JP5376284B2

JP5376284B2 - Interferometry method and interferometer

Info

Publication number: JP5376284B2
Application number: JP2008168227A
Authority: JP
Inventors: 志強劉
Original assignee: Nikon Corp
Current assignee: Nikon Corp
Priority date: 2008-06-27
Filing date: 2008-06-27
Publication date: 2013-12-25
Anticipated expiration: 2028-06-27
Also published as: JP2010008243A

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interference measuring method which enables high-accuracy measurement of a measuring object plane of a parallel plane plate. <P>SOLUTION: The method has a first step of irradiating a prescribed reference plane 6a and the measuring object plane 5a of the parallel plane plate 5, with two kinds of coherent lights which do not interfere with each other and having wavelengths different from each other; a second step of detecting an interference fringe formed, on the basis of reflected lights from the reference plane 6a and the measuring object plane 5a irradiated with the two kinds of coherent lights; and a third step of measuring the height of the measuring object plane 5a, on the basis of an optical intensity distribution of the detected interference fringe. In the method, the relation of the wavelength in the two kinds of coherent lights and an optical path between the reference plane 6a and the measuring object plane 5a are so adjusted as to remove the effect of reflected light from a plane 5b, on the opposite side to the measuring object plane 5a in the parallel plane plate 5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

本発明は、主に平行平面板の表面高さを測定する干渉測定方法および干渉計に関する。 The present invention mainly relates to an interference measuring method and an interferometer for measuring the surface height of a plane parallel plate.

従来の干渉計では、測定対象物の被測定面および基準板の参照面にレーザー光等の可干渉光を照射し、被測定面において反射して得られる測定光と参照面において反射して得られる参照光とが干渉して得られる干渉縞をイメージセンサーにより検出する（例えば、特許文献１を参照）。そして、イメージセンサーから出力される信号強度分布（すなわち、干渉縞の光強度分布）に基づいて被測定面の高さを測定し、被測定面の形状が求められる。 Conventional interferometers are obtained by irradiating the surface to be measured of the measurement object and the reference surface of the reference plate with coherent light such as laser light and reflecting the measurement light obtained by reflection on the surface to be measured and the reference surface. An interference fringe obtained by interference with the reference light to be detected is detected by an image sensor (see, for example, Patent Document 1). Then, the height of the measured surface is measured based on the signal intensity distribution (that is, the light intensity distribution of the interference fringes) output from the image sensor, and the shape of the measured surface is obtained.

ところが、干渉計のイメージセンサーには、被測定面と反対側の面において反射した光や干渉計内の他の光学素子の表面で反射した光（以下、これらの光をノイズ光と称する）も入射するため、測定精度が低下し、場合によっては測定が不可能になる。特に、平行平面板の表面形状を測定する場合、被測定面で反射して得られる測定光以外に、被測定面と反対側の面で反射したノイズ光によっても干渉が生じるので、通常のコヒーレント長が長い可干渉光を用いた干渉計では測定しにくい。そのため、このような場合、コヒーレント長が短い可干渉光を使用して干渉測定を行う。
特開２００５−２４９５７６号公報 However, the image sensor of the interferometer also includes light reflected on the surface opposite to the surface to be measured and light reflected on the surface of other optical elements in the interferometer (hereinafter, these lights are referred to as noise light). Because of incidence, measurement accuracy decreases, and in some cases, measurement is impossible. In particular, when measuring the surface shape of a plane parallel plate, interference occurs due to noise light reflected on the surface opposite to the measurement surface, in addition to the measurement light obtained by reflection on the measurement surface. It is difficult to measure with an interferometer using long coherent light. Therefore, in such a case, interference measurement is performed using coherent light having a short coherent length.
JP 2005-249576 A

しかしながら、コヒーレント長が短い可干渉光を使用して干渉測定を行う場合、ノイズ光による干渉を抑えることはできるが、測定に用いる干渉縞を得ることも困難になる。 However, when performing interference measurement using coherent light having a short coherent length, interference due to noise light can be suppressed, but it is difficult to obtain interference fringes used for measurement.

本発明は、このような問題に鑑みてなされたものであり、平行平面板の被測定面に対して高精度な測定を行うことが可能な干渉測定方法および干渉計を提供することを目的とする。 The present invention has been made in view of such problems, and an object of the present invention is to provide an interference measurement method and an interferometer capable of performing highly accurate measurement on a measurement target surface of a parallel flat plate. To do.

このような目的達成のため、本発明を例示する態様に従えば、所定の参照面および測定対象物である平行平面板の被測定面に、互いに干渉しない異なる波長を有する２種類の可干渉光を照射する第１のステップと、前記２種類の可干渉光が照射された前記参照面および前記被測定面からの反射光に基づいて形成される干渉縞を検出する第２のステップと、前記検出した干渉縞の光強度分布に基づいて前記被測定面の高さを測定する第３のステップとを有し、前記第１のステップにおいて、前記平行平面板における前記被測定面と反対側の面からの反射光の影響を排除するように、前記２種類の可干渉光のうち一方の波長をλ ₁ としてｋ ₁ ＝（２×π）／λ ₁ とし、他方の波長をλ ₂ としてｋ ₂ ＝（２×π）／λ ₂ とし、前記被測定面と前記反対側の面との間の光学距離をｄ _on とし、任意の整数をＭとしたとき、次式
（ｋ ₂ ×ｄ _on ）＝（ｋ ₁ ×ｄ _on ）＋π＋（２×π×Ｍ）
の関係を満たすように前記異なる波長同士の関係を調整するとともに、前記参照面と前記被測定面との間の光学距離をｄ _ro とし、任意の整数をＮとしたとき、次式
（ｋ ₂ ×ｄ _ro ）＝（ｋ ₁ ×ｄ _ro ）＋（２×π×Ｎ）
の関係を満たすように前記参照面と前記被測定面との間の光学距離を調整した状態で、前記２種類の可干渉光を照射し、前記異なる波長同士の関係および、前記参照面と前記被測定面との間の光学距離が相違する複数の測定条件について、前記第１のステップにおける前記２種類の可干渉光の照射と、前記第２のステップにおける前記干渉縞の検出とを行い、前記第３のステップにおいて、前記複数の測定条件について前記検出した干渉縞の光強度分布に基づいて、前記被測定面の高さに応じた前記被測定面からの反射光の位相分布と前記干渉縞の光強度分布との関係に関する連立方程式を解くことにより、前記被測定面の高さを測定することを特徴とする干渉測定方法が提供される。 In order to achieve such an object, according to an embodiment of the present invention, two types of coherent light having different wavelengths that do not interfere with each other on a predetermined reference surface and a measurement target surface of a plane parallel plate that is a measurement object. A second step of detecting interference fringes formed based on the reference surface irradiated with the two types of coherent light and the reflected light from the surface to be measured, A third step of measuring the height of the surface to be measured based on the detected light intensity distribution of the interference fringes, and in the first step, on the opposite side of the parallel surface plate to the surface to be measured. to eliminate the influence of the reflected light from the surface, the _{k 1 = (2 × π)} / λ 1 and the wavelength of one of the two coherent light as lambda _1, k the other wavelength as lambda ₂ _{2 = (2 × π) /} λ 2 and then, the opposite surface to the surface to be measured When the optical distance between the d _on, any integer and is M, the following equation
(K ₂ × d _on ) = (k ₁ × d _on ) + π + (2 × π × M)
When the relationship between the different wavelengths is adjusted so as to satisfy the following relationship, the optical distance between the reference surface and the surface to be measured is d _ro , and an arbitrary integer is N, the following equation:
(K ₂ × d _ro ) = (k ₁ × d _ro ) + (2 × π × N)
In the state where the optical distance between the reference surface and the surface to be measured is adjusted so as to satisfy the relationship, the two types of coherent light are irradiated , the relationship between the different wavelengths, and the reference surface and the For a plurality of measurement conditions having different optical distances from the surface to be measured, the two types of coherent light irradiation in the first step and the detection of the interference fringes in the second step are performed. In the third step, the phase distribution of the reflected light from the surface to be measured and the interference according to the height of the surface to be measured based on the light intensity distribution of the detected interference fringes for the plurality of measurement conditions There is provided an interference measuring method characterized by measuring the height of the surface to be measured by solving simultaneous equations relating to the relationship with the light intensity distribution of the fringes .

また、本発明を例示する態様に従えば、所定の参照面および測定対象物である平行平面板の被測定面に、互いに干渉しない異なる波長を有する２種類の可干渉光を照射する照明部と、前記２種類の可干渉光が照射された前記参照面および前記被測定面からの反射光に基づいて形成される干渉縞を検出する検出部と、前記検出部により検出された干渉縞の光強度分布に基づいて前記被測定面の高さを測定する測定部とを備え、前記照明部は、前記平行平面板における前記被測定面と反対側の面からの反射光の影響を排除するように前記異なる波長同士の関係を調整する第１の調整部および、前記影響を排除するように前記参照面と前記被測定面との間の光学距離を調整する第２の調整部を有し、前記第１の調整部は、前記２種類の可干渉光のうち一方の波長をλ ₁ としてｋ ₁ ＝（２×π）／λ ₁ とし、他方の波長をλ ₂ としてｋ ₂ ＝（２×π）／λ ₂ とし、前記被測定面と前記反対側の面との間の光学距離をｄ _on とし、任意の整数をＭとしたとき、次式
（ｋ ₂ ×ｄ _on ）＝（ｋ ₁ ×ｄ _on ）＋π＋（２×π×Ｍ）
の関係を満たすように前記異なる波長同士の関係を調整し、前記第２の調整部は、前記参照面と前記被測定面との間の光学距離をｄ _ro とし、任意の整数をＮとしたとき、次式
（ｋ ₂ ×ｄ _ro ）＝（ｋ ₁ ×ｄ _ro ）＋（２×π×Ｎ）
の関係を満たすように前記参照面と前記被測定面との間の光学距離を調整し、前記異なる波長同士の関係および、前記参照面と前記被測定面との間の光学距離が相違する複数の測定条件について、前記照明部による前記２種類の可干渉光の照射と、前記検出部による前記干渉縞の検出とが行われ、前記測定部は、前記複数の測定条件について前記検出部により検出された干渉縞の光強度分布に基づいて、前記被測定面の高さに応じた前記被測定面からの反射光の位相分布と前記干渉縞の光強度分布との関係に関する連立方程式を解くことにより、前記被測定面の高さを測定することを特徴とする干渉計が提供される。 In addition, according to an embodiment illustrating the present invention, an illumination unit that irradiates two types of coherent light having different wavelengths that do not interfere with each other on a predetermined reference surface and a measurement target surface of a parallel flat plate that is a measurement object; , A detection unit for detecting interference fringes formed based on reflected light from the reference surface and the surface to be measured irradiated with the two types of coherent light, and light of the interference fringes detected by the detection unit A measurement unit that measures the height of the surface to be measured based on an intensity distribution, and the illumination unit eliminates the influence of reflected light from a surface opposite to the surface to be measured in the parallel flat plate. A first adjustment unit that adjusts the relationship between the different wavelengths, and a second adjustment unit that adjusts the optical distance between the reference surface and the surface to be measured so as to eliminate the influence , The first adjusting unit is configured to transmit the two types of coherent light beams. The one wavelength _{k 1 = (2 × π)} / λ 1 and then as _{_{λ 1, k 2 = (2}} × π) / λ 2 and the other wavelength as lambda _2, the surface of the measurement surface and the opposite side Where d _on is the optical distance between and an arbitrary integer is M,
(K ₂ × d _on ) = (k ₁ × d _on ) + π + (2 × π × M)
The relationship between the different wavelengths is adjusted so as to satisfy the relationship, and the second adjustment unit sets the optical distance between the reference surface and the surface to be measured as d _ro and an arbitrary integer as N. When
(K ₂ × d _ro ) = (k ₁ × d _ro ) + (2 × π × N)
The optical distance between the reference surface and the surface to be measured is adjusted so as to satisfy the relationship, and the relationship between the different wavelengths and the optical distance between the reference surface and the surface to be measured are different. With respect to the measurement conditions, irradiation of the two types of coherent light by the illumination unit and detection of the interference fringes by the detection unit are performed, and the measurement unit detects the plurality of measurement conditions by the detection unit Solving simultaneous equations related to the relationship between the phase distribution of reflected light from the surface to be measured and the light intensity distribution of the interference fringes according to the height of the surface to be measured, based on the light intensity distribution of the interference fringes Provides an interferometer that measures the height of the surface to be measured .

本発明によれば、平行平面板の被測定面に対して高精度な測定を行うことができる。 ADVANTAGE OF THE INVENTION According to this invention, a highly accurate measurement can be performed with respect to the to-be-measured surface of a plane parallel plate.

以下、図面を参照して本発明の実施形態について説明する。本実施形態の干渉計１は、図１に示すように、測定対象物である平行平面板５の表面形状を測定するためのものであり、平行平面板５を支持するステージ１０と、ステージ１０上の平行平面板５に可干渉光を照射する照明部２０と、干渉縞を検出する検出部３０と、平行平面板５の表面形状を求める演算処理部４０とを主体に構成される。 Hereinafter, embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, the interferometer 1 of the present embodiment is for measuring the surface shape of a plane parallel plate 5 that is a measurement object, and includes a stage 10 that supports the plane parallel plate 5, and a stage 10. The illumination unit 20 that irradiates coherent light to the upper plane-parallel plate 5, the detection unit 30 that detects interference fringes, and the arithmetic processing unit 40 that obtains the surface shape of the plane-parallel plate 5 are mainly configured.

平行平面板５は、ガラスや樹脂等の透明な材料を用いて板状に形成され、平面である平行平面板５の表面が本実施形態における被測定面５ａとなる。また、平行平面板５における被測定面５ａの反対側には、当該被測定面５ａと平行な平面である裏面５ｂが形成される。 The plane parallel plate 5 is formed in a plate shape using a transparent material such as glass or resin, and the surface of the plane parallel plane plate 5 is the surface to be measured 5a in this embodiment. In addition, a back surface 5b that is a plane parallel to the measured surface 5a is formed on the opposite side of the measured surface 5a in the parallel flat plate 5.

ステージ１０は、被測定面５ａが上方を向くように平行平面板５を水平に支持する。また、ステージ１０は、ステージ１０上の平行平面板５を照明部２０の光軸に沿って上下移動させることができるようになっている。また、ステージ１０と照明部２０との間には、透明の基準板６が取り付け機構１５を用いて照明部２０の光軸に沿って上下移動可能に水平に取り付けられおり、基準板６の表面に参照面６ａが形成される。 The stage 10 supports the parallel flat plate 5 horizontally so that the surface to be measured 5a faces upward. Further, the stage 10 can move the parallel flat plate 5 on the stage 10 up and down along the optical axis of the illumination unit 20. A transparent reference plate 6 is horizontally mounted between the stage 10 and the illumination unit 20 so as to be movable up and down along the optical axis of the illumination unit 20 by using an attachment mechanism 15. A reference surface 6a is formed.

照明部２０は、光源側から順に、第１の光源２１と、第１のビームエキスパンダー２２と、ミラー２３と、偏光ビームスプリッター２６と、ビームスプリッター２７とを有して構成される。第１の光源２１として周波数安定化レーザーが用いられ、第１の光源２１は、波長が一定の可干渉光（レーザー光）を発光する。第１の光源２１から発せられた第１の可干渉光は、第１のビームエキスパンダー２２によって平行光束に広げられ、ミラー２３で反射して偏光ビームスプリッター２６を透過する。偏光ビームスプリッター２６を透過した第１の可干渉光は、ｐ偏光となってビームスプリッター２７を透過し、参照面６ａに達する。 The illumination unit 20 includes, in order from the light source side, a first light source 21, a first beam expander 22, a mirror 23, a polarization beam splitter 26, and a beam splitter 27. A frequency stabilized laser is used as the first light source 21, and the first light source 21 emits coherent light (laser light) having a constant wavelength. The first coherent light emitted from the first light source 21 is spread into a parallel light beam by the first beam expander 22, reflected by the mirror 23, and transmitted through the polarization beam splitter 26. The first coherent light that has passed through the polarization beam splitter 26 becomes p-polarized light, passes through the beam splitter 27, and reaches the reference surface 6a.

また、照明部２０は、第２の光源２４および、第２のビームエキスパンダー２５を有しており、第２の光源２４として波長可変レーザーが用いられ、第２の光源２４が発光する可干渉光（レーザー光）の波長を変えることができるようになっている。第２の光源２４から発せられた第２の可干渉光は、第２のビームエキスパンダー２５によって平行光束に広げられ、偏光ビームスプリッター２６で反射する。偏光ビームスプリッター２６で反射した第２の可干渉光は、ｓ偏光となってビームスプリッター２７を透過し、参照面６ａに達する。 The illumination unit 20 includes a second light source 24 and a second beam expander 25. A wavelength variable laser is used as the second light source 24, and coherent light emitted from the second light source 24 is used. The wavelength of (laser light) can be changed. The second coherent light emitted from the second light source 24 is spread into a parallel light beam by the second beam expander 25 and reflected by the polarization beam splitter 26. The second coherent light reflected by the polarization beam splitter 26 becomes s-polarized light, passes through the beam splitter 27, and reaches the reference surface 6a.

このとき、参照面６ａに達する第１の可干渉光はｐ偏光であり、第２の可干渉光はｓ偏光であるため、第１の可干渉光および第２の可干渉光は互いに干渉しない。このように、照明部２０は、互いに干渉しない異なる波長を有する２種類の可干渉光を参照面６ａおよび被測定面５ａに照射することが可能である。 At this time, since the first coherent light reaching the reference surface 6a is p-polarized light and the second coherent light is s-polarized light, the first coherent light and the second coherent light do not interfere with each other. . Thus, the illuminating unit 20 can irradiate the reference surface 6a and the measured surface 5a with two types of coherent light having different wavelengths that do not interfere with each other.

ところで、参照面６ａに達した第１または第２の可干渉光の一部は、参照面６ａで反射する。参照面６ａで反射した光は、ビームスプリッター２７で反射して検出部３０へ導かれる。一方、参照面６ａに達した第１または第２の可干渉光の残りは、参照面６ａを透過し、平行平面板５の被測定面５ａで反射する。被測定面５ａで反射した光は、再び参照面６ａを透過し、ビームスプリッター２７で反射して検出部３０へ導かれる。 Incidentally, a part of the first or second coherent light reaching the reference surface 6a is reflected by the reference surface 6a. The light reflected by the reference surface 6 a is reflected by the beam splitter 27 and guided to the detection unit 30. On the other hand, the remainder of the first or second coherent light that has reached the reference surface 6 a is transmitted through the reference surface 6 a and reflected by the surface to be measured 5 a of the parallel flat plate 5. The light reflected by the surface to be measured 5 a passes through the reference surface 6 a again, is reflected by the beam splitter 27, and is guided to the detection unit 30.

検出部３０は、物体側（ビームスプリッター２７側）から順に、結像レンズ３１と、イメージセンサー３２とを有して構成される。ビームスプリッター２７で反射して検出部３０へ導かれた光は、結像レンズ３１を透過してイメージセンサー３２に達し、イメージセンサー３２の検出面上に、参照面６ａからの反射光と被測定面５ａからの反射光とが互いに干渉して複雑な干渉縞が形成される。イメージセンサー３２は、このように形成された干渉縞を検出し、その検出信号を演算処理部４０へ出力する。演算処理部４０は、イメージセンサー３２から入力された検出信号の信号強度分布、すなわち、イメージセンサー３２により検出された干渉縞の光強度分布に基づいて、被測定面５ａの高さを測定する（詳細は後述する）。 The detection unit 30 includes an imaging lens 31 and an image sensor 32 in order from the object side (beam splitter 27 side). The light reflected by the beam splitter 27 and guided to the detection unit 30 passes through the imaging lens 31 and reaches the image sensor 32. On the detection surface of the image sensor 32, the reflected light from the reference surface 6a and the measured light are measured. The reflected light from the surface 5a interferes with each other to form complex interference fringes. The image sensor 32 detects the interference fringes formed in this way, and outputs the detection signal to the arithmetic processing unit 40. The arithmetic processing unit 40 measures the height of the measured surface 5a based on the signal intensity distribution of the detection signal input from the image sensor 32, that is, the light intensity distribution of the interference fringes detected by the image sensor 32 ( Details will be described later).

以上のように構成される干渉計１を用いた干渉測定方法について、図２に示すフローチャートを参照しながら説明する。まず、照明部２０により、参照面６ａおよび被測定面５ａに、互いに干渉しない異なる波長を有する第１および第２の可干渉光を照射する（ステップＳ１０１）。この照明工程において、第１の光源２１から発せられた第１の可干渉光は、第１のビームエキスパンダー２２によって平行光束に広げられ、ミラー２３で反射して偏光ビームスプリッター２６を透過する。偏光ビームスプリッター２６を透過した第１の可干渉光は、ｐ偏光となってビームスプリッター２７を透過し、参照面６ａに達する。一方、第２の光源２４から発せられた第２の可干渉光は、第２のビームエキスパンダー２５によって平行光束に広げられ、偏光ビームスプリッター２６で反射する。偏光ビームスプリッター２６で反射した第２の可干渉光は、ｓ偏光となってビームスプリッター２７を透過し、参照面６ａに達する。 An interference measurement method using the interferometer 1 configured as described above will be described with reference to the flowchart shown in FIG. First, the illumination unit 20 irradiates the reference surface 6a and the surface to be measured 5a with first and second coherent lights having different wavelengths that do not interfere with each other (step S101). In this illumination process, the first coherent light emitted from the first light source 21 is spread into a parallel light beam by the first beam expander 22, reflected by the mirror 23, and transmitted through the polarization beam splitter 26. The first coherent light that has passed through the polarization beam splitter 26 becomes p-polarized light, passes through the beam splitter 27, and reaches the reference surface 6a. On the other hand, the second coherent light emitted from the second light source 24 is spread into a parallel light beam by the second beam expander 25 and reflected by the polarization beam splitter 26. The second coherent light reflected by the polarization beam splitter 26 becomes s-polarized light, passes through the beam splitter 27, and reaches the reference surface 6a.

このようにして参照面６ａに達した第１および第２の可干渉光の一部は、参照面６ａで反射する。参照面６ａで反射した光は、ビームスプリッター２７で反射して検出部３０へ導かれる。一方、参照面６ａに達した第１および第２の可干渉光の残りは、参照面６ａを透過し、平行平面板５の被測定面５ａで反射する。被測定面５ａで反射した光は、再び参照面６ａを透過し、ビームスプリッター２７で反射して検出部３０へ導かれる。 Part of the first and second coherent light that has reached the reference surface 6a in this way is reflected by the reference surface 6a. The light reflected by the reference surface 6 a is reflected by the beam splitter 27 and guided to the detection unit 30. On the other hand, the remainder of the first and second coherent light that has reached the reference surface 6 a is transmitted through the reference surface 6 a and reflected by the surface to be measured 5 a of the parallel flat plate 5. The light reflected by the surface to be measured 5 a passes through the reference surface 6 a again, is reflected by the beam splitter 27, and is guided to the detection unit 30.

そこで、検出部３０により、第１および第２の可干渉光が照射された参照面６ａおよび被測定面５ａからの反射光に基づいて形成される干渉縞を検出する（ステップＳ１０２）。この検出工程において、ビームスプリッター２７で反射して検出部３０へ導かれた光は、結像レンズ３１を透過してイメージセンサー３２に達し、イメージセンサー３２の検出面上に、参照面６ａからの反射光（以下、参照光と称する）と被測定面５ａからの反射光（以下、測定光と称する）とが互いに干渉して複雑な干渉縞が形成される。イメージセンサー３２は、このように形成された干渉縞を検出し、その検出信号を演算処理部４０へ出力する。 Therefore, the detection unit 30 detects interference fringes formed on the basis of the reflected light from the reference surface 6a and the measured surface 5a irradiated with the first and second coherent light (step S102). In this detection step, the light reflected by the beam splitter 27 and guided to the detection unit 30 passes through the imaging lens 31 and reaches the image sensor 32, and the light from the reference surface 6 a is detected on the detection surface of the image sensor 32. Reflected light (hereinafter referred to as reference light) and reflected light from the surface 5a to be measured (hereinafter referred to as measurement light) interfere with each other to form complex interference fringes. The image sensor 32 detects the interference fringes formed in this way, and outputs the detection signal to the arithmetic processing unit 40.

ところで、平行平面板５に達した第１および第２の可干渉光は、前述したように被測定面５ａで反射するが、全てが被測定面５ａで反射するわけではなく、一部は被測定面５ａ（平行平面板５）を透過して平行平面板５の裏面５ｂで反射する。平行平面板５の裏面５ｂで反射したノイズ光は、再び被測定面５ａおよび参照面６ａを透過し、ビームスプリッター２７で反射して検出部３０へ導かれ、結像レンズ３１を透過してイメージセンサー３２に達する。そのため、イメージセンサー３２の検出面上においては、参照光および測定光に加えてノイズ光も干渉し、これらが干渉して形成される干渉縞から測定光の波面と参照光の波面との差を検出するのは通常不可能である。 Incidentally, the first and second coherent lights that have reached the plane-parallel plate 5 are reflected by the surface to be measured 5a as described above, but not all of them are reflected by the surface to be measured 5a. The light passes through the measurement surface 5a (parallel flat plate 5) and is reflected by the back surface 5b of the parallel flat plate 5. The noise light reflected by the back surface 5b of the plane-parallel plate 5 is transmitted again through the measurement target surface 5a and the reference surface 6a, reflected by the beam splitter 27, guided to the detection unit 30, and transmitted through the imaging lens 31 to form an image. The sensor 32 is reached. Therefore, on the detection surface of the image sensor 32, noise light interferes in addition to the reference light and the measurement light, and the difference between the wavefront of the measurement light and the reference light is determined from the interference fringes formed by the interference. It is usually impossible to detect.

ただし、波長可変レーザーを用いた第２の光源２４から発光される第２の可干渉光の波長を、平行平面板５の被測定面５ａと裏面５ｂとの間隔に対応して正しく選べば、ノイズ光の影響を排除することができる。ここで、その原理を説明する。まず、被測定面５ａに対応する平面座標を（ｘ，ｙ）とし、参照光の波面の複素振幅分布をＥ_r（ｘ，ｙ）とし、測定光の波面の複素振幅分布をＥ_o（ｘ，ｙ）とし、ノイズ光の波面の複素振幅分布をＥ_n（ｘ，ｙ）としたとき、第１の可干渉光によって形成された干渉縞の光強度分布Ｉ₁（ｘ，ｙ）は、次の（１）式のように表わされる。 However, if the wavelength of the second coherent light emitted from the second light source 24 using the wavelength tunable laser is correctly selected corresponding to the distance between the measured surface 5a and the back surface 5b of the parallel flat plate 5, The influence of noise light can be eliminated. Here, the principle will be described. First, the plane coordinate corresponding to the surface to be measured 5a is set to (x, y), the complex amplitude distribution of the wavefront of the reference light is E _r (x, y), and the complex amplitude distribution of the wavefront of the measurement light is E _o (x , Y) and the complex amplitude distribution of the wavefront of the noise light is E _n (x, y), the light intensity distribution I ₁ (x, y) of the interference fringes formed by the first coherent light is It is expressed as the following equation (1).

なお、＊は複素共役であることを示す。また、Ｉ_r（ｘ，ｙ）＝Ｅ_r ^*（ｘ，ｙ）×Ｅ_r（ｘ，ｙ）であり、Ｉ_o（ｘ，ｙ）＝Ｅ_o ^*（ｘ，ｙ）×Ｅ_o（ｘ，ｙ）であり、Ｉ_n（ｘ，ｙ）＝Ｅ_n ^*（ｘ，ｙ）×Ｅ_n（ｘ，ｙ）であるので、（１）式は、次の（２）式のように表わされる。 Note that * indicates a complex conjugate. Further, I _r (x, y) = E _r ^* (x, y) × E _r (x, y), and I _o (x, y) = E _o ^* (x, y) × E _o (x , Y) and I _n (x, y) = E _n ^* (x, y) × E _n (x, y), the expression (1) is expressed as the following expression (2). It is.

さらに、平行平面板５における被測定面５ａと裏面５ｂとの間の光学距離をｄ_onとし、参照面６ａと被測定面５ａとの間の光学距離をｄ_roとし、第１の可干渉光の波長をλ₁としてその波数をｋ₁＝（２×π）／λ₁とすると、複素振幅と波数の関係から、（２）式は、次の（３）式のように表わされる。 Further, the optical distance between the measured surface 5a and the back surface 5b of the parallel flat plate 5 is d _on , the optical distance between the reference surface 6a and the measured surface 5a is d _ro, and the first coherent light. If the wavelength of λ ₁ is λ ₁ and the wave number is k ₁ = (2 × π) / λ ₁ , Equation (2) is expressed as the following Equation (3) from the relationship between the complex amplitude and the wave number.

なお、ψ_r（ｘ，ｙ）、ψ_o（ｘ，ｙ）、およびψ_n（ｘ，ｙ）はそれぞれ、参照光、測定光、およびノイズ光の位相分布である。また、Ｉ_ro（ｘ，ｙ）＝｜Ｅ_r｜×｜Ｅ_o｜であり、Ｉ_rn（ｘ，ｙ）＝｜Ｅ_r｜×｜Ｅ_n｜であり、Ｉ_on（ｘ，ｙ）＝｜Ｅ_o｜×｜Ｅ_n｜である。 Ψ _r (x, y), ψ _o (x, y), and ψ _n (x, y) are phase distributions of the reference light, the measurement light, and the noise light, respectively. Further, I _ro (x, y) = | E _r | × | E _o |, I _rn (x, y) = | E _r | × | E _n |, and I _on (x, y) = | E _o | × | E _n |.

また、第２の可干渉光の波長をλ₂としてその波数をｋ₂＝（２×π）／λ₂とすると、第１の可干渉光の場合と同様にして、第２の可干渉光によって形成された干渉縞の光強度分布Ｉ₂（ｘ，ｙ）は、次の（４）式のように表わされる。 If the wavelength of the _second coherent light is λ ₂ and its wave number is k ₂ = (2 × π) / λ ₂ , the _second coherent light is the same as in the case of the first coherent light. The light intensity distribution I ₂ (x, y) of the interference fringes formed by is expressed by the following equation (4).

ここで、波長可変レーザーを用いた第２の光源２４を操作して、第２の可干渉光の波長を次の（５）式を満足するように調整する。 Here, the second light source 24 using the wavelength tunable laser is operated to adjust the wavelength of the second coherent light so as to satisfy the following expression (5).

さらに、取り付け機構１５等を用いて、参照面６ａが形成された基準板６を駆動し、参照面６ａの位置を次の（６）式を満足するように調整する。 Further, the reference plate 6 on which the reference surface 6a is formed is driven using the attachment mechanism 15 or the like, and the position of the reference surface 6a is adjusted so as to satisfy the following expression (6).

そうすると、（４）〜（６）式より、第２の可干渉光によって形成された干渉縞の光強度分布Ｉ₂（ｘ，ｙ）は、次の（７）式のように表わされる。 Then, from the equations (4) to (6), the light intensity distribution I ₂ (x, y) of the interference fringes formed by the second coherent light is expressed as the following equation (7).

（３）式および（７）式より、第１の可干渉光によって形成された干渉縞の光強度分布Ｉ₁（ｘ，ｙ）と第２の可干渉光によって形成された干渉縞の光強度分布Ｉ₂（ｘ，ｙ）との和は、次の（８）式のように表わされる。 From the expressions (3) and (7), the light intensity distribution I ₁ (x, y) of the interference fringes formed by the first coherent light and the light intensity of the interference fringes formed by the second coherent light. The sum with the distribution I ₂ (x, y) is expressed by the following equation (8).

（８）式によれば、ノイズ光に関する項がなくなるため、ノイズ光の影響を排除することができる。なお、（８）式において、Ｉ_n（ｘ，ｙ）の項が残るが、これは被測定面５ａによって変化しない成分であり、測定に影響を及ぼさない。そこで、照明工程において、（５）式および（６）式の条件を満足する状態で第１および第２の可干渉光を照射し、検出工程において、第１および第２の可干渉光によって生じる干渉縞をイメージセンサー３２により検出する。このとき、第１および第２の可干渉光の光量を互いに同じ光量にしておくことが好ましい。 According to the equation (8), since there is no term relating to noise light, the influence of noise light can be eliminated. In the equation (8), the term I _n (x, y) remains, but this is a component that does not change depending on the surface to be measured 5a, and does not affect the measurement. Therefore, in the illumination process, the first and second coherent lights are irradiated in a state satisfying the conditions of the expressions (5) and (6), and the first and second coherent lights are generated in the detection process. The interference fringes are detected by the image sensor 32. At this time, it is preferable that the first and second coherent light beams have the same light amount.

そして、演算処理部４０は、イメージセンサー３２から入力された検出信号の信号強度分布、すなわち、イメージセンサー３２により検出された第１および第２の可干渉光によって生じる干渉縞の光強度分布（Ｉ₁（ｘ，ｙ）+Ｉ₂（ｘ，ｙ））に基づいて、被測定面５ａの高さを測定する（ステップＳ１０３）。なお、（８）式において未知の変数が複数あるため、この測定工程において、演算処理部４０は、例えば（５）式および（６）式の条件を満足する複数の測定条件において検出した干渉縞の光強度分布に基づく（８）式に関する連立方程式を解くことにより、ψ_o（ｘ，ｙ）を算出し、算出したψ_o（ｘ，ｙ）から被測定面５ａの高さを求める。 The arithmetic processing unit 40 then detects the signal intensity distribution of the detection signal input from the image sensor 32, that is, the light intensity distribution (I of interference fringes generated by the first and second coherent light detected by the image sensor 32. _{Based on 1} (x, y) + I ₂ (x, y)), the height of the measured surface 5a is measured (step S103). Since there are a plurality of unknown variables in equation (8), in this measurement step, the arithmetic processing unit 40 detects interference fringes detected under a plurality of measurement conditions that satisfy the conditions of equations (5) and (6), for example. Ψ _o (x, y) is calculated by solving simultaneous equations related to the equation (8) based on the light intensity distribution, and the height of the measured surface 5a is obtained from the calculated ψ _o (x, y).

このように、本実施形態の干渉計１および干渉測定方法によれば、平行平面板５における被測定面５ａと反対側の面５ｂからの反射光（ノイズ光）の影響を排除するように、第１および第２の可干渉光における波長の関係および参照面６ａと被測定面５ａとの間の光学距離を調整するため、ノイズ光による干渉を抑えることが可能になり、平行平面板５の被測定面５ａに対して高精度な測定を行うことができる。 Thus, according to the interferometer 1 and the interference measurement method of the present embodiment, the influence of the reflected light (noise light) from the surface 5b opposite to the surface to be measured 5a in the parallel flat plate 5 is excluded. Since the wavelength relationship in the first and second coherent light and the optical distance between the reference surface 6a and the surface to be measured 5a are adjusted, it is possible to suppress interference due to noise light. High-precision measurement can be performed on the surface to be measured 5a.

このとき、（５）式および（６）式の条件を満足するように調整を行えば、確実にノイズ光の影響を排除することができる。 At this time, if the adjustment is performed so as to satisfy the conditions of the expressions (5) and (6), the influence of the noise light can be surely eliminated.

また、第１の可干渉光を周波数安定化レーザーにより照射するとともに、第２の可干渉光を波長可変レーザーにより照射し、波長可変レーザーを用いて第１および第２の可干渉光における波長の関係を調整することで、２つの可干渉光のうち一方の波長を安定化することが可能になり、平行平面板５の被測定面５ａに対してより高精度な測定を行うことができる。 In addition, the first coherent light is irradiated by the frequency stabilized laser, the second coherent light is irradiated by the wavelength tunable laser, and the wavelength of the first and second coherent light is changed using the wavelength tunable laser. By adjusting the relationship, it becomes possible to stabilize one wavelength of the two coherent lights, and it is possible to perform measurement with higher accuracy on the surface to be measured 5a of the plane parallel plate 5.

なお、上述の実施形態において、第１および第２の可干渉光によって生じる干渉縞を１つのイメージセンサー３２により同時に検出しているが、これに限られるものではく、例えば、第１の可干渉光によって生じる干渉縞および第２の可干渉光によって生じる干渉縞をそれぞれ、２つのイメージセンサーにより検出するか、１つのイメージセンサーにより異なるタイミングで検出し、それぞれの検出信号を合成するようにしてもよい。なおこのとき、第１および第２の可干渉光の光量が互いに異なっている場合には、検出信号の合成時に第１および第２の可干渉光の光量が同じ場合となるように補正することも可能である。 In the above-described embodiment, the interference fringes generated by the first and second coherent light are detected simultaneously by one image sensor 32. However, the present invention is not limited to this. The interference fringes generated by the light and the interference fringes generated by the second coherent light are detected by two image sensors or detected by different timings by one image sensor, and the respective detection signals are synthesized. Good. At this time, if the light amounts of the first and second coherent lights are different from each other, correction is made so that the light amounts of the first and second coherent lights are the same when the detection signals are combined. Is also possible.

また、上述の実施形態において、２つの光源を用いて第１および第２の可干渉光を照射しているが、これに限られるものではく、１つの光源から第１および第２の可干渉光を抽出してそれぞれ照射するようにしてもよい。 In the above-described embodiment, the first and second coherent lights are emitted using two light sources. However, the present invention is not limited to this, and the first and second coherent lights are emitted from one light source. Light may be extracted and irradiated.

干渉計の概略構成図である。It is a schematic block diagram of an interferometer. 干渉測定方法を示すフローチャートである。It is a flowchart which shows the interference measuring method.

符号の説明Explanation of symbols

１干渉計
５平行平面板（５ａ被測定面、５ｂ裏面）
６基準板（６ａ参照面）
１５取り付け機構（第２の調整部）
２０照明部
２１第１の光源（周波数安定化レーザー）
２４第２の光源（波長可変レーザーおよび第１の調整部）
３０検出部３２イメージセンサー
４０演算処理部 1 Interferometer 5 Parallel plane plate (5a surface to be measured, 5b back surface)
6 Reference plate (reference surface 6a)
15 Mounting mechanism (second adjusting part)
20 Illumination unit 21 First light source (frequency stabilized laser)
24 2nd light source (wavelength variable laser and 1st adjustment part)
30 Detection Unit 32 Image Sensor 40 Arithmetic Processing Unit

Claims

所定の参照面および測定対象物である平行平面板の被測定面に、互いに干渉しない異なる波長を有する２種類の可干渉光を照射する第１のステップと、
前記２種類の可干渉光が照射された前記参照面および前記被測定面からの反射光に基づいて形成される干渉縞を検出する第２のステップと、
前記検出した干渉縞の光強度分布に基づいて前記被測定面の高さを測定する第３のステップとを有し、
前記第１のステップにおいて、前記平行平面板における前記被測定面と反対側の面からの反射光の影響を排除するように、
前記２種類の可干渉光のうち一方の波長をλ ₁ としてｋ ₁ ＝（２×π）／λ ₁ とし、他方の波長をλ ₂ としてｋ ₂ ＝（２×π）／λ ₂ とし、前記被測定面と前記反対側の面との間の光学距離をｄ _on とし、任意の整数をＭとしたとき、次式
（ｋ ₂ ×ｄ _on ）＝（ｋ ₁ ×ｄ _on ）＋π＋（２×π×Ｍ）
の関係を満たすように前記異なる波長同士の関係を調整するとともに、
前記参照面と前記被測定面との間の光学距離をｄ _ro とし、任意の整数をＮとしたとき、次式
（ｋ ₂ ×ｄ _ro ）＝（ｋ ₁ ×ｄ _ro ）＋（２×π×Ｎ）
の関係を満たすように前記参照面と前記被測定面との間の光学距離を調整した状態で、前記２種類の可干渉光を照射し、
前記異なる波長同士の関係および、前記参照面と前記被測定面との間の光学距離が相違する複数の測定条件について、前記第１のステップにおける前記２種類の可干渉光の照射と、前記第２のステップにおける前記干渉縞の検出とを行い、
前記第３のステップにおいて、前記複数の測定条件について前記検出した干渉縞の光強度分布に基づいて、前記被測定面の高さに応じた前記被測定面からの反射光の位相分布と前記干渉縞の光強度分布との関係に関する連立方程式を解くことにより、前記被測定面の高さを測定することを特徴とする干渉測定方法。 A first step of irradiating two kinds of coherent light having different wavelengths that do not interfere with each other on a predetermined reference surface and a measurement target surface of a plane parallel plate that is a measurement object;
A second step of detecting interference fringes formed on the basis of the reflected light from the reference surface and the surface to be measured irradiated with the two types of coherent light;
A third step of measuring the height of the surface to be measured based on the light intensity distribution of the detected interference fringes,
In the first step, so as to eliminate the influence of reflected light from the surface opposite to the surface to be measured in the plane parallel plate ,
The two wavelengths of one of the coherent light is _{k 1 = (2 × π)} / λ 1 as _{_{λ 1, k 2 = (2}} × π) / λ 2 and the other wavelength as lambda _2, wherein When the optical distance between the surface to be measured and the opposite surface is d _on and an arbitrary integer is M, the following equation:
(K ₂ × d _on ) = (k ₁ × d _on ) + π + (2 × π × M)
While adjusting the relationship between the different wavelengths so as to satisfy the relationship ,
When the optical distance between the reference surface and the surface to be measured is d _ro and an arbitrary integer is N, the following formula
(K ₂ × d _ro ) = (k ₁ × d _ro ) + (2 × π × N)
In the state where the optical distance between the reference surface and the surface to be measured is adjusted so as to satisfy the relationship, the two types of coherent light are irradiated ,
For the plurality of measurement conditions in which the relationship between the different wavelengths and the optical distance between the reference surface and the surface to be measured are different, the two types of coherent light irradiation in the first step, and the first Detecting the interference fringes in step 2,
In the third step, the phase distribution of the reflected light from the surface to be measured and the interference according to the height of the surface to be measured based on the light intensity distribution of the detected interference fringes for the plurality of measurement conditions An interference measurement method , comprising: measuring a height of the surface to be measured by solving simultaneous equations relating to a light intensity distribution of stripes .

前記第１のステップにおいて、前記２種類の可干渉光のうち一方を周波数安定化レーザーにより照射するとともに、他方を波長可変レーザーにより照射し、
前記波長可変レーザーを用いて前記異なる波長同士の関係を調整することを特徴とする請求項１に記載の干渉測定方法。 In the first step, one of the two types of coherent light is irradiated with a frequency stabilized laser, and the other is irradiated with a wavelength tunable laser,
The interference measurement method according to claim 1 , wherein the relationship between the different wavelengths is adjusted using the wavelength tunable laser.

所定の参照面および測定対象物である平行平面板の被測定面に、互いに干渉しない異なる波長を有する２種類の可干渉光を照射する照明部と、
前記２種類の可干渉光が照射された前記参照面および前記被測定面からの反射光に基づいて形成される干渉縞を検出する検出部と、
前記検出部により検出された干渉縞の光強度分布に基づいて前記被測定面の高さを測定する測定部とを備え、
前記照明部は、前記平行平面板における前記被測定面と反対側の面からの反射光の影響を排除するように前記異なる波長同士の関係を調整する第１の調整部および、前記影響を排除するように前記参照面と前記被測定面との間の光学距離を調整する第２の調整部を有し、
前記第１の調整部は、前記２種類の可干渉光のうち一方の波長をλ ₁ としてｋ ₁ ＝（２×π）／λ ₁ とし、他方の波長をλ ₂ としてｋ ₂ ＝（２×π）／λ ₂ とし、前記被測定面と前記反対側の面との間の光学距離をｄ _on とし、任意の整数をＭとしたとき、次式
（ｋ ₂ ×ｄ _on ）＝（ｋ ₁ ×ｄ _on ）＋π＋（２×π×Ｍ）
の関係を満たすように前記異なる波長同士の関係を調整し、
前記第２の調整部は、前記参照面と前記被測定面との間の光学距離をｄ _ro とし、任意の整数をＮとしたとき、次式
（ｋ ₂ ×ｄ _ro ）＝（ｋ ₁ ×ｄ _ro ）＋（２×π×Ｎ）
の関係を満たすように前記参照面と前記被測定面との間の光学距離を調整し、
前記異なる波長同士の関係および、前記参照面と前記被測定面との間の光学距離が相違する複数の測定条件について、前記照明部による前記２種類の可干渉光の照射と、前記検出部による前記干渉縞の検出とが行われ、
前記測定部は、前記複数の測定条件について前記検出部により検出された干渉縞の光強度分布に基づいて、前記被測定面の高さに応じた前記被測定面からの反射光の位相分布と前記干渉縞の光強度分布との関係に関する連立方程式を解くことにより、前記被測定面の高さを測定することを特徴とする干渉計。 An illumination unit that irradiates two kinds of coherent light having different wavelengths that do not interfere with each other on a predetermined reference surface and a measurement target surface of a plane parallel plate that is a measurement object;
A detection unit that detects interference fringes formed based on reflected light from the reference surface and the measurement target surface irradiated with the two types of coherent light;
A measurement unit that measures the height of the surface to be measured based on the light intensity distribution of the interference fringes detected by the detection unit;
The illumination unit eliminates the influence by a first adjustment unit that adjusts the relationship between the different wavelengths so as to eliminate the influence of reflected light from the surface opposite to the surface to be measured in the plane parallel plate. A second adjusting unit for adjusting an optical distance between the reference surface and the surface to be measured ,
The first adjusting unit sets one wavelength of the two types of coherent light to λ ₁ and sets k ₁ = (2 × π) / λ _1, and sets the other wavelength to λ ₂ and k ₂ = (2 × When π) / λ ₂ , the optical distance between the measured surface and the opposite surface is d _on , and an arbitrary integer is M, the following equation:
(K ₂ × d _on ) = (k ₁ × d _on ) + π + (2 × π × M)
Adjusting the relationship between the different wavelengths so as to satisfy the relationship
When the optical distance between the reference surface and the surface to be measured is d _ro and an arbitrary integer is N, the second adjustment unit has the following formula:
(K ₂ × d _ro ) = (k ₁ × d _ro ) + (2 × π × N)
Adjusting the optical distance between the reference surface and the surface to be measured so as to satisfy the relationship:
For the plurality of measurement conditions in which the relationship between the different wavelengths and the optical distance between the reference surface and the surface to be measured are different, the illumination unit emits the two types of coherent light, and the detection unit. Detection of the interference fringes is performed;
The measurement unit, based on a light intensity distribution of interference fringes detected by the detection unit for the plurality of measurement conditions, and a phase distribution of reflected light from the measurement surface according to the height of the measurement surface An interferometer that measures the height of the surface to be measured by solving simultaneous equations related to the relationship with the light intensity distribution of the interference fringes .

前記照明部は、前記２種類の可干渉光の光源として、前記２種類の可干渉光のうち一方を照射する周波数安定化レーザーと、他方を照射する波長可変レーザーとを有し、
前記第１の調整部は、前記波長可変レーザーを利用して前記異なる波長同士の関係を調整することを特徴とする請求項３に記載の干渉計。 The illumination unit includes a frequency stabilization laser that irradiates one of the two types of coherent light as a light source of the two types of coherent light, and a wavelength tunable laser that irradiates the other.
The interferometer according to claim 3 , wherein the first adjustment unit adjusts a relationship between the different wavelengths by using the wavelength tunable laser.