Sputtered Ru and Ru-C films 5 nm thick are employed in the Cu/barrier/SiO2/Si system, and their performances as the diffusion barrier as well as the seed layer for direct Cu electroplating are investigated in parallel. Based on the sheet resistance measurement and energy dispersive X-ray line profiles, the 5 nm Ru-C film can retard the diffusion of Cu after a prolonged (30 min) annealing up to 700 degrees C, while the Ru film is an effective barrier up to only 400 degrees C. Direct electroplating of Cu is successfully carried out on both Ru and Ru-C films, which proves that Ru-C is a Cu seed layer in addition to being a robust diffusion barrier. The microstructural characteristics of ultrathin Ru and Ru-C films are also examined, indicating that the superior barrier performance of the 5 nm Ru-C film is associated with its inferior crystallinity and resistance to agglomeration at elevated temperatures.