JPH08104532A - Heat treating method of quartz glass - Google Patents

Abstract

PURPOSE: To provide a heat treating method hardly causing deterioration of an initial transmittance of a high purity transparent synthetic quartz glass body in ultraviolet and vacuum ultraviolet regions. CONSTITUTION: The occurrence of an absorption at a wavelength region of <=200nm due to a defective structure which is caused by reduction of fundamental structure of Si-Si or Si-O-Si is prevented by holding the high purity transparent synthetic quartz glass body in a thermostatic state in >=10<-10> S.cm<-1> to <=10<-6> S.cm<-1> electric conductivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for quartz glass used in optical systems for vacuum ultraviolet lasers in general.

[0002]

2. Description of the Related Art Conventionally, an exposure apparatus called a stepper has been used in an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit onto a wafer such as silicon. The light source of this stepper is g-line (436 nm) to i-line (365 nm), and further K
Shorter wavelengths are being promoted to rF (248 nm) and ArF (193 nm) excimer lasers.

Generally, the optical glass used as an illumination system or a projection lens of a stepper has a low light transmittance in a wavelength region shorter than the i-line, and therefore synthetic quartz glass or CaF ₂ (fluorite) is used instead of the conventional optical glass. It has been proposed to use a fluoride single crystal such as stone. The optical system mounted on the stepper is composed of a combination of many lenses, and even if the amount of decrease in transmittance per lens is small, it is added up by the number of lenses used, and the amount of light on the irradiation surface is increased. As a result, the optical material is required to have a high transmittance. Further, as the used wavelength becomes shorter, the image forming performance becomes extremely poor even by the slightest unevenness in the refractive index distribution (refractive index inhomogeneity).

As described above, quartz glass used as an optical body for photolithography in the ultraviolet region is required to have high transmittance of ultraviolet rays and high homogeneity of refractive index. However, commercially available synthetic quartz glass is insufficient in quality such as initial transmittance, homogeneity of refractive index, and resistance to ultraviolet rays, and cannot be used for precision optical instruments as described above. It was Therefore, secondary treatment for homogenization (Japanese Patent Publication No. 03-17775, Japanese Patent Laid-Open No. 64-28240) and improvement of homogeneity and laser resistance by heat treatment in pressurized hydrogen gas (Japanese Patent Laid-Open No. 03-109233). Etc. have been proposed.

[0005]

However, in the conventional secondary treatment by heating and pressure, the light transmittance after treatment may be lower than the initial transmittance in the ultraviolet region, particularly in the vacuum ultraviolet region of 200 nm or less, Even if a high refractive index homogeneity was obtained, it was not sufficient in terms of light transmittance as an optical body for ArF excimer laser light (193 nm).

Further, even if synthetic quartz glass having the same initial transmittance is heat-treated under the same conditions of temperature, pressure, atmosphere, etc., the same light transmittance may not always be obtained. An object of the present invention is to provide a heat treatment method which does not reduce the initial transmittance of the high-purity transparent synthetic quartz glass body in the ultraviolet and vacuum ultraviolet regions.

[0007]

The inventors of the present invention have conducted extensive studies on the cause of the decrease in light transmittance due to heat treatment, and as a result, the controlling factor is the theory of the amount of oxygen contained in quartz glass. It was found that this was due to the large deviation from the composition SiO ₂ ( ₂ O for 1 Si).

Then, as a result of further research to maintain the theoretical composition, it is possible to prevent the decrease in the light transmittance in the ultraviolet and vacuum ultraviolet regions due to the heat treatment by adjusting the electric conductivity at the time of constant temperature of the heat treatment. It turned out to be possible.
Furthermore, when a quartz glass body having an absorption in the ultraviolet to vacuum ultraviolet region is used, by adjusting the conductivity,
It turned out that the absorption could be eliminated.

Therefore, the present invention provides a high-purity transparent synthetic quartz glass.
Russ body 10^-TenS ・ cm^-1More than 10^-6S ・ cm ^-1Constant at the following conductivity
Heat treatment of quartz glass characterized by keeping in a warm state
Provide a way. The feature of the present invention is that quartz during heat treatment and isothermal
By specifying the conductivity of glass, the heat treatment
The basic structure of Si-Si and Si-O-Si that was generated by
Absorption of structural defects created in the wavelength range below 200 nm.
It is possible to prevent the generation of yield. This
The quartz glass has a light transmittance of ultraviolet or 200 nm or less.
Pull to a level that can be used with precision optical instruments in the vacuum ultraviolet region.
Can be raised. Especially using ArF excimer laser
In case of a stepper, the composition of quartz glass can be optimized.
Very useful as absorption can be eliminated by and
Is.

[0010]

[Function] Generally, there are two kinds of electric conductivity of a solid substance, that is, electronic conductivity and ionic conductivity, and one of them is dominant depending on the substance. For example, in the case of metal, which shows electric conduction by the movement of electrons, the electron conduction is dominant, and
Ionic conduction is predominant in substances that show electrical conduction due to the movement of Na ⁺ and Cl ⁻ ions, such as saline.

In the case of quartz glass, electron conduction is essentially dominant, but in reality, it is said that the conductivity depends on the amount of metal impurities contained therein. Among them, alkali metal impurities have a clear correlation with conductivity depending on the amount (R. Bruckner, J. Non-Crysta.
lline Solids, 123 (5), 177-216 (1970)). by the way,
At present, the amount of metallic impurities in silica glass, which is handled as an optical body for photolithography in the ultraviolet region, has been reduced to the ppb order. Therefore, when comparing the ionic conduction due to impurities with the intrinsic electronic conduction, it is considered that the electronic conduction is much larger and the contribution of the ionic conduction to the conductivity of the silica glass is almost negligible.
That is, the conductivity of silica glass with reduced metal impurities can be considered to be equivalent to the conductivity due to electron conduction of its essential composition.

However, like other oxides, the ratio of Si to O in quartz glass changes non-stoichiometrically depending on the synthesis conditions, heat treatment conditions, etc. Therefore, it is better to express it as SiO _2−x or SiO _{2 + y.} Be accurate. These SiO _2-x , Si
The relationship of O _{2 + y} is as follows.

[0013]

Embedded image

This equilibrium reaction depends on the temperature and the ambient oxygen pressure from the law of mass action and the relational expression with Gibbs free energy. Therefore, if the oxygen pressure and temperature are determined, one composition is inevitably determined. In equation (1), when X = 0 or Y = 0, the equilibrium state with the theoretical composition is shown.

In the silica glass having an arbitrary ratio of Si and O, the equilibrium of the formula (1) shifts to the right to generate oxygen deficiency defects such as Si-Si, and the balance of the formula (1) shifts to the left. Si-O-O
-Oxygen excess defects such as Si are generated. The generated electrons and holes are reflected in the conductivity of quartz glass. Further, an oxygen deficiency / excessive defect is generated, so that an absorption band is generated in the ultraviolet region.

From the above, it can be seen that the ratios of Si and O of silica glass having the same conductivity are the same, and the characteristics of the same light transmittance can be obtained. Therefore, with any Si
Quartz glass with O ratio is heated in an oxygen partial pressure atmosphere and the conductivity of the quartz glass is monitored.
Quartz glass close to the equilibrium composition can be obtained by performing heat treatment in which the equilibrium reaction converges to a constant value in the range of 0 ^-10 S · cm ^{−1 to} 10 ⁻⁶ S · cm ⁻¹ . However, it is difficult to confirm the equilibrium state actually, but the same effect can be obtained by completing the heat treatment by creating a constant temperature state in which the temperature rising / falling and oxygen partial pressure are adjusted so that the conductivity can be maintained within the above range. Obtainable.

Regarding the conductivity of quartz glass, if the temperature is less than 500 ° C., the electron conduction becomes very small, so that it is difficult to observe by the method used in the present invention, and the reliability of the measured value becomes low. Will end up. For this reason, the value obtained at 500 ° C. or higher is preferable. Also, the pressure is 5kgG
When it is higher, the deviation from the equilibrium state becomes more significant as compared with normal pressure, so heat treatment at 5 kgG or less is preferable.

The heat treatment is usually carried out by raising the temperature to the holding temperature, holding it for a certain period of time, and then gradually lowering the temperature. Although the present invention is carried out in a conventional heat treatment configuration, maintaining conductivity is a top priority. Therefore, in an arbitrary oxygen partial pressure atmosphere, the conductivity of the quartz glass is monitored as the temperature rises, and the conductivity is likely to exceed the upper limit of the range of the present invention before the predetermined holding temperature is reached, that is, during the temperature rising process. In that case, the temperature is interrupted or the oxygen partial pressure is manipulated to maintain the conductivity. The holding time is empirically determined in consideration of the reliability of the diameter and conductivity of the quartz glass.

According to the present invention, it is possible to ensure quality that can be used in precision optical instruments in the ultraviolet or vacuum ultraviolet region of 200 nm or less. In particular, by using a high-purity transparent synthetic quartz glass body having no distortion and no striae and a highly uniform refractive index distribution as the starting material for the heat treatment of the present invention, it can be used in precision optical instruments such as steppers. Laser wavelength (193
Quartz glass with a high transmittance of 99.9% or more and a highly uniform refractive index distribution can be obtained.

[0020]

【Example】

[Example 1] A high-purity quartz glass ingot uses high-purity silicon tetrachloride as a raw material, and an oxygen gas and a hydrogen gas are mixed and burned by a quartz glass burner, and the raw material gas is diluted with a carrier gas from a central portion. Then, it was ejected, deposited on the target, melted, and synthesized. During synthesis, the raw material gas is reacted with water generated by combustion of the surrounding oxygen gas and hydrogen gas, and vitrified and deposited on the target made of an opaque quartz glass plate below the burner. Rotates and oscillates at a constant cycle,
Further, the quartz glass ingot was obtained by simultaneously lowering the position of the ingot portion at the same distance from the burner.

Regarding these quartz glass ingots,
When the OH group content was measured by infrared absorption spectroscopy (the amount of absorption by the OH group at 1.38 μm is measured), it was 1200 ppm. In addition, when the chlorine and sodium contents were examined by activation analysis, they were 20 ppm and 5 ppb, respectively. Furthermore, contained metal impurities (Mg, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Co, M
Quantitative analysis of n) was carried out by inductively coupled plasma emission spectroscopy, and it was found that the concentration was 20 ppb or less and the purity was high.

A test piece was cut from this quartz glass ingot.
Produce a sample for transmittance measurement and put it in the vacuum ultraviolet region.
Transmittance from 100 nm to 240 nm was measured using a spectrophotometer.
As a result, it was 99.9% at 193 nm. However, this stone
The variation of refractive index of British glass ingot is △ n = 2.6x10
^-FiveSince it was an optically inhomogeneous quartz glass,
Make a processed sample from the ingot and perform homogenization heat treatment
went. That is, the treated sample is SiO₂Made of powder
N in the matrix₂Atmosphere, 5kg / cm²With a heater under pressure
Temperature is maintained at 1900 ℃ for 2 hours, and then at 50 ℃ / hr.
Heat treated. As a result, the refractive index of the processed sample varies.
△ n is 2.0x10^-6(See JP-A-5-116969).
In this way, there is no distortion or striae, and it is a highly pure and transparent synthetic material of high homogeneity.
A quartz glass body could be obtained.

With respect to the transmittance of this treated sample, a test piece was cut out by the same operation as above and measured. As a result, a broad absorption band was generated around 200 nm, and the transmittance at 193 nm was reduced to 99.0%. (Fig. 2-II). Therefore, heat treatment was performed according to the present invention. The treated sample was placed in a furnace and sealed. After evacuating with a vacuum pump once, N ₂ gas was introduced into the furnace to 1 atm and the temperature was raised to 1600 ° C. at a rate of 200 ° C./hr. The conductivity of the sample at this point is 10 ⁻⁷ S · cm
It was ^-1 . The method of measuring the conductivity is shown in FIG. The sample was treated for 60 hours in this atmosphere and at a temperature, and the conductivity of the sample was measured immediately before entering the cooling process and found to be ⁶ × 10 ⁻⁶ S · cm ⁻¹ . The sample was taken out by cooling to 500 ° C. at a cooling rate of 10 ° C./hr and then allowing to cool to room temperature. The atmosphere at this time was kept the same as that of the treatment. When the sample taken out was polished and the transmittance was measured, the 10 mm internal transmittance at 193 nm was 99.9% or more (Fig. 2-I). In addition, other optical properties (distortion, striae, homogeneity) were not impaired. [Embodiment 2] Striae and distortion are eliminated by the same method as in Embodiment 1,
A high-purity transparent synthetic quartz glass having a highly uniform refractive index distribution was prepared. The transmittance of this treated sample was cut out from the test piece by the same operation as above, and the measurement showed that a broad absorption band was generated around 200 nm, and the transmittance at 193 nm was
Since it had fallen to 99.0%, heat treatment was performed according to the present invention. That is, this treated sample was left to stand in a furnace and sealed. Once exhausted with a vacuum pump, Ar gas is fed into the furnace.
It was introduced to 1 atm and heated up to 1500 ° C at a rate of 200 ° C / hr. The conductivity of the sample at this point was 8 × 10 ⁻⁸ S · cm ⁻¹ . This atmosphere, subjected to 60hr at a temperature, where the conductivity of the sample immediately prior to entering the cooling process was measured, 3x10 ^-7
It was S · cm ⁻¹ . The sample was taken out by cooling to 500 ° C. at a cooling rate of 10 ° C./hr and then allowing to cool to room temperature. The atmosphere at this time was kept the same as that of the treatment. When the sample taken out was polished and the transmittance was measured, the internal transmittance at 10 mm at 193 nm was 99.9% or more as in Example 1. Moreover, other optical properties were not impaired. [Comparative Example 1] In the same manner as in Example 1, a high-purity transparent quartz glass having no striae and no distortion and a highly uniform refractive index distribution was prepared. The transmittance of this treated sample was cut out from the test piece by the same operation as above and measured. As a result, a broad absorption band was generated around 200 nm, and the transmittance at 193 nm was reduced to 99.0%. This treated sample was placed in a furnace and sealed. After evacuation with a vacuum pump once, Ar gas was introduced into the furnace to 1 atm and the temperature was raised to 1600 ° C. at a rate of 200 ° C./hr. The conductivity of the sample at this point is 1x10 ^-6 S
It was cm ^-1 . Perform 120hr treatment in this atmosphere and temperature,
When the conductivity of the sample was measured immediately before entering the cooling process, it was ¹ × 10 ⁻⁵ S · cm ⁻¹ . 500 at a cooling rate of 10 ℃ / hr
The sample was taken out after cooling to 0 ° C. and then allowing to cool to room temperature. The atmosphere at this time was kept the same as that of the treatment. When the sample taken out was polished and the transmittance was measured, the 10 mm internal transmittance at 193 nm was 99.2%, and disappearance of absorption could not be confirmed. Other optical properties were intact.

[0024]

As described above, according to the present invention,
The secondary treatment is carried out by performing the homogenization treatment such as pressurization and heating while the electric conductivity of the quartz glass is between 10 ^-10 S ・ cm ^{-1 and} 10 ^-6 S ・ cm ^-1. 200nm of Si-Si etc.
Generation of structural defects that have absorption in the following wavelength bands can be suppressed, and as a result, the light transmittance of UV or vacuum UV light of 200 nm or less can be improved to a level that can be used in precision optical equipment.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a method of measuring conductivity during heat treatment of a sample.

FIG. 2 is a schematic diagram showing an example of the transmittance of a treated sample, where I is based on a comparative example and II is based on an example.

[Explanation of symbols]

 1 ... DC power supply 2 ... Ammeter 3 ... Quartz glass sample 4 ... Heat treatment container 5 ... Electrode

Claims (2)

[Claims] 1. A high-purity transparent synthetic quartz glass body of 10 ^-10 S · cm
^-1 or more and 10 ^-6 S · cm ^-1 or less, a method for heat treatment of quartz glass, characterized in that it is kept in a constant temperature state. 2. The heat treatment method according to claim 1, wherein the isothermal state is at a temperature of 500 ° C. or higher under an arbitrary oxygen partial pressure.
A heat treatment method for quartz glass, which is characterized in that the pressure is not higher than kgG.