On the bimodal lognormal distribution of electromigration lifetimes

The accurate prediction of electromigration-limited lifetime based on high current density and temperature stressing is one of the key issues in reliability methodology. Especially the choice of the underlying distribution model is of importance since extrapolated life times for operation conditions depend on it. Two sets of electromigration lifetime data gathered from submicron via-line structures an presented which do not show log-normal failure behaviour. They can be fitted assuming a bimodal lognormal distribution. The intention of this paper is to characterize different bimodal lognormal distributions, to review some statistical basics and to give a proposal to handle bimodal failure distributions for lifetime projection.