362
P. Liu
dimensions. Similar to Cu interconnects in typical Si die, this TSV related interconnects are also using damascene process. Passivation layers and barrier metals are
used as well for the electroplated Cu TSV and its connected metal layers. The EM
fundamentals that summarized from Si die Cu damascene interconnects will be also
applicable to TSV related interconnects.
12.4.1 EM for Cu Damascene Interconnects
EM for Cu damascene interconnects in Si die has been well reported [45–47].
Different from Al based interconnects that are grain boundary dominated failure,
EM damage in Cu based metal lines is interface dominated. The interfaces at via
sidewalls or between Cu line and barrier/passivation layer are the fast diffusion paths
and hence the failure locations. Several key EM factors summarized from Cu damascene interconnects, which are believed also applicable to TSV related interconnects
are listed as follows:
1. Current crowding: because of the non-uniformity of interconnect dimensions
and complexity of routing, the current density is not uniform along the interconnects. It has been found that higher current density can be achieved at the
inner corner of a bend conductor, where the EM failure starts to happen [48].
Current crowding can also induce local self heat increase and generate the thermal
gradient and thermal mechanical stress gradient; both are the key driving forces
for Cu diffusion hence can impact the EM failure also.
2. Influence of passivation layer: since Cu EM is interfacial diffusion dominated,
the passivation layer plays very important role on the EM damage. Ways that can
improve the Cu/passivation adhesion and slow down the interfacial Cu diffusion
will help to improve the EM lifetime. It has been reported that the EM MTTF
for Cu interconnects coated with SiN x is about 12 times longer than those coated
with SiC [46]. This correlates very well with the experimental results that the
debond energy for Cu/SiC is much less than Cu/SiN [49].
3. Reservoir effect: Extensions of metal lines next to via connections (also described
as overhang region or reservoir) can delay the EM failure of interconnects in Si
die because the extensions serve as reservoir for void growth [50]. Lower levels of
stress and vacancy concentration in the extension are believed to contribute to the
better EM performance. Gan et al. [47] calculated the current density distributions
with different length of M2 extensions in dual-damascene Cu interconnects.
They observed a low current density zone in the M2 extension corners above
the cathode, which retards the void migration into the extensions. Based on this,
they proposed that there is a critical extension length beyond which increasing
extension lengths will not lead to longer EM lifetimes.
4. Thermal mechanical stress impact: as described in Blech effect, stress gradient
can be the driving force for atomic flux [51]. The stress gradient in Blech effect
arises from electron wind force induced vacancy density difference between
P. Liu
dimensions. Similar to Cu interconnects in typical Si die, this TSV related interconnects are also using damascene process. Passivation layers and barrier metals are
used as well for the electroplated Cu TSV and its connected metal layers. The EM
fundamentals that summarized from Si die Cu damascene interconnects will be also
applicable to TSV related interconnects.
12.4.1 EM for Cu Damascene Interconnects
EM for Cu damascene interconnects in Si die has been well reported [45–47].
Different from Al based interconnects that are grain boundary dominated failure,
EM damage in Cu based metal lines is interface dominated. The interfaces at via
sidewalls or between Cu line and barrier/passivation layer are the fast diffusion paths
and hence the failure locations. Several key EM factors summarized from Cu damascene interconnects, which are believed also applicable to TSV related interconnects
are listed as follows:
1. Current crowding: because of the non-uniformity of interconnect dimensions
and complexity of routing, the current density is not uniform along the interconnects. It has been found that higher current density can be achieved at the
inner corner of a bend conductor, where the EM failure starts to happen [48].
Current crowding can also induce local self heat increase and generate the thermal
gradient and thermal mechanical stress gradient; both are the key driving forces
for Cu diffusion hence can impact the EM failure also.
2. Influence of passivation layer: since Cu EM is interfacial diffusion dominated,
the passivation layer plays very important role on the EM damage. Ways that can
improve the Cu/passivation adhesion and slow down the interfacial Cu diffusion
will help to improve the EM lifetime. It has been reported that the EM MTTF
for Cu interconnects coated with SiN x is about 12 times longer than those coated
with SiC [46]. This correlates very well with the experimental results that the
debond energy for Cu/SiC is much less than Cu/SiN [49].
3. Reservoir effect: Extensions of metal lines next to via connections (also described
as overhang region or reservoir) can delay the EM failure of interconnects in Si
die because the extensions serve as reservoir for void growth [50]. Lower levels of
stress and vacancy concentration in the extension are believed to contribute to the
better EM performance. Gan et al. [47] calculated the current density distributions
with different length of M2 extensions in dual-damascene Cu interconnects.
They observed a low current density zone in the M2 extension corners above
the cathode, which retards the void migration into the extensions. Based on this,
they proposed that there is a critical extension length beyond which increasing
extension lengths will not lead to longer EM lifetimes.
4. Thermal mechanical stress impact: as described in Blech effect, stress gradient
can be the driving force for atomic flux [51]. The stress gradient in Blech effect
arises from electron wind force induced vacancy density difference between