Electromigration in Al(Cu) two-level structures: Effect of Cu and kinetics of damage formation

The electromigration characteristics and kinetics of damage formation for Al(Cu,Si) line segments on a continuous W line and Al(Cu)/W two-level interconnect structures have been investigated. The mass transport as a function of temperature was measured using a drift-velocity technique. The flux divergence at the line/stud contact was found to be responsible for formation of open failure in the interconnect structure, as shown by a direct correlation observed between mass depletion at the contact and resistance increase of the line/stud chain. The depletion of Al at the stud contact is preceded by an incubation period during which Cu is swept out a threshold distance from the cathode of the line. This leads to a damage formation process which is controlled by both Cu electromigration along grain boundaries and dissolution of the Al2Cu precipitates. This is distinctly different from single-level interconnects measured using a conventional electromigration test site. Measurements of the mean failure lifetime in the two-level interconnect yield an activation energy of 0.58 eV for Al, in contrast to 0.78 and 0.83 eV for Al(0.5 wt % Cu) and Al(2 wt % Cu), respectively. The activation energies of the electromigration drift velocity were found to be 0.86 and 0.68 eV for Cu and Al in Al(2 wt % Cu, 3 wt % Si), respectively. These results enable one to infer that the kinetic process is controlled by electromigration of Cu along grain boundaries instead of by dissolution of the Al2Cu precipitates.