Modeling and characterization of electromigration failures in IC metallization systems and copper metallization for ULSI application.

摘要

Electromigration characteristics and failure mechanism under DC, pulsed-DC and AC stress conditions have been studied in different materials (e.g., Al-based alloys, Cu metallization, and TiN and TiW barrier layer materials) and different structures (e.g., TiN/Al-alloy/TiN multilayered interconnects and W-plug and Al-via structures). Models have been developed to project the electromigration lifetime under different stress conditions. In via reliability studies, we found that although W-plug can eliminate the step coverage problem, this structure is not ideal from electromigration reliability point of view because the intermetallic contact represents a worst atomic flux divergence location. Al-via structure is much more reliable than W-plug structure with respect to electromigration.;Experimental and simulation results have demonstrated that the damage healing observed in TiN/Al-alloy/TiN multilayered interconnects under low current density stress was caused by stress gradients induced during Al-alloy electromigration instead of thermomigration induced by joule heating. Models have been developed to clarify the lifetime dependencies on interconnect line length and width for multilayered interconnects. The experimental results also show that the failure observed in TiN and TiW barrier layer materials was not caused by electromigration, but instead was due to thermomigration caused by temperature gradients in the test structure. The activation energy of this thermally-activated process for TiN was found to be 1.5eV. A 10-year lifetime is projected if the hottest spot temperature in a TiN layer is kept below 408;It has been experimentally verified that Cu has a much longer electromigration lifetime than Al-based alloys. The electromigration failure mechanism for Cu was found to be the same as Al-based alloys. The effects of complete and partial TiN encapsulation on the integration of Cu metallization into IC processes have been studied, and the results demonstrated that TiN is an effective diffusion barrier for Cu. By using the complete encapsulation Cu can be fully incorporated into ULSI device applications without degrading their electrical properties and gate oxide integrity.;Models have been developed to project the electromigration lifetime under time-varying stress conditions. The AC lifetime has been experimentally found to be orders of magnitude longer than DC lifetime because of the damage healing effect. The damage healing effectiveness (