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We demonstrated fabrication of Cu-TSV using all-wet process, which has a high potential to apply for the via-last Cu-TSV process. A Co-W alloy diffusion barrier layer was formed on SiO2 layer by electroless plating at 60 °C with Pd nanoparticle catalyst. The barrier layer was thin and continuous throughout a 2 φ × 24 μm TSV and its adhesion strength on SiO2 was as high as that of PVD-Ta. The Co-W alloy layer has undergone an interdiffusion test, which showed a high resistance against Cu diffusion into SiO2 layer. Then, seed layer was formed by electroless Cu deposition on the Co-W alloy layer through displacement reaction. These results reveal a feasibility of all-wet fabrication in a high aspect ratio TSV process.