Effect of texture and elastic anisotropy of copper microstructure on reliability

Copper in spite of being face centered cubic crystal has significant mechanical anisotropy. The elastic constants of copper vary considerably for different crystallographic orientations. Typically, the copper metal conductor lines in integrated circuits are polycrystalline in nature. The polycrystalline microstructure is known to impact the reliability and is yet to be thoroughly understood. In this work we used Voronoi tessellation to model the polycrystalline microstructure for the copper metal lines in test structures. Each of the grains was then assigned an orientation with distinct probabilistic texture with (111) as the preferred orientation and corresponding anisotropic elastic constants based on the assigned orientation. By subjecting the test structure through a thermal stress, we observed over 70% variation in hydrostatic stresses along the grain boundaries depending on the orientation, dimensions, surroundings, and location of the grains. This may introduce new weak points within the metal interconnects leading to failure. Hence, inclusion of microstructures and corresponding anisotropic properties for copper grains is crucial to conduct a realistic study of both the stress voiding and electromigration phenomena, especially at smaller nodes where the anisotropic effects are significant.