JP3793106B2 - Rectil - Google Patents

Description

【００２９】
として決定しても良いし、又、ピーク位置を微分法を使って求める等の手法も考えられる。
【００３０】
このようなＴＴＬオートフォーカスシステムの長所は、投影光学系の周囲の温度変化、大気圧変化、露光光線による投影光学系の温度上昇等によって生じる投影光学系のピント位置（焦点位置）の経時変化を常時計測し補正をかけられるという点である。
【００３１】
【発明が解決しようとする課題】
従来このようなＴＴＬオートフォーカスの計測に使われるマークにはラインアンドスペースの繰り返しパターンを有するマークが良く用いられている。これは例えば図１３の様にレチクル上で実素子（回路）パターン描かれた領域の外（ＫＭ，ＫＳ）に設けられる。そして、それらの線幅は投影光学系の解像限界に近い寸法のものである。その理由は、実際の回路パターン転写に際して、最も焦点深度の浅いのは投影光学系の解像限界付近のパターンであるという点、また、最良像面位置はパターン線幅によって異なるので、その意味からも最もフォーカスに敏感な最小線幅に対して像面を決定したい、という要求からである。
【００３２】
しかしながら、このような細い線幅を使用した場合、投影光学系の収差によっては図１１に示したＡＦ信号波形が図１２のようにくずれてしまい、ＡＦ計測不能となる場合があった。これは予め決められた出力値でスライスをするスライス法の場合、一つのスライス値に対して３点以上の交点（Ｚ１，Ｚ２，Ｚ３）が発生し、ベストピント位置（深度中心）を求められなくなるからである。さらには、実際のパターン転写が始まると、縮小投影光学系の受ける熱の為に益々その波形くずれが悪化する傾向にあった。
【００３３】
本発明の目的は、投影光学系の残存収差、或いは、露光による熱収差の影響を受けにくい、安定したフォーカス計測が可能となる、投影露光装置に用いるレチクルの提供にある。
【００３４】
【課題を解決するための手段】
上記目的を達成するための本発明のレチクルの好ましい形態は、投影露光装置に用いるレチクルであって、回路パターンと、該回路パターンが描かれた領域外に設けられた前記投影露光装置の投影光学系の像面位置を検出するための繰り返しパターンと、を有し、前記繰り返しパターンの周期Ｐは、前記投影光学系の開口数をＮＡ、投影倍率をβ、露光波長をλとした時、
２λ／（ＮＡβ）＜Ｐ＜１６λ／（ＮＡβ）
を満たすことを特徴とする。
【００３８】
上記レチクルの更に好ましい形態は、前記繰り返しパターンの周期Ｐは、
２．５λ／（ＮＡβ）＜Ｐ＜６λ／（ＮＡβ）
を満たすことを特徴とする。
【００３９】
【発明の実施の形態】
我々の解析結果によると、ＡＦ計測不能となる原因は投影光学系の持つ残存収差であり、露光時に発生する熱収差の影響である事が判った。図１４に基づいて、以下に詳細を述べる。
【００４０】
図１４はレチクル上の回路パターンが投影光学系によって像面上に縮小結像される一般的な結像関係を示す図である。レチクルは不図示の照明系（左側）によって照明される。ここに一般的なＴＴＬＡＦ用マーク（投影光学系の解像限界に近い線幅のラインアンドスペースの繰り返しパターン）が存在すると多数の回折光が発生するが、このうち投影光学系の瞳面を通過してウエハやステージ基準マーク上への再結像に寄与するのは主に０次光と±１次光である。ちなみに、実際の照明０次光束はレチクルに対して角度的にある広がりを持っている。そのために±１次光以外の高次回折光の一部も投影光学系の瞳面を通過し結像に寄与すると考えられるが、この効果は補足的にしか利かない若しくは無視できる。したがって、以下の説明はいわゆる空間的コヒーレンス度が０以外の一般的露光照明システムについても成り立つ。
【００４１】
一方、投影光学系には残存収差が存在する。これは設計上と製造上の両方で発生し得るものであり、球面収差、コマ収差、非点収差等、個々のレンズの偏芯及びレンズ間隔に起因するものも含む。
【００４２】
図１５ではその一例としてＮＡの４乗に比例する４次の球面収差が発生している場合を示している（横軸は瞳上の座標であり、投影光学系の最大開口数（ＮＡ）を１．０として正規化してある。縦軸は波面収差量（λ）である）。この様な投影光学系の最適ピント位置を求めるには、ウエハ面をわざとデフォーカスさせて収差のバランスをとる。すなわち、理論上デフォーカスによって発生する波面収差はＮＡの２乗に比例するので、これと残存球面収差とをキャンセルさせる。図１５では、瞳の最外周（ＮＡ１．０）の位置で両方の収差の和を０としている。この場合、最終的に最適ピント位置で発生する残存収差量（ＷＡ）は次式で与えられる。
【００４３】
ＷＡ＝Ｓ×（ＮＡ）^４−Ｓ×（ＮＡ）^２・・・・式（１）
但し、Ｓの値は瞳面最外周で発生するレンズ固有の残存収差量であり、図１５では実際的な値として１．０λとした。
【００４４】
式（１）で発生する残存収差の最大値は同式を微分すれば容易に求まり、ＮＡ＝０．７の位置でＷＡ＝０．２５λである。各瞳座標位置（ＮＡ）での残存波面収差量を図１６に示す。
【００４５】
以上は投影光学系を片道通過した時、つまり、ウエハ面、ないしは、それと概ね同一ピント面にあるウエハもしくは基準マーク上での残存収差量を示した。ところが、図７で述べたコンフォーカルなＴＴＬＡＦ方式では計測光は投影光学系を往復する。光学理論に従えば、この場合、コマ収差、デイストーション等のいわゆる非対称収差は相殺されて０になるのに対して、球面収差、像面湾曲、そして、非点収差等の対称収差は２倍になる。（図１６参照。）
片道、往復いずれの場合にも残存波面収差の量はＮＡ＝０．６から０．８の範囲で大きく、それ以外では小さい。また、収差曲線の変化率はＮＡ＝０．８以上の領域で大きい。このことは最良像面位置がわずかにずれただけでもこの領域の残存収差が大きく変動する事を意味している。つまり、安定性まで考慮すると、収差発生量の小さい領域はＮＡ＜０．５の領域である、といえる。
【００４６】
次にマークの±１次回折光が縮小投影光学系の瞳面上を透過する位置について述べる。
【００４７】
縮小投影光学系の開口数をＮＡ、ウエハ上への投影倍率をβ、露光波長をλとすると、レチクル上のＴＴＬＡＦマーク（デューティ比１：１のラインアンドスペースパターン）が解像限界線幅（＝λ／（２ＮＡβ））である時、この±１次回折光はレンズ瞳面上でその最外周の位置（ＮＡ＝１．０）に分布する。また、マーク線幅がこの２倍より大きい時、その回折光はＮＡ＝０．５の内側の位置に分布する。
【００４８】
以上、縮小投影光学系の残存収差とＴＴＬＡＦマーク回折光の分布状態について説明してきたが、これから次の事が言える。
【００４９】
つまり、縮小投影光学系の残存収差が発生している場合に、その解像限界に近い線幅のマークでＴＴＬＡＦ計測を行うと、マークの回折光が瞳面上で残存収差の影響を受け、その結像特性が悪化する。その影響度は、コンフォーカルなＴＴＬＡＦ系の場合、計測光路が縮小投影光学系を往復するので倍加される。その結果、計測精度が劣化したり、ひどい場合には、計測不能に陥ってしまう。さらに、パターン転写が進むと露光光の照射によって投影光学系の温度が上昇し、いわゆる、熱収差が発生する。実際の熱収差にはいろんな収差が存在するが、その主なものは球面収差である事が判っている。オートフォーカス信号はこれらの収差が重なって益々その波形くずれを悪化させてしまう。
【００５０】
一方、線幅の太いマークでＴＴＬＡＦ計測を行うと、図１１に示すような基準平面ミラーの光軸方向の位置の移動に対する、検出光量の変化が小さくなり、即ち検出感度が減少して、検出精度が劣化する。
【００５１】
我々が検討した結果、ＴＴＬＡＦ計測を良好に行うためのＴＴＬＡＦマークのラインとスペースの繰り返し周期Ｐは、
２λ／（ＮＡβ）＜Ｐ＜１６λ／（ＮＡβ）
を満たすことが条件となる。また、更に安定した計測を行うためには、
２．５λ／（ＮＡβ）＜Ｐ＜６λ／（ＮＡβ）
を満足する繰り返し周期Ｐを決定すると良い。
【００５２】
（実施例１）
図１（Ａ）は本発明第一の実施例であり、ＴＴＬＡＦ計測に用いるレチクル上のマークを示している。縦線のラインアンドスペースパターンで、ラインとスペースの繰り返し周期が２λ／（ＮＡβ）より広い。このマークを図７のシステムに適用すれば本発明の効果が達成される。図１（Ａ）ではラインとスペースのデユーテイ比を１：１としているが、必ずしもこの値である必要はない。一般には、このデユーテイ比を変えると、回折光の強度は変化するが、その瞳面上での位置は変わらないからである。
【００５３】
図１（Ｂ）は投影光学系の瞳面上での、その回折光の分布状態を示している。つまり、横軸上の黒いドットが±１次回折光を示していて、最大ＮＡ値（瞳半径）の０．５倍の位置より光軸側言い換えれば内側に存在する。これに対して、ＮＡが０．６から０．８のリング状の領域（図中点線の範囲）は残存波面収差が大きい領域である。発生量は図１６を参照の事。ＴＴＬＡＦマークの回折光が収差の小さい領域を選んで通過している事が判る。例えば、ＮＡ＝０．６，λ＝０．３６５μｍ，β＝１／５の露光レンズの場合には、ラインとスペースの繰り返し周期は約６μｍより大きくなる。
【００５４】
（実施例２）
図２（Ａ）は本発明第二の実施例であり、同じく横線のラインアンドスペース群である。図２（Ｂ）はこれの瞳面上分布であり、この場合、回折光は縦軸上に存在する。
【００５５】
（実施例３）
図３（Ａ）は本発明第三の実施例であり、同じく斜め４５度線のラインアンドスペース群である。図３（Ｂ）はこれの瞳面上分布であり、この場合、回折光は斜め４５度線上に存在する。
【００５６】
（実施例４）
図４（Ａ）は本発明第四の実施例である。このパターンは実素子回路パターンの内で、本発明の条件を満たすパターンの一例として取り上げた。縦線と横線の集合体であって、それらの回折光は図４（Ｂ）にあるように瞳面上で縦軸と横軸上に分布する。実素子中にこの様なパターンがあれば、それを模索してＴＴＬＡＦマークとして利用する事ができる。これにより、図１３のようにレチクル上にＴＴＬＡＦマークを指定して設ける必要がなくなる。
【００５７】
（実施例５）
図５は本発明第五の実施例である。これまでの実施例で用いた検出システムでは、最終的なＡＦ検出用光路は投影光学系９を往復２回通過していた。本実施例ではこれが１回しか通らない光路で検出する。すなわち、照明系から発した光束はレチクルパターン８を通過後、投影光学系９の作用で基準平面ミラー１３上に結像する。
【００５８】
基準平面ミラー１３上には透過部と不透過部とでパターニングされたマーク１１０（基準マーク）が形成されている。このレチクルマーク８と基準マーク１１０の形状はたとえば図１から図３のパターンのうちのいずれかを用いる。レチクルパターン８と基準マーク１１０の両方を通過した光束のみが集光光学系６０に検出される。そして、その光量は両者の合焦状態に依存して変化する。
【００５９】
尚、本実施例では、レチクルの上から照明し、ウエハと概ね共役な面（基準マーク面）側で受光しているが、本発明の範囲はこの構成にかぎらない。逆に、ウエハ側から照明して、レチクル側で受光してもよい。
【００６０】
（実施例６）
図６は本発明第六の実施例である。これまでの実施例で用いた検出システムでＡＦ計測光路としては、レチクルを照明し、それを透過した光束が投影光学系９を往復２回通過した後、再びレチクルを透過して受光されていた。本実施例では逆にＡＦ計測用光束がウエハ基準マーク１１０を照明し、それを透過した光束が投影光学系９を通過した後レチクル面で反射して再び投影光学系９と同マーク１１０を透過して受光される構成である。
【００６１】
すなわち、図６で述べた基準平面ミラー１３（マーク付）を照明し、もどり光を受光するために、図５で示した焦点面検出系６０とこれに照明系とを付加した光学ユニット２７をウエハステージ側に持つ。
【００６２】
本実施例の場合、ＴＴＬＡＦ用マーク１１０は基準平面ミラー１３上に形成されていて、レチクル面はこの像の単なる反射面としての機能しかない。したがって、レチクルの最適パターンとなりうるのはパターンの無い完全なクロム反射面か完全なガラス部である。そのような反射面がレチクル上に無い場合には、できるだけ焦点面計測に影響をあたえない実素子領域を最適パターンとして選択する。
【００６３】
尚、図７や図５の実施例において、ＴＴＬＡＦ計測用の照明光は計測マークを透過照明していた。本発明の及ぶ範囲はこれに限らない。つまり、レチクル、または、ウエハ基準マークを、例えば、ハーフミラー等を介して落射照明し、その反射光を露光レンズに導く構成もまったく同様に本発明の及ぶ範囲内である。
【００６４】
次に上記説明した露光装置を利用したデバイスの生産方法の実施例を説明する。
【００６５】
図１７は微小デバイス（ＩＣやＬＳＩ等の半導体チップ、液晶パネル、ＣＣＤ、薄膜磁気ヘッド、マイクロマシン等）の製造のフローを示す。ステップ１（回路設計）では半導体デバイスの回路設計を行なう。ステップ２（マスク製作）では設計した回路パターンを形成したマスクを製作する。一方、ステップ３（ウエハ製造）ではシリコン等の材料を用いてウエハを製造する。ステップ４（ウエハプロセス）は前工程と呼ばれ、上記用意したマスクとウエハを用いて、リソグラフィ技術によってウエハ上に実際の回路を形成する。次のステップ５（組み立て）は後工程と呼ばれ、ステップ４によって作製されたウエハを用いて半導体チップ化する工程であり、アッセンブリ工程（ダイシング、ボンディング）、パッケージング工程（チップ封入）等の工程を含む。ステップ６（検査）ではステップ５で作製された半導体デバイスの動作確認テスト、耐久性テスト等の検査を行なう。こうした工程を経て半導体デバイスが完成し、これが出荷（ステップ７）される。
【００６６】
図１８は上記ウエハプロセスの詳細なフローを示す。ステップ１１（酸化）ではウエハの表面を酸化させる。ステップ１２（ＣＶＤ）ではウエハ表面に絶縁膜を形成する。ステップ１３（電極形成）ではウエハ上に電極を蒸着によって形成する。ステップ１４（イオン打込み）ではウエハにイオンを打ち込む。ステップ１５（レジスト処理）ではウエハに感光剤を塗布する。ステップ１６（露光）では上記説明した露光装置によってマスクの回路パターンをウエハに焼付露光する。ステップ１７（現像）では露光したウエハを現像する。ステップ１８（エッチング）では現像したレジスト像以外の部分を削り取る。ステップ１９（レジスト剥離）ではエッチングが済んで不要となったレジストを取り除く。これらのステップを繰り返し行なうことによって、ウエハ上に多重に回路パターンが形成される。
【００６７】
本実施例の製造方法を用いれば、従来は製造が難しかった高集積度の半導体デバイスを低コストに製造することができる。
【００６８】
【発明の効果】
以上、述べてきた様に、本発明は次の様な効果を生む。
１．露光投影レンズの残存収差や露光によって発生するレンズの熱収差の影響を受けることなくＴＴＬＡＦ計測が可能である。
２．露光の経過に応じて常に安定したＴＴＬＡＦ計測が可能となる。
【図面の簡単な説明】
【図１】（Ａ）本発明第一の実施例で、ラインアンドスペース群のＴＴＬＡＦマークを示す図である。
（Ｂ）（Ａ）のマークの瞳面上での回折光分布を示す図である。
【図２】（Ａ）本発明第二の実施例で、ラインアンドスペース群のＴＴＬＡＦマークを示す図である。
（Ｂ）（Ａ）のマークの瞳面上での回折光分布を示す図である。
【図３】（Ａ）本発明第三の実施例で、ラインアンドスペース群のＴＴＬＡＦマークを示す図である。
（Ｂ）（Ａ）のマークの瞳面上での回折光分布を示す図である。
【図４】（Ａ）本発明第四の実施例で、ＴＴＬＡＦマークとして利用できる実素子パターンの一例を示す図である。
（Ｂ）（Ａ）のマークの瞳面上での回折光分布を示す図である。
【図５】本発明第五の実施例を示す図である。
【図６】本発明第六の実施例を示す図である。
【図７】従来例を示す図である。
【図８】コンフォーカルＴＴＬＡＦでの集光状態を示す図（ベストピント時）である。
【図９】コンフォーカルＴＴＬＡＦでの集光状態を示す図（デフォーカス時）である。
【図１０】図７におけるＴＴＬＡＦ受光光路の説明図である。
【図１１】光軸方向の位置と検出系出力の関係を示す図である。
【図１２】光軸方向の位置と検出系出力の関係を示す図である。
【図１３】レチクルの説明図である。
【図１４】像解像の模式図である。
【図１５】縮小投影光学系の残存収差の説明図である。
【図１６】縮小投影光学系の残存収差量を示す図である。
【図１７】微小デバイスの製造フローを示す図である。
【図１８】ウエハプロセスを示す図である。
【符号の説明】
１ 水銀ランプ
２ 楕円ミラー
３ オプティカルインテグレター
４ コンデンサーレンズ
５ ミラー
６ フィールドレンズ
７ レチクル
８ 縮小投影光学系
９ ウエハ
１０ ｘｙｚステージ
１１ 投光光学系
１２ 検出光学系
１３ 結像レンズ
１４ 視野絞り
１５ 集光レンズ
１６ 受光素子
１７ 基準平面ミラー
１８ 焦点面検出系
１９ オートフォーカス制御系
２０ 駆動系
２１ パターン部
２２ 透過部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a reticle used in a projection exposure apparatus, and particularly in the field of semiconductor element manufacturing, a projection called a stepper having a so-called autofocus function, an automatic focus adjustment function when repeatedly reducing and exposing a reticle circuit pattern onto a semiconductor wafer surface. The present invention relates to a reticle used in an exposure apparatus.
[0002]
[Prior art]
In recent years, an imaging (projection) optical system having a high resolving power has been required in a projection exposure apparatus due to the demand for miniaturization and high integration of patterns of semiconductor elements, LIS elements, VLSI elements, etc. As the imaging optical system increases in NA, the focal depth of the imaging optical system is becoming shallower.
[0003]
In addition, the wafer must allow some thickness variation and bending from the viewpoint of planar processing technology. In general, correction of wafer bending is performed by placing the wafer on a wafer chuck that has been processed so as to guarantee flatness on the order of submicrons, and vacuum-adsorbing the back surface of the wafer. However, variations in the thickness of one wafer, suction method, and wafer deformation caused by the progress of the process cannot be corrected no matter how much the wafer plane is corrected.
[0004]
For this reason, since the wafer has irregularities in the screen area where the reticle pattern is subjected to reduced projection exposure, the effective depth of focus of the optical system is further reduced.
[0005]
Therefore, in the reduction projection exposure apparatus, an effective automatic focusing method for making the wafer surface coincide with the focal plane (the image plane of the projection optical system) is an important theme.
[0006]
As a method for detecting the wafer surface position of a conventional reduced projection exposure apparatus, a method using an air microsensor, a light beam is incident on the wafer surface from an oblique direction without using a projection exposure optical system, and the positional deviation amount of the reflected light is determined. A detection method (optical system) is known.
[0007]
On the other hand, in this type of projection exposure apparatus, the focal position is changed due to a change in ambient temperature of the projection optical system, a change in atmospheric pressure, a temperature increase due to light rays applied to the projection optical system, or a temperature increase due to heat generation of the apparatus including the projection optical system The (image plane position) moves and must be corrected. Therefore, the focus position of the projection optical system can be determined by measuring the ambient temperature change and atmospheric pressure change with a detector, or measuring the temperature change and partial atmospheric pressure change in the projection optical system with a detector. It was calculated and corrected.
[0008]
However, this method does not directly measure the focus position of the projection optical system, so the detection error of the detector that measures the temperature and atmospheric pressure, the temperature change amount, and the atmospheric pressure change amount can be used to determine the focus position of the projection optical system. There is a drawback that it is impossible to detect the focus position of the projection optical system with high accuracy due to an error included in the calculation formula which is an approximate formula when calculating and correcting the above.
[0009]
As a method for overcoming such a problem, a so-called through-the-lens autofocus system (TTLAF) has been devised in which an exposure lens is directly passed to detect the focus surface. FIG. 7 shows a conventional example disclosed in Japanese Patent Laid-Open No. 1-286418.
[0010]
In FIG. 7, reference numeral 7 denotes a reticle, which is held on a reticle stage 70. The circuit pattern on the reticle 7 is reduced to 1/5 on the wafer 9 on the xyz stage 10 by the reduction projection lens 8 to form an image, and exposure is performed. In FIG. 7, a reference plane mirror 17 in which the upper surface of the wafer 9 and the mirror surface substantially coincide is disposed at a position adjacent to the wafer 9. The reason for using the reference plane mirror 17 instead of using a wafer coated with an actual resist is that the resist is not fooled.
[0011]
The xyz stage 10 can move in the optical axis direction (z) of the projection lens 8 and in a plane orthogonal to this direction, and can of course be rotated around the optical axis.
[0012]
The reticle 7 is illuminated in the screen area where the circuit pattern is transferred by the illumination optical system shown by 1 to 6 in FIG.
[0013]
The light emitting part of the mercury lamp 1 which is an exposure light source is located at the first focal point of the elliptical mirror 2, and the light emitted from the mercury lamp 1 is condensed at the second focal position of the elliptical mirror 2. An optical integrator 3 having its light incident surface positioned at the second focal position of the elliptical mirror 2 is placed, and the light exit surface of the optical integrator 3 forms a secondary light source. The light emitted from the optical integrator 3 serving as the secondary light source is bent at an angle of 90 ° by the mirror 5 via the condenser lens 4. Reference numeral 55 denotes a filter for selectively extracting light having an exposure wavelength, and reference numeral 56 denotes a shutter for controlling exposure. The exposure light reflected by the mirror 5 illuminates the screen area on the reticle 7 where the circuit pattern is transferred via the field lens 6. In the present embodiment, the mirror 5 is configured to partially transmit exposure light, for example, 5 to 10%. The light that has passed through the mirror 5 reaches a photodetector 50 for monitoring fluctuations of the light source and the like through a lens 52 and a filter 51 that transmits the exposure wavelength and cuts excess light for photoelectric detection.
[0014]
In the figure, reference numerals 11 to 12 form a known off-axis autofocus optical system. Reference numeral 11 denotes a light projecting optical system. A light beam that is non-exposure light emitted from the light projecting optical system 11 intersects the optical axis of the reduction projection lens 8. It is assumed that light is collected and reflected at a point on the reference plane mirror 17 (or the upper surface of the wafer 9). The light beam reflected by the reference plane mirror 17 enters the detection optical system 12. Although not shown, a position detection light receiving element is arranged in the detection optical system 12, and the reflection point of the light beam on the position detection light receiving element and the reference plane mirror 17 is arranged to be conjugate. The positional deviation in the optical axis direction of the reduction projection lens 8 of the reference plane mirror 17 is measured as the positional deviation of the incident light beam on the position detection light receiving element in the detection optical system 12.
[0015]
The positional deviation from the predetermined reference plane of the reference plane mirror 17 measured by the detection optical system 12 is transmitted to the autofocus control system 19. The autofocus control system 19 gives a command to the drive system 20 for driving the xyz stage 10 on which the reference plane mirror 17 is fixed. When the focus position is detected by TTL, the autofocus control system 19 drives the reference mirror 17 up and down in the optical axis direction (z direction) of the projection lens 8 in the vicinity of the predetermined reference position. Further, the position control of the wafer 9 during exposure (the wafer 9 is arranged at the position of the reference plane mirror 17 in FIG. 7) is also performed by the autofocus control system 19.
[0016]
Next, the focus position detection optical system of the reduction projection lens 8 according to the present invention will be described. 8 and 9, reference numeral 7 denotes a reticle, and reference numeral 21 denotes a pattern portion formed on the reticle 7, which has a light shielding property. Reference numeral 22 denotes a light shielding part sandwiched between the pattern parts 21. Here, when the focus position (image plane position) of the reduction projection lens 8 is detected, the xyz stage 10 moves in the optical axis direction of the reduction projection lens 8.
[0017]
It is assumed that the reference plane mirror 17 is positioned on the optical axis of the reduction projection lens 8 and the reticle 7 is illuminated by the illumination optical systems 1 to 6.
[0018]
First, the case where the reference plane mirror 17 is on the focal plane of the reduction projection lens 8 will be described with reference to FIG. The exposure light that has passed through the transmission part 22 on the reticle 7 is condensed and reflected on the reference plane mirror 17 via the reduction projection lens 8. The reflected exposure light follows the same optical path as the forward path, is condensed on the reticle 7 via the reduction projection lens 8, and passes through the light projecting section 22 between the pattern sections 21 on the reticle 7. At this time, the exposure light is not vignetted on the pattern portion 21 on the reticle 7, and the entire light beam passes through the transmission portion of the pattern portion 21.
[0019]
Next, the case where the reference plane mirror 17 is at a position shifted from the focal plane of the reduction projection lens 8 will be described with reference to FIG. The exposure light that has passed through the transmission part of the pattern part 21 on the reticle 7 reaches the reference plane mirror 17 via the reduction projection lens 8, but the reference plane mirror 17 is not on the focal plane of the reduction projection lens 8. The exposure light is reflected by the reference plane mirror 17 as a spread light beam. That is, the reflected exposure light follows an optical path different from the forward path, passes through the reduction projection lens 8, and is not condensed on the reticle 7, but corresponds to the amount of deviation from the focal plane of the reduction projection lens 8 of the reference plane mirror 17. It reaches the reticle 7 as a luminous flux having a spread. At this time, a part of the light beam is vignetted by the pattern portion 21 on the reticle 7 and the entire light beam cannot pass through the light projecting unit 22. That is, there is a difference in the amount of light reflected through the reticle when it matches the focus surface and when it does not.
[0020]
The optical path after the light beam of the exposure light reflected by the reference plane mirror 17 described with reference to FIGS. 8 and 9 passes through the reticle 7 will be described with reference to FIG.
[0021]
The exposure light transmitted through the reticle 7 passes through the field lens 6 and reaches the mirror 5. Since the mirror 5 has a transmittance of about 5 to 10% with respect to the exposure light as described above, a part of the exposure light reaching the mirror 5 passes through the mirror 5 and passes through the imaging lens 13 to the field stop 14. Condensed on the surface. At this time, the surface on which the pattern of the reticle 7 exists and the field stop 14 are in a conjugate position via the imaging lens 13.
[0022]
The exposure light that has passed through the opening of the field stop 14 enters the light receiving element 16 through the condenser lens 15.
[0023]
If necessary, a filter 51 that selectively transmits only exposure light is disposed on the front surface of the light receiving element 16, and an electric signal corresponding to the amount of incident exposure light is output.
[0024]
Hereinafter, a method of detecting the focus position (image plane position) of the reduction projection lens 8 using the signal output of the light receiving element 16 will be described.
[0025]
It is assumed that the xyz stage 10 with the reference plane mirror 17 is driven by the drive system 20 in the optical axis direction of the reduction projection lens 8 around the measurement zero point preset by the off-axis autofocus detection system 12. At this time, the position signal (autofocus measurement value z) of the reference plane mirror 17 measured by the autofocus detection system 12 at each position and the exposure light reflected by the reference plane mirror 17 are received by the light receiving element 16. The relationship between outputs obtained from the focal plane (image plane) detection system 18 by receiving light and converting it into an electrical signal is as shown in FIG. At this time, the signal of the detection system 18 is corrected by the signal from the reference light quantity detection system 53 by, for example, normalizing the signal of the detection system 18 with the signal of the detection system 53 in order to eliminate the influence of fluctuation of the light source 1. And
[0026]
When the reference plane mirror 17 is positioned on the focal plane of the reduction projection optical system 8, the output of the focal plane detection system 18 shows a peak value. Autofocus measurement value z at this time ₀ Then, the reduced projection lens 8 is used as the focus position of the projection optical system 8 when the wafer 9 is exposed (or the measured value z). ₀ The focus position set in advance is corrected based on. ).
[0027]
The focus position of the projection lens 8 determined in this way becomes the reference position of the off-axis autofocus detection system. The actual best printing position of the wafer is a value obtained by giving an offset from the reference position in consideration of values such as the thickness of the wafer coating and the amount of the step. For example, when the wafer is exposed using a multilayer resist process, only the uppermost portion of the multilayer needs to be burned, so that the resist surface of the wafer and the reference position are substantially coincident. On the other hand, when the exposure light sufficiently reaches the substrate with a single layer resist, the focus of the wafer matches not the resist surface but the substrate surface. In this case, there is an offset of 1 μm or more between the resist surface and the reference position. Things are not uncommon. Such an offset amount is unique to the process and is given as an offset different from that of the projection exposure apparatus. It is sufficient for the apparatus itself to be able to accurately determine the focus position of the projection lens 8 itself by the method of the present invention, and the offset amount is relative to the autofocus control system 19 and the drive system 20 only when necessary. What is necessary is just to input beforehand via the system controller not shown of a projection exposure apparatus.
[0028]
This focus position z ₀ This detection may be determined by the peak of the output of the focal plane detection system 18, but various other methods are conceivable. For example, in order to increase the sensitivity of detection, a certain level of slice level SL is set with respect to the peak output, and the autofocus measurement value z when the output of this slice level SL is indicated. ₁ , Z ₂ Knowing the focus position
[Outside 1]

[0029]
It is also possible to determine the peak position using a differential method.
[0030]
The advantage of such a TTL autofocus system is that the temporal change in the focus position (focal position) of the projection optical system caused by a change in the temperature around the projection optical system, a change in atmospheric pressure, a temperature rise in the projection optical system due to exposure light, and the like. It is a point that it can always measure and correct.
[0031]
[Problems to be solved by the invention]
Conventionally, a mark having a repetitive line and space pattern is often used as a mark used for such TTL autofocus measurement. This is provided outside (KM, KS) outside the region where the actual element (circuit) pattern is drawn on the reticle as shown in FIG. These line widths have dimensions close to the resolution limit of the projection optical system. The reason is that in actual circuit pattern transfer, the shallowest depth of focus is the pattern near the resolution limit of the projection optical system, and the best image plane position depends on the pattern line width. This is because the image plane needs to be determined for the minimum line width that is most sensitive to the focus.
[0032]
However, when such a thin line width is used, depending on the aberration of the projection optical system, the AF signal waveform shown in FIG. 11 may be distorted as shown in FIG. In the case of a slicing method in which slicing is performed with a predetermined output value, three or more intersections (Z1, Z2, Z3) are generated for one slice value, and the best focus position (depth center) can be obtained. Because it disappears. Furthermore, when the actual pattern transfer starts, the waveform distortion tends to worsen due to the heat received by the reduction projection optical system.
[0033]
An object of the present invention is to provide a reticle for use in a projection exposure apparatus that can perform stable focus measurement and is less susceptible to the residual aberration of a projection optical system or thermal aberration due to exposure.
[0034]
[Means for Solving the Problems]
To achieve the above purpose Me A preferred embodiment of the reticle of the present invention is a reticle used in a projection exposure apparatus, A circuit pattern and a projection exposure apparatus provided outside the region where the circuit pattern is drawn. Repetitive pattern for detecting image plane position of projection optical system When, The period P of the repetitive pattern is NA when the numerical aperture of the projection optical system is NA, the projection magnification is β, and the exposure wavelength is λ.
2λ / (NAβ) <P <16λ / (NAβ)
It is characterized by satisfying.
[0038]
the above In a more preferred embodiment of the reticle, the period P of the repetitive pattern is
2.5λ / (NAβ) <P <6λ / (NAβ)
It is characterized by satisfying.
[0039]
DETAILED DESCRIPTION OF THE INVENTION
According to our analysis results, it was found that the cause of AF measurement inability was the residual aberration of the projection optical system, and the influence of thermal aberration generated during exposure. Details will be described below with reference to FIG.
[0040]
FIG. 14 is a diagram showing a general imaging relationship in which the circuit pattern on the reticle is reduced and formed on the image plane by the projection optical system. The reticle is illuminated by an illumination system (not shown) (left side). If there is a general TTLAF mark (a line-and-space repetitive pattern with a line width close to the resolution limit of the projection optical system), a large amount of diffracted light is generated. Of these, it passes through the pupil plane of the projection optical system. Thus, it is mainly the 0th order light and the ± 1st order light that contribute to the re-imaging on the wafer and the stage reference mark. Incidentally, the actual illumination zero-order light beam has a certain angular spread with respect to the reticle. For this reason, it is considered that a part of higher-order diffracted light other than ± 1st-order light also passes through the pupil plane of the projection optical system and contributes to image formation. However, this effect is only supplementary or negligible. Therefore, the following description is valid for a general exposure illumination system having a so-called spatial coherence degree other than zero.
[0041]
On the other hand, residual aberration exists in the projection optical system. This can occur in both design and manufacturing, and includes those caused by decentering of individual lenses and lens spacing such as spherical aberration, coma and astigmatism.
[0042]
FIG. 15 shows an example in which a fourth-order spherical aberration proportional to the fourth power of NA occurs (the horizontal axis is a coordinate on the pupil, and the maximum numerical aperture (NA) of the projection optical system is The vertical axis represents the amount of wavefront aberration (λ). In order to obtain the optimum focus position of such a projection optical system, the wafer surface is intentionally defocused to balance aberrations. That is, theoretically, the wavefront aberration generated by defocus is proportional to the square of NA, and this and the remaining spherical aberration are canceled. In FIG. 15, the sum of both aberrations is set to 0 at the position of the outermost periphery (NA 1.0) of the pupil. In this case, the residual aberration amount (WA) finally generated at the optimum focus position is given by the following equation.
[0043]
WA = S × (NA) ⁴ -S x (NA) ² .... Formula (1)
However, the value of S is the amount of residual aberration inherent in the lens that occurs at the outermost periphery of the pupil plane, and in FIG.
[0044]
The maximum value of the residual aberration generated in the equation (1) can be easily obtained by differentiating the equation, and WA = 0.25λ at a position where NA = 0.7. FIG. 16 shows the amount of residual wavefront aberration at each pupil coordinate position (NA).
[0045]
The above shows the amount of residual aberration when passing one way through the projection optical system, that is, on the wafer surface or on the wafer or reference mark substantially on the same focal plane. However, in the confocal TTLAF method described with reference to FIG. 7, the measurement light reciprocates through the projection optical system. According to optical theory, in this case, so-called asymmetric aberrations such as coma and distortion are canceled out to zero, while symmetric aberrations such as spherical aberration, field curvature, and astigmatism are doubled. become. (See FIG. 16.)
The amount of residual wavefront aberration is large in the range of NA = 0.6 to 0.8 in both cases of one-way and reciprocation, and small in other cases. Further, the change rate of the aberration curve is large in a region where NA = 0.8 or more. This means that the residual aberration in this region varies greatly even if the best image plane position is slightly shifted. That is, in consideration of stability, it can be said that the region where the amount of aberration is small is a region where NA <0.5.
[0046]
Next, the position at which the ± first-order diffracted light of the mark passes through the pupil plane of the reduction projection optical system will be described.
[0047]
When the numerical aperture of the reduction projection optical system is NA, the projection magnification onto the wafer is β, and the exposure wavelength is λ, the TTLAF mark (line and space pattern with a duty ratio of 1: 1) on the reticle is the resolution limit line width ( = Λ / (2NAβ)), this ± first-order diffracted light is distributed at the outermost peripheral position (NA = 1.0) on the lens pupil plane. When the mark line width is larger than twice, the diffracted light is distributed at a position inside NA = 0.5.
[0048]
As described above, the residual aberration of the reduction projection optical system and the distribution state of the TTLAF mark diffracted light have been described. The following can be said from this.
[0049]
That is, when residual aberration of the reduction projection optical system occurs, when TTLAF measurement is performed with a mark having a line width close to the resolution limit, the diffracted light of the mark is affected by the residual aberration on the pupil plane, The imaging characteristics deteriorate. In the case of a confocal TTLAF system, the influence is doubled because the measurement optical path reciprocates through the reduction projection optical system. As a result, when the measurement accuracy is deteriorated or severe, measurement is impossible. Further, as the pattern transfer proceeds, the temperature of the projection optical system rises due to exposure light exposure, and so-called thermal aberration occurs. There are various types of aberration in actual thermal aberration, but it is known that the main one is spherical aberration. The autofocus signal further deteriorates the waveform distortion due to the overlap of these aberrations.
[0050]
On the other hand, when TTLAF measurement is performed with a mark having a large line width, the change in the detected light amount with respect to the movement of the position of the reference plane mirror in the optical axis direction as shown in FIG. Accuracy deteriorates.
[0051]
As a result of our study, the repetition period P of the TTLAF mark line and space for good TTLAF measurement is
2λ / (NAβ) <P <16λ / (NAβ)
It is a condition to satisfy. In order to perform more stable measurement,
2.5λ / (NAβ) <P <6λ / (NAβ)
It is preferable to determine a repetition period P that satisfies
[0052]
Example 1
FIG. 1A is a first embodiment of the present invention, and shows marks on a reticle used for TTLAF measurement. In a vertical line and space pattern, the repetition cycle of lines and spaces is wider than 2λ / (NAβ). If this mark is applied to the system of FIG. 7, the effect of the present invention is achieved. In FIG. 1A, the line to space duty ratio is 1: 1, but this value is not necessarily required. In general, changing the duty ratio changes the intensity of the diffracted light, but does not change its position on the pupil plane.
[0053]
FIG. 1B shows the distribution state of the diffracted light on the pupil plane of the projection optical system. That is, the black dots on the horizontal axis indicate ± first-order diffracted light, and are present on the optical axis side, in other words, on the inner side than the position of 0.5 times the maximum NA value (pupil radius). On the other hand, a ring-shaped region (range indicated by a dotted line in the figure) having an NA of 0.6 to 0.8 is a region having a large residual wavefront aberration. See Fig. 16 for the amount generated. It can be seen that the diffracted light of the TTLAF mark passes through a region having a small aberration. For example, in the case of an exposure lens with NA = 0.6, λ = 0.365 μm, and β = 1/5, the line and space repetition period is greater than about 6 μm.
[0054]
(Example 2)
FIG. 2A shows a second embodiment of the present invention, which is also a horizontal line and space group. FIG. 2B shows the distribution on the pupil plane. In this case, diffracted light exists on the vertical axis.
[0055]
Example 3
FIG. 3A shows a third embodiment of the present invention, which is also a line and space group having an oblique 45-degree line. FIG. 3B shows the distribution on the pupil plane. In this case, the diffracted light exists on a 45-degree oblique line.
[0056]
(Example 4)
FIG. 4A shows a fourth embodiment of the present invention. This pattern is taken as an example of a pattern that satisfies the conditions of the present invention among the actual element circuit patterns. It is an aggregate of vertical lines and horizontal lines, and the diffracted light thereof is distributed on the vertical and horizontal axes on the pupil plane as shown in FIG. If there is such a pattern in an actual element, it can be searched and used as a TTLAF mark. This eliminates the need to designate and provide a TTLAF mark on the reticle as shown in FIG.
[0057]
(Example 5)
FIG. 5 shows a fifth embodiment of the present invention. In the detection systems used in the embodiments so far, the final AF detection optical path has passed through the projection optical system 9 twice. In the present embodiment, this is detected by an optical path that passes only once. That is, the light beam emitted from the illumination system forms an image on the reference plane mirror 13 by the action of the projection optical system 9 after passing through the reticle pattern 8.
[0058]
On the reference plane mirror 13, a mark 110 (reference mark) patterned by a transmission part and a non-transmission part is formed. As the shapes of the reticle mark 8 and the reference mark 110, for example, one of the patterns shown in FIGS. 1 to 3 is used. Only the light beam that has passed through both the reticle pattern 8 and the reference mark 110 is detected by the condensing optical system 60. The amount of light changes depending on the in-focus state of both.
[0059]
In this embodiment, illumination is performed from above the reticle and light is received on the surface (reference mark surface) that is substantially conjugate with the wafer. However, the scope of the present invention is not limited to this configuration. Conversely, illumination may be performed from the wafer side and light may be received on the reticle side.
[0060]
(Example 6)
FIG. 6 shows a sixth embodiment of the present invention. In the detection system used in the embodiments so far, as the AF measurement optical path, the reticle is illuminated, and the light beam passing through the reticle passes through the projection optical system 9 twice, and then passes through the reticle again to be received. . In the present embodiment, conversely, the AF measurement light beam illuminates the wafer reference mark 110, and the light beam transmitted therethrough is reflected by the reticle surface after passing through the projection optical system 9 and again transmitted through the mark 110 as well as the projection optical system 9. Thus, the light is received.
[0061]
That is, in order to illuminate the reference plane mirror 13 (with a mark) described in FIG. 6 and receive the return light, the optical unit 27 to which the focal plane detection system 60 and the illumination system shown in FIG. Hold on the wafer stage side.
[0062]
In the case of this embodiment, the TTLAF mark 110 is formed on the reference plane mirror 13, and the reticle surface functions only as a reflection surface of this image. Therefore, the perfect pattern of the reticle can be a perfect chrome reflecting surface without a pattern or a perfect glass part. When such a reflective surface is not on the reticle, an actual element region that does not affect the focal plane measurement as much as possible is selected as the optimum pattern.
[0063]
In the embodiment shown in FIGS. 7 and 5, the illumination light for TTLAF measurement transmits and illuminates the measurement mark. The scope of the present invention is not limited to this. That is, the configuration in which the reticle or wafer reference mark is incident-light illuminated via, for example, a half mirror and the reflected light is guided to the exposure lens is also within the scope of the present invention.
[0064]
Next, an embodiment of a device production method using the above-described exposure apparatus will be described.
[0065]
FIG. 17 shows a flow of manufacturing a microdevice (a semiconductor chip such as an IC or LSI, a liquid crystal panel, a CCD, a thin film magnetic head, a micromachine, etc.). In step 1 (circuit design), a semiconductor device circuit is designed. In step 2 (mask production), a mask on which the designed circuit pattern is formed is produced. On the other hand, in step 3 (wafer manufacture), a wafer is manufactured using a material such as silicon. Step 4 (wafer process) is called a pre-process, and an actual circuit is formed on the wafer by lithography using the prepared mask and wafer. The next step 5 (assembly) is called a post-process, and is a process for forming a semiconductor chip using the wafer produced in step 4, and is a process such as an assembly process (dicing, bonding), a packaging process (chip encapsulation), or the like. including. In step 6 (inspection), the semiconductor device manufactured in step 5 undergoes inspections such as an operation confirmation test and a durability test. Through these steps, the semiconductor device is completed and shipped (step 7).
[0066]
FIG. 18 shows a detailed flow of the wafer process. In step 11 (oxidation), the wafer surface is oxidized. In step 12 (CVD), an insulating film is formed on the wafer surface. In step 13 (electrode formation), an electrode is formed on the wafer by vapor deposition. In step 14 (ion implantation), ions are implanted into the wafer. In step 15 (resist process), a photosensitive agent is applied to the wafer. In step 16 (exposure), the circuit pattern of the mask is printed onto the wafer by exposure using the exposure apparatus described above. In step 17 (development), the exposed wafer is developed. In step 18 (etching), portions other than the developed resist image are removed. In step 19 (resist stripping), unnecessary resist after etching is removed. By repeating these steps, multiple circuit patterns are formed on the wafer.
[0067]
By using the manufacturing method of this embodiment, it is possible to manufacture a highly integrated semiconductor device that has been difficult to manufacture at low cost.
[0068]
【The invention's effect】
As described above, the present invention produces the following effects.
1. TTLAF measurement can be performed without being affected by the residual aberration of the exposure projection lens and the thermal aberration of the lens caused by exposure.
2. Stable TTLAF measurement is always possible as exposure progresses.
[Brief description of the drawings]
FIG. 1A is a diagram showing TTLAF marks of a line and space group in the first embodiment of the present invention.
(B) It is a figure which shows the diffracted light distribution on the pupil plane of the mark of (A).
FIG. 2A is a diagram showing TTLAF marks of a line and space group in the second embodiment of the present invention.
(B) It is a figure which shows the diffracted light distribution on the pupil plane of the mark of (A).
FIG. 3A is a diagram showing TTLAF marks of a line and space group in a third embodiment of the present invention.
(B) It is a figure which shows the diffracted light distribution on the pupil plane of the mark of (A).
FIG. 4A is a diagram showing an example of an actual element pattern that can be used as a TTLAF mark in the fourth embodiment of the present invention.
(B) It is a figure which shows the diffracted light distribution on the pupil plane of the mark of (A).
FIG. 5 is a diagram showing a fifth embodiment of the present invention.
FIG. 6 is a diagram showing a sixth embodiment of the present invention.
FIG. 7 is a diagram showing a conventional example.
FIG. 8 is a diagram showing a condensing state with confocal TTLAF (at the time of best focus).
FIG. 9 is a diagram (at the time of defocusing) showing a condensing state with confocal TTLAF.
10 is an explanatory diagram of a TTLAF light receiving optical path in FIG. 7; FIG.
FIG. 11 is a diagram illustrating a relationship between a position in the optical axis direction and a detection system output.
FIG. 12 is a diagram showing a relationship between a position in the optical axis direction and a detection system output.
FIG. 13 is an explanatory diagram of a reticle.
FIG. 14 is a schematic diagram of image resolution.
FIG. 15 is an explanatory diagram of residual aberrations in the reduction projection optical system.
FIG. 16 is a diagram illustrating a residual aberration amount of the reduction projection optical system.
FIG. 17 is a diagram showing a manufacturing flow of a microdevice.
FIG. 18 is a diagram showing a wafer process.
[Explanation of symbols]
1 Mercury lamp
2 Elliptical mirror
3 Optical Integral Letter
4 Condenser lens
5 Mirror
6 Field lens
7 Reticle
8 Reduction projection optical system
9 Wafer
10 xyz stage
11 Projection optics
12 Detection optical system
13 Imaging lens
14 Field stop
15 Condensing lens
16 Light receiving element
17 Reference plane mirror
18 Focal plane detection system
19 Autofocus control system
20 Drive system
21 Pattern part
22 Transmission part

Claims (2)

A reticle used in a projection exposure apparatus,
It has a circuit pattern, a repetition pattern for detecting the image plane position of the projection optical system of the projection exposure apparatus provided outside a region the circuit pattern drawn and,
The period P of the repetitive pattern is NA when the numerical aperture of the projection optical system is NA, the projection magnification is β, and the exposure wavelength is λ.
2λ / (NAβ) <P <16λ / (NAβ)
A reticle characterized by satisfying The period P of the repetitive pattern is
2.5λ / (NAβ) <P <6λ / (NAβ)
The reticle according to claim 1, wherein:

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