5.3 Effect of Current Crowding and Microstructure

Electromigration degradation and failure in modern interconnect structures are the result of several physical phenomena acting simultaneously. In the case of dual-damascene technologies, both geometry and microstructure of the interconnect play an important role in determining the development of electromigration degradation. Different types of failure can arise from the effects each of these factors. An impact of the geometry can be observed in the case of a dual-damascene interconnect structure where the liner separates two copper regions and interrupts the material transport. This configuration leads to material depletion and void formation. Furthermore, when the dimensions of modern interconnects are on a scale where the material properties at the microscopic and atomistic levels are gaining importance, the impact of current crowding on the development of electromigration-induced voiding can be significant. The scaling of geometrical features enhances the impact of microstructure of the metal interconnect on the electromigration failure. The microstructure introduces a diversity of possible electromigration paths and local mechanical properties depending on the crystal orientation of the grains. Grain boundaries act as fast diffusivity paths and vacancy recombination sites. The diffusion of vacancies in the grain boundary is faster compared to diffusion in the grain bulk, because a grain boundary generally exhibits a larger diversity of point defect migration mechanisms. Formation energies and migration barriers of the atoms inside the grain boundaries are on average lower than those for the crystal lattice [143].

In this work, state-of-the-art models of electromigration are used to investigate the impact of current crowding and microstructure in electromigration performance in simple single layer dual-damascene copper structures. In particular, the electromigration performance of a standard linear structure is compared to a L-shaped geometry. Both interconnect structures are analyzed and the results are compared with the original experimental observations provided by Croes [39].