Negative ions in argon–oxygen discharges

The temporal behavior of negative oxygen ions in argon/oxygen mixtures is investigated in a pulsed inductively excited modified GEC reactor. The negative ion densities are measured by laser photodetachment of negative ions in combination with a Langmuir probe as well as by absorption of photons due to photodetachment with cavity ring-down spectroscopy. An unexpected high density of negative oxygen ions was found at high plasma densities. The high density of negative oxygen ions may account for the high oxidation rates of silicon in microwave discharges. The main loss mechanism for negative oxygen ions at plasma densities above 5·1010 cm−3 is mutual neutralization with positive ions. Measurements of atomic oxygen densities show that collisions of the atoms with negative ions are a minor loss process for the negative ions. According to predictions of common models it is unlikely that the density of the negative ions is high at such plasma densities. It is, therefore, necessary to invoke an additional production channel for the negative ions in order to explain the observed high negative ion density. It is likely that negative ions are generated to a considerable amount by dissociative attachment to excited oxygen molecules too. Two possible reactants for this generation can be considered, namely Rydberg oxygen molecules or metastable oxygen molecules O2M(A3Σu+, C3Δu, c1Σu−).