JP2000281359A - Production of synthetic quartz glass and control of hydrogen molecule concentration in the synthetic quartz glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical member wherein the concentration of dissolved hydrogen molecules is in an appropriate range by forming silica fine particles in a flame by ejecting a silicon compound and a combustion gas while keeping the temp. of a furnace within a specific temp. range and depositing synthetic quartz glass on a base body. SOLUTION: The temp. in the furnace is kept within ±25 deg.C of a prescribed temp. The concentration of dissolved hydrogen molecules contained in the synthetic quartz glass is preferably set to be in an appropriate range, concretely, the range of >=1×1017 and <=5×1018 molecule/cm3 in order to suppress lowering of transmissivity and formation of compaction by irradiation of light. Such synthetic quartz glass ingot is obtained by always keeping the temp. of the furnace within the temp. deviation range of 50 deg.C from a prescribed temp. (the prescribed temp. ±25 deg.C). The temp. in the furnace is kept constant by feeding back the measured temp. in the furnace and controlling the flow rate of combustion gas according to the temp. in the synthetic furnace, measured with a thermocouple, or the like. The concentration of hydrogen molecules depends on the temp. in the furnace.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing synthetic quartz glass suitable for use in an optical system of an ultraviolet laser having a wavelength of 300 nm or less such as excimer laser lithography, and a method for estimating the concentration of dissolved hydrogen in synthetic quartz glass. .

[0002]

2. Description of the Related Art Conventionally, optical lithography techniques for exposing and transferring an integrated circuit pattern on a silicon wafer include:
An exposure apparatus called a stepper has been used. With the recent miniaturization of integrated circuits, exposure light sources for steppers have changed from i-line (365 nm) to KrF excimer laser (248 nm), Ar
Shortening of the wavelength to F excimer laser (193 nm) is being promoted. Here, the optical glass that has been generally used in a stepper using light in a wavelength region longer than the i-line as a light source has a significantly reduced light transmittance in a wavelength region shorter than the i-line, and a wavelength region shorter than 250 nm. In this case, most optical glasses do not transmit light. For this reason, in a stepper using an excimer laser as a light source, it has been proposed to use an optical member made of a single crystal of fluoride such as quartz glass or fluorite instead of conventional optical glass. Quartz glass and fluorite are widely used materials for optical systems in the ultraviolet and vacuum ultraviolet regions.

[0003] Optical materials used in stepper optical systems, particularly projection optical systems, are required to have very high transmittance at the wavelength of the light source. In the projection optical system of a stepper, the total length may be about 1000 mm. At this time, if an attempt is made to achieve a throughput of 80% or more (transmittance through the entire optical system) with the projection optical system, the inside of the optical material used will be reduced. The absorption coefficient of absorption is required to be 0.002 cm-1 or less.

Further, in order to expose an integrated circuit pattern with high resolution and in a wide range, very high refractive index homogeneity is required. For example, the diameter is required to be 10 ^-6 order or less within a diameter of about 200 mm. Synthetic quartz glass is used as the quartz glass satisfying the above conditions. In particular, synthetic quartz glass manufactured by a manufacturing method called a direct method in a gas phase synthesis method has high purity and high homogeneity even in a large diameter, so it is used for a stepper optical system using an excimer laser. It is expected as a member.

As described above, the optical system member of the stepper is required to have high transmittance and high refractive index homogeneity over the operation period of the stepper. However,
In general, synthetic silica glass has a structure of E 'center (≡Si., Where ≡ indicates three single bonds with three oxygen atoms by irradiation of ultraviolet light, and indicates that it is a radical. 215 nm), which is caused by a structural defect called “shown”, the transmittance is significantly lowered by irradiation with light in the ultraviolet region. The precursor of the E ′ center includes a {Si—Si} bond, which is a defect in the basic structure, a ≡Si—H bond and a ≡Si—bond due to impurities taken into synthetic quartz glass.
Cl bonding and the like are considered.

Further, a synthetic silica glass is irradiated with light in an ultraviolet region including an excimer laser to cause a phenomenon called compaction, which causes a region having a non-uniform refractive index. The compaction is a phenomenon in which a bond of a basic structure is cut and recombined in a portion irradiated with high-energy ultraviolet light, and a dense and high-density region is locally formed to increase a refractive index.

One of the indices for maintaining the transmittance and refractive index homogeneity of a synthetic quartz glass at a high value over the operation period of a stepper is a synthetic quartz glass measured by Raman spectroscopy or infrared spectroscopy. Dissolved hydrogen molecule concentration. The dissolved hydrogen molecule concentration in the synthetic quartz glass used as a member for the optical system generally differs greatly depending on the manufacturing method, and the dissolved hydrogen molecule concentration of the synthetic quartz glass manufactured by the VAD method is on the order of 10 ¹⁶ molecules / cm ³ or less. In contrast, the concentration of dissolved hydrogen molecules in the synthetic quartz glass produced by the direct method is higher than 10 ¹⁶ molecules / cm ³ -10
It is said to be on the order of ¹⁸ molecules / cm ³ .

[0008]

In order for the synthetic quartz glass to maintain high transmittance and refractive index homogeneity over the operation period of the stepper, it is necessary to reduce structural defects caused by irradiation with ultraviolet light. If the concentration of dissolved hydrogen molecules in the synthetic quartz glass is low, the absorption at the E ′ center caused by the irradiation of ultraviolet light is not reduced, so that the concentration of hydrogen molecules dissolved in the synthetic quartz glass is 1 × 10 ¹⁷ molecules / cm ^3. The above is considered preferable.

As a method for increasing the concentration of dissolved hydrogen in synthetic quartz glass, the synthesis conditions are optimized at the stage of synthesis,
There are a method of obtaining synthetic quartz glass with a high hydrogen molecule concentration, and a method of containing hydrogen molecules by heat treatment in a hydrogen atmosphere after synthesis.The latter method may include impurities during the heat treatment. It is desirable to increase the concentration.

[0010] On the other hand, if the concentration of dissolved hydrogen molecules is too high in the synthesis step, the ≡Si-
Synthetic quartz glass with many H bonds can be obtained. In such a synthetic quartz glass, the transmittance decreases and compaction occurs due to light irradiation. Therefore, the dissolved hydrogen molecules in the synthetic quartz glass in the synthesis stage need to be in an appropriate concentration range. An appropriate range is, for example, 1 × 10 ¹⁷ molecules / cm ³ or more and 5 ×
It is 10 ¹⁸ molecules / cm ³ or less.

In the synthetic quartz glass ingot manufactured by the direct method, the synthesis situation fluctuates part by part due to external factors. Therefore, the concentration of dissolved hydrogen molecules is not uniform in the whole synthetic quartz glass ingot but varies depending on the synthesis situation. To produce a concentration distribution. Therefore, even in a synthetic quartz glass manufactured under the same synthesis conditions or in the same synthetic quartz glass ingot, there may be a portion where the hydrogen molecule concentration is not in the proper range.

Although the concentration of dissolved hydrogen molecules in a synthetic quartz glass ingot can be measured by cutting out a sample from each position of the completed synthetic quartz glass ingot, it is not rational from an economic viewpoint. As described above, the value of the hydrogen molecule concentration of the entire synthetic quartz glass ingot cannot be obtained except for the method of directly measuring the dissolved hydrogen molecule concentration after the conventional synthesis, and the dissolved hydrogen molecule concentration is within an appropriate range over the entire ingot. It was difficult to obtain synthetic quartz glass.

Therefore, in the present invention, by estimating and estimating the dissolved hydrogen molecule concentration of the synthetic quartz glass ingot,
It is an object of the present invention to provide a method for producing a synthetic quartz glass ingot having an appropriate hydrogen molecule concentration range.

[0014]

As described above, in order to optimize the value of the dissolved hydrogen molecule concentration of the synthetic quartz glass ingot, it is not a method of obtaining the value of the hydrogen molecule concentration after the synthesis by direct batch measurement but a synthesis method. There is a need for a method for continuously monitoring physical quantities that correlate with both the concentration of dissolved hydrogen molecules and the state of synthesis.

Conventionally, as a method of monitoring the synthesis status,
2. Description of the Related Art The shape of a synthetic quartz glass ingot, which is a measure of the supply of heat required for hydrolysis and subsequent deposition of quartz glass fine particles and vitrification, has been monitored. If the ingot is low in temperature and high in viscosity, the amount of oxyhydrogen gas (combustion gas) is increased, and if the ingot is high in temperature and low in viscosity, it is decreased. Become

In the present invention, in particular, as a result of studying physical properties showing a high correlation with the dissolved hydrogen concentration in the synthetic quartz glass, the phenomenon that the dissolved hydrogen molecule concentration becomes extremely small when the furnace temperature is low is considered. They found that monitoring the furnace temperature was an effective tool for estimating the concentration of dissolved hydrogen molecules. From the above, by measuring the furnace temperature in the furnace, it is possible to estimate the viscosity and temperature of the ingot and their distribution and correlate them with the production amount and diffusion rate of dissolved hydrogen molecules, and to estimate the concentration of dissolved hydrogen molecules Was reached. Since the furnace temperature varies depending on the position in the furnace, it is preferable to measure the furnace temperature at a plurality of locations in the furnace. Since the in-furnace temperature measurement is continuously performed during the synthesis period of the ingot, the concentration of dissolved hydrogen molecules in the ingot can be continuously estimated. Furthermore, since the concentration of dissolved hydrogen molecules can be estimated during the synthesis period, the synthesis conditions such as the amount of oxyhydrogen gas are adjusted so that the viscosity and temperature of the ingot and their distribution are appropriate, and the dissolved quartz glass ingot is dissolved. It is possible to control the hydrogen molecule concentration in an appropriate range.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a synthetic quartz glass synthesized by a direct method which is one of the gas phase synthesizing methods, without relying on the direct measurement of the concentration of hydrogen molecules dissolved in the synthetic quartz glass. A method for estimating the concentration, in which the temperature in the furnace during the synthesis period is monitored to estimate the viscosity and temperature of the ingot and their distribution, and to correlate them with the amount of hydrogen molecules produced in the synthetic quartz glass, the diffusion rate, etc. Is a method for estimating the concentration of dissolved hydrogen molecules by In addition, the furnace temperature is continuously monitored during the synthesis period of the ingot, and the synthesis conditions are adjusted so that the viscosity and temperature of the ingot and the distribution thereof become appropriate. This is a method for producing synthetic quartz glass in which the concentration of dissolved hydrogen molecules in an ingot is controlled within an appropriate range.

As described above, since the temperature in the synthesis furnace during the production of the synthetic quartz glass has a high correlation with the concentration of dissolved hydrogen molecules in the synthetic quartz glass, the temperature in the synthesis furnace is set within a predetermined temperature ± 10 ° C. , It is possible to suppress variations in the concentration of dissolved hydrogen in the obtained synthetic quartz glass. In particular, in the case of synthetic quartz glass used as an optical system member of an optical device using an excimer laser as a light source, the concentration of dissolved hydrogen molecules in the synthetic quartz glass is reduced in order to suppress the decrease in transmittance and the occurrence of compaction due to light irradiation. It is preferable to set it to an appropriate range, specifically 1 × 10 ¹⁷ molecules / cm ³ or more and 5 × 10 ¹⁸ molecules / cm ³ or less. Is always kept within a predetermined temperature range within 50 ° C. (predetermined temperature ± 25 ° C.). This temperature range varies depending on the measurement position of the furnace temperature,
For example, if the temperature is between the synthetic surface and the side surface of the quartz glass ingot, the furnace temperature is always 1200 to 1250 ° C.
Keep within the range. When the temperature in the furnace becomes 1200 ° C. or lower, the concentration of dissolved hydrogen molecules decreases.

In order to maintain the furnace temperature within a certain temperature range, it is necessary to feed back the measured furnace temperature to the synthesis conditions. For example, the combustion gas is changed according to the temperature in the synthesis furnace measured by a thermocouple. By controlling the flow rate, the temperature in the furnace can be kept within a certain temperature range. FIG.
Shows the hydrogen molecule concentration distribution of the ingot estimated using the method of the present invention, and the hydrogen molecule concentration of the ingot measured by a conventional method (a method in which a sample is cut out from a plurality of portions of the ingot after synthesis and subjected to direct batch measurement). . According to this, there is a correlation between the temperature inside the synthesis furnace and the hydrogen molecule concentration obtained by performing direct batch measurement, that is, it is possible to estimate the dissolved hydrogen molecule concentration from the measurement result of the furnace temperature. It can be seen (the estimated hydrogen molecule concentration is indicated by a broken line).

FIG. 2 shows a hydrogen molecule concentration distribution of quartz glass synthesized by using the manufacturing method of the present invention. According to this, it is possible to optimize the hydrogen molecule concentration distribution by predicting the hydrogen molecule concentration from the measurement result of the furnace temperature and adjusting the synthesis conditions accordingly. 1 and 2, the measured value of the hydrogen molecule concentration was obtained by Raman spectrophotometry. Hydrogen molecule concentration was determined from the ratio of the intensities of the peaks due to H-H bonds appears at 4315cm ^-1, and the intensity of the peak due to the basic structure of the quartz glass appear on the 800 cm ^-1.

[0021]

<Example 1> Synthetic quartz glass was synthesized using silicon tetrachloride as a raw material using an apparatus shown in FIG. Silicon tetrachloride was supplied from a silicon tetrachloride supply device using oxygen as a carrier gas, and was vaporized by baking. A mass flow controller was used for flow control. The raw material gas is ejected from a quartz glass burner central tube with a quintuple structure, and this is hydrolyzed in a flame by oxygen and hydrogen supplied from the tube outside the burner, and rotates and swings at a constant cycle Fine quartz glass particles were deposited on the target and vitrified to form a quartz glass ingot. At this time, simultaneous lowering kept the upper surface of the ingot at a fixed position from the burner. During the synthesis period, the temperature was measured at eight locations in the furnace using a thermocouple, and the change over time was visualized and recorded by a computer.

The concentration distribution of dissolved hydrogen molecules in the ingot was estimated from the time-dependent changes in the furnace temperature of the quartz glass ingot thus synthesized. In addition, test pieces were cut out from several places in the direction of growth of the center of rotation of the ingot and polished to obtain samples for measuring the concentration of dissolved hydrogen molecules. <Example 2> A high-purity quartz glass ingot was synthesized by the same synthesis method as in Example 1, using silicon tetrachloride as a raw material. During the synthesis period, the concentration of dissolved hydrogen molecules in the ingot is estimated based on the furnace temperatures monitored at eight locations inside the furnace, and the amount of oxyhydrogen gas is increased or decreased so that the dissolved hydrogen molecule concentration is in an appropriate range. The furnace temperature was controlled to the target temperature ± 10 ° C. A test piece was cut out from several places in the direction of growth of the ingot in the direction of the rotation center and polished to obtain a sample for measuring the concentration of dissolved hydrogen molecules.

In this embodiment, the value of the dissolved hydrogen molecule concentration was determined by examining the correlation between the values from a plurality of thermocouples provided in the furnace and the dissolved hydrogen molecule concentration. The temperature in the furnace obtained by the thermocouple at the height between the composite surface and the side surface of the ingot was used as a representative value. FIG.
As is clear from the comparison between the estimated value of the dissolved hydrogen molecule concentration in Example 1 and the measured value of the dissolved hydrogen molecule concentration (actually measured value), the estimated value of the dissolved hydrogen molecule concentration according to the present invention well corresponds to the actual hydrogen molecule concentration. Thus, the method of the present invention for estimating the concentration of dissolved hydrogen molecules in synthetic quartz glass was appropriate.

The concentration of dissolved hydrogen molecules in the ingot when synthesis is performed by feeding back the measurement result of the furnace temperature to the flow rate of the combustion gas according to the second embodiment is within an appropriate range over the entire ingot. The synthetic quartz glass produced by the above-mentioned method had sufficient quality as an optical member for ultraviolet light.

[0025]

According to the present invention, it is possible to easily estimate the concentration of dissolved hydrogen molecules at an arbitrary position on a synthetic quartz glass, and it is not necessary to directly measure the concentration of dissolved hydrogen molecules, and the synthetic quartz glass optics having an appropriate hydrogen molecule concentration range is unnecessary. A member can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing a measured value and an estimated value of a temporal change in a furnace temperature during a synthesis period of a quartz glass synthesized in Example 1, and a distribution of a dissolved hydrogen molecule concentration in an ingot.

FIG. 2 is a graph showing a measured value and an estimated value of a temporal change in a furnace temperature during a synthesis period of a quartz glass synthesized in Example 2 and a concentration distribution of dissolved hydrogen molecules in an ingot.

FIG. 3 is a schematic diagram of a quartz glass synthesizing apparatus used in an example of the present invention. 1. Quartz tube burner made of quartz glass 2. Quartz glass ingot 3. Quartz glass target 4. Refractory 5. Furnace frame 6. Hearth plate 7. Thermocouple

Claims (4)

[Claims] 1. A method for producing a synthetic quartz glass in which a silicon compound and a combustion gas are jetted into a synthesis furnace to generate silica fine particles in a flame and deposited on a substrate. A method for producing synthetic quartz glass, characterized in that the temperature of the synthetic quartz glass is kept within a predetermined temperature ± 25 ° C. to thereby suppress the variation in the concentration of dissolved hydrogen in the synthetic quartz glass obtained. 2. The method for producing synthetic quartz glass according to claim 1, wherein a thermocouple is used as a method for measuring the temperature in the synthesis furnace. 3. The method for producing synthetic quartz glass according to claim 2, wherein the flow rate of the combustion gas is controlled in accordance with the temperature in the synthesis furnace measured by the thermocouple. The temperature inside the synthesis furnace is set to a predetermined temperature ± 25 ° C.
A method for producing synthetic quartz glass, characterized in that the temperature is kept within the range. 4. A method for estimating a dissolved hydrogen concentration in a synthetic quartz glass obtained by ejecting a silicon compound and a combustion gas into a synthesis furnace to generate silica fine particles in a flame and depositing the fine particles on a substrate. Estimating the dissolved hydrogen concentration in the synthetic quartz glass by monitoring the temperature in the synthetic furnace during the production of the synthetic quartz glass and estimating the dissolved hydrogen concentration in the synthetic quartz glass obtained by the monitored temperature. Method.