Grain Size And Cap Layer Effects On Electromigration Reliability Of Cu Interconnects: Experiments And Simulation

This paper combined experiments and simulation to investigate the grain size and cap layer effects on electromigration (EM) reliability of Cu interconnects. First the statistical distribution of EM lifetime and failure modes were examined for in laid Cu interconnects of large and small grain structures with two different cap layers of SiCN vs. CoWP. The CoWP cap was found to significantly improve the EM lifetime due to the suppression of the interfacial mass transport as a result of strengthening of the Cu/cap interface bonding. In addition, the grain size was observed to affect the EM reliability significantly, particularly for the CoWP capped structures. Resistance traces and failure analysis revealed two distinct failure modes: mode I with voids formed near the cathode via corner and mode II with voids formed in the trench several microns away from the cathode via. It was found that large grain size and strong cap interface reduced the mass transport rate and the void diffusion in the Cu line, leading to a longer EM lifetime and a higher proportion of mode II failures. A statistical simulation of EM lifetimes was also applied to Cu interconnects with grain structures generated by the Monte Carlo method. The simulation results for different grain sizes and cap interfaces are in good agreement with the experimental observations.