JP5627077B2 - Method for heat treatment of synthetic silica glass body and optical member made of synthetic silica glass - Google Patents

Description

  The present invention relates to a heat treatment method for a synthetic silica glass body capable of effectively removing strain and purifying the synthetic silica glass body, and a method for producing an optical member made of synthetic silica glass including the heat treatment method for the synthetic silica glass body Further, the present invention relates to a synthetic silica glass optical member which is manufactured by the method for manufacturing a synthetic silica glass optical member and exhibits excellent light transmittance for a long period of time with irradiation of ultraviolet rays, particularly ArF excimer laser light. Specifically, the present invention particularly relates to a lens and other optical components of a lithography exposure apparatus using an excimer laser for manufacturing a semiconductor chip, a narrowing prism used for an excimer laser body, and ArF. Synthetic silica glass optical member suitably used for general optical members, lenses, beam splitters, prisms, photomasks, pellicles, etc. used for excimer laser light, and synthetic silica glass bodies suitably used for the production thereof It relates to a heat treatment method.

In recent years, along with the miniaturization of LSI design rules, ArF excimer laser lithography as an exposure technique, in particular, an exposure apparatus using a liquid immersion technique has been rapidly spread. Silica glass members for an ArF excimer laser exposure apparatus have further quality. Improvement is demanded. List the generally required optical properties:
1) High light transmittance at 193.4 nm 2) High refractive index homogeneity 3) Low birefringence 4) Stable against ArF excimer laser irradiation (laser resistance)
Etc.

  In general, synthetic silica glass has been used in many fields as an ultraviolet transmissive material because of its good ultraviolet transparency, and there is no exception in ArF exposure apparatuses, and synthetic silica glass is used as almost all transmissive materials. Has been used. However, in recent years, combined use of immersion technology has led to an increase in the NA (numerical aperture) of the optical system, and there is a demand for light transmission characteristics in the deep ultraviolet to vacuum ultraviolet range of synthetic silica glass optical members that affect the thermal aberration characteristics of lenses. The characteristics are becoming particularly severe. Alkali metals, alkaline earth metals, transition metals, and the like are generally known as factors that affect light transmission characteristics.

Among these influencing factors, the Na concentration in the silica glass has a large influence, and in order to achieve an internal transmittance of 193.4 nm or more of 99.81% or more, the Na weight concentration must be suppressed to 0.2 ppb or less. We have newly discovered that it is necessary to use the latest evaluation equipment.
Ultra high-purity synthetic silica glass is used as a material for optical members, and heat treatment (annealing) for strain removal is required to reduce refractive index homogeneity and birefringence. It is a common method to maintain a high temperature in the furnace for a long time. However, there has been a problem of process contamination in which metal impurities such as alkali metals released or moved from furnace materials, jigs and atmospheres of the heating furnace used are taken into the silica glass surface layer.

  In order to improve the process contamination of the alkali metal impurities as described above, many studies and developments have been made so far. For example, in Patent Document 1, in the thermal annealing treatment of a flat cylindrical optical member, a cutting margin of 30 mm or more in both the radial direction and the thickness direction is provided in advance, and after the thermal annealing treatment, the portion is cut to form Na. An optical member having a concentration of 20 ppb or less is obtained. In Patent Document 2, when a plate-like substrate of 153 × 153 × 6.4 mm is produced from a 16 × 16 × 30 cm prismatic molding block, the Na concentration in the substrate is 2.3 to 4.3 ppb, but 17 × When a plate substrate of 153 x 153 x 6.4 mm is produced from a 17 x 25 cm prism block molding block, it is <0.3 to 1.9 ppb, and by taking many cut parts, the inside of the prism block after thermal annealing treatment The technique for preventing the diffusion of Na element into the film is exactly the same as in Patent Document 1. In this method, there is a problem that the economy of the part to be discarded cannot be ignored.

  In Patent Document 3, an attempt is made to reduce the amount of Na contamination by thermally annealing the treated product in a quartz glass muffle in an electric furnace. In order to satisfy the internal transmittance of 99.78% and the internal transmittance of 99.82% or more, a grinding amount of about 2 mm for the outer surface layer is required. That is, although an expensive high-purity furnace member is used, Na element contamination on the surface layer portion of the processed product made of synthetic silica glass is not completely prevented.

Patent Document 4 describes an example of thermal annealing in synthetic silica powder having an average dissolved hydrogen molecule concentration of 1 × 10 ¹⁹ molecules / cm ³ or more and an Na concentration of 30 ppb or less. The Na concentration of the processed product is 2 ppb, which is an insufficient level for light transmittance. That is, it is impossible to reduce the Na concentration of the workpiece to 2 ppb or less simply by focusing on the Na concentration and the hydrogen content concentration of the coated powder.
Conventionally, halogen gas such as hydrochloric acid gas and chlorine gas has been used as a method for purifying Na element in silica glass. These gases are toxic to the human body, and the economy for eliminating the toxicity has been a problem.

Japanese Patent No. 42114496 JP 2001-302275 A Japanese Patent Laid-Open No. 10-279322 JP 2001-270725 A

  The present invention relates to a method for heat treatment of a synthetic silica glass body capable of effectively removing strain and purifying the synthetic silica glass body, a method for producing an optical member made of synthetic silica glass including the heat treatment method of the synthetic silica glass body, and An object of the present invention is to provide a synthetic silica glass optical member that is manufactured by the method for manufacturing a synthetic silica glass optical member and exhibits excellent light transmittance for a long period of time with respect to irradiation with ultraviolet rays, particularly ArF excimer laser light. .

In order to solve the above-mentioned problems, a heat treatment method for a synthetic silica glass body according to the present invention is a heat treatment method for removing strain and purifying a synthetic silica glass body, which is an object to be processed, on the outer surface of the object to be processed. All are covered in a contact state with SiO ₂ powder having an Al concentration of 10 ppm to 30 ppm and an Na concentration of less than 50 ppb, and a heat treatment is performed at a temperature range of 900 ° C. to 1300 ° C. for 30 minutes to 800 hours.

As the SiO ₂ -based powder, it is preferable to use natural quartz powder in which the Al concentration and the Na concentration satisfy the above-described conditions. Further, it is preferable to use α-quartz crystalline natural quartz powder as the natural quartz powder.

  The method for producing an optical member made of synthetic silica glass of the present invention includes the heat treatment method for a synthetic silica glass body of the present invention.

The synthetic silica glass optical member of the present invention covers all of the outer surface of the synthetic silica glass body, which is the object to be processed, in a contact state with SiO ₂ powder having an Al concentration of 10 ppm to 30 ppm and an Na concentration of less than 50 ppb . A method for producing an optical member made of synthetic silica glass, including a heat treatment method for synthetic silica glass body, wherein the synthetic silica glass body is subjected to heat treatment in a temperature range of 900 ° C. to 1300 ° C. for 30 minutes to 800 hours to remove strain and purify the synthetic silica glass body. in a synthetic silica glass optical member that will be produced,
When the Na concentration is less than 0.5 ppb in all regions, the difference between the maximum value and the minimum value is 0.1 ppb or less, the OH group is 100 ppm or less, the birefringence is 0.5 nm / cm or less, and the internal transmittance is 99.82% or more. Oh it is,
The synthetic silica glass body is characterized Rukoto produced by soot method or a plasma method.

As a result of intensive studies on reducing the Na concentration in the synthetic silica glass body, the present inventors have buried the synthetic silica glass body in a SiO ₂ powder whose Al and Na contents are controlled within a predetermined range ( In other words, when the synthetic silica glass body is covered with the above-mentioned predetermined SiO ₂ powder and subjected to heat treatment, the Na concentration in the surface portion of the synthetic silica glass body becomes 0.1 ppb or less. The synthetic silica glass body is buried in a SiO ₂ powder with the Al and Na contents outside the predetermined ranges in other words (in other words, the synthetic silica glass body is contacted by the SiO ₂ powder outside the predetermined range. It was found that the Na concentration did not become 2 ppb or less even after heat treatment. Examples of the SiO ₂ powder in which the Al and Na contents are controlled within a predetermined range include α-quartz crystalline natural quartz powder.

This phenomenon is natural silica powder (alpha-quartz) in the crystal, Al ions ^{(Al 3+)} is replaced with Si ions ^{(Si 4+),} it becomes AlO ₄ tetrahedra, AlO ₄ tetrahedra vertices (oxygen ions) This is considered to be an effect of so-called Al-isomorphic substitution, in which one more negative (-) bond is always activated negatively (-). That is, Na ⁺ ions which are monovalent cations are captured by negatively charged Al ions. This time, the present inventors have performed Na ⁺ ion movement between the natural quartz powder (α-quartz crystalline) and the synthetic silica glass (amorphous) foreign material, and the natural quartz powder (α-quartz crystalline) side. The present inventors have found a novel technical fact that Na ⁺ ions are trapped by Al ions.

  It is preferable that the OH group concentration of the synthetic silica glass body, which is an object to be heat-treated in the heat treatment method of the present invention, is set to 100 ppm or less. Examples of the method for producing the synthetic silica glass body to be treated include a soot method (VAD method, OVD method) and a plasma method. Synthetic silica glass produced by the direct method is known to have an OH group concentration of 500 ppm or more, ionic bonds with Na ions due to OH groups, and a low migration diffusion coefficient in silica glass. It is not suitable for use as a synthetic silica glass body that is an object to be treated in the heat treatment method.

In the heat treatment method for a synthetic silica glass body of the present invention, as a synthetic silica glass body to be processed, a porous base material produced from a SiCl ₄ raw material by a soot method is used in a helium gas atmosphere or under a vacuum of 1 Pa or less in 1300. An ingot obtained by carrying out a transparent vitrification treatment by heating at a temperature of not lower than ° C., or an optical member that is heat-molded into a predetermined shape can be used. In the heat treatment method of the present invention, it is desirable to immerse in 10% or more of hydrofluoric acid for 10 minutes or more in order to remove dirt adhering to the surface of the object to be treated before heat treatment, Other cleaning means may be used because it is cleaning for removing the deposits.

In the heat treatment method of the present invention, it is desirable that the Na concentration of the SiO ₂ powder covering the workpiece is less than 50 ppb, preferably 40 ppb or less, particularly 30 ppb or less. In order to form an AlO ₄ tetrahedron as an Al-isomorphous substitution and trap Na ⁺ ions, it is desirable that the Al concentration is 10 ppm or more and 30 ppm or less with a so-called natural quartz powder of α-quartz crystalline. In order to increase the specific surface area with the silica glass contact surface, the natural quartz powder should have a particle size of 1000 μm or less, preferably about 5 to 500 μm. The Na concentration is preferably adjusted by heat treatment in a hydrogen chloride gas atmosphere.

In the heat treatment method of the present invention, it is preferable that the work of covering the object to be treated with the SiO ₂ powder is performed using a lidded container having a structure described later. The material of the container with lid is more preferably made of fused silica glass or synthetic silica glass having a thermal conductivity close to that of the SiO ₂ powder to be used in order to maintain the heat uniformity of the object to be processed. Further, the SiO ₂ powder is not directly exposed to the metal impurities diffusing from the furnace body during the heat treatment, and the container and the lid have an effect of reducing the amount of the metal impurities mixed into the SiO ₂ powder.

  According to the heat treatment method for a synthetic silica glass body of the present invention, it is possible to effectively remove strain and purify the synthetic silica glass body. According to the method for producing an optical member made of synthetic silica glass of the present invention, an optical member made of synthetic silica glass can be produced effectively. The optical member made of synthetic silica glass of the present invention has an advantage of exhibiting excellent light transmittance for a long period of time with irradiation of ultraviolet rays, particularly ArF excimer laser light.

In the heat treatment method of the synthetic silica glass body of the present invention, it is a vertical cross-sectional explanatory view showing an example of a state in which a synthetic silica glass body as an object to be processed is covered in a contact state with SiO ₂ powder in a lidded container. It is sectional drawing which follows the II-II line of FIG. It is a vertical sectional view which shows the sampling position of the synthetic silica glass body which is the to-be-processed object in Examples 1-3 and Comparative Examples 1-2. It is sectional drawing which follows the IV-IV line of FIG. It is sectional drawing which follows the VV line of FIG. 3 is a graph showing a temperature profile of heat treatment in Example 1.

In the following, referring to the attached drawings, an embodiment in which the work of coating the object to be treated with SiO ₂ powder in contact with the synthetic quartz glass body according to the present invention is carried out using a lidded container. I will explain. However, this embodiment is merely an example or explanation, and does not limit the scope of the present invention.

In the heat treatment method of the present invention, the work of coating in contact with the object to be processed by SiO ₂ quality powder is preferably carried out using a container with a lid, SiO ₂ quality of the object to be processed by the container with lid The operation | work which coat | covers with a powder in a contact state is demonstrated based on FIGS. 1-2.

  1 to 2, reference numeral 10 denotes a container with a lid, and the container 10 with a lid has a cylindrical shape similar to a flat columnar synthetic silica glass body 50 that is an object to be treated. . The lidded container 10 includes a container body 15 including a circular wall 12 and a disk-like bottom plate 14, a disk-like lid 16, and an inner lid 18. The annular wall 12 and the bottom plate 14 may be integrated or separate.

It is the Na concentration and Al concentration of natural quartz powder, which is a coated SiO ₂ powder, but the Na concentration and Al concentration can be purified to the required Na concentration and Al concentration by performing thermal purification treatment in a hydrochloric acid atmosphere. is there. Although it is the thermal purification treatment conditions of natural quartz powder, it is preferable to carry out at a treatment temperature of 1100 to 1300 ° C., a treatment time of 1 to 20 hours, and an atmospheric hydrogen chloride gas concentration of 50 to 100%. Typical metal impurity elements purified by this thermal purification treatment include Na, K, Li, Mg, Ca, Fe, Cu, Al and the like.

The SiO ₂ powder 20 is filled in the lidded container 10 so as to cover the entire surface of the flat cylindrical synthetic silica glass body 50 in a contact state. The filling thickness of the SiO ₂ powder 20 filled between the side surface 50a of the cylindrical synthetic silica glass body 50 and the annular wall 12 is preferably 5 to 150 mm. Further, the SiO ₂ powder is filled between the upper surface 50 b of the flat cylindrical synthetic silica glass body 50 and the inner lid 18 and between the lower surface 50 c of the flat cylindrical synthetic silica glass body 50 and the bottom plate 14. The filling thickness of 20 is desirably 5 to 100 mm. The filling amount of the SiO ₂ powder 20 is adjusted according to the birefringence characteristics required for the processed material.

  In the heat treatment method of the present invention, the synthetic silica glass body 50, which is an object to be treated, is subjected to a heat treatment at a temperature range of 900 ° C. to 1300 ° C. for 30 minutes to 800 hours, thereby removing the strain of the synthetic silica glass body 50. Although the purpose of the purification treatment is achieved, it is possible to achieve the purpose more suitably by considering the following heat treatment conditions corresponding to each purpose.

  The purpose of the strain removal heat treatment of the synthetic silica glass body 50 as the object to be treated is to reduce the birefringence of the synthetic silica glass body 50 to 0.2 nm / cm or less, so that the cooling from 1200 ° C. to 1000 ° C. is performed. Is carried out at 3 ° C./hr, preferably 1 ° C./hr.

  Since the purpose of the purification heat treatment of the workpiece 50 is to reduce the Na concentration of the silica glass workpiece to 0.2 ppb or less, it is desirable to perform the treatment at 1100 to 1200 ° C. for 100 to 500 hours.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it is needless to say that these examples are shown by way of example and should not be interpreted in a limited manner.
The physical property value measuring methods and pretreatment methods described in Examples and Comparative Examples are as follows.

1) Sampling of workpieces after heat treatment:
As shown in FIGS. 3 to 5, the size of the cylindrical body or flat cylindrical synthetic silica glass 50 from three locations, that is, the surface center portion 0 to 5 mm of the side surface 50a, the surface center portion 0 to 5 mm of the upper surface 50b, and the center portion 50d. Cut out a sample of 10 mm × 10 mm × 5 mm and sampled to obtain a side center sample 52, an upper surface center sample 54, and a center sample 56, respectively.

2) Na, Al metal impurity concentration measurement of the object to be treated and SiO ₂ powder:
By ICP-mass spectrometry. The lower limit of detection of Na element is 0.1 ppb.

3) Internal transmittance measurement at 193.4 nm:
The transmittance of 193.4 nm was measured with a Cary 4000 ultraviolet / visible spectrophotometer manufactured by Varian. The apparent transmittance (T / T) at a thickness of 10 mm is obtained by using 90.68% obtained by subtracting 0.18% known as a loss in Rayleigh scattering from 90.86% which is the theoretical transmittance of silica glass at 193.4 nm. 90.68) × 100. The sample cut out from the workpiece before the transmittance measurement was subjected to a hydrogen atmosphere heat treatment at 400 ° C. for 4 weeks, and doped so that the hydrogen molecule concentration became 1 to 2 × 10 ¹⁷ mole / cm ³ . Structural type defects were filled with hydrogen.

4) Measurement of birefringence:
Birefringence at a wavelength of 632.8 nm was measured with an Exicor 350AT manufactured by HINDS Instruments. The measurement light was incident in the direction perpendicular to the surface of the cylindrical body.

(Experimental example 1)
Natural quartz powder (trade name: IOTA-STD, Unimin) is subjected to thermal purification at a treatment temperature of 1250 ° C, a thermal purification treatment time of 5 hours, and a hydrogen chloride atmosphere concentration of 100%, and chemical analysis of metal impurity concentrations before and after treatment. Went. The results are shown in Table 1.

Example 1
As a synthetic silica glass body treatment object, a cylindrical body made of synthetic silica glass having a diameter of 100 mm and a thickness of 100 mm (OH group concentration before heat treatment: 15 ppm) manufactured by the VAD method was prepared. It was 0.1 ppb or less when Na density | concentration of this synthetic silica glass body processed material was measured. As the coating powder, the natural quartz powder (α-quartz crystalline natural quartz powder having a particle size of 53 to 71 μm, an Al concentration of 16 ppm, and an Na concentration of 5 ppb) obtained in Experimental Example 1 was used. Table 2 shows the conditions of the workpiece and the coated powder.

The synthetic silica glass body treatment object is covered with a lid-like container shown in FIG. 1 filled with coating powder (a container body having an outer diameter of 350 mm × height 200 mm × thickness 5 mm, outer It was embedded in the above-mentioned coated powder so as to be located at the approximate center in the container body of a diameter 350 mm × thickness 5 mm outer lid, outer diameter 330 mm × thickness 3 mm inner lid).
The object to be treated was brought to 1150 ° C. over 12 hours and 50 minutes, then held at 1150 ° C. for 45 hours, and then gradually cooled to 1050 ° C. at 1 ° C./hr. FIG. 6 shows the heat treatment conditions of Example 1.

  The Na concentration of the side surface center sample 52, the upper surface center sample 54, and the center portion sample 56 obtained from the synthetic silica glass body treated material after the heat treatment was measured. Moreover, the transmittance | permeability in 193.4 nm of the side surface center sample 52 obtained from the to-be-processed object and the upper surface center sample 54 was measured, and the internal transmittance was calculated | required from the transmittance | permeability T% measured value. Furthermore, the birefringence and OH group concentration of the object to be processed after the heat treatment were measured. The results are shown in Table 3. In addition, the result of the transmittance | permeability showed the average value of the side surface center sample 52 and the upper surface center sample 54. FIG.

(Example 2)
A synthetic silica glass flat cylindrical body having a diameter of 320 mm and a thickness of 55 mm, which is heated and deformed to the size of a lens material for an exposure apparatus, which is a typical optical member to which the present invention is applied, as a synthetic silica glass object (OH group concentration before heat treatment 15 ppm) was prepared. It was 0.1 ppb or less when Na density | concentration of this synthetic silica glass body processed material was measured. As the coating powder, the natural quartz powder (α-quartz crystalline natural quartz powder having a particle size of 53 to 71 μm, an Al concentration of 16 ppm, and an Na concentration of 5 ppb) obtained in Experimental Example 1 was used. Table 2 shows the conditions of the workpiece and the coated powder.

  The synthetic silica glass body to be treated is covered with a lid having the same shape as the lidded container shown in FIG. 1 filled with coating powder (a container body having an outer diameter of 360 mm × height 200 mm × thickness 10 mm, an outer The outer cover of diameter 360 mm × thickness 10 mm, inner lid of outer diameter 330 mm × thickness 3 mm) was embedded in the above-mentioned coated powder so as to be located at the approximate center in the container body. The subsequent heat treatment conditions and measurement of the object to be processed were performed in the same manner as in Example 1. The results are shown in Table 3.

Example 3
Natural quartz powder (trade name: IOTA-STD, manufactured by Unimin) is purified by heat treatment at 1200 ° C. for 2 hours in a 100% hydrochloric acid atmosphere to obtain α-quartz having a particle size of 53 to 71 μm, an Al concentration of 15 ppm, and an Na concentration of 20 ppb. Crystalline natural quartz powder was obtained.
An experiment was conducted in the same manner as in Example 1 except that the obtained natural quartz powder was used instead of the natural quartz powder obtained in Experimental Example 1 as the coating powder. Table 2 shows the conditions of the workpiece and the coated powder. Table 3 shows the measurement results of the workpieces after the heat treatment.

(Comparative Example 1)
A synthetic silica glass cylinder (OH group concentration: 15 ppm) having a diameter of 120 mm and a thickness of 72 mm manufactured by the VAD method was prepared as a synthetic silica glass body treatment object. It was 0.1 ppb or less when Na density | concentration of this synthetic silica glass body processed material was measured. As a coating powder, an unpurified α-quartz crystalline natural quartz powder (trade name: IOTA-STD, Unimin) having a particle size of 53 to 71 μm, an Al concentration of 15 ppm, and an Na concentration of 150 ppb is used. It was. Table 2 shows the conditions of the workpiece and the coated powder.

  The synthetic silica glass body treatment object is covered with a lid-like container shown in FIG. 1 filled with coating powder (a container body having an outer diameter of 350 mm × height 200 mm × thickness 5 mm, outer The outer lid (diameter 350 mm × thickness 5 mm, outer diameter 330 mm × thickness 3 mm inner lid) was embedded in the covering so as to be located at the approximate center in the container body. The subsequent heat treatment conditions and measurement of the object to be processed were performed in the same manner as in Example 1. The results are shown in Table 3.

  As is apparent from Table 3, in the synthetic silica glass body that is the object to be treated in Example 1, the Na concentration in all three samples was 0.1 ppb or less. The internal transmittance of the upper surface of 0 to 5 mm and the side surface center surface of 0 to 5 mm was 99.82%. The film showed sufficient UV transparency as an optical member for a lithography apparatus using an ArF excimer laser as a light source. Further, the birefringence maximum value of the cylindrical body was 0.2 nm / cm, and the value was very good.

  In the synthetic silica glass body of Example 2, the size of the object to be processed is 320 mm in diameter × 55 mm in thickness, which is the size of a member for a lithography apparatus, but the Na concentration, internal transmittance, and maximum birefringence are all examples. It was equivalent to 1. That is, it was shown that the present invention can be applied to the size of an actual product.

In the synthetic silica glass body of Example 3, the Na concentration of the natural quartz powder covering the workpiece was 20 ppb, but the Na concentration of 0 to 5 mm on the surface of the workpiece was 0.1 ppb or less. Therefore, in the present invention, as described above, the action of trapping Na ⁺ ions contained in the object to be processed by the AlO ₄ tetrahedron (oxygen ions) in natural quartz powder is performed by Al-isomorphous substitution. It was confirmed.

In the synthetic silica glass body of Comparative Example 1, when the unpurified natural quartz powder in which the Na concentration of the natural quartz powder covering the object to be treated is 150 ppb was used, the Na concentration of the surface of the object to be treated of 0 to 5 mm was 2.0 ppb. And high concentration. The internal transmittance of the upper surface of 0 to 5 mm and the side center surface of 0 to 5 mm was a low value of 99.78%. The Al concentration in the unpurified natural quartz powder is 15 ppm, which is the same concentration as in Example 1. However, the AlO ₄ tetrahedron (oxygen ions) in the natural quartz powder captures Na ⁺ ions before heat treatment. It was confirmed that no occurred. The effects of the present invention are apparent from the above comparative examples.

10: Container with lid, 12: annular wall, 14: disk-shaped bottom plate, 15: container body, 16: disk-shaped lid, 18: inner lid, 20: SiO ₂ powder, 50: cylindrical synthetic silica glass body, 50a : Side surface of cylindrical synthetic silica glass, 50b: upper surface of cylindrical synthetic silica glass, 50c: lower surface of cylindrical synthetic silica glass, 50d: center portion of cylindrical synthetic silica glass, 52: side center sample, 54: upper surface center sample 56: Center sample.

Claims (7)

A heat treatment method for removing strain and purifying a synthetic silica glass body that is an object to be treated, wherein all of the outer surface of the object to be treated is made of SiO ₂ powder having an Al concentration of 10 ppm to 30 ppm and an Na concentration of less than 50 ppb. coated with contact, heat treatment method of the synthetic silica glass body and performing heat treatment for 30 minutes to 800 hours at a temperature range of 900 ° C. to 1300 ° C.. _2. The heat treatment method for a synthetic silica glass body according to claim 1, wherein the SiO ₂ powder is natural quartz powder.   3. The method for heat treatment of a synthetic silica glass body according to claim 2, wherein the natural quartz powder is an α-quartz crystalline natural quartz powder.   The manufacturing method of the optical member made from a synthetic silica glass characterized by including the heat processing method of the synthetic silica glass body of any one of Claims 1-3. All of the outer surface of the synthetic silica glass body to be treated is covered with SiO ₂ powder having an Al concentration of 10 ppm to 30 ppm and an Na concentration of less than 50 ppb in a contact state, and a temperature range of 900 ° C. to 1300 ° C. for 30 minutes. A synthetic silica glass optical member manufactured by a method of manufacturing a synthetic silica glass optical member including a heat treatment method of a synthetic silica glass body that performs strain removal and purification of the synthetic silica glass body by performing heat treatment for ˜800 hours. There,
When the Na concentration is less than 0.5 ppb in all regions, the difference between the maximum value and the minimum value is 0.1 ppb or less, the OH group is 100 ppm or less, the birefringence is 0.5 nm / cm or less, and the internal transmittance is 99.82% or more. Oh it is,
Synthetic silica glass optical member, wherein the synthetic silica glass body is characterized Rukoto produced by soot method or a plasma method. 6. The synthetic silica glass optical member according to claim 5, wherein the SiO ₂ powder is natural quartz powder.   The synthetic silica glass optical member according to claim 6, wherein the natural quartz powder is α-quartz crystalline natural quartz powder.

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