Energy Distribution of Negative Oxygen Ions Emitted from YBaCuO and Iron Garnet Targets by dc- and rf-Magnetron Sputtering

Abstract

The energy distribution of negative O⁻ species sputtered from an YBaCuO and an iron-garnet target in dc- and rf-magnetron discharges has been analyzed by means of an energy-dispersive mass spectrometer in the range of noble-gas pressures from 0.05 to 4.2 Pa. Evidence is presented that energetic 0- ions originate essentially from two sources: (a) those formed directly by sputtering from the target surface and accelerated across the sheath in the full cathode-potential gradient; and (b) those created by electron attachment to neutral oxygen atoms near the sheath boundary to the plasma and accelerated in the remaining potential gradient of the sheath. On their path through the plasma these energetic O⁻ ions are eventually annihilated by electron stripping and elastic scattering dominating in the pressure × distance regime above ∼0.1 Pa × m. The yield Y_O- of type-a O⁻ ions increases with decreasing oxygen-vacancy density. Thus, doping of the noble-gas plasma with up to ∼1 vol% oxygen results in enhanced yield of 0- ions, while doping with hydrogen reduces the formation of O⁻ species but gives rise to the creation of OH- ions. Furthermore, Y_O-increases with increasing ionicity of the oxides constituting the target, like BaO, exhibiting a larger difference in electronegativity between anions and cations. Thus, we find a between 1 and 2 orders of magnitude less yield of O⁻ ions from a GdBiFeGaAl-gamet, as compared to the YBaCuO target, confirming the dissociative behavior of the BaO bond.