Experimental and computational investigations of the effect of the electrode gap on capacitively coupled radio frequency oxygen discharges

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.

Section: Striation Disruption and Electron Heating Mode Transitionsupporting

confidence: 68%

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

Wang

Wen

Zhang

et al. 2019

“…Transitions between these power absorption modes have been observed in different gases by changing external control parameters. For instance, in oxygen CCPs, a transition from the DA-mode to the α-mode has been found to be induced by changing the gas pressure [33,34], the driving frequency [35,36], the driving voltage waveform [7,28,[36][37][38], as well as the gap distance [34,39]. All of the above electron power absorption modes are based on electron motion along the direction perpendicular to the electrodes.…”

Section: Introductionmentioning

confidence: 99%

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

Wang

Wen

Hartmann

et al. 2020

Self Cite

The influence of a uniform magnetic field parallel to the electrodes on radio frequency capacitively coupled oxygen discharges driven at 13.56 MHz at a pressure of 100 mTorr is investigated by one-dimensional Particle-in-Cell/Monte Carlo collision (1D PIC/MCC) simulations. Increasing the magnetic field from 0 to 200 G is found to result in a drastic enhancement of the electron and the O + 2 ion density due to the enhanced confinement of electrons by the magnetic field. The time and space averaged O − ion density, however, is found to remain almost constant, since both the dissociative electron attachment (production channel of O − ) and the associative electron detachment rate due to the collisions of negative ions with oxygen metastables (main loss channel of O − ) are enhanced simultaneously. This is understood based on a detailed analysis of the spatio-temporal electron dynamics.The nearly constant O − density in conjunction with the increased electron density causes a significant reduction of the electronegativity and a pronounced change of the electron power absorption dynamics as a function of the externally applied magnetic field. While at low magnetic fields the discharge is operated in the electronegative Drift-Ambipolar (DA) mode, a transition to the electropositive α-mode is induced by increasing the magnetic field. Meanwhile, a strong electric field reversal is generated near each electrode during the local sheath collapse at high magnetic fields, which locally enhances the electron power absorption. A model of the electric field generation reveals that the reversed electric field is caused by the reduction of the electron flux to the electrodes due to their trapping by the magnetic field.The consequent changes of the plasma properties are expected to affect the applications of such discharges in etching, deposition and other semiconductor processes.

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Vass¹,

Wilczek²,

Lafleur³

et al. 2020

Self Cite

A thorough understanding of the energy transfer mechanism from the electric field to electrons is of utmost importance for optimization and control of different plasma sources and processes. This mechanism, called electron power absorption, involves complex electron dynamics in electronegative capacitively coupled plasmas (CCPs) at low pressures, that are still not fully understood. Therefore, we present a spatio-temporally resolved analysis of electron power absorption in low pressure oxygen CCPs based on the momentum balance equation derived from the Boltzmann equation. Data are obtained from 1d3v Particle-In-Cell / Monte Carlo Collision simulations. In contrast to conventional theoretical models, which predict 'stochastic/collisionless heating' to be important at low pressure, we observe the dominance of Ohmic power absorption. In addition, there is an attenuation of ambipolar power absorption at low pressures due to the strong electronegativity, and the presence of electropositive edge regions in the discharge, which cause a high degree of temporal symmetry of the electron temperature within the RF period.