Surface dependent electron and negative ion density in SF6/argon gas mixtures

Electron and negative ion densities were measured in an inductively driven plasma containing mixtures of SF6 and Argon. The electron and negative ion density were measured as functions of the induction coil power, pressure, bias power, and SF6/argon ratio. To investigate the influence of surface material, the rf biased electrode was covered with a silicon wafer or a fused silica (SiO2) wafer. Line integrated electron density was determined using a microwave interferometer, and absolute negative ion densities in the center of plasma were inferred using laser photodetachment spectroscopy. Voltage and current at the induction coil and rf biased electrode were also measured for both surfaces as functions of induction coil power, pressure, rf bias, and SF6/argon ratio. For the range of induction powers, pressures, and bias powers investigated, the electron density had a maximum of 5×1012 cm−2 (line-integrated) or approximately 5×1011 cm−3. Over this same range the negative ion density had a maximum of 2×1011 cm−3, and was always less than the electron density. For most conditions, the negative ion density above the oxide surface was a factor of 5 to 10 larger than the density above the silicon surface. In contrast, the electron density above the oxide surface was equal to or slightly higher than the density above the silicon surface. Surface dependent changes in the induction coil and rf bias voltage and current were also observed.