WO2012038209A1 - Process for producing a quartz glass crucible having a transparent inner layer of synthetically produced quartz glass - Google Patents

Abstract

The invention is based on the object of specifying a cost-effective process for producing a quartz glass crucible having an inner layer of transparent, synthetically produced quartz glass which is distinguished by a long service life. According to the invention, this object is achieved by a process comprising the following process steps: (a) a gas-permeable crucible substrate having an inner side is produced by solidifying at least the surface of a layer of SiO₂ particles, (b) a porous SiO₂ soot layer is deposited on at least a partial area of the inner side of the crucible substrate by vapour deposition, and (c) the porous SiO₂ soot layer and at least part of the crucible substrate are subjected to vacuum-assisted sintering by means of an arc and under a vacuum which acts over the wall of a vacuum fusion mould, to form the quartz glass crucible and the inner layer of transparent quartz glass.

Description

 Process for the preparation of a quartz glass crucible with a transparent inner layer of synthetically produced quartz glass

description

The invention relates to a process for the production of a quartz glass crucible with a transparent inner layer of synthetically produced quartz glass.

Quartz glass crucibles are used for receiving the semiconductor melt when pulling single crystals, in particular from silicon, according to the so-called Czochralski method. The wall of such a quartz glass crucible is usually formed by an opaque outer layer, which is provided with an inner layer of transparent, bubble-free as possible quartz glass.

The transparent inner layer is in contact with the melt during the drawing process and is subject to high mechanical, chemical and thermal loads. Bubbles remaining in the inner layer grow under the influence of temperature and pressure and eventually burst, causing debris and impurities to enter the melt, resulting in a lower yield of dislocation-free single crystal.

In order to reduce the corrosive attack of the melt and thus to minimize the release of impurities from the crucible wall, the inner layer is therefore as homogeneous as possible and low in bubbles.

In addition, in the course of the progressive miniaturization of semiconductor wafers, the demands on the purity of the semiconductor crystal and thus also on the purity of the quartz glass crucibles are constantly increasing. State of the art

From DE 10 2008 030 310 B3 a method for producing a quartz glass crucible is known in which in a vacuum melt mold by means of a mold template, a rotationally symmetrical, bony-shaped granular layer of me- formed chemically solidified quartz sand with a layer thickness of about 12 mm, and then on this an inner granulation layer of synthetic quartz glass powder is also formed using a mold template. The synthetic quartz glass powder has particle sizes in the range of 50 to 120 pm. The graining layers are then sintered from inside to outside by means of an arc ignited in the interior of the molten metal. A transparent inner layer is obtained on an opaque crucible preform.

The synthetic quartz glass powder is produced, for example, by granulation of a suspension of pyrogenically produced SiO ₂ powder. In the process, a suspension is produced from the loose SiO ₂ soot dust and this is processed by wet granulation into Si0 ₂ granules. These are sintered after drying and cleaning by heating in a chlorine-containing atmosphere to a dense Quarzglaskörnung. Homogenizing and granulating the suspension can lead to intensive contact with walls of equipment or grinding media, which can lead to an entry of impurities in the granules.

This disadvantage avoids the process known from US Pat. No. 3,741,796 A for producing a crucible which consists entirely of synthetic SiO ₂ . This Si0 ₂ particles are produced by flame hydrolysis of SiCl ₄ and deposited by means of several oxyhydrogen burners on a rotating graphite mandrel. The oxyhydrogen burners thereby generate flame temperatures in the range of 1500 ° C, which thermally precoat the Si0 ₂ soot layer, so that a green strength is achieved, which makes it possible to remove the greenish green body after cooling from the mandrel and bring in a heating furnace for the purpose of complete vitrification.

The sintering of the pre-compacted green body in a separate heating furnace generates additional equipment, time and energy and is therefore cost-intensive and non-productive. This disadvantage avoids the method described in JP 1 1 -01 1956 A, which also corresponds to the type mentioned above. To produce a quartz glass crucible having an inner layer of synthetically produced quartz glass, it is proposed to provide a quartz glass crucible preform, to rotate it with its crucible opening pointing downwards about an axis of rotation and to produce an inner layer of quartz glass on its inside by means of vapor deposition. For this purpose, a detonating gas burner is used, to which oxygen, hydrogen and a silicon-containing starting material are supplied and the burner flame is directed into the crucible interior. In the oxyhydrogen flame Si0 ₂ particles are produced and deposited on the inside of the crucible preform and thereby glazed by means of the oxyhydrogen gas flame directly to the inner layer.

Technical task

The inner layer thus produced consists of high-purity, synthetic quartz glass. However, due to the manufacturing process, the quartz glass of the inner layer contains a high content of hydroxyl groups, which is accompanied by a comparatively low viscosity. High temperatures during the crystal pulling process, the known crucible therefore can not withstand long.

The invention is therefore based on the object to provide a cost-effective method for producing a quartz glass crucible with an inner layer of transparent, low-bubble and pure quartz glass, which also has a long service life.

Presentation of the invention

This object is achieved according to the invention by a method comprising the following method steps:

(a) producing an inner gas-permeable crucible substrate by solidifying at least the surface of a particle layer of Si0 ₂ particles, (b) depositing a porous Si0 ₂ soot layer (21) on at least a partial area of the inside of the crucible substrate by vapor deposition, and

(c) vacuum assisted sintering of the porous Si0 ₂ soot layer (21) and at least a portion of the crucible substrate by means of an arc and under a vacuum applied over the wall of the vacuum melt mold to form the quartz glass crucible and the inner layer of transparent quartz glass.

On the inside of a vacuum melt mold, a garnet-shaped layer of SiO ₂ particles, such as quartz sand or SiO ₂ soot particles, is produced, which obtains a certain mechanical strength by solidification and solidified as a whole or at least in the region of its free surface. This solidified layer is referred to herein as a "crucible substrate".

The crucible substrate has a bottom, which is connected via a curved transition region with a cylindrical circumferential side wall. Floor, transition area and side wall define the inside of the crucible and the inside of the crucible.

The mechanical strength of the crucible substrate may be low. Their inside only has to be solidified to the extent that in the subsequent process step, namely the vapor deposition to produce a porous Si0 ₂ - Sootschicht, the Si0 ₂ -Sootteilchen find a sufficiently solid substrate that is not blown away by the deposition process. It is essential, however, that the solidification is not such that the crucible substrate becomes impermeable to gas. This will be explained in more detail below.

On the inside of the crucible substrate, a porous Si0 ₂ soot layer is produced by means of gas phase deposition. Si0 ₂ particles are formed by hydrolysis or pyrolysis of a silicon-containing starting compound in a reaction zone and are deposited on the crucible substrate inner side to form the porous Si0 ₂ soot layer. The soot layer covers the entire inside or a part thereof, but at least the transition area. It is important that the Si0 ₂ -Sootschicht - apart from an optional existing, dense skin layer, which will be described in more detail below - has an open porosity. This is obtained by keeping the surface temperature of the soot layer at a low temperature during the deposition process, which prevents immediate dense sintering of the deposited SiO ₂ particles. The surface temperature can be adjusted for example by the distance of the reaction zone to the surface. Suitable surface temperatures can be determined by a few experiments.

The porosity of the crucible substrate and the soot layer allows, on the one hand, post-treatment, such as drying of the layer and loading with dopants, and, on the other hand, vacuum-assisted sintering in a vacuum melt mold by means of a plasma flame (also referred to herein as an "arc") Both vacuum-assisted sintering and the use of an arc - represent proven and productive process measures that allow a particularly fast, reproducible and cost-effective crucible production.

However, both measures are only possible if the crucible substrate is porous. Because when sintering the soot layer, there is a significant reduction in the layer volume, which can easily lead to the inclusion of bubbles. Bubble-free dense sintering of the soot layer by means of an arc therefore requires the simultaneous suction of gas from the soot layer, ie the simultaneous application of a vacuum to the outer wall of the soot layer.

A glazing furnace for sintering the soot layer is not required, so that the expenditure on equipment and energy is eliminated. Since no oxyhydrogen flame is used for vacuum-assisted sintering, the disadvantage of loading with hydroxyl groups of the inner layer is also eliminated. The inner layer obtained after sintering of the porous soot layer is transparent and largely bubble-free. Due to the initial porosity of the crucible substrate, it is interlocked and fused with it, precluding delamination. If the vacuum-assisted sintering is carried out in a water-poor - ideally an anhydrous - Um- a relatively low hydroxyl group content of preferably less than 200 ppm by weight is also obtained.

For solidifying the Si0 ₂ particle layer, for example, this can be densified thermally, for example by heating by means of laser (C0 ₂ laser) or heating torch, for example a flame hydrolysis burner, as it is also used for depositing the Si0 ₂ -Sootschicht. However, the solidification of the particle layer according to method step (a) is particularly preferably carried out by thermal compression by means of an arc.

In this case, a particle layer can be produced in the usual way on the wall of the rotating vacuum melt mold and then heated by means of an arc and thermally compressed to the porous crucible substrate. For the production of the crucible substrate, inexpensive quartz granules of natural quartz raw material can be used. In this way, a rapid and inexpensive production of the crucible substrate is made possible. Since an arc is also used in vacuum-assisted sintering according to process step (c), this manner of compacting the particle layer to the crucible substrate requires no system change in the heating method.

As an alternative or in addition to the mentioned thermal densification, the solidification of the particle layer according to method step (a) comprises a mechanical pressing of the particle layer or an application of an SiO ₂ -pick on the particle layer.

The mechanical pressing takes place for example in the production of the particle layer using a tool, such as a spatula, as it is also used for forming the particle layer. As a result, a substantially uniform pre-compression over the entire thickness of the particle layer is achieved. When using a Si0 ₂ Slicker contained in the slurry fine Si0 ₂ particles clog the pores of the particle layer, so that essentially sets a near-surface compression. The average density of the porous soot layer is preferably in the range of 10 to 35% of the density of quartz glass, more preferably in the range of 15 to 30% of the density of quartz glass. This is based on a density of undoped quartz glass of 2.21 g / cm ³ . Low soot densities make bubble-free vitrification of the soot layer difficult. This applies to densities of less than 15% and in particular at densities of less than 10%. Very high densities of more than 30%, in particular more than 35%, can reduce the effectiveness of a subsequent gas phase treatment, for example a dehydration treatment, and easily lead to inhomogeneities both within the soot layer and in the vitrified layer obtained therefrom.

It has proven useful if the Si0 ₂ soot layer according to process step (b) is produced with a layer thickness in the range of 5 mm to 50 mm.

At a layer thickness of less than 5 mm results after sintering, a thin inner layer, which can be removed quickly when using the crucible. Layer thicknesses of more than 50 mm are difficult to vitrify and extend the heating time due to their heat-insulating effect.

The vacuum assisted sintering can be divided into two phases. In the initial phase, a high temperature is generated in the crucible interior, which is sufficient for sintering the soot layer. But it is usually applied no or at most a slight negative pressure to the suction of gases from the

Melting atmosphere in the porous areas to avoid. The actual sintering phase begins after formation of a dense skin on the soot layer, which reduces the suction of gas from the mold interior. Only then is the negative pressure set to the desired value in the sintering phase. The applied in this stage of the process vacuum is hereinafter also referred to as "full vacuum".

In that regard, it has been found to be beneficial if the soot layer prior to vacuum assisted sintering has an upper soothetome having a thickness of less than 5 mm with a density of more than 50% of the density of quartz glass.

The pre-compressed soothaut acts as a barrier against the suction of gas from the mold interior. It also has increased sintering activity, which facilitates subsequent dense sintering, allowing early application of full vacuum and accelerating the vitrification of the underlying porous areas. It is not necessary that the uppermost soothaut be completely sealed. A soothaut with a low gas permeability can also be helpful. The compacted soot skin is produced in the soot deposition process or in a separate process step before the vacuum-assisted sintering. To generate the compaction, a laser or an arc can be used. Preferably, however, the production of the soot layer and the precompression in the region of the upper soot skin are effected by means of a soot deposition burner. The soot deposition burner produces a reaction zone in the form of a burner flame in which SiO ₂ soot particles are formed. The flame pressure can be used to accelerate the SiO ₂ soot particles formed in the reaction zone in the direction of the inner side of the crucible substrate to be coated. In order to effect the desired compaction of the upper soot skin, the temperature of the burner flame is only slightly increased or the distance to the surface of the soot layer is reduced, so that a slight increase in temperature on the surface of the soot occurs, leading to a compaction up to a completely glazed one Layer can lead. It is not necessary that SiO ₂ particles continue to be formed in the burner flame. In "vacuum assisted sintering," a vacuum is created from the molten-forming wall that engages the soot layer over the porous regions of the crucible substrate The sintering atmosphere within the crucible plays an important role until a dense surface layer forms on the free inner surface of the soot layer until then the gases contained in the atmosphere reach the porous regions of the soot layer and the crucible substrate. This effect is prevented in a preferred procedure in which the soot layer has a vitreous skin less than 0.5 mm thick prior to vacuum assisted sintering.

The glassy skin is dense and prevents the gas from being drawn into the soot layer from the interior of the trough and allows the application of the full vacuum immediately after its formation.

In a particularly preferred variant of the method it is provided that the SiO ₂ soot layer is subjected to a drying process for reducing the hydroxyl group content, wherein within a crucible substrate interior an atmosphere of a dry gas is adjusted, and the dry gas is heated and from the interior through the porous soot layer is pulled outward.

The reduction of the hydroxyl group content leads to a comparatively higher viscosity of the quartz glass of the inner layer, which has a favorable effect on the service life of the quartz glass crucible. The drying process may occur before or during the sintering of the soot layer. It comprises, for example, a vacuum treatment of the soot layer at elevated temperature (<300 mbar, preferably in the temperature range from 500 to 1000 ° C.) or a treatment with a reactive drying gas, for example a halogen-containing drying gas. Preferably, however, a thermal drying method is used which uses inert, dry gas which is heated and drawn from the interior through the porous soot layer to the outside. The heating of the gas can also take place within the hot or still hot soot layer and the crucible substrate. The temperature of the heated inert gas is preferably at least 800 ° C. As a result, the average hydroxyl group content in the quartz glass of the inner layer can be set to less than 150 ppm by weight.

The sintering of the soot layer is preferably carried out in a low-hydrogen atmosphere - such as helium. Thus, the formation of new hydroxyl groups by reaction of oxygen or oxides with hydrogen is prevented, so that low hydroxyl group contents can be set in the quartz glass of the inner layer even without thermal or reactive drying, preferably of less than 200 ppm by weight. A hydroxyl group content of less than 200 ppm by weight leads to a sufficiently high viscosity of the quartz glass of the inner layer, so that it can withstand long treatment periods at high temperature.

Helium is characterized by a high diffusion rate in quartz glass. Therefore, bubbles filled with helium do not form during sintering of the soot layer or they can still be dissolved during the sintering process. In this way, a particularly low-bubble inner layer is also achieved.

To prepare the SiO ₂ soot layer, the known methods for chemical vapor deposition are fundamentally suitable, provided that a porous soot layer is obtained. Preferably, the porous Si0 ₂ soot layer according to method step (b) is produced by a method in which the crucible substrate is rotatable about a central axis, and has a bottom portion and a peripheral side wall portion connected to the bottom portion with an upper edge, and that the depositing porous Si0 ₂ soot layer according to method step (b) by means of a deposition is performed at around the central axis of the crucible rotating substrate by being moved from the bottom region starting under description of an helical travel path along the Seitenwandbe- Reich towards the top.

In this case, a soot layer is deposited on the inside of the crucible substrate rotating about its central axis starting from the bottom region, by moving the deposition burner along the side wall in the direction of the upper edge. In this case, the deposition burner describes a helical movement path along the side wall, wherein the soot layer is produced in the desired thickness in a single pass. The soot layer produced in this way is homogeneous and substantially free of coaxial stratifications which run parallel to the deposition surface, so that a delamination of the soot layer is counteracted. When it comes to a particularly high productivity, a method variant is preferred in which the deposition of the porous Si0 ₂ soot layer according to process step (b) by means of a burner arrangement having a plurality of Abscheidebrenner. As a rule, the inside of the quartz glass crucible is to be cleaned before delivery. For this etching methods are common. In the method according to the invention, however, a high surface quality was achieved from the outset, which requires no etching treatment or at most a little intensive etching treatment. Preferably, after the sintering according to method step (c), a layer thickness of less than 0.5 mm, which as a rule has not been produced by sintering under full vacuum and therefore contains bubbles, is etched away from the inner layer.

embodiment

The invention will be explained in more detail with reference to embodiments and a drawing. 1 shows a schematic representation of a procedure for producing a crucible preform,

Figure 2 shows a procedure for depositing a soot layer on the inside of the crucible preform

FIG. 3 shows a procedure for vacuum-assisted sintering of soot layer and crucible preform for producing the quartz glass crucible, FIG. 4 shows a further procedure for producing a crucible preform,

FIG. 5 shows a further procedure for depositing a soot layer on the inside of the crucible preform, and

FIG. 6 shows a further procedure for vacuum-assisted sintering of

Soot layer and crucible preform for the production of the quartz glass crucible. The melting apparatus according to FIG. 1 comprises a vacuum molten metal mold 1 having an inner diameter of 75 cm and a height of 50 cm, which is rotatable about the central axis 2. Into the interior 3 of the melt mold 1 protrude a cathode and an anode (electrodes 5) made of graphite, which - as indicated by the direction arrows 7 - within the mold 1 in all directions are movable.

The melt mold 1 can be evacuated by means of a vacuum device and for this purpose has a plurality of passages 8, via which a vacuum applied to the outside of the melt mold 1 can penetrate into the interior 3. The passages 8 are each closed with a plug 10 made of porous graphite, which prevents the escape of Si0 ₂ grain from the interior 3.

In the following, the production of a crucible preform for a 28-inch quartz glass crucible will be explained by way of example with reference to FIG. Crystalline granules of natural, purified by hot chlorination quartz sand, with a particle size in the range of 90 pm to 315 pm is filled in the about its longitudinal axis 2 rotating vacuum melt mold 1. Under the action of the centrifugal force and by means of a shaping template, a rotationally symmetrical, bony granular layer 4 of mechanically solidified quartz sand is formed on the inner wall of the molten mold 1. The average layer thickness of the granulation layer 4 is about 15 mm. The height of the graining layer 4 in the side wall area corresponds to the height of the melt shape, ie about 50 cm.

For thermal densification of the Si0 ₂ -Körnungsschicht 4, the electrodes 5 are inserted into the interior 3 and ignited between the electrodes 5, an arc 6. In this case, the electrodes 5 are brought into the lateral position shown in FIG. 1 and subjected to low power in order to solidify the granulation layers 4 in the region of the side wall to the extent that a certain agglomeration of the granulation is produced but the open porosity is retained. For thermally compacting the granulation layer 4 in the region of the bottom the electrodes 5 are brought about their longitudinal axis 2 in a central position while rotating the melt mold 1 and lowered downwards.

In this way, a thermally consolidated, but still gas-permeable crucible preform 20 (Figure 2) is obtained, which is a crucible substrate according to the invention 5. During compaction, in the region of the inner side 9, a complete dense sintering may occur locally, but this is harmless as long as the gas permeability of the crucible preform 20 as a whole is ensured. Otherwise, the densely sintered surface areas of the inside 9 must be subsequently removed, for example by grinding or etching.

After cooling, the crucible preform 20 is removed from the melt mold 1, leaving a bed of unsintered quartz glass grain in the melt mold 1. The outside of the removed crucible preform 20 is ground. It has a bottom portion 27, which is connected via a curved transition region with a cylindrical side wall 28. The wall of the

15 Gel preform 20 has a total thickness of 10 mm and is almost open-pored and gas-permeable.

On the inside of the crucible preform 20, a SiO ₂ soot layer 21 is then deposited, as shown schematically in FIG. For this purpose, the crucible preform 20 is mounted upside down, with the crucible opening facing downwards, in a holding frame 20, which is rotatable about an axis of rotation. The rotation axis 23 is inclined in the embodiment at an angle of 30 ° C to the vertical.

By means of a conventional flame hydrolysis burner 24, to which oxygen and hydrogen are supplied as fuel gases and octamethylcyclo tetrasiloxane (OMCTS) as the silicon-containing starting material, a soot layer 21 is applied to the interior

25 side of the rotating crucible preform 20 produced. For this purpose, the deposition burner 24 is moved from the base region 27 along the side wall 28 in the direction of the upper edge 26, as indicated by the directional arrow 25. The deposition burner 24 describes a helical movement path along the side wall 28. The thermally compressed crucible preform 20 represents

30 ready for a suitable, mechanically strong basis for the soot layer. On the inside of the crucible preform 20, a uniformly thick, open-pored SiO ₂ soot layer 21 with an average thickness of about 10 mm is produced in this way, which is essentially free of coaxial stratifications and which has a density of 25% of the density of quartz glass , During the deposition process, the surface temperature in the region of the forming soot layer 21 is at a maximum of 1250 ° C. In order to achieve a higher compression by 80% (the density of quartz glass) in a thin surface layer of about 2 mm, the surface of the finished soot layer 21 is finally traversed with the deposition burner 24 without particle deposition, producing a surface temperature which is higher by about 100 ° C. becomes.

Subsequently, the gas-permeable crucible preform 20, together with the porous soot layer 21 with compacted surface layer, is vitrified in a vacuum-assisted sintering process. The sintering takes place in the same device as the production of the crucible preform 20 and is shown schematically in FIG. For this purpose, the crucible preform 20 together with the soot layer 21 compacted close to the surface is again inserted into the melt mold 1 and the gap between the inside of the melt mold and the outside of the crucible preform 20 is completely filled again with the quartz glass grain. The electrodes 5 are positioned in the melt mold 1 rotating about their longitudinal axis 2 in the vicinity of the soot layer 21 and an arc 6 is ignited between the electrodes 5. 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 of dense but bubble-containing quartz glass with a thickness of approximately 0.5 mm is produced on the soot layer 21, favored by the denser surface layer in this area.

After formation of the dense skin layer, a full vacuum (100 mbar absolute pressure) is applied via the passages 8 in the bottom region and in the lower wall region, as indicated by the directional arrows 11. When vacuum assisted glazing, a melt front migrates from inside to outside through the entire soot layer 21 and a part of the crucible preform 20.

The soot layer 21 vitrifies thereby to a transparent and highly pure inner layer without appreciable blistering (apart from the thin skin layer). Once the melt front is about 4 cm from the melt mold wall, evacuation is stopped. As a result, the rear side of the crucible preform 20 and of the remaining granulation bed glazes in the bottom and lower side wall region to form opaque, bubble-containing quartz glass. The vitrification is stopped shortly before the enamel front reaches the wall of the enamel mold 1. From the sintered layer, the skin layer produced during sintering, which has a higher bubble content, is subsequently removed. For this purpose, a layer thickness of about 0.4 mm is removed by etching in hydrofluoric acid.

The inner layer of the quartz glass crucible thus produced has an average thickness of 3 mm. It is smooth, low in bubbles and has a hydroxyl group content of 180 ppm by weight. It is firmly connected to the former crucible preform 20, which forms a transparent and an opaque outer area of the quartz glass crucible.

Unless identical reference numbers are used in FIGS. 4 to 6 as in FIGS. 1 to 3, identical or equivalent components of the device are thereby designated. In that regard, reference is made to the above explanations to the figures 1 to 3.

The melting device according to FIG. 4 corresponds to that of FIG. 1. To produce a crucible preform for a 28-inch quartz glass crucible, a rotationally symmetrical, garnet-shaped granulation layer 4 having a thickness of approximately 15 mm of crystalline granulation is formed on the inner wall of the melt mold 1 by means of a shaping template and mechanically consolidated as described above with reference to FIG.

The inside 9 of the graining layer 4 is sprayed with a suspension of deionized water and Si0 ₂ particles. The Si0 ₂ particles are synthetically produced, essentially spherical particles with bimodal Particle size distribution, wherein a first maximum of the distribution is about 0.5 pm and a second maximum at about 40 pm. The solids content of the suspension is 65% by weight.

The spherical Si0 ₂ particles partially fill the interspaces of the graining layer 4. They have a paste-like effect and lead to a certain compaction and solidification of the granulation layer 4 in a surface region 44 having a thickness of 3 to 5 mm, but the gas permeability of the resulting preform 40 is maintained. This thus represents a porous crucible substrate with mechanically solidified surface area 44 in the sense of the invention.

On the inside of the crucible preform 40, a SiO ₂ soot layer 41 is then deposited, as shown schematically in FIG. The crucible preform 40 remains in the melt mold 1, which rotates about its axis of rotation 2 during the deposition process. The soot layer 41 is produced on the inner side 9 of the rotating crucible preform 40 by means of a conventional flame hydrolysis burner 24, to which oxygen and hydrogen are supplied as fuel gases and octamethylcyclotetrasiloxane (OMCTS) as silicon-containing starting material. With the deposition burner 24, the soot layer 41 is deposited from the bottom region for this purpose by moving the deposition burner 24 along the side wall 28 in the direction of the upper edge 26, as the directional arrow 25 indicates. In this case, the deposition burner 24 describes a helical movement path along the side wall. The compacted surface area 44 represents a suitable, mechanically strong basis for the soot layer 41.

During the deposition process, the surface temperature in the region of the forming soot layer 41 is at a maximum of 1250 ° C. On the inner side 9 of the crucible preform 40, a uniformly thick, open-pored SiO ₂ soot layer 41 with an average thickness of about 10 mm is produced in this way, which is free of laminations and has a density of 25% of the density of quartz glass.

The subsequent sintering of the internally coated crucible preform 40 takes place in the same melt mold 1 and is shown schematically in FIG. In advance, the crucible preform 40 together with the soot layer 41 is dried. For this purpose, a high-temperature atmosphere of helium is generated in the mold interior 3 by introducing helium and igniting an arc 6 between the electrodes 5, so that the temperature in the mold interior increases to about 800 ° C. By applying a vacuum via the passages 8 in the bottom region and in the lower wall region, the hot helium gas is then drawn out of the mold interior through the crucible preform 40, so that the gas contained in the interstices of the granulation layer 4 is exchanged. After switching off the suction, the electrodes are briefly applied with a power of 600 kW (300 V, 2000 A), so that sets a further increase in temperature in the mold interior 3, due to the soot layer 41, a skin layer of dense, but bubble-containing quartz glass is formed with a thickness of about 0.5 mm. After formation of the dense skin layer, a full vacuum (100 mbar absolute pressure) is applied, as indicated by the directional arrows 11. In vacuum-assisted vitrification, a melt front migrates from inside to outside through the entire soot layer 41 and part of the crucible preform 40.

The soot layer 41 vitrifies thereby to a transparent and highly pure inner layer without appreciable blistering (apart from the thin skin layer). Once the melt front is about 4 cm from the melt mold wall, evacuation is stopped. As a result, the rear side of the crucible preform 40 and the remaining graining layer glazes in the bottom and lower sidewall regions to form opaque, bubble-containing quartz glass. The vitrification is stopped shortly before the enamel front reaches the wall of the enamel mold 1.

From the sintered layer, the skin layer produced during sintering, which has a higher bubble content, is subsequently removed. For this purpose, a layer thickness of about 0.4 mm is removed by etching in hydrofluoric acid. The inner layer of the quartz glass crucible thus produced has an average thickness of 3 mm. It is smooth, low in bubbles and has a hydroxyl group content of 130 ppm by weight. It is firmly connected to the former crucible preform 40 which forms a transparent and an opaque exterior of the quartz glass crucible.

Claims

claims Process for the production of a quartz glass crucible with a transparent inner layer of synthetically produced quartz glass, comprising the following process steps: (a) producing a gas-permeable crucible substrate (20; 40) having an inner side (9) by solidifying at least the surface of a particle layer (4) of Si0 ₂ particles, (b) depositing a porous Si0 ₂ soot layer (21; 41) on at least one partial area of the inner side (9) of the crucible substrate (20; 40) by vapor deposition gas, and (c) vacuum assisted sintering of the porous Si0 ₂ soot layer (21; 41) and at least a portion of the crucible substrate (20; 40) by means of an arc (6) and under a vacuum applied over the wall of a vacuum melt mold to form the Quartz glass crucible and the inner layer of transparent quartz glass. A method according to claim 1, characterized in that the solidification of the particle layer (4) according to process step (a) is carried out by thermal compression, preferably by means of arc (6). The method of claim 1 or 2, characterized in that solidification of the particle layer (4) according to process step (a) mechanical pressing of the particle layer (4) or an application of a Si0 ₂ -Schlickers on the particle layer (4). Method according to one of the preceding claims, characterized in that the porous Si0 ₂ soot layer (21; 41) according to process step (b) with an average density in the range of 10 to 35% of the density of quartz glass, preferably in the range of 15 to 30 % of the density of quartz glass is generated. 5. The method according to any one of the preceding claims, characterized in that the Si0 ₂ soot layer (21; 41) according to process step (b) is produced with a layer thickness in the range of 5 mm to 50 mm. 6. A method according to any one of the preceding claims, characterized in that the soot layer (21; 41) prior to the vacuum-assisted sintering comprises an upper soot skin having a thickness of less than 5 mm with a density of more than 50% of the density of quartz glass having. 7. The method according to claim 5, characterized in that the generation of the soot layer (21) and the pre-compression in the region of the upper Soothaut by means of a soot-Abscheidebrenners (24) take place. 8. The method according to any one of the preceding claims, characterized in that the soot layer (21; 41) before the vacuum-assisted sintering has a glassy skin with a thickness of less than 0.5 mm. 9. Method according to one of the preceding claims, characterized in that the SiO ₂ soot layer (21) is subjected to a drying process for reducing the hydroxyl group content, wherein an atmosphere of a dry gas is set within a crucible substrate interior (3), and the dry gas is heated and drawn from the interior through the porous soot layer (41) to the outside. 10. The method according to claim 9, characterized in that the average hydroxyl group content in the quartz glass of the inner layer is adjusted to less than 150 ppm by weight. 1 1 .Method according to one of the preceding claims, characterized in that the crucible substrate (20) about a central axis (2) is rotatable, and a bottom portion and connected to the bottom portion circumferential side wall portion having an upper edge, and that the deposition of porous Si0 ₂ soot layer (21) according to process step (b) by means of a Abscheidebrenners (24) at around the central axis (2) rotating crucible substrate (20) takes place by this from the bottom region beginning with description of a helical movement path along the side wall portion is moved in the direction of the upper edge. 12. The method according to any one of claims 1 to 10, characterized in that the deposition of the porous Si0 ₂ soot layer (21) according to process step (b) by means of a burner arrangement takes place, which has a plurality of Abscheidebrenner.