Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

F- negative ions were detected by direct observation of the weak photodetachment absorption continuum below 364.5 nm by cavity ring-down spectroscopy. The negative ions were generated in a modified industrial dielectric plasma etch reactor, with 2+27 MHz dual frequency capacitive excitation in ArC F4 O2 and Ar C4 F8 O2 gas mixtures. The F- signal was superimposed on an unidentified absorption continuum, which was diminished by O2 addition. The F- densities were in the range of (0.5-3) × 1011 cm-3, and were not significantly different for single (27 MHz) or dual (2+27 MHz) frequency excitation, not confirming recent modeling predictions.