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Novel nano-fabrication technique utilizing ion beam
Nuclear Instruments and Methods in Physics Research B 206 (2003) 482–485 www.elsevier.com/locate/nimb
Novel nano-fabrication technique utilizing ion beam Noriko Nitta, Masafumi Taniwaki
*
Department of Environmental Systems Engineering, Kochi University of Technology, Tosayamada-cho, Kochi 782-8502, Japan
Abstract Novel nano-fabrication technique is proposed on the basis of knowledge of anomalous behavior in ion-implanted GaSb and InSb at low temperatures. An initial array of hollows or voids is fabricated precisely on or under the substrate surface by focussed ion beam (FIB) and the structure is developed self-organizationally utilizing movement of point defects induced by ion implantation at relatively low temperatures. The precision in this technique is dominated mainly by extension of irradiation damage. Structure of 20-nm scale is fabricated at present technical level and that of 5-nm scale will be realized by improving the accuracy of FIB. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 81.05.Ea; 85.40.Ux; 85.40.Ry Keywords: Focussed ion beam; Void; Ion implantation; Self-organization; Cellular structure; Nano-fabrication
1. Introduction Nano-technology has been developed most drastically in semiconductor device process. Particularly, for the vertical direction of the substrate, stacking of mono-atomic layers is realized by molecular beam epitaxy (MBE) deposition method. Nevertheless, for patterning in the plane of the substrate, nano-fabrication is not realized due to the limit of precision in photo-lithography. In order to realize nano-fabrication in a plane, utilization of self-organization and self-assembly in materials is tried. For example, self-assembled quantum-dots are fabricated utilizing misfit strain in lattice mismatched hetero-epitaxial layers [1]. * Corresponding author. Tel.: +81-887-57-2504; fax: +81887-57-2520. E-mail address: [email protected] (M. Taniwaki).
Recently it was found that a cellular structure is developed on the GaSb and InSb surfaces ionimplanted at a low temperature and it was shown that this phenomenon is due to the behavior of induced point defects [2–5]. On the basis of this knowledge, we propose a new nano-fabrication technique using ion beam in this article. In this technique, an initial pattern is prepared artificially by focussed ion beam (FIB) on or under the substrate surface and the pattern is developed selforganizationally utilizing movement of point defects which are induced by ion implantation at relatively low temperatures.
2. Point defects creation by ion implantation and void formation The incidence of energetic heavy ions into material creates a lot of vacancies and interstitials by
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00802-4
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
483
Interstitial Vacancy
(a) low temperature
(b) intermediate temperature
(c) high temperature
Fig. 1. Behavior of point defects induced to supersaturation: (a) interstitials migrate and vacancies remain at low temperatures, (b) vacancies migrate and form voids at intermediate temperatures and (c) both of point defects with high mobility are annihilated and void formation does not occur at high temperatures.
displacing constituent atoms from crystallographic sites. One Snþ ion accelerated at 60 kV creates about 3200 vacancies and the same number of interstitials in a compound semiconductor GaSb, according to TRIM calculation [6]. These free point defects migrate and interact in the material, and finally most of the induced point defects disappear by recombination, migration to sinks and formation of secondary defects. Voids are often formed in irradiated materials due to the difference in mobility between vacancies and interstitials. This is roughly shown in Fig. 1. Even at sufficiently low temperatures, interstitials migrate from the implanted region and vacancies escaped from recombination with interstitials remain there. At intermediate temperatures, vacancies migrate and they form voids in the implanted region. At high temperatures where both of point defects migrate, induced vacancies and interstitials nearly completely annihilate by recombination or migration to sinks.
3. Cellular structure formed by ion implantation Taniwaki et al. and Nitta et al. found that a cellular structure is formed in GaSb and InSb ionimplanted at a low temperature, which is illustrated in Fig. 2 [2–4]. The formation will have occurred because the substrate temperature is low enough to depress the void formation by induced vacancies. In spite of the poor mobility of vacancies, they form voids where they are accumulated
Fig. 2. Cellular structure formed on GaSb surface implanted by 60 keV Snþ to a dose of 8.9 1014 ions/cm2 at 153 K. The cells are not uniform and typical values about their dimensions are shown.
over a critical concentration. Therefore the voids are ordered at nearly uniform intervals and near the depth corresponding to the ion projection range. Those voids absorb vacancies induced by subsequent implantation and grow to the surface direction, leading to self-organizational formation of the cellular structure. The cellular structure in GaSb has fine dimensions; holes with 50 nm diameter and 250 nm depth are arranged being partitioned by walls with 10 nm thickness [3,4]. Such fine structure self-organizationally formed is not fabricated by conventional planar technique, however we have a problem for its application to the electronic device process. It is lacking in regularity, that is, the arrangement of cells is irregular
484
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
and the cell diameters are not uniform, therefore, improvement of the regularity is needed.
4. Proposal of novel nano-fabrication We propose to prepare a regular initial array of defects on or under the substrate prior to the ion implantation process at a low temperature. At present, for fabrication of an initial array, ion beam is most suitable because the beam of charged particles is easily and precisely controlled. In FIB technique, the precision of 0.5 nm in positioning and the beam diameter of 5 nm is already realized [7]. The first method for preparing an initial pattern is shown in Fig. 3, where an initial array of hollows is fabricated by sputtering using conventional FIB. Each hollow is engraved by sputtering of scanning focussed ions where the relatively low acceleration voltage is preferred (Fig. 3(a)). Second step is de-
velopment of the cellular structure from the starting array of hollows by ion-implantation at a low temperature depressing void formation (about 150 K in GaSb). The point defects created in the walls between hollows do not contribute to development of the defect structure, because they are annihilated almost completely by recombination of vacancies and interstitials or by movement to the surface sink. Under the hollows, the highly mobile interstitials migrate far and vacancies which escaped recombination remain there (Fig. 3(b)). The walls develop by the interstitial atoms migrating from the surround and the remaining vacancies move to the hollowed surface during implantation, which deepen the hollows. In such a manner, a cellular structure partitioned by thin walls is fabricated self-organizationally as shown in Fig. 3(c). In the second method as shown in Fig. 4, the starting process is to arrange the voids under the surface. For this a large projection ion range is needed, therefore, relatively high acceleration
Fig. 3. Proposed nano-fabrication technique: (a) first an array of hollows is precisely engraved on the surface by FIB, (b) subsequently point defects are induced by ion implantation at a low temperature and (c) the pattern is developed self-organizationally by movement of the point defects.
Fig. 4. Another idea for nano-fabrication: (a) voids are created regularly under the surface by FIB accelerated at relatively high voltage and (b) and (c) by movement of the point defects at a low temperature, the cellular structure is developed self-organizationally.
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
voltage of ions is preferred. Implanted heavy ions create a lot of vacancies and interstitials by cascade collision; one Gaþ ion accelerated to 30 kV creates about 1550 vacancies and the same number of interstitials in GaSb, according to TRIM code [6]. Though the interstitials migrate far from the implanted cascade region, the vacancies with a low mobility remain and form voids there. Therefore, by irradiating the FIB on the points at uniform intervals, an initial array of voids is obtained under the surface. Subsequent process for developing the cell structure is the same as that in the previous method. Ion implantation at a low temperature creates interstitials and vacancies. Some of interstitials migrate and develop the partitioning walls, and the remaining vacancies are absorbed by voids. The top surfaces of the voids will be burst with their growth and ion irradiation. As a result, regular cellular structure is obtained on the substrate. 5. Precision in the fabrication The factors dominating the precision in the cell structure are the accuracy in positioning beam and the beam diameter. At present, those are 0.5 and 5 nm, respectively, which depend on the apparatus and are expected to improve technically for future [7]. In addition to those, the extension of irradiation damage in materials by ion implantation affects the precision. This is not artificially con-
485
trolled because irradiation damage is accumulation of accidental affairs in material. The damage in materials depends primarily on ion species (mass), acceleration voltage and ion dose. Implanted Gaþ ion projection range is extended to 15 nm in lateral direction at acceleration voltage of 30 kV ordinarily used in FIB fabrication, and the extension of 5 kV Gaþ is 5 nm [6]. According to the above discussion, the precision by the proposed fabrication technique is 20 nm which is a summation of the beam diameter and the extension of irradiation damage at present technical level. It is expected to be close 5 nm, extension of damage irradiated by 5 kV ions, by improving the ability of FIB.
References [1] R. Notzel, J. Temmyo, T. Furuta, T. Takamura, Appl. Phys. Lett. 65 (1994) 457. [2] M. Taniwaki, Y. Hayashi, T. Yoshiie, in: Y. Koiwa, K. Otsuka, T. Miyazaki (Eds.), Proceedings of International Conferene on Solid–Solid Phase Transformations, Kyoto, 1999, Japan Institute of Metals, Sendai, 1999, p. 45. [3] N. Nitta, M. Taniwaki, T. Suzuki, Y. Hayashi, Y. Satoh, T. Yoshiie, J. Jpn. Inst. Met. 64 (2000) 1141. [4] N. Nitta, M. Taniwaki, T. Suzuki, Y. Hayashi, Y. Satoh, T. Yoshiie, Mater. Trans. 43 (2002) 674. [5] N. Nitta, M. Taniwaki, Y. Hayashi, T. Yoshiie, J. Appl. Phys. 92 (2002) 1799. [6] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] T. Kaito, J. Mater. Sci. Soc. Jpn. 38 (2001) 184.
Novel nano-fabrication technique utilizing ion beam Noriko Nitta, Masafumi Taniwaki
*
Department of Environmental Systems Engineering, Kochi University of Technology, Tosayamada-cho, Kochi 782-8502, Japan
Abstract Novel nano-fabrication technique is proposed on the basis of knowledge of anomalous behavior in ion-implanted GaSb and InSb at low temperatures. An initial array of hollows or voids is fabricated precisely on or under the substrate surface by focussed ion beam (FIB) and the structure is developed self-organizationally utilizing movement of point defects induced by ion implantation at relatively low temperatures. The precision in this technique is dominated mainly by extension of irradiation damage. Structure of 20-nm scale is fabricated at present technical level and that of 5-nm scale will be realized by improving the accuracy of FIB. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 81.05.Ea; 85.40.Ux; 85.40.Ry Keywords: Focussed ion beam; Void; Ion implantation; Self-organization; Cellular structure; Nano-fabrication
1. Introduction Nano-technology has been developed most drastically in semiconductor device process. Particularly, for the vertical direction of the substrate, stacking of mono-atomic layers is realized by molecular beam epitaxy (MBE) deposition method. Nevertheless, for patterning in the plane of the substrate, nano-fabrication is not realized due to the limit of precision in photo-lithography. In order to realize nano-fabrication in a plane, utilization of self-organization and self-assembly in materials is tried. For example, self-assembled quantum-dots are fabricated utilizing misfit strain in lattice mismatched hetero-epitaxial layers [1]. * Corresponding author. Tel.: +81-887-57-2504; fax: +81887-57-2520. E-mail address: [email protected] (M. Taniwaki).
Recently it was found that a cellular structure is developed on the GaSb and InSb surfaces ionimplanted at a low temperature and it was shown that this phenomenon is due to the behavior of induced point defects [2–5]. On the basis of this knowledge, we propose a new nano-fabrication technique using ion beam in this article. In this technique, an initial pattern is prepared artificially by focussed ion beam (FIB) on or under the substrate surface and the pattern is developed selforganizationally utilizing movement of point defects which are induced by ion implantation at relatively low temperatures.
2. Point defects creation by ion implantation and void formation The incidence of energetic heavy ions into material creates a lot of vacancies and interstitials by
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00802-4
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
483
Interstitial Vacancy
(a) low temperature
(b) intermediate temperature
(c) high temperature
Fig. 1. Behavior of point defects induced to supersaturation: (a) interstitials migrate and vacancies remain at low temperatures, (b) vacancies migrate and form voids at intermediate temperatures and (c) both of point defects with high mobility are annihilated and void formation does not occur at high temperatures.
displacing constituent atoms from crystallographic sites. One Snþ ion accelerated at 60 kV creates about 3200 vacancies and the same number of interstitials in a compound semiconductor GaSb, according to TRIM calculation [6]. These free point defects migrate and interact in the material, and finally most of the induced point defects disappear by recombination, migration to sinks and formation of secondary defects. Voids are often formed in irradiated materials due to the difference in mobility between vacancies and interstitials. This is roughly shown in Fig. 1. Even at sufficiently low temperatures, interstitials migrate from the implanted region and vacancies escaped from recombination with interstitials remain there. At intermediate temperatures, vacancies migrate and they form voids in the implanted region. At high temperatures where both of point defects migrate, induced vacancies and interstitials nearly completely annihilate by recombination or migration to sinks.
3. Cellular structure formed by ion implantation Taniwaki et al. and Nitta et al. found that a cellular structure is formed in GaSb and InSb ionimplanted at a low temperature, which is illustrated in Fig. 2 [2–4]. The formation will have occurred because the substrate temperature is low enough to depress the void formation by induced vacancies. In spite of the poor mobility of vacancies, they form voids where they are accumulated
Fig. 2. Cellular structure formed on GaSb surface implanted by 60 keV Snþ to a dose of 8.9 1014 ions/cm2 at 153 K. The cells are not uniform and typical values about their dimensions are shown.
over a critical concentration. Therefore the voids are ordered at nearly uniform intervals and near the depth corresponding to the ion projection range. Those voids absorb vacancies induced by subsequent implantation and grow to the surface direction, leading to self-organizational formation of the cellular structure. The cellular structure in GaSb has fine dimensions; holes with 50 nm diameter and 250 nm depth are arranged being partitioned by walls with 10 nm thickness [3,4]. Such fine structure self-organizationally formed is not fabricated by conventional planar technique, however we have a problem for its application to the electronic device process. It is lacking in regularity, that is, the arrangement of cells is irregular
484
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
and the cell diameters are not uniform, therefore, improvement of the regularity is needed.
4. Proposal of novel nano-fabrication We propose to prepare a regular initial array of defects on or under the substrate prior to the ion implantation process at a low temperature. At present, for fabrication of an initial array, ion beam is most suitable because the beam of charged particles is easily and precisely controlled. In FIB technique, the precision of 0.5 nm in positioning and the beam diameter of 5 nm is already realized [7]. The first method for preparing an initial pattern is shown in Fig. 3, where an initial array of hollows is fabricated by sputtering using conventional FIB. Each hollow is engraved by sputtering of scanning focussed ions where the relatively low acceleration voltage is preferred (Fig. 3(a)). Second step is de-
velopment of the cellular structure from the starting array of hollows by ion-implantation at a low temperature depressing void formation (about 150 K in GaSb). The point defects created in the walls between hollows do not contribute to development of the defect structure, because they are annihilated almost completely by recombination of vacancies and interstitials or by movement to the surface sink. Under the hollows, the highly mobile interstitials migrate far and vacancies which escaped recombination remain there (Fig. 3(b)). The walls develop by the interstitial atoms migrating from the surround and the remaining vacancies move to the hollowed surface during implantation, which deepen the hollows. In such a manner, a cellular structure partitioned by thin walls is fabricated self-organizationally as shown in Fig. 3(c). In the second method as shown in Fig. 4, the starting process is to arrange the voids under the surface. For this a large projection ion range is needed, therefore, relatively high acceleration
Fig. 3. Proposed nano-fabrication technique: (a) first an array of hollows is precisely engraved on the surface by FIB, (b) subsequently point defects are induced by ion implantation at a low temperature and (c) the pattern is developed self-organizationally by movement of the point defects.
Fig. 4. Another idea for nano-fabrication: (a) voids are created regularly under the surface by FIB accelerated at relatively high voltage and (b) and (c) by movement of the point defects at a low temperature, the cellular structure is developed self-organizationally.
N. Nitta, M. Taniwaki / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 482–485
voltage of ions is preferred. Implanted heavy ions create a lot of vacancies and interstitials by cascade collision; one Gaþ ion accelerated to 30 kV creates about 1550 vacancies and the same number of interstitials in GaSb, according to TRIM code [6]. Though the interstitials migrate far from the implanted cascade region, the vacancies with a low mobility remain and form voids there. Therefore, by irradiating the FIB on the points at uniform intervals, an initial array of voids is obtained under the surface. Subsequent process for developing the cell structure is the same as that in the previous method. Ion implantation at a low temperature creates interstitials and vacancies. Some of interstitials migrate and develop the partitioning walls, and the remaining vacancies are absorbed by voids. The top surfaces of the voids will be burst with their growth and ion irradiation. As a result, regular cellular structure is obtained on the substrate. 5. Precision in the fabrication The factors dominating the precision in the cell structure are the accuracy in positioning beam and the beam diameter. At present, those are 0.5 and 5 nm, respectively, which depend on the apparatus and are expected to improve technically for future [7]. In addition to those, the extension of irradiation damage in materials by ion implantation affects the precision. This is not artificially con-
485
trolled because irradiation damage is accumulation of accidental affairs in material. The damage in materials depends primarily on ion species (mass), acceleration voltage and ion dose. Implanted Gaþ ion projection range is extended to 15 nm in lateral direction at acceleration voltage of 30 kV ordinarily used in FIB fabrication, and the extension of 5 kV Gaþ is 5 nm [6]. According to the above discussion, the precision by the proposed fabrication technique is 20 nm which is a summation of the beam diameter and the extension of irradiation damage at present technical level. It is expected to be close 5 nm, extension of damage irradiated by 5 kV ions, by improving the ability of FIB.
References [1] R. Notzel, J. Temmyo, T. Furuta, T. Takamura, Appl. Phys. Lett. 65 (1994) 457. [2] M. Taniwaki, Y. Hayashi, T. Yoshiie, in: Y. Koiwa, K. Otsuka, T. Miyazaki (Eds.), Proceedings of International Conferene on Solid–Solid Phase Transformations, Kyoto, 1999, Japan Institute of Metals, Sendai, 1999, p. 45. [3] N. Nitta, M. Taniwaki, T. Suzuki, Y. Hayashi, Y. Satoh, T. Yoshiie, J. Jpn. Inst. Met. 64 (2000) 1141. [4] N. Nitta, M. Taniwaki, T. Suzuki, Y. Hayashi, Y. Satoh, T. Yoshiie, Mater. Trans. 43 (2002) 674. [5] N. Nitta, M. Taniwaki, Y. Hayashi, T. Yoshiie, J. Appl. Phys. 92 (2002) 1799. [6] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] T. Kaito, J. Mater. Sci. Soc. Jpn. 38 (2001) 184.