Through-Si-via (TSV) filling with electrodeposited Cu was performed with a pulse current consisting of very a short duty cycle to achieve 3D interconnection in high density integrated circuit (IC) devices. Low frequency was unacceptable in this condition, because a highly acidic electrolyte attacked the seed layer and Cu deposit obtained with low current efficiency during comparably longer off time period. An increased frequency critically enhanced the filling rate of Cu, which was mainly related to the improvement of the current efficiency and lower dissolution rate. In pulse conditions, high density nanoscale twin structure and strain fields were observed, which were caused by induced stress during the on-time because of high peak current density. Application of pulse deposition reduced thermal extrusion of Cu that was related to the imperfect microstructure of the deposited Cu.
Thermal stress induced by annealing the Cu filling of through-silicon vias (TSVs) requires further investigation as it can inhibit the performance of semiconductor devices. This study reports the filling behavior of TSVs prepared using direct current and pulse current Cu electrodeposition with and without pre-annealing. The thermal extrusion of Cu inside the TSVs was studied by observing the extrusion behavior after annealing and the changes in grain orientation using scanning electron microscopy and electron backscatter diffraction. The bottom-up filling ratio achieved by the direct current approach decreased because the current was used both to fill the TSV and to grow bump defects on the top surface of the wafer. In contrast, pulse current electrodeposition yielded an improved TSV bottom-up filling ratio and no bump defects, which is attributable to strong suppression and thin diffusion layer. Moreover, Cu deposited with a pulse current exhibited lesser thermal extrusion, which was attributed to the formation of nanotwins and a change in the grain orientation from random to (101). Based on the results, thermal extrusion of the total area of the TSVs could be obtained by pulse current electrodeposition with pre-annealing.
The effect of Cu dissolution during pulse electrodeposition process on the generation of high density nanoscale twin structure was investigated. During off-time, the density of nuclei was reduced by acidity of electrolytes. The reduction in the density of nuclei could suppress the formation of nanocrystalline structure, which is commonly observed at pulse electrodeposition, and the twin structure was generated to reduce the accumulated strain energy during on-time in grains. We confirmed the effect of the dissolution of Cu on the twin generation by measuring the mechanical strength and electrical resistivity of Cu deposits obtained at different pH, with which, the dissolution rate of Cu was controlled.
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