The impact of accelerated electrons on electronegativity in an inductively coupled oxygen plasma with a DC-biased grid

Abstract

The electronegativity in an inductively coupled oxygen plasma with a DC-biased grid is investigated. The electronegativity increases by a factor of 3 at 25 mTorr, as the grid voltage decreases from 0 to -20 V, and then slightly decreases under -20 V. It is found that the variation of electronegativity below the grid is determined by the electrons accelerated by the grid sheath, whose energy is controlled by the grid voltage. To understand this behavior, the particle balance equations are used considering negative ion O-, neutral species ( O2(a1 Delta g), O2(b1 Sigma g+), O(3P) and O(1D)), and positive ions ( O+ and O2+), based on the estimated energy distributions of the accelerated electrons. The initial rise in electronegativity is attributed to larger decrease in electron density compared to that of O-, while the subsequent slight decrease results from a more pronounced reduction in O- density. The major generation and loss reactions of O- are examined, and the variations in electronegativity with DC bias are different with pressures. At 5 mTorr, the electronegativity remains almost constant over the entire grid voltage range. These findings demonstrate that grid-based control of electron energy enables control of electronegativity.