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

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.

High-power impulse magnetron sputtering with a pulsed O₂ flow control [1,2] was used for reactive depositions of stoichiometric ZrO₂ films with gradient ZrO_x 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 ZrO_x 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 ZrO_x 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+ZrO_x interlayer) films and that the adhesion of the ZrO₂ films is substantially higher when the gradient ZrO_x interlayers are used. The ZrO₂ films with the gradient ZrO_x interlayers exhibited an enhanced resistance to cracking, a lower compressive stress (0.8 GPa) and a high hardness (15-16 GPa).

References

[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)

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.

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.

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.

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

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.

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.