Low-Temperature Plasma Oxidation of Aluminum by Ar-O2 Mixtures in a Dielectric-Barrier Discharge Reactor

Room-temperature growth of oxide layers on aluminum in highly diluted mixtures of oxygen with argon (O-2 molar fractions 20 ppm <= x(O2) <= 500 ppm, partial pressures 2 Pa <= p(O2) <= 50 Pa) flowing through a dielectric-barrier discharge (DBD) reactor is studied, including oxidation in the pre- and post-discharge regions (PrD, PoD) adjacent to the main DBD. Three different mechanisms of plasma-enhanced oxidation were found to prevail, depending on the location of the sample: (1) In the close PrD region, up to 1 cm upstream from the discharge, accelerated growth of Al2O3 is due to the irradiation of the sample sur-face by highly energetic (9.8 eV) argon excimer radiation in the presence of O-2. (2) In the remote PoD, a few cm downstream from the DBD, oxidation can largely be attributed to oxygen atoms, with number densities typically between 1 and 5 x 10(14) cm(-3). Here, analysis in terms of Cabrera-Mott (CM) theory results in CM potentials between - 1.5 and - 2.1 V. (3) In the DBD itself both O atoms and VUV photons generally play an important role but, under special conditions, an additional oxidation mode can be identified, characterized by a much larger limiting thickness: While, in general, oxide growth by O atoms and/or VUV photons virtually stops at thicknesses X between 5 and 6 nm, much thicker oxide films can be achieved in the downstream region of the main DBD, with thicknesses growing with the length of the DBD zone. Tentatively, we attribute this observation to negative oxygen ions O-m(-) (1 <= m <= 3) accumulating in the gas while passing the reactor. Any direct electrical effects of the discharge process on the oxidation can probably be neglected