Electromigration Failure Controlled by Aging-like Gradual Resistance Shift in Co-Capped Cu Interconnects

Gradual resistance (R) increasing as high as ~13% before R jump was found during electromigration (EM) stress of Co-capped Cu interconnects. Such R shift persisted after removing the stress, and it showed to increase with both temperature and current, which indicates it can't be ascribed either to joule heating or to annealing effect. Failure analysis showed no killer void was found to occupy the whole line cross section while R shift reached ~10%. Therefore, such aging-like R shift should be a second EM failure mode aside of voiding. Analysis on the time dependence of the R shift showed it tended to obey ~t
^0.5
as the stress time (t) increased, which implies diffusion-controlled dynamics may be responsible. Moreover, from the temperature and current density dependence of the R shift, an activation energy of ~1.07 eV and a current density exponent of ~1.28 was extracted, the former of which is close to a half of that of Co diffusion in Cu. It further suggests (Dt)
^0.5
dependence of the R shift and it is thus proposed the underlying mechanism might be current assisted Co diffusion. Also, the activation energy and current density exponent are separately close to a half of those calculated from the time to failure of EM (E
a
~2.23 eV, n~2.44), being consistent with the proposed current assisted diffusion mechanism. So to monitor the R shift by ~10000s was enough for rapid determination of EM E
a
& n in such Co-capped Cu interconnects.