Dielectric and Scaling Effects on Electromigration for Cu Interconnects

Summary

In summary, multi-link statistical test structures were used to study the dielectric and scaling effects on EM reliability for Cu interconnects. Experiments were performed on dual-damascene Cu interconnects integrated with oxide, CVD low-k, porous MSQ, and organic polymer ILD. The EM activation energy for Cu structures was found to be between 0.8 and 1.0 eV, indicating mass transport is dominated by diffusion at the Cu/SiN_x cap-layer interface, independent of ILD. Compared with oxide, the decrease in lifetime and (jL)_c observed for low-k structures can be attributed to less dielectric confinement in the low-k structures. An effective modulus B obtained by finite element analysis was used to account for the dielectric confinement effect on EM. The scaling effect was investigated using a statistical approach as a function of line width and barrier thickness for three line widths: 0.25, 0.175, and 0.125 µm corresponding to the 180-, 130-, and 90 nm nodes. Results revealed an intrinsic scaling effect based on the observed line width dependence of the strong-mode EM statistics, which is as expected. However, process-related issues were observed leading to decreasing early failure lifetime and reliability degradation for the 0.125-µm interconnects. The jL_c product was found to decrease with decreasing barrier thickness and the trend can be attributed to a decreasing confinement effect, which was estimated using an effective elastic modulus.