Pulse-Modulated RF Thermal Plasma for Advanced Materials Processing

Abstract

Thermal plasma has been used for various kinds of materials processing. For further extension of the variable process parameters and the applications of thermal plasma, a time-domain controlled generation was developed of the pulse-modulated RF induction plasma, which has been successfully generated by the present authors, for the first time, with a sufficiently high electric power for the advanced materials processing. At the first stage, the steep overshoot and undershoot of the coil current amplitude of oscillating frequency of 1 MHz were recognized at the time instances of pulse on and off, giving a limited range of stable generation. Two solutions were found for suppressing the magnitude of overshoot and undershoot. One is the introduction of shimmer level, which is a ratio of coil current magnitude between pulse-on and -off periods. The other is that the rise and decay times of RF current was made a little slow in time domain. Consequently, when the Ar-H2 plasma was generated under 750 Torr with a high power level of 17 kW, the low power level went down to about 4 kW, i.e. below 1/4 of the higher level, with which such low power level, the continuous plasma generation was unable to be sustained. The increased difference between the high and lower power levels was large enough to give rise to the non-equilibrium condition in plasma temperature and composition at the instances of pulse on and off. Such unique condition is expected to offer a unique physico-chemical condition for advanced materials processing. The range of well-controlled generation of the pulse-modulated plasma was determined for Ar-H2 plasma with various parameters. It was recognized that the duration of pulse-on and -off times as well as the shimmer current level are the important parameters, which determine the degree of deviation from equilibrated condition in high pressure thermal plasma.