Low-temperature bonding of copper pillars for all-copper chip-to-substrate interconnections

A copper-to-copper bonding process was developed for an all-copper, chip-to-substrate interconnect technology. High aspect ratio polymer molds for electroplating were formed using a photodefinable polymer on both the chip and the substrate surfaces. Copper pillars were fabricated by electroplating metal in the polymer molds. The chip-to-substrate all-copper connections were formed by joining the two pillars with electroless copper plating followed by an anneal process. The copper-to-copper bonding of the high aspect ratio pillars does not require the use of solder or other noncopper metals. Mechanical shear force measurements were used to characterize the bonding process as a function of annealing conditions. Excellent bond strength of the electrolessly joined pillars was achieved with a 250 degrees C anneal, with the bond strength of the copper pillar interconnects exceeding 148 MPa. High aspect ratio pillars can provide mechanical compliance, and the electroless fabrication method compensates for pillar misalignment and nonplanarity of the bonded surfaces. (c) 2006 The Electrochemical Society.