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Metal Cylindrical Sieve (MCS) for plasma confinement and low sputtering nitrogen plasma immersion ion implantation
Journal Pre-proofs Full Length Article Metal Cylindrical Sieve (MCS) for Plasma Confinement and Low Sputtering Nitrogen Plasma Immersion Ion Implantation Carla Silva, Mario Ueda, Carina Barros Mello PII: DOI: Reference:
S0169-4332(19)34049-8 https://doi.org/10.1016/j.apsusc.2019.145232 APSUSC 145232
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
14 October 2019 11 December 2019 30 December 2019
Please cite this article as: C. Silva, M. Ueda, C. Barros Mello, Metal Cylindrical Sieve (MCS) for Plasma Confinement and Low Sputtering Nitrogen Plasma Immersion Ion Implantation, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145232
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Metal Cylindrical Sieve (MCS) for Plasma Confinement and Low Sputtering Nitrogen Plasma Immersion Ion Implantation Carla Silva1, Mario Ueda1, Carina Barros Mello1 1
Associated Laboratory of Plasma and Materials, National Institute for Space Research – INPE, Av. dos Astronautas, 1758, São José dos Campos, 12227-010, Brazil Abstract Nitrogen Plasma Immersion Ion Implantation (N-PIII) was performed in the Metal Cylindrical Sieve (MCS) configuration that consists of Stainless Steel 308 L (SS308L) spiral spring wound wire. Two N-PIII regimes were obtained: one using a current of 5 A (with ends open), in which an intense sputtering of the wire occurs while for currents above 12 A (with one side closed), the sputtering is diminished substantially. This behavior at different powers in MCS can be explained in terms of the sheath overlapping between the spiral spring paths. The plasma density near the wire is insufficient to overcome the sheath overlapping between the spiral spring paths when the current is low, which leads to high sputtering of the wire surface. However, in high current, sheath overlapping is avoided and high ion implantation condition is achieved. FEG-SEM analyses of the silicon sample placed on the screen plate in front of MCS mouth indicated a high deposition of the sputtered material at low current in contrast with low deposition at high current. These deposition free results allow us to plan efficient N-PIII treatments of different types and sizes of metal springs, grids, rings, and wires, with high performance, for industrial and scientific applications.
Keywords: Ion implantation, surface modification, hollow cathode discharge, deposition in metallic tubes, SS308L MCS tubes.
1
INTRODUCTION Treatments inside metallic tubes are attracting much attention of the plasma immersion ion implantation (PIII) community recently, due to their possible applications in highperformance tubing for oil companies, chemical, and food industries as well as for fuel pipes of fluids or corrosive gases [1]. These types of treatments can be carried out using hollow cathode discharges (HCD) as the plasma source given their better plasma confinement and higher density capabilities, which allow possibilities of achieving significantly better results than those obtained by conventional PIII processes, as higher uptake of implanted species and deposition rate as well as much thicker treated coatings [2-4]. Despite the various advantages of treatments inside metallic tubes when compared to the treatments in planar substrates, very high sputtering occurs inside the tubes, especially for the small diameter ones [5]. In view of these deleterious effects, a limitation in the tube treatments was found, since depending on the application, the high sputtering inside the tube could modify unfavorably the inner wall surface and therefore there is the possibility of not meeting the requirements for a particular tube application. Studies of these effects have to be performed, investigating ways to minimize the high sputtering that occurs during the treatment. A paper was published recently in which the authors used a grid-type configuration to performing Diamond Like-Carbon (DLC) deposition [6]. In their paper, it is shown that for DLC deposition, the result is more effective for the grid-type support than when using the conventional cylindrical tube. This is due to the high rate of sputtering inside the tube and also the difficulty of the mobility of the ions and the field distribution in such a configuration. Therefore, we decided to test a new tube configuration, the MCS configuration, combining the advantages of a grid type support with cylindrically closed support with enhanced plasma confinement. In this way, the electric field penetrated more easily the openings of the tubular cylinder and the ions had greater mobility through the sieve, making the PIII treatment of wire wall more effective. Therefore, preliminary results of an investigation to achieve better conditions of N-PIII treatments based on a newly developed Metal Cylindrical Sieve scheme producing highly confined plasma with either high or low sputtering of the wall are described in this paper.
2
MATERIALS AND METHODS: The PIII device used in the present study consists basically of a cylindrical discharge vacuum chamber of 30 liters (Fig. 1) and a RUP-6 high power high voltage source to pulse the samples at a maximum nominal power of 13 kW [7]. As can be seen in the figure, the MCS configuration adopted in the present experiments consists of stainless steel 308L wire with 0.15 cm diameter wound in a spiral spring shape with 4.0 cm tube diameter, 20 cm of length and pitch of about 1 cm. Since the MCS device does not hold straight by itself, these wires were wrapped around so that they were cylindrical in shape, and two clamping plates of stainless steel SS304 were used so that the MCS was in the final spiral spring shape and with a pitch of 1 cm. This configuration was fixed in the upper part of the chamber (aiming to minimize the occurrence of arcs) by means of a stainless steel ring attached to the surface of the MCS [8] and connected to high voltage feedthrough. In the first experimental condition, the MCS was used with two open ends which resulted in low current discharge (5 A). In the second experimental condition, one side of the MCS was closed (with a lid), and in this set-up, it was possible to achieve high currents (12 A). Additionally, in both cases, an electrically grounded screen plate of SS304 was used in front of the tube mouth where 1/3 or half of the silicon wafer substrate was fixed to it, aiming to detect/measure the two-dimensional deposition patterns of the materials ejected from the tube (Fig. 2. The screen plate was placed at 4 cm distance from the tube mouth. These silicon substrate samples were previously subjected to cleaning procedures before the PIII treatment to avoid any contamination that could affect the quality of the deposited material. Argon ion bombardment was used to cleaning the samples surface and tube forming wire/support structure, for 10 minutes using the following pulser conditions: 5.2x10-2 mbar, 6 A, 1.5 kV, 500 Hz and 20 µs and then, the nitrogen implantation was carried out for up to 60 minutes. Nitrogen was implanted with a pulse repetition rate of 500 Hz and pulse duration of 20 µs. The pulse voltage and the current were adjusted to maintain the treatment temperature relatively constant (measured with an optical pyrometer). These high voltages (H. V.) pulser conditions applied to the MCS, are described in Table I. The surfaces of silicon wafer were analyzed by field emission gun scanning electron microscopy (FEG-SEM) and the surface of the stainless steel plate and samples were analyzed by X-ray diffraction (XRD). 3
RESULTS AND DISCUSSION Previous studies of PIII treatments of interior of metallic tubes using hollow cathode discharges indicated very high sputtering effects, especially in small tubes with diameters less than 4 cm Ø [9]. In view of these results, for a direct comparison between open wall and continuous tubes, we have adopted a similar dimension tubular structure (with 4 cm Ø) for MCS. High-density plasmas that can be seen from the images of the confined plasmas, in Fig. 3, are formed inside the Metal Cylindrical Sieve by means of the hollow cathode discharge. The high-density plasma was confined inside the MCS but a part of it flows out from the open end of the MCS toward the grounded screen plate (Fig. 2). This is a quite different idea from the cage configuration often used in plasma nitriding (cage configuration) [10]. Pictures of lateral and frontal images of the plasma are shown in the same figure, as well as a picture including the Si wafer piece in front of the open tube mouth. The MCS configuration tube with 4 cm Ø has a significantly smaller surface area compared to the continuous one of the same diameter (about 1/5 to 1/10). In this way, it was naturally expected a strong reduction of wire sputtering and therefore deposition of the material expelled from the MCS. However, when performing the N-PIII treatments in MCS, we verified that although we had adopted a low current treatment of 5 A for the first condition studied, an intense sputtering occurred in this case, as can be seen in the photo of the silicon wafer and SS304 plate placed in front of the tube (Fig. 4). This unexpected result could be explained as the following: although the tubular radius of the MCS under test of 4 cmØ is sufficiently large to avoid sheath overlapping for plasma densities obtained for 5 A discharges, the sheath sizes obtained between the wire paths for such currents are exceedingly large compared to the distance between the paths. Therefore, the overlapping of the sheaths and the sputtering occurs in the wire in this case [11,12]. In the experiments performed here, we observed a high sputtering effect (Fig. 4a) in the treatment without a lid when a low current was applied to the MCS. Such effects are strongly ascribed to the dependency between the sputtered particles and the incidence angle of the ions: lower angles of incidence result in a higher sputtering yield [13]. Therefore we can suppose that the electric field is likely to be tangential along the wire since the sputtering was more pronounced as compared to the implantation (a schematic drawing of the electric field configuration in the MCS when 4
sheath overlapping occurred is shown in Fig. 5a) during the pulse. One way to minimize the occurrence of the sheath overlapping is to employ the scheme of one closed end with lid in the MCS and operate it with high powers, as used in this experiment. There was a very strong mark on the stainless steel plate and silicon wafer piece. The circular mark left was about twice the size of the 4 cm ∅ tube diameter (Fig. 4a). The SS304 screen plate was electrically grounded and placed about 4 cm away from the MCS mouth. It had a dark circular ring (sputtered material) on the outside of the mark and a light blue inner circle (both the screen and wafer) caused by the sheath overlap that occurs at low currents. By adopting higher current (12 A), we found that this phenomenon is supplanted which leads to a lower rate of sputtering and, consequently, a lower deposition on the silicon sample and SS304 plate (Fig. 4b). In this high current case, the E-fields are mostly perpendicular to the wire, as can be seen in Fig. 5b, favoring the ion implantation instead of sputtering. In this case, the deposition marks on the silicon wafer pieces and on the screen plate are reduced. In addition, by observing the deposition pattern for this case, we can see that there is a clear mark left on the silicon surface by the plasma hitting the silicon holding steel clips. This fact also confirms the generation of higher energy plasma for 12 A treatment compared to the 5 A case. In the previous work with a continuous tube of 1.1 cm in diameter, mapping was performed in order to know the conditions where the plasma sheath overlap and the conditions where this phenomenon was superseded. We could then identify which current range we should work without this phenomenon. For the case of this new MCS configuration, we see that a similar phenomenon occurred, but we did not perform a systematic mapping of working currents. However, we know from previous experiments [9] that if we increase the current the plasma sheath overlapping is avoided. The average thicknesses of depositions versus distances from the center of the circles on the silicon wafer were measured by FEG. In this analysis, silicon strips of 0.75 cm were sliced into pieces that were observed by FEG-SEM at the cross-sections in different points and their thicknesses averaged which are shown as data points on the graph of Fig. 6. We can see that the deposition is not uniform for the lower current condition (5 A). It is normal that the center of the silicon blade (1.5 cm) has more deposition; however, we see that in other regions there are also increases in thickness. This is due to three factors, the first, the plasma non-uniformity, the second due to the fact that spacing between the 5
wires that make up the MCS are not exactly of 1 cm, due to the difficulty of shaping them, and the third due to the sheath overlapping between the wires. So these three factors will greatly influence this thickness difference. For the condition of higher current (12 A) the phenomenon of sheath overlap has already been supplanted and, consequently, the deposition rate is lower, as we can see in the graph. In this treatment current we also verified a uniformity of thickness, but much smaller deposition than for the low current case. Therefore, the analysis of the deposition thickness on the silicon wafer pieces led us to confirm that two main different phenomena are occurring in the experiments presented above: a) For very low currents as 5 A, the phenomenon of sheath overlapping between the wire paths is occurring and b) Above 12 A, substantial nitrogen implantation is expected but also a thin deposition of the sputtered materials from the wires of MCS occurred. One inconvenience found in this new N-PIII scheme is that, as we increased the current in MCS, the vacuum chamber walls are heated considerably, in contrast to no heating for continuous wall cylinder tubes. For continuous tubes, the heating is high only in regions of the vacuum chamber where the plasma hits while flowing out from the opened mouths of the tubes. In MCS, the plasma spreads out from the sieve cylinder and brightens the chamber interior, heating it almost uniformly. To reduce the heating of the chamber, four powerful fans are used for cooling of our vacuum chamber used for N-PIII, which allows PIII and PIII&D (Plasma Immersion Ion Implantation and Deposition) treatments for more than 2 h. To perform the XRD analysis in parts of the MCS, cut pieces of the MCS clamping plate were used. These XRD results are shown in Fig. 7. For the treatment condition of 5 A, only the formations of CrN and α’ phases were observed. This occurred due to the overlapping of the plasma sheath, which prevented effective nitrogen implantation in this condition (lower temperatures as seen in Table I). On the other hand, for the treatment performed at 12 A, we can verify that the iron nitride layer was formed, indicating much effective nitrogen implantation (higher temperatures described in Table I). Below are presented some advantages of the Metal Cylindrical Sieve configuration: A) Plasma escaping from the open wall tube could be used to treat stainless steel or other type wires (as NiTi wires) using the nitrogen as working gas when the plasma density at points near the wires is high enough to overcome the overlapping of the 6
sheaths. The deposition of DLC on wires may be feasible also in this configuration, perhaps without the need to overcome the overlapping of the sheaths in that case; B) Discharge is very easy to be started and also to be maintained in the MCS configuration. The range of plasma operation in this configuration is very large; C) Plasma can be easily viewed from outside the tube through a transparent window. Just as it will be easy to hang and see the samples being treated inside the MCS tube (in contrast to the continuous wall tubes), in high-density plasma in the central region of the hollow cathode; D) It can be seen clearly from our results that the light emission of the plasma inside the tube is very intense, therefore much denser than the outside of it; E) Of course, with the leakage of plasma through the wires, it will heat the chamber much more than in the case of the continuous tube; F) And from what we saw on the target plate for the low current condition in MCS, there is a significant sputtering of the wires. This may be good (for deposition on the target) or bad (because of the high sputtering contaminating the target under treatment), depending on the application. G) When we perform the treatment inside continuous tubes the ions and the electric field will be perpendicular to the tube, so the ions will bombard the surface of the tube with high energy, causing the material sputtering from the inner side of the tube wall beyond the implantation. Auger profile for Ti-6Al-4V sample treated in continuous tubes showed a 23% nitrogen implantation with a depth of approximately 1.6 µm, but the iron contamination was 90% with a depth of 1 µm, that is, despite the implantation being excellent we have the problem of sputtered materials coming from the tube [14]. Therefore, for the treatment inside the MCS with high currents, the patterns of depositions are much thinner compared with continuous tubes because besides having a smaller surface area exposed, there is the spacing between the wires, which allows the ions to flow at different angles and the plasma fills in the interior of the tubular region resulting in high density confined plasma. GDOES profile for the Ti-6Al-4V sample treated in continuous tubes of SS304 showed 40% nitrogen implantation with a depth of approximately 250 nm, but iron contamination was 8% with depths of 75 nm. Here, with MCS, nitrogen implantation was low compared to the continuous tube (the data will be presented in a more complete article), but this happened because the treatment time used was only one hour. New treatments will be performed with longer times, aiming at increasing the depth of nitrogen implantation. 7
CONCLUSION: MCS is a new plasma source and sample fixation system built-up by light components in which high-efficiency PIII and PIII&D treatments can be performed. It is a source with lower cost and lighter than that using continuous tubes. It requires no special gas injection system because it is a transparent structure for gas feeding, and for this reason, components or samples are visible from outside the vacuum system, making it easier to follow the PIII and PIII&D treatments inside the open wall tube. The high deposition PIII&D treatment could lead to interesting multi-element film deposition for different applications. Experiments are been continued in this same subject and good nitrogen implantations in the wires, in the samples placed in the interior of the open wall tube as well as in the samples on the screen in front of the plasma, have been observed. These results will be reported soon in the oncoming paper. ACKNOWLEDGMENT This research work is supported by CNPq and Brazilian Ministry of Science Technology and Innovation and Communication (MCTIC).
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REFERENCES [1] R. Wei. Development of new technologies and practical applications of plasma immersion ion deposition (PIID). Surface and Coatings Technology 2010, 204:2864-74. [2] M. Ueda, C. Silva, N. M. Santos, G. B. Souza. Plasma immersion ion implantation (and deposition) inside metallic tubes of different dimensions and configurations. Nuclear Instruments and Methods in Physic Research B 2017, http://dx.doi.org/10.1016/j.nimb.2017.03.073. [3] S. Muhl, A. Pérez. The use of hollow cathode in deposition processes: A critical review. Thin Solid Films 2015,579: 174-98. [4] M. Ueda, A. R; Silva, E. J. D. M. Pillaca, S. F. M. Mariano, R. M. Oliveira, J. O. Rossi, C. M. Lepienski, L. Pichon. Review of Scientific Instruments 2016, 87: 0139028. [5] M. Ueda, C. Silva, A. R. Marcondes, H. Reuther, G. B. Souza. Recent experiments on plasma immersion ion implantation (and deposition) using discharges inside metal tubes. Surf. Coat. Technol., 2018, https://doi.org/10.1016/j.surfcoat.2018.05.009 [6] S. Flege, R. Hatada, A. Derepa, C. Dietz, W. Ensinger and K. Baba, Review of Scientific Instruments 88., 096106 (2017). [7] RUP-6 datasheet from GBS Elektronik, GBS Elektronik GmbH. [8] S. F. M. Mariano, M. Ueda. Hollow cathode effects observed in magnetically confined plasmas used for deposition of DLC films via PIII&D in tubes. Applied Surface Science. 2019; 465: 824-832 [9] M. Ueda, C. Silva, G. B. Souza, S. F. M. Mariano. Overcoming sheaths overlapping in a small diameter metallic tube with one end closed and using high density plasma from a high power pulsed hollow cathode discharge. AIP Advances, 2018. [10] C. Alves Jr, F. O.de Araújo, K. J. B. Ribeiro, J. A. P. da Costa, R. R. M. Sousa, R. S. de Sousa. Use of cathodic cage in plasma nitriding. Surface and Coatings Technology. 2006; 201: 2450-2454. [11] T. E. Sheridan, J. A. Goree. Analytic Expression for the Electric Potential in the Plasma Sheath. IEEE Trans. Plasma Sci. 1989, 17:884-888. [12] C. Yi, B. Park, S. B. Kim, W. Namkung, and M. Cho. Analysis of grid electrode assisted plasma based ion implantation system and potential application to ion beam assisted deposition for insulator substrates. Thin Solid Films. 2018, 660:380-385. [13] I. Sugai, M. Oyaizu, Y. Takeda, H. Kawakami, H. Hattori, K. Kawasaki. Influence 9
of carbon material and sputtering angles on stripper foil lifetime. Nuclear Instruments and Methods in Physics Research A. 2010, 613:448-452. [14] M. Ueda, C. Silva, G. B. Souza, L. Pichon, H. Reuther. High Temperature Plasma Immersion Ion Implantation Using Hollow Cathode Discharges in Small Diameter Metal Tubes. Journal of Vacuum Science & Technology B. 2019, 37:042902.
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TABLE Table I – Nitrogen Plasma Immersion Ion Implantation (N-PIII) treatment conditions using Metal Cylindrical Sieve.
1
Without Lid
Working pressure (mbar) 5.3x10-2
2
With Lid
7.2x10-2
Condition #
Repetition rate (Hz)
Pulse length (µs)
Voltage (kV)
Current (A)
Temperature (ºC)
500
20
4.1
5
402
500
20
3.3
12
560
11
List of figure captions
Fig. 1 Set-ups of the vacuum chamber of 20 liters used in the treatments of the Metal Cylindrical Sieve (inside). Fig. 2 Schematic drawing of the experimental set-up (a) and the expected frontal image of the deposition pattern on a whole Si wafer (b). Fig. 3 Photos of MCS in the PIII system, with the presence of plasma: a) side view, b) front view and c) back view. Fig. 4 Two-dimensional images of deposited materials coming from the 4.0 cmØ sieve, with the Si wafer piece placed at 4 cm distance from the tube mouth, under the PIII conditions: (a) 5 A and (b) 12 A. Fig. 5 Schematic of the E-Field line and the ion paths for the: a) 5 A and b) 12 A. Fig. 6 Average thicknesses (in cm) of depositions on the silicon wafer piece measured by FEG versus distances from the center of the marking circles (in cm) of the deposition at 0.0 cm. Fig. 7 X-Ray Diffraction of SS304 support, for reference and for different current cases.
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FIGURES Figure 1 -
Figure 2 –
(a)
(b)
Figure 3 –
13
Figure 4 – (b)
(a)
Figure 5 –
Figure 6 –
14
Figure 7 –
15
Highlights - Surface treatments have been carried out using Nitrogen Plasma Immersion Ion Implantation inside metal cylindrical sieve. - In high current regime, sheath overlapping is avoided and high ion implantation condition is achieved. - The high deposition Plasma Immersion Ion Implantation and Deposition treatment could lead to interesting multi-element film (high entropy alloy) deposition for different applications.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Author Contributions The submitted manuscript details our recent work about a new configuration of plasma immersion ion implantation (PIII), the metal cylindrical sieve (MCS), with the tubular shape of 4.0 cm diameter, 20 cm of length and path spacing of 1 cm. This new configuration combines the advantages of a grid type support (the low rate of sputtering inside the grid type and also the facility of the mobility of the ions and the field distribution in such a configuration) with a cylindrically closed support with an enhanced plasma confinement. The deposition free results obtained in this new PIII configuration allow us to plan efficient N-PIII treatments of different types and sizes of metal springs, grids, rings, and wires, with high performance, for industrial and scientific applications. On the other hand, the high deposition PIII&D treatment could lead to interesting multi-element film deposition for different applications.
18
S0169-4332(19)34049-8 https://doi.org/10.1016/j.apsusc.2019.145232 APSUSC 145232
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
14 October 2019 11 December 2019 30 December 2019
Please cite this article as: C. Silva, M. Ueda, C. Barros Mello, Metal Cylindrical Sieve (MCS) for Plasma Confinement and Low Sputtering Nitrogen Plasma Immersion Ion Implantation, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145232
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Metal Cylindrical Sieve (MCS) for Plasma Confinement and Low Sputtering Nitrogen Plasma Immersion Ion Implantation Carla Silva1, Mario Ueda1, Carina Barros Mello1 1
Associated Laboratory of Plasma and Materials, National Institute for Space Research – INPE, Av. dos Astronautas, 1758, São José dos Campos, 12227-010, Brazil Abstract Nitrogen Plasma Immersion Ion Implantation (N-PIII) was performed in the Metal Cylindrical Sieve (MCS) configuration that consists of Stainless Steel 308 L (SS308L) spiral spring wound wire. Two N-PIII regimes were obtained: one using a current of 5 A (with ends open), in which an intense sputtering of the wire occurs while for currents above 12 A (with one side closed), the sputtering is diminished substantially. This behavior at different powers in MCS can be explained in terms of the sheath overlapping between the spiral spring paths. The plasma density near the wire is insufficient to overcome the sheath overlapping between the spiral spring paths when the current is low, which leads to high sputtering of the wire surface. However, in high current, sheath overlapping is avoided and high ion implantation condition is achieved. FEG-SEM analyses of the silicon sample placed on the screen plate in front of MCS mouth indicated a high deposition of the sputtered material at low current in contrast with low deposition at high current. These deposition free results allow us to plan efficient N-PIII treatments of different types and sizes of metal springs, grids, rings, and wires, with high performance, for industrial and scientific applications.
Keywords: Ion implantation, surface modification, hollow cathode discharge, deposition in metallic tubes, SS308L MCS tubes.
1
INTRODUCTION Treatments inside metallic tubes are attracting much attention of the plasma immersion ion implantation (PIII) community recently, due to their possible applications in highperformance tubing for oil companies, chemical, and food industries as well as for fuel pipes of fluids or corrosive gases [1]. These types of treatments can be carried out using hollow cathode discharges (HCD) as the plasma source given their better plasma confinement and higher density capabilities, which allow possibilities of achieving significantly better results than those obtained by conventional PIII processes, as higher uptake of implanted species and deposition rate as well as much thicker treated coatings [2-4]. Despite the various advantages of treatments inside metallic tubes when compared to the treatments in planar substrates, very high sputtering occurs inside the tubes, especially for the small diameter ones [5]. In view of these deleterious effects, a limitation in the tube treatments was found, since depending on the application, the high sputtering inside the tube could modify unfavorably the inner wall surface and therefore there is the possibility of not meeting the requirements for a particular tube application. Studies of these effects have to be performed, investigating ways to minimize the high sputtering that occurs during the treatment. A paper was published recently in which the authors used a grid-type configuration to performing Diamond Like-Carbon (DLC) deposition [6]. In their paper, it is shown that for DLC deposition, the result is more effective for the grid-type support than when using the conventional cylindrical tube. This is due to the high rate of sputtering inside the tube and also the difficulty of the mobility of the ions and the field distribution in such a configuration. Therefore, we decided to test a new tube configuration, the MCS configuration, combining the advantages of a grid type support with cylindrically closed support with enhanced plasma confinement. In this way, the electric field penetrated more easily the openings of the tubular cylinder and the ions had greater mobility through the sieve, making the PIII treatment of wire wall more effective. Therefore, preliminary results of an investigation to achieve better conditions of N-PIII treatments based on a newly developed Metal Cylindrical Sieve scheme producing highly confined plasma with either high or low sputtering of the wall are described in this paper.
2
MATERIALS AND METHODS: The PIII device used in the present study consists basically of a cylindrical discharge vacuum chamber of 30 liters (Fig. 1) and a RUP-6 high power high voltage source to pulse the samples at a maximum nominal power of 13 kW [7]. As can be seen in the figure, the MCS configuration adopted in the present experiments consists of stainless steel 308L wire with 0.15 cm diameter wound in a spiral spring shape with 4.0 cm tube diameter, 20 cm of length and pitch of about 1 cm. Since the MCS device does not hold straight by itself, these wires were wrapped around so that they were cylindrical in shape, and two clamping plates of stainless steel SS304 were used so that the MCS was in the final spiral spring shape and with a pitch of 1 cm. This configuration was fixed in the upper part of the chamber (aiming to minimize the occurrence of arcs) by means of a stainless steel ring attached to the surface of the MCS [8] and connected to high voltage feedthrough. In the first experimental condition, the MCS was used with two open ends which resulted in low current discharge (5 A). In the second experimental condition, one side of the MCS was closed (with a lid), and in this set-up, it was possible to achieve high currents (12 A). Additionally, in both cases, an electrically grounded screen plate of SS304 was used in front of the tube mouth where 1/3 or half of the silicon wafer substrate was fixed to it, aiming to detect/measure the two-dimensional deposition patterns of the materials ejected from the tube (Fig. 2. The screen plate was placed at 4 cm distance from the tube mouth. These silicon substrate samples were previously subjected to cleaning procedures before the PIII treatment to avoid any contamination that could affect the quality of the deposited material. Argon ion bombardment was used to cleaning the samples surface and tube forming wire/support structure, for 10 minutes using the following pulser conditions: 5.2x10-2 mbar, 6 A, 1.5 kV, 500 Hz and 20 µs and then, the nitrogen implantation was carried out for up to 60 minutes. Nitrogen was implanted with a pulse repetition rate of 500 Hz and pulse duration of 20 µs. The pulse voltage and the current were adjusted to maintain the treatment temperature relatively constant (measured with an optical pyrometer). These high voltages (H. V.) pulser conditions applied to the MCS, are described in Table I. The surfaces of silicon wafer were analyzed by field emission gun scanning electron microscopy (FEG-SEM) and the surface of the stainless steel plate and samples were analyzed by X-ray diffraction (XRD). 3
RESULTS AND DISCUSSION Previous studies of PIII treatments of interior of metallic tubes using hollow cathode discharges indicated very high sputtering effects, especially in small tubes with diameters less than 4 cm Ø [9]. In view of these results, for a direct comparison between open wall and continuous tubes, we have adopted a similar dimension tubular structure (with 4 cm Ø) for MCS. High-density plasmas that can be seen from the images of the confined plasmas, in Fig. 3, are formed inside the Metal Cylindrical Sieve by means of the hollow cathode discharge. The high-density plasma was confined inside the MCS but a part of it flows out from the open end of the MCS toward the grounded screen plate (Fig. 2). This is a quite different idea from the cage configuration often used in plasma nitriding (cage configuration) [10]. Pictures of lateral and frontal images of the plasma are shown in the same figure, as well as a picture including the Si wafer piece in front of the open tube mouth. The MCS configuration tube with 4 cm Ø has a significantly smaller surface area compared to the continuous one of the same diameter (about 1/5 to 1/10). In this way, it was naturally expected a strong reduction of wire sputtering and therefore deposition of the material expelled from the MCS. However, when performing the N-PIII treatments in MCS, we verified that although we had adopted a low current treatment of 5 A for the first condition studied, an intense sputtering occurred in this case, as can be seen in the photo of the silicon wafer and SS304 plate placed in front of the tube (Fig. 4). This unexpected result could be explained as the following: although the tubular radius of the MCS under test of 4 cmØ is sufficiently large to avoid sheath overlapping for plasma densities obtained for 5 A discharges, the sheath sizes obtained between the wire paths for such currents are exceedingly large compared to the distance between the paths. Therefore, the overlapping of the sheaths and the sputtering occurs in the wire in this case [11,12]. In the experiments performed here, we observed a high sputtering effect (Fig. 4a) in the treatment without a lid when a low current was applied to the MCS. Such effects are strongly ascribed to the dependency between the sputtered particles and the incidence angle of the ions: lower angles of incidence result in a higher sputtering yield [13]. Therefore we can suppose that the electric field is likely to be tangential along the wire since the sputtering was more pronounced as compared to the implantation (a schematic drawing of the electric field configuration in the MCS when 4
sheath overlapping occurred is shown in Fig. 5a) during the pulse. One way to minimize the occurrence of the sheath overlapping is to employ the scheme of one closed end with lid in the MCS and operate it with high powers, as used in this experiment. There was a very strong mark on the stainless steel plate and silicon wafer piece. The circular mark left was about twice the size of the 4 cm ∅ tube diameter (Fig. 4a). The SS304 screen plate was electrically grounded and placed about 4 cm away from the MCS mouth. It had a dark circular ring (sputtered material) on the outside of the mark and a light blue inner circle (both the screen and wafer) caused by the sheath overlap that occurs at low currents. By adopting higher current (12 A), we found that this phenomenon is supplanted which leads to a lower rate of sputtering and, consequently, a lower deposition on the silicon sample and SS304 plate (Fig. 4b). In this high current case, the E-fields are mostly perpendicular to the wire, as can be seen in Fig. 5b, favoring the ion implantation instead of sputtering. In this case, the deposition marks on the silicon wafer pieces and on the screen plate are reduced. In addition, by observing the deposition pattern for this case, we can see that there is a clear mark left on the silicon surface by the plasma hitting the silicon holding steel clips. This fact also confirms the generation of higher energy plasma for 12 A treatment compared to the 5 A case. In the previous work with a continuous tube of 1.1 cm in diameter, mapping was performed in order to know the conditions where the plasma sheath overlap and the conditions where this phenomenon was superseded. We could then identify which current range we should work without this phenomenon. For the case of this new MCS configuration, we see that a similar phenomenon occurred, but we did not perform a systematic mapping of working currents. However, we know from previous experiments [9] that if we increase the current the plasma sheath overlapping is avoided. The average thicknesses of depositions versus distances from the center of the circles on the silicon wafer were measured by FEG. In this analysis, silicon strips of 0.75 cm were sliced into pieces that were observed by FEG-SEM at the cross-sections in different points and their thicknesses averaged which are shown as data points on the graph of Fig. 6. We can see that the deposition is not uniform for the lower current condition (5 A). It is normal that the center of the silicon blade (1.5 cm) has more deposition; however, we see that in other regions there are also increases in thickness. This is due to three factors, the first, the plasma non-uniformity, the second due to the fact that spacing between the 5
wires that make up the MCS are not exactly of 1 cm, due to the difficulty of shaping them, and the third due to the sheath overlapping between the wires. So these three factors will greatly influence this thickness difference. For the condition of higher current (12 A) the phenomenon of sheath overlap has already been supplanted and, consequently, the deposition rate is lower, as we can see in the graph. In this treatment current we also verified a uniformity of thickness, but much smaller deposition than for the low current case. Therefore, the analysis of the deposition thickness on the silicon wafer pieces led us to confirm that two main different phenomena are occurring in the experiments presented above: a) For very low currents as 5 A, the phenomenon of sheath overlapping between the wire paths is occurring and b) Above 12 A, substantial nitrogen implantation is expected but also a thin deposition of the sputtered materials from the wires of MCS occurred. One inconvenience found in this new N-PIII scheme is that, as we increased the current in MCS, the vacuum chamber walls are heated considerably, in contrast to no heating for continuous wall cylinder tubes. For continuous tubes, the heating is high only in regions of the vacuum chamber where the plasma hits while flowing out from the opened mouths of the tubes. In MCS, the plasma spreads out from the sieve cylinder and brightens the chamber interior, heating it almost uniformly. To reduce the heating of the chamber, four powerful fans are used for cooling of our vacuum chamber used for N-PIII, which allows PIII and PIII&D (Plasma Immersion Ion Implantation and Deposition) treatments for more than 2 h. To perform the XRD analysis in parts of the MCS, cut pieces of the MCS clamping plate were used. These XRD results are shown in Fig. 7. For the treatment condition of 5 A, only the formations of CrN and α’ phases were observed. This occurred due to the overlapping of the plasma sheath, which prevented effective nitrogen implantation in this condition (lower temperatures as seen in Table I). On the other hand, for the treatment performed at 12 A, we can verify that the iron nitride layer was formed, indicating much effective nitrogen implantation (higher temperatures described in Table I). Below are presented some advantages of the Metal Cylindrical Sieve configuration: A) Plasma escaping from the open wall tube could be used to treat stainless steel or other type wires (as NiTi wires) using the nitrogen as working gas when the plasma density at points near the wires is high enough to overcome the overlapping of the 6
sheaths. The deposition of DLC on wires may be feasible also in this configuration, perhaps without the need to overcome the overlapping of the sheaths in that case; B) Discharge is very easy to be started and also to be maintained in the MCS configuration. The range of plasma operation in this configuration is very large; C) Plasma can be easily viewed from outside the tube through a transparent window. Just as it will be easy to hang and see the samples being treated inside the MCS tube (in contrast to the continuous wall tubes), in high-density plasma in the central region of the hollow cathode; D) It can be seen clearly from our results that the light emission of the plasma inside the tube is very intense, therefore much denser than the outside of it; E) Of course, with the leakage of plasma through the wires, it will heat the chamber much more than in the case of the continuous tube; F) And from what we saw on the target plate for the low current condition in MCS, there is a significant sputtering of the wires. This may be good (for deposition on the target) or bad (because of the high sputtering contaminating the target under treatment), depending on the application. G) When we perform the treatment inside continuous tubes the ions and the electric field will be perpendicular to the tube, so the ions will bombard the surface of the tube with high energy, causing the material sputtering from the inner side of the tube wall beyond the implantation. Auger profile for Ti-6Al-4V sample treated in continuous tubes showed a 23% nitrogen implantation with a depth of approximately 1.6 µm, but the iron contamination was 90% with a depth of 1 µm, that is, despite the implantation being excellent we have the problem of sputtered materials coming from the tube [14]. Therefore, for the treatment inside the MCS with high currents, the patterns of depositions are much thinner compared with continuous tubes because besides having a smaller surface area exposed, there is the spacing between the wires, which allows the ions to flow at different angles and the plasma fills in the interior of the tubular region resulting in high density confined plasma. GDOES profile for the Ti-6Al-4V sample treated in continuous tubes of SS304 showed 40% nitrogen implantation with a depth of approximately 250 nm, but iron contamination was 8% with depths of 75 nm. Here, with MCS, nitrogen implantation was low compared to the continuous tube (the data will be presented in a more complete article), but this happened because the treatment time used was only one hour. New treatments will be performed with longer times, aiming at increasing the depth of nitrogen implantation. 7
CONCLUSION: MCS is a new plasma source and sample fixation system built-up by light components in which high-efficiency PIII and PIII&D treatments can be performed. It is a source with lower cost and lighter than that using continuous tubes. It requires no special gas injection system because it is a transparent structure for gas feeding, and for this reason, components or samples are visible from outside the vacuum system, making it easier to follow the PIII and PIII&D treatments inside the open wall tube. The high deposition PIII&D treatment could lead to interesting multi-element film deposition for different applications. Experiments are been continued in this same subject and good nitrogen implantations in the wires, in the samples placed in the interior of the open wall tube as well as in the samples on the screen in front of the plasma, have been observed. These results will be reported soon in the oncoming paper. ACKNOWLEDGMENT This research work is supported by CNPq and Brazilian Ministry of Science Technology and Innovation and Communication (MCTIC).
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REFERENCES [1] R. Wei. Development of new technologies and practical applications of plasma immersion ion deposition (PIID). Surface and Coatings Technology 2010, 204:2864-74. [2] M. Ueda, C. Silva, N. M. Santos, G. B. Souza. Plasma immersion ion implantation (and deposition) inside metallic tubes of different dimensions and configurations. Nuclear Instruments and Methods in Physic Research B 2017, http://dx.doi.org/10.1016/j.nimb.2017.03.073. [3] S. Muhl, A. Pérez. The use of hollow cathode in deposition processes: A critical review. Thin Solid Films 2015,579: 174-98. [4] M. Ueda, A. R; Silva, E. J. D. M. Pillaca, S. F. M. Mariano, R. M. Oliveira, J. O. Rossi, C. M. Lepienski, L. Pichon. Review of Scientific Instruments 2016, 87: 0139028. [5] M. Ueda, C. Silva, A. R. Marcondes, H. Reuther, G. B. Souza. Recent experiments on plasma immersion ion implantation (and deposition) using discharges inside metal tubes. Surf. Coat. Technol., 2018, https://doi.org/10.1016/j.surfcoat.2018.05.009 [6] S. Flege, R. Hatada, A. Derepa, C. Dietz, W. Ensinger and K. Baba, Review of Scientific Instruments 88., 096106 (2017). [7] RUP-6 datasheet from GBS Elektronik, GBS Elektronik GmbH. [8] S. F. M. Mariano, M. Ueda. Hollow cathode effects observed in magnetically confined plasmas used for deposition of DLC films via PIII&D in tubes. Applied Surface Science. 2019; 465: 824-832 [9] M. Ueda, C. Silva, G. B. Souza, S. F. M. Mariano. Overcoming sheaths overlapping in a small diameter metallic tube with one end closed and using high density plasma from a high power pulsed hollow cathode discharge. AIP Advances, 2018. [10] C. Alves Jr, F. O.de Araújo, K. J. B. Ribeiro, J. A. P. da Costa, R. R. M. Sousa, R. S. de Sousa. Use of cathodic cage in plasma nitriding. Surface and Coatings Technology. 2006; 201: 2450-2454. [11] T. E. Sheridan, J. A. Goree. Analytic Expression for the Electric Potential in the Plasma Sheath. IEEE Trans. Plasma Sci. 1989, 17:884-888. [12] C. Yi, B. Park, S. B. Kim, W. Namkung, and M. Cho. Analysis of grid electrode assisted plasma based ion implantation system and potential application to ion beam assisted deposition for insulator substrates. Thin Solid Films. 2018, 660:380-385. [13] I. Sugai, M. Oyaizu, Y. Takeda, H. Kawakami, H. Hattori, K. Kawasaki. Influence 9
of carbon material and sputtering angles on stripper foil lifetime. Nuclear Instruments and Methods in Physics Research A. 2010, 613:448-452. [14] M. Ueda, C. Silva, G. B. Souza, L. Pichon, H. Reuther. High Temperature Plasma Immersion Ion Implantation Using Hollow Cathode Discharges in Small Diameter Metal Tubes. Journal of Vacuum Science & Technology B. 2019, 37:042902.
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TABLE Table I – Nitrogen Plasma Immersion Ion Implantation (N-PIII) treatment conditions using Metal Cylindrical Sieve.
1
Without Lid
Working pressure (mbar) 5.3x10-2
2
With Lid
7.2x10-2
Condition #
Repetition rate (Hz)
Pulse length (µs)
Voltage (kV)
Current (A)
Temperature (ºC)
500
20
4.1
5
402
500
20
3.3
12
560
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List of figure captions
Fig. 1 Set-ups of the vacuum chamber of 20 liters used in the treatments of the Metal Cylindrical Sieve (inside). Fig. 2 Schematic drawing of the experimental set-up (a) and the expected frontal image of the deposition pattern on a whole Si wafer (b). Fig. 3 Photos of MCS in the PIII system, with the presence of plasma: a) side view, b) front view and c) back view. Fig. 4 Two-dimensional images of deposited materials coming from the 4.0 cmØ sieve, with the Si wafer piece placed at 4 cm distance from the tube mouth, under the PIII conditions: (a) 5 A and (b) 12 A. Fig. 5 Schematic of the E-Field line and the ion paths for the: a) 5 A and b) 12 A. Fig. 6 Average thicknesses (in cm) of depositions on the silicon wafer piece measured by FEG versus distances from the center of the marking circles (in cm) of the deposition at 0.0 cm. Fig. 7 X-Ray Diffraction of SS304 support, for reference and for different current cases.
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FIGURES Figure 1 -
Figure 2 –
(a)
(b)
Figure 3 –
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Figure 4 – (b)
(a)
Figure 5 –
Figure 6 –
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Figure 7 –
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Highlights - Surface treatments have been carried out using Nitrogen Plasma Immersion Ion Implantation inside metal cylindrical sieve. - In high current regime, sheath overlapping is avoided and high ion implantation condition is achieved. - The high deposition Plasma Immersion Ion Implantation and Deposition treatment could lead to interesting multi-element film (high entropy alloy) deposition for different applications.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Author Contributions The submitted manuscript details our recent work about a new configuration of plasma immersion ion implantation (PIII), the metal cylindrical sieve (MCS), with the tubular shape of 4.0 cm diameter, 20 cm of length and path spacing of 1 cm. This new configuration combines the advantages of a grid type support (the low rate of sputtering inside the grid type and also the facility of the mobility of the ions and the field distribution in such a configuration) with a cylindrically closed support with an enhanced plasma confinement. The deposition free results obtained in this new PIII configuration allow us to plan efficient N-PIII treatments of different types and sizes of metal springs, grids, rings, and wires, with high performance, for industrial and scientific applications. On the other hand, the high deposition PIII&D treatment could lead to interesting multi-element film deposition for different applications.
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