Effect of phase separation on mechanical strength of co-sputtering Cu(Ti) thin film in chip-level 3DIC bonding

In the present study, Cu bonds were fabricated through face-to-face thermocompression by co-sputtering Cu(Ti) films. The diffusion behaviors and bonding strengths at various Ti concentrations were analyzed. Phase separation occurred when Ti diffused toward the substrate and Cu moved to the surface with limited oxidation. A high affinity between Ti and O causes a concentration gradient, which acts as the driving force for Ti diffusion. A Cu(Ti) film containing 15 at% Ti with a bonding time of 60 min at 400 °C exhibited the highest bonding strength with no void caused by phase separation.