Thermal Compression Cu-Cu bonding using electroless Cu and the evolution of voids within bonding interface

Direct Cu-Cu bonding is successfully bonded at 250 °C, 5MPa under 10−2 torr for 15min with the use of electroless fabricated Cu. Several effected factors on bonding interface including temperature, pressure, surface roughness, and time are studied.Direct Cu-Cu bonding in Cu pillar bump is currently used to replace traditional solder bump due to the capability of scaling down pitch, better electrical and mechanical properties. Among all methods to fabricate Cu, electroless plating possesses the advantages of simple fabrication process, high uniformity and low cost. Moreover, the autocatalytic behavior of electroless deposition shows a high level of competence on massive production of uniform Cu layer without the use of external electrical energy under atmospheric environment, which is beneficial to the industries. Therefore, it is worthy of developing Cu-Cu bonding process using electroless fabricated Cu for future three-dimensional (3D) integration applications.In this study, Cu films are first deposited on silicon substrates. Chemical Mechanical Polishing (CMP) process is used to reduce the surface roughness of electroless Cu for comparing the bonding interface of different roughness. The effects of temperature, external pressure, surface roughness, and bonding time are studied to optimize the bonding parameters. Through prolonged annealing under 10−2 torr, the void ratio of the bonded joints can be further reduced. Several factors which contribute to the reduction of interfacial voids are studied and their mechanisms are delivered. To sum, a newly Cu- Cu bonding using electroless fabricated Cu is developed and optimized.