Electromigration-Induced Failures in VLSI Interconnects

Electromigration in thin films has been identified as one of the primary failure mechanisms limiting the reliability of film interconnections. Most of the electromigration testing has been carried out with Al and Al-alloy film conductors as they happen to be most widely used on integrated circuits. A general expression for the median time to failure (MTF) is as follows: Aj¿n exp (Q/kT) where (1) "A" is a parameter depending on the geometry, physical characteristics of film, protective coating, and substrate, (2) "j" is the current density A/cm2, and (3) "n" is the exponent: 1 < n < 3, Q is the activation energy, "T" is the average temperature of the conductor and "k" is Boltzmann\´s constant: MTF data are observed to obey lognormal distribution with a dispersion parameter ¿. Predicted interconnection failure rates at use conditions of current density and chip temperature are found to vary over several orders of magnitude as they strongly depend on accelerated life test data and "n" the exponent of current density in the MTF equation. Since electromigration-induced failure rates increase with usage time, in contrast to the usual assumption of constant failure rates of ICs, the current density design guidelines must comprehend the failure rate goals as a function of usage time for specified operating temperatures and the selection of conductor material to design-in reliability.