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Fabrication of 10 µm-scale conductive Cu patterns by selective laser sintering of Cu complex ink
Optics & Laser Technology 88 (2017) 128–133
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Fabrication of 10 µm-scale conductive Cu patterns by selective laser sintering of Cu complex ink
crossmark
⁎
Hyungsuk Mina, Byoungyoon Leeb, Sooncheol Jeongb, Myeongkyu Leea, a b
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea R & D Center, Changsung Co. Ltd., Incheon 405-100, Republic of Korea
A R T I C L E I N F O
A BS T RAC T
Keywords: Cu complex ink Selective laser sintering Patterning Metallization
A Cu complex ink was synthesized using copper formate as a precursor and its potential for laser patterning was investigated. The Cu ink was spin-coated onto a substrate and the coated film was space-selectively sintered using a nanosecond-pulsed ultraviolet laser. The unexposed Cu ink could be removed from the film by rinsing it with the dispersing agent used to synthesize the ink, disclosing a conductive Cu pattern. A minimum resistivity of 8.46×10−5 Ω cm was obtained for the Cu lines with 10–20 µm widths. The feasibility of this method for metallization was demonstrated by fabricating a complex Cu electric circuit on an indium tin oxide-coated glass substrate. The selective laser sintering approach provides a simple, cost-effective alternative to conventional lithography for the production of electrode or metallization patterns.
1. Introduction Metallization is an essential process in the manufacturing of all electronic devices. For many applications within the microelectronic industry, the metallization patterns have dimensions on the order of micrometers. In this respect, direct-write techniques such as inkjet printing [1–3], screen printing [4–6], gravure, laser printing [7–10], and roll-to-roll printing have attracted growing interest as alternatives to conventional lithography that requires high-cost facilities and multiple process steps. Ink-based printed electronics is a promising low-cost, scalable method to fabricate a variety of electronic components and patterns. Metallization utilizing metal inks generally consist of two steps: selective deposition by printing (i.e., patterning) and subsequent sintering. The ink contains some additives, which play a role to improve printing characteristics and adhesion to the substrate. The sintering process renders conductivity to the printed ink. While Ag and Au are still dominant source materials for the metal inks, Cu is a very promising alternative because it exhibits fairly high bulk conductivity but is much cheaper than such noble metals. Various types of Cu inks have been synthesized, with their electrical behaviors investigated [11–15]. A common method to sinter printed inks is heat treatment in a furnace. However, the typical thermal sintering is an energy and time-consuming process. The involved long sintering time and high thermal load are especially problematic for plastic substrates that may be easily damaged at high temperatures. Some fast alternatives have thus been explored and they include electrical sintering [16], microwave sintering [17], and optical sintering using either a ⁎
flash lamp [18–20] or laser [21–28]. A great advantage of the laser sintering, compared to the other methods, is that it enables sintering and patterning to be simultaneously accomplished. In this synchronized process, a metal-ink film coated onto a substrate is selectively sintered using a focused laser beam and the unexposed leftover of the film is removed by rinsing it with a solvent. This can reveal a conductive metallic pattern. Since the line width of the pattern is fundamentally proportional to the laser beam size, the laser selective sintering can provide better resolution than the inkjet and screen printing processes whose resolutions are typically ~50 µm. A recent report by Zenou et al. [21] showed 6.7 µmCu line patterns generated by selective laser sintering of a Cu nanoparticle ink. However, the microstructure and electrical property of the fabricated patterns were not presented in the report. Niittynen et al. [25] compared laser and intense pulsed light sintering for inkjetprinted copper nanoparticle layers. Although the investigated Cu patterns were prepared by inkjet printing, a conductivity of more than 20% of that of bulk copper material was obtained with both sintering methods. Kwon et al. [26] recently obtained Cu line patterns of 20 µm width by selective laser sintering of copper nanoparticle paste coated on a polymer substrate. However, the electrical resistivity of the fabricated pattern, ~1.67×10−4 Ω m, was fairly high. Kang et al. [27] reported a single-step fabrication of copper electrode by laser-induced direct local reduction of copper oxide nanoparticles. Although a minimum line width of ~11 µm was achievable, the fabricated line pattern was discrete containing sporadic voids. Many different types of Cu inks have so far been developed to
Corresponding author. E-mail address: [email protected] (M. Lee).
http://dx.doi.org/10.1016/j.optlastec.2016.09.021 Received 22 July 2016; Received in revised form 23 August 2016; Accepted 14 September 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 88 (2017) 128–133
H. Min et al.
uted to the fact that Cu complexes are more resistant to oxidation than nanoparticles and that the Cu complex ink can thus be sintered without a strong reducing atmosphere such as H2. Nevertheless, little has been known on the laser patterning behaviors of Cu complex inks. Important features such as minimum line width, conductivity, and scalability have yet to be investigated. We synthesized a Cu formate-based complex ink and examined its potential for laser-direct patterning. In this paper, we show that highly conductive Cu patterns can be fabricated by selectively sintering the synthesized ink with an ultraviolet (UV) pulsed laser. It was found that spatial filtering of the laser beam is a crucial factor influencing the pattern quality. Void-free continuous patterns with 10– 20 µm line widths were obtained with a minimum resistivity of 8.46×10−5 Ω cm. The feasibility of this selective sintering method for metallization was demonstrated by fabricating a complex Cu electric circuit on a glass substrate. Fig. 1. Schematic of the optical setup used for laser-direct patterning.
2. Methods
produce conductive Cu films. Of course, the sintering mechanism and condition depend on the type of the ink. Cu complex inks have been extensively investigated in recent years [11–13,15,28]. This is attrib-
A Cu complex ink was synthesized using copper formate as a precursor. Copper (II) oxide (CuO) and formic acid (HCOOH) were mixed to form the copper formate: (Cu(COOH)2). The synthesized ink
Fig. 2. (a) Schematic illustration of the patterning mechanism, (b) digital camera images showing the evolution of a Cu ink layer into a conductive pattern, (c) light emission from a series of LEDs attached to a Cu electrode pattern.
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Fig. 3. SEM images of Cu line patterns produced on an ITO/glass substrate. The scan rate was fixed at 2 mm/s. The used laser power was (a) 1.8 W, (b) 2.4 W, (c) 3.6 W, and (d) 4.8 W.
the dispersing agent used when the ink was synthesized. This made only the sintered parts of the film remain on the substrate, thereby revealing a conductive pattern. The digital camera images of Fig. 2(b) shows how a Cu ink layer is evolved into a conductive pattern on the PI substrate. A Cu electrode pattern was formed on a PI substrate to see whether the process is applicable for the metallization of flexible substrates. The Cu pattern consisted of two electrodes: an L-shaped electrode and an inverted L-shaped electrode. A series of white light emitting diodes (LEDs) were then connected to the electrode pattern using a conductive silver adhesive. Fig. 2(c) shows light emitted from the attached LEDs. Even though the substrate was bent to a curvature shown in the figure, no mechanical failure or performance degradation was observed. This implies that the laser-processed Cu electrodes remain firmly adhesive to the substrate. Naked eyes could not detect any discernible change in the LED intensity under this qualitatively performed bending test. The Cu patterns shown in Fig. 2 were fabricated using a defocused laser beam that had a spot size of ~0.5 mm on the film surface, where the sample was vertically displaced from the focal plane by 4 cm. To find out the minimum line width available with this process, we first carried out patterning by directly focusing the output laser beam onto the film surface. That is, the beam expander and aperture depicted in Fig. 1 were not used at first. When the whole film is sintered in a lineby-line fashion using a laser beam, the power and scan rate of the beam are two major variables that determine the microstructure and resistivity of the sintered film. It was found that when conductive patterns are to be formed by selective sintering, the beam profile is a critical factor strongly influencing the morphology and uniformity of the pattern. When the coated film was scanned using a laser spot directly focused from the output beam, the central region of the sintered line patterns was always ablated regardless of the used substrates. Fig. 3 shows the results obtained with an ITO/glass substrate. As the laser power increased, the width of the produced line also increased. Once the scan rate is fixed, more energy will be absorbed by the film as the laser power increases. This widens the sintered region due to thermal diffusion. The formation of an ablated or hollow structure in the line
was dispersed in isopropyl alcohol (IPA). More detailed synthetic process may be referred to earlier reports [15,24]. The synthesized Cu ink was coated onto a substrate by spin coating. Three different types of substrates (polyimide (PI), slide glass, and indium tin oxide (ITO)-coated glass) were used in this study. The coated film was kept inside an oven at 70 °C for 10 min for drying. The PI substrate (type: IF70, thickness: 50 µm, stable temperature range: < 400 °C) was supplied from SKC Inc. The ITO/glass substrate (thickness=0.7 mm), supplied from Samsung Corning Inc., has an ITO thickness of 145 nm and a sheet resistance of 10 Ω/sq. Fig. 1 is a schematic of the optics setup used for laser-direct patterning. A nanosecond-pulsed UV laser at 355 nm (model: Coherent AVIA 355-5) was employed as the laser source. The pulse width and maximum output power of the laser are < 20 ns and 4.2 W, respectively. While the repetition rate is more or less variable, it was operated at 30 kHz. The Gaussian-profile output beam (1/e-diameter=2.85 mm) was enlarged by a beam expander (8X). Then, the beam was spatially filtered using a circular aperture of 3 mm diameter so that only the central part of the expanded beam can go into a galvanometric scanner. The galvanometric scanner was combined with an F-theta lens (focal length=205 mm) to generate and steer a uniform laser spot on the sample surface. The used galvanometer has a maximum speed of 65,000 mm/s. The laser power and scan rate were independently varied. Nitrogen gas was blown into the sample region exposed by the laser spot in order to prevent potential oxidation. The gas gun was stationed on a computer-controlled translation stage, which was made to move in the same trajectory as the laser spot. Structural analysis was carried out by scanning electron microscopy (SEM, model: JEOL JSM7001F, 15 kV). 3. Results and discussion Fig. 2(a) schematically illustrates the patterning mechanism. A Cu ink layer was deposited onto a substrate by spin coating, followed by drying at 70 °C. The dried film, which was still sticky, was selectively sintered using a laser beam. The unexposed leftover of the coated ink could be rinsed away by immersing the whole film in IPA, which was 130
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Fig. 4. Ablation-free Cu lines formed on (a) polyimide, (b) ITO/glass substrate. Insets show their microstructures.
Fig. 5. (a) Uniform line pattern formed on a bare glass substrate, (b) Cross-sectional image of a line pattern, (c) I-V curve measured for a Cu line formed on a glass substrate.
the case where the intensity and scan rate of the beams are identical, the film is irradiated for a longer time as the beam size increases. This may induce significantly different microstructures. In order to derive the resistivity of a line pattern, its thickness should be known. When the PI sample was cut using a knife for crosssectional analysis, the pattern edge was easily smeared out. This made it difficult to precisely define the thickness. For the pattern fabricated on an ITO/glass substrate, its resistivity cannot be derived simply by measuring the sheet resistance, because the underlying ITO layer is also conducting. Therefore, Cu line patterns were produced on bare glass substrates. While the available line width was reduced to ~12 µm on this insulating substrate, apparently less compact microstructures were obtained (Fig. 5(a)). It seems that these are all related to thermal diffusion through the ITO layer. To produce fine patterns, the heat generated by a scanning laser beam should be highly localized. Since ITO is a better thermal conductor than glass, localized heating will be more difficult on the ITO-coated substrate when all other conditions are the same. This might have increased the attainable line width. However, further detailed analyses will be necessary to find out the precise cause of the different microstructures. Fig. 5(b) shows the cross-sectional SEM image of a Cu line pattern produced on a glass substrate. The resistivity of the line pattern was deduced from the I-V relation measured using a probe with two tips separated by 1 mm. Fig. 5(c) shows an I-V curve. The resistivity was calculated using the slope of the obtained I-V curve and the pattern width and thickness. A minimum resistivity of
center has also been reported in the works carried out with Cu nanoparticle inks and CuO solutions [21,27]. This is attributable to the nonuniform spatial profile associated with a Gaussian beam. Its center and edge exhibit a very large difference in the local intensity. Although the center ablation could be mitigated more or less by independent adjustments of the laser power and scan rate, it was not completely avoided once the output beam was incident into the galvanometer without spatial filtering. The output beam was thus expanded by a beam expander (8X), followed by spatial filtering with an aperture. The laser spot size at focus was calculated to be ~30 µm. The spatial filtering worked well to prevent the line center from being ablated, as shown in Fig. 4. Ablation-free Cu patterns with a minimum line width of ~15 µm could be fabricated on both PI and ITO/glass substrates. This line width was achieved at the conditions of output power=1.8 W and scan rate=2 mm/s for PI and 3.6 W and 1.5 mm/s for ITO/glass. The actual laser powers arriving at the film surface will be lower than these values because a substantial portion of the output beam was blocked. The available line width was smaller than the estimated laser spot size. Although the beam was spatially filtered, its local intensity will be still higher at the beam center. Sintering is likely to occur only when the local intensity is above a certain threshold value. If this is the case, the conductive line may have a smaller width than the laser spot size. The microstructures of the line patterns were different from those obtained when the whole film was laser-sintered using an unfocused beam [24]. The dissimilar microstructures seem related to the different beam size and irradiation conditions. Even in
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Fig. 6. (a) Digital camera picture of a Cu circuit pattern fabricated on an ITO/glass substrate, (b) graphic image of the electric circuit used to produce the Cu pattern shown in “a”. (c) Optical micrograph of the fabricated Cu pattern, (d)–(g) SEM images taken at different positions of the pattern.
8.46×10−5 Ω cm was obtained. This value is about 50 times the bulk resistivity of Cu (1.76×10−6 Ω cm). The minimum resistivity of the line pattern was also 5-fold higher than the value achievable without patterning. This resistivity discrepancy may be accounted for by two reasons. Although the electrical resistivity is an intrinsic material property, it may actually be feature size-dependent in the case of patterned thin films [29]. Compared to the large-area sintering, the laser patterning process utilizes a tightly focused, much smaller beam. A slight change in the process parameters (laser power and scan rate, etc.) may lead to significantly different results, often causing detrimental effects. Therefore, the degree of freedom in changing the process variables is more limited. The line width and resistivity given in this study represent those achievable without any void in the film and damage to the substrate. ITO-coated glasses are the most widely used transparent conducting substrates for modern electronic devices such as flat panel displays, touch screen panels, organic light emitting diodes, and solar cells. These devices are generally operated by metallic electrodes. Thus, the
development of a simple, economical metallization process on the ITOglass substrate is of technological significance. To demonstrate the feasibility of the aforementioned method for metallization, a complex Cu pattern was fabricated on an ITO/glass substrate. Fig. 6(a) shows a digital camera picture of the fabricated pattern. To produce this pattern, the graphic image of an electric circuit shown in Fig. 6(b) was converted to a bitmap image composed of pixels. Each pixel has a black/white depth that ultimately determines the laser exposure time. The circuit pattern was reproduced over an area of 15 mm×12 mm and the used pixel density was 2400 dpi. The calculated laser spot size was approximately 30 µm and the exposure time on each pixel was set to be either 20 ms or 0. Since each pixel has a side of 10.6 µm, the designated pixel density is high enough, considering the laser spot size. As compared in Fig. 6(a) and (b), the fabricated Cu pattern was consistent with the original circuit image. Fig. 6(c) is a reflection-mode optical microscopic image of the circuit pattern. Some SEM images taken at different positions of the pattern are shown in Fig. 6(d)–(g). This complex circuit was inadvertently selected to verify the 132
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characteristics of screen-printed Ag thin film, Electron Mater. Lett. 8 (2012) 309. [7] J. Bohandy, B. Kim, F. Adrian, Metal deposition from a supported metal film using an excimer laser, J. Appl. Phys. 60 (1986) 1538. [8] H. Shin, H. Lee, J. Sung, M. Lee, Parallel laser printing of nanoparticulate silver thin film patterns for electronics, Appl. Phys. Lett. 92 (2008) 233107. [9] C. Visser, R. Pohl, C. Sun, G. Romer, V. Huis in’t, D. Lohse, Toward 3D printing of pure metals by laser-induced forward transfer, Adv. Mater. 27 (2015) 4087. [10] M. Tseng, et al., Fabrication of plasmonic devices using femtosecond laser-induced forward transfer technique, Nanotechnology 23 (2012) 444013. [11] B. Lee, Y. Kim, S. Yang, I. Jeong, J. Moon, A low-cure-temperature copper nano ink for highly conductive printed electrodes, Curr. Appl. Phys. 9 (2009) E157. [12] A. Yabuki, S. Tanaka, Electrically conductive copper film prepared at low temperature by thermal decomposition of copper amine complexes with various amines, Mater. Res. Bull. 47 (2012) 4107. [13] S. Kim, J. Lee, Y. Choi, D. Yeon, Y. Byun, Effect of copper concentration in printable copper inks on film fabrication, Thin Solid Films 520 (2012) 2731. [14] K. Ida, M. Tomonari, Y. Sugiyama, Y. Chujyo, T. Tokunaga, T. Yonezawa, K. Kuroda, K. Sasaki, Behavior of Cu nanoparticles ink under reductive calcination for fabrication of Cu conductive film, Thin Solid Films 520 (2012) 2789. [15] D. Shin, S. Woo, H. Yem, M. Cha, S. Cho, M. Kang, S. Jeong, Y. Kim, K. Kang, Y. Piao, A self-reducible and alcohol-soluble copper-based metal–organic decomposition ink, ACS Appl. Mater. Interfaces 6 (2014) 3312. [16] M. Allen, M. Aronniemi, T. Mattila, A. Alastalo, K. Ojanpera, M. Suhonen, H. Seppa, Electrical sintering of nanoparticle structures, Nanotechnology 19 (2008) 175201. [17] J. Perelaer, B. de Gans, U. Schubert, Ink-jet printing and microwave sintering of conductive silver tracks, Adv. Mater. 18 (2006) 2101. [18] H. Hwang, W. Chung, H. Kim, In situ monitoring of flash-light sintering of copper nanoparticle ink for printed electronics, Nanotechnology 23 (2012) 485205. [19] S. Park, H. Kim, Flash light sintering of nickel nanoparticles for printed electronics, Thin Solid Films 550 (2014) 575. [20] S. Joo, S. Park, C. Moon, H. Kim, A highly reliable copper nanowire/nanoparticle ink pattern with high conductivity on flexible substrate prepared via flash lightsintering technique, Acs. Appl. Mater. Interfaces 7 (2015) 5674. [21] M. Zenou, O. Ermak, A. Saar, Z. Kotler, Laser sintering of copper nanoparticles, J. Phys. D: Appl. Phys. 47 (2014) 025501. [22] L. Schade, S. Franzka, S. Hardt, H. Wiggers, N. Hartmann, Sintering of thin titanium dioxide nanoparticle films via photothermal processing with ultraviolet continuous-wave lasers, Appl. Surf. Sci. 278 (2013) 336. [23] T. Kumpulainen, J. Pekkanen, J. Valkama, J. Laakso, R. Ruokko, M. Mantysalo, Low temperature nanoparticle sintering with continuous wave and pulse lasers, Opt. Laser Technol. 43 (2011) 570. [24] J. Lee, B. Lee, S. Jeong, Y. Kim, M. Lee, Microstructure and electrical property of laser-sintered Cu complex ink, Appl. Surf. Sci. 307 (2014) 42. [25] J. Niittynen, E. Sowade, H. Kang, R. Baumann, M. Mantysalo, Comparison of laser and intense pulsed light for inkjet-printed copper nanoparticle layers, Sci. Rep. 5 (2015) 8832. [26] J. Kwon, H. Cho, H. Eom, H. Lee, Y. Suh, H. Moon, J. Shin, S. Hong, S. Ko, Lowtemperature oxidation-free selective laser sintering of Cu nanoparticle paste on a polymer substrate for the flexible touch panel applications, ACS Appl. Mater. Interfaces 8 (2016) 11575. [27] B. Kang, S. Han, J. Kim, S. Ko, M. Yang, One-step fabrication of copper electrode by laser-induced direct local reduction and agglomeration of copper oxide nanoparticle, J. Phys. Chem. C 115 (2011) 23664. [28] H. Min, B. Lee, S. Jeong, M. Lee, Laser-direct process of Cu nano-ink to coat highly conductive and adhesive metallization patterns on plastic substrate, Opt. Lasers Eng. 80 (2016) 12. [29] Y. Tao, Y. Tao, L. Wang, B. Wang, Z. Yang, Y. Tai, High-reproducibility, flexible conductive patterns fabricated with silver nanowire by drop or fit-to-flow method, Nanoscale Res. Lett. 8 (2013) 147.
applicability of the presented approach for metallization. Since the laser irradiation had to be carried out in a spot-by-spot fashion, it took rather a long time of 3000 s to finish the process. A simpler pattern that consists only of vertical and horizontal lines would be produced in a much shorter time, because a laser spot can be scanned over the film in this case. 4. Conclusion The development of a simple, economical metallization process on transparent substrates is of technological significance. In this study, we synthesized a Cu complex ink using copper formate as a precursor and investigated its potential for laser-direct patterning. A Cu ink film spincoated onto a substrate was selectively sintered using a nanosecondpulsed UV laser beam. The unexposed leftover of the coated ink could be removed by rinsing the whole film in IPA, the dispersing agent of the synthesized ink. This revealed a conductive Cu pattern. A minimum resistivity of 8.46×10−5 Ω cm was obtained for the Cu lines with 10– 20 µm widths. To demonstrate the feasibility of this method for metallization, a complex Cu electric circuit was fabricated on an ITOcoated glass substrate using a laser spot steered by a high-speed galvanometer. The fabricated Cu pattern was in good agreement with the circuit image to be duplicated. This selective-sintering approach provides a simple, cost-effective alternative to conventional photolithography for the production of electrode or metallization patterns. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A15053000). The authors declare no competing financial interest. References [1] B. Sirringhaus, T. Kawase, R. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E. Woo, High-resolution inkjet printing of all-polymer transistor circuits, Science 290 (2000) 2123. [2] B. Ridley, B. Nivi, J. Jacobson, All-inorganic field effect transistors fabricated by printing, Science 286 (1999) 746. [3] L. Li, Y. Guo, X. Zhang, Y. Song, Inkjet-printed highly conductive transparent patterns with water based Ag-doped graphene, J. Mater. Chem. A 2 (2014) 19095. [4] W. Liao, T. Chou, Fabrication of a planar-form screen-printed solid electrolyte modified Ag/AgCl reference electrode for application in a potentiometric biosensor 2006, Anal. Chem. 78 (2006) 4219. [5] V. Narayanan, The fire-through processed screen-printed Ag thick film metal contacts formed on an electrochemically etched porous silicon antireflection coating of silicon solar cells, RSC Adv. 3 (2013) 3618. [6] K. Kim, W. Myung, S. Jung, Effects of sintering conditions on microstructure and
133
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Fabrication of 10 µm-scale conductive Cu patterns by selective laser sintering of Cu complex ink
crossmark
⁎
Hyungsuk Mina, Byoungyoon Leeb, Sooncheol Jeongb, Myeongkyu Leea, a b
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea R & D Center, Changsung Co. Ltd., Incheon 405-100, Republic of Korea
A R T I C L E I N F O
A BS T RAC T
Keywords: Cu complex ink Selective laser sintering Patterning Metallization
A Cu complex ink was synthesized using copper formate as a precursor and its potential for laser patterning was investigated. The Cu ink was spin-coated onto a substrate and the coated film was space-selectively sintered using a nanosecond-pulsed ultraviolet laser. The unexposed Cu ink could be removed from the film by rinsing it with the dispersing agent used to synthesize the ink, disclosing a conductive Cu pattern. A minimum resistivity of 8.46×10−5 Ω cm was obtained for the Cu lines with 10–20 µm widths. The feasibility of this method for metallization was demonstrated by fabricating a complex Cu electric circuit on an indium tin oxide-coated glass substrate. The selective laser sintering approach provides a simple, cost-effective alternative to conventional lithography for the production of electrode or metallization patterns.
1. Introduction Metallization is an essential process in the manufacturing of all electronic devices. For many applications within the microelectronic industry, the metallization patterns have dimensions on the order of micrometers. In this respect, direct-write techniques such as inkjet printing [1–3], screen printing [4–6], gravure, laser printing [7–10], and roll-to-roll printing have attracted growing interest as alternatives to conventional lithography that requires high-cost facilities and multiple process steps. Ink-based printed electronics is a promising low-cost, scalable method to fabricate a variety of electronic components and patterns. Metallization utilizing metal inks generally consist of two steps: selective deposition by printing (i.e., patterning) and subsequent sintering. The ink contains some additives, which play a role to improve printing characteristics and adhesion to the substrate. The sintering process renders conductivity to the printed ink. While Ag and Au are still dominant source materials for the metal inks, Cu is a very promising alternative because it exhibits fairly high bulk conductivity but is much cheaper than such noble metals. Various types of Cu inks have been synthesized, with their electrical behaviors investigated [11–15]. A common method to sinter printed inks is heat treatment in a furnace. However, the typical thermal sintering is an energy and time-consuming process. The involved long sintering time and high thermal load are especially problematic for plastic substrates that may be easily damaged at high temperatures. Some fast alternatives have thus been explored and they include electrical sintering [16], microwave sintering [17], and optical sintering using either a ⁎
flash lamp [18–20] or laser [21–28]. A great advantage of the laser sintering, compared to the other methods, is that it enables sintering and patterning to be simultaneously accomplished. In this synchronized process, a metal-ink film coated onto a substrate is selectively sintered using a focused laser beam and the unexposed leftover of the film is removed by rinsing it with a solvent. This can reveal a conductive metallic pattern. Since the line width of the pattern is fundamentally proportional to the laser beam size, the laser selective sintering can provide better resolution than the inkjet and screen printing processes whose resolutions are typically ~50 µm. A recent report by Zenou et al. [21] showed 6.7 µmCu line patterns generated by selective laser sintering of a Cu nanoparticle ink. However, the microstructure and electrical property of the fabricated patterns were not presented in the report. Niittynen et al. [25] compared laser and intense pulsed light sintering for inkjetprinted copper nanoparticle layers. Although the investigated Cu patterns were prepared by inkjet printing, a conductivity of more than 20% of that of bulk copper material was obtained with both sintering methods. Kwon et al. [26] recently obtained Cu line patterns of 20 µm width by selective laser sintering of copper nanoparticle paste coated on a polymer substrate. However, the electrical resistivity of the fabricated pattern, ~1.67×10−4 Ω m, was fairly high. Kang et al. [27] reported a single-step fabrication of copper electrode by laser-induced direct local reduction of copper oxide nanoparticles. Although a minimum line width of ~11 µm was achievable, the fabricated line pattern was discrete containing sporadic voids. Many different types of Cu inks have so far been developed to
Corresponding author. E-mail address: [email protected] (M. Lee).
http://dx.doi.org/10.1016/j.optlastec.2016.09.021 Received 22 July 2016; Received in revised form 23 August 2016; Accepted 14 September 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 88 (2017) 128–133
H. Min et al.
uted to the fact that Cu complexes are more resistant to oxidation than nanoparticles and that the Cu complex ink can thus be sintered without a strong reducing atmosphere such as H2. Nevertheless, little has been known on the laser patterning behaviors of Cu complex inks. Important features such as minimum line width, conductivity, and scalability have yet to be investigated. We synthesized a Cu formate-based complex ink and examined its potential for laser-direct patterning. In this paper, we show that highly conductive Cu patterns can be fabricated by selectively sintering the synthesized ink with an ultraviolet (UV) pulsed laser. It was found that spatial filtering of the laser beam is a crucial factor influencing the pattern quality. Void-free continuous patterns with 10– 20 µm line widths were obtained with a minimum resistivity of 8.46×10−5 Ω cm. The feasibility of this selective sintering method for metallization was demonstrated by fabricating a complex Cu electric circuit on a glass substrate. Fig. 1. Schematic of the optical setup used for laser-direct patterning.
2. Methods
produce conductive Cu films. Of course, the sintering mechanism and condition depend on the type of the ink. Cu complex inks have been extensively investigated in recent years [11–13,15,28]. This is attrib-
A Cu complex ink was synthesized using copper formate as a precursor. Copper (II) oxide (CuO) and formic acid (HCOOH) were mixed to form the copper formate: (Cu(COOH)2). The synthesized ink
Fig. 2. (a) Schematic illustration of the patterning mechanism, (b) digital camera images showing the evolution of a Cu ink layer into a conductive pattern, (c) light emission from a series of LEDs attached to a Cu electrode pattern.
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Fig. 3. SEM images of Cu line patterns produced on an ITO/glass substrate. The scan rate was fixed at 2 mm/s. The used laser power was (a) 1.8 W, (b) 2.4 W, (c) 3.6 W, and (d) 4.8 W.
the dispersing agent used when the ink was synthesized. This made only the sintered parts of the film remain on the substrate, thereby revealing a conductive pattern. The digital camera images of Fig. 2(b) shows how a Cu ink layer is evolved into a conductive pattern on the PI substrate. A Cu electrode pattern was formed on a PI substrate to see whether the process is applicable for the metallization of flexible substrates. The Cu pattern consisted of two electrodes: an L-shaped electrode and an inverted L-shaped electrode. A series of white light emitting diodes (LEDs) were then connected to the electrode pattern using a conductive silver adhesive. Fig. 2(c) shows light emitted from the attached LEDs. Even though the substrate was bent to a curvature shown in the figure, no mechanical failure or performance degradation was observed. This implies that the laser-processed Cu electrodes remain firmly adhesive to the substrate. Naked eyes could not detect any discernible change in the LED intensity under this qualitatively performed bending test. The Cu patterns shown in Fig. 2 were fabricated using a defocused laser beam that had a spot size of ~0.5 mm on the film surface, where the sample was vertically displaced from the focal plane by 4 cm. To find out the minimum line width available with this process, we first carried out patterning by directly focusing the output laser beam onto the film surface. That is, the beam expander and aperture depicted in Fig. 1 were not used at first. When the whole film is sintered in a lineby-line fashion using a laser beam, the power and scan rate of the beam are two major variables that determine the microstructure and resistivity of the sintered film. It was found that when conductive patterns are to be formed by selective sintering, the beam profile is a critical factor strongly influencing the morphology and uniformity of the pattern. When the coated film was scanned using a laser spot directly focused from the output beam, the central region of the sintered line patterns was always ablated regardless of the used substrates. Fig. 3 shows the results obtained with an ITO/glass substrate. As the laser power increased, the width of the produced line also increased. Once the scan rate is fixed, more energy will be absorbed by the film as the laser power increases. This widens the sintered region due to thermal diffusion. The formation of an ablated or hollow structure in the line
was dispersed in isopropyl alcohol (IPA). More detailed synthetic process may be referred to earlier reports [15,24]. The synthesized Cu ink was coated onto a substrate by spin coating. Three different types of substrates (polyimide (PI), slide glass, and indium tin oxide (ITO)-coated glass) were used in this study. The coated film was kept inside an oven at 70 °C for 10 min for drying. The PI substrate (type: IF70, thickness: 50 µm, stable temperature range: < 400 °C) was supplied from SKC Inc. The ITO/glass substrate (thickness=0.7 mm), supplied from Samsung Corning Inc., has an ITO thickness of 145 nm and a sheet resistance of 10 Ω/sq. Fig. 1 is a schematic of the optics setup used for laser-direct patterning. A nanosecond-pulsed UV laser at 355 nm (model: Coherent AVIA 355-5) was employed as the laser source. The pulse width and maximum output power of the laser are < 20 ns and 4.2 W, respectively. While the repetition rate is more or less variable, it was operated at 30 kHz. The Gaussian-profile output beam (1/e-diameter=2.85 mm) was enlarged by a beam expander (8X). Then, the beam was spatially filtered using a circular aperture of 3 mm diameter so that only the central part of the expanded beam can go into a galvanometric scanner. The galvanometric scanner was combined with an F-theta lens (focal length=205 mm) to generate and steer a uniform laser spot on the sample surface. The used galvanometer has a maximum speed of 65,000 mm/s. The laser power and scan rate were independently varied. Nitrogen gas was blown into the sample region exposed by the laser spot in order to prevent potential oxidation. The gas gun was stationed on a computer-controlled translation stage, which was made to move in the same trajectory as the laser spot. Structural analysis was carried out by scanning electron microscopy (SEM, model: JEOL JSM7001F, 15 kV). 3. Results and discussion Fig. 2(a) schematically illustrates the patterning mechanism. A Cu ink layer was deposited onto a substrate by spin coating, followed by drying at 70 °C. The dried film, which was still sticky, was selectively sintered using a laser beam. The unexposed leftover of the coated ink could be rinsed away by immersing the whole film in IPA, which was 130
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Fig. 4. Ablation-free Cu lines formed on (a) polyimide, (b) ITO/glass substrate. Insets show their microstructures.
Fig. 5. (a) Uniform line pattern formed on a bare glass substrate, (b) Cross-sectional image of a line pattern, (c) I-V curve measured for a Cu line formed on a glass substrate.
the case where the intensity and scan rate of the beams are identical, the film is irradiated for a longer time as the beam size increases. This may induce significantly different microstructures. In order to derive the resistivity of a line pattern, its thickness should be known. When the PI sample was cut using a knife for crosssectional analysis, the pattern edge was easily smeared out. This made it difficult to precisely define the thickness. For the pattern fabricated on an ITO/glass substrate, its resistivity cannot be derived simply by measuring the sheet resistance, because the underlying ITO layer is also conducting. Therefore, Cu line patterns were produced on bare glass substrates. While the available line width was reduced to ~12 µm on this insulating substrate, apparently less compact microstructures were obtained (Fig. 5(a)). It seems that these are all related to thermal diffusion through the ITO layer. To produce fine patterns, the heat generated by a scanning laser beam should be highly localized. Since ITO is a better thermal conductor than glass, localized heating will be more difficult on the ITO-coated substrate when all other conditions are the same. This might have increased the attainable line width. However, further detailed analyses will be necessary to find out the precise cause of the different microstructures. Fig. 5(b) shows the cross-sectional SEM image of a Cu line pattern produced on a glass substrate. The resistivity of the line pattern was deduced from the I-V relation measured using a probe with two tips separated by 1 mm. Fig. 5(c) shows an I-V curve. The resistivity was calculated using the slope of the obtained I-V curve and the pattern width and thickness. A minimum resistivity of
center has also been reported in the works carried out with Cu nanoparticle inks and CuO solutions [21,27]. This is attributable to the nonuniform spatial profile associated with a Gaussian beam. Its center and edge exhibit a very large difference in the local intensity. Although the center ablation could be mitigated more or less by independent adjustments of the laser power and scan rate, it was not completely avoided once the output beam was incident into the galvanometer without spatial filtering. The output beam was thus expanded by a beam expander (8X), followed by spatial filtering with an aperture. The laser spot size at focus was calculated to be ~30 µm. The spatial filtering worked well to prevent the line center from being ablated, as shown in Fig. 4. Ablation-free Cu patterns with a minimum line width of ~15 µm could be fabricated on both PI and ITO/glass substrates. This line width was achieved at the conditions of output power=1.8 W and scan rate=2 mm/s for PI and 3.6 W and 1.5 mm/s for ITO/glass. The actual laser powers arriving at the film surface will be lower than these values because a substantial portion of the output beam was blocked. The available line width was smaller than the estimated laser spot size. Although the beam was spatially filtered, its local intensity will be still higher at the beam center. Sintering is likely to occur only when the local intensity is above a certain threshold value. If this is the case, the conductive line may have a smaller width than the laser spot size. The microstructures of the line patterns were different from those obtained when the whole film was laser-sintered using an unfocused beam [24]. The dissimilar microstructures seem related to the different beam size and irradiation conditions. Even in
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Fig. 6. (a) Digital camera picture of a Cu circuit pattern fabricated on an ITO/glass substrate, (b) graphic image of the electric circuit used to produce the Cu pattern shown in “a”. (c) Optical micrograph of the fabricated Cu pattern, (d)–(g) SEM images taken at different positions of the pattern.
8.46×10−5 Ω cm was obtained. This value is about 50 times the bulk resistivity of Cu (1.76×10−6 Ω cm). The minimum resistivity of the line pattern was also 5-fold higher than the value achievable without patterning. This resistivity discrepancy may be accounted for by two reasons. Although the electrical resistivity is an intrinsic material property, it may actually be feature size-dependent in the case of patterned thin films [29]. Compared to the large-area sintering, the laser patterning process utilizes a tightly focused, much smaller beam. A slight change in the process parameters (laser power and scan rate, etc.) may lead to significantly different results, often causing detrimental effects. Therefore, the degree of freedom in changing the process variables is more limited. The line width and resistivity given in this study represent those achievable without any void in the film and damage to the substrate. ITO-coated glasses are the most widely used transparent conducting substrates for modern electronic devices such as flat panel displays, touch screen panels, organic light emitting diodes, and solar cells. These devices are generally operated by metallic electrodes. Thus, the
development of a simple, economical metallization process on the ITOglass substrate is of technological significance. To demonstrate the feasibility of the aforementioned method for metallization, a complex Cu pattern was fabricated on an ITO/glass substrate. Fig. 6(a) shows a digital camera picture of the fabricated pattern. To produce this pattern, the graphic image of an electric circuit shown in Fig. 6(b) was converted to a bitmap image composed of pixels. Each pixel has a black/white depth that ultimately determines the laser exposure time. The circuit pattern was reproduced over an area of 15 mm×12 mm and the used pixel density was 2400 dpi. The calculated laser spot size was approximately 30 µm and the exposure time on each pixel was set to be either 20 ms or 0. Since each pixel has a side of 10.6 µm, the designated pixel density is high enough, considering the laser spot size. As compared in Fig. 6(a) and (b), the fabricated Cu pattern was consistent with the original circuit image. Fig. 6(c) is a reflection-mode optical microscopic image of the circuit pattern. Some SEM images taken at different positions of the pattern are shown in Fig. 6(d)–(g). This complex circuit was inadvertently selected to verify the 132
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applicability of the presented approach for metallization. Since the laser irradiation had to be carried out in a spot-by-spot fashion, it took rather a long time of 3000 s to finish the process. A simpler pattern that consists only of vertical and horizontal lines would be produced in a much shorter time, because a laser spot can be scanned over the film in this case. 4. Conclusion The development of a simple, economical metallization process on transparent substrates is of technological significance. In this study, we synthesized a Cu complex ink using copper formate as a precursor and investigated its potential for laser-direct patterning. A Cu ink film spincoated onto a substrate was selectively sintered using a nanosecondpulsed UV laser beam. The unexposed leftover of the coated ink could be removed by rinsing the whole film in IPA, the dispersing agent of the synthesized ink. This revealed a conductive Cu pattern. A minimum resistivity of 8.46×10−5 Ω cm was obtained for the Cu lines with 10– 20 µm widths. To demonstrate the feasibility of this method for metallization, a complex Cu electric circuit was fabricated on an ITOcoated glass substrate using a laser spot steered by a high-speed galvanometer. The fabricated Cu pattern was in good agreement with the circuit image to be duplicated. This selective-sintering approach provides a simple, cost-effective alternative to conventional photolithography for the production of electrode or metallization patterns. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A15053000). The authors declare no competing financial interest. References [1] B. Sirringhaus, T. Kawase, R. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E. Woo, High-resolution inkjet printing of all-polymer transistor circuits, Science 290 (2000) 2123. [2] B. Ridley, B. Nivi, J. Jacobson, All-inorganic field effect transistors fabricated by printing, Science 286 (1999) 746. [3] L. Li, Y. Guo, X. Zhang, Y. Song, Inkjet-printed highly conductive transparent patterns with water based Ag-doped graphene, J. Mater. Chem. A 2 (2014) 19095. [4] W. Liao, T. Chou, Fabrication of a planar-form screen-printed solid electrolyte modified Ag/AgCl reference electrode for application in a potentiometric biosensor 2006, Anal. Chem. 78 (2006) 4219. [5] V. Narayanan, The fire-through processed screen-printed Ag thick film metal contacts formed on an electrochemically etched porous silicon antireflection coating of silicon solar cells, RSC Adv. 3 (2013) 3618. [6] K. Kim, W. Myung, S. Jung, Effects of sintering conditions on microstructure and
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