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 364.5nm by cavity ring-down spectroscopy. The negative ions were generated in a modified industrial dielectric plasma etch reactor, with 2+27MHz dual frequency capacitive excitation in Ar∕CF4∕O2 and Ar∕C4F8∕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)×1011cm−3, and were not significantly different for single (27MHz) or dual (2+27MHz) frequency excitation, not confirming recent modeling predictions.