Composite crucible, and preparation method

BACKGROUND OP THE INVENTION

1. Field of the Invention

The present invention relates to a composite crucible to be used for pulling up monocrystalline silicon (single crystal silicon ). More specifically, the present invention relates to a composite crucible having superior thermal resistance suitable for pulling up large-size monocrystalline silicon in which a quartz glass as an inner layer and a carbonaceous material as an outer layer are integrated to form a composite structure, and to a preparation method thereof.

2. Description of the Prior Art

Monocrystalline silicon namely single crystal silicone for semiconductor materials is prepared generally by the Czochralski method in which monocrystalline silicon is pulled up from molten polycrystalline silicon . In the method, a quartz glass crucible is used as a material for a vessel for holding the molten polycrystalline silicon therein.

Recently, there has been required a large-size crucible in accordance with the requirements of large-sized wafers. Using a large-size crucible for pulling up large-size monocrystalline silicon , the period for pulling up monocrystalline silicon becomes longer than that in the conventional ones. In addition, the used temperature is higher than the conventional one, thus particularly, the thermal resistance thereof becomes a problem. There may be considered a counter measure to increase strength by thickening the glass crucible, but the thickness of the glass crucible is limited due to its production procedure. Thus, if the tendency for a large size increases, the mechanical strength will become insufficient in crucibles of conventional structure, which causes deformation of the crucible when used at a high temperature, or which may cause a fracture when handled.

Generally, the quartz glass crucible is used by fitting to a crucible-form susceptor , that is, an outer crucible mounted on a pulling machine. This susceptor is made of carbon. Carbon is used as a thermally resistant material having a high thermal conductivity. However, since there are some partial gaps between the quartz glass crucible and the carbon susceptor to cause loose fitting thereof in the conventional ones, some problems such as swelling, inside falling over or sinking down of the quartz glass crucible wall may be caused. Further,if the fitting between the quartz glass crucible and the carbon susceptor is poor, the coefficient of thermal conductivity is lowered and conduction of heat becomes non-uniform

Further, the crucible itself is required to be large in size for pulling up large monocrystalline silicon , which is in turn inevitably accompanied with a large-size carbon susceptor. Thus, in order to make assembling thereof easy, a divided type has been used. Such divided type susceptor has such a problem that gaps are formed at seams, which causes fracture of the quartz glass crucible, or the like.

Further, since the quartz glass crucible should be fitted to the carbon susceptor every time when used in the conventional method, the operation thereof is troublesome and also contamination may be easily produced at a contacting face of the quartz glass crucible and the carbon susceptor, which may cause non-uniform heat conduction.

On the other hand, there have been proposed several susceptors in which the conventional graphite material is substituted with carbon fiber reinforced carbon (Japanese Patent Application Laid-Open No. Hei 09-263482, etc.) as the improved carbon susceptors. These susceptors have similar problems to the above-mentioned ones, since they are structured as separate bodies from the quartz glass crucibles and they are used after being fitted to the quartz glass crucible.

As disclosed in JP-A-4-21586 quartz glass crucibles have been prepared by melting and shaping quartz glass in a carbon crucible as a mold. After the production the glass crucible formed is removed from the mold and the product obtained is a crucible containing of a single layer of quartz glass.

JP-A-2-188489 discloses a method for regenerating a quartz glass crucible in which protrusions on the inner surface of the quartz crucible are removed by mechanical grinding or chemical etching and the periphery of the ground part or etched part is heat treated and smoothed. The crucible to be regenerated is a single layer quartz glass crucible.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-mentioned problems inherent in the conventional quartz glass crucibles. Therefore it is the object of the invention to provide a crucible for pulling up monocrystalline silicon with high reliability, and to provide a preparation method, in which the crucible has superior mechanical strength, and thus is suitable as a large-size crucible without any deformation when used at a high temperature, and avoids any problem due to insufficient adhesion between the susceptor and the quartz glass crucible since a conventional carbon susceptor is not necessarily used. Thus a high single crystallization yield can be attained.

That is, the present invention relates to a composite crucible for pulling up monocrystalline silicon, comprising an inner layer made of quartz glass, and an outer layer made of a carbonaceous material, wherein the carbonaceous material of the outer layer and the quartz glass of the inner layer are integrally formed, and wherein the outer layer comprises a laminate of carbon fibers having an average roughness of the surface in contact with the quartz glass in the inner layer, of 0.1 to 1.0 mm and an average distance between concave and convex parts of 0.2-5 mm.

In a preferred embodiment the outer layer comprises a laminate of carbon fibers having a gas permeability of 0.5 cm/sec or more.

Also, the present invention relates to a method for preparing a composite crucible for pulling up monocrystalline silicon having an inner quartz glass layer and an outer layer of a laminate of carbon fibers, the.method comprising: <ul id="ul0001" list-style="none" compact="compact"><li>providing a carbon fiber crucible as an outer circumferential part, said carbon fiber crucible comprising a laminate of carbon fibers having an average roughness of the surface in contact with the quartz glass of the inner layer of 0.1 to 1.0 mm and an average distance between concave and convex parts of 0.2 to 5 mm, supplying quartz powders to the inner surface of the carbon crucible while rotating it to be heated and melted; and</li><li>vitrifying the quartz layer up to the inner surface of the carbon crucible to integrally form the quartz glass layer and the carbon crucible layer, thereby obtaining the composite crucible having a carbon fiber outer layer integrally formed with a quartz glass inner layer.</li></ul>

The preparation method as to said method comprises the steps of: depositing quartz powders on the inner surface of the carbon crucible to be heated and melted; and supplying quartz powders gradually under heating to deposit a melted quartz powder on the quartz layer, thereby obtaining a composite crucible having a transparent glass layer formed in the inner surface thereof.

Further, the preparation method as to said method comprising the steps of: using a carbon fiber crucible having gas permeability as an outer circumferential part; depositing quartz powders on an inner surface of the carbon crucible while rotating it to be heated and fused; reducing the pressure in the crucible through the carbon crucible upon the heating and fusing; sucking and removing the gas contained in the quartz layer; and extruding bubbles contained in the quartz glass layer to form a transparent glass layer on an inner surface of the carbon crucible, thereby obtaining the composite crucible in which the quartz glass layer and the carbon crucible layer are integrally formed.

BRIEF DESCRIPTION OF THE DRAWING

Fig.1 is a longitudinal cross-sectional schematic view showing a composite crucible according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is described concretely with reference to a constituting example illustrated in the drawing.

(I)Composite crucible

Fig.1 is a longitudinal cross-sectional schematic view showing a composite crucible according to the present invention. The composite crucible according to the present invention is a crucible used for pulling up monocrystalline silicon which comprises a carbonaceous material as an outer layer 10 and a quartz glass as an inner layer 20 integrally formed with the carbonaceous material layer 10.

The carbonaceous material forming the outer layer of the composite crucible according to the present invention means a solid carbon material such as graphite and carbon fiber. The carbonaceous material is superior in thermal resistance and has material strength at a high temperature. Therefore, a composite crucible with superior shape stability at a high temperature can be obtained by forming the outer layer of the crucible with carbonaceous material. Therefore, it is unnecessary to use the conventional carbon susceptor. Further, the thickness of the carbonaceous material to form the outer layer is not limited. It may be of any thickness if it is sufficient for maintaining strength of the crucible when used at a high temperature.

As the above-mentioned carbonaceous material, there can be used graphite material molded in a crucible shape, but it is preferable to use laminated carbon fibers or the like molded in a crucible shape. For example, carbon fiber reinforced carbon (CFRC, C-C composite) or the like is preferable in which carbon fibers are blended into matrix carbon or carbon fibers, then hardened with carbon. As the carbon fiber reinforced carbons, there are known those prepared by different preparation methods such as a filament winding method and a laminating method with carbon fiber cloth or felt. Any carbon prepared by any method can be used, and any combination thereof may also be used.

Since the above-mentioned CFRC materials, etc. have a low coefficient of thermal expansion close to that of quartz glass, both are not peeled off by change in the temperature. Further, CFRC materials etc. are lighter than pure carbon materials such as graphite, the former ones are superior in strength and abrasion resistance, etc. Therefore, a composite crucible with high strength, light weight and superior abrasion resistance can be obtained without any peeling off between the outer layer and the inner layer, by forming the outer layer from such carbon fiber reinforced carbon. Further, junction strength can be increased by using a roughly woven cloth at the junction surface between the outer layer and the inner layer.

In concrete terms, it is appropriate that an average roughness (an arithmetic average roughness) of the surface jointed with the quartz glass of the inner layer being 0.1-1.0 mm and that an average distance of concave and convex parts being 0.2-5 mm. If the surface roughness being so, melted glass can enter among carbon fibers to obtain a composite crucible having high joint strength.

Further, bubbles contained in the melted glass layer to form the inner layer of the crucible can be sucked and removed into the outer layer by reducing pressure via the outer layer of the carbonaceous material during preparation of the crucible by using a carbonaceous material having gas permeability as an outer layer of the composite crucible. Since the above-mentioned carbon fiber reinforced carbon (CFRC) material can have appropriate gas permeability, it is also suitable for such fusing under the reduced pressure. Thus, gas permeability of the fibrous composite carbon material obtained by laminating carbon fibers, impregnating carbon or resinous binder in fibers and sintering integrally can be controlled when the filling amounts of carbon and resin binders being adjusted.

In concrete terms, the preferable gas permeability of carbon fibers in the present invention is 0.5 cm/sec or more. Carbonaceous materials having bulk specific gravity of 1.3-1.6 are preferable. Herein, the above-mentioned gas permeability is expressed by a value of gas amount passed through the material (carbon fiber) per unit time (cm³/cm²/sec = cm/sec), when air pressure of 1 kg/cm² is applied at 0°C and 1 atom.

The quartz glass layer in the crucible inner layer can be formed by accumulating quartz powders on the inner circumferential surface of the carbonaceous material in a crucible form (referred to as carbon crucible) at a specified thickness, then heating and fusing to vitrify. Since the surface of the quartz glass layer is contacted with molten silicon , it is preferable to use quartz powders with high purity containing low impurities as possible in order to prevent mixing of impurities into melted silicon . Further, if there are much bubbles in the quartz glass layer at the inner surface side of the crucible, single crystallization of silicon may be prevented during pulling up, which may cause polycrystallization. Thus, it is preferable to remove bubbles near the surface as possible. In order to remove bubbles at the inner surface side of the crucible, there may be used the following methods: (i) to supply quartz powders gradually with heating and make accumulation on the inner surface of the crucible with vitrification; or as described above (ii) to use a gas permeable carbon crucible having an outer layer of the carbon fibers, etc., to reduce the pressure via the outer layer during heating and fusing of quartz powders and to suck gas in the quartz layer and remove off outside.

For the conventional quartz glass crucible, the thickness of the total crucible wall is about 7-15 mm depending on an opening diameter. However, for the quartz glass crucible according to the present invention, the quartz glass layer in the inner layer can be made thinner than that in the conventional one since the outer layer of the quartz glass crucible according to the present invention is made of carbonaceous material with high strength. In concrete terms, the thickness of quartz glass in the inner layer may be decreased by about 1 mm or less when monocrystalline silicon is pulled. Therefore, the quartz glass layer may be about 2 ∼10mm in thickness, which is somewhat thicker than the loss of thickness, in order not to expose the outer layer of the carbonaceous material.

The above-mentioned composite crucible according to the present invention is superior in shape stability at a high temperature without any trouble such as deformation of the crucible when used at a high temperature, since the outer layer is formed integrally with the quartz glass layer of the inner layer by means of the carbon material such as carbon fiber reinforced carbon. Further, it is suitable for a large-size crucible, since it is superior in shape stability.

Further, scattering in coefficient of thermal conductivity in the composite crucible according to the present invention is decreased as well as single crystallization rate and quality of single crystal can be increased, since the conventional carbon susceptor is not necessary and the quartz glass layer is fused integrally with carbonaceous material.

Further, the production cost of the crucible can be much decreased, since the quartz glass layer at the inner circumferential side in the crucible can be formed much thinner than that of the conventional one and the quartz glass layer can be regenerated easily.

(II) Preparation method

For preparing the composite crucible according to the present invention by a rotating molding method, a carbon crucible to form the outer layer can be utilized as a molding mold. That is, quartz powders forming the inner layer are introduced into a rotating carbon crucible that has been made in a crucible form from carbon fibers (CFRC), etc. and maintained at an inner circumferential surface of the carbon crucible by utilizing centrifugal force. Subsequently it is heated and fused by arc discharge or the like from the inner surface side of the crucible as a heat source to fuse up to the inner surface of the carbon crucible at which quartz powders being contacted and to vitrify. Since the CFRC material used to form the carbon crucible has fine concave and convex parts due to fibers on the surface, vitrified quartz enters into fine gaps on the carbon crucible surface, thus to obtain a composite crucible having the quartz glass layer and the carbon crucible firmly integrated upon cooling of the quartz glass. As described above concretely, it is appropriate that the average roughness is 0.1-1.0 mm and the distance between concave and convex parts is 0.2-5 mm.

In the above-mentioned preparation method with using the carbon crucible made of carbon fibers, the composite crucible having the formed transparent glass layer almost without bubble can be prepared by supplying quartz powders gradually with heating, fusing to vitrify and then accumulating on the inner surface of crucible while the carbon crucible is rotated.

Alternatively, bubbles in the quartz layer can be removed by depositing quartz powders in uniform thickness on the inner surface of the rotating carbon crucible to heat and melt, and reducing the pressure and sucking to remove gas contained in the quartz layer to the outside via the carbon crucible. Heating temperature and time, as well as pressure decreasing amount and time can be determined according to the preparation conditions such as the opening diameter of the crucible, quality of quartz powders, accumulation thickness and bubble content rate.

(III) Regenerating method

Since the outer layer of the above-mentioned composite crucible according to the present invention is made of the carbonaceous material such as CFRC, the quartz glass part of the inner layer can be easily regenerated. In concrete terms, for example, in the case that there are damages, fine concave and convex parts or cracks or the like on the inner surface of the prepared composite crucible or in the case that the inner surface of the crucible is melted and lost by molten silicone when the used quartz glass crucible loses smoothness of the inner surface, the composite crucible according to the present invention can be regenerated by heating the crucible from its inner surface side to melt again the surface of the quartz glass layer and to embed fused glass in the damages or cracks on the inner surface.

Additionally, the quartz glass layer in the inner layer can be regenerated by substituting it with a fresh one totally. That is, the total quartz glass layer is removed by applying impact on the composite crucible and fresh quartz powders are accumulated on the inner surface of the remaining carbon crucible, heating and fusing it up to the inner surface of the carbon crucible which is contacted by quartz powders. Thereby, it is vitrified, thus the quartz glass layer and the carbon crucible layer are integrated to regenerate the composite crucible.

EXAMPLES

Example 1

Quartz powders were accumulated on an inner surface of a rotating carbon crucible made of carbon fiber reinforced carbon (CFRC material) (roughness of the inner surface: 0.3mm, average distance of concave and convex parts; 3mm). Then, they were heated at about 2000° C by arc discharge. After 5 minutes, synthetic quartz powders were gradually supplied for 10 minutes to the quartz glass layer with heating of arc discharge. Thereby, a composite crucible was obtained, in which the bubble-containing quartz glass layer having a thickness of about 3mm on the inner side of the carbon outer layer and the transparent glass layer without bubble having a thickness of about 1.5mm on the upper side are integrated in the carbon crucible.

After the quartz glass crucible was heated at about the pulling up temperature of a silicon single crystal (about 1500°C) for 24 hours, it was subjected to a cooling test 5 times. Thereby, any peeling off was not observed between the outer layer and the inner layer. Further, when monocrystalline silicon was pulled by using the composite crucible, almost all amount of the melt could be pulled in single crystallized form, and the quality of monocrystalline silicon was good up to tail, to obtain more superior results than in the case of a conventional quartz glass crucible.

Example 2

Quartz powders were accumulated on the inner surf ace of a rotating carbon crucible made of carbon fiber reinforced carbon (gas permeability; 1.8 cm/sec, bulk specific gravity; 1.45) and heated at about 2,000°C for 10 minutes by arc discharge from the surface side of the quartz powders, to melt and vitrify the total layer of the quartz powders contacting the inner surface of the carbon crucible. Further, the pressure on the outer surface of the carbon crucible was decreased to 0.2atm. and maintained for 5 minutes. After heating, the crucible was allowed to cool in still, to obtain a composite crucible (inner diameter; 580mm) in which the outer layer (thickness: 5mm) was made of carbon fiber reinforced carbon (CFRC material), the inner layer (thickness: 6mm) was made of a transparent quartz glass, and both were integrated. The bubble content rate in an about 1 mm thickness inner surface side in the quartz glass layer was 0.01%.

After the quartz glass crucible was heated at about the pulling up temperature of a silicon single crystal (about 1500°C) for 24 hours, it was subjected to a cooling test by leaving at room temperature 5 times, by which any peeling off was not observed between the outer layer and the inner layer. Further, monocrystalline silicon was pulled by using the composite crucible, almost all amount of the melt could be pulled in single crystallized forms, and the quality of monocrystalline silicon was good up to tail, to obtain more superior results than in the case of a conventional quartz glass crucible.

Example 3

A composite crucible was prepared under similar conditions to Example 2 except that carbon fibers having gas permeability of 0.6 cm/sec, an average roughness of 0.5 mm and an average distance between concave and convex parts of 2 mm were used for the part contacted with the quartz glass of the inner layer as the carbon fibers of the outer layer. In the composite crucible, any glass was not peeled off from the fiber layer even though it had fallen from a height of 20 cm. Further,the single crystallization rate was higher than that of the conventional quartz glass crucible.

On the other hand, when a composite crucible was prepared similarly to Example 2 except that carbon fibers having an average roughness of 0.01 mm and gas permeability of 0.4 cm/sec instead of the above-mentioned carbon fibers as a comparative sample, bubbles in the quartz glass layer were not sucked sufficiently because of low gas permeability of carbon fibers, thus the bubbles remained in the quartz glass layer. Therefore, the single crystallization rate was lower than in Example 2. Further, the quartz glass layer and the carbon fiber layer in the composite crucible were peeled off when it was fallen from a height of 20 cm.