| 1. |
- Brenning, Nils, et al.
(författare)
-
A unified treatment of self-sputtering, process gas recycling, and runaway for high power impulse sputtering magnetrons
- 2017
-
Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 26:12
-
Tidskriftsartikel (refereegranskat)abstract
- The combined processes of self-sputter (SS)-recycling and process gas recycling in high power impulse magnetron sputtering (HiPIMS) discharges are analyzed using the generalized recycling model (GRM). The study uses experimental data from discharges with current densities from the direct current magnetron sputtering range to the HiPIMS range, and using targets with self-sputter yields Y-SS from approximate to 0.1 to 2.6. The GRM analysis reveals that, above a critical current density of the order of J(crit) approximate to 0.2 A cm(-2), a combination of self-sputter recycling and gas-recycling is generally the case. The relative contributions of these recycling mechanisms, in turn, influence both the electron energy distribution and the stability of the discharges. For high self-sputter yields, above Y-SS approximate to 1, the discharges become dominated by SS-recycling, contain few hot secondary electrons from sheath energization, and have a relatively low electron temperature T-e. Here, stable plateau values of the discharge current develop during long pulses, and these values increase monotonically with the applied voltage. For low self-sputter yields, below Y-SS approximate to 0.2, the discharges above J(crit) are dominated by process gas recycling, have a significant sheath energization of secondary electrons and a higher T-e, and the current evolution is generally less stable. For intermediate values of YSS the discharge character gradually shifts between these two types. All of these discharges can, at sufficiently high discharge voltage, give currents that increase rapidly in time. For such cases we propose that a distinction should be made between 'unlimited' runaway and 'limited' runaway: in unlimited runaway the current can, in principle, increase without a limit for a fixed discharge voltage, while in limited runaway it can only grow towards finite, albeit very high, levels. For unlimited runway Y-SS > 1 is found to be a necessary criterion, independent of the amount of gas-recycling in the discharge.
|
|
| 2. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
-
Foundations of DC plasma sources
- 2017
-
Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 26:12
-
Tidskriftsartikel (refereegranskat)
|
|
| 3. |
- Huang, Shuo, et al.
(författare)
-
Dual frequency capacitively coupled chlorine discharge
- 2015
-
Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 24:1
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of the control parameters of both high and low frequency sources on a dual-frequency capacitively coupled chlorine discharge is systematically investigated using a hybrid approach, which consists of a particle-in-cell/Monte Carlo simulation and a volume-averaged global model. The high frequency current density is varied from 20 to 80Am-2, the driving high frequency is varied from 27.12 to 100MHz, and the driving low frequency is varied from 1 to 13.56MHz, while the low frequency current density is kept at 1Am-2. The discharge pressure is maintained at 10mTorr. Key plasma parameters (including the electron heating rate, the electron energy probability function, the ion flux, the ion energy, and angular distributions) are explored and their variations with the control parameters are analyzed and compared with other discharge chemistries. As the high frequency current increases, the electron heating is enhanced in the sheath region and is diminished in the bulk region, showing a transition of the electron heating from the drift-ambipolar mode to the α mode. The fluxes of ions and high-energy Cl2 molecules reaching the surface decrease with an increase in the driving high frequency, and the average sheath potential is approximately inversely proportional to the driving high frequency. The electron heating rate, the fluxes of and Cl+ ions reaching the surface, and the average sheath potential show little dependence on the driving low frequency, while the profile of the ion energy distribution evolves from a broad bimodal profile to a narrow single-peak profile as the driving low frequency increases, which corresponds to the transition of the discharge from the intermediate frequency regime to the high frequency regime.
|
|
| 4. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
-
The role of the metastable O2(b) and energy-dependent secondary electron emission yields in capacitively coupled oxygen discharges
- 2016
-
Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 25:5
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of including the singlet metastable molecule O2(b) in the discharge model of a capacitively coupled rf driven oxygen discharge are explored. We furthermore examine the addition of energy-dependent secondary electron emission yields from the electrodes to the discharge model. The one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used for this purpose, with the oxygen discharge model considering the species O2,(X3Σg -)O2(a1Σg),O2(b1Σg +), O(3P), O(1D), O2 +, O+, O-, and electrons. The effects on particle density profiles, the electron heating rate profile, the electron energy probability function and the sheath width are explored including and excluding the metastable oxygen molecules and secondary electron emission. Earlier we have demonstrated that adding the metastable O2(a1Σg) to the discharge model changes the electron heating from having contributions from both bulk and sheath heating to being dominated by sheath heating for pressures above 50 mTorr. We find that including the metastable O2(b1Σg +) further decreases the ohmic heating and the effective electron temperature in the bulk region. The effective electron temperature in the electronegative core is found to be less than 1 eV in the pressure range 50-200 mTorr which agrees with recent experimental findings. Furthermore, we find that including an energy-dependent secondary electron emission yield for -ions has a significant influence on the discharge properties, including decreased sheath width.
|
|
| 5. |
- Butler, Alexandre, et al.
(författare)
-
On three different ways to quantify the degree of ionization in sputtering magnetrons
- 2018
-
Ingår i: Plasma sources science & technology. - : IOP PUBLISHING LTD. - 0963-0252 .- 1361-6595. ; 27:10
-
Tidskriftsartikel (refereegranskat)abstract
- Quantification and control of the fraction of ionization of the sputtered species are crucial in magnetron sputtering, and in particular in high-power impulse magnetron sputtering (HiPIMS), yet proper definitions of the various concepts of ionization are still lacking. In this contribution, we distinguish between three approaches to describe the degree (or fraction) of ionization: the ionized flux fraction F-flux, the ionized density fraction F-density, and the fraction a of the sputtered metal atoms that become ionized in the plasma (sometimes referred to as probability of ionization). By studying a reference HiPIMS discharge with a Ti target, we show how to extract absolute values of these three parameters and how they vary with peak discharge current. Using a simple model, we also identify the physical mechanisms that determine F-flux, F-density, and a as well as how these three concepts of ionization are related. This analysis finally explains why a high ionization probability does not necessarily lead to an equally high ionized flux fraction or ionized density fraction.
|
|
| 6. |
- Giono, Gabriel, et al.
(författare)
-
Non-Maxwellian electron energy probability functions in the plume of a SPT-100 Hall thruster
- 2018
-
Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 27:1
-
Tidskriftsartikel (refereegranskat)abstract
- We present measurements of the electron density, the effective electron temperature, the plasma potential, and the electron energy probability function (EEPF) in the plume of a 1.5 kW-class SPT-100 Hall thruster, derived from cylindrical Langmuir probe measurements. The measurements were taken on the plume axis at distances between 550 and 1550 mm from the thruster exit plane, and at different angles from the plume axis at 550 mm for three operating points of the thruster, characterized by different discharge voltages and mass flow rates. The bulk of the electron population can be approximated as a Maxwellian distribution, but the measured distributions were seen to decline faster at higher energy. The measured EEPFs were best modelled with a general EEPF with an exponent a between 1.2 and 1.5, and their axial and angular characteristics were studied for the different operating points of the thruster. As a result, the exponent a from the fitted distribution was seen to be almost constant as a function of the axial distance along the plume, as well as across the angles. However, the exponent a was seen to be affected by the mass flow rate, suggesting a possible relationship with the collision rate, especially close to the thruster exit. The ratio of the specific heats, the. factor, between the measured plasma parameters was found to be lower than the adiabatic value of 5/3 for each of the thruster settings, indicating the existence of non-trivial kinetic heat fluxes in the near collisionless plume. These results are intended to be used as input and/or testing properties for plume expansion models in further work.
|
|
| 7. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
-
Electron heating mode transitions in a low pressure capacitively coupled oxygen discharge
- 2019
-
Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing. - 0963-0252 .- 1361-6595. ; 28:4
-
Tidskriftsartikel (refereegranskat)abstract
- Using particle-in-cell Monte Carlo collision simulations we have demonstrated an electron heating mode transition from drift-ambipolar (DA) mode to α-mode in the capacitively coupled oxygen discharge as the operating pressure and/or electrode separation are increased. Here we explore further the transition as pressure and electrode separation are varied. At fixed electrode spacing of 45 mm when operating at low pressure (10 mTorr) the electron heating is a combination of DA- and α-mode heating while at higher pressures (>30 mTorr) electron heating in the sheath regions dominates. When varying the electrode spacing from 25 to 60 mm at fixed operating pressure at the higher pressures (50 mTorr) the electron heating is a combination of DA- and α-mode heating for small electrode spacing and it transitions to pure α-mode heating as the electrode spacing is increased. We relate the transition to increased electronegativity and generation of drift and ambipolar electric field within the electronegative core when the discharge pressure is low or electrode spacing is small.
|
|
| 8. |
- Gudmundsson, Jon Tomas, et al.
(författare)
-
The frequency dependence of the discharge properties in a capacitively coupled oxygen discharge
- 2018
-
Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing (IOPP). - 0963-0252 .- 1361-6595. ; 27:2
-
Tidskriftsartikel (refereegranskat)abstract
- We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the evolution of the charged particle density profiles, electron heating mechanism, the electron energy probability function (EEPF), and the ion energy distribution in a single frequency capacitively coupled oxygen discharge, with driving frequency in the range 12-100 MHz. At a low driving frequency and low pressure (5 and 10 mTorr), a combination of stochastic (a-mode) and drift ambipolar (DA) heating in the bulk plasma (the electronegative core) is observed and the DA-mode dominates the time averaged electron heating. As the driving frequency or pressure are increased, the heating mode transitions into a pure a-mode, where electron heating in the sheath region dominates. At low pressure (5 and 10 mTorr), this transition coincides with a sharp decrease in electronegativity. At low pressure and low driving frequency, the EEPF is concave. As the driving frequency is increased, the number of low energy electrons increases and the relative number of higher energy electrons (> 10 eV) increases. At high driving frequency, the EEPF develops a convex shape or becomes bi-Maxwellian.
|
|
| 9. |
- Niyonzima, S., et al.
(författare)
-
Low-energy collisions between electrons and BeD
- 2018
-
Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 27:2
-
Tidskriftsartikel (refereegranskat)abstract
- Multichannel quantum defect theory is applied in the treatment of the dissociative recombination and vibrational excitation processes for the BeD+ ion in the 24 vibrational levels of its ground electronic state (X (1)Sigma(+), v(i)(+) = 0 ... 23). Three electronic symmetries of BeD** states ((2)Pi, (2)Sigma(+), and (2)Delta) are considered in the calculation of cross sections and the corresponding rate coefficients. The incident electron energy range is 10(-5)-2.7 eV and the electron temperature range is 100-5000 K. The vibrational dependence of these collisional processes is highlighted. The resulting data are useful in magnetic confinement fusion edge plasma modeling and spectroscopy, in devices with beryllium based main chamber materials, such as ITER and JET, and operating with the deuterium-tritium fuel mix. An extensive rate coefficients database is presented in graphical form and also by analytic fit functions whose parameters are tabulated in the supplementary material.
|
|
| 10. |
- Proto, A., et al.
(författare)
-
The role of surface quenching of the singlet delta molecule in a capacitively coupled oxygen discharge
- 2018
-
Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing (IOPP). - 0963-0252 .- 1361-6595. ; 27:7
-
Tidskriftsartikel (refereegranskat)abstract
- 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 O-2(a(1)Delta(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 0.000 01-0.1 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 alpha-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 alpha-mode electron heating is observed for a surface quenching coefficient of 0.001 or smaller.
|
|
