Optimizing utilization efficiencies in electronegative discharges: The importance of the impedance phase angle

We have investigated the operating conditions that result in the greatest u tilization efficiencies (UEs) of NF3, CF4, and C2F6 in a capacitively coupl ed GEC reference cell. We have also independently measured the rf electrica l characteristics and optical emission spectra of the plasmas. To avoid ina dvertently 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 sa me amount of power was dissipated within each discharge. For the NF3 plasma s, argon was used as a diluent and both the NF3 concentration and reactor p ressure were varied. For the CF4 and C2F6 based plasmas, the gas compositio ns were fixed (86 mol % CF4/O-2 and 50 mol % C2F6/O-2) and the reactor pres sure was varied. The greatest NF3 UEs occurred within a narrow range of NF3 partial pressures. The greatest CF4 and C2F6 UEs occurred within a narrow range of reactor pressures. For all mixtures, operating conditions that yie lded the highest UEs also yielded the brightest plasmas, the lowest impedan ce magnitudes, the greatest fraction of current flowing to the grounded ele ctrode, and impedance phase angles within a narrow window centered near phi (pe)=-40 degrees. Within this region, plasma power is most efficiently util ized to dissociate the source gas and excite the atoms that emit light. Col lapsed plasmas, observed for high pressure highly electronegative condition s, exhibited very low UEs. At optimal operating conditions the UE of the fl uorinated source gases were found to decrease in the order: NF3> C2F6> CF4. The results of this study suggest that the baseline corrected fluorine ato m emission intensity (703.7 nm), the magnitude of the discharge impedance, or phase angle of the discharge impedance could be monitored to determine t he relative fluorinated source gas UE in an arbitrary plasma reactor as the operating conditions are varied. The concept of an ideal NF3 partial press ure could prove to be a useful strategy to prevent the formation of collaps ed plasmas at high reactor pressures while maintaining high NF3UEs. (C) 199 9 American Institute of Physics. [S0021- 8979(99)08821-0].