Ruthenium (Ru) is regarded as a potential electrode material on ultra high-k SrTiO3 dielectric films for future high-density metal-insulator-metal (MIM) capacitors to be used in e.g. next generation DRAM devices. To achieve high quality Ru films with conformal growth and precise thickness control, atomic layer deposition (ALD) of Ru has been investigated. In previously developed ALD processes for Ru the relatively low growth per cycle and the precursors low vapor pressure are an issue. Moreover, thermal ALD of Ru using O2 gas generally results in a pronounced nucleation delay and high surface roughness. It has been hypothesized that the use of an O2 plasma could possibly improve the film nucleation by providing oxygen radicals from the gas phase. The current work aimed at developing thermal and plasma-assisted ALD processes for Ru using the novel CpRu(CO)2Et precursor and O2 as reactant on a TiN substrate. The material properties of the obtained Ru films were characterized extensively using several ex situ techniques. A fundamental understanding of the reaction mechanism of both ALD processes was obtained by monitoring reaction by-products in situ. Thermal and plasma-assisted ALD both resulted in Ru films with similar properties once the film growth started. The obtained polycrystalline Ru films with a low resistivity (20 ??·cm) and a high growth per cycle (1 Å/cycle) meet the requirements for the resistivity and thickness set by the industry to follow the roadmap for MIM capacitors. The nucleation behavior of the ALD processes was, however, different. The Ru films nucleated after ~ 40 ALD cycles for the plasma-based process, whereas the thermal process showed a nucleation delay of ~ 80 cycles. Nevertheless, despite the improved nucleation, the plasma-assisted ALD Ru films had a higher surface roughness than the thermal ALD films. The film composition and surface reactions were examined in order to investigate the differences in nucleation and roughness development. TOF-SIMS measurements showed that more oxygen was present in the plasma-assisted ALD Ru films. In comparison to thermal ALD, also more oxygen was incorporated in the entire TiN substrate for plasma-assisted ALD films. Mass spectrometry provided insight into the reaction products formed (mainly CO, CO2, and H2O) and, therefore, into the surface chemistry ruling both ALD processes. It showed that more chemisorbed oxygen is available on the surface for the plasma-assisted ALD process. By combining these results, a reaction mechanism is proposed for these O2 based ALD processes to understand the differences observed in nucleation and roughness development of the Ru films.