Controlled oxidation of alkyl monolayers grafted onto flat Si(111) in an oxygen plasma of low power density

Alkyl monolayers anchored to an atomically flat hydrogenated (111)-oriented silicon surface through covalent Si-C bonding have been submitted to an oxygen plasma treatment of controlled power density. The chemical state of the surface was monitored in situ in real time in the plasma cell using multiple-internal- reflection infrared spectroscopy and ex situ by X-ray photoelectron spectroscopy. While plasma treatments of moderate power densities (̃0.1-1 W/cm³) lead to complete removal of the organic monolayer, using very-low power densities (̃10 mW cm^-3) leads to selective oxidation of the organic chains with negligible etching of the monolayer and a very limited oxidation of the silicon surface. Successive formation of alcohol, ketone, and carboxyl functions is observed before reaching a quasi steady state on an hour time scale. Modelization of the plasma shows that the dominant reactive species at these low power densities is the singlet oxygen molecule (¹Δ _g). A molecular-scale reaction mechanism is proposed, and a simple kinetic model is derived. A major conclusion is that reaching a quasi steady state is attributable to the very low amount of atomic oxygen in the plasma and to the densification of the layer upon incorporation of oxygen-bearing groups.