Sub-micron aligned Cu-Cu direct bonding for TSV stacking

Cu-Cu direct bonding facilitates fine-pitch interconnection with low electrical resistivity and high electromigration (EM) resistance. However, reliable Cu-Cu bonds result only from high temperature, high pressure and long process time mainly because of its tendency to generate a native oxide that negatively impacts device reliability. Presently, high process temperature is one of the major bottlenecks of Cu-Cu thermo-compression bonding. We developed the optically-aligned, low-temperature Cu-Cu thermo-compression bonding process with sub-micron alignment accuracy. The quantitative analyses of the interfacial adhesion energies and seam voids of Cu-Cu bonds, performed with varying process parameters, showed that bonding temperature and post-bond annealing have the most significant influence on bond properties. By optimizing experimental parameters, we could achieve, even with a short bonding time, the sufficient interfacial adhesion energy (≥ 5 J/m² for subsequent processes) with no interfacial seam voids. Postbond annealing performed at ≥ 250°C drastically improves the interfacial adhesion energy. SmartView™ alignment, enabling the face-to-face Cu-Cu bonding of non-IR transparent wafers, allows less than 0.2 μm (3σ) and 1.0 μm (3σ) of the pre-bond and post-bond alignment accuracies, respectively.