Abstract:
A method of making a release coating includes the following steps: forming a mixture that includes (a) solid components comprising (i) 20-99% silicon by weight and (ii) 1-80% silicon nitride by weight and (b) a solvent; applying the mixture to an inner portion of a crucible or graphite board adapted to form an ingot or wafer comprising silicon; and annealing the mixture in a nitrogen atmosphere at a temperature ranging from 1000 to 2000° C. The invention may also relate to release coatings and methods of making a silicon ingot or wafer including the use of a release coating.

Description:
GOVERNMENT RIGHTS STATEMENT 
     This invention was made with Government support under the Department of Energy Contract No. DE-AC36-99GO10337. Subcontract ZAX-5-33628-06. The Government has certain rights to the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention may relate to a release coating comprising a mixture of silicon nitride and silicon powder. In certain embodiments, the release coating may inhibit or prevent the formation of a silicon carbide (SiC) alloy that can act as a bond or adhering agent between a silicon part and a graphite part. 
     Graphite has many properties that may make it a useful material on which to grow silicon parts. But in some instances, silicon can bond to the graphite, forming a SiC alloy. 
     The semiconductor industry may use quartz crucibles to line graphite parts. In some instances, this works well, such as where the interface between the quartz and the silicon may be discarded. Where silicon parts need to be released—for example, in ingot casting or other wafer casting processes—mixtures of Si 3 N 4  or SiO 2  may be used as release coatings. See U.S. Pat. No. 4,218,418. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an embodiment, there is a method of making a release coating including the following steps. First, a mixture is formed that includes (a) solid components comprising (i) 20-99% silicon by weight and (ii) 1-80% silicon nitride by weight and (b) a solvent. Second, the mixture is applied to an inner portion of a crucible or graphite board adapted to form an ingot or wafer comprising silicon. And third, the applied mixture is annealed in a nitrogen atmosphere at a temperature ranging from 1000 to 2000° C. 
     In another embodiment, there is a method of making an ingot or wafer comprising silicon. The method includes the following steps. First, a mixture is formed that includes (a) solid components comprising (i) 30-90% silicon by weight and (ii) 10-70% silicon nitride by weight and (b) water. Second, the mixture is applied to a crucible or graphite board adapted to form the ingot or wafer comprising silicon. Third, the applied mixture is annealed in a nitrogen atmosphere at a temperature ranging from 1300 to 1500° C. Fourth, silicon is poured into the crucible or graphite board and melted. Fifth, the crucible or graphite board is cooled until the silicon solidifies. And sixth, the ingot or wafer is removed from the crucible or graphite board. 
     In another embodiment, there is a colloidal mixture for preparing a release coating. The colloidal mixture consists essentially of: (a) a solid component comprising (i) 30-90% silicon by weight and (ii) 10-70% silicon nitride by weight; and (b) a liquid component comprising water. The colloidal mixture has a liquids-to-solids weight ratio of the solid component to liquid component of 3:1 to 10:1. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  partially illustrates a crucible within the heating chamber of a casting furnace, in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At least in certain exemplary embodiments, the present invention may prevent or inhibit a bond from forming between silicon and carbon, may introduce little to no impurities, and may be relatively inexpensive. In certain exemplary embodiments, the present invention may be used in the manufacture of silicon wafers for use in solar energy-related applications, including, but not limited to, photovoltaic modules. See U.S. Patent App. Pub. Nos. 2005/0176218 and 2007/0034251. The silicon wafer may be doped with one or more n-type or p-type dopants, including, for example, elements in group III and/or group V of the Periodic Table. Suitable dopants may include, for example, boron, arsenic, phosphorus and gallium. 
     As used herein, “silicon” means pure silicon and silicon comprising dopants or other impurities. Furthermore, as used herein, “silicon ingot” shall include silicon wafers or any other suitable shape or form of solidified silicon. 
     Crucibles and casting furnaces are well known in the art of manufacturing silicon wafers, and any suitable combination of configuration and operational parameters may be used in connection with at least certain embodiments of the present invention.  FIG. 1 , for example, illustrates a crucible  100  within the cylindrical heating chamber defined by the heaters  102  of a casting furnace. The crucible  100  is defined, in part, by a disc  106  (e.g., a molybdenum or graphite disc) which itself is supported by rods  112  (e.g., graphite rods) mounted on a support plate  114  (e.g., a graphite support plate) on the bottom of the heating chamber. A heat exchanger  116  (e.g., a helium cooled molybdenum heat exchanger), extends through openings in the center of the plate  114  and engages the center of disc  106 . Although  FIG. 1  is not drawn to scale, crucible  100  may be about 6 inches in height and diameter and its cylindrical wall  104  and base  24  may be about 0.15 inches thick. 
       FIG. 1  illustrates that the crucible  100  contains a cylindrical wall  104  (e.g., a graphite wall). The shape of silicon ingot  108  is influenced by the particular shape of the wall  104  and disc  106 . As illustrated in  FIG. 1 , there is a boundary layer  110  comprising a release agent between silicon wafer and the wall  104  and disc  106 . The boundary layer  110  need not be coterminous with the height of the silicon ingot. In the embodiment of  FIG. 1 , for instance, boundary layer  110  extends further up walls  104  than silicon ingot  108 . Furthermore, the boundary layer need not have a uniform or substantially uniform thickness and need not coat all or substantially all portions of walls  104  and/or disc  106 . 
     The silicon—whether pure or impure (e.g., containing dopants, impurities, etc.)—may be melted in the crucible, then solidified to form the ingot  108 . Alternatively, the silicon may be at least partially melted when poured into the crucible  100 , then solidified. The solidification process may include the use of a “seed.” After the silicon is at least partially or mostly solidified, the temperature of the crucible wall  104  will be decreased, e.g., below the silicon melting temperature, and the crucible  100  and solidified ingot  108  will then be cooled. 
     In an embodiment described herein, the boundary layer  110  comprising a release agent may inhibit or prevent the formation of a bond between the silicon ingot  108  and the contacting portions of the crucible walls  104 . In an embodiment, the release agent comprises a mixture of silicon nitride, silicon, and solvent (e.g., water). Preferably, this mixture may be approximately homogenous after mixing. This mixture, which may comprise a colloidal suspension, is sprayed on or applied to at least some of the graphite part(s) of the crucible, then preferably annealed at a temperature in the range of 1000 to 2000° C. (more preferably 1300 to 1500° C.) in a nitrogen atmosphere. In a preferred embodiment, the suspension is applied to substantially all of the graphite that would contact silicon. In some embodiments, the nitrogen atmosphere may also comprise other gases, preferably inert gases such as argon, etc. 
     In another embodiment, the release coating may be used in connection with a wafer casting process involving a graphite board having a machined mold. In an embodiment, for example, there may be a graphite board measuring 115 cm long, 30 cm wide, and 0.5 cm thick. The graphite board may be machined to have 7 molds on each side. The molds may be, for example, 0.1 cm deep and 16 cm 2 . The board may be first coated with a release coating as described herein, then silicon powder may be poured into the molds on the board. The board with the silicon may be pushed through the furnace with a temperature profile such that the silicon first melts and then crystallizes. 
     One suitable temperature profile according to an exemplary embodiment, for example, may be as follows. The peak temperature set point is about 1500° C. The melting process may take about 3 to 5 min from room temperature to 1415° C., and the crystallization process may take about 1 to 2 min to about 1400° C. The silicon may not exceed the melting point of silicon, which is 1415° C. The process of melting silicon (i.e., changing from solid to liquid) requires a large amount of energy (e.g., changing the temperature from 1414° C. to 1415° C. requires the same amount of energy as heating the solid silicon from room temperature to 1414° C.). This may cause the temperature to be stable at the melting point until all silicon has melted. In a preferred embodiment, all of the silicon is not melted, and thus the temperature of the silicon may not exceeds 1415° C. Once the wafer is formed, it cools down from 1400° C. to about 200° C. in about 10 min. 
     After the silicon wafers are removed, the board may be rotated so that the process may take place on the opposite side of the board—i.e., so that another batch of silicon may be melted and formed into silicon wafers. This process may be repeated numerous times (e.g., for several hours) until a new layer of the release coating needs to be applied. 
     The boundary layer may inhibit or prevent molten silicon metal from forming a silicon carbide (SiC) alloy, which may bond the silicon ingot to the graphite parts of the crucible or graphite board. By inhibiting or preventing the formation of SiC, the release coating may allow or facilitate the formed silicon ingot to be relatively easily removed from the crucible or graphite board. After formation, the ingot may be cut into individual slices or layers. 
     In an embodiment, the release coating comprises (a) silicon powder, (b) silicon nitride (Si 3 N 4 ), and optionally (c) a solvent. In a preferred embodiment, the solvent comprises water, although other suitable solvents may be used, including those, for example, that evaporate prior to contacting molten silicon (e.g., those that evaporate during the annealing process). Other suitable solvents may include organic or inorganic solvents, including, for example, alcohols such as ethanol, propanol, etc. Other additives, furthermore, may be included in the release coating, such as surfactants, etc. In an embodiment, the solvent-to-solids weight ratio is preferably in a range of 3:1 to 10:1, and all subranges therebetween, is more preferably 4:1 to 6:1, and all subranges therebetween, and is most preferably 5:1. 
     In an embodiment, the release coating comprises 20-99% silicon by weight of the solid components, and all subranges therebetween. In another embodiment, the release coating comprises 30-90% silicon by weight of the solid components, and all subranges therebetween. In yet another embodiment, the release coating comprises 50-85% silicon by weight of the solid components, and all subranges therebetween. In yet another embodiment, the release coating comprises 65-75% silicon by weight of the solid components, and all subranges therebetween. 
     The silicon may comprise a powder with a particle size distribution D 50  of less than 50 μm (i.e., 50% of the particles have a diameter less than 50 μm). Larger polycrystalline silicon powder particles—e.g., particles having a mean size larger than 100 μm—may be ground or pulverized to a smaller size. In an embodiment, the silicon may be a byproduct from a silicon grinding operation. 
     In an embodiment, the release coating comprises 1-80% silicon nitride by weight of the solid components, and all subranges therebetween. In another embodiment, the release coating comprises 10-70% silicon nitride by weight of the solid components, and all subranges therebetween. In yet another embodiment, the release coating comprises 15-50% silicon nitride by weight of the solid components, and all subranges therebetween. In yet another embodiment, the release coating comprises 25-35% silicon nitride by weight of the solid components, and all subranges therebetween. 
     The silicon nitride may comprise a powder with the tradename SicoNide® made by Vesta Ceramics. A suitable silicon nitride powder may have, for example, a particle size distribution D 50  of less than 1 μm (i.e., 50% of the particles have a diameter less than 1 μm) and a specific surface area of greater than 6 m 2 /g. 
     It is believed that the silicon nitride may promote the conversion of silicon powder to silicon nitride in the presence of nitrogen at high temperatures. Silicon powder may be generally less expensive than silicon nitride powder. In certain embodiments, the mixture of silicon and silicon nitride may include other ingredients, so long as those other ingredients do not substantially inhibit the formation of silicon nitride when annealed in a nitrogen atmosphere (i.e., do not materially affect the properties of the resultant release coating). 
     It should be noted that all numbers and numerical ranges described and claimed herein are approximate and include some degree of variation. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.