Controlled generation of pulse-modulated RF plasmas for materials processing

Pulse-modulated radio frequency (RF) inductively coupled plasmas for materials processing applications were generated using a voltage-control-type power source, and the operating ranges for controlled generation were extended to sufficiently low shimmer power levels. When the plasma was generated at atmospheric pressure and a high power level of 17 kW, the low power level typically went down to about 4 kW for an Ar-H-2 plasma and 3 kW for an Ar-N-2 plasma, levels too low to sustain the continuous plasmas. The overshoot and undershoot at the beginnings of power change were reduced considerably by using an exponential voltage control signal. Spectroscopic measurements of the radiation intensity of the Ar atomic spectral line (751.5 nm) showed that the plasma temperatures varied with time and that the characteristic times of the plasmas depended on the operating conditions and the position in the plasma generator. The characteristic times in the discharge zone may be largely determined by the competition among ionization, recombination and convection in the pulsed plasmas. The characteristic times estimated using an electron transportation model are reasonably in line with those determined from measured emission intensities. The difference between the plasma properties at the higher and lower power levels was large enough to give rise to the nonequilibrium states at the instances of pulse-on and pulse-off, and to the increase in the concentration of chemically active radical species. This offers a unique physico-chemical condition for materials processing. The ranges of controlled generation were determined for the Ar-H-2 and Ar-N-2 plasmas at pressures from 27 to 101 kPa.