Novel and Facile Approach to the Fabrication of Metal-Patterned

Chem. Mater. 2007, 19, 2299-2303

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Novel and Facile Approach to the Fabrication of Metal-Patterned Dielectric Substrates Luke M. Davis* and David W. Thompson Department of Chemistry, College of William and Mary, Williamsburg, Virginia 23187 ReceiVed October 4, 2006. ReVised Manuscript ReceiVed December 23, 2006

Both native and organic monolayer protected nanometer-sized silver particles were used to metallize patterns printed on glass and polyimide substrates by commercial permanent ink marking pens. The metal particles were selectively fixed onto the inked patterns by direct physical contact; the particles did not adhere to the free substrate surface. The patterns were made highly conductive by sintering or by mechanical buffing. The procedure allows the fabrication of patterned electrical circuits with minimal effort and a resolution of approximately 0.3 mm.

Introduction The fabrication of printed circuits by metal depositionmask-etch protocols is now a sophisticated science with resolution reaching into the submicrometer range.1,2 However, there is still a need for printed circuitry of modest resolution which can be constructed or repaired with ease.3 In this paper we present a facile technique for the construction of lower-technology, metal-patterned glass or polyimide substrates with a line resolution of approximately 300 µm. The approach of this paper for preparing the patterned, metallized substrates rests on the following line of reasoning. It is well-known that readily available, commercial permanent marking inks4 adhere strongly when written onto a variety of materials including metals, glasses, and polyimides. Indeed, if permanent ink binds strongly to the surfaces of metals and dielectric materials, then permanent ink might be used as an “adhesive” to bind a metal to a dielectric substrate such as glass or polyimide, resulting in a ternary “metal-ink (as the adhesive)-dielectric (glass or polyimide)” layered system. Specifically, in view of the common and substantial “ink-on-metal” adhesion, we thought that it would be possible to invert the process and apply dry nanometer-sized metal particles onto a permanent ink pattern written on a dielectric substrate. That is, we thought it possible to obtain significant “metal-on-ink” adhesion in addition to the much more common strong binding involved with the writing of “ink-on-metal.” It seemed that the best test of this hypothesis would be the use of nanometer-sized metal particles or nanometer-sized monolayer protected metal particles due to their relatively high effective surface areas. Thus, our methodology was to draw a pattern on a glass or * To whom correspondence should be addressed. E-mail: [email protected].

(1) Coombs, C. F. Printed Circuits Handbook, 5th ed.; McGraw-Hill Professional: New York, 2001. (2) Jawitz, M. W. Printed Circuit Board Materials Handbook; McGrawHill Professional: New York, 1997. (3) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822-10823. (4) For example: Sanford Ultra Fine Point Permanent Marker, Staedtler Lumocolor 313 Fine Point, Securline Universal Marker, Pentel Markathon Permanent Marker, etc.

polyimide substrate with a permanent marking ink and then brush, with modest pressure, that is, contact print, metal nanoparticles across the inked substrate. The metal nanoparticles would adhere only to the ink surface and would not bind to the unmasked portions of the “non-adhesive” glass and polyimide substrates. Because it is well-established that metal nanoparticles sinter at lower temperatures, modest thermal treatment of the direct contact printed metal nanoparticles would give a highly electrically conductive path. It has been experimentally and theoretically established by Kovacs and Vincett5,6 (Xerox, Canada) that atomic and nanometer-sized metal particles, with their large surface area to volume ratios and high surface free energies relative to organic materials such as polymers, would not only adhere strongly to polymers but would slowly reach a thermodynamic minimum when embedded in a softened polymer, either heated above the glass transition temperature or solvent softened. Also, metal ion complexes with organic ligands and organic protected metal clusters and organic particles usually obtain a thermodynamic minimum when the particles are partially embedded in the polymer. Thus, knowing that polymers are ubiquitous components of modern inks7 (even though specific formulations are proprietary), the working hypothesis of this paper is that nanometer-sized metal particles adhere to selected inks that contain a polymer fraction. Here we present the results of efforts to prepare silver patterns by physically contacting nanometer-sized silver particles of different origins with lines and patterns drawn with a permanent marker pen ink of the Sanford Corporation (Oak Brook, IL). The metal patterns were characterized by scanning electron microscopy (SEM) and conductivity measurements. (5) Kovacs, G. J.; Vincett, P. S. Thin Solid Films 1984, 111, 65-81. (6) Kovacs, G. J.; Vincett, P. S. J. Colloid Interface Sci. 1982, 90, 335351. (7) Permanent ink formulations are proprietary. However, it is well-known that modern inks of this type contain polymers such as polyacrylates, polyurethanes, polyesters, and copolymers of such. Kunjappu, J. T. Personal communication. Kunjappu, J. T. Essays in Ink Chemistry; Nova Science Publishers, New York, 2001.

10.1021/cm062372y CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007

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Results and Discussion Fabrication of Silvered Patterns. Three distinct samples of silver nanoparticles were utilized to fabricate metallized patterns: (1) A, commercially available and proprietary organic coated particles with particles sizes < 100 nm; (2) B, polydisperse unprotected 10-500 nm (diameter) particles synthesized in dimethylacetamide (DMAc)8,9 with an average size of approximately 120 nm with 58% of the particles sizes less than 100 nm, 73% less than 150 nm, 83% less than 200 nm, and 91% less than 250 nm; and (3) C, dodecylamine protected particles in the 5-10 nm range.10 These three samples adhered well onto the inked patterns but not onto the substrates and were fully efficacious in producing conductive designs. Additional metal particle syntheses that yielded large silver particles and/or aggregates in the 0.5 to several micrometers range did not adhere well to inked patterns; silver nanowires protected with polyvinylpyrrolidone11,12 with diameters of approximately 100 nm and lengths of several tens of micrometers or more (i.e., large anisotropic particles) did not adhere in any efficacious manner. Thus, it seems important to have metallic silver particles with a significant fraction of the particles having diameters of less than 100 nm. That is, it is important to have particles with the high effective surface areas that accompany very small particles to enhance binding to the ink. Recently, Li et al.10 reported a straightforward synthesis of dodecylamine protected silver nanoparticles (sample C in this work). In addition to small particle size, 5-10 nm, these amine protected particles were found to give highly conductive surfaces when applied (from solution followed by slow solvent evaporation) to a substrate and heated in the 120-140 °C range. By far, the class of organic molecules dominating the “metal protected cluster” area are organic thiols,13,14 but we chose to use the less common amine protected cluster approach of Li et al. because thiol protected clusters require much higher temperatures, in the 250-350 °C range, to exfoliate the thiol and leave bare metal.15 Both of the protected silver nanoparticle samples, A and C, produced well-formed metallized surfaces. It is unclear whether this is primarily due to the prevention of aggregation of silver nanoparticles by the organic monolayer or some organic-organic interaction between the organic monolayer and the organic polymer(s) in the ink aiding the adhesion process. Because the DMAc-reduced, non-protected particles (B as described in Experimental Section) strongly adhered to the ink, we suggest prevention of aggregation is the (8) Pastoriza-Santos, I.; Liz-Marzan, L. M. Langmuir 1999, 15, 948951. (9) Pastoriza-Santos, I.; Liz-Marzan, L. M. Tailoring the morphology and assembly of silver nanoparticles formed in DMF. Nanoparticle Assemblies and Superstructures; CRC Press, LLC: Boca Raton, FL, 2006; pp 525-550. (10) Li, Y.; Wu, Y.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 32663267. (11) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736-4745. (12) Chen, D.; Gao, L. J. Cryst. Growth 2004, 264, 216-222. (13) Brust, M.; Kiely, C. J. Colloids Surf., A 2002, 202, 175-186. (14) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42-50. (15) Wuelfing, W. P.; Zamborini, F. P.; Templeton, A. C.; Wen, X.; Yoon, H.; Murray, R. W. Chem. Mater. 2001, 13, 87-95.

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dominant factor in giving metallized surfaces of high quality. Small particle size appears to be a key factor in the adhesion process. To enhance the accuracy and reproducibility of four point probe resistivity measurements, relatively large area silvercoated samples, rather than narrow (0.3 mm) circuit lines, were first prepared. Thus, glass petrographic slides (27 × 46 mm) were covered fully with the adhesive ink from a Sanford Sharpie permanent marker. With the same purpose, the slides were wiped clean with acetone prior to the application of the permanent ink; no further preparation of the substrate was necessary. Two different ink coating methodologies were employed. First, overlapping lines, parallel with the short (27 mm) dimension of the slide, were drawn until the slide was covered. Second, the non-marking end of a Fine Sharpie was cut off, and the fibrous ink-storage reservoir was removed. This was compressed to extract the ink, which was then diluted by adding three volumes of 95% ethanol. The diluted ink was transferred to the slide using a pipet. Approximately 0.5 mL of diluted ink coated the slide. After both application methods, the slide was then allowed to dry at room temperature. Once dry, the silver nanopowder was applied by sprinkling it onto the substrate and gently wiping it across the surface with either the cosmetic puff or the piece of latex glove. Bearing down in this direct contact application was unnecessary; particle adhesion to the inked surface occurs with minimal pressure. Excess silver nanopowder could be collected to minimize waste. The amount of silver applied to a given sample could be varied. The application could stop after a pass or two, leaving the surface only partially metallized and not continuously conductive, or several additional passes with silver could be made, resulting in a surface layer that was fully conductive. Narrow (ca. 0.3 mm) line-patterned films were also made using an Extra Fine or Ultra Fine Sharpie permanent marker. Any pattern could be drawn with the marker on the substrate (glass plate, polyimide film, or epoxy circuit board). This pattern was then silvered by direct contact of the nanopowder across the patterned area with the nanoparticles adhering only to the inked portions of the substrate. Enhancement of Electrical Conductivity. The initial silvered surface was generally conductive after three to four silver nanoparticle passes with the applicator under only modest pressure. The conductivity of the silvered patterns and coatings increases by one to two orders of magnitude either by buffing with a non-abrasive material or by heating the silvered patterns as can be see from Table 1 and Figure 1. With final silver film thicknesses of 1-3 µm after sintering or buffing, the sheet resistivity values of 1 Ω/square or less would be a volume resistivity of 1 × 10-4 to 3 × 10-4 Ω‚ cm. Thus, the silvered patterns would be within a factor of approximately 100 of the volume resistivity of bulk polycrystalline silver. Preliminary work indicates that the silvered surfaces can also be readily electrochemically plated with copper metal to increase further conductivity and current density. The sheet resistivity was examined as a function of sintering temperature for slides prepared with the commercial protected particles, sample A, and prepared with the unpro-

Metal-Patterned Dielectric Substrates Fabrication

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Table 1. Sheet Resistivity as a Function of Post-Fabrication Modification by Heating or by Mechanical Buffing of the Silvered Surface for Various Samplesa sheet resistivity for silver-coated petrographic slides (Ω/square) sample number

as applied, 295 K

373 K

423 K

473 K

523 K

573 K

A1 A2 A3 A4 B1 B2 B3 B4

4 24 19 38 7 3 6 7

4 20 34 not heated 7 2 4 not heated

0.6 7 10 not heated 3 1.5 2 not heated

0.1 1 3 not heated 1 0.60 1 not heated

0.06 1 3 not heated 1 1 1 not heated

0.06 0.3 2 not heated 0.06 0.03 0.2 not heated

after buffing, 295 K not buffed not buffed not buffed 1 not buffed not buffed not buffed 0.8

a A1 through A4 utilized the as-received silver nanopowder from Sigma-Aldrich. B1 through B4 used silver nanoparticles from the second heating in the DMAc reduction. All values are in Ω/square.

Figure 1. Sheet resistivity as a function of temperature for selected thermally cured slides. The samples are the thermally treated slides of Table 1. Legend: A-1 (×), A-2 (+), A-3 (1), B-1 (4), B-2 (b), B-3 (0).

tected particles prepared in DMAc, sample B. Metallized slides were heated to 300 °C in a Blue M forced-air oven with the slides being with drawn at 50 °C intervals. Table 1 and Figure 1 display the results of selected temperature dependence runs. As can be seen clearly in the figure, the general trend is toward increased conductivity with increased sintering temperature. The amine-protected clusters described by Li et al.,10 when applied to an ink-patterned substrate, were initially not conductive, as expected owing to dielectric organic coating and the low conductivities observed via a hopping mechanism for dodecyl chain monolayer protected clusters.16 However, after heating at 100 °C for 6 min, the sheet resistivity increased to 2.0 × 103 Ω/square with the dodecylamine protecting groups being partially lost from the metal. After subsequent heating of the same sample for 3 min at 180 °C, the resistivity dropped to 45 Ω/square. This is in agreement with Li et al.’s findings for spin-casting these nanoparticles as a film onto a silicon wafer and heating them at low temperatures between 120 °C and 140 °C.10 Electron Microscopy for Samples Prepared from Commercial Silver Nanoparticles: A. The forms and interactions of the metal nanoparticles bonded onto the inked patterns under different conditions were characterized by (16) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548.

SEM. First, the as-received silver organic-protected nanoparticles from Sigma-Aldrich (denoted A in Table 1, Figure 1, and Experimental Section) were imaged as displayed in Figure 2,A1. Micrographs of the purchased nanoparticles, A, after direct contact application to the Sanford ink were compared to micrographs of the A nanoparticles prior to application (Figure 2,A2). Little or no deformation of the particles occurred with mechanical application. After vigorous buffing of the direct contact applied nanoparticles, micrographs (Figure 2,A3) demonstrated that the strong mechanical pressure increased aggregation sizes of the silver nanoparticles. Figure 3 displays SEM micrographs which show the evolution of large metal nanoparticles over the course of heating from the ambient temperature to 300 °C. The micrographs clearly show sintering of the silver nanoparticles begins to take place in the 150-200 °C range. By 300 °C there is significant growth in particle size and network structure and conductivity as seen in Table 1. This relatively low-temperature particle size growth is the result of the high surface area to volume ratio coupled with the high surface energy of the silver nanoparticles. Adhesion. The adhesion of the silver, commercial sample A and DMAc sample B silver particles, to inked substrate was fair to excellent, as verified by the inability to brush or wipe off the silver and by the Scotch-tape test. 3M Scotch Tape was applied to the silvered surface, pressed down, and then peeled off. “Zero” adhesion is demonstrated when no metal remains after removal of the tape, that is, when all of the metal comes off with the tape. “Poor” adhesion is demonstrated when a large amount of the silver comes off with the tape, such that the conductivity of the metallized surface is interrupted. “Fair” adhesion is demonstrated when a moderate amount of silver comes off with the tape, such that the metallized surface is visibly disturbed but continuity of conductivity remains. “Good” adhesion is demonstrated when only a superficial amount silver comes off with the tape, such that the metallized surface is not noticeably disturbed. “Excellent” adhesion is demonstrated when no silver comes off. Tape could be applied in the primary direction of brushing (with the grain) or perpendicular to that direction (against the grain). Table 2 displays adhesion data for selected samples. As can be seen, samples heated to 300 °C generally demonstrated higher adhesion than samples which were mechanically buffed to increase their conductivity. The adhesion of

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Figure 2. Micrograph on the left (A1) is the as-received silver nanopowder from Sigma-Aldrich (particles designated A in Table 1 and Figure 1) dropped onto a SEM stub having a copper adhesive. The micrograph on the right (A2) is the same nanopowder, after application to dried Sanford ink without subsequent buffing. The A3 micrograph is of the same sample in A2 except after vigorous subsequent buffing. The scale bar (100 nm) applies to all three images.

the silver after the thermal cure is fair to excellent, as compared to the poor to fair adhesion of the buffed samples. Not displayed in the table is the adhesion of as-fabricated or as-brushed silvered surfaces with no mechanical buff or thermal treatment; their adhesion was poor to fair. The data of Table 2 make it clear that heating the samples increases the adhesion of silver to the ink, in addition to increasing the conductivity. Mechanical buffing appears to affect only the conductivity. Conclusion We have demonstrated that nanometer-sized silver particles, both native and organic protected, can be selectively bound to a nonvolatile ingredient of permanent marker ink on a glass or polyimide surface to form electrically conductive patterns with a resolution of approximately 0.3 mm when mechanically buffed, or better, when heated. The protocol reported herein leaves a surface which is native metal, which is in contrast to silver conductive pastes which are composites of silver embedded in a permanent polymeric phase. Experimental Section Materials. Fine, Extra Fine, and Ultra Fine Sanford Sharpie Permanent Ink Markers were purchased locally. Glass petrographic slides (27 × 46 mm) were obtained from Buehler, Ldt. (Lake Bluff, IL). Commercial Kapton film was a gift of R. L. Kiefer; 3,3′,4,4′-

Benzophenonetetracarboxylic dianhydride (BTDA)/4,4′-Oxydianiline (4,4′-ODA) polyimide film was prepared as reported by Southward et al.17 DMAc, 99.8% anhydrous, was obtained from Sigma-Aldrich (Milwaukee, WI). Silver(I) nitrate was acquired from Fisher Scientific (Pittsburgh, PA). Silver “nanopowder” (