Low-energy xenon ion sputtering of ceramics investigated for stationary plasma thrusters

The low-energy sputtering of boron nitride, magnesium oxide, boron nitride and aluminum nitride (BNAIN), and boron nitride and silicon oxide (BNSiO2) by xenon ions of bombarding energies 350, 500 eV, and 1 keV was studied experimentally. In order to measure the ion current without being significantly disturbed by slow ions, only planar probes were used during short duration sputtering experiments (of the order of 10 h). Moreover, slow ion current contribution was estimated by numerical simulations and subtracted from each ion current measurement. It was found that the ion-beam incidence effect on sputtering yields was not as important as for theoretical results or experimental results on quasinonrough solid surfaces, for which it is possible to observe a more pronounced angular dependence of the sputtering yield. This phenomenon is due to surface irregularities of ceramic materials and because of surface roughness the macroscopic sputtering yield should actually result from the convolution of the microscopic sputtering yield by the angular distribution of surface facet incidences. The irregular surface structure of ceramics like BNSiO2 or BNAIN seems to be sputtered differently due to a differential erosion from grain to grain and from grain to surrounding matrix. This uneven erosion may be explained by the wide angular distribution of facet incidences of surface microprofiles and the various binding energies in the selvage of material. Finally, the dependence of sputtering yield at normal incidence on ion energy, in the range 0.35-1 keV, is almost a linear one. (C) 1999 American Vacuum Society. [S0734-2101(99)05406-8].