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Temperature effect on physical properties and surface morphology of printed silver ink during continuous laser scanning sintering
International Journal of Heat and Mass Transfer 108 (2017) 1960–1968
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Temperature effect on physical properties and surface morphology of printed silver ink during continuous laser scanning sintering Ik-sang Lee a, Kyongtae Ryu a, Kyung-Hoon Park a, Yoon-Jae Moon a,b, Jun-Young Hwang b, Seung-Jae Moon a,⇑ a b
Department of Mechanical Engineering, Hanyang University, 04763 Seoul, South Korea Korea Institute of Industrial Technology, 15588 Ansan, South Korea
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 14 May 2016 Accepted 20 November 2016
Keywords: Laser sintering Inkjet printing Silver nanoparticles Specific resistance Neck growth Surface morphology
a b s t r a c t The temperature effect on the electrical characteristics and surface morphology of printed silver ink was investigated for laser irradiation with various laser scanning speeds and intensities. Inkjet-printed silver lines onto a glass substrate were irradiated with a 532 nm continuous wave. The electrical resistance was measured after laser sintering with various scanning speeds and intensities. The temperature field was invoked in the printed silver ink line by laser scanning sintering and this was predicted by numerical analysis. This temperature field show a quasi-steady state behavior since laser irradiation can be considered as a moving heat source. Field emission scanning electron microscope images were then examined with calculated temperatures of printed silver ink. The relation of surface morphology with temperature field were discussed. The effect of a laser scanning speed on the specific resistance of the conductive ink line was intensively investigated. Ó 2016 Published by Elsevier Ltd.
1. Introduction Printed electronics has attracted a growing interest due to process simplicity and cost-effectiveness. The printed electronics replaces a conventional photolithography that has a highly complicated process [1–3]. Inkjet printing, which is one of the printing methods, is an especially promising technology to fabricate microstructures. Inkjet printing can simplify manufacturing processes into two steps: printing and thermal treatment. A vacuum chamber are not required for printing process because this process can be performed at ambient pressure. Moreover, contamination can be minimized because the patterns can be drawn by inkjet printing without chemical-consuming etching process [3]. Metal nanoparticles jetted out from a printer nozzle should be precisely placed on the substrate to pattern an electrical circuit. Therefore, prevention of the nozzle clogging is crucial for high quality printing. For this purpose, organic surfactants are contained in metal nanoparticle ink to prevent agglomeration among particles during inkjet printing. However, these surfactants cause high resistance to the printed conductive ink line. Moreover, point contacts among nanoparticles disturb the flow of the electrons and provide elec-
⇑ Corresponding author. E-mail address: [email protected] (S.-J. Moon). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.095 0017-9310/Ó 2016 Published by Elsevier Ltd.
tron scattering in nanoscale structure. They result in the resistance increase of the conductive ink line. Thus, inkjet printing technology needs extra process to enhance electrical conductivity to remove the surfactants and to form connections among particles. This extra process is well known as sintering process. The neck growth can establish a percolation network for electrons through nanoparticle sintering. These neck formation will lead to the grain growth by further supply of heat. The mechanism of coalescence among nanoparticles can be explained in three steps: (i) the nanoparticles begin to aggregate as the temperature rises, (ii) the nanoparticles begin to join together, and neck formation among particles begins and is driven by surface diffusion to minimize the surface area for densification. Multiple grains was formed by agglomeration among particles, (iii) grain growth can be accelerated as the temperature rises [4]. Various nanoparticle sintering methods have been applied for this research field by many groups of researchers by applying temperature field in the printed ink. Oven or furnace have been used for thermal sintering of nanoparticle ink [5–6]. In the oven sintering, uniform temperature field can be achieved. Thus, the relation between electrical properties and applied temperature can be clearly revealed through the investigation of surface morphology. Even though large area substrate can be sintered in an oven or furnace, oven sintering needs long heating time over a few tens of minutes. High temperature can cause thermal damage on glass
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or polymer substrates. Therefore, the oven sintering is difficult to apply to the temperature-sensitive substrate such as polyethylene terephthalate (PET) and polyimide (PI). Laser [7], plasma [8], microwave [9], and electrical sintering [10] methods can be chosen to minimize thermal damage on the substrate. Especially laser irradiation can selectively heat patterned lines in micro- and nanoelectronics with high scanning speed. However, few works investigated the effect of temperature field caused by laser scanning irradiation on the physical properties and surface morphology. Chiolerio et al. [11] reveals that specific electrical resistance of the ink lines for the radio frequency identification device (RFID) is lower than that treated by a traditional hot-plate sintering when the printed Ag nanoparticle ink was sintered by a continuous laser. Lee et al. [12] adopted 532 nm continuous-wave laser to heat printed silver nanoparticle ink at ambient temperature. They obtained temperature fields invoked by a single laser shot in the printed silver nanoparticle ink lines to estimate the thermal conductivity of the ink. They claimed that the porosity of the sintered nanoparticle ink has an effect on the thermal conductivity of the sintered silver nanoparticle ink. Moon et al. [13] carried out the laser sintering experiment with continuous-wave laser scanning. The scanned beam can track printed patterns in the application of laser sintering system. They concluded that the long laser irradiation time at high laser intensity increased the specific resistance of Ag nanoparticles due to the introduction of large pores in the ink. The surface morphology and electrical property of the ink were affected by laser irradiation time and intensity. From their results, the laser irradiation time and scanning speeds must affect the electrical properties and surface morphology though they did not provide temperature information of the laser sintered ink. In this work, the physical properties and surface morphology of the silver nanoparticle ink were examined with temperature information when the printed silver line was sintered with various laser intensities and scanning speeds. The temperature field of the lasersintered printed silver nanoparticle line was estimated using a three-dimensional numerical analysis. Temperature field information is required to understand sintering mechanism since temperature is one of main factors to highly affect mass transport among silver nanoparticles. The resulting morphology evolution affects the sintered electrical properties of silver nanoparticle ink. Surface morphology of the silver microstructures were investigated by field-emission scanning electron microscope (FESEM).
2. Experiment details The conductive ink (ANP, DGP 40LT-15C) is composed of 31 wt% spherical silver nanoparticles (with an average particle size of 50 nm). Triethylene glycol monoethyl ether (TGME) was used as dispersant to prevent the agglomeration of silver nanoparticles. The Ag nanoparticle ink was printed on the glass substrate (Samsung-Corning, Eagle XG 2000) via an inkjet printer (Dimatix, DMP-2831). Two pads were printed on the substrate to measure the specific resistance of conductive ink line at both ends of line, and the pads were baked at 250 °C for 30 min to minimize the error of measurement caused by the high resistance of the pads. The pattern line dimension is 130 3000 0.36 lm3 as shown in Fig. 1(a). The initial resistances of the conductive ink line were 242 ± 58 X. Fig. 1(b) shows the schematic diagram of the laser sintering system adopted in this work. A 532 nm continuous wave laser (Coherent Verdi V5) is used as a light source that had a maximum output power of 5 W. A plano-convex spherical lens with a focal length of 500 mm was used to focus the circular laser beam on the conductive ink line. The focused beam diameter was measured by a camera (Coherent, LaserCam-HR) and BeamView-USB Analyzer Software (Coherent). Beam diameter was 276 ± 47 lm. The power
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Fig. 1. (a) Pattern shape of printed silver nanoparticle ink and (b) schematic diagram of laser sintering setup.
of the laser beam was measured with a FieldMax-power meter (Coherent). The laser scanning speed was controlled using the XY moving stage and LabVIEW 2010 (National Instruments). Laser intensities were 255, 521, 1005, 3000, 5011, and 7215 W/cm2 and scanning speeds were 3000, 300, and 30 lm=s. After continuous wave (CW) laser sintering, the final resistance of the sintered conductive ink samples were obtained at ambient temperature using L4411A digital multimeter (Agilent). The cross-sectional area of the sintered silver nanoparticle ink line was then measured at three different points with an Alpha step IQ (Kla Tencor), and the average value was then obtained to determine the specific resistance of the silver nanoparticle ink. FESEM images were monitored to investigate the surface morphology of silver conductive ink line. Finally, image processing (Image Xpert) was performed to examine the porosity of the sintered silver nanoparticle ink with 10,000 FESEM image. 3. Numerical simulation The temperature of conductive ink line was estimated by solving three-dimensional heat conduction equations using the COMSOL Multiphysics 4.2 finite-elements solver. The threedimensional heat conduction equation is as follows [14]:
qi C p;i
@T i @ @T i @ @T i @ @T i ki ki ki ¼ þ þ þ Si ; i ¼ 1; 2; @x @y @z @t @x @y @z ð1Þ
" # Q0 ðx v tÞ2 y2 þ 2 expðy=cÞ; S2 ¼ 0; S1 ¼ 2 ½1 RðTÞ c exp pa a2 a ð2Þ where subscript i indicates each part of the model: i = 1 for printed silver nanoparticle ink line, and i = 2 for glass substrate. qi, Ci, and ki are the density, heat capacity, and thermal conductivity of Ag ink line and glass substrate, respectively. Si indicates a heat-source term by laser absorption. Q0, R(T), a, and c are the total input power, reflection coefficient, laser beam radius, and absorption depth at a wavelength of 532 nm, respectively. The refractive index and the extinction coefficient of silver nanoparticle ink was measured by an ellipsometer (HORIBA, MM-16) after spin-coated samples at 250 rpm sintered in the furnace at 323, 373, 423, 473, and 523 K for 1800 s. Reflectivity coefficient was calculated from measured optical properties of silver nanoparticle at a wavelength of 532 nm as shown in Fig. 2. The maximum optical penetration depth was about 20.1 nm. The conductive ink line would be considered as an infinite absorbing medium because the maximum optical penetration depth was small compared with the thickness of the
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I.-s. Lee et al. / International Journal of Heat and Mass Transfer 108 (2017) 1960–1968 Table 1 Parameters and values used in the temperature numerical solution. Parameter
Value
Unit
h
10 5.67 108 5400 245 0.01–0.02 1.05 2600 840 0.92 140 300 2.4 0.5
W/m2K W/m2K4 kg/m3 J/kgK – W/mK kg/m3 J/kgK – lm nm cm mm
r qink Cp,ink
eink
kglass
qglass Cp,glass
eglass
wink tink wglass tglass
Fig. 2. Reflectivity of the silver ink sintered at various sintering temperatures.
conductive ink line, which was about 360 nm. The normal reflectivity can be calculated from following equation [15]:
RðTÞ ¼
ðn1 n2 Þ2 þ j22 ðn1 þ n2 Þ2 þ j22
of conductive ink line and glass substrate were 24,122 and 63,374, respectively. The number of these meshes was verified with an invariance of calculated temperature field. From Fig. 4, thermal conductivity of the printed ink was obtained from previously published data [16] as a function of temperature.
kðTÞ ¼
;
ð3Þ
where n1 is the refractive index of the air and n2 and j2 are the refractive index and the extinction coefficient of silver nanoparticle ink at a wavelength of 532 nm, respectively. Fig. 3 shows the three-dimensional numerical simulation model of the conductive ink line printed on the glass substrate. The heat convection and radiation boundary conditions were applied at the top surfaces of the conductive ink line and glass substrate. The initial temperature of the conductive ink line and glass substrate was 300 K. The sides of the glass substrate were assumed to be thermally insulated by symmetry. Physical parameters utilized for the temperature estimation were shown in Table 1. The meshes
Fig. 3. Three-dimensional numerical solution model of the patterned conductive ink line on glass substrate.
0:06 expð0:014TÞ; 0 6 kðTÞ < 375:5 375:5;
kðTÞ P 375:5
½W=m K:
ð4Þ
The upper limit of thermal conductivity of the ink was set to be 375.5 W/m.K since the maximum value is 375.5 W/m.K from the previous work. This value is less than bulk Ag thermal conductivity of 429 W/m.K due to enclosed air among Ag nanoparticles [16]. The coefficient of determination for this fitting is about 0.90. 4. Results and discussion Calculated transient top surface maximum temperatures of the conductive silver ink with various laser intensities and scanning speeds were shown in Fig. 5. In Fig. 5 laser irradiation is initiated at t = 0. These temperature curves show a plateau after an initial temperature rise by a laser irradiation. This laser scanning sintering is identical with a moving heat source conduction heat transfer problem. When the heat source is moving on a solid that is long enough compared to the penetration depth to heat transfer field, the temperature distribution around the heat source becomes independent of time rapidly [17]. The maximum temperatures of the silver nanoparticle ink sintered at a laser intensity of 255 W/cm2 at the plateau were 358, 374, and 391 K for laser
Fig. 4. Thermal conductivity of the patterned conductive ink. Black scatters are estimated data from Ref. [16] and solid line is logarithmically fitted curve.
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Fig. 5. Maximum surface temperature of the conductive silver-ink line with various laser intensities at a scanning speed of (a) 3000 lm/s, (b) 300 lm/s, and (c) 30 lm/s.
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scanning speeds of 3000, 300, and 30 lm/s, respectively. The temperatures of the silver nanoparticle ink sintered at the laser intensity of 7215 W/cm2 at the flat region were approximately 722, 736, and 777 K for laser scanning speeds of 3000, 300, 30 lm/s as shown in Fig. 5(a–c). Those temperatures were lower than the melting temperature of bulk silver of 1235 K [17]. Thus, the heat conductive equation without phase change can be adopted for the simulation. The temperature of the conductive ink line increased as laser intensity increased and scanning speed decreased. Fig. 6 shows temperature profile through (a)- and (b)-directions on the scanned spot of the printed ink irradiated by a laser scanning at 0.6 s. This time is almost just before the initiation of the plateau region. The scanning speed was 0.3 mm/s and the laser intensity was 3000 W/cm2. In overall, these parabolic temperature profile are caused by a Gaussian beam profile of the heating laser. The temperature difference of 72 K between the peak point and the lowest point is observed in the (a)-direction as shown in Fig. 5(a) and the difference through (b)-direction was 3 K as presented in in Fig. 5(b). Because the temperature profile is known to be stretched to the scanning directions in the moving heat source problem, the difference in the (b)-direction profile is much smaller than that in the (a)-direction. This smaller temperature difference in the (b)-direction means that the printed ink pattern in the width direction is sintered at the approximately uniform temperature. Fig. 7 depicts the specific resistance of a conductive silver ink line measured at an ambient temperature after scanning laser sintering with various intensities and scanning speeds. The specific resistance of various laser scanning speeds shows a rapid decreasing behavior until a laser intensity reached to 1005 W/cm2. The minimum specific resistance with a laser scanning speed of 3000 lm/s was 4.32 108 X m at a laser intensity of 3000 W/cm2. The minimum specific resistance with a laser scanning speed of 300 lm/s was 4.2 108 X m at a laser intensity of 5011 W/cm2. The minimum specific resistance with a laser scanning speed of 30 lm/s was 4.26 108 Xm at a laser intensity of 3000 W/cm2. These values are approximately 2.67 times that of bulk silver (1.59 108 Xm). However, the specific resistance of the conductive ink line at a laser scanning speed of 30 lm/s began to increase from the laser intensity of 3000 W/cm2. The specific resistance with a laser scanning speed of 30 lm/s increased to 1.06 107 Xm at a laser intensity of 7215 W/cm2. Similar tendency has been reported by Lee et al. [18]. They heated the printed Ag ink sample irradiated by a single shot laser for 60 s without a scanning system. They reported that the specific resistance of the printed silver nanoparticle ink sintered at an intense laser intensity is higher due to the surface morphology with large pores in the texture. The increase of the specific resistance at a high laser intensity is known to be related with surface morphology of ink [13,18]. This will be discussed with FESEM images. FESEM images (X50,000) indicate that the top surface morphology of conductive ink line sintered with various laser intensities and scanning speeds of 3000, 300, and 30 lm/s as shown in Figs. 8–10. Presented temperatures are obtained from plateau of the temperature curves shown in Fig. 5. Generally, the sintering process of metal nanoparticle inks is composed of three steps [19]: (i) evaporation of surfactants, (ii) removal of surfactants and agglomeration among metal nanoparticles by van der Waals force, and (iii) neck formation and grain growth by diffusions. Diffusions consist of surface diffusion, lattice diffusion, throughlattice diffusion, and grain boundary diffusion. Surface and lattice diffusions from particle surface to contact other particle surface cause coarsening, neck formation, and growth of the particles without densification. When these diffusions dominate, coarsening
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Fig. 6. Temperature distributions with a scanning speed of 3000 lm/s and laser intensity of 3000 W/cm2 at 0.6 s.
Fig. 7. Specific resistances of the silver nanoparticle ink as various laser intensities and scanning speeds.
leads to the production of a porous structure. Grain boundary diffusion and through-lattice diffusion from the grain boundary to the contact surface (neck), which is the densifying mechanisms, lead to neck growth, densification, and shrinkage among particles. The neck formation is also driven by a reduction of surface energy due to atom diffusion and mass transport. The surface energy reduction caused by neck formation was reported to affect the formation of grain boundary [20,21]. The initiation of binding among nanoparticles with a scanning speed of 3000 lm/s was monitored at a low laser intensity of 255 W/cm2 and temperature of 358 K as shown in Fig. 8(a). Agglomeration and neck formation among silver nanoparticles formed at laser intensity of 521 W/cm2 as shown in Fig. 8(b). These agglomeration and neck growth among silver nanoparticles were noticeably accelerated by diffusions at laser intensities of 1005 and 3000 W/cm2 as depicted in Fig. 8(c-d) [20]. The temperatures
of the conductive ink line at these conditions were 476 and 600 K, respectively. Grains were formed by remarkable neck growth among silver nanoparticles at a laser intensity of 5011 W/cm2 and temperature of 685 K in Fig. 8(e). Grain size was approximately 600 nm at this temperature. Grains grew much larger up to 700 nm as the temperature increased to 722 K. However, pores were formed and cracks between grain boundaries propagated due to the activation of shrinkage and densification among silver nanoparticles as shown in Fig. 8(f) [21]. The minimum and maximum crack length were about 100 and 900 nm, respectively. As shown in Fig. 9(a–c), neck formation and grain growth were activated at a scanning speed of 300 lm/s as laser intensities varied from 255 to 1005 W/cm2 and the temperature increased from 374 to 493 K. Grains sintered with a laser scanning speed of 300 lm/s began to grow at a laser intensity of 3000 W/cm2 and temperature of 631 K, while small pores were formed as illustrated in Fig. 9(d). Those grain sizes were approximately 400 nm. Grains with approximately 600 nm were formed and the generation of a large pore with a size of 300 nm at a laser intensity of 5011 W/cm2 and temperature of 711 K is shown in Fig. 9(e). Grains grow much larger due to diffusions activated by the high temperature of 746 K, but longer cracks of around 2 lm sizes between grain boundaries also occurred as shown in Fig. 9(f). This tendency of neck growth, pore growth, and crack formation with various laser intensities was remarkably shown at a scanning speed of 30 lm/s. Necks among silver nanoparticles were densely formed at laser intensities of 255, 521, and 1005 W/cm2 in Fig. 10(a–c). Grain growth in the printed ink sintered with a laser scanning speed of 30 lm/s were noticeably enhanced at a laser intensity of 3000 W/cm2 and temperature of 665 K as shown in Fig. 10(d). This grain size was about 700 nm. Larger pores and cracks among silver nanoparticles were noticeably found at a laser intensity of 5011 W/cm2 and temperature of 743 K as shown in Fig. 10(e). Pore and crack size were approximately 300 and 400 nm, respectively. In Fig. 10(f), pore diameter was almost
I.-s. Lee et al. / International Journal of Heat and Mass Transfer 108 (2017) 1960–1968
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Fig. 8. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 3000 lm/s.
Fig. 9. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 300 lm/s.
1.5 lm when laser intensity and temperature increased to 7215 W/cm2 and 777 K, respectively. These results show that the calculated temperatures corresponding to laser intensities was dominant to the surface morphology of the conductive ink line as shown in Figs. 8–10. Neck growth among silver nanoparticles noticeably occurred from 400 K, and percolation network was established by this neck growth for electron flow. Large grain formation was noticed at the temperatures between 606 K and 711 K. The minimum specific resistance were obtained with large grain formation at the temperatures of 606 K (3000 lm/s and 3000 W/cm2), 711 K (300 lm/s and 5011 W/cm2), and 665 K (30 lm/s and 3000 W/cm2). Pores and cracks were grown from the temperature of 711 K with larger grain growth. As the laser scanning speed is slower, the sintering temperature increased at the same laser intensity. At the temperatures of 743 K (5011 W/cm2) and 777 K (5011 W/cm2) with the slowest scanning speed of 30 lm/s, large pores are clearly identified. Those large pores results in the increase of the specific resistance by narrowing the electron path.
The mechanism of pore and crack formation was presented in Fig. 11. Grains grow by the total sum of the surface energy minimization [22]. The atoms on the grain surface are vigorously activated and the surface tension (csv) by the solid–vapor interface was applied at a junction between grain boundaries while another surface tension (cgb) was applied to the plane of the grain boundary. As the csv increased by increasing laser intensity, cgb would also increase for the equilibrium with increased csv. The increased cgb caused cracks through the grain boundaries. Eventually, silver nanoparticles would be separated as an island due to the propagation of the cracks along the grain boundary [13,21]. To confirm a wide pore distribution of conductive silver ink line, 10,000 FESEM images were taken to cover larger areas as shown in Fig. 12. Small pores with a laser scanning speed of 3000 lm/s were formed at laser intensities of 5011 and 7215 W/cm2. Small pores with a laser scanning speed of 300 lm/s were also formed at laser intensities of 3000 and 5011 W/cm2. Pores with a laser scanning speed of 300 lm/s were grown to about 1.5 lm at a laser intensity of 7215 W/cm2 with larger pores especially observed in
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Fig. 10. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 30 lm/s.
Fig. 11. Schematic diagram of crack formation at a junction of pore and grain boundaries.
Fig. 12. 10,000 FESEM images of the printed silver nanoparticle ink with various laser intensities and scanning speeds.
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the overall surface of the conductive silver ink line with a scanning speed of 30 lm/s due to a high sintering temperature greater than 743 K and long irradiation time by low scanning speed. These larger pores would affect the specific resistance increase of the conductive ink line. Therefore, the relation of porosity and specific resistance of the conductive ink line will be discussed. Fig. 13 shows the porosity and specific resistance measured at ambient temperature with various laser intensities and scanning
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speeds. To measure the porosity of the conductive silver ink line, image processing was applied to the FESEM images presented in Fig. 12. Rectangular parts, which represent porosity, were considered as pores and cracks using light and shade difference. Porosity was approximately 0% for three scanning speed conditions at the low intensities. The specific resistance of the conductive ink line decreased with various intensities in a range without pores. The specific resistance of the conductive ink line tended to increase as porosity increased. The specific resistance of the conductive ink line with a scanning speed of 3000 lm/s increased from 4.34 108 Xm with a laser intensity of 3000 W/cm2 (600 K) to 4.8 108 Xm with a laser intensity of 7215 W/cm2 (722 K) when porosity was 2% as shown in Fig. 13(a). At a scanning speed of 300 lm/s, porosity slightly increased from a laser intensity of 1005 W/cm2 (493 K). However, the amount of the porosity was small compared with the grain growth rate. The minimum specific resistance with a scanning speed of 300 lm/s was 4.2 108 Xm at the porosity of 2.2%. The specific resistance of the conductive ink line increased from 4.20 108 to 4.62 108 Xm when porosity increased from 2% with a laser intensity of 5011 W/cm2 (711 K) to 5% with the laser intensity of 7215 W/cm2 (746 K) as shown in Fig. 13(b). The specific resistances of the conductive ink line with a scanning speed of 30 lm/s were 6.04 108 Xm and 1.06 107 Xm, respectively when porosities were 10.32% and 17.33% as shown in Fig. 13(c). The relation between porosity increase and specific resistance increase is well agreed with previous result reported by Lee et al. [18]. Electron paths for free electrons formed by larger neck formation and grain growth would decrease the specific resistance. However, noticeably grown pores formed by coarsening and shrinkage among particles would lead to increase of the specific resistance of the conductive ink line [23]. 5. Conclusion The temperature effect on the physical properties and morphology change of silver nanoparticle ink according to scanning speed change was studied. The estimated temperature of the lasersintered conductive ink line was dependent on the laser intensities and scanning speeds. The calculated temperature were found to affect the surface morphology of the micro structure of the conductive ink line. Neck growth among silver nanoparticles was noticeably found from the temperature of 400 K. Large grains among silver nanoparticles were shown from a temperature of about 606 K. However, pores were grown by densification and shrinkage among nanoparticles at a high temperature. Cracks propagated by tensions at the junction of pore and grain boundaries. Noticeable increase of porosity caused by shrinkage densification among silver nanoparticles led to increase of the specific resistance of silver nanoparticle ink. As the laser scanning speed decreases, the laser dwell time increases. This could result in the temperature rise of the printed ink that affects the surface morphology significantly. Acknowledgment This work was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20141010101850). This work was partially supported by an Industrial Technology Innovation program grant No. 10052802 funded by the Ministry of Trade, Industry and Energy of the Korean Government. References
Fig. 13. Relation of porosity and specific resistance of silver nanoparticle ink with various laser intensities at scanning speeds of (a) 3000, (b) 300, and (c) 30 lm/s.
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Temperature effect on physical properties and surface morphology of printed silver ink during continuous laser scanning sintering Ik-sang Lee a, Kyongtae Ryu a, Kyung-Hoon Park a, Yoon-Jae Moon a,b, Jun-Young Hwang b, Seung-Jae Moon a,⇑ a b
Department of Mechanical Engineering, Hanyang University, 04763 Seoul, South Korea Korea Institute of Industrial Technology, 15588 Ansan, South Korea
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 14 May 2016 Accepted 20 November 2016
Keywords: Laser sintering Inkjet printing Silver nanoparticles Specific resistance Neck growth Surface morphology
a b s t r a c t The temperature effect on the electrical characteristics and surface morphology of printed silver ink was investigated for laser irradiation with various laser scanning speeds and intensities. Inkjet-printed silver lines onto a glass substrate were irradiated with a 532 nm continuous wave. The electrical resistance was measured after laser sintering with various scanning speeds and intensities. The temperature field was invoked in the printed silver ink line by laser scanning sintering and this was predicted by numerical analysis. This temperature field show a quasi-steady state behavior since laser irradiation can be considered as a moving heat source. Field emission scanning electron microscope images were then examined with calculated temperatures of printed silver ink. The relation of surface morphology with temperature field were discussed. The effect of a laser scanning speed on the specific resistance of the conductive ink line was intensively investigated. Ó 2016 Published by Elsevier Ltd.
1. Introduction Printed electronics has attracted a growing interest due to process simplicity and cost-effectiveness. The printed electronics replaces a conventional photolithography that has a highly complicated process [1–3]. Inkjet printing, which is one of the printing methods, is an especially promising technology to fabricate microstructures. Inkjet printing can simplify manufacturing processes into two steps: printing and thermal treatment. A vacuum chamber are not required for printing process because this process can be performed at ambient pressure. Moreover, contamination can be minimized because the patterns can be drawn by inkjet printing without chemical-consuming etching process [3]. Metal nanoparticles jetted out from a printer nozzle should be precisely placed on the substrate to pattern an electrical circuit. Therefore, prevention of the nozzle clogging is crucial for high quality printing. For this purpose, organic surfactants are contained in metal nanoparticle ink to prevent agglomeration among particles during inkjet printing. However, these surfactants cause high resistance to the printed conductive ink line. Moreover, point contacts among nanoparticles disturb the flow of the electrons and provide elec-
⇑ Corresponding author. E-mail address: [email protected] (S.-J. Moon). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.11.095 0017-9310/Ó 2016 Published by Elsevier Ltd.
tron scattering in nanoscale structure. They result in the resistance increase of the conductive ink line. Thus, inkjet printing technology needs extra process to enhance electrical conductivity to remove the surfactants and to form connections among particles. This extra process is well known as sintering process. The neck growth can establish a percolation network for electrons through nanoparticle sintering. These neck formation will lead to the grain growth by further supply of heat. The mechanism of coalescence among nanoparticles can be explained in three steps: (i) the nanoparticles begin to aggregate as the temperature rises, (ii) the nanoparticles begin to join together, and neck formation among particles begins and is driven by surface diffusion to minimize the surface area for densification. Multiple grains was formed by agglomeration among particles, (iii) grain growth can be accelerated as the temperature rises [4]. Various nanoparticle sintering methods have been applied for this research field by many groups of researchers by applying temperature field in the printed ink. Oven or furnace have been used for thermal sintering of nanoparticle ink [5–6]. In the oven sintering, uniform temperature field can be achieved. Thus, the relation between electrical properties and applied temperature can be clearly revealed through the investigation of surface morphology. Even though large area substrate can be sintered in an oven or furnace, oven sintering needs long heating time over a few tens of minutes. High temperature can cause thermal damage on glass
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or polymer substrates. Therefore, the oven sintering is difficult to apply to the temperature-sensitive substrate such as polyethylene terephthalate (PET) and polyimide (PI). Laser [7], plasma [8], microwave [9], and electrical sintering [10] methods can be chosen to minimize thermal damage on the substrate. Especially laser irradiation can selectively heat patterned lines in micro- and nanoelectronics with high scanning speed. However, few works investigated the effect of temperature field caused by laser scanning irradiation on the physical properties and surface morphology. Chiolerio et al. [11] reveals that specific electrical resistance of the ink lines for the radio frequency identification device (RFID) is lower than that treated by a traditional hot-plate sintering when the printed Ag nanoparticle ink was sintered by a continuous laser. Lee et al. [12] adopted 532 nm continuous-wave laser to heat printed silver nanoparticle ink at ambient temperature. They obtained temperature fields invoked by a single laser shot in the printed silver nanoparticle ink lines to estimate the thermal conductivity of the ink. They claimed that the porosity of the sintered nanoparticle ink has an effect on the thermal conductivity of the sintered silver nanoparticle ink. Moon et al. [13] carried out the laser sintering experiment with continuous-wave laser scanning. The scanned beam can track printed patterns in the application of laser sintering system. They concluded that the long laser irradiation time at high laser intensity increased the specific resistance of Ag nanoparticles due to the introduction of large pores in the ink. The surface morphology and electrical property of the ink were affected by laser irradiation time and intensity. From their results, the laser irradiation time and scanning speeds must affect the electrical properties and surface morphology though they did not provide temperature information of the laser sintered ink. In this work, the physical properties and surface morphology of the silver nanoparticle ink were examined with temperature information when the printed silver line was sintered with various laser intensities and scanning speeds. The temperature field of the lasersintered printed silver nanoparticle line was estimated using a three-dimensional numerical analysis. Temperature field information is required to understand sintering mechanism since temperature is one of main factors to highly affect mass transport among silver nanoparticles. The resulting morphology evolution affects the sintered electrical properties of silver nanoparticle ink. Surface morphology of the silver microstructures were investigated by field-emission scanning electron microscope (FESEM).
2. Experiment details The conductive ink (ANP, DGP 40LT-15C) is composed of 31 wt% spherical silver nanoparticles (with an average particle size of 50 nm). Triethylene glycol monoethyl ether (TGME) was used as dispersant to prevent the agglomeration of silver nanoparticles. The Ag nanoparticle ink was printed on the glass substrate (Samsung-Corning, Eagle XG 2000) via an inkjet printer (Dimatix, DMP-2831). Two pads were printed on the substrate to measure the specific resistance of conductive ink line at both ends of line, and the pads were baked at 250 °C for 30 min to minimize the error of measurement caused by the high resistance of the pads. The pattern line dimension is 130 3000 0.36 lm3 as shown in Fig. 1(a). The initial resistances of the conductive ink line were 242 ± 58 X. Fig. 1(b) shows the schematic diagram of the laser sintering system adopted in this work. A 532 nm continuous wave laser (Coherent Verdi V5) is used as a light source that had a maximum output power of 5 W. A plano-convex spherical lens with a focal length of 500 mm was used to focus the circular laser beam on the conductive ink line. The focused beam diameter was measured by a camera (Coherent, LaserCam-HR) and BeamView-USB Analyzer Software (Coherent). Beam diameter was 276 ± 47 lm. The power
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Fig. 1. (a) Pattern shape of printed silver nanoparticle ink and (b) schematic diagram of laser sintering setup.
of the laser beam was measured with a FieldMax-power meter (Coherent). The laser scanning speed was controlled using the XY moving stage and LabVIEW 2010 (National Instruments). Laser intensities were 255, 521, 1005, 3000, 5011, and 7215 W/cm2 and scanning speeds were 3000, 300, and 30 lm=s. After continuous wave (CW) laser sintering, the final resistance of the sintered conductive ink samples were obtained at ambient temperature using L4411A digital multimeter (Agilent). The cross-sectional area of the sintered silver nanoparticle ink line was then measured at three different points with an Alpha step IQ (Kla Tencor), and the average value was then obtained to determine the specific resistance of the silver nanoparticle ink. FESEM images were monitored to investigate the surface morphology of silver conductive ink line. Finally, image processing (Image Xpert) was performed to examine the porosity of the sintered silver nanoparticle ink with 10,000 FESEM image. 3. Numerical simulation The temperature of conductive ink line was estimated by solving three-dimensional heat conduction equations using the COMSOL Multiphysics 4.2 finite-elements solver. The threedimensional heat conduction equation is as follows [14]:
qi C p;i
@T i @ @T i @ @T i @ @T i ki ki ki ¼ þ þ þ Si ; i ¼ 1; 2; @x @y @z @t @x @y @z ð1Þ
" # Q0 ðx v tÞ2 y2 þ 2 expðy=cÞ; S2 ¼ 0; S1 ¼ 2 ½1 RðTÞ c exp pa a2 a ð2Þ where subscript i indicates each part of the model: i = 1 for printed silver nanoparticle ink line, and i = 2 for glass substrate. qi, Ci, and ki are the density, heat capacity, and thermal conductivity of Ag ink line and glass substrate, respectively. Si indicates a heat-source term by laser absorption. Q0, R(T), a, and c are the total input power, reflection coefficient, laser beam radius, and absorption depth at a wavelength of 532 nm, respectively. The refractive index and the extinction coefficient of silver nanoparticle ink was measured by an ellipsometer (HORIBA, MM-16) after spin-coated samples at 250 rpm sintered in the furnace at 323, 373, 423, 473, and 523 K for 1800 s. Reflectivity coefficient was calculated from measured optical properties of silver nanoparticle at a wavelength of 532 nm as shown in Fig. 2. The maximum optical penetration depth was about 20.1 nm. The conductive ink line would be considered as an infinite absorbing medium because the maximum optical penetration depth was small compared with the thickness of the
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I.-s. Lee et al. / International Journal of Heat and Mass Transfer 108 (2017) 1960–1968 Table 1 Parameters and values used in the temperature numerical solution. Parameter
Value
Unit
h
10 5.67 108 5400 245 0.01–0.02 1.05 2600 840 0.92 140 300 2.4 0.5
W/m2K W/m2K4 kg/m3 J/kgK – W/mK kg/m3 J/kgK – lm nm cm mm
r qink Cp,ink
eink
kglass
qglass Cp,glass
eglass
wink tink wglass tglass
Fig. 2. Reflectivity of the silver ink sintered at various sintering temperatures.
conductive ink line, which was about 360 nm. The normal reflectivity can be calculated from following equation [15]:
RðTÞ ¼
ðn1 n2 Þ2 þ j22 ðn1 þ n2 Þ2 þ j22
of conductive ink line and glass substrate were 24,122 and 63,374, respectively. The number of these meshes was verified with an invariance of calculated temperature field. From Fig. 4, thermal conductivity of the printed ink was obtained from previously published data [16] as a function of temperature.
kðTÞ ¼
;
ð3Þ
where n1 is the refractive index of the air and n2 and j2 are the refractive index and the extinction coefficient of silver nanoparticle ink at a wavelength of 532 nm, respectively. Fig. 3 shows the three-dimensional numerical simulation model of the conductive ink line printed on the glass substrate. The heat convection and radiation boundary conditions were applied at the top surfaces of the conductive ink line and glass substrate. The initial temperature of the conductive ink line and glass substrate was 300 K. The sides of the glass substrate were assumed to be thermally insulated by symmetry. Physical parameters utilized for the temperature estimation were shown in Table 1. The meshes
Fig. 3. Three-dimensional numerical solution model of the patterned conductive ink line on glass substrate.
0:06 expð0:014TÞ; 0 6 kðTÞ < 375:5 375:5;
kðTÞ P 375:5
½W=m K:
ð4Þ
The upper limit of thermal conductivity of the ink was set to be 375.5 W/m.K since the maximum value is 375.5 W/m.K from the previous work. This value is less than bulk Ag thermal conductivity of 429 W/m.K due to enclosed air among Ag nanoparticles [16]. The coefficient of determination for this fitting is about 0.90. 4. Results and discussion Calculated transient top surface maximum temperatures of the conductive silver ink with various laser intensities and scanning speeds were shown in Fig. 5. In Fig. 5 laser irradiation is initiated at t = 0. These temperature curves show a plateau after an initial temperature rise by a laser irradiation. This laser scanning sintering is identical with a moving heat source conduction heat transfer problem. When the heat source is moving on a solid that is long enough compared to the penetration depth to heat transfer field, the temperature distribution around the heat source becomes independent of time rapidly [17]. The maximum temperatures of the silver nanoparticle ink sintered at a laser intensity of 255 W/cm2 at the plateau were 358, 374, and 391 K for laser
Fig. 4. Thermal conductivity of the patterned conductive ink. Black scatters are estimated data from Ref. [16] and solid line is logarithmically fitted curve.
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Fig. 5. Maximum surface temperature of the conductive silver-ink line with various laser intensities at a scanning speed of (a) 3000 lm/s, (b) 300 lm/s, and (c) 30 lm/s.
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scanning speeds of 3000, 300, and 30 lm/s, respectively. The temperatures of the silver nanoparticle ink sintered at the laser intensity of 7215 W/cm2 at the flat region were approximately 722, 736, and 777 K for laser scanning speeds of 3000, 300, 30 lm/s as shown in Fig. 5(a–c). Those temperatures were lower than the melting temperature of bulk silver of 1235 K [17]. Thus, the heat conductive equation without phase change can be adopted for the simulation. The temperature of the conductive ink line increased as laser intensity increased and scanning speed decreased. Fig. 6 shows temperature profile through (a)- and (b)-directions on the scanned spot of the printed ink irradiated by a laser scanning at 0.6 s. This time is almost just before the initiation of the plateau region. The scanning speed was 0.3 mm/s and the laser intensity was 3000 W/cm2. In overall, these parabolic temperature profile are caused by a Gaussian beam profile of the heating laser. The temperature difference of 72 K between the peak point and the lowest point is observed in the (a)-direction as shown in Fig. 5(a) and the difference through (b)-direction was 3 K as presented in in Fig. 5(b). Because the temperature profile is known to be stretched to the scanning directions in the moving heat source problem, the difference in the (b)-direction profile is much smaller than that in the (a)-direction. This smaller temperature difference in the (b)-direction means that the printed ink pattern in the width direction is sintered at the approximately uniform temperature. Fig. 7 depicts the specific resistance of a conductive silver ink line measured at an ambient temperature after scanning laser sintering with various intensities and scanning speeds. The specific resistance of various laser scanning speeds shows a rapid decreasing behavior until a laser intensity reached to 1005 W/cm2. The minimum specific resistance with a laser scanning speed of 3000 lm/s was 4.32 108 X m at a laser intensity of 3000 W/cm2. The minimum specific resistance with a laser scanning speed of 300 lm/s was 4.2 108 X m at a laser intensity of 5011 W/cm2. The minimum specific resistance with a laser scanning speed of 30 lm/s was 4.26 108 Xm at a laser intensity of 3000 W/cm2. These values are approximately 2.67 times that of bulk silver (1.59 108 Xm). However, the specific resistance of the conductive ink line at a laser scanning speed of 30 lm/s began to increase from the laser intensity of 3000 W/cm2. The specific resistance with a laser scanning speed of 30 lm/s increased to 1.06 107 Xm at a laser intensity of 7215 W/cm2. Similar tendency has been reported by Lee et al. [18]. They heated the printed Ag ink sample irradiated by a single shot laser for 60 s without a scanning system. They reported that the specific resistance of the printed silver nanoparticle ink sintered at an intense laser intensity is higher due to the surface morphology with large pores in the texture. The increase of the specific resistance at a high laser intensity is known to be related with surface morphology of ink [13,18]. This will be discussed with FESEM images. FESEM images (X50,000) indicate that the top surface morphology of conductive ink line sintered with various laser intensities and scanning speeds of 3000, 300, and 30 lm/s as shown in Figs. 8–10. Presented temperatures are obtained from plateau of the temperature curves shown in Fig. 5. Generally, the sintering process of metal nanoparticle inks is composed of three steps [19]: (i) evaporation of surfactants, (ii) removal of surfactants and agglomeration among metal nanoparticles by van der Waals force, and (iii) neck formation and grain growth by diffusions. Diffusions consist of surface diffusion, lattice diffusion, throughlattice diffusion, and grain boundary diffusion. Surface and lattice diffusions from particle surface to contact other particle surface cause coarsening, neck formation, and growth of the particles without densification. When these diffusions dominate, coarsening
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Fig. 6. Temperature distributions with a scanning speed of 3000 lm/s and laser intensity of 3000 W/cm2 at 0.6 s.
Fig. 7. Specific resistances of the silver nanoparticle ink as various laser intensities and scanning speeds.
leads to the production of a porous structure. Grain boundary diffusion and through-lattice diffusion from the grain boundary to the contact surface (neck), which is the densifying mechanisms, lead to neck growth, densification, and shrinkage among particles. The neck formation is also driven by a reduction of surface energy due to atom diffusion and mass transport. The surface energy reduction caused by neck formation was reported to affect the formation of grain boundary [20,21]. The initiation of binding among nanoparticles with a scanning speed of 3000 lm/s was monitored at a low laser intensity of 255 W/cm2 and temperature of 358 K as shown in Fig. 8(a). Agglomeration and neck formation among silver nanoparticles formed at laser intensity of 521 W/cm2 as shown in Fig. 8(b). These agglomeration and neck growth among silver nanoparticles were noticeably accelerated by diffusions at laser intensities of 1005 and 3000 W/cm2 as depicted in Fig. 8(c-d) [20]. The temperatures
of the conductive ink line at these conditions were 476 and 600 K, respectively. Grains were formed by remarkable neck growth among silver nanoparticles at a laser intensity of 5011 W/cm2 and temperature of 685 K in Fig. 8(e). Grain size was approximately 600 nm at this temperature. Grains grew much larger up to 700 nm as the temperature increased to 722 K. However, pores were formed and cracks between grain boundaries propagated due to the activation of shrinkage and densification among silver nanoparticles as shown in Fig. 8(f) [21]. The minimum and maximum crack length were about 100 and 900 nm, respectively. As shown in Fig. 9(a–c), neck formation and grain growth were activated at a scanning speed of 300 lm/s as laser intensities varied from 255 to 1005 W/cm2 and the temperature increased from 374 to 493 K. Grains sintered with a laser scanning speed of 300 lm/s began to grow at a laser intensity of 3000 W/cm2 and temperature of 631 K, while small pores were formed as illustrated in Fig. 9(d). Those grain sizes were approximately 400 nm. Grains with approximately 600 nm were formed and the generation of a large pore with a size of 300 nm at a laser intensity of 5011 W/cm2 and temperature of 711 K is shown in Fig. 9(e). Grains grow much larger due to diffusions activated by the high temperature of 746 K, but longer cracks of around 2 lm sizes between grain boundaries also occurred as shown in Fig. 9(f). This tendency of neck growth, pore growth, and crack formation with various laser intensities was remarkably shown at a scanning speed of 30 lm/s. Necks among silver nanoparticles were densely formed at laser intensities of 255, 521, and 1005 W/cm2 in Fig. 10(a–c). Grain growth in the printed ink sintered with a laser scanning speed of 30 lm/s were noticeably enhanced at a laser intensity of 3000 W/cm2 and temperature of 665 K as shown in Fig. 10(d). This grain size was about 700 nm. Larger pores and cracks among silver nanoparticles were noticeably found at a laser intensity of 5011 W/cm2 and temperature of 743 K as shown in Fig. 10(e). Pore and crack size were approximately 300 and 400 nm, respectively. In Fig. 10(f), pore diameter was almost
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Fig. 8. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 3000 lm/s.
Fig. 9. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 300 lm/s.
1.5 lm when laser intensity and temperature increased to 7215 W/cm2 and 777 K, respectively. These results show that the calculated temperatures corresponding to laser intensities was dominant to the surface morphology of the conductive ink line as shown in Figs. 8–10. Neck growth among silver nanoparticles noticeably occurred from 400 K, and percolation network was established by this neck growth for electron flow. Large grain formation was noticed at the temperatures between 606 K and 711 K. The minimum specific resistance were obtained with large grain formation at the temperatures of 606 K (3000 lm/s and 3000 W/cm2), 711 K (300 lm/s and 5011 W/cm2), and 665 K (30 lm/s and 3000 W/cm2). Pores and cracks were grown from the temperature of 711 K with larger grain growth. As the laser scanning speed is slower, the sintering temperature increased at the same laser intensity. At the temperatures of 743 K (5011 W/cm2) and 777 K (5011 W/cm2) with the slowest scanning speed of 30 lm/s, large pores are clearly identified. Those large pores results in the increase of the specific resistance by narrowing the electron path.
The mechanism of pore and crack formation was presented in Fig. 11. Grains grow by the total sum of the surface energy minimization [22]. The atoms on the grain surface are vigorously activated and the surface tension (csv) by the solid–vapor interface was applied at a junction between grain boundaries while another surface tension (cgb) was applied to the plane of the grain boundary. As the csv increased by increasing laser intensity, cgb would also increase for the equilibrium with increased csv. The increased cgb caused cracks through the grain boundaries. Eventually, silver nanoparticles would be separated as an island due to the propagation of the cracks along the grain boundary [13,21]. To confirm a wide pore distribution of conductive silver ink line, 10,000 FESEM images were taken to cover larger areas as shown in Fig. 12. Small pores with a laser scanning speed of 3000 lm/s were formed at laser intensities of 5011 and 7215 W/cm2. Small pores with a laser scanning speed of 300 lm/s were also formed at laser intensities of 3000 and 5011 W/cm2. Pores with a laser scanning speed of 300 lm/s were grown to about 1.5 lm at a laser intensity of 7215 W/cm2 with larger pores especially observed in
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Fig. 10. FESEM images of the printed silver nanoparticle ink with various laser intensities at a scanning speed of 30 lm/s.
Fig. 11. Schematic diagram of crack formation at a junction of pore and grain boundaries.
Fig. 12. 10,000 FESEM images of the printed silver nanoparticle ink with various laser intensities and scanning speeds.
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the overall surface of the conductive silver ink line with a scanning speed of 30 lm/s due to a high sintering temperature greater than 743 K and long irradiation time by low scanning speed. These larger pores would affect the specific resistance increase of the conductive ink line. Therefore, the relation of porosity and specific resistance of the conductive ink line will be discussed. Fig. 13 shows the porosity and specific resistance measured at ambient temperature with various laser intensities and scanning
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speeds. To measure the porosity of the conductive silver ink line, image processing was applied to the FESEM images presented in Fig. 12. Rectangular parts, which represent porosity, were considered as pores and cracks using light and shade difference. Porosity was approximately 0% for three scanning speed conditions at the low intensities. The specific resistance of the conductive ink line decreased with various intensities in a range without pores. The specific resistance of the conductive ink line tended to increase as porosity increased. The specific resistance of the conductive ink line with a scanning speed of 3000 lm/s increased from 4.34 108 Xm with a laser intensity of 3000 W/cm2 (600 K) to 4.8 108 Xm with a laser intensity of 7215 W/cm2 (722 K) when porosity was 2% as shown in Fig. 13(a). At a scanning speed of 300 lm/s, porosity slightly increased from a laser intensity of 1005 W/cm2 (493 K). However, the amount of the porosity was small compared with the grain growth rate. The minimum specific resistance with a scanning speed of 300 lm/s was 4.2 108 Xm at the porosity of 2.2%. The specific resistance of the conductive ink line increased from 4.20 108 to 4.62 108 Xm when porosity increased from 2% with a laser intensity of 5011 W/cm2 (711 K) to 5% with the laser intensity of 7215 W/cm2 (746 K) as shown in Fig. 13(b). The specific resistances of the conductive ink line with a scanning speed of 30 lm/s were 6.04 108 Xm and 1.06 107 Xm, respectively when porosities were 10.32% and 17.33% as shown in Fig. 13(c). The relation between porosity increase and specific resistance increase is well agreed with previous result reported by Lee et al. [18]. Electron paths for free electrons formed by larger neck formation and grain growth would decrease the specific resistance. However, noticeably grown pores formed by coarsening and shrinkage among particles would lead to increase of the specific resistance of the conductive ink line [23]. 5. Conclusion The temperature effect on the physical properties and morphology change of silver nanoparticle ink according to scanning speed change was studied. The estimated temperature of the lasersintered conductive ink line was dependent on the laser intensities and scanning speeds. The calculated temperature were found to affect the surface morphology of the micro structure of the conductive ink line. Neck growth among silver nanoparticles was noticeably found from the temperature of 400 K. Large grains among silver nanoparticles were shown from a temperature of about 606 K. However, pores were grown by densification and shrinkage among nanoparticles at a high temperature. Cracks propagated by tensions at the junction of pore and grain boundaries. Noticeable increase of porosity caused by shrinkage densification among silver nanoparticles led to increase of the specific resistance of silver nanoparticle ink. As the laser scanning speed decreases, the laser dwell time increases. This could result in the temperature rise of the printed ink that affects the surface morphology significantly. Acknowledgment This work was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20141010101850). This work was partially supported by an Industrial Technology Innovation program grant No. 10052802 funded by the Ministry of Trade, Industry and Energy of the Korean Government. References
Fig. 13. Relation of porosity and specific resistance of silver nanoparticle ink with various laser intensities at scanning speeds of (a) 3000, (b) 300, and (c) 30 lm/s.
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