Cu-Cu Bonding using Optimized Copper Nitride Passivation for 3D Packaging Applications

Abstract

3D packaging is able to keep the scaling in semiconductor market. Increased and shorter interconnects achieved by vertical stacking have benefits such as improved performance, reduced signal delay, and small form factor. In order to obtain high-quality 3D packaging applications, the integration of heterogeneous devices through bonding technologies is very important. Low temperature and pressure are essential during the bonding process because most of logic/memory devices has many metal and low-k dielectric layers, which are vulnerable to thermal budget and mechanical stress. Therefore, a small amount of solder with a low melting point is presently used on the top of the copper pillar for mass production. But, solder creates an intermetallic compound with copper at the bonding interface and cannot be applied to fine pitch patterns due to their reflow characteristics. Thus, Cu is emerged as a promising interconnect, but Cu-Cu bonding has few challenges because copper is easily oxidized and has a high melting point. In this paper, copper nitride, which prevents oxidation of the copper surface and promotes low temperature bonding, was studied by two step Ar/N2 plasma treatment. The optimum thickness of copper nitride passivation was derived using the design of experiment. It was also found that the copper nitride layer was almost decomposed at a temperature of 200°C.