JPH08283026A - Production of silica glass - Google Patents

Abstract

PURPOSE: To obtain a silica glass having uniform optical properties by reducing the effect of change of atmospheric pressure. CONSTITUTION: The silica glass is produced by flame-hydrolyzing an evaporated silicon compound and if necessary, an evaporated doping compound in oxyhydrogen flame to generate a silica fine particle and a doping material fine particle and sticking and depositing the fine particles on a rotating speed rod. In this method, the vapor phase pressure in each of raw material vessels is kept to >=1kg/cm<2> to <2kg/cm<2> gauge pressure in a liquid raw material supply method for supplying a gaseous mixture of a carrier gas and the raw material vapor to a burner by filling each liquid raw material of the silicon compound and the compound for doping in each of the raw material vessels and blowing the carrier gas in each of the liquid raw material to evaporate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silica glass type optical fiber preform or a silica type glass useful as an LSI photomask substrate.

[0002]

2. Description of the Related Art In a production facility for silica glass, which becomes an optical fiber core member, in addition to gaseous raw materials such as hydrogen and oxygen, SiCl _{4 which} is a raw material for SiO ₂ glass and a refractive index distribution for forming a refractive index in the glass are used. GeCl ₄ is used as a raw material for dope in the production as a liquid raw material. In the vapor phase axial deposition (VAD) method, each liquid raw material is vaporized and supplied to an oxyhydrogen flame burner to form a glass particulate deposit. Conventionally,
In these methods, SiCl ₄ and GeCl ₄ are filled in each raw material container, and a carrier gas whose flow rate is controlled, for example, Ar gas, is bubbled into the liquid glass raw material in each raw material container to obtain each desired raw material carrier gas. It was vaporized and diffused inside, and each raw material vapor was generated as a mixed gas with a carrier gas and supplied to the burner. This is conceptually shown in FIG.

In FIG. 2, 11 is a carrier gas flow rate controller, 12 is a raw material container (container, the same applies hereinafter), 13 is a raw material liquid,
14 is a pressure gauge, 15 is an atmospheric pressure sensor, 16 is a correction calculator, and P in the gas phase inside the container is a two-component system of carrier gas and raw material vapor diffused in the carrier gas.
Is the total pressure in the container 12, φ is the degree of saturation, t is the container temperature,
When p (t) is the saturated vapor pressure of the raw material at the temperature t, the partial pressure of the raw material vapor in the container 12 is represented by φp (t), and the partial pressure of the carrier gas in the container 12 is P−φp (t. )
Therefore, q is the carrier gas flow rate and Q is
Where is the flow rate of the accompanying raw material vapor, the following equation holds. Q: q = partial vapor partial pressure φp (t): carrier gas partial pressure P−φp (t) (1) That is, the entrained raw material vapor flow rate Q is calculated by the following equation (2). Q = q × φp (t) / {P−φp (t)} (2)

To keep the raw material vapor flow rate Q constant,
In the equation (2), the values of q, φ, p (t) and P may be held constant. The carrier gas flow rate q is accurately controlled by a mass flow controller (MFC). Further, as a method of keeping the saturation φ constant, there is a method disclosed in Japanese Patent Publication No. 61-1378. According to this, a container filled with the same liquid raw material is divided into front and rear stages, connected in series by a bubbling carrier gas conduit, and maintained at a high temperature due to the temperature of the container in the front stage, so that the saturation φ is approximately 100. A bubbler two-stage system that holds the percentage is disclosed.

In the equation (2), as described above, the carrier gas flow rate q is controlled by the MFC, so that in addition to high accuracy, the response is fast, but φp (t) / {P−φp (t )} Term is determined from multiple factors and the response is extremely slow. Therefore, in the carrier gas system, the raw material vapor flow rate Q is
The ideal control method is to keep the concentration of the raw material vapor at a constant value by controlling and stabilizing various conditions in the gas phase in the container, and controlling the carrier gas flow rate.

However, in the VAD method, the flow resistance of the raw material vapor flow path in the vessel to the flame burner path is small, and the vapor phase pressure in the vessel is 0.01 kg / cm ² gauge or less, and the fluctuation of atmospheric pressure remains unchanged in the vapor phase. Since the pressure varies, even if the carrier gas flow rate q, the saturation φ, and the temperature t in the container are held constant as in the conventional supply method, the gas phase pressure P in the container is π as the atmospheric pressure. When it changes to → π ± Δπ, the gas phase pressure in the container also changes to P → P ± Δπ, and the entrained vapor flow rate Q changes by ΔQ represented by the equation (3). ΔQ = q × φp (t) [1 / {P ± Δπ−φp (t)} −1 / {P−φp (t)}] (3)

Conventionally, when the atmospheric pressure π fluctuates in this way, the concentration term φp (t) / {P- in the equation (2) due to this fluctuation.
This was dealt with by changing the carrier gas flow rate term q so as to cancel the fluctuation of φp (t)} (Japanese Patent Publication No. 4-3373).
No. 8 bulletin). According to it, the atmospheric pressure sensor in FIG.
Δπ detected in 15 is transmitted to the correction calculator 16,
In the arithmetic unit 16, the degree of concentration fluctuation φp (t) [1 / {P ± Δπ−
[phi] p (t)}-1 / {P- [phi] p (t)}] is immediately calculated, and the new set value of the carrier gas flow rate increased or decreased accordingly is transmitted to the carrier gas flow rate controller 11. In this way, by decreasing or increasing the set value of the carrier gas flow rate q in accordance with the increase or decrease of the gas phase concentration,
The raw material vapor flow rate was kept constant.

[0008]

However, if the correction range is large when the carrier gas flow rate q is reset, a drastic change in the carrier gas flow rate is caused, and the carrier gas flow rate changes greatly. Causes a temperature fluctuation of the oxyhydrogen flame, and the bulk density of the surface of the deposited base material is different before and after the change, resulting in inhomogeneity. This is one of the causes of poor characteristics. Further, a large variation in the carrier gas flow rate due to the automatic resetting of the carrier gas flow rate may cause a variation in the saturation of the raw material vapor in the vapor phase in the container. However, it is difficult to predict the saturation fluctuation range due to the atmospheric pressure fluctuation, and if the saturation fluctuation occurs, the raw material vapor supply flow rate may not be accurately corrected. Therefore, a bubbler two-stage method for maintaining 100% saturation is considered to be effective, but this method has the problem that the device becomes complicated.

[0009]

As a result of various studies conducted by the present inventors to solve the above problems, the higher the pressure P of the gas phase in the container is set, the more the atmospheric pressure fluctuation rate Δπ / π becomes. Focusing on the fact that the influence on the raw material vapor flow rate fluctuation rate ΔQ / Q is small, the pressure P was studied to complete the present invention. The present invention is based on the vaporized silicon compound and the necessity. The vaporized dope compound,
In a method for producing a silica-based glass, which comprises subjecting silica fine particles and fine particles of a dope material to flame hydrolysis in an oxyhydrogen flame and depositing and depositing these fine particles on a rotating seed rod, Liquid raw materials for dope compounds are filled in respective raw material containers, carrier gas is blown into these liquid raw materials through a carrier gas blowing pipe to be vaporized, and a mixed gas of the carrier gas and raw material vapor is supplied to a burner. The gist of the method is a method for producing a silica-based glass, which comprises maintaining a gas phase pressure in each raw material container at a gauge pressure of 1 kg / cm ² or more and less than 2 kg / cm ² .

The gas phase pressure in the raw material container is 1 kg / c gauge pressure.
If it is less than m ² , the effect is not sufficient and the gauge pressure is 2 kg / c.
If it is more than m ² , the line pressure is too high and there is a problem in practical use. Therefore, it is necessary to set this value to a gauge pressure of 1 kg / cm ² or more and less than 2 kg / cm ² , preferably a gauge pressure of 1.0 to 1.6 kg / cm ² Is good. Hereinafter, the present invention will be specifically described. FIG. 1 is an explanatory view of the present invention, in which 1 is a carrier gas flow controller, 2
Is a liquid raw material container (container), 3 is a liquid raw material, 4 is a pressure gauge,
5 is an atmospheric pressure sensor, 6 is a correction calculator, 7 is a throttle valve, 8
Is a carrier gas blowing pipe, 9 is a raw material vapor supply pipe, 1
0 represents a liquid material supply pipe, respectively. With the above-mentioned configuration, the present invention differs from the conventional method in that in the present invention, the throttle valve 7 is installed in the raw material gas flow path in the vessel-flame burner path to adjust the gas phase pressure to a gauge pressure of 1 kg / cm ² or more and 2 kg / cm ² It is a point less than.

A second aspect of the present invention is a method for maintaining the saturation of the raw material vapor in the vapor phase in the container at a constant value of 99% or more, which is the vaporized silicon compound and if necessary. D is the diffusion coefficient of the raw material vapor component of the vaporized dope compound in the carrier gas component, and r is the carrier gas ejection radius of the carrier gas blowing pipe.
_O and the carrier gas jet depth in the liquid are Le
, D ・ Le / (r _O ² √ (r _O・ g)) (g; gravitational acceleration)
The gist is the method for producing the above silica-based glass having a value of 2.0 or more. This will be described with reference to FIG. 1. A container 2 is filled with a liquid raw material of a silicon compound or a dope compound, and a carrier gas is blown into the liquid raw material through a blowing pipe 8. And is flame-hydrolyzed to be glass fine particles. In the present invention, in order to maintain the saturation of the raw material vapor at a constant value of 99% or more, the structure of the carrier gas blowing pipe 8 is Where D is the diffusion coefficient of the silicon compound or the compound for doping in the carrier gas component, r _O is the carrier gas ejection radius of the carrier gas blowing pipe, and Le is the carrier gas ejection depth in the liquid. (r _O ² √
(r _O · g)) (g; gravitational acceleration) is a method for producing the above silica-based glass, wherein the value is 2.0 or more.
If the value of D ・ Le / (r _O ² √ (r _O・ g)) is less than 2.0, there is a problem that the degree of saturation of the raw material vapor in the gas phase in the container is less than 99%, so this value is It is necessary to set the value to more than 2.0, preferably 2.0 to 4.0.

Examples of silica-based glass obtained by the method of the present invention include silica glass for optical fiber preforms and LSI photomask substrates. The doping compound of the present invention is a compound that is doped to adjust the refractive index of the optical fiber preform, and examples thereof include well-known compounds such as Ge, Ti, and fluorine.

[0013]

The first aspect of the present invention will be described by the following equation (4) introduced from the equations (2) and (3). ΔQ / Q = −P / {P−φp (t)} × (π / P) × (Δπ / π) (4) That is, the raw material vapor flow rate Q, which is the reaction condition in the oxyhydrogen flame, and the carrier gas Since the flow rate q is controlled to a constant value, the concentration term φp (t) / {P-φp (t)} shown on the right side of equation (2) becomes Q / q and becomes constant. Become.
Therefore, it is necessary to determine the gas phase concentration in the container in this way and keep it at a constant value. In the equation (4), the setting condition φp (t) / {P−φp (t)} value is constant and therefore P
/ {P-φp (t)} Under the condition that the value is constant (> 1), the π / P value becomes smaller as the gas phase pressure P in the container becomes larger than the atmospheric pressure fluctuation rate Δπ / π. Therefore, it is explained that ΔQ / Q becomes small. However, since the upper limit of the line pressure of the carrier gas supplied is generally about 2 kg / cm ² , the practical upper limit of the gas phase pressure P in the container is 2 gauge pressure.
It is less than kg / cm ² .

On the other hand, the gas phase pressure P in the container is a gauge pressure of 1 kg / cm ²
When the pressure is maintained at the above pressure, an automatic opening / closing valve by a two-stage bubbler system is required to prevent the backflow of the liquid raw material from the gas blowing pipe, but the two-stage bubbler system inevitably complicates the device. A second aspect of the present invention is a method of maintaining a constant saturation value of 99% or more by using only one vaporizer without using the two-stage bubbler method. While the bubbles generated by blowing the carrier gas into the raw material liquid in the container rise in the liquid phase, the raw material vapor component diffuses and permeates into the bubbles, and the mixed gas of the raw material vapor and the carrier gas reaching the gas phase. , The ratio of the raw material vapor in the carrier gas is further increased by diffusion from the gas-liquid interface before passing through the gas phase and escaping out of the container.

The degree of saturation in the bubble stage, such as the contact time θ determined by the bubble diameter Dp determined by the carrier gas injection pipe ejection radius r _O in the container and the carrier gas ejection depth Le in the liquid of the injection pipe, As a result of investigating these factors because they are determined by the container structure factors, the diffusion coefficient D of the raw material vapor component in the carrier gas component, the bubble diameter Dp, and the contact time θ (bubble rise time) are related by the equation (5). It was concluded that if so, the saturation of the raw material vapor in the gas phase could maintain a constant value above 99.5%. D · θ / (Dp / 2) ² > 0.85 ... (5) Further, the contact time θ is obtained from the equation for float separation in the liquid of the carrier gas jet depth Le in the carrier gas injection pipe in the liquid. It is given by the formula (6) by dividing by the bubble rising speed u, and the bubble rising speed u is expressed by the formula (7) from the literature. θ = Le / u… (6) u = √ (3.03Dp · g)… (7) However, 500 <Re≡Dp · u · ρ / μ <100,000 g; Gravitational acceleration Dp; Bubble diameter μ; Liquid viscosity ρ; Liquid density (Reference; Chemical Engineering Handbook, Chapter 14.4, page 1,101) Next, equation (8) is obtained from equations (5) and (6). D · θ / (Dp / 2 ) 2 = D · (Le / u) / (Dp / 2) 2> 0.85 ... (8) Furthermore Equation (7), equation (9) is obtained from (8). D · Le / (Dp ² √ (Dp · g))> 0.37 ... (9) It is assumed that the bubble diameter Dp is proportional to the carrier gas jet radius r ₀ of the carrier gas blowing pipe, and carrier gases of various design specifications. As a result of designing and examining the blow-in pipe, the formula (10) was derived from the formula (9). D · Le / (r _O ² √ (r _O · g))> 2.0 (10)

[0016]

EXAMPLES The present invention will be described below with reference to examples and comparative examples. Example 1 For liquid SiCl ₄ and liquid GeCl _{4 as} core raw materials, the liquid raw material supply devices shown in FIG. 1 were separately used and Ar gas was used as a carrier gas at a flow rate of 300 SCCM (standard volume flow rate (CC) per minute, respectively) Same) and 30 SCCM, 250S flow rate
CCM SiCl ₄ vapor and 3.0 SCCM GeCl ₄ vapor are used as H ₂ gas.
20SLM (standard volume flow rate per minute (L), same hereafter), O ₂ gas
It is sent to the core burner together with 20SLM and flame-hydrolyzed to Si.
Fine particles of O ₂ and GeO ₂ were generated and deposited on a rotating seed rod. The blowing Ar gas as a carrier gas at a flow rate 500SCCM using a liquid material supply apparatus shown in another figure 1 also liquid SiCl ₄ in the cladding material at the same time, the SiCl ₄ vapor flow rate 410SCCM H ₂ gas 20 SLM, O ₂ gas 20
Send to the clad burner with SLM for flame hydrolysis
SiO ₂ fine particles were generated and deposited on the seed rod to prepare a porous glass base material. At this time, SiCl ₄ and Ge for core
For Cl ₄ and SiCl ₄ for cladding, adjust each throttle valve 7 to maintain the pressure in each raw material container at 1.01 kg / cm ² gauge, and to keep SiCl ₄ and GeCl ₄ for core and SiCl ₄ for cladding.
_The degree of saturation of the raw material vapor in each raw material container of ₄ was maintained at 100%. The fluctuation range of atmospheric pressure during this period is 1.033 to 1.053 kg /
Although it was an absolute value of cm ^2, each raw material vapor flow rate was kept constant by adjusting the carrier gas flow rate by the carrier gas flow rate controller 1. The porous glass preform thus obtained was dehydrated at 1,200 ° C. and sintered at 1,500 ° C. to produce five single mode optical fiber preforms each having a diameter of 30 mm and a length of 500 mm. Refractive index difference in the longitudinal direction Δ
When the change in n was examined, the results shown in Table 1 were obtained.

COMPARATIVE EXAMPLE 1 Example 1 was repeated except that the throttle valve 7 was not used. The results are shown in Table 1. The pressure inside each container at this time was 0.01 kg / cm ² gauge, and the fluctuation range of atmospheric pressure during this time was 1.033 to 1.053 kg / cm ² absolute value.

[0018]

[Table 1] From the results in Table 1, it can be seen that Example 1 has a smaller variation δΔn in Δn than Comparative Example 1.

Example 2 Example 2 was performed in the same manner as in Example 1 except that the carrier gas flow rate control by the atmospheric pressure sensor was not performed. The fluctuation range of atmospheric pressure during this period is 1.033 to 1.053 kg.
It was an absolute value of / cm ² . Table 2 shows the results.

[0020]

[Table 2] From the results in Table 2, it can be seen that the variation of Δn δΔn is small and therefore the influence of atmospheric pressure fluctuation is small.

Example 3 In Example 1, SiCl _{4 as a} raw material for core and cladd
Carrier gas injection pipe, carrier gas outlet radius r ₀ = 0.002m, carrier gas outlet depth Le = 0.25m
When using the above item, D = 5.3 × 10 ^-6 m ^{2 /} sec., G = 9.8 m ² /
When calculating the value of D ・ Le / (r _O ² √ (r _O・ g)) as sec., this value is 2.4, and the saturation of SiCl ₄ vapor in the gas phase of the container at this time is 100. %Met. For comparison, the carrier gas blowing tube was set to r ₀ = 0.003m, Le = 0.2m, D
・ The value of Le / (r _O ² √ (r _O · g)) was 0.7, and the saturation of the SiCl ₄ vapor in the gas phase of the container at this time was 90%.

[0022]

EFFECTS OF THE INVENTION According to the present invention, the influence of the vapor phase pressure fluctuation in the container due to the atmospheric pressure fluctuation is reduced, and the saturation of the raw material vapors of the silicon compound and the doping compound in the gas phase is 99%.
By maintaining the above high constant value and supplying it to the burner, it was possible to manufacture a silica-based glass having uniform optical characteristics.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a liquid raw material supply device for explaining the present invention.

FIG. 2 is a vertical cross-sectional view showing a conventional liquid raw material supply device.

[Explanation of symbols]

 1, 11 ... Carrier gas flow controller 2, 12 ... Liquid raw material container (container) 3, 13 ... Liquid raw material 4, 14 ... Pressure gauge 5, 15 ... Atmospheric pressure sensor 6, 16 ... Correction calculator 7 ... Throttle valve 8 , 18 ... Carrier gas injection pipe 9, 19 ... Raw material vapor supply pipe 10, 20 ... Liquid raw material supply pipe

Claims (2)

[Claims] 1. A seed rod for rotating a vaporized silicon compound and optionally a vaporized dope compound by flame hydrolysis in an oxyhydrogen flame to generate silica fine particles and fine particles of a dope material, and rotating these fine particles. In a method for producing a silica-based glass, which comprises depositing and depositing on the above, a liquid raw material of a silicon compound and a liquid raw material of a dope compound are filled in respective raw material containers, and a carrier gas is blown into a carrier gas into the liquid raw material. In the liquid raw material supply method of supplying the mixed gas of the carrier gas and the raw material vapor to the burner by further bubbling with bubbling to vaporize the gas phase pressure in each raw material container, the gauge pressure is 1 kg / cm ² or more and less than 2 kg / cm ^2. The method for producing a silica-based glass, characterized in that 2. A diffusion coefficient D in a carrier gas component of a raw material vapor component of a vaporized silicon compound and, if necessary, a vaporized doping compound, a carrier gas jet port radius of a carrier gas blowing pipe is r _O , When the depth of the carrier gas jet in the liquid is Le,
The method for producing a silica-based glass according to claim 1, wherein a value of D · Le / (r _O ² √ (r _O · g)) (g is gravitational acceleration) is 2.0 or more.