Energy distribution of negative O− and OH− ions emitted from YBaCuO and iron garnet targets by dc and rf magnetron sputtering

Abstract

Energy-dispersive mass spectrometry has been used to analyze the energy distribution of O− and OH− species ejected from YBaCuO and iron-garnet targets by Ar+, Kr+, and Xe+ bombardment in H2- and in O2-doped dc and rf magnetron plasmas at 0.05–4.2 Pa pressure. The orifice of the plasma monitor was at 70 mm from the erosion groove underneath the plasma ring. The energy spectra of O− and OH− ions are found to exhibit two major peaks: a sharp one at typically 20 eV termed A and a sharp (dc case) or broad (rf case) peak termed B at higher energies. Peak-A ions may be formed near the edge of the cathode sheath by electron attachment to sputtered neutral oxygen atoms accelerated in the remaining potential gradient of the sheath. Peak-B ions are shown to be accelerated from the target surface to kinetic energies given by the potential gradient across the cathode sheath. In the case of rf magnetron plasmas the total flux of O− and OH− ions associated with the peak B steeply increases with pressure up to ∼0.6 Pa for argon, ∼0.4 Pa for krypton, and ∼0.3 Pa for xenon, concomitant with a shift of the mean particle energy from ≳100 eV at 0.1 Pa to <35 eV at these characteristic pressures. This behavior may be explained by charge-exchange collisions within the rf sheath. At pressures beyond this maximum the total flux of negative ions declines due to electron detachment in regions remote from the plasma which may be caused by collisions with noble-gas atoms and Maxwellian electrons, and by charge transfer to positive noble-gas ions. At pressures beyond several Pa elastic scattering is the dominant loss mechanism of energetic atomic oxygen species. Doping of the noble-gas plasma at the vol % level by oxygen or hydrogen causes severe changes of the O− and OH− yield resulting from a change in the density of oxygen vacancies in the target surface. For argon, krypton, and xenon the yield of O− ions is ≳20 times higher with the YBaCuO target, as compared to the iron-garnet target, due to the larger electronegativity of the BaO bond. From these data optimum conditions for magnetron sputtering of YBaCuO films are derived, as summarized in the conclusions.