Fabrication of Silver Patterns on Polyimide Films Based on Solid

LETTER pubs.acs.org/Langmuir

Fabrication of Silver Patterns on Polyimide Films Based on Solid-Phase Electrochemical Constructive Lithography Using Ion-Exchangeable Precursor Layers Kensuke Akamatsu,*,† Yurina Fukumoto,† Tomoki Taniyama,† Takaaki Tsuruoka,† Hiroshi Yanagimoto,‡ and Hidemi Nawafune† †

Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojimaminami, Chuo-ku, Kobe 650-0047, Japan ‡ Magnetic Material & Surface Modification Department, Metallic & Inorganic Material Engineering Division, Toyota Motor Corporation, Toyota-cho, Toyota, Aichi 471-8572, Japan

bS Supporting Information ABSTRACT: We report a fully additive-based electrochemical approach to the site-selective deposition of silver on a polyimide substrate. Using a cathode coated with ion-doped precursor polyimide layers, patterns of metal masks used as anodes were successfully reproduced at the cathode
precursor interface through electrochemical and ion-exchange reactions, which resulted in the generation of silver patterns on the polyimide films after subsequent annealing and removal from the substrate. Excellent interfacial adhesion was achieved through metal nanostructures consisting of interconnecting silver nanoparticles at the metal
polymer interface, which are electrochemically grown “in” the precursor layer. This approach is a resist- and etch-free process and thus provides an effective methodology toward lower-cost and high-throughput microfabrication.

’ INTRODUCTION Metallized polymers are very important in the field of microelectronics engineering as flexible electrodes,1
11 and the demand for the development of polymer metallization strategies has been rapidly growing in the field of various electronic applications, such as liquid-crystal displays, electronic paper, and solar cells. Polyimide films have been widely used to date for such applications as low-k substrates because of their chemical and thermal stability and excellent dielectric properties.12 Therefore, various processes for the metallization of polyimide films and the adhesion of metals on polyimide substrates have been investigated for the development of flexible circuit elements in microelectronic applications.13
20 The conventional approach to the fabrication of metal circuits on polyimide substrates utilizes a subtractive-based patterning strategy for prelaminated metal on polyimide films by lithographic methods, but this approach requires stringent environmental control, costly equipment, and complex multistep processes such as resist coating, lithography, and etching. To sustain the demand for generating multichip packaging systems for future electronic devices, it would be exceedingly useful to develop an additive-based strategy with high-throughput capability that would allow the direct site-selective metallization of flexible low-k substrates. In this context, direct metal deposition processes such as the fabrication of metallic patterns on polymer substrates using ion-doped precursor films have been r 2011 American Chemical Society

investigated. A variety of direct deposition techniques have been introduced by several groups, including ours, on the basis of photoinduced chemistry,21
24 thermal treatment,25
27 and selective chemical reduction28
31 for polyimides and other functional polymers. We have also reported a chemical metallization strategy that utilizes soft lithography with postchemical mechanical polishing, which enables the fabrication of metal damascene patterns on the polyimide substrate.32 The use of ion-doped precursors has triggered the development of a new concept in polymer metallization (e.g., metallic thin films can be embossed through the diffusion of metallic ions from the interior of a substrate, which is essentially different from conventional vacuum, electrochemical, and electroless metal deposition processes). By taking advantage of the ion-exchangeable nature of the precursors used for the generation of metal/ polymer heterostructures, we demonstrate in this study how it is possible to merge the direct metallization technique using iondoped precursors with electrochemical lithography to fabricate metal patterns on polyimide substrates. Electrochemical lithography is a powerful strategy that is used to construct micro- and nanopatterns of materials by scanning probe and imprinting methods in a direct-write fashion.33
41 As a proof-of-concept Received: July 5, 2011 Revised: September 2, 2011 Published: September 08, 2011 11761

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Figure 1. (A) Schematic diagram of the proposed electrochemical lithography process using a silver ion-doped precursor layer. (B) Schematic representation of the process for the fabrication of polyimide films with metallic patterns. (C) Photograph of a sample for the electrochemical lithography process. (D) Optical microscope image of square silver patterns deposited on an ITO substrate using a Pt mesh grid as the anode. The image was taken from the anode side where the precursor film remains. (E) SEM image of polyimide film with silver patterns. The film was obtained after electrochemical patterning, the extraction of remaining silver ions via acid treatment, and annealing at 300 °C for 30 min to initiate the imidization of the precursor film, followed by removal of the film from the ITO substrate. The film in the image is bent, which demonstrates the flexibility of the polyimide film with a metallic pattern. (F) Current
voltage (I
V) curve of the deposited silver line with a length of 1.2 mm, a width of 100 μm, and a thickness of 200 nm.

study, we herein present the successful electrochemical deposition of silver patterns at the interface between an electrode and ion-doped precursor layers in a completely additive-based manner. This strategy offers an opportunity to translate solid-phase electrochemical lithography into a high-throughput, cost-effective process for the fabrication of metal circuits on polymer substrates.

’ EXPERIMENTAL SECTION The proposed process is schematically presented in Figure 1. At the core of this process is the use of a poly(amic acid) thin layer as a precursor of polyimide that acts as a solid electrolyte for silver ions. Pyromellitic dianhydride (PMDA) and a 4,40 -oxidianiline (ODA)-type poly(amic acid) film was spin coated onto an indium tin oxide (ITO)/ glass substrate, followed by heat treatment in vacuum at 60 °C for 5 h to obtain an ion-exchangeable precursor film with a thickness of 4.0 μm. The silver ions were doped though an ion-exchange reaction by immersing the precursor films on the ITO substrate into a 200 mM aqueous silver nitrate solution for 10 h. The number of silver ions in the precursor films was quantified using inductively coupled plasma (ICP) spectroscopy (SPS7700, Seiko Instruments). For the electrochemical lithography process, a small Pt mesh grid was brought into contact with the precursor films to be patterned. Before the grid was placed, a few drops of distilled water were used to coat the film surface via pipet, which accelerates the electrochemical reaction. The deposition of metals was conducted using an electrochemical probe system (HSV-100, Hokuto Denko) with gold probes as electrodes (Figure 1C). The process was performed in chronoamperometry mode in which the potential between the anode and cathode was kept constant and the current was monitored.

The cross-sectional microstructure of the deposited silver films was observed by transmission electron microscopy (TEM; JEM-1400, Jeol, operated at 120 kV). Samples for cross-sectional TEM observation were prepared by embedding the films removed from the ITO substrates into epoxy resin, followed by curing and sectioning into approximately 100-nm-thick slices using a microtome (Leica, Ultracut R).

’ RESULTS AND DISCUSSION PMDA-ODA-type poly(amic acid) was used as a polymer electrolyte, which bears carboxylic acid groups that can act as ionexchangeable sites for silver ions. After the ion-exchange reaction using silver nitrate solution, the quantity of silver ions doped into the precursors was found to be ca. 3.4 μmol cm
2 by ICP spectroscopy, which corresponds to 8.6 mol L
1 calculated using the thickness of the precursor layer. This treatment provides a highly concentrated solid electrolyte that was subjected to the following electrochemical deposition experiments. Upon application of an electrical bias to the ITO substrate as a cathode and a Pt grid as an anode, the electrochemical reaction takes place at the cathode
precursor interface underneath the anode and at the contact points of the anode
precursor interface. In a typical experiment, patterns were deposited with a driving potential of 2.0 V for 2.0 s on a 4-μm-thick poly(amic acid) layer. At the cathode
precursor interface, an appreciable potential drop causes the reduction of silver ions on the cathode surface. The formation of silver patterns on the ITO surface was clearly observed, as shown by the optical microscope images in Figure 1D. These images were taken from the anode side where the precursor film remains and demonstrate the successful deposition of silver patterns, the features of which reflect those 11762

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Figure 2. Current
time curve of the electrochemical reaction for the fabrication of silver patterns (at 2.0 V for 2.0 s).

of the Pt grid used as the anode. No electrochemical reaction occurred without water dropped on the film, irrespective of the electrical bias applied to the electrode. This suggests that the reduction of silver ions and the oxidation of water occur at the cathode and anode, respectively, in this solid-state electrochemical reaction system (Figure 1A). The carboxylate anions formed at the cathode
precursor interface can act as further ion-exchange sites that induce the diffusion of silver ions in the precursor layer and their further reduction to form silver nanoparticles that grow and become a thin film as the reaction continues. The oxidation of water that occurs at the anode surface generates protons, which then bind to carboxylate anions to form carboxylic acid groups. The current
time curve in Figure 2 shows a constant current during the electrochemical reaction for 2.0 s and demonstrates that the diffusion of ions is faster than the reduction of silver ions so that the ion-exchange reaction could reach equilibrium during the electrochemical reaction. The average current efficiency for the reduction of silver ions to metallic silver was estimated to be ca. 95.7%, which was calculated using the amount of electricity measured from the electrochemical reaction and the amount of silver metal deposited.42 The feature size is dependent on the thickness of the precursor layer (i.e., the distance between the electrodes), which is related to the diffusion length of the ions in the precursor layer. Figure 3 shows optical microscope images of Pt grid patterns used as the anode and deposited silver patterns using precursors with different thicknesses. The pattern features obtained using 4-μm-thick precursors are almost the same as those of the Pt grid. However, the width of the patterns deposited from 10-μm-thick precursors was slightly larger than that of the Pt grid. These results demonstrate that the use of thinner precursors yields higher-resolution patterns. Figure 4 shows the effect of the applied voltage and reaction time on the features of deposited silver patterns. The width of deposited silver patterns is found to increase slightly as the applied voltage increases, whereas the width remains unchanged with the reaction time (although the amount of deposited silver increases). In addition, the pattern features are not dependent on the initial silver concentration as shown in Figure 5. From these results, we suggest the following reduction process mechanism: the reduction of silver ions preferentially takes place on the ITO surface (cathode) underneath the Pt grid (anode) because the lowest resistivity of the ion-doped precursor layer between two electrodes is achieved by the closest distance for our present configuration, and the current passes perpendicularly through the equipotential plane between the electrodes. The increase in precursor thickness causes a spread in the equipotential plane across the cathode surface, leading to an increase in the width of

Figure 3. Optical microscope images of (A) the Pt mesh grid used as the anode and silver patterns deposited from (B) 4-μm-thick and (C) 10-μm-thick precursor layers. Note that the thinner precursor layer results in finer features. The lower micrographs show enlarged images. The scale bar is 200 μm.

the deposited silver patterns (Figure 3). In addition, because the increase in the applied voltage increase the opportunity for the silver ions to be reduced at a position on the cathode that is far from the position just underneath the edge of the anode Pt grid, the width of the silver lines increases with increasing applied voltage. The results that the width of the silver lines does not depend on the reaction time and concentration of initial silver ions (both experiments were carried out at constant voltage and precursor thickness) may support this consideration. However, further detailed experimental study is needed and is currently underway to elucidate the reaction mechanism fully. In the current unoptimized system, the line width could be controlled from 20 μm to many millimeters, depending on the features of metal electrodes used as anodes.43 Flexible polyimide films with silver patterns were prepared by extracting remaining (unreduced) silver ions, followed by annealing in an inert atmosphere and the subsequent removal of the films from the ITO substrates (Figure 1B). The samples obtained after electrochemical patterning were immersed in a 10% aqueous solution of acetic acid for 30 min. The resulting samples were annealed at 130 °C for 30 min and then at 300 °C for 30 min in a nitrogen atmosphere. As a result of the dehydration reaction during annealing, the poly(amic acid) precursor was converted into a polyimide film, as confirmed by the appearance of carbonyl stretching vibrational modes for imide rings in the infrared spectrum (results not shown). After subsequent immersion of the annealed samples into a dilute aqueous hydrogen fluoride solution for 15 min, the films were removed from the ITO substrate, successfully providing free-standing polyimide films with silver patterns on their surfaces. The SEM images in Figure 1E show that the square patterns were maintained upon annealing and removal from the substrate. The pattern on a polyimide film with a bent structure is shown, which demonstrates the flexibility of the sample. Figure 1F shows the I
V curve of the deposited silver film with a length of 1.2 mm, a width of 100 μm, and a thickness of 200 nm in which the linear dependence of the current on the applied voltage is observed. The resistivity was estimated to be ca. 2.7 μΩ cm for the film, the value of which is slightly larger than that of bulk silver (1.6 μΩ cm) but suitable for various electronic applications. 11763

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Figure 4. (A
C) Optical microscope images of silver patterns deposited for 2 s at (A) 1.0, (B) 3.0, and (C) 4.0 V. (D
F) Optical microscope images of silver patterns deposited at 2.0 V for (D) 1.0, (E) 1.5, and (F) 5.0 s. The lower micrographs show enlarged images of the upper micrographs. All data was obtained for samples with 4-μm-thick precursor layers. The scale bar is 200 μm.

Figure 5. Optical microscope images of silver patterns deposited at 2.0 V for 2 s using samples with initial silver ion concentrations of (A) 0.78, (B) 1.8, and (C) 2.5 μmol cm
2, obtained after immersing precursor films into an aqueous silver nitrate solution for 5, 30, and 60 min, respectively. The scale bar is 200 μm.

These results demonstrate that the electrochemical lithography process enables the metallization of ion-doped precursor films in the fabrication of flexible polyimide dielectrics with metal circuit overcoatings. One of the most important issues concerning metal/polymer heterojunctions is ensuring adhesion between the metal and underlying polymer substrate.13,14,19,20 In the present study, the deposited silver films readily passed the Scotch tape test, which indicates the excellent adhesion of the electrochemically fabricated silver/polyimide junction. To confirm the reason for the adhesion of this heterointerface, cross-sectional TEM samples were observed (Figure 6). A silver layer with a thickness of ca. 200 nm was observed only on the cathode side, and no isolated silver species (nanoparticles or clusters) were evident within the 4-μm-thick polyimide layer.44 The enlarged TEM image (Figure 6B) shows the dense granular microstructure of the deposited silver film, which has a relatively smooth surface and a composite interlayer consisting of small metal nanoparticles and polyimide. This granular structure was also observed by planview SEM of the silver film obtained after dissolving unreduced silver ions via acid treatment and removal of the poly(amic acid) layer by washing the film with an aqueous sodium carbonate solution (Figure S3). The deposited film consisted of silver nanoparticles in the range of 5
20 nm, which is consistent with that observed from cross-sectional TEM (Figure 6B). These

Figure 6. (A) Cross-sectional TEM image of a polyimide film with a deposited silver layer. The film was obtained using the same procedure as that for the sample shown in Figure 3. (B) Enlarged image of the deposited silver layer shown in A.

nanostructures could play a critical role in achieving good adhesion because of the increased contact area (and thus work of adhesion) between silver nanoparticles and polyimide. We previously reported that granular structures consisting of a metalrich composite interlayer deposited at the interface provide good adhesion, which can be produced by a light-induced reduction22 or the chemical reduction of doped metal ions.45 However, these photo- and chemical reduction processes also resulted in isolated metal nanoparticles inside the polymer substrate as a result of the penetration of the incident light deeper into the polymer and the diffusion of the reducing reagent into the polymer via swelling, which could not contribute to the interfacial adhesion of the heterojunction. In the present case, however, the electrochemical reduction of silver ions can take place only at the interface of the metal (initially the ITO surface) and precursor layer so that metal thin films can be grown as continuous films in which each of the grains forms a continuous electrical connection. The nanogranular structure thus formed by the present electrochemical reaction is therefore much more desirable and reliable in terms of the effective adhesion of the heterointerface than those structures previously developed using photochemical and chemical reduction schemes.

’ CONCLUSIONS Electrochemical lithography combined with a direct metallization process using ion-doped precursors was successfully 11764

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Langmuir demonstrated. The basis for this process is the local electrochemical reduction of doped metallic ions in a polymer matrix. The reduction of silver ions at the cathode
precursor interface induces the formation of silver thin films with granular microstructures that leads to effective adhesion between the silver and polyimide films. This strategy is extendable to other metals46 and offers some unique advantages that will provide a resist-free, additive-based patterning methodology for fabricating flexible circuit elements, which is otherwise difficult using current lithography-based subtractive methods. We anticipate that the continuous development of this direct electrochemical patterning approach will facilitate studies on solid-phase electrochemical deposition processes using ion-doped precursors and will contribute to the development of new electrochemical lithography processes as a general tool for the fabrication of flexible printed circuit boards.

’ ASSOCIATED CONTENT

bS

Supporting Information. Optical, TEM and SEM images of deposited metal layer obtained after electrochemical deposition in different experimental conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a grant-in-aid for scientific research (23686107) to K.A. from MEXT, Japan. ’ REFERENCES (1) Loo, Y. L.; Willet, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654–7655. (2) McAlpine, M. C.; Friedman, R. S.; Lieber, C. M. Nano Lett. 2003, 3, 443–445. (3) Boncheva, M.; Ferrigno, R. D.; Bruzewicz, A.; Whitesides, G. M. Angew. Chem., Int. Ed. 2003, 42, 3368–3378. (4) Nicolas-Debarnot, D.; Pascu, M.; Vasile, C.; Poncin-Epaillard, F. Surf. Coat. Technol. 2006, 200, 4257–4265. (5) Charbonnier, M.; Romand, M.; Goepfert, Y.; Leonard, D.; Bouadi, M. Surf. Coat. Technol. 2006, 200, 5478–5486. (6) Greco, P.; Cavallini, M.; Stoliar, P.; Quiroga, S. D.; Dutta, S.; Zacchini, S.; Carmela lapalucci, M.; Morandi, V.; Milita, S.; Merli, P. G.; Biscarini, F. J. Am. Chem. Soc. 2008, 130, 1177–1182. (7) Sugiyama, T.; Iimori, Y.; Baba, K.; Watanabe, M.; Honma, H. J. Electrochem. Soc. 2009, 156, D360–D363. (8) Ravagnan, L.; Divitini, G.; Rebasti, S.; Marelli, M.; Piseri, P.; Milani, P. J. Phys. D: Appl. Phys. 2009, 42, 082002. (9) Inagaki, N. Polym. Int. 2009, 58, 585–593. (10) Serban, D. A.; Greco, P.; Melinte, S.; Vlad, A.; Dutu, C. A.; Zacchini, S.; Carmela lapalucci, M.; Biscarini, F.; Cavallini, M. Small 2009, 5, 1117–1122. (11) Higashitani, K.; McNamee, C. E.; Nakayama, M. Langmuir 2011, 27, 2080–2083. (12) Polyimides: Fundamental Aspects and Technological Applications; Ghosh, M. K., Mittal, L., Eds.; Marcel Dekker: New York, 1996. (13) Strunskus, T.; Grunze, M.; Kochendoerfer, G.; W€oll, C. Langmuir 1996, 12, 2712–2725. (14) Faupel, F.; Willecke, R.; Thran, A. Mater. Sci. Eng. 1998, R22, 1–55.

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(15) Ranucci, E.; Sandgren, S.; Andronova, N.; Albertsson, A. C. J. Appl. Polym. Sci. 2001, 82, 1971–1985. (16) Murdey, R.; Stuckless, J. T. J. Am. Chem. Soc. 2003, 125, 3995–3998. (17) Southward, R. E.; Thompson, D. W. Chem. Mater. 2004, 16, 1277–1284. (18) Davis, L. M.; Thompson, D. S.; Dean, C. J.; Pevzner, M.; Scott, J. L.; Broadwater, S. T.; Thompson, D. W.; Southward, R. E. J. Appl. Polym. Sci. 2007, 103, 2409–2418. (19) Park, S. C.; Yoon, S. S.; Nam, J. D. Thin Solid Films 2008, 516, 3028–3035. (20) Park, J. B.; Oh, J. S.; Gil, E. L.; Kyoung, S. J.; Lim, J. T.; Yeom, G. Y. J. Electrochem. Soc. 2010, 157, D614–D619. (21) Ikeda, S.; Akamatsu, K.; Nawafune, H. J. Mater. Chem. 2001, 11, 2919–2921. (22) Akamatsu, K.; Ikeda, S.; Nawafune, H. Langmuir 2003, 19, 10366–10371. (23) Korchev, A. S.; Bozack, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10–11. (24) Yin, D.; Horiuchi, S. Langmuir 2005, 21, 9532–9358. (25) Qi, S.; Wu, D.; Bai, Z.; Wu, Z.; Yang, W.; Jin, R. Macromol. Rapid Commun. 2006, 27, 372–376. (26) Qi, S.; Wu., Z.; Wu, D.; Wang, W.; Jin, R. Langmuir 2007, 23, 4878–4885. (27) Qi, S.; Wu, Z.; Wu, D.; Wang, W.; Jin, R. Chem. Mater. 2007, 19, 393–401. (28) Wang, T. C.; Chen, B.; Rubner, M. F.; Cohen, R. E. Langmuir 2001, 17, 6610–6615. (29) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822–10823. (30) Shibata, M.; Kuribayashi, M.; Uda, T. J. Appl. Polym. Sci. 2004, 94, 2164–2169. (31) Kuribayashi, M.; Narita, Y.; Shibata, M. J. Appl. Polym. Sci. 2006, 99, 1627–1632. (32) Matsumura, Y.; Enomoto, Y.; Tsuruoka, T.; Akamatsu, K.; Nawafune, H. Langmuir 2010, 26, 12448–12454. (33) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097– 1099. (34) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 424–429. (35) Hoeppener, S.; Maoz, R.; Sagiv, J. Nano Lett. 2003, 3, 761–767. (36) Lee, M.; O’Hayre, R.; Prinz, F. B. Appl. Phys. Lett. 2004, 85, 3552–3554. (37) Hsu, K. H.; Schultz, P. L.; Ferreira, P. M.; Fang, N. X. Nano Lett. 2007, 7, 446–451. (38) Lee, S. H.; Ishizaki, T.; Saito, N.; Takai, O. Surf. Sci. 2007, 601, 4206–4211. (39) Simeone, F. C.; Albonetti, C.; Cavallini, M. J. Phys Chem. C 2009, 113, 18987–18994. (40) Deiss, F.; Combellas, C.; Fretigny, C.; Sojic, N.; Kanoufi, F. Anal. Chem. 2010, 82, 5169–5175. (41) Cavallini, M.; Simeone, F. C.; Borgatti, F.; Albonetti, C.; Morandi, V.; Sangregorio, C.; Innocenti, C.; Pineider, F.; Annese, E.; Panaccione, G.; Pasquali, L. Nanoscale 2010, 2, 2069–2072. (42) The amount of electricity was measured from the integrated area in the current
time curve obtained after electrochemical reaction at 2.0 V for 10 s (4.2  10
2 C), and the amount of silver deposited (i.e., reduced silver ions) was measured by ICP analysis (4.1  10
8 mol) for the sample after electrochemical deposition, the extraction of remaining ions by acid treatment, and the removal of the poly(amic acid) layer from the deposited silver film using sodium carbonate solution. (43) The current limit of the pattern width is 20 μm (Supporting Information, Figure S1) and indeed should be improved by using the higher-resolution metal patterns as the anode, thinner precursors, and a lower applied voltage. (44) We also conducted TEM observation for a sample obtained after ion extraction and no heat treatment (Figure S2A) and for a sample 11765

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obtained after heat treatment and no ion extraction (Figure S2B). No silver nanoparticles were observed under the deposited silver film for the sample without heat treatment, although many small silver nanoparticles were observed for the sample without ion extraction treatment, demonstrating the complete extraction of the remaining silver ions after treatment using a 10% aqueous solution of acetic acid for 30 min (Supporting Information). (45) Ikeda, S.; Yanagimoto, H.; Akamatsu, K.; Nawafune, H. Adv. Funct. Mater. 2007, 17, 889–897. (46) We have extended our process to the deposition of copper and nickel thin films using aqueous copper sulfate and nickel sulfate solutions. Although the quantity of doped divalent ions is half that of monovalent silver ions, these ions were successfully reduced by electrochemical lithography to form metallic thin films and patterns on polyimide substrates (Supporting Information, Figure S4).

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