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

Wednesday, April 22, 2015 1:30 PM in Room Sunset
Wednesday Afternoon

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1:30 PM Invited F2-2-1 Laser Diagnostics of Particle Dynamics in HiPIMS Plasmas
Catalin Vitelaru (National Institute for Optoelectronics-INOE 2000, Romania); Daniel Lundin (Université Paris-Sud 11, France); Vasile Tiron (Alexandru Ioan Cuza University, Romania); Nils Brenning (Royal Institute of Technology, Sweden); Gheorghe Popa (Alexandru Ioan Cuza University, Romania); Tiberiu Minea (Université Paris-Sud 11, France)

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.

2:10 PM F2-2-3 Correlation Between Ion Transport and Plasma Oscillations in DC and HiPIMS Discharges
Ante Hecimovic, Volker Schulz-von der Gathen, Jörg Winter, Achim von Keudell (Ruhr-Universität Bochum, Germany)

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.

2:30 PM F2-2-4 Dynamics and Potential Structure of Ionization Zones in Magnetron Discharges
Matjaž Panjan, Yuchen Yang, Jason Liu, André Anders (Lawrence Berkeley National Laboratory, USA)

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

[6] A. Anders, M. Panjan, R. Franz, J. Andersson, P. Ni, Appl. Phys. Lett., 103 (2013) 144103
2:50 PM F2-2-5 Plasma Characterization of Sputtered Aluminum with a MF Superimposed HIPIMS Process from Industrial Sized Rotatables
Holger Gerdes, Ralf Bandorf, Günter Bräuer (Fraunhofer Institute for Surface Engineering and Thin Films, Germany); Michael Mark (MELEC GmbH, Germany); Thomas Schütte (PLASUS, Germany)

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).

Different operating modes on the target and at resulting properties at the substrate were investigated. At the target site electric properties like voltage and current, as well as optical emission spectroscopy were investigated. The results will be correlated with the measurement of the ion and electron density, investigated by Langmuir probe. No significant reduction in deposition rate with respect to the applied average power to the cathode was observed, even by adding HIPIMS. For a moderate peak current density of up to 0.2 A/cm² significant modifications of the resulting electron and ion current was observed. While the MF process showed an electron density of 0.3*1017/m3, the superimposed process led to a density of 1.6*1017/m3. Similar behavior was observed for the ion density, increasing from 0.8*1017/m3 in MF operation to 3.8*1017/m3 with superimposed HIPIMS.
3:10 PM F2-2-6 Controlled Reactive High-power Impulse Magnetron Sputtering - Experiments and Modelling
Jaroslav Vlcek, Tomas Kozak, Jiri Rezek (University of West Bohemia, Czech Republic)

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).

3:30 PM F2-2-7 Analysis of Ion Energy Distribution at the Substrate during a HPPMS (Cr, Al)N Process using Energy Resolved Mass Spectrometer and Retarding Field Analyser
Kirsten Bobzin, Tobias Brögelmann, Ricardo Brugnara, Stephan Chromy (RWTH Aachen University, Germany)
The ion energy is known to have a strong influence on the properties of coatings deposited by physical vapor deposition (PVD). Therefore, the ion energy distribution (IEDF) specially measured at the substrate side is of great interest for understanding the coating growth. In PVD coating processes the ion energy at the substrate can be adjusted by applying a negative voltage (bias) to the substrate table. In the present work, mass integrated measurements of the IEDF were carried out during a high power pulse magnetron sputtering (HPPMS) (Cr,Al)N process using a Cr-target with 20 plugs of Al within an industrial scale PVD coating unit. The HPPMS cathode was operated with different average powers (1, 3 and 5 kW) and pulse lengths (40 and 200 µs) at constant frequency of 500 Hz. In a first step, measurements of the IEDF using mass spectrometer (MS) and retarding field energy analyser (RFEA) were carried out. By comparing the measured IEDFs with these two different methods some differences had been found. At high peak power, i. e. short pulse duration and high average power, a much greater high energetic fraction in the IEDF between 10 and 50 eV were found in the results of the RFEA in comparison to the MS measurements. However, the effective ion temperatures between 10 and 50 eV calculated relatively from RFEA and MS measurements show a slight difference of a factor of approx. 1.3. In a second step, measurements were done using biased RFEA with a substrate bias of -100 V. The comparison of the IEDF measurements with and without substrate bias using the RFEA shows a significant change in the shape of the IEDF. Using a substrate bias the IEDF showed a nearly gaussian shape, while without a substrate bias the shape of the IEDF is asymmetric and in the high energetic range of 10 to 50 eV it showed a maxwellian distribution.
3:50 PM F2-2-8 Electronic Properties Correlated to Vacancy Model in Nickel Oxide Thin Films Deposited by Reactive HiPIMS Discharge
Julien Keraudy (IRT Jules Verne, France); Axel Ferrec, Antoine Goullet, Pierre-Yves Jouan (IMN Jean Rouxel, France)

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 EF-EV 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)

4:10 PM F2-2-9 Investigation of Plasma Conditions and Film Growth during Reactive HiPIMS of HfO2 Films
Amber Reed (Air Force Research Laboratory and University of Dayton, USA); Jianjun Hu (University of Dayton Research Institute and Air Force Research Laboratory, USA); Jennifer Wohlwend (UTC; Air Force Research Laboratory, USA); Rachel Naguy (Air Force Research Laboratory, USA); John Bultman (University of Dayton Research Institute; Air Force Research Laboratory, USA); Christopher Muratore (University of Dayton, USA); Patrick Shamberger (Texas A & M University, USA); Andrey Voevodin (Air Force Research Laboratory, USA)
Hafnium oxide thin films have been produced by reactive HiPIMS from a hafnium target in an oxygen-argon background. The effect of total pressure, oxygen partial pressure and target-substrate distance on film microstructure was investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Film microstructure was strongly dependent on total pressure, with pressures below 20 mTorr resulting in smooth (RMS roughness < 1.0 nm) amorphous films and higher pressures resulting in nanocrystalline films with the (-111) orientation parallel to the substrate surface. Growth of the HfO2 films in O2/Ar mixture was also sensitive to increases in O2 partial pressure. For oxygen/argon flow rates above 0.06, an onset of plasma instability was observed, likely caused by heating of the target due to oxide formation on its surface. This study explores the plasma conditions during deposition and their relationship to film growth. Time-resolved current measurements combined with energy-resolved mass spectroscopy and optical emission spectroscopy (OES) at the substrate are used to identify the flux of particles and their energies at the substrate surface. Time-resolved target current measurements and calculations of the heat of formations for HfO2 are applied to determine if the observed target heating is the result of runaway or surface chemical reaction.
4:30 PM F2-2-10 Magnetically Enhanced Hipims
Xiubo Tian, Junfeng Gao, Chunzhi Gong (Harbin Institute of Technology, China); Paul Chu (City University of Hong Kong, China)

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., 200oC.

4:50 PM F2-2-11 Plasma Pretreatment of Tungsten Carbide and Steels by High Power Impulse Magnetron Sputtering
Arutiun P. Ehiasarian, Anna Oniszczuk, Thomas Morton (Sheffield Hallam University, UK); Carl-Fredrik Carlstrom, Mats Ahlgren (Sandvik Coromant, Sweden)

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.

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