Effects of Mechanical Properties on the Circuit‐Level Reliability of Cu/low‐k Metallization Available to Purchase

The use of insulating over‐layers and conducting refractory metal liners with Cu, along with the introduction of low‐k materials with low stiffness, results in important differences in the mechanisms of electromigration‐induced mechanical failure of Cu‐based integrated circuit metallization compared to Al‐based metallization. Despite this, the same electromigration test structures and circuit‐level reliability assessment methodologies are often applied to Cu as those developed and used for Al. Unlike in Al technology, in Cu technology, measured electromigration lifetimes and critical line‐length current‐density products for immortality depend on whether test segments terminate in vias above or below the segments. Also, the critical stress for void nucleation is lower for Cu embedded in SiO2 than for Al embedded in SiO2. As a result, Cu must have a better test‐level reliability to achieve the same circuit‐level reliability as Al. The critical stress for void nucleation is expected to further decrease as lower‐k, and therefore lower modulus, dielectrics and thinner liners are used. In addition, this evolution is expected to lead to lower stresses for Cu extrusion into the dielectric or rupture of the liner at the base of the vias. Lowered resistance to Cu extrusion will degrade reliability. Lower resistance to liner rupture is also likely to degrade reliability, though quantitative analysis of this effect requires further characterization of the lifetimes of multi‐segment interconnect trees.