Evidence of Cu diffusion paths was found here using fine scanning electron microscopy failure analysis after electromigration failures in single damascene interconnect structures. For the polycrystalline grain structures, it was established that grain-boundary diffusion occurred, although experimental electromigration activation energy values were ranging from 0.65 to 0.80eV. Narrow lines with quasi-bamboo microstructures exhibited interface diffusion located at upper surface of Cu damascene e.g., SiN capping layer interface, with activation energy values ranging from 0.80 to 1.06eV. Due to the compression stress state in Cu under electromigration and oxide cracking, respectively, SiN cap de-bonding occurred at the upper corners of the damascene trench and resulted in Cu extrusions. For both microstructures, this failure mode provided decreased extrapolated lifetimes. Electromigration experiments on two-level damascene test structures provided an upper limit of the Blech length threshold product of jLth<4000A/cm. The data were compared with available literature results and to studies of AlCu.

Evidence of Grain-Boundary Versus Interface Diffusion in Electromigration Experiments in Copper Damascene Interconnects. L.Arnaud, T.Berger, G.Reimbold: Journal of Applied Physics, 2003, 93[1], 192-204