Line depletion electromigration characteristics of Cu interconnects

Specific details of fabrication process and geometry of Cu interconnects result in different electromigration (EM) fail modes. This paper discusses EM characteristics of line depletion mode stress, i.e. for the case of electrons flowing into a Cu line through a Cu diffusion barrier to cause voiding in the line. For electrons flowing from a W via, for example to a Cu line above, redundancy exists due to the overlap of line bottom liner over the top of the via. When electrons flow from a via above down to a Cu line, the redundancy characteristics can be very different for different via/line layouts and result in different EM fail distributions. The solid contact between via and the liner of the line below can result in tight fail distributions, while weak contact or lack of contact between the via and the liner of the line below can cause broad (high sigma), or even multi-mode fail distributions. A few examples and their implications on robust interconnect design are presented. The relation between void size and liner redundancy characteristics is also discussed.