JP5365248B2 - Silica glass containing TiO2 and optical member for EUV lithography - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide TiO<SB>2</SB>-SiO<SB>2</SB>glass whose coefficient of linear thermal expansion in the range of the time of irradiation with EUV energy rays is substantially zero when used as an optical member of an exposure tool for EUVL and which has extremely high surface smoothness. <P>SOLUTION: The TiO<SB>2</SB>-containing silica glass has a TiO<SB>2</SB>content of from 7.5 to 12 mass%, a temperature at which a coefficient of linear thermal expansion is 0 ppb/&deg;C falling within the range of from 40 to 110&deg;C, and a standard deviation (&sigma;) of a stress level of striae of 0.03 MPa or lower. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

The present invention relates to silica glass containing TiO ₂ (hereinafter referred to as TiO ₂ —SiO ₂ glass), and more particularly, TiO ₂ —SiO ₂ glass used as an optical system member of an exposure apparatus for EUV lithography. About. The EUV (Extreme Ultra Violet) light referred to in the present invention refers to light in the wavelength band of the soft X-ray region or the vacuum ultraviolet region, specifically, light having a wavelength of about 0.2 to 100 nm. .

  Conventionally, in an optical lithography technique, an exposure apparatus for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has been widely used. As integrated circuits become highly integrated and highly functional, miniaturization of integrated circuits advances, and the exposure apparatus is required to form a high-resolution circuit pattern on the wafer surface with a deep focal depth. Short wavelength is being promoted. As an exposure light source, an ArF excimer laser (wavelength 193 nm) has begun to be used, proceeding from the conventional g-line (wavelength 436 nm), i-line (wavelength 365 nm) and KrF excimer laser (wavelength 248 nm). Further, in order to cope with next-generation integrated circuits in which the line width of the circuit pattern is 70 nm or less, immersion exposure technology and double exposure technology using ArF excimer laser are considered promising. Is expected to cover only the 45 nm generation.

  In such a flow, it is considered that lithography technology using light having a wavelength of 13 nm, typically EUV light (extreme ultraviolet light) as an exposure light source, can be applied over generations having a circuit pattern line width of 32 nm or later. It is attracting attention. The image forming principle of EUV lithography (hereinafter abbreviated as “EUVL”) is the same as that of conventional photolithography in that a mask pattern is transferred using a projection optical system. However, since there is no material that transmits light in the EUV light energy region, the refractive optical system cannot be used, and all the optical systems are reflective optical systems.

  The optical system member of the exposure apparatus for EUVL is a photomask or a mirror, but (1) a base material, (2) a reflective multilayer film formed on the base material, and (3) an absorption formed on the reflective multilayer film. It is basically composed of body layers. As the reflective multilayer film, it is considered to form a Mo / Si reflective multilayer film in which Mo layers and Si layers are alternately stacked. For the absorber layer, Ta and Cr are considered as film forming materials. Has been. As the base material, a material having a low linear thermal expansion coefficient is required so that distortion does not occur even under EUV light irradiation, and glass having a low linear thermal expansion coefficient has been studied.

TiO ₂ —SiO ₂ glass is known as an ultra-low thermal expansion material having a smaller coefficient of thermal expansion (CTE) than quartz glass, and the linear thermal expansion coefficient is controlled by the TiO ₂ content in the glass. Therefore, a zero expansion glass having a linear thermal expansion coefficient close to 0 can be obtained. Therefore, TiO ₂ —SiO ₂ glass has a possibility as a material used for an optical member of an exposure apparatus for EUVL.

In a conventional method for producing TiO ₂ —SiO ₂ glass, first, a silica precursor and a titania precursor are converted into vapor forms and mixed. The mixture in vapor form is introduced into a burner and thermally decomposed to become TiO ₂ —SiO ₂ glass particles. The TiO ₂ —SiO ₂ glass particles are deposited in a refractory container and melted there at the same time as the deposition to become TiO ₂ —SiO ₂ glass. Patent Document 1 discloses a method of obtaining a mask substrate after forming a TiO ₂ —SiO ₂ porous glass body into a glass body.

However, in the TiO ₂ —SiO ₂ glass produced by these methods, periodic fluctuations in the TiO ₂ / SiO ₂ composition ratio occurred, which appeared as stripe striae at a pitch of 10 to 200 μm. When TiO ₂ —SiO ₂ glass is used as an optical member for EUV lithography, it is necessary to polish the glass so that the surface thereof has ultrahigh smoothness. However, in the TiO ₂ —SiO ₂ glass, the portions with different TiO ₂ / SiO ₂ composition ratios differ in mechanical and chemical properties of the glass depending on the composition ratio, so the polishing rate is not constant, and the polished glass It is difficult to finish so that the surface has ultra-high smoothness. Polishing striped striae TiO ₂ —SiO ₂ glass with a pitch of 10 to 200 μm generates “swells” on the glass surface with a pitch similar to the striae pitch, thereby obtaining ultra-high smoothness. Is very difficult.

In order to obtain ultra-high smoothness, TiO ₂ —SiO ₂ glass having a small variation in the composition ratio of TiO ₂ / SiO ₂ is preferable, but the inventors of the present application disclosed a porous TiO ₂ —SiO ₂ glass body in Patent Document 2. the rotation speed of the seed rod in the obtaining step, a result of detailed studies about the relation between the striae of transparent TiO ₂ -SiO ₂ glass body, the greater the rotational speed of the seed rod, the transparent TiO ₂ -SiO ₂ glass It has been found that the variation in TiO ₂ concentration in the body is reduced and the striae are reduced. Also disclosed is a TiO ₂ —SiO ₂ glass characterized in that the fluctuation range (Δn) of the refractive index is 2 × 10 ⁻⁴ or less in a range of 30 mm × 30 mm in at least one plane.

  Patent Document 3 discloses titania-containing silica glass and extreme ultraviolet light optical elements having a low level of striations, methods for producing them, and the like.

  The inventors of the present application disclosed in Patent Document 4 that there is a relationship between the virtual temperature and the range of the zero expansion temperature range, that is, the relationship between the virtual temperature and ΔT. It is disclosed that ΔT becomes narrower and ΔT becomes wider when the fictive temperature becomes lower.

US Patent Application Publication No. 2002/157421 JP 2004-315351 A JP 2005-519349 JP 2005-104820 A

  In order to increase the throughput of the exposure apparatus for EUVL, it is effective to increase the energy of the EUV light used for exposure. In this case, the temperature of the member may rise more than expected. Specifically, since there is a possibility that the temperature rises to 40 to 110 ° C., it is preferable that substantially zero expansion occurs at these temperatures. This is to prevent the pattern pitch from changing in the case of a photomask or the like, and to prevent the shape from changing in the case of a stepper mirror or the like.

It is known that the linear thermal expansion coefficient of TiO ₂ —SiO ₂ glass changes depending on the TiO ₂ concentration contained. (See, for example, PC Schultz and HT Smith, in: RW Douglas and B. Ellis, Amorphous Materials, Willy, New York, p. 453 (1972)).

Therefore, by adjusting the TiO ₂ content of the TiO ₂ —SiO ₂ glass, the temperature at which the zero expansion occurs can be adjusted. Specifically, in the conventional TiO ₂ —SiO ₂ glass having zero expansion at 22 ° C., the TiO ₂ concentration is about 7% by mass, but the TiO ₂ —SiO used when the throughput of the exposure apparatus for EUVL is increased. _{In the case of 2} glass, in order to achieve zero expansion at a temperature of 40 ° C. or higher, a TiO ₂ concentration of about 7.5% by mass or more is necessary, and the TiO ₂ concentration needs to be increased.

To increase the TiO ₂ concentration in the TiO ₂ -SiO ₂ glass, in the prior art described _above, it is necessary to increase the relative amount of the titania precursor as a raw material of the TiO 2 -SiO ₂ glass. The titania precursor generally has a higher boiling point than the silica precursor, and is likely to form dew during the transportation to the burner after being made into a vapor form. For this reason, in the above-described prior art, if the relative amount of the titania precursor is large, dew condensation occurs during conveyance, and there arises a problem that the TiO ₂ / SiO ₂ composition ratio is changed in the finally obtained glass. . Moreover, even without condensation occurs, that TiO ₂ concentration is increased, the fluctuation range of the TiO ₂ / SiO ₂ compositional ratio is increased, different sites TiO ₂ / SiO ₂ compositional ratio, mechanical glass by the composition ratio In addition, the polishing rate is not constant because of different chemical properties. For this reason, the problem that the glass which has ultrahigh smoothness cannot be obtained arises.

In order to solve the above-described problems of the prior art, the present invention has a thermal expansion characteristic suitable as an optical system member for an exposure apparatus using high EUV energy light for the purpose of increasing the throughput, and has an ultra-high smoothness. and to provide a TiO ₂ -SiO ₂ glass can be imparted sex. More specifically, when used as an optical system member of an EUVL exposure apparatus, a TiO ₂ —SiO ₂ glass having an extremely high linearity and a linear thermal expansion coefficient when irradiated with high EUV energy light is substantially zero. The purpose is to provide.

In the present invention, the fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C., 0.03MPa der below, in the range of 30 mm × 30 mm in the standard deviation of the physical stress level (sigma) at least one plane Ri, 30 ppb / ° C. the average linear thermal expansion coefficient of 20 to 100 ° C. is -30ppb / ℃ or higher The following silica glass containing TiO ₂ is provided.

In the present invention, the fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, and the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C. , the maximum roughness of the stress level of striae (PV) is Ri 30 mm × 30 mm der 0.2MPa or less in the range of at least one plane, 20-100 average linear thermal expansion coefficient of ° C. is -30ppb / ℃ or higher A silica glass that is 30 ppb / ° C. or lower is provided.

In the present invention, the fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, and the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C. , fluctuation width in refractive index ([Delta] n) is Ri der 4 × 10 ^-4 or less in the range of 30 mm × 30 mm in at least one plane, 20-100 average linear thermal expansion coefficient of ° C. is -30ppb / ℃ than 30 ppb / Provided is a silica glass containing TiO ₂ having a temperature of 0 ° C. or lower .

The TiO ₂ —SiO ₂ glass of the present invention preferably has a temperature width (ΔT) exceeding 6.6 ° C. at which the linear thermal expansion coefficient (CTE) becomes 0 ± 5 ppb / ° C.

The TiO ₂ —SiO ₂ glass of the present invention preferably has a temperature width (ΔT) at which the linear thermal expansion coefficient (CTE) is 0 ± 5 ppb / ° C. is 7.8 ° C. or more .

The TiO ₂ —SiO ₂ glass of the present invention preferably has no inclusion.

Furthermore, the TiO ₂ —SiO ₂ glass of the present invention can be used for an optical member for EUV lithography, and the optical member for EUV lithography using the TiO ₂ —SiO ₂ glass of the present invention has a surface smoothness (rms). It is preferable that it is 3 nm or less.
The present invention is a method for producing the above TiO ₂ —SiO ₂ glass, characterized in that the titania precursor is vaporized by bubbling, and the temperature of the pipe carrying the titania precursor is increased as the temperature proceeds to the burner. A method for producing silica glass is provided.
The present invention is a method for producing the above TiO ₂ —SiO ₂ glass, characterized in that the temperature fluctuation range of a pipe carrying a titania precursor is controlled to be within ± 1 ° C. by PID control. Provide a method.
The present invention is a method for producing the above TiO ₂ —SiO ₂ glass, characterized in that the gas flow rate in the titania precursor pipe is 0.1 m / sec or more in terms of atmospheric pressure. A method for producing glass is provided.
The present invention provides a method for producing the above-described TiO ₂ —SiO ₂ glass, characterized by providing a gas stirring mechanism before supplying the silica precursor and the titania precursor to the burner. provide.
The present invention, after a manufacturing method of the TiO ₂ -SiO ₂ glass, silica glass shaped body formed in a predetermined shape and held for more than 2 hours at a temperature of 600 to 1200 ° C., 5 ° C. / hr or less The method for producing silica glass is characterized in that the temperature is lowered to 700 ° C. or lower at an average temperature drop rate of 5 ° C.

The TiO ₂ —SiO ₂ glass of the present invention can be used for EUVL because it has a very small change in dimensions and shape from room temperature with respect to a temperature rise upon irradiation with high EUV energy light, and an ultra-high smooth surface can be obtained. It is extremely suitable as an optical system member for an exposure apparatus.

FIG. 1 is a graph plotting the relationship between CTE and temperature.

In the present invention, the TiO ₂ content is 7.5 to 12% by mass. Within this range, the temperature at which the linear thermal expansion coefficient (CTE) becomes 0 ppb / ° C. (cross-over temperature; COT) tends to be in the range of 40 to 110 ° C. Specifically, when the TiO ₂ content is less than 7.5% by mass, the COT tends to be less than 40 ° C. Further, if the TiO ₂ content exceeds 12% by mass, the COT tends to exceed 110 ° C., or tends to negatively expand in the range of −150 to 200 ° C. In addition, crystals such as rutile are likely to precipitate and bubbles may remain. The TiO ₂ content is preferably 11% by mass or less, more preferably 10% by mass or less. The TiO ₂ content is preferably 8% by mass or more, more preferably 8.5% by mass or more.

  In the present invention, COT is in the range of 40 to 110 ° C. for the purpose of preventing changes in dimensions and shapes due to temperature changes of optical system members such as mirrors when EUVL is performed. More preferably, it is 45-100 degreeC, Most preferably, it is 50-80 degreeC.

  In the present invention, the standard deviation (σ) of the striae stress level is preferably 0.03 MPa or less in a range of 30 mm × 30 mm in at least one plane. If it exceeds 0.03 MPa, the surface roughness after polishing becomes large, and ultrahigh smoothness may not be obtained. More preferably, it is 0.02 MPa or less, and particularly preferably 0.01 MPa or less.

In the present invention, the maximum roughness (PV) of the striae stress level is preferably 0.2 MPa or less in a range of 30 mm × 30 mm in at least one plane. If it exceeds 0.2 MPa, the portions having different TiO ₂ / SiO ₂ composition ratios have different polishing rates because the mechanical and chemical properties of the glass differ depending on the composition ratio. For this reason, the surface roughness after polishing becomes large, and there is a possibility that ultra-high smoothness cannot be obtained. More preferably, it is 0.17 MPa or less, More preferably, it is 0.15 MPa or less, Most preferably, it is 0.1 MPa or less.

  In the present invention, the root mean square (RMS) of the striae stress level is preferably 0.2 MPa or less in a range of 30 mm × 30 mm in at least one plane. When the pressure is 0.2 MPa or less, the roughness of the surface after polishing becomes small, and ultra-high smoothness is easily obtained. More preferably, it is 0.17 MPa or less, More preferably, it is 0.15 MPa or less, Most preferably, it is 0.1 MPa or less.

The striae stress of TiO ₂ —SiO ₂ glass can be obtained from the following equation by obtaining a retardation by measuring a region of about 1 mm × 1 mm using a known method, for example, a birefringence microscope.

Δ = C × F × n × d
Here, Δ is retardation, C is a photoelastic constant, F is stress, n is a refractive index, and d is a sample thickness.

The stress profile is obtained by the above method, and the standard deviation (σ), the maximum roughness (PV), and the root mean square (RMS) can be obtained therefrom. More specifically, a cube of about 40 mm × 40 mm × 40 mm, for example, is cut out from the transparent TiO ₂ —SiO ₂ glass body, sliced and polished at a thickness of about 1 mm from each surface of the cube, and 30 mm × 30 mm × 0.00 mm. A 5 mm plate-like TiO ₂ —SiO ₂ glass block is obtained. With a birefringence microscope, helium neon laser light is vertically applied to the 30 mm x 30 mm surface of the glass block, and the magnification is increased to a level at which the striae can be observed sufficiently. The in-plane retardation distribution is examined and converted into a stress distribution. To do. When the striae pitch is fine, it is necessary to reduce the thickness of the plate-like TiO ₂ —SiO ₂ glass block to be measured.

In the present invention, the refractive index fluctuation range (Δn) in the range of 30 mm × 30 mm in at least one plane is preferably 4 × 10 ⁻⁴ or less. If it exceeds 4 × 10 ⁻⁴ , the surface roughness after polishing becomes large, and ultrahigh smoothness may not be obtained. More preferably, it is 3.5 × 10 ⁻⁴ or less, and further preferably 3 × 10 ⁻⁴ or less.

In particular, in order to impart ultra high smoothness such as surface smoothness (rms) ≦ 1 nm, the refractive index fluctuation range (Δn) is preferably 2 × 10 ⁻⁴ or less, more preferably 1.5 × 10 ⁻⁴ or less. More preferably, it is 1 × 10 ⁻⁴ or less, particularly preferably 0.5 × 10 ⁻⁴ or less.

A method for measuring the refractive index fluctuation range Δn can be measured by using a known method, for example, an optical interferometer. More specifically, a cube of about 40 mm × 40 mm × 40 mm, for example, is cut out from the transparent TiO ₂ —SiO ₂ glass body, sliced and polished at a thickness of about 0.5 mm from each surface of the cube, and 30 mm × 30 mm × A 0.2 mm plate-like TiO ₂ —SiO ₂ glass block is obtained. Using a small-diameter Fizeau interferometer, only light of a specific wavelength is extracted from white light using a filter on the 30 mm x 30 mm surface of this glass block, and is vertically expanded to expand the magnification so that the striae can be sufficiently observed. Then, the in-plane refractive index distribution is examined, and the refractive index fluctuation range Δn is measured. When the striae pitch is fine, it is necessary to reduce the thickness of the plate-like TiO ₂ —SiO ₂ glass block to be measured.

  When striae is evaluated using the birefringence microscope or the optical interferometer, the size of one pixel in the CCD may not be sufficiently small compared to the width of the striae, and the striae may not be detected. is there. In this case, it is preferable to divide the entire range of 30 mm × 30 mm into a plurality of minute areas of about 1 mm × 1 mm, for example, and perform measurement in each minute area.

In the TiO ₂ —SiO ₂ glass of the present invention, the difference between the maximum value and the minimum value of TiO ₂ concentration in a range of 30 mm × 30 mm on one surface is preferably 0.06% by mass or less, and 0.04% by mass. The following is more preferable. When the content is 0.06% by mass or less, the surface roughness after polishing becomes small, and ultra-high smoothness is easily obtained.

There are several methods for producing the TiO ₂ —SiO ₂ glass of the present invention as follows. First, TiO ₂ —SiO ₂ glass particles (soot) obtained by flame hydrolysis or thermal decomposition of a silica precursor and a titania precursor, which are glass forming raw materials, are deposited and grown by a soot method, and porous TiO _{2 is} grown. get a ₂ -SiO ₂ glass body. Next, the obtained porous TiO ₂ —SiO ₂ glass body is heated to a densification temperature or higher under reduced pressure or a helium atmosphere, and further heated to a transparent vitrification temperature or higher to obtain a TiO ₂ —SiO ₂ glass. There is a manufacturing method. Depending on how to make the soot method, there are an MCVD method, an OVD method, a VAD method, and the like.

Further, there is a production method for obtaining a TiO ₂ —SiO ₂ glass body by hydrolyzing and oxidizing a silica precursor and a titania precursor as glass forming raw materials in an oxyhydrogen flame at 1800 to 2000 ° C. by a direct method. .

  In the present specification, the densification temperature refers to a temperature at which the porous glass body can be densified until voids cannot be confirmed with an optical microscope. The transparent vitrification temperature refers to a temperature at which crystals cannot be confirmed with an optical microscope and transparent glass is obtained.

At this time, in order to obtain TiO ₂ —SiO ₂ glass, it is necessary to increase the relative amount of titania precursor as a raw material in order to increase the TiO ₂ concentration. The titania precursor generally has a higher boiling point than the silica precursor, and not only easily forms dew during the transfer to the burner after being converted to a vapor form, but also increases the TiO ₂ concentration, thereby increasing the TiO ₂ / SiO ₂ composition. The fluctuation range of the ratio tends to be large.

In order to obtain the TiO ₂ —SiO ₂ glass of the present invention with small striae, it is necessary to firmly control the temperature of the piping for conveying the raw material, particularly the piping for conveying the titania precursor. When the titania precursor is vaporized at a high concentration by bubbling, the inventors of the present application are effective in reducing striae by setting the temperature of the piping to be higher than the bubbling temperature and increasing the temperature as it proceeds to the burner. I found out that If there is a low temperature portion, the gas volume temporarily decreases in the low temperature portion, causing unevenness in the concentration of the titania precursor introduced to the burner.

The inventors of the present application have also found that piping temperature fluctuations cause striae. For example, in a pipe that transports TiCl ₄ at 0.5 m / sec, when the temperature of the gas in the 2 m length of the pipe fluctuates at 130 ° C. ± 1.5 ° C. in a cycle of 30 seconds, 0.1% by mass Composition fluctuation occurs. Therefore, in order to obtain the TiO ₂ —SiO ₂ glass of the present invention, it is preferable that the temperature fluctuation range of the pipe carrying the titania precursor is within ± 1 ° C. by PID control. More preferably, the temperature fluctuation range is within ± 0.5 ° C. Further, it is preferable that the temperature fluctuation range is not more than ± 1 ° C. by the PID control, not only for the pipe for carrying the titania precursor but also for the pipe for carrying the silica precursor, and the temperature fluctuation range is ± 0.5. More preferably, the temperature is within the range of ° C. In order to warm the pipe, it is preferable to wrap a flexible heater such as a ribbon heater or a rubber heater around the pipe in order to warm the pipe uniformly. To make the pipe more uniform, the pipe and heater are made of aluminum foil. It is preferable to cover. The outermost layer is preferably covered with a heat insulating material such as urethane or heat-resistant fiber cloth. In addition, in order to reduce composition fluctuations, it is better to increase the gas flow rate in the piping. The volume in terms of atmospheric pressure at that temperature is preferably 0.1 m / sec or more, more preferably 0.3 m / sec or more, still more preferably 0.5 m / sec or more, and particularly preferably 1 m / sec or more.

In order to supply gas uniformly, it is preferable to provide a gas stirring mechanism before supplying the silica precursor and titania precursor to the burner. There are two types of agitation mechanisms: a mechanism that subdivides and merges gases with components such as a static mixer and a filter, and a mechanism that supplies fine fluctuations by introducing gas into a large space. In order to obtain the TiO ₂ —SiO ₂ glass of the present invention, it is preferable to produce the glass using at least one of the stirring mechanisms, and it is more preferable to use both. Of the stirring mechanism, it is preferable to use both a static mixer and a filter.

In the TiO ₂ —SiO ₂ glass of the present invention, the average linear thermal expansion coefficient at 20 to 100 ° C. is preferably 60 ppb / ° C. or less. Thereby, when high energy EUV light is irradiated, even if the temperature of an optical member changes from room temperature to high temperature, the change of a dimension or a shape can be made small. More preferably, it is 50 ppb / ° C. or less, further preferably 40 ppb / ° C. or less, and particularly preferably 30 ppb / ° C. or less. On the other hand, when COT is high temperature, the average linear thermal expansion coefficient at 20 to 100 ° C. tends to be negative, but for the same reason, the absolute value of the average linear thermal expansion coefficient at 20 to 100 ° C. is smaller. The average linear thermal expansion coefficient at 20 to 100 ° C. is preferably −120 ppb / ° C. or higher. More preferably, it is -100 ppb / degrees C or more, More preferably, it is -60 ppb / degrees C or more. When it is desired to reduce the change in size and shape when irradiated with high-energy EUV light, the average linear thermal expansion coefficient at 20 to 100 ° C. is preferably −50 ppb / ° C. or more, more preferably −40 ppb. / ° C. or higher, particularly preferably −30 ppb / ° C. or higher.

In the TiO ₂ —SiO ₂ glass of the present invention, the temperature width (ΔT) at which the linear thermal expansion coefficient (CTE) is 0 ± 5 ppb / ° C. is preferably 5 ° C. or more. When ΔT is 5 ° C. or more, when TiO ₂ —SiO ₂ glass is used as an optical system member of an EUVL exposure apparatus, thermal expansion of the optical system member during EUV light irradiation is suppressed. More preferably, it is 6 degreeC or more, More preferably, it is 8 degreeC or more. It is particularly preferable that ΔT is 15 ° C. or higher because CTE can achieve 0 ± 5 ppb / ° C. in a temperature range of 50 to 80 ° C.

The TiO ₂ —SiO ₂ glass of the present invention preferably has a TiO ₂ content of 7.5 to 12% by mass and a fictive temperature of 1100 ° C. or lower. When the fictive temperature is 1100 ° C. or less, the average linear thermal expansion coefficient of 20 to 100 ° C. tends to be 60 ppb / ° C. or less, and TiO ₂ —SiO ₂ glass is used as an optical system member of an EUVL exposure apparatus. In addition, thermal expansion due to temperature changes of the optical system member during EUV light irradiation is suppressed.

  The fictive temperature is more preferably 1000 ° C. or lower, and further preferably 950 ° C. or lower. In order to further reduce the average linear thermal expansion coefficient of 20 to 100 ° C., the fictive temperature is preferably 900 ° C. or less, and more preferably 850 ° C. or less. A temperature of 800 ° C. or lower is particularly preferable.

The COT of TiO ₂ —SiO ₂ glass, the average coefficient of linear thermal expansion of 20 to 100 ° C., and ΔT are the known methods such as the laser interferometric thermal dilatometer of the coefficient of linear thermal expansion (CTE) of TiO ₂ —SiO ₂ glass. , And can be obtained by plotting the relationship between CTE and temperature as shown in FIG.

In order to obtain the TiO ₂ —SiO ₂ glass of the present invention having a fictive temperature of 1100 ° C. or lower, the TiO ₂ —SiO ₂ glass molded body formed into a predetermined shape was held at a temperature of 600 to 1200 ° C. for 2 hours or more. Thereafter, a method of lowering the temperature to 500 ° C. or less at an average temperature decrease rate of 10 ° C./hr or less is effective. In order to further lower the fictive temperature, it is preferable to decrease the temperature at a rate of 5 ° C./hr or less, and more preferable to decrease the temperature at a rate of 3 ° C./hr or less. If the temperature is lowered at a slower average temperature reduction rate, a lower fictive temperature is achieved. For example, if the temperature is lowered at a rate of 1 ° C./hr or less, the fictive temperature can be 900 ° C. or less. The temperature can be reduced by cooling at a cooling rate of 5 ° C./hr or more in other temperature ranges.

The fictive temperature of TiO ₂ —SiO ₂ glass can be measured by a known procedure. In Examples described later, the fictive temperature of TiO ₂ —SiO ₂ glass was measured by the following procedure.

With respect to the mirror-polished TiO ₂ —SiO ₂ glass, an absorption spectrum is obtained using an infrared spectrometer (in the examples described later, Magna 760 manufactured by Nikolet is used). At this time, the data interval is set to about 0.5 cm ⁻¹ , and the average value obtained by scanning 64 times is used as the absorption spectrum. In the infrared absorption spectrum thus obtained, the peak observed in the vicinity of about 2260 cm ⁻¹ is attributed to the overtone of stretching vibration due to the Si—O—Si bond of the TiO ₂ —SiO ₂ glass. Using this peak position, a calibration curve is created with glass having the same fictive temperature and the same composition, and the fictive temperature is obtained. Alternatively, the reflection spectrum of the surface is similarly measured using a similar infrared spectrometer. In the infrared reflection spectrum thus obtained, the peak observed in the vicinity of about 1120 cm ⁻¹ is caused by stretching vibration due to the Si—O—Si bond of the TiO ₂ —SiO ₂ glass. Using this peak position, a calibration curve is created with glass having the same fictive temperature and the same composition, and the fictive temperature is obtained. Note that the shift of the peak position due to the change in the glass composition can be extrapolated from the composition dependency of the calibration curve.

When the TiO ₂ —SiO ₂ glass of the present invention is used as an optical member of an exposure apparatus for EUVL, making the TiO ₂ / SiO ₂ composition ratio uniform in the glass is a linear thermal expansion coefficient in the glass. This is important in reducing variation.

The TiO ₂ —SiO ₂ glass of the present invention preferably has a fictive temperature variation of 50 ° C. or less, more preferably 30 ° C. or less. If the variation in the fictive temperature exceeds the above range, there may be a difference in the linear thermal expansion coefficient depending on the location.

  In this specification, “variation of virtual temperature” is defined as the difference between the maximum value and the minimum value of virtual temperature within 30 mm × 30 mm in at least one plane.

The variation in fictive temperature can be measured as follows. A transparent TiO ₂ —SiO ₂ glass body molded into a predetermined size is sliced into a 50 mm × 50 mm × 1 mm TiO ₂ —SiO ₂ glass block. By measuring the fictive temperature according to the method described above at a pitch of 10 mm on the 50 mm × 50 mm surface of the TiO ₂ —SiO ₂ glass block, the variation in fictive temperature of the formed TiO ₂ —SiO ₂ glass body is obtained.

In order to produce the TiO ₂ —SiO ₂ glass of the present invention, a production method including the following steps (a) to (e) can be employed.

(A) Step TiO ₂ —SiO ₂ glass fine particles obtained by flame hydrolysis of a silica precursor and a titania precursor, which are glass forming raw materials, are deposited and grown on a substrate to form a porous TiO ₂ —SiO ₂ glass body. Let it form. The glass forming raw material is not particularly limited as long as it is a gasifiable raw material. Examples of the silica precursor include chlorides such as SiCl ₄ , SiHCl ₃ , SiH ₂ Cl ₂ , and SiH ₃ Cl, SiF ₄ , SiHF ₃ , fluorides such as SiH ₂ F _2, SiBr _4, bromides such as SiHBr _3, halogenated silicon compounds such as iodide such as SiI _{4, R n Si (OR) 4-} n ( wherein R is 1 to 4 carbon atoms Wherein n is an integer of 0 to 3. A plurality of Rs may be the same or different from each other.), And titania precursors include halogens such as TiCl ₄ and TiBr _4. titanium compound, _{R n Ti (OR) 4-} n ( here R is an alkyl group having 1 to 4 carbon atoms, n represents an integer. multiple R of 0-3 may be the same or different from each other.) It includes alkoxy titanium represented. In addition, Si and Ti compounds such as silicon titanium double alkoxide can be used as the silica precursor and titania precursor.

  As a base material, a seed rod made of quartz glass (for example, a seed rod described in Japanese Patent Publication No. 63-24973) can be used. Moreover, you may use not only rod shape but a plate-shaped base material.

  When supplying the glass forming raw material, it is preferable to stabilize the supply of the glass raw material gas by controlling the aforementioned piping temperature and gas flow rate.

  Furthermore, it is preferable to provide the above-mentioned glass source gas stirring mechanism in the gas supply system.

By either of these, the striae level of the TiO ₂ —SiO ₂ glass can be reduced, and the striae stress level and the refractive index fluctuation range can be reduced to a predetermined value or less.

As described above, the present inventors have made a detailed study on the relationship between the rotational speed of the seed rod and the striae of the transparent TiO ₂ —SiO ₂ glass body in the stage of obtaining the porous TiO ₂ —SiO ₂ glass body. As a result, it has been found that the larger the number of rotations of the seed rod, the smaller the variation in TiO ₂ concentration in the transparent TiO ₂ —SiO ₂ glass body, and the smaller the striae (see Patent Document 2).

In the present invention, in addition to stabilizing the supply of the raw material, it is preferable to perform the rotation speed of the seed rod when forming the porous TiO ₂ —SiO ₂ glass body at 25 rotations / minute or more, and 50 rotations / minute. It is more preferable to carry out at the above, more preferably at 100 revolutions / minute or more, and particularly preferably at 250 revolutions / minute or more.

In addition to stabilizing or homogenizing the supply of the raw material in the vapor form, the TiO ₂ —SiO ₂ glass with even smaller striae can be obtained by rotating the seed rod at a high speed.

(B) Step The porous TiO ₂ —SiO ₂ glass body obtained in the step (a) is heated to a densification temperature under reduced pressure or in a helium atmosphere to obtain a TiO ₂ —SiO ₂ dense body. The densification temperature is usually 1250 to 1550 ° C, and particularly preferably 1300 to 1500 ° C.

(C) Step TiO ₂ —SiO ₂ dense body obtained in step (b) is heated to a transparent vitrification temperature to obtain a transparent TiO ₂ —SiO ₂ glass body. The transparent vitrification temperature is usually 1350 to 1800 ° C, particularly preferably 1400 to 1750 ° C.

  The atmosphere is preferably an atmosphere of 100% inert gas such as helium or argon, or an atmosphere mainly composed of an inert gas such as helium or argon. The pressure may be reduced pressure or normal pressure. In the case of reduced pressure, 13000 Pa or less is preferable.

(D) Step The transparent TiO ₂ —SiO ₂ glass body obtained in the step (c) is heated to a temperature equal to or higher than the softening point and formed into a desired shape to obtain a molded TiO ₂ —SiO ₂ glass body. The molding process temperature is preferably 1500 to 1800 ° C. Below 1500 ° C., the viscosity of the transparent TiO ₂ —SiO ₂ glass is high, so that substantially no self-weight deformation occurs, and the growth of cristobalite, which is the crystalline phase of SiO ₂ , or the rutile or anatase of the crystalline phase of TiO ₂ Growth may occur and so-called devitrification may occur. If it exceeds 1800 ° C., the sublimation of SiO ₂ may not be negligible.

  The steps (c) and (d) can be performed continuously or simultaneously.

(E) a step (d) obtained in the step shaped TiO ₂ -SiO ₂ glass body was held for 1 hour or more at a temperature of 600 to 1200 ° C., 700 ° C. or less at 10 ° C. / hr or less of the average cooling rate An annealing process for lowering the temperature is performed to control the fictive temperature of the TiO ₂ —SiO ₂ glass. Alternatively, the formed TiO ₂ —SiO ₂ glass body obtained in the step (d) at 1200 ° C. or higher is annealed to an average temperature decrease rate of 60 ° C./hr or less up to 500 ° C. to obtain a TiO ₂ —SiO ₂ glass Control the virtual temperature. After cooling down to 500 ° C. or lower, it can be allowed to cool. The atmosphere in this case is preferably an atmosphere of 100% inert gas such as helium, argon or nitrogen, an atmosphere containing such an inert gas as a main component, or an air atmosphere, and the pressure is preferably reduced or normal pressure.

In order to achieve a lower fictive temperature, it is effective to cool the temperature region near the annealing point or strain point of the glass at a slower cooling rate. In addition, unevenness of the polished surface due to striae is caused not only by differences in mechanical and chemical properties of glass due to fluctuations in the composition ratio of TiO ₂ / SiO ₂ , but also by stresses caused by differences in linear thermal expansion coefficient due to composition differences. It is. Therefore, it is effective to cool the temperature region near the annealing point or strain point of the glass at a slower cooling rate for the purpose of reducing stress between striae and suppressing formation of unevenness after polishing. Specifically, in the cooling profile of step (e), the slowest cooling rate is preferably 10 ° C./hr or less, more preferably 5 ° C./hr or less, further preferably 3 ° C./hr or less, particularly preferably. Is 1 ° C./hr or less.
In particular, in order to achieve a lower fictive temperature, it is effective to cool a temperature region near the annealing point of glass (for example, annealing point ± 25 ° C.) at a slower cooling rate. Further, in order to further reduce the stress between striae and further suppress the formation of unevenness after polishing, the temperature range near the strain point of glass (for example, strain point ± 25 ° C.) is cooled at a slower cooling rate. It is effective.

The TiO ₂ —SiO ₂ glass of the present invention preferably has no inclusion. The inclusion is a foreign substance or bubble present in the glass. Foreign matter may be generated by contamination or crystal precipitation in the glass production process. In order to eliminate inclusions such as foreign matters and bubbles, it is necessary to suppress contamination particularly in step (a) and to accurately control the temperature conditions in steps (b) to (d) in the above production process. It is.

The optical member of the EUVL exposure apparatus using the TiO ₂ —SiO ₂ glass of the present invention can easily obtain an ultra-high smooth surface.

The surface smoothness (rms) of the optical member of the exposure apparatus for EUVL using the TiO ₂ —SiO ₂ glass of the present invention is preferably 3 nm or less. More preferably, it is 2 nm or less, More preferably, it is 1.5 nm or less, Most preferably, it is 1 nm or less.

  Surface smoothness (rms) is measured by the following method.

  The surface shape of the area | region used as an optical member is measured with the non-contact surface shape measuring device (Zygo NewView5032) with respect to the glass surface which carried out mirror polishing. A 2.5 × objective lens is used for the measurement. The measured surface shape is divided for each 2 × 2 mm square area, and the rms value is calculated as the smoothness. In calculating the rms value, data processing is performed using a bandpass filter having a wavelength of 10 μm to 1 mm, and undulation components having wavelengths other than the same wavelength range are removed.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to this.
Examples 2 , 3 and 5 are examples, examples 1 and 4 are reference examples, and others are comparative examples.

[Example 1]
TiCl ₄ and SiCl ₄ as glass-forming raw material for TiO ₂ -SiO ₂ glass, was mixed after each is gasified, TiO ₂ -SiO to subjecting the mixture to heat hydrolysis in an oxyhydrogen flame (flame hydrolysis) _{(2) A} porous TiO ₂ —SiO ₂ glass body is formed by depositing and growing glass fine particles on a seed rod rotating at a rotational speed of 250 rpm (step (a)).

By performing PID control on the supply pipes of SiCl ₄ and TiCl ₄ , the gas temperature fluctuation range in the supply pipe is within ± 0.5 ° C. The gas flow rate in the pipe is 3.04 m / sec. The temperature of the pipe is set higher than the bubbling temperature, and is set so that the temperature rises as it proceeds to the burner. A source gas stirring mechanism is provided before supplying SiCl ₄ and TiCl ₄ to the burner.

Since the obtained porous TiO ₂ —SiO ₂ glass body is difficult to handle as it is, it is kept in the atmosphere at 1200 ° C. for 6 hours and then removed from the seed rod.

After that, the porous TiO ₂ —SiO ₂ glass body was placed in an electric furnace capable of controlling the atmosphere, depressurized to 1300 Pa at room temperature, heated to 1450 ° C. in a helium gas atmosphere, and held at this temperature for 4 hours. Thus, a dense TiO ₂ —SiO ₂ body is obtained (step (b)).

The obtained TiO ₂ —SiO ₂ dense body is heated to 1700 ° C. in an argon atmosphere using a carbon furnace to obtain a transparent TiO ₂ —SiO ₂ glass body (step (c)).

The obtained transparent TiO ₂ —SiO ₂ glass body is heated to 1750 ° C. and formed into a desired shape to obtain a formed TiO ₂ —SiO ₂ glass body (step (d)).

  The obtained glass is held at 1100 ° C. for 10 hours, cooled to 500 ° C. at a rate of 3 ° C./hr, and allowed to cool to the atmosphere (step (e)).

[Example 2]
In the step (a) of Example 1, a TiO ₂ —SiO ₂ glass body is obtained in the same manner as in Example 1 except that the supply amount of TiCl ₄ is increased and the rotation speed of the seed bar is 25 rpm.

[Example 3]
In the step (a) of Example 1, the supply amount of TiCl ₄ is increased, the rotation speed of the seed bar is set to 25 rpm, and a raw material gas stirring mechanism is provided before supplying SiCl ₄ and TiCl ₄ to the burner. A TiO ₂ —SiO ₂ glass body is obtained in the same manner as in Example 1 except that it is not.

[Example 4]
In the same manner as in Example 1 except that the supply amount of TiCl ₄ is slightly increased in the step (a) of Example 1 and the cooling rate is 10 ° C./hr instead of slow cooling in the step (e). get a ₂ -SiO ₂ glass body.

[Example 5]
TiCl ₄ and SiCl ₄ as glass-forming raw material for TiO ₂ -SiO ₂ glass, was mixed after each is gasified, TiO ₂ -SiO to subjecting the mixture to heat hydrolysis in an oxyhydrogen flame (flame hydrolysis) _{(2) A} porous TiO ₂ —SiO ₂ glass body is formed by depositing and growing glass fine particles on a seed rod rotating at a rotational speed of 25 rpm (step (a)). By performing PID control on the supply pipes of SiCl ₄ and TiCl ₄ , the gas temperature fluctuation range in the supply pipe is within ± 0.5 ° C. The temperature of the pipe is set higher than the bubbling temperature, and is set so that the temperature rises as it proceeds to the burner.

Since the obtained porous TiO ₂ —SiO ₂ glass body is difficult to handle as it is, it is kept in the atmosphere at 1200 ° C. for 6 hours and then removed from the substrate.

Thereafter, the porous TiO ₂ —SiO ₂ glass body was placed in an electric furnace capable of controlling the atmosphere, and after reducing the pressure to 1300 Pa at room temperature, water was put into a glass bubbler and bubbled with He gas at an atmospheric pressure of 100 ° C. While introducing water vapor into the furnace together with He gas, the atmosphere is kept at 1000 ° C. under normal pressure for 4 hours to perform OH doping.

Then, after raising the temperature to 1450 ° C. under the same atmosphere to obtain a TiO ₂ -SiO ₂ densified body containing OH and held at this temperature for 4 hours ((b) step).

The obtained OH-containing TiO ₂ —SiO ₂ dense body is heated to 1700 ° C. in an argon atmosphere using a carbon furnace to obtain a transparent TiO ₂ —SiO ₂ glass body containing OH ((c)). Process).

The obtained transparent TiO ₂ —SiO ₂ glass body containing OH was heated to a temperature above the softening point (1750 ° C.) and molded into a desired shape, and a molded TiO ₂ —SiO ₂ glass body containing OH was obtained. To obtain (step (d)).

  The obtained glass is held at 1100 ° C. for 10 hours, the temperature is lowered to 900 ° C. at a rate of 10 ° C./hr, the temperature is lowered to 700 ° C. at a rate of 1 ° C./hr, and further to 500 ° C. at a rate of 10 ° C./hr The temperature is lowered and allowed to cool to the atmosphere (step (e)).

[Example 6]
In the process (a) of Example 4, the supply amount of TiCl ₄ is reduced, the rotation speed of the seed bar is set to 25 rpm, and the heater temperature is controlled not by PID control but by ON / OFF control. ₄ and TiCl ₄ supply piping had a gas temperature fluctuation range of ± 2 ° C. or more, and was the same as in Example 4 except that no source gas stirring mechanism was provided before supplying SiCl ₄ and TiCl ₄ to the burner. Thus, a TiO ₂ —SiO ₂ glass body is obtained.

[Example 7]
In the process (a) of Example 4, the supply amount of TiCl ₄ is reduced, the rotation speed of the seed rod is 25 rpm, and the heater temperature control is not ON / OFF control but PID control, and SiCl ₄ In addition, a TiO ₂ —SiO ₂ glass body is obtained in the same manner as in Example 4 except that the gas temperature fluctuation range in the supply pipe of TiCl ₄ is ± 2 ° C. or more.

[Example 8]
In the step (a) of Example 4, there is a part where the supply amount of TiCl ₄ is slightly reduced, the rotation speed of the seed rod is 25 rpm, and there is a part where the temperature is lower than the previous part in a part of the pipe. The temperature control is not PID control but ON / OFF control, and the gas temperature fluctuation range in the supply pipes of SiCl ₄ and TiCl ₄ is ± 2 ° C. or more, before supplying SiCl ₄ and TiCl ₄ to the burner. A TiO ₂ —SiO ₂ glass body is obtained in the same manner as in Example 4 except that no material gas stirring mechanism is provided.

[Example 9]
Corning ULE # 7972, known as zero expansion TiO ₂ —SiO ₂ glass.

  The measurement results of the physical properties of the glasses prepared in Examples 1 to 9 are summarized in Table 1. In addition, about an evaluation method, it carries out according to the above-mentioned measuring method, respectively. The COT in Table 1 is derived by obtaining the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. from the curve in FIG. ΔT in Table 1 is derived from the temperature range in which the linear thermal expansion coefficient is −5 to 5 ppb / ° C. from the curve of FIG.

  As is apparent from Table 1, in Examples 1 to 5 where COT is in the range of 40 to 110 ° C., the linear thermal expansion coefficient at the time of irradiation with high EUV energy light becomes almost zero and is stable in a wide temperature range. Therefore, it is suitable for an optical system member of an EUVL exposure apparatus.

  The evaluation results of the striae levels of the glasses of Examples 1 to 9 are summarized in Table 2. The evaluation method is as follows. In addition, the surface smoothness (rms) of Examples 1 to 5 is 1 nm or less, and the smoothness (rms) of Example 8 is 3 nm or more.

As is apparent from Table 2, the standard deviation σ of the striae stress level is 0.03 MPa or less, the maximum roughness PV is 0.2 MPa or less, and the refractive index fluctuation range is 2 × 10 ⁻⁴ or less. Examples 1 to 5 are suitable for an optical member of an EUVL exposure apparatus because an ultra-high smooth surface can be obtained even though the TiO ₂ concentration is higher than that of the glasses of Examples 6 to 9.

As is clear from Examples 1 to 5 in Table 2, 1) stabilization of the supply of glass raw material gas when supplying the glass forming raw material, 2) installation of a stirring mechanism for the glass raw material gas supply system, and 3) the seed rod Any of the three striae reduction methods of high-speed rotation has a standard deviation σ of the striae stress level of 0.03 MPa or less, a maximum roughness PV of 0.2 MPa or less, and a refractive index fluctuation range of 2 × 10 ⁻⁴ or less. It is effective to make it.

  The silica glass and optical member of the present invention are suitable for an exposure apparatus for EUV lithography.

Claims (13)

The fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C., and the stress level of striae standard deviation (sigma) is 0.03MPa der below, in the range of 30 mm × 30 mm in at least one plane of is, average linear thermal expansion coefficient of 20 to 100 ° C. is not more than 30 ppb / ° C. or higher -30ppb / ℃, Silica glass containing TiO ₂ . The fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C., and the stress level of striae maximum roughness (PV) is der 0.2MPa or less in the range of 30 mm × 30 mm in at least one plane is, average linear thermal expansion coefficient of 20 to 100 ° C. is not more than -30ppb / ℃ than 30 ppb / ° C. of , silica glass containing TiO _2. The fictive temperature is 1000 ° C. or less, the TiO ₂ content is 8.5 to 12% by mass, the temperature at which the linear thermal expansion coefficient is 0 ppb / ° C. is in the range of 40 to 110 ° C., and the refractive index fluctuation range ([Delta] n) is Ri der 4 × 10 ^-4 or less in the range of 30 mm × 30 mm in at least one plane, the average linear thermal expansion coefficient of 20 to 100 ° C. is not more than 30 ppb / ° C. or higher -30ppb / ℃, TiO _2. Silica glass containing ₂ . The silica glass containing TiO _{2 according} to any one of claims 1 to 3 , wherein a temperature width (ΔT) at which a linear thermal expansion coefficient (CTE) is 0 ± 5 ppb / ° C is more than 6.6 ° C. The silica glass containing TiO _{2 according} to any one of claims 1 to 3 , wherein a temperature width (ΔT) at which a linear thermal expansion coefficient (CTE) is 0 ± 5 ppb / ° C is 7.8 ° C or more . The silica glass containing TiO _{2 according} to any one of claims 1 to 5, wherein there is no inclusion. EUV lithography optical member using the silica glass containing TiO ₂ according to claim 1.   The optical member for EUV lithography according to claim 7, wherein the surface smoothness (rms) is 3 nm or less. Temperature as a method for producing a silica glass containing TiO ₂ according to claim 1, the titania precursor was vaporized by bubbling and proceeds the temperature of the piping for transporting the titania precursor to a burner A method for producing silica glass, wherein A method for producing a silica glass containing TiO ₂ according to claim 1, characterized in that within ± 1 ° C. The temperature fluctuation width a pipe for conveying the titania precursor by the PID control A method for producing silica glass. A method for producing a silica glass containing TiO ₂ according to any one of claims 1 to 6, and 0.1m / sec or more gas flow velocity in the piping of the titania precursor in a volume at the time of conversion atmospheric pressure A method for producing silica glass, characterized in that: A method for producing a silica glass containing TiO ₂ according to claim 1, wherein providing a stirring mechanism of the gas prior to feeding the silica precursor and a titania precursor to a burner A method for producing silica glass. A method for producing a silica glass containing TiO ₂ according to claim 1, after holding for 2 hours or more silica glass shaped body formed in a predetermined shape at a temperature of 600 to 1200 ° C. A method for producing silica glass, wherein the temperature is lowered to 700 ° C. or lower at an average temperature lowering rate of 5 ° C./hr or lower.

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