Power Impulse Magnetron Sputtering Discharge Research Articles

Enhanced extraction efficiency of the sputtered material from a magnetically assisted high power impulse hollow cathode

The high power impulse hollow cathode sputtering (HiPIHCS) technique was pioneered by researchers attempting to combine the high ionization degree of sputtered species in a high power impulse magnetron sputtering discharge with the highly energetic electrons from a hollow cathode (HC) in order to obtain high-density and high-energy fluxes of sputtered metal species. Hence, this novel technique may open new horizons in surface engineering, offering the ability to control, enhance and tailor coating properties by using additional electric and/or magnetic fields. However, there are still many issues to be solved, including the ionization degree of the sputtered material and the deposition rate, as well as the extraction efficiency. This work reports on the improvement of the extraction efficiency of the sputtered material in an HiPIHCS process when an external magnetic field is applied. The HiPIHCS discharge was operated with ultra-short pulses (5 μs) of high voltage (600–800 V) applied directly on the HC. The obtained experimental results show that the extraction efficiency of both neutral and ionized sputtered material increases at least by a factor of two when an external magnetic field of only 15 mT is applied in the HC discharge region.

Ion density evolution in a high-power sputtering discharge with bipolar pulsing

Time evolution of sputtered metal ions in high power impulse magnetron sputtering (HiPIMS) discharge with a positive voltage pulse applied after a negative one (regime called “bipolar pulse HiPIMS”—BPH) is studied using 2-D density mapping. It is demonstrated that the ion propagation dynamics is mainly affected by the amplitude and duration of the positive pulse. Such effects as ion repulsion from the cathode and the ionization zone shrinkage due to electron drift towards the cathode are clearly observed during the positive pulse. The BPH mode also alters the film crystallographic structure, as observed from X-ray diffraction analysis.

Influence of spokes on the ionized metal flux fraction in chromium high power impulse magnetron sputtering

High power impulse magnetron sputtering (HiPIMS) discharges are an excellent tool for deposition of thin films with superior properties. By adjusting the plasma parameters, an energetic metal and reactive species growth flux can be controlled. This control requires, however, a quantitative knowledge of the ion-to-neutral ratio in the growth flux and of the ion energy distribution function to optimize the deposited energy per incorporated atom in the film. This quantification is performed by combining two diagnostics, a quartz crystal microbalance (QCM) combined with an ion-repelling grid system (IReGS) to discriminate ions versus neutrals and a HIDEN EQP plasma monitor to measure the ion energy distribution function (IEDF). This approach yields the ionized metal flux fraction (IMFF) as the ionization degree in the growth flux. This is correlated to the plasma performance recorded by time resolved ICCD camera measurements, which allow to identify the formation of pronounced ionization zones, so called spokes, in the HiPIMS plasma. Thereby an automatic technique was developed to identify the spoke mode number. The data indicates two distinct regimes with respect to spoke formation that occur with increasing peak power, a stochastic regime with no spokes at low peak powers followed by a regime with distinct spokes at varying mode numbers at higher peak powers. The IMFF increases with increasing peak power reaching values of almost 80% at very high peak powers. The transition in between the two regimes coincides with a pronounced change in the IMFF. This change indicates that the formation of spokes apparently counteracts the return effect in HiPIMS. Based on the IMFF and the mean energy of the ions, the energy per deposited atom together with the overall energy flux onto the substrate is calculated. This allows us to determine an optimum for the peak power density around 0.5 kW cm−2 for chromium HiPIMS.

Effect of magnetic field on spoke behaviour in HiPIMS plasma

The objective of this paper is to study the effect of magnetic field strength on spoke behaviour in a high power impulse magnetron sputtering discharge. In three magnetic field configurations, a broad range of experimental conditions was investigated by high-speed camera imaging. The dual image feature of the employed camera enabled us to determine the shape and spoke mode number, as well as the velocity of the spokes. Five distinct spoke shapes were detected. For each magnetic field, a map was created relating the occurrence of the spoke shape with the discharge conditions. Similarly, the spoke mode number, as well as the spoke velocity, was found to be strongly dependent on the discharge conditions. Based on the optical emission results, it was proposed that the trailing edge of the triangular spoke is related to the production of secondary electrons created by argon ions on the spoke edges, while in the case of a round spoke shape, the secondary electrons are mainly created by multiple charged titanium ions in the centre of the spoke.

A unified treatment of self-sputtering, process gas recycling, and runaway for high power impulse sputtering magnetrons

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 YSS from ≈ 0.1 to 2.6. The GRM analysis reveals that, above a critical current density of the order of Jcrit ≈ 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 YSS ≈ 1, the discharges become dominated by SS-recycling, contain few hot secondary electrons from sheath energization, and have a relatively low electron temperature Te. 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 YSS ≈ 0.2, the discharges above Jcrit are dominated by process gas recycling, have a significant sheath energization of secondary electrons and a higher Te, 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 YSS > 1 is found to be a necessary criterion, independent of the amount of gas-recycling in the discharge.

Synergistic enhancement effect between external electric and magnetic fields during high power impulse magnetron sputtering discharge

As a newly emerging high ionization physical vapor deposition method, high power impulse magnetron sputtering (HIPIMS) has wide and promising prospects. Besides, the low efficiency of electron utilization and the ionization rate of the metallic particles still require to be further improved when utilized in practical industry applications. In the present work, a novel HIPIMS discharge mode that being enhanced synergistically by both the electric and magnetic fields were proposed. The synergistic enhancement discharge effect was investigated through a current sensor, plasma emission spectrometry, and higher electron utilization efficiency and sputtering particle ionization rates were obtained. The results demonstrated that, the application of external magnetic field significantly constrained the plasma and enhanced the uni-directionality. The target discharge current decreases remarkably under constant target pulse conditions, whereas the ion current collected by the substrates increases remarkably. This means that more ions participated in the film deposition process, and the number of absorbed ions and lost ions reduce significantly with the low electric potential and the scattering, respectively. The introduction of the external auxiliary anode changed the electric field and electric potential distribution in the discharging zone, which enhanced the discharging intensity, whereas the density and charge state of plasma in the system increased substantially. When the electric and magnetic fields are present at the same time, a synergistic enhancement effect occurs and interaction enhancement and the plasma density is significantly increased in all positions, which is approximately 5 times the conventional HIPIMS under charging.

Discharge state transition and cathode fall thickness evolution during chromium HiPIMS discharge

The temporal evolutions of target voltage and current waveforms under different pulse voltage and working pressure conditions were studied during Cr high power impulse magnetron sputtering discharges. Target voltage and current characteristics demonstrated that when the pulse width was set as 200 μs, HiPIMS discharge went through a four-stage sequence during each pulse, Townsend discharge, glow discharge, afterglow, and pulse-off stages. A discharge state transition in the glow discharge stage happened at high pulse voltage and working pressure conditions. Furthermore, the dependence of reduced cathode fall thickness pdc on pulse voltage, working pressure, and normalized current density j/p2 was presented. It was found that gas rarefaction leads to a change of relationship between pdc and j/p2. A noticeable increase of the cathode fall thickness caused by gas rarefaction has been found.

A study of the oxygen dynamics in a reactive Ar/O2 high power impulse magnetron sputtering discharge using an ionization region model

The oxygen dynamics in a reactive Ar/O2 high power impulse magnetron sputtering discharge has been studied using a new reactive ionization region model. The aim has been to identify the dominating physical and chemical reactions in the plasma and on the surfaces of the reactor affecting the oxygen plasma chemistry. We explore the temporal evolution of the density of the ground state oxygen molecule O2(X1Σg−), the singlet metastable oxygen molecules O2(a1Δg) and O2(b1Σg), the oxygen atom in the ground state O(3P), the metastable oxygen atom O(1D), the positive ions O2+ and O+, and the negative ion O−. We furthermore investigate the reaction rates for the gain and loss of these species. The density of atomic oxygen increases significantly as we move 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 oxidized target. 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 depositing stoichiometric TiO2 in the transition mode.

Process- and optoelectronic-control of NiOx thin films deposited by reactive high power impulse magnetron sputtering

In this contribution, based on the analyses of the discharge behavior as well as final properties of the deposited Ni-O films during reactive high power impulse magnetron sputtering discharge, we have demonstrated that monitoring the oxygen flow rate leads to 4 different regimes of discharge. Tuning the oxygen partial pressure allows deposition of a large range of chemical compositions from pure nickel to nickel-deficient NiOx (x > 1) in the poisoned mode. Investigation of the plasma dynamics by time-resolved optical emission spectroscopy suggests that the discharge behavior in the poisoned mode principally comes from the higher contribution of both oxygen and argon ions in the total ionic current, leading to a change in the ion induced secondary electron emission coefficient. Additionally, material characterizations have revealed that optoelectronic properties of NiOx films can be easily tuned by adjusting the O/Ni ratio, which is influenced by the change of the oxygen flow rate. Stoichiometric NiO films (O/Ni ratio ∼ 1) are transparent in the visible range with a transmittance ∼80% and insulating as expected with an electrical resistivity ∼106 Ω cm. On the other hand, increasing the O/Ni > 1 leads to the deposition of more conductive coating (ρ ∼ 10 Ω cm) films with a lower transmittance ∼ 50%. These optoelectronic evolutions are accompanied by a band-gap narrowing 3.65 to 3.37 eV originating from the introduction of acceptor states between the Fermi level and the valence band maximum. In addition, our analysis has demonstrated that nickel vacancies are homogeneously distributed over the film thickness, explaining the p-type of the films.

Investigation of plasma spokes in reactive high power impulse magnetron sputtering discharge

Spokes, localised ionisation zones, are commonly observed in magnetron sputtering plasmas, appearing either with a triangular shape or with a diffuse shape, exhibiting self-organisation patterns. In this paper, we investigate the spoke properties (shape and emission) in a high power impulse magnetron sputtering (HiPIMS) discharge when reactive gas (N2 or O2) is added to the Ar gas, for three target materials; Al, Cr, and Ti. Peak discharge current and total pressure were kept constant, and the discharge voltage and mass flow ratios of Ar and the reactive gas were adjusted. The variation of the discharge voltage is used as an indication of a change of the secondary electron yield. The optical emission spectroscopy data demonstrate that by addition of reactive gas, the HiPIMS plasma exhibits a transition from a metal dominated plasma to the plasma dominated by Ar ions and, at high reactive gas partial pressures, to the plasma dominated by reactive gas ions. For all investigated materials, the spoke shape changed to the diffuse spoke shape in the poisoned mode. The change from the metal to the reactive gas dominated plasma and increase in the secondary electron production observed as the decrease of the discharge voltage corroborate our model of the spoke, where the diffuse spoke appears when the plasma is dominated by species capable of generating secondary electrons from the target. Behaviour of the discharge voltage and maximum plasma emission is strongly dependant on the target/reactive gas combination and does not fully match the behaviour observed in DC magnetron sputtering.

A HiPIMS plasma source with a magnetic nozzle that accelerates ions: application in a thruster

We demonstrate a solid fuel electrodeless ion thruster that uses a magnetic nozzle to collimate and accelerate copper ions produced by a high power impulse magnetron sputtering discharge (HiPIMS). The discharge is initiated using argon gas but in a practical device the consumption of argon could be minimised by exploiting the self-sputtering of copper. The ion fluence produced by the HiPIMS discharge was measured with a retarding field energy analyzer (RFEA) as a function of the magnetic field strength of the nozzle. The ion fraction of the copper was determined from the deposition rate of copper as a function of substrate bias and was found to exceed 87%. The ion fluence and ion energy increased in proportion with the magnetic field of the nozzle and the energy of the ions was found to follow a Maxwell-Boltzmann distribution with a directed velocity. The effectiveness of the magnetic nozzle in converting the randomized thermal motion of the ions into a jet was demonstrated from the energy distribution of the ions. The maximum ion exhaust velocity of at least 15.1 km/s, equivalent to a specific impulse of 1543 s was measured which is comparable to existing Hall thrusters and exceeds that of Teflon pulsed plasma thrusters.

An efficient way to evidence and to measure the metal ion fraction in high power impulse magnetron sputtering (HiPIMS) post-discharge with Pt, Au, Pd and mixed targets

The proportion of metal ions in a high power impulse magnetron sputtering discharge is key information for the potential development of new materials and new layer architectures deposited by this technique. This paper aims to measure this proportion by using a homemade system consisting of a quartz crystal microbalance and a grid energy analyser assembly. Such a system yields relevant results on the composition of the post-discharge depending on the nature of the gas (Ar, Kr, Xe) and the target materials (Pt, Pd, Au, $\text{Pt}_{50}\text{Au}_{50}$ and $\text{Pt}_{5}\text{Pd}_{95}$). In our conditions, the highest proportion of metal ions in the post-discharge are obtained by using Ar gas and reaches 10 %, 12 %, 50 %, 19 % and 88 % for Pt, Au, Pd, $\text{Pt}_{50}\text{Au}_{50}$ and $\text{Pt}_{5}\text{Pd}_{95}$ targets, respectively.

Synthesis of hydrogenated diamondlike carbon thin films using neon–acetylene based high power impulse magnetron sputtering discharges

Hydrogenated diamondlike carbon (DLC:H) thin films exhibit many interesting properties that can be tailored by controlling the composition and energy of the vapor fluxes used for their synthesis. This control can be facilitated by high electron density and/or high electron temperature plasmas that allow one to effectively tune the gas and surface chemistry during film growth, as well as the degree of ionization of the film forming species. The authors have recently demonstrated by adding Ne in an Ar-C high power impulse magnetron sputtering (HiPIMS) discharge that electron temperatures can be effectively increased to substantially ionize C species [Aijaz et al., Diamond Relat. Mater. 23, 1 (2012)]. The authors also developed an Ar-C2H2 HiPIMS process in which the high electron densities provided by the HiPIMS operation mode enhance gas phase dissociation reactions enabling control of the plasma and growth chemistry [Aijaz et al., Diamond Relat. Mater. 44, 117 (2014)]. Seeking to further enhance electron temperature and thereby promote electron impact induced interactions, control plasma chemical reaction pathways, and tune the resulting film properties, in this work, the authors synthesize DLC:H thin films by admixing Ne in a HiPIMS based Ar/C2H2 discharge. The authors investigate the plasma properties and discharge characteristics by measuring electron energy distributions as well as by studying discharge current characteristics showing an electron temperature enhancement in C2H2 based discharges and the role of ionic contribution to the film growth. These discharge conditions allow for the growth of thick (>1 μm) DLC:H thin films exhibiting low compressive stresses (∼0.5 GPa), high hardness (∼25 GPa), low H content (∼11%), and density in the order of 2.2 g/cm3. The authors also show that film densification and change of mechanical properties are related to H removal by ion bombardment rather than subplantation.

An ionization region model of the reactive Ar/O2 high power impulse magnetron sputtering discharge

A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O2 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.

Influence of pulse frequency and bias on microstructure and mechanical properties of TiB2 coatings deposited by high power impulse magnetron sputtering

Abstract The high plasma density and large fraction of ionized species created in a high power impulse magnetron sputtering (HiPIMS) discharge add new measures to control the sputtering process. We have studied the sputtering of TiB2 coatings by HiPIMS from a compound target in an industrial system. How the degree of ionized species effects coating microstructure and mechanical properties has been investigated by varying the pulse frequency between 200 Hz and 1000 Hz while keeping the average power constant at 2 kW. The coatings have a B/Ti atomic ratio ≥ 2.5 and a microstructure exhibiting 001 textured nanocolumnar grains with an amorphous B tissue phase in grain boundaries. Lower frequencies provide higher degree of ionization, which does, however, increase the compressive residual stress in the coatings. This results in harder coatings and the highest hardness of 49 GPa is measured for the coating deposited at 200 Hz (− 3.8 GPa residual stress). A change in texture from random orientation to 001 texture is achieved when going from regular dc sputtering to HiPIMS at a floating bias. Superhard (H = 43 GPa) TiB2 coatings with a relatively low compressive stress of about − 1 GPa can be deposited by HiPIMS at 1000 Hz using floating bias.

Anomalous cross-B field transport and spokes in HiPIMS plasma

Localized light emission patterns observed during on time of a high power impulse magnetron sputtering (HiPIMS) discharge on a planar magnetron, known as spokes or ionization zones, have been identified as a potential source of anomalous cross-B field diffusion. In this paper experimental evidence is presented that anomalous diffusion is triggered by the appearance of spokes. The Hall parameter , product of the electron cyclotron frequency and the classical collision time, reduces from Bohm diffusion values ( and higher) down to the value of 3 as spokes appear, indicating anomalous cross-B field transport. A combination of intensified charge coupled device imaging and electric probe measurements reveals that the ions from the spokes are instantaneously diffusing away from the target. The ion diffusion coefficients calculated from a sideways image of the spoke are six times higher than Bohm diffusion coefficients, which is consistent with the reduction of the Hall parameter.

Low-temperature synthesis of thermochromic vanadium dioxide thin films by reactive high power impulse magnetron sputtering

Thermochromic (TC) vanadium dioxide thin films provide means for controlling solar energy throughput and can be used for energy-saving applications such as smart windows. One of the factors limiting the deployment of VO2 films in TC devices is the growth temperature τs. At present, temperatures in excess of 450°C are required, which clearly can be an impediment especially for temperature-sensitive substrates. Here we address the issue of high τs by synthesizing VO2 thin films from highly ionized fluxes of depositing species generated in high power impulse magnetron sputtering (HiPIMS) discharges. The use of ions facilitates low-temperature film growth because the energy of the depositing species can be readily manipulated by substrate bias. For comparison, films were also synthesized by pulsed direct current magnetron sputtering. Structural and optical characterization of VO2 thin films on ITO-coated glass substrates confirms previous results that HiPIMS allows τs to be reduced from ~500 to ~300°C. Importantly, we demonstrated that HiPIMS permits the composition and TC response of the films to be tuned by altering the energy of the deposition flux via substrate bias. An optimum ion energy of 100eV was identified, which points at a potential for further reduction of τs thereby opening new possibilities for industrially-relevant applications of VO2-based TC thin films. Weak TC activity was observed even at τs≈200°C in HiPIMS-produced films.

A comparative study of direct current magnetron sputtering and high power impulse magnetron sputtering processes for CNx thin film growth with different inert gases

Reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) discharges of carbon in different inert gas mixtures (N2/Ne, N2/Ar, and N2/Kr) were investigated for the growth of carbon-nitride (CNx) thin films. Ion mass spectrometry showed that energies of abundant plasma cations are governed by the inert gas and the N2-to-inert gas flow ratios. The population of ion species depends on the sputter mode; HiPIMS yields approximately ten times higher flux ratios of ions originating from the target to process gas ions than DCMS. Exceptional are discharges in Ne with N2-to-Ne flow ratios <20%. Here, cation energies and the amount of target ions are highest without influence on the sputter mode. CNx thin films were deposited in 14% N2/inert gas mixtures at substrate temperatures of 110°C and 430°C. The film properties show a correlation to the substrate temperature, the applied inert gas and sputter mode. The mechanical performance of the films is mainly governed by their morphology and composition, but not by their microstructure. Amorphous and fullerene-like CN0.14 films exhibiting a hardness of ~15GPa and an elastic recovery of ~90% were deposited at 110°C in reactive Kr atmosphere by DCMS and HiPIMS.

Are the argon metastables important in high power impulse magnetron sputtering discharges?

We use an ionization region model to explore the ionization processes in the high power impulse magnetron sputtering (HiPIMS) discharge in argon with a titanium target. In conventional dc magnetron sputtering (dcMS), stepwise ionization can be an important route for ionization of the argon gas. However, in the HiPIMS discharge stepwise ionization is found to be negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species, Penning ionization can be a significant ionization mechanism in the dcMS discharges, while in the HiPIMS discharge Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density in the HiPIMS discharge, and a higher electron temperature. Furthermore, we explore the ionization fraction and the ionized flux fraction of the sputtered vapor and compare with recent experimental work.

Design of magnetic field configuration for controlled discharge properties in highly ionized plasma

In the present article, the effect of magnetic field design on electron and ion properties in both a metallic Ti/Ar and a reactive Ti/Ar + O2 high power impulse magnetron sputtering (HiPIMS) discharges is investigated. For the purpose, a variable magnetron with defined imbalance and geometrical coefficients K and , respectively, was utilized. The electron density, the mean electron energy, the plasma potential, and the floating potential were determined by employing time-resolved Langmuir probe measurements, for four specified magnetic field configurations. Mass spectroscopy was used in order to determine the energy distribution function of metal (Ti+ , Ti2+) and gaseous (Ar+ , Ar2+ , O+ , O) ions. Analysis of the measured data shows that the magnetic field design dramatically affects the charged particles energy- and spatial-distribution, causing a change in the plasma properties. It is concluded that a well-determined configuration of the magnetic field is necessary in order to insure discharge stability and reproducibility.