1.4 Electromigration Failure

In an interconnect subjected to a high electrical current, a motion of metal atoms takes place in the same direction as the current flow [29]. This phenomenon is called electromigration, which is the process of mass transport due to current flow towards the anode of a metal line and is the most important cause of failure in interconnects.

Fundamentally, electromigration is a quantum mechanical effect caused by the action of the local electric field and the resulting scattering of conducting electrons on lattice atoms. The contributions of these two microscopic processes on the atoms result in the electromigration driving force, which acts on metal ions and causes their migration in the metal line. Beside this driving force, electromigration is a complex multiphysics problem which includes the actions of other mechanisms responsible for the material transport inside the metal, such as diffusion, thermo-migration, and stress-migration [20]. The driving force of diffusion is represented by the vacancy concentration gradient, while the driving forces of thermo- and stress-migration are gradients in temperature and mechanical stress, respectively.

In a typical interconnect composed of conducting and insulating dielectric materials, the mass transport leads to an accumulation of atoms at the anode end of the metal line, due to the constraints imposed by the surrounding layer which prevents metal migration into the dielectric [84]. At the same time, depletion of atoms occurs at the cathode end of the line. The material transport culminates in the build-up of mechanical stress in the structure which results in two distinctive failures. Compressive stress arises due to atom accumulation and causes the formation of a hillock at the anode end of the metal, degrading the surrounding dielectric material and resulting in a short circuit. On the other end of the line, depletion of atoms induces the development of tensile stress, leading to void nucleation. The tensile stress for void nucleation is lower than that for extrusion formation and can be easily reached in an interconnect under electromigration [64,118]. Therefore, the nucleation of a void is the dominant mechanism of electromigration failure rather than the formation of a hillock. Once the void has formed, its eventual propagation across the interconnect width can cause extremely high changes in the interconnect resistance and even open circuit failure.