Guidelines for structural and material-system design of a highly reliable 3D die-stacked module with copper through-vias

Design guidelines for a highly reliable three-dimensional die-stacked type module-namely, our 3D die-stacked module named "V-STACK", in which four ultra-thin dies (50 Pm thick) are vertically stacked and electrically interconnected through copper through-vias in the silicon dies-were devised through the following structural and material analyses. First, stress concentration during temperature cycling (due to the low-rigidity of the ultra-thin die) was investigated by parametric FEM calculations on a V-STACK-type module. The FEM results showed that the stress concentration is mainly caused by the combination of extremely low die rigidity and thermal mismatch between the underfill resin and the gold bumps. It was thus confirmed that controlling the coefficient of thermal expansion of the underfill resin eliminates the stress concentration in spite of the fact that the dies are ultrathin. Second, optimal combinations of upper and lower bump heights for higher interconnection reliability were determined. Consequently, it was found that several, pre-induced defects along the bonding surface greatly reduce interconnection reliability. Finally, the appropriate. aspect ratio of the copper through-via structure (i.e., for reducing the tensile stress in the surrounding SiO2) was determined from the viewpoint of thermal fatigue in a high-aspect-ratio via structure.