KR20010032101A - Optical component made of silica glass and method for the production thereof - Google Patents

Abstract

Optical devices for ultraviolet light transmission of wavelengths up to 250 nm made from synthetic quartz glass prepared by flame hydrolyzing a chlorine free silicon compound, deposited on a substrate in the form of fine SiO ₂ and immediately vitrified, have been known. On the basis of the optical element described above, an object of the present invention is to provide an optical element exhibiting long-term stability and particularly exhibiting deterioration behavior in which induced absorption is saturated at low levels for UV light transmission of 250 nm or less. For this reason, quartz glass has a hydrogen content of 5 x 10 ¹⁶ molecules / cm 3 or less. The manufacturing method of the optical device includes the production of synthetic hydrogen-containing quartz glass which synthesizes fine SiO ₂ by flame hydrolysis of a chlorine-free compound and deposits and vitrifies fine SiO ₂ on a substrate to form a hydrogen-containing quartz glass, The hydrogen containing quartz glass is subjected to hydrogen reduction treatment, and the hydrogen content is set to a value of 5 x 10 ¹⁶ molecules / cm 3 or less.

Description

Optical device made of quartz glass and its manufacturing method {OPTICAL COMPONENT MADE OF SILICA GLASS AND METHOD FOR THE PRODUCTION THEREOF}

The present invention relates to an optical element for transmission of ultraviolet light having a wavelength of 250 nm or less, made of synthetic quartz glass obtained by flame hydrolysis of a chlorine free silica compound, deposited on a substrate as fine SiO ₂ , and directly vitrified.

The present invention is 250 nm will hereinafter regarding UV transmission composite quartz glass optical element in the manufacturing method for a wavelength, a method is synthesis of fine SiO ₂ by flame hydrolysis of a chlorine free silica compounds, and particulate SiO ₂ on a substrate Synthesis of hydrogen containing quartz glass, which deposits and directly vitrifies to form hydrogen containing quartz glass.

Optical devices of synthetic quartz glass are particularly used for the transmission of high power ultraviolet laser beams, for example in the form of exposure lenses for fine etching equipment in the manufacture of highly integrated circuits in semiconductor chips, or optical fibers. Recent exposure systems for microetching equipment are equipped with excimer lasers that produce high energy pulsed UV light of 248 nm (KrF laser) or 193 nm (ArF laser). Such shortwave UV light causes defects in the optical device, resulting in absorption specific to the quality or type of each quartz glass. For example, the pattern of defects is observed where the induced absorption increases linearly with successive UV light, or where the saturation level is reached after the initial rise, or where the resulting defects accumulate and show sudden and strong absorption. The strong increase in absorption of the latter defect pattern is described in the literature as SAT defects.

Light resistance of quartz glass depends on structural properties such as density, refractive index distribution, homogeneity, or chemical composition. The relationship between the chemical composition of quartz glass and its behavior under damaging high power UV light is described, for example, in EP 401,845 A1. High purity quartz glass having a relatively high OH content in the range of 100 to 1000 wt.ppm and a relatively high hydrogen concentration of 5 × 10 ¹⁶ molecules / cm 3 per cm 3 (relative to the volume of quartz glass) has high light resistance. The beneficial action of hydrogen at light resistance can be explained that hydrogen helps to heal the defect and slow down the increase in light induced absorption. Because of the action of hydrogen, EP 401,845 A1 recommends charging hydrogen to optical devices that require high light resistance.

However, despite the similar chemical or structural properties of quartz glass, the deterioration pattern of the optical element may change if the quartz glass is obtained by other manufacturing methods, or even if there are chemical or structural differences, they must be observed. It does not cause. Therefore, the optical device according to the present invention is most characterized in its manufacturing method.

The method for producing synthetic quartz glass by flame hydrolysis of silica containing compounds can be distinguished by the method used to vitrify the deposited SiO ₂ particles as well as the starting material. SiCl ₄ is a frequently used starting material in the production of flame hydrolysis of synthetic quartz glass. However, other materials, such as chlorine-free silica, may also be used, including organic compounds such as silane or siloxane. The preparation of quartz glass using alkoxysilanes is described in EP 525,984 A1; The production of quartz glass using silosic acid is described in EP 463,045 A1. It is possible to produce quartz glass that is substantially free of chlorine based on the starting material.

The vitrification of SiO ₂ particles can proceed directly during deposition on the substrate, hereinafter referred to as ″ direct vitrification ″. In contrast, the soot method keeps the temperature very low during the deposition of SiO ₂ particles to form a porous soot body, but only minimal vitrification of the SiO ₂ particles occurs. It is necessary to further sinter the soot body for the vitrification and production of quartz glass. Both manufacturing methods produce dense, transparent and high purity quartz glass.

Optical devices of this kind for the UV light transmission of wavelengths below 300 nm, and methods for their preparation are known from EP 780,345 A1. The device described therein is obtained as follows. Synthetic quartz glass is deposited and directly vitrified on the substrate in the form of SiO ₂ particles by flame hydrolysis of the chlorine free polymethylsiloxy acid compound.

When exposed to pulsed light of an excimer laser (wavelength: 248 nm, power output: 360 mJ / cm 2), deterioration of an optical device obtained in such a manner is characterized by no SAT defects as compared with conventional devices. Conventional devices are also made of synthetic quartz glass, but chlorine-containing starting material (SiCl ₄ ) is used in its preparation. Thus, defects found in known optical devices may be the result of low chlorine content of 3 ppm or less.

Although light induced absorption in known optical devices manufactured according to the method described above does not exhibit SAT defects, it shows a relatively flat increase. However, it is not clear whether such an increase can end upon saturation, and even if any saturation level is reached, if any. However, in photonic applications where long-term stability is important, the decisive factor is not the initial increase in absorption, but the arrival of the saturation point and its absolute level.

Accordingly, it is an object of the present invention to provide an optical device having a long-term stability when transmitting UV light having a wavelength of 250 nm or less, and particularly exhibiting a deterioration pattern in which induced absorption is saturated at a low level. It is also an object of the present invention to provide a method for manufacturing such a device.

Such an object is achieved according to the invention on the basis of an optical element described earlier in which quartz glass has a hydrogen content of 5 x 10 ¹⁶ molecules / cm 3 or less.

The optical device according to the present invention is made of quartz glass from which the device is synthetically produced, the quartz glass is synthesized by flame hydrolysis of a chlorine free starting material, and the quartz glass is directly vitrified during deposition, and quartz The hydrogen content of the glass is characterized by not exceeding 5 × 10 ¹⁶ molecules / cm 3 (relative to the volume of the quartz glass).

Characteristic degradation patterns of optical devices fabricated in such a manner cause the induced absorption to increase rapidly initially but then to break off quickly at low levels of saturation. Such characteristic degradation aspect obviously depends on the manufacturing parameters as described above. One variable in device fabrication changes, for example, the soot method is used instead of direct vitrification for synthesis. What is particularly surprising is that the hydrogen content of the quartz glass must be less than 5 x 10 ¹⁶ molecules / cm 3 to achieve the desired light resistance. Hydrogen not only heals light-induced defects, but also creates defects that themselves become very noticeable under prolonged light irradiation. In the hydrogen content of 5 × 10 ¹⁶ molecules / cm 3 or less, not only the saturation level of photoinduced absorption is achieved, but also its saturation level is relatively low compared to quartz glass having a high hydrogen content.

Due to the presence of hydrogen during device fabrication, the quartz glass may contain hydrogen at concentrations higher than the above-mentioned upper limit of 5 × 10 ¹⁶ molecules / cm 3 after vitrification. Under such circumstances it is necessary to release hydrogen from the device.

The hydrogen content here and further below is the mean (the arithmetic mean of three or more measurement positions evenly distributed in the device) of the device volume. In optical devices in which such conditions are not satisfied, it is sufficient that the average hydrogen content is at least equal to or above the above-mentioned maximum value in the area where it is optically utilized.

Hydrogen content is determined by `` Khotimchenko '' et al. It is calculated | required by the Raman measurement described below. The hydrogen content is obtained by calculating the ratio of the surface intensity of the Raman absorption band at 4,135 cm ^-1 wave due to hydrogen molecules in quartz glass and the Raman absorption band at 800 cm ^-1 wave due to SiO ₂ . It is obtained by multiplying the ratio by the constant k (k = 1.22 × 10 ²¹ ). The result is the hydrogen concentration at 1 cm 3.

Currently, the measurable limit for determining the hydrogen level by such a measurement is 5 x 10 ¹⁵ molecules / cm 3. In particular, advantageous optical elements have a hydrogen content of 2 × 10 ¹⁶ molecules / cm 3 or less, in particular 5 × 10 ¹⁵ molecules / cm 3, such that the hydrogen content is below the current measurable limit. Such devices have very long lasting stability when exposed to high power UV light and very low saturation levels of UV light induced absorption.

Particularly advantageous aspects of degradation occur in optical devices where the quartz glass has an OH content of at least 400 wt. Ppm. Similar to the hydrogen content, the OH content reflects the mean value over the volume of the quartz glass and is spectroscopically determined.

Also advantageous is quartz glass with a chlorine content of at most 1 wt.ppm. The chlorine content also reflects the mean value over the volume of the quartz glass and is determined by the wet-chemical process. Chlorine or chlorine containing compounds are often used to remove hydroxy ions or contaminants from quartz glass. However, chlorine content of more than 1 wt.ppm may adversely affect the deterioration pattern of the optical device.

With regard to the process, the above technical object is achieved according to the present invention based on the above-described method of hydrogen reduction treatment of hydrogen-containing quartz glass to obtain a hydrogen content of 5 × 10 ¹⁶ molecules / cm 3 or less.

For the characteristics of the deterioration aspect of the optical element manufactured according to the present invention and the meaning of the expression "hydrogen content", reference is made to the above description. It is important to adjust the hydrogen content of the quartz glass to a value of 5 x 10 ¹⁶ molecules / cm 3 or less by the hydrogen reduction treatment in the manufacture of the optical device. By this treatment the hydrogen content can be adjusted in a defined manner, and thus reproducibly from the first high concentration to a concentration of 5 x 10 ¹⁶ molecules / cm 3 or less. Optical devices manufactured in such a manner have a reproducible deterioration pattern.

According to a preferred method, the hydrogen reduction treatment results in a hydrogen content of quartz glass of 2 × 10 ¹⁶ molecules / cm 3 or less, more preferably of 5 × 10 ¹⁵ molecules / cm 3 or less, i.e. below the current limit of the Raman measurement method. . Devices fabricated in such a manner are characterized by very good long term stability when exposed to high power UV light and very low saturation levels of light induction absorption.

Preferably, the hydrogen reduction treatment includes heat treatment of the quartz glass, treatment in vacuum, and / or treatment in a chemically reactive atmosphere. The above-described processing methods can be used alternatively or in duplicate. However, heat treatment in a hydrogen-free atmosphere such as heat treatment in an inert gas or vacuum (hereinafter referred to as 'tempering') is particularly effective for controlling the hydrogen content of quartz glass. The method applies to quartz glass which has a very high hydrogen content after vitrification and requires any additional treatment such as heat shaping or homogenizing. Hydrogen diffuses from the quartz glass blank during tempering, and the hydrogen in the region near the surface is first lost and then the hydrogen in the central region of the blank is lost. Therefore, it is important to sufficiently reduce the hydrogen content in areas where high optical loads are applied when intended to be used after fabricating the device from the blanks, and in general such areas are exactly the center areas of the blanks. During the removal of hydrogen, the thickness or wall thickness of the quartz glass blank must be taken into account in adjusting the temperature, tempering period and tempering atmosphere to remove hydrogen to the desired extent. Tempering of quartz glass is commonly used, for example, for the removal of mechanical stresses which adversely affect the properties of quartz glass. The tempering proposed here for the removal of hydrogen is such that the quartz glass according to the invention has a 5 × 10 ¹⁶ molecule / cm 3 or less, preferably 2 × 10 ¹⁶ molecules / cm 3 or less, more preferably 5 × 10 ¹⁵ molecules / cm 3 The purpose and result differ from the known tempering method in that it has the following average hydrogen content.

When the quartz glass containing hydrogen is formed into an intermediate product whose dimension of the smallest side does not exceed 100 mm, preferably 80 mm, and when the hydrogen reduction treatment is partially or completely performed on the intermediate product. The method is particularly efficient. Hydrogen reduction treatment is based on diffusion processes in quartz glass, such as, for example, external diffusion of hydrogen during tempering. Relative layer thickness is an important factor in the time taken for diffusion. For example, the lens of the microetching device may have a diameter of 250 mm and a thickness of 100 mm. The quartz glass blank therefore needs to have a diameter of 300 mm and a thickness of at least 100 mm. Proper hydrogen reduction treatment in such devices will require a very long treatment time which is not economical. The diffusion process can be accelerated by increasing temperature, but there is a limit to raising the temperature because it leads to plastic deformation of the device or other thermally induced damage. To solve such a problem, an intermediate product is formed from the hydrogen containing quartz glass and its dimensions are selected so that hydrogen can be easily removed. The smallest lateral dimension is, for example, the outer diameter of the rod-shaped intermediate product, the wall thickness of the tubular product, or the thickness of the dish-shaped product. Such intermediates allow for hydrogen reduction treatment in economic terms. If the hydrogen can be completely reduced in the present invention by further treatment of the quartz glass, the method is sufficient to reduce but not reduce the hydrogen content in the intermediate product below the above maximum value. The device is manufactured from the intermediate product in the next step.

Homogenization of quartz glass was successful when forming intermediate products. Homogenizing treatment of quartz glass needs to remove the existing strand frequently. Quartz glass deforms plastic during homogenization. Hydrogen reduction treatment is proposed after shaping the intermediate product during such homogenization.

Hereinafter, an optical device and a method according to the present invention will be described in detail with reference to the drawings and embodiments. The single figure shows two absorption curves, one of which reflects the degradation aspect of the optical device according to the present invention, and the other reflects the degradation aspect according to the prior art. The number of laser pulses is shown on the X axis in FIG. 1, and the absorption coefficient (1 / cm for base e) is shown on the Y axis.

Hereinafter, the manufacture of an optical device according to the present invention will be described using an embodiment.

The disk-shaped board | substrate is arrange | positioned so that one of the flat surfaces may face vertically downward. A deposition burner having a center nozzle coaxially surrounded by four ring nozzles is disposed below the substrate. The deposition burner is towards the substrate and the substrate rotates about the central axis. Carrier gas (nitrogen) supplies methyltrimethoxysilane to the central nozzle of the deposition burner, while the other nozzles (in order from inner to outer) separate gas (nitrogen), oxygen and hydrogen to the outermost. . Oxygen and hydrogen react to form an oxyhydrogen flame, and methyltrimethoxysilane from the central nozzle is hydrolyzed and deposited on the substrate in the form of fine SiO ₂ . SiO ₂ deposited on the substrate is immediately vitrified by the heat of an oxyhydrogen flame during the formation of rod-shaped quartz glass blanks. Due to the starting materials used, the quartz glass blanks are substantially free of chlorine (chlorine content is less than 1 wt. Ppm).

The quartz glass blanks are then placed in a quartz glass rotating machine for homogenization and heated and twisted to about 2,000 ° C. Suitable homogenization methods are described in EP 673,888 A1. After repeated twisting of the quartz glass, the body has a diameter of 80 mm and a length of 800 mm, no three-dimensional strands, an average hydrogen content of about 5 × 10 ¹⁷ molecules / cm 3, and an OH content of about 900 It will be in the form of a round rod, vol.ppm. Round rods are hydrogen reduced by tempering at 1,100 ° C. for 200 hours under vacuum. The average hydrogen concentration of the round rod is then about 3 x 10 cm 3 molecules / cm 3.

Annular quartz glass blocks having an outer diameter of 240 mm and a length of 90 mm are made from round rods by heating molding at 1,700 ° C. and using nitrogen flushed molds.

After an additional tempering step in which the quartz glass block is heated to 1,100 ° C in atmospheric air and cooled at 1 ° C / hr, the strain double refraction is up to 2 nm / cm and the refractive index is the difference between the maximum and the minimum refractive index. Is uniform to a degree of 1 × 10 ⁻⁶ . The hydrogen content of the quartz glass block is no longer observed by the Raman method so that it is 5 × 10 ¹⁵ molecules / cm 3 or less, and the average OH content does not change at about 900 wt. Ppm. The quartz glass blocks produced in this way are suitable for direct use as blanks for the manufacture of optical lenses in microetch equipment.

Test samples having the degradation aspect shown in FIG. 1 were prepared by a modification method. In such a variant, quartz glass blocks were formed immediately after the rod was twisted by tempering without the procedure of hydrogen reduction treatment. Two cylindrical test samples (A and B) having dimensions of 10 × 10 × 40 mm were cut from the quartz glass blocks and then four long sides were polished.

The test sample (B) was then subjected to a conventional tempering program which was heated in air for 5 hours to a temperature of 800 ° C. The average hydrogen content of the test sample (B) after the tempering treatment was 1 × 10 ¹⁷ molecules / cm 3 and the average OH content was 900 wt.ppm.

The test sample (A) was hydrogen reduced and the treatment consisted of a treatment that heated up to a temperature of 800 ° C. similarly to sample (B) but with a duration of 15 hours. The average hydrogen content of the sample (A) after such tempering was 5 × 10 ¹⁵ molecules / cm 3 and the average OH content was 900 wt.ppm.

Test samples (A and B) were exposed to UV excimer laser light (wavelength = 193 nm, impulse energy = 100 mJ / cm ² , pulse repetition rate = 200 Hz) and at the same time the transmission at wavelength 193 nm was measured. The resulting absorption curve is shown in FIG. 1 and denoted as ″ A ″ for test sample A and ″ B ″ for test sample B. FIG.

The following deterioration pattern can be seen from the absorption curve.

The absorption curve ″ A ″ of the optical element according to the invention initially shows a large step and shows rapid heat drop of quartz glass, but after about 1,000,000 impulses, the degradation stops at the absolute value of the absorption coefficient of about 0.12 cm ⁻¹ . Very low saturation level under such conditions.

In contrast, the absorption curve ″ B ″ of the optical element according to the prior art, which is the test sample B, slowly increases at an almost constant rate. Sample B shows no saturation of the induced absorption up to 15,000,000 pulses. At 5,000,000 pulses, the absorption curves ″ A ″ and ″ B ″ are staggered. This shows that the transmission capability of the optical device according to the present invention is superior to that of other optical devices at a high impulse number. This shows that the optical device according to the present invention has an excellent deterioration pattern with respect to long term stability.

Claims (11)

In the optical device for transmitting UV light having a wavelength of 250 nm or less, which is produced by flame hydrolysis of a chlorine-free silica compound, deposited on a substrate in the form of fine SiO _{2, and made} of directly vitrified synthetic quartz glass, An optical device having a hydrogen content of 5 x 10 ¹⁶ molecules / cm 3 or less. The optical device according to claim 1, wherein the quartz glass has a hydrogen content of 2 x 10 ¹⁶ molecules / cm 3 or less. The optical device according to claim 1 or 2, wherein the quartz glass has a hydrogen content of 5 x 10 ¹⁵ molecules / cm 3 or less. The optical device according to any one of claims 1 to 3, wherein the quartz glass has an OH content of 400 wt. Ppm or more. The optical device according to claim 1, wherein the quartz glass has a chlorine content of at most 1 wt.ppm. 250, comprising preparing a synthetic hydrogen-containing quartz glass prepared by synthesizing particulate SiO ₂ through flame hydrolysis of a chlorine-free silica compound, and forming a hydrogen-containing quartz glass while depositing and vitrifying the fine SiO ₂ on a substrate. A method for producing a quartz glass optical element for UV light transmission having a wavelength of less than or equal to nm, wherein the hydrogen-containing quartz glass is subjected to a hydrogen reduction treatment and the content of hydrogen is 5 x 10 ¹⁶ molecules / cm 3 or less during the treatment. 6. The method according to claim 5, wherein the hydrogen content of the quartz glass in the hydrogen reduction treatment is at a level of 2 x 10 ¹⁶ molecules / cm 3 or less, preferably 5 x 10 ¹⁵ molecules / cm 3 or less. 8. A method according to claim 6 or 7, wherein the hydrogen reduction treatment comprises heat treatment, vacuum treatment and / or treatment in a chemically active atmosphere of quartz glass. The hydrogen-containing quartz glass of claim 1, wherein the hydrogen-containing quartz glass is formed of an intermediate product having a lateral length not exceeding 100 mm, preferably 80 mm, wherein the hydrogen reduction treatment is partially or completely applied to the intermediate product. Characterized in that it is carried out. 10. The method of claim 9, wherein the quartz glass is homogenized while the intermediate product is formed. 10. The method according to any one of claims 6 to 9, wherein the quartz glass is homogenized and the hydrogen reduction treatment occurs partially or completely before homogenization of the quartz glass.