JP2005306708A - Quartz crucible - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent exfoliation of crystal chips from the inner surface of a quartz crucible during use in order to improve the yield of a silicon single crystal. <P>SOLUTION: The number of brown marks causing exfoliation is reduced by forming a layer, which is highly reactive with a silicon melt in comparison with its inner layer, within a layer of 150 μm from the surface of the quartz crucible so as to make the erosion speed higher than the formation speed of a crystal nucleus. Concretely, the inner surface layer highly reactive with the silicon melt is formed from synthetic quartz glass or natural quartz glass containing alkali metals such as lithium, sodium or potassium and alkaline earth metals such as beryllium, magnesium, calcium, strontium or barium in an amount of ≤100 ppm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a quartz crucible having a high yield of single crystallization even when used for a long time for pulling up a silicon single crystal.

The quartz crucible is an important member that determines the yield and quality of a silicon single crystal as the only member in contact with the silicon melt. The yield of the single crystal is deteriorated when SiO ₂ pieces mixed from the quartz crucible adhere to the edge of the silicon single crystal and become dislocations. This is because a crystal called β-cristobalite is stable in the use temperature range of quartz glass, and easily undergoes phase transition to β-cristobalite called a brown mark by contact with a silicon melt. This β-cristobalite is known to cause peeling due to the difference in structure with the surrounding glass.

  This crystallization occurs on the surface of the quartz crucible that is in contact with the silicon melt, but also occurs where the interface energy is small, such as bubbles. Therefore, a method called a decompression method has been proposed as a measure for suppressing crystallization of β-cristobalite that occurs like bubbles (Patent Document 1-2). In this method, suction is performed with a pump from the outside of the mold, and melting is performed while degassing. Thereby, a transparent layer with few bubbles is formed on the inner surface of the quartz crucible.

  Moreover, the method of throwing in raw material silica from the upper part during melting and forming a transparent layer is also proposed (Patent Document 3). This is the same principle as the so-called Bernoulli method in which transparent quartz glass is conventionally produced by an oxyhydrogen flame. That is, quartz glass without bubbles is produced while degassing by lowering the viscosity of the quartz glass surface called a target, and dispersing and melting quartz particles on the surface. This transparent layer is effective for suppressing crystallization due to bursting of bubbles.

  In order to suppress crystallization, it is said that reducing alkali and alkaline earth metal is also effective (Patent Document 4).

  However, it is difficult to obtain a high-quality silicon single crystal by any of the above methods.

  Further, it has been proposed to use a synthetic crucible (Patent Document 5). According to this method, synthetic quartz glass has a large silica matrix and is irregular, so it is difficult to crystallize and can be used for a long time. However, since the synthetic crucible has a large silica matrix, it easily reacts with the silicon melt, and in a severe case, the liquid level is vibrated. Naturally, a large amount of oxygen is contained in the silicon single crystal, which may affect the quality. In particular, in pulling up a silicon single crystal for solar cells, there is a problem in that the oxygen concentration in the silicon ingot increases and the lifetime decreases.

  On the other hand, in contrast to the above-described studies and trials, non-uniform crystallization is a problem, but from the viewpoint that exfoliation does not occur if all are crystallized uniformly, barium carbonate is applied to the inner surface of the quartz crucible. An applied crucible has been proposed (Patent Documents 6-7). Barium belongs to an alkaline earth metal and becomes an impurity for crystallizing quartz glass. By this coating, the surface of the quartz crucible is crystallized during the temperature rise during the pulling of the silicon single crystal. However, in this method, it is considered that a large amount of barium is applied to the surface, so that the silicon melt is mixed in a large amount. In this regard, it is described that the amount of barium mixed into the silicon single crystal is very small because the segregation coefficient of barium is small.

  However, the present inventors have confirmed that when using this barium-coated crucible, the barium concentration and coating thickness must be changed depending on the pulling conditions. This indicates that the crystallization speed must be controlled moderately. For example, when pulling up a silicon single crystal of 16 inches by 5 inches and when pulling up a single crystal of 12 inches by 32 inches, it takes an initial stage. The temperature is different by 100 ° C. or more. Naturally, as is clear from the Arrhenius equation, the crystallization rate is greatly influenced by temperature. In order to crystallize the surface, a certain level of barium is required at low temperatures, and crystallization progresses at low concentrations when used at high temperatures.

  Recently, it has been found that for a quartz crucible coated with barium on its inner surface, the barium concentration must be controlled considerably in order to improve the single crystallization yield when a large-diameter ingot is pulled up. That is, in a large-diameter crucible, since the temperature distribution of the crucible itself is considerable, the single crystal yield cannot be increased unless the crystallization rate is controlled by adding a concentration distribution.

  And the method of adding a crystal accelerator in a silicon melt is also known (patent document 8). According to this, barium added to the silicon melt after the polysilicon is melted reacts with the inner surface of the quartz crucible to crystallize the crucible surface. In the method described above (Patent Documents 6-7), the surface of the quartz crucible is crystallized before the silicon is melted, whereas in this method, the surface of the quartz crucible is crystallized after the silicon is melted. However, this method is effective when the silicon melting temperature is very high, but when it is used at a low temperature such as a small-diameter crucible, the crystallization speed is slow and crystallization occurs unevenly. Since the peeling phenomenon occurs, the single crystallization rate is lowered.

  As for the method using a crystallization accelerator, a coating layer or a solid solution layer containing a group 2a element or a group 3b element such as barium as a crystallization accelerator is formed within a thickness of 1 mm on the inner surface of the quartz glass crucible. A method (Patent Document 9) has already been proposed, and as a method developed further, a method of doping an inner surface layer of 0.2 to 1.2 mm with a crystallization accelerator at a concentration of 50 to 200 ppm is also available. (Patent Document 10-11). In this proposal, a method of crystallizing the surface before silicon melting by doping barium on the inner surface and a method of suppressing crystallization by doping aluminum or titanium on the inner surface are proposed together. . However, in the former case, even if 50 to 200 ppm of barium is contained in the inner surface of the quartz crucible, the inner surface of the quartz crucible is not uniformly crystallized before the silicon is melted. Large diameter crucibles crystallize because the temperature rise time is long and the melting temperature is high, but small diameter crucibles often do not crystallize even when containing 50 to 200 ppm of barium. On the other hand, since the method described in Patent Documents 6-7 has a considerably high concentration, the inner surface is surely crystallized even with a small-diameter quartz crucible. Regarding a quartz crucible having an inner surface layer of 0.2 to 1.2 mm containing 50 to 200 ppm of aluminum or titanium, there is a question that the crystallization rate can basically be suppressed by aluminum or titanium. Conversely, when aluminum is added, the viscosity decreases and crystallization is promoted. When aluminum is mixed into silicon, it is taken into the silicon single crystal, and the resistance value of the silicon single crystal is significantly reduced. Since titanium enters the silica network like silicon and becomes an element unrelated to crystallization, crystallization cannot be slowed down.

  As described above, examinations and attempts have been made from various viewpoints, but high quality silicon single crystals have not necessarily been successfully formed at a high yield.

Although the method of crystallizing the inner surface of a quartz crucible has been considered promising among the conventional techniques, it is very important to control the concentration and distribution of the crystal accelerator when a large-diameter silicon ingot is pulled up. There is a problem that it is difficult. In addition, when a method of making it difficult to crystallize, such as synthetic quartz glass, there is a problem of liquid level vibration and a problem of cost.
U.S. Pat. No. 4,416,680 US Pat. No. 4,632,686 Japanese Patent Laid-Open No. 1-148818 JP-A-3-17817 JP 2002-154894 A US Pat. No. 5,976,247 Japanese Patent No. 3054362 US Pat. No. 6,461,427 Japanese Patent No. 3100836 US Pat. No. 6,641,663 Japanese Patent Laid-Open No. 2003-956783

  The present invention, based on the background as described above, eliminates conventional problems, suppresses the separation of crystal pieces from the inner surface of the quartz crucible, which lowers the yield of single crystallization, and enables single crystallization even when used for a long time. It is an object to provide a new quartz crucible for pulling a silicon single crystal with a good yield.

  In the present invention, first, in order to improve the yield of a silicon single crystal even when used for a long time, a layer within 150 μm from the surface of the quartz crucible and a layer having a higher reactivity with the silicon melt than the inner layer. It is characterized by being a quartz crucible for pulling up a silicon single crystal. Second, the layer having high reactivity with the silicon melt has a thickness within 50 μm from the surface of the quartz crucible, and third, the inner layer having high reactivity with the silicon melt is synthesized. It is characterized by being quartz glass or natural quartz glass containing alkali metals such as lithium, sodium and potassium and alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium at 100 ppm or less.

  According to the present invention, in the above-described quartz crucible, fourthly, the layer within 50 μm from the inner surface is a layer of natural quartz glass containing one or more alkaline earth metals in a range of 30 to 80 ppm. Fifth, the natural quartz glass layer is characterized by containing barium as an alkaline earth metal in the range of 40 to 60 ppm.

  The features as described above are based on the following findings obtained by the intensive studies of the present inventors.

  That is, first, the cristobalite piece peeled off from the surface of the quartz crucible is generated on the inner surface of the crucible immediately after contact with the silicon melt. The number of crystal nuclei does not change much over time. The cause of this crystal nucleus is presumed to be due to the impurities and structure in the quartz glass, but the clear cause is unknown. These crystal nuclei grow while in contact with the silicon melt. At the same time, the crystal nuclei are dissolved by reaction with the silicon melt, so that a ring-like pattern called a brown mark is formed by visual observation. When the number of brown marks increases, the number of the brown core grows with time. The central nucleus begins to peel off, and when this crystal piece adheres to the end of the silicon single crystal, it is polycrystallized to reduce the yield of the single crystal.

  As a result of intensive studies on this phenomenon, the present inventors have found that when the surface of the quartz crucible easily reacts with the silicon melt, the surface of the crucible is dissolved, and the number of brown marks is reduced. The present invention was completed by knowing that no peeling occurred.

  According to the quartz crucible of the present invention as described above, it is possible to suppress the exfoliation of crystal pieces from the inner surface of the quartz crucible and to improve the yield of silicon single crystallization by pulling up even after long use. Can do. That is, by forming a layer having a higher reactivity with the silicon melt than the inner layer in a layer within 150 μm from the surface of the quartz crucible, and more preferably within 50 μm, the rate of erosion is reduced rather than the generation of crystal nuclei. The number of brown marks that cause peeling can be reduced by increasing the speed. Specifically, the inner surface layer having high reactivity with the silicon melt is synthetic quartz glass, or alkali metals such as lithium, sodium, and potassium, and alkaline earth metals such as beryllium, magnesium, calcium, strontium, and barium. Is natural quartz glass containing 100 ppm or less. In this quartz crucible, the number of brown marks generated on the surface of the crucible is extremely small, and the inner surface of the brown mark is smooth, so that the silicon single crystallization yield can be greatly improved.

  In the present invention, as described above, the ultrathin layer on the inner surface of the quartz crucible and the layer having a thickness of 150 μm or less from the inner surface are more easily reacted with the silicon melt than the inner layer. This layer is preferably synthetic quartz glass or natural quartz glass containing alkali metal such as lithium, sodium and potassium and alkaline earth metal such as beryllium, magnesium, calcium, strontium and barium at 100 ppm or less.

  Synthetic quartz glass has an irregular structure and a large number of Si-O multi-membered rings, so that the reaction rate with the silicon melt is high. Therefore, the 150 μm layer on the inner surface dissolves in a short time. At that time, the crystal nuclei on the inner surface of the crucible generated by contact with the silicon melt dissolve and disappear.

  All the elements that enter the silica network reduce the viscosity of the quartz glass, so that the reaction rate with the silicon melt can be increased. That is, the same effect can be obtained even with alkali metals such as lithium, sodium and potassium and alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium.

  When the thickness of the layer having a high reaction speed with the silicon melt is within 150 μm, the thickness exceeds 150 μm. When the thickness exceeds 150 μm, alkali metals such as lithium, sodium and potassium and alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium This is because, in the case of natural quartz glass containing, the generation rate of crystal nuclei exceeds the melting rate and the number of crystal nuclei increases. Further, in the case of synthetic quartz glass, the liquid surface vibration cannot be stopped and the crystal seed cannot be necked. Considering that crystallization progresses as the thickness of the layer increases, the thickness of the layer having high reactivity with the silicon melt is preferably within 50 μm. The lower limit can be appropriately determined as a thickness for realizing the object of the present invention and a desired effect, for example, 5 μm or more from the viewpoint of formation.

  When the layer having a high reaction rate with the silicon melt is a natural quartz glass containing an alkali metal such as lithium, sodium or potassium and an alkaline earth metal such as beryllium, magnesium, calcium, strontium or barium, the concentration of each layer It is necessary to change the concentration depending on the difference in crystallization speed. In either case, it is desirable that the concentration be 100 ppm or less in the present invention. Furthermore, in the case of alkali metals such as lithium, sodium, and potassium, it is desirable that the content be 40 ppm or less, and further 10 ppm or less. In the case of alkaline earth metals such as beryllium, magnesium, calcium, strontium, and barium, it is preferably considered that the thickness of the above layer is within 50 μm and is in the range of 30-80 ppm. Further, when the alkaline earth metal is barium or strontium, the range of 40 to 60 ppm, particularly less than 50 ppm is desirable.

  When these contents are large, the devitrification due to the brown mark is large and cristobalite (crystal) is covered. This is because when the content is low, the crystallization rate is slow, the dissolution of the layer proceeds, and the crystal nucleus (Brown ring center) disappears. On the other hand, when the content is high, the crystallinity is fast and before dissolution. This is because the solubility is further lowered by crystallization. And, as crystallization progresses, defects due to peeling increase.

  Barium as an alkaline earth metal to be added has particularly remarkable effects in the present invention.

  The OH group also lowers the viscosity of quartz glass, so the reaction rate with the silicon melt is increased. However, since the diffusion rate of hydrogen atoms is fast and it is difficult to fix only to the crucible surface layer, the inner surface layer of the crucible is dissolved. Since it becomes large, it is not preferable.

  A method of forming a synthetic quartz glass layer on a layer of 150 μm or less on the inner surface of the quartz crucible or a method of forming a layer containing an alkali metal or alkaline earth metal can be performed by a conventionally known method.

  For example, natural quartz powder is first formed in a rotating mold before melting, and natural quartz powder containing synthetic powder, alkali metal, or alkaline earth metal is formed on the surface and melted. Moreover, you may sprinkle the quartz powder containing synthetic powder, an alkali metal, and an alkaline-earth metal during melting.

  An embodiment will be described next. Of course, the present invention is not limited to this embodiment.

<Example 1>
Into a rotating graphite mold with an inner diameter of 574 mm, 22 kg of IOTA-4 manufactured by Unimin Co., Ltd. was charged, and 60 g of synthetic quartz powder was uniformly formed on the inner surface. A voltage of 80 to 100 V was applied to a 2.0 inch graphite electrode, and a current of 2,000 to 2,500 A was passed. After 20 minutes, melting was finished, and a predetermined quartz crucible of 22 inches was manufactured by a known process. On the inner surface of the crucible, a reactive glass layer made of synthetic quartz powder is formed with a thickness of 50 μm.
<Example 2>
Into a rotating graphite mold having an inner diameter of 574 mm, 22 kg of IOTA-4 manufactured by Unimin Co., Ltd. was charged, and 60 g of natural quartz powder containing 40 ppm of barium uniformly was formed on the inner surface. A voltage of 80 to 100 V was applied to a 2.0 inch graphite electrode, and a current of 2,000 to 2,500 A was passed. After 20 minutes, melting was finished, and a predetermined quartz crucible of 22 inches was manufactured by a known process. On the inner surface of the crucible, a natural quartz glass layer containing 40 ppm of barium is formed with a thickness of 50 μm.
<Comparative Example 1>
22 kg of IOTA-4 manufactured by Unimin Co., Ltd. was put into a rotating mold having an inner diameter of 574 mm, a voltage of 80 to 100 V was applied to a 2.0 inch graphite electrode, and a current of 2,000 to 2,500 A was passed. 20 minutes after the start of melting, the melting was terminated and the crucible was taken out.
<Example 3>
100 kg of polysilicon was put into the crucibles of Examples 1 and 2 and the crucible of Comparative Example 1, and the 8-inch silicon single crystal was pulled up. After 40 hours, the pulled silicon single crystal was redissolved, and 8 inches of silicon was pulled again. This was repeated, and a total of 3 wires were raised for 120 hours. The single crystallization ratio is shown in Table 1. This single crystallization yield is not a value relative to the input weight of polycrystalline silicon but a value when the planned yield is 100%. That is, in this case, the single crystal weight ratio is 70 kg for the first, 65 kg for the second, and 80 kg for the third.

From the results in Table 1, it can be seen that the quartz crucibles of Example 1 and Example 2 are excellent in single crystal ratio. The single crystallization rate of the quartz crucible of Comparative Example 1 is extremely worse from the second.
<Example 4>
In the same manner as in Example 3, 100 kg of polysilicon was put into the crucibles of Example 2 and Comparative Example 1, and an 8-inch silicon single crystal was pulled up. After 40 hours, the quartz crucible surface was cooled and the surface roughness was measured. The number of brown marks and the surface roughness results are shown in FIGS. FIG. 2 also shows a micrograph of the surface of the white crystal.

It can be seen that the number of brown marks on the inner surface of the crucible is extremely reduced in Example 2 as compared with Comparative Example 1. Also, from the surface roughness results, it can be seen that the surface of the quartz crucible of Example 2 is smooth and no peeling occurs, whereas the surface of the crucible of Comparative Example 1 clearly shows a peeling.
<Example 5>
A barium-containing natural quartz glass layer was formed in the same manner as in Example 2. In this case, the relationship between the barium concentration, the layer thickness, and the number of brown marks (BR) was evaluated. The results are shown in Tables 2 and 3.

From the results of Tables 2 and 3, it is understood that when barium is used, the best results are obtained when the barium concentration is 30 ppm to less than 50 ppm and the layer thickness is 50 μm or less.

  Further, from the viewpoint of suppressing the number of BR to 10 or less, it can be seen that a barium concentration of 10 to 100 ppm and a layer thickness of 150 μm or less are desirable.

  FIG. 3 shows micrographs of the surface of white crystals with barium concentrations of 40 ppm (A), 100 ppm (B), and 200 ppm (C) when the layer thickness is 50 μm.

  A quartz crucible for pulling a silicon single crystal, which has a very high single crystal yield and low manufacturing cost, can be provided when a silicon single crystal ingot serving as a base of a silicon substrate used as an element such as a semiconductor or a solar cell is manufactured.

It is a comparison figure regarding the number of the brown marks of the quartz crucible produced in Example 2 and Comparative Example 1. It is the comparison figure and microscope picture regarding the roughness of the inner surface of the quartz crucible produced in Example 2 and Comparative Example 1. 6 is a photomicrograph showing an example in Example 5.

Claims (5)

  A quartz crucible for pulling a silicon single crystal formed by forming a layer having a higher reactivity with a silicon melt than an inner layer in a layer within 150 μm from the surface of the quartz crucible.   The quartz crucible for pulling a silicon single crystal according to claim 1, wherein the layer having high reactivity with the silicon melt has a thickness within 50 μm from the surface of the quartz crucible.   The inner surface layer having high reactivity with the silicon melt is synthetic quartz glass, or one or more alkali metals such as lithium, sodium and potassium and alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium. A quartz crucible for pulling a silicon single crystal according to claim 1 or 2, wherein the quartz crucible is natural quartz glass containing 100 ppm or less.   4. The quartz crucible for pulling up a silicon single crystal according to claim 3, wherein the layer within 50 [mu] m from the surface of the quartz crucible is a layer of natural quartz glass containing at least one alkaline earth metal in a range of 30 to 80 ppm.   The quartz crucible for pulling up a silicon single crystal according to claim 4, wherein the natural quartz glass layer contains barium in the range of 40 to 60 ppm as an alkaline earth metal.