Time-resolved radial uniformity of pulse-modulated inductively coupled O2/Ar plasmas*

Abstract

Time-resolved radial uniformity of pulse-modulated inductively coupled O2/Ar plasma has been investigated by means of a Langmuir probe as well as an optical probe in this paper. The radial uniformity of plasma has been discussed through analyzing the nonuniformity factor β (calculated by the measured n e, lower β means higher plasma radial uniformity). The results show that during the active-glow period, the radial distribution of n e exhibits an almost flat profile at the beginning phase, but it converts into a parabola-like profile during the steady state. The consequent evolution for β is that when the power is turned on, it declines to a minimum at first, and then it increases to a maximum, after that, it decays until it keeps constant. This phenomenon can be explained by the fact that the ionization gradually becomes stronger at the plasma center and meanwhile the rebuilt electric field (plasma potential and ambipolar potential) will confine the electrons at the plasma center as well. Besides, the mean electron energy (〈 ε 〉on) at the pulse beginning decreases with the increasing duty cycle. This will postpone the plasma ignition after the power is turned on. This phenomenon has been verified by the emission intensity of Ar (; = 750.4 nm). During the after-glow period, it is interesting to find that the electrons have a large depletion rate at the plasma center. Consequently, n e forms a hollow distribution in the radial direction at the late stage of after-glow. Therefore, β exhibits a maximum at the same time. This can be attributed to the formation of negative oxygen ion (O−) at the plasma center when the power has been turned off.