Appl Phys A (2009) 96: 933–938 DOI 10.1007/s00339-009-5262-7
Electrohydrodynamic micropatterning of silver ink using near-field electrohydrodynamic jet printing with tilted-outlet nozzle Doo-Hyeb Youn · Seong-Hyun Kim · Yong-Suk Yang · Sang-Chul Lim · Seong-Jin Kim · Su-Han Ahn · Hyo-Sun Sim · Seung-Myoung Ryu · Dong-Wook Shin · Ji-Beom Yoo
Received: 16 March 2009 / Accepted: 27 April 2009 / Published online: 14 May 2009 © Springer-Verlag 2009
Abstract This paper introduces for the first time near-field electrohydrodynamic jet printing with tilted-outlet nozzle to obtain the fine and highly conductive patterns of silver (Ag) ink. Line widths produced by near-field electrohydrodynamic jet printing are less than 6 µm, which is approximately twenty times smaller than that of inkjet printing. Under optimized Ag ink annealing ranges 3–9 min for 30 wt% at 150°C, we observed Ag line pattern resistivities as low as 7 × 10−6 ·cm. Ag ink conduction mechanisms were brought to light from microstructure analysis and postthermal-annealing examination of electrical characteristics. PACS 47.65.-d · 47.54.-r · 66.20.+d · 61.46.+w · 83.80.Gv 1 Introduction The ability to make fine micropatterns is of significant importance because it is key in the fabrication of high-density D.-H. Youn () · S.-H. Kim · Y.-S. Yang · S.-C. Lim IT Components and Materials Technology Research Division, Electronics and Telecommunications Research Institute, Daejeon 305-350, South Korea e-mail:
[email protected] S.-J. Kim Mitsubishi Cable Industries Ltd. 8, Nishino-cho, Amagasaki, Hyogo 660-0856, Japan S.-H. Ahn · H.-S. Sim · S.-M. Ryu DMK Co., Ltd. 17B-1L, 4 Gong-dan, Mosi-, jiksan-eup, Cheonan-si, Chungchengnam-do 305-350, South Korea D.-W. Shin · J.-B. Yoo School of Information and Communication Engineering and Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, South Korea
and conductive printed circuit boards (PCBs) [1, 2]. In direct-write approaches, patterns and structures can be produced without using masks, etching liquids, and photolithographic processes. The so-called direct writing technology is therefore low cost, high speed, contact free, and more environmentally friendly than conventional methods [3, 4]. As a direct write technology, inkjet printing is most frequently used for the fabrication of PCBs, organic thin-film transistor (OTFTs), and radio-frequency identification tags (RFIDs) [5–8]. However, inkjet printing methods have serious limitations on fabrication of fine (less than 50 µm in width), highly conductive (resistivities lower than 5 × 10−5 ·cm), and thick (greater than 0.5 µm) patterns for fabricating highdensity and high-speed electronic devices. In this study, to overcome the limitations of inkjet printing and gain a better understanding of the fabrication of high-density and highly conductive PCBs, we investigated near-field electrohydrodynamic (NFED) jet printing with tilted-outlet nozzle. NFED jet printing is a patterning method that uses a fine jet generated at the apex of the liquid cone of an electrospray in the cone-jet mode [9–14]. When liquid is forced through a nozzle where the air–liquid interface is electrically charged to about kilovolt potentials, the liquid meniscus takes the form of a stable cone, and the apex emits a microscopic jet. This fine jet, which is called a nanocolloid jet, consists of solid nanoparticles, liquid, and additives. After the jet impinges on a substrate to form a pattern, the nanoparticles remain on the substrate even after the liquid disappears by thermal evaporation. Consequently, NFED jet printing allows one to obtain fine patterns that consist of solid nanoparticles [15, 16]. The patterning of silver ink by this approach offers several advantages compared to inkjet printing. First, NFED jet printing can fabricate small micropatterns (less than 10
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Fig. 2 Schematic diagram of the near-field electrohydrodynamic jet printing
Fig. 1 Nozzle structures of the tilted-nozzle: (a) front view; (b) side view
µm in width) by preventing large droplet formation. Because the diameter of the tilted-outlet nozzle, become thin as it goes from the top to the end of a nozzle, is less than 10 µm at the apex of nozzle as shown in Fig. 1. Second, NFED jet printing has the merit of being able to fabricate fine patterns owing to the aforementioned cone-jet mode. Third, NFED jet printing can fabricate highly conductive thick patterns because NFED jet printing can pattern highly viscous ink. The conductivity of the printed metal pattern increases linearly with its thickness because of volume fraction of metal increases as already mentioned. For a given pattern width, nozzle diameters of NFED jet printing can be larger (>100 µm) than those (50 µm) used in inkjet printing hereafter referred as inkjet printing. These larger nozzles of NFED jet printing are less susceptible to blockage, which often plague inkjet printing processes. Thus a highly viscous Ag with weight percentage in excess of 20 wt% can be processed with relative ease. Tilted-outlet nozzle was used by Chen and coworkers for obtaining stable flow of cone-jet over the wide process range [17, 18]. However, the goal of this study was to deliver a tool for minimizing the diameter of jet flow and obtaining the fine patterns by means of NFED jet printing employing tilted-outlet nozzle.
2 Experimental details The experimental setup used for NFED jet printing is shown in Fig. 2. The system, fabricated from Tera-Leader Inc., consisted of a liquid supply system, an electrical system, and a movable stage system. The liquid supply system included a
syringe pump (kds-200, KD Scientific Inc.) with minimum flow rate of 0.1 µl/h. The nozzle diameters were 50 (inner) and 320 µm (outer). The Ag ink was injected downwards from the nozzle into a silicon substrate. The moving stage consisted of an X–Y moving part and a Y –Z moving robot, in which program controllable motion controller that communicates directly with a PC controlled the motion of substrate. The electrical system consisted of a high-voltage power supply (DC ∼30 kV) and two electrodes. The nozzle used for the Ag ink supply system served as the anode. The nozzle–substrate distance was maintained 3.0 mm during NFED jet printing. A commercial Ag ink (TEC-IJ-010 and TEC-PA-010 supplied from Ink-Tech. Inc.) was used. It consisted of ∼75 wt% Ag and 25 w% methanol. Other component of the ink was toluene (polymer resin) used to prevent Ag particle agglomeration. Once the Ag lines were jetted on to the silicon substrate, they were annealed at 150°C for 3, 6, and 9 min (for the 30 wt% of Ag ink) and for 10, 20, and 30 min (for the 15 wt% of Ag ink) by using a rapid thermal apparatus (RTA), respectively. The heating ramp of rapid thermal process was 10°C/min. Ag line patterns, 1.5 mm in width and 1.7 µm in height, were fabricated for measuring a resistance by means of a four-point probe method. The resistance of Ag line patterns was measured by passing a current through two of the four tips, usually the two outermost, and measuring the voltage drop across the two other tips. Further details of the four-point probe instrument and measurement procedure can be found elsewhere [19–21]. In this study, we introduced the tilted-outlet structure nozzle to obtain a fine pattern of a line width less than a few microns. The diameter of nozzle become thin as the diameter goes from the top to the apex of a nozzle. The microstructural evolution of the submicron Ag lines under var-
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Fig. 3 Microscopic nozzle structure of (a) a conventional rectangular nozzle, (b) a tilted-outlet nozzle. Microscopic image of printed silver patterns using (c) a conventional rectangular nozzle, (d) a tilted-outlet nozzle
ious annealing conditions was investigated in order to shed light on the post-annealing conduction mechanism.
3 Results and discussion The new nozzle structure of tilted-outlet nozzles, specially designed to fabricate fine Ag ink patterns, are shown in Fig. 1. A fine jet was obtained owing to the aforementioned cone-jet mode when ∼3 kV was applied between the nozzle and the grounded moving stage. The Ag ink jet reached the substrate without converting into spray. The flow rate of the syringe pump was maintained at 0.2 µl/h and the X–Y moving stage moved at the speed of 100 mm/s. The tilted-outlet nozzle reduces the diameter of the jet to micron size because the field distribution upon the tip decreases as the diameter of nozzle decreases. A key to achieving high resolution, from the standpoint of nozzle structure design, is the use of fine nozzles with sharp tips. Such nozzles lead directly to small droplets/streams. In addition, the low V values, result from electric-field-line focusing at the sharp tip, minimize the droplets size on the nozzle [22, 23]. The tilted-outlet nozzle used in this study effectively reduced the printed Ag line width from 63 (conventional nozzle) to 5.8 µm (tilted-outlet nozzle) as shown in Figs. 3c and d. This large decrease in droplet size, compared to that of the conventional rectangular nozzle (Fig. 3a), effectively
reduces the diameter of the Ag liquid jet because the magnitude of the applied electric field upon the sharp tip decreases as the diameter of nozzle decreases (Fig. 3b). To the best of our knowledge, this is one of the best results ever reported for the line width produced with conductive materials such as Ag inks. It is very difficult to form the micro-sized pattern for highly conductive liquids like Ag inks. The electrical relaxation time for highly conducting liquids is very short compared to liquids of low ion mobility, like, for example, polyvinyl alcohol (PVA) that results in faster charge accumulation on the jet surface, which overcomes the surface tension force. As a result, the Ag ink forms the straight jet instead of the conventional Taylor cone jet. The commercial nozzle which has the diameter less than 20 µm does not exist. Therefore it is very important to reduce the diameter of nozzle to obtain the micro-sized pattern by using a tilted-outlet nozzle. The micron-sized jets formed by the cone-jet mode are very difficult to stabilize. To obtain a stable jet, the growth rate of the jet should be decreased by applying high axial electric fields, which are imposed by the potential difference between the nozzle and the ground electrode. However, increasing the potential difference could not only increase the axial components of the electric fields but also increase the radial components. Increasing the radial components might amplify the growth rate of the cone jet and the diameter increase. The tilted-outlet nozzle used in this study effectively increased the axial electric fields but
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Fig. 4 Surface and the cross sectional scanning electron microscopy (SEM) images (a) and (d) as-deposited; (b) and (e) annealed at 150°C for 10 minutes; (c) and (f) annealed at 150°C for 20 minutes with RTA in N2 atmosphere
did not greatly change the radial electric fields, so the diameter of the cone-jet mode was reduced. Surface and the cross-sectional scanning electron microscopy (SEM) images of the 15 wt% Ag line after the following annealing conditions are displayed in Fig. 4: (a) and (d) as-deposited; (b) and (e) annealed at 150°C for 10 min; (c) and (f) annealed at 150°C for 20 min with RTA in N2 atmosphere. The as-received Ag particles were smaller than the annealed Ag particles. We have changed the annealing temperature from 50 to 150°C for 5 min in steps of 10°C. Particles sintered below 90°C showed no significant difference in particle shape compared with the asreceived ones (Figs. 4a and d). On the other hand, particles annealed at 150°C for 10 min show a dramatic increase in size (Figs. 4b and e). Increasing the annealing time from 10
to 20 min at 150°C resulted in gradually increasing particle sizes (Figs. 4c and f). The Ag pattern thickness decreased from 837 to 771 nm after annealing at 150°C. The surfaces of the Ag particles were connected, and dumbbell-type coalescence formed after annealing at 150°C. The size of the coalesced particles and Ag ink gradually increased with increasing annealing time. Electrical resistivity as a function of annealing time was also examined (Fig. 5). I –V characteristics of the Ag line pattern (15 wt% Ag content) were linearly ohmic. The DC resistance, calculated from the slope of the I –V line, fell from 44 (annealed at 150°C for 10 min) to 8 (annealed at 150°C for 30 min). Increasing the Ag volume fraction to 30 wt% as shown in the inset of Fig. 5, resulted in a reduction of DC resistance from 14 (annealed at 150°C for
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Fig. 5 I –V curves obtained for the Ag line pattern (15 wt% Ag content). The inset figure shows I –V curves for the Ag line pattern (30 wt% Ag content)
3 min) to 6 (annealed at 150°C for 9 min). This reduction was evident even with a shortened annealing time. Ag line pattern conductance increased with annealing time and Ag volume percentage. The lowest electrical resistivity of 7 × 10−6 ·cm was obtained (using four-point probe methods) from the pattern whose Ag weight percentage was 30 w%. The printed Ag pattern exhibited almost the same order of resistivity as the bulk Ag after the annealing process. The large decrease in resistivity after annealing is thought to correspond to the establishment of a physical contact between the conducting surfaces of the Ag particles. Figure 6 shows change in resistivity of Ag ink plotted as a function of annealing time in terms of coalescence, which occurred at the interface of Ag ink. Resistivities of the asdeposited patterns were 321 ·cm for 15 wt% and 17 ·cm for 30 wt%. The change in resistivity of the Ag particles sintered below 90°C was small and showed no significant difference for different annealing temperatures (50 to 90°C). On the other hand, the resistivity of particles annealed at 150°C for 10 min showed a dramatic decrease. Increasing annealing time from 10 to 20 min at 150°C results in a decrease in resistivity from 1.7 × 10−5 to 7 × 10−6 ·cm. Considering the above results, we assume that the Ag ink contact resistance depends on the following: resistance of the insulating layer (polymer resin) and the interfacial resistance between Ag particles. At the initial patterning stage, the resistivity of the as-printed pattern was very high because of the presence of the insulating polymer resin between Ag particles, which increased the resistance between them. An increase in volume percentage of Ag at the as-
Fig. 6 Resistivity change as a function of annealing temperatures. The open circle represents the resistivity of the as-deposited 15 wt% Ag pattern. The open rectangular represents the resistivity of the as-deposited 30 wt% Ag pattern. The open triangle represents the resistivity of the 15 wt% Ag pattern, annealed at 150°C for 20 minutes
deposited state results in decreased contact resistance between the Ag particles. This indicates that contact resistance between Ag particles has linear relationship with the distance between Ag particles because the distance between Ag particles decreases with the volume percentage of Ag particles increases. Even after evaporation of the polymer resin via annealing below 90°C, the resistivity of the Ag pattern was still high. This means that even when Ag particles are in physical contact with one another there is still a contact resistance at the interfacial tunneling barrier. After anneal-
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ing at above 100°C resistivity decreased abruptly because coalescence eliminated the interfacial tunneling barrier. The above observations indicate that complete formation of the conducting Ag ink proceeds via three stages: (1) shrinkage of Ag ink after evaporation of the insulating polymer resin under ambient air reduces the distance between Ag particles and induces the direct contact between them; (2) coalescence between Ag particles begins as the annealing temperature exceeds 100°C; (3) complete conduction pass formation through coalescence, which results in a sharp drop in resistivity. The size of the coalesced Ag particles gradually increased with increasing annealing time. The interfacial tunneling barrier between Ag particles diminished after annealing at above 100°C, electrons can then flow through the coalesced region between Ag particles. This reduces contact resistance. We thus propose that annealing will improve the electrical properties of the particles. The total contact resistance is minimized because Ag particle coalescence reduces total contact resistance.
4 Conclusions In this novel work, we have demonstrated that NFED jet printing using tilted-outlet nozzles can be used to fabricate very fine and highly conductive patterns deposited from Ag ink. We were able to obtain fine (5.8 µm in width) and conductive (7 × 10−7 ·cm) patterns. These results are very important for the eventual realization of high-density and highspeed PCBs. These obtained line widths are twenty times smaller than those produced by inkjet printing. The resistivity of the printed Ag line, annealed at below 90°C, did not change significantly. However, the resistivity of the printed Ag line, annealed at above 100°C, decreased abruptly due to annealing-induced Ag ink particle coalescence. A conductive pass was formed because of the coalescence. Acknowledgements This work was supported by a grant from the Ministry of Information and Communication in the South Korea, under project No. A1100-0601-0110. The authors would like to thank Dr. Sang-Yun Kim and Prof. Jung-Ho Hwang of the Yonsei University for helpful discussions; Mrs. Jung-Suck Lee of the Electronics
D.-H. Youn et al. and Telecommunications Research Institute (ETRI) for X-ray measurements, and Mrs. Sung-Ree Nam of ETRI for SEM Analysis.
References 1. H.M. Nur, J.H. Song, J. Mater. Sci., Mater. Electron. 13, 213 (2002) 2. P. Cooley, D. Wallac, B. Antohe, JALA 7, 33 (2002) 3. A.L. Dearden, P.J. Smith, D.-Y. Shin, N. Reis, B. Derby, P. O’Brien, Macromol. Rapid Commun. 26, 315 (2005) 4. K. Cheng, M.H. Yang, W.W.W. Chiu, C.Y. Huang, J. Chang, Y. Yang, Macromol. Rapid Commun. 26, 247 (2005) 5. Y. Noguchi, T. Sekitani, T. Someya, Appl. Phys. Lett. 89, 253507 (2006) 6. H. Sirringhaus, T. Kawase, R.H. Fiend, T. Shimoda, M. Inabasekaran, W. Wu, E.P. Woo, Science 290, 2123 (2000) 7. T. Kawase, S. Moriya, C.J. Newsome, T. Shimoda, Jpn. J. Appl. Phys. Part 1 44, 3649 (2005) 8. Y. Ishida, G. Nakagawa, T. Asano, Jpn. J. Appl. Phys. Part 1 46, 6437 (2007) 9. S.N. Jayasinghe, M.J. Edirisinghe, D.Z. Wang, Nanotechnology 15, 1519 (2004) 10. J.-U. Park, M. Hardy, S.J. Kang, K. Barton, K. Adair, D.K. Mukhopadhyay, Ch.Y. Lee, M.S. Strano, A.G. Alleyne, J.G. Georgiadis, P.M. Ferreira, J.A. Rogers, Nat. Mater. 6, 782 (2007) 11. D.Z. Wang, M.J. Edrisinghe, S.N. Jayasinghe, Jpn. J. Am. Ceram. Soc. 89, 1727 (2006) 12. D.Z. Wang, S.N. Jayasinghe, M.J. Edrisinghe, Z.B. Luklinska, J. Nanoparticle Res. 9, 825 (2007) 13. A. Jaworek, A.T. Sobczyk, J. Electrostatics 66, 197 (2008) 14. A. Jaworek, Power Technol. 176, 18 (2007) 15. H.F. Poon, Electrohydrodynamic printing, PhD thesis, Department of Chemical Engineering, Princeton University, 2002 16. J.H. Yu, S.Y. Kim, J. Hwang, Appl. Phys. A 89, 157 (2007) 17. C.H. Chen, M.H.J. Emond, E.M. Kelder, J. Schoonman, J. Aerosol. Sci. 30, 959 (1999) 18. S. Leeuwenburgh, J. Wolke, J. Schoonman, J. Jansen, J. Biomed. Mater. Res. A 66A, 330 (2003) 19. L. Gammelgaard, P. Boggild, J.W. Wells, K. Handrup, Ph. Hofmann, M.B. Balslev, J.E. Hansen, P.R.E. Petersen, Appl. Phys. Lett. 93, 093104 (2008) 20. J.W. Wells, J.F. Kallehauge, T.M. Hansen, P. Hofmann, Phys. Rev. Lett. 37, 206803 (2006) 21. J.W. Wells, J.F. Kallehauge, P. Hofmann, J. Phys., Condens. Matter 19, 176008 (2007) 22. J.U. Park, M. Hardy, S.J. Kang, K. Barton, K. Adair, C.Y. Lee, J.A. Rogers, Nat. Mater. 6, 782 (2007) 23. I.W. Lenggoro, B. Xia, K. Okuyama, Langmuir 18, 4584 (2002)
