JP4946960B2 - Synthetic quartz glass and manufacturing method thereof - Google Patents

Description

The present invention relates to a synthetic quartz glass for an optical member of an optical device using light having a wavelength of 180 nm or less as a light source and a method for producing the same. More specifically, a lens (projection system, illumination system), prism of an optical device using a Xe ₂ excimer lamp (wavelength 172 nm), a deuterium lamp (wavelength 170 to 400 nm), an F ₂ laser (wavelength 157 nm) or the like as a light source. The present invention relates to synthetic quartz glass for optical members used as optical component materials such as etalon, diffraction grating, photomask, pellicle (pellicle material, pellicle frame or both) and window material, and a method for producing the same.

Conventionally, in an optical lithography technique, an exposure apparatus for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has been widely used. As integrated circuits become highly integrated and highly functional, miniaturization of integrated circuits advances, and exposure apparatuses are required to form high-resolution circuit patterns on the wafer surface with a deep focal depth. Wavelengths are being promoted. The exposure light source is advanced from the conventional g-line (wavelength 436 nm) and i-line (wavelength 365 nm), and KrF excimer laser (wavelength 248 nm) and ArF excimer laser (wavelength 193 nm) are about to be used. Further, in order to cope with next-generation integrated circuits whose circuit pattern is 100 nm or less, it has begun to consider using an F ₂ laser (wavelength 157.6 nm) as an exposure light source.

  An optical member used in an optical apparatus using such a light source is required to be stable and excellent in light transmittance in a used wavelength region (hereinafter simply referred to as “vacuum ultraviolet light transmittance”).

  In order to improve the vacuum ultraviolet ray transmittance, the OH group content in the synthetic quartz glass needs to be within a predetermined range. For example, in JP-A-8-75901, the OH group content is 10 ppb to 100 ppm. There has been proposed a synthetic quartz glass having a fluorine content of 100 ppm or more.

  In JP-A-8-75901, as a method for producing such synthetic quartz glass, a silicon compound is hydrolyzed in a flame to obtain glass fine particles, and the glass fine particles are deposited to form porous quartz glass. Next, a method for producing synthetic quartz glass has been proposed, in which porous quartz glass is heat-treated in a fluorine-containing atmosphere and then transparentized to obtain fluorine-doped synthetic quartz glass. However, this method reduces the OH group content by heat-treating porous quartz glass in a fluorine-containing atmosphere, and oxygen-deficient defects may be generated in the subsequent transparent vitrification process, and vacuum Synthetic quartz glass excellent in ultraviolet transmittance cannot be stably produced.

  Moreover, in order to repair the oxygen-deficient defects generated in the process before the transparent vitrification process, a method of heat-treating the transparent synthetic quartz glass block in an oxygen gas-containing atmosphere has also been proposed in the publication, There are problems such as the fact that the treatment takes a very long time, oxygen excess type defects are generated, and the vacuum ultraviolet light transmittance and light resistance may be impaired.

  Further, in Japanese Patent Laid-Open No. 2001-89170, the porous quartz glass is porous in an atmosphere containing oxygen gas after the reduction treatment of the OH group or the fluorine doping treatment, or when performing these treatments. A method for repairing oxygen-deficient defects by treating quartz glass has been proposed. However, even in this case, oxygen-excess type defects may be generated, which is a problem.

JP-A-8-75901 JP 2001-89170 A

  An object of this invention is to provide the synthetic quartz glass which exhibits the outstanding vacuum ultraviolet-ray transmittance stably, and its manufacturing method.

  The inventors of the present invention have a process for reducing the OH group content in the porous quartz glass and a transparent vitrification process with respect to the content of oxygen-deficient defects present in the finally obtained synthetic quartz glass body. As a result of detailed examination of the effects of each of the above conditions, the type of electric furnace to be used for transparent vitrification of porous quartz glass and the atmosphere to be used for transparent vitrification are very important. It has been found that a fluorine-doped synthetic quartz glass containing no oxygen-deficient defects can be obtained by forming a transparent glass.

Therefore, the present invention provides a method for producing synthetic quartz glass for an optical member of an optical device using light having a wavelength of 180 nm or less as a light source.
(A) a step of depositing and growing quartz glass fine particles obtained by flame hydrolysis of a glass forming raw material on a substrate to form porous quartz glass;
(B) a step of reducing the OH group content of the porous quartz glass;
(C) treating the porous quartz glass in a fluorine compound-containing atmosphere and doping the porous quartz glass with fluorine;
(D) The porous quartz glass is made into a transparent glass by raising the temperature to 1300 ° C. or higher in a metal furnace using a material containing tungsten or molybdenum as a constituent material of a heater and a reflector ,
And a step of obtaining a transparent quartz glass body containing substantially no oxygen-deficient defects having a fluorine content of 300 ppm or more .

  According to the present invention, a synthetic quartz glass having a stable light transmittance at a wavelength of 157.6 nm can be obtained.

Process (a) of the present invention is a process for producing porous quartz glass.
The raw material for forming synthetic quartz glass is not particularly limited as long as it is a gasifiable raw material, but chlorides such as SiCl ₄ , SiHCl ₃ , SiH ₂ Cl ₂ , and SiCH ₃ Cl ₃ , SiF ₄ , SiHF ₃ , SiH _2. fluorides such as F _2, SiBr _4, bromides such as SiHBr _3, halogenated silicon compounds such as iodide such as SiI _4, or _{R n Si (oR) 4-} n ( wherein R is an alkyl of 1 to 4 carbon atoms Group, n is an integer of 0 to 3), and silicon compounds containing no halogen such as (CH ₃ ) ₃ Si—O—Si (CH ₃ ) ₃ .

Step (b) of the present invention is a step of reducing the OH group content.
In order to reduce the content of OH groups contained in the porous quartz glass, several methods are possible. Specifically, the following methods can be exemplified, but the present invention is not limited to these.
(Method 1) The porous quartz glass is held in a fluorine compound-containing atmosphere. In this case, fluorine doping is performed simultaneously.
(Method 2) The porous quartz glass is held in a chlorine compound-containing atmosphere.
(Method 3) The porous quartz glass is held in an atmosphere containing carbon monoxide.
(Method 4) The porous quartz glass is held in a hydrogen gas-containing atmosphere. In this case, the porous quartz glass is doped with hydrogen. Therefore, a doped hydrogen content reduction process is used in combination as necessary.

  Among these four methods, Method 1 or Method 4 is preferred because the residual chlorine content in the finally obtained synthetic quartz glass is low and the safety is high.

In the present invention, the average bulk density of the porous quartz glass when performing the OH group content control step is 1.6 g / cm ³ or less, and the bulk density distribution (that is, perpendicular to the growth axis direction of the porous quartz glass). In the cross section, the difference between the maximum and minimum bulk density in the region excluding 20 mm from the outer periphery is preferably 0.6 g / cm ³ or less. This is because the porous quartz glass is adjusted within the range of 1000 ° C. to 1500 ° C. between the process of producing the porous quartz glass or reducing the OH group content. Can be done by heating.

  In the OH group content control step, by setting the average bulk density of the porous quartz glass within the above range, the OH group content in the porous quartz glass can be sufficiently controlled, and the subsequent fluorine Fluorine dope can be sufficiently performed in the dope process. In addition, by setting the bulk density distribution within the above range, the distribution of the OH group content and fluorine content in the finally obtained synthetic quartz glass body is difficult to occur, and the uniformity of refractive index and light transmittance is improved. To do.

Step (c) of the present invention is a fluorine doping step.
In order to dope fluorine, the porous quartz glass is held in a fluorine compound-containing atmosphere. As the fluorine compound-containing atmosphere, an inert gas atmosphere containing 0.1 to 50% by volume of a fluorine-containing gas (for example, SiF ₄ , SF ₆ , CHF ₃ , CF ₄ , F _2, etc.) is preferable. The ambient temperature is preferably room temperature to 1300 ° C. Moreover, the atmospheric pressure is preferably 100 Pa to 101 kPa (101 kPa = atmospheric pressure). Furthermore, the holding time is preferably several tens of minutes to several tens of hours.

  In this case, since the porous quartz glass can be uniformly doped with fluorine in a short time, the fluorine-containing gas is maintained under reduced pressure (100 Torr (13.3 kPa) or less, particularly preferably 10 Torr (1.33 kPa) or less). It is preferable to introduce until a normal pressure is reached to obtain a fluorine compound-containing atmosphere. Further, when fluorine doping is performed at a high temperature of 400 ° C. or higher, reduced defects such as oxygen-deficient defects are likely to be generated. For this reason, when fluorine doping is performed at a high temperature of 400 ° C. or higher, it is preferable to hold porous quartz glass in an inert gas atmosphere containing oxygen gas in addition to the fluorine-containing gas to prevent the generation of reduced defects. .

Step (d) of the present invention is a transparent vitrification step.
Transparent vitrification is performed by holding porous quartz glass at a predetermined transparent vitrification temperature for a predetermined time. The transparent vitrification temperature is usually 1300 to 1600 ° C, particularly preferably 1350 to 1500 ° C. As an atmosphere at this time, an atmosphere containing 100% by volume of an inert gas such as helium or nitrogen, or an atmosphere containing an inert gas such as helium or nitrogen as a main component can be used. The pressure may be reduced pressure or normal pressure. In particular, helium gas can be used at normal pressure. In the case of reduced pressure, it is preferably 100 Torr (13.3 kPa) or less.

  As the electric furnace used for transparent vitrification, a heat insulating material, a sample container, a sample table, and an electric furnace whose heater material is not carbon, or a furnace other than a graphite furnace is used. Specifically, an electric furnace (hereinafter also referred to as a metal furnace) in which a material containing tungsten or molybdenum is used for a heater and a reflector (a heat reflection plate or a heat shield plate) can be used. In addition, an electric furnace (hereinafter, also referred to as a multi-furnace) in which disilicide molybdenum or silicon carbide is used as a heater and heat-resistant ceramic mainly composed of alumina is used as a heat insulating material can be used. When oxygen gas is contained in the atmosphere during transparent vitrification, it is preferable to use a multi-furnace.

  The synthetic quartz glass obtained by the method of the present invention is used to provide optical characteristics such as refractive index homogeneity and low birefringence necessary for an optical member for use as a lens for an exposure apparatus and other optical members. Each heat treatment (hereinafter referred to as optical heat treatment) such as homogenization, molding, and annealing needs to be appropriately performed. In an atmosphere of an inert gas such as nitrogen gas or argon gas, a temperature of 500 to 1200 ° C. and a pressure of 101 kPa (atmospheric pressure) to 1 Pa are maintained for several tens to several hundred hours, and annealing is performed. Although distorted structures such as three-membered ring structures and four-membered ring structures can be reduced, the more fluorine content in synthetic quartz glass, the shorter the annealing treatment, the more distorted structure in synthetic quartz glass. Can be reduced. The optical heat treatment can be performed after transparent vitrification.

  In the present invention, the OH group content in the synthetic quartz glass affects the light transmittance at a wavelength of 180 nm or less, and the initial vacuum ultraviolet transmittance decreases as the OH group content increases. The content is preferably 1 ppm or less, and particularly preferably 0.1 ppm or less.

  In the present invention, fluorine in the synthetic quartz glass has the effect of substituting OH groups to reduce the OH group content, and also has the effect of reducing distorted structures such as three-membered ring structures and four-membered ring structures. Specifically, the synthetic quartz glass of the present invention preferably contains 100 ppm or more, particularly 300 ppm or more of fluorine.

  In the present invention, oxygen-deficient defects (≡Si—Si≡ (≡ represents Si—O bond; the same applies hereinafter), oxygen-rich defects (≡Si—O—O—Si≡), Since ≡SiH bonds, dissolved oxygen molecules, and the like adversely affect vacuum ultraviolet light transmittance and light resistance, it is preferable not to contain them substantially.

  In particular, the oxygen deficiency defect has an absorption band centered at a wavelength of 163 nm, and therefore it is particularly preferable that the oxygen deficiency defect is not substantially contained. Further, the oxygen excess type defect not only has a broad absorption band from a wavelength of 155 to 180 nm, but also generates NBOHC when irradiated with ultraviolet rays. NBOHC is a cause of red fluorescence during ultraviolet irradiation, and has absorption bands near 180 nm and 260 nm, so that light transmittance in a wide wavelength range of 155 to 300 nm is impaired. As described above, it is particularly preferable that oxygen-excess defects are not substantially contained.

In the present invention, a distorted structure such as a three-membered ring structure or a four-membered ring structure in the synthetic quartz glass tends to lower the light transmittance at a wavelength of 165 nm or less, and therefore it is preferable that the number is less. Scattering peak intensity _{I 2} of the specific scattering peak intensity of 495cm ^-1 in the laser Raman spectrum is _{I 1} and 606 cm ^-1 is, _I 1 / _{I 0} ≦ 0.59 respectively scattering peak intensity _{I 0} of 440 cm ^-1 , I ₂ / I ₀ ≦ 0.15 is preferable.

  In the present invention, chlorine in the synthetic quartz glass deteriorates the light transmittance and light resistance in the vacuum ultraviolet region, so that the content thereof is preferably small. Specifically, the chlorine content in the synthetic quartz glass is preferably 10 ppm or less, particularly preferably 5 ppm or less, and further not substantially contained.

  In the present invention, alkali metals (Na, K, Li, etc.), alkaline earth metals (Mg, Ca, etc.), transition metals (Fe, Ni, Cr, Cu, Mo, W, Al, Ti, etc.) in synthetic quartz glass are used. A metal impurity such as Ce) not only lowers the transmittance from the ultraviolet region to the vacuum ultraviolet region, but also causes a decrease in ultraviolet resistance, so that its content is preferably as small as possible. Specifically, the total content of metal impurities is preferably 100 ppb or less, particularly preferably 50 ppb or less.

  Furthermore, the synthetic quartz glass obtained by the method of the present invention may be effective if it contains hydrogen molecules in order to improve ultraviolet resistance. Specifically, the synthetic quartz glass is heat-treated in a hydrogen-containing atmosphere at a temperature of 600 ° C. or lower to diffuse and contain hydrogen molecules in the synthetic quartz glass.

Hydrogen molecules have a function of repairing paramagnetic defects such as E ′ center and NBOHC caused by ultraviolet irradiation and suppressing generation of absorption bands at wavelengths of 180 to 300 nm. When used as an optical member of an optical device using light having a wavelength of 180 to 250 nm as a light source, it is preferable to contain 1 × 10 ¹⁷ molecules / cm ³ or more of hydrogen molecules.

However, the hydrogen molecules in the synthetic quartz glass body have an action of promoting the generation of oxygen-deficient defects (≡Si—Si≡) during ultraviolet irradiation, and the defects have an absorber centered at a wavelength of 163 nm. When used as an optical member of an optical device using light of 155 to 180 nm as a light source, the hydrogen molecule content in the synthetic quartz glass is 1 × 10 ¹⁷ molecules / cm ³ or less, depending on the application and use conditions. It may be preferable.

[Examples 1 to 8]
The glass forming raw materials shown in Table 1, namely, silicon tetrachloride or hexamethyldisilazane (HMDS) are hydrolyzed in an oxyhydrogen flame, and the formed SiO ₂ fine particles are deposited on the substrate to have a diameter of 350 mm and a length of 600 mm. Porous silica glass (average bulk density = 0.5 g / cm ³ , bulk density distribution = 0.3 g / cm ³ ) was prepared. This porous quartz glass was installed in an electric furnace capable of controlling the atmosphere, and the steps (b) and (c) were carried out under the conditions shown in Table 1, to reduce the OH group content of the porous quartz glass and to dope fluorine Went. In carrying out the steps (b) and (c), the porous quartz glass is heated to a predetermined temperature described in Table 1 at a pressure of 150 Pa or less, and then a predetermined gas is introduced, The atmosphere.

  Subsequently, porous quartz glass was put into an electric furnace (metal furnace) composed of a tungsten rod heater and a tungsten reflector, and a transparent quartz glass body (diameter 180 mm, length 400 mm) was produced under the conditions shown in Table 1.

[Examples 9 to 11]
Silicon tetrachloride is hydrolyzed in an oxyhydrogen flame, and the formed SiO ₂ fine particles are deposited on a substrate to form porous quartz glass having a diameter of 350 mm and a length of 600 mm (average bulk density = 0.5 g / cm ³ , Bulk density distribution = 0.3 g / cm ³ ) was produced. This porous quartz glass was installed in an electric furnace capable of controlling the atmosphere, and the steps (b) and (c) were carried out under the conditions shown in Table 1, to reduce the OH group content of the porous quartz glass and to dope fluorine Went. In carrying out the steps (b) and (c), the porous quartz glass is heated to a predetermined temperature described in Table 1 at a pressure of 150 Pa or less, and then a predetermined gas is introduced to a predetermined atmosphere. It was. The porous quartz glass was put into an electric furnace (multi-furnace) composed of a disilicide molybdenum heater and an alumina heat insulating material, and a transparent quartz glass body (diameter 180 mm, length 400 mm) was produced under the conditions shown in Table 1.

[Examples 12 to 14]
Silicon tetrachloride is hydrolyzed in an oxyhydrogen flame, and the formed SiO ₂ fine particles are deposited on a substrate to form porous quartz glass having a diameter of 350 mm and a length of 600 mm (average bulk density = 0.5 g / cm ³ , Bulk density distribution = 0.3 g / cm ³ ) was produced. This porous quartz glass was installed in an electric furnace capable of controlling the atmosphere, and the steps (b) and (c) were carried out under the conditions shown in Table 1, to reduce the OH group content of the porous quartz glass and to dope fluorine Went. In carrying out the steps (b) and (c), the porous quartz glass is heated to a predetermined temperature described in Table 1 at a pressure of 150 Pa or less, and then a predetermined gas is introduced to a predetermined atmosphere. It was. The porous quartz glass was put into an electric furnace (graphite furnace) composed of a carbon heater and a carbon heat insulating material, and a transparent quartz glass body (diameter 180 mm, length 400 mm) was produced under the conditions shown in Table 1.

The transparent quartz glass body obtained in each example was set in a carbon crucible having an inner diameter of 240 mm, and the crucible was heated to 1750 ° C. in argon gas, 100 vol%, 1 atm in an electric furnace. For 10 hours to form a transparent quartz glass body.
The following evaluation was performed about the sample obtained in each case.

(OH group content evaluation)
A 20 mm × 20 mm × 30 mm thick sample was cut out from the vicinity of the center of the evaluation sample, and two 20 mm square surfaces were mirror-polished. Subsequently, the vicinity of the center of the sample for evaluation was measured with an infrared spectrophotometer, and the OH group content was determined from the absorption peak at a wavelength of 2.7 μm (JP Williams et.al., Ceramic Bulletin, 55 (5 ), 524, 1976). The detection limit by this method is 0.1 ppm.

(Fluorine content evaluation)
A sample having a weight of about 5 g was cut out from the vicinity of the center of the sample for evaluation, and the fluorine content was analyzed by the fluorine ion electrode method. The analysis method of fluorine content is as follows. According to the method described in Journal of the Chemical Society of Japan, 1972 (2), 350, synthetic quartz glass was heated and melted with anhydrous sodium carbonate, and distilled water and hydrochloric acid (1: 1 by volume) were added to the resulting melt. A sample solution was prepared. The electromotive force of the sample solution was used as a fluorine ion selective electrode and a reference electrode. 945-220 and no. Each of 945-468 was measured with a radiometer, and the fluorine content was determined based on a calibration curve prepared in advance using a fluorine ion standard solution. The detection limit by this method is 10 ppm.

(Evaluation of chlorine content)
A 20 mm × 20 mm × 10 mm thick sample was cut out from the vicinity of the center of the evaluation sample, and one surface of 20 mm square was mirror-polished. The mirror-polished surface was subjected to fluorescent X-ray analysis to determine the chlorine content in the synthetic quartz glass. The detection limit by this method is 10 ppm.

(Evaluation of internal light transmittance at a wavelength of 157.6 nm)
A 20 mm × 20 mm × 5 mm sample and a 20 mm × 20 mm × 30 mm sample were cut out from the center of the evaluation sample, and two surfaces of 20 mm square were each mirror-polished, and the temperature of the sample was kept at 25 ° C., vacuum ultraviolet spectroscopy The light transmittance at a wavelength of 157.6 nm was measured under a nitrogen atmosphere with a photometer (UV201M manufactured by Spectrometer Co., Ltd.). The internal light transmittance T _157.6 at a wavelength of 157.6 nm was determined according to the following formula (1) from the light transmittances T ₁ and T ₂ of two types of samples having a thickness of 5 mm and a thickness of 30 mm.

(Evaluation of presence of oxygen-deficient defects)
Using a sample prepared by internal light transmittance evaluation at a wavelength of 157.6 nm, the wavelength was measured by a vacuum ultraviolet spectrophotometer (“UV201M” manufactured by Spectrometer Co., Ltd., the same shall apply hereinafter) with the sample temperature maintained at 25 ° C. The light transmittance at ₁₆₃ nm was measured in a nitrogen atmosphere, the internal transmittance T ₁₆₃ at ₁₆₃ nm was calculated from the formula (2), and calculated from the OH group concentration C _OH (ppm) in the quartz glass by the formula (3). The presence or absence of oxygen-deficient defects was evaluated by comparison with the value _Tid .

  If there is an oxygen-deficient defect, there is an absorption band centered at 163 nm, which is lower than the value calculated from the above equation (3).

(Evaluation of presence of oxygen-rich defects)
A 20 mm square × 10 mm thick synthetic quartz glass sample is prepared from the vicinity of the center of the sample for evaluation, and the OH group content in the sample is measured with an infrared spectrophotometer. Next, the sample is kept at 100% hydrogen gas, 101 kPa, 1000 ° C. for 30 hours, cooled to room temperature, and then the OH group content in the sample is measured again by the same method. The amount of change in the OH group content in the sample before and after the heat treatment was calculated. If the amount of change was 1 ppm or less, it was determined that the sample did not contain oxygen-excess defects.

The results are shown in Table 2. Examples 1 to 8 are Examples, Examples 12 to 14 are Comparative Examples, and Examples 9 to 11 are Reference Examples . From Examples 1 to 11, it can be seen that the internal transmittance at a wavelength of 157.6 nm is high.

Claims (3)

In the method for producing synthetic quartz glass for an optical member of an optical device using light having a wavelength of 180 nm or less as a light source,
(A) a step of depositing and growing quartz glass fine particles obtained by flame hydrolysis of a glass forming raw material on a substrate to form porous quartz glass;
(B) a step of reducing the OH group content of the porous quartz glass;
(C) treating the porous quartz glass in a fluorine compound-containing atmosphere and doping the porous quartz glass with fluorine;
(D) The porous quartz glass is made into a transparent glass by raising the temperature to 1300 ° C. or higher in a metal furnace using a material containing tungsten or molybdenum as a constituent material of a heater and a reflector ,
And a step of obtaining a transparent quartz glass body substantially free of oxygen-deficient defects having a fluorine content of 300 ppm or more .   2. The method for producing a synthetic quartz glass optical member according to claim 1, wherein the glass is transparent vitrified under reduced pressure.   The method for producing synthetic quartz glass according to claim 1, wherein the glass is heated to a temperature of 1300 ° C. to 1500 ° C. to form a transparent glass.