JPH08711B2 - Optical glass - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical glass mainly used for high power light, and particularly to YAG (1064n).
m), Ar laser (350 to 515 nm), KrF (24
8 nm) or ArF (193 nm) excimer laser light and other optical glasses used for high-power lasers.

[0002]

2. Description of the Related Art In recent years, excimer lasers and other high-power lasers have been attracting attention as being used in lithography technology for manufacturing LSI, technology utilizing photochemical reaction, processing technology for cutting and grinding, and laser nuclear fusion technology. Are gathering. And this kind of high power laser is transmitted, transmitted, refracted,
Attempts have been made to apply silica glass optical bodies as lenses, prisms, filters, etc. for reflection, absorption, and interference. However, when the light in the visible wavelength range of about 700 to 600 nm acts on the silica glass constituting the various optics, or when the light in the ultraviolet wavelength range of about 360 nm to about 160 nm acts, the structure of the glass is particularly important. It is easily damaged.

This is because when a high-power laser is irradiated for a long time, an absorption band of about 630 nm called so-called NBOHC (non-bridge, oxygen, hole, center) and an absorption band of about 215 nm called E'center are different from each other. An absorption band of approximately 260 nm is generated, which results in approximately 750-500 nm and approximately 360 nm to approximately 1
It reduces the transmittance of 60 nm ultraviolet light and deteriorates the optical characteristics. Therefore, it is structurally extremely difficult to improve the durability of silica glass against a high-power laser in the above wavelength range.

Furthermore, the KrF or ArF excimer laser in the short ultraviolet region of about 250 nm or less has the strongest energy as compared with other ultraviolet light, and the silica glass is irradiated with the stronger optical energy by irradiation of the excimer laser. It has been confirmed that it is easily damaged.

Therefore, the present inventors have used synthetic silica glass of high purity and high homogeneity (trade name SUPRASIL-P10, manufactured by Shin-Etsu Quartz Co., Ltd.) as a starting base material, and the light transmittance by ultraviolet laser irradiation in the base material. A technique is proposed in which a sufficient amount of argon or other rare gas is contained to suppress the above. (Japanese Patent Application No. 2-184048).

[0006]

However, it has been found that even with the above technique, the desired laser resistance cannot be obtained as the output power of the irradiation laser is increased. Analyzing the reason, silica glass for high-power laser is premised on high purity and high homogeneity, so synthetic silica glass, especially transparent synthetic silica glass can not be used, These synthetic silica glasses undergo high-temperature synthesis using an oxyhydrogen flame in a short time regardless of whether they are produced by the oxyhydrogen flame hydrolysis method or the CVD soot method, and therefore the equilibration reaction is not sufficiently carried out and structurally Is not stable enough.

Therefore, in the above-mentioned technique, synthetic silica glass is used as a starting base material, but by doping a rare gas into the glass body, the above-mentioned drawbacks are eliminated and laser resistance is improved. Technology is a coping therapy that assumes the existence of an unstable structure, and does not necessarily lead to a fundamental solution.

In the prior application, the silica glass is heated at a temperature of 850 to 1150 ° C. below the annealing point, but even if the heating and pressurizing treatment is performed at a temperature below the annealing point as described above. There is a drawback in that it is difficult to produce thick optical glass such as prisms and lenses because the content of rare gas that can be doped is limited.

In view of the above-mentioned drawbacks of the prior art, the present invention stabilizes the glass structure by applying a predetermined heat treatment to the synthetic quartz glass while using the synthetic quartz glass, thereby improving the high output laser resistance. The purpose is to improve the characteristics of the optical glass itself.

[0010]

The process leading to the present invention will be described step by step. First, as mentioned above, in order to obtain laser resistance, it is necessary to be highly pure and highly homogeneous and transparent, and such conditions cannot be obtained except for synthetic silica glass, but synthetic silica glass can be used in a short time. As described above, the equilibration reaction is not sufficiently performed and the structure is not sufficiently stable because of high-temperature synthesis.

It has been found by the present inventor that many glass structures having structurally unstable three-membered ring and four-membered ring structure are included because such an equilibration reaction is not sufficiently carried out. Therefore, the present inventor has further researched, and the three- and four-membered ring structure can be reduced by a pressure heat treatment capable of increasing the absolute refractive index of the glass body, and a stable six-membered ring structure is obtained. It has been found that it can increase.

Furthermore, the present inventor sets the absolute refractive index nd (d-line of sodium) in the glass body to approximately 1.461 or more, so that even when the high power laser is irradiated, the laser is substantially emitted. It was confirmed by experiments that the laser resistance without lowering the transmittance can be secured. That is,
In the present invention, the transparent synthetic silica glass body is subjected to remelting treatment under a high pressure at a temperature of a softening point (1600 ° C.) or higher in a rare gas atmosphere, and more specifically, the transparent synthetic silica glass body. The softening point (160
(0 ° C.) or higher and 2000 Kgf / cm ² pressure to perform remelting treatment by a hot isostatic pressing method so that the glass body contains a rare gas element and the absolute refraction of the glass body is increased. The characteristic is that the rate (nd) is set to approximately 1.461, specifically 1.4615 or more.

However, even if the above construction is adopted, sufficient high output laser resistance cannot be obtained. Therefore, the second characteristic is that the pressure heat treatment is performed in a rare gas atmosphere so that the silica glass body is doped with a rare gas to strengthen the glass network structure and improve laser resistance.

That is, in the above technique, the rare gas is doped into the gap of the member ring structure to dope the rare gas,
It is possible to suppress the breaking of inter-element bonds by laser irradiation, and to facilitate degassing to the outside of harmful gases that are decomposed and excited by oxygen gas or other ultraviolet light contained during the synthesis or during the subsequent heat treatment. did.

If gas doping is performed using nitrogen gas instead of the rare gas, even if the absolute refractive index is improved, some nitrogen compound is generated in the glass and the transmittance is lowered as shown in the experimental example described later. Is likely to occur and preferable laser resistance cannot be obtained. Therefore, rare gas doping is an essential requirement.

In this case, the rare gas dope is not subjected to pressure heat treatment at a low temperature of 850 to 1150 ° C., which is lower than the slow cooling point of silica glass as in the prior art, but is 1600 to 1700 or higher which is higher than the softening point. Since the pressure heating treatment is performed in the temperature range of 1, the gas content and the dopeable thickness can be set to be large as compared with the above-mentioned conventional technique, and more preferable laser resistance can be realized.

By the way, the above-mentioned characteristics are considered in consideration of laser resistance, and therefore, in order to obtain preferable characteristics as an optical glass, it is necessary to consider high homogeneity as well. Especially, when the fluctuation range of the refractive index (Δn) and the strain amount become large,
When the lens, prism, etc. are manufactured, the optical characteristics deteriorate, making it difficult to perform accurate drawing when incorporated in a fine drawing device such as a stepper, and for irradiating the lens with a high-power laser for a long time. If you do, it may lead to damage. That is, by maintaining the strain amount in the glass within 5 (nm / cm) and the refractive index fluctuation range (Δn) within 2 × 10 ⁻⁶ , an optical glass satisfying the above requirements can be obtained. I can do things.

[0018]

Example First, a high-purity silicon tetrachloride was prepared by distilling the raw material silicon tetrachloride to remove impurities and then stored in a stainless steel container with a fluororesin lining. A plurality of high-purity silica glass ingots were synthesized by the direct flame method of oxyhydrogen flame hydrolysis (hereinafter referred to as the direct method). Next, these ingots were drawn into rod-shaped bodies with a constant diameter, and then kneaded and homogenized by the horizontal floating zone melting method (FZ method), and striae were not recognized in the three directions and the light use area (clear aperture) was observed.
Cha -) cutting the silica glass body with suppressed refractive index variation width ([Delta] n) within 2 × 10 ^-6 in diameter and grinding 100φ
Several test pieces of 100 mm x ^h were prepared. (Hereinafter referred to as the first test piece) Further, the ingot is cut and ground as it is without being kneaded by the FZ method, and the diameter is 100φ x ^h 100 mm.
Several test pieces were also prepared. (Hereinafter referred to as the second test piece)

Next, each of the above two kinds of test pieces was placed in a platinum-rhodium crucible and subjected to a hot isostatic pressing method (HIP processing method) under a high pressure atmosphere of 2000 kg / cm ² of 100% argon gas.
After maintaining the temperature of 1750 ℃ for 3 hours and remelting, the slow cooling rate was almost 100 ℃ / hr based on the temperature / pressure curve based on Fig.1.
While gradually cooled to 900 ° C. while maintaining and pressure reduction rate in correspondence with the cooling rate to 1300Kg at 50~100Kg / cm ² .hr / cm ²
Step down to. Then, while the pressure of 1300 kg / cm ² is maintained, the heat treatment temperature is lowered to 200 ° C., and then the pressure is gradually released after a while. Also at the heating temperature, the material is gradually cooled to 900 ° C. and then naturally cooled. An experimental example manufactured from the first test piece is referred to as Example 1 and an experimental example manufactured from the second test piece is referred to as Example 2.

On the other hand, 2000 kg / cm ^{2 of} 100% nitrogen gas
In the high-pressure atmosphere, the remelting treatment is performed under the same temperature and pressure conditions as in Example 1 (Comparative Example 1).

Next, the physical properties of each sample are measured.
First, in order to measure the rare gas content, each sample is cut and polished to prepare a sample for measuring a rare gas emission amount of 20 × 20 × ^t 1 mm, and the quartz chamber in which the sample is set is placed in a vacuum atmosphere. After that, the temperature is raised to 1000 ° C. at 4 ° C./min, and then the temperature is maintained at 1000 ° C. for 2 hours. The noble gas released at that time is introduced into a quadrupole mass spectrometer to measure the amount released. As a result, the rare gas content was about 5 × 10 ²⁰ atoms / m ² in Examples 1 and ² , while no rare gas could be detected in Comparative Example 1.

Next, the Japan Optical Glass Industry Association standard "JOGIS1"
4 ”The amount of strain was measured based on the method for measuring the strain of the optical glass. In each of Example 1 and Comparative Example 1, it was 5 (nm /
cm), the refractive index fluctuation width (Δn) was within 2 × 10 ⁻⁶ , and striae were not seen in the triaxial directions. However, in Example 2, the strength of striae was re-melted. Became weaker,
The strain is 10 (nm / cm) and the fluctuation range of the refractive index (Δn) is 1 × 10
^-5 or more.

Next, regarding each of the above-mentioned Examples and Comparative Examples, 40 × 30 × ^t 10 mm for evaluation of excimer laser resistance, double-sided mirror-finished sample, and 5 × 10 × ^t 20 mm for Raman scattering measurement and absolute refractive index measurement, A three-sided mirror-finished sample was prepared and evaluated as follows. First, in the evaluation of the KrF excimer laser resistance, the irradiation condition was a pulse energy density of about 90.
0 (mj / cm2.pulse) and high output, frequency 100 (H
z), and the number of irradiation pulses was set to 1 × 10 ⁶ (pulses), and changes in transmittance in the 140 nm to 900 nm wavelength region before and after laser irradiation were examined.

As a result, in the samples of Examples 1 and 2, the transmittance was not substantially reduced. Especially, 5.8 eV (about 21
4 nm) has a transmittance of 91 before and after laser irradiation.
The apparent transmittance was within ± 0.5 (%), which was within the range of variation within the accuracy of the measuring device. However, in the sample of Comparative Example 1, the apparent transmittance before irradiation of 91% decreased to 80% after irradiation. Next, in the evaluation of the ArF excimer laser resistance, the irradiation conditions were pulse energy density of about 200 (mj / cm ² .pulse) frequency of 100 (Hz) and irradiation pulse number of 1 × 10 ⁶ (pulses) before and after laser irradiation. The change in transmittance of silica glass in the 140 nm to 900 nm region was examined.

As a result, the apparent transmittances of 91% of the samples of Examples No. 1 and 2 at 5.8 eV did not show any decrease after laser irradiation, but the samples of Comparative Example 1 However, the preferable laser resistance could not be obtained, which was significantly decreased from 91% to 70% before and after the laser irradiation. The cause of this is presumed to be that some nitrogen compound was generated in the glass.

As a result, in the case of a short wavelength ultraviolet laser, it was confirmed that the presence of a rare gas is particularly effective, and in terms of laser resistance, the strain and the refractive index fluctuation width (Δn) do not have a particular effect. Understood

Next, the wavelength of the sample is 588 nm.
The absolute refractive index n _d at was measured. As a result, in each of Examples 1 and 2 and Comparative Example 1, n _d = 1.4615, and the refractive index of the test piece before the remelting treatment was n _d = 1.4585.
Therefore, it was confirmed that the refractive index of the samples of each example increased by 0.003.

The scattering peak (800 ^-1 cm) of the fundamental vibration between silicon and oxygen was measured by the laser Raman scattering measurement method for the samples of the respective examples and the samples before the treatment.
If 495 the D ₁ line and the peak of the D ₂ line of 606 (cm ^-1) of (cm ^-1) was measured and was determined the intensity ratio based on the following 1 and 2 where the sample pretreatment R1 = 0.485
0.503 R2 = 0.155 to 0.160, while R1 = 0.405 R for the sample of Example 1.
2 = 0.110, for the sample of Example 2 R1 = 0.10.
It was confirmed that 400 R2 = 0.110 and the sample of Comparative Example 1 were significantly reduced to R1 = 0.415 R2 = 0.115. ^{_{R1 = I 1 (495cm -1)}} / I 0 (800cm -1) (1) formula ^{_{R2 = I 2 (606cm -1)}} / I 0 (800cm -1) (2) Formula I _1: 495cm ^-1 scattering peak Intensity I ₂ : 606 cm ^-1 scattering peak intensity I ₀ : 800 cm ^-1 scattering peak intensity

Next, in order to investigate the influence on the absolute refractive index or the member ring structure and the excimer laser resistance, the temperature conditions were fixed at the same conditions as in Example 1, and the argon gas 100% pressure condition was 10 kg / cm ² . After re-melting and maintaining for a predetermined time, the pressure condition is maintained and the slow cooling rate is about 100.
The heat treatment temperature is lowered to 200 ° C. while maintaining the re-melting pressure while maintaining the re-melting pressure, and gradually releasing the pressure for a while. A treatment experiment was carried out for cooling (Comparative Example 2).

Then, a sample in the central region of each of the comparative examples was taken, and the KrF excimer laser resistance was evaluated in the same manner as in Example 1. As a result, the apparent transmittance before irradiation of 5.8 eV was obtained. The sample of 91% decreased to around 30% in the sample of Comparative Example 2 after laser irradiation.

Next, when the absolute refractive index n _{d at} the wavelength of 588 nm was measured for the samples of the respective comparative examples, n _d = 1.4585 in Comparative example 2, which was almost the same as the refractive index of the test piece before heat treatment. there were.

The scattering peak (800 cm) due to the fundamental vibration between silicon and oxygen measured by the laser Raman scattering method.
^-1 ) and 495 (cm ^-1 ) D ₁ line and 606 (cm ^-1 ) D2 line
The peak of the line was measured, and the intensity ratio was determined based on the above equations 1 and 2, and in Comparative Example 2, R1 = 0.495.
R2 = 0.160 and no reduction was observed compared to the sample before treatment.

Therefore, it is possible to significantly improve the laser resistance by improving the absolute refractive index or reducing the ratio of the unstable glass structure of the three-membered ring and the four-membered ring on the premise of the presence of the rare gas. It was also confirmed that it could be done.

As can be understood from the above description, the present invention is premised on the presence of a rare gas, and the increase of the absolute refractive index in the synthetic silica glass and the corresponding instability of the three-membered and four-membered ring are caused. By reducing the glass ring structure ratio and increasing the density, it is possible to significantly improve laser resistance. It has various remarkable effects.

[Brief description of drawings]

FIG. 1 is a time series curve diagram of temperature and pressure showing a heat treatment state in an example of the present invention.

Claims (3)

[Claims] 1. An optical glass formed of transparent synthetic silica glass , wherein the transparent synthetic silica glass body is softened in a rare gas atmosphere.
Remelting at a temperature above the point (1600 ° C) and under high pressure
An optical glass characterized in that the glass body contains a rare gas element and the absolute refractive index (nd) of the glass body is set to approximately 1.461 or more by performing a treatment. 2. The strain amount in the glass is 5 (nm / cm)
The optical glass according to claim 1, wherein the refractive index fluctuation range (Δn) is maintained within 2 × 10 ^−6. 3. A laser resistant optical glass formed of transparent synthetic silica glass, wherein the transparent synthetic silica glass body is heated to a temperature of a softening point (1600 ° C.) or higher in a rare gas atmosphere and at 2000 Kgf / c.
Perform remelting process by hot isostatic pressing at m ² pressure
According to the optical glass , the glass body contains a rare gas element, and the absolute refractive index (nd) of the glass body is set to 1.4615 or more.