JP2003292328A - Synthetic quartz glass and heat treatment method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treatment method for synthetic quartz glass, by which quartz glass having a low birefringence value and optical homogeneity can be obtained, and to provide the synthetic quartz glass obtained by the method. <P>SOLUTION: In the method, synthetic quartz glass is held at a constant temperature in a range of its strain point or more to its annealing point or below for 20 hours or more and then cooled. By this heat treatment method, the quartz glass which has the low birefringence value to light in the wavelength region of ≤400 nm, especially to vacuum ultraviolet light of ≤200 nm such as an F<SB>2</SB>excimer laser beam, or the like, and which has optical homogeneity, can be obtained. <P>COPYRIGHT: (C)2004,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 which is effective for producing an optical member for lithography used in a wavelength region of 400 nm or less, particularly in a vacuum ultraviolet region, and a heat treatment method for the synthetic quartz glass.

[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 stepper lens material 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 transparency of light from the light source, but also the optical homogeneity such that the intensity of the transmitted light is uniform. Has become.

In recent years, LSIs have become more and more multifunctional and have higher performance, and techniques for highly integrating devices on a wafer have been researched and developed. In order to achieve high integration of the device, it is necessary to obtain a high resolution capable of transferring a fine pattern, and the resolution can be expressed by equation (1). R = k1 × λ / NA (1) R: resolution k1: coefficient λ: light source wavelength NA: numerical aperture

According to equation (1), the means for obtaining high resolution is 2
Can be considered. 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 main wavelength of ultraviolet rays used as a light source is 248 nm (KrF), but 193n
The transition to m (ArF) is urgent, and in the future, the transition to 157 nm (F ₂ ) will be very effective.

As a material used in the 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 is only transparent. However, material strength, coefficient of thermal expansion, substrate for lens or reticle. There are many problems that must be overcome at a practical level, such as surface polishing technology for use as. Therefore, it is considered that synthetic quartz glass will continue to play a very important role as a material for forming substrates for steppers and reticles in the future.

However, even in the case of quartz glass having high UV transparency, the transparency gradually decreases in the vacuum ultraviolet region of 200 nm or less, and 140 nm which is an absorption region due to the essential structure of quartz glass. It will not be able to transmit light in the vicinity. The permeability up to the essential absorption region is determined by the unstable structure and defect structure in the silica glass.

The unstable structure is a structure having an unstable bond angle due to the Si-O-Si bond, which is the basic skeleton of quartz glass, 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 nm is taken as an example, there are Si—Si bonds and Si—OH bonds as defect structures that affect the transmittance. The Si-Si bond is called an oxygen deficiency type defect and has a central wavelength of absorption at 163 nm. Since this oxygen-deficient defect is also a precursor of Si / defect structure showing an absorption band at 215 nm, F ₂ (157n
m), of course, KrF (248 nm) and Ar
It is also very problematic when F (193 nm) is used 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-mentioned three-membered ring and four-membered ring structures and defect structures as much as possible. In order to solve this, 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 this in a fluorine compound gas atmosphere has been taken.

By this method, the quartz glass is doped with fluorine, and it is known that the three-membered ring structure and the four-membered ring structure are reduced by the fluorine doping. Further, fluorine doping can eliminate Si—OH bonds in the quartz glass and generate Si—F bonds. S
The i-F bond has a large binding energy, is a strong bond, and has no absorption band at 150 to 170 nm. As a result, the quartz glass doped with fluorine by the above method exhibits high transparency to vacuum ultraviolet rays of F ₂ (157 nm). However, when the synthetic quartz glass thus obtained is molded to form a substrate, optical nonuniformity such as extremely high birefringence in the substrate surface is often exhibited.

When an optically non-uniform substrate is used for a reticle or the like, the transferred image is partly 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 is a method for heat-treating synthetic quartz glass capable of obtaining quartz glass having a low birefringence amount and being optically homogeneous, and a synthetic quartz obtained thereby. Intended to provide glass.

[0014]

Means for Solving the Problems and Modes for Carrying Out the Invention In order to achieve the above-mentioned object, the inventors of the present invention diligently studied the heat treatment conditions of vitrified synthetic quartz glass, and as a result, improved birefringence by the following method. Made it possible. As a result, 400 nm or less, especially 200 n such as ArF and F ₂
It has been found that an optically homogeneous quartz glass having a high transmittance for vacuum ultraviolet light of m or less and a low birefringence amount can be obtained.

That is, the present invention provides the following method for producing synthetic quartz glass. (1) A method for heat-treating synthetic quartz glass, which comprises holding the synthetic quartz glass at a constant temperature in a temperature range from a strain point to a slow cooling point for 20 hours or more, and then cooling the synthetic quartz glass, (2)
After the first heat treatment and cooling described in (1), the second heat treatment is held at a constant temperature lower than the heat treatment temperature described in (1) for 20 hours or more, and then cooled. Quartz glass heat treatment method, (3) The cooling rate from the execution temperature of the first heat treatment to 500 ° C. is 10 on average.
0 ° C./Hr or more, (1) or the heat treatment method of synthetic quartz glass according to (2), characterized in that the synthetic quartz glass is fluorine-doped (1) A heat treatment method for synthetic quartz glass according to any one of (1) to (3), and a synthetic quartz glass obtained by the method according to any one of (5), (1) to (4),
(6) The synthetic quartz glass according to (5), which has a birefringence of 10 nm / cm or less.

The present invention will be described in more detail below.
The present invention has a particularly high transmittance of vacuum ultraviolet light, and relates to a heat treatment method of an optically homogeneous synthetic quartz glass, in this case, in order to increase the transmittance of vacuum ultraviolet light,
As the quartz glass, it is preferable to use synthetic quartz glass doped with fluorine atoms. This is because fluorine doping can reduce unstable bond states and defective structures in quartz glass. In addition, since the Si—F bond generated by fluorine doping has a large bond energy, it has good ultraviolet resistance.

In the present invention, synthetic birefringence is reduced, and in particular, in birefringence of fluorine-doped silica glass having a high transmittance in the wavelength region of 400 nm or less, particularly in the vacuum ultraviolet region, the birefringence is reduced by heat treatment under a condition different from the conventional one. And improve optical homogeneity. That is, conventionally, heat treatment has been performed in order to remove strains and the like due to thermal stress in the quartz glass. As a method thereof, the quartz glass is heated for a certain period of time at or above the annealing point and then gradually cooled to below the strain point. here,
The strain point is the temperature at which the viscosity of quartz glass reaches 10 ^13.5 Pas, and viscous flow does not substantially occur at this temperature, and strain below this temperature cannot be removed. The annealing point is a temperature at which the viscosity becomes 10 ¹² Pas, and the internal strain generated by glass processing can be removed in about 15 minutes (amorphous silica material application handbook, Realize Co., Ltd.). . That is, as a conventional method, the strain is removed by holding it at a high temperature such that the strain can be removed in 15 minutes, and gradually cooled so as not to generate new strain during cooling.

This method can reduce the birefringence of ordinary silica glass synthesized by the direct method or the soot method, but it is not always possible to reduce the birefringence of fluorine-doped silica glass for F ₂ excimer laser. It was not always possible to reduce the refraction. The reason for this is not clear,
The present inventors consider as follows. If Si-F bonds are present in quartz glass at high temperature, some of the Si-F bonds are broken. Fluorine is so reactive that free fluorine atoms may recombine with other bonds, but even if they do not, a distribution of fluorine concentrations will result. This distribution of fluorine concentration becomes the density distribution in quartz glass,
Generate distortion. If this phenomenon is applied to the conventional heat treatment method, the thermal strain can be removed while the material is held at a temperature higher than the annealing point, but if this is slowly cooled, the above-mentioned Si—F bond disconnection / recombination occurs. It also occurs during slow cooling, resulting in new strain. Also, the higher the temperature, the more S
Since the cleavage of the i-F bond is promoted, strain is likely to occur.

On the other hand, in the heat treatment method of the present invention, as the first heat treatment, the synthetic quartz glass is kept at a constant temperature for 20 hours or more in the temperature range from the strain point to the annealing point and then cooled. In this case, after completion of the heat treatment and cooling, it is preferable that the second heat treatment is maintained at a constant temperature lower than the heat treatment temperature for 20 hours or more and then cooled.

Here, as in the present invention, when the temperature is kept above the strain point and below the slow cooling point for a certain period of time and then cooled (rapidly cooled), the thermal strain is removed from above the strain point, and it is compared with the conventional heat treatment temperature. Since the temperature is low, the breaking of the Si-F bond is minimized, and the rapid cooling also minimizes the breaking / rebonding of the Si-F bond. Although it may be feared that thermal strain will occur due to rapid cooling, the heat treatment temperature of the present invention is a temperature lower than the slow cooling point and is a temperature at which viscous flow is not large, so its influence is very small.

The details of the present invention will be explained by taking quartz glass for F ₂ excimer laser as an example. First, when producing fluorine-doped quartz glass, the method is to supply oxygen gas, hydrogen gas, and silica-producing raw material gas from a burner to a reaction zone, and in this reaction area, silica fine particles are produced by flame hydrolysis of the silica-producing raw material gas. At the same time, the silica fine particles are deposited on the base material rotatably arranged in the reaction zone to prepare a porous silica base material, and the base material is heated and melted in an atmosphere containing a fluorine compound gas to form a quartz glass. obtain. 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.

Further, a fluorine compound gas is supplied from the burner to the reaction zone to prepare a fluorine-containing porous silica matrix,
This may be vitrified.

Known silica compounds such as chlorosilanes such as silicon tetrachloride, alkoxysilanes such as tetramethoxysilane, and disilanes such as hexamethyldisilane are used as the raw material gas for producing silica. In consideration, Cl-free alkoxysilane is preferable. SiF as the fluorine compound gas
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.

Appropriate conditions are selected for the vitrification temperature and time within the range of 1200 to 1700 ° C. depending on the concentration of the fluorine compound gas in the vitrification atmosphere and the density of the porous silica matrix. Prior to vitrification, a dehydration step of heating the porous silica matrix 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, slow cooling or cooling.

The synthetic quartz glass thus obtained is molded, and an optical member for lithography is manufactured through steps such as heat treatment, cutting, and polishing. In the present invention and the conventional method, among them, Different heat treatment methods.

Conventionally, quartz glass is heated and held at a temperature above its annealing point using an electric furnace, and then gradually cooled at a rate of about 10 ° C./hr or less. The range is lower than the conventional one and is higher than the strain point and lower than the annealing point. Preferably, the temperature is closer to the strain point, specifically, the strain point is within ± 50 ° C. Below the strain point, the thermal strain generated during molding cannot be sufficiently removed, and above the annealing point, the Si—F bond is easily broken. Further, the heat treatment temperature of the present invention is lower than the annealing point, but regarding strain removal, since the temperature is lower than that when exceeding the annealing point, viscous flow in quartz glass is small. It is compensated by setting it longer. The heat treatment time of the present invention is preferably 20 hours or longer, particularly preferably 50 hours or longer. The upper limit is appropriately selected, but is usually 100 hours or less. Although the quartz glass is cooled after the heat treatment, unlike the conventional method, gradual cooling is not performed in the present invention. For example, when the holding at the heat treatment temperature is completed, the heater of the electric furnace is turned off and the furnace is cooled as it is. You may introduce an inert gas into the furnace at the time of cooling, take out the quartz glass from the furnace to cool it, or open the electric furnace and let it cool with the electric furnace. In order to implement, it is necessary to take special equipment and measures other than the electric furnace.

For cooling, it is necessary to quench rapidly to a temperature at which the Si--F bond in the quartz glass is not broken, but as a result of the study by the present inventors, it was found that quenching to 500 ° C. is sufficient. Therefore, cooling is preferably performed at an average rate of 100 ° C./Hr or more from the heat treatment temperature to 500 ° C. Also, faster cooling is preferred. From 500 ° C. to room temperature, it is not necessary to consider the effect of breaking the Si—F bond in the quartz glass, and therefore any of rapid cooling, standing cooling, and slow cooling may be performed.

When the heat treatment is performed again after the above heat treatment is completed, the birefringence may have a better value. this is,
It is considered that the first heat treatment causes a slight thermal strain, which is removed by the reheat treatment.
In this case, the second heat treatment temperature is lower than the first (first) treatment temperature, and preferably the strain point or higher. The heat treatment time is 1 to 100 hours, especially 2
0 to 50 hours are preferable, and the cooling conditions and the cooling method after the heat treatment are preferably the same as above.

The quartz glass thus obtained becomes an optical member for lithography after being subjected to grinding / cutting processing, polishing, etc. after heat treatment. In the case of a member obtained as a result, for example, a substrate for a reticle, the following physical properties are preferable. Transmittance measured by spectrophotometer, 157.6n
If m, it is 80.0% or more, preferably 83.0% or more, and more preferably 84.0% or more. The transmittance distribution at 157.6 nm is preferably 1.0% or less. 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 10 nm / cm or less, more preferably 5 nm / cm.
The following is more suitable, and more preferably 1 nm / cm or less.
Since the amount of birefringence has wavelength dependence, the measured value can be converted into the amount of birefringence such as 157.6 nm which is the wavelength used by the F ₂ laser and 193.4 nm which is the wavelength used by the ArF excimer laser ( Physics and
Chemistry of Glasses19
(4) 1978).

[0032]

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. Further, the conditions such as the heat treatment temperature of the quartz glass described in this example do not mean that the present invention is limited to the range.

[Example 1] H ₂ gas, O ₂ gas, and tetramethoxysilane as a raw material were supplied from a burner, and a porous silica preform was produced by hydrolysis with an oxyhydrogen flame. This porous silica base material was heated to 1500 ° C. in a mixed atmosphere of SiF ₄ and He to obtain a synthetic quartz glass ingot. The strain point and annealing point of this quartz glass are each 9
It was 20 ° C and 1000 ° C.

The obtained quartz glass ingot is set to 150 m.
After heat-molding to a size of m square, it was cut into several samples, and one of them was heat-treated in an electric furnace. When the birefringence before heat treatment was measured, it was 30 nm / cm or less in the sample plane. Heat treatment temperature is 980 ℃
The holding time was 100 hours. After the heat treatment was completed, the heater of the electric furnace was turned off and the furnace was cooled as it was. The average rate of cooling the furnace to 500 ℃ is 120 ℃ / H
It was r. When the birefringence of the sample was measured, it was 5 nm / cm or less within the sample plane. The results are shown in Table 1.

Example 2 The sample heat-treated in Example 1 was heat-treated again in an electric furnace. Heat treatment temperature is 93
The holding time was 50 hours at 0 ° C. After the heat treatment,
The heater of the electric furnace was turned off and the furnace was cooled as it was. The average rate of cooling the furnace to 500 ° C is 120 ° C /
It was Hr. When the birefringence of the sample was measured, it was 2 nm / cm or less within the sample plane. The results are shown in Table 1.

[Comparative Example 1] One sample before the heat treatment prepared in Example 1 was heat-treated in an electric furnace. When the birefringence before the heat treatment was measured, it was 28 nm / cm or less within the sample plane. The heat treatment temperature was 1050 ° C., and the holding time was 10 hours. After that, after gradually cooling to 800 ° C. at a rate of 3 ° C./Hr and from 800 ° C. to 500 ° C. at a rate of 10 ° C./Hr, the heater of the electric furnace was turned off and cooled in the furnace as it was. When the birefringence of the sample was measured, it was 15 nm / cm or less within the sample plane. The results are shown in Table 1.

[Comparative Example 2] One sample before the heat treatment prepared in Example 1 was heat-treated in an electric furnace. When the birefringence before heat treatment was measured, it was 30 nm / cm or less in the sample plane. Heat treatment temperature is 800 ° C, holding time is 10
It was set to 0 hours. After that, the temperature was gradually cooled to 500 ° C. at a rate of 10 ° C./Hr, the heater of the electric furnace was turned off, and the furnace was cooled as it was. When the birefringence of the sample was measured,
It was 30 nm / cm or less in the sample plane, and there was almost no change. The results are shown in Table 1.

[0038]

[Table 1]

[0039]

Industrial Applicability According to the heat treatment method for synthetic quartz glass of the present invention, optically homogeneous quartz having a low birefringence amount in a wavelength region of 400 nm or less, particularly for vacuum ultraviolet light of 200 nm or less such as for F ₂ excimer laser. Glass can be obtained.

Claims (6)

[Claims] 1. A heat treatment method for synthetic quartz glass, which comprises holding the synthetic quartz glass at a constant temperature within a temperature range from a strain point to a slow cooling point for 20 hours or more, and then cooling the synthetic quartz glass. 2. After carrying out the first heat treatment and cooling according to claim 1, after holding as a second heat treatment at a constant temperature lower than the heat treatment temperature according to claim 1 for 20 hours or more,
A method for heat-treating synthetic quartz glass, characterized by cooling. 3. The heat treatment method for synthetic quartz glass according to claim 1, wherein the cooling rate from the execution temperature of the first heat treatment to 500 ° C. is 100 ° C./Hr or more on average. 4. The synthetic quartz glass is fluorine-doped, and any one of claims 1 to 3 is characterized.
A method for heat-treating synthetic quartz glass according to the item. 5. A synthetic quartz glass obtained by the method according to any one of claims 1 to 4. 6. The synthetic quartz glass according to claim 5, which has a birefringence of 10 nm / cm or less.