High Power Impulse Magnetron Sputtering Discharge Research Articles

Spatially resolved optical emission analysis of spokes in HiPIMS utilising Al, Cr, Cu, Ti, and W targets

Investigating spokes in high-power impulse magnetron sputtering discharge requires non-invasive diagnostic methods to characterise accurately spoke properties. A fast photodiode and a cylindrical Langmuir probe were employed to synchronise the moment of acquisition of the optical emission spectrum with the position of a passing spoke. This study provides statistical data analysis to bring insights into spoke characteristics in a non-reactive argon atmosphere, employing aluminium, chromium, copper, titanium, and tungsten targets. Utilising different target materials, the objective is to describe basic parameters such as shape, length, and propagation velocity of spokes and also analyse spoke inner parameters such as floating potential and spectral emission, under nearly identical experimental conditions. From the optical emission, the most prominent species within the spoke were determined. Additionally, the mechanism governing spoke movement was described using a phenomenological model.

2D analysis of sputtered species transport in high-power impulse magnetron sputtering (HiPIMS) discharge

Among the numerous advantages of the high-power impulse magnetron sputtering (HiPIMS) technique, the most important is the enhanced ionization degree of sputtered species contributing to the film growth. Consequently, the quality of deposited thin films is highly improved. Still, the optimization process is challenging due to the complexity associated with the intricate transport of the sputtered species, ionized or neutrals. The scarce knowledge available on the spatial distribution of these species when operating a HiPIMS discharge makes the quantitative prediction of any deposition feature particularly difficult. In this paper, we discuss the influence of experimentally controllable quantities, such as gas pressure and target current density, on the transport of sputtered titanium in non-reactive (argon) HiPIMS, namely, on the behavior of metal atoms and metal ion fluxes intercepting the substrate. Systematic quantitative measurements were performed in a diameter normal plane on a circular planar target. Hence, the 2D spatial distribution of the ionized flux fraction (IFF) and the total flux of titanium sputtered particles (deposition rate) are evaluated by biasing a quartz crystal microbalance equipped with an electron magnetic filter. The wide range of parameters we examined allows us to predict and optimize the flux of sputtered species based on complete mapping of the IFF of sputtered particles.

Understanding ion and atom fluxes during high-power impulse magnetron sputtering deposition of NbCx films from a compound target

A combination of time-averaged mass spectroscopy (MS), time-averaged optical emission spectroscopy (OES), and plasma transport modeling was employed to understand the transport processes of ions and atoms in high-power impulse magnetron sputtering discharges resulting in changes in the stoichiometry of NbCx films during their deposition from a stoichiometric NbC compound target at different repetition frequencies and duty cycles. Mass spectrometry showed that the fluxes of ions originating from the elements of the target increase with increasing pulse power density (and decreasing pulse length) due to an increasing electron density and, thus, electron-impact ionization probability. Due to the higher ionization energy and much lower ionization cross section of C (compared to Nb), it was found that the contribution of C+ ions to the deposition flux is practically negligible. Additionally, OES tracked the densities of ions and atoms at different distances from the target. The OES analysis revealed that the atom densities decreased as the pulse power density increased. In contrast, the ion densities exhibited an increase, which is consistent with the findings of MS. Using the data from MS, OES, and modeling, we were able to estimate the fluxes of atoms to the substrate. Our observations demonstrated a transition from C-rich toward Nb-rich flux of film-forming species with increasing pulse power density, corresponding to changes in the film composition. We further discuss the role of internal plasma processes that are responsible for this transition.

Corundum-structured AlCrNbTi oxide film grown using high-energy early-arriving ion irradiation in high-power impulse magnetron sputtering

Multicomponent or high-entropy oxide films are of interest due to their remarkable structure and properties. Here, energetic ion irradiation is utilized for controlling the phase formation and structure of AlCrNbTi oxide at growth temperature of 500°C. The ion acceleration is achieved by using a high-power impulse magnetron sputtering (HiPIMS) discharge, accompanied by a 10 μs-long synchronized substrate bias (Usync), to minimize the surface charging effect and accelerate early-arriving ions, mainly Al+, O+, Ar2+, and Al2+. By increasing the magnitude of Usync from −100 V to −500 V, the film structure changes from amorphous to single-phase corundum, followed by the formation of high-number-density stacking faults (or nanotwins) at Usync = −500 V. This approach paves the way to tailor the high-temperature-phase and defect formation of oxide films at low growth temperature, with prospects for use in protective-coating and dielectric applications.

Pulse length dependence of a reactive high power impulse magnetron (HiPIMS) discharge

The pulse length dependence of a reactive high power impulse magnetron sputtering (HiPIMS) discharge with a tungsten cathode in an argon+oxygen gas mixture gas was investigated. The HiPIMS discharge is operated with a variable pulse length of 20–500 µs. Discharge current measurements, optical emission spectroscopy of neutral Ar, O, and W lines, and energy-resolved ion mass spectrometry are employed. A pronounced dependence of the discharge current on pulse length is noted while the initial discharge voltage is maintained constant. Energy-resolved mass spectrometry shows that the oxygen-to-tungsten (O+/W+) and the tungsten oxide-to-tungsten (WO+/W+) ion ratio decreases with pulse length due to target cleaning. Simulation results employing the SDTrimSP program show the formation of a non-stoichiometric sub-surface compound layer of oxygen which depends on the impinging ion composition and thus on the pulse length.

Angular distribution of titanium ions and neutrals in high-power impulse magnetron sputtering discharges

The angular dependence of the deposition rates due to ions and neutrals in high-power impulse magnetron sputtering (HiPIMS) discharges with a titanium target were determined experimentally using a magnetically shielded and charge-selective quartz crystal microbalance (or ionmeter). These rates have been established as a function of the argon working gas pressure, the peak discharge current density, and the pulse length. For all explored cases, the total deposition rate exhibits a heart-shaped profile and the ionized flux fraction peaks on the discharge axis normal to the cathode target surface. This heart-shaped pattern is found to be amplified at increasing current densities and reduced at increased working gas pressures. Furthermore, it is confirmed that a low working gas pressure is beneficial for achieving high deposition rates and high ionized flux fractions in HiPIMS operation.

Impact of self-sputtering in high power impulse magnetron sputtering (HiPIMS) with helium

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal.

Particle-based simulation of atom and ion transport in HiPIMS: effect of the plasma potential distribution on the ionized flux fraction

We present a three-dimensional particle-based computer simulation of high-power impulse magnetron sputtering (HiPIMS) discharges which enables us to simulate the transport of atoms and ions in the discharge and the corresponding plasma parameters. The simulation requires a definition of the plasma potential and electron density distribution (not calculated self-consistently), for which parametric analytical formulae were devised. A numerical algorithm is used to constrain the simulation by an experimental target current waveform, which ensures that the simulation results are closely tied to the experimental discharge conditions. Simulations of a HiPIMS discharge with Ti target show the capability to calculate the spatial distributions of target material atoms and ions and also to quantify the process-gas rarefaction. We evaluated, among others, the ion return probability and the ionized fraction of the target material flux onto the substrate for various values of the potential difference across the magnetic presheath in front of the target racetrack, which is responsible for attracting most of the plasma ions towards the target. It is shown that this parameter of the plasma potential distribution strongly affects the ion return probability and, thus, it must be known quite precisely to reliably predict the ionized flux fraction on the substrate. Other parameters, such as the composition of the ion flux onto the target are less sensitive. The simulation can be run in a reasonably short time and can easily be extended by adding more plasma species (excited states or doubly ionized species) and their interactions.

Insights on film growth conditions on a floating substrate during reactive Ar/O2 bipolar high power impulse magnetron sputter deposition

We report several effects found in reactive bipolar high-power impulse magnetron sputtering (abbreviated as bipolar HiPIMS or BPH) discharges at the substrate level. The role of the negative peak current and the positive pulse voltage, U, on the deposition flux at the electrically insulating substrate is discussed and the results are compared to conventional DC discharges. In particular, the normalized energy flux, ϕ n , delivered to the substrate is found higher under BPH conditions at U = +300 V ( eV·nm−3) as compared to the HiPIMS ( eV·nm−3) and DCMS ( eV·nm−3) regimes which is assumed to be provoked by an intensification of the positive ion bombardment. Accordingly, the mean energy and the count of positively charged metal and gaseous ions increase in line with U as measured by energy-resolved mass spectrometry. Contrariwise, our analysis has shown a significant decrease (≈20 %) of negative oxygen ion count under BPH conditions.

MAGNETRON SPUTTER DEPOSITION OF TUNGSTEN COATINGS IN DEUTERIUM-HELIUM MIXTURES

The electrical parameters of a high-power impulse magnetron sputtering (HiPIMS) discharge operating in H<sub>2</sub>/He mixtures were studied using different values of the pulse width, repetition frequency, and power. Deposition of tungsten (W) coatings was carried out in a magnetron discharge with a thermally insulated W target in two regimes at different pulse widths (80 and 500 µs) and frequencies (2 and 0.5 kHz). The average discharge power was fixed at P<sub>d</sub> = 1500 W. Fusion-relevant molecular deuterium/helium (He) and hydrogen/He gas mixtures with a 90/10 flow ratio were used in the experiments.

Target ion and neutral spread in high power impulse magnetron sputtering

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 ξtn remains roughly the same, while transport parameters for ions ξti vary greatly. Furthermore, the relative ion-to-neutral transport factors, ξti/ξtn, 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.

Time-resolved ion energy distribution functions during a HiPIMS discharge with cathode voltage reversal

The effect on the ion energy distribution function (IEDF) of plasma produced during a high-power impulse magnetron sputtering (HiPIMS) discharge as the pulse conditions are varied is reported. Pressure was varied from 0.67–2.00 Pa (5–15 mTorr), positive kick pulses up to 200 V tested with a constant 4 μs delay between negative and positive cycles. The results demonstrate that the resulting plasma during the positive cathode voltage reversal is the result of expansion through the largely neutral gas species between the end of the magnetic trap of the target and the workpiece. The plasma potential rises on similar time scale with the evolution of a narrow peak in the IEDF close to the applied bias. The peak of the distribution function remains narrow close to the applied bias irrespective of pulse length, and with only slight pressure dependence. One exception discovered is that the IEDF contains a broad high energy tail early in the kick pulse due to acceleration of ions present beyond the trap from the main pulse separate from the ionization front that follows.

Copper tungsten oxide (CuxWOy) thin films for optical and photoelectrochemical applications deposited by reactive high power impulse magnetron co-sputtering

Copper tungsten oxide films are deposited with the help of reactive high power impulse magnetron sputtering (HiPIMS) in an argon/oxygen gas mixture. Two magnetrons, one equipped with a tungsten target and the other with a copper target, are employed. The HiPIMS discharge is operated with a repetition frequency of f=100 Hz. Pulse widths of 100 and 20 μs separated by 25 μs are chosen for the tungsten and copper target, respectively. Films deposited on two different glass substrates [soda lime glass and fluorine doped tin oxide (FTO) coated glass] are characterized by energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, x-ray diffraction, Raman spectroscopy, and ellipsometry. Photoelectrochemical activity was investigated by linear voltammetry. The composition and crystal structure of as-deposited and annealed films are found to depend on the deposition conditions. Annealed films deposited on FTO glass are composed of WO3 and CuWO4 or Cu2WO4 crystal phases. Films deposited on soda lime glass are subject to sodium diffusion into the films during annealing and the formation of Na2W2O7 and Na2W4O13 phases.

Effect of carbon content on the structural, mechanical and corrosion properties of TiC films deposited using a HiPIMS discharge

TiCx thin films have been deposited on steel substrates using High Power Impulse Magnetron Sputtering (HiPIMS) under Ar-C2H2 atmosphere. The effect of varying the acetylene content on the deposition process, and on the structural, mechanical and corrosion resistance properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, scratch testing, potentiodynamic polarization, electrochemical impedance measurements (EIS), and salt spray. This study showed that an increase of the carbon ratio affected the deposited TiC coatings morphology and mechanical properties, with hardness values reaching up to 25 GPa. The near-stoichiometric coating exhibited the highest hardness while maintaining improved toughness, resistance to plastic deformation and resistance against crack propagation thanks to their nanocomposite structure. In contrast, corrosion tests showed that the understoichiometric coatings exhibited the best corrosion resistance in a 3.5 % wt NaCl medium. The drop of the corrosion performances of TiCx coatings deposited at higher acetylene flow rate was linked to a growth-related defects extending throughout the coating thickness, formed due to micro-droplet ejection from the target as evidenced by SEM analysis.

Dynamic features of the electron drift and electron properties in a HiPIMS discharge

Information on the evolution of electron properties during high-power impulse magnetron sputtering (HiPIMS) operation of planar magnetrons enables the study of fundamental physical processes. In this work, incoherent Thomson scattering is implemented for the non-invasive, spatiotemporally-resolved characterization of electron properties and drifts in the HiPIMS regime of a planar magnetron. In the ionization region of argon and helium plasmas, the azimuthal electron drifts are directly measured perpendicular to the magnetic field and are found to evolve according to a changing balance of E × B and diamagnetic electron drifts, while radial electron drifts, measured parallel to the magnetic field, can be attributed to plasma expansion/contraction and centrifugal forces. The evolutions of electron density and temperature in the afterglow plasma phase show the existence of two time scales for the variation of plasma properties. These characterizations provide detailed information on electron properties and dynamics in regions of the magnetic trap ordinarily inaccessible to invasive diagnostics.

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.

Parameters and ion mass-to-charge composition of a high-power impulse magnetron discharge with electron injection

We have explored the effect of electron injection on the burning conditions and parameters of a high-power impulse magnetron sputtering discharge, including the ion mass-to-charge composition of the discharge plasma. We find that with stabilized discharge burning voltage, electron injection leads to a significant increase in the discharge current and sustains discharge operation in the low pressure regime, but also leads to a lower fraction of metal ions in the plasma. By using a reflecting electrode, conditions are created that provide transport of the injected electron flux directly into the region of intense ionization, and one can preserve the fraction of metal ions in the plasma corresponding to the self-sputtering mode, and also decrease the limiting operating pressure.

Niobium thin film thickness profile tailoring on complex shape substrates using unbalanced biased High Power Impulse Magnetron Sputtering

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength.

Effect of geometric position on the film properties for a complex-shaped substrate in HiPIMS discharge: Experiment and simulation

The purpose of this paper is to explore the effect of geometric position on the film properties for a complex-shaped substrate in high-power impulse magnetron sputtering (HiPIMS) discharge. The substrate is a trapezoidal prism, whose base has four inner corners of 60°, 120°, 75°, and 105°. A negative bias is added to this trapezoidal prism during the high-density discharge to deposit TiAlSiN films. The chemical compositions, microstructures and mechanical properties of the films at different area of the substrate are analyzed using the energy dispersive spectroscopy, X-ray diffraction, scanning electron microscope, nano-indentor and Vickers indentation tester. Systematic investigations demonstrate that the films properties have prominent differences on various planes of the trapezoidal-prism sample, due to the so-called shadowing effect. Compared with the measurements on the plane perpendicular to the target surface, there is a higher average hardness and stronger toughness on the plane facing to the target surface. However, the values of both the hardness and toughness are the lowest on the plane facing away from the target surface. Moreover, even for the same plane, the enhanced mechanical properties as well as a smoother surface and denser microstructure appear in the edge regions, with respect to that in the planar center regions. To understand these interesting phenomena, a two-dimensional particle-in-cell/Monte Carlo collision simulation (2D PIC-MCC) and Transport of Ions in Matter method (TRIM) are employed to explore the ion dynamics at the different sites of the sample. Simulation results suggest that a higher ion flux density and larger re-sputtering rate may contribute to the improved film properties in the edge regions. These results in this paper are important for broadening the industrial applications of high ion fraction plasma sources in irregular structures, especially for cutting tools.

Dynamics of bipolar HiPIMS discharges by plasma potential probe measurements

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 μs) 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.