ICMCTF2016 Session F2: High Power Impulse Magnetron Sputtering (HiPIMS)

Thursday, April 28, 2016 8:00 AM in Room Royal Palm 1-3

Thursday Morning

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8:00 AM F2-1 Reactive High-Power Impulse Magnetron Sputtering: Modeling and Applications
Tomas Kozak, Jaroslav Vlcek (University of West Bohemia, Czech Republic)

Reactive magnetron sputtering has become a standard process for deposition of oxide and nitride films. In the last years, high-power impulse magnetron sputtering (HiPIMS) has been used to prepare various optically transparent non-conductive metal oxides with a higher film density, index of refraction and lower surface roughness. However, there are still substantial problems with arcing on the poisoned target, especially at high target power densities, and with low deposition rates. Recently, we have been able to perform stable reactive HiPIMS depositions of highly optically transparent and densified ZrO2, Ta2O5 and HfO2 films using a pulsed reactive gas flow control and an optimized reactive gas inlet. Moreover, we achieved significantly higher deposition rates compared to typical continuous dc magnetron depositions.

Due to the large number of process input parameters and the complexity of discharge processes involved, reactive HiPIMS is not fully understood theoretically. We have, therefore, developed a parametric model of the reactive HiPIMS process which allows us to understand the relationships between the measured discharge and deposition characteristics and the processes on the target and substrate surfaces. The model simulates the time evolution of the target and substrate composition during individual HiPIMS pulses and on longer time scales. Specific features of the HiPIMS discharges, such as gas rarefaction and ionization in front of the sputtered target, high dissociation of the reactive gas molecules in the discharge and backward flux of the ionized sputtered metal and reactive gas atoms onto the target are included.

We simulated depositions of ZrO2 films under conditions ranging from continuous dc sputtering to HiPIMS with the average target power density of 2 kWcm-2 in a pulse and compared the simulation results with our experiments. The model predicts significantly lower compound coverage of the target in HiPIMS regimes than in the continuous dc sputtering regime at the same partial pressure of oxygen which results in an increased deposition rate. Not only the increased target current density in the pulse, but also the change in the internal discharge parameters must be taken into account to explain the experimental results. We show that to achieve stoichiometric films on the substrate, the increased dissociation of oxygen in a HiPIMS discharge is desirable. The limits of the model and challenges in reactive HiPIMS modeling will be discussed.

8:40 AM F2-3 Property Evaluation of CrSiN Coatings Fabricated by a Superimposed High Power Impulse Magnetron Sputtering System
Chih-Yuan Cheng, Jyh-Wei Lee (Ming Chi University of Technology, Taiwan, Republic of China)
High power impulse magnetron sputtering (HIPIMS) technique has been developed for more than 15 years. It is characterized by its ultra-high peak current and peak power density to obtain unique thin film properties, such as high hardness, good adhesion and tribological performance. However, its low deposition rate makes it hard to be applied in industries. In this work, a superimposed high power impulse magnetron sputtering system was used to deposit CrSiN thin films on Si wafer and AISI420 stainless steel substrates. The duty cycles of superimposed HIPIMS power were adjusted to evaluate its effect on the deposition rate, microstructure and mechanical properties of coating. The phase of each coating was studied by means of the X-ray diffractometer. The microstructures of thin films were examined by the field-emission scanning electron microscopy. Atomic force microscopy was used to characterize the surface morphology. The nanoindentation and scratch tests were used to evaluate the hardness and adhesion properties of thin films, respectively. It was found that the deposition rate increased greatly due to the proper parameter adjustment of superimposed HIPIMS. Effects of duty cycles of superimposed HIPIMS system on the microstructure, hardness and adhesion properties of CrSiN coatings were discussed in this work.
9:00 AM F2-4 Reactive High-power Impulse Magnetron Sputtering of ZrO2 Films with Gradient ZrOx Interlayers on Pretreated Steel Substrates
Alexandr Belosludtsev, Jaroslav Vlcek, Jiri Houska, Stanislav Haviar, Radomir Cerstvy, Jiri Rezek (University of West Bohemia, Czech Republic)

High-power impulse magnetron sputtering with a pulsed O2 flow control [1,2] was used for reactive depositions of stoichiometric ZrO2 films with gradient ZrOx interlayers onto floating Si, glass and steel substrates at low substrate temperatures (less than 150 °C). The depositions were performed using a strongly unbalanced magnetron with a planar Zr target of 100 mm diameter in Ar+O2 gas mixtures at the total pressure close to 2 Pa. The repetition frequency was 500 Hz at the average target power density of about 37 Wcm-2 during a deposition of the ZrO2 films and in the range from 30 Wcm-2 to 37 Wcm-2 during a deposition of the gradient ZrOx interlayers with a controlled increase in x from 0 to 2. The voltage pulse duration was 200 μs (duty cycle of 10 %). Two kinds of the gradient ZrOx interlayers with different depth profiles of x were deposited using the feed-back pulsed O2 flow control. Prior to deposition, a modification of the substrate surfaces (etching, shallow implantation and ion mixing) was performed by pulsed magnetron sputtering of the Zr target in Ar gas at the same pressure of 2 Pa, a voltage pulse duration of 50 μs, a peak target power density of 220 Wcm-2 in a pulse, a dc substrate bias from 965 V to 620 V in a pulse and the substrate temperatures less than 150 °C for 10 min to enhance adhesion of the zirconium oxide films to steel substrates. It was shown that the pretreatment of the steel substrates is the necessary condition for the adhesion of the zirconium oxide (both pure ZrO2 and ZrO2+ZrOx interlayer) films and that the adhesion of the ZrO2 films is substantially higher when the gradient ZrOx interlayers are used. The ZrO2 films with the gradient ZrOx interlayers exhibited an enhanced resistance to cracking, a lower compressive stress (0.8 GPa) and a high hardness (15-16 GPa).


[1] J. Vlcek, J.Rezek, J.Houska, T. Kozak, J. Kohout, Benefits of the controlled reactive high-power impulse magnetron sputtering of stoichiometric ZrO2 films, Vacuum 114 (2015) 131

[2] J. Vlcek, A. Belosludtsev, J. Rezek, J. Houska, J. Capek, R. Cerstvy, S. Haviar, High-rate reactive high-power impulse magnetron sputtering of hard and optically transparent HfO2 films, Surf. Coat. Technol. (2015, doi:10.1016/j.surfcoat.2015.08.024)

9:20 AM F2-5 Target Poisoning in Mixed Ar, N2 and CH4 Atmosphere in HIPIMS and DC Magnetron Sputtering Modes
AnnaWiktoria Oniszczuk (Sheffield Hallam University, UK); Carl-Fredrik Carlstrom, Mats Ahlgren (Sandvik Coromant R&D, Sweden); Arutiun Ehiasarian (Sheffield Hallam University, UK)

Reactive sputter deposition in a mixed Ar, N2, and CH4 atmosphere is a widely used industrial process for the production of metal carbonitride thin films. Target poisoning in this complex environment is insufficiently described. In this study we analyse the discharge parameters, partial pressures and plasma composition during poisoning and compare High Power Impulse Magnetron Sputtering and DC Magnetron Sputtering Discharges and TiAl and V targets.

Vanadium target poisoning resulted in a 2-fold increase in total pressure, a 50 % increase in discharge voltage/current ratio, a 5 fold drop in V I optical emission intensity and a 10 fold drop in V1+ and Ar1+ fluxes obtained from energy-resolved mass spectroscopy.

A step-wise target poisoning process was observed by plasma sampling mass spectroscopy and discharge voltage/current ratio analyses. In the initial stages, the target consumed preferentially carbon-containing radicals and developed coverage by a carbon-rich material. As the partial pressure of the reactive gas was increased, nitrogen radicals were absorbed building up nitrogen-rich material. The behaviour was independent of target material and sputtering mode.

TiAl targets in DC mode poisoned at lower reactive gas flows and exhibited narrower hysteresis than V due to the higher reactivity of the target material. The voltage/current ratio of TiAl targets went through a minimum with flow, while for V target it increased with flow.

For HIPIMS both targets poisoned earlier and the hysteresis was narrower than in DC mode. This was observed in trends of the partial pressure, the voltage/current ratio and ion fluxes of metals and reactive gasses. These effects are due to higher reactivity of the plasma as evidenced by higher fluxes of N1+ and N21+ ions and radicals containing H, C and N. DC and HIPIMS operation exhibited opposing behaviour in the voltage/current ratio during poisoning of the target.

Pathways for poisoning and resulting ion fluxes are discussed.

9:40 AM F2-6 The Origin of the Suppressed Hysteresis in HiPIMS: Experiment and Model
Jiří Čapek (University of West Bohemia, NTIS, Czech Republic); Stanislav Kadlec (HVM Plasma Ltd, Czech Republic)
The main issue that must be faced in reactive sputtering is the sustainment of the operational point during a deposition process. It is desirable to operate in the transition mode between the metal and the compound state of the target to obtain high deposition rates and optimum film properties. However, this goal is frequently difficult to reach due to abrupt changes in the discharge conditions caused by the hysteresis effect commonly observed during reactive deposition.

Reduced or even eliminated hysteresis effect have been reported in reactive high power impulse magnetron sputtering (HiPIMS), as compared to other sputtering techniques utilizing a low instantaneous target power density (e.g., direct current or pulsed mid-frequency magnetron sputtering) operated at the same average discharge power. However, the understanding of the phenomenon is still inadequate.

In the present work, we systematically investigate the hysteresis behavior of a discharge above a pair of Ti targets in Ar + O2 and Ar + N2 working atmospheres during bipolar dual magnetron HiPIMS. When the pulsing conditions have been gradually changed from mid-frequency operation to HiPIMS at a constant average target power density in a period of 7.6 W cm-2, narrower or even disappearing jumps in the pressure and voltage have been observed with HiPIMS. The negative slope of reactive gas sorption as a function of the reactive gas partial pressure is clearly lower in the case of HiPIMS resulting in a lower critical pumping speed which implies disappearing hysteresis.

Moreover, we present a model of reactive HiPIMS combining the Berg – type model of reactive sputtering and the global HiPIMS model of Christie offering deeper understanding of the investigated phenomenon. The model shows that the most important effect explaining the experimental data is the return of the ionized metal to the target and resulting covering of the reacted target parts by a more metallic surface. This effectively lowers the target coverage by the reaction product at a given partial pressure. The model also predicts some interesting and technically relevant effects. When the sputtering yield of the compound from the target is relatively high (factor > 0.2 to sputtering yield of metal, e.g. TiN to Ti), the deposition rate of reactive HiPIMS is always lower than in mid–frequency reactive sputtering for the same composition of the deposited film. In contrast, when the compound sputtering yield is much lower (factor < 0.10), a higher deposition rate of reactive HiPIMS is predicted in the transition regime compared to mid-frequency reactive sputtering for some interval of deposited film composition.

10:00 AM F2-7 Feedback Control of Reactive High Power Impulse Magnetron Sputtering by Measuring the Pulse Peak Current and Adjusting the Pulse Frequency
Tetsuhide Shimizu (Tokyo Metropolitan University, Japan); MichelleMarie Villamayor, RommelPaulo Viloan (Linköping University, Sweden); Daniel Lundin (CNRS, Université Paris-Sud, France); Ulf Helmersson (Linköping University, Sweden)
The present study proposes a simple and cost effective strategy to stabilize the sputtering process in the transition zone during reactive high-power impulse magnetron sputtering (HiPIMS). As an input parameter the peak current value, in the pulse, was used from which a feedback control system was implemented that automatically regulates the pulse frequency, and thereby the average sputtering power, to maintain a given pulse peak current. That this is an effective way for controlling reactive HiPIMS were demonstrated for Hf sputtered in Ar/N2 and for Ti sputtered in Ar/O2 gas mixtures. For these materials systems it was show that the discharge current waveform is an excellent indicator of variation in target surface state from compound to metallic mode. By activating the reactive HiPIMS peak current regulation mode, oxygen partial pressure could be maintained constant when varying the O2 flows from 0.35 to 0.6 sccm, under conditions where a strong hysteresis was seen not using the regulation. In addition, both Hf–N and Ti-O films deposited using peak current regulation exhibit a stable stoichiometry, crystal structure and a nearly constant power-normalized deposition rate over the entire reactive gas flow range investigated. The physical reasons for the change in the current pulse waveform for different process conditions for metal nitrides and oxides are discussed.
10:20 AM F2-8 Silicon Substrate Measurement In High-Power Impulse Magnetron Sputtering
Takayuki Ohta, Katsuhiro Hattori (Meijo University, Japan); Akinori Oda (Chiba Institute of Technology, Japan); Hiroyuki Kousaka (Nagoya University, Japan)
In high-power impulse magnetron sputtering (HiPIMS), short-pulse high-voltage is applied to the target and realizes high density plasma to promote the ionization of sputtered atoms. The substrate during plasma processes is heated due to the energy influx by the collision of charged species, the chemical reaction with neutral species, and the heat radiation. The substrate temperature influences the film quality, the heating mechanism of the substrate in HiPIMS has not been reported in detail.We have reported the substrate temperature measurement method using optical low-coherence interferometry, which is non-contact and high- speed method. The substrate temperature was calculated from the optical thickness of the substrate which is the peak interval of interference signals generated by the reflected lights from top and bottom surfaces of the substrate. In this study, we measured the silicon substrate temperature using optical low-coherence interfero- metry in HiPIMS and discuss the heating mechanisms of the substrate.

substrate temperature measurement system was equipped with HiPIMS chamber. The target material was Ti and the distance between the substrate and the target was 60mm. Ar is used as the sputtering gas, and the flow rate was 10sccm. The operating pressure was 1Pa, applied voltage was -500V, the pulse frequency was 500Hz. The pulse widths were changed to be 50µsec, 100µsec and 300µsec.

Temporal variation of silicon substrate temperature was measured. The silicon substrate temperature increased quickly after the plasma was turned on. The silicon substrate temperature decreased just after the plasma turned off. In addition, the silicon substrate temperature increased with increasing the pulse width, and the temperature for the pulse width 300µsec showed the highest value. The total power applied to the target increases with increasing the pulse width under same pulse frequency due to increase in the duty ratio. Thus, the ionization of sputtered species was promoted, resulting that many ions collide to the substrate and heat it.

10:40 AM F2-9 Magnetron Sputtering: Illuminating Physics of Spokes in Continuous and Pulsed Discharges
Matjaž Panjan (Jožef Stefan Institute, Slovenia); André Anders (Lawrence Berkeley National Laboratory, USA); Ludvik Martinu (Polytechnique Montreal, Canada)

Magnetron sputtering discharges have been generally regarded as azimuthally uniform plasma sources. Over the last few years this view has changed. Imaging by high-speed cameras revealed formation of dense plasma structures which were given name spokes or ionization zones. Depending on the discharge conditions the magnetron plasma produces periodic or quasi-periodic patterns with one or more spokes. In many cases spokes exhibit an arrowhead-like shape that points in the E×B direction. Spokes have been initially observed in pulsed discharges (i.e., HiPIMS) [1-3] and later on also in continuously run discharges (i.e., DCMS) [4,5]. In general, spokes in DCMS have a longer azimuthal length, whereas in HiPIMS they are more numerous and azimuthally shorter. Dynamics of spokes is complex and it strongly varies with discharge conditions (i.e., pressure and discharge current) but there is a general difference in the motion of spokes in low- and high-current discharges. In a low-current DCMS discharges spokes move in the -E×B direction, while in a high-current DCMS or HiPIMS discharges they move in the E×B direction [6].

In this talk we will review our understanding of the spokes phenomenon. We will examine processes which govern formation, sustainability, organization and dynamics of the spokes. It will be shown that the presence of spokes significantly alters spatial distribution of the plasma potential and, as a consequence, causes azimuthal dependence of processes associated with the sustainability of the discharge, including: electron-gas collisions, secondary electron emission and sputtering. Properties of spokes and pattern formation will therefore be discussed with respect to these azimuthally non-homogeneous processes. Understanding the spoke phenomenon is not only important for the fundamental plasma research but also for the deposition of thin film. We will show that spokes play an important role in the transport of electrons and ions in the magnetron discharges [7]. Better understanding of the charge transport, particularly of metal ions, should therefore be beneficial for various applications where microstructural control of thin film is essential.

[1] A. Kozyrev et al., Plasma Physics Reports 37 (2011) 621

[2] A. Anders et al., J. Appl. Phys. 111 (2012) 053304

[3] A.P. Ehiasarian et al., Appl. Phys. Lett. 100 (2012) 114101

[4] A. Anders et al., IEEE T. Plasma. Sci. 99 (2014) 1

[5] M. Panjan et al., Plasma Sources Sci. Technol. 23 24 (2015) 065010

[6] Y. Yang et al., Appl. Phys. Lett. 105 (2014) 254101

[7] M. Panjan et al., Plasma Sources Sci. Technol. 23 (2014) 025007

11:20 AM F2-11 HIPIMS - Stable Reactive Process With High Deposition Rate
Wojciech Gajewski, Mateusz Baran, Piotr Rozanski, Andrzej Klimczak (TRUMPF Huettinger Sp. z o. o., Poland); Ludwik Zajac (Warsaw University of Technology, Poland); Pawel Lesiuk (TRUMPF Huettinger Sp. z o. o., Poland)

Due to the high ionization efficiency of the sputtered target material the High Power Impulse Magnetron Sputtering (HiPIMS) technology is winning interest and acceptance for industrial scale production of films with superior microstructure and properties. Of greatest importance for coating production are the reactive processes, which are, however, facing few challenges such as system specific hysteresis effect or lower deposition rate compared to DC magnetron sputtering.

This paper demonstrates the application of a specially designed control method based on the Ultra Fast Digital Processing cooperating with a new water-cooled HiPIMS power supply. Such solution allows to surmount the low deposition rate problem as well as has a fundamental importance for short and long-time process stability. It will be also shown, that efficient arc detection and suppression in the HiPIMS power supply ensures stable operation and repeatability of coating properties produced with enormous high instantaneous power density. Finally, the mechanical and optical properties of functional oxide coatings will be analyzed to emphasize the benefits of the developed control method and its ability to significantly increase deposition rates in HiPIMS sputtering processes.

11:40 AM F2-12 HiPIMS/DCMS Hybrid Co-sputtering of CNx Thin Films using Heavy Metal Ion Bombardment
Konstantinos Bakoglidis, Susann Schmidt, Lars Hultman, Grzegorz Greczynski (Linköping University, IFM, Thin Film Physics Division, Sweden)
Carbon nitride (CNx) is an interesting member of the family of diamond-like carbon (DLC) materials. CNx films are already used as protective low-friction layers in disk drives and their tribological response, with the high elasticity and wear resistance as well as low friction, make them good candidates also for use in other demanding applications. However, for substrates such as martensitic steels or high-density polymers, the growth temperatures are limited to not more than ~ 150 °C. Low CNx growth temperatures aid a featureless morphology of the coatings. Under such conditions, CN0.16 films with a density of 2.3 g/cm3, and a hardness of 13 GPa were produced. In order to densify the CNx structure further and thus enhance its film properties, we investigated a new way to fabricate dense CNx films, avoiding high substrate temperatures, by employing heavy metal ion bombardment during CNx growth. In this study, Tantalum (Ta) and graphite targets were used in reactive hybrid co-sputter processes, where the Ta target was sputtered in high power impulse magnetron sputtering (HiPIMS) mode, to serve as a source of Ta+/Ta2+ ion irradiation, while two graphite targets were simultaneously operated in direct current magnetron sputtering (DCMS) mode. CNx films were deposited in an industrial deposition chamber using a N2/Ar flow ratio of 0.16 and a substrate temperature of ~ 150 °C. The negative substrate bias voltage, Vs, synchronized with the metal portion of the HiPIMS pulse, was varied from floating potential (~ 20 V) up to 250 V, in order to manipulate energy and momentum transfer to the growing film surface. The Ta content in all Ta-CNx films was 8 at%. X-ray photoelectron spectroscopy of the Ta-CNx films revealed TaN bond formation, owing to lower heat of formation of TaN than that of TaC. The reference CNx film presented N-C bonds, assigned to sp2 hybridized N (pyridine-like structure), and substitutional bonded N in a 3-fold coordinated C matrix. A decrease of the substitutional bonded N to 3-fold coordinated C atoms was observed upon Ta+/Ta2+ irradiation, while the pyridine-like structures of the Ta-CNx films remained unaffected. The hardness of our reference CNx film increased from 9 GPa to 11 GPa when Ta+/Ta2+ irradiation was used, at Vs = 20 V. With increasing Vs, thus increasing energy and momentum of incident metal ions, the hardness of the Ta-CNx films exhibits an increase from 11 GPa with Vs = 20 V to 16 GPa with Vs = 250 V. This effect was attributed to an increased film density, as confirmed by X-ray reflectivity.
12:00 PM F2-13 Plasma Spokes in Reactive High Power Impulse Magnetron Sputtering
Ante Hecimovic, Carles Corbella Roca (Ruhr-Universität Bochum, Germany); Volker Schulz-von der Gathen, Achim von Keudell (Ruhr Universität Bochum, Germany)

Periodic plasma emission patterns observed in HiPIMS, referred to as spokes or ionisation zones, have been extensively investigated in a non reactive HiPIMS discharges. The spokes rotate in direction of ExB drift, and the mechanism driving the spokes formation has been explained by the localised generation of secondary electrons (SE) believed to be responsible for observed anomalous cross B field transport. The target poising occurring when reactive gas is introduced strongly affects the sputter yield and the secondary electron sputter yield. We present the evolution of the spokes along the hysteresis curve during a reactive HiPIMS discharge for different target material/reactive gas combinations.

This work is funded by the DFG within the framework of the SFB-TR 87.

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