Effect of Microstructure and Anisotropy of Copper on Reliability in Nanoscale Interconnects

The mechanical behavior of copper is highly anisotropic. Although it is a face centered cubic crystal, the elastic constants vary considerably for different crystallographic orientations. Typically, the copper metal conductor lines in integrated circuits are polycrystalline in nature. In this paper, we utilize Voronoi tessellation to model the polycrystalline microstructure for the copper metal lines in test structures and then assign textured orientation to each grain and assign corresponding anisotropic elastic constants based on the assigned orientation. By subjecting the test structure through a thermal stress, we observe over 10x variation in normal 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 where normal stresses can be very high depending on the orientation of the grains leading to delamination and accumulation sites for vacancies. Hence, inclusion of microstructures and corresponding anisotropic properties for copper grains is critical to conduct a realistic study of both stress voiding and electromigration phenomena, especially at smaller nodes where the anisotropic effects are significant. Further, a comparison between stress levels in test structures with SiCOH and SiO2 as the inter level dielectric was conducted.