High Power Impulse Magnetron Sputtering Plasma Research Articles

A field programmable gate array based Langmuir probe system for measurement of plasma parameters at 500kHz in a high-power impulse magnetron sputtering plasma.

By utilizing Field Programmable Gate Arrays in a configuration similar to that of the Mirror Langmuir Probe, it is possible to bias a single probe at three precise voltages in sequence. These voltages can be dynamically adjusted in real-time based on the measured plasma electron temperature to ensure the transition region is always sampled. The first results have been obtained by employing this method and have generated real-time outputs of electron temperature, ion saturation current, and floating potential on a low temperature pulsed-DC magnetron at 500kHz. These results are in good agreement with the analysis of a conventionally swept Langmuir probe. This probe is designed with the intention of being implemented on MAST-U to aid in the study of exhaust physics and enable further investigation into filamentary behavior.

Analysis of a second peak of electron density observed in high-power impulse magnetron sputtering plasma using a Langmuir probe

Plasma characteristics of a high-power impulse magnetron sputtering (HIPIMS) for copper deposition were investigated using a time-resolved Langmuir probe to explore HIPIMS discharge physics. Various discharge frequencies and pulse widths were employed while operating the HIPIMS in a constant current mode. Waveforms of the HIPIMS cathode current remained constant throughout the HIPIMS voltage pulse. It was found that electrons exhibited a bi-Maxwellian energy distribution both during and after the HIPIMS pulse. After the HIPIMS pulse, plasma density built up to a second peak while the bulk electron temperature quickly decreased. By examining the effect of pulse width and discharge frequency on the temperature of hot electrons through Langmuir I–V curves, it is suggested that the hot electron ionization contributed to the occurrence of the second peak.

Spoke-resolved electron density, temperature and potential in direct current magnetron sputtering and HiPIMS discharges

Spokes are long wavelength oscillations observed in the magnetized region of direct current magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS), as well as other discharges. Spokes rotate in front of the cathode with velocities between about 2 km s−1 and 15 km s−1, making it difficult to perform quantitative measurements. This is overcome by synchronizing Langmuir probe measurements to the movement of spokes in DCMS to obtain the probe current–voltage (I–V) characteristic without averaging out the spoke influence. The I–V curves are then evaluated using magnetized probe theory, revealing the strong plasma parameter modulations, caused by the spokes. The plasma density was found to oscillate between 2.5 × 1016 m−3 and 1.7 × 1017 m−3, which corresponds to a modulation strength of more than 70% or an almost seven times increase of density. In good agreement with previous work, a plasma potential minimum of −55 V is found ahead of the spoke followed by a sudden increase to about 2 V inside the spoke. The electron temperature was found to oscillate between 3 eV and 7 eV. On top of that oscillation, electrons experience a sudden energy increase as they move inside the spoke, crossing the potential jump at the leading edge for the spoke. On basis of these observations a model is presented to explain spokes in DCMS. These results are then compared to HiPIMS spokes under otherwise similar conditions. The plasma parameter modulation found for HiPIMS is much weaker than for DCMS, which is explained by the higher collision frequency for electrons in HiPIMS plasmas.

Time-resolved electron energy distribution functions at the substrate during a HiPIMS discharge with cathode voltage reversal

The time-dependent plasma properties of a high-power impulse magnetron sputtering plasma are investigated which include a positive ‘kick’ pulse on the sputtering target 2 μs after the main negative pulse, this reversing the voltage on the cathode. At a substrate 15 cm distant from the magnetron, the time-dependent electron energy distribution function (EEDF), plasma potential, potential commute time and plasma diffusion properties are measured using a single Langmuir probe. Results show that the positive pulse on the target expels plasma and raises the plasma potential across the chamber on the order of 1 to 2 μs, which is the time scale of the electron diffusion. The EEDF at the substrate fits a Druyvesteyn distribution during the main negative pulse rising slightly in average energy over time. The distribution is still Druyvesteyn and at the very start of the positive pulse, but then loses the higher energy electrons and drops in average electron energy as the positive pulse progresses. A Boltzmann equation solver, BOLSIG+, was used to predict the EEDF at the substrate during the positive pulse and it agrees best with the measurements assuming a value of 0.2 Td for the E/N (electric field/gas number density).

Role of Energetic Ions in the Growth of fcc and ω Crystalline Phases in Ti Films Deposited by HiPIMS

Titanium (Ti), due to its excellent properties, is widely exploited in thin film technology that usually leads to the production of hcp (α-phase) Ti films. In this work, we show how to control the nucleation of fcc and hexagonal (ω-phase) crystalline phases in Ti films by adding a proper Ti ion flux to the film-forming species. To this aim, films with different thicknesses are grown by High Power Impulse Magnetron Sputtering (HiPIMS) and conventional Direct Current Magnetron Sputtering (DCMS) as a reference. Furthermore, HiPIMS depositions with different substrate bias voltage Us(0 V, -300 V and -500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples, where the species are mostly atoms, exhibit the Ti α-phase only and show a tensile residual stress decreasing with thickness. As far as HiPIMS samples are concerned, films deposited in low energy ion conditions (Us=0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (Us= -300 V and -500 V) show the nucleation of the Ti ω-phase for thicknesses greater than 260 and 330 nm, respectively. A compressive-tensile-compressive (CTC) behavior is observed for residual stresses as thickness increases. The formation of these unusual Ti phases is discussed considering the different deposition conditions.

Time-resolved optical emission spectroscopy of a unipolar and a bipolar pulsed magnetron sputtering discharge in an argon/oxygen gas mixture with a cobalt target

Reactive high power impulse magnetron sputtering (HiPIMS) of a cobalt cathode in pure argon gas and with different oxygen admixtures was investigated by time-resolved optical emission spectroscopy (OES) and time-integrated energy-resolved mass spectrometry. The HiPIMS discharge was operated with a bipolar pulsed power supply capable of providing a large negative voltage with a typical pulse width of 100 μs followed by a long positive pulse with a pulse width of about 350 μs. The HiPIMS plasma in pure argon is dominated by Co+ ions. With the addition of oxygen, O+ ions become the second most prominent positive ion species. OES reveals the presence of Ar I, Co I, O I, and Ar II emission lines. The transition from an Ar+ to a Co+ ion sputtering discharge is inferred from time-resolved OES. The enhanced intensity of excited Ar+* ions is explained by simultaneous excitation and ionisation induced by energetic secondary electrons from the cathode. The intensity of violet Ar I lines is drastically reduced during HiPIMS. Intensity of near-infrared Ar I lines resumes during the positive pulse indicating an additional heating mechanism.

Evidence of ion energy distribution shift in HiPIMS plasmas with positive pulse

The deposition of a dense and void-free coating in complex geometries can be obtained with High Power Impulse Magnetron Sputtering (HiPIMS) by negatively biasing the substrate. This accelerates the incoming metal ions that consequently densify the film. However, a biased substrate can be a technical challenge for large objects that are generally kept grounded. HiPIMS with a positive voltage pulse after the main negative one is a potential alternative to be still fully investigated. In this letter, we report on the first measurements of a dominant population of energetic ions in HiPIMS with a positive pulse. Time-integrated energy distribution functions of the background gas ions (Ar+) and target ions (Nb+) are measured with and without the positive pulse. The peak of low energy ions at ∼2 eV is shown to disappear by varying the duration of a +50 V pulse. For a sufficiently long duration of 100 μs, energy peaks in the range of ∼30 eV are observed for both Ar+ and Nb+. Negligible ion fluxes at the energies below 10 eV are measured for 250 μs, indicating an effective acceleration of ions towards the substrate. Time-resolved ion flux measurements provide insights on the positive pulse effect on the ion fluxes.

The spoke origin based on the coupling-induced wave model in high-power impulse magnetron sputtering plasma

Spokes, identified as the source of anomalous across-B field transport during the high-power impulse magnetron sputtering (HiPIMS) discharge, have been considered as a result of plasma instability. This paper investigates the evolution of the coupling between two azimuthal waves in HiPIMS plasma by revisiting the dispersion relation. A coupling-induced wave model is proposed for the origin of the spoke. The ion sound wave can be strongly coupled with a Doppler-shifted electron Bernstein wave, inducing a long-scale (cm size) electric field oscillation waving at the frequency difference. The spoke appears to be a collective behavior of ion rearrangement guided by the difference frequency wave along the azimuthal direction. The direct comparison of spoke characteristics, such as rotating velocity, frequency and dynamic features, given by this model with experimental and simulation results from the literature shows a good qualitative agreement. Moreover, the evolution of spoke behavior following on from the model are presented, including rotating velocity and mode number along the racetrack. Their variation tendency matches well with experimental findings, from the literature.

The Electrical and Structural Properties of Nitrogen Ge1Sb2Te4 Thin Film

The present work describes the process of obtaining thin films of Ge1Sb2Te4 by means of the short-pulse High Power Impulse Magnetron Sputtering (HiPIMS) technique. The Ge1Sb2Te4 (GST-124) and nitrogen Ge1Sb2Te4 (nitrogen GST-124) films were obtained in HiPIMS plasma ignited in Ar/GST-124 and Ar/N2/GST-124, respectively. In particular, the possibility of tailoring the electrical properties of films for applications in the phase change memory (PCM) cells was investigated. The I–V measurements performed in a voltage sweeping mode on GST-124 and nitrogen GST-124 show that the threshold switching voltage varies as a function of nitrogen level in HiPIMS plasma. Amorphous-to-crystalline trigonal phase transition of the films was induced by thermal annealing, and structural changes were identified using X-ray diffraction and Raman scattering spectroscopy. The most intense bands appeared for the annealed layers in the range of 138–165 cm−1, for GST-124, and 138–150 cm−1 for nitrogen GST-124, respectively.

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.

Probing the electron density in HiPIMS plasmas by target inserts

High power impulse magnetron sputtering (HiPIMS) is a versatile technology to deposit thin films with superior properties. During HiPIMS, the power is applied in short pulses of the order of 100 μs at power densities of kW to a magnetron target creating a torus shaped dynamic high density plasma. This plasma torus is not homogeneous, but individual ionization zones become visible, which rotate along the torus with velocities of 10 km . Up to now, however, any direct measurement of the electron density inside these rotating ionization zones is missing. Here, we probe the electron density by measuring the target current locally by using small inserts embedded in an aluminium target facing the plasma torus. By applying simple sheath theory, a plasma density of the order of at the sheath edge can be inferred. The plasma density increases with increasing target current. In addition, the dynamics of the local target current variation is consistent with the dynamics of the traveling ionization zone causing a modulation of the local current density by 25%.

Composite targets in HiPIMS plasmas: Correlation of in-vacuum XPS characterization and optical plasma diagnostics

In-vacuum characterization of magnetron targets after High Power Impulse Magnetron Sputtering (HiPIMS) has been performed by X-ray photoelectron spectroscopy (XPS). Al-Cr composite targets (circular, 50 mm diameter) mounted in two different geometries were investigated: an Al target with a small Cr disk embedded at the racetrack position and a Cr target with a small Al disk embedded at the racetrack position. The HiPIMS discharge and the target surface composition were characterized in parallel for low, intermediate, and high power conditions, thus covering both the Ar-dominated and the metal-dominated HiPIMS regimes. The HiPIMS plasma was investigated using optical emission spectroscopy and fast imaging using a CCD camera; the spatially resolved XPS surface characterization was performed after in-vacuum transfer of the magnetron target to the XPS chamber. This parallel evaluation showed that (i) target redeposition of sputtered species was markedly more effective for Cr atoms than for Al atoms; (ii) oxidation at the target racetrack was observed even though the discharge ran in pure Ar gas without O2 admixture, the oxidation depended on the discharge power and target composition; and (iii) a bright emission spot fixed on top of the inserted Cr disk appeared for high power conditions.

Species transport on the target during high power impulse magnetron sputtering

High Power Impulse Magnetron Sputtering (HiPIMS) is a prominent technique to deposit superior materials due to the very energetic growth flux. The origin of this energetic growth flux is believed to be an electric potential structure inside localized ionization zones, the so-called spokes, in the HiPIMS plasma, which rotate in the E × B direction along the racetrack. The measurement of this electric potential or of the electric fields surrounding this ionization zone is extremely challenging due to the very high local power density that obstructs any traditional probe diagnostics. Here, we use a marker technique on the magnetron target to analyze the lateral transport of a target material on a HiPIMS target. We show that the target material is predominantly transported in the E × B direction irrespective of the presence of spokes. However, only when spokes are present, we observe also an enhanced transport in the opposite E × B direction. This is explained by the large electric field at the trailing edges of spokes.

Fast charge exchange ions in high power impulse magnetron sputtering of titanium as probes for the electrical potential

High power impulse magnetron sputtering (HiPIMS) plasmas exhibit a high ionization fraction of the sputtered material and ions with high kinetic energies, which produce thin films with superior quality. These ion energy distribution functions (IEDF) contain energetic peaks, which are believed to be linked to a distinct electrical potential hump inside rotating localized ionization zones, so called spokes, at target power densities above 1 kW cm−2. Any direct measurement of this electrical potential structure is, however, very difficult due to the dynamic nature of the spokes and the very high local power density, which hampers the use of conventional emissive probes. Instead, we use a careful analysis of the IEDFs for singly and doubly charged titanium ions from a HiPIMS plasma at varying target power density. The energy peaks in the IEDFs measured at the substrate depend on the point of ionization and any charge exchange collisions on the path between ionization and impact at the substrate. Thereby, the IEDFs contain a convoluted information about the electrical potential structure inside the plasma. The analysis of these IEDFs reveal that higher ionization states originate at high target power densities from the central part of the plasma spoke, whereas singly charged ions originate from the perimeter of the plasma spoke. Consequently, we observe different absolute ion energies with the energy of Ti2+ being slightly higher than two times the energy of Ti+. Additional peaks are observed in the IEDFs of Ti+ originating from charge exchange reactions from Ti2+ and Ti3+ with titanium neutrals. Based on this analysis of the IEDFs, the structure of the electrical potential inside a spoke is inferred yielding = 25 V above the plasma potential, irrespective of target power density.

Characterization of electromagnetic fluctuations in a HiPIMS plasma

A high power impulse magnetron sputtering (HiPIMS) plasma has been characterized by means of Langmuir probes measuring floating potential, electron temperature and density. Moreover, magnetic sensors have been employed in the analysis of the magnetic fluctuations. A low frequency electro-magnetic structure has been detected to propogate in the HiPIMS plasma at a velocity of about 5 km s−1 in the direction of the drift. It has been characterized in terms of time and space periodicities for different plasma conditions. It exhibits common features with the so-called rotating spokes, observed in various HiPIMS plasmas. The increasing relevance of the electromagnetic nature of the fluctuating structure at high power levels is highlighted.

Performance of HIPIMS deposited CrN/NbN nanostructured coatings exposed to 650 °C in pure steam environment

In the current work, 4 μm thick CrN/NbN coating utilising nanoscale multilayer structure with bi-layer thickness of Δ = 2.9 nm has been used to protect 9 wt% Cr steels such as P92 widely used in steam power plants. The uniquely layered coatings have a combination of nitrides of chromium and niobium which are not only resistant to aqueous corrosion and corrosion-erosion and have excellent tribological properties, but also have oxidation resistance in dry air up to a temperature of 850 °C. The novel High Power Impulse Magnetron Sputtering (HIPIMS) deposition technology has been used to deposit CrN/NbN with enhanced adhesion (critical load of scratch adhesion LC2 = 80 N) and a very dense microstructure as demonstrated by Transmission Electron Microscopy (TEM) imaging. These superior coating properties are achieved due to the unique high metal ion content (up to 90%) in the HIPIMS plasma, which allows particle acceleration and trajectory control by external electrical and magnetic fields thus delivering highly energetic material flux on the condensing surface.P92 bare and coated samples were oxidised at 650 °C in 100% steam atmosphere up to 2000 h, in order to simulate the future operation conditions of steam turbines employed in power plants. The oxidation kinetics was evaluated by mass gain measurements. Under these conditions CrN/NbN provided reliable protection of the P92 steel. The paper also discusses the effect of growth defects and high temperature crack formation analysed by Scanning Electron Microscopy and Focused Ion Beam-Scanning Electron Microscopy techniques (SEM and FIB-SEM, respectively) on the high temperature corrosion resistance in pure steam atmosphere thus revealing the coatings potential failure mechanisms.

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.

Electronic Bond Structure of Carbon Nitride Thin Film Deposited by HiPIMS and dc Magnetron Plasma

We report the difference in electronic bond structure and surface chemical property of amorphous carbon nitride (a-CNx) film as a function of N/C which has been deposited by high power impulse magnetron sputtering (HiPIMS) and DC magnetron (dc-MS) plasma. The spectroscopic analyses suggest that the presence of oxide layer is the major cause for the surface charging in the core level peak. The core electron peaks (C1s, N1s and O1s) in x-ray photoelectron spectroscopy (XPS) show higher surface charging in HiPIMS compare to the film deposited by dc-MS plasma. The core peaks are moved gradually toward its virgin position as the etching duration is increased from 1 min to 80 mins. The films deposited by HiPIMS retain a higher N/C ratio following ion etching as compared to those deposited by DC magnetron sputtering suggesting denser films with a higher degree of cross-linking.

A combinatorial comparison of DC and high power impulse magnetron sputtered Cr2AlC

Using a combinatorial approach, Cr, Al and C have been deposited onto sapphire wafer substrates by High Power Impulse Magnetron Sputtering (HiPIMS) and DC magnetron sputtering. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray diffraction were employed to determine the composition and microstructure of the coatings and confirm the presence of the Cr2AlC MAX phase within both coatings. One location in both the DCMS and HiPIMS coatings contained only MAX phase Cr2AlC. The electrical resistivity was also found to be nearly identical at this location and close to that reported from the bulk, indicating that the additional energy in the HiPIMS plasma was not required to form high quality MAX phase Cr2AlC.

A review comparing cathodic arcs and high power impulse magnetron sputtering (HiPIMS)

High power impulse magnetron sputtering (HiPIMS) has been at the center of attention over the last years as it is an emerging physical vapor deposition (PVD) technology that combines advantages of magnetron sputtering with various forms of energetic deposition of films such as ion plating and cathodic arc plasma deposition. It should not come as a surprise that many extension and variations of HiPIMS make use, intentionally or unintentionally, of previously discovered approaches to film processing such as substrate surface preparation by metal ion sputtering and phased biasing for film texture and stress control. Therefore, in this review, an overview is given on some historical developments and features of cathodic arc and HiPIMS plasmas, showing commonalities and differences. To limit the scope, emphasis is put on plasma properties, as opposed to surveying the vast literature on specific film materials and their properties.