Graphite Crucible for Silicon Crystal Production

Background

[0001] Rising demand for energy and limited fossil fuel reserves are increasingly driving demand for alternative energy sources. One particularly important type of alternative energy is solar power, and specifically, the use of photovoltaic cells to produce electricity.

[0002] Most photovoltaic cells are made of crystalline silicon which is manufactured in a variety of methods. One common method is through a directional solidification system (DSS) process wherein silicon feedstock is charged in a quartz crucible and heated until the contents of the crucible are melted. Thermal energy is then drawn from the bottom of the crucible. The melt experiences a temperature gradient and the solidification begins at the bottom. Crystals grow upwardly with grain boundaries forming parallel to the solidification direction. To obtain a directional solidification the solidification heat must flow through the growing layer of solid silicon. Therefore, the temperature at the lower part of the crucible should be decreased in coordination with the increase in solid silicon thickness to maintain a steady growth rate.

Brief Summary

[0003] According to one aspect, a graphite crucible for processing silicon includes a bottom wall including a bottom wall interior facing surface. A plurality of side walls extend upwardly from the bottom wall, each side wall including a side wall interior facing surface. A protective coating is provided on the bottom wall interior facing surface and the side wall interior facing surface. The side walls have a coefficient of thermal expansion perpendicular to the solidification direction that is less than 95% of the coefficient of thermal expansion of the silicon processed therein. Also, the side walls and the bottom wall have a thru-plane thermal conductivity from about 90 to about 160 W/m- K at room temperature.

Brief Description of the Drawings

[0004] Figure 1 is a front schematic view of the directional solidification assembly.

[0005] Figure 2 is a top plan view of the crucible. [0006] Figure 3 is a section view of the crucible of Figure 2 taken along the line

A-A.

[0007] Figure 4 is a section view of a crucible.

[0008] Figure 5 is a front schematic view of an alternate configuration for the directional solidification assembly.

Detailed Description

[0009] With reference now to Fig. 1, a directional solidification assembly is shown and generally indicated by the numeral 10. The assembly 10 includes a thermally insulated enclosure 12 within which is positioned a crucible 14. One or more heating elements 16 are positioned within enclosure 12 proximate to one or more sides of crucible 14. In the embodiment shown in Fig. 1, two heaters are employed, with one on opposed sides of the crucible 14. However, it should be appreciated that more or fewer heaters may be employed in a variety of locations within enclosure 12. For example, in other embodiments, a heater is positioned next to each side of the crucible 14. In still other embodiments, one or more heaters may be positioned proximate to the crucible top, crucible bottom or both. These top and/or bottom heater(s) may be in conjunction with or in the alternative to side positioned heaters.

[0010] Crucible 14 is positioned on, and in thermal contact with a base plate 18.

Base plate 18 supports the weight of crucible 14 and also functions as a heat sink to draw thermal energy from the bottom of crucible 14. Base plate 18 may advantageously be a graphite material. In one embodiment, base plate 18 is substantially the same dimension as the bottom surface of crucible 14. In other embodiments the base plate 18 is less than about 120% but greater than 100% of the dimensions of the bottom surface of crucible 14. In still further embodiments, the base plate 18 is less than about 110% but greater than 100% of the dimensions of the bottom surface of crucible 14. In this or other embodiments, the base plate 18 is greater than about 850 mm square. In other embodiments, the base plate is greater than about 900 mm square. Base plate 18 may be from, about 15 mm to about 50 mm thick. In other embodiments, base plate 18 may be from about 20 mm to about 30 mm thick.

[0011] When producing directionally solidified silicon, polysilicon 15 is melted within crucible 14 or is melted and added to crucible 14. Thereafter, heating elements 16 and the heat sink function provided by base plates 18 control the temperature of the silicon 15 charged in crucible 14.

[0012] In one embodiment, base plate 18 is selectively actively cooled during the solidification process. Active cooling may be provided by, for example, drawing a liquid or gas cooling medium either into contact with and/or interior to the base plate 18. In other embodiments, base plate 18 is not actively cooled. In still other embodiments, base plate 18 rests on a heat exchanger in the form of an immobile graphite block. In this embodiment, the graphite block may be from greater than about 1 meter square and between about 50 mm to about 200 mm thick. In other embodiments, the graphite block may be from about 75 to about 125 mm thick.

[0013] Heating elements 16 are controlled so that thermal energy is drawn from the molten silicon at the bottom of the crucible 14 (through base plate 18). Thus, the solidification process begins at the bottom of the crucible 14 and directionally solidifies to the top of crucible 14. Once the silicon ingot is formed, the silicon is removed from the crucible 14 for further processing. A complete ingot formation cycle is referred to herein as a heat. Each crucible 14 may be used for multiple heats. In one embodiment, the crucible 14 is used for at least 20 heats. More advantageously, the crucible 14 is used for at least 30 heats. Still more advantageously the crucible 14 is used for at least 40 heats.

[0014] The crucible 14 may be generally rectangular or square. As shown in Figs. 2 and 3, crucible 14 includes four side walls 20 and a bottom wall 22. Each of the four sidewalls 20 includes an inner face 24 and an outer face 26. Because the silicon ingot solidifies within crucible 14, inner faces 24 are disposed at an angle Θ other than perpendicular to bottom wall 22 to enable removal of the silicon ingot. In one embodiment, inner faces 24 are disposed at greater than about 1 degree angle from perpendicular relative to bottom wall 22. In other embodiments, inner faces 24 are disposed at a greater than about 2 degree angle from perpendicular relative to bottom wall 22. In still other embodiments, inner faces 24 are disposed at a greater than about 3 degree angle from perpendicular relative to bottom wall 22. In still further embodiments, inner faces 24 are disposed at a greater than about 4 degree angle from perpendicular relative to bottom wall 22. In these or other embodiments, inner faces 24 are disposed at an angle from about 1 degrees to about 5 degrees. In still further embodiments, inner faces 24 are disposed at an angle from about 2 degrees to about 4 degrees.

[0015] A corner 28 is formed between adjacent inner faces 24. Another corner 30 is formed between each inner face 24 and the bottom wall 22. Corners 28 and 30 may include a radius. In one embodiment, the radius is from about 5 mm to about 20 mm. In other embodiments the radius is from about 8 mm to about 15 mm. In still a still further embodiment, the radius is from about 10 mm to about 12 mm.

[0016] In one embodiment, crucible 14 has a vertical height of greater than about

350 mm. In other embodiments, crucible 14 has a vertical height of greater than about 400 mm. In still further embodiments, the crucible 14 has a vertical height of greater than about 500 mm. In still further embodiments the crucible has a vertical height greater than about 600 mm. In these or other embodiments, the crucible may have a height between about 400 mm and about 800 mm.

[0017] In one embodiment the bottom wall 22 is a quadrilateral having at least one side greater than about 700 mm. In other embodiments the bottom wall has at least one side greater than about 800 mm. In still further embodiments, the bottom wall has at least one side greater than about 1000 mm. In these or other embodiments, the bottom wall 22 is in the form of a square.

[0018] In one embodiment, the side walls 20 have a thickness of from about 15 mm to about 50 mm. In other embodiments, the side walls 20 have a thickness from about 20 mm to about 40 mm. In still other embodiments, the side walls 20 have a thickness from about 20 mm to about 25 mm. In one embodiment, the bottom wall 22 has a thickness of from about 15 mm to about 50 mm. In other embodiments, the bottom wall 22 has a thickness from about 20 mm to about 40 mm. In still other embodiments, the bottom wall 22 has a thickness from about 20 mm to about 25 mm.

[0019] In one embodiment, the directional solidification assembly 10 may be used in the absence of a base plate 18. In such an embodiment, the bottom wall 22 may have a thickness of from about 25 mm to about 75 mm. In other embodiments, the bottom wall 22 has a thickness from about 30 mm to about 40 mm. In other embodiments, the bottom wall 22 has a thickness from about 35 mm to about 65 mm. In still other embodiments, the bottom wall 22 has a thickness from about 45 mm to about 55 mm. In still further embodiments, the bottom wall has a thickness that is at least about 1.5 times greater than the thickness of the side walls. In still further embodiments, the bottom wall has a thickness that is at least about 2 times the thickness of the side walls.

[0020] The crucible 14 advantageously includes a thin layer of coating material

32 on inner faces 24 and the upwardly facing surface 25 of bottom wall 22. Material 32 advantageously has a thickness of from about 50 μιη to about 1 mm. More advantageously, material 32 has a thickness of from about 150 μιη to about 400 μιη. Coating material 32 may function as a release agent, to ease the removal of the silicon ingot from crucible 14 after solidification. Material 32 may further protect the crucible from silicon penetration and the formation of SiC within the interior and exterior of walls 20 and 22 which may lead to premature failure. Coating material 32 is advantageously silicon nitride S1 ₃ N4. Coating material 32 may be applied by spraying with a fine mist nozzle with a controlled number of spray passes, drying, and sintering in an oven. Alternately, material 32 may be applied by drain casting, whereby the crucible is filled with a silicon nitride slurry for a controlled amount of time resulting in a fine layer of powder coating. The crucible is then emptied and the coating remains on the wall to be dried and sintered. Alternately, the material 32 may be painted on faces 24 and 25 with a brush or roller, then dried and sintered. The coating material 32 is advantageously permanent and will not require reapplication for the life of the crucible 14. However, depending on use conditions, coating material 32 may be reapplied after each heat. In other embodiments, the coating material 32 is reapplied after every other heat. In still other embodiments, the coating material 32 is reapplied after every third heat. In still other embodiments, the coating material 32 is reapplied every fourth heat.

[0021] A lip 34 may be provided at the top of side walls 20. Lip 34 provides a laterally extending surface which may be used to capture and/or lift the crucible 14. Though the drawings show a lip 34 extending from each side wall 20, it should be appreciated that, alternately, lip 34 may extend from only two, opposed side walls 20. In other embodiments, the crucible 14 may not include a lip extending from any side walls.

[0022] With reference now to Fig. 4, an alternate crucible configuration is shown.

Crucible 14 of Fig. 4 is substantially similar to crucible 14 of Figs 1-3 except that outer faces 26 of side walls 20 are oriented at an angle β other than perpendicular from bottom wall 22. In one embodiment, outer faces 26 are disposed at greater than about 1 degree angle from perpendicular relative to bottom wall 22. In other embodiments, outer faces 26 are disposed at a greater than about 2 degree angle from perpendicular relative to bottom wall 22. In still other embodiments, outer faces 26 are disposed at a greater than about 3 degree angle from perpendicular relative to bottom wall 22. In still further embodiments, outer faces 26 are disposed at a greater than about 4 degree angle from perpendicular relative to bottom wall 22. In these or other embodiments, outer faces 26 are disposed at an angle from about 1 degrees to about 5 degrees. In still further embodiments, outer faces 26 are disposed at an angle from about 2 degrees to about 4 degrees. In still further embodiments, the inner face 24 and outer face 26 are substantially parallel. In this or other embodiments, the side walls 20 may have a substantially uniform thickness from the bottom to the top of the side wall 20. As can be seen, such a configuration of crucible 14 may allow multiple crucibles 14 to be efficiently machined from an extruded cylindrical stock with reduced waste by using coring machining techniques.

[0023] The room-temperature coefficient of thermal expansion (hereinafter

"CTE") of the crucible 14 affects life and ease of silicon removal and is therefore particularly consequential in the direction perpendicular to solidification (i.e. in the plane parallel to the bottom wall). Thus, if extruded stock is the base material, the against-grain CTE is of particular consequence. However, if molded stock is the base material, the with- grain CTE is of particular consequence. In one embodiment, the crucible 14 has a coefficient of thermal expansion perpendicular to the solidification direction that is less than 95% of the CTE of the silicon processed therein (CTE of Si at room temperature is about 3.5 x 10 ^"6 /°C). Even more advantageously, the crucible 14 has a CTE in the direction perpendicular to solidification of less than 85% of the CTE of the silicon processed therein. Still more advantageously, the crucible 14 has a CTE in the direction perpendicular to solidification of less than 75% of the silicon processed therein. In these or other embodiments the crucible 14 exhibits a CTE in the direction perpendicular to solidification of from about 1.0 x 10 ^"6 /°C to about 3.325 x 10 ^"6 /°C. In another embodiment, the CTE in the direction perpendicular to solidification is from about 2 x 10 ^{" 6} /°C to about 3.0 x 10 ^"6 /°C. In still further embodiments, the CTE in the direction perpendicular to solidification is from about 3.0 x 10 ^"6 /°C to about 3.325 x 10 ^"6 /°C.

[0024] Advantageously the crucible 14 has a thru-plane (i.e. parallel to heat flow and solidification) thermal conductivity of from about 80 to about 200 W/m-K at room temperature. In other embodiments, the thermal conductivity is from about 90 to about 160 W/m-K at room temperature. In other embodiments, the thermal conductivity is from about 120 to about 130 W/m- K at room temperature.

[0025] Advantageously the crucible 14 has a with-grain flexural strength of from between 15 and 22 MPa. In other embodiments, the with-grain flexural strength is from between about 17 and about 20 MPa. In this or other embodiments, the against-grain flexural strength is advantageously between about 17 and about 24 MPa. In other embodiments, the against-grain flexural strength is from between about 19 and about 21 MPa.

[0026] Advantageously the coating material 32 provides a substantially gas impermeable layer that effectively prevents silicon from contacting the graphite material of crucible 14. The coating material advantageously exhibits a gas permeability of less than about 0.01 Darcy. Even more advantageously, the coating material exhibits a gas permeability of less than about 0.005 Darcy. Still more advantageously, the coating material exhibits a gas permeability of less than about 0.002 Darcy. However, the graphite material of crucible 14 also advantageously exhibits a gas permeability of less than about 0.1 Darcy. Even more advantageously, the graphite material of crucible 14 exhibits a gas permeability of less than about .05 Darcy. Still more advantageously, the graphite material of crucible 14 exhibits a gas permeability of less than about 0.03 Darcy. Even more advantageously, the graphite material of crucible 14 exhibits a gas permeability of less than about 0.01 Darcy. Still more advantageously, the graphite material of crucible 14 exhibits a gas permeability of less than about 0.002 Darcy. In one embodiment the graphite material of crucible 14 exhibits a gas permeability of from between about .002 Darcy and about .1 Darcy. In still further embodiments, the graphite material of crucible 14 exhibits a gas permeability of from between about .01 and about .05 Darcy. The relatively low permeability of the crucible graphite material provides added safety and improved life should a failure or degradation of the coating material occur.

[0027] Advantageously, the graphite material of crucible 14 includes a bulk density of from between about 1.0 and about 2.5 g/cm ³ . In still further embodiments, the graphite material of crucible 14 includes a bulk density of from between about 1.5 and about 2.0 g/cm ³ . In still further embodiments, the graphite material of crucible 14 includes a bulk density of from between about 1.5 and about 2.0 g/cm ³ . In still further embodiments, the graphite material of crucible 14 includes a bulk density of from between about 1.65 and about 1.85 g/cm ³ .

[0028] Advantageously, the graphite material of crucible 14 includes a specific resistance of from between about 6.0 and about 10.0 μΩιη. In still further embodiments, the graphite material of crucible 14 includes a specific resistance of from between about 7.0 and about 9.0 μΩιη. In still further embodiments, the graphite material of crucible 14 includes a specific resistance of from between about 8.0 and about 9.0 μΩιη.

[0029] Advantageously the crucible 14 has a with- grain compressive strength of from between 40 and 60 MPa. In other embodiments, the with-grain compressive strength is from between about 48 and about 53 MPa. In this or other embodiments, the against-grain compressive strength is advantageously between about 45 and about 65 MPa. In other embodiments, the against-grain compressive strength is from between about 50 and about 55 MPa.

[0030] Crucible 14 is preferably a graphite material. The graphite material may be formed by first combining a filler, binder and additional optional ingredients. In one embodiment, the filler is a calcined petroleum coke. The binder may be, for example, a coal tar pitch. Other fillers may include, for example, recycled graphite. In one embodiment the calcined petroleum coke is crushed, sized and mixed with a coal-tar pitch binder and optionally one or more fillers and/or other ingredients to form a blend.

[0031] The mix is then formed into an article of green stock by either, extrusion though a die, molding in a conventional forming mold or through isomolding. The mold may form the green stock in substantially final form and size, although some machining of the final article is typically needed.

[0032] After extrusion, the green stock is heat treated by baking at a temperature of between about 700 °C and about 1100 °C, more preferably between about 800 °C and about 1000 °C to carbonize the pitch binder to solid pitch coke, which gives the article permanency of form. The bake cycle is performed in the substantial absence of air to avoid oxidation at a rate of about 1 °C to about 5 °C rise per hour to the final temperature. After baking, the carbonized stock may be impregnated one or more times with coal tar pitch or petroleum pitch, or other types of pitches or resins known in the industry, to deposit additional coke in any open pores of the stock to reach the desired strength and density. Each impregnation is then followed by an additional baking step.

[0033] After baking, the carbonized stock is graphitized. Graphitization is performed by heating the carbonized article to a final temperature of from between about 2500 °C to about 3400 °C for a time sufficient to cause the carbon atoms in the coke and pitch coke binder to transform from a poorly ordered state into the substantially crystalline structure of graphite. Advantageously, graphitization is performed by maintaining the carbonized stock at a temperature of at least about 2700 °C, and more advantageously at a temperature of from between about 2700 °C and about 3200 °C. At these high temperatures, non-carbon elements are volatilized and escape as vapors. The time required for maintenance at the graphitization temperature is from, for example, about 5 minutes to about 240 minutes. Once graphitization is completed, as discussed above, the graphitized article can be machined to reach the final crucible form disclosed above.

[0034] In one embodiment, shown in Fig. 1, the crucible 14 may be advantageously used in directional solidification assembly 10 without support plates being positioned between heating elements 16 and/or base plate 18. However, as shown in Fig. 5, it should be appreciated that crucible 14 may be used in a directional solidification assembly that includes support plates 40 positioned between heating element 16 and/or a support plate 42 positioned between base plate 18 and crucible 14.

[0035] The various embodiments described herein can be practiced in any combination thereof. The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence that is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.