| 81. |
- Hasan, Mohammad I., et al.
(författare)
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Modeling the extraction of sputtered metal from high power impulse hollow cathode discharges
- 2013
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 22:3, s. 035006-
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Tidskriftsartikel (refereegranskat)abstract
- High power impulse hollow cathode sputtering is studied as a means to produce high fluxes of neutral and ionized sputtered metal species. A model is constructed for the understanding and optimization of such discharges. It relates input parameters such as the geometry of the cathode, the electric pulse form and frequency, and the feed gas flow rate and pressure, to the production, ionization, temperature and extraction of the sputtered species. Examples of processes that can be quantified by the use of the model are the internal production of sputtered metal and the degree of its ionization, the speed and efficiency of out-puffing from the hollow cathode associated with the pulses, and the gas back-flow into the hollow cathode between pulses. The use of the model is exemplified with a special case where the aim is the synthesis of nanoparticles in an expansion volume that lies outside the hollow cathode itself. The goals are here a maximum extraction efficiency, and a high degree of ionization of the sputtered metal. It is demonstrated that it is possible to reach a degree of ionization above 85%, and extraction efficiencies of 3% and 17% for the neutral and ionized sputtered components, respectively.
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| 82. |
- Helmersson, Ulf, et al.
(författare)
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A novel pulsed high-density plasma process for nanoparticle synthesis
- 2012
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Ingår i: Technical Proceedings of the 2012 NSTI Nanotechnology Conference and Expo, NSTI-Nanotech 2012. - 9781466562745 ; , s. 368-370
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Konferensbidrag (refereegranskat)abstract
- This work presents a new technique to produce nanoparticles in a controlled manor using highly ionized plasmas that will ionize the source material [1]. The advantage of ionizing the source material is that it will increase the trapping onto negatively charged nanoparticles (NPs) that should result in a significant increase in productivity. In this work we have performed both simulations and experiments. The experiments were performed using high power impulses to generate high plasma densities. The dense plasma yields a high degree of ionization of the sputtered metal species. Solid metal cylinders of Cu, Ag, Ti and Mo were used as hollow cathodes for the synthesis of NPs. By tuning the process parameters, pulsing energy, pulsing frequency, etc., the particle size can range from of 5 nm to 700 nm in diameter.
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| 83. |
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| 84. |
- Huo, Chunqing, et al.
(författare)
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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-
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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.
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| 85. |
- Huo, Chunqing
(författare)
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Modeling and Experimental Studies of High Power Impulse Magnetron Sputtering Discharges
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- HiPIMS, high power impulse magnetron sputtering, is a promising technology that has attracted a lot of attention, ever since it was introduced in 1999. A time-dependent plasma discharge model has been developed for the ionization region (IRM) in HiPIMS discharges. As a flexible modeling tool, it can be used to explore the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, the gas rarefaction and refill processes, the heating mechanisms, and the self-sputtering process etc.. The model development has proceeded in steps. A basic version IRM I is fitted to the experimental data from a HiPIMS discharge with 100 μs long pulses and an aluminum target (Paper I). A close fit to the experimental current waveform, and values of density, temperature, gas rarefaction, as well as the degree of ionization shows the general validity of the model. An improved version, IRM II is first used for an investigation of reasons for deposition rate loss in the same discharge (Paper II). This work contains a preliminary analysis of the potential distribution and its evolution as well as the possibility of a high deposition rate window to optimize the HiPIMS discharge. IRM II is then fitted to another HiPIMS discharge with 400 μs long pulses and an aluminum target and used to investigate gas rarefaction, degree of ionization, degree of self-sputtering, and the loss in deposition rate (Paper III). The most complete version, IRM III is also applied to these 400 μs long pulse discharges but in a larger power density range, from the pulsed dcMS range 0.026 kW/up to 3.6 kW/where gas rarefaction and self-sputtering are important processes. It is in Paper IV used to study the Ohmic heating mechanism in the bulk plasma, couple to the potential distribution in the ionization region, and compare the efficiencies of different mechanisms for electron heating and their resulting relative contributions to ionization. Then, in Paper V, the particle balance and discharge characteristics on the road to self-sputtering are studied. We find that a transition to a discharge mode where self-sputtering dominates always happens early, typically one third into the rising flank of an initial current peak. It is not driven by process gas rarefaction, instead gas rarefaction develops when the discharge already is in the self-sputtering regime. The degree of self-sputtering increases with power: at low powers mainly due to an increasing probability of ionization of the sputtered material, and at high powers mainly due to an increasing self-sputter yield in the sheath.Besides this IRM modeling, the transport of charged particles has been investigated byiv measuring current distributions in HiPIMS discharges with 200 μs long pulses and a copper target (Paper VI). A description, based on three different types of current systems during the ignition, transition and steady state phase, is used to analyze the evolution of the current density distribution in the pulsed plasma. The internal current density ratio (Hall current density divided by discharge current density) is a key transport parameter. It is reported how it varies with space and time, governing the cross-B resistivity and the mobility of the charged particles. From the current ratio, the electron cross-B (Pedersen) conductivity can be obtained and used as essential input when modeling the axial electric field that was the subject of Papers II and IV, and which governs the back-attraction of ions.
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| 86. |
- Huo, Chunqing
(författare)
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Modeling High Power Impulse Magnetron Sputtering Discharges
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- HiPIMS, high power impulse magnetron sputtering, is a promising technology that has attracted a lot of attention ever since its appearance. A time-dependent plasma discharge model has been developed for the ionization region in HiPIMS discharges. As a flexible modeling tool, it can be used to explore the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, and the gas rarefaction and refill processes. The model development has proceeded in steps. A basic version IRM I is fitted to the experimental data from a HiPIMS discharge with 100 μs long pulses and an aluminum target. A close fit to the experimental current waveform, and values of density, temperature, gas rarefaction, as well as the degree of ionization shows the validity of the model. Then an improved version is first used for an investigation of reasons for deposition rate loss, and then fitted for another HiPIMS discharge with 400 μs long pulses and an aluminum target to investigate gas rarefaction, degree of ionization, degree of self sputtering, and the loss in deposition rate, respectively. Through these results from the model, we could analyse further the potential distribution and its evolution as well as the possibility of a high deposition rate window to optimize the HiPIMS discharge. Besides this modeling, measurements of HiPIMS discharges with 100 μs long pulses and a copper target are made and analyzed. A description, based on three different types of current systems during the ignition, transition and steady phase, is used to describe the evolution of the current density distribution in the pulsed plasma. The internal current density ratio is a key transport parameter. It is reported how it varies with space and time, governing the cross-B resistivity and the energy of the charged particles. From the current ratio the electron cross-B transport can be obtained and used as essential input when modeling the axial electric field, governing the back-attraction of ions.
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| 87. |
- Huo, Chunqing, et al.
(författare)
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On sheath energization and Ohmic heating in sputtering magnetrons
- 2013
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Ingår i: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 22:4, s. 045005-
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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.
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| 88. |
- Huo, Chunqing, et al.
(författare)
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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-
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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.
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| 89. |
- Huo, Chunqing, et al.
(författare)
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Particle-balance models for pulsed sputtering magnetrons
- 2017
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 50:35
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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.
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| 90. |
- Hurtig, Lars Tomas Gustav, et al.
(författare)
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Investigation Into Relativistic Magnetic Flux Amplification
- 2016
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Ingår i: IEEE Transactions on Plasma Science. - : IEEE. - 0093-3813 .- 1939-9375. ; 44:1, s. 2-6
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Tidskriftsartikel (refereegranskat)abstract
- Amplification of magnetic flux and electric polarization fields caused by a plasma streaming at relativistic velocity into a magnetic field is discussed. It is shown that the electrostatic polarization field that arises in a plasma beam streaming across magnetic field lines at relativistic velocities will cause amplification of the magnetic flux. This effect is in complete contrast to the expulsion of the magnetic field from the plasma interior that can be expected in high beta(K) plasmas, where beta(K) is the kinetic energy density in the plasma stream divided by the energy density in the magnetic field. The amplification is shown to be caused by the relativistic motion of the space charge layers setting up the polarization field. 3-D electromagnetic particle-in-cell simulations that support this theory are presented.
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