THE TRANSITION FROM SYMMETRICAL TO ASYMMETRIC DISCHARGES IN PULSED 13.56-MHZ CAPACITIVELY COUPLED PLASMAS

The behavior of a rapidly pulsed radio-frequency capacitively coupled parallel plate reactor has been investigated using time-resolved volta ge probe, microwave interferometer and optical emission techniques. Th e reactor was operated with 50 mTorr of argon and 100 W rf power (meas ured at the generator) at 13.56 MHz supplied to the 100-mm-diam powere d electrode, with pulse durations of 25 and 100 mu s. For low repetiti on rates (50 Hz) the voltage envelope has a characteristic form which has been entitled the ''Bird's Head,'' There is no plasma present at t he beginning of the pulse, so that an initial breakdown phase occurs. This phase lasts about 600 ns, after which time the plasma density is sufficiently high for the Debye length to enter the gap between the el ectrodes and for sheaths to form on the electrodes. In asymmetric para llel plate reactors the blocking capacitor in the matching circuit cha rges such that the powered electrode acquires a continuous negative vo ltage offset (the so-called de bias). In this system the charging time of the capacitor is longer than the rise time of the rf voltage. Cons equently, for the first few mu s of the pulse the discharge is symmetr ic (no de bias) and confined between the rf rind the adjacent earthed electrode. As the bias voltage increases the discharge fills more of t he reactor and becomes asymmetric. The rate at which the blocking capa citor charges (due to net electron current from the plasma to the powe red electrode) is controlled by the Bohm-criterion limited flux of ion s to the earthed walls of the reactor, as shown by particle-in-cell si mulations in H. B. Smith, C. Charles, and R. W. Boswell, J. Appl. Phys . 82, 561 (1997). At high repetition rates (20 kHz) the plasma density is hardly modulated, there is no breakdown or symmetric phase, and on ly the electron temperature and de bias are modulated. The conditions which lead to a symmetric discharge phase are defined. A simple analyt ical model is developed to describe the temporal evolution of the plas ma density and electron temperature. The model is in good qualitative agreement with the observations, and predicts an average electron ener gy of 10's of eV during the first few mu s of the symmetric discharge. (C) 1997 American Institute of Physics.