Characterization of electron and negative ion densities in fluorocarbon containing inductively driven plasmas

Electron and negative ion densities were measured in inductively coupled discharges containing C4F8. In addition, the identity of the negative ions in C2F6, CHF3, and C4F8 containing discharges was investigated with a photodetachment experiment utilizing a microwave resonant cavity structure. 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 the 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, and rf bias. For the range of induction powers, pressures, and bias power investigated, the electron density peaked at 6×1012 cm−2 (line integrated), or approximately 6×1011 cm−3. The negative ion density peaked at approximately 2.2×1011 cm−3. In most cases, the trends in the electron and negative ion densities were independent of the wafer material. However, a maximum in the negative ion density as a function of induction coil power was observed above a silicon wafer. The maximum is attributed to a power-dependent change in the density of one or more of the potential negative ion precursor species. A microwave resonant cavity structure was developed to identify the negative ions using laser photodetachment spectroscopy. The technique was demonstrated for inductively coupled discharges containing C4F8, C2F6, and CHF3. Scanning the laser wavelength over the range of the F− photodetachment energy indicated that while the dominant negative ion appeared to be F−, weak evidence for other molecular negative ions was observed. Unlike traditional microwave cavity techniques, this method offers the possibility of spatial resolution.