JP3972374B2 - Silica glass annealing method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an annealing method for obtaining silica glass with high homogeneity, and more particularly to an annealing method in which silica glass is heat-treated by being placed in a container, embedded in a powder, or surrounded by a plate. Is.
[0002]
[Prior art]
In recent years, there has been an increasing demand for improving the properties of silica glass. In particular, for homogeneity, which is one of the basic physical properties of glass, there is a strong demand for improved properties. Silica glass has a wide range of applications, and optical applications such as prisms and lenses require homogeneity, but they are mounted on lenses and satellites used in excimer laser steppers used in semiconductor manufacturing. Corner cubes (cube corner prisms) are required to have particularly high homogeneity.
[0003]
Conventionally, for silica glass for optics, first, a high-quality part is selected from a glass ingot, and a glass material having a size suitable for a target object is cut out from this part. The glass material is then heated and held at a temperature slightly higher than the temperature defined as the annealing point for several hours to several tens of days to remove strains that are present inside the glass and impair homogeneity. Next, a method of performing annealing by cooling to a temperature slightly lower than the temperature defined as the strain point, taking a long time of several tens of hours to sometimes several months so that no new strain is caused by cooling. Was common.
[0004]
However, even when annealing is performed for a long time as described above, a phenomenon is observed in which strain is observed near the surface of the glass material, and homogeneity deteriorates in this portion. This is because, in the cooling process, a temperature difference occurs between the surface portion and the central portion of the glass lump, and distortion occurs near the surface due to the difference in thermal expansion. Therefore, when trying to obtain a glass block with high homogeneity, perform cooling for a longer time so as not to cause distortion on the surface portion, or avoid that portion even if strain remains near the surface. That is, in order to be able to use it by removing a portion where the strain exists, a method of giving a margin to a required dimension and annealing a larger glass material must be taken.
[0005]
Regardless of which method is used, there is a problem that it takes a long time to obtain a highly homogeneous glass lump, resulting in poor throughput or reduced product yield.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide an annealing method capable of obtaining a silica glass lump having high homogeneity in a short time and with a high yield.
[0007]
[Means for Solving the Problems]
As a result of diligent research to solve the above-mentioned problems, the present inventors have found that silica glass lump is treated with silica or siliceous, for example, silica alumina, in order to anneal without leaving strain on the surface of the silica glass lump. If it is covered with an oxide or a material whose thermal conductivity is not significantly different from that of silica, and if a substantial surface is formed outside the glass lump, the strain will enter the cover and not enter the silica glass lump. The headline and the present invention have been completed.
[0008]
That is, in the annealing method provided by the present invention, in order to give the silica glass lump the state as described above, the silica glass lump is placed in a container made of silica or the like, or embedded in a powder, or It is characterized in that it is annealed by surrounding or sandwiching with a plate.
[0009]
[Action]
Hereinafter, the present invention will be described in detail.
[0010]
A feature of the present invention is to provide an annealing method for obtaining a silica glass lump having high homogeneity in a short time with good yield. High homogeneity in glass means that there is no mixing of bubbles and foreign matters, there is no bias in composition, there is no local variation in the refractive index represented by striae, and mechanical and thermal distortions. It means that there is no more than a certain amount. In general, high-homogeneity glass is premised on the absence of inhomogeneous parts such as bubbles, foreign substances, compositional deviation, and striae, and the degree of homogeneity is determined by how much distortion is present in the glass.
[0011]
Therefore, the strain removal operation by annealing is a very important operation for obtaining highly homogenous glass. As described above, the annealing is performed at a temperature of several to several tens of degrees Celsius than the decooling point (temperature at which the viscosity of the glass is log η = 12.0 / Pa · s). Raise the temperature to a high temperature, hold at that temperature for a certain period of time, and then several to several tens of degrees Celsius from the temperature to the strain point (the temperature at which the viscosity is log η = 13.5 / Pa · s), depending on the case The main operation is to cool slowly to a low temperature around 100 ° C.
[0012]
Here, each temperature, holding time, and cooling rate are determined by the composition and size of the glass. If the cooling rate is too high during annealing, a temperature difference occurs between the surface portion and the central portion. Then, the shrinkage accompanying cooling precedes the surface portion. This causes distortion in the vicinity of the glass surface where the temperature gradient is particularly large. Strain present in the glass contributes to the loss of the homogeneity of the glass. Therefore, the annealing operation for increasing the homogeneity of the glass is performed at a very slow cooling rate so that a certain temperature difference between the central portion and the surface portion does not occur during cooling after heating. It takes place for a long time. However, if the size of the glass lump becomes larger than a certain level, it becomes difficult to keep the temperature difference between the center and the surface during cooling within an allowable range, and distortion occurs in the outer periphery due to the reasons described above. . In general, a portion where distortion exists cannot be used as highly homogeneous glass, and can only be removed by grinding or the like.
[0013]
Therefore, if the generation of strain on the surface portion can be prevented, the yield and productivity of the highly homogeneous glass are dramatically improved. In the annealing method of the present invention, the surface of a glass lump is covered with a container made of silica or the like, powder or plate, and subjected to heating and cooling treatments. Silica is preferable as the material covering the glass block. Further, even if it is other than silica, it is stably present at the highest temperature in the annealing operation and does not react with silica glass, and its thermal conductivity at 100 ° C. is 0.004 or more and less than 0.080 (cal Any material that is s ⁻¹ · cm ⁻¹ · ° C. ⁻¹ ) According to this method, the outer peripheral portion of a container or the like covering the silica glass block becomes the surface, and the surface of the glass block is not a substantial surface. Therefore, even if the temperature gradient during cooling is large on the surface of the container or the like, the temperature gradient is gentle in the vicinity of the surface of the glass lump, and the strain amount falls within the allowable limit. As described above, according to the annealing method of the present invention, a highly homogeneous glass can be manufactured with much better yield and productivity than the conventional method.
[0014]
【Example】
The invention is further illustrated by the following examples.
[0015]
The homogeneity evaluation method is as follows. In the portion where the distortion occurs, the refractive index changes. Therefore, the homogeneity was measured by measuring the refractive index distribution (Δn). Here, the smaller the width of the refractive index distribution (Δn value), and the larger the effective region, the better the homogeneity if the Δn value is the same. A Fizeau interferometer was used for precise measurement of the refractive index distribution.
[0016]
Example 1
Silica glass lump of length x width x height 100mm x 100mm x 100mm was put in a container made of silica glass with outer dimensions of 200mm x 200mm x 200mm and inner dimensions of about 100mm x 100mm x 100mm and set in a muffle furnace . The temperature in the furnace was increased from room temperature to 1230 ° C. at a rate of 100 ° C./hour in the air atmosphere, held at 1230 ° C. for 7 hours, decreased to 1000 ° C. at a rate of 48 ° C./hour, and subsequently increased to 900 ° C. at 90 ° C. The temperature was lowered at a rate of ° C./hour, further lowered to 660 ° C. at 240 ° C./hour, and then cooled in the furnace.
[0017]
When the refractive index distribution of the glass block cooled to room temperature was measured, the effective area was 100 mm × 100 mm, and Δn = 2.0 × 10 ⁻⁶ .
[0018]
Example 2
A silica glass lump of the same size as that of Example 1 is obtained by adding 40 mm of quartz sand (average particle diameter of about 1 mm) to the bottom of a container made of silica glass having an outer size of 200 mm × 200 mm × 200 mm and an inner size of about 180 mm × 180 mm × 180 mm. The container was placed in the middle of the thickness and filled with quartz sand up to the top of the container in the gap with the container, covered and set in a muffle furnace. The temperature in the furnace is increased from room temperature to 1230 ° C. at a rate of 100 ° C./hour, held at 1230 ° C. for 7 hours, lowered to 1000 ° C. at a rate of 48 ° C./hour, and subsequently to 900 ° C. The temperature was lowered at a rate of 90 ° C./hour, further lowered to 660 ° C. at 240 ° C./hour, and then cooled in the furnace.
[0019]
When the refractive index distribution of the glass block cooled to room temperature was measured, Δn = 2.6 × 10 ⁻⁶ at an effective area of 100 mm × 100 mm and Δn = 2.0 × 10 ⁻⁶ at 100 mm × 90 mm.
[0020]
Comparative Example 1
A silica glass lump of the same size as in Example 1 was set in a muffle furnace. The inside of the furnace was heated from room temperature to 1230 ° C. at a rate of 100 ° C./hour, held at 1230 ° C. for 7 hours, lowered to 1000 ° C. at a rate of 48 ° C./hour, and subsequently to 900 ° C. at 90 ° C./hour. The temperature was lowered at a rate of 240 ° C./hour to 660 ° C., and then cooled in the furnace. The atmosphere in the furnace was an atmospheric atmosphere throughout.
[0021]
When the refractive index distribution of the glass block cooled to room temperature was measured, Δn = 9.8 × 10 ⁻⁶ at an effective area of 100 mm × 100 mm, and Δn = 2.0 × 10 ⁻⁶ at 50 mm × 50 mm.
[0022]
Example 3
A silica glass plate having a size of 160 mm ^φ × 100 mm ^t was sandwiched from above and below by two silica glass plates of 160 mm ^φ × 50 mm ^t and set in a muffle furnace. The inside of the furnace was heated from room temperature to 1230 ° C. at a rate of 100 ° C./hour, held at 1230 ° C. for 7 hours, lowered to 1000 ° C. at a rate of 48 ° C./hour, and subsequently to 900 ° C. at 90 ° C./hour. The temperature was then lowered to 660 ° C. at 240 ° C./hour, and then cooled in the furnace. The atmosphere in the furnace was an atmospheric atmosphere throughout. Was subjected to measurement of the refractive index distribution of the glass mass was cooled to room temperature, Δn = 3.0 × 10 ^-6 in the effective region 160 mm ^phi, was Δn = 2.0 × 10 ^-6 at 150 mm ^phi.
[0023]
Example 4
A silica glass plate of the same size as that of Example 3 is made by adding quartz sand (average particle size of about 1 mm) to the bottom of a container made of silica glass having an outer size of 200 mm × 200 mm × 200 mm and an inner size of about 180 mm × 180 mm × 180 mm. The quartz sand was filled in the gap between the container and the top of the container, and the container was covered and set in a muffle furnace. The inside of the furnace was heated from room temperature to 1230 ° C. at a rate of 100 ° C./hour, held at 1230 ° C. for 7 hours, lowered to 1000 ° C. at a rate of 48 ° C./hour, and subsequently to 900 ° C. at 90 ° C./hour. The temperature was lowered at a rate of 240 ° C./hour to 660 ° C., and then cooled in the furnace. The atmosphere in the furnace was an atmospheric atmosphere throughout. When the refractive index distribution of the glass block cooled to room temperature was measured, Δn = 2.0 × 10 ⁻⁶ with an effective area of 160 ^mmφ .
[0024]
Comparative Example 2
A silica glass plate of the same size as in Example 3 was set in a muffle furnace. The temperature in the furnace is increased from room temperature to 1230 ° C. at a rate of 100 ° C./hour, held at 1230 ° C. for 7 hours, lowered to 1000 ° C. at a rate of 48 ° C./hour, and subsequently to 900 ° C. The temperature was lowered at a rate of 90 ° C./hour, further lowered to 660 ° C. at 240 ° C./hour, and then cooled in the furnace. Was subjected to measurement of the refractive index distribution of the glass mass was cooled to room temperature, Δn = 9.0 × 10 ^-6 in the effective region 160 mm ^phi, was Δn = 2.0 × 10 ^-6, at 150 mm ^phi.
[0025]
【The invention's effect】
As is apparent from the above description, according to the annealing method of the present invention, a highly homogeneous silica glass lump with less strain can be produced with good yield, and its usefulness is extremely high.

Claims (2)

In the annealing method of the silica glass, the time of heating and cooling process of the annealing, the sheet Rikagarasu surface, at ^{100 ℃ 0.004 (cal · s -1} · cm -1 · ℃ -1) or more, 0.080 (cal · A method of annealing silica glass, characterized by covering a substance , a powder, or a plate made of a material having a thermal conductivity of less than s ^-1 · cm ^-1 · ° C ^-1 ) . 2. The method for annealing silica glass according to claim 1, wherein the container, powder or plate is made of silica.