JP3714853B2

JP3714853B2 - Planar shape measuring method in phase shift interference fringe simultaneous imaging device

Info

Publication number: JP3714853B2
Application number: JP2000197483A
Authority: JP
Inventors: 川崎　　和彦; 宏配野
Original assignee: Mitutoyo Corp
Current assignee: Mitutoyo Corp
Priority date: 2000-06-30
Filing date: 2000-06-30
Publication date: 2005-11-09
Anticipated expiration: 2020-06-30
Also published as: JP2002013919A

Description

【０００１】
【発明の属する技術分野】
本発明は、被検面と参照面からの反射光が光学的に無干渉状態にある原光束を複数の分枝原光束に分割し、夫々分枝原光束に異なる固定的光学位相差を与えて干渉させ、複数の撮像装置で同時撮像を行う位相シフト干渉縞同時計測装置に関する。
【０００２】
【従来の技術】
従来、図１に示すような位相シフト干渉縞同時計測装置が本出願人による特願平１１−１３６８３１号出願で提案されている。
即ち、同位相シフト干渉縞同時計測装置においては、レーザ光源１からのレーザ光束はレンズ２よりビーム径を拡大され、ビームスプリッタ３を透過してコリメートレンズ４にて平行光束とされる。
そして、この平行光束は参照面５で反射された参照光と参照面５，１／４波長板６を透過し被検面７で反射された試料光を生成するが、この参照光と試料光は直交する直線偏光で光学的無干渉状態にある。
【０００３】
また、ビームスプリッタ３で反射された参照光と試料光は、１／４波長板８でそれぞれ互いに回転方向の異なる円偏光状態となり三分光プリズム９で３つの分枝光束に分割される。それぞれの分枝光束の光路上には偏光板１０〜１２が配置され、光軸に対してほぼ直交する面内における偏光板の透過軸角度が設定され、固定的光学位相差を与えた分枝位相シフト干渉縞が発生し、これらの分枝位相シフト干渉縞が撮像装置１３〜１５により同時に撮像される。
【０００４】
【発明が解決しようとする課題】
つまり、この位相シフト干渉縞同時計測装置では、偏光板１０〜１２の透過軸の正確な角度の設定により、分枝位相シフト干渉縞に固定的光学位相差が与えられるが、偏光板の透過軸角度を精度よく設定することは一般に困難であり、また、材質の不均一性などから偏光板が全領域において光学的に全く均一に機能することは期待できない。
さらに、計画された固定的光学位相差と実測上での３枚の分枝位相シフト干渉縞間の固定的光学位相差は異なるのが現実であり、高精度な位相シフト干渉縞同時撮像装置を製作することは難しい。
【０００５】
本発明の目的は、前述したような位相シフト干渉縞同時撮像装置の問題に鑑み、偏光板の透過軸の設置誤差や偏光板自体の材質の不均一性などにより、分枝位相シフト干渉縞間の固定的光学位相差に相違があっても、充分に解析できる位相シフト干渉縞画像を得ることができる平面形状計測方法を得るにある。
【０００６】
【課題を解決するための手段】
この目的を達成するため、本発明は、複数の分枝原光束のそれぞれに与えた固定的光学位相差を撮像装置間の各点ごとに予め計測し、計測によって得られた固定的光学位相差を被検面起伏形状算出時に使用することで、位相シフト干渉縞同時撮像装置の大幅な高精度化を図ることを提案するものである。
つまり、本発明においては、
レーザ光源からのコヒーレント光束を参照面と被検面に照射し、前記参照面及び前記被検面のそれぞれからの反射光である参照光と試料光の偏光面を偏光光学素子を介在させて互いに直交させることにより、光学的無干渉状態となした原光束を生成する観測光学系と、前記原光束を複数に分光した分枝原光束に分け、前記分枝原光束のそれぞれに偏光光学素子を介して異なる固定的光学位相差を与えた複数の分枝位相シフト干渉縞を発生させ、前記被検面の観測範囲にある一つの位置がそれぞれの分枝観測座標系において同一位置になるよう位置の整合させ、分枝光束ごとに設けられた撮像装置でこれらの干渉縞に対応する画像データを取得し、前記被検面の観測範囲の平面起伏形状を位相シフト法を用いて数値データとして再現させる位相シフト干渉縞同時撮像装置において、
前記参照光と前記試料光との間に相対的な光学的位相差を別途与えたときに前記各撮像装置で得られる分枝ごとの位相シフト干渉縞画像データから位相シフト法を用いて平面起伏形状を分枝光束ごとに数値データとして算出し、異なる分枝の平面起伏形状間で位置的に整合された点における前記数値データの相対的な差から前記固定的光学位相差を各点ごとに求め、位相シフト法における干渉縞の各点ごとの位相算出過程にて同固定的光学位相差を使用する位相シフト干渉縞同時撮像装置における平面形状計測方法が提案される。
【０００７】
後述する本発明の好ましい実施例の説明においては、
1)前記参照光と前記試料光との間に相対的な光学的位相差を別途与える際、前記レーザ光源の波長をわずかづつ変化させることにより前記光学的位相差を発生させる方法、
2)前記参照光と前記試料光との間に相対的な光学的位相差を別途与える際、前記参照面あるいは前記被検面のどちらか一方を光軸に沿ってわずかづつ平行移動させることにより、前記光学的位相差を発生させる方法、
3)前記参照光と前記試料光との間に相対的な光学的位相差を別途与える際、前記参照面と前記被検面との間の光路に１より大きい屈折率をもつ無反射透過体であって、互いに厚みが異なる少なくとも１枚の平行板を挿入することにより前記光学的位相差を発生させる方法、
4)前記参照光と前記試料光との間に相対的な光学的位相差を別途与える際、前記参照面と前記被検面との間の光路に１より大きい屈折率を持つ無反射透過体であって、参照面及び被検面に向かい合う２面が平行でない光学楔を挿入し、光軸に対してほぼ略直交する面内において光学楔を楔方向に移動させ前記光学的位相差を発生させる方法、
5)前記参照光と前記試料光との間に相対的な光学的位相差を別途与える際、前記参照面を被検面との間に液晶を配置し、液晶の電気的な制御により屈折率を可変し、所定の光学的位相差を発生させる方法、
【０００８】
そして、本発明の実施例の説明においては、
1)各点ごとに求めた前記固定的光学位相差の他点との差異が、許容範囲である各点の集合ごとに整理された前記固定的光学位相差の値であるもの、
2)各点ごとに求めた前記固定的光学位相差の値から得た単純平均または中央値または２乗平均値が、固定的光学位相差の代表値として、各点の位置に関係なく全領域に用いられるもの、
3)前述した各平面形状計測方法を具体化するための、波長をわずかづつ変化できるレーザ光源、光軸に沿ってわずかづつ平行移動できる前記参照面あるいは前記被検面、前記参照面と前記被検面との間の光路に位置される１より大きい屈折率の無反射透過体平行板、前記参照面と前記被検面との間の光路に位置される１より大きい屈折率をもちかつ楔方向に移動できる光学楔、前記参照面と前記被検面との間の光路に位置されかつ電気的制御で屈折率を変化できる液晶を組み込まれた位相シフト干渉縞同時撮像装置
も説明される。
【０００９】
【発明の実施の形態】
本発明の平面形状計測方法は、図１に示した位相シフト干渉縞同時撮像装置において、複数の分枝原光束のそれぞれに与えた固定的光学位相差を撮像装置間の各点ごとに予め計測し、計測によって得られた光学的位相差を被検面起伏形状算出時に使用することを特徴とするものである。
【００１０】
本発明の平面形状計測方法を具体的に説明すると、図１の位相シフト干渉縞同時撮像装置において、被検面７を観測したときに撮像装置１３〜１５で得られる光学的に位相シフトした分枝位相シフト干渉縞は次式で表される。
【数１】

【００１１】
ここで、Ｂ（ｘ，ｙ）、Ａ（ｘ，ｙ）はそれぞれ３枚の分枝位相シフト干渉縞のバイアス及び振幅を、φ（ｘ，ｙ）は参照面に対する被検面の相対起伏形状を表す干渉縞の位相を、α（ｘ，ｙ）、β（ｘ，ｙ）は偏光板の透過軸角度などによって発生する固定的光学位相差をそれぞれ表す。
また、（１−１式）〜（１−３式）のα（ｘ，ｙ）、β（ｘ，ｙ）は、図１に示す光学系の構成要素の機械的設置誤差、光学的屈折率、反射率、透過率の不均一性の影響を受け、計画値とは異なった値となるので、このまま干渉縞位相算出にα（ｘ，ｙ）、β（ｘ，ｙ）の計画値を用いると算出位相に大きな誤差がでることが解る。
この誤差の影響を防ぐために３枚の分枝位相シフト干渉縞における固定的光学位相差α（ｘ，ｙ）とβ（ｘ，ｙ）を予め各点（ｘ，ｙ）ごとに計測し、計画値ではなく計測した固定的光学位相差α（ｘ，ｙ）、β（ｘ，ｙ）を被検面起伏形状演算時に使用することで、本発明では、位相シフト干渉縞同時撮像装置の高精度化を図ることができる。
【００１２】
一方、参照光と試料光の間に光学的位相差δ_i を与えたときに、得られる分枝ごとの位相シフト干渉縞は次式で表されることになる。
【数２】

【００１３】
これらの（２−１式）〜（２−３式）において、δ_i を任意に変化させて分枝ごとに３枚以上の分枝位相シフト干渉縞を得れば、分枝原光束ごと独立に被検面起伏形状を算出できる。例えば、δ_i を干渉縞位相１周期２πを等しく分割する値
δ_i ＝ｉ２π／Ｎ；ｉ＝１，２，３，・・・Ｎ
とした場合には、各分枝ごとの被検面起伏形状は次式より得られるわけである。
【００１４】
【数３】

式（３−１式）、（３−２式）、（３−３式）の左辺の関係から、お互いの差をとると、固定的光学位相差α（ｘ，ｙ）とβ（ｘ，ｙ）が被検面観測領域内の各点にて算出できる。（３−１式）〜（３−３式）は、別途与える光学位相差δ_i を多くとれば、空気揺らぎや振動などの偶発的な誤差による計測誤差は軽減され、より高精度な固定的光学位相差α（ｘ，ｙ）とβ（ｘ，ｙ）の決定が可能になることを示している。
【００１５】
次に、分枝位相シフト干渉縞間の固定的光学位相差を計測するために、参照光と試料光に光学位相差を別途与える本発明の方法を原理的に説明する。
参照面に対する被検面の距離がｄ（ｘ，ｙ）のときに、撮像装置１３で得られる干渉縞は、（１−１式）において、

【数４】

ここに、Ｉ（ｘ，ｙ）は干渉縞強度、Ｂ（ｘ，ｙ）、Ａ（ｘ，ｙ）はそれぞれバイアス、振幅、λはレーザ光源１の波長を表す。
【００１６】
（４式）において、波長λを微少量Δλ_i 変化させたときの干渉縞は、
【数５】

として表現できる。
【００１７】
ここで、
【数６】

である。
【００１８】
したがって、撮像装置１３，撮像装置１４，撮像装置１５によって得られる干渉縞は、それぞれ
【数７】

で表わされる。
【００１９】
よって、δ_i ＝Ｃ・Δλ_i に相当する量、レーザ光源１の波長をΔλ_i だけ変化させ、参照光と試料光に光学位相差を別途付加して分枝ごとに被検起伏形状を計測して数値データを得、分枝ごとの数値データの差をとることで、固定的光学位相差を被検面計測領域内の各点にて計測することができる。
【００２０】
また、参照光と試料光に光学位相差を与えるには、図２に示すように、被検面７を光軸方向にΔｄ_i 平行移動させるか、または、図３に示すように、参照面５を光軸方向にΔｄ_i 平行移動させても、同様に、分枝位相シフト干渉縞間の固定的光学位相差を計測することができる。
これらの場合の分枝位相シフト干渉縞は次式で表される。
【数８】

つまり、（６−１式）、（６−２式）、（６−３式）からは、

に相当する変位量Δｄ_i を与えたときに、各分枝位相シフト干渉縞ごとに（３−１式）、（３−２式）、（３−３式）と同様の算出を行えば、各分枝ごとに被検面起伏形状が数値データとして得られ、分枝ごとの数値データ間の差をとることで固定的光学位相差を計測することができることが理解される。
【００２１】
また、参照光と試料光に光学位相差を与えるには、前記参照面５を被検面７との間の光路に１より大きい屈折率をもつ無反射透過体で厚みの異なる平行板を挿入しても、分枝位相シフト干渉縞間の固定的光学位相差を計測することができる。
【００２２】
例えば、図４に示すように屈折率がｎで、厚みがそれぞれｌ₁ 、ｌ₂ の平行板を用い、
平行板を挿入していない状態を δ₁
厚みｌ₁ の平行板挿入時を δ₂
厚みｌ₂ の平行板挿入時を δ₃
としても、
図５に示すように、屈折率がそれぞれｎ₁ 、ｎ₂ で、厚みが同じｌの平行板を用いて、
平行板を挿入していない状態を δ₁
屈折率ｎ₁ の平行板挿入時を δ₂
屈折率ｎ₂ の平行板挿入時を δ₃
としても、
参照光と試料光に光学的位相差を別途与えることが可能で、分枝位相シフト干渉縞間の固定的光学位相差を計測することができる。
【００２３】
また、参照光と試料光に光学的位相差を別途与える場合、図６に示すように、前記参照面５を被検面７との間の光路に１より大きい屈折率を持つ無反射透過体で、参照面５と被検面７と向かい合う２面が平行でない光学楔２０を挿入し、同光学楔２０を光軸と概略直行する面内において楔方向に移動させ、分枝位相シフト干渉縞間の固定的光学位相差を計測することも可能である。
【００２４】
また、図７に示すように、液晶２１を参照面５と被検面７の間に挿入し、制御装置２２により電圧などの電気的な制御により同液晶２１の屈折率を可変させることにより、δ₁ 、δ₂ 、δ₃ に相当する光学的位相差を発生させても、分枝位相シフト干渉縞間の固定的光学位相差の計測は可能である。
【００２５】
前述したように、各分枝原光束ごとに得られる被検面起伏形状間の差から算出したα（ｘ，ｙ）、β（ｘ，ｙ）をそのまま固定的光学位相差として位相シフト干渉縞同時撮像装置における位相シフト法に適用することもできるが、データ処理をさらに容易にする本発明の方法を次に示す。
図８はα（ｘ，ｙ）のデータ中のあるｙ₁ における位置ｘと固定的光学位相差の１次元のデータα（ｘ，ｙ₁ ）の関係を示し、同図中、グラフはα（ｘ，ｙ₁ ）が観測領域内の位置ｘに対して値が異なることを意味している。
本発明の場合、α（ｘ，ｙ₁ ）に対してある許容範囲ｔを設定し、その範囲内にある数値データ群をひとまとめに整理して代表値で置き換え、α’（ｘ，ｙ₁ ）で図示したような関係を作成する。この考えを２次元ｘ、ｙのデータ群に応用し、ある許容範囲ｔ内にある領域のデータ群を代表値で置き換えて、α’（ｘ，ｙ）を作成する。
また、固定的光学位相差β（ｘ，ｙ）に対しても同様の処理を施してβ’（ｘ，ｙ）を作成しておけば、α（ｘ，ｙ）、β（ｘ，ｙ）を用いてｘ、ｙの各点ごとに異なる計算を行なう場合に対して、α’（ｘ，ｙ）、β’（ｘ，ｙ）を使用すれば、計算コストを削減することができる。
【００２６】
本発明においては、図９に示すように、α（ｘ，ｙ）、β（ｘ，ｙ）を単純平均値や中央値α’、β’で置き換えるか、あるいは、図１０に示すように、２乗平均から算出したα’（ｘ，ｙ）、β’（ｘ，ｙ）を用いれば、位相シフト干渉縞同時撮像装置における被検面起伏形状算出の際の計算コストをさらに削減できる。
【００２７】
【発明の効果】
以上の説明から明らかなように、本発明によれば、可動部を持たないという特徴から得られる位相シフト干渉縞同時撮像装置の誤差要素の再現性の良さを利用して、光学系固有の固定的光学位相差α（ｘ，ｙ）、β（ｘ，ｙ）を高精度に算出し、この値を被検面起伏形状計測時に使用することにより、被検面起伏形状計測の際の測定精度の大幅な向上を期待できる。
【図面の簡単な説明】
【図１】本発明による位相シフト干渉縞同時撮像装置の光学系の概念図である。
【図２】同位相シフト干渉縞同時撮像装置における第１の光学的位相差付与手段の説明図である。
【図３】同位相シフト干渉縞同時撮像装置における第２の光学的位相差付与手段の説明図である。
【図４】同位相シフト干渉縞同時撮像装置における第３の光学的位相差付与手段の説明図である。
【図５】同位相シフト干渉縞同時撮像装置における第４の光学的位相差付与手段の説明図である。
【図６】同位相シフト干渉縞同時撮像装置における第５の光学的位相差付与手段の説明図である。
【図７】同位相シフト干渉縞同時撮像装置における第６の光学的位相差付与手段の説明図である。
【図８】本発明による固定的光学位相差の算出方法の説明図である。
【図９】単純平均あるいは中央値による固定的光学位相差の算出方法の説明図である。
【図１０】２乗平均による固定的光学位相差の算出方法の説明図である。
【符号の説明】
１レーザ光源
２レンズ
３ビームスプリッタ
４コリメータレンズ
５参照面
６１／４波長板
７被検面
８１／４波長板
９３分光プリズム
１０〜１２偏光板
１３〜１５撮像装置
１６〜１９平行板
２０光学楔
２１液晶
２２制御装置[0001]
BACKGROUND OF THE INVENTION
The present invention divides an original light beam in which reflected light from a test surface and a reference surface is optically non-interfering into a plurality of branched original light beams, and gives different fixed optical phase differences to the branched original light beams, respectively. The present invention relates to a phase shift interference fringe simultaneous measurement apparatus that performs simultaneous imaging with a plurality of imaging apparatuses.
[0002]
[Prior art]
Conventionally, a phase shift interference fringe simultaneous measurement apparatus as shown in FIG. 1 has been proposed in Japanese Patent Application No. 11-136831 filed by the present applicant.
That is, in the same phase shift interference fringe simultaneous measurement apparatus, the laser beam from the laser light source 1 has a beam diameter larger than that of the lens 2, passes through the beam splitter 3, and is converted into a parallel beam by the collimator lens 4.
Then, the parallel light beam generates the reference light reflected by the reference surface 5 and the sample light that passes through the reference surface 5 and the quarter-wave plate 6 and is reflected by the surface 7 to be measured. Are orthogonally polarized light and in an optical non-interference state.
[0003]
Further, the reference light and the sample light reflected by the beam splitter 3 are circularly polarized in mutually different rotation directions by the quarter wavelength plate 8 and are divided into three branched light beams by the three spectroscopic prism 9. The polarizing plates 10 to 12 are arranged on the optical path of each branched light beam, the transmission axis angle of the polarizing plate is set in a plane substantially orthogonal to the optical axis, and the branched optical phase difference is given. Phase shift interference fringes are generated, and these branched phase shift interference fringes are simultaneously imaged by the imaging devices 13-15.
[0004]
[Problems to be solved by the invention]
That is, in this phase shift interference fringe simultaneous measurement apparatus, a fixed optical phase difference is given to the branched phase shift interference fringes by setting an accurate angle of the transmission axis of the polarizing plates 10 to 12, but the transmission axis of the polarizing plate It is generally difficult to set the angle with high accuracy, and it is not expected that the polarizing plate functions optically and uniformly in the entire region due to the non-uniformity of the material.
Furthermore, the actual fixed optical phase difference between the planned fixed optical phase difference and the actually measured three branched phase shift interference fringes is different. It is difficult to produce.
[0005]
The object of the present invention is to solve the problem of the simultaneous imaging device for phase shift interference fringes as described above, due to the installation error of the transmission axis of the polarizing plate and the non-uniformity of the material of the polarizing plate itself. Even if there is a difference in the fixed optical phase difference, a planar shape measuring method that can obtain a phase shift interference fringe image that can be sufficiently analyzed is obtained.
[0006]
[Means for Solving the Problems]
In order to achieve this object, the present invention measures the fixed optical phase difference given to each of the plurality of branched original light beams in advance for each point between the imaging devices, and obtains the fixed optical phase difference obtained by the measurement. Is used when calculating the undulation shape of the surface to be tested, and it is proposed that the phase shift interference fringe simultaneous imaging apparatus is greatly improved in accuracy.
That is, in the present invention,
A reference surface and a test surface are irradiated with a coherent light beam from a laser light source, and the polarization planes of the reference light and the sample light that are reflected from each of the reference surface and the test surface are interposed with a polarization optical element. The optical beam is divided into an observation optical system that generates an original light beam that is in an optically non-interfering state by orthogonality, and a branched original light beam that is obtained by dividing the original light beam into a plurality of light beams. A plurality of branch phase shift interference fringes with different fixed optical phase differences are generated via the position, and one position in the observation range of the test surface is the same position in each branch observation coordinate system The image data corresponding to these interference fringes is acquired by an imaging device provided for each branched light beam, and the planar undulation shape of the observation range of the test surface is reproduced as numerical data using the phase shift method Phase In shift interference fringe simultaneous imaging device,
Plane undulation using a phase shift method from phase shift interference fringe image data for each branch obtained by each imaging device when a relative optical phase difference is separately provided between the reference light and the sample light. The shape is calculated as numerical data for each branched light beam, and the fixed optical phase difference is calculated for each point from the relative difference of the numerical data at the points that are positionally matched between the planar undulating shapes of different branches. A planar shape measuring method in a phase shift interference fringe simultaneous imaging apparatus that uses the same fixed optical phase difference in the phase calculation process for each point of the interference fringes in the phase shift method is proposed.
[0007]
In the description of the preferred embodiments of the invention described below,
1) A method of generating the optical phase difference by slightly changing the wavelength of the laser light source when separately giving a relative optical phase difference between the reference light and the sample light,
2) When separately providing a relative optical phase difference between the reference light and the sample light, by moving one of the reference surface and the test surface slightly along the optical axis, , A method of generating the optical phase difference,
3) A non-reflective transmitter having a refractive index greater than 1 in the optical path between the reference surface and the test surface when a relative optical phase difference is separately provided between the reference light and the sample light. A method of generating the optical phase difference by inserting at least one parallel plate having different thicknesses,
4) A non-reflecting transparent body having a refractive index greater than 1 in the optical path between the reference surface and the test surface when a relative optical phase difference is separately provided between the reference light and the sample light. An optical wedge is inserted in which the two surfaces facing the reference surface and the test surface are not parallel, and the optical wedge is moved in the wedge direction in a plane substantially perpendicular to the optical axis to generate the optical phase difference. How to
5) When a relative optical phase difference is separately provided between the reference light and the sample light, a liquid crystal is disposed between the reference surface and the test surface, and the refractive index is controlled by electrical control of the liquid crystal. A method of generating a predetermined optical phase difference,
[0008]
And in the description of the embodiments of the present invention,
1) The difference between other points of the fixed optical phase difference obtained for each point is the value of the fixed optical phase difference arranged for each set of points that is an allowable range,
2) The simple average, median, or mean square value obtained from the value of the fixed optical phase difference obtained for each point is used as a representative value of the fixed optical phase difference, regardless of the position of each point. Used for
3) A laser light source capable of changing the wavelength slightly to embody each of the above-described planar shape measurement methods, the reference surface or the test surface that can be translated little by little along the optical axis, the reference surface and the object to be measured A non-reflective transparent parallel plate with a refractive index greater than 1 positioned in the optical path between the test surface and a wedge having a refractive index greater than 1 positioned in the optical path between the reference surface and the test surface A phase shift interference fringe simultaneous imaging device incorporating an optical wedge that can move in the direction and a liquid crystal that is positioned in the optical path between the reference surface and the test surface and that can change the refractive index by electrical control is also described.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The planar shape measurement method of the present invention measures in advance a fixed optical phase difference given to each of a plurality of branched original light beams at each point between the imaging devices in the phase shift interference fringe simultaneous imaging device shown in FIG. The optical phase difference obtained by the measurement is used when calculating the undulation shape of the test surface.
[0010]
The planar shape measuring method of the present invention will be described in detail. In the phase shift interference fringe simultaneous imaging apparatus shown in FIG. 1, the amount of optical phase shift obtained by the imaging apparatuses 13 to 15 when the test surface 7 is observed. The branch phase shift interference fringes are expressed by the following equation.
[Expression 1]

[0011]
Here, B (x, y) and A (x, y) are the bias and amplitude of each of the three branched phase shift interference fringes, and φ (x, y) is the relative undulation shape of the test surface with respect to the reference surface. Where α (x, y) and β (x, y) represent fixed optical phase differences generated by the transmission axis angle of the polarizing plate.
In addition, α (x, y) and β (x, y) in (1-1) to (1-3) are mechanical installation errors and optical refractive indexes of the components of the optical system shown in FIG. Since the value is different from the planned value due to the influence of the non-uniformity of the reflectance and the transmittance, the planned values of α (x, y) and β (x, y) are used for calculating the interference fringe phase as they are. It can be seen that there is a large error in the calculated phase.
In order to prevent the influence of this error, the fixed optical phase differences α (x, y) and β (x, y) in the three branched phase shift interference fringes are measured in advance for each point (x, y), and the plan is made. By using the measured fixed optical phase differences α (x, y), β (x, y) instead of the values when calculating the surface undulation shape of the test surface, in the present invention, the high accuracy of the phase shift interference fringe simultaneous imaging device is obtained. Can be achieved.
[0012]
On the other hand, when an optical phase difference δ _i is given between the reference light and the sample light, the obtained phase shift interference fringe for each branch is expressed by the following equation.
[Expression 2]

[0013]
In these formulas (2-1) to (2-3), if δ _i is arbitrarily changed to obtain three or more branched phase shift interference fringes for each branch, each branch original beam is independent. The surface undulation shape can be calculated. For example, δ _i is a value obtained by equally dividing the interference fringe phase 1 period 2π δ _i = i2π / N; i = 1, 2, 3,... N
In this case, the undulation shape of the test surface for each branch is obtained from the following equation.
[0014]
[Equation 3]

From the relationship between the left sides of the formulas (3-1), (3-2), and (3-3), the difference between the fixed optical phase differences α (x, y) and β (x, y) can be calculated at each point in the test surface observation area. In (Equation 3-1) to (Equation 3-3), if the optical phase difference δ _i given separately is increased, measurement errors due to accidental errors such as air fluctuations and vibrations can be reduced, and more accurate fixed It shows that the optical phase differences α (x, y) and β (x, y) can be determined.
[0015]
Next, in principle, the method of the present invention for separately providing an optical phase difference between the reference light and the sample light in order to measure the fixed optical phase difference between the branched phase shift interference fringes will be described.
When the distance of the test surface with respect to the reference surface is d (x, y), the interference fringes obtained by the imaging device 13 are (1-1)

[Expression 4]

Here, I (x, y) represents interference fringe intensity, B (x, y) and A (x, y) represent bias and amplitude, and λ represents the wavelength of the laser light source 1, respectively.
[0016]
In (Formula 4), the interference fringes when the wavelength λ is changed by a small amount Δλ _i is
[Equation 5]

Can be expressed as
[0017]
here,
[Formula 6]

It is.
[0018]
Therefore, the interference fringes obtained by the imaging device 13, the imaging device 14, and the imaging device 15 are expressed as follows:

It is represented by
[0019]
Therefore, the amount corresponding to δ _i = C · Δλ _i and the wavelength of the laser light source 1 are changed by Δλ _i , and the optical phase difference is separately added to the reference light and the sample light to measure the undulation shape for each branch. Thus, by obtaining numerical data and taking the difference of the numerical data for each branch, a fixed optical phase difference can be measured at each point in the measurement surface measurement region.
[0020]
Further, in order to give an optical phase difference between the reference light and the sample light, as shown in FIG. 2, the test surface 7 is translated by Δd _{i in the} optical axis direction, or as shown in FIG. 5 even in the optical axis direction is moved [Delta] d _i parallel, likewise, it is possible to measure the fixed optical phase difference between the branched phase shift interference fringe.
The branched phase shift interference fringes in these cases are expressed by the following equations.
[Equation 8]

In other words, from (Expression 6-1), (Expression 6-2), and (Expression 6-3),

When a displacement amount Δd _i corresponding to is given, calculation similar to (Equation 3-1), (Equation 3-2), and (Equation 3-3) is performed for each branch phase shift interference fringe. It is understood that the test surface undulation shape is obtained as numerical data for each branch, and the fixed optical phase difference can be measured by taking the difference between the numerical data for each branch.
[0021]
Further, in order to give an optical phase difference between the reference light and the sample light, a parallel plate with a different thickness is inserted in the optical path between the reference surface 5 and the test surface 7 with a non-reflective transparent material having a refractive index larger than 1. Even so, the fixed optical phase difference between the branched phase shift interference fringes can be measured.
[0022]
For example, as shown in FIG. 4, a parallel plate having a refractive index of n and thicknesses of l ₁ and l ₂ is used.
Δ _{1 when the} parallel plate is not inserted
The time parallel plate inserted in the thickness l ₁ [delta] ₂
When a parallel plate with thickness l ₂ is inserted, δ ₃
Even
As shown in FIG. 5, using parallel plates having refractive indexes n ₁ and n ₂ and the same thickness l,
Δ _{1 when the} parallel plate is not inserted
When a parallel plate with a refractive index n ₁ is inserted, δ ₂
When a parallel plate with a refractive index n ₂ is inserted, δ ₃
Even
An optical phase difference can be separately given to the reference light and the sample light, and a fixed optical phase difference between the branched phase shift interference fringes can be measured.
[0023]
Further, in the case where an optical phase difference is separately given to the reference light and the sample light, as shown in FIG. 6, a non-reflective transmission body having a refractive index larger than 1 in the optical path between the reference surface 5 and the test surface 7 Then, an optical wedge 20 in which the two surfaces facing the reference surface 5 and the test surface 7 are not parallel is inserted, and the optical wedge 20 is moved in the wedge direction in a plane substantially perpendicular to the optical axis, so that a branched phase shift interference fringe is obtained. It is also possible to measure a fixed optical phase difference between them.
[0024]
Further, as shown in FIG. 7, by inserting the liquid crystal 21 between the reference surface 5 and the test surface 7 and changing the refractive index of the liquid crystal 21 by electrical control such as voltage by the control device 22, Even if optical phase differences corresponding to δ ₁ , δ ₂ , and δ ₃ are generated, the fixed optical phase difference between the branched phase shift interference fringes can be measured.
[0025]
As described above, α (x, y) and β (x, y) calculated from the difference between the test surface undulation shapes obtained for each branched original light beam are used as the fixed optical phase difference as it is as the phase shift interference fringes. Although it can be applied to the phase shift method in the simultaneous imaging apparatus, the method of the present invention for further facilitating data processing will be described below.
FIG. 8 shows the relationship between the position x at a certain y ₁ in the data of α (x, y) and the one-dimensional data α (x, y ₁ ) of the fixed optical phase difference. x, y ₁ ) means that the value is different from the position x in the observation region.
In the case of the present invention, a certain allowable range t is set for α (x, y ₁ ), numerical data groups within the range are arranged together and replaced with representative values, and α ′ (x, y ₁ ) The relationship as shown in Fig. 1 is created. This idea is applied to a two-dimensional x, y data group, and a data group in a region within a certain allowable range t is replaced with a representative value to create α ′ (x, y).
Further, if β ′ (x, y) is generated by performing the same process on the fixed optical phase difference β (x, y), α (x, y), β (x, y) If α ′ (x, y) and β ′ (x, y) are used in contrast to the case where different calculations are performed for each point of x and y using, the calculation cost can be reduced.
[0026]
In the present invention, as shown in FIG. 9, α (x, y) and β (x, y) are replaced with simple average values and median values α ′ and β ′, or as shown in FIG. If α ′ (x, y) and β ′ (x, y) calculated from the mean square are used, it is possible to further reduce the calculation cost when calculating the undulated shape of the test surface in the phase shift interference fringe simultaneous imaging apparatus.
[0027]
【The invention's effect】
As is apparent from the above description, according to the present invention, by utilizing the good reproducibility of the error element of the phase shift interference fringe simultaneous imaging device obtained from the feature of having no moving part, the optical system is fixed. Measurement accuracy at the time of measurement of the test surface undulation shape by calculating the optical phase difference α (x, y), β (x, y) with high accuracy and using this value at the measurement of the test surface undulation shape A significant improvement can be expected.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of an optical system of a phase shift interference fringe simultaneous imaging apparatus according to the present invention.
FIG. 2 is an explanatory diagram of a first optical phase difference providing unit in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 3 is an explanatory diagram of second optical phase difference providing means in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 4 is an explanatory diagram of a third optical phase difference providing unit in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 5 is an explanatory diagram of a fourth optical phase difference providing unit in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 6 is an explanatory diagram of a fifth optical phase difference providing unit in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 7 is an explanatory diagram of a sixth optical phase difference providing unit in the same phase shift interference fringe simultaneous imaging apparatus;
FIG. 8 is an explanatory diagram of a method for calculating a fixed optical phase difference according to the present invention.
FIG. 9 is an explanatory diagram of a method for calculating a fixed optical phase difference based on a simple average or a median value.
FIG. 10 is an explanatory diagram of a method for calculating a fixed optical phase difference by means of root mean square.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Laser light source 2 Lens 3 Beam splitter 4 Collimator lens 5 Reference surface 6 1/4 wavelength plate 7 Test surface 8 1/4 wavelength plate 9 3 Spectral prism 10-12 Polarizing plate 13-15 Imaging apparatus 16-19 Parallel plate 20 Optical wedge 21 Liquid crystal 22 Control device

Claims

A reference surface and a test surface are irradiated with a coherent light beam from a laser light source, and the polarization planes of the reference light and the sample light that are reflected from each of the reference surface and the test surface are interposed with a polarization optical element. An observation optical system that generates an original light beam that is in an optically non-interfering state by being orthogonal to each other;
Dividing the original light beam into a plurality of branched original light beams, and generating a plurality of branched phase shift interference fringes each having a different fixed optical phase difference via a polarization optical element to each of the branched original light beams, Images corresponding to these interference fringes with an imaging device provided for each branched light beam, with the positions aligned so that one position in the observation range of the test surface is the same position in each branch observation coordinate system In the phase shift interference fringe simultaneous imaging device that acquires data and reproduces the planar undulation shape of the observation range of the test surface as numerical data using the phase shift method,
Plane undulation using a phase shift method from phase shift interference fringe image data for each branch obtained by each imaging device when a relative optical phase difference is separately provided between the reference light and the sample light. The shape is calculated as numerical data for each branched light beam, and the fixed optical phase difference is calculated for each point from the relative difference of the numerical data at the points that are positionally matched between the planar undulating shapes of different branches. A planar shape in a phase shift interference fringe simultaneous imaging device characterized in that the same optical phase difference is used in the phase calculation process for each point of the interference fringe of the phase shift method in the phase shift interference fringe simultaneous observation device Measurement method.

The optical phase difference is generated by changing a wavelength of the laser light source little by little when separately providing a relative optical phase difference between the reference light and the sample light. A planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 1.

When separately providing a relative optical phase difference between the reference light and the sample light, by moving one of the reference surface and the test surface little by little along the optical axis, 2. The planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 1, wherein an optical phase difference is generated.

When a relative optical phase difference is separately provided between the reference light and the sample light, the non-reflective transmission body has a refractive index greater than 1 in the optical path between the reference surface and the test surface. 2. The planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 1, wherein the optical phase difference is generated by inserting at least one parallel plate having different thicknesses.

When a relative optical phase difference is separately provided between the reference light and the sample light, the non-reflective transmission member has a refractive index greater than 1 in the optical path between the reference surface and the test surface. Then, an optical wedge is inserted into which the two surfaces facing the reference surface and the test surface are not parallel, and the optical wedge is moved in the direction of the wedge within a plane substantially perpendicular to the optical axis to generate the optical phase difference. The planar shape measuring method in the phase shift interference fringe simultaneous imaging device according to claim 1.

When a relative optical phase difference is given between the reference light and the sample light, a liquid crystal is disposed between the reference surface and the test surface, and the refractive index is varied by electrical control of the liquid crystal. 2. The planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 1, wherein a predetermined optical phase difference is generated.

The difference between the fixed optical phase difference obtained for each point and other points is a value of the fixed optical phase difference arranged for each set of points that is an allowable range. A planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 1.

The simple average value, median value, or mean square value obtained from the value of the fixed optical phase difference obtained for each point is used as a representative value of the fixed optical phase difference in the entire region regardless of the position of each point. The planar shape measuring method in the phase shift interference fringe simultaneous imaging apparatus according to claim 7, wherein the planar shape measuring method is used.

The laser light source according to claim 2, wherein the wavelength can be changed little by little, and the reference surface or the test surface according to claim 3, wherein the reference surface or the test surface can be moved slightly along the optical axis. 5. The nonreflective transparent parallel plate according to claim 4, having a refractive index greater than 1 positioned in the optical path, having a refractive index greater than 1 positioned in the optical path between the reference surface and the test surface and a wedge. The optical wedge according to claim 5, which is movable in a direction, and the liquid crystal according to claim 6, wherein the optical wedge is positioned in an optical path between the reference surface and the test surface and the refractive index can be changed electrically Phase shift interference fringe simultaneous imaging device.