JP2002121038A - Heat treatment method for synthetic quartz glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat treatment method which lowers the virtual temperature of synthetic quartz glass, shortens the treatment time and enhances productivity. SOLUTION: The synthetic quartz glass having a target virtual temperature is rapidly obtained by setting >=1 intermediate virtual temperatures within a temperature range between an initial virtual temperature and the target virtual temperature in order to lower the virtual temperature of the synthetic quartz glass, holding the synthetic quartz glass at the intermediate heat treatment temperature corresponding to the intermediate heat treatment temperature and stepwise lowering the intermediate virtual temperature, then holding the synthetic quartz glass at the final heat treatment temperature.

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 producing synthetic quartz glass suitable for use in an optical system using a laser having an ultraviolet wavelength, and more particularly, to a lens, a mirror, and a mirror for an optical system. The present invention relates to a heat treatment method for producing synthetic quartz glass used as an optical member that directly receives a laser having an ultraviolet wavelength, such as a prism.

[0002]

2. Description of the Related Art At present, the information technology industry is growing rapidly. For the growth of the information technology industry, an increase in the amount of information that can be handled is indispensable, and information processing apparatuses are required to have higher speed and higher integration. Research and development are being carried out from various directions for high speed and high integration of MPUs, etc., especially from the manufacturing side, for example, ultra-light interference such as deformation illumination method and phase shift mask method. Resolution lithography technology has been developed. On the other hand, the simplest technique for increasing the resolution of the lithography technique is to shorten the wavelength of a light source used in the lithography technique.

In using the lithography technology, mercury lamps (g-line (436 nm), i-line (36
5 nm)). Since g-rays and i-rays having relatively long wavelengths have low energy of the light source, the optical member itself is hardly damaged by light irradiation.
However, with the shortening of the wavelength of the light source, excimer lasers using KrF or ArF as an oscillation source are now being used.
Conventionally used optical members are easily damaged, and thus the use of quartz glass as a new optical member has become indispensable.

[0004] Quartz glass for optical members is required to have characteristics such as light transmittance and laser resistance. Excimer laser basically has optical transparency to quartz glass,
When impurities are mixed in the quartz glass, the light transmittance decreases. To improve the initial light transmittance, metal, OH
It is necessary to manufacture high-purity quartz glass by removing impurities such as bases and halogens. Therefore, a synthetic quartz glass obtained by hydrolyzing a high-purity silicon compound in an oxygen-hydrogen flame by a so-called VAD method is preferably used.

[0005] On the other hand, the mere removal of impurities cannot prevent a decrease in light transmittance due to the irradiation of the excimer laser, that is, does not improve the durability against laser (laser resistance). When the synthetic quartz glass is irradiated with a laser,
Due to its large energy density, the network structure of quartz glass is damaged, and is caused by the so-called E ′ center ({Si .;} indicates that it is bonded to three oxygen atoms, and • indicates an unpaired electron). Absorption band around Non-Bridg
A wide variety of absorption bands, such as an absorption band near a wavelength of 260 nm, caused by the ing Oxygen Hole Center (NBOHC, ≡Si-O.) are formed, and light transmittance is reduced. To prevent the formation of these defects, it is necessary to make the structure in the quartz glass stable.

For example, synthetic quartz glass has a stable basic structure called a six-membered ring structure in which six bonds represented by O—Si—O form a network structure. However, when synthesized at a high temperature for a short time by the VAD method or the like, structurally unstable ones are likely to be generated, and a three-membered ring structure,
Many unstable structures such as a membered ring structure exist. These unstable structures, when irradiated with excimer laser, form defects such as the E ′ center and NBOHC, and reduce the light transmittance of the synthetic quartz glass.

[0007]

SUMMARY OF THE INVENTION
When there are few unstable structures such as a four-membered ring structure and a four-membered ring structure, that is, when the synthetic quartz glass has a more stable structure, the fictive temperature, which is an index of the structural stability of the synthetic quartz glass, decreases. Therefore, in order to make the synthetic quartz glass have a more stable structure, it is only necessary to perform a process for lowering the virtual temperature. JP-A-9-241030 and JP-A-10-67526 disclose a quartz glass invention in which light transmittance is improved by defining a process for lowering a virtual temperature.

Although the synthetic quartz glass of these inventions has sufficient light transmittance by lowering the fictive temperature, a considerable time is required for heat treatment for lowering the fictive temperature. In other words, in the inventions described in the above publications, as the heat treatment for lowering the virtual temperature, the temperature is maintained at the same temperature as the virtual temperature. There was a problem of lack of sex.

An object of the present invention is to provide a synthetic quartz glass heat treatment method which can end the heat treatment in a shorter time and improve the productivity of the synthetic quartz glass, instead of the conventional heat treatment method for lowering the virtual temperature. .

[0010]

Means for Solving the Problems The present inventors have paid attention to the relationship among the virtual temperature, the heat treatment temperature, and the holding time when the virtual temperature is lowered by heat treatment of synthetic quartz glass.

Therefore, when the virtual temperature of the synthetic quartz glass (hereinafter referred to as an initial virtual temperature) is set to a target virtual temperature (hereinafter referred to as a target virtual temperature), the virtual temperature is
Heat treatment is performed so as to be a temperature between the initial virtual temperature and the target virtual temperature (hereinafter, referred to as an intermediate virtual temperature), and finally, a synthetic material having the target virtual temperature via the synthetic quartz glass having the intermediate virtual temperature. It has been found that the processing time can be shortened by obtaining quartz glass, as compared with a heat treatment in which the initial virtual temperature is directly set to the target virtual temperature.

[0012] At this time, it was confirmed that when the temperature was maintained at a temperature lower than the virtual temperature, the target virtual temperature was reached in a shorter time than when the temperature was maintained at the same temperature as the virtual temperature.

The present invention is a method for heat treating synthetic quartz glass for lowering the virtual temperature of synthetic quartz glass, wherein one or more intermediate virtual temperatures are set between an initial virtual temperature and a target virtual temperature. Holding the glass at an intermediate heat treatment temperature corresponding to the intermediate virtual temperature, sequentially decreasing the intermediate virtual temperature, and then holding the glass at the final heat treatment temperature to obtain a target virtual temperature; Method,
I will provide a.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The heat treatment method according to the present invention is a heat treatment method performed for lowering the fictive temperature of synthetic quartz glass. The virtual temperature is an index indicating the structural stability of the quartz glass, and the structure of the quartz glass is in an equilibrium state at a temperature indicated as the virtual temperature. Generally, it is said that the lower the fictive temperature, the more stable the structure of quartz glass, and the less the unstable structure such as the above-described three-membered ring structure or four-membered ring structure. Therefore, if the fictive temperature of the synthetic quartz glass is lowered and the unstable structure is reduced, a decrease in transmittance due to laser irradiation can be suppressed, and the durability of the synthetic glass itself can be improved.

In the heat treatment method according to the present invention, an intermediate virtual temperature is set within a temperature range between the initial virtual temperature and the target virtual temperature. As described above, the initial virtual temperature is a virtual temperature before heat treatment is performed, and the target virtual temperature is a virtual temperature to be finally obtained by heat treatment. For example, usually
The virtual temperature of the synthetic quartz glass as manufactured by the VAD method, that is, the initial virtual temperature, depends on the manufacturing conditions, but is 1200
Often above ° C. The intermediate fictive temperature is reduced step by step by heat treatment to obtain a synthetic quartz glass having a target fictive temperature set corresponding to the final product.

The term "intermediate fictive temperature" refers to a fictive temperature of synthetic quartz glass obtained intermediately before finally obtaining synthetic quartz glass in the practice of the present invention. The intermediate heat treatment temperature refers to a temperature maintained when heat treatment is performed step by step sequentially, and an intermediate virtual temperature is obtained by performing the heat treatment maintained at this temperature. The intermediate virtual temperature is set within a temperature range between the initial virtual temperature and the target virtual temperature, and the set number is one or more. The intermediate virtual temperature does not necessarily need to be set so as to minimize the holding time of the entire heat treatment. It is preferable to set the holding time to be the shortest, but it takes time to accumulate the data for that. The upper limit of the set number is not limited, but if the set number is too large, it becomes substantially the same as continuous slow cooling, which is not preferable. In principle, continuous slow cooling may be used.However, it is not easy to perform slow cooling while maintaining the surface and internal temperatures of the synthetic quartz glass uniform. Temperature may not be obtained. Considering practical aspects, the set number is preferably 40 steps or less. More preferably, the number of stages is 5 or less. Further, the temperature difference between the initial virtual temperature and the intermediate virtual temperature or between the two intermediate virtual temperatures is preferably 20 ° C. or more.

The intermediate heat treatment temperature is preferably set lower than the intermediate virtual temperature. Set 5-25 ° C lower by the temperature difference. When the temperature in this range is used, the virtual temperature can be reduced in a shorter time than when heat treatment is performed using the same temperature as the intermediate virtual temperature. If the temperature difference between the intermediate heat treatment temperature and the intermediate virtual temperature is less than 5 ° C., the holding time may be longer, and the intermediate heat treatment temperature may be higher than the intermediate virtual temperature depending on the accuracy of the temperature control of the heat treatment furnace. There is.
Also, when the temperature difference exceeds 25 ° C., the holding time may be prolonged.

As a result, a synthetic quartz glass having an intermediate virtual temperature is produced stepwise, a glass having the lowest intermediate virtual temperature is produced, and then maintained at the final heat treatment temperature. A synthetic quartz glass at a temperature can be obtained. The final heat treatment temperature refers to a heat treatment temperature of a heat treatment performed to obtain a target virtual temperature.
In this case, as in the relationship between the intermediate virtual temperature and the intermediate heat treatment temperature, the final heat treatment temperature is 5 to 25 ° C. higher than the target virtual temperature.
It is preferable to set it low.

When the heat treatment method according to the present invention is performed,
After passing through the stage of forming a synthetic quartz glass having an arbitrary intermediate virtual temperature, in order to confirm that the synthetic quartz glass of the set intermediate virtual temperature is actually obtained, after the heat treatment is completed in each stage, First, it is necessary to measure the fictive temperature of the synthetic quartz glass. After confirming that it has an intermediate virtual temperature, it is preferable to proceed to the next processing stage, but this is not practical.

Therefore, it is preferable to carry out the heat treatment method according to the present invention as follows. From the data accumulated in advance, the heat treatment temperature corresponding to the set intermediate virtual temperature and the holding time are determined. That is, the virtual temperature of the synthetic quartz glass manufactured under the normally used manufacturing conditions is set as the initial virtual temperature, and is set to an arbitrary virtual temperature lower than the virtual temperature (for example, several kinds of temperatures 20 to 50 ° C. lower than the initial virtual temperature). Data on the heat treatment temperature and the holding time required for the heat treatment is accumulated. Next, the virtual temperature thus obtained is set as the initial virtual temperature, and data of the heat treatment temperature and the holding time at which a lower virtual temperature is obtained are also stored. Using these data, considering the target virtual temperature corresponding to the target product, the schedule of the heat treatment temperature and the holding temperature at each stage is determined, and the heat treatment is performed. However, even if the holding time required to lower the temperature to an arbitrary virtual temperature is 15 minutes or less, it is preferable to hold the temperature for at least 15 minutes or more in order to maintain the temperature uniformity.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the heat treatment method according to the present invention, first, a synthetic quartz glass to be subjected to a heat treatment was produced by a VAD method. First, high-purity silicon tetrachloride (SiC
l ₄ ) Approximately 175 in oxygen / hydrogen flame while flowing gas.
Fine particle synthetic quartz glass was synthesized by a gas phase chemical reaction (hydrolysis reaction) at 0 ° C. The fine-grained synthetic quartz glass adheres and accumulates around a seed rod placed above the site where the gas phase chemical reaction occurs, forming a porous synthetic quartz glass (soot body) measuring 400 mm in diameter and 1500 mm in length. .

Subsequently, the soot body was heated at a dew point of -70.degree.
The heat treatment is performed at 1000 ° C. for 10 hours in a He atmosphere of 00 Pa. After the OH treatment, the film was further kept at 1550 ° C. for 6 hours to perform a clearing treatment. The synthetic quartz glass obtained by the transparency treatment became a rod having a diameter of about 150 mm and a length of about 1000 mm. The rod-shaped synthetic quartz glass was cut into a sample of 10 mm × 10 mm × 40 mm in length along the central axis from the center of the rod section.

Prior to carrying out the heat treatment method according to the present invention, various characteristics of the sample were examined. The OH group concentration in the synthetic quartz glass was 12 ppm or less as measured by an infrared absorption method. In addition, high-frequency inductively coupled plasma emission spectroscopy (ICP
Method), and as a result, it was found that each metal element was 1 ppb or less and the amount of impurity metals was extremely small.

The fictive temperature was measured by using a laser Raman scattering method. By comparing the data of the synthetic quartz glass whose fictive temperature is known in advance and the measurement data prepared as described above, it was found that the fictive temperature of the synthetic quartz glass of the present sample was 1220 ° C.

In the heat treatment, the synthetic quartz glass having the fictive temperature of 1220 ° C. (initial fictive temperature) is converted into a synthetic quartz glass having the fictive temperature of 1000 ° C. (target fictive temperature). The heat treatment was performed stepwise under the conditions shown in FIG. In Table 1, in Example 1, one intermediate heat treatment temperature was set, and in Example 2, two intermediate heat treatment temperatures were set, and heat treatment was performed. As a comparative example,
Heat treatment performed in one stage, that is, heat treatment without an intermediate heat treatment temperature was also performed. Table 1 also shows comparative examples.

[0026]

[Table 1] In Example 1, the intermediate virtual temperature was set to 1050 ° C, the intermediate heat treatment temperature was maintained at 1040 ° C for 20 hours, and then the two-stage heat treatment was performed at the final heat treatment temperature of 990 ° C for 80 hours to obtain the target virtual temperature. It is. At this time, the total time taken to obtain the synthetic quartz glass having the target virtual temperature is 10
It was 0 hours. Note that the finally obtained virtual temperature is 100
7 ° C and the target virtual temperature of 1000 ° C were in good agreement.

Similarly, in Example 2, two intermediate heat treatment temperatures of 1090 ° C. and 1030 ° C. were provided, and at each temperature,
Hold for 4 hours and 13 hours, then 9 as final heat treatment temperature
A three-stage heat treatment was performed at 90 ° C. for 78 hours. At this time, the total time required to obtain the synthetic quartz glass having the target virtual temperature was 95 hours.

On the other hand, in Comparative Examples 1 and 2 in which the final virtual temperature was reduced to 1000 ° C. without providing the intermediate virtual temperature, the temperature was maintained at 990 ° C. and 1000 ° C., respectively.
It took 140 hours and 200 hours respectively to obtain the synthetic quartz glass with the target virtual temperature.

From the above results, it is understood that when the intermediate virtual temperature is provided and the heat treatment is performed stepwise, the virtual temperature can be lowered to a desired temperature in a relatively short holding time.

[0030]

According to the heat treatment method for synthetic quartz glass according to the present invention, when performing the heat treatment, the synthetic quartz glass is maintained at the intermediate heat treatment temperature, the fictive temperature is reduced step by step, and the synthetic quartz glass having the target fictive temperature is obtained. Therefore, in order to directly obtain the target virtual temperature, the processing time can be shortened and the productivity can be increased as compared with a heat treatment method in which the target heat treatment temperature is maintained.

Continued on the front page (72) Inventor Kensuke Fukushima 1-8 Fuso-cho, Amagasaki-shi, Hyogo Sumitomo Metal Industries, Ltd. Electronics Technology Research Laboratory F-term (reference) 4G014 AH21 AH23

Claims (2)

[Claims] 1. A synthetic quartz glass heat treatment method for lowering a virtual temperature of a synthetic quartz glass, wherein one or more intermediate virtual temperatures are set between an initial virtual temperature and a target virtual temperature.
Synthetic quartz characterized by holding the synthetic quartz glass at an intermediate heat treatment temperature corresponding to the intermediate virtual temperature, sequentially decreasing the intermediate virtual temperature, and then maintaining the final heat treatment temperature to obtain a target virtual temperature. Heat treatment method for glass. 2. An intermediate heat treatment temperature lower than an intermediate virtual temperature.
The method according to claim 1, wherein the final heat treatment temperature is lower than the target virtual temperature.