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Research paper : Development of flexible-printable device processing technology (T. Kamata et al.)−179 Synthesiology - English edition Vol.1 No.3 (2009) can be obtained by liquid phase process. However, this is an oxidation reaction that normally requires high-temperature treatment at 500 °C or above after film preparation. It is known that catalysts is useful to reduce process temperature. However, this is not adequate for electronic device use because it often acts as impurities in the device. Therefore, we considered providing required energy locally using light energy, and tried to develop technology that would promote this reaction. As result, we succeeded in developing a new technique by introducing multiple-source photooxidation method (Figure 2)[1]. Points for this new technological development were: selection of material with combined species that were less likely to be subject to stress failure on reaction precursor in forming SiO2 film; selection of light source with sufficient energy to excite this combined species; and selection of separate light source sufficient to excite the active species to react with this precursor. Particularly, major point in appealing the significance of newly developed process is that the reaction can be promoted using a conventional light source such as lamp, rather than locally high-density energy source such as a laser. By keeping the maximum reaction temperature of the entire process to 200 °C or less, occurrence of defect due to contraction and expansion of film could be controlled, and as result, highly dense SiO2 film was obtained. The formed SiO2 film showed high resistivity over 1015 cm and high dielectric strength over 7 MV/cm. This technology is currently considered for preparation of a dielectric layer of TFT for display use, especially for displays with flexibility and large size.(2) Triaxial distributed pressure annealing methodEven for “flexible printable device processing technology,” there are major limitations in the specification depending on the technology used. For example, for electronic display of memo level information, the limitation of production cost is extremely severe. In this case, expensive material cannot be used. Low-temperature processing must be used and there is limit on usable materials, but it is necessary to show that technological application is possible under this condition. Therefore, we developed technology to print wiring and electrode with low-resistivity on a conventional plastic film such as PET using conventional conductive inks. Conductive ink is normally thermally annealed at 400 °C or higher to reduce resistivity after forming the printed pattern. Recently, using nanoparticles of metals has been suggested to lower the annealing temperature. However if the material cost increases by using nanoparticles, the aforementioned objective is defeated. Therefore, we worked on low-temperature annealing technology to obtain low resistivity with conventional ink, and succeeded a new technique to reduce process temperature by using pressure as alternative energy to heat (Figure 3). Here, the point was also how to lower the temperature required for reaction. In this technology, low temperature was achieved by using pressure energy. Pressure energy was locally applied as anisotropic energy instead of applying energy homogeneously. That is, energy was concentrated onto necessary area and was not dispersed to surrounding area that did not require it. As result, in printing pattern using conventional silver conductive paste, we obtained resistivity 6×10-6 cm at reaction temperature 120 °C or less (for comparison, resistivity of bulk silver =1.6×10-6 cm). Considering that heating to 200 °C or more was needed to obtain similar resistivity even when silver nanoparticle paste was used, it can be seen that pressure energy is extremely effective in lower temperature in annealing.3.3 Printable device manufacture processThe greatest hurdle in developing printable device processing technology is whether the process specification requirement of being “printable” and the device specification requirement of being high performance can be both realized. For device performance, whether processing accuracy can be (25)−Fig. 2 Development of multi-source photooxidation method for forming dielectric layer by low-temperature printing.Fig. 3 Development of triaxial distributed pressure annealing method for forming low-temperature printed conductive pattern.Si heat oxidized filmDevelopment of this technologyConventionally applied SiO2 filmLeak current densityJ(A/cm2) (MV/cm)E24681010-1110-510-910-110-710-3020040060080010001×10-410-61×10-5This technologyMaximum temperature for PET filmBulk AgCeramicsType inkHigh-temperature annealingType inkNanoparticle dispersalAg inkResin typeAnnealing temperature / ℃ Resistivity / Ω･cm

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