JP3237043B2 - Heat treatment method for quartz glass and synthetic quartz glass - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heat-treating quartz glass used in an optical system of a general vacuum ultraviolet laser.

[0002]

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

In general, 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, so that synthetic silica glass or CaF ₂ (fluorescent) is used instead of conventional optical glass. It has been proposed to use a single crystal of fluoride such as stone). The optical system mounted on the stepper is composed of a combination of many lenses, and even if the transmittance reduction per lens is small, it is integrated by the number of lenses used, and the light amount on the irradiation surface Therefore, high transmittance is required for the optical material. Further, as the wavelength used becomes shorter, the imaging performance becomes extremely poor even with a very small unevenness of the refractive index distribution (nonuniformity of the refractive index).

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 UV resistance, and cannot be used in precision optical instruments as described above. Was. For this reason, secondary treatment for homogenization (Japanese Patent Publication No. 03-17775, JP-A-64-28240) and improvement of homogeneity and laser resistance by heat treatment in pressurized hydrogen gas (JP-A-03-109233) Etc. have been proposed.

[0005]

However, in the conventional secondary treatment by heating and pressurizing, the light transmittance after the 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 high homogeneity of the refractive index was obtained, it was not sufficient in terms of light transmittance as an optical body for ArF excimer laser light (193 nm).

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

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the cause of the decrease in light transmittance due to the heat treatment, and found that the dominant factor is the theory of the amount of oxygen contained in quartz glass. It was found that this was due to a large deviation from the composition SiO ₂ (two Os per Si).

Therefore, as a result of further study to maintain the theoretical composition, it was found that by adjusting the conductivity at a constant temperature of the heat treatment, a decrease in the light transmittance in the ultraviolet and vacuum ultraviolet regions due to the heat treatment was prevented. It turned out to be possible.
Furthermore, in the case of using a quartz glass body having absorption in the ultraviolet to vacuum ultraviolet region, by adjusting the conductivity,
It has been found that the absorption can be eliminated.

Accordingly, the present invention provides a high purity transparent synthetic quartz gas.
Lath body 10^-TenS ・ cm^-1More than 10^-6S ・ cm ^-1Constant with the following conductivity
Heat treatment of quartz glass characterized by holding at a temperature
Provide a way. The feature of the present invention is that
By specifying the conductivity of the glass,
Reduces basic structure of Si-Si and Si-O-Si generated by
Absorption in the wavelength band of 200 nm or less, such as structural defects
The point is that the generation of income can be prevented. This
And the light transmittance of quartz glass
Pull down to a level that can be used with precision optical equipment in the vacuum ultraviolet region.
Can be lifted. Especially using ArF excimer laser
In the case of a stepper, optimize the composition of quartz glass.
Very useful because the absorption can be eliminated by
It is.

[0010]

[Effect] Generally, there are two types of conductivity of a solid substance, an electronic conductivity and an ionic conductivity, and one of them is dominant depending on the substance. For example, those that show electrical conduction by the movement of electrons, such as metals, are dominated by electron conduction,
Na ⁺ and Cl as saline ^- shows the electrical conductivity by the movement of ions are dominant ionic conduction.

[0011] In the case of quartz glass, electron conduction is essentially dominant, but it is said that the electrical conductivity is actually affected by the amount of metal impurities contained therein. Among them, alkali metal impurities have a clear correlation with conductivity depending on their amount (R. Bruckner, J. Non-Crysta
lline Solids, 123 (5), 177-216 (1970)). by the way,
At present, the amount of metal impurities in quartz glass, which is treated as an optical body for photolithography in the ultraviolet region, has been reduced to the order of ppb. For this reason, when comparing the ionic conduction due to impurities with the essential electronic conduction, the electron conduction is much larger, and the contribution of the ionic conduction to the conductivity of the quartz glass is considered to be almost negligible.
In other words, the conductivity of quartz glass with reduced metal impurities can be considered to be equivalent to the conductivity due to electronic conduction of its essential composition.

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

[0013]

Embedded image

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

In quartz glass having an arbitrary ratio of Si to O, the equilibrium of the formula (1) shifts to the right to generate oxygen deficiency defects such as Si-Si, and the equilibrium 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, the generation of oxygen deficiency / excessive defects generates an absorption band in an ultraviolet region.

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

When the conductivity of quartz glass is lower than 500 ° C., the electron conduction becomes very small. Therefore, it is difficult to observe the conductivity by the method used in the present invention, and the reliability of the measured value becomes low. Would. For this reason, it is preferable that the value is obtained at 500 ° C. or higher. The pressure is 5kgG
If the pressure is higher, the deviation from the equilibrium state becomes remarkable as compared with the normal pressure. Therefore, the heat treatment at 5 kgG or less is preferable.

The heat treatment usually comprises a temperature operation in which the temperature is raised to a holding temperature, held for a certain time, and then gradually lowered. Although the present invention is carried out by a conventional heat treatment configuration, the highest priority is given to maintaining conductivity. Therefore, in any oxygen partial pressure atmosphere, the conductivity of the quartz glass accompanying the temperature rise is monitored, and the conductivity is likely to exceed the upper limit of the range of the present invention before reaching the predetermined holding temperature, that is, during the temperature rise process. In such a case, the conductivity is maintained by interrupting the temperature rise or operating the oxygen partial pressure. The holding time is determined empirically, taking into account the reliability of the diameter and conductivity of the quartz glass.

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

[0020]

EXAMPLES Example 1 A high-purity quartz glass ingot uses high-purity silicon tetrachloride as a raw material, mixes and burns oxygen gas and hydrogen gas with a burner made of quartz glass , and discharges the raw material gas from the center. Diluted with carrier gas and ejected, deposited on target,
It was melted and synthesized. At the time of synthesis, the raw material gas is reacted with water generated by the combustion of the surrounding oxygen gas and hydrogen gas, and is vitrified and deposited on a target formed of an opaque quartz glass plate below the burner. A quartz glass ingot was obtained by rotating and oscillating at a constant cycle, and simultaneously lowering the ingot to keep the position of the ingot at the same distance from the burner.

For these quartz glass ingots,
The OH group content was measured by infrared absorption spectroscopy (measuring the amount of absorption by 1.38 μm OH groups) and found to be 1200 ppm. The contents of chlorine and sodium determined by activation analysis were 20 ppm and 5 ppb, respectively. Furthermore, the contained metal impurities (Mg, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Co, M
When the quantitative analysis of n) was performed by inductively coupled plasma emission spectroscopy, the concentrations were found to be 20 ppb or less and high purity.

A test piece is cut from the quartz glass ingot.
And prepare a sample for transmittance measurement.
Using a spectrophotometer, measured the transmittance from 100 nm to 240 nm
As a result, it was 99.9% at 193 nm. However, this stone
British ingots have a refractive index variation of △ n = 2.6x10
^-FiveThe optically heterogeneous quartz glass of
Produce a processed sample from the ingot and perform heat treatment for homogenization.
went. That is, the processed sample is_TwoMade of powder
In the matrix, N_TwoAtmosphere, 5kg / cm^TwoHeater under pressure
Temperature, hold at 1900 ° C for 2 hours, then cool at 50 ° C / hr
Heat treated. As a result, variations in the refractive index of the processed sample
Key is 2.0x10^-6(See JP-A-5-116969).
In this way, high-purity transparent synthesis of high homogeneity without distortion and striae
A quartz glass body was obtained.

The transmittance of this treated sample was measured by cutting out a test piece by the same operation as described above and measuring it. 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. Once evacuated with a vacuum pump, N ₂ gas was introduced into the furnace up 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 Scm
It was ^-1 . The method of measuring the conductivity is shown in FIG. The sample was treated in this atmosphere and at the same temperature for 60 hours, and the conductivity of the sample was measured immediately before the cooling process. The result was ⁶ × 10 ⁻⁶ S · cm ⁻¹ . The sample was cooled to 500 ° C. at a temperature decreasing rate of 10 ° C./hr, then allowed to cool to room temperature, and a sample was taken out. At this time, the atmosphere was kept the same as that during the processing. The sample taken out was polished and measured for transmittance. As a result, the internal transmittance at 10 mm at 193 nm was 99.9% or more (FIG. 2-I). In addition, other optical properties (strain, striae, homogeneity) were not impaired. [Embodiment 2] In the same manner as in Embodiment 1, there was no striae and no distortion.
A high-purity transparent synthetic quartz glass having a highly homogeneous refractive index distribution was prepared. The transmittance of this treated sample was measured by cutting out a test piece in the same manner as above, and a broad absorption band was generated at around 200 nm.
Since it was reduced to 99.0%, heat treatment was performed according to the present invention. That is, this treated sample was allowed to stand in a furnace and sealed. Once evacuated with a vacuum pump, Ar gas was introduced into the furnace.
It was introduced up 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 cooled to 500 ° C. at a temperature decreasing rate of 10 ° C./hr, then allowed to cool to room temperature, and a sample was taken out. At this time, the atmosphere was kept the same as that during the processing. The sample taken out was polished and measured for transmittance. As in Example 1, the internal transmittance at 10 mm at 193 nm was 99.9% or more. Also, 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 uniform refractive index distribution was prepared. The transmittance of this treated sample was measured by cutting out a test piece by the same operation as described above, and a broad absorption band was generated at around 200 nm, and the transmittance at 193 nm was reduced to 99.0%. The treated sample was left standing in a furnace and sealed. Once evacuated with a vacuum pump, Ar gas was introduced into the furnace up to 1 atm and the temperature was raised to 1600 ° C at a rate of 200 ° C / hr. At this point the conductivity of the sample is 1x10 ^-6 S
cm ^-1 . 120 hours treatment in this atmosphere and temperature,
When the conductivity of the sample was measured immediately before the cooling process, it was ¹ × 10 ⁻⁵ S · cm ⁻¹ . 500 at 10 ° C / hr cooling rate
C., and then allowed to cool to room temperature, and a sample was taken out. At this time, the atmosphere was kept the same as that during the processing. When the sample taken out was polished and the transmittance was measured, the internal transmittance at 10 mm at 193 nm was 99.2%, and no disappearance of absorption was confirmed. Other optical properties were not impaired.

[0024]

As described above, according to the present invention,
Secondary treatment is performed by performing homogenization treatment by applying pressure, heating, etc., when the conductivity of the quartz glass is between 10 ^-10 S · cm ^{-1 and} 10 ^-6 S · cm ^-1. 200nm for Si-Si etc.
The generation of structural defects having absorption in the following wavelength bands is suppressed, and as a result, the light transmittance of ultraviolet or vacuum ultraviolet light of 200 nm or less can be improved to a level that can be used in precision optical instruments.

[Brief description of the drawings]

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

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

[Explanation of symbols]

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

Continuation of front page (56) References JP-A-5-116969 (JP, A) JP-A-1-197335 (JP, A) JP-A-62-158121 (JP, A) JP-A-3-109233 (JP) JP-A-64-28240 (JP, A) JP-B-47-2272 (JP, B1) JP-B-34-6987 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB Name) C03B 32/00-32/02 C03B 20/00

Claims (4)

(57) [Claims] (1) A high-purity transparent synthetic quartz glass body is 10 ^-10 S · cm.
^-1 or 10 ^-6 S · cm ^-1 or less conductivity, heat treatment method of the quartz glass, characterized in that to hold the constant temperature state. 2. The heat treatment method according to claim 1, wherein the constant temperature state is a temperature of 500 ° C. or more under an arbitrary oxygen partial pressure.
A heat treatment method for quartz glass, wherein the pressure is not more than kgG. 3. A high-purity transparent synthetic quartz glass body of 10 ^-Ten S ・ cm
^-1 More than 10 ^-6 S ・ cm ^-1 Keep the temperature constant with the following conductivity
Obtained, free of oxygen deficiency defects and oxygen excess defects
Synthetic quartz glass. 4. A high purity transparent synthetic quartz glass body ^-Ten S ・ cm
^-1 More than 10 ^-6 S ・ cm ^-1 Keep the temperature constant with the following conductivity
Resulting synthesis with 10mm internal transmittance at 193nm of 99.9% or more
Quartz glass.