Evidence of grain-boundary versus interface diffusion in electromigration experiments in copper damascene interconnects

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