Effect of silicon nitride capping layer on via electromigration and failure criterion methodology in multilevel interconnection

Electromigration of via-terminated Al(Cu) stripes in two-level and four-level interconnect structures was studied. We found that the top-level metals adjacent to the dielectric capping layers exhibited the longest electromigration lifetimes and sustained the highest critical current densities. Comparison of capping layers with and without SiN suggests that the electromigration lifetime enhancement is due to the mechanical constraint of the SiN capping layer, which has a larger Young's modulus than that of the oxide. We discuss various cases of current shunting in terms of pure edge, mixed and pure random depletions. We deduce the failure criteria of upper and lower metal levels based on current shunting effect, and compare them with microanalysis results.