JP2004029458A

JP2004029458A - Projection optical system and stepper

Info

Publication number: JP2004029458A
Application number: JP2002186661A
Authority: JP
Inventors: Kenji Suzuki; 鈴木　健司
Original assignee: Nikon Corp
Current assignee: Nikon Corp
Priority date: 2002-06-26
Filing date: 2002-06-26
Publication date: 2004-01-29

Abstract

<P>PROBLEM TO BE SOLVED: To provide a projection optical system and a stepper in which the number of reflection mirrors constituting the projection optical system is reduced, the correction of aberration is excellent, and NA and an exposure region are large. <P>SOLUTION: First to fourth reflection mirrors 4-7 constituting the projection optical system 1 are constituted so as to have aspherical convex and concave reflection surfaces and the aspherical surface is constituted of the reflection surface indicated by the formula of the aspherical surface of an even-numbered order ≥20th order. Thus, in a coaxial projection optical system constituted of four reflection mirrors, the aberration is reduced, the NA and the exposure region are enlarged and even a fine pattern formed on a first object surface 2 is accurately formed on a second object surface 3. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、感光基板上にレチクルのパターンの縮小像を投影形成する投影光学系、及びこの投影光学系を備えた縮小投影露光装置に関する。
【０００２】
【従来の技術】
半導体用縮小投影露光装置の開口数（以下、「ＮＡ」と呼ぶ。）及び使用波長は、半導体素子の高密度化、対象線幅の細線化に伴って年々大口径化、短波長化する傾向にある。使用する光線の波長は水銀灯のｉ線（波長３６５．０１５ｎｍ）から、ＫｒＦエキシマレーザー（波長２４８ｎｍ）へ移り、ＡｒＦエキシマレーザー（波長１９３ｎｍ）を光源とした縮小投影露光装置も実用化されている。しかし、近年においては、パターンの微細化の要求がさらに強まっており、Ｆ２エキシマレーザー（波長１５７ｎｍ）を経て、さらに波長の短いＥＵＶ光（極端紫外光、波長１３ｎｍ付近）を光源として用いた縮小投影露光装置が次世代の半導体リソグラフィの有力手段として研究されている。
【０００３】
このような縮小投影露光装置に用いられる投影レンズの硝材は、透過率の問題からＦ２エキシマレーザーを光源として使用したものが限界であり、これより波長が短い光源を利用する場合は、反射鏡で構成された反射屈折光学系を用いて縮小投影露光装置を構成する必要がある。また、高解像度を実現するためには、この光学系の収差を良好に保ったままＮＡと露光領域の十分に大きいものを得る必要があり、そのため、反射屈折光学系を構成する反射鏡の枚数が増加する傾向にあった。
【０００４】
【発明が解決しようとする課題】
しかしながら、ＥＵＶ光のような短波長の光線を照射すると反射鏡で反射せず、透過ないし吸収されてしまうため、反射鏡の反射面に反射促進膜を形成してＥＵＶ光を反射をさせなければならないが、現状では十分な反射率を持った反射促進膜が得られていないことから、実際の露光を考えると反射促進膜で露光光の吸収による熱が発生し、光学系の性能が熱変動により悪化する可能性があり、また、反射促進膜での露光光の吸収を考慮して光源の光量を大きくすることが必要となる。このため、反射屈折光学系に多数の反射鏡を用いることができず、結果として収差を補正しつつ、ＮＡと露光領域が十分に大きいものを得ることが難しいという問題があった。
【０００５】
これらの問題を解決するために、反射鏡を光軸に対してシフトして（投影光学系の光軸から反射鏡の光軸を平行に移動する）配置することや、チルトして（投影光学系の光軸と反射鏡の光軸の角度を変える）配置することで、光学系の改善を図る方法が研究されている。しかし、これらの方法は実際の製造において、反射鏡のチルト量やシフト量を正確に計測して光学系を組み立てなければならないため、生産性が悪いという問題があった。また、反射鏡を従来の２０次以下の偶数次非球面の式で表わされる反射面で形成して、この反射鏡を用いた共軸の投影光学系では収差の補正が難しく、そのためＮＡ及び露光領域の確保が難しいという問題もあった。
【０００６】
本発明はこのような問題に鑑みなされたものであり、投影光学系を構成する反射鏡の反射面を従来の光学設計で用いられていたものよりも高次の偶数次の非球面の式を用いて表される形状で形成することにより、収差の補正を良好に行い、ＮＡ及び露光領域の大きな投影光学系及びこの投影光学系を備えた縮小投影露光装置を提供することを目的とする。
【０００７】
【課題を解決するための手段】
前記課題を解決するために本発明に係る投影光学系は、それぞれ所定形状の反射面を有する４枚の反射鏡からなる縮小投影装置を用いて、第１物体面上の物体の縮小像を第２物体面上に投影形成するように構成され、反射鏡が非球面形状の反射面を有し、この非球面形状が次式（１）で表されるように構成する。
【０００８】
【数１】

【０００９】
なお、非球面形状の式（１）において、多項式の次数ｉで表される非球面の次数２ｉが２０次以上であることが好ましい。
【００１０】
また、本発明に係る投影光学系は、露光用照明光源と、照明光学系とを備え、露光用照明光源の円周部の光を、照明光学系で円弧状のスリット光として第１物体面に照射し、縮小投影装置を用いて、第１物体面上の物体の縮小像を第２物体面上に投影形成するように構成され、この投影光学系の開口数が０．１６以上であり、第２物体面上に投影形成される縮小像の領域を形成する円弧の弦の長さが２２ｍｍ以上であり、その縮小像の領域の高さが１．５ｍｍ以上であるように構成されることが好ましい。
【００１１】
本発明に係る縮小投影露光装置は、レチクルに露光光を照射し、レチクルに形成されたパターンの像を投影光学系を介して感光基板上に投影するように構成される。
【００１２】
【発明の実施の形態】
以下、本発明の好ましい実施形態について図面を参照して説明する。まず、図１を用いて本発明に係る投影光学系の構成について説明する。図１は、本発明に係る投影光学系の横断面の光路図であり、光束の幅は横断面のみを表している。投影光学系１は、第１物体面２と第２物体面３の間に配設されており、第１の反射鏡４、第２の反射鏡５、第３の反射鏡６及び第４の反射鏡７で構成されている。第１物体面２を出た光は、第１の反射鏡４、第２の反射鏡５、第３の反射鏡６、第４の反射鏡７の順に反射して第２物体面３上に第１物体面２上の物体の縮小像を投影して結像形成する。ここで、平坦性を維持し、倍率を確保するためには、少なくとも１枚の凸面鏡と１枚の凹面鏡が必要であり、また収差を補正するためには非球面であることが望ましいが、本実施例においては、第１の反射鏡４は非球面の凹面状の反射面を有しており、第２の反射鏡５は非球面の凸面状の反射面を有しており、第３の反射鏡６は非球面の凸面状の反射面を有しており、第４の反射鏡７は非球面の凹面状の反射面を有して構成されている。
【００１３】
また、第１〜４の反射鏡４〜７は投影光学系１の光軸に対して同軸に配設されており、第１の反射鏡４と第３の反射鏡６は反射面が第１物体面を向くように配設されており、また、第２の反射鏡５と第４の反射鏡７は反射面が第２物体面を向くように配設されている。このとき、第１物体面２から第２物体面３に向かって、第４の反射鏡７、第２の反射鏡５、第１の反射鏡４、第３の反射鏡６の順で並んで配設されている。
【００１４】
次に、上述した投影光学系１を用いて構成され、半導体製造工程の一つである光リソグラフィ工程で使用される縮小投影露光装置について、図２を参照して説明する。光リソグラフィ工程で使用される縮小投影露光装置は、原理的には写真製版と同じであり、レチクル（第１物体面）上に精密に描かれたデバイスパターンを、フォトレジストを塗布した半導体ウエハやガラス基板等の感光基板（第２物体面）上に光学的に投影して転写するものである。
【００１５】
この縮小投影露光装置１０は、反射型のレチクル１３に露光用照明光源１１からの光を照明光学系１２を通してスリット状の露光光にして照射し、レチクル１３に形成されたパターンの一部の像を上述した投影光学系１を通して半導体ウエハ１７に投影し、レチクル１３と半導体ウエハ１７とを投影光学系１に対して１次元方向（Ｙ軸方向）に相対走査することによって、レチクル１３のパターンの全体を半導体ウエハ１７上の複数のショット領域の各々にステップ・アンド・スキャン方式で転写するものである。本実施例の露光光としては１３．４ｎｍのＥＵＶ光を使用している。なお、図２においては、投影光学系１の光軸方向をＺ軸とし、このＺ軸と直交する方向であって、レチクル１３及び半導体ウエハ１７の操作方向をＹ軸とし、これらＹＺ軸と直交する紙面垂直方向をＸ軸とする。なお、レチクル１３と半導体ウエハ１７とはＹ軸方向に相対走査すると記述したが、Ｘ軸方向に対しても走査可能であることは言うまでも無い。
【００１６】
ここで、本実施例における露光光の形状は、図３に示すように、露光用照明光源１１からの照明光３０の円周部をスリット状にして利用しており、第２物体面３上に投影結像されて形成される露光領域３０ａとなる。これは照明光３０を反射鏡で反射させた場合、本発明のように共軸な反射光学系では反射鏡による光線の遮光により、光軸付近の領域は使うことが出来ず、光軸から離れた周辺部しか使うことができない。また、このことから収差の補正も周辺部のスリット状の使用領域だけ収差が補正されていれば良い。
【００１７】
レチクル１３は、少なくともＹ軸方向に沿って移動可能なレチクル支持台１４に支持されており、半導体ウエハ１７はＸＹＺ軸方向に沿って移動可能な載置台１８に載置されている。これらのレチクル支持台１４及び載置台１８の移動には、それぞれに接続されたレチクル支持台駆動部１５及び載置台駆動部１９により駆動される。露光動作の際には、照明光学系１２よりレチクル１３に対してＥＵＶ光を照射し、投影光学系１に対してレチクル１３及び半導体ウエハ１７を、投影光学系１の縮小倍率により定まる所定の速度比で移動させる。これにより、半導体ウエハ１７上の所定のショット領域内には、レチクル１３上のパターンの像が走査露光される。
【００１８】
最後に、本発明に係る投影光学系の数値実施例について説明する。投影光学系１の構造としては、上述した図１の通りであり、第２物体面３上に投影形成される露光領域３０ａの形状は図３に示したスリット状をしている。なお、本実施例における第１〜４の反射鏡４〜７の反射面は、上述した式（１）で表され、光軸に対して回転対称な非球面形状をしている。なお、距離ｒを中心接平面のＸＹ軸で表したものは式（２）であり、また、正規化された放射座標ρは式（３）で表される。このとき、式（３）におけるＲは反射鏡の反射面の規格化半径を表している。
【００１９】
【数２】
ｒ^２＝ｘ^２＋ｙ^２　　　　　　　　　　　　　　　　　　　　　　　（２）
【００２０】
【数３】
ρ＝ｙ／Ｒ　　　　　　　　　　　　　　　　　　　　　　　　　　（３）
【００２１】
なお、本実施例における露光光（ＥＵＶ光）の波長は１３．４ｎｍ、非球面の次数が２２０、縮小倍率が１／４倍、露光領域３０ａの幅（露光領域を形成する円弧の弦の長さ）が２２ｍｍで高さが１．５ｍｍ、投影光学系１の像側のＮＡが０．１６となっている。
【００２２】
本実施例の投影光学系１は、微細なパターンを第２物体面上に形成するために、第２物体面上に縮小投影される第１物体面上の物体の像の歪曲収差を５ｎｍ以下に抑えるように設計しており、この条件の下で、投影光学系１を構成する第１〜４の反射鏡４〜７の反射面の非球面形状を表す式（１）の非球面の次数を２２０次とする理由について図４を用いて説明する。本実施例では、第２物体面に投影される第１物体面上の物体の縮小像を精度良く結像するためには、第２物体面上に投影される像の歪曲収差だけでなく波面収差も抑えなければならず、この波面収差の平均自乗根（ＲＭＳ）を３０ｍλより小さくする必要がある（ここで、１ｍλは像を形成する光の１波長の千分の一の長さを表している）。図４に示す通り第１〜４の反射鏡４〜７の反射面の非球面形状を表す式（１）の非球面の次数を大きくするとそれに合わせて波面収差が小さくなり、２２０次以上になると３０ｍλ以下になる。このため本実施例では非球面の次数として２２０を採用している。
【００２３】
表１〜５に、投影光学系１の諸元を示す。表１において、曲率半径には各反射面の近似区曲率半径（単位：ｍｍ）が示されており、中心厚には各面間隔（単位：ｍｍ）が示されている。なお、曲率半径の符号は第１物体面２側に向けて凹となる場合を正としており、中心厚は一つ前の面からの間隔で反射面の前後で符号が逆転するものとする。また、表２〜５には、第１〜４の反射鏡４〜７の非球面データ（第２ｉ次の非球面係数α_ｉ）を示す。
【００２４】
【表１】

【００２５】
【表２】

【００２６】
【表３】

【００２７】
【表４】

【００２８】
【表５】

【００２９】
上述したような構成を有する投影光学系１によって、第１物体面２上の物体の像を第２物体面３上に投影形成することができる。次に、上記諸元の投影光学系１にて図３に示した照明光３０の円周部をスリット状にして投影したときに第２物体面３上に形成される第１物体面２上の物体の縮小像（露光領域３０ａ）の歪曲収差（単位：ｎｍ）及び波面収差（単位：ｍλ）を表６及び表７に示す。なお、本実施例における第２物体面３上での照明光３０の像高（Ｙ軸方向高さ）は３９．５ｍｍであり、露光領域３０ａはこの像高のうち、表７に示す波面収差が３０ｍλ以下になる範囲、つまり３８．０〜３９．５ｍｍの範囲を利用しているため、表６及び表７はその範囲に対応する部分の歪曲収差及び波面収差を示している。上述したとおり、歪曲収差は５ｎｍ以下であり、また、波面収差は３０ｍλ以下であり良好な結果となっている。
【００３０】
【表６】

【００３１】
【表７】

【００３２】
次に、第２物体面３上に形成される縮小像のコマ収差を、図３の露光領域３０ａの像高（Ｙ軸上の位置）に対応してグラフにしたものを図５に示す。図５において、それぞれの像高の位置に対するグラフのうち、左側のグラフがＹ軸方向の収差を表しており、縦軸にＹ軸方向の収差（ＥＹ　単位：μｍ）を取り、横軸に瞳における光線のＹ軸方向の位置（ＰＹ　単位：μｍ）を取っている。右側のグラフがＸ軸方向の収差を表しており、縦軸にＸ軸方向の収差（ＥＸ　単位：μｍ）を取り、横軸に瞳における光線のＸ軸方向の位置（ＰＸ　単位：μｍ）を取っている。
【００３３】
以上のように、投影光学系１を構成する反射鏡を非球面の凸面状及び凹面状の反射面を有するように構成し、かつ、その非球面の次数を２０次以上にすることにより、４枚の反射鏡でも収差が少なく、かつ、十分ＮＡ及び露光領域が大きい精度の良い露光を行うことが可能となる。
【００３４】
【発明の効果】
本発明による投影光学系によれば、投影光学系を構成する反射鏡を非球面の凸面状及び凹面上の反射面を有するように構成し、かつ、その非球面形状を表す式の非球面の次数を２０次以上で構成することにより、４枚の反射鏡で構成された共軸の投影光学系で、収差が少なくかつＮＡ及び露光領域を大きくすることができ、レチクル上に形成された微細なパターンでも精度良く感光基板上に形成することができる。
【図面の簡単な説明】
【図１】本発明に係る投影光学系の横断面の光路図である。
【図２】本発明に係る縮小投影露光装置のブロック図である。
【図３】本発明に係る投影光学系の露光領域を表す図である。
【図４】本発明に係る投影光学系における非球面の次数と波面収差の関係を表すグラフである。
【図５】本発明に係る投影光学系のコマ収差を表すグラフである。
【符号の説明】
１　投影光学系
２　第１物体面
３　第２物体面
４　第１の反射鏡
５　第２の反射鏡
６　第３の反射鏡
７　第４の反射鏡
１０　縮小投影露光装置
１３　レチクル
１７　半導体ウエハ（感光基板）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a projection optical system for projecting and forming a reduced image of a reticle pattern on a photosensitive substrate, and a reduction projection exposure apparatus including the projection optical system.
[0002]
[Prior art]
The numerical aperture (hereinafter, referred to as "NA") and the wavelength used of the reduction projection exposure apparatus for semiconductors tend to be larger and shorter year by year with the increase in the density of semiconductor elements and the thinning of the target line width. It is in. The wavelength of the light beam used shifts from the i-line of a mercury lamp (wavelength 365.015 nm) to a KrF excimer laser (wavelength 248 nm), and a reduction projection exposure apparatus using an ArF excimer laser (wavelength 193 nm) as a light source is also in practical use. However, in recent years, there has been an increasing demand for finer patterns, and reduction projection using an F2 excimer laser (wavelength: 157 nm) and EUV light having a shorter wavelength (extreme ultraviolet light, near 13 nm) as a light source. Exposure apparatuses have been studied as a promising tool for next-generation semiconductor lithography.
[0003]
The glass material of the projection lens used in such a reduced projection exposure apparatus is limited to a glass material using an F2 excimer laser as a light source due to the problem of transmittance. When a light source having a shorter wavelength is used, a reflecting mirror is used. It is necessary to configure a reduction projection exposure apparatus using the configured catadioptric optical system. Also, in order to realize high resolution, it is necessary to obtain an NA and a sufficiently large exposure area while maintaining good aberration of the optical system. Therefore, the number of reflecting mirrors constituting the catadioptric optical system is required. Tended to increase.
[0004]
[Problems to be solved by the invention]
However, when irradiated with a short wavelength light such as EUV light, the light is not reflected by the reflecting mirror but is transmitted or absorbed. Therefore, it is necessary to form a reflection promoting film on the reflecting surface of the reflecting mirror to reflect the EUV light. However, at present, a reflection enhancing film with sufficient reflectivity has not been obtained, so when considering actual exposure, heat is generated by the absorption of exposure light in the reflection enhancing film, and the performance of the optical system fluctuates due to thermal fluctuation. In addition, it is necessary to increase the light amount of the light source in consideration of the absorption of the exposure light by the reflection promoting film. For this reason, a large number of reflecting mirrors cannot be used in the catadioptric system, and as a result, there is a problem that it is difficult to obtain a lens having a sufficiently large NA and exposure area while correcting aberrations.
[0005]
In order to solve these problems, the reflecting mirror is shifted with respect to the optical axis (moving the optical axis of the reflecting mirror in parallel from the optical axis of the projection optical system) or tilted (projection optical system). A method of improving the optical system by arranging (changing the angle between the optical axis of the system and the optical axis of the reflector) has been studied. However, these methods have a problem in that the productivity is poor because the optical system must be assembled by accurately measuring the tilt amount and the shift amount of the reflecting mirror in actual manufacturing. Further, the conventional reflecting mirror is formed by a conventional reflecting surface represented by an even-order aspherical surface of order 20 or less, and it is difficult to correct aberration by a coaxial projection optical system using the reflecting mirror. There was also a problem that it was difficult to secure an area.
[0006]
The present invention has been made in view of such a problem, and the reflecting surface of a reflecting mirror constituting a projection optical system is expressed by a higher order even-numbered aspherical surface than that used in a conventional optical design. An object of the present invention is to provide a projection optical system having a large NA and an exposure area, and a reduction projection exposure apparatus including the projection optical system, by forming a shape represented by using such a shape, thereby correcting aberrations well.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, a projection optical system according to the present invention uses a reduction projection device including four reflection mirrors each having a reflection surface of a predetermined shape to form a reduced image of an object on a first object surface into a second image. The projection mirror is formed on two object planes, the reflecting mirror has an aspherical reflecting surface, and the aspherical shape is configured to be represented by the following equation (1).
[0008]
(Equation 1)

[0009]
Note that, in the aspherical surface expression (1), the order 2i of the aspherical surface represented by the order i of the polynomial is preferably 20 or more.
[0010]
Also, the projection optical system according to the present invention includes an illumination light source for exposure and an illumination optical system, and converts the light of the circumferential portion of the illumination light source for exposure into an arc-shaped slit light by the illumination optical system on the first object plane. , And a reduced image of the object on the first object plane is projected and formed on the second object plane using the reduced projection apparatus, and the numerical aperture of the projection optical system is 0.16 or more. The length of the chord of the arc forming the area of the reduced image projected and formed on the second object plane is 22 mm or more, and the height of the area of the reduced image is 1.5 mm or more. Is preferred.
[0011]
The reduction projection exposure apparatus according to the present invention is configured to irradiate a reticle with exposure light and project an image of a pattern formed on the reticle onto a photosensitive substrate via a projection optical system.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. First, the configuration of the projection optical system according to the present invention will be described with reference to FIG. FIG. 1 is an optical path diagram of a transverse section of a projection optical system according to the present invention, and the width of a light beam represents only the transverse section. The projection optical system 1 is disposed between the first object plane 2 and the second object plane 3, and includes a first reflecting mirror 4, a second reflecting mirror 5, a third reflecting mirror 6, and a fourth reflecting mirror 6. It comprises a reflecting mirror 7. The light that has exited the first object plane 2 is reflected on the first reflecting mirror 4, the second reflecting mirror 5, the third reflecting mirror 6, and the fourth reflecting mirror 7 in this order, and is reflected on the second object plane 3. A reduced image of the object on the first object plane 2 is projected to form an image. Here, at least one convex mirror and one concave mirror are required in order to maintain flatness and ensure magnification, and it is desirable that the lens be aspherical in order to correct aberrations. In the embodiment, the first reflecting mirror 4 has an aspherical concave reflecting surface, the second reflecting mirror 5 has an aspherical convex reflecting surface, and the third reflecting mirror 5 has a third reflecting mirror. The reflecting mirror 6 has an aspherical convex reflecting surface, and the fourth reflecting mirror 7 has an aspherical concave reflecting surface.
[0013]
Further, the first to fourth reflecting mirrors 4 to 7 are arranged coaxially with respect to the optical axis of the projection optical system 1, and the first reflecting mirror 4 and the third reflecting mirror 6 have a reflecting surface of the first reflecting mirror. The second reflecting mirror 5 and the fourth reflecting mirror 7 are arranged so that the reflecting surfaces face the second object surface. At this time, from the first object plane 2 to the second object plane 3, the fourth reflecting mirror 7, the second reflecting mirror 5, the first reflecting mirror 4, and the third reflecting mirror 6 are arranged in this order. It is arranged.
[0014]
Next, a reduction projection exposure apparatus configured using the above-described projection optical system 1 and used in an optical lithography process, which is one of the semiconductor manufacturing processes, will be described with reference to FIG. The reduction projection exposure apparatus used in the optical lithography process is in principle the same as photolithography, in which a device pattern precisely drawn on a reticle (first object plane) is formed on a semiconductor wafer coated with a photoresist, This is to optically project and transfer onto a photosensitive substrate (second object surface) such as a glass substrate.
[0015]
The reduction projection exposure apparatus 10 irradiates a reflection type reticle 13 with light from an illumination light source 11 for exposure through an illumination optical system 12 into slit-like exposure light, thereby forming an image of a part of a pattern formed on the reticle 13. Is projected onto the semiconductor wafer 17 through the above-described projection optical system 1, and the reticle 13 and the semiconductor wafer 17 are relatively scanned in a one-dimensional direction (Y-axis direction) with respect to the projection optical system 1, thereby forming a pattern of the reticle 13. The whole is transferred to each of a plurality of shot areas on the semiconductor wafer 17 by a step-and-scan method. In this embodiment, 13.4 nm EUV light is used as exposure light. In FIG. 2, the optical axis direction of the projection optical system 1 is the Z axis, and the direction perpendicular to the Z axis is the Y axis, and the operation direction of the reticle 13 and the semiconductor wafer 17 is the Y axis. The direction perpendicular to the plane of the drawing is the X axis. Although the reticle 13 and the semiconductor wafer 17 are described as being relatively scanned in the Y-axis direction, it is needless to say that the reticle 13 and the semiconductor wafer 17 can be scanned also in the X-axis direction.
[0016]
Here, as shown in FIG. 3, the shape of the exposure light in the present embodiment uses the circumference of the illumination light 30 from the exposure illumination light source 11 in a slit shape, and is used on the second object plane 3. The exposure area 30a is formed by projection and image formation on the exposure area 30a. This is because, when the illumination light 30 is reflected by a reflecting mirror, in a coaxial reflecting optical system as in the present invention, an area near the optical axis cannot be used due to light blocking by the reflecting mirror, and the distance from the optical axis is increased. Can only be used around the periphery. From this, it is sufficient that the aberration is corrected only in the peripheral slit-shaped use area.
[0017]
The reticle 13 is supported on a reticle support 14 movable at least along the Y-axis direction, and the semiconductor wafer 17 is mounted on a mounting table 18 movable along the XYZ-axis directions. The reticle support table 14 and the mounting table 18 are moved by a reticle support table driving unit 15 and a mounting table driving unit 19 connected thereto, respectively. In the exposure operation, the illumination optical system 12 irradiates the reticle 13 with EUV light, and the projection optical system 1 moves the reticle 13 and the semiconductor wafer 17 at a predetermined speed determined by the reduction magnification of the projection optical system 1. Move by ratio. As a result, an image of the pattern on the reticle 13 is scanned and exposed in a predetermined shot area on the semiconductor wafer 17.
[0018]
Finally, numerical examples of the projection optical system according to the present invention will be described. The structure of the projection optical system 1 is as shown in FIG. 1 described above, and the shape of the exposure area 30a projected and formed on the second object plane 3 has the slit shape shown in FIG. The reflecting surfaces of the first to fourth reflecting mirrors 4 to 7 in the present embodiment are represented by the above-described formula (1), and have an aspherical shape that is rotationally symmetric with respect to the optical axis. Expression (2) expresses the distance r by the XY axis of the center tangent plane, and normalized radiation coordinate ρ is expressed by Expression (3). At this time, R in Expression (3) represents a normalized radius of the reflecting surface of the reflecting mirror.
[0019]
(Equation 2)
r ² = x ² + y ² (2)
[0020]
[Equation 3]
ρ = y / R (3)
[0021]
In this embodiment, the wavelength of the exposure light (EUV light) is 13.4 nm, the order of the aspheric surface is 220, the reduction magnification is 1/4, and the width of the exposure area 30a (the length of the chord of the arc forming the exposure area). ) Is 22 mm, the height is 1.5 mm, and the NA of the projection optical system 1 on the image side is 0.16.
[0022]
In order to form a fine pattern on the second object plane, the projection optical system 1 of the present embodiment reduces the distortion of the image of the object on the first object plane that is reduced and projected on the second object plane to 5 nm or less. Under these conditions, the order of the aspherical surface of the expression (1) representing the aspherical shape of the reflecting surfaces of the first to fourth reflecting mirrors 4 to 7 constituting the projection optical system 1 is set. Will be described with reference to FIG. In this embodiment, in order to accurately form a reduced image of an object on the first object plane projected on the second object plane, not only the distortion of the image projected on the second object plane but also the wavefront The aberration must also be suppressed, and the root mean square (RMS) of the wavefront aberration needs to be smaller than 30 mλ (where 1 mλ represents one thousandth of a wavelength of light forming an image). ing). As shown in FIG. 4, when the order of the aspherical surface of the expression (1) representing the aspherical shape of the reflecting surfaces of the first to fourth reflecting mirrors 4 to 7 is increased, the wavefront aberration is reduced accordingly, and when the order is 220 or higher. 30 mλ or less. For this reason, in this embodiment, 220 is adopted as the order of the aspherical surface.
[0023]
Tables 1 to 5 show the specifications of the projection optical system 1. In Table 1, the radius of curvature indicates the approximate radius of curvature (unit: mm) of each reflecting surface, and the center thickness indicates the distance between the surfaces (unit: mm). Note that the sign of the radius of curvature is positive when it becomes concave toward the first object surface 2 side, and the sign of the center thickness is reversed before and after the reflecting surface at an interval from the immediately preceding surface. Tables 2 to 5 show aspherical surface data (2i-th order aspherical surface coefficient α _i ) of the first to fourth reflecting mirrors 4 to 7.
[0024]
[Table 1]

[0025]
[Table 2]

[0026]
[Table 3]

[0027]
[Table 4]

[0028]
[Table 5]

[0029]
With the projection optical system 1 having the above-described configuration, an image of an object on the first object plane 2 can be projected and formed on the second object plane 3. Next, on the first object plane 2 formed on the second object plane 3 when the projection optical system 1 of the above specifications projects the illumination light 30 shown in FIG. Table 6 and Table 7 show the distortion (unit: nm) and wavefront aberration (unit: mλ) of the reduced image (exposure region 30a) of the object. Note that the image height (height in the Y-axis direction) of the illumination light 30 on the second object plane 3 in this embodiment is 39.5 mm, and the exposure area 30a has the wavefront aberration shown in Table 7 among the image heights. Is in the range of 30 mλ or less, that is, the range of 38.0 to 39.5 mm, and Tables 6 and 7 show the distortion and the wavefront aberration of the portion corresponding to the range. As described above, the distortion is 5 nm or less, and the wavefront aberration is 30 mλ or less, which is a good result.
[0030]
[Table 6]

[0031]
[Table 7]

[0032]
Next, FIG. 5 shows a graph of the coma aberration of the reduced image formed on the second object plane 3 corresponding to the image height (position on the Y axis) of the exposure area 30a in FIG. In FIG. 5, among the graphs for the respective image height positions, the graph on the left side shows the aberration in the Y-axis direction, the vertical axis shows the aberration in the Y-axis direction (EY unit: μm), and the horizontal axis shows the pupil. The position (PY unit: μm) of the light beam in the Y-axis direction is taken. The graph on the right side shows the aberration in the X-axis direction, the vertical axis shows the aberration in the X-axis direction (EX unit: μm), and the horizontal axis shows the position (PX unit: μm) of the ray in the pupil in the X-axis direction. taking it.
[0033]
As described above, by configuring the reflecting mirror constituting the projection optical system 1 to have an aspherical convex and concave reflecting surface, and by setting the order of the aspherical surface to 20 or more, 4 Even with a single reflecting mirror, it is possible to perform accurate exposure with a small aberration and a large NA and large exposure area.
[0034]
【The invention's effect】
According to the projection optical system of the present invention, the reflecting mirror constituting the projection optical system is configured to have a reflecting surface on the convex surface and the concave surface of the aspheric surface, and the aspheric surface of the expression representing the aspheric shape is used. By setting the order to 20 or more, a coaxial projection optical system composed of four reflecting mirrors can reduce the aberration, increase the NA and the exposure area, and reduce the fineness formed on the reticle. Even a simple pattern can be accurately formed on a photosensitive substrate.
[Brief description of the drawings]
FIG. 1 is an optical path diagram of a cross section of a projection optical system according to the present invention.
FIG. 2 is a block diagram of a reduction projection exposure apparatus according to the present invention.
FIG. 3 is a diagram illustrating an exposure area of the projection optical system according to the present invention.
FIG. 4 is a graph showing the relationship between the order of the aspheric surface and the wavefront aberration in the projection optical system according to the present invention.
FIG. 5 is a graph showing coma aberration of the projection optical system according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Projection optical system 2 1st object plane 3 2nd object plane 4 1st reflection mirror 5 2nd reflection mirror 6 3rd reflection mirror 7 4th reflection mirror 10 Reduction projection exposure apparatus 13 Reticle 17 Semiconductor wafer (photosensitive substrate)

Claims

In a projection optical system, a reduced image of an object on a first object plane is projected and formed on a second object plane by using a reduction projection apparatus including four reflecting mirrors each having a reflecting surface of a predetermined shape.
The reflecting mirror has an aspherical reflecting surface, and this aspherical surface has the following formula:

A projection optical system characterized by being represented by:

2. The projection optical system according to claim 1, wherein, in the expression of the aspheric surface shape, the order 2i of the aspheric surface represented by the order i of the polynomial is 20 or more.

An illumination light source for exposure and an illumination optical system are provided,
The light of the circumferential portion of the exposure illumination light source is irradiated on the first object surface as arc-shaped slit light by the illumination optical system, and the reduction projection device is used to illuminate the object on the first object surface. In a projection optical system for projecting and forming a reduced image on a second object plane,
The numerical aperture is 0.16 or more,
2. The chord length of an arc forming an area of a reduced image projected and formed on the second object plane is 22 mm or more, and the height of the area is 1.5 mm or more. Or the projection optical system according to 2.

A reduction projection exposure apparatus that irradiates a reticle with exposure light and projects an image of a pattern formed on the reticle onto a photosensitive substrate via the projection optical system according to any one of claims 1 to 3.