JPS6283325A - Production of quartz glass having high purity - Google Patents

Abstract

PURPOSE:To vitrify a porous quartz glass parent material having a large caliber to transparent glass without impairing the parent material and to obtain quartz glass having high purity contg. no defects such as bubbles by calcining previously to make the distribution of the density in the diametral direction of the parent material more uniform prior to the vitrification of the porous quartz glass parent material. CONSTITUTION:In a stage of vitrifying by heating a porous quartz glass parent material which has been grown by depositing on an end of a seed quartz glass bar, the porous glass parent material is calcined previously at 1,150-1,350 deg.C to adjust the distribution of the density in the diametral direction of the parent material, then the parent material is vitrified by calcining at 1,400-1,500 deg.C. It is preferred that the above-described calcination is carried out to cause contraction of the parent material by 10-45% in the diametral direction by inserting the porous parent material into a heating furnace slowly while revolving the parent material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing high-purity quartz glass.

[Prior Art] Conventionally, as one method for producing synthetic quartz glass, a porous quartz glass ifi material is formed by a gas phase reaction,
The method used is to heat this base material and vitrify it. For example, a method called the VAD method is used to manufacture synthetic quartz glass as a material for optical fibers.

This method uses silicon compounds and hydrogen from the burner.

A raw material gas such as oxygen is supplied to a vertically suspended seed rod, and silica fine particles generated by hydrolyzing silicon compounds such as silicon tetrachloride in an oxyhydrogen flame are placed at the lower end of a seed rod made of quartz or the like. After adhesion and deposition to form a porous silica glass TL material (hereinafter sometimes referred to as porous base material or base material).

This method uses a heater installed at the top to create transparent glass. This method has the advantage that the formation of a porous IfJ material and the formation of transparent glass can be performed continuously. The porous base material formed by this method has a roughly cylindrical shape.

[Problems to be Solved by the Invention] When attempting to manufacture large-sized quartz glass for photomask substrates, etc. using this method, it is necessary to make the porous quartz glass base material large, and a large eight-piece However, the flame emitted from the burner has a high temperature at the center, but a lower temperature at the outer periphery than the center, and the density at the outer periphery of the porous quartz glass matrix formed by this temperature difference is lower than that at the center. For example, when a cylindrical porous quartz glass base material with a diameter of 30 cm is produced by the above method, the density of the outer periphery, where silica fine particles adhere at a lower temperature than the center, is 0.1 to 0. .15g/cc and 0.15g/cc in the center.
35 to 0.453/cc. When the above-mentioned cylindrical porous base material having such a density distribution is heated to 1400 to 1500°C and attempted to be made into transparent glass, cracks occur from the outer periphery of the base material during firing and damage occurs, and the transparent loft A defect or a problem occurring in multiple values was found.

One of the causes of such defects is that when a porous quartz glass base material is sintered to become transparent glass, the base material is
Although shrinkage of 50 to 80% occurs in the axial direction, in a base material with the above density distribution, the shrinkage progresses unevenly between the center and the outer periphery, so if you try to vitrify a large base material with a diameter of 30 cm in one step, It is thought that it will not be able to withstand rapid thermal contraction and cracks will occur from the outer periphery. Furthermore, with large-diameter base materials, the temperature difference between the center and the outer periphery becomes large during firing, and if you try to make the glass transparent in one step, the sintering of the outer periphery, which has a low density and is exposed to high temperatures, will progress quickly, causing the inside of the base material to sinter. It is thought that the air bubbles contained in the loft become loose at the outer periphery, causing defects such as air bubbles to occur in the loft.

It is possible to make the radial density distribution of the porous quartz glass base material constant as described above to equalize the shrinkage of the base material and prevent the occurrence of defects, but it is difficult to make the temperature distribution of the flame uniform. Therefore, it was extremely difficult to obtain a large-diameter porous quartz glass base material with a constant radial density distribution.

[Means for Solving the Problems] The present invention solves the problems of the prior art described in ``-'', converts a large-diameter porous quartz glass base material into transparent glass without damaging it, and transforms the resulting quartz glass into transparent glass. It was invented as a result of research with the aim of providing a vitrification method for porous quartz glass base material that does not contain defects such as air bubbles. Deposits at one end of the
When converting the grown porous quartz glass base material into transparent vitrification, the porous quartz glass LI material is preliminarily heated to 1150 to 13
Production of high-purity quartz glass characterized by calcination (also referred to as pre-firing) at 50°C to adjust the density distribution in the radial direction of the base material, and then firing at 1400 to 1450°C to form transparent glass. It is about the method.

In the present invention, the porous base material is manufactured by, for example, an apparatus as shown in FIG. That is, hydrogen and oxygen are supplied from the pumps 1 and 2 to the multi-tube burner 5 through the mass flow controller 3.4. Further, gases of silicon compounds such as silicon tetrachloride, trichlorosilane, and silicon tetrabromide are supplied from the tank 6 to the multi-tube burner 5 by a pump 7 through a heat exchanger 8. The multi-tube chamber 5 forms an oxyhydrogen flame in the reaction chamber 9 to hydrolyze the silicon compound to form silica fine particles. Although not shown in the figure, inert gases such as nitrogen and argon can also be used.
It is used as a carrier gas for these silicon compounds or as an air curtain in an oxyhydrogen flame.

The chemical formula for this hydrolysis reaction when the silicon compound is silicon tetrachloride is as follows.

2 H2+ oz→2H20・・・・・・・・・(1)
2 H2O'+ 5iGla → SiOノ
+ 4) 101 ・・・ ・・・ ・・・ (2)
The silica fine particles adhere to and deposit on the lower end of the quartz seed rod 10 suspended vertically in the reaction chamber 9, and are sequentially eroded to form a large-diameter porous quartz glass base material 11. Note that MCI generated by the reaction is brought into contact with the NaOH solution in a countercurrent flow in the washing tower 13 and is absorbed and removed.

In the present invention, the porous quartz glass base material manufactured by the above-described method is preliminarily heated to
The porous quartz glass base material is calcined in a temperature range of 0 to 1350°C to adjust the density distribution in the radial direction. It is preferable to carry out this calcining in a clean atmosphere, and if necessary, air pre-treated with a filter, nitrogen, or other inert gas may be introduced in a heating furnace within the above temperature range. The firing time varies depending on the temperature, the length of the furnace temperature zone described later, etc., but the radial shrinkage rate of the porous quartz glass base material after calcination is 10 to 45z, preferably 25 to 3
You can choose from conditions within the range of 5z. If the shrinkage rate is less than 1 oz, the density of the outer periphery of the porous quartz glass base material will not be sufficiently increased and non-uniform shrinkage will occur during transparent vitrification, resulting in breakage of the base material or defects on the outer periphery of the base material. Furthermore, when the yield exceeds 45%, the density at the outer periphery of the porous quartz glass base material becomes too high, and the internal degassing becomes incomplete, causing defects such as bubbles in the transparent glass. Cause. For the above reasons, the shrinkage rate of the porous quartz glass base material during calcination is 10 to 45, preferably 35.
It is necessary to control within the range of ~402.

In the present invention, the above-mentioned calcination of the base material is carried out by rotating the base material in a heating furnace (not shown) heated to a relatively low temperature such that the base material is not damaged, for example, as in the embodiment described later. This can be carried out by inserting the base material while the base material is being rotated, then gradually raising the temperature to a temperature in the range of 1150 to 1350°C, and maintaining the base material at this temperature for the required time while rotating the base material. In addition, for example, a heating furnace is provided with a temperature gradient that increases from the bottom to the top of the furnace, or the temperature gradient is further increased so that a partial area in the upper part of the heating furnace becomes a uniform temperature region where the temperature is almost uniform in the vertical direction. In a heating furnace provided with a temperature distribution, the base material produced by the method described above is first inserted into the lower part of the heating furnace (not shown) in which the calcination is performed, and the seed rod is moved to the upper part of the heating furnace while rotating. Alternatively, the calcining may be carried out gradually from the upper part of the base material, and then the calcined base material is extracted from the upper part of the heating furnace.

After performing such calcination, the porous quartz glass base material is transferred to the furnace in which the calcination was performed or to another heating furnace, and is sintered immediately or after a predetermined period of time to become transparent vitrification. This main sintering is carried out in a heating furnace in an atmosphere containing He gas of at least 70% or more, preferably 80 to 902.
The time for zero-strand sintering performed in the temperature range of 00 to 1500°C varies depending on the temperature and degree of Hee, but is suitably 1.5 to 4 hours.

In the present invention 1, the heating furnace for converting the porous quartz glass base material into transparent vitrification (hereinafter, the above-mentioned main sintering and transparent vitrification will simply be referred to as transparent vitrification) increases from 2 parts to 1 part. It is preferable to create a temperature gradient. In this way, the temperature of the porous quartz glass base material can be gradually increased when the porous quartz glass base material is inserted into the furnace from above. It is also possible to prevent air bubbles from remaining in the transparent glass due to sudden temperature changes. The temperature gradient at this time is such that the upper part of the heating furnace is
Around 200℃, lower temperature of furnace 1400-1500℃
During transparent vitrification, it is optimal to stop the descent when the vicinity of the lower end of the seed rod reaches a high temperature range of 1400° C. or higher. In this way, the porous quartz glass base material can be completely transformed into transparent glass while the base material is supported by the seed rod.

In this way, the base material can be turned into transparent vitrification without exposing the seed rod for a long time to the high temperature range of 1400°C or higher, which softens the quartz seed rod, which softens the seed rod. It prevents oats from falling and can support large diameter quartz glass rods without falling. Furthermore, air bubbles that flow out due to transparent vitrification can escape to the upper porous layer that has not yet been transparent vitrified, so it is possible to prevent air bubbles from being contained in the obtained quartz glass. .

In the present invention, the heating during calcination and transparent vitrification is performed by gradually inserting the porous quartz glass base material into the heating furnace while rotating it, and by rotating the porous quartz glass base material at zero rotation, the base material can be heated uniformly. , it is possible to prevent the base material from deforming due to non-uniform shrinkage during firing.

Further, in the present invention, the heating furnace for converting the base material into transparent vitrification is provided with a temperature gradient that increases from the top to the bottom in the heating furnace as described above. The partial region may have a temperature distribution such that the temperature is approximately equal in the vertical direction and becomes a uniform temperature region.

Example A cylindrical porous quartz glass base material (diameter 30 mm) was formed on a quartz seed rod manufactured using the apparatus shown in FIG.
C layer, length 0.5 m, average density in the center (3 cm in diameter from the center) is 0.45 g/cc, and average density in the surface (4 cm from the surface) is 0.23/cc). It is inserted into a heating furnace (not shown) maintained at 300℃ while rotating slowly, and N2 gas is supplied at 3m3/H from the bottom of the furnace.
1300 at a heating rate of 250°C/Hr while supplying at r.
The temperature was raised to 0.degree. C. and held under these conditions for 60 minutes. This is the calcination of the base material. The shrinkage rate in the radial direction of the porous quartz glass base material after calcination is approximately 35%, and the average density at the center is 0.6 g/7 cc, and the average density at the outer periphery is 0.68 g/7 cc.
It was cc. Immediately after calcining, as shown in FIG. The sample was inserted from above into a heating furnace 21 maintained in a helium atmosphere with a helium concentration of 1.5%. This heating furnace 21 has a temperature gradient of about 1200°C in the upper part and about 1430°C in the lower part, and slowly rotates a class 0 rod IO which is controlled so that the uniform temperature range above 1400°C is about 20cm. while letting 1
The porous quartz glass base material 11 was inserted into the heater 22 from its lower end by descending at a speed of 00 Sokaku/Hr (second
Figure (a)).

This porous quartz glass base material 11 was gradually degassed from the lower end and turned into transparent glass, forming transparent glass 23 with a diameter of approximately 13 cm (FIG. 2(b)). The surface layer of the fused silica glass (approx. 51 mm thick)
There were no defects in the inner center, and the number of bubbles generated on this surface was as small as 0.8 on average per 1 cm2 of surface area.

On the other hand, when transparent vitrification was performed by the same method as above without performing calcination, cracks occurred in the upper porous glass part when the base material had become transparent vitrification by about % from the lower end.
The bottom fell off from this part. As a result of evaluating the internal defects of this transparent vitrified part, we found that the surface layer (approximately 3 cm
Although no defects were found in the inner central part from +s thickness), an average of 20 defects were found in this surface layer per 3 ICl.

In addition, the holding time during calcination using the method described above is approximately 180%.
When the radial shrinkage rate of the porous quartz glass base material was increased to 5H and the shrinkage rate in the radial direction of the porous quartz glass base material was increased to 5H, transparent vitrification was performed using the above method, and as a result, transparent vitrification was possible without cracking the base material. However, the surface layer of this quartz glass (
Although the number of defects was as low as 0.8 defects per 1 c 2 (approximately 5 layers thick), about 0.6 to 1 defect per 1 kg of quartz glass was also observed in the inner part from the surface layer.

[Effects of the Invention] As explained above, according to the present invention, before a porous quartz glass base material is subjected to a transparent vitrification treatment by sintering, it is calcined in advance at a temperature lower than the temperature of the transparent vitrification treatment to form a porous quartz glass base material. Since the quartz glass base material has a uniform density distribution in the radial direction, it is possible to prevent cracks from occurring due to rapid shrinkage. Furthermore, the occurrence of internal defects such as bubbles due to differences in the radial density distribution of the porous quartz glass base material can be significantly reduced, resulting in improved quality and yield.

In particular, the method of the present invention is most suitable for transparent vitrification of a large-diameter porous quartz glass base material, the above-mentioned effects are remarkable, and high-purity quartz glass can be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory view showing an example of an apparatus for obtaining a porous quartz glass base material, and FIGS. 2(a) and 2(b) are explanatory views showing an example of the vitrification method according to the present invention.

Claims (3)

[Claims] (1) When heating the porous quartz glass base material deposited and grown on one end of the quartz glass seed rod to turn it into transparent vitrification,
A high-purity quartz characterized by preliminarily calcining a porous glass base material at 1150 to 1350°C to adjust the density distribution in the radial direction of the base material, and then firing at 1400 to 1500°C to form transparent glass. Glass manufacturing method. (2) The method for producing high-purity quartz glass according to claim 1, wherein the calcination shrinks the porous quartz glass base material by 10 to 45% in the radial direction. (3) A patent characterized in that the calcination is performed by gradually inserting the porous base material into a heating device while rotating the porous quartz glass base material, thereby shrinking the porous quartz glass base material by 10 to 45% in the radial direction. A method for producing high-purity quartz glass according to claim 1.