JPH0222137A - Production of synthetic quartz preform - Google Patents

Abstract

PURPOSE:To obtain a high-quality preform in the absence of bubbles in a transparent material by surrounding a burner flame stream with a sealing gas stream and controlling fine glass particles in an atmosphere between the flame stream and the sealing gas stream so as to provide a specific value or below of concentration thereof in producing a synthetic quartz preform by a vapor axial deposition method. CONSTITUTION:A synthetic quartz preform 5 is produced by a vapor axial deposition method. In the process, the following constitution is adopted. That is a flame stream 4 of a burner 3 arranged under a reaction vessel 1 is surrounded with a sealing gas stream 13 (jetted from a nozzle 11) and the flow rate of the sealing gas stream 13 or feed rate of a glass raw material fed from the burner 3 is regulated to control the concentration of fine glass particles suspended in the atmosphere between the flame stream 4 and the sealing gas stream 13 to <=10g/Nm<3>.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a porous base material of synthetic quartz used for communications, IC circuit manufacturing, etc.
The present invention relates to a method for synthesizing a high quality porous base material in the VAD) method.

(Conventional technology and its issues) Recent developments in information communication and systemization have been remarkable, and it can be said that we have entered an era of mass consumption of advanced communication equipment and highly integrated circuit semiconductors (ICs). There is a long-awaited ability to more economically manufacture synthetic quartz photomasks used in the circuit exposure process.

Various methods have been proposed to date for manufacturing quartz-based optical fiber base materials and synthetic quartz photomask base materials, but the VAD method has particularly high reaction efficiency and allows for larger base material dimensions. For these reasons, it has excellent mass productivity and meets the above requirements. That is, in the VAD method, as shown in FIG. 5, for example, 5iC14, G
Frit gas such as eCl4 and 0□, N2 and N2
, gas, etc. are supplied, and a hydrolysis reaction shown in the following formula 0 is performed in an oxyhydrogen flame to generate glass particles.

5iC14 (GeCI 4) + 211 t +Oz
→S iOz (GeOz) + 4HC1- ■ The glass particles generated by this reaction ride the flame stream 4 of the burner 3 and are deposited at the same time as they collide with the seed rod 2, which is the starting base material, and this deposition is continuously repeated. Then, a porous synthetic quartz glass base material (hereinafter abbreviated as base material J) 5 grows in the vertical (long axis) direction.

Surplus glass particles (
(hereinafter abbreviated as "Hyum") 6 is a reaction vessel l
After floating and staying in the reaction vessel 1, most of it is discharged to the outside of the vessel through the exhaust pipe 7 along with the atmospheric gas inside the reaction vessel 1.

The base material produced by this method is porous with a porosity of approximately 80%, so generally, this base material is immediately heated to defoam it to make a transparent glass lump (hereinafter abbreviated as "transparent material"). ) has been adopted.

However, when the base material is made transparent by this method, air bubbles may be generated in the transparent material, resulting in a defective product. As disclosed in JP-A No. 62-162642, some of the fumes combine with each other to form secondary particles during the synthesis of the base material, and then adhere to the surface of the base material. It is said that the part to which particles are attached is formed due to insufficient sintering during transparency.

The following three methods have been proposed as measures to prevent bubble generation.

The first method is to reduce the amount of fume floating and staying in the reaction container by improving the conditions for discharging atmospheric gas from inside the reaction container. For example, JP-A-62-
As disclosed in Japanese Patent No. 17037, an exhaust port is placed close to the base material on the extension line of the burner flame, and fume that starts to scatter without being deposited on the base material is immediately exhausted.
Alternatively, as disclosed in Japanese Unexamined Patent Publication No. 62-27344, a ring-shaped exhaust pipe with an intake port on the inner periphery is provided near the growth end of the base material, and fumes generated in the same manner as in the above method are removed. Methods such as immediate exhaustion have been proposed.

However, with this method of improving the exhaust conditions, when the raw material gas supply rate is increased to increase the synthesis rate of the base material, the exhaust cannot be fully exhausted, and the number of Huyum stars floating and staying in the reaction vessel increases significantly. Bubbles are generated.

The second method, as disclosed in JP-A No. 62-162642, is a method in which the reaction vessel is heated to prevent fume from adhering to the inner wall of the vessel and to suppress the generation of secondary particles. However, as will be described later, the inventors' investigation revealed that sources of secondary particles include, in addition to the inner wall of the container, the outer surfaces of the seed rod and burner, and the agglomeration of fume in the atmosphere. This method is not sufficiently effective against sources other than the inner wall of the reaction vessel.

As a third method, as disclosed in JP-A No. 61-97141, when heating the synthesized base material to make it transparent, the base material is calcined at 1100 to 1300'c in advance. After adjusting the radial density distribution of the base material, 1400 ~
This is a method of baking at 1600°C to make it transparent.

That is, when calcination is not performed, there is a density difference between the low-density base material surface layer and the relatively high-density base material interior, and high-density silica particles (secondary particles) enter the base material surface layer. Therefore, a density difference occurs between the material and the surrounding base material, and uniform sintering and defoaming do not occur in the gap between transparency.

Therefore, bubbles with a diameter of about several tens of micrometers are generated in the surface layer 5 to 10 mm from the surface of the transparent material. However, when this is calcined, the entire base material is slightly sintered and the density distribution in the radial direction of the base material becomes constant, and when it becomes transparent, the shrinkage of the base material becomes uniform and no bubbles are generated in the surface layer. It is said that

This third method is effective for improving the low-density surface layer inside the base material, but improving the internal structure requires calcination conditions at high temperatures or for a long time. The thing is,
Partial transparency of the surface layer is caused, and on the contrary, non-uniformity of the surface layer occurs.

Therefore, it is not possible to eliminate all defects by improving the transparency method disclosed in JP-A-61-97141, which involves calcining before the transparency treatment.

As described above, conventional measures to prevent bubble generation include methods such as immediately discharging the generated fume out of the reaction vessel, suppressing the generation of secondary particles, or improving the transparency conditions.

On the other hand, in order to improve the gas flow in the atmosphere around the burner flame, the following method has been proposed by changing the structure of the burner.

The method is disclosed in Japanese Unexamined Patent Application Publication No. 62-143838 and No. 61
As disclosed in Japanese Patent Publication No. 162637, a hood is provided to surround the flame along the longitudinal direction of a burner with a multi-tube structure that stands vertically, and a hood is provided to surround the flame on the side (tip part) of the flame that is closer to the base material. He, N,, 11 so as to cover the outer periphery of
Gas such as □ is jetted out from the tip of the hood to stabilize the flow of the burner flame.

These inventions aim to improve the transmission efficiency of optical fibers by creating a base material in which the refractive index distribution in the radial direction of the base material is sharp, that is, the refractive index changes rapidly near the boundary between the core portion with a high refractive index and the cladding portion with a low refractive index. Specifically, the gas flow around only the tip of the base material in the burner flame is made to flow in parallel with the burner flame flow.
By covering the outer periphery of the burner flame with this gas flow and the hood, the burner flame is isolated from the atmosphere surrounding the flame, thereby achieving stabilization of the burner flame flow.

However, these inventions cannot improve the gas flow within the reaction vessel, but are not effective against quality deterioration due to secondary particles adhering to the synthesized base material.

The present invention solves the above-mentioned problems of the prior art and provides a method for manufacturing a high-quality synthetic quartz matrix in which bubbles are not generated in the transparent material.

(Means for Solving the Problems) The present invention was accomplished based on basic research into the causes of internal bubble formation.

That is, the present inventors observed the burner flame flow and the floating and retention status of fume during synthesis, electron microscope observation of the internal cross section of the base material, and observation of air bubbles in the transparent material after heating and degassing the same base material. When the burner flame flow is disturbed, a large amount of fume is generated and deposited on the inner wall of the reaction vessel, the outer periphery of the burner, the viewing window, the side of the seed rod, etc., and this fume deposit layer forms the gas flow (hereinafter referred to as (referred to as "leak gas flow") or by contact with the sliding part as the seed rod is pulled up, or the fume aggregates in the atmosphere inside the container to generate secondary particles, and these secondary particles are released into the flame. It has been found that when mixed, it fuses to the bottom or side surface of the base material, and the melting point changes into bubbles when it becomes transparent.

Furthermore, this observation shows that when the leak gas flow around the flame flow is flowing along the flame flow, the amount of accumulated fume decreases, the generation of secondary particles decreases, and the generated secondary particles decrease.
It has been found that the secondary particles move along with the leak gas flow, do not mix with the flame flow, and have the effect of preventing bubble formation.

In addition, the mechanism by which the fusion point of the secondary particles becomes a bubble is that the secondary particles are heated and sintered by the flame in the flame and immediately after the base material is fused, and become particles with high density, compared to the fused secondary particles mentioned above. It was found that the periphery of the low-density fused secondary particles was turned into fibers during the transparentization process, creating voids or bubbles.

The inventors of the present invention have developed a method for economically manufacturing a high-quality base material in which no air bubbles are generated inside the transparent material, as described below, based on the basic study on the bubble generation mechanism as described above. .

That is, the first aspect of the present invention is a method for manufacturing a synthetic quartz base material by a vapor phase shafting method, in which a flame stream of a burner disposed below a reaction vessel is surrounded by a sealing gas stream;
By adjusting the flow rate of the sealing gas or the supply rate of the glass raw material supplied from the burner, the concentration of fume floating in the atmosphere between the flame flow and the sealing gas flow is reduced to Log/N III 3 or less. The second aspect of the present invention is that in the first method, sealing is performed from an annular opening provided on the outer periphery of the burner or from a large number of nozzle holes arranged annularly on the outer periphery of the burner. A third method is to inject gas, and a third method is to create a sealing gas flow using gas leaking from an annular opening provided on the outer periphery of the burner or from a large number of nozzle holes arranged annularly on the outer periphery of the burner. The gist of the present invention is to control the sealing gas flow by configuring the reactor and adjusting the amount of exhaust from the inside of the reaction vessel.

In the present invention, the fume concentration in the atmosphere between the burner flame flow and the sealing gas flow is set to 10 g/h+' (however:
According to the research and experiments conducted by the present inventors, as shown in Figure 2, under the conditions where the hume concentration is less than 10 g/Nm'', the This is because the amount of bubbles inside the transparent material decreases accordingly, and if the fume concentration is Ig/Nm3 or less, there are no bubbles. The reason for this relationship is that the amount of secondary particles of fume, which is the main cause of bubble generation mentioned above, mixed into the flame is almost constant when the concentration of fume around the flame exceeds Log/Nm. This is presumed to be because when the concentration is below Ig/Nm3, it decreases as the concentration decreases, and when it is below Ig/Nm'', it disappears.

In addition, this Figure 2 shows the atmosphere between the burner flame flow and the sealing gas flow and the fume concentration in the bulk atmosphere in the reaction vessel by changing the sealing gas flow rate using the AD device shown in Figure 1, which will be described later. FIG. 3 is a diagram showing the results of measuring the relationship between the amount of bubbles inside the transparent material and the amount of internal bubbles in the transparent material after the synthesized base material was subjected to the transparent treatment.

(Example) The method of the present invention will be explained below based on the example shown in FIGS. 1 to 4.

l) FIG. 1 is an explanatory diagram showing a first example of the method of the present invention,
In FIG. 1, reference numeral 11 denotes a nozzle for supplying sealing gas, which surrounds the burner 3 on the outer periphery of the burner 3, and whose tip is located at a position further back from the tip of the burner 3 with respect to the base material 5; Reference numeral 2 denotes a tube for sampling atmospheric gas, the tip of which is inserted between the flame stream 4 of the burner 3 and the sealing gas stream 13 injected from the nozzle 11; The gas in the surrounding area is sucked, the fume is collected using a filter 14, and the fume is weighed to measure the hume concentration.

Further, the fume concentration in the bulk atmosphere within the reaction vessel 1 can also be measured in the same manner by inserting a tube 15 similar to the tube 12 into the reaction vessel 1.

Next, we will discuss the characteristics of the first example of the method of the present invention, which are related to the first example of the method of the present invention among the conventional methods, by
A comparison will be made with the methods described in Japanese Patent Nos. 143838 and 62-62637 (hereinafter abbreviated as the "hood method").

- The behavior of the burner flame during the base material synthesis process using the VAD method in boat fishing is that when the supply rate of raw material gas such as 5iC1n is increased, the diameter of the glass particle generation zone in the flame increases, and the glass particle generation zone is The appropriate distance between the burner and the base material becomes longer. Therefore, in the hood method, it is necessary to change the diameter and length of the hood according to changes in the raw material gas supply rate. On the other hand, in the method of the present invention, the nozzle for supplying seal gas is arranged at a position recessed from the tip of the burner, so there is no need to change the diameter or length of the nozzle. Furthermore, the hood method has the disadvantage that if the length of the hood is increased as the raw material gas supply rate increases, fume tends to accumulate near the tip of the hood. On the other hand, in the case of the method of the present invention, the nozzle arrangement conditions can be kept constant even when the raw material gas supply rate is increased, so it is an excellent method that can produce a high-quality base material without the risk of fume accumulation.

Results of continuous operation of Examples ■ to ■ using the VAD device shown in FIG. 1 under the conditions shown in Table 1 below, and results of operations using the conventional VAD device shown in FIG. The results are also shown in Table 1.

Note that the transparentization treatment of the base material was also performed under the conditions shown in Table 1.

In this example, the fume concentration of the atmosphere between the sealing gas stream and the burner flame stream was controlled by increasing or decreasing the feed rate (g/min) of sealing gas Meteor or 5iC14. From Table 1, the more sealing gas meteors there are, the more
It is also recognized that the lower the supply rate of 5iC14, the lower the fume concentration in the atmosphere around the flame stream. In the case of the example, under all conditions, the fume concentration in the atmosphere around the flame flow is low, less than 1Og/Nm'', and the amount of internal bubbles in the transparent material is less than 8 bubbles/kg ingot, indicating high quality. That is clear.

On the other hand, in the case of the comparative example, the fume concentration in the atmosphere around the flame flow exceeds 10g/Nm'', and the amount of internal bubbles in the transparentizing material is also large, which is not desirable in terms of quality.

In the embodiment, N2 gas was used as the sealing gas, but other inert gases such as Ar gas and compressed air may also be used.

Part 2) FIG. 3 is an explanatory diagram showing a second example of the method of the present invention,
Reference numeral 16 in FIG. 3 is an air leak tube installed around the outer periphery of the burner 3. The fume concentration is measured by inserting gas sampling tubes 18 and 19 into the leak gas flow 17 that has flowed in from the tube 16 at the outer periphery of the flame flow 4 and into the bulk atmosphere inside the reaction vessel l. The gas in the bulk atmosphere inside and outside the leak gas flow 17 is sucked, and the fume is collected by the filter 22, weighed, and divided by the amount of gas sucked to measure the concentration of the fume.

Note that the exhaust pipe 7 is equipped with an exhaust volume regulator 20 and an exhaust volume measuring device 2.
1 is provided so that the amount of exhaust gas and thus the amount of air leaked from the tube 16 can be adjusted in accordance with the concentration value of fume in the leak gas flow 17.

Using the VAD device shown in FIG. 3, the fume concentration in the leak gas flow 17 at the outer periphery of the flame flow 4 and in the bulk atmosphere is measured by changing the exhaust volume, and the synthesized base material 5 is transparentized after being transparentized. The results of investigating the relationship with the amount of internal bubbles in the material are shown in FIG. 4, and representative examples thereof are shown in Table 3. 5th for comparison
Table 3 also shows the case where the conventional VAD device shown in the figure is used.

The synthesis conditions and transparency treatment conditions for the base material 5 are shown in Table 2 below.

Table 3 Examples and Comparative Examples of the Method of the Present Invention From these results, if the fume concentration in the leak gas stream 17 is less than Log/Nm'' (however, the concentration in the dry gas excluding HCl2 and moisture), the hume concentration As the concentration decreases, the amount of internal bubbles in the transparent material decreases, and the same concentration decreases to about 3
It was found that internal bubbles disappear when the amount is less than 100 g/Nm''.

That is, the flow rate of the leak gas flow 17 flowing in from the tube 16 is adjusted by changing the exhaust volume, and the concentration of fume in the leak gas flow 17 at the outer periphery of the flame flow 4 is kept below Log/Nm', preferably below 3 g/Nm'. By suppressing the amount of bubbles inside the transparent material, the amount of bubbles inside the transparent material can be reduced or eliminated.

(Effects of the Invention) As explained above, according to the present invention, a nozzle for supplying seal gas is provided around the burner, and a means for suppressing the fume concentration around the flame flow of the burner is used, or a tube for near leakage is provided around the outer periphery of the burner. By controlling the fume concentration at the outer periphery of the flame flow of the burner by adjusting the near-leak meteor by adjusting the exhaust gas, it is possible to easily obtain a high-quality base material in which no internal bubbles are generated after the transparentization treatment.

[Brief explanation of the drawing]

Figure 1 is a schematic explanatory diagram of the VAD device used in the first method of the present invention, and Figure 2 is the flame flow in the example and comparative example of the first method of the present invention conducted under the conditions shown in Table 1. A diagram showing the relationship between the fume concentration in the surrounding atmosphere and the amount of bubbles inside the transparent material,
FIG. 3 is a diagram similar to FIG. 1 in the case of the second method of the present invention,
Figure 4 shows the fume concentration and transparency in the gas flow at the outer periphery of the flame flow and in the bulk atmosphere in the reaction vessel in the Example and Comparative Example of the second method of the present invention carried out under the conditions shown in Table 2. FIG. 5, which is a diagram showing the relationship between the amount of internal bubbles in the chemical agent and the amount of internal bubbles, is a schematic diagram of a VAD device used in the conventional method. l is the reaction vessel, 3 is the burner, 4 is the flame stream, 5 is the base material, 6
11 is a nozzle, 12 is a tube, 13 is a sealing gas flow, 16 is a tube, 17 is a leak gas flow,
2o is the displacement regulator. Fig. 1 Fig. 2 Hz-4, 11/4-) Inner'fF foremilk amount (ImaK addition/conversion coat) Fig. Ω

Claims (3)

[Claims] (1) In a method of manufacturing a synthetic quartz base material by the vapor phase axial method, the flame stream of a burner disposed below the reaction vessel is surrounded by a sealing gas stream, and the flow rate of this sealing gas or the burner is The concentration of glass fine particles suspended in the atmosphere between the flame flow and the sealing gas flow was adjusted to 10 g/Nm^ by adjusting the supply speed of the glass raw material to be supplied.
1. A method for producing a synthetic quartz base material, characterized in that the synthetic quartz base material is controlled to 3 or less. (2) The method for producing a synthetic quartz base material according to claim 1, characterized in that the sealing gas is injected from an annular opening provided on the outer periphery of the burner or from a large number of nozzle holes arranged annularly on the outer periphery of the burner. (3) Forming a sealing gas flow using gas leaking from an annular opening provided on the outer periphery of the burner or from a large number of nozzle holes arranged annularly around the outer periphery of the burner, and adjusting the exhaust volume from inside the reaction vessel. 2. The method of manufacturing a synthetic quartz base material according to claim 1, wherein the sealing gas flow is controlled by: