Ru-based alloy thin films were prepared by atomic layer deposition (ALD) at a deposition temperature of 225 °C for use as a diffusion barrier for seedless Cu interconnects. Ru-Mn alloy thin films were grown by a repetition of super-cycles consisting of multiple Ru and Mn ALD sub-cycles. Ru and Mn ALD was performed using a sequential supply of a Ru precursor [1-Isopropyl-4-methylbenzene (cyclohexa-1, 3-diene) Ru (0)] and NH3 plasma, and a Mn precursor [bis(1,4-di-isopropyl-1,3-diazabutadienyl)Mn(II)] and H2 plasma, respectively. Secondary ion mass spectrometry (SIMS) indicated that Mn had been incorporated successfully into the Ru film by the periodic addition of Mn ALD sub-cycles during the ALD-Ru process. Plan-view transmission electron microscopy (TEM) showed that the grain size was reduced obviously from ∼15 nm (pristine Ru) to ∼8 nm (Ru-Mn alloy) and continuous columnar grain growth of the ALD-Ru film was prevented by the addition of the periodic Mn ALD sub-cycles. After annealing at 500 °C, the formation of a MnSiOx diffusion barrier was observed between the Ru-Mn film and underlying SiO2 surface by both cross-sectional view TEM and energy dispersive spectroscopy analysis and confirmed by SIMS depth profile. X-ray diffraction showed that the ultrathin ALD Ru-Mn thin film (∼5 nm) prevented Cu diffusion up to an annealing temperature of 600 °C, whereas the ALD-Ru thin film with the same thickness failed to prevent Cu diffusion only after annealing at 500 °C. Finally, the capability of the direct plating of Cu was demonstrated on the annealed ALD Ru-Mn film.