Effects of Ti insertion between Cu and TiN layers on reliability in Cu/Ti/TiN/Ti layered damascene interconnects

Abstract

We investigated the effects of Ti insertion between Cu and TiN layers on electromigration (EM) in Cu/Ti/TiN/Ti layered damascene interconnects. The Cu damascene interconnects with Ti insertion show an EM lifetime of up to 100 times longer than those without Ti insertion, depending on the inserted Ti thickness. The lifetime improves as Ti thickness increases from 10 to 20 nm. Ti insertion increased the sheet resistance of the layered film after annealing and enhanced the adhesion of Cu to underlayer TiN. The resistance change was mainly caused by formation of Cu–Ti reaction products resulting in reduced cross-sectional area in the Cu film, and was also caused by Ti incorporation into the Cu film. Ti insertion, however, does not have any effect on Cu film microstructure such as median grain size, standard deviation of grain size distribution, and texture. During EM testing, the resistance in Cu interconnects with Ti insertion initially decreases, then rises gradually, and finally increases rapidly until failure occurs. This behavior is probably caused by Ti redistribution in Cu during the EM testing. EM lifetime consists mainly of the time when the interconnect resistance is lower than the initial resistance, that is, the time before void growth occurs (incubation time). Improved Cu adhesion to underlayer TiN is an important factor because the void growth rate of Cu interconnects during EM testing is considerably decreased by Ti insertion. The void growth rate, however, does not depend on Ti thickness and Ti surface treatment before Cu deposition. As a result, EM lifetime is mainly governed by the incubation time, and the void growth rate is determined by Cu EM through the Cu/barrier interface path.