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Development and characterization of toroidal magnetron sputtering system for thin films deposition
Thin Solid Films 518 (2010) 6650–6653
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Development and characterization of toroidal magnetron sputtering system for thin films deposition Young-Woo Kim ⁎, Yonghyun Kim, Daechul Kim, Jong-Sik Kim, Jung-Sik Yoon, Suk Jae Yoo, Bongju Lee Convergence Plasma Research Center National Fusion Research Institute, Gwahangno113, Yuseong-Gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Available online 2 April 2010 Keywords: Toroidal magnetron sputtering system Auxiliary magnet Low voltage discharge Thin films
a b s t r a c t Newly designed magnetron sputtering system called the toroidal magnetron sputtering (TMS) system was developed and evaluated to decrease the damage of thin films surface. On the basis of magnetic field simulation, it was constructed and debugged for the plasma generation of high ionization rate near a target surface. Magnetic field distributions were measured by a magnetic field measurement probe. Auxiliary magnets were added by means of the magnetic field simulation at the corners of the TMS source. As a result, the TMS source could be operated at lower voltage. In the case of the TMS source with the auxiliary magnets, the sputtering yield times the target current density also increased at the same DC power. In this study, we investigated the possibility of employing the TMS system for thin films deposition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental apparatus
Magnetron sputtering is one of the efficient physical vapor deposition (PVD) processes for industrially important coatings, i.e. hard coatings, wear resistant coatings, corrosion resistant coatings, decorative coatings, and so on [1–5]. Magnetron sputter has been widely used in many processes because it has several advantages over other deposition systems such as improvement of uniformity in large area, easy control of film thickness and good possibility of compound material deposition [6,7]. In general sputtering system, however, the sputter target is face to face with the substrate and high energetic particles from the sputter target are injected to all areas of the substrate. Therefore, the film surface is damaged by the high energetic particles, which can lead to a serious degradation of the film quality. Also, the substrate is intensely heated by thermal radiation and the bombardment of high energy electrons from the target [8]. In this study, novel sputtering system, aiming at the lower damage of film surface, was constructed based on a regular tetragonal structure consisting of four sputter targets. Experiments were carried out for various magnet structures to verify the efficiency of a new sputter system and the optimum experimental conditions using the auxiliary magnet were examined. Also, the characteristics of the discharge condition with a different power applied to the sputter target were investigated. These results showed the possibility of employing the TMS system for thin films deposition.
As shown schematically in Fig. 1, the TMS system with high current and low voltage for high-rate deposition has been newly designed and constructed in-house. The TMS source chamber was made of stainless steel 267 mm long, 267 mm wide and 100 mm high as shown in Fig. 1 (a). The TMS source had a regular tetragonal structure which consisted of four parallel sputter targets 190 mm apart as shown in Fig. 1(b). Two Nd-Fe-B permanent magnet bars, 160 mm × 12 mm × 12 mm, were placed inside of a single sputter target to create lines of magnetic flux. The magnetic field strength was 4200 Gs at a surface of the magnet bars. These permanent magnet bars were arranged in a square shape in order to surround the regular tetragon-shaped sputter targets. Two permanent magnet bars were magnetized vertically to the surface of the target in the opposite directions from each other. The upper magnets were the North Pole and the lower magnets were the South Pole. Consequently, the lines of the magnetic flux were perpendicular to the electric field and parallel to the surface of the target. So, the E × B drift motion of the electrons was turned counterclockwise near the target surface. As a result of the trapping of electrons near the target surface, high-density plasma was formed around the target. The TMS source was water-cooled to prevent demagnetizing of the permanent magnet during sputtering. In Fig. 1, the length of the permanent magnet (160 mm) was smaller than the target (195 mm). Due to the smaller magnet length, the discontinuity of the magnetic field was formed at the corner of the sputter. Thus, adoption of the auxiliary magnets was necessary to get better magnetic field configuration at the corner. The gap between the auxiliary magnets, which were made up of Nd-Fe-B permanent magnets was equivalent to that of the magnet bars. The magnetic field strength was 5700 Gs at the surface of the auxiliary magnet. Unlike
⁎ Corresponding author. E-mail address: [email protected] (Y.-W. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.145
Y.-W. Kim et al. / Thin Solid Films 518 (2010) 6650–6653
6651
Fig. 1. Schematic diagram of the toroidal magnetron sputtering system: (1) source chamber; (2) permanent magnet bars; (3) auxiliary magnet; (4) auxiliary magnet block (5) sputter target; (6) anode.
the TMS source, a coolant wasn't necessary for the auxiliary magnets because the block of the auxiliary magnets wasn't directly exposed to the plasma and was heated only by conduction. For example, when sputtering power was 2 kW and sputtering time was 2 h, the block's temperature was measured below 50 °C. In this paper, we presented the improved results compared with no auxiliary magnets. Before construction of the TMS system, the magnetic field simulation was performed using a computer simulation program and compared to the experimental results. According to the computer simulation results, the auxiliary magnets had a large effect on the magnetic field configuration. Fig. 2 shows the strength of the magnetic field (B∥) with and without the auxiliary magnet at 50 mm from the top plate of the TMS source (center of the sputter target). In the case of the TMS source with the auxiliary magnets, the magnetic field strength became smooth compared with no auxiliary magnet. On the other hand, the E × B drift motion of the electrons is unstable due to the discontinuous distribution of the magnetic field at the corner when there was no auxiliary magnet. As a result, high-density plasma wasn't formed around the corner and the plasma discharge was also unstable. Sputtering experiment was performed for Cr deposition on Si wafer and glass plate. The TMS source was connected to a processing chamber. Prior to introducing the process gas, the TMS system was evacuated to a base pressure of ∼ 10−6 Torr by a turbo molecular pump and the chamber was filled with argon to a working pressure of 1–50 mTorr. The sputter target biases were 500–2000 W for sputtering Cr target. The z-axis position of the substrate was the perpendicular distance from the midpoint of the sputter target. The magnetic field distributions and I–V characteristics of the TMS source were measured. Also, the as-deposited Cr thin films were investigated using AFM and the film thickness was measured by α-step and SEM.
sputter target. It was observed that the strength of the magnetic field was relatively uniform between the edge and the midpoint of the sputter target.
3. Results and discussion Fig. 3 shows the distribution of the magnetic field strength (B∥) with and without the auxiliary magnet near the sputter target surface. The coordinate origin of the x-axis position was the midpoint of the single sputter target. These data were obtained experimentally from the magnetic field measurement probe. The probe position was 10 mm from the target surface. As shown in the figure, when the TMS source had the auxiliary magnet, the strength of the magnetic field became large enough to confine the electrons at the edge of the
Fig. 2. Configuration of the magnetic field strength (B∥) (a) with and (b) without the auxiliary magnet using a computer simulation.
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Y.-W. Kim et al. / Thin Solid Films 518 (2010) 6650–6653
Fig. 3. Distribution of the magnetic field strength (B∥) with and without the auxiliary magnet near the sputter target surface.
In Fig. 4, the relation between the discharge current and the discharge voltage was investigated to confirm the discharge characteristics of the TMS source with a Cr target for various gas pressures. The discharge current varied between 0.5 and 7 A in 0.2 A step to
Fig. 4. Current–voltage (I–V) characteristics of the TMS source (a) with and (b) without the auxiliary magnet at various operating pressures.
acquire I–V trace. It was found that the discharge voltage increased with decreasing the working pressure. When the auxiliary magnets were used in the TMS source, the discharge voltage was kept nearly constant despite increasing the discharge current. Also, the discharge voltage was much lower than that of the TMS source with no auxiliary magnets. Therefore, the TMS source with the auxiliary magnets not only generates high-density plasma near the target at the same power, but it will also reduce the arcing problem. Fig. 5 represents the deposition rate of Cr thin films with increasing the gas pressure for various substrate positions. It was obtained from the constant DC power of 2 kW (target power density of 5.13 W/cm2) and substrate positions of 10, 12.5 and 15 cm, respectively. The z-axis position of the substrate was counted from the midpoint of the sputter target. In the case of the TMS source with the auxiliary magnet, as shown in the figure, the deposition rate of Cr thin films increased about 10% below a gas pressure of 10 mTorr because the current density was much higher than that of the TMS source with no auxiliary magnets. It was also observed the deposition rate of Cr thin films reached approximately 150 nm/min for a gas pressure of 10 mTorr. The deposition rate of the TMS system was indirectly compared via other papers [9–12], whereupon it was found to be much higher than that of the conventional magnetron sputtering system at a similar power density. According to the AFM results, a root mean square (RMS) roughness of the Cr thin films was measured between 2 nm and 8 nm in either case (data isn't shown here). The RMS roughness increased with increasing the gas pressure and began to decrease after a gas pressure of about 10 mTorr. These results showed that the RMS roughness curves were similar to the configuration of the deposition rate as a function of gas pressure. It was also shown that the RMS roughness of the thin films increased with decreasing the substrate position. However, the roughness change by the auxiliary magnet has not been cleared until now. So it is necessary to do further experiment and research. From a theoretical standpoint, the deposition rate was directly proportional to the sputtering yield and the ion flux (Dsput = (γsputΓiAt) / (nf As)) where γsput is the sputtering yield and Γi is the incident ion flux. The sputtering yield depends on the target bias voltage and the ion flux depends on the target current density. The sputtering yield times the target current density increased slightly at the same DC power in the case of the TMS source with the auxiliary magnets and is shown in Fig. 6. Because the discharge voltage related to the sputtering yield decreased despite the increase of the current density, the deposition rate seemed to improve somewhat (corresponding to results in Figs. 4 and 5). And the TMS source with the auxiliary magnets can produce stable plasma compared to the TMS source with no auxiliary magnet.
Fig. 5. Deposition rate of Cr thin films with and without the auxiliary magnet for the substrate positions as a function of gas pressure.
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operated at low voltage and high current for high-rate deposition. In addition, in the case of the TMS source with the auxiliary magnets, the sputtering yield times the target current density also increased at the same DC power. These results revealed that the TMS system was very suitable for thin films deposition. However, re-design of the sputter structure, improvement of the magnetic field configuration and more detailed analysis of the thin films needs to be done for a more successful use of the TMS source.
Acknowledgement This research was supported by the R&D Program through the National Fusion Research Institute of Korea (NFRI) funded by the Ministry of Education, Science and Technology (MEST).
References Fig. 6. Sputtering yield times current density with and without the auxiliary magnet as a function of gas pressure under the same DC power.
4. Conclusions In order to improve the disadvantages of the conventional magnetron sputter such as substrate heating by thermal radiation and the bombardment of high energy electrons, damage of film surface by high energetic particles, etc., a new type of the sputtering system should be developed. In this work, the TMS source with a regular tetragonal structure consisting of four sputter targets was developed and evaluated. By computer simulation and measurement of the magnetic field distribution, the auxiliary magnet was applied to the TMS source. I–V measurement confirmed that the TMS system was
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P.J. Kelly, R.D. Arnell, Vacuum 56 (2000) 159. K.H. Nam, M.J. Jung, J.G. Han, Surface Coatings Technology 131 (2000) 222. Q. Yang, L.R. Zhao, P.C. Patnaik, X.T. Zeng, Wear 261 (2006) 119. B. Subramanian, M. Jayachandran, Materials Letters 62 (2008) 1727. M. Nose, T. Nagae, M. Yokota, S. Saji, M. Zhou, M. Nakada, Surfing Coatings Technology 116 (1999) 296. Brian Chapman, Glow Discharge Processes, JOHN WILEY & SONS, New York, USA, 1980, p. 177. Joh A. Thornton, Deposition Technologies for Films and Coatings, NOYES, New Jersey, USA, 1982, p. 170. M. Naoe, S. Yamanaka, Y. Hoshi, IEEE Transactions on Magnetics MAG-16 (1980) 646. J.H. Bin, K.H. Nam, J.-H. Boo, J.G. Han, Journal of the Korean Institute Surface Engineering 34 (1980) 409. J.M. Purswani, T. Spila, D. Gall, Thin Solid Films 515 (2006) 1166. D. Kim, Transactions on Electrical and Electronic Materials 10 (2009) 165. K.H. Kim, I.H. Son, K.B. Song, S.H. Kong, M.J. Keum, S. Nakagawa, M. Naoe, Applied Surface Science 169–170 (2001) 410.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Development and characterization of toroidal magnetron sputtering system for thin films deposition Young-Woo Kim ⁎, Yonghyun Kim, Daechul Kim, Jong-Sik Kim, Jung-Sik Yoon, Suk Jae Yoo, Bongju Lee Convergence Plasma Research Center National Fusion Research Institute, Gwahangno113, Yuseong-Gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Available online 2 April 2010 Keywords: Toroidal magnetron sputtering system Auxiliary magnet Low voltage discharge Thin films
a b s t r a c t Newly designed magnetron sputtering system called the toroidal magnetron sputtering (TMS) system was developed and evaluated to decrease the damage of thin films surface. On the basis of magnetic field simulation, it was constructed and debugged for the plasma generation of high ionization rate near a target surface. Magnetic field distributions were measured by a magnetic field measurement probe. Auxiliary magnets were added by means of the magnetic field simulation at the corners of the TMS source. As a result, the TMS source could be operated at lower voltage. In the case of the TMS source with the auxiliary magnets, the sputtering yield times the target current density also increased at the same DC power. In this study, we investigated the possibility of employing the TMS system for thin films deposition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental apparatus
Magnetron sputtering is one of the efficient physical vapor deposition (PVD) processes for industrially important coatings, i.e. hard coatings, wear resistant coatings, corrosion resistant coatings, decorative coatings, and so on [1–5]. Magnetron sputter has been widely used in many processes because it has several advantages over other deposition systems such as improvement of uniformity in large area, easy control of film thickness and good possibility of compound material deposition [6,7]. In general sputtering system, however, the sputter target is face to face with the substrate and high energetic particles from the sputter target are injected to all areas of the substrate. Therefore, the film surface is damaged by the high energetic particles, which can lead to a serious degradation of the film quality. Also, the substrate is intensely heated by thermal radiation and the bombardment of high energy electrons from the target [8]. In this study, novel sputtering system, aiming at the lower damage of film surface, was constructed based on a regular tetragonal structure consisting of four sputter targets. Experiments were carried out for various magnet structures to verify the efficiency of a new sputter system and the optimum experimental conditions using the auxiliary magnet were examined. Also, the characteristics of the discharge condition with a different power applied to the sputter target were investigated. These results showed the possibility of employing the TMS system for thin films deposition.
As shown schematically in Fig. 1, the TMS system with high current and low voltage for high-rate deposition has been newly designed and constructed in-house. The TMS source chamber was made of stainless steel 267 mm long, 267 mm wide and 100 mm high as shown in Fig. 1 (a). The TMS source had a regular tetragonal structure which consisted of four parallel sputter targets 190 mm apart as shown in Fig. 1(b). Two Nd-Fe-B permanent magnet bars, 160 mm × 12 mm × 12 mm, were placed inside of a single sputter target to create lines of magnetic flux. The magnetic field strength was 4200 Gs at a surface of the magnet bars. These permanent magnet bars were arranged in a square shape in order to surround the regular tetragon-shaped sputter targets. Two permanent magnet bars were magnetized vertically to the surface of the target in the opposite directions from each other. The upper magnets were the North Pole and the lower magnets were the South Pole. Consequently, the lines of the magnetic flux were perpendicular to the electric field and parallel to the surface of the target. So, the E × B drift motion of the electrons was turned counterclockwise near the target surface. As a result of the trapping of electrons near the target surface, high-density plasma was formed around the target. The TMS source was water-cooled to prevent demagnetizing of the permanent magnet during sputtering. In Fig. 1, the length of the permanent magnet (160 mm) was smaller than the target (195 mm). Due to the smaller magnet length, the discontinuity of the magnetic field was formed at the corner of the sputter. Thus, adoption of the auxiliary magnets was necessary to get better magnetic field configuration at the corner. The gap between the auxiliary magnets, which were made up of Nd-Fe-B permanent magnets was equivalent to that of the magnet bars. The magnetic field strength was 5700 Gs at the surface of the auxiliary magnet. Unlike
⁎ Corresponding author. E-mail address: [email protected] (Y.-W. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.145
Y.-W. Kim et al. / Thin Solid Films 518 (2010) 6650–6653
6651
Fig. 1. Schematic diagram of the toroidal magnetron sputtering system: (1) source chamber; (2) permanent magnet bars; (3) auxiliary magnet; (4) auxiliary magnet block (5) sputter target; (6) anode.
the TMS source, a coolant wasn't necessary for the auxiliary magnets because the block of the auxiliary magnets wasn't directly exposed to the plasma and was heated only by conduction. For example, when sputtering power was 2 kW and sputtering time was 2 h, the block's temperature was measured below 50 °C. In this paper, we presented the improved results compared with no auxiliary magnets. Before construction of the TMS system, the magnetic field simulation was performed using a computer simulation program and compared to the experimental results. According to the computer simulation results, the auxiliary magnets had a large effect on the magnetic field configuration. Fig. 2 shows the strength of the magnetic field (B∥) with and without the auxiliary magnet at 50 mm from the top plate of the TMS source (center of the sputter target). In the case of the TMS source with the auxiliary magnets, the magnetic field strength became smooth compared with no auxiliary magnet. On the other hand, the E × B drift motion of the electrons is unstable due to the discontinuous distribution of the magnetic field at the corner when there was no auxiliary magnet. As a result, high-density plasma wasn't formed around the corner and the plasma discharge was also unstable. Sputtering experiment was performed for Cr deposition on Si wafer and glass plate. The TMS source was connected to a processing chamber. Prior to introducing the process gas, the TMS system was evacuated to a base pressure of ∼ 10−6 Torr by a turbo molecular pump and the chamber was filled with argon to a working pressure of 1–50 mTorr. The sputter target biases were 500–2000 W for sputtering Cr target. The z-axis position of the substrate was the perpendicular distance from the midpoint of the sputter target. The magnetic field distributions and I–V characteristics of the TMS source were measured. Also, the as-deposited Cr thin films were investigated using AFM and the film thickness was measured by α-step and SEM.
sputter target. It was observed that the strength of the magnetic field was relatively uniform between the edge and the midpoint of the sputter target.
3. Results and discussion Fig. 3 shows the distribution of the magnetic field strength (B∥) with and without the auxiliary magnet near the sputter target surface. The coordinate origin of the x-axis position was the midpoint of the single sputter target. These data were obtained experimentally from the magnetic field measurement probe. The probe position was 10 mm from the target surface. As shown in the figure, when the TMS source had the auxiliary magnet, the strength of the magnetic field became large enough to confine the electrons at the edge of the
Fig. 2. Configuration of the magnetic field strength (B∥) (a) with and (b) without the auxiliary magnet using a computer simulation.
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Y.-W. Kim et al. / Thin Solid Films 518 (2010) 6650–6653
Fig. 3. Distribution of the magnetic field strength (B∥) with and without the auxiliary magnet near the sputter target surface.
In Fig. 4, the relation between the discharge current and the discharge voltage was investigated to confirm the discharge characteristics of the TMS source with a Cr target for various gas pressures. The discharge current varied between 0.5 and 7 A in 0.2 A step to
Fig. 4. Current–voltage (I–V) characteristics of the TMS source (a) with and (b) without the auxiliary magnet at various operating pressures.
acquire I–V trace. It was found that the discharge voltage increased with decreasing the working pressure. When the auxiliary magnets were used in the TMS source, the discharge voltage was kept nearly constant despite increasing the discharge current. Also, the discharge voltage was much lower than that of the TMS source with no auxiliary magnets. Therefore, the TMS source with the auxiliary magnets not only generates high-density plasma near the target at the same power, but it will also reduce the arcing problem. Fig. 5 represents the deposition rate of Cr thin films with increasing the gas pressure for various substrate positions. It was obtained from the constant DC power of 2 kW (target power density of 5.13 W/cm2) and substrate positions of 10, 12.5 and 15 cm, respectively. The z-axis position of the substrate was counted from the midpoint of the sputter target. In the case of the TMS source with the auxiliary magnet, as shown in the figure, the deposition rate of Cr thin films increased about 10% below a gas pressure of 10 mTorr because the current density was much higher than that of the TMS source with no auxiliary magnets. It was also observed the deposition rate of Cr thin films reached approximately 150 nm/min for a gas pressure of 10 mTorr. The deposition rate of the TMS system was indirectly compared via other papers [9–12], whereupon it was found to be much higher than that of the conventional magnetron sputtering system at a similar power density. According to the AFM results, a root mean square (RMS) roughness of the Cr thin films was measured between 2 nm and 8 nm in either case (data isn't shown here). The RMS roughness increased with increasing the gas pressure and began to decrease after a gas pressure of about 10 mTorr. These results showed that the RMS roughness curves were similar to the configuration of the deposition rate as a function of gas pressure. It was also shown that the RMS roughness of the thin films increased with decreasing the substrate position. However, the roughness change by the auxiliary magnet has not been cleared until now. So it is necessary to do further experiment and research. From a theoretical standpoint, the deposition rate was directly proportional to the sputtering yield and the ion flux (Dsput = (γsputΓiAt) / (nf As)) where γsput is the sputtering yield and Γi is the incident ion flux. The sputtering yield depends on the target bias voltage and the ion flux depends on the target current density. The sputtering yield times the target current density increased slightly at the same DC power in the case of the TMS source with the auxiliary magnets and is shown in Fig. 6. Because the discharge voltage related to the sputtering yield decreased despite the increase of the current density, the deposition rate seemed to improve somewhat (corresponding to results in Figs. 4 and 5). And the TMS source with the auxiliary magnets can produce stable plasma compared to the TMS source with no auxiliary magnet.
Fig. 5. Deposition rate of Cr thin films with and without the auxiliary magnet for the substrate positions as a function of gas pressure.
Y.-W. Kim et al. / Thin Solid Films 518 (2010) 6650–6653
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operated at low voltage and high current for high-rate deposition. In addition, in the case of the TMS source with the auxiliary magnets, the sputtering yield times the target current density also increased at the same DC power. These results revealed that the TMS system was very suitable for thin films deposition. However, re-design of the sputter structure, improvement of the magnetic field configuration and more detailed analysis of the thin films needs to be done for a more successful use of the TMS source.
Acknowledgement This research was supported by the R&D Program through the National Fusion Research Institute of Korea (NFRI) funded by the Ministry of Education, Science and Technology (MEST).
References Fig. 6. Sputtering yield times current density with and without the auxiliary magnet as a function of gas pressure under the same DC power.
4. Conclusions In order to improve the disadvantages of the conventional magnetron sputter such as substrate heating by thermal radiation and the bombardment of high energy electrons, damage of film surface by high energetic particles, etc., a new type of the sputtering system should be developed. In this work, the TMS source with a regular tetragonal structure consisting of four sputter targets was developed and evaluated. By computer simulation and measurement of the magnetic field distribution, the auxiliary magnet was applied to the TMS source. I–V measurement confirmed that the TMS system was
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
P.J. Kelly, R.D. Arnell, Vacuum 56 (2000) 159. K.H. Nam, M.J. Jung, J.G. Han, Surface Coatings Technology 131 (2000) 222. Q. Yang, L.R. Zhao, P.C. Patnaik, X.T. Zeng, Wear 261 (2006) 119. B. Subramanian, M. Jayachandran, Materials Letters 62 (2008) 1727. M. Nose, T. Nagae, M. Yokota, S. Saji, M. Zhou, M. Nakada, Surfing Coatings Technology 116 (1999) 296. Brian Chapman, Glow Discharge Processes, JOHN WILEY & SONS, New York, USA, 1980, p. 177. Joh A. Thornton, Deposition Technologies for Films and Coatings, NOYES, New Jersey, USA, 1982, p. 170. M. Naoe, S. Yamanaka, Y. Hoshi, IEEE Transactions on Magnetics MAG-16 (1980) 646. J.H. Bin, K.H. Nam, J.-H. Boo, J.G. Han, Journal of the Korean Institute Surface Engineering 34 (1980) 409. J.M. Purswani, T. Spila, D. Gall, Thin Solid Films 515 (2006) 1166. D. Kim, Transactions on Electrical and Electronic Materials 10 (2009) 165. K.H. Kim, I.H. Son, K.B. Song, S.H. Kong, M.J. Keum, S. Nakagawa, M. Naoe, Applied Surface Science 169–170 (2001) 410.