3.1. Reactors’ Parameters and Characteristics
In general, the vacuum chambers that are applied to the coating of tools and components are constantly evolving. However, the industry already presents a wide range of solutions in this field [32
]. The emergence and development of dedicated software that is easy and quick to use through remote control, have contributed to the technological evolution of PVD reactors [74
]. Manual labour has been replaced by technology because the main purpose is to make the equipment more autonomous and automatic. This will reduce maintenance and management costs, and, on the other hand, increase production by making the investment more profitable.
One of the great advantages of this type of reactors and technology is its ability to deposit a wide range of films into parts with complex geometries of different materials, making the process quite flexible. Loading and unloading workpieces is a simple task since access to the coating areas is extremely easy. Currently, the characteristics of the reactors contribute to its handling. In summary, the main characteristics and parameters of the reactors can be seen in Figure 5
It is essential to highlight the importance of the useful diameter of the vacuum chambers, because this parameter will limit the working pressures, and, consequently, the substrates size. The chamber diameter can vary between 400 and 850 mm [74
] but it can reach up to 2500 mm under customer request [74
]. Cycle time is relatively fast, for example for a deposition of 3 μm, usually, takes between 5–6 h. Regarding the number of satellites, this can reach twenty, being more common the use of six or ten satellites.
The rotation systems of the substrates are very important in an industrial context. Its efficiency can reduce costs and improve film properties, with the rotation speed being determinant in the deposition sequence of the layer. Studies have shown that this effect is reflected in the morphologic properties of the coated substrates [60
As described in Section 1
, in the last years, new pulsed techniques with many potentialities have emerged. The technique with the greatest impact on its development, taking into account all sputtering techniques, is undoubtedly the one using Magnetron. However, it can also be used Diode, Triode, Ion Beam, and Reactive Sputtering systems. The studies on the evolution of the sputtering magnetron technique in DC and RF contribute to the emergence of techniques, such as dual magnetron sputtering (DMS), reactive bipolar pulsed dual magnetron sputtering (BPDMS), modulated pulsed power magnetron sputtering (MPPMS), HiPIMS, dual anode sputtering (DAS), among others.
One of the cheaper power supplies with easy process control is the DC power supply and hence is the most used although the sputter yield is generally much lower [79
]. This makes the DC power source the most used in magnetron or pulsed systems. Its major disadvantage is the low rate of ionization. Studies show that only about a fraction of 1% of the target species is sprayed ionized [17
]. The DC source only applies when the targets are made of conductive materials. On the other hand, the RF source is only applicable to the use of non-conductive or low conductivity targets. An alternative to using DC and RF sources is the Mid Frequency source (MF). To maintain the plasma in the sputtering process, an alternating high-frequency signal is applied, which allows the current to pass through the target, thus avoiding the accumulation of charges.
The dual magnetron sputtering (DMS) process uses MF power supply and it has been widely used for reactive deposition. It has become increasingly sophisticated, being usually used in systems for industrial applications using magnetron rotation [80
]. In particular, this method is characterized by a different composition of the targets and way as the film grows. Surface oxidation is one of the sensitive aspects of this technique. To counter oxidation, it is necessary to take into account parameters, such as reactive gas partial pressure, voltage, and sputter rate [80
]. Furthermore, in order to receive DC power and apply pulsed-DC power sources in the magnetrons, components need to be configured to switch power. This is possible while using a pulsed power supply [80
DAS is a technology that allows switching from the commonly used alternating current—mid frequency (AC-MF) mode to a DC power process to reduce the heat load on the substrate.
The BPDMS technique is followed by the DMS technique. The interesting fact about this technique is that it also uses an MF source like DAS. Rizzo et al. [82
] used in their study an MF band of 80–350 kHz. Using this technique, it was possible to prevent arc formation and its results showed a high deposition rate of around 0.044 μm min−1
using ZrN coating.
The deposition rate is always the focus of improvement when one thinks to upgrade a reactor for industrial purposes. The need of the industry thus obliges it, and, in that sense, in the last years, studies have been conducted also following industrial needs. Some recent investigations have been focused on the increase of spray ionization, on process stability, on new segmented targets, on gas flow optimization of different gases, on the bias influence, on obtaining better absorbers, among others. However, in order to obtain low-cost absorbers as compared to industrial techniques, a laboratory-tested sputtering unit was tested and the results pointed out that the deposition rates were low [83
]. On the other hand, in studies regarding the gas flow in sprayed zirconia coatings on flat substrates, the deposition rate results reached 20 μm h−1
, which represents a good deposition rate. Other studies regarding the influence of bias voltage and gas flow showed that the temperature increase in the substrate and the application of a bias voltage resulted in a decreased deposition rate. For example, having a substrate temperature of 650 °C and applying a bias voltage at −10 V, it is possible to obtain a deposition rate of 20 μm h−1
. However, the deposition rate is reduced to 5 μm h−1
if −20 V bias voltage is applied [84
Another approach in an industrial context using the reactor CemeCon®
CC800/9 is the study of Weirather et al. [86
]. They used the Reactive Pulsed DC magnetron sputtering technique with triangle-like segmented targets. That work contributed to show the potential of this technique in an industrial context, reducing the costs in thin film deposition. Cr1−x
N (0.21 ≤ x
≤ 0.74) was used as a coating material, having obtained low friction values of 0.4 and wear coefficients up to 1.8 × 10−16
, in order to obtain good results regarding the tribological properties. The maximum hardness obtained was 25.2 GPa, which proved to be a good result.
A study carried out regarding the plastics industry compared conventional DC, MF pulsed and HiPIMS techniques considering the deposition rate and coating’s hardness. It is noteworthy that the complex geometry of the injection moulding tools was an additional challenge in this study, taking into account the three technologies that were used. For the three different technologies, five different targets configurations were used, varying the chemical composition of the (Cr1−x
)N coatings. The HiPIMS technique provided the best results for aluminium deposition rate, which was reflected in an increase from 1.32 to 1.67 μm h−1
. In this case, the deposition rates of DC and MF coatings decrease from about 2.45 to 1.30 μm h−1
. On the other hand, chromium deposition rate presented the worst results for the HiPIMS technique as compared to DC and MF ones. The morphology, surface, and roughness that were obtained by the HiPIMS technique showed almost constant coatings behaviour [87
HiPIMS technology allows for combining technologies, such as cathodic arc plasma deposition and ion plating, with this being its greatest advantage [88
]. Although this type of reactor appeared in the 1990s, with the evolution of sputtering magnetron technology, just in recent years it has known more interest in its improvement and in exploring its potentialities. Since then, it has been used in the improvement of the spray ionization through the pulsed power that influences the plasma conditions and the coating’s properties. When compared to conventional magnetron sputtering, the studies about this technique have shown significant improvements in coating structure, properties, and adhesion [89
]. On the other hand, the combination of HiPIMS and DC-Pulsed also shows evidence in improving adhesion and morphology while using TiSiN coating [91
]. Although versatile, it is necessary to have some care in the process and in the evaluation of results, given the difficulty in obtaining consistent and repeatable results [92
One variation of the HiPIMS is the power pulses method MPPMS. This technique uses a pulsed high peak target power density for a short period of time and creates high-density plasma with an elevated degree of ionization of the sputtered species [93
Deep oscillation magnetron sputtering (DOMS) is another variant of HiPIMS. A study that was carried out using this system led to seeking a relation between the ionization of the sputtered species and thin film properties [94
]. This investigation had the purpose of identifying the mechanisms which influence the shadowing effect in this technique. To effectively reduce the atomic shadows, it was necessary to accelerate the chromium ions in the substrate sheath in the DOMS, which reduces significantly the high angle component of its collision. A high degree of ionization allows the deposition of dense and compact films without the need for the bombardment of high-energy particles during the coating growing process.
Plasma enhanced magnetron sputtering (PEMS) is an advanced version of conventional DC magnetron sputtering (DCMS). In conventional MS, the discharge plasma is generated in front of the magnetrons, as can be seen in Figure 6
a. On the other hand, PEMS assisted deposition has the advantage of generating an independent plasma through impact ionization by accelerating electrons that are emitted from hot filaments in the chamber, which expands through the entire vacuum chamber, as shown in the illustration of Figure 6
b. Lin et al. [95
] carried out a comparative study between the techniques DCMS and HiPIMS with and without PEMS assistance regarding the deposition of TiSiCN nanocomposite coatings, concluding that PEMS assistance improves the microstructure and mechanical properties of coatings that are produced by DOMS or DCMS, as well as the reduction of residual stress.
The receptivity of the industry to the HiPIMS technique has been very positive bearing in mind the range of reactor power supply. Emerging technologies allow gains around the 30% in the ionization rate and higher charge states of the target ions. This high degree of ionization results in increased advantages of some coating properties, such as improved adhesion and the possibility of consistently covering surfaces with complex geometry [79
]. Thus, the scientific community has focused on the development of high power magnetic pulsed technologies, since these results are very interesting concerning the industrial context.
3.2. Improvements and Applications of External Devices
Studies show a great interest in using the HiPIMS technique due to its versatility in the production of the PVD coating. This technique has as a disadvantage the deposition rate, which is lower when compared with the conventional sputtering DC. This factor needs to be improved. Some studies have been developed around this concern, trying to overcome the above-mentioned problems by the use of external devices, such as magnetic fields, although the improvements have not been significant yet.
In order to increase the deposition rate of thin films and improve the performance of HiPIMS, Li et al. [97
] tested two different vacuum chamber approaches, using five different substrates positions: 0°, 45°, 90°, 135°, and 180°, based in the magnetron cathode in both studies. The first study was focused on the application of an external unbalanced magnetic field. This method indicates that, in the 0° angle substrate position, a substantially higher ion current in the substrate was reported. An increase in plasma density in the substrate region has occurred, showing that this method achieved the expected results. Following the first goal to increase the deposition rate, the second work focuses on more simplified and efficient ion discharge using external electric and magnetic fields with the auxiliary anode. To optimize the magnetic field distribution, the authors used a coaxial electromagnetic coil. This method allowed for a better distribution of the electric field and electric potential in the reactor, increased discharging, plasma intensification, and uni-directionality. The amplitude of the plasma density was five times greater in all positions when compared to the discharging without outer-field HiPIMS [98
]. Figure 7
shows the vacuum system during HiPIMS discharge, measuring the ionic current of the substrate in different positions regarding both studies.
Other examples using an auxiliary magnetic field as external device showed more ambitious results, presenting increased deposition rates between 40% and 140% when considering the inclusion of an external device with different types of targets that were chosen due to their relevance in technological applications. The results were compared with the HiPIMS process without the external device under similar experimental conditions (working gas pressure, average power). Figure 8
shows the configuration of the setup used. However, it is possible to improve the system, as described by the researchers that are involved in that work. It was shown a great potential for deposition improvement in HiPIMS through the control of the magnetic field and pulse configuration [99
Using magnetic fields, Ganesan et al. [100
] showed that it is possible to increase the deposition rate, guiding the ion flux in the direction of the substrate with the application of an external magnetic field using a solenoidal coil, excited with a DC current pulse. This is the scheme that is used in this study, as depicted in Figure 9
; it is possible to see in the centre of the chamber the additional solenoidal coil that provides an external magnetic field.
The study shows evidence of intensification in the ionization zone that increases the plasma extension and density, leading to an increase in the deposition rate through the combination of magnetic and reactor magnetron fields. This evidence can be interpreted in Figure 10
, where (a) represents a conventional situation of deposition by the generation of transporting ions in neutral (N) and ionized fluxes (I) plasmas, those are deposited on the substrate. The same happens on (b), although applying the magnetic field. The ionization zone will be extended, activating additional ions that will be directed to the substrate, increasing the deposition rate. The results show that an increased maximum peak current and/or in the power density corresponds to a significant improvement in the pulverized ions flow. It has further been found that an increase of about 25% in peak current is seen when a 150 A magnetic field at the start of the HiPIMS pulse is used, inducing a 25% increase in the rate of the target ion emission as compared to the case where no external magnetic field is applied [100
In order to improve Cu films, Wu et al. [101
] studied the utilization of a modified HiPIMS system using a positive kick voltage after an initial negative pulse, being possible to control the magnitude and the pulse width of the reverse pulse. This result is interpreted by a bipolar pulsed effect that was studied in detail in this type of deposition. Figure 11
shows the results that were obtained in the deposition of Cu films using three different kick pulses, comparing three different systems: DC magnetron sputtering (DCMS), conventional HiPIMS, and Bipolar Pulsed. It was found that the increase in the voltage amplitude and pulse width of the kick pulse can promote an increase of the deposition rate relatively to the conventional HiPIMS, but even so, the deposition rate that was achieved by the DCMS process showed to be higher. To conclude, the HiPIMS bipolar pulse shows great potential and this new approach can improve Cu film properties such as electronic conductivity and adhesion. However, in order to achieve deposition rates higher than DCMS, the substrates positioning needs to be planned in the centre of the reactor, where the deposition rate is more effective.
The combination of two or more vacuum chambers in improving the process and its efficiency should be considered. This approach can be seen in the recent work of Bras et al. [102
], which simulates an industrial in-line vacuum production of solar cells using as a deposit compound the copper indium gallium selenide (CIGS). The use of the sputtering technique in the industrial context applied in the production of solar cells was demonstrated. In this study, automated arms were used to load and unload the cells. The system presents a process sequence using two vacuum chambers and 25 cathodic spray stations, which has its own heating and helium cooling arrangement. Following the simplified process in Figure 12
, it can be seen: (a) in chamber A sputtering stations 1 to 5, where the substrate cleaning and absorber metal layers are carried out; (b) in the transition chamber A to B, the heating station increases the substrate temperature to improve deposition; (c) in chamber B, sputtering stations 6–18 promote the deposition of CIGS layers and the substrate is then rapidly cooled down in the intermediate chamber, and, finally, back to the chamber A; and, (d) sputtering stations 19–25 are producing the buffer layer in an oxygen-containing atmosphere. To finalize the cells, the addition of resistive transparent conductive oxide (TCP) bilayers is needed. The efficiency was demonstrated for cells with a total area of 1 cm2
and 225 cm2
with values of 15.1% and 13.2%, respectively.
3.3. Considerations Using CFD Simulation
To study the phenomena that occur in the PVD method, numerical simulation models are usually used. These simulation methods help to solve complex engineering problems in scientific and/or industrial contexts. The most common numerical approaches are finite elements methods (FEM) and computational fluid dynamics (CFD). FEM studies are commonly centred on mechanical properties and CFD is usually focused on process concerns. Initially, studies have been focused mainly on material properties but now the trend is to study the PVD reactor in an industrial context using the simulation, avoiding the cost of stopping the equipment [103
]. However, the use of numerical methods allows obtaining only an approximate but not exact solution. It is also necessary to have a critical analysis of the results that were obtained through the models because their approximation can introduce errors. Comparing the results that were obtained by the models with experimental results is desirable [105
]. FEM helps in studying the phenomenon related to the substrate and coating on their mechanical properties, such as strength, brittleness, adhesion, and performance, among others [31
]. CFD is typically focused on the study of fluid flow dynamics, anodic chamber performance prediction, thermal evaluation in a reactor’s design, input temperature, the velocity of distribution of the species into the reactor, pressure, and others [109
]. The quality of the coatings that were obtained in commercial PVD processes is of great importance and therefore its optimization. Thus, it is necessary to take into account the discharge characteristics to know the motion of the neutral gas flow inside the reactor chamber.
Monte Carlo method (DSMC) models are a class of computational algorithms that provide approximate solutions and are widely used in research of thin films [105
Bobzin et al. [28
], used direct simulation Monte Carlo method (DSMC) models in CFD analysis to characterized and it assess gas behaviour in the PVD coating process using the Knudsen number (Kn) by means of different approaches: for Kn ≤ 0.1 the gas flow is described by the Navier–Stokes equations and for Kn > 0.1 a kinetic approach was used by the Boltzmann equation. In order to validate the model, they used an argon neutral gas flow and molecular nitrogen gas in an industrial scale reactor CemeCon 800/9®
typically used for DC-MS and HiPIMS processes. Considering the developed CFD model, they conclude that it presents limitations in the transition flow regime. To accurately predict flow characteristics, only the kinetic model should be considered. The benefits of each model and the comparison between them were studied and showed that the advances in simulation lead to a detailed analysis on the PVD processes of the formation of coatings that are capable of complying with industrial requirements.
Kapopara et al. [29
], predicted the gases concentration and distribution (argon and nitrogen), density profiles, velocity profiles, and pressure profiles across the sputtering chamber. They conclude that the locations of both gas inlet port and substrate have a crucial influence on the gas distribution inside the chamber. With this study, it was possible to propose a modification of the reactor geometry for a better gas flow over the substrate. This research showed that the CFD simulation has a great potential and its influence is growing over the time on the PVD reactors studies. After the modulation phase, it allows varying parameters, being a strong advantage in an industrial context due to the capacity of predicting the final results regarding the different phenomena that occur in the reactor during the deposition process. The main goal is a reduction in the production time, with a consequent reduction in costs, maintaining the quality standards.
] also used the DSMC to study neutral gas simulation on the influence of rotating spokes on gas rarefaction in HiPIMS. This different approach helps to understand the gas dynamics in the harsh discharge condition. It was concluded that the influence of a rotating plasma ionization zone is limited by a segmented time-modulated sputtering inlet distribution [115
To conclude, the CFD modelling has been carried out to analyze gas flow and its mixing behaviour within the chamber reactor. However, for a better approximation of a real situation, it is important to use different models and compare them. The models defined and studied are important on the advance of geometry and parameters changes in the reactors that can be simulated, also taking into account external devices, in order to improve the process.