Atomic layer deposition (ALD) of Ru films under various precursor and oxidant supply conditions was carried out at 320°C on silicon wafers. The Ru precursor used was bis(ethylcyclopentadienyl) ruthenium [Ru(EtCp)2], and oxygen gas was used as the oxidant. The Ru precursor pulse time, oxygen pulse time, and oxygen flow rate were varied, and the effects of these parameters on the morphology of the deposited films were analyzed using scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The films showed hollow shell-like bumps, indicating that film growth began with a reaction in the gas phase or in physically adsorbed states. A mismatch between the thermal expansion coefficients of Si and Ru or the existence of residues on the wafer surfaces was suspected to be responsible for the hollow shell-like structures. However, the results of energy-dispersive X-ray spectroscopy (EDS) proved that the latter was not the case.