Electromigration describes the phenomenon of a directed movement of atoms in metals triggered by the flow and scattering of electrons. Experiments indicate that the interconnect lifetime reduces as the cross-sections decrease. The new generation of interconnects, with their reduced size, are more influenced by the fast diffusivity paths such as the interfaces to the surrounding layers and the grain boundaries in copper. In these fast diffusivity paths a larger fraction of atoms is transported due to electromigration, which causes significant variation in the performance and electromigration degradation. To produce more reliable interconnects, models are needed that address these aspects and are used to introduce new designs and materials.
The electromigration lifetime depends highly on the variability of material properties. For a better understanding of the physical processes, atomistic simulations based on molecular dynamics are used. Atomistic properties are configurations of atoms at the grain boundaries and at the interface to the surrounding materials.
Molecular dynamics is a computer simulation approach determining the physical movements of atoms. These atoms are modeled as particles with an exact position and velocity. The trajectories of the atoms are evaluated by numerically solving the Newton's differential equations. The forces are defined through a multi-body potential energy. To include in the molecular dynamic simulations the forces due to electromigration, first principal methods are employed. For this purpose a tool based on WIEN2k was developed to enable the calculation of the forces for different atomic configurations.
All these results are used to verify the continuum electromigration models used nowadays and to enhance them further. The parameters for those models are extracted from the atomistic simulations. Finally, in order to test the models against experimental accelerated electromigration tests, 3D continuum model simulations are carried out.