JP2009053148A - Multi-wavelength interferometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-wavelength interferometer capable of measuring at high precision with accurately specifying a composed wavelength determined by a plurality of different wavelengths by measuring wavelength difference among a plurality of different wavelengths with a simple constitution. <P>SOLUTION: Multi-wavelength interference measurement is performed using emitted light having different wavelengths λ<SB>1</SB>, λ<SB>2</SB>(λ<SB>1</SB>>λ<SB>2</SB>). A reflection member 20 for measuring wavelength is provided with a reflection face 20A and 20B which are arranged mutually separately at a light path length 1a. The light reflected by a beam splitter 19 and reflected by each of the reflection faces 20A and 20B reaches a photographing device 21 to form interference fringe. By acquiring these interference fringe images using different wavelengths λ<SB>1</SB>, λ<SB>2</SB>respectively, the phase of each interference fringe is computed. Using these computed phases, the composed wavelength Λ is computed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a multiwavelength interferometer that performs interference measurement of an object to be measured using light of a plurality of different wavelengths.

  An interferometer is known as an apparatus for measuring a planar shape or the like with high accuracy. The interferometer generates interference fringes by synthesizing the reflected light (reference light) from the reference surface serving as a reference and the reflected light (measurement light) from the object to be measured. It measures the relative shape of the object to be measured. However, what can be uniquely obtained by interference measurement is a fractional component of less than one wavelength excluding an integral multiple component (stripe order) of the wavelength of the measurement light. The fringe order portion can be obtained by a method such as phase unwrapping. However, if there is a discontinuous step (step between adjacent pixels of the image sensor) that is more than half the wavelength of the measurement light on the measurement surface, it is impossible to determine the height difference at that discontinuous portion. There is a problem that the measurement cannot be performed.

As a solution to this problem, there is known a method (hereinafter referred to as “multi-wavelength interferometry”) in which interference measurement is performed using a plurality of different wavelengths as measurement light, and the phases of interference fringes measured by the respective wavelengths are synthesized. (For example, refer to Patent Document 1). For example, when two different wavelengths λ ₁ and λ ₂ (λ ₁ > λ ₂ ) are used, it is possible to measure discontinuous steps up to half of the combined wavelength Λ = λ ₁ × λ ₂ / (λ ₁ −λ ₂ ). Thus, the measurement range can be made larger than that of normal interference measurement. For example, when λ ₁ = 633 nm and λ ₂ = 632 nm, the shape of an object to be measured having a discontinuous step of about 0.2 mm can be measured, and the measurement range is 300 times or more that of interference measurement using a single wavelength. Can be obtained. The smaller the difference between λ ₁ and λ ₂ , the larger the measurement range can be obtained.

In the multi-wavelength interferometry, if the wavelength difference (λ ₁ −λ ₂ ) deviates from the set value, the combined wavelength Λ changes and affects the measurement accuracy, so the wavelength difference (λ ₁ −λ ₂ ) It is necessary to control so as not to shift. In particular, when the wavelength difference (λ ₁ −λ ₂ ) is set to be small in order to increase the measurement range, an error from the set value of the wavelength difference (λ ₁ −λ ₂ ) greatly affects the measurement accuracy.
JP 7-181005 A

  The present invention measures the wavelength difference between a plurality of different wavelengths with a simple configuration and accurately specifies a synthetic wavelength determined by the plurality of different wavelengths, thereby enabling multi-wavelength interference that enables highly accurate measurement. The purpose is to provide a total.

  In order to achieve the above object, an interferometer according to the present invention divides light emitted from the light source into measurement light and reference light, which is configured to emit light having a plurality of different wavelengths. A light splitting / combining member that combines the reference light reflected by the reference surface and the measurement light reflected by the object to be measured to be combined light, and an imaging unit that captures interference fringes by the combined light, In a multi-wavelength interferometer that calculates the shape of the object to be measured based on the phase of interference fringes obtained from each of outgoing lights having different wavelengths and the combined wavelength of the plurality of different wavelengths, the optical path length difference is known. A plurality of interference fringes obtained by causing a reflection member having one reflection surface and a second reflection surface, and a plurality of interference fringes obtained by causing the plurality of outgoing lights having different wavelengths to enter the reflection member, respectively, Based on the phase of the interference fringes Characterized by comprising a calculation unit for calculating a wavelength.

  According to the present invention, the wavelength difference between a plurality of different wavelengths can be measured with a simple configuration, and the combined wavelength determined by the plurality of different wavelengths can be accurately specified, enabling high-precision measurement. A multi-wavelength interferometer can be provided.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  1ST EMBODIMENT FIG. 1: has shown the whole structure of the multiwavelength interferometer which concerns on the 1st Embodiment of this invention. This multiwavelength interferometer includes a laser light source 11, a collimating optical system 12, a beam splitter 13, a reference surface 14, an imaging lens 16, an imaging device 17, a monitor 18, a beam splitter 19, and wavelength observation. It is comprised from the reflective member 20, the image pick-up element 21, the monitor 22, and the computer 100. FIG. As a functional block diagram, the computer 100 includes a phase calculation unit 101 that calculates the phase of interference fringes for measuring the object to be measured W, a shape calculation unit 102 that calculates the shape of the object to be measured W, and a reflection for wavelength observation. It is expressed as including a phase calculation unit 103 that calculates the phase of interference fringes due to reflected light from the member 20 and a synthetic wavelength calculation unit 104.

  The laser light source 11 is a light source that emits light of at least two or more coherent single wavelengths. The collimating optical system 12 has a function of converting the emitted light beam emitted from the laser light source 11 into a parallel light beam. The beam splitter 13, the reference surface 14, the imaging lens 16, and the imaging device 17 have the same configuration as a known multiwavelength interferometer. That is, the beam splitter 13 separates the light beam emitted from the laser light source 11 into measurement light directed to the object to be measured W and reference light directed to the reference surface 14, and the measurement light and the reference surface reflected by the object to be measured W. 14, the combined light is generated by combining the reference light reflected by 14.

  The combined light passes through the imaging lens 16 and forms an image on the light receiving surface of the imaging device 17. An interference fringe image formed by the combined light is formed on the imaging surface of the imaging device 17, and this interference fringe image is displayed on the monitor 18. By analyzing the interference fringe image captured by the imaging device 17 by the computer 100, the surface property data of the object W to be measured can be obtained. A CCD image pickup device can be used as the image pickup device 17, but not limited to this, various devices such as a photodiode array can be used as long as the light and darkness of interference fringes can be observed.

  Here, the measurement principle of a normal single-wavelength interferometer and the measurement principle of a multi-wavelength interferometer will be briefly described.

  The intensity I (x, y) of the interference fringes obtained by the light source having the wavelength λ is expressed by the following [Equation 1].

  Where B (x, y) is the interference fringe bias at point (x, y), A (x, y) is the amplitude at point (x, y), and φ (x, y) + 2πn / λ is the point (x , Y) (n is the fringe order, φ (x, y) is the fractional part). From the interference fringe intensity I (x, y), the actual phase φ (x, y) + 2πn / λ cannot be determined uniquely, but can be determined uniquely by an integer multiple of the wavelength λ. That is, only φ (x, y) which is a fractional component of less than one wavelength excluding the fringe order n. The fringe order n can be obtained by trial and error using a phase unwrapping method or the like. However, if the workpiece W has a discontinuous step exceeding half the wavelength λ, the workpiece W cannot be measured.

The multi-wavelength interferometer according to the embodiment of the present invention obtains an interference fringe image for each wavelength by using outgoing lights having different wavelengths λ ₁ and λ ₂ . At this time, the period of change of the phase difference obtained at each wavelength coincides with the period represented by the combined wavelength Λ = λ ₁ · λ ₂ / | λ ₁ −λ ₂ |. Within the range of the combined wavelength Λ, the fringe order can be determined by the phase difference between the two wavelengths. For this reason, a large measurement range can be obtained as compared with interference measurement using a single wavelength.

In the multiwavelength interferometer as in the present embodiment, the combined wavelength Λ changes when the difference (wavelength difference) between the different wavelengths λ ₁ and λ ₂ changes. Even when a laser having high wavelength stability or the like is used as a light source, the wavelength changes due to a temperature change or the like, and the combined wavelength Λ also changes. Performing interference measurement along the set value without knowing that the synthetic wavelength Λ has changed leads to a decrease in measurement accuracy. In particular, when the wavelength difference between the wavelengths λ ₁ and λ ₂ is reduced in order to widen the measurement range, a slight change in the wavelength leads to a large change from the set value of the combined wavelength Λ.

  Therefore, in the multiwavelength interferometer of this embodiment, the combined wavelength Λ is calculated with the following configuration, and the shape of the workpiece W is measured using the calculated combined wavelength Λ. That is, the multi-wavelength interferometer of this embodiment includes a beam splitter 19 between the collimating optical system 12 and the beam splitter 13, and a wavelength observation reflecting member 20 is installed in the branch optical path of the beam splitter 19. . An imaging device 21 is provided on the side opposite to the wavelength observation reflecting member 20 with the beam splitter 19 in between. An imaging optical system may be provided between the beam splitter 19 and the imaging device 21.

As shown in FIG. 1, the wavelength observing reflecting member 20 is composed of reflecting surfaces 20A and 20B spaced apart by an optical path length la. The light reflected by the beam splitter 19 and reflected by the reflecting surfaces 20A and 20B reaches the imaging device 21 with an optical path length difference of 2 × la. When lights having different optical path differences are combined by the beam splitter 19 and incident on the imaging device 21, an interference fringe image based on the optical path difference is captured. Such interference fringe images are acquired using different wavelengths λ ₁ and λ ₂ , respectively, and the phase of each interference fringe is calculated by the phase calculation unit 103 of the computer 100. The synthetic wavelength Λ can be calculated by the synthetic wavelength calculation unit 104 using the calculated phase. The specific calculation procedure will be described below.

When the phases at the points where there are interference fringes obtained by the imaging device 21 in the case of each of the wavelengths λ ₁ and λ ₂ are φ ₁ (x, y) and φ ₂ (x, y), respectively, λ ₁ , λ ₂ , The relationship between φ ₁ and φ ₂ can be expressed as the following [Equation 2] and [Equation 3] (the portions of (x, y) are omitted for simplification).

Here, n ₁ and n ₂ are stripe orders and indicate how many times the optical path length difference la is the wavelength. Taking the difference between [Equation 2] and [Equation 3], the following equation [Equation 4] is obtained.

When the values of the wavelengths λ ₁ and λ ₂ and the optical path length difference la are in the relationship of [Equation 5], the fringe orders n ₁ and n ₂ do not change between the wavelength λ ₁ and the wavelength λ ₂ ( n ₁ = n ₂ ).

For example, if λ ₁ = 632 nm and λ ₂ = 633 nm and the distance la is about 0.1 mm or less, it can be considered that n ₁ = n ₂ . In this case, by deleting the second term of [Equation 4] and modifying it, the combined wavelength Λ can be calculated as in the following [Equation 6]. As described above, according to this embodiment, since the accurate combined wavelength Λ can be calculated in real time and with a simple configuration such as the wavelength observation reflecting member 20, the measurement accuracy of the multiwavelength interferometer is lowered. This can be prevented.

In general, the absolute values of n ₁ and n ₂ can be determined if la is given the same accuracy as the wavelengths λ ₁ and λ ₂ . However, if it can be considered that n ₁ = n ₂ as described above, there is no need to obtain the absolute values of n ₁ and n ₂ . Even if la is larger than 0.1 mm, if la is priced to such an extent that the relative relationship between the stripe orders n ₁ and n ₂ can be determined, [Equation 4], [ The synthetic wavelength Λ can be calculated based on Equation 6]. In this case, the second term on the right side of [Equation 4] remains, but since the second term is a constant, there is no problem in calculating the combined wavelength Λ.

  In the above embodiment, the wavelength observation reflection member 20 is described as having the reflection surfaces 20A and 20B spaced apart by the path length difference La, but the wavelength observation member 20 is limited to this. It is not a thing. For example, as shown in FIG. 2, the reflection surface 20A is disposed on the transmission side of the beam splitter 19 when viewed from the laser light source 11, and the reflection surface 20B is disposed on the reflection side of the beam splitter 19, and the optical path length difference between the two is la. It may be made to become. Further, a parallel flat plate as shown in FIG. 3 may be used as the wavelength observation reflecting member 20. In this case, the front surface side functions as the reflecting surface 20A and the back surface side functions as the reflecting surface 20B, and the optical path length difference between them is set to la.

  FIG. 4 shows another configuration example of the wavelength observation reflecting member 20. The wavelength observing reflecting member 20 in FIG. 4 includes a planar reflecting surface 20A and a material having a reflecting surface 20B having a step-like height changing surface. As a result, the optical path length difference between the reflecting surface 20A and the reflecting surface 20B changes stepwise as la, lb, lc, and ld (la <lb <lc <ld). By providing a plurality of optical path length differences in this way, it is possible to obtain a wide measurement range and at the same time calculate the combined wavelength Λ with high resolution.

As is clear from the above [Equation 2] and [Equation 3], changes in the phases [phi] ₁ and [phi] ₂ of the interference fringes calculated when the wavelengths [lambda] ₁ and [lambda] _{2 of the} light emitted from the laser light source 11 change. The amount depends on the optical path length difference la between the reflecting surfaces 20A and 20B of the wavelength observation reflecting member 20. Wavelength lambda ₁ and a large optical path length difference _la, phase phi _1, the amount of change phi ₂ becomes large with respect to a change in lambda _2, the wavelength lambda _1, a change in lambda ₂ is within the predetermined range, the synthetic wavelength Λ It can be calculated with high resolution. However, as the amount of change in the phases φ ₁ and φ ₂ is large, the fringe orders n ₁ and n ₂ are also easily changed. When the fringe orders n ₁ and n ₂ change, it becomes difficult to determine the amount of change in the wavelengths λ ₁ and λ ₂ and thus the combined wavelength Λ. On the other hand, when la is small, the amount of change in the phases φ ₁ and φ ₂ with respect to changes in the wavelengths λ ₁ and λ ₂ is small, and the interference fringe signal can be obtained in a state where the fringe orders n ₁ and n ₂ do not change. Can be widened. However, the calculation accuracy of the combined wavelength Λ is reduced by the small change in the phases φ ₁ and φ ₂ .

  When the reflection member 20 for wavelength observation of FIG. 4 is used, the amount of change in the fringe order n is grasped by comparing the changes in the phase φ measured in the different optical path length differences la, lb, lc, and ld. This makes it possible to obtain a wide measurement range and at the same time calculate the composite wavelength Λ with high resolution. That is, it is possible to obtain the same effect as when a plurality of wavelength sensors having different measurement ranges and resolutions are prepared.

  FIG. 5 shows another example of the wavelength observation reflecting member 20 having a plurality of optical path length differences. This wavelength observation reflecting member 20 is formed of only one transparent material such as glass, and the surface formed in a stepped shape is the reflecting surface 20A, and the planar reflecting surface on the back surface side is the reflecting surface 20B.

  Second Embodiment FIG. 6 shows the overall configuration of a multiwavelength interferometer according to a second embodiment of the present invention. In the multi-wavelength interferometer of this embodiment, the wavelength observation reflecting member 20 is installed in an imaging region of an imaging device that captures interference fringes generated by reflected light from the object W to be measured, and is parallel by the collimating optical system 12. Both are comprised in the illumination range of a light beam. Accordingly, the interference fringe images P <b> 1 and P <b> 2 of the object to be measured W and the wavelength observation reflecting member 20 are captured by the same imaging device 17 and displayed in parallel on the monitor 18. In this embodiment, since the separate beam splitter 19 and the imaging device 21 are not required as in the first embodiment, the configuration can be simplified, and the cost and size of the device can be reduced. Can do.

  [Third Embodiment] FIG. 7 shows an overall configuration of a multi-wavelength interferometer according to a third embodiment of the present invention. This embodiment is the second embodiment in that the wavelength observation reflecting member 20 is installed adjacent to the object W to be measured, and both are included in the illumination range of the parallel light flux by the collimating optical system 12. It is common with the form.

  However, this embodiment is different from the second embodiment in that a configuration for performing interference measurement by a so-called phase shift method is provided. That is, the multiwavelength interferometer of this embodiment includes a λ / 4 plate 31 disposed between the object to be measured W and the reference surface 14, and a beam splitter 32 that divides the combined light obtained by combining the reference light and the measurement light into three. , Three polarizing plates 33A to 33C having different transmission axis directions by 45 ° and imaging devices 17A to 17C are provided.

  In this configuration, the measurement light reflected by the object to be measured W and passed through the λ / 4 plate 31 and the reference light reflected by the reference surface 14 become linearly polarized light whose polarization directions are orthogonal to each other, and are combined by the beam splitter 13. To become synthetic light. The combined light is divided into three by the beam splitter 32 and then passes through the three polarizing plates 33A to 33C.

The three lights that have passed through the polarizing plates 33A to 33C are lights that are 90 ° out of phase with each other, and are projected onto the imaging devices 17A to 17C. According to this embodiment, the phase of the interference fringes can be shifted by the polarizing plates 33A to 33C, and a plurality of interference fringe images can be simultaneously acquired by the imaging devices 17A to 17C. Similarly to the above-described embodiment, the wavelength of the light emitted from the laser light source 11 is switched between the wavelengths λ ₁ and λ ₂ to perform multi-wavelength interference. Is possible. The synthetic wavelength Λ can be obtained by analyzing the interference fringe image by the wavelength observation reflecting member 20 arranged adjacent to the object W to be measured, as in the above embodiment.

  In this embodiment, it is also possible to observe the interference image of the wavelength observation reflecting member by installing separate beam splitters and imaging devices (19, 21) as in the first embodiment. is there. In FIG. 7, the interferometer based on the optical phase shift method using the polarizing plates 33 </ b> A to 33 </ b> C has been described. However, the present invention can also be applied to an interferometer that performs phase shift by physically moving the reference surface. It is. Moreover, although the example which prepares three imaging device 17A-C was demonstrated, it is not restricted to this, It is also possible to image only with one imaging device by switching by an optical path switching member. By using such a phase shift method, the shape of the workpiece W can be analyzed with higher accuracy than the interferometer of the above embodiment.

  Although the embodiments of the invention have been described above, the present invention is not limited to these embodiments, and various modifications and additions can be made without departing from the spirit of the invention. For example, although the wavelength observation reflecting member 20 has been described as having two planar reflecting members 20A and 20B in the above-described embodiment, it may be necessarily planar if it has a predetermined optical path length difference. For example, it may be spherical.

  In the above-described embodiment, the interference fringes obtained by causing the light reflected by the reflection surfaces 20A and 20B to interfere with each other are imaged. Instead, the reflection surface 20A and the reference surface 14 are captured. You may make it image the interference fringe obtained by making the light reflected by each of these interfere, and the interference fringe obtained by making the light reflected by each of the reflective surface 20B and the reference surface 14 interfere. Even in this case, the reflecting surfaces 20A and 20B are given a known optical path difference, and the two interference fringes have a difference according to the difference in the optical path difference, so that the same effect can be obtained.

  In addition to the reference surface 14, as shown in FIG. 8, for example, a beam splitter 41 disposed in the optical path toward the wavelength observation reflecting member 20, and a second reference surface 42 disposed in the optical path branched thereby. Can also be provided. At this time, the interference fringes obtained by causing the reflected light on the reflecting surface 20A and the reflected light on the second reference surface 42 to interfere with each other, and the reflected light on the reflecting surface 20B and the reflected light on the second reference surface 42 are caused to interfere with each other. The optical system can be configured to image the interference fringes obtained in this way.

1 shows an overall configuration of a multiwavelength interferometer according to a first embodiment of the present invention. Another configuration example of the wavelength observation reflecting member 20 shown in FIG. 1 is shown. Another configuration example of the wavelength observation reflecting member 20 shown in FIG. 1 is shown. Another configuration example of the wavelength observation reflecting member 20 shown in FIG. 1 is shown. Another configuration example of the wavelength observation reflecting member 20 shown in FIG. 1 is shown. The whole structure of the multiwavelength interferometer which concerns on the 2nd Embodiment of this invention is shown. The whole structure of the multiwavelength interferometer which concerns on the 3rd Embodiment of this invention is shown. The modification of embodiment of this invention is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Laser light source, 12 ... Collimating optical system, 13 ... Beam splitter, 14 ... Reference surface, 16 ... Imaging lens, 17 ... Imaging apparatus, 18 ... Monitor, DESCRIPTION OF SYMBOLS 19 ... Beam splitter, 20 ... Reflective member for wavelength observation, 20A, 20B ... Reflecting surface, 21 ... Imaging element, 22 ... Monitor, 31 ... λ / 4 plate, 32. .. Beam splitter, 33A to C ... Polarizing plate, 100. Computer, 101 ... Phase calculation unit, 102 ... Shape calculation unit, 103 ... Phase calculation unit, 104 ... Calculation of combined wavelength Department.

Claims (8)

A light source configured to emit a plurality of different wavelengths of emitted light;
A light splitting and combining member that divides the emitted light emitted from the light source into measurement light and reference light, and combines the reference light reflected by the reference surface and the measurement light reflected by the object to be measured into a combined light;
An imaging unit for imaging interference fringes by the combined light, and
In the multi-wavelength interferometer that calculates the shape of the object to be measured based on the phase of the interference fringes obtained by each of the plurality of different wavelengths of emitted light and the combined wavelength of the plurality of different wavelengths,
A reflecting member having a first reflecting surface and a second reflecting surface whose optical path length difference is known;
The plurality of interference fringes obtained by causing the plurality of outgoing lights having different wavelengths to enter the reflecting member are imaged by the imaging unit, and the combined wavelength is calculated based on the phases of the plurality of captured interference fringes. A multi-wavelength interferometer comprising: a calculation unit.   2. The multiwavelength interferometer according to claim 1, wherein the reflection member is a plane-parallel plate having a front surface and a back surface as the first reflection surface and the second reflection surface.   In the reflecting member, the first reflecting surface or the second reflecting surface is formed in a step shape, and the optical path length difference between the first reflecting surface and the second reflecting surface is one or more steps. The multi-wavelength interferometer according to claim 1, wherein the multiwavelength interferometer changes in a step shape.   The reflecting member is disposed in an imaging region of the imaging unit that images an interference fringe generated by reflected light from the object to be measured, and an image of the interference fringe by the reflecting member is an interference fringe by the object to be measured. The multi-wavelength interferometer according to claim 1, wherein the multi-wavelength interferometer is picked up by a common image pickup device together with an image.   A phase shift imparting unit that imparts a phase shift to the combined light, and configured to execute the phase shift method by imaging a plurality of interference fringes obtained for different amounts of phase shift by the imaging unit. 1. The multiwavelength interferometer according to 1.   The multi-wavelength interferometer according to claim 1, wherein the imaging unit captures an interference fringe obtained by causing the reflected light on the first reflecting surface and the reflected light on the second reflecting surface to interfere with each other.   The interference fringes obtained by causing the reflected light on the first reflecting surface and the reflected light on the reference surface to interfere with each other, and obtained by causing the reflected light on the second reflecting surface and the reflected light on the reference surface to interfere with each other. The multi-wavelength interferometer according to claim 1, wherein the multi-wavelength interferometer is configured to image an interference fringe with the imaging unit.   Interference fringes obtained by causing the reflected light on the first reflecting surface and the reflected light on the second reference surface provided separately from the reference surface to interfere with each other, and the reflected light on the second reflecting surface and the second The multi-wavelength interferometer according to claim 1, wherein interference fringes obtained by causing reflected lights on a reference surface to interfere with each other are imaged by the imaging unit.