WO2013041661A1

WO2013041661A1 - Loading silicon in a crucible

Info

Publication number: WO2013041661A1
Application number: PCT/EP2012/068611
Authority: WO
Inventors: Tyke Naas; Gero Wolfgang NEUROTH
Original assignee: Rec Wafer Pte. Ltd.
Priority date: 2011-09-21
Filing date: 2012-09-21
Publication date: 2013-03-28
Also published as: TW201331430A; GB201116336D0; GB2494893A

Abstract

A process for loading a crucible with silicon and a loaded crucible for use in a crystallisation process are provided. The crucible is loaded with two different form factors of silicon, which allows a higher packing density to be achieved than if only a single form factor were used. Moreover, to prevent cracking of the crucible due to shrinkage during the crystallisation process, a peripheral region is provided in which only silicon of the smaller form factor is disposed.

Description

LOADING SILICON IN A CRUCIBLE

Field of the Invention The present invention relates to the packing of semiconductor material within a crucible. In particular, the present invention relates to the packing of silicon in a crucible for use in a crystallisation process.

Background to the Invention

Crystalline silicon finds many uses in the modern world, particular in the fields of semiconductor electronics and photovoltaics. The majority of crystalline silicon wafers are now being produced for use in solar cells. Two common processes for producing crystalline silicon are the Czochralski method and the Bridgman method. Both processes develop crystalline silicon from a molten silicon ingot. In the case of the Czochralski process, the crystalline silicon is formed by the slow withdrawal of a seed crystal from the molten ingot, while in the Bridgman method a directional solidification process allows the crystalline silicon to be formed as the silicon ingot solidifies.

In order to form crystalline silicon, therefore, it is necessary first to produce a molten bath of substantially pure silicon. This is typically achieved by introducing silicon feedstock into a crucible and heating the crucible under controlled conditions. Sufficiently pure silicon for use as a feedstock in this process is typically, though not exclusively, formed using one of two processes: the Siemens method in which silicon is deposited from monosilane or trichlorosilane gas at high temperature on initial seed rods of pure silicon, thereby forming relatively large rods of polycrystalline silicon; and fluidised bed processes which result in granules of polycrystalline silicon. Silicon rods formed by the Siemens process are often broken into smaller rods or into irregular shaped chunks before being used as feedstock material. Another source of silicon feedstock material may come in the form of recycled material from previous crystallisation processes. One important factor in the cost effectiveness of crystalline silicon production processes is the amount of silicon that can be processed in each batch. In particular, this will depend on the amount of silicon feedstock that is melted in the crucible to form the silicon ingot. For a given crucible size, this will depend on the packing density of silicon feedstock that can be placed in the crucible. This depends on the form factor of the silicon feedstock and the shape of the crucible itself. It is therefore desired to increase the density of the silicon feedstock that can be placed in a crucible for use in developing crystalline silicon.

Summary of the Invention According to a first aspect of the present invention, there is provided a method for loading a crucible with silicon for use in a crystallisation process, comprising:

1 ) placing silicon of a first form factor in a central region of the crucible, the central region being separated from a wall of the crucible by a peripheral region;

2) providing silicon of a second form factor in the central region and the peripheral region, thereby creating a silicon feedstock layer across the crucible comprising silicon of both the first and second form factors, wherein the silicon of the first form factor is larger than the silicon of the second form factor.

By using at least two different form factors of silicon in each feedstock layer, the present invention can achieve a greater packing density than would be achieved by either form factor in isolation. In this case, the term form factor refers to a class of silicon pieces in terms of their physical geometry, including size and/or shape. Each class of silicon pieces need not be identical, although they will typically be obtained from a similar source. In particular, the silicon of the second, smaller form factor can fill gaps that appear between the silicon of the first, larger form factor. Preferably, the silicon of the first form factor is larger by average volume than the silicon of the second form factor. In addition, the peripheral region of the present invention ensures that only silicon of the second, smaller form factor is within a predetermined distance from the wall of the crucible. The peripheral region is a predetermined area of the crucible in which no silicon of the first form factor is placed. It is found that this can reduce the risk of damage to the crucible during the crystallisation process. In particular, the crucible is liable to deform by shrinking under the high temperatures required by silicon crystallisation processes. As a result, dense packing of silicon feedstock within the crucible can result in excessive pressure on the crucible walls, resulting in damage to these walls. The present invention addresses this problem by ensuring that only silicon of the second, smaller form factor is disposed adjacent to the walls of the crucible. It has been found that this allows the silicon feedstock to adjust to the contraction of the crucible, thereby reducing the risk of damage to the crucible as it contracts. In preferred embodiments, the silicon of the second form factor is granular silicon, and preferably the granular silicon is substantially spherical. Granular silicon, particularly, if spherical, can act in a fluid-like manner, ensuring that adjustment of the silicon feedstock layer to take account of the deformation of the crucible during the crystallisation process can occur relatively easily.

In preferred embodiments, the granular silicon has a diameter of less than 5mm. Nevertheless, in other embodiment the diameter of the granular silicon may vary, although it will preferably be in the range of 0.1 mm to 20mm, more preferably between 0.15 and 15mm, yet more preferably between 0.15 and 10mm.

Granular silicon is preferably formed using a fluidised bed reactor (FBR) process. This is an efficient and reliable process for the formation of granular silicon which has good properties for both dense packing of the crucible and avoiding damage to the crucible during the crystallisation process.

In preferred embodiments, the silicon of the first form factor comprises one or both of silicon chunks or silicon rods. Moreover, the silicon of the first form factor may comprise silicon which is a product of the Siemens process. The Siemens process typically produces relatively large rods of polycrystalline silicon. All or some of these rods may be broken down into smaller rods and/or irregular shaped chunks of silicon. Silicon rods used in the present invention will typically have an average diameter of at least 40mm, and may preferably have an average diameter between 40mm and 140mm. The length of the silicon rods may be selected as appropriate, though typically will fall between 150mm and 2000mm, and in preferred embodiments is in the range of 150mm to 400mm. In other circumstances, the length of the silicon rods may differ, and may fall, for example, in the range of 500mm to 2000mm. Silicon chunks will typically have a size between 3mm and 200mm across their largest dimension. In some preferred embodiments, the size distribution of silicon chunks is such that at least 95% of such bodies have a size between 10mm and 100mm across their largest dimension. As an alternative or in addition to the Siemens process, silicon of the first form factor may also be a product of other processes. For example, silicon of the first form factor may be recycled silicon obtained from earlier crystallisation processes. The silicon of the first form factor may also be so-called UMG silicon (upgraded metallurgical silicon)

Preferably, the method further comprises creating one or more further silicon feedstock layers across the crucible by repeating steps 1 ) and 2). In this way, the silicon feedstock within the crucible can be built up layer by layer. This reduces, for example, any risk that silicon of the first form factor will contaminate the peripheral region by unwanted spreading to that region before the silicon of the second form factor has been used to fill it. Accordingly, the arrangement of the silicon of each form factor within the crucible can be closely controlled.

Preferably, the peripheral region has a width at least equal to the expected shrinkage of the crucible during the crystallisation process. More preferably, the peripheral region may have a width of at least double the expected shrinkage of the crucible during the crystallisation process. As such, as the crucible shrinks during the crystallisation process, its walls will not encounter the silicon of the first form factor. In some preferred embodiments, the peripheral region has a width of at least 0.5% of the width of the crucible, more preferably at least 1 % of the width of the crucible, and yet more preferably at least 1.5% or 2% of the width of the crucible. In preferred embodiments, the peripheral region has a width of at least 5mm. To some extent, increasing the width of the peripheral region reduces the risk of damage to the crucible. However, this also reduces the packing density of the silicon feedstock. Accordingly, is has been found that it is preferable that the width of the barrier layer does not exceed 15mm.

In preferred embodiments, the method further comprises, prior to step 1 ), forming a protective layer of silicon across the bottom of the crucible. The protective layer is preferably formed of silicon of the second form factor, preferably granular silicon. Alternatively, the protective layer may be formed of other silicon form factors, such as substantially planar pieces of silicon or pieces of silicon having at least one planar face, if desired. The protective layer reduces the risk of damage to the floor of the crucible as the silicon feedstock layers are introduced. This finds particular utility in preferred embodiments in which the crucible is provided with a release coating on its inner surface, since such a release coating is often easily damaged. The protective layer typically has a height of around 1 mm, although the height may be chosen as appropriate. For example, in some instances the protective layer may have a height of several centimetres. The crucible is preferably formed of slip-cast vitreous silica. This material is able to withstand the necessary temperatures for silicon crystallisation processes.

According to a second aspect of the present invention, there is provided a loaded crucible for use in a silicon crystallisation process, the crucible comprising: one or more silicon feedstock layers, each silicon feedstock layer comprising a central region separated from a wall of the crucible by a peripheral region, wherein silicon of a first form factor is disposed only in the central region and silicon of a second form factor is disposed in central region and in the peripheral region, and wherein the silicon of the first form factor is larger than the silicon of the second form factor.

The second aspect provides a loaded crucible for use in a silicon crystallisation process which benefits from a high packing density of silicon feedstock while reducing any potential risk of damage to the crucible as it shrinks during the crystallisation process. Preferred features of the first aspect may be equally applied to the second aspect as appropriate. Brief Description of the Drawings

Preferred embodiments of the present invention will now be described by reference to the accompanying drawings, in which:

Figure 1 shows a cross-section of a loaded crucible for use in a crystallisation process in accordance with a first preferred embodiment of the present invention;

Figures 2A to 2H show the cross section of crucible during various steps in a process for loading the crucible with silicon feedstock; and

Figure 3 shows a cross-section of a loaded crucible for use in a crystallisation process in accordance with a second preferred embodiment of the present invention;

Detailed Description Referring to Figure 1 , there is provided a crucible 10 loaded with silicon feedstock 20 for use in a silicon crystallisation process. The silicon feedstock 20 comprises granular silicon 22 and silicon bodies 24. In this context, the term silicon bodies 24 is used to refer to pieces of silicon having a relatively large form factor. The silicon bodies 24 are in this case silicon chunks.

The crucible 10 shown in Figure 1 is prepared for use in a directional solidification crystallisation process based on the Bridgman or Vertical Gradient Freeze methods. In such processes, the crucible 10 filled with silicon feedstock is heated in a furnace until the silicon feedstock melts. This creates a molten silicon ingot within the furnace. The silicon ingot is then allowed to cool in such a way that it gradually solidifies, starting at one face of the ingot (typically the bottom) and finishing at a diametrically opposed face. In this way, a solidification front travels through the ingot, allowing crystalline structure to form as the ingot solidifies.

The material from which the crucible 10 is formed must be able to withstand the temperatures used during the crystallisation process, typically around 1420 to 1460 degrees Celsius. Moreover, the crucible material should be chosen so as to avoid contamination of the silicon feedstock/ingot. In the preferred embodiment, the crucible is formed of a slip-cast vitreous silica. Using slip-cast vitreous silica has been found to be an economical way of producing appropriate crucibles, particularly crucibles having a square cross section. Moreover, a silicon nitride containing release coating is provided on the inner surface of the crucible 10 to allow for easy release of the solidified ingot after the process is complete.

Additional reinforcement of the crucible 10 is provided by supports 12. These supports 12 are provided to secure the walls of the crucible 10 in place during the high temperatures required for the crystallisation process. A base 14 is provided on which the supports 12 and the crucible 10 are placed.

As mentioned above, the silicon feedstock comprises both granular silicon 22 and silicon bodies 24. The granular silicon 22 and the silicon bodies 24 have different form factors. In particular, the silicon bodies 24 have a larger average size by volume than the granular silicon 22. The combination of different form factors of silicon within the silicon feedstock 20 allows a higher packing density to be achieved than if only a single form factor were used. In particular, gaps between the larger form factor silicon bodies 24 can be filled by the relatively small granular silicon 22.

The granular silicon 22 is typically obtained using a fluidised bed reactor (FBR) process. This process involves thermal pyrolysis of a silicon-containing gas, typically monosilane or trichlorosilane, conducted under fluidised bed conditions in which deposition of pyrolysis products on a silicon seed particle occurs to produce an essentially smooth spherical polycrystalline silicon granule or particle. Accordingly, the granular silicon 22 typically comprises a plurality of spherical granules. In preferred embodiments, each granule has a diameter of up to 5mm, although it is recognised that larger diameter granules may be used.

The silicon bodies 24 have a larger form factor than the granular silicon 22 by average volume. The silicon bodies 24 may comprise silicon from a number of different sources. For instance, the silicon bodies 24 may contain silicon obtained via the Siemens process. The Siemens process can be understood to involve the thermal pyrolysis of a silicon-containing gas, typically monosilane or trichlorosilane, and deposition of the pyrolysis products on a filament to produce a large rod of silicon. The diameter of the silicon rod produced in this way may vary, but will typically be in the order of 10cm to 20cm. The length of the rod will also vary, but typical examples have a length of between 15cm and 40cm.

The silicon bodies 24 may include entire rods obtained during the Siemens process, or may alternatively/additionally include sections of such rods. In the preferred embodiment shown in Figure 1 , the silicon bodies 24 are chunks of silicon that have been broken from such rods and/or have been obtained from other sources. The broken chunks of silicon shown in Figure 1 are of irregular size and shape. In the preferred embodiment, the broken chunks of silicon that make up the silicon bodies 24 have a largest dimension of between 3mm and 10cm.

The silicon bodies 24 may also be obtained from sources other than the Siemens process. For example, the silicon bodies 24 may comprise one or more pieces of recycled material. Recycled material can be obtained from previous operations of the crystallisation process. For example, the recycled material may be obtained from parts of the solidified silicon ingot that are not suitable for use in their intended purpose. The recycled material may be obtained from tops, tails and side-cuts of solidified silicon that are not used for wafers, but after cleaning can be used as feedstock. The silicon bodies 24 may be also be obtained from alternative silicon purification processes other than the Siemens or FBR methods where appropriate.

While the higher packing density that is realised by combining silicon of different form factors in the silicon feedstock 20 is beneficial in terms of the quantity of crystalline silicon that can be produced from a single crucible 10, it has been found that higher density packing can create a risk of damage to the crucible 10 during the crystallisation process. This is due to the reaction of the crucible 10 to the temperatures it undergoes during the crystallisation process. In particular, it has been found that the crucible 10 is likely to undergo a degree of shrinkage during the crystallisation process. Typically, shrinkage is caused by residual sintering when the crucible 10 is exposed to the high temperatures required during the crystallisation process. This can damage the crucible 10, leading to cracks, if such shrinkage is inhibited by the silicon feedstock 20.

To limit the risk of damage, the larger silicon bodies 24 are placed only in a central region of the crucible 10 separated from the walls of the crucible 10 by a peripheral region in which only granular silicon 22 is disposed. By ensuring that only the smaller elements of the silicon feedstock are disposed adjacent to the walls of the crucible 10, the silicon feedstock 20 is able to adjust more readily to the contraction of the crucible 10. Accordingly, the silicon bodies 24 should be separated from the walls of the crucible 10 by a distance at least equal to the expected shrinkage of the crucible during the crystallisation process. In typical examples, this may require the peripheral region to have a width of at least 0.5% of the width of the crucible, or preferably at least 1 % of the width of the crucible, yet more preferably at least 1.5% or 2% of the width of the crucible.

A greater distance between the silicon bodies 24 and the wall of the crucible will reduce the risk of damage to the crucible 10 further. However, this benefit comes at the expense of a reduced packing density in the region which contains only granular silicon 22. Accordingly, the width of the peripheral region may preferably less than 10% of the width of the crucible, more preferably less than 5% of the width of the crucible. For example, where the crucible has a width of 750mm, the width of the peripheral region may be 15mm. Alternatively, where the crucible has a width of 800mm, the width of the peripheral region may be 20mm. In other preferred embodiments, the width of the peripheral region between the silicon bodies 24 and the walls of the crucible may be in the range of 5mm to 15mm.

The process of loading the crucible 10 can be understood with reference to Figures 2A to 2E. Figure 2A shows an empty crucible 10, prior to any silicon feedstock 20 being placed within the crucible 10. The first step of the loading process is to provide a protective layer of granular silicon on the bottom of the crucible 10, as shown in Figure 2B. This layer protects the crucible 10, and in particular the ingot release coating, from damage during later steps in the loading process. Although in the preferred embodiment the protective layer is formed of granular silicon, it may be formed of other form factors of silicon if appropriate. For example, flat pieces of recycled silicon, or pieces of silicon presenting at least one flat face, may be placed against the bottom of the crucible 10 to protect the release coating. The protective layer of silicon typically has a height in the order of 1 cm or up to several centimetres.

The next step of the process is to place a layer of silicon bodies across a central region of the crucible 10, the central region being separated from the side walls of the crucible 10 by the peripheral region. In a typical example, the peripheral region has a width w of between 5cm and 15cm. Figure 2C shows the appearance of the crucible 10 after the layer of silicon bodies 24 has been disposed in the central region. The height h of the layer of silicon bodies 24 is typically determined by the size of those bodies. In particular, it is often undesirable to place silicon bodies on top of each other in a single layer, since this inhibits the stability of the structure, and may therefore cause the silicon bodies 24 to fall into the peripheral region. Thus, the height h of the layer will typically be similar to the size of the silicon bodies 24. Granular silicon 22 is then poured into the crucible 10, filling the gaps between the silicon bodies 24 and also filling the peripheral region up to the level of the top of the layer of silicon bodies 24. This completes a feedstock layer which comprises both granular silicon 22 and silicon bodies 24, as illustrated in Figure 2D.

Further feedstock layers can be added in the same way, by firstly disposing silicon bodies 24 on the existing silicon feedstock layer in a central region and then filling the gaps between the silicon bodies 24 and the peripheral region with granular silicon 22. The addition of further layers can be seen in Figures 2E to 2H. For optimal packing density the silicon bodies are arranged so that they touch each other.

In the embodiment shown in Figures 1 and 2A to 2H, the silicon bodies 24 are irregular chunks of silicon, which may be obtained from breaking down the silicon rods created in the Siemens process or from recycling silicon that has been through a previous crystallisation process, for example. However, in other embodiments, the silicon bodies 24 may include regularly shaped pieces of silicon, such as the rods that are produced in the Siemens process. Silicon bodies 24 may also be obtained through other processes, such as the UMG (upgraded metallurgical silicon) process.

Figure 3 shows a crucible 10 in which the silicon bodies 24 comprise both rods of silicon and silicon chunks. The silicon bodies 24 and granular silicon 22 are placed in the crucible in the same manner as described with reference to Figures 2A to 2H. In the embodiment of Figure 3, the rods of silicon are disposed so as to form two a two-dimensional dense hexagonal packing arrangement. Silicon chunks are used to fill gaps between the silicon rods in the central region, but are not placed in the peripheral region. The peripheral regions contain only granular silicon 22, which is also disposed in the gaps between the silicon rods and silicon chunks in the central region.

Variations and modifications to the above embodiments are possible without departing from the scope of the present invention. For example, the exact constitution of the silicon bodies 24, as can the height h of the silicon feedstock layers or the width w of the peripheral region. Moreover, although the crucible 10 described above is designed for use in a directional solidification crystallisation process, the present invention may also find utility in other crystallisation processes, such as in the Czochralski method.

Other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known and which may be used instead of, or in addition to, features described herein. Features that are described in the context of separate embodiments may be provided in combination in a single embodiment. Conversely, features which are described in the context of a single embodiment may also be provided separately or in any suitable sub-combination.

Claims

1. A method for loading a crucible with silicon for use in a crystallisation process, comprising the steps of:

2) providing silicon of a second form factor in the central region and the peripheral region, thereby creating a silicon feedstock layer across the crucible comprising silicon of both the first and second form factors,

wherein the silicon of the first form factor is larger than the silicon of the second form factor.

2. A method according to claim 1 , wherein the silicon of the second form factor is granular silicon.

3. A method according to claim 2, wherein the granular silicon is spherical.

4. A method according to claim 3, wherein the granular silicon has a diameter of less than 5mm.

5. A method according to any one of claims 2 to 4, wherein the granular silicon is the product of a fluidised bed reactor process.

6. A method according to any one of the preceding claims, wherein the silicon of the first form factor comprises silicon chunks.

7. A method according to any one of the preceding claims, wherein the silicon of the first form factor comprises silicon rods.

8. A method according to any one of the preceding claims, wherein the silicon of the first form factor is the product of a Siemens process.

9. A method according to any one of the preceding claims, wherein the silicon of the first form factor is recycled from a silicon ingot production process

10. A method according to any one of the preceding claims, wherein the silicon of the first form factor is a UMG solar silicon feedstock

1 1. A method according to any one of the preceding claims, wherein the silicon of the first form factor is a mixture of two or more of silicon rods, silicon chunks, recycled silicon or UMG silicon

12. A method according to any one of the preceding claims, further comprising creating one or more further silicon feedstock layers across the crucible by repeating steps 1 ) and 2).

13. A method according to any one of the preceding claims, wherein the peripheral region has a width at least equal to the expected shrinkage of the crucible during the crystallisation process.

14. A method according to any one of the preceding claims, wherein the peripheral region has a width of at least 0.5% of the width of the crucible.

15. A method according to any one of the preceding claims, wherein the peripheral region has a width of at least 5mm.

16. A method according to any one of the preceding claims, wherein the peripheral region has a width of less than 15mm.

17. A method according to any one of the preceding claims, further comprising, prior to step 1 ), forming a protective layer of silicon across the crucible.

18. A loaded crucible for use in a silicon crystallisation process, the crucible comprising: one or more silicon feedstock layers, each silicon feedstock layer comprising a central region separated from a wall of the crucible by a peripheral region,

wherein silicon of a first form factor is disposed only in the central region and silicon of a second form factor is disposed in central region and in the peripheral region, and