Study of the Effect and Mechanism of a Cap Layer in Controlling the Statistical Variation of Via Extrusion

This work examines the effect of a metallic cap layer in controlling via extrusion and explores the underlying mechanisms. Ta was deposited as the cap material, which was very effective in reducing the statistical spread of via extrusion. The correlation between extrusion and microstructure of the vias was investigated and compared for the reference uncoated vias and the Ta-capped vias. Thermo-mechanical characterization as well as TEM characterization of the Ta cap/via interface were also carried out. Void formation at grain junctures were observed. The results suggest that mass transport through grain boundaries plays an important role in causing the statistical variation of extrusion, which can be effectively suppressed by the Ta cap. Void formation was also reduced by the cap layer. Additional factors affecting extrusion, including interfacial diffusion and dislocation glide, were also discussed.