JP4814855B2 - Silica glass crucible - Google Patents

Description

  The present invention relates to a silica glass crucible for pulling up a single crystal by the Czochralski method (hereinafter referred to as “CZ method”).

  The CZ method is widely used for the growth of silicon single crystals. In this method, the seed crystal is brought into contact with the surface of the silicon melt contained in the silica glass crucible, the crucible is rotated, and the seed crystal is pulled upward while rotating the seed crystal in the opposite direction. A single crystal is formed at the lower end of the substrate.

By the way, although the silica glass crucible is increasing in size year by year, the increase in the size of the silica glass crucible has an advantage that the amount of polycrystalline silicon loaded can be increased and the throughput is improved.
However, on the other hand, it must be used in harsh environments such as a longer melting time and a larger output of the heating carbon heater, and the silica glass crucible is greatly adversely affected.

For example, a conventional silica glass crucible has a low viscosity value in a high temperature range, and it has been difficult to maintain its shape for a long time in a thermal environment of 1400 ° C. or higher. Therefore, problems have occurred in the single crystal pulling process, such as fluctuations in the melt surface of the molten silicon due to deformation of the silica glass crucible and reduction in the single crystallization rate.
In order to solve this problem, as shown in FIG. 5, Patent Document 1 discloses a three-layer structure in which an outer layer 51 is an Al-added quartz layer, an intermediate layer 52 is a natural quartz layer, and an inner layer 53 is a high-purity synthetic quartz layer. A quartz glass crucible 50 is disclosed.

According to the crucible 50, the outer layer 51 is crystallized into cristobalite when heated constantly at 1200 ° C. or higher in the heating temperature raising process of the silicon single crystal pulling process.
In the growth process of cristobalite, the viscosity is improved, and high durability can be obtained by crystallization, so that the problems such as deformation and breakage of the crucible as described above can be solved.
JP 2000-247778 A

However, in the quartz glass crucible 50 disclosed in Patent Document 1, the progress of crystallization from the outer surface toward the inner layer side is slow, and the Al-added quartz layer of the outer layer 51 is all crystallized, so that it has sufficient durability. It took a long time to secure the sex.
In addition, the crucible may be deformed during a long time when the crystallization is completed.

As a method for solving the problems such as early securing of durability and prevention of deformation as described above, increasing the Al concentration of the Al-added silica layer of the outer layer 51 can be mentioned.
That is, if the Al concentration of the Al-added silica layer is high, crystallization of the outer layer 51 proceeds rapidly after the start of heating. For this reason, crystallization can be completed before deformation of the crucible occurs.

  However, if the crystallization of the outer layer 51 is too fast, the adhesion to the carbon susceptor that holds the crucible 50 is deteriorated. As a result, the support of the crucible 50 by the carbon susceptor becomes unstable, which adversely affects the pulling of the silicon single crystal. There was a problem of affecting.

  The present invention has been made under the circumstances as described above, and in a silica glass crucible, while ensuring the durability, it is possible to improve the stable support of the crucible and obtain a sufficient single crystallization rate. An object is to provide a silica glass crucible.

In order to solve the above-mentioned problems, a silica glass crucible according to the present invention is a silica glass crucible for pulling up a single crystal by the Czochralski method, and a first outer layer made of a crystallization accelerator-added silica glass, A second outer layer formed of silica glass containing crystallization accelerator and formed on the inner side of the first outer layer, and the concentration of the crystallization accelerator is three times higher than that of the first outer layer; It is characterized in that it has an opaque intermediate layer made of a natural raw material silica glass and an inner transparent layer made of a natural raw material silica glass or a synthetic raw material silica glass. .
With this configuration, after the start of pulling, the outer layer can be crystallized (cristobalite) by heating at a high temperature without slowing down, and the entire crucible and the carbon that supports it can be supported. Adhesion stability with the susceptor is improved, and sufficient durability can be obtained at an early stage.

A bottom portion having a first curvature, a bottom corner formed around the bottom portion and having a second curvature, and a side portion extending upward from the bottom corner, and from the crucible upper end to the side portion The first outer layer, the second outer layer, the opaque intermediate layer, and the transparent inner layer up to a predetermined range based on a curvature change point between the bottom portion and the bottom corner, which are below, Preferably, the bottom is formed of an opaque outer layer formed continuously from the opaque intermediate layer and the transparent inner layer.
With this configuration, in the initial stage of starting the pulling of the silicon single crystal, the bottom portion (opaque outer layer) in a predetermined range where the outer layer does not exist can be softened and more closely attached to the carbon susceptor that supports the crucible.
Therefore, according to the silica glass crucible according to the present invention, the single crystallization rate of the silicon single crystal can be improved.

The crystallization accelerator concentration in the first outer layer is preferably set in the range of 30 to 100 ppm, and the crystallization accelerator concentration in the second outer layer is preferably set in the range of 90 to 300 ppm. .
By setting the concentration in this way, the crystallization speed of the outer layer can be made substantially constant, and sufficient durability can be obtained by crystallization together with crucible stabilization.

The predetermined range in which the lower ends of the first outer layer and the second outer layer are formed is 0 ° on a straight line connecting the center of curvature of the first curvature and the curvature change point, and the straight line is It is desirable to be within a range rotated ± 5 ° around the center of curvature.
The thickness dimensions of the first outer layer and the second outer layer are preferably set in the range of 0.5 to 2.0 mm.
By comprising in this way, after a single crystal pulling start, a bottom part and a carbon susceptor can fully be stuck, and the support of a crucible can be stabilized.
Moreover, the outer layer for obtaining sufficient durability can be formed.
Examples of the crystallization accelerator include Al, Ba, Ca, and K, which can be selected from any of these or a combination of these, but most preferably only Al. .

  According to the present invention, in a silica glass crucible, a silica glass crucible capable of ensuring sufficient durability and improving the adhesion stability of the entire crucible and the carbon susceptor supporting the crucible and obtaining a sufficient single crystallization rate. Obtainable.

Hereinafter, embodiments of a silica glass crucible according to the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a silica glass crucible 1 according to the present invention. FIG. 2 is a partially enlarged cross-sectional view of the silica glass crucible of FIG.
This silica glass crucible 1 is used, for example, in a single crystal pulling apparatus (not shown), and is used while being held by a carbon susceptor (not shown) in the apparatus.
That is, in the single crystal pulling apparatus, the raw material silicon is melted in the silica glass crucible 1 and the silicon single crystal is pulled from the melt.

The silica glass crucible 1 is formed to have a diameter (caliber) of 810 mm, for example, and as shown in FIG. The outer layer 2 is formed up to a predetermined range based on the curvature change point therebetween.
The outer layer 2 includes, for example, an Al-added silica glass layer 2a (first outer layer) to which Al is added as a crystallization accelerator, and a high Al-added silica glass layer 2b (second second) to which Al is added so that the Al concentration becomes higher. The outer layer is formed of two layers.

Further, an opaque intermediate layer 3 made of natural raw silica glass is formed inside the outer layer 2.
Furthermore, a transparent inner layer 4 made of high-purity synthetic raw silica glass (or natural raw silica glass) that contacts molten silicon when the silicon single crystal is pulled is formed inside the intermediate layer 3.

  The term “opaque” as used herein means a state in which a large number of pores are inherent in silica glass and apparently cloudy. The natural raw silica glass means a silica glass produced by melting a natural raw material such as quartz, and the synthetic raw silica glass means a synthetic raw material synthesized by, for example, hydrolysis of silicon alkoxide. This means silica glass produced in the same manner.

As described above, the outer layer 2 is not formed on the entire outer side of the crucible. That is, the outer layer 2 is not formed on the crucible bottom 9, and the opaque outer layer 5 made of natural raw silica glass continuously formed from the opaque intermediate layer 3 and the transparent made of synthetic raw silica glass (or natural raw silica glass). A two-layer structure formed by the inner layer 4 is formed.
More specifically, as shown in the cross-sectional view of FIG. 2, the bottom 9 has a radius R curvature (first curvature) in the crucible, and the bottom corner 8 has a radius r curvature (second curvature). Curvature). Moreover, the crucible side part 7 is formed linearly in the vertical direction.

  Here, the outer layer 2 is formed downward from the crucible upper end 6, and the lower end of the outer layer 2 has a curvature change point P between the bottom 9 (curvature radius R) and the bottom corner 8 (curvature radius r). It is formed up to a predetermined range as a reference. More specifically, the lower end of the outer layer 2 is rotated by ± 5 ° around the curvature center C, with 0 ° on the straight line L connecting the curvature center C of the radius R and the curvature change point P. It is formed to within the range.

This is because, when the lower direction (bottom direction) is a positive direction, if the lower end of the outer layer 2 is formed to a range larger than + 5 °, crystallization (cristobalite formation) proceeds rapidly, making it difficult to obtain desired adhesion. This is because the support of the crucible by the carbon susceptor becomes unstable and the silicon single crystallization rate tends to decrease.
On the other hand, if the lower end of the outer layer 2 is formed in a range smaller than −5 °, the proportion of the outer layer 2 in the entire crucible is small and the durability tends to be low.

  As shown in FIG. 3, the outer Al-added silica glass layer 2a of the outer layer 2 has a thickness dimension et1 at the crucible side portion 7 and a thickness dimension et2 at the crucible bottom corner 8 of 0.5 to 0.5. It is formed to 3 mm (preferably 1 to 2 mm). This is because if the thickness is larger than 3 mm, cracks may occur due to the generation of large stress due to the difference in thermal expansion between the crucible wall layers. This is because it is difficult to obtain.

  Further, in the outer layer 2, the inner high Al-added silica glass layer 2b has a thickness dimension ft1 at the crucible side portion 7 and a thickness dimension ft2 at the crucible bottom corner 8 of 0.5 to 3 mm (preferably 1 to 3). 2 mm). This is because if the thickness is larger than 3 mm, the crystallized layer is too thick, so that it is difficult to obtain adhesion to the carbon crucible, and if it is smaller than 1 mm, it is difficult to obtain a contribution to improving durability. is there.

  Further, as described above, the Al concentration of the inner high Al-added silica glass in the outer layer 2 is made three times or more higher than the Al concentration of the outer Al-added silica glass. This is because when the crucible is heated, the crystallization speed that proceeds from the outer surface toward the inner side is not lowered, and is made substantially constant. That is, the crystallization of the crucible proceeds from the outside to the inside as described above, but the crystallization speed becomes slower toward the inside. Therefore, the lowering of the speed can be compensated by increasing the concentration of Al added to the inner silica glass layer in the outer layer 2 (in comparison with the outer layer).

Specifically, in the outer layer 2, the Al concentration of the outer Al-added silica glass layer 2a is 30 to 100 ppm (preferably 50 to 80 ppm). This is because if the Al concentration is higher than 100 ppm, crystallization is likely to proceed rapidly, and crystallization may be completed before the adhesion between the crucible and the carbon susceptor is stabilized. Moreover, it is because there exists a possibility that a crack may arise by generation | occurrence | production of the big stress resulting from a thermal expansion difference etc.
On the other hand, when the Al concentration is lower than 30 ppm, the crystallization rate is slow, and it is difficult to obtain desired durability.

The Al concentration of the inner high Al-added silica glass layer 2b is 90 to 300 ppm (preferably 150 to 250 ppm). This is because when the Al concentration is higher than 300 ppm, crystallization is likely to proceed rapidly, and crystallization may be completed before the adhesion between the crucible and the carbon susceptor is stabilized.
On the other hand, if it is lower than 90 ppm, the crystallization rate is slow, and it is difficult to make the crystallization rate of the outer layer 2 constant.

The opaque intermediate layer 3 made of natural silica glass is formed such that the thickness dimension mt1 at the crucible side portion 7 is 3 mm or more, and the thickness dimension mt2 at the crucible bottom corner 8 is 6 mm or more.
Moreover, the thickness dimension mt3 of the opaque outer layer 5 of the crucible bottom 9 formed continuously from the opaque intermediate layer 3 is formed to be 6 mm or more.

This is because when the thickness of the opaque intermediate layer 3 in the three-layer portion is smaller than 3 mm, the crystallization accelerator compound of the opaque intermediate layer 3 is prevented from being scattered by the irregular flow of the arc flame during the melting of the silica glass powder. This is because the crystallization accelerator compound in the outer layer 2 passes through the opaque intermediate layer 3 and is mixed into the transparent inner layer 4, and the concentration of the crystallization accelerator in the transparent inner layer 4 may increase.
Further, if the thickness dimension mt2 of the opaque intermediate layer 3 at the crucible bottom corner 8 and the thickness dimension mt3 of the opaque outer layer 5 at the crucible bottom section 9 are smaller than 6 mm, it is difficult to obtain sufficient durability.

The transparent inner layer 4 is a transparent layer substantially free of bubbles formed by melting synthetic raw silica glass (or natural raw silica glass) having a metal impurity content of 1 ppm or less for each of Na, K, and Al. is there.
In the transparent inner layer 4, the thickness dimension it1 at the crucible side portion 7, the thickness dimension it2 at the crucible bottom corner 8, and the thickness dimension it3 at the crucible bottom portion 9 are all formed to a thickness of 3 mm or more.
This is because if the thickness of the transparent inner layer 4 is smaller than 3 mm, the concentration of the crystallization accelerator on the inner surface of the transparent inner layer 4 in contact with the silicon melt cannot be made sufficiently low, for example, 1 ppm or less.

According to the silica glass crucible 1 having such a structure, at the initial stage of starting the pulling of the silicon single crystal, the bottom portion 9 (the opaque outer layer 5) in which the outer layer 2 made of Al-added silica glass and high Al-added silica glass does not exist. ) Softens and comes into close contact with the carbon susceptor that supports the crucible 1.
Thereafter, crystallization (cristobalite) of the outer layer 2 such as the bottom corner 8 and the side portion 7 in a predetermined range proceeds by high temperature heating from the start to the completion of the pulling.

  Here, since the Al concentration (90 to 300 ppm) of the high Al-added silica glass layer 2b is formed three times or more higher than the Al concentration (30 to 100 ppm) of the Al-added silica glass layer 2a, the inner layer side from the outer surface A decrease in the rate of crystallization that progresses toward is suppressed. That is, in the outer layer 2, since the inner Al concentration is formed high, crystallization is further promoted by the Al compound, and the crystallization rate becomes substantially constant without decreasing.

When the crystallization of the outer layer 2 is completed, the adhesion stability and durability between the entire crucible 1 and the carbon susceptor that supports the crucible 1 are improved.
Therefore, while ensuring the durability of the crucible 1, the adhesion stability of the support by the carbon susceptor can be obtained, and the single crystallization rate of the silicon single crystal can be improved.

Next, a method for producing the silica glass crucible 1 having the above structure will be described.
A silica glass crucible 1 is manufactured using a silica glass crucible manufacturing apparatus 10 as shown in FIG. The crucible molding die 11 of the silica glass crucible manufacturing apparatus 10 includes an inner member 12 made of a gas permeable member such as a die having a plurality of through holes or a highly purified porous carbon die. In addition, a ventilation portion 13 is provided on the outer periphery thereof, and a holding body 14 that holds the inner member 12 is configured.

A rotating shaft 15 connected to a rotating means (not shown) is fixed to the lower portion of the holding body 14, and supports the crucible forming die 11 rotatably. The ventilation portion 13 is connected to an exhaust passage 17 provided in the center of the rotary shaft 15 through an opening 16 provided in the lower portion of the holding body 14, and the exhaust passage 17 is connected to a decompression mechanism 18. Has been.
An arc electrode 19 for arc discharge, an Al-added raw material supply nozzle 20, a high Al-added raw material supply nozzle 21, a natural silica powder supply nozzle 22, and a high-purity synthetic silica powder supply nozzle are disposed on the upper part facing the inner member 12. 23 is provided.

The Al-added silica powder used for the outer layer 2 can be obtained as follows. For example, an aqueous solution of Al nitrate prepared by dissolving Al nitrate (Al (NO ₃ ) ₃ ) in water in such an amount that the Al concentration of the silica powder becomes a desired concentration is added to the silica powder and stirred. After stirring, heat treatment is performed at 800 to 1100 ° C. for the purpose of dehydration and acid removal. Thereby, Al addition silica powder is obtained.

When the silica glass crucible manufacturing apparatus 10 is used to manufacture the silica glass crucible 1 using the Al-added silica powder thus obtained, a rotary drive source (not shown) is operated to rotate the rotary shaft 18 in the direction of the arrow, thereby The crucible molding die 11 is rotated at high speed.
Next, Al-added silica glass powder (Al concentration of 30 to 100 ppm (preferably 50 to 80 ppm)) is supplied from the Al-added raw material supply nozzle 20 into the crucible molding die 11. The supplied Al-added silica glass powder is pressed to the inner surface side of the inner member 12 by centrifugal force, and is formed as an Al-added silica glass layer 2 a of the outer layer 2.

Subsequently, the high Al-added silica glass powder (90 to 300 ppm (preferably 150 to 300 ppm)) having a concentration three times higher than the Al concentration of the Al-added silica glass powder from the high Al-added raw material supply nozzle 21 in the rotating crucible molding die 11. ~ 250 ppm)). The supplied high Al-added silica glass powder is pressed against the inner surface side of the Al-added silica glass layer 2a by centrifugal force to form the high Al-added silica glass layer 2b of the outer layer 2.
Here, the outer layer 2 is positioned at the lower end as described with reference to FIG. 2, and the bottom is removed. That is, the Al-added silica glass layer 2a and the high Al-added silica glass layer 2b are removed from the curvature changing point to the bottom of the silica glass crucible 1 to be manufactured.

  Next, the natural silica powder is supplied to the natural silica powder supply nozzle 22 so that the opaque intermediate layer 3 and the opaque outer layer 5 having a thickness of 3 mm or more on the inner surface side of the outer layer 2 and a thickness of 6 mm or more at the bottom corner and the bottom are formed. Supply from. The supplied natural silica powder is pressed against the inner surface side of the outer layer 2 and the bottom of the inner member 12 by centrifugal force, and is molded as a molded body of the opaque intermediate layer 3 and the opaque outer layer 5.

Next, high-purity synthetic silica with metal impurity contents of Na, K and Al of 1 ppm or less each so that a transparent layer having a thickness of 3 mm or more is formed on the inner surface side of the opaque intermediate layer 3 and the opaque outer layer 5. Powder is supplied from the high-purity synthetic silica powder supply nozzle 23 (or natural silica powder is supplied from the natural silica powder supply nozzle 22).
The supplied high-purity synthetic silica powder is pressed against the inner surfaces of the opaque intermediate layer 3 and the opaque outer layer 5 by centrifugal force, and is molded as a molded body of the transparent inner layer 4.

In this way, a crucible molded body having two outer layers 2, an opaque intermediate layer 3, an opaque outer layer 5 and a transparent inner layer 4 having different Al concentrations is obtained.
Further, the inside of the inner member 12 is depressurized by the operation of the decompression mechanism 18, the arc electrode 19 is energized and heated from the inside of the crucible molded body, and the transparent inner layer 4, the opaque intermediate layer 3, the opaque outer layer 5 and the outer layer of the crucible molded body. 2 is melted to produce a silica glass crucible 1.

  As described above, according to the present embodiment, two outer layers 2 to which, for example, Al is added as a crystallization accelerator are formed, and the Al concentration in the inner layer is higher than the Al concentration in the outer layer. The The outer layer 2 is formed downward from the upper end of the crucible, and the lower end of the outer layer 2 is formed up to a predetermined range based on the curvature change point of the bottom.

With this structure, in the initial stage of starting the pulling of the silicon single crystal, the bottom portion (the opaque outer layer 5) in a predetermined range where the outer layer 2 does not exist can be softened and brought into close contact with the carbon susceptor that supports the crucible.
In addition, after the start of pulling, the outer layer 2 can be crystallized (cristobalite) by heating at a high temperature without decreasing the speed, and the adhesion stability between the entire crucible and the carbon susceptor that supports the crucible is improved. At the same time, sufficient durability can be obtained early.

  Therefore, according to the silica glass crucible 1 according to the present invention, the adhesion stability of the support by the carbon susceptor can be obtained while ensuring the durability, and the single crystallization rate of the silicon single crystal can be improved.

  Next, the silica glass crucible according to the present invention will be further described based on examples. In this example, the silica glass crucible having the structure described in the above embodiment was manufactured, and the effect was verified by performing heat treatment.

In Examples 1 to 6, the silica glass crucible having the layer structure shown in FIG. 1 was manufactured, and the silicon single crystal was actually pulled up, whereby the durability of the crucible and the single crystal crystallization rate (DF) (DF) Rate). In Example 7 and the conventional example, a silica glass crucible having a two-layer structure of all outer layers including the bottom of the crucible was manufactured, the silicon single crystal was pulled up, and the durability of the crucible and the DF ratio of the single crystal were measured.
Further, in the silica glass crucible having the layer structure shown in FIG. 1, Comparative Examples 1 and 2 were set according to the concentration of Al added to the outer layer and the inner layer in the outer layer and the combination thereof.

Table 1 shows the results of Examples 1 to 7 and Comparative Examples 1 and 2.
The durability of the crucible was evaluated by the change in crucible height before and after use, that is, the ratio (%) of subsidence deformation.
Specifically, the durability is obtained by subtracting deformation ratio (%) = ((product height before use−product height after use) / product height before use) × 100, and the value is 7% or less. If it is higher than 7%, it is indicated by △.
Further, the DF ratio is indicated by “◯” if it is higher than 95%, indicated by “Δ” if it is 90% to 95%, and indicated by “x” if it is lower than 90%.
The overall evaluation shown in Table 1 is that the durability and the DF ratio are both ◯ and total ◯, if either is △, the total △, if both are △, the total ×, even if the durability is ◯, the DF ratio If it is x, it was set as total x.

As shown in Table 1, good results could be obtained when the Al concentration in the second outer layer (inner side) was three times or more of the Al concentration in the first outer layer (outer side). In that case, the Al concentration of the first outer layer (outer side) was 30 to 100 ppm, and the Al concentration of the second outer layer (inner side) was 90 to 300 ppm.
Also, when an Al-added outer layer was formed at the bottom (Example 7), good results were obtained as a comprehensive evaluation although the DF ratio was slightly low.

  From the experimental results of the above examples, according to the silica glass crucible according to the present invention, the adhesion stability of the support by the carbon susceptor is obtained while ensuring the durability of the crucible, and the single crystallization rate of the silicon single crystal is increased. It was confirmed that it could be improved.

  The present invention relates to a silica glass crucible for pulling a single crystal by the Czochralski method, and is suitably used in the semiconductor manufacturing industry and the like.

FIG. 1 is a cross-sectional view of a silica glass crucible according to the present invention. FIG. 2 is a partially enlarged cross-sectional view of the silica glass crucible of FIG. FIG. 3 is a partially enlarged cross-sectional view for explaining the thickness dimension of each layer of the silica glass crucible of FIG. FIG. 4 is a cross-sectional view of a silica glass crucible manufacturing apparatus for manufacturing the silica glass crucible of FIG. FIG. 5 is a cross-sectional view of a conventional silica glass crucible having a three-layer structure.

Explanation of symbols

1 silica glass crucible 2 outer layer 2a Al-added silica glass layer (first outer layer)
2b High Al-added silica glass layer (second outer layer)
DESCRIPTION OF SYMBOLS 3 Opaque intermediate layer 4 Transparent inner layer 5 Opaque outer layer 6 Crucible top 7 Side 8 Bottom corner 9 Bottom 10 Silica glass crucible manufacturing equipment

Claims (5)

In a silica glass crucible for pulling up a single crystal by the Czochralski method,
A first outer layer comprising a crystallization accelerator-added silica glass;
A second outer layer formed of crystallization accelerator-added silica glass formed on the inner side of the first outer layer and having a concentration of the crystallization accelerator three times higher than that of the first outer layer;
An opaque intermediate layer formed inside the second outer layer and made of natural silica glass;
A silica glass crucible formed inside the opaque intermediate layer and having a transparent inner layer made of natural raw silica glass or synthetic raw silica glass. The crystallization accelerator concentration in the first outer layer is set in a range of 30 to 100 ppm,
2. The silica glass crucible according to claim 1, wherein the crystallization accelerator concentration in the second outer layer is set in a range of 90 to 300 ppm.   3. The silica glass crucible according to claim 1, wherein thickness dimensions of the first outer layer and the second outer layer are both set in a range of 0.5 to 2.0 mm. . A bottom portion having a first curvature; a bottom corner formed around the bottom portion and having a second curvature; and a side portion extending upward from the bottom corner;
The first outer layer, the second outer layer, and the opaque intermediate layer up to a predetermined range based on a curvature change point between the bottom and the bottom corner, which are below the side portion, from the upper end of the crucible And the transparent inner layer,
The silica glass crucible according to any one of claims 1 to 3, wherein the bottom portion is formed of an opaque outer layer formed continuously from the opaque intermediate layer and the transparent inner layer. The predetermined range in which the lower ends of the first outer layer and the second outer layer are formed is
5. The straight line connecting the curvature center of the first curvature and the curvature change point is set to 0 °, and the straight line is within a range rotated ± 5 ° around the curvature center. The described silica glass crucible.

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