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 PdOx formation is observed for the same time period with the sample shadowed. Growth of PdOx 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. PdOx formation does not appear to exceed one to two atomic layers of Pd for conditions explored herein.

Colloidal transfer printing.

Many fields of research have adopted self-assembly of colloidal spheres as an easy and reliable method to produce macroscopic structures with nanoscale periodicity. The field of soft lithography in particular has used colloidal self-assembly to fabricate lithographic masks and templates. We developed a colloidal lithography method that uses the colloidal assembly directly to produce submicrometer topographic and chemical surface patterns. The method does not require any specialized equipment, making it particularly useful in biological and chemical laboratories without lithography expertise. The technique involves the curing and solvent removal of a self-assembled colloidal crystal from an inorganic surface. The result is a triangular array of polymer features with submicrometer periodicity that covers square centimeters of surface area. The feature size and spacing is easily controlled, and the features serve as reactive sites for biomolecule immobilization.