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Grain structure of silicon solidified from an inductive cold crucible
Materials Science and Engineering, A173 (1993) 63-66
63
Grain structure of silicon solidified from an inductive cold crucible S. S e r v a n t a, B. Pillin b, D . Sarti a a n d F. D u r a n d b aphotowatt int. SA, 33 rue St HonorO, ZA Champ Fleuri, 38300 Bourgoin (France) bMadylam, ENSHMG, BP 95, 38402 St Martin d'H~res Cedex (France)
Abstract Some billets of multicrystalline silicon were semi continuously solidified, using an inductive cold crucible. The outer surface of the billet shows that electromagnetic pressure limits the contact. The grain structure is entirely columnar. Grain orientation and size varies along the thermal fluxes lines. The classical grain selection mechanism is adapted to pure silicon.
1. Inductive cold crucible applied to multicrystailine silicon processing Presently, the industrial route based on massive ingots is economically competitive with respect to thin film techniques for the production of polycrystalline silicon. In this respect, the use of an inductive cold crucible has several advantages over the usual graphite or silica crucible. The main advantages are that the crucible is reusable and that the process is semicontinuous. The application of an inductive cold crucible to massive silicon processing has been considered for many years [1-3]. More recently, explicitly for photovoltaic application, Kaneko et al. [4] developed a process for semicontinuous remelting and solidification of massive silicon billets, based on the inductive cold crucible, and derived from the so called 4C process for continuous remelting and solidification of titanium scrap [5]. In the present version, the crucible is built in copper and divided into water-cooled isolated segments. Induction currents are developed directly in the conductive part of the charge in the so-called "skin depth". Consequently the heat source by the Joule effect is localized in the vicinity of the charge outer surface. Moreover, induced currents interact with the local magnetic field, giving Laplace-Lorentz forces, which have two effects: • a pressure-like effect which tends to repel the charge from the crucible, thus suppressing or limiting pollution effects • in the bulk of the liquid part, a stirring effect, which accelerates heat and mass transfer. 0921-5093/93/$6.00
The present paper gives observations on the grain structure of massive silicon billets solidified semicontinuously from an inductive cold crucible. In the discussion, current ideas on columnar solidification mechanism are adapted to pure silicon.
2. Experimental methods The experimental equipment is designed for continuous remelting and solidification of billets under an argon protective atmosphere (Fig. 1). The process starts on a cylinder of massive silicon, used as "primer". It is placed inside the induction coil, from which it is separated by the segmented copper cold crucible.
l
S
~ ..~,,-COIL
CRUCIBLE SILICON
Fig. 1. Schematic representation of a silicon billet in an inductive cold crucible, showing (left) grain structure in the silicon billet (1, chill zone; 2, transition zone; 3, central zone) and (right): 4, heat flow lines in the solid ingot; 5, liquid stream line in the melt; 6, Laplace-Lorentz forces. © 1993 - Elsevier Sequoia. All rights reserved
64
S. Servant et al.
/
Silicon is a semiconducting material. From room temperature up to approximately 1000 K, silicon acts as a non-conducting material. So the primer cylinder must be preheated until its electrical conductivity is sufficient. Due to the repelling effect, the liquid outer surface takes the shape of a dome (Fig. 1, see also Fig.3). In normal processing condition, the billet is withdrawn continuously at approximately 1 mm rain I, and the liquid zone is fed by input material, usually broken particles of silicon. A series of billets has been cast in different conditions Celectrical power, current frequency, withdrawal velocity, cooling conditions for the billet, etc.). Two crucibles were used; one with a circular cross-section and an inner diameter of 102 mm, the second square, 60 m i n x 6 0 ram. Two cooling conditions were used: these were either direct
Grain structure of silicon
transfer in the ambient gas by convection and radiation or cooling controlled in an oven. In this case, the temperature of the surface of the billet and a vertical gradient are imposed at the end of the crucible. After solidification, the billets are submitted to surface examination and crack detection. Some of them are sawn transversely and longitudinally, and the section is mechanically polished and etched. A saturated sodium hydroxide solution, at its boiling temperature, is used as etchant for macrographs. Micrographs are obtained after using a Sirtl etching, or after anodic oxidation [6].
3. Billet outer surface
The outer surface of the billet is shaped by the contact between the liquid and the crucible. In good operating conditions, the surface is smooth, without marks of tearing. The position of the slits between crucible segments is clearly marked, The skin between slot marks is formed from periodical, 2D ripple marks, as for a turtle shell. The relative depth is low, less than 0.1 mm. For square billets, the edges are rectangular, as was the inner surface of the crucible, but slightly rounded, and marked by the turtle platelets. Some of the edges can become irregular if the crucible is not perfectly centred with respect to the coil.
4. Grain structure and defects
lcm
Fig. 2. Outer surface of silicon billets. (a) Square section billet, showing: 1, irregular edge; 2, regular edge. (b) Circular section billet, showing: 1, white lines corresponding to crucible slits.
Fig. 3. Top of a circular cross-section of a billet.
On macrographs, completed by micrographs after anodic oxidation, the grain structure appears to be entirely columnar (Fig. 4). The grain orientation varies continuously. It starts perpendicularly to the crucible wall, then slopes upward and ends parallel to the withdrawal direction. Different zones can be distinguished according to grain size and orientation:
Fig. 4. Transverse cut of circular-section billets in steady-state casting for (a) direct cooling conditions and (b) controlled cooling conditions, showing: A, chill zone; B, transition zone; C, central zone.
S. Servant et al.
/
• a chill zone with very fine columnar structure oriented perpendicular to the cold crucible wall: typical grain size perpendicular to grain growth direction is about 0.1 mm. • a transition zone with larger grains: grain size is about 2 or 3 mm. • a central zone with the largest grains oriented parallel to the withdrawal direction: the grain size in the central zone at steady state depends upon the thermal conditions of billet cooling. With controlled cooling, grain size is about 3 or 4 mm v s . 5 or 8 mm under direct cooling (Fig. 4). An estimate of the shape of the solid-liquid front was sketched as the surface orthogonal to the mean line tangent to the grain boundaries (Fig. 5, mark D). In this case, the depth of the front is about 0.4 times the billet diameter. In the central zone, the front is approximately plane. An addition of antimony was used to mark one front position. This latter was detected by the difference in electrical resistivity. The corresponding boundary (Fig. 5, mark E) is comparable to the above surface. The depth of the front is 0.42 times the billet diameter. In the central zone, the curvature is greater for mark E than for mark D, and there is no planar part. Dislocation pits are revealed by Sirtl etchant. The dislocation density is higher on the side of the billet than in the bulk (Fig. 6). A first estimate of dislocation density on macrographs leads to values in the range of 106 cm-2. The distribution of defects is irregular from one grain to another. According to Andonov [6], colour contrast resulting from anodic oxidation of silicon can be related to different crystal orientation. In the very first milli-
Fig. 5. Longitudinal cut of a circular-section billet, showing: A, chill zone; B, transition zone; C, central zone; D, shape of the front drawn perpendicularly to grain growth direction; E, shape of the front detected by resistivity change after an antimony addition.
Grain structure of silicon
65
metres near the outer surface, grains are distributed in a large colour palette. In the bulk, only one colour prevails in practice. Numerous twins are clearly noticeable by colour changes in the grain. Each colour cannot, however, be associated with a crystallographic plane because the relationship depends on the impurity density for the same etching conditions.
5. Discussion
The material considered here is practically pure silicon. Its level of impurities (in particular, oxygen) is very low. If solutal undercooling is evaluated by application of current formulas, its value is practically nil. So the only thermodynamical affinity source to be considered here is thermal undercooling. Silicon is a non-metal element. This characteristic is supported by its high value of melting entropy. This leads to its high tendency to form a faceted solid-liquid interface. The withdrawal rate (approximately 17 p m s -1) is relatively high for lateral growth, which is normally associated with a faceted interface. In the case considered here, micrographs suggest that the classical twin-groove mechanism plays an important role. The grain structure is typically columnar. Our observations show that the transverse size of the crystals increases by a factor of 20 or more. The grains structure starts with very small crystals formed at the three-fold line, where the solid-liquid interface contacts the crucible, and also the liquid-flee surface. At the beginning, the grains seem to be equiaxed, Progressively, they become more and more elongated, and their transverse size also increases. This latter fact is evidence that a selection mechanism is operating. According to Walton and Chalmers [7], the typical columnar mechanism involves dendritic growth, because grain elimination is operated only from the
Fig. 6. Dislocation pits in the bulk of a square-section billet.
66
S. Servant et al.
/
Grain structure of silicon
Fig. 7. Micrographof a transverse zone close to the outer surface of a square billet, anodic oxidation.
grain boundary deviation, which can result from the lateral extension of secondary-plus-tertiary dendrite arms. The above authors observed also a grain size increase in cellular growth conditions, but this is much slower. In the case of pure silicon, a regime of dendritic growth is possible [8]. The starting situation can be a platelet developed from three crystals A, B and C, associated in twin position with the same (111) twinplane direction. This platelet is able to grow rapidly in the {112} direction by the twin-groove mechanism. In fact, something like a primary dendrite axis can be formed by the classical repeated growth of such twin platelets. What is important is that secondary and tertiary branches can be formed by the same mechanism in the equivalent crystallographic directions of the considered (111) plane, and also of other equivalent planes. Although this mechanism is typically faceted, it allows lateral development of the favourably oriented crystals, so grain boundary deviation can result, leading to selection. Bubbles have been observed inside multicrystalline silicon [8]. They are attributed to the slight misorientation which can exist between different dendrite arms. If such a mechanism operates in our billets, it is a source mechanism for the numerous dislocations revealed by chemical etching. Therefore, a better understanding of the dendritic-columnar solidification mechanism of silicon could be a way to improve the quality of the material.
Now concerning the billet outer surface, the mark facing the crucible slits is a colour contrast, but there is no protrusion. This point clearly demonstrates that the liquid-free surface is shaped by the electromagnetic pressure. This prevents the liquid from flowing between the crucible segments. The turtle platelets reflect the effect of surface instabilities, possibly related to the successive perturbations due to solid particle feeding on the liquid dome. Finally, a better understanding of local hydrodynamic and electromagnetic behaviour should provide some rules for a closer control of the casting conditions.
References 1 J. Reboux, US 3,461,215, Augvst 12, 1969, Affi. March 27, 1967. 2. W.E Menashi,J. Wenkus, EC.T. Int. Affi. 8,001,489, July 24, 1980. Affi.January 18, 1979, 2 off. 3. T.F.Ciszek, J. E lectrochem. Soc., 132(1985) 963. 4. K. Kaneko, T. Misawa and K. Tabata, Proc. Int. PVSEC 5, Kyoto, Japan, 1990, p. 201. 5. M. Garnier, I. Leclercq,P. Paillere and J.E Wadier, Proc. Sixth Int. Iron and Steel Congress, Nagoya, Japan, ISIJ, 1990, p. 260. 6. P. Andonov, Revue Phys. Appl., 17(1982) 657. 7. D. Walton and B. Chalmers, Trans. Met. Soc. AIME, 215 (1959) 447. 8. O. Raaness and O. Sunde, Proc. Conf. Silicon for Chemical Indust~, Geiranger, Norway, 1992, p. 53.
63
Grain structure of silicon solidified from an inductive cold crucible S. S e r v a n t a, B. Pillin b, D . Sarti a a n d F. D u r a n d b aphotowatt int. SA, 33 rue St HonorO, ZA Champ Fleuri, 38300 Bourgoin (France) bMadylam, ENSHMG, BP 95, 38402 St Martin d'H~res Cedex (France)
Abstract Some billets of multicrystalline silicon were semi continuously solidified, using an inductive cold crucible. The outer surface of the billet shows that electromagnetic pressure limits the contact. The grain structure is entirely columnar. Grain orientation and size varies along the thermal fluxes lines. The classical grain selection mechanism is adapted to pure silicon.
1. Inductive cold crucible applied to multicrystailine silicon processing Presently, the industrial route based on massive ingots is economically competitive with respect to thin film techniques for the production of polycrystalline silicon. In this respect, the use of an inductive cold crucible has several advantages over the usual graphite or silica crucible. The main advantages are that the crucible is reusable and that the process is semicontinuous. The application of an inductive cold crucible to massive silicon processing has been considered for many years [1-3]. More recently, explicitly for photovoltaic application, Kaneko et al. [4] developed a process for semicontinuous remelting and solidification of massive silicon billets, based on the inductive cold crucible, and derived from the so called 4C process for continuous remelting and solidification of titanium scrap [5]. In the present version, the crucible is built in copper and divided into water-cooled isolated segments. Induction currents are developed directly in the conductive part of the charge in the so-called "skin depth". Consequently the heat source by the Joule effect is localized in the vicinity of the charge outer surface. Moreover, induced currents interact with the local magnetic field, giving Laplace-Lorentz forces, which have two effects: • a pressure-like effect which tends to repel the charge from the crucible, thus suppressing or limiting pollution effects • in the bulk of the liquid part, a stirring effect, which accelerates heat and mass transfer. 0921-5093/93/$6.00
The present paper gives observations on the grain structure of massive silicon billets solidified semicontinuously from an inductive cold crucible. In the discussion, current ideas on columnar solidification mechanism are adapted to pure silicon.
2. Experimental methods The experimental equipment is designed for continuous remelting and solidification of billets under an argon protective atmosphere (Fig. 1). The process starts on a cylinder of massive silicon, used as "primer". It is placed inside the induction coil, from which it is separated by the segmented copper cold crucible.
l
S
~ ..~,,-COIL
CRUCIBLE SILICON
Fig. 1. Schematic representation of a silicon billet in an inductive cold crucible, showing (left) grain structure in the silicon billet (1, chill zone; 2, transition zone; 3, central zone) and (right): 4, heat flow lines in the solid ingot; 5, liquid stream line in the melt; 6, Laplace-Lorentz forces. © 1993 - Elsevier Sequoia. All rights reserved
64
S. Servant et al.
/
Silicon is a semiconducting material. From room temperature up to approximately 1000 K, silicon acts as a non-conducting material. So the primer cylinder must be preheated until its electrical conductivity is sufficient. Due to the repelling effect, the liquid outer surface takes the shape of a dome (Fig. 1, see also Fig.3). In normal processing condition, the billet is withdrawn continuously at approximately 1 mm rain I, and the liquid zone is fed by input material, usually broken particles of silicon. A series of billets has been cast in different conditions Celectrical power, current frequency, withdrawal velocity, cooling conditions for the billet, etc.). Two crucibles were used; one with a circular cross-section and an inner diameter of 102 mm, the second square, 60 m i n x 6 0 ram. Two cooling conditions were used: these were either direct
Grain structure of silicon
transfer in the ambient gas by convection and radiation or cooling controlled in an oven. In this case, the temperature of the surface of the billet and a vertical gradient are imposed at the end of the crucible. After solidification, the billets are submitted to surface examination and crack detection. Some of them are sawn transversely and longitudinally, and the section is mechanically polished and etched. A saturated sodium hydroxide solution, at its boiling temperature, is used as etchant for macrographs. Micrographs are obtained after using a Sirtl etching, or after anodic oxidation [6].
3. Billet outer surface
The outer surface of the billet is shaped by the contact between the liquid and the crucible. In good operating conditions, the surface is smooth, without marks of tearing. The position of the slits between crucible segments is clearly marked, The skin between slot marks is formed from periodical, 2D ripple marks, as for a turtle shell. The relative depth is low, less than 0.1 mm. For square billets, the edges are rectangular, as was the inner surface of the crucible, but slightly rounded, and marked by the turtle platelets. Some of the edges can become irregular if the crucible is not perfectly centred with respect to the coil.
4. Grain structure and defects
lcm
Fig. 2. Outer surface of silicon billets. (a) Square section billet, showing: 1, irregular edge; 2, regular edge. (b) Circular section billet, showing: 1, white lines corresponding to crucible slits.
Fig. 3. Top of a circular cross-section of a billet.
On macrographs, completed by micrographs after anodic oxidation, the grain structure appears to be entirely columnar (Fig. 4). The grain orientation varies continuously. It starts perpendicularly to the crucible wall, then slopes upward and ends parallel to the withdrawal direction. Different zones can be distinguished according to grain size and orientation:
Fig. 4. Transverse cut of circular-section billets in steady-state casting for (a) direct cooling conditions and (b) controlled cooling conditions, showing: A, chill zone; B, transition zone; C, central zone.
S. Servant et al.
/
• a chill zone with very fine columnar structure oriented perpendicular to the cold crucible wall: typical grain size perpendicular to grain growth direction is about 0.1 mm. • a transition zone with larger grains: grain size is about 2 or 3 mm. • a central zone with the largest grains oriented parallel to the withdrawal direction: the grain size in the central zone at steady state depends upon the thermal conditions of billet cooling. With controlled cooling, grain size is about 3 or 4 mm v s . 5 or 8 mm under direct cooling (Fig. 4). An estimate of the shape of the solid-liquid front was sketched as the surface orthogonal to the mean line tangent to the grain boundaries (Fig. 5, mark D). In this case, the depth of the front is about 0.4 times the billet diameter. In the central zone, the front is approximately plane. An addition of antimony was used to mark one front position. This latter was detected by the difference in electrical resistivity. The corresponding boundary (Fig. 5, mark E) is comparable to the above surface. The depth of the front is 0.42 times the billet diameter. In the central zone, the curvature is greater for mark E than for mark D, and there is no planar part. Dislocation pits are revealed by Sirtl etchant. The dislocation density is higher on the side of the billet than in the bulk (Fig. 6). A first estimate of dislocation density on macrographs leads to values in the range of 106 cm-2. The distribution of defects is irregular from one grain to another. According to Andonov [6], colour contrast resulting from anodic oxidation of silicon can be related to different crystal orientation. In the very first milli-
Fig. 5. Longitudinal cut of a circular-section billet, showing: A, chill zone; B, transition zone; C, central zone; D, shape of the front drawn perpendicularly to grain growth direction; E, shape of the front detected by resistivity change after an antimony addition.
Grain structure of silicon
65
metres near the outer surface, grains are distributed in a large colour palette. In the bulk, only one colour prevails in practice. Numerous twins are clearly noticeable by colour changes in the grain. Each colour cannot, however, be associated with a crystallographic plane because the relationship depends on the impurity density for the same etching conditions.
5. Discussion
The material considered here is practically pure silicon. Its level of impurities (in particular, oxygen) is very low. If solutal undercooling is evaluated by application of current formulas, its value is practically nil. So the only thermodynamical affinity source to be considered here is thermal undercooling. Silicon is a non-metal element. This characteristic is supported by its high value of melting entropy. This leads to its high tendency to form a faceted solid-liquid interface. The withdrawal rate (approximately 17 p m s -1) is relatively high for lateral growth, which is normally associated with a faceted interface. In the case considered here, micrographs suggest that the classical twin-groove mechanism plays an important role. The grain structure is typically columnar. Our observations show that the transverse size of the crystals increases by a factor of 20 or more. The grains structure starts with very small crystals formed at the three-fold line, where the solid-liquid interface contacts the crucible, and also the liquid-flee surface. At the beginning, the grains seem to be equiaxed, Progressively, they become more and more elongated, and their transverse size also increases. This latter fact is evidence that a selection mechanism is operating. According to Walton and Chalmers [7], the typical columnar mechanism involves dendritic growth, because grain elimination is operated only from the
Fig. 6. Dislocation pits in the bulk of a square-section billet.
66
S. Servant et al.
/
Grain structure of silicon
Fig. 7. Micrographof a transverse zone close to the outer surface of a square billet, anodic oxidation.
grain boundary deviation, which can result from the lateral extension of secondary-plus-tertiary dendrite arms. The above authors observed also a grain size increase in cellular growth conditions, but this is much slower. In the case of pure silicon, a regime of dendritic growth is possible [8]. The starting situation can be a platelet developed from three crystals A, B and C, associated in twin position with the same (111) twinplane direction. This platelet is able to grow rapidly in the {112} direction by the twin-groove mechanism. In fact, something like a primary dendrite axis can be formed by the classical repeated growth of such twin platelets. What is important is that secondary and tertiary branches can be formed by the same mechanism in the equivalent crystallographic directions of the considered (111) plane, and also of other equivalent planes. Although this mechanism is typically faceted, it allows lateral development of the favourably oriented crystals, so grain boundary deviation can result, leading to selection. Bubbles have been observed inside multicrystalline silicon [8]. They are attributed to the slight misorientation which can exist between different dendrite arms. If such a mechanism operates in our billets, it is a source mechanism for the numerous dislocations revealed by chemical etching. Therefore, a better understanding of the dendritic-columnar solidification mechanism of silicon could be a way to improve the quality of the material.
Now concerning the billet outer surface, the mark facing the crucible slits is a colour contrast, but there is no protrusion. This point clearly demonstrates that the liquid-free surface is shaped by the electromagnetic pressure. This prevents the liquid from flowing between the crucible segments. The turtle platelets reflect the effect of surface instabilities, possibly related to the successive perturbations due to solid particle feeding on the liquid dome. Finally, a better understanding of local hydrodynamic and electromagnetic behaviour should provide some rules for a closer control of the casting conditions.
References 1 J. Reboux, US 3,461,215, Augvst 12, 1969, Affi. March 27, 1967. 2. W.E Menashi,J. Wenkus, EC.T. Int. Affi. 8,001,489, July 24, 1980. Affi.January 18, 1979, 2 off. 3. T.F.Ciszek, J. E lectrochem. Soc., 132(1985) 963. 4. K. Kaneko, T. Misawa and K. Tabata, Proc. Int. PVSEC 5, Kyoto, Japan, 1990, p. 201. 5. M. Garnier, I. Leclercq,P. Paillere and J.E Wadier, Proc. Sixth Int. Iron and Steel Congress, Nagoya, Japan, ISIJ, 1990, p. 260. 6. P. Andonov, Revue Phys. Appl., 17(1982) 657. 7. D. Walton and B. Chalmers, Trans. Met. Soc. AIME, 215 (1959) 447. 8. O. Raaness and O. Sunde, Proc. Conf. Silicon for Chemical Indust~, Geiranger, Norway, 1992, p. 53.