Experimental and numerical investigations of the characteristics of electron density in O2/Ar pulsed planar-coil-driven inductively coupled plasmas

The characteristics of electron density (n(e)) in pulsed inductively coupled O-2/Ar plasmas are investigated by means of a time-resolved hairpin probe and a two-dimensional (2D) hybrid model. A decrease in n(e) is found at the beginning of active-glow in the discharges with high pulse frequencies (i.e., 2 and 5kHz with 50% duty cycle). The period of this n(e) decrement becomes shorter when decreasing the pulse frequency (i.e., 22 mu s for 5kHz but 11.5 mu s for 2kHz in the experimental results), and it finally becomes zero in 1 kHz discharge. Combined with the 2D hybrid model, the decrease in n(e) can be attributed to (i) the large consumption rate of electrons [mainly via the dissociative attachment of O-2, O 2 ( a 1 Delta g ), and O 2 M to generate O-] at the probe position and (ii) the axial electron flux toward the coils that arises at the start of active-glow. Also, hardly any of the high-energy electrons that are generated near the coils reach the probe position (P1) because of their short electron energy relaxation length (smaller than the reactor length L=10cm). Consequently, electron generation via ionization becomes unimportant at P1, and therefore, the increase in electron density during active-glow is dominated by the axial electron flux (toward the substrate). However, the temporal variation of electron density at P2 (close to the coils) differs greatly from that at P1 because the ionization processes dominate the electron generation during active-glow. The formation of the n(e) peak after the power is turned off can be attributed to the detachment of O-.