Method for fabricating last level copper-to-C4 connection with interfacial cap structure

The present invention relates to a method for fabricating a semiconductor device with a last level copper-to-C4 connection that is essentially free of aluminum. Specifically, the last level copper-to-C4 connection comprises an interfacial cap structure containing CoWP, NiMoP, NiMoB, NiReP, NiWP, and combinations thereof. Preferably, the interfacial cap structure comprises at least one CoWP layer. Such a CoWP layer can be readily formed over a last level copper interconnect by a selective electroless plating process.