Method for producing heat-resistant synthetic quartz glass

Abstract: To provide a method of producing dense, highly heat-resistant and high-purity synthetic quartz glass, a treatment of a silica porous body with a density of 0.1 g/cm 3 to 0.5 g/cm 3 and the variation of density of less than 30% is suggested, comrising a first heat-treatment in an ammoniacal atmosphere, a second heat-sintering in non-oxidizing atmosphere, and further heat-treatment at a temperature in the range of 1400 °C to 2000 °C under an increased pressure of 500 kg/cm 2 or more in a non-oxidizing atmosphere.

Description: Industrial Field of the Application The present invention relates to a method of producing heat-resistant, nitrogen-doped synthetic quartz glass used as material for a high-purity heat-resistant structural member or as material for a quartz glass substrate of a thin-film transistor using polycrystalline silicon. Prior Art Conventionally quartz glass was used as material for a furnace tube in a fabrication process of a semiconductor device such as LSI ( process tube or liner tube ), a wafer boat, or other jigs, since it is highly pure and does not change its characteristics and the original form of a body thereof even at a temperature as high as about 1000 °C in a heat-treatment. In recent years, the heat-treatment for LSI has come to be conducted at about 1200 °C and therefore high-purity silicon carbide heat-resistant material has been used in place of quartz glass, which is subject to viscosity deformation. The silicon carbide heat-resistant material unavoidably includes a trace of impurities by nature from a method of producing it and the impurities included in the heat-resistant material has then become a great problem to fabrication of super LSI with a large packing density of elements as a contamination source. In view of the problem, quartz glass, which is easier for purifying, has, therefore, become to be reevaluated and improvement on the heat-resistance is strongly demanded. As an example of a method of producing highly heat-resistant quartz glass, there is provided one as described in publication of Unexamined patent application No. Sho 63-236722. In the method, elements of impurities that become nuclei inducing cristobalitization in quartz glass material are introduced into the surface region and the quartz glass is thus conferred with not only high heat-resistance but also barrier to stop the impurities from intrusion, since a cristobalite layer is formed in the surface region. This method is effective in an aspect to produce quartz glass with improved heat-resistance. When this quartz glass is long used, however, a fault is apparent that the region occupied by a cristobalite layer grows, so that the quartz glass becomes brittle and apt to produce dusts, or in some case suffers occurrence of cracks. As a method to solve the fault, a method was proposed that nitrogen atoms were introduced into quartz glass to substitute stronger Si-N bonds for Si-O bonds. In more details, this method comprises a step of conducting heat-treatment of a silica porous body made of silicon dioxide synthesized by vapor phase synthesis methods such as VAD method ( Vapor-Phase Axial Deposition ), OVD method ( Outside Vapor Deposition ), MCVD method ( Modified Chemical Vapor Deposition ) at a temperature at which densifying by sintering does not occur ( 1200 °C or lower ) in a furnace filled with a ammoniacal atmosphere and another step, which follows the former, of sintering by heating at a temperature higher than the aforementioned temperature so as to obtain dense nitrogen-doped quartz glass. In order to improve the heat-resistance of the quartz glass produced by these methods, that is, to be 12.5 dPa s or more in viscosity (log) at 1280 °C, where the viscosity (log) at 1280 °C is an index for heat-resistance and the 12.5 dPa s or more is of the natural quartz glass produced by the electric fusion method, much of nitrogen should be doped and that is a fault in the proposed methods, since nitrogen gas and hydrogen gas arise in the silica porous body thus highly doped with nitrogen by decomposition during sintering of the body and the gases cause to generate bubbles to thereby invite difficulty in obtaining dense quartz glass. Problem the Invention Seeks to Solve In light of such present circumstances of technology, the present inventor has found after a long and intense study a fact that highly heat-resistant synthetic quartz glass is producible by sintering a porous silica body in a non-oxidizing atmosphere and further heating the sintered porous body at a high temperature under an increased pressure ( hereafter referred to as HIP treatment ) and thus completed the present invention. It is an object to provide a method of producing dense, highly heat-resistant synthetic quartz glass. It is annother object to provide a method of producing high-purity synthetic quartz glass having a viscosity ( log ) of 12.5 dPas or more at 1280 °C. Means to Solve the Problem The present invention to achieve the objects above-mentioned pertains to a method of producing heat-resistant, nitrogen-doped synthetic quartz glass where after a silica porous body is heated in an ammonia containing atmosphere, it is sintered by heating in a non-oxidizing atmosphere and then further heated at a temperature in the range of 1400 °C to 2000 °C under a pressure of 500 kg/cm or more in a non-oxidizing atmosphere, whereby the porous silica body has a density prior to sintering of 0,1 g/cm 3 to 0,5 g/cm 3 and a density variation of less than 30%. The silica porous body above mentioned is that which, in synthetic methods developed and put into practical use for the purpose of producing optical fibres such as MCVD method, OVD method, VAD method, silicon chloride such as silicon tetrachloride is vaporized, the vapor is subjected to hydrolysis in an oxidizing atmosphere such as a flame, the silica fine particles thus produced are piled up to a loose body, and finally it is sintered by heating to be transformed to a extremely high-purity amorphous silica porous body (hereafter referred to as synthetic quartz glass base material). When the above-mentioned synthetic quartz glass base material is heat-treated in an ammoniacal atmosphere, some of the oxygen-atoms (O) in the SiO 2 network is replaced by nitrogen-atoms (N) and as a result N is doped in the quartz glass base material. In this doping treatment N is fixed through the following reaction taking place: Si-OH + NH 3 → Si-NH 2 + H 2 O Through the above-mentioned treatment nitrogen can be doped at a concentration in the range of 2000 ppm to 20000 ppm. As a gas containing ammonia used in the treatment, for example, ammonia gas itself or its mixtures with a inert gas such as nitrogen, helium or argon are named. Ammonia is most suitable as a doping agent, because it is readily available in the state of high-purity and H 2 and H 2 O which are products from the decomposition are removed away out of the reaction system with ease. Nitrogen gas can also be considered a candidate for the doping agent, but is rather not suitable due to difficulty in replacement for oxygen. The nitrogen-doped synthetic quartz glass base material is then sintered by heating and in this step the NH 2 fixed as described above is separated free as N 2 or H 2 . These gases are kept to remain inside closed bubbles formed in the course of sintering to disturb densifying of quartz glass by thermal expansion. The separation of nitrogen vigorously takes place at a temperature of sintering of 1200 °C or more, but if the base material is heated up to 1700 °C, which is the temperature at which densifying starts, at a rate of 200 °C/h or less, before micro-pores of the base material are closed, the decomposition gases flee out of the bulk of the base material and as a result generation of bubbles is prevented. In the course of sintering, if parts with high densities are present inside the bulk of the base material, a gas of N 2 or H 2 , which is trying to flee, encounters an obstacle against fleeing, is thus confined inside, and it expands during heating at a temperature of sintering to become bubbles. From that reason, it is important that a density of the synthetic quartz glass base material should be set in the range of 0.1 g/cm 3 to 0.5 g/cm 3 and the varaition of density in the range of less 30%. A porous body can not hold its original shape in the range of density lower than the lower limit and on the other hand nitrogen doping to the porous body becomes hard in the range of density higher than the higher limit. When the variation of density falls outside the limiting range of less than 30%, distribution of nitrogen becomes uneven in the bulk and thus improvement on heat-resistance is not realized. The atmosphere is regulated preferably under a reduced pressure, in particular at a vacuum degree of 13 Pa or less, in the heat-sintering step. In sum under the above indicated conditions, a synthetic quartz glass that does practically not contain any bubble and is densified. However, bubble generation has often occurred in the bulk of the synthetic quartz glass produced by the above mentioned process during a further heat-treatment at a temperature in the operational temperature range. When, following the above-mentioned heat-treatment, an HIP treatment was conducted under conditions of a pressure in the range of 500 kg/cm 2 or more, preferably between 1000 kg/cm 2 and 2000 kg/cm 2 , and a temperature in the range of 1400 °C to 2000 °C, no bubble arose within the operational temperature range. Such extinction of bubbles is considered to caused by phenomena that N 2 and H 2 present in the state of molecules are dispersed in the bulk of quartz glass or revive the original Si-N bonds again by the HIP treatment. The synthetic quartz glass as thus obtained by heat-sintering was further improved on the heat-resistance by a second HIP treatment. In an HIP treatment at a temperature of 1400 °C or lower, however, the effect of a pressure can reach only to the surface region due to excessively high viscosity of the synthetic quartz and thus the above-mentioned reaction does not occur. Beyond 2000 °C upwards of the heat-treatment, sublimation of glass unfavorably arises. In an HIP treatment, under a pressure of less than 500 kg/cm 2 , the above-mentioned effect is not observed and on the other hand a super-high pressure more than 2000 kg/cm 2 is not acceptable from an industrial view point due to excess in cost. In the following the invention will be described by reference to examples. The measurements of physical characteristics in the examples are conducted in conformity with the following instructions. (1) Heat-Resistance: to cut a glass body into strips and evaluate a viscosity at 1280 °C by the beam-bending method. (2) Nitrogen Concentration: to measure by the inert-gas fusion thermal conductivity method. (3) Specific Gravity: to measure by an Archimedian method. (4) Heat-Bubble Test: to heat and soften by an oxygen-hydrogen burner and observe generation of bubbles. Example 1: High-purity synthetic quartz glass with an density of 0.2 g/cm 3 and an overall variation of density of 10% produced in vapor phase by a VAD method, as shown in Table 1, was heat-treated at 1000 °C for 4 hours in an atmosphere composed of 50 vol% of NH 3 and 50 vol% of N 2 . In the obtained base material included was 0.3 wt% of nitrogen. After the above-mentioned nitrogen-doped synthetic quartz glass base material was heated-up to 1200 °C from room temperature at a rate of 10 °C /min. under a reduced pressure of about 7 Pa, secondly further heated-up to 1600 °C from the 1200 °C at a rate of 2 °C /min and finally kept at the last temperature for 2 hours, a heater was disconnected and the base material was left to be cooled to room temperature to be then taken out. The thus treated synthetic quartz glass did not include any bubble and was dense at the same time. The synthetic quartz glass was inserted in an HIP-treatment furnace placing in a carbon vessel, heated-up to 1600 °C from room temperature at a rate of 400 °C/h in an argon atmosphere, and at the same time the pressure was increased to 1600 kg/cm 2 from 1 kg/cm 2 at a rate of 400 kg/cm 2 hr. Thereafter the synthetic glass was held at the 1600 °C under the1600 kg/cm 2 for 5 hours, in sequence the heating was stopped to cool down the synthesized-quartz glass to room temperature and the synthetic quartz glass was taken out. The synthetic quartz glass taken out included no bubble and was densified. Samples were prepared by cutting to measure the physical characteristics, which are shown in Table 1. Comparative Example 1: Nitrogen-doped synthetic quartz glass was produced in a similar manner to that in Example 1 except no HIP treatment being conducted. The synthetic quartz glass thus obtained had an appearance of being dense, but when the synthetic quartz glass was cut to strips to conduct a heat-bubble test, generation of bubbles was great. the physical characteristics of the synthetic quartz glass are also shown in Table 1. Comparative Examples 2 and 3: Quartz glass porous bodies treated by ammoniacal treatment in a similar manner to that in Example 1 were first heated-up to 1600 °C from room temperature at a rate of 10 °C/min. under a reduced pressure of about 7 Pa, then held at the temperature for 2 hours, and thereafter cooled down to room temperature by disconnecting a heater. No part of the thus obtained synthesized-quartz glass bodies was dense throughout the bulk and cloudiness remained in the inner bulk. A heat-bubble test was conducted on the synthetic quartz glass, which is resulted in very many bubbles arising. The other of the nitrogen-doped synthetic quartz glass bodies was given a HIP treatment ( Comparative Example 3 ). The synthetic quartz glass thus treated was further subjected to heat-bubble test and generation of bubbles was observed. The physical characteristics were measured on the respective nitrogen-doped synthetic quartz glass bodies and they are shown in Table 1. Comparative Examples 4 and 5 Synthetic quartz glass base material with an apparent density of 0.6 g/cm 3 and an overall density dispersion of 50% produced in vapor phase by a VAD method which uses a plurality of burners was treated in an ammoniacal atmosphere in a similar manner to that in Example 1 and then sintered. No part of the nitrogen-doped synthetic quartz glasses thus obtained was dense throughout the bulk and included parts of cloudiness in the bulk. A heat-bubble test was conducted on the synthetic quartz glass and the result was generation of very many bubbles. An HIP treatment, as conducted in Example1, was conducted on part of the nitrogen-doped synthetic quartz glass ( Comparative Example 5 ). A heat-bubble test was conducted on this synthetic quartz glass and bubbles were observed. The physical characteristics of the respective nitrogen-doped synthetic quartz glass samples were measured. The results are shown in Table 1. Comparative Examples 6 and 7: Synthetic quartz glass base material as shown in Table 1 with an apparent density of 0.6 g/cm 3 and an overall density dispersion of 50% produced in vapor phase by a VAD method which uses a plurality of burners was treated in an ammoniacal atmosphere in a similar manner to that in Example 1, then heated-up to 1600 °C from room temperature at a rate of 10 °C/min. under a reduced pressure of about 66 Pa, further held at the 1600 °C for 2 hours and lastly a heater was disconnected to cool the synthetic quartz glass to room temperature. No part of the nitrogen-doped synthetic quartz glass was dense throughout the bulk and parts of cloudiness were observed in the inside. A heat-bubble test was conducted on the nitrogen-doped synthetic quartz glass and generation of very many bubbles was found. An HIP treatment as conducted in Example 1 was further conducted on part of the nitrogen-doped synthetic quartz glass as Comparative Example 7. The result was still generation of bubbles. The physical characteristics of the respective nitrogen-doped synthetic quartz glass samples were measured and are shown in Table 1. Viscosity 1280°C (poise) Specific Gravity (g/cm 3 ) Metallic impurities (ppm) After Sintering by Heating Heat-Bubble Test N Na K Li Ca Fe Al OH Example 1 12.5 2.22 3000 < 0.1 < 0,1 < 0.1 < 0.2 < 0.1 < 5 0 Densified No Bubble Comparative Example 1 12.3 2.21 3000 < 0.1 < 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Densified Very many Bubbles Comparative Example 2 12.3 2.21 3000 < 0.1 < 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Very many Bubbles Comparative Example 3 12.5 2.22 3000 0.1 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Bubbles Comparative Example 4 12.3 2.21 3000 < 0.1 < 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Very many Bubbles Comparative Example 5 12.5 2.22 3000 0.1 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Bubbles Comparative Example 6 12.3 2.21 3000 < 0.1 < 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Very many Bubbles Comparative Example 7 12.5 2.22 3000 0.1 0.1 < 0.1 < 0.2 < 0.1 < 5 0 Cloudy Bubbles As seen from the above table, quartz glass produced by a method according to the present invention has a high viscosity at a high temperature and is dense enough to be processed at a high temperature. On the other hand, quartz glasses in the comparative examples have parts of cloudiness in the bulk and generation of bubbles was observed at a temperature in the operational temperature range. Effects of the Invention According to a method of the present invention, nitrogen-doped quartz glass with a high heat-resistance of 12.5 dPas or more at 1280 °C can be produced. In addition to that, since the raw material is produced by a vapor-synthesis method and therefore is a super-high-purity silica, the nitrogen-doped quartz glass according to the present invention never be a contamination source to semiconductor devices in working, even it is used as structural members for semiconductor-device fabrication facilities.

Claims: A method for producing heat-resistant, nitrogen-doped, synthetic quartz glass, comprising the following steps providing a synthetic, porous silica body, heating the porous silica body in an ammonia-containing atmosphere, sintering the thus obtained nitrogen-doped porous silica body into a sintered nitrogen-doped silica body by heating in a non-oxidizing atmosphere, characterized in that the porous silica body has a density prior to sintering of 0,1 g/cm 3 to 0,5 g/cm 3 and a density variation of less than 30 %, and that the sintered nitrogen-doped silica body is heated to a temperature of 1400 °C to 2000 °C under a pressure of 500 kg/cm 2 or more in a non-oxidizing atmosphere. A method for producing heat-resistant synthetic quartz glass as claimed in claim 1, characterized in that the density of the silica porous body before is sintering is 0.1 g/cm 3 to 0.5 g/cm 3 and the variation of density is less than 30%. A method of producing heat-resistant synthetic quartz glass as claimed in claim 1, characterized in that the sintering by heating is conducted at a pressure of 1 x 10 -1 torr or less and at a temperature in the range of 1200 °C to 1700 °C whereby the heating rate to the temperature is regulated to be 200 °C/h or less.

Inventors: FUJINOKI AKIRA, SATO TATSUHIRO, FUJINOKI, AKIRA, SATO, TATSUHIRO
Applicants: HERAEUS QUARZGLAS, SHINETSU QUARTZ PROD, HERAEUS QUARZGLAS GMBH & CO. KG, SHIN-ETSU QUARTZ PRODUCTS CO., LTD.
Application year: 1995
Main classification: C03C3
IPC codes: C03B20, C03B19, C03B32, C03B8