JP2003192363A - Synthetic quartz glass and method for producing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain optically homogeneous synthetic quartz glass having a high transmittance to a vacuum ultraviolet light of a wave length of less than 400 nm, especially less than 200 nm for F<SB>2</SB>excimer laser and having a small amount of birefringence. <P>SOLUTION: A method for producing a synthetic quartz glass comprises heat-treating a synthetic quartz glass material containing fluorine atoms under an atmosphere containing a fluorine compound is provided. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synthetic quartz glass useful as an optical member for lithography used in the vacuum ultraviolet region and a method for producing the same.

[0002]

2. Description of the Related Art Synthetic silica glass plays a major role as an optical member for lithography in semiconductor manufacturing because of its high ultraviolet light transmittance. The role of synthetic quartz glass in a lithographic apparatus is a lens material for steppers and a reticle (photomask) substrate material used in the exposure and transfer processes of circuit patterns on a silicon wafer.

The stepper device is composed of an illumination system section, a projection lens section, and a wafer driving section, and the light emitted from the light source is supplied onto the reticle by the illumination system as light of uniform illuminance.
The projection lens unit has a role of accurately and reducing the circuit pattern on the reticle to form an image on the wafer.

The quality required for these materials is not only high in the light transmission from the light source, but also the optical homogeneity is very important such that the intensity of the transmitted light is uniform. ing. In recent years, LSIs have become more multifunctional and have higher performance, and high integration technology of devices on a wafer has been researched and developed.

For high integration of the device, it is necessary to obtain a high resolution capable of transferring a fine pattern. In this case, the resolution can be expressed by the following equation (1). R = k1 × λ / NA (1) R: resolution k1: coefficient λ: wavelength NA of light source: numerical aperture

According to the above equation (1), there are two possible means for obtaining high resolution. One is to increase the numerical aperture. However, as the numerical aperture increases, the depth of focus decreases, and the current situation is considered to be almost the limit. Another method is to shorten the wavelength of the light source. At present, the mainstream wavelength of ultraviolet rays used as a light source is 248.3 nm (KrF).
The shift to 3.4 nm (ArF) is urgent, and in the future, the shift to 157.6 nm (F ₂ ) will be very effective.

As a material used in a so-called vacuum ultraviolet region having a wavelength of 200 nm or less, it is considered that calcium fluoride single crystal can be used as long as it has transparency, but it is used as a material strength, a coefficient of thermal expansion and a substrate for a reticle. There are many problems that need to be overcome at a practical level, such as surface polishing technology for doing so.

Therefore, it is considered that synthetic quartz glass will continue to play a very important role as a material constituting a substrate for a reticle in the future.

However, even with synthetic quartz glass having a high ultraviolet ray transmittance, the transmittance gradually decreases in the vacuum ultraviolet region of 200 nm or less, and in the absorption region due to the essential structure of synthetic quartz glass. At around 140 nm, light cannot be transmitted.

The permeability up to the essential absorption region is determined by the unstable structure and defect structure in the synthetic quartz glass.

The unstable structure is a Si-O-Si bond which is a basic skeleton of synthetic quartz glass and has an unstable bond angle, and has a 3-membered ring structure and a 4-membered ring structure. When these are irradiated with a laser, they are opened by the energy and a defect structure is generated.

Regarding the defect structure, for example, when an F ₂ excimer laser having a light source wavelength of 157.6 nm is taken as an example, Si-Si bonds and Si-OH bonds are present as defect structures affecting the transmittance. The Si-Si bond is called an oxygen deficiency type defect and has an absorption center wavelength of 163 nm.

Since this oxygen-defective defect is also a precursor of Si / defect structure showing an absorption band at 215 nm,
₂ (157.6 nm), of course, KrF (2
It is also very problematic when using 48.3 nm) or ArF (193.4 nm) as the light source. Further, the Si-OH bond has an absorption band near 160 nm.

Therefore, in order to realize high vacuum ultraviolet ray transparency, it is necessary to reduce the above three-membered ring and four-membered ring structures and defect structures as much as possible.

In order to solve this problem, in the conventional research, a method of producing a porous silica base material by flame hydrolysis of a silica raw material gas, and melting and vitrifying it in a fluorine compound gas atmosphere has been taken. It was

By this method, fluorine is doped into the synthetic quartz glass, and it is known that doping of fluorine reduces the three-membered ring structure and the four-membered ring structure.
In addition, Si-O in synthetic quartz glass is doped with fluorine.
The H bond can be eliminated and a Si—F bond can be generated. The Si—F bond has a large binding energy and is a strong bond, and on top of that, it has no absorption band at 150 to 170 nm. As a result, the synthetic quartz glass doped with fluorine by the above method exhibits high transparency to vacuum ultraviolet rays of F ₂ (157.6 nm).

However, when the synthetic quartz glass thus obtained is molded to form a substrate, optical nonuniformity such as extremely high birefringence in the plane of the substrate is often exhibited. When such an optically non-uniform substrate is used as a reticle or the like, the transferred image is partially blurred, and it becomes difficult to use it as a material. Therefore, it is desired to establish a method for producing synthetic quartz glass that is optically homogeneous in addition to having high transparency.

The present invention has been made in order to meet the above-mentioned demands, and provides a synthetic quartz glass having a high transparency to vacuum ultraviolet light, a low birefringence amount, and an optically homogeneous synthetic glass, and a method for producing the same. The purpose is to provide.

[0019]

MEANS FOR SOLVING THE PROBLEMS AND BEST MODE FOR CARRYING OUT THE INVENTION In order to achieve the above-mentioned object, the present inventors have diligently studied the heat treatment conditions of vitrified synthetic quartz glass, and as a result, the synthetic quartz containing a fluorine atom By subjecting the glass material to a heat treatment in an atmosphere containing a fluorine compound, in this case, by removing the outer peripheral portion of the synthetic quartz glass material in advance before the heat treatment, 400 nm or less, especially ArF
The inventors have found that an optically homogeneous synthetic quartz glass having a high transparency to a vacuum ultraviolet light of 200 nm or less such as F ₂ and F ₂ and having a low birefringence amount can be obtained, and completed the present invention. .

That is, the present invention provides the following synthetic quartz glass and a method for producing the same. (1) A synthetic quartz glass manufacturing method, characterized in that a synthetic quartz glass material containing fluorine atoms is heat-treated in an atmosphere containing a fluorine compound, and (2) an outer peripheral portion of the synthetic quartz glass material to be heat-treated is removed in advance. (1) The synthetic quartz glass obtained by the method according to (1) or (2), which has a birefringence of 10 nm / cm or less.

The present invention will be described in more detail below.
The present invention relates to a fluorine-containing synthetic quartz glass having a high transmittance for vacuum ultraviolet light and being optically homogeneous, and a method for producing the same.

Here, in order to increase the transmittance of vacuum ultraviolet light, it is necessary to dope the synthetic quartz glass with fluorine atoms. Due to the fluorine doping, unstable bonding states and defect structures in the synthetic quartz glass are caused. Can be reduced. In addition, since the Si-F bond generated by doping has a large bond energy, it has good ultraviolet resistance.

Therefore, in the present invention, first, the fluorine-doped synthetic quartz glass material is manufactured. In this case, as the method, oxygen gas, hydrogen gas and silica-producing raw material gas are supplied from the burner to the reaction zone, and silica fine particles are produced by flame hydrolysis of the silica-producing raw material gas in this reaction zone, and the reaction zone A method of obtaining a synthetic quartz glass material by depositing the above silica fine particles on a rotatably arranged base material to prepare a porous silica base material and heating and melting the base material in an atmosphere containing a fluorine compound gas Can be adopted. As the method itself, known methods and conditions can be adopted, and for example, the flow rate of oxygen gas, hydrogen gas, silica production raw material gas, etc. can be selected within a normal flow rate range. Alternatively, a fluorine compound gas may be supplied from a burner to the reaction zone to prepare a fluorine-containing porous silica base material and vitrify it.

Known silica compounds such as chlorosilanes such as silicon tetrachloride and alkoxysilanes such as tetramethoxysilane and disilanes such as hexamethyldisilane are used as the raw material gas for producing silica. Considering the above, an alkoxysilane containing no Cl is preferable. As a fluorine compound gas, Si
F ₄ , CHF ₃ , CF ₄ etc. may be selected. The heating / melting atmosphere is the above-mentioned fluorine compound gas, an inert gas such as helium or argon, or a mixed atmosphere thereof. Here, the vitrification temperature and time are appropriately selected within the range of 1200 to 1700 ° C. depending on the concentration of the fluorine compound gas in the vitrification atmosphere, the density of the porous silica matrix and the like. Before vitrification, a dehydration step of heating the porous silica base material at a temperature lower than the vitrification temperature may be carried out. The heating atmosphere in this case is also the above-mentioned fluorine compound gas, an inert gas such as helium or argon, or a mixed atmosphere thereof.

After vitrification, it is cooled to room temperature in the same furnace by rapid cooling, gradual cooling or cooling, but it is also preferable to perform cooling in an atmosphere containing a fluorine compound gas.

The fluorine doping amount in the synthetic quartz glass material containing fluorine atoms obtained in this way is 0.
1 wt% or more, more preferably 0.1 to 2.4 wt%,
More preferably, it is 0.3 to 1.5% by weight.

The fluorine-doped synthetic quartz glass material thus obtained can be molded, and an optical member for lithography can be manufactured through steps such as heat treatment, cutting and polishing. In the present invention, however, an atmosphere containing a fluorine compound is used. The fluorine-doped synthetic quartz glass material is heat-treated below.

That is, conventionally, the annealing treatment for removing the strain of the synthetic quartz glass is carried out in the atmosphere, but under such an environment, Si--F in the synthetic quartz glass is used.
The bond is cut by heat, and an altered layer is formed on the outer periphery. The presence of this altered layer makes it difficult to remove the strain in the synthetic quartz glass even by annealing.

In the present invention, the strain is effectively removed by performing the annealing treatment in the environment in which the generation of the altered layer is suppressed. That is, annealing is performed in a fluorine compound-containing atmosphere.

The fluorine compound in this case is SiF ₄ , C
HF ₃ , CF ₄ or the like is selected, and the annealing atmosphere is the above-mentioned fluorine compound gas atmosphere or a mixed atmosphere of the fluorine compound gas and an inert gas such as helium or argon.

The concentration of the fluorine compound during annealing is preferably equal to or higher than the concentration of fluorine atoms in the synthetic quartz glass or the concentration during the production (doping) of the synthetic quartz glass.

Further, by removing the deteriorated layer which has already been formed by hot molding or the like before the annealing treatment,
It is possible to obtain a further strain removing effect.

The depth of the altered layer varies depending on the fluorine atom concentration of the synthetic quartz glass and heat treatment conditions, but is usually several mm from the surface, and is usually removed by grinding or cutting about 1 to 10 mm.

Regarding the temperature condition of the annealing, a certain time (usually 1
It is good to heat and hold, and then gradually cool to below the strain point, but since the slow cooling point and strain point change depending on the fluorine atom concentration in the synthetic quartz glass, it is necessary to set an appropriate temperature. There is. It is usually in the range of 1200 ° C or lower. The slow cooling rate up to the strain point or lower is preferably 10 ° C./Hr or lower.

As described above, according to the present invention, the birefringence is reduced by heat-treating the synthetic quartz glass having a high transmittance in the wavelength region of 400 nm or less, particularly in the vacuum ultraviolet region, by the fluorine doping under the condition different from the conventional one. , To improve optical homogeneity.

That is, conventionally, the heat treatment is performed in the synthetic quartz glass.
It has been performed to remove strains and the like due to thermal stress. So
As a method of, at a certain temperature above the annealing point of synthetic quartz glass
It is heated for a while and gradually cooled to a strain point or lower. here,
The strain point is that the viscosity of synthetic quartz glass is 10^13.5Pa · s
Temperature at which viscous flow actually occurs.
However, below this temperature, the strain in the glass cannot be removed. Well
Also, the annealing point has a viscosity of 10 ¹²The temperature is Pa · s,
Temperature at which internal strain generated by glass processing can be removed in about 15 minutes
(Amorphous silica material application handbook, Ltd.)
Expression company Realize Inc.).

That is, the conventional heat treatment method is 15
The strain is removed by holding it at a high temperature such that the strain can be removed in minutes, and gradually cooled over time so that new strain does not occur during cooling.

This method can reduce the birefringence of the ordinary synthetic quartz glass synthesized by the direct method or the soot method, but with respect to the synthetic quartz glass doped with fluorine such as for F ₂ excimer laser, It was not always possible to reduce the birefringence. The present inventors consider the reason for this as follows.

When the synthetic quartz glass containing fluorine is exposed to a high temperature, the Si--F bond in the synthetic quartz glass is partially broken, and the concentration of fluorine atoms in that portion is lowered. Since the bond breakage easily occurs in the outer peripheral portion of the synthetic quartz glass, the fluorine concentration in the outer peripheral portion becomes lower than in the central portion of the synthetic quartz glass. Actually EPMA (Electron Pro
When the surface of the synthetic quartz glass subjected to the heat treatment was analyzed by be Micro Analyzer) analysis, the fluorine atom concentration on the surface was below the detection limit.

The outer peripheral portion of the synthetic quartz glass has a much higher viscosity than the central portion due to the low concentration of fluorine atoms, and the annealing point and strain point described above also become hot depending on the viscosity. If such a highly viscous layer (hereinafter referred to as an altered layer) exists on the outer periphery of the synthetic quartz glass, when annealing is performed for the purpose of strain removal, the outer peripheral portion is cured first during slow cooling. The internal stress is not sufficiently released and remains in the synthetic quartz glass. Further, under such a situation, it is not only difficult to release the stress and remove the strain, but also a new strain may be generated due to the altered layer on the outer periphery. When actually measuring the birefringence of synthetic quartz glass, it is very often the case that synthetic quartz glass having a high birefringence even after annealing has a region having a high birefringence in its outer peripheral portion.

The generation of the altered layer occurs when the synthetic quartz glass is exposed to a high temperature. However, in the present invention, since the annealing treatment for removing the strain is performed in the fluorine compound-containing atmosphere, the Si--F bond in the synthetic quartz glass is formed. Even if it is cut by heat, it is in an environment where it is likely to be recombined. Further, since the fluorine concentration in the annealing atmosphere is high, it is possible to suppress the diffusion of fluorine from the synthetic quartz glass. Therefore, generation of an altered layer can be suppressed, and the effect of removing strain by annealing can be sufficiently exerted.

Further, when synthetic quartz glass is hot-molded in the atmosphere, it is highly likely that an altered layer is formed on the outer periphery, but the outer periphery of this synthetic quartz glass should be removed by grinding or cutting. Thereby, annealing in a fluorine compound-containing atmosphere can be performed more effectively.

The synthetic quartz glass thus obtained can be used as an optical member for lithography through grinding, cutting and polishing after heat treatment. as a result,
The following values are preferable for the physical properties of the obtained member, for example, a substrate for a reticle.

The transmittance was measured by a spectrophotometer and
If it is 7.6 nm, it is 83.0% or more, preferably 85.
0% or more. The transmittance distribution is 157.6 nm at 1.
0% or less is preferable. It is more preferably 0.5% or less, still more preferably 0.3% or less.

The amount of birefringence is He--Ne having a wavelength of 633 nm.
It is measured by an optical heterodyne method using a laser, and the value is preferably 10 nm / cm or less, more preferably 5
nm / cm or less, more preferably 1 nm / cm or less. Since the amount of birefringence depends on the wavelength, the measured value can be converted into the amount of birefringence such as the wavelength used for the F ₂ laser of 157.6 nm and the wavelength used for the ArF excimer laser of 193.4 nm (Physics). and
Chemistry of Glasses19
(4) 1978).

[0046]

EXAMPLES The present invention will be specifically described below by showing Examples and Comparative Examples, but the present invention is not limited to the following Examples. The conditions such as the heat treatment temperature of the synthetic quartz glass described in this example do not mean that the present invention is limited to the range.

[Example 1] A porous silica base material was replaced with SiF ₄
And He mixed gas atmosphere (SiF ₄ concentration: 10 vol
%) To produce a fluorine-containing synthetic quartz glass ingot, which was heat-molded to a size of 180 mm square. The birefringence of the molded ingot in the center 125 mm square was measured and found to be 30 nm / cm or more. Then
This molded ingot was annealed. The annealing conditions are the same concentration of Si as when vitrifying the porous silica base material.
Hold in a mixed atmosphere of F ₄ and He at 1100 ° C. for 10 hours, and keep the SiF ₄ concentration constant up to 500 ° C.
It was slowly cooled at a rate of ° C / Hr. When the birefringence after annealing is compared in the same region as before annealing, 10 nm / cm
It was below. The results are shown in Table 1.

[Example 2] Similar to Example 1, a porous silica matrix was heated and melted in a mixed gas atmosphere of SiF ₄ and He to produce a fluorine-containing synthetic quartz glass ingot, which was 180 mm square in size. It was heat molded into. When the birefringence in the central 125 mm square of the molded ingot was measured,
It was 30 nm / cm or more. Then, the outer periphery of the molded ingot was removed by 3 mm on the entire surface, and then annealed. The annealing condition is 1100 ° C. in a mixed atmosphere of SiF ₄ and He of the same concentration as when vitrifying the porous silica base material.
At 10 ° C for 10 hours, keeping the SiF ₄ concentration constant at 5
The mixture was gradually cooled to 00 ° C at a rate of 10 ° C / Hr. When comparing the birefringence after annealing in the same region as before annealing,
It was 5 nm / cm or less. The results are shown in Table 1.

[Example 3] As in Example 1, a porous silica base material was heated and melted in a mixed gas atmosphere of SiF ₄ and He to produce a fluorine-containing synthetic quartz glass ingot, which was 180 mm square in size. It was heat molded into. When the birefringence in the central 125 mm square of the molded ingot was measured,
It was 30 nm / cm or more. Next, the outer periphery of the molded ingot was removed by 10 mm on the entire surface and then annealed. The annealing conditions are such that the SiF ₄ concentration is higher than that when the porous silica base material is vitrified (SiF ₄ concentration: 20 vol).
%) And held at 1100 ° C. for 10 hours and gradually cooled to 500 ° C. at a rate of 5 ° C./Hr while keeping the SiF ₄ concentration constant. When the birefringence after annealing was compared in the same region as before annealing, it was 3 nm / cm or less. The results are shown in Table 1.

Comparative Example 1 A molded ingot of synthetic silica glass containing fluorine was prepared under the same conditions as in Example 1, and the birefringence within the 125 mm square in the center of the molded ingot was measured and found to be 30 nm / cm or more. This was annealed under the same conditions as in Example 1 except that it was in the atmosphere. When the birefringence after annealing was compared in the same region as before annealing, the birefringence was still 30 nm / cm or more. In addition, in the measurement region, a part where the birefringence increased due to annealing was also seen. The results are shown in Table 1.

[0051]

[Table 1]

[0052]

Industrial Applicability According to the method for producing synthetic quartz glass of the present invention, the transmittance is high and the birefringence amount is low with respect to vacuum ultraviolet light having a wavelength region of 400 nm or less, particularly 200 nm or less for F ₂ excimer laser. It is possible to obtain synthetic quartz glass that is homogeneous in nature.

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Claims (3)

[Claims] 1. A method for producing synthetic quartz glass, which comprises subjecting a synthetic quartz glass material containing fluorine atoms to a heat treatment in an atmosphere containing a fluorine compound. 2. The method for producing synthetic quartz glass according to claim 1, wherein the outer peripheral portion of the synthetic quartz glass material to be heat-treated is previously removed. 3. The synthetic quartz glass obtained by the method according to claim 1, which has a birefringence of 10 nm / cm or less.