JPH0912323A - Quartz glass member suppressed from becoming dense due to irradiation of uv ray - Google Patents

Abstract

PURPOSE: To obtain such a synthesized quartz glass suitable for an optical system of photolithography that the structure is suppressed from becoming dense due to irradiation of UV rays in a specified wavelength region by controlling the conc. of OH groups in the quartz glass member to a specified value or smaller and adding hydrogen molecules to the member. CONSTITUTION: This quartz glass member has a low OH group conc. as <=200ppm. Even when the quartz glass member is irradiated with UV rays or excimer laser beams in a specified wavelength region of <=400nm for a long time, the increase in the refractive index, generation of strain and recession of the surface can be suppressed. Therefore, a lens comprising this glass member shows no deterioration of focusing performance even when the glass is irradiated with UV rays for a long time. Moreover, by adding hydrogen molecules to the glass member to obtain >=1×10<17> molecules/cm<2> hydrogen molecule density, the glass member is more suppressed from becoming dense.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quartz glass member used in an ultraviolet optical system.

[0002]

2. Description of the Related Art Conventionally, in an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit on a wafer such as silicon, an exposure apparatus called a stepper is used.
The light source of this stepper is g-line (436 nm) to i-line (365 nm), and further KrF with the recent high integration of LSI.
(248 nm) and ArF (193 nm) excimer lasers are being shortened in wavelength. In general, optical glass used as an illumination system of a stepper or a projection lens has a low light transmittance in a wavelength region shorter than the i-line, and thus synthetic quartz glass is used instead of conventional optical glass.

Synthetic quartz glass is mainly used for direct method, soot method,
It is classified into the plasma method. All of these belong to a manufacturing method called a gas phase synthesis method. First, in the direct method, a silicon compound such as silicon tetrachloride is used as a raw material, and the raw material is hydrolyzed with an oxygen-hydrogen flame to form quartz glass fine particles (soot), which are rotated, rocked, and lowered. In this method, a lump of quartz glass is obtained by performing deposition, melting, and clarification on a target all at once. In addition, the soot method uses silicon tetrachloride as a raw material, hydrolyzes the raw material with an oxyhydrogen flame to form soot, deposits it on the target to obtain a soot mass, and then makes it transparent by secondary treatment. It is a method of obtaining a quartz glass block. Furthermore, the plasma method uses silicon tetrachloride as a raw material, and oxidizes the raw material with a high-frequency plasma flame of a mixture of oxygen and argon to form soot, which is deposited on the target that is rotating and pulling down, melting, and transparent. It is a method of obtaining a lump of quartz glass by performing crystallization at once.

When these synthetic quartz glass are used in an optical system for optical lithography, very high quality is required for the synthetic quartz glass optical member in order to expose an integrated circuit pattern in a large area with high resolution. . For example, the refractive index distribution of the member is required to be 10 ^{−6 order} or less in a very large diameter of about 200 mm. Also, reducing the amount of birefringence, that is, reducing the internal strain of the optical member, is important for the resolution of the optical system, as is improving the homogeneity of the refractive index distribution.

[0005] In general, an amorphous material such as glass is isotropic in all properties unless stress is present inside the material and strained. However, distorted glass is an anisotropic body, and a phenomenon called birefringence appears optically.
Birefringence is a phenomenon in which two refracted lights are obtained when one incident light passes through an optically anisotropic body. This refracted light is linearly polarized light whose vibrating planes are orthogonal to each other, and since the refraction indices thereof are different, the phases of these two refracted lights are different. At this time, the amount of birefringence is a phase difference between two refracted lights when light passes through a substance for a unit length. Usually, the strain amount of glass is defined as this birefringence amount. The presence of such distortion in the optical member adversely affects the resolution of the optical system.

For this reason, a synthetic quartz glass used in an image forming optical system of an optical lithography apparatus is of high quality with good refractive index homogeneity and little distortion.

[0007]

However, even if high-quality synthetic silica glass with respect to such homogeneity and strain of the refractive index is irradiated with high-power ultraviolet light or excimer laser light for a long time, the E ′ center 215nm absorption band and NBOHC (Non-Bridging Oxygen) caused by structural defects called
In some cases, the 260 nm absorption band due to a structural defect called a hole center) appeared and the transmittance in the ultraviolet region decreased remarkably. Furthermore, at the same time, the rise of the refractive index and the occurrence of distortion,
Since a phenomenon such as a dent on the surface appears, there is a problem that the resolution of the optical system is significantly deteriorated. Each of these phenomena is that when the quartz glass is irradiated with ultraviolet rays, the refractive index of the irradiated part becomes higher than the refractive index of the unirradiated part, and a large strain is generated near the boundary between the irradiated part and the unirradiated part. This means that the irradiated portion of the quartz glass surface is recessed as compared with the unirradiated portion.

Regarding the rise of the refractive index, the generation of strain, and the dent on the surface, there have been some reports to date. However, even though each phenomenon was discussed independently, it was not clear about the mechanism of their occurrence, their interrelationships, and how they could be suppressed. .

The present invention provides a synthetic quartz glass most suitable for an optical system of optical lithography by suppressing the increase of the refractive index, the occurrence of distortion, and the depression of the surface due to the irradiation of ultraviolet rays, which are the drawbacks of the prior art. The purpose is to

[0010]

Means for Solving the Problems The inventors of the present invention have earnestly studied the cause of the increase in the refractive index, the generation of strain, and the depression of the surface due to the irradiation of ultraviolet rays. As a result, it was found that these characteristics are caused by the phenomenon that the structure of quartz glass is densified, that is, the density is increased, which is caused by ultraviolet irradiation. Then, in order to suppress such densification, it was concluded that the OH group concentration contained in the silica glass should be lowered. Therefore, the present invention is a quartz glass member having an OH group concentration of 200 ppm or less, and is characterized in that densification of the structure due to ultraviolet irradiation is suppressed.

Further, as a result of further investigation, it was found that the addition of hydrogen molecules into the member can further suppress the densification. Therefore, the present invention is OH
A quartz glass member having a base concentration of 200 ppm or less and a hydrogen molecule concentration of 1 × 10 ¹⁷ molecules / cm ³ or more. As described above, if the increase in density caused by ultraviolet irradiation can be suppressed, a quartz glass optical member that does not deteriorate the imaging performance of the lens even when irradiated with ultraviolet light or excimer laser light of short wavelength and high output for a long period of time. Can be provided.

Such a quartz glass optical member is made of ArF.
Excimer laser with one pulse energy density 200 mJ / cm ²
The difference between the density after 6 × 10 ⁵ pulse irradiation and the density before irradiation is 3 × 10 ⁻⁵ g / cm ³ or less.

[0013]

[Function] There have been some reports so far about the rise of the refractive index, the occurrence of strain, and the surface dent when the quartz glass is irradiated with strong ultraviolet rays or radiation.
As described in the prior art, such a phenomenon leads to performance deterioration of the optical system for ultraviolet rays. However, it has not been clarified about the mechanism of these phenomena, their relation to each other, and how to suppress their physical property changes.

The present inventors first investigated the interrelationship of these phenomena. Various excimer laser irradiation conditions were set, and irradiation experiments were conducted on various types of quartz glass, and the data of the refractive index rise, strain, and surface dents were measured. The amount of increase in the refractive index was obtained by measuring the refractive index difference between the part irradiated with the excimer laser and the part not irradiated with the Fizeau interferometer using a He—Ne laser as a light source. The strain amount was determined as the maximum strain amount by measuring the strain amount generated near the boundary between the excimer laser-irradiated portion and the non-irradiated portion with an automatic birefringence measuring device using a He-Ne laser as a light source. This device measures the phase difference (amount of birefringence) of two refracted lights generated by a phenomenon called birefringence when one incident light passes through a distorted glass when the light passes through a unit length. However, it is a device for measuring the spatial distribution of the birefringence amount in the region of the member. Usually, the strain amount of glass is defined as this birefringence amount. Further, the amount of surface depression was determined by measuring the amount of depression generated on both the incident surface and the exit surface of the excimer laser light of the sample using a diamond needle contact surface shape measuring instrument, and calculated as the sum.

As a result, as shown in FIG. 2, it was found that there is a one-to-one correlation between the amount of increase in refractive index and the amount of strain. Furthermore, as shown in FIG. 3, it was found that a one-to-one correlation also holds between the refractive index increase amount and the surface depression amount. From this, it was concluded that the increase in the refractive index, the occurrence of strain, and the surface depression were caused by the same phenomenon, that is, the densification of the silica glass structure due to the excimer laser irradiation. Bonding inside the quartz glass by excimer laser irradiation (eg, ≡Si-O-Si≡, ≡Si-OH, etc.
≡Si does not represent a triple bond, but represents that Si is bonded to three oxygens. It is considered that, when the rearrangement of (1) occurs and the densification of the structure progresses, the refractive index increases as the density increases. Since densification of such a structure progresses in the irradiated portion, a large stress is generated near the boundary with the unirradiated portion where the densification does not proceed,
As a result, it appears as distortion. Furthermore, since the progress of densification is accompanied by volume contraction, it is considered that the surface of the irradiated portion is recessed as compared with the unirradiated portion.

The increase in density due to ultraviolet irradiation can be measured by the Archimedes method. As a result of investigating the influence of the increase in density due to ultraviolet irradiation on the imaging performance of the optical system, the present inventors have found that the density and irradiation after irradiation of ArF excimer laser with 6 × 10 ⁵ pulses at one pulse energy density of 200 mJ / cm ² It was found that the image forming performance of the ArF excimer laser stepper projection optical system is not deteriorated by using a silica glass optical member whose difference from the previous density is 3 × 10 ⁻⁵ g / cm ³ or less.

Next, the present inventors have studied a method for suppressing the above-mentioned densification. As a result, it was found that suppression of densification can be achieved by reducing the OH group concentration in quartz glass. That is, it was found that the presence of the OH group facilitates the rearrangement of the bond of the quartz glass. The mechanism by which the presence of the OH group promotes the densification further is not necessarily clear, but it is estimated based on the studies by the present inventors.
For example, the following reactions associated with excimer laser irradiation, ≡Si-OH HO-Si≡ → ≡Si-O-Si≡ + H ₂ O (1), are presumed to lead to structural densification. .

Next, due to the inclusion of hydrogen molecules,
The mechanism of suppression of densification will be estimated based on the results of the study by the present inventors. Γ in quartz glass containing hydrogen molecules
It is said that when irradiated with a ray, a reaction such as ≡Si-O-Si≡ + H ₂ → ≡Si-OH H-Si≡ (2) occurs and the refractive index decreases due to the presence of a large amount of OH groups. There is. That is, it is said that the density decreases. Therefore, when the quartz glass contains hydrogen molecules, the inventors of the present invention substantially densify the material with ultraviolet irradiation and simultaneously reduce the density due to the formation of OH groups. I have been conducting research, thinking that it may be possible to suppress densification. As a result, they have found that the presence of hydrogen molecules actually suppresses the generation of strain. At this time, if hydrogen molecules are present in the photolysis process shown in (1), it is presumed that the hydrogen is stabilized by being terminated by ≡Si-OH H-Si≡ before changing to a more dense structure.

The effect of suppressing the densification due to the decrease of the OH group concentration is not found in any quartz glass, but has a certain structure, that is, an incomplete structure as described below. No, it is effective only for quartz glass that has a structure close to ideal. As an example of the incomplete structure, ≡Si-Si≡ bond which is an oxygen deficient type defect, ≡Si-OO-Si≡ bond which is an oxygen excess type defect, etc. have been proposed. , It is also necessary that there be no incomplete structure due to such deviation from the stoichiometric ratio. That is, oxygen deficiency type defect absorption band (7.6, 5.0 eV
Absorption band) is substantially not included. The quartz glass of the present invention has A
When irradiated with the rF excimer laser, the oxygen excess defect absorption band (4.8 eV absorption band) is not substantially generated. Due to the absence of these defects, vacuum ultraviolet, ultraviolet, visible,
In the transmittance measurement with an infrared spectrophotometer, g-line (436 nm)
~ I-line (365nm), further KrF (248nm) wavelength internal transmittance 99.9% or more, ArF (193nm) wavelength approximately 99.8%
The above can be obtained.

Conventionally, for example, in the synthetic quartz glass of the VAD method, when an OH group concentration is reduced in the manufacturing process, oxygen deficiency type defects are often generated.
Such an oxygen-deficient defect was a cause of large densification accompanying excimer laser irradiation, even though the OH group concentration was low. On the contrary, synthetic quartz glass produced by, for example, the plasma method can theoretically obtain synthetic quartz glass containing no OH group, but since it contains a large amount of oxygen-excessive defect structure, the excimer laser is also used. Large densification progressed with irradiation. Furthermore, in the field of quartz glass thin films, waveguides, and silica-based fibers, densification of quartz glass by UV irradiation is actively used, and refractive index control by UV irradiation, waveguide fabrication, fiber grating fabrication, etc. Has been applied to. Rather, research is being carried out in these fields in a more efficient manner, that is, irradiation with ultraviolet rays of the same energy causes greater densification. As a result, recently, it has been revealed that reduction treatment of a thin film and a fiber causes densification more efficiently. Quartz glass of such thin films and fibers does not have an ideal structure, but contains, for example, an oxygen deficient defect structure (7.6 eV and 5.0 eV absorption bands are observed), so that large densification may proceed. It is clear. As described above, in the silica glass having a structure close to the ideal as in the present invention, that is, in the silica glass having no structural defect, O
By reducing the H group concentration, the effect of suppressing densification due to ultraviolet irradiation is exhibited.

The silica glass obtained in the present invention is useful not only for the projection lens of the excimer laser stepper, but also for an illumination system, an ultraviolet precision optical element such as an etalon.

[0022]

Example 1 Using high-purity silicon tetrachloride as a raw material, a quartz glass burner was used to mix and burn oxygen gas and hydrogen gas, and the raw material gas was diluted with a carrier gas (usually oxygen gas) from the center. The silica glass particles (soot) were deposited on the target to form a porous body of silica glass. This porous body is heated to 1000 in an inert gas atmosphere.
After heat treatment at ℃ for 20 hours, heat treatment at 1000 ℃ in vacuum for 20 hours. Then, at 1600 ℃ in an inert gas atmosphere,
A heat treatment for transparency was performed for a time. After obtaining a transparent quartz glass ingot in this manner, a test piece for ArF excimer laser irradiation having a shape with a diameter of 60 mm and a thickness of 10 mm was cut out, and two surfaces facing each other in the thickness direction were optically polished. This is Example 1. The OH group concentration of Example 1 was 10 ppm. In addition, the hydrogen molecule concentration is the detection limit (5 ×
10 ¹⁶ molecules / cm ³ ) or less.

The OH group concentration was calculated by measuring the amount of absorption at 1.38 μm. The hydrogen molecule concentration was measured by a laser Raman spectrophotometer. The intensity at 800 cm ^-1 and 4135 cm ^-1 of the Raman scattered light in the direction perpendicular to the sample generated when irradiated with an Ar ⁺ laser (output 400 mW) was measured and the intensity ratio was taken. One pulse energy density: 200 mJ / cm ² / puls was applied to the test piece of Example 1 thus created by ArF excimer laser light.
e, Repetition frequency: 100Hz, Beam shape: 0.7 × 0.7cm ²
A 6 × 10 ⁵ pulse irradiation test was performed in order to measure the amount of increase in refractive index and the amount of increase in density. The amount of increase in refractive index was measured by a Fizeau interferometer using a He-Ne laser (633 nm) as a light source.
It was measured as the difference in refractive index between the irradiated part and the unirradiated part. Moreover, the amount of density increase was measured by the Archimedes method. As a result, the refractive index increase amount of Example 1 was 2.5 × 10 ⁻⁶ and the density increase amount was 1.3 × 10 ⁻⁵ g / cm ³ . The amount of increase in refractive index of Example 1 was 5 × 10 ⁻⁶ or less, and the amount of increase in density was 3 × 10 ⁻⁵ g / c.
Since it is less than m ³ , very good results were obtained.

[0024]

Example 2 The test piece of Example 2 is manufactured by substantially the same process as that of Example 1, but the difference from the process of Example 1 is that the heat treatment is not performed in vacuum. Example 2
The OH group concentration of the test piece of was 145 ppm. The hydrogen molecule concentration was below the lower limit of detection (5 × 10 ¹⁶ molecules / cm ³ ).

The thus prepared test piece of Example 2 was subjected to an irradiation test of ArF excimer laser under the same conditions as in Example 1 to measure the amount of increase in refractive index. As a result, the amount of increase in refractive index in Example 2 was 4.4 × 10 ⁻⁶ and the amount of increase in density was 2.2 × 10 ⁻⁵ g / cm ³ , and very good results were obtained.

[0026]

[Comparative Example 1] The quartz glass ingot of the comparative example uses high-purity silicon tetrachloride as a raw material, and an oxygen gas and a hydrogen gas are mixed and burned by a quartz glass burner to burn the raw material gas from the center portion into a carrier gas ( Usually, it was diluted with oxygen gas) and ejected, deposited on a target, melted, and synthesized. At this time, the mixing ratio of oxygen gas and hydrogen gas was set to O ₂ / H ₂ = 0.2. 60m diameter from the obtained quartz glass ingot
An ArF excimer laser irradiation test piece having a shape of m and a thickness of 10 mm was cut out, and two surfaces facing each other in the thickness direction were optically polished. Next, the test piece was dehydrogenated by vacuum heat treatment. The treatment temperature was 700 ° C. and the treatment time was 60 hours. The test piece of Comparative Example 1 was produced by such a procedure. The OH group concentration of Comparative Example 1 was 600 ppm. The hydrogen molecule concentration was below the detection limit.

The test piece of Comparative Example 1 thus prepared was subjected to an ArF excimer laser irradiation test under the same conditions as in Example 1 to measure the amount of increase in refractive index. As a result, the amount of increase in refractive index of Comparative Example 1 was 8.5 × 10 ⁻⁶ and the amount of increase in density was 4.3 × 10 ⁻⁵ g / cm ³ . Therefore, the refractive index increase is 5 × 10 ^-6 or more, and the density increase is 3 × 10 ^-5 g / cm ³
From the above, Comparative Example 1 was found to be defective.

[0028]

[Comparative Example 2] While the test piece of Comparative Example 2 is prepared in substantially the Comparative Example 1 and the same process is different from the comparative example 1 step, the mixing ratio of oxygen gas and hydrogen gas O ₂ / H ₂ = 0.4 has been set. The OH group concentration of the test piece of Comparative Example 2 was 960 ppm. In addition, the hydrogen molecule concentration is the detection limit (5 × 10 ¹⁶ molecul
es / cm ³ ) or less.

The test piece of Comparative Example 1 thus prepared was subjected to an ArF excimer laser irradiation test under the same conditions as in Example 1 to measure the amount of increase in refractive index. As a result, the amount of increase in refractive index of Comparative Example 2 was 14 × 10 ⁻⁶ , and the amount of increase in density was
It was obtained as 7 × 10 ⁻⁵ g / cm ³ . Therefore, since the refractive index increase amount is 5 × 10 ⁻⁶ or more and the density increase amount is 3 × 10 ⁻⁵ g / cm ³ or more, Comparative Example 2 was found to be defective.

As described above, Examples 1 and 2 and Comparative Examples 1 and 2
The results obtained by the ArF excimer laser irradiation test with respect to are shown in FIG. As shown in FIG. 1, the amount of increase in the refractive index due to ArF excimer laser irradiation strongly depends on the OH group concentration.
It was 5 × 10 ⁻⁶ or less, and it was found that a very good result was obtained. Note that, for example, 1E-05 on the vertical axis of FIG. 1 is 1
It means × 10 ^-5 .

[0031]

Example 3 The test piece of Example 1 was heat-treated at 500 ° C. for 60 hours in a 100% hydrogen gas atmosphere. This test piece was referred to as Example 3. As a result, the hydrogen molecule concentration of the test piece of Example 3 was 2 × 10 ¹⁸ molecules / cm ³ . OH
The base concentration did not change before and after the heat treatment. The test piece of Example 3 was subjected to an ArF excimer laser irradiation test under the same conditions as in Example 1 to measure the amount of increase in refractive index. As a result, the amount of increase in refractive index of Example 3 was 1.9 × 10 ⁻⁶ and the amount of increase in density was 1.3 × 10 ⁻⁵ g / cm ³ . Since the amount of increase in the refractive index of Example 3 was 5 × 10 ⁻⁶ or less and the amount of increase in density was 3 × 10 ⁻⁵ g / cm ³ or less, very good results were obtained. That is, it was found that the presence of hydrogen molecules further suppressed the densification as compared with the case of Example 1.

[0032]

[Embodiment 4] In the silica glass optical member of the present invention, the maximum refractive index difference in the excimer laser irradiation region is Δn ≦ 2 × 10.
An ArF excimer laser stepper projection lens was produced using a member having a characteristic that the maximum birefringence was ^-6 and the maximum birefringence was 2 nm / cm or less. Then, the resolution of the obtained projection optical system reached 0.2 μm in line and space, and good imaging performance as an ArF excimer laser stepper could be obtained.

As described above, good image forming performance needs to be maintained while the ArF excimer laser stepper is in operation, but since the operation period is expected to be 10 years or longer, it is confirmed by an actual device. You cannot do it. Therefore, it was examined whether or not the silica glass optical member of the present invention satisfies the densification allowable amount required for the lens member of the projection optical system calculated in the optical design stage.

Ar applied to the lens member of the projection optical system
The energy density per pulse of the F excimer laser light is 0.2 mJ / cm ² / pulse on average. At this energy density, when the ArF excimer laser stepper is operated with a pulse number of 1 × 10 ¹¹ pulses corresponding to the actual operating time, the fluctuation of the refractive index is required to be 1 × 10 ⁻⁶ or less. Within this allowable range, the imaging performance of the projection optical system of the ArF excimer laser stepper is not hindered.

In order to investigate how the refractive index of the silica glass optical member of the present invention actually fluctuates under the above operating conditions, pulse energy density dependency and pulse number dependency of the refractive index increase amount with irradiation. Need to investigate. Therefore, as a result of investigating the quartz glass optical member of the present invention, the amount of increase in the refractive index due to irradiation is
It was found to increase in proportion to the power of 1.3. It was also found that the increase in the refractive index with irradiation increases in proportion to the 0.6th power of the pulse number.

Based on this data, the amount of increase in the refractive index of the silica glass optical member of the present invention under the actual operating conditions of the ArF excimer laser stepper projection optical system was predicted. First, regarding the pulse energy density, the amount of increase in the refractive index under the actual operating conditions was calculated from the amount of increase in the refractive index under the irradiation test conditions performed in Examples 1 and 2 as {(0.2 mJ / cm ² / pulse) / (200 mJ / cm ² / pulse)} ^1.3 = 1.25 × 10 ^-4 times. Regarding the number of pulses, the amount of increase in the refractive index under the actual operating conditions was calculated from the amount of increase in the refractive index under the irradiation test conditions performed in Examples 1 and 2 as {(1 × 10 ¹¹ pulses) / (6 × 10 ⁵ pulses) )} ^0.6 = 1.4 × 10 ³ times. Therefore, the amount of increase in the refractive index of the silica glass optical member of the present invention under actual operating conditions is (5 × 10 ⁻⁶ or less) × 1.25 × 10 ⁻⁴ × 1.4 × 10 ³ = 0.9 × 10 ⁻⁶ or less, and the ArF It has been found that the excimer laser stepper projection optical system satisfies the densification allowance required for the lens member.

[0037]

According to the present invention, the densification of the quartz glass structure due to the irradiation of ultraviolet rays is suppressed, and the imaging performance of the lens is not deteriorated even by the irradiation of ultraviolet rays or excimer laser light of short wavelength and high output for a long period of time. It has become possible to provide a quartz glass optical member.

[Brief description of the drawings]

FIG. 1 is a diagram showing a correlation between a refractive index increase amount by ArF excimer laser irradiation and an OH group concentration.

FIG. 2 is a diagram showing a correlation between a refractive index increase amount and a maximum strain amount due to ArF excimer laser irradiation.

FIG. 3 is a diagram showing a correlation between a refractive index increase amount by ArF excimer laser irradiation and a dent amount on a sample surface.

Claims (4)

[Claims] 1. A quartz glass optical member having an OH group concentration of 200 ppm or less, wherein the densification of the structure due to ultraviolet irradiation in a specific wavelength region of 400 nm or less is suppressed. 2. The quartz glass optical member according to claim 1, wherein the hydrogen molecule concentration is 1 × 10 ¹⁷ molecules / cm ³ or more. 3. The difference between the density after irradiation of 6 × 10 ⁵ pulses of ArF excimer laser with one pulse energy density of 200 mJ / cm ² and the density before irradiation is 3 × 10 ⁻⁵ g / cm ³ or less. A quartz glass optical member characterized by. 4. The quartz glass optical member according to claim 1, 2 or 3, which is substantially free of oxygen-deficient defects.