Influence of trace tungsten contents on thin-film properties and electromigration behaviors of electroless-deposited cobalt interconnect lines

Electroless-plated cobalt alloys with tungsten typically exceeding 5 at.% are the common barrier materials for copper metallization. As cobalt is a new interconnect material for advanced microchips, it is worth studying the advanced cobalt-interconnect metallization process that involves extremely diluted cobalt-tungsten (CoW) alloys with an eliminated impurity resistivity and enhanced electromigration reliability. To this aim, we fabricated three different CoW nanolines with 0.03, 0.06 and 0.11 at.% of W using an electroless process, and examined their electromigration behaviors under accelerated stressing conditions, using Co lines as a control. All three W contents, especially 0.06 at.%, mitigate Co electromigration. The mechanism for the electromigration mitigation is due to an improvement of the film's nanomechanical and surface wetting properties, as well as grain-structure refinements. A full account of the relationship between the film properties, grain structures and electromigration testing results is given, reasonably explaining the cause of the reliability optimization.