Statistical study for electromigration reliability in dual-damascene Cu interconnects

This paper summarizes results from recent studies on electromigration (EM) reliability of Cu dual-damascene interconnects using a statistical approach. First, mass transport in Cu damascene structure was discussed, where the activation energies for oxide, porous MSQ (methylsilsesquioxane), and organic polymer dielectrics were found to be in the range of 0.8-1.0 eV, suggesting mass transport is dominated by interfacial diffusion at the Cu and SiN_x cap-layer interface regardless of the dielectric material. Then the impact of low-k inter-layer dielectrics on EM characteristics was investigated by measuring the critical product of current density and conductor length, (jL)_c. Compared with oxide, the weaker mechanical strength of low-k dielectrics reduces (jL)_c due to less confinement and a smaller back flow stress gradient Δσ/L in Cu/low-k interconnects. Extrinsic failure due to interfacial delamination was observed in Cu/organic polymer interconnects, which caused further decrease in (jL)_c. Complementing EM tests, Monte Carlo simulation was developed based on the weakest link approximation to separate the bimodal failure distribution into two individual lognormal distributions and deduce the characteristics of the weak-mode (early) and the strong-mode failures. Failure analysis using FIB confirmed the bimodal failure behavior with void formation at the cathode via bottom found to be responsible for the early failures.