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We demonstrate the design and fabrication of a mode (de)multiplexer based on the tapered mode-selective coupler. The proposed device is directly inscribed in the substrate material by a needle-type liquid micro-dispenser and a three-axis robot stage using commercially available UV-curable resins. The fabricated mode (de)multiplexer has an excess loss less than 0.3 dB, a coupling ratio higher than 0.93, and an extinction ratio higher than 23 dB within a broadband of 100 nm around 1550 nm. The near field patterns of LP 11 mode at different wavelengths were successfully observed using an infrared chargecoupled-device camera. The fabrication tolerance on the spacing, core diameter deviation, and core circularity were investigated and the results imply that our fabrication process can satisfy the requirements on it. The proposed method is capable of fabricating functional devices for high-speed on-board optical interconnects.

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We have previously developed a novel plating-free bumping process using Cu nanopaste and Injection Molded Solder (IMS) technology. In the present study, we investigated the further detail about the microstructural and mechanical properties of sintered Cu nanoparticles formed into a pillar shape. By analyzing cross-sections of Cu nanoparticle pillars sintered in various conditions, we clarified how the sintering conditions affect the microstructural features, including the size and numbers of Cu grains and voids inside sintered Cu nanoparticles. In addition, we conducted the shear testing for the obtained Cu pillars to evaluate relationships between the mechanical strength and the microstructural features. We found that the results of the shear testing were consistent with the microstructural features of the sintered Cu nanoparticles. Finally, we injected molten solder onto the Cu nanoparticle pillars to evaluate the overall feasibility of the developed process. It was confirmed that the molten solder injected by IMS process has good wettability against the sintered Cu nanoparticles, which resulted in the successful bump formation without solder missing. In addition, The IMC layer between the sintered Cu nanoparticles and injected solder was formed well. These results proved the quality of microbumps fabricated by the novel bumping process using Cu nanopaste and IMS.