JP2008019139A - Crucible protective sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crucible protective sheet which improves workability by preventing damage and inclination of an internal crucible and can prevent SiC formation and thickness reduction of an external crucible by maintaining gas shielding even if compressibility is high. <P>SOLUTION: The crucible 1 comprises an internal crucible 2, an external crucible 3, and a sheet 4 composed of exfoliated graphite which is disposed between the crucibles 2 and 3. The sheet has gas permeability of less than 1.0×10<SP>-4</SP>cm<SP>2</SP>/s and compressibility of 20% or more when compressed with pressure of 34.3 MPa in the thickness direction. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a crucible protective sheet. A crucible is used for manufacturing a silicon single crystal or the like by the CZ method. Such crucibles include an outer crucible heated by a heater or the like and an inner crucible in which raw materials such as silicon single crystals are accommodated. Usually, a quartz crucible is used for the inner crucible due to problems with silicon reactivity and purity, and a graphite crucible is used for the outer crucible due to problems with purity, heat resistance and strength. Used in the production of silicon single crystals and the like with the crucible inserted into the outer crucible.
However, the quartz inner crucible is very fragile and prone to breakage and must be handled very carefully when inserted into the outer crucible. Further, when the crucible is cooled after the production of silicon single crystal or the like is finished, there is a problem that damage such as cracking occurs in both the crucibles due to the difference in thermal expansion coefficient between the inner crucible and the outer crucible. Further, there is a problem that SiO gas or the like generated from the quartz crucible reacts with the outer crucible to generate SiC or thinning of the outer crucible.
In order to solve such a problem, a member for protecting both the crucibles is provided between the outer crucible and the inner crucible.
The present invention relates to a crucible protective sheet that is disposed between the outer crucible and the inner crucible and used to protect both crucibles.

Conventionally, various sheets have been developed to be disposed between an outer crucible and an inner crucible, and to protect the damage between the two (for example, Patent Documents 1 to 5).
Patent Documents 1 and 2 disclose a sheet or fabric in which the surface of a carbon fiber member such as a woven fabric made of carbon fiber is coated with pyrolytic carbon, and these sheets and the like are disposed between an outer crucible and an inner crucible. A technique for interposing is disclosed. These sheets and the like have a certain degree of flexibility, so that they function as cushioning materials, and because the pyrolytic carbon coated on the surface reacts with SiO gas generated from the inner crucible, etc. There is a description that the reaction of the outer crucible can be prevented.

Since the carbon fiber members of Patent Documents 1 and 2 are made of carbon fiber and have a certain degree of flexibility, they can be deformed along the outer surface of the inner crucible or the inner surface of the outer crucible. However, since the carbon fiber members of Patent Documents 1 and 2 are not so compressible, they cannot sufficiently absorb the impact when the inner crucible is inserted into the outer crucible, and are generated between both crucibles when the crucible is cooled. It is not so effective for relaxation of expansion and contraction stress.
Further, even if pyrolytic carbon coated on the surface is reacted with SiO gas or the like, not all SiO gas and the like can be reacted, so there is SiO gas or the like that passes through the carbon fiber member. And since there are many gaps between the fibers, the gas shielding property cannot be expected so much. Certainly, a certain degree of gas shielding can be provided by coating pyrolytic carbon. However, if the coating is increased in order to improve the gas shielding property, the flexibility is lost. Then, there is a high possibility of breakage during the interpolation work. Conversely, if the coating is reduced in order to maintain flexibility, a large number of gaps remain between the fibers, so that the gas shielding property cannot be sufficiently improved.

As a material superior in flexibility and compressibility than the carbon fiber member, there is an expanded graphite sheet in the form of expanded graphite, and techniques for using the expanded graphite sheet as a protective sheet for a crucible are also disclosed in Patent Documents 3 to 5. Has been.
Patent Document 3 describes that the expanded graphite sheet is a material having flexibility and a high compressibility and a high recovery rate, and is therefore useful for relieving expansion and contraction stress due to thermal shock and thermal expansion. Moreover, since the expanded graphite sheet is a highly anisotropic material having very low gas permeability, Patent Document 3 also describes that there is resistance to gas permeability.

  However, in Patent Documents 3 to 5, only the thickness and impurity concentration of the expanded graphite sheet are considered in specifying the properties of the expanded graphite sheet, and both the compressibility and gas permeability are considered. Absent.

Japanese Patent Laid-Open No. 2001-261481 JP 2002-226292 A Japanese Patent No. 2528285 JP 2003-267881 JP 2004-75521 A

  In view of the above circumstances, an object of the present invention is to provide a crucible protective sheet that can prevent damage to the inner crucible and suppress the formation of SiC in the outer crucible.

The protective sheet for crucibles of the first invention is a sheet made of expanded graphite disposed between both crucibles in a crucible having an inner crucible and an outer crucible, and has a gas permeability of 1.0 × 10 ⁻⁴ cm. ^It is smaller than ² / s, and it is characterized in that the compression rate is 20% or more when pressed and compressed with a pressing force of 34.3 MPa from the thickness direction.
The protective sheet for crucibles of the second invention is characterized in that, in the first invention, the sheet has a thickness of 0.2 mm or more and 0.6 mm or less.
The crucible protective sheet according to the third invention is characterized in that, in the first invention, when the pressure is compressed with a pressure of 34.3 MPa from the thickness direction, the restoration rate is 5% or more.
The crucible protective sheet of the fourth invention is characterized in that, in the first invention, the ash content is 10 massppm or less.
A crucible protective sheet according to a fifth aspect of the present invention is characterized in that a plurality of sheets according to claim 1, 2, 3 or 4 are laminated.

According to the first invention, since the compression rate is high, the effect of preventing breakage is enhanced when the inner crucible is inserted, and workability can be improved. Moreover, even if the bottom surface of the inner crucible is uneven, the sheet serves as a cushioning material, so that the inclination of the inner crucible in the outer crucible can be prevented. In addition, even if the compression rate is high, the gas permeability is kept to such an extent that the permeation of the SiO gas generated when the inner crucible is heated, so that the outer crucible is made SiC or thinned. Can be prevented.
According to the 2nd invention, it is formed in the thickness which can maintain cushioning properties, even if it compresses at the time of insertion of an inner crucible, without losing a softness. Therefore, the sheet can be prevented from cracking when the inner crucible is inserted, and the expansion and contraction stress generated between the crucibles when the crucible is cooled can be reduced.
According to the third invention, since the restoring property is high, even if the gap between the crucibles fluctuates due to the difference in the amount of expansion and contraction generated between the both crucibles, the gap between the two can be filled with the sheet. And the cushioning property of the seat can be maintained.
According to the fourth invention, since the ash content in the sheet is small, it is possible to prevent the silicon from being contaminated. Therefore, the pulled silicon single crystal can be of higher quality.
According to the fifth invention, since a plurality of sheets are formed so as to overlap each other, the strength can be improved even if the thickness of the sheets is small, and the buffering force is increased. Therefore, the sheet can be prevented from cracking and the cushioning property can be improved. And gas-shielding property can also be improved.

Next, an embodiment of the present invention will be described with reference to the drawings.
Before describing the crucible protection sheet of the present invention, a device in which this sheet is used will be briefly described.
FIG. 1A is a schematic explanatory diagram of equipment for producing a silicon single crystal and the like, and FIG. 1B is an enlarged explanatory diagram of a crucible.
In FIG. 1, reference numeral 1 is a crucible for accommodating polycrystalline silicon which is a raw material such as a silicon single crystal. A heater 5 is disposed around the crucible, and the crucible 1 is heated by radiant heat from the heater 5. For this reason, when the crucible 1 is heated by the heater 5, the polycrystalline silicon is heated and melted by the heat conduction from the crucible 1, so that the seed crystal is brought into contact with the melted silicon and pulled up to obtain a silicon single crystal. Can be manufactured.
The above crucible 1 is usually composed of an inner crucible 2 made of quartz and an outer crucible 3 made of graphite, but when the outer crucible 2 is inserted into the outer crucible 3, the inner crucible 2 is suppressed. In order to prevent breakage and to prevent damage due to the difference in the thermal expansion coefficient of the materials forming the crucibles 2 and 3 when the crucible 1 is cooled after the production of the silicon single crystal or the like is finished, In addition, the sheet 4 is disposed. The crucible protection sheet of the present invention is used for the sheet 4 (FIG. 1B).

Here, the following properties are required for the crucible protective sheet of the present invention used in the above-described state.
(1) Having shock absorption when inserting the inner crucible into the outer crucible (shock absorption).
(2) To prevent the outer crucible from being thinned and made into SiC by the SiO gas generated from the inner crucible or the like (gas shielding property).
(3) When the crucible is cooled, the difference in deformation amount of both the crucibles caused by the difference in thermal expansion coefficient between the material of the outer crucible (graphite) and the material of the inner crucible (quartz) can be absorbed (deformation amount absorption ability). .

The crucible protective sheet of the present invention is formed from expanded graphite, has a gas permeability of less than 1.0 × 10 ⁻⁴ cm ² / s, and is compressed by pressing with a pressing force of 34.3 MPa from the thickness direction. In some cases, since the compression rate is 20% or more, all of the above properties (1) to (3) can be satisfied.
Below, the relationship between each parameter of the crucible protection sheet of the present invention and the properties (1) to (3) will be described.

First, the protective sheet for crucible of the present invention formed expanded graphite formed in a sheet form by immersing natural graphite or quiche graphite in a liquid such as sulfuric acid or nitric acid and then performing heat treatment at 400 ° C. or higher. Is.
Expanded graphite has a flocculent or fibrous shape, that is, its axial length is longer than its radial length. For example, its axial length is about 1 to 3 mm and its radial direction. The length is about 300 to 600 μm. And in the crucible protective sheet of the present invention, the above expanded graphites are entangled with each other.
In addition, although the protective sheet for crucibles of this invention may be formed only with expanded graphite as mentioned above, binders, such as a phenol resin and a rubber component, may be mixed a little (for example, about 5%).

(Shock absorption and deformation absorption)
The crucible protective sheet of the present invention formed from expanded graphite as described above has a compressibility of 20% or more when pressed and compressed with a pressure of 34.3 MPa from the thickness direction. The compression rate is a value obtained by dividing the thickness of the sheet when pressure-compressed with the above-mentioned pressure by the thickness of the sheet when no pressure is applied.
If the crucible protective sheet of the present invention has the compression rate as described above, even when a force is applied in the direction of pressing the inner crucible against the outer crucible when the inner crucible is inserted into the outer crucible, The force can be absorbed by compressive deformation. That is, since the crucible protective sheet can absorb the impact generated when the inner crucible is inserted, the inner crucible can be prevented from being damaged, and the workability of the insertion work can be improved.
In addition, if the crucible protective sheet has a sufficient compression ratio, even if the bottom surface of the inner crucible has irregularities, the irregularities are in a state where the sheet is difficult to squeeze. Then, the space between the outer surface of the inner crucible and the inner surface of the outer crucible can be filled with the sheet. Therefore, when the inner crucible is inserted into the outer crucible, the inner crucible can be prevented from tilting even if the inner crucible has irregularities on the bottom surface, resulting in the inner crucible being tilted in the outer crucible. Molten silicon liquid can be prevented from leaking.

In addition, even if the crucible protective sheet has the compression rate as described above, if the thickness is too thin, it is not possible to take a sufficient cushioning. In other words, the sheet may not be able to absorb the impact when the inner crucible is inserted, or may not be able to adhere to the outer surface of the inner crucible and the inner surface of the outer crucible.
Further, when the crucible protective sheet is sandwiched between the inner crucible and the outer crucible, the crucible protective sheet is bent and deformed so as to be in close contact with the bottom surface of the inner crucible and the inner surface of the outer crucible. At this time, if the strength of the sheet itself is weak or the flexibility is small, even when the crucible protective sheet has the compression rate as described above, when sandwiched between the inner crucible and the outer crucible, The sheet itself may be cracked, chipped or torn.

However, if the thickness of the crucible protective sheet is 0.2 to 0.6 mm, sufficient cushioning can be taken and the space between both crucibles can be filled with the sheet. In addition, if the bulk density is set to 0.5 to 1.6 Mg / m ³ , the sheet has a certain degree of strength, and therefore, even if the sheet is deformed, cracks and the like can be prevented.
Therefore, in the crucible protective sheet having the compression ratio as described above, if the thickness is 0.2 to 0.6 mm and the bulk density is 0.5 to 1.6 Mg / m ³ , the crack of the sheet is maintained while maintaining the shock absorption and the deformation absorption capacity. And the like can be prevented.
In particular, if the protective sheet for the crucible is 0.4 to 0.6 mm in thickness and 0.5 to 1.5 Mg / m ³ , cracking of the sheet can be prevented more reliably, and the impact absorption and deformation capacity absorption capabilities can be prevented. Can be made higher, which is preferable.

Moreover, if the protective sheet for the crucible is 0.2 to 0.6 mm, especially 0.4 to 0.6 mm, the sheet is maintained in a state where it can be further deformed even after the inner crucible is placed in the outer crucible. Is done. Then, when the entire crucible is cooled after the production of single crystal silicon, even if the shrinkage amount of the outer crucible becomes larger than the shrinkage amount of the inner crucible due to the difference in the thermal expansion coefficient of the material, the difference in shrinkage amount. The crucible protection sheet can be absorbed. That is, since the expansion and contraction stress generated between the crucibles during the crucible cooling can be relaxed, the crucible can be prevented from being damaged during the crucible cooling.
If a plurality of sheets are used in an overlapping manner, the strength can be improved even if the thickness of a single sheet is small, and the buffer size increases. Therefore, the sheet can be prevented from cracking and the cushioning property can be improved.
Furthermore, a single sheet may be used in a stacked manner, or a multilayer sheet formed by previously stacking a plurality of sheets may be used.

Furthermore, when the crucible protective sheet is pressed and compressed from the thickness direction with a pressure of 34.3 MPa, if the recovery rate is 5% or more, the sheet maintains cushioning even after the post-compression load is removed. Can be kept. Then, even if the gap between the two crucibles fluctuates due to the difference in expansion and contraction between the two crucibles during the manufacturing process of the silicon single crystal, the fluctuation of the gap can be restored within the range in which the protective sheet for the crucible can restore its thickness. If so, the space between the inner crucible and the outer crucible can always be filled with a sheet, which is preferable.
The restoration rate is a value obtained by dividing the thickness of the sheet when the pressure is compressed with the above-mentioned pressure by the thickness of the sheet when the pressure is removed.

(Gas shielding)
The crucible protective sheet of the present invention is adjusted so that the gas permeability is smaller than 1.0 × 10 ⁻⁴ cm ² / s.
Although the gas permeability tends to increase as the compression rate increases (see FIG. 5B), if the gas permeability is set to be smaller than 1.0 × 10 ⁻⁴ cm ² / s, the crucible is used. Even if the compression ratio of the protective sheet is in the above-described range, permeation of the sheet of SiO gas generated when the inner crucible is heated can be suppressed. Then, since the SiO gas which permeate | transmitted the sheet | seat reacts with an outer crucible and it can prevent that an outer crucible turns into SiC, the lifetime of an outer crucible can be lengthened and the manufacturing cost of a silicon single crystal can be reduced. Can do.

Moreover, when SiO gas permeate | transmits the protective sheet for crucibles, it flows in between a sheet | seat and an outer crucible, and this SiO gas is heated. Then, a convection of SiO gas is generated between the sheet and the outer crucible, and the outer crucible may be scraped by the convection, and the outer crucible may be thinned. In particular, this phenomenon is often observed in the R part of the crucible (curved part, see A part of FIG. 1B), and the crucible where the thinning proceeds is broken.
However, if the gas permeability is smaller than 1.0 × 10 ⁻⁴ cm ² / s, the amount of gas flowing between the crucible protection sheet and the outer crucible is reduced. It can prevent the outer crucible from thinning.

Even if the crucible protective sheet has the gas permeability as described above, it is difficult to completely prevent the gas permeation. However, if the crucible protective sheet has the compression rate and the restoration rate as described above, the space between the inner crucible and the outer crucible can always be filled with the crucible protective sheet. Then, since there is no gap for convection of SiO gas between both crucibles, the effect of suppressing the thinning of the outer crucible can be enhanced.
Furthermore, since the portion where the convection of SiO gas is generated is the portion where the side surface and bottom surface of the crucible are connected (A portion in FIG. 1B), the crucible of the present invention is only required to prevent such convection. The protective sheet may be provided only near the portion. Further, if a plurality of sheets are stacked or a multilayer sheet composed of a plurality of sheets is used, the gas shielding property can be enhanced even if the gas shielding property of one sheet is not so high.

The compression ratio and the restoration ratio when the protective sheet for crucible of the present invention was pressure-compressed with a pressing force of 34.3 MPa from the thickness direction were examined.
Measurements were made on the relationship between bulk density, compression rate and restoration rate when the bulk density was 0.1, 0.5, 0.8, 1.0, 1.2, 1.5, and 1.8 Mg / m ³ in a protective sheet for crucibles with a thickness of 0.5 mm. confirmed. The compression ratio is evaluated by the ratio of the sheet thickness during pressure compression to the sheet thickness before pressure compression, and the restoration ratio is obtained by removing the pressurization pressure after compression against the sheet thickness before pressure compression. The sheet thickness was evaluated as a percentage.
The sheet is adjusted with halogen gas so that the ash content is 10 ppm or less.

  As shown in FIG. 2A, it can be confirmed that as the bulk density increases, the compression rate decreases and the restoration rate increases. Further, when the relationship between the compression rate and the restoration rate is confirmed, as shown in FIG. 2B, it can be confirmed that the restoration rate decreases as the compression rate increases as a whole. That is, it can be confirmed that the compression rate and the restoration rate are in a trade-off relationship.

In the crucible protective sheet of the present invention, the relationship between the sheet thickness, the flexibility and the buffering property was confirmed. Flexibility and shock-absorbing properties were confirmed by inserting the inner crucible into the outer crucible in a state where the crucible protective sheet was placed on the inner surface of the outer crucible, and checking the sheet for damage.
The sheet had a bulk density of 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, 1.7 Mg / m ³ and a thickness of 0.1, 0.2, 0.4, 0.6, 1.0 m.
The evaluation is ◎ when the flexibility is good, the sheet is not cracked, torn, or chipped, and excellent in buffering properties, and when the flexibility and buffering properties are acceptable, ○, When either the flexibility or the buffering property is bad, it is indicated by Δ, and when both are bad, it is indicated by ×.
In addition, IG-110 (inner diameter φ500, height 490 mm) manufactured by Toyo Tanso Co., Ltd. was used for the outer crucible, and a quartz crucible (outer diameter φ480, height 500 mm) was used for the inner crucible.
The sheet is adjusted with a halogen gas so that the ash content is 10 ppm or less.

In a sheet made of expanded graphite, the strength of the sheet generally increases as the bulk density increases. However, when the sheet thickness is too thin (0.1 mm), as shown in FIG. Even if the value is increased, sufficient strength cannot be obtained. For this reason, if a bending force is applied to the sheet when the crucible is attached, tearing, cracking, chipping or the like occurs. Moreover, there is not enough buffer white.
On the other hand, when the thickness of the sheet is too thick (1 mm), there is sufficient cushioning and sufficient strength, but workability is not deteriorated, but flexibility is deteriorated. If a bending force is applied to the sheet at the time of mounting, cracks and chips will occur.

In addition, when the bulk density is small, the flexibility is low due to insufficient strength, and when the sheet thickness is 1 mm, when a bending force is applied to the sheet, cracking or chipping occurs. Conversely, when the bulk density is 1.7 Mg / m ³ , the compressibility is low, and even if the thickness is increased, there is no sufficient buffering force.

If the sheet thickness is 0.2 to 0.6 mm and the bulk density is 0.5 to 1.5 Mg / m ³ , the sheet strength can be increased while maintaining sufficient flexibility and maintaining flexibility. In particular, when the thickness is 0.4 to 0.6 mm, the cushioning force increases and the sheet strength increases, which is preferable. Further, when a plurality of such sheets are stacked, the sheets themselves are thin, so that they are not cracked or chipped, and the cushioning properties and gas shielding properties can be further improved.

The compressibility and gas permeability of the crucible protective sheet of the present invention were examined.
The measurement confirmed the compressibility and gas permeability when the bulk density was 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, and 1.7 Mg / m ³ in the crucible protective sheet having a thickness of 0.5 mm.
The compression ratio was evaluated by the ratio of the sheet thickness during pressure compression to the sheet thickness before pressure compression when compressed with pressure of 34.3 MPa from the thickness direction, as in Example 1. .
Gas permeability was measured by the following method.
(1) In a pair of sealed chambers CA and CB communicated with each other, a passage (diameter 10 mm) communicating both chambers CA and CB is disposed so as to be closed with the protective sheet for crucible (diameter 30 mm) of the present invention. In other words, air does not flow between the pair of sealed chambers CA and CB unless passing through the crucible protection sheet.
(2) From this state, both chambers CA and CB are evacuated until the atmospheric pressure in both chambers CA and CB reaches 1.0 × 10 ⁻⁴ Pa. Then, while evacuating one chamber CA is continued, N ₂ gas is supplied until the inside of the other chamber CB reaches a predetermined pressure (1.0 × 10 ⁵ Pa).
(3) When the inside of the other chamber CB reaches a predetermined pressure (1.0 × 10 ⁵ Pa), the evacuation in one chamber CA is stopped. Then, N ₂ gas gradually flows from the other chamber CB to the one chamber CA according to the gas permeability of the protective sheet for crucible, which takes a pressure difference between the two chambers CA and CB. Rises.
(4) Then, after the evacuation in one chamber CA is stopped, the pressure increase rate in one chamber CA for about 100 seconds is measured, and the gas permeability K (cm ² / s) was calculated.
K = Q · L / (P · A)
Q is the gas flow rate (Pa · cm ² / s), P is the pressure difference (Pa) between both chambers CA and CB, A is the gas permeability area of the crucible protective sheet, that is, both chambers CA and CB are It is the area (cm ² ) of the passage that communicates.
Further, the gas flow rate Q is calculated from the pressure increase rate in the one chamber CA and the volume of the one chamber CA for about 100 seconds after the evacuation in the one chamber CA is stopped.

As shown in FIG. 4, it can be confirmed that as the bulk density increases, the gas permeability decreases, in other words, the gas shielding performance increases as the bulk density increases.
Further, as shown in FIG. 5, when the relationship between the compressibility and the gas permeability is confirmed, it can be confirmed that the gas permeability becomes smaller (the gas shielding property becomes higher) as the compressibility becomes larger. That is, it can be confirmed that the compression ratio is in a trade-off relationship with the gas shielding property.

  The protective sheet for crucible of the present invention is suitable for a sheet used for protecting an outer crucible and an inner crucible in the production of a silicon single crystal or the like by the CZ method.

(A) is schematic explanatory drawing of the equipment which manufactures a silicon single crystal etc., (B) is an enlarged explanatory view of a crucible. It is the figure which showed the relationship between the compression rate in a protective sheet for crucibles, and a decompression | restoration rate. It is the figure which showed the thickness of the sheet | seat in a crucible protective sheet, and the relationship between a flexibility and a buffering property. It is the figure which showed the relationship between the bulk density in a protective sheet for crucibles, a compressibility, and gas permeability. It is the figure which showed the relationship between the compressibility in a protective sheet for crucibles, and gas permeability.

Explanation of symbols

1 crucible 2 inner crucible 3 outer crucible 4 crucible protective sheet

Claims (5)

A sheet of expanded graphite disposed between both crucibles in an inner crucible and an outer crucible,
The gas permeability is less than 1.0 × 10 ⁻⁴ cm ² / s,
A crucible protective sheet characterized by having a compression ratio of 20% or more when pressed and compressed with a pressure of 34.3 MPa from the thickness direction. The crucible protection sheet according to claim 1, wherein the sheet has a thickness of 0.2 mm to 0.6 mm. The crucible protection sheet according to claim 1, wherein the restoration rate is 5% or more when compressed and compressed with a pressure of 34.3 MPa from the thickness direction. The protective sheet for crucible according to claim 1, wherein the ash content is 10 massppm or less. A protective sheet for crucible, wherein a plurality of the sheets according to claim 1, 2, 3, or 4 are laminated.

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