The median-times-to-failure (t50's) for straight
dual-damascene via-terminated copper interconnect structures, tested under
the same conditions, depend on whether the vias connect down to underlaying
leads (metal 2, M2, or via-below structures) or connect up to overlaying
leads (metal 1, M1, or via-above structures). Experimental results for a
variety of line lengths, widths, and numbers of vias show higher t50's for
M2 structures than for analogous M1 structures. It has been shown that
despite this asymmetry in lifetimes, the electromigration drift velocity is
the same for these two types of structures, suggesting that fatal void
volumes are different in these two cases. A numerical simulation tool based
on the Korhonen model has been developed and used to simulate the conditions
for void growth and correlate fatal void sizes with lifetimes. These
simulations suggest that the average fatal void size for M2 structures is
more than twice the size of that of M1 structures. This result supports an
earlier suggestion that preferential nucleation at the
Cu/Si3N4 interface in both M1 and M2 structures
leads to different fatal void sizes, because larger voids are required to
span the line thickness in M2 structures while smaller voids below the base
of vias can cause failures in M1 structures. However, it is also found that
the fatal void sizes corresponding to the shortest-times-to-failure (STTF's)
are similar for M1 and M2, suggesting that the voids that lead to the
shortest lifetimes occur at or in the vias in both cases, where a void need
only span the via to cause failure. Correlation of lifetimes and critical
void volumes provides a useful tool for distinguishing failure
mechanisms.