rus
Issue #5/2022
A.G.Kolesnikov, Yu.A.Kryukov, N.V.Gorbunov, A.Kh.Abduev, A.Sh.Asvarov, A.K.Akhmedov,
LOW TEMPERATURE FORMATION OF BORON CARBIDE THIN FILMS ENRICHED BORON ISOTOPE 10B
LOW TEMPERATURE FORMATION OF BORON CARBIDE THIN FILMS ENRICHED BORON ISOTOPE 10B
DOI: https://doi.org/10.22184/1993-8578.2022.15.5.290.299
Thin-film neutron converters based on boron carbide B4C, enriched with the 10B isotope, applied to thin substrates of aluminum, aluminum foil and polymer films of a large area, are a promising material for creating new neutron detectors. The application of B4C films by magnetron sputtering on such bases is greatly complicated by the need to heat the substrates to a temperature of 400 °C or more, which can lead to their deformation. It is shown that the use of ion assistance in the process of magnetron deposition of B4C leads to the possibility of forming films of nano-crystalline structure with high strength and flexibility, even when the temperature drops to 50 °C, and the use of an Al sublayer increases adhesion. B4C thin films were obtained on 0.5 × 100 × 100 mm2 and 0.5 × 280 × 400 mm2 aluminum substrates in an argon atmosphere at temperatures of 400, 200 and 50 °C. The structure, composition and refractive index of films on silicon wafers are studied. The flexibility of the resulting films and the low formation temperature make it possible to create neutron converters from 10B4C on thin polymer bases.
Thin-film neutron converters based on boron carbide B4C, enriched with the 10B isotope, applied to thin substrates of aluminum, aluminum foil and polymer films of a large area, are a promising material for creating new neutron detectors. The application of B4C films by magnetron sputtering on such bases is greatly complicated by the need to heat the substrates to a temperature of 400 °C or more, which can lead to their deformation. It is shown that the use of ion assistance in the process of magnetron deposition of B4C leads to the possibility of forming films of nano-crystalline structure with high strength and flexibility, even when the temperature drops to 50 °C, and the use of an Al sublayer increases adhesion. B4C thin films were obtained on 0.5 × 100 × 100 mm2 and 0.5 × 280 × 400 mm2 aluminum substrates in an argon atmosphere at temperatures of 400, 200 and 50 °C. The structure, composition and refractive index of films on silicon wafers are studied. The flexibility of the resulting films and the low formation temperature make it possible to create neutron converters from 10B4C on thin polymer bases.
Теги: boron carbide film structure magnetron deposition of films neutron detector thin-film neutron converter карбид бора магнетронная нанесение пленок нейтронный детектор структура пленки тонкопленочный конвертер нейтронов
INTRODUCTION
Nowadays, neutron radiations have an application in various fields of science and technology: biology, medicine, engineering and space research, condensed matter study, nuclear and high-energy physics. Thermal and cold neutrons have been used as probes for non-destructive study of archaeological artifacts, as well as in nuclear safety and cargo control with detection of radioactive radiation and explosives. The main measuring instrument in these areas is a neutron detector. Developments of neutron detectors are carried out practically in all main world neutron centers. Direct detection of neutrons is impossible due to the lack of an electric charge. Registration is carried out with special substances capable of producing secondary charged particles and gamma quanta. This secondary radiation is registered using standard methods of charged particles detection. A substance used to convert neutron radiation into charged particles is called a neutron converter. There are few such substances, and one of them is boron isotope 10B. When a neutron is captured by a 10B nucleus, a nuclear decay takes place producing an alpha particle and a lithium nucleus, moving in opposite directions with high energy.
A promising direction for the development of neutron detectors is the thin-film neutron converters based on boron carbide B4C enriched with 10B isotope deposited on an aluminum substrate. During the conversion of neutron some of them leaves the converter and can be registered and the others disappear in the converter material and in the substrate. Reducing the thickness of the converter film increases the part of the detected particles, but reduces their total number, i.e. reduces efficiency. The optimal thickness of the converter layer is determined by the ranges of the particles in the B4C converter, and as shown in [1–2] should not exceed 3 microns. To increase the efficiency of the detector, several converter layers or an inclined arrangement of layers can be used. Various types of detectors using B4C thin-film converters have been developed and are being used [3–6]. At the research stations of the scientific mega installation of the European Spallation Source (ESS), Lund, Sweden, detectors based on solid-state boron converters are planned to be used as the main type of neutron detectors [7].
There are a number of requirements to the boron carbide thin-film coatings used as a neutron converter: high adhesion to the substrate, uniformity of thickness, dense structure (the smallest number of pores), and minimum amount of impurities. The production of films with the required characteristics, especially on low-melting aluminum, is complicated by the physical properties of boron carbide – high melting point, fragility and intolerance of heat stroke. As an example of a successful solution to the problem of developing a film application technology, we can cite the joint development of ESS and Linkoping University, Sweden [8]. The paper [8] publishes the results of the application of boron carbide 10B4C using magnetron sputtering when the substrate is heated to 500 °C. This method makes it possible to obtain high-quality B4C films on aluminum and other metal substrates. The application of B4C films to polymer substrates requires smaller temperature of the chamber and the substrate, which leads to a decrease in adhesion and deterioration of the film quality. The Swedish team investigated methods for obtaining a stable film at a substrate temperature of 100 °C [9]. Magnetron sputtering on plastics and thin (less than 0.05 mm) metal foils requires further reduction of the substrate temperature.
The authors describes a method for producing fine homogeneous boron carbide 10B4C thin films with high adhesion to aluminum substrates of a large area by the magnetron sputtering with a decreased substrate temperature from 200 °C to 50 °C, the analysis of the structure and the chemical composition of the resulting films.
METHODS
Basically, the thin-film coatings of boron carbide at low temperatures were made by magnetron sputtering with ion-assisted deposition. The work was carried out on the VSR-300 vacuum chamber (assembled by ROBVAK, Fryazino, Russia), reconstructed for magnetron sputtering by specialists of the Dubna State University together with Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR, Dubna). In the center of the U-shaped chamber of the VSR-300 with a size of 400 (W) × 700 (H) mm was installed two balanced magnetrons with indirect cooling of the B4C and Al targets with a size of 5 × 100 × 400 mm each and an ion source for the substrate cleaning and ion assistance was placed between them. The film is sprayed onto the inner surface of the cylindrical drum, where 4 substrates with a size of 400 × 280 mm made of aluminum sheet with a thickness of 0.5 mm or aluminum foil or high–temperature polymer films – Mylar or Kapton in special mandrels can be fixed. In the central part of the drum was placed "witnesses", which are 0.2 mm thick silicon plates polished in the crystal plane <100>. The magnetron’s magnetic system is based on Nd-Fe-B permanent magnets with the 3 × 16 mm horizontal shunt made of St 37 steel which is needed to increase the width of the magnetron`s erosion to 10 mm. The magnetic field above the target surface is about 0.1 T. The target –substrate distance is 80 mm. A heating element placed in the upper part of the chamber to heat the chamber and the substrates up to 250 °C.
Test depositions of natural natB4C were made at the upgraded "MAGNETRON" installation (Votkinsk, Russia) of the Center for High Technologies and Nanostructures of the Institute of Physics of the Dagestan Federal Research Center (IP DFRC RAS, Makhachkala, Russia). A composite target made of natB4C with a size of 120 × 65 × 3 mm is bonded onto a copper base mounted in an unbalanced magnetron of the second type (magnetic field unbalanced to the sides) with an unbalance coefficient of 1.2. The magnetron’s magnetic system is made of Sm-Co permanent magnets. The magnetic field induction above the target surface is 700 Gs, and at a distance of 5 mm – 350 Gs (measured by the NOVOTEST MF-1 magnetometer). The distance from the target to substrate is 120 mm. In order to heat the reverse side of the substrate up to 400 °C the heating elements are installed inside the cylindrical drum on which the substrates are placed. Substrates: "witness" – silicon KEF (40 Ohm · cm) and aluminum 0.5 × 100 × 100 mm.
Adhesion of the film was evaluated with a sticky tape, according to the percentage of the film torn from the substrate. To make a scanning electron microscope (SEM) images of the boron carbide film, silicon "witnesses" were used, split along a notch from the reverse side.
RESULTS
Magnetron deposition of boron carbide on aluminum substrates was tested at the "MAGNETRON" installation. A film of natural boron carbide is sputtered on a 100 × 100 mm aluminum substrate heated to 400 °C at two pressures of argon working gas – 0.75 and 3.75 mTorr and the same magnetron discharge power of 233W (2 ÷ 3 W/cm2). The current and voltage are 370 mA, 630 V and 570 mA, 410 V, respectively. Transparent films of dark brown tone of approximately the same thickness (930 nm and 900 nm, respectively, the second is lighter) with good adhesion and uniformity of thickness were obtained. The deposition time was 360 minutes. The film sputtering quality was evaluated using "witness samples" – small silicon peace attached to an aluminum substrate. Fig.1 for both samples on a silicon substrate shows SEM images of films cleavage on lateral view and at an angle of 45°, made with a scanning electron microscope. If the film of the first sample (Fig.1a) has a homogeneous structure and minimal surface relief, then the film of the second sample (Fig.1b) has a clear columnar structure, probably due to the clustering of the flow of sprayed boron carbide from the gas phase, as a result of which a columnar structure is formed, which is generated by islands during the deposition of cluster flows growing in the form of columns that end on the surface with bulgy inhomogeneity.
The experiments with boron carbide deposition at lower temperatures were performed at the VSR-300 installation. To improve adhesion, the surface of the substrate is activated by a beam of working gas ions, and an aluminum sub layer was added to the substrate. Images of the sample No.1 at an angle of 45° and 90° to the surface plane of the silicon "witness" with the film are shown in Fig.2. During the deposition of aluminum – 7 minutes and B4C – 5 hours at a substrate temperature of 200 °C, an argon pressure of 1.4 mTorr and magnetron power of 2 kW for Al-magnetron, and 1.7 kW for B4C magnetron, a dark brown film with good adhesion was obtained. The Al sub layer thickness is 380 nm and a B4C layer thickness is 1030 nm. The image clearly shows a columnar structure with a boundary between layers of aluminum and boron carbide. The boron carbide has a layered structure because of the periodic passage of the substrate through the sputter area due to the rotation of the drum with the substrate holder.
By using irradiation of the film deposited on the substrate with a stream of ions (ion-assisted deposition), it is possible not only to improve adhesion to the substrate, but also to obtain a coating with a different structure and properties.
Fig.3 shows the fine-crystalline tightly packed structure of the boron carbide layer of sample No.14, deposited with ion assistance. At the same time, the aluminum layer was also formed as a fine-crystalline, but different structure. When a silicon plate is fractured, the fracture of the silicon "witness" film layers passes, apparently, along the boundaries of the crystal grains, which creates a complex relief of each layer, especially the Al layer with larger crystals. The film has a dark gray color with a greenish tinged and the thickness of B4C is 450 nm.
When the substrate temperature drops to 50 °C, The structure of the boron carbide layer does not change and the adhesion of the film does not deteriorate when the substrate temperature decreased to 50 °C. The ion flux does not have the same effect on the growth of the aluminum layer as at 200 °C, and a columnar structure is formed. An example of such a film growth is sample No.16. Figure 4 shows an SEM image of sample No.16 obtained using ion assisted at 50 °C. Figure 5 shows a SEM image of the film in the same position with a large magnification. The fine-crystalline densely packed structure of the layer B4C with a thickness of 500 nm, the transition layer Al + B4C with a thickness of 70 nm and the columnar structure of the Al layer with a thickness of 280 nm are visible. The color of the film is dark gray.
SEM image of sample No.16, a chipped from three sides silicon with film deposited on it, is shown in Fig.6. The B4C layer does not chip off along the edge of the silicon wafer, as it is observed with the aluminum layer (see Fig.4), but at some distance. At the site of the split, the B4C layer forms pieces that crumble like tempered glass (circled in Fig.6). Apparently, this is due to the hardness of the B4C layer, exceeding the adhesion to the Al layer.
Figure 7 shows the energy-dispersive X-ray spectra of the B4C film of samples No. 1, No. 14 and No. 16. In comparison with sample No.1 make without ion assistance, films of the samples No.14 and No.16, made with ion assistance, have lower oxygen, but increased argon percentage and the ratio of boron and carbon is changed.
Figure 8 shows the measured refractive index and light absorption index for a boron carbide film with a thickness of 850 nm in the range of 300 nm – 1000 nm. The value of the refractive index for the visible light is more than 2.5. The absorption of visible light in the material is also high. As the wavelength decreases in the ultraviolet region, the refractive index decreases with a rapid increase in absorption.
DISCUSSION
The ion assistance significantly changes the structure of the B4C film: the columnar structure disappears, and the films have a dense structure consisting of most likely from non-oriented Nano crystals. The films obtained with ion-assisted deposition become less transparent and their color with a thickness of 500 nm becomes black, in contrast to brown color of thicker films (1030 nm) obtained without assistance. The surface of the B4C film, sputtered at a temperature of 200 °C, is covered with "crater" (Fig.3), which is probably due to impact on the heated surface by the flow of argon ions. This is confirmed by the fact that the surfaces of the film deposited at a temperature of 50 °C have greater smoothness and lack of "crater" (Fig.4, 5). Highly likely, for the same reason a columnar structure of the Al-layer is formed at 50 °C, since there is not enough energy to destroy the column growth of crystallites. The increase in argon content in the composition of the film is likely to be due to the use of ionic assistance when the B4C layer is deposited. Reducing the fraction of oxygen and changes in quantitative indicators of the fractions of boron and carbon is likely to be explained by an increase in the density of the B4C layer. The increase in density indicates also a high refractive factor. The rapid increase in the absorption coefficient with a decrease in the wavelength in the ultraviolet region is likely due to the B4C film structure consisting of the dense packed Nano scale crystals.
CONCLUSIONS
Dense B4C films with high adhesion to silicon and aluminum substrates were obtained by magnetron ion-assisted sputtering. The deposition of B4C films at a temperature of 50 °C makes it possible to apply film to aluminum foil, organic materials, plastics and polymers, including Mylar and Kapton films, which can be used to create neutron detectors. A new thin-film functional material based on B4C has been obtained.
ACKNOWLEDGEMENTS
The authors are grateful to Gorin Anatoly Vasilyevich for valuable assistance in the design of the magneton and an ion source and work on sputtering B4C films, as well as Apel Pavel Yuryevich and Orelovich Oleg Leonidovich from the Center for Applied Physics Laboratory of Flerov Laboratory of Nuclear Reaction JINR (CAP FLNR JINR) for help in the study of the surface of B4C films.
This work was performed using equipment of MIPT Shared Facilities Center, of CAP FLNR JINR and of Institute of Physics Dagestan Federal Research Center Russian Academy of Science (IP DFRC RAS).
The work was carried out with the financial support of the Russian Federation represented by the Ministry of Science and Higher Education, agreement No. 075-10-2021-115 of 13 October, 2021 (internal number 15.СИН.21.0021).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. 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.
Nowadays, neutron radiations have an application in various fields of science and technology: biology, medicine, engineering and space research, condensed matter study, nuclear and high-energy physics. Thermal and cold neutrons have been used as probes for non-destructive study of archaeological artifacts, as well as in nuclear safety and cargo control with detection of radioactive radiation and explosives. The main measuring instrument in these areas is a neutron detector. Developments of neutron detectors are carried out practically in all main world neutron centers. Direct detection of neutrons is impossible due to the lack of an electric charge. Registration is carried out with special substances capable of producing secondary charged particles and gamma quanta. This secondary radiation is registered using standard methods of charged particles detection. A substance used to convert neutron radiation into charged particles is called a neutron converter. There are few such substances, and one of them is boron isotope 10B. When a neutron is captured by a 10B nucleus, a nuclear decay takes place producing an alpha particle and a lithium nucleus, moving in opposite directions with high energy.
A promising direction for the development of neutron detectors is the thin-film neutron converters based on boron carbide B4C enriched with 10B isotope deposited on an aluminum substrate. During the conversion of neutron some of them leaves the converter and can be registered and the others disappear in the converter material and in the substrate. Reducing the thickness of the converter film increases the part of the detected particles, but reduces their total number, i.e. reduces efficiency. The optimal thickness of the converter layer is determined by the ranges of the particles in the B4C converter, and as shown in [1–2] should not exceed 3 microns. To increase the efficiency of the detector, several converter layers or an inclined arrangement of layers can be used. Various types of detectors using B4C thin-film converters have been developed and are being used [3–6]. At the research stations of the scientific mega installation of the European Spallation Source (ESS), Lund, Sweden, detectors based on solid-state boron converters are planned to be used as the main type of neutron detectors [7].
There are a number of requirements to the boron carbide thin-film coatings used as a neutron converter: high adhesion to the substrate, uniformity of thickness, dense structure (the smallest number of pores), and minimum amount of impurities. The production of films with the required characteristics, especially on low-melting aluminum, is complicated by the physical properties of boron carbide – high melting point, fragility and intolerance of heat stroke. As an example of a successful solution to the problem of developing a film application technology, we can cite the joint development of ESS and Linkoping University, Sweden [8]. The paper [8] publishes the results of the application of boron carbide 10B4C using magnetron sputtering when the substrate is heated to 500 °C. This method makes it possible to obtain high-quality B4C films on aluminum and other metal substrates. The application of B4C films to polymer substrates requires smaller temperature of the chamber and the substrate, which leads to a decrease in adhesion and deterioration of the film quality. The Swedish team investigated methods for obtaining a stable film at a substrate temperature of 100 °C [9]. Magnetron sputtering on plastics and thin (less than 0.05 mm) metal foils requires further reduction of the substrate temperature.
The authors describes a method for producing fine homogeneous boron carbide 10B4C thin films with high adhesion to aluminum substrates of a large area by the magnetron sputtering with a decreased substrate temperature from 200 °C to 50 °C, the analysis of the structure and the chemical composition of the resulting films.
METHODS
Basically, the thin-film coatings of boron carbide at low temperatures were made by magnetron sputtering with ion-assisted deposition. The work was carried out on the VSR-300 vacuum chamber (assembled by ROBVAK, Fryazino, Russia), reconstructed for magnetron sputtering by specialists of the Dubna State University together with Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR, Dubna). In the center of the U-shaped chamber of the VSR-300 with a size of 400 (W) × 700 (H) mm was installed two balanced magnetrons with indirect cooling of the B4C and Al targets with a size of 5 × 100 × 400 mm each and an ion source for the substrate cleaning and ion assistance was placed between them. The film is sprayed onto the inner surface of the cylindrical drum, where 4 substrates with a size of 400 × 280 mm made of aluminum sheet with a thickness of 0.5 mm or aluminum foil or high–temperature polymer films – Mylar or Kapton in special mandrels can be fixed. In the central part of the drum was placed "witnesses", which are 0.2 mm thick silicon plates polished in the crystal plane <100>. The magnetron’s magnetic system is based on Nd-Fe-B permanent magnets with the 3 × 16 mm horizontal shunt made of St 37 steel which is needed to increase the width of the magnetron`s erosion to 10 mm. The magnetic field above the target surface is about 0.1 T. The target –substrate distance is 80 mm. A heating element placed in the upper part of the chamber to heat the chamber and the substrates up to 250 °C.
Test depositions of natural natB4C were made at the upgraded "MAGNETRON" installation (Votkinsk, Russia) of the Center for High Technologies and Nanostructures of the Institute of Physics of the Dagestan Federal Research Center (IP DFRC RAS, Makhachkala, Russia). A composite target made of natB4C with a size of 120 × 65 × 3 mm is bonded onto a copper base mounted in an unbalanced magnetron of the second type (magnetic field unbalanced to the sides) with an unbalance coefficient of 1.2. The magnetron’s magnetic system is made of Sm-Co permanent magnets. The magnetic field induction above the target surface is 700 Gs, and at a distance of 5 mm – 350 Gs (measured by the NOVOTEST MF-1 magnetometer). The distance from the target to substrate is 120 mm. In order to heat the reverse side of the substrate up to 400 °C the heating elements are installed inside the cylindrical drum on which the substrates are placed. Substrates: "witness" – silicon KEF (40 Ohm · cm) and aluminum 0.5 × 100 × 100 mm.
Adhesion of the film was evaluated with a sticky tape, according to the percentage of the film torn from the substrate. To make a scanning electron microscope (SEM) images of the boron carbide film, silicon "witnesses" were used, split along a notch from the reverse side.
RESULTS
Magnetron deposition of boron carbide on aluminum substrates was tested at the "MAGNETRON" installation. A film of natural boron carbide is sputtered on a 100 × 100 mm aluminum substrate heated to 400 °C at two pressures of argon working gas – 0.75 and 3.75 mTorr and the same magnetron discharge power of 233W (2 ÷ 3 W/cm2). The current and voltage are 370 mA, 630 V and 570 mA, 410 V, respectively. Transparent films of dark brown tone of approximately the same thickness (930 nm and 900 nm, respectively, the second is lighter) with good adhesion and uniformity of thickness were obtained. The deposition time was 360 minutes. The film sputtering quality was evaluated using "witness samples" – small silicon peace attached to an aluminum substrate. Fig.1 for both samples on a silicon substrate shows SEM images of films cleavage on lateral view and at an angle of 45°, made with a scanning electron microscope. If the film of the first sample (Fig.1a) has a homogeneous structure and minimal surface relief, then the film of the second sample (Fig.1b) has a clear columnar structure, probably due to the clustering of the flow of sprayed boron carbide from the gas phase, as a result of which a columnar structure is formed, which is generated by islands during the deposition of cluster flows growing in the form of columns that end on the surface with bulgy inhomogeneity.
The experiments with boron carbide deposition at lower temperatures were performed at the VSR-300 installation. To improve adhesion, the surface of the substrate is activated by a beam of working gas ions, and an aluminum sub layer was added to the substrate. Images of the sample No.1 at an angle of 45° and 90° to the surface plane of the silicon "witness" with the film are shown in Fig.2. During the deposition of aluminum – 7 minutes and B4C – 5 hours at a substrate temperature of 200 °C, an argon pressure of 1.4 mTorr and magnetron power of 2 kW for Al-magnetron, and 1.7 kW for B4C magnetron, a dark brown film with good adhesion was obtained. The Al sub layer thickness is 380 nm and a B4C layer thickness is 1030 nm. The image clearly shows a columnar structure with a boundary between layers of aluminum and boron carbide. The boron carbide has a layered structure because of the periodic passage of the substrate through the sputter area due to the rotation of the drum with the substrate holder.
By using irradiation of the film deposited on the substrate with a stream of ions (ion-assisted deposition), it is possible not only to improve adhesion to the substrate, but also to obtain a coating with a different structure and properties.
Fig.3 shows the fine-crystalline tightly packed structure of the boron carbide layer of sample No.14, deposited with ion assistance. At the same time, the aluminum layer was also formed as a fine-crystalline, but different structure. When a silicon plate is fractured, the fracture of the silicon "witness" film layers passes, apparently, along the boundaries of the crystal grains, which creates a complex relief of each layer, especially the Al layer with larger crystals. The film has a dark gray color with a greenish tinged and the thickness of B4C is 450 nm.
When the substrate temperature drops to 50 °C, The structure of the boron carbide layer does not change and the adhesion of the film does not deteriorate when the substrate temperature decreased to 50 °C. The ion flux does not have the same effect on the growth of the aluminum layer as at 200 °C, and a columnar structure is formed. An example of such a film growth is sample No.16. Figure 4 shows an SEM image of sample No.16 obtained using ion assisted at 50 °C. Figure 5 shows a SEM image of the film in the same position with a large magnification. The fine-crystalline densely packed structure of the layer B4C with a thickness of 500 nm, the transition layer Al + B4C with a thickness of 70 nm and the columnar structure of the Al layer with a thickness of 280 nm are visible. The color of the film is dark gray.
SEM image of sample No.16, a chipped from three sides silicon with film deposited on it, is shown in Fig.6. The B4C layer does not chip off along the edge of the silicon wafer, as it is observed with the aluminum layer (see Fig.4), but at some distance. At the site of the split, the B4C layer forms pieces that crumble like tempered glass (circled in Fig.6). Apparently, this is due to the hardness of the B4C layer, exceeding the adhesion to the Al layer.
Figure 7 shows the energy-dispersive X-ray spectra of the B4C film of samples No. 1, No. 14 and No. 16. In comparison with sample No.1 make without ion assistance, films of the samples No.14 and No.16, made with ion assistance, have lower oxygen, but increased argon percentage and the ratio of boron and carbon is changed.
Figure 8 shows the measured refractive index and light absorption index for a boron carbide film with a thickness of 850 nm in the range of 300 nm – 1000 nm. The value of the refractive index for the visible light is more than 2.5. The absorption of visible light in the material is also high. As the wavelength decreases in the ultraviolet region, the refractive index decreases with a rapid increase in absorption.
DISCUSSION
The ion assistance significantly changes the structure of the B4C film: the columnar structure disappears, and the films have a dense structure consisting of most likely from non-oriented Nano crystals. The films obtained with ion-assisted deposition become less transparent and their color with a thickness of 500 nm becomes black, in contrast to brown color of thicker films (1030 nm) obtained without assistance. The surface of the B4C film, sputtered at a temperature of 200 °C, is covered with "crater" (Fig.3), which is probably due to impact on the heated surface by the flow of argon ions. This is confirmed by the fact that the surfaces of the film deposited at a temperature of 50 °C have greater smoothness and lack of "crater" (Fig.4, 5). Highly likely, for the same reason a columnar structure of the Al-layer is formed at 50 °C, since there is not enough energy to destroy the column growth of crystallites. The increase in argon content in the composition of the film is likely to be due to the use of ionic assistance when the B4C layer is deposited. Reducing the fraction of oxygen and changes in quantitative indicators of the fractions of boron and carbon is likely to be explained by an increase in the density of the B4C layer. The increase in density indicates also a high refractive factor. The rapid increase in the absorption coefficient with a decrease in the wavelength in the ultraviolet region is likely due to the B4C film structure consisting of the dense packed Nano scale crystals.
CONCLUSIONS
Dense B4C films with high adhesion to silicon and aluminum substrates were obtained by magnetron ion-assisted sputtering. The deposition of B4C films at a temperature of 50 °C makes it possible to apply film to aluminum foil, organic materials, plastics and polymers, including Mylar and Kapton films, which can be used to create neutron detectors. A new thin-film functional material based on B4C has been obtained.
ACKNOWLEDGEMENTS
The authors are grateful to Gorin Anatoly Vasilyevich for valuable assistance in the design of the magneton and an ion source and work on sputtering B4C films, as well as Apel Pavel Yuryevich and Orelovich Oleg Leonidovich from the Center for Applied Physics Laboratory of Flerov Laboratory of Nuclear Reaction JINR (CAP FLNR JINR) for help in the study of the surface of B4C films.
This work was performed using equipment of MIPT Shared Facilities Center, of CAP FLNR JINR and of Institute of Physics Dagestan Federal Research Center Russian Academy of Science (IP DFRC RAS).
The work was carried out with the financial support of the Russian Federation represented by the Ministry of Science and Higher Education, agreement No. 075-10-2021-115 of 13 October, 2021 (internal number 15.СИН.21.0021).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. 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.
Readers feedback
