Time-resolved current and voltage measurements on a pulsed rf inductively coupled plasma†

Time resolved current and voltage measurements have been made on a pulsed radio frequency (rf) inductively coupled plasma (ICP) at 13.56 MHz in argon. Measurements were made on the rf coil using a high-voltage probe, a Rogowski current probe, and a high-performance digital oscilloscope. Relative phase information was also obtained so that time resolved rf power measurements could be made. Due to the inductive nature of the load, measurement of the phase had to be better than 0.6 mrad at 13.56 MHz in order that the power measurements were accurate to 10%. This accuracy in phase measurement was achieved by careful positioning of the probes and by establishing accurate phase calibration procedures. The power was calculated by three methods: discrete Fourier transform, integral of the current voltage product over N periods, and least-squares fits of a sine wave to the measured data. Time-resolved measurements of the system complex impedance, power loss in the ICP planar coil, and the actual amount of rf power delivered to the plasma were made. These measurements give details during plasma breakdown and show the transition from capacitive to inductive discharge. The results are compared with both time-resolved plasma emission and time-resolved Langmuir probe measurements.