Stages of damage formation by electromigration in line/stud structures

Abstract

The electromigration characteristics and kinetics of damage formation for Ti/Al(Cu)/Ti‐W two‐level interconnect structures have been investigated. The mass transport as a function of temperature was measured using drift‐velocity and resistance techniques. Progressive void growth at the cathode end was found to be responsible for open, or high resistance, failure of the line/stud structures. The dominant mass transport path in on‐chip interconnects from 0.7 to 2.0 μm in width is along the edges of line (interfacial diffusion). Lifetime was similar in bamboo and multigrain lines. The solute Cu at Al/Al oxide interfaces and grain boundaries drastically reduces the Al motion. The observed mass transport in Al(Cu) lines can be separated into three stages: incubation, slow Al migration, and steady‐state. The incubation period is the time necessary to sweep out Cu solute beyond a critical length from the cathode end. The slow Al motion is the period when the depletion zone of Cu at the cathode end slightly exceeds the critical length at which the stress‐induced flow of Al can no longer compensate the electromigration flux. As the test continues, the Cu depletion zone at the cathode end increases beyond the critical length and a final steady‐state similar to electromigration in pure Al lines is reached. Using the proposed model, the electromigration mean lifetime is related to current density, jm, where m is between −2 to −1.