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Silicon Info: Multicrystalline Ingot Growth

A multicrystalline structure with grains on the order of mm to cm in width and approximately columnar along the solidification direction is characteristic of directional solidification (DS) or casting in crucibles, and also of electromagnetic semicontinuous casting.  Casting of multigrain silicon was reported more than 80 years ago (Allen, 1913). In 1960, large multicrystalline-silicon domes and plates 330 mm in diameter by 50 mm thick were formed via ambit casting.  Early applications were for infrared optics and other non-semiconductor uses.  A summary of early silicon casting approaches was compiled by Runyan (1965).  Si casting into graphite molds as an alternative to CZ growth for PV applications was reported by Fischer and Pschunder (1976).
 

Directional Solidification/Casting

DS can be carried out in a separate crucible after (or as) silicon is poured into it from a melting crucible, as indicated in the figure above.  This process is usually referred to as silicon casting.  Or, the silicon can be melted and directionally solidified in the same crucible (e.g., the bottom crucible).  This technique is referred to as directional solidification.  It is simpler than casting, because no melt pouring is involved, but there are longer reaction times at high temperature between the melt and the crucible, as well as longer turnaround times.  Because melting and solidification are decoupled in casting, higher throughputs are possible.  But the process and equipment are more complex.

DS of silicon in the same quartz (fused silica) crucible used to melt it would seem to be a logical approach for large-grained ingot production, but sticking and thermal expansion mismatch between the solidified Si and the crucible lead to significant cracking problems.  This was alleviated to some extent by deliberately weakening the inner wall of the quartz crucible (Khattak and Schmid, 1978).  High-density graphite crucibles were introduced for DS as a way of avoiding the cracking (Ciszek, 1979).  Most crucible-based commercial methods of growing multicrystalline Si ingots for PV consumption use either the casting method or DS in the melting crucible.  The crucibles are either graphite or quartz (often coated with Si3N4 or other compounds to discourage sticking or enable reuse).  The actual growth process is DS in either method.

Multicrystalline ingots as large as 690 mm x 690 mm in cross section and weighing as much as 240 kg are grown in total cycle times of 56 hours.  The resultant throughput is 4.3 kg/h.  Thus, the larger area, compared to CZ crystals, more than offsets the somewhat lower linear growth rates leading to higher throughputs for DS by a factor of ~3.   Either induction heating or resistance heating can be used.  The energy consumption for DS is in the range of 8-15 kWh/kg.  Unlike CZ growth, the solid/liquid interface is submerged in DS, and precipitates or slag at the melt surface do not disrupt growth.  DS is a simpler process requiring less skill, manpower, and equipment sophistication than CZ growth, which can make it a lower cost process.  However, there are also drawbacks.  There are numerous crystal defects (grain boundaries and dislocations) due to the multicrystalline structure.  Impurity contents can be higher depending on the crucibles used, and portions of the bottom, sides, and top surface of the ingot are discarded. So, the lower cost of DS is at the expense of solar cell efficiency.  DS solar cells are about 85% as efficient as CZ cells.  The best efficiency of small cells, with sophisticated processing, is 18.6%.  Typical large production cell efficiencies are 13%-14%, with good consistency. 

Electromagnetic  Casting

Electromagnetic casting (EMC), shown here, has some similarities to the casting and DS methods just described, but also has several unique features that change the ingot properties and warrant a separate discussion.   The method was first applied to semicontinuous silicon ingot casting by Ciszek (1985, 1986).  EMC is based on induction-heated cold-crucible melt confinement, except that unlike the conventional cold crucible, there is no crucible bottom.  A parallel, vertical array of close-spaced, but not touching, water-cooled, conducting fingers is attached at one end to a water-cooling manifold.  The other end of each finger is closed.  An internal distribution system carries cooling water to the tip and back again.  The shape of the region enclosed by the close-spaced fingers determines the cross section of the cast ingot, and a wide variety of shapes are possible (circular, hexagonal, square, rectangular, etc.).  Silicon is melted on a vertically moveable platform (typically graphite) located within the finger array.  The melting is accomplished by induction heating after suitable preheating.  The induction coil, placed outside the finger array, induces a current to flow on the periphery of each finger, around the finger's vertical axis.  Like a high-frequency transformer, each finger in turn induces a current to flow in the periphery of the silicon charge, about its vertical axis.  The silicon is heated by its resistance to the current flow.  There is Biot and Savart-law repulsion between the current flowing in the periphery of the silicon melt and the currents flowing in the fingers, because they are induced to flow in opposite directions at any particular instant in the RF cycle.  Thus, the melt is repulsed from the water-cooled fingers.  The open-bottom arrangement allows the platform to be withdrawn downward, solidifying the molten silicon, while new melt is formed by introducing feed material from the top.  In this way, a semicontinuous casting process can be carried out. 

A variety of feed silicon geometries can be used (melts, rods, pellets, scrap, etc.).  Because the interface is submerged, feed perturbances or slag at the melt surface do not affect the solidification front.  Ingot lengths of 3 m have been demonstrated.  The cross section of the ingots has evolved over years of development and is currently about 350 mm x 350 mm. The cold fingers allow steep thermal gradients and fast growth speeds (~1.5-2.0 mm/min), even in ingots with large cross sections.  But they also cause a steeply curved interface that is concave toward the melt.  Thus, grains are neither as columnar nor as large as in conventional DS.  The average grain size is on the order of 1.5 mm in large ingots.  This decreases A variety of feed silicon geometries can be used (melts, rods, pellets, scrap, etc.).  Because the interface is submerged, feed perturbances or slag at the melt surface do not affect the solidification front.  Ingot lengths of 3 m have been demonstrated.  The cross section of the ingots has evolved over years of development and is currently about 350 mm x 350 mm. The cold fingers allow steep thermal gradients and fast growth speeds (~1.5-2.0 mm/min), even in ingots with large cross sections.  But they also cause a steeply curved interface that is concave toward the melt.  Thus, grains are neither as columnar nor as large as in conventional DS.  The average grain size is on the order of 1.5 mm in large ingots.  This decreases t, but the relatively high purity and freedom from oxygen and carbon impurities (O < 6x1015; C < 8x1016) largely offset the grain-size effect, so that solar cell efficiencies of about 14%-15% are obtained on 15 cm x 15 cm cells.  The throughput of EMC is the highest of any ingot growth technique - up to approximately 30 kg/h.  The power consumption is about 12 kWh/kg. 
__________________________
Allen, T.B. (1913) U.S. Patent 1,073,560.
Ciszek, T.F. (1979) J. Crystal Growth 46, 527.
Ciszek, T.F. (1985) J. Electrochemical Soc. 132, 963.
Ciszek, T.F. (1986) U.S. Patent 4,572,812.
Fischer, H., and Pschunder, W. (1976) IEEE 12th Photovoltaic Specialists Conf. Record, IEEE, New York, 86.
Khattak, C.P., and Schmid, F. (1978) IEEE 13th Photovoltaic Specialists Conf. Record, IEEE, New York, 137.

Runyan, W.R. (1965) "Silicon Semiconductor Technology" McGraw-Hill Book Company, New York. 

 


Siliconsultant, P.O. Box 1453, Evergreen, CO 80437 USA                                   e-mailted_ciszek@siliconsultant.com

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This page was last updated on June 19, 2016