Metal–metal bonding process using metallic copper nanoparticles prepared in aqueous solution

A method for preparing metallic Cu nanoparticles in aqueous solution is proposed, and metal–metal bonding with the use of the nanoparticles is demonstrated. The metallic Cu nanoparticle colloid solution was prepared by mixing a 0.01 M aqueous solution of copper salt (copper chloride (CuCl2), copper nitrate (Cu(NO3)2), or copper acetate ((CH3COO)2Cu) (copper source) and a 0.2–1.0 M hydrazine aqueous solution (reducing reagent) in the presence of 0.0005 M citric acid and 0.005 M cetyltrimethylammonium bromide (stabilizers). The metallic Cu nanoparticles with average sizes in the range of 54–64 nm were prepared for all the copper salts and the hydrazine concentrations examined. A stage and a plate of metallic Cu were successfully bonded under annealing at 400 °C and pressurizing at 1.2 MPa for 5 min in H2 gas with the help of the metallic Cu particles. The shear strength with the use of (CH3COO)2Cu was large compared to CuCl2 and Cu(NO3)2. A shear strength required for separating the bonded Cu substrates reached the maximum value at a hydrazine concentration of 0.6 M, and it decreased beyond the concentration. Consequently, the largest shear strength of 37.7 MPa was achieved using (CH3COO)2Cu at the 0.6 M hydrazine concentration.