JP4531904B2 - Optical material for ultraviolet ray and method for producing the same - Google Patents

Description

【０１６２】
表中の初期透過率及び照射後低下透過率は以下の式から計算した。
【０１６３】
初期透過率（％）＝（Ｔ₁／Ｔ₀）×１００
Ｔ₁＝サンプル透過パルス強度(Ｆ₂レーザ照射開始時)
Ｔ₀＝ブランクパルス強度
照射後低下透過率ΔＴ＝Ｔ_A−Ｔ_B
Ｔ_A＝（Ｔ₁／Ｔ₀）×１００
Ｔ_B＝（Ｔ₂／Ｔ₀）×１００
Ｔ₁＝サンプル透過パルス強度(Ｆ₂レーザ照射開始時)
Ｔ₀＝ブランクパルス強度
Ｔ₂＝サンプル透過パルス強度(Ｆ₂レーザ１０ｍＪ／ｃｍ²，
１×１０⁶pulses照射後)
実施例９
実施例６で得られたガラスからさらに１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＭ１、Ｍ２とした。
【０１６４】
サンプルＭ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、
照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約５５％に低下したが、そのまま照射を続けても、それ以上の透過率低下は見られなかった。
【０１６５】
サンプルＭ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下した。サンプルＭ２は、照射数１０⁶ショットで透過率が初期透過率の約８３％に低下したが、その後１０⁷ショットまで照射を続けたところ、それ以上の透過率低下は見られなかった。
【０１６６】
実施例１０
実施例７で得られたガラスからさらに１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＮ１、Ｎ２とした。
【０１６７】
サンプルＮ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約６０％に低下したが、そのまま照射を続けても、それ以上の透過率低下は見られなかった。
【０１６８】
サンプルＮ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下した。サンプルＮ２は、照射数１０⁶ショットで透過率が初期透過率の約８５％に低下したが、その後１０⁷ショットまで照射を続けたところ、それ以上の透過率低下は見られなかった。
【０１６９】
実施例１１
実施例８で得られたガラスからさらに１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＯ１、Ｏ２とした。
【０１７０】
サンプルＯ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約５０％に低下したが、そのまま照射を続けても、それ以上の透過率低下は見られなかった。
【０１７１】
サンプルＯ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下した。サンプルＯ２は、照射数１０⁶ショットで透過率が初期透過率の約８０％に低下したが、その後１０⁷ショットまで照射を続けたところ、それ以上の透過率低下は見られなかった。
【０１７２】
比較例５
比較例３で得られたガラスからさらに１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＰ１、Ｐ２とした。
【０１７３】
サンプルＳ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約４５％に低下した。そのまま照射を続けると、透過率は徐々に低下し、一定となることはなかった。
【０１７４】
サンプルＰ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下し、照射数１０⁶ショットで透過率が初期透過率の約７９％に低下し、その後１０⁷ショットまで照射を続けたところ、透過率は徐々に低下し、安定することはなかった。
【０１７５】
比較例６
比較例４で得られたガラスからさらに１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＱ１、Ｑ２とした。
【０１７６】
サンプルＳ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約４５％に低下した。そのまま照射を続けると、透過率は徐々に低下し、一定となることはなかった。
【０１７７】
サンプルＱ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下し、照射数１０⁶ショットで透過率が初期透過率の約７９％に低下し、その後１０⁷ショットまで照射を続けたところ、透過率は徐々に低下し、安定することはなかった。
【０１７８】
比較例7
四塩化ケイ素を原料とし、これを酸水素火炎中で加水分解させ、１６００℃以上の温度で直接堆積ガラス化させる直接法により、合成シリカガラスを得た。得られたガラスの大きさは、直径２００ｍｍ長さ１２００ｍｍであった。ガラスに含まれるＯＨ基量を赤外吸収スペクトルの２．７３μｍにおける吸収強度から求めたところ、７００ｐｐｍであった。また、得られたガラスの一部を３８％フッ酸中に溶解して塩素の定量分析をおこなったところ、ガラス中の塩素濃度は検出限界以下の１５０ｐｐｍであった。さらにＩＣＰ質量分析計を用いてガラス中に含まれる不純物の定量分析をおこなったところ、アルカリおよびアルカリ土類金属、遷移金属、その他の金属不純物濃度は、全ての元素について１０ｐｐｂ未満であり、それらの合計量は５０ｐｐｂ以下であった。
【０１７９】
得られたガラスから１０×１０×３０ｍｍの試験片を２個切り出し、２面を光学研磨し、それぞれサンプルＲ１、Ｒ２とした。
【０１８０】
サンプルＲ１に波長１７２ｎｍの誘電体バリア放電エキシマランプの光を真空中で照射したところ、照射時間の増加とともにランプ光の透過率が低下した。透過率は照射開始後約１００時間で初期透過率の約７５％に低下した。そのまま照射を続けると、透過率は徐々に低下し、一定となることはなかった。
【０１８１】
サンプルＲ２に波長１９３ｎｍのＡｒＦエキシマレーザーを、エネルギー密度１００ｍＪ／ｃｍ²・パルス、繰り返し周波数１００Ｈｚで照射した。レーザー光の透過率は、照射時間の増加とともに低下し、照射数１０⁶ショットで透過率が初期透過率の約９２％に低下し、その後１０⁷ショットまで照射を続けたところ、透過率は徐々に低下し、安定することはなかった。
【０１８２】
図１にサンプルＭ〜Ｒにおけるエキシマランプの照射時間による透過率の変化、図２にサンプルＭ〜ＲにおけるＡｒＦエキシマレーザーショット数による透過率の変化を示す。
【０１８３】
以上の実施例９〜１１及び比較例８〜１０の図１〜２より明らかな様に、ガラス中のＯＨ基濃度を１〜７０ｐｐｍとすることによって、エキシマランプやエキシマレーザーなどの紫外線を一定量照射した後は、紫外線照射による透過率の低下は起こらず、経時変化の無い安定した透過率を示すシリカガラスを得ることができる。
【０１８４】
【発明の効果】
以上説明したごとく本発明によれば、スート法で合成したスートを透明化する前にＮ₂あるいは還元性ガス雰囲気下でスート中の会合したＯＨ基同士のみの縮合が生じるような条件で充分時間をかけて熱処理することにより、仮想温度分布が小さく、不安定な前駆体がの除去され、スート内部での欠陥生成も抑制された、ガラス中のＯＨ基濃度が１〜７０ｐｐｍ、Ｃｌ濃度が１ｐｐｍ未満であり、アルカリおよびアルカリ土類金属、遷移金属、その他の金属不純物濃度は各元素が１０ｐｐｂ未満、それらの合計量が５０ｐｐｂ以下であり、波長１６０〜３００ｎｍの紫外線を照射し続けても１７２〜２００ｎｍにおける透過率が４０％以上、波長１５７ｎｍという短波長の光に対して６０％以上という高い透過率を示す優れた特性の合成シリカガラスからなる紫外線用光学材料を提供することができる。
【０１８５】
また、熱処理による構造変化が少ないため、屈折率の均質性が良い材料が容易に得られるばかりでなく、コンパクションも小さいため、Ｆ₂レーザーを用いたリソグラフィー用光学素材として適している。このほか、ＯＨ基含有量が低いため、放射線耐性が良く、放射能を持った原子炉内を観察するためのイメージファィバーや放射線暴露下での通信用光ファイバーとしても利用が可能である。また、高温での粘度が高いために半導体製造用の超高純度素材としても利用が可能である。
【０１８６】
さらに、熱サイクルによる寸法変化が少ないことから、ポリシリコンＴＦＴ用の基板材料としても使用可能である。また、ガス放出量が極めて少なく、劣化も少ないために各種ランプ用管球材料としても利用できる。
【図面の簡単な説明】
【図１】サンプルＭ〜Ｒにおけるエキシマランプの照射時間による透過率の変化を示す図である。
【図２】サンプルＭ〜ＲにおけるＡｒＦエキシマレーザーショット数による透過率の変化を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical material for ultraviolet rays comprising a transparent silica glass used for lithography, photo-CVD, optical cleaning, etc. using a light source in a wavelength region of approximately 160 to 300 nm, and a method for producing the same. More specifically, excimer lasers in the vacuum ultraviolet to ultraviolet region, lenses with high transmittance for excimer lamp light, photomask substrates for VLSI, prisms, window materials, optical materials for ultraviolet rays made of transparent silica glass such as lamp materials, In particular, F with a wavelength of 157 nm₂The present invention relates to an optical material for ultraviolet rays made of transparent silica glass for excimer laser.
[0002]
[Prior art]
Each silica glass currently used has its merits and demerits, and is used according to the purpose. For example, KrF having a wavelength of 248 nm or an optical material for an ArF excimer laser having a wavelength of 193 nm has a sufficiently high OH group content or a high-purity synthetic silica glass subjected to hydrogen treatment ( For example, JP-A-4-97922). This is intended to improve the laser resistance by increasing the amount of SiOH to make the glass network structure flexible. That is, in order to prevent the generation of defects due to the formation of precursors of defect structures and the breakage of distorted Si-O-Si bonds, it is necessary to reduce the number of distorted bonds themselves. By increasing the amount of SiO₂The distortion of the skeleton structure is reduced and the generation of defects is prevented. Conventionally, glass for an ArF excimer laser was manufactured by synthesizing so as not to form such an unstable molecule and used. However, it is known that the transmission property of vacuum ultraviolet light is better when the amount of OH groups in the silica glass is smaller, and when the wavelength used is in the vacuum ultraviolet region (hereinafter referred to as VUV) having a shorter wavelength, Silica glass containing a large amount of OH groups has an Xe wavelength of 172 nm because the absorption by UV hinders the transmission of the ultraviolet light._{2 *}Excimer lamp or F of 157nm wavelength₂Not suitable as an optical material for excimer laser.
[0003]
On the other hand, in addition to the method of increasing the amount of OH groups, a method of dissolving hydrogen by hydrogen treatment has been considered preferable for preventing the skeletal structure from being broken.
[0004]
However, H₂Not only does it have a problem with durability after release, but it reacts with the skeleton structure to produce a ≡Si—HHO—Si≡ structure, so that the durability is remarkably deteriorated when used for a long time. When used as a lamp tube material, H in the tube₂, H₂As O is released, a defect structure is generated, and thus problems such as a decrease in lamp light intensity accompanying a decrease in light transmittance have been raised.
[0005]
In addition to these, a method of doping fluorine as an optical material for VUV has been proposed. Doping with a sufficiently large amount of fluorine not only improves the UV resistance in the same way as a material containing a large amount of OH groups, but also does not shift the vacuum UV transmittance absorption edge to the long wavelength side due to the OH groups, so transmission of vacuum ultraviolet light A material having excellent characteristics can be obtained. In order to impart sufficient excimer laser resistance to glass by this method, it is necessary to dope several hundred ppm or more of fluorine, but since fluorine functions to lower the refractive index, striae and refractive index distribution Will occur.
[0006]
Optical non-uniformity such as striae cannot be made uniform by methods such as heat treatment and melt extension, and fluorine-doped silica glass has absorption bands of 7.6 eV and 5.0 eV accompanying heat treatment. Will be generated. For example, when used as an optical material for an excimer laser, a part of fluorine is liberated as the laser is irradiated, and F is highly etched from the surface.₂Is discharged as a negative effect on the device. For these reasons, F-doped silica glass has a problem as an optical material for vacuum ultraviolet, such as a stepper projection system lens that requires a large diameter and high homogeneity.
[0007]
Therefore, if the OH group content is appropriately reduced, the transmittance in the vacuum ultraviolet region is improved, but conversely, the decrease in transmittance (change with time) due to ultraviolet irradiation becomes significant. Thus, when the transmittance decreases with time, not only the absolute value of the transmittance but also the stability problem arises. For example, when used as a window material or condensing lens for optical CVD or ultraviolet dry cleaning using an excimer lamp of 172 nm, defects are gradually generated by continuing to irradiate ultraviolet rays, particularly VUV light having a wavelength of less than 200 nm. However, it was usually accompanied by a decrease in the ultraviolet transmittance in proportion to the irradiation time.
[0008]
From the above, the content of OH groups is small, there is no defect that absorbs ultraviolet rays, and there is no precursor structure of the defect, and no defect or precursor structure is generated even if heat treatment or VUV light is continuously irradiated. Although it can be considered that synthetic silica glass exhibits excellent ultraviolet transmittance, an optical material for ultraviolet rays made of such synthetic silica glass has not yet been put into practical use.
[0009]
[Problems to be solved by the invention]
The object of the present invention is to transmit the transmittance in the ultraviolet region, particularly F₂It is an optically stable material with good transmittance for the excimer laser oscillation wavelength of 157 nm, and it does not change compaction or light transmittance, and absorbs light even when heat treatment at high temperature or irradiation with VUV light continues. It is an object of the present invention to provide an optical material for ultraviolet rays made of a synthetic silica glass which does not cause the occurrence of deterioration or homogeneity.
[0010]
[Means for Solving the Problems]
As a result of various studies for obtaining a stable ultraviolet optical material, the inventors of the present application have made porous silica produced by depositing soot obtained by hydrolyzing silicon tetrachloride in an oxyhydrogen flame. In a manufacturing method (hereinafter abbreviated as soot method) of synthetic silica glass that is heat-treated at a relatively low temperature of 1100 to 1450 ° C. and then vitrified at a higher temperature, a porous silica (hereinafter referred to as a soot method) The soot (which is abbreviated as soot) is densified over a sufficient amount of time before being transparent, and then further slowly transparentized, so that the fictive temperature (hereinafter referred to as TF) distribution in the glass (hereinafter referred to as ΔTF) is obtained. As a result, the present inventors have found that a stable optical material for ultraviolet rays having a high transmittance at a wavelength of 157 nm can be obtained. By this method, ΔTF ≦ 50 ° C., OH group content is 30 to 40 ppm, Cl concentration is less than 1 ppm, alkali and alkaline earth metals, transition metals, and other metal impurity concentrations are less than 10 ppb for each element. The optical material for ultraviolet rays which consists of synthetic silica glass whose total amount is 50 ppb or less and whose transmittance | permeability of the ultraviolet-ray with a wavelength of 157 nm is 10 mm or more in optical path length 10mm is obtained.
[0011]
By adjusting the conditions of the heat treatment among the above production methods, a synthetic silica glass having an ultraviolet light transmittance of 157 nm and an optical path length of 10 mm and 60% or more can be obtained.
[0012]
As an optical material for ultraviolet rays, it is necessary that absorption does not occur due to irradiation for a long time. It is known that the ≡Si structure called E 'center caused by ultraviolet irradiation has an absorption band having a peak at 215 nm. As other causes of the absorption band, oxygen vacancies ≡Si..., Si≡ having a peak at a wavelength of 245 nm and hydrogen-related E ′ centers (Eβ ′ centers) having a peak at 230 nm are known. When these defect structure precursors are present in a material for producing an optical material, an absorption band is generated by ultraviolet irradiation.
[0013]
In addition to this, as a cause of the generation of absorption bands, there is also defect generation due to the fracture of strained Si—O—Si bonds. For example, when a distorted ≡Si—O—Si≡ structure exists,
≡Si-O-Si≡ → ≡Si ... Si≡ (1)
As a result, a defect is generated. At this time, ≡Si—O. Produced together with the E ′ center is called a non-bridging oxygen vacancy defect (NBOHC) and causes an absorption band of 625 nm and 260 nm.
[0014]
As another cause of the generation of the absorption band, the present inventors have proposed ≡Si—H H—O—Si≡. This precursor is considered to be produced by a reaction between a distorted Si—O—Si bond and hydrogen gas during glass synthesis.
[0015]
When this is irradiated with ultraviolet rays,
≡Si—H H—O—Si≡ → ≡Si · H—O—Si≡ + H (2)
Produces an E 'center.
[0016]
In order to prevent the formation of these precursors and the generation of defects due to the rupture of distorted Si—O—Si bonds, the distorted bonds themselves may be reduced. As one means for that purpose, by increasing the amount of Si-OH, SiO 2₂It was shown that the generation of defects can be prevented by reducing the distortion of the skeletal structure.
[0017]
However, OH groups have absorption in the vacuum ultraviolet region, and introduction of a large amount of OH groups to reduce strain simultaneously leads to a decrease in transmittance with respect to vacuum ultraviolet rays. Therefore, the inventors of the present application have a light transmittance for ultraviolet rays, particularly F.₂What kind of synthetic silica glass should be used as an optical material for lamps with good excimer laser transmittance at a wavelength of 157 nm, little deterioration, and little structural change due to heat treatment at high temperatures? When the synthetic silica glass obtained by various production methods was examined, the following was found.
[0018]
(1) A method of producing a synthetic silica glass by carrying out hydrolysis of silicon tetrachloride in an oxyhydrogen flame at a relatively low temperature to synthesize a porous body composed of silica fine particles, and then forming a sintered glass at a higher temperature. The vacuum ultraviolet transmission characteristics of the synthetic silica glass (OH group = 80 ppm) obtained by (hereinafter abbreviated as the soot method) is directly deposited into glass by oxyhydrogen flame hydrolysis of silicon tetrachloride to produce a synthetic silica glass. This is superior to that of synthetic silica glass (OH group = 1200 ppm) produced by the method (hereinafter abbreviated as direct method).
[0019]
(2) Even if it is a synthetic silica glass obtained by the soot method, the vacuum ultraviolet transmission property of the glass (OH group ≦ 1 ppm) obtained by dehydration treatment with chlorine (including chlorine-containing compound gas) is directly It is inferior to that of the synthetic silica glass (OH group = 1200 ppm) obtained by the method.
[0020]
(3) Synthetic silica glass obtained by the soot method, which has not been subjected to chlorination and dehydrated with a reducing gas (OH group ≦ 10 ppm). It is inferior to that of synthetic silica glass by the method (OH group = 80 ppm).
[0021]
From these facts, the following can be considered. First, in the synthetic silica glass, the OH group content is greatly different between those obtained by the soot method and those obtained by the direct method. OH groups are known to have absorption in the vacuum ultraviolet region, and the difference in the amount of OH groups is considered to be the difference in transmission characteristics in the vacuum ultraviolet region.
[0022]
Next, the difference between the synthetic silica glasses obtained by the same soot method is considered as follows. The silica porous matrix obtained by the soot method has a moisture of the order of%, but the glass obtained by sintering this has a moisture (OH group) of the order of ppm. This is because the OH groups present on the surface of the glass fine particles are associated with each other as the sintering proceeds.₂This is thought to be due to dissociation as O. When a gas having a dehydrating effect is present in the atmosphere when this sintering proceeds, the OH group concentration can be reduced within a short time because any OH group on the surface is replaced or taken away. it can.
[0023]
However, it is considered that at this time, defects or precursors thereof are generated in the glass structure, and at the same time, Si—O—Si bonds forming the glass skeleton are also distorted. When distortion occurs in the bond, it is considered that defects are generated as described above, and as a result, the transmission characteristics of VUV light are impaired. This distortion of coupling can be considered by TF and ΔTF.
[0024]
Here, ΔTF indicates the difference between the highest and the lowest TF in the glass. TF is an index indicating the glass structure, that is, the angle of Si—O—Si bond, and varies greatly depending on the thermal history. The difference in TF indicates that the structure of SiO2 constituting the glass is different, that is, the bond angle of a regular tetrahedron made of SiO2 is different. A large TF distribution indicates that the bond angle distribution is also large, which means that the bond distortion is large. That is, when ΔTF is large, there are many bonds with smaller stability, and bonds with lower stability have a stronger Si—O—Si bond strain and are likely to be precursors of defects. Therefore, by reducing ΔTF, bonding distortion is relaxed, defect precursors are less likely to be generated, and VUV light transmission characteristics are improved.
[0025]
In order to reduce ΔTF, it is necessary to heat the porous silica material for a sufficiently long time and to allow the sintering to proceed slowly. The silica glass having a small ΔTF thus obtained has very little bond distortion, does not undergo abrupt OH group substitution or deprivation, and dehydration proceeds only by condensation of associated OH groups. By taking a sufficient amount of time, the end result is a glass with very few OH groups, and such a glass is a stable glass with excellent VUV transmission properties, few defects and its precursors.
[0026]
Elements that inhibit VUV transmission characteristics other than the amount of OH groups and the amount of bond strain (= ΔTF) and their stability include Cl and metal impurities.
[0027]
When Cl is contained, SiCl becomes a precursor, E ′ center is generated, and since SiCl has absorption in the wavelength region of vacuum ultraviolet rays, it is preferable that the material does not substantially contain Cl and is preferably less than 1 ppm. .
[0028]
Metal impurities are also a cause of direct absorption band generation, and materials that are substantially not included are preferable in order to promote generation of defect structures such as E ′ centers. Specifically, the total sum of metal impurities is preferably 50 ppb or less. The concentration of each element is preferably less than 10 ppb.
[0029]
On the other hand, as a stable optical material, a material having a high absolute value of transmittance and a small change with time is desirable. However, as a practical problem, there is a case where a certain decrease in transmittance is unavoidable. Even in this case, if the transmittance does not change with time, it can be used as a stable optical material. For example, in the case where the irradiation time is long, even if a decrease in transmittance is observed at the initial stage of irradiation, if the optical material is such that the decrease in transmittance stops as light irradiation continues and becomes stable, an optical component in advance Before processing, it may be used after being sufficiently irradiated with ultraviolet rays.
[0030]
In the soot method, a precursor of defects is generated when the surface of the soot is fused during vitrification. However, since this precursor is localized on the surface of the soot, the number of defects generated therefrom is limited.
[0031]
Therefore, glass obtained by the soot method generates defects by irradiation with ultraviolet rays, but since the amount of generation is limited, new defects can be obtained by irradiating with ultraviolet rays for a time sufficient to saturate the amount of defects. Will not be generated. Such glass has a constant ultraviolet transmittance after ultraviolet irradiation (no change over time). The wavelength of the vacuum ultraviolet rays at this time is preferably 165 to 300 nm, and more preferably 170 to 250 nm. With ultraviolet light having a longer wavelength than this, the processing time for stabilization exceeds a realistic scale. On the other hand, a new defect structure is likely to be generated with ultraviolet light having a shorter wavelength. Although it is most preferable to directly irradiate ultraviolet rays having a wavelength to be used, it is preferable to select an appropriate method in consideration of processing throughput and the cost of the light source.
[0032]
Specific light sources for stabilizing the transmittance include excimer lasers such as KrF and ArF, and Xe._{2 *}Excimer lamps and mercury lamps are exemplified.
[0033]
The present invention will be specifically described.
[0034]
The glass-forming compound may be a high-purity silicon compound that can be gasified.
Using these raw materials, glass is produced by a method comprising the following steps.
[0035]
(1) A step of forming a porous silica base material by subjecting a glass-forming raw material to flame hydrolysis, and depositing vitrified silica fine particles on a starting member.
(2) A step of holding the porous silica base material for a certain period of time in a temperature range below the transparent vitrification temperature in an appropriate gas atmosphere.
(3) A step of obtaining a quartz glass body by transparentizing the heat-treated porous silica base material.
[0036]
ΔTF and OH content can be controlled by appropriately controlling the heat treatment temperature and the heat treatment time and / or the atmosphere in step (2). Examples of means for that purpose include, for example, a method of heat treatment in a nitrogen gas atmosphere for a sufficiently long time, a relatively short treatment time, and a suitable concentration of CO and N₂Mixed gas or H₂And N₂A method of heat treatment in a mixed gas is preferable, but a CO gas-containing atmosphere is preferable from the viewpoint of effects, and a method of heat treatment in a nitrogen gas atmosphere is preferable from the viewpoint of easy handling of the processing gas. The heat treatment time varies depending on the size of the object to be treated, but if the size is about 3 cm square when it becomes glass, the effect can be obtained by the heat treatment for about 3 hours. When the reducing gas is used, it is effective for reducing OH, but tends to generate oxygen-deficient defects in the glass due to the longer processing time. Therefore, it is desirable to keep the processing time in the reducing gas atmosphere within 32 hours. When the heat treatment temperature is lower than 1100 ° C., a sufficient effect for preventing the formation of defects on the surface of the soot due to soot densification cannot be obtained, and when it exceeds 1450 ° C., transparent vitrification proceeds due to long-time heat treatment. Therefore, it is necessary to set it as 1100-1450 degreeC. In this case, when the temperature is lower than 1300 ° C., the effect of de-OH is not sufficient.₂In this mixed gas, the defect structure inside the soot can be mitigated while controlling the OH group concentration by performing heat treatment in a processing gas having a high CO ratio. Then, by the step (3) of heat treatment at 1450 to 1600 ° C., defects that occur when irradiated with ultraviolet rays reach saturation in a short time, and there is no decrease in transmittance even for subsequent continuous irradiation, etc. A transparent silica glass having a certain amount of defects can be obtained.
[0037]
More specifically, in the step (2), the temperature that does not lead to transparency, that is, 1200 ° C. to 1350 ° C., preferably a higher temperature range is effective, and the maximum temperature is 1250 ° C. to 1350 ° C. for 16 hours. 216 hours or less, preferably 24 hours or more and 144 hours or less, in order to remove the unstable precursors on the surface and the inside of the porous silica base material effectively and uniformly to prevent the generation of internal defects. Preferably, heat treatment (first heat treatment) is performed for 32 hours or more and 144 hours or less. The reason why it was set to 216 hours or less was that it was vitrified by heat treatment longer than this, and 144 hours or less was dependent on the combination conditions with the rate of temperature increase, but the treatment effect was as good as 144 hours or more. This is because.
[0038]
The atmosphere at this time is N₂Alternatively, He or a gas obtained by diluting the gas with another inert gas is used. By using the reducing gas, a more effective OH reduction is expected. However, since a defect occurs in the glass due to the long-time treatment, the treatment in the reducing gas atmosphere is performed for 32 hours of the above treatment time. It is desirable to keep within.
[0039]
The rate of temperature rise to the heat treatment temperature is also from 100 ° C./h to 1500 ° C./h, preferably from 100 ° C. to 1000 ° C./h, more preferably from 100 ° C./h to 500 ° C./h, from room temperature to 500 ° C. h or less, 500 to 1000 ° C., 50 ° C./h or more and 1000 ° C./h or less, preferably 50 ° C. or more and 500 ° C./h or less, more preferably 50 ° C./h or more and 200 ° C./h or less, 1000 to 1200 ° C. 5 ° C./h or more and 200 ° C./h or less, preferably 5 ° C. or more and 100 ° C./h or less, more preferably 5 ° C./h or more and 50 ° C./h or less, and 1200 to 1350 ° C. from 1 ° C./h to 100 ° C. / H or less, preferably 1 ° C. or more and 50 ° C./h or less, more preferably 1 ° C./h or more and 40 ° C./h or less.
[0040]
At this time, the temperature rise to 1000 to 1200 ° C., particularly 1200 to 1350 ° C. is high in temperature, and the progress of sintering on the surface of the porous silica base material is accelerated, so that it is effective to raise the temperature more slowly under the above conditions. Is.
[0041]
By the first heat treatment, the average bulk density of the porous silica base material is 1.4 g / cm.^ThreeOr more, preferably 1.8 g / cm^ThreeOr more, more preferably 2.0 g / cm^ThreeMore than that and 2.20 g / cm^ThreeThe radial density distribution Δρ (highest bulk density−lowest bulk density) is 0.4 g / cm.^ThreeOr less, preferably 0.3 g / cm^ThreeOr less, more preferably 0.2 g / cm^ThreeArrange as follows.
[0042]
In the subsequent transparent treatment in the step (3), the porous silica base material that has finished the first heat treatment has a transparent temperature set between 1450 ° C. and 1550 ° C. in a temperature region of 1000 ° C. or higher. Until it reaches 150 ° C./h, preferably 120 ° C./h or less, more preferably 100 ° C./h or less.
[0043]
The synthetic silica glass thus obtained has ΔTF ≦ 50 ° C. and an OH group content of 1 to 40 ppm dehydrated only by the condensation of associated OH groups in the soot, and this is an ultraviolet ray having a wavelength of 157 nm. It has been found that the transmittance exhibits an excellent property of 60% or more at an optical path length of 10 mm. F₂1x10 excimer laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence is 1 nm / cm or less. In this case, compaction refers to a change in refractive index and a change in surface caused by a high density phenomenon induced by ultraviolet excimer laser light.
[0044]
In addition, when Cl is contained, SiCl becomes a precursor, E ′ center is generated, and SiCl has absorption in the wavelength region of 157 nm, so that it is substantially free of Cl, and less than 1 ppm. preferable. A metal impurity is also a cause of direct absorption band generation, and a material that is substantially not included is preferable in order to promote generation of a defect structure such as an E ′ center. Specifically, the total sum of metal impurities is preferably 50 ppb or less. The concentration of each element is preferably less than 10 ppb.
[0045]
The synthetic silica glass thus obtained has excellent vacuum ultraviolet light transmission and durability, which is unprecedented, stable against heat treatment, and resistant to irradiation with gamma rays, etc. Is also high.
[0046]
The homogeneity is thought to be due to the fact that a refractive index distribution with no difference between inside and outside can be obtained by gradually heating and heating the porous silica base material over a sufficient time, but the difference in refractive index (Δn) The homogeneity expressed is Δn ≦ 2 × 10 in the range of φ = 200 mm.^-6A synthetic silica glass was obtained.
[0047]
Moreover, since the thing whose viscosity is as high as a fused silica glass is obtained, it can be used for various uses, such as a structural material for semiconductor manufacture besides an optical material.
[0048]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.
[0049]
Example 1
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous base material is N in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1300 ° C. in an atmosphere.
[0050]
The temperature rise to 1300 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and 5 ° C./h to 1200 to 1300 ° C. The holding time at 1300 ° C. was 32 hours.
[0051]
The average bulk density of the treated porous silica matrix is 2.10 g / cm.^ThreeMet.
[0052]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0053]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was less than 50 ppb.
[0054]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0055]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples A1 and A2, respectively.
[0056]
The amounts of OH groups contained in Samples A1 and A2 were determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, and were 40 and 30 ppm, respectively.
[0057]
The fictive temperature (TF) of samples A1 and A2 is 1120 cm in the infrared reflection spectrum.^-1When obtained from the nearby peak position, the difference (= virtual temperature distribution) ΔTF between the two was 10 ° C.
[0058]
F with a wavelength of 157 nm on the optical polishing surface of sample A1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 65%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0059]
Example 2
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material was heat-treated at 1300 ° C. in a helium gas atmosphere in a uniform heating electric furnace equipped with a furnace core tube.
[0060]
The temperature rise to 1300 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and 5 ° C./h to 1200 to 1300 ° C. The holding time at 1300 ° C. was 38 hours.
[0061]
The average bulk density of the treated porous silica matrix is 2.10 g / cm.^ThreeMet.
[0062]
Next, the heat-treated porous base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C./h. The glass was lowered at such a speed as to become transparent glass.
[0063]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was less than 50 ppb.
[0064]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0065]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples B1 and B2, respectively.
[0066]
The amounts of OH groups contained in Samples B1 and B2 were determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, and were 40 and 30 ppm, respectively.
[0067]
The virtual temperature (TF) of samples B1 and B2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0068]
F with a wavelength of 157 nm on the optical polishing surface of sample B1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 65%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0069]
Example 3
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1200 ° C. in an atmosphere.
[0070]
The temperature rise to 1200 ° C. was 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and the holding time at 1200 ° C. was 84 hours.
[0071]
The average bulk density of the treated porous silica matrix is 2.15 g / cm.^ThreeMet.
[0072]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0073]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was less than 50 ppb.
[0074]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0075]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (100 mm position from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples C1 and C2, respectively.
[0076]
When the amount of OH groups contained in Samples C1 and C2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 20 and 10 ppm, respectively.
[0077]
The fictive temperature (TF) of samples C1 and C2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0078]
F having a wavelength of 157 nm on the optical polishing surface of sample C1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 75%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0079]
Example 4
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material was subjected to heat treatment at 1200 ° C. in a helium gas atmosphere in a uniform heating electric furnace equipped with a furnace core tube.
[0080]
The temperature rise to 1200 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and the holding time at 1200 ° C. is 96 ° C. It was time.
[0081]
The average bulk density of the treated porous silica matrix is 2.15 g / cm.^ThreeMet.
[0082]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0083]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0084]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0085]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples D1 and D2.
[0086]
When the amount of OH groups contained in Samples D1 and D2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 15 and 5 ppm, respectively.
[0087]
The virtual temperature (TF) of samples D1 and D2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0088]
F with a wavelength of 157 nm on the optical polishing surface of sample D1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 80%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0089]
Example 5
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1300 ° C. in an atmosphere.
[0090]
The temperature rise to 1300 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and 5 ° C./h to 1200 to 1300 ° C. The holding time at 1300 ° C. was 38 hours.
[0091]
The average bulk density of the treated porous silica matrix is 2.10 g / cm.^ThreeMet.
[0092]
Next, the heat-treated porous silica base material is heated in a zone-heating electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 50 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0093]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0094]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0095]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples E1 and E2, respectively.
[0096]
When the amount of OH groups contained in Samples E1 and E2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 30 and 20 ppm, respectively.
[0097]
The fictive temperature (TF) of samples E1 and E2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0098]
F with a wavelength of 157 nm on the optical polishing surface of sample E1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 70%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0099]
Comparative Example 1
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1150 ° C. in an atmosphere.
[0100]
The temperature was raised to 1150 ° C. at a rate of 500 ° C./h up to 500 ° C., 100 ° C./h from 500 to 1000 ° C., and 10 ° C./h from 1000 to 1150 ° C., and the holding time at 1150 ° C. was 32 It was time.
[0101]
The average bulk density of the treated porous silica matrix is 0.70 g / cm.^ThreeMet.
[0102]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0103]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0104]
The homogeneity in the radial direction of the obtained glass is Δn = 8.0 × 10 in the region of φ = 200 mm.^-6Met.
[0105]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples F1 and F2, respectively.
[0106]
When the amount of OH groups contained in Samples F1 and F2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 170 and 100 ppm, respectively.
[0107]
The fictive temperature (TF) of samples F1 and F2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was found to be 80 ° C. as determined from the nearby peak position.
[0108]
F with a wavelength of 157 nm on the optical polishing surface of sample F1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 5%. 1 × 10 laser⁶The transmittance decrease at 157 nm after the pulse irradiation is 50%, and the refractive index difference before and after the irradiation, which is an index of compaction, is 10 × 10.^-6The amount of change in birefringence was 10 nm / cm.
[0109]
Comparative Example 2
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1300 ° C. in an atmosphere.
[0110]
The temperature rise to 1300 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and 5 ° C./h to 1200 to 1300 ° C. The holding time at 1300 ° C. was 12 hours.
[0111]
The average bulk density of the treated porous silica matrix is 1.20 g / cm.^ThreeMet.
[0112]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0113]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0114]
The homogeneity in the radial direction of the obtained glass is Δn = 5.0 × 10 in the region of φ = 200 mm.^-6Met.
[0115]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and two opposing surfaces were optically polished to obtain samples G1 and G2, respectively.
[0116]
The amounts of OH groups contained in Samples G1 and G2 were determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, and were 100 and 80 ppm, respectively.
[0117]
The fictive temperature (TF) of samples G1 and G2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 60 ° C. as determined from the nearby peak position.
[0118]
F with a wavelength of 157 nm on the optical polishing surface of sample G1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 30%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is 40%, and the refractive index difference before and after irradiation, which is an index of compaction, is 5 × 10.^-6The amount of change in birefringence was 5 nm / cm.
[0119]
Comparative Example 3
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1300 ° C. in an atmosphere.
[0120]
The temperature rise to 1300 ° C. is 500 ° C./h to 500 ° C., 100 ° C./h to 500 to 1000 ° C., 10 ° C./h to 1000 to 1200 ° C., and 5 ° C./h to 1200 to 1300 ° C. The holding time at 1300 ° C. was 16 hours.
[0121]
The average bulk density of the treated porous silica matrix is 1.40 g / cm.^ThreeMet.
[0122]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 200 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0123]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0124]
The homogeneity of the obtained glass in the radial direction is as follows: Δn = 6.0 × 10 6^-6Met.
[0125]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples H1 and H2, respectively.
[0126]
When the amount of OH groups contained in Samples H1 and H2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 90 and 60 ppm, respectively.
[0127]
The fictive temperature (TF) of samples H1 and H2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was found to be 70 ° C. as determined from the nearby peak position.
[0128]
F with a wavelength of 157 nm on the optical polishing surface of sample H1₂Excimer laser with an energy density of 10 mJ / cm²-When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 35%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is 40%, and the refractive index difference before and after irradiation, which is an index of compaction, is 7 × 10.^-6The amount of change in birefringence was 7 nm / cm.
[0129]
Comparative Example 4
Purified silicon tetrachloride was hydrolyzed in a oxyhydrogen flame by a known method to prepare a porous silica base material having a diameter of 400 mm and a length of 1000 mm. This porous silica base material is subjected to N heating in a uniform heating type electric furnace equipped with a furnace core tube.₂Heat treatment was performed at 1300 ° C. in an atmosphere.
[0130]
The temperature was raised to 1300 ° C. at a rate of 1800 ° C./h up to 1000 ° C., 500 ° C./h up to 1000-1200 ° C., and 200 ° C./h up to 1200-1300 ° C., and the holding time at 1300 ° C. was 16 It was time.
[0131]
The average bulk density of the treated porous silica matrix is 1.30 g / cm^ThreeMet.
[0132]
Next, the heat-treated porous silica base material is heated in a zone heating type electric furnace equipped with a furnace core tube in a helium gas atmosphere at a temperature of 1500 ° C., and the temperature increase rate of the porous silica base material is 100 ° C. / Transparent vitrification was performed by pulling down at a speed such that h.
[0133]
When a portion of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0134]
The homogeneity of the obtained glass in the radial direction is as follows: Δn = 6.0 × 10 6^-6Met.
[0135]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and two opposing surfaces were optically polished to obtain samples I1 and I2, respectively.
[0136]
When the amount of OH groups contained in Samples I1 and I2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 100 and 70 ppm, respectively.
[0137]
The fictive temperature (TF) of samples I1 and I2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was found to be 70 ° C. as determined from the nearby peak position.
[0138]
F with a wavelength of 157 nm on the optical polishing surface of sample I1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 30%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is 30%, and the refractive index difference before and after irradiation, which is a compaction index, is 6 × 10.^-6The amount of change in birefringence was 6 nm / cm.
[0139]
Example 6
N in uniform heating electric furnace₂After heat treatment at 1350 ° C. for 16 hours in a gas atmosphere, N was maintained while maintaining the temperature at 1350 ° C.₂Heat treatment was performed in a gas atmosphere for 38 hours, and the average bulk density of the treated porous silica base material was 2.18 g / cm.^ThreeA transparent silica glass was obtained in the same manner as in Example 1 except that it was changed.
[0140]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the concentrations of alkali and alkaline earth metals, transition metals, and other metal impurities were less than 10 ppb for all elements. .
[0141]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0142]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples J1 and J2, respectively.
[0143]
The amounts of OH groups contained in Samples J1 and J2 were determined from an absorption intensity of 2.73 μm using an infrared spectrophotometer, and were 30 and 20 ppm, respectively.
[0144]
The virtual temperature (TF) of samples J1 and J2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0145]
F having a wavelength of 157 nm on the optical polishing surface of sample J1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 70%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0146]
Example 7
N in uniform heating electric furnace₂After processing in a gas atmosphere for 16 hours, CO / N₂Heat treatment was performed in a mixed gas atmosphere (mixed volume ratio = 10: 90) for 8 hours, and the average bulk density of the treated porous silica base material was 1.80 g / cm.^ThreeA transparent silica glass was obtained in the same manner as in Example 1 except that it was changed.
[0147]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0148]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0149]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and two opposing surfaces were optically polished to obtain samples K1 and K2, respectively.
[0150]
The amounts of OH groups contained in samples K1 and K2 were determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, and were 40 and 30 ppm, respectively.
[0151]
The fictive temperature (TF) of samples K1 and K2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0152]
F with a wavelength of 157 nm on the optical polishing surface of sample K1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 74%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0153]
Example 8
Process atmosphere in a uniform heating electric furnace is H₂/ N₂The mixed gas (mixed volume ratio = 10: 90) was used, the heat treatment time was 15 hours, and the average bulk density of the porous silica base material after treatment was 1.40 g / cm.^ThreeA transparent silica glass was obtained in the same manner as in Example 1 except that it was changed.
[0154]
When a part of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was less than 1 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the alkali and alkaline earth metal, transition metal, and other metal impurity concentrations were less than 10 ppb for all elements, The total amount was 50 ppb or less.
[0155]
The homogeneity in the radial direction of the obtained glass is Δn = 1.0 × 10 in the region of φ = 200 mm.^-6Met.
[0156]
A 10 × 10 × 30 mm test piece was cut out from the radial center and the outside (position of 100 mm from the center) of the obtained glass, and the two opposing surfaces were optically polished to obtain samples L1 and L2, respectively.
[0157]
When the amount of OH groups contained in Samples L1 and L2 was determined from the absorption intensity of 2.73 μm using an infrared spectrophotometer, they were 45 and 35 ppm, respectively.
[0158]
The fictive temperature (TF) of samples L1 and L2 is 1120 cm in the infrared reflection spectrum.^-1The ΔTF was 10 ° C. as determined from the nearby peak position.
[0159]
F with a wavelength of 157 nm on the optical polishing surface of sample L1₂Excimer laser with an energy density of 10 mJ / cm²When irradiated with pulse and repetition frequency of 1000 Hz, the transmittance was 65%. 1 × 10 laser⁶The transmittance decrease at 157 nm after pulse irradiation is less than 1%, and the refractive index difference before and after irradiation, which is a compaction index, is 1 × 10.^-6Hereinafter, the amount of change in birefringence was 1 nm / cm or less.
[0160]
Table 1 below shows the heating conditions, clearing conditions, OH concentration, Cl concentration, ΔTF, initial transmittance of 157 nm, and laser resistance of samples A to L as indices indicating laser resistance.₂1x10 excimer laser⁶The degree of transmittance decrease at 157 nm after pulses irradiation was shown.
[0161]
[Table 1]

[0162]
The initial transmittance and the reduced transmittance after irradiation in the table were calculated from the following formulas.
[0163]
Initial transmittance (%) = (T₁/ T₀) × 100
T₁= Sample transmitted pulse intensity (F₂(At the start of laser irradiation)
T₀= Blank pulse intensity
Decreased transmittance after irradiation ΔT = T_A-T_B
T_A= (T₁/ T₀) × 100
T_B= (T₂/ T₀) × 100
T₁= Sample transmitted pulse intensity (F₂(At the start of laser irradiation)
T₀= Blank pulse intensity
T₂= Sample transmitted pulse intensity (F₂Laser 10mJ / cm²,
1 × 10⁶(after pulses irradiation)
Example 9
Two test pieces of 10 × 10 × 30 mm were further cut out from the glass obtained in Example 6, and the two surfaces were optically polished to obtain samples M1 and M2, respectively.
[0164]
When the sample M1 was irradiated with light of a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in a vacuum,
As the irradiation time increased, the transmittance of the lamp light decreased. The transmittance decreased to about 55% of the initial transmittance about 100 hours after the start of irradiation, but no further decrease in transmittance was observed even if the irradiation was continued as it was.
[0165]
An ArF excimer laser with a wavelength of 193 nm is applied to the sample M2 with an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The laser beam transmittance decreased with increasing irradiation time. Sample M2 has 10 irradiations⁶The transmittance dropped to about 83% of the initial transmittance after the shot.⁷When irradiation was continued until the shot, no further reduction in transmittance was observed.
[0166]
Example 10
Two test pieces of 10 × 10 × 30 mm were further cut out from the glass obtained in Example 7, and the two surfaces were optically polished to obtain samples N1 and N2, respectively.
[0167]
When the sample N1 was irradiated with light from a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in vacuum, the transmittance of the lamp light decreased with increasing irradiation time. The transmittance decreased to about 60% of the initial transmittance about 100 hours after the start of irradiation, but no further decrease in transmittance was observed even if the irradiation was continued as it was.
[0168]
An ArF excimer laser with a wavelength of 193 nm is applied to the sample N2 at an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The laser beam transmittance decreased with increasing irradiation time. Sample N2 has 10 irradiations⁶The transmittance dropped to about 85% of the initial transmittance after the shot, but then 10%⁷When irradiation was continued until the shot, no further reduction in transmittance was observed.
[0169]
Example 11
Two test pieces of 10 × 10 × 30 mm were further cut out from the glass obtained in Example 8, and two surfaces were optically polished to obtain samples O1 and O2, respectively.
[0170]
When the sample O1 was irradiated with light from a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in a vacuum, the transmittance of the lamp light decreased as the irradiation time increased. The transmittance decreased to about 50% of the initial transmittance about 100 hours after the start of irradiation, but even if the irradiation was continued as it was, no further decrease in the transmittance was observed.
[0171]
An ArF excimer laser with a wavelength of 193 nm is applied to the sample O2 at an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The laser beam transmittance decreased with increasing irradiation time. Sample O2 has 10 irradiations⁶The transmittance dropped to about 80% of the initial transmittance after the shot.⁷When irradiation was continued until the shot, no further reduction in transmittance was observed.
[0172]
Comparative Example 5
Two test pieces of 10 × 10 × 30 mm were further cut out from the glass obtained in Comparative Example 3, and the two surfaces were optically polished to obtain samples P1 and P2, respectively.
[0173]
When the sample S1 was irradiated with light from a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in a vacuum, the transmittance of the lamp light decreased as the irradiation time increased. The transmittance dropped to about 45% of the initial transmittance about 100 hours after the start of irradiation. When irradiation was continued as it was, the transmittance gradually decreased and did not become constant.
[0174]
An ArF excimer laser with a wavelength of 193 nm is applied to the sample P2 with an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The transmittance of laser light decreases with increasing irradiation time, and the number of irradiation is 10⁶The transmittance drops to about 79% of the initial transmittance in the shot, and then 10%⁷When irradiation was continued until the shot, the transmittance gradually decreased and did not stabilize.
[0175]
Comparative Example 6
Two test pieces of 10 × 10 × 30 mm were further cut out from the glass obtained in Comparative Example 4, and the two surfaces were optically polished to obtain samples Q1 and Q2, respectively.
[0176]
When the sample S1 was irradiated with light from a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in a vacuum, the transmittance of the lamp light decreased as the irradiation time increased. The transmittance dropped to about 45% of the initial transmittance about 100 hours after the start of irradiation. When irradiation was continued as it was, the transmittance gradually decreased and did not become constant.
[0177]
An ArF excimer laser with a wavelength of 193 nm is applied to sample Q2 with an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The transmittance of laser light decreases with increasing irradiation time, and the number of irradiation is 10⁶The transmittance drops to about 79% of the initial transmittance in the shot, and then 10%⁷When irradiation was continued until the shot, the transmittance gradually decreased and did not stabilize.
[0178]
Comparative Example 7
Synthetic silica glass was obtained by a direct method using silicon tetrachloride as a raw material, hydrolyzing it in an oxyhydrogen flame, and directly depositing vitrification at a temperature of 1600 ° C. or higher. The size of the obtained glass was 200 mm in diameter and 1200 mm in length. When the amount of OH groups contained in the glass was determined from the absorption intensity at 2.73 μm in the infrared absorption spectrum, it was 700 ppm. Further, when a portion of the obtained glass was dissolved in 38% hydrofluoric acid and subjected to quantitative analysis of chlorine, the chlorine concentration in the glass was 150 ppm below the detection limit. Furthermore, when quantitative analysis of impurities contained in the glass was performed using an ICP mass spectrometer, the concentrations of alkali impurities, alkaline earth metals, transition metals, and other metal impurities were less than 10 ppb for all elements. The total amount was 50 ppb or less.
[0179]
Two test pieces of 10 × 10 × 30 mm were cut out from the obtained glass, and two surfaces were optically polished to obtain samples R1 and R2, respectively.
[0180]
When the sample R1 was irradiated with light from a dielectric barrier discharge excimer lamp having a wavelength of 172 nm in a vacuum, the transmittance of the lamp light decreased as the irradiation time increased. The transmittance decreased to about 75% of the initial transmittance about 100 hours after the start of irradiation. When irradiation was continued as it was, the transmittance gradually decreased and did not become constant.
[0181]
An ArF excimer laser with a wavelength of 193 nm is applied to the sample R2 with an energy density of 100 mJ / cm.²-Irradiated with a pulse, repetition frequency of 100 Hz. The transmittance of laser light decreases with increasing irradiation time, and the number of irradiation is 10⁶The transmittance drops to about 92% of the initial transmittance in the shot, and then 10%⁷When irradiation was continued until the shot, the transmittance gradually decreased and did not stabilize.
[0182]
FIG. 1 shows the change in transmittance with the irradiation time of the excimer lamp in samples M to R, and FIG. 2 shows the change in transmittance with the number of ArF excimer laser shots in samples M to R.
[0183]
As is clear from FIGS. 1 and 2 of Examples 9 to 11 and Comparative Examples 8 to 10 described above, by setting the OH group concentration in the glass to 1 to 70 ppm, a certain amount of ultraviolet rays such as excimer lamps and excimer lasers are emitted. After irradiation, there is no decrease in transmittance due to ultraviolet irradiation, and silica glass showing a stable transmittance without change with time can be obtained.
[0184]
【The invention's effect】
As described above, according to the present invention, before the soot synthesized by the soot method is made transparent, N₂Alternatively, heat treatment is performed for a sufficient amount of time under conditions such that condensation of only the associated OH groups in the soot occurs in a reducing gas atmosphere, so that the fictive temperature distribution is small and unstable precursors are removed. The generation of defects inside the soot is suppressed, the OH group concentration in the glass is 1 to 70 ppm, the Cl concentration is less than 1 ppm, and the concentration of alkali and alkaline earth metals, transition metals and other metal impurities is 10 ppb for each element. Less than 50 ppb, and the transmittance at 172 to 200 nm is 40% or more even for continuous irradiation with ultraviolet rays having a wavelength of 160 to 300 nm, and it is as high as 60% or more for short wavelength light of 157 nm. An optical material for ultraviolet rays comprising a synthetic silica glass having excellent properties exhibiting transmittance can be provided.
[0185]
In addition, since the structural change due to heat treatment is small, not only a material with a good refractive index homogeneity can be easily obtained, but also the compaction is small.₂It is suitable as an optical material for lithography using a laser. In addition, since the OH group content is low, the radiation resistance is good, and it can be used as an image fiber for observing the inside of a nuclear reactor having radioactivity or an optical fiber for communication under radiation exposure. Moreover, since the viscosity at high temperature is high, it can be used as an ultra-high purity material for semiconductor production.
[0186]
Furthermore, since there are few dimensional changes by a thermal cycle, it can be used also as a substrate material for polysilicon TFTs. Further, since the amount of released gas is extremely small and the deterioration is small, it can be used as a bulb material for various lamps.
[Brief description of the drawings]
FIG. 1 is a diagram showing a change in transmittance according to excimer lamp irradiation time in samples M to R;
FIG. 2 is a diagram showing a change in transmittance according to the number of ArF excimer laser shots in samples M to R.

Claims (13)

From a high-purity silicon compound, obtained by heating a porous silica base material produced by depositing a soot produced by the soot method in which the silicon compound is pyrolyzed by flame, and substantially free of impurities other than OH groups An optical material for ultraviolet rays made of synthetic silica glass, wherein the difference between the maximum and minimum fictive temperatures in the glass is 50 ° C. or less and the transmittance of ultraviolet rays having a wavelength of 157 nm is 60% or more when the optical path length is 10 mm. The optical material for ultraviolet rays according to claim 1, wherein the OH group content is 1 to 70 ppm, the Cl concentration is less than 1 ppm, the metal impurity concentration is less than 10 ppb for each element, and the total amount thereof is 50 ppb or less. An optical material for ultraviolet rays. 3. The synthetic silica glass according to claim 1, wherein the transmittance of ultraviolet light having a wavelength of 172 to 200 nm is 40% or more at an optical path length of 10 mm even when irradiated with ultraviolet light having a wavelength of 160 to 300 nm for 1 hour. An optical material for ultraviolet rays. The optical material for ultraviolet rays according to claim 1, wherein the homogeneity represented by the refractive index difference (Δn) of the synthetic silica glass is Δn ≦ 2 × 10 ⁻⁶ within the range of φ = 200 mm. Optical material for ultraviolet rays. When the first heat treatment is performed on the porous silica base material obtained by the soot method at a temperature of 1200 ° C. or more and 1350 ° C. or less, the temperature is from 100 ° C./h to 1500 ° C./h, 500 to 1000 ° C. from room temperature to 500 ° C. Up to 50 ° C / h to 1000 ° C / h, up to 1000-1200 ° C from 5 ° C / h to 200 ° C / h, up to 1200-1350 ° C from 1 ° C / h to 100 ° C / h After the heat treatment is performed, the second heat treatment is gradually inserted into the zone heated to a temperature of 1450 ° C. or higher and 1550 ° C. or lower so that the rate of temperature rise is 10 ° C./h or higher and 150 ° C./h or lower. transparentizing performs a heat treatment, according to claim 1 to 4, the average bulk density of the silica porous preform having been subjected to the first heat treatment is equal to or less than 2.10 g / cm ³ or more 2.20 g / cm ³ In any of Method of manufacturing an optical material for an outside line. 6. The method for producing an optical material for ultraviolet rays according to claim 5, wherein the first heat treatment of the porous silica base material is performed in a nitrogen gas atmosphere. 6. The method for producing an optical material for ultraviolet rays according to claim 5, wherein the first heat treatment of the porous silica base material is carried out in an atmosphere containing hydrogen gas. 6. The method for producing an optical material for ultraviolet rays according to claim 5, wherein the first heat treatment of the porous silica base material is carried out in an atmosphere containing carbon monoxide gas. 6. The method for producing an ultraviolet optical material according to claim 5, wherein the holding time at the maximum temperature in the first heat treatment is 16 hours or more and 216 hours or less. The manufacturing method of an ultraviolet optical material according to claim 5, the atmosphere for performing first heat treatment, a He gas atmosphere, or is the He gas atmosphere diluted with an inert gas other than He gas containing N ₂ The manufacturing method of the optical material for ultraviolet rays characterized by the above-mentioned. The optical material for ultraviolet rays according to any one of claims 1 to 4, which is suitable for irradiating ultraviolet rays having a wavelength of 157 nm to 300 nm. In the ultraviolet optical material according to claim 11, ultraviolet _{F 2} excimer laser, ArF excimer laser, KrF excimer laser, Xe ₂ excimer lamp, Xe lamp, _{D 2} lamp, is selected from low pressure Hg lamp, and KrCl excimer lamp An optical material for ultraviolet rays that is irradiated from a radiation source. The optical material for ultraviolet rays according to claim 12 is an excimer lamp (Xe ₂ , KrCl, KrBr, KrI etc.), a metal halide lamp, a mercury lamp, a short arc mercury lamp, a D ₂ lamp, an H ₂ lamp, and a xenon lamp. An optical material for ultraviolet rays, characterized in that it is a cylindrical material and spherical material for TFT, a TFT TFT substrate material, a radiation-resistant optical fiber material, a photomask material, or a soft X-ray reflective mirror substrate material.

Images