Electrical optimization of plasma-enhanced chemical vapor deposition chamber cleaning plasmas

Fluorinated gas discharges are widely used by the semiconductor industry in etching and chamber cleaning applications, but the performance of these discharges varies in unpredictable ways, for unknown reasons believed to be electrical in origin. To investigate possible mechanisms for this behavior, we have measured the electrical characteristics of NF₃/Ar, CF₄/O₂/Ar, and C₂F₆/O₂/Ar chamber cleaning plasmas at 6.7–267 Pa in a 13.56 MHz, capacitively coupled, parallel-plate reactor, using radio-frequency current and voltage probes and optical emission spectroscopy. From the measurements, power losses in the external circuitry surrounding the discharge were determined. Furthermore, using the measurements and equivalent circuit models, the mechanisms by which power was absorbed within the discharge itself were investigated. Power was absorbed most efficiently at particular values of the discharge impedance. These optimal impedances occur in the middle of a transition from capacitive impedances at low pressures to resistive impedances at high pressures. These results illustrate that the plasma impedance is a useful parameter for monitoring and optimizing plasma processes in highly electronegative gases. © 1998 American Vacuum Society.