Coating processes – Mold coating
Reexamination Certificate
2001-04-06
2002-12-10
Chen, Bret (Department: 1762)
Coating processes
Mold coating
C427S226000, C427S236000, C427S387000, C427S393600, C427S397700, C427S427000, C106S038220, C106S287110, C106S287130
Reexamination Certificate
active
06491971
ABSTRACT:
FIELD OF INVENTION
The invention relates to preparation and application of release coatings for crucibles used in the handling of molten materials that are solidified in the crucible and then removed as ingots, and more particularly to release coatings for crucibles used in the directional solidification of polycrystalline silicon.
BACKGROUND
Crucibles of fused-silica (quartz) are typically used in directional solidification of polycrystalline silicon. Quartz is chosen primarily for high-purity and availability. There are problems in using quartz, however, as a crucible for the production of silicon by this method.
Silicon in its molten state will react with the quartz crucible that is in contact with it. Molten silicon reacts with quartz to form silicon monoxide and oxygen. Oxygen will contaminate the silicon. Silicon monoxide is volatile, and will react with the graphite components inside the furnace. Silicon monoxide reacts with graphite to form silicon carbide and carbon monoxide. The carbon monoxide will then react with the molten silicon, forming additional volatile silicon monoxide and carbon. Carbon will contaminate the silicon.
The reaction between quartz and silicon promotes adhesion of the silicon to the crucible. This adhesion, combined with a difference in coefficients of thermal expansion between the two materials, creates stress in the silicon ingot, causing it to crack on cooling. It is known in the art that a release coating applied to the inside of the crucible in the area of contact with the ingot can prevent the reaction between silicon and quartz that leads to ingot contamination and cracking. To be effective, the release coating must prevent the silicon from reacting with the quartz crucible, and must not adversely contaminate the silicon either by itself or from contaminants within it.
A variety of materials and techniques are described in the literature, which attempt to solve the problem of reaction and adhesion of the crucible in contact with molten material. For example, U.S. Pat. No. 4,256,530 by Schmid et al., suggests coating the outside of a quartz crucible with a refractory material, to prevent reaction with adjacent carbon components. The coating does not contact the molten silicon. The method of preparing and applying the coating are, however, undisclosed.
U.S. Pat. No. 5,431,869 by Kumar, et. al., describes a multi-component release agent of silicon nitride and calcium chloride for silicon processing using a graphite crucible. The silicon nitride coating is applied as a slurry in an organic binder and solvent. The method of preparation and application are largely undisclosed. It is suggested that the binder can be removed after the coating, but the details are undisclosed. The calcium chloride portion is introduced with the bulk silicon, rather than as a coating, to the silicon-nitride coated crucible. The use of silicon nitride alone is described as unfavorable as a crucible coating for directional solidification of silicon.
U.S. Pat. No. 4,741,925 by Chaudhuri, et. al., describes a silicon nitride coating for crucibles applied by chemical vapor deposition at 1250 degrees Centigrade. U.S. Pat. No. 3,746,569 discloses the pyrolysis formation of a silicon nitride coating on the walls of a quartz tube. The process requires application temperatures at least 800 degrees C, and tempering at 1250 degrees Centigrade. U.S. Pat. No. 4,218,428 by Schmid, et. al., describes a technique of forming a glass layer inside a silica crucible by rapid heating to prevent cracking of silicon during melt-processing.
U.S. Pat. No. 3,660,075 by Harbur et al., discloses a coating of niobium carbide or yttrium oxide on a graphite crucible for melting fissile materials. The niobium carbide is applied by chemical vapor deposition, while the yttrium oxide is applied as a colloidal suspension in an aqueous inorganic solution. Details such as the method of preparation and application are largely undisclosed. U.S. Pat. No. 3,613,633 by Anderson, describes a heated rotating crucible used to hold articles to be coated. The crucible facilitates the containment of an “evaporant” which coats the articles therein. The crucible itself is not, however, used to contain molten material.
Reference is made in “Liquid Encapsulated Bridgman (LEB) Method for Directional Solidification of Silicon Using Calcium Chloride”, by P. S. Ravishankar, Journal of Crystal Growth, 94 (1989) 62-68, to the coating of a silica crucible with silicon nitride. However, no method is detailed for preparing and applying the coating. Furthermore, the resulting ingot quality using this coating is described as poor, due to particle nucleation leading to poor grain-growth and low solar cell efficiency.
Saito, et. al., in “A Reusable Mold in Directional Solidification for Silicon Solar Cells”, Solar Energy Materials, vol 9, (1983) pg 337-345, and in “A New Directional Solidification Technique for Polycrystalline Solar Grade Silicon”, Conf. Record of 15 th PV Specialists Conference, 1981, p 576-580, describes a coating of silicon nitride powder which is brushed onto a quartz, silicon carbide coated carbon or silicon nitride sintered mold. The powder is suspended in an organic solvent, which is evaporated by heating. Methods of preparation and application are not detailed, except that the coating needs to be at least 150 microns thick.
Saito reports, “The [silicon nitride] powder was mixed together with a suitable amount of organic solvent, such as liquid polyvinylalcohol, to form a slurry. The slurry was coated by a brush on the inner crucible walls. Then, the crucible was heated in an air ambient at 600 C. for 30 minutes to burn out the organic solvent. The coated layer thus obtained had good mechanical strength against scratching.” However, no method is detailed for preparing and applying the coating. Brushing, we have found, is a difficult way to obtain a uniform coating.
Scaling a laboratory process such as Saito's up to production requirements is also problematic. Saito's crucible was only a few inches across, and contained only 225 g of molten material, while the present technology requires crucibles over two feet across, and contains over 240 kg of molten material. The difference in size and weight makes the physical demands on the coating and the coating process much more profound.
Other publications that mention crucible coatings, usually of silicon nitride, for directional solidification of silicon, but do not discuss methods of preparation or details of application, include: “HEM Technology for Photovoltaic Applications”, Khattak et al, 6th IPSEC Conference, New Delhi, India, 1992, p 117-124; “Growth and Characterization of 200 kg Multicrystalline Silicon Ingots by HEM”, 26th IEEE PVSC Conference, Anaheim, Calif., Sep. 29-30 1997; “Growth of 240 kg Multicrystalline HEM Silicon Ingots”, 2nd WCPEC Conference, Vienna, Austria, Jul. 6-10 1998; “High Efficiency Solar Cells Using HEM Silicon”, First WCPEC Conference, Dec. 5-9, Hawaii, 1994 p 1351-1355; “Characteristics of HEM Silicon in a Reusable Crucible”, 23rd IEEE PV Specialists Conference, Louisville, Ky., May 10-14, 1993, p 73-77; “Analysis and Control of the Performance-Limiting Defects in HEM-Grown Silicon for Solar Cells”, Material Research Society Symposium Proceedings, 1995, v 378 p 767-776; “Lifetime Improvement of Multicrystalline Silicon”, Habler et al., 14 th EPVSE Conference, Barcelona, Spain, Jun. 30-Jul. 4, 1997, p 720-723; “
3
D Distribution Study of Impurities into a Polix Ingot”, Borne et al., 13th European PV Conference, Nice, France, 1995, p 1340-1343; “Study and Conditioning of Defect Areas in Eurosil Multicrystalline Silicon”, Acciarri et al., 13th European PV Conference, Nice, France, 1995, p 1336-1339; and “Selection of a Crucible Material in Contact with Molten Silicon”, Revel et al., 5th EC PV SEC, Athens, Greece, Oct. 17-21 1983, p 1037-1042.
Prior art references include specific references to powdered mold release agents for application to crucibles in the directional solidification of silicon. In addition, th
Chandra Mohan
Costantini Michael A
Hariharan Alleppey V.
Matthei Keith
Chen Bret
Crockford Kirsten A.
G.T. Equipment Technologies, Inc
Maine Vernon C
Maine & Asmus
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