Statistics of electromigration early failures in Cu/oxide dual-damascene interconnects

Electromigration (EM) study at temperatures between 325 and 400 degreesC and current densities between 1.0 and 2.0 MA/cm(2) has determined the failure time characteristics and failure behavior of submicron dual-damascene Cu/oxide interconnects. The test structures used are based on statistical concepts potentially suitable to address the "early failure" issue in submicron interconnects and are purposely designed to examine failures occurring only in dual-damascene interconnects. A combination of single and repeated (N = 1, 10, 50, and 100) serial chains of nominally identical interconnects are used in conjunction with statistical analysis based on "weakest-link" [1] concepts to identify differences in the failure distribution as larger collections of interconnect elements are sampled. In total, nearly 10,000 interconnect have been tested using this configuration. Through the use of these multiply-linked interconnect ensembles [2-5], statistical evidence of two distinct ("weak-" and "strong-mode") failure modes in dual-damascene Cu/oxide interconnects is first reported. The bimodal failures have also been identified with distinct void formation mechanisms that appear characteristic of the dual-damascene interconnect architecture. The weaker mode is found to be void formation within the dual-damascene via, while the stronger mode is associated with voiding that occurs in the dual-damascene trench. The weak mode activation energy is found to be about 1.0 eV and seems consistent with void formation that is controlled by interface diffusion between the Cu metal and its Ta diffusion barrier. These observations using this type of testing methodology confirm the utility of the multi-link approach in electromigration reliability analysis and the detection of "early failures" [3].