Reactive sputtering is widely used for thin film deposition of various compounds such as oxides and nitrides of metal and non-metal elements. In reactive sputtering processes, reactive gases such as oxygen are added to discharge of a sputtering system and chemical reactions between gaseous species and sputtered materials from the target form chemical compounds that are deposited on the substrate. In reactive sputtering processes, chemical compounds are also formed on the target surface and the thickness of the compound layer affects the sputtering rate of the target, which in turn can affect the film deposition rates and even film qualities. Therefore, for deposition of high quality thin films, control of compound formation on the sputter target is considered to be of significant importance.

Compound-layer formation on a target must be caused by influx of reactive ions and free radicals from the plasma despite the constant sputtering of the target surface. Details of such surface reaction processes have not been well understood to the extent that such knowledge could be readily used for the control of reactive sputter deposition processes.

In this work, we have focused on reactive sputtering deposition processes of SiO₂ thin films and examined the oxide layer formation on a Si target, using multi-beam injection experiments with beams of radicals and mass-analyzed energetic ions as well as molecular dynamics (MD) simulations of surface reactions. It has been found that oxygen ions, which penetrate the substrate more deeply than argon ions at the same incident energy, cause constant oxidation of the surface whereas the flux ratio of low-energy charge-neutral oxygen radicals to argon ions can also significantly affect the thickness of surface oxide. These results can be used to facilitate the control of plasma conditions for reactive sputter deposition processes.