Wafer thinning, bonding, and interconnects induced local strain/stress in 3D-LSIs with fine-pitch high-density microbumps and through-Si vias

Mechanical strain/stress and crystal defects are produced in extremely thin wafers (thickness ~10 μm) of 3D-LSIs not only during wafer thinning, but also after wafer bonding using fine-pitch, high-density microbumps and curing. Furthermore, the metal of through-Si via (TSV) and microbump not only becomes the cause of contamination, but also induces strain/stress (due to the difference in the co-efficient of thermal expansion (CTE) between Si and metal) in thinned Si substrate. X-ray photoelectron spectroscopy (XPS) results showed that the crystal quality of Si is highly deteriorated in the ultra-poly ground (UPG) surface after wafer thinning and stress relief. Micro-Raman spectroscopy (μRS) data revealed that a local tensile strain amount to 1.8 GPa was induced by 4×4 μm² square sized Si microbumps in 10 μm-thick LSI wafers after bonding and curing. We have noticed that this locally induced strain/stress caused more than 10% change in the ON current of p-MOS transistor. CuSn microbumps have also induced strain/stress at Si wafer surface, and it penetrates deeper for larger bump size and wider for smaller bump pitch.