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

Abstract

F⁻ negative ions were detected by direct observation of the weak photodetachmentabsorption continuum below 364.5nm by cavity ring-down spectroscopy. The negative ions were generated in a modified industrial dielectricplasmaetch 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.