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Plasma resistant aluminum oxide coatings for semiconductor processing apparatus by atmospheric aerosol spray method
Surface & Coatings Technology 205 (2010) S125–S128
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Plasma resistant aluminum oxide coatings for semiconductor processing apparatus by atmospheric aerosol spray method Hoomi Choi a, Kwangsu Kim a, Heesung Choi c, Sangwoo Kang d, Juyoung Yun d, Yonghyeon Shin d, Taesung Kim a,b,⁎ a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea Samsung Electro-Mechanics Co., Maetan 3-dong, Yeongtong-gu, Suwon, Republic of Korea d Vacuum Center, Korea Research Institute of Standards and Science, Doryong-dong, Yuseong, Daejeon, Republic of Korea b c
a r t i c l e
i n f o
Available online 28 June 2010 Keywords: Aluminum oxide coating Film formation Atmospheric aerosol spray Plasma resistance Semiconductor device
a b s t r a c t To decrease the amount of contaminant particles generated during semiconductor manufacturing processes, coatings that can prevent erosion on the inner surfaces and parts of the chamber are required. In this study, plasma resistant dense Al2O3 film was formed on a silicon substrate through the atmospheric aerosol spray method (AAS). AAS is a novel powder spray method, which can form a film under atmospheric pressure and low temperature conditions. It can also form a highly functional film on any type of substrate using metal or non metal powders. The film performance can be evaluated by important film properties such as porosity, crystal structure, hardness, surface roughness, electrical characteristics and erosion properties. Among these, erosion property is especially important for chamber protection in dry etching or plasma oriented equipment. Therefore, we analyzed the thickness, porosity, and structure of a film on a sample section using SEM. Furthermore, we compared the surface morphology and erosion rate of the deposited film before and after erosion according to plasma exposure time through AFM and a surface profiler. With the particles in the size range from 3 to 30 μm, Al2O3 film of 1 to a few hundred micrometers in thickness was formed to have relatively low porosity and dense structure. Moreover, the Al2O3 film was formed by AAS good quality as a plasma resistant coating in terms of surface uniformity and plasma erosion resistivity. These results imply that the films formed by AAS can endure fluorine and oxygen plasma etching environment in semiconductor processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With the development of semiconductor technology, processes have required highly pure environment. Foremost, the control of contamination particle generation during the semiconductor manufacturing processes has become a critical issue in the minimization of the pattern dimension. Semiconductor processes such as dry etching, sputter, and CVD are exposed to chemical gas and intense plasma condition which can cause erosions on the wall and parts of the chamber. Such erosions generate contamination particles which demand frequent maintenance of equipment and decrease the yield ratio due their deposition [1,2]. Al2O3 is an interesting coating material, especially for use as a wear resistant layer [3,4]. Because of the highly plasma erosion resistant
⁎ Corresponding author. School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea. Tel.: +82 31 290 7466. E-mail addresses: [email protected] (H. Choi), [email protected] (K. Kim), [email protected] (H. Choi), [email protected] (T. Kim). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.046
characteristics of Al2O3, higher plasma erosion resistant, many studies have been done on the formation of Al2O3 films by use of coating technologies such as the anodizing and the plasma thermal spray method. However, these methods have some disadvantages with respect to the duration time, the surface morphology of the formed film, and the high operating temperature condition. It is well known that micro–nano particles can effectively act as a bridge between bulk materials and molecular structures. They have a high surface area to volume ratio, which provides large contact area and dense inner structure, resulting in better electrical properties. Powder spray method is one of the coating technologies that can take advantage of the characteristics of these particles, having relatively low limitation on the shape of object. Many kinds of powder spray processes are used in various industries, such as the high velocity oxygen fuel spray, arc wire spray, cold spray, and the aerosol deposition. The plasma thermal spray mentioned above is also one of the conventional powder spray methods. These conventional methods have side effects. Thermal spray methods operate at high temperature and high pressure conditions, which deform the substrate and the coated layer has both porosity and non melted
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Fig. 1. Schematic diagram of the atmospheric aerosol spray apparatus.
powder [5]. In cold spraying, a high speed operating gas stream produces accelerated particles, which can move at velocities in the range of 500–1200 m/s in a De Laval type nozzle [6]. This process also deforms the substrate because of the large amount of impaction produced. Moreover, it is specific to metal coating. A relatively recent coating technology is the aerosol deposition method. This method is based on shock loading solidification by use of the pressure difference between two vacuum chambers. This system mainly uses ceramic powder and has high speed deposition rate [7,8]. However, the low pressure condition is not cost effective. In this paper, to form a plasma resistant dense Al2O3 coating layer, atmospheric aerosol spray method (AAS) was used. AAS is a novel powder spray method which operates at atmospheric pressure and low temperature condition. Fig. 2. X-ray diffraction patterns of Al2O3 powder used for atmospheric aerosol spray method.
2. Experimental methods 2.1. Atmospheric aerosol spray method (AAS)
2.2. Sample preparation
AAS is a kind of powder spray method that can be used for both metal and non metal film formation. The AAS system consists of an operating gas, several parallel powder feeders, a furnace, a supersonic nozzle, a three-axis stage, and a deposition chamber as shown in Fig. 1. The film formation process is relatively simple. First, the operating gas is made to flow by controlling the regulator and the nozzle orifice diameter at the same time and flow goes through the tube and powder feeder, which is filled with a specific powder. Particles are suspended in the operating gas and accelerated by the flow. An additional heating step for more conductivity of particles and acceleration is available below the melting point of the powder material by furnace. Finally, accelerated particles are impacted on any kind of substrate. From this impaction, deposited particles form a film. A deposition chamber exists to protect against pollution from the surrounding. Film formation depends on several parameters including the distance between the nozzle exit and substrate, exit gas velocity, spray time, and particle diameter. General experimental condition of each parameter is given in Table 1. The film formation mechanism for AAS method is not clarified yet but plastic deformation for ductile metal particles and destruction by grain boundary for brittle ceramic particles are estimated.
A commercially available Al2O3 powder (purity 99.7%, SigmaAldrich Co., Korea) was used for the AAS process. The particle size of the powder was below 10 μm. Fig. 2 shows the XRD patterns of the powder. The XRD patterns of the powder are described as the rhombohedral phase and silicon wafer (4 in., P doped, b0.005 Ω) was used as the substrate. To prepare a dense Al2O3 film on the substrate by AAS, we performed an experiment to find the optimum condition. Based on the result, samples were prepared to be of square shape. The deposited film area was 10 mm × 50 mm.
Table 1 Experimental parameters of atmospheric aerosol spray method. Parameters
Values
Shape of nozzle orifice Spray distance Operating gas temperature Exit gas velocity Size of particle diameter Substrate materials Substrate thickness
Rectangular, circle 1–15 mm 300–1100 K 100–800 m/s 0.25–45 μm Ceramic, wafer, CCL 0.4–1 mm
2.3. Analysis methods To evaluate the film performance, field emission scanning electron microscope (FE-SEM, JEOL JSM-7401F), surface profiler (Alpha step IQ), and atomic force microscope (AFM, Veeco diNNOVA 840012-711) were used and ICP (Inductively Coupled Plasma) type plasma etching system (Plasmart Co., Mini-Plasma Station) was used to observe the plasma resistant characteristics. Half of the sample was covered with kapton tape to protect from the plasma surrounding. The plasma etching conditions in this paper is summarized in Table 2 and plasma exposure time is controlled from 10 to 30 min. Three same samples for each test were prepared and averaged their results. We
Table 2 Plasma etching conditions. Parameters
Condition
RF power (W) RF power, bias (W) SF6 (sccm) O2 (sccm) Ar (sccm) Pressure (mTorr)
700 200 30 5 10 20
H. Choi et al. / Surface & Coatings Technology 205 (2010) S125–S128
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Fig. 3. SEM micrograph of the cross-section of the Al2O3 film formed on the silicon wafer substrate by atmospheric aerosol spray method at room temperature and pressure.
Fig. 5. Etched depths of silicon wafer and Al2O3 film according to plasma exposure time.
compared the difference of surface morphology, roughness, and film thickness before and after plasma exposure. Silicon wafer was used as the reference and exposed under same conditions as the samples were.
that partial or total melting of the particles does not occur during collision [10]. To clarify the film formation mechanism more accurately, controlled experiments are required. 3.2. Plasma resistance
3. Results and discussion 3.1. Film formation In Fig. 3, the cross-sectional SEM image shows the dense inner structure of the deposited Al2O3 coated film without post processing at high temperature (N300 °C) and vacuum operating condition. Relatively low porosity and strongly bonded interface between the substrate and deposited film are depicted. The thickness of the film is about 4 μm and the deposition rate of AAS for this case is estimated to be 5 μm/min. This deposition rate is of higher value than those of the general methods such as aerosol deposition, and evaporation [9]. It is expected that dense inner structure is able to prevent erosion longer. However the mechanism for AAS method is not carried out yet. The inner structure consisted of grains of size smaller than the 10 μm particles. This means that the particles are crushed on the substrate at impaction. From the published investigations, it may be concluded
Fig. 4 shows the change of the surface morphology according to plasma exposure time. Morphology difference of the substrate is shown in (a) to (c) and the change of surface of the film is shown in (d) to (f). Small areas that were attacked look like craters on both the silicon and Al2O3 coated surfaces due to the dense plasma etching. The damaged area enlarged with the increase of the exposure time. One different result between the silicon wafer and the coated surface was the surface roughness. The initial surface roughness of the Al2O3 coated film measured by the tapping mode AFM with 15 × 15 μm2 scan area and 10 nm tip size was about 250 nm (Ra). However, the roughness decreased with the enlargement of the etched area. It is estimated that a convex surface reacts with plasma easily than a relatively concave surface. So with respect to the surface roughness, a film made by the AAS method has profitability. Fig. 5 shows the eroded depth of the reference substrate and deposited film after plasma exposure in the condition given in the
Fig. 4. SEM micrograph of the surface morphologies of silicon wafer and Al2O3 film (4 μm thickness) after plasma exposure; (a) silicon wafer, 10 min, (b) silicon wafer, 20 min, (c) silicon wafer, 30 min, (d) Al2O3, 10 min, (e) Al2O3, 20 min, and (f) Al2O3, 30 min.
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4. Conclusion
Fig. 6. Effect of the plasma exposure time on the surface roughness of silicon wafer and Al2O3 film.
table. The etched depth of the reference substrate and deposited film increased almost linearly with time up to 30 min. The superiority of the deposited film by AAS is depicted on this graph. The maximum etched depth is 19 μm for the reference silicon wafer and 900 nm for the deposited film. The deposited substrate was etched 20 times slower than non-treated substrate at same condition. There is no standardized test and evaluation method for erosion by plasma surrounding. However etching rate of Al2O3 coated substrate was 29 nm/min. This value is relatively smaller than the etching rate of Al2O3 thin film (over 50 nm/min) by atomic layer deposition from the previous research [11]. Although this research was performed under different plasma etching condition, it could be an evidence of superior plasma resistant characteristics of Al2O3 coating by using AAS. Fig. 6 shows the surface roughness of the samples after plasma exposure under the same condition as in Fig. 5. The surface roughness of the silicon wafer linearly increases with plasma exposure time, but that of the coated substrate shows the opposite. This result agreed with the above SEM analysis result. The reactive surface area between the coated film and the plasma surrounding is reduced with the decrease of the surface roughness. So, a lower etching rate for the coated area due to decreased reaction is expected. In conclusion, the film coated by the AAS method showed excellent plasma resistance.
Plasma resistant dense Al2O3 films were formed on a silicon substrate through the novel powder spray method, atmospheric aerosol spray method (AAS). The film was deposited at atmospheric pressure and low temperature condition by impaction of accelerated particles on the substrate. The thickness of the film for evaluation was about 4 μm and the deposition rate was estimated to be 5 μm/min. This deposition rate was relatively higher than that achieved by general film formation methods. It is found that dense inner structure and strongly bonded interface can be formed by AAS method. To evaluate the plasma resistant characteristics of the deposited film, ICP type plasma chamber system was used. Prepared samples were exposed to severe etching condition. As a result, the areas that were attacked by plasma looked like craters on both the silicon substrate and the coated surface. The damaged area enlarged with the increase of the exposure time. The eroded depth of the sample after plasma exposure to the etched depth of the sample increased almost linearly with exposure time. The Al2O3 coated substrate was 20 times more resistant against plasma surrounding. Moreover, the surface roughness of the silicon substrate linearly increased with plasma exposure time but the coated substrate became uniform. Therefore a lower etching rate for the coated area is expected due to the decreased reaction. The Al2O3 film made by the AAS method is suitable and effective for application in the plasma resistant coating industry. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (2010-0015035). References [1] [2] [3] [4] [5] [6] [7] [8]
S. Geha, R. Carlile, J. O'Hanlon, G. Selwyn, J. Appl. Phys. 72 (1992) 374. D. Samsonov, J. Goree, J. Vac. Sci. 17 (1999) 2835. Z. Yin, S. Tao, X. Zhou, C. Ding, J. Phys. D Appl. Phys. 40 (2007) 7090. Zywitzki, K. Goedicke, H. Morgner, Surf. Coat Technol. 151–152 (2002) 14. P. Fauchais, J. Phys. D Appl. Phys. 37 (2004) R86. H. Assadi, F. Gartner, T. Stoltenhoff, H. Kreye, Acta Mater. 51 (2003) 4379. J. Akedo, J. Am. Ceram. Soc. 89 (2006) 1834. J. Iwasawa, R. Nishimizu, M. Tokita, M. Kiyohara, K. Uematsu, J. Am. Ceram. Soc. 90 (2007) 2327. [9] O. Zaslavskii, Powder Metall. Met. Ceram. 48 (2009) 53. [10] J. Akedo, J. Therm, Spray Technol. 17 (2008) 181. [11] D. Kim, J. Yeo, C. Kim, Thin Solid Films 459 (2004) 122.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Plasma resistant aluminum oxide coatings for semiconductor processing apparatus by atmospheric aerosol spray method Hoomi Choi a, Kwangsu Kim a, Heesung Choi c, Sangwoo Kang d, Juyoung Yun d, Yonghyeon Shin d, Taesung Kim a,b,⁎ a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea Samsung Electro-Mechanics Co., Maetan 3-dong, Yeongtong-gu, Suwon, Republic of Korea d Vacuum Center, Korea Research Institute of Standards and Science, Doryong-dong, Yuseong, Daejeon, Republic of Korea b c
a r t i c l e
i n f o
Available online 28 June 2010 Keywords: Aluminum oxide coating Film formation Atmospheric aerosol spray Plasma resistance Semiconductor device
a b s t r a c t To decrease the amount of contaminant particles generated during semiconductor manufacturing processes, coatings that can prevent erosion on the inner surfaces and parts of the chamber are required. In this study, plasma resistant dense Al2O3 film was formed on a silicon substrate through the atmospheric aerosol spray method (AAS). AAS is a novel powder spray method, which can form a film under atmospheric pressure and low temperature conditions. It can also form a highly functional film on any type of substrate using metal or non metal powders. The film performance can be evaluated by important film properties such as porosity, crystal structure, hardness, surface roughness, electrical characteristics and erosion properties. Among these, erosion property is especially important for chamber protection in dry etching or plasma oriented equipment. Therefore, we analyzed the thickness, porosity, and structure of a film on a sample section using SEM. Furthermore, we compared the surface morphology and erosion rate of the deposited film before and after erosion according to plasma exposure time through AFM and a surface profiler. With the particles in the size range from 3 to 30 μm, Al2O3 film of 1 to a few hundred micrometers in thickness was formed to have relatively low porosity and dense structure. Moreover, the Al2O3 film was formed by AAS good quality as a plasma resistant coating in terms of surface uniformity and plasma erosion resistivity. These results imply that the films formed by AAS can endure fluorine and oxygen plasma etching environment in semiconductor processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With the development of semiconductor technology, processes have required highly pure environment. Foremost, the control of contamination particle generation during the semiconductor manufacturing processes has become a critical issue in the minimization of the pattern dimension. Semiconductor processes such as dry etching, sputter, and CVD are exposed to chemical gas and intense plasma condition which can cause erosions on the wall and parts of the chamber. Such erosions generate contamination particles which demand frequent maintenance of equipment and decrease the yield ratio due their deposition [1,2]. Al2O3 is an interesting coating material, especially for use as a wear resistant layer [3,4]. Because of the highly plasma erosion resistant
⁎ Corresponding author. School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea. Tel.: +82 31 290 7466. E-mail addresses: [email protected] (H. Choi), [email protected] (K. Kim), [email protected] (H. Choi), [email protected] (T. Kim). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.06.046
characteristics of Al2O3, higher plasma erosion resistant, many studies have been done on the formation of Al2O3 films by use of coating technologies such as the anodizing and the plasma thermal spray method. However, these methods have some disadvantages with respect to the duration time, the surface morphology of the formed film, and the high operating temperature condition. It is well known that micro–nano particles can effectively act as a bridge between bulk materials and molecular structures. They have a high surface area to volume ratio, which provides large contact area and dense inner structure, resulting in better electrical properties. Powder spray method is one of the coating technologies that can take advantage of the characteristics of these particles, having relatively low limitation on the shape of object. Many kinds of powder spray processes are used in various industries, such as the high velocity oxygen fuel spray, arc wire spray, cold spray, and the aerosol deposition. The plasma thermal spray mentioned above is also one of the conventional powder spray methods. These conventional methods have side effects. Thermal spray methods operate at high temperature and high pressure conditions, which deform the substrate and the coated layer has both porosity and non melted
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H. Choi et al. / Surface & Coatings Technology 205 (2010) S125–S128
Fig. 1. Schematic diagram of the atmospheric aerosol spray apparatus.
powder [5]. In cold spraying, a high speed operating gas stream produces accelerated particles, which can move at velocities in the range of 500–1200 m/s in a De Laval type nozzle [6]. This process also deforms the substrate because of the large amount of impaction produced. Moreover, it is specific to metal coating. A relatively recent coating technology is the aerosol deposition method. This method is based on shock loading solidification by use of the pressure difference between two vacuum chambers. This system mainly uses ceramic powder and has high speed deposition rate [7,8]. However, the low pressure condition is not cost effective. In this paper, to form a plasma resistant dense Al2O3 coating layer, atmospheric aerosol spray method (AAS) was used. AAS is a novel powder spray method which operates at atmospheric pressure and low temperature condition. Fig. 2. X-ray diffraction patterns of Al2O3 powder used for atmospheric aerosol spray method.
2. Experimental methods 2.1. Atmospheric aerosol spray method (AAS)
2.2. Sample preparation
AAS is a kind of powder spray method that can be used for both metal and non metal film formation. The AAS system consists of an operating gas, several parallel powder feeders, a furnace, a supersonic nozzle, a three-axis stage, and a deposition chamber as shown in Fig. 1. The film formation process is relatively simple. First, the operating gas is made to flow by controlling the regulator and the nozzle orifice diameter at the same time and flow goes through the tube and powder feeder, which is filled with a specific powder. Particles are suspended in the operating gas and accelerated by the flow. An additional heating step for more conductivity of particles and acceleration is available below the melting point of the powder material by furnace. Finally, accelerated particles are impacted on any kind of substrate. From this impaction, deposited particles form a film. A deposition chamber exists to protect against pollution from the surrounding. Film formation depends on several parameters including the distance between the nozzle exit and substrate, exit gas velocity, spray time, and particle diameter. General experimental condition of each parameter is given in Table 1. The film formation mechanism for AAS method is not clarified yet but plastic deformation for ductile metal particles and destruction by grain boundary for brittle ceramic particles are estimated.
A commercially available Al2O3 powder (purity 99.7%, SigmaAldrich Co., Korea) was used for the AAS process. The particle size of the powder was below 10 μm. Fig. 2 shows the XRD patterns of the powder. The XRD patterns of the powder are described as the rhombohedral phase and silicon wafer (4 in., P doped, b0.005 Ω) was used as the substrate. To prepare a dense Al2O3 film on the substrate by AAS, we performed an experiment to find the optimum condition. Based on the result, samples were prepared to be of square shape. The deposited film area was 10 mm × 50 mm.
Table 1 Experimental parameters of atmospheric aerosol spray method. Parameters
Values
Shape of nozzle orifice Spray distance Operating gas temperature Exit gas velocity Size of particle diameter Substrate materials Substrate thickness
Rectangular, circle 1–15 mm 300–1100 K 100–800 m/s 0.25–45 μm Ceramic, wafer, CCL 0.4–1 mm
2.3. Analysis methods To evaluate the film performance, field emission scanning electron microscope (FE-SEM, JEOL JSM-7401F), surface profiler (Alpha step IQ), and atomic force microscope (AFM, Veeco diNNOVA 840012-711) were used and ICP (Inductively Coupled Plasma) type plasma etching system (Plasmart Co., Mini-Plasma Station) was used to observe the plasma resistant characteristics. Half of the sample was covered with kapton tape to protect from the plasma surrounding. The plasma etching conditions in this paper is summarized in Table 2 and plasma exposure time is controlled from 10 to 30 min. Three same samples for each test were prepared and averaged their results. We
Table 2 Plasma etching conditions. Parameters
Condition
RF power (W) RF power, bias (W) SF6 (sccm) O2 (sccm) Ar (sccm) Pressure (mTorr)
700 200 30 5 10 20
H. Choi et al. / Surface & Coatings Technology 205 (2010) S125–S128
S127
Fig. 3. SEM micrograph of the cross-section of the Al2O3 film formed on the silicon wafer substrate by atmospheric aerosol spray method at room temperature and pressure.
Fig. 5. Etched depths of silicon wafer and Al2O3 film according to plasma exposure time.
compared the difference of surface morphology, roughness, and film thickness before and after plasma exposure. Silicon wafer was used as the reference and exposed under same conditions as the samples were.
that partial or total melting of the particles does not occur during collision [10]. To clarify the film formation mechanism more accurately, controlled experiments are required. 3.2. Plasma resistance
3. Results and discussion 3.1. Film formation In Fig. 3, the cross-sectional SEM image shows the dense inner structure of the deposited Al2O3 coated film without post processing at high temperature (N300 °C) and vacuum operating condition. Relatively low porosity and strongly bonded interface between the substrate and deposited film are depicted. The thickness of the film is about 4 μm and the deposition rate of AAS for this case is estimated to be 5 μm/min. This deposition rate is of higher value than those of the general methods such as aerosol deposition, and evaporation [9]. It is expected that dense inner structure is able to prevent erosion longer. However the mechanism for AAS method is not carried out yet. The inner structure consisted of grains of size smaller than the 10 μm particles. This means that the particles are crushed on the substrate at impaction. From the published investigations, it may be concluded
Fig. 4 shows the change of the surface morphology according to plasma exposure time. Morphology difference of the substrate is shown in (a) to (c) and the change of surface of the film is shown in (d) to (f). Small areas that were attacked look like craters on both the silicon and Al2O3 coated surfaces due to the dense plasma etching. The damaged area enlarged with the increase of the exposure time. One different result between the silicon wafer and the coated surface was the surface roughness. The initial surface roughness of the Al2O3 coated film measured by the tapping mode AFM with 15 × 15 μm2 scan area and 10 nm tip size was about 250 nm (Ra). However, the roughness decreased with the enlargement of the etched area. It is estimated that a convex surface reacts with plasma easily than a relatively concave surface. So with respect to the surface roughness, a film made by the AAS method has profitability. Fig. 5 shows the eroded depth of the reference substrate and deposited film after plasma exposure in the condition given in the
Fig. 4. SEM micrograph of the surface morphologies of silicon wafer and Al2O3 film (4 μm thickness) after plasma exposure; (a) silicon wafer, 10 min, (b) silicon wafer, 20 min, (c) silicon wafer, 30 min, (d) Al2O3, 10 min, (e) Al2O3, 20 min, and (f) Al2O3, 30 min.
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4. Conclusion
Fig. 6. Effect of the plasma exposure time on the surface roughness of silicon wafer and Al2O3 film.
table. The etched depth of the reference substrate and deposited film increased almost linearly with time up to 30 min. The superiority of the deposited film by AAS is depicted on this graph. The maximum etched depth is 19 μm for the reference silicon wafer and 900 nm for the deposited film. The deposited substrate was etched 20 times slower than non-treated substrate at same condition. There is no standardized test and evaluation method for erosion by plasma surrounding. However etching rate of Al2O3 coated substrate was 29 nm/min. This value is relatively smaller than the etching rate of Al2O3 thin film (over 50 nm/min) by atomic layer deposition from the previous research [11]. Although this research was performed under different plasma etching condition, it could be an evidence of superior plasma resistant characteristics of Al2O3 coating by using AAS. Fig. 6 shows the surface roughness of the samples after plasma exposure under the same condition as in Fig. 5. The surface roughness of the silicon wafer linearly increases with plasma exposure time, but that of the coated substrate shows the opposite. This result agreed with the above SEM analysis result. The reactive surface area between the coated film and the plasma surrounding is reduced with the decrease of the surface roughness. So, a lower etching rate for the coated area due to decreased reaction is expected. In conclusion, the film coated by the AAS method showed excellent plasma resistance.
Plasma resistant dense Al2O3 films were formed on a silicon substrate through the novel powder spray method, atmospheric aerosol spray method (AAS). The film was deposited at atmospheric pressure and low temperature condition by impaction of accelerated particles on the substrate. The thickness of the film for evaluation was about 4 μm and the deposition rate was estimated to be 5 μm/min. This deposition rate was relatively higher than that achieved by general film formation methods. It is found that dense inner structure and strongly bonded interface can be formed by AAS method. To evaluate the plasma resistant characteristics of the deposited film, ICP type plasma chamber system was used. Prepared samples were exposed to severe etching condition. As a result, the areas that were attacked by plasma looked like craters on both the silicon substrate and the coated surface. The damaged area enlarged with the increase of the exposure time. The eroded depth of the sample after plasma exposure to the etched depth of the sample increased almost linearly with exposure time. The Al2O3 coated substrate was 20 times more resistant against plasma surrounding. Moreover, the surface roughness of the silicon substrate linearly increased with plasma exposure time but the coated substrate became uniform. Therefore a lower etching rate for the coated area is expected due to the decreased reaction. The Al2O3 film made by the AAS method is suitable and effective for application in the plasma resistant coating industry. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (2010-0015035). References [1] [2] [3] [4] [5] [6] [7] [8]
S. Geha, R. Carlile, J. O'Hanlon, G. Selwyn, J. Appl. Phys. 72 (1992) 374. D. Samsonov, J. Goree, J. Vac. Sci. 17 (1999) 2835. Z. Yin, S. Tao, X. Zhou, C. Ding, J. Phys. D Appl. Phys. 40 (2007) 7090. Zywitzki, K. Goedicke, H. Morgner, Surf. Coat Technol. 151–152 (2002) 14. P. Fauchais, J. Phys. D Appl. Phys. 37 (2004) R86. H. Assadi, F. Gartner, T. Stoltenhoff, H. Kreye, Acta Mater. 51 (2003) 4379. J. Akedo, J. Am. Ceram. Soc. 89 (2006) 1834. J. Iwasawa, R. Nishimizu, M. Tokita, M. Kiyohara, K. Uematsu, J. Am. Ceram. Soc. 90 (2007) 2327. [9] O. Zaslavskii, Powder Metall. Met. Ceram. 48 (2009) 53. [10] J. Akedo, J. Therm, Spray Technol. 17 (2008) 181. [11] D. Kim, J. Yeo, C. Kim, Thin Solid Films 459 (2004) 122.