Disruption of self-organized striated structure induced by secondary electron emission in capacitive oxygen discharges

Abstract

Self-organized striated structure has been observed experimentally and numerically in CF 4 plasmas in radio-frequency capacitively coupled plasmas (RF CCPs) recently (Liu et al 2016 Phys. Rev. Lett. 116 255002). In this work, the striated structure is investigated in a capacitively coupled oxygen discharge with the introduction of the effect from the secondary electron emission, based on a particle-in-cell/Monte Carlo collision model (PIC/MCC). As we know, the transport of positive and negative ions plays a key role in the formation of striations in electronegative gases, for which, the electronegativity needs to be large enough.As the secondary electron emission increases, electrons in the sheaths gradually contribute more ionization to the discharge. Meanwhile, the increase of the electron density, especially in the plasma bulk, leads to an increased electrical conductivity and a reduced bulk electric field, which would shield the ions' mobility. These changes result in enlarged striation gaps.And then, with more emitted electrons, obvious disruption of the striations is observed accompanied with a transition of electron heating mode. Due to the weakened field, the impact ionization in the plasma bulk is attenuated, compared with the enhanced ionization caused by secondary electrons. This would lead to the electron heating mode transition from striated (STR) mode to γ-mode. Besides, our investigation further reveals that γ-mode is more likely to dominate the discharge under high gas pressures or driving voltages.

Electron power absorption dynamics in magnetized capacitively coupled radio frequency oxygen discharges

“…In the simulations, the dissociative attachment is the only source of O − ions. The loss of O − is due to four different reactions in our code (see [55]). Under the conditions studied here, the associative electron detachment is the main loss channel of O − causing more than 80% of the total loss at B = 0 G and to an even higher percentage at B > 0 G. Figure 6 A c c e p t e d M a n u s c r i p t distributions of the electron density, the mean electron energy, and the dissociative electron attachment as well as the associative electron detachment rate as a function of the magnetic field.…”

“…In our code, we trace electrons as well as O − and O + 2 ions. The reactions and cross sections we implemented in our code can be found in [55]. Metastable O 2 (a 1 ∆ g ) molecules have been found to play A c c e p t e d M a n u s c r i p t particle power absorption dynamics [37,56].…”

Electron power absorption dynamics in a low pressure radio frequency driven capacitively coupled discharge in oxygen

“…Note that the oxygen reaction set used in this current study is signficantly more extensive than the one used by Vass et al [32] where the only metastable state included is the singlet metastable molecule O 2 ( 1 ∆ g ). To estimate the density of the singlet metastable molecule O 2 ( 1 ∆ g ) they assume a homogeneous spatial density where a balance between electron impact excitation and quenching at the electrodes [36,37].…”

Electron power absorption in low pressure capacitively coupled electronegative oxygen radio frequency plasmas

“…In this work, we use the same analysis as in [5] for single-frequency low pressure electronegative oxygen CCPs. There has been a growing interest in oxygen CCP discharges [21][22][23][24][25][36][37][38][39][40][41][42][43][44][45][46][47]. The electron power absorption dynamics in oxygen has been studied in [21][22][23][24][25]47].…”