Thin Ruthenium (Ru) films were deposited on native SiO2 on Si substrate by thermal atomic layer deposition (ALD) using dicarbonyl-bis(5-methyl-2,4-hexanediketonato)Ru(II) precursor with O-2 and H2O as reactant. In the case of the O-2 process, increase in the O2 pulse led to oxidation of the film into RuO2. In contrast, purely metallic Ru thin film without significant oxidation was formed using H2O, even with extended H2O pulse. Using H2O, growth rate of similar to 0.75 angstrom/cycle and electrical resistivity of similar to 18 mu Omega cm for metallic Ru were obtained at temperatures as low as 250 degrees C. Through density functional theory (DFT) calculations, it is shown that adsorption of the Ru precursor would spontaneously occur on metallic Ru surface. Ru ALD using H2O reactant is performed on SiO2 powder to successfully form Ru@SiO2 core-shell structure, demonstrating applicability of this process to substrates with morphological complexity, which often require extended exposures of reactants.