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Research paper : Development of laser-assisted inkjet printing technology (A. Endo et al.)−12−Synthesiology - English edition Vol.4 No.1 (2011) However, as the ejecting frequency of the inkjet head is increased, the meniscus (boundary between the ink and air) formed at the nozzle orifice vibrates during discharge making the continous discharge very unstable. Moreover, problems such as unstable discharge at various vibration modes occur due to the pressurization and decompression of the ink in the injector of the inkjet head. Therefore, it is necessary to simultaneously optimize the various parameters such as the droplet diameter, orifice diameter, volume change of the actuator, and physical properties of the ink (surface tension, viscosity, etc.). In the current inkjet technology, it is extremely difficult to significantly increase the ejecting frequency to several ten kHz without altering the pattern formation[7].In the conventional inkjet wiring technology, the fine-sizing of the wires and the throughput have a trade-off relationship, and, to overcome this, a breakthrough in the technology to control the wet-spread after the droplet lands on the substrate was needed.3.2 Conventional methods for controlling the wet-spreadOur research was also focused to control the wet-spreading of ink that lands on the substrate, which is an essential issue of inkjet printing technology. Summarized below are the methods for controlling the wet-spreading of ink in the R&Ds so far, and why the throughput cannot be further improved by further optimizations of these methods(Fig. 4).1) Reduction of resistivity by improvements of inkBefore considering the methods to control the wet-spreading of ink, it may be possible to decrease the wire resistance by replacing the ink material with one with lower resistivity. However, it is clear that the ink resistivity cannot be lowered below the resistivity of the metals contained in the ink. Specifically, the resistivity of the currently commercially-available nanoparticle silver ink is 2−5 x 10−6 ·cm, which is higher than the 1.6 x 10−6 ·cm of silver metal. The room for reducing resistivity by improvements in ink material is relatively small. Therefore, we did not set the reduction of wire resistance through ink improvement as our development approach.2) Use of ink with high viscosity; decreased diameter of dropletsWhen high viscosity ink is used, as mentioned before, there are reductions of the ejecting frequency or decreased throughput, because of the meniscus issue and the vibration mode that inhibits the stable discharge. There is also the problem of nozzle getting clogged easily. By reducing the nozzle diameter in order to reduce the droplet size decreased throughput cannot be avoided and the possibility of nozzle clogging increases.3) Substrate surface treatment to control the wet-spreading after landingAlthough the method of substrate surface treatment may possibly control the wet-spreading of ink while reducing the width of wire, the surface treatment agent may reduce the adhesiveness of the ink and wire. For example, when the wire is formed using the aqueous-solvent conducting ink on a flexible substrate such as water-repellent polyimide, the adhesiveness may be greately affected by the surface treatment[8]. To avoid this and to increase the adhesiveness of ink, patterning is done on the hydrophilic and hydrophobic surfaces using a mask, and then applying the ink only to the hydrophilic surface[9]. However, this causes an increase in the manufacturing process steps and the total improvement of throughput cannot be achieved.4) Increased drying speed of ink by heating the substrateWhen the substrate is heated, there are serious problems such as the nozzle drying due to the heat radiation from the substrate which causes clogging, and cracks and gaps are formed in the wire as bumping occurs when the ink lands on the substrate. Therefore, this method has no good practical applications.The fine-sizing of the wire width by controlling the wet-spreading of ink and the achievement of high throughput of the wiring process were at a trade-off relationship. This was a technological issue that could not be overcome with conventional technology.3.3 Ink drying method using the laser energyTo realize both the fine-sizing of the wire width and high throughput that were in trade-off relationship, we developed a process technology from a totally new approach. As the method to control the wet-spreading of ink, we did not select the methods that were extensions of conventional solutions such as the use of high viscosity ink, small nozzle bore, or substrate surface treatment, but we selected the method of providing energy assistance to the discharged droplet to Fig. 4 Conventional methods to control the ink wet-spreading and the effect finally obtainedPractically impossibleDecrease throughputControl heat in micro-regionsPatterning on hydrophilic substrate surfaceIncrease ink supply per unit lengthDecrease discharge frequencySolution methods:Nozzle clogging; bumping in wire and dotDecrease adhesiveness of formed wireDevelop small bore inkjet headMatch inkjet headIssues to be solved:Heat substrateSurface treatment of substrateDecrease size of dropletIncrease viscosity of inkSolution methods:How to control ink wet-spreadingLimitation due to bulk valueHow to obtain practical wire resistance valueImprove resistivity by improvement of ink

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