Complex transients of input power and electron density in pulsed inductively coupled discharges

Abstract

Time-dependent studies of pulsed inductively coupled Ar and Ar/CF4 discharges are presented in this work. By using a time-resolved power diagnosis system, i.e., a Langmuir probe and a Hairpin probe, the temporal evolutions of input power and electron density are measured. In the initial pulse stage, the input power exhibits two peaks, which are related to the properties of the source and the plasma, respectively. In addition, an overshoot of the electron density is observed in the initial pulse stage at high powers (500–800 W) and low pressures (1–10 mTorr), and the overshoot becomes weaker by increasing pressure (10–80 mTorr) or decreasing input power (200–500 W). This can be explained by the dependence of the power transfer efficiency on pressure and input power, as well as the balance between the electron production and loss rates. When the power is turned off, the electron density and the input power exhibit a peak at the initial afterglow period, due to the release of charges from capacitors and inductors in the radio frequency power source. In Ar/CF4 discharges, the plasma responds to the changes in the input power more quickly than in Ar discharges, so it takes a shorter time to reach the ionization equilibrium. This may be caused by more ionization channels, larger ionization cross section, and lower ionization thresholds in Ar/CF4 plasmas.