Bonding of copper pillars using electroless Ni plating

High bonding temperature and force are known to cause physical damage to the device. The objective of this present study is to develop a new low-temperature and pressureless bonding method for Cu-to-Cu interconnects using electroless Ni plating. Two plating methods were employed to test the feasibility of electroless bonding of copper pillars. The first method was carried out in a stirred plating bath with a stirred rate of 500 rpm with and without surfactant additives. The second method was performed in a microfluidic fixture, in which the plating solution was directly pumping into the test vehicles. Preliminary results show that a seam-free bonding joint of dome-shaped copper pillars could be obtained with the use of the microfluidic fixture, while a continuous seam was observed after bonding in a stirred bath with the addition of Triton X-100. In addition, an elongated disc-shaped void was present in the flat-topped copper pillars after bonding in both methods.