A study was made of the stress migration and electromigration interaction in lower (MX structure) and upper metal (MX+1 structure) of dual-damascene Cu/low-κ interconnects. It was found that both mechanisms were dependent. Statistical analysis showed that the electromigration failure time was affected by the presence of residual stress induced by stress migration. This effect was more severe in the lower metal, where the electromigration median-time-to-failure (t50) for the majority of samples could be degraded by 30 to 60%. For the upper metal of Cu interconnects, the t50 was degraded by about 10%. The reliability implication of the residual stress in copper interconnects on the electromigration was further investigated with various failure analysis techniques and three-dimensional finite element simulation. It was proposed that stress migration could influence electromigration when there was significant amount of vacancy accumulation due to stress migration in the cathode area which accelerates electromigration nucleation time. In the case of the MX structure, the experimental results showed that stress migration and electromigration interaction occurred exactly below the via at the MX cathode side, leading to abrupt failures. On the other hand, in MX+1 structure, vacancies were likely to accumulate at the edge of upper metal lead during stress migration tests, thus accelerating the failure during subsequent electromigration test. A failure mechanism model for stress evolution and void formation was proposed to provide insight into the interaction between these two failure mechanisms.

The Effect of Stress Migration on Electromigration in Dual Damascene Copper Interconnects. A.Heryanto, K.L.Pey, Y.K.Lim, W.Liu, N.Raghavan, J.Wei, C.L.Gan, M.K.Lim, J.B.Tan: Journal of Applied Physics, 2011, 109[1], 013716