KR20190020238A - Qurtz glass vaccum melting apparatus and melting method - Google Patents

Abstract

An upper heating type quartz glass vacuum melting apparatus of the present invention includes: a cylindrical crucible which is formed in a container shape to receive a quartz glass powder injected thereinto so as to be melted; an insulating material which covers an outer portion of the crucible; a passage for an upper injection port (11), which is formed in the top surface of the insulating material (20); a passage for a lower exhaust port (12), which is formed in the bottom surface of the insulating material; an upper heater (40) which is formed between an upper portion of the crucible (50) and the top surface of the insulating material (20); and a lower heater (30) which is formed between the bottom surface of the crucible (50) and a lower surface of the insulating material (20). Argon and nitrogen gas are injected into the upper injection port, a pump is connected to the lower exhaust port to form a vacuum, the crucible is porous such that the argon and nitrogen gas pass through the crucible, the upper and lower heaters each have a gap formed therein such that the argon and nitrogen gas pass through the upper and lower heaters, and the argon and nitrogen gas injected into the upper injection port sequentially pass through the quartz glass powder or a melt of the quartz glass powder within the crucible, the lower surface of the crucible (50), and the lower exhaust port. The present invention provides manufacturing apparatus and method which minimize cracking of large diameter high purity transparent quartz glass, prevent the large diameter high purity transparent quartz glass from being contaminated with impurities, and can manufacture the large diameter high purity transparent quartz glass with good productivity. Therefore, the present invention can manufacture large diameter quartz glass with high uniformity by suppressing the impurities to 5 ppm or lower and reducing the generation of bubbles in a quartz melt.

Description

Top heating quartz glass vacuum melting apparatus and method {QURTZ GLASS VACCUM MELTING APPARATUS AND MELTING METHOD}

The present invention relates to a top heating quartz glass vacuum melting apparatus and method, and in particular, it is possible to manufacture a large diameter quartz glass with a diameter of 1,000 mm and a height of 500 mm or more while using an electric heating method, and efficiently produce bubbles while producing a large diameter quartz glass. The present invention relates to a quartz glass vacuum melting apparatus and method which can be removed to enable the production of high purity quartz glass.

As a conventional method for producing transparent quartz glass, there are known methods of melting a quartz glass powder by vacuum melting using a heating furnace and melting by an oxyhydrogen flame. In addition, the quartz powder uses a natural product and has excellent heat resistance, but in order to be applied to a process such as a semiconductor, there is a problem in that a necessity of a strict manufacturing apparatus of high purity of the surrounding material may arise. Moreover, although the high purity treatment of the said natural crystal powder is performed, it is not only easy to suppress OH <^-> 5 ppm or less easily by the conventional method only.

Therefore, the quartz glass by the VAD method which sinters the deposit of synthetic amorphous silica powder, or the sol-gel method which sinters the amorphous silica powder obtained by the hydrolysis of the alkoxide silicate, etc. is examined. However, when synthetic amorphous silica powder is used, high purity is possible, but heat resistance is also lowered, and properties are poor compared to quartz glass using natural crystal raw materials.

In order to solve this problem, a method of crystallizing and manufacturing amorphous silica as a proposed method is also proposed. However, a problem of containing impurities is caused by mixing a crystallization accelerator such as alkali or alumina. In addition, such impurities may cause a decrease in transparency of the product.

Of course, the Republic of Korea published patent (Korean Patent Publication No. 10-2008-0097260) has a "halogen use gas Cl2, gas concentration 200cc / min or more, heat treatment temperature 950 ℃ or more, heat treatment time 10 hours or more to remove the OH group in the quartz glass H2 as a halogen removal gas, gas concentration of 200 cc / min or more, heat treatment temperature of 950 ° C. or more, residual halogen removed at a heat treatment time of 10 hours or more; slow cooling to a virtual temperature of 1050 ° C. or less; uniform OH and halogen content The method for controlling the OH content in the quartz glass characterized in that it has a. "And the Republic of Korea Patent (Republic of Korea Registration No .: 10-1378748)" inside at 10mm thickness for ultraviolet light of wavelength 245nm. The transmittance is 95% or more, the OH content is 5 ppm or less, the Li, Na, K, Mg, Ca, and Cu content is less than 0.1 ppm, respectively, and the viscosity at 1215 ° C is 1011.5 Pa.s Molten quartz glass, characterized in that. Is presented. "

However, the above patented technology does not provide a technology capable of producing a large diameter quartz glass with a diameter of 1,000 mm and a height of 500 mm or more, but efficiently removing bubbles to enable the production of high purity quartz glass.

As a result, for the production of high-purity quartz emulsions with large diameters, there is an urgent need to develop high-purity quartz glass manufacturing techniques having a desirable temperature gradient while minimizing impurities and removing bubbles.

Prior Art 1: Republic of Korea Patent Publication Number: 10-2008-0097260 (November 5, 2008) Prior Art 2: Republic of Korea Registration Number: 10-1378748 (March 21, 2014)

The present invention has been made to solve the problems of the prior art, it is possible to manufacture a large diameter quartz glass of 1,000mm in diameter, 500mm or more in height while removing the bubbles efficiently, thereby minimizing the occurrence of cracks and at the same time reduce the impurities It is an object to provide a technique that enables the production of large diameter quartz glass.

The object is a crucible having a cylindrical shape and a container, and a heat insulating material surrounding the outside of the crucible is provided, and the inside of the crucible is injected to melt the quartz glass powder, and an upper injection hole is formed on the upper surface of the heat insulating material 20. A passage 11 is made, and a lower exhaust port 12 is formed in a lower surface of the heat insulating material, and an upper heater 40 is provided between the upper portion of the crucible 50 and the upper surface of the heat insulating material 20. The lower heater 30 is provided between the lower surface of the crucible 50 and the lower surface of the heat insulator 20, argon and nitrogen gas are injected into the upper inlet, and a pump is connected to the lower exhaust port. The vacuum is made, the crucible is porous and the argon and nitrogen gas is passed, the gap between the upper heater and the lower heater, respectively, the argon and nitrogen gas is passed, Argon and nitrogen gas injected from the upper injection hole is achieved by passing through a melt of quartz glass powder or quartz glass powder inside the crucible, passing through the lower surface of the crucible 50, and passing through the lower exhaust port.

In addition, as another embodiment of the present invention, a crucible having a cylindrical shape and a container and a heat insulating material surrounding the outside of the crucible are provided, and the inside of the crucible is injected to melt the quartz glass powder, and the heat insulating material 20 A passage of the upper injection hole 11 is made in the upper surface of the), and a passage of the lower exhaust port 12 is made in the lower surface of the heat insulating material, between the upper portion of the crucible 50 and the upper surface of the heat insulating material 20. The top heating quartz glass vacuum melting method in which the upper heater 40 is provided and the lower heater 30 is provided between the lower surface of the crucible 50 and the lower surface of the heat insulator 20 is characterized in that the quartz glass powder The first step is to put the inside of the crucible 50, start exhausting the vacuum is made by the pump connected to the lower exhaust port 12, and to heat up the upper heater and the lower heater to increase the temperature, Step 2 in which the upper part of the raw material, which is the quartz glass powder present in the crucible 50, is melted before the lower part of the raw material, and the whole raw material of the quartz glass powder present in the crucible 50 is melted, and the molten state is maintained for a predetermined time. 3 steps of making, and turning off the power of the upper heater and the lower heater, and then injecting nitrogen to the normal pressure, and the argon and nitrogen gas injected from the upper inlet is quartz glass powder in the crucible Alternatively, the mixture passes through the melt of the quartz glass powder, passes through the lower surface of the crucible 50, and passes through the lower exhaust port.

The present invention provides a manufacturing apparatus and method capable of producing a large-caliber high-purity transparent quartz glass with a minimum of cracks and contaminating impurities without satisfactory productivity, so that impurities can be suppressed to 5 ppm or less, and bubbles are generated in the quartz melt. It is possible to provide a device and a method for manufacturing a high uniformity large diameter quartz glass by reducing the

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure of the Example which shows the quartz glass manufacturing apparatus of the conventional electric melting method.
2 is a view showing the structure of the top heating quartz glass vacuum melting apparatus of the present invention.
Figure 3 is a view showing the structure of the crucible and the heat insulating material in the quartz glass manufacturing apparatus of the present invention.
4 is a view of an embodiment showing a manufacturing method for producing the quartz glass of the present invention.

Hereinafter, an embodiment of the present invention will be described in detail with respect to the top heating quartz glass vacuum melting apparatus and method. In addition, detailed description can be abbreviate | omitted about the normal technique which is necessary for demonstrating this invention.

In the present invention uses a natural quartz powder (powder), the particles of the powder used is less than 200㎛ (may be less than 200㎛ in 1㎛), but can also be used in addition to the quartz powder having a particle size Do.

The vacuum melting method to which the present invention is applied is mainly applied to manufacturing a quartz glass cylinder or increasing the diameter by resizing an acid-hydrogen ingot.

In order to manufacture high-purity quartz glass for use in semiconductor processes and various optical processes, large diameters are required to be large, and there are problems in that fewer bubbles are generated and cracks are kept to a minimum.

In addition, impurities also need to be minimized. In particular, metallic impurities such as Li, Na, Mg, Ca, K, Al, Ti, Cr, Ni, Zn, Zr, Mo, Fe, and the like are preferably 20 ppm or less ((from the above metals). The content of Al, which has the highest concentration, should be 10 ppm or less.)), And the content of other impurities including Cl ₂ and the like should be 10 ppm or less, and the content of OH groups should be 5 ppm or less. In addition, it is preferable to maintain the viscosity value in 1200 degreeC about 1012.0. In other words, when the content of the specific impurity is high, the tendency of the light transmittance in the ultraviolet region is decreased, so that impurity control is necessarily required.

On the other hand, in the case of the high-purity transparent quartz glass is less than 1012.0 poise viscosity value at 1200 ℃ is deformed during the heat treatment of 1200 ℃ necessary for the semiconductor process, so the inconvenience that may cause the defect occurs, more than 1013.0 poise It is good to do this, but these specific methods are not suggested. However, the present invention provides a manufacturing apparatus that can solve such a problem by providing a method for maintaining a desirable warm head gradient by providing a heater at the top and the bottom.

In particular, in order to perform a uniform quartz glass melting process in the present invention, it is sometimes necessary to measure the viscosity of quartz glass, and unlike ordinary glass, quartz glass has a high melting point and high temperature viscosity, making it difficult to measure viscosity at high temperatures. . Viscosity measurement methods include beam bending method, cantilever beam bending method, torsion method, and falling ball method. Beam bending method and cantilever beam method can measure the viscosity of 109 ~ 1016 Pa · s at 1100 ~ 1500 ℃, Torsion method can measure the viscosity of 105 ~ 108 Pa · s at 1500 ~ 2200 ℃, and falling ball method The viscosity measurement of 104 Pa.s or more is possible at 2300 degreeC or more. Only the beam bending method and the cantilever beam bending method are available for measuring the viscosity to check the heat resistance of quartz glass used at high temperature. Among them, in the present invention, the viscosity of the quartz glass is measured using a cantilever beam bending method which can be easily measured.

In addition, the thermal expansion coefficient of quartz glass was measured using TDA (Thermal Dilatometric Analyzer). The specimen for measuring the coefficient of thermal expansion was produced in the form of a rod 5 mm in diameter x 50 mm in length. The measurement temperature was in the range of room temperature to 500 ° C., and the temperature increase rate was 5 ° C./min. To determine the relationship between the coefficient of thermal expansion and the OH content, the measured value may be obtained by varying the OH content of the samples. Generally, the coefficient of thermal expansion of quartz glass is 0.45x10 ^-6 / K, or 0.55x10 ^-6 / K. Since the coefficient of thermal expansion tends to increase as the content of OH groups increases, By minimizing the coefficient of thermal expansion of the quartz glass by minimizing it can be suggested a method for producing a high quality quartz glass of high quality.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure of the Example which shows the quartz glass manufacturing apparatus of the conventional electric melting method.

That is, Fig. 1 is a quartz glass manufacturing apparatus showing a conventional method of the electrofusion method to which the present invention is applied, and is a method of melting quartz crystal with electricity in the chamber 10. It contains little moisture.

In general, the conventional melting method is a method of electrically melting by providing the side heater 35 of the chamber 10, and injects quartz powder into the crucible 50 and applies the side heater 40 around the crucible. Melting the quartz powder by applying heat through. Then, the insulating wall 20 is provided between the chamber and the heater to increase the thermal energy efficiency. Here, as the material of the heat insulation wall 20, a fire brick or a ceramic material is used.

In addition, the pump 80 may be used to make a vacuum using the discharge port of the chamber, and the filter 70 may be provided at the discharge port of the chamber.

Although the atmosphere of the quartz glass melting production process is not particularly limited, it is preferable to perform the process in a vacuum atmosphere or in a helium gas atmosphere to minimize the air present in the crystals after crystallization. In order to perform the process in a vacuum atmosphere, it is preferable to maintain a vacuum degree of 10 ⁻¹ torr or less, preferably 10 ⁻² torr or less.

In addition, it is preferable to maintain a temperature at a suitable level (temperature at which melting point above the glass transition temperature is considered) in which the heating temperature of the quartz glass process is not overheated, preferably l350 ° C or more and 1650 ° C or less. More preferably, they are 1450 degreeC or more and 1600 degrees C or less. That is, since the crystal | crystallization of the molded object melt-processed at the said process is a Kurisutobarito structure, it is preferable that heating temperature is more than melting | fusing point.

Therefore, in some cases, 1713 ° C or more may be required, and 1750 ° C or more, more preferably 1800 ° C or more may be required in order to completely dissolve. Porridge, 1800 ℃-2200 ℃ process temperature also needs to be maintained.

In addition, the above process is preferably free of contamination of impurities, and through the optimization process, high purity quartz glass can be manufactured without performing impurity removal.

In order to achieve the purpose of high-purity quartz glass for use in semiconductor processes, metal impurities such as Li, Na, Mg, Ca, K, Al, Ti, Cr, Ni, Zn, Zr, Mo, Fe, etc. should be 20 ppm or less. (The content of Al, which has the highest concentration among the metals, should be 10 ppm or less.) In addition, the content of other impurities including Cl2 and the like should be 10 ppm or less, and the content of OH groups should be 5 ppm or less. In addition, it is preferable to maintain the viscosity value in 1200 degreeC about 1012.0.

However, in the past, the OH group content is also in most cases of 20 ppm or less, and it is very difficult to present a method for maintaining 5 ppm or less in reality. And the viscosity value of quartz glass under high temperature is improved, and the viscosity value in 12OO degreeC is about 1012.0 poise.

As a result, the exhaust port 60 is directly connected to the upper portion to significantly reduce the pollution degree. In addition, a vacuum pump 80 is connected to the exhaust port 60. The filter 70 is further provided in the middle of the exhaust port to filter the contaminants of the steam generated in the crucible, thereby minimizing the degree of contamination in the chamber.

On the other hand, argon (Ar) and nitrogen (N ₂ ), which are inert gases, are supplied from the side of the apparatus as shown in FIG. 1, that is, quartz glass is produced in an inert gas atmosphere.

2 is a view showing the structure of the top heating quartz glass vacuum melting apparatus of the present invention.

As shown in the drawing, the extinguisher 20 is provided outside the crucible 50 for melting the quartz powder, the upper heater 40 on the crucible 50, the lower heater 30 on the bottom of the crucible 50 is Each is provided.

And, in the prior art, also inert gases such as argon (Ar) and nitrogen in the apparatus side, as in embodiment of the 1 (N ₂₎ The surface is not injected, in the present invention the inert gas is argon (Ar) and nitrogen from the upper apparatus (N ₂₎ Is injected.

That is, the heat insulating material 20 is located outside the crucible 50 in which the quartz glass powder is melted inside, and the heat insulating material is surrounded by the chamber 10. In addition, an injection hole 11 is provided in the chamber 10. Argon (Ar) and nitrogen (N ₂ ) is injected from the inlet (11). The lower exhaust port 12 is provided at the lower part of the chamber, and exhaust occurs at the lower exhaust port 12.

In addition, although not shown in FIG. 2, the exhaust port 12 is connected to the pump 80 to make a vacuum as shown in the embodiment of FIG. 1, and a filter 70 is provided between the pump and the exhaust port to reduce contamination.

In addition, the crucible in the present invention is prepared using porous graphite. In addition, the present invention has a feature that the diameter of the crucible should be larger than the height of the crucible. That is, it aims at manufacturing large diameter quartz glass with a diameter of 1,000 mm and a height of 500 mm or more.

As a result, the prior art is difficult to manufacture a large diameter quartz glass ingot because the melting using a side heater, the raw material powder of the central portion of the quartz glass ingot is unmelted, a large amount of bubbles are generated.

On the other hand, the vacuum melting method is mainly used to manufacture a quartz glass cylinder, or to increase the diameter by resizing the acid-hydrogen ingot, the improved vacuum melting method of the present invention is melted in the upper portion as the melt is lowered by the difference in density By suggesting a method of melting, while producing a large-diameter quartz glass has the effect of significantly reducing the bubble.

Figure 3 is a view showing the structure of the crucible and the heat insulating material in the quartz glass manufacturing apparatus of the present invention.

As shown in FIG. 3, the crucible 50 has a cylindrical shape and quartz glass powder is introduced into the crucible 50, and the crucible 50 has a cylindrical container shape in which a lower surface is formed and an upper surface is opened. . In addition, the heat insulating material 20 has a structure surrounding the crucible and both sides, an upper surface, and a lower surface are formed. In this case, a passage of the upper injection hole 11 is made in the upper surface of the heat insulating material 20, and the lower part of the heat insulating material is provided. On the surface, a passage of the lower exhaust port 12 is made.

In addition, an upper heater 40 is provided between the upper portion of the crucible 50 and the upper surface of the heat insulating material 20, and the lower heater 30 is disposed between the lower surface of the crucible 50 and the lower surface of the heat insulating material 20. ) Is provided.

In addition, the crucible 50 is made of porous graphite so that the inert gas is passed through, and the upper heater 40 has a gap in which the inert gas can move, and the inert gas can also move in the lower heater 30, respectively. There is a gap.

4 is a view of an embodiment showing a manufacturing method for producing the quartz glass of the present invention.

As shown in FIG. 4, quartz glass is manufactured by a four step process from (A) to (D).

As shown in (A) of FIG. 4, quartz glass powder is introduced into the crucible 50, and the exhaust gas is started by a pump connected to the lower exhaust port 12, and the upper and lower heaters are heated to heat the temperature. Will increase.

At this time, the degree of vacuum is about 10 torr, and the temperature is heated so that the upper and lower temperatures are 1600.

4B shows that the upper part of the raw material, which is the quartz glass powder present in the crucible, is melted before the lower part of the raw material. The temperature of the top of the raw material is 1800 ℃, the temperature of the bottom of the raw material is 1600 ℃,

FIG. 4C is a view showing that the entire raw material of quartz glass powder present in the crucible is melted. At this time, the temperature of the upper portion of the raw material is 1850 ℃, the temperature of the lower portion of the raw material is 1650 ℃, and keeps 7hr (hour) in this state.

4 (D) is turned off by cutting off the power of the upper heater and the lower heater, and then nitrogen is injected to the atmospheric pressure.

In (B), when the quartz glass powder starts to melt from the top of the raw material and becomes a melt, the weight becomes heavier than that of the powder, so that the soluate sinks under the crucible. In addition, the pressure of the inert gas injected from the inlet 11 above the crucible has an effect of allowing the melt to sink better.

On the other hand, during the melting of the quartz glass powder, the bubble layer is raised to form a quartz glass that is significantly reduced in the overall bubble. In particular, the bubble layer is further raised in the step (C) maintained for 7 hours.

Then, when an inert gas is injected from the injection port 11 existing in the upper portion, the inert gas is passed through the melt of the quartz glass powder or the quartz glass powder present in the crucible, passes through the lower surface of the crucible, and passes through the lower exhaust port 12. .

Tables 1 to 4 are experimental example values showing changes in bubble generation according to each manufacturing process.

And Table 1 is an experimental value showing the change in the number of bubbles according to the process temperature.

Table 1.

At the process temperature of 1800 ° C, only a small portion of the upper part of the raw material powder was melted, and at the process temperature of 1950 ° C or more, the whole melted, but a crack was generated by the reaction of the quartz glass ingot and the Mo crucible. At the process temperatures of 1850 ° C and 1900 ° C, total melting was confirmed and bubble layers were generated.

Therefore, 1850 degreeC and 1900 degreeC were the most ideal process temperature, and especially 1900 degreeC was the bubble number 6.1 (ea / 50mm <^2> ).

Table 2 is an experimental value showing the change in the number of bubbles with the melting temperature holding time.

Table 2.

As shown in Table 2, it can be seen that the number of bubbles decreases as the holding time increases, and cracks are formed by bonding to Mo crucibles under 7hr holding conditions at a process temperature of 1900 ° C. In addition, it is judged that the optimum condition with the bubble number of 3.2ea / 50mm ² at the holding temperature of the process temperature of 1850 ° C and 7hr.

Therefore, when the process temperature is maintained at 1850 ° C. and maintained at 7 hr, quartz glass having a small number of bubbles is produced.

Table 3 is an experimental value showing the change in the number of bubbles depending on the lower temperature.

Table 3.

As shown in Table 3, it can be seen that the number of bubbles decreases as the lower temperature is increased, and the change in the number of bubbles is not large at the lower temperature of 1650 ° C. or higher.

Table 4 is an experimental value showing the change in the number of bubbles according to the process vacuum.

Table 4.

In Table 4, it can be seen that the number of bubbles decreases as the degree of vacuum decreases, and the optimum number of bubbles is 0.5ea / 50mm ² at a vacuum degree of 10torr.

Cylindrical or ingot-type quartz glass obtained through the above process has very high transparency, and thus has an absorption coefficient of 2 × 10 ⁻³ cm ⁻¹ at a wavelength of 245 nm light and a Raman Raman spectrum. The ratio of scattering peak intensities at is at I2250 / I800 (2250 cm ⁻¹ (I2250) to the scattering peak intensity 800 cm ⁻¹ (I800)).

In the present invention, the process is performed in a vacuum atmosphere by operating the vacuum pump 80 to make a high vacuum of 10torr level, and the size of the bubbles in the quartz glass is maintained at about 100 to 150 μm because the process is performed in a high vacuum atmosphere. Can be.

In addition, the content of the OH group is 5 ppm or less, and the value of the thermal expansion coefficient is set to 0.15x10 ^-6 / K, thereby enabling the production of high quality quartz glass.

That is, through the present invention, not only the production of high-quality uniform quartz glass is possible, but also the effect of extending the life of the heater and the heat insulating agent.

10: chamber 20: insulation
11: upper inlet 12: lower exhaust port
30: lower heater 40: upper heater
50: crucible 35: side heater
60: exhaust port 70: filter
80: vacuum pump

Claims (2)

A cylindrical and container-shaped crucible and a heat insulating material surrounding the outside of the crucible are provided, and the inside of the crucible is injected to melt the quartz glass powder,
The upper surface of the heat insulating material 20 is made of a passage of the upper injection hole 11, the lower surface of the heat insulating material is made of a passage of the lower exhaust port 12,
An upper heater 40 is provided between the upper portion of the crucible 50 and the upper surface of the heat insulating material 20, and a lower heater 30 is disposed between the lower surface of the crucible 50 and the lower surface of the heat insulating material 20. Equipped,
Argon and nitrogen gas is injected into the upper inlet, a pump is connected to the lower exhaust port to create a vacuum,
The crucible is porous through the argon and nitrogen gas,
A gap exists in each of the upper heater and the lower heater to allow the argon and nitrogen gas to pass through,
Argon and nitrogen gas injected from the upper injection port passes through the melt of the quartz glass powder or quartz glass powder in the crucible, passes through the lower surface of the crucible 50, and passes through the lower exhaust port, characterized in that the upper heated quartz glass vacuum Melting device. A cylindrical and container-shaped crucible and a heat insulating material surrounding the outside of the crucible are provided, and the inside of the crucible is injected to melt the quartz glass powder,
The upper surface of the heat insulating material 20 is made of a passage of the upper injection hole 11, the lower surface of the heat insulating material is made of a passage of the lower exhaust port 12,
An upper heater 40 is provided between the upper portion of the crucible 50 and the upper surface of the heat insulating material 20, and a lower heater 30 is disposed between the lower surface of the crucible 50 and the lower surface of the heat insulating material 20. The top heating quartz glass vacuum melting method provided is,
A step of injecting quartz glass powder into the crucible 50, starting exhausting vacuum by a pump connected to the lower exhaust port 12, and heating the upper heater and the lower heater to increase the temperature;
Step 2 in which the raw material upper portion of the quartz glass powder present in the crucible 50 is melted before the lower portion of the raw material,
A third step of melting the entire raw material of quartz glass powder present in the crucible 50 and maintaining the molten state for a predetermined time;
And shutting off the power of the upper heater and the lower heater to turn off, and then injecting nitrogen to atmospheric pressure.
Argon and nitrogen gas injected from the upper injection port passes through the melt of the quartz glass powder or quartz glass powder inside the crucible, passes through the lower surface of the crucible 50, and passes through the lower exhaust port vacuum vacuum Melting method.

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