Experimental observation on electronegativity variation in inductively coupled oxygen plasma with DC biased grid

A plasma with a very low electron temperature (~ 0.5 eV) is generated by a DC biased grid in an oxygen inductively coupled plasma. The electronegativity of this plasma below the grid is measured as the grid voltage decreases (0 V to -40 V) by using the ratio of electron and ion saturation currents. The electronegativity is high (5.6 to 7.5) even at the very low electron temperature (0.4 to 0.3 eV). The primary reason for this high electronegativity is that electrons are sufficiently accelerated by the grid sheath voltages to induce significant dissociative ionization reactions below the grid, which require a high threshold energy (17 eV). Consequently, the generation of negative ions is substantial. The particle balance equation for negative ions is studied by further investigating the generation and loss reactions of negative ions, such as dissociative attachment and detachment reactions caused by electron-negative ion collisions.