Role of Interfacial Reaction in Atomic Layer Deposition of TiO2 Thin Films Using Ti(O-iPr)2(tmhd)2 on Ru or RuO2 Substrates

Abstract

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 (RuO2) substrate at a growth temperature of 370 °C. Anatase-TiO2 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 O3 pulse step promoted the formation of rutile TiO2.