JPH05116969A - Production of quartz glass - Google Patents

Abstract

PURPOSE:To obtain the quartz glass of a large mass which is used as, for example, an optical lens of a large aperture diameter, has about <=DELTAn=10<-6> and is optically homogeneous, as the homogeneity is improved over the wide range and the formation of a deteriorated property layer is suppressed in the quartz glass. CONSTITUTION:The optically heterogeneous quartz glass having about DELTAn=10<-5> fluctuation in refractive index is used as a sample 1. SiO2 powder which is a matrix 2 is placed on the front and rear surfaces and flanks thereof and is put into an outer mold 3. The powder is heated up under 0 to 10kg/cm<2> compression and is held for a while at a specified temp.; thereafter, the temp. is lowered.

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 quartz glass useful as an optical element (eg lens) used in an apparatus such as ultraviolet lithography.

[0002]

2. Description of the Related Art It is essential that a glass of this kind has a highly uniform refractive index distribution. The main reason why the refractive index distribution of glass is not uniform is that it contains a granular structure caused by incomplete melting of raw material particles when synthesizing glass and growth streaks called striae introduced by fluctuations of glass. It is in. These factors result in a visually inhomogeneous distribution in which the dispersion (homogeneity) of the refractive index is approximately Δn = 10 ⁻⁴ . For example, the glass used for a camera lens may be homogeneous to the extent that no inhomogeneity is visually observed. Therefore, in order to remove the inhomogeneous distribution of about Δn = 10 ^−4, general optical glass having a relatively low softening point (for example, phosphate glass) is reheated to a high temperature and mechanically stirred. By doing so, homogenization is performed.

However, in quartz glass, when heated to normal temperature and high temperature, the sublimation becomes remarkable before the viscosity decreases to a degree sufficient for mechanical stirring, so that it is difficult to homogenize by stirring. . Therefore, a homogenization method is used in which heat treatment is performed in a high-pressure argon gas atmosphere instead of mechanical stirring. For example, by heating to 1800 ° C or higher in an argon gas (Ar) atmosphere having a pressure of 2 atm or more, the inhomogeneous distribution of the refractive index such as striae that is visually recognized is removed (Patent Publication 3- (See No. 17775). The homogeneity of the glass thus homogenized is Δn =
It is about 10 ^-5 .

By the way, quartz glass is used for an optical element used in a recent ultraviolet lithography apparatus because a high transmittance in the ultraviolet region is required.

[0005]

Recently, there has been a demand for this quartz glass to be optically uniform with a variation in the refractive index of about Δn = 10 ⁻⁶ or less. Therefore, attempts have been made to obtain optically homogeneous quartz glass by adjusting the conditions of heat treatment (pressure, treatment temperature, cooling time, etc.).

For example, when heat treatment is performed by raising the temperature to 1800 ° C. to 2200 ° C. under atmospheric pressure, the nonuniform distribution of the refractive index in the central portion of the sample (quartz glass before heat treatment) is reduced. However, this time, a heterogeneous portion called an altered layer of about Δn = 10 ⁻⁴ is formed around the sample. Since this portion can be used as a lens, only the central portion is scraped and used, but this causes the following problems. Quartz glass, which is a sample, is expensive, and if only the central portion of the sample is used as a lens, the final product will be even more expensive. In recent years, in ultraviolet lithography, it is necessary to increase the resolution of the lens, and it is urgent to increase the lens diameter to increase the NA. However, in the case of homogenizing only the central portion, it is necessary to manufacture a large-diameter lens. I can't. The deteriorated layer may adversely affect the distribution of the refractive index in the central portion.

An object of the present invention is to solve such a problem and to provide an optically homogeneous (Δn
= 10 ⁻⁶ or less) to produce a large lump of quartz glass.

[0008]

The present inventors investigated the cause of this altered layer. First, when the temperature distribution of the sample when the temperature of the heat treatment was lowered was examined, it was found that it had a temperature distribution as shown in FIG. 1 (a). In FIG. 1, the line in the sample shows an isotherm. After the heat treatment, it is desirable that the temperature of the entire sample be lowered uniformly. However, even if the rate of temperature decrease is sufficiently slow, the temperature decrease rate is different between the outside and inside of the sample, so the temperature distribution as shown in Fig. 1 (a) after temperature decrease Is generated, which appears as a refractive index distribution. In particular, the number of isotherms increases in the peripheral portion of the sample when viewed from the thickness direction (optical axis direction when used as a lens), and an altered layer having a large refractive index value is formed in this portion.

Further, it has been found that the altered layer is also formed by gas release from the inside of the sample, gas diffusion from the atmosphere, or diffusion of impurities present in the outer mold for heat treatment into the sample. Therefore, when the present inventors perform heat treatment,
It has been found that by placing the sample in a master mold made of SiO ₂ powder or lumps, improvement of homogeneity is realized in a wide range of the quartz glass as the sample. Furthermore, by using such a master mold, it was possible to perform the heat treatment without directly contacting the sample with the outer mold and the atmosphere, and it was found that the formation of the altered layer was suppressed, and the present invention was accomplished. ..

Therefore, the first aspect of the present invention is "optically inhomogeneous quartz glass having a refractive index variation Δn = 10 ^-5 ".
In a matrix made of SiO ₂ powder or lumps, 0-10 kg / cm ²
"Δn = 10 ^-6 characterized by heat treatment under pressure
A method for producing "optically homogeneous quartz glass of a degree or less" is provided (Claim 1). A second aspect of the present invention is characterized in that in the above manufacturing method, the treatment temperature is 1800 ° C. or higher and 2200 ° C. or lower, and the atmosphere is He, N ₂ , Ar, H ₂ or a mixed gas thereof. A method for producing "homogeneous quartz glass" is provided (Claim 2).

[0011]

The quartz glass as the sample and the SiO _{2 as the} matrix are both in a molten state when kept at a high temperature, and their physical properties are very close to each other. Therefore, in the present invention, it is presumed that the temperature distributions of the sample and the mother mold during the temperature decrease are as shown in FIG. 1 (b). In other words, the part with a steep temperature gradient (the part with many isotherms) that seems to form an altered layer is the Si of the matrix.
It is closer to O ₂ and the sample quartz glass is considered to be homogenized over a wide range from the central part to the peripheral part.

[0012] Alternatively, it is presumed that a large number of minute air bubbles are mixed in the matrix, but the temperature distribution in the sample is relaxed by the heat insulating effect of these air bubbles. In any case, after heat treatment according to the invention, an optically homogeneous quartz glass is obtained over almost the entire sample. The shape of SiO ₂ used for the matrix is, for example,
It may be a synthetic quartz powder produced by the sol-gel method, cullet (crushed), or a once melted lump. It should be noted that a lump of SiO ₂ once melted is preferable because the volume change of the matrix is small.

Further, if impurities such as Na and Ca are mixed in the matrix SiO ₂ and diffuse into the sample and contaminate the sample, the purity of SiO ₂ is equal to or higher than that of the sample. Is desirable. There is no problem as the treatment condition as long as the treatment temperature (holding temperature) is equal to or higher than the softening point, but it is preferably 150 ° C. or more higher than the softening point in view of productivity depending on the treatment time. Generally speaking, an absolute value of about 1800 to 2200 ° C is preferable. The holding time and the cooling rate depend on the size of the sample. The atmosphere to be treated is preferably an inert gas such as He, N ₂ , Ar, H ₂ or a mixed gas thereof.

Since the matrix is in a molten state when it is held at a high temperature, the sample and the matrix are usually put in an outer mold made of carbon graphite. Further, a felt made of carbon fiber may be used on the inner wall of the outer die in order to prevent the outer die and the mother die from being separated after the heat treatment. The temperature distribution in the sample at the time of cooling, which is one of the causes of the non-uniform distribution of the refractive index, can be reduced to some extent (the number of isotherms is reduced) by slowing the cooling rate. But,
Reducing the temperature decrease rate leads to holding the sample at a high temperature for a long time, and there is a risk of devitrification of the sample. -In terms of industry, production efficiency decreases. For this reason, about 5 to 200 ° C./hour is preferable.

It should be noted that the quartz glass manufacturing method of the present invention cannot remove the granular structure and striae where Δn = about 10 ⁻⁴ . Therefore, as a raw material (sample), Δn = 1
Quartz glass of about 0 ^-5 is used. The quartz glass obtained by the manufacturing method of the present invention is used as a lens for an ultraviolet lithography apparatus, and is also effective for various purposes such as a camera lens and a prism.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these.

[0017]

[Embodiment 1] FIG. 2 is a schematic cross-sectional view of a manufacturing apparatus for carrying out this embodiment, and 4 is a heater and 5 is a heating furnace provided with a heat insulating material. The inner wall of the outer mold made of carbon graphite is covered with carbon fiber felt 6. The sample is quartz glass with a diameter of 200 mm and t50 mm. The sample was set in the outer mold so that 20 mm on the upper and lower surfaces and 100 mm on the side surfaces of the SiO ₂ powder were present. Set the outer mold in the heating furnace, N ₂ atmosphere, 5.0k
The temperature was raised by a heater under pressure of g / cm ² . The temperature was maintained at 1900 ° C for 2 hours and then lowered at 50 ° C / h. This sample was annealed for the purpose of removing internal strain, and then homogeneity was measured by an interferometer. As a result, formation of an altered layer was hardly confirmed, and Δn of the φ150 mm portion of the sample was
It has changed from 2.6 × 10 ^-5 to 2.0 × 10 ^-6 .

[0018]

[Example 2] Two samples in the same initial state (φ200 mm, t150
mm) was set in the outer mold so that the SiO ₂ powder was 50 mm and 100 mm on the side surface, respectively, and the temperature was raised under N ₂ atmosphere and 5.0 kg / cm ² pressure. After holding at 1900 ° C for 2 hours, the temperature was lowered at 50 ° C / h. This sample was annealed for the purpose of removing internal strain and then measured for homogeneity with an interferometer.
The homogeneity Δn of the mm part is 2.6 × 10 ^-5 , 5.5
It became x 10 ^-6 and 3.0 x 10 ^-6 .

[0019]

Example 3 FIG. 3 shows the product obtained as a result of performing the conventional method (the method of FIG. 1 (a)) and the method of the example with various diameters / heights of the samples. It is a graph which shows (DELTA) n. In this graph, the horizontal axis is the ratio of diameter to height and the vertical axis is homogeneity. According to this graph, it can be seen that even if the sample is thick, Δn is about 10 ⁻⁶ or less, and optically homogeneous quartz glass can be obtained.

[0020]

As described above, according to the present invention, it is possible to reduce the temperature distribution in the sample when the temperature is lowered by changing the shape of the matrix. Therefore, regardless of the diameter / height of the sample, quartz glass that is optically homogeneous throughout the sample is obtained. For example, even if a sample having a large diameter is used, an altered layer is not formed around the sample and Δn = 10 ⁻⁶
Sub-substantially optically homogeneous quartz glass is obtained over almost the entire sample, making it possible to manufacture large diameter lenses.

Further, although the altered layer portion sometimes emits fluorescence when irradiated with an excimer laser, the manufacturing method of the present invention provides quartz glass that does not emit fluorescence.

[Brief description of drawings]

FIG. 1 is a diagram showing a temperature distribution when the temperature of quartz glass is lowered.

FIG. 2 is a schematic sectional view of a manufacturing apparatus for carrying out an embodiment of the present invention.

FIG. 3 is a graph showing the measurement results of homogeneity.

[Explanation of symbols for main parts]

1 sample (quartz glass before heat treatment) 2 mother die (SiO ₂ ) 3 outer die 4 heater 5 heating furnace 6 felt

Claims (2)

[Claims] 1. A matrix made of SiO ₂ powder or agglomerates of “optically inhomogeneous quartz glass with a refractive index variation Δn = 10 ⁻⁵ ” applied at 0-10 kg / cm ² . A method for producing "optically homogeneous quartz glass with Δn = about 10 ^-6 or less", characterized by performing heat treatment under pressure. 2. In claim 1, the treatment temperature is 1800 ° C. or higher and 2200 ° C. or lower, and the atmosphere is He, N ₂ , Ar,
A method for producing "homogeneous quartz glass", which is H ₂ or a mixed gas thereof.