A basic process to fabricate Ru/rutile-Co-doped TiO2/Ru capacitors for 20 nm-technology generation of DRAMs and beyond was developed. The aim of this study is that the basic process provides foresights into EOT and leakage-current density requirements. We chose rutile-TiO2 for the insulators to meet the requirement of relative permittivity and Ru to suppress leakage-current density. We found the as-deposited lower electrode of Ru crystallized into the rutile phase of TiO2 with a relative permittivity of 109 due to the similarity of crystal structures and lattice constants between rutile-TiO2 and rutile-RuO2, which was generated by oxidizing Ru with ambient oxygen. Furthermore, we confirmed that doping elements that had a small-ionic radius, such as Co, decreased leakage-current density. The dependence of the leakage current of Ru/Co-doped-TiO2/Ru capacitors on temperature and analysis of the band structure with X-ray photoelectron spectroscopy revealed that leakage-current density is determined by a balance between thermionic current and tunneling current through the Schottky barrier. Through calculations using a theoretical equation for these currents, the optimum percentage of Co was estimated to range from 0.3-0.6 % to meet requirement of leakage current for 20 nm-DRAMs. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.040201jes]