On the effect of grain boundary segregation on electromigration driving force in thin metal films

Electromigration (EM) describes flux of atomic diffusion induced by electron and hole ''winds'', i.e. by fluxes of electrical charge carriers. In use conditions of thin metal film lines referred to as interconnects, electric current induced diffusion proceeds mainly along grain boundaries (GB), and fast grain boundary electromigration (GBEM) has been recognized as a major failure mechanism of modern integrated circuits. Despite intensive studies concentrated mainly on the kinetic (diffusivity) aspects of the problem, control and prevention of GBEM remains a challenge, both scientific and technological. We explore a novel line of attack on the problem. The approach outlined addresses to the driving force of GBEM and considers how it can be reduced, and even eliminated, through equilibrium GB segregation (GBS) of specially selected impurities. We consider this possibility by the example of copper, that is the most promising substitute for the traditional aluminum interconnects, though it may suffer from GBEM problems. The estimates based on simple uniformly-charged-slab GB model suggest that a submonolayer coverage with electronegative impurities, e.g. sulfur in copper, may result in hole conductance at GB, and hence, in partial or full compensation of the electron-wind, as the grain-boundary electromigration driving force, by the hole-wind.