Nanoinks in inkjet metallization

Current Opinion in Colloid & Interface Science 19 (2014) 155–162

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Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis

Nanoinks in inkjet metallization — Evolution of simple additive-type metal patterning Henry J. Gysling CatAssays, PO Box 15430, Rochester, NY 14615, United States

a r t i c l e

i n f o

Article history: Received 11 March 2014 Received in revised form 17 March 2014 Accepted 21 March 2014 Available online 4 April 2014 Keywords: Metallization Selective metallization Inkjet Nanometals Nanometal inks E-jet printing Electrohydrodynamic jet printing Electroless plating

a b s t r a c t Recent advances in the development of stable dispersions of nanophase metal particles have allowed the direct fabrication of metal patterns (e.g., printed circuits, RFID tags, touch screens, etc.) by simple additive type inkjet processes. Such processes replace the more costly and less environmentally friendly subtractive lithographic type photoprocesses involving selective etching of photoresists and metal layers and more complex additive type process using photocatalysts for patterned metal deposition by electroless plating processes and inkjet patterning of metal catalyst or catalyst precursor for subsequent metallization by electroless plating. The recent development of electrohydrodynamic jet printing (e-jet printing), in which the ink drop is ejected under the influence of an electric field, has allowed a significant resolution increase vs. conventional inkjet printing with a piezoelectric head (printing resolution of ca. 100 nm for e-jet printing vs. ca. 20 μm for inkjet printing). © 2014 Elsevier Ltd. All rights reserved.

1. The evolution of solution metal deposition processes — subtractive to additive The synthesis of stable nanophase metal dispersions [1–5], coupled with the development of ink jet printing technology [6–11], and, more recently, electrohydrodynamic jet (e-jet) printing [13–23] has led to the development of convenient new fabrication processes for printed circuits and related conductive metal patterns by inkjet processes [24–26•]. These additive type processes provide a significant advantage over the earlier subtractive type processes. The conventional fabrication of printed circuits uses a multistep photolithographic subtractive type process involving several chemical steps — e.g., coat a metal layer, typically copper, with a photoresist, imagewise expose the photoresist layer through a mask to provide a solubility difference in the exposed and unexposed areas of the photopolymer (both positive and negative working photoresists are known), solvent etch the photopolymer to selectively expose the underlying metal layer, and finally solvent etch the exposed metal layer to provide the desired printed circuit [27–31]. In addition to the time-consuming nature of such processes, the disposal of the etched materials also imposes a significant process cost due to environmental regulations. A metal patterning process with only one etch used the following steps [32]:

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http://dx.doi.org/10.1016/j.cocis.2014.03.013 1359-0294/© 2014 Elsevier Ltd. All rights reserved.

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deposit a layer of photoresist on a substrate form a pattern on the photoresist by light exposure through a mask deposit a layer of metal nanoparticles on the patterned photoresist remove the photoresist and overlying metal nanoparticles on the photoresist - sinter the remaining nanoparticles to form a metallic pattern. Another lithographic one-etch process [33] provides the direct ultraviolet (UV) imprinting of silver patterns using an acrylate-based resin incorporating a nano-silver colloid, with post-imprinting steps involving heat treatment to sinter the nanosilver particles and wet etching. The electrical resistivity for 60 wt.% Ag loading was roughly 2.5 times higher than that of bulk silver. A second generation technology for printed circuits provides an additive process but also has severe chemical restrictions. In such processes a photocatalyst, typically a Pd(2+) compound, is uniformly coated on a suitable substrate, imagewise exposed to appropriate radiation (through a mask or using laser writing) to generate a pattern of metal catalyst, and the printed circuit is then generated by treatment of the imaged substrate with an electroless plating solution [34–39]. These solutions, comprising a metal ion, a complexing agent and a reducing agent (the commercial plating solutions generally also incorporate other addenda to promote the formation of bright metal deposits, etc.) are thermodynamically unstable with respect to metal deposition but kinetically stable for some period of time (minutes to days, depending on the particular combination of the three main

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H.J. Gysling / Current Opinion in Colloid & Interface Science 19 (2014) 155–162

components) [40–43]. A wide variety of such metal electroless plating solutions are known in the literature, as well as being commercially available (e.g., Cu [44–50], Ni [51–57] (the Ni deposited from these plating solutions can incorporate P and B from the reducing agent (e.g., NaH2PO2 [51] or amine boranes (e.g., dimethylamine borane (Me2(H) NBH3) [52]) used in the plating baths), Ag [58–61], Au [62–66], Pd [67–71], Pt [72–74], Co [75–77], Rh [78–80], Sn [81–83]). Commercial electroless plating solutions are typically supplied as two solutions which are mixed just prior to use (1 — metal compound and complexing agent and 2 — reducing agent). The most common systems of this type use palladium(2+) complexes as the photoelement, since Pd(0) is an excellent catalyst for a wide variety of electroless plating solutions, and Pd(2+) compounds readily undergo photoreduction to give the active Pd(0) catalyst for metal deposition from the metal plating solution. An example of such an additive system for fabrication of printed circuits on a flexible support is a polyethyleneterphthalate film (Estar® [84]) coated with potassium palladium oxalate, K2[Pd(C2O4)2], [34] in a thin cross-linked gelatin layer on this film base. The thin surface gelatin layer, which is water insoluble but water permeable allowing penetration of the aqueous plating solution, also provides Pd(2+) binding sites for the photocatalyst which is coated onto the film from an aqueous solution. Surface modification of polymeric substrates such as the polyimide Kapton® [85] and polyethyleneterphthalate (PET) (Kodak Estar® [84]; DuPont Mylar® [86]) by chemical treatment [87,88], plasma or corona discharge [89–95] to provide binding sites for Pd(2 +) have also been reported. Although these additive type systems eliminate the need for disposal of etched photoresist and metal, they require very careful selection of reducing agent in the plating solution to avoid spontaneous reduction of Pd(2+) in the background area (i.e., the areas not exposed to radiation to effect the Pd(2+) to Pd(0) photoreduction) which would give unwanted metallization [34]. In addition, the use of electroless plating solutions in the final metallization step requires careful control of the process conditions (reagent concentrations, temperature and pH). These processes, however, require no thermal annealing treatment after the metal deposition as is generally the case for metal patterns deposited by inkjet processes using nanophase metal inks [24–26•]. A third generation process eliminated the uniform photocatalyst layer by inkjet addressing the substrate with an ink comprising an aqueous dispersion of catalytic metal nanoparticles [96] or metal catalyst precursor compounds [97], with electroless plating used to generate the final metal pattern. This process eliminates the problem of background metallization by unwanted reduction of a uniform photocatalyst layer [34] but still requires the use of an electroless plating solution for the final step of the metallization. The ultimate solution to selective solution metallization has now been achieved, and commercialized — the use of inks comprising dispersions of nanophase metals to directly provide, after suitable thermal processing, the final metal pattern [24–26•]. Such processes eliminate the need for separate steps for catalyst patterning (by patterned photoreduction of metal catalyst precursor [34] or inkjet delivery of a metal catalyst [96] or catalyst precursor [97]) and final metallization with electroless plating solutions associated with the earlier additive type processes. The critical material science components of these new direct metallization processes are: 1. — inks comprising stable dispersion of the nanometals and 2. — suitable surface modification and thermal stability of the substrate to provide the required adhesion of the inkjet delivered nanometal and final thermal processing to provide the desired metal conductivity. Current commercial inkjet technology can pattern relatively large areas on a resolution scale of 100 μm. Largeformat 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. The most recent refinement of inkjet metallization is electrohydrodynamic jet (e-jet) printing technology [12–23] which delivers the ink drops under the influence of an electric field and can provide a

significant increase in resolution of the metal patterns compared to conventional inkjet printing — e.g., resolutions of ca. 100 nm vs. ca. 20 μm for inkjet printing. The concentration of ions allows the tip of the cone to break away and form droplets that are just a fraction of the volume of the cone. The use of e-jet printers, that currently can precisely print dots of various materials 250 nm in diameter, offers the possibility to rapidly fabricate complex nanoscale structures out of various materials. Recent work by the Rogers group at the University of Illinois [19], for example, demonstrated e-jet printing that can pattern large areas with block-copolymers based on poly(styrene-block-methylmethacrylate) to give geometries with diameters and linewidths in the sub-500 nm range, line edge roughness as small as ~45 nm, and thickness uniformity and repeatability that can approach molecular length scales (~2 nm). This review provides an overview of the evolution of patterned metallization processes over the past 30 years, from the early chemically complex subtractive photolithographic methods [27–31] to the current processes using commercial nanometal inks for the totally additive fabrication of metal patterns and interconnects in applications such as printed circuits, RFID antennas, touch screens, solar cells, thin film transistors, electroluminescent devices and OLED displays. An excellent comprehensive journal review of inkjet metallization processes has also been recently published [26•]. 2. Fabrication of metal patterns by ink jet delivery of metal catalyst precursor or metal catalyst nanoparticles followed by electroless plating The three key enabling technologies for the development of inkjet metallization processes have been the development of synthetic methodology for metal nanoparticles (inorganic synthesis) and stable dispersions of these materials (colloid chemistry) [1] and substrate surface modification to provide good adhesion of the metal to the substrate, often a polymer film for flexible electronics [87–95]. Surface modification of polymeric films for use as flexible substrates in inkjet metallization processes (and coatable electronics, in general) includes the coating of a thin film of a second functional polymer on the film base. Gel-subbed Estar® (polyethyleneterphthalate, PET), available from Eastman Kodak Co., for example, has a thin layer of cross-linked gelatin on the surface that provides binding sites for metal ions such as Pd(2 +) [34,84]. PET is also available from DuPont as Mylar® [86]. Chemical etching [87,88] and surface plasma treatments [89–95] have been widely used to give surfaces that provide binding sites for metallization catalysts as well as good adhesion of metal coatings. Since thermal annealing is generally used to process inkjet deposited metal nanoparticles (to eliminate surface stabilizing agents, allow aggregation of the metal nanoparticles and provide high conductivity [26•]), the thermal stability of the polymeric film base is also critical — e.g., PET can be used up to ca. 120 °C and Kapton®, a DuPont polyimide [85] can be used up to ca. 150 °C. Two and three step modifications of inkjet metallization involving final metallization by electroless plating catalyzed by an inkjetpatterned catalyst include the following variations: 1. inkjet delivery of metallization catalyst followed by electroless plating of the final metal pattern (immersion of the substrate with inkjet patterned catalyst into an electroless plating bath) [96]; 2. inkjet delivery of a metallization catalyst precursor followed by electroless plating metallization (immersion of the substrate with an inkjet patterned catalyst precursor, e.g., Pd(2+), into the electroless plating bath with the patterned catalyst precursor being initially reduced to the active metallic catalyst by the reducing agent of the electroless plating bath) [97]; 3. inkjet delivery of metallization catalyst and 2 components of the electroless plating bath.


In such processes three inks are sequentially delivered to the substrate — the metallization catalyst, followed by inks containing the two components of the electroless plating solution [40–43]. In these processes the reducing agent and the metal ion components of the electroless plating solution are in separate inks to provide stable inks and avoid spontaneous metal deposition. Since these two components are in separate inks, a metal ion complexing agent used in conventional electroless plating to provide stability is often omitted; 4. a variation of process #3 using two inks with the catalyst or catalyst precursor included in one of the two inks containing the electroless plating solution components. The most common metal used to seed a substrate with catalytic nanoparticles is palladium. This choice is based on the wellestablished catalytic activity of this metal for initiation of metal deposition from a wide variety of electroless plating solutions [30,40–43, 98–100], its high stability to oxidation, and the ease of reduction of Pd(2 +) to Pd(0) (i.e., in systems involving patterned photoreduction of a Pd(2+) compound to the active Pd(0) catalyst [34] or inkjet delivery of a Pd(2+) catalyst precursor [97,101]). In addition to inkjet patterning of a metal catalyst precursor (e.g., Pd(2 +) or some other metal ion that on reduction gives catalytically active metal nuclei), inkjet delivery of preformed catalyst nanoparticles has also been used [24–26•]. The critical size of Pd nanoclusters (i.e., the number, n, of atoms in a Pdn nanocluster), has been measured to be ca. 4–6 atoms for a nickel electroless plating solution [98]. In a modification of this process a polymeric receiving layer for Pd(2+) (a positively charged polymeric polyelectrolyte poly(allylamine hydrochloride), PAH), is inkjet patterned on a polyethyleneterphthalate substrate, followed by immersion of the patterned substrate in an aqueous solution of K2PdCl4 and final metallization in a r.t. nickel electroless plating solution. The patterned nickel metal deposition is catalyzed by the Pd(0) formed in-situ by the reduction of the patterned PAH-bound Pd(2 +) catalyst precursor by the dimethylamine borane reducing agent of the nickel electroless plating solution [102]. The nickel thickness ranged from 0.01 to 1 μm and was determined by the plating time (e.g., after 12 min. of plating, the Ni deposition was ca. 900 Å. and the electrical resistivity of the Ni wire was measured to be about 1.45 × 10−6 Ω m). 2.1. Use of inkjeted Pd(2+) metallization catalyst precursors & metal electroless plating Inkjet metallization processes have used inks containing Pd(2 +) compounds which function as Pd(0) catalyst precursors, the active Pd(0) catalyst being formed by thermal, chemical or photochemical reduction (using subsequent inkjet delivery of a reducing agent, or, more conveniently, by the reducing agent in an electroless plating solution). A process for production of printed circuits and RFID tracking labels on flexible supports, such as polyester or polyimide (e.g., Kapton® [85]), first generates a pattern of Pd(2+) catalyst precursor by printing, e.g., such as by gravure or inkjet printing, an ink containing a Pd(2+) complex, followed by thermal decomposition of the coated Pd(2+) precursor to give the active Pd(0) nanoparticle catalyst. The final conductive metal pattern, corresponding to the Pd(0) catalyst pattern, is produced by metal electroless plating (e.g., Cu, Ag, Au, Ni) [103]. This technology allows roll-to-roll and other forms of continuous high throughput manufacturing of the metal patterns. Printing resolution of this process can be less than 100 μm, the resolution being determined by the specific chemistry, processing (e.g., Pd(2+) complex and concentration, support surface) and printer parameters used. A similar process [104] uses the reducing agent of the electroless plating bath to reduce the inkjeted Pd(2 +) pattern to Pd(0), which then functions as a catalyst for copper electroless plating. A related process for printed metal patterns, patented by Hewlett Packard, inkjets a Pd(2 +) aliphatic amine complex, e.g.,

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[Pd(CH3CH(NH2)CH2NH2)2]Cl2, onto a substrate (e.g., silicon, polyethylene terephthalate (e.g., DuPont Mylar®), polyimides (e.g., DuPont Kapton®)), followed by inkjet addressing the Pd(2+) pattern with a reducing agent solution, e.g., formic acid, with subsequent thermal processing generating a Pd(0) catalyst pattern [105]. The final conductive metal pattern is generated by metal electroless plating (e.g., Cu — using commercial Envision EC-2123, available from Enthone, Inc. [50c]). 2.2. Use of inkjeted metal nanoparticles as metallization catalysts 2.2.1. Nanophase Pdn inks High resolution conductive nickel lines have been fabricated by a 2 step metallization process in which an aqueous ink containing catalytic Pd nanoparticles stabilized by styrene-N-isopropylacrylamide cooligomer is inkjet patterned on a flexible PET substrate followed by Ni electroless plating [106]. Busato and co-workers demonstrated the printing of palladium on a polyimide film for electroless copper plating using a Hewlett-Packard Deskjet 1220C printer [107]. Large area microscale copper patterning on paper has been achieved by inkjet patterning a commercial Pd–Sn colloid catalyst (Cataposit 44®, Rohm & Haas [108]), which strongly binds to cellulose fibers, and subsequent copper electroless plating [109]. With plating at 30 °C for 15 min all of the isolated Cu particles grew sufficiently large to merge with neighboring particles, establishing a coalesced metal network required for good electrical conductivity (resistance for a thickness of 1.5 μm is 9.9 × 10−8 Ω m). Growth of electroless Cu between cellulose fibers of the paper mechanically interlocks the metal deposit to the substrate allowing the plated Cu to readily pass a tape adhesion peel test. Paper substrate has the advantages of being lightweight, flexible, and inexpensive and is supported by broad implementation of paper-handling technology. Paper, being fibrous in nature, is also readily adapted to use in fiber- or fabric-based composite materials where strength and other properties are provided by carbon, aramid, or glass fibers. An early DuPont patent described the use of an aqueous ink containing surfactant-stabilized palladium black nanoparticles to inkjet pattern this catalyst for the subsequent copper electroless plating metallization [110]. 2.2.2. Nanophase Ptn inks Shah et al. [111] used inkjet printing on plastic transparency sheets to pattern a platinum colloid as a catalyst for electroless copper metallization using a commercial Hewlett-Packard 51626A printer. This process gave highly conductive lines 100 μm wide by 0.2–2 μm high (printer drop size = 140 pL). 2.2.3. Inkjeted copper nanoparticles as catalysts for electroless plating metallization The DuPont patent described above [110] also claims the inkjet patterning of nano-copper particles which function as catalyst centers for subsequent copper electroless plating to produce a conductive metal pattern. 3. Fabrication of metal patterns by inkjet delivery of metal precursors or metal nanoparticles The ideal totally additive solution-patterned metallization process is direct inkjet fabrication using metal precursor compounds or metal nanoparticles in the ink formulation. As in the above hybrid systems involving inkjet delivery of a catalyst followed by electroless plating of the final conductive metal pattern, surface chemistry to provide good adhesion and thermal stability of the substrate compatible with any thermal annealing treatment of the initially inkjeted metal precursor compound or nanoparticle pattern are critical. This metallization technology has been widely described in the literature [24–26•] and a number of nanometal inks for such metallization processes are now

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commercially available (e.g., Cu [112,113], Ag [114–120], and Au [121–124]). Inkjet metallization processes with various metal inks are briefly reviewed below to illustrate the scope of the technology.

glycol solvent, an alcohol and a diol monoether. The ink is characterized by excellent jettability and freedom from nozzle-plate flooding. Ink surface tensions are in the range of 34 to 37 mN/m and viscosities are less than 41 cP.

3.1. Copper patterning 3.2. Silver patterning Inkjet printing of a copper hexanoate–isopropanol solution, followed by thermal conversion of the Cu(2+) precursor to copper metal at 200 °C, has been used for additive source/drain metallization for amorphous silicon thin-film transistors [125]. A subsequent anneal at 200 °C in forming gas (15 vol.% H2 in N2) was used to convert any copper oxide in the deposit to metallic copper, giving a resistivity of ca. 10 μΩ cm [126]. This is claimed to be first example of direct printing of a transistor material using a digital nonimpact technique (an Epson Stylus 400 piezoelectric inkjet office printer was used). Conductive copper traces on paper and poly(vinylidene-fluoride) (PVDF) have also been fabricated by a reactive inkjet processing multiple-cycle sequential delivery of an aqueous copper citrate solution followed by an aqueous sodium borohydride solution [127]. More oxidation resistant nickel traces have been deposited by a similar 2 ink process (NiSO4 and NaBH4 inks). The best conductivity of printed copper lines was 1.8 × 106 S m−1 and that of nickel was 2.2 × 104 S m−1, about 1/30 and 1/600 of their bulk metal counterparts, respectively. The low conductivities have been attributed to inclusion of salt by-products in the metal deposits [127]. The Wiley group at Duke University has reported the synthesis of a variety of metal nanowires which can be incorporated into ink formulations for solution metal patterning [128]. This work has included copper nanowires [129,130] and, more significantly, copper–nickel nanowires (copper shelled with nickel, 20 mol% Ni) that have significantly improved electrical conductivity and resistance to corrosion than copper or silver, but not indium tin oxide, ITO [131]. The CuNi material is claimed, however, to provide a significant advantage vs. ITO, the most common optically transparent conductor used in the electronics industry, in processing (i.e., the CuNi nanowires can be solution processed for coatable electronics vs. a more expensive vapor deposition process for ITO), cost (indium is ca. $700 per kg and the main source is China), and cracking resistance (ITO is somewhat brittle). Inks for the fabrication of copper traces are commercially available. NovaCentrix's Metalon ICI Series of Copper Oxide Reduction Inks [113] contain copper oxide nano-particles in aqueous dispersions. After printing, the inks are converted to copper by using the energy from a PulseForge tool to in-situ reduce the oxide to metallic copper. The result is a mesoporous copper structure with excellent conductivity at a fraction of the cost of a comparable silver-based deposition. These copper inks require an overcoat after curing to prevent long-term re-oxidation of the copper metal. Intrinsiq Materials [112] has commercialized a copper nanoparticle based ink (CI-002) which consists of coated pure copper nanoparticles dispersed in solvent. The specialized coating stabilizes the copper and prevents the ink from agglomerating, ensuring a long shelf life. Compatible with standard commercially available inkjet equipment, sintered by means of broadband flash or laser, the copper ink deposits and adheres to a variety of substrates including paper, plastic and glass. For applications where broad area illumination is preferred, a broadband flash system is the preferred technique while for applications requiring extremely narrow linewidths, sintering on metal substrates, and other specialized energetics, laser sintering may be preferred. Intrinsiq provides a precision laser sintering system (Intrinsiq LAPS-60 system) for lab scale and developmental efforts in applications for which laser sintering is required. This system is 2-dimensionally controlled with an IR laser for sintering of fine lines (bthan 5 μm) for applications requiring very fine lines, significant penetration depth, or sintering on metal substrates. Intrinsiq also has developed thermally sinterable inks. A recent Intrinsiq patent [132] describes an ink composition for printing conductive layers on a variety of substrates, the ink composition comprising polymer encapsulated copper nanoparticles, a primary

Conductive silver patterns have been fabricated using an inkjet printing device with two ink channels: (1) a silver(1 +) ammonia complex, [Ag(NH3)4]NO3 — a Ag(0) precursor, and (2) formaldehyde reductant solution [133]. These 2 inks were separately ejected, mixed, and reacted on the substrate to produce smooth and continuous silver lines of 90 μm in width with an average film thickness of 200 nm and comprising Ag nanoparticle grains of 50 to 200 nm. The electrical conductivity of the resulting silver lines is 6% of bulk silver at room temperature and, after sintering at 150 °C for an hour, 14%. A commercial drop-on-demand inkjet printer has been used to print a silver neodecanoate solution in xylene onto glass substrates [134]. The normal print head was replaced by a single 60 mm diameter nozzle piezoelectric driven inkjet print head. Conductive silver tracks were obtained by heat treatment of the inkjet printed Ag(0) precursor salt at temperatures ranging from 125 °C to 200 °C in air. Resistivity values were found to have dropped to two to three times the theoretical resistivity of bulk silver after temperatures of 150 °C and above were used. A standard office ink-jet thermal-head printer was used to sequentially print an aqueous silver salt solution and an ascorbic acid reductant solution to give nanosized silver patterns, composed of particles in the size range 10–200 nm [135]. The deposited layers of silver had high electrical conductance, up to 1.89 × 105 S m−1, and sheet resistance up to 0.5 Ω/□. The Wiley group has reported silver nanowires with independently controlled lengths and diameters using a polyol synthesis by controlling the reaction temperature and time and prepared optically transparent conductive films by solution spraying processing with these materials [136,137]. Suggested applications for these nanowires include conducting nanowire films for flexible displays, organic light emitting diodes and thin-film solar cells. Xerox has patented [138] an ink composition suitable for printing conductive lines on various substrates comprising silver nanoparticles, a hydrocarbon solvent, and an alcohol co-solvent. The silver nanoparticles are stabilized with a carboxylic acid or an organoamine and preferably have a silver content of 75–85% by weight and particle size of 1–20 nm. The preferred ink compositions have weight percent of the silver nanoparticles, along with any stabilizer, of 40–60%. Conductive lines fabricated from these inks, after annealing at ca. 210 °C for 30 min., have a width of about 100 μm or less and have electronic applications such as thin film transistors (TFTs), light-emitting diodes (LEDs), RFID tags, photovoltaics, etc. The use of silver in such applications provides much lower cost than gold and much better environmental stability than copper. A Kodak patent [139] describes the use of silver halide aqueous gelatin dispersions, such as used in conventional photographic technology [140], for the production of nano-silver inks. The silver particles produced from this well-established silver halide dispersion technology by treatment with reducing agents (e.g., silver halide “developers”) provide controlled properties of size, morphology and size distribution of the silver nanoparticles for use in manufacturing of conductive inks. Conductive silver tracks (25 μm wide) on polyimide have been inkjet printed by reduction of the droplet size to ~ 2 pL for a standard 10 pL printhead by tailoring the waveform and heating the print cartridge and the substrate to 55 °C [141]. Sintering of inkjet printed silver tracks on polyimide (PI), polyethyleneterphthalate (PET) and photographic paper at room temperature using an aqueous nanosilver ink with polyvinyl pyrrolidone stabilizer has also been accomplished using a camera flash-lamp, which makes use of the enhanced photothermal effect in silver


nanoparticles [142]. The thermal conductivity of the substrate had an important effect on the microstructure and resistance of the silver tracks after the flashing. A drop-on-demand piezoelectric inkjet printing process has been used to print passive microwave circuitry on a Corning 7740 glass substrate [143]. Using a 10 pL drop volume, 50 μm resolution was achieved. The conductivity of the sintered silver structures was 1/6 that of bulk silver after sintering at a temperature much lower than the melting point of bulk silver. The DC resistance of the sintered silver was comparable to that for electroplated and etched copper and printed coplanar lines had losses of 1.62 dB/cm at 10 GHz and 2.65 dB/cm at 20 GHz. Several commercial sources supply silver-based nanoinks for inkjet metallization. Advanced Nano Products silver inks such as Silverjet DGP and DGH series [114] can easily produce electrode circuits with fine line widths comparable to those using photolithography. Silverjet DGP and DGH series were developed for low and high sintering temperatures according to their purpose and can be used with most of major commercial inkjet heads (Dimatix, Konica-Minolta, SEMCO). ULVAC, Inc. [115] supplies nano-silver inks that are produced by a gas evaporation method. These newly developed inks, consisting of metal nanoparticles dispersed stably in organic solvents, can directly inkjet print electroconductive fine patterns. The “Ag Series” inks form Ag films at a curing temperature of 230 °C (solvents: toluene and tetradecane). The “L-Ag Series” inks (solvent: toluene), with an average Ag particle size = 3 nm, require a curing temperature as low as 150 °C. InkTec Corp. IJ Series Silver Inks [116] are stable even in long-term storage and can be applied to various substrates due to their low temperature sintering (130–150 °C) for conductive traces. They produce silver lines with sheet resistivities less than 160 mΩ/□. Aqueous nanosilver inks suitable for inkjet printing on a variety of substrates are also available from NovaCentrix (Metalon® nanosilver inks) [118]. These inks require sintering temperatures of 100–110 °C for 20–30 min. NanoMas Technologies NanoSilver™ Conductive Inks [119] are useful for printing electronic devices on plastics or papers. The particle size of the silver nanocrystals in these inks is b10 nm, allowing low processing temperatures (less than 135–160 °C) for a typical printed resistivity of 2.4 × 10−8 Ω m (1.5 × bulk silver). Harima Chemicals Nano Paste® series of conductive silver pastes contain stable distributions of nano-sized silver particles (7–12 nm) in ink-form for ink-jet printing metallic silver (sintering temperatures: 120–500 °C) [120].

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3.3. Gold patterning Although gold is considerably more expensive than the other metals used for conductive traces, Cu and Ni, its high work function and excellent stability can provide advantages, especially for applications involving exposure to extreme/corrosive environments. Efforts are being directed to provide conductive materials with both good chemical stability and low cost — the nickel-shelled copper nanowires developed in Wiley's laboratory at Duke University being such an example [131]. Subramanian's laboratory [144] has studied the fabrication of gold nanowires by ink jet delivery (40 pL drops) of thiol-stabilized nanogold particles in toluene solution (prepared by the borohydride reduction of H[AuCl4] in a 2-phase reaction [3]). The optimized stable ink used 1.5 nm gold nanoparticles stabilized by C6H13SH. Thermal annealing of these deposited nanoparticles at 150 °C provided conductive wires (40–160 μm line widths, 1 μm thick, sheet resistance of 0.03 Ω/□) with conductivities ca. 70% of bulk gold, making the process suitable for electronics on plastic applications. In subsequent work [145] with this nanogold ink (α-terpineol solvent; ink drop size ca. 60 pL), the sintering temperature was reduced to 120 °C to give conducting lines. Sintering at 180 °C gave a resistivity of 5 μΩ cm using a standard four-point test structure. This value is significantly lower than for the common alloybased solders used in packaging applications (i.e., Pb–Sn (16.5 μΩ cm) and Sn–Ag–Cu (12 μΩ cm)). Additionally, electromigration studies showed that these sintered Au patterns have excellent robustness, suggesting that this nanogold ink is a promising candidate for nextgeneration lead-free solders in packaging applications with polymeric substrates. Patterning these nanogold dispersion with inkjet printing technology can produce ultradense interconnection with b20 μm pitch. Pulsed laser-based curing of a printed Au nanoparticle ink (3–7 nm Au NP in toluene; purchased from ULVAC Corp. [121]), combined with moderate substrate heating, provide microconductors at low enough temperatures appropriate for polymeric substrates [146••]. The printed line width was shown to be dependent on the substrate temperature (e.g., under comparable printing conditions, r.t. printing gave a line width of 140 μm and 70–80 μm when the substrate was heated at 90 °C). An optimized microsecond pulsed laser treatment of the deposited Au ink allowed melting and coalescence of the Au NPs at lower substrate temperature than continuous thermal annealing (200 °C vs. 400 °C; the melting temperature for bulk Au is 1064 °C). This lower

Fig. 1. Schematic of nanoink printing and curing system. © 2005 ASME. Reproduced by permission of American Society of Mechanical Engineers from ref. [146].

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substrate temperature allowed fabrication of conductive microlines on polyimide with 3–4 times higher resistivity than the bulk Au value without damaging the polymeric substrate while maintaining good adhesion. The printing/curing system used in this work is shown in Fig. 1. Nanogold inks are commercially available. ULVAC, Inc. [121] supplies nano-gold inks that are produced by the gas evaporation method. These newly developed inks, consisting of metal nanoparticles dispersed stably in organic solvents (toluene and cyclododecane) provide electroconductive fine patterns directly by ink-jet printing, after curing at 250 °C. NanoGold™ Conductive Inks for printing electronic devices on plastics or papers are available from NanoMas Technologies [123]. These inks, which have Au nanocrystal sizes b 10 nm and narrow size distributions, require processing temperatures less than 200 °C (e.g., Au traces with a printed resistivity of 1.0 × 10−7 Ω m (4× bulk gold) were produced with 190 °C annealing). Harima Chemicals NGP-1 Gold Nano Paste® contains stable 7 nm gold particles in an ink suitable for inkjet printing metallic patterns (sintering temperatures: 250 °C) [124]. A report on opportunities for nanogold inks in the electronics industry has been recently published [147]. 4. Hydroelectrodynamic jet printing Electrohydrodynamic jet printing is a relatively new process [12–23] in which, for metallization processes, the ink solution of a soluble metal compound (metal precursor) or dispersion of stabilized metal nanoparticles is ejected from a small capillary nozzle which is connected to a high voltage with the substrate connected to ground. Under the electric field, the ink ejected from the nozzle takes a conical shape (Taylor cone) and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets, the droplets getting smaller until they are deposited on the substrate as a uniform thin layer. Electrohydrodynamic jet (E-jet) printing is emerging as a highresolution alternative to other forms of direct solution-based fabrication approaches, such as inkjet printing [12–23,148–151]. In inkjet printing the droplets generated are larger than the nozzle's diameter and even if a fine, e.g., 65 μm dia., inkjet nozzle is used, the deposited droplets are well over 100 μm in diameter. In contrast, the electrohydrodynamic process allows much coarser nozzles (200–500 μm in internal diameter) to be used, with deposited droplets less than 50 μm diameter [152,153]. In an early study [150] an electrohydrodynamic direct-write device, generating ca. 26 μm droplets of an ethanol dispersion with 15 nm Au particles, was used to deposit the ink onto silicon wafers. The gold particles organized in a 75 μm wide central region surrounded by two regions, each ca. 50 μm wide, at the edge of which two further gold tracks, each ca. 7 μm existed. A later paper [151] reported e-jet printing of highly conductive silver lines with widths less than 6 μm, which was approximately twenty times smaller than that of inkjet printing. Optimized Ag ink annealing (3–9 min. at 150 °C) gave Ag line pattern resistivities as low as 7 × 10−6 Ω · cm. 5. Summary – future directions As described in this review, the technology for patterning metal traces (and, indeed, other organics and inorganics used in the electronics and other industries) has undergone a great advance over the past 30 years. From the time-consuming, complex, and environmentally unfriendly subtractive methods based on photolithography we are now at the dawn of a new age of printing electronics from solution on a variety of substrates, including flexible film base. This technological revolution has resulted from significant developments in hardware (i.e., drop-on-demand piezoelectric inkjet printers that can pattern with high resolution and, more recently, e-jet printers) and chemistry (reliable large-scale synthesis of nanophase materials and their stable dispersions for printing inks and other solution-based coating processes), and polymer surface modification techniques to provide binding to solution-coated catalysts and good adhesion of conductive metal

traces and other functional materials. As large companies have reduced their R&D efforts during this period, a major driver for this technological advance has been academic research, especially in the area of nanophase materials, with many small innovative companies being spun off from this university work to quickly commercialize these exciting opportunities. Scientists from the R&D staffs at large companies, with both broad technical expertise and an understanding of the commercialization process, have also been important in driving the commercialization of this evolving technology at small start-up companies. This printing technology, along with the increasing commercial availability of a wide range of nanophase materials, offer the ability to fabricate complex electronic devices under ambient conditions using relatively inexpensive printers in continuous manufacturing processes vs. very expensive equipment for conventional vapor phase deposition processes which require high vacuum conditions, and often, expensive and pyrophoric chemicals. This quickly evolving technology promises to unleash an expansion of more innovative small companies that can utilize these new technologies to commercialize, in relatively short time frames, a wide spectrum of new processes for new devices. Opportunities for continuous medical monitoring with inexpensive printed sensors worn on the body that can transmit patient data to your doctor or medical center are an especially promising application for this technology. This technology, coatable electronics, holds great promise for developing a broad spectrum of mass-produced devices/products to improve our lives as well as provide a major expanding manufacturing sector for US economic development. Small start-ups using innovative new technology and the ability to move quickly from R&D to the marketplace will be major participants in this developing sector.

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