JP2004286639A

JP2004286639A - Method and instrument for measuring thickness

Info

Publication number: JP2004286639A
Application number: JP2003080176A
Authority: JP
Inventors: Mitsuo Koike; 三夫小池
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 2003-03-24
Filing date: 2003-03-24
Publication date: 2004-10-14
Anticipated expiration: 2023-03-24
Also published as: JP3950074B2

Abstract

<P>PROBLEM TO BE SOLVED: To simply and precisely measure a thickness of a very thin sample. <P>SOLUTION: This method of the present invention comprises a process for calculating sample thickness dependency of at least one diffraction spot intensity ratio drawn out based on scattering cross-sectional area and crystal structure of a measured substance, and based on an electron beam emitting direction, a process for convergence-emitting an electron beam to a desired area of the measured substance, and for measuring two or more of diffraction spot intensities corresponding to the diffraction spot intensity ratio out of a plurality of diffraction spots corresponding to a crystal face space and the direction of the transmission-diffracted electron beam, so as to find the diffraction spot intensity ratio, and a process for determining the thickness of the sample to conform a diffraction spot intensity ratio drawn out by calculation with the diffraction spot intensity ratio obtained by an experiment. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は極薄試料の厚さを測定する方法およびそれに用いる測定装置に関する。
【０００２】
【従来の技術】
高加速電子線が透過し得るような極薄試料の厚さを測定する方法としては、透過電子顕微鏡による収束電子線回折法および電子エネルギー損失分光法を利用した方法が用いられている。
【０００３】
収束電子線回折法を用いる具体的な手段は次の通りである。まず、高加速電子線が透過し得る厚さの被測定物の所望な位置に、角θ１（θ１＞＞０）で収束された電子線を照射する。照射された高加速電子の一部またはその多くは被測定物内で弾性および非弾性散乱されて試料裏面へ透過し、蛍光板上またはＣＣＤカメラを介してモニター上に収束電子線回折像を結像する。厚さ測定では２波回折条件で励起された回折斑点内に観察される縞状コントラストの強度変動から計算して求める。しかし、このときの照射電子線は収束角θ１にて収束されているため回折像上に現れる回折斑点は角θ１とカメラ長とに依存した有限の面積を有する。また回折斑点内のコントラストを正確に測定する必要があるためには相応の面積が必要である。これら条件を満たすためには、ダイレクトスポットと２波条件で励起された回折スポットが十分に離れていること、つまり指数の大きい回折斑点を選択する必要があり、また測定のためには電子線入射方位を２波回折条件に設定するなどの実験上の制約が多く、簡便に用いることは困難である。
【０００４】
また、電子エネルギー損失分光法を用いた具体的な厚さ測定方法は、次の通りである。まず、高加速電子線が透過し得る厚さの被測定物の所望な位置に、電子線を照射する。照射された高加速電子の一部は被測定物内で非弾性散乱され、そのエネルギーを損失する。試料を透過した電子線を、電子線分光器を用いてエネルギー分解することにより透過電子線のエネルギー分解スペクトルを取得する。取得したスペクトルの中でエネルギーを損失していない電子によって形成されるピーク（ゼロロスピーク）は損失エネルギー量０ｅＶを中心としてガウス分布していると仮定することができるため、そのピーク高さおよび半値幅とからゼロロスピークを規定することができる。これらスペクトルの全強度とゼロロスピークの強度との差分を求めることにより非弾性散乱電子の割合が得られる。一方、被測定物内において高加速電子が非弾性散乱される確立は物質によって異なり、各々の物質には高加速電子が非弾性散乱されてから次の非弾性散乱されるまでの平均自由行程という距離が存在する。平均自由行程は結晶性の物質であれば電子線の照射方位によって変化する物理量である。ここで先に述べた被測定物に電子を照射して非弾性散乱される電子の割合から、平均自由行程で規格化された厚さを求めることができる。
但し、電子エネルギー損失分光法を用いた厚さ測定では被測定物の平均自由行程を事前に求めておく必要があるが、その物理量を実験的に求めることは容易ではない。
【０００５】
さらにそれぞれの厚さ測定法には被測定物質に依存する測定可能な厚さ範囲が存在する。例えば、収束電子線回折法を用いた厚さ測定では収束電子線回折像を明瞭なコントラストで取得するためには相応の厚さが必要であるが、その一方では２波回折条件が成立するためには試料厚さを抑制する必要があるなど、非常に制約された条件下での膜厚しか測定することが出来ない。さらに、電子エネルギー損失分光法では、エネルギーを損失していない電子数に対してエネルギー損失電子数が少な過ぎたり、多重散乱電子が増加したりすると測定誤差が大きくなるという問題が生じる。
【０００６】
また、Ｘ線回折現象を利用した多結晶膜の膜厚測定方法として、試料に入射角を固定してＸ線を照射し、Ｘ線検出器を所定の角度範囲に走査して基板からの任意の回折ピークの回折Ｘ線の回折角と強度を測定し、この手順を入射角を変更して複数回繰り返し、得られた回折角度、回折強度、入射角及び回折角のデータから、厚さを算出することが知られている（特許文献１参照）。
しかしながら、この方法によれば、１試料について電子線照射角を変更しながら、極めて多数のデータを取得する必要があり、作業効率が悪かった。
【０００７】
【特許文献１】特開平４−１９４６１１号公報
【０００８】
【発明が解決しようとする課題】
以上に説明したように、従来極薄い試料厚さを測定する方法としては収束電子線回折法を用いた方法、および電子エネルギー損失分光法を用いた方法が行われているが、その適用厚さ範囲には制約があり、例えば厚さ数ｎｍの試料への適用は困難であった。
本発明による方法は、このような従来の測定方法における問題点を解消するためになされたもので、試料の像を観察中に測定することができ、簡便に且つ高精度な厚さ測定が可能とすることを目的とする。
【０００９】
【課題を解決するための手段】
第１の本発明は、被測定物の散乱断面積および結晶構造および電子線照射方位とから導出される少なくともひとつの回折スポット強度比の試料厚さ依存性を計算する工程と、
被測定物質の所望の領域に電子線を収束照射し、透過回折された電子線の結晶面間隔および方位に対応する複数の回折スポットの中から前記回折強度比に対応する少なくとも２つ以上の所望の回折スポットの強度を測定し、回折スポットの強度比を求める工程と、
計算によって導出された回折スポットの強度比が実験によって得られた回折スポットの強度比と一致するような試料厚さを決定する工程とからなることを特徴とする厚さ測定方法である。
【００１０】
第２の本発明は、被測定物の散乱断面積および結晶構造および電子線照射方位とから導出される少なくともひとつの回折スポット強度比の試料厚さ依存性を計算する工程と、
被測定物質の所望の領域に電子線を収束照射し、透過回折された電子線の結晶面間隔および方位に対応する複数の回折スポットの中から上記回折強度比に対応する少なくとも２つ以上の所望の回折スポットの強度を測定し、回折スポットの強度の大小関係を求める工程と、
計算によって導出された回折スポットの強度比と実験によって得られた回折スポットの強度の相対的関係とが一致する領域を試料厚さ範囲として決定する工程とからなることを特徴とする厚さ測定方法である。
【００１１】
前記第１及び第２の本発明において、前記回折スポット強度比の実測値を得るために照射した高加速電子線が、被測定物の晶帯軸からずれた方位から入射された場合に、透過回折された高加速電子線が形成する回折パターンにおいて、ダイレクトスポットを挟んで対称位置に現れた回折スポット対の強度比から晶帯軸のズレを検知する過程と、晶帯軸のズレを検知した際には、計算によって導出される回折スポット強度比を試料厚さｔおよび晶帯軸ズレ角θという二つのパラメータを有する関数として扱うことによって測定値を補正することが好ましい。
【００１２】
また、前記第１及び第２の本発明において、前記被測定物の電子線照射領域が単一の結晶で形成されていない場合に、透過回折された高加速電子線が形成する回折パターンから、バックグラウンドとなる非晶質相に起因するハレーションの削除、および複数の結晶粒からの回折波の分離を行う過程により、測定精度を向上させる手段を有することが好ましい。
【００１３】
第３の本発明は、被測定物質の散乱断面積および結晶構造および電子線照射方位とから導出される少なくともひとつの回折スポット強度比の試料厚さ依存性を計算する第１の演算処理装置と、
高加速電子線を用いた透過電子顕微鏡から出力される被測定試料の透過回折像を取得するための撮像装置と、前記撮像装置の制御および撮像装置で取得した画像を記録保存し、前記画像から前記回折スポット強度比を算出するための第２の演算制御装置と、
前記第１の演算処理装置によって算出した回折スポット強度比と、前記第２の演算処理装置によって算出した前記回折スポット強度比とを比較し前記比測定物質の厚さを決定する第３の演算処理装置からなることを特徴とする厚さ測定装置である。
【００１４】
【発明の実施の形態】
以下本発明を実施の形態によって詳細に説明する。
本発明は電子線を透過し得る被測定物の所望な箇所に高加速電子線を照射し、透過回折された電子線の回折スポットの強度を用いて厚さを測定する方法であり、従来法に対して比較的簡便に測定が可能であるとともに、原理的には適応範囲の制限がないという特徴を有する。
【００１５】
上記本発明において、高加速電子線とは、具体的には、数ｋｅＶ以上の加速電圧で出射された電子線で、被測定試料の厚さよりも同物質における照射電子線の平均自由行程の方が長くなる程度のエネルギーを有するものが好ましい。また、本発明の方法を適用するのに適した試料としては、結晶構造を有している物質層であって、照射電子線による平均自由行程よりも薄い厚さの層として自立する膜を構成できるものであれば、どのような材料であっても使用することができるが、Ｓｉのような半導体単結晶や、ＳｉＣなどのようなセラミックス等が適している。
【００１６】
［測定方法の第１の実施の形態］
以下本発明方法の第１の実施の形態について説明する。
被測定物質に照射される電子線が回折されて生じる回折スポットについて、結晶構造、物質構成原子の散乱断面積、照射電子線方位、及び、照射電子線の加速電圧の情報から当該試料の任意の回折スポット強度についての試料厚さ依存性を計算により算出して参照情報とする。この算出の手順については、Ｂｌｏｃｈ波法（文献Ｈ．Ｂｅｔｈｅ，Ａｎｎ，Ｐｈｙｓ．，８７（１９２８），５５参照）やＭｕｌｔｉｓｌｉｃｅ法（文献ＣｏｗｌｙａｎｄＭｏｏｄｉｅ，ＡｃｔａＣｒｙｓｔ．，１０（１９５７），６０９−６１９参照）と呼ばれるシミュレーション法を使用することができる。
一方、同じ結晶構造の物質からなる試料について、同じ照射電子線方位、照射電子線の加速電圧を用いて回折スポット強度を実測する。
このようにして得られるスポット強度の実測値と、上記過程で算出した試料厚さをパラメータとする回折スポット強度の参照情報のデータ群とを比較して、試料厚さを決定するものである。
【００１７】
具体的には、照射電子線の強度Ｉ_０を基準としたときの各回折スポットの強度Ｉ_ｉは結晶構造および照射電子線方位および被測定物質厚さおよび照射電子線の加速電圧および物質構成原子の散乱断面積とから計算にて求め（算出の手順については、上記文献参照）、膜厚を変数とする回折スポット強度関数のデータ群（参照情報）として蓄積する。
【００１８】
一方、電子線を透過し得る被測定物質の所望の箇所に比較的並行な高加速電子線を照射する。高加速電子の一部または多くは被測定物質内で散乱または回折されて透過するので、この透過した高加速電子を、蛍光板上またはＣＣＤカメラのような撮像装置を介したモニター上に透過電子回折像として形成する。このようにして得られた透過電子回折像には、被測定物質が結晶性の物質であれば結晶構造および照射電子線方位に対応した回折スポットが現れる。
【００１９】
ただし、本発明をＣＣＤカメラのような撮像装置を具備しない装置に適用する場合、回折像のスポット強度をリアルタイムに測定することができない。そのために、観測する回折スポット強度を相対的に比較することにより強度計算値との整合性を確認することで、簡易的にその場評価を可能にすることができる。またそのような相対比較においては、測定精度の観点からも比較対照スポット数を多く設定することが好ましい。
【００２０】
上記手順によって、被測定物質の設定条件下での回折スポットの厚さ依存性のデータを計算によって算出し、一方、厚さが未知の同じ被測定物質について、回折スポットを測定し、両者を比較することにより、厚さを判定することができる。
【００２１】
ところで、電子顕微鏡によって形成された像を、感光性フィルムに焼き付けて形成された画像を元に、回折スポット強度を算出する場合、感光性フィルムのダイナミックレンジが回折スポット強度のレンジより狭い場合には、感光性フィルムの画像からは確度の高い強度の実測はできない。このような場合には、得られる電子線回折像をディジタル画像に変換し、回折像内の回折斑点の径が、回折強度に相関している性質を利用し、この回折スポットの径を測定することによって回折強度を判定することができる。
【００２２】
なお、前記回折スポット強度比の実測値を得るために照射した高加速電子線が、被測定物の晶帯軸からずれた方位から入射された場合に、回折スポット強度に影響を及ぼすため、これによる影響を排除する必要がある。具体的には、透過回折された高加速電子線が形成する回折パターンにおいて、ダイレクトスポットを挟んで対称位置に現れた回折スポット対の強度比から晶帯軸のズレを検知する過程と、晶帯軸のズレを検知した際には、計算によって導出される回折スポット強度比を試料厚さｔおよび晶帯軸ズレ角θという二つのパラメータを有する関数として扱うことによって測定値を補正することができる。
【００２３】
また、前記被測定物の電子線照射領域が単一の結晶で形成されていない場合に、透過回折された高加速電子線が形成する回折パターンから、バックグラウンドとなる非晶質相に起因するハレーションの削除、および複数の結晶粒からの回折波の分離を行うことによって、測定精度を向上させることができる。
【００２４】
具体的には、次のような手法によって実施する。
まず、結晶相である被測定物が測定箇所において非晶質相と重複している場合、回折像には結晶相に起因する回折スポットと非晶質相に起因するハローパターンとが現れる。そのときの被測定物の厚さが照射電子線の平均自由行程よりも短ければ、被測定物にて透過散乱された高加速電子は結晶相、非晶質相のいずれか一方とのみ相互作用したものと考えられる。よって、回折スポットとハローパターンとは明確に分離することが可能となる。またハローパターンは通常中心スポットから外へ連続的に変化しているため、結晶相と非晶質相が重複した測定箇所から得られる回折像における回折スポットの強度は、結晶相による本来の回折スポット強度とハローパターンの強度の足し合わせとなる。よって、回折スポット間のハローパターンの強度変化からスポットと重複している領域にハローパターンの強度を見積もり、及び差し引くことで結晶相に起因する回折スポット強度のみを測定することが可能となる。
【００２５】
さらに、回折像に複数のネットパターンが現れている場合には、まず電子線照射領域内に存在する結晶粒の厚さ方向の重複の有無を確認し、厚さ方向の重複な無いことを確認できた場合は、複数確認されたネットパターンの中から、晶帯軸からのずれが少ない、回折スポット強度が測定しやすいなど、評価に適したパターンのみを選択し、上記単一パターンの場合と同様の測定を行うことで試料厚さを求めることが可能である。
一方、試料の厚さ方向への重複によって複数のネットパターンが表れている場合は、各々のパターンを識別した後、それぞれについて上記単一パターンの場合と同等の測定を行うことで各結晶の厚さを求め、その測定された各結晶相の厚さの総和として試料厚さを求めることが可能である。
【００２６】
［測定方法の第２の実施の形態］
上記測定方法の第１の実施の形態においては、電子線を照射して透過回折されたスポット強度と、これを、撮像装置を用いて記録及び測定した回折スポット強度との間に線形性があるとして、その強度比を算出して厚さ判定に使用しているが、必ずしも常に透過回折スポット強度と、撮像装置によって記録測定される回折スポット強度との間に線形の関係が成立するとは限らない。このような場合には、回折スポット強度の比を比較することができない。そこで、２つ以上の回折スポット強度の大小関係を判定し、この大小の相対的関係を満足する領域の厚さを被測定物質の厚さの範囲として判定すること好ましい。
【００２７】
［測定装置の実施の形態］
次に、上記方法に用いることのできる測定装置を説明する。
この発明の測定装置は、高加速電子線を照射することのできる透過型電子顕微鏡と、観測される像を観測するＣＣＤカメラや、銀塩カメラのような撮像装置と、該撮像装置を制御し、この撮像装置から出力される画像を記録保存し、かつこの画像から任意の回折スポット強度を算出するための第１の演算処理装置と、被測定物質の回折スポット強度をシミュレーションし、その結果を参照情報のデータ群として保存するための第２の演算処理装置と、前記実測したスポット強度と、前記参照情報データ群とを比較して、被測定物質の厚さを判定する第３の演算処理装置からなるものである。
上記測定装置において、第１、第２、第３の演算処理装置は、同一の演算処理装置を用い、ソフトウエアによって上記機能を実現するように設定することが好ましい。
【００２８】
【実施例】
以下、本発明の測定方法を実施例により詳細に説明する。
（実施例１）
本実施例は、被測定物質として薄片化されたシリコン基板の［１１０］方位から電子線を照射した場合の実施例である。本実施例の電子線の透過回折像を図１に示す。同回折像は電子顕微鏡による像をフィルムを用いて撮影されたものであるため、回折スポット強度はフィルムのダイナミックレンジを超えている。このようなフィルムを用いて記録された回折像からは正確な回折強度を測定することは困難であるが、各回折スポットは強度に応じた有限の面積を有する回折斑点として記録されている。そこで、回折斑点径と回折強度との関係はリニアではないが、相対的な強度比較の指標として用いることが可能である。ここで、図１の電子線回折像をディジタル画像に変換し、各回折スポット径を測定した結果を表１に示す。
【００２９】
【表１】

【００３０】
表１に示した回折斑点径は回折波の強度を反映しているため、ここでは回折斑点径ｒの大小により回折強度Ｉの強弱を判定する。その結果、各回折波の相対的な強度関係を導くことが可能となり、特に下記式（１）の関係が成立していることからシリコン単結晶の晶帯軸が下記式（２）で示される方向へ傾斜していることが判る。この晶帯軸の傾斜量は下記式（３）程度であると見積もられた。
次にＢｌｏｃｈ波法により晶帯軸の傾斜量をも考慮して計算した回折波強度を図２に示す。
一方、表１から得られる下記式（４）で表される３つの回折方位の各回折強度の関係は、下記式（５）であることから、測定領域の試料厚さｔは、図２の破線で示した厚さ範囲２３〜２６ｎｍであると決定することができる。
【００３１】
【数１】

【００３２】
（実施例２）
本実施例は、炭化珪素４Ｈ−ＳｉＣ単結晶における厚さ測定例である。図３に４Ｈ−ＳｉＣの［１１０］方位から電子線を照射して得られた電子線回折像を示す。上記実施例１と同様に回折斑点の径から回折強度を行う。回折斑点径は図３から測定した結果、Ｉ_００１＝６、Ｉ_００３＝７であった。
一方、Ｂｌｏｃｈ波法により計算された各回折波の強度を図４に示す。計算値によるとＩ_００１＜Ｉ_００３となるのは図４において破線で示した領域に限られる。このことから図３の回折像を取得した領域の試料厚さｔは５〜１７ｎｍの範囲であったと判断される。
【００３３】
本実施例は感光フィルムに記録された電子線回折像を用いたため、回折波の強度を直接測定することは出来ないが、そのような場合にも、例えば回折斑点径のように回折強度との相関を有するパラメータを用いて強度比較を行うことにより、測定物の厚さ範囲を求めることが可能である。
【００３４】
【発明の効果】
以上説明したとおり、本手法を用いることにより高加速電子線を透過し得るほどの極薄試料の厚さを、観察を妨げることなく簡便、且つ高精度に測定することが可能となる。
【図面の簡単な説明】
【図１】シリコン単結晶の電子線回折像を示す写真。
【図２】Ｂｌｏｃｈ波法により計算されたＳｉ単結晶による電子線回折波の強度を示すグラフ。
【図３】炭化珪素単結晶の電子線回折像を示す写真。
【図４】Ｂｌｏｃｈ波法により計算されたＳｉＣ単結晶による電子線回折波の強度を示すグラフ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for measuring the thickness of an extremely thin sample and a measuring device used for the method.
[0002]
[Prior art]
As a method for measuring the thickness of an ultrathin sample through which a high-acceleration electron beam can pass, a method using a convergent electron diffraction method using a transmission electron microscope and an electron energy loss spectroscopy is used.
[0003]
Specific means using the convergent electron beam diffraction method are as follows. First, an electron beam converged at an angle θ1 (θ1 >> 0) is applied to a desired position of an object to be measured having a thickness through which a highly accelerated electron beam can pass. Some or many of the irradiated high-acceleration electrons are elastically and inelastically scattered within the object to be measured and transmitted to the back of the sample, forming a focused electron beam diffraction image on a fluorescent screen or on a monitor via a CCD camera. I do. In the thickness measurement, the thickness is obtained by calculating from the intensity fluctuation of the striped contrast observed in the diffraction spot excited under the two-wave diffraction condition. However, since the irradiation electron beam at this time is converged at the convergence angle θ1, the diffraction spot appearing on the diffraction image has a finite area depending on the angle θ1 and the camera length. Further, in order to accurately measure the contrast in the diffraction spot, a corresponding area is required. In order to satisfy these conditions, it is necessary that the direct spot and the diffraction spot excited under the two-wave condition are sufficiently separated, that is, a diffraction spot having a large index must be selected. There are many experimental restrictions such as setting the azimuth to two-wave diffraction conditions, and it is difficult to use it simply.
[0004]
Further, a specific thickness measuring method using electron energy loss spectroscopy is as follows. First, an electron beam is applied to a desired position of a measured object having a thickness through which a highly accelerated electron beam can pass. Some of the irradiated high-acceleration electrons are inelastically scattered in the object to be measured and lose their energy. The electron beam transmitted through the sample is energy-resolved using an electron beam spectrometer to obtain an energy-resolved spectrum of the transmitted electron beam. The peak (zero-loss peak) formed by electrons that have not lost energy in the acquired spectrum can be assumed to have a Gaussian distribution centered on a loss energy amount of 0 eV. From this, a zero loss peak can be defined. By calculating the difference between the total intensity of these spectra and the intensity of the zero loss peak, the ratio of inelastic scattered electrons can be obtained. On the other hand, the probability that highly accelerated electrons are inelastically scattered in an object to be measured depends on the material. For each material, the mean free path from the time when the highly accelerated electrons are inelastically scattered until the next time when they are inelastically scattered Distance exists. The mean free path is a physical quantity that changes depending on the irradiation direction of an electron beam in a crystalline substance. Here, the thickness standardized by the mean free path can be obtained from the ratio of the electrons which are inelastically scattered by irradiating the object with the electrons described above.
However, in the thickness measurement using the electron energy loss spectroscopy, it is necessary to obtain the mean free path of the measured object in advance, but it is not easy to experimentally determine the physical quantity.
[0005]
Further, each thickness measurement method has a measurable thickness range depending on the substance to be measured. For example, in thickness measurement using a focused electron beam diffraction method, an appropriate thickness is necessary to obtain a focused electron beam diffraction image with clear contrast, but on the other hand, a two-wave diffraction condition is satisfied. Can only measure the film thickness under very restricted conditions, such as the need to suppress the sample thickness. Further, in the electron energy loss spectroscopy, there is a problem that a measurement error increases if the number of energy-loss electrons is too small or the number of multiple scattered electrons increases with respect to the number of electrons that have not lost energy.
[0006]
In addition, as a method of measuring the thickness of a polycrystalline film using the X-ray diffraction phenomenon, a sample is irradiated with X-rays at a fixed incident angle, and an X-ray detector is scanned within a predetermined angle range to arbitrarily scan a sample from a substrate. The diffraction angle and intensity of the diffracted X-ray of the diffraction peak are measured, and this procedure is repeated a plurality of times by changing the incident angle. From the obtained diffraction angle, diffraction intensity, incident angle and diffraction angle data, the thickness is determined. It is known to calculate (see Patent Document 1).
However, according to this method, it is necessary to acquire an extremely large number of data while changing the electron beam irradiation angle for one sample, and the working efficiency is low.
[0007]
[Patent Document 1] JP-A-4-194611
[Problems to be solved by the invention]
As described above, as a method of measuring an extremely thin sample thickness, a method using a convergent electron beam diffraction method and a method using electron energy loss spectroscopy have been conventionally performed. The range is limited, and for example, it has been difficult to apply to a sample having a thickness of several nm.
The method according to the present invention has been made in order to solve such problems in the conventional measuring method, and the image of the sample can be measured during the observation, so that the thickness can be measured easily and with high accuracy. It is intended to be.
[0009]
[Means for Solving the Problems]
The first invention is a step of calculating a sample thickness dependency of at least one diffraction spot intensity ratio derived from a scattering cross section and a crystal structure and an electron beam irradiation direction of an object to be measured,
A desired region of the substance to be measured is converged and irradiated with an electron beam, and at least two or more desired spots corresponding to the diffraction intensity ratio are selected from a plurality of diffraction spots corresponding to the crystal plane spacing and orientation of the transmitted and diffracted electron beam. Measuring the intensity of the diffraction spot of, and determining the intensity ratio of the diffraction spot,
Determining the sample thickness such that the intensity ratio of the diffraction spots derived by calculation matches the intensity ratio of the diffraction spots obtained by experiment.
[0010]
The second present invention is a step of calculating the sample thickness dependence of at least one diffraction spot intensity ratio derived from the scattering cross section and the crystal structure and the electron beam irradiation direction of the measured object,
A desired region of the substance to be measured is converged and irradiated with an electron beam, and at least two or more desired spots corresponding to the above-described diffraction intensity ratio are selected from a plurality of diffraction spots corresponding to the crystal plane spacing and orientation of the transmitted and diffracted electron beam. Measuring the intensity of the diffraction spot of, to determine the magnitude relationship of the intensity of the diffraction spot,
Determining a region where the intensity ratio of the diffraction spot derived by calculation and the relative relationship of the intensity of the diffraction spot obtained by experiment coincide with each other as a sample thickness range. It is.
[0011]
In the first and second aspects of the present invention, when a high-acceleration electron beam irradiated to obtain an actual measured value of the diffraction spot intensity ratio is incident from an azimuth deviated from a crystal zone axis of an object to be measured, In the diffraction pattern formed by the diffracted high-acceleration electron beam, the process of detecting the shift of the zone axis and the shift of the zone axis were detected from the intensity ratio of the pair of diffraction spots appearing symmetrically across the direct spot. In this case, it is preferable to correct the measured value by treating the diffraction spot intensity ratio derived by calculation as a function having two parameters of the sample thickness t and the zone deviation angle θ.
[0012]
In the first and second aspects of the present invention, when the electron beam irradiation region of the object to be measured is not formed of a single crystal, a diffraction pattern formed by a high-acceleration electron beam diffracted by transmission is: It is preferable to have means for improving measurement accuracy by removing halation caused by an amorphous phase serving as a background and separating diffraction waves from a plurality of crystal grains.
[0013]
According to a third aspect of the present invention, there is provided a first arithmetic processing device for calculating a sample thickness dependency of at least one diffraction spot intensity ratio derived from a scattering cross section, a crystal structure, and an electron beam irradiation direction of a substance to be measured. ,
An imaging device for acquiring a transmission diffraction image of a sample to be measured output from a transmission electron microscope using a high-acceleration electron beam, and controls and controls the imaging device and records and saves an image acquired by the imaging device. A second arithmetic and control unit for calculating the diffraction spot intensity ratio,
A third arithmetic processing for comparing the diffraction spot intensity ratio calculated by the first arithmetic processing device with the diffraction spot intensity ratio calculated by the second arithmetic processing device to determine the thickness of the ratio measurement substance; A thickness measuring device comprising a device.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to embodiments.
The present invention is a method of irradiating a high-acceleration electron beam to a desired portion of an object to be measured that can transmit an electron beam, and measuring the thickness using the intensity of a diffraction spot of the transmitted and diffracted electron beam. However, it is characterized in that it can be measured relatively easily, and that there is no restriction on the applicable range in principle.
[0015]
In the present invention, the high-acceleration electron beam is, specifically, an electron beam emitted at an acceleration voltage of several keV or more, and the direction of the mean free path of the irradiation electron beam of the same substance is larger than the thickness of the sample to be measured. It is preferable that the material has energy enough to increase the length. In addition, a sample suitable for applying the method of the present invention is a material layer having a crystal structure, which is a self-supporting film having a thickness smaller than the mean free path by the irradiation electron beam. Any material can be used as long as it can be used, but a semiconductor single crystal such as Si or a ceramic such as SiC is suitable.
[0016]
[First Embodiment of Measurement Method]
Hereinafter, a first embodiment of the method of the present invention will be described.
Regarding the diffraction spot generated by diffraction of the electron beam irradiated on the substance to be measured, the crystal structure, the scattering cross section of the constituent atoms of the substance, the irradiation electron beam direction, and the information of the acceleration voltage of the irradiation electron beam, the arbitrary information of the sample is obtained. The dependence of the diffraction spot intensity on the sample thickness is calculated and used as reference information. For the calculation procedure, the Bloch wave method (see H. Bethe, Ann, Phys., 87 (1928), 55) and the Multislice method (Cowley and Moody, Acta Cryst., 10 (1957), 609-619). Simulations) can be used.
On the other hand, for a sample made of a material having the same crystal structure, the diffraction spot intensity is actually measured using the same irradiation electron beam direction and the same acceleration voltage of the irradiation electron beam.
The actual measurement value of the spot intensity obtained in this way is compared with the data group of the reference information of the diffraction spot intensity using the sample thickness calculated in the above process as a parameter to determine the sample thickness.
[0017]
Specifically, the diffraction spot intensity I _i is the acceleration voltage and material constituent atoms of the crystal structure and electron beam irradiation direction and the substance to be measured thickness and electron beam irradiation time relative to the intensity I ₀ of the electron beam irradiation (For the calculation procedure, refer to the above document), and accumulate as a data group (reference information) of the diffraction spot intensity function using the film thickness as a variable.
[0018]
On the other hand, a relatively parallel high-acceleration electron beam is applied to a desired portion of the substance to be measured which can transmit the electron beam. Some or many of the high-acceleration electrons are scattered or diffracted in the substance to be measured and are transmitted, and the transmitted high-acceleration electrons are transmitted to a fluorescent screen or a monitor via an imaging device such as a CCD camera. Form as an image. In the transmission electron diffraction image thus obtained, if the substance to be measured is a crystalline substance, a diffraction spot corresponding to the crystal structure and the direction of the irradiated electron beam appears.
[0019]
However, when the present invention is applied to an apparatus without an imaging device such as a CCD camera, the spot intensity of a diffraction image cannot be measured in real time. Therefore, the in-situ evaluation can be simply performed by confirming the consistency with the calculated intensity value by relatively comparing the diffraction spot intensities to be observed. In such a relative comparison, it is preferable to set a large number of comparison control spots from the viewpoint of measurement accuracy.
[0020]
According to the above procedure, the data on the thickness dependence of the diffraction spot under the set conditions of the substance to be measured is calculated by calculation, while the diffraction spot is measured for the same substance to be measured whose thickness is unknown, and the two are compared. By doing so, the thickness can be determined.
[0021]
By the way, when calculating the diffraction spot intensity based on the image formed by printing the image formed by the electron microscope on the photosensitive film, when the dynamic range of the photosensitive film is narrower than the range of the diffraction spot intensity, However, it is not possible to measure the intensity with high accuracy from the image of the photosensitive film. In such a case, the obtained electron beam diffraction image is converted into a digital image, and the diameter of the diffraction spot in the diffraction image is used to measure the diameter of the diffraction spot using the property of being correlated with the diffraction intensity. Thus, the diffraction intensity can be determined.
[0022]
In addition, when the high-acceleration electron beam irradiated to obtain the actual measured value of the diffraction spot intensity ratio is incident from an azimuth deviated from the zonal axis of the object to be measured, it influences the diffraction spot intensity. Need to eliminate the effects of Specifically, in a diffraction pattern formed by a high-acceleration electron beam that has been transmitted and diffracted, a process of detecting a shift of a crystal zone axis from an intensity ratio of a pair of diffraction spots appearing symmetrically across a direct spot; When an axis deviation is detected, the measured value can be corrected by treating the diffraction spot intensity ratio derived by calculation as a function having two parameters of the sample thickness t and the zone deviation angle θ. .
[0023]
Further, when the electron beam irradiation region of the object to be measured is not formed of a single crystal, the electron beam irradiation region is caused by an amorphous phase serving as a background from a diffraction pattern formed by a high-acceleration electron beam that has been transmitted and diffracted. By removing halation and separating diffraction waves from a plurality of crystal grains, measurement accuracy can be improved.
[0024]
Specifically, the following method is used.
First, when an object to be measured, which is a crystalline phase, overlaps with an amorphous phase at a measurement site, a diffraction image shows a diffraction spot caused by the crystalline phase and a halo pattern caused by the amorphous phase. If the thickness of the measured object at that time is shorter than the mean free path of the irradiated electron beam, the highly accelerated electrons transmitted and scattered by the measured object interact with only one of the crystalline phase and the amorphous phase. It is thought that it was done. Therefore, the diffraction spot and the halo pattern can be clearly separated. In addition, since the halo pattern usually changes continuously from the center spot to the outside, the intensity of the diffraction spot in the diffraction image obtained from the measurement point where the crystal phase and the amorphous phase overlap is the original diffraction spot due to the crystal phase. It is the sum of the strength and the strength of the halo pattern. Therefore, it is possible to estimate only the intensity of the halo pattern in a region overlapping with the spot from the intensity change of the halo pattern between the diffraction spots, and to measure only the intensity of the diffraction spot caused by the crystal phase by subtracting the intensity.
[0025]
Furthermore, if multiple net patterns appear in the diffraction image, first check the presence or absence of overlap in the thickness direction of the crystal grains existing in the electron beam irradiation area, and confirm that there is no overlap in the thickness direction If successful, select only the patterns suitable for evaluation, such as a small deviation from the crystal zone axis and easy measurement of the diffraction spot intensity, from the plurality of confirmed net patterns. By performing the same measurement, the sample thickness can be obtained.
On the other hand, when a plurality of net patterns appear due to overlap in the thickness direction of the sample, after identifying each pattern, the same measurement as in the case of the above-described single pattern is performed for each of the patterns to thereby determine the thickness of each crystal. It is possible to determine the sample thickness as the sum of the thicknesses of the measured crystal phases.
[0026]
[Second Embodiment of Measurement Method]
In the first embodiment of the measurement method, there is linearity between the spot intensity transmitted and diffracted by irradiating an electron beam and the diffraction spot intensity recorded and measured by using an imaging device. Although the intensity ratio is calculated and used for thickness determination, a linear relationship is not always established between the transmitted diffraction spot intensity and the diffraction spot intensity recorded and measured by the imaging device. . In such a case, the ratio of the diffraction spot intensities cannot be compared. Therefore, it is preferable to determine the magnitude relationship between the two or more diffraction spot intensities, and determine the thickness of the region satisfying the magnitude relationship as the range of the thickness of the substance to be measured.
[0027]
[Embodiment of measuring device]
Next, a measuring device that can be used in the above method will be described.
The measurement apparatus of the present invention controls a transmission electron microscope capable of irradiating a highly accelerated electron beam, an imaging apparatus such as a CCD camera or a silver halide camera for observing an observed image, and controls the imaging apparatus. A first processing unit for recording and storing an image output from the imaging device, and calculating an arbitrary diffraction spot intensity from the image; and simulating the diffraction spot intensity of the substance to be measured, and arranging the result. A second processing unit for storing the data as a reference information data group, and a third calculation processing for comparing the actually measured spot intensity with the reference information data group to determine the thickness of the substance to be measured It consists of a device.
In the measurement device, it is preferable that the first, second, and third arithmetic processing units use the same arithmetic processing unit and are set so as to realize the above functions by software.
[0028]
【Example】
Hereinafter, the measuring method of the present invention will be described in detail with reference to examples.
(Example 1)
The present embodiment is an embodiment in which an electron beam is irradiated from a [110] direction of a sliced silicon substrate as a substance to be measured. FIG. 1 shows a transmission diffraction image of the electron beam of this embodiment. Since the diffraction image is obtained by photographing an image by an electron microscope using a film, the diffraction spot intensity exceeds the dynamic range of the film. Although it is difficult to accurately measure diffraction intensity from a diffraction image recorded using such a film, each diffraction spot is recorded as a diffraction spot having a finite area corresponding to the intensity. Therefore, the relationship between the diffraction spot diameter and the diffraction intensity is not linear, but can be used as an index for relative intensity comparison. Here, the results of converting the electron beam diffraction image of FIG. 1 into a digital image and measuring the diameter of each diffraction spot are shown in Table 1.
[0029]
[Table 1]

[0030]
Since the diffraction spot diameter shown in Table 1 reflects the intensity of the diffraction wave, here, the magnitude of the diffraction intensity I is determined based on the size of the diffraction spot diameter r. As a result, it is possible to derive the relative intensity relation of each diffraction wave, and in particular, since the relation of the following equation (1) is established, the crystal zone axis of the silicon single crystal is expressed by the following equation (2) It turns out that it inclines in the direction. The amount of inclination of the crystal zone axis was estimated to be about the following equation (3).
Next, FIG. 2 shows the diffraction wave intensity calculated by the Bloch wave method in consideration of the tilt amount of the crystal zone axis.
On the other hand, since the relationship between the respective diffraction intensities of the three diffraction directions represented by the following equation (4) obtained from Table 1 is the following equation (5), the sample thickness t in the measurement area is as shown in FIG. It can be determined that the thickness range indicated by the broken line is 23 to 26 nm.
[0031]
(Equation 1)

[0032]
(Example 2)
This embodiment is an example of measuring the thickness of a silicon carbide 4H-SiC single crystal. FIG. 3 shows an electron diffraction image obtained by irradiating an electron beam from the [110] direction of 4H—SiC. The diffraction intensity is determined from the diameter of the diffraction spot as in the first embodiment. As a result of measuring the diffraction spot diameter from FIG. 3, I ₀₀₁ = 6 and I ₀₀₃ = 7.
On the other hand, FIG. 4 shows the intensity of each diffracted wave calculated by the Bloch wave method. According to the calculated values, I ₀₀₁ <I ₀₀₃ is limited to the region shown by the broken line in FIG. From this, it is determined that the sample thickness t in the region where the diffraction image of FIG. 3 was obtained was in the range of 5 to 17 nm.
[0033]
In this embodiment, since the electron beam diffraction image recorded on the photosensitive film was used, the intensity of the diffracted wave could not be directly measured, but even in such a case, for example, the diffraction intensity such as the diffraction spot diameter was used. By performing the intensity comparison using the parameters having the correlation, the thickness range of the measured object can be obtained.
[0034]
【The invention's effect】
As described above, by using this method, it is possible to easily and accurately measure the thickness of an ultrathin sample that can transmit a highly accelerated electron beam without obstructing observation.
[Brief description of the drawings]
FIG. 1 is a photograph showing an electron diffraction image of a silicon single crystal.
FIG. 2 is a graph showing the intensity of an electron diffraction wave by a Si single crystal calculated by the Bloch wave method.
FIG. 3 is a photograph showing an electron diffraction image of a silicon carbide single crystal.
FIG. 4 is a graph showing the intensity of an electron diffraction wave by a SiC single crystal calculated by the Bloch wave method.

Claims

Calculating the sample thickness dependence of at least one diffraction spot intensity ratio derived from the scattering cross section and crystal structure of the object to be measured and the electron beam irradiation direction,
A desired region of the substance to be measured is converged and irradiated with an electron beam, and at least two or more desired spots corresponding to the diffraction intensity ratio are selected from a plurality of diffraction spots corresponding to the crystal plane spacing and orientation of the transmitted and diffracted electron beam. Measuring the intensity of the diffraction spot of, and determining the intensity ratio of the diffraction spot,
Determining the sample thickness such that the intensity ratio of the diffraction spots derived by calculation matches the intensity ratio of the diffraction spots obtained by experiment.

Calculating the sample thickness dependence of at least one diffraction spot intensity ratio derived from the scattering cross section and crystal structure of the object to be measured and the electron beam irradiation direction,
A desired region of the substance to be measured is converged and irradiated with an electron beam, and at least two or more desired spots corresponding to the above-described diffraction intensity ratio are selected from a plurality of diffraction spots corresponding to the crystal plane spacing and orientation of the transmitted and diffracted electron beam. Measuring the intensity of the diffraction spot of, to determine the magnitude relationship of the intensity of the diffraction spot,
Determining a region where the intensity ratio of the diffraction spot derived by calculation and the relative relationship of the intensity of the diffraction spot obtained by experiment coincide with each other as a sample thickness range. .

When the high-acceleration electron beam irradiated to obtain the actually measured value of the diffraction spot intensity ratio is incident from an azimuth deviated from the crystal zone axis of the object to be measured, the diffraction formed by the transmission-diffraction high-acceleration electron beam is formed. In the pattern, the process of detecting the deviation of the zone axis from the intensity ratio of the pair of diffraction spots appearing symmetrically with respect to the direct spot, and the process of detecting the deviation of the zone axis when calculating the deviation of the zone axis The thickness measurement method according to claim 1, wherein the measured value is corrected by treating the intensity ratio as a function having two parameters of a sample thickness t and a zone misalignment angle θ.

When the electron beam irradiation region of the object to be measured is not formed of a single crystal, a halation caused by an amorphous phase serving as a background is obtained from a diffraction pattern formed by a high-acceleration electron beam that has been transmitted and diffracted. 3. The thickness measuring method according to claim 1, further comprising means for improving measurement accuracy by removing and diffracting diffraction waves from a plurality of crystal grains.

A first arithmetic processing unit for calculating the sample thickness dependence of at least one diffraction spot intensity ratio derived from the scattering cross section and crystal structure of the substance to be measured and the electron beam irradiation direction,
An imaging device for acquiring a transmission diffraction image of a sample to be measured output from a transmission electron microscope using a high-acceleration electron beam, and controls and controls the imaging device and records and saves an image acquired by the imaging device. A second arithmetic and control unit for calculating the diffraction spot intensity ratio,
A third arithmetic processing for comparing the diffraction spot intensity ratio calculated by the first arithmetic processing device with the diffraction spot intensity ratio calculated by the second arithmetic processing device to determine the thickness of the ratio measurement substance; A thickness measuring device comprising a device.