WO2013160603A1

WO2013160603A1 - Method for manufacturing a crucible made of silicon nitride

Info

Publication number: WO2013160603A1
Application number: PCT/FR2013/050893
Authority: WO
Inventors: Haavard Soerheim; Arve Solheim; Adrien Vincent
Original assignee: Saint-Gobain Centre De Recherches Et D'etudes Europeen; Saint-Gobain Ceramic Materials As
Priority date: 2012-04-24
Filing date: 2013-04-23
Publication date: 2013-10-31
Also published as: FR2989680A1; FR2989680B1

Abstract

The invention relates to a method for manufacturing a ceramic crucible, in particular to be used for melting and crystallizing a silicon ingot, including the following steps: manufacturing an uncoated silicon crucible made of a material essentially consisting of silicon nitride, the open porosity of which is between 15 and 60% and the median pore diameter of which is between 0.1 and 10 micrometers, said silicon nitride further including at least 3 wt % of oxygen; partially oxidizing an assembly of silicon nitride grains having a median diameter of between 0.3 and 30 micrometers, such that said silicon nitride grains include between 5 and 10 wt % of oxygen; and applying a coating layer onto said surfaces of the uncoated ceramic crucible, said coating layer consisting of said partially-oxidized silicon nitride grains. The invention further relates to a crucible that can be produced according to such a method.

Description

SILICON NITRIDE CREUSET AND METHOD OF MANUFACTURING SAME

The invention relates to a crucible that can be used in particular for the manufacture by melting and crystallization of silicon ingots. Such ingots are particularly used in the field of photovoltaics, for obtaining polycrystalline silicon substrates 10 used in certain photovoltaic cells.

The use of crucibles for the manufacture of polycrystalline silicon ingots has long been known, as well as the problems to be solved for obtaining such ingots.

It is in particular described in many patent applications the use of different refractory materials and found inert for obtaining such crucibles. Among these materials, mention may be made of graphite,

Silicon, especially in quartz form, boron nitride or silicon nitride.

In particular, it is described in US2009 / 0119882 or WO 2006/02779, the use of silicon oxide crucibles. The use of such crucibles poses

However, the problem of the reactivity of the molten silicon with the silica constituting the crucible. Such a reaction is translated after crystallization by points of strong adhesion between the crucible and the ingot, the complete destruction of the crucible may be necessary to recover

The ingot. In addition, the difference in the coefficients of thermal expansion between the crucible material and the silicon can also cause stresses in the ingot, which can even crack during cooling. Also, the oxygen contained in the crucible is a pollution factor of silicon ingots, whose final purity is nevertheless an essential element for their good use in the manufacture of photovoltaic cells.

It is alternatively described, for example in the application US 2009/0119882, the use of a coating comprising a mixture of silicon nitride and silicon oxide on the walls of the crucible, to avoid strong bonding zones between the crystallized ingot and crucible.

However, the tests carried out by the applicant company showed that high levels of contamination of the ingot by oxygen occurred if the crucible itself contained too much oxygen, even with a layer of silicon nitride interposed between the walls of the crucible and the melt. By high levels of contamination, reference is made here to their future use in photovoltaic devices and the degree of purity required for such an application.

According to other publications, for example applications WO2007 / 148986 or WO2004 / 016835, it is proposed to use silicon nitride crucibles, in particular obtained by reactive sintering (often called RBSN for Reaction Bonded Silicon Nitride). To avoid the appearance of strong bonding zones between the crucible and the crystallized silicon ingot, it is described in the application WO 2004/016835 to affix a silicon nitride coating on the walls of the crucible, itself already being in silicon nitride. Deposition of such a coating of silicon nitride on a crucible made of the same material already shaped, however, poses problems of sintering and ultimately adhesion of the surface layer, as described in the publication WO 2004 / 053207 Al, paragraphs [008] and [0009], again with a risk of pollution of the silicon ingot. To solve such a problem, the application WO 2004/053207 A1 proposes a thermal spraying of a mixture of silicon metal, silicon nitride and silica, supposed to lead to low final contamination of the ingot. However, this solution is relatively expensive and requires specific tools, difficult to implement and adjust on an industrial site of production.

No. 4,099,924 discloses a crucible for growing a silicon crystal in which a protective layer of silicon oxynitride is deposited by CVD while the crucible is maintained at high temperature, that is to say included between 800 and 1500 ° C, for a duration of the order of several hours and in the presence of N ammon ammonia and nitrous oxide ₂₀ and a stream of hydrogen. Such a process necessarily leads to contamination of the crucible itself by these different species, which in turn may contaminate the ingot, even in the presence of the silicon oxynitride barrier.

The object of the present invention is first of all to provide a method for manufacturing an ingot making it possible to find the best compromise between the economic profitability of said process, the purity of the silicon ingot finally obtained and the ease with which it is possible to extract the latter from the mold formed by the crucible.

By economic profitability is meant that the crucible according to the invention is relatively simple to manufacture and does not require the implementation of specific tools but also that it is reusable to ensure multiple cycles of melting-crystallization of silicon ingots. It is also understood by this term that the process does not require special, expensive and / or complex equipment to use when implemented on an industrial scale.

By ease of recovery, it is understood within the meaning of the present invention that the ingot finally obtained is easily "extractable" from the mold, that is to say that there is no or very few strong bonding areas between said mold and the solidified ingot. In addition the surface of the crucible must have a high resistance to abrasion so as not to be damaged during the loading of the crucible with silicon granulate before heating the reactor.

By purity of the silicon ingot, it is meant that the ingots obtained successively on different melting-crystallization cycles, and from the same crucible, has a purity sufficient for the desired use, that is to say the production of semiconductors for photovoltaic cells.

To solve the technical problem as described above, the experiments carried out by the applicant company have shown that it was necessary to adjust a whole series of characteristics concerning both the crucible and the coating disposed on said crucible and intended to avoid the appearance of strong bond zones between the crucible and the solidified ingot.

Thus, and more particularly, according to a first aspect of the invention, it has been found that, contrary to a majority opinion, the best results are obtained when the crucible is made of a silicon nitride material which is free or virtually free. of oxygen and that the coating is itself essentially silicon nitride, but in a previously partially oxidized form, in a controlled manner. Thus, we have seen surprisingly that the presence of a coating of silicon nitride but a limited proportion of which is oxidized does not result in an unacceptable pollution of the solidified ingot, although this oxygen is necessarily in contact with the molten silicon material, whereas the presence of even a very small portion of oxygen in the core material of the crucible led on the contrary to an unacceptable pollution of the silicon ingot by the oxygen during melting-recrystallization cycles, even if this core material is never directly in contact with the melt.

According to another aspect of the invention, it has been found that the physical characteristics of the median diameter of the particles used for the manufacture of the crucible and the coating must be adjusted accordingly, so as to obtain the desired effect and in particular to allow the obtaining "reusable" crucibles, that is to say likely to allow from the same crucible successive fusions-recrystallizations of several silicon ingots without problems of breakage, appearance of bonding zones and / or excessive impurity levels in the final ingots.

More specifically, the present invention thus relates to a method of manufacturing a ceramic crucible particularly useful for the melting and crystallization of a silicon ingot comprising the following steps: - manufacture of a bare ceramic crucible having a bottom and walls whose surfaces define an internal volume for the melting / crystallization of said ingot, said crucible being made of a material consisting essentially of silicon nitride whose open porosity is between 15 and 60% by volume, preferably between 20 and 40 % by volume, and the median pore diameter by volume is between 0.1 and 10 micrometers, preferably between 0.2 and 5 micrometers, said silicon nitride further comprising less than 3% by weight of oxygen, preferably less than 2% by weight of oxygen,

partial oxidation of a set of silicon nitride grains having a median diameter of between 0.3 and 30 microns, preferably between 0.5 and 5 microns, such that said silicon nitride grains comprise between 5 and 10 microns; % by weight of oxygen,

- Applying a coating layer on said surfaces of the bare ceramic crucible, said coating layer consisting of said partially oxidized silicon nitride grains.

The average thickness of the deposited layer is preferably less than 1000 micrometers, preferably less than 500 micrometers, and more preferably less than 100 micrometers.

The application of the coating layer is advantageously carried out according to the invention without firing, that is to say without the need to effecter a sintering of said layer on the surface of the crucible. For the purposes of the present invention, "baking" means a heat treatment at a temperature above 800 ° C. In particular, the application of the coating layer is carried out according to the invention at a temperature which is preferably less than 600 ° C or even less than 500 ° C, or even lower than 300 ° C. According to a particularly advantageous mode of implementation of the method, a simple removal treatment of the water present in said layer, in particular by heating at a temperature below 200 ° C is implemented.

For the purposes of the present invention, the term "material essentially consisting of silicon nitride" is intended to mean a material consisting essentially of silicon nitride and having an elemental oxygen content of the material is less than 3% and preferably less than 2%.

The material may also optionally comprise other elements in the form of unavoidable impurities, such as C, Cl, F, Fe, Cu, Ca, Na, K, Mg, Cr, Co, W, Ni, Ti, Zr, P , B, these impurities being inherent to its manufacturing process.

For the purposes of the present invention, porosities and pore diameters are measured by conventional mercury porosimetry techniques.

The percentages of oxygen in the silicon nitride constituting the bare crucible or the coating layer are measured after sampling according to conventional techniques and in particular using a LECO type device TC436 (melting under inert gas according to the well-known techniques). known to those skilled in the art).

The median diameter of the set of grains used for forming the coating is defined and obtained according to the invention according to conventional techniques by a particle size distribution characterization by means of a laser granulometer.

For the purposes of the present invention, the term "median diameter" of a set of grains or particles, the percentile D ₅₀ , that is to say the size dividing the particles in first and second populations equal in mass, these first and second populations comprising only particles having a diameter greater than or equal to the median size, or a diameter less than the median size.

Advantageously, the coating layer is applied from a mixture of said oxide grains with a solvent such as water, so as to obtain a slip comprising 20 to 80% by weight of said grains, the remainder being the solvent, said slip being then deposited on the surface of the bottom and the walls by a technique preferably selected from tempering, immersion, spin coating (spin coating ), brushing, spraying, scraping (doctor blade), brushing (painting) and finally drying at a temperature evaporating the solvent, said temperature being below 200 ° C. The oxidation treatment is preferably a heat treatment carried out under an oxidizing atmosphere, in particular in air, at a temperature of between 900 and 1400 ° C., in particular between 1000 ° C. and 1200 ° C. The duration of the oxidation thermal treatment is preferably between 0.5 and 10 hours. The rate of rise in temperature before reaching the treatment temperature is typically between 20 and 500 ° C / hour, preferably between 100 and 300 ° C / hour. These parameters are advantageously adjusted according to the particle size characteristics of the starting powder.

The silicon nitride grains used to form the crucible are preferably RBSN grains. The invention also relates to a crucible for the melting / crystallization of a silicon ingot obtainable by said process, having a bottom and walls whose surfaces define an interior volume for said crystallization of said ingot, said crucible comprising:

a core made of a material consisting of silicon nitride comprising less than 3%, preferably less than 2% by weight of oxygen, whose open porosity is between 15 and 60% by volume, preferably between 20 and 40% by volume and the median pore volume diameter is between 0.1 and 10 micrometers, preferably between 0.2 and 5 micrometers, and

a coating layer at least on said surfaces defining the interior volume of the crucible, said layer consisting of partially oxidized silicon nitride grains, said silicon nitride grains having a median size of between 0.3 and 30 microns, preferably between 0.5 and

5 micrometers, and comprising between 5 and 10% by weight of oxygen.

It is verified according to the invention that the median size of the oxidized grains constituting the coating corresponds substantially to the median diameter of the silicon nitride grains initially used in the crucible manufacturing process, in particular by means of electron microscope micrographs, in accordance with the techniques well known in the field.

According to advantageous modes:

the average thickness of the coating layer is less than 1000 micrometers, preferably less than 500 micrometers,

the average thickness of the coating layer consisting of partially oxidized silicon nitride grains is between 10 and 500 times the median size of said grains,

the median size of the partially oxidized silicon nitride grains is between 0.5 and 5 times the median pore diameter of the material constituting the crucible. The examples which follow are given purely by way of illustration and in no way limit the scope of the present invention. Example 1 (comparative):

A silicon nitride crucible is obtained by reactive sintering from a silicon powder whose median grain diameter is of the order of 75 microns. The powder is placed in a mold sized accordingly. The silicon metal powder is compacted by vibration in the mold. A circular crucible is obtained by reactive sintering of the compacted powder at 1300 ° C in a furnace maintained under a nitrogen atmosphere, to allow the conversion of silicon to silicon nitride. It has the following dimensions:

- outer diameter 280mm,

- inside diameter 265mm,

- internal height 200mm,

- external height (bottom thickness included) 220 mm.

The crucible thus obtained has an open porosity of the order of 30% by volume and a median pore diameter of about 1 micrometer at the core of the material constituting the crucible.

The surface of the walls and the bottom of the crude crucible thus obtained are spray-coated SATAjet 1000 B RP Nozzle 1.0 0.6 / QCC of a mixture comprising a weight part of a silicon nitride powder for a part weight of demineralised water, in the presence of 0.5% by weight of RT Vanderbilt CN grade Darvan dispersant relative to the introduced mass of silicon nitride. The silicon nitride powder used, sold by the UBE company under the reference SN-E05, is characterized by a median particle diameter of the order of 0.6 microns. for a specific surface area of about 5 m ² / gram. In order to obtain a homogeneous mixture before spraying, this mixture was homogenized and dispersed in a jar with an alumina ball, the mass of balls being equal to the mass of the mineral powder of the mixture, at 100 rotations per minute for 2 hours.

The coated crucible is then dried in air for 4 hours at 110 ° C. and then heated at 1100 ° C. for 4 hours in air to obtain sufficient strength and adhesion of the layer to the walls of the crucible.

The median size of the grains of silicon nitride constituting the coating is verified on images obtained by electron microscopy according to the following protocol:

a crucible coating section is made so as to obtain a view of the surface of the coating,

an image acquisition is carried out by an SEM scanning electron microscope, in order to obtain an image showing at least 50 grains in the horizontal direction and at least 50 grains in the vertical direction according to the plane of the image,

if necessary, the raw SEM image is processed by a thresholding technique to obtain binarized images,

the area of each grain is measured in the image plane considered and an "equivalent diameter" is determined, on the basis of a perfect disk of the same area, and a distribution curve in number of grains can thus be obtained ,

the median size of the grains constituting the coating is determined on the basis of all the data obtained by the SEM images, the median size being the median equivalent diameter dividing the particles into first and second populations equal in number, these first and second populations with only particles having either an equivalent diameter greater than or equal to the median size, or a median diameter less than the median size. The coated crucible thus obtained is used for the melting of a powder and the crystallization of a silicon ingot under the usual conditions, comprising heating at 1500 ° C. under argon and then cooling slowly to a temperature of the order 1350 ° C. Very few areas of adhesion, relatively small, are observed and the ingot can finally be detached from the crucible without particular difficulties.

The interstitial oxygen content is also measured by measurement techniques using infrared spectrophotometry according to the SEMI standard MF1188-1107 and on the basis of ten samples taken at mid-height of the ingot. The value of oxygen retained is the average of these 10 samples.

Significant contamination of the ingot is observed from this first use of the crucible.

The main characteristics of the process and measurements made on the ingot are shown in Table 1. Example 2 (comparative):

A second crucible is manufactured using the procedure described in Example 1, but this time the coating is obtained from an initial mixture of powders consisting of 80% by weight of a powder of silicon nitride particles of diameter. median 0.6 microns and 20% weight of fumed silica WACKER reference HDK T40 and having a specific surface of the order of 400m ² / g. The slurry of this mixture is then applied to the working surfaces of the crucible, previously heated to 120 ° C. with water and nitric acid to obtain a pH of 2.3.

The coated crucible is then dried under air for 4 hours at 110 ° C. and then heated at 900 ° C. for 4 hours, in air, in order to obtain the mechanical strength and the sufficient adhesion of the layer to the walls of the crucible. It is observed that a simple calcination treatment at 600 ° C. or up to 800 ° C. is insufficient to obtain a coating having an abrasion resistance sufficient for the application.

The coated crucible thus obtained is used for the melting of a powder and the crystallization of a silicon ingot under the same conditions as for the previous example.

As for the previous example, very few adhesion zones, relatively small, are observed and the ingot can ultimately also be detached from the crucible without particular difficulties.

Oxygen contamination is also measured according to the technique previously described. Oxygen ingot contamination is also observed from this first use of the crucible, although less important than for example 1.

The main characteristics of the process and measurements made on the ingot are shown in Table 1.

Example 3 (according to the invention):

A second crucible is manufactured by using the procedure described in Example 1, but this time the silicon nitride powder SN-E05 used for the coating is partially partially oxidized in air at a temperature of 1100 ° C. for 4 hours. The surface of the walls and the bottom of the raw crucible preheated to 120 ° C., are coated by paint with a mixture comprising a weight part of the oxidized silicon nitride powder thus obtained for a part by weight of demineralized water, but this time without the need to add a dispersing agent or an acid.

The coating is dried at 110 ° C under air for 4 hours. This time, it is observed that no firing step is necessary to obtain sufficient adhesion of the oxidized silicon nitride coating layer to the porous walls of the crucible.

The coated crucible thus obtained is used for the melting of a powder and the crystallization of a silicon ingot under the same conditions as for the preceding examples.

Very few areas of adhesion, relatively narrow, are observed and the ingot can finally be detached from the crucible without particular difficulties.

Oxygen contamination is measured according to the technique previously described. This time it is observed a contamination of the ingot by the oxygen significantly lower than that obtained for Example 1.

Example 4 (comparative):

In this example, the procedure is the same as Example 3 according to the invention but is used for coating a silicon nitride powder with a median diameter of 0.1 micrometers.

The crucible is used for the melting of a powder and the crystallization of a silicon ingot under the same conditions as for the previous examples.

After solidification, the crucible has zones of adhesion with the ingot so that the crucible does not can more directly be reused after demolding, unlike the crucible coated according to the invention.

The main characteristics of the process and measurements made on the ingot are shown in Table 1 which follows:

1 2 3 4

Example

(comparative) (comparative) (invention) (comparative)

Crucible type RBSN Type RBSN Type RBSN Type RBSN

Before cooking

Oxygen in the

crucible

(% weight)

Measure LECO 1,2 1,2 1,2 1,2 TC436 on

sample

taken to heart

Composition of 100% Si ₃ N ₄ 20% Si0 ₂ 100% S1 ₃ N ₄ 100% S1 ₃ N ₄ coating 80% S1 ₃ N ₄ preoxidized preoxidized

Median size

grains of 0.6 μπι Ο, βμπι (Si ₃ N ₄₎ 0.6 μπι 0.1 μπι coating

Oxygen level

grains of

<2% - 7%> 7% coating

(% weight)

Cooking 1100 ° C / air 900 ° C / air none none

After cooking

Oxygen in the

crucible after

baking

(% weight)

3,4 1,2 1,2 1,2 Measure LECO

TC346 on

sample

taken to heart

Strong areas

No No No Yes of membership

Contamination in

oxygen at mid-height of Base 100 -10% -40% (not measured) ingot after

baking

Table 1

Claims

1. A method of manufacturing a ceramic crucible particularly useful for the melting and crystallization of a silicon ingot comprising the following steps:

- Manufacture of a bare ceramic crucible having a bottom and walls whose surfaces define an internal volume for the melting / crystallization of said ingot, said crucible being made of a material consisting essentially of silicon nitride whose open porosity is between 15 and 60% and the median pore volume diameter is between 0.1 and 10 microns, said silicon nitride further comprising less than 3% by weight of oxygen,

partial oxidation of a set of silicon nitride grains having a median diameter of between 0.3 and 30 microns, such that said silicon nitride grains comprise between 5 and 10% by weight of oxygen,

2. Method according to one of claims 1 or 2 wherein the application of the coating layer is carried out without cooking.

3. Method according to one of claims 1 or 2 wherein the coating layer is applied from a mixture of said oxide grains with a solvent such as water, so as to obtain a slip comprising 20 to 80% by weight. weight of said grains, the remainder being the solvent, said slip being subsequently deposited on the bottom surface and the walls by a technique selected from dipping, dipping, spin coating, brushing, spraying, scraping, brushing and finally drying at an evaporating temperature the solvent, said temperature being less than 200 ° C.

4. Method according to one of the preceding claims, wherein the oxidized silicon nitride grains are obtained by an air heat treatment of silicon nitride grains at a temperature between 900 and 1400 ° C, for a period of 0 , 5 to 10 hours.

5. Method according to one of the preceding claims, wherein the grains of silicon nitride used to form the crucible are RBSN grains.

6. A method of manufacturing a silicon ingot, comprising the steps of melting high purity silicon at about 1500 ° C in a ceramic crucible obtained according to one of the preceding claims, to solidify a silicon ingot and to unmold said ingot of the crucible.

A crucible capable of being made from a process according to one of the preceding claims having a bottom and walls whose surfaces define an interior volume, comprising:

a core made of a material consisting of silicon nitride comprising less than 3% oxygen, whose open porosity is between 15 and 60% by volume and the median pore diameter by volume is between 0.1 and 10; micrometers and, a coating layer on said surfaces defining the interior volume of the crucible, said layer consisting of partially oxidized silicon nitride grains, said silicon nitride grains having a median size of between 0.3 and

30 micrometers, and comprising between 5 and 10% by weight of oxygen.

8. Crucible according to the preceding claim, wherein the silicon nitride constituting the core material comprises less than 2% oxygen.

9. The crucible according to one of claims 7 or 8, wherein the median pore diameter is between 0.2 and 5 micrometers.

10. Crucible according to one of claims 7 to 9, wherein the open porosity is between 20 and 40%.

11. Crucible according to one of claims 7 to 10, wherein the median size of the partially oxidized silicon nitride grains of the coating layer is between 0.5 and 5 micrometers.

12. Crucible according to one of claims 7 to 11 wherein the average thickness of the coating layer is less than 1000 micrometers, preferably less than 500 micrometers.

13. Crucible according to one of claims 7 to 12 wherein the average thickness of the coating layer consisting of partially oxidized silicon nitride grains is between 10 and 500 times the median size of said grains.

14. Crucible according to one of claims 7 to 13 wherein the median size of the partially oxidized silicon nitride grains is between 0.5 and 5 times the median pore diameter of the material constituting the crucible.