Modelling and simulation were used to judge the effect of additives or dopants on Cu interconnects. A virtual simulation procedure was based on the assumption that grain-boundary diffusion predominated during Cu electromigration. The procedure investigated dopant segregation to a grain boundary, bulk diffusion, dopant and Cu self-diffusion at the grain boundary and the effect of the dopant upon Cu diffusion at the grain boundary. Defect formation and migration energies, as well as activation energies, were calculated by using state-of-the-art ab initio methods. Two primary requirements for a dopant to be effective were identified. These were dopant-blocking and dopant-dragging mechanisms. In dopant-blocking mechanisms, the desired dopant occupied grain-boundary interstitial sites and blocked fast diffusion pathways for Cu. In the case where Cu atoms occupied grain-boundary interstitial sites, the desired dopants segregated to nearby substitutional sites and dragged the fast-diffusing Cu. Early experimental results confirmed the model prediction for several dopants. The mean time to failure was increased by more than 60% for dopant concentrations as low as 0.01at% in Cu.

Modelling and Simulation Design of Advanced Cu Alloy Interconnects. C.L.Liu: Journal of Applied Physics, 2002, 91[9], 6089-94