FIELD OF INVENTION
[0002] The invention disclosed herein relates, in general, to manufacturing of multi-crystalline silicon. More specifically, the present invention relates to coatings for crucibles used in manufacturing of multi-crystalline silicon.
BACKGROUND
[0003] Manufacturing of multi-crystalline silicon (mc-Si) ingots, typically for photovoltaic use, by a directional solidification process is a promising low cost alternative to Czochralski growth process. The directional solidification process involves melting of silicon ingots into molten form and thereafter, progressively solidifying the molten silicon to form mc-Si. This process is carried out in a crucible that may be placed inside a furnace
[0004] The crucibles used in the directional solidification process to form mc-Si are usually made of fused silica or quartz. Silica is chosen primarily for its high-purity and easy availability. However, using silica has its own share of problems. Silicon in molten form reacts with silica of the crucible to form silicon monoxide and oxygen. The oxygen can react with the molten silicon to form Si02 and thereby contaminates the mc-Si being formed. Further, the silicon monoxide formed is volatile, and reacts with graphite components inside the furnace to form silicon carbide and carbon monoxide. The carbon monoxide may then react with the silicon in molten form, forming additional volatile silicon monoxide and carbon that can further contaminate the silicon. Furthermore, the reaction between silica and silicon promotes adhesion of the silicon to the crucible.
[0005] One technique that can be used to reduce this contamination is to line an internal surface of the crucible with silicon nitride. However, even use of the silicon nitride coating has problems; for example, the silicon nitride coating is mechanically weak and can peel or flake off during or even before use, resulting in contamination. Further, the silicon nitride coating can also undergo solid state diffusion into the silicon after solidification. Furthermore, even the silicon nitride coating includes transition metal impurities that dissolve into the silicon in molten form leading to significant metal contamination in the mc-Si.
[0006] In light of the above discussion, there is a need for an improvement in the coatings for a crucible being used in a directional solidification process, which enable a reduction in contamination of the silicon during the directional solidification process.
BRIEF DESCRIPTION OF FIGURES
[0007] The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention may best be understood by reference to the following description, taken in conjunction with the accompanying drawings. These drawings and the associated description are provided to illustrate some embodiments of the invention, and not to limit the scope of the invention.
[0008] FIG. 1 illustrates a perspective view of a refractory container including a coating, in accordance with an embodiment of the present invention;
[0009] FIG. 2 illustrates a flowchart describing a method of forming a coating for use in a refractory container, in accordance with an embodiment of the present invention;
[0010] FIG. 3 illustrates a flowchart describing another method of forming a coating for use in a refractory container, in accordance with another embodiment of the present invention;
[0011] FIG. 4 illustrates a table indicating trace metallic element content in silicon nitride before and after treatment with EDTA, in accordance with another embodiment of the present invention; and
[0012] FIG. 5 illustrates a perspective view of a refractory container including a coating, in accordance with another embodiment of the present invention.
[0013] Those with ordinary skill in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the
dimensions of some of the elements in the figures may be exaggerated, relative to other elements, in order to improve the understanding of the present invention.
[0014] There may be additional structures described in the foregoing application that are not depicted on one of the described drawings. In the event such a structure is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification.
SUMMARY
[0015] The invention provides a coating composition to be applied on a portion of an internal surface of a crucible. The crucible is suitable for directional solidification of silicon. The directional solidification process includes melting polycrystalline silicon into molten silicon and thereafter crystallizing the molten silicon to form multi-crystalline silicon. The coating composition includes silicon nitride and EDTA in a weight ratio ranging from 4:1 to.20:l. The silicon nitride in the coating composition acts as a releasing agent and prevents adhesion between the molten silicon and the crucible and EDTA chemically bonds with metallic impurities present in either the silicon nitride or the crucible and inactivates the metallic impurities and prevents transfer of the metallic impurities from the silicon nitride or the crucible into the molten silicon. Use of EDTA cleans the silicon nitride and the crucible and significantly reduces metal contamination in the multi-crystalline silicon being manufactured.
[0016] The present invention provides a coating composition for application on at least a portion of an internal surface of a refractory container. The refractory container being suitable for melting polycrystalline material into a molten material and thereafter crystallizing the molten material. Further, the coating composition includes a releasing agent and a chelating agent in a weight ratio ranging from 4:1 to 20:1. The releasing agent prevents adhesion between the molten material and the refractory container and the chelating agent chemically bonds with metallic impurities present in one of the releasing agent and the refractory container. This bonding
enables inactivating the metallic impurities and prevents transfer of the metallic impurities from the releasing agent and the refractory container into the molten material.
[0017] In some embodiments, the releasing agent is silicon nitride.
[0018] In some embodiments, chelating agent is one of Ethylene diamine tetraacetic acid (EDTA), Ethylene glycol tetraacetic acid (EGTA) and Dimercaprol.
[0019] In some embodiments, the coating composition is formed by mixing the releasing agent and the chelating agent in a neutral solvent to form a solution. Thereafter, the solution is allowed to stand for 12 to 24 hours, whereby impurities in the solution are allowed to sediment and separate. Subsequently, the solution is filtered to remove the impurities and obtain a filtrate that is dried to obtain the coating'composition,
[0020] In some embodiments, the coating composition can be formed by dissolving the releasing agent and the chelating agent in a neutral solvent.
[0021] In some embodiments, the polycrystalline material is polycrystalline silicon.
DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0022] Before describing the present invention in detail, it should be observed that the present invention utilizes a combination of method steps and apparatus components related to protective coating for a crucible. Accordingly the apparatus components and the method steps have been represented where appropriate by conventional symbols in the drawings, showing only specific details that are pertinent for an understanding of the present invention so as not to obscure the disclosure with details that will be readily apparent to those with ordinary skill in the art having the benefit of the description herein.
[0023] While the specification concludes with the claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood
from a consideration of the following description in conjunction with the drawings, in which like reference numerals are carried forward.
[0024] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.
[0025] The terms "a" or "an", as used herein, are defined as one or more than one. The term "another", as used herein, is defined as at least a second or more. The terms "including" and/or "having" as used herein, are defined as comprising (i.e. open transition). The term "coupled" or "operatively coupled" as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.
[0026] Referring now to the drawings, there is shown in FIG. 1 a refractory container 100 in accordance with an embodiment of the present invention. The refractory container 100 is suitable for melting an ingot to a molten material, and thereafter crystallizing the molten material to form a multi-crystalline ingot. In an embodiment, the material of the ingot can be a semiconductor material, for example, but not limited to, silicon.
[0027] The refractory container 100 is formed of any material suitable for directional solidification of the semiconductor material. For example, the refractory container 100 may be formed from a material selected from silica, silicon carbide, graphite, boron nitride, other ceramic material, mixtures thereof and composites thereof. In addition, the material used for forming the refractory container 100 is capable of withstanding temperatures at which the semiconductor material is melted and solidified. For example, the material for forming the refractory container 100 is suitable for withstanding temperatures of at least above 300° C,
preferably temperatures of at least above 1000° C and yet more preferably temperatures of at least above 1600° C. Further, in an embodiment, the refractory container 100 can be a crucible 100. The crucible 100 being suitable for melting polycrystalline silicon to molten silicon, and thereafter crystallizing the molten silicon to form a multi-crystalline silicon.
[0028] Further, the refractory container 100 is shown to be defined by an external surface 102 and an internal surface 104. Further, in FIG. 1 the refractory container 100 is illustrated to have a bottom end, a top end and, a sidewall that extends from the bottom end to the top end. While the refractory container 100 is illustrated with four sidewalk in FIG. 1, it should be appreciated that the refractory container 100 may include fewer than four sidewalls or may include more than four sidewalls without departing from the scope of the present invention. Also, corners between the sidewalls may be connected to each other at any angle suitable for forming an enclosure of the refractory container 100 and may be sharp as illustrated in FIG. 1 or may be rounded. In some embodiments, the refractory container 100 can include only one sidewall that is generally cylindrical or spherical in shape. Further, the refractory container 100 is illustrated to be open, i.e., the top end of the refractory container 100 is not shown to include a cover. However, it should be appreciated that the refractory container 100 may have a cover (not shown) at the top end opposite the bottom end without departing from the scope of the present invention. Further, it should also be understood that any refractory container shapes and dimensions may be used in the present invention without departing from the scope of the present invention.
[0029] Another exemplary embodiment of a refractory container in accordance with the present invention is depicted in FIG. 5. A refractory container 500 is shown to be defined by an external surface 502 and an internal surface 504. In the figure corners between sidewalls defining the refractory container are shown to be connected to each other at an angle of 90°.
[0030] The refractory container 100 is also shown to include a coating 106. The coating 106 is shown to be applied on the internal surface 104 of the refractory container 100. The coating 106 covers atjeast a portion of the internal surface 104. In an embodiment, the coating 106 is
applied on a portion of the internal surface 104 of the refractory container 100 that comes in contact with the molten semiconductor material.
[0031] The coating 106, in accordance with an embodiment of the present invention, includes a releasing agent and a chelating agent. In an embodiment, the releasing agent and the chelating agent are present in a weight ratio ranging from 4:1 to 20:1, preferably in a ratio ranging from 8:1 to 16:1. [0032] The releasing agent enables inhibition or prevention of adhesion between the molten material and the refractory container 100. The releasing agent provides a slip effect between the refractory container 100 and the niolten material. Material for the releasing agent is chosen such that the releasing agent adheres to the crucible but does not adhere to the molten material. In an embodiment, the releasing agent can be silicon nitride. The silicon nitride used as the releasing agent can be in any of α or ß crystallographic phase silicon nitride or amorphous silicon nitride.
[0033] Further, the chelating agent chemically bonds with metallic impurities present in at least one of the releasing agent and the refractory container 100. This bonding inactivates the metallic impurities and prevents transfer of the metallic impurities from the releasing agent and the refractory container 100 into the molten material. In an embodiment, the chelating agent can be Ethylene diamine tetraacetic acid (EDTA). EDTA is a polyamino carboxylic acid having the following structure:
(Formula Removed)
[0034] In real life applications, EDTA, being a hexadentate ligand, sequesters metal ions such as Ca2+, Fe3+, Ti2+, Cr3+ and Ni2+, by binding the metal ions. The metal ions on being bound by EDTA form a co-ordination complex and remain in solution but exhibit diminished reactivity. This prevents the metal impurities present in the releasing agent like silicon nitride and the material forming the refractory container 100, from reacting with the molten material. Example, of a metal-EDTA bond can be seen below, wherein 'M' is a metal ion:
(Formula Removed)
[0035] Other examples of chelating agent include, but are not limited to, Ethylene glycol tetraacetic acid (EGTA) and Dimercaprol.
[0036] The method of forming the coating 106 has been described in detail in conjunction with FIGS. 2 and 3.
[0037] Moving on to FIG. 2, there is shown a flowchart describing a method 200 of forming a coating composition, for example the coating 106, for use in a refractory container, for example the refractory container 100. For the purpose of this description, the method 200 is explained in conjunction with the coating 106 and the refractory container 100. However, it will be readily apparent to those ordinarily skilled in the art that the method 200 can also be applied, without deviating from the scope of the invention, for any other refractory container. Moreover, the invention is not limited to the order in which the steps are listed in the method 200. In addition, the method 200 can contain a greater or fewer numbers of steps than those shown in FIG. 2.
[0038] The method 200 is initiated at step 202. Thereafter at step 204, the releasing agent and the chelating agent are mixed with a neutral solvent. For example, in real life applications, EDTA can be mixed with silicon nitride in distilled water. The EDTA and the silicon nitride completely dissolve in distilled water to form a solution, wherein the distilled water acts as a solvent and the EDTA and the silicon nitride are solutes. Further, the distilled water is in a quantity sufficient enough for dissolving the EDTA and the silicon nitride completely. Thereafter, at step 206, the solution is allowed to stand in a beaker for about 12 to 24 hours. In an embodiment, the step 206 may be repeated two to three times. At the end of this step impurities in the solution sediment and separate.
[0039] Subsequently, at step 208, the solution is filtered to obtain a filtrate and remove the impurities which had separated at step 206. In an embodiment, the solution may be filtered using a filter having a pre-defined porosity, for example a Grade 4 filter having a porosity of 20 - 25 microns, the filter enabling separation of the sedimented impurities from the solution and obtaining the filtrate. The filtrate includes a homogeneous mixture of the releasing agent and the chelating agent in the neutral solvent. Thereafter the filtrate is dried to obtain a dried powder, and the dried power is taken as the coating 106 for application on the refractory container 100. The filtrate may be dried at room temperature or any other suitable drying methods. Thereafter, the method 200 is terminated at the step 210.
[0040] Moving on to FIG. 3, there is shown a flowchart describing another method 300 of forming a coating composition, for example the coating 106, for use in a refractory container, for
example the refractory container 100. For the purpose of this description, the method 300 is explained in conjunction with the coating 106 and the refractory container 100. However, it will be readily apparent to those ordinarily skilled in the art that the method 300 can also be applied, without deviating from the scope of the invention, for any other refractory container. Moreover, the invention is not limited to the order in which the steps are listed in the method 300. In addition, the method 300 can contain a greater or fewer numbers of steps than those shown in FIG. 3.
[0041] The method 300 is initiated at step 302. Thereafter at step 304, the releasing agent and the chelating agent are mixed with a neutral solvent. For example, in real life applications, EDTA can be mixed with silicon nitride in distilled water to form a solution, wherein the distilled water is in a quantity sufficient enough for dissolving the EDTA and the silicon nitride completely. Thereafter at the step 306, the solution is used as the coating 106 for application on the refractory container 100. Thereafter, the method 300 is terminated at the step 308.
[0042] The coating 106 may be applied to the inner surface 104 of the refractory container 100, using any commonly known method in the art. For example the coating 106 may be applied by any one of a brush, a roller, dispensing, spin-coating, spray coating.
[0043] In an embodiment, the coating 106 once applied is allowed to dry in air at room temperature in a ventilated area. Thereafter step of applying the coating may be repeated with drying in-between to build up a layered coating to achieve a required coating thickness. Subsequently, the layered coating is baked in a furnace to a temperature between 800 to 1000° C for 8 to 10 hours to affix the coating 106 to the refractory container 100. For the purpose of this description, the method of applying the coating 106 to the refractory container 100 has been explained using the above method. However, it will be readily apparent to those ordinarily skilled in the art that the method of applying the coating 106 can contain a greater or fewer numbers of steps than those listed. Further, it should be appreciated that the method of application of the coating 106 to the refractory container 100 described above is an exemplary process that has been presented for easy understanding of the invention and not to limit the scope of the present invention.
[0044] In real life applications, 350 gm of commercial silicon nitride powder can be used as a coating in a refractory container. According to the invention, this coating also includes 0.1 Mole of EDTA (~37.25g). Content of trace elements in the commercial silicon nitride powder before and after treating with EDTA in accordance with an embodiment of the present invention has been illustrated in a table shown in FIG.4.
[0045] Various embodiments, as described above, provide an improved coating composition having several advantages. An advantage of using a mixture of EDTA and silicon nitride, in accordance with an embodiment of the present invention, is that both silicon nitride and EDTA are water soluble, resulting in elimination of need for a binder or a surfactant. However, it should be appreciated that in an embodiment, the coating composition may include a binder or a surfactant.
[0046] The primary advantage of using EDTA is that it removes most transition metal ions and main group metal ions from the silicon nitride. EDTA is versatile and can form a ligand with metal ions (Fe3+, Ti2+, Ni2+, Cr2+, Cu2+ and Ca2+). Furthermore, after treatment with EDTA, a parent compound will have less metal impurity concentration. Similarly, EDTA used in the present invention reduces the metal impurities present in the silicon nitride and minimize any effect on minority carrier diffusion length and solar cell efficiency.
[0047] Additionally, the chelating agents used in accordance with the present invention are easily available and addition of chelating agents does not cause any addition product formation. Also, since EDTA is an organic additive, EDTA evaporates along with the sequestered metal ions at a temperature of 250 °C, thereby providing efficient cleaning.
[0048] While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is not to be limited by the foregoing examples, but is to be understood in the broadest sense allowable by law.
[0049] All documents referenced herein are hereby incorporated by reference.

CLAIMS
What is claimed is:
1. A coating composition for application on at least a portion of an internal surface of a refractory container, said refractory container suitable for melting polycrystalline material into a molten material and thereafter crystallizing said molten material, said coating composition comprising a releasing agent and a chelating agent in a weight ratio ranging from 4:1 to 20:1, wherein said releasing agent prevents adhesion between said molten material and said refractory container, and further wherein said chelating agent chemically bonds with metallic impurities present in at least one of said releasing agent and said refractory container, thereby inactivating said metallic impurities and preventing transfer of said metallic impurities from at least one of said releasing agent and said refractory container into said materials.
2. The coating composition of claim 1, wherein said releasing agent is silicon nitride.
3. The coating composition of claim 1, wherein said chelating agent is one of Ethylene diamine tetraacetic acid (EDTA), Ethylene glycol tetraacetic acid (EGTA) and Dimercaprol.
4. The coating composition of claim 1, wherein said coating composition is formed by:
mixing said releasing agent and said chelating agent in a neutral solvent, thereby forming a solution, wherein said neutral solvent is present in an amount sufficient to dissolve said releasing agent and said chelating agent;
allowing said solution to stand for 12 to 24 hours, whereby allowing impurities in said solution to sediment and separate;
filtering said solution to remove said impurities and obtain a filtrate; and
drying said filtrate to obtain said coating composition.
5. The coating composition of claim 1, wherein said coating composition is formed by dissolving said releasing agent and said chelating agent in a neutral solvent, said neutral solvent being present in an amount sufficient to dissolve said releasing agent and said chelating agent.
6. The coating composition of claim 1, wherein said polycrystalline material is polycrystalline silicon.