Optimizing Utilization Efficiencies in Electronegative Discharges: The Importance of the Impedance Phase Angle

Published

November 1, 1999

Author(s)

W R. Entley, J G. Langan, B S. Felker, Mark A. Sobolewski

Abstract

We have investigated the operating conditions that result in the greatest utilization efficiencies (UEs) of NF₃, CF₄, and C₂F₆ in a capacitively coupled GEC reference cell. We have also independently measured the rf electrical characteristics and optical emission spectra of the plasmas. To avoid inadvertently attributing changes in the UE, discharge impedance, rf currents, or atomic emission intensities to parasitic losses in the matching network or rf delivery system, the rf generator was adjusted to ensure that the same amount of power was dissipated within each discharge. For the NF₃ plasmas, argon was used as a diluent and both the NF₃ concentration and reactor pressure were varied. For the CF₄ and C₂F₆ based plasmas, the gas compositions were fixed (86 mol % CF₄/O₂ and 50 mol % C₂F₆/O₂) and the reactor pressure was varied. The greatest CF₄ and C₂F₆ UEs occurred within a narrow range of reactor pressures. For all mixtures, operating conditions that yielded the highest UEs also yielded the brightest plasmas, the lowest impedance magnitudes, the greatest fraction of current flowing to the grounded electrode, and impedance phase angles within a narrow window centered near diameter}_pe = -40 C. Within this region, plasma power is most efficiently utilized to dissociate the source gas and excite the atoms that emit light. Collapsed plasmas, observed for high pressure highly electronegative conditions, exhibited very low UEs. At optimal operating conditions the UE of the fluorinated source gases were found to decrease in the order: NF₃ > C₂F₆ > CF₄. The results of this study suggest that the baseline corrected fluorine atom emission intensity (703.7 nm), the magnitude of the discharge impedance, or phase angle of the discharge impedance could be monitored to determine the relative fluorinated source gas UE in an arbitrary plasma reactor as the operating conditions are varied. The concept of an ideal NF₃ partial pressure could prove to be a useful strategy to prevent the formation of collapsed plasmas at high reactor pressures while maintaining high NF₃ UEs.

Citation

Journal of Applied Physics

Volume

86

Pub Type

Journals

Keywords

C₂F₆, CF₄, destruction, discharge, efficiency, electronegative, impedance, mass spectrometry, NF₃

Citation

Entley, W. , Langan, J. , Felker, B. and Sobolewski, M. (1999), Optimizing Utilization Efficiencies in Electronegative Discharges: The Importance of the Impedance Phase Angle, Journal of Applied Physics (Accessed April 16, 2024)