Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Oxidation of metal layer surfaces due to atomic layer deposition (ALD) of thin metal oxide films is studied. For this, dense and smooth metal films of Pt, Ta and Hf were grown by off-axis sputtering and were in-situ transferred into the ALD system. Oxide films like HfO2, TiO2 and Al2O3 with a thickness of about 2 to 3 nm were deposited between 250 and 300degC by means of plasma assisted ALD utilizing a remote oxygen plasma as oxygen source. The 'built-in' oxygen gradients formed at different metal/ALD metal oxide interfaces were analysed by angle resolved X-ray photoelectron spectroscopy. The Pt metal films show no evidence of oxidation due to the PE-ALD process, independent of the grown metal oxide. In contrast, the thickness of the metal oxide layers formed at the surfaces of the Hf and Ta metal films depends on the ALD surface reactions as well as on the differences in oxidation enthalpy of the substrate metal and the metal oxide. The results are important for the design of high-density memristor arrays based on OxRAM devices.