Effect of Co-Reactants on Interfacial Oxidation in Atomic Layer Deposition of Oxides on Metal Surfaces

We have examined the atomic layer deposition (ALD) of Al2O3 using TMA as the precursor and t-BuOH and H2O as the co-reactants, focusing on the effects of the latter on both the ALD process and the possible modification of the underlying substrate. We employed a quartz crystal microbalance (QCM) to monitor ALD in situ and in real time, and the deposited thin films have been characterized using X-ray photoelectron spectroscopy, spectroscopic ellipsometry, X-ray reflectivity, and atomic force microscopy. Growth of thin films of Al2O3 using TMA and either t-BuOH or H2O as the co-reactant at T = 285 °C produces thin films of similar physical properties (density, stoichiometry, minimal carbon incorporation), and the growth rate per cycle is similar for the two co-reactants at this temperature. At a lower temperature of T = 120 °C, the behavior is starkly different, where growth occurs with H2O but not with t-BuOH. At either process temperature, we find no evidence for significant coverages of a long-lived tert-butoxy species from the reaction of t-BuOH. Deposition of thin films of Al2O3 on metal surfaces of Cu and Co has been examined for evidence of interfacial oxidation. While growth with either co-reactant does not lead to the oxidation of the underlying Cu substrate, use of H2O leads to the oxidation of Co, but use of t-BuOH as the co-reactant does not. Thermodynamic factors may affect the early stages of growth, as Al species will likely scavenge all free O species. In contrast, at later times, diffusion of species through the deposited Al2O3 thin film could result in oxidation at the Al2O3|metal interface, a process that is strongly hindered in the case of t-BuOH due to its size. This observation highlights the importance of the choice of the co-reactant concerning ALD of oxides on metal surfaces.