Since electromigration has been recognized as an important risk for interconnect reliability, engineers have been searching for strategies to reduce or completely eliminate its effects. Independent of interconnect technology, there are basically two possible ways how to contest electromigration. The first one is choosing the appropriate materials or combination of these materials to produce preferable properties. Such efforts led from originally aluminum interconnects to aluminum-copper alloys, and later to pure copper interconnects. This method of designing interconnect structures to increase reliability also encompasses choosing the materials that surround the interconnect metal. The second strategy for the control of electromigration behavior is the introduction of special geometrical features. The basic idea is to avoid dwindling material due to material transport in specific interconnect structures. The most widely applied examples of such strategies are material reservoirs and redundant vias.
Using two or more via contacts between interconnect levels has proven to be a very promising geometrical strategy for preventing electromigration. This is because the nucleation of a void underneath the first via is supposed to relax the stress under the second one as well, suppressing the driving force for the formation of a new void.
Most recently, the electromigration model has been implemented in the Finite Element Diffusion and Oxidation Simulator (FEDOS) for three-dimensional geometries. All important driving forces have been taken into account in the vacancy transport equation. Furthermore, anisotropic material transport generated by the stress has also been implemented. Fast diffusivity paths for material transport, such as grain boundaries and material interfaces, have also been included in the model, allowing the study of the impact of each individual path on the electromigration phenomenon.