Electroless Copper Deposition with Pyramidal Micro-cones Morphology for Low-temperature Cu-Cu Bump Interconnections

Interconnection between IC chip and substrate is the core function of microelectronic packaging. Direct copper-to-copper interconnection has received extensive attention owing to its superior electrical and mechanical properties. At present, the bonding temperature of Cu-Cu interconnection is generally as high as 300°C -400°C, which is unfavorable for the integrity of thermally fragile ICs. There have been methods of forming non-solder metal interconnections between chip- and substrate-side metallizations using electroless Cu or Ni deposition in recent years, which provide strong bonding strength through low-temperature metal growth process and interfacial merging. However, the possible presence of interfacial voids caused by insufficient cupric ion supply and peripheral closure under high deposition rate has limited the practical productivity of electroless interconnection to a low value. In this study, in pursuit of a fast and reliable electroless interconnection process, we propose the method of depositing Cu with a special micro-cone morphology for forming all-copper interconnections at the rate of>15μm/h. By using certain crystallization modifier, the growth of Cu deposit can be tuned to highly anisotropic such that sharp and fast-growing micro-cones can be formed and the merging of opposite Cu bumps can take place through simultaneous fusing at multiple locations, leaving sufficient space for the electroless deposition solution to flow through to allow subsequent merging. A micro fluidic channel device was utilized to pump the plating solution, which enhanced the ion supply and guaranteed high growth rate of Cu. The cross-sectional morphology of the joints was examined microscopically, the electrical performance, as well as the mechanical strength of the joints, were also evaluated. It was found that high shear force joints of 1N per joint was obtained through deposition time of 1h. The merging mechanism of the micro-cone-morphology Cu layer is discussed. This method is therefore demonstrated as a potential candidate for the high-quality fine-pitch interconnection process, especially for 3D integration.