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Characteristics of a low-energy ion beam used for deposition
Surface & Coatings Technology 188–189 (2004) 404 – 408 www.elsevier.com/locate/surfcoat
Characteristics of a low-energy ion beam used for deposition T. Matsumotoa,b,*, K. Mimotoc, M. Kiuchia, K. Matsuyamac, T. Sadahiroc, M. Okuboc, S. Sugimotoc, S. Gotoc a
National Institute of Advanced Industrial Science and Technology (AIST), Special Division for Green Life Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan c Science and Technology Center for Atoms, Molecules and Ions Control, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Available online 8 October 2004
Abstract In ion beam deposition, particularly in the energy range below 100 eV, it is very important to investigate the interactions between incident ions and a surface. The high-energy neutrals included in the low-energy ion beam may influence the film formation; thus, it is necessary to detect the accurate amount of high-energy neutrals and discuss those effects. In this study, we measured the ratio of high-energy neutrals to low-energy ions using Faraday cups to eliminate ions from the ion beam, and estimated the characteristics of the ion beam. As the result, the percentage of high-energy neutrals contained in low-energy ion beam was 1.5%, and the cause of the generation of high-energy neutrals was mainly the collision between high-energy ions and inner wall of the slit. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion beam deposition; High-energy neutral; Faraday cup
1. Introduction Ion beam techniques are very useful for the surface modification of materials. In the case of a low-energy (below 100 eV) ion beam, the ions are deposited on the substrate surface resulting in film formation. The interaction between the ions and the surface is a non-thermal equilibrium reaction; thus, interesting materials, which are generally hard to grow by the thermal equilibrium processes, are possible to be crystallized using this technique. For instance, iron films formed by a low-energy ion beam deposition are more corrosion resistant than stainless steels, well-oriented diamond films could be grown on Si substrates, and hetero-epitaxial 3C-SiC thin films were crystallized on Si substrates at low temperatures [1–6]. In * Corresponding author. National Institute of Advanced Industrial Science and Technology (AIST), Special Division for Green Life Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. Tel.: +81 72 751 9535; fax: +81 72 751 9637. E-mail address: [email protected] (T. Matsumoto). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.035
order to discuss the mechanism of this technique in detail, it is very important to investigate the characteristics of a lowenergy ion beam. Especially, the high-energy neutrals included in the low-energy ion beam might affect or damage the film formation. In this investigation, we measured the high-energy neutrals included in the low-energy ion beam by high-energy neutral detectors consisting of modified Faraday cups and discussed the generation of the highenergy neutrals.
2. Experimental The low-energy ion beam deposition system contains a Freeman-type ion source, a beam transportation section with a mass selector and a deposition chamber [7]. As the lowenergy ion beam was strongly affected by space-charge effect, the ions generated from the ion source were extracted at high energy of 20–30 keV. The high-energy ion beam was mass selected by the sector magnet and decelerated below 100 eV in front of the substrate. However, as the result of
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
the charge exchange reaction between high-energy ions and residual gases in a drift tube, high-energy neutrals were generated. As the high-energy neutrals were not able to be controlled by magnetic and electric field, they would damage the film formation. In order to remove the highenergy neutrals from the ion beam, the ion beam was deflected by an angle of 78 by electrostatic field and transported to the deposition chamber. However, some highenergy neutrals would be regenerated after deflection and irradiated to the substrate with the low-energy ion beam. We performed measurements of the high-energy neutrals included in the low-energy ion beam using high-energy neutrals detectors consisting of modified Faraday cups designed by our group. As the neutrals do not have charges, they cannot be detected by the ampere meter. Therefore, we fabricated two high-energy neutral detectors consisting of modified Faraday cups and measured the current by secondary electron emission to estimate the flux of the high-energy neutrals. The high-energy neutral detectors have an electrostatic reflector or a magnetic deflector to reject the low-energy ions. At first, we fabricated the high-energy neutral detector with an ion reflector. We show the schematic diagram of the high-energy neutral detector (type A) in Fig. 1(a). The first and second electrodes were made of molybdenum. For elimination of the effect by secondary electron emission, we selected molybdenum as the electrode material because of its low coefficient of secondary electron emission. The first electrode is grounded to shield the internal electric field to reflect low-energy ions from the external electric field. The low-energy ions and high-energy neutrals pass through the aperture of the first electrode. The low-energy ions are reflected by the second electrode at over several hundred volts. However, the high-energy neutrals are not affected by the electrostatic field and pass through the second electrode. Finally, only high-energy neutrals are irradiated to the target materials (copper) and make the secondary electron emission. This reaction makes current in the circuit, and the existence of the high-energy neutrals could be confirmed. The coefficient of secondary electron emission between Cu and Ar+ with 20 keV is 1.6 [8]. Thus, the number of the high-energy neutrals is calculated using the coefficient of secondary electron emission. Secondarily, we fabricated the other high-energy neutral detector with an ion deflector by a magnetic field. The highenergy neutrals measurement by the different type of detectors makes the experiment more reliable. We showed the schematic diagram of the high-energy neutral detector (type B) in Fig. 1(b). The low-energy ions and high-energy neutrals pass through the aperture of the first electrode. The low-energy ions are deflected by a magnetic field (magnetic flux density=0.3 T at the center) and suppressed by the second electrode. However, the high-energy neutrals are not affected by the magnetic field and pass through the second and third electrodes. Finally, the high-energy neutrals are irradiated to the target plate (Cu) and make the secondary
405
Fig. 1. The schematic diagram of the high-energy neutral detectors, (a) with the reflector by electrostatic field, (b) with the deflector by magnetic field.
electron emission. The generated secondary electrons are captured by the collector of the third electrode at a positive potential (22.5 V). The first, second and third electrodes were made of molybdenum. The secondary electron emission causes current in the circuit, and it makes possible high-energy neutrals detection.
3. Results Using the high-energy neutral detector (type A), the highenergy neutrals included in the low-energy ion beam were measured. The low-energy Ar+ ion beam (ion energy: 100– 300 eV) was irradiated to the high-energy neutral detector,
406
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
and the current at each electrode was measured with the variation of the second electrode voltage. The correlation between the bias voltage of the second electrode and the current at the target was shown in Fig. 2(a). The incident ion energy was 200 eV and the pressure in the deposition chamber was 4.210 6 Pa. According to the increase of the bias voltage, the current at the target decreased and remained 20–30 nA. As the low-energy ions with 200 eV were reflected by the second electrode at over 200 V, the current at the bias voltage over 200 V originated from secondary electron emission by the irradiation of the high-energy neutrals. In order to make sure that the current at the target originated from the high-energy neutrals, we investigated the correlation between the pressure in the deposition chamber
Fig. 3. The high-energy neutrals measurements by the high-energy neutral detector with the reflector by magnetic field (type B). (a) The dependence of the current at the target on the pressure of the deposition chamber. (b) The ratio of high-energy neutrals to low-energy ions and its dependence on the pressure of the deposition chamber.
Fig. 2. The high-energy neutrals measurements by the high-energy neutral detector with the reflector by electrostatic field (type A). (a) The correlation between the second electrode voltage and the current at the target. (b) The dependence of the current at the target on the pressure of the deposition chamber. (c) The ratio of high-energy neutrals to low-energy ions and its dependence on the pressure of the deposition chamber.
and the current at the target. In Fig. 2(b), as the pressure was higher, the low-energy ion current was lower and the highenergy neutrals current was higher. In the case of high pressure, the low-energy ions were decreased by the collision with residual gases, while the high-energy neutrals were increased by the charge exchange reaction. The flux ratio of the high-energy neutrals to the low-energy ions was shown in Fig. 2(c). As the pressure in the deposition chamber was higher, the high-energy neutrals were increased. In the case of the deposition chamber pressure: 4.210 6 Pa, the number of the high-energy neutrals was 2.51010 particle/ (mm2 s), and the percentage of the high-energy neutrals included in the low-energy ions was 1.3%. Secondarily, using the high-energy neutrals detector (type B), we measured the number of high-energy neutrals included in the low-energy ions. We showed the correlation between the pressure in the deposition chamber and the current at the target in Fig. 3(a). We controlled the pressure by introduction of nitrogen gases into the deposition chamber. In the case of a high pressure in the deposition chamber, the current of low-energy ions was decreased,
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
407
Fig. 4. The comparison of the calculation with the experimental results.
while the target current was increased. In Fig. 3(b), we show the flux ratio of the high-energy neutrals to the low-energy ions. On the condition of a high pressure over 110 3 Pa, the ratio of the high-energy neutrals to the low-energy ions became higher. The flux of high-energy neutrals included in the low-energy ion beam was 2.81010 particle/(mm2 s) at the pressure of 2.610 6 Pa, and the percentage of the highenergy neutrals was 1.5%.
4. Discussion The results of the number of the high-energy neutrals generated from charge exchange were compared to the
calculated curve (Fig. 4). According to increase of the pressure, the calculated curve was consistent with the experimental data. In the condition of high pressure, the collision between ions and residual gases occurred and the high-energy neutrals were generated from the charge exchange. However, in the low-pressure range, the results of the experiments were far from the calculated curve. This meant that the main cause of the high-energy neutrals generation was different from the collision between ions and residual gases. The schematic diagram of the part from the deflector to the decelerator was shown in Fig. 5. The graphite slit (/=4 mm) performed as the protector which prevented high-energy neutrals induction into the deposition chamber. Moreover, it played the limiter of the operating gases
Fig. 5. The schematic diagram of the part of the apparatus between the deflector and decelerator.
408
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
conduction to the deposition chamber. We consider that the high-energy neutrals generation would occur due to the collision between the ion beam and the inner wall of the slit.
and inner wall of the slit rather than the charge exchange reaction between the high-energy ions and residual gases in the condition of film formation.
5. Conclusions
References
We measured the high-energy neutrals, which were included in the low-energy ion beam and irradiated the substrate during film formation. In order to detect neutral particles in the ion beam, we fabricated high-energy neutral detectors consisting of modified Faraday cups with an ion reflector by electric field and the other with an ion deflector by magnetic field. Using two detectors for measurement, we confirmed that the current measured by the high-energy neutral detectors was assigned to the high-energy neutrals radiation. As the result, the percentage of high-energy neutrals contained in low-energy ion beam was 1.5%. We determined that the cause of the generation of high-energy neutrals was mainly the collision between high-energy ions
[1] K. Miyake, K. Ohashi, H. Takahashi, T. Minemura, Surf. Coat. Technol. 65 (1994) 208. [2] J.L. Robertson, S.C. Moss, Y. Lifshitz, S.R. Kasi, J.W. Rabalais, G.D. Lempert, E. Rapoport, Science 243 (1989) 1047. [3] H. Ohno, J.A. van den Berg, S. Nagai, D.G. Armour, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 148 (1999) 673. [4] S.P. Withrow, K.L. More, R.A. Zuhr, T.E. Haynes, Vacuum 39 (1989) 1065. [5] T. Matsumoto, M. Kiuchi, S. Sugimoto, S. Goto, Surf. Sci. 493 (2001) 426. [6] N. Tsubouchi, A. Chayahara, A. Kinomura, Y. Horino, Appl. Phys. Lett. 77 (2000) 654. [7] T. Matsumoto, K. Mimoto, S. Goto, M. Ohba, Y. Agawa, M. Kiuchi, Rev. Sci. Instrum. 71 (2000) 1168. [8] P.C. Zalm, L.J. Beckers, Surf. Sci. 152/153 (1985) 135.
Characteristics of a low-energy ion beam used for deposition T. Matsumotoa,b,*, K. Mimotoc, M. Kiuchia, K. Matsuyamac, T. Sadahiroc, M. Okuboc, S. Sugimotoc, S. Gotoc a
National Institute of Advanced Industrial Science and Technology (AIST), Special Division for Green Life Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan c Science and Technology Center for Atoms, Molecules and Ions Control, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Available online 8 October 2004
Abstract In ion beam deposition, particularly in the energy range below 100 eV, it is very important to investigate the interactions between incident ions and a surface. The high-energy neutrals included in the low-energy ion beam may influence the film formation; thus, it is necessary to detect the accurate amount of high-energy neutrals and discuss those effects. In this study, we measured the ratio of high-energy neutrals to low-energy ions using Faraday cups to eliminate ions from the ion beam, and estimated the characteristics of the ion beam. As the result, the percentage of high-energy neutrals contained in low-energy ion beam was 1.5%, and the cause of the generation of high-energy neutrals was mainly the collision between high-energy ions and inner wall of the slit. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion beam deposition; High-energy neutral; Faraday cup
1. Introduction Ion beam techniques are very useful for the surface modification of materials. In the case of a low-energy (below 100 eV) ion beam, the ions are deposited on the substrate surface resulting in film formation. The interaction between the ions and the surface is a non-thermal equilibrium reaction; thus, interesting materials, which are generally hard to grow by the thermal equilibrium processes, are possible to be crystallized using this technique. For instance, iron films formed by a low-energy ion beam deposition are more corrosion resistant than stainless steels, well-oriented diamond films could be grown on Si substrates, and hetero-epitaxial 3C-SiC thin films were crystallized on Si substrates at low temperatures [1–6]. In * Corresponding author. National Institute of Advanced Industrial Science and Technology (AIST), Special Division for Green Life Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. Tel.: +81 72 751 9535; fax: +81 72 751 9637. E-mail address: [email protected] (T. Matsumoto). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.035
order to discuss the mechanism of this technique in detail, it is very important to investigate the characteristics of a lowenergy ion beam. Especially, the high-energy neutrals included in the low-energy ion beam might affect or damage the film formation. In this investigation, we measured the high-energy neutrals included in the low-energy ion beam by high-energy neutral detectors consisting of modified Faraday cups and discussed the generation of the highenergy neutrals.
2. Experimental The low-energy ion beam deposition system contains a Freeman-type ion source, a beam transportation section with a mass selector and a deposition chamber [7]. As the lowenergy ion beam was strongly affected by space-charge effect, the ions generated from the ion source were extracted at high energy of 20–30 keV. The high-energy ion beam was mass selected by the sector magnet and decelerated below 100 eV in front of the substrate. However, as the result of
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
the charge exchange reaction between high-energy ions and residual gases in a drift tube, high-energy neutrals were generated. As the high-energy neutrals were not able to be controlled by magnetic and electric field, they would damage the film formation. In order to remove the highenergy neutrals from the ion beam, the ion beam was deflected by an angle of 78 by electrostatic field and transported to the deposition chamber. However, some highenergy neutrals would be regenerated after deflection and irradiated to the substrate with the low-energy ion beam. We performed measurements of the high-energy neutrals included in the low-energy ion beam using high-energy neutrals detectors consisting of modified Faraday cups designed by our group. As the neutrals do not have charges, they cannot be detected by the ampere meter. Therefore, we fabricated two high-energy neutral detectors consisting of modified Faraday cups and measured the current by secondary electron emission to estimate the flux of the high-energy neutrals. The high-energy neutral detectors have an electrostatic reflector or a magnetic deflector to reject the low-energy ions. At first, we fabricated the high-energy neutral detector with an ion reflector. We show the schematic diagram of the high-energy neutral detector (type A) in Fig. 1(a). The first and second electrodes were made of molybdenum. For elimination of the effect by secondary electron emission, we selected molybdenum as the electrode material because of its low coefficient of secondary electron emission. The first electrode is grounded to shield the internal electric field to reflect low-energy ions from the external electric field. The low-energy ions and high-energy neutrals pass through the aperture of the first electrode. The low-energy ions are reflected by the second electrode at over several hundred volts. However, the high-energy neutrals are not affected by the electrostatic field and pass through the second electrode. Finally, only high-energy neutrals are irradiated to the target materials (copper) and make the secondary electron emission. This reaction makes current in the circuit, and the existence of the high-energy neutrals could be confirmed. The coefficient of secondary electron emission between Cu and Ar+ with 20 keV is 1.6 [8]. Thus, the number of the high-energy neutrals is calculated using the coefficient of secondary electron emission. Secondarily, we fabricated the other high-energy neutral detector with an ion deflector by a magnetic field. The highenergy neutrals measurement by the different type of detectors makes the experiment more reliable. We showed the schematic diagram of the high-energy neutral detector (type B) in Fig. 1(b). The low-energy ions and high-energy neutrals pass through the aperture of the first electrode. The low-energy ions are deflected by a magnetic field (magnetic flux density=0.3 T at the center) and suppressed by the second electrode. However, the high-energy neutrals are not affected by the magnetic field and pass through the second and third electrodes. Finally, the high-energy neutrals are irradiated to the target plate (Cu) and make the secondary
405
Fig. 1. The schematic diagram of the high-energy neutral detectors, (a) with the reflector by electrostatic field, (b) with the deflector by magnetic field.
electron emission. The generated secondary electrons are captured by the collector of the third electrode at a positive potential (22.5 V). The first, second and third electrodes were made of molybdenum. The secondary electron emission causes current in the circuit, and it makes possible high-energy neutrals detection.
3. Results Using the high-energy neutral detector (type A), the highenergy neutrals included in the low-energy ion beam were measured. The low-energy Ar+ ion beam (ion energy: 100– 300 eV) was irradiated to the high-energy neutral detector,
406
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
and the current at each electrode was measured with the variation of the second electrode voltage. The correlation between the bias voltage of the second electrode and the current at the target was shown in Fig. 2(a). The incident ion energy was 200 eV and the pressure in the deposition chamber was 4.210 6 Pa. According to the increase of the bias voltage, the current at the target decreased and remained 20–30 nA. As the low-energy ions with 200 eV were reflected by the second electrode at over 200 V, the current at the bias voltage over 200 V originated from secondary electron emission by the irradiation of the high-energy neutrals. In order to make sure that the current at the target originated from the high-energy neutrals, we investigated the correlation between the pressure in the deposition chamber
Fig. 3. The high-energy neutrals measurements by the high-energy neutral detector with the reflector by magnetic field (type B). (a) The dependence of the current at the target on the pressure of the deposition chamber. (b) The ratio of high-energy neutrals to low-energy ions and its dependence on the pressure of the deposition chamber.
Fig. 2. The high-energy neutrals measurements by the high-energy neutral detector with the reflector by electrostatic field (type A). (a) The correlation between the second electrode voltage and the current at the target. (b) The dependence of the current at the target on the pressure of the deposition chamber. (c) The ratio of high-energy neutrals to low-energy ions and its dependence on the pressure of the deposition chamber.
and the current at the target. In Fig. 2(b), as the pressure was higher, the low-energy ion current was lower and the highenergy neutrals current was higher. In the case of high pressure, the low-energy ions were decreased by the collision with residual gases, while the high-energy neutrals were increased by the charge exchange reaction. The flux ratio of the high-energy neutrals to the low-energy ions was shown in Fig. 2(c). As the pressure in the deposition chamber was higher, the high-energy neutrals were increased. In the case of the deposition chamber pressure: 4.210 6 Pa, the number of the high-energy neutrals was 2.51010 particle/ (mm2 s), and the percentage of the high-energy neutrals included in the low-energy ions was 1.3%. Secondarily, using the high-energy neutrals detector (type B), we measured the number of high-energy neutrals included in the low-energy ions. We showed the correlation between the pressure in the deposition chamber and the current at the target in Fig. 3(a). We controlled the pressure by introduction of nitrogen gases into the deposition chamber. In the case of a high pressure in the deposition chamber, the current of low-energy ions was decreased,
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
407
Fig. 4. The comparison of the calculation with the experimental results.
while the target current was increased. In Fig. 3(b), we show the flux ratio of the high-energy neutrals to the low-energy ions. On the condition of a high pressure over 110 3 Pa, the ratio of the high-energy neutrals to the low-energy ions became higher. The flux of high-energy neutrals included in the low-energy ion beam was 2.81010 particle/(mm2 s) at the pressure of 2.610 6 Pa, and the percentage of the highenergy neutrals was 1.5%.
4. Discussion The results of the number of the high-energy neutrals generated from charge exchange were compared to the
calculated curve (Fig. 4). According to increase of the pressure, the calculated curve was consistent with the experimental data. In the condition of high pressure, the collision between ions and residual gases occurred and the high-energy neutrals were generated from the charge exchange. However, in the low-pressure range, the results of the experiments were far from the calculated curve. This meant that the main cause of the high-energy neutrals generation was different from the collision between ions and residual gases. The schematic diagram of the part from the deflector to the decelerator was shown in Fig. 5. The graphite slit (/=4 mm) performed as the protector which prevented high-energy neutrals induction into the deposition chamber. Moreover, it played the limiter of the operating gases
Fig. 5. The schematic diagram of the part of the apparatus between the deflector and decelerator.
408
T. Matsumoto et al. / Surface & Coatings Technology 188–189 (2004) 404–408
conduction to the deposition chamber. We consider that the high-energy neutrals generation would occur due to the collision between the ion beam and the inner wall of the slit.
and inner wall of the slit rather than the charge exchange reaction between the high-energy ions and residual gases in the condition of film formation.
5. Conclusions
References
We measured the high-energy neutrals, which were included in the low-energy ion beam and irradiated the substrate during film formation. In order to detect neutral particles in the ion beam, we fabricated high-energy neutral detectors consisting of modified Faraday cups with an ion reflector by electric field and the other with an ion deflector by magnetic field. Using two detectors for measurement, we confirmed that the current measured by the high-energy neutral detectors was assigned to the high-energy neutrals radiation. As the result, the percentage of high-energy neutrals contained in low-energy ion beam was 1.5%. We determined that the cause of the generation of high-energy neutrals was mainly the collision between high-energy ions
[1] K. Miyake, K. Ohashi, H. Takahashi, T. Minemura, Surf. Coat. Technol. 65 (1994) 208. [2] J.L. Robertson, S.C. Moss, Y. Lifshitz, S.R. Kasi, J.W. Rabalais, G.D. Lempert, E. Rapoport, Science 243 (1989) 1047. [3] H. Ohno, J.A. van den Berg, S. Nagai, D.G. Armour, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 148 (1999) 673. [4] S.P. Withrow, K.L. More, R.A. Zuhr, T.E. Haynes, Vacuum 39 (1989) 1065. [5] T. Matsumoto, M. Kiuchi, S. Sugimoto, S. Goto, Surf. Sci. 493 (2001) 426. [6] N. Tsubouchi, A. Chayahara, A. Kinomura, Y. Horino, Appl. Phys. Lett. 77 (2000) 654. [7] T. Matsumoto, K. Mimoto, S. Goto, M. Ohba, Y. Agawa, M. Kiuchi, Rev. Sci. Instrum. 71 (2000) 1168. [8] P.C. Zalm, L.J. Beckers, Surf. Sci. 152/153 (1985) 135.