Role of interfacial reaction in atomic layer deposition of TiO₂ thin films using Ti(O- i Pr)₂(tmhd)₂ on Ru or RuO ₂ substrates

The role of the interfacial reactions in the atomic layer deposition of TiO₂ films was examined using titanium diisopropoxide bis(tetramethylheptadionate) (Ti(O-iPr)₂(tmhd)₂) as the titanium precursor, and H₂O or O₃ as the oxygen sources when the films were grown on ruthenium (Ru) or ruthenium dioxide (RuO ₂) substrate at a growth temperature of 370 °C. Anatase-TiO ₂ films with a dielectric constant of 32 were grown on ruthenium substrates when H₂O was used, whereas rutile-TiO₂ films with a much higher dielectric constant (89) were grown when O₃ was used as the oxygen source. The rutile-TiO₂ film was grown with the aid of an in-situ-formed thin RuO₂ layer on the surface by the strong oxidation power of O₃, which has structural similarity to the rutile-TiO₂ film, as has been reported previously with different precursor and growth temperature [S. K. Kim et al. Appl. Phys. Lett. 2004, 85, 4112]. The initial and steady-state growth rates of the TiO₂ films were strongly dependent on the oxygen source. O₃ induced a substrate-enhanced growth mode at the initial growth step (<25 cycles), whereas H₂O resulted in a linear growth mode, regardless of the number of cycles, and a lower growth rate at the steady regime, because of its lower reactivity toward tmhd ligands. A substrate-enhanced growth mode was also observed on the RuO₂ substrate, regardless of the oxygen source. Interestingly, RuO₂ was reduced completely to metallic Ru during TiO₂ film growth. Oxygen atoms that come from the reduction of the RuO₂ substrate induced substrate-enhanced growth, and the reduction process was limited kinetically by the growth temperature. Reduction of the RuO₂ substrate occurred at the early stages of TiO₂ film growth. Therefore, it played a key role in determining the phase of the growing TiO₂ films. Reoxidation of the reduced Ru substrate during the O ₃ pulse step promoted the formation of rutile TiO₂.