JP2004184225A

JP2004184225A - Double refraction measuring instrument, method for detecting axial orientation of double refraction sample and method for calibrating the instrument

Info

Publication number: JP2004184225A
Application number: JP2002351400A
Authority: JP
Inventors: Tomoyuki Fukazawa; 知行深沢; Mitsuru Sano; 充佐野; Yuichi Miyoshi; 有一三好; Takachika Fukuda; 崇哉福田
Original assignee: Jasco Corp
Current assignee: Jasco Corp
Priority date: 2002-12-03
Filing date: 2002-12-03
Publication date: 2004-07-02

Abstract

<P>PROBLEM TO BE SOLVED: To provide a double refraction measuring instrument which detects an accurate axial orientation of a sample without rotating the sample, and also to provide a method for detecting the axial orientation of the sample. <P>SOLUTION: The double refraction measuring instrument 10 is equipped with: a polarizer 14 for permitting the light from a light irradiation means 12 to transmit to linearly polarize the same; and an optical elastic modulator 16 for imparting the linearly polarized light, and constituted so that a sample 18 is irradiated with the modulated light, and the double refraction of the sample 18 is measured by the output light from the sample 18. This double refraction measuring instrument 10 is also equipped with an analyzer 20 for permitting the output light from the sample 18 to transmit and an analyzer rotating means 22 for altering the azimuth of the axial orientation of the analyzer 20 with respect to the axial orientation of the polarizer 14, and the double refraction of the sample is measured on the basis of a plurality of different azimuths by the analyzer rotating means 22 to detect the axial orientation of the sample 18. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は複屈折測定装置、特に複屈折試料の軸方位検出機構及び方法の改良、また、複屈折測定装置のキャリブレーション方法に関する。
【０００２】
【従来の技術】
物質の光学的性質が異方性をもつ場合、その物質の屈折率は二以上の値をとる。このような物質に光が入射すると、一つの入射光に対して常光及び異常光の二つの屈折光が得られる。この現象を複屈折と呼ぶ。複屈折の現象は、光学異方性を持つ物質が振動面の異なる直線偏光に対して異なる屈折率を持つことから理解される。位相速度の速い光の振動方向は進相軸、位相速度の遅い光の振動方向は遅相軸と呼ばれる。複屈折測定は、上記の進相軸または遅相軸の方位を決定し、進相軸方向に振動面を持つ光と遅相軸方向に振動面をもつ光の位相差（リターデーション）を測定することで行う。
【０００３】
例えば特許文献１では、ＰＥＭを用いた複屈折解析装置が記載され、低角（５度以下）入射のビームスプリッタを用いることで検出チャンネルとして反射と透過の２チャンネルを持ち、試料の回転無しに進相軸方位とリターデーションを測定している。また装置の安定性を維持する目的で、試料室のドリフト周期、部屋のドリフト周期などに応じてキャリブレーションを繰返し行っている。装置の安定性が崩れる主な原因はＰＥＭの変調振幅が環境温度の変動に依存してドリフトする為である。
【０００４】
【特許文献１】
米国特許第６２６８９１４号公報
【非特許文献１】
深沢知行、「真空紫外域の複屈折測定」、光技術コンタクト、社団法人日本オプトメカトロニクス協会、平成１４年４月、第４０巻、第４号、ｐ．２１〜２６
【０００５】
【発明が解決しようとする課題】
特許文献１の軸方位検出方法は、２チャンネルの光路を形成するため使用されるビームスプリッタの特性に対して、（１）入射角５度では反射により偏光状態が変わらない、（２）透過により偏光状態が変わらない、という２つの仮定を前提としている。そこで真空紫外域の波長領域を考えるとき、実用的な透過媒体がＭｇＦ_２やＣａＦ_２などのフッ化物に限定されて来る。しかも、従来等方的と考えられて来たＣａＦ_２などの立方晶系の結晶でも、次世代の光リソグラフィーの波長とされる波長１５７ｎｍでは１１ｎｍ／ｃｍ程度の大きな複屈折を持つ事（例えば、非特許文献１参照）から、等方的でなくなるため上記の（２）の前提が崩れてしまう。つまり真空紫外領域において、特許文献１の装置では試料の正確な軸方位検出が行えない。
本発明の目的は、試料を回転する必要がなく、かつ正確な軸方位検出を可能にする複屈折測定装置及び試料の軸方位検出方法を提供することにある。
【０００６】
【課題を解決するための手段】
上記目的を達成するため、本発明の複屈折測定装置は、光照射手段からの光を透過し直線偏光にする偏光子と、該直線偏光の光に変調を与える光弾性変調子と、を備え、前記変調された光を試料に照射し、該試料からの出力光により試料の複屈折を測定する複屈折測定装置において、前記試料からの出力光が透過する検光子と、前記偏光子の軸方位に対する前記検光子の軸方位の方位角を変更するための検光子回転手段と、を備え、前記検光子回転手段により複数の異なる方位角で測定を行うことで、前記試料の軸方位を検出することを特徴とする。
上記の複屈折測定装置において、前記複数の異なる方位角が、約０度及び約４５度の二つの角度であることが好適である。
【０００７】
本発明の複屈折試料の軸方位検出方法は、偏光子からの光を光弾性変調子により変調し、該変調された光を試料に照射し、該試料からの出力光を検光子に通して検出することで前記試料の複屈折を測定する複屈折測定装置において、前記偏光子の軸方位に対する前記検光子の軸方位の方位角を変更して測定を行うことで前記試料の軸方位を検出することを特徴とする。
上記の複屈折試料の軸方位検出方法において、前記方位角として約０度と約４５度の二つの方位角で測定を行うことで前記試料の軸方位を検出することが好適である。
【０００８】
また、本発明の複屈折測定装置のキャリブレーション方法は、偏光子からの光を光弾性変調子により変調し、該変調された光を試料に照射し、該試料からの出力光を検光子に通して検出することで前記試料の複屈折を測定する複屈折測定装置において、前記試料として４分の１波長板を用いた時の検出信号を基準として、検出信号の較正を行うことを特徴とする。
【０００９】
【発明の実施の形態】
以下に本発明の実施形態の構成を、図面を参照しながら説明する。図１は本発明の複屈折測定装置の概略構成を示したものである。
図１の複屈折測定装置１０は、特定の波長域の光を照射する光照射手段１２（光源２６、分光器２８）と、光照射手段１２からの光を透過し直線偏光にする偏光子１４と、該偏光子１４を透過した光の偏光状態を変調させるための光弾性変調子１６（ＰＥＭ）と、試料１８からの出力光を通す検光子２０と、該検光子２０を透過した光を検出する光検出手段（ＰＭＴ２４）と、を備えている。ここでの偏光子１４は複屈折性偏光子であり、例えばローションプリズム等が使用される。偏光子１４が複像性を持つことから偏光子１４を透過した光は常光と異常光とに分離され、分離した光は互いに直交する直線偏光となる。これらの二つの光をそれぞれ試料測定のための測定光、参照光として使用する。また、光照射手段１２は図２に示したように、光源（Ｄ_２ランプ３０）にフィルタ３２を備えた構成にしてももちろん良い。
【００１０】
偏光子１４を透過して直線偏光となった光はＰＥＭ１６を透過し、二つの互いに直交する振動面を持つ直線偏光間に位相差δを生じさせ、一般には楕円偏光となる。このＰＥＭ１６は制御手段（ＰＥＭコントローラー４０）に接続され、ＰＥＭコントローラー４０によってＰＥＭ１６に周波数ｆの交流電圧が加えられ、その結果上記の位相差δもδ＝δ_０ｓｉｎ（２πｆｔ）のように変調される（ｔ：時間、δ_０：振幅）。
【００１１】
このように変調された測定光は試料１８に照射され、その試料１８からの出力光（今の場合は試料からの透過光のことであるが反射光を測定する構成としても良い）を検光子２０に透過させる。検光子２０を透過した光は光検出手段（ＰＭＴ２４）により検出される。この検出信号はロックインアンプ３８へと送られ、参照信号をもとに検出信号の周波数ｆまたは／及び２ｆの成分を抽出し演算手段（ＣＰＵ４４）へと送る。このようにＣＰＵ４４を介して得られる測定データはパーソナルコンピュータ４５により所望のデータ処理が行われ、またパーソナルコンピュータ４５に記憶された手順に従い試料１８の軸方位やリターデーションなどが求められる。検光子２０には、その軸方位を変更するための検光子回転手段２２が設けられており、偏光子１４の軸方位に対する検光子２０の軸方位の角度を変更することができる。検光子２０の軸方位角を変更して測定を行うことにより試料１８を回転することなく、試料１８の軸方位を決定することができ、また試料１８の複屈折の測定も行うことができる。好適には、検光子２０の軸方位を０度と４５度の２通りの角度で測定を行えばよい。試料１８の軸方位検出およびリターデーション測定の詳細については後述する。
【００１２】
このように本発明によれば、試料を回転せずに位相差が求まるので、露光器用のレンズ硝材などの大きな試料に対して軸検出が容易になる。また、搬送機構を付けて連続測定を行うような自動化システムでも、試料の回転機構が不要となるためシステムの構築が容易となる。
光弾性変調子１６を透過した参照光は、参照光検光子３４を通って参照光用検出器（ＰＭＴ３６）で検出される。参照光の検出信号はＰＥＭコントローラー４０へＰＥＭモニタ信号として送られる。また、測定光に関する信号処理に用いられる参照信号としてロックインアンプ３８へも送られる。ＰＥＭモニタ信号を受け取ったＰＥＭコントローラー４０はその信号を基にしてＰＥＭの駆動電圧を制御する。例えばＰＥＭモニタ信号に含まれる周波数２ｆ成分の振幅と直流成分の大きさの比が一定になるように駆動電圧を制御し、光弾性変調子１６を周波数ｆで振動させる。参照光に従い光弾性変調子を制御するという構成により、光弾性変調子の変調振幅を環境温度の変動を受けずに一定に保持することができる。ＣＰＵ４４からＰＥＭコントローラー４０へは、波長に応じた交流電圧の振幅の上限に関する情報が送られる。なお、ＣＰＵ４４はドライバー４２、検光子回転手段２２も制御し、分光器の波長走査の制御や検光子の軸角度の制御を行うようにもできる。
【００１３】
図３は上記の構成で波長１５７ｎｍでのＣａＦ_２の複屈折を約１時間測定し続けた結果を表すグラフである。縦軸がリターデーション（ｎｍ／ｃｍ）の値を示し、横軸が時間を示している。図３のグラフから明らかなように、約１時間の間リターデーションはほとんど一定値を保っており、本発明の複屈折測定装置は、真空波長域でも十分に安定であることが確認された。
つまり、本発明での複屈折測定装置は真空紫外の波長に対しても十分正確な軸方位検出および複屈折の測定を行うことができる。
【００１４】
次に本発明の軸方位検出方法について説明する。
＜軸方位検出方法＞
図２に示した複屈折測定装置の光路図を参照して説明する。図１に対応する部分には同一符号を用い、説明を省略する。図２に描かれた角度はそれぞれの光学素子の軸方位を示している（偏光子１４の軸方位を基準にとっている）。光照射手段１２から照射された光は複屈折性偏光子１４によって参照光と測定光とに分離される。この参照光と測定光は互いに垂直な偏光方向を持つ直線偏光である。
【００１５】
偏光子１４により直線偏光とされた測定光は、光弾性変調子１６によって２つの軸方向の偏光成分間に位相差δを与えられ変調をうける。以上の説明をストークスベクトルとミュラー行列を用いて表すと以下のようになる。まず、０度方位の偏光子を表すミュラー行列Ｐ_０は下記の数１で表される。
【００１６】
【数１】

また、θ方位の直線位相子（ＰＥＭ１６に対応）のミュラー行列は数２のようになる。
【００１７】
【数２】

ここで、θは進相軸方位、δは上記の位相差を示している。次に光照射手段１２からの光である自然光を表すストークスベクトル^ｔ（１，０，０，０）（ｔは転置を示す）を上記の数１の行列に右から掛け、さらにその結果を数２の行列に右から掛けると、係数（１／２）を除いて、数３で示されたＰＥＭによって変調された照射光Ｓが得られる。
【００１８】
【数３】

数３から偏光子とＰＥＭの成す角θは４５度が最適であることがわかる。しかし、この角度に限定する必要はない。例えば２２．５度とすると測定光Ｓの第４成分は０．７ｓｉｎδとなり、検出信号は３割小さくなる。また、４０度では４５度の場合と比べて９８．５％の大きさとなり、１．５％だけ損するのみであり問題はない。ここでは、簡単のためθ＝（１／４）πの時を考える。このとき照射光Ｓは
【数４】

と表される。次に進相軸方位がθ（上記のＰＥＭ１６の軸方位θとは無関係）、位相差Δの複屈折試料Ｂのミュラー行列は、
【数５】

と表される。ここで、ｔａｎΨは、０度方位（ｐ偏光）の直線偏光の透過係数の大きさと９０度方位（ｓ偏光）の直線偏光の透過係数の大きさの比を表している。照射光Ｓが試料に照射され、試料から出力光Ｃが出射される。この出力光Ｃは数４のベクトルを数５の行列に右から掛けることで得られ、数６のように表される。
【００１９】
【数６】

【００２０】
この出力光Ｃが検光子２０を通り光検出器であるＰＭＴ２４（フォトマル）によって検出される。この検知信号を表す式は、数６で示される出射光Ｃを、検光子を表すミュラー行列に掛け、その結果得られるストークスベクトルの第１成分として求められる。数６から分かるように試料の軸方位θと位相差Δの２つのパラメータを求める為には、検光子を複像とするか、または検光子の軸方位を２方位取って情報を増やす必要がある。ここでは、検光子の透過軸方位を２方位取る場合について説明する。
【００２１】
まず検光子２０の軸方位を０度にして測定を行い、この時の検知信号をＩ_０とする。検知信号Ｉ_０を表す式は数６を数１で表されるミュラー行列Ｐ_０に掛け、その第１成分として求められ、下記の数７のようになる。
【数７】

【００２２】
次に検光子２０の軸方位を４５度にして測定を行い、４５度方位での検知信号Ｉ_４５を検出する。検知信号Ｉ_４５は、数６の出射光Ｃを、４５度方位偏光子を示すミュラー行列Ｐ_４５に掛けて得られる。
【数８】

【００２３】
ここで、ＰＥＭ１６による位相差δはδ＝δ_０ｓｉｎ（２πｆｔ）のように変調されており、これを数７と数８のｃｏｓδ，ｓｉｎδの部分に代入する。そして良く知られているようにべッセル関数を用いて展開すると、ｃｏｓδは定数またはｃｏｓ｛２ｎ・２πｆｔ｝に比例した項の和として表され、ｓｉｎδはｓｉｎ｛（２ｎ−１）２πｆｔ｝に比例した項の和として表される（ｎ＝１，２…）。この周波数ｆを持つｓｉｎ（２πｆｔ）に比例した成分が上でも述べた検出信号のｆ成分であり、ここではｆ（Ｉ）と書いて表す。そこで、ｆ成分の比ｆ（Ｉ_０）／ｆ（Ｉ_４５）を取ると、
【数９】

となる。この数９の逆関数から試料の進軸軸方位θが−９０≦２θ≦９０の範囲で求まる。したがって、試料を回転すること無しに試料の進行軸方位を決定することができる。
【００２４】
また、このようにして求まった複屈折試料の軸方位θからさらに複屈折試料の位相差Δが求められる。そのため、数７のｆ成分ｆ（Ｉ_０）を取ると、
【数１０】

のようになる。特に試料に２色性が無い場合（Ψ＝４５°）の場合にはｆ（Ｉ_０）は
【数１１】

となり、進相軸方位θが既知なので、数１１より位相差Δが求まる。この位相差はΔ＝（２π／λ）Γの関係でリターデーションΓと結びついている。ここでλは測定に使う光の波長を表している。
【００２５】
＜キャリブレーション方法＞
複屈折測定装置のキャリブレーション方法を述べる。装置のキャリブレーションは、試料として４分の１波長板を用いて行う。４分の１波長板は位相差ΔがΔ＝±（π／２）・（４ｍ＋１）、（ただしｍ＝０，１，２，…）の条件を満たすので、｜ｓｉｎΔ｜＝１となる。そこで、４分の１波長板を試料として用いた測定を行うと、上記の数１０よりその時の検知信号の大きさは、検知信号のｆ成分の振幅の上限を与えることが分かる。このようにして求められた信号の振幅の上限をフルスケールとして基準にとり、検知信号のスケールの較正を行う。このキャリブレーションによって、測定波長を変えたときや、雰囲気温度の変化などにも対応できる。
【００２６】
【発明の効果】
本発明の複屈折測定装置および軸方位検出方法によれば、試料を回転する必要無しに、複屈折試料の軸方位や複屈折位相差を求めることができる。
また本発明の複屈折測定装置および複屈折試料の軸方位検出方法によれば、真空紫外域でも安定した測定を行うことが可能となる。
さらに、本発明の複屈折測定装置のキャリブレーション方法によればより安定した複屈折測定を行うことが可能である。
【図面の簡単な説明】
【図１】本発明の複屈折測定装置の概略構成図
【図２】本発明の複屈折測定装置の光路図
【図３】複屈折試料の測定結果を示すグラフ
【符号の説明】
１０…複屈折測定装置
１２…光照射手段
１４…偏光子
１６…光弾性変調子
１８…試料
２０…検光子
２２…検光子回転手段[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a birefringence measuring device, particularly to an improved mechanism and method for detecting the axial orientation of a birefringent sample, and to a method for calibrating a birefringence measuring device.
[0002]
[Prior art]
When the optical properties of a substance have anisotropy, the refractive index of the substance takes two or more values. When light is incident on such a substance, two refracted lights of ordinary light and extraordinary light are obtained for one incident light. This phenomenon is called birefringence. The phenomenon of birefringence is understood from the fact that substances having optical anisotropy have different refractive indexes for linearly polarized light having different vibration planes. The vibration direction of light having a high phase velocity is called a fast axis, and the vibration direction of light having a low phase velocity is called a slow axis. In birefringence measurement, the direction of the above-mentioned fast axis or slow axis is determined, and the phase difference (retardation) between light having a vibration plane in the fast axis direction and light having a vibration plane in the slow axis direction is measured. Do it by doing.
[0003]
For example, Patent Literature 1 describes a birefringence analyzer using a PEM, and has two channels of reflection and transmission as detection channels by using a low-angle (5 degrees or less) incident beam splitter without rotating the sample. The fast axis direction and retardation are measured. Further, in order to maintain the stability of the apparatus, calibration is repeatedly performed according to the drift cycle of the sample chamber, the drift cycle of the room, and the like. The main cause of the instability of the device is that the modulation amplitude of the PEM drifts depending on the fluctuation of the environmental temperature.
[0004]
[Patent Document 1]
US Pat. No. 6,268,914 [Non-Patent Document 1]
Tomoyuki Fukasawa, "Measurement of Birefringence in Vacuum Ultraviolet Region", Optical Technology Contact, Japan Opto-Mechatronics Association, April 2002, Vol. 40, No. 4, p. 21-26
[0005]
[Problems to be solved by the invention]
The axial azimuth detecting method disclosed in Patent Document 1 has the following characteristics: (1) polarization state does not change due to reflection at an incident angle of 5 degrees, and (2) transmission due to characteristics of a beam splitter used to form a two-channel optical path. Two assumptions are made that the polarization state does not change. Therefore, when considering the wavelength region in the vacuum ultraviolet region, practical transmission media are limited to fluorides such as MgF ₂ and CaF ₂ . Moreover, even a cubic crystal such as CaF ₂ which has been conventionally considered to be isotropic has a large birefringence of about 11 nm / cm at a wavelength of 157 nm, which is the wavelength of next-generation photolithography (for example, (See Non-Patent Document 1), it is no longer isotropic, so the premise of (2) above is broken. That is, in the vacuum ultraviolet region, the device of Patent Document 1 cannot accurately detect the axial orientation of the sample.
An object of the present invention is to provide a birefringence measuring apparatus and a method for detecting the axial direction of a sample, which do not require the sample to be rotated, and enable accurate axis direction detection.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the birefringence measurement apparatus of the present invention includes a polarizer that transmits light from a light irradiation unit and converts the light into linearly polarized light, and a photoelastic modulator that modulates the linearly polarized light. Irradiating the sample with the modulated light, a birefringence measuring apparatus for measuring the birefringence of the sample by the output light from the sample, an analyzer through which the output light from the sample is transmitted, and an axis of the polarizer Analyzer rotation means for changing the azimuth of the analyzer's axis azimuth with respect to the azimuth, and measuring at a plurality of different azimuths with the analyzer rotation means to detect the axis azimuth of the sample It is characterized by doing.
In the above-described birefringence measurement apparatus, it is preferable that the plurality of different azimuths are two angles of about 0 degree and about 45 degrees.
[0007]
The method of detecting the axial orientation of a birefringent sample of the present invention modulates light from a polarizer with a photoelastic modulator, irradiates the sample with the modulated light, and passes output light from the sample through an analyzer. In a birefringence measurement device that measures the birefringence of the sample by detecting, the axis azimuth of the analyzer is measured by changing the azimuth of the axis azimuth of the analyzer with respect to the axis azimuth of the polarizer. It is characterized by doing.
In the above-described method for detecting the axial direction of a birefringent sample, it is preferable that the axial direction of the sample is detected by performing measurement at two azimuthal angles of about 0 degree and about 45 degrees.
[0008]
Further, the calibration method of the birefringence measurement apparatus of the present invention modulates light from a polarizer with a photoelastic modulator, irradiates the modulated light to a sample, and outputs output light from the sample to an analyzer. In a birefringence measuring device for measuring the birefringence of the sample by detecting the same, the detection signal is calibrated based on the detection signal when a quarter-wave plate is used as the sample. I do.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a configuration of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of a birefringence measuring apparatus of the present invention.
The birefringence measuring apparatus 10 shown in FIG. 1 includes a light irradiating unit 12 (light source 26 and a spectroscope 28) that irradiates light in a specific wavelength range, and a polarizer 14 that transmits light from the light irradiating unit 12 to linearly polarize the light. A photoelastic modulator 16 (PEM) for modulating the polarization state of the light transmitted through the polarizer 14, an analyzer 20 through which output light from the sample 18 passes, and a light transmitted through the analyzer 20. Light detecting means (PMT 24) for detection. Here, the polarizer 14 is a birefringent polarizer, and for example, a lotion prism or the like is used. Since the polarizer 14 has a double image property, the light transmitted through the polarizer 14 is separated into ordinary light and extraordinary light, and the separated lights become linearly polarized light orthogonal to each other. These two lights are used as measurement light and reference light for sample measurement, respectively. Further, as the light irradiating means 12 shown in FIG. 2, of course may have a configuration having a filter 32 to the light source (D ₂ lamp 30).
[0010]
The light that has passed through the polarizer 14 and has become linearly polarized light has passed through the PEM 16 and causes a phase difference δ between two linearly polarized lights having vibration planes that are orthogonal to each other, and generally becomes elliptically polarized light. The PEM 16 is connected to control means (PEM controller 40), and the PEM controller 40 applies an AC voltage having a frequency f to the PEM 16, so that the phase difference δ is also modulated as δ = δ ₀ sin (2π ft). (T: time, δ ₀ : amplitude).
[0011]
The measurement light thus modulated is applied to the sample 18, and the output light from the sample 18 (in this case, transmitted light from the sample, but may be configured to measure reflected light) may be used as an analyzer. 20. The light transmitted through the analyzer 20 is detected by a light detecting means (PMT 24). This detection signal is sent to the lock-in amplifier 38, and the frequency f and / or 2f component of the detection signal is extracted based on the reference signal and sent to the arithmetic means (CPU 44). The measurement data obtained through the CPU 44 is subjected to desired data processing by the personal computer 45, and the axial direction, retardation, and the like of the sample 18 are obtained according to the procedure stored in the personal computer 45. The analyzer 20 is provided with an analyzer rotating means 22 for changing the axis direction, and can change the angle of the axis direction of the analyzer 20 with respect to the axis direction of the polarizer 14. By changing the axis azimuth of the analyzer 20 and performing measurement, the axis azimuth of the sample 18 can be determined without rotating the sample 18, and the birefringence of the sample 18 can also be measured. Preferably, the measurement may be performed at two angles of the axial direction of the analyzer 20 of 0 degree and 45 degrees. The details of the axial orientation detection and the retardation measurement of the sample 18 will be described later.
[0012]
As described above, according to the present invention, since the phase difference is obtained without rotating the sample, the axis can be easily detected for a large sample such as a lens glass material for an exposure device. Further, even in an automated system in which a transfer mechanism is attached and continuous measurement is performed, the construction of the system becomes easy because a rotating mechanism of the sample is not required.
The reference light transmitted through the photoelastic modulator 16 passes through the reference light analyzer 34 and is detected by the reference light detector (PMT 36). The reference light detection signal is sent to the PEM controller 40 as a PEM monitor signal. Further, it is also sent to the lock-in amplifier 38 as a reference signal used for signal processing on the measurement light. The PEM controller 40 receiving the PEM monitor signal controls the driving voltage of the PEM based on the signal. For example, the drive voltage is controlled so that the ratio between the amplitude of the frequency 2f component and the magnitude of the DC component included in the PEM monitor signal is constant, and the photoelastic modulator 16 is oscillated at the frequency f. With the configuration in which the photoelastic modulator is controlled in accordance with the reference light, the modulation amplitude of the photoelastic modulator can be kept constant without fluctuation of the environmental temperature. Information about the upper limit of the amplitude of the AC voltage according to the wavelength is sent from the CPU 44 to the PEM controller 40. Note that the CPU 44 can also control the driver 42 and the analyzer rotating means 22 to control the wavelength scanning of the spectroscope and the axis angle of the analyzer.
[0013]
FIG. 3 is a graph showing the result of continuously measuring the birefringence of CaF ₂ at a wavelength of 157 nm for about one hour in the above configuration. The vertical axis indicates the value of retardation (nm / cm), and the horizontal axis indicates time. As is clear from the graph of FIG. 3, the retardation remains almost constant for about one hour, confirming that the birefringence measuring apparatus of the present invention is sufficiently stable even in a vacuum wavelength region.
In other words, the birefringence measuring device according to the present invention can perform sufficiently accurate axial azimuth detection and birefringence measurement even for a vacuum ultraviolet wavelength.
[0014]
Next, the axial direction detecting method of the present invention will be described.
<Axis orientation detection method>
This will be described with reference to the optical path diagram of the birefringence measuring device shown in FIG. Parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. The angles depicted in FIG. 2 indicate the axis directions of the respective optical elements (based on the axis directions of the polarizer 14). The light irradiated from the light irradiation means 12 is separated by the birefringent polarizer 14 into reference light and measurement light. The reference light and the measurement light are linearly polarized lights having polarization directions perpendicular to each other.
[0015]
The measurement light converted into linearly polarized light by the polarizer 14 is given a phase difference δ between the polarization components in the two axial directions by the photoelastic modulator 16 and undergoes modulation. The above description is expressed as follows using Stokes vector and Mueller matrix. First, a Mueller matrix P ₀ representing a polarizer having a 0-degree azimuth is represented by the following Equation 1.
[0016]
(Equation 1)

The Mueller matrix of the linear phase shifter in the θ direction (corresponding to PEM16) is as shown in Expression 2.
[0017]
(Equation 2)

Here, θ indicates the fast axis direction, and δ indicates the above phase difference. Next, a Stokes vector ^t (1,0,0,0) (t indicates transpose) representing natural light, which is light from the light irradiating means 12, is multiplied from the right by the above-described matrix of Equation 1, and the result is further expressed as When the matrix of 2 is multiplied from the right, the irradiation light S modulated by the PEM shown in Expression 3 is obtained except for the coefficient (1/2).
[0018]
[Equation 3]

From Equation 3, it is understood that the optimum angle θ between the polarizer and the PEM is 45 degrees. However, it is not necessary to limit to this angle. For example, if it is 22.5 degrees, the fourth component of the measurement light S is 0.7 sin δ, and the detection signal is reduced by 30%. At 40 degrees, the size is 98.5% as compared with the case of 45 degrees, and there is no problem because only 1.5% is lost. Here, for the sake of simplicity, the case of θ = (1/4) π is considered. At this time, the irradiation light S is given by

It is expressed as Next, the Mueller matrix of the birefringent sample B having a fast axis direction θ (irrelevant to the axis direction θ of the PEM 16 described above) and a phase difference Δ is:
(Equation 5)

It is expressed as Here, tanΨ represents the ratio of the magnitude of the transmission coefficient of linearly polarized light having a 0-degree azimuth (p-polarized light) to the magnitude of the transmission coefficient of linearly polarized light having a 90-degree azimuth (s-polarized light). Irradiation light S is applied to the sample, and output light C is emitted from the sample. The output light C is obtained by multiplying the matrix of Equation 4 by the matrix of Equation 5 from the right, and is expressed as in Equation 6.
[0019]
(Equation 6)

[0020]
The output light C passes through the analyzer 20 and is detected by a PMT 24 (photomultiplier) as a photodetector. The expression representing this detection signal is obtained by multiplying the outgoing light C shown in Expression 6 by the Mueller matrix representing the analyzer, and obtaining the result as the first component of the resulting Stokes vector. As can be seen from Equation 6, in order to obtain the two parameters of the sample axis direction θ and the phase difference Δ, it is necessary to increase the information by forming the analyzer as a double image or taking the analyzer in two directions. is there. Here, a case in which the analyzer has two transmission axis directions will be described.
[0021]
First axial orientation of the analyzer 20 and perform the measurement at 0 °, the detection signal at this time is I _0. The equation representing the detection signal I ₀ is obtained by multiplying Equation 6 by the Mueller matrix P ₀ represented by Equation 1 and obtaining the first component thereof, as shown in Equation 7 below.
(Equation 7)

[0022]
Then the axial orientation of the analyzer 20 and perform the measurement at 45 degrees, for detecting the detection signal I ₄₅ at 45 degree orientation. The detection signal I ₄₅ is obtained by multiplying the output light C of Expression 6 by a Mueller matrix P ₄₅ indicating a 45-degree azimuth polarizer.
(Equation 8)

[0023]
Here, the phase difference δ by the PEM 16 is modulated as δ = δ ₀ sin (2πft), and this is substituted into the cos δ and sin δ parts of Equations 7 and 8. Then, as is well known, when expanded using a Bessel function, cos δ is expressed as a constant or a sum of terms proportional to cos {2n · 2πft}, and sinδ is proportional to sin {(2n−1) 2πft}. (N = 1, 2,...). The component proportional to sin (2πft) having the frequency f is the f component of the above-described detection signal, and is represented by f (I) here. Therefore, taking the ratio f (I ₀ ) / f (I ₄₅ ) of the f component,
(Equation 9)

It becomes. From the inverse function of Equation 9, the axial axis direction θ of the sample is obtained in the range of −90 ≦ 2θ ≦ 90. Accordingly, the direction of the traveling axis of the sample can be determined without rotating the sample.
[0024]
Further, the phase difference Δ of the birefringent sample is further obtained from the axial orientation θ of the birefringent sample thus obtained. Therefore, taking the f component f (I ₀ ) of _equation 7,
(Equation 10)

become that way. In particular, when the sample has no dichroism (Ψ = 45 °), f (I ₀ ) is given by

Since the fast axis azimuth θ is known, the phase difference Δ is obtained from Expression 11. This phase difference is linked to the retardation で in a relation of Δ = (2π / λ) Γ. Here, λ represents the wavelength of light used for measurement.
[0025]
<Calibration method>
A method of calibrating a birefringence measuring device will be described. The calibration of the apparatus is performed using a quarter-wave plate as a sample. Since the quarter-wave plate satisfies the condition that the phase difference Δ is Δ = ± (π / 2) · (4m + 1) (where m = 0, 1, 2,...), | SinΔ | = 1. Therefore, when measurement is performed using a quarter-wave plate as a sample, it can be seen from the above Equation 10 that the magnitude of the detection signal at that time gives the upper limit of the amplitude of the f component of the detection signal. The scale of the detection signal is calibrated based on the upper limit of the amplitude of the signal obtained in this manner as a full scale. This calibration can cope with a change in the measurement wavelength, a change in the ambient temperature, and the like.
[0026]
【The invention's effect】
According to the birefringence measuring device and the axial azimuth detecting method of the present invention, the axial azimuth and the birefringence phase difference of the birefringent sample can be obtained without having to rotate the sample.
Further, according to the birefringence measuring apparatus and the method for detecting the axial orientation of a birefringent sample of the present invention, stable measurement can be performed even in the vacuum ultraviolet region.
Further, according to the calibration method of the birefringence measuring device of the present invention, more stable birefringence measurement can be performed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a birefringence measuring device of the present invention. FIG. 2 is an optical path diagram of a birefringent measuring device of the present invention. FIG. 3 is a graph showing measurement results of a birefringent sample.
DESCRIPTION OF SYMBOLS 10 ... Birefringence measuring device 12 ... Light irradiation means 14 ... Polarizer 16 ... Photoelastic modulator 18 ... Sample 20 ... Analyzer 22 ... Analyzer rotation means

Claims

A polarizer that transmits light from the light irradiation unit and converts the light into linearly polarized light, and a photoelastic modulator that modulates the linearly polarized light, irradiates the sample with the modulated light, In a birefringence measurement device that measures the birefringence of a sample using output light,
An analyzer through which output light from the sample passes;
Analyzer rotation means for changing the azimuth of the axis azimuth of the analyzer with respect to the axis azimuth of the polarizer,
A birefringence measurement apparatus, wherein the analyzer rotation means performs measurement at a plurality of different azimuth angles to detect the axial azimuth of the sample.

The birefringence measurement device according to claim 1,
The birefringence measurement device, wherein the plurality of different azimuths are two angles of about 0 degree and about 45 degrees.

The light from the polarizer is modulated by a photoelastic modulator, the modulated light is irradiated on a sample, and the output light from the sample is passed through an analyzer and detected to measure the birefringence of the sample. In a refractometer,
A method for detecting the axial direction of a birefringent sample, wherein the axial direction of the sample is detected by changing the azimuth of the axial direction of the analyzer with respect to the axial direction of the polarizer.

The method for detecting the axial orientation of a birefringent sample according to claim 3,
An axial azimuth detecting method for a birefringent sample, wherein the axial azimuth of the sample is detected by measuring at two azimuthal angles of about 0 degree and about 45 degrees as the azimuthal angle.

The light from the polarizer is modulated by a photoelastic modulator, the modulated light is irradiated on a sample, and the output light from the sample is passed through an analyzer and detected to measure the birefringence of the sample. In a refractometer,
A method for calibrating a birefringence measurement apparatus, comprising: calibrating a detection signal based on a detection signal when a quarter-wave plate is used as the sample.