Synthetic fused silica member

There is disclosed a synthetic fused silica member used at a wavelength of 200 nm or less which has a hydroxyl group-content of 1 to 50 ppm and a fluorine-content of 100 to 1000 ppm, and contains no chlorine, and has a birefringence of the synthetic fused silica member of 2 nm/cm or less. The synthetic fused silica member can be used as a synthetic fused silica substrate for photomask, or in an optical system wherein a fluorine excimer laser is used as a light source. There can be provided a synthetic fused silica member which can efficiently transmit a light having a wavelength as 200 nm or less, especially fluorine excimer laser (157 nm), and does not suffer from lowering of transmittance due to damage.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a synthetic fused silica member used, for
 example, in an exposure apparatus (optical aligner) for producing IC in
 which a light source providing light having a wavelength of 200 nm or less
 such as fluorine excimer laser or the like is used, especially a synthetic
 fused silica substrate for photomask.
 2. Description of the Related Art
 Along with a recent increase in the degree of integration of semiconductor
 device, there has been required for improving accuracy of micromachining
 in so-called photolithography technique. In order to make fine patterns,
 it is necessary to use a light source providing a light having shorter
 wavelength, and higher energy. Therefore, high transmittance and
 durability to light having short wavelength have been required in optical
 member to be used.
 Conventionally, there have been used synthetic fused silica or fluorite
 (CaF.sub.2) as a photomask, lens or the like in an exposure apparatus for
 producing semiconductor IC or the like in which a light source providing
 light having short wavelength, as in ultraviolet region or vacuum
 ultraviolet region, such as excimer laser or the like is used. Because,
 only synthetic fused silica and fluoride crystal as represented by
 fluorite can be practically used as the material having high transmittance
 to light having wavelength such as KrF (248 nm), ArF (193 nm) excimer
 laser.
 The characteristics of the synthetic fused silica are different depending
 on methods for producing it. For example, there are the synthetic fused
 silica having high hydroxyl group-content which is produced from
 oxyhydrogen flame by "a direct method" and those having low hydroxyl
 group-content produced by "a soot method" wherein silica sintered body is
 molten.
 As described above, along with increase in a degree of integration of
 semiconductor devices, a light source providing a light having a shorter
 wavelength, represented by KrF (248 nm) or ArF (193 nm) excimer laser has
 been used, and as a result, a photon energy of the light has been
 increased. When a synthetic fused silica is irradiated with a light having
 a short wavelength and high photon energy, structural defects which can be
 substantial damages may be caused in the glass, and the transmittance may
 be lowered. There is proposed the material in which significant lowering
 of transmittance is not caused, even when it is irradiated with a light
 having short wavelength and high photon energy (see, for example, in
 Japanese Patent KOKAI Hei 7-291635).
 However, recently, optical member for light in the vacuum ultraviolet
 region which has shorter wavelength as 200 nm or less, especially for
 fluorine excimer laser (157 nm), have been developed. If the optical
 member used for such a light source having short wavelength can be made of
 a synthetic fused silica, it would be quite advantageous, since the
 technology is an extension of the conventional optical exposure
 technology, and only slight change of processes, for example, change of
 light source is necessary. Namely, the conventional photolithography
 technology can be prolonged.
 However, the above-mentioned range of the wavelength is close to essential
 absorption edge of transmittance of the synthetic fused silica.
 Accordingly, if it is pure SiO.sub.2, it is in principle transparent in
 the range of the wavelength up to 125 nm (see Material Technology-Highly
 Functional Glass, published by Tokyo University publishing department).
 However, as described above, an actual synthetic fused silica contains
 hydroxyl groups, chlorine, metal impurity, or the like depending on the
 method for production, and absorption due to them has a subtle affect on
 the transmittance.
 On the other hand, fluorite may be used as other material than synthetic
 fused silica that is excellent in transmittance in the vacuum ultraviolet
 region. However, it has a coefficient of thermal expansion that is
 double-digits higher than that of synthetic fused silica. For example,
 when it is used as a substrate for a photomask, accuracy of dimension gets
 worse due to affect of thermal expansion, which may lead to blur of
 patterns. Furthermore, it is disadvantageous that water cannot be used in
 the steps for polishing a member made of fluorite to be transparent, since
 fluorite is deliquescent. Accordingly, it is difficult to be machined.
 SUMMARY OF THE INVENTION
 The present invention has been accomplished to solve the above-mentioned
 problems. An object of the present invention is to provide a synthetic
 fused silica member which can efficiently transmit a light having a
 wavelength of 200 nm or less, especially light from fluorine excimer laser
 (157 nm), and is not liable to sustain damage causing lowering of
 transmittance.
 To achieve the above mentioned object, the present invention provides a
 synthetic fused silica member used at a wavelength of 200 nm or less which
 has a hydroxyl group-content of 1 to 50 ppm and a fluorine-content of 100
 to 1000 ppm, and contains no chlorine.
 When the synthetic fused silica which has a hydroxyl group-content and a
 fluorine-content in the above range, and contains no chlorine is used as
 an optical member for light having a wavelength of 200 nm or less,
 especially for fluorine excimer laser (wavelength of 157 nm), it can
 transmit the light well, and it is durable for long hours of use.
 In that case, the synthetic fused silica member can be produced so that
 birefringence may be 2 nm/cm or less.
 Thereby, light can be uniformly transmitted, and therefore, blur of
 patterns is not caused on exposure, for example, in photolithography.
 The synthetic fused silica of the present invention is very useful when it
 is used as a synthetic fused silica substrate for photomask used at a
 wavelength of 200 nm or less. It can be used in an optical system wherein
 a fluorine excimer laser is used as a light source.
 Accordingly, the present invention is practically very useful from a
 practical standpoint, since it can prolong the conventional
 photolithography technique.
 According to the present invention, the synthetic fused silica member
 contains a desired amount of hydroxyl groups and a desired amount of
 fluorine, and no chlorine, and therefore it can efficiently transmit light
 having a wavelength of 200 nm or less, especially the fluorine excimer
 laser (157 nm), and is durable for long hours of use. Furthermore, since
 it may have birefringence of 2 nm/cm or less, it can transmit the light
 uniformly, and does not cause blur of pattern, for example, in
 photolithography.
 Accordingly, a technology such as photo lithography or the like using a
 fluorine excimer laser as a light source can be conducted without changing
 design used in conventional methods, namely prolongation of the technology
 can be achieved, and therefore, the present invention is significantly
 excellent from the practical standpoint.
 DESCRIPTION OF THE INVENTION AND EMBODIMENT
 The present invention will now be described more in detail. However, the
 invention is not limited thereto.
 The inventors of the present invention have made a lot of experiments, and
 found that if a synthetic fused silica member has a hydroxyl group-content
 of 1 to 50 ppm and a fluorine-content of 100 to 1000 ppm, and contains no
 chlorine, and preferably has birefringence of 2 nm/cm or less, there can
 be improved a transmittance and resistance to the light having a
 wavelength of 200 nm or less, especially a fluorine excimer laser, and
 therefore the member can be used as an optical member for the light, and
 the exposure technology can be prolonged. Then, the inventors have further
 studied the conditions to complete the present invention.
 The synthetic fused silica member of the present invention used for the
 light having a wavelength of 200 nm or less has a hydroxyl group-content
 of 1 to 50 ppm, preferably 1 to 30 ppm. If the hydroxyl group-content is
 more than 50 ppm, transmittance at 157 nm is 10% or less, and is not
 practically useful. It is preferable that the hydroxyl group-content is as
 low as possible, since transmittance in the wavelength region of 200 nm or
 less depends on the hydroxyl group-content in the glass. However, if
 .ident.Si--Si.ident. bonding is formed in glass structure while hydroxyl
 group content is decreased, there may be caused absorption due to the
 bonding near the wavelength region of 160 nm, which may lead to
 significant lowering of transmittance of a fluorine excimer laser (157
 nm). Accordingly, the hydroxyl group content is defined not to be lower
 than 1 ppm. 1 to 30 ppm of the hydroxyl group content can ensure a desired
 transmittance.
 The synthetic fused silica member according to the present invention used
 at a wavelength of 200 nm or less contains 100 to 1000 ppm, preferably 300
 to 800 ppm, more preferably 400 to 600 ppm of fluorine. When the
 fluorine-content is less than 100 ppm, light resistance will be degraded,
 which may cause lowering of transmittance during radiation by fluorine
 excimer laser (157 nm). When the fluorine-content is more than 1000 ppm,
 .ident.Si--Si.ident. bonding is liable to be formed in glass structure so
 that the transmittance of fluorine excimer laser may be less than 10%. 300
 to 800 ppm, preferably 400 to 600 ppm of the fluorine-content can ensure
 desired transmittance and light resistance.
 Preferably, the synthetic fused silica member of the present invention has
 birefringence of 2 nm/cm or less. Although it depends on a thickness of
 the synthetic fused silica member, when the birefringence is more than 2
 nm/cm, for example, uniform exposure cannot be conducted, so that blur of
 patterns may be caused. When the birefringence is 2 nm/cm or less, such
 blur of pattern is not caused.
 The synthetic fused silica member of the present invention is also
 characterized in that it contains no chlorine. If chlorine is contained in
 the synthetic fused silica, there is caused absorption around 160 nm due
 to Si --Cl bonding, which lowers transmittance of fluorine excimer laser
 (157 nm). Accordingly, chlorine-content should be as low as possible. For
 example, it is preferably not more than the lower limit of the detectable
 value in radioactivity analysis (0.1 ppm).
 The above-described synthetic fused silica member that has a desired
 hydroxyl group-content and a desired fluorine-content, and contains no
 chlorine can be produced, for example, as follows.
 A mixture of a silane compound containing no chlorine and a fluoro compound
 containing no chlorine is introduced in oxyhydrogen flame to form a porous
 silica sintered body. Then, the porous silica sintered body was heated and
 molten in vacuum or in an atmosphere of an inert gas to form transparent
 glass, and thereby the synthetic fused silica can be obtained, which is
 then mechanically machined to be in a desired form to provide a synthetic
 fused silica member.
 The hydroxyl group-content and the fluorine-content can be controlled by
 controlling a mixing ratio of the silane compound and the fluoro compound
 to be supplied, a bulk density of the porous silica sintered body, or the
 like. The synthetic fused silica thus produced contains no chlorine, since
 the silane compound and the fluoro compound that contain no chlorine are
 used as a raw material.
 The silane compound that contains no chlorine used in the present invention
 can be a compound represented by the following formula (1):
EQU R.sup.1.sub.n Si(OR.sup.2).sub.4-n (1)
 wherein R.sup.1 and R.sup.2 are methyl, ethyl, propyl, butyl, and n is an
 integer of 0 to 4.
 Examples of the silane compounds include: tetramethoxy silane, tetraethoxy
 silane, methyl trimethoxy silane or the like.
 The fluoro compound that contains no chlorine used in the present invention
 can be a compound represented by the following formula (2):
EQU C.sub.p H.sub.q F.sub.r (2).
 Wherein p, q, r are integers, provided that they satisfy the formulae:
 1.ltoreq.p.ltoreq.3, 0.ltoreq.q.ltoreq.7, 1.ltoreq.r.ltoreq.8.
 Examples of the fluoro compounds include: CF.sub.4, CHF.sub.3, C.sub.2
 F.sub.6, C.sub.3 F.sub.8, or the like. SiF.sub.4 or SF.sub.6 can also be
 used as the fluoro compound.
 The silane compound containing no chlorine and the fluoro compound
 containing no chlorine mentioned above are previously mixed, is introduced
 in oxyhydrogen flame, and subjected to flame hydrolysis or oxidative
 destruction to yield silica fine particles containing fluorine, which are
 then deposited on the heat resistant carrier to form a porous silica
 sintered body.
 A mixing ratio of the silane compound and the fluoro compound (as a ratio
 of fluoro atoms/(silica atoms and fluoro atoms)) is 5 to 70 atomic %,
 preferably 5 to 60 atomic %, especially 10 to 50 atomic %.
 When the ratio of fluoro atoms/(silica atoms and fluoro atoms) is less than
 5 atomic %, fluorine-content in the glass is liable to be 100 ppm or less.
 When the ratio of fluoro atoms/(silica atoms and fluoro atoms) is more
 than 70 atomic %, fluorine-content in the glass may be more than 1000 ppm.
 In that case, hydroxyl group-content will be significantly lowered due to
 dehydration effect of fluorine, and as a result, .ident.Si--Si.ident.
 bonding is caused in the glass structure, and absorption at 160 nm is
 caused, resulting in lowering of transmittance.
 The bulk density of the porous silica sintered body is 0.3 to 0.6
 g/cm.sup.3, preferably 0.4 to 0.5 g/cm.sup.3. If it is out of the above
 range, hydroxyl groups cannot be completely removed during vitrification,
 and will be more than 50 ppm in some cases.
 The synthetic fused silica substrate for a photomask can be made by the
 following method using the synthetic fused silica thus obtained that
 contains the desired amount of hydroxyl groups, the desired amount of
 fluorine and no chlorine.
 First, the synthetic fused silica is placed in a mold made of
 heat-resistant material such as carbon or the like, and heated and molten
 in vacuum or in an atmosphere of an inert gas at a temperature of
 1700.degree. C. to 1800.degree. C., and then formed, for example, in a
 size of 5 to 6 inches cube, or larger by hot forming. The glass ingot thus
 formed is subjected to anneal treatment at 1000.degree. C. to 1200.degree.
 C. for one hour or more, and cooled to 900.degree. C. to 1000.degree. C.
 slowly, for example, at a rate of 15.degree. C./hour or less to remove
 heat distortion in the glass. By such an anneal treatment, distortion
 remaining in the glass can be removed, so that a birefringence may be 2
 nm/cm or less. Finally, a slicing step and a polishing step are conducted
 to provide the synthetic fused silica substrate for a photomask.
 The synthetic fused silica substrate for a photomask contains 1 to 50 ppm
 of hydroxyl groups, 100 to 1000 ppm of fluorine and no chlorine, and has a
 birefringence of 2 nm/cm or less. Accordingly, it is a quite useful
 synthetic fused silica substrate, as it can efficiently transmit light
 having a wavelength of 200 nm or less, especially fluorine excimer laser
 (157 nm), is durable for long hours of use, can transmit light uniformly,
 and does not cause blur of pattern on exposure.

EXAMPLE
 The following examples and comparative examples are being submitted to
 further explain the present invention. These examples are not intended to
 limit the scope of the present invention.
 Example, Comparative Example
 Furon 23 (trade name, manufactured by Showa Denko Co. Ltd.) as a fluoro
 compound was mixed with 1000 g/hr of tetramethoxy silane
 (Si(OCH.sub.3).sub.4) as a raw material silane compound with varying a
 mixing ratio (5 to 70 atomic %), and then was introduced into an
 oxyhydrogen flame formed by introducing a hydrogen gas at 5 Nm.sup.3 /hr
 and an oxygen gas at 6 Nm.sup.3 /hr into a quartz multiple pipe burner, to
 form silica fine particles containing fluorine. The particles were
 deposited on a quartz carrier by spraying it on the carrier rotating at 20
 rpm, and then the deposited quartz carrier was withdrawn at a constant
 rate to provide a porous silica sintered body having a diameter of 300 mm
 and length of 500 mm. The porous silica sintered body had a bulk density
 of 0.3 to 0.6 g/cm.sup.3.
 Then, the porous silica sintered body was placed in a vacuum furnace, and
 heated in He atmosphere and under atmospheric pressure, with increasing a
 temperature from room temperature to 1250.degree. C. at a temperature
 increase rate of 10.degree. C./hour and maintaining at the temperature for
 10 hours. Subsequently, the temperature was increased to 1500.degree. C.
 at a temperature increase rate of 3.degree. C./min to form a transparent
 glass. Thus, there was produced a transparent fused silica ingot having a
 diameter of 150 mm, a length of 300 mm and a weight of 11.6 kg.
 The resulting ingot was placed in a carbon prism mold having an inner size
 of 6.7 inches in a vacuum furnace, and heated in an inert gas atmosphere
 with increase a temperature from a room temperature to 1700-1800.degree.
 C. over about two hours, and maintaining the temperature for about one
 hour, and was cooled to a room temperature by turning the electric power
 off. The resulting ingot had a size of 170 mm square and a thickness of
 180 mm.
 It was then heated in an atmospheric furnace with increasing a temperature
 from a room temperature to 1100.degree. C. over 3 hours, and maintaining
 the temperature for 10 hours in order to remove the remaining heat
 distortion. Then, it was cooled with decreasing the temperature to
 950.degree. C. at a temperature decreasing rate of 1.degree. C./hour, and
 allowed to be cool in an atmosphere by turning the electric power off.
 It was then subjected to surface grinding in order to make a right angle at
 the edge of the outer surface. The birefringence of the resulting
 synthetic fused silica block was measured with birefringence evaluation
 system (manufactured by OAK SEISAKUSYO, type ADR200B), and found to be
 about 1 nm/cm. It was confirmed that distortion was removed by annealing
 treatment.
 Then, the synthetic fused silica block was sliced to provide a substrate
 having a thickness of 6.5 mm, the surface of it was then polished to be
 transparent. The synthetic fused silica substrate having a dimension of
 152 mm square and a thickness of 6.4 mm was thereby obtained.
 A hydroxyl group-content of the resulting synthetic fused silica substrate
 for a photomask was obtained from an absorption peak at a wavelength of
 2.7 micron. On the other hand, a fluorine-content thereof was determined
 by cutting the substrate, powdering it, and irradiating it with thermal
 neutron for 2 minutes by radioactivity analysis (a nuclear reactor:
 TRIGA-11 type), and quantification was carried out with a Ge detector.
 The transmittance of the fluorine excimer laser (157 nm) was measured with
 vacuum ultraviolet spectrometer (manufactured by Nihon Bunko Co. Ltd.,
 VUV-200 type).
 The results were shown in Table 1.

hydroxyl transmittance
 group- Fluorine- of fluorine
 content Content excimer laser
 (ppm) (ppm) (%)
 Example 1 30 400 40
 Example 2 10 800 80
 Comparative not 2000 0
 Example 1 detected
 Comparative 300 800 0.1
 Example 2
 Table 1 shows that the substrate of Example of which hydroxyl group-content
 and fluorine-content were in the range defined according to the present
 invention had practically useful transmittance of fluorine excimer laser.
 On the other hand, the substrate of Comparative Example of which hydroxyl
 group-content and the fluorine-content were not in the range defined
 according to the present invention hardly transmitted the fluorine excimer
 laser. The chlorine-content as measured with radioactivity analysis was
 less than the lower limit of the detectable value in either of the
 substrates.
 The present invention is not limited to the above-described embodiment. The
 above-described embodiment is a mere example, and those having the
 substantially same structure as that described in the appended claims and
 providing the similar action and effects are included in the scope of the
 present invention.
 In the embodiment explained above, the synthetic fused silica containing no
 chlorine was produced using a silane compound containing no chlorine and a
 fuloro compound containing no chlorine. The present invention is not
 limited thereto. For example, the synthetic fused silica can be produced
 using a silane compound and a fluoro compound containing chlorine such as
 tetrachlorosilane, and then chlorine can be removed by chlorine
 dehydration treatment, to provide a desired synthetic fused silica member.