JP2001058839A - Production of synthetic quartz glass for ultraviolet light optics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing synthetic quartz glass which can control the generation of fluorescent light especially on the irradiation of ultraviolet light, by thermally treating the synthetic quartz glass at a specific temperature in an atmosphere having a hydrogen pressure in a specific range for a time not less than a time for reaching a quartz glass structure-determining temperature. SOLUTION: This method for producing synthetic quartz glass for ultraviolet light optics comprises thermally treating quartz glass at a temperature of >=1,400 deg.C for a longer time than a time until reaching a structure-determining temperature, namely longer than the relaxation time, in a hydrogen atmosphere having a low pressure of 1/10 to 1/1,000 atm. The thermal treatment temperature may be >=1,400 deg.C, namely higher than the softening point of ordinary synthetic quartz glass, but is preferably 1,600 to 1,900 deg.C in order to shorten the relaxation time for reaching the structure- determining temperature to stably complete the treatment in a short time. The low pressure hydrogen atmosphere may be not only a pressure-reduced atmosphere comprising only hydrogen but also a hydrogen atmosphere diluted with an inert gas such as Ar, He, Ne, or nitrogen.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing synthetic quartz glass for ultraviolet optics, and more particularly, to a method for producing an optical device member for short wavelength ultraviolet light such as an excimer laser in the field of semiconductor device production. More particularly, the present invention relates to a method for producing synthetic silica glass for ultraviolet optics in which the generation of fluorescence upon irradiation with ultraviolet light is suppressed.

[0002]

2. Description of the Related Art In recent years, in response to the miniaturization of semiconductor elements, KrF (2
Short wavelength ultraviolet rays such as 48 nm) and ArF (193 nm) excimer lasers have been used.
Many synthetic quartz glass members are used as optical members of these devices. Synthetic quartz glass is an excellent material having a high transmittance for such short-wavelength ultraviolet rays, but has been found to be damaged by irradiation with the short-wavelength ultraviolet laser light. As a result of the damage, it emits fluorescence in the visible region having a peak near 650 nm.

[0003] The fluorescence is a serious problem in practical use because it is close to the wavelength of a He-Ne laser beam used for adjusting an optical system. Therefore, reduction of the generation of fluorescence is an essential condition for quartz glass for optical use. The behavior and mechanism of the generation of fluorescence are described in, for example, Physical Review, Vol.
82 (1993) (N. Kuzuu, Y. Komastu, M. Murahara
Authors, 48, 6952-6956 (1994).
(By N.Kuzuu, Y.Matsumoto, M.Murahara), Journal
Volume 68 of the Applied Physics, 3584
To 3591 (1990) (by K. Awazu, H. Kawazoe) and the like. As a cause of the fluorescence, oxygen deficiency caused by excess oxygen inevitably present in glass, so-called,
It has been pointed out that generation of non-bonded oxygen-hole trapping centers (NBOHC) and ozone generation have been pointed out.

It is known that this fluorescence can be reduced by controlling the ratio of oxygen and hydrogen supplied to the oxyhydrogen flame burner used in synthesizing glass. For example, Japanese Patent Application Laid-Open No. 2-64645 discloses that silicon tetrachloride is used. A method has been proposed for reducing the generation of fluorescence by making the ratio (H ₂ / O ₂ ) of the hydrogen gas and the oxygen gas supplied to the burner more hydrogen-excess than the stoichiometric ratio when hydrolyzing oxyhydrogen with an oxyhydrogen flame. I have.
Japanese Patent Application Laid-Open No. 6-199531 discloses that when silica tetrachloride is hydrolyzed in an oxyhydrogen flame in which the oxygen / hydrogen ratio exceeds the stoichiometric amount of hydrogen to synthesize quartz glass, an inert gas is used. By adjusting the reaction conditions of the burner containing the gas and the exhaust conditions of the exhaust gas, the silanol group (S
There has been proposed a synthetic quartz glass for optics in which the above-mentioned generation of fluorescence is suppressed when excimer laser light is irradiated with an —OH group concentration such as iOH) of 1000 ppm or more.

[0005]

However, the oxygen / hydrogen supply ratio in an oxyhydrogen flame at the time of glass synthesis depends on the quartz synthesis speed, the concentration of -OH groups remaining in the glass, the light transmittance, etc. Because it affects many characteristics that are required in the field, finding the optimal conditions while considering these effects requires a great deal of work and delicate skill, and the range of selection is limited. Is done. On the other hand, a method has been already proposed in which glass is treated in a high-concentration hydrogen atmosphere of 1300 ° C. or less to react hydrogen with excess oxygen to inactivate the glass (Japanese Patent Application Laid-Open No. Hei 1-2201664).
No.).

However, this treatment method is unstable, and requires high concentration of hydrogen, ie, high pressure (partial pressure). Cost, time, labor and cost. Thus, even in view of the situation, if it is possible to manufacture and provide a synthetic quartz glass for optics in which the above-mentioned fluorescence is suppressed during irradiation with short-wavelength ultraviolet rays such as an excimer laser by a simple process and provide it, it will have an impact on the industry. The benefits are great.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is a synthetic quartz glass used as an optical member of an exposure machine for manufacturing a semiconductor such as a stepper, which emits short wavelength ultraviolet light such as an excimer laser. When irradiated, quartz glass for ultraviolet optics in which visible light fluorescence near 650 nm, which is usually generated by light irradiation damage, is significantly reduced,
It is an object of the present invention to provide a method for processing and manufacturing in a simple and safe manner.

[0008]

According to the present invention, the structure of synthetic quartz glass is determined at a temperature of 1400 ° C. or more in an atmosphere having a hydrogen pressure of 1/10 to 1/1000 atm. Time to reach temperature (relaxation time)
There is provided a method for producing synthetic silica glass for ultraviolet optics, wherein the method is heat-treated for the above time.

Further, according to the present invention, as a preferred embodiment of the above-mentioned manufacturing method, there is provided a method of manufacturing quartz glass for ultraviolet optics, wherein the atmosphere comprises a mixed gas of an inert gas and hydrogen. Is done. Further, according to the present invention,
A method for producing a synthetic quartz glass for ultraviolet optics, wherein the inert gas is at least one selected from argon, helium, neon and nitrogen.

The method for producing a synthetic quartz glass for ultraviolet optics of the present invention is characterized in that the quartz glass is produced at a temperature of 1400 ° C. or more and in the above-mentioned controlled low-pressure hydrogen atmosphere at the time of producing the glass. It is said that the heat treatment is performed for a period of time equal to or longer than the time required to reach the structure determination temperature, that is, the relaxation time or longer.
Heat treatment under specific conditions is a structural feature.

The temperature for determining the structure of glass is a factor introduced as a parameter indicating the structural stability of quartz glass, as described below. The density fluctuation of quartz glass at room temperature, that is, structural stability, is determined by the density of quartz glass in a molten state at high temperature, the density when the structure is frozen near the glass transition point in the cooling process, and the structure. You. That is, the density, the thermodynamic density corresponding to the temperature at which the structure was frozen, and the structure are preserved even at room temperature. The temperature at which the density and structure are frozen is called the structure determination temperature.

By performing the heat treatment under the above-mentioned specific conditions of the present invention during the production of quartz glass, the synthetic quartz glass for optics produced by an ordinary method is produced at the time of irradiation with ultraviolet rays.
m can effectively suppress the visible region fluorescence.

The fluorescence generated from the quartz glass upon irradiation with ultraviolet light such as excimer laser light is red, and it is considered that excess oxygen in the glass is involved as described in, for example, the above-mentioned known documents. It is considered that the reaction of generating the fluorescent center is, for example, as follows. {Si—O—O—Si} + ultraviolet light → 2 {Si—O · (NBOHC) (1) On the other hand, when the quartz glass having such excess oxygen is hydrogen-treated, ₂ ← → {Si—OH + HO—Si} (2) The excess oxygen is changed into an inactive —OH group, and the generation of fluorescence is suppressed. In order for this reaction to proceed stably at a practical rate, a microscopic geometrical shape such as a bond angle of Si—O and a size occupied by a molecule, that is, a molecular structural change must occur. It is thought that there is. Therefore, in order for the above equation (2) to proceed to the right, it is necessary to rearrange the entire molecular group around the excess oxygen and stabilize the structure. Otherwise, the reaction product becomes unstable, and the reverse reaction in which equation (2) proceeds to the left also becomes dominant, so that the reaction equilibrium is approached, and the above inactivation of excess oxygen cannot be sufficiently performed. In order to cause the reaction of the formula (2) to proceed to the right as much as possible, a treatment with high-concentration hydrogen is required. Moreover, if the dissolved oxygen is reduced by the subsequent heat treatment,
The reaction proceeds further to the left, approaching the state before the treatment.

Therefore, the rearrangement of the whole molecule around the excess oxygen and the stabilization of the structure in the reaction become possible within a very short time, that is, at a glass softening temperature of 1400 ° C. or more, the structure determination temperature is increased. As a result of the heat treatment until the temperature reaches the relaxation time (relaxation time), it was found that the reaction proceeded stably even in an atmosphere with an extremely low hydrogen concentration and the fluorescence was reduced. If the heat treatment temperature is lower than the strain point of the glass, the relaxation time becomes infinite. Therefore, even if the hydrogen treatment is performed for a long time, it is naturally confirmed by experiments that the above effects cannot be expected at a low hydrogen concentration. Was done. Therefore, the heat treatment temperature in the method of the present invention is at least the softening temperature of quartz glass, that is, 1400 ° C. or more, and preferably 1600 ° C. or more.
° C or higher. The higher the heat treatment temperature, the better from the viewpoint of the reaction rate.
If the temperature exceeds 100 ° C., other inconveniences such as evaporation of SiO ₂ and reaction with the heat treatment container material occur, so that heat treatment at a temperature of 1900 ° C. or less is preferable.

According to the method of the present invention, a stable effect can be obtained rather than the conventional method in which the reaction is performed at a temperature of about 1150 ° C. using high pressure or high concentration of hydrogen. Since there is no need to handle hydrogen, there is no danger of explosion and the like, handling is extremely easy, and there is a large time and labor saving effect.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail and specifically. In the manufacturing method of the present invention, the synthetic quartz glass to be treated is typically a so-called direct synthetic quartz glass obtained by hydrolyzing silicon tetrachloride with an oxyhydrogen flame. However, a sufficient effect can be obtained by applying the present invention to quartz glass of another synthetic method such as VAD synthetic quartz glass.

In the method of the present invention, quartz glass is
At temperatures above 00 ° C., longer than the time to reach its structure determination temperature, ie longer than its relaxation time, 1/1
Heat treatment is performed in a low pressure (partial pressure) hydrogen atmosphere of 0 to 1/1000 atm. The heat treatment temperature is 1400 ° C. or higher, that is, a temperature higher than the softening point of ordinary synthetic quartz glass can sufficiently achieve the object, but the relaxation time for reaching the structure determination temperature is shortened as much as possible, In order to complete the treatment in a short time and stably, it is preferably carried out at 1600 ° C. or higher. If the temperature exceeds 1900 ° C., inconveniences such as evaporation of quartz and reaction with the container material occur, so that the heat treatment temperature is preferably set to 1900 ° C. or less.

The relationship between the heat treatment temperature and the relaxation time of quartz glass is described, for example, by W. Primak,
Physics and Chemistry of Glass (Ph
ysics and Chemistry of Glass), 24 vol. 1,8 ~
18 (1983), for example, 1400
At about ° C, it is about 1 minute. Although not specifically described in the above materials, at 1600 ° C., about 0.1
Minutes.

However, the time required for the reaction between hydrogen and excess oxygen slightly varies depending on the size of the glass to be treated and the pressure (partial pressure) of hydrogen. In order to complete the reaction, it is preferable to allow some time for the heat treatment time. From this viewpoint, when the hydrogen pressure (partial pressure) is 1/10 to 1/1000 atm at the heat treatment temperature of 1400 to 1600 ° C., the relaxation time If the temperature is 1600 ° C or more, the hydrogen pressure is 5
/ 10 to 5/1000 atm, relaxation time to about 5 minutes, 5
At a low concentration of less than / 1000 atm, the relaxation time is preferably maintained for about 30 minutes. In the heat treatment at a high temperature (1600 ° C. or higher) at which the glass is completely softened and fluidized, the shape can be maintained by using a graphite container or the like, so that quartz glass can be formed at the same time, which is convenient.

In the method of the present invention, the hydrogen gas pressure is 1
The pressure must be at least / 1000 atm. At lower pressures, the effect decreases rapidly. Such a hydrogen pressure (concentration) is not limited to a reduced-pressure hydrogen atmosphere, that is, an atmosphere containing only hydrogen gas, but also includes argon, helium, neon, krypton, and the like.
It can be sufficiently achieved even in a hydrogen atmosphere diluted with an inert gas such as xenon and nitrogen. When using a mixed atmosphere with an inert gas, the hydrogen partial pressure of the atmosphere gas may be set to a predetermined value, and the total pressure does not affect the fluorescence generation preventing effect of the quartz glass at all. There is an advantage that it is effective in suppressing evaporation of quartz glass. However, if the total pressure exceeds 1 atm (10 ⁵ Pa), a pressure-resistant furnace is required,
The pressure is preferably 1 atm or less from the viewpoints of furnace construction cost and simplicity of operation.

[0021]

EXAMPLES Examples 1 to 9 and Comparative Examples 1 to 5 1 kg of a synthetic quartz glass ingot obtained by hydrolyzing silicon tetrachloride with an oxyhydrogen flame was used as a sample. This sample was placed in a circular graphite container having an inner diameter of 120 mm and a height of a rim of 50 mm, and placed in an electric furnace equipped with a carbon heating element and capable of heating in an inert and reducing atmosphere from vacuum to 1 atm and heat-treated.
The total pressure of the atmosphere was 1 atm, and a mixture gas of Ar and hydrogen was used at the ratio shown in Table 1. In addition, the heat treatment temperature and time were each set under the conditions described in Table 1. The sample after the heat treatment was cut into a prism of 12 × 12 × 50 mm, and the surface was optically polished. The optically polished sample using an ArF excimer laser, after 10 ⁵ shots at an energy density of 100 mJ / cm ^2, using a fluorescence spectrophotometer (the Hitachi Ltd. 650-10 type), 25
The intensity of fluorescence at 640 nm under excitation light of 5 nm was measured. On the other hand, for a glass sample that was not heat-treated, the fluorescence intensity after laser irradiation was measured by the same method, and the value was set to 1, and the effect was evaluated using the ratio to the fluorescence intensity of the heat-treated sample. Shown in No fluorescence was observed in the sample without laser irradiation.

[0022]

[Table 1]

In Comparative Examples 1 and 2 shown in Table 1, it was recognized that the processing time was insufficient, and strong fluorescence was emitted. In Comparative Examples 3 and 4, it was recognized that high-pressure and long-time treatment was required, though no strong fluorescence was emitted.

Further, as can be seen from Table 1, at a temperature of 1400 ° C. or more, the treatment was carried out in an atmosphere containing 0.001 to 0.1 atm of hydrogen for a time sufficient to reach the structure determination temperature. In the glass, no fluorescence of 640 nm was detected (Examples 1 to 8). Alternatively, it was hardly detected (Example 9). However, as Comparative Example 5, the glass heat-treated in an atmosphere of pure argon containing no hydrogen was:
Three times the fluorescence of the untreated product was generated.

Example 10 The same quartz glass samples as in Examples 1 to 9 were heat-treated in the electric furnace used in Examples 1 to 9. The heat treatment was performed at 1700 ° C. for 15 minutes, and the pressure of atmospheric hydrogen was controlled by a method of reducing the pressure with a vacuum pump while flowing hydrogen into the furnace. The obtained glass was treated in the same manner as in Examples 1 to 9, and the fluorescence after laser irradiation was examined. When the hydrogen pressure is 1, 0.1, 0.01, 0.
Under the processing conditions up to 002 and 0.001 (1/1000) atm, no fluorescence was observed. However, 2
When treated under a reduced pressure of / 10,000 atmospheres, 60% of the fluorescence of the untreated sample was observed.

[0026]

As described above, according to the present invention, the generation of red fluorescence due to the irradiation of ultraviolet rays can be stably removed, and the pressure of hydrogen used can be extremely small, so that there is no danger of explosion or fire. , The equipment required for safety measures is simplified, the cost is low, and the processing time is short,
Further, the operation can be processed very simply. Further, according to the present invention, since the shaping of the glass shape and the fluorescence removing operation can be performed at the same time, the practical advantage is extremely large.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Naoki Tsuji 30 Soya, Hadano-shi, Kanagawa F-term in Hadano Plant, Toshiba Ceramics Co., Ltd. 4G014 AH00

Claims (3)

[Claims] 1. A synthetic quartz glass having a hydrogen pressure of 1/10
A heat treatment at a temperature of 1400 ° C. or more in an atmosphere of 1 to 1000 atmospheres for a time longer than a time required to reach the structure determining temperature of the quartz glass (relaxation time) or more. A method for producing synthetic quartz glass. 2. The method according to claim 1, wherein the atmosphere comprises a mixed gas of an inert gas and hydrogen. 3. The method according to claim 2, wherein the inert gas is at least one selected from argon, helium, neon and nitrogen.