High density Cu-Sn TLP bonding for 3D integration

3D die stacking is a key technology for enabling 3D integration wherein two or more dies are stacked on top of each other with vertical interconnections. This result in high speed interconnects with reduced noise and crosstalk as compared to wire bonded assemblies. 3D integration may require sequential stacking of multiple dies without disturbing the previously bonded die. This can be achieved by transient liquid phase (TLP) bonding where the melting point of the intermetallic formed after the bonding is much higher than that of the solder itself. In this paper, we demonstrate the feasibility of narrow pitch TLP bonding for the Cu-Sn system in die stacking applications. Furthermore, we explore several process options for cost reduction, throughput enhancement and thermal budget minimization. More than 90% yielding devices are achieved on 40 mum pitch peripheral array and 100 mum pitch area array dies at 250degC using both flux and No flow UnderFill (NUF) using both die-to-die and die-to-wafer setup. Preliminary bonding results at temperature less than 200degC are also presented.