In addition to metal precursors and ALD operating temperature, the nature of the initial substrate plays an important role in the growth of metals films, especially at the initial stages, or nucleation, of thin film growth. The investigation of the initial stages of metal ALD growth is necessary for the complete understanding of the reaction mechanisms between the metal precursor and substrates. Silicon (Si) and SiO2 covered wafers are commonly used as substrates for metal deposition in semiconductor industry. Usually, an Si substrate is cleaned by dipping in diluted HF solution, followed by deionized water rinsing and dry N2 blowing and the wafer is immediately loaded into the ALD chamber to prevent the formation of native oxide. An SiO2 substrate goes through a similar process that includes dipping in acetone, isopropyl alcohol and deionized water rinsing and dry N2 blowing. In this way, the relevant substrate terminations are H:Si(100)/H:Si(001) and OH:SiO2(001). In addition to Si and SiO2 wafers, previous studies have deposited Ru thin films on TaN/SiO2/Si or TiN/SiO2/Si substrates , which is used for the conformal Cu seed layers on high aspect ratio via holes and trenches. Among the experimental studies for PE-ALD of Ru on these substrates, the observed incubation period (the number of cycles before growth is observed) varies from report to report as listed in Table . For Ru(EtCp)2, an incubation period of more than 500 ALD cycles on TiN substrate is observed when using N2/NH3 plasma at 323K, while the incubation time is reduced to 120 ALD cycles when switching to N2/H2 plasma at 323K. This is different from other literature reports in which NH3-based plasmas are used to deposit Ru on TiN substrate without long incubation periods. In the same study, when a different metal precursor, methylcyclopentadienylpyrrolyl ruthenium ((MeCpPy)Ru), is applied, the reported incubation period on TiN substrate is less than 10 ALD cycles for both N2/NH3 and N2/H2 plasmas at 323K. When the substrate is changed to SiO2 substrate, a longer incubation time is observed with N2/H2 plasma and Ru(EtCp)2.