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Japanese Journal of Applied Physics 52 (2013) 05DB14 http://dx.doi.org/10.7567/JJAP.52.05DB14

Comparison of Reactive Inkjet Printing and Reactive Sintering to Fabricate Metal Conductive Patterns Soorathep Kheawhom and Kamolrat Foithong Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand E-mail: [email protected] Received October 31, 2012; accepted February 28, 2013; published online May 20, 2013 Two methods to fabricate metal conductive patterns including reactive inkjet printing and reactive sintering were investigated. The silver printed lines were prepared from reactive inkjet printing of silver nitrate and L-ascorbic acid. Alternatively, the silver lines were prepared by the reactive sintering process of ethylene glycol vapor at 250  C and formic acid vapor at 150  C. In reactive printing, we investigated the effect of the number of printing cycles and the effect of silver nitrate concentration on the properties of the conductive patterns obtained. In reactive sintering, we investigated the usage of formic acid and ethylene glycol as reducing agents. The effect of reactive sintering time on the properties of the conductive patterns obtained was studied. As compared to reactive inkjet printing, the reactive sintering process gives more smooth and contiguous pattern resulting in lower resistivity. The resistivity of the silver line obtained by ethylene glycol vapor reduction at 250  C for 30 min was 12  cm, which is about eight times higher than that of bulk silver. In contrast, the copper lines were fabricated by reactive inkjet printing and reactive sintering using various conditions of formic acid, ethylene glycol and hydrogen atmosphere, the copper lines printed have no conductivity due to the formation of copper oxide. # 2013 The Japan Society of Applied Physics

1. Introduction

Printed electronic has attracted lots of attentions because it is a simple process of making a desired conductive pattern on various substrates with low cost, eco-friendship, and simple process.1–11) Inkjet printing is one of the promising printing techniques. It is an additive technology requiring rather low investments, and it can be operated under ambient conditions. The conductive inks adopted for the inkjet printing are either based on suspensions of metallic nanoparticles12–21) or solutions of metal-ions/metal–organic compounds,22–31) which are of interest in this work. The main drawback of the nanoparticle inks lies on the fact that the nanoparticles show a tendency to agglomerate and thus can clog the inkjet nozzle.28) Moreover, metal nanoparticles are generally synthesized using the complicated processes, and various toxic wastes are generated throughout these steps. Organic compounds used as capping agents are inevitably present in nanoparticle inks because of the high surface energy of metal nanoparticles. Thus, high curing temperature is required to get rid of these organic compounds,23) and hence limits the application of low cost polymer substrates. Moreover, the formation of porosity on printed patterns resulting from decomposition of capping agents during curing process can decrease the conductivity of the patterns printed. In comparison, metal-ion inks can be processed at lower temperature because metal ion inks do not require organic compounds to maintain the stability. Further, as metal nanoparticle inks contain metal element in a solid form, the particles of printed patterns are likely accumulated at the edge and depleted at the center, showing a so-called coffee ring effect. This coffee ring effect makes non-uniform surface structure of printed patterns. The formation of porosity and non-uniform surface structure on printed line patterns can be avoided by using metal-ion or metal–organic compound inks. Metal-ion and metal–organic compound inks require a post processing upon which the metal-ion precursors are converted to metal element and then sintered to a contiguous conductive pattern. The technique where printing and metal-

ion reduction processes occur simultaneously is classified as reactive printing. In contrast, in reactive sintering, the metalion reduction and sintering processes occur at the same time. The reactive printing has been demonstrated in various applications such as printing of polyurethanes for rapid prototyping24) and printing of silver pattern.22) Bidoki et al.22) used aqueous silver nitrate as metal precursor and ascorbic acid as reducing agent. In their approach, the ascorbic acid was printed first onto either paper or cotton, followed by the silver nitrate. Conductivities that were 0.3% of bulk silver were obtained. They also pointed out that the copper patterns could be formed by using this technique but it did not work in practice due to the formation of copper oxide. Later, Li et al.25) demonstrated reactive printing of copper and nickel lines. The copper ink was an aqueous copper citrate and the reducing agent was a solution of sodium borohydride. Conductivities of the patterns obtained were 3% of bulk copper. They also applied the same technique on reactive printing of nickel lines. Conductive patterns can be fabricated by printing metalion inks and sequentially reactive sintering. In this case, sintering process is done in a reducing atmosphere simultaneously with reduction process. Huang et al.29) synthesized silver-ion ink prepared by silver nitrate, ammonium hydroxide and dextrose, which is a weak reducing agent. The reduction and sintering were carried out at 500  C for 60 min. The resistivities of silver patterns obtained were about 3.1  cm, which is as twice as the resistivity of bulk silver. Although, the low resistivity patterns could be obtained, the processing temperature required is too high for the application of polymer substrates. The application of Ag(I)-2-[2-(2-methoxyethoxy)ethoxy]acetate as particle-free inkjet ink was reported.27) The conversion of Ag(I)-2-[2-(2-methoxyethoxy)ethoxy]acetate to elemental silver follows a two-step decomposition at temperature between 200 and 300  C. In case of polymer substrates, thermal sintering at 130  C and a combination of simultaneous heating and UV irradiation of the samples could be applied. The conductivities of the printed patterns on poly(ehtylene terephthalate) (PET) are about 18% of bulk silver.

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Nie et al.30) also demonstrated using silver citrate-amine complex as silver conductive ink. The patterns thermally treated reached the lowest resistivity of 17  cm after curing at 150  C for 50 min, and 3.1  cm after curing at 230  C. Preparation of copper pattern at low temperature was carried out by thermal decomposition in a nitrogen atmosphere using copper formate-amine complex ink.31) Blended amines of octylamine and dibutylamine produced patterns with low resistivity of 5  cm achieved by calcination at 140  C. Goo et al.26) demonstrated inkjet printing of copper complex ink on polymer substrates. The printed patterns were simultaneously reduced and sintered by thermal treatment in hydrogen atmosphere at 200  C. The patterns with resistivity of 62.26  cm were obtained by treatment at 200  C for 1 h. This research aims to study the use of silver salt in a form of silver nitrate as conductive ink to fabricate conductive patterns by using two different techniques including reactive printing and reactive sintering processes. The paper is organized as follows. In Sect. 2, the experimental details including reactive printing procedure, procedure of reactive sintering by formic acid vapor, procedure of reactive sintering by ethylene glycol vapor, and the characterization techniques used in this work. In Sect. 3, all experimental results are presented and discussed. In reactive printing, we investigated the effect of the number of printing cycles and the effect of silver nitrate concentration on the properties of the conductive patterns obtained. In reactive sintering, we investigated the usage of formic acid and ethylene glycol as reducing agents. The effect of reactive sintering time on the properties of the conductive patterns obtained was studied. Finally, we conclude the paper in Sect. 4.

Fig. 1. Two point probe pattern.

Fig. 2. Experimental setup of formic acid vapor reduction.

2. Experimental Procedure

All printing experiments were carried out with a commercially available multi-color drop-on-demand inkjet printer (Epson Stylus Photo T60), and each droplet volume is approximately 1.5 pL. The metal salt solution was poured into the ink cartridge and printed as a two-point probe pattern as shown in Fig. 1.

Fig. 3. Experimental setup of ethylene glycol vapor reduction.

2.1 Preparation of silver lines by reactive inkjet printing

The same printing cycle procedure was repeated several times to obtain a multi-layer conductive pattern. Firstly, 1 M of L-ascorbic acid was printed as a reducing agent on PET substrate at room temperature and then overprinted by silver nitrate solution (5, 7, and 10 M). After that, the pattern printed was baked at 150  C for 30 min. The influences of 5, 10, and 15 printing cycles and 5, 7, and 10 M of silver nitrate concentration on the properties of conductive patterns obtained were studied. 2.2 Preparation of silver lines by reactive sintering 2.2.1 Formic acid vapor reduction

7 M of silver nitrate solution with five printing cycles on PET substrate was carried out. The pattern printed was placed on a hotplate inside a glass dish with formic acid and heated to 150  C. Figure 2 shows the diagram of the experimental system used. The pattern printed was held at

this temperature for 30 to 120 min to convert to a conductive silver line. It was then naturally cooled down to room temperature inside the glass dish. 2.2.2 Ethylene glycol vapor reduction

This experiment was performed at 250  C, which is above the boiling point of ethylene glycol. Thus, a polyimide (PI) film was used as the substrate for this experiment. The PI film was firstly treated by oxygen plasma at working power 200 W, back pressure 15 Pa, 40 sccm of oxygen gas flow rate and treatment time for 3 s. The pattern printed by using 5 printing cycles of 7 M of silver nitrate on treated PI film was reduced by ethylene glycol vapor at 250  C for 15 to 60 min. Figure 3 shows the experimental setup used in this study. The silver conductive patterns fabricated were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) to confirm the crystal structure and

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Fig. 4. SEM images of silver lines of (a) 5, (b) 10, and (c) 15 cycles reactive printing.

Fig. 5. XRD patterns of silver lines of 5, 10, and 15 cycles reactive

morphology, respectively. Finally, the resistivities of all conductive patterns were measured by two-point probe at room temperature.

printing.

3. Results and Discussion 3.1 Effect of printing cycle on reactive inkjet printing process

The SEM images of silver lines obtained by reactive inkjet printing of 5 M silver nitrate and 1 M L-ascorbic acid are shown in Fig. 4. The reactive inkjet printing process produced silver particles. The ranges in diameter of silver particles obtained by 5, 10, and 15 printing cycles were 100– 300, 100–400, and 100–500 nm, respectively. Non-contiguous pattern was observed in Fig. 4(a). By increasing the number of printing cycles, the pattern obtained was more smooth and contiguous because of the agglomeration and coalescence of the small particles, as observed in Figs. 4(b) and 4(c). Silver conductive lines fabricated with different number of printing cycles showed no significant differences in morphology. The silver line obtained by five cycles shows highest electrical resistivity. The patterns obtained by 10 and 15 cycles show decreasing in electrical resistivity as 353 and 46.2  cm, respectively. The lowest resistivity obtained in this work is about 30 times lower than that of bulk silver, which is quite low compared with the values reported in other studies by using a similar system.28) The XRD patterns of silver line are shown in Fig. 5. One strong peak located at 37.42 and three weak peaks located at 43.68, 63.87, and 76.80 were detected. These are attributed to the (111), (200), (220), and (311) diffraction planes of the facecentered cubic structure of metallic silver. It is confirmed that the silver ions were reduced by ascorbic acid to silver element on the surface of printed patterns. 3.2 Effect of silver nitrate concentration on reactive inkjet printing process

The effect of silver nitrate concentration can be noticed from morphology of silver lines as shown in Fig. 6. The contiguous pattern could not be formed due to too low concentration of silver in the solution at 5 M. Silver lines fabricated with higher concentration of silver nitrate present a considerably different morphology from those fabricated

Fig. 6. SEM images of silver lines by five cycles of reactive printing of silver nitrate at concentration of (a) 5, (b) 7, and (c) 10 M.

from 5 M of silver nitrate. In place of interconnected particles, coral-like deposits were formed on the surface. The silver lines prepared from the 7 and 10 M solutions are contiguous with resistivity of 107 and 120  cm, respectively. An increase in silver nitrate concentration also increases the metal content in the printed pattern. However, by using higher concentration of silver salt, larger particles were obtained. These explanations are confirmed by SEM images of microstructure of the conductive patterns fabricated. The cavities were found all over the printed areas, which is very likely due to vigorous reaction between silver ion and reducing agent. Hence, the porosity and resistivity of the printed patterns increased. 3.3 Preparation of silver line from reactive sintering

The SEM images of silver line obtained by formic acid vapor reduction are shown in Fig. 7. The silver particles started to coalesce as the duration time exceeds 60 min. The resistivities of the silver patterns obtained by reactive sintering time for 30, 60, 90, and 120 min were 39900,

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Fig. 7. SEM images of silver lines by five cycles of reactive sintering with formic acid vapor at 150  C for (a) 30, (b) 60, (c) 90, and (d) 120 min. Fig. 9. XRD patterns of silver lines by reactive sintering with formic acid vapor at 150  C for 120 min.

Fig. 8. SEM images of silver lines by five cycles of reactive sintering with ethylene glycol vapor at 250  C for (a) 15, (b) 30, (c) 45, and (d) 60 min.

7620, 22.9, and 18.6  cm, respectively. The resistivity of the conductive patterns obtained drastically decreased with reactive sintering time. The minimum resistivity was obtained by 120 min reactive sintering time. For the reactive sintering process of ethylene glycol, the silver particles started to coalesce as the duration time exceeds 30 min as shown in Fig. 8. By increasing the reactive sintering time to 45 and 60 min, the grains of the silver patterns grew significantly. The resistivities of the printed patterns decreased from 712  cm at 15 min to 12  cm at 30 min. The lowest resistivity obtained is about eight times higher than that of bulk silver. It could be explained that the resistivity is dependent of the contact resistance between silver particles. The XRD patterns of silver lines obtained by reactive sintering with formic acid vapor and ethylene glycol vapor show peak of metallic silver as they can be seen in Figs. 9 and 10. It is confirmed that silver nitrate solution was reduced to silver particles by reactive sintering. 3.4 Preparation of copper line from reactive inkjet printing and reactive sintering

The copper lines were fabricated by reactive inkjet printing and reactive sintering using various conditions of formic

Fig. 10. XRD patterns of silver lines by reactive sintering with ethylene glycol vapor at 250  C for 30 min.

acid, ethylene glycol and hydrogen atmosphere. Due to the low percentage of copper content in the copper salt solution, the small particles and discontiguous patterns occurred after reduction and sintering process. Hence, the small copper particles immediately oxidized with oxygen in an atmosphere as it was previously reported by Li et al.23) Thus, the copper lines printed have no conductivity. 4. Conclusions

In reactive inkjet printing, the silver line was fabricated by sequentially printing reducing agent and silver nitrate solution. The lowest resistivity of silver line obtained by using 5 M of silver nitrate solution and 15 printing cycles was 46.2  cm. Alternatively, continuous and smooth patterns of silver lines were fabricated by the reactive sintering process of formic acid vapor and ethylene glycol vapor. The resistivity of the silver line obtained by formic acid vapor reduction at 150  C for 120 min was 18.6  cm.

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The resistivity of the silver line obtained by ethylene glycol vapor reduction at 250  C for 30 min was 12  cm, which is about eight times higher than that of bulk silver. As compared to reactive inkjet printing, the reactive sintering process gives more smooth pattern resulting in lower resistivity. In contrast, the copper lines were fabricated by reactive inkjet printing and reactive sintering using various conditions of formic acid, ethylene glycol and hydrogen atmosphere. The copper lines printed have no conductivity due to the formation of copper oxide.

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Acknowledgments

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This work was supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (EN636A) and Integrated Innovation Academic Center: IIAC Chulalongkorn University Centenary Academic Development Project (CU56-EN08), and Special Task Force for Activating Research (STAR), Chulalongkorn University Centenary Academic Development Project.

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