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Study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition
Vacuum 84 (2010) 1111–1117
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition W.C. Lang a, b, *, J.Q. Xiao a, b, J. Gong a, C. Sun a, R.F. Huang b, L.S. Wen a a b
Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Hunan YKH Surface Engineering Co., Ltd, Changsha 410013, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 August 2009 Received in revised form 4 January 2010 Accepted 12 January 2010
Cathode spot motion and macroparticles (MPs) reduction on related films are the two main issues in the application of the vacuum arc deposition (VAD). In the present work, an axisymmetric magnetic field (AMF) was applied to the cathode surface to investigate the influence of the AMF on the cathode spot motion and the MPs reduction on TiN films. The results show that the AMF affected the cathode spot motion by redistributing the dense plasma connected with the initiation of the new spot. With increasing AMF, there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. Based on the results of FEM simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion in the AMF was discussed. The morphology, detailed size distribution, and roughness of the resultant TiN films were systematically investigated. Fewer and smaller MPs ejection is observed with an increase in the transverse component of AMF. The effect of the AMF on the MPs reduction on TiN films was discussed, and the results were compared with the theoretical predictions. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Vacuum arc deposition Axisymmetric magnetic field Cathode spot motion Macroparticles reduction
1. Introduction Vacuum arc deposition (VAD) has been widely utilized in the deposition of various kinds of thin solid films due to the excellent characteristics of the arc plasma produced from an active cathode spot that emits ions of cathode material, as well as electrons [1–3]. Together with the arc plasma, macroparticles (MPs) with various sizes and shapes are also emitted due to the violent plasma–liquid pool interactions in the cathode spot [4,5]. MPs adhering to the prepared films deteriorate the quality and the performance of the films; thus, MPs contamination is usually regarded as the most important technological problem and has become a major obstacle in VAD process. In order to eliminate MPs, several methods connected with different aspects of the MPs process have been adopted, such as magnetically ‘steered arcs’ [6,7], magnetic filters [8–10] and negative substrate biasing [11]. Among these methods, the external magnetic field-enhanced VAD is an approach to reduce MPs generation and can be considered as a positive means because it solves the problem at the origin of the process. The cathode spot can be steered by an external magnetic field because an external * Corresponding author. Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. E-mail address: [email protected] (W.C. Lang). 0042-207X/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.01.037
magnetic field can influence and change the distribution of dense plasma generated by the cathode spot [12]. The typical magnetic field configuration in the so-called ‘steered arc’ is an arched magnetic field, which has been studied intensively [6,7,13]. However, this magnetic field configuration leads to a narrow groove left on the target, resulting in a poor utilization of the target material and this configuration is only suitable for a relatively large and thin target. For a small and thick target (R ¼ 30–40 mm, H > 25 mm), which is commonly used on an industrial scale, a different magnetic field configuration should be employed and given more attention, such as an axisymmetric magnetic field (AMF). For magnetic field-enhanced VAD, the cathode spot motion and the distribution of MPs on related films are the two main issues. Cathode spot motion is the key factor; it affects the physical characteristics of the vacuum arc plasma, the utilization of the cathode material, the emission of MPs and the quality of subsequent films containing these MPs. Therefore, it is important to investigate the mechanism of cathode spot motion in an external magnetic field. To date, intensive studies of cathode spot dynamics under different magnetic field components in a vacuum have been performed theoretically [14–17]. Much research has focused on finding the smallest spatial structures and fastest temporal events. The structure and dynamics of cathode spots are still the subject of discussions and research [18]. Such dynamics also should be understood
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practically under a compound magnetic field, such as in AMF, for industrial applications. Therefore, we will not to search for the finer structure of cathode spot, that is, our study does not focus on the microstructure of the cathode spot but the macroscopic behavior of the cathode spot in a specific compound magnetic field and on a cathode with specific geometry. Specifically, we study the macroscopic behavior of the cathode spot steered by an axisymmetric magnetic field on a small and thick target. In the present work, an AMF produced by using an adjustable electromagnetic coil enclosed with a coaxial cylinder of magnetically soft material was applied on the cathode surface to steer the cathode spot motion, and TiN films were deposited with different AMF intensities. Based on the results of the finite element method (FEM) simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion at different AMF intensities was discussed. The effect of the AMF on the reduction of the MPs on the surface of the resultant TiN films was also investigated, and the results were compared with the theoretical predictions.
2. Experimental A schematic diagram of the experimental apparatus is shown in Fig. 1. A water-cooled titanium cathode (99.99% purity) of 64 mm in diameter and 30 mm in thickness with an inclined wall shoulder was mounted inside a vacuum chamber, which acted as the anode. An adjustable electromagnetic coil enclosed with a coaxial cylinder of magnetically soft material, which was protected by using a deposited nickel coating, was located behind the cathode to produce the magnetic field steering the cathode spot motion. A circular-shaped magnetic flux guide placed around the cathode surface was utilized to increase the transverse magnetic component. TiN films were deposited on 1Cr18Ni9Ti stainless-steel specimens with different AMF intensities. The deposition time was 30 min, and the arc current was controlled at 60 A. The images of cathode spot motion was captured from a dynamic video by a highspeed camera, the exposure time was set as 1/1000 s to get an obvious spot trace. A reflector with 45 angle of the cathode surface was employed to reflect the light of cathode spot into the camera lens, which was placed close to the observation window. A transparent protective layer that can be replaced was employed to avoid the observation window being coated. The distribution of the AMF was simulated and analyzed by using the FEMM 4.2 software package. The magnetic field’s
Fig. 1. Schematic illustration of the AMF enhanced VAD equipment.
intensity could be adjusted by altering the current to the electromagnetic coil. The transverse magnetic flux density with the value (which varies in the range of 0–35 G) at the bottom of the edge of the cathode was chose as the parameter to evaluate the MPs reduction. The morphology of the TiN films was observed by using a scanning electron microscope (SEM, Hitachi S-4200). A quantitative analysis of the number and the size distribution of Mps was carried out by using an image analysis system (SISC IAS V8.0). This system has the ability to adjust the threshold of the intensity of the pictures, pick up the MPs by the color and brightness contrast between the MPs and the background of the films, and that could measure the diameter and the area of each particle automatically. 3. Results 3.1. Magnetic flux distribution Fig. 2 shows the magnetic flux distribution obtained by using the FEM simulation at two different situations. It can be seen that magnetic flux exhibited an axisymmetric distribution, the intensity of magnetic flux decreased from a large value near the cathode to a small value far away from the cathode, and the magnetic flux intersected with the target surface at acute angles directed to the edge of the cathode. Fig. 2(b) and (a) presents the magnetic field configurations around the cathode target with or without a circular-shaped magnetic flux guide placed around the cathode surface. The magnetic flux guide could cause a concentration and enhancement of the magnetic flux and could increase the transverse magnetic component. The distributions of the transverse and the normal components of different magnetic flux density on the cathodic target surface are plotted in Fig. 3(a) and (b), respectively. The transverse component of the magnetic flux density increased gradually from zero at the center of the target to a relatively larger value at the edge of the target. In contrast, the normal component of the magnetic flux density decreased from the maximum value at the center of the target to a relatively smaller value at the edge of the target. The transverse magnetic flux density at the bottom of the inclined wall shoulder, which was measured with a magnetometer (SHT-V), increased from 0 G to 35 G by altering the current to the electromagnetic coil. 3.2. Cathode spot movement Cathode spots are highly unusual physical objects and notoriously difficult to measure, but one could image the light emission from a spot. Therefore, the images of cathode spot motion (macroscopic) were captured from a dynamic video by a highspeed camera. A cathode spot is an assembly of emission centers showing fractal properties in spatial and temporal dimensions [18]. To get an obvious spots trace (a group of spot ignitions but not one spot event), the exposure time was set as 1/1000 s because long exposure time would smear out the image of a fast changing object and show the overall trajectory. A schematic illustration and transient cathode spot morphologies with different AMF intensities are shown in Fig. 4. The results show that increasing the AMF intensity strongly influences the cathode spot motion. In the case of a weak AMF, a big bright cathode spot moves randomly and slowly on the cathode surface, typical images of which are shown in Fig. 4(a) and (a0 ). With increasing AMF, there is an increasing tendency for the cathode spot to refine, rotate and drift toward the cathode target edge, exhibiting a chrysanthemum structure, the cathode spot pattern changes from a big bright spot to a longer and thinner line, which is shown in Fig. 4 (b) and (b0 ). An increase in the transverse magnetic field (TMF), B//, intensity can accelerate the rotational velocity of the
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Fig. 2. Magnetic flux distribution obtained by using FEM simulation: (a) without a circular-shaped magnetic flux guide and (b) with a circular-shaped magnetic flux guide.
cathode spot and increase the arc voltage. With a relatively strong AMF (B// z 30 G), the cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new cathode spot is ignited, splits, and is extinguished repeatedly on the cathode surface, which can be observed at intervals of about 0.5 s, as shown in Fig. 4(c) and (c0 ). The increase in the AMF causes an increase in the arc voltage, which is observed in the experiment. The arc voltage increase can be attributed to the distortion and elongate of the plasma channel due to the increasing rotational velocity of the cathode spot. Another reason for arc voltage increase is the raised resistance between the cathode and the anode owing to the electrons loss in the plasma channel resulting from the electron drift confined by the curved magnetic field configuration around the cathode. A stable arc discharge requires sufficient electrons transport between the anode and the cathode, an increase in the arc voltage can compensate the electrons loss in the plasma channel.
3.3. MPs reduction Typical SEM micrographs of the TiN films with different magnetic field intensities are shown in Fig. 5(a)–(d). Fig. 6 presents an overall size distribution of MPs on TiN films. It can be seen that MPs densities and sizes are greatly decreased with increasing the
magnetic field intensity. The SEM micrographs of the TiN films show that in the case of a weak AMF, MPs are mostly large liquid particles. While for a relatively strong AMF, MPs are mostly small solidified particles. Fig. 7 shows the variation in the number of MPs classified by their diameters with transverse magnetic field. For comparison, a histogram showing the total number of MPs collected larger than different diameters is also plotted in Fig. 8. The most striking feature is the MPs’ decreased slope over several orders of magnitude changes less, this indicates a fractal characteristic of MPs distribution on films. The dependence of the TiN film roughness (Ra) on the TMF is plotted in Fig. 9. It is clear that reducing MPs could significantly improve the surface quality of films.
4. Analysis and discussion A key to seeking the mechanism of cathode spot motion is to look at the life cycle of a spot, in particular the physical mechanism of the spot discharge and the condition for the occurrence of a spot. According to the emission process of electrons from the cathode, an arc discharge is characterized by a collective electron emission mechanism, which can generate large numbers of electrons. A continuous large amounts electrons emission is the key factor for a self-sustaining arc discharge. It is more effective to increase the
Fig. 3. Distribution of different magnetic field intensity on the target surface: (a) transverse component (b) normal component.
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Fig. 4. Schematic illustration and static images of the cathode spots motion under different magnetic field intensities: (a, a0 ) TMF ¼ 0 G (b, b0 ) TMF ¼ 15 G (c, c0 ) TMF ¼ 30 G.
temperature on a small concentrated area to obtain higher electron current with a cathode near room temperature (also called cold cathode). Therefore, electrons emission occurs in VAD process (where cathode is directly or indirectly water-cooled) is localized in fragments of hot, micron-size, non-stationary cathode spot, which
determined the physical characteristics of arc plasma and properties of subsequent films. The formation of cathode spot is a fundamental characteristic of the vacuum arc discharge with a cold cathode, the processes in a cathode spot are very complex due to the very small scale and
Fig. 5. SEM micrographs of the MPs on the surfaces of TiN films for different magnetic fields. (a) TMF ¼ 0 G; (b) TMF ¼ 10 G; (c) TMF ¼ 20 G; and (d) TMF ¼ 30 G.
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Fig. 8. Total numbers of MPs collected with diameter larger than different diameters for different magnetic fields. Fig. 6. Overall size distribution of MPs on the surface of TiN films.
very short characteristic time of the spot. To seek the mechanism of cathode spot motion in AMF, the key factor affecting the occurrence, maintanence and movement of a spot should be studied by analyzing the physical mechanism of the spot discharge, especially the condition of continuous emission of a large amount of electrons. To generate a large amount of electrons, it means that there have lots of electrons can overcome the potential barrier at the cathode surface. Only thermionic emission cannot provide so many electrons from the micro-size cathode spot, an electric field that deforms the potential barrier should be formed above the cathode surface, and any field enhancement factors promote the emission of more electrons. When a spot is ignited, the cathode material suffers a transition to dense plasma at the spot due to the processes of effective ionization and ion acceleration in collisions of accelerated electrons with evaporated atoms [19]. Most of the applied voltage drops in a very thin sheath near the cathode due to the presence dense plasma, and the adjacent dense plasma influences the effective work function of the cathode surface. Hence, even a small voltage drop of the order of 20 V, which is a characteristic of arc discharges, can create an electric field at the cathode surface strong enough to cause field emission of electrons. Therefore, the dense plasma with adjacent sheath is associated with a strong electric field, and this strong electric field gives rise to a continuous large amount of
Fig. 7. Variation in the number of MPs classified by their diameters with the transverse magnetic field.
electron emission, which is the key factor to a self-sustaining arc discharge. The sheath thickness is of critical importance to the ignition of an emission center because it determines the surface electric field, which needs to be sufficiently high to cause thermal runaway at this location. The contracted sheath thickness caused by the dense plasma depends on the plasma density, which follows Child’s law [20], and different dense plasma densities can cause different increase in the electric field strength, which may cause different enhancements of electron emission. The probability of the largest amount of electron emission appears at the location that has the largest dense plasma density. After a spot is ignited, a small molten pool of liquid cathode material is also formed on the cathode surface due to the high-power density in the cathode spot, and the molten pool grows in size rapidly. The increasing molten pool size and temperature result in a decrease in the current density and an increase in the resistance of the cathode material at the spot area. The region of the cathode bulk directly under the cathode spot is more resistive than all other areas or parts of the cathode. Hence, if there were alternative, less resistive way for the current to flow, the current would switch to the new path [18]. Therefore, the heat production and the original spot will cool down and the process terminates eventually. During this cycle, at least a new spot, which takes over the current from the original spot, must be formed and
Fig. 9. Dependence of the TiN film roughness (Ra) on the TMF.
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activated to maintain the discharge. The probability of initiation of a new cathode spot will occur at a place where the electric field is sufficient to extract a large electron current to fulfill the ignited condition such as the minimum ‘chopping’ current [21]. The spot is established when this current is large enough to heat the spot to fusion and even to ebullition; then, the spot is sustained as a result of the presence of the dense plasma. A continuous rebuilding of the dense plasma occurs since the repetitive formation of ionized cathode material originates from newly developed spots. The repeated processes, new daughter spots forming at the edge of previous parent spots, give rise to an apparent spot motion across the cathode surface. The probability of initiation of new cathode spots can be affected by the way of the dense plasma distributed over the surface, and occurs at the place with the largest dense plasma density. Today, it is firmly established that cathode spot motion is not motion of matter, but a process associated with a rapid sequence of individual ignition and extinction of active emission sites [22], and an external magnetic field B strongly influences the behavior of cathode spot. Intensive studies of cathode spot dynamics under different magnetic field components in a vacuum have been performed theoretically by researchers [14–17]. It has been demonstrated that the cathode spot moves at random on the cathode surface in the absence of a magnetic field while it moves in a direction perpendicular to the field lines in the reverse direction to the Ampere rule (‘retrograde’ direction) when a magnetic field is applied parallel to the cathode surface, and follows the ‘acute angle rule’ under the condition of a sharp-angled magnetic field. Though there are many models to explain the cathode spot motion under different magnetic field components, especially in a transverse magnetic field, the key factors are the dense plasma distribution and redistribution on the cathode surface. Let us continue by grabbing this key factor firmly to understand the motion of cathode spot in an AMF. In our experiment, the cathode spot moved at random on the cathode surface in the case of a weak AMF. This was reasonable because the effect of a weak AMF on the plasma was relatively small, and the redistribution of dense plasma was affected by other factors. Initially the dense plasma mainly had a symmetry shape above the molten pool. It was obvious that the dense plasma cannot retain its symmetry definitely due to some kind of instability, such as expanded plasma jet, unstable gas flow and ions diffusion. Thus, this will result in asymmetry distribution of dense plasma. Finally, in view of the cathode spot growth, the current density was too low to sustain the existence of original spot, a new cathode spot would ignited at the place with the largest dense plasma density. Furthermore, spot process also depended on the surface conditions, including the presence of a dielectric layer and geometric enhancement, which promoted the buildup of surface charge and ignition of new spots. All these factors were instable, fluctuant and had a random distribution; therefore, the probability of initiation of a new cathode spot would occur on the target surface randomly and would exhibit a stochastic cathode spot motion. In an imposed magnetic field, the cathode spot motion could be defined by three superimposed components. These components were the random walk, the retrograde motion and the Robson drift. The random walk component was always present and depends only on the cathode material. In our experiment, the AMF was composed of two components: transverse magnetic field and normal magnetic filed. With increasing AMF, the increasing transverse magnetic field component had a deterministic effect on the plasma and this effect caused the largest dense plasma density always occur in the ‘retrograde’ direction, leading to a preferential initiation of the new cathode spot on the retrograde side. The axisymmetric distribution of transverse magnetic field component caused
a rotational cathode spot motion tendency, the strength of which and the rotational velocity of cathode spot depended on the transverse magnetic filed (TMF), B//, intensity. The acute angles formed by AMF intersected with the target surface directed to the edge of the cathode, increasing the tendency of Robson drift toward the cathode target edge. Therefore, there was an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge, combined with the cathode spot random walk and the restriction effect of electric current inside the cathode on the expanded cathode spot, leading to an exhibited chrysanthemum structure. With a relatively strong AMF (B// z 30 G), the tendency of rotational arc cathode spot motion and Robson drift was strong enough that the random walk could be neglected. The cathode spot would escape from cathode surface if there were not any confine on the smooth cathode surface. In our experiment, a cathode with an inclined wall shoulder was employed. Fig. 10 shows the magnetic flux distribution on the inclined wall shoulder of the cathode. On the one hand, the magnetic flux intersects with the inclined wall shoulder and forms acute angles directed to cathode surface, avoiding the cathode spot escaping from cathode surface. On the other hand, the magnetic flux intersects with the target surface at acute angles directed to the edge of the cathode, restricting the drift of cathode spot to the center of cathode surface. Due to these combined factors, the arc cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. The high rotational velocity of confined cathode spot could lead to the spilt of arc sometimes, which was also observed and illustrated in Fig. 4(c) and (c0 ). The stable confined rotational cathode spot motion generates high-density dense plasma above the edge of the cathode surface while the plasma density above the center of the cathode surface is relatively low. Therefore, the dense plasma diffuses from the edge to the center of the cathode surface, increasing the plasma density above the center of the cathode surface. A new spot may be ignited at the center of the cathode surface when the plasma density is sufficient to induce effective electron emission to fulfill the ignition condition. Under the combined effects of the strong tendency for rotational cathode spot motion and Robson drift, the ignited spot extends rapidly to the edge of the cathode surface, exhibiting a helical form, which was shown in Fig. 4(c0 ). The repetitive processes of diffusion, ignition, and extension leads to a repeating ignition, split, and extinguishment of the cathode spot motion on the cathode surface, which can be observed at intervals of about 0.5 s. After a spot is ignited, various sizes and shapes of MPs are also emitted. The generation of MPs is the most severe issue in vacuum arc deposition. Though the specific mechanism of the formation of the MPs is unclear, it can straightforward believe that a small molten pool formed on the cathode surface coupled with the timevarying plasma pressure produces the ejection of MPs. The MPs
Fig. 10. Magnetic flux distribution on the inclined wall shoulder of the cathode.
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generation is related to the formation of a molten pool of liquid cathode material formed as a result of thermal runaway and the explosive plasma formation. MPs form when the molten pool of liquid cathode material yields to the large plasma pressure. The volume of the molten pool of liquid cathode material is determined by the local energy input associated with heat conduction and ion bombardment in the cathode volume. The amount of local energy input depends on the spot residence time associated with the spot velocity, which varies from one place to another, and on the probability of traversing areas previously heated. An increase in the AMF can increase the rotational velocity of cathode spot and decrease the spot residence time. A shorter interaction time of the dense plasma with the liquid cathode material at the spot location can generate fewer MPs ejection. In the case of a relatively weak magnetic field, the cathode spot moves slowly on the target, resulting in a relatively large-volume molten pool between the dense plasma and the cathode bulk, which causes the emission of more large liquid MPs. However, an increase in the intensity of the transverse component of the AMF could accelerate the rotational velocity of the cathode spot and cause a shorter interaction time of the plasma with the cathode material at the spot location, resulting in a reduced MPs size and reduced amount of MPs emission. The distribution of MPs indicated a fractal characteristic associated with the fractal nature of a cathode spot [23]. It is clear that reducing MPs could significantly improve the surface quality of films, so it deserves more research. 5. Conclusions This paper describes the mechanism of cathode spot motion in the presence of an AMF and the MPs reduction on TiN films prepared by using VAD. Cathode spot motion is not motion of matter but is a process associated with a rapid sequence of individual ignitions and extinctions of active emission sites. The probability of initiation of a new cathode spot is affected by the way that dense plasma distributed over the surface and occurs at the location with the largest dense plasma density. In the case of a weak AMF, the cathode spot moves at random on the cathode surface due to the relatively small effect of the weak AMF on the plasma, and the redistribution of dense plasma is affected by fluctuating factors. With increasing AMF, the increasing tendency of rotational cathode spot motion and Robson drift lead to an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. This is combined with cathode spot random walk, leading to chrysanthemum structure. With a relatively strong AMF, the
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cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new cathode spot is ignited, splits, and is extinguished repeatedly on the cathode surface, which can be observed at intervals of about 0.5 s. The molten pool formed on the cathode surface, coupled with the time-varying dense plasma pressure to the cathode, produces the ejection of MPs. An increase in the AMF could increase the velocity of rotational cathode spot motion, decrease the spot residence time, and generate fewer MPs ejections. The distribution of MPs indicated a fractal characteristic associated with the fractal nature of the cathode spot. Reducing MPs can significantly improve the surface quality of films, so further research is warranted. We believe that this article brings some new insight into the VAD process. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (863 Program), No. 2006AA03Z521, Nano-composite Coatings of Ultra-fine Grained Hard Metals. The authors would be grateful to D. C. Meeker with Foster-Miller Inc. USA for his free provision and kind advice about the FEMM 4.2 software package. References [1] Kimblin CW. J Appl Phys 1974;45:5235. [2] Hantzsche E. IEEE Trans Plasma Sci 2003;31:799. [3] Boxman RL, Martin PJ, Sanders DM, Handbook of vacuum arc science and technology; 1995. [4] Boxman RL, Goldsmith S. Surf Coat Technol 1992;52:39. [5] Anders S, Anders Andre, Brown IG. IEEE Trans Plasma Sci 1993;21:440. [6] Swift PD. J Phys D Appl Phys 1996;29:2025. [7] Ramalingam S, Qi CB, Kim K, U.S. patent: 4673477; 1987. [8] Karpov DA. Surf Coat Technol 1997;96:22. [9] Martin PJ, Bendavid A. Thin Solid Films 2004;394:1. [10] Boxman RL, Beilis II. IEEE Trans Plasma Sci 2005;33:1618. [11] Huang Mei Dong, Lin Guo Qiang, Dong Chuang. Surf Coat Technol 2003;176:109. [12] Ju¨ttner B. J Phys D Appl Phys 2000;33:2025. [13] Walke PJ, New R, Care CM. Surf. Coat. Technol 1993;59:126. [14] Tanberg R. Nature 1929;124:371. [15] Ju¨ttner B. J Phys D Appl Phys 1995;516:28. [16] Beilis II. IEEE Trans Plasma Sci 2001;29:657. [17] Beilis II. Appl Phys Lett 2002;181:3936. [18] Anders A. Cathodic arcs: from fractal spots to energetic condensation; 2008. [19] Boxman RL, Zhitomirsky VN. Rev Sci Instrum 2006;77:6748. [20] Forrester AT. Large ion beams. New York: Wiley; 1988. [21] Smeets RPP. IEEE Trans Plasma Sci 1989;17:303. [22] Anders A. IEEE Trans Plasma Sci 2005;33:1456. [23] Anders A. Thin Solid Films 2006;502:22.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition W.C. Lang a, b, *, J.Q. Xiao a, b, J. Gong a, C. Sun a, R.F. Huang b, L.S. Wen a a b
Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Hunan YKH Surface Engineering Co., Ltd, Changsha 410013, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 August 2009 Received in revised form 4 January 2010 Accepted 12 January 2010
Cathode spot motion and macroparticles (MPs) reduction on related films are the two main issues in the application of the vacuum arc deposition (VAD). In the present work, an axisymmetric magnetic field (AMF) was applied to the cathode surface to investigate the influence of the AMF on the cathode spot motion and the MPs reduction on TiN films. The results show that the AMF affected the cathode spot motion by redistributing the dense plasma connected with the initiation of the new spot. With increasing AMF, there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. Based on the results of FEM simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion in the AMF was discussed. The morphology, detailed size distribution, and roughness of the resultant TiN films were systematically investigated. Fewer and smaller MPs ejection is observed with an increase in the transverse component of AMF. The effect of the AMF on the MPs reduction on TiN films was discussed, and the results were compared with the theoretical predictions. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Vacuum arc deposition Axisymmetric magnetic field Cathode spot motion Macroparticles reduction
1. Introduction Vacuum arc deposition (VAD) has been widely utilized in the deposition of various kinds of thin solid films due to the excellent characteristics of the arc plasma produced from an active cathode spot that emits ions of cathode material, as well as electrons [1–3]. Together with the arc plasma, macroparticles (MPs) with various sizes and shapes are also emitted due to the violent plasma–liquid pool interactions in the cathode spot [4,5]. MPs adhering to the prepared films deteriorate the quality and the performance of the films; thus, MPs contamination is usually regarded as the most important technological problem and has become a major obstacle in VAD process. In order to eliminate MPs, several methods connected with different aspects of the MPs process have been adopted, such as magnetically ‘steered arcs’ [6,7], magnetic filters [8–10] and negative substrate biasing [11]. Among these methods, the external magnetic field-enhanced VAD is an approach to reduce MPs generation and can be considered as a positive means because it solves the problem at the origin of the process. The cathode spot can be steered by an external magnetic field because an external * Corresponding author. Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. E-mail address: [email protected] (W.C. Lang). 0042-207X/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.01.037
magnetic field can influence and change the distribution of dense plasma generated by the cathode spot [12]. The typical magnetic field configuration in the so-called ‘steered arc’ is an arched magnetic field, which has been studied intensively [6,7,13]. However, this magnetic field configuration leads to a narrow groove left on the target, resulting in a poor utilization of the target material and this configuration is only suitable for a relatively large and thin target. For a small and thick target (R ¼ 30–40 mm, H > 25 mm), which is commonly used on an industrial scale, a different magnetic field configuration should be employed and given more attention, such as an axisymmetric magnetic field (AMF). For magnetic field-enhanced VAD, the cathode spot motion and the distribution of MPs on related films are the two main issues. Cathode spot motion is the key factor; it affects the physical characteristics of the vacuum arc plasma, the utilization of the cathode material, the emission of MPs and the quality of subsequent films containing these MPs. Therefore, it is important to investigate the mechanism of cathode spot motion in an external magnetic field. To date, intensive studies of cathode spot dynamics under different magnetic field components in a vacuum have been performed theoretically [14–17]. Much research has focused on finding the smallest spatial structures and fastest temporal events. The structure and dynamics of cathode spots are still the subject of discussions and research [18]. Such dynamics also should be understood
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practically under a compound magnetic field, such as in AMF, for industrial applications. Therefore, we will not to search for the finer structure of cathode spot, that is, our study does not focus on the microstructure of the cathode spot but the macroscopic behavior of the cathode spot in a specific compound magnetic field and on a cathode with specific geometry. Specifically, we study the macroscopic behavior of the cathode spot steered by an axisymmetric magnetic field on a small and thick target. In the present work, an AMF produced by using an adjustable electromagnetic coil enclosed with a coaxial cylinder of magnetically soft material was applied on the cathode surface to steer the cathode spot motion, and TiN films were deposited with different AMF intensities. Based on the results of the finite element method (FEM) simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion at different AMF intensities was discussed. The effect of the AMF on the reduction of the MPs on the surface of the resultant TiN films was also investigated, and the results were compared with the theoretical predictions.
2. Experimental A schematic diagram of the experimental apparatus is shown in Fig. 1. A water-cooled titanium cathode (99.99% purity) of 64 mm in diameter and 30 mm in thickness with an inclined wall shoulder was mounted inside a vacuum chamber, which acted as the anode. An adjustable electromagnetic coil enclosed with a coaxial cylinder of magnetically soft material, which was protected by using a deposited nickel coating, was located behind the cathode to produce the magnetic field steering the cathode spot motion. A circular-shaped magnetic flux guide placed around the cathode surface was utilized to increase the transverse magnetic component. TiN films were deposited on 1Cr18Ni9Ti stainless-steel specimens with different AMF intensities. The deposition time was 30 min, and the arc current was controlled at 60 A. The images of cathode spot motion was captured from a dynamic video by a highspeed camera, the exposure time was set as 1/1000 s to get an obvious spot trace. A reflector with 45 angle of the cathode surface was employed to reflect the light of cathode spot into the camera lens, which was placed close to the observation window. A transparent protective layer that can be replaced was employed to avoid the observation window being coated. The distribution of the AMF was simulated and analyzed by using the FEMM 4.2 software package. The magnetic field’s
Fig. 1. Schematic illustration of the AMF enhanced VAD equipment.
intensity could be adjusted by altering the current to the electromagnetic coil. The transverse magnetic flux density with the value (which varies in the range of 0–35 G) at the bottom of the edge of the cathode was chose as the parameter to evaluate the MPs reduction. The morphology of the TiN films was observed by using a scanning electron microscope (SEM, Hitachi S-4200). A quantitative analysis of the number and the size distribution of Mps was carried out by using an image analysis system (SISC IAS V8.0). This system has the ability to adjust the threshold of the intensity of the pictures, pick up the MPs by the color and brightness contrast between the MPs and the background of the films, and that could measure the diameter and the area of each particle automatically. 3. Results 3.1. Magnetic flux distribution Fig. 2 shows the magnetic flux distribution obtained by using the FEM simulation at two different situations. It can be seen that magnetic flux exhibited an axisymmetric distribution, the intensity of magnetic flux decreased from a large value near the cathode to a small value far away from the cathode, and the magnetic flux intersected with the target surface at acute angles directed to the edge of the cathode. Fig. 2(b) and (a) presents the magnetic field configurations around the cathode target with or without a circular-shaped magnetic flux guide placed around the cathode surface. The magnetic flux guide could cause a concentration and enhancement of the magnetic flux and could increase the transverse magnetic component. The distributions of the transverse and the normal components of different magnetic flux density on the cathodic target surface are plotted in Fig. 3(a) and (b), respectively. The transverse component of the magnetic flux density increased gradually from zero at the center of the target to a relatively larger value at the edge of the target. In contrast, the normal component of the magnetic flux density decreased from the maximum value at the center of the target to a relatively smaller value at the edge of the target. The transverse magnetic flux density at the bottom of the inclined wall shoulder, which was measured with a magnetometer (SHT-V), increased from 0 G to 35 G by altering the current to the electromagnetic coil. 3.2. Cathode spot movement Cathode spots are highly unusual physical objects and notoriously difficult to measure, but one could image the light emission from a spot. Therefore, the images of cathode spot motion (macroscopic) were captured from a dynamic video by a highspeed camera. A cathode spot is an assembly of emission centers showing fractal properties in spatial and temporal dimensions [18]. To get an obvious spots trace (a group of spot ignitions but not one spot event), the exposure time was set as 1/1000 s because long exposure time would smear out the image of a fast changing object and show the overall trajectory. A schematic illustration and transient cathode spot morphologies with different AMF intensities are shown in Fig. 4. The results show that increasing the AMF intensity strongly influences the cathode spot motion. In the case of a weak AMF, a big bright cathode spot moves randomly and slowly on the cathode surface, typical images of which are shown in Fig. 4(a) and (a0 ). With increasing AMF, there is an increasing tendency for the cathode spot to refine, rotate and drift toward the cathode target edge, exhibiting a chrysanthemum structure, the cathode spot pattern changes from a big bright spot to a longer and thinner line, which is shown in Fig. 4 (b) and (b0 ). An increase in the transverse magnetic field (TMF), B//, intensity can accelerate the rotational velocity of the
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Fig. 2. Magnetic flux distribution obtained by using FEM simulation: (a) without a circular-shaped magnetic flux guide and (b) with a circular-shaped magnetic flux guide.
cathode spot and increase the arc voltage. With a relatively strong AMF (B// z 30 G), the cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new cathode spot is ignited, splits, and is extinguished repeatedly on the cathode surface, which can be observed at intervals of about 0.5 s, as shown in Fig. 4(c) and (c0 ). The increase in the AMF causes an increase in the arc voltage, which is observed in the experiment. The arc voltage increase can be attributed to the distortion and elongate of the plasma channel due to the increasing rotational velocity of the cathode spot. Another reason for arc voltage increase is the raised resistance between the cathode and the anode owing to the electrons loss in the plasma channel resulting from the electron drift confined by the curved magnetic field configuration around the cathode. A stable arc discharge requires sufficient electrons transport between the anode and the cathode, an increase in the arc voltage can compensate the electrons loss in the plasma channel.
3.3. MPs reduction Typical SEM micrographs of the TiN films with different magnetic field intensities are shown in Fig. 5(a)–(d). Fig. 6 presents an overall size distribution of MPs on TiN films. It can be seen that MPs densities and sizes are greatly decreased with increasing the
magnetic field intensity. The SEM micrographs of the TiN films show that in the case of a weak AMF, MPs are mostly large liquid particles. While for a relatively strong AMF, MPs are mostly small solidified particles. Fig. 7 shows the variation in the number of MPs classified by their diameters with transverse magnetic field. For comparison, a histogram showing the total number of MPs collected larger than different diameters is also plotted in Fig. 8. The most striking feature is the MPs’ decreased slope over several orders of magnitude changes less, this indicates a fractal characteristic of MPs distribution on films. The dependence of the TiN film roughness (Ra) on the TMF is plotted in Fig. 9. It is clear that reducing MPs could significantly improve the surface quality of films.
4. Analysis and discussion A key to seeking the mechanism of cathode spot motion is to look at the life cycle of a spot, in particular the physical mechanism of the spot discharge and the condition for the occurrence of a spot. According to the emission process of electrons from the cathode, an arc discharge is characterized by a collective electron emission mechanism, which can generate large numbers of electrons. A continuous large amounts electrons emission is the key factor for a self-sustaining arc discharge. It is more effective to increase the
Fig. 3. Distribution of different magnetic field intensity on the target surface: (a) transverse component (b) normal component.
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Fig. 4. Schematic illustration and static images of the cathode spots motion under different magnetic field intensities: (a, a0 ) TMF ¼ 0 G (b, b0 ) TMF ¼ 15 G (c, c0 ) TMF ¼ 30 G.
temperature on a small concentrated area to obtain higher electron current with a cathode near room temperature (also called cold cathode). Therefore, electrons emission occurs in VAD process (where cathode is directly or indirectly water-cooled) is localized in fragments of hot, micron-size, non-stationary cathode spot, which
determined the physical characteristics of arc plasma and properties of subsequent films. The formation of cathode spot is a fundamental characteristic of the vacuum arc discharge with a cold cathode, the processes in a cathode spot are very complex due to the very small scale and
Fig. 5. SEM micrographs of the MPs on the surfaces of TiN films for different magnetic fields. (a) TMF ¼ 0 G; (b) TMF ¼ 10 G; (c) TMF ¼ 20 G; and (d) TMF ¼ 30 G.
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Fig. 8. Total numbers of MPs collected with diameter larger than different diameters for different magnetic fields. Fig. 6. Overall size distribution of MPs on the surface of TiN films.
very short characteristic time of the spot. To seek the mechanism of cathode spot motion in AMF, the key factor affecting the occurrence, maintanence and movement of a spot should be studied by analyzing the physical mechanism of the spot discharge, especially the condition of continuous emission of a large amount of electrons. To generate a large amount of electrons, it means that there have lots of electrons can overcome the potential barrier at the cathode surface. Only thermionic emission cannot provide so many electrons from the micro-size cathode spot, an electric field that deforms the potential barrier should be formed above the cathode surface, and any field enhancement factors promote the emission of more electrons. When a spot is ignited, the cathode material suffers a transition to dense plasma at the spot due to the processes of effective ionization and ion acceleration in collisions of accelerated electrons with evaporated atoms [19]. Most of the applied voltage drops in a very thin sheath near the cathode due to the presence dense plasma, and the adjacent dense plasma influences the effective work function of the cathode surface. Hence, even a small voltage drop of the order of 20 V, which is a characteristic of arc discharges, can create an electric field at the cathode surface strong enough to cause field emission of electrons. Therefore, the dense plasma with adjacent sheath is associated with a strong electric field, and this strong electric field gives rise to a continuous large amount of
Fig. 7. Variation in the number of MPs classified by their diameters with the transverse magnetic field.
electron emission, which is the key factor to a self-sustaining arc discharge. The sheath thickness is of critical importance to the ignition of an emission center because it determines the surface electric field, which needs to be sufficiently high to cause thermal runaway at this location. The contracted sheath thickness caused by the dense plasma depends on the plasma density, which follows Child’s law [20], and different dense plasma densities can cause different increase in the electric field strength, which may cause different enhancements of electron emission. The probability of the largest amount of electron emission appears at the location that has the largest dense plasma density. After a spot is ignited, a small molten pool of liquid cathode material is also formed on the cathode surface due to the high-power density in the cathode spot, and the molten pool grows in size rapidly. The increasing molten pool size and temperature result in a decrease in the current density and an increase in the resistance of the cathode material at the spot area. The region of the cathode bulk directly under the cathode spot is more resistive than all other areas or parts of the cathode. Hence, if there were alternative, less resistive way for the current to flow, the current would switch to the new path [18]. Therefore, the heat production and the original spot will cool down and the process terminates eventually. During this cycle, at least a new spot, which takes over the current from the original spot, must be formed and
Fig. 9. Dependence of the TiN film roughness (Ra) on the TMF.
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activated to maintain the discharge. The probability of initiation of a new cathode spot will occur at a place where the electric field is sufficient to extract a large electron current to fulfill the ignited condition such as the minimum ‘chopping’ current [21]. The spot is established when this current is large enough to heat the spot to fusion and even to ebullition; then, the spot is sustained as a result of the presence of the dense plasma. A continuous rebuilding of the dense plasma occurs since the repetitive formation of ionized cathode material originates from newly developed spots. The repeated processes, new daughter spots forming at the edge of previous parent spots, give rise to an apparent spot motion across the cathode surface. The probability of initiation of new cathode spots can be affected by the way of the dense plasma distributed over the surface, and occurs at the place with the largest dense plasma density. Today, it is firmly established that cathode spot motion is not motion of matter, but a process associated with a rapid sequence of individual ignition and extinction of active emission sites [22], and an external magnetic field B strongly influences the behavior of cathode spot. Intensive studies of cathode spot dynamics under different magnetic field components in a vacuum have been performed theoretically by researchers [14–17]. It has been demonstrated that the cathode spot moves at random on the cathode surface in the absence of a magnetic field while it moves in a direction perpendicular to the field lines in the reverse direction to the Ampere rule (‘retrograde’ direction) when a magnetic field is applied parallel to the cathode surface, and follows the ‘acute angle rule’ under the condition of a sharp-angled magnetic field. Though there are many models to explain the cathode spot motion under different magnetic field components, especially in a transverse magnetic field, the key factors are the dense plasma distribution and redistribution on the cathode surface. Let us continue by grabbing this key factor firmly to understand the motion of cathode spot in an AMF. In our experiment, the cathode spot moved at random on the cathode surface in the case of a weak AMF. This was reasonable because the effect of a weak AMF on the plasma was relatively small, and the redistribution of dense plasma was affected by other factors. Initially the dense plasma mainly had a symmetry shape above the molten pool. It was obvious that the dense plasma cannot retain its symmetry definitely due to some kind of instability, such as expanded plasma jet, unstable gas flow and ions diffusion. Thus, this will result in asymmetry distribution of dense plasma. Finally, in view of the cathode spot growth, the current density was too low to sustain the existence of original spot, a new cathode spot would ignited at the place with the largest dense plasma density. Furthermore, spot process also depended on the surface conditions, including the presence of a dielectric layer and geometric enhancement, which promoted the buildup of surface charge and ignition of new spots. All these factors were instable, fluctuant and had a random distribution; therefore, the probability of initiation of a new cathode spot would occur on the target surface randomly and would exhibit a stochastic cathode spot motion. In an imposed magnetic field, the cathode spot motion could be defined by three superimposed components. These components were the random walk, the retrograde motion and the Robson drift. The random walk component was always present and depends only on the cathode material. In our experiment, the AMF was composed of two components: transverse magnetic field and normal magnetic filed. With increasing AMF, the increasing transverse magnetic field component had a deterministic effect on the plasma and this effect caused the largest dense plasma density always occur in the ‘retrograde’ direction, leading to a preferential initiation of the new cathode spot on the retrograde side. The axisymmetric distribution of transverse magnetic field component caused
a rotational cathode spot motion tendency, the strength of which and the rotational velocity of cathode spot depended on the transverse magnetic filed (TMF), B//, intensity. The acute angles formed by AMF intersected with the target surface directed to the edge of the cathode, increasing the tendency of Robson drift toward the cathode target edge. Therefore, there was an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge, combined with the cathode spot random walk and the restriction effect of electric current inside the cathode on the expanded cathode spot, leading to an exhibited chrysanthemum structure. With a relatively strong AMF (B// z 30 G), the tendency of rotational arc cathode spot motion and Robson drift was strong enough that the random walk could be neglected. The cathode spot would escape from cathode surface if there were not any confine on the smooth cathode surface. In our experiment, a cathode with an inclined wall shoulder was employed. Fig. 10 shows the magnetic flux distribution on the inclined wall shoulder of the cathode. On the one hand, the magnetic flux intersects with the inclined wall shoulder and forms acute angles directed to cathode surface, avoiding the cathode spot escaping from cathode surface. On the other hand, the magnetic flux intersects with the target surface at acute angles directed to the edge of the cathode, restricting the drift of cathode spot to the center of cathode surface. Due to these combined factors, the arc cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. The high rotational velocity of confined cathode spot could lead to the spilt of arc sometimes, which was also observed and illustrated in Fig. 4(c) and (c0 ). The stable confined rotational cathode spot motion generates high-density dense plasma above the edge of the cathode surface while the plasma density above the center of the cathode surface is relatively low. Therefore, the dense plasma diffuses from the edge to the center of the cathode surface, increasing the plasma density above the center of the cathode surface. A new spot may be ignited at the center of the cathode surface when the plasma density is sufficient to induce effective electron emission to fulfill the ignition condition. Under the combined effects of the strong tendency for rotational cathode spot motion and Robson drift, the ignited spot extends rapidly to the edge of the cathode surface, exhibiting a helical form, which was shown in Fig. 4(c0 ). The repetitive processes of diffusion, ignition, and extension leads to a repeating ignition, split, and extinguishment of the cathode spot motion on the cathode surface, which can be observed at intervals of about 0.5 s. After a spot is ignited, various sizes and shapes of MPs are also emitted. The generation of MPs is the most severe issue in vacuum arc deposition. Though the specific mechanism of the formation of the MPs is unclear, it can straightforward believe that a small molten pool formed on the cathode surface coupled with the timevarying plasma pressure produces the ejection of MPs. The MPs
Fig. 10. Magnetic flux distribution on the inclined wall shoulder of the cathode.
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generation is related to the formation of a molten pool of liquid cathode material formed as a result of thermal runaway and the explosive plasma formation. MPs form when the molten pool of liquid cathode material yields to the large plasma pressure. The volume of the molten pool of liquid cathode material is determined by the local energy input associated with heat conduction and ion bombardment in the cathode volume. The amount of local energy input depends on the spot residence time associated with the spot velocity, which varies from one place to another, and on the probability of traversing areas previously heated. An increase in the AMF can increase the rotational velocity of cathode spot and decrease the spot residence time. A shorter interaction time of the dense plasma with the liquid cathode material at the spot location can generate fewer MPs ejection. In the case of a relatively weak magnetic field, the cathode spot moves slowly on the target, resulting in a relatively large-volume molten pool between the dense plasma and the cathode bulk, which causes the emission of more large liquid MPs. However, an increase in the intensity of the transverse component of the AMF could accelerate the rotational velocity of the cathode spot and cause a shorter interaction time of the plasma with the cathode material at the spot location, resulting in a reduced MPs size and reduced amount of MPs emission. The distribution of MPs indicated a fractal characteristic associated with the fractal nature of a cathode spot [23]. It is clear that reducing MPs could significantly improve the surface quality of films, so it deserves more research. 5. Conclusions This paper describes the mechanism of cathode spot motion in the presence of an AMF and the MPs reduction on TiN films prepared by using VAD. Cathode spot motion is not motion of matter but is a process associated with a rapid sequence of individual ignitions and extinctions of active emission sites. The probability of initiation of a new cathode spot is affected by the way that dense plasma distributed over the surface and occurs at the location with the largest dense plasma density. In the case of a weak AMF, the cathode spot moves at random on the cathode surface due to the relatively small effect of the weak AMF on the plasma, and the redistribution of dense plasma is affected by fluctuating factors. With increasing AMF, the increasing tendency of rotational cathode spot motion and Robson drift lead to an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. This is combined with cathode spot random walk, leading to chrysanthemum structure. With a relatively strong AMF, the
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cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new cathode spot is ignited, splits, and is extinguished repeatedly on the cathode surface, which can be observed at intervals of about 0.5 s. The molten pool formed on the cathode surface, coupled with the time-varying dense plasma pressure to the cathode, produces the ejection of MPs. An increase in the AMF could increase the velocity of rotational cathode spot motion, decrease the spot residence time, and generate fewer MPs ejections. The distribution of MPs indicated a fractal characteristic associated with the fractal nature of the cathode spot. Reducing MPs can significantly improve the surface quality of films, so further research is warranted. We believe that this article brings some new insight into the VAD process. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (863 Program), No. 2006AA03Z521, Nano-composite Coatings of Ultra-fine Grained Hard Metals. The authors would be grateful to D. C. Meeker with Foster-Miller Inc. USA for his free provision and kind advice about the FEMM 4.2 software package. References [1] Kimblin CW. J Appl Phys 1974;45:5235. [2] Hantzsche E. IEEE Trans Plasma Sci 2003;31:799. [3] Boxman RL, Martin PJ, Sanders DM, Handbook of vacuum arc science and technology; 1995. [4] Boxman RL, Goldsmith S. Surf Coat Technol 1992;52:39. [5] Anders S, Anders Andre, Brown IG. IEEE Trans Plasma Sci 1993;21:440. [6] Swift PD. J Phys D Appl Phys 1996;29:2025. [7] Ramalingam S, Qi CB, Kim K, U.S. patent: 4673477; 1987. [8] Karpov DA. Surf Coat Technol 1997;96:22. [9] Martin PJ, Bendavid A. Thin Solid Films 2004;394:1. [10] Boxman RL, Beilis II. IEEE Trans Plasma Sci 2005;33:1618. [11] Huang Mei Dong, Lin Guo Qiang, Dong Chuang. Surf Coat Technol 2003;176:109. [12] Ju¨ttner B. J Phys D Appl Phys 2000;33:2025. [13] Walke PJ, New R, Care CM. Surf. Coat. Technol 1993;59:126. [14] Tanberg R. Nature 1929;124:371. [15] Ju¨ttner B. J Phys D Appl Phys 1995;516:28. [16] Beilis II. IEEE Trans Plasma Sci 2001;29:657. [17] Beilis II. Appl Phys Lett 2002;181:3936. [18] Anders A. Cathodic arcs: from fractal spots to energetic condensation; 2008. [19] Boxman RL, Zhitomirsky VN. Rev Sci Instrum 2006;77:6748. [20] Forrester AT. Large ion beams. New York: Wiley; 1988. [21] Smeets RPP. IEEE Trans Plasma Sci 1989;17:303. [22] Anders A. IEEE Trans Plasma Sci 2005;33:1456. [23] Anders A. Thin Solid Films 2006;502:22.