EP3966368A1

EP3966368A1 - Method and crucible for producing particle-free and nitrogen-free silicon ingots by means of targeted solidification, silicon ingot, and the use of the crucible

Info

Publication number: EP3966368A1
Application number: EP20724460.9A
Authority: EP
Inventors: Christian Reimann; Matthias Trempa; Stanislaus Schwanke; Christian KRANERT; Jochen Friedrich
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2019-05-06
Filing date: 2020-05-05
Publication date: 2022-03-16
Also published as: DE102019206489A1; JP2022531716A; US20220213616A1; WO2020225244A1

Abstract

The present invention relates to a method and to a crucible for producing particle-free and nitrogen-free silicon ingots by means of targeted solidification, in which method a crucible is provided, the inner surface of the crucible having a coating containing Si_xN_y over its full surface or at least in regions, which coating is coated with a protective layer containing SiO_x in order to reduce or prevent the introduction of nitrogen and Si_xN_y particles into the silicon. The invention also relates to a silicon ingot, which is virtually free from nitrogen or Si_xN_y particles.

Description

Method and crucible for the production of particle and nitrogen-free silicon ingots by means of directional solidification, silicon ingot

and the use of the crucible. The present invention relates to a method and a crucible for the produc- tion of particle- and nitrogen-free silicon ingots by means of directional solidification, in which a crucible is provided, the inner surface of the crucible over the entire surface or at least partially Si _x N _y (in particular Si ₃ N ₄ ) containing coating, which is coated with a SiO _x (with 1 <x <2) containing protective layer to reduce or avoid the entry of nitrogen and Si _x N _y particles into the silicon. The invention also relates to a silicon ingot which is virtually free of nitrogen or Si _x N _y particles. Because of its physical properties, monocrystalline silicon is very suitable for use as a mirror substrate. For the production of large silicon blocks from which components such as mirror substrates with dimensions of approx. 90 x 60 x 20 cm ³ (LxWxH) can be prepared, only the method of directional solidification comes into question, which so far has been used almost exclusively for the production of multicrystalline or quasi-single crystalline silicon blocks for use in photovoltaic taik is used. Processes for the production of silicon single crystals that are otherwise widespread, such as the Czochralski process or the floating zone process, are ruled out from the start due to the limited crystal dimensions.

The quasi-monocrystalline technology by means of directional solidification

The production of silicon blocks is established (quasi-mono technology). Here, single-crystal silicon plates are placed on the bottom of the crucible, from which the single-crystal block is solidified in the crucible. The term quasi-monocrystalline comes from the fact that multicrystalline growth occurs in the outermost edge area of the silicon blocks.

A process-related problem, however, is the nitrogen and carbon contamination of the silicon melt. The nitrogen is introduced into the silicon melt via the chemical dissolution and mechanical erosion of the Si _x N _y crucible coating (Si _x N _y particles). Most of the carbon gets into the silicon melt via the furnace atmosphere. In both cases, when the solubility limits are exceeded in the silicon melt, SiC or Si _x N _y particles are formed, which are then incorporated into the crystal during the crystallization process. In addition, SiC and Si _x N _y precipitates can form in the solidified silicon as a result of diffusion processes. Experience has shown that the diameter of such particles is in the range from a few micrometers to approx. 50 μm. In addition, SiC filaments or Si _x N _y needles, which are in a similar area in cross section to the particles, can be up to several millimeters in length. During the conditioning process of the component surfaces (polishing), these particles cause scratch structures on the surface when they are torn out of the surface or leave behind hole-like depressions.

Accordingly, these particles / precipitations in the silicon material must be kept very small (nm range) or completely avoided in order to ensure that the above-mentioned silicon components, in particular mirror substrates, with the required to be able to produce the highest surface quality.

A few technological approaches that address this problem are already known from the PV industry. An argon countercurrent can be built up over the silicon melt surface via a suitable gas management system, which prevents the entry of the gaseous carbon monoxide (CO) into the

Silicon melt and thus the formation of SiC precipitates are reduced (WO 2009100694 Al). However, the introduction of carbon cannot be completely avoided, so that there is always a certain residual risk of SiC formation in the melt or the crystal.

It is also known that with the help of electromagnetic fields, the melt convection can be influenced in such a way that an et wae supersaturation of nitrogen or carbon before the

Silicon crystal growth front is broken down into the less contaminated melt volume in a short time before a SiC or Si _x N _y precipitation occurs (DE102010041061 B4). This indirect method for avoiding the SiC and Si ₃ N ₄ precipitations is very effective, but not trivial, relatively expensive and, for geometric reasons, not applicable to any crystallization furnace. In addition, with the quasimono process, care must be taken that, due to excessive convection of the melt, the germination plates at the bottom of the crucible are not too strong or uneven

be melted, which makes the application of electromagnetic stirring for this technology difficult.

However, the measures mentioned do not prevent the introduction of nitrogen / Si _x N _y particles via the contact of the Si _x N _y coating with the

Silicon melt.

On the basis of this, the object of the present invention was to provide a method for producing silicon ingots, the silicon ingots being essentially free of nitrogen and Si _x N _y particles.

This object is achieved with the method with the features of claim 1, the crucible with the features of claim 18 and the silicon ingot with the features of claim 25. In claim 29 a use according to the invention is specified. According to the invention a method for the production of particle and stick material-free silicon ingots by means of directional solidification is provided in which

a) a crucible is provided, the inner surface of the crucible having a coating containing Si _x N _{y over the} entire surface or at least in some areas, which with an SiO _x (with 1 <x <2) contain the protective layer to reduce or avoid nitrogen entry and Si _x N _y particle entry into the silicon is coated, b) the crucible is filled with silicon raw material,

c) the silicon raw material in the crucible to form a silicon melt

is melted, and

d) the silicon melt is subjected to a directional solidification, whereby particle-free and nitrogen-free silicon is formed.

Surprisingly, it was found that the entry of nitrogen and Si _x N _y (in particular Si ₃ N ₄ ) can be prevented or at least significantly reduced by applying an additional protective layer to the surface of the Si _x N _y crucible coating. This protective layer as such consists of, essentially consists of or contains high-purity nanometer or micrometer-sized SiO _x particles, in particular SiO particles in an aqueous suspension which is sprayed onto the existing Si _x N _y layer. The spray parameters are to be selected in such a way that the underlying Si _x N _y layer, in particular an Si ₃ N ₄ layer, is not damaged.

The SiO _x protective layer prevents direct contact of the silicon melt with the Si _x N _y coating and thus both the chemical dissolution reaction between silicon and Si _x N _y and the direct erosion of the SixNy coating due to the movement of the melt. After solidification, the SiO _x layer forms a firm bond with the silicon block due to its wetting behavior. The interface of the block and the crucible is consequently at the boundary between SiO _x and Si _x N _y layer, within the Si _x N _y layer or at the interface between Si _x N _y layer and crucible (depending on the adhesion properties of the Si _x N _y layer).

It is preferred that the protective layer by means of spraying, a brush Process, a coating process, and / or a dipping process of a suspension containing SiO _x on the Si _x N _y- containing coating and the moist protective layer produced in this way containing SiO _{x is} dried. The suspension preferably contains 5 to 90% by weight SiO _x , in particular colloidal SiO _x , and 95 to 10% by weight of a suspension agent, preferably an alcohol or water, particularly preferably deionized water.

Another preferred embodiment provides that the protective layer is applied to the Si _x N _y- containing coating by means of a spray method, a brush method, a painting method and / or a dipping method of a suspension containing Si and the moist protective layer produced in this way containing Si is dried and / or is oxidized. The suspension here preferably contains 5 to 90% by weight of Si and 95 to 10% by weight of a suspending agent, preferably an alcohol or water, particularly preferably deionized water. The Si layer is preferably oxidized under an air atmosphere or an inert gas atmosphere enriched with oxygen at a temperature of 800 and 1400 ° C., preferably at a temperature of 1050 and 1200 ° C., to form an SiO _x layer. The duration of the oxidation is preferably in the range from 0.5 h to 12 h.

It is preferred that when the suspension containing SiO _x or Si is applied, the crucible or the coating containing Si _x N _{y has} a temperature of 10 ° C to 200 ° C, preferably a temperature of 20 ° C to 100 ° C.

The SiO _{x of} the protective layer preferably has at least one of the following properties:

• an iron content of 0 ppm to 5 ppm, preferably 0.5 to 2 ppm,

• an aluminum content of 0 ppm to 20 ppm, preferably 0.5 to 15 ppm,

• a content of metals other than iron and aluminum of 0 ppm to 5 ppm, preferably 0.5 to 2 ppm,

• a particle size d50 of 0.05 to 100 miti, preferably 0.5 to 50 miti, particularly preferably from 1 to 30 miti,

• a particle size d90 of 0.01 to 200 miti, preferably 0.05 to 100 miti, particularly preferably from 0.1 to 50 miti. The particle size can be determined using established laser scattering and laser diffraction methods.

The particle size of the SiO _x must be selected to be very small in order to enable the most dense possible layer. This is the only way to prevent a sufficient barrier effect against the diffusion of nitrogen through the protective layer. It was also found that a protective layer made of a monolayer of SiO _x does not have a sufficient barrier effect, since the arrangement as a monolayer does not allow an adequate barrier effect against diffusion.

The crucible preferably contains or consists of a material which is selected from the group consisting of SiC, C, BN, pBN, Si _x N _y , SiO _x , and mixtures and combinations thereof.

Another preferred embodiment provides that the coating containing Si _x N _{y has} at least one of the following properties:

• a content of up to 100% by weight, preferably 60% by weight to 100% by weight, particularly preferably 80% by weight to 100% by weight, Si _x N _y

• a thickness of 10 pm to 2500 pm, preferably from 100 pm to 1000 pm, particularly preferably from 300 pm to 600 pm,

• a square mean roughness value R _q from 1 pm to 100 pm, preferably from 5 pm to 20 pm,

• an adhesive strength on the crucible bottom of 0.5 MPA to 5 MPa, preferably from 1 MPa to 3 MPa,

• a porosity of 0.5% to 60%, preferably from 1% to 40%.

The square mean roughness value R _q can be determined according to

With / "= profile line length and z = values in the z-direction of the roughness profile, determined in accordance with DIN EN ISO 4287: 2010-07. The adhesive strength can be determined according to E DIN EN ISO 4624: 2014-06: tear-off test to assess the adhesive strength or DIN EN ISO 2409: 2013-06: cross-cut test. The porosity can be determined using

Mercury porosimetry or BET measurement according to DIN-ISO 9277.

The coating containing Si _x N _{y is} preferably produced by applying a suspension containing Si _x N _{y over the} entire surface or at least in some areas to the inner surface of the crucible and drying the moist Si _x N _y coating produced in this way.

The suspension containing Si _x N _y preferably has a composition with the following components:

10% by weight to 60% by weight Si _x N _y ,

30% by weight to 80% by weight organic solvent or water,

0 wt% to 30 wt% silicon,

0.5% by weight to 10% by weight dispersant,

0.01% by weight to 0.2% by weight defoamer, as well as

0.05 wt% to 2 wt% organic binder,

the proportions of the components adding up to 100% by weight.

The suspension containing Si _x N _y is preferably applied by means of a spray method, a brush method, a brush method, and / or a dipping method. When the suspension containing Si _x N _{y is applied,} the crucible should have a temperature of preferably 10 ° C to 200 ° C, preferably a temperature of 20 ° C to 100 ° C.

Another preferred variant of the method according to the invention provides that after step a) and before step b) at least one seed plate, in particular as a base plate, is introduced into the crucible. This serves to nucleat the silicon in a direction perpendicular to the bottom of the crucible. This seed plate is preferably formed from mono- or multi-crystalline silicon, i. E. consists or contains mono- or multi-crystalline silicon. The material of the seed plate preferably has an orientation (100, 110 or 111) perpendicular to the seed plate in the direction of crystal growth.

With regard to the dimensioning of the seed plate, this is given by the surface of the crucible with regard to the maximum possible area. Indeed a smaller area of the seed plate is preferably chosen so that several seed plates can be arranged in the crucible. The fiction, contemporary seed plates can preferably have a square shape (eg

10 x 10 cm, 20 x 20 cm, 30 x 30 cm), a rectangular shape (e.g. 100 cm x 10 cm, 100 cm x 20 cm, 100 x 30 cm) or a circular shape (e.g. with 200 or 300 mm diameter). Several germination plates should then cover the bottom surface of the pan as well as possible. A further preferred embodiment provides that several seed plates are arranged in a grid on the bottom of the crucible, e.g. as 3 x 3 grid or 4 x 4 grid with 200 or 300 mm diameter of the round germination plates.

The thickness of the at least one seed plate is preferably in the range from 1 to 10 cm, particularly preferably in the range from 3 to 7 cm.

According to the invention, a crucible for the production of particle- and nitrogen-free silicon ingots by means of directional solidification is also provided, the inner surface of the crucible having a coating containing Si _x N _{y over the} entire surface or at least in areas, on which a protective layer containing SiO _x for reducing or Avoidance of nitrogen entry and Si _x N _y entry of particles into the silicon is deposited.

According to the invention, it was found that the SiO _x layer must lie in a certain thickness range which essentially depends on the rate of dissolution / erosion of the SiO _x layer in the respective furnace / crystal growth process.

If the layer is applied too thinly, it can be completely eroded and the positive effect does not occur because the contact of the Si melt with the Si _x N _y layer is restored. If the layer is applied too thick, cracks can form in the silicon block during cooling due to the above-mentioned solid bond and the different expansion coefficients of SiO _x and silicon. A layer thickness of 200-500 μm after the crystallization process has proven to be ideal. The originally applied layer thickness should therefore be in the range of 200-500 pm + layer thickness eroded in the process. It is therefore preferred that the protective layer containing SiO _{x has} a thickness of 10 to 2000 μm, particularly preferably 50 to 1000 μm.

Another preferred embodiment provides that the SiO _x containing Protective layer has a square center rough value R _q from 1 to 250 mih, preferably from 5 to 150 mih.

It is further preferred that the protective layer containing SiO _{x has} a porosity after coating the Si _x N _y layer of 20 to 80%, preferably of 30% to 70%.

According to the invention, a silicon ingot with a nitrogen concentration of <1E16 at / cm ³ , preferably <5E15 at / cm ³ , particularly preferably <1E15 at / cm ^{3 is also} provided.

The silicon block preferably has an Si _x N _y particle density of <10 / cm ³ , preferably of <5 / cm ³ .

The silicon ingot can preferably be produced according to the above-described method according to one of claims 1 to 9.

The ingot preferably consists or consists essentially of monocrystalline, quasi-monocrystalline or multicrystalline silicon.

The subject according to the invention is to be described in more detail using the following figures and examples, without restricting them to the specific forms shown here.

FIG. 1 uses a diagram to show the nitrogen concentration in a silicon ingot according to the invention over the ingot height, each measured in the ingot center

Embodiment

In a first embodiment, a series of laboratory crystallization experiments were carried out (Si initial weight 1.1 kg, ingot dimensions 100 mm in diameter, 60 mm in height). First, a reference was grown without a SiO2 protective layer. The resulting nitrogen concentration in the ingot, measured by FTIR, is [N]> 1E16 at / cm ³ (cf. FIG. 1), which is in the range of the nitrogen solubility limit and thus leads to the formation of precipitation. If a very thin SiO2 protective layer (2U) is now applied, the concentration over the majority of the block already falls below the detection limit of the FTIR measurement method of 1E15 at / cm ³ . A value of ~ 2E15 at / cm ³ can only be measured at the end of the block, which is an indication of the dissolution of the Si02-

Layer represents. With an increased thickness (4U or 8U), practically no nitrogen can be measured by means of FTIR, ie constantly below 1E15 at / cm ³ , and Si3N4 precipitation is reliably avoided. In addition, it can be assumed that no Si3N4 particles got into the melt, since otherwise they would have dissolved until nitrogen solubility was reached and would therefore have had to be detected with nitrogen. In the case of an SiO2 coating with a thickness of 20U, cracks occurred in the lower block bottom area due to the different thermal expansion of the SiO2 layer and the Si block.

In comparison to the experiments with the SiO2 layer, a further experiment with forced melt convection is shown in Fig. 1. The nitrogen values show that it is possible to completely avoid the formation of precipitates, but not nitrogen entry.

Claims

1. Process for the production of particle- and nitrogen-free silicon

Ingots by means of directional solidification, in which

a) a crucible is provided, the inner surface of the crucible having a coating containing Si _x N _{y over the} entire surface or at least in some areas, which is coated with a protective layer containing SiO _x to reduce or avoid the entry of nitrogen and Si _x N _y particles the silicon is coated, b) the crucible is filled with silicon raw material,

c) the silicon raw material is melted in the crucible to form a silicon melt, and

2. The method according to claim 1,

characterized in that the protective layer is applied to the coating containing Si _x N _y by means of a spray method, a brush method, a brush method and / or a dipping method of a suspension containing SiO _x and the moist protective layer containing SiO _x produced in this way is dried .

3. The method according to claim 2,

characterized in that the suspension contains 5 to 90 wt.% SiO _x , in particular colloidal SiO _x , and 95 to 10 wt.% of a suspending agent, preferably an alcohol or water, particularly preferably deionized water.

4. The method according to claim 1,

characterized in that the protective layer is applied to the Si _x N _y containing coating by means of a spray method, a brush method, a painting method and / or a dipping method and the moist protective layer produced in this way containing Si is dried and / or is oxidized.

5. The method according to claim 4,

characterized in that the suspension contains 5 to 90% by weight of Si and 95 to 10% by weight of a suspending agent, preferably an alcohol or water, particularly preferably deionized water.

6. The method according to claim 4 or 5,

characterized in that the Si layer is oxidized under an air atmosphere or an oxygen-enriched inert gas atmosphere at a temperature of 800 and 1400 ° C, preferably at a temperature of 1050 and 1200 ° C to form an SiO _x layer.

7. The method according to claim 6,

characterized in that the duration of the oxidation is in the range from 0.5 h to 12 h.

8. The method according to any one of claims 2 to 7,

characterized in that when the SiO _x or Si-containing suspension is applied, the crucible or the Si _x N _y- containing coating has a temperature of 10 ° C to 200 ° C, preferably a temperature of 20 ° C to 100 ° C.

9. The method according to any one of the preceding claims,

characterized in that the SiO _{x has} at least one of the following properties: • an iron content of 0 ppm to 5 ppm, preferably 0.5 to 2 ppm,

• an aluminum content of 0 ppm to 20 ppm, preferably 0.5 to 15 ppm,

5 • a content of metals other than iron and aluminum of 0 ppm to 5 ppm, preferably 0.5 to 2 ppm,

• a particle size d50 of 0.05 to 100 miti, preferably 0.5 to 50 miti,

• a particle size d90 of 0.01 to 200 miti, preferably 0.05 to

10 100 miti.

10. The method according to any one of the preceding claims, characterized in that the crucible contains or consists of a material which is selected from the group consisting of

15 SiC, C, BN, pBN, Si _x N _y , SiO _x , and mixtures and combinations thereof.

11. The method according to any one of the preceding claims, characterized in that the coating containing Si _x N _y

20 has at least one of the following properties:

• a thickness of 10 μm to 2500 μm, preferably from 100 μm to 1000 μm, particularly preferably from 300 μm to 600 μm,

25 • a square mean roughness value R _q from 1 pm to 100 pm, preferably from 5 pm to 20 pm,

• a porosity of 0.5% to 60%, preferably from 1% to 40%.

BO

12. The method according to any one of the preceding claims,

characterized in that loading the generation of the Si _x N _y containing coating is effected by that a Si _x N _y containing suspension is applied to the inner surface of the crucible bottom and containing thus produced moist Si _x N _y coating is dried.

13. The method according to any one of the preceding claims,

characterized in that the suspension containing Si _x N _{y has} a composition with the following components:

10% by weight to 60% by weight Si _x N _y ,

30% by weight to 80% by weight organic solvent or water, 0% by weight to 30% by weight silicon,

0.5% by weight to 10% by weight dispersant,

0.01% by weight to 0.2% by weight defoamer, as well as

0.05 wt% to 2 wt% organic binder,

the proportions of the components adding up to 100% by weight.

14. The method according to any one of the preceding claims,

characterized in that

The application of the suspension containing Si _x N _{y takes place} by means of a spraying process, a brushing process, a painting process, and / or a dipping process, and / or

• when the suspension containing Si _x N _{y is applied} , the crucible has a temperature of 10 ° C. to 200 ° C., preferably a temperature of 20 ° C. to 100 ° C., and / or

• the suspension containing Si _x N _y is additionally applied to at least one further inner surface of the crucible, preferably to all inner surfaces of the crucible.

15. The method according to any one of the preceding claims,

characterized in that after step a) and before step b) at least one seed plate is arranged on the bottom of the crucible.

16. The method according to the preceding claim,

characterized in that the at least one seed plate consists of or contains mono- or multi-crystalline silicon, the at least one seed plate preferably having an orientation (100, 110 or 111) perpendicular to the seed plate in the crystal growth direction.

17. The method according to any one of claims 11 or 12,

characterized in that the at least one seed plate has a square shape, in particular 10 x 10 cm, 20 x 20 cm, 30 x 30 cm, a rectangular shape, in particular 100 cm x 10 cm, 100 cm x 20 cm, 100 x 30 cm, or has a round shape, in particular with a diameter of 200 or 300 mm, and / or the at least one seed plate has a thickness of 1 to 10 cm.

18. Crucible for the production of particle- and nitrogen-free silicon ingots by means of directional solidification, the inner surface of the crucible having a Si _x N _y containing coating over the entire area or at least in some areas, on which a protective layer containing SiO _x for reduction or Avoidance of nitrogen entry and Si _x N _y - particle entry into the silicon is deposited.

19. Crucible according to claim 18

characterized in that the protective layer containing SiO _{x has} a thickness of 10 to 2000 μm, preferably 50 to 1000 μm.

20. Crucible according to one of claims 18 or 19,

characterized in that the protective layer containing SiO _{x has a} square center rough value R _q of 1 to 250 μm, preferably of 5 to 150 miti.

21. Crucible according to one of claims 18 to 20,

characterized in that the protective layer containing SiO _{x has} a porosity after coating the Si _x N _y layer of 20 to 80%, preferably from 30% to 70%.

22. Crucible according to one of claims 16 to 21,

characterized in that the crucible has at least one seed plate at the bottom of the crucible.

23. Crucible according to the preceding claim,

24. Crucible according to one of claims 22 or 23,

25. Silicon ingot with a nitrogen concentration <1E16 at / cm ³ , preferably <5E15 at / cm ³ , particularly preferably <1E15 at / cm ³ .

26. Silicon ingot according to claim 25,

characterized in that the silicon ingot has a Si _x N _y - Particle density of <10 / cm ³ , preferably of <5 / cm ³ .

27. Silicon ingot according to claim 25 or 26,

characterized in that the silicon ingot can be produced by the method according to any one of claims 1 to 17.

28. Silicon ingot according to one of claims 25 to 27,

characterized in that the silicon ingot consists of monocrystalline, quasi-monocrystalline or multicrystalline silicon.

29. Use of a crucible according to one of claims 18 to 24 for the production of silicon ingots by means of directional solidification.