High temperature electromigration behavior of cobalt lines observed by in situ transmission electron microscopy

Abstract

As industrial practices shift away from Cu as the initial back-end-of-line interconnect material due to size limitations, new candidate metals are being tested and characterized. Electromigration resistance is particularly important in ultra-narrow lines and an in situ study provides unique insight into the formation and progression of electromigration damage. This, in turn, helps to inform device design so that electromigration resistant circuits can be produced efficiently. In this work, the authors demonstrate an in situ transmission electron microscopy technique for electromigration analysis of Cu replacement metals in microelectronic interconnects. Using this method, candidate metal lines can be tested at high current densities, ∼5×106 A/cm2, at controllable temperatures over the range of 300–1000 °C. In this work, cobalt lines are tested in the range of the effective valence inversion temperature. The analysis examines void nucleation, growth, and migration as a function of temperature and line geometry. We find that there is a relative insensitivity of failure time to operating temperature, with samples tested between approximately 600 and 900 °C having roughly equivalent failure times. We ascribe this result to a combination of linewidth effects and a decrease in the magnitude of the effective valence approaching the inversion temperature. Failure mechanism is also not affected by temperature in this range, with the primary determining factor being the linewidth and corresponding availability of grain boundaries for diffusive mass transport. We also observe increased lifetimes of devices with uniform temperatures compared to those in which large thermal gradients exist.