Exposure to an oxygen plasma is a standard approach for the etching of carbon-based thin films providing high rates. More recently it has also gained considerable interest regarding the removal of redeposited tritium-containing carbon films in magnetic-confinement nuclear fusion experiments. To gain insight into the underlying mechanisms we study the erosion of hard amorphous hydrocarbon films (a-C:H) during exposure to quantified particle fluxes by means of in situ ellipsometry.
From an oxygen plasma various species reach the surface; besides molecular oxygen, these are oxygen ions and radicals. In contrast to the latter species, molecular oxygen by itself does not erode carbon below ~600 K. In our particle beam experiment we observe, however, strong erosion if the surface is simultaneously bombarded by ions—in our case Ar+ ions at 20–800 eV—with maximum yields up to 11 eroded C per Ar+. The basic effect was observed earlier by Vietzke et al. with 5 keV Ar+ and graphite as target as is briefly mentioned in [1]. In our energy range the yield increases with increasing energy. When increasing the flux of thermal O2 the rate grows drastically and is enhanced over pure physical sputtering by roughly an order of magnitude at an oxygen/ion ratio of 10000 and an Ar+ energy of 400 eV. Cooling of the surface from 400 K down to 110 K increases the erosion rate further. The latter result is explained in terms of an ion-induced surface reaction in which adsorbed oxygen forms volatile oxides. A simple rate equation model explains the basic features of the flux, energy, and temperature dependences. The observed synergistic interaction appears to generate an important contribution to the overall carbon erosion rate in typical O2 plasmas.
[1] E. Vietzke, T. Tanabe, V. Philipps, et al., J. Nucl. Mater. 145–147, 425 (1987).
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