued for patterning NiB on display glass substrates and were inspired by the work of Hidber et al. in which a PDMS stamp was inked with a solution of colloidal Pd particles and printed onto a substrate treated with silanes.28,29 The process for depositing a blanket film of NiB (left panel in Figure 1) starts with grafting silanes onto the glass.69-72 The grafted layer serves to immobilize a colloidal Pd/Sn catalyst from solution to the surface. We specifically use EDA-Si because (64) Bo¨ltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877-879. (65) Delamarche, E.; Geissler, M.; Vichiconti, J.; Graham, W. S.; Andry, P. A.; Flake, J. C.; Fryer, P. M.; Nunes, R. W.; Michel, B.; O’Sullivan, E. J.; Schmid, H.; Wolf, H.; Wisnieff, R. L. Langmuir, in press. (66) Delamarche, E.; Geissler, M.; Magnuson, R. H.; Schmid, H.; Michel, B. Langmuir, in press. (67) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. Langmuir, submitted for publication, 2003. (68) The NiB layers deposited on EDA-Si-treated display glass substrates were usually ∼150-nm thick, contained 0.25% of B, and had a specific resistivity of 25 µΩ cm and 13 µΩ cm for the as-plated and annealed films, respectively. (69) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (70) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (71) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. A.; Ocko, B. N.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852-5861. (72) Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1992.

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Figure 1. Three methods based on µCP can direct the ELD of NiB on a glass substrate. The ELD of a blanket film of NiB itself comprises several processing steps, which are all performed in solution. As ELD initiates only if a catalyst is present on the substrate, patterning the NiB layer is possible by either microcontact-printing a linker between the catalyst and the glass, the catalyst itself, or a catalyst deactivator. These three methods are collectively called “print & plate” and are described in detail in this article.

(i) it chemisorbs to glass via its silane group,73 (ii) its amino functionalities can bind the colloids in acidic environments, and (iii) it allows a simple treatment of large glass substrates to produce high-quality, well-adhering NiB layers.65 Immersing the EDA-Si-treated glass substrate into an acidic solution containing Pd/Sn colloids constitutes the activation step of the process. This is followed by an acceleration step, in which the activated substrate is exposed to an aqueous solution of HBF4. ELD of NiB proceeds by immersing the activated sample into a Ni plating bath containing dimethylamine borane (DMAB) as the reducing agent.74,75 Microcontact-printing EDA-Si onto glass constitutes the first variant of the “print & plate” method. An alternative is to derivatize the glass in a uniform manner with EDASi and to print Pd/Sn colloids to activate the substrate where desired. The last patterning method relies on derivatizing the entire glass substrate with EDA-Si and the colloids homogeneously, and subsequently deactivating the catalyst, after the acceleration step and before ELD, by microcontact-printing alkanethiols. 2. Printing the Linker 2.1. Strategy. The sequence of steps involved in the “print & plate” process using silane linkers such as EDASi is illustrated in Figure 2. The stamp is first inked with EDA-Si and placed onto a glass substrate for ∼30 s to pattern EDA-Si on the glass. The printed glass substrate is immersed into an acidic solution of a colloidal Pd/Sn catalyst (50% diluted in HCl) for 30 s to immobilize Pd/Sn (73) EDA-Si molecules can hydrolyze or oligomerize in solution while still being able to chemisorb to the glass surface and bind Pd/Sn colloids. (74) Lelental, M. J. Electrochem. Soc. 1973, 120, 1650-1654. (75) Riedel, W. Electroless Nickel Plating; Finishing Publications Ltd.: Stevenage, Hertfordshire, U.K., 1991.

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Figure 2. Procedure for the selective ELD of NiB using microcontact printing of EDA-Si onto a glass substrate. EDASi is inked onto a hydrophilized PDMS stamp and then microcontact-printed onto a glass substrate to act as a linker between a colloidal Pd/Sn catalyst deposited from solution and the substrate. The pattern of EDA-Si on the surface, ideally, defines the electroless-deposited NiB patterns.

colloids over the printed regions of the substrate. The activated substrate is rinsed with deionized (DI) water upon removal from the colloid solution, immersed into an accelerator solution containing HBF4 (Accelerator 19H, Shipley, diluted by 10 with DI water) for 30 s, and rinsed with water. Deposition of NiB can then take place when the accelerated substrate is immersed in the ELD bath. As EDA-Si and other silanes bearing polar groups are inconvenient inks for µCP because they have a rather low affinity for PDMS stamps and are prone to hydrolysis and oligomerization, their inking onto stamps is the key step of this method. We discuss several methods including wetinking,5 vapor-phase inking, and contact-inking76,77 to ink the stamp with such silanes below. 2.2. Inking of EDA-Si and its Effect on the Ni Patterns. Wet-Inking. We started to ink stamps using solutions as it is conventionally done in µCP: the patterned face of hydrophilic or hydrophobic stamps was covered entirely with a solution of EDA-Si in ethanol/water 95:5 (v/v) having a concentration ranging from 0.1 to 1.0 vol %. After ∼1 min of inking, the stamp was blown dry with a stream of N2 and immediately used for printing. The strike of the ELD of NiB in general was fast and homogeneous, and the resulting Ni structures were (76) Libioulle, L.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 1999, 15, 300-304. (77) Geissler, M.; Bernard, A.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. J. Am. Chem. Soc. 2000, 122, 6303-6304.

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Figure 3. Optical microscope images of NiB structures illustrating an important problem when EDA-Si or similar silanes are microcontact-printed onto glass to direct the ELD of NiB. The ∼120-nm-thick NiB structures are homogeneous and dense enough to block light, but a diffuse background composed of NiB grains present on the substrate near the printed regions compromises the contrast and the resolution of the pattern. This background occurred in most of our experiments; its intensity varied depending on the type of silane linker printed, its concentration in the ink solution, and the inking method. Here, a hydrophilic stamp was inked with a 1.0% solution of EDA-Si in ethanol/water 95/5 (v/v) for 1 min and then printed onto the glass substrate for 30 s.

accurate for this range of ink concentrations. However, the contrast of these patterns was usually compromised by the presence of a diffuse background of NiB grains around the patterns, irrespective of the ink concentration used, Figure 3. Some of the samples had an asymmetrical background in which the patterns were smeared mostly in one direction (see below). We also noticed the presence of defective regions in the Ni patterns. We attribute these defects to nonuniform drying of the ink on the PDMS stamp under the stream of N2. The concentration of EDA-Si in the ink solution had a direct effect on the diffuse background: high concentrations (∼1%) of EDA-Si resulted in a stronger background, and lower concentrations (∼0.1%) resulted in a less intense background. EDA-Si concentrations higher than 1% led to broadening of the plated Ni structures, and concentrations below 0.01% were insufficient to obtain a homogeneous strike. These observations confirmed the importance the ink concentration has for the characteristics of the pattern.78 In terms of homogeneity and defect density of the final Ni patterns, hydrophilic PDMS stamps generally yielded better results than hydrophobic stamps. Inspecting freshly inked and dried hydrophilic stamps with an optical microscope revealed an inhomogeneous distribution of EDA-Si on the stamp. EDA-Si does not diffuse into PDMS but remains on the surface of the stamp; wetting and (78) We observed that NiB features produced with this “print & plate” method tended to detach from the glass substrate in the plating bath when the film thickness was reaching 100-150 nm.


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Table 1. Effect of the Concentration of EDA-Si in the Ink Solutiona on the Printed EDA-Si Layer [EDA-Si] in ethanol/water (vol %)

thickness of EDA-Si as printedb (nm)

thickness of EDA-Si after rinsing with waterc (nm)

loss of EDA-Si during rinsing (%)

0.025 0.05 0.1 1.0

0.55 0.80 1.85 5.20

0.35 0.50 1.00 1.20

36 38 46 77

a Equal volumes of EDA-Si dissolved in ethanol/water (v/v ) 95/5) were spin-coated onto a planar Au inker pad for contactinking of the stamp. b The thickness of the EDA-Si layers was determined by ellipsometry measurements using Si/SiO2 wafers as substrates for printing. c The microcontact-printed substrates were dipped into a beaker containing ∼200 mL of DI water.

drying phenomena define the amount of EDA-Si present in the various regions of the PDMS pattern to a large extent. We failed to improve the distribution of EDA-Si on the stamp by varying the drying conditions. Rinsing of inked stamps with ethanol or water to remove excess EDA-Si was not successful either because too many EDASi molecules were washed off the stamp. Adding surfactants79 to the ink solution did not suppress drying artifacts for hydrophilic stamps, whereas a slight improvement was achieved for hydrophobic stamps. On the basis of these findings, we evaluated other inking procedures to improve the homogeneity of the ink on the stamp. Vapor-Phase Inking. Inking a stamp for microcontactprinting silanes onto glass or Si/SiO2 wafers through the gas phase is an alternative to wet-inking and should be free of wetting and drying artifacts. We verified this possibility by placing a stamp on top of a beaker containing a few milliliters of EDA-Si and warming the beaker to 35-45 °C to accelerate the evaporation of EDA-Si molecules.80,81 We found that this simple inking method was sufficient to cover stamps with enough EDA-Si to activate glass substrates with Pd/Sn colloids. The corresponding electroless-deposited Ni patterns were free of defects and accurate, but in most cases a diffuse background formed by Ni grains was still observed. Contact-Inking. We tried to ink stamps locally without employing bulk liquids and in a more controlled fashion using an inker pad.76 Solutions of EDA-Si with concentrations ranging from 0.025 to 1.0 vol % in ethanol/water were spin-coated onto a planar inker pad.82 The stamp was then inked by contact with the inker pad for typical durations of 30-60 s. The Ni patterns produced with this inking method were similar to those obtained using vaporphase inking. A diffuse background of NiB grains was again present on the plated samples. 2.3. Origin of the Electroless-Deposited Background. We performed experiments using contact-inked stamps, Si/SiO2 wafers as substrates, and ellipsometry to characterize the printed films to gain a better understanding of the phenomena occurring during inking and printing of EDA-Si, Table 1. Planar hydrophilic PDMS stamps were inked for 30 s. The thickness of the printed films increased with the concentration of EDA-Si in the solution used to prepare the inker pad. Previous studies (79) We used nonionic (poly(ethylene oxide) derivatives) and anionic (sodium dodecyl sulfate, SDS) surfactants at a concentration of ∼0.1% (wt) in the ink solutions containing EDA-Si. (80) Jo¨nsson, U.; Olofsson, G.; Malmqvist, M.; Ro¨nnberg, I. Thin Solid Films 1985, 124, 117-123. (81) Duchet, J.; Chabert, B.; Chapel, J. P.; Ge´rard, J. F.; Chovelon, J. M.; Jaffrezic-Renault, N. Langmuir 1997, 13, 2271-2278. (82) A 15-nm-thick layer of Au evaporated on a Si wafer was used as inker pad because EDA-Si does not react with the Au surface.

Figure 4. AFM images of EDA-Si patterns on a glass substrate (A) as printed and (B) after printing and rinsing with water. The coarse features decorating the as-printed EDA-Si film disappeared during rinsing, and EDA-Si molecules spread to the adjacent zones on the substrate. This spreading of EDA-Si may lead to the immobilization of some Pd/Sn catalyst and thus to the plating of NiB grains in these regions. Here, a 1.0% solution of EDA-Si in ethanol/water 95/5 (v/v) was spin-coated onto a Si wafer covered with 15 nm of Au for contact-inking the stamp. Inking and printing lasted for 30 s each, and the printed substrate was rinsed with ∼100 mL of water for ∼10 s.

showed that a layer of EDA-Si grafted from a 1.0% aqueous solution onto a Si wafer has a thickness of ∼0.35 to 0.4 nm.65 The values reported in Table 1 reveal that multilayers of EDA-Si were present on the inker pad and transferred to the substrate for all concentrations used to prepare the inker pad. Rinsing the printed substrates thoroughly with water noticeably diminished the thickness of the printed EDA-Si film, suggesting that a part of the printed EDA-Si molecules was weakly bound to the substrate. Experiments using X-ray photoelectron spectroscopy (XPS) confirmed this observation (data not shown). We suspect that in all of our inking experiments, multilayers of EDA-Si were present on the stamp and transferred to the substrate as well. It seemed therefore reasonable to assume that the fraction of EDA-Si printed and weakly bound to the substrates83,84 was responsible for the ubiquitous background of NiB particles after ELD. We verified this hypothesis by using atomic force microscopy (AFM) to study the morphology of the EDA-Si patterns printed on glass before and after rinsing with water. The AFM image in Figure 4A reveals the presence of a ∼4.5-nm-thick layer of EDA-Si that was microcontact(83) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstro¨m, I. J. Colloid Interface Sci. 1991, 147, 103-118. (84) Vrancken, K. C.; Possemiers, K.; van der Voort, P.; Vansant, E. F. Colloids Surf., A 1995, 98, 235-241.

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printed onto a glass substrate with high accuracy and contrast. This suggests that EDA-Si molecules do not diffuse away from the areas of contact between the stamp and the substrate during printing. Figure 4B illustrates that exposing the printed substrate to water displaces a fraction of the printed molecules to regions adjacent to the areas of contact: the thickness of the EDA-Si layer is reduced to ∼2.6 nm, and EDA-Si is detected in nonprinted regions. A small amount of EDA-Si on glass can bind a noticeable amount of Pd/Sn colloids.65 We believe that immersing the printed substrates in the solution of catalyst smears the printed pattern and induces ELD of NiB grains next to the printed regions. Our efforts to suppress this background formation by reducing the concentration of EDA-Si in the ink met with failure. Even the lowest possible concentration of EDA-Si (the minimum concentration required for a good strike) led to faint background oriented in the direction of immersion of the sample in the solution of Pd/Sn. 2.4. Attempts to Suppress Background Formation. Confronted with the problem that a fraction of the printed EDA-Si is not covalently bound to the glass substrate and can be displaced from the printed areas, we tried to reduce the amount of printed, weakly bound EDA-Si. Cleaning the Glass Substrates before Printing. The glass substrates were used as received in all of the abovementioned experiments. Glass has a high surface free energy and is prone to contamination in noncontrolled environments. We verified whether some contamination of the glass surface could possibly slow the grafting process of printed EDA-Si molecules and increase their mobility after printing. We cleaned the glass substrates with an O2-based plasma to assert this assumption. An O2 plasma efficiently cleans glass and increases the number of surface silanol groups (-Si-OH) that can react with EDA-Si molecules. Unfortunately, such a plasma treatment did not suppress the appearance of the deposited background, nor was cleaning the substrates with both polar and nonpolar solvents helpful. Blocking the Nonprinted Areas of the Glass Substrate. A strategy to prevent the migration of EDA-Si molecules away from the printed areas could be to block the nonprinted regions of the glass with silanes that do not bind catalysts. Such a strategy is often used to pattern Au surfaces with both reactive and nonreactive SAMs62,63,85 but would be impractical here because of the crossreactivity between EDA-Si and blocking silanes such as alkyltrichlorosilanes deposited from the vapor phase. Varying the pH of the EDA-Si Solution. Trimethoxysilanes can hydrolyze and oligomerize in solution.72,86 As these reactions are pH dependent, and oligomerized EDASi molecules might be less mobile on the glass surface during immersion in the solution of catalyst owing to their higher molecular weight (MW) and larger number of anchoring groups, we varied the pH of the EDA-Si solutions used to prepare the inker pad from pH 2 to 12. We found no significant reduction of the plated Ni background, which suggests that either an improved reactivity between the silanes and the glass87 or the use of polymeric silanes might be necessary. Improving the Cross-Linking of the Printed EDA-Si to the Glass. An obvious strategy to reduce the diffuse background is to improve the yield of the grafting of printed (85) Delamarche, E.; Geissler, M.; Wolf, H.; Michel B. J. Am. Chem. Soc. 2002, 124, 3834-3835. (86) Okumoto, S.; Fujita, N.; Yamabe, S. J. Phys. Chem. A 1998, 102, 3991-3998. (87) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 69686976.

Geissler et al.

EDA-Si to the glass substrate. First, we increased the time between printing and activation of the sample in the catalyst solution from 1 min to 3 h to allow a more extensive cross-linking of the -Si(OH)3 moieties of printed EDA-Si molecules to glass. Alternatively, we tried baking of the printed samples at 50-150 °C for 15 min.83,84 The background never decreased during these experiments, however. In fact, it became even worse when a baking temperature of 150 °C was used. This indicated that the excess of EDA-Si printed on the substrate could not be entirely confined to the printed regions. For this reason, we attempted to remove excess EDA-Si after the printing step. Removing Excess of Printed EDA-Si. Rinsing the printed glass substrates with a variety of polar and nonpolar solvents such as ethanol, acetone, toluene, or heptane did not prevent the formation of background on the plated samples. Rinsing the printed glass with solutions containing tetramethoxysilane (TMOS) did not help either. We selected TMOS, hoping that this silane could block the nonprinted areas of the glass while reacting with the silanol functions of EDA-Si. These unsatisfactory results could be due to the slow grafting of TMOS to the glass substrate and the possible physisorption of EDA-Si to the glass via its amino groups. Removing Excess EDA-Si in Vacuum. We exposed the printed samples to vacuum in an attempt to remove the excess EDA-Si by evaporation. An applied pressure of ∼10-2 mbar for 30 min yielded no improvement. We did not try more stringent conditions because our aim is to develop a convenient route to the patterned metallization of substrates. Removing Excess EDA-Si by Contact. In this approach, we placed a hydrophilic PDMS layer on the printed substrate and removed it after 30-60 s. The resulting Ni patterns were impressive and promising, although a faint background was still discernible. The PDMS layer used to remove some of the excess EDA-Si was also activated with Pd/Sn, accelerated, and finally plated with NiB. Interestingly, an accurate Ni pattern formed on this PDMS layer. This resembles experiments based on contact-inking planar stamps in which high-quality patterns can be formed.77 We identified this method as the most effective strategy to remove excess ink from the final substrate, yet it was not sufficient to suppress the faint background of NiB entirely, even when the process was repeated several times on the same substrate. We then evaluated different types of ink molecules that have Pd/Sn-binding amino groups and that could have differing chemisorption characteristics with glass substrates. 2.5. Using Other Aminosilane Linkers. Amino alkoxysilane-based inks with varying numbers of anchoring groups and different molecular weights are compared using a standard inking and printing procedure, Table 2. All silanes had a concentration of 0.05 vol % in ethanol/ water 95:5 (v/v) and were spin-coated onto a Au inker pad that served to contact-ink hydrophilic stamps for 30 s. The NiB patterns corresponding to these experiments were examined using an optical microscope. The electrolessdeposited background was always present; it was particularly strong for (3-aminopropyl)triethoxysilane (APTS) and EDA-Si and weakest for 3-(trimethoxysilyl)propyl modified polyethylenimine (PEI-Si). The correlation between the molecular weight of these aminosilanes and their tendency to induce a plated background on the substrate is probably marginal. Moreover, we believe that inconsistent aging takes place in these alkoxysilane ink solutions, which may lead to undefined transfer of ink to the substrates, giving rise to variable plated backgrounds. Varying the conditions of inking and printing proved to


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Table 2. Evolution of the NiB Background Observed for Different Aminosilanes Printed onto Glass Substratesa aminosilane

no. of anchoring groups

MW (g mol-1)

backgroundb

3-aminopropyl-methyl-diethyoxysilane (3-aminopropyl)triethoxysilane (APTS) (3-diethylaminopropyl)-trimethoxysilane bis[3-(triethoxysilyl)propyl]amine (bis-APTS) 3-(2-aminoethylamino)propyl-methyl-dimethoxysilane (Me-EDA-Si) 3-(2-aminoethylamino)-propyltrimethoxysilane (EDA-Si) bis[(3-trimethoxysilyl)propyl]-ethylenediamine (bis-EDA-Si) 3-(trimethoxysilyl)propyl modified polyethylenimine (PEI-Si)

1 1 1 2 1 1 2 na

191.35 221.37 235.40 313.47 206.36 222.36 384.62 >400

+++ +++ +++ ++ +++ +++ ++ +

a Equal amounts of solutions containing 0.05% of the aminosilane in ethanol/water (v/v ) 95/5) were spin-coated onto a planar Au inker pad for contact-inking the stamp. b The background refers to the electroless deposition of NiB grains in the nonprinted regions of the substrate and the findings are of qualitative nature: + + + strong background, + + medium background, + low background.

be partially successful. Some NiB patterns prepared from a solution of bis[(3-trimethoxysilyl)propyl]-ethylenediamine (bis-EDA-Si) with originally pH 2-3 were homogeneous and uniform over their entire area and did not exhibit the severe macroscopic defects that were frequently observed with pH-unadjusted ink solutions. The NiB patterns were well-defined also on the microscopic length scale. These results, however, suffered from limited reproducibility, presumably because too many parameters contributed to the final quality of the plated patterns. 2.6. Summary on Printing the Linker. We investigated microcontact printing of EDA-Si and of a variety of related compounds onto glass to direct the immobilization of Pd/Sn colloids and hence the ELD of NiB on glass. Clearly, this method is challenging. A PDMS stamp does not seem to take up EDA-Si conveniently from a solution of ink so that a homogeneous coverage of the stamp during inking is difficult to achieve using wet-inking. Alternative inking methods such as evaporation or contact inking might therefore be preferable. EDA-Si, and alkoxy silanes in general, are challenging inks for µCP: they can hydrolyze, condense, and oligomerize; multilayers of silanes are easily inked and subsequently transferred onto the substrate. Silanes do not adsorb onto glass in a selflimiting manner like alkanethiols do on Au. A fraction of the printed ink molecules is covalently bound to the glass surface, whereas a significant amount remains physisorbed. This is problematic because physisorbed EDA-Si molecules are washed off during the subsequent immersion step and can readsorb on the glass surface where they bind Pd/Sn colloids and induce the ELD of NiB grains in undesired areas. 3. Printing the Catalyst 3.1. Strategy. The method described here may circumvent the problems encountered with the above approach: it consists of homogeneously derivatizing a glass substrate with EDA-Si. Patterning is achieved by inking a hydrophilic PDMS stamp with an acidic solution of Pd/Sn colloids and printing the catalyst onto the EDASi-treated glass surface, Figure 5. Previous studies of inking and microcontact-printing polar Pd-based complexes showed that it is preferable to use hydrophilized PDMS stamps to improve the affinity between the complexes and the stamp.30,35 We treated stamps with an O2 plasma for this reason but found that this treatment alone was not sufficient to take up Pd/Sn colloids in a homogeneous manner from an acidic solution during inking: the distribution of the colloids on the stamp showed drying artifacts similar to those found when wet-inking PDMS stamps with EDA-Si (see 2.1.). Hence, we added a positively charged polyelectrolyte to the hydrophilic stamp to improve the inking of the stamp with the colloids. 3.2. Inking and Printing of Pd/Sn Colloids. We employed Cartaretin F4 (Clariant), which is a cationic

Figure 5. Microcontact-printing catalysts on an insulating glass substrate provides a method for selective ELD of metals. One method explored here in depth starts by hydrophilizing a PDMS stamp, treating it with a polyelectrolyte such as Cartaretin, and inking it with an acidic solution of Pd/Sn colloids. The colloids on the stamp are accelerated before the printing step. The catalytic colloids transfer from the stamp to a glass surface derivatized with EDA-Si during the printing step. ELD of NiB proceeds in those areas of the glass substrate where the Pd/Sn catalyst has been printed.

polyelectrolyte possessing many quaternary ammonium groups. Such cationic polyelectrolytes can adsorb onto surfaces and effectively recover colloids from solution.88,89 We immersed the hydrophilic stamps into a 1.0 vol % solution of Cartaretin in water for 1 min and rinsed them copiously with water. Immediately following this treatment, the stamps were inked by immersing them into a solution of Pd/Sn colloids (diluted by 2 with concentrated HCl) for 1 min. The stamps were then rinsed with DI water and immersed into the acceleration solution (using the accelerator #1019 from Fidelity without dilution) for 1 min.90 Printing the colloids onto an EDA-Si-pretreated glass substrate took between 10 s and 1 min. EDA-Si was previously grafted to the glass, using a 1.0 vol % solution (88) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (89) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624.

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in ethanol/water 9:1 (v/v) (10-min exposure time), which was followed by rinsing the substrate with DI water, drying it with a stream of N2, and baking it at 150 °C for 10 min. The printed substrate was rinsed first with a 7.6 M solution of HCl for ∼15 s, then with DI water, and finally immersed into the NiB plating bath. We observed that the affinity of the Pd/Sn colloids for the Cartaretin-treated stamp varied with the pH of the solution used to treat the stamp with the polyelectrolyte. The best results for ELD of NiB were obtained when stamps were treated with a solution of Cartaretin having a pH of 6-8. Too many Pd/Sn colloids were immobilized on stamps treated with solutions of Cartaretin having a pH >10, which compromised the conformal contact between the stamp and the substrates during printing. Treating stamps with solutions of Cartaretin having a pH 400 nm. NiB deposits thinner than 150 nm passed the “tape test” as plated but did not pass this test when they were annealed at 400 °C for 1 h.93 Using a home-built two-point probe instrument, the specific resistivity of the micron-sized NiB patterns was determined to be 16-18 µΩ cm after the annealing step. 3.5. XPS Measurements. We analyzed the surface chemistry of the stamp and the substrate at the various steps of this “print & plate” strategy to understand better the inking and printing of Cartaretin and the Pd/Sn colloids, Figure 8. The hydrophilized PDMS stamp exhibits the Si 2s and 2p, C 1s, and O 1s signals expected from the O2-plasma treatment,30,35 Figure 8A. Immobilizing a layer (92) Xia, Y.; Qin, D.; Whitesides, G. M. Adv. Mater. 1996, 8, 10151017. (93) Evans, W. T.; Schlesinger, M. J. Electrochem. Soc. 1994, 141, 78-82.


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Figure 7. (A) Optical microscope images of NiB microstructures that were plated over the regions of a glass substrate microcontactprinted with Pd/Sn colloids. These NiB patterns have high contrast and resolution, are 370-nm thick, and totally block the transmission of light. (B) The three-dimensional detail of one line from (A) measured using an AFM confirms the high quality of the NiB and exhibits the slightly rounded profile that is typical for electroless-deposited metals. (C) Photograph of a 6′′ glass substrate, which was derivatized with EDA-Si, microcontact-printed in its center with Pd/Sn colloids using a stamp bearing a 4′′ pattern, and immersed into a NiB ELD bath. The inner part of the pattern contains four fields with test structures ranging from 2 to 80 µm in width. The periphery of the stamp has alignment marks and support structures, which however could not prevent a partial collapse of the stamp in these regions during printing.

of Cartaretin F4 is accompanied by the appearance of a N 1s peak (397.0 eV) and a Cl 2p peak (197.4 eV) in the spectrum. After immersing the stamp into the Pd/Sn catalyst solution, the corresponding Pd 3d (338.2 eV) and Sn 3d signals (490.2 eV) are detected. The HBF4 acceleration step dissolves part of the Sn shell around the Pd core of the colloids, which decreases the Sn 3d signal intensity. Printing of the stamp onto the EDA-Si-derivatized glass leads to a complete transfer of the Pd to the substrate, and only a small amount of Sn remains on the stamp. XPS spectra of the substrate obtained after each step are shown in Figure 8B. The spectrum of the bare glass substrate shows Si 2p and 2s, C 1s, O 1s, and Ba 3d peaks. The formation of a layer of EDA-Si on the glass during grafting is evidenced by the small N 1s signal (398.7 eV), as already shown in previous studies.65 The Sn and Pd 3d signals observed on the substrate after printing are consistent with the signals found on the stamp after the acceleration step. The relative amount of Sn compared to Pd diminishes after rinsing the printed substrate with HCl, as expected. The plated material consists of Ni (main signal is Ni 2p at 858.3 eV) covered with an oxide (O 1s peak at 532.0 eV). No B is detected because of the small fraction of B in the deposit (0.25%)93,94 and the low atomic sensitivity of this element to X-rays. 3.6. Reusing Stamps. Using µCP for an application requires that stamps can be reused many times, as it is the case for photomasks in photolithography. Making a stamp hydrophilic, inking it first with a polyelectrolyte and then with an acidic solution of colloids, and exposing the inked stamp to a solution containing HBF4 might deteriorate the stamp after each use. We reused some (94) Mallory, G. O. Plating 1971, 58, 319-327.

stamps for up to 10 inking and printing cycles. After each printing step, the overall inking procedure was repeated but without applying an O2-plasma treatment to the stamp again. The quality of the electroless-deposited NiB patterns was invariant, but we noticed that the stamp darkened with every inking step. This was due to an accumulation of Cartaretin and Pd/Sn in the recessed regions of the pattern of the stamp. Such an accumulation is not desirable because it could compromise the contact of the stamp with the substrate and the contrast of the printed pattern. Cleaning the stamp regularly after a limited number of cycles might be necessary, but all attempts to clean used stamps with aqueous solutions containing surfactants such as SDS, Tween 40, or Triton X 100 at different pH values were unsuccessful. 3.7. Summary on Printing the Catalyst. Our second method to “print & plate” is based on printing a colloidal Pd/Sn catalyst onto a glass substrate derivatized with EDA-Si. Covering hydrophilic PDMS stamps with a thin layer of polyelectrolyte helped inking the stamps homogeneously, and rinsing the printed substrates with HCl before ELD was crucial to obtaining high-quality deposited NiB patterns. Performing the acceleration step on the stamp instead of activating the catalyst on the printed substrate improved the affinity of the colloidal Pd/Sn for the EDA-Si layer grafted on the glass substrates and resulted in a uniform transfer of the catalytic particles to the substrate. This method is suited to electroless-deposit uniform NiB films up to >400 nm in thickness with submicrometer accuracy and over 4′′ areas. Overall, however, the inking procedure used in this method is cumbersome because it differs too much from inking a

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Figure 8. XPS spectra taken on a planar stamp (A) and on the glass substrate (B) during the various processing steps for microcontact-printing a colloidal Pd/Sn catalyst onto an EDASi-treated glass substrate (see text for details).

PDMS stamp once with a solution of alkanethiols dissolved in ethanol and using the stamp many times without reinking or even cleaning it. We consequently investigated a third method as an attempt to circumvent the limitations encountered when microcontact-printing a linker for a catalyst or a colloidal catalyst to a substrate. 4. Printing a Deactivator 4.1. Strategy. The two preceding methods were based on defining catalytically active patterns in those areas where the stamp comes in contact with the glass substrate. The inverse method is to suppress catalytic activity in the corresponding regions of a substrate by microcontactprinting a deactivating ink. Because many reagents that can deactivate a catalyst are organic compounds, there should be a good chance to identify a more suitable ink for µCP with this method than with the previous ones. Commercial ELD baths contain inhibitors (also called stabilizers, poisons, or anticatalysts) that prevent uncontrolled plating when contaminants, for example, are dragged in the bath.95,96 Sulfur compounds such as (95) Gutzeit, G. Plating 1960, 47, 63-70. (96) Feldstein, N.; Amodio, P. R. J. Electrochem. Soc. 1970, 117, 11101113.

Geissler et al.

Figure 9. Microcontact-printing a deactivating alkanethiol such as ECT onto a glass substrate treated with EDA-Si and activated with Pd/Sn colloids provides a third method to direct the ELD of NiB on the glass.

thiourea are commonly used as stabilizers because they coordinate catalytic or metallic particles suspended in ELD baths and prevent them from growing by suppressing hydrogenation/dehydrogenation catalysis.96,97 We based our third “print & plate” patterning method on microcontact-printing ECT to selectively deactivate a Pd/Sn catalyst that covers a glass surface homogeneously, Figure 9. We assumed that alkanethiols would be an excellent deactivator for this method because they strongly coordinate Pd,98 and their alkyl chains can help to screen the colloids from reactants in the bath. ECT seems particularly useful because it forms an excellent resist when printed onto Pd43-45 or other coinage metals and can be microcontact-printed with high accuracy because it diffuses less at room temperature than shorter alkanethiols do.39 The derivatization and activation of the glass substrates followed the well-established sequence of processing steps. The glass was derivatized first by immersion into a 1.0 vol % solution of EDA-Si in DI water for 10 min, then rinsed with water and dried with a stream of N2. The back of each substrate was covered with an adhesive foil (Powatec, Cham, Switzerland) prior to grafting of EDASi. This foil ensured that ELD of NiB takes place on only (97) Feldstein, N.; Lancsek, T. S. J. Electrochem. Soc. 1971, 118, 869-874. (98) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: New York, 1984.


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Table 3. Characterization of ECT Monolayers on Colloidal Pd/Sn Immobilized on EDA-Si-Treated Glass Substratesa

substrate Pd/Sn on EDA-Si/glass Pd/Sn on EDA-Si/glass Pd/Sn (accelerated)c on EDA-Si/glass Pd/Sn (accelerated)c on EDA-Si/glass Pd/Sn (accelerated)c on EDA-Si/glass

method to form the monolayer

[ECT] in ethanol (mM)

adsorption time (s)

immersion

0.2

30

immersion printingd

0.2 0.2

30 30

θadv (deg)b

θrec (deg)b

∆θ (deg)

relative intensity of the XPS C1s signal

8 9 16 82 114

12 45 44

0.2 0.2 0.47 0.67 1

relative intensity of the XPS Pd 3d signal 0.55 0.55 1 0.93 0.76

a The substrates were activated by immersion into a 50% solution of the Pd/Sn catalyst in concentrated HCl for 30 s in all experiments. Average contact angles determined with DI water as the probe liquid. c The substrate was immersed into a 10% solution of accelerator in DI water for 30 s. d ECT was microcontact-printed using a planar PDMS stamp.

b

one face of the substrate. We used solutions containing 50 and 2% of the Pd/Sn catalyst in HCl, into which the preconditioned glass substrates were immersed for 30 s. After rinsing them with DI water, the samples were immersed into a 10% aqueous solution of accelerator for 30 s, rinsed with DI water, and dried in a stream of N2. The substrates were then used immediately for printing. We inked the stamps by covering them with a 0.2 or 0.3 mM solution of ECT in ethanol for 1 min, followed by drying with a stream of N2. The stamps were then placed onto the activated substrate for 20 s to 1 min. The printed substrates were finally immersed into the Ni plating bath. The protection foil was removed from the back of each sample after plating, but it was also possible to remove it before printing or plating. 4.2. Covering Pd/Sn Colloids with ECT. We first investigated whether ECT can adsorb from solution onto a homogeneous layer of immobilized Pd/Sn colloids before microcontact-printing it, Table 3. An EDA-Si-treated glass surface covered with a layer of immobilized Pd/Sn colloids is very hydrophilic. Immersing such a surface into a 0.2 mM solution of ECT in ethanol for 30 s does not change its wetting properties. This suggests that no ECT molecules adsorbed onto the colloids, which is confirmed by the invariant C 1s and Pd 3d XPS signals (reported in Table 3). The XPS signal associated with C and Pd increases after acceleration, whereas the surface remains well hydrophilic. This is consistent with the removal of some of the Sn from the shell of the colloids, which occurs during the acceleration step. Immersion of the accelerated sample into a solution of ECT clearly renders its surface substantially more hydrophobic. Because the substrate was rinsed with ethanol after the immersion step, we attribute this observationsaccompanied by an increase of the C 1s signalsto the chemisorption of ECT molecules from solution to the Pd that was exposed after the acceleration step. In addition, the Pd XPS signal became slightly attenuated by the adsorbed layer of ECT. The monolayer formed under these conditions is probably far from being complete: accelerated Pd/Sn colloids on a pretreated glass surface do not represent an ideal substrate for self-assembling a perfect monolayer. In fact, the measured advancing contact angle of 82° is 35° lower than the average advancing contact angle of a monolayer of ECT self-assembled on a thin, evaporated Pd layer under similar conditions. Interestingly, microcontact-printing ECT onto accelerated Pd/Sn colloids resulted in a surface with an advancing contact angle of 114°, which is close to that of a SAM of ECT on evaporated Pd. The relatively large hysteresis between the advancing and receding contact angles may originate from the roughness of the substrate and from defects in the layer of ECT. The substantial increase of the C 1s signal and decrease of the Pd 3d signal confirm the better coverage of the Pd/Sn colloids by ECT molecules on

this substrate. However, when immersed into the Ni plating bath, the ECT molecules microcontact-printed on this sample were not able to block the ELD of NiB for more than a few seconds. These results indicate that removing some Sn from the colloids using the acceleration step helps to bind ECT to Pd and block it, although insufficiently.99 4.3. Adjusting the Density of the Colloids on the Substrate. Blocking a catalyst to metallize a substrate selectively using ELD can be challenging: if a small region of the substrate remains catalytically active after the printing step, the contrast and accuracy of the plated patterns may be compromised.43 A critical density of catalytic sites on a surface must be reached to initiate the ELD of a metal from a bath; this threshold in catalytic activity depends on the nature of the catalyst used, the bath, and the plating conditions. Reducing the density of the catalytic sites on the surface without compromising the strike of ELD lowers the number of catalytic particles that must be deactivated by µCP and reduces the risk of leaving some catalyst still active after printing. We reduced the concentration of the Pd/Sn colloids from 50 to 2% in the acidic activation solution for this reason.65 Microcontact-printing ECT onto such a surface (always done after accelerating the catalyst) efficiently blocked the ELD of NiB in the printed regions. 4.4. Characterization of the Plated NiB Patterns. A variety of NiB test patterns was fabricated using the procedure described above. The optical microscope image in Figure 10A reveals that macroscopic and micrometersized areas are efficiently blocked independent of their geometrical characteristics. The Ni structures formed in the test arrays are plated with excellent uniformity and without visible defects. It is equally possible to plate adjacent large and small areas with high accuracy, Figure 10B. The AFM image in Figure 10C reveals that the NiB electroless-deposited in the nonprinted regions of the glass substrate is smooth and consists of small grains, which reflects that the underlying catalytic layer was homogeneous and smooth as well.100 Obtaining such a high-quality NiB layer might be important for some applications in which, for example, conformal coverage of the NiB by another layer may be necessary. We found that using this approach to plate Ni deposits whose thickness exceeded 150 nm was challenging: isolated structures tended to lose adhesion to the substrate during ELD. This maximum thickness is three times lower than the thickness of NiB that could be plated by printing the catalyst. We do not yet entirely understand this difference. (99) Increasing the concentration of ECT in the ink solution to 0.3 mM improved the blocking properties of the monolayer that was formed during printing, but the monolayer could still not suppress plating of NiB completely. (100) Brandow, S. L.; Dressick, W. J.; Marrian, C. R. K.; Chow, G.M.; Calvert, J. M. J. Electrochem. Soc. 1995, 142, 2233-2243.

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Figure 10. NiB patterns plated onto a catalytically active glass substrate that was selectively deactivated by microcontactprinting ECT. (A) ELD of NiB can be blocked entirely in macroscopic areas, whereas the nonprinted regions preserve their catalytic activity. The arrays reveal macroscopic uniformity, and the thickness of the Ni structures in the quadrants is 40 ( 10 nm, on average. (B) These Ni lines have high contrast and accuracy, are 80 ( 20 nm thick, and can block light efficiently. (C) AFM image taken on the surface of a 80-nm-thick NiB structure. The deposit has similar characteristics as a blanket film of NiB plated under similar ELD conditions: it has an RMS roughness of 1.2 nm and consists of 40-80-nm-wide grains. This relative smoothness of the deposit reflects the homogeneity of the catalytic layer underneath. These EDA-Si-treated glass substrates were activated by immersion into a 2% solution of catalyst in HCl for 30 s, followed by the usual acceleration step (10% of Accelerator 19H in DI water) for 30 s. Stamps were inked with a 0.3 mM solution of ECT in ethanol for 1 min, dried, and printed onto the glass substrates for 1 min.

4.5. Limits of Resolution. We performed experiments using stamps having high-resolution features (

Direct Patterning of NiB on Glass Substrates Using Microcontact

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