Diagnostics of Fluorine Negative Ions by Laser Photodetachment Combined with a Heated Probe in High-Density CF₄ Plasmas

This paper reports the absolute density and reaction kinetics of fluorine negative ions (F - ) in a low-pressure, high-density CF 4 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 CF 4 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 ×10 12 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.