Copper interconnect electromigration behavior in various structures and precise bimodal fitting

Abstract

Copper interconnect electromigration (EM) was examined in various structures and three low-k materials (k = 2.65-3.6) using advanced back-end-of-line (BEOL) technology. Various structures for testing were designed to identify the EM failure modes, and extensive failure analysis was carried out to reveal failure in the test structures. A strong current dependence of the electromigration lifetime in three-level via-terminated metal lines was shown. Moreover, different processing approaches lead to different EM behaviors and related failure modes. The weak links of the interconnect system were identified by evaluating various test structures. Multimodality in the electromigration behavior of Cu dual-damascene interconnects was studied. Both superposition and weak-link models were used for the statistical determination of lifetimes of each failure model (statistical method). Results correlated to the lifetimes of the respective failure models physically identified according to resistance time evolution behavior (physical method). A good agreement was achieved. The activation energies of failure modes were calculated. Results of this study suggest that the interface of Cu interconnects is the key factor in EM performance.