Electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge produced in an O₂ and Ar/O₂/C₄F₈ gas mixture

Abstract

This paper presents an experimental study of electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge. The plasma is produced in an O 2 and Ar/O 2 /C 4 F 8 gas mixture at 2 Pa gas pressure. Laser photo-detachment combined with a microwave resonance probe is used to diagnose the absolute densities of multiple negative ion species, F − and O − . In pure oxygen discharge, both the quasi-steady state electron and negative ion densities increase during the active glow with an increase in RF power level. In the afterglow plasma, the electron density decay rate is ∼10-15 μs, whereas the O − decays more slowly with a decay time constant in the range of 30-35 μs. At a low RF power, a peak in the O − density is observed in the early afterglow plasma. In the Ar/O 2 /C 4 F 8 gas mixture, the quasi-steady state electron density in the active glow plasma increases with RF power, whereas the O − density decreases and F − density increases. In comparison to pure O 2 discharge, in the Ar/O 2 /C 4 F 8 gas mixture the electron density decay is faster. In the afterglow the F − density is ten times higher than the O − density. These results suggest a change in the dominant negative ion production mechanism within the discharge pulse from dissociative electron attachment of C x F y in the active on-phase to electron attachment to fluorine atoms in the afterglow plasma.

Time-varying mechanism of ion composition in a pulse-modulated Ar/C4F8/O2 dual-frequency capacitively coupled plasma

“…On the other hand, from k rec » 1 × 10 −7 cm 3 s −1 and the measured ion density of n » , which is reasonable based on reported theoretical and experimental results in Ar/C 4 F 8 /O 2 ,29) Cl 2 , 50) SiH 4 ,…”

“…Sirse et al 29,30) evaluated the applied power dependence of nand n e in a CCP with 60 or 100 MHz using an Ar/C 4 F 8 /O 2 gas flow ratio which is similar to ours. According to their experimental data, in this study using VHF = 40 MHz, a can be considered approximately 1 at most.…”

“…and O − . On the basis of the electron energy dependence of the dissociative attachment cross section for each negative ion [26][27][28] and the negative-ion measurement via laser photodetachment in a CCP 29,30) with similar gas composition ratio to our study, the F − ion produced through reaction (7) in Table I was chosen as the major negative ions in this analysis and the other negative ions were not considered. Some of the positive ions were lost via recombination with negative ions, according to reaction pathways and rate constants, which are shown in Table I as reactions ( 8)- (12).…”

“…It was reported that in discharges with the C 4 F 8 gas, electronegativity decreases with increasing the discharge power because the increase in n e promotes C 4 F 8 dissociation and induces suppression of negative-ion production through dissociative attachment of C 4 F 8 . 35,55) Sirse et al 29) evaluated the RF (100 MHz) power dependence of n e and nin a pulsed Ar/C 4 F 8 /O 2 CCP using time-resolved measurement. According to their data, it was confirmed that an increase in RF power (100 W → 200 W) reduces a by several tens of percent.…”

Electron temperature measurements with a hairpin resonator probe in a pulsed low pressure capacitively coupled plasma

“…A simple parameterization of forward power is used to vary n e in order to explore the measurement technique, with lighter curves corresponding to higher forward power rang-ing from P fwd = 8-28 W. In figure 9(a), we see that n e takes a familiar shape, with a quick rise in n e at power-on around t = 0 μs and approaching a steady state value around t = 300 μs. At power-off, n e exhibits a slow decay (relative to T e ) with τ n = 150-250 μs, as one would expect from literature [11,12,[31][32][33][34]. A spike in n e occurs within 5 μs of power off, previously explained as probe sheath collapse manifesting as an observed increase in n e [35].…”