  In this work, we investigated the effect of pitch distance on the statistical
variation and morphology of extrusion in Cu TSVs and the underlying mechanisms.
Extrusion statistics were obtained from TSV samples with two different pitch
distances. A notable increase in the magnitude of extrusion was observed in
vias with smaller pitch, yet the extrusion spread was largely unaffected. The
morphologies of the extruded vias were characterized and categorized, and
finite element analysis was carried out to study the effect of pitch distance
on stress and deformation. The results suggested that the overlapping of stress
fields from neighboring vias resulted in larger stress in the small-pitch vias,
which subsequently led to higher extrusion. The morphologies observed in the
extruded vias were related to the operation of different deformation mechanisms
under the combined effect of stress and microstructure. The statistical spread
of via extrusion, which was similar in both groups of vias, was related to the
stochastic nature of the via microstructure. By using a thin cap layer of Ta to
suppress the vacancy sources at the via top surface, the adverse effect of the
pitch distance was minimized and a pronounced reduction of extrusion was
achieved in vias of both pitch distances.
