EM and SM induced degradation dynamics in copper interconnects studied using electron microscopy and X-ray microscopy

In addition to statistically relevant standard reliability tests and lifetime analysis, the study of solid-state physical degradation mechanisms for a limited number of representative samples is needed to understand weaknesses in the interconnect technology and to exclude reliability-related failure in Cu interconnects. We present dynamic studies of damage mechanisms in on-chip Cu interconnects caused by electromigration (EM) and stress migration (SM). Scanning electron microscopy (SEM) and synchrotron-based transmission X-ray microscopy (TXM) are applied to visualize the void evolution, electron backscatter diffraction (EBSD) in the SEM and conical dark-field (CDF) analysis in the transmission electron microscope (TEM) are applied to characterize the Cu microstructure. In case of EM, our experiments show that voids are formed at interfaces and grain boundaries, often far away from vias in via/line interconnect structures. Due to the gradient of the electrical potential, Cu atoms migrate along weak pathways for material transport. Depending on the interface strength, voids that virtually move along interfaces or grain boundaries over large distances into the opposite direction of the current flow, i. e. toward the cathode end of the line, have been visualized. In case of SM, our experiments do not show void movement over large distances during the stress-induced voiding (SIV). In via/line interconnect structures we rather observe that voids are formed directly beneath the via, i. e. in wide Cu line at the edge of the via bottom. It is concluded that voids are originally formed at the site where eventually the catastrophic failure occurs. During SM tests, Cu atoms migrate from regions of low tensile (or high compressive) stress to regions of high tensile stress, and simultaneously, vacancies migrate along the stress gradient (within a limited range of some μm) in the opposite direction, to the location where vias connect wide Cu lines. For both EM and SM, the driving forces for atomic transport depend strongly on the particular geometry of the tested structure, but also on interface bonding and metal microstructure.