Temporary passivation of Cu for low temperature (< 300°C) 3D wafer stacking

Self-assembled monolayer (SAM) of alkane-thiol is applied to provide temporary passivation on Cu surface for protection against oxidation. The thermal stability of SAM is improved by storage at low temperature and in inert ambient. Surface analysis shows that SAM is effective in retarding surface oxidation and the degree of protection is higher for SAM with longer chain length. Wafer-to-wafer bonding result shows shear strength enhancement in the Cu-Cu bond when the appropriate chain length is used. Longer chain is less susceptible to thermal desorption hence no improvement in the bond strength is obtained.