Electroless Ni Plating to Compensate for Bump Height Variation in Cu–Cu 3-D Packaging

The feasibility of electroless Ni plating to compensate the Cu bump height variation in Cu-Cu 3-D packaging was investigated. After fabricating Cu pillar bump and Cu micro-stud structure on the top and bottom chips, respectively, Cu-Cu thermocompression bonding and then electroless Ni plating were sequentially carried out and evaluated using dies shear force, daisy chain resistance, and microstructure. It is found that thermocompression bonding parameter optimization and Ar plasma surface cleaning increased the Cu-Cu bond strength. The electroless Ni plating can compensate for the bump height variation of 1.9 μm in a chip with filling the gap between the Cu-pillar bump and Cu-micro-stud structure. In addition, the electroless plated Ni reduced the daisy chain resistance with the increase of the contact area. The resistance of bump daisy chains after electroless Ni plating was measured to be about 15% lower than that obtained after Cu-Cu thermocompression bonding.