Electron and Negative Ion Densities in C2F6 and CHF3 Containing Inductively Coupled Discharges

Electron and negative ion densities have been measured in inductively coupled discharges containing C2F6 and CHF3. Line integrated electron density was determined using a microwave interferometer, negative ion densities were inferred using laser photodetachment spectroscopy, and electron temperature was determined using a Langmuir probe. For the range of induction powers, pressures and bias power investigated, the electron density peaked at 9 x 1012 cm-2 (line-integrated) or approximately 9 x 1011 cm-3. The negative ion density peaked at approximately 1.3 x 1011 cm-3. A maximum in the negative ion density as a function of induction coil power was observed. The maximum is attributed to a power dependent change in the density of one or more of the potential negative ion precursor species since the electron temperature did not depend strongly on power. The variation of photodetachment with laser wavelength indicated that the dominant negative ion was F-. Measurement of the decay of the negative ion density in the afterglow of a pulse modulated discharge was used to determine the ion-ion recombination rate for CF4, C2F6 and CHF3 discharges