ELECTROMIGRATION IN STRESSED THIN-FILMS

Multilayered thin-film circuits in microelectronic devices are confined in dielectrics for insulation between layers. The confinement produces mechanical stress in the circuit. During operation, the stress can enhance or retard electromigration. This interaction has been analyzed using thermodynamics of irreversible processes and the result applied to short-stripe electromigration experiments. It is shown that the calculated critical length of Al stripes, below which no electromigration damage occurs, agrees well with observation. The critical length is temperature insensitive and allows the effective electromigration-charge number to be determined independently, uncoupled to the diffusivity. In the analysis, a simple expression is given for calculating the effective electromigration-charge number of nearly-free-electron metals on the basis of the ballistic model of scattering.