Diagnostics of Fluorine Negative Ions by Laser Photodetachment Combined with a Heated Probe in High-Density CF 4 Plasmas

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Abstract
This paper reports the absolute density and reaction kinetics of fluorine negative ions (F-) in a low-pressure, high-density CF4 plasma excited by helicon-wave discharge. Probe-assisted laser photodetachment has been adopted for the measurement of the F- density. To avoid the deposition of insulated fluorocarbon films on the probe surface, the probe tip was heated to approximately 1000 K. For a CF4 gas pressure of 2 mTorr, the ratio of the F- density to the electron density was n -/n e ≃0.06 in the active discharge plasma with a plasma density of 1.8 ×1012 cm-3. In the afterglow, the electron density decreased rapidly with a decay time constant of ≃10 µs, while the F- density had a peak at ∼15 µs after the termination of the rf power. The ratio n -/n e increased up to ∼5.7 in the afterglow. The production and loss processes of F- are discussed based on the experimental observations.