Measurements of time resolved rf impedance of a pulsed inductively coupled Ar plasma

An rf impedance meter was developed to measure the time resolved rf power and impedance characteristics of a square-wave time modulated inductively coupled Ar plasma. The impedance meter consisted of a voltage/current sensor head and a signal processing circuit unit. The voltage and current sensors were a capacitive voltage divider and a magnetic flux pick-up coil, respectively. Bridge rectifiers were used to determine the voltage and current amplitudes, while the relative phase signal was extracted using a double balanced mixer. The meter was calibrated against a commercial impedance meter under continuous wave operation. A multilayer feedforward neural network was developed for the phase angle estimation, and an accuracy of around +/- 0.1 degrees was obtained. Measurements of the rf impedance and absorbed power of an inductively coupled Ar plasma under square-wave time modulation reveal that the time dependence of the ion saturation current follows the rf power closely. A spike in the real part of the rf impedance was observed at the beginning of the modulation pulse due to the transition from capacitive to inductive coupling. At a lower modulation frequency about 0.7 ms is needed to attain a steady state compared with cw operation in this experiment.