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Purification of metallurgical-grade silicon by slagging and impurity redistribution
Solar Cells, 10 (1983) 109 - 118
109
PURIFICATION OF METALLURGICAL-GRADE SILICON BY SLAGGING AND IMPURITY REDISTRIBUTION
H. M. LIAW and F. SECCO D'ARAGONA
Semiconductor Research and Development Laboratory, Motorola Inc.-Semiconductor Products Sector, 5005 East McDowell Road, Phoenix, A Z 85008 (U.S.A.) (Received March 17, 1983; accepted April 5, 1983)
Summary The purification of metallurgical-grade silicon (MG-Si) was carried out by slagging and impurity redistribution through repeated melting and pulling. A Czochralski crystal puller was used for performing both functions. The slags consisting of CaO-SiO2 and CaO-MgO-SiO2 were found to be effective for extracting aluminum from the MG-Si melts. The repeated melting and pulling method was effective for the removal of other metallic impurities. When slagging and multiple pulling were combined, the thricepulled ingots contained an impurity concentration equivalent to that of semiconductor-grade silicon with the exception of boron and phosphorus.
1. Introduction The direct purification of metallurgical-grade silicon (MG-Si) was carried out as early as the 1950s and 1960s [1 -3]. Reactive gases such as halogen or hydrogen chloride were used to treat the molten silicon. This technique failed to produce the high purity silicon required by the semiconductor industry. The need for low cost silicon for solar cell applications has regenerated the interest in this topic. Chu e t al. [4] have demonstrated a reduction in the concentration of aluminum in MG-Si by the treatment of the melt with a halogen or hydrogen halide. Hunt e t al. [5] have developed purification techniques using reactive gas treatment followed by unidirectional solidification. Kotval and Strock [6] used several steps for the purification of MG-Si: (1) dissolution and recrystallization from an aluminum solution; (2) the leaching of crystallized silicon platelets with HC1; (3) the purification of silicon platelets by slagging and unidirectional solidification. Both Hunt et al. and Kotval and Strock used the Bridgman technique for the unidirectional solidification. The purpose of this paper is to report the use of the Czochralski crystal-puUing technique to perform slagging and unidirectional solidification. 0379-6787/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
110
2. Purification procedures A commercial silicon crystal puller modified with the attachment of a gate valve was used for this work. MG-Si was loaded into the quartz crucible in the crystal puller. On melting of the MG-Si charge the insoluble impurities would float on the melt surface. This prevented pulling an ingot with large grain sizes. The slagging process was then applied. The slagging process adds insoluble materials into the silicon melt. The purpose of the slagging is twofold: (1) to dissolve the insoluble impurities which float on the melt and (2) to extract soluble impurities from the silicon melt. A thorough stirring was necessary to ensure good mixing between the slag and silicon phases. The slag was then manipulated to adhere to the crucible wall. This created a clean surface at the center region of the melt. A single-crystal seed was dipped and an ingot was pulled with the normal Czochralski crystal-growing procedures.
3. Slagging study The slagging is based on the principle of liquid-liquid extraction. This technique has been used in the steel industry. The slag used for the extraction of impurities from the molten silicon should fulfill the following criteria. (1) It should be immiscible with molten silicon. (2) Its melting point should be at a temperature approximately equal to the melting point of silicon. (3) It should not contaminate the silicon melt. (4) It should have a greater chemical affinity to impurities than to silicon. The first criterion can be fulfilled by certain metallic oxides. Since the transition metals and heavy metals contribute to the deep level traps in silicon, their oxides have been excluded from consideration. Alkali metals contribute to mobile ions in silicon; their oxides have also been excluded. Alkaline earth oxides were chosen for investigation. Since a quartz (SiO2) crucible is used for containing the silicon melt, the oxide added into the crucible inevitably forms mixed oxides with the quartz. When the binary phase diagrams [7] of SiO2-alkaline earth oxide systems were considered, it was found that CaO-SiO2 has t w o eutectic points near the melting point of silicon. One occurs at 46 at.% SiO2 with a eutectic temperature at 1460 °C, and the other at 63 at.% SiO2 with a eutectic temperature at 1436 °C. Three ternary oxide systems which have eutectic points in the range 1400 - 1500 °C were also chosen. They are SiO2-CaO-MgO, SiO2-BaO-CaO and SiO2-BaO-MgO. In all cases the phase diagrams show that the eutectic temperatures occur at the compositions for which the t w o alkaline earth oxides are in approximately the same proportion. It was not known whether the selected oxide systems could fulfill criteria (3) and (4). Experimental work was carried o u t to test their feasibility and effectiveness for the purification of molten silicon.
111
3.1. The contamination test Semiconductor-grade silicon (SG-Si) was used as a testing vehicle to study whether the silicon melt would be contaminated b y adding a slag. The silicon melts were d o p e d with b o r o n so that the pulled ingots would yield a resistivity of 2 ~ cm at the seed end of crystals. Any deviation of the resistivity from this value indicates the contamination of silicon by the slag. Table 1 lists the results of the experiments. As shown in this table the alkaline earth carbonates were added instead of their oxides. The carbonates decomposed quickly into oxides and carbon dioxide which evaporated from the melt. The results of the experiments can be summarized as follows. (1) CaO-SiO2 slag does not contaminate silicon melts. The silicon melt surface remains clean after the addition of this slag. Single crystals can be grown from the silicon melt treated with this slag. (2) CaO-BaO-SiO2 slag gives n-type contamination. Single crystals cannot be grown from the silicon melts treated with this slag. (3) BaO-MgO-SiO2 slag gives p-type contamination. Single crystals cannot be grown from the silicon melts treated with this slag. (4) CaO-MgO-SiO2 slag does not contaminate silicon melts. However, the ingot pulled from the silicon melts treated with this slag was polycrystalline in structure. TABLE 1 Slag constituents, type and resistivity of ingots pulled from slag-treated semiconductorgrade silicon melts
Slag constituent
Type of ingots
Resistivity of ingots
Remarks
(~ cm) (1) None
p
2.1
Single crystal
(2) 1% CaCO3
p
1.99
Single crystal
(3) 1% CaCO3, 0.5% BaCO3
p at the seed end n at the tang end
2.9 1.9
Polycrystal
(4) 1% MgCO3, 1% BaC03
p
0.22
Polycrystal
(5) 15% MgCO3, 1% CaC03
p
1.71
Polycrystal
3.2. Purification test Both CaO--SiO2 and CaO-MgO-SiO2 which do not contaminate silicon melts were used for the purification test. MG-Si was used for this study. The effectiveness of purification was evaluated by analyzing the impurity concentration in the pulled ingots as well as in the slags. Table 2 lists the results of treatment with CaO-SiO2 slag. This table shows that the aluminum concentration in silicon ingots decreases with the increase in the a m o u n t of CaCO3 added. The results agree with the electrical measurements of the silicon
112 TABLE 2 Concentration of impurities a in metallurgical-grade amounts of CaCO 3 Element
Impurity concentration (wt.ppm) for following amounts o f CaC03 added to the Si melt
silicon melts treated by various
Impurity concentration (wt.ppm) in the slag after slagging with 1% (CaO Si02) in the Si mell
0.12%
0.88%
1.0%
Fe Mn Cr Ca Sr Mg Ni V Ti Cu Zr
> 1000 330 100 100 1 1 100 < 10 330 33 100
> 1000 330 100 33 3 <1 100 33 330 33 100
> 1000 100 33 33 <1 <1 10 10 33 < 10 10
> 1000 33 33 > 1000 100 100 10 < 10 330 < 10 10
AI B Co
I000 <3 10
330 <3 < 10
i00 <3 < 10
> i000 <3 < 10
aAnalyzed using emission spectroscopy. TABLE 3 Resistivity of silicon pulled from the melts treated with various amounts of CaCO3 A m o u n t o f CaC03 (%)
Resistivity ( ~ cm)
0.12 0.88 1.0 2.0
0.069 0.078 0.082 0.105
ingots, w h i c h s h o w t h a t t h e resistivity o f silicon ingots increased with t h e increase in t h e a m o u n t o f CaCOs a d d e d t o t h e m e l t (Table 3). The analysis o f C a O - S i O ~ a f t e r slagging s h o w s t h a t t h e a l u m i n u m c o n c e n t r a t i o n increased t o greater t h a n 1 0 0 0 w t . p p m . F r o m these results it can be c o n c l u d e d t h a t C a O - S i O 2 slag is effective f o r t h e r e m o v a l o f a l u m i n u m f r o m t h e MG-Si melts. H o w e v e r , little e f f e c t was seen o n t h e o t h e r impurities. The effectiveness o f using Ca-MgO--SiO2 f o r p u r i f i c a t i o n was also studied. Table 4 s h o w s t h e c o n c e n t r a t i o n o f impurities in t h e slags a f t e r t h e e x t r a c t i o n o f impurities f r o m MG-Si melts. A high c o n c e n t r a t i o n o f alumin u m , iron a n d s t r o n t i u m in t h e slags suggests t h a t this slag is effective f o r t h e e x t r a c t i o n o f these impurities. T h e resistivity o f silicon ingots pulled f r o m
114
(a)
(b)
(c)
(d)
Fig. 1. (a) Scanning electron micrograph showing the boundary between the slag (CaO + SiO2) and the silicon phases; (b) X-ray image of iron radiation; (c) X-ray image of calcium radiation; (d) X-ray image of aluminum radiation.
4. Impurity redistribution The major impurities in MG-Si are metals which have a segregation coefficient of 10 -2 or lower. Therefore it can be predicted that impurity redistribution on solidification will be an effective m e t h o d of purification. The solidification was conducted using the Czochralski pulling method. In theory the ingot pulled from the molten silicon will have an a m o u n t of metal impurities at least two orders of magnitude lower than that in the original
115
(a)
(b)
(c)
(d)
Fig. 2. (a) Scanning electron micrograph showing the boundary between the slag (CaO + M g O + SiO2) and the silicon phases; (b) X-ray image of calcium radiation; (c) X-ray image of aluminum radiation; (d) X-ray image of magnesium radiation.
MG-Si. The majority of impurities will remain in the residual melt which cannot be pulled from the b o t t o m of the crucible. By remelting the pulled ingots in a new crucible and resolidification, the repulled (twice-pulled) ingots will contain an a m o u n t of metal impurities four orders of magnitude less than that in the original MG-Si. The process can be repeated to produce thrice-pulled ingots, which will contain an amount of metal impurities six orders of magnitude lower than that in the original MG-Si. Experimental work was carried o u t using a Hamco 800 crystal puller. The mass of melt in each stage of the crystal pullings was in the range 4 - 6 kg. The slagging process was applied prior to the first pulling. The melt surfaces
116 TABLE 5 Comparison of maximum impurity concentration in once-, twice- and thrice-pulled ingots analyzed using spark source mass spectrometry Element
Maximum impurity concentration (wt.ppm) One pull Seed end
Cd Mo Zr
Two pulls Tang end
0.16 0.10
0.44 0.96
--
13.00
Three pulls
Seed end
Tang end
Seed end
Tang end
< 0.16 < 0.19
< 0.16 < 0,19
< 0.16 --
< 0.16 --
--
--
0.04
--
Cu Ni Fe Mn
0.02 0.53 2.40 0.63
0.05 19.00 560.0 29.00
0.01 0.97 0.28 0.23
0.02 0.97 0.11 0.41
0.01 0.96 < 0.09 0.63
-1.20 0.24 0.32
Ti Ca
0.08 --
120.00 49.00
0.03 0.37
0.02 0.38
< 0.02 0.20
0.02 0.56
K C] P At Mg Na C B
-2.30 1.60 36.0 0.27 0.20 12.0 0.42
0.19 < 0.01 0.74 5.20 0.05 0.04 0.80 1.20
0.08 < 0.01 0.74 8.20 -0.05 0.59 1.20
-< 0.01 0.45 0.02 0.08 -0.30 0.60
-0.01 1.20 0.03 0.16 0.21 0.83 1.30
-2.40 3.70 3300 36.00 0.82 28 0.73
were clean w h e n pulled crystals were used f o r the remelts. T h e slagging was n o t necessary f o r t h e s e c o n d and t h i r d pulling processes o f ingots. T h e experimental results are t a b u l a t e d in Table 5, in which t h e m a x i m u m i m p u r i t y c o n c e n t r a t i o n f o u n d in the once-, twice- and thrice-pulled ingots is listed. T h e c o n c e n t r a t i o n o f s o m e impurities at the seed end o f once-pulled crystals is as low as a f r a c t i o n o f a weight p a r t per million. This is primarily due t o t h e low segregation c o e f f i c i e n t o f these m e t a l s and t h e low c o n c e n t r a t i o n in t h e starting MG-Si. In o r d e r t o see t h e effectiveness o f each additional crystal pulling, t h e change in c o n c e n t r a t i o n o f iron, manganese, t i t a n i u m and a l u m i n u m was investigated. By c o m p a r i s o n o f t h e i r c o n c e n t r a t i o n s at the tang end o f once- and twice-pulled ingots listed in Table 5, it is f o u n d t h a t the twice-pulled ingots c o n t a i n an i m p u r i t y c o n c e n t r a t i o n t w o to t h r e e orders o f m a g n i t u d e less t h a n t h a t o f t h e once-pulled ingots. A r e d u c t i o n in c o n c e n t r a t i o n o f a similar m a g n i t u d e f r o m twice-pulled t o thrice-pulled ingots is f o u n d o n l y f o r a l u m i n u m . T h e crystal p e r f e c t i o n o f ingots is i m p r o v e d b y increasing the n u m b e r o f crystal pulls. T h e o n c e - p u l l e d crystals are always p o l y c r y s t a l l i n e in structure. T h e seed end o f t h e ingots exhibits a large-grain s t r u c t u r e , while the tang e n d has a lamellar s t r u c t u r e . T h e m a j o r i t y o f twice-pulled ingots are single crystals. H o w e v e r , a cellular s t r u c t u r e was occasionally observed at t h e tang end o f ingots. No such s t r u c t u r e has been f o u n d at t h e tang end o f t h e thrice-pulled ingots.
117 Crystals pulled from MG-Si are p type. This agrees with the chemical analysis of ingots (Table 1 ) that the sum o f the boron and aluminum concentrations i$ greater than the phosphorus concentration. Increasing the number of crystal pulls does not change the conductivity type of the ingots. The majority carrier mobility is increased with an increasing number of crystal pulls. Typically the mobility for the once-pulled ingots is in the range 80 90 cm 2 V -~ s-l; the mobility is increased to 100 cm 2 V -~ s-1 for twice-pulled crystals and to 120 - 150 cm 2 V -1 s -~ for thrice-pulled ingots. 5. Discussion Aluminum and iron are the two major impurities in MG-Si. Their concentrations are in the range 0.2%- 1.0%. The concentration of each remaining impurity is hundreds of parts per million or less. The segregation coefficient of aluminum is 3 X 10 -2 and that of iron is 6.4 X 10 -6. Segregation coefficients of other impurities are less than 10 -3 , with the exception of boron and phosphorus. Therefore purification by unidirectional solidification alone cannot reduce the aluminum concentration to an acceptable value. By the use o f repeated melting and pulling of ingots for four times, the aluminum concentration can be reduced to 0.81 X 10 -s (i.e. 10 -1 × (3 × 10-2) 4) which is equivalent to the concentration in the SG-Si. The experimental results show t h a t the aluminum concentration in the thrice-pulled ingots is already as low as 2 X 10 -s. This suggests t h a t the slagging using CaO-SiO2 at the first pull has reduced the aluminum concentration by one to two orders of magnitude. Some limitations exist for the slagging process. One is t h a t the a m o u n t of slag cannot be too excessive; otherwise the slag will erode through the quartz crucible and the silicon melt will leach through the crucible. An a t t e m p t was made to use a graphite crucible and CaO-SiO2 slag was added. The experiment was not successful for pulling a large-grained ingot because the slag did not adhere to the graphite crucible. The other limitation is t h a t continuous multiple extraction cannot be applied to the slagging of silicon melts. Table 5 shows that certain impurities at the tang end of once-pulled crystals are in concentrations two to three orders of magnitude higher than those at the seed end. This is primarily because the tang end has a lamellar structure due to the constitutional supercooling of the melt. If constitutional supercooling was avoided or the lamellar structure portion of the ingots was n o t used for remelting, the purity in the twice-pulled ingots would be improved considerably. In this case the twice-pulled ingots may reach the purity of SG-Si. Boron a n d phosphorus are the impurities which cannot be reduced to the same level as t h a t in the SG-Si. Since these two impurities will n o t contribute to the presence of deep level traps in silicon, the silicon wafers produced by this m e t h o d will be suitable to use as substrates for epitaxial solar cells and other semiconductor devices.
118 Acknowledgments The a u t h o r s wish to t h a n k Dr. K e n t Hansen f o r his s u p p o r t o f this w o r k and a critical reading o f t h e m a n u s c r i p t . T h a n k s are also due t o D o n D o w n and Angel Hoge for t h e e x p e r i m e n t a l w o r k a n d Sally Hill for t y p i n g the manuscript.
References 1 2 3 4
J. Strauss, U.S. Patent 2,866, 701, 1958. Wacker-Chemie G.m.b.H., Br. Patent 922,879, 1963. E. Enk and J. Nickl, F.R.G. Patent 1,098,931, 1961. T. L. Chu, G. A. Van der Leeden and H. I. Yuo, J. Electrochem. Soc., 125 (1978) 661. 5 L. P. Hunt, V. D. Dosaj, J. R. McCormick and L. D. Crossman, Solar Energy, Electrochemical Society, Princeton, NJ, 1976. 6 P. S. Kotval and H. B. Strock, U.S. Patent 4,124,410, 1978. 7 E. M. Levin, C. R. Robbins and H. F. McMurdie, Phase Diagrams for Ceramics, American Ceramic Society, Columbus, OH, 1st edn., 1964, 2nd edn., 1969.
109
PURIFICATION OF METALLURGICAL-GRADE SILICON BY SLAGGING AND IMPURITY REDISTRIBUTION
H. M. LIAW and F. SECCO D'ARAGONA
Semiconductor Research and Development Laboratory, Motorola Inc.-Semiconductor Products Sector, 5005 East McDowell Road, Phoenix, A Z 85008 (U.S.A.) (Received March 17, 1983; accepted April 5, 1983)
Summary The purification of metallurgical-grade silicon (MG-Si) was carried out by slagging and impurity redistribution through repeated melting and pulling. A Czochralski crystal puller was used for performing both functions. The slags consisting of CaO-SiO2 and CaO-MgO-SiO2 were found to be effective for extracting aluminum from the MG-Si melts. The repeated melting and pulling method was effective for the removal of other metallic impurities. When slagging and multiple pulling were combined, the thricepulled ingots contained an impurity concentration equivalent to that of semiconductor-grade silicon with the exception of boron and phosphorus.
1. Introduction The direct purification of metallurgical-grade silicon (MG-Si) was carried out as early as the 1950s and 1960s [1 -3]. Reactive gases such as halogen or hydrogen chloride were used to treat the molten silicon. This technique failed to produce the high purity silicon required by the semiconductor industry. The need for low cost silicon for solar cell applications has regenerated the interest in this topic. Chu e t al. [4] have demonstrated a reduction in the concentration of aluminum in MG-Si by the treatment of the melt with a halogen or hydrogen halide. Hunt e t al. [5] have developed purification techniques using reactive gas treatment followed by unidirectional solidification. Kotval and Strock [6] used several steps for the purification of MG-Si: (1) dissolution and recrystallization from an aluminum solution; (2) the leaching of crystallized silicon platelets with HC1; (3) the purification of silicon platelets by slagging and unidirectional solidification. Both Hunt et al. and Kotval and Strock used the Bridgman technique for the unidirectional solidification. The purpose of this paper is to report the use of the Czochralski crystal-puUing technique to perform slagging and unidirectional solidification. 0379-6787/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
110
2. Purification procedures A commercial silicon crystal puller modified with the attachment of a gate valve was used for this work. MG-Si was loaded into the quartz crucible in the crystal puller. On melting of the MG-Si charge the insoluble impurities would float on the melt surface. This prevented pulling an ingot with large grain sizes. The slagging process was then applied. The slagging process adds insoluble materials into the silicon melt. The purpose of the slagging is twofold: (1) to dissolve the insoluble impurities which float on the melt and (2) to extract soluble impurities from the silicon melt. A thorough stirring was necessary to ensure good mixing between the slag and silicon phases. The slag was then manipulated to adhere to the crucible wall. This created a clean surface at the center region of the melt. A single-crystal seed was dipped and an ingot was pulled with the normal Czochralski crystal-growing procedures.
3. Slagging study The slagging is based on the principle of liquid-liquid extraction. This technique has been used in the steel industry. The slag used for the extraction of impurities from the molten silicon should fulfill the following criteria. (1) It should be immiscible with molten silicon. (2) Its melting point should be at a temperature approximately equal to the melting point of silicon. (3) It should not contaminate the silicon melt. (4) It should have a greater chemical affinity to impurities than to silicon. The first criterion can be fulfilled by certain metallic oxides. Since the transition metals and heavy metals contribute to the deep level traps in silicon, their oxides have been excluded from consideration. Alkali metals contribute to mobile ions in silicon; their oxides have also been excluded. Alkaline earth oxides were chosen for investigation. Since a quartz (SiO2) crucible is used for containing the silicon melt, the oxide added into the crucible inevitably forms mixed oxides with the quartz. When the binary phase diagrams [7] of SiO2-alkaline earth oxide systems were considered, it was found that CaO-SiO2 has t w o eutectic points near the melting point of silicon. One occurs at 46 at.% SiO2 with a eutectic temperature at 1460 °C, and the other at 63 at.% SiO2 with a eutectic temperature at 1436 °C. Three ternary oxide systems which have eutectic points in the range 1400 - 1500 °C were also chosen. They are SiO2-CaO-MgO, SiO2-BaO-CaO and SiO2-BaO-MgO. In all cases the phase diagrams show that the eutectic temperatures occur at the compositions for which the t w o alkaline earth oxides are in approximately the same proportion. It was not known whether the selected oxide systems could fulfill criteria (3) and (4). Experimental work was carried o u t to test their feasibility and effectiveness for the purification of molten silicon.
111
3.1. The contamination test Semiconductor-grade silicon (SG-Si) was used as a testing vehicle to study whether the silicon melt would be contaminated b y adding a slag. The silicon melts were d o p e d with b o r o n so that the pulled ingots would yield a resistivity of 2 ~ cm at the seed end of crystals. Any deviation of the resistivity from this value indicates the contamination of silicon by the slag. Table 1 lists the results of the experiments. As shown in this table the alkaline earth carbonates were added instead of their oxides. The carbonates decomposed quickly into oxides and carbon dioxide which evaporated from the melt. The results of the experiments can be summarized as follows. (1) CaO-SiO2 slag does not contaminate silicon melts. The silicon melt surface remains clean after the addition of this slag. Single crystals can be grown from the silicon melt treated with this slag. (2) CaO-BaO-SiO2 slag gives n-type contamination. Single crystals cannot be grown from the silicon melts treated with this slag. (3) BaO-MgO-SiO2 slag gives p-type contamination. Single crystals cannot be grown from the silicon melts treated with this slag. (4) CaO-MgO-SiO2 slag does not contaminate silicon melts. However, the ingot pulled from the silicon melts treated with this slag was polycrystalline in structure. TABLE 1 Slag constituents, type and resistivity of ingots pulled from slag-treated semiconductorgrade silicon melts
Slag constituent
Type of ingots
Resistivity of ingots
Remarks
(~ cm) (1) None
p
2.1
Single crystal
(2) 1% CaCO3
p
1.99
Single crystal
(3) 1% CaCO3, 0.5% BaCO3
p at the seed end n at the tang end
2.9 1.9
Polycrystal
(4) 1% MgCO3, 1% BaC03
p
0.22
Polycrystal
(5) 15% MgCO3, 1% CaC03
p
1.71
Polycrystal
3.2. Purification test Both CaO--SiO2 and CaO-MgO-SiO2 which do not contaminate silicon melts were used for the purification test. MG-Si was used for this study. The effectiveness of purification was evaluated by analyzing the impurity concentration in the pulled ingots as well as in the slags. Table 2 lists the results of treatment with CaO-SiO2 slag. This table shows that the aluminum concentration in silicon ingots decreases with the increase in the a m o u n t of CaCO3 added. The results agree with the electrical measurements of the silicon
112 TABLE 2 Concentration of impurities a in metallurgical-grade amounts of CaCO 3 Element
Impurity concentration (wt.ppm) for following amounts o f CaC03 added to the Si melt
silicon melts treated by various
Impurity concentration (wt.ppm) in the slag after slagging with 1% (CaO Si02) in the Si mell
0.12%
0.88%
1.0%
Fe Mn Cr Ca Sr Mg Ni V Ti Cu Zr
> 1000 330 100 100 1 1 100 < 10 330 33 100
> 1000 330 100 33 3 <1 100 33 330 33 100
> 1000 100 33 33 <1 <1 10 10 33 < 10 10
> 1000 33 33 > 1000 100 100 10 < 10 330 < 10 10
AI B Co
I000 <3 10
330 <3 < 10
i00 <3 < 10
> i000 <3 < 10
aAnalyzed using emission spectroscopy. TABLE 3 Resistivity of silicon pulled from the melts treated with various amounts of CaCO3 A m o u n t o f CaC03 (%)
Resistivity ( ~ cm)
0.12 0.88 1.0 2.0
0.069 0.078 0.082 0.105
ingots, w h i c h s h o w t h a t t h e resistivity o f silicon ingots increased with t h e increase in t h e a m o u n t o f CaCOs a d d e d t o t h e m e l t (Table 3). The analysis o f C a O - S i O ~ a f t e r slagging s h o w s t h a t t h e a l u m i n u m c o n c e n t r a t i o n increased t o greater t h a n 1 0 0 0 w t . p p m . F r o m these results it can be c o n c l u d e d t h a t C a O - S i O 2 slag is effective f o r t h e r e m o v a l o f a l u m i n u m f r o m t h e MG-Si melts. H o w e v e r , little e f f e c t was seen o n t h e o t h e r impurities. The effectiveness o f using Ca-MgO--SiO2 f o r p u r i f i c a t i o n was also studied. Table 4 s h o w s t h e c o n c e n t r a t i o n o f impurities in t h e slags a f t e r t h e e x t r a c t i o n o f impurities f r o m MG-Si melts. A high c o n c e n t r a t i o n o f alumin u m , iron a n d s t r o n t i u m in t h e slags suggests t h a t this slag is effective f o r t h e e x t r a c t i o n o f these impurities. T h e resistivity o f silicon ingots pulled f r o m
114
(a)
(b)
(c)
(d)
Fig. 1. (a) Scanning electron micrograph showing the boundary between the slag (CaO + SiO2) and the silicon phases; (b) X-ray image of iron radiation; (c) X-ray image of calcium radiation; (d) X-ray image of aluminum radiation.
4. Impurity redistribution The major impurities in MG-Si are metals which have a segregation coefficient of 10 -2 or lower. Therefore it can be predicted that impurity redistribution on solidification will be an effective m e t h o d of purification. The solidification was conducted using the Czochralski pulling method. In theory the ingot pulled from the molten silicon will have an a m o u n t of metal impurities at least two orders of magnitude lower than that in the original
115
(a)
(b)
(c)
(d)
Fig. 2. (a) Scanning electron micrograph showing the boundary between the slag (CaO + M g O + SiO2) and the silicon phases; (b) X-ray image of calcium radiation; (c) X-ray image of aluminum radiation; (d) X-ray image of magnesium radiation.
MG-Si. The majority of impurities will remain in the residual melt which cannot be pulled from the b o t t o m of the crucible. By remelting the pulled ingots in a new crucible and resolidification, the repulled (twice-pulled) ingots will contain an a m o u n t of metal impurities four orders of magnitude less than that in the original MG-Si. The process can be repeated to produce thrice-pulled ingots, which will contain an amount of metal impurities six orders of magnitude lower than that in the original MG-Si. Experimental work was carried o u t using a Hamco 800 crystal puller. The mass of melt in each stage of the crystal pullings was in the range 4 - 6 kg. The slagging process was applied prior to the first pulling. The melt surfaces
116 TABLE 5 Comparison of maximum impurity concentration in once-, twice- and thrice-pulled ingots analyzed using spark source mass spectrometry Element
Maximum impurity concentration (wt.ppm) One pull Seed end
Cd Mo Zr
Two pulls Tang end
0.16 0.10
0.44 0.96
--
13.00
Three pulls
Seed end
Tang end
Seed end
Tang end
< 0.16 < 0.19
< 0.16 < 0,19
< 0.16 --
< 0.16 --
--
--
0.04
--
Cu Ni Fe Mn
0.02 0.53 2.40 0.63
0.05 19.00 560.0 29.00
0.01 0.97 0.28 0.23
0.02 0.97 0.11 0.41
0.01 0.96 < 0.09 0.63
-1.20 0.24 0.32
Ti Ca
0.08 --
120.00 49.00
0.03 0.37
0.02 0.38
< 0.02 0.20
0.02 0.56
K C] P At Mg Na C B
-2.30 1.60 36.0 0.27 0.20 12.0 0.42
0.19 < 0.01 0.74 5.20 0.05 0.04 0.80 1.20
0.08 < 0.01 0.74 8.20 -0.05 0.59 1.20
-< 0.01 0.45 0.02 0.08 -0.30 0.60
-0.01 1.20 0.03 0.16 0.21 0.83 1.30
-2.40 3.70 3300 36.00 0.82 28 0.73
were clean w h e n pulled crystals were used f o r the remelts. T h e slagging was n o t necessary f o r t h e s e c o n d and t h i r d pulling processes o f ingots. T h e experimental results are t a b u l a t e d in Table 5, in which t h e m a x i m u m i m p u r i t y c o n c e n t r a t i o n f o u n d in the once-, twice- and thrice-pulled ingots is listed. T h e c o n c e n t r a t i o n o f s o m e impurities at the seed end o f once-pulled crystals is as low as a f r a c t i o n o f a weight p a r t per million. This is primarily due t o t h e low segregation c o e f f i c i e n t o f these m e t a l s and t h e low c o n c e n t r a t i o n in t h e starting MG-Si. In o r d e r t o see t h e effectiveness o f each additional crystal pulling, t h e change in c o n c e n t r a t i o n o f iron, manganese, t i t a n i u m and a l u m i n u m was investigated. By c o m p a r i s o n o f t h e i r c o n c e n t r a t i o n s at the tang end o f once- and twice-pulled ingots listed in Table 5, it is f o u n d t h a t the twice-pulled ingots c o n t a i n an i m p u r i t y c o n c e n t r a t i o n t w o to t h r e e orders o f m a g n i t u d e less t h a n t h a t o f t h e once-pulled ingots. A r e d u c t i o n in c o n c e n t r a t i o n o f a similar m a g n i t u d e f r o m twice-pulled t o thrice-pulled ingots is f o u n d o n l y f o r a l u m i n u m . T h e crystal p e r f e c t i o n o f ingots is i m p r o v e d b y increasing the n u m b e r o f crystal pulls. T h e o n c e - p u l l e d crystals are always p o l y c r y s t a l l i n e in structure. T h e seed end o f t h e ingots exhibits a large-grain s t r u c t u r e , while the tang e n d has a lamellar s t r u c t u r e . T h e m a j o r i t y o f twice-pulled ingots are single crystals. H o w e v e r , a cellular s t r u c t u r e was occasionally observed at t h e tang end o f ingots. No such s t r u c t u r e has been f o u n d at t h e tang end o f t h e thrice-pulled ingots.
117 Crystals pulled from MG-Si are p type. This agrees with the chemical analysis of ingots (Table 1 ) that the sum o f the boron and aluminum concentrations i$ greater than the phosphorus concentration. Increasing the number of crystal pulls does not change the conductivity type of the ingots. The majority carrier mobility is increased with an increasing number of crystal pulls. Typically the mobility for the once-pulled ingots is in the range 80 90 cm 2 V -~ s-l; the mobility is increased to 100 cm 2 V -~ s-1 for twice-pulled crystals and to 120 - 150 cm 2 V -1 s -~ for thrice-pulled ingots. 5. Discussion Aluminum and iron are the two major impurities in MG-Si. Their concentrations are in the range 0.2%- 1.0%. The concentration of each remaining impurity is hundreds of parts per million or less. The segregation coefficient of aluminum is 3 X 10 -2 and that of iron is 6.4 X 10 -6. Segregation coefficients of other impurities are less than 10 -3 , with the exception of boron and phosphorus. Therefore purification by unidirectional solidification alone cannot reduce the aluminum concentration to an acceptable value. By the use o f repeated melting and pulling of ingots for four times, the aluminum concentration can be reduced to 0.81 X 10 -s (i.e. 10 -1 × (3 × 10-2) 4) which is equivalent to the concentration in the SG-Si. The experimental results show t h a t the aluminum concentration in the thrice-pulled ingots is already as low as 2 X 10 -s. This suggests t h a t the slagging using CaO-SiO2 at the first pull has reduced the aluminum concentration by one to two orders of magnitude. Some limitations exist for the slagging process. One is t h a t the a m o u n t of slag cannot be too excessive; otherwise the slag will erode through the quartz crucible and the silicon melt will leach through the crucible. An a t t e m p t was made to use a graphite crucible and CaO-SiO2 slag was added. The experiment was not successful for pulling a large-grained ingot because the slag did not adhere to the graphite crucible. The other limitation is t h a t continuous multiple extraction cannot be applied to the slagging of silicon melts. Table 5 shows that certain impurities at the tang end of once-pulled crystals are in concentrations two to three orders of magnitude higher than those at the seed end. This is primarily because the tang end has a lamellar structure due to the constitutional supercooling of the melt. If constitutional supercooling was avoided or the lamellar structure portion of the ingots was n o t used for remelting, the purity in the twice-pulled ingots would be improved considerably. In this case the twice-pulled ingots may reach the purity of SG-Si. Boron a n d phosphorus are the impurities which cannot be reduced to the same level as t h a t in the SG-Si. Since these two impurities will n o t contribute to the presence of deep level traps in silicon, the silicon wafers produced by this m e t h o d will be suitable to use as substrates for epitaxial solar cells and other semiconductor devices.
118 Acknowledgments The a u t h o r s wish to t h a n k Dr. K e n t Hansen f o r his s u p p o r t o f this w o r k and a critical reading o f t h e m a n u s c r i p t . T h a n k s are also due t o D o n D o w n and Angel Hoge for t h e e x p e r i m e n t a l w o r k a n d Sally Hill for t y p i n g the manuscript.
References 1 2 3 4
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