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

Electron and negative ion densities were measured in inductively coupled di scharges containing C4F8. In addition, the identity of the negative ions in C2F6, CHF3, and C4F8 containing discharges was investigated with a photode tachment experiment utilizing a microwave resonant cavity structure. To inv estigate the influence of surface material, the rf-biased electrode was cov ered with a silicon wafer or a fused silica (SiO2) wafer. Line-integrated e lectron density was determined using a microwave interferometer, and absolu te negative ion densities in the center of the plasma were inferred using l aser photodetachment spectroscopy. Voltage and current at the induction coi l 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 peake d at 6x10(12) cm(-2) (line integrated), or approximately 6x10(11) cm(-3). T he negative ion density peaked at approximately 2.2x10(11) cm(-3). In most cases, the trends in the electron and negative ion densities were independe nt 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 p hotodetachment spectroscopy. The technique was demonstrated for inductively coupled discharges containing C4F8, C2F6, and CHF3. Scanning the laser wav elength over the range of the F- photodetachment energy indicated that whil e the dominant negative ion appeared to be F-, weak evidence for other mole cular negative ions was observed. Unlike traditional microwave cavity techn iques, this method offers the possibility of spatial resolution. (C) 2001 A merican Institute of Physics.