Cluster interactions and stress evolution during electromigration in confined metal interconnects

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Abstract
In narrow metal interconnects used in advanced integrated circuits, electromigration flux divergences occur at the intersection between polycrystalline cluster segments (where grain boundaries offer a fast diffusion path), and bamboo segments (where there are no grain boundaries along the line length). In confined, passivated metal interconnects, these flux divergences are linked to the evolution of significant mechanical stresses in the metal. A quasisteady state stress distribution builds up quickly in the cluster segments and remains unchanged until the stress profiles between cluster segments begin to overlap, and the clusters begin to ‘‘interact.’’ A significant increase in stress above the quasisteady state can result from cluster interactions, increasing the potential for electromigration and stress-induced damage. If the cluster separation is small, this stress increase can occur on a time scale which is short compared to the stress evolution of the interconnect line as a whole.

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