TIME-RESOLVED POWER AND IMPEDANCE MEASUREMENTS OF PULSED RADIOFREQUENCY DISCHARGES

The power to a pulsed RF discharge in the Gaseous Electronics Conference reference reactor has been measured with sub-microsecond time resolution. There are large swings in the power delivered to the cell as a function of time during the first 5-10 mu s of the transient turn-on and turn-off. These power oscillations are in large part caused by the passive circuitry between the current-voltage sensors and the electrode assembly of the cell and have been modelled using an equivalent network with reasonable accuracy. They make it difficult to determine the plasma impedance at very early times during the discharge turn-on. This passive circuitry can also store RF energy and release it both to the plasma and to the RF generator at plasma turn-off. The power delivered to a 500 mTorr argon discharge does not reach steady state until approximately 500 mu s after the discharge turn-on even though the initial power oscillations end within the first 10 mu s. This longer term power variation appears to be related to a contraction of the electrode sheaths as well as to an electron density increase and electron temperature decrease during this time. As the sheath contracts, less of the RF field penetrates to the body of the glow and the electron atom collision frequency, which is related to the electron average energy, decreases by a factor of two.