Direct-Write Patterning Palladium Colloids as a Catalyst for Electroless

Langmuir 2009, 25, 1785-1789

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Direct-Write Patterning Palladium Colloids as a Catalyst for Electroless Metallization for Microwave Composites Dan Zabetakis,* Peter Loschialpo, Doug Smith, Michael A. Dinderman, and Walter J. Dressick U.S. NaVal Research Laboratory, Washington, D.C. 20735 ReceiVed October 10, 2008. ReVised Manuscript ReceiVed NoVember 21, 2008 Patterning of metal colloids by inkjet printing on paper is demonstrated as a precursor to electroless metallization. The development of the metal pattern is followed in terms of the conductivity and mass of the metal deposited and is shown to have critical phase behavior. The utility of this technique for large-area microscale patterning is demonstrated. Sample patterns of frequency-selective surface designs were manufactured and shown to conform to computationally modeled expectations in the microwave regime.

Introduction Inkjet printing of metal colloids as a precursor for electroless metallization is a little-used but promising technology for the fabrication of patterned metal films. Leveraging the commercial development of low-cost, high-resolution printing technology, direct-write by inkjet can enable large-scale, reliable reproduction of patterns on many substrates for numerous applications.1 Shah et al.2 used inkjet printing on plastic transparency sheets to pattern platinum colloids as a catalyst for electroless copper metallization using a commercial Hewlett-Packard 51626A printer. Guo et al.3 employed inkjet printing with an Epson Stylus Color 670 printer of layered polyelectrolytes followed by specific binding of palladium ion as a catalyst for electroless nickel deposition. Recently, Busato and co-workers demonstrated the printing of palladium on a polyimide film for electroless copper plating using a Hewlett-Packard Deskjet 1220C printer.4 A main attraction of employing existing commercial inkjet technology is the ease with which patterns on the scale of 100 µm can be applied to relatively large areas. Large-format commercial printers commonly range up to 4 ft in print width, and industrial, high-throughput printers can accommodate textiles in 96 in. formats or greater. One area where such technology can find application is in the field of microwave composites. As an example, we manufacture a set of frequency-selective surfaces (FSSs). An FSS is a member of a class of structures that selectively influence propagating electromagnetic waves as they pass through. In general, an FSS will allow the passage of some frequencies but reject (reflect) others. They can be designed to exhibit a variety of frequencydependent characteristics, including low-pass, high-pass, bandstop, and band-pass filters. This is generally achieved by the use of conductive patterns on a nonconductive substrate. Different performance characteristics are realized by varying the shape and size of the patterns.5 Additionally, this technique has been used to test and validate a model for the interaction of microwaves with randomly oriented fibers on a plane.16 * To whom correspondence should be addressed. (1) Calvert, P. Chem. Mater. 2001, 13, 3299–3305. (2) Shah, P.; Kevrekidis, Y.; Benziger, J. Langmuir 1999, 15, 1584–1587. (3) Guo, T.-F.; Chang, S.-C.; Pyo, S.; Yang, Y. Langmuir 2002, 18, 8142– 8147. (4) Busato, S.; Belloli, A.; Ermanni, P. Sens. Actuators, B 2007, 123, 840–846. (5) Wu, T. K. Frequency SelectiVe Surface and Grid Array; John Wiley and Sons: New York, 1995.

10.1021/la803356y

In this paper we demonstrate the use of inkjet printing to pattern a commercial palladium-tin catalyst for electroless copper metallization on paper. We show that such a technique can produce low-resistance copper tracings and can be used to manufacture functional large-scale structures for use in microwave technology.

Experimental Section Direct-Write Printing. Print cartridges were prepared using an HP97 tricolor cartridge. The ink was removed through a needle, and copious washing with water was carried out using a peristaltic pump. Following the wash, the cartridge was emptied of as much water as possible to avoid dilution of the catalyst. A commercial Pd-Sn electroless catalyst, Cataposit 44 (Rohm & Haas), used as received, was pumped into each ink chamber until full. The cartridge was placed in a Hewlett-Packard Deskjet 9800 printer and tested for catalyst flow for each color. Patterns were designed using CorelDraw and printed on bright white uncoated bond paper (13 in. × 19 in., 24 lb, Graytex Papers). Samples were stored in air at room temperature before plating. Electroless Copper Metallization. Paper sheets were first immersed in water to fully and evenly hydrate the fibers. Plating of the patterns was carried out using Fidelity 1025 electroless copper at 30-40 °C for the time specified in the text. Large samples were supported by being clamped in an acrylic frame during the plating process. When complete, the samples were soaked in water (three changes) to remove the residual electroless bath, followed by drying in a model 583 gel dryer (Bio-Rad) to achieve flatness. Conductivity Measurements. For measurements of resistance up to about 100 MΩ, an Agilent 34401A multimeter was used. For low-level measurements, voltage was supplied by an Agilent E3644A dc power supply, while current and voltage across the sample were measured by a Keithley 6512 electrometer and a Keithley 617 electrometer, respectively. Copper Determination. The amount of metal deposited was analyzed by colorimetric EDTA titration.6 Individual plated lines of 1.1 mm by 15.5 cm were cut into sections and placed in 1-2 mL of 1 M nitric acid. The acid was heated until the copper was removed from the paper. The solution was buffered with 2-(N-morpholino)ethanesulfonic acid/NaOH (MES) to about pH 6.6 and titrated with a standard ethylenediaminetetraacetic acid (EDTA) solution. 1-(2Pyridylazo)-2-naphthol (PAN) was used as the indicator. Frequency-Selective Surface Design. FSSs can be designed to exhibit a variety of radio frequency characteristics. Typical patterns consist of an array of conductive unit cells that are repeated on a (6) Harris, D. C. QuantitatiVe Chemical Analysis, 6th ed.; W. H. Freeman and Co.: New York, 2003; Chapter 13.

This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 01/05/2009

1786 Langmuir, Vol. 25, No. 3, 2009 two-dimensional planar surface. Alternately, an FSS may consist of an array of slots or gaps in an otherwise conductive surface. Two fundamental FSS patterns are shown in Figure 4, consisting of an array of isolated conducting squares (low-pass) and a continuous array of conducting stripes (high-pass). The expected frequency response of FSS structures is computationally determined by the method-of-moments (MoM), which provides an exact numerical solution of Maxwell’s equations for a given geometry.7,8 This allows us to predict the interactions between a specific conductive pattern and an incident electromagnetic radiation source from basic principles.9 Each unit cell of the pattern for the MoM calculations is tessellated from an array of subwavelength triangles. The unit cells repeat out to infinity in both orthogonal directions of the planar FSS surface. A 0.01 cm thick slab having an electrical permittivity is included to model the effect of the cellulose backing. Current flow across each of the triangular edges is computed to determine the FSS response to an incident wave. The conductive pattern is taken to be infinitesimal and is characterized by a frequency-independent (dc) surface resistivity that is determined from our measurements for each sample. This assumption of a frequency-independent surface resistivity is strictly valid for uniform conducting layers. Owing to the rough texture of the cellulose and the consequent uneven distribution of the conductive coating, the metallized surface has a small inductive reactive component that is frequency-dependent. Although negligible for most of the samples used in our comparisons, the 5 dB discrepancy in sample k, shown in Figure 6, is attributed to the reactive component of the metallized surface. Transmission Measurements. Analytically, the performance of an FSS is treated for a plane wave at normal incidence. As this is not practical in the laboratory, a focused lens system is used to provide a spatially limited beam that limits diffraction from sample edges and boosts the dynamic range via this concentration of energy.10,11 The lens system consists of two horn-lens combinations, one used to transmit and the other to receive energy. The 2 ft diameter lenses are composed of Rexolite and are configured to produce a converging beam. The width of this beam is largely controlled by the beam parameters created by the broad-band horns as well as the lens aperture-horn separation. The sample is placed at the center of this waist. To be consistent with the assumption of a plane wave excitation of the FSS, the “beam” must have a flat phase profile over its width and have a width significantly larger than the unit cell size of the FSS. For the particular lens-horn combination used these parameters are met for amplitudes within 3 dB of the peak amplitude. For example, at 10 GHz the spot size is 5 in. Given the characteristic unit cell size of 0.5 in., the dimension of the width is large enough to properly characterize the FSS. The frequency range used in this study extends from 2 GHz, with a spot size in excess of 12 in., to 19 GHz, with a spot size of approximately 4 in. The amplitude and phase of the transmitted and reflected energy were measured using an Agilent 8510C network analyzer. Results are presented as transmission loss in negative decibels (where a value of -10 dB represents 10% transmission, -20 dB equals 1% transmission, and so on).

Results and Discussion A direct-write device for metal colloids was achieved by replacing the ink from a commercial inkjet cartridge with a commercial Pd-Sn electroless catalyst, which binds strongly to cellulose fibers.13,14 Patterns were designed by computer and printed on plain paper. The paper was then subjected to electroless (7) Mittra, R.; Chan, C. H.; Cwik, T. Proc. IEEE 1988, 76, 1593–1615. (8) Harrington, R. F. Field Computation by Moment Methods; Wiley-IEEE Press: New York, 1993. (9) Smith, D. L.; Medgyesi-Mitschang, L. N.; Forester, D. W. PIERS 2005, 51, 27–48. (10) Musil J.; Zacek F. MicrowaVe Measurements of Complex PermittiVity by Free Space Methods and Their Applications; Elsevier: New York, 1986; pp 44-60. (11) Goldsmith, P. F. In Infrared and Millimeter WaVes; Button, K. J., Ed.; Academic Press: New York, 1982; Vol. 6, Chapter 5.

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Figure 1. Examples of large-scale, high-resolution patterning of conductive copper tracings. (A) Full-size samples are shown fixed in a 7.5 in. aperture for measurement. Overall, the printed sheet is approximately 11.5 by 11.5 in. The picture shows a sample of gapped hexagons, while on the right is an example of a Hilbert pattern, which is composed of a single continuous line (overlapped at the edges by the aperture). (B) Electron micrograph detail of a sample composed of splitring resonators. The rings are about 4.5 mm in diameter. The bar is 100 µm. The bright copper metallization can be seen overlying the network of cellulose fibers that comprise the paper.

copper metallization, washed, and dried flat. Examples of patterns produced on a surface of about 1 ft2 are shown in Figure 1A, demonstrating the ease with which complex patterns can be made by this method. Details of the plated surface can be seen in Figure 1B, which shows an electron micrograph of a sheet patterned with split-ring resonators. The sharpness of edges in the pattern is not high, but is matched by the roughness of the paper substrate, which may be limiting for this application. The cellulose fibers may also be slightly mobilized in the aqueous plating bath and washing steps and reach a different final postion after drying. Although for some applications a microscopically smooth and sharp feature is essential, for many others, such as microwave technology, it is not as important. To assess the development of the metal plating, a series of experiments were done. Sets of Pd-Sn lines, 1.1 mm by 15.5 cm, were patterned onto paper and metallized at a temperature of 33 °C. Samples were plated for specific times between 1 and 11 min, removed, washed, and dried, after which the electrical resistance of each line was measured. The results are plotted in Figure 2. The data show critical phase behavior, where the resistance does not change very much for an extended period of time, followed by a rapid reduction once a critical threshold is reached. Resistance of the samples dropped 9 orders of magnitude between the 10th and 11th minute of plating. This implies that there would be great difficulty in producing tracings of a specific intermediate resistance with reproducibility. However, in the general case most electromagnetic technology, such as the FSSs demonstrated here, would require highly conductive lines rather than an intermediate value.

Direct-Write Patterning Palladium Colloids

Figure 2. Critical phase behavior of the development of copper metal on printed lines. Groups of lines were printed with Pd-Sn colloid onto paper, followed by electroless metallization. A photograph of one sample is shown as an inset where the copper metal appears dark against the white paper background. Resistance of the lines was measured as a function of the plating time. Error bars are shown and represent the standard deviation of the resistance among the five individual lines from each sample.

Such behavior is not unexpected, given the mechanism for electroless metal deposition.12 In general, each Pd-Sn colloid particle acts as a catalytic center for initiation of Cu deposition on the catalyzed surface. Growth of Cu metal occurs in an isotropic fashion from each particle until the Cu completely covers the Pd/Sn particle. Isotropic Cu deposition then continues from the Cu surface in an autocatalytic fashion until Cu metal fronts from adjacent particles coincide and merge on the substrate surface. Thereafter, lateral Cu growth is impeded and Cu deposition continues in an anisotropic manner perpendicular to the substrate surface, increasing the effective thickness of the Cu plate. In terms of the electrical conductivity of the electroless Cu film as a function of the plating time, this model requires very low conductivity (on the order of 1010 Ω for our paper-based materials) for the Pd-Sn-catalyzed substrate prior to and during the early stages of Cu deposition. During these times (11 min), all of the isolated Cu particles have grown sufficiently large to merge with neighboring particles, establishing a coalesced metal network required for good electrical conductivity. Continued immersion of the Cu bath leads to anisotropic growth of Cu perpendicular to the surface of the substrate, increasing the Cu film thickness and thereby decreasing its resistance further until the film eventually exhibits the conductivity expected for a bulk deposit of electroless Cu. In the intermediate plating time interval (i.e., 10-11 min), the interconnecting networks of particles rapidly form. Critical phase behavior suggests that a printed line can be viewed as a group of elements connected in series. Individually, these elements will undergo critical behavior, so the distribution of resistance of sections in an intermediately plated sample will be expected to reflect this principle. Therefore, as a validation of the critical behavior, the plated lines from the 9 min sample in Figure 2 (40 ( 2 GΩ resistance overall) were cut into 1 cm sections and measured independently. Measurements were not corrected for contact resistance and were limited to those samples having a resistance of less than about 120 MΩ. The data are shown in Figure 3, sorted low-to-high so that the critical behavior may be observed. A clear discontinuity in the distribution is (12) Ohno, I. Mater. Sci Eng. 1991, A146, 33–49.

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Figure 3. Variation of resistance in a sample plated for 9 min. The lines from the 9 min sample in Figure 2 were cut into 1 cm sections, and resistance was measured individually. Samples with resistance above about 120 MΩ were not determined. Samples were sorted low-to-high on the basis of the measured resistance to show the distribution.

observed in the range of 104 to 107 Ω resistance. Elements with resistance below about 10 000 Ω (down to about 1 Ω, including contact resistance) are those that have passed the critical threshold of metal deposition. The overall conductivity of the line is dominated by those elements that have not yet reached the threshold and have resistances of >10 MΩ/cm. The amount and thickness of Cu metal deposited for each line are critically important in determining the electrical resistance of the line for plating times after which metal network coalescence has occurred. Table 1 shows the data from an analysis of the rate and mass of metal deposition. Sets of lines were printed as above and metallized for 11 and 15 min at 35 °C. The copper was then removed from the paper by dissolution in nitric acid and determined by EDTA titration with PAN as the colorimetric end-point indicator.6 Of particular note is the fact that over the times studied the reaction rate increases. Since electroless plating is an autocatylic process, this is expected and indicates that the plating baths are not nearing exhaustion. This is due to the relatively low surface area of these printed samples, as compared to the plating of dispersed cellulose fibers which exhaust the bath readily.13 Thickness was determined assuming a uniform flat coating of the determined mass over the patterned area. The resistivity implied for the 15 min sample by the measured resistance (9.0 ( 0.6 Ω) and presumptive thickness of 1.5 µm is 9.9 × 10-8 Ω m, which is about 6 times higher than the literature value for copper (1.7 × 10-8 Ω m).15 The difference is most likely due to the roughness of the substrate as compared to the thickness of the metal. The 11 min sample was taken near the sharp drop in conductivity as seen in Figure 2 and did not yield precise resistance values (two of the five lines had overall resistance >120 MΩ), so a resistivity calculation was not attempted. Although substrate roughness prevents a direct measurement of the coating thickness, it provides a definite advantage for Cu adhesion. Growth of electroless Cu between cellulose fibers of the paper mechanically interlocks the metal deposit to the substrate. As a result the plated Cu readily passes a tape adhesion peel test. Consequently, the integrity and conductivity of the (13) Zabetakis, D.; Dinderman, M.; Schoen, P. AdV. Mater. 2005, 17, 734– 738. (14) Dinderman, M. A.; Dressick, W. J.; Kostelansky, C. N.; Price, R. R.; Qadri, S. B.; Schoen, P. E. Chem. Mater. 2006, 18, 4361–4368. (15) Lide, D. R. Handbook of Chemistry and Physics; 75th ed.; CRC Press: Boca Raton, FL, 1994; pp 12-40. (16) Loschialpo, P.; Smith, D.; Zabetakis, D. J. Appl. Phys. 2008, 104, 104903.

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Figure 4. Specification of the frequency-selective surface samples. Photographs of samples c (low-pass) and e (high-pass) are shown, together with a schematic identifying the relevant dimensions. Note that although the unit cell is defined differently, the samples are essentially opposites of each other. Below are tables showing the physical dimensions for the samples measured and an estimate of the surface resistance (Ω/square) used in the method-of-moments calculation. Table 1. Metal Deposition onto Lines of 15.5 cm by 1.1 mm at 35 °C As Determined by EDTA Titration mass, mg thickness, µm rate, (µg/min)/cm2 resistivity, Ω m

11 min

15 min

1.3 ( 0.3 0.85 70

2.3 ( 0.1 1.5 90 9.9 × 10-8

patterns are maintained even after flexing of the paper, providing a significant processing advantage in the manufacture of advanced composites. To demonstrate the utility of our process, this direct-write technique was used to produce a class of composites known as frequency-selective surfaces in both the low-pass and high-pass forms for the microwave regime between 2 and 19 GHz. These materials function as filters that allow the passage of some wavelengths while reflecting others. Figure 4 shows the details of the samples tested. A low-pass design is composed of an array of metal squares separated by a grid of nonconductive lines. A high-pass design is the opposite pattern, with a grid of metal lines and an array of nonconducting squares. Representative samples of both types were manufactured by printing the appropriate pattern with Pd-Sn colloid on paper and metallizing with copper as described. Resistivity of the conducting areas is given in Figure 4. To verify the performance of these samples, the designs were analyzed by the method-of-moments to determine the expected function of transmission with frequency. The results of the low-pass samples are shown in Figure 5. With small conducting patches, the sheet is very transmissive and does not attenuate electromagnetic waves within the range of 2-19 GHz. Samples with larger patches begin to attenuate energy, first at higher frequencies and then across the range of interest. In all cases the transmission is greater at longer wavelengths (lower frequencies) than at shorter wavelengths, as typical for a low-pass FSS. The low-pass character of these sheets is due to the capacitively coupled gaps between the conductive patches. The high impedance of these gaps limits the current flow at low frequencies. Current flows more readily at higher frequencies as the capacitive component of the impedance

Figure 5. Direct-write manufacture of low-pass filters. Samples were produced as specified in the text and tested for transmission over the range of 2-19 GHz. Sample identities are as given in Figure 4. Solid lines are physical measurements. Symbols are method-of-moments computations.

decreases, thereby increasing the sheet’s reflectivity. The experimental results closely match the computational predictions. Figure 6 shows the results for the high-pass FSS designs. By contrast, these samples transmit the higher frequencies and reject the lower ones. Again, agreement between computation and experiment is very good, except for the highly attenuating samples j and k. Sample k significantly underperforms the theoretical expectation, while both samples fail to agree over the range of 4-6 GHz. The failure at the low frequencies may be explained by the fact that at these longer wavelengths the spot size of the instrument is larger, with fringes that may pass outside the physical dimensions of the sample. This effect is only noticed when the attenuation of transmission is on a very high order. The deviation of sample k across the frequency range may be indicative of an inductive component in the electromagnetic behavior of the samples. The method-of-moments implementation we used assumes a purely resistive nature of the composites and neglects any inductive aspect. An inductive component may arise from the nonuniformity of the conductive layer deposited on the textured surface of the sample (see Figure 1). Current flow is constrained to the narrow paths along the fibers. For most samples,

Direct-Write Patterning Palladium Colloids

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Conclusion

Figure 6. Direct-write manufacture of high-pass filters. Samples were produced as specified in the text and tested for transmission over the range of 2-19 GHz. Sample identities are as given in Figure 4. Samples e-h are in order, descending from the top. Solid lines are physical measurements. Symbols are method-of-moments computations.

Figure 7. Analysis of a complex pattern by manufacture-and-test. A sample was designed and produced as described. A photograph of the sample is shown as an inset, where the conductive copper is darkcolored. The transmission of the sample as a function of frequency is shown as a solid line.

any inductive component is insignificant. However, sample k, with a high fill factor, may not be adequately described by the simple model. One benefit of the technique of direct-write is the ease of analysis of complex patterns that lack simple geometry and simple repeated unit cells and/or possess acute, irregular features difficult to model. Figure 7 shows the production and analysis of such a complex pattern. The design is a me´lange of split-ring resonators, patches, and lines surrounded by nonconducting space and set within a conducting background which is irregular, with acute features, and lacking in symmetry. Analytically or computationally difficult, this sample is easily produced, metallized, and tested. As expected the design demonstrates a series of transmission dropouts representing resonances on top of a complex frequency-dependent background function.

Electroless metallization on paper can be used to examine the fundamental nature of conductivity of thin metal films on rough surfaces. While the bulk electrical transport properties of a metal are achieved when there is continuity and a direct line of metal from one measurement point to another, the situation will be very different when the substrate is rough on a scale greater than the thickness of the film and the film thickness itself is not many times greater than the electron mean free path. For electroless depositions there will be not only the issue of a nondirect line of transport (as the metal follows the microstructure of the cellulose fibers) but also discontinuities at metal grain boundaries as the growth process achieves a complete surface coverage. In addition, work by Munoz and co-workers has shown that there will be effects at the atomic scale and nanoscale which depend on the fine structure of the substrate surface.17 What we have shown in this work is that the development of conductivity through electroless metallization proceeds with a critical phase behavior where long-range conductivity develops very suddenly following an extended period of little change. Our results show that on the length scale of centimeters the electrical transport properties of a rough film are determined by the “exceptional elements”. For example, during the early phase of metallization, no change in resistance is observed regardless of the amount of metal deposited. However, as soon as the first one or few continuous paths are established, the resistance drops very rapidly to a minimal value. Conversely, the distribution of resistance measured in subelements of a line (Figure 3) shows that even if some elements have single-digit resistance values the overall resistance will be determined by those elements that are most restrictive to electrical transport. Therefore, while a single path is sufficient to yield conductivity in an otherwise incomplete film, a single gap is sufficient to prevent conductivity in a largely complete film. A rough fiber-based substrate will enhance these effects. Nevertheless, complete conductive films may be achieved by an adequate metallization process. Direct-write of metal colloids is an effective means to generate complex conductive patterns of reasonable resolution for large area coverage. The use of commercial printers, combined with commercial electroless metallization technology, facilitates the development of functional composites. The use of paper as a substrate may seem unusual, but has several advantages. Paper is lightweight, flexible, and inexpensive and is supported by broad implementation of paper-handling technology. One additional major advantage is that paper, being fibrous in nature, is readily adapted to use in fiber- or fabric-based composite materials. Functionalized paper can be incorporated into composites where strength and other properties are provided by carbon, aramid, or glass fibers without critically weakening the structure or introducing the possibility of delamination. These behaviors indicate a significant potential for the increased use of cellulosebased composites for advanced microwave technology and related applications. LA803356Y (17) Munoz, R. C. J. Mol. Catal. A 2005, 228, 163–175.