Copper (Cu) dual damascene interconnects with self-formed MnSi xO y barrier layer using Cu-Manganese (Mn) alloy seed layer is successfully fabricated for the first time. No delamination is found in the chemical mechanical polishing process probably because of better adhesion strength between MnSi xO y barrier and dielectric. More than 90% yield is obtained for 1 million via chain. Microstructure analysis by transmission electron microscopy shows that an approximately 2nm thick and continuous MnSi xO y layer is formed at the interface between Cu and dielectric of the via and trench and that no barrier at the via bottom. This via structure without the bottom barrier provides essential advantages of reducing via resistance, significant via-electromigration lifetime improvement due to no flux divergence site at the via and excellent stress-induced voiding performance.