JP3510224B2 - Silica glass optical material for projection lens used in vacuum ultraviolet lithography and projection lens - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to silica glass optical materials and projection lenses, and more particularly to wavelengths 155-195.
TECHNICAL FIELD The present invention relates to a silica glass optical material for a projection lens and a projection lens used for vacuum ultraviolet lithography of nm (mainly an excimer laser, an exposure apparatus for making an ultrahigh-density integrated circuit using an excimer lamp as a light source).

[0002]

2. Description of the Related Art Conventionally, ultraviolet rays from a mercury lamp such as g-line and i-line have been used as a light source of an optical lithography apparatus for drawing an integrated electronic circuit pattern on a silicon wafer. The g-line and i-line have a limited resolution, and an excimer laser having a shorter wavelength is drawing attention, and an optical lithography apparatus using a KrF excimer laser (248 nm) has been developed and is in the stage of implementation. However, it is expected that the degree of integration of semiconductor devices will further increase in the near future, which requires a light source capable of drawing a fine pattern with a line width of 0.1 μm or less. As the light source, ArF excimer laser (193 nm) is mainly used.
Cl excimer laser (175nm), F ₂ excimer laser (157n
m) and other high-output vacuum ultraviolet rays with wavelengths of 155 to 195 nm have been considered, and their development has begun. However, since the high-output vacuum ultraviolet ray has a higher output than the ultraviolet ray used in the conventional photolithography apparatus, the optical material irradiated with the ultraviolet ray has a decreased transmittance, an increased refractive index, distortion, and fluorescence. Damage such as generation and generation of microcracks suddenly occurs and the projection lens cannot function. For these reasons, excimer lasers, which are high-power vacuum ultraviolet rays,
There has been a strong demand for the development of highly homogeneous optical materials with little damage to excimer lamps.

[0003]

As an optical material satisfying the above-mentioned demands, the one disclosed in JP-A-6-227827 is known. That is, the optical material disclosed in the above publication is a transparent material obtained by heating a porous quartz glass body formed by depositing and growing fine quartz glass particles obtained by flame hydrolysis of a glass forming raw material on a substrate. In quartz glass, OH in the transparent quartz glass
It is characterized in that the group content is 10 ppm or less, the halogen content is 400 ppm or more, and the hydrogen content is also contained.

Further, as an optical material which meets the above demand, the present inventors have disclosed in Japanese Patent Publication No. 6-48734 that the hydrogen gas concentration is at least 5 × 10 ¹⁶ (molecules / cm ³ ) or more and the OH group concentration is 1 or more.
Optical components for laser light of 00wtppm or more
No. 13 discloses that the hydrogen gas concentration is at least 5 × 10 ¹⁶ (molecul
es / cm ³ ) or more, the OH group concentration is 50 wtppm or more, and the refractive index fluctuation distribution based on the OH group concentration distribution cancels the refractive index fluctuation distribution based on the fictive temperature, and there is substantially no refractive index fluctuation distribution. A glass optical body was proposed.

The above-mentioned conventional silica glass optical materials have been used for excimer lasers and excimer lamps of 195 to 250 nm, and in the case of excimer light of 195 nm or less, for thin members such as photomasks. It was satisfying in some cases.

However, a projection lens made of silica glass is used for drawing a circuit pattern using the excimer light, and such a projection lens has a diameter of 200 mm × thickness.
Since it becomes a large optical element exceeding 30 mm, when using the above conventional silica glass optical material as a projection lens, a non-uniform distribution is likely to occur in the contained hydrogen molecules and OH group concentration,
Under the irradiation of an excimer laser or an excimer lamp, the transmittance and the refractive index change, and the optical characteristics are likely to deteriorate. Further, if the silica glass optical material is contained in the silica glass optical material at a high concentration of more than 100 wtppm in the OH group concentration, the initial transmittance in the vacuum ultraviolet region becomes low, and the durability is likely to deteriorate. That is, the optical material proposed in the above publication has a problem that the initial transmittance in the 155 to 195 nm region is low and the durability is insufficient.

In the optical material disclosed in Japanese Patent Laid-Open No. 6-227827, all halogens are used.
Among halogens, Cl or the like has a serious problem that paramagnetic defects are easily generated by irradiation with ultraviolet rays and the performance of the optical material is deteriorated such as transmittance in a target spectral region. Therefore, as a result of the inventors of the present invention continued diligent research, while making the impurity concentration contained in the optical material more highly pure than the optical material described in the above publication, the OH group concentration and the hydrogen molecule concentration in a specific range, and A synthetic silica glass optical material having high transmittance, high homogeneity, and excellent durability is obtained by selecting especially fluorine from halogens and further containing the elemental fluorine in a certain concentration or more in an axisymmetric concentration distribution. The inventors have found that they can be obtained and completed the present invention.

That is, the present invention provides a silica glass optical material having a high initial transmittance with respect to vacuum ultraviolet rays having a wavelength of 155 to 195 nm, high precision, high durability, and excellent homogeneity, a method for producing the same, and a projection lens. The purpose is to provide.

[0009]

The above-mentioned object can be achieved by the constitution described in any one of the following (1) to (9) of the present invention. (1) Li, Na, and K are 1 wtp each in a silica glass optical material for large-scale projection lenses with a diameter of more than 200 mm, which is used for vacuum ultraviolet lithography with a wavelength of 155 to 195 nm.
pb or less, Ca and Mg are each 0.5 wtppb or less, Cr, Fe, Ni,
Mo and W used as heat-resistant metal elements are each 0.1
Ultra high purity of less than wtppb, 1-10 wtppm of OH group, F
Of 100 to 10,000 wtppm, and H ₂ of 1 × 10 ¹⁷ to 1 × 10 ¹⁹ molecule / cm ³ , F / OH value of 50 to 1000, and dissolved water molecule concentration of 1 × 10 ¹⁷ molecule / cm ³ or less, the F concentration distribution is axially symmetric with respect to the central axis of the cylindrical silica glass optical material, gradually increases or decreases from the central portion of the optical material toward the outer peripheral portion, and is adjacent to each other at an interval of 1 cm. A silica glass optical material characterized in that the difference Δf in fluorine concentration between measurement points is 10 wtppm or less. (2) The silica glass optical material according to (1), wherein the F / OH value is 50 to 100. (3) The F concentration axisymmetric distribution curve is a parabola or an ellipse 2.
The silica glass optical material according to the above (1) or (2), which approximates the following curve. (4) The silica glass optical material according to any one of (1) to (3) above, wherein the F concentration fluctuation width ΔF is within 50 wtppm. (5) The amount of Cl contained is 10 wtppm or less (1)
~ The silica glass optical material according to any one of (4). (6) The silica glass optical material according to any one of (1) to (5) above, wherein the H ₂ concentration fluctuation width ΔH ₂ is within 1 × 10 ¹⁷ molecules / cm ³ . (7) The silica glass optical material according to any one of (1) to (6) above, which has a refractive index fluctuation width Δn of 2 × 10 ⁻⁶ or less. (8) The strain amount is 1 nm / cm or less (1) to (7)
One of silica glass optical material. (9) A projection lens used in vacuum ultraviolet lithography, which uses the silica glass optical material according to any one of (1) to (8).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the silica glass optical material of the present invention, due to the combination of physical properties of ultra-high purity, OH group-containing, fluorine axisymmetric concentration distribution-containing and dissolved hydrogen gas, further excimer laser resistance and excimer lamp resistance can be obtained. With the improvement, homogeneity is achieved, and it is particularly suitable as a material for a large projection lens having a diameter of more than 200 mm.

The reason why the above-mentioned physical property combination is necessary is as follows. For ultra-high purity, by reducing the impurity metal concentration in silica glass, the transmittance in the vacuum ultraviolet region can be improved and the energy absorption during ultraviolet irradiation can be reduced. Li, Na, and K are each less than 1 wtppb, and Ca and Mg are each 0.
5 wtppb or less, and Cr, Fe, Ni, Mo and W are preferably 0.1 wtppb or less. Li, Na, K, Ca, and Mg are contained as impurities in various heat-resistant ceramics such as alumina, and are easily polluted elements during silica glass production.
Fe, Ni, Mo and W are compositions of plant metal structural materials, especially M
o and W are used as heat-resistant metallic elements and are also likely to become polluting elements.

To achieve ultra-high purity, for example, the following means may be used. That is, 2 liquid silicon compound raw materials
The impurity concentration is reduced by repeating the distillation treatment three to three times. The high-purity silicon compound raw material subjected to the distillation treatment is stored in a Teflon resin-lined container to prevent contamination of impurities. Also, when the raw material is used, it is introduced into the synthesizer through a Teflon resin lining pipe. High-purity alumina is used as the heat-resistant material of the synthesizer. Furthermore, the graphite mold used for heat molding is also high-purity graphite. High-purity alumina is also used as the heat-resistant material of the electric furnace to be annealed. Thus, the objective is achieved by making all the devices and jigs highly pure.

The OH group is the terminal end of the glass network structure, and by containing an appropriate amount, the structure is relaxed, and Si-OS
The bond angle of i is brought close to a stable value to reduce the ionization effect by the excimer laser irradiation. However, when the OH group is contained in a high concentration, it causes a decrease in transmittance in the vacuum ultraviolet region.
Therefore, the OH group content is preferably 1 to 10 wtppm.

Similar to the OH group, F becomes the terminal end of the glass network structure, and unlike other halogens, even if F is contained in a high concentration, it does not cause a decrease in the transmittance in the vacuum ultraviolet region.
However, glass that does not contain any OH groups and contains only high concentration of F decomposes by heat treatment, and produces a 7.6 eV (about 163 nm) absorption band due to F ₂ gas generation and oxygen deficiency type generation. Therefore,
It is important to contain OH groups and F at the same time to suppress thermal decomposition of glass and generation of oxygen-deficient defects.

From this point of view, the total amount of OH group and F amount is 1
01 wtppm or more and F / OH 50 to 1000, especially 50 to 1
It is preferable to satisfy 00. In this case, 1-10 OH groups
wtppm, F 100 to 10,000 wtppm, especially 200 to 2,000 wtppm
It is preferably contained.

The optical material of the present invention does not substantially contain halogens other than F. Particularly, in the case of Cl, the vacuum ultraviolet region of the glass, that is, the excimer light, is irradiated by irradiation with an excimer laser or excimer lamp. The content is preferably 10 wtppm or less because it causes a decrease in the transmittance in the wavelength range.

Regarding F, it is preferable that the silica glass optical body has an axisymmetric concentration distribution with respect to the central axis and the F concentration fluctuation is gentle. That is, it is preferable that the F concentration variation curve has no inflection point. This F concentration fluctuation width ΔF
Is preferably within 50 wtppm, and particularly preferably within 30 wtppm. For example, in the case of a cylindrical silica glass body for lenses, so-called lens blanks, it is preferable that the concentration distribution of F viewed from the axial direction is axially symmetric and gradually increases or decreases from the central portion to the outer peripheral portion of the lens blanks. Since F has a function of lowering the refractive index of silica glass, in the present invention, a highly accurate lens blank can be obtained by making the F concentration fluctuation have an axially symmetric distribution. At the same time, as described above, the F concentration fluctuation range Δ
By controlling F within a fixed value, a more uniform lens blank can be obtained. There is no particular lower limit to ΔF, but it is currently about 10 wtppm.

The fluorine concentration fluctuation range ΔF is, for example,
In a cylindrical silica glass optical material with a diameter of 250 mm and a thickness of 50 mm, 25 points of F concentration were measured at 10 mm intervals in the diameter direction when viewed from the axis of rotational symmetry, and the difference between the maximum and minimum values of 25 points of F concentration. Is calculated to obtain the fluctuation range (ΔF) of fluorine concentration in the entire optical material. At this time, 1 cm of the above measurement point
The difference Δf in fluorine concentration between adjacent measurement points is 10 wtp
It is preferably pm / cm or less.

Further, the axisymmetric distribution curve of the above F concentration is
It is preferably a parabolic (see FIG. 1) or elliptic (see FIG. 2) quadratic curve represented by the following equation. y = lx ² + ax: Distance from symmetry axis (mm) y: F concentration (wtppm) l: Constant a: F concentration at symmetry axis position (wtppm) (yb) ² / n + x ² / m = 1 x: Distance from the axis of symmetry (mm) y: F concentration (wtppm) m: 1/2 of the major axis length of the ellipse n: 1/2 of the minor axis length of the ellipse b: F at the axis of symmetry Concentration (wtppm)

Dissolved hydrogen gas, that is, hydrogen molecules in the optical material, is called E'center (referred to as e-prime center and exhibits an absorption band of about 215 nm) and NBOH center (referred to as non-bridging, oxygen, and hole center) by UV irradiation. nm and an absorption band of about 630 nm), and its content is 1 × 10 ¹⁷ to 1 ×.
It is preferably 10 ¹⁹ molecule / cm ³ , particularly 5 × 10 ^{17 to} 5 × 10 ¹⁸ molecule / cm ³ .

Dissolved water molecules, that is, water molecules dissolved in silica glass, undergo a photochemical reaction by irradiation with vacuum ultraviolet rays from an excimer laser or excimer lamp, and are decomposed into hydrogen and oxygen. The generated oxygen absorbs vacuum ultraviolet rays and thus reduces the transmittance of silica glass at 155 to 195 nm. Furthermore, oxygen is converted to ozone by absorption of vacuum ultraviolet rays, and an approximately 260 nm absorption band called the ozone band is generated, which promotes optical damage to the silica glass. The dissolved water molecule concentration is preferably 1 × 10 ¹⁷ molecule / cm ³ or less. There is no particular lower limit, but it is about 1 × 10 ¹⁶ molecule / cm ³ at present.

In the silica glass optical material of the present invention, the refractive index fluctuation width Δn is preferably 2 × 10 ⁻⁶ or less. Within the above range, it can be used for an optically highly homogeneous lens, prism, etc. incorporated in an optical lithography apparatus. Δn is 2 × 10 ^- it is necessary is that the ⁶ following at least one direction striae-free assumption. Further, by setting Δn to be 2 × 10 ⁻⁶ or less, it becomes possible to dope the hydrogen gas at a uniform concentration. It is presumed that the reason for this is that the glass having a large Δn value has a large F concentration fluctuation range and the hydrogen gas concentration is affected by the F concentration. Further, in order to set Δn to 2 × 10 ⁻⁶ or less, the fluorine concentration distribution should be an axisymmetric distribution, the fluctuation range of the concentration should be within 50 wtppm, and the temperature distribution in the silica glass body should be precisely maintained before annealing. Will be required. There is no particular lower limit to Δn, but currently it is 3 × 10
^-7 has been achieved.

In the silica glass optical material of the present invention, the strain amount is preferably 1 nm / cm or less. By setting the strain amount to 1 nm / cm or less, it can be used for an optical highly homogeneous lens incorporated in an optical lithography apparatus. In order to achieve this value, it is necessary to make the fluorine concentration distribution an axisymmetric distribution and to set the concentration fluctuation width ΔF within 50 wtppm, and to anneal the silica glass while maintaining a precise temperature distribution. Becomes Although there is no particular lower limit to the amount of strain, at present, about 0.3 nm / cm has been achieved.

Next, a method for producing the silica glass optical material of the present invention will be described. In producing the silica glass optical material, first, a silicon compound as a raw material is used to synthesize an OH group-containing white soot body having a substantially cylindrical shape by a flame hydrolysis method.

The above silicon compounds include SiCl ₄ , SiHC
l ₃ , SiH ₂ Cl ₂ , SiCH ₃ Cl ₃ , Si (CH ₃ ) ₂ Cl ₂ , Si (CH ₃ ) ₃ (OC
H), Si (CH ₃ ) ₂ (OCH) ₂ , Si (CH ₃ ) (OCH) ₃ , SiF ₄ , SiHF ₃ , S
iH ₂ F _{2 and the} like are preferable. As the flame, an oxyhydrogen flame,
Propane oxygen flame and the like are preferred.

Next, the obtained OH group-containing white soot body is heat-treated in a fluorine-containing gas atmosphere and fluorine-doped to obtain a soot body containing OH groups and fluorine. As fluorine-containing gas, SiF ₄ , CHF ₃ , SF _6, etc. 0.1 to 100 vo
It is preferable to use a gas containing l.%. The processing temperature is preferably 400 to 1200 ° C., and the processing pressure is preferably about 0.1 to 10 kgf / cm ² .

Then, the white soot body is subjected to a transparent vitrification treatment. This treatment is preferably carried out in a reduced pressure atmosphere having a temperature of about 1400 to 1600 ° C. and a pressure of 1 kgf / cm ² or less, particularly 0.1 to 0.01 kgf / cm ² . In particular, by not using a high vacuum atmosphere, the release of fluorine from the outer portion of the material is prevented, and the fluorine concentration gradient in the inner and outer portions of the material is relaxed. The atmosphere may contain He.

Subsequently, flame heating molding is performed to form a round rod-shaped transparent silica glass body, and flame heating is sequentially performed from one end to the other end of the round rod-shaped transparent silica glass body to perform band-melting rotary stirring treatment. It is preferable that this zone-melting rotary stirring treatment is carried out reciprocally, and particularly preferably 2 to 4 reciprocally. These processes are USP2904713, USP3128166, USP312
8169, USP3483613. As a result, the fluorine concentration distribution in the material has a rotational axis symmetrical distribution. The round rod-shaped transparent silica glass body preferably has a diameter of about 30 to 90 mm and a length of about 1 to 3 m.

Next, a vacuum forming furnace 10 provided with a forming frame 14 having a cylindrical forming cavity 12 as shown in FIG. 3 is prepared. The diameter of the molding cavity 12 is 200-4
It is preferable that the height is 00 mm and the height is about 400 to 2000 mm. The mold wall forming this cavity 12 is preferably made of high-purity graphite or the like. The round rod-shaped transparent silica glass body S is placed in the molding cavity 12.
The rotation axis is arranged so as to coincide with the central axis of the cylindrical molding cavity (that is, vertical state), and heating is performed in this state. The heating temperature is about 1700 to 2000 ° C. At this time, the above-mentioned round rod-shaped transparent silica glass body S is not substantially compressed or pressed from above, and is melted and settled only by gravity and waits until it is housed in the molding cavity. At this time, if pressed and forcedly pressed, the shaft of the round rod-shaped transparent silica glass body S may be bent,
Since it is no longer vertical, the fluorine concentration distribution in the material is not symmetrical about the rotation axis. At this time, a thin lid 16 made of graphite or the like may be arranged on the round rod-shaped transparent silica glass body S as shown in FIG.

After this, an annealing process is performed. Atmosphere is generally used as the atmosphere for this treatment, and other inert gas atmospheres can also be used. The treatment temperature is 1000 to 1200 ℃, hold for 1 to 100 hours, then gradually cool down to 880 ℃ or less at 1 ℃ / hr to 5 ℃ / hr, and further to room temperature 5 ℃ / hr to 20 ℃ / hr Slowly cool. By this annealing treatment of a sufficient length, it is possible to sufficiently remove the strain and sufficiently reduce the water content.

Finally, hydrogen gas doping treatment is performed by hydrogen molecule-containing atmosphere heat treatment. As the hydrogen molecule-containing atmosphere, it is preferable to use 100% hydrogen gas or a mixed gas atmosphere of a rare gas such as Ar and hydrogen gas. Processing temperature is 100
It is preferably from 1000 to 1000 ° C, particularly from 200 to 600 ° C. When the temperature is higher than the above temperature range, the reducing action becomes strong and oxygen deficiency type defects are generated, and when the temperature is low, it takes too much time to diffuse and dissolve hydrogen gas in the transparent glass body.

The processing pressure is about 1 kgf / cm ² to 10 atmospheric pressure.
0 kgf / cm ² is preferred. Saturated dissolved concentration of hydrogen gas in transparent glass body at 1 kgf / cm ² at 100% hydrogen gas is about 1 × 10 ¹⁷ 〜
4 × 10 ¹⁷ molecule / cm ³ , 10 kgf / cm ² , 100 kgf / cm ² 1 × 10 ¹⁸ to 4 × 10 ¹⁸ and 1 × 10 ¹⁹ to 4 × 10 ¹⁹ molecule / cm ^{3 respectively}
Is.

By performing the annealing treatment in a hydrogen molecule-containing gas atmosphere, the annealing treatment and the hydrogen gas doping treatment can be simultaneously performed. The obtained material has its outer surface ground to form a lens blank having a desired shape. A desired lens is obtained by further polishing the outer surface.

[0034]

Example First, a substantially columnar OH group-containing white soot body was synthesized by an oxyhydrogen flame hydrolysis method using ultra-high-purity silicon tetrachloride SiCl ₄ as a raw material. Then, the obtained white soot body containing OH groups was subjected to 1 kg / c in a gas atmosphere containing 1% of SiF _4.
The white soot body was fluorine-doped by heat treatment under conditions of m ² (approximately the same as atmospheric pressure) and 600 to 800 ° C. At this time, the heat treatment temperature and the treatment time were variously changed to change the OH group and the F amount in each of the examples and the comparative examples as shown in the table.

Next, the above white soot body is treated with 0.1 kgf / cm ²
Was heated at a temperature of 1500 to 1600 ° C. in a He-containing reduced-pressure atmosphere, and the above white soot body was vitrified into a transparent glass. Subsequently, a round rod-shaped transparent silica glass body having a diameter of 60 mm and a length of 1.5 m is heated by propane gas flame heating, and then both ends of the glass body are grasped, and band melting is performed by twisting while heating with propane gas flame from one end. 1 by performing 2 rounds of rotation stirring processing
It was a transparent glass body free of direction striae. Heating the material
It was performed at about 2000 ° C.

As shown in FIG. 3, the above-mentioned round rod-shaped transparent silica glass body was molded by making its rotation axis overlap with the center axis of the molding cavity of the high-purity graphite molding frame in the vacuum molding furnace. It was placed in a cavity and melted to obtain a glass molded body having a size, a diameter of 270 mm, and a thickness of 70 mm.

Then, the molded body was surface-etched with a hydrofluoric acid solution, and then placed in an electric furnace of a high-purity alumina refractory material and a molybdenum disilicide heater, and was placed in an air atmosphere at 11
After holding at 50 ℃ for 20 hours, slowly cool to 800 ℃ at 2 ℃ / hr,
After that, the electric furnace power was turned off and naturally cooled at 10 ° C / hr to 20 ° C / hr for annealing treatment.

After that, the above-mentioned glass molded body was placed in an electric furnace of a stainless steel jacket, tungsten, and a mesh heater, and under a 100% hydrogen atmosphere, 400 ° C., 1 kgf /
Hydrogen gas doping was performed under a pressure of cm ² or 10 kgf / cm ² . Finally, the outer surface of the glass molding was ground to a diameter of 25
The silica glass lens blanks of the example of a flat cylinder having a thickness of 0 mm and a thickness of 50 mm were used.

On the other hand, a silica glass lens blank of Comparative Example was prepared as follows. In Comparative Example 1, after a white columnar soot body containing an OH group was synthesized under the same conditions as in Example, SiF ₄ 100% gas atmosphere, 1 kgf / cm ² ,
Heat treatment was performed at 900 ° C. to perform OH group-free and F dope. Then, as in the example, transparent vitrification, band-melting rotary stirring treatment, annealing treatment, and hydrogen gas doping treatment were performed.

In Comparative Example 2, a glass in which hydrogen gas was not dissolved was obtained without performing the hydrogen gas doping treatment as compared with the Examples.

Comparative Example 3 is a glass which is not subjected to the band-melting rotary stirring treatment as compared with the Examples.

In Comparative Example 4, F-doping treatment was not performed as compared with the Examples, but instead, Cl ₂ 100% gas atmosphere 1 kgf /
Cl doping was performed at 900 cm ² at cm ² .

In Comparative Example 5, the F doping treatment was not performed as compared with the Examples.

With respect to the samples of the above-mentioned Examples and Comparative Examples, in addition to the above-mentioned OH group concentration, at OH group concentration fluctuation width ΔOH, F concentration and F concentration fluctuation width ΔF, F concentration distribution and at measurement points adjacent to each other, Difference in fluorine concentration per cm of Δf, Cl
The concentration, the dissolved hydrogen concentration and the dissolved hydrogen concentration fluctuation width ΔH _{2, the} refractive index fluctuation width Δn, the strain amount, the H ₂ O concentration, and the transmittance before and after the light irradiation from the laser and the lamp were measured. The results are shown in each table. The fluorine concentration distributions of Examples 1 and 5 and Comparative Examples 1 and 3 are shown in FIGS. Further, Table 3 shows the content of impurities in the glass of the samples of Examples 1 to 5.

The methods for measuring the physical properties of the above Examples and Comparative Examples are as follows. (I) Method for measuring OH group concentration DM DODD and DB FRASER, Optical determination o
f OH in fused silica, Journal of Applied Physics,
Vol. 37 (1966) p. 3911 Measuring methods described in the literature. (Ii) Method for measuring OH group concentration fluctuation width ΔOH In a cylindrical silica glass optical material having a diameter of 250 mm and a thickness of 50 mm, 25 points of OH group concentration are measured at 10 mm intervals in the diameter direction as viewed from the axis of rotational symmetry. From the maximum and minimum OH group concentration at 25 points, the OH group concentration fluctuation range (Δ
OH) is a measurement method for calculating the OH group average concentration from the arithmetic average value of the OH group concentrations at 25 points. (Iii) Measuring method of hydrogen molecule concentration VK KHOTIMCHENKO, et al., Determining the content
of hydrogen dissolved in quartz glass using the m
ethods of Raman scattering and mass spectrometry,
Journal of Applied Spectroscopy, Vo.46, No.6 (198
7) The measuring method described in the literature of pp.632-635. (Iv) Measuring method of hydrogen molecule concentration fluctuation width ΔH _{2 In} a cylindrical silica glass optical material with a diameter of 250 mm and a thickness of 50 mm, measure H ₂ concentration at 25 points at 10 mm intervals in the diameter direction when viewed from the axis of rotational symmetry. . From the maximum and minimum values of H ₂ concentration at 25 points, the fluctuation range of H ₂ concentration in the entire optical material (ΔH ₂ )
Is a measurement method for calculating the H ₂ average concentration from the arithmetic mean value of the H ₂ concentration at 25 points. (V) Method of measuring chlorine concentration A method of measurement by turbidimetric method in which AgNO _{3 is} added after decomposition with HF aqueous solution. (Vi) Method of measuring fluorine concentration A method of measuring by the ion electrode method after decomposing with an aqueous NaOH solution. (Vii) Method of measuring fluorine concentration fluctuation width ΔF In a cylindrical silica glass optical material having a diameter of 250 mm and a thickness of 50 mm, F concentration is measured at 25 points at 10 mm intervals in the diameter direction as viewed from the axis of rotational symmetry. Fluctuation range of fluorine concentration (ΔF) in the entire optical material from the maximum and minimum F concentration at 25 points
, The method of calculating the average concentration value from the arithmetic average value of the F concentration of 25 points. (Viii) Difference in fluorine concentration Δf between adjacent measuring points Δf 25 in the measuring method of fluorine concentration fluctuation width ΔF in (vii) above.
1cm from the measurement point of the point The maximum of the difference Δf (wtppm / cm) of the fluorine concentration per 1cm at the adjacent measurement points. (Ix) Impurity measurement in silica glass Na, K, Mg, Ca, Fe are atomic absorption spectrophotometric methods, L
i, Cr, Ni, Mo, W are measured by plasma mass spectrometry (IC
P-MS method). (X) Measuring method of refractive index fluctuation width (Δn) Measuring method by optical interferometry using He-Ne laser (633 nm) as a light source. However, the value in the 230 mm diameter area is shown. (Xi) Method of measuring birefringence amount (strain amount) A retardation measuring method using a polarizing plate strain meter. However,
The value in the area of 230 mm in diameter is shown. (Xii) Measuring method of transmittance at 193 nm before and after IrF excimer laser irradiation Size 30 × 20 × thickness 10 mm, double-sided mirror-polished sample, wavelength 193 nm, wavelength half-length width 3 nm, pulse life half-width
17 nsec, energy density 30 mJ / cm ² / shot, frequency 100
Immediately after laser irradiation with 1 × 10 ⁶ shots in Hz
A measurement method that measures the transmittance at 193 nm after 1 minute. (Xiii) Method for measuring the transmittance at a wavelength of 172 nm after irradiation with a Xe ₂ excimer lamp Size 30 × 20 × thickness 10 mm, wavelength 172 nm, half wavelength width at half maximum 14 nm, lamp energy density 10 mW / cm for a sample finished by double-sided mirror polishing A measurement method that measures the transmittance at 172 nm 1 minute immediately after irradiation with ² for 14 days. (Xiv) Measurement of molecular concentration Y. Morimoto, et.al., Analysis of gas release from
vitreous silica, Journal of Non-crystalline solid
s, Vol. 1, No. 139 (1992) pp. 35-46 1
Measuring method of water vapor emission under vacuum at 000 ℃.

[Table 1]

[Table 2]

[Table 3]

From the table, the effect of the present invention is clear. That is, the glasses of Examples 1 to 5 showed high transmittance even after the irradiation with excimer light, and the Δn value was 1 × 10 ⁻⁶ or less and showed high homogeneity.

On the other hand, the glass of Comparative Example 1 does not contain an OH group and contains 5000 wtppm of F, so that F is decomposed by various heat treatments to release F ₂ gas, and the 7.6 eV absorption band. And the excimer light resistance was poor.

In Comparative Example 2, since the glass was not subjected to hydrogen gas doping treatment and hydrogen gas was not dissolved, the excimer light resistance was poor and the transmittance was largely reduced.

In Comparative Example 3, since the glass was not subjected to the band-melting rotary stirring treatment, the values of ΔF and ΔH ₂ were larger than the others, and the value of Δn was also a large value. Also, the excimer light resistance changes greatly depending on the glass part,
There was a difference in the decrease in transmittance.

In Comparative Example 4, a glass obtained by not performing F-doping and instead performing Cl-doping in a 100% Cl ₂ gas atmosphere, containing 1000 wtppm of Cl and being F-free, has a 7.6 eV. When an excimer lamp was used, an absorber was formed, and the transmittance of the excimer lamp light was poor from the beginning. On the other hand, when the excimer laser light was irradiated, an E ′ center was generated and the transmittance was drastically reduced.

In Comparative Example 5, OH was not subjected to the F doping treatment.
Since it is a glass containing 200 wtppm of base, the excimer lamp resistance was poor.

[Brief description of drawings]

FIG. 1 is a graph showing a preferable fluorine concentration fluctuation curve (parabola) in the silica glass optical material of the present invention.

FIG. 2 is a graph showing a preferable fluorine concentration fluctuation curve (elliptic curve) in the silica glass optical material of the present invention.

FIG. 3 is a diagram for explaining a columnar forming step of the silica glass optical material in the method for producing a silica glass optical material of the present invention.

FIG. 4 is a graph showing a fluorine concentration fluctuation curve of the silica glass lens blanks of Example 1.

FIG. 5 is a graph showing a fluorine concentration fluctuation curve of a silica glass lens blank of Example 5.

FIG. 6 is a graph showing a fluorine concentration fluctuation curve of a silica glass lens blank of Comparative Example 1.

FIG. 7 is a graph showing a fluorine concentration fluctuation curve of a silica glass lens blank of Comparative Example 3.

[Explanation of symbols]

10: Vacuum forming furnace 11: High-purity graphite heater 12: Molding cavity 14: High-purity graphite molding form 16: High-purity graphite lid S: Round rod-shaped transparent silica glass body

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H01L 21/027 H01L 21/30 515D

Claims (9)

(57) [Claims] 1. A silica glass optical material for use in vacuum ultraviolet lithography having a wavelength of 155 to 195 nm and having a diameter of more than 200 mm, for a projection lens, wherein Li, Na and K are each 1 wtppb or less, and Ca and Mg are each 0.5. wtppb or less, Cr, F
e, Ni, Mo and W used as refractory metal elements
Is ultra high purity of 0.1 wtppb or less for each and 1-10 wt% of OH group
ppm, F 100-10,000 wtppm, and H ₂ 1 × 10 ¹⁷ -1
X10 ¹⁹ molecules / cm ³ contained, F / OH value is 50-1000,
The dissolved water molecule concentration is 1 × 10 ¹⁷ molecules / cm ³ or less, and F
The concentration distribution is axially symmetric with respect to the central axis of the cylindrical silica glass optical material, gradually increases or decreases from the central portion of the optical material toward the outer peripheral portion, and the fluorine between adjacent measurement points at an interval of 1 cm. A silica glass optical material, characterized in that the concentration difference Δf is 10 wtppm or less. 2. The F / OH value is 50 to 100.
Silica glass optical material. 3. The silica glass optical material according to claim 1, wherein the curve of the F concentration axis symmetric distribution is approximate to a parabolic or elliptic quadratic curve. 4. The silica glass optical material according to claim 1, wherein the F concentration fluctuation width ΔF is within 50 wtppm. 5. The silica glass optical material according to claim 1, wherein the content of Cl is 10 wtppm or less. 6. A fluctuation range of H ₂ concentration ΔH ₂ of 1 × 10 ¹⁷ molecule / cm ³
The silica glass optical material according to any one of claims 1 to 5, which is within the range. 7. The silica glass optical material according to claim 1, wherein the refractive index fluctuation width Δn is 2 × 10 ⁻⁶ or less. 8. A strain amount of 1 nm / cm or less.
One of silica glass optical material. 9. A projection lens used in vacuum ultraviolet lithography using the silica glass optical material according to claim 1.