JP4011455B2

JP4011455B2 - Sample observation method using transmission electron microscope

Info

Publication number: JP4011455B2
Application number: JP2002309710A
Authority: JP
Inventors: 功長沖
Original assignee: Hitachi High Technologies Corp
Current assignee: Hitachi High Tech Corp
Priority date: 2002-10-24
Filing date: 2002-10-24
Publication date: 2007-11-21
Anticipated expiration: 2022-10-24
Also published as: JP2004146192A

Description

【０００１】
【発明の属する技術分野】
本発明は、透過電子顕微鏡を用いた試料観察方法に関し、特に非点補正を簡便に行うことのできる試料観察方法に関する。
【０００２】
【従来の技術】
透過電子顕微鏡は、試料の内部を透過した透過電子線を結像するもので、試料の内部構造を観察することができる。透過電子顕微鏡によると、材料の格子欠陥や粒界の構造、析出物の大きさや分布、あるいは格子像を観察することができる。透過電子顕微鏡の磁界レンズには、磁極の精度、磁気材料の不均一、磁極の軸の精度等に起因して非点収差が生じる。非点収差は、通常、相互に４５゜ずらせて配置した２組の四極子を独立して励起することにより調整される。しかし、この調整には熟練を要する。
【０００３】
特許文献１には、電子線を光軸を中心として対称的に繰り返し傾斜させながらその方位角を順次変化させて試料に照射することにより、非点の方向とその大きさを検出すること、検出された非点の大きさと方向に関する画像を画面に表示し、表示画面中に同時に表示されたカーソルを動かすことにより非点補正指令信号を入力し、演算手段により、入力されたカーソルの移動方位角を２倍し、その２倍の方位角とカーソルの移動量に対応する大きさを有する２次元ベクトルをＸ、Ｙ方向に分解し、それぞれに基づいてＸ側非点補正用信号とＹ側非点補正信号を形成することが記載されている。
【０００４】
【特許文献１】
特開平７−２６２９５２号公報
【０００５】
【発明が解決しようとする課題】
特許文献１に記載の非点補正方法は、画面表示された非点の大きさと方向を表す画像に対してオペレータが非点補正信号を入力する必要がある。すなわち、非点補正に当たってオペレータの介在を必要とし、自動化できないという問題がある。
【０００６】
本発明は、透過電子顕微鏡を用いた試料の拡大像観察時に、非点量を自動的に検出し、検出した非点量に基づいて非点を自動的に補正することのできる試料観察方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記目的を達成するために、本発明の非点補正機構を備える透過電子顕微鏡による試料観察方法は、光軸に対して所定角度傾斜した電子線を複数の異なる方位角方向から試料に照射し、複数の試料透過電子像を撮像するステップと、試料透過電子像間の移動量を求め、複数の試料透過電子像の移動軌跡によって形成される楕円を求めるステップと、複数の試料透過電子像の移動軌跡を前記楕円と同じ面積の真円とするために必要な非点補正機構の制御量を求めるステップと、求めた制御量によって非点補正機構を制御するステップとを含むことを特徴とする。
【０００８】
本発明の非点補正機構を備える透過電子顕微鏡による試料観察方法は、また、光軸に対して所定角度傾斜した電子線を非点補正機構の複数の非点補正方向に関してそれぞれ正負２方向から試料に照射し、複数の試料透過電子像を撮像するステップと、各非点補正方向に関して正負２方向に傾斜した電子線による２つの試料透過電子像間の移動量を求めるステップと、互いに直交する２つの非点補正方向に関して求められた２つの移動量を楕円の長軸と短軸としたとき、前記２つの試料透過電子像の移動量を当該楕円と同じ面積の真円の半径とするために必要な非点補正機構の制御量を求めるステップと、求めた制御量によって非点補正機構を制御するステップとを含むことを特徴とする。
２つの試料透過電子像間の移動量は、当該２つの試料透過電子像間の相関強度から計算することができる。
【０００９】
非点補正機構は相互に４５゜ずらして配置した第１の四極子と第２の四極子を有するものとすることができ、非点補正機構を制御するステップは、第１の四極子と第２の四極子に対して個別に行うことができる。
【００１０】
前記試料観察方法において、光軸に対して傾斜角ゼロの電子線を試料に照射したときの試料透過電子像と、光軸に対して所定角度傾斜した電子線を試料に照射したときの試料透過電子像との間の移動量を求めるステップと、２つの試料透過電子像間の移動量からフォーカス補正に必要な対物電流補正値を求めるステップと、求めた対物電流補正値を対物電流値に加算してフォーカス補正を行うステップとを更に備えるのが好ましい。
【００１１】
【発明の実施の形態】
以下、図面を参照して本発明の実施の形態を説明する。
図１は、本発明で用いる透過型電子顕微鏡の一例の概略機能ブロック図である。なお、偏向コイルの段数は問わないが、ここでは一般的な２段の偏向コイルを用いた透過電子顕微鏡について説明する。
【００１２】
電子銃１から放出されて加速された電子線は第１照射レンズ２及び第２照射レンズ３によって集束され、第１偏向コイル４及び第２偏向コイル５で偏向され、対物レンズ６で結像されて、試料ステージ６０に保持された試料に照射される。試料を透過した電子線は、非点補正機構を構成するＸ側非点補正コイル７及びＹ側非点補正コイル８を通り、第１中間レンズ９及び第２中間レンズ１０によって拡大された後、第１投射レンズ１１及び第２投射レンズ１２で更に拡大されて、シンチレータ４８上に試料の透過像が形成される。シンチレータ４８で光像に変換された試料の透過電子像は、ＴＶカメラ（撮像装置）４９によって撮像される。ＴＶカメラ４９からの映像信号はＴＶカメラ制御部４７、画像取り込みインターフェース５１を介してマイクロプロセッサ３５に取り込まれ、処理された後、ＣＲＴコントローラ３８で制御されるＣＲＴ３９に表示される。マイクロプロセッサ３５は、ＤＡＣ２４〜３４を介して、電子顕微鏡の各レンズ２〜１２に給電する励磁電源１３〜２３を制御する。また、マイクロプロセッサ３５には、バスを介してハードディスク等の記憶装置３６、演算装置３７、倍率切替用ロータリーエンコーダ４２、入力用ロータリーエンコーダ４３、キーボード４４、ＲＡＭ４５、ＲＯＭ４６、マウス５０等が接続されている。ロータリーエンコーダ４２，４３はＩ／Ｆ４０，４１を介してバスに接続されている。
【００１３】
図２は、試料を保持する試料ステージの概念図である。試料ステージ６０はメッシュ状の試料面５５を備え、試料は試料面５５上に保持される。また、試料ステージ６０は、マイクロプロセッサ３５からの指令のもとに試料ステージコントローラ５７によって駆動制御されるＸ方向駆動モータ５８及びＹ方向駆動モータ５９によってＸ方向及びＹ方向に駆動される。
【００１４】
図３は、偏向コイルの概略図である。試料に対する傾斜角αでの電子線照射は、２段の偏向コイル４，５により電子線を傾斜して偏向角αとなる偏向データをＲＯＭ４６よりＤＡＣ２６，２７に出力し、ＤＡＣ２６，２７から励磁電源１５，１６にアナログ信号を出力して偏向コイル４，５に電流を流すことにより行うことができる。
以下に、本発明の透過電子顕微鏡を用いた試料観察方法において用いる要素技術について説明する。
【００１５】
（１）２つの画像間の移動量検出
図４に示す画像相関の例を用いて、２つの画像間７１，７２の移動量（ずれ量）検出方法について説明する。透過電子顕微鏡像７１をＭ×Ｎの画素数で記憶装置３６に登録画像f1(m,n)として記録する。次に、透過電子顕微鏡像７１に対して平行移動した透過電子顕微鏡像７２をＭ×Ｎの画素数で記憶装置に参照画像f2(m,n)として記録する。但し、どちらも自然画像とし、ｍ＝０，１，２，…，Ｍ−１；ｎ＝０，１，２，…，Ｎ−１である。
【００１６】
登録画像f1(m,n)及び参照画像f2(m,n)の離散フーリエ画像F1(u,v)，F2(u,v)は、それぞれ次の式(1)、(2)で定義される。但し、ｕ＝０，１，２，…，Ｍ−１；ｖ＝０，１，２，…，Ｎ−１であり、A(u,v)，B(u,v)は振幅スペクトル、θ(u,v)，φ(u,v)は位相スペクトルである。
【００１７】
【数１】

【００１８】
位相相関では、２つの画像間で画像の平行移動があった場合には、相関のピークの位置が移動量だけずれる。以下に移動量の導出方法を説明する。まず、原画像f2(m,n)がｍ方向にｒ′だけ移動したとして、f4(m,n)＝f2(m+r',n)とする。前記式(2)を式(3)のように変形する。
【００１９】
【数２】

振幅スペクトルB(u,v)を定数とすることにより、画像のコントラストに依存しない位相画像となる。f4の位相画像F'(u,v)は、次の式(4)となる。
【００２０】
【数３】

位相画像F'1(u,v)にF'2(u,v)の複素供役を乗ずることによって、次の式(5)で表される合成画像H14(u,v)が得られる。
【００２１】
【数４】

相関強度画像G14(r,s)は、合成画像H14(u,v)を逆フーリエ変換することによって次の式(6)となる。
【００２２】
【数５】

【００２３】
式(6)より、２つの画像間でｍ方向に位置ずれ量ｒ′が存在する場合、相関強度画像のピークの位置は−ｒ′だけずれる。また、位相成分で相関計算するため、２つの画像で明るさやコントラストに違いがあっても移動量の計算が行える。２つの画像間でｍ方向に位置ずれ量が存在する場合は、相関強度画像の中心よりΔG(pixel)の位置にピークが発生する。例えば、図４に示すように、２つの画像７１，７２間でｍ方向に２pixelのずれがあると、合成位相画像７３は２周期の波になる。これを逆フーリエ変換すると相関強度画像７４となり、中心から２pixelずれた位置にピーク７５が発生する。このΔG(pixel)は検出器の受光面での移動量に相当し、ΔGを試料面上の移動量Δｘに変換する。検出器の受光面の径をL、受光面上での透過電子顕微鏡の倍率をM、検出器の受光面の画素数をLmとすると、試料面上の移動量Δｘは次式(7)で計算される。
Δｘ＝ΔG(pixel)×L／Lm(pixel)／M …(7)
Δｘは２つ画像７１，７２間の試料面上での移動量となる。Ｙ方向の移動量も、同様にして計算できる。
【００２４】
（２）非点量検出及び非点補正
次に、本発明による非点量検出方法及び非点補正方法について説明する。
図５は、非点補正機構が備える非点補正コイルの配置を示す概念図である。非点補正コイルは、互いに４５゜の角度で交差するＸ側非点補正コイルとＹ側非点補正コイルとを組み合わせて構成されている。図５（ａ）にＸ側非点補正コイル７の配置を略示し、図５（ｂ）にＹ側非点補正コイル８の配置を略示する。便宜上、Ｘ側非点補正コイル７の非点補正方向をＸ_A方向、Ｙ_A方向と表記し、Ｙ側非点補正コイル７の非点補正方向をＸ_B方向、Ｙ_B方向と表記する。Ｘ_A方向とＹ_A方向は直交し、Ｘ_B方向とＹ_B方向も直交している。また、Ｘ_A方向とＸ_B方向とは４５゜回転した関係にある。Ｘ側非点補正コイル７に流す電流Ｉ_AとＹ側非点補正コイル８に流す電流Ｉ_Bを独立に調整することによって、電子ビーム６５が持っている非点収差を補正することができる。
【００２５】
いま、試料に入射する照射電子線を光軸に対して一定の角度αだけ傾斜させ、その傾斜した照射電子線の入射方向（方位角）を光軸の周りに変化させていく場合を考える。このとき、非点収差がない電子光学系の場合、焦点があっていれば透過電子線像は不動であり、焦点が合っていなければ透過電子線像は図６（ａ）に示すように円の軌跡を描いて移動する。一方、電子光学系に非点収差があれば、透過電子線像は図６（ｂ）に示すように一般に楕円軌跡を描いて移動する。従って、光軸に対する傾斜角αを一定にした照射電子線を異なる方位角方向から照射したときの透過電子線像の移動量を検出することにより、非点量を求めることができる。本発明は、このような原理に基づいて非点量を算出する。また、算出した非点量に基づいて非点補正コイル７，８に流す電流を制御することにより、非点の自動補正を可能にする。
【００２６】
ここで、図７に示すように、画像の移動量を定義する。
A_X：照射電子線を非点補正方向Ｘ_A方向に傾斜した時の画像の非点補正方向Ｘ_A方向の移動量
A_Y：照射電子線を非点補正方向Ｙ_A方向に傾斜した時の画像の非点補正方向Ｙ_A方向の移動量
B_X：照射電子線を非点補正方向Ｘ_B方向に傾斜した時の画像の非点補正方向Ｘ_B方向の移動量
B_Y：照射電子線を非点補正方向Ｙ_B方向に傾斜した時の画像の非点補正方向Ｙ_B方向の移動量
電子線を傾斜して像の移動軌跡を検出すると、非点がない場合には移動軌跡は真円を描くが、非点があると楕円を描く。本発明による非点補正は、面積を変えずに楕円の軌跡を真円にすることにより動作する。
【００２７】
（３）フォーカス補正
図８に示すレイダイアグラムを用いて、フォーカス補正について説明する。図８は、対物レンズ６の焦点が試料面５５に合っている場合と合っていない場合のレイダイアグラムを示したものである。
【００２８】
図８（ａ）は対物レンズ６の焦点が試料面５５に合っている場合を表し、この場合には、照射電子線５２を傾斜角ゼロで試料に照射したときと、傾斜角αで試料に照射したときとで、対物レンズ６によって像面５６に形成される試料の透過電子線像は移動しない。しかし、対物レンズ６の焦点が試料面５５に合っていない場合には、図８（ｂ）に示すように、試料の透過電子線像は像面５６から離れた仮想像面５３に結像することになるため、試料に照射する電子線５２を傾斜角ゼロの状態から傾斜角αに傾斜すると、像面５６で観察される試料の透過電子線像はΔXtだけ移動する。この像の移動量ΔXtを測定すると、焦点位置５４からのデフォーカス量Δｆを計算することができる。
【００２９】
具体的には、次のような操作によりフォーカス補正を行うことができる。透過像をＭ×Ｎの画素数で記憶装置３６にf1(m,n)として記録する。次に、図３に示すように、照射電子線を傾斜角αで試料に照射した透過像をＭ×Ｎの画素数で記憶装置にf2(m,n)として記録する。上記に述べた手法と同様に２つの画像間の試料面上での移動量Δｘを計算する。但し、この移動量Δｘには球面収差による像の移動量δが入っており、デフォーカスによる移動量は、Δｘからδを引いたものである。試料面上でのδは球面収差Ｃｓと偏向角αにより次の式(8)のように表される。
δ＝Ｃｓ・α³ …(8)
以上より、デフォーカスによる像の移動量ΔXtは次式(9)で表される。
ΔXt＝Δｘ−δ …(9)
移動量ΔXtからデフォーカスΔfは次式(10)で計算される。
Δf＝ΔXt／α …(10)
【００３０】
次に、デフォーカス量Δfを対物電流値補正値ΔIに変換する。ΔfとΔIの間には、Cを定数として次式(11)で表される関係がある。従って、式(11)の関係で求まる対物電流補正値ΔIを対物電流値に加算することで、試料に焦点を合わせることができる。
ΔI＝√（C／Δf） …(11)
以下に、本発明による試料観察方法の例について説明する。
【００３１】
図９は、本発明による非点補正の手順を示すフローチャートである。
最初、ステップ１１において、非点補正値を初期化する。すなわち、出力する電流値を０にする。次に、ステップ１２に進み、非点補正コイルによる非点補正方向に電子線を傾斜し、画像を記憶する。具体的には、非点補正方向であるＸ_A方向（図５参照）及びＸ_A方向と直交するＹ_A方向の２方向に照射電子線を傾斜し、それぞれの透過電子線像を記憶する。更に、非点補正方向であるＸ_B方向及びＸ_B方向と直交するＹ_B方向の２方向に照射電子線を傾斜し、それぞれの透過電子線像を記憶する。次に、ステップ１３に進み、記憶した画像から各非点補正方向について、照射電子線を傾斜したことによる透過電子線像の移動量を計測する。最後に、ステップ１４において、計測した移動量より移動軌跡が描く楕円を求め、更に、移動軌跡をそれと同じ面積の真円とするために必要な非点補正コイル電流の補正量を求め、ＤＡＣ２９，３０（図１参照）に入力して非点補正を行う。以下に、各ステップの詳細を説明する。なお、ステップ１２では、電子線を傾斜する代わりに、試料ステージ６０を傾斜してもよい。
【００３２】
ステップ１２における、照射電子線を傾斜し、透過電子線像を記憶する処理について説明する。非点補正方向であるＸ_A方向に関し、最初、角度αだけマイナス方向に照射電子線を傾斜し、透過電子線像A_X_Image(-)を記憶する。同様に、Ｙ_A方向に関し、角度αだけマイナス方向に照射電子線を傾斜し、透過電子線像A_Y_Image(-)を記憶する。次に、非点補正方向であるＸ_B方向に関し、角度αだけマイナス方向に照射電子線を傾斜し、透過電子線像B_X_Image(-)を記憶する。同様に、Ｙ_B方向に関し、角度αだけマイナス方向に照射電子線を傾斜し、透過電子線像B_Y_Image(-)を記憶する。
【００３３】
続いて、非点補正方向であるＸ_A方向に関し、角度αだけプラス方向に照射電子線を傾斜し、透過電子線像A_X_Image(+)を記憶する。同様に、Ｙ_A方向に関し、角度αだけプラス方向に照射電子線を傾斜し、透過電子線像A_Y_Image(+)を記憶する。次に、非点補正方向であるＸ_B方向に関し、角度αだけプラス方向に照射電子線を傾斜し、透過電子線像B_X_Image(+)を記憶する。同様に、Ｙ_B方向に関し、角度αだけプラス方向に照射電子線を傾斜し、透過電子線像B_Y_Image(+)を記憶する。なお、照射電子線を傾斜する方向、すなわち８枚の透過電子線像を撮像する順番は必ずしもここに記載した順番である必要はない。
【００３４】
ステップ１３においては、上記８枚の透過電子線像A_X_Image(-)，A_Y_Image(-)，B_X_Image(-)，B_Y_Image(-)とA_X_Image(+)，A_Y_Image(+)，B_X_Image(+)，B_Y_Image(+)を用いて、上述の位相限定相関法により、Ｘ_A方向の移動量（移動画素数）A_X とＹ_A方向の移動量（移動画素数）A_Y、及びＸ_B方向の移動量（移動画素数）B_XとＹ_B方向の移動量（移動画素数）B_Yを計算する。
【００３５】
次に、上述のようにして求められた移動量A_X，A_Y，B_X，B_Yにそれぞれ対応する試料面上での移動量A_x，A_y，B_x，B_yを下式のように計算する。下式において、T_Magは透過電子顕微鏡の倍率、CametaMagはカメラ取り込み倍率、Resolusionは分解能(128 または 256)、rRadianはμｍ／pixelを表す。
(A)A_x = A_X/(T_Mag*CametaMag*Resolusion*rRadian) (μｍ)
(B)A_y = A_Y/(T_Mag*CametaMag*Resolusion*rRadian) (μｍ)(C)B_x = B_X/(T_Mag*CametaMag*Resolusion*rRadian) (μｍ)(D)B_y = B_Y/(T_Mag*CametaMag*Resolusion*rRadian) (μｍ)
【００３６】
ステップ１４では、Ｘ側非点補正コイル７による補正量、Ｙ側非点補正コイル８による補正量をそれぞれ計算し、それを非点補正ＤＡＣを介して非点補正コイル７，８に加算する。以下では、Ｘ側非点補正コイル７の加算分（μｍ）をA_set、Ｙ側非点補正コイル８の加算分（μｍ）をB_setと記載する。
【００３７】
最初に、Ｘ側非点補正コイル７による補正について、場合分けをして説明する。
(1) A_x=A_y（A_x= A_y=0）の場合、Ｘ側非点補正コイル７による補正を行わない。
(2) A_x＞A_y、A_y≠0の場合、A_Set = A_x - (A_x*A_y)^1/2 とする。
(3) A_x＜A_y、A_x≠0の場合、A_Set = A_x + (A_x*A_y)^1/2 とする。
(4) A_x≠0、A_y=0の場合、A_Set = A_x/2とする。
(5) A_x=0、A_y≠0の場合、A_Set = A_y/2とする。
【００３８】
次に、Ｙ側非点補正コイル８による補正について、同様に場合分けをして説明する。
(1) B_x=B_y（B_x= B_y=0）の場合、Ｙ側非点補正コイル８による補正を行わない。
(2) B_x＞B_y、B_y≠0の場合、B_Set = B_x - (B_x*B_y)^1/2 とする。
(3) B_x＜B_y、B_x≠0の場合、B_Set = B_x + (B_x*B_y)^1/2 とする。
(4) B_x≠0、B_y=0の場合、B_Set = B_x/2とする。
(5) B_x=0、B_y≠0の場合、B_Set = B_y/2とする。
【００３９】
Ｘ側非点補正コイル７の加算分A_set（μｍ）と非点補正ＤＡＣ２９への入力値、及びＹ側非点補正コイル８の加算分B_set（μｍ）と非点補正ＤＡＣ３０への入力値との関係は予め分かっているので、その関係に基づいて非点補正ＤＡＣ２９，３０への入力値を求め、励磁電源１８，１９を制御する。この制御によって非点補正が行われる。
【００４０】
図１０は、非点補正とフォーカス補正を行って試料観察を行う手順の例を示すフローチャートである。
図１０のステップ２１からステップ２４までは図９のステップ１１からステップ１４と同じであるため、説明を省略する。ステップ２１〜２４にて非点補正を行ったのち、自動フォーカス合わせを行って試料の透過電子顕微鏡像を撮像する。
【００４１】
フォーカス合わせは、上述した手順によって行うことができる。すなわち、ステップ２５において、照射電子線を傾斜角ゼロで試料に照射した透過像をＭ×Ｎの画素数で記憶装置にf1(m,n)として記録すると共に、傾斜角αで試料に照射した透過像をＭ×Ｎの画素数で記憶装置にf2(m,n)として記録し、ステップ２６において、２つの画像f1(m,n)，f2(m,n)間の試料面上での移動量（ずれ量）Δｘを計算する。ステップ２８において、この移動量Δｘをもとに、上式(8)〜(11)に従って対物電流補正値ΔIを求める。そして、ステップ２９において、求めた対物電流補正値ΔIを対物電流値に加算することで試料に焦点を合わせることができる。移動量がゼロになるまで、ステップ２８からステップ２６の手順を繰り返してフォーカス合わせを行う。
【００４２】
傾斜角αでの電子線照射は、図３に示すように、２段の偏向コイル４，５により電子線を傾斜して偏向角αとなる偏向データをＲＯＭ４６よりＤＡＣ２６，２７に出力し、ＤＡＣ２６，２７から励磁電源１５，１６にアナログ信号を出力して偏向コイル４，５に電流を流すことにより行うことができる。また、対物電流補正値ΔIはＤＡＣ２８に出力してレンズデータをアナログ信号に変換する。ＤＡＣ２８は、励磁電源１７にアナログ信号を出力して対物レンズ（コイル）６に電流を出力させる。なお、ステップ２５では、電子線を傾斜させる代わりに、試料ステージ６０を傾斜してもよい。
【００４３】
【発明の効果】
本発明によると、電子顕微鏡の非点補正を自動的に行うことが可能となり、電子顕微鏡の操作性が大きく向上する。
【図面の簡単な説明】
【図１】本発明で用いる透過型電子顕微鏡の一例の概略機能ブロック図。
【図２】試料ステージの説明図。
【図３】偏向コイルの説明図。
【図４】画像相関の説明図。
【図５】非点補正コイルの配置を示す概念図。
【図６】照射電子線を変化させたときの透過電子線像の移動軌跡の説明図。
【図７】画像の移動量を定義する図。
【図８】透過電子顕微鏡のレイダイアグラム。
【図９】本発明による非点補正の手順を示すフローチャート。
【図１０】非点補正とフォーカス補正を行って試料観察を行う手順の例を示すフローチャート。
【符号の説明】
１：電子銃、２：第１照射レンズ、３：第２照射レンズ、４：第１偏向コイル、５：第２偏向コイル、６：対物レンズ、７：Ｘ側非点補正コイル、８：Ｙ側非点補正コイル、９：第１中間レンズ、１０：第２中間レンズ、１１：第１投射レンズ、１２：第２投射レンズ、１３〜２３：励磁電源、２４〜３４：ＤＡＣ、３５：マイクロプロセッサ、３６：記憶装置、３７：演算装置、３８：ＣＲＴコントローラ、３９：ＣＲＴ、４０，４１：Ｉ／Ｆ、４２：倍率切替用ロータリーエンコーダ、４３：入力用ロータリーエンコーダ、４４：キーボード、４５：ＲＡＭ、４６：ＲＯＭ、４７：ＴＶカメラ制御部、４８：シンチレータ、４９：ＴＶカメラ、５０：マウス、５１：画像取込みインターフェース、５３：仮想像面、５４：焦点位置、５５：試料面、５６：像面、５７：試料ステージコントローラ、５８：Ｘ方向駆動モータ、５９：Ｙ方向駆動モータ、６０：試料ステージ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a sample observation method using a transmission electron microscope, and more particularly to a sample observation method that can easily perform astigmatism correction.
[0002]
[Prior art]
The transmission electron microscope forms an image of a transmission electron beam that has passed through the inside of the sample, and the internal structure of the sample can be observed. According to the transmission electron microscope, it is possible to observe the lattice defects of the material, the structure of grain boundaries, the size and distribution of precipitates, or the lattice image. Astigmatism occurs in the magnetic lens of a transmission electron microscope due to the accuracy of the magnetic pole, the nonuniformity of the magnetic material, the accuracy of the axis of the magnetic pole, and the like. Astigmatism is usually adjusted by exciting two sets of quadrupoles arranged 45 ° apart from each other independently. However, this adjustment requires skill.
[0003]
Patent Document 1 detects the direction and size of an astigmatism by irradiating a sample by sequentially changing its azimuth while tilting an electron beam symmetrically about the optical axis. An image related to the size and direction of the astigmatism displayed is displayed on the screen, and the astigmatism correction command signal is input by moving the cursor displayed simultaneously on the display screen. Is doubled, and a two-dimensional vector having a size corresponding to the double azimuth angle and the amount of movement of the cursor is decomposed in the X and Y directions. The formation of a point correction signal is described.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 7-262952
[Problems to be solved by the invention]
The astigmatism correction method described in Patent Document 1 requires an operator to input an astigmatism correction signal for an image representing the size and direction of the astigmatism displayed on the screen. That is, there is a problem that it is not possible to automate astigmatism correction by requiring operator intervention.
[0006]
The present invention provides a sample observation method capable of automatically detecting astigmatism during a magnified image observation of a sample using a transmission electron microscope and automatically correcting astigmatism based on the detected astigmatism. The purpose is to provide.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a sample observation method using a transmission electron microscope provided with an astigmatism correction mechanism of the present invention irradiates a sample from a plurality of different azimuth directions with an electron beam inclined at a predetermined angle with respect to the optical axis, A step of capturing a plurality of sample transmission electron images, a step of obtaining a movement amount between the sample transmission electron images, a step of obtaining an ellipse formed by a movement locus of the plurality of sample transmission electron images, and a movement of the plurality of sample transmission electron images The method includes a step of obtaining a control amount of the astigmatism correction mechanism necessary for making the locus a perfect circle having the same area as the ellipse, and a step of controlling the astigmatism correction mechanism by the obtained control amount.
[0008]
The sample observation method using a transmission electron microscope having the astigmatism correction mechanism according to the present invention is also configured such that an electron beam inclined at a predetermined angle with respect to the optical axis is taken from two positive and negative directions with respect to a plurality of astigmatism correction directions of the astigmatism correction mechanism. And a step of obtaining a plurality of sample transmission electron images, a step of obtaining a movement amount between two sample transmission electron images by an electron beam inclined in two positive and negative directions with respect to each astigmatism correction direction, and 2 orthogonal to each other When the two movement amounts obtained for the two astigmatism correction directions are the major axis and the minor axis of the ellipse, the movement amount of the two sample transmission electron images is set to the radius of a perfect circle having the same area as the ellipse. The method includes a step of obtaining a necessary control amount of the astigmatism correction mechanism and a step of controlling the astigmatism correction mechanism by the obtained control amount.
The amount of movement between two sample transmission electron images can be calculated from the correlation strength between the two sample transmission electron images.
[0009]
The astigmatism correction mechanism may have a first quadrupole and a second quadrupole that are offset by 45 ° from each other, and the step of controlling the astigmatism correction mechanism includes the first quadrupole and the second quadrupole. This can be done individually for two quadrupoles.
[0010]
In the sample observation method, the sample transmission electron image when the sample is irradiated with an electron beam having an inclination angle of zero with respect to the optical axis, and the sample transmission when the sample is irradiated with an electron beam inclined at a predetermined angle with respect to the optical axis A step of obtaining a movement amount between the electron image, a step of obtaining an objective current correction value necessary for focus correction from the movement amount between the two sample transmission electron images, and adding the obtained objective current correction value to the objective current value. And a step of performing focus correction.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic functional block diagram of an example of a transmission electron microscope used in the present invention. Although the number of deflection coils is not limited, here, a transmission electron microscope using a general two-stage deflection coil will be described.
[0012]
The electron beam emitted from the electron gun 1 and accelerated is focused by the first irradiation lens 2 and the second irradiation lens 3, deflected by the first deflection coil 4 and the second deflection coil 5, and imaged by the objective lens 6. Then, the sample held on the sample stage 60 is irradiated. The electron beam that has passed through the sample passes through the X-side astigmatism correction coil 7 and the Y-side astigmatism correction coil 8 that constitute the astigmatism correction mechanism, and is enlarged by the first intermediate lens 9 and the second intermediate lens 10. Further enlarged by the first projection lens 11 and the second projection lens 12, a transmission image of the sample is formed on the scintillator 48. A transmission electron image of the sample converted into an optical image by the scintillator 48 is captured by a TV camera (imaging device) 49. The video signal from the TV camera 49 is captured by the microprocessor 35 via the TV camera control unit 47 and the image capturing interface 51, processed, and then displayed on the CRT 39 controlled by the CRT controller 38. The microprocessor 35 controls the excitation power supplies 13 to 23 that supply power to the lenses 2 to 12 of the electron microscope via the DACs 24 to 34. The microprocessor 35 is connected to a storage device 36 such as a hard disk, a calculation device 37, a magnification switching rotary encoder 42, an input rotary encoder 43, a keyboard 44, a RAM 45, a ROM 46, a mouse 50, and the like via a bus. Yes. The

rotary encoders

42 and 43 are connected to the bus via the I /

Fs

40 and 41.
[0013]
FIG. 2 is a conceptual diagram of a sample stage for holding a sample. The sample stage 60 includes a mesh-like sample surface 55, and the sample is held on the sample surface 55. The sample stage 60 is driven in the X direction and the Y direction by an X direction drive motor 58 and a Y direction drive motor 59 that are driven and controlled by a sample stage controller 57 in accordance with a command from the microprocessor 35.
[0014]
FIG. 3 is a schematic diagram of the deflection coil. Electron beam irradiation at a tilt angle α to the sample is performed by tilting the electron beam by the two-stage deflection coils 4 and 5 and outputting deflection data to the deflection angle α from the ROM 46 to the

DACs

26 and 27. This can be done by outputting an analog signal to 15 and 16 and passing a current through the deflection coils 4 and 5.
The elemental technology used in the sample observation method using the transmission electron microscope of the present invention will be described below.
[0015]
(1) Detection of movement amount between two images A method of detecting the movement amount (deviation amount) between two

images

71 and 72 will be described using the example of image correlation shown in FIG. The transmission electron microscope image 71 is recorded as a registered image f1 (m, n) in the storage device 36 with the number of pixels of M × N. Next, the transmission electron microscope image 72 translated with respect to the transmission electron microscope image 71 is recorded as a reference image f2 (m, n) in the storage device with the number of pixels of M × N. However, both are natural images, and m = 0, 1, 2,..., M−1; n = 0, 1, 2,.
[0016]
The discrete Fourier images F1 (u, v) and F2 (u, v) of the registered image f1 (m, n) and reference image f2 (m, n) are defined by the following equations (1) and (2), respectively. The Where u = 0, 1, 2,..., M−1; v = 0, 1, 2,..., N−1, A (u, v) and B (u, v) are amplitude spectra, θ (u, v) and φ (u, v) are phase spectra.
[0017]
[Expression 1]

[0018]
In phase correlation, when there is a parallel movement of an image between two images, the position of the correlation peak is shifted by the amount of movement. A method for deriving the movement amount will be described below. First, assuming that the original image f2 (m, n) has moved by r ′ in the m direction, f4 (m, n) = f2 (m + r ′, n). Equation (2) is transformed into Equation (3).
[0019]
[Expression 2]

By setting the amplitude spectrum B (u, v) as a constant, a phase image independent of the contrast of the image is obtained. The phase image F ′ (u, v) of f4 is expressed by the following equation (4).
[0020]
[Equation 3]

By multiplying the phase image F′1 (u, v) by the complex role of F′2 (u, v), a composite image H14 (u, v) represented by the following equation (5) is obtained.
[0021]
[Expression 4]

The correlation intensity image G14 (r, s) is expressed by the following equation (6) by performing inverse Fourier transform on the composite image H14 (u, v).
[0022]
[Equation 5]

[0023]
From equation (6), when there is a positional shift amount r ′ in the m direction between two images, the peak position of the correlation intensity image is shifted by −r ′. In addition, since the correlation calculation is performed using the phase component, the movement amount can be calculated even if there is a difference in brightness or contrast between the two images. When there is a positional shift amount in the m direction between two images, a peak occurs at a position of ΔG (pixel) from the center of the correlation strength image. For example, as shown in FIG. 4, if there is a deviation of 2 pixels in the m direction between the two

images

71 and 72, the composite phase image 73 becomes a wave of two cycles. When this is subjected to inverse Fourier transform, a correlation intensity image 74 is obtained, and a peak 75 is generated at a position shifted by 2 pixels from the center. This ΔG (pixel) corresponds to the amount of movement on the light receiving surface of the detector, and ΔG is converted into the amount of movement Δx on the sample surface. When the diameter of the light receiving surface of the detector is L, the magnification of the transmission electron microscope on the light receiving surface is M, and the number of pixels on the light receiving surface of the detector is Lm, the movement amount Δx on the sample surface is expressed by the following equation (7). Calculated.
Δx = ΔG (pixel) × L / Lm (pixel) / M (7)
Δx is the amount of movement on the sample surface between the two

images

71 and 72. The amount of movement in the Y direction can be calculated in the same way.
[0024]
(2) Astigmatism Detection and Astigmatism Correction Next, an astigmatism detection method and an astigmatism correction method according to the present invention will be described.
FIG. 5 is a conceptual diagram showing the arrangement of astigmatism correction coils provided in the astigmatism correction mechanism. The astigmatism correction coil is configured by combining an X side astigmatism correction coil and a Y side astigmatism correction coil that intersect each other at an angle of 45 °. FIG. 5A schematically shows the arrangement of the X-side astigmatism correction coil 7, and FIG. 5B schematically shows the arrangement of the Y-side astigmatism correction coil 8. For convenience, the astigmatism correction directions of the X-side astigmatism correction coil 7 are denoted as X _A direction and Y _A direction, and the astigmatism correction directions of the Y-side astigmatism correction coil 7 are denoted as X _B direction and Y _B direction. X _A direction and the Y _A direction are orthogonal and are also orthogonal X _B direction and the Y _B direction. Further, the X _A direction and the X _B direction are in a relationship rotated by 45 °. Astigmatism of the electron beam 65 can be corrected by independently adjusting the current I _A flowing through the X-side astigmatism correction coil 7 and the current I _B flowing through the Y-side astigmatism correction coil 8.
[0025]
Now, consider a case where the irradiation electron beam incident on the sample is inclined by a certain angle α with respect to the optical axis, and the incident direction (azimuth angle) of the inclined irradiation electron beam is changed around the optical axis. At this time, in the case of an electron optical system having no astigmatism, the transmitted electron beam image does not move if it is in focus, and if it is not in focus, the transmitted electron beam image is a circle as shown in FIG. Draw a trajectory and move. On the other hand, if the electron optical system has astigmatism, the transmission electron beam image generally moves while drawing an elliptical locus as shown in FIG. Therefore, the astigmatism amount can be obtained by detecting the amount of movement of the transmission electron beam image when the irradiation electron beam having a constant inclination angle α with respect to the optical axis is irradiated from different azimuth directions. The present invention calculates the astigmatism based on such a principle. Further, by controlling the current flowing through the astigmatism correction coils 7 and 8 based on the calculated astigmatism amount, the astigmatism can be automatically corrected.
[0026]
Here, as shown in FIG. 7, the moving amount of the image is defined.
A_X: movement amount of astigmatic correction direction X _A direction of the image when the tilted electron beam irradiation astigmatic correction direction X _A direction
A_Y: movement amount of astigmatic correction direction Y _A direction of the image when the tilted electron beam irradiation astigmatic correction direction Y _A direction
B_x: movement amount of astigmatic correction direction X _B direction of the image when the tilted electron beam irradiation astigmatism correction direction X _B direction
B_Y: When tilted astigmatism correction direction Y _B direction movement amount electron beam image when tilted electron beam irradiation astigmatic correction direction Y _B direction for detecting the movement trajectory of the image, if there is no astigmatic The movement trajectory draws a perfect circle, but if there is an astigmatism, it draws an ellipse. The astigmatism correction according to the present invention operates by making an elliptical locus into a perfect circle without changing the area.
[0027]
(3) Focus Correction Focus correction will be described using the ray diagram shown in FIG. FIG. 8 shows a ray diagram when the objective lens 6 is focused on the sample surface 55 and not.
[0028]
FIG. 8A shows a case where the objective lens 6 is focused on the sample surface 55. In this case, the sample is irradiated with the irradiation electron beam 52 at an inclination angle of zero, and at the inclination angle α. The transmitted electron beam image of the sample formed on the image plane 56 by the objective lens 6 does not move when irradiated. However, when the objective lens 6 is not focused on the sample surface 55, the transmission electron beam image of the sample is formed on a virtual image surface 53 away from the image surface 56, as shown in FIG. Therefore, when the electron beam 52 irradiating the sample is tilted from the tilt angle zero state to the tilt angle α, the transmitted electron beam image of the sample observed on the image plane 56 moves by ΔXt. By measuring the movement amount ΔXt of the image, the defocus amount Δf from the focal position 54 can be calculated.
[0029]
Specifically, focus correction can be performed by the following operation. The transmitted image is recorded as f1 (m, n) in the storage device 36 with the number of pixels of M × N. Next, as shown in FIG. 3, a transmission image obtained by irradiating the sample with an irradiation electron beam at an inclination angle α is recorded as f2 (m, n) in the storage device with the number of pixels of M × N. Similar to the method described above, the movement amount Δx between the two images on the sample surface is calculated. However, this movement amount Δx includes an image movement amount δ due to spherical aberration, and the movement amount due to defocus is obtained by subtracting δ from Δx. Δ on the sample surface is expressed by the following equation (8) by the spherical aberration Cs and the deflection angle α.
δ = Cs · α ³ (8)
From the above, the image movement amount ΔXt due to defocusing is expressed by the following equation (9).
ΔXt = Δx−δ (9)
The defocus Δf is calculated from the movement amount ΔXt by the following equation (10).
Δf = ΔXt / α (10)
[0030]
Next, the defocus amount Δf is converted into an objective current value correction value ΔI. Between Δf and ΔI, there is a relationship represented by the following equation (11), where C is a constant. Therefore, it is possible to focus on the sample by adding the objective current correction value ΔI obtained by the relationship of Expression (11) to the objective current value.
ΔI = √ (C / Δf) (11)
Below, the example of the sample observation method by this invention is demonstrated.
[0031]
FIG. 9 is a flowchart showing the procedure of astigmatism correction according to the present invention.
First, in step 11, the astigmatism correction value is initialized. That is, the output current value is set to zero. Next, proceeding to step 12, the electron beam is inclined in the astigmatism correction direction by the astigmatism correction coil, and the image is stored. Specifically, the irradiation electron beam is tilted in two directions, ie, the X _A direction (see FIG. 5), which is the astigmatism correction direction, and the Y _A direction orthogonal to the X _A direction, and the respective transmission electron beam images are stored. Furthermore, by tilting the electron beam irradiation in two directions of the Y _B direction orthogonal to the X _B direction and X _B direction, which is the astigmatism correction direction and stores each of the transmission electron beam image. Next, the process proceeds to step 13, and the movement amount of the transmission electron beam image due to the tilting of the irradiation electron beam is measured for each astigmatism correction direction from the stored image. Finally, in step 14, the ellipse drawn by the movement trajectory is obtained from the measured movement amount, and further, the correction amount of the astigmatism correction coil current required to make the movement locus a perfect circle of the same area is obtained, and the DAC 29, Astigmatism correction is performed at 30 (see FIG. 1). Details of each step will be described below. In step 12, the sample stage 60 may be tilted instead of tilting the electron beam.
[0032]
The process of tilting the irradiation electron beam and storing the transmission electron beam image in step 12 will be described. It relates X _A direction, which is the astigmatism correction direction, initially, an angle α only to tilt the electron beam irradiation in the negative direction, transmission electron beam image A_X_Image - storing (). Similarly, it relates Y _A direction, an angle α only to tilt the electron beam irradiation in the negative direction, transmission electron beam image A_Y_Image - storing (). Then relates X _B direction astigmatism correction direction, an angle α only to tilt the electron beam irradiation in the negative direction, transmission electron beam image B_X_Image - storing (). Similarly, with respect to the Y _B direction, the irradiation electron beam is tilted in the minus direction by an angle α, and a transmission electron beam image B_Y_Image (−) is stored.
[0033]
Subsequently relates X _A direction, which is the astigmatism correction direction, an angle α only to tilt the electron beam irradiation in a positive direction, and stores the transmission electron beam image A_X_Image the (+). Similarly, it relates Y _A direction, an angle α only to tilt the electron beam irradiation in a positive direction, and stores the transmission electron beam image A_Y_Image the (+). Then relates X _B direction astigmatism correction direction, an angle α only to tilt the electron beam irradiation in a positive direction, and stores the transmission electron beam image B_X_Image the (+). Similarly, with respect to the Y _B direction, the irradiation electron beam is tilted in the plus direction by an angle α, and a transmission electron beam image B_Y_Image (+) is stored. Note that the direction in which the irradiation electron beam is tilted, that is, the order in which the eight transmitted electron beam images are captured does not necessarily have to be the order described here.
[0034]
In step 13, the eight transmission electron beam images A_X_Image (-), A_Y_Image (-), B_X_Image (-), B_Y_Image (-) and A_X_Image (+), A_Y_Image (+), B_X_Image (+), B_Y_Image ( +) was used to by the above-described phase only correlation method, X amount of movement direction _a (the number of pixels moved) A_X and amount of movement Y _a direction (the number of pixels moved) A_Y, and X amount of movement _B direction (moving pixels Number) B_X and Y The amount of movement (number of moving pixels) B_Y in the _B direction is calculated.
[0035]
Next, the movement amounts A_x, A_y, B_x, and B_y on the sample surface corresponding to the movement amounts A_X, A_Y, B_X, and B_Y obtained as described above are calculated as follows. In the following formula, T_Mag represents the magnification of the transmission electron microscope, CametaMag represents the camera capture magnification, Resolusion represents the resolution (128 or 256), and rRadian represents μm / pixel.
(A) A_x = A_X / (T_Mag * CametaMag * Resolusion * rRadian) (μm)
(B) A_y = A_Y / (T_Mag * CametaMag * Resolusion * rRadian) (μm) (C) B_x = B_X / (T_Mag * CametaMag * Resolusion * rRadian) (μm) (D) B_y = B_Y / (T_Mag * CametaMag * Resolusion * rRadian) (μm)
[0036]
In step 14, the correction amount by the X-side astigmatism correction coil 7 and the correction amount by the Y-side astigmatism correction coil 8 are respectively calculated and added to the astigmatism correction coils 7 and 8 via the astigmatism correction DAC. Hereinafter, the addition amount (μm) of the X-side astigmatism correction coil 7 is referred to as A_set, and the addition amount (μm) of the Y-side astigmatism correction coil 8 is referred to as B_set.
[0037]
First, the correction by the X-side astigmatism correction coil 7 will be described for each case.
(1) When A_x = A_y (A_x = A_y = 0), correction by the X-side astigmatism correction coil 7 is not performed.
(2) When A_x> A_y and A_y ≠ 0, A_Set = A_x− (A_x * A_y) ^1/2 .
(3) When A_x <A_y and A_x ≠ 0, A_Set = A_x + (A_x * A_y) ^1/2 .
(4) When A_x ≠ 0 and A_y = 0, A_Set = A_x / 2.
(5) When A_x = 0 and A_y ≠ 0, A_Set = A_y / 2.
[0038]
Next, the correction by the Y-side astigmatism correction coil 8 will be described in different cases.
(1) When B_x = B_y (B_x = B_y = 0), correction by the Y-side astigmatism correction coil 8 is not performed.
(2) When B_x> B_y and B_y ≠ 0, B_Set = B_x− (B_x * B_y) ^1/2 .
(3) When B_x <B_y and B_x ≠ 0, B_Set = B_x + (B_x * B_y) ^1/2 .
(4) When B_x ≠ 0 and B_y = 0, B_Set = B_x / 2.
(5) When B_x = 0 and B_y ≠ 0, B_Set = B_y / 2.
[0039]
The addition amount A_set (μm) of the X side astigmatism correction coil 7 and the input value to the astigmatism correction DAC 29, and the addition amount B_set (μm) of the Y side astigmatism correction coil 8 and the input value to the astigmatism correction DAC 30 Since the relationship is known in advance, the input values to the

astigmatism correction DACs

29 and 30 are obtained based on the relationship, and the

excitation power supplies

18 and 19 are controlled. Astigmatism correction is performed by this control.
[0040]
FIG. 10 is a flowchart illustrating an example of a procedure for performing sample observation by performing astigmatism correction and focus correction.
Steps 21 to 24 in FIG. 10 are the same as steps 11 to 14 in FIG. After performing astigmatism correction in steps 21 to 24, automatic focusing is performed to take a transmission electron microscope image of the sample.
[0041]
Focusing can be performed according to the procedure described above. That is, in step 25, a transmission image obtained by irradiating the sample with an irradiation electron beam at an inclination angle of zero is recorded as f1 (m, n) in the storage device with the number of pixels of M × N, and the sample is irradiated at an inclination angle α. The transmitted image is recorded as f2 (m, n) in the memory device with the number of pixels of M × N, and in step 26, the two images f1 (m, n) and f2 (m, n) on the sample surface are recorded. The movement amount (deviation amount) Δx is calculated. In step 28, the objective current correction value ΔI is obtained according to the above equations (8) to (11) based on the movement amount Δx. Then, in step 29, the object can be focused by adding the obtained objective current correction value ΔI to the objective current value. Until the amount of movement becomes zero, the procedure from step 28 to step 26 is repeated to perform focusing.
[0042]
In the electron beam irradiation at the inclination angle α, as shown in FIG. 3, the deflection data to be the deflection angle α by tilting the electron beam by the two-stage deflection coils 4 and 5 is output from the ROM 46 to the

DACs

26 and 27. , 27 to output an analog signal to the

excitation power sources

15 and 16 and cause a current to flow through the deflection coils 4 and 5. The objective current correction value ΔI is output to the DAC 28 to convert lens data into an analog signal. The DAC 28 outputs an analog signal to the excitation power source 17 and causes the objective lens (coil) 6 to output a current. In step 25, the sample stage 60 may be tilted instead of tilting the electron beam.
[0043]
【The invention's effect】
According to the present invention, astigmatism correction of the electron microscope can be automatically performed, and the operability of the electron microscope is greatly improved.
[Brief description of the drawings]
FIG. 1 is a schematic functional block diagram of an example of a transmission electron microscope used in the present invention.
FIG. 2 is an explanatory diagram of a sample stage.
FIG. 3 is an explanatory diagram of a deflection coil.
FIG. 4 is an explanatory diagram of image correlation.
FIG. 5 is a conceptual diagram showing the arrangement of astigmatism correction coils.
FIG. 6 is an explanatory diagram of a movement locus of a transmission electron beam image when an irradiation electron beam is changed.
FIG. 7 is a diagram for defining a moving amount of an image.
FIG. 8 is a ray diagram of a transmission electron microscope.
FIG. 9 is a flowchart showing the procedure of astigmatism correction according to the present invention.
FIG. 10 is a flowchart showing an example of a procedure for performing sample observation by performing astigmatism correction and focus correction.
[Explanation of symbols]
1: electron gun, 2: first irradiation lens, 3: second irradiation lens, 4: first deflection coil, 5: second deflection coil, 6: objective lens, 7: X side astigmatism correction coil, 8: Y Side astigmatism correction coil, 9: first intermediate lens, 10: second intermediate lens, 11: first projection lens, 12: second projection lens, 13-23: excitation power source, 24-34: DAC, 35: micro Processor: 36: Storage device, 37: Arithmetic unit, 38: CRT controller, 39: CRT, 40, 41: I / F, 42: Rotary encoder for switching magnification, 43: Rotary encoder for input, 44: Keyboard, 45: RAM, 46: ROM, 47: TV camera control unit, 48: scintillator, 49: TV camera, 50: mouse, 51: image capture interface, 53: virtual image plane, 54: focus position, 55 Sample surface, 56: the image plane, 57: sample stage controller, 58: X-direction driving motor, 59: Y-direction driving motor, 60: sample stage

Claims

In a sample observation method using a transmission electron microscope having an astigmatism correction mechanism,
Wherein the four directions is astigmatic correction direction that due to astigmatism correction mechanism, a step of a predetermined angle tilt an electron beam from the optical axis is irradiated on the sample, imaging a specimen transmission electron image for the four directions,
Obtaining a movement amount between the sample transmission electron images in the four directions, and obtaining information on an ellipse formed by a movement locus of the plurality of sample transmission electron images;
Obtaining a control amount of the astigmatism correction mechanism necessary to make the movement trajectory of the plurality of sample transmission electron images a perfect circle having the same area as the ellipse;
And a step of controlling the astigmatism correction mechanism according to the obtained control amount.

In a sample observation method using a transmission electron microscope having an astigmatism correction mechanism,
Irradiating the specimen with electron beams inclined at a predetermined angle with respect to the optical axis from two positive and negative directions with respect to the four directions which are the astigmatism correction directions of the astigmatism correction mechanism , and capturing a sample transmission electron image in the four directions ; ,
Obtaining a movement amount between two sample transmission electron images by an electron beam inclined in two positive and negative directions with respect to the four astigmatism correction directions;
When the two movement amounts obtained with respect to the two astigmatism correction directions orthogonal to each other among the four astigmatism correction directions are the major axis and the minor axis of the ellipse, the movement amounts of the two sample transmission electron images are expressed as follows. Obtaining a control amount of the astigmatism correction mechanism necessary for setting the radius of a perfect circle having the same area as the ellipse;
And a step of controlling the astigmatism correction mechanism according to the obtained control amount.

3. The sample observation method according to claim 2 , wherein the amount of movement between the two sample transmission electron images is calculated from the correlation strength between the two sample transmission electron images.

4. The sample observation method according to claim 1, wherein the astigmatism correction mechanism includes a first quadrupole element and a second quadrupole element arranged so as to be shifted from each other by 45 °, and the astigmatism correction function. A sample observation method, wherein the step of controlling the mechanism is performed individually for the first quadrupole and the second quadrupole.

In the sample observation method according to any one of claims 1 to 4,
Between the sample transmission electron image when the sample is irradiated with an electron beam with a tilt angle of zero with respect to the optical axis, and the sample transmission electron image when the sample is irradiated with an electron beam inclined at a predetermined angle with respect to the optical axis Determining the amount of movement;
Obtaining an objective current correction value necessary for focus correction from the amount of movement between the two specimen transmission electron images;
And a step of performing focus correction by adding the obtained objective current correction value to the objective current value.

An electron gun,
A sample stage for holding the sample;
A lens system including an objective lens that irradiates a sample held on the sample stage with an accelerated electron beam emitted from the electron gun;
A deflection system for deflecting an electron beam applied to the sample;
An astigmatism correction mechanism;
Wherein the four directions is astigmatic correction direction that due to astigmatism correction mechanism, and inclined at a predetermined angle with the electron beam from the optical axis is irradiated on the sample, and means for imaging the specimen transmission electron image for the four directions,
Means for obtaining a movement amount between the sample transmission electron images in the four directions, and obtaining information on an ellipse formed by a movement locus of the plurality of sample transmission electron images;
Means for obtaining a control amount of the astigmatism correction mechanism necessary to make the movement trajectory of the plurality of sample transmission electron images a perfect circle having the same area as the ellipse;
And a means for controlling the astigmatism correction mechanism according to the obtained control amount.

An electron gun,
A sample stage for holding the sample;
A lens system including an objective lens that irradiates a sample held on the sample stage with an accelerated electron beam emitted from the electron gun;
A deflection system for deflecting an electron beam applied to the sample;
An astigmatism correction mechanism;
Means for irradiating the sample from two positive and negative directions with respect to four directions , which are the astigmatism correction directions of the astigmatism correction mechanism , with an electron beam inclined at a predetermined angle with respect to the optical axis, and imaging a sample transmission electron image in the four directions ; ,
Means for obtaining a movement amount between two sample transmission electron images by an electron beam inclined in two positive and negative directions with respect to the four astigmatism correction directions;
When the two movement amounts obtained with respect to the two astigmatism correction directions orthogonal to each other among the four astigmatism correction directions are the major axis and the minor axis of the ellipse, the movement amounts of the two sample transmission electron images are expressed as follows. Means for obtaining a control amount of the astigmatism correction mechanism necessary for setting the radius of a perfect circle having the same area as the ellipse;
And a means for controlling the astigmatism correction mechanism according to the obtained control amount.