Driving Frequency Fluctuations in Pulsed Capacitively Coupled Plasmas

We report time resolved measurements of the RF current, voltage and complex impedance for pulsed plasmas through electropositive (Ar) and electronegative (CF₄, O₂) gases and gas mixtures. In addition, we report measurements of the effective frequency versus time at various locations within the RF circuitry. The frequency is found to fluctuate away from that sourced by the RF generator when the plasma re-ignites. Plasma re-ignition induces abrupt impedance changes due to the re-formation of the plasma sheath and bulk. These fast changes in the plasma impedance cause the measured changes in the voltage and current frequencies. As a result, the frequency of the RF power at the plasma electrodes was found to be as much as 250 kHz different from that being sourced by the RF generator for short periods of time. These frequency fluctuations are of particular interest to the application of frequency tuned matching networks.