A fabrication process has been developed and characterized to create all-copper chip-to-substrate input/output connections. Electroless copper plating followed by low-temperature annealing in a nitrogen environment was used to create an all-copper bond between copper pillars. The ability to fuse the two copper surfaces at modest temperature and pressure is demonstrated. The bond strength for the all-copper structure exceeded 165 MPa after annealing at 180°C. During the anneal process, a significant microstructural transformation in the bonded copper-copper interface was observed. The changes were correlated to an increase in the bond strength. The process was characterized with respect to in-plane misalignment of bond sites. Significant planar misalignment, greater than the diameter of the pillars, could be tolerated. Through-plane mismatches between the pillars ͑pillar gap͒ as large as 65 m could be overcome, resulting in good pillar-to-pillar bonding. Successful silicon-on-silicon and silicon-on-FR-4 bonding was achieved with no degradation of the organic board.

A fabrication technique using electroless copper deposition has been used to produce all-copper chip-to-substrate connections. This process replaces solder by electrolessly joining copper pillars on a chip and substrate. Previously, solid copper-to-copper bonding was demonstrated using a known electroless copper bath followed by low temperature annealing at 180°C for 1 h in a nitrogen environment. Although the process feasibility was demonstrated, it was inherently slow and required excessive process time. In this paper, an acceleration-suppression approach to copper plating was used to achieve a rapid deposition of high quality copper in enclosed regions. Elevated temperature was used for acceleration along with poly͑ethylene glycol͒ ͑PEG͒ suppression. High temperature increased the transport of reactants and products in spatially restricted regions, and the addition of PEG provided control of the deposition rate. This allowed a kinetically controlled deposition while still maintaining good quality copper deposits without excessive porosity. Plating rates as high 6 m/h in the spatially restricted region between mated pillars were achieved.