Cu film surface reduction through formic acid vapor/solution for 3-D interconnection

Copper has been was widely applied on packaging technique because of the high conductivity, high thermal conduction index and low cost. It is hard to make Cu-Cu direct bonding for 3-D interconnection because Cu oxide is easily generated on copper surface. Formic acid is an organic acid with simple molecule formula, which may decompose to hydrogen and react with oxide to reduce Cu surface. In this study, formic acid vapor and solution were performed to reduce the surface of copper film for Cu low temperature direct bonding, respectively. After formic acid vapor/solution reduction, fewer oxide and higher roughness on the surface of copper film were obtained. With the increase of the formic acid vapor reduction time, the surface roughness and particle size become larger at 200⁰C. The copper film was corroded by formic acid solution, which make the surface roughness and particle size of copper film increase significantly for the solution reduction. Making use of the formic acid solution, better surface reduction and rougher surface for Cu film were acquired. The surface reduction used vapor reduction is not so fine compared to the reduction of the formic acid solution, but smoother than the same reduced by formic acid solution. The copper to copper direct bonding was realized at 200⁰C after formic acid vapor/solution reduction. Finally, the bonding interface was analyzed by TEM.