JP2006329975A - Interferometer and method of calibrating the interferometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interferometer capable of attaining higher accuracy of measurement regardless of a using environment by enabling a user to easily measure dispersions of bias and amplitude of interference fringe intensities of interference fringes obtained from a plurality of imaging elements and deviations from a phase design value in a periodic base or before measurement. <P>SOLUTION: For the wavelength of laser beam from a wavelength-variable laser 301, wave length shift quantity Δλ is changed in at least three ways, and different interference fringes are taken by one CCD camera 316, 318 or 320 for the three different wavelength shift quantities Δλ, and interference fringe intensities thereof are acquired. According to this, biases B<SB>i</SB>(x,y), amplitudes A<SB>i</SB>(x,y) and phases F<SB>i</SB>(x,y) can be calculated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an interferometer and an interferometer calibration method.

  There is a phase shift method as an analysis method of the interference fringes obtained by the interferometer. The phase shift method is a method of calculating the shape of the measurement object by acquiring a plurality of interference fringe images by shifting the phase of the interference fringes by a method such as displacing the reference surface in the optical axis direction. Since high accuracy is obtained, it is currently applied to many interferometers.

  However, since this phase shift method acquires interference fringes while displacing the reference surface, it takes time for measurement. In addition, since the object to be measured needs to be settled while acquiring the interference fringes, it can only be used in a special environment that eliminates fluctuations such as vibration.

  On the other hand, for example, Patent Documents 1 and 2 disclose a method of simultaneously imaging an interference fringe optically shifted in phase using a plurality of image sensors. In this document, the combined light obtained by combining the reference light and the object light is divided into three by a beam splitter, and the three divided lights are passed through three polarizing plates having different polarization directions and projected onto three image sensors. A phase shift method is disclosed. According to this method, the number of interference fringe images required for the arithmetic processing by the phase shift method can be instantaneously captured, so that the measurement speed can be increased. Also, measurement under vibration conditions is possible.

  However, in order to measure the measurement object with high accuracy by the methods of Patent Documents 1 and 2, it is premised that the biases and the amplitudes of the corresponding points of the three phase-shifted interference fringe images are equal. However, it is difficult to equalize the bias and amplitude between the three interference fringe images due to the effect of the splitting strength error in the beam splitter and the setting error of the fast axis and slow axis of the λ / 4 plate. The accuracy is reduced.

  Also, if there is an installation error of the polarization axes of the three polarizing plates, the phase shift amount between the three interference fringe images will be different from the design value, so for the interference fringes generated by the measurement object surface, The phase shift amount as designed according to the measurement principle cannot be obtained, and the measurement accuracy is lowered.

In order to improve the measurement accuracy, it is necessary to manufacture optical components constituting the interferometer with high accuracy and to adjust the interferometer using a high-precision adjustment mechanism. However, the bias value, amplitude value, and phase changed immediately after the interferometer is manufactured change due to changes in geometric dimensions due to changes in the interferometer housing over time and temperature fluctuations, and changes in the performance of optical components. Resulting in. Therefore, it is preferable to allow the user to easily price these values regularly or in an environment where the user uses the interferometer. This is because measurement by an interferometer that does not select the use environment at high speed and high accuracy is realized.
JP-A-2-287107 JP-A-11-136831

  The present invention provides an interference fringe obtained from a plurality of image sensors in an interferometer that simultaneously acquires interference fringes whose phases are shifted by an optical technique. It is possible to easily measure the deviation periodically or before measurement, to achieve high accuracy of measurement, and to provide an interferometer and a method for calibrating the interferometer that can be used in any environment. Objective.

  In order to achieve the above object, an interferometer according to the present invention includes a wavelength tunable light source configured to be capable of changing the wavelength of emitted light, and divides the emitted light into measurement light and reference light, as well as reference A light splitting / combining member that combines the reference light reflected by the surface and the measurement light reflected by the measurement object to be combined light; a light splitting unit that splits the combined light into a plurality of split lights; A plurality of phase shift optical members that give a predetermined phase difference between each of the divided lights, an imaging unit that takes a plurality of interference fringe images formed by each of the plurality of divided lights whose phases are shifted, and A wavelength control unit that controls the wavelength tunable light source to change the wavelength of the emitted light in plural ways and a part of the measurement object are used, or a calibration substrate is installed instead of the measurement object And the wavelength control unit When the length is changed in a plurality of ways, images of a plurality of interference fringes obtained by each of the plurality of divided lights are picked up by the respective imaging units, and based on the interference fringe strengths of the plurality of picked-up interference fringes And an arithmetic unit for calculating the bias, amplitude and phase shift amount of the interference fringe intensity of each of the plurality of divided lights.

  In order to achieve the above object, an interferometer calibration method according to the present invention divides outgoing light emitted from a light source into measurement light and reference light, and reflects the reference light reflected by a reference surface and the measurement object. The plurality of interference fringe images obtained by synthesizing the reflected light again to obtain combined light, dividing the combined light into a plurality of divided lights, and giving a predetermined phase difference between each of the plurality of divided lights In the interferometer calibration method for measuring the shape of the measurement object by analyzing the above, using a part of the measurement object, or installing a calibration substrate instead of the measurement object, A step of changing the wavelength of the emitted light in a plurality of ways, a step of imaging a plurality of types of interference fringes obtained by each of the plurality of divided lights based on a change in the wavelength of the emitted light, and the imaged interference Take the fringe interference fringe intensity And, the plurality of divided light of each of the bias, characterized by comprising the step of calculating the amplitude and phase shift amount.

  According to this invention, it becomes possible for the user to easily measure the interference fringe intensity bias and amplitude variation of a plurality of interference fringe images, and the deviation of the phase from the design value periodically or before measurement. As a result, high accuracy of measurement can be achieved, and an interferometer and interferometer calibration method can be provided regardless of the use environment.

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

  FIG. 1 shows the overall configuration of the interferometer according to the first embodiment of the present invention. The interferometer of this embodiment includes a light projecting system including a wavelength tunable laser 301, a condenser lens 302, a pinhole 303, and a collimator lens 304. The wavelength tunable laser 301 is configured to change the wavelength of the emitted light in a plurality of ways, and the wavelength is controlled by the computer 401 via the wavelength controller 402. The computer 401 is installed with software for analyzing the captured interference fringe image and calculating the shape of the measurement object. Further, the computer 401 outputs a control signal for controlling the wavelength of the emitted light of the wavelength tunable laser 301 when executing the calibration operation of the interferometer. The wavelength controller 402 controls the wavelength of the wavelength tunable laser 301 based on this control signal.

  The outgoing light emitted from the light projecting system is split into measurement light and reference light that are orthogonal linearly polarized light by a polarizing beam splitter 305 as a light splitting and combining member. The measurement light and the reference light are converted from linearly polarized light to circularly polarized light by the quarter wavelength plate 308 and the quarter wavelength plate 306, respectively, and then reflected by the measurement object 501 and the reference surface 307 fixed to the jig 321. To do. The reflected measurement light and reference light are converted by the quarter-wave plates 308 and 306, respectively, into linearly polarized light that is orthogonal to each other and differing in vibration direction by 90 degrees from the incident light, and then combined by the polarization beam splitter 305. . The combined light is converted by the quarter wavelength plate 310 into left-handed circularly polarized light and right-handed circularly-polarized light.

  The interferometer includes an imaging lens 311 and a light splitting member including beam splitters 312 and 313 and a reflecting prism 314. The three divided lights divided by the light dividing member are projected onto the CCD cameras 316, 318, and 320 via the polarizing plates 315, 317, and 319, respectively.

  The polarizing plates 315, 317, and 319 have different transmission axis directions by 45 degrees, and the phases of the three divided lights that have passed through these are made to differ by 90 degrees.

  If the corresponding points of the interference fringes by the three divided lights have the same bias and amplitude, the shape of the measurement object can be measured by the following calculation procedure. That is, the difference between the image signals of the interference fringe images by the three divided lights is obtained, and the cosine signal S and sine signal C are obtained. Then, the arc tangent arctan (S / C) of S / C is calculated. Thereby, the phase difference between the reference light and the measurement light is specified, and thereby the shape of the measurement object can be measured. Since the detailed description of this measurement method is also made in Patent Document 1, it is omitted here.

  However, the bias and the amplitude may not be equal between corresponding points of each interference fringe image due to various errors from the time of shipment of the optical members constituting the interferometer and subsequent errors due to changes with time. When the bias and the amplitude are not equal, the bias component is not erased even if the difference between the image signals is calculated, and the amplitude component is not erased when calculating the arctangent. Also, the phase shift amount relatively given between the interference fringe images cannot be given an exact amount as designed for the same reason. Therefore, the above calculation procedure cannot specify the phase difference between the reference light and the measurement light with high accuracy.

  Therefore, in the interferometer of this embodiment, the interference fringe intensity bias, amplitude, and phase shift amount of the interference fringes of the CCD cameras 316, 318, and 320 are measured and the interferometer is calibrated by the following method. To do. First, the calibration substrate 502 is fixed to the jig 321 instead of the measurement object 501. The calibration substrate 502 need not have a known shape as long as the interference fringe intensity bias, amplitude, and phase shift amount are measured.

  However, by using the calibration substrate 502 whose shape is known, the interference fringes are analyzed to calculate the shape of the calibration substrate 502, and the difference between the calculated value and the true value of the shape of the calibration substrate 502 is calculated, It is possible to grasp systematic errors that occur systematically with the interferometer. It becomes possible to correct the measurement result of the measuring object W based on the grasped systematic error.

  The calibration substrate 502 is irradiated with the light emitted from the wavelength tunable laser 301 by changing its wavelength in a plurality of ways, and an interference fringe image is taken by each CCD camera 316, 318, 320 for each different wavelength.

In general, the interference fringe intensity I _i (i = 1, 2, 3, where i = 1 represents the CCD camera 316 and i = 2 represents the CCD camera 318 obtained by one CCD camera 316, 318 or 320. , I = 3 represents the CCD camera 320), respectively, can be expressed by the following [Equation 1].

Here, B _i (x, y) represents a bias, A _i (x, y) represents an amplitude, and (x, y) represents the coordinates of each corresponding point. Further, ΔL (x, y) is a difference between the optical path length L1 (x, y) of the reference light and the optical path length L2 (x, y) of the measurement light, which is divided by the polarization beam splitter 305 and recombined. It is. Further, δΦ _i (x, y) is the phase of the interference fringe intensity I ₁ , I ₂ , I ₃ , and different values are given by the polarizing plates 315, 317, and 319 in increments of 90 °.

When the wavelength λ of the light emitted from the wavelength tunable laser 310 is changed by the wavelength shift amount Δλ, the interference fringe intensity I _i is expressed by the following [Equation 2].

  This [Equation 2] can be further expressed as follows.

That is, when the wavelength is varied from λ by the wavelength shift amount Δλ, different interference fringe images captured by the CCD cameras 316, 318, or 320 are represented by G · Δλ (G = −2π / λ ² ) can be expressed as waveforms having different phases.

Therefore, by changing the wavelength shift amount Δλ in at least three ways, for example, in the case of at least three different wavelength shift amounts Δλ, one CCD camera 316, 318, or 320 captures different interference fringes, By obtaining the interference fringe intensity, the bias B _i (x, y), the amplitude A _i (x, y), and the phase F _i (x, y) can be calculated. From the calculated phase F _i (x, y), the phase shift amount δφ _i (x, y) of each split light can also be calculated. The values of the bias B _i (x, y) and the like calculated in this way are stored in the storage unit 403, and are used for calculating the shape with high accuracy in the shape measurement of the measurement object 501, as will be described later.

The N wavelengths (N ≧ 3) may have any size and interval as long as they have different values, but as an example, N wavelength shift amounts Δλ _j as shown in [Expression 4]. Can be set to a value obtained by dividing one period (2π) into N equal parts.

Under this condition, the following [Equation 5] is calculated to calculate the bias B _i (x, y), the amplitude A _i (x, y), and the phase F _i (x, y).

The phase δφ _i (x, y) can also be calculated from F _i (x, y). In this embodiment, instead of calculating δφ _i (x, y), the phase of the interference fringe intensity of the interference fringe imaged by the CCD camera 316 (i = 1) is used as the reference phase, and the following [several 6], the relative phase shift amounts α (x, y) and β (x, y) with respect to the reference phase are calculated and stored. When measuring the relative shape of the measurement object with respect to the reference surface, the calculation based on the relative phase shift amounts α (x, y) and β (x, y) is calculated as will be apparent from the following description. Can be simplified.

Next, the bias B _i (x, y), the amplitude A _i (x, y), and the phase shift amounts α (x, y), β (x, y) obtained by measuring the calibration substrate 502 in this way. A method for correcting the measurement object 501 and correcting the measurement object 501 with high accuracy will be described.

First, the intensities of various kinds of light generated when the measurement object 501 or the calibration substrate 502 is measured are defined as shown in FIGS. The intensity of the reference light reflected by the reference surface 307 is a (x, y), and the intensity of the reference light reflected by the beam splitter 312, the beam splitter 313, and the reflecting prism 314 is a ₁ (x, y), a ₂ (x , Y), a ₃ (x, y). These are the same when the measurement object 501 is measured and when the calibration substrate 502 is measured.

Further, when measuring the calibration substrate 502, the intensity of the measurement light reflected by the calibration substrate 502 is b (x, y), and the intensity of the measurement light reflected by the beam splitter 312, the beam splitter 313, and the reflection prism 314, respectively. It is defined as b ₁ (x, y), b ₂ (x, y), b ₃ (x, y). a _i (x, y) and b _i (x, y) have a relationship as shown in the following [Equation 7] with the bias B _i (x, y) and the amplitude A _i (x, y).

Bias _B i by measurement of the calibration substrate 502 (x, y), the amplitude _A i (x, y) the value of is obtained, from two equations of the following equation _{7], a i (x,} y) , B _i (x, y) can also be specified.

Further, when measuring the measurement object 501, the intensity of the measurement light reflected by the measurement object 501 is b ′ (x, y), and the intensity of the measurement light reflected by the beam splitter 312, the beam splitter 313, and the reflection prism 314, respectively. Are defined as b ₁ ′ (x, y), b ₂ ′ (x, y), b ₃ ′ (x, y). The intensity ratio of the measurement light b ′ (x, y) to the measurement light b (x, y) is defined as a reflectance ratio γ (x, y) as shown in [Equation 8] below.

Similarly, the relationship shown in the following [Equation 9] holds for the intensity b ₁ ′ (x, y) of the measurement light after the light splitting.

The bias B _i ′ (x, y) and the amplitude A _i ′ (x, y) when measuring the measurement object 501 are obtained by using the above b _i (x, y) and the reflectance ratio γ (x, y). Then, it is expressed as the following [Equation 10].

Therefore, the interference fringe intensity I _i (x, y) obtained by the CCD cameras 316, 318, and 320 when measuring the measurement object 501 is expressed by the following [Equation 11].

I _i ′ (x, y) is obtained from the image signals of the CCD cameras 316, 318, 320, and φ (x, y) is set as follows using this I _i ′ (x, y), for example, It can be obtained algebraically.

First, the above [Equation 11] is transformed into the following [Equation 12] with I _i ′ (x, y) −a _i (x, y) as the left side.

This [Equation 12] is solved for γ (x, y), (γ (x, y)) ^1/2 · sin φ, (γ (x, y)) ^1/2 · cos φ.

If the determinant shown in [Equation 14] below is not 0, β, [Equation 13] is changed to γ (x, y), (γ (x, y)) ^1/2 · sinφ, (γ ( x, y)) ^1/2 · cos φ can be solved.

When (γ (x, y)) ^1/2 · sinφ, (γ (x, y)) ^1/2 · cosφ is obtained, the arctangent of the ratio between the two is taken as in the following [Equation 15]. Thus, φ can be obtained.

  Since this [Equation 15] is established uniformly regardless of the magnitude of the reflectance ratio γ, even if γ ≠ 1, that is, a calibration substrate 502 having a reflectance different from that of the measurement object 501 is used. , The measured value can be calibrated without being affected by it. Further, the measured value can be calibrated without obtaining the numerical value of γ. Incidentally, the reflectance ratio γ (x, y) can be obtained from the following equation.

  FIG. 4 shows the overall configuration of the interferometer according to the second embodiment of the present invention. As shown in FIG. 4, the interferometer of this embodiment captures an interference fringe image using measurement light and reference light divided into three by beam splitters 312 and 313 and a reflecting prism 314 with one CCD camera 322. Constitutes an interferometer.

  FIG. 5 shows only the part after the lens 311 in the configuration of the interferometer according to the third embodiment of the present invention. Since other parts are the same as those in the above embodiment, a detailed description is omitted. In the interferometer of this embodiment, measurement light and reference light are divided by a diffraction grating 323 instead of the beam splitters 312 and 313 and the reflecting prism 314, and a phase difference is given by polarizing plates 315, 317, 319 and 324. Is adopted. The imaging device may be one CCD camera 325 as shown in FIG. 5 or a CCD camera provided for each divided light as shown in FIG.

  FIG. 6 shows the overall configuration of an interferometer according to a fourth embodiment of the present invention. As shown in FIG. 6, the interferometer of this embodiment employs a format in which the measurement light and the reference light are divided into a plurality of parts by a high-density phase array element 460 and a phase difference is given to the divided lights. . The high-density phase array element 460 is the same as that disclosed in Japanese Patent Application Laid-Open No. 2004-138617 filed by the applicant of the present application, and the configuration thereof will be briefly described below. The high-density phased array element 460 includes a birefringent substrate 462 such as a low-order quartz wavelength plate. Birefringent substrate 462 has a surface 469 and a fast axis 468. The slow axis is directed in a direction perpendicular to the fast axis 468.

In the birefringent substrate 462, the thickness of the pattern 461 (portions 466a to 466d) formed in, for example, a rectangular shape for each corresponding pixel group is different in four ways (T ₀ , T ₉₀ , T ₁₈₀ , T ₂₇₀ ). This produces an interference fringe image having four different relative phase displacements (P ₀ , P ₉₀ , P ₁₈₀ , P ₂₇₀ ). The birefringent substrate 461 is covered with a planar layer 462 that does not give a thickness-dependent phase.

  Even in this structure, it is extremely difficult to manufacture and arrange each optical element without error, and it is expected that deviations from the design values of bias, amplitude variation and phase shift amount of a plurality of interference fringe image tubes will occur. The Therefore, the calibration method as described above can be applied also in the fourth embodiment.

  Next, a fifth embodiment of the present invention will be described with reference to FIG. In the first to fourth embodiments, the measurement object 501 has no reflection from the back surface or can be ignored (for example, the measurement object 501 is (1) a metal mirror surface, (2) a surface. It is applicable to a wedge-type glass substrate that is relatively inclined between the back surface and the back surface, and (3) a glass substrate that has a rough surface processed on the back surface. When reflection from the back surface cannot be ignored, the reflected light becomes noise light, and accurate measurement of interference fringe bias, amplitude, phase shift amount, etc. becomes difficult or impossible.

  On the other hand, the interferometer and the calibration method thereof according to the fifth embodiment have the interference fringe signal bias, amplitude, etc., even for a measurement object for which reflected light (noise light) from the back surface of the measurement object cannot be ignored. This is different from the above embodiment in that it can be measured.

  FIG. 8 shows a configuration example of the interferometer according to the present embodiment. Here, it is assumed that the calibration substrate 502A corresponding to the measurement object is installed in the interferometer, and not only the measurement light from the front surface 502S, which is the measurement target surface, but also the noise light from the back surface 502R cannot be ignored. The interferometer shown in FIG. 8 has the same optical system structure as that of the first embodiment (FIG. 1). The same is true in that the wavelength of the wavelength tunable laser 301 is made variable by the wavelength tunable controller 402.

However, in this embodiment, in order to reduce the influence of noise light from the back surface of the measurement object, not only the wavelength can be changed but also each wavelength has a certain wavelength width (wavelength scanning width). Is different from the above-described embodiment. In the following, as shown in FIG. 9, the wavelength λ of the light emitted from the wavelength tunable laser 310 is in a range of ± Δp / 2 with at least three types of wavelengths λ ₀ , λ ₀ + Δλ ₁ , and λ ₀ + Δλ ₂ as the central wavelengths ( In the following description, it is assumed that the wavelength controller 402 is configured so that it can be varied with the wavelength scanning width Δp). However, the wavelength does not need to change with a uniform width around the center wavelength, and it is sufficient if the wavelength varies with a predetermined wavelength scanning width based on a certain wavelength (reference wavelength). The amount of variation around the reference wavelength may be different between the + side and the − side, or may be varied only from the reference wavelength to either the + side or the − side.

  Then, each of the CCD cameras 316, 318, and 320 integrates and receives light due to the change in wavelength. Here, the integrated light reception means that the light is received by changing the wavelength λ over a certain wavelength width within the exposure time Te of the CCD camera. Accordingly, the bias, amplitude, and phase shift amount of the interference fringes can be measured without being affected by noise light from the back surface 502R. Hereinafter, the principle of this integrated light reception will be described in detail.

When the noise light from the back surface 502R cannot be ignored, the interference fringe intensity I _i ′ obtained by one of the CCD cameras 316, 318, or 320 of the combined light of the reference light, the measurement light, and the noise light is It is expressed by the following formula. The reference light, the measurement light, and the noise light are shown in a simplified manner with an amplitude of 1 when expressed in complex amplitude. Although the interference fringe image has a two-dimensional light intensity distribution, the light intensity at a certain point is simplified and shown one-dimensionally in the following equations.

Here, L ₁ , L ₂ , and L ₃ represent the optical path lengths of the reference light, the measurement light, and the reflected light from the back surface of the object, respectively. K represents the wave number (= 2π / λ).
Assuming that the difference between L ₂ and L _{1 and} the difference between L ₃ and L ₁ are ΔL ₂₁ and ΔL ₃₁ , respectively, [Equation 17] can be expressed as the following equation.

When the wavelength is changed with the wavelength scanning width Δp around the center wavelength λ within the exposure time Te of the CCD camera, an integrated interference fringe intensity I _i is obtained as the integrated intensity of the interference fringe image generated by each wavelength. The integrated interference fringe intensity I _i is expressed by the following equation.

The wavelength scanning width Δp is preferably set within the range represented by the following equation.

The wavelength scanning width Δp is related to the coherence distance Δl. By setting Δp to satisfy the above [Equation 20], the optical path difference ΔL ₃₁ can be made larger than the coherent distance Δl, and the optical path difference ΔL ₂₁ can be made smaller than the coherent distance Δl. As a result, the influence of noise light, which is reflected light from the back surface 502R, is reduced or eliminated, while the reflected light (measurement light) from the front surface 502S can be obtained without disappearing.

Now consider the alignment of the measuring object W as the reference light optical path length L ₂ of the optical path length L ₁ measured light are substantially equal has been made. In this case, among the terms in [Equation 19], the term including only ΔL ₂₁ as in the second term has a substantially constant intensity regardless of the wavelength, and has an intensity that varies depending on the shape of the measurement object W.

On the other hand, the term including only ΔL ₃₁ like the third term varies in intensity as the wavelength is scanned, but when integrated, the intensity becomes an average intensity and becomes an intensity unrelated to ΔL ₃₁ . Therefore, the third term is a term representing the bias in the interference fringe intensity I. The constant term of the first term is also a term representing the bias. The fourth and fifth terms are intensities obtained by integrating the bias component with the intensities that change according to the shape of the measurement object W. Therefore, [Equation 19] can be simplified and expressed as the following equation.

However, B _i represents a bias, and A _i represents an amplitude. Therefore, even if there is back-side reflected noise light, an integrated interference fringe image can be obtained by performing wavelength scanning over the wavelength scanning width Δp with the center wavelength λ as the center, and the influence of back-side reflection. Measurement can be performed except for.

Further, such measurement is performed at each of three or more central wavelengths λ ₀ , λ ₀ + Δλ ₁ ,... Λ ₀ + Δλ _j , and the interference fringes are imaged by the CCD cameras 316, 318, and 320. Thus, it is possible to measure the bias, amplitude, and phase shift amount of the interference fringes while reducing the influence of noise light from the back surface of the measurement object 501. A method for measuring the bias, amplitude, and phase shift amount will be described in detail below.

The integrated interference fringe intensity I _ij obtained by each CCD camera 316, 318 and 320 takes into account the difference in the phase shift amount Δφ _i ,

It is expressed. When the wavelength λ _j is changed from λ ₀ by Δλ _j and Δλ _j is sufficiently smaller than λ ₀ , the approximation can be made by the following equation.

This equation can be further modified as follows.

That is, when the center wavelength λ is shifted to λ ₀ , λ ₀ + Δλ ₁ ,..., Λ ₀ + Δλ _j , the integrated interference intensity I _ij obtained by one CCD camera 316, 318 or 320 has a phase of GΔλ. It can be expressed as a waveform whose phase is different by _j . Therefore, the wavelength shift amount Δλ _j is changed in at least three ways, and at least three different wavelength shift amounts are picked up by the three CCD cameras 316, 318, and 320, and as in the above embodiment, The bias B _i , the amplitude A _i , the phase shift amount δφ _{i and the} like can be calculated. Further, the phase shift amount is calculated as in [Equation 4] described in the first embodiment, and the bias B _i , amplitude A _i , phase shift amount δφ _i and the like are calculated as in [Equation 5]. You can also
As an example, when one central wavelength is λ ₀ = 1 μm, and a glass substrate having a thickness of 4 mm and a refractive index of about 1.5 is used as a measurement object and a calibration substrate, Δp = 0.1 nm, Δλ ₁ = 0. The wavelength may be controlled so as to be 2 nm, which can be easily implemented with a commercially available wavelength tunable laser.

The embodiment of the interferometer calibration method has been described by installing a calibration substrate instead of the measurement target. When a plurality of objects are measured, the interferometer may be calibrated using one or several of them instead of installing and calibrating the calibration substrate. Also, when scanning and measuring the shape of an object that is larger than the observation area of the interferometer, calibrate using a partial area of the object, and use the calibration value obtained at that time to perform other measurements. The region may be scanned and measured. In any case, the problem and method to be solved are exactly the same except that the calibration substrate need not be used separately.
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.

1 shows a configuration of an interferometer according to a first embodiment of the present invention. In the interferometer shown in FIG. 1, the intensities of various kinds of light generated when the calibration substrate 502 is measured are defined. In the interferometer shown in FIG. 1, the intensities of various kinds of light generated when the measurement object 501 is measured are defined. The structure of the interferometer which concerns on the 2nd Embodiment of this invention is shown. 3 shows a configuration of an interferometer according to a third embodiment of the present invention. The structure of the interferometer which concerns on the 4th Embodiment of this invention is shown. 7 shows the structure of the high-density phased array element 460 of FIG. 7 shows a configuration of an interferometer according to a fifth embodiment of the present invention. It is a graph which shows the range of the wavelength radiate | emitted from the wavelength variable laser 301 of 5th Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 301 ... Tunable laser, 302 ... Condensing lens, 303 ... Pinhole, 304 ... Collimator lens, 305 ... Polarizing beam splitter, 306, 308, 310 ... 1/4 wavelength 307 ... reference plane, 311 ... imaging lens, 312, 313 ... beam splitter, 314 ... reflective prism, 315, 317, 319, 324 ... polarizing plate, 316, 318, 320, 322, 325 ... CCD camera, 321 ... jig, 323 ... diffraction grating, 501 ... measurement object, 401 ... computer, 402 ... wavelength controller, 403 ... Storage unit 501... Measurement object, 502.

Claims (10)

A wavelength tunable light source configured to change the wavelength of the emitted light,
A light splitting and combining member that divides the emitted light into measurement light and reference light, and combines the reference light reflected by the reference surface and the measurement light reflected by the measurement object into a combined light;
A light splitting means for splitting the combined light into a plurality of split lights;
A plurality of phase shift optical members that give a predetermined phase difference between each of the plurality of split lights;
An imaging unit that captures a plurality of interference fringe images formed by each of the plurality of divided lights whose phases are shifted;
A wavelength controller that controls the wavelength tunable light source to change the wavelength of the emitted light in a plurality of ways;
When a part of the measurement object or a calibration substrate is installed instead of the measurement object and the wavelength control unit changes the wavelength of the emitted light in a plurality of ways, the plurality of split lights A plurality of interference fringe images obtained by each of the plurality of interference fringe images obtained by each of the plurality of divided lights based on the interference fringe strengths of the plurality of interference fringes thus captured. An interferometer comprising: an arithmetic unit that calculates a bias, an amplitude, and a phase shift amount.   The wavelength control unit changes the wavelength of the emitted light in N ways so that the phase of each of the plurality of divided lights changes to a size obtained by dividing one period (2π) into N (N ≧ 3). The interferometer according to claim 1. The calculation unit obtains interference fringe intensity I _ij (j = 1 to N) of interference fringes for each of the plurality of divided lights at each of N wavelengths, and based on the data of the interference fringe intensity. The interferometer of claim 2, wherein the bias, amplitude and phase are calculated.   In the phase shift optical member, one birefringent material is divided into a plurality of portions, and the birefringent material is formed so that the thickness of the birefringent material is different for each of the plurality of portions. The interferometer according to claim 1. The wavelength control unit is configured to perform control to vary the wavelength of the emitted light with a predetermined wavelength scanning width within an exposure time of the imaging unit with reference to a plurality of reference wavelengths.
The interferometer according to claim 1, wherein the imaging unit is configured to capture an interference fringe image obtained by a change in wavelength of the emitted light within the exposure time.   The wavelength scanning width is such that the optical path difference between the reflected light from the back surface of the measurement object and the reference light is larger than the coherence distance, and the optical path difference between the measurement light and the reference light is acceptable. The interferometer according to claim 5, wherein the interferometer is set to be smaller than the interference distance. The emitted light emitted from the light source is divided into measurement light and reference light, and the reference light reflected by the reference surface and the reflected light reflected by the measurement object are combined again to form composite light, and the combined light Interferometer calibration method for measuring the shape of an object to be measured by analyzing a plurality of interference fringe images obtained by dividing a plurality of divided lights and giving a predetermined phase difference between each of the plurality of divided lights In
A part of the measurement object or a step of installing a calibration substrate instead of the measurement object;
Changing the wavelength of the emitted light in multiple ways;
Imaging a plurality of interference fringes obtained by each of the plurality of split lights based on a change in wavelength of the emitted light;
A method for calibrating an interferometer, comprising: obtaining an interference fringe intensity of the picked-up interference fringe and calculating a bias, an amplitude, and a phase shift amount of each of the plurality of divided lights. The calibration substrate has a known shape,
Analyzing the interference fringes to calculate the shape of the calibration substrate, and grasping systematic errors systematically generated by the interferometer based on the difference between the calculated value and the true value of the shape of the calibration substrate; ,
The interferometer calibration method according to claim 7, further comprising: correcting a measurement result of the measurement object based on the systematic error. The step of changing the wavelength of the emitted light in a plurality of ways, with a plurality of reference wavelengths as a reference, changing the wavelength with a predetermined wavelength scanning width within an exposure time for imaging,
The interferometer calibration method according to claim 7, wherein the imaging step captures an interference fringe image obtained by a change in wavelength of the emitted light within the exposure time. The wavelength scanning width is such that the optical path difference between the reflected light from the back surface of the measurement object and the reference light is larger than the coherence distance, and the optical path difference between the measurement light and the reference light is acceptable. The interferometer calibration method according to claim 9, wherein the interferometer calibration method is set to be smaller than the interference distance.