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Crystal growth of high-purity multicrystalline silicon using a unidirectional solidification furnace for solar cells
ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 1572–1576
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth of high-purity multicrystalline silicon using a unidirectional solidification furnace for solar cells B. Gao n, X.J. Chen, S. Nakano, K. Kakimoto Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 December 2009 Received in revised form 20 January 2010 Accepted 25 January 2010 Communicated by P. Rudolph Available online 1 February 2010
An improved furnace was designed to reduce the carbon impurity of multicrystalline silicon at unidirectional solidification process. Global simulations of oxygen and carbon transport in the improved furnace showed that the carbon concentration in the crystal can be reduced to a negligible value in the order of 1014 atom/cm3; simultaneously, the oxygen concentration in the crystal can also be reduced by at least 30%. Therefore, the present design can markedly reduce the back transfer of CO from graphite components of the furnace. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Impurities A1. Computer simulation B2. Semiconducting silicon B3. Solar cells
1. Introduction Multicrystalline silicon has now become the main material in the photovoltaic market because of its low production cost and because of the relative high conversion efficiency of solar cells made from this material. The unidirectional solidification method is a cost-effective technique for large-scale production of multicrystalline silicon material. Similar to the Czochralski method, the unidirectional solidification is connected with transport of impurities [1]. Carbon is one of the major impurities in multicrystalline silicon. If the carbon concentration exceeds 1 1016 atom/cm3, it will markedly influence the precipitation of oxygen during thermal annealing of crystals and during device processing of the wafers cut from these crystals [2–8]. Oxygen precipitation is known to act as intrinsic gettering sites for impurities and to affect the mechanical strength of the wafer [2,9,10]. When the concentration of carbon exceeds its solubility limit in silicon, it will precipitate to form silicon carbide (SiC) particles, which can cause severe ohmic shunts in solar cells and result in nucleation of new grains in silicon ingots [11]. Carbon, oxygen and SiC particles in a solidified silicon ingot can cause significant deterioration of the conversion efficiency of solar cells. Therefore, effective control of carbon concentration in a crystal is
n
Corresponding author. Tel.: + 81 92 583 7744; fax: + 81 92 583 7743. E-mail addresses: [email protected], [email protected] (B. Gao). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.01.034
required for the production of a high-quality crystal. In this study, an improved unidirectional solidification furnace was suggested to produce crystals with lower carbon concentration. The global simulation of coupled oxygen and carbon transports within this furnace was used to validate the feasibility of its improved design.
2. Mechanism of carbon incorporation The basic configurations of unidirectional solidification furnaces are well known [12–15]. The graphite components include the crucible, heat shields, heaters and pedestal. They are the main sources of carbon elements in a grown crystal. The basic processes of carbon incorporation into a crystal are shown in Fig. 1. First, the silica crucible (SiO2) is dissolved and the oxygen and silicon atoms penetrate into the melt. Secondly the dissolved oxygen atoms are transported to the gas/melt interface and evaporate as SiO gas. Then the SiO gas is carried away by the argon gas flow and reacts with all of the graphite components to produce gas-phase CO. After that the resultant CO is transported back to the melt surface by diffusion or convection and dissolves within the melt. Finally, the C and O atoms are segregated into the crystal [2]. For fully understanding the mechanism of carbon incorporation, a global simulation of coupled oxygen and carbon transport within the unidirectional solidification furnace was carried out. The argon gas flow above the melt under the conditions of inlet flow rate of 0.8 l/min, inlet static temperature of 350 K and outlet
ARTICLE IN PRESS B. Gao et al. / Journal of Crystal Growth 312 (2010) 1572–1576
1573
11
(3) CO
SiO
gas tube
16
heater
12
3
4
h (4)
SiO (2)
C
O
CO
crucible
(5) C
O
2
Si (1)
10
13
1
silicon melt
5
SiO2
9
(5) O
15
crystal
Fig. 1. Basic processes of carbon incorporation into a crystal.
17
14
A
6
8
0.0089 m/s B
7
1
melt
2
crystal
3-4
crucible
5-6
pedestal
7-11
heat shield
12-15
heater
16
gas inlet
17
gas outlet
Fig. 3. Improved design of a unidirectional solidification furnace.
0.00038 m/s
3. Design optimization and comparison of results 3.1. Simulation
melt
The gas tube is extended to form a cover as shown in Fig. 3. The cover material can be made of tungsten or molybdenum, both having high melting points. The distance h between the cover and the melt surface is variable. Global simulation of coupled oxygen and carbon transports was carried out for h= 28, 49 and105 mm. The third case (h =105 mm) corresponds to the old furnace that has no cover. All of the calculations show the same inlet and outlet conditions. 3.2. Carbon concentration distribution
Fig. 2. Flow above the melt.
static pressure of 0.1 atm is shown in Fig. 2. There is a recirculation flow above the melt surface and a main convection flow passing through the top of the crucible. The back-diffusion of CO from the outside of crucible into the melt is hindered by the top convection flow. Therefore, if the top convection flow is strengthened, in addition to elimination of the recirculation flow, CO transportation into the melt might be dramatically reduced. It can be seen from Fig. 2 that CO is transported into the melt mainly by diffusion flux that passes through the area AB. If the diffusion area is markedly reduced, CO flux into the melt can be significantly reduced. An optimized design according to the above analysis is given in the following section.
The carbon concentration distributions (CO in gas and C atoms in melt) for the old furnace can be referred to the paper [16]. The basic order of CO concentration in gas is 10 9 mol/cm3. The carbon concentration in the melt is 1018 atom/cm3, which is close Here, we use the relation to 10 6 mol/cm3. 1 mol = 0.622 1024.Thus, for the old furnace, the carbon concentration in the melt is much larger than that in the gas. For two new furnace cases of different h values, the C distributions are shown in Fig. 4(a), (b). The carbon concentrations in the melt show almost the same order as those in the gas. In the cases of distances of 28 and 49 mm and both the basic orders of carbon concentrations in the melt are 10 9 mol/cm3. Thus, smaller carbon concentrations are obtained in the melt compared to that in the old case (10 6 mol/cm3) [16]. If the segregation relation for carbon atoms is considered, the carbon concentration for a chosen position in the crystal is given
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10 10 12 9 7 5
4
Level C (mol/cm3) 20 2.47x10-09 19 2.35x10-09 18 2.22x10-09 17 2.10x10-09 16 1.98x10-09 15 1.85x10-09 14 1.73x10-09 13 1.61x10-09 12 1.48x10-09 11 1.36x10-09 10 1.24x10-09 9 1.11x10-09 8 9.88x10-10 7 8.65x10-10 6 7.41x10-10 5 6.18x10-10 4 4.94x10-10 3 3.71x10-10 2 2.47x10-10 1 1.24x10-10
Level C (mol/cm3)
5 5 12
5
3
2
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
6.14x10-09 5.83x10-09 5.52x10-09 4.91x10-09 4.30x10-09 3.99x10-09 3.68x10-09 3.38x10-09 3.07x10-09 2.76x10-09 2.45x10-09 2.15x10-09 1.84x10-09 1.53x10-09 1.23x10-09 1.09x10-09 9.21x10-10 6.14x10-10 4.62x10-10 3.07x10-10
Fig. 4. (a) Carbon concentration distribution (CO in gas and C in melt) for the improved furnace at h= 28 mm.(b) Carbon concentration distribution (CO in gas and C in melt) for the improved furnace at h=49 mm.
increases, the strength of the convection flow decreases and thus the gradient region of CO is shifted towards the melt surface. Therefore, strengthening the convection gas flow above the melt surface is a good method for reducing carbon flux into the melt.
carbon concentration (atom/cm3)
1018
improved furnace
1017
old furnace 3.3. Oxygen concentration distribution
1016
1015
1014
1013
0
20
40 60 80 distance h (mm)
100
120
Fig. 5. Comparison of carbon concentrations in the crystal for the improved and old furnaces.
in Fig. 5. The basic order of carbon concentration in the crystal for the old furnace was 1017 atom/cm3. Compared to that, the basic order for the improved furnace is 1013–1014 atom/cm3, which is of a negligible quantity in crystal growth. Fig. 5 shows that the carbon concentration decreases as the distance h becomes smaller. This distance has a limiting value which is specified by the expansion height of the Si-melt solidification process. For understanding the decrease of the carbon concentration, carbon concentrations ranging from 1.00 10 17 to 1.50 10 12 mol/cm3 for the new furnaces are shown in Fig. 6. All of the carbon concentrations near the melt surface are the same. However, the gradients of carbon concentration are quite different. The case of a large distance corresponds to a large gradient of carbon concentration above the melt surface and thus the flux of carbon into the melt becomes strong. The distance between the cover and the melt surface affects the strength of the convection flow. As the distance
The oxygen distributions in the melt for the old furnace can be referred to the paper [16]. The oxygen distribution for the improved case of a distance of 49 mm is presented in Fig. 7. The oxygen concentration in most of the melt volume for the improved furnace is less than that for the old furnace. First the oxygen originates from the wall of the crucible and is then transported to the melt surface by convection flow. At the melt surface, the oxygen evaporates as SiO gas. In the improved furnace, however, more SiO is taken away because of stronger convection flow above the melt surface. A comparison of oxygen concentrations for a chosen position in the crystal is shown in Fig. 8. In the case of h=49 mm, the oxygen concentration is reduced by 30% compared to that in the old furnace. It is understandable that a smaller distance corresponds to a stronger convection flow intensity, which brings away more oxygen from the melt surface. Therefore, strengthening the convection flow at the melt surface proves to be also favorable for more oxygen transport away from the melt.
4. Some discussion The carbon concentration can be reduced with the gas shield design. However, the gas shield design will affect the heat transfer between the heaters and silicon melt. It is possible that the temperature distribution is no longer sufficient for crystal growth. A higher temperature of heaters will be needed for the same growth rate without the gas shield. Higher temperature of heaters can cause an increase in the gas reaction outside the crucible. Therefore, the carbon concentration in gas might be higher than that in the old case. However, considering the strong resistance of CO back-diffusion of our design, it might have only a small effect on carbon concentration in the crystal.
ARTICLE IN PRESS B. Gao et al. / Journal of Crystal Growth 312 (2010) 1572–1576
1575
Level CO (mol/cm3)
2
Level CO (mol/cm3)
1 3 20 13 2
8 9
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
1.50x10-12 1.42x10-12 1.34x10-12 1.26x10-12 1.18x10-12 1.11x10-12 1.03x10-12 9.47x10-13 8.68x10-13 7.89x10-13 7.11x10-13 6.32x10-13 5.53x10-13 4.74x10-13 3.95x10-13 3.16x10-13 2.37x10-13 1.58x10-13 7.90x10-14 1.00x10-17
Fig. 6. C concentration distribution (CO) ranging from 1 10 17 to 1.50 10 12 mol/cm3 above the melt for the improved furnace at (a) h= 28 mm, (b) h= 49 mm.
14
2
9
4
4
11
1 8
Level O(atom/cm3)
1.50x10-12 1.42x10-12 1.34x10-12 1.26x10-12 1.18x10-12 1.11x10-12 1.03x10-12 9.47x10-13 8.68x10-13 7.89x10-13 7.11x10-13 6.32x10-13 5.53x10-13 4.74x10-13 3.95x10-13 3.16x10-13 2.37x10-13 1.58x10-13 7.90x10-14 1.00x10-17
14
3
7
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
9.00x10+17 8.50x10+17 8.00x10+17 7.50x10+17 7.00x10+17 6.50x10+17 6.00x10+17 5.50x10+17 5.00x10+17 4.50x10+17 4.00x10+17 3.50x10+17 3.00x10+17 2.50x10+17 2.00x10+17 1.50x10+17 1.00x10+17 5.00x10+16
Fig. 7. Oxygen concentration distribution in the melt for the improved furnace at h=49 mm.
5 oxygenc oncentration, 1017 (atom/cm3)
20
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
improved furnace
4.5
old furnace 4
3.5
3
2.5
20
40
60 80 distance h (mm)
100
120
Fig. 8. Comparison of oxygen concentrations in the crystal for the improved and old furnaces.
The choice of material of the gas shield is important. Some depositions such as SiC particles are possible if the gas SiO reacts with the gas shield. A drop of deposited material can affect the quality of silicon growth. Therefore, the material of the gas shield should have chemical inertia and a high melting point so that the gas SiO has almost no reaction with it.
present improvement enables the production of a high-purity multicrystalline silicon crystal in a unidirectional solidification furnace.
References 5. Conclusions An improved unidirectional solidification furnace with modified gas tube was designed. Global simulations of coupled oxygen and carbon transport show that the carbon concentration in the crystal can be reduced from 1017 to 1014 atom/cm3. The oxygen concentration can also be reduced by at least 30%. Therefore, the
[1] H. Matsuo, R.B. Ganesh, S. Nakano, L.J. Liu, Y. Kangawa, K. Arafune, Y. Ohshita, M. Yamaguchi, K. Kakimoto, J. Cryst. Growth 310 (2008) 2204. [2] D.E. Bornside, R.A. Brown, J. Electrochem. Soc. 142 (1995) 2790. [3] T. Fukuda, M. Koizuka, A. Ohsawa, J. Electrochem. Soc. 141 (1994) 2216. [4] Q. Sun, K.H. Yao, J. Lagowski, H.C. Gatos, J. Appl. Phys. 67 (1990) 4313. [5] M. Ogino, Appl. Phys. Lett. 41 (1982) 847. [6] B.O. Kolbesen, Aggregation phenomena of point defects in silicon, in: E. Sirtl, J. Goorissen (Eds.), The Electrochemical Society Proceedings Series, vol. 83–4, , 1983, pp. 155–175.
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[7] U. Goesele, Oxygen, carbon, hydrogen and nitrogen in crystalline silicon, in: J.C. Mikkelsen Jr., S.P. Peaton, J.W. Corbett, S.J. Pennycook (Eds.), Mater. Res. Soc. Symp. Proc., MRS, Pittsburgh, PA, 1986, p. 419. [8] F. Shimura, in: Semiconductor Silicon Crystal Technology, Academic Press, New York, 1989 148. [9] T. Abe, Silicon materials, in: N.G. Einspruch, H. Huff (Eds.), VLSI Electronics Microstructure Science Series, vol. 12, Academic Press, Inc., New York, 1985, p. 3. [10] U. Goesele, T.Y. Tan, Appl. Phys. A 28 (1982) 79.
[11] J. Bauer, O. Breitenstein, J.P. Rakotoniaina, in: Proceedings of 21st EUPVSEC, September, Dresden, Germany, 2006, p. 1115. [12] L.J. Liu, S. Nakano, K. Kakimoto, J. Cryst. Growth 303 (2007) 165. [13] H. Miyazawa, L.J. Liu, S. Hisamatsu, K. Kakimoto, J. Cryst. Growth 310 (2008) 1034. [14] X.J. Chen, S. Nakano, L.J. Liu, K. Kakimoto, J. Cryst. Growth 310 (2008) 4330. [15] S. Nakano, L.J. Liu, X.J. Chen, H. Matsuo, K. Kakimoto, J. Cryst. Growth 311 (2009) 1051. [16] B. Gao, S. Nakano, K. Kakimoto, J. Electrochem. Soc. 157 (2010) H153.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystal growth of high-purity multicrystalline silicon using a unidirectional solidification furnace for solar cells B. Gao n, X.J. Chen, S. Nakano, K. Kakimoto Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 December 2009 Received in revised form 20 January 2010 Accepted 25 January 2010 Communicated by P. Rudolph Available online 1 February 2010
An improved furnace was designed to reduce the carbon impurity of multicrystalline silicon at unidirectional solidification process. Global simulations of oxygen and carbon transport in the improved furnace showed that the carbon concentration in the crystal can be reduced to a negligible value in the order of 1014 atom/cm3; simultaneously, the oxygen concentration in the crystal can also be reduced by at least 30%. Therefore, the present design can markedly reduce the back transfer of CO from graphite components of the furnace. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Impurities A1. Computer simulation B2. Semiconducting silicon B3. Solar cells
1. Introduction Multicrystalline silicon has now become the main material in the photovoltaic market because of its low production cost and because of the relative high conversion efficiency of solar cells made from this material. The unidirectional solidification method is a cost-effective technique for large-scale production of multicrystalline silicon material. Similar to the Czochralski method, the unidirectional solidification is connected with transport of impurities [1]. Carbon is one of the major impurities in multicrystalline silicon. If the carbon concentration exceeds 1 1016 atom/cm3, it will markedly influence the precipitation of oxygen during thermal annealing of crystals and during device processing of the wafers cut from these crystals [2–8]. Oxygen precipitation is known to act as intrinsic gettering sites for impurities and to affect the mechanical strength of the wafer [2,9,10]. When the concentration of carbon exceeds its solubility limit in silicon, it will precipitate to form silicon carbide (SiC) particles, which can cause severe ohmic shunts in solar cells and result in nucleation of new grains in silicon ingots [11]. Carbon, oxygen and SiC particles in a solidified silicon ingot can cause significant deterioration of the conversion efficiency of solar cells. Therefore, effective control of carbon concentration in a crystal is
n
Corresponding author. Tel.: + 81 92 583 7744; fax: + 81 92 583 7743. E-mail addresses: [email protected], [email protected] (B. Gao). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.01.034
required for the production of a high-quality crystal. In this study, an improved unidirectional solidification furnace was suggested to produce crystals with lower carbon concentration. The global simulation of coupled oxygen and carbon transports within this furnace was used to validate the feasibility of its improved design.
2. Mechanism of carbon incorporation The basic configurations of unidirectional solidification furnaces are well known [12–15]. The graphite components include the crucible, heat shields, heaters and pedestal. They are the main sources of carbon elements in a grown crystal. The basic processes of carbon incorporation into a crystal are shown in Fig. 1. First, the silica crucible (SiO2) is dissolved and the oxygen and silicon atoms penetrate into the melt. Secondly the dissolved oxygen atoms are transported to the gas/melt interface and evaporate as SiO gas. Then the SiO gas is carried away by the argon gas flow and reacts with all of the graphite components to produce gas-phase CO. After that the resultant CO is transported back to the melt surface by diffusion or convection and dissolves within the melt. Finally, the C and O atoms are segregated into the crystal [2]. For fully understanding the mechanism of carbon incorporation, a global simulation of coupled oxygen and carbon transport within the unidirectional solidification furnace was carried out. The argon gas flow above the melt under the conditions of inlet flow rate of 0.8 l/min, inlet static temperature of 350 K and outlet
ARTICLE IN PRESS B. Gao et al. / Journal of Crystal Growth 312 (2010) 1572–1576
1573
11
(3) CO
SiO
gas tube
16
heater
12
3
4
h (4)
SiO (2)
C
O
CO
crucible
(5) C
O
2
Si (1)
10
13
1
silicon melt
5
SiO2
9
(5) O
15
crystal
Fig. 1. Basic processes of carbon incorporation into a crystal.
17
14
A
6
8
0.0089 m/s B
7
1
melt
2
crystal
3-4
crucible
5-6
pedestal
7-11
heat shield
12-15
heater
16
gas inlet
17
gas outlet
Fig. 3. Improved design of a unidirectional solidification furnace.
0.00038 m/s
3. Design optimization and comparison of results 3.1. Simulation
melt
The gas tube is extended to form a cover as shown in Fig. 3. The cover material can be made of tungsten or molybdenum, both having high melting points. The distance h between the cover and the melt surface is variable. Global simulation of coupled oxygen and carbon transports was carried out for h= 28, 49 and105 mm. The third case (h =105 mm) corresponds to the old furnace that has no cover. All of the calculations show the same inlet and outlet conditions. 3.2. Carbon concentration distribution
Fig. 2. Flow above the melt.
static pressure of 0.1 atm is shown in Fig. 2. There is a recirculation flow above the melt surface and a main convection flow passing through the top of the crucible. The back-diffusion of CO from the outside of crucible into the melt is hindered by the top convection flow. Therefore, if the top convection flow is strengthened, in addition to elimination of the recirculation flow, CO transportation into the melt might be dramatically reduced. It can be seen from Fig. 2 that CO is transported into the melt mainly by diffusion flux that passes through the area AB. If the diffusion area is markedly reduced, CO flux into the melt can be significantly reduced. An optimized design according to the above analysis is given in the following section.
The carbon concentration distributions (CO in gas and C atoms in melt) for the old furnace can be referred to the paper [16]. The basic order of CO concentration in gas is 10 9 mol/cm3. The carbon concentration in the melt is 1018 atom/cm3, which is close Here, we use the relation to 10 6 mol/cm3. 1 mol = 0.622 1024.Thus, for the old furnace, the carbon concentration in the melt is much larger than that in the gas. For two new furnace cases of different h values, the C distributions are shown in Fig. 4(a), (b). The carbon concentrations in the melt show almost the same order as those in the gas. In the cases of distances of 28 and 49 mm and both the basic orders of carbon concentrations in the melt are 10 9 mol/cm3. Thus, smaller carbon concentrations are obtained in the melt compared to that in the old case (10 6 mol/cm3) [16]. If the segregation relation for carbon atoms is considered, the carbon concentration for a chosen position in the crystal is given
ARTICLE IN PRESS 1574
B. Gao et al. / Journal of Crystal Growth 312 (2010) 1572–1576
10 10 12 9 7 5
4
Level C (mol/cm3) 20 2.47x10-09 19 2.35x10-09 18 2.22x10-09 17 2.10x10-09 16 1.98x10-09 15 1.85x10-09 14 1.73x10-09 13 1.61x10-09 12 1.48x10-09 11 1.36x10-09 10 1.24x10-09 9 1.11x10-09 8 9.88x10-10 7 8.65x10-10 6 7.41x10-10 5 6.18x10-10 4 4.94x10-10 3 3.71x10-10 2 2.47x10-10 1 1.24x10-10
Level C (mol/cm3)
5 5 12
5
3
2
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
6.14x10-09 5.83x10-09 5.52x10-09 4.91x10-09 4.30x10-09 3.99x10-09 3.68x10-09 3.38x10-09 3.07x10-09 2.76x10-09 2.45x10-09 2.15x10-09 1.84x10-09 1.53x10-09 1.23x10-09 1.09x10-09 9.21x10-10 6.14x10-10 4.62x10-10 3.07x10-10
Fig. 4. (a) Carbon concentration distribution (CO in gas and C in melt) for the improved furnace at h= 28 mm.(b) Carbon concentration distribution (CO in gas and C in melt) for the improved furnace at h=49 mm.
increases, the strength of the convection flow decreases and thus the gradient region of CO is shifted towards the melt surface. Therefore, strengthening the convection gas flow above the melt surface is a good method for reducing carbon flux into the melt.
carbon concentration (atom/cm3)
1018
improved furnace
1017
old furnace 3.3. Oxygen concentration distribution
1016
1015
1014
1013
0
20
40 60 80 distance h (mm)
100
120
Fig. 5. Comparison of carbon concentrations in the crystal for the improved and old furnaces.
in Fig. 5. The basic order of carbon concentration in the crystal for the old furnace was 1017 atom/cm3. Compared to that, the basic order for the improved furnace is 1013–1014 atom/cm3, which is of a negligible quantity in crystal growth. Fig. 5 shows that the carbon concentration decreases as the distance h becomes smaller. This distance has a limiting value which is specified by the expansion height of the Si-melt solidification process. For understanding the decrease of the carbon concentration, carbon concentrations ranging from 1.00 10 17 to 1.50 10 12 mol/cm3 for the new furnaces are shown in Fig. 6. All of the carbon concentrations near the melt surface are the same. However, the gradients of carbon concentration are quite different. The case of a large distance corresponds to a large gradient of carbon concentration above the melt surface and thus the flux of carbon into the melt becomes strong. The distance between the cover and the melt surface affects the strength of the convection flow. As the distance
The oxygen distributions in the melt for the old furnace can be referred to the paper [16]. The oxygen distribution for the improved case of a distance of 49 mm is presented in Fig. 7. The oxygen concentration in most of the melt volume for the improved furnace is less than that for the old furnace. First the oxygen originates from the wall of the crucible and is then transported to the melt surface by convection flow. At the melt surface, the oxygen evaporates as SiO gas. In the improved furnace, however, more SiO is taken away because of stronger convection flow above the melt surface. A comparison of oxygen concentrations for a chosen position in the crystal is shown in Fig. 8. In the case of h=49 mm, the oxygen concentration is reduced by 30% compared to that in the old furnace. It is understandable that a smaller distance corresponds to a stronger convection flow intensity, which brings away more oxygen from the melt surface. Therefore, strengthening the convection flow at the melt surface proves to be also favorable for more oxygen transport away from the melt.
4. Some discussion The carbon concentration can be reduced with the gas shield design. However, the gas shield design will affect the heat transfer between the heaters and silicon melt. It is possible that the temperature distribution is no longer sufficient for crystal growth. A higher temperature of heaters will be needed for the same growth rate without the gas shield. Higher temperature of heaters can cause an increase in the gas reaction outside the crucible. Therefore, the carbon concentration in gas might be higher than that in the old case. However, considering the strong resistance of CO back-diffusion of our design, it might have only a small effect on carbon concentration in the crystal.
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1575
Level CO (mol/cm3)
2
Level CO (mol/cm3)
1 3 20 13 2
8 9
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
1.50x10-12 1.42x10-12 1.34x10-12 1.26x10-12 1.18x10-12 1.11x10-12 1.03x10-12 9.47x10-13 8.68x10-13 7.89x10-13 7.11x10-13 6.32x10-13 5.53x10-13 4.74x10-13 3.95x10-13 3.16x10-13 2.37x10-13 1.58x10-13 7.90x10-14 1.00x10-17
Fig. 6. C concentration distribution (CO) ranging from 1 10 17 to 1.50 10 12 mol/cm3 above the melt for the improved furnace at (a) h= 28 mm, (b) h= 49 mm.
14
2
9
4
4
11
1 8
Level O(atom/cm3)
1.50x10-12 1.42x10-12 1.34x10-12 1.26x10-12 1.18x10-12 1.11x10-12 1.03x10-12 9.47x10-13 8.68x10-13 7.89x10-13 7.11x10-13 6.32x10-13 5.53x10-13 4.74x10-13 3.95x10-13 3.16x10-13 2.37x10-13 1.58x10-13 7.90x10-14 1.00x10-17
14
3
7
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
9.00x10+17 8.50x10+17 8.00x10+17 7.50x10+17 7.00x10+17 6.50x10+17 6.00x10+17 5.50x10+17 5.00x10+17 4.50x10+17 4.00x10+17 3.50x10+17 3.00x10+17 2.50x10+17 2.00x10+17 1.50x10+17 1.00x10+17 5.00x10+16
Fig. 7. Oxygen concentration distribution in the melt for the improved furnace at h=49 mm.
5 oxygenc oncentration, 1017 (atom/cm3)
20
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
improved furnace
4.5
old furnace 4
3.5
3
2.5
20
40
60 80 distance h (mm)
100
120
Fig. 8. Comparison of oxygen concentrations in the crystal for the improved and old furnaces.
The choice of material of the gas shield is important. Some depositions such as SiC particles are possible if the gas SiO reacts with the gas shield. A drop of deposited material can affect the quality of silicon growth. Therefore, the material of the gas shield should have chemical inertia and a high melting point so that the gas SiO has almost no reaction with it.
present improvement enables the production of a high-purity multicrystalline silicon crystal in a unidirectional solidification furnace.
References 5. Conclusions An improved unidirectional solidification furnace with modified gas tube was designed. Global simulations of coupled oxygen and carbon transport show that the carbon concentration in the crystal can be reduced from 1017 to 1014 atom/cm3. The oxygen concentration can also be reduced by at least 30%. Therefore, the
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