In-situ SEM electromigration studies were performed at
fully embedded via/line interconnect structures to visualize the
time-dependent void evolution in inlaid copper interconnects. Void
formation, growth and movement, and consequently interconnect degradation,
depend on both interface bonding and copper microstructure. Two phases are
distinguished for the electromigration-induced interconnect degradation
process: In the first phase, agglomerations of vacancies and voids are
formed at interfaces and grain boundaries, and voids move along weak
interfaces. In the second phase of the degradation process, they merge into
a larger void which subsequently grows into the via and eventually causes
the interconnect failure. Void movement along the copper line and void
growth in the via are discontinuous processes, whereas their step-like
behavior is caused by the copper microstructure. Directed mass transport
along inner surfaces depends strongly on the crystallographic orientation of
the copper grains. Electromigration lifetime can be drastically increased by
changing the copper/capping layer interface. Both an additional CoWP coating
and a local copper alloying with aluminum increase the bonding strength of
the top interface of the copper interconnect line, and consequently,
electromigration-induced mass transport and degradation processes are
reduced significantly.