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OES studies of plasmoids distribution during the coating deposition with the use of the Impulse Plasma Deposition method controlled by the gas injection
Vacuum 128 (2016) 259e264
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
OES studies of plasmoids distribution during the coating deposition with the use of the Impulse Plasma Deposition method controlled by the gas injection* Katarzyna Nowakowska-Langier a, *, Rafal Chodun b, Krzysztof Zdunek b, Sebastian Okrasa b, Roch Kwiatkowski a, Karol Malinowski a, c, Elzbieta Skladnik-Sadowska a, Marek J. Sadowski a, c a
National Centre for Nuclear Research (NCBJ), Material Physics Department, Andrzeja Soltana 7, 05-400 Otwock-Swierk, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland c Institute of Plasma Physics and Laser Microfusion, Hery 23, 01-497 Warsaw, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 2 March 2016 Accepted 3 March 2016 Available online 8 March 2016
The article presents the results concerning the OES characterization of plasmoids generated by the Impulse Plasma Deposition method modified by the use of gas injection (Gas Controlled IPD). Both of the electrodes of the coaxial accelerator: the tube (cathode) and the rod (anodeethe source of the vapors) are permanently connected with the capacitors charged to the high voltage of few kV value. During the coatings deposition both the electro-eroded vapors of the anode material (Ti) and the working gas (N2) form the plasmoids which are accelerated by the Ampere force in the coaxial accelerator and ejected to the vacuum chamber in the direction of non-heated substrates forming there the TiN coating. Periodic gas injections which make pressure fluctuations in the accelerator space in the range of critical values and cause the discharge to initiate or vanish accordingly with the chosen frequency. The promising practical effects of the use of gas injection for controlling the plasma processes during the coatings deposition by the GCIPD method was previously proved. On the basis of the present studies one can state that the GCIPD0 plasmoids containing multiple ions both of metallic as well as gas plasmoids constituents are more energetic as compared to the IPD case. Contrary to the IPD during the GCIPD the titanium fraction of the plasmoids structure overtakes the nitrogen one which seems to be important to the coating growth mechanism. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Impulse plasma deposition Gas controlled impulse plasma deposition Plasmoids structure Optical emission spectroscopy of plasmoids
1. Introduction The origin of the Impulse Plasma Deposition (IPD) method is based on the assumption that non-equilibrium plasma is a favorable environment for a synthesis of phases, in particular those characterized by high barriers of nucleation [1]. In this kind of plasma, the homogenous nucleation on plasma ions is expected and this kind of nucleation is thermodynamically stabilized by ultra-fine particles of the new phase [2]. The clusters, which are present in non-equilibrium plasma and can act as critical nuclei, are
* The paper was presented at the 9th Symposium on Vacuum based Science and Technology, held on November 17e19, 2015 in Kolobrzeg Poland. * Corresponding author. E-mail address: [email protected] (K. Nowakowska-Langier).
http://dx.doi.org/10.1016/j.vacuum.2016.03.002 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
components of a particle stream delivering to the surface of a substrate where the coating grows. The clusters' participation during the process of a coating growth makes this mechanism to be a mix of the atomic and cluster type of growth. In the first case, the nucleation at the substrate surface dominates and the further growth of crystallites occurs with morphology dependent by the thermal activation. In the second case, the mechanism of coating growth is determined by kinetics and clusters dispersion reaching the boundary region and the cluster coalescence processes conditioned by thermal activation [1,3]. In search of an efficient method of highly ionized and thermodynamically non-equilibrium plasma generation, attention was turned onto the coaxial accelerator consisting of two concentric electrodes in the form of: a positively biased axial rod as an anode, surrounded by a grounded tube as a cathode. This specific
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construction of the coaxial accelerator causes the forming of ! ! ! magnetic pressure ð j H Þ, where j e interelectrode current, ! H e concentric magnetic field generated by the current flow accelerating the plasma along the accelerator length [4,5]. The source of electric energy in the standard version of an IPD device is a 50e200 $ 104 F capacitor charged to a 1e6 $ 103 V of voltage [1]. The discharge of the capacitor occurs in the inter-electrode space at a constant 101e102 Pa pressure and is controlled by a threeelectrode spark ignitron. The amplitude of attenuated current waveform in the electric circuit of coaxial accelerator can reach the orders of 103e105 A during the time of 104 s range [1]. From a practical point of view, the IPD method is similar to the HiPIMS methodethe high power magnetron sputtering with respect to high cathodic current of 102 A and the discharge time of 104 s [12,13]. The electric circuit of plasma sourceemagnetron in HiPIMS method is practically identical with the electrical circuit used in IPD method. It is worth noting that there is one significant difference between the IPD and HiPIMS methodsethe IPD method actively uses the magnetic pressure to attach the plasmoids at a very high drift velocity. The pack of plasma (plasmoid) generated in that way has features of arc discharge and the form of a radial symmetric thin current sheet which is accelerated by magnetic pressure up to velocity at the accelerator outlet of the order of 104 m/s [6,7]. The substrate is not intentionally heated from any external heat source, but the dissipation of kinetic energy of the plasmoid at the surface of the substrate raises the temperature of the substrate for a very short period of time up to 2000 K [8]. This thermal peak disperses heat emissions and phonons of the substrate material excitation. This temperature is enough to temporarily heat activation of the substrate surface and the emitted heat is conducted through the material of the substrate resulting from the heat conductivity of solids (rate of temperature decrease e 106 K/s [9]). The consecutive plasmoids are generated periodically with a frequency of 101e100 Hz. The kinetics of growth count in relation to the number of plasmoids, not to the duration of the deposition process like in other methods of plasma surface engineering, which gives the IPD method a digital status as a sum of assumed amount of complete processes of plasma surface engineering. An indicative relative increase of thickness of layers is calculated on about 0.8e1 nm per shot, with a perpendicular fixed substrate holder [10]. The practical advantages of IPD method has been verified in industry to prolong the service life of cutting tools made of SW7M HSS steel coated by titanium nitride coating [11]. Recently we proposed a modification of IPD method relying on the use of the pulsing pressure of working gas to control the process of generating the plasmoids (instead of operation of spark ignitron generating the plasmoids under constant pressure achieved by continuous gas flow) [14]. This pressure changes periodically with specific frequencies applied at the ranges of threshold values: from the 104e103 Pa when the discharge does not initiate, to the 101 Pa allowing the discharge to initiate and spread in the working gas. In these conditions the spark ignitron has been eliminated from the electric circuit of the plasma accelerator and voltage is applied to electrodes permanently. The process of generation and acceleration of plasmoids is controlled by changeable gas concentration only, resulting from periodical gas injections in the interelectrode space of accelerator (Gas Controlled Impulse Plasma Deposition, GCIPD). The concept and interpretation of the physical effects of this gaseous mode in IPD method application has been widely described in our previous work [14,15]. It can be assumed that the application of the gaseous mode, characterized by a dynamically changeable concentration of working gas molecules in the range of threshold values, contributes to the preservation of the kinetic energy of plasma particles by decreasing the probability of energy dissipation by inelastic collisions with neutral molecules.
This assumption corresponds well with the results of our previous research concerning the TiO2 thin films [16], deposited on glass and silicon unbiased and unheated substrates by pulsed magnetron sputtering method (PMS) [17,18] and modified by applying the gas controlled PMS method e GIMS (Gas Injection Magnetron Sputtering) [19,20]. We were able to deposit the rutile films by GIMS method, while the PMS favors the anatase phase. As is known, the presence of the rutile-crystal structure in titanium oxide films is controlled by heating and/or biasing the substrate [21]. The practical result of “gaseous” modification of IPD method (GCIPD) was the achievement of high durability of cutting tool inserts made of SW7M HSS coated by TiN layers. These layers were deposited by using the coaxial accelerator made of titanium and nitrogen as a working gas periodically injected in doses into the inter-electrode space of accelerator. The durability of coated inserts was 1600% better in relation to the uncoated ones, what should be considered as remarkable for TiN coatings [14]. The aim of the presented study was the spectral characterization of plasmoids generated by coaxial accelerator working in gaseous mode, which can be helpful in the interpretation of dynamic phenomena of plasma interactions with substrate during the growth of TiN coatings deposited by GCIPD method providing this significant growth of cutting tools durability.
2. Experimental part In our experiments the pulsed plasma was generated by a coaxial accelerator equipped with two cylindrical electrodes made of titanium in the GCIPD apparatus. A schematic view of the apparatus is shown in Fig. 1. The apparatus used in this experiment was equipped with a special gas-injection system [14], contrary to the standard version of the IPD device [1]. In the presented system a pulsed plasma was generated after the injection of the working gas into the inter-electrode region of the coaxial accelerator. Nitrogen was used as a working gas. In our experiment plasma was generated at different technological conditions: e discharge voltage: 3, 4 kV and battery capacity 100 or 200 mF. The main experimental parameters are shown in Table 1. Measurements of optical emission spectra were carried out by means of a Mechelle®900 optical spectrometer that are characterized by the very short exposition time of measurements. During the experiments it operated in the wavelength range from 300 nm to 600 nm at the exposition time (texp) equals 20 ms e during the time resolved measurements and 100 ms during the measurements of whole spectrum of plasma generated in specific conditions. Exposition was triggered from a current rise measured by 8000 A Series Infinium Oscilloscope from Rogowski coil with sensitivity 1 kA:1 V applied to the electrical circuit of internal electrode. During those experiments the optical
Fig. 1. Schematic view of the IPD apparatus and experiment setup.
K. Nowakowska-Langier et al. / Vacuum 128 (2016) 259e264 Table 1 Parameters of pulsed plasma generations. Internal electrode material
Ti
Working gas Battery capacity [mF] Discharge voltage [kV] Pressure [Pa] Estimated Energy stored in the capacitor [kJ] Duration of the pulse [ms]
N2 100, 200 3, 4 101 e 103 0.45e1.6 ~250
collimator was oriented perpendicular to the plasma stream axis and was placed at a distance of 60 mm from the accelerator outlet. A schematic arrangement of the experiment is shown in Fig. 1. Spectroscopic measurements were performed under the typical conditions of the TiN layers synthesis process. 3. Results and discussions The optical emission spectra of the free-propagating Ti/N2 plasmoids generated during the GCIPD for different parameters are shown in Fig. 2. The peak value of the pulse energy introduced to the plasma process was calculated from following equation: E ¼ (CU2)/2, where E pulse energy, Cecapacity of battery, Uedischarge voltage. The values of discharge energy were calculated, taking into account that after each consecutive discharge the capacitor was not fully unloaded. The remaining voltage after the discharge was up to 200 V. As one can see, these spectra were
Fig. 2. Emission spectra of pulsed plasma.
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related to the optical emission of the vapors of the internal electrode material (titanium) and the working gas (nitrogen) species, Ti II, Ti III, Ti IV and N II, N III, respectively. Additionally, the weak lines from carbon and oxygen were also observed. The intensity of the lines is dependent on process parameters and increase with increasing the pulse energy so the most intensive spectrum was obtained during the discharge with the energy peak equals 1.6 kJ. Plasmoids are characterized by the presence of the multiple ions of both main plasma constituents: titanium and nitrogen. It is worth noting that the presence of the multiple ions were observed previously for the IPD during different condition of plasmoids generation [22,23], during the continuous working gas flow and the dynamic pressure in the vacuum chamber of about of 20 Pa with the discharge energy as high as 10 kJ. Contrary to the present studies, in the cited observations [22,23] the presence of the multiple Ti ions were detected for more energetic plasmoids. Hitherto for much higher energetic conditions discharge than those used at presently described studies. The authors of the cited paper [22,23] reported the appearance of the Ti IV line at the pulse energy as high as 10 kJ while at energy level of 2.18 kJ very close to the energy level used in our experiment, only the twofold ionization level of the titanium ions highest energetic state of the Ti ions was Ti III. It is interesting, that with a decrease of the plasmoids' energy, some spectral lines disappear almost completely, while the intensity of certain lines decreases more slowly. For the interpretation of these observations, the spectral lines of the various components were selected and their intensity are presented in Fig. 3. Although the intensities of emission lines do not evaluate the number of species in plasma, the ratios of intensities the excited species compared to each other show estimated relative populations [24]. It is known from the literature that the structure of the plasmoids in the IPD method is composed of zones of two main fractions ions slightly separated in time and space: the fraction originating from the internal electrode erosion products located mainly in the central part of the plasmoid and of the ionized working gas located mainly in outer plasmoid regions [25]. The results presented in Fig. 3 show that in conditions of relatively low energy, spectral lines of plasma particles of “metallic” fraction are much less intense than the spectral lines of “working gas” particles. It seems that energy introduced by discharge is primary dissipated for working gas ionization and if the energy is relatively low it may be not enough for a successive Ti internal electrode erosion. With an increase of pulse energy, the intensity of spectral lines of titanium vapors increase faster than those of the
Fig. 3. Intensity of chosen lines of registered spectra.
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Fig. 4. Current waveforms measured during experiments.
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Fig. 5. Time resolved distribution of pulsed plasma generated under different discharge energies: 900 kJ (left graph) and 1600 kJ (right graph).
nitrogen lines. This probably means that a higher energy discharge enhances more intensive erosion of the internal electrode. While measuring the coaxial accelerator i(t) waveforms for the GCIPD we found that for the higher discharge energy a specific reversely directed peak has been visible for the central part of the first or even the second half of the waveform (for the highest energy). The i(t) waveforms of electric circuit of GCIPD is presented in Fig. 4. The presence of such a peak in the current waveform has not been observed before in the case of the IPD [26]. SkladnikSadowska et al. [27] while studying the evolution of the plasmoids generated in the IBIS device e.s. in the conditions similar to those of the GCIPD interpreted the presence of the reversely directed peak located in the second part of the first half of the current waveform as being the result of internal electrode erosion (“erosion regime” e fast regime) [27]. If so the current waveforms measured by us have shown the energy dependent “erosion regime” mode of the coaxial accelerator work during the GCIPD. Fig. 5 shows the time evolution of the titanium/nitrogen plasmoids emitted from the coaxial accelerator during the GCIPD obtained at two different discharge energies, 900 and 1600 kJ respectively. In the case of the titanium electrode erosion products, the intensity of titanium ions is greater in the early stages for the face region of the plasmoids while the intensity of the nitrogen ions increases with time. Fig. 6 summarize the results given at Fig. 5 showing clearly that during the GCIPD the titanium fraction in the plasmoids overtakes the nitrogen one. On the basis of our present studies one can conclude that the erosion vs. energy relation illustrated by the i(t) characteristics (Fig. 4) corresponds well with the time evolution of the titanium and nitrogen ions fractions of the plasmoids (Fig. 3) showing that during the GCIPD the titanium ions are dominated in plasmoid front. Such a conclusion is in strong contradiction to the corresponding results obtained previously for the IPD [25]. Taking into account the results of our studies the following mechanism of plasmoids forming during the GCIPD could be proposed. When the pulse valve is opened the pressure of the working gas (nitrogen) rapidly increases from the it initial value of about 104e103 Pa to the value suitable for initiation of the discharge of about of 102e101 Pa; as the source of the electric energy serves the capacitors bank charged to the voltage of order of 103 V. Because of the capacitors discharging initiated by the pressure increase the current sheet is generated and accelerated by the magnetic pressure. The current sheet dynamics are correlated with the dynamics of the working gas particles changes. In contrast to the operating at a constant pressure of the order of 101 Pa, the GCIPD
should characterized by lesser concentration of the cold gas molecules. Thus the GCIPD's plasmoid should lose less of its total energy during the interaction with cold gas than for IPD and can be effectively used not only for stronger acceleration of the current sheet but also for the enhancing of the titanium electrode erosion (as a result of the complex mechanism [26]). Due to the nonuniform time e space distribution of the plasma particles density during the GCIPD, it is more likely that the maximum of density corresponds to the plasmoid itself while before and after the plasmoid density of particles is much lower. This means that, in the contrast to the IPD, during the GCIPD there is no cold gas swept and pressed at the front of the plasmoid which could be further exited and ionized (and therefore detected by the OES studies as a gas fraction located the plasmoid front like in the case of the IPD). It seems this is the reason of the different structure of plasmoid generated during the GCIPD and IPD. It is worthy to cite here the results of the computer simulation of plasmoids dynamics during the IPD and GCIPD presented in Ref. [28]. The results showed that the electron
Fig. 6. Time resolved distribution of spectral lines of pulsed plasma generated by 1600 kJ discharge.
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temperature for the GCIPD is a five time larger than the electron temperature obtained for the IPD and could be as high as 15 eV for the capacitor voltage of 4 kV (not published yet our Langmuire probe measurements brought the similar values of about of 10 eV for the capacitor voltage of 2 kV). The results of computer simulation seem to support the concept of plasma energy loss caused by non e elastic collisions of particles with neutral molecules during the IPD. As a consequence of the proposed mechanism of plasmoids distribution during the GCIPD the plasmoid impact at the substrate surface probably should be different to that postulated before for the IPD. Because of the presence of the Ti fraction at the front of the plasmoid and probable higher dispersion and kinetic energy of the plasmoid during the GCIPD in comparison to the IPD, the surface bombarded by the energetic Ti ions first. It might cause the energetic activation of the surface region (the subplantation of the Ti atoms is possible [6,15]) enabling the mechanism of growth of the coating not only by the cluster type like for the IPD [1] but also by the atomic one constituted as typical for the other plasma surface engineering methods (e.g. for magnetron sputtering). The presence of the atomic growth mechanism during the IPD when the gas injection was used had been discussed before on the basis of the structural study of the coatings [14]. 4. Conclusion This article describes the emission spectra of plasmoids periodically generated in the coaxial accelerator under the modified IPD methodeGas Controlled IPD (GCIPD). In contrast to the previous study reported in the literature concerning the IPD method in which the working gas was dosed in a continuous manner, the pressure in the coaxial accelerator and the vacuum chamber had a fixed dynamic value for approximately of 20 Pa and discharge initiation was controlled by the three-electrode spark ignitron., the GCIPD's coaxial accelerator is directly connected to the charged capacitors and, as well as the whole vacuum chamber, is initially pumped out to the pressure of order of 104 Pa. Then the working gas is injected straight to the plasma accelerator which makes the gas discharge capable to generate the plasmoid. Our results reported previously showed that the practical effect of the IPD modification was the significant increase of the service life of the cutting tools coated by the TiN coatings even as high as 16-fold in comparison to uncoated tools while those coated by the IPD resulted by the increase of few times. The presented thorough study of the spatiotemporal structure of GCIPD0 plasmoids, based on OES measurements, showed that the plasmoids generated and distributed under condition of periodically changing pressure of working gas (in the range of threshold pressure values for which the gas discharges are initiated and vanished) are probably more energetic than the plasmoids generated and distributed during the IPD. We believe that this contributes in more effective energy exchange during the plasma processes in the case of GCIPD in comparison to the IPD. Additionally, the presence of heavier metallic (Ti) ions on the GCIPD0 plasmoids front instead of working gas ions as during the IPD activates the surface of the substrate more effectively. Taking into account the previously reported results and the present ones, in our opinion, the idea of using the gas injection as a control factor for plasma processes of coating deposition opens up the new opportunities for coating deposition among the plasma methods of surface engineering. Acknowledgements This work was supported by the Polish State National Science
Centre within the Project 2013/09/B/ST8/02418. References [1] K. Zdunek, Concept, techniques, deposition mechanism of impulse plasma depositiondA short review, Surf. Coat. Tech. 201 (2007) 4813e4816. [2] M. Sokolowski, A. Sokolowska, B. Gokieli, A. Michalski, A. Rusek, Z. Romanowski, Reactive plasma pulse crystallization of diamond and diamond-like carbon, J. Cryst. Growth 47 (1979) 421e426. [3] K. Nowakowska-Langier, K. Zdunek, Nanostructured alloy layers with magnetic properties obtained by the impulse plasma deposition, Plasma Process. Polym. 6 (S1) (2009) S826eS829. [4] P. Hart, Plasma acceleration with coaxial electrodes, Phys. Fluid. 5 (1962) 38e47. [5] M. Rabinski, K. Zdunek, Snow plow model of IPD discharge, Vacuum 70 (2003) 303e308. [6] A. Werbowy, A. Olszyna, K. Zdunek, et al., Peculiarities of thin film deposition by means of reactive impulse plasma assisted chemical vapor deposition (RIPACVD) method, Thin Solid Films 459 (2004) 161e164. [7] J. Baranowski, L. Jakubowski, M. Rabinski, K. Zdunek, Studies of simultaneous X-ray emission and ion beams in the IPD plasma accelerator, Czechoslov. J. Phys. 52 (2002) 188e193. [8] Z. Romanowski, M. Wronikowski, Specific sintering by temperature impulses as a mechanism of formation of a TiN layer in the reactive pulse plasma, J. Mater. Sci. 27 (1992) 2619e2622. [9] K. Zdunek, Mechanism of crystallization of multicomponent metallic coatings using the impulse plasma method, J. Mater. Sci. 26 (16) (1991) 4433e4438. [10] K. Zdunek, K. Nowakowska-Langier, R. Chodun, S. Okrasa, M. Rabinski, Impulse plasma in surface engineering-a review, J. Phys. Conf. Ser. 564 (1) (2014) 012007. [11] K. Zdunek, Combined impulse-stationary impulse plasma deposition, Surf. Coat. Technol. 98 (1998) 1448e1454. [12] K. Macak, V. Kouznetsov, J. Schneider, U. Helmersson, I. Petrov, Ionized sputter deposition using an extremely high plasma density pulsed magnetron discharge, J. Vac. Sci. Technol. A18 (2000) 1533e1537. [13] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Ionized physical vapor deposition (IPVD): a review of technology and applicationsreview article, Thin Solid Films 513 (2006) 1e24. [14] K. Zdunek, K. Nowakowska-Langier, R. Chodun, M. Kupczyk, P. Siwak, Properties of TiN coatings deposited by the modified IPD method, Vacuum 85 (2010) 514e517. [15] K. Nowakowska-Langier, K. Zdunek, R. Chodun, S. Okrasa, R. Kwiatkowski, On coating adhesion during impulse plasma deposition, Phys. Scr. T161 (2014) 014063. [16] L. Skowronski, K. Zdunek, K. Nowakowska-Langier, R. Chodun, M. Trzcinski, M. Kobierski, M.K. Kustra, A.A. Wachowiak, W. Wachowiak, T. Hiller, A. Grabowski, L. Kurpaska, M.K. Naparty, Characterization of microstructural, mechanical and optical properties of TiO 2 layers deposited by GIMS and PMS methods, Surf. Coat. Technol. 282 (25) (2015) 16e23. [17] W.M. Posadowski, Pulsed magnetron sputtering of reactive compounds, Thin Solid Films 343e344 (1999) 85e89. [18] J. Musil, P. Baroch, J. Vl cek, K.H. Nam, J.G. Han, Reactive magnetron sputtering of thin films: present status and trends, Thin Solid Films 475 (1e2) (2005) 208e218. [19] K. Zdunek, K. Nowakowska-Langier, J. Dora, R. Chodun, Gas injection as a tool for plasma process control during coating deposition, Surf. Coat. Technol. 228 (2013) S367eS373. [20] K. Zdunek, K. Nowakowska-Langier, R. Chodun, J. Dora, S. Okrasa, E. Talik, Optimization of gas injection conditions during deposition of AlN layers by novel reactive GIMS method, Mater. Pol. 32 (2) (2014) 171e175. rquez, [21] L.M. Franco, M. Fazio, E. Halac, D. Vega, E. Heredia, A. Kleiman, A. Ma Phase transformation of TiO2 thin films in function bias voltage, Proc. Mater. Sci. 8 (2015) 39e45. [22] B. Kulakowska-Pawlak, Spatially resolved spectroscopy of an impulse plasma for thin film deposition, Plasma Sour. Sci. Technol. 18 (2009) 035015 (7pp). [23] B. Kulakowska, W. Zyrnicki, A spectroscopic study of a pulse plasma used for production and deposition of tin films, J. Less-common Met. 166 (1990) 81e93. [24] R. Ganesan, et al., The role of pulse length in target poisoning during reactive HiPIMS: application to amorphous HfO2, Plasma Sour. Sci. Technol. 24 (3) (2015) 035015. [25] J. Walkowicz, J. Sekula, Space-time diagnostics of reactive impulse plasma, IEEE Trans. Plasma Sci. 15 (1987) 603e608. [26] K. Zdunek, T. Karwat, Distribution of magnetic field in the coaxial accelerator of impulse plasma, Vacuum 47 (11) (1996) 1391e1394. [27] M.J. Sadowski, J. Baranowski, E. Skladnik-Sadowska, V.N. Borisko, O.V. Byrka, V.I. Tereshin, A.V. Tsarenko, Characterization of pulsed plasma-ion streams emitted from RPI-type devices applied for material engineering, Appl. Surf. Sci. 238 (2004) 433e437. ski, R. Chodun, K. Nowakowska-Langier, K. Zdunek, Computational [28] M. Rabin modelling of discharges within the impulse plasma deposition accelerator with a gas valve, Phys. Scr. T161 (2014) 014049.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
OES studies of plasmoids distribution during the coating deposition with the use of the Impulse Plasma Deposition method controlled by the gas injection* Katarzyna Nowakowska-Langier a, *, Rafal Chodun b, Krzysztof Zdunek b, Sebastian Okrasa b, Roch Kwiatkowski a, Karol Malinowski a, c, Elzbieta Skladnik-Sadowska a, Marek J. Sadowski a, c a
National Centre for Nuclear Research (NCBJ), Material Physics Department, Andrzeja Soltana 7, 05-400 Otwock-Swierk, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland c Institute of Plasma Physics and Laser Microfusion, Hery 23, 01-497 Warsaw, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 2 March 2016 Accepted 3 March 2016 Available online 8 March 2016
The article presents the results concerning the OES characterization of plasmoids generated by the Impulse Plasma Deposition method modified by the use of gas injection (Gas Controlled IPD). Both of the electrodes of the coaxial accelerator: the tube (cathode) and the rod (anodeethe source of the vapors) are permanently connected with the capacitors charged to the high voltage of few kV value. During the coatings deposition both the electro-eroded vapors of the anode material (Ti) and the working gas (N2) form the plasmoids which are accelerated by the Ampere force in the coaxial accelerator and ejected to the vacuum chamber in the direction of non-heated substrates forming there the TiN coating. Periodic gas injections which make pressure fluctuations in the accelerator space in the range of critical values and cause the discharge to initiate or vanish accordingly with the chosen frequency. The promising practical effects of the use of gas injection for controlling the plasma processes during the coatings deposition by the GCIPD method was previously proved. On the basis of the present studies one can state that the GCIPD0 plasmoids containing multiple ions both of metallic as well as gas plasmoids constituents are more energetic as compared to the IPD case. Contrary to the IPD during the GCIPD the titanium fraction of the plasmoids structure overtakes the nitrogen one which seems to be important to the coating growth mechanism. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Impulse plasma deposition Gas controlled impulse plasma deposition Plasmoids structure Optical emission spectroscopy of plasmoids
1. Introduction The origin of the Impulse Plasma Deposition (IPD) method is based on the assumption that non-equilibrium plasma is a favorable environment for a synthesis of phases, in particular those characterized by high barriers of nucleation [1]. In this kind of plasma, the homogenous nucleation on plasma ions is expected and this kind of nucleation is thermodynamically stabilized by ultra-fine particles of the new phase [2]. The clusters, which are present in non-equilibrium plasma and can act as critical nuclei, are
* The paper was presented at the 9th Symposium on Vacuum based Science and Technology, held on November 17e19, 2015 in Kolobrzeg Poland. * Corresponding author. E-mail address: [email protected] (K. Nowakowska-Langier).
http://dx.doi.org/10.1016/j.vacuum.2016.03.002 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
components of a particle stream delivering to the surface of a substrate where the coating grows. The clusters' participation during the process of a coating growth makes this mechanism to be a mix of the atomic and cluster type of growth. In the first case, the nucleation at the substrate surface dominates and the further growth of crystallites occurs with morphology dependent by the thermal activation. In the second case, the mechanism of coating growth is determined by kinetics and clusters dispersion reaching the boundary region and the cluster coalescence processes conditioned by thermal activation [1,3]. In search of an efficient method of highly ionized and thermodynamically non-equilibrium plasma generation, attention was turned onto the coaxial accelerator consisting of two concentric electrodes in the form of: a positively biased axial rod as an anode, surrounded by a grounded tube as a cathode. This specific
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construction of the coaxial accelerator causes the forming of ! ! ! magnetic pressure ð j H Þ, where j e interelectrode current, ! H e concentric magnetic field generated by the current flow accelerating the plasma along the accelerator length [4,5]. The source of electric energy in the standard version of an IPD device is a 50e200 $ 104 F capacitor charged to a 1e6 $ 103 V of voltage [1]. The discharge of the capacitor occurs in the inter-electrode space at a constant 101e102 Pa pressure and is controlled by a threeelectrode spark ignitron. The amplitude of attenuated current waveform in the electric circuit of coaxial accelerator can reach the orders of 103e105 A during the time of 104 s range [1]. From a practical point of view, the IPD method is similar to the HiPIMS methodethe high power magnetron sputtering with respect to high cathodic current of 102 A and the discharge time of 104 s [12,13]. The electric circuit of plasma sourceemagnetron in HiPIMS method is practically identical with the electrical circuit used in IPD method. It is worth noting that there is one significant difference between the IPD and HiPIMS methodsethe IPD method actively uses the magnetic pressure to attach the plasmoids at a very high drift velocity. The pack of plasma (plasmoid) generated in that way has features of arc discharge and the form of a radial symmetric thin current sheet which is accelerated by magnetic pressure up to velocity at the accelerator outlet of the order of 104 m/s [6,7]. The substrate is not intentionally heated from any external heat source, but the dissipation of kinetic energy of the plasmoid at the surface of the substrate raises the temperature of the substrate for a very short period of time up to 2000 K [8]. This thermal peak disperses heat emissions and phonons of the substrate material excitation. This temperature is enough to temporarily heat activation of the substrate surface and the emitted heat is conducted through the material of the substrate resulting from the heat conductivity of solids (rate of temperature decrease e 106 K/s [9]). The consecutive plasmoids are generated periodically with a frequency of 101e100 Hz. The kinetics of growth count in relation to the number of plasmoids, not to the duration of the deposition process like in other methods of plasma surface engineering, which gives the IPD method a digital status as a sum of assumed amount of complete processes of plasma surface engineering. An indicative relative increase of thickness of layers is calculated on about 0.8e1 nm per shot, with a perpendicular fixed substrate holder [10]. The practical advantages of IPD method has been verified in industry to prolong the service life of cutting tools made of SW7M HSS steel coated by titanium nitride coating [11]. Recently we proposed a modification of IPD method relying on the use of the pulsing pressure of working gas to control the process of generating the plasmoids (instead of operation of spark ignitron generating the plasmoids under constant pressure achieved by continuous gas flow) [14]. This pressure changes periodically with specific frequencies applied at the ranges of threshold values: from the 104e103 Pa when the discharge does not initiate, to the 101 Pa allowing the discharge to initiate and spread in the working gas. In these conditions the spark ignitron has been eliminated from the electric circuit of the plasma accelerator and voltage is applied to electrodes permanently. The process of generation and acceleration of plasmoids is controlled by changeable gas concentration only, resulting from periodical gas injections in the interelectrode space of accelerator (Gas Controlled Impulse Plasma Deposition, GCIPD). The concept and interpretation of the physical effects of this gaseous mode in IPD method application has been widely described in our previous work [14,15]. It can be assumed that the application of the gaseous mode, characterized by a dynamically changeable concentration of working gas molecules in the range of threshold values, contributes to the preservation of the kinetic energy of plasma particles by decreasing the probability of energy dissipation by inelastic collisions with neutral molecules.
This assumption corresponds well with the results of our previous research concerning the TiO2 thin films [16], deposited on glass and silicon unbiased and unheated substrates by pulsed magnetron sputtering method (PMS) [17,18] and modified by applying the gas controlled PMS method e GIMS (Gas Injection Magnetron Sputtering) [19,20]. We were able to deposit the rutile films by GIMS method, while the PMS favors the anatase phase. As is known, the presence of the rutile-crystal structure in titanium oxide films is controlled by heating and/or biasing the substrate [21]. The practical result of “gaseous” modification of IPD method (GCIPD) was the achievement of high durability of cutting tool inserts made of SW7M HSS coated by TiN layers. These layers were deposited by using the coaxial accelerator made of titanium and nitrogen as a working gas periodically injected in doses into the inter-electrode space of accelerator. The durability of coated inserts was 1600% better in relation to the uncoated ones, what should be considered as remarkable for TiN coatings [14]. The aim of the presented study was the spectral characterization of plasmoids generated by coaxial accelerator working in gaseous mode, which can be helpful in the interpretation of dynamic phenomena of plasma interactions with substrate during the growth of TiN coatings deposited by GCIPD method providing this significant growth of cutting tools durability.
2. Experimental part In our experiments the pulsed plasma was generated by a coaxial accelerator equipped with two cylindrical electrodes made of titanium in the GCIPD apparatus. A schematic view of the apparatus is shown in Fig. 1. The apparatus used in this experiment was equipped with a special gas-injection system [14], contrary to the standard version of the IPD device [1]. In the presented system a pulsed plasma was generated after the injection of the working gas into the inter-electrode region of the coaxial accelerator. Nitrogen was used as a working gas. In our experiment plasma was generated at different technological conditions: e discharge voltage: 3, 4 kV and battery capacity 100 or 200 mF. The main experimental parameters are shown in Table 1. Measurements of optical emission spectra were carried out by means of a Mechelle®900 optical spectrometer that are characterized by the very short exposition time of measurements. During the experiments it operated in the wavelength range from 300 nm to 600 nm at the exposition time (texp) equals 20 ms e during the time resolved measurements and 100 ms during the measurements of whole spectrum of plasma generated in specific conditions. Exposition was triggered from a current rise measured by 8000 A Series Infinium Oscilloscope from Rogowski coil with sensitivity 1 kA:1 V applied to the electrical circuit of internal electrode. During those experiments the optical
Fig. 1. Schematic view of the IPD apparatus and experiment setup.
K. Nowakowska-Langier et al. / Vacuum 128 (2016) 259e264 Table 1 Parameters of pulsed plasma generations. Internal electrode material
Ti
Working gas Battery capacity [mF] Discharge voltage [kV] Pressure [Pa] Estimated Energy stored in the capacitor [kJ] Duration of the pulse [ms]
N2 100, 200 3, 4 101 e 103 0.45e1.6 ~250
collimator was oriented perpendicular to the plasma stream axis and was placed at a distance of 60 mm from the accelerator outlet. A schematic arrangement of the experiment is shown in Fig. 1. Spectroscopic measurements were performed under the typical conditions of the TiN layers synthesis process. 3. Results and discussions The optical emission spectra of the free-propagating Ti/N2 plasmoids generated during the GCIPD for different parameters are shown in Fig. 2. The peak value of the pulse energy introduced to the plasma process was calculated from following equation: E ¼ (CU2)/2, where E pulse energy, Cecapacity of battery, Uedischarge voltage. The values of discharge energy were calculated, taking into account that after each consecutive discharge the capacitor was not fully unloaded. The remaining voltage after the discharge was up to 200 V. As one can see, these spectra were
Fig. 2. Emission spectra of pulsed plasma.
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related to the optical emission of the vapors of the internal electrode material (titanium) and the working gas (nitrogen) species, Ti II, Ti III, Ti IV and N II, N III, respectively. Additionally, the weak lines from carbon and oxygen were also observed. The intensity of the lines is dependent on process parameters and increase with increasing the pulse energy so the most intensive spectrum was obtained during the discharge with the energy peak equals 1.6 kJ. Plasmoids are characterized by the presence of the multiple ions of both main plasma constituents: titanium and nitrogen. It is worth noting that the presence of the multiple ions were observed previously for the IPD during different condition of plasmoids generation [22,23], during the continuous working gas flow and the dynamic pressure in the vacuum chamber of about of 20 Pa with the discharge energy as high as 10 kJ. Contrary to the present studies, in the cited observations [22,23] the presence of the multiple Ti ions were detected for more energetic plasmoids. Hitherto for much higher energetic conditions discharge than those used at presently described studies. The authors of the cited paper [22,23] reported the appearance of the Ti IV line at the pulse energy as high as 10 kJ while at energy level of 2.18 kJ very close to the energy level used in our experiment, only the twofold ionization level of the titanium ions highest energetic state of the Ti ions was Ti III. It is interesting, that with a decrease of the plasmoids' energy, some spectral lines disappear almost completely, while the intensity of certain lines decreases more slowly. For the interpretation of these observations, the spectral lines of the various components were selected and their intensity are presented in Fig. 3. Although the intensities of emission lines do not evaluate the number of species in plasma, the ratios of intensities the excited species compared to each other show estimated relative populations [24]. It is known from the literature that the structure of the plasmoids in the IPD method is composed of zones of two main fractions ions slightly separated in time and space: the fraction originating from the internal electrode erosion products located mainly in the central part of the plasmoid and of the ionized working gas located mainly in outer plasmoid regions [25]. The results presented in Fig. 3 show that in conditions of relatively low energy, spectral lines of plasma particles of “metallic” fraction are much less intense than the spectral lines of “working gas” particles. It seems that energy introduced by discharge is primary dissipated for working gas ionization and if the energy is relatively low it may be not enough for a successive Ti internal electrode erosion. With an increase of pulse energy, the intensity of spectral lines of titanium vapors increase faster than those of the
Fig. 3. Intensity of chosen lines of registered spectra.
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Fig. 4. Current waveforms measured during experiments.
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Fig. 5. Time resolved distribution of pulsed plasma generated under different discharge energies: 900 kJ (left graph) and 1600 kJ (right graph).
nitrogen lines. This probably means that a higher energy discharge enhances more intensive erosion of the internal electrode. While measuring the coaxial accelerator i(t) waveforms for the GCIPD we found that for the higher discharge energy a specific reversely directed peak has been visible for the central part of the first or even the second half of the waveform (for the highest energy). The i(t) waveforms of electric circuit of GCIPD is presented in Fig. 4. The presence of such a peak in the current waveform has not been observed before in the case of the IPD [26]. SkladnikSadowska et al. [27] while studying the evolution of the plasmoids generated in the IBIS device e.s. in the conditions similar to those of the GCIPD interpreted the presence of the reversely directed peak located in the second part of the first half of the current waveform as being the result of internal electrode erosion (“erosion regime” e fast regime) [27]. If so the current waveforms measured by us have shown the energy dependent “erosion regime” mode of the coaxial accelerator work during the GCIPD. Fig. 5 shows the time evolution of the titanium/nitrogen plasmoids emitted from the coaxial accelerator during the GCIPD obtained at two different discharge energies, 900 and 1600 kJ respectively. In the case of the titanium electrode erosion products, the intensity of titanium ions is greater in the early stages for the face region of the plasmoids while the intensity of the nitrogen ions increases with time. Fig. 6 summarize the results given at Fig. 5 showing clearly that during the GCIPD the titanium fraction in the plasmoids overtakes the nitrogen one. On the basis of our present studies one can conclude that the erosion vs. energy relation illustrated by the i(t) characteristics (Fig. 4) corresponds well with the time evolution of the titanium and nitrogen ions fractions of the plasmoids (Fig. 3) showing that during the GCIPD the titanium ions are dominated in plasmoid front. Such a conclusion is in strong contradiction to the corresponding results obtained previously for the IPD [25]. Taking into account the results of our studies the following mechanism of plasmoids forming during the GCIPD could be proposed. When the pulse valve is opened the pressure of the working gas (nitrogen) rapidly increases from the it initial value of about 104e103 Pa to the value suitable for initiation of the discharge of about of 102e101 Pa; as the source of the electric energy serves the capacitors bank charged to the voltage of order of 103 V. Because of the capacitors discharging initiated by the pressure increase the current sheet is generated and accelerated by the magnetic pressure. The current sheet dynamics are correlated with the dynamics of the working gas particles changes. In contrast to the operating at a constant pressure of the order of 101 Pa, the GCIPD
should characterized by lesser concentration of the cold gas molecules. Thus the GCIPD's plasmoid should lose less of its total energy during the interaction with cold gas than for IPD and can be effectively used not only for stronger acceleration of the current sheet but also for the enhancing of the titanium electrode erosion (as a result of the complex mechanism [26]). Due to the nonuniform time e space distribution of the plasma particles density during the GCIPD, it is more likely that the maximum of density corresponds to the plasmoid itself while before and after the plasmoid density of particles is much lower. This means that, in the contrast to the IPD, during the GCIPD there is no cold gas swept and pressed at the front of the plasmoid which could be further exited and ionized (and therefore detected by the OES studies as a gas fraction located the plasmoid front like in the case of the IPD). It seems this is the reason of the different structure of plasmoid generated during the GCIPD and IPD. It is worthy to cite here the results of the computer simulation of plasmoids dynamics during the IPD and GCIPD presented in Ref. [28]. The results showed that the electron
Fig. 6. Time resolved distribution of spectral lines of pulsed plasma generated by 1600 kJ discharge.
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temperature for the GCIPD is a five time larger than the electron temperature obtained for the IPD and could be as high as 15 eV for the capacitor voltage of 4 kV (not published yet our Langmuire probe measurements brought the similar values of about of 10 eV for the capacitor voltage of 2 kV). The results of computer simulation seem to support the concept of plasma energy loss caused by non e elastic collisions of particles with neutral molecules during the IPD. As a consequence of the proposed mechanism of plasmoids distribution during the GCIPD the plasmoid impact at the substrate surface probably should be different to that postulated before for the IPD. Because of the presence of the Ti fraction at the front of the plasmoid and probable higher dispersion and kinetic energy of the plasmoid during the GCIPD in comparison to the IPD, the surface bombarded by the energetic Ti ions first. It might cause the energetic activation of the surface region (the subplantation of the Ti atoms is possible [6,15]) enabling the mechanism of growth of the coating not only by the cluster type like for the IPD [1] but also by the atomic one constituted as typical for the other plasma surface engineering methods (e.g. for magnetron sputtering). The presence of the atomic growth mechanism during the IPD when the gas injection was used had been discussed before on the basis of the structural study of the coatings [14]. 4. Conclusion This article describes the emission spectra of plasmoids periodically generated in the coaxial accelerator under the modified IPD methodeGas Controlled IPD (GCIPD). In contrast to the previous study reported in the literature concerning the IPD method in which the working gas was dosed in a continuous manner, the pressure in the coaxial accelerator and the vacuum chamber had a fixed dynamic value for approximately of 20 Pa and discharge initiation was controlled by the three-electrode spark ignitron., the GCIPD's coaxial accelerator is directly connected to the charged capacitors and, as well as the whole vacuum chamber, is initially pumped out to the pressure of order of 104 Pa. Then the working gas is injected straight to the plasma accelerator which makes the gas discharge capable to generate the plasmoid. Our results reported previously showed that the practical effect of the IPD modification was the significant increase of the service life of the cutting tools coated by the TiN coatings even as high as 16-fold in comparison to uncoated tools while those coated by the IPD resulted by the increase of few times. The presented thorough study of the spatiotemporal structure of GCIPD0 plasmoids, based on OES measurements, showed that the plasmoids generated and distributed under condition of periodically changing pressure of working gas (in the range of threshold pressure values for which the gas discharges are initiated and vanished) are probably more energetic than the plasmoids generated and distributed during the IPD. We believe that this contributes in more effective energy exchange during the plasma processes in the case of GCIPD in comparison to the IPD. Additionally, the presence of heavier metallic (Ti) ions on the GCIPD0 plasmoids front instead of working gas ions as during the IPD activates the surface of the substrate more effectively. Taking into account the previously reported results and the present ones, in our opinion, the idea of using the gas injection as a control factor for plasma processes of coating deposition opens up the new opportunities for coating deposition among the plasma methods of surface engineering. Acknowledgements This work was supported by the Polish State National Science
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