Semiconductor devices are formed with a Cu or Cu alloy interconnect encapsulated by a substantially uniform MnO or Al2O3 layer. Embodiments include forming an opening having side surfaces and a bottom surface in a dielectric layer, forming a barrier layer on the side surfaces and the bottom surface of the opening and on an upper surface of the dielectric layer, treating the barrier layer with an oxygen plasma to form dangling oxygen atoms on the barrier layer, depositing a seed layer on the barrier layer, and filling the opening with Cu or a Cu alloy.