JP2004026587A

JP2004026587A - High homogeneous synthetic quartz glass for vacuum ultraviolet light, manufacture method of the same and mask substrates for vacuum ultraviolet light using this

Info

Publication number: JP2004026587A
Application number: JP2002186599A
Authority: JP
Inventors: Hideharu Horikoshi; 堀越　秀春
Original assignee: Tosoh Corp; Tohos SGM KK
Current assignee: Tosoh Corp; Tohos SGM KK
Priority date: 2002-06-26
Filing date: 2002-06-26
Publication date: 2004-01-29
Anticipated expiration: 2022-06-26
Also published as: JP4213413B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low cost synthetic quartz glass for vacuum ultraviolet light which has excellent homogeneity and is manufactured using existing general equipment without carrying out excessive and needless modification of a quartz glass synthesized by the soot method (VAD method) using a glass forming raw material such as SiCl<SB>4</SB>and a manufacturing method of the same and a mask substrate for vacuum ultraviolet light using the same. <P>SOLUTION: The synthetic quartz glass for vacuum ultraviolet light has ≥70% transmissivity per 10 mm thickness in 163 mm wave length, the difference (refractive index distribution) of ≤5×10<SP>-6</SP>per 10 mm thickness between the maximum refractive index and the minimum refractive index in a region of 200 mm diameter and ≤5 nm/cm birefringence. The manufacturing method of the same and the mask substrate for vacuum ultraviolet light using the same are provided. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は波長２００ｎｍ以下の真空紫外光照射に対して優れた透過率および均質性を有する、真空紫外光用合成石英ガラス、その製造方法及びこれを加工してなる真空紫外光用マスク基板に関するものである。
【０００２】
【従来の技術】
近年、ＬＳＩの高集積化と共に、集積回路パターンも微細化の一途をたどり、０．２５μｍ以下の超微細パターンが描画された超ＬＳＩの量産化が行われ始めている。このような超微細パターンを得るには、それを描画する露光光源を短波長化する必要があり、エキシマレーザー光を光源とするステッパーが開発され、既にＫｒＦエキシマレーザー光（波長２４８ｎｍ）を光源とするステッパーが量産化され、より波長の短いＡｒＦエキシマレーザー光（波長１９３ｎｍ）や、Ｆ_２エキシマレーザー光（波長１５７ｎｍ）など真空紫外光を光源とするステッパーが注目を集めている。
【０００３】
この真空紫外域においても十分な透過率と均質性を示す光学材料としては、合成石英ガラスや螢石などが挙げられるが、中でも高純度のケイ素化合物を原料として製造した合成石英ガラスは、２００ｎｍ以下の真空紫外領域でも高い透過率と均質性を示す事から、真空紫外光を光源とするリソグラフィー工程の光学材料として広く用いられている。
【０００４】
しかしながら、従来の合成石英ガラスは真空紫外域の短波長光を照射すると、構造欠陥が誘起され、透過率及び均質性が低下する問題を有し、特にＦ_２エキシマレーザーを光源とする超ＬＳＩのリソグラフィーに用いられる光学材料としては問題があった。
【０００５】
真空紫外光照射耐性を改善する手段の一つとして、例えば特開平７−４３８９１号公報あるいは特開平９−１２４３３７号公報においては、合成石英ガラス内のＨ_２（水素分子）含有量を高め、真空紫外線照射によって生じた欠陥をＨ（水素原子）により修復し、照射耐性を向上される方法が提案されている。しかし、Ｈで修復された≡Ｓｉ−Ｈ結合により均質性が悪化してしまう。また、≡Ｓｉ−Ｈ結合に真空紫外光が照射される事で、いわゆるＥ’センター（≡Ｓｉ・）などの新たな欠陥が生成する問題があった。さらに、Ｈ_２を含浸させるための工程が必要となるなど製造コストが高くなるという問題点があった。
【０００６】
合成石英ガラスの原料として、一般に四塩化ケイ素（ＳｉＣｌ_４）が用いられるが、この場合、Ｃｌが合成石英ガラス中に≡Ｓｉ−Ｃｌの形態で残留し、透過率及び均質性に悪影響を及ぼす事が知られている。この問題を解決するため、例えば特開平８−３１７２３号公報ではアルコキシシランの様なＣｌを含有しない原料を用いる方法が提案されている。この方法では、ＳｉＣｌ_４を用いた場合に比べ原料コストが高くなる問題点と共に、原料中のＣが合成石英ガラス中に残留する事で真空紫外光照射耐性に悪影響を及ぼす問題があった。また、金属不純物は透過率及び均質性を著しく低下させる事が知られているが、Ｃｌには合成雰囲気中に存在する金属不純物を塩化物として系外に除去する働きがある。そのためＣｌを含まない原料を用いた場合、金属不純物が増加し透過率及び均質性の低下を招いてしまうことになる。
【０００７】
近年、真空紫外光照射に対する耐性が優れた合成石英ガラスを得る方法として、特開平８−６７５３０号公報、特開平８−７５９０１号公報などに開示されている様に、高濃度のＦを含有した合成石英ガラスが提案されている。本方法では製造時に生じた欠陥および、真空紫外光照射などによって生じた欠陥は≡Ｓｉ−Ｈなどに比べて結合エネルギーの大きい≡Ｓｉ−Ｆの形態で修復されるため、欠陥修復後さらに真空紫外光を照射しても新たな欠陥は生成せず、真空紫外光照射耐性に優れているとされている。しかし、石英ガラス中に≡Ｓｉ−Ｆ結合が存在すると、石英ガラスの構造に歪みが生じ、均質性に悪影響を及ぼす。また、アニール、成型等の熱処理を行うと、Ｆが遊離して新たな構造欠陥を生成するため、透過率及び均質性が悪化する問題があった。
【０００８】
このＦを含有した合成石英ガラスを得る方法としては、ＳｉＣｌ_４などを原料とする一般的方法で製造した合成石英ガラスに後からＦを含浸させる方法（特開平１３−１９４５０号公報）および、ＳｉＦ_４などのＦ含有化合物を原料として合成石英ガラスを製造する方法（特開平１２−２６４６７１号公報）などがある。しかし、工業化を図る場合、いずれの方法も高濃度のＦ化合物を取り扱うため、作業が煩雑となり危険性が高いと共に、原料コストが高く、また副生する弗酸に対して人体および環境への影響面から様々な対策が必要となるなど、製造コストが高くなる問題もあった。
【０００９】
【発明が解決しようとする課題】
以上説明したように、これまでは主として、Ｈ_２及びＦ含浸処理などの後処理や、原料にＣｌを含有しないアルコキシシランやＦ化合物を使用するなど、合成石英ガラスの基本構造や製造方法を大幅に変更する事で真空紫外光照射耐性向上を目指し開発が行われていた。その結果、透過率の改善には一応の効果が見られたものの、屈折率分布や複屈折量等の均質性に問題が生じていた。また、特殊で高価な原料を用いたり、製造設備の大規模な改造が必要になると共に製造工程が複雑となるなど、作業性および製造コストの面などで新たな課題が生じていた。超ＬＳＩの量産化に対して、真空紫外光用光学素材の開発は必要不可欠であり、真空紫外光照射特性に優れ、高品質でかつ経済的な光学材料とその製造方法が強く望まれている。
【００１０】
本発明は、ＳｉＣｌ_４等のガラス形成原料を用いたスート法（ＶＡＤ法）で合成された合成石英ガラスに対して、過大で不必要な修飾を行う事なく、既存の汎用設備を使用して、安価でかつ均質性に優れた真空紫外光用合成石英ガラス及びその製造方法を提供し、さらにこの真空紫外光用合成石英ガラスを用いた真空紫外光用マスク基板を提供する事を目的とする。
【００１１】
【課題を解決するための手段】
本発明者らは、上記課題を解決するため合成石英ガラスの諸物性と、真空紫外光照射における透過率及び均質性との相関について鋭意検討を行った結果、合成石英ガラスに含有されるＯＨ基含有量とＦ元素含有量が透過率及び均質性に対して特に重要であり、それぞれの値を特定の範囲に制限する事で、透過率及び均質性に優れた真空紫外光用合成石英ガラスを得る事が出来る事を見出した。さらに、この合成石英ガラスを真空紫外光用マスク基板として用いた場合、特に優れた性能を示す事を見出し、本発明を完成するに至った。
【００１２】
すなわち本発明は、波長１６３ｎｍにおける透過率が厚さ１０ｍｍあたり７０％以上で、直径２００ｍｍの領域における最大屈折率と最小屈折率の差（屈折率分布）が厚さ１０ｍｍあたり５×１０^−６以下かつ、複屈折量が５ｎｍ／ｃｍ以下である真空紫外光用合成石英ガラスである。さらにその中でも、ＯＨ基含有量が１〜２０ｐｐｍ、Ｆ元素含有量が１０〜３０ｐｐｍである事を特徴とする真空紫外光用合成石英ガラスである。
【００１３】
また、ガラス形成原料を酸水素火炎中で火炎加水分解して得られたシリカ微粒子をターゲット上に堆積させスート体を合成し、その後の熱処理で透明ガラス化するいわゆるスート法において、スート体の熱処理を還元性ガス雰囲気で行った後、さらにＦ化合物ガス雰囲気で熱処理を行い、最後にＯ_２含有雰囲気で透明ガラス化処理を行う事を特徴とする、前記物性の真空紫外光用合成石英ガラスの製造方法及び、前記物性の真空紫外光用合成石英ガラスを、真空紫外光用マスク基板として使用する用途も本願発明の範囲に含まれるものである。
【００１４】
以下本発明を詳細に説明する。
【００１５】
ＯＨ基は真空紫外域で吸収を示すため、ＯＨ基含有量が少ない程真空紫外域での透過率は上昇する。従って、ＯＨ基濃度は２０ｐｐｍ以下にする必要がある。ただし、ＯＨ基には、ＳｉＯ_４四面体が連なって形成されている石英ガラスの網目構造を安定化させる作用があるため、ＯＨ基濃度が１ｐｐｍ未満になると石英ガラスの構造が不安定になることがあり、熱処理等により容易に欠陥が生成して透過率及び均質性が悪化することがある。このため、石英ガラス中に１ｐｐｍ以上のＯＨ基を含有することが好ましい。
【００１６】
酸水素火炎中で合成されたシリカ微粒子は、通常、数１０００ｐｐｍ程度のＯＨ基を含有する。ＯＨ基は真空紫外域で吸収を示すため、スート体はＨ_２ガス、Ｆ化合物ガス等の還元性ガス雰囲気で熱処理する事で、ＯＨ基濃度の低減化が行われる。しかし、Ｆ化合物ガスで脱ＯＨ基処理した場合、石英ガラスに対するＦ元素の反応性が高いめ、ＯＨ基濃度の制御が困難になると共に、過剰のＦが石英ガラスを形成する≡Ｓｉ―Ｏ―Ｓｉ≡結合のＯ原子と反応して、≡Ｓｉ―Ｓｉ≡の酸素欠乏型欠陥を生成する。この欠陥は、１６３ｎｍを中心とする真空紫外域に吸収を示すため、真空紫外域で透過率が低下する。また、酸素欠乏型欠陥により結合に歪みが生じるため、均質性が悪化する事が分かった。従って、スート体の脱ＯＨ基処理はＨ_２ガス等のＦ化合物以外の還元性ガスで行うのが好ましい。
【００１７】
Ｆ化合物以外の還元性ガスで処理した石英ガラスであっても、Ｆ化合物ガスで処理した場合に比べてはるかに低濃度ではあるが、脱ＯＨ基処理により酸素欠乏欠陥等が生成し、真空紫外域の透過率が低下すると共に、均質性も悪化することがある。脱ＯＨ基処理により生成したこの微量の欠陥は、脱ＯＨ基処理後、さらにＦ元素含有ガス雰囲気で熱処理する事で、≡Ｓｉ−Ｆ結合の型で修復できる。この結合は真空紫外光照射に対して吸収を示さない。しかし、既に記述したように、欠陥修復のためのＦ元素含有ガス処理において、過剰なＦが石英ガラス中に取り込まれると、≡Ｓｉ−ＯＨあるいは≡Ｓｉ−Ｏ−Ｓｉ≡と反応して、新たな欠陥を生成する。また、≡Ｓｉ−Ｆ結合は構造に歪みを生じさせるため、多量に存在すると均質性が悪化する。このため、石英ガラス中に含有するＦ濃度は１０〜３０ｐｐｍが好ましい。Ｆ濃度を所望の範囲とするためには、脱ＯＨ基処理後、２００℃〜７００℃、０．０１〜１ｖｏｌ％Ｆ化合物ガス含有雰囲気で処理する必要がある。脱ＯＨ基処理の前にＦ化合物ガスで処理を行うと、スート体中に多量に含まれるＯＨ基と反応するため、Ｆ濃度の制御が困難となり過剰のＦが含浸される。透明ガラス化処理後にＦ化合物ガスで処理しても、所望の濃度以下のＦしか含浸できない。従って、Ｆ化合物ガスによる熱処理は、仮焼処理後に行う必要がある。
【００１８】
真空紫外光照射に悪影響を及ぼす酸素過剰型欠陥生成を抑制するため、通常Ｈ_２過剰の条件でスート合成が行われる。この時、Ｈ_２の一部は石英ガラス中に取り込まれる。石英ガラス中のＨ_２は熱処理や真空紫外光照射等により生じた欠陥と反応して、≡Ｓｉ−Ｈ結合を生成する。≡Ｓｉ−Ｈ結合は石英ガラスの網目構造に歪みを生じさせるため、均質性に悪影響を及ぼす。さらに、≡Ｓｉ−Ｈ結合は通常の≡Ｓｉ−Ｏ−Ｓｉ≡結合と比べて結合エネルギーが小さく、そのため真空紫外光照射により比較的容易に解裂し欠陥生成の原因になると考えられる。また、Ｈ_２過剰になるとその還元作用により、酸素欠乏型欠陥≡Ｓｉ−Ｓｉ≡が生成し、真空紫外光照射特性が悪化する。このため、Ｈ_２及び≡Ｓｉ−Ｈ結合の含有量は少ない方が好ましい。Ｈ_２及び≡Ｓｉ−Ｈ濃度が１×１０^１７個／ｃｍ^３以下であれば、真空紫外光照射特性の良好な石英ガラスが得られる事が分かった。
【００１９】
金属不純物（アルカリ金属、アルカリ土類金属、遷移金属、その他の金属）は真空紫外域に吸収を示すため、その含有量は出来るだけ少ない方が望ましい。金属不純物含有量が多くなると、真空紫外域での透過率及び均質性が低下するだけでなく、真空紫外光照射により金属不純物に起因する欠陥が生成し、さらに透過率及び均質性を悪化させる原因となる。透過率および均質性に影響する金属不純物とその含有量について検討した。その結果、Ｌｉ，Ｎａ，Ｋなどのアルカリ金属、Ｍｇ，Ｃａなどのアルカリ土類金属、Ｔｉ，Ｃｒ，Ｆｅ，Ｎｉ，Ｃｕ，Ｚｒ，Ｍｏなどの遷移金属、その他Ａｌなどの金属が真空紫外光照射特性に悪影響を及ぼす事が分った。その含有量と真空紫外特性との関係から、金属不純物の総量が３０ｐｐｂ以下のときに高い性能が得られる事が分かった。
【００２０】
合成石英ガラスでは一般的に、取り扱いやすさ、価格の点などから原料にＳｉＣｌ_４が用いられる。このため、合成された石英ガラスには、Ｃｌが残存する可能性があり、このＣｌはガラス内部で直接Ｓｉと結合して≡Ｓｉ−Ｃｌの形態で存在していると考えられる。この≡Ｓｉ−Ｃｌ結合は、真空紫外光照射により容易に解裂し欠陥の原因となるため、Ｃｌ含有量は出来るだけ低い方が好ましい。Ｃｌ含有量が１０ｐｐｍ以下であれば、真空紫外光用合成石英ガラスとして十分満足出来る性能が得られる。
【００２１】
次に本発明の合成石英ガラスの製造方法について説明する。本発明の真空紫外光用合成石英ガラスの製造方法は、運転操作性、生産性、品質安定性、コスト等の点からスート法が好ましい。以下、スート法について具体的に説明する。
【００２２】
スート法では、例えば、多重管構造の石英ガラス製バーナーの中心からＳｉＣｌ_４などの原料を供給し、その外側の管からＨ_２およびＯ_２を供給して原料を火炎加水分解してシリカ微粒子（スート）を合成する。このスートは多量のＯＨ基を含有するため、第１の熱処理として、１１００℃〜１３５０℃の温度範囲で、還元性ガス雰囲気で処理を行い、ＯＨ基濃度を適切な範囲まで低減させる。第１の熱処理の温度が１３５０℃より高いと、ガラス化が起こり、ＯＨ基濃度の低減が不充分となる。逆に温度が１１００℃以下であると脱ＯＨ基速度が遅く、処理時間が長くなるため実用的でない。第１の熱処理を行った後、２００℃〜７００℃、０．０１〜１ｖｏｌ％Ｆ化合物ガス雰囲気で、第１の熱処理により生成した欠陥修復のための第２の熱処理を行う。この処理を行う際の、Ｆ化合物ガスの濃度と処理温度の制御が重要である。Ｆ化合物ガス濃度が０．０１％より低いと、Ｆ濃度が１０ｐｐｍより低くなるため欠陥の修復が完全に行われない。濃度が１％より高いとＦ元素が過剰になり、石英ガラスの構造である≡Ｓｉ−Ｏ−Ｓｉ≡結合との反応が起こり新たな欠陥が生成する。処理温度が、２００℃より低いと反応が進行しない。温度が７００℃より高いと、Ｆ元素の反応性が高まり、≡Ｓｉ−Ｏ−Ｓｉ≡との反応が起こるため新たな欠陥が生成する。Ｆ化合物ガスとしては、取り扱いの点等から、ＳｉＦ_４等が挙げられるが、Ｆ元素を含むガスであれば特に限定しない。続いて、１３５０℃〜１５５０℃、Ｏ_２含有雰囲気で第３の熱処理を行い、透明ガラス化する。第３の熱処理を、Ｏ_２含有雰囲気で行うのは、この熱処理で酸素欠乏型欠陥が生成するのを抑制するためである。温度が１３５０℃より低いと透明ガラス化が起こらない。１５５０℃より高温だと、ガラス化速度が速過ぎるため、構造に歪み及び欠陥が生成してしまう。
【００２３】
原料は、取り扱いおよび入手が容易で、かつ安価であるなどの点からＳｉＣｌ_４が望ましい。しかし、本発明は特にこれに限定されるものではなく、原料中にＣｌを含有していれば、ＳｉＣｌ_４以外の原料を用いても良い。原料にＳｉＣｌ_４などのＣｌ含有ケイ素化合物を使用する事で、特別な処理を行う事なく金属不純物含有量を３０ｐｐｂ以下にする事ができる。
【００２４】
原料にＳｉＣｌ_４の様なＣｌ含有物を用いた場合、スート中にＣｌが残留するが、この残留したＣｌは、第１の熱処理の際、ＯＨ基と共に除去されるため特別な処理を行う事なく、Ｃｌ濃度を１０ｐｐｍ以下にする事ができる。
【００２５】
以上記述した条件で合成石英ガラスを製造すれば、原料に高価で副生物の処理設備が必要なＦ化合物を使用したり、Ｈ_２濃度を高めるための特別な処理設備を設置する必要がないため、汎用的な製造方法、製造設備により、安価で優れた真空紫外光照射特性を有する、真空紫外光用合成石英ガラスを得る事が可能である。
【００２６】
このようにして合成した石英ガラスを、所定の形状に加工、研磨して真空紫外光用マスク基板として使用した場合、優れた均質性及び紫外線光照射特性を示し、真空紫外光用マスク基板としての使用に特に適している。
【００２７】
【実施例】
以下の実施例により本発明を具体的に説明するが、本発明はこれら実施例に何等限定されるものではない。尚、評価は以下の方法によった。
【００２８】
１６３ｎｍの透過率は、真空紫外分光光度計を用いて測定した。
【００２９】
各試料の含有成分の定量方法は以下の通りである。
【００３０】
ＯＨ基含有量は約２．７μｍの吸収からＩＲ測定法により定量した。仮想温度はＩＲスペクトルの約２２６０ｃｍ^−１にあらわれる吸収の位置から計算により求めた。
【００３１】
Ｈ_２含及び≡Ｓｉ−Ｈ含有量は、ラマン分光測定法で定量した。Ｈ_２及び≡Ｓｉ−Ｈ対応するピークは、それぞれ約４１５０ｃｍ^−１及び約２２５０ｃｍ^−１にあらわれ、このピークの面積強度と石英ガラスの基本構造による約８００ｃｍ^−１のピークの面積強度との比からＨ_２及び≡Ｓｉ−Ｈ含有量を算出した。
【００３２】
Ｆ及びＣｌ含有量は、得られた石英ガラスをアルカリ溶融してイオンクロマト法で求めた。
【００３３】
金属不純物含有量はＩＣＰ質量分析法で求めた。
【００３４】
実施例１〜４
原料にＳｉＣｌ_４を使用して、スート法により合成石英ガラスインゴットを製造した。石英ガラス製バーナーの中心管から原料を供給し、バーナーの外管からＨ_２およびＯ_２を供給してスートを合成した。このスートを１ｖｏｌ％（容積％）Ｈ_２ガス雰囲気、１２００℃で５時間熱処理（第１の熱処理）して脱ＯＨ基処理を行った。その後、０．１％ＳｉＦ_４雰囲気、５００℃で２時間熱処理（第２の熱処理）し、最後に、２０％Ｏ_２雰囲気、１４００℃で５時間熱処理（第３の熱処理）して透明石英ガラスインゴットを得た。このインゴットからテストピースを切り出し、実施例１の評価用試料とした。
【００３５】
実施例２、３及び４の試料も実施例１の試料と同様にして作製した。実施例１〜４の試料の製造条件を表１に示す。
【００３６】
【表１】

表２に各試料の評価結果の一覧（ＯＨ基濃度、Ｆ濃度、Ｈ_２濃度、≡Ｓｉ−Ｈ濃度、Ｃｌ濃度、金属不純物濃度、透過率、屈折率分布、複屈折量）を示す。
【００３７】
【表２】

表２に示すように、本発明の範囲の合成石英ガラスである実施例の試料は、真空紫外光用光学材料として優れた性能を持つ合成石英ガラスであった。
【００３８】
さらに、実施例１で作製したインゴットの一部をさらに、４００℃、１００％Ｈ_２雰囲気で処理して、試料を作製した。この試料のＯＨ基濃度は１２ｐｐｍ、Ｆ濃度は２５ｐｐｍ、Ｈ_２濃度は３．２×１０^１７個／ｃｍ^３、≡Ｓｉ−Ｈ濃度は２．５×１０^１７個／ｃｍ^３、Ｃｌ濃度は＜１０ｐｐｍ、金属不純物濃度２０ｐｐｂ、透過率は６８％、屈折率分布は３．８×１０^−６、複屈折量は６．５ｎｍ／ｃｍであり、真空紫外光用光学材料として適さないものであった。
【００３９】
また、実施例２で作製したインゴットの一部をさらにＯＨ基雰囲気で処理して、試料を作製した。この試料のＯＨ基濃度は２６ｐｐｍ、Ｆ濃度は１５ｐｐｍ、Ｈ_２濃度は＜１×１０^１７個／ｃｍ^３、≡Ｓｉ−Ｈ濃度は＜１×１０^１７個／ｃｍ^３、Ｃｌ濃度は＜１０ｐｐｍ、金属不純物濃度２５ｐｐｂ、透過率は６３％、屈折率分布は４．５×１０^−６、複屈折量は３．９ｎｍ／ｃｍであり、真空紫外光用光学材料として適さないものであった。
【００４０】
比較例１
第１の熱処理を１０００℃で行う以外は、実施例１と同様な条件で合成した石英ガラスインゴットからテストピースを切り出し、比較例１の試料とした。比較例１のＯＨ基含有量は、３５ｐｐｍであった。この試料の、初期透過率は５３％と低く、真空紫外光用光学材料として適さないものであった。
【００４１】
比較例２
第１の熱処理を１４５０℃とする以外は、実施例１と同様な条件で合成した石英ガラスインゴットからテストピースを切り出し、比較例２の試料とした。比較例２の条件では透明な石英ガラスが得られなかった。
【００４２】
比較例３
第２の熱処理を５％ＳｉＦ_４ガス雰囲気で行う以外は、実施例１と同様な条件で合成した石英ガラスインゴットからテストピースを切り出し、比較例３の試料とした。この試料のＦ含有量は５５ｐｐｍであった。この試料の複屈折料は、６．８ｎｍ／ｃｍと大きく、真空紫外光用光学材料として適さないものであった。
【００４３】
比較例４
第２の熱処理を１００℃で行う以外は、実施例１と同様な条件で合成した石英ガラスインゴットからテストピースを切り出し、比較例４の試料とした。この試料のＦ濃度は１０ｐｐｍ未満であった。この試料の透過率は６１％と低く、真空紫外光用光学材料として適さないものであった。
【００４４】
比較例５
第２の熱処理を８００℃で行う以外は、実施例１と同様な条件で合成した石英ガラスインゴットからテストピースを切り出し、比較例４の試料とした。この試料の透過率は７７％と高透過率であったが、Ｆ濃度が特許請求の範囲外であるため、屈折率分布及び複屈折量が大きく均質性に劣り、真空紫外光用光学材料として適さないものであった。
【００４５】
比較例６
第３の熱処理を１３００℃で行う以外は、実施例１と同様な条件で合成した。比較例６の条件では透明な石英ガラスが得られなかった。
【００４６】
比較例７
第３の熱処理を１６００℃で行う以外は、実施例１と同様な条件で合成した。比較例７の条件では透明な石英ガラスが得られなかった。
【００４７】
【発明の効果】
本発明によれば、安価でかつ均質性に優れた真空紫外光用合成石英ガラス及びこれを用いた真空紫外光用マスク基板の提供が可能となった。
【００４８】
本発明の方法によれば、石英ガラス中のＯＨ基、Ｆ、Ｈ_２及び≡Ｓｉ−Ｈの各濃度を制御する事で構造を安定化するため、安価で取り扱いの容易なＳｉＣｌ_４などのＣｌを含有した原料の使用が可能である。さらに、Ｃｌには金属不純物除去効果があるため金属不純物濃度低減のための特別な処理が不要となり製造コストが削減出来る。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a synthetic quartz glass for vacuum ultraviolet light having excellent transmittance and homogeneity for irradiation with vacuum ultraviolet light having a wavelength of 200 nm or less, a method for producing the same, and a mask substrate for vacuum ultraviolet light obtained by processing the same. It is.
[0002]
[Prior art]
In recent years, with increasing integration of LSIs, integrated circuit patterns have been steadily miniaturized, and mass production of super LSIs on which ultrafine patterns of 0.25 μm or less are drawn has begun. In order to obtain such an ultra-fine pattern, it is necessary to shorten the wavelength of an exposure light source for writing the pattern. A stepper using an excimer laser beam as a light source has been developed, and a KrF excimer laser beam (wavelength: 248 nm) has already been used as a light source. stepper to is mass-produced, shorter wavelength ArF excimer laser light (wavelength 193 nm) or the stepper as a light source of vacuum ultraviolet light such as F ₂ excimer laser light (wavelength 157 nm) have attracted attention.
[0003]
Synthetic quartz glass and fluorite are examples of optical materials that exhibit sufficient transmittance and homogeneity even in the vacuum ultraviolet region. Among them, synthetic quartz glass produced using a high-purity silicon compound as a raw material is 200 nm or less. Because of its high transmittance and homogeneity even in the vacuum ultraviolet region, it is widely used as an optical material in a lithography process using vacuum ultraviolet light as a light source.
[0004]
However, the conventional synthetic silica glass is irradiated with short-wavelength light in the vacuum ultraviolet region, structural defects are induced, a problem that transmittance and homogeneity is reduced, in particular ultra LSI as a light source an F ₂ excimer laser There have been problems with optical materials used for lithography.
[0005]
As one of means for improving the resistance to irradiation with vacuum ultraviolet light, for example, JP-A-7-43891 or JP-A-9-124337 discloses a method of increasing the H ₂ (hydrogen molecule) content in a synthetic quartz glass to reduce the vacuum. A method has been proposed in which defects caused by ultraviolet irradiation are repaired by H (hydrogen atoms) to improve irradiation resistance. However, the homogeneity is deteriorated by the ≡Si—H bond repaired by H. In addition, there is a problem that a new defect such as a so-called E ′ center (≡Si.) Is generated by irradiating the ≡Si—H bond with vacuum ultraviolet light. Further, there is a problem that the production cost is increased, for example, a step for impregnating with H ₂ is required.
[0006]
Generally, silicon tetrachloride (SiCl ₄ ) is used as a raw material of synthetic quartz glass. In this case, Cl remains in the synthetic quartz glass in the form of ≡Si—Cl, which adversely affects transmittance and homogeneity. It has been known. To solve this problem, for example, Japanese Patent Application Laid-Open No. 8-31723 proposes a method using a Cl-free raw material such as alkoxysilane. In this method, the raw material cost is higher than when SiCl ₄ is used, and there is a problem that C in the raw material remains in the synthetic quartz glass and adversely affects the resistance to irradiation with vacuum ultraviolet light. It is known that metal impurities significantly lower the transmittance and the homogeneity, but Cl has the function of removing metal impurities present in the synthesis atmosphere as chlorides out of the system. Therefore, when a raw material containing no Cl is used, metal impurities increase, resulting in a decrease in transmittance and homogeneity.
[0007]
In recent years, as a method for obtaining a synthetic quartz glass having excellent resistance to vacuum ultraviolet light irradiation, as disclosed in JP-A-8-67530 and JP-A-8-75901, a high concentration of F is contained. Synthetic quartz glass has been proposed. In this method, defects generated during manufacturing and defects generated by irradiation with vacuum ultraviolet light are repaired in the form of ≡Si-F having a larger binding energy than エネルギー Si-H or the like. No new defects are generated even when irradiated with light, and it is said to be excellent in vacuum ultraviolet light irradiation resistance. However, if a ≡Si—F bond is present in the quartz glass, the structure of the quartz glass is distorted, which adversely affects the homogeneity. Further, when heat treatment such as annealing and molding is performed, F is liberated to generate a new structural defect, and thus there has been a problem that transmittance and homogeneity deteriorate.
[0008]
As a method of obtaining the synthetic quartz glass containing F, a method of impregnating synthetic quartz glass produced by a general method using SiCl ₄ or the like with F later (Japanese Patent Laid-Open No. 13-19450) and a method of obtaining SiF _For example, there is a method of producing a synthetic quartz glass using an F-containing compound such as No. ₄ as a raw material (JP-A No. 12-264671). However, in the case of industrialization, all methods use high concentrations of F compounds, which complicates the operation and increases the danger. In addition, the cost of raw materials is high, and the effect of by-product hydrofluoric acid on the human body and the environment is reduced. There were also problems such as the need for various countermeasures from the viewpoint of increased manufacturing costs.
[0009]
[Problems to be solved by the invention]
As described above, the basic structure and manufacturing method of synthetic quartz glass have been greatly improved, such as post-treatments such as H ₂ and F impregnation, and the use of alkoxysilanes and F compounds that do not contain Cl as raw materials. The development was aimed at improving the resistance to vacuum ultraviolet light irradiation by changing to. As a result, although a certain effect was seen in improving the transmittance, there was a problem in the homogeneity of the refractive index distribution, the amount of birefringence, and the like. In addition, new problems have arisen in terms of workability and manufacturing cost, such as the use of special and expensive raw materials, the necessity of large-scale modification of manufacturing equipment, and the complicated manufacturing process. For mass production of VLSI, development of optical materials for vacuum ultraviolet light is indispensable, and there is a strong demand for high-quality and economical optical materials with excellent vacuum ultraviolet light irradiation characteristics and economical manufacturing methods. .
[0010]
The present invention uses existing general-purpose equipment without excessively and unnecessarily modifying synthetic quartz glass synthesized by a soot method (VAD method) using a glass-forming raw material such as SiCl _4. To provide a synthetic quartz glass for vacuum ultraviolet light which is inexpensive and excellent in homogeneity and a method for producing the same, and further to provide a mask substrate for vacuum ultraviolet light using the synthetic quartz glass for vacuum ultraviolet light. .
[0011]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on the correlation between various physical properties of synthetic quartz glass and the transmittance and homogeneity in vacuum ultraviolet light irradiation in order to solve the above-described problems. The content and the F element content are particularly important for transmittance and homogeneity, and by limiting each value to a specific range, a synthetic quartz glass for vacuum ultraviolet light excellent in transmittance and homogeneity can be obtained. I found that I can get it. Furthermore, they found that when this synthetic quartz glass was used as a mask substrate for vacuum ultraviolet light, they showed particularly excellent performance, and completed the present invention.
[0012]
That is, in the present invention, the transmittance at a wavelength of 163 nm is 70% or more per 10 mm thickness, and the difference (refractive index distribution) between the maximum refractive index and the minimum refractive index in a region of 200 mm diameter is 5 × 10 ⁻⁶ or less per 10 mm thickness. Further, it is a synthetic quartz glass for vacuum ultraviolet light having a birefringence of 5 nm / cm or less. Further, among them, the synthetic quartz glass for vacuum ultraviolet light is characterized in that the OH group content is 1 to 20 ppm and the F element content is 10 to 30 ppm.
[0013]
Further, in a so-called soot method in which silica fine particles obtained by flame hydrolysis of a glass-forming raw material in an oxyhydrogen flame are deposited on a target to synthesize a soot body, and then vitrified into a transparent glass by a subsequent heat treatment, the soot body is subjected to heat treatment. Is performed in a reducing gas atmosphere, and then heat-treated in an F-compound gas atmosphere, and finally a vitrification treatment is performed in an O ₂ -containing atmosphere. The manufacturing method and the use of using the synthetic quartz glass for vacuum ultraviolet light as the mask substrate for vacuum ultraviolet light are also included in the scope of the present invention.
[0014]
Hereinafter, the present invention will be described in detail.
[0015]
Since OH groups show absorption in the vacuum ultraviolet region, the transmittance in the vacuum ultraviolet region increases as the OH group content decreases. Therefore, the OH group concentration needs to be 20 ppm or less. However, since the OH group has an effect of stabilizing the network structure of the quartz glass formed of a series of SiO ₄ tetrahedrons, the structure of the quartz glass becomes unstable when the OH group concentration is less than 1 ppm. In some cases, defects are easily generated by heat treatment or the like, and the transmittance and the homogeneity may be deteriorated. For this reason, it is preferable that quartz glass contains 1 ppm or more of OH groups.
[0016]
Silica fine particles synthesized in an oxyhydrogen flame usually contain about several thousand ppm of OH groups. Since the OH group shows absorption in the vacuum ultraviolet region, the soot body is heat-treated in a reducing gas atmosphere such as H ₂ gas or F compound gas to reduce the OH group concentration. However, when the OH group treatment is performed with an F compound gas, the reactivity of the F element with the quartz glass is high, so that it is difficult to control the OH group concentration, and excess F forms quartz glass. It reacts with O atoms of Si≡ bonds to generate oxygen-deficient defects of {Si—Si}. Since this defect shows absorption in a vacuum ultraviolet region centered on 163 nm, the transmittance is reduced in the vacuum ultraviolet region. Also, it was found that the homogeneity deteriorated because the bond was distorted due to the oxygen-deficient defect. Therefore, it is preferable to perform the deOH-group treatment of the soot body with a reducing gas other than the F compound such as H ₂ gas.
[0017]
Even though the quartz glass treated with a reducing gas other than the F compound has a much lower concentration than that treated with the F compound gas, oxygen deficiency defects and the like are generated by the deOH group treatment, and the vacuum ultraviolet As the transmittance of the region decreases, the homogeneity may also deteriorate. This minute amount of defects generated by the deOH group treatment can be repaired in the form of a ≡Si—F bond by performing a heat treatment in a gas atmosphere containing an F element after the deOH group treatment. This bond shows no absorption for vacuum ultraviolet light irradiation. However, as described above, in the treatment with the F element-containing gas for repairing a defect, if excessive F is taken into quartz glass, it reacts with {Si-OH or {Si-O-Si} to form a new gas. Create a defect. In addition, since the ≡Si—F bond causes a strain in the structure, the homogeneity is deteriorated when it is present in a large amount. Therefore, the F concentration contained in the quartz glass is preferably 10 to 30 ppm. In order to adjust the F concentration to a desired range, it is necessary to perform the treatment in a 200 ° C. to 700 ° C., 0.01 to 1 vol% F compound gas-containing atmosphere after the deOH group treatment. If the treatment with the F compound gas is performed before the deOH group treatment, it reacts with a large amount of the OH group contained in the soot body, so that the control of the F concentration becomes difficult and the excess F is impregnated. Even if it is treated with an F compound gas after the transparent vitrification treatment, only F having a desired concentration or less can be impregnated. Therefore, the heat treatment with the F compound gas needs to be performed after the calcination treatment.
[0018]
To suppress adverse effects oxygen excess type defects generated in the vacuum ultraviolet light irradiation, soot synthesis is usually carried out in H ₂ excess condition. At this time, part of H ₂ is taken into the quartz glass. H ₂ in the quartz glass reacts with a defect generated by heat treatment, vacuum ultraviolet light irradiation, or the like to generate a ≡Si—H bond. The ≡Si—H bond causes a distortion in the network structure of quartz glass, which adversely affects homogeneity. Furthermore, the ≡Si—H bond has a smaller binding energy than the normal ≡Si—O—Si≡ bond, and therefore it is considered that the 解 Si—H bond is relatively easily cleaved by irradiation with vacuum ultraviolet light to cause defect generation. Further, when H ₂ becomes excessive, an oxygen-deficient defect {Si—Si} is generated due to the reducing action, and the vacuum ultraviolet light irradiation characteristics deteriorate. For this reason, it is preferable that the contents of H ₂ and ≡Si—H bonds are small. It was found that when the concentration of H ₂ and −Si—H was 1 × 10 ¹⁷ / cm ³ or less, quartz glass having good vacuum ultraviolet light irradiation characteristics could be obtained.
[0019]
Metal impurities (alkali metals, alkaline earth metals, transition metals, and other metals) exhibit absorption in the vacuum ultraviolet region, so that their content is preferably as small as possible. When the content of metal impurities increases, not only does the transmittance and homogeneity in the vacuum ultraviolet region decrease, but defects due to metal impurities are generated by irradiation with vacuum ultraviolet light, which further deteriorates the transmittance and homogeneity. It becomes. Metal impurities affecting the transmittance and homogeneity and their contents were investigated. As a result, alkali metals such as Li, Na and K, alkaline earth metals such as Mg and Ca, transition metals such as Ti, Cr, Fe, Ni, Cu, Zr and Mo, and other metals such as Al are vacuum ultraviolet light. It was found that the irradiation characteristics were adversely affected. From the relationship between the content and the vacuum ultraviolet characteristics, it was found that high performance was obtained when the total amount of metal impurities was 30 ppb or less.
[0020]
In general, synthetic quartz glass uses SiCl ₄ as a raw material in terms of ease of handling, cost, and the like. For this reason, Cl may remain in the synthesized quartz glass, and it is considered that this Cl is directly bonded to Si inside the glass and exists in the form of ≡Si—Cl. Since the ≡Si—Cl bond is easily cleaved by irradiation with vacuum ultraviolet light and causes defects, the Cl content is preferably as low as possible. When the Cl content is 10 ppm or less, sufficiently satisfactory performance as a synthetic quartz glass for vacuum ultraviolet light can be obtained.
[0021]
Next, a method for producing the synthetic quartz glass of the present invention will be described. The soot method is preferred as the method for producing the synthetic quartz glass for vacuum ultraviolet light of the present invention from the viewpoints of operability, productivity, quality stability, cost, and the like. Hereinafter, the soot method will be specifically described.
[0022]
In the soot method, for example, a raw material such as SiCl ₄ is supplied from the center of a quartz glass burner having a multi-tube structure, and H ₂ and O ₂ are supplied from a tube outside the burner to flame-hydrolyze the raw material to produce silica fine particles ( Soot). Since this soot contains a large amount of OH groups, the first heat treatment is performed in a reducing gas atmosphere in a temperature range of 1100 ° C. to 1350 ° C. to reduce the OH group concentration to an appropriate range. If the temperature of the first heat treatment is higher than 1350 ° C., vitrification occurs, and the reduction of the OH group concentration becomes insufficient. On the other hand, if the temperature is 1100 ° C. or lower, the rate of deOH group is low and the processing time is long, which is not practical. After the first heat treatment, a second heat treatment for repairing a defect generated by the first heat treatment is performed in a 200 to 700 ° C., 0.01 to 1 vol% F compound gas atmosphere. When performing this treatment, it is important to control the concentration of the F compound gas and the treatment temperature. When the concentration of the F compound gas is lower than 0.01%, the F concentration is lower than 10 ppm, so that the defect cannot be completely repaired. When the concentration is higher than 1%, the F element becomes excessive, and a reaction with the {Si—O—Si} bond, which is a structure of quartz glass, occurs, and a new defect is generated. If the treatment temperature is lower than 200 ° C., the reaction does not proceed. When the temperature is higher than 700 ° C., the reactivity of the F element increases, and a reaction with {Si—O—Si} occurs, so that a new defect is generated. As the F compound gas, SiF _{4 and the} like are cited in terms of handling and the like, but there is no particular limitation as long as the gas contains the F element. Subsequently, a third heat treatment is performed at 1350 ° C. to 1550 ° C. in an O ₂ -containing atmosphere to form a transparent glass. The third heat treatment is performed in an O ₂ -containing atmosphere in order to suppress generation of oxygen-deficient defects due to the heat treatment. When the temperature is lower than 1350 ° C., vitrification does not occur. If the temperature is higher than 1550 ° C., the vitrification rate is too high, and distortion and defects are generated in the structure.
[0023]
The raw material is desirably SiCl ₄ because it is easy to handle and obtain and is inexpensive. However, the present invention is not particularly limited to this, and a raw material other than SiCl ₄ may be used as long as the raw material contains Cl. By using a Cl-containing silicon compound such as SiCl ₄ as a raw material, the metal impurity content can be reduced to 30 ppb or less without special treatment.
[0024]
When a Cl-containing material such as SiCl ₄ is used as a raw material, Cl remains in the soot, and the remaining Cl is removed together with OH groups during the first heat treatment, so that special treatment must be performed. In addition, the Cl concentration can be reduced to 10 ppm or less.
[0025]
If synthetic quartz glass is manufactured under the above-described conditions, it is not necessary to use an expensive F compound as a raw material, which requires processing equipment for by-products, or to install special processing equipment for increasing the H ₂ concentration. By using general-purpose manufacturing methods and manufacturing equipment, it is possible to obtain a synthetic quartz glass for vacuum ultraviolet light having excellent vacuum ultraviolet light irradiation characteristics at low cost.
[0026]
When the quartz glass synthesized in this manner is processed and polished into a predetermined shape and used as a mask substrate for vacuum ultraviolet light, it exhibits excellent uniformity and ultraviolet light irradiation characteristics, and is used as a mask substrate for vacuum ultraviolet light. Particularly suitable for use.
[0027]
【Example】
The present invention will be specifically described with reference to the following examples, but the present invention is not limited to these examples. In addition, evaluation was based on the following method.
[0028]
The transmittance at 163 nm was measured using a vacuum ultraviolet spectrophotometer.
[0029]
The method of quantifying the components contained in each sample is as follows.
[0030]
The OH group content was determined by IR measurement from an absorption of about 2.7 μm. The fictive temperature was calculated from the position of the absorption appearing at about 2260 cm ^-1 in the IR spectrum.
[0031]
H ₂ content and ≡Si-H content were quantified by Raman spectrometry. The peaks corresponding to H ₂ and ≡Si—H appear at about 4150 cm ⁻¹ and about 2250 cm ⁻¹ , respectively. From the ratio of the area intensity of this peak to the area intensity of about 800 cm ⁻¹ due to the basic structure of quartz glass, It was calculated H ₂ and ≡Si-H content.
[0032]
The content of F and Cl was determined by ion chromatography after the obtained quartz glass was alkali-melted.
[0033]
The metal impurity content was determined by ICP mass spectrometry.
[0034]
Examples 1-4
A synthetic quartz glass ingot was manufactured by a soot method using SiCl ₄ as a raw material. The raw material was supplied from the central tube of the quartz glass burner, and H ₂ and O ₂ were supplied from the outer tube of the burner to synthesize soot. This soot was subjected to a heat treatment (first heat treatment) at 1200 ° C. for 5 hours in a 1 vol% (volume%) H ₂ gas atmosphere to perform an OH removal treatment. Thereafter, a heat treatment is performed for 2 hours at 500 ° C. in a 0.1% SiF ₄ atmosphere (second heat treatment), and finally, a heat treatment is performed for 5 hours at 1400 ° C. in a 20% O ₂ atmosphere (third heat treatment). Got an ingot. A test piece was cut out from the ingot and used as a sample for evaluation in Example 1.
[0035]
The samples of Examples 2, 3 and 4 were produced in the same manner as the sample of Example 1. Table 1 shows the manufacturing conditions of the samples of Examples 1 to 4.
[0036]
[Table 1]

List of evaluation results of each sample in Table 2 shows (OH group concentration, F concentration, _{H 2} concentration, ≡Si-H concentration, Cl concentration, the metal impurity concentration, transmittance, refractive index distribution, the birefringence amount).
[0037]
[Table 2]

As shown in Table 2, the sample of the example which was a synthetic quartz glass within the scope of the present invention was a synthetic quartz glass having excellent performance as an optical material for vacuum ultraviolet light.
[0038]
Further, a part of the ingot produced in Example 1 was further treated at 400 ° C. in a 100% H ₂ atmosphere to produce a sample. The OH group concentration of this sample is 12 ppm, the F concentration is 25 ppm, the H ₂ concentration is 3.2 × 10 ¹⁷ / cm ³ , the ≡Si—H concentration is 2.5 × 10 ¹⁷ / cm ³ , and the Cl concentration is < The content was 10 ppm, the metal impurity concentration was 20 ppb, the transmittance was 68%, the refractive index distribution was 3.8 × 10 ⁻⁶ , and the birefringence was 6.5 nm / cm, which was not suitable as an optical material for vacuum ultraviolet light. .
[0039]
Further, a part of the ingot produced in Example 2 was further treated in an OH-based atmosphere to produce a sample. OH group concentration of the sample 26 ppm, F concentration is 15 ppm, _{H 2} concentration is <1 × ^{10 17} atoms / cm ^3, ≡Si-H concentration is <1 × ^{10 17} atoms / ^cm 3, Cl concentration <10 ppm, The metal impurity concentration was 25 ppb, the transmittance was 63%, the refractive index distribution was 4.5 × 10 ⁻⁶ , and the birefringence was 3.9 nm / cm, which was not suitable as an optical material for vacuum ultraviolet light.
[0040]
Comparative Example 1
A test piece was cut out from a quartz glass ingot synthesized under the same conditions as in Example 1 except that the first heat treatment was performed at 1000 ° C., to obtain a sample of Comparative Example 1. The OH group content of Comparative Example 1 was 35 ppm. The initial transmittance of this sample was as low as 53%, and was not suitable as an optical material for vacuum ultraviolet light.
[0041]
Comparative Example 2
A test piece was cut out from a quartz glass ingot synthesized under the same conditions as in Example 1 except that the first heat treatment was performed at 1450 ° C., to obtain a sample of Comparative Example 2. Under the conditions of Comparative Example 2, transparent quartz glass was not obtained.
[0042]
Comparative Example 3
A test piece was cut out from a quartz glass ingot synthesized under the same conditions as in Example 1 except that the second heat treatment was performed in a 5% SiF ₄ gas atmosphere to obtain a sample of Comparative Example 3. The F content of this sample was 55 ppm. The birefringent of this sample was as large as 6.8 nm / cm, and was not suitable as an optical material for vacuum ultraviolet light.
[0043]
Comparative Example 4
A test piece was cut out from a quartz glass ingot synthesized under the same conditions as in Example 1 except that the second heat treatment was performed at 100 ° C., to obtain a sample of Comparative Example 4. The F concentration of this sample was less than 10 ppm. The transmittance of this sample was as low as 61%, and was not suitable as an optical material for vacuum ultraviolet light.
[0044]
Comparative Example 5
A test piece was cut out from a quartz glass ingot synthesized under the same conditions as in Example 1 except that the second heat treatment was performed at 800 ° C., to obtain a sample of Comparative Example 4. Although the transmittance of this sample was as high as 77%, the F concentration was out of the range of claims, so that the refractive index distribution and the amount of birefringence were large and the homogeneity was inferior, and as an optical material for vacuum ultraviolet light, It was not suitable.
[0045]
Comparative Example 6
Synthesis was performed under the same conditions as in Example 1 except that the third heat treatment was performed at 1300 ° C. Under the conditions of Comparative Example 6, no transparent quartz glass was obtained.
[0046]
Comparative Example 7
Synthesis was performed under the same conditions as in Example 1 except that the third heat treatment was performed at 1600 ° C. Under the conditions of Comparative Example 7, transparent quartz glass was not obtained.
[0047]
【The invention's effect】
According to the present invention, it has become possible to provide a synthetic quartz glass for vacuum ultraviolet light which is inexpensive and excellent in homogeneity and a mask substrate for vacuum ultraviolet light using the same.
[0048]
According to the method of the present invention, since the structure is stabilized by controlling the concentrations of OH groups, F, H ₂ and ≡Si—H in quartz glass, Cl such as SiCl _{4 is} inexpensive and easy to handle. It is possible to use a raw material containing. Further, since Cl has an effect of removing metal impurities, a special treatment for reducing the metal impurity concentration is not required, and the manufacturing cost can be reduced.

Claims

The transmittance at a wavelength of 163 nm is 70% or more per 10 mm in thickness, and the difference (refractive index distribution) between the maximum refractive index and the minimum refractive index in a region of 200 mm in diameter is 5 × 10 ⁻⁶ or less per 10 mm in thickness, and the amount of birefringence. Is 5 nm / cm or less, synthetic quartz glass for vacuum ultraviolet light.

The synthetic quartz glass for vacuum ultraviolet light according to claim 1, wherein the content of the OH group is 1 to 20 ppm and the content of the F element is 10 to 30 ppm.

The synthetic quartz glass according to claim 1, wherein the H ₂ content is 1 × 10 ¹⁷ pieces / cm ³ or less, and the ΔSi—H content is 1 × 10 ¹⁷ pieces / cm ³ or less. Synthetic quartz glass for vacuum ultraviolet light.

The synthetic quartz glass for vacuum ultraviolet light according to claim 1, wherein the content of Cl is 10 ppm or less and the content of metal impurities is 30 ppb or less.

The silica glass forming raw material is hydrolyzed in an oxyhydrogen flame to synthesize silica fine particles (soot), and the obtained soot is deposited on a target to form a porous silica base material (soot body). The method for producing synthetic quartz glass for vacuum ultraviolet light according to any one of claims 1 to 4, which is a method for producing quartz glass by a so-called soot method, in which a body is heated to form a transparent glass, which comprises the following steps.
(A) Using a Cl-containing compound as a raw material, synthesizing silica fine particles (soot) in an oxyhydrogen flame, depositing the obtained soot to form a soot body, and (b) heating the soot body at a temperature of 1100. Process (calcining) in a reducing gas atmosphere at ~ 1350 ° C (first heat treatment)
(C) a step of treating the obtained calcined body in a temperature of 200 to 700 ° C. and an atmosphere of 0.01 to 1 vol% F compound gas (second heat treatment)
(D) A step of forming the treated calcined body into a transparent vitreous at a temperature of 1350 to 1550 ° C. in an atmosphere containing O ₂ gas (third heat treatment)

The method for producing a synthetic quartz glass for vacuum ultraviolet light according to claim 5, wherein the reducing gas is a gas other than the F compound gas.

A vacuum ultraviolet light mask substrate obtained by processing the synthetic silica glass for vacuum ultraviolet light according to claim 1.