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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., QC 20240627

The influence of pulse length, working gas pressure, and peak discharge current density on the deposition rate and ionised flux fraction in high power impulse magnetron sputtering discharges of copper is investigated experimentally using a charge-selective (electrically biasable) magnetically shielded quartz crystal microbalance (or ionmeter). The large explored parameter space covers both common process conditions and extreme cases. The measured ionised flux fraction for copper is found to be in the range from ≈10% to 80%, and to increase with increasing peak discharge current density up to a maximum at ≈ 1.25 A cm − 2 , before abruptly falling off at even higher current density values. Low working gas pressure is shown to be beneficial in terms of both ionised flux fraction and deposition rate fraction. For example, decreasing the working gas pressure from 1.0 Pa to 0.5 Pa leads on average to an increase of the ionised flux fraction by ≈ 14 percentage points (pp) and of the deposition rate fraction by ≈ 4 pp taking into account all the investigated pulse lengths., QC 20240108

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., Funding Agencies|Icelandic Research Fund [196141]; Swedish Research Council [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

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., QC 20230110

The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3-2 Pa), HiPIMS peak current (10-70 A corresponding to 0.5-3.5 A cm(-2)), HiPIMS pulse length (5-60 mu s) and the amplitude of the positive voltage U (+) applied during the positive pulse (0-150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall., Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFOMat-LiU) [2009-00971]; Eurostars program (IonDrive) [E!114277]; Ministry of Education Youth and Sports of Czech RepublicMinistry of Education, Youth & Sports - Czech Republic [LM2018097]; Czech Science FoundationGrant Agency of the Czech Republic [GA19-00579S]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [51805102, 52165021]

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., Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

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., Funding Agencies|Free State of Saxony; European Regional Development Fund [100336119]; Icelandic Research Fund [196141]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

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., Funding Agencies|Icelandic Research Fund [196141]; Free State of Saxony; European Regional Development Fund [100336119]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

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., Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2018-04139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

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., Funding Agencies|Free State of Saxony; European Regional Development FundEuropean Commission [100336119]; Icelandic Research Fund [196141]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 201804139]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

The hydrogen evolution reaction (HER) is a key process in electrochemical water splitting. To lower the cost and environmental impact of this process, it is highly motivated to develop electrocatalysts with low or no content of noble metals. Here, we report on an ingenious synthesis of hybrid PtxNi1-x electrocatalysts in the form of a nanoparticle-nanonetwork structure with very low noble metal content. The structure possesses important features such as good electrical conductivity, high surface area, strong interlinking, and substrate adhesion, which render an excellent HER activity. Specifically, the best performing Pt0.05Ni0.95 sample demonstrates a Tafel slope of 30 mV dec-1 in 0.5 M H2SO4 and an overpotential of 20 mV at a current density of 10 mA cm-2 with high stability. The impressive catalytic performance is further rationalized in a theoretical study, which provides insight into the mechanism on how such small platinum content can allow for close-to-optimal adsorption energies for hydrogen.

High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique, providing a high flux of metal ions to the substrate. However, one of the disadvantages for industrial use of this technique is a reduced deposition rate compared to direct current magnetron sputtering (dcMS) at equal average power. This is mainly due to a high target back-attraction probability of the metal ions with typical values in the range 70%-90% during the pulse. In order to reduce this effect, we focused on the contribution of ion fluxes available immediately after each HiPIMS pulse; a time 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 magnetic trap and travel toward the substrate. To quantify the proposed mechanism, we studied the effect of HiPIMS pulse duration on the outward flux of film-forming species in titanium discharges, which are known to exhibit more than 50% reduction in deposition rate compared to dcMS. By shortening the HiPIMS pulse length, it was found that the contribution to the outward flux of film-forming species from the afterglow increases significantly. For example, HiPIMS discharges at a constant peak current density of about 1.10 A cm(-2) showed a 45% increase of the deposition rate, by shortening the pulse duration from 200 to 50 mu s. Ionized flux fraction measurements, using a gridless quartz crystal micro-balance-based ion meter, showed that this increase of the deposition rate could be achieved without compromising the ionized flux fraction, which remained approximately constant. The key to the achieved optimization of HiPIMS discharges lies in maintaining a high peak discharge current also for short pulse lengths to ensure sufficient ionization of the sputtered species., QC 20210621

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., QC 20210601

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., QC 20210301

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., QC 20200525

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., QC 20200525

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., QC 20200608

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 attr, QC 20240705

The most striking difference between HiPIMS and other magnetron sputtering discharges, in terms of the plasma process itself, lies in the high-power discharge pulses applied and the large discharge currents generated. We therefore start this chapter on the physics of HiPIMS by exploring the current composition at the target surface and the physical and chemical mechanisms operating at different stages of the discharge pulse and afterglow, which give rise to large discharge currents. Of particular interest is how internal process features such as gas rarefaction, ionization of the sputtered species, self-sputter recycling, and working gas recycling can be influenced by (as well as influence) the choice of pulse length, repetition frequency, applied power density, magnetic field strength and topology, target material, working gas, and so on. Using our understanding of the physics behind the discharge pulse, we then turn to discussing several key aspects in non-reactive and reactive HiPIMS, which includes dealing with the much debated issues of deposition rate as well as loss and transport of charged particles. The latter topic will, by necessity, also address plasma instabilities in HiPIMS., QC 20200610