Segregation of copper in dilute aluminum-copper alloys for interconnects

Abstract

It is well known that the resistance to electromigration of aluminum-based interconnects is enhanced by alloying with copper. The mechanism is thought to involve the segregation of copper to the grain boundaries. The copper concentration at grain boundaries has been measured in well annealed alloys, with the compositions A1-0.5 wt% Cu and Al-2 wt%Cu using a VG HB5 STEM. The grain structure was stabilized by an anneal at 673 K for one hour and the grain boundaries were loaded with copper by an anneal for 90 hours at 503 K. It was found that the copper concentration varied from boundary to boundary. With the assumption that the copper was concentrated in a single layer the saturation concentration of copper was found to be in the range 0.025 to 0.25. This corresponds to an enhancement of the copper concentration relative to that in the grain interiors by a factor in the range between 3 and 30. The higher value approaches the copper concentration which would correspond to a monolayer of theta phase. Extensive surface segregation, amounting to a monolayer equivalent was also detected.