ICMCTF2015 Session F2-2: High Power Impulse Magnetron Sputtering (HiPIMS)

The continuous development of new versions of the magnetron process is pushing towards novel technological applications, also bringing forward the need of fundamental understanding. The High Power Impulse Magnetron Sputtering (HiPIMS) is gathering great interest in the last years. The HiPIMS specificity resides in its temporal behavior, on a time scale of nanoseconds for electrons instabilities (‘spokes’), ranging from microsecond to hundreds of microseconds for heavy species, during the high power pulse, and extending to milliseconds in the afterglow phase. The temporal dimension of the process requires time resolved measurements to get an insight on the undergoing processes.

Laser spectroscopy is generally used to diagnostic the density, the temperature or the velocity distribution function of investigated species. Its formidable effectiveness comes from the narrowness of this radiation and the ability to deliver the exact energy required for probing the desired transition, usually implying the ground state or metastables. The main types of laser sources used so far for this purpose are tunable diode lasers, optical parametric oscillators (OPO-s) and dye lasers.

The investigation of inhomogeneous time evolving discharges, like the HiPIMS, requires a different approach compared to continuous and/or homogeneous plasmas, and specific techniques have been adapted for it.

This contribution focuses on the main developed methods to perform time and space resolved measurements by laser based diagnostics, using different types of laser sources, pointing out the main advantages and weaknesses of each technique illustrated by typical results. Hence, the temporal and spatial behavior of the most relevant species is revealed, e.g. metal atoms and ions, inert or reactive gas atoms, etc. It is shown that the high dynamics of density and temperature of metal atoms is mainly related to transport phenomena, occurring mostly in the afterglow phase. The dynamics of metal ions is related to atom transport and ionization phenomena in the discharge volume. The density and temperature of gas metastable atoms are governed by the production and loss mechanisms through electron impact and to gas rarefaction during the pulse. The main focus involves the processes occurring during the high power pulse and immediately after it. As for the reactive species it is shown that both volume and surface phenomena play an important role, with different weights in different discharge regions.

Recent findings show that plasma oscillations are commonly found in magnetrons, regardless of the power supply and power levels obtained. We present a comprehensive investigation of the oscillation properties in terms of amplitude, frequency and rotation direction using the 12 flat probes, installed azimuthally around the circular magnetron. The investigated discharge conditions encompass both DC and HiPIMS discharges with current density ranging from 0.5mA/cm² to 5A/cm². The results exhibit a wide spectrum of frequencies ranging from 250 Hz to 200 kHz.

When the flat probes are negatively biased, the ion saturation current is collected and measured. The correlation between the observed oscillations and the ions transported away from the target allows establishing a qualitative understanding of the ion transport for wide range of discharge currents in DC and HiPIMS discharges. The results are compared with the Hall parameter, a measure commonly used to evaluate the cross-field transport, reducing from 16 in DC discharges to values of around 2 in HiPIMS discharge.

Organization of plasma into so-called ionization zones or spokes has been first observed in HiPIMS discharges [1-3]. Measurements of charge transport in continuously operated magnetron suggested that ionization zones also form in DCMS discharges [4]; indeed this was recently observed by imaging with high-speed camera [5]. DCMS discharges run at very low currents (few mA on 3 inch target) form a single ionization zone with elongated arrowhead shape that points in the direction of electron drift. The number of zones increases sequentially from two to three and more as discharge current and/or pressure are increased; in some cases multiple zones form symmetrical patterns.

In the present work we used two high-speed ICCD cameras triggered in sequence to study dynamics of ionization zones in DCMS discharges (limited studies were also performed for HiPIMS discharges). Images were correlated with probes positioned around the perimeter of magnetron's racetrack, which measured time-dependent floating potential and ion current. Analysis of images and probe measurements shows that the speed of ionization zone and the direction of their movement depends on the discharge current and gas pressure. At very low discharge currents ionization zones rotate in the direction opposite to the E×B electron drift. At particular discharge conditions almost stationary, standing wave plasma configuration forms. Only at very high discharge currents (several amperes) ionization zones move in the E×B direction as was previously reported for HiPIMS discharges.

In our previous work we suggested that ionization zones are regions of negative-positive-negative space charge distribution, which results in formation of potential hump and electric fields parallel to the target [6]. It is believed that traveling electric fields play important role in anomalous transport of charge across magnetic field lines [4, 6]. Probe measurements and images acquired in this work will therefore be discussed in the context of traveling potential hump model.

[1] A. Kozyrev, N. Sochugov, K. Oskomov, A. Zakharov, A. Odivanova, Plasma Phys. Rep., 37 (2011) 621

[2] A. Anders, P. Ni, A. Rauch, J. Appl. Phys., 111 (2012) 053304

[3] A.P. Ehiasarian, A. Hecimovic, T. de los Arcos, R. New, V. Schulz-von der Gathen, M. Boke, J. Winter, Appl. Phys. Lett., 100 (2012) 114101

[4] M. Panjan, R. Franz, A. Anders, Plasma Sources Sci. Technol., 23 (2014) 025007

[5] A. Anders, P. Ni, J. Andersson, IEEE T Plasma Sci., (2014) 1

High power impulse magnetron sputtering HIPIMS is often referred to suffer from low deposition rates. To compensate the rate loss, the parallel use of another target running with a conventional discharge is reported. Alternatively superposition of conventional and HIPIMS operation for one cathode is available. The question for such combination is: how much influence has the ionized part of the process on the plasma and film properties?

Within this presentation a novel approach for combining Mid-frequency and HIPIMS will be presented. The set-up consists of a dual rotatable (length 550 mm, equipped with aluminum) and a mid-frequency (MF) power generator connected to them. This MF generator is running in a bipolar mode. Furthermore two HIPIMS power supplies are connected to one of the rotatables and to a separated anode. These power supplies are running in a unipolar mode. This setup is giving a high deposition rate (MF-process) and can influence the growth of the film by supporting a higher flux of ionized particles to the substrate (HIPIMS process).

High-power impulse magnetron sputtering (HiPIMS) with a pulsed reactive gas (oxygen) flow control was used for high-rate reactive depositions of densified, highly optically transparent, stoichiometric zirconium dioxide films. The depositions were performed using a strongly unbalanced magnetron with a planar zirconium target of 100 mm diameter in argon-oxygen gas mixtures at the argon pressure of 2 Pa. The repetition frequency was 500 Hz at the deposition-averaged target power density from 5 Wcm^-2 to 53 Wcm^-2 with the duty cycles from 2.5% to 10%. Typical substrate temperatures were less than 130°C during the depositions of films on a floating substrate at the distance of 100 mm from the target. Usual deposition rates, being around 10 nm/min, were achieved for the target power density of 5 Wcm^-2. An optimized location of the oxygen gas inlets in front of the target and their orientation toward the substrate surface made it possible to improve quality of the films due to minimized arcing on the sputtered target and to enhance their deposition rates up to 120 nm/min for a deposition-averaged target power density of 52 Wcm^-2 and a voltage pulse duration of 200 µs[1,2].

To understand complicated processes during reactive HiPIMS of dielectric films, we have developed a parametric model. The model takes into account specific features of the HiPIMS discharges, namely gas rarefaction in front of the sputtered target, backward flux of the ionized sputtered metal atoms and reactive gas atoms onto the target, and high degree of dissociation of reactive gas molecules in the flux onto the target and substrate. Moreover, a local overfilling of the reactive gas in front of the reactive gas inlet is considered. The model makes it possible to calculate the time-dependent compositions of the compound layers on the target and substrate surfaces in a pulse period, and the number of metal atoms, forming the compound layers, which are deposited onto the substrate per second.

We used this model to clarify the experimental results achieved by us for the controlled reactive HiPIMS of zirconium dioxide films.

References

[1] J. Vlcek, J. Rezek, J. Houska, R. Cerstvy, R. Bugyi, Process stabilization and a significant enhancement of the deposition rate in reactive high-power impulse magnetron sputtering of ZrO₂ and Ta₂O₅ films, Surf. Coat. Technol.236 (2013) 550.

[2] J. Vlcek, J. Rezek, J. Houska, T. Kozák, J. Kohout, Benefits of the controlled reactive high-power impulse magnetron sputtering of dielectric oxide films, Vacuum(2014, submitted).

Nickel Oxide (NiO) is a p-type semiconductor which has recently attracted a great deal of attention. Indeed, despite his simple crystallographic structure, NiO have proven to be an attractive material for hot topics: 1) renewable energy as a holes extractor in solar cells; 2) new generation of nonvolatile resistive random access memory devices as a prototypical Mott Insulator and 3) gas sensor as a result of his excellent chemical response. Among the numerous methods to synthesize NiO films, reactive magnetron sputtering discharge is considered to be the most widely used. Recently, D. T. NGuyen et al [1] have showed the feasibility to synthesized NiO films with HiPIMS. Based on Ellipsometry and XPS characterization, the authors show that by varying the pulse duration, they can control precisely the stoichiometry and the opto-electronic properties of the films. However, despite the large number of research focus on this material, no investigations have been reported on the correlation between plasma and films properties.

The first part of this talk is dedicated to detect precisely the transition from metallic to oxide regime. Surprisingly, by using optical emission spectroscopy (OES) and EDX/XPS measurements, we have estimated four distinct regions (plasma composition and chemical composition of the film) reported by the oxidation curve measured with time-integrated voltage waveforms. This first result proves the good synergy between plasma diagnostic and thin film characterization. In a second part, influence of oxygen partial pressure on the electrical properties of the as-deposited materials was studied. Based on XPS and Ellipsometry measurements, we have well reconstructed the band diagram of all films. Indeed, by increasing the O₂ ratio, we have observed a decrease of the E_F-E_V distance (from 0.92 eV to 0.68 eV) which leads to an improvement of the p-type behavior, confirmed by C-AFM measurements when the collected current increase exponentially with O₂ partial pressure. Elevation of electrical conductivity with the O₂ ratio was explained using vacancy model. Increase in the nickel vacancy results in an increase in the number Ni³⁺ ions (confirmed by XPS) and the hole concentration of the nickel oxide films. Moreover, higher amount of nickel vacancy with O₂ ratio was also confirmed by the presence of magnetic anomalies in NiO thin film deposited at higher oxygen content. NiO thin films (28%) exhibit a weak ferromagnetic behavior which is attributed to the presence of Ni³⁺ within the NiO lattice [2].

[1] D.T. Nguyen et al, Surf & Coat Technol 250 (2014) 21-25

[2] I. Sugiyama, Nature Nanotechnology 8, 266–270 (2013)

High power impulse magnetron sputtering (Hipims) has proven to be an effective tool to obtain smooth functional films. Hipims is characterized by high plasma density induced by pulsed higher power applied to the target in magnetron sputtering. A Hipims process may be substantially enhanced by external magnetic field. In our Hipims system, a coil is equipped around the magnetron target to induce strong EXB effect. The substrate current may be increased by a factor of 2 or more if a proper current flows through the coil, accompanied by an intensified glow discharge. TiAl target is utilized for the deposition of AlTiN. The external magnetic field leads to a thicker film and smooth surface. This is an interesting and inspiring result. In conventional Hipims processes, a decreased deposition rate is often complained by the industrial users. The critical loads of 50N for AlTiN and 70N for TiN are easily achieved using this set up, even at a low processing temperature, e.g., 200^oC.

Coated cutting tools are used for the majority of today's manufacturing operations. In a given cutting operation, the adhesion of the coating to the substrate is directly related to the lifetime of tools. Adhesion is commonly enhanced by the use of gaseous plasma to preclean the substrate and present a surface free of oxides for the growth of the coating. Metal plasmas are often more efficient due to the shallow implantation of metal into the substrate which enhances the wettability of the surface during nucleation of coatings of the same material.

The effects of metal ion implantation on the depth and chemistry of the interface and the microstructure of the surface are not sufficiently understood due to the relatively constrained parameter space available from conventional metal ion sources.

In this experiment tungsten carbide (WC), high speed steel and stainless steel were treated in the environment of a High Power Impulse Magnetron Sputtering plasma. The plasma chemistry was evaluated quantitatively by a combination of optical emission spectroscopy and plasma-sampling energy-resolved mass spectroscopy. Ion fluxes and deposition rates were measured simultaneously to obtain ion-to-neutral ratios. The measurements confirmed a strong rarefaction of the gas and indicated that rarefaction of the metal species may take place as well. Both single- and double-charged metal ions were detected. No significant delay between the gas and metal plasma was observed within a pulse.

The plasma diagnostics results were used as input to modelling calculations of penetration depth and chemistry near the substrate surface. Metal ions were found to penetrate approximately 4 nm into the WC substrate. The maximum implanted content of metal was found to increase as plasma became metal ion dominated and the metal ionisation degree increased.

Surface roughness of polished substrates increased due to the pretreatment as observed by atomic force microscopy, whereas as-received surfaces showed negligible differences. The etching removed preferentially smaller grains leaving behind a stronger substrate. Grain boundaries were also preferentially etched and the waviness factor was used to quantify the difference between samples. The etching rates corresponded to the ion flux to the substrate.

The mechanisms linking the plasma chemistry, surface chemistry and the adhesion of the coatings are discussed. Optimal parameters for improved adhesion are determined.