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

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The characteristics of electron density (ne) in pulsed inductively coupled O2/Ar plasmas are investigated by means of a time-resolved hairpin probe and a two-dimensional (2D) hybrid model. A decrease in ne is found at the beginning of active-glow in the discharges with high pulse frequencies (i.e., 2 and 5 kHz with 50% duty cycle). The period of this ne decrement becomes shorter when decreasing the pulse frequency (i.e., 22 μs for 5 kHz but 11.5 μs for 2 kHz in the experimental results), and it finally becomes zero in 1 kHz discharge. Combined with the 2D hybrid model, the decrease in ne can be attributed to (i) the large consumption rate of electrons [mainly via the dissociative attachment of O2, O2(a1Δg), and O2M 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 = 10 cm). 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 ne peak after the power is turned off can be attributed to the detachment of O−.