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Progress in ingot and foil casting of silicon
562
Joumal of Crystal Growth 79 (1986) 562-571 North-Holland, Amsterdam
PROGRESS IN INGOT AND FOIL CASTING OF SILICON D. HELMREICH and E. SIRTL Heliotronie GmbH, P.O. Box 1129, D-8263 Burghausen, Fed. Rep. of Germany
Ingot and foil casting are efficient and low-cost processes used to produce crystalline silicon for terrestrial photovoltaic applications. Two modifications - the SILSO® technique and the ramp assisted foil casting technique (RAFT) - are reviewed and compared.
1. Introduction To meet the requirements of a future "solar" silicon industry, unconventional growth methods have to be assessed for with their large-scale potential with particular regard to practicability, productivity, production costs and last but not least, material quality. In the mid-seventies it was recognized that monocrystalline silicon may be replaced by melt-grown multicrystalline silicon. In the meantime it has been demonstrated that such material with an average grain size greater than a certain threshold value can be used successfully for solar cells with promising trends in terms of solar cell efficiency. There are various approaches to the preparation of coarsegrained multicrystalline silicon sheet: (1) silicon layers on foreign or appropriate silicon substrates; (2) self-supporting silicon slices prepared from ingots by cutting and wafering; (3) self-supporting silicon sheet prepared directly by ribbon or foil growth. This paper deals with representatives of the second and third groups which are under development at Wacker-Heliotronic: the Wacker ingot casting process (SILSO*) and the ramp assisted foil-casting technique (RAFT). In principle, both techniques are ideally suited for mass production. In a continuously operated ingot technique the output is limited only by the rate of tilting the moulds. In the case of RAFT the output is governed by the pulling rate. In neither case is
linear crystallization velocity the primary obstacle for a high output.
2. Ingot casting 2.1. S I L S O ® technique
In contrast to unconventional ingot techniques based on the modified Bridgman-Stockbarger technique (i.e., the ubiquitous crystallization process (UCP), heat exchanger method (HEM), Polyx, directional solidification (DS)) the SILSO ~ process has the advantage that the individual processing steps are decoupled with respect to location. As an example, melting and crystallization are carried out in different receptacles as well as at different places. This decoupling offers the possibility of optimising the individual processing steps, irrespective of process sequence; i.e., the different processing steps can be adjusted to achieve the best performances and material quality. The basic procedure for the SILSO ® technique has already been given in detail in refs. [1,2]. The sequence of the processing steps consists of: (a) mould preconditioning; (b) melt preparation; (c) casting; (d) crystallization; (e) annealing of crystallized ingot; (f) cooling down of annealed ingot. The melt preparation takes place in a con-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
trolled atmosphere of inert gas at reduced pressure to prevent excessive contamination of the silicon melt with carbon and oxygen. The enrichment of the silicon melt with oxygen and carbon occurs during the melting down procedure. Oxygen is introduced mainly through the reaction between the silicon melt and quartz crucible. Carbon is introduced via the gas phase as CO. An adequate melting cycle and a melt preparation phase ahead of the crystallization cycle are the basis for a material with low O and C contents. The melt energy for melting is generated through induction heating: the RF energy (with a frequency of 2-4 kHz) is coupled directly into the graphite crucible which acts as a susceptor for the actual melt crucible. Different crucible materials (quartz, graphite, silicon nitride) have been tested for their applicability in this process. Quartz crucibles have proved best with respect to melt contamination, flexibility of geometry, dimension, and costs. The moulds are assembled from graphite elements so that a square shaped inner cross-section is obtained. Preconditioning of the moulds in a separate preheating station assures the correct adjustment of temperature level and temperature distribution. This helps to develop a homogeneous chilled layer of solid silicon before the silicon can react with the mould walls. In this way it is possible to use the mould repeatedly and to suppress melt contamination. Although a multicrystalline ingot is produced deliberately, the linear growth rate should not exceed a critical threshold value of about 1 mm/min. An increase in output, however, is definitely established by increasing the cross-section of the solidifying mass. In addition, purification effects based on segregation are substantially improved by lower growth rates. At present silicon ingots with a weight of 120 kg and a cross-section of 43 × 43 cm2 are produced semi-continuously with a crystallization rate of about 0.5 kg/min. In future continuously operating equipment the individual processing steps will be further disentangled with a clear separation in time and space of both silicon and mould transfer.
563
2.2. Cutting and slicing An integrated line for the low-cost production of crystalline silicon wafers based on an economical ingot casting technique has to be supplemented by efficient cutting and slicing techniques
[31. The shaping and sectioning of large sized ingots into columns with a cross-section of 10 × 10 cm2 is best carried out by using a band saw. It allows the machining of bigger dimensions as compared with the conventional OD sawing disks. In addition, it reduces the kerf loss drastically. The slicing of the preshaped columns is done by a multi-blade slurry (MBS) sawing technique. In contrast to "conventional" MBS technology we use a high reciprocation cutting speed and an increased feed force. This results in a cutting rate which is higher by more than an order of magnitude compared to that attained with "conventional" MBS sawing.
2.3. Material properties The advances in SILSO ~ casting offer the possibility of preparing large-sized ingots with improved characteristics. Optimization of the casting and crystallization processes helped to improve the crystal structure and to lower the defect density. Figs. la and lb show the crystal structure of a 100 kg ingot with 43 × 43 cm2 cross-section in horizontal and vertical sections. An effective control of the nucleation of crystallites has been achieved through seeding with a SILSO ~ wafer at the bottom of the mould as well as through adequate preheating of the mould and heat transfer (see figs. 2a and 2b). Further improvements refer to a decrease and homogenization of the impurity content and the reduction of thermally induced stresses (the latter through the introduction of an in-situ annealing step). The concentration of the impurities carbon and oxygen has now reached values of < 0.2 x 1017/cm3 which is far below their respective saturation concentrations at the melting point of silicon. Fig. 3 shows the oxygen content and photovoltaic efficiency as a function of ingot height. High photovoltaic efficiencies are related to low
564
D. Helmreieh, E. Sirtl / Progress in ingot and foil casting of silicon
Fig. 1. (a) Horizontally and (b) vertically cut slice of 100 kg ingot with 43 X43 cm2 cross-section and 25 cm height. Marker represents
10 cm. o x y g e n levels a n d vice versa. A n i n v e s t i g a t i o n of the d e p e n d e n c e of PV perf o r m a n c e o n the degree of d o p i n g showed that the resistivity r a n g e b e t w e e n 0.5 a n d 1 o h m c m s h o u l d b e p r e f e r r e d over lower resistivities [4].
A s a result of recent a d v a n c e m e n t s in solar cell processing, PV p e r f o r m a n c e s as high as 16% A M 1.5 have b e e n o b t a i n e d a l r e a d y even with n o n - o p t i m i z e d S I L S O ® s a m p l e s with n o t yet r e d u c e d c a r b o n a n d o x y g e n c o n t e n t [5].
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
565
Fig. 2. Crystal structure at the bottom of 100 kg ingot (as per detail of fig. 1 b): (a) without, (b) with melt preparation [4]. The thickness of either plate is approprimately 1 cm.
AM 1.5 ( % ) ~
--
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\
\
\
\
100
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\
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Bigger charges and larger dimensions as well as an improved geometry of the ingot with a flattened top area and sharper edges have led to a m i n i m u m of material loss during the subsequent cutting processes. As a result of the above improvements it has been possible to raise the conversion efficiency to a value of 50 W / k g silicon feed stock.
6
\
/
I
3. Foil casting
/ \
/
8 --
[0]
( 1017/cm31 -~D"
Fig. 3. Oxygen content [O] and PV efficiency 7/ as a function of ingot height. Solid lines: with melt preparation; dashed lines: without melt preparation.
A large variety of techniques has been proposed for the direct production of self-supporting silicon wafers. Most of these techniques under development have not survived because of difficulties with respect to basic process performance, material quality a n d / o r costs.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
566
a
10x1Ocm2
silicon wafers
,nspect,on
regeneration
feed stock
W
heater
--
--silicon melt
--
--V_/_~J
heater
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layer/
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-1 silicon foil
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¢¢ ¢ solid/iiql
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Fig. 4. Schematic diagram of the ramp assisted foil casting technique (RAFT): (a) general; (b) detail at the solid/liquid interface.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
567
Fig. 5. The crystallinity of RAFT foil depends on local heat transfer (expressed by heat transfer coefficient h) and thermal properties of ramp material. (a) Surface and (b) cross-section of RAFT foil (thickness - 350/zm) with uncontrolled nucleation. (c) Surface and (d) cross-section of RAFT foil (thickness - 350/zm) with restricted nucleation [7].
3.1. Ramp assisted foil-casting technique (RAFT) RAFT profits by the much more efficient heat removal through heat conduction as compared to the heat removal through heat radiation in the case of other ribbon techniques. In addition, RAFT uses an inclined solid/liquid interface (SLI), which allows very high growth rates of about 15000 m m / m i n combined with a moderate temperature gradient perpendicular to the surface of the solidified silicon. As a result of the inclined SLI the actual linear crystallization rate is about two orders of magnitude lower (150 mm/min), but still more than two orders of magnitude higher than that applied in the SILSO ® process. The trade offs between increased throughput via increased growth rate and material quality show that an increase of growth rate should not be exaggerated. Therefore, RAFT is performed at a
reasonable growth rate which is essential with respect to crystallinity, thermal stress, defects and impurities. The basic procedure of RAFT has been published already in ref. [6]. The schematic diagram of RAFT shown in fig. 4 indicates that the slices begin to separate from the ramps at a certain stage during the cooling-off period. The ramps can be reused many times before the expendable coating has to be regenerated. The slices, now obtainable with an area of 100 × 100 mm2, can be used directly in standard solar cell processing. The silicon wafers (thickness 0.4 mm) are prepared from semiconductor-grade feed stock with a fixed growth rate (6000 mm/min). The gas atmospheres in the input and output conditions are set to minimize the incorporation of impurities (mainly carbon and oxygen).
568
D. Helmreieh, E. Sirtl / Progress in ingot and foil casting of silicon
3.2. M a t e r i a l properties
Characteristics of R A F T material have already been discussed [7]. In this paper the quality criteria of R A F T are discussed in the light of process criteria and contrasted against SILSO ® characteristics. The typical crystalline structure of R A F T foil is columnar with low-energy grain boundaries nearly perpendicular to the surface. The size of individual grains depends on the probability of heterogeneous nucleation which is influenced by the rate of local heat removal (expressed in terms of a heat transfer coefficient), the thermal properties of the ramp-material and the initial temperature (see fig. 5). Due to plastic deformation during the cooling phase silicon foils show a considerable dislocation density within the individual grains (figs. 6a and 6b). The dislocation density can be reduced drastically by heat treatment. In a high C / h i g h O situation the dislocation density decreases from some 106/cm 2 to below 103/cm 2 with a 2 h / 1000°C anneal (figs. 6c and 6d). In a low C / h i g h O situation a one step anneal shows no effect, in this case a pre-annealing step for precipitation nucleation seems to be necessary. As a consequence of the high crystallization rate, the segregation coefficient keff tends towards 1 which means negligible purification. From this it follows that higher standards for the starting material have to be met. The main impurities in R A F T foil are oxygen and carbon, which are introduced during the casting process from the silica crucible (O) and via the gas phase (C). T E M examinations of annealed R A F T foil show an array of submicroscopic precipitations along the dislocations (fig. 7). The nature of such precipitates has not yet been identified directly. From their etching behaviour, however, SiC precipitation is strongly indicated. Present work shows that a drastic lowering of both C and O concentrations is possible with certain precautions taken during the melting and casting steps. Photovoltaic data of test cells from R A F T foils are still hampered by unstabilized process data in RAFT. The best PV efficiencies obtained here (10.3% AM 1.5; 5 × 5 cm 2) are comparable to
Fig. 6. Dislocation density of RAFT foil with high C/high O (thickness - 350 /Lrn).(a) Surface and (b) cross-sectionof as grown material. (c) Surface and (d) cross-sectionof RAFT foil annealed at 1000oC for 2 h.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
569
Fig. 7. TEM picture (dark field; g = (220)) of annealed RAFT foil showing precipitation arrays along dislocations. Courtesy H. Gottschalk [8]. Marker represents 0.5 ~m.
those obtained with standard SILSO ® prepared b y the same device procedure (8.5-10.9% AM 1.5; 10 X 10 cm2) being applied in our own test laboratory.
4. Limitations of SILSO ® technique and RAFT 4.1. S I L S O ®
In an ultimate version of a continuously operating production equipment the output should be limited only by the casting cycle.' Melting and crystallization with subsequent stepwise annealing and cooling procedures should not be limiting
factors because the chronological sequence can be transformed into a local sequence (see fig. 8a). In the first case, the individual processing steps are carried out strictly one after another at one place; in the second case they are carried out partly in parallel at different places related to the respective processing step. A casting cycle of 120 kg/0.5 h converted into an annual output of 70 MW in solar cells can be projected for one equipment unit. The bottleneck in ingot technology is cutting and slicing. The annual output of an MBS saw with two working stations is projected to be about "2 MW". As the linear crystallization rate does not influence the output, process variables can be opti-
D. Helmreich, E. Sirtl / Progress in ingot and foil costing of si#con
570
ingot
feed stock
INGOT
TEEHNIO.UE
wafer
X feed
stock
SHEET TECHNIQUE
F~g. 8. The interconnection of individual processing steps: (a) with the processing steps loosely linked together (ingot technique), (b) with the processing steps tightly linked together (sheet technique). M = melting, C = casting, X = crystallization, R = recharging of feed stock, P = preconditioning of mould. Bold-face arrows indicate material flow.
mized independently to improve the impurity and defect situation in SILSO ® material. High output and increased performance help to reduce the production costs for a SILSO ® wafer for the following reasons: (a) As a result of a low linear crystallization rate effective segregation can be considered an inherent purification step, thus allowing the quality (specification) of the feed stock material to be reduced. (b) Based on a high quality product both material yield and PV performance will be improved. Output and production costs will be influenced accordingly. (c) High throughput has a direct impact on costs due to reduced costs of depreciation and personnel.
"
4.2. R A F T
In this technique the individual processing steps are coupled very strongly with respect to time and place which means that its limitations are determined by the weakest link in the chain (see fig. 8b). At our present state of experience such a weak link can be seen here in the kinetics of crystallization with its scientific uncertainties in the formation and growth of nuclei. As a certain grain size is required, nucleation has to be minimized by avoiding heterogeneous seeding and the crystallization rate should, in addition, not exceed a certain threshold value. A prerequisite to the first point is a minimum quality of processing atmosphere, substrate, and feedstock material. The second point requires an appropriate crystallization (viz. pulling) rate. A reasonable growth rate is about 300 m m / s (higher rates result in a microcrystalline to amorphous material with inadequate PV properties). Such a growth rate is related to an estimated output of about 70 " M W " / y e a r per equipment unit. As slicing is not necessary in this process, feed stock consumption can be reduced drastically. To summarize, RAFT requires higher standards of feed stock and processing. On the other hand it needs less feed stock material and avoids complex processing of the as-grown slices. The SILSO * process needs additional processing steps (cutting/slicing) and a higher quantity of feedstock material but offers considerable advantages with an almost perfect theoretical impurity segregation and the possibility of disengaging the individual processing steps.
Acknowledgements The work described here has been supported by the Bundesministerium ftir Forschung und Technologie (BMFT) of the Federal Republic of Germany. References [1] B. Authier, in: Advances in Solid State Physics, Vol. 18, Ed. J. Treusch (Vieweg, Braunschweig, 1978) p. 1.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon [2] J. Dietl, D. Helmreich and E. Sirtl, in: Crystals: Growth, Properties and Applications, Vol. 5, Ed. J. Grabmaier (Springer, Berlin, 1981) p. 43; see also D. Helmreich, in: Silicon Processing for Photovoltalcs, Vol. II, Eds. C.P. Khattak and K.V. Ravi (Elsevier, Amsterdam, 1986). [3] W. Ermer, D. Helmreich, A. Moritz and K. Mi~hlbaner, in: Proc. 6th European Community Photovoltaic Solar Energy Conf., London, 1985 (Reidel, Dordrecht, 1985) p. 971. [4] C. Gessert, D. Helmreich and M. Peterat, ref. [3], p. 891.
571
[5] M.A. Green, S. Narayanan and S.R. Wenham, Internal Report of Solar Photovoltaic Laboratory, University of New South Wales, Australia (Dec. 1985), to be pubfished. [6] D. Helmreieh and J. Geissler, in: Proc. 5th European Community Photovoltaic Solar Energy Conf., Athens (Reidel, Dordrecht, 1983) p. 955. [7] J. Geissler, D. Helmreich and R. Wahlich, in: Proc. 18th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1986 (IEEE, New York, 1~86). [8] H. Gottschalk, to be pubfished.
Joumal of Crystal Growth 79 (1986) 562-571 North-Holland, Amsterdam
PROGRESS IN INGOT AND FOIL CASTING OF SILICON D. HELMREICH and E. SIRTL Heliotronie GmbH, P.O. Box 1129, D-8263 Burghausen, Fed. Rep. of Germany
Ingot and foil casting are efficient and low-cost processes used to produce crystalline silicon for terrestrial photovoltaic applications. Two modifications - the SILSO® technique and the ramp assisted foil casting technique (RAFT) - are reviewed and compared.
1. Introduction To meet the requirements of a future "solar" silicon industry, unconventional growth methods have to be assessed for with their large-scale potential with particular regard to practicability, productivity, production costs and last but not least, material quality. In the mid-seventies it was recognized that monocrystalline silicon may be replaced by melt-grown multicrystalline silicon. In the meantime it has been demonstrated that such material with an average grain size greater than a certain threshold value can be used successfully for solar cells with promising trends in terms of solar cell efficiency. There are various approaches to the preparation of coarsegrained multicrystalline silicon sheet: (1) silicon layers on foreign or appropriate silicon substrates; (2) self-supporting silicon slices prepared from ingots by cutting and wafering; (3) self-supporting silicon sheet prepared directly by ribbon or foil growth. This paper deals with representatives of the second and third groups which are under development at Wacker-Heliotronic: the Wacker ingot casting process (SILSO*) and the ramp assisted foil-casting technique (RAFT). In principle, both techniques are ideally suited for mass production. In a continuously operated ingot technique the output is limited only by the rate of tilting the moulds. In the case of RAFT the output is governed by the pulling rate. In neither case is
linear crystallization velocity the primary obstacle for a high output.
2. Ingot casting 2.1. S I L S O ® technique
In contrast to unconventional ingot techniques based on the modified Bridgman-Stockbarger technique (i.e., the ubiquitous crystallization process (UCP), heat exchanger method (HEM), Polyx, directional solidification (DS)) the SILSO ~ process has the advantage that the individual processing steps are decoupled with respect to location. As an example, melting and crystallization are carried out in different receptacles as well as at different places. This decoupling offers the possibility of optimising the individual processing steps, irrespective of process sequence; i.e., the different processing steps can be adjusted to achieve the best performances and material quality. The basic procedure for the SILSO ® technique has already been given in detail in refs. [1,2]. The sequence of the processing steps consists of: (a) mould preconditioning; (b) melt preparation; (c) casting; (d) crystallization; (e) annealing of crystallized ingot; (f) cooling down of annealed ingot. The melt preparation takes place in a con-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
trolled atmosphere of inert gas at reduced pressure to prevent excessive contamination of the silicon melt with carbon and oxygen. The enrichment of the silicon melt with oxygen and carbon occurs during the melting down procedure. Oxygen is introduced mainly through the reaction between the silicon melt and quartz crucible. Carbon is introduced via the gas phase as CO. An adequate melting cycle and a melt preparation phase ahead of the crystallization cycle are the basis for a material with low O and C contents. The melt energy for melting is generated through induction heating: the RF energy (with a frequency of 2-4 kHz) is coupled directly into the graphite crucible which acts as a susceptor for the actual melt crucible. Different crucible materials (quartz, graphite, silicon nitride) have been tested for their applicability in this process. Quartz crucibles have proved best with respect to melt contamination, flexibility of geometry, dimension, and costs. The moulds are assembled from graphite elements so that a square shaped inner cross-section is obtained. Preconditioning of the moulds in a separate preheating station assures the correct adjustment of temperature level and temperature distribution. This helps to develop a homogeneous chilled layer of solid silicon before the silicon can react with the mould walls. In this way it is possible to use the mould repeatedly and to suppress melt contamination. Although a multicrystalline ingot is produced deliberately, the linear growth rate should not exceed a critical threshold value of about 1 mm/min. An increase in output, however, is definitely established by increasing the cross-section of the solidifying mass. In addition, purification effects based on segregation are substantially improved by lower growth rates. At present silicon ingots with a weight of 120 kg and a cross-section of 43 × 43 cm2 are produced semi-continuously with a crystallization rate of about 0.5 kg/min. In future continuously operating equipment the individual processing steps will be further disentangled with a clear separation in time and space of both silicon and mould transfer.
563
2.2. Cutting and slicing An integrated line for the low-cost production of crystalline silicon wafers based on an economical ingot casting technique has to be supplemented by efficient cutting and slicing techniques
[31. The shaping and sectioning of large sized ingots into columns with a cross-section of 10 × 10 cm2 is best carried out by using a band saw. It allows the machining of bigger dimensions as compared with the conventional OD sawing disks. In addition, it reduces the kerf loss drastically. The slicing of the preshaped columns is done by a multi-blade slurry (MBS) sawing technique. In contrast to "conventional" MBS technology we use a high reciprocation cutting speed and an increased feed force. This results in a cutting rate which is higher by more than an order of magnitude compared to that attained with "conventional" MBS sawing.
2.3. Material properties The advances in SILSO ~ casting offer the possibility of preparing large-sized ingots with improved characteristics. Optimization of the casting and crystallization processes helped to improve the crystal structure and to lower the defect density. Figs. la and lb show the crystal structure of a 100 kg ingot with 43 × 43 cm2 cross-section in horizontal and vertical sections. An effective control of the nucleation of crystallites has been achieved through seeding with a SILSO ~ wafer at the bottom of the mould as well as through adequate preheating of the mould and heat transfer (see figs. 2a and 2b). Further improvements refer to a decrease and homogenization of the impurity content and the reduction of thermally induced stresses (the latter through the introduction of an in-situ annealing step). The concentration of the impurities carbon and oxygen has now reached values of < 0.2 x 1017/cm3 which is far below their respective saturation concentrations at the melting point of silicon. Fig. 3 shows the oxygen content and photovoltaic efficiency as a function of ingot height. High photovoltaic efficiencies are related to low
564
D. Helmreieh, E. Sirtl / Progress in ingot and foil casting of silicon
Fig. 1. (a) Horizontally and (b) vertically cut slice of 100 kg ingot with 43 X43 cm2 cross-section and 25 cm height. Marker represents
10 cm. o x y g e n levels a n d vice versa. A n i n v e s t i g a t i o n of the d e p e n d e n c e of PV perf o r m a n c e o n the degree of d o p i n g showed that the resistivity r a n g e b e t w e e n 0.5 a n d 1 o h m c m s h o u l d b e p r e f e r r e d over lower resistivities [4].
A s a result of recent a d v a n c e m e n t s in solar cell processing, PV p e r f o r m a n c e s as high as 16% A M 1.5 have b e e n o b t a i n e d a l r e a d y even with n o n - o p t i m i z e d S I L S O ® s a m p l e s with n o t yet r e d u c e d c a r b o n a n d o x y g e n c o n t e n t [5].
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
565
Fig. 2. Crystal structure at the bottom of 100 kg ingot (as per detail of fig. 1 b): (a) without, (b) with melt preparation [4]. The thickness of either plate is approprimately 1 cm.
AM 1.5 ( % ) ~
--
20t
\k \
o o
\ !
\
\
\
\
100
I
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12
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\\
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,[o]---_~
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Bigger charges and larger dimensions as well as an improved geometry of the ingot with a flattened top area and sharper edges have led to a m i n i m u m of material loss during the subsequent cutting processes. As a result of the above improvements it has been possible to raise the conversion efficiency to a value of 50 W / k g silicon feed stock.
6
\
/
I
3. Foil casting
/ \
/
8 --
[0]
( 1017/cm31 -~D"
Fig. 3. Oxygen content [O] and PV efficiency 7/ as a function of ingot height. Solid lines: with melt preparation; dashed lines: without melt preparation.
A large variety of techniques has been proposed for the direct production of self-supporting silicon wafers. Most of these techniques under development have not survived because of difficulties with respect to basic process performance, material quality a n d / o r costs.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
566
a
10x1Ocm2
silicon wafers
,nspect,on
regeneration
feed stock
W
heater
--
--silicon melt
--
--V_/_~J
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preconditioning consumable
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b
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o¢
¢¢ ¢ solid/iiql
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I--2 ~
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Fig. 4. Schematic diagram of the ramp assisted foil casting technique (RAFT): (a) general; (b) detail at the solid/liquid interface.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon
567
Fig. 5. The crystallinity of RAFT foil depends on local heat transfer (expressed by heat transfer coefficient h) and thermal properties of ramp material. (a) Surface and (b) cross-section of RAFT foil (thickness - 350/zm) with uncontrolled nucleation. (c) Surface and (d) cross-section of RAFT foil (thickness - 350/zm) with restricted nucleation [7].
3.1. Ramp assisted foil-casting technique (RAFT) RAFT profits by the much more efficient heat removal through heat conduction as compared to the heat removal through heat radiation in the case of other ribbon techniques. In addition, RAFT uses an inclined solid/liquid interface (SLI), which allows very high growth rates of about 15000 m m / m i n combined with a moderate temperature gradient perpendicular to the surface of the solidified silicon. As a result of the inclined SLI the actual linear crystallization rate is about two orders of magnitude lower (150 mm/min), but still more than two orders of magnitude higher than that applied in the SILSO ® process. The trade offs between increased throughput via increased growth rate and material quality show that an increase of growth rate should not be exaggerated. Therefore, RAFT is performed at a
reasonable growth rate which is essential with respect to crystallinity, thermal stress, defects and impurities. The basic procedure of RAFT has been published already in ref. [6]. The schematic diagram of RAFT shown in fig. 4 indicates that the slices begin to separate from the ramps at a certain stage during the cooling-off period. The ramps can be reused many times before the expendable coating has to be regenerated. The slices, now obtainable with an area of 100 × 100 mm2, can be used directly in standard solar cell processing. The silicon wafers (thickness 0.4 mm) are prepared from semiconductor-grade feed stock with a fixed growth rate (6000 mm/min). The gas atmospheres in the input and output conditions are set to minimize the incorporation of impurities (mainly carbon and oxygen).
568
D. Helmreieh, E. Sirtl / Progress in ingot and foil casting of silicon
3.2. M a t e r i a l properties
Characteristics of R A F T material have already been discussed [7]. In this paper the quality criteria of R A F T are discussed in the light of process criteria and contrasted against SILSO ® characteristics. The typical crystalline structure of R A F T foil is columnar with low-energy grain boundaries nearly perpendicular to the surface. The size of individual grains depends on the probability of heterogeneous nucleation which is influenced by the rate of local heat removal (expressed in terms of a heat transfer coefficient), the thermal properties of the ramp-material and the initial temperature (see fig. 5). Due to plastic deformation during the cooling phase silicon foils show a considerable dislocation density within the individual grains (figs. 6a and 6b). The dislocation density can be reduced drastically by heat treatment. In a high C / h i g h O situation the dislocation density decreases from some 106/cm 2 to below 103/cm 2 with a 2 h / 1000°C anneal (figs. 6c and 6d). In a low C / h i g h O situation a one step anneal shows no effect, in this case a pre-annealing step for precipitation nucleation seems to be necessary. As a consequence of the high crystallization rate, the segregation coefficient keff tends towards 1 which means negligible purification. From this it follows that higher standards for the starting material have to be met. The main impurities in R A F T foil are oxygen and carbon, which are introduced during the casting process from the silica crucible (O) and via the gas phase (C). T E M examinations of annealed R A F T foil show an array of submicroscopic precipitations along the dislocations (fig. 7). The nature of such precipitates has not yet been identified directly. From their etching behaviour, however, SiC precipitation is strongly indicated. Present work shows that a drastic lowering of both C and O concentrations is possible with certain precautions taken during the melting and casting steps. Photovoltaic data of test cells from R A F T foils are still hampered by unstabilized process data in RAFT. The best PV efficiencies obtained here (10.3% AM 1.5; 5 × 5 cm 2) are comparable to
Fig. 6. Dislocation density of RAFT foil with high C/high O (thickness - 350 /Lrn).(a) Surface and (b) cross-sectionof as grown material. (c) Surface and (d) cross-sectionof RAFT foil annealed at 1000oC for 2 h.
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Fig. 7. TEM picture (dark field; g = (220)) of annealed RAFT foil showing precipitation arrays along dislocations. Courtesy H. Gottschalk [8]. Marker represents 0.5 ~m.
those obtained with standard SILSO ® prepared b y the same device procedure (8.5-10.9% AM 1.5; 10 X 10 cm2) being applied in our own test laboratory.
4. Limitations of SILSO ® technique and RAFT 4.1. S I L S O ®
In an ultimate version of a continuously operating production equipment the output should be limited only by the casting cycle.' Melting and crystallization with subsequent stepwise annealing and cooling procedures should not be limiting
factors because the chronological sequence can be transformed into a local sequence (see fig. 8a). In the first case, the individual processing steps are carried out strictly one after another at one place; in the second case they are carried out partly in parallel at different places related to the respective processing step. A casting cycle of 120 kg/0.5 h converted into an annual output of 70 MW in solar cells can be projected for one equipment unit. The bottleneck in ingot technology is cutting and slicing. The annual output of an MBS saw with two working stations is projected to be about "2 MW". As the linear crystallization rate does not influence the output, process variables can be opti-
D. Helmreich, E. Sirtl / Progress in ingot and foil costing of si#con
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ingot
feed stock
INGOT
TEEHNIO.UE
wafer
X feed
stock
SHEET TECHNIQUE
F~g. 8. The interconnection of individual processing steps: (a) with the processing steps loosely linked together (ingot technique), (b) with the processing steps tightly linked together (sheet technique). M = melting, C = casting, X = crystallization, R = recharging of feed stock, P = preconditioning of mould. Bold-face arrows indicate material flow.
mized independently to improve the impurity and defect situation in SILSO ® material. High output and increased performance help to reduce the production costs for a SILSO ® wafer for the following reasons: (a) As a result of a low linear crystallization rate effective segregation can be considered an inherent purification step, thus allowing the quality (specification) of the feed stock material to be reduced. (b) Based on a high quality product both material yield and PV performance will be improved. Output and production costs will be influenced accordingly. (c) High throughput has a direct impact on costs due to reduced costs of depreciation and personnel.
"
4.2. R A F T
In this technique the individual processing steps are coupled very strongly with respect to time and place which means that its limitations are determined by the weakest link in the chain (see fig. 8b). At our present state of experience such a weak link can be seen here in the kinetics of crystallization with its scientific uncertainties in the formation and growth of nuclei. As a certain grain size is required, nucleation has to be minimized by avoiding heterogeneous seeding and the crystallization rate should, in addition, not exceed a certain threshold value. A prerequisite to the first point is a minimum quality of processing atmosphere, substrate, and feedstock material. The second point requires an appropriate crystallization (viz. pulling) rate. A reasonable growth rate is about 300 m m / s (higher rates result in a microcrystalline to amorphous material with inadequate PV properties). Such a growth rate is related to an estimated output of about 70 " M W " / y e a r per equipment unit. As slicing is not necessary in this process, feed stock consumption can be reduced drastically. To summarize, RAFT requires higher standards of feed stock and processing. On the other hand it needs less feed stock material and avoids complex processing of the as-grown slices. The SILSO * process needs additional processing steps (cutting/slicing) and a higher quantity of feedstock material but offers considerable advantages with an almost perfect theoretical impurity segregation and the possibility of disengaging the individual processing steps.
Acknowledgements The work described here has been supported by the Bundesministerium ftir Forschung und Technologie (BMFT) of the Federal Republic of Germany. References [1] B. Authier, in: Advances in Solid State Physics, Vol. 18, Ed. J. Treusch (Vieweg, Braunschweig, 1978) p. 1.
D. Helmreich, E. Sirtl / Progress in ingot and foil casting of silicon [2] J. Dietl, D. Helmreich and E. Sirtl, in: Crystals: Growth, Properties and Applications, Vol. 5, Ed. J. Grabmaier (Springer, Berlin, 1981) p. 43; see also D. Helmreich, in: Silicon Processing for Photovoltalcs, Vol. II, Eds. C.P. Khattak and K.V. Ravi (Elsevier, Amsterdam, 1986). [3] W. Ermer, D. Helmreich, A. Moritz and K. Mi~hlbaner, in: Proc. 6th European Community Photovoltaic Solar Energy Conf., London, 1985 (Reidel, Dordrecht, 1985) p. 971. [4] C. Gessert, D. Helmreich and M. Peterat, ref. [3], p. 891.
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[5] M.A. Green, S. Narayanan and S.R. Wenham, Internal Report of Solar Photovoltaic Laboratory, University of New South Wales, Australia (Dec. 1985), to be pubfished. [6] D. Helmreieh and J. Geissler, in: Proc. 5th European Community Photovoltaic Solar Energy Conf., Athens (Reidel, Dordrecht, 1983) p. 955. [7] J. Geissler, D. Helmreich and R. Wahlich, in: Proc. 18th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1986 (IEEE, New York, 1~86). [8] H. Gottschalk, to be pubfished.