Three terminal ‘dotted-I’ interconnect structures, with vias at both ends
and an additional via in the middle, were tested under a variety of test
conditions. Failures (mortalities) were observed even when segments were
tested under conditions that would not have led to failure in two-terminal
structures. Mortalities were found in right segments with jL values as low as 1250 A/cm, which is lower when compared to the
immortality condition (jL)cr of 3700 A/cm reported for similar via-terminated structures.
Moreover, we found that the mortality of a dotted-I segment is dependent on
the direction and magnitude of the current in the adjacent segment. These
results suggest that there is not a definite value of the jL product that defines true immortality in individual segments
that are part of an interconnect tree. More importantly, the critical
jL value for a single segment of Cu interconnects may be reduced
or increased by an adjacent segment. Therefore independently determined
(jL)cr values cannot be directly applied to
interconnects with branched segments, but rather the magnitude as well as
the direction of the current flow in the adjoining segments must be taken
into consideration in evaluating the immortality of interconnect segments in
an interconnect network.