SILICON INGOT GROWTH CRUCIBLE WITH PATTERNED PROTRUSION STRUCTURED LAYER

Abstract

A crucible for growing silicon ingots may include a vessel having a bottom wall and side walls surrounding an inner portion of the vessel. A coating layer is applied to inner surfaces of the bottom wall and the side walls, the coating layer including a temperature-resistant material compatible with ingot growth from molten silicon such as silicon nitride. A patterned protrusion layer is applied at the inner surface of the bottom wall, which includes a matrix consisting of a temperature-resistant material compatible with ingot growth from molten silicon such as silicon nitride. Furthermore, the patterned protrusion layer includes particles of a nucleation enhancing material such as silica, the particles locally protruding from the matrix. The protruding particles may generate a pattern of multiple nucleation points during crystal growth of the ingot. Due to such multiple nucleation points, a dislocation density defect propagation towards a top may be reduced during crystal growth such that, e.g., solar cells produced with wafers sliced from the resulting ingot may have an improved conversion efficiency.

Claims

1. A crucible for growing silicon ingots, the crucible comprising: a vessel having a bottom wall and side walls surrounding an inner portion of the vessel; a coating layer applied to inner surfaces of the bottom wall and the side walls, the coating layer comprising a temperature-resistant material compatible with ingot growth from molten silicon; a patterned protrusion layer applied at the inner surface of the bottom wall, the patterned protrusion layer comprising a matrix consisting of silicon nitride and further comprising particles of a nucleation enhancing material which is adapted for forming a wetting agent when in contact with a liquid silicon melt, the particles locally protruding from the matrix.

2. The crucible of claim 1, wherein the patterned protrusion layer is applied to the inner surface of the bottom wall exclusively.

3. The crucible of claim 1, wherein the particles of the nucleation enhancing material are one of Silica (SiO2) sand particles, Silicon Carbide (SiC) particles and Carbon (C) particles.

4. The crucible of claim 1, wherein the particles of the nucleation enhancing material have sizes of between 20 m and 2 mm.

5. The crucible of claim 1, wherein the patterned protrusion layer has a thickness of between 0.3 mm and 3 mm.

6. The crucible of claim 1, wherein the particles of the nucleation enhancing material protruding from the matrix are comprised in the patterned protrusion layer with an areal density of between 1 to 10 cm.sup.2.

7. The crucible of claim 1, wherein the coating layer has a thickness of between 0.1 mm and 1 mm.

8. The crucible of claim 1, wherein the temperature-resistant material comprised in the coating layer is silicon nitride.

9. The crucible of claim 1, wherein the coating layer is applied using a first slurry comprising silicon nitride powder.

10. The crucible of claim 1, wherein the patterned protrusion layer is applied using a second slurry comprising silicon nitride powder and particles of the nucleation enhancing material.

11. A method of preparing a crucible (1) for growing silicon ingots, the method comprising: providing a vessel having a bottom wall and side walls surrounding an inner portion of the vessel; applying a coating layer to inner surfaces of the bottom wall and the side walls, the coating layer comprising a temperature-resistant material compatible with ingot growth from molten silicon; applying a patterned protrusion layer to the inner surface of the bottom wall, the patterned protrusion layer comprising a matrix consisting of silicon nitride and further comprising particles of a nucleation enhancing material which is adapted for forming a wetting agent when in contact with a liquid silicon melt, wherein the patterned protrusion layer is applied in such manner and the particles are adapted such that the particles locally protrude from the matrix.

12. The method of claim 11, wherein the coating layer is applied using a first slurry comprising silicon nitride powder and the patterned protrusion layer is applied using a second slurry comprising silicon nitride powder and particles of the nucleation enhancing material.

13. The method of claim 11, wherein the first slurry has a lower viscosity than the second slurry.

14. The method of claim 11, wherein the first slurry has a lower density than the second slurry.

15. The crucible of claim 1, wherein the particles of the nucleation enhancing material have sizes of between 100 m and 1 mm.

16. The crucible of claim 1, wherein the patterned protrusion layer has a thickness of between 1 mm and 2 mm.

17. The crucible of claim 1, wherein the particles of the nucleation enhancing material protruding from the matrix are comprised in the patterned protrusion layer with an areal density of between 7 to 10 cm.

18. The crucible of claim 1, wherein the coating layer has a thickness of between 0.4 mm and 0.5 mm.

19. The crucible of claim 1, wherein the coating layer is applied using a first slurry comprising silicon nitride powder and further comprising a binding agent, a dispersing agent and deionised water.

20. The crucible of claim 1, wherein the patterned protrusion layer is applied using a second slurry comprising silicon nitride powder and particles of the nucleation enhancing material and further comprising a binding agent, a dispersing agent and deionised water.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0054] In the following, embodiments of the invention will be described herein with reference to the enclosed drawing. However, neither the drawing nor the description shall be interpreted as limiting the invention.

[0055] FIG. 1 shows a sectional view through a crucible according to an embodiment of the present invention.

[0056] The FIGURE is only a schematic representation and not to scale.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0057] FIG. 1 shows a cross section through a crucible 1 according to an embodiment of the present invention. The crucible 1 is of a square type, i.e. has a box form or cuboid form. Other geometries are possible for the crucible.

[0058] The crucible 1 comprises a pot-like vessel 3 having a bottom wall 5 and side walls 7. The bottom wall 5 is generally rectangular and horizontal whereas the side walls 7 are rectangular and substantially vertical. The bottom wall 5 and the side walls 7 may form an integral component forming the entire vessel 3. Alternatively, the bottom wall 5 and the side walls 7 may be separate components and may be mounted together in order to form the entire vessel 3. The walls 5, 7 may be formed using sheet-like components. For example, the walls 5, 7 may be made with fused silica sheets. Typically, the bottom wall 5 is several tens of centimetres long and several tens of centimetres wide. The side walls 7 are typically several tens of centimetres high and several tens of centimetres wide. The bottom wall 5 and the side walls 7 typically have a thickness in a range of a few millimetres up to a few centimetres, for example between 3 mm and 10 cm.

[0059] The bottom wall 5 and the side walls 7 surround an inner portion 9 of the vessel 3. Therein, the vessel 3 is preferably open at its top.

[0060] Inner surfaces of the bottom wall 5 and the side walls 7 are coated with a thin coating layer 11. The coating layer 11 comprises or consists of silicon nitride as a temperature-resistant material. The coating layer 11 may therefore resist the very high temperatures of a silicon melt being cast into the vessel 3. Preferably, the coating layer 11 has a thickness of 400 to 500 m. Preferably, the coating layer 11 is substantially homogeneous, i.e. does not comprise macroscopic particles other than the silicon nitride powder particles used for forming the coating layer 11. Particularly, the coating layer 11 may have a macroscopically homogeneous thickness and may have a macroscopically smooth, preferably planar surface. The portions of the coating layer 11 covering the side walls 7 of the vessel 3 are preferably not covered by any other layer, i.e. are exposed towards the inner portion 9 of the vessel 3.

[0061] On the bottom wall 5, an additional layer is applied on top of the coating layer 11. This additional layer is a patterned protrusion layer 13 preferably covering the entire inner surface of the bottom wall 5. Accordingly, at the bottom wall 5, the coating layer 11 is not exposed but is covered by the overlying patterned protrusion layer 13.

[0062] As visible in the enlarged view of FIG. 1, the patterned protrusion layer 13 comprises a matrix 15 into which multiple particles 17 are embedded. Therein, the matrix 15 comprises silicon nitride as a temperature-resistant material. The particles 17 consist of a nucleation enhancing material such as silica. The particles 17 protrude beyond the upper exposed surface of the matrix 15. Thereby, the protruding particles 17 form a kind of pattern with nucleation enhancing tips protruding towards the inner portion 9 of the vessel 3.

[0063] The silica particles 17 may have typical sizes in a range of 100 m to 1 mm. A pattern size, i.e. a lateral average distance between neighbouring protruding particles 17, may be in a typical range of 1 to 2 mm. A protruding height of the particles 17 may be in a range from 1 to 2 mm.

[0064] Finally, steps of a method for preparing or fabricating a crucible 1 are explained with respect to an exemplary embodiment.

[0065] In order to prepare or fabricate the crucible 1, first, a vessel 3 with a bottom wall 5 and side walls 7 is provided.

[0066] Then, a first silicon nitride coating slurry may be prepared by mixing high pure silicon nitride powder, deionised water, binding agent and dispersing agent. Such first slurry may then for example be sprayed or deposited in other ways such as screen printing on the inner surfaces of the bottom wall 5 and the side walls 7 of the square type fused silica crucible 1. Therein, the first slurry may be applied with a desired specified coating thickness of for example between 400 and 500 m. The coating is typically done at elevated coating temperatures of between 40 and 50 C.

[0067] After the coating layer 11 has been applied in such manner, a second slurry is prepared with silicon nitride powder, deionised water, a binding agent and a wetting agent. Therein, the wetting agent may be formed by particles 17 made of a nucleation enhancing material such as silica sand particles. Preferably, this second slurry is applied only to the inner surface of the bottom wall 5 or of the coating layer 11 previously applied thereon.

[0068] While the first slurry has a relatively low density of for example 1.37 g/cm and a low viscosity of for example 2.96 cP, the second slurry has a higher density of for example 1.96 g/cm and a higher viscosity of about 700 cP.

[0069] After having applied the coating layer 11 and the patterned protrusion layer 13 onto the inner surfaces of the walls 5, 7 of the vessel 3, the coated crucible 1 is baked with high temperature and open atmospheric condition. At such conditions, the first and second slurries are solidified and form a dense coating layer 11 and patterned protrusion layer 13, respectively.

[0070] Silicon ingots grown with such crucible 1 may show reduced dislocation density defects. As a result thereof, solar cells produced with silicon wafers sliced from such ingots may have an improved conversion efficiency.

[0071] Finally, it should be noted that terms such as comprising do not exclude other elements or steps and the a or an does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

LIST OF REFERENCE SIGNS

[0072] 1 crucible [0073] 3 vessel [0074] 5 bottom wall [0075] 7 side wall [0076] 9 inner portion [0077] 11 coating layer [0078] 13 patterned protrusion layer [0079] 15 matrix [0080] 17 particles