WO2000039038A1

WO2000039038A1 - Method for producing optical quartz glass for excimer lasers

Info

Publication number: WO2000039038A1
Application number: PCT/EP1999/010282
Authority: WO
Inventors: Quarzglas Gmbh & Co. Kg Heraeus; Ltd. Shin-Etsu Quartz Products Co.
Original assignee: Heraeus Quarzglas GmbH and Co KG; Shin Etsu Quartz Products Co Ltd
Current assignee: Heraeus Quarzglas GmbH and Co KG; Shin Etsu Quartz Products Co Ltd
Priority date: 1998-12-25
Filing date: 1999-12-22
Publication date: 2000-07-06
Anticipated expiration: 2001-06-25
Also published as: JP4011217B2; JP2000191329A

Abstract

An object of the present invention is to provide a method for producing a large optical quartz glass for use in excimer lasers, which exhibits high transmittance to excimer laser radiations and which is yet resistant to lasers. This problem can be solved by a method for producing an optical quartz glass for use in excimer lasers, comprising a step of forming a porous silica preform by depositing soot-like silica formed by flame hydrolysis of a high-purity volatile silicon compound, a step of obtaining a quartz glass body by heating the porous silica preform in an oxidizing atmosphere and vitrifying the resulting body into a transparent body, and a step of performing a heat treatment in a reducing atmosphere comprising hydrogen. The oxidizing heat treatment generates oxygen defects, the hydrogen treatment results in a silica body doped with hydrogen molecules and free of said defects.

Description

Patent Application

Heraeus Quarzglas GmbH & Co. KG Shin Etsu Quartz Products Co., Ltd.

Method for Producing Optical Quartz Glass for Excimer Lasers

Detailed Description of the Invention

[0001]

Industrial Field of Application

The present invention relates to a method for producing an optical quartz glass for use in excimer lasers, and in further detail, it relates to a method for producing an optical quartz glass for use in the optical system of a lithographic system using an excimer laser radiation as the light source; in a still further detail, it relates to a method for producing a synthetic quartz glass for use in the optical system of an ArF excimer laser lithographic system, such as a lens, a prism, or a beam splitter.

[0002]

Prior Art

With the recent increase in the degree of integration in LSIs (large scale integrated circuits), the integrated circuit patterned on wafers are becoming finer, and mass production of Ultra LSIs provided with ultrafine patterns in the order as fine as quarter micron (0.25μm) or even more is now under way. To obtain such ultrafine patterns, it is necessary to use aligner light sources having still shorter wavelength, and, steppers using excimer laser radiation as the light source had been developed. Steppers equipped with KrF excimer laser radiation (248 nm in wavelength) is already put into practical use, and steppers using ArF excimer laser radiation (193 nm in wavelength) as the light source is attracting much attention as a promising stepper of the next generation. As a glass material which exhibits sufficiently high transmittance in the short wavelength region of the KrF excimer laser and ArF excimer laser radiations, there can be mentioned a quartz glass, fluorite, etc. Particularly among them, a synthetic quartz glass prepared by fusion and vitrification of a product obtained by flame hydrolysis of a high purity silicon compound and the like is preferred as an optical material for lithography using excimer laser radiation as the light source, because it exhibits high transmittance in the short wavelength region of 260 nm or less.

[0003]

Problems the Invention is to Solve

However, if excimer laser radiations such as those of the KrF excimer lasers and ArF excimer lasers are irradiated for a long duration of time to the synthetic quartz glass above, paramagnetic defects known as E' centers or NBOHC (non-bridging oxygen hole centers) generate inside the synthetic quartz glass as to impair long time stability, because the excimer lasers yield high energy pulse radiations with a life of about 20 nanoseconds. As a means to solve such problems, in JP-A-Hei3-88742 (the term „JP-A-„ as referred herein signifies „an unex- amined published Japanese patent application") is proposed a method of doping the synthetic quartz glass body with hydrogen. However, this method may lead to a generation of reducing defects in the synthetic quartz glass during its production process, and these defects easily undergo decomposition upon irradaition of the excimer laser radiation and yield paramagnetic defects. The resulting defects then generate an undesirable absorption band at a wavelength of 215 nm, thereby considerably deteriorating the transmittance of excimer laser radiations, particularly, that of the ArF excimer laser radiations. The paramagnetic defects above are the oxygen deficient type defects which generate as a result of the combination of Si in the quartz glass structure (SiO₂) with less than stoichiometric quantity of oxygen. In JP-A-Hei6-166528 is proposed a method of preventing the generation of these parametric defects from occurring, which comprises subjecting the quartz glass to an oxidation treatment in the temperature range of from 600 to 1500 °C under an atmosphere containing oxygen, followed by a treatment in the temperature range of from 300 to 600 °C under an atmosphere containing hydrogen. This method certainly is a remedy for preventing paramagnetic defects from occurring; however, since it incorporates a thermal treatment performed in a plurality of steps, impurities which diffuse from the outside lower the transmittance of the synthetic quartz glass due to contamination. It is a still other disadvantage that the process requires a long duration of time in producing a large scale quartz glass member, because the diffusion rate of hydrogen in the synthetic quartz glass body remains too low in a heat treatment that is performed in such a low temperature range of from 300 to 600 °C. In particular, with the recent trend in the increase of exposure area, thick and large aperture optical systems are required; in such cases, hydrogen treatment in the above temperature range requires several months or longer, and this makes the process industrially unfeasible. Further, in Japanese Patent No. 2566151 (JP-A-Hei1 -197335) is proposed a method similar to above, which comprises producing quartz glass by subjecting a bulk of synthetic quartz glass to an oxidation treatment that is followed by hydrogen treatment. However, this production method also suffers a disadvantage such that the oxygen deficient defects cannot be sufficiently removed from the thick and large aperture optical members which meet to the recent requirements, because the method requires an oxidation treatment of a bulk quartz glass. In the light of such circumstances, there is a keen demand for a proposal of an optical member suitable for thick and large aperture optical systems having excellent transmittance of excimer laser radiations, particularly, ArF excimer laser radiations, and yet, having superior resistance against laser radiations.

Accordingly, the present inventors intensively continued studies, and, as a result, they have found that, to efficiently recover the reducing defects which generate in a synthetic quartz glass during hydrogen treatment, it is effective to first form oxygen excessive defects inside the quartz glass, and to then treat the resulting product in hydrogen. Further, in the treatment above, the soot-like silica generated by flame hydrolysis of a high purity volatile silicon compound is deposited to obtain a porous silica preform (referred to hereinafter as „a soot preform"), and the soot preform is subjected to an oxidation treatment while elevating the treatment temperature to 800 °C or higher. In this manner, oxygen sufficiently diffuses into the inner side of a large glass body as to generate oxygen excessive defects, and by subjecting the resulting product to a hydrogen treatment at a temperature higher than 600 °C, it was found that an optical quartz glass free from generation of reducing defects and doped with hydrogen molecules at a high concentration can be obtained with high transmittance for excimer laser radiations, particularly, ArF excimer laser radiations, still having excellent resistance against laser radiations. The present invention has been accomplished based on these findings. That is:

[0005] An object of the present invention is to provide a method for producing an optical quartz glass for excimer lasers having high transmittance and yet having excellent resistance against laser radiations. [0006]

Another object of the present invention is to provide a method for producing a thick and large aperture optical quartz glass for ArF excimer lasers having high transmittance for ArF excimer laser radiations, and yet having excellent resistance against laser radiations.

[0007]

Means for Solving the Problems

The object above is accomplished by a method for producing an optical quartz glass for use in excimer lasers, comprising a step of forming a porous silica preform by depositing sootlike silica formed by flame hydrolysis of a high-purity volatile silicon compound, a step of obtaining a quartz glass body by heating the porous silica preform in an oxidizing atmosphere and vitrifying it into a transparent body, and a step of performing a heat treatment in a reducing atmosphere.

[0008]

As high purity volatile silicon compounds, there can be mentioned silicon tetrachloride, methyl trimethoxysilane, tetramethoxysilane, etc., and the soot-like silica produced by subjecting them to flame hydrolysis is deposited on a rotating heat-resistant base body to form a porous silica preform. The porous silica preform is then heated in an oxidizing atmosphere for vitrification to obtain a transparent body. The heating is preferably performed in a temperature range not lower than 1400 °C, but not higher than 1600 °C. Preferably, the porous silica preform is pre-sintered in a temperature range of from 1000 to 1400°C in an oxidizing atmosphere for densification, and then heated in a temperature range of from 1400 to 1600°C in an inert gas atmosphere. By thus treating in an oxidizing atmosphere, a quartz glass body having oxygen excessive defects is produced. The oxygen excessive defects are the defects that are formed by the combination of Si in the quartz glass structure (SiO₂) with oxygen at an amount not less than the stoichiometric quantity. As the gas for use in the oxidizing atmosphere, there can be used a mixed gas comprising oxygen with an inert gas, such as gaseous He or N₂, but particularly preferred is gaseous He. In the mixed gas above, gaseous oxygen preferably accounts for 30 % by volume or more but less than 100 %. If the oxygen content is less than 30 % by volume, the oxygen deficient defects account too low as result in an insufficient recovery of the reducing defects. It is not preferred to use 100 % gaseous oxygen because bubbles may remain in the quartz glass. Subsequent to this treatment in an oxidizing atmosphere, a homoge- nization treatment, or a homogenization treatment and molding (referred to hereinafter as „a homogenization treatment and the like") is performed in order to highly homogenize the optical member. The layered structure, as well as the striae as the like in the three directions, which generates during the deposition of the soot-like silica is removed by the homogenization treatment and the like, thereby resulting in a highly homogeneous quartz glass having a refractive index distribution (Δn) of 2 x 10^"6 or lower, or a birefringence of 2 nm/cm or lower. However, since the homogenization treatment is performed by maintaining the product at a high temperature of 1600 °C or higher for a long duration of time in a refractory furnace, contamination of the quartz glass occurs because impurities diffuse into the quartz glass from the furnace material, jigs, and the atmosphere. In particular, the contamination attributed to Na is serious, because the incorporation of Na greatly impairs the transmittance for ArF excimer laser radiations. In this context, in JP-A-Hei7-267662 is proposed a homogenization treatment which is performed free of furnace materials. Molding of the quartz glass is performed after the homogenization treatment, in which the quartz glass body is shaped into an optical material having the desired shape of, for example, a column, a disk, or a tetrahedron. The molding is performed, in general, by heating the quartz glass body in a crucible at a temperature not lower than 1600°C or higher to allow the body to deform in accordance with its own weight or forcibly. Thus, the quartz glass molding undergoes contamination again due to the thermal diffusion of Na during the molding process.

[0009]

In the process according to the present invention, a heat treatment under a reducing atmosphere is performed subsequent to the heat treatment in an oxidizing atmosphere and the homogenization treatment and the like described above. As the reducing atmosphere usable for the treatment, mentioned is a gaseous hydrogen atmosphere, or an atmosphere of mixed gas containing hydrogen and an inert gas. The heating temperature is in a range of 600 to 1500 °C, and preferably, the range is from 800 to 1000°C. If the temperature should be lower than 600 °C, the effect of the treatment is low; if the treatment should be performed at a temperature higher than 1500 °C, an increase in the effect cannot be expected. Even if such a high temperature heat treatment is performed under a reducing atmosphere, the quartz glass not only remains free from reducing defects, but also is doped with hydrogen molecules at a high concentration, thereby resulting in a product having stability against the irradiation of excimer laser radiations. Since the stability against the irradiation of excimer laser radiations increases in proportion to the content of hydrogen, it is preferred to apply pressure during the heat treatment under the reducing atmosphere. The pressure is preferably applied at 1 atm or higher but not higher than 10 atm. If the applied pressure should exceed 10 atm, the treatment is categorized as a hydrogen treatment under a high temperature and high pressure; hence, it happens that such a treatment should follow some regulations under a law. The oxygen excessive defects that are contained in the quartz glass molding undergo reduction during the treatment under the reducing atmosphere to newly generate OH groups and produce hydrogen molecules that are doped in the quartz glass. The concentration of the hydrogen molecules is preferably 2 x 10¹⁷ molecules/cm³ or higher. An annealing treatment follows the heat treatment under the reducing atmosphere, and this annealing treatment is performed by maintaining in the temperature range of from 1100 to 1200 °C in air for a long duration of 10 hours or longer. After the annealing treatment, the temperature is gradually lowered to 600 °C at a cooling rate of 50 ° C/hour, and then left to natural cooling. A cooling rate higher than the range above is not preferred because cracks may generate during the cooling. Although the annealing treatment is performed at a low temperature in the vicinity of the cooling point (1120°C) of the quartz glass, impurities which diffuse out from the furnace material and the atmosphere and the like contaminates the quartz glass in a manner similar to the case of homogenization treatment and molding. If the content of Na in the quartz glass molding falls in a range of from 24 to 60 ppb due to the contamination during the homogenization treatment, molding, or annealing treatment, the transmittance can be recovered by irradiating a continuous ultraviolet radiation of 260 nm or shorter for a long duration of time; particularly, in case of an ArF excimer laser radiation, the internal absorption can be recovered to within 0.2 %. As the lamp for use irradiating the continuous ultraviolet radiation above, there can be employed a low vapor pressure mercury lamp radiating a radiation having a principal wavelength of 253.7 nm and 184.9 nm, a Xe excimer lamp which radiates a light having a wavelength of 172 nm, or a KrCI excimer lamp which radiates a light having a wavelength of 222 nm. The luminance of the ultraviolet radiation is preferably 1 mW/cm³ or higher, and the duration of irradiation is preferably 50 hours or longer.

[0010] Embodiment of the Invention

Preferred embodiments according to the present invention are decribed below by way of examples, but it should be understood that the present invention is by no means limited thereto.

[0011]

Examples

The physical properties described below in Examples and Comparative Examples are values obtained by the measuring methods as follows.

i) Distribution of refractive indices: Measurements were made using a Fizeau's interferometer.

n) Birefringence: Measurements were made in accordance with crossed nicol method.

m) Striae: Visual observation.

IV) Internal transmittance of light 193 nm in wavelength: Apparent transmittance T % was obtained at a thickness of 10 mm, and a reduced value was calculated in accordance with the equation (T/90.68) x 100, where 90.68 % is a value obtained by subtracting the loss, i.e., 0.18 %, known to be attributed to Rayleigh scattering, from the theoretical transmittance 90.86 % of the quartz glass for a light 193 nm in wavelength.

v) Na concentration: The value was obtained by a measurement using flameless atomic absorption spectroscopy.

[0012]

Example 1

Soot-like silica was prepared by means of flame hydrolysis comprising introducing silicon tetrachloride accompanied by gaseous oxygen into an oxyhydrogen flame, and was deposited on a rotating heat-resistant base body to obtain a soot body. The soot body thus obtained yielded a low density on the outer side thereof, whereas the inner side exhibited a relatively high density. The average bulk density of the soot body was found to be 1.2 g/cm³. The resulting soot body was subjected to pre-sintering so that it may yield a homogeneous density by perfor- ming heating at 1000 °C for a duration of 8 hours in an oxidizing atmosphere consisting of 80 % by volume of oxygen and 20 % by volume of He. The bulk density of the soot body was increased to 1.5 g/cm³ by the pre-sintering, and the refractive index distribution was decreased. Then, the temperature inside the furnace was elevated to 1450°C in an oxidizing atmosphere consisting of 50 % by volume of oxygen and 50 % by volume of He, and by pulling up, the soot body was vitrified to obtain a transparent body . The OH concentration of the thus obtained quartz glass body was found to be 20 ppm, but no hydrogen was detected. The resulting quartz glass body was mounted on a graphite crucible, and was heated to 1800°C under a nitrogen-purged atmosphere so as to obtain a molding by its own weight into a disk having an outer diameter of 250 mm and a height of 100 mm.

[0013]

Of the thus obtained molding, the portion that was brought into contact with the peripheral graphite was sufficiently removed by HF etching treatment, and the resulting molding was subjected to hydrogen loading in a high pressure hydrogen treatment furnace by heating it at 1100 °C for a duration of 24 hours under a hydrogen pressure of 10 atm. Subsequent to the treatment, annealing treatment of the molding was performed. The annealing treatment was performed by maintaining it at 1150 °C for a duration of 20 hours under a hydrogen pressure of 1 atm, followed by cooling to 600 °C at a cooling rate of 5 °C/hour, and, from that temperature, the molding was allowed to cool naturally to carry out the strain removal operation. The refractive index distribution (Δn) was found to be 1 x 10^"6, and the birefringence was 1 nm/cm or lower. No striae in three directions nor layer structure was observed. The concentration of OH groups was 65 ppm, and that of the hydrogen molecules was 2.5χ 10¹⁸ molecules/cm³. A specimen 60 mm in diameter and 10 mm in height was cut out of the molding to measure the transmittance thereof. Then, ArF excimer laser radiation was irradiated to the same specimen at an energy density per pulse of 20mJ/cm² and a frequency of 200 Hz to observe the change in transmittance and in refractive index. The results are given in Table 1.

[0014]

Example 2

Soot-like silica was prepared by means of flame hydrolysis comprising introducing high purity methyltrimethoxysilane accompanied by gaseous argon into an oxyhydrogen flame, and was deposited on a rotating heat-resistant base body to obtain a soot body. The average bulk density of the soot body was found to be 1.3 g/cm³. The resulting soot body was subjected to pre-sintering by performing heating at 900 °C for a duration of 10 hours in an oxidizing atmosphere consisting of 80 % by volume of oxygen and 20 % by volume of He. The bulk density of the soot body was increased to 1.5 g/cm³ by the pre-sintering, and the refractive index distribution of the entire body was decreased. Then, the temperature inside the furnace was elevated to 1450 °C in an oxidizing atmosphere consisting of 50 % by volume of oxygen and 50 % by volume of He, and by pulling up, the soot body was vitrified to obtain a transparent body. The OH concentration of the thus obtained quartz glass body was found to be 30 ppm, but no hydrogen was detected. Similar to the case described in Example 1 , the resulting quartz glass body was shaped to the same size as in Example 1 , where the portion that was brought into contact with the peripheral graphite was sufficiently removed by HF etching treatment. Subsequent to the treatment, the resulting molding was subjected to hydrogen loading in a high pressure hydrogen treatment furnace by heating it at 1000°C for a duration of 32 hours under a hydrogen pressure of 5 atm. After the treatment, annealing treatment was performed on the molding. The annealing treatment was performed by maintaining it at 1150°C for a duration of 20 hours under a hydrogen pressure of 1 atm. This was followed by cooling to 600°C at a cooling rate of 5 °C/hour, and, strain removal operation was carried out from that temperature by allowing the molding to be cooled naturally. The refractive index distribution

was found to be 1 x 10^"6, and the birefringence was 1 nm/cm or lower. No striae in three directions nor layer structure was observed. The concentration of OH groups was 80 ppm, and that of the hydrogen molecules was 1.0 x 10¹⁸ molecules/cm³. A specimen 60 mm in diameter and 10 mm in height was cut out of the molding to measure the transmittance thereof. Then, ArF excimer laser radiation was irradiated to the same specimen to observe the change in transmittance and in refractive index. The results are given in Table 1.

[0015]

Example 3

Soot-like silica was prepared by means of flame hydrolysis comprising introducing high purity tetramethoxysilane accompanied by gaseous argon into an oxyhydrogen flame, and was deposited on a rotating heat-resistant base body to obtain a soot body. The average bulk density of the soot body was found to be 1.3 g/cm³. The resulting soot body was subjected to pre- sintering by performing heating at 900 °C for a duration of 10 hours in an oxidizing atmosphere consisting of 80 % by volume of oxygen and 20 % by volume of He. The bulk density of the soot body was increased to 1.5 g/cm³ by the pre-sintering, and the refractive index distribution of the entire body was decreased. Then, the temperature inside the furnace was elevated to 1450 °C in an atmosphere consisting of 100 % by volume of He, and by pulling up, the soot body was vitrified to obtain a transparent body. The OH concentration of the thus obtained quartz glass body was found to be 25 ppm, but no hydrogen was detected. Similar to the case described in Example 1 , the resulting quartz glass body was shaped to the same size as in Example 1 , where the portion that was brought into contact with the peripheral graphite was sufficiently removed by HF etching treatment. Subsequent to the treatment, the resulting molding was subjected to hydrogen loading in a high pressure hydrogen treatment furnace by heating it at 1000 °C for a duration of 32 hours under a hydrogen pressure of 5 atm. After the treatment, annealing treatment was performed on the molding. The annealing treatment was performed by maintaining it at 1150 °C for a duration of 20 hours under atmosphere. This was followed by cooling to 600 °C at a cooling rate of 5 °C/hour, and, strain removal operation was carried out from that temperature by allowing the molding to be cooled naturally. The refractive index distribution (ΔΠ) was found to be 1 _x 10⁶, and the birefringence was 1 nm/cm or lower. No striae in three directions nor layer structure was observed. The concentration of OH groups was 80 ppm, and that of the hydrogen molecules was 1.0 _x 10¹⁸ molecules/cm³. A specimen 60 mm in diameter and 10 mm in height was cut out of the molding to measure the transmittance thereof. Then, ArF excimer laser radiation was irradiated to the same specimen to observe the change in transmittance and in refractive index. The results are given in Table 1.

[0016]

Table 1

[0017]

From Table 1 above, it can be seen that the quartz glass obtained by the production method according to the present invention remains without significant decrease in transmittance and no compaction is observed thereon even after an ArF excimer laser radiation is irradiated for a long duration of time. The compaction herein refers to the shrinking of the quartz glass on irradiating a laser radiation. If the compaction takes place, the refractive index increases and leads to the deterioration of image forming characteristics of the optical system such as the lenses of an aligner.

[0018]

The production method according to the present invention enables a highly laser- resistant optical quartz glass which also has a high transmittance for excimer laser radiations. In particular, the present invention is effective in the production of thick optical quartz glass having large diameter for use in ArF excimer lasers, which is stable and has no drop in transmit tance even when it is irradiated by an ArF excimer laser radiation for a long duration of time.

Claims

1. A method for producing an optical quartz glass for use in excimer lasers, comprising a step of forming a porous silica preform by depositing soot-like silica formed by flame hydrolysis of a high purity volatile silicon compound, a step of obtaining a quartz glass body by heating the porous silica preform in an oxidizing atmosphere and vitrifying it into a transparent body, and a step of performing a heat treatment in a reducing atmosphere.

2. A method for producing an optical quartz glass for use in excimer lasers as claimed in Claim 1 , wherein the heating temperature in the oxidizing atmosphere is in a range of from 800 °C to 1600 °C.

3. A method for producing an optical quartz glass for use in excimer lasers as claimed in Claim 1 , wherein the step of obtaining a quartz glass body comprises densifying the porous silica preform by heating it in an oxidizing atmosphere in a temperature range of from 800 to 1400 °C, and then heating it in an inert gas atmosphere in a temperature range of from 1400 °C to 1600 °C.

4. A method for producing an optical quartz glass for use in excimer lasers as claimed in any of Claims 1 to 3, wherein the oxygen concentration of the oxidizing atmosphere is set in the range of 30 % by volume or higher but less than 100 % by volume.

5. A method for producing an optical quartz glass for use in excimer lasers as claimed in any of Claims 1 to 4, wherein the quartz glass body vitrified into a transparent body is molded and then heat treated in a reducing atmosphere.

6. A method for producing an optical quartz glass for use in excimer lasers as claimed in any of Claims 1 to 5, wherein the heat treatment in the reducing atmosphere is performed in the temperature range of from 600 to 1200°C under a pressure of 1 atm or higher.

7. A method for producing an optical quartz glass for use in excimer lasers as claimed in any of Claims 1 to 6, wherein hydrogen molecules are doped at a density of 1 x 10¹⁷ molecules/cm³ or more in the heat treatment performed under the reducing atmosphere.

8. A method for producing an optical quartz glass for use in excimer lasers as claimed in any of Claims 1 to 7, wherein, after the heat treatment in the reducing atmosphere, annealing treatment is performed by maintaining the temperature in the range of from 1100 to 1200 °C in air, and then gradual cooling is performed at a gradually cooling rate of 50 °C/hour or less.