A Vacuum Ultraviolet-Enhanced Oxidation Mechanism for Pd: Near-Surface Oxidation for Atomic Layer Etching

Density functional theory (DFT) is used to better understand the oxidation of Pd metal using vacuum ultraviolet (VUV) light co-exposed with O₂, which is known to produce O and O₃. The oxidation of Pd metal arising from O, O₂, and O₃ is assessed on bare Pd, Pd with a 0.25 monolayer of adsorbed atomic O, and Pd with increasing O incorporation into the substrate. DFT calculations are complemented experimentally by co-exposing 20 nm Pd films to 1 Torr of O₂ and VUV photons (6.5 < hν < 11.3 eV) from a D₂ lamp at temperatures ranging from 50 to 200 °C and times from 30 s to 40 min. Oxidation of Pd is characterized using in situ X-ray photoelectron spectroscopy. Co-exposures at 50 °C and 1 Torr O₂ are performed with the Pd illuminated by the VUV light and shadowed from the VUV light in attempting to select for the oxidant that impinges on the Pd surface and causes oxidation. Results suggest that atomic O incident from the gas phase is responsible for oxidation of Pd, as no PdO formation is observed for the same time period with the sample shadowed. Growth of PdO via O diffusion is studied with the nudged elastic band method. Atomic O diffusion through Pd has an activation energy barrier of ∼2.87 eV with respect to a surface O. This decreases to ∼1.80 eV once the 0.25 monolayer of O occupies the surface. The extent of Pd oxidation is limited to the near-surface Pd region for all times and temperatures investigated. PdO formation does not appear to exceed one to two atomic layers of Pd for conditions explored herein.