Methods of Boron-carbon Deposited Film Removal - ScienceDirect

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ScienceDirect Physics Procedia 71 (2015) 58 – 62

18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015

Methods of boron-carbon deposited film removal A. Airapetov*, V. Terentiev, A. Voituk, A. Zakharov National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31,115409, Moscow, Russia

Abstract Boron carbide was proposed as a material for in-situ renewable protecting coating for tungsten tiles of the ITER divertor. It is necessary to develop a method of gasification of boron-carbon film which deposits during B4C sputtering. In this paper the results of the first stage investigation of gasification methods of boron-carbon films are presented. Two gasification methods of films are investigated: interaction with the ozone-oxygen mixture and irradiation in plasma with the working gas composed of oxygen, ethanol, and, in some cases, helium. The gasification rate in the ozone-oxygen mixture at 250 °C for B/C films with different B/C ratio and carbon fiber composite (CFC), was measured. For B/C films the gasification rate decreased with increasing B/C ratio (from 45nm/h at B/C=0.7 to 4nm/h at B/C=2.1; for CFC – 15 μm/h). Films gasification rates were measured under ion irradiation from ethanol-oxygen-helium plasma at different temperatures, with different ion energies and different gas mixtures. The maximum obtained removal rate was near 230 nm/h in case of ethanol-oxygen plasma and at 150 °ɋ of the sample temperature. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)

Physics Institute) .

Keywords: boron; carbon; films; boron carbide; deposition; ozone; oxygen; ethanol; removing; chemical sputtering; ion irradiation; gasification.

* Corresponding author. Tel.: +7(495) 788-56-99, add. 9993. E-mail address: [email protected]

1875-3892 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.312

A. Airapetov et al. / Physics Procedia 71 (2015) 58 – 62

1. Introduction Tungsten is selected as a material for ITER divertor tiles. Investigations show that plasma irradiation with high power density leads to surface erosion, cracking, blistering, macroscopic particle emission, etc. These processes are described by Buzi et al.(2014), Krasheninnikov et al. (2011), Maier et al.(2013), Arakcheev et al. (2015), Budaev et al. (2015), Klimov et al. (2011), Kurnaev et al.(2015), Würz et al. (2002) and others. These processes result in accelerated destruction of tungsten tiles, deposition of tungsten layers and tungsten dust formation, which could trap tritium, and increase in tungsten impurity in plasma to unacceptable values. These problems could be solved by using a protective coating for tungsten tiles. This protective coating must be in-situ renewable, have low sputtering yields, and products of its sputtering must be removable from the chamber without its ventilation. The last one is necessary to obtain a renewable coating. In any case, during the work, a protective coating will be eroded and produce dust, deposited layers, which could accumulate tritium. The best way to remove erosion products is their gasification. One of the probable materials for a protective coating is boron carbide (B4C) proposed by Buzhinskij et al.(1996) and investigated in Azizov et al.(2015) and Begrambekov et al. (2011). It has good properties as PFM: low Z, in-situ renewable, low sputtering, high melting temperature and others, but we don’t have a technique for gasification of deposited B-C films (gasification could be realized by hydrogen ion sputtering, for example, but it gives a low removal rate, it was shown by Annen et al. (1996)). In this work the results of the first stage of development of the boron-carbon film gasification method are presented. Two methods of gasification of films are investigated: by interaction with the ozone-oxygen mixture and under irradiation of plasma with the working gas composed of oxygen, ethanol, and, in some cases, helium. 2. Boron-carbon film preparing and analyzing Deposited boron-carbon layers were used for the experiments. Boron-carbon layers were produced by sputtering a graphite target with B4C layer by ions of deuterium-argon plasma. Pressures of argon and deuterium were the same and were equal to 7×10-4 Torr. BC films with different B/C ratios were prepared by sputtering targets with a different degree of B4C coverage. Stainless steel 12ɏ18ɇ10Ɍ (0.12% C, 18% Cr, 10% Ni, less than 1% Ti) sheets were used as substrates. Thickness of films and removing rates were measured by the gravimetric method and recalculated from the density of 2.5 g/cm3. Films with different thickness were used in experiments: in the first method – 200-300 nm, and 600-800 nm in the second. B/C ratios of samples after deposition and after film removal were measured by the Oxford Instruments energy dispersive spectroscopy (EDS) X-act device mounted on the TESCAN Vega 3 scanning electron microscope with electron energies equal to 5 keV and 30 keV. Amount of trapped deuterium was measured by thermodesorptional spectroscopy on the thermal desorption stand described by Airapetov et al. (2011). Typical amount of deuterium in films was 20-25 at%. 3. Boron-carbon layer gasification in the ozone-oxygen mixture The first method is an exposition in the ozone-oxygen mixture. It is perspective for removing carbon films, as described by Davis et al. (2009) and Moormann et al. (2000) and it is an “upgrade version” of thermal oxidation Alegre et al. (2013), Möller et al. (2014) and Tanarro et al.(2009). For obtaining the ozone-oxygen mixture a special ozone generator was established. It is based on the barrier discharge in oxygen. Pressure of gas mixture was 1 bar, ozone concentration of 10 at.% was obtained. Ozone concentration was measured by the UV absorption method. The ozone-oxygen mixture went through a glass working vessel which contained samples. Heating of the samples was provided by the external heater. The vessel was pumped to 1 Torr residual pressure before experiments. The experiments were performed with B-C films of different B/C ratios. For comparison a pure carbon material was also used. The gasification rate of a carbon fiber composite was 15 μm/hour at temperature of 250 °ɋ. The gasification rate of B/C films in the same condition is presented in Fig.1.

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The removal rate which is less than 30 nm/hour is very small for the gravimetric method. In this case EDS measurements were used. In our case paths of electrons for the analysis in material are significantly longer than film thickness. Concerning this, EDS gives information about chemical characterization of a layer consisting of a film and a substrate. We can obtain information about the decreasing thickness of a film by measuring the increasing substrate signal. The removal rate could be calculated using this measurements and the initial thickness of the film is obtained by gravimetric measurements.

Fig. 1. Removal rate in dependence on ȼ/ɋ ratio in the ozone-oxygen mixture.

One can see (Fig.1) that the removal rate of a B/C film is decreasing with increasing boron concentration. The EDS analysis shows that during experiments the amount of carbon atoms as well as boron atoms is decreasing. This fact shows that gasification of boron occurs. Probably, boron interacted with ozone and produced boron oxide, which interacted with water from the residual gas or with deuterium from a film with production of evaporable boric acid. During interaction between film and gas mixture the content of carbon in film was decreasing and the oxygen content was increasing. In case of a B/C film with the initial ratio of B/C=3.5, after 6 hours in the ozone-oxygen mixture with 1% of ozone carbon concentration went down to ȼ/ɋ=10. 4. Boron-carbon film gasification under plasma irradiation It is known, that one of the volatile boron containing substances is triethyl borate. It could be obtained by interaction of boron oxide or boric acid with ethanol. It is also known, that carbon atoms from carbonaceous materials could interact with oxygen ions and atoms and produce carbon oxides. Based on these facts, a method of carbon-boron film gasification was offered. This method is implemented by film irradiation with ions from plasma which contains oxygen and ethanol. A plasma device was used for experiments. A gas discharge was initiated in a three electrode system (heated cathode, irradiated sample and anode). The residual gas pressure was 5×10-6 Torr. The working mixture was composed of ethanol, oxygen and, in some of experiments, helium. Films with ȼ/ɋ= 3.7±0.3 were used for experiments. Several series of experiments were made. The removal rate in dependence on the sample temperature is presented in Fig.2a and in dependence on the ethanol pressure in Fig.2b. One can see that each of dependencies has a maximum. If the removing process is based only on physical sputtering, temperature dependence would have no peak. One can suggest, that in this case both physical and chemical processes were observed. Also we can see, that the removal rate by ion irradiation from helium-oxygen plasma is lower that by ions from ethanol-containing plasma. Experiments with plasma in the oxygen-ethanol mixture were performed also (oxygen pressure – 0.5×10-3 Torr, ethanol pressure – 1×10-3 Torr, temperature – 150 °ɋ). The removable rate was measured under different conditions: the mean ion flux (0.6×1016 and 1×1016 atoms/sm2s, sample bias voltage of 200 V) and sample bias voltages (150


and 200 V, with the mean ion flux 0.6×1016 atoms/sm2s). The removal rate was almost equal at different ion fluxes, and decreased two times with decreasing the sample bias voltage from 200 V to 150 V. The highest removal rate was 230±40 nm/hour and was observed without helium, with following parameters: oxygen pressure – 0.5×10-3 Torr, ethanol pressure – 1×10-3 Torr, temperature – 150 °ɋ, ion flux, sample bias voltage-200V. The obtained removal rate is ten times higher, than that was obtained in case of removing by hydrogen ECR plasma in [10] for B/C films with the same B/C ratio.

Fig. 2. (a) Removal rate (nm/h) in dependence on temperature (oxygen pressure – 0,8×10-3 Torr, helium pressure – 6×10-3 Torr, ethanol pressure – 1×10-3 Torr, mean ion flux 1×1016 atoms/sm2s, sample bias voltage of 200 V). (b) Removal rate (nm/h) in dependence on ethanol pressure (oxygen pressure – 0,4×10-3 Torr, helium pressure – 2×10-3 Torr, temperature – 150 °ɋ, mean ion flux – 1×1016 atoms/cm2s, sample bias voltage of 200 V).

5. Conclusion The results of the first stage investigation of boron-carbon film gasification are presented. The first method involved exposition boron-carbon films in the ozone-oxygen mixture at the elevated temperature. This method shows the high removal rate for carbon materials and boron-carbon films with low concentration of boron. The removal rate is decreasing with increasing the amount of boron in a film. During exposition both carbon and boron are removed. An advantage of this method is the possibility of removing films from gaps and shadow regions. Disadvantages are high working pressure (1 bar) of the ozone-oxygen gas mixture, low removal rate for layers with high boron concentration, and decreasing of efficiency with increasing the distance from ozone source to layer, related with ozone destruction. The second method was performed by irradiation of layers with ions from oxygen and ethanol gas mixture plasma. Advantages of this method are: possibility to remove boron-carbon films with high boron concentration (up to ȼ:ɋ=4:1), high removal rate (in relation to the first method and some other), and using a low pressure discharge. Disadvantage of this method is a fact, that this method is plasmic and it could not give good results for gaps and shadow regions in its initial state. At the next stages of this work the authors suggest to continue the investigation of this method of gasification to increase the removing rate of boron-carbon layers and upgrading the method for using in gaps and shadow regions.

Acknowledgements This work was supported by Competitiveness Growth Program of the Federal Autonomous Educational Institution of Higher Professional Education National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) and by the Ministry of Education and Science of the Russian Federation (agreement contract ʋ14.575.21.0049 of 27.06.14).

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