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Etching of polycrystalline copper by oblique injection of argon ion
Applied Surface Science 237 (2004) 321–325 www.elsevier.com/locate/apsusc
Etching of polycrystalline copper by oblique injection of argon ion Takashi Taguchia,*, Yuji Yamauchia, Yuko Hirohataa, Tomoaki Hinoa, Masana Nishikawab a
Department of Nuclear Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan b Science Solutions International Laboratory, Inc., 2-21-7 Naka-cho, Meguro-ku, Tokyo 153-0065, Japan Available online 11 September 2004
Abstract In order to miniaturize large scaled IC (LSI) and/or produce a multi-layered LSI device, copper wire with a smooth surface is required. For reduction of the surface roughness, ion etching with oblique injection was applied for polycrystalline copper. Argon ion irradiation was conducted by changing the incident angle and ion energy. After irradiation, many blisters of various sizes were observed. During normal ion injection, the etched amount of copper increased with ion energy and the average height of blisters also increased. Then, blister formation can be adjusted by ion energy. As the incident angle increased, the blister size became smaller and the surface became smooth compared with normal ion injection. This result suggests that preferential sputtering occurred for the protuberant parts of blisters. In particular, when the incident angle is 67.58, the average height of blisters became half of the normal value. The present result shows that oblique ion irradiation contributes to the reduction of surface roughness. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Cf Keyword: Ion etching
1. Introduction Polycrystalline copper (Cu) has been employed as a wiring material for large scale integration (LSI) devices [1]. In order to miniaturize LSI and/or produce a multi-layered LSI device, Cu wiring with a smooth * Corresponding author. Tel.: +81 11 706 6662; fax: +81 11 706 6662. E-mail address: [email protected] (T. Taguchi).
surface is required [2]. The wet process, chemicalmechanical polishing (CMP), has been widely used to make the surface smooth. However, the production process can be simplified if a dry process, such as ion etching, is applied, instead of a wet process. In the case of ion etching, it is known that blister formation takes place and surface smoothness is limited by such blister formation. For reduction in the roughness produced by blisters, the use of oblique ion injection may be suitable since the protuberant part of blisters is preferentially sputtered. It has been reported that
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.159
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T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
with different incident angles. In the present study, we systematically examined the surface morphology of Cu after Ar ion irradiation with different ion energy and injection angles.
2. Experiments and results
Fig. 1. Schematic illustration of ECR plasma irradiation device.
surface morphology of Cu is changed by ion etching [3,4], and a ripple or conical structure was observed in the case of Ar ion etching with different incident angles [5,6]. It is also well known that surface morphology changes by incident angle, energy and fluence of ion, ion species, target temperature and so on [7]. In order to observe the effect of oblique ion injection on the surface smoothness, the surface morphology has to be examined after ion injection
The Cu sample was mechanically polished and cleaned by using an ultrasonic bath with ethanol. The initialroughnessoftheCusamplewas3.5 nm.Afterthat, the sample was irradiated by Ar+ ions at room temperature. The surface morphology of the irradiated surface was observed by using a scanning electron microscope, SEM, and an atomic force microscope, AFM. First, the ion energy dependence of surface morphology was examined. An ECR plasma irradiation device shown in Fig. 1 was used for the Ar ion irradiation. The ion energy was adjusted by changing the negative bias voltage on the Cu sample. The negative bias voltage used was from 0.1 to 1 kV, corresponding to the maximum ion energy in the range from 0.1 to 1 keV, respectively. During the ion irradiation, the incident angle of Ar ions was in a normal direction, namely perpendicular to the surface. The ion fluence was kept the same, 1.5 1017 Ar/cm2. Weight loss after the ion irradiation was measured by using a microbalance. The AFM images of irradiated Cu are shown in Fig. 2. It is known that blisters are
Fig. 2. AFM images of polycrystalline copper after Ar ion irradiation with normal incidence.
T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
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Fig. 3. Ar ion energy dependence on weight loss (a), and surface density, average diameter and height of blisters (b).
easily produced by argon ion irradiation [8]. In the present argon ion irradiation, the formation of many blisters with different sizes was also observed on the irradiated sample. When the ion energy was increased, the size, the height and the surface density of the blisters became larger. The maximum size of the blisters was approximately the size of a crystal grain. Fig. 3a shows ion energy dependence on weight loss. When the ion energy was 100 eV, there was little weight loss. In the case of 1 keV, the etched amount was 18 mg/cm2. The etched amount increased with ion energy since the implantation depth became shallow. Fig. 3b shows the surface density, average diameter and height of the blisters as a function of ion energy. When ion energy was 100 eV, the average diameter and the height of the blisters were 85 and 6 nm, respectively. The surface density of blisters was 2.4 109/cm2. The grain size of the Cu sample etched with a mixed liquid of NH4OH and H2O2 was measured based on the AFM observation. The size was approximately 70 nm. The average diameter of the blister was roughly the same as the crystal grain size as mentioned above. As ion energy increased, the average height, average diameter and surface density of the blisters increased. In particular, the increase in average height was large, i.e. the average height for the ion energy of 1 keV was 33 nm, which was five times larger than that for 100 eV. This result shows that the blister structure can be changed by ion energy. Secondary, the incident angle dependence on the surface morphology was examined. In this experi-
ment, we used an ion gun as shown in Fig. 4. The incident angle of the Ar+ ion was changed by rotating the sample. The incident angle u, was 0, 22.5, 45 and 67.58 respectively. Here, the incident angle is defined by the difference of angle with that of normal incidence. The argon ion energy was the same, 1 keV. After the ion irradiation, weight loss was measured using a microbalance. The ion fluence was in the range from 0.9 1017 to 1.2 1017 Ar/cm2. The weight losses for the injection with u = 0, 22.5, 45 and 67.5, were 16, 18, 16 and 20 mg/cm2, respectively. Therefore, the weight loss was roughly the same. The AFM images of the surface after irradiation with different incident angles are shown in Fig. 5. In the case of the 08 incident angle, the blister size was observed to be large. As the incident angle increased, the blister size became small and the surface became smooth. Fig. 6 shows the surface density, average diameter and height of blisters as a function of the
Fig. 4. Ar ion irradiation apparatus for oblique ion injection.
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Fig. 5. AFM images of polycrystalline copper after oblique ion irradiation at different angles.
Fig. 6. Surface density, average blister diameter and height of blisters as a function of incident angle of Ar ion.
incident angle of Ar ion. In the case of 08, the average diameter and the height of the blisters were 125 and 35 nm, respectively. The surface density of blisters was 8 109/cm2. As the incident angle increased, the average height of blisters decreased while the average diameter and the surface density of the blisters remained roughly the same. In the case of u = 67.58, the average height of blisters was 18 nm. This value was half of the value for the case of normal incidence. The reason is explained by preferential sputtering for the protuberant parts of blisters, since the ion fluence for the protuberant part becomes large. The cross sections of blisters were measured in many positions.
Fig. 7. Profiles of blister height after ion irradiations with incident angles of 08 (a), and 67.58 (b).
Fig. 7 shows a typical cross sectional view of blisters for the cases of normal and oblique injections. For the case of u = 08, the profile of blister height was roughly symmetrical. On the other hand, the profile of blister height became non-symmetrical when the part facing towards the ions was significantly etched.
3. Conclusion The surface smoothness after oblique ion injection of polycrystalline copper was investigated. The Ar+ ion irradiation was conducted by changing the ion
T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
energy and the incident angle. After the ion irradiation, the surface smoothness was observed by using AFM and SEM. When the ion injection was perpendicular to the surface, the diameter, height and surface density of blisters increased with ion energy. The etched amount of copper increased with ion energy, since the target copper atom became shallow. This result suggests that the blister structure or surface roughness can be adjusted by both ion energy and ion fluence. As the incident angle increased, the blister height became small while the diameter and the surface density remained roughly the same. The improved smoothness is due to preferential sputtering for the protuberant part of the blister. In the present experiment, the roughness in the height was as low as 20 nm when the energy of Ar ion was 1 keV and the incident angle was 67.58. This height is approximately
325
half the value obtained by the present method, CMP. Thus, the process using oblique ion injection may simplify the production process for LSI and MEMS.
References [1] M. Migitaka, New LSI Process Engineering, Ohmsya, Tokyo, 1992. [2] K. Maeda, VLSI and CVD, Makishoten, Tokyo, 1997. [3] T. Yamauchi, S. Tokura, S. Yamanaka, M. Miyake, J. Nucl. Mater. 155-157 (1988) 438. [4] B. Terreault, J. Nucl. Mater 93-94 (1980) 707. [5] R. Shimizu, Jpn. J. Appl. Phys. 13 (1974) 228. [6] S. Rusponi, G. Costantini, C. Boragno, U. Valbusa, Phys. Rev. Lett. 81 (1998) 2735. [7] M. Kaminsky, Radiat. Eff. Solid Surf. (1976) 136. [8] Y. Yamauchi, T. Taguchi, Y. Hirohata, T. Hino, M. Nishikawa, Vacuum, (2004), in press.
Etching of polycrystalline copper by oblique injection of argon ion Takashi Taguchia,*, Yuji Yamauchia, Yuko Hirohataa, Tomoaki Hinoa, Masana Nishikawab a
Department of Nuclear Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan b Science Solutions International Laboratory, Inc., 2-21-7 Naka-cho, Meguro-ku, Tokyo 153-0065, Japan Available online 11 September 2004
Abstract In order to miniaturize large scaled IC (LSI) and/or produce a multi-layered LSI device, copper wire with a smooth surface is required. For reduction of the surface roughness, ion etching with oblique injection was applied for polycrystalline copper. Argon ion irradiation was conducted by changing the incident angle and ion energy. After irradiation, many blisters of various sizes were observed. During normal ion injection, the etched amount of copper increased with ion energy and the average height of blisters also increased. Then, blister formation can be adjusted by ion energy. As the incident angle increased, the blister size became smaller and the surface became smooth compared with normal ion injection. This result suggests that preferential sputtering occurred for the protuberant parts of blisters. In particular, when the incident angle is 67.58, the average height of blisters became half of the normal value. The present result shows that oblique ion irradiation contributes to the reduction of surface roughness. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Cf Keyword: Ion etching
1. Introduction Polycrystalline copper (Cu) has been employed as a wiring material for large scale integration (LSI) devices [1]. In order to miniaturize LSI and/or produce a multi-layered LSI device, Cu wiring with a smooth * Corresponding author. Tel.: +81 11 706 6662; fax: +81 11 706 6662. E-mail address: [email protected] (T. Taguchi).
surface is required [2]. The wet process, chemicalmechanical polishing (CMP), has been widely used to make the surface smooth. However, the production process can be simplified if a dry process, such as ion etching, is applied, instead of a wet process. In the case of ion etching, it is known that blister formation takes place and surface smoothness is limited by such blister formation. For reduction in the roughness produced by blisters, the use of oblique ion injection may be suitable since the protuberant part of blisters is preferentially sputtered. It has been reported that
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.159
322
T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
with different incident angles. In the present study, we systematically examined the surface morphology of Cu after Ar ion irradiation with different ion energy and injection angles.
2. Experiments and results
Fig. 1. Schematic illustration of ECR plasma irradiation device.
surface morphology of Cu is changed by ion etching [3,4], and a ripple or conical structure was observed in the case of Ar ion etching with different incident angles [5,6]. It is also well known that surface morphology changes by incident angle, energy and fluence of ion, ion species, target temperature and so on [7]. In order to observe the effect of oblique ion injection on the surface smoothness, the surface morphology has to be examined after ion injection
The Cu sample was mechanically polished and cleaned by using an ultrasonic bath with ethanol. The initialroughnessoftheCusamplewas3.5 nm.Afterthat, the sample was irradiated by Ar+ ions at room temperature. The surface morphology of the irradiated surface was observed by using a scanning electron microscope, SEM, and an atomic force microscope, AFM. First, the ion energy dependence of surface morphology was examined. An ECR plasma irradiation device shown in Fig. 1 was used for the Ar ion irradiation. The ion energy was adjusted by changing the negative bias voltage on the Cu sample. The negative bias voltage used was from 0.1 to 1 kV, corresponding to the maximum ion energy in the range from 0.1 to 1 keV, respectively. During the ion irradiation, the incident angle of Ar ions was in a normal direction, namely perpendicular to the surface. The ion fluence was kept the same, 1.5 1017 Ar/cm2. Weight loss after the ion irradiation was measured by using a microbalance. The AFM images of irradiated Cu are shown in Fig. 2. It is known that blisters are
Fig. 2. AFM images of polycrystalline copper after Ar ion irradiation with normal incidence.
T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
323
Fig. 3. Ar ion energy dependence on weight loss (a), and surface density, average diameter and height of blisters (b).
easily produced by argon ion irradiation [8]. In the present argon ion irradiation, the formation of many blisters with different sizes was also observed on the irradiated sample. When the ion energy was increased, the size, the height and the surface density of the blisters became larger. The maximum size of the blisters was approximately the size of a crystal grain. Fig. 3a shows ion energy dependence on weight loss. When the ion energy was 100 eV, there was little weight loss. In the case of 1 keV, the etched amount was 18 mg/cm2. The etched amount increased with ion energy since the implantation depth became shallow. Fig. 3b shows the surface density, average diameter and height of the blisters as a function of ion energy. When ion energy was 100 eV, the average diameter and the height of the blisters were 85 and 6 nm, respectively. The surface density of blisters was 2.4 109/cm2. The grain size of the Cu sample etched with a mixed liquid of NH4OH and H2O2 was measured based on the AFM observation. The size was approximately 70 nm. The average diameter of the blister was roughly the same as the crystal grain size as mentioned above. As ion energy increased, the average height, average diameter and surface density of the blisters increased. In particular, the increase in average height was large, i.e. the average height for the ion energy of 1 keV was 33 nm, which was five times larger than that for 100 eV. This result shows that the blister structure can be changed by ion energy. Secondary, the incident angle dependence on the surface morphology was examined. In this experi-
ment, we used an ion gun as shown in Fig. 4. The incident angle of the Ar+ ion was changed by rotating the sample. The incident angle u, was 0, 22.5, 45 and 67.58 respectively. Here, the incident angle is defined by the difference of angle with that of normal incidence. The argon ion energy was the same, 1 keV. After the ion irradiation, weight loss was measured using a microbalance. The ion fluence was in the range from 0.9 1017 to 1.2 1017 Ar/cm2. The weight losses for the injection with u = 0, 22.5, 45 and 67.5, were 16, 18, 16 and 20 mg/cm2, respectively. Therefore, the weight loss was roughly the same. The AFM images of the surface after irradiation with different incident angles are shown in Fig. 5. In the case of the 08 incident angle, the blister size was observed to be large. As the incident angle increased, the blister size became small and the surface became smooth. Fig. 6 shows the surface density, average diameter and height of blisters as a function of the
Fig. 4. Ar ion irradiation apparatus for oblique ion injection.
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T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
Fig. 5. AFM images of polycrystalline copper after oblique ion irradiation at different angles.
Fig. 6. Surface density, average blister diameter and height of blisters as a function of incident angle of Ar ion.
incident angle of Ar ion. In the case of 08, the average diameter and the height of the blisters were 125 and 35 nm, respectively. The surface density of blisters was 8 109/cm2. As the incident angle increased, the average height of blisters decreased while the average diameter and the surface density of the blisters remained roughly the same. In the case of u = 67.58, the average height of blisters was 18 nm. This value was half of the value for the case of normal incidence. The reason is explained by preferential sputtering for the protuberant parts of blisters, since the ion fluence for the protuberant part becomes large. The cross sections of blisters were measured in many positions.
Fig. 7. Profiles of blister height after ion irradiations with incident angles of 08 (a), and 67.58 (b).
Fig. 7 shows a typical cross sectional view of blisters for the cases of normal and oblique injections. For the case of u = 08, the profile of blister height was roughly symmetrical. On the other hand, the profile of blister height became non-symmetrical when the part facing towards the ions was significantly etched.
3. Conclusion The surface smoothness after oblique ion injection of polycrystalline copper was investigated. The Ar+ ion irradiation was conducted by changing the ion
T. Taguchi et al. / Applied Surface Science 237 (2004) 321–325
energy and the incident angle. After the ion irradiation, the surface smoothness was observed by using AFM and SEM. When the ion injection was perpendicular to the surface, the diameter, height and surface density of blisters increased with ion energy. The etched amount of copper increased with ion energy, since the target copper atom became shallow. This result suggests that the blister structure or surface roughness can be adjusted by both ion energy and ion fluence. As the incident angle increased, the blister height became small while the diameter and the surface density remained roughly the same. The improved smoothness is due to preferential sputtering for the protuberant part of the blister. In the present experiment, the roughness in the height was as low as 20 nm when the energy of Ar ion was 1 keV and the incident angle was 67.58. This height is approximately
325
half the value obtained by the present method, CMP. Thus, the process using oblique ion injection may simplify the production process for LSI and MEMS.
References [1] M. Migitaka, New LSI Process Engineering, Ohmsya, Tokyo, 1992. [2] K. Maeda, VLSI and CVD, Makishoten, Tokyo, 1997. [3] T. Yamauchi, S. Tokura, S. Yamanaka, M. Miyake, J. Nucl. Mater. 155-157 (1988) 438. [4] B. Terreault, J. Nucl. Mater 93-94 (1980) 707. [5] R. Shimizu, Jpn. J. Appl. Phys. 13 (1974) 228. [6] S. Rusponi, G. Costantini, C. Boragno, U. Valbusa, Phys. Rev. Lett. 81 (1998) 2735. [7] M. Kaminsky, Radiat. Eff. Solid Surf. (1976) 136. [8] Y. Yamauchi, T. Taguchi, Y. Hirohata, T. Hino, M. Nishikawa, Vacuum, (2004), in press.