The equilibrium concentrations of the various gaseous and solid phases in metallorganic chemical vapor deposition of Ru thin films were calculated in the experimentally relevant temperature and oxygen partial pressure ranges. Although thermal decomposition of the precursor, bis(ethyl-pi-cyclopentadienyl)Ru [Ru(EtCp)(2)] required a sufficient amount of oxygen, experimental results showed that up to a certain concentration of oxygen, Ru metal was deposited without any detectable RuO2 impurity. Thermodynamic calculations showed that all the supplied oxygen was consumed to oxidize carbon and hydrogen, cracked from the precursor ligand, rather than Ru. Thus, metal films could be obtained. There was an optimum oxygen-to-precursor ratio at which the pure Ru phase could be obtained with minimum generation of not only carbon and RuO2 but also detrimental hydrogen. Ru thin films with minimal carbon and RuO2 contamination could be obtained by optimization of the oxygen supply at a low deposition temperature of 300 degrees C.