SwePub
Sök i SwePub databas

  Utökad sökning

Träfflista för sökning "WFRF:(Raadu Michael A.) "

Sökning: WFRF:(Raadu Michael A.)

  • Resultat 1-50 av 54
Sortera/gruppera träfflistan
   
NumreringReferensOmslagsbildHitta
1.
  • Rudolph, M., et al. (författare)
  • On the population density of the argon excited levels in a high power impulse magnetron sputtering discharge
  • 2022
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 29:2, s. 023506-
  • Tidskriftsartikel (refereegranskat)abstract
    • Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p*)(-6), where p * is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron-ion recombination.
  •  
2.
  • Antunes, V. G., et al. (författare)
  • Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges
  • 2023
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 32:7
  • 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.
  •  
3.
  • Rudolph, M., et al. (författare)
  • Influence of the magnetic field on the discharge physics of a high power impulse magnetron sputtering discharge
  • 2022
  • Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 55:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability.
  •  
4.
  • Babu, Swetha Suresh, et al. (författare)
  • Modeling of high power impulse magnetron sputtering discharges with tungsten target
  • 2022
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 31:6, s. 065009-
  • Tidskriftsartikel (refereegranskat)abstract
    • The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge with a tungsten target. The IRM gives the temporal variation of the various species and the average electron energy, as well as internal discharge parameters such as the ionization probability and the back-attraction probability of the sputtered species. It is shown that an initial peak in the discharge current is due to argon ions bombarding the cathode target. After the initial peak, the W+ ions become the dominating ions and remain as such to the end of the pulse. We demonstrate how the contribution of the W+ ions to the total discharge current at the target surface increases with increased discharge voltage for peak discharge current densities J (D,peak) in the range 0.33-0.73 A cm(-2). For the sputtered tungsten the ionization probability increases, while the back-attraction probability decreases with increasing discharge voltage. Furthermore, we discuss the findings in terms of the generalized recycling model and compare to experimentally determined deposition rates and find good agreement.
  •  
5.
  • Barynova, Kateryna, et al. (författare)
  • On working gas rarefaction in high power impulse magnetron sputtering
  • 2024
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 33:6
  • Tidskriftsartikel (refereegranskat)abstract
    • The ionization region model (IRM) is applied to explore working gas rarefaction in high power impulse magnetron sputtering discharges operated with graphite, aluminum, copper, titanium, zirconium, and tungsten targets. For all cases the working gas rarefaction is found to be significant, the degree of working gas rarefaction reaches values of up to 83%. The various contributions to working gas rarefaction, including electron impact ionization, kick-out by the sputtered species or hot argon atoms, and diffusion, are evaluated and compared for the different target materials, and over a range of discharge current densities. The relative importance of the various processes varies between different target materials. In the case of a graphite target with argon as the working gas at 1 Pa, electron impact ionization (by both primary and secondary electrons) is the dominating contributor to working gas rarefaction, with over 90% contribution, while the contribution of sputter wind kick-out is small < 10 %. In the case of copper and tungsten targets, the kick-out dominates, with up to ∼60% contribution at 1 Pa. For metallic targets the kick-out is mainly due to metal atoms sputtered from the target, while for the graphite target the small kick-out contribution is mainly due to kick-out by hot argon atoms and to a smaller extent by carbon atoms. The main factors determining the relative contribution of the kick-out by the sputtered species to working gas rarefaction appear to be the sputter yield and the working gas pressure.
  •  
6.
  • Brenning, Nils, et al. (författare)
  • A bulk plasma model for dc and HiPIMS magnetrons
  • 2008
  • Ingår i: PLASMA SOURCES SCIENCE and TECHNOLOGY. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 17:4, s. 045009-
  • Tidskriftsartikel (refereegranskat)abstract
    • A plasma discharge model has been developed for the bulk plasma (also called the extended presheath) in sputtering magnetrons. It can be used both for high power impulse magnetron sputtering (HiPIMS) and conventional dc sputtering magnetrons. Demonstration calculations are made for the parameters of the HiPIMS sputtering magnetron at Link "oping University, and also benchmarked against results in the literature on dc magnetrons. New insight is obtained regarding the structure and time development of the currents, the electric fields and the potential profiles. The transverse resistivity eta(perpendicular to) has been identified as having fundamental importance both for the potential profiles and for the motion of ionized target material through the bulk plasma. New findings are that in the HiPIMS mode, as a consequence of a high value of eta(perpendicular to), (1) there can be an electric field reversal that in our case extends 0.01-0.04m from the target, (2) the electric field in the bulk plasma is typically an order of magnitude weaker than in dc magnetrons, (3) in the region of electric field reversal the azimuthal current is diamagnetic in nature, i.e. mainly driven by the electron pressure gradient, and actually somewhat reduced by the electron Hall current which here has a reversed direction and (4) the azimuthal current above the racetrack can, through resistive friction, significantly influence the motion of the ionized fraction of the sputtered material and deflect it sideways, away from the target and towards the walls of the magnetron.
  •  
7.
  • 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.
  •  
8.
  • Brenning, Nils, et al. (författare)
  • Conditions for plasmoid penetration across abrupt magnetic barriers
  • 2005
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 12:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The penetration of plasma clouds, or plasmoids, across abrupt magnetic barriers (of the scale less than a few ion gyro radii, using the plasmoid directed velocity) is studied. The insight gained earlier, from detailed experimental and computer simulation investigations of a case study, is generalized into other parameter regimes. It is concluded for what parameters a plasi-noid should be expected to penetrate the magnetic barrier through self-polarization, penetrate through magnetic expulsion, or be rejected from the barrier. The scaling parameters are n(e), upsilon(o), B-perpendicular to, m(i), T-i, and the width w of the plasmoid. The scaling is based on a model for strongly driven, nonlinear magnetic field diffusion into a plasma which is a generalization of the earlier laboratory findings. The results are applied to experiments earlier reported in the literature, and also to the proposed application of impulsive penetration of plasmoids from the solar wind into the Earth's magnetosphere.
  •  
9.
  • Brenning, Nils, et al. (författare)
  • Faster-than-Bohm Cross-B Electron Transport in Strongly Pulsed Plasmas
  • 2009
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:22
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the empirical discovery of an exceptionally high cross-B electron transport rate in magnetized plasmas, in which transverse currents are driven with abruptly applied high power. Experiments in three different magnetic geometries are analyzed, covering several orders of magnitude in plasma density, magnetic field strength, and ion mass. It is demonstrated that a suitable normalization parameter is the dimensionless product of the electron (angular) gyrofrequency and the effective electron-ion momentum transfer time, omega(ge)tau(EFF), by which all of diffusion, cross-resistivity, cross-B current conduction, and magnetic field diffusion can be expressed. The experiments show a remarkable consistency and yield close to a factor of 5 greater than the Bohm-equivalent values of diffusion coefficient D-perpendicular to, magnetic-diffusion coefficient D-B, Pedersen conductivity sigma(P), and transverse resistivity eta(perpendicular to).
  •  
10.
  • Brenning, Nils, et al. (författare)
  • HiPIMS optimization by using mixed high-power and low-power pulsing
  • 2021
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The possibility to optimize a high-power impulse magnetron sputtering (HiPIMS) discharge through mixing two different power levels in the pulse pattern is investigated. Standard HiPIMS pulses are used to create the ions of the film-forming material. After each HiPIMS pulse an off-time follows, during which no voltage (or, optionally, a reversed voltage) is applied, letting the remaining ions in the magnetic trap escape towards the substrate. After these off-times, a long second pulse with lower amplitude, in the dc magnetron sputtering range, is applied. During this pulse, which is continued up to the following HiPIMS pulse, mainly neutrals of the film-forming material are produced. This pulse pattern makes it possible to achieve separate optimization of the ion production, and of the neutral atom production, that constitute the film-forming flux to the substrate. The optimization process is thereby separated into two sub-problems. The first sub-problem concerns minimizing the energy cost for ion production, and the second sub-problem deals with how to best split a given allowed discharge power between ion production and neutral production. The optimum power split is decided by the lowest ionized flux fraction that gives the desired film properties for a specific application. For the first sub-problem we describe a method where optimization is achieved by the selection of five process parameters: the HiPIMS pulse amplitude, the HiPIMS pulse length, the off-time, the working gas pressure, and the magnetic field strength. For the second sub-problem, the splitting of power between ion and neutral production, optimization is achieved by the selection of the values of two remaining process parameters, the HiPIMS pulse repetition frequency and the discharge voltage of the low-power pulse.
  •  
11.
  • Brenning, Nils, et al. (författare)
  • Optimization of HiPIMS discharges : The selection of pulse power, pulse length, gas pressure, and magnetic field strength
  • 2020
  • Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 38:3
  • Tidskriftsartikel (refereegranskat)abstract
    • In high power impulse magnetron sputtering (HiPIMS) operation, there are basically two goals: a high ionized flux fraction of the sputtered target material and a high deposition rate. In this work, it is demonstrated that the former always comes at the cost of the latter. This makes a choice necessary, referred to as the HiPIMS compromise. It is here proposed that this compromise is most easily made by varying the discharge current amplitude, which opens up for optimization of additionally four external process parameters: the pulse length, the working gas pressure, the magnetic field strength, and the degree of magnetic unbalance to achieve the optimum combination of the ionized flux fraction and the deposition rate. As a figure of merit, useful for comparing different discharges, ( 1 - beta t ) is identified, which is the fraction of ionized sputtered material that escapes back-attraction toward the cathode target. It is shown that a discharge with a higher value of ( 1 - beta t ) always can be arranged to give better combinations of ionization and deposition rate than a discharge with a lower ( 1 - beta t ). Maximization of ( 1 - beta t ) is carried out empirically, based on data from two discharges with Ti targets in Ar working gas. These discharges were first modeled in order to convert measured plasma parameters to values of ( 1 - beta t ). The combined effects of varying the different process parameters were then analyzed using a process flow chart model. The effect of varying the degree of unbalance in the studied range was small. For the remaining three parameters, it is found that optimum is achieved by minimizing the magnetic field strength, minimizing the working gas pressure, and minimizing the pulse length as far as compatible with the requirement to ignite and maintain a stable discharge.
  •  
12.
  • Brenning, Nils, et al. (författare)
  • Radiation from an electron beam in a magnetized plasma : Whistler mode wave packets
  • 2006
  • Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 111:A11
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental studies are reported of oscillations and radiation that is spontaneously excited by an electron beam which is shot along a diverging magnetic field into a plasma from a hot cathode. In the present study we focus on oscillations below the electron gyrofrequency, where we find that whistler mode radiation appears in the form of bursts, or wave packets, each with typically 0.1-1 mu s time duration, and which together cover typically a few percent of the full time. Wave packets are found in a broad frequency range of 7-40 MHz, while each individual wave packet is dominated by a single frequency. There is propagation along two routes: at the group velocity resonance cone angle, away from the central channel where the waves are excited, and in a channel along the magnetic field. Features of the whistler mode wave packets that are studied include (1) the statistics of amplitudes, frequencies, and time durations; (2) the propagation and decay of wave packets with different frequencies; (3) the group and phase velocities; and (4) how the wave packet production varies with the energy, and the current density, in the electron beam.
  •  
13.
  • Brenning, Nils, et al. (författare)
  • Radiation from an electron beam in magnetized plasma : excitation of a whistler mode wave packet by interacting, higher-frequency, electrostatic-wave eigenmodes
  • 2017
  • Ingår i: Plasma Physics and Controlled Fusion. - : Institute of Physics Publishing (IOPP). - 0741-3335 .- 1361-6587. ; 59:12
  • Tidskriftsartikel (refereegranskat)abstract
    • Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in the laboratory by an electron beam from a hot cathode, appear transiently, each with a time duration tau around similar to 1 mu s. The wave packets have a center frequency f(W) that is broadly distributed in the range 7 MHz < f(W) < 40 MHz. They are excited in a region with separate electrostatic (es) plasma oscillations at values of f(hf), 200 MHz < f(hf) < 500 MHz, that are hypothesized to match eigenmode frequencies of an axially localized hf es field in a well-defined region attached to the cathode. Features of these es-eigenmodes that are studied include: the mode competition at times of transitions from one dominating es-eigenmode to another, the amplitude and spectral distribution of simultaneously occurring es-eigenmodes that do not lead to a transition, and the correlation of these features with the excitation of whistler mode waves. It is concluded that transient coupling of es-eigenmode pairs at f(hf) such that vertical bar f(1, hf) - f(2, hf)vertical bar = f(W) < f(ge) can explain both the transient lifetime and the frequency spectra of the whistler-mode wave packets (f(W)) as observed in lab. The generalization of the results to bursty whistler-mode excitation in space from electron beams, created on the high potential side of double layers, is discussed.
  •  
14.
  • Brenning, Nils, et al. (författare)
  • The role of Ohmic heating in dc magnetron sputtering
  • 2016
  • Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing (IOPP). - 0963-0252 .- 1361-6595. ; 25:6
  • 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.
  •  
15.
  • Brenning, Nils, et al. (författare)
  • Understanding deposition rate loss in high power impulse magnetron sputtering : I. Ionization-driven electric fields
  • 2012
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 21:2, s. 025005-
  • Tidskriftsartikel (refereegranskat)abstract
    • The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields E-z directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields E-z in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a 'sweet spot' in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product Pi(Townsend) is proposed. A parametric analysis of Pi(Townsend) shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength.
  •  
16.
  • 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.
  •  
17.
  • Eliasson, H., et al. (författare)
  • Modeling of high power impulse magnetron sputtering discharges with graphite target
  • 2021
  • Ingår i: Plasma sources science & technology. - : IOP Publishing Ltd. - 0963-0252 .- 1361-6595. ; 30:11
  • 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.
  •  
18.
  • Gudmundsson, Jon Tomas, 1965-, et al. (författare)
  • An ionization region model of the reactive Ar/O-2 high power impulse magnetron sputtering discharge
  • 2016
  • Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 25:6
  • 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.
  •  
19.
  • Gudmundsson, Jon Tomas, 1965-, et al. (författare)
  • ON ELECTRON HEATING IN MAGNETRON SPUTTERING DISCHARGES
  • 2017
  • Ingår i: 2017 IEEE International Conference on Plasma Science (ICOPS). - : IEEE.
  • 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.
  •  
20.
  • Gudmundsson, Jon Tomas, et al. (författare)
  • The current waveform in reactive high power impulse magnetron sputtering
  • 2016
  • Ingår i: 2016 IEEE International Conference on Plasma Science (ICOPS). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781467396011
  • 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.
  •  
21.
  • Hajihoseini, Hamidreza, et al. (författare)
  • Sideways deposition rate and ionized flux fraction in dc and high power impulse magnetron sputtering
  • 2020
  • Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 38:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The sideways (radial) deposition rate and ionized flux fraction in a high power impulse magnetron sputtering (HiPIMS) discharge are studied and compared to a dc magnetron sputtering (dcMS) discharge, while the magnetic field strength | B | and degree of balancing are varied. A significant deposition of the film forming material perpendicular to the target surface is observed for both sputter techniques. This sideways deposition decreases with increasing axial distance from the target surface. The sideways deposition rate is always the highest in dc operation, while it is lower for HiPIMS operation. The magnetic field strength has a strong influence on the sideways deposition rate in HiPIMS but not in dcMS. Furthermore, in HiPIMS operation, the radial ion deposition rate is always at least as large as the axial ion deposition rate and often around two times higher. Thus, there are a significantly higher number of ions traveling radially in the HiPIMS discharge. A comparison of the total radial as well as axial fluxes across the entire investigated plasma volume between the target and the substrate position allows for revised estimates of radial over axial flux fractions for different magnetic field configurations. It is here found that the relative radial flux of the film forming material is greater in dcMS compared to HiPIMS for almost all cases investigated. It is therefore concluded that the commonly reported reduction of the (axial) deposition rate in HiPIMS compared to dcMS does not seem to be linked with an increase in sideways material transport in HiPIMS.
  •  
22.
  • Hajihoseini, H., et al. (författare)
  • Target ion and neutral spread in high power impulse magnetron sputtering
  • 2023
  • Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 41:1
  • 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.
  •  
23.
  • Hajihoseini, Hamidreza, et al. (författare)
  • The Effect of Magnetic Field Strength and Geometry on the Deposition Rate and Ionized Flux Fraction in the HiPIMS Discharge
  • 2019
  • Ingår i: Plasma. - : MDPI AG. - 2571-6182. ; 2:2, s. 201-221
  • Tidskriftsartikel (refereegranskat)abstract
    • We explored the effect of magnetic field strength (Formula presented.) and geometry (degree of balancing) on the deposition rate and ionized flux fraction (Formula presented.) in dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) when depositing titanium. The HiPIMS discharge was run in two different operating modes. The first one we refer to as “fixed voltage mode” where the cathode voltage was kept fixed at 625 V while the pulse repetition frequency was varied to achieve the desired time average power (300 W). The second mode we refer to as “fixed peak current mode” and was carried out by adjusting the cathode voltage to maintain a fixed peak discharge current and by varying the frequency to achieve the same average power. Our results show that the dcMS deposition rate was weakly sensitive to variations in the magnetic field while the deposition rate during HiPIMS operated in fixed voltage mode changed from 30% to 90% of the dcMS deposition rate as (Formula presented.) decreased. In contrast, when operating the HiPIMS discharge in fixed peak current mode, the deposition rate increased only slightly with decreasing (Formula presented.). In fixed voltage mode, for weaker (Formula presented.), the higher was the deposition rate, the lower was the (Formula presented.). In the fixed peak current mode, both deposition rate and (Formula presented.) increased with decreasing (Formula presented.). Deposition rate uniformity measurements illustrated that the dcMS deposition uniformity was rather insensitive to changes in (Formula presented.) while both HiPIMS operating modes were highly sensitive. The HiPIMS deposition rate uniformity could be 10% lower or up to 10% higher than the dcMS deposition rate uniformity depending on (Formula presented.) and in particular the magnetic field topology. We related the measured quantities, the deposition rate and ionized flux fraction, to the ionization probability (Formula presented.) and the back attraction probability of the sputtered species (Formula presented.). We showed that the fraction of the ions of the sputtered material that escape back attraction increased by 30% when (Formula presented.) was reduced during operation in fixed peak current mode while the ionization probability of the sputtered species increased with increasing (Formula presented.), due to increased discharge current, when operating in fixed voltage mode.
  •  
24.
  • Huo, Chunqing, et al. (författare)
  • Gas rarefaction and the time evolution of long high-power impulse magnetron sputtering pulses
  • 2012
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 21:4, s. 045004-
  • Tidskriftsartikel (refereegranskat)abstract
    • Model studies of 400 mu s long discharge pulses in high-power impulse magnetron sputtering have been made to study the gas dynamics and plasma chemistry in this type of pulsed processing plasma. Data are taken from an experiment using square voltage pulses applied to an Al target in an Ar atmosphere at 1.8 Pa. The study is limited to low power densities, < 0.5 kW cm(-2), in which the discharge is far away from the runaway self-sputtering mode. The model used is the ionization region model, a time-dependent plasma chemistry discharge model developed for the ionization region in magnetron sputtering discharges. It gives a close fit to the discharge current during the whole pulse, both an initial high-current transient and a later plateau value of constant lower current. The discharge current peak is found to precede a maximum in gas rarefaction of the order of Delta n(Ar)/n(Ar),(0) approximate to 50%. The time durations of the high-current transient, and of the rarefaction maximum, are determined by the time it takes to establish a steady-state diffusional refill of process gas from the surrounding volume. The dominating mechanism for gas rarefaction is ionization losses, with only about 30% due to the sputter wind kick-out process. During the high-current transient, the degree of sputtered metal ionization reaches 65-75%, and then drops to 30-35% in the plateau phase. The degree of self-sputtering (defined here as the metal ion fraction of the total ion current to the target) also varies during the pulse. It grows from zero at pulse start to a maximum of 65-70% coinciding in time with the maximum gas rarefaction, and then stabilizes in the range 40-45% during the plateau phase. The loss in deposition rate that can be attributed to the back-attraction of the ionized sputtered species is also estimated from the model. It is low during the initial 10-20 mu s, peaks around 60% during the high-current transient, and finally stabilizes around 30% during the plateau phase.
  •  
25.
  • Huo, Chunqing, et al. (författare)
  • On sheath energization and Ohmic heating in sputtering magnetrons
  • 2013
  • Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 22:4, s. 045005-
  • Tidskriftsartikel (refereegranskat)abstract
    • In most models of sputtering magnetrons, the mechanism for energizing the electrons in the discharge is assumed to be sheath energization. In this process, secondary emitted electrons from the cathode surface are accelerated across the cathode sheath into the plasma, where they either ionize directly or transfer energy to the local lower energy electron population that subsequently ionizes the gas. In this work, we present new modeling results in support of an alternative electron energization mechanism. A model is experimentally constrained, by a fitting procedure, to match a set of experimental data taken over a large range in discharge powers in a high-power impulse magnetron sputtering (HiPIMS) device. When the model is matched to real data in this way, one finding is that the discharge can run with high power and large gas rarefaction without involving the mechanism of secondary electron emission by twice-ionized sputtered metal. The reason for this is that direct Ohmic heating of the plasma electrons is found to dominate over sheath energization by typically an order of magnitude. This holds from low power densities, as typical for dc magnetrons, to so high powers that the discharge is close to self-sputtering, i.e. dominated by the ionized vapor of the sputtered gas. The location of Ohmic heating is identified as an extended presheath with a potential drop of typically 100-150V. Such a feature, here indirectly derived from modeling, is in agreement with probe measurements of the potential profiles in other HiPIMS experiments, as well as in conventional dc magnetrons.
  •  
26.
  • Huo, Chunqing, et al. (författare)
  • On the road to self-sputtering in high power impulse magnetron sputtering : particle balance and discharge characteristics
  • 2014
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 23:2, s. 025017-
  • Tidskriftsartikel (refereegranskat)abstract
    • The onset and development of self-sputtering (SS) in a high power impulse magnetron sputtering (HiPIMS) discharge have been studied using a plasma chemical model and a set of experimental data, taken with an aluminum target and argon gas. The model is tailored to duplicate the discharge in which the data are taken. The pulses are long enough to include both an initial transient and a following steady state. The model is used to unravel how the internal discharge physics evolves with pulse power and time, and how it is related to features in the discharge current-voltage-time characteristics such as current densities, maxima, kinks and slopes. The connection between the self-sputter process and the discharge characteristics is quantified and discussed in terms of three parameters: a critical target current density J(crit) based on the maximum refill rate of process (argon) gas above the target, an SS recycling factor Pi(SS-recycle), and an approximation alpha a of the probabilities of ionization of species that come from the target (both sputtered metal and embedded argon atoms). For low power pulses, discharge voltages UD <= 380V with peak current densities below approximate to 0.2A cm(-2), the discharge is found to be dominated by process gas sputtering. In these pulses there is an initial current peak in time, associated with partial gas rarefaction, which is followed by a steady-state-like plateau in all parameters similar to direct current magnetron sputtering. In contrast, high power pulses, with U-D >= 500V and peak current densities above J(D) approximate to 1.6Acm(-2), make a transition to a discharge mode where SS dominates. The transition is found not to be driven by process gas rarefaction which is only about 10% at this time. Maximum gas rarefaction is found later in time and always after the initial peak in the discharge current. With increasing voltage, and pulse power, the discharge can be described as following a route where the role of SS increases in four steps: process gas sputtering, gas-sustained SS, self-sustained SS and SS runaway. At the highest voltage, 1000V, the discharge is very close to, but does not go into, the SS runaway mode. This absence of runaway is proposed to be connected to an unexpected finding: that twice ionized ions of the target species play almost no role in this discharge, not even at the highest powers. This reduces ionization by secondary-emitted energetic electrons almost to zero in the highest power range of the discharge.
  •  
27.
  • Huo, Chunqing, et al. (författare)
  • Particle-balance models for pulsed sputtering magnetrons
  • 2017
  • Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:35
  • Tidskriftsartikel (refereegranskat)abstract
    • The time-dependent plasma discharge ionization region model (IRM) has been under continuous development during the past decade and used in several studies of the ionization region of high-power impulse magnetron sputtering (HiPIMS) discharges. In the present work, a complete description of the most recent version of the IRM is given, which includes improvements, such as allowing for returning of the working gas atoms from the target, a separate treatment of hot secondary electrons, addition of doubly charged metal ions, etc. To show the general applicability of the IRM, two different HiPIMS discharges are investigated. The first set concerns 400 μs long discharge pulses applied to an Al target in an Ar atmosphere at 1.8 Pa. The second set focuses on 100 μs long discharge pulses applied to a Ti target in an Ar atmosphere at 0.54 Pa, and explores the effects of varying the magnetic field strength. The model results show that -ions contribute negligibly to the production of secondary electrons, while -ions effectively contribute to the production of secondary electrons. Similarly, the model results show that for an argon discharge with Al target the contribution of Al+-ions to the discharge current at the target surface is over 90% at 800 V. However, at 400 V the Al+-ions and Ar+-ions contribute roughly equally to the discharge current in the initial peak, while in the plateau region Ar+-ions contribute to roughly of the current. For high currents the discharge with Al target develops almost pure self-sputter recycling, while the discharge with Ti target exhibits close to a 50/50 combination of self-sputter recycling and working gas-recycling. For a Ti target, a self-sputter yield significantly below unity makes working gas-recycling necessary at high currents. For the discharge with Ti target, a decrease in the B-field strength, resulted in a corresponding stepwise increase in the discharge resistivity.
  •  
28.
  • Hurtig, Tomas, et al. (författare)
  • The penetration of plasma clouds across magnetic boundaries : The role of high frequency oscillations
  • 2004
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 11:7, s. L33-L36
  • Tidskriftsartikel (refereegranskat)abstract
    • Experiments are reported where a collision-free plasma cloud penetrates a magnetic barrier by self-polarization. Two closely related effects, both fundamental for the penetration mechanism, are studied quantitatively: anomalous fast magnetic field penetration (two orders of magnitude faster than classical), and anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion). It is concluded that they are both mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process.
  •  
29.
  • Hurtig, T., et al. (författare)
  • The role of high frequency oscillations in the penetration of plasma clouds across magnetic boundaries
  • 2005
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 12:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Experiments are reported where a collissionfree plasma cloud penetrates a magnetic barrier by self-polarization. Three closely related effects, all fundamental for the penetration mechanism, are studied quantitatively: (1) anomalous fast magnetic field penetration (two orders of magnitude faster than classical), (2) anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion), and (3) the ion energy budget as ions enter the potential structure set up by the self-polarized plasma cloud. It is concluded that all three phenomena are closely related and that they are mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process. The fast magnetic field penetration occurs as a consequence of the anomalous resistivity caused by the wave field and the fast electron transport across magnetic field lines is caused by the correlation between electric field and density oscillations in the wave field. It is also found that ions do not lose energy in proportion to the potential hill they have to climb, rather they are transported against the dc potential structure by the same correlation that is responsible for the electron transport. The results obtained through direct measurements are compared to particle in cell simulations that reproduce most aspects of the high frequency wave field.
  •  
30.
  • Hurtig, T., et al. (författare)
  • Three-dimensional electrostatic particle-in-cell simulation with open boundaries applied to a plasma beam entering a curved magnetic field
  • 2003
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 10:11, s. 4291-4305
  • Tidskriftsartikel (refereegranskat)abstract
    • Three-dimensional electrostatic particle-in-cell simulations of a laboratory experiment with an elongated plasma cloud entering a curved magnetic field are presented. A moving grid is used to follow the plasma motion from a region with longitudinal magnetic field, through a transition region where the field curves, and into a region where the magnetic field has a constant angle of 45degrees to the flow direction. In order to isolate the physics from disturbing boundary effects a method to create open boundary conditions has been implemented. As a result the boundaries are essentially moved to infinity. The simulation reproduces and gives physical insight into several experimental results concerning the plasma's macroscopic behavior in the transition region, which have earlier been only partly understood. First, the deformation of the plasma from a cylinder to a slab; second, the formation of strong currents along the sides of the plasma cloud in the transition region, which continue into field-aligned currents in the (upstream) flow-parallel field region, and close across the magnetic field both in the front and in the back of the penetrating cloud; and, third, the formation of a potential structure including (in the transition region) magnetic-field-aligned electric fields, and (both in, and downstream of, the transition region) a potential trough structure in the plasma's rest frame. It is found that all these macroscopic phenomena are intimately linked and can be understood within one consistent physical picture. The basic driving mechanism is the azimuthal electric field that is induced when, in the plasma's rest frame, the transverse magnetic field grows in time. The plasma's response is complicated by the fact that penetrating plasma clouds are in a parameter range where currents are not related to electric fields by a local conductivity: the ion motion is instead determined by the macroscopic potential structure.
  •  
31.
  • Keraudy, Julien, et al. (författare)
  • Bipolar HiPIMS for tailoring ion energies in thin film deposition
  • 2019
  • Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 359, s. 433-437
  • Tidskriftsartikel (refereegranskat)abstract
    • The effects of a positive pulse following a high-power impulse magnetron sputtering (HiPIMS) pulse are studied using energy-resolved mass spectrometry. This includes exploring the influence of a 200 mu s long positive voltage pulse (U-rev = 10-150 V) following a typical HiPIMS pulse on the ion-energy distribution function (IEDF) of the various ions. We find that a portion of the Ti+ flux is affected and gains an energy which corresponds to the acceleration over the full potential U-rev. The Ar+ IEDF on the other hand illustrates that a large fraction of the accelerated Ar+, gain energies corresponding to only a portion of U-rev. The Ti+ IEDFs are consistent with the assumption that practically all the TO-, that are accelerated during the reverse pulse, originates from a region adjacent to the target, in which the potential is uniformly increased with the applied potential U-rev while much of the Ar+ originates from a region further away from the target over which the potential drops from U-rev to a lower potential consistent with the plasma potential achieved without the application of U-rev. The deposition rate is only slightly affected and decreases with U-rev, reaching 90% at U-rev = 150 V. Both the Ti IEDF and the small deposition rate change indicate that the potential increase in the region close to the target is uniform and essentially free of electric fields, with the consequence that the motion of ions inside the region is not much influenced by the application of U-rev. In this situation, Ti will flow towards the outer boundary of the target adjacent region, with the momentum gained during the HiPIMS discharge pulse, independently of whether the positive pulse is applied or not. The metal ions that cross the boundary in the direction towards the substrate, and do this during the positive pulse, all gain an energy corresponding to the full positive applied potential U-rev.
  •  
32.
  • Lundin, Daniel, et al. (författare)
  • A study of the oxygen dynamics in a reactive Ar/O high power impulse magnetron sputtering discharge using an ionization region model
  • 2017
  • Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 121:17
  • Tidskriftsartikel (refereegranskat)abstract
    • The oxygen dynamics in a reactive Ar/O2high power impulse magnetron sputtering discharge hasbeen studied using a new reactive ionization region model. The aim has been to identify thedominating physical and chemical reactions in the plasma and on the surfaces of the reactoraffecting the oxygen plasma chemistry. We explore the temporal evolution of the density of theground state oxygen molecule O2ðX1RgÞ, the singlet metastable oxygen molecules O2ða1DgÞandO2ðb1RgÞ, the oxygen atom in the ground state O(3P), the metastable oxygen atom O(1D), thepositive ions Oþ2and Oþ, and the negative ion O. We furthermore investigate the reaction ratesfor the gain and loss of these species. The density of atomic oxygen increases significantly as wemove from the metal mode to the transition mode, and finally into the compound (poisoned) mode.The main gain rate responsible for the increase is sputtering of atomic oxygen from the oxidizedtarget. Both in the poisoned mode and in the transition mode, sputtering makes up more than 80%of the total gain rate for atomic oxygen. We also investigate the possibility of depositingstoichiometric TiO2in the transition mode.
  •  
33.
  • Lundin, Daniel, et al. (författare)
  • Deposition rate loss in high power impulse magnetron sputtering : understanding through computational modeling
  • 2011
  • Ingår i: 54th Annual Technical Conference Proceedings, Chicago, IL April 16-21,2011. - : Society of Vacuum Coaters. ; , s. 172-177
  • Konferensbidrag (refereegranskat)abstract
    • A lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared to DC magnetron sputtering (DCMS) for the same average power is often reported. The invoked reason is in many cases back-attraction of ionized sputteredmaterial to the target, but other effects are also likely to appear. In this work experimental results on the plasma and discharge conditions from several different HiPIMS experiments have been analyzed using a global discharge model to get an overall understanding of the HiPIMS discharge. The objective has been to quantify mechanisms reducing the deposition rate by studying the ionization of sputtered material and the possibility of back-attraction of ionized species, as well as heating of the neutral process gas leading to gas rarefaction.
  •  
34.
  • Lundin, Daniel, et al. (författare)
  • Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering
  • 2009
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 18:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 mu s. Two different current regimes could clearly be distinguished during the pulses: (1) a high-current transient followed by (2) a plateau at lower currents. These results provide a link between the HiPIMS and the direct current magnetron sputtering (DCMS) discharge regimes. At high applied negative voltages the high-current transient had the characteristics of HiPIMS pulses, while at lower voltages the plateau values agreed with currents in DCMS using the same applied voltage. The current behavior was found to be strongly correlated with the chamber gas pressure, where increasing gas pressure resulted in increasing peak current and plateau current. Based on these experiments it is suggested here that the high-current transients cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high-voltage, lower current regime.
  •  
35.
  •  
36.
  • Raadu, Michael A. (författare)
  • Effective distribution functions for electrostatic waves in dusty plasmas with a dust-size distribution
  • 2001
  • Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 29:2, s. 182-185
  • Tidskriftsartikel (refereegranskat)abstract
    • The kinetic theory for the electrostatic modes of dusty plasmas with a distribution of grain sizes is examined. It is assumed that the size distribution predominantly decreases exponentially with the mass for large sizes, and that a power law prevails for small sizes. Thermodynamic equilibrium leads to Maxwellian distributions over velocity with a fixed temperature and continuously varying mass. Smaller particles have higher thermal velocity and dominate the tail of the velocity distribution. The contribution of the dust component to the dispersion function is found to be non-Maxwellian and is equivalent to that for a kappa (generalized Lorentzian) distribution of monosized particles, Known results for kappa distributions may be exploited, However, the nonlinear response of the charge density of the dust to an electrostatic potential is quite different to that of a monosized kappa distribution. In general, the definition of an effective dust distribution function for linearized electrostatic modes leads to a useful straightforward procedure to find the dispersion function. It is important to realize that the combined effects of velocity and size distribution can, in general, strongly modify the kinetic behavior of the plasma dust component.
  •  
37.
  • Raadu, Michael A. (författare)
  • Generalised Sagdeev potentials for dusty plasmas with varying grain charges
  • 2003
  • Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 68:4, s. 266-270
  • Tidskriftsartikel (refereegranskat)abstract
    • Grain charges in a dusty plasma are deter-mined by the random currents from the ambient plasma and vary with the local conditions. The charge on a slowly moving grain will be close to the locally determined equilibrium. given for zero net current to the grain. For a steady electrostatic structure (e.g., solitary wave, double layer) integrals of motion for grains with varying charge can then be found. (These integrals reduce to the total energy if the charge is constant. but in general the electrostatic term becomes an integral of the grain charge with respect to the potential.) Steady state solutions of Vlasov's equation are piecewise given by arbitrary functions of these integrals of motion. A generalised Sagdeev (Classical) potential can be found, which is. to within an added constant. equal to minus the sum of the total particle pressures (including that of the grains). This extends the well known equivalence found for conventional plasmas and dusty plasmas with constant grain charges. The analysis of dust acoustic solitary waves is modified by additional terms proportional to potential derivatives of the charge. A grain size distribution may be incorporated. The second derivative of the Sagdeev potential (leading to the generalised Bohm condition) is then Riven in terms of the same effective distribution function as found for linear electrostatic modes. Comparisons are made with several analyses of nonlinear electrostatic structures including dynamical charging.
  •  
38.
  • Raadu, Michael A. (författare)
  • Particle acceleration mechanisms in space plasmas
  • 2001
  • Ingår i: Physics and Chemistry of the Earth, Part C. - 1464-1917 .- 1873-4685. ; 26:03-jan, s. 55-59
  • Tidskriftsartikel (refereegranskat)abstract
    • There are many possible mechanisms for particle acceleration in space plasmas. Among these are two broadly defined types that may be characterised on the one hand by large scale electric field structures and on the other hand by stochastic mechanisms and enhanced wave activity. Double layers (DLs) are an example of the first type. They sustain a net potential difference, across which particles are accelerated. Crucial questions concern the generator maintaining the potential, the rate of energy transfer, equilibrium and matching conditions. Magnetic reconnection at an X-type neutral line is another example and shares many characteristics with DLs, such as the cutting of field lines and constraints set by external MHD motions. Particle acceleration by the growth of the modified two- stream instability (MTSI) is an example of the second type. The MTSI is driven by cross magnetic field ion motion and rapidly accelerates electrons. It has been seen as a link in the critical ionisation velocity (CIV) mechanism, which has been extensively studied both in experiments and theory. Crucial questions concern the evolution of the particle energy distribution, the wave source and the saturation of the driving instability.
  •  
39.
  • Raadu, Michael A., et al. (författare)
  • Shielding of a slowly moving test charge in a dusty plasma with dynamical grain charging
  • 2003
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 10:9, s. 3484-3491
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamical charging of grains in a dusty plasma enhances the shielding of test charges. Time scales for charging are determined by the ambient plasma parameters and the grain dimensions. They can be very short, approaching the ion plasma period for grain sizes of the order of an electron Debye length. For a slowly moving test charge the response potential is found as a power series in the test charge velocity. Collisional effects are included. Analytical expressions for the response potential, valid for all radial distances, are found up to second order in the test charge velocity. The first-order dynamical charging term is shown to be the consequence of the delay in the shielding due to the dynamics of the charging process. The remaining first-order terms are given by analytical expressions that yield the well known asymptotic power law forms for large distances.
  •  
40.
  • Raadu, Michael A., et al. (författare)
  • Test charge response of a dusty plasma with a grain size distribution
  • 2002
  • Ingår i: Physics Letters A. - 0375-9601 .- 1873-2429. ; 305, s. 79-86
  • Tidskriftsartikel (refereegranskat)abstract
    • The form of the grain size distribution strongly influences the linear dielectric response of a dusty plasma. For a class of size distributions there is an equivalence to a Lorentzian distribution of mono-sized particles. The electrostatic response to a slowly moving test charge, using a second order approximation, is found. The effects of collisions are investigated.
  •  
41.
  •  
42.
  •  
43.
  • Raadu, Michael A., et al. (författare)
  • Wake potential of a test charge using the stationary phase method
  • 2005
  • Konferensbidrag (refereegranskat)abstract
    • The linear response of a dusty (complex) plasma to a moving test charge can be determined using an appropriate plasma dielectric function and a three dimensional Fourier analysis. Many analytical results have been found for a slowly moving test charge. For intermediate and large velocities numerical methods of integration are commonly used. However general asymptotic results valid at large distances can be found, using a combination of the residue calculus applied to the zeroes of the dielectric function and the method of stationary phase for integration over a wave vector component. The method can be expressed in terms of conditions on the group and phase velocity of waves in the reference frame of the moving test charge. In particular for a given radial direction the asymptotic response is determined by wave vectors for which the group velocity is directed radially outwards. The analysis is close to that due to Kelvin for ship waves in deep water. An essential difference is that in the present case three spatial dimensions are involved instead of only two, and also that the dispersion relation for plasma waves is more complicated.
  •  
44.
  • Rudolph, Martin, et al. (författare)
  • On how to measure the probabilities of target atom ionization and target ion back-attraction in high-power impulse magnetron sputtering
  • 2021
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 129:3
  • Tidskriftsartikel (refereegranskat)abstract
    • High-power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique that provides a high flux of ionized target species for thin film growth. Optimization of HiPIMS processes is, however, often difficult, since the influence of external process parameters, such as working gas pressure, magnetic field strength, and pulse configuration, on the deposition process characteristics is not well understood. The reason is that these external parameters are only indirectly connected to the two key flux parameters, the deposition rate and ionized flux fraction, via two internal discharge parameters: the target atom ionization probability alpha (t) and the target ion back-attraction probability beta (t). Until now, it has been difficult to assess alpha (t) and beta (t) without resorting to computational modeling, which has hampered knowledge-based optimization. Here, we present a simple method to deduce alpha (t) and beta (t) based on measured deposition rates of neutrals and ions. The core of the method is a refined analytical model, which is described in detail. This approach is furthermore validated by independent calculations of alpha (t) and beta (t) using the considerably more complex ionization region model, which is a plasma-chemical global discharge model.
  •  
45.
  • Rudolph, Martin, et al. (författare)
  • On the electron energy distribution function in the high power impulse magnetron sputtering discharge
  • 2021
  • Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We apply the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model to study the electron kinetics of a high power impulse magnetron sputtering (HiPIMS) discharge. In the IRM the bulk (cold) electrons are assumed to exhibit a Maxwellian energy distribution and the secondary (hot) electrons, emitted from the target surface upon ion bombardment, are treated as a high energy tail, while in the OBELIX the electron energy distribution is calculated self-consistently using an isotropic Boltzmann equation. The two models are merged in the sense that the output from the IRM is used as an input for OBELIX. The temporal evolutions of the particle densities are found to agree very well between the two models. Furthermore, a very good agreement is demonstrated between the bi-Maxwellian electron energy distribution assumed by the IRM and the electron energy distribution calculated by the OBELIX model. It can therefore be concluded that assuming a bi-Maxwellian electron energy distribution, constituting a cold bulk electron group and a hot secondary electron group, is a good approximation for modeling the HiPIMS discharge.
  •  
46.
  • Rudolph, M., et al. (författare)
  • Operating modes and target erosion in high power impulse magnetron sputtering
  • 2022
  • Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 40:4, s. 043005-
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetron sputtering combines a glow discharge with sputtering from a target that simultaneously serves as a cathode for the discharge. The electrons of the discharge are confined between overarching magnetic field lines and the negatively biased cathode. As the target erodes during the sputter process, the magnetic field strengthens in the cathode vicinity, which can influence discharge parameters with the risk of impairing reproducibility of the deposition process over time. This is of particular concern for high-power impulse magnetron sputtering (HiPIMS) as the discharge current and voltage waveforms vary strongly with the magnetic field strength. We here discuss ways to limit the detrimental effect of target erosion on the film deposition process by choosing an appropriate mode of operation for the discharge. The goal is to limit variations of two principal flux parameters, the deposition rate and the ionized flux fraction. As an outcome of the discussion, we recommend operating HiPIMS discharges by maintaining the peak discharge current constant.
  •  
47.
  • Rudolph, Martin, et al. (författare)
  • Optimizing the deposition rate and ionized flux fraction by tuning the pulse length in high power impulse magnetron sputtering
  • 2020
  • Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 29:5
  • Tidskriftsartikel (refereegranskat)abstract
    • High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapour deposition technique. While HiPIMS provides a high flux of metal ions to the substrate, the disadvantage is a reduced deposition rate compared to direct current magnetron sputtering (dcMS) at equal average power. This is mainly due to the high target back-attraction probability of the metal ions with typical values in the range 70%-90% during the pulse. In this work, we investigate how to reduce this effect by quantifying the contribution of the metal ion flux after each HiPIMS pulse, a period also known as afterglow. Without a negative potential on the target at this stage of the HiPIMS process, the back-attracting electric field disappears allowing remaining ions to escape the ionization region. In order to analyze the fate of the film-forming ions, we extend the time-dependent ionization region model (IRM) by adding consideration of an afterglow. This approach allows to distinguish between fluxes from the ionization region during the pulse and during the afterglow. We show that by shortening the pulse length of a titanium HiPIMS discharge, the contribution to the outward flux of film-forming species from the afterglow increases significantly. The IRM predicts a gain in deposition rate of 46% and 47% for two discharges with different peak discharge currents, when using 40 mu s compared to 100 mu s-long pulses at the same average power. This is without compromising the ionized flux fraction that remains constant for the range of pulse lengths investigated here.
  •  
48.
  • Samuelsson, Mattias, et al. (författare)
  • On the film density using high power impulse magnetron sputtering
  • 2010
  • Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 205:2, s. 591-596
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence on thin film density using high power impulse magnetron sputtering (HIPIMS) has been investigated for eight different target materials (Al, Ti, Cr. Cu, Zr, Ag, Ta, and Pt). The density values as well as deposition rates have been compared to results obtained from thin films grown by direct current magnetron sputtering (DCMS) under the same experimental conditions. Overall, it was found that the HIPIMS deposited coatings were approximately 5-15% denser compared to the DCMS deposited coatings This could be attributed to the increased metal ion bombardment commonly seen in HIPIMS discharges, which also was verified using a global plasma model to assess the degree of ionization of sputtered metal One key feature is that the momentum transfer between the growing film and the incoming metal ions is very efficient due to the equal mass of film and bombarding species, leading to a less pronounced columnar microstructure As expected the deposition rates were found to be lower for HiPIMS compared to DCMS For several materials this decrease is not as pronounced as previously reported in the literature, which is shown in the case of Ta. Pt, and Ag with rate(HIPIMS)/rate(DCMS)-70-85%. while still achieving denser coatings.
  •  
49.
  • Shafiq, Mohammad, et al. (författare)
  • Delayed shielding of a test charge due to dynamical grain charging in a dusty plasma
  • 2004
  • Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 32, s. 627-631
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamical charging of grains in a dusty plasma modifies the plasma dielectric response function and the nature of the electrostatic wave modes. The grain charging leads to an additional shielding effect that acts in the same way as Debye shielding. Both the additional shielding and the charging rate are important in determining the response of a dusty plasma to a moving test charge. The dynamics of the charging can be approximated by using a time delay. An alternative analysis of the potential of a slowly moving test charge is performed introducing a delay operator for the grain charge response. The terms in the potential that depend on the charging dynamics involve a spatial shift given by the test charge velocity and the charging time. This gives a physical interpretation of earlier results which are identical to first order in the test charge velocity.
  •  
50.
  • Shafiq, Muhammad (författare)
  • Dusty plasma response to a moivng test charge
  • 2005
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This licentiate thesis reports analytical results for the electrostatic response to a test charge moving through dusty plasma. Two particular cases for a slowly moving test charge, namely, grain size distribution and grain charging dynamics are considered. Analytical results for the delayed shielding of a test charge due to dynamical grain charging in dusty plasma are also reported. In the first case, a dusty plasma in thermal equilibrium and with a distribution of grain sizes is considered. A size distribution is assumed which decreases exponentially with the grain mass for large sizes and gives a simple smooth reduction for small sizes. The electrostatic response to a slowly moving test charge, using a second order approximation is found and the effects of collisions are also investigated. It turns out that for this particular size distribution, there is a remarkably simple result that the resulting effective distribution for the electrostatic response is a kappa (generalized Lorentzian) distribution. In the second case, we present an analytical model for the shielding of a slowly moving test charge in a dusty plasma with dynamical grain charging for cases both with and without the collision effects. The response potential is treated as a power series in test charge velocity. Analytical expressions for the response potential are found up to second order in test charge velocity. The first-order dynamical charging term is shown to be the consequence of the delay in the shielding due to the dynamics of the charging process. It is concluded that the dynamical charging of the grains in a dusty plasma enhances the shielding of a test charge. To clarify the physics, a separate study is made where the charging is approximated by using a time delay. The resulting potential shows the delayed shielding effect explicitly. The terms in the potential that depend on the charging dynamics involve a spatial shift given by the test charge velocity and the charging time. This kind of work has relevance both in space and astrophysical plasmas.
  •  
Skapa referenser, mejla, bekava och länka
  • Resultat 1-50 av 54

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.

 
pil uppåt Stäng

Kopiera och spara länken för att återkomma till aktuell vy