| 1. |
- Antunes, V. G., et al.
(författare)
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Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges
- 2023
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 32:7
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Tidskriftsartikel (refereegranskat)abstract
- The high power impulse magnetron sputtering (HiPIMS) discharge brings about increased ionization of the sputtered atoms due to an increased electron density and efficient electron energization during the active period of the pulse. The ionization is effective mainly within the electron trapping zone, an ionization region (IR), defined by the magnet configuration. Here, the average extension and the volume of the IR are determined based on measuring the optical emission from an excited level of the argon working gas atoms. For particular HiPIMS conditions, argon species ionization and excitation processes are assumed to be proportional. Hence, the light emission from certain excited atoms is assumed to reflect the IR extension. The light emission was recorded above a 100 mm diameter titanium target through a 763 nm bandpass filter using a gated camera. The recorded images directly indicate the effect of the magnet configuration on the average IR size. It is observed that the shape of the IR matches the shape of the magnetic field lines rather well. The IR is found to expand from 10 and 17 mm from the target surface when the parallel magnetic field strength 11 mm above the racetrack is lowered from 24 to 12 mT at a constant peak discharge current.
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| 2. |
- Brenning, Nils, et al.
(författare)
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Spokes and charged particle transport in HiPIMS magnetrons
- 2013
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 46:8, s. 084005-
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Tidskriftsartikel (refereegranskat)abstract
- Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E x B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented.
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| 3. |
- Brenning, Nils, et al.
(författare)
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The role of Ohmic heating in dc magnetron sputtering
- 2016
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Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing (IOPP). - 0963-0252 .- 1361-6595. ; 25:6
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Tidskriftsartikel (refereegranskat)abstract
- Sustaining a plasma in a magnetron discharge requires energization of the plasma electrons. In this work, Ohmic heating of electrons outside the cathode sheath is demonstrated to be typically of the same order as sheath energization, and a simple physical explanation is given. We propose a generalized Thornton equation that includes both sheath energization and Ohmic heating of electrons. The secondary electron emission yield gamma(SE) is identified as the key parameter determining the relative importance of the two processes. For a conventional 5 cm diameter planar dc magnetron, Ohmic heating is found to be more important than sheath energization for secondary electron emission yields below around 0.1.
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| 4. |
- Eliasson, H., et al.
(författare)
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Modeling of high power impulse magnetron sputtering discharges with graphite target
- 2021
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Ingår i: Plasma sources science & technology. - : IOP Publishing Ltd. - 0963-0252 .- 1361-6595. ; 30:11
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Tidskriftsartikel (refereegranskat)abstract
- The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge in argon with a graphite target. Using the IRM, the temporal variation of the various species and the average electron energy, as well as internal parameters such as the ionization probability, back-attraction probability, and the ionized flux fraction of the sputtered species, is determined. It is found that thedischarge develops into working gas recycling and most of the discharge current at the cathode target surface is composed of Ar+ ions, which constitute over 90% of the discharge current, while the contribution of the C+ ions is always small (<5%), even for peak current densities close to 3 A cm(-2). For the target species, the time-averaged ionization probability is low, or 13-27%, the ion back-attraction probability during the pulse is high (>92%), and the ionized flux fraction is about 2%. It is concluded that in the operation range studied here it is a challenge to ionize carbon atoms, that are sputtered off of a graphite target in a magnetron sputtering discharge, when depositing amorphous carbon films.
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| 5. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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An ionization region model of the reactive Ar/O-2 high power impulse magnetron sputtering discharge
- 2016
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Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 25:6
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Tidskriftsartikel (refereegranskat)abstract
- A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O-2 high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. It is then applied to study the temporal behavior of the discharge plasma parameters such as electron density, the neutral and ion composition, the ionization fraction of the sputtered vapor, the oxygen dissociation fraction, and the composition of the discharge current. We study and compare the discharge properties when the discharge is operated in the two well established operating modes, the metal mode and the poisoned mode. Experimentally, it is found that in the metal mode the discharge current waveform displays a typical non-reactive evolution, while in the poisoned mode the discharge current waveform becomes distinctly triangular and the current increases significantly. Using the R-IRM we explore the current increase and find that when the discharge is operated in the metal mode Ar+ and Ti+ -ions contribute most significantly (roughly equal amounts) to the discharge current while in the poisoned mode the Ar+ -ions contribute most significantly to the discharge current and the contribution of O+ -ions, Ti+ -ions, and secondary electron emission is much smaller. Furthermore, we find that recycling of atoms coming from the target, that are subsequently ionized, is required for the current generation in both modes of operation. From the R-IRM results it is found that in the metal mode self-sputter recycling dominates and in the poisoned mode working gas recycling dominates. We also show that working gas recycling can lead to very high discharge currents but never to a runaway. It is concluded that the dominating type of recycling determines the discharge current waveform.
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| 6. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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Are the argon metastables important in high power impulse magnetron sputtering discharges?
- 2015
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 22:11
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Tidskriftsartikel (refereegranskat)abstract
- We use an ionization region model to explore the ionization processes in the high power impulse magnetron sputtering (HiPIMS) discharge in argon with a titanium target. In conventional dc magnetron sputtering (dcMS), stepwise ionization can be an important route for ionization of the argon gas. However, in the HiPIMS discharge stepwise ionization is found to be negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species, Penning ionization can be a significant ionization mechanism in the dcMS discharges, while in the HiPIMS discharge Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density in the HiPIMS discharge, and a higher electron temperature. Furthermore, we explore the ionization fraction and the ionized flux fraction of the sputtered vapor and compare with recent experimental work.
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| 7. |
- Gudmundsson, Jon Tomas, 1965-, et al.
(författare)
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ON ELECTRON HEATING IN MAGNETRON SPUTTERING DISCHARGES
- 2017
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Ingår i: 2017 IEEE International Conference on Plasma Science (ICOPS). - : IEEE.
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Summary form only given. The magnetron sputtering discharge is a highly successful tool for deposition of thin films and coatings. It has been applied for various industrial applications for over four decades. Sustaining a plasma in a magnetron sputtering discharge requires energy transfer to the plasma electrons. In the past, the magnetron sputtering discharge has been assumed to be maintained by cathode sheath acceleration of secondary electrons emitted from the target, upon ion impact. These highly energetic electrons then either ionize the atoms of the working gas directly or transfer energy to the local lower energy electron population that subsequently ionizes the working gas atoms. This leads to the well-known Thornton equation, which in its original form is formulated to give the minimum required voltage to sustain the discharge. However, recently we have demonstrated that Ohmic heating of electrons outside the cathode sheath is typically of the same order as heating due to acceleration across the sheath in dc magnetron sputtering (dcMS) discharges. The secondary electron emission yield γsee is identified as the key parameter determining the relative importance of the two processes. In the case of dcMS Ohmic heating is found to be more important than sheath acceleration for secondary electron emission yields below around 0.1. For the high power impulse magnetron sputtering (HiPIMS) discharge we find that direct Ohmic heating of the plasma electrons is found to dominate over sheath acceleration by typically an order of magnitude, or in the range of 87 - 99 % of the total electron heating. A potential drop of roughly 100 - 150 V, or 15 - 25% of the discharge voltage, always falls across the plasma outside the cathode sheath.
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| 8. |
- Gudmundsson, Jon Tomas, et al.
(författare)
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The current waveform in reactive high power impulse magnetron sputtering
- 2016
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Ingår i: 2016 IEEE International Conference on Plasma Science (ICOPS). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781467396011
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Konferensbidrag (refereegranskat)abstract
- Summary form only given. The understanding of the current waveform for the non-reactive HiPIMS discharge is now rather well established [1,2]. It is described by a rise in the current to an initial peak and then a drop followed by a stable plateau. The drop is a result of a strong gas compression due to the sudden large flux of atoms from the target. For the reactive HiPIMS discharge striking differences are observed and those seem to depend on the mode of operation, the reactive gas and the target material. The discharge current waveform changes in shape as well as in the peak value when the target surface enters the poisoned mode. For Ar/O2 discharge with Ti target the discharge current waveform varies with oxygen partial pressure and pulse repetition frequency [3]. For the higher repetition frequencies the familiar nonreactive current waveform is observed. As the repetition frequency is lowered there is an increase in the current which transits into a different waveform as the repetition frequency is decreased further. The waveform observed at low repetition frequency is similar to the one observed at high reactive gas flow rate. Similarly, the current waveform in the reactive Ar/N2 HiPIMS discharge with Ti target is highly dependent on the pulse repetition frequency and the current is found to increase significantly as the frequency is lowered [4]. However, the discharge current keeps its shape and it remains as for the non-reactive case as the current increases. These findings will be compared with results for various combinations of gas mixtures and targets found in the literature [5]. Furthermore, we explore the current waveform in reactive HiPIMS using the ionization region model (IRM) [6] of the reactive Ar/O2 discharge with a Ti target. We discuss the current waveform development and how the discharge composition varies between metal and poisoned mode.
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| 9. |
- Hajihoseini, H., et al.
(författare)
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Target ion and neutral spread in high power impulse magnetron sputtering
- 2023
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 41:1
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Tidskriftsartikel (refereegranskat)abstract
- In magnetron sputtering, only a fraction of the sputtered target material leaving the ionization region is directed toward the substrate. This fraction may be different for ions and neutrals of the target material as the neutrals and ions can exhibit a different spread as they travel from the target surface toward the substrate. This difference can be significant in high power impulse magnetron sputtering (HiPIMS) where a substantial fraction of the sputtered material is known to be ionized. Geometrical factors or transport parameters that account for the loss of produced film-forming species to the chamber walls are needed for experimental characterization and modeling of the magnetron sputtering discharge. Here, we experimentally determine transport parameters for ions and neutral atoms in a HiPIMS discharge with a titanium target for various magnet configurations. Transport parameters are determined to a typical substrate, with the same diameter (100 mm) as the cathode target, and located at a distance 70 mm from the target surface. As the magnet configuration and/or the discharge current are changed, the transport parameter for neutral atoms xi(tn) remains roughly the same, while transport parameters for ions xi(ti) vary greatly. Furthermore, the relative ion-to-neutral transport factors, xi(ti)/xi(tn), that describe the relative deposited fractions of target material ions and neutrals onto the substrate, are determined to be in the range from 0.4 to 1.1.
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| 10. |
- Kalered, Emil, et al.
(författare)
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On the work function and the charging of small (r ≤ 5 nm) nanoparticles in plasmas
- 2017
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Ingår i: Physics of Plasmas. - : American Institute of Physics (AIP). - 1070-664X .- 1089-7674. ; 24:1
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Tidskriftsartikel (refereegranskat)abstract
- The growth of nanoparticles (NPs) in plasmas is an attractive technique where improved theoretical understanding is needed for quantitative modeling. The variation of the work function W with size for small NPs, rNP≤ 5 nm, is a key quantity for modeling of three NP charging processes that become increasingly important at a smaller size: electron field emission, thermionic electron emission, and electron impact detachment. Here we report the theoretical values of the work function in this size range. Density functional theory is used to calculate the work functions for a set of NP charge numbers, sizes, and shapes, using copper for a case study. An analytical approximation is shown to give quite accurate work functions provided that rNP > 0.4 nm, i.e., consisting of about >20 atoms, and provided also that the NPs have relaxed close to spherical shape. For smaller sizes, W deviates from the approximation, and also depends on the charge number. Some consequences of these results for nanoparticle charging are outlined. In particular, a decrease in W for NP radius below about 1 nm has fundamental consequences for their charge in a plasma environment, and thereby on the important processes of NP nucleation, early growth, and agglomeration.
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