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- Wen, De-Qi, et al.
(author)
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Benchmarked and upgraded particle-in-cell simulations of a capacitive argon discharge at intermediate pressure : the role of metastable atoms
- 2021
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In: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:10
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Journal article (peer-reviewed)abstract
- The capacitive argon discharge operated in the intermediate pressure regime is studied by performing one-dimensional particle-in-cell Monte Carlo collision simulations. First, the basic object-oriented plasma device code (oopd1-v1) is strictly benchmarked against the well-established xpdp1 code over a wide range of pressure (0.05-15 Torr) and varying blocking capacitor of the external circuit (5-10(5) nF), and excellent agreement is obtained. The oopd1-v1 is upgraded to oopd1-v2 and oopd1-v3, by adding excited atoms modeled as time- and space-evolving fluid species. The metastable Ar-m, the radiative Ar(R), and the Ar(4p) manifold, and their roles in discharge equilibrium are explored. It is found that the presence of the metastable Ar-m enhances the plasma density by a factor of 3 at 1.6 Torr and even higher at pressures up to 5 Torr. At low pressure (0.05 Torr), electron impact ionization from the ground state atom dominates the ionization over the whole discharge region, while metastable pooling and step-wise ionization has small contribution. The proportion of metastable pooling ionization and step-wise ionization increases with increasing pressure and becomes the dominant ionization source at 5-15 Torr.
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| 3. |
- Wen, De Qi, et al.
(author)
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Field reversal in low pressure, unmagnetized radio frequency capacitively coupled argon plasma discharges
- 2023
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 123:26
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Journal article (peer-reviewed)abstract
- In general, the radio frequency (rf) electric field within a sheath points toward the metal electrode in low pressure, unmagnetized rf electropositive capacitively coupled plasma (CCP) glow discharges. This is due to the large ratio of electron to ion mobility and the formation of an ion sheath. In this work, we studied, using fully kinetic particle-in-cell simulations, a reversed electric field induced by the strong secondary electron emission during the phase of sheath collapse in a high-voltage rf-driven low pressure CCP glow discharge. We explored the transition behavior of the formation of field reversal as a function of driving voltage amplitude and found that field reversal starts to form at around 750 V, for a discharge with an electrode spacing of 4 cm at 10 mTorr argon pressure driven at 13.56 MHz. Accordingly, the energy distribution function of electrons incident on the electrode shows peaks from around 3 to 10 eV while varying the driving voltage from 150 to 2000 V, showing potentially beneficial effects in plasma material processing where relatively directional electrons are preferred to solely thermal diffusion electrons.
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| 4. |
- Wen, De Qi, et al.
(author)
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On the importance of excited state species in low pressure capacitively coupled plasma argon discharges
- 2023
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In: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 32:6
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Journal article (peer-reviewed)abstract
- In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1-10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.
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