- Email: [email protected]
Preparation of self-supporting boron films by sputtering with electron-beam-excited plasma
ARTICLE IN PRESS
Vacuum 74 (2004) 417–421
Preparation of self-supporting boron films by sputtering with electron-beam-excited plasma S. Ozawa*, M. Hamagaki Beam Physics and Engineering Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
Abstract In order to use boron as a target for projectile fragmentation in nuclear physics experiments, we developed a method of preparing self-supporting boron films by sputtering with an electron-beam-excited plasma. Boron films of 330 mg/cm2 (1.4 mm) were obtained by deposition for 1 h. The films were analyzed using an electron probe microanalyzer (EPMA). Quantities of impurities in the boron films were suppressed to below the EPMA detection limit. r 2004 Elsevier Ltd. All rights reserved. Keywords: Plasma; Sputtering; Boron film; Electron beam; Electron-beam-excited plasma
1. Introduction In nuclear physics experiments, self-supporting boron films are required as targets for projectile fragmentation, for example, in the 11B (d, 2He) 11 Be reaction [1]. Fig. 1 shows an image of the reaction. A purity of more than 90% is necessary for self-supporting films, and the measurement of the number of atoms (nuclei) in a unit area is more important than that of thickness in units of mm. Therefore, thicknesses of the films are expressed in units of g/cm2 instead of mm. In the present paper, we use g/cm2 as the unit with mm in parentheses, as calculated from density, for convenience’s sake. There are three commonly used methods of preparing boron films: vapor deposition [2], pressing, and sputtering [3,4]. Vapor deposition has an advantage in terms of the deposition rate, *Corresponding author. Tel.: +81-48-467-4397; fax: +8148-462-4719. E-mail address: [email protected] (S. Ozawa).
but it is difficult to prepare self-supporting films because the films prepared by this method tend to be low density and porous. This is because particles in vapor have a low energy. Nevertheless, self-supporting films must have sufficient strength to support their own weight. Pressing enables the easy preparation of high-purity films, but the method is only effective for a film thicker than 10 mg/cm2 (40 mm). Actually, self-supporting boron films prepared by the pressing method are used in nuclear physics experiment [1]. However, optimum thickness of boron target is different according to conditions of nuclear physics experiment. In some cases, target with thickness of thinner than 10 mg/cm2 is necessary. Sputtering enables the preparation of high-density films because the energy of sputtered particles is higher than in vapor deposition; however, the deposition rate of sputtering is lower than that of vapor deposition. Moreover, sputtering requires an external heater to heat the boron target in order to apply a bias voltage because boron has a high
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.046
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
418
electrical resistivity of 106 O cm at room temperature (20 C), which decreases considerably with an increase in temperature [5]. For example, an infrared lamp or a laser can be used as an external heater [3,4]. Electron-beam-excited plasma (EBEP) sputtering [6] has an advantage in sputtering boron; the high-density electron beam leads to an increase in plasma density and deposition rate. Electron density in the plasma is approximately 1013 cm 3. The deposition rate increases with an increase of the ion current which bombards the target. The boron target changes from an insulator to a
Fig. 1. Illustration of the
11
B (d, 2He)
11
Be reaction.
conductor upon electron and ion bombardment without an external heater such as a lamp or laser. In the present paper, we report a new method for preparing self-supporting films of pure boron by sputtering without an external heater. The purity of the films was measured using an electron probe microanalyzer (EPMA).
2. Experimental A schematic view of the experimental setup is shown in Fig. 2. The setup consists of three regions: the discharge region, acceleration region, and EBEP region. In each region, argon gas flows at an appropriate pressure. Electrons are emitted from the cathode made of LaB6, and discharge plasma produces from the electrons. Then the electron is accelerated by acceleration voltage (Va) in the acceleration region and penetrates into the EBEP region to produce high-density plasma in argon gas. The electron beam bombards the boron target and heats it. A bias voltage (Vt) is applied to the boron target after the target is sufficiently heated and has become a conductor. The target
Fig. 2. The experimental setup. The apparatus consists of three regions: discharge plasma region (1), electron-acceleration region (2) and EBEP region (3).
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
temperature is measured using an optical pyrometer. The entire vacuum chamber is made of Pyrex glass for insulation. The typical parameters of the EBEP system are shown in Table 1. The sputtering target is a boron tablet (f40 mm 5 mm) with a purity of 99%. The tablet is covered with carbon in order to supply bias voltage (see Fig. 3). The material applied with bias voltage is sputtered by Ar+ ions which are accelerated by the bias voltage when the material comes into contact with plasma. Hence all of materials, except the boron tablet, must be covered
Table 1 Typical parameters of the experimental apparatus Filament voltage (Vc) Filament current (Ic) Discharge voltage (Vd) Discharge current (Id) Acceleration voltage (Va) Electron beam current (Ia) Target bias voltage (Vt) Ion current (It) Gas pressure of region (1)-a Gas pressure of region (1)-b Gas pressure of region (2) Gas pressure of region (3) Base pressure of region (3)
4V 20 A 43 V 10 A 100 V 4A 280 V 0.8 A 10 Pa 1 Pa 1 10 2 Pa 0.1 Pa 1 10 4 Pa
Refer to Fig. 1 for the gas pressure of each region.
419
by an insulator to prepare high-purity boron films. As shown in Fig. 3, the carbon support and the feedthrough made of copper are covered with insulator to prevent them from being sputtered by Ar+ ions with bias voltage. The insulator cover for the target, which was made of hexagonal boron nitride, has an aperture of 28 mm because the diameter of the electron beam is approximately 30 mm. The feedthrough was covered with ceramic tubes. Three kinds of substrates, tantalum, Pyrex glass and silicon, were used in the experiment. The substrates are placed on a copper table the temperature of which is controlled from 20 C to 250 C. The temperature of the table is measured using a thermocouple (not shown in Fig. 2). The weight of the substrates was measured beforehand to determine the quantity of deposited boron. The shutter located above the table can be opened and closed by rotating the rod outside the vacuum chamber. The distance from the target to the substrate is 30 mm. The tilt angle of the target is approximately 20 . To stop deposition, the electron beam is stopped and the bias voltage was turned off. Then the films are left to cool for more than 1 h in vacuum. To be considered as self-supporting, the deposited boron film must be separate from the substrate. In general, water-soluble compounds are deposited on the substrate before films are deposited. When the substrate with water-soluble compounds and films is put into water, the film floats to the water surface because the watersoluble compounds dissolve. The prepared films are analyzed by EPMA and the purity is confirmed. The EPMA apparatus applies an electron beam to a sample and detects characteristic X-rays to identify the element contained in the sample. Five spectrum crystals were installed in the EPMA apparatus to detect impurity contents in boron film.
3. Results and discussion Fig. 3. Schematic of the target. Only the boron surface is sputtered by argon ions with energy equivalent to the applied voltage. Formerly, the carbon support was not covered by an insulator and was consequently sputtered by argon ions.
Boron is an insulator at room temperature, but it has an electrical resistivity of 0.1 O cm at 730 C [5]. When an electron beam with the energy of 100 eV and the current of 4 A bombards the target,
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Ar+ ions also bombard the target with the energy of a floating potential ( 70 V). The boron target was heated sufficiently to become a conductor by several minutes of electron beam bombardment. At this point, the target temperature was 1100 C, which was maintained until the end of deposition. Ar+ ions sputtering with the energy equivalent to the applied voltage enabled the boron target to keep a high temperature. We confirmed that the boron target changes from an insulator to a conductor without the use of an external heater. The present apparatus and method are effective for preparing boron films. In the present experiment, boron film of 330 mg/ cm2 (1.4 mm) was obtained on the silicon substrate after 1 h of deposition at Vt= 280 V and It=0.76 A. The deposition rate in the present experiment was deduced to be 0.4 nm/s, and the temperature of the substrate was 20 C. However, films exceeding 140 mg/cm2 (0.6 mm) started flaking on the Pyrex glass substrates, as were observed in the previous study [7], even though the sputtering rate and substrate temperature were the same as in the case of silicon substrates. Boron films on tantalum substrates also start flaking off at approximately this thickness in the present experiment, whereas boron films on silicon substrates can be much thicker. The surface of the glass substrate is smooth, but its thermal conductivity is low. The surface of the tantalum substrate is rough. The roughness and the thermal conductivity of the surface are thought to be more important than the kind of material. Silicon wafers have a smooth surface and high thermal conductivity. To prepare self-supporting boron films, LaCl3 and NiCl2 were tested for use in the present experiment. LaCl3 is commonly used as a watersoluble compound in the preparation of selfsupporting carbon films. However, it was found that LaCl3 is not useful for separating boron films from the substrate because La forms a compound with boron. NiCl2 was used for preparing selfsupporting boron films. Boron films on the tantalum substrate were prepared for EPMA. The boron film thickness was 70 mg/cm2 (0.3 mm). In the previous experiment [7], EPMA showed that the boron film contained carbon, nitrogen, and oxygen. In addition, we
must investigate copper which is included in the feedthrough, the components of the stainless steel (SUS304) used for the target-holding flange, and the components of the Pyrex glass vacuum chamber. We chose five kinds of spectrum crystals in order to detect all elements contained in the boron film. The typical EPMA spectra of the main elements, which were detected in the previous experiment [7], were shown in Fig. 4. The upper spectrum was obtained by diffraction though the LDE-1H crystal which detects carbon, nitrogen and oxygen. The LDE-2 crystal detects boron, carbon, nitrogen and oxygen, as does the NSTE crystal. An oxygen peak can be observed in the LDE-1H spectrum because the LDE-1H crystal has a high sensitivity which enables it to detect oxygen absorbed in the surface of the films. The oxygen peak cannot be observed in other spectra.
Fig. 4. EPMA spectra of boron film. Three spectra were obtained by diffraction of LDE-1H, LDE-2, and NSTE. In the spectra, positions of boron, carbon, oxygen, and nitrogen peaks are indicated by arrows.
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
Carbon and nitrogen are not observed in any of the spectra. By using other two crystals to detect other elements (copper, tantalum, and ingredients of stainless steel and Pyrex glass), only the tantalum peak was observed. This was because the energy of the electron beam (15 keV) from the EPMA apparatus was high enough to pass through the boron film and reach the tantalum substrate. These results show that the quantity of impurities in the boron films is less than the EPMA detection limit. A boron target without insulators was used in the previous experiment [7], and carbon content in boron film was more than 30%. After the carbon support of the boron tablet was covered with insulator, the amount of carbon in boron film was suppressed to below the EPMA detection limit. Naturally, the insulator cover is sputtered by Ar+ ions with energy equivalent to a floating potential. The floating potential of insulator cover is about 15 V, and the value is less than the floating potential of the boron target. It is because the insulator cover is not bombarded by the electron beam directly but bombarded by electrons in the Ar plasma. The floating potential ( 15 V) is generated by plasma of which electron temperature of 3 eV. Since the sputtering energy of 15 eV is about same as a threshold value of physical sputtering, the sputtering rate of the insulator cover is negligible compared with that of carbon with a bias voltage of Vt. In the previous experiment, we introduced nitrogen gas into vacuum chamber, and exposed films to atmosphere immediately after stopping deposition. As a result, it is thought that nitrogen and oxygen was existed in the boron films. In the present experiment, to suppress nitrogen and oxygen, films were left to cool for more than 1 h after turning off the bias voltage. The purity of almost 100% was achieved as a result of the following improvements to the preparing method: (1) the sputtering target was covered with boron nitride and (2) films were
421
left to cool for more than 1 h in vacuum after the deposition.
4. Conclusions We developed a new apparatus and method for preparing self-supporting boron films. A boron film of 330 mg/cm2 (1.4 mm) was obtained after 1 h of deposition without using an external heater. The system can be used to prepare films of other metals. The purity of the films satisfies the requirements for nuclear physics experiments. Carbon impurity of content was decreased by placing the target support made of carbon in an isolator. Nitrogen and oxygen were reduced by cooling the prepared films for more than 1 h after the deposition.
Acknowledgements The authors are grateful to H. Hasebe for preparing the substrates.
References [1] Ohnishi T, Sakai H, Okamura H, Niizeki T, Itoh K, Uesaka T, Satou Y, Sekiguchi K, Yakou K, Fukusaka S, Sakamoto N, Ohnuma H. Nucl Phys A 2001;687:38c–43c. [2] Kamimura K, Yoshimura T, Nagaoka T, Nakao M, Onuma Y, Makimura M. J Solid State Chem 2000;154:153–6. [3] Itoh Y, Kuriki S, Saidoh M, Nishikawa M. Jpn J Appl Phys 1994;33:5959–66. [4] Ohtsuka Y, Tsuji M, Kitamura Y, Ueda Y, Isobe M, Nishikawa M. J Nucl Mater 1998;258–263:735–9. [5] Goldsmith A, Waterman TE, Hirschhorn HJ. Handbook of thermophysical properties of solid materials, vol. 1. New York: Macmillan; 1961. p. 93. [6] Hamagaki M, Hara T. Jpn J Appl Phys 1994;33:4369–72. [7] Ozawa S, Hamagaki M. Proceedings of the 20th Symposium of Plasma Processing (SPP-20), 2003. p. 283.
Vacuum 74 (2004) 417–421
Preparation of self-supporting boron films by sputtering with electron-beam-excited plasma S. Ozawa*, M. Hamagaki Beam Physics and Engineering Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
Abstract In order to use boron as a target for projectile fragmentation in nuclear physics experiments, we developed a method of preparing self-supporting boron films by sputtering with an electron-beam-excited plasma. Boron films of 330 mg/cm2 (1.4 mm) were obtained by deposition for 1 h. The films were analyzed using an electron probe microanalyzer (EPMA). Quantities of impurities in the boron films were suppressed to below the EPMA detection limit. r 2004 Elsevier Ltd. All rights reserved. Keywords: Plasma; Sputtering; Boron film; Electron beam; Electron-beam-excited plasma
1. Introduction In nuclear physics experiments, self-supporting boron films are required as targets for projectile fragmentation, for example, in the 11B (d, 2He) 11 Be reaction [1]. Fig. 1 shows an image of the reaction. A purity of more than 90% is necessary for self-supporting films, and the measurement of the number of atoms (nuclei) in a unit area is more important than that of thickness in units of mm. Therefore, thicknesses of the films are expressed in units of g/cm2 instead of mm. In the present paper, we use g/cm2 as the unit with mm in parentheses, as calculated from density, for convenience’s sake. There are three commonly used methods of preparing boron films: vapor deposition [2], pressing, and sputtering [3,4]. Vapor deposition has an advantage in terms of the deposition rate, *Corresponding author. Tel.: +81-48-467-4397; fax: +8148-462-4719. E-mail address: [email protected] (S. Ozawa).
but it is difficult to prepare self-supporting films because the films prepared by this method tend to be low density and porous. This is because particles in vapor have a low energy. Nevertheless, self-supporting films must have sufficient strength to support their own weight. Pressing enables the easy preparation of high-purity films, but the method is only effective for a film thicker than 10 mg/cm2 (40 mm). Actually, self-supporting boron films prepared by the pressing method are used in nuclear physics experiment [1]. However, optimum thickness of boron target is different according to conditions of nuclear physics experiment. In some cases, target with thickness of thinner than 10 mg/cm2 is necessary. Sputtering enables the preparation of high-density films because the energy of sputtered particles is higher than in vapor deposition; however, the deposition rate of sputtering is lower than that of vapor deposition. Moreover, sputtering requires an external heater to heat the boron target in order to apply a bias voltage because boron has a high
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.046
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
418
electrical resistivity of 106 O cm at room temperature (20 C), which decreases considerably with an increase in temperature [5]. For example, an infrared lamp or a laser can be used as an external heater [3,4]. Electron-beam-excited plasma (EBEP) sputtering [6] has an advantage in sputtering boron; the high-density electron beam leads to an increase in plasma density and deposition rate. Electron density in the plasma is approximately 1013 cm 3. The deposition rate increases with an increase of the ion current which bombards the target. The boron target changes from an insulator to a
Fig. 1. Illustration of the
11
B (d, 2He)
11
Be reaction.
conductor upon electron and ion bombardment without an external heater such as a lamp or laser. In the present paper, we report a new method for preparing self-supporting films of pure boron by sputtering without an external heater. The purity of the films was measured using an electron probe microanalyzer (EPMA).
2. Experimental A schematic view of the experimental setup is shown in Fig. 2. The setup consists of three regions: the discharge region, acceleration region, and EBEP region. In each region, argon gas flows at an appropriate pressure. Electrons are emitted from the cathode made of LaB6, and discharge plasma produces from the electrons. Then the electron is accelerated by acceleration voltage (Va) in the acceleration region and penetrates into the EBEP region to produce high-density plasma in argon gas. The electron beam bombards the boron target and heats it. A bias voltage (Vt) is applied to the boron target after the target is sufficiently heated and has become a conductor. The target
Fig. 2. The experimental setup. The apparatus consists of three regions: discharge plasma region (1), electron-acceleration region (2) and EBEP region (3).
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
temperature is measured using an optical pyrometer. The entire vacuum chamber is made of Pyrex glass for insulation. The typical parameters of the EBEP system are shown in Table 1. The sputtering target is a boron tablet (f40 mm 5 mm) with a purity of 99%. The tablet is covered with carbon in order to supply bias voltage (see Fig. 3). The material applied with bias voltage is sputtered by Ar+ ions which are accelerated by the bias voltage when the material comes into contact with plasma. Hence all of materials, except the boron tablet, must be covered
Table 1 Typical parameters of the experimental apparatus Filament voltage (Vc) Filament current (Ic) Discharge voltage (Vd) Discharge current (Id) Acceleration voltage (Va) Electron beam current (Ia) Target bias voltage (Vt) Ion current (It) Gas pressure of region (1)-a Gas pressure of region (1)-b Gas pressure of region (2) Gas pressure of region (3) Base pressure of region (3)
4V 20 A 43 V 10 A 100 V 4A 280 V 0.8 A 10 Pa 1 Pa 1 10 2 Pa 0.1 Pa 1 10 4 Pa
Refer to Fig. 1 for the gas pressure of each region.
419
by an insulator to prepare high-purity boron films. As shown in Fig. 3, the carbon support and the feedthrough made of copper are covered with insulator to prevent them from being sputtered by Ar+ ions with bias voltage. The insulator cover for the target, which was made of hexagonal boron nitride, has an aperture of 28 mm because the diameter of the electron beam is approximately 30 mm. The feedthrough was covered with ceramic tubes. Three kinds of substrates, tantalum, Pyrex glass and silicon, were used in the experiment. The substrates are placed on a copper table the temperature of which is controlled from 20 C to 250 C. The temperature of the table is measured using a thermocouple (not shown in Fig. 2). The weight of the substrates was measured beforehand to determine the quantity of deposited boron. The shutter located above the table can be opened and closed by rotating the rod outside the vacuum chamber. The distance from the target to the substrate is 30 mm. The tilt angle of the target is approximately 20 . To stop deposition, the electron beam is stopped and the bias voltage was turned off. Then the films are left to cool for more than 1 h in vacuum. To be considered as self-supporting, the deposited boron film must be separate from the substrate. In general, water-soluble compounds are deposited on the substrate before films are deposited. When the substrate with water-soluble compounds and films is put into water, the film floats to the water surface because the watersoluble compounds dissolve. The prepared films are analyzed by EPMA and the purity is confirmed. The EPMA apparatus applies an electron beam to a sample and detects characteristic X-rays to identify the element contained in the sample. Five spectrum crystals were installed in the EPMA apparatus to detect impurity contents in boron film.
3. Results and discussion Fig. 3. Schematic of the target. Only the boron surface is sputtered by argon ions with energy equivalent to the applied voltage. Formerly, the carbon support was not covered by an insulator and was consequently sputtered by argon ions.
Boron is an insulator at room temperature, but it has an electrical resistivity of 0.1 O cm at 730 C [5]. When an electron beam with the energy of 100 eV and the current of 4 A bombards the target,
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Ar+ ions also bombard the target with the energy of a floating potential ( 70 V). The boron target was heated sufficiently to become a conductor by several minutes of electron beam bombardment. At this point, the target temperature was 1100 C, which was maintained until the end of deposition. Ar+ ions sputtering with the energy equivalent to the applied voltage enabled the boron target to keep a high temperature. We confirmed that the boron target changes from an insulator to a conductor without the use of an external heater. The present apparatus and method are effective for preparing boron films. In the present experiment, boron film of 330 mg/ cm2 (1.4 mm) was obtained on the silicon substrate after 1 h of deposition at Vt= 280 V and It=0.76 A. The deposition rate in the present experiment was deduced to be 0.4 nm/s, and the temperature of the substrate was 20 C. However, films exceeding 140 mg/cm2 (0.6 mm) started flaking on the Pyrex glass substrates, as were observed in the previous study [7], even though the sputtering rate and substrate temperature were the same as in the case of silicon substrates. Boron films on tantalum substrates also start flaking off at approximately this thickness in the present experiment, whereas boron films on silicon substrates can be much thicker. The surface of the glass substrate is smooth, but its thermal conductivity is low. The surface of the tantalum substrate is rough. The roughness and the thermal conductivity of the surface are thought to be more important than the kind of material. Silicon wafers have a smooth surface and high thermal conductivity. To prepare self-supporting boron films, LaCl3 and NiCl2 were tested for use in the present experiment. LaCl3 is commonly used as a watersoluble compound in the preparation of selfsupporting carbon films. However, it was found that LaCl3 is not useful for separating boron films from the substrate because La forms a compound with boron. NiCl2 was used for preparing selfsupporting boron films. Boron films on the tantalum substrate were prepared for EPMA. The boron film thickness was 70 mg/cm2 (0.3 mm). In the previous experiment [7], EPMA showed that the boron film contained carbon, nitrogen, and oxygen. In addition, we
must investigate copper which is included in the feedthrough, the components of the stainless steel (SUS304) used for the target-holding flange, and the components of the Pyrex glass vacuum chamber. We chose five kinds of spectrum crystals in order to detect all elements contained in the boron film. The typical EPMA spectra of the main elements, which were detected in the previous experiment [7], were shown in Fig. 4. The upper spectrum was obtained by diffraction though the LDE-1H crystal which detects carbon, nitrogen and oxygen. The LDE-2 crystal detects boron, carbon, nitrogen and oxygen, as does the NSTE crystal. An oxygen peak can be observed in the LDE-1H spectrum because the LDE-1H crystal has a high sensitivity which enables it to detect oxygen absorbed in the surface of the films. The oxygen peak cannot be observed in other spectra.
Fig. 4. EPMA spectra of boron film. Three spectra were obtained by diffraction of LDE-1H, LDE-2, and NSTE. In the spectra, positions of boron, carbon, oxygen, and nitrogen peaks are indicated by arrows.
ARTICLE IN PRESS S. Ozawa, M. Hamagaki / Vacuum 74 (2004) 417–421
Carbon and nitrogen are not observed in any of the spectra. By using other two crystals to detect other elements (copper, tantalum, and ingredients of stainless steel and Pyrex glass), only the tantalum peak was observed. This was because the energy of the electron beam (15 keV) from the EPMA apparatus was high enough to pass through the boron film and reach the tantalum substrate. These results show that the quantity of impurities in the boron films is less than the EPMA detection limit. A boron target without insulators was used in the previous experiment [7], and carbon content in boron film was more than 30%. After the carbon support of the boron tablet was covered with insulator, the amount of carbon in boron film was suppressed to below the EPMA detection limit. Naturally, the insulator cover is sputtered by Ar+ ions with energy equivalent to a floating potential. The floating potential of insulator cover is about 15 V, and the value is less than the floating potential of the boron target. It is because the insulator cover is not bombarded by the electron beam directly but bombarded by electrons in the Ar plasma. The floating potential ( 15 V) is generated by plasma of which electron temperature of 3 eV. Since the sputtering energy of 15 eV is about same as a threshold value of physical sputtering, the sputtering rate of the insulator cover is negligible compared with that of carbon with a bias voltage of Vt. In the previous experiment, we introduced nitrogen gas into vacuum chamber, and exposed films to atmosphere immediately after stopping deposition. As a result, it is thought that nitrogen and oxygen was existed in the boron films. In the present experiment, to suppress nitrogen and oxygen, films were left to cool for more than 1 h after turning off the bias voltage. The purity of almost 100% was achieved as a result of the following improvements to the preparing method: (1) the sputtering target was covered with boron nitride and (2) films were
421
left to cool for more than 1 h in vacuum after the deposition.
4. Conclusions We developed a new apparatus and method for preparing self-supporting boron films. A boron film of 330 mg/cm2 (1.4 mm) was obtained after 1 h of deposition without using an external heater. The system can be used to prepare films of other metals. The purity of the films satisfies the requirements for nuclear physics experiments. Carbon impurity of content was decreased by placing the target support made of carbon in an isolator. Nitrogen and oxygen were reduced by cooling the prepared films for more than 1 h after the deposition.
Acknowledgements The authors are grateful to H. Hasebe for preparing the substrates.
References [1] Ohnishi T, Sakai H, Okamura H, Niizeki T, Itoh K, Uesaka T, Satou Y, Sekiguchi K, Yakou K, Fukusaka S, Sakamoto N, Ohnuma H. Nucl Phys A 2001;687:38c–43c. [2] Kamimura K, Yoshimura T, Nagaoka T, Nakao M, Onuma Y, Makimura M. J Solid State Chem 2000;154:153–6. [3] Itoh Y, Kuriki S, Saidoh M, Nishikawa M. Jpn J Appl Phys 1994;33:5959–66. [4] Ohtsuka Y, Tsuji M, Kitamura Y, Ueda Y, Isobe M, Nishikawa M. J Nucl Mater 1998;258–263:735–9. [5] Goldsmith A, Waterman TE, Hirschhorn HJ. Handbook of thermophysical properties of solid materials, vol. 1. New York: Macmillan; 1961. p. 93. [6] Hamagaki M, Hara T. Jpn J Appl Phys 1994;33:4369–72. [7] Ozawa S, Hamagaki M. Proceedings of the 20th Symposium of Plasma Processing (SPP-20), 2003. p. 283.