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Figure 2 Depositing a NiB layer on glass using ELD requires derivatizing the glass substrate with silanes, binding Pd/Sn catalysts to the derivatized ...
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Langmuir 2003, 19, 5892-5897

Patterning NiB Electroless Deposited on Glass Using an Electroplated Cu Mask, Microcontact Printing, and Wet Etching Emmanuel Delamarche,* Matthias Geissler, Roy H. Magnuson,† Heinz Schmid, and Bruno Michel IBM Research, Zurich Research Laboratory, Sa¨ umerstrasse 4, 8803 Ru¨ schlikon, Switzerland Received January 30, 2003. In Final Form: April 11, 2003 We present a method to pattern an electroless-deposited metal layer based on electroplating a mask and patterning this mask using microcontact printing (µCP) and wet etching. This method starts by derivatizing a glass substrate with an amino-functionalized silane, 3-(2-aminoethylamino)propyltrimethoxysilane (EDASi) from solution and using the amine functions of the grafted silane to immobilize Pd/Sn colloids from an acidic suspension onto the glass. The Pd/Sn colloids initiate the electroless deposition (ELD) of a 150 nm thick NiB layer onto the glass. The as-deposited NiB has a specific resistivity of 22 µΩ cm and can be covered with 50 nm of an electroplated Cu layer. Microcontact printing a protective monolayer of eicosanethiol (ECT) onto the Cu and etching it selectively with a cyanide-based etch bath defines the pattern of the Cu mask. This pattern is transferred into the NiB underlayer using a sulfuric acid-based wet etch. The ECT monolayer, the Cu mask, and the Pd/Sn residue remaining on the glass were all removed, yielding the NiB structures. Every step was monitored with X-ray photoelectron spectroscopy (XPS) and the NiB structures were characterized using atomic force microscopy (AFM). Although it comprises a large number of steps, this patterning method is simple and yields high-quality NiB patterns. This method generalizes µCP to metals that are not directly printable, and it could be used instead of vacuumdepositing a metal and patterning it using photolithography.

1. Introduction Patterning metallic layers on surfaces using photolithography has many technological applications.1 It is useful for fabricating conductive lines and elements necessary in the microelectronic, data storage, and display industries. In a photolithographic process, a thin layer of a radiation-sensitive material is formed by spin-coating a liquid resist onto a substrate. Exposing the photoresist through an optical mask modifies the solubility properties of the resist. Depending on the exact chemistry of the resist used, the exposed or nonexposed areas are subsequently removed by dissolution. The patterned resist can protect an underlying metallic layer in a wet or dry etch process. The combination of photolithography and wet etching constitutes a photoengraving process (PEP). For some applications such as the fabrication of thin-film transistor liquid-crystal displays (TFT-LCDs), patterning a layer using vacuum deposition and photolithography is challenging and particularly costly. Typically, six or more TFT arrays are fabricated simultaneously on a large substrate, which is subsequently diced to yield individual arrays; this necessitates formidable equipment for handling the substrates, depositing materials, and performing photolithography and etching. An array of TFTs comprises a gate layer patterned on glass, insulating and semiconducting layers, the “data” layer, and a patterned indium tin oxide (ITO) layer.2,3 A given ITO pixel electrode is charged via its associated * Corresponding author. E-mail: [email protected]. † Present address: IBM Endicott, 1701 North Street, Endicott, NY 13760. (1) Semiconductor Devices. Physics and Technology; Sze, S. M., Ed.; Wiley & Sons: New York, 1985. (2) Kim, S. S. Inf. Display 2001, 17, 22-26. (3) Tsukuda, T. TFT/LCD. Liquid Crystals Addressed by Thin-Film Transistors; Japanese Technology Reviews, Vol. 29; OPA Amsterdam B. V.: Amsterdam, The Netherlands, 1996.

TFT by addressing a selected gate and data lines of the array. The electric charge stored in this capacitor defines the orientation of the liquid crystals and affects the amount of light transmitted. Fabricating the TFT array requires more than 40 processing steps;4 a blanket metallic layer alone must be sputtered several times. Here, we pattern a metallic layer that might be useful as the gate layer of TFT-LCDs by electroless deposition (ELD)5,6 and microcontact printing (µCP).7,8 This strategy is part of a larger effort in which we strive to implement these methods instead of vacuum deposition and PEP to fabricate the gate layer of TFT-LCDs having large active areas and high resolution. Our first step was to establish an ELD process for depositing NiB onto a 15 in. display glass substrate and patterning it using PEP9 to form the gates of an array of TFTs. This part was called “Plate & PEP” and provided the basis of our work to replace the PEP by µCP.9 The effort described here uses electroplating of a printable Cu layer on the NiB layer. The Cu layer is microcontact-printed with eicosanethiol (ECT), etched, and used as a mask to pattern the NiB underlayer. The (4) Colgan, E. G.; Alt, P. M.; Wisnieff, R. L.; Fryer, P. M.; Galligan, E. A.; Graham, W. S.; Greier, P. F.; Horton, R. R.; Ifill, H.; Jenkins, L. C.; John, R. A.; Kaufman, R. I.; Kuo, Y.; Lanzetta, A. P.; Latzko, K. F.; Libsch, F. R.; Lien, S.-C. A.; Millman, S. E.; Nywening, R. W.; Polastre, R. J.; Powell, C. G.; Rand, R. A.; Ritsko, J. J.; Rothwell, M. B.; Staples, J. L.; Warren, K. W.; Wilson, J. S.; Wright, S. L. IBM J. Res. Dev. 1998, 42, 427-444. (5) Electroless Plating: Fundamentals and Applications; Mallory, G., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (6) Riedel, W. Electroless Nickel Plating; ASM International: Metals Park, OH, 1991. (7) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (8) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (9) 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.

10.1021/la0341658 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/11/2003

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and Pd,25-28 for example, and the printed SAM can be used as an ultrathin resist to protect the substrate selectively from etchants in solution.24,29-34 This patterning technique can process large substrates provided that stamps can be made large and accurate enough.8,10,35 Potentially, it has a high throughput and can replace expensive photoexposure tools, spin coaters, resist strippers, and resist developers. µCP alone has been applied to pattern, for example, conductive structures for electronic devices36-41 and for novel types of displays.42 µCP and ELD have been combined in a few published examples in which a catalyst for ELD was inked on a stamp and printed onto an insulating surface.43-47 “Plate, iPlate & Print” combines these techniques in a different manner because we wanted to continue using alkanethiols as the ink to define the desired pattern rather than microcontact printing a catalyst. This has the advantage of using µCP in its most established mode and does not require altering

Figure 1. Brief outline of the “Plate, iPlate & Print” approach, in which we combine ELD and µCP for the fabrication TFT gate structures on a glass substrate by electroplating a printable mask to pattern the electroless-deposited gate layer. See text for details.

Cu mask is removed at the end of the process. We call this approach “Plate, iPlate & Print” (Figure 1). We will simplify this method later by electroless-depositing Cu directly on glass and patterning it using µCP, an approach that is referred to as “Plate & Print”.10 ELD involves the reduction and deposition of metallic ions from a bath to a substrate in the presence of a reducing agent and without applying an external current.5 ELD is an alternative to depositing metals from vacuum or electroplating them onto conductive substrates. A layer of catalyst must be placed on the substrate to initiate ELD before the deposition proceeds autocatalytically. Despite being difficult to establish for demanding technological processes, ELD appears to be superior to vacuum deposition because it has a potentially higher throughput and can process large substrates in parallel at low cost. µCP7 uses a micropatterned stamp, which is inked, dried, and placed onto a substrate, to localize a reaction between molecules from the ink and the substrate in the areas of contact. The stamp is made of poly(dimethylsiloxane) (PDMS), an elastomer, to ensure good contact between the stamp and the substrate during printing.11 It is prepared by curing liquid prepolymers for PDMS on a patterned mold. µCP can pattern self-assembled monolayers (SAMs) of alkanethiols on Au,7,12-17 Ag,18-22 Cu,23,24 (10) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. Langmuir (submitted). (11) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318. (12) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (13) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600-604. (14) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324-3334. (15) Libioulle, L.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 1999, 15, 300-304. (16) Rogers, J. A.; Bao, Z.; Makhija, A.; Braun, P. Adv. Mater. 1999, 11, 741-745.

(17) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862-6867. (18) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1994, 143, 1070-1079. (19) Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 2059-2067. (20) Xia, Y.; Venkateswaran, N.; Qin, D.; Tien, J.; Whitesides, G. M. Langmuir 1998, 14, 363-371. (21) Jackman, R. J.; Brittain, S. T.; Adams, A.; Prentiss, M. G.; Whitesides, G. M. Science 1998, 280, 2089-2091. (22) Xia, Y.; Qin, D.; Whitesides, G. M. Adv. Mater. 1996, 12, 10151017. (23) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601-603. (24) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (25) Carvalho, A.; Geissler, M.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2406-2412. (26) Love, J. C.; Wolfe, D. B.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 1576-1577. (27) Wolfe, D. B.; Love, J. C.; Paul, K. E.; Chabinyc, M. L.; Whitesides, G. M. Appl. Phys. Lett. 2002, 80, 2222-2224. (28) Geissler, M.; Kind, H.; Schmidt-Winkel, P.; Michel, B.; Delamarche, E. Langmuir, in press. (29) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (30) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332-2337. (31) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron. Microeng. 1996, 32, 255-268. (32) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 3257-3264. (33) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122-126. (34) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (35) 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. (36) 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. (37) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182-1191. (38) Loo, Y.-L.; Someya, T.; Baldwin, K. W.; Bao, Z.; Ho, P.; Dodabalapur, A.; Katz, H. E.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10252-10256. (39) Zschieschhang, U.; Klauk, H.; Halik, M.; Schmid, G.; Radlik, W.; Weber, W. IEEE Polytr. 2002 Conf. 191-195. (40) Park, S. K.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Kim, W. K. IEEE Trans. Electron. Devices 2002, 49, 2008-2015. (41) Kagan, C. R.; Breen, T. L.; Kosbar, L. L. Appl. Phys. Lett. 2001, 79, 3536-3538. (42) Rogers, J. A.; Bao, Z.; Baldwin, K.; Dodabalapur, A.; Crone, B.; Raju, V. R.; Kuck, V.; Katz, H.; Amundson, K.; Ewing, J.; Drzaic, P. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4835-4840. (43) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (44) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215. (45) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (46) Bittner, A. M.; Wu, C. X.; Kern, K. Adv. Funct. Mater. 2002, 12, 432-436. (47) Wu, X. C.; Bittner, A. M.; Kern, K. Langmuir 2002, 18, 49844988.

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the surface chemistry of stamps.48,49 This approach also benefits from having an electroplated Cu layer to provide the mask for a large variety of electroless-deposited metals or alloys. 2. Experimental Section Glass Substrates and Chemicals. All experiments were done on display glass substrates (1.1 mm thick, code 1737 from Corning, Japan). The substrates for ELD and electroplating experiments were up to 15 × 15 in.2, whereas the substrates for printing and etching experiments were typically 1 × 3 in.2 in size. All chemicals were of the best grade available and obtained from Fluka or Aldrich unless indicated otherwise. Water was deionized and had a resistivity > 18.2 MΩ cm-1. EDA-Si was from Gelest (Tullytown, PA). Pd/Sn colloids (#1018, received in an acidic solution and diluted to 50% with concentrated HCl) and the accelerator (#1019, used as a 10% solution in water) were both from Fidelity (Captree Chemical Corporation, Amytiville, NY). Deposition of the Metallic Layers. ELD of NiB was achieved by immersing the activated samples into a commercial bath (Niposit 468, Shipley, prepared as recommended) operated at 57 °C, having a pH of 7.2 (adjusted using ammonia), and with moderate stirring. The deposition rate of the bath at these conditions was ∼25 nm min-1. The Cu-electroplating bath was prepared by dissolving 1.1 g of CuSO4‚5H2O, 3 g of Na4P2O7, and 20 mg of NaH2PO4 in 120 mL of water. This bath had a pH of ∼9 and was used at 30 °C. Electroplating was done using a potentiostat (PAR EG&G 263A) with a 4 cm2 platinized titanium grid as the counter electrode and a Ag/AgCl reference electrode. ECT was from Robinson Brothers Ltd. (West Bromwich, U.K.) and used as received. Microcontact Printing. Stamps were made by curing the prepolymer components of PDMS (Sylgard 184, Dow Corning, Midland, MI) for at least 12 h at 60 °C on a 4 in. Si wafer covered with a patterned resist. Smaller stamps were cut from the stamp replicated on the 4 in. wafer and used for only one printing experiment. The stamp was inked for 30 s with a 0.2 mM solution of ECT in ethanol, blown dry with a stream of N2, and placed by hand on the Cu substrate for 30 s. Etching. The etch bath for microcontact-printed Cu was composed of 0.25 M KCN dissolved in a KCl/NaOH solution of pH 12 (caution: the pH 12 buffer must be prepared first and then KCN added to it to prevent the formation of HCN). Octanol was added to the bath (at saturation) to block defects in the microcontact-printed monolayer.24 NiB was etched using a 1.0 M solution of H2SO4 at room temperature. The Pd/Sn colloids were underetched in the regions where NiB was removed using a solution containing 10 vol % H2O2 and 1.2 g of KOH per 100 mL of water. Instrumentation. The samples were inspected using a Leica Polyvar optical microscope equipped with a CCD camera (Coolpix 990, Nikon), a Hitachi S-4000 SEM, or a Nanoscope III AFM (Digital Instruments, Santa Barbara, CA). XPS spectra were acquired on a Sigma Probe VG Scientific spectrophotometer operating at a base pressure of ∼2 × 10-9 mbar and equipped with a monochromatized Al KR source (E ) 1486.6 eV). The X-ray spot was focused at 400 µm for all experiments. We used ∼1 × 1 cm2 samples, and the analyzer was at an angle of 37° to the samples. Spectra were referenced to the O 1s peak of the glass substrates at 532 eV. Surveys were acquired with a pass energy of 80 eV (0.1 eV steps for 20 ms acquisition time). The samples were investigated using a charge compensation gun (∼0.25 µA emission current) and a pressure of Ar of ∼2 × 10-8 mbar in the analysis chamber.

3. Results and Discussion The process of electroless-depositing NiB, electroplating Cu on top of it, microcontact printing the Cu layer, and (48) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. Adv. Mater. 2001, 13, 1164-1167. (49) Delamarche, E.; Donzel, C.; Kamounah, F. S.; Wolf, H.; Geissler, M.; Stutz, R.; Schmidt-Winkel, P.; Michel, B.; Mathieu, H. J.; Schaumburg, K. Langmuir (submitted).

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etching both metallic layers selectively is described below step by step. The overall method has a relatively large number of steps, but these are all elementary and experimentally simple and yield NiB patterns of high quality. We divided this process into three parts for simplicity and present XPS spectra of the surface of the sample for each step. Before NiB can be deposited homogeneously over a glass substrate, it is necessary to derivatize the glass with a layer capable of binding catalysts for ELD (Figure 2). We selected EDA-Si as the “linker” between the glass and the Pd/Sn catalyst.9,50-52 EDA-Si was typically grafted within a few minutes from a 1% solution in water onto glass, and the derivatized glass substrate was rinsed with water, dried with a stream of N2, and baked at 150 °C for 10 min.9 Grafting EDA-Si onto a Si/SiO2 wafer using similar conditions resulted in a ∼0.5 nm thick layer.9 The composition of EDA-Si grafted on glass is shown in Figure 2B. The N 1s (at 399.9 eV) and C 1s (at 285.1 eV) signals are attributed to the grafted EDA-Si layer, and the O 1s (at 531.6 eV, serving as a reference for all spectra), Si 2s (at 153.5 eV), and Si 2p (at 101.8 eV) signals originate primarily from the glass and to a lesser extent from the grafted silane. The amine functions of EDA-Si serve to bind Pd/Sn colloids53-57 from an acidic solution (Figure 2C): the electrostatic interactions between the protonated amines with Pd/Sn colloids (dispersed in concentrated HCl) ensure the deposition of a few nanometer thick film of Pd/Sn particles on the substrate.9,50 This deposition was achieved within 30 s of immersion. Careful rinsing of the sample with water ensured that excess Pd/Sn particles were removed from the substrate. XPS readily detected Pd (Pd 3d at 338.0 eV) and Sn (Sn 3d at 490.9 eV), owing to the high sensitivity of this technique to these elements and their position at the surface of the sample (Figure 2C). An “acceleration” step is usually performed after deposition of the catalyst and before ELD to make Pd more accessible by species from the ELD bath;58 Pd/Sn colloids are composed of a Pd-rich core surrounded by Sn, and exposing the immobilized colloids to HBF4 in solution for 30 s removed a small fraction of the Sn, as revealed by the XPS spectrum in Figure 2D. There, the Pd signals, as well as the signals from the EDA-Si layer and the glass substrates, are slightly higher than those before the acceleration step. This step might be important for ELD baths having low activity or for plating on very large substrates to ensure a homogeneous start of the ELD and thus depositing a NiB layer of homogeneous thickness. Accelerated samples were rinsed with water and immersed in a NiB ELD bath heated to 60 °C. We selected a bath depositing Ni together with 0.25% B to obtain a gate layer of sufficient conductivity. We typically plated 150 nm of a NiB alloy having a specific resistivity of 22 µΩ cm and a rms roughness of ∼0.5 nm, in 6 min. Such a NiB layer is in principle (50) Brandow, S. L.; Dressick, W. J.; Marrian, C. R. K.; Chow, G.-M.; Calvert, J. M. J. Electrochem. Soc. 1995, 142, 2233-2243. (51) Kind, H.; Bittner, A.; Cavalleri, O.; Kern, K.; Greber, T. J. Phys. Chem. B 1998, 102, 7582-7589. (52) 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. (53) Matijevic´, E.; Poskanzer, A. M.; Zuman, P. Plat. Surf. Finish. 1975, 62, 958-965. (54) Sard, R. J. Electrochem. Soc. 1970, 117, 864-870. (55) de Minjer, C. H.; van der Boom, P. F. J. J. Electrochem. Soc. 1973, 120, 1644-1650. (56) Kim, J.; Wen, S. H.; Jung, D. Y.; Johnson, R. W. IBM J. Res. Dev. 1984, 28, 697-709. (57) van der Putten, A. M. T.; de Bakker, J.-W. G.; Fokkink, L. G. J. J. Electrochem. Soc. 1992, 139, 3475-3480. (58) Osaka, T.; Takematsu, H.; Nihei, K. J. Electrochem. Soc. 1980, 127, 1021-1029.

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Figure 3. A Cu mask is used to pattern an electroless-deposited NiB layer on glass. The Cu is electroplated and microcontactprinted with ECT, and the Cu in the nonprinted areas is selectively removed with a wet-etch process. XPS surveys reveal the surface chemistry of the sample at each step, in both the printed and nonprinted areas.

Figure 2. Depositing a NiB layer on glass using ELD requires derivatizing the glass substrate with silanes, binding Pd/Sn catalysts to the derivatized glass, activating the catalysts, and depositing NiB. The chemical composition of the surface of a sample after each step is inspected using XPS.

conductive enough for making the gate layer of TFT arrays having a resolution of ∼160 pixels per inch and a diagonal size of ∼12 in.9 as well as for electroplating Cu over large areas (up to 15 × 15 in.2). A thin layer of native oxide formed on the surface of the Ni deposit (Figure 2E). The as-plated NiB layer can be thermally annealed to release some of its internal stress and decrease its specific resistivity before or after electroplating Cu.6,59,60 (59) Parker, K.; Shah, H. Plating 1971, 58, 230-236.

The next series of steps was to electroplate Cu on Ni and to pattern the Cu mask (Figure 3). We removed the native oxide from Ni prior to electroplating Cu to prevent failure of adhesion between the two layers.61 The Cu mask was deposited from a pyrophosphate Cu bath62,63 in an electrochemical cell and using NiB on glass as the working electrode. We usually deposited Cu at potentials more negative than -0.5 V (vs Ag/AgCl) but kept the potential of the NiB electrode higher than -1.0 V in order not to compromise the adhesion of the NiB layer to the glass. Small samples (1 × 3 in.2) could be coated with a homogeneously thick Cu layer, whereas 15 × 15 in.2 glass plates covered with Ni had to be contacted via all four sides by means of a cathode frame to minimize the drop of potential toward the middle of the Ni electrode, which diminished the current density and the plating rate there. Coulometry served to monitor the (average) thickness of the Cu layer during plating, and 50 nm of Cu was typically deposited within 30 s. ELD of Cu on Ni is in principle possible by immersing the NiB layer directly into a Cu ELD bath. This method, called immersion plating, relies on the galvanic exchange of Ni from the surface by Cu2+ from the bath to form a first thin layer of Cu on NiB, after which the deposition of Cu becomes autocatalytic.64 Immersion plating of Cu was not successful here because this corrosion-like process led to rough Cu films, and the NiB layer had the tendency to lose its adhesion to the (60) Parker, K. ASTM Spec. Technol. Publ. (Test. Met. Inorg. Coat.) 1987, 947, 111-122. (61) Ellipsometry experiments suggested that the nickel oxide forming under ambient conditions on freshly evaporated Ni was thinner than 2 nm. (62) An alkaline copper pyrophosphate bath was used because acidic Cu electroplating solutions attacked the Ni too strongly. (63) Modern Electroplating; Schlesinger, M., Paunovic, M., Eds.; Wiley & Sons: New York, 2000; pp 122-133. (64) Chen, M.-S.; Dulcey, C. S.; Brandow, S. L.; Leonard, D. N.; Dressick, W. J.; Calvert, J. M.; Sims, C. W. J. Electrochem. Soc. 2000, 147, 2607-2610.

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Figure 4. Patterning the electroplated Cu mask on the NiB constitutes an important checkpoint of the “Plate, iPlate & Print” method reported here. This SEM image reveals that the ∼50 nm thick Cu was successfully printed and etched without affecting the NiB underlayer.

glass substrate. Attempts to accelerate the initial deposition of Cu and to activate the Ni with a thin layer of immersion-plated Pd failed as well. ELD of Au onto NiBcovered samples was successful and may provide an alternative to the electroplating of Cu. A thin oxide formed on the freshly electroplated Cu; see Figure 3F. It was preferable to remove this oxide layer under mildly acidic conditions before microcontact printing alkanethiols on the Cu layer. This was done by dipping electroplated samples into a 4% solution of HCl in water (per volume) for 15 s. Microcontact printing ECT onto the Cu substrate for 30 s yielded a SAM in the regions of contact. It was possible to inspect the chemical composition of the printed regions in millimeter wide areas and to compare it with that of the adjacent nonprinted regions (Figure 3G). The nonprinted region shows a decrease in the O 1s signal due to the oxide-stripping step. The printed area has little oxide and a significant amount of C 1s (at 285.6 eV), corresponding to the alkyl chains of the printed SAM. The pattern was transferred into the Cu layer using a CN-/O2-based etch within ∼3 min.24 The selectivity of this etch chemistry was good enough to form high-quality patterns with micrometer accuracy in 50 nm thick Cu (Figure 4). The NiB layer was not attacked by the CN-/O2-based etch, and the Cu mask does not seem to have defects. More selective etch chemistries could be used to etch thicker Cu layers having a strong gradient of thickness or being less protected by the printed SAM. In principle, metals other than Cu that can be microcontact printed with alkanethiols can also provide the mask.65 In practice, electroless and electroplating solutions of Au are very expensive and may form rougher films than Cu does. Au forming the mask would otherwise be interesting because Au and Ni have more orthogonal etching chemistry than Cu and Ni.66,67 Using an electroless-deposited Au mask, we could, for example, etch Ni selectively using (i) HCl/H2O2,68 (ii) 0.5 M HNO3, or (iii) a H3PO4/acetic acid/HNO3/water (75:16:4:5 in volume) mixture. Pd is expensive to use for ELD and electroplating, (65) Kim, E.; Kumar, A.; Whitesides, G. M. J. Electrochem. Soc. 1995, 142, 628-633. (66) Saubestre, E. B. In Modern Electroplating; Lowenheim, F. A., Ed.; John Wiley & Sons Inc.: New York, 1974; p 748. (67) CRC Handbook of Metals Etchants; Walker, P., Tarn, W. H., Eds.; CRC Press: Boca Raton, FL, 1991. (68) Etch times ranged from 15 s to 3 min for 150 nm of NiB (nonannealed) using 6 mL of H2O2 in 100 mL of water, 0.1-0.4 mL of HCl, and etching at room temperature with strong stirring.

Figure 5. Transferring the Cu mask into the underlying NiB deposit relies on selective wet-etching steps. Removing the Pd/ Sn residues together with the preconditioning layer and the Cu mask yields the final NiB structures and ends the “Plate, iPlate & Print” process. The wet-etch chemistries were carefully selected to minimize side reactions with the various layers.

and etching it with an acidic solution of ferrichloride would probably induce etching of the NiB underneath it. Ag tends to oxidize too much to form a convenient printable mask. We etched NiB in 1.0 M H2SO4 within 20 min; the etch rate of NiB was 5-10 nm min-1 at room temperature and with moderate stirring. Figure 5I shows the chemical composition of the surface after selectively etching NiB. A slight reduction in the amount of oxide present on the Cu mask is the only noticeable change in these parts of the sample. NiB dissolved entirely on the other side of the sample, leaving Pd and some Sn on the glass substrate. Most of the Sn was probably removed during this step, exposing the Pd-rich core of the colloids at the surface even more. The transmission of visible light in the regions where the NiB was removed was ∼95%, but it is desirable to improve the transparency of these regions for display applications by removing the Pd/Sn remaining on the glass. Selectively etching Pd/Sn in the presence of Ni is difficult, but immersing the sample in an aqueous solution of KOH/ H2O2 for 20 min results in the chemical decomposition of the EDA-Si layer on the glass and helps release the Pd/Sn particles into the solution. The XPS spectra in Figure 5J confirm that little Pd remained on the glass surface and, interestingly, the SAM of ECT was largely removed from the Cu mask during this oxidizing treatment. The transparency of the glass substrate has become >98% at this step in the process. Removing the Cu mask is optional, depending on the application sought. It might be necessary to remove it to fabricate a TFT because Cu is mobile in Si layers and can create deep-level recombination sites in the amorphous Si layer that forms the semiconductor of the TFT.1,69,70 A CN-/O2-based etch dissolved Cu directly because the ECT monolayer had already been removed in (69) Schmidt, J.; Aberle, A. G. J. Appl. Phys. 1999, 85, 3626-3633. (70) Sachdeva, R.; Istratov, A. A.; Weber, E. R. Appl. Phys. Lett. 2001, 79, 2937-2939.

Patterning NiB Electroless Deposited on Glass

Langmuir, Vol. 19, No. 14, 2003 5897

sponds to the design of the gate layer of an IBM “Roentgen” display.72 These displays are 16.3 in. in diagonal and feature a QSXGA resolution (2560 pixels × 2048 pixels), which is equivalent to a full-color resolution of 200 pixels per inch. The dashed rectangle in Figure 6A overlaps with three subpixels (or one pixel) having a size of 42 × 126 µm2 each. The NiB gate layer is well defined in terms of resolution and accuracy; the transparency of regions macroscopically free of NiB structures is >99% (for visible light), as measured using a spectrophotometer. The AFM image in Figure 6C reveals the smoothness and accuracy of a NiB line. Conductivity measurements done in various parts of the gate pattern did not reveal any electrical short, even for the closest structures of the pattern.73 Annealing the NiB alloy was done in an oven filled with N2 and at a temperature of 350-400 °C for 2 h.9 This treatment reduced the specific resistivity of the NiB to 13 µΩ cm and concluded the overall patterning process. 4. Conclusion A strategy to pattern a metal on a substrate by combining ELD, electroplating of a mask, and structuring the mask by means of µCP was presented here. The interest of this strategy lies in using a substrate that is well suited for µCP, such as an electroplated Cu mask, to pattern a nondirectly printable layer. Microcontact printing a mask not only enables extending µCP to a variety of substrates but might also provide an alternative to amplifying a printed resist or using selective etch systems to form well-patterned structures. This strategy could be particularly advantageous for the patterning of electrolessdeposited metals. ELD is an alternative to the deposition of metals from vacuum and is highly complementary to µCP: neither of these techniques requires expensive equipment; they can both process large substrates and are in principle economical techniques from a manufacturing point of view. Several questions associated with the practical use of this approach still remain. What are the best orthogonal etch systems between a specific mask and a specific base layer? What is the highest resolution achievable? Can this method be simplified? We think that simplifying this method is necessary when applying it to the fabrication of arrays of gates for making thin-film transistors, for example, and future work will specifically be to electroless deposit Cu onto glass and pattern it using µCP directly. Figure 6. Optical microscope images (A and B) and AFM image (C) of NiB gates patterned on glass (1 × 3 in.2 in size) using the “Plate, iPlate & Print” method. (A) The vertical NiB lines correspond to the gate lines of an array having 42 × 126 µm2 subpixels (equivalent to a resolution of 200 pixel per inch). The dashed square represents the area occupied by one pixel. (B) Light from the back of the sample is blocked by the NiB structures (this is required to prevent light-induced current in the TFTs) but is transmitted in the areas where Cu, NiB, and the Pd/Sn particles were removed. (C) The NiB layer was patterned with high resolution and without creating defects in the final NiB structures.

the previous step. A second exposure of the sample to KOH/H2O2 removed any remaining Pd, and a short immersion into HCl yielded a clean and almost oxide-free NiB layer (Figure 5K).71 The images in Figure 6 reveal the quality of the final NiB structures formed using the “Plate, iPlate & Print” process described above. The pattern shown here corre(71) A thin Ni oxide forms again before the measurement and is visible in the spectrum.

Acknowledgment. We are grateful to our colleagues of the Advanced Display Manufacturing Laboratory at the IBM Watson Research Center for their continuous help and support, to H. Kind, J. C. Flake, A. Bietsch, P. Schmidt-Winkel, H. Wolf, and D. Juncker for useful discussions, to R. Stutz for his help with the initial electroplating experiments, and to U. Drechsler for her help with the microfabrication of molds. We thank P. F. Seidler and the IBM Display Business Unit for their continuous support. LA0341658 (72) Schleupen, K.; Alt, P.; Andry, P.; Asaad, S.; Colgan, E.; Fryer, P.; Galligan, E.; Graham, W.; Greier, P.; Horton, R.; Ifill, H.; John, R.; Kaufman, R.; Kinoshita, H.; Kitahara, H.; Kodate, M.; Lanzetta, A.; Latzko, K.; Libertini, S.; Libsch, F.; Lien, A.; Mastro, M.; Millman, S.; Nunes, R.; Nywening, R.; Polastre, R.; Ritsko, J.; Rothwell, M.; Takasugi, S.; Warren, K.; Wilson, J.; Wisnieff, R.; Wright, S.; Yue, C. Proceedings of the Eighteenth International Display Research Conference, Seoul, South Korea, 1998. (73) The resistance between structures separated by ∼2 µm was consistently 50 MΩ.