| 1. |
- Brenning, Nils, et al.
(author)
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A unified treatment of self-sputtering, process gas recycling, and runaway for high power impulse sputtering magnetrons
- 2017
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In: Plasma sources science & technology. - : Institute of Physics (IOP). - 0963-0252 .- 1361-6595. ; 26:12
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Journal article (peer-reviewed)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.
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| 2. |
- Brenning, Nils, et al.
(author)
-
HiPIMS optimization by using mixed high-power and low-power pulsing
- 2021
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In: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:1
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Journal article (peer-reviewed)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.
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| 3. |
- Brenning, Nils, et al.
(author)
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Optimization of HiPIMS discharges : The selection of pulse power, pulse length, gas pressure, and magnetic field strength
- 2020
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In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 38:3
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Journal article (peer-reviewed)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.
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| 4. |
- Brenning, Nils, et al.
(author)
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Understanding deposition rate loss in high power impulse magnetron sputtering : I. Ionization-driven electric fields
- 2012
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In: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 21:2, s. 025005-
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Journal article (peer-reviewed)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.
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| 5. |
- Butler, Alexandre, et al.
(author)
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On three different ways to quantify the degree of ionization in sputtering magnetrons
- 2018
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In: Plasma sources science & technology. - : IOP PUBLISHING LTD. - 0963-0252 .- 1361-6595. ; 27:10
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Journal article (peer-reviewed)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.
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| 6. |
- Cemin, Felipe, et al.
(author)
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Low-energy ion irradiation in HiPIMS to enable anataseTiO(2) selective growth
- 2018
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In: Journal of Physics D. - : IOP PUBLISHING LTD. - 0022-3727 .- 1361-6463. ; 51:23
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Journal article (peer-reviewed)abstract
- High power impulse magnetron sputtering (HiPIMS) has already demonstrated great potential for synthesizing the high-energy crystalline phase of titanium dioxide (rutile Ti-O2) due to large quantities of highly energetic ions present in the discharge. In this work, it is shown that the metastable anatase phase can also be obtained by HiPIMS. The required deposition conditions have been identified by systematically studying the phase formation, microstructure and chemical composition as a function of mode of target operation as well as of substrate temperature, working pressure, and peak current density. It is found that films deposited in the metal and transition modes are predominantly amorphous and contain substoichiometric TiOx compounds, while in compound mode they are well-crystallized and present only O2- ions bound to Ti4+, i.e. pure TiO2. Anatase TiO2 films are obtained for working pressures between 1 and 2 Pa, a peak current density of similar to 1 A cm(-2) and deposition temperatures lower than 300 degrees C. Rutile is favored at lower pressures (amp;lt; 1 Pa) and higher peak current densities (amp;gt;2 A cm(-2)), while amorphous films are obtained at higher pressures (greater than or similar to 5 Pa). Microstructural characterization of selected films is also presented.
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| 7. |
- Crespi, Ângela Elisa, et al.
(author)
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Low resistivity amorphous carbon-based thin films employed as anti-reflective coatings on copper
- 2020
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 712
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Journal article (peer-reviewed)abstract
- Amorphous carbon-based coatings deposited on copper substrates by magnetron sputtering at different target-to-substrate distances were investigated. Films deposited at short distances as 2 cm presented the best results in terms of morphology, density, and resistivity. Ultraviolet near-infrared range spectrometry measurements determined total reflectance and ellipsometry, extinction coefficient, refraction index, and pseudo bandgap. Amorphous carbon films of 150 nm deposited at 2 cm reduced the total reflectance by up to 60 ± 5% in the near-infra-red range when compared to pure copper films. The addition of Fe* boosts the absorption of the coating reducing the total reflectance by up to 70 ± 5% in near-infrared. (Fe*: deposited from stainless-steel target used in direct-current magnetron sputtering). Also, Fe* reduces the electrical resistivity by a factor of 100 compared to that of pure amorphous carbon films. The reduction in total reflectance induced by the presence of the amorphous carbon-based films on copper depends, as expected, on light penetration depth and the absorption coefficient.
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| 8. |
- Lundin, Daniel, et al.
(author)
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Ti-Ar scattering cross sections by direct comparison of Monte Carlo simulations and laser-induced fluorescence spectroscopy in magnetron discharges
- 2013
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In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 46:17, s. 175201-
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Journal article (peer-reviewed)abstract
- A 3D Monte Carlo code (OMEGA) was developed to simulate the transport of sputtered atoms in a magnetron discharge operating in direct current mode. Collisions between the sputtered Ti atoms and the neutral process gas atoms (Ar) were modelled. Spatially resolved simulated velocity distributions of the sputtered particles parallel as well as perpendicular to the cathode surface for different operating pressures were recorded and benchmarked against experimentally obtained profiles using laser-induced fluorescence. New differential (angular and energy-dependent) cross sections for Ti-Ar elastic collisions were thereby obtained, which resulted in good agreement between modelled and experimental results. The differences with respect to commonly used extrapolated Ar-Ar cross sections to describe the Ti-Ar interaction are highlighted and discussed.
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| 9. |
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| 10. |
- Rudolph, Martin, et al.
(author)
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On how to measure the probabilities of target atom ionization and target ion back-attraction in high-power impulse magnetron sputtering
- 2021
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 129:3
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Journal article (peer-reviewed)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.
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| 11. |
- Rudolph, Martin, et al.
(author)
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On the electron energy distribution function in the high power impulse magnetron sputtering discharge
- 2021
-
In: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:4
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Journal article (peer-reviewed)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.
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| 12. |
- Vitelaru, Catalin, et al.
(author)
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Plasma reactivity in high-power impulse magnetron sputtering through oxygen kinetics
- 2013
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:10
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Journal article (peer-reviewed)abstract
- The atomic oxygen metastable dynamics in a Reactive High-Power Impulse Magnetron Sputtering (R-HiPIMS) discharge has been characterized using time-resolved diode laser absorption in an Ar/O-2 gas mixture with a Ti target. Two plasma regions are identified: the ionization region (IR) close to the target and further out the diffusion region (DR), separated by a transition region. The ls temporal resolution allows identifying the main atomic oxygen production and destruction routes, which are found to be very different during the pulse as compared to the afterglow as deduced from their evolution in space and time.
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