Plasma diagnostic studies of low-energy ion bombardment in r.f. magnetron sputter deposition

Abstract

The flux of positive ions bombarding the sample position during r.f. magnetron sputtering of ZnO in a pure oxygen plasma has been analysed in detail as to its mass and energy distribution for various discharge conditions in r.f. power range between 100 W and 1000 W and at chamber pressures from 0.2 Pa to 6 Pa using a plasma monitor. The incident ion flux varies from about 1 × 10 15 cm −2 s −1 to close to 1 × 10 16 cm −2 s −1 and was found to be determined primarily by the bulk plasma ion density in front of the sample surface, which was also measured. These values are comparable to those which were obtained for the arrival rate of sputtered particles. The bombarding ion flux is nearly completely governed by O 2 + ions and to a few per cent by O +. Moreover, two further groups of ionic species are present as traces coming clearly from sputtered material and residual gas including the latter's reaction products with the working gas. All the ions detected were found to show a similar ion bombardment energy distribution responding very sensitively in both the shape and the mean energy to changes of r.f. power and oxygen pressure, for example from having a single peak characteristic with a small high-energy tail to a saddle-shaped one with some asymmetry and mean energies between 20 eV and 60 eV. From the data obtained the internal processing parameter ion bombardment energy per arriving target particle was calculated to range from about 30 eV up to more than 120 eV. On the basis of these results r.f. magnetron sputter conditions can be exactly predicted. This enables the deposition process to be performed at a defined ion bombardment of the growing film.