Cu passivation with self-assembled monolayers for direct metal bonding in 3D integration

Direct metal bonding is a preferred fine-pitch technology for stacking of Si dies in 3D integration. Cu is a metal of choice for direct metal bonding because it is the most common metal for redistribution layer in advanced semiconductor manufacturing, Cu has high conductivity and it is a low cost candidate. However Cu oxidises very fast in air which makes the bonding procedure challenging. In this study we present the novel technique of Cu passivation with temporary protective self-assembled monolayer (SAM). X-ray photoelectron spectroscopy (XPS) analysis was used in order to carry out the chemical analysis of the Cu surface. Contact angle (CA) measurements provided the information about the monolayer formation. The influence of immersion time and storage conditions on the SAM passivation quality was examined. Storage of a coated Cu surface at low-temperature air conditions was found to be a promising technique for a long-term oxidation retarding. We summarize the key substrate parameters that influence SAM protective capability.