Application of a Metallic Cap Layer to Control Cu TSV Extrusion

In this study, we report the reduction of via extrusion for Cu through-silicon vias (TSVs) through the application of a metallic cap layer. The basic idea of this approach is based on suppressing the mass transport which causes via extrusion on the top surface of TSVs. Two materials, W and Co, were deposited as the cap materials. Experiments were carried out to characterize the extrusion behavior and shown that both materials were able to reduce the average and maximum extrusion height in the TSV sample comparing to a reference TSV sample without the cap layer. In addition, the results suggest that Co is more effective than W to suppress via extrusion, as Co is known to be a good diffusion barrier, while W is immiscible with Cu. The underlying mechanism for the cap layer effect is discussed.