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Research paper : Development of laser-assisted inkjet printing technology (A. Endo et al.)−13−Synthesiology - English edition Vol.4 No.1 (2011) increase the drying speed.As a method of injecting the energy directly into the discharged droplet without drying the nozzle or causing bumping, we devised a simple method in which drying is promoted by focusing the laser to the droplet and controlling the wet-spreading of ink (laser-assisted inkjet technology).The laser-assisted inkjet technology is a method where the ink solvent is evaporated and dried instantly by heat energy, as the focused laser beam is irradiated onto the droplet and the substrate at the same time the droplet discharged from the inkjet head lands on the glass substrate.Through the local assistance by laser energy, it was possible to reduce the nozzle clogging and damage to the substrate and to control of wet-spreading of ink through drying and higher viscosity.In this research, the droplet of about 25 m ~ 50 m diameter was discharged from the single head, and the area around the discharged droplet was irradiated using the carbon gas laser with wavelength of 10.6 m at continuous wave (CW) mode to form the wires.3.4 Objective of the technological development and its aimIn the wire forming technology using the inkjet, to reduce the wire resistance, it is necessary to control the wet-spreading and increase the wire thickness, or in other words, to improve the aspect ratio of the wire. Therefore, to solve both the high throughput and wire resistance reduction, we wanted to establish a process technology that enables formation of wire with high aspect ratio without recoating (Fig. 6).In the wire forming technology using the inkjet, the setting of the diameter of the discharged droplet is important. In the conventional inkjet technology, when the generally used droplet of 20 m is used for wire forming, the ink spreads out, and the width of the formed wire would be about 30~50 m even with substrate surface treatment[2]. Since recoating is necessary to reduce the wire resistance, the total time required to complete the patterning process is relatively high not only because of repeating coting process but also because of the waiting time necessary for the ink to dry before the next layer can be applied.If wire formation is done at droplet diameter of 10 m or less[10], the contribution of the surface area per unit volume increases according to the decreased size of the droplet[9], and the evaporation speed increases non-linearly during the flight of the droplet discharged from the inkjet head, making possible the control of the wet-spreading of the landed ink on the substrate surface and the wire formation of several m or less width can be realized. But, because of the thin thickness of the wire, recoating must be performed several times to reduce the wire resistance, and this decreased the throughput.To overcome the limitation of the conventional technology, the objective of laser-assisted inkjet technology was set to maintain the high throughput while realizing the wire width of 10 m or less that was considered impossible with conventional technology.The technological issue set was to reduce the wire width to smaller than the discharged droplet size by promoting drying with energy assistance, for droplets of about 25 m ~ 50 m in diameter. If the droplet size can be increased, the current inkjet head can be used, long-term stability and reliability can be obtained, the effect of airflow is reduced since the kinetic energy of the droplet increases, and the precision of the flying droplet landing on the substrate is increased. Moreover, by improving the landing precision, it would Fig. 5 Wire formation method by laser-assisted inkjet technologyFig. 6 Droplet size and wiring pattern targeted in the laser-assisted inkjet Forming directionFormed wireLaser headLaser beamGlass substrateLaser spotInk dropletInkjet nozzleInkjet headFastSlowLowHighForming speedWire resistanceWire width: 10 µm or lessWire width: 0.5~10 µmWire width: 30~50 µmDrying speed increases and wire width decreases by laser assistanceWire width increases due to wet-spreading of inkDrying speed increases and wire width decreases due to size effectCross-section of wiringTop view of wiringEffect on wire widthInk droplet size(droplet size 25~50 µm)Laser-assisted inkjet(droplet size 20 µm or more)Inkjet by conventional technology(droplet size 10 µm or less)Inkjet capable of discharging small droplet size

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