This work reports a systematic review of the studies of magnetron sputtering (MS) discharges and their utilities for the deposition of transparent coating oxide thin films like indium tin oxides (ITOs). It collates the overall information of plasma science, diagnostics, and chemistry and their usefulness in controlling the plasma process, film growth, and properties. It discusses studies on various MS systems and their capabilities and reports scientific aspects like the formation of instability and plasma flares to understand the various discharge phenomena. The study also discusses various issues, progress, and challenges in ITO films for industrial applications. In addition, this work highlights the importance of plasma parameters and energy flux on thin film growth and film properties.
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As discussed earlier, the study of aspects of plasma generation has been an enigmatic area for the industry. A plasma state can be achieved by applying a sufficiently high electric field to a gas or mixture of gases. 126&#;129 The MS device consists of two electrodes, the cathode and the anode. Electric power (rate of change of energy) in the form of DC, 109&#;112,121,123,125 pulsed DC, 102,115 or radio frequency (RF) power 105,129 is applied through the cathode to create an MS plasma process. Based on the nature of applied electric power, the MS devices are categorized as direct current (DC), 116,120,122 pulsed DC, 115 mid-frequency (MF), 113 and radio frequency (RF). 118,121,122,124,125 The progress in the study of MS devices led to the development of the high-power impulse magnetron sputtering (HIPIMS) 130 process. Based on the geometry and orientation of the cathodes, the design of MS devices is known as conventional MS (CMS) or planar MS, 99,108 dual MS (DMS), 102,113 facing target MS (FTMS), 105 cylindrical MS (CYMS), 107 and three-dimensional MS (3DMS). 99&#;101,103,104,106,107 Furthermore, MS devices utilize a closed magnetic field geometry. The MS devices are divided into balanced and unbalanced configurations based on the formation of magnetic fields. 129 Due to the presence of a magnetic field configuration, a high-density plasma can be generated close to the cathode, which is the target material to be sputtered for the deposition of the thin film. 130,131 The MS device provides advanced material processes with desired plasma control for the efficient energy transport required to grow the thin film. 131 Therefore, for the deposition of thin films using MS devices, it is necessary to study both plasma science and plasma chemistry, and the understanding of the plasma process is not straightforward due to the complexities. Additionally, different categories of studies involve the design and study of magnetron sputtering sources, magnetron discharges operated by different (DC, pulsed DC, RF, and HIPIMS) powers, plasma diagnostics, and the deposition of wide varieties of films for different applications. However, the present paper provides an integrated aspect of designing magnetron sources, magnetron discharges, and the formation of ionization zone/plasma flares; the importance of plasma diagnostics in plasma processes; and the synergy between plasma science and film growth and control of film properties. What follows are overviews of the overall aspects of MS devices and processes, which are crucial for thin film applications. What follows are overviews of the overall aspects of MS devices and processes, which are crucial for thin film applications.
The MS system deposits the thin films as a plasma processing unit, a processor, or a reactor. 126 The reactor is known as a plasma generation device or plasma source. 126&#;128 The processor generates an output by applying a combination of several inputs. The inputs include different parameters like electrical power, gas pressure, external bias, and thermal energy. The output becomes a thin film as the end product, which depends on the internal environment. The internal environment represents plasma, which is specified by the plasma parameters, neutrals, radicals, and light radiation. Plasma is thus an ionized gas that contains electrons, ions, radicals, neutral atoms, and radiation. The study on the mechanism of plasma generation and plasma parameters like electron density (n e ), electron temperature (T e ), plasma potential (V p ), and electron energy distribution function (EEDF) defines the physics of the processing reactor. 127&#;129 The generation of radicals and charged species using various electron impact reactions like excitation, dissociation, and ionization specifies the plasma chemistry of the processor. 126 Therefore, the entire plasma process for thin film deposition is quite complex because it goes through several stages before reaching the output. 102,126 The first step in a plasma-based process is to provide gas/precursor input to the plasma processor. 126 Following that, the gases, or neutrals, break down under the action of an electric field, which is one of the external parameters used to generate plasma inside the processor. The generated plasma results in various plasma physics and plasma chemistry phenomena, which can be useful in various applications. 126&#;129 Therefore, it is imperative to study what is happening inside the reactor, what is the ability/capacity to produce plasma and radicals, how it is making plasma or what is the mechanism, and how to know and measure various parameters of plasma, what are the different possible reactions that may occur during the plasma generation, and how it can be helpful for an intended application. Therefore, following the plasma generation, as a next step, it is necessary to perform plasma and radical diagnostics to determine plasma parameters and optimal conditions for the plasma process for intended applications. Careful design and approach are used so that the diagnostic must not disturb the plasma environment. Furthermore, understanding plasma physics is required to interpret the diagnostic results. 126&#;129 Therefore, studies involving both plasma physics and chemistry are required to comprehend the mechanism of plasma and radical generation, which will aid in achieving the best conditions for plasma processing. 126
The magnetron sputtering (MS) method, a plasma-based technique, has been extensively used for the deposition of a wide variety of thin films. 1&#;3 Particularly, transparent conductive oxide (TCO) 4 films like tin-doped indium oxide (ITO), Al or Ga-doped zinc oxides (AZO or GZO), F-doped SnO 2 (FTO), and zinc oxide (ZnO) have been recognized as materials with excellent optical and electrical properties for many such applications. Among these films, ITO films have been shown to produce very low resistivity or high conductivity along with a very high optical transmittance even at relatively lower film thickness. 3 With increasing demands in display devices and flat panel technologies, the deposition of thin films on flexible substrates (regarded as flexible films) has attracted significant attention in these fields. 1&#;3 The plasma-assisted MS process has become a good choice to develop flexible ITO films for industry applications. 3 Although several other methods like thermal evaporation, 5&#;10 electron beam (e-beam) evaporation, 11&#;30 molecular beam epitaxy (MBE), 31&#;36 activated reactive evaporation (RE), 37&#;42 pulsed laser deposition (PLD), 43&#;70 atomic layer epitaxy (ALE), 71&#;75 sol&#;gel, 76&#;95 chemical solution process, 96 and spray pyrolysis 97,98 have deposited ITO thin films, the film produced by MS instead has high purity, transmittance, and conductivity, making it more durable, reproducible, efficient, and appealing for use in industries. 99&#;125
Apart from considerable motivation for basic studies on MS plasmas, numerous investigations have focused on the aspect of plasma chemistry for intended applications. 100,108,130&#;132,189&#;193 The plasma parameters crucially affect the thin film&#;s growth, microstructure, and other properties. 189,190 Additionally, to understand the film growth and control the microstructure, studying the energy flux deposited on the substrate using a calorimetric probe is also essential. 100,131,189&#;194 During the thin film deposition, various processes in the plasmas include ionization, excitation, de-excitation, and dissociation. 189,190,192&#;197 These electron impact processes define the plasma chemistry that can control the film's growth and properties. 189,193,194
Numerous diagnostic methods have been used to understand the different scientific aspects. Various types of probe-based diagnostics, such as the LP, 136,138,142,148,165&#;174 B-dot probe, 175&#;179 and Hall sensor, 180 have been used to study the plasma parameters. Emissive probe diagnostic has been used in HIPIMS plasmas to study plasma potential distribution. 140 Sometimes, due to the presence of a strong magnetic field near the target region, it is not easy to measure plasma parameters using probe-based diagnostics. 144 To avoid this issue, some optical-based diagnostics, such as optical emission spectroscopy (OES), 181&#;186 optical absorption spectroscopy (OAS), 187,188 etc., have been used. The OES technique is a non-invasive, passive method based on recording light emitted due to the transition of excited species from a higher excited level to a lower level in plasma. 108 As the diagnostic methods are not straightforward, the interaction of the diagnostic probe with the plasma without disturbing the plasma behavior needs an understanding of the physics involved in the diagnostic methods. 126 In addition, there are recent and advanced studies on the excitation and ionization mechanisms of the ionization zones produced in HIPIMS plasmas using spectroscopic imaging. 143
Additionally, probing MS plasmas via electric probes like Langmuir probes (LP) or current probes showed oscillatory signals riding on the DC level, indicating the signatures of different kinds of instabilities. 144 Many studies have reported low and high-frequency electric fluctuations in MS discharges. 106,140,143&#;151 The key motivation for such studies is that researchers propose that collision cannot be the sole the reason for electron transport across magnetic fields. 152&#;164 Such an inference has resulted from the observation that the plasma potential (V p ) fall often occurs near the cathode. 106 On the other hand, theoretical models involving collisional transport 143,162 propose a regime of anode potential fall. In fusion devices, such electrostatics fluctuations induced by unstable modes and drift waves are responsible for the higher cross-field charge transport 161,163 and the degradation of plasma confinement. Additionally, several recent studies on instabilities have been based on HIPIMS and high DC power, which showed distinct plasma non-uniformities as the ionization zone spoke 152&#;159 and plasma flare 160,162,163 rotated along the magnetron racetrack due to the E × B 0 drift.
As discussed earlier, understanding the science of plasma generation and charge (particle) confinement is essential for designing and developing plasma devices called plasma sources. 109,113,115,125&#;131 Particularly, the MS devices use the configuration of a suitable magnetic field for trapping electrons above the target (or cathode) that allows for their operation at low pressure compared to that of an unmagnetized plasma. 131,132 Normally, the electrons have very small Larmor radii compared to those of ions, and ions are unmagnetized due to their larger radius of gyration about the field lines. 132 The magnetically trapped electrons cause the ionization (creation of both ions and electrons) near the target in the plasma; ions in plasmas accelerate into the target by the applied bias to cause the sputter etching or sputtering of the cathode atoms. An equal number of electrons (or electron flux) should reach the anode to maintain the discharge with the ion flux to the cathode. Therefore, electrons need to escape the magnetic fields to move to the anode. Numerous experiments in DC, pulse MS, 133&#;141 and HIPIMS 130,142,143 reported studies involving electron transport and the mechanism of charge particle transport.
Consequently, there is a need to study the plasma sources, plasma processes, and thin films deposited using these sources using both in situ plasma diagnostics and ex situ film analysis relevant to plasma applications. However, the present work only focuses on the MS processes and deposition of ITO films using the MS method. An overview of MS processes is discussed below.
Despite considerable advancement in various fields of plasma applications using plasma sources, there is still a need for large-area plasma sources for developing processes over large areas. 209&#;213 The other requirements in these processes are plasma and film uniformity. The design of advanced plasma sources and processes can also significantly influence plasma parameters such as electron temperature and plasma density. Furthermore, control over the plasma chemistry in such sources would be beneficial by monitoring and controlling plasma parameters and radicals. 189,214,215
As discussed earlier, plasma devices or sources specify the plasma physics and associated plasma chemistry. 126,129,189,195&#;197 Different plasma applications necessitate various types of plasma sources, and the applications need the design of plasma processes. Plasma sources operated at low operational frequencies, such as DC 109,121,123,125 and pulsed DC, 102,115 were used in physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD) processes for industrial applications. In addition, mid-frequency (MF), RF, VHF, UHF, and microwave were used in PVD, PECVD, and plasma etching applications. 99&#;102,113,125,148,198&#;201 Another category using low-frequency power is used for the plasma polymer process known as plasma polymerization on flexible substrates. Plasma polymer processes offer low temperatures and the high deposition rate conditions required for industries. 202&#;204 Another-type of plasma process based on pulsed plasmas that has provided widespread applications in several industries like medical, agricultural, and food is atmospheric pressure plasma (APP). 205&#;208
MS is gaining market share in various application areas that were previously dominated by the other PVD processes. Among the various applications of MS, interest in the area of microelectronics and flat panel displays using sputter deposition of thin films has grown firmly over the last few years. 1,102,213 The dedicated application in various areas necessarily involves a thorough understanding of the basic mechanism of magnetron sputtering. 131,208 MS is a physical vapor deposition (PVD) method, and it involves a cathode and anode as the two electrodes, as shown in Fig. 1 Figure 1 shows that the cathode is the negatively biased target material, and the substrate is placed at the anode. A suitable magnetic field configuration is incorporated near the target surface using a set of permanent magnets. 216 A balanced or unbalanced magnetron configuration is created based on the magnet/magnetic field design. 217,218 Sputtering is fundamentally a momentum transfer process, often referred to as atomic pool billiards, in which incident ions (cue balls) break up the close-packed rack of atoms (billiard balls). 131,208,216&#;218 A high voltage is applied between the cathode (sputtering target) and the anode (sometimes a chamber or wall that acts as the electrical ground) to ionize the gas in the plasma. An electron in the background of cosmic radiation gains energy from the applied electric field (E = applied bias voltage/distance) to start the ionization process (as Ar + e = Ar+ e + e) and maintain the plasma. 126 Under the influence of the applied electric potential, positive (Ar) ions are accelerated toward the negatively biased cathode, resulting in high energy collisions with the target surface as shown in Fig. 1 . Therefore, the ion bombardment etched the target atoms or the target material from the surface of the target; the process is called sputtering, and the method is familiar as MS. 1,126,130,131 The sputtered atoms/materials coming out of the target possess enough kinetic energy (gained from ions via collisions) to move toward the surface of the substrate. The generation/presence of dense plasma between the cathode target and substrate results in a highly ionized flux of the sputtered species. 129,219 The gas used for the MS is typically of a high molecular weight, such as argon or xenon, and plasma in the background allows for as many high-energy collisions as possible, which leads to higher deposition rates. 129 Additionally, secondary electrons that are generated from the ion bombardment on the target aid in sustaining the plasma in the chamber. 1,126,130,131,220 This whole MS process is characterized by a crucial parameter known as the sputter yield (S), 221 which is defined aswhere S is a measure of the efficiency of sputtering that depends on the energy of incident ions and a number of ions (the specified plasma parameter is ion density or plasma density) impinge on the target. Experimental values of S range from 10&#;10. However, for thin films, S has a narrow range of 0.1&#;10. 220,221
The magnetic field applied near the cathode target typically varies in the range of &#;2 to 40 mT. 129 Because of the incorporation of the magnetic field, secondary electrons are trapped in the cathode vicinity, 222 and they do not bombard the substrate, resulting in low substrate heating. As a result, the MS discharges are appropriate for deposition on heat-sensitive substrates. In addition, the unbalanced magnetic field configuration (with a weaker magnet at the center of the cathode than that of the external strong magnets) provides better confinement than the balanced configuration (with magnets of similar field strengths placed at both the center and outside). In an unbalanced magnetron configuration, plasma tends to form uniformity and extend up to the substrate region. MS deposition does not require melting or evaporation of the source material, which provides significant advantages over other PVD technologies like thermal evaporation. 1,129,131 A wide range of materials can be deposited regardless of melting temperature. MS methods offer several advantages, such as good film quality, step coverage, and conformity in depositing substrates with complex shapes. 223 Apart from ionizing the sputtered atoms and controlling the plasma properties, the ion energy at the substrate can be controlled with the help of biasing the substrate and a directional deposition or collimation of these ions with the plasma sheath adjacent to the substrate surface. 104,130,189 Self-ion bombardment of the growing film causes it to crystallize more. 224 However, there are various issues involved with the MS.
Additionally, the deposited pattern on the substrate depends on the working gas pressure and substrate position with respect to the target, known as the target-to-substrate distance (&#; ts ). Waits reported 231 an off-axis peak in the film thickness for &#; ts &#; r rt (racetrack radius) and an on-axis thick film for &#; ts &#; r rt . Furthermore, the optimum condition for film uniformity was obtained when the ratio r rt /&#; ts = 4/3. 231 In addition, for the ratio r rt /&#; ts = 4/3, the optimum condition for film uniformity was obtained. 231 Target utilization efficiency, deposition uniformity, and film properties are particularly interesting in industrial applications. 232 For a single cathode target, achieving a uniform coating on a larger substrate is difficult. To address this issue, industrial MS frequently consists of multiple long rectangular targets surrounded by a rotating substrate holder to achieve uniform deposition. 233 In addition, highly energetic particles or ions bombarded the growing film, as shown in Fig. 2 , can degrade the substrate and films. Figure 2 suggests that ions in the plasma can severely impart the substrate due to their larger mass compared to the electron. In this sense, the control of ion energy is highly desired by shaping the plasma through the design of a magnetic field. A high-density plasma up to the target surface could be extended by the unbalanced MS. Over several decades, different kinds of MS systems have been designed and used based on the issues of target utility, plasma and film uniformity over a larger area, and energy control of plasma species. These aspects are discussed below.
One of the key disadvantages of the magnetron sputtering system is that the plasma is not uniform over the target surface as it is inherently a nonuniform deposition source. Due to the magnetic confinement of the electrons, the plasma is localized and concentrated into a torus-shaped region in front of the target. This region has the highest plasma density; consequently, high ion current density and maximum ion bombardment of the target occur in this region. 225 The ion current density reaches its maximum at the radius where the magnetic field is tangent to the cathode surface. This leads to the formation of an erosion groove on the target surface, also known as racetrack formation. 226 The erosion groove will be helpful in determining the target utilization and efficiency of material usage. 227 In the case of conventional planar magnetron sputtering, 20%&#;30% of the target gets utilized. 226,227 Iseki has reported that the issue of low target efficiency in planar magnetron sputtering can be resolved with the help of rotating magnet assemblies. 228,229 The target utilization of up to 80% has been achieved using an asymmetric yoke magnet structure in the planar magnetron. 230
The information in Table I suggests different MS systems have different capabilities. However, these are all geometrical variations of the same principle: magnetically confined electrons near the cathode target. The MS technique, due to its various configurations, can be applied to a wide range of materials that can be deposited on a variety of substrates in various forms and shapes, and it is desired to achieve scalability in very large areas. However, the energy control of the ions and neutrals sometimes poses a major issue for substrates and films for large-area deposition. Positive ions (PI) gain direct kinetic energy from the discharge voltage (V) or cathode in MS devices and sputter the target materials. As a result, these high-energy ions from the target have the potential to strike the depositing films. This concern can be alleviated by confining these energetic ions by enforcing strong magnetic confinement near the target. 101 Because of the closed 3D geometry, the 3DMS (Three-Dimensional Magnetron Sputtering) configuration can provide better confinement than the CMS configuration. 106 The use of a 3DMS meets most of the requirements for fabricating high-quality thin films, including the generation of high plasma density at low electron temperature and plasma potential, lower discharge voltages with high current, and a low process temperature. 101 The standard rectangular magnet polarity can be arranged to generate a closed and cusped magnetic field configuration. 106 , Figure 3(f) presents a schematic representation of a 3-DMS system. The magnetron consists of several rectangular targets. The front and back sides of the system do not appear in the side view of Fig. 3(f) . The two rectangular-facing targets are placed with an N&#;S configuration of magnets. The fifth target with an S&#;N&#;S magnet configuration can be placed at the top [ Fig. 3(f) ]. The top target has an unbalanced magnetron, which assists in the presence of a magnetic field in the substrate regime. 99 The presence of a magnetic field near the substrate enables a high-density plasma near the film-growing substrate ( Table I ). 99&#;109
In contrast, the electric field E is radial, and the E × B drift paths go around the cylinder, either inside or outside, depending on the configuration. The cathode is an elongated source of the film-forming material to be deposited in the cylindrical post-cathode magnetron sputter device and provides uniform sputtering from the entire cathode target surface. Except for the hollow cathode magnetron (HCM) sputtering configuration, which is well suited for coating wires or fibers that can pass through it, these cylindrical configurations are rarely used at present.
Figures 3(d) and 3(e) represent two cylindrical magnetron sputtering (CYMS) systems with magnetic fields aligned in their axis (axial direction). 107,129 Figure 3(d) shows the cylindrical post-cathode configuration. Much of the early MS research focused on cylindrical magnetrons with an axial magnetic field. 241 These discharges were either cylindrical post configurations [ Fig. 3(d) ], with the inner cylinder serving as the cathode target, or cylindrical hollow configurations [ Fig. 3(e) ], with the outer cylinder serving as the cathode target. 242 The discharge is formed between two coaxial cylindrical surfaces in the post-cathode configuration, where the inner cylinder is the cathode and the outer cylinder is the anode. 129,243 The magnetic field in this configuration is normally quasi-one-dimensional and oriented parallel to the axis of the cylinder. As a result, magnetic field B is axial in the cylindrical configuration.
The FTMS system 105 shown in Fig. 3(c) utilizes two magnetrons facing each other at some distance. Permanent magnets were installed behind the water-cooled cathode targets to generate proper magnetic field lines that enter and leave the target surface perpendicularly. 105 The magnetrons can be either rectangular or circular. The electrons, which are tightly held by the Lorentz force, spiral with the helix pitch diameter, which is constantly changing toward the facing target. 211 When the two targets have the same negative bias voltages, the velocity component of the electron perpendicular to the magnetic field will slow down in the vicinity of the opposite target's dark space and then speed up in the opposite direction. As a result of the combined effect of the electric and gradient magnetic fields, electrons oscillate in the space between the two targets. This facing target configuration gives electrons a longer path length and more chances of colliding with gas atoms, and plasma will form in the space between the targets. 245 Due to facing magnetrons and closed magnetic field geometry, the issue of direct ion bombardment on the substrate is not severe in this case. In addition, relatively larger area targets with high target efficiency ( Table I ) can be used for moderate area deposition. FTS has many advantages over other magnetron sputtering techniques, including high sputtering efficiency, the deposition of films with low defect states, and a smooth surface. 246
Figure 3(b) represents a dual MS (DMS) 102,113 system, which utilizes two PMS systems. The targets are aligned in an inclined position to create a confined magnetic field. Due to the nature of the design, it provides a better (moderate) confinement than that of CMS, which leads to a moderate plasma density (10 9 &#;10 11 cm &#;3 in Table I ) in MS processes. One can realize the following two possible configurations: (i) if both magnetrons have the same polarity, that arrangement is called the mirror B field, and (ii) magnetrons differ by the polarity of magnets, which is called the closed B field. In &#;mirror B field&#; configurations, the discharge of magnetrons is repelled toward each other, although in &#;closed B field&#; configurations, discharges are mutually coupled and plasma is well confined near both cathodes' vicinity. 244 The deposition rate increases dramatically in a dual magnetron arrangement powered by two supplies. 113
Different kinds of magnetron configurations are used for the MS systems. Note that Figs. 1 and 2 discussed earlier are familiar as conventional magnetron sputtering (CMS) or planar magnetron sputtering (PMS). 99,108 These systems utilize only one target. The magnetic field can be either balanced or unbalanced for these systems. Figures 3(b) &#; 3(f) present a schematic of the other kinds of MS systems. In addition, Table I presents salient information on various MS systems for ease of understanding. The data, as presented in Table I , are collected from various works of literature. 99,101&#;108,113,129,238&#;240
(a) E × B 0 configuration of an MS system. In addition, shown in the figure is the visualization of E × B 0 drift and cross-field flux, (b) DMS system with two magnetrons aligned at an inclined angle to the vertical axis, (c) FTMS system showing two facing magnetrons with their targets positioned parallel to each other, (d) cylindrical MS system with post-cathode configuration, (e) MS system with cylindrical hollow configurations, and (f) 3-DMS system with inverted &#;U&#; shaped structure with a hollow cathode configuration.
(a) E × B 0 configuration of an MS system. In addition, shown in the figure is the visualization of E × B 0 drift and cross-field flux, (b) DMS system with two magnetrons aligned at an inclined angle to the vertical axis, (c) FTMS system showing two facing magnetrons with their targets positioned parallel to each other, (d) cylindrical MS system with post-cathode configuration, (e) MS system with cylindrical hollow configurations, and (f) 3-DMS system with inverted &#;U&#; shaped structure with a hollow cathode configuration.
Figure 3(a) presents the basic principles of the MS system. The figure suggests that positive ions (or positive charge) will be accelerated toward the cathode as the cathode is negatively biased. Therefore, the electric field ( E ) is in the direction perpendicular to the target. Simultaneously, the electric field is aligned perpendicular to the magnetic field (B 0 ) (represented by magnetic lines of forces in Figs. 1 and 2 ), shown by the dots in Fig. 3(a) above the target. Therefore, the electric field (E) and the magnetic field (B 0 ) vectors create an E × B 0 drift, as shown in Fig. 3(a) . This E × B 0 drift creates a Hall flux to guide the charged species to follow its direction. 234 The E × B 0 drift motion is azimuthal above the cathode target surface, producing azimuthal current density (Hall current/Hall flux) and discharge current density produced by the cross-field flux. Therefore, the MS system is an E × B 0 device producing current. As discussed earlier in subsection A (Sec. I ), due to the continuous flow acceleration of ions toward the biased cathode (target), an equal number of electrons would be expected to reach the anode. However, as electrons are magnetized due to their very small Larmor radii, plasma evolves with various instabilities to be discussed later. Consequently, electrons can travel across the magnetic field lines, which is known as cross-field transport. 234&#;237
In the early s, the planar magnetron sputtering discharge was introduced, 226 and this configuration is now well established for the sputter deposition of thin films of both metallic and compound materials. The magnetron assembly is the core of the magnetron discharge, as shown in Figs. 1 and 2 . In the magnetron assembly, the cathode target is attached to an array of static magnets ( Figs. 1 and 2 ). The magnetron assembly must be designed to conduct heat away from the cathode target efficiently. The magnetron incorporates the water inlet and outlet ( Fig. 1 ) to prevent overheating. The target is mounted in good thermal contact with the backing plate by clamping or soldering. The magnets are arranged in such a way that a central magnet forms one pole, and a magnet ring or a ring of magnets placed along the target's edge forms the second pole. This configuration is used in laboratories where the cathode target is small and circular, usually 5&#;15 cm in diameter. For industrial applications, the cathode targets are linear and larger by up to a few tens of centimeters.
As discussed earlier, considerable work has been done using different kinds of MS methods. Even though the key goal of these MS devices is the deposition of thin films, significant efforts are being made to study the mechanism of plasma generation and the formation of instabilities in magnetron discharges. What follows is a brief review of various scientific studies on MS plasmas.
Toward the end of the twentieth century (s), interestingly, there were several reported works on high-power pulsed glow and magnetron discharges. 250,251 The key motivation for introducing pulsing in magnetron/glow discharge is having a natural tendency of &#;arcing.&#; 252 The arcing effect is associated with the high current and low discharge voltage and is characterized by the formation of a cathode spot on the target. This arcing can cause the emission of microscopic droplets and degrade the coating quality. 253 It was demonstrated in the s that using mid-frequency pulsed sputtering may considerably lower the probability of arc formation. 253,254 The approach is based on the strategy of using a short pulse (<50 µs) for mid-frequency sputtering. 254 In short pulses, the current pulses are approximately triangular in shape, and the current has not reached the maximum that could develop for the given voltage. 255,256 As the pulse progresses, plasma density appreciably improves, and impedance falls in the plasma. Although several groups reported instabilities in HIPIMS magnetron plasma, instabilities are essential to the operation; they may not only facilitate the current transport but contribute to the dissociation and the ionization of gas and are helpful in the generation of multiple charge ions. 257
Apart from the MS systems discussed in Fig. 3 , another method known as high-power impulse magnetron sputtering (HIPIMS) has shown potential application in the deposition of thin films. HIPIMS is a relatively new PVD technology that combines MS and pulsed power technology. 130,153,247,248 In HIPIMS, power densities to the target during the pulse on time are much higher than those in conventional mid-frequency pulse sputtering. 130,247 Two definitions of HIPIMS are broadly considered to define the scope clearly. 130,153 According to the technical perspective, the first definition of HIPIMS is a kind of pulsed sputtering in which the peak power typically exceeds the time-averaged power by two orders of magnitude. This definition implies that long pauses exist in between pulses of very high amplitude, so the term &#;impulse&#; is justified in the terminology. Alternatively, the more physical definition is that HIPIMS is a category of pulsed sputtering in which a significant fraction of the sputtered atoms become ionized. 153 This definition indicates that self-sputtering can take place, which may or may not be sustained entirely by target ions. The goal is to ionize sputtered atoms so that ions are available for substrate etching and to aid in film growth, resulting in well-adherent coatings with desirable microstructures and properties. 249
Despite significant progress in the study of MS discharges, several issues still need further study to improve the MS systems' performance for different applications. As discussed in Sec. II E, a magnetron source utilizes magnetic fields in balanced or unbalanced configurations. In addition, as discussed earlier, in plasma, ions are unmagnetized due to their larger Larmor radii than electrons, and they continuously accelerate toward the cathode with the applied voltage. Therefore, to compensate for the flow of ions toward the cathode, the same number of electrons must reach the anode to sustain the discharge. Further, generating E × B0 drift assists electron transport across the magnetic field.152&#;164 Plasma non-uniformities are a major issue in MS systems106,152,153,155,159 due to the nature of varying magnets in MS systems and E × B0 drift. A magnetron is a current-producing device that generates instabilities due to the E × B0 drift.162,163 It inspires further research into the nature of instabilities and their role in MS plasma processes. In addition, plasma parameters play a significant role in the formation of plasma and the deposition of thin films.108,144 Therefore, various plasma parameters must be measured to understand their role in plasma generation, film growth, and film properties.108,144 Different diagnostics, like probe-based and optical, are required to understand the plasma behavior in magnetron discharges.106,152,153,155,159 Collective studies of plasma diagnostic and material properties would better understand the plasma and process control intended for the magnetron sputtering application.106,152&#;162
In a recent study,144 Sahu et al. carried out the LP measurements to investigate fluctuations in floating potential (FFP)145,146 by the time series analysis method in the CMS discharges operated by DC power. Figure 4 shows the frequency spectra acquired at various operating pressures. Results showed the presence of low-frequency oscillations in the 2&#;3 MHz range, which were expected to be generated by the high-frequency waves. It was proposed that low-frequency waves could be Landau damped by electrons via wave energy transfer to bulk electrons, whose thermal velocity (vth) approaches the wave phase velocity (vph).144 With increasing pressure from 3 to 6 mTorr, the instability frequencies shifted from 2 to 3 MHz. Furthermore, the oscillations flattened out, and the peaks disappeared at higher pressures (see Fig. 4). The flattering of the peak in the FFP at high pressures suggests ineffective Landau damping due to increased collisions.144 As shown in Table II, numerous studies were also performed to study such non-linear fluctuations. Table II shows that there are two categories of fluctuations with low- and high-frequency oscillations observed in different MS systems.
TABLE II.
Various studies on non-linear effect in magnetron plasmas.
Different frequency range
.
MS plasma sources and nature of applied power
.
Results
.
References
.
Low frequency oscillations (KHz) MS discharges using DC powers Observation of 10&#;500 kHz range fluctuations in DC MS 144, 146, and 147 3DMS 40&#;250 kHz range periodic and chaotic oscillations observed with discharge current up to 10 A 106 HIPIMS 20&#;100 kHz range large amplitude oscillations 140 High frequency oscillation (MHz) ICP assisted DC MS Oscillatory nature in the 2&#;5 MHz range was observed in ICP-assisted DC MS 148 CMS Fluctuations in the range of 2&#;3 MHz were seen at low-pressure discharges 144 RF MS Harmonics oscillations of high-frequency (MHz band) were observed in the power spectra 149 HIPIMS Broad frequency peaks &#;1 MHz were visible in the FFT time series analysis for pulsed operation 150 HIPIMS FFT of the FPFs showed instabilities at a frequency of 2.4 MHz 151 Different frequency range
.
MS plasma sources and nature of applied power
.
Results
.
References
.
Low frequency oscillations (KHz) MS discharges using DC powers Observation of 10&#;500 kHz range fluctuations in DC MS 144, 146, and 147 3DMS 40&#;250 kHz range periodic and chaotic oscillations observed with discharge current up to 10 A 106 HIPIMS 20&#;100 kHz range large amplitude oscillations 140 High frequency oscillation (MHz) ICP assisted DC MS Oscillatory nature in the 2&#;5 MHz range was observed in ICP-assisted DC MS 148 CMS Fluctuations in the range of 2&#;3 MHz were seen at low-pressure discharges 144 RF MS Harmonics oscillations of high-frequency (MHz band) were observed in the power spectra 149 HIPIMS Broad frequency peaks &#;1 MHz were visible in the FFT time series analysis for pulsed operation 150 HIPIMS FFT of the FPFs showed instabilities at a frequency of 2.4 MHz 151 View Large
Sahu et al.,106 in a three-dimensional MS (3-DMS) system, reported plasma flares and instabilities. They considered the similarity between the Hall thruster (HT)127 and the MS system for their design of 3-DMS. They thought of a design with the E × B0 configuration for their experiments, as shown in Fig. 5(a). Figure 5(a) shows the formation of electron drift in the E × B0 direction when the applied magnetic field (B0) is aligned perpendicular to the electric field (E). The figure also depicts the flux due to E × B0 drift as the Hall flux and the flux due to electron transport across the field as the cross-field flux. The E × B0 drift velocity127 acquires a value [=(E × B0)/B02] &#; E/B0 when the collision (electron-neutral) frequency (υen) &#; ωce, the electron&#;cyclotron frequency [=eB0/me, where me and e are, respectively, the mass and charge of the electron]. Furthermore, they consider the fact that the electron current associated with Hall flux or Hall current234 should not flow toward the wall. This would cause polarization of the charge species that could set up the Hall electric field (EH) (familiar as the Hall effect) in the direction parallel to the E × B0 direction. This field (EH) would further oppose the current. This Hall field, in turn, would produce the EH × B0 flux that would be in the direction opposite to the applied field [i.e., along the direction of the cross-field flux as depicted in Fig. 5(a)]. It would destroy the particle (e) confinement by the magnetic field and promote charge transport across the magnetic field. In this sense, the Hall effect is not desired in Hall thrusters to ensure electron confinement and reduce electron conductivity in the axial direction. Different designs of HTs, such as annular258 and linear (HT with walls along E × B0 directions),259,260 were studied, which showed little success due to poor confinement. Sahu et al.106 suggested a design that is either cylindrical or with closed geometry, as shown in Fig. 5(b), to enforce E × B0 to align along the tangential direction. They designed an inverted &#;U&#; shaped and hollow 3-DMS configuration, as shown in Fig. 5(c), which has a similarity with the geometry reported by Janes and Lowder.258
FIG. 5.
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(a) The geometry of E × B0 configuration, Hall flux, and cross-field flux, (b) structure with one end enclosed geometry, which specifies the direction of E × B0, (c) schematic of the 3-DMS system along with different diagnostic systems used for the experiments in 3-DMS, (d) 3-D sketch of with alternate polarity of the set of magnets of the magnetron and measured magnetic field contours in the center and substrate planes, and (e) 3-DMS discharge characteristics. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
FIG. 5.
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(a) The geometry of E × B0 configuration, Hall flux, and cross-field flux, (b) structure with one end enclosed geometry, which specifies the direction of E × B0, (c) schematic of the 3-DMS system along with different diagnostic systems used for the experiments in 3-DMS, (d) 3-D sketch of with alternate polarity of the set of magnets of the magnetron and measured magnetic field contours in the center and substrate planes, and (e) 3-DMS discharge characteristics. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
Close modal
The various design parameters for their 3-DMS can be found elsewhere (see Table I of Ref. 106). Figures 5(c) and 5(d), respectively, represent the side and three-dimensional view of the 3-DMS system, which was assembled as a hollow cathode. The 3-DMS, a combination of five rectangular ITO targets, had a cathode area of nearly cm2. Because of the closed and inverted &#;U&#; shaped geometry of the 3-DMS, the magnetic field and the E × B0 drifts are not trivial. Figure 5(c) also shows the combination of rectangular magnets with opposite polarity to generate a closed and cusped magnetic field. Two sets of opposite-facing rectangular targets were placed parallel [Fig. 5(d)] to design the closed racetrack. The design was considered to enable electrons to oscillate between the cusps and follow the field lines, causing their azimuthal drift.258, Figure 5(d) presents the contours of the magnetic field in planes at the center and substrate. The magnetic field profile at the central plane was approximately symmetric about the Z axis. The magnetic field strength on the target surface was &#;250&#;300 G. The design shown in Fig. 5(d) allowed the side or bottom racetrack to form a cusp magnetic field. The field produced from the magnets in the upper racetrack (top target) could arrive at the location of the substrate owing to an unbalanced configuration. The strength of the magnetic field at the substrate plane [shown in Fig. 5(d)] was in the range of 10&#;100 G. Sahu et al.106 conducted their experiments in the 3-DMS at a pressure of 4 mTorr using DC magnetron sputtering. The dimensions of the side target of the lower racetrack were 30 × 20 cm2 [Fig. 5(d)], whose dimensions were close to the electron-neutral mean-free path (Table I in Ref. 106) to assist efficient collisions inside the hollow regime of the 3-DMS. Figure 5(e) shows the discharge characteristic of the 3-DMS.106 It can be seen that the discharge could occur even at a lower voltage of &#;225 V due to the incorporation of a closed magnetic field in the 3-DMS discussed earlier. Three distinct regimes [Fig. 5(e)], such as low power density (PD), intermediate PD, and high PD corresponding to low, intermediate, and high discharge current (Id), were used for further experiments.
As shown in Fig. 5(c), an LP diagnostic was used to determine the plasma parameters and examine the FPFs. The FPFs measured at different power densities showed different natures of oscillations in the oscilloscope, as shown in Figs. 6(a)&#;6(c).106 FPFs shown in Figs. 6(a) and 6(b) at low power densities of 0.5 and 1 W/cm2 are periodic in nature. The oscillation frequency at power density 1 W/cm2 [Fig. 6(b)] is slightly higher than that at 0.5 W/cm2 [Fig. 6(a)]. However, the oscillations shown in Fig. 6(c) became chaotic as the power density further increased to 2 W/cm2. The FFT analysis of the corresponding oscillations was used to determine the power spectrum shown in Figs. 6(d)&#;6(f). The power spectrum showed the presence of strong and dominant frequencies and subharmonics of the dominant frequencies.
FIG. 6.
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(a)&#;(c) FPFs measured by LP at different discharge currents and power densities, and (d)&#;(f) corresponding FFT spectra showing leading frequency components in FPFs. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
FIG. 6.
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(a)&#;(c) FPFs measured by LP at different discharge currents and power densities, and (d)&#;(f) corresponding FFT spectra showing leading frequency components in FPFs. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
Close modal
The intensified charge-coupled device (ICCD) imaging using a PIMAX camera in combination with an image intensifier and an objective UV-Nikkor 105 mm was used to acquire frame images to investigate plasma flares in 3-DMS plasmas.106 The substrate location from the lower surface of the 3-DMS was viewed side-on [as shown in Fig. 5(c)] via a viewport and isolated by a shutter. The ICCD camera, having a resolution of 512 × 512 pixels, was focused [in the X&#;Z plane; see the axes in Fig. 5(d)] and synchronized with the 25 × 25 mm2 field of view to get a magnification of &#;20.5 pixels/mm. Figures 7(a)&#;7(c), respectively, show the plasma flare structures106 observed by the ICCD camera at different discharge currents. Figures show that the nature and distribution of ionization zones vary with discharge current. LP measurements were performed at the location of the substrate [see Fig. 5(c)] to measure the plasma parameters like plasma density, electron temperature, floating potential, and plasma potential. The relevant parameters are also given in the figures. At the low current density [Fig. 7(a)], the plasma density is low, and the formation of a C-like ionization zone is quite symmetric about the location z = 0 (lateral dimension). With increasing discharge currents [Figs. 7(b) and 7(c)], the zone structures appeared to be flattening and extending toward the substrate plane. Particularly at high current density [shown in Fig. 7(c)], two distinct zones were composed of flares of merging sections with approximately the same sizes and appearances. Notably, images captured by a standard camera had observations similar to those of ICCD measurements, as shown in Figs. 7(d)&#;7(f). The frame images obtained [Figs. 7(b) and 7(c)] by ICCD and camera [Figs. 7(e)&#;7(f)] showed the formation of ionization zones in between the location of the target and the substrate plane, and the observed plasma flares106 were slanted in space. It was observed from Figs. 7(a)&#;7(f) that the regimes of strong intensities (either deep red or green) originated from the lower surface, which was close to the location of gas feeding in the 3-DMS [see Fig. 5(c)]. One can visualize the nature of the ionization and excitation cross sections of atomic Ar,196 which are dependent on energy. Ar&#;s electron impact ionization cross-section data show that the maximum ionization occurs for energy <100 eV.196 The secondary electron can pick up energy equal to the discharge or bias voltage and lose the energy via collisions with other neutral atoms, ions, and electrons. Their collisions could lead to efficient atomic excitation and ionization until their energy decreases. These events could produce greater intensities [Fig. 7] in regimes where the secondary electrons lose their energy; ionization substantially increases the electron density.
FIG. 7.
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Frame images measured by ICCD showing distinct ionization zones in 3-DMS discharges at (a) low-, (b) intermediate-, and (c) high-discharge currents (and PDs); (d)&#;(f) corresponding images captured with an ordinary camera displaying flares similar to (a)&#;(c). [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
FIG. 7.
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Frame images measured by ICCD showing distinct ionization zones in 3-DMS discharges at (a) low-, (b) intermediate-, and (c) high-discharge currents (and PDs); (d)&#;(f) corresponding images captured with an ordinary camera displaying flares similar to (a)&#;(c). [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
Close modal
Additionally, the formation of ionization zones in MS discharges was beautifully studied by Anderson et al.163 using fast camera measurements (not shown) using interference filters. Their spectroscopic studies by camera with a short exposure time revealed that (i) the emission from the low-excitation-energy metallic atoms appeared as approximately regular and organized zones, which were extended across the magnetron racetrack, and (ii) the emission from the higher order energy ions was concentrated in the ionization zones. They are probably the first group to surmise that the ionization zone144 could be associated with a double layer that can lead to the formation of plasma flares, asymmetric emission [as shown in Figs. 7(a)&#;7(f)], and the scenario of localized ohmic heating. Motivated by their work, Sahu et al.106 conducted LP measurements to detect such double layers if they were present in their experiments. Sahu et al.106 used an L-shaped LP to measure the axial profile of various plasma parameters, as shown in Figs. 8(a)&#;8(c). In the figure, the distances 0 and 6 cm represent the location of the substrate (anode) and the lower surface of the 3-DMS (cathode). It was seen that between distances (x axis) of 4.0 and 4.5 cm,there was a rapid variation in the parameters, and then the variation saturated after the location of 4.5 cm. The sharp fall in plasma density and plasma potential was reported as the formation of the double layer in the 3-DMS discharges. Additionally, the formation of the double layer in their work106 was also found to be associated with low-frequency (&#;kHz range) instabilities (not shown), similar to the reports of different studies.143,144,235
FIG. 8.
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Axial profile of various plasma parameters exhibiting the formation of a double layer (shaded regime) with a significant fall in plasma density that was associated with a characteristic potential drop. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
FIG. 8.
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Axial profile of various plasma parameters exhibiting the formation of a double layer (shaded regime) with a significant fall in plasma density that was associated with a characteristic potential drop. [Reproduce with permission from Sahu et al., Phys Rev. Appl. 10, (). Copyright American Physical Society. See Ref. 106].
Close modal
Apart from the MS discharges in CMS and 3-DMS devices, the instabilities were also in HIPIMS MS plasmas.130, Figure 9 indicates instabilities in current and voltage waveforms, which were suggested by the formation of noise and waves as the HIPIMS pulses progressed. As reported by Anders,130 the peak power density (PD) averaged over the target area in a HIPIMS discharge can acquire a value of &#;107 W/m2. The high value of PD generates plasma with efficient ionization of sputtered atoms. This feature of ionization of sputtered atoms suggests that the ion of the sputtered material will additionally sputter the cathode target, which results in self-sputtering. This self-sputtering mode could operate in the self-sustained mode in plasmas when the ions of the target (sputtered) atoms are sufficiently created at a high rate in HIPIMS discharges. In addition, in self-sustained modes, Ar neutrals could be switched off. Notably, the flux of these sputtered atoms from the target can noticeably influence the neutral gas near the cathode (target). Because gas neutrals will be displaced nearby and heated due to collisions with these sputtered atoms, their density near the target will be significantly lowered.130 This reduction of neutral gas density is termed &#;rarefaction&#; in the HIPIMS plasmas.130 Anders showed that this rarefaction affects the current peaks. Anders observed oscillations in both voltage and current waveforms, as shown in Fig. 9. The pattern observed in the current waveform was depicted as the end of the runaway phase, a peak followed by a small reduction, which was expected to be due to the rarefaction of the neutral gas. As the gas density decreases in the plasma due to less Ar, the voltage waveform becomes noisier. Subsequently, the wave nature with growing amplitudes or instability was seen in the current waveform.
FIG. 9.
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Observation of noise, waves, and instabilities in current and voltage waveforms in HIPIMS discharges during the operation in pulse mode. [Reproduce with permission from A. Anders, Surf. Coat. Technol. 205, S1 (). Copyright Elsevier. See Ref. 130].
FIG. 9.
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Observation of noise, waves, and instabilities in current and voltage waveforms in HIPIMS discharges during the operation in pulse mode. [Reproduce with permission from A. Anders, Surf. Coat. Technol. 205, S1 (). Copyright Elsevier. See Ref. 130].
Close modal
Furthermore, Raman et al.143 performed a series of measurements using ICCD to study the role of the width of the racetrack on the formation of ionization zones/plasma spokes in HIPIMS discharges. Their work used Al targets with magnetron racetracks of different widths, e.g., 0.5, 1.1, and 1.6 cm, at various radial magnetic fields.143 Accordingly, the magnetrons with such racetrack configurations were named Narrow 1, Narrow 2, and Narrow 3, respectively. Figures 10(a)&#;10(c) present the observed zone structures at an operating condition of pulse = 500 µs, voltage = 650 V, average power = 250 W, and pressure = 20 mTorr. Figure 10(a) shows that the plasma appeared to be homogeneous at 20 µs soon after the beginning of the discharge, and the plasma appeared as contraction at 140 µs. The ionization zones/plasma spokes appeared at 160 µs and remained until 480 µs. The peak discharge current relevant to these measurements was 35 A. In the &#;Narrow 2&#; configuration [Fig. 10(b)], an intense and homogeneous plasma was observed at 20 and 120 µs. The distinct plasma spokes started evolving at 140 µs, which remained until 480 µs (i.e., the end of the pluses). It was noted that the peak discharge current was 85 A for this configuration. The images of the &#;Narrow 3&#; configuration, shown in Fig. 8(c), showed that in the initial instance at 20 µs, the plasma was homogeneous, and discrete plasma spokes started appearing at 40 µs from the beginning of the HIPIMS discharge. It was seen that the number of spokes diminished with time and disappeared in the end, i.e., at 480 µs. They also used a racetrack with a width of 2.7 cm, whose ICCD images are shown in Fig. 10(d). With a pulse width of 250 µs, a voltage of 600 V, an average power of 250 W, and a radial magnetic field of 450 G, this configuration was named the &#;Regular&#; racetrack configuration. Measurements using the ICCD images showed that, at the beginning of the discharge at 20 µs, the plasma looked intense and homogeneous. Then, distinct spokes appeared at 40 µs, and the number of plasma spokes reduced as time progressed.
FIG. 10.
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ICCD images showing the effect of racetrack configurations (a) Narrow 1, (b) Narrow 2, (c) Narrow 3, and (d) Regular during HIPIMS discharge with the time of pulse. The details are given in the text. [Reproduce with permission from Raman et al., Vacuum 156, 9 (). Copyright Elsevier. See Ref. 143].
FIG. 10.
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ICCD images showing the effect of racetrack configurations (a) Narrow 1, (b) Narrow 2, (c) Narrow 3, and (d) Regular during HIPIMS discharge with the time of pulse. The details are given in the text. [Reproduce with permission from Raman et al., Vacuum 156, 9 (). Copyright Elsevier. See Ref. 143].
Close modal
The spokes were present until 250 µs, i.e., the end of the pulse. It can be noted that the peak discharge current was &#;138 A in the experiments. Experiments involving these varieties of racetracks showed that, with decreasing the width of the racetrack, the time taken for the formation of spokes in HIPIMS discharges increased for a given combination of voltage and average power. In addition, at a low radial magnetic field [Fig. 10(d)], the time taken to generate plasma spokes increased for a set of HIPIMS voltage and average power. As discussed earlier, there are extensive investigations on plasma generation, the formation of flares/spokes, ionization zones, and instabilities in MS discharges operated by DC, RF, and HIPIMS powers. Table III summarizes the studies involving plasma flare/ionization zone/plasma spokes in MS discharges. Apart from the basic studies, MS discharges have been used to deposition a wide range of thin films for different applications. The following section highlights the studies on TCO films, particularly the deposition, and studies of ITO films using MS methods.
TABLE III.
Studies involving plasma flare/ionization zone/plasma spokes in magnetron plasmas.
Type/nature of MS discharge
.
Results
.
References
.
HIPIMS Formation of localized ionization zones in the MS plasmas, which propagate with a velocity of the order of &#;2 km s&#;1 158 RF MS Ionization drift turbulence was observed in weakly ionized plasmas by RF MS 149 DC CMS Azimuthally propagating waves were present in the CMS plasmas operated with DC power 133 HIPIMS Traveling ionization zones in HIPIMS were observed using fast ICCD camera imaging 163 DC MS In DC MS, when operating at lower gas pressures, the spokes were seen to be elongated and arrowhead-like shape, whereas at higher pressures, the spokes exhibit a less defined shape 161 HIPIMS In HIPIMS discharges, more positively charged ions with higher count rates in the medium energy range of their distributions were detected in (+ve) E × B than in (&#;ve) E × B direction, thus confirming the notion that ionization zones were associated with moving potential humps 157 HIPIMS Self-organized ionization zones and associated plasma flares were recorded with fast cameras in side-on view 152 DC MS A single drifting ionization zone has been presented, even down to the threshold current of &#;10 mA 164 DC 3-DMS The formation of instabilities was linked with plasma flares of several centimeters, driven by low to moderate current in a 3-DMS system operated by DC power 106 Type/nature of MS discharge
.
Results
.
References
.
HIPIMS Formation of localized ionization zones in the MS plasmas, which propagate with a velocity of the order of &#;2 km s&#;1 158 RF MS Ionization drift turbulence was observed in weakly ionized plasmas by RF MS 149 DC CMS Azimuthally propagating waves were present in the CMS plasmas operated with DC power 133 HIPIMS Traveling ionization zones in HIPIMS were observed using fast ICCD camera imaging 163 DC MS In DC MS, when operating at lower gas pressures, the spokes were seen to be elongated and arrowhead-like shape, whereas at higher pressures, the spokes exhibit a less defined shape 161 HIPIMS In HIPIMS discharges, more positively charged ions with higher count rates in the medium energy range of their distributions were detected in (+ve) E × B than in (&#;ve) E × B direction, thus confirming the notion that ionization zones were associated with moving potential humps 157 HIPIMS Self-organized ionization zones and associated plasma flares were recorded with fast cameras in side-on view 152 DC MS A single drifting ionization zone has been presented, even down to the threshold current of &#;10 mA 164 DC 3-DMS The formation of instabilities was linked with plasma flares of several centimeters, driven by low to moderate current in a 3-DMS system operated by DC power 106 View Large
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