The experimental approach into the influence of external inductance on the discharge characteristic of HiPIMS
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The main objective of the current paper is to describe the effect of external inductance (EI) on the current discharge waveforms of HiPIMS at different pulse-on time (Pon) and its relation with static deposition rate and topographical properties of deposited titanium thin films, which is investigated by scanning electron microscope and atomic force microscope. It has shown that the higher the EI, independent of the Pon, the higher the peak power is. The delay time also extensively increases when an EI is implemented into the circuit. However, the rise time does not have a linear dependency with the EI and its behavior changes to some extent at different Pon. By increasing the EI from zero to 30 mH at Pon = 60 μs, the peak power subsequently rises from 11 to 32 kW at constant time-average power. Meanwhile, the deposition rate decreases from 8.5 to 1.5 nm/min, which is mainly attributed to the metal ions return to the target surface and nonlinear dependency of sputtering yield with applied voltage. It was also revealed that the higher peak power has no special effect on the surface roughness of titanium thin films deposited by HiPIMS.
KeywordsDeposition rate External inductance HiPIMS Surface roughness
Magnetron sputtering (MS) is one of the most common methods to fabricate a variety of thin films with specific properties appropriate for industrial applications [1, 2, 3, 4, 5, 6, 7]. Unbalanced magnetron sputtering (UMS) and closed field unbalanced magnetron sputtering (CFUMS) are a step forward of classical MS in which they provide higher deposition rate, denser and more spacious plasma expanded to the substrate while the quality of thin films is also enhanced [6, 8, 9]. The enhancement of thin films quality is strongly connected to the ratio of the ion to atom transported into the substrate [10, 11], in which they not only have enough kinetic energy to be located on the thermodynamically lower energy place but also they can kick off loosely bonded atoms from the substrate surface, providing the negative bias voltage is applied. Although an advantage of such mentioned methods is to lack micron-size droplets [12, 13, 14], which is prevalent in cathodic arc physical vapor deposition (CAPVD), the deposition rate and the amount of ionization rate are inferior to that of CAPVD method. Much research has been carried out to improve the ionization rate of plasma and the deposition rate of coatings in the MS system [15, 16, 17, 18, 19, 20]. High-power impulse magnetron sputtering (HIPIMS) has recently attracted much interest owing to its capability to deposit droplet-free coatings with a high degree of ionization in the plasma that is desirable for some films [21, 22]. The high ionization is of interest because it gives rise to dense coatings as well as good adhesion on the substrate . Therefore, many researchers worldwide have been motivated to fully exploit the features of this method [24, 25, 26, 27, 28].
The current waveforms in HiPIMS are a fingerprint of the system, indicating the ionization rate of Ar, gas sputtering, self-sputtering, metal ionization, waves, and instabilities, and it has been covered in the literature under different conditions [29, 30, 31, 32]. For instance, in Mclain’s work  who studied the deposition rate of HiPIMS with respect to the arrangement of magnets, the shape of the current waveforms of HiPIMS, correspondingly with the plasma confinement nearby the cathode, has altered from runaway to plateau, resulting in distinct deposition rate. In another work by Zuo et al. , the current waveforms for a variety of target materials are discussed in which all have some humplike shape with distinct peak current that is related to the secondary electron emission yield. In another paper , the target current at different reactive gas pressures is considered, demonstrating the higher the gas pressure, the higher the peak current reaches. The correlation between the peak current in HiPIMS with the microstructure of coatings was explored by Alami et al. ; when the high peak current is achieved, the density and surface roughness of deposited films changed, whereas the deposition rate tremendously reduced. Additionally, the crystallinity of the films decreased so that no XRD peaks were finally detected. They reasoned that the high ionization rate in higher peak current leads to more ions reaching to the substrate, changing the films morphology. This is also reported by  that the ion-to-atom ratio near the substrate increased around 3 times when the peak current changed from 113 to 185 A. The current waveforms for different target materials showed that the peak current value and the current plateau are corresponding with the rarefaction (or sputter wind) and ionization rate of metals, respectively . The comparison of the current waveforms of HiPIMS and 1.0 A-DC-superimposed HiPIMS is indicative of the higher peak current in HiPIMS while the slope of the current exceeds for DC-HiPIMS . Revel et al.  have theoretically and experimentally investigated the effect of an external circuit with a given resistance on the current waveforms of HiPIMS. They observed that the lower external series resistor leads to the higher current plateau.
Despite the many studies on the features of current waveforms, the effect of an EI on the properties of current waveforms of HiPIMS has not been explored. Therefore, in this work, the influence of an EI on the current waveforms and static deposition rate and topographical properties of deposited titanium is addressed.
The experiments were carried out in a home-made vacuum deposition system that is described elsewhere . The HiPIMS system was also designed and fabricated in our laboratory. The power supply of HiPIMS has capabilities to produce pulses with Pon and frequency in the range of 20–500 µs and 100–1500 Hz, respectively. A DC power supply (1–1000 V and 1 A) was used to support the HiPIMS power, in which the maximum target voltage and current could increase up to 1000 V and 100 A, respectively.
The parameters for titanium thin films deposition and evolution of current waveforms
Target to substrate distance
− 100 V
5 × 10−5 Torr
Duty cycle (%)
3, 5, 7.5, 10, 15
The voltage was measured at two points: point A, which includes the summation of inductor and magnetron sputtering and point B, which includes only magnetron sputtering. One should note that the average power is calculated based on the voltage of point A, assuming the energy loss in the inductor is negligible. The deposition parameters of titanium thin films and those relating to extracting evolution of current waveforms data are also listed in Table 1. The topography of deposited thin films was evaluated by atomic force microscopy (AFM) in contact mode (park scientific instrument-CP auto probe). Field emission scanning electron microscope (FESEM) was also used to measure the film thickness so that the static deposition rate  is calculated.
Results and discussion
The rise time graph (Fig. 5c) exhibits that the longer the Pon, the longer the rise time is. However, the variation of rising time with EI is not well defined. Figure 5d shows that the increase of EI and Pon leads to longer delay time. When the EI is stronger, it absorbs more energy to allow the current passes through and consequently, the delay time becomes longer. As the applied voltage is lower for the higher Pon, the time for core saturation of inductor is longer and the delay time is extended.
Thin films properties
In this section, the deposition rate and surface roughness of Ti deposited layers are discussed.
When the higher EI was implemented into the HiPIMS circuit or the lower pulse-on time (Pon) was set, the peak power and the peak current increased. Note that the time-average power remained constant in all experiments and that is the reason why the peak current increased when the duration of pulse decreased. Furthermore, when the higher EI is used, the more extended delay time appears, and it means that the real on time of the pulse decreased, leading to a higher peak current and power.
When the value of EI increased from 0 to 30 mH, the peak power went up from 11 to 32 kW, whereas the deposition rate decreases from 8.5 to 1.5 nm/min which it stems from the higher return of metal ions into the cathode and the nonlinear dependency of sputtering yield of target with the voltage.
The variation of EI did not influence the surface topography that could be related to the high amount of ionization in HiPIMS regardless of EI; i.e., the large ratio of ion to atom in HiPIMS leads to the ions with sufficient energy transported to the substrate surface and overcoming the possible shadowing effect. Hence, the more increasing of ion to atom ratio does not change relatively the surface roughness.
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