KR970011334B1 - Apaque silica glass and its process thereof - Google Patents

Abstract

No summary

Description

Opaque silica glass and its manufacturing method

1 is a schematic diagram illustrating a method for producing an opaque silica glass according to the present invention using a silica glass mold.

* Explanation of symbols for main parts of the drawings

1: quartz grain 2: graphite mold

3: silica glass mold 4: electric furnace

5: heater

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to opaque silica glass having high purity and high heat resistance and excellent thermal barrier, and more particularly, opaque silica glass useful as light diffusing and thermal barrier materials used in furnaces. The present invention relates to a solid opaque silica glass capable of producing the member effectively and a method for producing the same.

Silica glass has been used for semiconductor industry and heat treating jig until now because of its high purity and excellent heat resistance. The important thing in the heat treatment furnace of the semiconductor industry is to keep the temperature gradient of the furnace constant. In order to cope with this purpose, the reactor tube is made from opaque silica glass containing 100,000 bubbles / cm 3 or less, and also disclosed in Japanese Utility Model Publication (JP-A-5-900). As disclosed in (Plat.) 1-162234, a heat shielding plate using opaque silica glass containing less than 6,000 bubbles / cm 3 is provided at both ends of a boat into which a semiconductor wafer is inserted.

Recently, a vertical furnace has become mainstream in the semiconductor industrial furnace, but in the vertical furnace, a metal frame is inserted at the bottom of the furnace, so that heat rays are scattered irregularly at the connection between the furnace and the metal frame. The temperature gradient in the furnace is not constant because the cooling unit provided to protect the sealing member that is or otherwise prevents the sealing member blocking between the flange of the furnace and the connection of the metal frame, and thus light scattering And providing thermal barriers.

Opaque silica glass has been preferably used as the light scattering and heat shielding material described above because of its excellent heat resistance and its thermal insulation.

Light scattering and thermal barrier members made of opaque silica glass described above are generally cut from solid opaque silica glass blocks in the form of plates in terms of work efficiency for the product, and quartz raw material (quratz raw). Material grains In particular, a method of filling rock crystal grains in heat-resistant molds and heating them in an electric furnace to sinter them has been used to prepare opaque silica glass blocks. However, in the conventional method, since a large hole is formed in the center of the block, a high purity opaque silica glass block containing uniform bubbles and having excellent light scattering and thermal barrier properties cannot be manufactured.

Moreover, the bubble density in the conventional opaque silica glass was less than 1000,000 bubbles / cm ⁴ as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 1-62234. When the light scattering member made of such opaque silica glass is used as a light scattering and thermal barrier member for a vertical furnace which is the mainstream in the semiconductor industry recently, it does not prevent irregular scattering of the heating wire occurring at the connection portion between the flange and the metal frame of the vertical furnace. However, the instability of the furnace temperature, which may affect the cooling unit provided to protect the sealing member, cannot be prevented. In addition, conventional opaque silica glass members cause large deformation at high temperatures. In particular, during the heat treatment of the silicon wafer heated to about 1000 ℃ or more due to a long heat treatment time, the thermal deformation is large, and the function of the heat shield and light scattering member is not sufficiently represented, thereby reducing the life of the furnace.

Therefore, in order to solve the above problems, the present inventors have thoroughly studied, and as a result, the thermal deformation of the opaque silica glass at high temperature is related to the density of bubbles occupied in the silica glass and the heat resistance of the silica glass. In order to increase the total bubble cross-sectional area per unit volume, the thermal deformation is reduced by reducing the diameter of the bubble and increasing the bubble density while decreasing the total volume of the bubbles occupied in the opaque silica glass, and also contained in the opaque silica glass. It was found that the heat resistance was improved by reducing the concentration of each of the sodium, phosphorus and OH groups below a certain range. Nitrogen elements doped in a specific range in the silica glass have been found to be particularly effective for improving heat resistance and particularly remarkable when synthetic quartz grains are used as raw material grains. It has also been found that the above opaque silica glass can be produced by sintering at a specific temperature using quartz raw material grains having a particular particle size. The present invention has been completed based on the above facts.

It is a first object of the present invention to provide a high purity opaque silica glass in which a large number of small independent bubbles are present therein and the bubbles are uniformly dispersed.

It is a second object of the present invention to provide a heat shield and a light scattering member having high heat resistance and excellent heat shielding properties.

It is a third object of the present invention to provide a method for producing the above high purity silica glass.

The present invention for achieving the above object has a density of 2.0 to 2.18g / cm3, the concentration of sodium and phosphorus elements in the silica glass is each 0.5ppm or less, the OH group concentration is 30ppm or less, and the following physical properties : (1) An opaque silica glass containing an independent bubble having a bubble diameter of 300 µm or less and (2) a bubble density of 100,000 to 1,000,000 bubbles / cm 3, and a method for producing the same.

Hereinafter, the present invention will be described in detail.

Opaque silica glass as defined in the present invention means opaque silica glass containing fine bubbles and is obtained by heating and sintering quartz raw material grains in a non-oxidizing atmosphere. High purity crystalline quartz grains having a particle size of 10 to 350 μm and high purity amorphous silica grains are used as raw materials for producing the opaque silica glass.

More preferably, high purity quartz grains having a sodium and phosphorus concentration of 0.5 ppm or less in the alkali metal, or chemically reducing high purity mineral crystal grains to reduce the concentration of sodium and phosphorus to 0.5 ppm or less, as disclosed in US Pat. No. 4,983,370. It is a quartz grain obtained by purification and a synthetic silica glass obtained by the soot method, and an amorphous silica grain having a concentration of sodium and phosphorus of 0.5 ppm or less and a particle size of 100 to 350 µm. In addition, the heat resistance is further increased by adjusting the nitrogen element concentration of the opaque silica glass to 50 to 500 ppm by doping the quartz raw material grains with ammonia. In particular, it is remarkable when using synthetic silica grains as raw material grains.

For example, an opaque silica glass having ammonia doped natural quartz grains and having a nitrogen element concentration adjusted to the above range has a viscosity of 10 ^12.8 poises at 1.260 ° C., which is the equivalent of opaque silica glass made from natural quartz grains. It exceeds 10 ^12.2 pois corresponding to the viscosity. Doping nitrogen element into the opaque silica glass described above is thought to result in Si-N bonds stronger than si-O bonds to improve the high temperature viscosity. In addition, doping of the nitrogen element allows NH groups to be present, and absorption at wavelengths of 3,400 cm ⁻¹ occurs to improve absorption in the infrared, which leads to further reduction in conduction plane of the hot wire.

The amount of nitrogen below 50 ppm cannot increase the high temperature viscosity, and an amount exceeding 500 ppm is undesirable because it causes nitrogen to be released during use. In addition, reducing the lithium concentration to 1.0 ppm or less is preferable to improve the heat resistance of the opaque silica glass.

An eletric fusion method, which provides a less OH group-mixed amount, is applied to melt the quartz raw material grains described above. When the OH group concentration is 30 ppm or less in the opaque silica glass, it is possible to prevent a decrease in viscosity in the silica glass at a high temperature that may occur due to the OH group.

The thermal conductivity of the silica glass is known to be dominant when the temperature rises above 1,000 ° C. The presence of bubbles in the silica glass causes radiation to occur at the temperature not only in the glass surface but also in the bubbles present in the glass. Therefore, the bubbles contained in the silica glass and their distribution have a great influence on the reflection and conduction of the radiation, so that not only the bubbles are uniformly distributed but also the heat shield and light scattering for the silicon wafer furnace. It is essential to increase the overall cross sectional area of the bubbles in the opaque silica glass when used as a member.

Uniform distribution of large bubbles in the opaque silica glass is effective to increase the overall cross-sectional area of the bubble. However, according to the results of the research conducted by the present inventors, it has been found that heat shields and light scattering members made of opaque silica glass containing large bubbles have high thermal deformation at high temperatures. Since the larger the total volume occupied by the bubbles contained in the opaque silica glass, the larger the thermal deformation at high temperature, the inventors found that decreasing the volume of the bubbles contained in the opaque silica glass and increasing the bubble density increased the total increase in the overall cross-sectional area of the bubbles. On the basis of the idea that it is caused, experiments are carried out to set the diameter of the bubble to 300 µm or less, preferably 20 to 180 µm, and the density to 100,000 to 1,000,000 bubbles / cm 3, thereby significantly preventing thermal interruption of the opaque silica glass. It was found to be improved. The opaque silica glass was measured for the transmission of light having a wavelength of 0.2 to 5㎛, it can be seen that for a sample having a thickness of 4mm transmittance is 10% or less.

The opaque silica glass according to the present invention needs to be limited to its density ratio of 2.0 to 2.18 g / cm 3 in order to control the strain by its own weight to a minimum level.

As described above, the opaque silica glass according to the present invention has a large bubble density and allows independent bubbles of small size to be uniformly distributed, provides excellent high temperature viscosity, and is excellent in heat resistance, light scattering and heat shielding. Therefore, the heat shield and the light scattering member made of the opaque silica glass according to the present invention do not become thermally deformed even when heat treated at a temperature exceeding 1000 ° C. for a relatively long time such as heat treatment of a silicon wafer.

The opaque silica glass of the present invention is produced by the following method.

That is, a high purity quartz grain having a particle size of 10 to 350 μm is filled into the heat-resistant mold, and then the quartz raw material grain is heated at a temperature of 50 to 150 ° C. lower than the temperature at which the quartz raw material grain is applied from normal temperature in a non-oxidizing atmosphere. Heating at a rate of temperature not exceeding 50 ° C./min, preferably at 10 to 40 ° C./min, and the temperature is then raised to a temperature 10 to 80 ° C. higher than the temperature at which the quartz raw material grains are melted. The temperature increase rate of 10 degrees C / min or less is gradually raised. Thereafter, the temperature is maintained for a predetermined time and then cooled to prepare an opaque silica glass of the present invention. Since the temperature increase rate and the holding time vary depending on the type and particle size of the quartz raw material grain, it is necessary to set the optimum temperature raising rate and the holding time within the range according to the quartz raw material grain. In particular, in order to produce a high quality opaque silica glass in which fine, independent bubbles are uniformly distributed, it is desirable to raise the temperature while providing a certain temperature gradient at the temperature of the top and bottom of the filling layer on the quartz raw material grains. The said temperature gradient is 10 degreeC or more.

The particle size of the quartz raw material grains used in the above-described manufacturing method is in the range of 10 to 350 µm. Particle sizes above this range cannot provide a constant bubble, and particle sizes below this range do not form bubbles. When sieving to a sieve having an opening of the particle size of 350 μm, it means that no particles remain in the sieve.

In addition, the heat resistance of the opaque silica glass in which nitrogen is introduced into the quartz raw material grain is improved. Heating the quartz raw material grains in a temperature range of 600 to 1,300 ° C. under an ammonia atmosphere is preferable as a doping method. When the treatment is performed as described above, the nitrogen element content of 50ppm to 600ppm is supplied to the opaque silica glass, and the high temperature heat resistance is improved.

In the production of the opaque silica glass, when the silica glass mold is inserted into a heat-resistant container and the quartz raw material grains are filled and dissolved therein, the quartz raw material shrinks by about 10 to 20% in the melting process, and the silica glass mold and the quartz A gap is generated between the raw material grain layers, and so-called grain dropping occurs in which quartz material grains fall from the top of the quartz material grain layer, thereby causing cracks in the opaque silica glass. However, the viscosity of the silica glass tube is low and it deforms to its own weight, filling the gap described above. Thus, the opaque silica glass obtained is free of cracks and can produce high quality opaque silica glass. In addition, since the silica glass mold protects the silica glass from the heat resistant container, the opaque silica glass is not affected by contaminants contained in the heat resistant mold (eg, graphite). Insertion of the silica glass mold may make the transmission distribution of the opaque silica glass constant in the lateral direction. It is believed that this is due to the fact that the silica glass has a lower ionic conductivity than graphite, so that abnormal temperature rise can be suppressed on the surface of the quartz raw material grain during dissolution. The mold formed by standing a quartz glass tube and a silica glass block is used as the silica glass mold described above. Molds made of high purity ceramics, especially high purity graphite molds, are suitable as heat resistant molds.

The laminar height of the quartz raw material grains filled in each mold is preferably set to 300 mm or less. If the above range is exceeded, the bubble may not be uniformly distributed in some cases.

Nitrogen gas is preferred as the non-oxidizing atmosphere used in the process of dissolving the opaque silica glass. Since dissolved in the non-oxidizing atmosphere gas, the active gas is replaced with an inert gas, and the active gas contained in the bubble is reduced.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.

Example

In this embodiment, the following measuring methods are used.

i) Thermal diffusion coefficient: The value was measured by a laser flash method.

ii) Specific heat: The value was measured by an adiabatic continuous method.

iii) Thermal conductivity: measured by heat ray method.

iv) Bubble density / bubble distribution: According to DIN 58927, a thin section of opaque silica glass having a certain volume is light-transmitted and the number of bubbles contained therein is counted, and the number is per cm 3 of the opaque silica glass. Change.

v) glass density: measured by Archimedes method.

vi) OH group concentration: measured by FT-IR value.

vii) apparent viscosity: cut the sample into strips of 3 × 1 × 50 mm, and keep the sample at 1,260 ° C. for 10 hours according to the beam bending method (supported at two points without load). When the value is calculated from the given strain.

viii) Nitrogen element concentration: measured by the inert gas melt conductivity method.

ix) The amount of gas in the bubble: obtained by measuring the gas coming out of the bubble after crushing the opaque silica glass into pieces by gas chromatographmass spectrometry.

The raw materials used in Examples 1 and 1 and Comparative Examples 1 to 3 are quartz grain A obtained by removing particles having a particle size of 250 μm or more and quartz grain B obtained by removing particles having a particle size of 180 μm or more. 1 shows the particle size distribution (the weight ratio of quartz grains remaining in the sieve having the mesh diameter shown in the particle size column when the used quartz grains are sieved).

TABLE 1

Example 1

The quartz grain A described above was charged into an electric furnace having a silica glass tube as a reaction tube, and heat-treated under hydrogen chloride / nitrogen of 50:50 at 1,200 ° C. for 1 hour to remove alkali metal. The purified quartz grain A was filled up to a depth of 100 mm in a high purity graphite vessel having an inner diameter of 200 mm × 200 mm in height, and mounted in a vacuum furnace to be 10 ⁻² torr or less by vacuum discharge to remove residual air between the particles. .

Nitrogen is then pressured into the vacuum of the furnace, at a rate of 20 ° C./minute from 1200 ° C. to 1200 ° C., 6.14 ° C./minute to 1,630 ° C., and 1,630 while introducing nitrogen at a flow rate of 51 / min. The temperature was raised from 1 ° C. to 1,750 ° C. at a rate of 0.34 ° C./min, and then held at 1,750 ° C. for 50 minutes. After vitrification, the furnace was turned off and slowly cooled.

The sample was cut from the opaque silica glass block thus obtained, and the thermal diffusivity, specific heat, thermal conductivity, bubble density, total bubble volume, glass density, and bubble distribution were measured for the sample. The measurement results of the former three values are shown in Table 2, and the rest are shown in Tables 3 and 4.

TABLE 2

As indicated in the results shown in Table 2, it can be seen that the opaque silica glass according to the present invention has a low thermal conductivity and excellent thermal barrier properties.

Example 2

After the alkali metal element was removed to quartz grain B under the same conditions as in Example 1, silica glass blocks were obtained and vitrified while adjusting the dissolution temperature in the same manner as in Example 1. Samples were cut from the opaque silica glass to measure bubble density, total bubble volume, glass density and bubble distribution. The measurement results are shown in Tables 3 and 4.

Comparative Example 1

After the alkali metal removal treatment was performed on the raw materials under the same conditions as in Example 1, it was filled with a graphite vessel and an opaque silica glass block was prepared in the same manner as in Example 1. The sample was cut from the opaque silica glass block thus obtained, and the bubble density, total bubble volume, glass density, and bubble distribution of the sample were measured. The results are shown in Tables 3 and 4.

Comparative Example 2

Quartz grain A was filled into the graphite mold as it was without alkali metal removal treatment and dissolved for vitrification under the same conditions as in Example 1 to obtain an opaque silica glass. A sample was prepared from the opaque silica glass and the bubble density, total bubble volume, glass density and bubble distribution of the sample were measured. The results are shown in Tables 3 and 4.

Comparative Example 3

After quartz grain A was subjected to an Alaki metal removal treatment, it was dissolved by heating from the inside with an arc flame in the air by rotational molding to prepare an opaque silica glass block. Samples were taken from the opaque silica glass blocks to measure bubble density, total bubble volume, glass density and bubble distribution. The results are shown in Tables 3 and 4.

Comparative Example 4

The raw material was sieved so as to classify only quartz grains having a particle size of 103 μm or less and an average particle size of 50 μm, and the alkali metal removal treatment was performed in the same manner as in Example 1. This was then heated to produce an opaque silica glass. Samples were taken from the opaque silica glass blocks to determine bubble density, total bubble volume, glass density and bubble distribution. The results are shown in Tables 3 and 4.

TABLE 3

TABLE 4

As shown in Tables 3 and 4, the opaque silica glass obtained by alkali removal treatment of quartz grains having a maximum particle size adjusted to a range of 110 μm or more and 250 μm or less has comparative examples of which the particle size is not controlled. The particle size is controlled, but the bubble volume is almost the same as that of the opaque silica glass in Comparative Example 2, which is not subjected to the alkali removal treatment, but the bubble density is large, thereby increasing the thermal barrier effect.

In addition, as shown in Table 4, the bubbles contained in the opaque silica glass obtained by using quartz grains having a particle size distribution satisfying the scope of the present invention are finer than the bubbles of the comparative examples and the distribution thereof is constant. The alkali metal element concentration, OH group concentration and apparent viscosity (1,260 ° C.) in the opaque silica glass prepared as described above in Examples 1 and 2 and Comparative Examples 1 to 3, respectively, are shown in Table 5 below.

TABLE 5

As can be seen in Table 5, the opaque silica glass having alkali metal element concentration and OH group degree of content satisfying each of the ranges defined in the present invention has high viscosity and excellent heat resistance at high temperature.

Samples of 4 mm thickness were taken from the opaque silica glass prepared in Examples 1 and 2 and the blocks obtained in Comparative Examples 1 to 4 and polished to the surface No. # 800, and then the surface was oxyhydrogen flame (oxyhydrogen flame). Heating was completed. The transmittances of the samples were measured at 0.2 to 5 μm with spectroporometers for ultraviolet, visible and infrared spectrophotometers. The results are shown in Table 6.

TABLE 6

As shown in Table 6, it can be seen that the opaque silica glass of the present invention has a low infrared transmittance and excellent thermal barrier properties.

Example 3

Natural quartz grains having a particle size of 100 to 300 um and an average particle size of about 230 um are filled in graphite molds having an inner diameter of 260 mm and a height of 300 mm and mounted in these vacuum furnaces to evacuate to 10 torr or less, thereby remaining air between the particles. Was removed. Then, nitrogen is blown into the vacuum of the furnace, and nitrogen is blown at a flow rate of 5/1 minutes, and the lower temperature of the quartz grain-layered bed is controlled at 1200 ° C. for 1 hour from room temperature, while the upper temperature is always 10 ° C. The temperature was raised to 70 ° C., and the temperature was raised from 1200 ° C. to 1630 ° C. for 70 minutes and from 1630 ° C. to 1720 ° C. for 350 minutes. Then, heating was performed for 50 minutes while maintaining the temperature of the entire quartz grain-layered layer at 1720 ° C., then the heating was stopped, cooled to room temperature, and the opaque silica glass block was taken out. In this way, the obtained opaque silica glass block was cut into a plate of about 8 m and the bubble density and bubble size were measured. The bubble density was 320,000 bubbles / cm 3 and the bubble size was in the range of 10 to 160 um. The heat shield member manufactured using the opaque silica glass of the present invention did not generate heat deformation even after heat treatment of 1000 ° C. or more for a long time such as heat treatment of a silicon wafer, and was able to maintain sufficient light diffusivity and heat shielding property.

Comparative Example 5

An opaque silica block was prepared by filling a graphite mold with the same high purity quartz grain as used in Example 3, heating it at the same temperature rising rate as Example 3 without giving a temperature difference, and then maintaining it at 1750 ° C. for 350 minutes. The opaque silica glass blocks thus obtained had blowholes near the top surface.

Comparative Example 6

The quartz grains were heated under the same atmosphere and elevated temperature conditions as in Comparative Example 5, and maintained at 1850 ° C. for 3 hours. Very fine bubbles collected at the top during complete dissolution, and the bubble density was low at the center and bottom. In particular, the bubbles at the bottom were large and a translucent layer formed.

Example 4

In order to charge quartz grain A having a specific surface area of 0.1 m 2 / g used in Example 1 into an electric furnace having a silica glass tube as a reaction tube and to purify the alkali metal and the elements of iron, magnesium and zirconium, 50:50 hydrogen chloride was used. Heat treatment was performed at 1200 ° C. under nitrogen for 1 hour. The purified quartz grain A was once again charged into a reaction tube and treated at 900 ° C. for 3 hours in an atmosphere of 50:50 ratio of ammonia / nitrogen to dope ammonia therein. The ammonia-doped quartz grains were filled in a high purity graphite vessel having an inner diameter ψ 200 mm × height 200 mm to a depth of 200 mm and mounted in a vacuum furnace to remove the air remaining between the particles by evacuating to 10 ⁻² torr or less. Then, nitrogen is blown into the vacuum in the furnace, at a rate of 20 ° C./minute from 1200 ° C. to 1200 ° C., 6.14 ° C./minute from 1200 ° C. to 1630 ° C., while blowing nitrogen at a flow rate of 5 1 / min. The temperature was raised from 1630 ° C. to 1750 at 0.34 ° C./min and then held at 1750 ° C. for 50 minutes to vitrify it. After vitrification, the electricity supply to the furnace was stopped and cooled slowly as it was. The sample was taken from the opaque silica glass block and measured. The bubble density was 478000 bubbles / cm 3, the total bubble volume was 8.8%, the apparent viscosity at 1260 ° C was 12.80 poise, the nitrogen element concentration was 500pp, and The OH group concentration was 0 ppm and the bubbles were in the range of 160 μm or less in diameter. The gas component in the bubble was at least 99% nitrogen. In addition, the transmittance in the infrared region was small and the thermal barrier property was excellent.

Example 5

A silica glass mold 31 having an outer diameter of 270 mm, a thickness of 4 mm and a height of 300 mm was inserted into the graphite mold 2 having an inner diameter of 270 mm and a height of 300 mm, as shown in FIG. An electric furnace equipped with a heater 5 and filled with quartz grain 1 having a particle size of 50 to 200 um and having an average particle size of 100 um and a bulk density of 1,4 g / cm 3. The air remaining between the particles was removed by attaching to (4) and evacuating to 10 to 2 torr or less.

Then, nitrogen is blown into the vacuum in the furnace, in which the nitrogen is blown at a flow rate of 5 1 / min, up to 1200 ° C. in 120 minutes, from 90 ° C. to 1200 ° C. to 1630 ° C., and from 1630 ° C. to 1750 ° C. It heated up in 240 minutes. After heating for 60 minutes while maintaining at 1750 ℃, the heating was stopped and cooled to room temperature to remove the opaque silica glass block. A disc of about 4 mm was taken from the thus obtained opaque silica glass block, and the surface was heated and finished with an oxyhydrogen flame to prepare a sample. The infrared transmittance was measured at 0.2 to 5 μm for the sample, and the transmittance was less than 5%. Abnormal foaming and inclusions were not found.

A sample having a thickness of 4 mm was taken from the opaque silica glass block and the opaque silica glass block prepared in Example 1, the surface was polished with # 800, and then the surface was heated with an oxyhydrogen flame to finish.

For the sample thus obtained, the conduction gradient was measured laterally at 2um every 1 cm from the center of the sample. The results are shown in Table 7.

TABLE 7

As can be seen from the results shown in Table 7, when the silica glass tube was inserted into the graphite mold, the uniformity in the conduction gradient in the lateral direction was improved as the diameter of the opaque silica glass was increased.

Claims (7)

The density is from 2.0 to 2,18 g / cm 3, the concentration of each of the sodium and phosphorus elements contained therein is 0.5 ppm or less, the concentration of OH groups contained therein is 30 ppm or less, and the bubbles contained therein are as follows. The same physical value: opaque silica glass, characterized in that it is independent having (1) a bubble diameter of less than 300um, and (2) a bubble density of 100,000 to 1,000,000 bubbles / cm 3. Its density is from 2.0 to 2.18 g / cm 3, the concentration of sodium and phosphorus elements contained in it is 0.5 ppm or less, the concentration of OH groups contained in it is 30 ppm or less, and the concentration of nitrogen element contained in it is 50 To 500 ppm, and the bubbles contained therein are independent opaque silicas having the following physical values: (1) a bubble diameter of 300 μm or less, and (2) a bubble density of 100,000 to 1,000,000 bubbles / cm 3. Glass The opaque silica glass according to claim 1 or 2, wherein the transmittance of light having a wavelength of 0.2 to 5 um passing through the opaque silica glass (thickness 4 mm) is 20% or less. Filling the heat-resistant mold with quartz raw material grains having a particle size of 10 to 350 um; Heating the grain from room temperature to a temperature in the range of 50 to 150 ° C. below the melting point of the quartz grain at a heating rate not exceeding 50 ° C./min; Slowly heating the grain to a temperature in the range of 10 to 80 ° C. above the melting point of the silica glass at a rate of 10 ° C./min or less; And maintaining the grain at the heating temperature and then cooling the grain. The method according to claim 4, wherein the quartz raw material grain is filled into a space formed by a silica glass mold frame inserted into the heat resistant container. 5. The method according to claim 4, wherein the temperature is raised while providing a temperature gradient at temperatures above and below the filling layer of the quartz raw material grain. The method of claim 6, wherein the temperature gradient is 10 ° C. or higher.