Statistical Analysis of Electromigration Lifetimes and Void Evolution

Electromigration (EM) failure statistics and the origin of the lognormal standard deviation (σ) for Copper (Cu) interconnects have been investigated by analyzing the statistics of EM lifetime and void size distributions at various stages during EM testing. Experiments were performed on 0.18μm wide Cu interconnects with tests terminated after certain amounts of resistance increases, or after a specified test time. Void size distributions of resistance-based, as well as time-based EM tests were obtained using focused ion beam (FIB) microscopy. The lifetime and void size distributions were found to follow lognormal distribution functions. The σ values of EM lifetime and time-based void size distributions decrease with higher percentages of resistance increase, reaching an asymptotic value of σ ∼ 0.14. In contrast, σ values of resistance-based void size distributions are significantly smaller and do not show an obvious dependence on time. The statistics of resistance-based void size distributions can be accounted for by considering geometrical variations of the void shape, while the characteristics of time-based void size distributions require consideration of kinetic aspects of the EM process. The σ values of EM lifetime distributions can be adequately simulated from experimentally obtained void size distributions, including geometrical and experimental factors of the EM experiment as well as kinetic aspects of the mass transport process, such as differences in interface diffusivity between the lines. The variation in diffusivities at the cathode ends of the lines arises from differences in the interface structure as a result of varying Cu grain orientations