COMPUTER-SIMULATION OF ELECTROMIGRATION IN THIN-FILM METAL CONDUCTORS

A model is presented for simulating electromigration in thin-film meta l conductors. The backfluxes are calculated explicitly in each of the grain boundaries using concentration and stress gradients resulting fr om the initial electromigration flux. The stress-dependent diffusivity tem is also directly included in the formulation. It is assumed that the main cause of the flux divergence is the grain structure of the co nductor and that these divergences occur at the triple-point junctions of the grain boundaries. Time-to-failure and classic resistometric an alyses of five conductors are performed. Results indicate that current -density exponent of n almost-equal-to 2 should be used in time-to-fai lure analysis. This is due to the localized stress migration and diffu sion acting against the electromigration force throughout the period o f the conductor lifetime. A direct correlation between the time to fai lure (TTF) and relative rate of resistance change R(rc) was found when all conductors and stress conditions were considered together. This r elationship is of the form TTF=0.223R(rc)-1.11 and indicates that resi stance measurements can be used in producing lifetime parameters for u se in providing reliability rules for conductor design.