Underbump metallurgy employing an electrolytic cu / electorlytic ni / electrolytic cu stack

Abstract

An electroless Cu layer is formed on each side of a packaging substrate containing a core, at least one front metal interconnect layer, and at least one backside metal interconnect layer. A photoresist is applied on both electroless Cu layers and lithographically patterned. First electrolytic Cu portions are formed on exposed surfaces of the electroless Cu layers, followed by formation of electrolytic Ni portions and second electrolytic Cu portions. The electrolytic Ni portions provide enhanced resistance to electromigration, while the second electrolytic Cu portions provide an adhesion layer for a solder mask and serves as an oxidation protection layer. Some of the first electrolytic Cu may be masked by lithographic means to block formation of electrolytic Ni portions and second electrolytic Cu portions thereupon as needed. Optionally, the electrolytic Ni portions may be formed directly on electroless Cu layers.