WO2011147860A1

WO2011147860A1 - Quartz glass crucible and method for producing same

Info

Publication number: WO2011147860A1
Application number: PCT/EP2011/058530
Authority: WO
Inventors: Walter Lehmann; Thomas Kayser
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg
Priority date: 2010-05-27
Filing date: 2011-05-25
Publication date: 2011-12-01
Also published as: DE102010021694A1; DE112011101800A5; CN102906037B; CN102906037A; JP5639264B2; DE112011101800B4; JP2013527113A

Abstract

In a known method for producing a quartz glass crucible, a vacuum fusion mould is provided, on the inside of which a crucible-shaped granular layer made of SiO₂ granules is formed, which layer has a base region and a side wall region. A skin made of low-bubble quartz glass is formed on at least one part of the porous granular layer. Gaseous components are removed from at least one part of the granular layer adjoining the skin by applying a vacuum, and the granular layer is vitrified, forming the quartz glass crucible having a crucible height H. In order to, proceeding therefrom, produce a quartz glass crucible which makes the preparation process easier and in which the risk of contamination by impurities in the silicon melt is low, according to the invention a bubble zone, which contains gas-filled bubbles having a specific bubble volume that is at least twice the specific volume of gas-filled bubbles in low-bubble quartz glass, is produced under the skin and adjoining same in an upper region of the granular layer which adjoins the lower region and extends to the full height H during the vitrification process by applying the vacuum in only or predominantly in a lower region of the granular layer which extends from the base region to no more than 0.8 times the crucible height H.\

Description

Quartz glass crucibles and process for its production

The invention relates to a quartz glass crucible for pulling single crystals, having a crucible height H and having an inner side crucible wall formed by a bottom and a quartz glass connected to the bottom side wall, the inside at least partially of a skin layer of dense quartz glass is covered. Furthermore, the invention relates to a method for producing a quartz glass crucible for pulling single crystals, comprising the following method steps:

(a) providing a vacuum melt mold having a wall having an inside, an outside, and through holes between outside and inside,

(b) forming a garnet-shaped porous granulation layer of SiO ₂ grain on the inside of the vacuum melt mold, the granulation layer having a bottom portion and a sidewall portion,

(c) forming a skin layer of low-bubble fused quartz glass on at least a portion of the porous graining layer; (d) removing gaseous components from at least a portion of the graining layer adjacent the skin layer by applying a negative pressure across the outside of the melt-forming wall;

(e) vitrifying the porous graining layer to form the quartz glass crucible having a crucible height H.

State of the art

Quartz glass crucibles are used to hold a silicon melt when pulling silicon single crystals by the so-called Czochralski method. In this process, polycrystalline, metallic silicon in the quartz glass Gel melted and brought a seed crystal of silicon monocrystalline from above to the enamel surface, so that forms enamel meniscus between crystal and melt. The monocrystal is slowly pulled up while rotating the crucible, whereby the silicon monocrystal grows on the seed crystal. This process is referred to below as the "startup process" or shortly as "startup".

The quartz glass crucibles are usually designed with a transparent inner layer on a pore-containing, opaque outer layer. The transparent inner layer is in contact with the silicon melt during the crystal pulling process and is subject to high mechanical, chemical and thermal loads. In order to reduce the corrosive attack of the silicon melt and thus to minimize the release of impurities from the crucible wall, the inner layer is as homogeneous as possible and low in bubbles.

To improve freedom from bubbles, crucible production methods with vacuum-assisted formation of the inner layer are known. In this case, a vacuum melting mold is used, the wall of which has through holes, which is thus porous or provided with a plurality of through holes, so that upon application of a negative pressure on the outside of the melt, gases can be withdrawn from the SiO ₂ grain layer to the outside. Such a vacuum manufacturing method is known from the unpublished EP 2 236 469 A1. Therein a quartz glass crucible with bubble-free inner layer of indefinite thickness and bubble-containing outer layer is described. Bubble content and bubble size of the outer layer increase upward from the bottom of the crucible via the sidewall. The transition between the area containing the high bubbles and the areas containing the low bubbles is not exactly defined, the embodiment schematically showing a division approximately at the center of the crucible. The production of the bubble-free inner layer is carried out by applying a vacuum. For the generation of the different bubble contents, the use of different quartz glass grains as starting material is proposed. Due to the higher bubble content of the upper wall section is the Reduced weight, so that there is a lower deformation of the quartz glass crucible when used as intended.

A vacuum manufacturing method according to the aforementioned type is known from DE 10 2008 030 310 B3. In this case, an outer grain layer of relatively coarse-grained quartz grain is first produced on the inner wall of the melt. On this another SiO ₂ -grain layer of finely divided, synthetically produced SiO ₂ powder is applied. The granulation layers are then heated under the action of an arc from the inner wall, thereby sintering the SiO ₂ granular layers into the quartz glass crucible with an opaque outer layer and a transparent inner layer. The finely divided grain acts as a mechanical barrier layer by hindering the suction of atmosphere from the crucible interior when applying the vacuum on the outer mold wall, so that forms a dense, glassy sealing layer quickly and without local inhomogeneities, the rapid application allows a stronger vacuum.

The inner layer of synthetically produced quartz glass ensures a low concentration of impurities in the area close to the melting point and has a favorable effect on the yield of pure and dislocation-free silicon monocrystal. It has been found, however, that crucibles having an inner layer of synthetic quartz glass tend to cause oscillations of the enamel surface compared to quartz glass crucibles made of naturally occurring quartz sand. Such oscillations may be caused or enhanced, for example, by the rotation of the melt and seed crystal or by the immersion of the seed crystal. They are especially detrimental to the piecing process by making it difficult, delaying or even preventing nucleation. This reduces the productivity and can go so far that the service life of the quartz glass crucible is already exceeded during the piecing process, or that dislocations are generated in the single crystal, which require a re-melting of the solidified silicon. In order to reduce oscillations in the enamel surface, EP 1 532 297 A1 proposes a quartz glass crucible which has a transparent inner layer made of synthetic quartz glass but which is interrupted at a height of the melt level at the beginning of the crystal pulling process by a transparent bubble zone of naturally occurring quartz glass is. This bubble zone extends in a range of at least 0.5 x H to 0.8 x H, where H represents the crucible height between the bottom of the bottom and the sidewall top edge.

The circumferential side wall region of the quartz glass crucible, which is at the level of the melt level at the beginning of the crystal pulling process, is also referred to below as the "attachment zone".

In another crucible according to EP 1 045 046 A1, which contributes to the reduction of oscillations in the enamel surface, it is provided that the inner wall in the region of the attachment zone is designed as a circumferential annular surface with a multiplicity of depressions. A similar quartz glass crucible is also known from JP 2004-250304 A. Here, a circumferential annular surface is provided for suppressing vibrations of the silicon melt, are contained in the bubbles with a volume fraction of 0.01 to 0.2%.

The roughened surface around the region of the attachment zone can assume all possible contact angles with respect to the silicon melt, which prevents in-phase wetting or non-wetting of the quartz glass surface and thus counteracts the occurrence of vibrations. On the other hand, for example, during cleaning operations, during transport or handling of the quartz glass crucible, contaminants may accumulate in the roughened area, which are released into the silicon melt during the crystal pulling process.

task

The invention has for its object to provide a quartz glass crucible, which facilitates the piecing process and in which the risk of contamination in the silicon melt is low. Furthermore, the invention has for its object to provide a method for the reproducible production of such a crucible.

General description of the invention

With regard to the method, this object is achieved on the basis of a method of the type mentioned above, that in an upper region of the granulation layer, which then extends to the lower part H to the full height, glazing below the skin layer and adjacent to this one Bubble zone is generated containing gas-filled bubbles with a specific bubble volume that is at least twice as large as the specific volume of gas-filled bubbles in low-bubble silica glass by applying the negative pressure in a lower region of the granulation layer, which extends from the bottom region to a maximum of 0, 8 times the crucible height H extends.

A circumferential bubble-containing zone (also referred to herein as a "bubble zone") is created in the side wall of the quartz glass crucible, which is at the level of the melt surface at the beginning of the single-crystal drawing process - ie in the region of the attachment zone Technically, the bubble-containing zone is covered by a - preferably thin - skin layer of dense quartz glass

Melt mold exposed to a high temperature atmosphere while glazing surface, or by on the graining layer, a thin glassy, dense layer is deposited. In the simplest case, the skin layer covers the entire inner wall of the granulation layer. The skin layer prevents impurities in rough surface areas, for example, during cleaning or further processing steps of the crucible, during transport or during installation in the crystal pulling device. The skin layer is so thin at least in the area of the attachment zone that, when the quartz glass crucible is used as intended, time is dissolved by the corrosive attack of the silicon melt, so that then the immediately adjacent bubble-containing zone of the side wall comes into direct contact with the surface of the silicon melt.

After exposure, the bubble zone shows the expected effect of reducing silicon melt oscillation. The bubble zone serves as a roughened surface area, which can take all possible contact angles with the melt and thus minimizes oscillations.

The bubble zone extends above and below the attachment zone. That is, it is located at a height of the quartz glass crucible sidewall that corresponds to the melt level height at the beginning of the crystal pulling process. This height is known before the beginning of the drawing process due to the inner volume of the quartz glass crucible and the filling volume of silicon melt.

The specific bubble content (bubble volume / cm ³ ) in the bubble zone is at least twice as high as in low-bubble quartz glass, as is the case, for example, in the skin layer. Bladder zone and low-bubble quartz glass are usually not directly adjacent to each other, but there is a smooth transition. In any case, the lower edge of the bubble zone is in the region of the crucible side wall, ie above the bottom. This upper bubble-containing partial area is also referred to here as the "upper area" of the side wall (or the grain layer), and the lower partial area as the "lower area" of the side wall (or the grain layer).

The bubble zone extends from its lower end, either over the entire remaining upper surface of the side wall or over only a part thereof. The thickness of the bubble zone - viewed in the radial direction - corresponds to either the wall thickness of the quartz glass crucible in this area (minus the skin zone) or a part thereof. As crucible height H - as usual - the distance between the bottom of the crucible bottom and the side wall top edge defined. Since the bubble zone has bubbles filled with gas, they can not disappear when the quartz glass crucible is heated.

The generation of the bubble zone initially covered by the skin layer is based on an inhomogeneous distribution of the suction over the crucible wall in the glazing process. In this case, the negative pressure is exclusively or predominantly applied in a lower region of the granulation layer, which is defined by extending from the bottom region to a maximum of 0.8 times the crucible height H.

In addition, several methods for producing the bubble zone are suitable.

In a first preferred variant of the method it is provided that the gas-filled bubbles are produced in the upper region of the granulation layer by supplying a gas to the upper region during or before vitrification of the granulation layer or by allowing an inflow of gas into the upper region ,

A gas is supplied to the upper portion of the granulation layer, whereas gases present in the lower portion are withdrawn due to the negative pressure treatment. The gas supply in the upper region of the granulation layer increases the gas content in this region, so that gas bubbles are produced underneath the skin layer during the simultaneous or subsequent glazing of this region. Suitable gases for the gas supply are, for example, nitrogen, oxygen, argon, mixtures of these gases or air.

To ensure continued gas supply, precautions are taken to prevent dense sintering of the upper edge of the granular layer during vitrification.

Alternatively, the gas is not actively supplied, but provisions are also made to prevent dense sintering of the upper edge of the granulation layer during vitrification and thus allow further inflow of gas into this region. For in vitrifying the granulation layer by generating a high-temperature atmosphere from the inside of the melt mold, the free surface areas of the granulation layer generally sinter rapidly, which prevents the further supply of gas. This applies in particular to the upper edge of the graining layer. Nevertheless, in order to ensure a continuous, passive or active gas supply to the upper region of the granulation layer during vitrification, the gas is supplied to the "backside" of the granulation layer facing away from the high-temperature atmosphere, namely via a gas-permeable wall section of the melt mold Graining layer preferably supplied by a adjacent to the upper region of the granulation layer, gas-permeable wall portion of the vacuum melt mold.

The gas permeability may advantageously be effected by bores in the molten wall ending at the granulation layer or by a ring of porous material, such as porous graphite, disposed in the upper region of the granulation layer and forming part of the melt mold.

In addition, it has been found useful if a grain barrier layer is provided between the upper and the lower region of the granulation layer, which hinders gas flow from the upper to the lower region. Gas is supplied continuously to the upper portion of the granulation layer during vitrification or once before or during vitrification. Particularly suitable are gases and gas mixtures which diffuse slowly in quartz glass, in particular nitrogen, argon, oxygen or air. The anti-graining layer increases the flow resistance to limit outflow of gas from the upper region to the lower region of the granulation layer underlying the suction.

The grit barrier layer is preferably designed as an annular intermediate layer within the granulation layer of SiO ₂ powder, which has a higher bulk density than the remaining SiO ₂ grain of the granulation layer. The graining barrier layer forms a closed powder layer between the upper and lower regions of the granulation layer. The powder layer consisting of particles with a comparatively higher bulk density sets the gas flow a higher flow resistance than coarser grain size. The outflow of gas from the upper region of the granulation layer as a result of the suction acting in the lower region is thus reduced.

In a further preferred variant of the method, it is provided that the gas-filled bubbles in the upper region of the granulation layer are produced by replacing gas present in the lower region of the granulation layer with helium, and that this gas exchange is hindered in the upper region of the granulation layer.

In this variant of the method, a difference between the bubbles from the lower region of the side wall and the upper region of the side wall is created by exchanging gases which are present only or predominantly in the lower region of the granulation layer by helium. Helium atoms are small in size and can diffuse comparatively quickly in quartz glass, which counteracts the formation of gas-filled pores in quartz glass.

Arrangements which prevent or prevent or reduce gas exchange in the upper region of the graining layer preferably consist of helium being supplied to the bottom region of the graining layer and, at the same time, being sucked off over the wall of the molten state due to the negative pressure applied in the lower region of the graining layer, before reaching it the upper portion of the graining layer.

In this way, the gas exchange is effected only or predominantly in the lower region of the granulation layer and avoided in the upper region of the granulation layer. In vitrification, the lower, helium-laden or evacuated region of the graining layer sinters into low-bubble or bubble-free quartz glass, whereas gas-filled bubbles remain in the upper region. The skin layer is on the one hand so thick that it is not simply rubbed off during handling and transport of the quartz glass crucible, but on the other hand so thin that it is already completely removed in the earliest possible stage of the melting process. In view of this, it has proved to be advantageous if a skin layer is produced which has a thickness in the upper region of the granulation layer in the range from 50 μm to 800 μm.

It has proven useful to apply the negative pressure in a lower region of the granulation layer which extends from the bottom region to a height which is at least 0.2 × H, preferably at least 0.4 × H. When applying a negative pressure in this area, a comparatively low-bubble quartz glass is obtained at least in the bottom and in the crucible side wall at a height of up to 0.2 × the crucible height H, preferably up to 0.4 × H. At the top, either the bubble zone or a transition zone to the bubble zone immediately follows. The width of the bubble zone below the melt level is advantageously as narrow as possible and as wide as necessary. Since the height of the attachment zone in the finished quartz glass crucible is generally known in advance, the bubble zone can be limited to this height range of the quartz glass crucible.

In view of this, it is advantageous if the side wall region is assigned a fictitious attachment zone at a height of between 0.5 × H to 0.95 × H, the bubble zone not exceeding 10 cm, preferably not more than 5 cm, below this attachment zone enough.

With regard to the quartz glass crucible, the abovementioned object starting from a quartz glass crucible of the type specified at the outset is achieved according to the invention in that the skin layer extends at the bottom and in a lower region of the side wall which extends from the bottom to a maximum of 0.8 times the cone height H, to a bubble-poor quartz glass, and in a region adjoining the lower region up to the full crucible height H, the upper region of the side wall having a thickness in the range from 50 μm to 800 μm, to a blister. senzone from a bubble-rich, gas-filled bubbles containing quartz glass adjacent.

In the quartz glass crucible according to the invention, a thin skin layer of dense quartz glass covers a circumferential, bubble-containing zone in the wall of the quartz glass crucible, which lies at the level of the melt surface at the beginning of the single-crystal drawing process, ie in the region of the attachment zone.

The skin layer prevents impurities from accumulating in the bubbles, for example during cleaning or further treatment steps of the crucible, during transport or during installation in the crystal pulling device. It is so thin that it is dissolved in the intended use of the quartz glass crucible within a short time by the corrosive attack of the silicon melt, so that then the immediately adjacent bubble-containing zone of the side wall comes into direct contact with the surface of the silicon melt. After exposure, the bubble zone thus serves as a roughened surface area, which can take all possible contact angles with the melt and thus minimize oscillations.The specific bubble volume in the bubble zone is at least twice as high as in low-bubble quartz glass of the skin layer Bladder content does not differ significantly or not at all from the skin layer and low-bubble quartz glass, and optically there is generally no difference between the low-bubble quartz glass of the skin layer and the low-bubble quartz glass in the bottom and the bottom side wall of the crucible.

The bubble zone lies in the region of the attachment zone, ie at a height of the quartz glass crucible sidewall which corresponds to the melt level height at the beginning of the crystal pulling process. This height is usually known before the beginning of the drawing process due to the inner volume of the quartz glass crucible and the volume of the silicon melt to be filled.

The bubble zone has bubbles filled with gas so that they do not disappear when the quartz glass crucible is heated. The quartz glass crucible according to the invention can be produced by means of the above-explained method according to the invention. After removal of the quartz glass crucible from the melt mold, the top edge is uneven and is abraded or cut off. The height H corresponds to the height of the crucible side wall after abrading or cutting off and also approximately to the height of the side wall region of the former granulation layer.

Skin layer has in the upper region of the side wall has a thickness in the range of 50 μιτι to 800 μιτι on.

At a thickness of less than 50 μιτι there is a risk that it is removed by abrasion during handling and transport of the quartz glass crucible. At thicknesses of more than 800 μιτι the removal of the skin layer through the melt during the crystal pulling process unnecessarily long times.

The transition from the bubble zone to the adjacent lower-bubble areas of the sidewall is usually not sharp but fluid and there is a transition area. In any case, the lower edge of the bubble zone lies in the crucible side wall, ie above the bottom of the crucible. The bubble zone extends from its lower end, starting either over the entire upper partial surface of the side wall or only over a partial section thereof.

The bubble zone is helpful only at the beginning of the crystal pulling process to reduce melt oscillation. In the later stages of the crystal pulling process, a bubble-containing surface is rather undesirable.

Therefore, the bubble zone ideally extends only at the height of the attachment zone, but not far below. In practice, it has been proven that the bubble zone extends at a height ranging from 0.4 x H to the upper edge of the pot.

In view of this, an embodiment of the quartz glass crucible according to the invention is preferred in which the side wall is assigned a fictitious attachment zone in a height between 0.5 × H to 0.95 × H, wherein the bubble zone not more than 10 cm, preferably not more than 5 cm, below this attachment zone.

The strength of the bubble zone - viewed in the radial direction - corresponds either to the wall thickness of the quartz glass crucible in this area (minus the skin tone) or a part thereof.

embodiment

The invention will be explained in more detail with reference to embodiments and a drawing. In a schematic representation shows

Figure 1 shows a first embodiment of an apparatus for producing a

Quartz glass crucible according to the invention,

Figure 2 shows a second embodiment of an apparatus for producing a

Quartz glass crucible according to the invention,

3 shows a third embodiment of a device for producing a

Quartz glass crucible according to the invention, and Figure 4 shows an embodiment of the quartz glass crucible according to the invention in a side view in section.

The melting apparatus shown schematically in Figure 1 comprises a

Melt form 1 made of metal with an inside diameter of 68 cm, a curved bottom and a side wall with a height of 50 cm. The melt mold 1 is rotatably mounted about its central axis 2. In the interior 3 of the mold 1 protrude electrodes 4 made of graphite, which are movable within the interior 3 in all directions in space indicated.

In the bottom region and in the region of the lower half of the wall of the molten mold 1, a plurality of passages 6 are provided, through which a vacuum applied to the outside of the molten mold 1 can pass through into the interior 3. In the upper third wall 17 of the mold 1 further passages 7 are present. seen, over which a gas can be directed towards the mold Innenraunn 3. The passages 7 open into a common annular groove 16 which is inserted into the upper side of the melt mold wall. The passages 6; 7 are each closed with a plug 1 1 of porous graphite, which prevents the escape of SiO ₂ grain from the interior 3.

Hereinafter, a procedure for producing a quartz glass crucible according to the invention will be explained in more detail with reference to the melting device shown in FIG.

In a first process step, crystalline granules of natural quartz sand purified by means of hot chlorination are introduced into the melt mold 1. The quartz sand has a grain size in the range of 90 μιτι to 315 μιτι. Under the action of the centrifugal force and using a shaping template, a rotationally symmetrical, garnet-shaped granulation layer 12 of mechanically solidified quartz sand is formed on the inner wall of the melt mold 1 rotating about the longitudinal axis 2. The layer thickness of the graining layer is approximately equal in the bottom area 8 and in the lower and upper side area 9, 10 and is approximately 12 mm. The height of the graining layer in the sidewall area corresponds to the height of the melt shape, ie 50 cm.

In a second method step, the electrodes 4 are positioned in the further about its longitudinal axis 2 rotating mold 1 in the vicinity of the granulation layer 12 and between the electrodes 4 ignited an arc 13.

The electrodes are thereby subjected to a power of 600 kW (300 V, 2000 A), so that a high-temperature atmosphere is established in the mold interior 3. In this way, a skin layer 14 of dense, transparent quartz glass with a thickness of about 0.5 mm is produced on the quartz granulation layer 12. In this case, the free top 5 of the granulation layer 12 is compressed.

After formation of the skin layer 14, a vacuum (100 mbar absolute pressure) is applied to the granulation layer 12 in a third method step via the passages 6 created in the bottom region 8 and in the lower wall region 9. At the same time, air is introduced via the passages 7 into the upper third 10 of the still porous granulation layer 12. The respective gas flows during the aspiration and introduction of air are indicated in FIGS. 1 to 3 by arrows. The air essentially remains in the upper third 10 of the granulation layer. Downwards, in the lower region 9 reaching air counteracts the suction through the vacuum in the lower wall region 9. As a result, relatively high loading of the SiO ₂ grain with air occurs in the upper region 10 of the granulation layer. In the lower region 9 and in the bottom region 8, the flow resistance of the granulation layer 12 in conjunction with the applied vacuum prevents the ingress of air into these regions 8; 9th

In order to further increase the flow resistance between the bottom region 8, the lower side region 9 on the one hand, and the upper side region 10 of the granulation layer on the other hand, the expected attachment zone is in the height range - this is a height of 2/3 H, where "H" is the final crucible height is - an annular intermediate layer 15 is provided, which consists of particularly fine-grained grain size with grain sizes in the range of 80 μιτι and which is characterized by a high flow resistance.

Bim vitrifying wanders a melt front from inside to outside through the granular layer 12. In the upper area 10-above the height H-a continuously bubble-containing, glazed zone (FIG. 4, bubble zone 41) is formed as a result of the greater air charge. This extends over the entire upper third of the wall, so a length of about 17 cm and it is completely covered by the inner skin 14. On the other hand, the lower region 9 and the bottom region of the granulation layer 12 initially glaze without appreciable bubble formation. With regard to transparency, there is no noticeable difference between the quartz glass of the skin layer 14 and the low-bubble quartz glass of the bottom and side wall. When the melt front is still about 4 cm from the melt mold wall, evacuation is stopped. As a result, the rear side of the granulation layer 12 also glazes in the bottom and lower side wall region to form opaque, bubble-containing quartz glass. Vitrification is stopped shortly before the enamel front reaches the enamel mold.

After vitrification, the former liner 15 marks a relatively sharp transition from bubble containing fused silica to a low bubble region. The bubble zone is considered to be the area in which the specific bubble volume below the skin layer is twice as high as in the skin layer. The lower edge of this zone is about 3 cm below the expected attachment zone.

Insofar as the same reference numerals as in FIG. 1 are used in FIGS. 2 to 4, identical or equivalent components and components are referred to, as explained in more detail above with reference to FIG.

In the melter schematically shown in Figure 2, a melt mold 21 is provided, which consists of a lower metal lower part 22, which defines the domed bottom and the two lower thirds of the side wall with a total height of 50 cm, which is also approximately the Height "H" of the produced quartz glass crucible corresponds.

On the lower part 22, an upper part in the form of a graphite ring 23 is fixed with a height of 17 cm and an inner diameter of 68 cm. The graphite ring 23 is made of porous graphite with a porosity of 25%. When the vacuum is applied, air is sucked into the upper granulation region 10 via the porous graphite ring 23. Apart from the graphite ring 23, which in this respect has a similar function as the annular groove 16 and the passages 7 of the melting device of Figure 1, the first and second embodiments of the melting device do not differ.

In the following, a procedure for producing a quartz glass crucible will be explained in more detail with reference to the melting device shown in FIG. In a first method step, a granulation layer 12 is formed in the melt mold 21 and provided with a skin layer 14 in a second method step, as described with reference to FIG.

In a third method step, a vacuum (100 mbar absolute pressure) is applied to the granulation layer 12 in the bottom region 8 and in the lower wall region 9 via the passages 6, and at the same time air passes through the porous graphite ring 23 into the upper wall region 10 due to the negative pressure air The graphite ring 23 not only serves as a molding element in forming the granulation layer 12, but also shields it against the heat of the arc 13, so that dense sintering of the granulation layer 12 from its side facing away from the plasma 13 is prevented.

As a result, air can flow into the upper region 10 of the granulation layer 12 via the graphite ring 23. Air flowing out into the lower region 9 is sucked off, so that the vacuum can not completely pass through into the upper region 10 of the granular layer 12 and a pressure gradient occurs between the upper region 10 and the lower region 9 of the granulation layer 12. This results in the upper region 10 of the granulation layer to a relative loading of the SiO ₂ grain with air. In the lower region 9 and in the bottom region 8, the flow resistance of the granulation layer 12 in conjunction with the applied vacuum reduces the penetration of air into these regions 8; 9th

When vitrifying the granulation layer 12, a continuously vesicular zone is formed in the upper region 10 as a result of the stronger air charge, which extends downwards over the upper third of the crucible height H, ie over a length of approximately 17 cm, and that of the inner skin 14 completely covered. On the other hand, the lower region 9 and the bottom region 8 of the granulation layer 12 vitrify without appreciable bubble formation as long as the vacuum is applied. The outer region of the quartz glass crucible is produced opaque throughout, as described above with reference to FIG. The transition of the bubble zone into the low-bubble or bubble-free area is fluid. The bubble zone is considered to be the area in which the specific bubble volume of the Skin layer 14 adjacent quartz glass is twice as high as in low-bubble silica glass of the skin layer 14th

The melting device shown schematically in FIG. 3 comprises a

Melt mold 31 made of metal with an inner diameter of 68 cm, a curved bottom and a side wall with a height "H" of 50 cm .The mold 31 is rotatably mounted about its center axis 2. In the interior 3 of the mold 31 are electrodes 4 made Graphite in all spatial directions movable, as indicated by the directional arrows 5.

In the region of the lower half of the wall of the mold 31, a plurality of passages 6 are provided, through which a vacuum applied to the outside of the mold 31 can pass inwards. Via a central opening 32 at the bottom of the melt mold 31, which is closed with a plug 33 made of porous graphite, the melt mold 31 helium can be supplied.

In the following, a procedure for producing a quartz glass crucible will be explained in more detail with reference to the melting device shown in FIG.

In a first method step, a granulation layer 12 is formed in the melt mold 21 and provided with a skin layer 14 in a second method step, as described with reference to FIG.

In a third method step, a vacuum is applied to the granulation layer 12 via the passages 6 in the lower wall region 9, and at the same time the granulation layer 12 is flooded without pressure via the central opening 32 with a gas mixture of helium. The supply of the gas mixture and the simultaneous suction cause an exchange of air through the He / H ₂ -Gasgemsich in the bottom and lower side region 8; 9 of the graining layer 12. The rinsing process is continued during the glazing. In the upper wall portion 10 of the porous graining layer 12, substantially the air previously contained remains.

When vitrifying the granulation layer 12 is formed in the upper region 10 due to the strong air charge from a consistently blistering bubble zone, which is about a height of about 17 cm from the upper edge extends downwards and which is completely covered by the inner skin 14.

On the other hand, the lower region 9 and the bottom region of the granulation layer 12 initially glaze without appreciable bubble formation. With regard to transparency, there is no noticeable difference between the quartz glass of the skin layer 14 and the low-bubble quartz glass of the bottom 8 and lower side wall region 9.

When the melt front is still about 5 cm from the melt mold wall, the introduction of helium and the suction are stopped. As a result, the outer sides of the granulation layer 12 also glaze in the bottom and lower side wall region into opaque, bubble-containing quartz glass, as described above with reference to FIG. The vitrification is stopped shortly before the enamel front reaches the enamel inner wall.

After removal from the melt mold, the upper, partially glazed edge of the former granulation layer is cut off and a quartz glass crucible 40 having a bubble zone 41, as shown schematically in FIG. 4, is obtained with all variants of the method explained in greater detail above. Its lower edge extends approximately 2 to 3 cm below a fictitious attachment zone which runs at a height "A." The bubble zone 41 is visually easily recognizable as an opaque region, and its specific bubble content is at least twice as high as in the skin layer 14. In this area, which is designated by "B", the skin layer 14 thus adjoins the bubble zone 41. The bubble zone 41 extends to the upper edge of the pot and extends over the entire wall

In the crucible bottom 42 and in the lower region of the side wall 43, the skin layer 14 adjoins a quartz glass having a similar low bubble content, as is present in the skin layer 14. This bubble-poor region is designated by the reference numeral 44 in FIG. The outer wall region of the quartz glass crucible 40 also consists of opaque, bubble-containing quartz glass in the bottom and lower side wall region. In Figure 4, the crucible height "H" is shown as a distance between the bottom of the bottom 42 and the upper edge 44 of the side wall.

When the quartz glass crucible is used as intended, the gas-filled bubbles are subject to bubble growth. After dissolution of the skin layer 14, a rough or wavy crucible surface is exposed in the region of the surface of the silicon melt. Because of this roughened crucible surface, any contact angle between the silicon melt and the wall results, so that melting vibrations are suppressed.

Claims

A method for producing a quartz glass crucible for pulling single crystals, comprising the following method steps:

(a) providing a vacuum melt mold (1; 21; 31) having a wall having an inside, an outside and through holes (6; 7) between outside and inside,

(b) forming a garnet-shaped, porous granulation layer (12) of SiO ₂ grain on the inside of the vacuum melt mold (1; 21; 31), wherein the granulation layer (12) has a bottom region (8) and a sidewall region (9 10),

(c) forming a skin layer (14) of low-bubble quartz glass on at least a part of the porous granulation layer (12),

(d) removing gaseous components from at least a portion of the graining layer (12) adjacent to the skin layer (14) by applying a negative pressure across the outside of the molten-metal wall (1; 21; 31),

(e) vitrifying the porous graining layer (12) to form the quartz glass crucible (40) with a crucible height H,

characterized in that in an upper annular region (10) of the granulation layer (12) which extends to the lower region (9) subsequently to the full height H, glazes below the skin layer (14) and adjacent thereto a bubble zone ( 41) containing gas-filled bubbles having a specific bubble volume of at least twice the specific volume of gas-filled bubbles in the low-bubble silica glass by applying the negative pressure in a lower portion (9) of the granulation layer (12). which extends from the bottom region (8) to a maximum of 0.8 times the crucible height H.

2. The method according to claim 1, characterized in that the gas-filled bubbles in the upper region (10) of the granulation layer (12) are produced by supplying a gas to the upper region (10) of the granulation layer (12) during or before vitrification or by allowing an influx of gas into the upper area (10).

3. The method according to claim 2, characterized in that the gas of the granulation layer (12) by a, to the upper region (10) of the granulation layer (12) adjacent, gas-permeable wall portion (17; 23) of the vacuum melt mold (21) supplied becomes.

4. The method according to claim 3, characterized in that the gas permeability of the wall portion (17) through bores (7; 16) is effected in the wall.

5. The method according to claim 3, characterized in that the gas permeability of the Wandungsabschnitts (23) is effected by a ring of porous material forming part of the wall of the vacuum-melt mold (21).

6. The method according to any one of claims 2 to 5, characterized in that between the upper (10) and the lower region (9) of the graining layer (12) a graining barrier layer (15) is provided, the gas flow from the upper (10) in hinders the lower area (9).

7. The method according to claim 6, characterized in that the graining barrier layer (15) is designed as an annular intermediate layer within the graining layer (12) of SiO ₂ powder having a higher bulk density than the remaining SiO ₂ grain of the graining layer (12) 8. The method according to claim 1, characterized in that the gas-filled bubbles in the upper region (10) of the granulation layer (12) are produced in that in the lower region (9) of the granulation layer (12) existing gases

Be replaced by helium gases, wherein the gas exchange in the upper region (10) of the granulation layer (12) is hindered.

9. The method according to claim 8, characterized in that the gas exchange in the lower region (9) of the granulation layer (12) and the obstruction of the 5 gas exchange in the upper region (10) of the granulation layer (12) are effected by the bottom region (8). helium is supplied to the granulation layer (12), which is sucked off via the wall of the melt mold (31) as a result of the negative pressure applied in the lower region (9) of the granulation layer (12), before reaching the upper region (10) of the granulation layer (12 ).

10 10.Verfahren according to any one of the preceding claims, characterized in that a skin layer (14) is generated, which in the upper region (10) of the granulation layer (12) has a thickness in the range of 50 μιτι to 800 μιτι.

11. A method according to any one of the preceding claims, characterized in that the negative pressure is applied in a lower region (9) of the granulation layer 15 (12) which extends from the bottom region (8) to a height which is at least 0.2 x H, preferably at least 0.4 x H, is.

12. The method according to any one of the preceding claims, characterized in that the side wall region (9; 10) is associated with a fictitious attachment zone (A) in a height between 0.5 x H to 0.95 x H, and that the bubble zone

20 (41) not more than 10 cm below this attachment zone (A) is sufficient.

13. A quartz glass crucible for pulling single crystals, with a crucible height H and with an inner side having a crucible wall formed by a bottom (42) and connected to the bottom (42) side wall (41, 43) made of quartz glass, wherein the inside at least partially from a skin

25 layer (14) is covered by dense quartz glass, characterized in that the skin layer (14) at the bottom (42) and in a lower portion (43) of the side wall extending from the bottom (42) to a maximum of 0.8 times the crucible height H, extends to a bubble-poor quartz glass, and in an upper area adjoining the lower area (43) up to the full crucible height H rich of the side wall has a thickness in the range of 50 μιτι to 800 μιτι having a bubble zone (41) of a bubble-rich, gas-filled bubbles containing quartz glass adjacent, wherein the skin layer (14) in the upper region of the side wall has a thickness in the range of 50 μιτι to 800 μιτι has.

14. Quartz glass crucible according to claim 13, characterized in that the side wall (41, 43) has a fictitious attachment zone (A) at a height between

0.5 x H to 0.95 x H, and that the bubble zone (41) does not extend more than 10 cm below this attachment zone (A).

15. Quartz glass crucible according to claim 13 or 14, characterized in that the bubble zone (41) extends at a height ranging from 0.4 x H to the upper edge of the crucible.