Long term efficacy of ultra-thin Ti passivation layer for achieving low temperature, low pressure Cu-Cu Wafer-on-Wafer bonding

In this paper, we report the efficiency of Cu surface passivation by optimally chosen ultra-thin layer of Ti resulting low temperature, low pressure CMOS compatible Wafer-on-Wafer (WoW) Cu-Cu thermo-compression bonding. Ultra-thin Ti layer is regarded as one of the promising passivation layer as the presence of the same prevents oxidation of copper over an extended period of time. This allows various semiconductor industries dealing with heterogeneous technologies which are geographically located at different places to sync up and carry out 3D integration through WoW bonding. The optimum thickness for achieving low temperature and low pressure bonding is found to be 3 nm. In this endeavor, efficiency of ultra-thin Ti passivation layer in preventing oxidation of Cu over a long period was analyzed using various characterization techniques. These include contact angle measurements and X-Ray Photoelectron Spectroscopy (XPS). The samples got successfully bonded even after their exposure to non-vacuum, ambient conditions and quality of the bonding was characterized using Cross-sectional Field Emission-Scanning Electron Microscopy (FE-SEM), Surface Acoustic Microscopy (SAM) and bond strength analysis by tensile Pull test. The results are similar to the passivated samples bonded just after the deposition indicating that Ti is an ideal candidate for passivating Cu over extended period of time.