JP2004014485A - Wafer defect inspection method and wafer defect inspection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a defect inspection device of a high resolution realizing the increase of inspection speed in a technology of inspecting defects of patterns, foreign matters, remnants, steps or the like on a wafer in the course of manufacturing process by electron beams. <P>SOLUTION: An electric field for decelerating electron beam is formed on the surface of a semiconductor 7 to be tested, and the electron beam with a certain area (plane-shaped electron beam), including energy component which can not reach the surface of the semiconductor tested due to deceleration electric field, are reflected at the very vicinity of the surface of the semiconductor to be tested to form an image with an image-forming lens 11, and the image of a plurality of areas of the surface of the semiconductor to be tested are obtained and stored in an image storage part. By comparing the images of the plurality of the areas, presence and position of defects in the areas are measured. <P>COPYRIGHT: (C)2004,JPO

Description

この場合に、ビーム電流と検査時間の関係を表したものが、図８である。なお、この関係は、Ｐｉｘ＝４０ｎｍで計算した。本発明が、他の方式と比較して圧倒的な短時間で検査可能であることがわかる。
【００４７】
次に、本発明の一実施例の構成について詳しく説明する。図９に、本発明の一実施例になる検査装置の構成を示す。本実施例による検査装置は、大別して、電子光学系１０１、試料室１０２、画像検出部１０３、画像処理部１０４および制御部１０５より構成されている。それぞれの部分について以下に説明する。
【００４８】
まず、電子光学系１０１について説明する。加速電源２３により負の高電位が与えられている電子源１から放出された加速電子ビームは、コンデンサレンズ２によって収束され、矩形の開口を有する絞り４を照射する。電子源１には、Ｚｒ／Ｏ／Ｗ型のショットキー電子源を用いた。大電流ビーム（例えば、１．５μＡ）で、かつエネルギー幅が１．５ｅＶの均一な面状電子ビームを安定に形成できる。そして、ビームセパレータ３によってウェハ７の方向に偏向される。ビームセパレータ３は、電子源１からの入射電子ビームと試料からのミラー電子ビームの光路を分離するためのものである。コンデンサレンズ２は、クロスオーバを対物レンズ６の焦点面に形成する。また、対物レンズ６によってウェハ７表面上に絞り４の像を形成するように絞りやレンズの配置を最適化してある。
【００４９】
これにより、ウェハ７表面に垂直な方向を向き、各電子同士の軌道がほぼ平行に揃ったビームで、かつ絞り４の開口形状に整形された面状の電子ビームが形成される。絞り４上での矩形絞り開口の大きさは、例えば１００μｍ角であり、対物レンズ６によってこれを１／２に縮小し、ウェハ７表面上では５０μｍ角の面状電子ビームが得られるようにした。この面状電子ビームは、照射系偏向器５によってウェハ７表面上の任意の位置に移動（または、走査）され得る。
【００５０】
対物レンズの前焦点面とクロスオーバ位置を完全に一致できなくてもある許容範囲内であれば問題ない。また、クロスオーバの大きさも理想的にはゼロであるが、実際は電子銃やコンデンサレンズの収差により有限の大きさを持っている。この大きさもある許容範囲であれば問題ない。このクロスオーバの位置を正確に制御し、しかも電子銃やコンデンサレンズの収差を十分に低減させた電子光学系では、試料への入射角の広がりは０．５ｍｒａｄ以下に抑えることができた。この入射角の広がりはミラー電子による試料表面の拡大像の分解能を決める要因のひとつであり以下の式で表される。
【００５１】
ｒ０＝β^２・Ｚｍ　…（３）
ここで、ｒ０は入射角の広がりで決まる分解能、βは最大入射半角、Ｚｍは電子を引き戻す電界が生じている距離である。
【００５２】
本実施例では、βは０．２５ｍｒａｄ、Ｚｍは５ｍｍである。これを（３）式に代入すると、ｒ０は０．３ｎｍとなり、本実施例では分解能に影響を与えないことがわかる。したがって、ビーム電流は必要に応じてもっと増やすことが可能である。
【００５３】
なお、分解能が３０ｎｍ程度でも半導体の欠陥検出には十分であると考えられるので、Ｚｍが５ｍｍとすれば、βは２．４ｍｒａｄまで許容できることになる。この場合は、対物レンズの前焦点面とクロスオーバの位置のずれやクロスオーバの大きさにはかなり余裕が生じる。
【００５４】
前焦点面でのビーム開き半角をαとし、対物レンズの焦点距離をｆ、クロスオーバの位置ずれをΔｆ、面状電子ビームの半径をＸとすれば、次の式が成り立つ。
【００５５】
Δｆ＝ｆ・β／α　　　　　　　　　　　　　　　　　　　　・・・（４）
α＝Ｘ／（２ｆ）　　　　　　　　　　　　　　　　　　　・・・（５）
式（４）（５）から、例えば対物レンズの焦点距離ｆが１０ｍｍで面状ビームの大きさＸを４０μｍとしたときは、クロスオーバ位置のずれΔｆが１０ｍｍ程度ずれても問題ない。これを前焦点面でのビーム直径に換算すると約４０μｍとなる。いずれにしろ、電子ビームのクロスオーバを対物レンズの前焦点面の近傍に配置させることで十分な分解能を得られることがわかる。
【００５６】
ここで、ビームセパレータ３について簡単に説明する。ビームセパレータ３は、電子源１から放出された電子ビームをウェハ７の方向に電子ビームを偏向し、一方、ウェハ７から引き戻されたミラー電子は電子源１の方向ではなく結像レンズ１１の方向に偏向する。このような作用の偏向器には磁場による偏向器が最適である。磁場による偏向作用は電子の入射方向によって偏向作用の方向が異なるからである。
【００５７】
また、実施例２として図１０を用いて後述するが、結像レンズの光軸と対物レンズ６の光軸を一直線上に配置した光学系の場合は電場と磁場を直行させて、下からのミラー電子は直進させ、上からの電子ビームのみに偏向作用を持たせるＥ×Ｂ偏向器を用いる。
【００５８】
ウェハ７、ウェハ（試料）移動ステージ８には、電源９により、電子源１よりも僅かに高い（絶対値の大きい）負電位を印加する。具体的には０．５〜５Ｖだけ負電位にするのが良い。あまり高い負電位にすると画像の分解能が劣化する。また、あまり小さい電位では表面の凹凸や電位等のわずかな変化を極端に強いコントラストとして画像化してしまい、真に必要な欠陥のみを検出することが困難となってしまう。
【００５９】
ウェハ７表面に垂直に向けられた電子ビームは、上記の負電位によってウェハ７の手前で減速されてウェハ７表面の電界によって上方に引き戻される。この電子はウェハ７の表面の情報を反映していることは既に説明した。このミラー電子は、対物レンズ６により焦点を結び、ビームセパレータにより結像系偏向器１０および結像レンズ１１の方向に偏向される。そして結像レンズ１１によりウェハ７表面の状態を電子像として結像させる。この電子像を拡大レンズ１３、１４によって蛍光板１５上に拡大投影させることによって、ウェハ７表面のパターンや帯電状態を反映した蛍光像（顕微鏡像）を得ることができる。
【００６０】
この電子像のコントラストと分解能を向上するために、クロスオーバ面にコントラストアパーチャ１２を挿入できるようになっている。このコントラストアパーチャ１２によって、ウェハ７表面電場で引き戻されたときに垂直方向から大きく外れた電子を取り除くことにより画像の分解能とコントラストが強調できる。
【００６１】
本発明の画像形成原理において、ウェハ表面の帯電の微妙な違いを検出するための感度や画像の分解能は面状電子ビームのエネルギー幅によって決まってくる。これをシミュレーションにより比較した結果が、図１０である。図４と同様のパターンを仮定し、電子ビームのエネルギー幅（ΔＥ）を２ｅＶと４ｅＶの二通りの場合について得られる画像を比較した。
【００６２】
この結果によると、４ｅＶのエネルギー幅を仮定した場合、パターン中央部の電位の異なる部分のコントラストが認められない。半導体の微細化の進展を考慮したとき、図１０に示したような微細パターンにおいて１Ｖ程度の帯電電位の違いを欠陥として検出することが必要となってくる。したがって、本発明において用いる電子ビームのエネルギー幅は２ｅＶ以下であることが望ましいことがわかる。
【００６３】
前述のように、Ｚｒ／Ｏ／Ｗショットキー電子源を用いた本実施例においては、エネルギー幅が１．５ｅＶであるので問題ない。たとえば、もっとエネルギー幅の大きい電子源を用いた場合には、電子ビームの光路上にエネルギーフィルタを設け、電子源から電子が放出されてから最終的に画像が形成されるまでの間に電子のエネルギー幅を２ｅＶ以下にする必要がある。エネルギーフィルタは電子源からウェハ７の間に設けることが望ましいが、ウェハ７からのミラー電子に対してエネルギーフィルタリングを実施しても同様の効果は得られる。
【００６４】
本発明では、電子ビームがウェハ７に衝突することがない。したがって、原則的にはウェハ７表面に絶縁膜が存在していても表面が帯電することはない。したがって、帯電させない状態で検査を実施すれば、検出できる欠陥は形状欠陥（形状が正常部と異なっているもの）のみである。
【００６５】
しかし、電子ビームを用いた半導体パターンの欠陥検査において、導通不良や、絶縁されるべきものがショートしている、またはリーク電流が正常部と比べて大きいというような、いわゆる電気的欠陥と呼ばれているものを電子ビームの照射によって帯電させ、その電位の違いによって発生するＳＥＭ画像の電位コントラストにより検出する方法が行われている。
【００６６】
このような欠陥を高感度に検出できるように、本発明では検査画像を取得する前にあらかじめ帯電制御専用の電子ビームを照射する予備帯電制御装置を備えている。この装置によりウェハ７をあらかじめ所定の電位に帯電させてから検査を実施すれば、形状欠陥だけでなく導通不良部のような電気的欠陥を検出できる。以下、この動作と構成について説明する。
【００６７】
図１１は、予備帯電制御装置の動作原理を説明する図である。電子源４１は、大電流の電子ビームをある程度の広さ（数百μｍ〜数十ｍｍ）を持った面から放出する電子源である。たとえば、カーボンナノチューブを束ねた電子源やタングステンフィラメント熱電子源、あるいはＬａＢ６電子源等を用いることが可能である。引出グリッド４２に引出電極４８により引出電圧を印加して電子源４１から電子ビーム４３を放出させる。電子ビームは、制御グリッド４４を通過して絶縁膜４６に照射される。これにより二次電子４５が放出する。
【００６８】
この二次電子は、絶縁膜４６の表面の電位を基準としておよそ２ｅＶのエネルギーを持っている。絶縁膜表面が基板４７の電位と同等であれば電子ビームの照射エネルギーは加速電源４９の電圧であり、この電圧は二次電子放出効率が１以上となるような値に設定しておく。一般的な半導体デバイス用の絶縁膜材料では５００Ｖで良い。このとき、二次電子放出効率が１より大きいため、絶縁膜表面は正に帯電していく。
【００６９】
制御グリッド４４には制御電源５０が接続されており任意の正または負の電圧を印加できるようになっているので、絶縁膜表面の電位が制御グリッド４４の設定電位よりも正になり、二次電子が絶縁膜表面に引き戻されるようになると絶縁膜表面の正への帯電が止まる。このとき、絶縁膜表面の帯電電位は制御グリッドの電位よりやや低い（約２Ｖ）正の電位で安定することになる。制御グリッドの電位と等しくならないのは、二次電子がエネルギーを持っているためである。以上のような原理によって、絶縁膜４６表面の電位を制御グリッド４４の電位によって制御することが可能となる。
【００７０】
図１２は、カーボンナノチューブ電子源を用いた予備帯電制御装置の構成である。電子源４１は、碍子５１により真空内に保たれた状態で保持され、電位を印加できるようになっている。制御グリッド４４はウェハ７に面して配置され、引出グリッド４２が電子源５１から電子を引き出す。
【００７１】
図１３は、ＬａＢ６電子源を利用した予備帯電制御装置の構成である。ＬａＢ６電子源を顕微鏡に用いる場合は、ウェーネルト電極を用いて電子放出直後にクロスオーバを形成させるが、この場合は光源が小さい必要がないので代わりに引出電極４２’を設けている。
【００７２】
試料室１０２内では、２次元（Ｘ、Ｙ）方向に移動可能な試料移動ステージ８上にウェハ７が載置され、ウェハ７には電源９により前述のように電子ビームの大部分がウェハ７に衝突しないような負電位が印加されている。試料移動ステージ８にはステージ位置測定器２７が付設され、ステージ位置をリアルタイムで正確に計測している。これは、ステージ８を連続移動させながら画像を取得するためである。このステージ位置測定器２７には、例えばレーザ干渉計（ｌａｓｅｒ　ｉｎｔｅｒｆｅｒｏｍｅｔｅｒ）が用いられる。
【００７３】
また、半導体試料（ウェハ）表面の高さを正確に計測するために、光学的な試料高さ測定器２６も取りつけられている。これには、例えば、ウェハ表面上の検査すべき領域に斜め方向から光を入射させ、その反射光の位置変化からウェハ表面の高さを計測する方式のものを用いることができる。この他、試料室１０２には、検査領域の位置決め用に用いられる光学顕微鏡３１も付設されている。
【００７４】
次に、試料移動ステージ８の整定時間について述べる。ステージ８の移動方法をステッフ゜・アンド・リピート方式とすると、ステージ８の整定時間は、ｍｓｅｃオーダが必要となるため、画像Ｓ／Ｎ比を向上させて画像取得時間を短縮してもステージ移動に時間がかかってしまい検査時間を短縮することができない。従って、ステージ８の移動方法は、ステージが常にほぼ等速で移動している連続移動方式とした。これによりステージの整定時間による検査時間の制約はなくなる。ただし、ステージ８が連続移動していると、同一場所の画像を形成するのに必要な時間である１ショットの間にもステージ８が移動して、試料表面上での照射位置が変化してしまう。そこで、１ショットの間に照射位置が変化しないように、照射系偏向器５により照射電子ビームをステージ８の移動に追従させるようにした。また、静止座標系である電子光学系から見ると、電子ビーム照射位置は移動するから結像レンズ１１により作られる像１２も移動してしまう。この移動が生じないようにするために、結像系偏向器１０を照射系偏向器５と連動動作させるようにした。
【００７５】
次に、画像検出部１０３について説明する。画像検出には、ミラー電子の像１２の拡大像を光学像に変換するための蛍光板１５と光学画像検出素子（例えばＣＣＤ素子）１７とを光ファイバー束１６により光学結合させる。これにより蛍光板１５上の光学像を光学画像検出素子１７の受光面上に結像させる。光ファイバー束１６は、細い光ファイバーを画素数と同じ本数束ねたものである。また、上記光ファイバー束１６の代わりに光学レンズを用い、光学レンズによって蛍光板１５上の光学像を光学画像検出素子（ＣＣＤ）１７の受光面上に結像させるようにしてもよい。蛍光板１５の両面には電極３００と透明電極３０１を設け、両電極間に透明電極３０１側が正の高電圧を印加して電子ビームの散乱を防いでいる。光学画像検出素子（ＣＣＤ）１７は、その受光面上に結像された光学像を電気的な画像信号に変換して出力する。出力された画像信号は、画像処理部１０４に送られ、そこで画像信号処理が行われる。
【００７６】
次に、画像検出素子（ＣＣＤ）の読み取り時間について述べる。本実施例では、ＣＣＤ１７に蓄積された電荷を１２８チャンネルの読み出し口から８Ｍライン／秒の読出速度で多チャンネル並列読み出しできるようにした。１チャンネル１ライン当たりの画素数は８で、１ライン当たりの読み出所要時間は１２５ｎｓｅｃである。従って、１画素当たりの読み出所要時間は１２５ｎｓｅｃ／８（画素）＝１６ｎｓｅｃとなる。これに対し、ＣＣＤからの画像データの読み出しが１チャンネル方式では、非常に高速での読み出しが必要となり、実現困難である。
【００７７】
本実施例では、ＣＣＤからの画像データの読み出し口を１２８チャンネルに分け、この１２８チャンネルで並列同時読み出しする方式とすることによって、１画素当たりの読み出し所要時間を１６ｎｓｅｃとし、十分実現可能な読出速度としている。これを模式的に示したのが、図１４である。
【００７８】
ＣＣＤ１７からの画像データの読出チャンネル数は１２８ｃｈであり、各チャンネル毎に８画素×１０２４ラインがあるから、このＣＣＤから一枚の画像データを読み出すに必要な時間は約１２５μｓｅｃとなる。すなわち、１ショット領域の画像信号を１２５μｓｅｃで取り込めることになり、画素サイズ５０ｎｍ、１ショット領域を５０μｍ角とすれば試料表面積１ｃｍ^２当たりの検査所要時間は５ｓｅｃとなる。
【００７９】
以上のように、画素サイズ５０ｎｍで検査した場合の従来方式による試料面積１ｃｍ^２当たりの検査所要時間約４００ｓｅｃに比べて、８０倍もの高速化が達成できた。また、本実施例では、検査所要時間を決めているのはＣＣＤ素子からの信号読み出し速度であるので、将来ＣＣＤ素子におけるより高速のデータ読み出し方式が実現されれば、さらなる検査の高速化が期待できる。
【００８０】
画像処理部１０４は、画像信号記憶部１８及び１９、演算部２０、欠陥判定部２１より構成されている。画像記憶部１８と１９は同一パターンの隣接部の画像を記憶するようになっており、両者の画像を演算部２０で演算して両画像の異なる場所を検出する。この結果を欠陥判定部２１により欠陥として判定しその座標を記憶する。なお、取り込まれた画像信号はモニタ２２により画像表示される。
【００８１】
装置各部の動作命令および動作条件は、制御部１０５内の制御計算機２９から入出力される。制御計算機２９には、予め電子ビーム発生時の加速電圧、電子ビーム偏向幅・偏向速度、試料ステージ移動速度、画像検出素子からの画像信号取り込みタイミング等々の諸条件が入力されている。ビーム制御系２８は、制御計算機２９からの指令を受けて、ステージ位置測定器２７、試料高さ測定器２６からの信号を基にして補正信号を生成し、電子ビームが常に正しい位置に照射されるように対物レンズ電源２５や走査信号発生器２４に補正信号を送る。ステージ制御系３０は、制御計算機２９から指令を受けて試料移動ステージ８を制御する。
【００８２】
次に、実際の検査手順について説明する。まず、光学顕微鏡３１と電子ビーム画像を用いてのアライメントの方法について説明する。ウェハ７をウェハ移動ステージ（Ｘ−Ｙ−θステージ）８上に載置し、光学顕微鏡３１の下へ移動する。モニタ２２によりウェハ７表面の光学顕微鏡画像を観察し、画面内の例えば中央に現れた任意のパターンを記憶する。この際、選択するパターンは電子ビーム画像上でも観察可能なパターンである必要がある。
【００８３】
次に、上記の光学顕微鏡画像を用いてウェハ７表面上の回路パターンがステージ移動方向と平行あるいは直交となるように、ウェハ移動ステージ８により回転補正を行う。回転補正時には、あるステージ位置におけるウェハ７表面上の回路パターンの任意のチップ内の任意のパターン部分の光学画像を取り込んでモニタ２２に表示させて、表示画面内の任意箇所にマーキングを付す。そして、その光学画像信号を記憶部１８に記憶させる。
【００８４】
次に、ウェハ７表面上の回路パターンの数チップ分の距離だけステージ８をｘ方向またはｙ方向に移動させ、新たなチップ内の先と同一のパターン部分の光学画像を取り込んでモニタ２２に表示させる。そして先のマーキング箇所に対応する箇所にやはりマーキングを付した後、その新たな光学画像信号を記憶部１９に記憶させる。次いで、演算部２０において、記憶部１８、１９に記憶された光学画像信号同士を比較演算して、両画像間でのマーキング箇所の位置ずれ量を算出する。このマーキング箇所の位置ずれ量と両画像間でのステージ移動量とから、ウェハ７の回転角度誤差を算出し、その分ステージ８を回転させて回転角度を補正する。以上の回転補正操作を数回繰り返して、回転角度誤差が所定値以下となるようにする。
【００８５】
さらに、光学顕微鏡画像を用いてウェハ７表面上の回路パターンを観察し、ウェハ上でのチップの位置やチップ間の距離（例えば、メモリセルのような繰り返しパターンの繰り返しピッチ）を予め測定し、その値を制御計算機２９に入力する。そして、ウェハ７表面上の被検査チップおよびそのチップ内の被検査領域をモニタ２２の光学顕微鏡画像上で設定する。光学顕微鏡画像は、比較的低倍率で観察が可能であり、また、ウェハ７表面の回路パターンが例えばシリコン酸化膜のような透明な膜で覆われている場合でもその下地まで観察可能である。したがって、チップ内回路パターンのレイアウト等が簡便に観察でき、検査領域の設定が簡便に行える。
【００８６】
次に、ウェハ７を電子光学系の下へ移動する。そこで、先に光学顕微鏡画像上で設定した被検査領域を含むと予想される領域の電子ビーム画像を取得する。この時、１ショット領域内に上記の被検査領域が入るようにする。この電子ビーム画像上においても、先の光学顕微鏡画像上においてマーキングしたのと同じ画面内に、先にマーキングした箇所のパターンが現れるようにステージ８を移動する。これにより、予め検査開始前に電子ビーム照射位置と光学顕微鏡観察位置との間の対応をつけ、かつ、画像取得位置を校正することができるようになる。そして、この電子ビーム画像上において、先に光学顕微鏡像上で行ったのと同様の操作を実施する。これにより、光学顕微鏡を用いての簡便な観察位置の確認や位置合わせ、および電子ビーム照射位置の調整が可能となる。
【００８７】
さらには、ある程度の回転補正も実施した後に、光学顕微鏡画像に比べて分解能が高く、高倍率画像を得ることのできる電子ビーム画像を用いてさらに高精度な回転補正ができるようになる。さらに、この電子ビーム画像を用いて、被検査領域または同一パターン領域を高倍率で高精度に観察確認・補正することができる。ただし、半導体ウェハ７の表面の全部（または一部）が絶縁物で覆われている場合には、絶縁物表面の帯電電位が基板電位と等しくなっていない可能性があるので画像取得前に呼び帯電制御装置３２によって表面の帯電電圧を制御しておく必要がある。
【００８８】
上記した検査条件の設定が完了したら、半導体ウェハ７表面上の被検査領域の一部を実際の検査条件と全く同一の条件で電子ビーム画像化し、被検査領域の材質や形状に依存した画像の明るさの情報およびそのばらつき範囲を算出しテーブルにして記憶する。そして、後の検査工程において該記憶テーブルを参照して実際に画像化検出された被検査領域内のパターン部分が欠陥であるか否かを判定する際の判定条件を決定する。
【００８９】
上記の手順によって被検査領域および欠陥判定条件の設定が完了したら、実際に検査を開始する。検査時には、試料（半導体ウェハ）７を搭載したステージ８はｘ方向に一定速度で連続移動する。その間、電子ビームは各１ショットの間ウェハ７表面上の同一照射領域（面積領域）を一定のショット時間（本実施例では、５０μｓｅｃ以上）照射する。ステージ８は連続移動しているので、電子ビームは照射系偏向器５によってステージ８の移動に追従して偏向走査させる。
【００９０】
電子ビームの照射領域あるいは照射位置は、ステージ８に設けられたステージ位置測定器２７、試料高さ測定器２６等により常時モニタされる。これらのモニタ情報が制御計算機２９に転送されて詳細に位置ずれ量が把握され、かつこの位置ずれ量はビーム制御系２８によって正確に補正される。これにより、パターンの比較検査に必要な正確な位置合わせが高速・高精度で行われ得る。
【００９１】
また、半導体ウェハ７の表面高さを、電子ビーム以外の手段でリアルタイムに測定し、電子ビームを照射するための対物レンズ６や結像レンズ１１の焦点距離をダイナミックに補正する。電子ビーム以外の手段としては例えば、レーザ干渉方式や反射光の位置変化を計測する方式等による光学式の高さ測定器２６である。これにより、常に被検査領域の表面に焦点のあった電子ビーム像を形成することができる。また、予め検査前にウェハ７の反りを測定しておき、その測定データを基に上記の焦点距離補正をするようにして、実検査時にはウエハ７の表面高さ測定を行う必要がないようにしてもよい。
【００９２】
電子ビームをウェハ７表面に向け、ミラー電子によりウェハ７表面上の所望の被検査領域（面積領域）についての拡大光学像を蛍光板１５上に形成する。この拡大光学像をＣＣＤ素子１７により電気的な画像信号に変換し、この画像信号を画像処理部１０４に取り込む。そして、制御計算機２９からの指令を受けて制御部２８により与えられた電子ビーム照射位置に対応した面積領域についての電子ビーム画像信号として、記憶部１８（または１９）に格納する。
【００９３】
半導体ウェハ７表面上に形成された同一設計パターンを有する隣接チップＡ、Ｂ間でのパターンの比較検査をする場合には、先ず、チップＡ内の被検査領域についての電子ビーム画像信号を取り込んで、記憶部１８内に記憶させる。次に、隣接するチップＢ内の上記と対応する被検査領域についての画像信号を取り込んで、記憶部１９内に記憶させながら、それと同時に、記憶部１８内の記憶画像信号と比較する。さらに、次のチップＣ内の対応する被検査領域についての画像信号を取得し、それを記憶部１８に上書き記憶させながら、それと同時に、記憶部１９内のチップＢ内の被検査領域についての記憶画像信号と比較する。このような動作を繰り返して、全ての被検査チップ内の互いに対応する被検査領域についての画像信号を順次記憶させながら、比較して行く。
【００９４】
上記の方法以外に、予め、標準となる良品（欠陥のない）試料についての所望の検査領域の電子ビーム画像信号を記憶部１８内に記憶させておく方法を採ることも可能である。その場合には、予め制御計算機２９に上記良品試料についての検査領域および検査条件を入力しておき、これらの入力データに基づき上記良品試料についての検査を実行し、所望の検査領域についての取得画像信号を記憶部１８内に記憶する。次に、検査対象となるウェハ７をステージ８上にロードして、先と同様の手順で検査を実行する。
【００９５】
そして、上記と対応する検査領域についての取得画像信号を記憶部１９内に取り込むと同時に、この検査対象試料についての画像信号と先に記憶部１８内に記憶された上記良品試料についての画像信号とを比較する。これにより上記検査対象試料の上記所望の検査領域についてのパターン欠陥の有無を検出する。なお、上記標準（良品）試料としては、上記検査対象試料とは別の予めパターン欠陥が無いことが判っているウェハを用いても良いし、上記検査対象試料表面の予めパターン欠陥が無いことが判っている領域（チップ）を用いても良い。例えば、半導体試料（ウェハ）表面にパターンを形成する際、ウェハ全面にわたり下層パターンと上層パターン間での合わせずれ不良が発生することがある。このような場合には、比較対象が同一ウェハ内あるいは同一チップ内のパターン同士であると、上記のようなウェハ全面にわたり発生した不良（欠陥）は見落とされてしまう。
【００９６】
しかし、本実施例によれば、予め良品（無欠陥）であることが判っている領域の画像信号を記憶しておき、この記憶画像信号と検査対象領域の画像信号とを比較するので、上記したようなウェハ全面にわたり発生した不良をも精度良く検出することができる。
【００９７】
記憶部１８、１９内に記憶された両画像信号は、それぞれ演算部２０内に取り込まれ、そこで、既に求めてある欠陥判定条件に基づき、各種統計量（具体的には、画像濃度の平均値、分散等の統計量）、周辺の画素間での差分値等が算出される。これらの処理を施された両画像信号は、欠陥判定部２１内に転送されて、そこで比較されて両画像信号間での差信号が抽出される。これらの差信号と、既に求めて記憶してある欠陥判定条件とを比較して欠陥判定がなされ、欠陥と判定されたパターン領域の画像信号とそれ以外の領域の画像信号とが分別される。
【００９８】
これまでに述べてきた検査方法および検査装置により、ウェハ７の表面の電位および形状の情報を反映した画像を形成し、対応するパターン領域についての画像信号を比較検査することによって、パターン欠陥の有無を検出することが可能となった。これにより、従来の電子ビームによる検査装置と比べ非常に高速な検査が可能になった。
【００９９】
（実施例２）
実施例１では、１ショットの電子ビーム照射領域の面積が５０μｍ×５０μｍとかなり大きいため、半導体試料の拡大像の周辺部に歪みが生じると云う問題や照射領域内でのビーム電流密度の均一性に問題が生じる場合がある。画像歪みや電流密度の不均一性が固定的に生じている場合には、光ファイバー束１６のファイバー素線配列に変化を付けることで補正可能である。また、画像信号の取得感度や画像処理に重みを付けることでも補正できるが、それらが時間的に変動する場合には、それらの方法では対応が困難となる。
【０１００】
本実施例では、１ショットの照射領域を５μｍ角として、１ショットの照射領域内では歪みや電流密度の不均一性の問題が生じないようにした。照射電子ビーム電流は１ショット当たり１μＡである。この時、電子ビームの照射時間は電子の結像効率ηを０．５とすると、先の（１）式より、１ショット当たりの照射時間ｔは、０．１８μｓｅｃとなる。ショット時間０．１８μｓｅｃで一つの照射領域（５μｍ角）を照射した後、電子ビームは照射系偏向器５により隣接する次の照射領域（５μｍ角）上に移動される。このようにして、次々に照射位置を移動して、ｘ方向１００μｍ×ｙ方向１００μｍの範囲全体を２０×２０＝４００ショットで照射する。
【０１０１】
この時、ＣＣＤ素子１７上には、各１ショット毎に、その時の電子ビーム照射位置に対応した位置に拡大像が得られ、電子ビームの走査による電子ビーム照射位置の移動に応じてＣＣＤ素子に得られる拡大像位置も移動して行く。この様子を示したのが、図１５である。
【０１０２】
ＣＣＤ素子１７には１０２４×１０２４画素のものを用いた。ＣＣＤ素子上での１画素はウェハ７表面上での５０ｎｍ角の領域に相当し、従って、ウェハ７表面上での１ショットの照射領域（５μｍ角）は、ＣＣＤ素子受光面上での１００×１００画素の領域（ＣＣＤ素子受光面全体の１／１００に相当する）となる。そして、ＣＣＤ素子の受光面全体で試料表面上の５０μｍ角の領域をカバーできるようにした。従って、試料表面上での５０μｍ角の領域の拡大像を得るためには、０．１８（μｓｅｃ）×１００（ショット）＝１８（μｓｅｃ）を要することとなる。
【０１０３】
上述のようにして、ウェハ７表面上の５０μｍ角の領域の画像を１８μｓｅｃでＣＣＤ上に形成させたら、ＣＣＤに蓄積された画像信号をデジタル信号として画像記憶部１８に記憶させる。試料表面上の隣接する次の領域の画像信号を取得するためにはステージ８を５０μｍ移動させる必要がある。このステージ移動には、先の実施例１の場合と同様、ステージ８を一定速度で連続移動（ｃｏｎｔｉｎｕｏｕｓ　ｍｏｖｉｎｇ）させる方式を採った。その際、照射電子ビームに対してステージ８があたかも静止しているかの状態になる様、照射系偏向器５によって照射電子ビームをステージ８の移動に追従させて偏向走査するようにした。これにより、ステージ８を移動・停止させる際に生じる無駄時間をゼロにした。
【０１０４】
この連続移動ステージ８への照射電子ビームを追従させながら走査するためには、ビーム制御系２８内で、ステージ位置測定器２７からの信号を参照して偏向信号の補正信号を計算し、この偏向補正信号を照射系の偏向器５に送り照射電子ビームの偏向を制御させる。さらに、電子ビームによる試料拡大像の歪みや位置ドリフト等に関する補正分も上記の偏向信号の補正信号に重畳させることにより、これらの補正も行うようにした。また、照射系偏向器５と連動して結像系偏向器１０も動作させて、ＣＣＤ上での試料拡大像の位置が上記のステージ追従によるビーム位置移動の影響を受けないようにした。これにより、ステージ移動による無駄時間を無くし、高速・高精度の検査を実現することができた。なお、上記以後の欠陥検査のための画像処理等については、先の実施例１の場合と同様である。
【０１０５】
以上説明した手順により検査を進めていくと、試料表面１ｃｍ^２当たりについての拡大像をＣＣＤ上に順次形成するに要する時間Ｔは０．７２ｓｅｃとなる。一方、先の実施例１の場合と同様に、ＣＣＤから一枚の画像（試料表面５０μｍ角についての画像）を読み出すのに１２５μｓｅｃが必要であるため、試料表面積１ｃｍ^２当たり５ｓｅｃが必要となる。ＣＣＤ素子における画像形成と画像信号の読み出しは並行して行われるから、検査に要する時間は、画像形成に要する時間と画像信号読み出しに要する時間との内何れか長い方の時間となる。本実施例では、画像形成所要時間より画像信号読み出し所要時間が長く、１ｃｍ^２当たり５ｓｅｃとなっており、従って、本実施例における試料表面積１ｃｍ^２当たりについての検査所要時間は５ｓｅｃとなる。
【０１０６】
以上の説明では、１ショットの電子ビーム照射領域を５μｍ角の大きさに固定した場合について例示したが、半導体ウェハ７表面でのパターン繰り返しピッチに応じて、この電子ビーム照射領域の大きさを可変できるようにしても良い。上述したように、本実施例では、１ショットの電子ビーム照射領域をより小さく設定している。したがって、各照射領域間のつなぎ部分に多少の歪みが生じたとしても、常に同一箇所に同程度の歪みが生じることになり、相互比較すべき二つの画像上での歪みの現れ方も等しくなるため、歪みによる誤検出の問題が無くなる。これにより、信頼性の高いパターン欠陥検査が実現できる。
【０１０７】
（実施例３）
本実施例では、試料表面画像を電気信号に変換する素子として、時間蓄積型のＣＣＤセンサを用いた。この素子はＴＤＩセンサと呼ばれるもので、光学式検査装置において一般的に使用されている。それ以外は、先の実施例２の場合と同様である。このＴＤＩセンサの動作概念を、図１６を参照して説明する。
【０１０８】
ＴＤＩセンサでは、各受光領域で受光した光の強度に応じて生成された電荷をｘ方向のラインに移動させて行くと同時に、その移動先で受光した光の強度に応じて生成された電荷を順次足し合わせて行くように動作する。そして、受光面の最終ラインに達した時点で電気信号として外部に出力する。従って、ｘ方向の電荷の移動速度と受光面上の画像のｘ方向の移動速度を同一にすることで、画像がセンサ上を移動する間の信号を積分して出力することになる。
【０１０９】
本実施例では、先の実施例１から実施例４の場合のＣＣＤセンサと同様に、信号読み出しを１２８チャンネルに分割しそれぞれ並行して読み出すことにより、読出速度を４Ｍライン／秒とした。また、受光領域の大きさは、ｘ方向に６４画素、ｙ方向に２０４８画素のものを用いた。１ラインのｘ方向長さは、試料表面上の５０ｎｍ、ｙ方向長さは約１００μｍに相当する。このとき、縦５０ｎｍ、横１００μｍの画像が４Ｍ／秒の速度で出力されることになるため、ステージの連続移動速度もこれと同じ速度（５０ｎｍ／２５０ｎｓｅｃ＝２００ｍｍ／ｓｅｃ）としている。このように、検査領域のｘ方向移動はステージ８を移動させることにより行う。
【０１１０】
一方、１ショットの照射領域は５μｍ角であるので、図１６のように、照射領域のｙ方向移動は電子ビームを走査して行う必要が生じる。すなわち、ステージ８がｘ方向に１ショット分（５μｍ）だけ移動する間に電子ビームをｙ方向に１００μｍ走査する必要がある。１ショットの所要時間を１．２５μｓｅｃとすると、ｙ方向に１００μｍ（２０ショット分）を走査するには２５μｓｅｃ必要となる。一方、ステージ８のｘ方向移動速度は２００ｍｍ／ｓｅｃであるから、ステージ８がｘ方向に丁度１ショット分（５μｍ）移動するに要する時間は２５μｓｅｃとなる。このように、ｘ方向に１ショット分（５μｍ）のステージ移動に要する時間とｙ方向に２０ショット分（１００μｍ）の電子ビーム走査に要する時間とを一致させ、無駄時間が生じるのを防いでいる。この方法によって試料表面積１ｃｍ^２の画像を取得するには、上述の５μｍ×１００μｍの単位走査領域についての走査所要時間（２５μｓｅｃ）の２×１０^５倍を要することになるので、試料表面積１ｃｍ^２当たりの検査所要時間は５ｓｅｃとなる。
【０１１１】
上述したように、本実施例では、ＴＤＩセンサの信号出力速度から決まるステージの移動速度が２００ｍｍ／ｓｅｃであるから、十分ステージ移動による検査領域のｘ方向移動が可能である。しかもその間に電子ビームの検査領域上ｙ方向走査のための十分な時間を確保できる。また、本実施例では、検査速度を決めているのはＴＤＩセンサの信号出力速度であるため、この信号出力速度が改善されれば、さらに高速での検査が実現できる。
【０１１２】
（実施例４）
本実施例は、ＳＥＭ画像が取得可能な電子光学系を採用したものである。図１７に、その構成を示す。
【０１１３】
電子源２０１、コンデンサレンズ２０２およびＳＥＭ用対物レンズ２３３は、ＳＥＭの電子光学系を構成する要素をそのまま採用している。電子源２０１は、Ｚｒ／Ｏ／Ｗショットキー電子源を採用した。この電子源から引出した電子はビームセパレータ２４３により偏向を受け、静電セクタ（ｅｌｅｃｔｒｏｓｔａｔｉｃ　ｓｅｃｔｏｒ　ｅｌｅｃｔｒｏｄｅ）型の電子偏向器２０５により大角度の変更を受けてビームセパレータ２０３に導かれ対物レンズ２０６に垂直入射する。電子ビームは対物レンズの前焦点面でクロスオーバを結んでおり対物レンズ２０６によりウェハ２０７の表面に垂直な方向にそろった面状電子ビームとなる。ウェハ２０７に印加する電圧や絞りの配置等に関しては実施例１と同等である。
【０１１４】
本実施例では、ウェハ検査後に検出した欠陥の画像を詳細に観察したい場合に装置からウェハ７を取り出さずに高分解能なＳＥＭ画像を観察できることが特徴である。すなわち、ビームセパレータ２４３を動作させずに電子ビームを直進させるようにし、同時にウェハ移動ステージ２０８によりウェハ２０７をＳＥＭ用対物レンズ２３３の光軸下に移動すればウェハ２０７の任意の位置の観察が実行できる。なお、図中、２１１は結像レンズ、２１３、２１４は拡大レンズ、２２２はＳＥＭ用コンデンサレンズ、２２８はビーム制御系、２３２は予備帯電制御装置、２５２は試料室、２６３は画像検出部を示す。
【０１１５】
この機能は、検出した欠陥の観察だけでなく検査前のウェハのパターン確認や検査条件設定、アライメント等でも活用できる。
【０１１６】
（実施例５）
本実施例は、試料に注入して正常パターンと欠陥パターンとの電位差を生じさせる電子線と、試料に当たらずに反射して結像する電子線とをほぼ同じ場所でほぼ同時に照射する手段を設けたものである。図１８にその構成を示す。電子源１より放出された電子線は、ビームセパレータ３によりウェハ７に垂直な光軸方向に偏向され、対物レンズ６により試料（ウェハ）表面に垂直な方向にそろった面状の入射電子線３０１が形成される。　試料（ウェハ）７には、電子線の加速電圧とほぼ同じ負の電位が試料印加電源９によって印加されている。試料印加電源９は円孔レンズ電極４１と試料７の間の電位差をも設定できるようになっており、円孔レンズ電極（ａｐｅｒｔｕｒｅ　ｌｅｎｓ　ｅｌｅｃｔｒｏｄｅ）４１と試料７の間の減速電界を可変できるような構成である。たとえば、形状欠陥を検出する場合には、減速電界を通常より強く設定して形状欠陥の検出感度を向上させることができる。円孔レンズ電極４１と試料７の間で入射電子線３０１は急速に減速されて、試料７の表面に形成された半導体パターン形状や帯電の状態を反映した電界により入射電子線３０１は試料に吸収されるか、試料の直上で向きを変えて反射電子線３０２となる。
試料７の表面に絶縁物が存在する場合には、入射電子線３０１を試料７に注入して、絶縁物が帯電した状態で画像を取得する。たとえば、図１９の断面図は、シリコン基板（Ｓｉ）３１０上に絶縁膜（ＳｉＯ２膜）３１１が形成されたウェハにコンタクトホール加工後タングステン（Ｗ）の電極部が埋め込まれた試料を示したものである。電極部は基板と低抵抗で導通している正常電極部３１２と、中央部には電極の底にあるＳｉＯ２の残渣（ｒｅｓｉｄｕｅ）により基板と高抵抗状態である不良電極部３１３がある。
一次電子線に対する二次電子の放出比（ｔｈｅ　ｍｅａｎ　ｎｕｍｂｅｒ　ｏｆ　ｓｅｃｏｎｄａｒｙ　ｅｌｅｃｔｒｏｎｓ　ｅｘｃｉｔｅｄ　ｐｅｒ　ｉｎｃｉｄｅｎｔ　ｅｌｅｃｔｒｏｎ　　）が１より小さい極低エネルギーの電子線を絶縁物試料に照射すると、絶縁物試料は負電荷（電子）が蓄積することにより負帯電する。例えば、ＳｉＯ２は約３０ｅＶ以下の照射エネルギーで、二次電子の放出比が１未満となり負帯電する。図１９（ａ）は絶縁膜３１１を負帯電させる場合の初期照射の状態を示したものである。試料７に照射する入射電子線３０１のエネルギーは、試料７が帯電しない初期状態において、照射エネルギーｅＶ０を含むエネルギー拡がり（ｅｎｅｒｇｙ　ｓｐｒｅａｄ）を持ったものとする。ここで、照射エネルギーは試料７に対する電子源１の電位差と照射電子線のエネルギー拡がりで定められるものである。入射電子線３０１を試料７に照射し続けると、絶縁膜３１１の負帯電は低エネルギーの入射電子線を反発するようになるまで進行し、絶縁膜３１１に進入する電子数と絶縁膜３１１から周辺へ逃げていく電子数が釣り合った電位まで負帯電して安定する。特に、絶縁膜３１１中に蓄積した電子が周辺部に逃げていかない場合には、絶縁膜３１１は入射電子線３０１が進入しなくなる電位まで負帯電して安定する。電極部もまた、進入する電子と電極部から周辺へ逃げていく電子数が釣り合うような電位になる。すなわち、正常電極部３１２に注入された電子は低抵抗で接触している基板３１０に流れ込むのでほとんど負帯電しないが、不良電極部３１３は抵抗値が大きく、周辺部へ逃げていく電子数が小さいので負帯電した状態となる。図１９（ｂ）に示すように、等電位面３１４はこれらの電極および絶縁物電位を反映し、Ｖ０の等電位面３１５が絶縁膜３１１および不良電極部３１３の直上近傍に形成されて、エネルギーＶ０より小さい入射電子線３０１を反射させるが、正常電極部３１２の直上近傍にはＶ０の等電位面３１５が形成されずに入射電子線３０１は正常電極部３１２に衝突するので、反射電子線３０２の結像像（ｉｍａｇｅ）は正常電極部３１２と不良電極部３１３の直上近傍の等電位面形状の差を反映した像となる。
図２１（ａ）は３ｘ３列の７０ｎｍ角プラグパターンの正常部が０Ｖ、中央パターンの不良部が−１Ｖ、ＳｉＯ２膜が−２Ｖに帯電した場合に、正常部電位０Ｖより１．２Ｖほど大きなエネルギーで照射した電子線が反射して結像したミラー結像像の模式図である。正常部は電子がパターンに衝突して吸収されるので暗パターン、高抵抗のオープン欠陥は負帯電しており電子は当たらずに反射して結像されるので明パターンのコントラストとして識別される。
ここで、絶縁物３１１の帯電電位は入射電子線３０１のエネルギーｅＶ０に依存するので、入射電子線３０１のエネルギーを変化させることによって、絶縁物３１１の帯電電位を自由に制御することができる。
また、試料の基板の電位（０Ｖ）に対してｅＶ１のエネルギーを持った入射電子線３０１はすべてＶ１の等電位面３１６で反射されるが、Ｖ１の等電位面３１６の形状は正常電極部３１２と不良電極部３１３の直上近傍では大きく異なるので、ｅＶ１のエネルギーを持った入射電子線３０１を反射させた反射電子線３０２の結像像でも正常電極部３１２と不良電極部３１３とを識別できる像を得ることができる。
絶縁膜の帯電電圧が安定するまでの時間の目安は以下のようになる。絶縁膜の上下に電極がある平行平板コンデンサと考えれば、絶縁膜の膜厚をＤ、電極面積をＤ、真空の誘電率をε０、絶縁膜の誘電率をεｒとおいて、コンデンサの静電容量（ｃａｐａｃｉｔａｎｃｅ　ｏｆ　ｃａｐａｃｉｔｏｒ）Ｃ＝ε０εｒＳ／Ｄで表わされる。たとえば、膜厚０．４μｍ、εｒ＝４の絶縁膜に照射面積２０μｍ角で１μＡの電流を照射すれば、１Ｖの電圧に帯電する時間はおおよそ３５ｎｓとなるので、一瞬の内に帯電して安定化する。絶縁膜が一瞬に帯電して安定しない場合には、図２０に示すように、検査方向（走査）方向の前方に電子線照射領域を画像取得領域より大きく取り、画像取得前に電子線をあらかじめ照射できる構成とした。また、電子線取得領域を画像取得領域とほぼ等しく取る場合には、画像取得領域に電子線を照射して、帯電が安定化した後に画像取得するように画像取得回路を制御する構成としても良い。
ここで、ウェハ７に当たらずに反射する反射電子線３０２とは別に、ウェハ（試料）７に入射電子線３０１が衝突することによって試料７から二次電子が発生するが、二次電子の放出角度分布（ａｎｇｕｌａｒ　ｄｉｓｔｒｉｂｕｔｉｏｎ　ｏｆ　ｓｅｃｏｎｄａｒｙ　ｅｌｅｃｔｒｏｎｓ）は図７に示すように分散した形となる。そこで、対物レンズ６の後焦点面（ｂａｃｋｆｏｃａｌ　ｐｌａｎｅ）近傍に設けたコントラストアパーチャ１２を小さくしてやれば、角度分散した二次電子はほとんどコントラストアパーチャ１２により吸収されるので、試料に当たらずに反射するビームのみで画像を形成することができる。
電気的欠陥を検出するためには、基板との接合状態によりパターン内の電子の移動方向を制御する必要もある。そこで、絶縁膜を負帯電だけでなく正帯電させることが必要である。絶縁膜３１１を正帯電させるためには、一次電子線に対する二次電子の放出比が１より大きいエネルギーの電子線を照射しなければならない。図１９（ｃ）は絶縁膜３１１を正帯電させる場合の初期照射の状態を示したものである。まず、例えば、まず試料電位を基準として、試料７にエネルギーｅＶｂのエネルギーの入射電子線３０１を照射する。例えば、Ｖｂとして５００Ｖを選べば、試料７からの２次電子放出比が１より大きくなるので、絶縁物３１１は正電位に帯電し、正電位に帯電したことによって生じる障壁（ｐｏｔｅｎｔｉａｌ　ｂａｒｒｉｅｒ）によって試料に引き戻される電子により電位が安定するまで帯電が進行する。このように絶縁物３１１および不良電極部３１３が正帯電した後に、図１９（ｄ）に示すように絶縁物パターンにも当たらないｅＶｐのエネルギーの電子を照射すれば、Ｖｐの等電位面３１７は正常電極部３１２と不良電極部３１３の直上近傍では大きく異なるので、正帯電部の電位の歪みを検出することができる。照射する入射電子線３０１のエネルギーを増やしていくと、絶縁膜３１１が正帯電しているので、パターン部より早く絶縁膜３１１に入射電子線３０１が衝突する。絶縁膜３１１に入射電子線３０１が衝突すると正帯電していた電位が緩和されてしまうので、試料に照射するエネルギーとしては、絶縁膜に電子線が衝突しないような条件に選ぶ必要がある。ただし、正帯電させる電子線と画像を取得する電子線を同時に試料７へ照射する場合には、画像を取得する電子線が絶縁物に衝突して得られる絶縁物への電子の注入と、正帯電させる電子線により得られる絶縁物の電子放出が平衡状態になるように絶縁物の正帯電の電位が安定するので、同時照射の場合には正帯電電位を制御することが可能である。
図２１（ｂ）　は３ｘ３列の電極の内、正常電極部３１２が０Ｖ、中央パターンの不良電極部３１３が＋１Ｖ、絶縁膜３１１が＋２Ｖに帯電した場合に、入射電子線３０１を上記の条件、すなわち正常電極部電位０Ｖより１．９Ｖほど小さなエネルギーで照射した条件で結像した場合のミラー結像像の模式図である。正常電極部と不良電極部のパターンの電位差を反映して、試料７と円孔レンズ４１の間に形成される等電位面は、正常パターン近傍と不良部のパターン近傍では異なる形状になる。試料に衝突しないで反射した電子線の軌道は、反射される位置における等電位面の形状に大きく依存し、正常パターン近傍で反射した電子線と不良部のパターン近傍で反射した電子線とでは、電子線の結像位置が大きく変化する。図２１（ｂ）は正常パターンの近傍で反射した電子の結像面付近に投影レンズの物面位置を合わせたものであり、正常パターンが明るいコントラストとなる像が形成される。また、欠陥パターンの前方で反射した電子の結像面付近に投影レンズの物面位置を合わせることも可能となり、その場合には図２１（ｂ）が反転した図２１（ａ）のような像を得ることができる。
上記の操作を１つの電子源で実現させるには、試料印加電源９と電子源電源２３との相対電圧、すなわち電子源電源２３あるいは試料印加電源９の印加電圧を周期的に切り替えることにより実現できる。例えば、図２２に示すように、試料印加電源電圧を基準として電子源電源２３に印加する電圧を周期的に切り替えるとする。まず、ステージ８あるいは照射系偏向器５を用いて、所定の検査領域に電子線を移動し、試料を正帯電させる電圧＋Ｖｂ、例えば５００Ｖを電子源電源２３から電子源１に印加して一定時間照射することにより、所定の検査領域を正帯電させる。次に、電子源電源２３に供給する電圧を画像取得する電圧−Ｖｐに切り替えて電子源１に印加することにより、試料ポテンシャルに対し−ｅＶｐほどエネルギーの小さいビームを所定の検査領域に照射して、試料が正帯電した状態で一定時間画像を取得することができる。検査領域を移動するたびにこの操作を周期的に繰り返すことにより、試料を正帯電させた状態で連続して画像を取得することができる。
【０１１７】
（実施例６）
試料直前で引き戻されて結像する電子線を発生する電子源とは別に、一次電子線に対する二次電子の放出比が１より大きいエネルギーの電子線を同時に照射するための電子源を新たに配置することで、試料を正帯電させると同時に画像を取得することが可能となる。図２３に本実施例の構成を示す。
電子源１はウェハ７よりわずかに正電位に置かれており、電子源１から放出された入射電子線３０１はウェハ７の直上で引き戻されて反射電子線３０２となる。一方、第二の電子源７１とウェハ７とは約５００Ｖの電位差を持ち、第二の電子源７１から放出された電子線３０３は試料に５００ｅＶのエネルギーで照射される。磁界プリズム（ｍａｇｎｅｔｉｃ　ｐｒｉｓｍ）７３はＶｓの電圧にフローティングされたシールド電極７４に覆われており、磁界プリズム７３に入射する電子線はｅＶｓのエネルギーだけ減速されて偏向を受け、磁界プリズム７３を出射する電子線はエネルギーｅＶｓだけ加速される。例えば、電子源１から出射した電子線３０１は、磁界プリズム７３の直前で約５００Ｖまで減速され、磁界プリズム７２に入射する。一方、第二の電子源７１から出射した電子線３０３は、磁界プリズム７３に入射する直前に約１０００Ｖまで減速され、磁界プリズム７３に入射する。ここでプリズム内の磁束密度を５ガウスに設定すれば、電子線３０１はサイクロトロン半径１５０ｍｍ、電子線３０２は半径約２１０ｍｍの軌道を描いて同一光軸３０４　に入射することにより、同一視野を異なるエネルギーの電子線で照射することができる。試料の直上近傍で引き戻された反射電子線３０２は、磁界プリズム７３に入射して入射電子線と反対方向に偏向された後、中間レンズ（ｉｎｔｅｒｍｅｄｉａｔｅ　ｌｅｎｓ）１３、投影レンズ（ｐｒｏｊｅｃｔｉｏｎ　ｌｅｎｓ）１４を通じて、蛍光版１５上に結像される。
＜実施例７＞
図２４に示す本実施例は、第二の電子源２０１および第二の照射レンズ２０２を配置して、第二の電子源２０１から出射した電子線３０３を円孔レンズ電極４１下部に衝突させて発生した弾性散乱電子３０４を試料７に照射できるような構成である。円孔レンズ電極４１下部に電子線を衝突させて発生した弾性散乱電子３０４の光軸方向のエネルギー成分が円孔レンズ電極４１と試料７の間に印加される電圧で減速されるエネルギーより大きければ、弾性散乱電子３０４は試料面に達する。例えば、弾性散乱電子３０４の方向分布が光軸４０２に対し３０度傾斜した方向にピークを持っていれば、第二の電子源２０１から生じる電子線３０３の加速電圧としては、円孔レンズ電極４１と試料７の間に印加される電圧に５００Ｖ／ＣＯＳ３０°＝　５７７Ｖ加えた電圧を与えてやれば、弾性散乱した電子３０４のピーク成分は試料７に約５００Ｖの加速電圧で照射できるので、試料７を正帯電させることができる。電極下部に衝突させる電子線３０３の角度および位置は、弾性散乱電子３０４が減速電界で軌道（ｔｒａｊｅｃｔｏｒｙ）を変えることを考慮して、結像させるための照射電子線３０１と照射位置がほぼ同じくなるように設定するが、アライナ２０４にビーム傾斜機能とビーム位置移動機能を持たせ、照射位置が調整できるようにしている。
＜実施例８＞
図２５に示す本実施例は、試料を帯電させる手段として光源２５１を配置して、試料に光線２５２を同時照射できるような構成である。試料７に試料７を構成する物質の仕事関数より大きなエネルギーの光線を照射すれば、試料７から光電子が放出されることによって、試料７を正帯電させることができる。図２６の断面図は、シリコン（Ｓｉ）上に絶縁膜（ＳｉＯ２膜）が形成されたウェハにコンタクトホール加工後タングステン（Ｗ）プラグが埋め込まれた試料７を示したものである。中央部のプラグ底部にＳｉＯ２の残渣に起因する高抵抗のオープン欠陥が存在する。
図２６（ａ）は絶縁膜３１１を正帯電させる場合の初期照射の状態を示したものである。試料７の電位に対して電子源１の電位は＋Ｖｐの正電位に置かれ、電子源１から出射した電子線３０１は円孔電極４１と試料７の間で減速されて、試料の直上近傍で方向を変えて反射電子線３０２となる。ＳｉＯ２の仕事関数はおおよそ９ｅＶであるので、光線２５２として９ｅＶよりエネルギーの高い紫外線を同時に試料に照射すれば、ＳｉＯ２で構成される絶縁膜３１１は光電子３０５を放出するので、図２６（ｂ）に示すように、絶縁膜３１１が正帯電して絶縁膜３１１直上近傍の等電位面３１４は湾曲するようになる。絶縁膜３１１の正帯電がさらに進行すると、入射電子線３０１の一部が絶縁膜３１１に達するようになり、絶縁膜３１１からの光電子３０５の放出量と入射電子線３０１の絶縁膜３１１への到達量が等しくなるような平衡状態で絶縁膜３１１の正帯電電位は安定する。この状態で反射された電子線３０２を結像すると、絶縁膜３１１の部分が暗くなるとともに、絶縁膜近傍の電位歪を反映するような像が形成されるので、パターン正常部と異常部とのコントラストが高い像を取得できる。光源２５１から出射した光線２５２の光軸４０４と照射電子線３０１の光軸４０２とは、試料上で概一致するように調整されているが、光線２５２の試料７での照射領域を照射電子線３０１の照射領域より広く取ることによって、ステージを移動させて連続的に画像を取得する際には、一定時間光線２５２を照射して試料を正帯電させた状態で試料７に照射電子線３０１を照射することができる。同時に発生した光電子３０５は反射電子線の光路内に配置されたコントラストアパーチャ１２により吸収させることができるので、コントラストの高い結像電子像を得ることができる。
＜実施例９＞
本実施例では、ビームセパレータ３として、Ｅ×Ｂ型偏向器（Ｅ×Ｂ　ｄｅｆｌｅｃｔｏｒ）２組と球面型の静電型プリズム（ｅｌｅｃｔｒｏｓｔａｔｉｃ　ｐｒｉｓｍ）２組を組み合わせて、入射電子線と反射電子線を分離する構成を用いた。その構成図を図２７に示す。この構成では試料から反射した反射電子線はＥ×Ｂ型偏向器５４を直進するように設定されており、入射電子線３０１は２組のＥ×Ｂ型偏向器を逆方向に動作させるとともに、２組の静電型プリズムも逆方向に動作させてレンズ結像条件を最適に選ぶことにより、照射系および結像系の収差を低減できる構成である。さらに、結像系および照射系をステージに対して垂直に並べられる構成となっているので、機械振動などによる電子光学系への影響を最小限に低減できる効果がある。以下、構成の詳細を説明する。試料から反射した反射電子線に対しては、Ｅ×Ｂ型偏向器５４は磁界と電界の作用が打ち消すように動作して、反射電子線はＥ×Ｂ型偏向器５４内を直進する。さらに、反射電子線の像点をＥ×Ｂ型偏向器５４近傍に置くことによりＥ×Ｂ型偏向器５４で発生するレンズ収差を小さくするようにしている。電子源から出た入射電子線３０１はコンデンサレンズによって光軸４０１上の結像点４０３に結像した後、Ｅ×Ｂ型偏向器５１に入射して、約１５度偏向される。偏向した入射電子線３０１は静電型プリズム５２に入射して、円形軌道を描き、所定の角度回転しながら、集束レンズ作用を受ける。静電型プリズム５２を出た入射電子線３０１は静電型プリズム５３に入射し、反対方向に回転しながら集束作用を受ける。静電型プリズム５３を出射した入射電子線３０１はＥ×Ｂ型偏向器５４に入射し、Ｅ×Ｂ型偏向器５１とは反対方向に約１５度偏向を受けて、照射系の対物レンズ光軸４０２に入射して、結像点４０４に結像する。ここで、静電型プリズム５１と結像点４０３との間の距離と静電型プリズム５３と結像点４０４の間の距離をほぼ等しく置き、静電型プリズム５１をほぼ平行で出射した入射電子線３０１が静電型プリズム５３にほぼ平行に入射する条件になるように上記静電型プリズムの動作条件を選べば、この２組の静電型プリズムの収差はほぼ補正される。
たとえば、回転角２００度、半径５０ｍｍの静電型プリズムに静電型プリズム５１とコンデンサレンズ結像点４０３との間の距離約１７０ｍｍから入射エネルギー１０ｋｅＶで入射電子線３０１が入射するとプリズム出射時には入射電子線３０１はほぼ平行となり、静電型プリズム５４出射時にはほぼ対称の位置、静電型プリズム５３と結像点４０４の間の距離約１７０ｍｍの位置に集束する。この条件では、静電型プリズム５１と静電型プリズム５４の収差は打ち消しあってほぼ零になるので、試料に高輝度な入射電子線３０１を照射することが可能となる。＜実施例１０＞
本実施例は試料高さや傾斜角が変動しても正確な検査ができる手段を設けたものである。大口径のウェハは、ウェハのそり等による変形により試料の高さや傾斜角度が面内で大きく変動してしまうので、試料高さが変動しても結像位置が変動しない手段と、試料傾斜角が変動しても常に試料面に垂直に入射電子線が入射するための手段が必須である。
図２８はウェハが傾斜した場合にも、試料に垂直に入射電子線３０１が入射するための制御方法を説明するものである。
ｘｙｚ直交座標系において、ｚ方向を光軸に取り、通常はｚ＝０のｘｙ平面内にある試料表面が、ｚ＝０を通り、ｘ方向にθだけ傾斜している面内にあるとすると、試料７と円孔レンズ４１電極間の距離をＬと置けば、電界強度のｚ方向成分Ｅｚおよびｘ方向成分Ｅｘはそれぞれ、Ｅｚ≒Ｅ、Ｅｘ≒（１−ｚ／Ｌ）Ｅで近似される。ｘ方向およびｚ方向について電子の運動方程式を解けば、入射電子線３０１が試料７に一番近づいた状態で試料７にほぼ垂直に入射する条件は、円孔レンズ４１を通過する角度βが
【０１１８】
【数１】

をおおむね満たすことであることがわかる。
さらに、円孔レンズ４１の焦点距離は、円孔レンズ４１の円孔の直径がＬに対して十分小さければｆ＝４Ｌと近似できるので、このような条件においては、円孔レンズ４１に入射する入射電子線３０１の角度を２ｓｉｎθだけ傾ければ良いことがわかる。
そこで、たとえば、対物レンズ７と円孔レンズ４１の間にアライナ４２を配置して、試料がｘ方向にθ傾いた場合には、概２θだけ傾斜するように作用させれば、常に試料に垂直に入射電子線３０１を照射することができる。ここで、このアライナ４２は入射電子線３０１と反射電子線３０２とで同じ方向に作用するように静電型のアライナを用いるほうが望ましい。
試料高さの変動に対する補正は以下のように行う。円孔レンズ４１の焦点距離ｆは、ｆ＝４Ｌで近似され、試料の下Ｌ／３の位置に試料から反射した反射電子線の虚像（ｖｉｒｔｕａｌ　ｉｍａｇｅ）が形成される。対物レンズ７の物面位置（ｏｂｊｅｃｔ　ｐｌａｎｅ）はこの円孔レンズ４１の虚像位置（ｖｉｒｔｕａｌ　ｉｍａｇｅ　ｐｌａｎｅ）となるので、試料高さが変動すると円孔レンズ４１の虚像位置が変動し、対物レンズ（ｏｂｊｅｃｔｉｖｅ　ｌｅｎｓ）７の像面位置も変動してしまうことになる。そこで、試料高さからＬを算出して、円孔レンズの虚像位置すなわち対物レンズの物面位置を算出して、常に対物レンズの像面位置が一定となるように対物レンズの強度（ｌｅｎｓ　ｅｘｃｉｔａｔｉｏｎ）を制御すれば，試料高さが変動しても常に像がボケないように制御することができる。
半導体ウェハ７の高さ分布は、あらかじめ検査前にウェハ内の数十点以上の高さ計測点について、高さ測定器２６などの高さ計測手段を用いて計測される。試料の特定点の傾斜角は隣接した高さ計測点との高度差を、隣接した高さ計測点までの距離で割ることにより求めることができる。隣接した複数の高さ計測点との傾斜角計測値の平均を取ることにより、計測値の制度を向上させることができる。任意の検査位置に対応する傾斜角データは上記高さ測定点の傾斜角データを補間することにより求めることができる。検査時にはこの検査位置座標に対応したウェハ傾斜データに基づいて、アライナ４２の強度を制御することにより、ウェハ面内で常に試料に垂直に入射電子線３０１を照射することが可能となる。
【０１１９】
【発明の効果】
本発明の実施例によれば、電子ビームを用いて、半導体装置等の試料表面の微細なパターン欠陥およびオープン、ショート、リーク等の電気的欠陥を高感度、高分解能で、かつ高速に画像化して検査することのできる検査方法および検査装置を実現する。
【図面の簡単な説明】
【図１】本発明の第１の実施例になる検査装置の概略構成を示す図。
【図２】本発明の原理を説明する図。
【図３】本発明の原理を説明する図。
【図４】本発明で得られる欠陥の画像例を示す図。
【図５】従来例において画像形成に利用できる電子の効率を説明する図。
【図６】従来例において画像形成に利用できる電子の効率を説明する図。
【図７】従来例と本発明の画像形成に利用できる電子の効率の違いを説明する図。
【図８】従来例と本発明のコントラスト（ａ）と検査時間（ｂ）を比較する図。
【図９】本発明の第１の実施例の構成を説明する図。
【図１０】電子のエネルギー幅と欠陥検出感度の関係を説明する図
【図１１】本発明の第１の実施例における予備帯電制御装置の原理を説明する図。
【図１２】本発明の第１の実施例における予備帯電制御装置の一構成を説明する図。
【図１３】本発明の第１の実施例における予備帯電制御装置の他の構成を説明する図。
【図１４】本発明の第１の実施例における画像信号検出手段を説明する図。
【図１５】本発明の第２の実施例における画像信号検出手段を説明する図。
【図１６】本発明の第３の実施例における画像信号検出手段を説明する図。
【図１７】本発明の第４の実施例における電子光学系の構成を説明する図。
【図１８】本発明の第５の実施例の構成を説明する図。
【図１９】本発明の第５の実施例における帯電制御を説明する図
【図２０】本発明の第５の実施例における画像取得領域と電子線照射領域を説明する図。
【図２１】本発明の第５の実施例における欠陥の画像例を示す図。
【図２２】本発明の第５の実施例における試料の印加電圧の制御を説明する図。
【図２３】本発明の第６の実施例を説明する図。
【図２４】本発明の第７の実施例を説明する図。
【図２５】本発明の第８の実施例を説明する図。
【図２６】本発明の第８における帯電制御を説明する図。
【図２７】本発明の第９の実施例を説明する図。
【図２８】本発明の第１０の実施例を説明する図。
【符号の説明】
１：電子源、２：コンデンサレンズ、３：ビームセパレータ、４：絞り、５：照射系偏向器、６：対物レンズ、７：試料（ウェハ）、８：ウェハ移動ステージ、９：電源、１０：結像系偏向器、１１：結像レンズ、１２：コントラストアパーチャ、１３：拡大レンズ、１４：拡大レンズ、１５：蛍光板、１６：光ファイバー束、１７：ＣＣＤ、１８：画像記憶部、１９：画像記憶部、２０：演算部、２１：欠陥判定部、２２：モニタ、２３：加速電源、２４：走査信号発生器、２５：対物レンズ電源、２６：試料高さ測定器、２７：ステージ位置測定器、２８：ビーム制御系、２９：制御計算機、３０：ステージ制御系、３１：光学顕微鏡、３２：予備帯電制御装置、１０１：電子光学系、１０２：試料室、１０３：画像検出部、１０４：画像処理部、１０５：制御部、２０１：電子銃、２０２：コンデンサレンズ、２０３：ビームセパレータ、２０５：偏向器、２０６：対物レンズ、２０７：試料（ウェハ）、２０８：ウェハ移動ステージ、２１１：結像レンズ、２１３、２１４：拡大レンズ、２２２：ＳＥＭ用コンデンサレンズ、２２８：ビーム制御系、２３２：予備帯電制御装置、２３３：ＳＥＭ用対物レンズ、２４３：ビームセパレータ、２５２：試料室、２６３：画像検出部。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for inspecting a surface state of a sample (such as a semiconductor device).
[0002]
[Prior art]
As a method of detecting a defect of a circuit pattern formed on a wafer by a comparative inspection of an image in a manufacturing process of a semiconductor device, a so-called SEM method in which a point-shaped electron beam is scanned is used. Methods for comparing and inspecting patterns are described in JP-A-59-192943, "J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991)", and "J. Vac.Sci.Tech.B, Vol.10, No.6, pp.2804-2808 (1992), SPIEVol.2439, pp.174-183, and JP-A-05-258703. ing.
[0003]
These techniques are used for detecting minute etching residues or minute pattern defects having a resolution equal to or lower than the resolution of an optical microscope, and detecting electric defects such as non-opening defects of minute conduction holes. In these techniques, it is necessary to obtain a pattern image at a very high speed in order to obtain a practical inspection speed. Then, in order to secure an S / N ratio (signal to noise ratio) of an image acquired at high speed, a beam current of 100 times or more (10 nA or more) of a normal scanning electron microscope is used.
[0004]
Further, “Japanese Unexamined Patent Publication No. 7-249393”, “Japanese Unexamined Patent Publication No. 10-1974627”, Japanese Unexamined Patent Application Publication No. 2000-340160, and “Japanese Unexamined Patent Application Publication No. 11-108864” The semiconductor wafer is irradiated with an electron beam, and is reflected without being irradiated (imping?) On the wafer due to formation of back scattered electrons, secondary electrons, or a reversing electric field (primary field). (Reflect) Describes an apparatus for performing high-speed inspection by a so-called projection method such as forming an image of electrons by a lens.
[0005]
A mirror microscope is known as a technique for obtaining an image of the outermost surface of a sample by applying a potential to the sample to create a situation in which the sample is not irradiated with an electron beam but is reflected by an electric field near the surface, thereby obtaining an image of the outermost surface of the sample (for example, , "Rheinhold Godhardt, ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 94, pp. 81-150").
[0006]
[Problems to be solved by the invention]
In the conventional inspection technique using the above-described electron beam, the following problems remain in the SEM method and the projection method of reflected electrons and secondary electrons.
[0007]
First, in the case of the SEM method, the electron beam is made to have a larger current than that of a general SEM in order to form an image maintaining an S / N ratio that can be inspected. However, since the "point beam" is scanned in a plane (two-dimensionally) on the surface of the sample by narrowing the electron beam into a point, there is a limit in increasing the speed (reducing the inspection time).
[0008]
In addition, there is a limit in increasing the current of the electron beam due to the brightness of the electron source to be employed, a space charge effect, and the like. For example, to obtain a resolution of about 0.1 μm, the theoretical limit of the electron beam current is about several hundred nA, and in practice, only about 100 nA can be used. The S / N ratio of an image is determined by the number of electrons used to form an image, that is, the product of the beam current value and the time required for image acquisition. In order to secure an S / N ratio of an image in which the image processing can normally operate, when the beam current value is 100 nA and 0.1 μm, the area of the sample surface is 1 cm. ² Requires more than 100 seconds to test
[0009]
On the other hand, in the projection method, an electron beam having a larger current can be irradiated at a time than in the SEM method, and an image can be acquired at a time. Therefore, it can be expected that an image can be formed at an ultra-high speed as compared with the SEM method. However, the emission angle distribution of the secondary electrons is spread over a wide angle, and the energy is also spread at about 1 to 10 eV. It is reported in LSI Test Symposium / 1999 Proceedings, P142 that sufficient resolution cannot be obtained unless most secondary electrons are cut off when forming an enlarged image of a sample by forming such electrons. It can be easily determined from FIG. 6 described above. This shows the negative sample applied voltage for accelerating the secondary electrons emitted from the sample and the imaging resolution of the secondary electrons. According to this, when the sample applied voltage is -5 kV, the resolution is approximately 0.2 μm.
[0010]
Then, not all the emitted secondary electrons can be used for image formation. For example, in the calculation of the cited document, a beam having an opening angle of 1.1 mrad or less is used on the image plane after passing through the objective lens. . The secondary electrons within the range of the opening angle are only about 10% of the whole. Furthermore, the energy width of the secondary electrons used for imaging is calculated at 1 eV, but the energy width of the emitted secondary electrons is actually emitted with a width of several eV or more. Side tails exist up to about 50 eV. When only electrons having an energy width of at most 1 eV are extracted from the secondary electrons having such a wide energy distribution, the number is further reduced to a fraction.
[0011]
As described above, even if an image is collectively formed by irradiating a large current with an electron beam as an area beam, it is difficult to secure the S / N ratio of the image because the ratio of electrons that can actually contribute to image formation is low. Therefore, it is impossible to shorten the inspection time as much as expected. Similarly, in the case of using the backscattered electrons, the emission amount is smaller by two orders of magnitude than the irradiation beam current, and it is difficult to achieve both high resolution and high speed as in the case of the secondary electrons.
[0012]
Regarding the above-mentioned mirror microscope technology, there has been no report on a method or an apparatus for applying this technology to inspection of a semiconductor wafer. In addition, there is no restriction on the directionality of the trajectory of the electron beam directed to the sample, and the electron beam is irradiated to the sample at a wide angle, and the resolution is about sub-μm, which is insufficient for observing the current semiconductor. Was.
[0013]
The present invention has been made in view of the above points, and has as its object to provide a defect inspection method and a defect inspection apparatus for detecting a defect portion in a pattern formed on a wafer at high resolution and at high speed. And
[0014]
[Means for Solving the Problems]
The object of the present invention can be achieved by the following method.
[0015]
That is, a plurality of irradiation areas (area areas) on the surface of the sample are not "point beams", but "area beams (seat beams)" (plane-shaped electrons) having a two-dimensional spread. Beam). A negative potential is applied to the wafer. This negative potential is set to a value such that most electron beams are returned near the outermost surface of the wafer. Specifically, the potential is set to a negative potential higher by 0.5 V to 5 V than the potential of the electron source (cathode or electron source). This causes the electrons that have been pulled back to form an image (hereinafter, electrons that have been pulled back by the electric field without colliding with the sample will be referred to as “retracted electrons” or “mirror electrons”). Then, enlarged images of the plurality of irradiation regions are sequentially formed, the enlarged images of the plurality of irradiation regions are converted into electrical image signals, and the image signals of the plurality of irradiation regions are compared with each other. This makes it possible to detect a pattern defect in each of the irradiation regions with high resolution and at high speed.
[0016]
As a typical configuration example of the present invention, first, a pattern defect inspection method according to the present invention applies a planar electron beam having a two-dimensional spread to a plurality of irradiations of a sample surface to which a negative potential is applied. Irradiating the plurality of irradiation areas sequentially to form an image of the electrons drawn back near the surface of the sample without colliding with the sample, thereby sequentially forming enlarged electron images of the plurality of irradiation areas, By converting the enlarged electronic image of the region into an electric image signal and comparing the image signals of the plurality of irradiation regions with each other, a pattern defect formed on the sample is detected. I do. Here, the planar electron beam is configured to irradiate each of the plurality of irradiation regions so that the traveling directions thereof are substantially parallel to the sample surface and are incident substantially perpendicularly.
[0017]
In addition, the wafer defect inspection apparatus according to the present invention includes a first electron optical system that irradiates an electron beam from an electron source as a planar electron beam having a two-dimensional spread to a plurality of irradiation regions on a sample surface; Means for generating an electric field such that electrons forming the electron beam irradiated toward the sample are drawn back near the outermost surface of the sample, and forming an image of the electrons drawn back from the plurality of irradiation areas to form an image. A second electron optical system for forming enlarged images of the plurality of irradiation regions; image signal detecting means for converting the enlarged images of the plurality of irradiation regions into electrical image signals for detection; Image signal processing means for comparing image signals of a plurality of irradiation regions on the sample surface with each other to detect a pattern defect in each irradiation region, and detecting a pattern defect formed on the sample It is characterized in that the cormorants configuration.
[0018]
Further, the pattern defect inspection apparatus according to the present invention comprises: an electron beam irradiation optical system for irradiating an electron beam from an electron source as a planar electron beam having a two-dimensional spread to a plurality of irradiation regions on a sample surface; Means for generating an electric field such that all or a part of the electrons forming the electron beam are pulled back immediately before colliding with the surface of the sample, and forming an image of the electrons drawn back from the plurality of irradiation regions to form an image. An imaging optical system for forming an enlarged image of a plurality of irradiation regions; and a sample for mounting the sample and moving the sample so that each of the plurality of irradiation regions is irradiated with the electron beam. A moving stage, an image signal detecting unit that converts each of the enlarged images of the plurality of irradiation areas into an electric image signal and detects the image signals, and converts the detected image signals of the plurality of irradiation areas on the sample surface to each other. Compare to, and characterized by having an image signal processing means for detecting a pattern defect by determining whether differences exist above a predetermined threshold at each irradiation area.
[0019]
Further, the pattern defect inspection apparatus according to the present invention is characterized in that an electron optical system for acquiring a SEM image of the sample is provided at a predetermined position in the moving direction of the sample moving stage.
[0020]
The first electron optical system or the electron beam irradiation optical system is disposed between an electron gun that generates an electron beam, a condenser lens that focuses the electron beam emitted from the electron gun, and the condenser lens and the sample. An objective lens and an electron beam deflecting mechanism. The condenser lens focuses the electron beam on a focal plane on the electron source side of the objective lens, and the traveling direction is substantially parallel to the sample surface. A planar electron beam that is incident substantially perpendicularly is formed to irradiate a plurality of irradiation areas on the sample surface.
Further, when a circular electrode (aperture electron) facing the sample largely acts as an electron lens, the objective lens focuses the electron beam on a focal plane of a circular lens (aperture lens). With this arrangement, it is possible to form a planar electron beam that is incident substantially perpendicularly with the traveling direction being substantially parallel to the sample surface.
[0021]
The means for generating an electric field such that electrons irradiated toward the sample are pulled back on the outermost surface of the sample is constituted by, for example, a power supply for applying a predetermined negative potential to a conductive sample holder that holds the sample. In the case of a sample such as a semiconductor having an insulating film on its surface, a second electron gun is provided at a position distant from the optical axis of the electron beam for image formation, and pre-charge control for irradiating the sample with electrons before image formation. Means. Further, a grid is provided between the sample and the second electron gun to which a potential can be applied, and means for irradiating the sample with electrons using the second electron gun while controlling the voltage applied to the grid is provided.
In order to inspect a conduction defect such as a short circuit or a disconnection of a pattern of a semiconductor sample, a defective pattern can be detected by detecting a voltage difference between a normal pattern and a defective pattern caused by the preliminary irradiation. However, if the resistance of the defect pattern is relatively small, the accumulated charges escape to the surroundings, so that the potential difference between the normal pattern and the defect pattern caused by the preliminary irradiation is not maintained for a long time. In this case, there should be provided a means for injecting electrons into the sample to generate a potential difference between the normal pattern and the defect pattern, and a means for irradiating the electrons which are reflected and imaged without hitting the sample at almost the same place and almost simultaneously. Is required. In order to meet the demand for negatively charging the defective pattern and the insulating film pattern, the sample is irradiated with an electron beam having an extra energy eVn with respect to the sample potential so that the insulator is charged to the negative voltage Vn and stabilized. When the image is obtained by the above, the image can be obtained in a state where the defective pattern and the insulator pattern are negatively charged. The requirement for positively charging the defective pattern and the insulating film pattern can be solved by simultaneously or alternately irradiating a beam for acquiring an image and an electron beam or light for positively charging the sample.
[0022]
Further, by setting the sample moving stage (sample stage) so as to continuously move the sample at substantially the same speed, it is possible to realize high-speed defect inspection. In this case, it is needless to say that by monitoring the position of the sample moving stage, it is necessary to control the electron beam irradiation area on the sample surface to be the same for a predetermined time. That is, a stage position measuring mechanism that measures the position of the continuously moving sample moving stage in real time is provided, and a position change that occurs during the stage movement due to the continuous movement is measured by the stage position measuring mechanism. By feeding back to the electron beam deflecting mechanism of the beam irradiation optical system, the positional relationship between the electron beam and the sample moves at a substantially constant speed in a predetermined direction.
[0023]
Further, the image signal detecting means projects an enlarged electronic image of the irradiation area formed by the second electron optical system or the imaging optical system onto a phosphor screen (optical screen) to thereby form an optical image (optical image). ), And forms this optical image on an optical image detecting element via an optical lens or an optical fiber. Alternatively, the magnified electronic image formed by the optical system may be directly formed on an image sensing element having sensitivity for electron. Note that a charge-coupled device (CCD) or a device that integrates and outputs a time-delayed optical signal (TDI: Time Delayed Integration sensor) can be used as the image detection device. The reading of the detection signal from the image detecting element is performed in a method of reading in multiple channels in parallel.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0025]
(Example 1)
FIG. 1 shows the minimum components necessary for explaining the operation principle of the present invention. The electron beam emitted from the electron source 1 is converged by the condenser lens 2 and forms a crossover around the beam separator 3 and on the front focal plane of the objective lens. The electron beam is deflected by the beam separator 3 to an optical axis perpendicular to the wafer 7. The beam separator 3 has a deflecting effect only on the electron beam from above. For example, an E × B deflector in which an electric field and a magnetic field are perpendicular to each other is used. The electron beam deflected by the beam separator 3 is formed by the objective lens 6 into a planar electron beam aligned in a direction perpendicular to the surface of the sample (wafer).
[0026]
A negative potential substantially equal to or slightly higher than the acceleration voltage of the electron beam is applied to the sample (wafer) 7 by the power supply 9, and the shape of the semiconductor pattern formed and the state of charging are formed on the surface of the wafer 7. The reflected electric field is formed. Due to this electric field, most of the planar electron beam is pulled back immediately before colliding with the wafer 7 and rises in the direction and intensity reflecting the pattern information of the wafer 7.
[0027]
The retracted electron beam is converged by the objective lens 6, and the beam separator 3 rises vertically as it has no deflecting effect on the electron beam traveling from below. An image of the surface of the wafer 7 is formed on 103. As a result, a structural change such as a local change in the charged potential and unevenness on the surface of the wafer 7 is formed as an image. This image is converted into an electric signal and sent to the image processing unit 104.
[0028]
In order to detect a defect in the semiconductor pattern formed on the wafer 7, the image processing unit 104 compares an image with a peripheral pattern part having the same shape or a previously acquired image of a defect-free part, and determines whether the defect is different. The location where it is is stored as a defect. The wafer 7 is mounted on a stage (not shown), and the stage is continuously moved by the stage control system 30. The stage control system 30 and the beam control system 28 are linked, and continuously move the image acquisition area while finely adjusting the position of the electron beam with a deflector (not shown) as the stage moves.
[0029]
When an insulator exists on the surface of the wafer 7, the surface potential of the wafer 7 cannot be determined only by the power supply 9. For this purpose, a preliminary charging control device 32 is provided as a function for charging the potential on the wafer surface to a desired potential. This is a device for controlling the charged potential on the surface of the wafer 7 by irradiating the wafer 7 with an electron beam while applying a voltage to a grid electrode close to the surface of the wafer 7. The operation principle and structure will be described later. When the surface of the wafer 7 is charged in advance before the inspection, the wafer is passed under the preliminary charging control device 32 to set the inspection area to a desired charging voltage, and then is passed immediately below the objective lens 6 to acquire an image. To do it.
[0030]
Next, the principle of imaging defects on the wafer surface under the condition that the electron beam is pulled back on the surface of the wafer 7 will be described.
[0031]
FIG. 2 schematically shows a state in which the electron beam 201 vertically incident on the equipotential line 205 near the outermost surface of the wafer 7 is pulled back. Due to the defect 202 existing on the surface of the wafer 7, the equipotential line 205 has an uneven shape at the position where the defect exists. The electron beam perpendicularly incident thereon is not pulled back vertically, but is drawn back at an angle as shown and enters the lens 204. The lens 204 shows the operation of the objective lens 6 and the imaging lens 11 with one equivalent lens. FIG. 2 shows that when an image is formed on the image forming surface 203 by this lens, the electron beam from the portion of the defect 202 is concentrated on one portion of the image forming surface, and the portion becomes brighter than the surroundings. It is possible to detect the presence of the defect and the position of the defect from this image.
[0032]
FIG. 3 shows a simulation result of the equipotential line 305 near the wafer surface and the trajectory 306 of the electron beam in the present invention. A conductive material portion 312 (white) and an insulating film portion 311 (hatched portion) having a size of 70 nm are present in the pattern cross section, only the central conductive material portion (conductive portion) 313 is 1 V, and the other conductive material portions are It was assumed to be 0V. That is, it is assumed that only the center conductive material portion is insufficiently electrically connected to the substrate, and is positively charged by 1 V compared to the surroundings.
[0033]
The potential of the electron source was +1 eV based on the potential of the wafer. That is, when the potential applied to the wafer is 0 V, the electron beam is reflected on the +1 V equipotential surface and becomes an imaging electron beam. In this case, it is shown that the electron beam is strongly affected by the disturbance of the equipotential lines formed by the patterns having different potentials at the center, and the vertically incident beam is reflected at a large angle.
[0034]
FIG. 4 shows the calculated density of the electron beam pulled back by the electric field on the wafer surface. As shown in the “structure” column in the figure, the portions arranged in 3 × 3 are conductive materials, and only the central potential is different from the surrounding by 1V. The lower figure shows the electron beam trajectory calculation with the energy width (ΔE) of the electron beam set to 2 eV in this case, and plots the electrons returned from the wafer surface. Higher dot density indicates higher electron density. It is shown that electrons are concentrated in the central part and there is a high electron density part, which indicates that a change of 1 V of a 70 nm fine pattern can be detected by the principle described with reference to FIG. 2, that is, a conduction failure defect can be detected. ing.
[0035]
Here, a description will be given of how the present invention improves the inspection speed by an order of magnitude as compared with the method of scanning a point beam or the method of imaging secondary electrons. In the case of an image forming apparatus using an electron beam, what limits the speed of image acquisition ultimately results in an S / N ratio required for an image. The S / N ratio of an image is determined by the number of electrons used to form the image. Also, the required S / N ratio is determined by the magnitude of the contrast that the defect to be detected causes in the image. That is, if the defect contrast is the signal C, the noise needs to be smaller than that. The noise N is defined by the 3σ value of the signal. The σ value is determined by the shot noise of the number of irradiated electrons, and is the square root (√S) of the number of electrons S irradiated per pixel (pixel). Therefore, the noise N is 3√S.
[0036]
In the case where the electron beam collides with the wafer 7 and the secondary electrons generated at that time are detected as in the related art, there is a stochastic process called secondary electron emission from the sample. Assuming that the secondary electron emission is a Poisson process, the noise N is N = (3√2) √S. For example, if the defect contrast C is 5% of the average signal amount S, C = 0.05 × S, and the noise N needs to be N ≦ 0.05 × S. Become. 1cm based on this idea ² Obtaining the inspection time T per unit is as follows.
T = (0.01 / x) ² ・ T
= (1.6e-19 · 0.01 ² ・ (3√2) ² ) / (I · η · C ² ・ Pix ² …… (1)
Note that t = ((1.6e-19 · (3√2) ² ) / (I · η · C ² )) ・ (X ² / Pix ² )
Here, t is the time during which the electron beam must stay at the same location to make the noise smaller than the contrast C. That is, in the case of the SEM method, this is the time during which the electron beam probe irradiates one pixel. In the case of surface beam irradiation, it is a time during which the beam needs to be directed to a certain point, and this time is called a shot time.
[0037]
Pix is the required resolution, x is the length of one side of the area beam (same as the pixel size or Pix in the case of the SEM type), I is the beam current, and η is the efficiency of electrons that can be used for image formation. On the other hand, in the present invention, since the electron beam does not collide with the surface of the wafer 7 and is merely scattered by the electric field, there is no stochastic process associated with secondary electron emission. Therefore, √2 in equation (1) becomes unnecessary, and equation (2) is obtained.
T = (0.01 / x) ² ・ T
= (1.6e-19 · 0.01 ² ・ (3) ² ) / (I · η · C ² ・ Pix ² ) (2)
In addition, t = ((1.6e-19 · (3) ² ) / (I · η · C ² )) ・ (X ² / Pix ² )
Here, η and C in each system are estimated. In the case of the SEM method, almost the same number of secondary electrons as the irradiated electron beam are emitted, and almost 100% of the emitted secondary electrons can be taken into the detector.
[0038]
On the other hand, in the secondary electron projection system in which a planar electron beam is irradiated on the wafer 7 and the generated secondary electrons are imaged, the secondary electrons having a very limited vertical component among the emitted secondary electrons are used. If the image is not formed only by itself, the resolution is deteriorated. This will be described with reference to FIGS.
[0039]
FIG. 5 shows the resolution of an image with respect to the emission half angle β of secondary electrons or reflected electrons that contribute to image formation. That is, it is the half-open angle of the electrons taken into the imaging system. For example, it shows that the resolution when an image is formed by secondary electrons having an emission angle of 100 mrad or less is about 100 nm. As the calculation conditions, the energy of the electrons applied to the wafer is 500 eV, the surface of the wafer is under a strong electric field of 5 kV / mm, and the energy width of the secondary electrons is 5 eV. Although the energy distribution of the secondary electrons spreads over 10 eV or more, only the component of ± 2.5 eV centered on the emission energy of 2 eV is used for imaging. This corresponds to about 1/2 of all secondary electrons.
[0040]
The energy width was set to 1 eV, considering only the elastic scattered electrons as the reflected electrons. From these figures, it is necessary to set the emission angle to 25 mrad in order to make the resolution of the secondary electrons, for example, 40 nm. In this case, the probability that the secondary electrons are scattered within the emission angle β is about 0.1%. . Assuming that the secondary electron emission efficiency (yield) (the ratio of the number of secondary electrons to the number of irradiated electrons) is approximately 1, η in the case of the secondary electron imaging type is ×× 0.001 × 1 = 0. 0005.
[0041]
On the other hand, in order to set the resolution to 40 nm in the case of backscattered electrons, the sample emission angle (β) is 80 mrad, and the probability of the presence of backscattered electrons within that angle is 0.2% from FIG. The emission efficiency of the backscattered electrons (the ratio of the number of backscattered electrons to the number of irradiated electrons) is based on the reference document “Image Formation in Low-Voltage Scanning Electron Microscopy, SPIE, Bellingham, p. It is about 0.02 to 0.03. Therefore, η in the backscattered electron imaging type has a considerably small value of 0.002 × 0.025 = 5e−5.
[0042]
On the other hand, in the present invention, since the electron beam is bounced vertically upward on the flat wafer surface, the opening angle of the beam is very small (several mrad), which is equal to the angular variation of the irradiation beam. FIG. 7 is an explanatory diagram for further understanding the description so far.
[0043]
As shown in the left diagram of FIG. 7, the secondary electrons are emitted from the sample into a vacuum with a spread of 180 degrees, whereas in the present invention, as shown in the right diagram, all the electrons are almost right above. As a result, the irradiated electrons can be effectively used as an image. On the other hand, when the surface has irregularities or a potential distribution, it travels upward at a certain angle instead of the vertical direction. In this case, the ratio of electrons directly contributing to the image formation is reduced, but the change in the angle itself is a factor for forming an image on the wafer surface, so that the contrast is increased. In other words, this is equivalent to increasing the contrast C of the defect, which is advantageous for defect detection.
[0044]
After all, in the present invention, η and C are linked, and if η is limited, C will increase accordingly. Strictly speaking, the type of pattern differs depending on the type of defect. In this case, however, a signal that can be detected as an image at a defective portion is の of all electrons. The remaining half signal contributes to the contrast. Therefore, η = 0.5 and C = 0.5.
[0045]
Table 1 summarizes the above.
[0046]
[Table 1]

FIG. 8 shows the relationship between the beam current and the inspection time in this case. Note that this relationship was calculated at Pix = 40 nm. It can be seen that the present invention can be inspected in an overwhelmingly short time as compared with other methods.
[0047]
Next, the configuration of an embodiment of the present invention will be described in detail. FIG. 9 shows a configuration of an inspection apparatus according to one embodiment of the present invention. The inspection apparatus according to the present embodiment is roughly divided into an electron optical system 101, a sample chamber 102, an image detection unit 103, an image processing unit 104, and a control unit 105. Each part will be described below.
[0048]
First, the electron optical system 101 will be described. An accelerating electron beam emitted from the electron source 1 to which a negative high potential is given by the accelerating power supply 23 is converged by the condenser lens 2 and irradiates the aperture 4 having a rectangular opening. As the electron source 1, a Zr / O / W type Schottky electron source was used. A uniform planar electron beam having a large current beam (for example, 1.5 μA) and an energy width of 1.5 eV can be stably formed. Then, the light is deflected by the beam separator 3 in the direction of the wafer 7. The beam separator 3 separates the optical paths of the incident electron beam from the electron source 1 and the mirror electron beam from the sample. The condenser lens 2 forms a crossover on the focal plane of the objective lens 6. Further, the arrangement of the stop and the lens is optimized so that an image of the stop 4 is formed on the surface of the wafer 7 by the objective lens 6.
[0049]
As a result, a planar electron beam which is oriented in a direction perpendicular to the surface of the wafer 7 and whose orbits of the respective electrons are substantially parallel to each other and which is shaped into the aperture shape of the diaphragm 4 is formed. The size of the rectangular aperture on the aperture 4 is, for example, 100 μm square, and is reduced to に よ っ て by the objective lens 6 so that a 50 μm square planar electron beam can be obtained on the surface of the wafer 7. . This planar electron beam can be moved (or scanned) to an arbitrary position on the surface of the wafer 7 by the irradiation system deflector 5.
[0050]
Even if the front focal plane of the objective lens and the crossover position cannot be completely matched, there is no problem as long as it is within a certain allowable range. Also, the magnitude of the crossover is ideally zero, but actually has a finite magnitude due to aberrations of the electron gun and the condenser lens. There is no problem if this size is within a certain allowable range. In the electron optical system in which the position of the crossover was accurately controlled and the aberrations of the electron gun and the condenser lens were sufficiently reduced, the spread of the incident angle on the sample could be suppressed to 0.5 mrad or less. The spread of the incident angle is one of the factors that determine the resolution of the enlarged image of the sample surface due to the mirror electrons, and is expressed by the following equation.
[0051]
r0 = β ² ・ Zm… (3)
Here, r0 is the resolution determined by the spread of the incident angle, β is the maximum half angle of incidence, and Zm is the distance in which an electric field that pulls back electrons is generated.
[0052]
In this embodiment, β is 0.25 mrad and Zm is 5 mm. By substituting this into equation (3), r0 is 0.3 nm, which indicates that this embodiment does not affect the resolution. Therefore, the beam current can be further increased as needed.
[0053]
Note that even if the resolution is about 30 nm, it is considered to be sufficient for detecting a defect in a semiconductor. Therefore, if Zm is 5 mm, β can be tolerated up to 2.4 mrad. In this case, there is considerable margin in the deviation of the crossover position from the front focal plane of the objective lens and the size of the crossover.
[0054]
Assuming that the half angle of the beam opening on the front focal plane is α, the focal length of the objective lens is f, the displacement of the crossover is Δf, and the radius of the planar electron beam is X, the following equation is established.
[0055]
Δf = f · β / α (4)
α = X / (2f) (5)
From Equations (4) and (5), for example, when the focal length f of the objective lens is 10 mm and the size X of the planar beam is 40 μm, there is no problem even if the crossover position shift Δf is shifted by about 10 mm. This is approximately 40 μm when converted to a beam diameter at the front focal plane. In any case, it is understood that sufficient resolution can be obtained by disposing the crossover of the electron beam near the front focal plane of the objective lens.
[0056]
Here, the beam separator 3 will be briefly described. The beam separator 3 deflects the electron beam emitted from the electron source 1 toward the wafer 7, while the mirror electrons pulled back from the wafer 7 are directed toward the imaging lens 11 instead of the electron source 1. To deflect. A deflector using a magnetic field is most suitable for the deflector having such an operation. This is because the direction of the deflection action by the magnetic field differs depending on the incident direction of the electrons.
[0057]
Further, as will be described later with reference to FIG. 10 as Example 2, in the case of an optical system in which the optical axis of the imaging lens and the optical axis of the objective lens 6 are arranged in a straight line, the electric field and the magnetic field are made to go straight, An E.times.B deflector is used to make the mirror electrons go straight and to deflect only the electron beam from above.
[0058]
To the wafer 7 and the wafer (sample) moving stage 8, a negative potential slightly higher (absolute value) than that of the electron source 1 is applied by the power supply 9. Specifically, it is preferable to set the negative potential to 0.5 to 5 V. If the potential is too high, the resolution of the image is degraded. Also, if the potential is too low, slight changes in surface irregularities and potentials are imaged as extremely strong contrast, making it difficult to detect only the truly necessary defects.
[0059]
The electron beam directed perpendicular to the surface of the wafer 7 is decelerated in front of the wafer 7 by the above-mentioned negative potential, and is pulled upward by the electric field on the surface of the wafer 7. As described above, the electrons reflect information on the surface of the wafer 7. The mirror electrons are focused by the objective lens 6 and deflected by the beam separator in the directions of the imaging system deflector 10 and the imaging lens 11. Then, the state of the surface of the wafer 7 is formed as an electronic image by the imaging lens 11. By enlarging and projecting this electronic image onto the fluorescent plate 15 by the magnifying lenses 13 and 14, a fluorescent image (microscope image) reflecting the pattern and charged state of the surface of the wafer 7 can be obtained.
[0060]
In order to improve the contrast and resolution of the electronic image, a contrast aperture 12 can be inserted in the crossover plane. The contrast aperture 12 enhances the resolution and contrast of an image by removing electrons that are largely deviated from the vertical direction when pulled back by the electric field on the surface of the wafer 7.
[0061]
In the image forming principle of the present invention, the sensitivity and the resolution of an image for detecting a subtle difference in charge on the wafer surface are determined by the energy width of the planar electron beam. FIG. 10 shows the result of comparing the results by simulation. Assuming the same pattern as in FIG. 4, images obtained in two cases where the electron beam energy width (ΔE) is 2 eV and 4 eV were compared.
[0062]
According to this result, when an energy width of 4 eV is assumed, no contrast is observed in a portion having a different potential at the central portion of the pattern. In consideration of the progress of miniaturization of semiconductors, it is necessary to detect a difference in charged potential of about 1 V as a defect in a fine pattern as shown in FIG. Therefore, it is understood that the energy width of the electron beam used in the present invention is desirably 2 eV or less.
[0063]
As described above, in this embodiment using the Zr / O / W Schottky electron source, there is no problem because the energy width is 1.5 eV. For example, when an electron source with a larger energy width is used, an energy filter is provided on the optical path of the electron beam, and the electrons are emitted from the electron source until the image is finally formed. The energy width needs to be 2 eV or less. The energy filter is desirably provided between the electron source and the wafer 7, but the same effect can be obtained even if energy filtering is performed on mirror electrons from the wafer 7.
[0064]
In the present invention, the electron beam does not collide with the wafer 7. Therefore, in principle, even if an insulating film exists on the surface of the wafer 7, the surface is not charged. Therefore, if the inspection is carried out in a state in which charging is not performed, the only defects that can be detected are shape defects (those having a shape different from that of the normal part).
[0065]
However, in a defect inspection of a semiconductor pattern using an electron beam, a so-called electrical defect such as a continuity failure, a short-circuit of an object to be insulated, or a leak current larger than that of a normal part is called. There is a method in which a charged object is charged by irradiation with an electron beam and is detected based on a potential contrast of an SEM image generated by a difference in the potential.
[0066]
In order to detect such a defect with high sensitivity, the present invention includes a preliminary charging control device that irradiates an electron beam dedicated to charging control before acquiring an inspection image. If the inspection is performed after the wafer 7 is charged to a predetermined potential by this apparatus in advance, it is possible to detect not only the shape defect but also the electric defect such as the conduction defect portion. Hereinafter, this operation and configuration will be described.
[0067]
FIG. 11 is a diagram illustrating the operation principle of the preliminary charging control device. The electron source 41 is an electron source that emits a high-current electron beam from a surface having a certain width (several hundred μm to several tens of mm). For example, an electron source in which carbon nanotubes are bundled, a tungsten filament thermoelectron source, a LaB6 electron source, or the like can be used. An extraction voltage is applied to the extraction grid 42 by the extraction electrode 48 to cause the electron source 41 to emit the electron beam 43. The electron beam passes through the control grid 44 and irradiates the insulating film 46. Thereby, secondary electrons 45 are emitted.
[0068]
The secondary electrons have an energy of about 2 eV with reference to the potential of the surface of the insulating film 46. If the surface of the insulating film is equal to the potential of the substrate 47, the irradiation energy of the electron beam is the voltage of the acceleration power supply 49, and this voltage is set to a value so that the secondary electron emission efficiency becomes 1 or more. The voltage may be 500 V for a general insulating film material for a semiconductor device. At this time, since the secondary electron emission efficiency is greater than 1, the insulating film surface becomes positively charged.
[0069]
Since a control power supply 50 is connected to the control grid 44 so that any positive or negative voltage can be applied, the potential of the insulating film surface becomes more positive than the set potential of the control grid 44, When the electrons are pulled back to the insulating film surface, the positive charging of the insulating film surface stops. At this time, the charging potential on the surface of the insulating film is stabilized at a positive potential slightly lower (about 2 V) than the potential of the control grid. The reason why the potential is not equal to the potential of the control grid is that secondary electrons have energy. According to the above principle, the potential of the surface of the insulating film 46 can be controlled by the potential of the control grid 44.
[0070]
FIG. 12 shows the configuration of a preliminary charging control device using a carbon nanotube electron source. The electron source 41 is held in a state of being kept in a vacuum by the insulator 51 so that a potential can be applied. The control grid 44 is arranged facing the wafer 7, and the extraction grid 42 extracts electrons from the electron source 51.
[0071]
FIG. 13 shows a configuration of a preliminary charging control device using a LaB6 electron source. When a LaB6 electron source is used for a microscope, a crossover is formed immediately after electron emission using a Wehnelt electrode. In this case, since the light source does not need to be small, an extraction electrode 42 'is provided instead.
[0072]
In the sample chamber 102, the wafer 7 is mounted on a sample moving stage 8 that can move in two-dimensional (X, Y) directions. A negative potential is applied so as not to collide with. A stage position measuring device 27 is attached to the sample moving stage 8 to accurately measure the stage position in real time. This is for acquiring an image while continuously moving the stage 8. As the stage position measuring device 27, for example, a laser interferometer is used.
[0073]
In order to accurately measure the height of the semiconductor sample (wafer) surface, an optical sample height measuring device 26 is also provided. For this purpose, for example, a method in which light is obliquely incident on a region to be inspected on the wafer surface and the height of the wafer surface is measured from a change in the position of the reflected light can be used. In addition, the sample chamber 102 is also provided with an optical microscope 31 used for positioning the inspection area.
[0074]
Next, the settling time of the sample moving stage 8 will be described. If the moving method of the stage 8 is a step-and-repeat method, the settling time of the stage 8 needs to be on the order of msec. Therefore, even if the image S / N ratio is improved and the image acquiring time is shortened, the stage is moved. It takes time and the inspection time cannot be shortened. Therefore, the moving method of the stage 8 is a continuous moving method in which the stage always moves at a substantially constant speed. This eliminates the restriction on the inspection time due to the settling time of the stage. However, if the stage 8 moves continuously, the stage 8 also moves during one shot, which is the time required to form an image at the same place, and the irradiation position on the sample surface changes. I will. Therefore, the irradiation electron beam is caused to follow the movement of the stage 8 by the irradiation system deflector 5 so that the irradiation position does not change during one shot. Further, when viewed from the electron optical system which is a stationary coordinate system, the electron beam irradiation position moves, so that the image 12 formed by the imaging lens 11 also moves. In order to prevent this movement, the imaging system deflector 10 is operated in conjunction with the irradiation system deflector 5.
[0075]
Next, the image detection unit 103 will be described. For image detection, a fluorescent plate 15 for converting an enlarged image of the mirror electron image 12 into an optical image and an optical image detection element (for example, a CCD element) 17 are optically coupled by an optical fiber bundle 16. Thus, the optical image on the fluorescent screen 15 is formed on the light receiving surface of the optical image detecting element 17. The optical fiber bundle 16 is a bundle of thin optical fibers of the same number as the number of pixels. Further, an optical lens may be used instead of the optical fiber bundle 16, and an optical image on the fluorescent plate 15 may be formed on the light receiving surface of the optical image detecting element (CCD) 17 by the optical lens. An electrode 300 and a transparent electrode 301 are provided on both sides of the fluorescent plate 15, and the transparent electrode 301 side applies a high positive voltage between the two electrodes to prevent scattering of the electron beam. The optical image detecting element (CCD) 17 converts the optical image formed on the light receiving surface into an electric image signal and outputs it. The output image signal is sent to the image processing unit 104, where image signal processing is performed.
[0076]
Next, the reading time of the image detecting element (CCD) will be described. In this embodiment, the charge accumulated in the CCD 17 can be read in parallel from a 128-channel readout port at a readout speed of 8 M lines / sec. The number of pixels per line per channel is 8, and the readout time per line is 125 nsec. Therefore, the required reading time per pixel is 125 nsec / 8 (pixel) = 16 nsec. On the other hand, in the one-channel system for reading image data from the CCD, very high-speed reading is required, which is difficult to realize.
[0077]
In the present embodiment, the readout port of image data from the CCD is divided into 128 channels, and the system is configured to perform parallel and simultaneous readout on the 128 channels, so that the required readout time per pixel is 16 nsec, and the readout speed that can be sufficiently realized is achieved. And This is schematically shown in FIG.
[0078]
The number of channels for reading image data from the CCD 17 is 128 channels, and each channel has 8 pixels × 1024 lines. Therefore, the time required to read one image data from this CCD is about 125 μsec. That is, the image signal of one shot area can be captured in 125 μsec, and if the pixel size is 50 nm and the one shot area is 50 μm square, the sample surface area is 1 cm. ² The required inspection time per hit is 5 seconds.
[0079]
As described above, a sample area of 1 cm according to the conventional method when inspecting with a pixel size of 50 nm ² In comparison with the required inspection time of about 400 seconds, a speedup of 80 times was achieved. In this embodiment, the time required for the inspection is determined by the signal read speed from the CCD element. Therefore, if a higher-speed data read method in the CCD element is realized in the future, a further increase in inspection speed is expected. it can.
[0080]
The image processing unit 104 includes image signal storage units 18 and 19, a calculation unit 20, and a defect determination unit 21. The image storage units 18 and 19 store images of adjacent portions of the same pattern, and the two images are calculated by the calculation unit 20 to detect different locations of the two images. The result is determined as a defect by the defect determination unit 21 and its coordinates are stored. The captured image signal is displayed on the monitor 22 as an image.
[0081]
The operation command and operation condition of each unit of the device are input and output from the control computer 29 in the control unit 105. Various conditions such as an acceleration voltage at the time of generation of an electron beam, an electron beam deflection width / deflection speed, a sample stage moving speed, and an image signal fetch timing from an image detection element are input to the control computer 29 in advance. The beam control system 28 receives a command from the control computer 29, generates a correction signal based on signals from the stage position measuring device 27 and the sample height measuring device 26, and irradiates the electron beam to a correct position at all times. The correction signal is sent to the objective lens power supply 25 and the scanning signal generator 24 as described above. The stage control system 30 controls the sample moving stage 8 in response to a command from the control computer 29.
[0082]
Next, an actual inspection procedure will be described. First, a method of alignment using the optical microscope 31 and the electron beam image will be described. The wafer 7 is placed on a wafer moving stage (XY-θ stage) 8 and moved below the optical microscope 31. An optical microscope image of the surface of the wafer 7 is observed by the monitor 22, and an arbitrary pattern appearing in the center of the screen, for example, is stored. At this time, the pattern to be selected needs to be a pattern that can be observed even on an electron beam image.
[0083]
Next, rotation correction is performed by the wafer moving stage 8 using the optical microscope image so that the circuit pattern on the surface of the wafer 7 is parallel or orthogonal to the stage moving direction. At the time of rotation correction, an optical image of an arbitrary pattern portion in an arbitrary chip of a circuit pattern on the surface of the wafer 7 at a certain stage position is fetched and displayed on the monitor 22, and an arbitrary position on the display screen is marked. Then, the optical image signal is stored in the storage unit 18.
[0084]
Next, the stage 8 is moved in the x direction or the y direction by a distance corresponding to several chips of the circuit pattern on the surface of the wafer 7, and an optical image of the same pattern portion in a new chip is captured and displayed on the monitor 22. Let it. Then, after the marking corresponding to the previous marking location is also given, the new optical image signal is stored in the storage unit 19. Next, the arithmetic unit 20 compares the optical image signals stored in the storage units 18 and 19 to calculate the positional deviation amount of the marking location between the two images. The rotation angle error of the wafer 7 is calculated from the positional deviation amount of the marking portion and the stage movement amount between the two images, and the rotation angle is corrected by rotating the stage 8 accordingly. The above rotation correction operation is repeated several times so that the rotation angle error becomes equal to or less than a predetermined value.
[0085]
Further, the circuit pattern on the surface of the wafer 7 is observed using the optical microscope image, and the positions of the chips on the wafer and the distance between the chips (for example, the repetition pitch of a repetition pattern such as a memory cell) are measured in advance. The value is input to the control computer 29. Then, a chip to be inspected on the surface of the wafer 7 and a region to be inspected in the chip are set on an optical microscope image on the monitor 22. The optical microscope image can be observed at a relatively low magnification, and even when the circuit pattern on the surface of the wafer 7 is covered with a transparent film such as a silicon oxide film, it is possible to observe the underlayer. Therefore, the layout of the circuit pattern in the chip can be easily observed, and the inspection area can be easily set.
[0086]
Next, the wafer 7 is moved below the electron optical system. Therefore, an electron beam image of a region expected to include the region to be inspected previously set on the optical microscope image is acquired. At this time, the above-mentioned inspection area is made to fall within one shot area. Also on this electron beam image, the stage 8 is moved so that the pattern of the previously marked portion appears on the same screen as that marked on the previous optical microscope image. This makes it possible to establish a correspondence between the electron beam irradiation position and the optical microscope observation position before starting the inspection, and to calibrate the image acquisition position. Then, on this electron beam image, the same operation as previously performed on the optical microscope image is performed. This makes it possible to easily confirm and position the observation position using the optical microscope and adjust the electron beam irradiation position.
[0087]
Furthermore, after a certain degree of rotation correction is performed, a higher-precision rotation correction can be performed using an electron beam image having a higher resolution than an optical microscope image and capable of obtaining a high-magnification image. Further, using this electron beam image, the inspection area or the same pattern area can be observed and confirmed / corrected with high magnification and high precision. However, when the entire surface (or a part) of the semiconductor wafer 7 is covered with the insulator, the charging potential on the insulator surface may not be equal to the substrate potential. It is necessary to control the charging voltage on the surface by the charging control device 32.
[0088]
After the setting of the inspection conditions described above, a part of the inspection area on the surface of the semiconductor wafer 7 is converted into an electron beam image under exactly the same conditions as the actual inspection conditions, and an image depending on the material and shape of the inspection area is formed. The brightness information and its variation range are calculated and stored in a table. Then, a determination condition for determining whether or not the pattern portion in the inspection area actually imaged and detected is a defect is determined by referring to the storage table in a subsequent inspection process.
[0089]
When the setting of the area to be inspected and the defect determination conditions is completed by the above procedure, the inspection is actually started. At the time of inspection, the stage 8 on which the sample (semiconductor wafer) 7 is mounted continuously moves at a constant speed in the x direction. During that time, the electron beam irradiates the same irradiation area (area area) on the surface of the wafer 7 for one shot for a fixed shot time (50 μsec or more in this embodiment). Since the stage 8 is continuously moving, the electron beam is deflected and scanned by the irradiation system deflector 5 following the movement of the stage 8.
[0090]
The irradiation area or irradiation position of the electron beam is constantly monitored by a stage position measuring device 27, a sample height measuring device 26, and the like provided on the stage 8. These pieces of monitor information are transferred to the control computer 29, and the positional deviation amount is grasped in detail. The positional deviation amount is accurately corrected by the beam control system 28. Thus, accurate positioning required for pattern comparison inspection can be performed at high speed and high accuracy.
[0091]
Further, the surface height of the semiconductor wafer 7 is measured in real time by means other than the electron beam, and the focal length of the objective lens 6 and the imaging lens 11 for irradiating the electron beam is dynamically corrected. The means other than the electron beam is, for example, an optical height measuring device 26 using a laser interference method or a method for measuring a change in the position of reflected light. Thus, an electron beam image in which the surface of the inspection area is always focused can be formed. Further, the warpage of the wafer 7 is measured in advance before the inspection, and the focal length is corrected based on the measurement data so that the surface height of the wafer 7 need not be measured during the actual inspection. You may.
[0092]
The electron beam is directed to the surface of the wafer 7, and an enlarged optical image of a desired inspection area (area area) on the surface of the wafer 7 is formed on the fluorescent screen 15 by mirror electrons. The magnified optical image is converted into an electric image signal by the CCD element 17, and the image signal is taken into the image processing unit 104. Then, in response to a command from the control computer 29, the image data is stored in the storage unit 18 (or 19) as an electron beam image signal for an area corresponding to the electron beam irradiation position given by the control unit 28.
[0093]
When performing a comparative inspection of patterns between adjacent chips A and B having the same design pattern formed on the surface of the semiconductor wafer 7, first, an electron beam image signal for a region to be inspected in the chip A is fetched. , In the storage unit 18. Next, an image signal for the inspection area corresponding to the above in the adjacent chip B is fetched and stored in the storage unit 19, and at the same time, compared with the stored image signal in the storage unit 18. Further, while obtaining an image signal for a corresponding inspection area in the next chip C and storing the image signal in the storage unit 18 by overwriting, at the same time, storing the image signal in the storage area 19 for the inspection area in the chip B Compare with the image signal. By repeating such an operation, comparison is performed while sequentially storing the image signals of the corresponding test regions in all the test chips.
[0094]
In addition to the above method, it is also possible to adopt a method in which an electron beam image signal of a desired inspection area of a standard non-defective (no defect) sample is stored in the storage unit 18 in advance. In this case, the inspection area and the inspection conditions for the non-defective sample are previously input to the control computer 29, and the inspection for the non-defective sample is executed based on the input data, and the acquired image for the desired inspection area is obtained. The signal is stored in the storage unit 18. Next, the wafer 7 to be inspected is loaded on the stage 8, and the inspection is performed in the same procedure as above.
[0095]
Then, at the same time as capturing the acquired image signal for the inspection area corresponding to the above into the storage unit 19, the image signal for this inspection target sample and the image signal for the non-defective sample previously stored in the storage unit 18 are obtained. Compare. Thus, the presence or absence of a pattern defect in the desired inspection area of the inspection target sample is detected. In addition, as the standard (non-defective) sample, a wafer different from the inspection target sample, which is known to have no pattern defect in advance, may be used, or the inspection target sample surface may have no pattern defect in advance. A known area (chip) may be used. For example, when a pattern is formed on the surface of a semiconductor sample (wafer), a misalignment defect between the lower layer pattern and the upper layer pattern may occur over the entire surface of the wafer. In such a case, if the objects to be compared are patterns in the same wafer or in the same chip, the above-described defect (defect) occurring over the entire surface of the wafer is overlooked.
[0096]
However, according to the present embodiment, the image signal of the area known to be good (no defect) is stored in advance, and the stored image signal is compared with the image signal of the inspection target area. Such a defect occurring over the entire surface of the wafer can be accurately detected.
[0097]
The two image signals stored in the storage units 18 and 19 are respectively fetched into the calculation unit 20, where various statistics (specifically, the average value of the image density) are based on the defect determination conditions already obtained. , A statistic such as variance), a difference value between neighboring pixels, and the like. The two image signals that have been subjected to these processes are transferred to the defect determination unit 21, where they are compared and a difference signal between the two image signals is extracted. These difference signals are compared with the defect determination conditions that have already been obtained and stored, and defect determination is performed, and the image signal of the pattern area determined to be defective and the image signal of the other area are separated.
[0098]
With the inspection method and the inspection apparatus described above, an image reflecting the information on the potential and the shape of the surface of the wafer 7 is formed, and the image signal of the corresponding pattern area is compared and inspected to determine whether there is a pattern defect. Can be detected. As a result, an extremely high-speed inspection can be performed as compared with a conventional inspection apparatus using an electron beam.
[0099]
(Example 2)
In the first embodiment, since the area of the one-shot electron beam irradiation region is as large as 50 μm × 50 μm, the problem that distortion occurs at the periphery of the enlarged image of the semiconductor sample and the uniformity of the beam current density in the irradiation region May cause problems. If image distortion or non-uniformity of current density is fixedly generated, it can be corrected by changing the arrangement of the optical fibers of the optical fiber bundle 16. Also, correction can be made by weighting the image signal acquisition sensitivity and image processing. However, when these change with time, it is difficult to cope with these methods.
[0100]
In the present embodiment, the irradiation area of one shot is set to 5 μm square so that the distortion and the non-uniformity of the current density do not occur in the irradiation area of one shot. The irradiation electron beam current is 1 μA per shot. At this time, assuming that the electron beam irradiation time is an electron imaging efficiency η of 0.5, the irradiation time t per shot is 0.18 μsec from the above equation (1). After irradiating one irradiation area (5 μm square) with a shot time of 0.18 μsec, the electron beam is moved by the irradiation system deflector 5 to the next adjacent irradiation area (5 μm square). In this way, the irradiation position is successively moved, and the entire area of 100 μm in the x direction × 100 μm in the y direction is irradiated with 20 × 20 = 400 shots.
[0101]
At this time, an enlarged image is obtained on the CCD element 17 for each shot at a position corresponding to the electron beam irradiation position at that time, and the CCD element is moved to the CCD element in accordance with the movement of the electron beam irradiation position by scanning of the electron beam. The obtained enlarged image position also moves. FIG. 15 shows this state.
[0102]
The CCD element 17 used had a size of 1024 × 1024 pixels. One pixel on the CCD element corresponds to an area of 50 nm square on the surface of the wafer 7. Therefore, an irradiation area of one shot (5 μm square) on the surface of the wafer 7 is 100 × on the light receiving surface of the CCD element. This is an area of 100 pixels (corresponding to 1/100 of the entire light receiving surface of the CCD element). The entire light receiving surface of the CCD element was able to cover a 50 μm square area on the sample surface. Therefore, in order to obtain an enlarged image of a 50 μm square area on the sample surface, 0.18 (μsec) × 100 (shots) = 18 (μsec) is required.
[0103]
As described above, when an image of a 50 μm square area on the surface of the wafer 7 is formed on the CCD in 18 μsec, the image signal stored in the CCD is stored in the image storage unit 18 as a digital signal. In order to acquire an image signal of the next adjacent area on the sample surface, it is necessary to move the stage 8 by 50 μm. As in the case of the first embodiment, a method of continuously moving the stage 8 at a constant speed (continuous moving) was employed for the stage movement. At that time, the irradiation system deflector 5 performs deflection scanning while following the movement of the stage 8 so that the stage 8 is stationary as to the irradiation electron beam. Thereby, the dead time generated when moving and stopping the stage 8 is reduced to zero.
[0104]
In order to scan while irradiating the continuous movement stage 8 with the irradiation electron beam, a correction signal of the deflection signal is calculated in the beam control system 28 with reference to the signal from the stage position measuring device 27, and the deflection signal is calculated. The correction signal is sent to the deflector 5 of the irradiation system to control the deflection of the irradiation electron beam. Further, the correction for the distortion of the magnified image of the sample due to the electron beam, the position drift, and the like are also superimposed on the correction signal of the deflection signal, thereby performing these corrections. The imaging system deflector 10 was also operated in conjunction with the irradiation system deflector 5, so that the position of the sample enlarged image on the CCD was not affected by the beam position movement due to the following of the stage. As a result, wasted time due to stage movement was eliminated, and high-speed and high-accuracy inspection was realized. Note that the image processing and the like for the subsequent defect inspection are the same as those in the first embodiment.
[0105]
As the inspection proceeds according to the procedure described above, the sample surface is 1 cm ² The time T required to sequentially form enlarged images of hits on the CCD is 0.72 sec. On the other hand, as in the case of the first embodiment, since it takes 125 μsec to read one image (image of a sample surface of 50 μm square) from the CCD, the sample surface area is 1 cm. ² 5 seconds are required. Since image formation and image signal readout in the CCD element are performed in parallel, the time required for inspection is the longer of the time required for image formation and the time required for image signal readout. In this embodiment, the time required for reading an image signal is longer than the time required for image formation, and is 1 cm. ² 5 sec per sample, and therefore, the surface area of the sample in this example was 1 cm. ² The required inspection time for the hit is 5 seconds.
[0106]
In the above description, the case where the electron beam irradiation region of one shot is fixed to the size of 5 μm square is exemplified. However, the size of the electron beam irradiation region can be changed according to the pattern repetition pitch on the surface of the semiconductor wafer 7. You may be able to. As described above, in the present embodiment, the electron beam irradiation area of one shot is set smaller. Therefore, even if a slight distortion occurs at the joint between the irradiation areas, the same degree of distortion always occurs at the same location, and the appearance of the distortion on the two images to be compared with each other becomes equal. Therefore, the problem of erroneous detection due to distortion is eliminated. Thus, highly reliable pattern defect inspection can be realized.
[0107]
(Example 3)
In this embodiment, a time accumulation type CCD sensor is used as an element for converting a sample surface image into an electric signal. This element is called a TDI sensor, and is generally used in an optical inspection device. Other than that, it is the same as the case of the second embodiment. The operation concept of the TDI sensor will be described with reference to FIG.
[0108]
In the TDI sensor, the electric charge generated according to the intensity of the light received in each light receiving area is moved to the line in the x direction, and at the same time, the electric charge generated according to the intensity of the light received at the destination is moved. It works so that they are added sequentially. Then, when the light reaches the last line on the light receiving surface, the light is output to the outside as an electric signal. Therefore, by making the moving speed of the charge in the x direction and the moving speed of the image on the light receiving surface in the x direction the same, a signal during the movement of the image on the sensor is integrated and output.
[0109]
In the present embodiment, as in the case of the CCD sensors in the first to fourth embodiments, the signal reading is divided into 128 channels and read in parallel with each other, so that the reading speed is set to 4 M lines / sec. The size of the light receiving area was 64 pixels in the x direction and 2048 pixels in the y direction. The length of one line in the x direction corresponds to 50 nm on the sample surface, and the length in the y direction corresponds to about 100 μm. At this time, since an image having a length of 50 nm and a width of 100 μm is output at a speed of 4 M / sec, the continuous moving speed of the stage is set to the same speed (50 nm / 250 nsec = 200 mm / sec). As described above, the inspection area is moved in the x direction by moving the stage 8.
[0110]
On the other hand, since the irradiation area of one shot is 5 μm square, it is necessary to move the irradiation area in the y direction by scanning the electron beam as shown in FIG. That is, it is necessary to scan the electron beam by 100 μm in the y direction while the stage 8 moves by one shot (5 μm) in the x direction. Assuming that the time required for one shot is 1.25 μsec, it takes 25 μsec to scan 100 μm (for 20 shots) in the y direction. On the other hand, since the moving speed of the stage 8 in the x direction is 200 mm / sec, the time required for the stage 8 to move just one shot (5 μm) in the x direction is 25 μsec. As described above, the time required for moving the stage by one shot (5 μm) in the x direction is matched with the time required for electron beam scanning for 20 shots (100 μm) in the y direction, thereby preventing waste time from being generated. . The sample surface area is 1 cm by this method. ² In order to acquire the image of (2), the required scanning time (25 μsec) for the unit scanning area of 5 μm × 100 μm is 2 × 10 ⁵ 2 times as much as the sample surface area of 1 cm ² The required inspection time per hit is 5 seconds.
[0111]
As described above, in the present embodiment, since the stage moving speed determined by the signal output speed of the TDI sensor is 200 mm / sec, the inspection area can be sufficiently moved in the x direction by moving the stage. In addition, a sufficient time for scanning the electron beam in the inspection area in the y direction can be secured during that time. Further, in this embodiment, since the inspection speed is determined by the signal output speed of the TDI sensor, if the signal output speed is improved, an even higher-speed inspection can be realized.
[0112]
(Example 4)
This embodiment employs an electron optical system capable of acquiring an SEM image. FIG. 17 shows the configuration.
[0113]
For the electron source 201, the condenser lens 202, and the SEM objective lens 233, the elements constituting the electron optical system of the SEM are employed as they are. As the electron source 201, a Zr / O / W Schottky electron source was adopted. Electrons extracted from this electron source are deflected by a beam separator 243, changed by a large angle by an electrostatic sector type electron deflector 205, guided to a beam separator 203, and vertically incident on an objective lens 206. I do. The electron beam forms a cross-over at the front focal plane of the objective lens, and becomes a planar electron beam aligned in a direction perpendicular to the surface of the wafer 207 by the objective lens 206. The voltage applied to the wafer 207 and the arrangement of the diaphragm are the same as those in the first embodiment.
[0114]
This embodiment is characterized in that a high-resolution SEM image can be observed without taking out the wafer 7 from the apparatus when it is desired to observe the image of the defect detected after the wafer inspection in detail. That is, if the electron beam is made to travel straight without operating the beam separator 243, and at the same time the wafer 207 is moved below the optical axis of the SEM objective lens 233 by the wafer moving stage 208, an arbitrary position of the wafer 207 can be observed. it can. In the figure, 211 is an image forming lens, 213 and 214 are magnifying lenses, 222 is a condenser lens for SEM, 228 is a beam control system, 232 is a preliminary charging control device, 252 is a sample chamber, and 263 is an image detecting unit. .
[0115]
This function can be used not only for observing the detected defect but also for confirming the pattern of the wafer before inspection, setting inspection conditions, alignment, and the like.
[0116]
(Example 5)
This embodiment employs a means for irradiating an electron beam which is injected into a sample to generate a potential difference between a normal pattern and a defective pattern, and an electron beam which is reflected and forms an image without hitting the sample at almost the same place and almost simultaneously. It is provided. FIG. 18 shows the configuration. The electron beam emitted from the electron source 1 is deflected by the beam separator 3 in the direction of the optical axis perpendicular to the wafer 7, and the objective lens 6 forms a plane incident electron beam 301 aligned in the direction perpendicular to the surface of the sample (wafer). Is formed. The sample (wafer) 7 is applied with a negative potential substantially equal to the electron beam acceleration voltage by a sample application power supply 9. The sample application power supply 9 can also set the potential difference between the circular hole lens electrode 41 and the sample 7 and can vary the deceleration electric field between the circular lens electrode 41 and the sample 7. Configuration. For example, when detecting a shape defect, the deceleration electric field can be set stronger than usual to improve the shape defect detection sensitivity. The incident electron beam 301 is rapidly decelerated between the circular lens electrode 41 and the sample 7, and the incident electron beam 301 is absorbed by the sample by an electric field reflecting the shape of the semiconductor pattern formed on the surface of the sample 7 and the state of charging. Alternatively, the reflected electron beam 302 changes its direction immediately above the sample.
When an insulator is present on the surface of the sample 7, the incident electron beam 301 is injected into the sample 7, and an image is obtained with the insulator charged. For example, the cross-sectional view of FIG. 19 shows a sample in which an electrode portion of tungsten (W) is buried after processing a contact hole in a wafer in which an insulating film (SiO 2 film) 311 is formed on a silicon substrate (Si) 310. It is. The electrode portion includes a normal electrode portion 312 that is electrically connected to the substrate with low resistance, and a defective electrode portion 313 that is in a high resistance state with the substrate due to a residue of SiO2 at the bottom of the electrode at the center.
When an electron beam with an extremely low energy having an emission ratio of secondary electrons to a primary electron beam (the mean number of second electrons excited per incident electron) of less than 1 is applied to the insulator sample, the insulator sample becomes negatively charged (electron). It becomes negatively charged by accumulation. For example, with the irradiation energy of about 30 eV or less, SiO2 has a secondary electron emission ratio of less than 1 and is negatively charged. FIG. 19A shows the state of initial irradiation when the insulating film 311 is negatively charged. It is assumed that the energy of the incident electron beam 301 applied to the sample 7 has an energy spread including the irradiation energy eV0 in an initial state in which the sample 7 is not charged. Here, the irradiation energy is determined by the potential difference of the electron source 1 with respect to the sample 7 and the energy spread of the irradiation electron beam. When the sample 7 is continuously irradiated with the incident electron beam 301, the negative charge of the insulating film 311 proceeds until repelling the low-energy incident electron beam, and the number of electrons entering the insulating film 311 and the surrounding area from the insulating film 311. The number of electrons escaping to the substrate is negatively charged to a balanced potential and stabilized. In particular, when the electrons accumulated in the insulating film 311 do not escape to the peripheral portion, the insulating film 311 is negatively charged to a potential at which the incident electron beam 301 does not enter and is stabilized. The electrode section also has a potential at which the number of electrons entering and the number of electrons escaping from the electrode section to the periphery are balanced. That is, the electrons injected into the normal electrode portion 312 flow into the substrate 310 which is in contact with low resistance, so that they are hardly negatively charged. However, the defective electrode portion 313 has a large resistance value and the number of electrons escaping to the peripheral portion is small. Therefore, a negatively charged state is obtained. As shown in FIG. 19B, the equipotential surface 314 reflects these electrode and insulator potentials, and the equipotential surface 315 of V0 is formed immediately above the insulating film 311 and the defective electrode portion 313, and the energy Although the incident electron beam 301 smaller than V0 is reflected, the incident electron beam 301 collides with the normal electrode portion 312 without forming the equipotential surface 315 of V0 immediately above the normal electrode portion 312. Is an image reflecting the difference between the equipotential surface shapes near immediately above the normal electrode portion 312 and the defective electrode portion 313.
FIG. 21A shows that when a normal portion of a 70 nm square plug pattern of 3 × 3 rows is charged to 0 V, a defective portion of the center pattern is charged to −1 V, and the SiO 2 film is charged to −2 V, the energy is 1.2 V larger than the normal portion potential of 0 V FIG. 5 is a schematic diagram of a mirror image formed by reflecting and forming an electron beam irradiated in the step (a). In the normal part, a dark pattern and a high-resistance open defect are negatively charged because electrons collide with and are absorbed by the pattern, and are reflected and imaged without hitting the electrons, so that they are identified as a bright pattern contrast.
Here, since the charging potential of the insulator 311 depends on the energy eV0 of the incident electron beam 301, the charging potential of the insulator 311 can be freely controlled by changing the energy of the incident electron beam 301.
In addition, all the incident electron beams 301 having energy of eV1 with respect to the potential (0V) of the substrate of the sample are reflected by the equipotential surface 316 of V1, but the shape of the equipotential surface 316 of V1 is the normal electrode portion 312. And the vicinity immediately above the defective electrode portion 313, the image can be used to identify the normal electrode portion 312 and the defective electrode portion 313 even in the image formed by the reflected electron beam 302 reflecting the incident electron beam 301 having the energy of eV1. Can be obtained.
The standard of the time until the charging voltage of the insulating film is stabilized is as follows. Assuming a parallel plate capacitor having electrodes above and below the insulating film, the capacitance of the capacitor is defined as D, the electrode area is D, the dielectric constant of vacuum is ε0, and the dielectric constant of the insulating film is εr. (Capacitance of capacitator) C = ε0εrS / D For example, when an insulating film having a thickness of 0.4 μm and εr = 4 is irradiated with a current of 1 μA in an irradiation area of 20 μm square, the charging time to a voltage of 1 V is approximately 35 ns. Become When the insulating film is instantaneously charged and is not stable, as shown in FIG. 20, an electron beam irradiation area is set to be larger than the image acquisition area ahead of the inspection direction (scanning) direction, and the electron beam is preliminarily acquired before the image acquisition. Irradiation was adopted. Further, when the electron beam acquisition area is set to be substantially equal to the image acquisition area, the image acquisition area may be irradiated with an electron beam, and the image acquisition circuit may be controlled so as to acquire an image after charging is stabilized. .
Here, apart from the reflected electron beam 302 which is reflected without hitting the wafer 7, secondary electrons are generated from the sample 7 when the incident electron beam 301 collides with the wafer (sample) 7. The angular distribution of the secondary electrons is distributed as shown in FIG. Therefore, if the contrast aperture 12 provided near the back focal plane of the objective lens 6 is reduced, the secondary electrons dispersed at an angle are almost absorbed by the contrast aperture 12, so that the reflected beam does not hit the sample. An image can be formed only by the above.
In order to detect an electrical defect, it is necessary to control the moving direction of electrons in the pattern depending on the bonding state with the substrate. Therefore, it is necessary to charge the insulating film not only negatively but also positively. In order to positively charge the insulating film 311, an electron beam having an energy at which the emission ratio of secondary electrons to primary electron beams is greater than 1 must be applied. FIG. 19C shows an initial irradiation state when the insulating film 311 is positively charged. First, for example, first, the sample 7 is irradiated with the incident electron beam 301 having energy eVb with reference to the sample potential. For example, if 500 V is selected as Vb, the secondary electron emission ratio from the sample 7 becomes larger than 1, so that the insulator 311 is charged to a positive potential, and the insulator 311 is charged by a positive barrier to the sample. The charging proceeds until the potential is stabilized by the electrons drawn back to the substrate. After the insulator 311 and the defective electrode portion 313 are positively charged in this way, if electrons with energy of eVp that does not hit the insulator pattern are irradiated as shown in FIG. 19D, the equipotential surface 317 of Vp becomes Since there is a great difference between the normal electrode section 312 and the area immediately above the defective electrode section 313, distortion of the potential of the positively charged section can be detected. When the energy of the incident electron beam 301 to be irradiated is increased, the incident electron beam 301 collides with the insulating film 311 earlier than the pattern portion because the insulating film 311 is positively charged. When the incident electron beam 301 collides with the insulating film 311, the positively charged potential is relaxed. Therefore, it is necessary to select the energy for irradiating the sample under such conditions that the electron beam does not collide with the insulating film. However, when simultaneously irradiating the sample 7 with an electron beam to be positively charged and an electron beam to acquire an image, the electron beam to acquire an image collides with the insulator to inject electrons into the insulator, and Since the positively charged potential of the insulator is stabilized so that the electron emission of the insulator obtained by the electron beam to be charged is in an equilibrium state, it is possible to control the positively charged potential in the case of simultaneous irradiation.
FIG. 21B shows that, when the normal electrode portion 312 is charged to 0 V, the defective electrode portion 313 of the central pattern is charged to +1 V, and the insulating film 311 is charged to +2 V, the incident electron beam 301 is subjected to the above-mentioned conditions. That is, it is a schematic diagram of a mirror image formed when the image is formed under irradiation conditions with energy smaller than the normal electrode potential of 0 V by about 1.9 V. Reflecting the potential difference between the patterns of the normal electrode portion and the defective electrode portion, the equipotential surface formed between the sample 7 and the hole lens 41 has different shapes near the normal pattern and the defective portion. The trajectory of the electron beam reflected without colliding with the sample greatly depends on the shape of the equipotential surface at the position where the electron beam is reflected, and between the electron beam reflected near the normal pattern and the electron beam reflected near the defective portion, The imaging position of the electron beam changes greatly. FIG. 21B shows a state in which the object plane position of the projection lens is adjusted to the vicinity of the image plane of the electrons reflected in the vicinity of the normal pattern, and an image in which the normal pattern has a bright contrast is formed. Further, it is also possible to adjust the object plane position of the projection lens to the vicinity of the image plane of the electrons reflected in front of the defect pattern. In this case, the image shown in FIG. Can be obtained.
The above operation can be realized by one electron source by periodically switching the relative voltage between the sample application power source 9 and the electron source power source 23, that is, the application voltage of the electron source power source 23 or the sample application power source 9. . For example, as shown in FIG. 22, it is assumed that the voltage applied to the electron source power supply 23 is periodically switched based on the sample applied power supply voltage. First, the electron beam is moved to a predetermined inspection area using the stage 8 or the irradiation system deflector 5, and a voltage + Vb, for example, 500 V for positively charging the sample is applied from the electron source power supply 23 to the electron source 1 for a predetermined time. By irradiation, a predetermined inspection area is positively charged. Next, by switching the voltage supplied to the electron source power supply 23 to the voltage -Vp for acquiring an image and applying the voltage to the electron source 1, a beam having energy as small as -eVp with respect to the sample potential is irradiated to a predetermined inspection area. In addition, an image can be acquired for a certain period of time while the sample is positively charged. By repeating this operation every time the inspection area is moved, images can be continuously obtained while the sample is positively charged.
[0117]
(Example 6)
Apart from an electron source that generates an electron beam that is pulled back immediately before the sample and forms an image, a new electron source for simultaneously irradiating an electron beam with an energy with an emission ratio of secondary electrons to the primary electron beam of more than 1 is newly arranged. By doing so, it becomes possible to obtain an image while simultaneously charging the sample positively. FIG. 23 shows the configuration of this embodiment.
The electron source 1 is placed at a slightly higher potential than the wafer 7, and the incident electron beam 301 emitted from the electron source 1 is pulled back just above the wafer 7 to become a reflected electron beam 302. On the other hand, the second electron source 71 and the wafer 7 have a potential difference of about 500 V, and the electron beam 303 emitted from the second electron source 71 irradiates the sample with energy of 500 eV. The magnetic prism 73 is covered with a shield electrode 74 floating at a voltage of Vs. The electron beam incident on the magnetic prism 73 is decelerated by eVs energy, is deflected, and exits the magnetic prism 73. The electron beam is accelerated by energy eVs. For example, the electron beam 301 emitted from the electron source 1 is decelerated to about 500 V immediately before the magnetic field prism 73 and enters the magnetic field prism 72. On the other hand, the electron beam 303 emitted from the second electron source 71 is decelerated to about 1000 V immediately before entering the magnetic field prism 73 and enters the magnetic field prism 73. Here, if the magnetic flux density in the prism is set to 5 gauss, the electron beam 301 enters the same optical axis 304 along a path with a cyclotron radius of 150 mm and the electron beam 302 with a radius of about 210 mm, so that the same field of view has a different energy. Irradiation with an electron beam. The reflected electron beam 302 pulled back immediately above the sample enters the magnetic field prism 73 and is deflected in a direction opposite to the incident electron beam, and then passes through an intermediate lens 13 and a projection lens 14. An image is formed on the fluorescent screen 15.
<Example 7>
In the present embodiment shown in FIG. 24, the second electron source 201 and the second irradiation lens 202 are arranged, and the electron beam 303 emitted from the second electron source 201 is caused to collide with the lower part of the circular lens electrode 41. The configuration is such that the sample 7 can be irradiated with the generated elastic scattered electrons 304. If the energy component in the optical axis direction of the elastic scattered electrons 304 generated by colliding the electron beam with the lower part of the hole lens electrode 41 is larger than the energy decelerated by the voltage applied between the hole lens electrode 41 and the sample 7 The elastic scattered electrons 304 reach the sample surface. For example, if the directional distribution of the elastically scattered electrons 304 has a peak in a direction inclined by 30 degrees with respect to the optical axis 402, the acceleration voltage of the electron beam 303 generated from the second electron source 201 will be the circular lens electrode 41 If a voltage obtained by adding 500 V / COS 30 ° = 577 V to the voltage applied between the sample 7 and the sample 7 is applied, the peak component of the elastically scattered electrons 304 can be irradiated to the sample 7 at an acceleration voltage of about 500 V. Can be positively charged. The angle and the position of the electron beam 303 that collides with the lower part of the electrode are substantially the same as the irradiation electron beam 301 for forming an image in consideration of the fact that the trajectory of the elastic scattered electrons 304 is changed by the deceleration electric field. However, the aligner 204 has a beam tilting function and a beam position moving function so that the irradiation position can be adjusted.
Example 8
The present embodiment shown in FIG. 25 has a configuration in which a light source 251 is arranged as a means for charging a sample, and the sample can be irradiated with a light beam 252 simultaneously. When the sample 7 is irradiated with a light beam having an energy larger than the work function of the substance constituting the sample 7, photoelectrons are emitted from the sample 7, whereby the sample 7 can be positively charged. The cross-sectional view of FIG. 26 shows Sample 7 in which a tungsten (W) plug is buried after processing a contact hole in a wafer in which an insulating film (SiO 2 film) is formed on silicon (Si). At the bottom of the plug at the center, there is a high-resistance open defect due to the residue of SiO2.
FIG. 26A shows the state of initial irradiation when the insulating film 311 is positively charged. The potential of the electron source 1 is set to a positive potential of + Vp with respect to the potential of the sample 7, and the electron beam 301 emitted from the electron source 1 is decelerated between the hole electrode 41 and the sample 7, and is immediately above the sample. The direction changes to become the reflected electron beam 302. Since the work function of SiO 2 is approximately 9 eV, if the sample is simultaneously irradiated with ultraviolet rays having energy higher than 9 eV as the light beam 252, the insulating film 311 made of SiO 2 emits photoelectrons 305. As shown, the insulating film 311 is positively charged, and the equipotential surface 314 immediately above the insulating film 311 is curved. When the positive charging of the insulating film 311 further progresses, a part of the incident electron beam 301 reaches the insulating film 311, and the amount of the photoelectrons 305 emitted from the insulating film 311 and the amount of the incident electron beam 301 reaching the insulating film 311. The positively charged potential of the insulating film 311 is stabilized in an equilibrium state where the amounts are equal. When the reflected electron beam 302 is imaged in this state, the portion of the insulating film 311 becomes dark and an image reflecting potential distortion near the insulating film is formed. An image with high contrast can be obtained. The optical axis 404 of the light beam 252 emitted from the light source 251 and the optical axis 402 of the irradiation electron beam 301 are adjusted so as to substantially coincide with each other on the sample. When the stage is moved and the image is continuously acquired by taking the irradiation area wider than the irradiation area 301, the irradiation electron beam 301 is applied to the sample 7 while the sample 7 is irradiated with the light beam 252 for a certain period of time and the sample is positively charged. Can be irradiated. The photoelectrons 305 generated at the same time can be absorbed by the contrast aperture 12 arranged in the optical path of the reflected electron beam, so that a high-contrast imaging electron image can be obtained.
<Example 9>
In this embodiment, as the beam separator 3, two sets of E × B deflectors (E × B deflectors) and two sets of spherical electrostatic prisms (electrostatic prisms) are combined to reduce the incident electron beam and the reflected electron beam. A separating configuration was used. FIG. 27 shows the configuration diagram. In this configuration, the reflected electron beam reflected from the sample is set so as to go straight through the E × B deflector 54, and the incident electron beam 301 operates the two sets of E × B deflectors in opposite directions, The two sets of electrostatic prisms are also operated in the opposite directions to optimally select the lens imaging conditions, thereby reducing the aberrations of the irradiation system and the imaging system. Further, since the image forming system and the irradiation system are arranged vertically to the stage, the effect on the electron optical system due to mechanical vibration and the like can be reduced to a minimum. Hereinafter, the configuration will be described in detail. With respect to the reflected electron beam reflected from the sample, the E × B deflector 54 operates so as to cancel the effects of the magnetic field and the electric field, and the reflected electron beam goes straight in the E × B deflector 54. Further, by placing the image point of the reflected electron beam near the E × B deflector 54, the lens aberration generated in the E × B deflector 54 is reduced. The incident electron beam 301 emitted from the electron source forms an image on an image point 403 on the optical axis 401 by a condenser lens, and then enters the E × B deflector 51 and is deflected by about 15 degrees. The deflected incident electron beam 301 enters the electrostatic prism 52, draws a circular trajectory, and receives a converging lens action while rotating by a predetermined angle. The incident electron beam 301 that has exited the electrostatic prism 52 enters the electrostatic prism 53 and undergoes a focusing action while rotating in the opposite direction. The incident electron beam 301 emitted from the electrostatic prism 53 is incident on the E × B deflector 54, is deflected by about 15 degrees in the opposite direction to the E × B deflector 51, and is irradiated with the objective lens light of the irradiation system. The light enters the axis 402 and forms an image at the image forming point 404. Here, the distance between the electrostatic prism 51 and the image forming point 403 and the distance between the electrostatic prism 53 and the image forming point 404 are set substantially equal, and the incident light exiting the electrostatic prism 51 almost in parallel. If the operating conditions of the electrostatic prism are selected so that the electron beam 301 is incident on the electrostatic prism 53 almost in parallel, the aberrations of the two sets of electrostatic prisms are substantially corrected.
For example, when the incident electron beam 301 enters the electrostatic prism having a rotation angle of 200 degrees and a radius of 50 mm at an incident energy of 10 keV from a distance of about 170 mm between the electrostatic prism 51 and the condenser lens imaging point 403, the incident light exits the prism. The electron beam 301 becomes substantially parallel, and converges at a position substantially symmetrical when the electrostatic prism 54 is emitted, that is, at a position at a distance of about 170 mm between the electrostatic prism 53 and the imaging point 404. Under this condition, the aberrations of the electrostatic prism 51 and the electrostatic prism 54 cancel each other out and become almost zero, so that it is possible to irradiate the sample with the incident electron beam 301 with high luminance. <Example 10>
This embodiment is provided with a means for performing an accurate inspection even if the sample height or the inclination angle fluctuates. For a large-diameter wafer, the height and tilt angle of the sample fluctuate greatly in the plane due to deformation due to the warp of the wafer, so that the imaging position does not change even if the sample height fluctuates. It is necessary to provide a means for always allowing the incident electron beam to be perpendicularly incident on the sample surface even if the value fluctuates.
FIG. 28 illustrates a control method for causing the incident electron beam 301 to be perpendicularly incident on the sample even when the wafer is tilted.
In the xyz rectangular coordinate system, if the z direction is taken as the optical axis, and the sample surface normally in the xy plane at z = 0 is in a plane passing z = 0 and inclined by θ in the x direction, When the distance between the sample 7 and the electrode of the circular lens 41 is set to L, the z-direction component Ez and the x-direction component Ex of the electric field strength are approximated by Ez ≒ E and Ex ≒ (1-z / L) E, respectively. You. Solving the equations of motion of the electrons in the x direction and the z direction, the condition that the incident electron beam 301 enters the sample 7 almost perpendicularly in the state of being closest to the sample 7 is that the angle β passing through the circular lens 41 is
[0118]
(Equation 1)

Is generally satisfied.
Further, the focal length of the circular lens 41 can be approximated to f = 4L if the diameter of the circular hole of the circular lens 41 is sufficiently smaller than L, so that the light enters the circular lens 41 under such conditions. It can be seen that the angle of the incident electron beam 301 should be inclined by 2 sin θ.
Therefore, for example, if the aligner 42 is arranged between the objective lens 7 and the circular hole lens 41 and the sample is inclined by about 2θ when the sample is inclined by θ in the x direction, it is always perpendicular to the sample. Can be irradiated with the incident electron beam 301. Here, it is preferable to use an electrostatic aligner so that the incident electron beam 301 and the reflected electron beam 302 act in the same direction.
The correction for the change in the sample height is performed as follows. The focal length f of the circular lens 41 is approximated by f = 4L, and a virtual image of a reflected electron beam reflected from the sample is formed at a position L / 3 below the sample. Since the object plane position of the objective lens 7 is the virtual image plane of the circular lens 41, when the sample height changes, the virtual image position of the circular lens 41 changes, and the objective lens is objective. The image plane position of (lens) 7 will also fluctuate. Therefore, L is calculated from the sample height, the virtual image position of the circular lens, that is, the object plane position of the objective lens is calculated, and the intensity (lens exitation) of the objective lens is set such that the image plane position of the objective lens is always constant. ) Can be controlled so that the image is not always blurred even if the sample height changes.
The height distribution of the semiconductor wafer 7 is measured in advance by using height measuring means such as the height measuring device 26 at several tens or more height measuring points in the wafer before inspection. The inclination angle of a specific point of the sample can be obtained by dividing the altitude difference between adjacent height measurement points by the distance to the adjacent height measurement point. By taking the average of the inclination angle measurement values with a plurality of adjacent height measurement points, the accuracy of the measurement values can be improved. The inclination angle data corresponding to an arbitrary inspection position can be obtained by interpolating the inclination angle data of the height measurement point. At the time of inspection, by controlling the intensity of the aligner 42 based on the wafer tilt data corresponding to the inspection position coordinates, it is possible to always irradiate the sample with the incident electron beam 301 vertically in the wafer plane.
[0119]
【The invention's effect】
According to the embodiment of the present invention, by using an electron beam, fine pattern defects on the surface of a sample such as a semiconductor device and electrical defects such as open, short, and leak are imaged with high sensitivity, high resolution, and high speed. An inspection method and an inspection apparatus that can be inspected by using the inspection method are realized.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an inspection apparatus according to a first embodiment of the present invention.
FIG. 2 illustrates the principle of the present invention.
FIG. 3 illustrates the principle of the present invention.
FIG. 4 is a diagram showing an example of a defect image obtained by the present invention.
FIG. 5 is a diagram illustrating the efficiency of electrons that can be used for image formation in a conventional example.
FIG. 6 is a diagram illustrating the efficiency of electrons that can be used for image formation in a conventional example.
FIG. 7 is a view for explaining a difference in efficiency of electrons that can be used for image formation according to the related art and the present invention.
FIG. 8 is a diagram comparing the contrast (a) and the inspection time (b) of the conventional example and the present invention.
FIG. 9 is a view for explaining the configuration of the first embodiment of the present invention.
FIG. 10 is a diagram for explaining a relationship between an electron energy width and defect detection sensitivity.
FIG. 11 is a view for explaining the principle of the preliminary charging control device according to the first embodiment of the present invention.
FIG. 12 is a diagram illustrating a configuration of a pre-charging control device according to the first embodiment of the present invention.
FIG. 13 is a diagram for explaining another configuration of the preliminary charging control device according to the first embodiment of the present invention.
FIG. 14 is a diagram illustrating an image signal detection unit according to the first embodiment of the present invention.
FIG. 15 is a diagram illustrating an image signal detecting unit according to a second embodiment of the present invention.
FIG. 16 is a diagram illustrating an image signal detecting unit according to a third embodiment of the present invention.
FIG. 17 is a diagram illustrating a configuration of an electron optical system according to a fourth embodiment of the present invention.
FIG. 18 is a view for explaining the configuration of a fifth embodiment of the present invention.
FIG. 19 is a diagram illustrating charging control according to a fifth embodiment of the present invention.
FIG. 20 is a diagram illustrating an image acquisition area and an electron beam irradiation area according to a fifth embodiment of the present invention.
FIG. 21 is a diagram showing an example of a defect image according to a fifth embodiment of the present invention.
FIG. 22 is a view for explaining control of a voltage applied to a sample according to a fifth embodiment of the present invention.
FIG. 23 is a view for explaining a sixth embodiment of the present invention.
FIG. 24 is a view for explaining a seventh embodiment of the present invention.
FIG. 25 is a diagram for explaining an eighth embodiment of the present invention.
FIG. 26 is a diagram illustrating charging control according to an eighth embodiment of the present invention.
FIG. 27 is a diagram illustrating a ninth embodiment of the present invention.
FIG. 28 is a view for explaining a tenth embodiment of the present invention.
[Explanation of symbols]
1: electron source, 2: condenser lens, 3: beam separator, 4: aperture, 5: irradiation system deflector, 6: objective lens, 7: sample (wafer), 8: wafer moving stage, 9: power supply, 10: Imaging system deflector, 11: imaging lens, 12: contrast aperture, 13: magnifying lens, 14: magnifying lens, 15: fluorescent plate, 16: optical fiber bundle, 17: CCD, 18: image storage unit, 19: image storage , 20: calculation unit, 21: defect determination unit, 22: monitor, 23: acceleration power supply, 24: scanning signal generator, 25: objective lens power supply, 26: sample height measurement device, 27: stage position measurement device, 28: beam control system, 29: control computer, 30: stage control system, 31: optical microscope, 32: pre-charging control device, 101: electron optical system, 102: sample chamber, 103: image detector, 104: image Processing unit, 105: control unit, 201: electron gun, 202: condenser lens, 203: beam separator, 205: deflector, 206: objective lens, 207: sample (wafer), 208: wafer moving stage, 211: imaging Lenses, 213 and 214: magnifying lens, 222: condenser lens for SEM, 228: beam control system, 232: pre-charge control device, 233: objective lens for SEM, 243: beam separator, 252: sample chamber, 263: image detection Department.

Claims (16)

When illuminated toward the sample surface, a planar electron beam whose planar shape has a two-dimensional spread is applied to a plurality of sample surfaces to which a negative voltage is applied with reference to the potential of the electrode facing the sample surface. By sequentially irradiating the irradiation area, of the electrons forming the electron beam, the electrons pulled back near the surface of the sample without colliding with the surface of the sample are imaged on the image forming means, and the image forming means is used. A plurality of electronic images respectively corresponding to the plurality of irradiation regions are converted into a plurality of electrical image signals, and the plurality of image signals are compared with each other to detect a pattern defect formed on the sample. And a pattern having substantially the same shape exists in each of the plurality of irradiation areas. The planar electron beam is configured to irradiate each of the plurality of irradiation regions so that the traveling direction is substantially parallel to the sample surface and is incident substantially perpendicularly. 1. The defect inspection method according to 1. 2. The defect inspection method according to claim 1, further comprising a charging step for charging the surface of the sample to a desired potential prior to the irradiation with the electron beam. 2. The defect inspection method according to claim 1, further comprising a charging step for charging the sample surface to a desired potential in parallel with the irradiation of the electron beam. The charging step uses the same optical system as the electron beam irradiating means, and is configured to provide a desired potential difference between an electron source for irradiating the electron beam and the sample, and a first potential difference ( The surface of the sample is charged to a desired potential by giving a potential difference value of 0, and then a second potential difference (potential difference value) different from the first potential difference value is applied. 5. The defect inspection method according to claim 4, wherein the incident electron beam is pulled back near the surface of the sample without colliding with the surface of the sample. The first potential difference is applied between the electrode and the sample during the time from time t1 to time t2, and the second potential difference is applied between the time t3 and time t4 (where t1ｔt2, t3 ≠ t4). Is applied between the electrode and the sample, and the potential difference to be applied between the electrode and the sample is applied alternately in the order of the first potential difference and the second potential difference, and the potential difference is applied. 6. The defect inspection method according to claim 5, wherein the value is periodically changed. 2. The defect inspection method according to claim 1, wherein the energy value of the electron beam sequentially irradiating the irradiation area on the sample surface is a value necessary for positively charging the insulating film on the sample surface. When irradiated from the electron source and directed toward the sample surface, the planar electron beam whose planar shape has a two-dimensional spread is converted to a negative voltage with reference to the potential of the electrode facing the sample surface. A first electron optical system for sequentially irradiating a plurality of irradiation regions on the sample surface to which
Means for generating an electric field near the sample surface such that electrons forming the electron beam irradiated toward the surface are pulled back near the sample surface,
A second electron which forms an electron beam and which is drawn back near the surface of the sample without colliding with the surface of the sample, on an imaging means provided on an extension of the electron trajectory; Optical system,
Image signal detection means for converting a plurality of electronic images respectively corresponding to the plurality of irradiation areas into a plurality of electrical image signals using the imaging means,
By comparing the plurality of image signals with each other, having an image signal processing means for detecting a defect formed in the sample,
A defect inspection apparatus, wherein a pattern having substantially the same shape exists in each of the plurality of irradiation areas. The first electron optical system has at least an electron gun for generating an electron beam, a condenser lens for focusing the electron beam emitted from the electron gun, and an objective lens disposed between the condenser lens and the sample. Then, the focal point of the electron beam is arranged near the electron source side focal plane (focal @ plane) of the objective lens by the condenser lens, and the traveling direction of the planar electron beam with respect to the sample surface is substantially reduced. 9. The defect inspection apparatus according to claim 8, wherein each of the plurality of irradiation regions is irradiated so as to be incident substantially perpendicularly in parallel. A stage position measuring mechanism that measures the position of the sample moving stage that moves continuously in real time, and a position change that occurs during the stage movement due to the continuous movement is measured by the stage position measuring mechanism, and the electron beam irradiation is performed. 9. The apparatus according to claim 8, wherein a positional relationship between the electron beam and the sample moves at a substantially constant speed in a predetermined direction by feeding back to the electron beam deflecting mechanism of an optical system. Pattern defect inspection equipment. 9. The inspection apparatus according to claim 8, further comprising a charging unit for charging the surface of the sample to a desired potential prior to the irradiation with the electron beam. 9. The inspection apparatus according to claim 8, further comprising a charging unit for charging the surface of the sample to a desired potential in parallel with the irradiation of the electron beam. When the charging unit for charging the sample surface to a desired potential uses the same optical system, a power supply for giving a desired potential difference between the electron source for irradiating the electron beam and the sample is provided. The sample surface is charged to a desired potential by applying a first potential difference (including a case where the value of the potential difference is 0), and then the value is different from the value of the first potential difference. By applying a second potential difference (including a case where the value of the potential difference is 0; however, both the first and second potential differences do not become 0), the electrons constituting the electron beam incident thereon are converted into the sample. The defect inspection apparatus according to claim 11, wherein the sample is pulled back near the surface of the sample without colliding with the surface. 9. The defect inspection apparatus according to claim 8, wherein the energy value of the electrons sequentially irradiated to the plurality of irradiation regions on the sample surface is a value necessary for positively charging the insulating film on the sample surface. First means for generating a planar light beam having a planar shape having a two-dimensional spread when incident on the sample at a desired incident energy when emitted from a light source and illuminating the sample surface,
A planar electron beam generated from an electron source and having a two-dimensional spread when irradiated toward the sample surface is decelerated near the sample surface, and collided with the sample surface. Second means for irradiating so that at least a part thereof is pulled back in the vicinity of the sample surface,
An electron optical system that forms an electron beam on the imaging means provided on an extension of the electron trajectory, of the electrons forming the electron beam, the electrons pulled back near the surface of the sample without colliding with the surface of the sample; ,
Image signal detecting means for converting a plurality of electronic images respectively corresponding to the plurality of irradiation areas into electrical first, second and third image signals using the image forming means;
By comparing the plurality of image signals with each other, having an image signal processing means for detecting a defect formed in the sample,
A defect inspection apparatus, wherein a pattern having substantially the same shape exists in each of the plurality of irradiation areas. 16. The defect inspection apparatus according to claim 15, wherein the desired incident energy is energy required to positively charge at least a part of the surface of the sample.

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