JPH0478567B2 - - Google Patents

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, one method for producing synthetic quartz glass has been to form a porous quartz glass base material through a gas phase reaction, and then heat this base material to vitrify it. . For example, regarding the production of synthetic quartz glass as a material for optical fibers, VAD
A method known as the law is used. In this method, raw material gases such as silicon compounds, hydrogen, and oxygen are supplied from a burner to a vertically suspended seed rod, and silicon compounds such as silicon tetrachloride are hydrolyzed in an oxyhydrogen flame to produce silica fine particles. is attached and deposited on the lower end of a seed rod made of quartz or the like to form a porous quartz glass base material (hereinafter sometimes referred to as porous base material or base material), and then This method uses a heater to create transparent glass.
This method has the advantage that formation of a porous base material and transparent vitrification can be performed continuously. The porous base material formed by this method has a substantially cylindrical shape. [Problems to be Solved by the Invention] When using this method to manufacture large quartz glass for photomask substrates, etc., it is necessary to make the porous quartz glass base material large and a large burner is used. The flame emitted from the burner has a high temperature at the center, but a lower temperature at the outer periphery than the center. Due to this difference in temperature, the density at the outer periphery of the porous quartz glass base material formed is lower than that at the center. There is a tendency to 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 particles adhere at a lower temperature than the center, is 0.1 to 0.2 g/cc, and the density of the center is 0.1 to 0.2 g/cc.
It becomes 0.35-0.45g/cc. When an attempt is made to heat the above-mentioned cylindrical porous base material with such a density distribution to 1400 to 1500°C to make it into transparent vitrification, cracks may occur from the outer periphery of the base material during firing, resulting in breakage, or the transparent rod may be damaged. A problem was discovered in which a large number of defects occurred. 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 shrinks by 50 to 60% in the radial and axial directions, but with the base material having the above density distribution, the shrinkage progresses unevenly between the center and the outer periphery, so a large base material with a diameter of 30 cm is If an attempt was made to vitrify it, it would not be able to withstand rapid thermal contraction, and cracks would appear from the outer periphery. Furthermore, with large-diameter base materials, there is a large temperature difference between the center and the outer periphery during firing, and if you try to make it transparent in one step, the sintering of the outer periphery, which has a lower density and is heated to a higher temperature, will proceed more quickly. The air bubbles contained inside become difficult to escape to the outer periphery,
This is thought to be the cause of defects such as bubbles occurring during the rod. By keeping the radial density distribution of the porous quartz glass base material constant as described above, the shrinkage of the base material is made uniform,
Although it is possible to prevent the occurrence of defects, it is difficult to make the temperature distribution of the flame uniform, and it is extremely difficult to obtain a large-diameter porous quartz glass base material with a constant density distribution in the radial direction. Ta. [Means for Solving the Problems] The present invention solves the problems of the prior art described above, converts a large-diameter porous quartz glass base material into transparent glass without damaging it, and eliminates air bubbles in the resulting quartz glass. It was invented as a result of research with the aim of providing a vitrification method for a porous quartz glass base material that does not include the disadvantages of quartz glass. When heating a porous quartz glass base material to turn it into transparent glass, the porous quartz glass base material is first rotated and gradually inserted into a heating device, and then calcined in the range of 1200 to 1350°C to give the base material a diameter. The base material is shrunk by 10 to 45% in the direction without reaching a saturation shrinkage rate to adjust the density distribution in the radial direction of the base material, and then fired at a temperature of 1400 to 1500°C to form transparent glass. The present invention provides a method for manufacturing high-purity quartz glass. In the present invention, the porous base material is manufactured using, for example, an apparatus as shown in FIG. That is, hydrogen and oxygen are supplied from cylinders 1 and 2 to multi-tube burner 5 through mass flow controllers 3 and 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 burner 5 forms an oxyhydrogen flame in the reaction chamber 9 and hydrolyzes the silicon compound to form silica fine particles.
Although not shown, an inert gas such as nitrogen or argon is also supplied to the burner 5 and used as a carrier gas for these silicon compounds or as an air curtain in the oxyhydrogen flame. The chemical formula for this hydrolysis reaction when the silicon compound is silicon tetrachloride is as follows. 2H ₂ +O ₂ →2H ₂ O ...(1) 2H ₂ O+SiCl ₄ →SiO ₂ +4HCl ...(2) These silica particles adhere to the lower end of the quartz seed rod 10 suspended vertically in the reaction chamber 9. - It is deposited and grows sequentially to form a large-diameter porous quartz glass base material 11. In addition, HCl generated by the reaction is
It comes 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 produced by the method described above is pre-calcined at a temperature range of 1200-1350°C before being turned into transparent glass by firing at a temperature range of 1400-1500°C. to adjust the radial density distribution of the porous quartz glass matrix. This calcination is preferably carried out in a clean atmosphere, and is carried out in a heating furnace within the above temperature range while introducing air, nitrogen or other inert gas that has been previously treated with a filter or the like as necessary. The calcination time varies depending on the temperature and the length of the temperature-uniforming zone in the furnace, which will be described later, but the radial shrinkage rate of the porous quartz glass base material after calcination is preferably 10 to 45%.
You can choose from conditions ranging from 25% to 40%. Here, when the diameter at the center in the longitudinal direction of the porous base material before calcination is Do, and the same diameter of the base material after calcination is D, radial shrinkage rate (%) = [(Do −
D)/Do]×100(%). If the shrinkage rate is less than 10%, 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 the transparent vitrification process, resulting in breakage of the base material or defects on the outer periphery of the transparent quartz glass. become the cause. Moreover, when the shrinkage rate exceeds 45%, the surface layer of the calcined porous quartz glass becomes a substantially complete silica glass layer and its density increases, and in the transparent vitrification process performed after calcining, It becomes difficult to remove internal air bubbles, and defects such as air bubbles may remain in the transparent glass.
Alternatively, it takes a long time to completely eliminate these defects, making it substantially difficult to produce high-quality, high-purity quartz glass. The shrinkage rate in the present invention is not the saturated shrinkage rate at the calcination temperature. The present invention does not perform calcination until the saturated shrinkage rate at the calcination temperature;
As shown in the examples of the present invention, shrinkage of the porous base material due to calcination is interrupted within an appropriate range. FIG. 3 shows a graph of the calcination temperature of the porous quartz glass base material and the radial saturation shrinkage rate at that temperature. In the present invention, calcination is carried out at a temperature range of 1200 to 1350°C so that the porous quartz glass base material shrinks by 10 to 45% in the radial direction without reaching the saturation shrinkage rate. According to Figure 3, for example 1200
The saturated shrinkage rate at °C is about 14%, and the saturated shrinkage rate at 1350°C is about 55%. In the claimed invention
At a predetermined calcination temperature selected from the range of 1200 to 1350°C, calcination is interrupted at the stage when the saturated shrinkage rate as shown in Figure 3 is not reached. It is possible to take methods such as controlling. In the present invention, the above-mentioned base material is calcined using a heating furnace (not shown) heated to a relatively low temperature that does not damage the base material, for example, as in the embodiment described later.
Insert the base material inside while rotating, then gradually
This can be carried out by raising the temperature to a temperature in the range of 1200 to 1350°C and maintaining the base material at that temperature for a required time while rotating the base material. In addition, for example, a heating furnace with a temperature gradient that increases from the bottom to the top of the furnace, or a heating furnace with the above-mentioned temperature gradient and a partial area in the upper part of the heating furnace that 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 can 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 at a temperature range of 1400 to 1500° C. in a heating furnace in an atmosphere containing at least 70% or more preferably 80 to 90% He gas. The time for main sintering varies depending on the temperature and He concentration, but 1.5 to 4 hours is appropriate. In the present invention, 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) has a temperature gradient that increases from the top to the bottom. It is preferable that the In this way, when the porous quartz glass base material is inserted into the heating furnace from above, the temperature of the base material can be gradually increased, and the sudden temperature change can also prevent air bubbles from forming in the transparent glass. can be prevented from remaining. Since the vitrification temperature of the porous quartz glass base material is 1400°C or higher, the temperature gradient at this time is approximately 1200°C at the upper part of the heating furnace and 1400 to 1500°C at the lower part of the furnace.
℃ is appropriate, and 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 during transparent vitrification. 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. This will soften the quartz seed rod.
It is possible to make the base material transparent without exposing the seed rod to high temperature ranges above ℃ for a long time.This prevents the seed rod from softening and changing due to heat, and allows large-diameter quartz glass rods to be made without falling. I can support it. Furthermore, air bubbles that flow out due to transparent vitrification can escape to the upper porous layer that has not yet been transparent vitrified, making it possible to prevent air bubbles from being contained in the obtained quartz glass. . In the present invention, heating during calcination and transparent vitrification is performed by gradually inserting the porous quartz glass base material into a heating furnace while rotating it. By rotating, the base material can be heated uniformly, and it is possible to prevent the base material from being deformed due to uneven 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 is a uniform temperature region. Example: A cylindrical porous quartz glass base material formed on a quartz seed rod manufactured using the apparatus shown in Fig. 1 {diameter 30 cm, length 0.5 m, center part (diameter 3 cm from the center) The average density of the surface part (4 cm from the surface part) is 0.45 g/cc, and the average density of the surface part (4 cm from the surface part) is 0.2 g/cc.
cc} into a heating furnace (not shown) maintained at 300℃ while slowly rotating it, and insert it from the bottom of the furnace.
While supplying N ₂ gas at 3 m ³ /Hr, the temperature was raised to 1300℃ at a rate of 250℃/Hr.
Hold for minutes. This is the calcination of the base material. The diameter of this porous quartz glass base material after calcination is 19.5 cm, the shrinkage rate in the radial direction is approximately 35%, the average density at the center is 0.6 g/cc, and the average density at the outer periphery is 0.68 g/cc. It was hot. Immediately after the 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 an upper temperature of approximately 1200°C and a lower temperature of approximately 1430°C.
It has a temperature gradient of ℃, and the isothermal temperature range of 1400℃ or more is about 20.
It is controlled to be cm. The seed rod 10 was slowly rotated and lowered at a speed of 100 mm/hr, and the porous quartz glass base material 11 was inserted into the heater 22 from its lower end (FIG. 2a). This porous quartz glass base material 11 is gradually defoamed from the lower end to become transparent glass, and has a diameter of approximately 13 cm.
It became a transparent glass 23 with a length of about 31 cm (Fig. 2b). The quartz glass that has been made transparent in this way has no defects in its surface layer (approximately 5 mm thick) and its inner center, and the average number of bubbles generated on this surface is 0.8 per 1 cm ² of surface area. There weren't many. On the other hand, when transparent vitrification was performed using the same method as above without performing calcination, the base material was reduced by about 1/2 inch from the lower end.
When the glass became transparent and vitrified for about 3 minutes, a crack occurred in the upper porous glass part, and the lower part fell off from this part. As a result of evaluating the internal defects of this transparent vitrified part, although there were no defects in the inner center from the surface layer (approximately 3 cm thick),
This surface layer had an average of 20 defects per cm ² . In addition, as a result of using the method described above to increase the holding time during calcination to approximately 180 minutes and increasing the radial shrinkage rate of the porous quartz glass matrix to 51%, the matrix was made transparent by the method described above. It was possible to make the material transparent and vitrified without any cracks. However, although the surface layer of this quartz glass (approximately 5 mm thick) had only a few defects, about 0.8 defects per ¹ cm2,
From the surface layer to the inner part, quartz glass per 1 kg
Approximately 0.6 to 1 defect was observed. When the above holding time is about 180 minutes, the calcination temperature is 1300℃ as mentioned above, and the above shrinkage rate at this time is 51%.
is equal to the saturated shrinkage rate of 51% at 1300°C shown in FIG. That is, it can be seen that if the porous quartz glass base material is shrunk to the saturated shrinkage rate at that temperature by calcination, bad results will result. In addition, a cylindrical porous quartz glass base material formed in the same manner as above {diameter 30 cm, length 0.5 m, average density at the center (3 cm diameter from the center) is 0.45
g/cc, the average density of the surface part (4 cm from the surface part) is 0.2 g/cc}, the shrinkage rate by changing the calcination temperature and holding time, the success or failure of transparent vitrification, and the disadvantages of transparent vitrified quartz glass I looked into it. The above results are summarized in Table 1 as Examples Nos. 1 to 7, including cases where calcination was not performed. In the table, the surface layer is a layer approximately 5 mm thick from the surface. Further, the number of defects in the surface layer is determined by the number per unit surface area of the surface layer, and the number of internal defects is determined by the number per unit weight of transparent vitrified quartz glass.

【table】

[Table] Example No. 2 had many defects in the surface layer and was rejected. Further, the defects in the surface layer in Example No. 3 were 2 to 3 defects/cm ² unevenly distributed on the very surface, which did not pose a problem in terms of product yield, and it can be considered that a good product was obtained. In addition, the internal defect in Example No. 6 is 0.1 to 0.2
Considering the weight of the quartz glass, which is approximately 9 kg, the number of pieces/kg does not pose a problem in terms of product yield, and it can be considered that a good product was obtained. In all of Examples No. 3 to No. 6, substantially good products were obtained, but in all cases, the calcination was interrupted before the saturated shrinkage rate was reached. The shrinkage rate in the case of No. 7 is 51%, but as mentioned above, it can be seen from FIG. 3 that this is almost equal to the saturated shrinkage rate (about 51%) at 1300°C. In this case, although transparent vitrification is possible, the number of internal defects poses a problem in terms of product yield, which is an inconvenient result. That is, Example No. 7 shows that it is necessary to avoid performing calcination shrinkage to a saturated shrinkage rate. [Effects of the Invention] As explained above, according to the present invention, before the porous quartz glass base material is subjected to transparent vitrification treatment by sintering, it is pre-calcined at a temperature lower than the temperature for transparent vitrification treatment. Since the density distribution in the radial direction of the porous quartz glass base material is made uniform, 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. 2a and 2b are explanatory views showing an example of the vitrification method according to the present invention.
FIG. 3 is a graph showing the relationship between the calcination temperature and the radial saturation shrinkage rate of the porous quartz glass base material. 10... Seed rod, 11... Porous quartz glass base material, 21... Heating furnace, 22... Heater, 23...
High purity quartz glass.

Claims (1)

[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, the porous quartz glass base material is first rotated and gradually inserted into the heating device for 1200 ~ After calcining in the range of 1350℃ and shrinking the base material in the radial direction by 10 to 45% without reaching the saturation shrinkage rate to adjust the density distribution of the base material in the radial direction, A method for manufacturing high-purity quartz glass, which is characterized by firing in a range to make it transparent.