To broaden the process window of the conventional sputtered TiN diffusion barrier and to find an appropriate bottom electrode for high-density capacitors, we investigated the effect of a thin RuOx oxidized layer formed at the surface of each Ru and RuOx film pre-annealed at 400 °C on the oxidation behaviour of the TiN diffusion barrier at the temperature range of 400–550 °C. Not only did the Ru/TiN/n+poly-Si/Si structure retain its structure up to 550 °C, but also a partially ohmic behaviour was exhibited up to 600 °C without an increase in resistivity, whereas after pre-annealing at 400 °C, the degradation of the RuOx/TiN/n+poly-Si/Si system started. In the latter case, the microstructure of the RuOx film was an embedded porous amorphous RuO2 crystalline phase and with no formation of thin RuOx layer at the surface of the RuOx film, resulting from free diffusion of oxygen and then oxidation of the TiN diffusion barrier. In the former case, after pre-annealing at 400 °C, the addition of oxygen at the surface of the Ru film leads to the diffusion path complexes. In addition, after post-annealing at 450 °C, the conductive RuO2 phase, which can act as an oxygen-sink layer, forms at the surface of the Ru film. Therefore, as a bottom electrode structure for high-density capacitors, the Ru/TiN/n+poly-Si/Si contact system is more effective than the RuOx/TiN/n+poly-Si/Si contact system.