Modelling of the reservoir effect on electromigration lifetime

Abstract

Electromigration behaviour in W-plug/metal stripe structures is different from conventional metal-strip structures because there is a blocking boundary formed by the immobile W-plug in the contact/via. Electromigration failures occur more readily close to the W-plug than in metal-strip structures because metal ions are forced away from the contacts/vias by electric current, blocking the contacts/vias area. Several works have reported electromigration lifetime of multiple level interconnects to be influenced by the presence of a reservoir around the contacts/vias. Reservoirs are metal parts that are not or are hardly conducting current that act as a source to provide atoms for the area around the blocking boundary where the atoms migrate away due to the electric current. Interconnect lifetime can be prolonged by using the reservoirs, called the "reservoir effect". 2D simulation of the effects of reservoirs has been performed. The stress build-up during electromigration in the contact area can be simulated for several configurations, separating the effects of overlap, total reservoir area, the reservoir layout directions (vertical and horizontal), number of contacts/vias and contact/via placement. It is very useful for IC design rules to estimate which parameters are important for IC reliability. In this study, we considered the critical stress that the metal line can sustain before void formation as failure criterion. The failure time is determined by the time to reach the critical stress.