Vertical Interconnections by Electroless Au Deposition on Electroless Ni Immersion Au Surface Finish

Electroless Au is used to vertically connect copper pillars with electroless Ni immersion Au surface finish on two stacked chips by a microfluidic electroless interconnection process. A very low bonding temperature of 50 degrees C and a pressure-less bonding process can be achieved. The vertical interconnections are formed by a forced flow of electroless Au plating solution through a microchannel so that the reduced Au atoms self-assemble between the gaps of facing copper pillars to complete the interconnections. The deposited Au grains span the entire gap across the two copper pillars, suggesting that the rate-limiting step is more likely to be the nucleation rather than the growth of the Au grains. Four-point probe measurements show that the average resistance of the electroless-Au-bonded interconnections is low. Mechanical shear testing reveals that the bonding of the interconnections is strong. Furthermore, it is demonstrated that this process is capable of accommodating a high degree of pillar misalignment.