JP2002087833A - Optical quartz glass for uv and process for producing the same glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide optical quartz glass for UV which shows excellent initial transmittance and smaller transmittance reduction by irradiation with respect to an excimer laser beam such as that of KrF or ArF and also to provide a process for producing the optical quartz glass. SOLUTION: This optical quartz glass has a 10-30 ppm OH group concentration expressed in terms of a mass ratio, a <=10 ppm difference between the maximum and minimum OH group concentrations within the glass, a <=0.10 ppm/mm concentration gradient, a >=1018 mol/cm3 hydrogen content and a refractive index of 1.50860 at 248.25 nm wavelength. The production process comprises steps of preparing a synthetic quartz glass material by subjecting a silica porous body to OH group removal treatment at 1,100-1,500 deg.C under reduced pressure and thereafter subjecting the treated material to sintering to make a material transparent, subjecting the prepared synthetic quartz glass material to compression at 1,500-1,700 deg.C so as to provide a >=40% degree of compression, subjecting, after completion of the compression, the compressed material to strain removal treatment that comprises cooling the compressed material to <=600 deg.C at a 1-10 deg.C/hr cooling rate, and subsequently subjecting the cooled material to hydrogen-doping treatment in a hydrogen atmosphere under a <=1 MPa pressure at 400-1,000 deg.C for 10-200 hr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent synthetic quartz glass used for an optical device utilizing a high-power laser beam in the ultraviolet region such as an excimer laser beam and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, with the demand for miniaturization and high-density of semiconductor elements, circuit patterns on wafers have become ultrafine, and the light used in photolithography has a shorter wavelength from ultraviolet to vacuum ultraviolet. Light is being used. Lenses, prisms for ultraviolet light,
As an optical material such as a window, an etalon plate, or a lithography mask for LSI manufacture, quartz glass having excellent light transmittance in this wavelength region has been conventionally applied. If quartz glass contains a large amount of impurities, it absorbs light of a specific wavelength or emits fluorescent light. For this purpose, high-purity synthetic quartz glass is used.

However, the light used shifts to the shorter wavelength side and has a high energy density of KrF (wavelength: 248 nm).
When m) or ArF (wavelength: 193 nm) excimer laser light is applied, the synthetic quartz glass is also damaged, the transmittance is reduced, and the service life is shortened. This is because the bond between silicon and oxygen constituting the glass is cut or cut and recombined at another position, thereby damaging the glass structure itself.

[0004] Various measures have been reported to reduce the transmittance of quartz glass with repeated passage of time due to short-wavelength ultraviolet light having a strong ionizing action. It may be controlled to an appropriate amount and may contain hydrogen. OH groups and hydrogen have the effect of repairing damaged parts and reduce the decrease in transmittance.

[0005] For example, in the invention disclosed in Japanese Patent Publication No. 6-53593, quartz glass having an OH group content of 100 ppm or more and a hydrogen content of 5 × 10 ¹⁶ mol is used as a quartz glass used for a high-output laser beam having a wavelength of 250 nm or less. / cm ³ or more. This publication also regulates that there is no striae and that all of the impurities such as alkali metal elements, alkaline earth metal elements and transition metal elements are at most 50 ppb or less.

[0006] Each metallic element of impurities must be kept as small as possible because it reduces the light transmittance in the ultraviolet region, and tetrachloride, which is a raw material for producing synthetic quartz glass by an oxyhydrogen flame hydrolysis method. It can be reduced by sufficiently purifying a silicon compound such as silicon. Also, striae are not preferable because they increase the fluctuation of the refractive index, and the above publication states that striae is removed because a uniform concentration distribution cannot be obtained during hydrogen gas doping treatment for increasing the hydrogen content. Can be removed by the floating zone melting method.

As described above, the presence of the OH group has an effect of suppressing a decrease in transmittance due to irradiation with ultraviolet light having a short wavelength. However, as the concentration increases, the heat resistance decreases, the concentration distribution tends to be non-uniform, and striae also occur accordingly. In addition, it has the effect of shifting the lower limit wavelength at which ultraviolet light does not pass, that is, the absorption edge to the longer wavelength side, and lowers the initial transmittance of short wavelength ultraviolet light such as ArF excimer laser light. Therefore, it is necessary to select the OH group concentration range depending on the purpose.

The main methods for producing synthetic glass by oxygen-hydrogen flame hydrolysis of a silicon compound include a direct method of synthesizing and melting SiO _{2 in} a flame to obtain a dense and transparent quartz glass, and a decomposition reaction. and once deposited a SiO ₂ particles make a porous body, which heat sintered soot method of clearing densified (or VAD-Vapor-phase Axial
Deposition-method). The OH group concentration can be controlled to some extent by changing the composition of the oxyhydrogen flame and the supply ratio of the silicon compound thereto. In the case of the direct method, the concentration tends to be high, but in the case of the VAD method, the sintering of the obtained porous body can be changed variously by means such as further reducing the pressure.

When the porous body is heat-treated under reduced pressure by the VAD method, the OH group concentration can be greatly reduced. In this case, however, a part of the bond to become Si-O-Si becomes a Si-Si bond. When this bond increases, the absorption peak having a center near 163 nm increases, and the absorption edge shifts to the longer wavelength side. On the other hand, for example, Japanese Patent Application Laid-Open No. Hei 8-91867 discloses an invention in which heating is performed in an oxygen-containing atmosphere prior to heat treatment under reduced pressure to reduce the OH group concentration. In this case, the porous body is heat-treated at 1000 to 1300 ° C. in an atmosphere containing oxygen of 10 vol% or more, and then at 1100 to 1400 ° C. under reduced pressure.
The company claims to perform de-OH treatment for 10 hours or more, and then perform sintering for transparency. In this way, the absorption edge is expanded to the shorter wavelength side to improve the initial transmittance for ultraviolet light having a wavelength longer than 155 nm, but it is not clear how the transmittance is reduced by irradiation with ultraviolet light.

[0010]

The quartz glass for optics has a transparency or an initial transmittance as high as possible with respect to the ultraviolet rays to be applied, and has little damage due to the transmission or irradiation of the light. The decline must be kept to a minimum. An object of the present invention is to provide a synthetic quartz glass having a high initial transmittance with respect to excimer laser beams such as ArF having a wavelength of 193 nm and KrF having a wavelength of 248 nm, and having a small decrease in transmittance due to use, and a method for producing the same.

[0011]

Means for Solving the Problems The inventors of the present invention have made a synthetic quartz glass mainly intended for excimer laser light having a wavelength of 248 nm, which has a high initial transmittance and a very small decrease in transmittance with an increase in the irradiation amount. Various production methods for obtaining the same were studied.

First, it was considered that the lower the OH group concentration, the higher the initial transmittance of ultraviolet light having a short wavelength. Thus, the porous body was degassed at 1300 ° C. for 5 hours under a reduced pressure inert atmosphere of 5 × 10 ⁴ Pa. After performing the OH group treatment, transparent sintering was performed in a vacuum. As a result, it was found that the OH group concentration was 30 ppm or less, the absorption near 163 nm was eliminated, and the initial transmittance of short-wavelength ultraviolet rays could be sufficiently improved.

However, when investigating such synthetic quartz glass, it was found that the initial transmittance was not always good,
Some were found to have a large decrease in transmittance due to ultraviolet irradiation. As a result of further investigation on quartz glass with poor properties, it was found that such inferior glass has a large difference depending on the location of the OH group concentration and a high refractive index. Met.

It is not always clear why the distribution of the OH group concentration and the refractive index affect the transmittance of ultraviolet rays and the deterioration due to irradiation. However, it is considered that the production conditions under which the distribution of the OH group concentration becomes uniform and the refractive index becomes low have produced favorable results. Therefore, various studies have been carried out using these two as guidelines for selecting the production conditions.

In the case of the VAD method, the concentration of the central portion along the axial direction of the soot body formed in a columnar shape is high and the concentration of the peripheral portion is low when the deOH group treatment is performed.
That is, the concentration distribution on the cross section perpendicular to the axis has a large difference in elevation. In addition to the difference in density between the central part and the peripheral part, a part with a high density or a part with a low density may be found locally. If the distance between the high density part and the low density part is known, a density gradient can be obtained.

Examination of the relationship between the magnitude of such a concentration gradient and the deterioration due to irradiation revealed that the smaller the concentration gradient, the less the deterioration. Such a large concentration gradient appears in most cases in a direction from the center to the periphery. Therefore, in order to reduce such a concentration gradient as much as possible, when the temperature, time, or atmospheric pressure of the deOH group treatment is changed, the OH group concentration itself may be too low, or it may take a long time to remove the OH group. However, the concentration distribution could not always be stably uniformed.

However, in the concentration distribution in a direction perpendicular to the axis in a cross section parallel to the axis, the difference between the highest value and the lowest value is relatively small. Then, next, compression processing was performed by using a quartz glass column-shaped preform material that had been subjected to transparent sintering, in a direction perpendicular to the axial direction. The density value such as the maximum value and the minimum value cannot be changed by compressive deformation, but the density gradient in a cross section perpendicular to the axis can be reduced.

Further, in the cooling process after the compression working, as a result of slow cooling for the purpose of removing distortion, reduction in transmittance decrease due to irradiation which is much larger than expected due to reduction in concentration gradient, and refraction It was found that a reduction in the rate and homogenization were obtained.

Normally, the refractive index of quartz glass produced by the VAD method with ultraviolet light having a wavelength of 248 nm is about 1.50870 to 1.5890. On the other hand, the refractive index was significantly reduced by lowering the OH group concentration, adding compression, and then gradually cooling. It is not clear why the decrease in the refractive index has an effect, but the reduction in the transmittance due to laser irradiation tends to decrease as the refractive index decreases. It is said that the refractive index decreases when the virtual temperature decreases, for example, and it is considered that the reduction in the virtual temperature increases the stabilization of the quartz glass structure.
It is speculated that the reduction of the H-group concentration and the slow cooling for hot working and subsequent strain relief may have improved the glass structure of quartz.

However, a decrease in the OH group concentration may still cause some instability in suppressing a decrease in transmittance due to irradiation. This could be compensated for by making the hydrogen concentration sufficiently high.

As described above, it was possible to obtain a synthetic quartz having an initial transmittance as high as possible, less damage due to the irradiation of the light, and a decrease in transmittance due to long-term use being suppressed as much as possible. Therefore, we further clarified the range of characteristics of this synthetic quartz glass and the limits of its manufacturing conditions,
The present invention has been completed. The gist of the present invention is as follows.

(1) The concentration of OH groups is 10 to 30 ppm by mass,
The difference between the highest concentration and the lowest concentration is 10 ppm or less, and the concentration gradient is 0.1
0 ppm / mm or less, hydrogen is 10 ¹⁸ mol / cm ³ or more, and wavelength 248.
An optical silica glass for ultraviolet light, having a refractive index of 1.50860 or less for 25 nm light.

(2) A porous body composed of SiO ₂ fine particles obtained by hydrolysis of a silicon compound in an oxyhydrogen flame is subjected to
A synthetic quartz glass material that has been dehydrated at 100 to 1500 ° C and then clarified and sintered at a temperature of 1500 to 1700 ° C
% Of compression processing, and 1-10
Perform a strain relief process of cooling to a temperature of 600 ° C or less at a cooling rate of ° C / hr, and then perform a hydrogen doping process for 10 to 200 hours in a temperature range of 400 to 1000 ° C in a hydrogen atmosphere of 1 MPa or less. (1) The method for producing an optical quartz glass for ultraviolet light according to the above (1).

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The concentration of OH groups in the synthetic quartz glass of the present invention is 10 to 30 ppm by mass ratio, and the difference between the highest concentration and the lowest concentration in a quartz glass block as one component is different. 10 ppm or less, concentration gradient existing in the block is 0.10 ppm
/ mm or less. The reason why the OH group concentration is set to 10 to 30 ppm is that if the OH group concentration exceeds 30 ppm, the UV absorption edge of the synthetic quartz glass shifts to the longer wavelength side, and the wavelength 193 nm
This is because the initial transmittance also decreases with the ArF excimer laser light. Further, when the OH group concentration is less than 10 ppm, a large number of oxygen deficiency defects in which the oxygen of the Si—O—Si bond has been released are generated, and the decrease in transmittance due to irradiation with ultraviolet rays is large.

In the quartz glass block which is one component of the optical member to be used, the difference between the maximum and minimum OH group concentrations is 10 ppm or less, and the concentration gradient between adjacent portions having different concentrations is 0.10 ppm / The reason why the thickness is not more than mm is that in quartz glass having a concentration difference or a concentration gradient exceeding this range, the transmittance is greatly reduced due to the irradiation of a strong short-wavelength excimer laser beam.

The hydrogen content in the synthetic quartz glass is 10 ¹⁸ mol /
cm ³ or more. This is because, if the hydrogen content is less than 10 ¹⁸ mol / cm ³ , the transmittance may be significantly reduced by laser light irradiation. Since the OH group concentration is made as low as possible, it is presumed that the effect of suppressing the decrease in transmittance due to the OH group is compensated for by increasing the hydrogen content. Although the upper limit of the hydrogen content is not particularly limited, the doping from a normal atmosphere has a limit of about 10 ²⁰ mol / cm ³ .

The alkali metal elements of impurities (Li, Na,
K), the content of the alkaline earth metal elements (Mg, Ca) and the content of the transition metal elements (Ti, Cr, Fe, Ni, Cu) are each desirably 50 ppb or less by mass. Each of these elements has an effect of increasing the refractive index, and tends to promote a decrease in transmittance due to laser beam irradiation. Such effects can be almost ignored when the content is 50 ppb or less.

The refractive index of the synthetic quartz glass by ultraviolet rays having a wavelength of 248 nm is assumed to be 1.50860 or less. This is because in the case of glass having a refractive index of more than 1.50860, a decrease in transmittance due to laser beam irradiation is large, which is not preferable. Since the lower the refractive index is, the lower the transmittance decrease due to laser beam irradiation is, the lower limit is not particularly limited. However, even if various reductions are performed by hot working or heat treatment, 1.50830
It is only low to the extent.

In the hydrolysis reaction of the high-purity gas by the oxyhydrogen flame, impurities such as alkali metal elements, alkaline earth metal elements and transition metal elements are not mixed.
In addition, since these impurities are not reduced, it is necessary to use a sufficiently purified silicon compound raw material to reduce these impurities in synthetic quartz glass.

In order to adjust the OH group concentration to 10 to 30 ppm, the porous body is heated to 1100 to 1500 ° C. under reduced pressure to carry out a deOH group treatment. This is because it is not easy to lower the temperature to 30 ppm or less at a temperature lower than 1100 ° C., and vitrification proceeds at a temperature higher than 1500 ° C., and the temperature may be lower than 10 ppm. The atmosphere when the pressure is reduced is preferably a rare gas such as helium or argon, or an inert gas such as nitrogen to suppress the contamination of impurities.
It is desirable to set the pressure to about × 10 ⁴ Pa. This is 7 × 10
^At a pressure exceeding ⁴ Pa, the reduction of the OH group concentration is not sufficient, and at a pressure of less than 2.5 × 10 ⁴ Pa, the concentration distribution is poor,
This is because the H group concentration tends to be too low.

If the deOH group treatment only lowers the average concentration in glass, the pressure can be reduced and the treatment can be performed in a short time. However, in order to make the OH group concentration distribution after the treatment as uniform as possible, it is preferable to carry out the treatment slowly over a long time without greatly reducing the atmospheric pressure as described above. That is, in order to make the difference between the highest concentration and the lowest concentration 10 ppm or less, and the concentration gradient in the glass to 0.10 ppm / mm or less, the deOH group treatment is 5 to It should be done in about 100 hours. This is because the above-mentioned concentration distribution cannot be obtained in less than 5 hours, and the treatment exceeding 100 hours not only lowers the productivity. The OH group concentration may be too low. Preferably 10 to 50 hours
The treatment temperature and the atmospheric pressure of the deOH group are appropriately selected within the above range so as to be 10 to 30 ppm.

Transparency can be achieved by sintering at 1550 ° C. or higher, which is usually performed. The columnar preform material after transparency is
Perform hot compression processing of 40% or more in the axial direction. The processing temperature in that case is 1500 to 1700 ° C.

By setting the working degree of the compression working to 40% or more, the concentration gradient in the concentration distribution of OH groups in the quartz glass can be finally reduced to 0.10 ppm / mm or less. A decrease in transmittance due to irradiation with ultraviolet light of a wavelength is reduced. This compression processing has the effect of lowering the refractive index to 1.50860 or less as well as lowering the concentration gradient simply by geometric deformation, and the lowering of the refractive index strengthens the tendency of suppressing the decrease in transmittance due to laser beam irradiation. When the working ratio of compression is less than 40%, such an effect is insufficient. The working ratio is not particularly limited as long as it is 40% or more, but is preferably up to about 80% as a range in which implementation is not difficult. If the processing temperature is 1500 ° C or lower, deformation is not easy, and if it exceeds 1700 ° C, fluidization starts and processing becomes difficult.

The working ratio of compression is a value obtained by dividing the height reduced by compression by the height before compression in%, and can be expressed as the rate of increase of the cross-sectional area in the direction perpendicular to the compression direction. Is the same.

From the high temperature state at the end of the compression working, 600
Slowly cool to 1 ° C at a cooling rate of 1 to 10 ° C / hr. this is,
This is in order to sufficiently remove the distortion caused by the compression processing.
If cooled faster than 10 ° C / hr, the strain may not be sufficiently removed and the strain may be re-introduced depending on the shape.
On the other hand, even if the cooling is performed slowly at a rate of less than 1 ° C./hr, no further effect can be obtained only by increasing the cooling time. Also
After reaching a temperature below 600 ° C., the cooling rate has no effect on the strain relief.

The hydrogen content in the synthetic quartz glass is 10 ¹⁸ mol /
cm ³ or more, but to contain hydrogen, almost 1 at normal pressure
Heating at 400 to 1000 ° C. for 10 hours or more in a hydrogen atmosphere of 00% is required. If the heating temperature is less than 400, it takes too much time to reach the target concentration, which is not practical. If the temperature exceeds 1000 ° C., the solubility of hydrogen in quartz glass decreases, so that the hydrogen content cannot be increased. Therefore, the temperature is set in the range of 400 to 1000C. Even if the heating time is long, it is difficult to expect a further increase in the content, so that the heating time is up to about 200 hours. In addition, if the pressure is not normal pressure but a pressurized hydrogen atmosphere, the time for inclusion can be shortened.
It is better to be Pa or less. This is because the processing time can be shortened as the pressure is increased, but if the pressure exceeds 1 MPa, much cost is required for strengthening the structure of the apparatus and ensuring durability, and the practicality is poor.

[0037]

EXAMPLE A high-purity silicon tetrachloride material was used as a raw material, and a porous SiO ₂ particle was obtained by hydrolysis in an oxygen / hydrogen flame. After subjecting the porous body to dehydration treatment under different conditions, it was subjected to a transparency treatment at 1550 ° C. in a He atmosphere of less than 1 Pa to obtain a preform material of a cylindrical synthetic quartz glass having a diameter of 200 mm.

The preform material was heated to perform compression working in the direction of the center axis of the cylinder, and after working, was cooled from the temperature to 600 ° C. by controlling the cooling rate, followed by hydrogen doping. The temperature, pressure,
Table 1 summarizes the time, the temperature of the compression processing, the degree of processing, the cooling rate after the compression processing, the hydrogen doping conditions, and the like.

[0039]

[Table 1]

A specimen was cut out from the obtained quartz glass material in a cross section parallel to the compression axis in a wafer shape, and the OH group concentration at each position of the cross section was measured by an infrared absorption method. Regarding the OH group concentration, no local uneven distribution was observed in any case, and the OH group concentration was highest in the central portion and lowest in the peripheral portion. Therefore, the OH group concentration at three or four positions was measured on the line from the center to the periphery except for the outermost edge where the product could not be collected, and the maximum OH concentration gradient was determined.

The concentration of hydrogen molecules was determined by laser Raman scattering measurement. In addition, the refractive index is 24
It was measured by the minimum declination method using 8 nm KrF excimer laser light, and the average value was obtained. For the measurement of deterioration due to ultraviolet transmission or irradiation, a block having a shape of a cube having a side of 20 mm was cut out, and a test piece in which two opposing parallel surfaces perpendicular to the axial direction of the original quartz glass material were mirror-polished was used. Using a KrF excimer laser light generator and an ultraviolet spectrophotometer,
Light is transmitted in the direction perpendicular to the polished surface,
Lasers of various energy densities were irradiated, and the maximum energy density at which the transmittance reduction after irradiation of 3 × 10 ¹⁰ shots was 0.1% was determined.

Table 2 shows the results of examination of the quartz glass materials of the test numbers shown in Table 1. Test Nos. 2 and 4 in which the required amount of OH group concentration is contained, the concentration gradient is small, the hydrogen concentration is sufficiently high, and the refractive index is small, are higher in initial transmittance than other glass materials, It can be seen that the resistance to deterioration is large.

Table 1 shows the method of producing the quartz glass of Test No. 2 or 4 having excellent initial transmittance and laser irradiation resistance.
It is apparent that the respective conditions of the slow cooling and the hydrogen doping after the processing are within the ranges defined by the present invention.

[0044]

[Table 2]

[0045]

The synthetic quartz glass of the present invention is made of KrF or A
It has excellent initial transmittance with respect to high-output ultraviolet light from an rF excimer laser or the like, and has excellent durability against deterioration of optical characteristics due to transmission of ultraviolet light. This synthetic quartz glass can be effectively used particularly for an optical system of a photolithography for a super LSI in which the wavelength of light used is short and the output is increasing.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takashi Arakawa 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Inside Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratory (72) Inventor Kazuhiro Minagawa 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Electronics Technology Laboratory F-term (reference) 4G014 AH21 AH23 4G062 AA04 BB02 CC07 MM31 MM40 NN16 NN20 5F046 CB10 CB12 CB17 CB19

Claims (2)

[Claims] 1. The concentration of an OH group is 10 to 30 ppm by mass, the difference between the highest concentration and the lowest concentration is 10 ppm or less, and the concentration gradient is 0.10 ppm.
/ mm or less, hydrogen concentration is 10 ¹⁸ mol / cm ³ or more, and wavelength 248.
An optical silica glass for ultraviolet light, having a refractive index of 1.50860 or less for 25 nm light. 2. A porous body composed of SiO ₂ fine particles obtained by hydrolyzing a silicon compound in an oxygen-hydrogen flame is reduced under reduced pressure to 1100 to 1
A synthetic quartz glass material that has been dehydrated at 500 ° C and then transparentized and sintered is subjected to compression processing of 40% or more in a temperature range of 1500 to 1700 ° C, and 1 to 10 ° C / Perform strain relief processing to cool to a temperature of 600 ° C or less at a cooling rate of hr, and then in a hydrogen atmosphere of 1 MPa or less for 400
The method for producing optical silica glass for ultraviolet light according to claim 1, wherein hydrogen doping treatment is performed for 10 to 200 hours in a temperature range of -1000 ° C.