Effect of AR-N2 plasma treatment on copper surface for Cu-to-Cu wafer bonding

3D packaging technology offers great benefits such as reduced power consumption, improved performance, and reduced form factor. Among three key processes in 3D packaging a low temperature Cu-to-Cu wafer bonding is the subject of interest in this study. To accommodate high conductivity, non-formation of intermetallic compound, fine pitch connectivity, high pin count, and low cost, Cu-to-Cu wafer bonding is becoming increasingly important in advanced IC device packaging manufacturing. However, for high bonding quality, Cu-to-Cu wafer bonding requires high temperature process above 400^oC, which is not allowed in IC device packaging manufacturing. In this study the effect of Ar-N2 plasma treatment on Cu surface was investigated for low temperature Cu-to-Cu wafer bonding applications. Ar gas is used in a plasma ignition and the activation of Cu surface by ion bombardments, and the purpose of N2 gas was to passivate Cu surface from contaminations such as -O or -OH. The Cu/Ti/SiO2/Si specimens were fabricated on 8-inch Si wafers. Then various Ar-N2 plasma treatments were performed on Cu wafer surface. After the Ar-N2 plasma treatments, electrical and structural properties were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscope, and 4-point probe measurements. It has been confirmed that Ar-N2 plasma treatment can provide copper oxide removal and copper nitride passivation at the topmost Cu surface.