The Effect of Pitch Distance on the Statistics and Morphology of Through-Silicon Via Extrusion

Abstract: 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.