Selective Electroless Nickel Plating on Polyelectrolyte Multilayer

Sep 22, 2001 - A more universal, yet still patternable, platform for selective electroless plating would be useful, especially one that is less depend...
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Selective Electroless Nickel Plating on Polyelectrolyte Multilayer Platforms T. C. Wang,† B. Chen,‡,§ M. F. Rubner,*,‡ and R. E. Cohen*,† Departments of Chemical Engineering and Materials Science and Engineering and the Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 22, 2001. In Final Form: August 6, 2001 Polyelectrolyte multilayers (PEMs) fabricated by the layer-by-layer adsorption of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are used to selectively bind palladium catalysts for electroless nickel plating. Depending on the pH conditions used during multilayer processing and the polyelectrolyte adsorbed last, the surface of the PAH/PAA-based multilayers can be tuned to contain primarily PAA, primarily PAH, or a mixture of PAA and PAH functionalities (i.e., carboxylic acids or amines). The palladiumcomplex ions of tetraaminepalladium chloride (Pd(NH3)4Cl2) or sodium tetrachloropalladate (Na2PdCl4) bind selectively by ion exchange only to surfaces of PAA or PAH, respectively. Substrates that are coated with regions of PAA-topped and PAH-topped multilayers, fabricated at a PAH pH of 7.5 and a PAA pH of 3.5, bind [Pd(NH3)4]2+ only on the PAA regions. Upon immersion into an electroless nickel plating bath, only the PAA-topped regions of the substrate, activated by the Pd complex, promote nickel plating while the PAH-topped regions resist plating. By using [PdCl4]2-, the PAH regions are selectively plated instead. Moreover, the facile patternability of PEMs is demonstrated using the inkjet printing of a PAA solution onto a PAH-topped multilayer film. Tetraaminepalladium ion binding and nickel plating are confined to the printed PAA pattern. Because of their surface tunable capabilities and therefore ease of selective activation, conformal coverage, facile processing, adherent coating, and patternability, PAH/PAA-based multilayers are ideal platforms for selective electroless plating.

Introduction Electroless plating of metal, particularly copper or nickel, is an important industrial technique for metallizing insulators (e.g., plastics) and objects with geometries that are difficult to coat by electroplating.1 One important example is the use of electroless copper and nickel in printed circuit boards to form interconnects.2 In contrast to electroplating, where an applied current supplies electrons to reduce a high-oxidation-state metal precursor, the basis of electroless plating is an autocatalytic redox reaction. The important material prerequisite for initiating metal deposition is the presence of an appropriate catalytic surface. For many materials (e.g., insulators), the surface is not inherently catalytic and must be activated prior to electroless plating. For example, a standard technique for creating a catalytic surface on an insulator involves a cleaning and etching step and an activation step.1 First, the substrate is treated with a solution of chromium oxide and sulfuric acid, which removes any impurities and etches the surface for better adhesion of the catalyst and plated metal. Then, a palladium and tin mixture is deposited onto the cleaned surface by reduction. These surfaceconditioning steps are tedious and harsh, and they can limit the selection of materials to be plated and make it difficult to selectively activate surfaces without resorting to photolithography. Recently, non-photolithographic approaches to preparing patterned catalytic surfaces for electroless plating have * Corresponding authors. † Department of Chemical Engineering. ‡ Department of Materials Science and Engineering. § Current address: Molecular Dynamics, 928 Arques Ave., Sunnyvale, CA 94086. (1) Riedel, W. Electroless Nickel Plating; Finishing Publications: Metals Park, 1991; Chapters 1, 3, and 9. (2) Clark, R. H. Handbook of Printed Circuit Manufacturing; Van Nostrand Reinhold: New York, 1985; Chapter 15.

been developed. Using microcontact printing (µCP), functionalized Pd colloids or complexes are deposited onto a treated surface that can bind the Pd, in these cases an organosilane layer or a titanium coating, respectively.3,4 Metal plating only occurs where the stamp deposits the Pd. The main drawback is that the Pd must be suitably stabilized or functionalized to interact and be compatible with the µCP stamp and the surface. Inkjet printing has also been used to pattern Pt colloids as catalysts.5 However, the quality and adhesion of the electrolessly plated metal were strongly dependent on the surface material. A more universal, yet still patternable, platform for selective electroless plating would be useful, especially one that is less dependent on the substrate material and amenable to both facile, large-scale plating and small-scale patterning. Polyelectrolyte multilayers (PEMs) are thin films formed from two oppositely charged polyelectrolytes, alternately adsorbed onto a surface one layer at a time. PEMs have attracted much attention over the past decade as easy to fabricate, robust thin films with tunable architectures (i.e., film composition and physical and chemical microstructure).6 Besides polyelectrolytes, this simple and versatile layer-by-layer process has been extended to multilayers of polymers formed by hydrogen bonding,7,8 inorganic materials,9-12 dendrimers,13 and (3) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (4) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (5) Shah, P.; Kevrekidis, Y.; Benziger, J. Langmuir 1999, 15, 15841587. (6) For reviews, see (a) Decher, G. Science 1997, 277, 1232-1237. (b) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430-442. (c) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (7) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 27172725.

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biological molecules.14-16 A broad range of applications, including light-emitting diodes,17 photovoltaics,18 optical sensors,19 bioinert coatings,20 and nonlinear optics,21,22 has been explored. Moreover, besides the commonly used flat glass and silicon substrates, PEMs easily coat a variety of other materials and substrate geometries, including inorganic particles,23 polymer colloids,24 enzyme crystals,25 biological surfaces,20 various polymers,26-29 and the inner walls of fused silica capillaries30 and plastic microfluidic channels.31 Finally, selective deposition of PEMs onto µCPpatterned self-assembled monolayers has also been demonstrated.32 While much of the work on PEMs has focused on polyelectrolytes that are fully ionized in solution (e.g., sulfonated polystyrene and poly(diallyldimethylammonium)),6 PEMs of weak polyelectrolytes have a unique and ultimately useful feature in that their architecture depends on the polyelectrolyte degree of ionization. The chemical and physical microstructure of PEMs based on poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) can be tuned by the pH of the polyelectrolyte solution.33,34 The degree of ionization of PAA (effective pKa within the PEM is33 ∼4, as compared to pKa ) 4.7 for aqueous acetic acid35) depends strongly on pH in the range of 2.5-5.5. PAH, on the other hand, remains fully ionized (8) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360-1363. (9) Keller, S. W.; Kim, H. N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817-8818. (10) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370-373. (11) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (12) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195-6203. (13) He, J.-A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C.; Kumar, J.; Samuelson, L.; Balogh, L.; Tomalia, D. A.; Tripathy, S. K. Chem. Mater. 1999, 11, 3268-3274. (14) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396-5399. (15) Hong, J.-D.; Lowack, K.; Schmitt, J.; Decher, G. Prog. Colloid Polym. Sci. 1993, 93, 98-102. (16) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117-6123. (17) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501-7509. (18) Mattoussi, H.; Rubner, M. F.; Zhou, F.; Kumar, J.; Tripathy, S. K.; Chiang, L. Y. Appl. Phys. Lett. 2000, 77, 1540-1542. (19) Lee, S.-H.; Kumar, J.; Tripathy, S. K. Langmuir 2000, 16, 1048210489. (20) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355-5362. (21) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304-8309. (22) Wang, X. G.; Balasubramanian, S.; Li, L.; Jiang, X. L.; Sandman, D. J.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451-459. (23) Gittins, D. I.; Caruso, F. Adv. Mater. 2000, 12, 1947-1949. (24) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf., A 1998, 137, 253266. (25) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485-1488. (26) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78-86. (27) Delcorte, A.; Bertrand, P.; Wischerhoff, E.; Laschewsky, A. Langmuir 1997, 13, 5125-5136. (28) Phuvanartnuruks, V.; McCarthy, T. J. Macromolecules 1998, 31, 1906-1914. (29) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354-1359. (30) Graul, T. W.; Schlenoff, J. B. Anal. Chem. 1999, 71, 4007-4013. (31) Barker, S. L. R.; Tarlov, M. J.; Canavan, H.; Hickman, J. J.; Locascio, L. E. Anal. Chem. 2000, 72, 4899-4903. (32) (a) Hammond, P. T.; Whitesides, G. M. Macromolecules 1995, 28, 7569-7571. (b) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237-7244. (c) Clark, S. L.; Montague, M.; Hammond, P. T. Supramol. Sci. 1997, 4, 141-146. (33) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309-4318. (34) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 42134219.

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until the pH becomes basic (effective pKa is34 > 7, as compared to pKa ) 10.6 for aqueous ethylamine35). The interaction between PAA and PAH at different pHs leads to intricate yet tunable PEM assemblies. Several physical and chemical features can be controlled, including the adsorbed polymer layer thicknesses, which can be varied from 0.5 to 8 nm, the bulk and surface compositions (i.e., amount of PAH relative to PAA), and the number of carboxylic acid groups contained within the film and on its surface. For example, the effect of pH on bulk composition has been exploited to fabricate a PAH/PAAbased multilayer that undergoes a transformation from nonporous to porous structure upon exposure to specific pH conditions.36 While layer thicknesses can be varied in strong polyelectrolyte systems using ionic strength, the other physicochemical manipulations are not addressable with strong polyelectrolytes. Surface compositions can be readily tailored using pH to exhibit the characteristics of the last adsorbed polyelectrolyte, either PAH or PAA, or to show mixed properties of both PAH and PAA regardless of which polyelectrolyte is last adsorbed. At a relatively high PAH pH and low PAA pH (e.g., 7.5 and 3.5, respectively), the surface is dominated by PAA when PAA is the last layer and by PAH when PAH is last. PAA dominance is exemplified by a very low contact angle and high absorbance of a cationic dye, methylene blue.34 On the other hand, when the pH of both polyelectrolytes is comparable, the surface exhibits properties that are less dependent on the last polyelectrolyte adsorbed. At a low pH of 2.5, the contact angle and methylene blue absorbance suggest that a large fraction of carboxylic acid groups are at the surface regardless of whether PAA or PAH is the last polyelectrolyte adsorbed.33 At higher pHs, the surface is composed of significant fractions of both polyelectrolytes. The ability to control the content of carboxylic acid groups is of particular importance in employing and manipulating these PEMs as soft, easily processible matrices for templating inorganic materials. We have recently demonstrated the in situ synthesis of inorganic nanoparticles within multilayers based on PAH and PAA.29 By retaining nonionized carboxylic acid functionalities within the PEMs through low processing pHs, the multilayers can subsequently act as proton exchangers for metal cations. Further chemistry, for example, reduction or sulfidation, nucleates and grows zerovalent metal or diatomic semiconductor particles (e.g., silver or lead sulfide). In this paper, we demonstrate the use of PEMs as a platform for the selective electroless plating of nickel. Key features of PEMs based on PAH and PAA, specifically nanoscale control over the internal architecture (e.g., degree of interpenetration), control over surface composition (e.g., carboxylic acid content), facile and environmentally friendly processing, conformal coating onto a wide variety of surfaces independent of size, and facile patternability, make them ideal, readily activated, and selective platforms for electroless plating. With the ability to dramatically alter multilayer surface functionalities with a single layer of polyelectrolyte, surfaces that can and cannot bind a Pd complex can be prepared. A PAAdominant surface binds a positively charged Pd complex, while a PAH-dominant surface resists binding. With a negatively charged Pd complex, the PAH-dominant surface (35) Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; pp 487 and 733. (36) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023.

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binds the catalyst. Electroless nickel plating is selectively promoted only on the PAA or PAH surface and inhibited on the other with only one polyelectrolyte layer difference. PAH/PAA multilayer-coated substrates that contain regions of PAA or PAH outermost layers are used to direct plating only to the PAA or PAH surfaces. In addition, inkjet printing is used to selectively deposit a PAA layer onto a PAH-topped multilayer, where only the PAA pattern becomes electrolessly plated. Experimental Section PAH (Mw ) 70 000), tetraaminepalladium chloride (Pd(NH3)4Cl2), and nickel sulfate hexahydrate were obtained from SigmaAldrich (St. Louis, MO). PAA (Mw ) 90 000) was obtained from Polysciences (Warrington, PA). Dimethylamine borane (DMAB) was obtained from Acros Organics (Fair Lawn, NJ), sodium citrate and lactic acid were obtained from Alfa Aesar (Ward Hill, MA), and sodium tetrachloropalladate (Na2PdCl4) was obtained from Strem (Newburyport, MA). All chemicals were used without further purification. Deionized water (>18 MΩ cm, Millipore Milli-Q) was exclusively used in all aqueous solutions and rinsing procedures. PAH/PAA-based multilayers were fabricated on glass microscope slides or polystyrene (PS) tissue-culture substrates (coronatreated, Nalge Nunc International, Naperville, IL) using an automated Zeiss HMS slide stainer as previously described.33,34 Glass substrates were degreased in a detergent solution followed by air plasma treatment (5 min at 100 W, Harrick Scientific PDC-32G plasma cleaner/sterilizer) prior to deposition. PS substrates were used as received. PAH aqueous solutions (10-2 M by repeat unit) were adjusted to pH 7.5 ( 0.1 with 1 M NaOH, and PAA aqueous solutions (10-2 M by repeat unit) were adjusted to pH 3.5 ( 0.1 with 1 M HCl. Other pH conditions were similarly obtained by adding the appropriate amount of acid or base. Multilayers were formed by first immersing substrates into the PAH solution for 15 min followed by three 2 min immersions into water as rinsing steps. The substrates then were immersed into the PAA solution for 15 min followed by identical rinsing steps. The adsorption and rinsing steps were repeated until the desired number of bilayers was obtained. One bilayer is defined operationally as a single adsorption of PAH followed by an adsorption of PAA. The PEM was finally dried in air and stored under ambient conditions. An Epson Stylus Color 980 inkjet printer, modified to accommodate hard substrates and to print polyelectrolyte solutions, was used for patterning the multilayer surface. Patterns were created electronically on a PC using a standard drawing program and sent to the printer using the manufacturer-supplied printer driver. Immediately prior to printing, a PAH-topped multilayer film fabricated on a glass substrate was heated to approximately 60 °C for 15 min with a heat lamp. A PAA solution of the same concentration and pH used in the multilayer fabrication was printed at 2880 dpi onto the PAH-topped multilayer. A standard formulation for the electroless nickel bath was used, consisting of 40 g/L nickel sulfate, 20 g/L sodium citrate, 10 g/L lactic acid, and 1 g/L DMAB in water.1 A nickel stock solution of all components except the DMAB reductant was prepared in advance. A DMAB aqueous solution was prepared separately. The stock solutions were prepared for a 4:1 volumetric proportion of nickel to reductant stocks in the final electroless bath. They were mixed as needed and adjusted to a pH of 6.8 ( 0.3 with ammonium hydroxide. Stock solutions were used within a week of preparation, after which they were discarded. The PAH/PAA-based multilayer film was electrolessly nickel plated in the following steps: (i) 10 s immersion in dilute, aqueous Pd(NH3)4Cl2 solution (5 mM), (ii) 2 min rinse in water, (iii) immersion in the electroless nickel bath at room temperature for up to 13.5 min, and (iv) after the desired level of plating is reached, copious rinsing with water followed by air-drying. When Na2PdCl4 (10 mM, aqueous) was used, the plating procedure differed only in the rinsing step (ii). Instead of a single rinse between the Pd complex and the electroless nickel bath, three rinsing steps were employedsless than 5 s immersion in water, then in dilute acid solution (pH 2.5 with HCl or H2SO4) for 1 min, and again less than 5 s in water.

Figure 1. Chemical structures of the palladium salts used as electroless nickel plating catalysts and the polyelectrolytes used for PEM fabrication. For transmission electron microscopy (TEM) imaging, multilayer films deposited on PS substrates were cut direction normal to the film plane using a RMC MT-X ultramicrotome with a diamond knife (Diatome, Fort Washington, PA) at room temperature. Approximately 50 nm thick cross sections of the samples were obtained. Cut sections were floated onto a trough of deionized water, immediately picked up with copper TEM grids, and blotted dry. TEM was performed on ultramicrotomed samples using a JEOL JEM-2000FX operated at 200 kV. While imaging was performed, samples were tilted until the electron beam was oriented normal to the cross-sectional plane. Resistivity was measured using the van der Pauw four-point probe method.37 Fine copper wires as electrodes were attached to the nickel surface using silver paint (Ernest F. Fullam, Latham, NY). A current was applied by a Hewlett-Packard 3245A universal source, and the voltage was measured using a HewlettPackard 34401A multimeter. Adhesion was determined qualitatively by a Scotch-tape peel test, in which a piece of tape was firmly applied to a nickel-coated surface and then removed.3 Good adhesion was indicated by the lack of any nickel film on the peeled tape. PEM film and nickel plating thicknesses on a glass substrate were measured using a Tencor P10 Profilometer.

Results and Discussion To demonstrate the versatility and selectivity of PEM platforms for electroless nickel deposition, two catalyst systems were employedsa salt of a positively charged Pd complex, Pd(NH3)4Cl2, and a salt of a negatively charged Pd complex, Na2PdCl4. Chemical structures for the Pd catalysts and the polyelectrolytes are shown in Figure 1. The Pd(NH3)4Cl2 experiments are of primary focus and will be discussed first. To investigate the surface binding affinity of PAH/PAA-based multilayers for [Pd(NH3)4]2+, PEMs were fabricated at a pH of 3.5 for both polyelectrolyte solutions. At this pH combination, the surface of the PEM is expected to contain a significant concentration of carboxylic acid groups regardless of the polyelectrolyte adsorbed last.33 PEMs were fabricated with 5 layers of PAA alternating with 5 or 6 layers of PAH (i.e., 5 or 5.5 bilayers of PAH/PAA). Five bilayers of PAH and PAA adsorbed at pH 3.5, referred to as (PAH3.5/PAA3.5)5 throughout the paper, have a PAA last adsorbed layer, and 5.5 bilayers, or (PAH3.5/PAA3.5)5.5, have a PAH last layer. Multiple bilayers were adsorbed to ensure complete coverage and elimination of any substrate surface effects. It has been previously shown in the literature that after approximately three bilayers, depending on the deposition conditions and substrate material, the surface properties of PEMs become independent of the number of bilayers and the substrate.38 Once fabricated, the PEM-coated substrates were activated and electrolessly nickel plated. A shiny, metallic (37) van der Pauw, L. J. Philips Res. Rep. 1958, 13, 1-9. (38) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249-1255.

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Figure 2. Cross-sectional TEM image of electrolessly plated nickel on a multilayer film of (PAH3.5/PAA3.5)5 supported on a PS substrate. The PEM, approximately 30 nm thick, does not show any contrast (i.e., dark color) against the PS substrate except for the nickel within it.

luster developed on the PEMs regardless of the outermost layer, PAA or PAH. At the electroless bath conditions, pH 6.8 and room temperature, and catalyst immersion times employed, a uniform, reflective surface developed in about 10 min. A 100 nm thick, conductive nickel coating was obtained (approximately 2 × 10-6 Ω m; published values are (5-13) × 10-8 Ω m for DMAB-based electroless nickel with greater than 2.5 µm thicknesses39). Moreover, the nickel coating exhibited excellent adhesion to the PEMcoated surface, regardless of substrate material (i.e., glass or PS). Multilayer films formed from PAH and PAA solutions at pH 3.5 have surfaces that are rich in both polyelectrolytes because of extensive interpenetration. In addition, a high concentration of carboxylic acid protons is retained because of the low processing pH. Upon immersion of the PEM-coated substrate into the Pd-salt solution, protons of the PAA carboxylic acid groups exchange for [Pd(NH3)4]2+. In previous work with proton-exchange membranes, other researchers have shown that a related platinum complex, [Pt(NH3)4]2+ (chloride salt), exchanges for protons and reduces to form Pt nanoparticles.40-42 The excellent adhesion of the nickel to the PEM platform is noteworthy, particularly in comparison to traditional electroless plating where harsh surface pretreatment conditions are often used to etch and roughen the surface to obtain good adhesion of the plated metal. In PEM fabrication, mild substrate surface treatments (e.g., detergent cleaning of glass) are enough to promote the adsorption of the polyelectrolytes. Even bare surfaces treated to have a negative charge for polycation adsorption (e.g., corona-treated PS) are relatively ineffective at binding the [Pd(NH3)4]2+ catalyst. Nickel plated much more slowly and incompletely on bare PS than on PEMcoated substrates. Moreover, the resulting metal film adhered poorly to PS, in many places, flaking off during the plating procedure. No plating occurred at all on bare glass substrates. A cross-sectional TEM image of a (PAH3.5/PAA3.5)5 multilayer fabricated on a PS substrate and coated with electrolessly plated nickel, shown in Figure 2, indicates that there is some growth of the metal film into the PEM. This penetration of the metal layer improves adhesion to the PEM. Another possible source for improved adhesion is the decreased stress of metal films on the PEM as compared to the bare substrate. In an aqueous environment, and even in ambient air, PEMs absorb significant amounts of water and behave like (39) Safranek, W. H. The Properties of Electrodeposited Metals and Alloys, 2nd ed.; American Electroplaters and Surface Finishers Society: Orlando, 1986; Chapter 23. (40) Fedkiw, P. S.; Her, W.-H. J. Electrochem. Soc. 1989, 136, 899900. (41) Fedkiw, P. S. U.S. Patent 4,959,132, 1990. (42) Liu, R.; Her, W.-H.; Fedkiw, P. S. J. Electrochem. Soc. 1992, 139, 15-23.

Figure 3. (a) Schematic (not drawn to scale) of a (PAH7.5/ PAA3.5) multilayer on glass with regions of PAH or PAA as the outermost layer and (b) top-down photograph of this multilayer after electroless nickel plating.

ionically cross-linked hydrogels.36,43-45 This plasticization of the PEM imparts a degree of mobility and conformational accommodation of the surface during metal plating. To generate surfaces that could resist, as well as bind, the positively charged Pd complex, PEMs were fabricated from a PAH solution of pH 7.5 and a PAA solution of pH 3.5, denoted (PAH7.5/PAA3.5). At these pH conditions, the PEM surface is predominantly the last adsorbed polyelectrolyte.33 Three regions of PAA- or PAH-topped multilayers, 5, 5.5, and 6 bilayers, were created on the same substrate by simply immersing the substrate only partway into the final polyelectrolyte solution. A schematic of the final PEM structure is shown in Figure 3a. This type of structure not only provided a single sample with both catalyst binding and resistant surfaces but also gave an indication of the selectivity at the interface between the PAH-topped and the PAA-topped multilayers. After immersion of the entire substrate into the [Pd(NH3)4]2+ solution and the electroless nickel bath, a shiny, metallic luster developed only on the portions of the substrate with a PAA top layer while no metal deposition occurred on the PAH-topped region. A photograph of the sample after plating is shown in Figure 3b. The interface between the plated PAA-topped region and the unplated PAH-topped region remained sharp after electroless nickel deposition. For most PEMs based on two polyelectrolytes, a degree of interpenetration occurs between adsorbed polyelectrolyte layers such that the bulk of the film is fully chargecompensated and can be viewed as a macroscopically homogeneous blend of the two polyelectrolytes.6a,46,47 However, in solution, the surface of PEMs is charged to a certain extent.26,33 This is often implicit when discussing PEMs; there must be some heterogeneity between the fully charge-compensated bulk and the active surface to facilitate further polyelectrolyte adsorption for building a multilayer. In fact, with PAH/PAA-based multilayers, surface properties can show a strong dependence on the (43) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (44) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623. (45) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006-2013. (46) Kellogg, G. J.; Mayes, A. M.; Stockton, W. B.; Ferreira, M.; Rubner, M. F.; Satija, S. K. Langmuir 1996, 12, 5109-5113. (47) Baur, J. W.; Rubner, M. F.; Reynolds, J. R.; Kim, S. Langmuir 1999, 15, 6460-6469.

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last polyelectrolyte adsorbed at certain solution adsorption conditions, as previously mentioned. (PAH7.5/PAA3.5) multilayers have surfaces enriched almost completely in PAH or PAA depending on the last polyelectrolyte adsorbed. This type of architecture results from the PEM fabrication process in which an adsorbing polyelectrolyte is in a partially ionized state and the previously adsorbed polyelectrolyte is in a highly ionized state. When PAA is adsorbed at pH 3.5, it is only partially ionized in solution and hence a relatively large amount (i.e., thick layer) of PAA is adsorbed to pair with the highly ionized, previously adsorbed PAH. The adsorption of a PAH layer takes place at pH 7.5, where all of the remaining carboxylic acid groups on the previously adsorbed PAA (i.e., those not already bound to PAH within the multilayer) are ionized. In turn, a relatively large amount of PAH is required for charge compensation. The result is thick layers of PAH and PAA, approximately 9.0 and 5.9 nm, respectively.34 There are relatively few unbound carboxylic acid groups within the interior of the (PAH7.5/PAA3.5) multilayers as determined by in situ silver nanoparticle formation. A low concentration of silver nanoparticles, prepared using the ion-exchange and reduction methodology described earlier,29 was observed (results not shown). Because of the thick layers, the surface is almost completely enriched in the last adsorbed polyelectrolyte. Moreover, a PAA last layer will retain a large fraction of nonionized carboxylic acid groups, while a PAH last layer will be relatively free of any carboxylic acid groups from the PAA layer beneath. In previous published work, methylene blue was found to adsorb onto multilayers only when PAA was the outermost layer, as mentioned earlier.34 Because the catalyst binding selectivity of (PAH7.5/ PAA3.5) multilayers is determined solely by the last adsorbed polyelectrolyte layer, Pd-complex binding is confined to the outermost layer by limiting the mass transport of [Pd(NH3)4]2+ into the bulk of the multilayer through short exposure times, typically less than 10 s. Only multilayers with PAA as the outermost layer exhibit accessible acid groups, and, hence, [Pd(NH3)4]2+ is bound only on PAA surfaces. PEM regions with PAH as the outermost layer do not have these ion-exchangeable groups. Hence, electroless plating occurs selectively on the Pd-activated, PAA regions of the surface. Longer exposure to the Pd complex permits access to the carboxylic acid groups, albeit at low concentrations, within the interior of the PEM, leading to nonselective surface plating regardless of the outermost layer being PAA or PAH. In addition to the ability to easily coat large areas conformally and uniformly, PEMs can also be patterned using high-resolution yet inexpensive (i.e., non-photolithographic) techniques. Drop-on-demand inkjet printing is one such technique that has recently been used to print polymers for light-emitting devices48,49 and transistor circuits.50 Inkjet printing was used in this work to print a PAA solution at pH 3.5 onto a (PAH7.5/PAA3.5)5.5 multilayer (i.e., PAH outermost layer) coated on a glass substrate. After the entire printed sample was immersed in the [Pd(NH3)4]2+ catalyst and the electroless nickel solution, only the PAA pattern became plated as shown in Figure 4. Patterned line widths are approximately 200 µm. The plated lines were not removed by Scotch tape, and they were conductive. To reduce the coalescence of the printed drops and thereby improve the pattern quality (48) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl. Phys. Lett. 1998, 72, 519-521. (49) Bharathan, J.; Yang, Y. Appl. Phys. Lett. 1998, 72, 2660-2662. (50) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123-2126.

Wang et al.

Figure 4. Patterned, conductive nickel lines (width ∼200 µm) electrolessly plated on inkjet-printed PAA on a (PAH7.5/ PAA3.5)5.5 multilayer supported on a glass substrate.

and resolution, the PEM-coated substrate was heated to enhance drop drying after printing. This process, however, was clearly not optimized for high-resolution patterning. Further optimization of the printing conditions (e.g., faster dry times and more precise control over the amount of material printed) should further improve spatial resolution. Nevertheless, the inkjet printing of polyelectrolyte solutions demonstrates a facile method for patterning PEMs. It also shows the effectiveness of the (PAH7.5/ PAA3.5) multilayer platform in selectively binding and resisting the Pd catalyst through the choice of outermost polyelectrolyte. To our knowledge, this is the first time that inkjet printing has been successfully used to pattern PEMs. So far, the focus of this discussion has been on the positively charged catalyst, [Pd(NH3)4]2+, and its interaction with PAA. Alternatively, a negatively charged Pdcomplex salt, Na2PdCl4, could be used to direct plating onto the PAH surface rather than the PAA. With the (PAH7.5/PAA3.5) multilayer platform, excellent selectively and adhesion of the plated nickel film were confined to the PAH surface. Because this Pd-complex salt dissociates in aqueous solution to [PdCl4]2-, it is expected to bind to the ammonium groups of PAH. Others have shown this type of interaction with an acidified (i.e., positively charged) polyethyleneimine block copolymer;51 a Ptcomplex analogue has also been shown to bind with cationic hydrogel/surfactant complexes.52 The main processing difference between the two catalysts is the additional dilute acid wash of the PEM-covered substrate after immersion in the Na2PdCl4 solution. Without this acid wash step, plating selectivity between the PAH and the PAA surfaces is poor (i.e., plating occurs not only on the PAH but also on the PAA surface). This suggests that [PdCl4]2- is binding nonselectively (i.e., nonelectrostatically) to the PAA surface. One hypothesis is that this Pd complex undergoes ligand exchange with ionized carboxylates on the PAA by replacing one or more of its chlorides; carboxylates are known to be good ligands for transition metals.53 Upon reprotonation of the PAA carboxylates in the acid wash, the reaction is reversed and [PdCl4]2- desorbed. Finally, while we have prepared stable PAH/PAA-based multilayers34 at pHs as low as 2.5 and as high as 9.0 and subjected some of them to temperatures as high as 50 °C in an aqueous environment without observing de-adher(51) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Co¨lfen, H.; Antonietti, M. Inorg. Chim. Acta 1998, 280, 348-354. (52) Svergun, D. I.; Shtykova, E. V.; Kozin, M. B.; Volkov, V. V.; Dembo, A. T.; Shtyhova, E. V., Jr.; Bronstein, L. M.; Platonova, O. A.; Yahunin, A. N.; Valetsky, P. M.; Khokhlov, A. R. J. Phys. Chem. B 2000, 104, 5242-5250. (53) Cotton, W. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley: New York, 1988; Chapter 12.

Selective Electroless Nickel Plating

ence from the substrate (results not shown), PEMs based on weak polyelectrolytes are susceptible to destabilization (i.e., desorption of polyelectrolytes) at extreme pH conditions.45 Therefore, the electroless nickel bath formulation used in all our experiments was chosen for its mild temperature and pH requirements. While some common electroless metal baths requiring high alkalinity and temperature may be incompatible with these PEMs, PAH/ PAA-based multilayers can be stabilized to resist extreme pH conditions by chemical cross-linking; PAA carboxylate and PAH ammonium groups undergo a condensation reaction at elevated temperatures to form amide linkages.54 Even without cross-linking, we have also successfully plated copper on PAH/PAA-based multilayers from a moderately basic, hypophosphite-based electroless bath (results not shown). We believe that selective plating on PEM platforms is generalizable to the electroless deposition of other metals. Summary and Conclusions We have shown that PEMs based on the weak polyelectrolytes PAH and PAA are promising platforms for promoting selective electroless nickel plating. By extending our ion-exchange methodology for preparing metal nanoparticles within PAH/PAA-based multilayers29 to PAH/PAA surfaces, a Pd-complex ion can be bound onto the surface to promote electroless plating. The PEM surface can be rendered selective or nonselective toward catalyst binding by choosing the appropriate pH conditions during PEM fabrication and through the use of appropriately short activation times. PEMs fabricated at low pHs, such as 3.5, for both PAH and PAA solutions, have surfaces in which the two polyelectrolytes are sufficiently interpenetrated such that the catalyst binds and nickel plates regardless of the polyelectrolyte last adsorbed. By choosing a pH combination that minimizes interpenetration of PAH and PAA at the multilayer (54) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978-1979.

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surface, such as PAH at pH 7.5 and PAA at pH 3.5, a surface dominated by PAH or PAA can be created. PAAtopped multilayers will selectively bind a positively charged catalyst, [Pd(NH3)4]2+ and promote electroless plating while PAH-topped multilayers remain unplated. Using a negatively charged catalyst, [PdCl4]2-, the PAH and PAA roles are reversed. In addition to uniformly plating large areas of PAA- or PAH-topped multilayers, selective plating of fine features can be accomplished on PEMs using inkjet printing. An inkjet-produced PAA pattern on a PAH-topped multilayer selectively binds the positively charged catalyst and plates nickel. In conclusion, the excellent selectivity of nickel plating for the PAA-rich multilayer surface over the PAH-rich surface when using Pd(NH3)4Cl2, the excellent adhesion of the nickel film to the PEM-coated substrate, and the mild conditions under which the substrate is activated all indicate that PEMs are an effective foundation for selective electroless metal plating. Finally, while this paper has focused on electroless deposition on PEM surfaces, in a forthcoming paper,55 we demonstrate that long ionexchange times and fine control over the electroless deposition kinetics facilitate the controlled electroless deposition of nickel within PAH/PAA-based multilayers, analogous to previous published work in block copolymer systems.56 Acknowledgment. We thank Hartmut Rudmann at MIT and Prof. Yang Yang and Shun-Chi Chang at UCLA for their assistance in modifying the inkjet printer. This work was supported by the MIT MRSEC Program of the National Science Foundation under award No. DMR 9400334 and made use of shared experimental facilities at the MIT Center for Materials Science and Engineering. LA010755Z (55) Wang, T. C.; Rubner, M. F.; Cohen, R. E. To be submitted for publication. (56) Boontongkong, Y.; Cohen, R. E.; Rubner, M. F. Chem. Mater. 2000, 12, 1628-1633.