DE102004038602B3 - Process for making quartz glass for use in the manufacture of lamps and semiconductors involves melting glass by electrically heating in a vacuum, the glass containing a specified concentration of temperature-stable hydroxyl groups - Google Patents

Abstract

In a process to make quartz glass for use in the manufacture of lamps and semiconductors, the quartz glass is melted by electrical heating and contains a concentration of temperature-stable hydroxyl groups in the range 20 Wt. ppm and 400 Wt. ppm. The glass is heated in a vacuum of 10-2> bar to 1040[deg]C for 40 hours, during which it remains no further than 5 mm from a diffusion route.

Description

The Invention relates to electro-melted, synthetic quartz glass, especially for the use in lamp and semiconductor manufacturing.

• (a) Herstellen einer Suspension amorpher SiO₂-Primärteilchen, die eine mittlere Teilchengröße von weniger als 5 μm aufweisen,
• (b) Granulieren der Suspension unter Bildung von porösem SiO₂-Granulat mit einer spezifischen Oberfläche (nach BET) von mindestens 20 m²/g,
• (c) Trocknen des SiO₂-Granulats,
• (d) Verdichten und Reinigen des getrockneten SiO₂-Granulats durch Erhitzen in halogenhaltiger Atmosphäre unter Bildung eines verdichteten SiO₂-Granulats mit einer spezifischen Oberfläche (nach BET) von maximal 40 m²/g, und
• (e) Schmelzen des verdichteten SiO₂-Granulats unter Bildung von Quarzglas

Furthermore, the invention relates to a method for producing a synthetic quartz glass, comprising the method steps:

• (a) preparing a suspension of amorphous SiO ₂ primary particles having an average particle size of less than 5 μm,
• (b) granulating the suspension to form porous SiO ₂ granules having a BET specific surface area of at least 20 m ² / g,
• (c) drying the SiO ₂ granules,
• (D) compacting and cleaning the dried SiO ₂ granules by heating in a halogen-containing atmosphere to form a compacted SiO ₂ granules having a specific surface area (according to BET) of at most 40 m ² / g, and
• (e) melting the compacted SiO ₂ granules to form quartz glass

Such a method for the production of synthetic quartz glass for use in semiconductor manufacturing is known from DE 199 62 451 C known. It is about the production of opaque quartz glass. For this purpose, a molded article of synthetic SiO ₂ granules is first produced and this is then vitrified.

The SiO ₂ granules used for this purpose have a specific surface area (BET) between 1.5 m ² / g and 40 m ² / g and a tamped density of at least 0.8 g / cm ³ . It consists of porous agglomerates and SiO ₂ primary particles which have an average particle size in the range of 0.5 to 5 μm. The SiO ₂ granulate is produced from an aqueous suspension by wet granulation and then thermally precompressed in a pass at a temperature of about 500 ° C in chlorine-containing atmosphere. By this treatment, the granules are purified and simultaneously freed of hydroxyl groups.

The Yield and electrical performance of semiconductor devices hang essentially on the extent to which it is in semiconductor manufacturing succeeds, contaminations of the semiconductor material by impurities prevent them from acting as "semiconductors." Contaminations of the semiconductor material are, for example, by those in the manufacturing process caused used equipment. Due to the chemical resistance of quartz glass opposite many substances used in the manufacturing process and its relative high temperature resistance, such equipment is often made Quartz glass. The purity of the quartz glass is therefore high and constantly Increasing demands made of natural quartz glass Raw material only conditionally to fulfill are. Increasingly, the equipment is therefore with specially purified or processed layers provided or it is synthetically produced Quartz glass is used, which is characterized by a high purity.

It But it has been shown that, despite the high purity of synthetic Quartz glass contamination of the semiconductor material by the quartz glass equipment not always reduce to the expected extent. Especially with synthetic Quartz glass results from natural compared to quartz glass Raw material the problem that impurities in the form of cations can diffuse rapidly; one also calls synthetic quartz glass in this context as a "contamination highway".

by virtue of its transparency in the ultraviolet spectral range is quartz glass also for Lamp tubes and other components, such as cover plates, used for UV applications. To mention Here are in particular mercury lamps and Excimerstrahler. Particularly short-wave UV radiation caused during use of the quartz glass component radiation damage, resulting in a discoloration and in a deterioration of the optical transmission express what is called solarization. As a cause for the increase Absorption is governed by ODC (Oxygen Deficiency Centers) Center) in the UV wavelength range absorb, here being two in equilibrium Differentiates types referred to in the literature as ODC I and ODC II become.

The invention has for its object to provide a quartz glass, which is characterized by a low tendency to contamination with respect to semiconductor materials and by a low solarization tendency and to provide a simple and inexpensive method for the reproducible production of such a quartz glass.

With regard to the quartz glass, this object is achieved according to the invention in that it is an electro-fused, synthetic quartz glass which has a concentration of temper-stable hydroxyl groups in the range from 20 ppm by weight to 400 ppm by weight, the hydroxyl groups which are stable in temperature being characterized thereby, that they are not removed by heating the quartz glass to a temperature of 1040 ° C under a vacuum of 10 ^-2 mbar and for a period of 40 hours over a diffusion distance of 5 mm.

Of the Hydroxyl group content of quartz glass is chemically determined bonded and not chemically bound hydroxyl groups (OH groups) together. The OH groups of the former species, in the following as "temper stable Hydroxyl "called be, can not be by the above annealing treatment remove. The OH groups of the latter type, however, by the above-mentioned temperature treatment is "tempered out" of the quartz glass. These hydroxyl groups are hereafter referred to as "metastable Hydroxyl "called.

It It has been found that a certain content of hydroxyl groups the Reduce the transport of impurities (especially by "diffusion") in the quartz glass can. This result is surprising as hydroxyl-containing quartz glass compared to hydroxyl-free quartz glass a lower viscosity has, what usually the diffusion of impurities in quartz glass at high temperatures facilitated. It turned out, however, that this diffusion-inhibiting Effect of hydroxyl groups not clearly with the total hydroxyl group content of the quartz glass can be correlated. Only the chemically bound, lead stable temperature hydroxyl groups to effectively hinder the transport of semiconductors - in particular of ferrous contaminants - in quartz glass, whereas the metastable hydroxyl group content is ineffective in this regard is or even unfavorable can affect. This mechanism is obviously based on one chemical or charge-induced binding of the diffusing impurities on the temperature-stable hydroxyl groups.

The synthetic quartz glass according to the invention therefore has a concentration of temperature-stable hydroxyl groups in the range of at least 20 ppm by weight and at most 400 ppm by weight. For the purposes of this invention, the "OH-content which remains after heating a quartz glass component with a thickness of 10 mm in the component is defined as" temperature-stable hydroxyl group content "if the heating takes place at a temperature of 1040 ° C. over a period of 40 hours and under vacuum (10 ^-2 mbar). Accordingly, "metastable hydroxyl groups" are understood as meaning those hydroxyl groups which can be removed from the quartz glass component as a result of such an annealing treatment.

According to "R. Brückner, Silicon Dioxide; Encyclopedia of Applied Physics, Vol. 18 (1997), pp. 101-131 ", quartz glass types can be differentiated depending on the raw material used and the method of its processing into quartz glass The quartz glass type I is quartz glass of electrically melted quartz crystals. This quartz glass typically has an OH content of less than 5 ppm. Type II quartz glass is produced by melting quartz crystals in the oxyhydrogen flame (H ₂ / O ₂ ) .This quartz glass has a higher OH content of between 150 and 300 ppm due to its manufacture Synthetic quartz glass is produced either by flame hydrolysis, by plasma enhanced oxidation, or by sol-gel techniques (types III, IV, and VII) Range from less than 0.1 ppm to about 1000 ppm In the context of the present invention, in particular the types Va and Vb of I INTEREST. The quartz glass type Va is quartz glass, which is melted in an electric fusion of pegmatitic quartz (quartz in conjunction with other minerals) in a crucible under a hydrogen-containing atmosphere. The quartz glass thus obtained typically has an OH content of 100 ppm. However, by outgassing at high temperature (10 hours at 1080 ° C), the OH content can be reduced to a range of <1 ppm to 15 ppm, thereby obtaining the quartz glass type Vb.

The quartz glass according to the invention is produced by electrolytic melting and is distinguished by a comparatively high content of temper-stable hydroxyl groups of more than 20 ppm by weight for electrofused quartz glass. This quartz glass does not fall under any of the quartz glass types described above according to Brückner. As explained in more detail below with reference to the description of the method according to the invention, this relatively high OH content is made possible for example by starting from a raw material in which a predetermined content of OH groups can be introduced and fixed on the basis of its defined specific surface area , which then in the subsequent electro fusion process to Bil Contribution of stable temperature hydroxyl groups.

in the However, the atmosphere in the electrofusion process also has a view of this for making the quartz glass effects. It should be noted that very high temperatures around 2000 ° C are required for the melting of quartz glass are what the selection of suitable crucible materials essentially limited to the materials graphite, molybdenum and tungsten. none These materials are at high temperatures to oxygen resistant, so that the melting atmosphere usually contains a protective gas, that too - like for example, hydrogen - reducing can work.

It has been shown that for the production of the quartz glass according to the invention with a high content of temperature-stable hydroxyl groups a comparatively favorable reducing melting atmosphere is advantageous, which also both Types of oxygen deficit centers reduced or eliminated, so that the quartz glass according to the invention also by a high UV radiation resistance distinguished.

There are also the synthetic invention Quartz glass at the for the semiconductor production of typical temperature treatments little Pollution to the environment by obstructing their diffusion. It is therefore especially for the production of equipment for use in semiconductor manufacturing suitable.

The viscosity of quartz glass decreases with the OH content. The upper limit of 400 Ppm by weight for the Content of stable-temperature hydroxyl groups is determined by the minimum viscosity of the quartz glass certainly.

Preferably is the concentration of hydroxyl groups a maximum of 500 ppm by weight. Because It has been shown that the cheap Effect of the temperature-stable hydroxyl groups with respect to the bond of impurities, especially iron impurities, at very high hydroxyl group concentrations of more than 500 ppm by weight the lower high-temperature viscosity of the quartz glass at least partially nullified. With regard to viscosity reduction it does not matter if the hydroxyl groups are stable to temperature or metastable hydroxyl groups.

in the In view of this, it has proved to be particularly favorable when the concentration at temperature-stable hydroxyl groups in the range between 30 ppm by weight and 200 ppm by weight.

Below of 30 ppm by weight makes the effect of the stable-temperature hydroxyl groups little in relation to the transport of impurities in the quartz glass noticeable while a high hydroxyl group content - for example, from the stated upper limit of 200 ppm by weight - the High temperature viscosity of the quartz glass, resulting in deformation of the quartz glass when used as intended to lead can.

advantageously, is the concentration of metastable hydroxyl groups maximum 100 ppm by weight, preferably at most 50 ppm by weight.

Of the carries metastable OH content not to fix impurities and it worsens the High temperature viscosity of the quartz glass. Preference is given to the concentration of the metastable Hydroxyl groups therefore kept as low as possible. A concentration in the range of the specified upper limit of 100 ppm by weight, preferably maximum 50 ppm by weight, but is generally acceptable.

A further improvement is achieved if the quartz glass according to the invention has an Al ₂ O ₃ content in the range between 5 ppm by weight and 20 ppm by weight. Al ₂ O ₃ in the stated concentration range increases the viscosity of quartz glass and thus its high temperature stability. As a result, the viscosity reduction, which is accompanied by the required concentration of temperature-stable hydroxyl groups, positively counteracts supportive.

The synthetic quartz glass within the meaning of the present invention preferably in the form of a wall thickness having quartz glass component before, wherein the concentration of stable-temperature hydroxyl groups on the Wall thickness by a maximum of +/- 10 % deviates from the mean.

It has been found that even by means of flame melting a quartz glass component with a content of stable-temperature hydroxyl groups above 20 ppm by weight can be produced. However, due to the hydrogen content of the flame, quartz glass components obtained by means of flame melting generally have a higher hydroxyl group content in the region of their surface than in the interior. In contrast, in the case of the electrically molten quartz glass according to the present invention, it is possible to obtain a component thickness over the component wall thickness ke to set homogeneous hydroxyl group distribution. A homogeneous distribution of the hydroxyl groups leads to an optical homogeneity, in particular with regard to the stress birefringence and an undisturbed transmission of the quartz glass. For these reasons, flame-melted quartz glass is considered to be less suitable for the above-mentioned applications, and is therefore not the subject of the present invention.

It has also proven itself when the inventive synthetic Quartz glass in the form of a layer formed on a quartz glass component is.

Already a layer of the quartz glass according to the invention on a quartz glass component has the above explained Effects of reducing the diffusion of impurities on. The layer thickness is preferably at least 200 microns.

With regard to the method, the above-mentioned technical problem is solved according to the invention by the fact that the SiO ₂ granulate compacted according to process step (d) experiences an occupancy of its specific surface with hydroxyl groups before or during the melting according to process step (e), and that the melting according to process step (e) takes place at least temporarily in a hydrogen-containing atmosphere in an electrically heated furnace, such that in the quartz glass a target concentration of temperature-stable hydroxyl groups in the range from 20 ppm by weight to 400 ppm by weight becomes.

The melting of the SiO ₂ granules according to process step (e) is to be carried out so that thereafter in the quartz glass, a content of stable-temperature hydroxyl groups in the range of said target concentration is present. However, the previous method steps also contribute to the setting of this hydroxyl group content, in particular the method steps (b), (c) and (d), as will be explained in more detail below.

In the method according to the invention is based on a suspension containing amorphous SiO ₂ primary particles, from which by agglomeration in a suspension, a porous SiO ₂ granules is formed, which has a BET surface area of at least 20 m ² / g. The large inner and outer surface serves as a support for hydroxyl groups.

Of the Drying step according to process step (c) leads to a significant reduction in the hydroxyl group content of the granules, without the specific surface changed significantly becomes. A certain residual moisture is retained.

For cleaning and compacting after process step (d), the granules are heated. This leads to a significant reduction of the free specific surface area (according to BET). It is essential that the BET surface area before melting is below 40 m ² / g in order to achieve the desired concentration of OH groups which are stable to tempering, and that the granules have at least one opportunity for their specific surface area with hydroxyl groups before the formation of a melt saturate at least in large part. The saturation occurs during the process step (d) and / or during the process step (s) and / or between the two process steps by the granules is brought to air, for example.

The method according to the invention is thus distinguished from the known method in that the SiO ₂ granules before melting in accordance with process step (e) in a temperature treatment either receives a defined specific surface to which OH groups can be fixed, which in turn in the melting process the desired concentration of stable-temperature hydroxyl groups in the range of 20 ppm by weight and 400 ppm by weight, or has been completely compressed under conditions which make it possible to fix a predetermined content of stable-temperature hydroxyl groups in the compacted granules, which then In the following melting process leads to a quartz glass with the desired content of stable-temperature OH groups.

Decisive for the later content of stable-tempered hydroxyl groups in the quartz glass is when using a pre-compacted but still porous SiO ₂ granules whose specific surface, and in a fully compressed SiO ₂ granules whose firmly fixed initial hydroxyl group content.

At a specific surface area of more than 40 m ² / g, the granules are so hygroscopic that in the quartz glass melted therefrom easily results in an excessively high content of hydroxyl groups which impairs the high-temperature strength.

This temperature treatment, which simultaneously causes a cleaning and compacting of the SiO ₂ granules and thus a reduction of the specific surface, allows the reproducible production of synthetic quartz glass, which is characterized by a content of stable-temperature hydroxyl groups in the range of said target concentration. The effect of the temperature-stable hydroxyl groups with regard to the hindrance of the diffusion of impurities in quartz glass, in particular iron-containing impurities, has already been described above on the basis of the quartz glass according to the invention.

The uniform occupancy the individual granules grains ensured with hydroxyl groups a homogeneous course of OH concentration over the wall of it melted quartz glass body.

at the hydrogenous atmosphere it is pure hydrogen or a hydrogen and Inert gas (noble gas or nitrogen) containing gas mixture. In the Normally, the hydrogen concentration is more than 90% by volume.

In a preferred procedure, the specific surface area of the SiO ₂ granules is coated with hydroxyl groups in a water-containing atmosphere before melting according to process step (e). It has been found that the hydroxyl groups which are incorporated into the quartz glass during or after the compacting of the SiO ₂ granulate according to process step (d) in a water-containing atmosphere form essentially temperable-and non-metastable-hydroxyl groups. The hydrous atmosphere is produced by introducing into the atmosphere once, several times or continuously water, steam or a starting material which forms water in chemical reaction with a reactant, such as hydrogen in combination with oxygen present, the introduction of water also being effected by Delivery of hydroxyl groups can result, which adhere to the SiO ₂ granules as residual moisture. In the simplest case, air forms the atmosphere for occupying the SiO ₂ granules with hydroxyl groups.

By heating in a water-containing atmosphere, the content of stable-temperature hydroxyl groups in the range of the target concentration is substantially prepared. Normally, sufficient moisture content of the SiO ₂ granulate after the drying step (c) is sufficient to adjust the water content of the atmosphere. In addition to this, however, before or during the compression step (d), the atmosphere may be supplied once, several times or continuously with water, steam or a starting substance which forms water in a chemical reaction with a reactant, for example hydrogen. The required treatment time, temperature and atmosphere depend on the specific surface of the SiO ₂ granules and their particle sizes and also on the process control in the subsequent melting step, and are to be determined for each individual case by a person skilled in the art by means of a few experiments.

A sufficient occupancy of the SiO ₂ granules with hydroxyl groups prior to the melting process eliminates special measures to create the required concentration of stable-temperature hydroxyl groups only during the melting process and thus facilitates the process control.

The compacted granules are present as a granular starting material and will be in the final Electro-melting process further processed to quartz glass. The granules For example, it can be melted in a crucible to give a shaped body or in a Tiegelziehverfahren continuously to a strand to be pulled. In process step (e), the electrofusion process under a hydrogen-containing atmosphere, the final adjustment takes place the content of stable-temperature hydroxyl groups.

It has been shown to melt when in a pure water atmosphere with low Dew point temper stable hydroxyl groups are no longer appreciable Scope are built into the quartz glass, but arise from the at the granule surface adhering OH groups temper stable hydroxyl groups. thats why when using a pure water atmosphere (to protect the crucible material) the occupancy of the surface with hydroxyl groups in the preceding process steps crucial. Alternatively contains the melting atmosphere a certain amount of oxygen or water, resulting in an installation of hydroxyl groups in the molten quartz glass contributes otherwise existing reducing effect of the melting atmosphere something mitigates, and thereby - with the above mentioned Effects on UV radiation resistance of quartz glass - the Formation of oxygen deficit centers avoids in the quartz glass.

It has proved to be particularly favorable if, during compacting and cleaning of the dried SiO ₂ granules in method step (d), a compacted SiO ₂ granulate having a specific surface area (BET) between 1.5 m ² / g and 30 m ² / g and with a tamped density of at least 0.75 g / cm ^{3 is} generated.

At a specific surface area of less than 1.5 m ² / g, it is not possible to raise the content of temper-stable hydroxyl groups in the quartz glass to a value above 20 ppm by weight, unless the extent to which the precompressed SiO ₂ slurry is already sufficiently high Has hydroxyl group content.

The said tamped density primarily ensures the flowability of the granules, which has an effect on process step (e). However, the BET specific surface area has significant influence on the "occupation ability" of the granules with OH groups and thus on the content of anneal-hydroxyl after the melting process. The specific surface area of the SiO ₂ granulate is made by the BET method (DIN 66132) and the tamped density is determined according to DIN / ISO 787 Part 11.

advantageously, the compaction takes place in accordance with the method step (d) at a temperature in the range between 800 ° C and 1350 ° C.

Compacting at a comparatively low temperature below 800 ° C results in a SiO ₂ granulate which, while compacted, is still porous and therefore prone to extensive fixation of hydroxyl groups, which tends to result in too high a total hydroxyl group content. At a treatment temperature of 1350 ° C or more results (with sufficiently long treatment time and depending on the granule size) a fully compacted, vitrified product with a specific surface area of less than 1.5 m ² / g, which is used as a carrier for OH groups for the purpose of formation of stable-temperature hydroxyl groups in process step (e) is not suitable. In the latter case, it is necessary that the granules are vitrified individually in a water-containing atmosphere, so that the required content of stable-temperature hydroxyl groups in the individual vitrified granules is already fixed prior to their melting.

The Compacting and cleaning according to process step (d) is preferably carried out in a chlorine-containing atmosphere.

By treating the SiO ₂ granules in halogen-containing, in particular in a chlorine-containing atmosphere, both metastable hydroxyl groups and temperature-stable hydroxyl groups are largely removed, as well as impurities which form volatile chlorine compounds at the treatment temperature. This improves the purity of the quartz glass, increases the viscosity and lowers the devitrification tendency. The chlorine-containing atmosphere contains chlorine and / or a chlorine compound for this purpose.

It has proved its worth, if the melting according to process step (e) in an atmosphere takes place, which has a dew point in the range between -35 ° C and -70 ° C, preferably above -65 ° C, has.

By adjusting the dew point of the hydrogen-containing melt atmosphere can the incorporation of hydroxyl groups in the quartz glass can be influenced. A melting atmosphere with a dew point above -35 ° C can because of its high water content the crucible or nozzle material badly affected in the smelting furnace. At dew points below of -70 ° C is the effect melting atmosphere due to their low water content greatly reducing what the Training of oxygen deficit centers favors.

It has proved to be advantageous if the SiO ₂ granules after drying according to process step (c) has a hydroxyl group concentration of less than 3 ppm by weight.

As a result of the low OH content results for the subsequent process steps for adjusting the temperature-stable OH content - especially for a subsequent densification of the SiO ₂ granules - a defined starting situation, and it is avoided that superfluous or unfavorable impacting metastable OH groups contained in the quartz glass.

An essential aspect of the invention is that the SiO ₂ granulate is formed from amorphous SiO ₂ primary particles. It has been shown that not all of this SiO ₂ primary particles are completely suitable, for example, indicate by "sol-gel process" produced SiO ₂ primary particles a high metastable hydroxyl group content of the must be laboriously removed Therefore, preferably SiO _2. - Primary particles, which are formed by flame hydrolysis or oxidation of inorganic silicon compounds, which have a mean particle size of less than 0.2 microns and they are characterized by a large free surface by agglomeration of a plurality of such primary particles, the porous SiO ₂ granules The known granulation methods can be used for the granulation, in particular build-up granulation, spray granulation, and press granulation (such as, for example, extrusion) The mean particle size of the primary particles is referred to as so-called D ₅₀ value determined according to ASPM C1070.

As already explained above, a certain Al ₂ O ₃ doping in the quartz glass causes an increase in viscosity. In view of this, it has proved favorable that the suspension of amorphous SiO ₂ primary particles according to process step (a) is added pyrogenically produced Al ₂ O ₃ or an aluminum salt. The pyrogenically produced Al ₂ O ₃ is present in finely distributed form as nanoparticles, so that a uniform and homogeneous distribution of the dopant is established in the suspension. Soluble halides are preferably used as the aluminum salt. Alternatively or additionally, it has also been found useful to carry out the drying or compacting of the SiO ₂ granulate in an atmosphere containing an aluminum halide. In this case, an Al ₂ O ₃ doping of the quartz glass is achieved via the gas phase during process step (c) and / or process step (d).

following The invention will be explained in more detail with reference to embodiments. When single figure shows

1 a bar graph showing the content of hydroxyl groups of different quartz glass samples before and after an austempering process and the measured average free diffusion length.

The bar chart of 1 shows the total content of hydroxyl groups in ppm by weight for different quartz glass samples, before (1st bar) and after (2nd bar) an annealing treatment. The third bar indicates the mean free diffusion length in μm measured on the respective quartz glasses, for which the measuring method is described in more detail below. The ordinate dimension of the left side of the diagram refers to the hydroxyl group content, and that of the right side to the mean free diffusion _length l _diff . 1 shows the measurement results listed in detail in Table 1 in graphical form.

The quartz glass samples were melted from different particulate SiO ₂ starting substances, wherein the melting was carried out either in a Tiegelziehverfahren under comparable melting conditions or in a flame melting process. On the quartz glass samples thus obtained, the contents of OH groups were determined before and after an annealing process. This annealing process and the other process steps for the production of the quartz glass are described below with reference to the information in Table 1 and the representation in 1 explained in more detail:

Preparation and characterization of SiO ₂ starting substances

As starting materials for melting a quartz glass body, either SiO ₂ granules (samples 1 to 9) or SiO ₂ grains of crystalline, natural quartz were used (samples 10 and 11).

Each of Samples 1 to 9 is quartz glass of SiO ₂ granules produced by a conventional wet granulation method using a mixer. For this purpose, an aqueous suspension of amorphous, nanoscale, produced by flame hydrolysis of SiCl ₄ pyrogenic SiO ₂ particles having a BET specific surface area of 60 m ² / g are formed. The average particle size of the SiO ₂ primary particles is in the range below 100 nm. The suspension is removed from the suspension with continuous stirring until it disintegrates into spherical granules with outer diameters in the range between 160 μm and 1000 μm. The granules thus obtained are dried at about 400 ° C in a rotary kiln to a residual moisture content below 3 ppm by weight. With regard to the subsequent further treatment of the SiO ₂ granules, the quartz glass samples 1 to 9 differ.

In Sample 1, the starting granules were then strongly precompressed in the rotary kiln at a temperature of about 1420 ° C to a BET surface area of about 3 m ² / g, whereas the starting granules in samples 2 and 3 at 700 ° C only slightly pre-compressed (BET surface area around 40 m ² / g) was.

Highly precompressed granules as in sample 1 have a tamped density of 1.1 g / cm ³ and an average grain diameter of about 320 μm. The total content of the impurities Li, Na, K, Mg, Ca, Fe, Cu, Cr, Mn, Ti and Zr is less than 200 parts by weight ppb. As an alternative to pre-compaction in the rotary kiln, precompression and purification are carried out in a fluidized bed. In any case, the specific surface area of the granulate is reduced from previously 100 m ² / g to less than 20 m ² / g.

Samples 4 to 9 are quartz glass made of SiO ₂ granules, in which the individual granules of the granules were first precompressed in the same way as the granules in Sample 1, and which were then completely vitrified under different atmospheres. The heating profile during vitrification of the granules each comprises a heating to 1000 ° C with a heating rate of 5 ° C / min and a hold time of 120 min, a heating to 1400 ° C at a heating rate of 5 ° C / min and a further hold time of 120 min. After this treatment, the individual granules are vitrified in themselves. The grains are individually present without being fused to a mass.

For samples 4 and 5, vitrification was carried out at 1400 ° C. under vacuum (residual pressure for sample 4: 10 ^-2 mbar, residual pressure for sample 5: 10 ^-3 mbar), and for samples 6 and 7 under helium. In the case of sample 8, vitrification of the individual granules was carried out under air, at a flow rate of 0.1 m ³ / h, and in the case of sample 9 under hydrogen, at a flow rate of 10 m ³ / h.

A special feature is the sample no. 7. This is a doped with Al ₂ O ₃ quartz glass. For this purpose, the SiO ₂ granules were produced by means of a conventional wet granulation method using a mixer, as described above, wherein the suspension was additionally added AlCl ₃ , in an amount that the resulting Al ₂ O ₃ in the quartz glass 10 ppm by weight , based on the amount of SiO ₂ . The drying, compression and vitrification of the granules is carried out as in the sample 6 under helium.

Samples 10 and 11 are fused quartz glass of natural quartz crystal which has been cleaned by hot chlorination at high temperature (about 1050 ° C). The BET specific surface area of these samples is well below 1 m ² / g.

fusion for producing a quartz glass body

The particulate SiO ₂ starting materials (granules, grain size) were subsequently melted down into quartz glass bodies.

at Samples 1 to 7 and Sample 10, this was a Tiegelziehverfahren used. For this purpose, in each case an approach of about 100 kg in a molybdenum crucible under a hydrogen atmosphere melted and over a bottom opening of the crucible pulled down as a pipe string. The dew point of the hydrogen atmosphere was at each of these samples at about -55 ° C.

Out The granules of samples 8 and 9 were each in an amount of 500 g in a closed, evacuable chamber in one Mo crucible under hydrogen-containing atmosphere Fused silica blocks melted. The granules were in the same plant, in which also the Vitrification of the individual grains took place from the holding temperature of 1400 ° C with a heating rate a temperature of 2000 ° C heated. Upon melting of samples 8 and 9, hydrogen became used with a low dew point of -64 ° C, resulting in the process pressure of 2 bar corresponds to a water content of 3 ppm.

Out the quartz grain the sample 11 was produced by melting in the oxyhydrogen flame a quartz glass body. This was the starting grain by means of the well-known Verneuil method in a blast gas flame interspersed and vitrified on a substrate to a rod-shaped quartz glass body.

Determination of the OH content and tempering treatment

The total content of hydroxyl groups (C _{OH (t)} + C _{OH (m)} ) was determined spectroscopically on prepared samples of the quartz glass bodies produced as described above. Subsequently, samples of the same quartz glass material were subjected to an annealing treatment to determine the concentration of stable-temperature hydroxyl groups (C _{OH (r)} ). For this purpose, 10 mm thick slices of the glass were heated under vacuum (10 ^-2 mbar) to a temperature of 1040 ° C during a period of 40 hours. This treatment removes the existing metastable OH groups. By definition, the hydroxyl group concentration measured corresponds to the content of temperature-stable hydroxyl groups.

Determination of contamination tendency

by virtue of This measurement was for the quartz glass samples have the results given in Table 1.

In column 3 of Table 1, under C _{OH (t)} + C _{OH (m),} the total hydroxyl group content of the respective Quartz glass sample listed. This is composed of metastable hydroxyl groups C _{OH (m)} and temperature-stable hydroxyl groups C _{OH (t)} . l _Diff (last column) indicates the diffusion length of the minority charge carriers measured in the semiconductor material in conjunction with the respective sample. Ideally, l _Diff is about 200 μm (line A in _FIG 1 ). A value above 100 μm is still considered acceptable (line B in FIG 1 ).

The quartz glass sample 1 produced from partially glazed granules with a total hydroxyl group content of 250 ppm by weight and a content of temper stable OH groups of 191 ppm by weight is characterized by a comparatively large mean free diffusion length l _Diff of 140 μm. This quartz glass can be readily used for manufacturing apparatuses for semiconductor production.

Although the quartz glass samples 2 and 3 melted from low-density granules have average free diffusion _lengths l _diff in the same order of magnitude as sample 1; however, these glasses are of limited usefulness for high temperature applications due to their high total hydroxyl group content and concomitant low viscosity.

For quartz glass samples 4 to 9 melted from pre-glazed SiO ₂ granules, the total hydroxyl group content is significantly dependent on the conditions prevailing during vitrification of the granules.

Samples 4 and 5 differ only in the amount of negative pressure applied during vitrification of the granules. While with sample 4 with a vacuum of 10 ^-2 mbar during the vitrification a content of temper stable OH groups of even 69 ppm by weight, and consequently an acceptable value of 110 μm for the average diffusion length, the result is somewhat lower Residual pressure of 10 ^-3 mbar for sample 5 such a low content of stable-tempered OH groups in the fused quartz glass, that for the average diffusion length l _Diff results in a value below 100 microns, which is too low for an effective inhibition of the diffusion of Contamination is considered.

The vitrification of the granules under helium proves to be less problematic in terms of setting a suitable content of stable-temperature hydroxyl groups. Both samples 6 and 7 thereof show a sufficiently high concentration and a relatively long mean free diffusion length l _Diff, wherein the doped with alumina Sample 7 both in terms of the mean free diffusion length l _Diff as well as with respect to the high temperature of the quartz glass to be particularly advantageous proves.

The smelting of the under air-glazed SiO ₂ -Granultas the sample 8 under a hydrogen-containing atmosphere leads to a surprisingly low total hydroxyl group content, of which the content of metastable OH groups is about 30 ppm by weight and to an acceptable value for the middle free diffusion _length l _diff of 100 microns leads. The low overall hydroxyl group content of the fused quartz glass body melted despite the vitrification of the granules in air is due to the applied low-OH melting technique, in which the melt atmosphere is almost free of water. Such low-OH melting techniques, in which an incorporation of OH groups by reducing atmosphere (hydrogen atmosphere with low dew point) is avoided, are widely used to protect the commonly used melting vessels or fusible links of tungsten or molybdenum in the melting of quartz glass.

In the case of the SiO ₂ granulate of sample 9 which has been vitrified under hydrogen, the same melting technique leads to a glass unfavorable with regard to suitability for semiconductor production. In this quartz glass, the already low total hydroxyl group content of 60 ppm by weight proves to be almost completely austemperbar. The resulting content of metastable OH groups of only 3 ppm by weight is too low to contribute to an acceptable value for the average free diffusion length l _Diff . In the case of the quartz glass of sample 9, l _Diff is 60 μm. The sample 9 is typical of a quartz glass, which is obtained by an overall low-OH process technology.

Of the Comparing the quartz glasses melted to samples 8 and 9, that both by the pretreatment of the granules, and by the leadership the melting process, the content of stable-temperature OH groups significantly can be influenced. The quartz glass obtained from Sample 8 shows despite the low-OH melting process acceptable Results from previous, OH-enriching vitrification treatment of the granules under air.

The quartz glass of grain size of natural quartz crystal according to sample 10 is characterized by an OH content of about 110 ppm by weight, which, however, is almost completely austemperable. The content of stable-temperature hydroxyl groups is only 16 ppm by weight, and the measured at this quartz glass average free diffusion length l _Diff is below 50 microns.

The fact that this low value for l _{Diff is} not attributable solely to the content of impurities in the natural raw material is confirmed by sample 11, in which the quartz granules were melted in a flame melting process. Apparently, the oxyhydrogen flame used in this case contributes to a comparatively high total hydroxyl group content of the fused quartz glass of 160 ppm by weight, which also proves to be stable in temperature as a whole. For this silica glass, an acceptable mean free path of about 130 μm was determined. However, it turns out that the temperature-stable OH groups are distributed unevenly over the thickness of the quartz glass rod obtained by the Verneuil method and enriched in the region of the surface, so that the concentration of stable-temperature hydroxyl groups deviates by more than 10% from the mean over the wall thickness ,

Claims (16)

Electro-fused, synthetic quartz glass, in particular for use in the lamp and the Semiconductor manufacturing, which has a concentration of stable-temperature hydroxyl groups in the range between 20 ppm by weight and 400 ppm by weight, wherein the temperature-stable hydroxyl groups are characterized in that they by heating the quartz glass to a temperature of 1040 ° C under a vacuum of 10 ^-2 mbar and not be removed over a diffusion path of 5 mm for a period of 40 hours. Synthetic quartz glass according to claim 1, characterized characterized in that the concentration of hydroxyl groups maximum 500 ppm by weight. Synthetic quartz glass according to claim 1, characterized characterized in that the concentration of stable-temperature hydroxyl groups in the range between 30 ppm by weight and 200 ppm by weight. Synthetic quartz glass according to one of the preceding Claims, characterized in that the concentration of metastable hydroxyl groups at most 100 ppm by weight, preferably at most 50 ppm by weight. Synthetic quartz glass according to one of the preceding claims, characterized in that it has an Al ₂ O ₃ content in the range between 5 ppm by weight and 20 ppm by weight. Synthetic quartz glass according to one of the preceding Claims, characterized in that it has in the form of a wall thickness Quartz glass component is present, the concentration of stable to temperature Hydroxyl groups over the wall thickness by a maximum of +/- 10 % deviates from the mean. Synthetic quartz glass according to one of claims 1 to 5, characterized in that it is in the form of a layer on a Quartz glass component is formed. A process for the production of synthetic quartz glass according to any one of claims 1 to 7, comprising the process steps: (a) preparing a suspension of amorphous SiO ₂ primary particles having a mean particle size of less than 5 μm, (b) granulating the suspension to form porous SiO ₂ granules having a BET specific surface area of at least 20 m ² / g, (c) drying the SiO ₂ granules, (d) compacting and purifying the dried SiO ₂ granules to form a compacted SiO ₂ granules with a specific surface area (according to BET) of less than 40 m ² / g, and (e) melting the compacted SiO ₂ granules to form quartz glass, characterized in that the SiO ₂ granules compacted according to process step (d) before or during the melting according to process step (e) experiences an occupancy of its specific surface with hydroxyl groups, and that the melting according to process step (e) at least temporarily e takes place in a hydrogen-containing atmosphere in an electrically heated furnace, such that in the quartz glass, a target concentration of temperature-stable hydroxyl groups in the range of 20 ppm by weight to 400 ppm by weight is set. A method according to claim 8, characterized in that the assignment of the specific surface of the SiO ₂ granules with hydroxyl groups in a water-containing atmosphere prior to melting takes place after process step (e). A method according to claim 8 or 9, characterized in that when compacting and cleaning the dried SiO ₂ granules in process step (d) a compacted SiO ₂ granules having a specific surface area (BET) between 1.5 m ² / g and 30 m ² / g and with a tamped density of at least 0.75 g / cm ^{3 is} generated. Method according to one of the preceding method claims, characterized in that the compression according to process step (d) is at a temperature in the range between 800 ° C and 1350 ° C. Method according to one of the preceding method claims, characterized characterized in that the compacting and cleaning according to method step (d) in a chlorine-containing atmosphere he follows. Method according to one of the preceding method claims, characterized characterized in that the melting according to process step (e) in an atmosphere takes place, which has a dew point in the range between -35 ° C and -70 ° C, preferably above -65 ° C, has. Method according to one of the preceding method claims, characterized in that the SiO ₂ granules after drying has a residual moisture content of less than 3 wt .-%. Method according to one of the preceding method claims, characterized in that SiO ₂ primary particles are used, which are formed by flame hydrolysis or oxidation of inorganic silicon compounds. Method according to one of the preceding method claims, characterized in that the suspension of amorphous SiO ₂ primary particles according to process step (a) pyrogenically prepared Al ₂ O ₃ or an aluminum salt, preferably in the form of a halide, is added.