The influence of surface properties on the plasma dynamics in radio-frequency driven oxygen plasmas: Measurements and simulations

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.

Section: Methodsmentioning

confidence: 99%

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

Vass¹,

Wilczek²,

Lafleur³

et al. 2020

“…Gordiets et al [51] and Kutasi et al [52] use wall quenching coefficient for O 2 (a 1 ∆ g ) of 2×10 −5 and for O 2 (b 1 Σ + g ) of 2×10 −2 for a quartz tube in their models of flowing N 2 /O 2 dc glow discharge and Ar/O 2 surface-wave microwave discharge, respectively. By comparing the 1D fluid simulations to phase and space resolved optical emission (PROES) measurements Greb et al [53] determine the wall quenching coefficient for O 2 (a 1 ∆ g ) to be 1 × 10 −5 for stainless steel and 3 × 10 −3 for teflon. A comparison of PIC/MCC simulation with experimental findings using PROES for a CCP with aluminum electrodes and electrode spacing of 2.5 cm suggests a wall quenching coefficient of 0.006 [35].…”

Section: B Wall Quenching Coefficientsmentioning

confidence: 99%

The role of surface quenching of the singlet delta molecule in a capacitively coupled oxygen discharge

Proto¹,

Guðmundsson²

2018

We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the influence of the surface quenching of the singlet delta metastable molecule O2(a 1 ∆g) on the electron heating mechanism, and the electron energy probability function (EEPF), in a single frequency capacitively coupled oxygen discharge. When operating at low pressure (10 mTorr) varying the surface quenching coefficient in the range 0.00001 -0.1 has no influence on the electron heating mechanism and electron heating is dominated by drift-ambipolar (DA) heating in the plasma bulk and electron cooling is observed in the sheath regions. As the pressure is increased to 25 mTorr the electron heating becomes a combination of DA-mode and α−mode heating, and the role of the DA-mode decreases with decreasing surface quenching coefficient. At 50 mTorr electron heating in the sheath region dominates. However, for the highest quenching coefficient there is some contribution from the DA-mode in the plasma bulk, but this contribution decreases to almost zero and pure α−mode electron heating is observed for a surface quenching coefficient of 0.001 or smaller.

“…It is also known that surface interaction probabilities for reactive neutral species can strongly influence the properties and charged particle dynamics of low pressure plasma sources [38,[57][58][59][60][61][62]. A prominent example is the role played by the surface quenching probability of the molecular oxygen metastable molecule O 2 (a g 1 D ) in determining the electronegativity (i.e.…”

Section: Introductionmentioning

confidence: 99%

“…A prominent example is the role played by the surface quenching probability of the molecular oxygen metastable molecule O 2 (a g 1 D ) in determining the electronegativity (i.e. the ratio of the negative ion density to the electron density) of oxygen plasmas [38,48,57,59,63]. This dependence occurs as a result of the high rate coefficient for electron detachment from O -ions in collisions with O 2 (a g 1 D ) molecules in comparison with the equivalent reaction with ground state molecular oxygen [64].…”

Section: Introductionmentioning

confidence: 99%

Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation

Gibson

Gans

2017

Self Cite

The charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15-20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity.