The study prepared Ru/RuNx bilayer barriers on mesoporous "SiO"2 (mp-"SiO"2) dielectric layers for direct Cu electroplating applications using in situ two-step plasma-enhanced atomic chemical vapor deposition (PEALD). For the 5 nm thick Ru/RuNx bilayer deposited at 200C, obvious thermal decomposition begins at temperatures lower than 400C. Copper can be successfully electroplated on the as-deposited Ru/RuNx bilayer, and the Cu/Ru/RuNx/mp- "SiO"2 film stack can withstand thermal treatment at temperatures up to 500C without significant physical and chemical degradations according to TEM and SIMS analyses. The study shows that the electroplated Cu layer behaves like a passivation layer that improves the thermal stability of the Ru/RuNx barrier during the thermal annealing. Pull-off tensile test shows that interfaces in the Cu/Ru/RuNx/mp- "SiO"2 film stack have good adhesion strength, but delamination occurs at the interface between the Ru/RuNx bilayer and the mp-"SiO"2 layer at 600C, resulting in Cu and Ru diffusion into the dielectric layer. The study has demonstrated that the PEALD Ru/RuNx bilayer structure prepared using the in situ two-step approach is suitable for the seedless Cu electroplating process in nanometer scale interconnect technology.