(Invited) Copper Surface Passivation for Low Temperature Cu-to-Cu Bonding Applications

Abstract

As the IC devices are rapidly scaled down, IC integration has faced several technical bottlenecks: further miniaturization, power delivery and distribution, signal/power integrity, and heat dissipation. In this regard, chip or wafer stacking processes have attracted attention to improve an interconnect delay and an inter-die bandwidth, reduce a form factor, and decrease manufacturing costs. Vertically stacking technology not only enhances device performance, but also can realize heterogeneous packaging systems for the next generation heterogeneous integration. Of the three unit processes in the vertical stacking system, Cu-to-Cu bonding has emerged as a core technology for efficient power density, high bandwidth, and fine-pitch connecton.Various Cu-to-Cu bonding studies have been reported, such as thin metal passivation on Cu surface, self-assembled monolayer that passivates Cu surface with alkane-thiol, surface activated bonding that removes surface oxide layers and contaminants by plasma treatment, and direct bonding interconnect that enables Cu/SiO2 hybrid bonding. Despite many studies, Cu surface oxidation and high bonding temperature are still major issues to be resolved in high volume manufacturing. Sustainable, oxidization-free copper surface prior to Cu-to-Cu bonding is critical, especially since chip-to-chip bonding must still be performed in the air. Therefore, in this study, the copper surface passivation using the two-step plasma treatment has been investigated.The Cu-to-Cu bonding process of less than 300℃ was investigated using the copper nitride passivation. The copper nitride passivation layer was formed on 8-inch Si/SiO2/50nm Ti/1μm Cu using the two-step Ar-N2 plasma treatment process. The purpose of two step plasma treatment is first to remove copper oxides and any contaminants through Ar plasma, activate the Cu surface, and then form a copper nitride layer through N2 plasma. This metastable passivation of couple nanometer thickness prevents further oxidation and reduces surface roughness. In addition, since a copper nitride tends to decompose at 300oC or lower it does not adversely affect Cu bonding interface. The two-step plasma treatment was optimized to improve Cu bonding quality, and optimization was systematically carried out by the response surface methodology (RSM) based on a central composite design (CCD) in design of experiment (DOE). It has been found that copper nitride passivation lasts at lasts for 1 week.. The two-step plasma treatment showed the Cu2O removal and Cu4N formation on the Cu surface. Under the optimized two step Ar-N2 plasma treatment at 300℃, Cu-to-Cu bonding quality has significantly improved compared to the Cu-to-Cu bonding without any surface treatment. The interfacial strength measured by 4-point bending test and shear test and electrical measurements will be discussed.