Tin (Sn)-based solders have been the most commonly used lead-free solder for joining microelectronics devices and components. Even at a low process temperature (200-250° C.), the reactions between the solder and copper (Cu) bump, Under Bump Metallizations (UBMs), such as copper (Cu), nickel (Ni), silver (Ag), palladium (Pd), etc., are very aggressive and can cause serious reliability concerns. The formation of intermetallic compounds (IMCs) results in degradation of mechanical properties due to their brittle nature, and a decrease in electrical conductivity due to their higher resistivity than pure metals. The Kirkendall voids which are formed as a consequence of the imbalanced inter-diffusion between different metals also deteriorate the mechanical and electrical properties of the joint. These are the reasons why copper (Cu) has been suggested as an alternative bonding material instead of Sn-based solder. Indeed, Cu to Cu homogeneous joining provides a solution to avoid concerns about complex metallic reactions and the accompanying issues, consequently achieving a high reliability bonding. Nevertheless, there are limitations of Cu to Cu direct bonding. To achieve reliable Cu—Cu thin-film bonding, high temperature and high pressure are required, hence applications to semiconductor processes or electronics packaging is limited.
USPTO Backgrounds
USPTO Backgrounds