Crucible materials for silicon crystal growth

Abstract

The present invention relates to a crucible (1) for containing molten semiconductor material (5) having an inner surface layer (2) arranged to contact the molten semiconductor material (5), wherein the inner surface layer (2) comprises at least 95 % SrF2 by weight of the layer, such as at least 99 % SrF2 by weight of the layer. The present invention further relates to its method for manufacturing, its use, and a crystallization process for a semiconductor material, such as silicon.

Description

[0001] CRUCIBLE MATERIALS FOR SILICON CRYSTAL GROWTH

[0002] Technical field

[0003] The present invention relates to crucibles for containing molten semiconductor material, their manufacture, and use in crystallization of semiconductor materials.

[0004] Background

[0005] With increasing focus on climate change issues, and humanity having an increasing energy demand worldwide, sustainable energy supplies are required. The solar cell market is expanding fast. Solar cells may be manufactured using photovoltaic silicon crystals. To obtain the photovoltaic silicon crystals for solar cells, silicon ingots are cast or Czochralski-grown in crucibles made from high-purity quartz, i.e., high-purity silica (SiCh). Quartz crucibles are made by fusing together high-purity quartz sand. Crucibles used for silicon crystal growth are expensive, but they still need to be discarded after having been used just a few times, due to cracks and impurities. Quartz also undergoes crystallographic changes, from trigonal to hexagonal to cubic, upon heating and cooling. These crystallographic changes may provide microcracks in crucibles, especially upon being repeatedly subjected to high- temperature processes, such as silicon crystal growth. This is a reason for quartz crucibles not being reusable more than 4-5 times before risking cracking during silicon crystal growth of e.g., solar cell materials. The crucible, upon risk of cracking, then becomes a discarded material.

[0006] Today, crucibles for use in the production of photovoltaic silicon crystals are exclusively made from a high-purity silica (SiO2). The high purity silica used today is coming from one specific mine. The mined silica, which is to be converted to crucibles, is then exported and processed into high-purity crucibles, e.g., large high-purity crucibles, such as having a diameter of about 1 m. The world’s increasing demand for solar cells to strengthen energy delivery, thus, faces problematic supply chain risks. To date, there are, unfortunately, no other known or commercially available materials that can survive contact with molten silicon for long periods of time, and be reusable.

[0007] Thus, there is a need to find new solutions to be able to produce crucibles which are able to withstand high processing temperatures, prolonged process cycles, and be reusable multiple times in processing of high-temperature demanding compositions or compounds. Specifically, there is a need to find new solutions to be able to produce crucibles which are able to process silicon containing materials into photovoltaic silicon crystals usable for solar cell production.

[0008] Summary

[0009] The present invention relates to provision of ways to obtain crucibles which are able to process semiconductor materials to crystal structures, such as silicon containing materials into silicon crystals usable for solar cells. The present invention provides a way to avoid unwanted changes in crystal structure, which may cause micro cracks, and thus increase the lifetime of produced crucibles due to improved reusability. The scope of protection of the present invention is defined by the enclosed set of claims.

[0010] In one aspect the present invention relates to a crucible for containing molten semiconductor material having an inner surface layer arranged to contact the molten semiconductor material, wherein the inner surface layer comprises strontium fluoride, SrF2.

[0011] According to one embodiment, the inner surface layer comprises at least 95 % SrF2 by weight of the layer, such as at least 99 % SrF2 by weight of the layer.

[0012] According to one embodiment, the inner surface layer further comprises impurities at an amount of less than 5 % by weight of the inner surface layer, such as by less than 1 % by weight of the inner surface material.

[0013] According to one embodiment, the impurities are selected from the list of barium, calcium, magnesium, sodium, and the rare earth elements. According to one embodiment, the inner surface layer further comprises silicon dioxide, SiCh.

[0014] According to one embodiment, the inner surface layer is surface textured.

[0015] According to one embodiment, the crucible further comprises a support body adjacent to the inner surface layer.

[0016] According to one embodiment, the inner surface layer is arranged as an insert adapted to fit inside an opening in the support body.

[0017] According to one embodiment, the inner surface layer is arranged as a lining on the support body.

[0018] According to one embodiment, the support body comprises graphite or SiO2.

[0019] According to one embodiment, the crucible is provided with a measuring device configured to measure a temperature of the molten semiconductor material.

[0020] According to one embodiment, the inner surface layer defines an open chamber adapted to contain a molten semiconductor material, wherein an opening of the chamber has a diameter in the range of from 1 to 150 cm.

[0021] According to one embodiment, the open chamber has a height in the range of from 1-100 cm.

[0022] In one aspect the present invention relates to a crystallization process for a semiconductor material, comprising

[0023] - providing a molten semiconductor material in a crucible as defined in any one of the preceding claims,

[0024] - growing a crystalline material from said molten semiconductor material.

[0025] According to one embodiment, the crystalline material is a monocrystalline material.

[0026] According to one embodiment, the growing involves Czochralski crystal growth. According to one embodiment, the crystal or crystalline material is a polycrystalline material.

[0027] According to one embodiment, the molten semiconductor material is silicon, Si.

[0028] In one aspect the present invention relates to use of a crucible having an inner surface layer comprising strontium fluoride for containing a molten semiconductor material.

[0029] In one aspect the present invention relates to a method for manufacturing a crucible for containing molten semiconductor material in a crystallization process comprising

[0030] - providing a SrF2 powder,

[0031] - compacting the SrF2 powder in a die to form a crucible-shaped body such that the SrF2 powder is provided at an inner surface of the crucible-shaped body,

[0032] - sintering the crucible-shaped body to form a crucible having an inner surface layer comprising SrF2.

[0033] According to one embodiment, providing a SrF2 powder comprises synthesizing the SrF2 powder from a strontium containing ore.

[0034] According to one embodiment, the sintering is performed in air.

[0035] According to one embodiment, the sintering is performed at a temperature in the range of 600-1000°C.

[0036] According to one embodiment, the sintering is performed for a period in the range of 2-8 hours.

[0037] Brief description of the drawings

[0038] Figure 1 is showing a schematic view of a SrF2 layered crucible.

[0039] Figure 2 is showing a schematic view of a SrF2 layered crucible used in a process.

[0040] Figure 3 is showing a SrF2 containing solid pellet after sintering.

[0041] Figure 4 is showing a cross sectioned SrF2 layered graphite crucible with formed silicon button. Figure 5 is showing a clean, shiny silicon button comprising crystalline silicon.

[0042] Figure 6 is showing a cross section of the silicon button in Fig 5.

[0043] Detailed description

[0044] Many industrial processes require high temperature resistant containers that are chemically inert and resistant to thermal shock.

[0045] A crucible is a type of container that is used for high-temperature applications, such as melting, calcination, or combustion of materials. It may be made of pure silica, which is a heat-resistant and chemically inert material that can withstand high temperatures without deforming or reacting with the materials being heated. However, a problem with silica crucibles remains with content of impurities, and crystal changes therein, which may form cracks upon subjection to high temperatures, such as above 1200°C (e.g., upon thermal cycling up to 1500°C), and repeated use.

[0046] The present invention relates to a crucible having an inner surface layer comprising strontium fluoride, SrF2. The crucible may be suitable for containing molten semiconductor material. The inner surface layer may be arranged to be in contact with the molten semiconductor material, when in use, e.g., during a crystallization process.

[0047] The inner surface layer may comprise at least 95 % SrF2 by weight of said layer, such as at least 97 %, at least 98 %, at least 99 %, at least 99. 5 %, or at least 99.9 % SrF2 by weight of said layer. High amount of SrF2 is beneficial.

[0048] The inner surface layer further comprises impurities in an amount of less than 5 % by weight of the inner surface layer, such as less than 4 %, less than 3 %, less than 2 %, less than 1 %, less than 0.5 %, or less than 0.1 % by weight of the inner surface material. Low amount of impurities is preferred. The impurities may be selected from the list of barium, calcium, magnesium, sodium, the rare earth elements, or any combination thereof. The rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The impurities may be selected from group of barium, calcium, magnesium, sodium, scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or any combination thereof, e.g., selected from the group of barium, calcium, magnesium, sodium, lanthanum, cerium, praseodymium, neodymium, or any combination thereof. The impurities may be selected from the group of barium, calcium, magnesium, sodium, lanthanum, cerium, or any combination thereof.

[0049] The inner surface layer may further comprise silicon dioxide, SiCh. If the inner surface layer comprises silicon dioxide it may originate from a high- purity silica powder. With high-purity silica powder is meant a powder having a SiO2 content of at least 99.9 %, e.g., at least 99.97 %, >99.997 %, >99.999 % by weight. The inner surface layer may comprise silicon dioxide in an amount of at most 5 % by weight of the inner surface material, such as at most 4 %, at most 3 %, at most 2 %, at most 1 %, or at most 0.5 %, by weight of the inner surface material.

[0050] The inner surface layer may be surface textured. By including a surface textured inner layer of the crucible this may aid in lowering the wettability between molten semiconductor material, such as molten silicon, and the SrF2 containing inner layer of the crucible.

[0051] The present crucible has an inner surface layer comprising strontium fluoride, SrF2. The crucible may further comprise a support body. The present crucible may be made of the inner surface layer, i.e. , is fully made of the material of the inner surface layer, or the crucible may be made of the inner surface layer and a support body. The crucible may comprise the support body adjacent to the inner surface layer. The inner surface layer may be provided or contained in the support body. The inner layer may be enclosed by the support body, apart for an opening intended to receive semiconductor feedstock material to be processed and for removal of the treated semiconductor material after processing. The use of a support body may e.g., be done for safety reasons, in case of a crucible rupture. The inner surface layer may be arranged as an insert adapted to fit inside an opening in the support body. The inner surface layer may be arranged as a lining on the support body. If the crucible comprises a support body, the inner surface layer may be provided in an interior cavity of the support body, and having an opening in one end for provision of semiconductor material to be processed in the crucible. The inner surface layer may define an open chamber adapted to contain a molten semiconductor material. The opening of the chamber or opening of the interior cavity may have a diameter in the range of from 1 to 150 cm, such as 5-150 cm, or 10-150 cm.

[0052] The open chamber or interior cavity may have a height in the range of from 1-100 cm, such as 5-100 cm, or 10-100 cm.

[0053] The present crucible may be made with or without internal tapers. Thus, the diameter of the open chamber may be the same along a partial or the height of the crucible, or may be tapered. The crucibles may be prepared in different sizes and shapes, such as cylindrical, conical, hemi-spherical, semi-spherical, hemi-ellipsoidal, semi-ellipsoidal, hexagonal, or flat-bottomed, to fit different requirements.

[0054] The support body may comprise graphite, SiC>2, alumina, zirconia, boron nitride, ceramic, or any combination thereof. The support body may comprise graphite or SiCh. The support body may essentially be made of graphite or SiCh.

[0055] The crucible may be provided with a measuring device configured to measure a temperature of the molten semiconductor material. Thermocouples may be used as measuring device. Embedding measuring devices, such as thermocouples, into the crucible is done to more accurately record the temperature during the crystal growth process. The measuring devise may be incorporated into the inner surface layer or the support body of a crucible, preferably into the inner surface layer. The crucibles obtainable herein may e.g., be reused in semiconductor crystallization, such as silicon crystallization, at least 10 times, such as at least 12 or 15 times.

[0056] The present crucibles may also be recycled upon end of life, which then are pulverized and recycled back into new crucibles.

[0057] Herein is also provided use of a crucible having an inner surface layer comprising strontium fluoride for containing a molten semiconductor material. The crucible used is preferably the present crucible presented herein.

[0058] Herein is also provided disclosure of a method for manufacturing a crucible for containing molten semiconductor material in a crystallization process, said method comprising the steps of:

[0059] - providing a SrF2 powder,

[0060] - compacting the SrF2 powder in a die to form a crucible-shaped body such that the SrF2 powder is provided at an inner surface of the crucibleshaped body,

[0061] - sintering the crucible-shaped body to form a crucible having an inner surface layer comprising SrF2.

[0062] After compacting or sintering, the compacted or sintered SrF2 containing crucible-shaped body may be provided with a support body. When compacting the SrF2 powder in a die to form a crucible-shaped body, it may be shaped to be adapted to be introduced into and fit a support body. It may be introduced into the support body before or after sintering or the SrF2 containing crucible-shaped body. The forming of the crucible-shaped body may after sintering require additional working, to become the crucible adapted to receive a feed of semiconductor material to be used and processed therein. For example, a cavity may need to be provided as an open chamber adapted to receive any semiconductor feed material for use in a crystallization process.

[0063] The step of providing a SrF2 powder may comprise synthesizing the SrF2 powder from a strontium containing ore. SrF2 is rare in nature, therefore it needs to be synthesized from available strontium ores. SrF2 may thus made from strontianite or celestine mineral reacted with nitric acid and then doubledisplaced with sodium fluoride. The synthesis for producing SrF2 may be done according to the following reactions:

[0064] SrCO3+ 2HNO3Sr(NO3)2+ H2O + CO2

[0065] The SrF2powder provided may be a dried SrF2powder.

[0066] Sintering is often chosen as a shaping process for materials with high melting points. Sintering is a process that applies pressure and heat to fuse metal powders without melting the metal powders to the point of liquefaction. On a molecular level, powder sintering fuses the atoms in the metal across the boundaries of the particles, creating one solid piece. Herein, the wordings sintering and fusing are considered to be interchangeable.

[0067] The sintering step may be performed on the compacted SrF2containing crucible-shaped body as it is after the compacting, or the compacted SrF2containing crucible-shaped body may be provided to a support body before the sintering step. Thus, a support body may either be provided with a sintered SrF2containing crucible-shaped body, or a compacted SrF2containing crucible-shaped body, and the compacted SrF2body and support body are sintered together. It may be preferred to provide a compacted and sintered SrF2containing crucible-shaped body before providing it to a support body to obtain the crucible having an inner surface layer comprising SrF2.

[0068] The sintering step may be performed in air. The sintering may be performed at a temperature in the range of 600-1000°C, such as at about 650-950°C, or about 700-900°C. The sintering may be performed for a period in the range of 2-8 hours, e.g., for about 3-7 h, or about 4-6 h.

[0069] The after sintering, formed crucible may be further heat-treated in air. This treatment may be made to relieve any residual stresses in the material. Such a heat treatment may be made at about 180-250°C, such as at about 190-220°C.

[0070] The crucible obtained after sintering, and optional further heat treatment, may be further shaped and / or sized using finishing techniques, such as anyone of machining, lathing, burnishing, milling, and drilling. These shaping and / or sizing step may include providing a cavity or open chamber, e.g., by drilling, in the SrF2 containing part of the crucible, to allow usage within crystallization processing of semiconductor materials. Machining may also be performed in order to develop surface texturing, e.g., grooves, indentations, channels or dimples, on or of the inner surface layer, such as at the interface between crucible and semiconductor melt, when in use in a crystallization process.

[0071] Herein is also provided a crystallization process for a semiconductor material, comprising:

[0072] - providing a molten semiconductor material in a crucible as defined above,

[0073] - growing a crystalline material from said molten semiconductor material.

[0074] The crystalline material may be a monocrystalline material, or a polycrystalline material. The growing step may involve Czochralski crystal growth The Czochralski method is a crystal growth technology that starts with insertion of a small seed crystal into a melt in a crucible, pulling the seed upwards to obtain a single crystal. Czochralski crystal growth may involve a semiconductor material seed, such as a silicon seed, being dipped into a molten semiconductor material, e.g., also being Si, and pulled upwards, wherein also a rotation may be provided upon the upwards pulling. A rotation of the obtained crystal may be provided upon the upwards pulling of the crystal from the melt in the crucible. Alternatively, or simultaneously, the crucible may be rotated upon provision of the crystal, e.g., the crucible may be rotating in the opposite direction of the rotation of the crystal being formed and removed from the semiconductor melt. A solid single crystal of a semiconductor material is thus withdrawn and removed from the crucible, e.g., in the shape of a long cylinder. As a result, semiconductor material crystals, e.g., Si crystals, may actually be formed not in the melt of the semiconductor material in the crucible itself, but directly above the semiconductor material melt.

[0075] The semiconductor material being crystallized using the present crucible may be any type of semiconductor material, e.g., silicon, and / or germanium. If the semiconductor material is silicon, it may be doped with germanium, n- or p-type dopants. Dopants may be n-type, such as arsenic, phosphorus, antimony or bismuth, or p-type, such as boron, gallium, aluminium or indium. The dopants may be added in small quantities to the silicon. If silicon is used as semiconductor material, silicon crystals can be obtained, which may be non-doped or doped. The molten semiconductor material used may be silicon, Si.

[0076] The obtained silicon crystals may be used for solar cells, such as photovoltaic solar cells; laser diodes; electronic circuits; and others.

[0077] It is to be noted that the present crucibles comprising SrF2 may during use be operating at temperatures both below 1480°C. The crystalline material from the molten semiconductor material may be grown in the crucible at a temperature of at most 1480°C, such as at a temperature of at most 1470°C, or at about 1400-1470°C, or at about 1420-1460°C, or at about 1420-1430°C.

[0078] Enclosed figures are used to illustrate the present invention and are not to be interpreted as limiting the present invention.

[0079] Figure 1 shows a crucible 1 having a support body 7 and a SrF2 inner surface layer 2. The crucible 1 has an open chamber or cavity 3, which is adapted to be provided with semiconductor material 4 during use. Upon initiation of a crystallization process the provided semiconductor material may be chunks of silicon 4 to be processed.

[0080] Figure 2 shows a crucible 1 having a support body 7 and a SrF2 inner surface layer 2. The crucible 1 has an open chamber or cavity 3. During use of the crucible 1 semiconductor material is melted and subjected to crystallization. Molten semiconductor material, e.g., Si, is disclosed as 5.

[0081] Solid Si single crystal being withdrawn from the melt by Czochralski growth is a crystalline material disclosed as 6.

[0082] Example

[0083] Synthesis method to produce SrF2

[0084] SrF2 powder was synthesized in the lab, according to the synthesis reactions:

[0085] SrCO3+ 2HNO3Sr(NO3)2+ H2O + CO2 followed by washing in a Buchner filter funnel, and thereafter the powder was dried. Fine, pure, micron-sized SrF2 was thus obtained.

[0086] Sintering method to consolidate SrF2

[0087] The fine, pure, micron-sized SrF2 powder was readily sintered in air at temperatures of 800°C for 4-6 hours, leading to solid pellets with minimal porosity (see figure 3). Natural shrinkage of 25-30 vol% occurs as the pellet densifies from pressed powder to a solid object and this methodology can be used to make large, near-net shape crucibles with centimetre-thick walls. The fact that SrF2 sintering is so effective at low temperature and in air, also makes this method very advantageous from a cost perspective.

[0088] In addition, fine, pure, micron-sized SrF2 powder was compacted to a pellet, and was readily sintered in air at temperatures of 800°C for 4-6 hours. The obtained compacted and sintered SrF2 pellet was thereafter pressed into a cavity of a graphite support body. A hole was thereafter drilled into the SrF2 filled cavity of the graphite support body to provide an open chamber leading to a SrF2 layered graphite crucible. The crucible was then used for growing a crystalline silicon material, see figure 4. The crucible was provided with silicon as semiconductor material feed and was heated above the melting point of silicon to form a molten Si button. This test was performed to see if and, in such case, how the crucible would interact or react with a semiconductor material provided thereto.

[0089] Fig 4 shows a sectioned a sectioned graphite crucible after processing, where the sintered SrF2 layer is still intact, and the molten silicon button has solidified as a round button. The silicon button was crystalline, more precisely polycrystalline.

[0090] Figure 5 shows a clean, shiny crystalline silicon button after easy removal from the SrF2 layer.

[0091] Figure 6 shows the silicon button sectioned in half to reveal a clean unreacted interface. The cut button clearly shows a homogenous cross section, and no evidence of reactions or chemical pick-up of Sr or F from the crucible it was made in. The silicon button was tested using Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). There was zero concentration of strontium or fluorine detected. Thus, no transfer or pick-up of crucible materials as impurities was detected.

Claims

CLAIMS1 . A crucible (1 ) for containing molten semiconductor material (5) having an inner surface layer (2) arranged to contact the molten semiconductor material (5), wherein the inner surface layer (2) comprises at least 95 % SrF2 by weight of the layer, such as at least 99 % SrF2 by weight of the layer.

2. The crucible (1 ) according to claim 1 , wherein the inner surface layer (2) further comprises impurities at an amount of less than 5 % by weight of the inner surface layer (2), such as by less than 1 % by weight of the inner surface material.

3. The crucible (1 ) according to claim 2, wherein the impurities are selected from the list of barium, calcium, magnesium, sodium, and the rare earth elements.4 The crucible (1 ) according to any one of the preceding claims, wherein the inner surface layer (2) further comprises silicon dioxide, SiO2.

5. The crucible (1 ) according to any one of the preceding claims, wherein the inner surface layer (2) is surface textured.

6. The crucible (1 ) according to any one of the preceding claims, wherein the crucible (1 ) further comprises a support body (7) adjacent to the inner surface layer (2).

7. The crucible (1 ) according to claim 6, wherein the inner surface layer (2) is arranged as an insert adapted to fit inside an opening in the support body (7).

8. The crucible (1) according to claim 6, wherein the inner surface layer (2) is arranged as a lining on the support body (7).

9. The crucible (1 ) according to any one of claim 6 to 8, wherein the support body (7) comprises graphite or SiCh.

10. The crucible (1 ) according to any one of the preceding claims, wherein the crucible (1 ) is provided with a measuring device configured to measure a temperature of the molten semiconductor material (5).11 . The crucible (1 ) according to any one of the preceding claims, wherein the inner surface layer (2) defines an open chamber (3) adapted to contain a molten semiconductor material (5), wherein an opening of the chamber (3) has a diameter in the range of from 1 to 150 cm.

12. The crucible (1 ) according to any one of the preceding claims, wherein the open chamber (3) has a height in the range of from 1-100 cm.

13. A crystallization process for a semiconductor material, comprising- providing a molten semiconductor material (5) in a crucible (1) as defined in any one of the preceding claims,- growing a crystalline material (6) from said molten semiconductor material (5).

14. The method according to claim 13, wherein the crystalline material (6) is a monocrystalline material.

15. The method according to claim 14, wherein the growing involves Czochralski crystal growth.

16. The method according to claim 13, wherein the crystalline material (6) is a polycrystalline material.

17. The method according to claim any one of claims 13-16, wherein the molten semiconductor material (5) is silicon, Si.

18. Use of a crucible (1 ) having an inner surface layer (2) comprising at least 95 % SrF2 by weight of the layer for containing a molten semiconductor material (5).

19. A method for manufacturing a crucible (1 ) for containing molten semiconductor material (5) in a crystallization process, the method comprising- providing a SrF2 powder,- compacting the SrF2 powder in a die to form a crucible-shaped body such that the SrF2 powder is provided at an inner surface of the crucible-shaped body,- sintering the crucible-shaped body to form a crucible (1) having an inner surface layer (2) comprising at least 95 % SrF2 by weight of the layer.

20. The method according to claim 19, wherein providing a SrF2 powder comprises synthesizing the SrF2 powder from a strontium containing ore.21 . The method according to any one of claims 19 or 20, wherein the sintering is performed in air.

22. The method according to any one of claims 19 to 21 , wherein the sintering is performed at a temperature in the range of 600-1000°C.

23. The method according to any one of claims 19 to 22, wherein the sintering is performed for a period in the range of 2-8 hours.

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