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Morphological evolution of surfaces irradiated by gas cluster ion beams during thin film deposition
Nuclear Instruments and Methods in Physics Research B 237 (2005) 449–454 www.elsevier.com/locate/nimb
Morphological evolution of surfaces irradiated by gas cluster ion beams during thin film deposition S. Inoue b
a,b,* ,
N. Toyoda b, H. Tsubakino a, I. Yamada
b
a Graduated School of Engineering, University of Hyogo, Syosya, Himeji, Hyogo 671-2201, Japan Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2, Kouto, Kamigori, Ako-gun, Hyogo 678-1205, Japan
Available online 26 July 2005
Abstract Ta2O5 films were deposited with O2 cluster ion beam assisted deposition at various incidence angles h, between 0 and 80 from surface normal. The surface morphology and cross-sectional images were studied. The film structure was significantly affected by incidence angle. When h was between 0 and 30, dense and flat Ta2O5 films were formed. However, in the case of h between 30 and 60, ripples were formed on the surface whose wave vector was in the incidence direction and the film surface was rough. On the other hand, when h was above 70, the wave vector of the ripple was rotated to perpendicular and surface roughness decreased to the same value at normal incidence. Ripples formed during thin film assisted deposition were similar to surface morphological evolution during the sputtering process. 2005 Published by Elsevier B.V. PACS: 81.15.Jj; 42.79.Wc Keywords: Gas cluster ion beam; GCIB assisted deposition; Ta2O5; Morphological evolution
1. Introduction Ta2O5 dielectric films are widely used as highly refractive index films in multiple layer coatings. Its refractive index is about 2.1 at a wavelength of 550 nm and is transparent at wavelengths between * Corresponding author. Address: LASTI, University of Hyogo, 3-1-2, Kouto, Kamigori, Ako, Hyogo 678-1205, Japan. Tel.: +81 791 58 1419; fax: +81 791 58 0242. E-mail address: [email protected] (S. Inoue).
0168-583X/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.05.022
0.35 lm and 10 lm. When these films are used for narrow band pass filters such as dense wavelength division multiplexing (DWDM) filters, it requires quite high quality Ta2O5 films. Very flat surface to reduce scatter loss, high refractive index n, low extinction coefficient k and dense film structure to achieve good environmental stability are required. To achieve these properties, ion assisted processes are usually used. However, these techniques present problems such as damage effects induced energetic
450
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
ion bombardment, and therefore, low energy ion beam irradiation is required. However, it is difficult to transport high-current and low energy ion beam because of the space charge effects. We have developed gas cluster ion beam process, which can realize equivalently low energy irradiation. The gas cluster ion consists of several thousands of gas atoms or molecules. Therefore, when a gas cluster ion consisting of 1000 atoms is accelerated with a total energy of 10 keV, each constituent atom has the kinetic energy of 10 eV. Thus, extremely low energy irradiation can be realized. There are various applications of gas cluster ion beam (GCIB) such as shallow implantation, surface smoothing and thin film formation [1,2]. For thin film formation, hard carbon films, tin-doped indium oxide (ITO) films and optical thin films (Ta2O5, SiO2 and Nb2O5) have been studied [3–5]. In the case of optical thin film formation, it has been reported that Ta2O5 films formed by O2 GCIB assisted deposition have a high refractive index, low absorption, high density, a very smooth surface and good environmental stability [6]. In addition, a surface smoothing effect was observed during thin film formation with O2 GCIB assisted deposition. When multi-layer films were formed with conventional process, once a rough interface or nodule formed during deposition, surface roughness increased with an increasing number of
layers. We have reported that O2 GCIB assisted deposition can solve this problem. With GCIB assist irradiation, quite flat surface and interface can be formed even on a rough surface. The origin of this smoothing effect is not the sputtering effect because the energy is too low for sputtering, but rather it was explained that the lateral motion of deposited material filled valleys preferentially so that the deposited film realized a smooth surface [7]. All these experiments however, were performed by irradiation of O2 GCIB at normal incidence. In the case of sputtering experiments with Ar GCIB, the surface roughness and morphology were significantly influenced by the incident angle of Ar GCIB [8]. It was expected that the incidence angle would affect the thin film deposition with O2 GCIB in the same manner. In this work we studied the effect of incidence angle of the assisting O2 cluster ion beam deposition.
2. Experiment Fig. 1 shows a schematic diagram of O2 GCIB assisted deposition system. This system consists of three parts, namely gas cluster source, ion source and deposition chamber. Neutral O2 cluster beams were formed in the source chamber with supersonic expansion using a Laval nozzle. The
Fig. 1. Schematic diagram of O2 cluster ion beam assisted deposition system.
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
source gas was pure O2 gas (99.999%) and the inlet gas pressure in the nozzle was 20 atm. After expansion of the gas, the cluster-containing core of the gas stream was extracted by a skimmer from the molecular flow. In the ion source chamber, neutral O2 clusters were ionized by electron bombardment and accelerated up to 30 keV. Monomer ions were completely removed by the permanent magnet filter located after the ionizer. The deposition chamber has two separate electron beam (EB) evaporators for Ta2O5 and SiO2. The O2 GCIB was irradiated during EB-evaporation. Substrate temperature was about 100 C due to radiation heating from the EB-evaporator. The O2 GCIB was irradiated at varying incidence angles h between 0 and 80 from the surface normal by tilting the substrate. The Ta2O5 film thickness was 200 nm. The acceleration energy and the ion current density of O2 GCIB were 7 keV and 1.0 lA/cm2, respectively. This condition was the optimal condition for Ta2O5 film formation at normal irradiation [5,6]. Total ion current was increased with increasing h to obtain the same ion current density at each incident angle. To study the effect of tilting during GCIB irradiation, films were also produced at identical evaporation and tilting conditions, but without GCIB assistance. After deposition, the surface morphology and roughness were observed by atomic force microscopy (AFM) with a scan area of 1.0 lm2. Crosssectional image was observed by field emission scanning electron microscopy (FE-SEM). To study the effect of ion bombardment on surface morphology by O2 GCIB in the energy range which is known to cause sputtering, an O2 GCIB was irradiated on Ta2O5 film produced with EBevaporation at the various angle h. The acceleration energy was 10 keV and the ion dose was 5.0 · 1016 ions/cm2, respectively.
3. Results and discussion 3.1. Surface smoothing effect caused by sputtering with O2 cluster ion beam irradiation At first, the sputtering effect by O2 GCIB was studied. Ta2O5 film deposited on Si with EB-depo-
451
sition was irradiated by O2 cluster ion beam at various angles h. The acceleration energy and ion dose were 10 keV and 5.0 · 1016 ions/cm2, respectively. In Fig. 2, the surface roughness and surface morphologies are plotted as a function of the incidence angle h. The average roughness of the initial Ta2O5 film was 1.0 nm. When the film was irradiated at the 0, as shown in Fig. 2(a), the film was smoothed and its surface roughness was 0.4 nm. For h between 45 and 60, the film was roughed by irradiation and ripples appeared on the surface with a wave vector in the incidence direction, as shown in Fig. 2(b) and its surface roughness was about 4.1 nm at 60. When h was above 60, the surface roughness decreased with increasing h and the film was smoothed by irradiation and tiny ripples appeared on the surface, but its direction is perpendicular to the direction of incoming ions, as shown in Fig. 2(c). The surface roughness at 80 was about 0.4 nm, almost the same as obtained with normal irradiation. These results indicate that the surface smoothing effect caused by sputtering was significantly affected by the tilting angle h and this would also affect the thin film formation. 3.2. Thin film formation with O2 GCIB assist irradiation at oblique incidence Next, the incidence angle dependence of assisting O2 GCIB on film formation process was studied. Ta2O5 films were deposited with O2 cluster ion beam assisted deposition at various incidence angles h. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively. The film thickness was 200 nm. Fig. 3 shows the incidence angle dependence of O2 cluster ion on the surface roughness of Ta2O5 films. The surface roughness of Ta2O5 films deposited without O2 cluster ion beam was about 1.0 nm. There was no significant dependence of film properties on incident angles when there was no O2 GCIB assistance. When the incidence angle of O2 GCIB was below 30, the surface roughness was below 0.5 nm. When h was between 30 and 60, the surface roughness increased and had a maximum around 60. For h values between 60 and 80, the surface roughness decreased with increasing h, and when h has reached 80, the film was
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Fig. 2. Incidence angle dependence of surface roughness and surface morphologies of Ta2O5 film irradiated with O2 cluster ion beam. The acceleration energy and ion dose were 10 keV and 5.0 · 1016 ions/cm2, respectively. Arrows show the incidence direction of GCIB.
Fig. 3. Incidence angle dependence of average roughness of Ta2O5 film deposited with O2 GCIB assisted deposition. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively.
smoothed almost to the same value as normal irradiation. These tendencies were similar in the case of sputtering, as shown in Fig. 2.
The series of micrographs in Fig. 4 show the surface morphology and the cross-sectional images of Ta2O5 films formed with O2 cluster ion assisted deposition observed with AFM and FE-SEM. The incidence angles h were 0, 60 and 80, respectively. When the incidence angle was between 0 and 30, there was no significant difference in both surface morphologies and cross-sectional images compared with normal incidence. Results at 30 are not shown, but were similar to those at 0. At these incidence angles, very flat surfaces and dense films were realized (Figs. 4(a) and (b)). Fig. 4(c) shows the surface morphology at 60. When the incidence angle h was between 30 and 60, ripples were formed with the wave vector in the incidence direction, as shown in Fig. 4(c) and when h was 80, ripples were formed on the film surface with a wave vector perpendicular to the incidence direction (Fig. 4(f)). This tendency was also observed at 70 (not shown). These tendencies were similar to the morphological evolution caused by sputtering as shown in Fig. 2. Figs. 4(d) and (e) show cross-sectional images at the incidence angle of 60 along the line A–B and C–D, respectively. The line A–B is parallel to the
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
453
Fig. 4. The surface morphologies and the cross-sectional images of Ta2O5 film deposited with O2 GCIB assisted deposition at various incidence angles h. Arrows show the incidence direction of GCIB. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively.
incidence direction of GCIB and the line C–D is perpendicular to it. Figs. 4(g) and (h) show the microstructures at the incident angle of 80. A porous and columnar structure was observed for all the images. However, there was significant difference in the cross-sectional images between the parallel and perpendicular lines to the incidence direction. These results mean that thin film growth was significantly influenced by the incidence angle of cluster ions and ripples were formed during deposition.
Kitani et al. reported incidence angle dependence of the surface smoothing effects with Ar cluster ion beam irradiation [8] and the same tendency was observed as shown in Fig. 2, because of these experiments were performed under the condition, which the acceleration energy was sufficiently high to cause sputtering of materials. However, as shown in Figs. 3 and 4, the same tendency was observed at the acceleration energy of 7 keV, which did not cause sputtering [7]. In the case of
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S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
ripple formation in Ar cluster ion sputtering, it was explained by the fact that the one-directional motion of the atoms was enhanced due to oblique incidence irradiation. In film formation, the same motion of the atoms most probably happen and consequently, the film grew with columnar structure and ripples were formed on the surface. From these results which was shown above, normal irradiation of GCIB is the optimum condition to form flat and dense film.
flat films were formed the same as with normal incidence. These tendencies were similar to surface morphological evolution caused by sputtering. In the case of h above 30, a porous and columnar structure was observed even though dense film structure can form at incident angles below 30. Therefore, normal incidence irradiation is the optimum condition to form dense and flat film.
References 4. Summary In this paper, we discussed the surface morphological evolution of Ta2O5 film at oblique incidence of O2 GCIB during film formation. From AFM observations, when the incident angle h was below 30, dense and flat films were formed with average roughness lower than 0.5 nm. When h was between 30 and 60, the surface roughness was increased suddenly and film was roughed by O2 GCIB irradiation, and a ripple appeared with a surface wave vector was parallel to the incident direction. In the region of h above 60, ripple was appeared and the wave vector rotated to perpendicular and the surface roughness was decreased with increasing h. When h was 80, quite
[1] I. Yamada, J. Matsuo, Z. Insepov, T. Aoki, T. Seki, N. Toyoda, Nucl. Instr. and Meth. B 164–165 (2000) 944. [2] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 231. [3] K. Miyauchi, T. Kitagawa, N. Toyoda, S. Matsui, K. Mochiji, T. Mitamura, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 893. [4] J. Matsuo, H. Katsumata, E. Minami, I. Yamada, Nucl. Instr. and Meth. B 161–163 (2000) 952. [5] N. Toyoda, Y. Fujiwara, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 875. [6] K. Shirai, Y. Fujiwara, R. Takahashi, N. Toyoda, S. Matsui, T. Mitamura, M. Terasawa, I. Yamada, Jpn. J. Appl. Phys. 41 (2002) 4291. [7] Y. Fujiwara, N. Toyoda, K. Mochiji, T. Mitamura, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 870. [8] H. Kitani, N. Toyoda, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 121 (1997) 489.
Morphological evolution of surfaces irradiated by gas cluster ion beams during thin film deposition S. Inoue b
a,b,* ,
N. Toyoda b, H. Tsubakino a, I. Yamada
b
a Graduated School of Engineering, University of Hyogo, Syosya, Himeji, Hyogo 671-2201, Japan Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2, Kouto, Kamigori, Ako-gun, Hyogo 678-1205, Japan
Available online 26 July 2005
Abstract Ta2O5 films were deposited with O2 cluster ion beam assisted deposition at various incidence angles h, between 0 and 80 from surface normal. The surface morphology and cross-sectional images were studied. The film structure was significantly affected by incidence angle. When h was between 0 and 30, dense and flat Ta2O5 films were formed. However, in the case of h between 30 and 60, ripples were formed on the surface whose wave vector was in the incidence direction and the film surface was rough. On the other hand, when h was above 70, the wave vector of the ripple was rotated to perpendicular and surface roughness decreased to the same value at normal incidence. Ripples formed during thin film assisted deposition were similar to surface morphological evolution during the sputtering process. 2005 Published by Elsevier B.V. PACS: 81.15.Jj; 42.79.Wc Keywords: Gas cluster ion beam; GCIB assisted deposition; Ta2O5; Morphological evolution
1. Introduction Ta2O5 dielectric films are widely used as highly refractive index films in multiple layer coatings. Its refractive index is about 2.1 at a wavelength of 550 nm and is transparent at wavelengths between * Corresponding author. Address: LASTI, University of Hyogo, 3-1-2, Kouto, Kamigori, Ako, Hyogo 678-1205, Japan. Tel.: +81 791 58 1419; fax: +81 791 58 0242. E-mail address: [email protected] (S. Inoue).
0168-583X/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.05.022
0.35 lm and 10 lm. When these films are used for narrow band pass filters such as dense wavelength division multiplexing (DWDM) filters, it requires quite high quality Ta2O5 films. Very flat surface to reduce scatter loss, high refractive index n, low extinction coefficient k and dense film structure to achieve good environmental stability are required. To achieve these properties, ion assisted processes are usually used. However, these techniques present problems such as damage effects induced energetic
450
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
ion bombardment, and therefore, low energy ion beam irradiation is required. However, it is difficult to transport high-current and low energy ion beam because of the space charge effects. We have developed gas cluster ion beam process, which can realize equivalently low energy irradiation. The gas cluster ion consists of several thousands of gas atoms or molecules. Therefore, when a gas cluster ion consisting of 1000 atoms is accelerated with a total energy of 10 keV, each constituent atom has the kinetic energy of 10 eV. Thus, extremely low energy irradiation can be realized. There are various applications of gas cluster ion beam (GCIB) such as shallow implantation, surface smoothing and thin film formation [1,2]. For thin film formation, hard carbon films, tin-doped indium oxide (ITO) films and optical thin films (Ta2O5, SiO2 and Nb2O5) have been studied [3–5]. In the case of optical thin film formation, it has been reported that Ta2O5 films formed by O2 GCIB assisted deposition have a high refractive index, low absorption, high density, a very smooth surface and good environmental stability [6]. In addition, a surface smoothing effect was observed during thin film formation with O2 GCIB assisted deposition. When multi-layer films were formed with conventional process, once a rough interface or nodule formed during deposition, surface roughness increased with an increasing number of
layers. We have reported that O2 GCIB assisted deposition can solve this problem. With GCIB assist irradiation, quite flat surface and interface can be formed even on a rough surface. The origin of this smoothing effect is not the sputtering effect because the energy is too low for sputtering, but rather it was explained that the lateral motion of deposited material filled valleys preferentially so that the deposited film realized a smooth surface [7]. All these experiments however, were performed by irradiation of O2 GCIB at normal incidence. In the case of sputtering experiments with Ar GCIB, the surface roughness and morphology were significantly influenced by the incident angle of Ar GCIB [8]. It was expected that the incidence angle would affect the thin film deposition with O2 GCIB in the same manner. In this work we studied the effect of incidence angle of the assisting O2 cluster ion beam deposition.
2. Experiment Fig. 1 shows a schematic diagram of O2 GCIB assisted deposition system. This system consists of three parts, namely gas cluster source, ion source and deposition chamber. Neutral O2 cluster beams were formed in the source chamber with supersonic expansion using a Laval nozzle. The
Fig. 1. Schematic diagram of O2 cluster ion beam assisted deposition system.
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
source gas was pure O2 gas (99.999%) and the inlet gas pressure in the nozzle was 20 atm. After expansion of the gas, the cluster-containing core of the gas stream was extracted by a skimmer from the molecular flow. In the ion source chamber, neutral O2 clusters were ionized by electron bombardment and accelerated up to 30 keV. Monomer ions were completely removed by the permanent magnet filter located after the ionizer. The deposition chamber has two separate electron beam (EB) evaporators for Ta2O5 and SiO2. The O2 GCIB was irradiated during EB-evaporation. Substrate temperature was about 100 C due to radiation heating from the EB-evaporator. The O2 GCIB was irradiated at varying incidence angles h between 0 and 80 from the surface normal by tilting the substrate. The Ta2O5 film thickness was 200 nm. The acceleration energy and the ion current density of O2 GCIB were 7 keV and 1.0 lA/cm2, respectively. This condition was the optimal condition for Ta2O5 film formation at normal irradiation [5,6]. Total ion current was increased with increasing h to obtain the same ion current density at each incident angle. To study the effect of tilting during GCIB irradiation, films were also produced at identical evaporation and tilting conditions, but without GCIB assistance. After deposition, the surface morphology and roughness were observed by atomic force microscopy (AFM) with a scan area of 1.0 lm2. Crosssectional image was observed by field emission scanning electron microscopy (FE-SEM). To study the effect of ion bombardment on surface morphology by O2 GCIB in the energy range which is known to cause sputtering, an O2 GCIB was irradiated on Ta2O5 film produced with EBevaporation at the various angle h. The acceleration energy was 10 keV and the ion dose was 5.0 · 1016 ions/cm2, respectively.
3. Results and discussion 3.1. Surface smoothing effect caused by sputtering with O2 cluster ion beam irradiation At first, the sputtering effect by O2 GCIB was studied. Ta2O5 film deposited on Si with EB-depo-
451
sition was irradiated by O2 cluster ion beam at various angles h. The acceleration energy and ion dose were 10 keV and 5.0 · 1016 ions/cm2, respectively. In Fig. 2, the surface roughness and surface morphologies are plotted as a function of the incidence angle h. The average roughness of the initial Ta2O5 film was 1.0 nm. When the film was irradiated at the 0, as shown in Fig. 2(a), the film was smoothed and its surface roughness was 0.4 nm. For h between 45 and 60, the film was roughed by irradiation and ripples appeared on the surface with a wave vector in the incidence direction, as shown in Fig. 2(b) and its surface roughness was about 4.1 nm at 60. When h was above 60, the surface roughness decreased with increasing h and the film was smoothed by irradiation and tiny ripples appeared on the surface, but its direction is perpendicular to the direction of incoming ions, as shown in Fig. 2(c). The surface roughness at 80 was about 0.4 nm, almost the same as obtained with normal irradiation. These results indicate that the surface smoothing effect caused by sputtering was significantly affected by the tilting angle h and this would also affect the thin film formation. 3.2. Thin film formation with O2 GCIB assist irradiation at oblique incidence Next, the incidence angle dependence of assisting O2 GCIB on film formation process was studied. Ta2O5 films were deposited with O2 cluster ion beam assisted deposition at various incidence angles h. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively. The film thickness was 200 nm. Fig. 3 shows the incidence angle dependence of O2 cluster ion on the surface roughness of Ta2O5 films. The surface roughness of Ta2O5 films deposited without O2 cluster ion beam was about 1.0 nm. There was no significant dependence of film properties on incident angles when there was no O2 GCIB assistance. When the incidence angle of O2 GCIB was below 30, the surface roughness was below 0.5 nm. When h was between 30 and 60, the surface roughness increased and had a maximum around 60. For h values between 60 and 80, the surface roughness decreased with increasing h, and when h has reached 80, the film was
452
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
Fig. 2. Incidence angle dependence of surface roughness and surface morphologies of Ta2O5 film irradiated with O2 cluster ion beam. The acceleration energy and ion dose were 10 keV and 5.0 · 1016 ions/cm2, respectively. Arrows show the incidence direction of GCIB.
Fig. 3. Incidence angle dependence of average roughness of Ta2O5 film deposited with O2 GCIB assisted deposition. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively.
smoothed almost to the same value as normal irradiation. These tendencies were similar in the case of sputtering, as shown in Fig. 2.
The series of micrographs in Fig. 4 show the surface morphology and the cross-sectional images of Ta2O5 films formed with O2 cluster ion assisted deposition observed with AFM and FE-SEM. The incidence angles h were 0, 60 and 80, respectively. When the incidence angle was between 0 and 30, there was no significant difference in both surface morphologies and cross-sectional images compared with normal incidence. Results at 30 are not shown, but were similar to those at 0. At these incidence angles, very flat surfaces and dense films were realized (Figs. 4(a) and (b)). Fig. 4(c) shows the surface morphology at 60. When the incidence angle h was between 30 and 60, ripples were formed with the wave vector in the incidence direction, as shown in Fig. 4(c) and when h was 80, ripples were formed on the film surface with a wave vector perpendicular to the incidence direction (Fig. 4(f)). This tendency was also observed at 70 (not shown). These tendencies were similar to the morphological evolution caused by sputtering as shown in Fig. 2. Figs. 4(d) and (e) show cross-sectional images at the incidence angle of 60 along the line A–B and C–D, respectively. The line A–B is parallel to the
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
453
Fig. 4. The surface morphologies and the cross-sectional images of Ta2O5 film deposited with O2 GCIB assisted deposition at various incidence angles h. Arrows show the incidence direction of GCIB. The acceleration energy and ion current density were 7 keV and 1.0 lA/cm2, respectively.
incidence direction of GCIB and the line C–D is perpendicular to it. Figs. 4(g) and (h) show the microstructures at the incident angle of 80. A porous and columnar structure was observed for all the images. However, there was significant difference in the cross-sectional images between the parallel and perpendicular lines to the incidence direction. These results mean that thin film growth was significantly influenced by the incidence angle of cluster ions and ripples were formed during deposition.
Kitani et al. reported incidence angle dependence of the surface smoothing effects with Ar cluster ion beam irradiation [8] and the same tendency was observed as shown in Fig. 2, because of these experiments were performed under the condition, which the acceleration energy was sufficiently high to cause sputtering of materials. However, as shown in Figs. 3 and 4, the same tendency was observed at the acceleration energy of 7 keV, which did not cause sputtering [7]. In the case of
454
S. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 237 (2005) 449–454
ripple formation in Ar cluster ion sputtering, it was explained by the fact that the one-directional motion of the atoms was enhanced due to oblique incidence irradiation. In film formation, the same motion of the atoms most probably happen and consequently, the film grew with columnar structure and ripples were formed on the surface. From these results which was shown above, normal irradiation of GCIB is the optimum condition to form flat and dense film.
flat films were formed the same as with normal incidence. These tendencies were similar to surface morphological evolution caused by sputtering. In the case of h above 30, a porous and columnar structure was observed even though dense film structure can form at incident angles below 30. Therefore, normal incidence irradiation is the optimum condition to form dense and flat film.
References 4. Summary In this paper, we discussed the surface morphological evolution of Ta2O5 film at oblique incidence of O2 GCIB during film formation. From AFM observations, when the incident angle h was below 30, dense and flat films were formed with average roughness lower than 0.5 nm. When h was between 30 and 60, the surface roughness was increased suddenly and film was roughed by O2 GCIB irradiation, and a ripple appeared with a surface wave vector was parallel to the incident direction. In the region of h above 60, ripple was appeared and the wave vector rotated to perpendicular and the surface roughness was decreased with increasing h. When h was 80, quite
[1] I. Yamada, J. Matsuo, Z. Insepov, T. Aoki, T. Seki, N. Toyoda, Nucl. Instr. and Meth. B 164–165 (2000) 944. [2] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 231. [3] K. Miyauchi, T. Kitagawa, N. Toyoda, S. Matsui, K. Mochiji, T. Mitamura, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 893. [4] J. Matsuo, H. Katsumata, E. Minami, I. Yamada, Nucl. Instr. and Meth. B 161–163 (2000) 952. [5] N. Toyoda, Y. Fujiwara, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 875. [6] K. Shirai, Y. Fujiwara, R. Takahashi, N. Toyoda, S. Matsui, T. Mitamura, M. Terasawa, I. Yamada, Jpn. J. Appl. Phys. 41 (2002) 4291. [7] Y. Fujiwara, N. Toyoda, K. Mochiji, T. Mitamura, I. Yamada, Nucl. Instr. and Meth. B 206 (2003) 870. [8] H. Kitani, N. Toyoda, J. Matsuo, I. Yamada, Nucl. Instr. and Meth. B 121 (1997) 489.