TWI844545B - Device for producing molten silicon - Google Patents

Abstract

The invention relates to a device (10) for producing molten silicon (11), comprising an enclosure (12) and comprising in the enclosure: a crucible (15) intended to receive oxidised silicon particle powder (19), the crucible comprising an inner volume (16) intended to contain silicon in the molten state and silica and a channel for evacuating (28) silicon in the molten state outside of the inner volume, the crucible comprising at least two holes (32), the straight section of each hole having a maximum dimension greater than or equal to a value of between 1mm and 10mm, or at least one slot of which the straight section has a maximum dimension greater than or equal to a value of between 1mm and 10mm connecting the inner volume to the crucible; and a heating system (20) surrounding at least partially the crucible.

Description

Molten Silicon Device

The invention relates to a device for producing molten silicon and a facility for purifying silicon comprising the device, in particular for producing an electric energy generating cell by means of a photovoltaic effect.

At present, most of the silicon used for photovoltaic applications is produced by chemical processes similar to those used to produce silicon for electronic applications. These chemical methods are well understood, but require very large investments and lead to increased production costs. The pressure on the cost of photovoltaic energy has led to a search for alternative purification methods to the chemical ones. Such methods consist in producing molten silicon from a silicon particle powder that can correspond to waste from sawmilling of silicon blocks in the microelectronics or photovoltaic industry, from processes for grinding silicon or from processes for producing polycrystalline silicon by means of fluid beds, which molten silicon can then be recrystallized to form silicon blocks.

However, it is difficult to make such a melt of silicon granule powder in a conventional melting device comprising a crucible in which the powder can be introduced, melted and then recrystallized. Indeed, silicon granules can contain a high oxygen content, between 1% and 5% by mass, due to the silica layer that naturally forms on the surface of the granules. Indeed, at the melting temperature of silicon, silica does not melt, tending to become a thicker paste and forming a sponge-like structure. Moreover, the filling rate of the crucible with silica granule powder is low, making the ingot produced after the recrystallization of the molten silicon brittle.

It is therefore an object of embodiments to provide an apparatus for producing molten silicon from silicon oxide particle powder that overcomes at least some of the disadvantages of the above-mentioned apparatuses.

According to another object of the embodiment, the apparatus for producing molten silicon makes it possible to separate silicon dioxide from silicon of silicon oxide particle powder.

According to another object of the embodiment, the apparatus for producing molten silicon makes it possible to produce molten silicon continuously or semi-continuously.

According to another object, an apparatus for producing molten silicon is productive and suitable for operation on an industrial scale.

Thus, embodiments provide an apparatus for producing molten silicon, comprising a housing and comprising in the housing: a crucible for receiving silicon oxide granular powder, the crucible comprising a content volume for containing silicon and silicon dioxide in a molten state and an exhaust channel for molten silicon outside the content volume, the crucible comprising at least two holes or at least one slot connecting the content volume to the crucible, the straight section of each hole having a maximum dimension greater than or equal to a value between 1 mm and 10 mm, the straight section of the slot having a maximum dimension greater than or equal to a value between 1 mm and 10 mm; and a heating system at least partially surrounding the crucible.

According to an embodiment, one of the holes is located above the other hole.

According to an embodiment, the minimum dimension of the straight section of each hole or slot varies from 0.5 mm to 5 mm.

According to an embodiment, the minimum and maximum dimensions of the straight section of the exhaust channel vary from 1 mm to 50 mm.

According to an embodiment, the crucible includes a bottom and a side wall and holes or slots opened in the side wall.

According to an embodiment, the minimum distance between the holes closest to the bottom or between the slot and the bottom is greater than 10% of the height of the crucible.

Embodiments provide facilities including: An apparatus for producing molten silicon as described above; A system for providing the apparatus to a crucible, the apparatus being used to produce molten silicon in a silicon oxide particle powder; and A system for solidifying the molten silicon provided by the apparatus, the apparatus being used to produce molten silicon in a silicon block.

According to an embodiment, the solidification system includes an additional crucible that receives molten silicon provided by means of a device for generating molten silicon and heating elements of the additional crucible.

The same elements have been designated by the same references in the different figures. For reasons of clarity, only the elements of the apparatus for producing molten silicon that are necessary for an understanding of the described embodiments have been represented and described in detail in the different figures. In the following description, the modifiers "lower", "higher", "towards the top" and "towards the bottom" are used relative to the axis D, which is considered to be vertical. However, it is clear that the axis D can be slightly tilted with respect to the vertical (for example, an angle less than or equal to 20°). Further, the expressions "substantially", "approximately", "approximately" and "about" mean "almost 10%", preferably "almost 5%". In addition, when the expressions "substantially", "approximately", "approximately" and "approximately" are used for angles, they mean "almost 10°", preferably "almost 5°".

In the present invention, at least one vertical slot and/or at least two holes (one above the other) have at least the function of allowing the continuous passage of molten silicon through the drain channel. In particular, if silicon dioxide blocks the passage of the drain channel (for example in the lower part of the crucible near the bottom of the crucible), the higher holes or the higher part of the slot allow maintaining unobstructed passage of the drain channel.

The present invention is therefore not limited to slots or holes that are precisely aligned along the vertical D axis. A vertical slot is defined as a slot that is primarily vertically oriented. However, it may be tilted at an angle of less than 30°, preferably less than 20°, more preferably less than 10° with respect to the vertical D axis.

The holes located one above the other do not need to be aligned along the vertical D axis. They can, for example, be arranged in an alternating manner.

The term "particles" as used in the context of this application must be understood in a broad sense and corresponds not only to small particles that are somewhat spherical, but also to angular particles, flat particles, particles in the form of flakes, particles in the form of fibers, or fiber-like particles, etc. It is understood that the "size" of a particle within the context of this application means the smallest cross-sectional dimension of the particle. For example, in the case of particles in the form of fibers, the size of the particle corresponds to the diameter of the fiber.

By the expression "average size" of the particles, this means according to the present application the size which is larger than 50% of the particles by volume and the size which is smaller than 50% of the particles by volume. This corresponds to d50. The particle size analysis of the particles can be measured by laser particle size analysis using, for example, a Malvern Mastersizer 2000.

An example of an apparatus for producing molten silicon will now be described, which is particularly used to obtain silicon blocks with sufficient purity for direct use in the production of photovoltaic products. However, molten silicon can also be used to obtain silicon blocks that have a purity (lower than the level required for direct use in the production of photovoltaic products) and are then processed to have sufficient purity to produce photovoltaic products.

FIG. 1 shows a first embodiment of an apparatus 10 for producing molten silicon 11.

The device 10 comprises a gas-tight housing 12, which is formed by a gas-tight wall 13, which isolates the housing 12 from the outside. At least one opening (not shown) is provided through the wall 13 and makes it possible to communicate the content volume of the housing 12 with the outside. The device 10 can include a system for providing (not shown) a neutral gas or a mixture of neutral gases (e.g. argon or helium) in the housing 12.

The device 10 includes a furnace 14 for melting silicon, which is placed in an outer shell 12. The furnace 14 includes a crucible 15, which defines an internal volume 16. The crucible 15 includes a bottom 17 and side walls 18. According to an embodiment, the crucible 15 is made of a material that is a good thermal conductor. A good thermal conductor is a material with a thermal conductivity greater than or equal to 5 W/(m*K). For example, the crucible 15 is made of graphite. According to an embodiment, the crucible 15 is further made of a material that is a good electrical conductor. A good electrical conductor is a material with an electrical conductivity greater than or equal to 1000 S/m. According to another embodiment, the crucible 15 is made of a material that is not a good thermal conductor, or even a good thermal insulator. A good thermal insulator is a material with a thermal conductivity lower than or equal to 5 W/(m*K). For example, the crucible 15 is made of silicon oxide, silicon nitride or silicon carbide.

The crucible 15 comprises, for example, a circular base with axis D, whose outer diameter can vary from 100 mm to 800 mm. The crucible 15 has, for example, a height that varies from 100 mm to 800 mm. The crucible 15 rests on a support (not shown). The support can be made of refractory material, for example of refractory concrete associated with a stack of materials that ensures good thermal insulation of the bottom of the crucible 15. When in operation, a silicon oxide granular powder 19 is injected into the crucible 15.

The device 10 further comprises a system 20 for heating the silicon present in the crucible 15. According to an embodiment, the heating system 20 is an induction heating system. The heating system 20 comprises, for example, a coil 22 surrounding the crucible 15. The coil 22 can be hollow and comprise an inner opening 24 for cooling the coil 22 by the circulation of a cooling liquid. The crucible 15 can thus be surrounded by a thermally and electrically isolating wall 26, for example made of flexible or rigid graphite felt. In particular, an isolating cover (not shown) can cover the crucible 15, the cover comprising an opening for introducing silicon oxide granular powder 19 into the crucible 15. The isolation support, wall 26, and lid facilitate maintaining a uniform temperature and reducing heat losses in the crucible 15. Maintaining the liquid silicon at a desired temperature in the crucible 15 is achieved by generating an electric current in the crucible 15 (when it is made of an electrically conductive material), and/or in the silicon, which is induced by the coil 22. In a variation, heating of the crucible 15 and the silicon contained in the crucible 15 can be achieved by an electric heating system that includes a resistor that is positioned around the crucible 15 and is thermally isolated from the housing 12 by a thermal isolation element.

Crucible 15 comprises a channel 28 for evacuating molten silicon 11 present in the content volume 16 of crucible 15. Preferably, the evacuation channel 28 extends through the bottom 17 and/or into the side wall 18 of the crucible 15 and is opened by an orifice 30 of a nose 31, which is arranged on the lower face of the crucible 15. The evacuation channel 28 can be substantially vertical. The evacuation channel 28 can have a circular, square, or rectangular straight section. The minimum dimension of the straight section of the evacuation channel 28 varies from 1 mm to 30 mm. The maximum dimension of the straight section of the evacuation channel 28 varies from 10 mm to 50 mm. For example, the evacuation channel 28 can have a circular straight section having a diameter greater than 5 mm, preferably varying from 10 mm to 15 mm. The crucible 15 comprises at least two holes 32 (which open on the side wall 18 of the crucible 15), preferably at least three holes, more preferably at least four holes, each hole connecting the content volume 16 of the crucible 15 to the emptying channel 28. The holes 32 can be arranged along a substantially vertical column. Each hole 32 can have a circular, square, or rectangular straight section. The maximum dimension of the straight section of each hole 32 varies from 5 mm to 15 mm and the minimum dimension of the straight section of each hole 32 varies from 0.5 mm to 5 mm. According to an embodiment, one of the holes 32 opens on the side wall 18 and is substantially tangential to the bottom 17. In the case where the emptying channel 28 and the holes 32 have a circular straight section, the diameter of each hole 32 is preferably smaller than or equal to the diameter of the emptying channel 28. According to another embodiment, the holes 32 are replaced by vertical or inclined slots. The straight section of the slot has a maximum dimension varying from 5 mm to 15 mm and a minimum dimension varying from 0.5 mm to 5 mm. According to another embodiment, the holes 32 or at least some of the holes 32 are located on the bottom 17 of the crucible 15.

The faces of the crucible 15 which define the nose 31 only form an angle between them which is less than 120°. The device 10 can comprise elements for heating the nose 31 (not represented).

The operation of the apparatus 10 is as follows. An inert atmosphere is maintained in the housing 12. According to an embodiment, the pressure in the housing 12 is between 0.1 atm (10132.5 Pa) and 1 atm (101325 Pa). Preferably, the pressure in the housing is substantially equal to the atmospheric pressure. A powder 19 of silicon oxide particles is introduced into the crucible 15 by means not shown in FIGS. 1 and 2. According to an embodiment, the average size of the silicon oxide particles is less than 300 μm, preferably between 100 nm and 100 μm. The provision of the silicon oxide particle powder 19 to the crucible 15 can be carried out according to a continuous flow or in successive batches separated by periods in which no powder is provided. For a fluid supplying the silicon oxide particle powder 19 at less than 5 kg/h, discontinuous supply of the increased fluid makes it possible to optimally distribute the silicon oxide particle powder 19 on the bottom 17 of the crucible 15.

Inside the crucible 15, the silicon oxide granular powder 19 and the molten silicon 11 are heated directly by a system 20 for heating and/or irradiating the walls of the crucible 15 and/or by controlling the walls and the bottom of the crucible 15 above the melting temperature of silicon of 1400° C., preferably above 1600° C. The silicon present in the molten powder separates from the silicon dioxide, which forms an agglomerate 33 in the crucible 15. The molten silicon 11 flows through the holes 32 and the channel 28, for example in the form of molten silicon droplets 34, through the orifice 30. Advantageously, the straight section of the emptying channel 28 is dimensioned sufficiently to make it possible to reduce the pressure required to push the gas bubbles into the emptying channel 28. The fact that the faces of the crucible 15 defining the nose 31 only form an angle of less than 120° therebetween makes it possible that the silicon does not flow laterally to the partition wall 26, but falls off in the form of droplets 34. According to an embodiment, the distance between the droplets 34 escaping through the orifice 30 and the adjacent partition wall 26 is greater than 5 mm, in order to avoid allowing the silicon to penetrate the partition wall 26.

The silicon dioxide remains in the crucible 15 and is accumulated. The holes 32 are distributed between the bottom 17 and the top of the crucible 15 so that when the holes 32 are filled with silicon dioxide, the molten silicon 11 can flow through the next hole 32. The fact that the diameter of the hole 32 is smaller than or equal to the diameter of the drain channel 28 makes it possible to avoid silicon dioxide flowing through the hole 32 in the drain channel 28 and not causing a blockage in the drain channel 28.

The apparatus 10 can include a system for stirring the molten silicon 11 in the crucible 15. The stirring can be performed at least in part by induction when the heating system 20 is inductively heated. The frequency of the current supplied to the induction coil 22 can therefore be adjusted to facilitate stirring of the molten silicon in the crucible 15.

According to an embodiment, in the case where the crucible 15 is made of graphite, an electronic grade silicon coating can be placed in the crucible 15, so that after melting of this silicon coating and before the silicon oxide granular powder 19 is introduced into the crucible 15, a carbon silicide layer can be formed on the inner wall of the crucible 15. This makes it possible to avoid the graphite of the crucible 15 from reacting with the silicon oxide granular powder 19 melted in the crucible 15. The formation of the carbon silicide layer can be accelerated by temporarily reducing the pressure in the housing 12 to below 10 mbar, preferably below 1 mbar. According to another embodiment, the carbon silicide coating is formed on the inner wall of the crucible 15, which is made of graphite by another method such as chemical vapor deposition (CVD).

When the production of molten silicon is finished, after cooling, the silicon dioxide can be removed by mechanical means. A coating can be placed in the crucible 15 to prevent the silicon dioxide from sticking to the crucible 15, the coating being composed of at least one material such as graphite, carbon silicide, silicon oxide and silicon nitride.

FIG. 2 is a cross-sectional view of another embodiment of an apparatus 35 for producing molten silicon 11. Apparatus 35 includes all of the elements of apparatus 10 shown in FIG. 1, with the difference that the hole 32 in side wall 18 closest to the opening of bottom 17 is located at a distance from bottom 17 that is greater than 10% of the height of the crucible. For example, hole 32 is located in the upper half of crucible 15.

During operation, some of the molten silicon 11 remains in the crucible 15, so that the silicon dioxide aggregate 33 floats on the molten silicon 11 and is not in contact with the crucible 15. The silicon dioxide can thus be removed by mechanical means while the crucible 15 is above the melting temperature of silicon. The method for producing molten silicon can be performed continuously without cooling the crucible 15. In the absence of stirring of the molten silicon in the crucible 15, the height of the bath of molten silicon 11 present in the crucible 15 can be about 10 mm. In the case where stirring of the molten silicon in the crucible 15 is performed, the height of the bath of molten silicon present in the crucible 15 can be approximately the diameter of the crucible 15, for example, approximately 200 mm.

FIG. 3 shows an embodiment of a facility 40 for producing silicon ingots or blocks. The facility 40 comprises the device 10 or 35 for producing molten silicon shown in FIGS. 1 and 2 and further comprises a housing 42 formed by a gas-tight wall 44 which isolates the housing 42 from the outside. Openings (not shown) can be provided through the wall 44 of the housing 42 and allow the housing 42 to communicate with the outside. Openings 46 can be provided through the wall 13 of the housing 12 and allow the internal volume of the housing 12 to communicate with the internal volume of the housing 42. The device 40 includes a damper 48 sealed at the height of the opening 46 to seal the internal volume 16 of the housing 12 from the internal volume of the housing 42. The damper 48 is, for example, in the form of a casement or a slide, actuated by a mechanism (not shown).

The facility 40 can include a system 50 for providing silicon oxide granule powder to the melting furnace 14. The supply system 50 can include an airtight tank 52 and an airtight system 54 for providing the silicon oxide granules provided by the tank 52, which includes, for example, a vibrating feeder or a worm screw rotating feeder.

The facility 40 further includes a system 56 in the housing 42 for solidifying molten silicon provided by the melting furnace 14. The system 56 can include a crucible 58, wherein the molten silicon is solidified to obtain silicon blocks. The system 56 can further include a heating element 60, which is disposed at the top of the crucible 58. The heating element 60 can include a resistor. The system 56 can further include a cooling and/or heating element 62 disposed below the crucible 58 to obtain solidification of silicon in the crucible 58 directed from the bottom to the top. The cooling element 62 can include a pipe in which a cooling liquid circulates. A thermal isolation wall 64 can be disposed around the crucible 58, the heating element 60, and the heating/cooling element 62. In particular, a cover 66 made of a thermally insulating material (e.g., graphite or silicide) having holes 68 can be placed on the crucible 58 to prevent the spattering of molten silicon from contacting the heating element 60.

The installation 40 can include at least one vacuum pump (not shown) connected to each housing 12 and 42. The pump is adapted to establish a controlled atmosphere inside the housing 12 or 42. According to another embodiment, the installation 40 can include, for each housing 12 and 42, a vacuum pump (not shown) connected to the housing 12 or 42, and means for injecting one or more inert gases into each housing 12 or 42, in order to maintain a controlled atmosphere (possibly different) inside the housing 12 and 42.

In operation, the crucible 58 is filled with molten silicon produced by the device 10 for producing molten silicon, and a controlled solidification of the molten silicon present in the crucible 58 is carried out, for example, there is a solidification front of silicon that progresses from the bottom to the top. The heating element 60 provided at the top of the crucible 58 can be used to advantageously heat the nose 31 of the crucible 58.

Various embodiments with various variations have been described above. It will be noted that a person skilled in the art can combine various elements of these various embodiments and variations without demonstrating any advancement. Specific embodiments of the invention have been described. Various variations and modifications will be apparent to a person skilled in the art. In particular, although FIG. 3 shows an embodiment of a system 56 in which molten silicon is provided for guided solidification of silicon, the molten silicon can provide a crucible for subsequent processing (wherein the silicon is solidified without guided solidification), for example, purification, which is performed on a block of silicon.

10, 35: device 11: molten silicon 12, 42: airtight shell 13: airtight wall 14: furnace 15, 58: crucible 16: content volume 17: bottom 18: side wall 19: silicon oxide granular powder 20: heating system 22: coil 24: inner opening 26: isolation wall 28: channel 30: orifice 31: nose 32, 68: hole 33: accumulation 34: droplet 40: facility 44: airtight wall 46: opening 48: damper 50: supply system 52: supply slot 56: system 60, 62: heating element 64: thermal isolation wall 66: cover

These features and advantages and others are described in detail in the following description of specific embodiments made in a non-limiting manner in relation to the accompanying drawings, in which: Figures 1 and 2 are cross-sectional, partial and schematic views of embodiments of an apparatus for producing molten silicon; and Figure 3 is a cross-sectional, partial and schematic view of an embodiment of an apparatus for producing silicon ingots.

10: Device

11: Molten Silicon

12: Airtight outer shell

13: Airtight wall

14: Furnace

15: Crucible

16: Content accumulation

17: Bottom

18: Side wall

19: Silicon oxide granular powder

20: Heating system

22: Coil

24: Inner opening

26: Isolation wall

28: Channel

30: Orifice

31: Nose

32: Holes

33: Accumulation

34: Droplets

Claims (7)

A device for producing silicon in a molten state, comprising a housing and in the housing: a crucible, the crucible being used to receive silicon oxide granular powder, the crucible comprising a content volume and a channel for evacuation, the content volume being used to contain the silicon in the molten state and silicon dioxide, the channel being used to evacuate the silicon in the molten state out of the content volume, the crucible comprising: - at least two holes connecting the content volume to the evacuation channel in a side wall of the crucible, each hole having a straight section having a maximum dimension greater than or equal to a value between 1 mm and 10 mm, one of the holes being located above the other hole, or - At least one vertical slot, a straight section of the at least one vertical slot having a maximum dimension greater than or equal to a value between 1 mm and 10 mm; and a heating system at least partially surrounding the crucible. A device as described in claim 1, wherein a minimum dimension of the straight section of each hole or the slot varies from 0.5 mm to 5 mm. A device as described in any one of items 1 to 2 of the patent application scope, wherein the drain channel (28) has a straight section with a minimum dimension and a maximum dimension, and the minimum dimension and the maximum dimension of the straight section of the drain channel (28) vary from 1 mm to 50 mm. A device as described in any one of items 1 to 2 of the patent application, wherein the crucible includes a bottom and wherein the hole or the slot opens to the side wall. The device as described in claim 4, wherein a minimum distance between the holes closest to the bottom or between the slot and the bottom is greater than 10% of a height of the crucible. A device (10; 35) for producing molten silicon (11) as described in any one of items 1 to 5 of the patent application scope, the device also includes: A system for providing the crucible of the device for producing molten silicon in silicon oxide particle powder; and a system for solidifying the molten silicon provided by the device for producing molten silicon in a silicon block. The apparatus as described in claim 6, wherein the solidification system includes an additional crucible and an element for heating the additional crucible, the additional crucible receiving the molten silicon provided by the device for producing molten silicon.