Electromigration characterization of aluminum -copper alloy interconnects

Electromigration is the mass transport of atoms in a material due to elevated temperatures and an applied electric field. Electromigration-induced failure has increasingly become a reliability concern in the integrated circuit industry. As this process occurs material can be depleted resulting in void formation and an accumulation can result in hillock formation.;When modeling electromigration in interconnects Black's Law is utilized to model a mean time to failure as a function of current density and temperature with additional parameters such as a current exponent, n, and an activation energy, Ea. Using this equation to extrapolate interconnect lifetimes from accelerated testing conditions to normal operating conditions requires the determination of n and Ea. It will be shown that the choice of failure criterion when performing accelerated electrical testing does have an effect on the error associated with the parameters determined for use in Black's Law. In addition, a limit on the applied current density when using Black's Law will be examined.;The addition of relatively small amounts of copper (1--2wt%) has repeatedly been shown to improve device lifetimes. Through the use of an SEM with EDS capabilities we have measured the copper concentration as a function of length for interconnects after several accelerated stress time periods. This has shown that a redistribution of copper takes place during the initial incubation stage of electromigration. In addition, the open circuit failure location on the stressed interconnects corresponds to locations of reduced copper concentration.;A study of macroscopic, electromigration-induced damage has been performed using a standard optical microscope to measure the relative reflectance of an interconnect as a function of time during an accelerated test. A strong correlation has been observed between relative reflectance and resistance as a function of time.