Cu–Cu Bond Quality Enhancement Through the Inclusion of a Hermetic Seal for 3-D IC

In this paper, we investigate the formation of hermetic seal, electrical contact, and mechanical support simultaneously through Cu-Cu bonding. Two types of test vehicles are designed and fabricated using wafer-to-wafer Cu thermocompression bonding to characterize the hermeticity and electrical contact of the Cu-Cu bonds. Excellent helium-leak rate is found and it is 30× lower than the reject limit (5×10^-8atm.cm³/sec) with the application of seal ring with a width of ≥20 μm. This result shows an outstanding bonding quality against harsh environment for hermetic encapsulation in a 3-D integration application. In addition, the electrical stability of Kelvin structure is greatly improved with the inclusion of the Cu-Cu seal. FIB analysis shows that the Cu-Cu bond surrounded by the Cu-Cu seal results in remarkable reduction in imperfections at the bonding interface. Finally, high-density Cu-Cu bonding structures exhibit advantageous stability with the protection of the hermetic seal. This promotes Cu-Cu bonding as a reliable option for wafer level 3-D integration.