JP2014173900A - Measuring device - Google Patents

Abstract

A technique advantageous in measuring the position of a test surface with a plurality of wavelengths with high accuracy is provided.
[Solution]
The measuring device for measuring the position of the test surface includes a first wavelength set using the first wavelength reference element, and a second wavelength reference element having a wavelength dependency on an environmental change larger than the first wavelength reference element. An irradiation unit that irradiates the test surface and the reference surface with light including a second wavelength set using the test light; test light from the test surface irradiated with the light; and the light is irradiated A detection unit that detects a phase difference with reference light from the reference surface, and a control unit that determines a position of the test surface using a combined wavelength of the first wavelength and the second wavelength. The control unit is configured to change the wavelength of the second wavelength before and after the change when the optical path length difference between the test light and the reference light is changed. The amount of change in phase difference at the second wavelength and the optical path of the test light at the first wavelength The refractive index, the refractive index of the optical path in the second wavelength, determined on the basis of.
[Selection] Figure 1

Description

  The present invention relates to a measuring apparatus that measures the position of a surface to be measured.

  As a measuring device for measuring the position of the surface to be measured, a combined wavelength type optical interference measuring device using a combined wavelength obtained by combining a plurality of wavelengths is known. The light wave interference measurement device irradiates the test surface and the reference surface with light having a plurality of wavelengths, and detects an interference signal between the reflected light reflected by the test surface and the reference light reflected by the reference surface. The distance between the test surface and the reference surface is measured. Thereby, the position of the test surface can be determined. In such a measuring apparatus, in order to measure the position of the test surface with high accuracy, it is important to improve the accuracy of each wavelength used for measurement.

  Patent Document 1 discloses a method for guaranteeing FSR (free spectrum region) of an etalon by setting a plurality of wavelengths to different transmission spectra of one Fabry-Perot etalon and detecting the beat frequency of two wavelengths with a frequency counter. Is disclosed. By guaranteeing the etalon's FSR in this way, the accuracy of each wavelength can be improved by the FSR and the number of transmission spectra. Patent Document 2 discloses a method of determining an unknown wavelength in a Michelson interferometer using reference light having a known wavelength and light to be measured having an unknown wavelength. In Patent Document 2, for each of the reference light and the light to be measured, an interference signal is generated using a fixed mirror and a moving mirror, and based on a change in the interference signal when the moving mirror is moved at a constant speed. The unknown wavelength can be determined.

Japanese Patent No. 4000195 Japanese Patent No. 2604052

  In the method disclosed in Patent Document 1, beat frequencies of two wavelengths set in the transmission spectrum of a Fabry-Perot etalon are detected by a frequency counter. Since the measurement range of the frequency counter is generally about several tens GHz, it is difficult to detect the beat frequency with the frequency counter when the frequency interval between the two wavelengths is to be separated to, for example, about several hundred GHz. It can be. In addition, the Fabry-Perot etalon has environment dependency such as temperature and wavelength dispersion depending on the medium used in the Fabry-Perot etalon, but the method disclosed in Patent Document 1 does not consider them. Therefore, for example, when the surrounding environment (temperature) changes, an error may occur in the measurement result of the position of the test surface.

  In the method disclosed in Patent Document 2, since the change in the interference signal is measured by clock pulses, it is assumed that the moving mirror (surface to be measured) moves at a constant speed. Therefore, for example, when the movement of the test surface is not constant, it may be difficult to determine an unknown wavelength.

  Accordingly, an object of the present invention is to provide a technique that is advantageous in measuring the position of the test surface with high accuracy using a plurality of wavelengths.

  In order to achieve the above object, a measurement apparatus according to one aspect of the present invention is a measurement apparatus that measures the position of a test surface, and includes a first wavelength set using a first wavelength reference element, and an environment. An irradiating unit that irradiates the test surface and the reference surface with light including a second wavelength that is set using a second wavelength reference element that has a wavelength dependency on the change of the second wavelength reference element that is greater than the first wavelength reference element; A detection unit that detects a phase difference between the test light from the test surface irradiated with light and the reference light from the reference surface irradiated with the light; the first wavelength and the second wavelength; A control unit that determines a position of the test surface using a synthetic wavelength of the first and second wavelengths, and the control unit changes the optical path length difference between the test light and the reference light by changing the wavelength of the second wavelength. The amount of change in phase difference at the first wavelength and the second wave before and after the change when And the amount of change in phase difference in the refractive index of the optical path of the test light at the first wavelength, and the refractive index of the optical path in the second wavelength, obtained based on, characterized in that.

  According to the present invention, for example, it is possible to provide a technique that is advantageous in measuring the position of a test surface with a plurality of wavelengths with high accuracy.

It is a figure which shows the structure of the measuring device of 1st Embodiment. It is a figure for demonstrating the method to set the wavelength of the light inject | emitted from the light source. It is a flowchart which shows the method of calibrating the synthetic | combination wavelength of a reference | standard wavelength and a scanning wavelength. It is a flowchart which shows the method of calibrating a synthetic | combination wavelength in consideration of the influence degree of the chromatic dispersion on the optical path of test light. It is a figure which shows the relationship between the interference order of etalon, and FSR. It is a flowchart which shows the method of calibrating a synthetic | combination wavelength in real time, when measuring the position of a to-be-tested surface. It is a figure which shows the relationship between the change of the phase difference in 1st wavelength, and time.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In addition, in each figure, the same reference number is attached | subjected about the same member thru | or element, and the overlapping description is abbreviate | omitted.

<First Embodiment>
The configuration of the measuring apparatus 100 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of a measurement apparatus 100 according to the first embodiment. The measurement apparatus 100 according to the first embodiment is a measurement apparatus that measures the position of the test surface 7 by measuring the distance between the test surface 7 and the reference surface 9, and includes an irradiation unit 10 and a detection unit. 11, an environment measurement unit 13, and a control unit 12. The irradiation unit 10 includes light sources 1 and 3, a gas cell 5 as a first wavelength reference element, a Fabry-Perot etalon 2 as a second wavelength reference element, a wavelength control unit 4, and a polarization beam splitter 8. The detection unit 11 includes phase detectors 11 a and 11 b and detects a phase difference between the reference light reflected by the reference surface 9 and the test light reflected by the test surface 7.

  The light emitted from the light source 1 enters the beam splitter 81a and is separated (branched) into two lights. The light reflected by the beam splitter 81a is further separated into two lights by the beam splitter 81b, and enters the gas cell 5 as the first wavelength reference element and the Fabry-Perot etalon 2 as the second wavelength reference element, respectively. On the other hand, the light transmitted through the beam splitter 81 a is incident on the polarization beam splitter 8. The light emitted from the light source 3 enters the beam splitter 81a via the mirror 84a and is separated (branched) into two lights. The light transmitted through the beam splitter 81a passes through the beam splitter 81b, and enters the Fabry-Perot etalon 2 as the second wavelength reference element via the mirror 84b. On the other hand, the light reflected by the beam splitter 81 a enters the polarization beam splitter 8. Here, the light sources 1 and 3 in the first embodiment use, for example, semiconductor lasers of separate elements, but are not limited thereto, and a plurality of semiconductor lasers are used as in the multi-wavelength light source used for optical communication. May be integrated as a single element. In this case, for example, it is advantageous in terms of cost and dimensions.

  The light emitted from the light source 1 enters the gas cell 5 and the amount of light after passing through the gas cell 5 is detected by the detector 83a. In the first embodiment, a wavelength in the vicinity of 1.5 μm is used as the wavelength of the light emitted from the light source 1. Therefore, acetylene is used as the gas enclosed in the gas cell 5. In addition to acetylene, carbon monoxide, hydrogen cyanide, or the like that can be used in a wavelength band near 1.5 μm is used as the sealing gas. Each of these gases has a different wavelength band and center wavelength accuracy, and therefore may be selected as necessary. Further, the light emitted from the light source 1 and the light emitted from the light source 3 are incident on the Fabry-Perot etalon 2, and after passing through the Fabry-Perot etalon 2, are separated again by the spectroscopic element 82a. The amount of light after passing through the Fabry-Perot etalon 2 is detected by the detector 83b in the light emitted from the light source 1, and detected by the detector 83c in the light emitted from the light source 3, respectively. In the first embodiment, a dichroic mirror is used as the spectroscopic element 82a. In addition to the dichroic mirror, for example, a prism, a bulk type diffraction grating, or an arrayed waveguide type diffraction grating can be used as the spectroscopic element 82a, and it may be selected from necessary wavelength resolution and cost.

  Here, a method for setting the wavelength of the light emitted from the light source 1 and the wavelength of the light emitted from the light source 3 using the gas cell 5 and the Fabry-Perot etalon 2 will be described with reference to FIG. FIG. 2 is a diagram for explaining a method of setting the wavelengths of light emitted from the light source 1 and the light source 3 using the gas cell 5 and the Fabry-Perot etalon 2. 2A shows a transmission spectrum of the gas cell 5, FIG. 2B shows a transmission spectrum of the Fabry-Perot etalon 2, and FIG. 2C shows a spectrum of light emitted from the light source 1 and the light source 3, respectively. The wavelength of the light emitted from the light sources 1 and 3 is set by the wavelength control unit 4, and the wavelength control unit 4 controls the light source 1, the light source 3, and the Fabry-Perot etalon 2.

The wavelength control unit 4 controls the light source 1 using the signal from the detector 83 a so as to stabilize the wavelength of the light emitted from the light source 1 to the reference wavelength λ ₁ that is the absorption line of the gas cell 5. For stabilization of the wavelength of light emitted from the light source 1, for example, the wavelength controller 4 adjusts the wavelength of light emitted from the light source 1 so that the amount of light detected by the detector 83a is within an allowable range. Is done. Wavelength control unit 4 at the same time, by using the signal of the detector 83 b, one of the transmission spectrum of the Fabry-Perot etalon 2 so as to coincide with the reference wavelength lambda _1, and controls the optical path length of the Fabry-Perot etalon 2. Here, it is assumed that the internal medium of the Fabry-Perot etalon 2 is a vacuum, and the optical path length is controlled by changing the temperature of the Fabry-Perot etalon. Further, the wavelength controller 4 controls the light source 3 so as to stabilize the wavelength of the light emitted from the light source 3 to one of the transmission spectrum of the Fabry-Perot etalon 2 using the signal of the detector 83c. Do. For stabilization of the wavelength of the light emitted from the light source 3, for example, the wavelength controller 4 adjusts the wavelength of the light emitted from the light source 3 so that the amount of light detected by the detector 83c is within an allowable range. Is done. Here, light emitted from the light source 3 can be stabilized at any wavelength lambda ₂ and wavelength lambda _3, and can be scanned between the wavelength lambda ₂ and wavelength lambda _3. Hereinafter, the wavelengths (λ ₂ and λ ₃ ) of light emitted from the light source 3 are referred to as scanning wavelengths.

The light emitted from the light source 1 and transmitted through the beam splitter 81a and the light emitted from the light source 3 and reflected by the beam splitter 81a are incident on the polarization beam splitter 8 and separated into two lights as described above. (Branch). The light reflected by the polarization beam splitter 8 is reflected by the corner cube as the reference surface 9 and enters the polarization beam splitter 8 again as reference light. The reference light is reflected by the polarization beam splitter 8 and enters the spectroscopic element 82b. On the other hand, the light transmitted through the polarization beam splitter 8 is reflected by the corner cube as the test surface 7 (test object) and enters the polarization beam splitter 8 again as test light. The test light passes through the polarization beam splitter 8 and enters the spectroscopic element 82b. The reference light and the test light incident on the spectroscopic element 82b are separated into light emitted from the light source ₁ (wavelength λ ₁ ) and light emitted from the light source 3 (wavelength λ ₂ or wavelength λ ₃ ), and phase detection is performed. It enters into the detection part 11 comprised with the instruments 11a and 11b. The light emitted from the light source 1 enters the phase detector 11a, and the phase of the interference signal between the reference light and the test light in the light emitted from the light source 1, that is, the phase difference between the reference light and the test light. Detected. The light emitted from the light source 3 enters the phase detector 11b, and the phase of the interference signal between the reference light and the test light in the light emitted from the light source 3, that is, the phase difference between the reference light and the test light. Detected. Here, in the first embodiment, a dichroic mirror is used as the spectroscopic element 82b, similarly to the spectroscopic element 82a. In addition to the dichroic mirror, for example, a prism, a bulk type diffraction grating, or an arrayed waveguide type diffraction grating can be used as the spectroscopic element 82b, and it may be selected from necessary wavelength resolution and cost.

The control unit 12 detects the wavelength of the light emitted from the light source ₁ (wavelength λ ₁ ), the wavelength of the light emitted from the light source 3 (wavelength λ ₂ or λ ₃ ), and the phase detectors 11a and 11b. The distance between the reference surface 9 and the test surface 7 is determined using the phase difference. Thereby, the position of the test surface 7 can be determined. In the measurement apparatus 100 according to the first embodiment, the environment measurement unit 13 that measures the environment on the optical path of the test light is disposed in the vicinity of the test surface 7. The environment measurement unit 13 includes, for example, a temperature sensor, an atmospheric pressure sensor, and a humidity sensor, and can measure temperature, atmospheric pressure, and humidity as information on the environment on the optical path of the test light. The control unit 12 uses the information (temperature, atmospheric pressure, and humidity) measured by the environment measurement unit 13 to determine the refractive index (atmospheric refraction) on the optical path of the test light using a known dispersion equation (for example, Edren's equation). Rate). The refractive index determined in this way is used when the measurement wavelength is calibrated, which will be described later.

In the measuring apparatus 100 configured as described above, the wavelength of light emitted from the light sources 1 and 3 is stabilized (set) using the gas cell 5 and the Fabry-Perot etalon 2. Here, the characteristics of the gas cell and the Fabry-Perot etalon will be described. The gas cell has a merit that it can be used as a wavelength reference because the transmission spectrum has a low dependence on the surrounding environment, that is, the transmission spectrum is stable. On the other hand, since the transmission spectrum is determined according to the gas type, the gas cell has a demerit that it cannot be changed to a desired wavelength and a narrow spectral interval cannot be created with a single gas. The Fabry-Perot etalon has the advantage that the spectral interval can be arbitrarily set by designing the mirror interval and the etalon medium, and the spectral interval can be changed by temperature control or the like. On the other hand, the Fabry-Perot etalon has a demerit that the dependence of the transmission spectrum on the change in the surrounding environment is high, that is, the transmission spectrum is unstable and cannot be used as a wavelength reference. In the measurement apparatus 100 according to the first embodiment, the wavelength of light emitted from the light source 1 is set as the reference wavelength λ ₁ by the gas cell 5 using the characteristics of the gas cell and the Fabry-Perot etalon. Further, the wavelength of the light emitted from the light source 3 is set by the Fabry-Perot etalon 2 on the basis of the reference wavelength λ ₁ as the scanning wavelength (λ ₂ or λ ₃ ).

However, as described above, the Fabry-Perot etalon 2 is highly dependent on the transmission spectrum with respect to changes in the surrounding environment, that is, the transmission spectrum can be easily changed by a temperature change. Therefore, an error may occur in the wavelength of the light emitted from the light source 3 (scanning wavelength (λ ₂ or λ ₃ )). If there is an error in the scanning wavelength in this way (when the wavelength is indeterminate), an error occurs in the combined wavelength of the reference wavelength and the scanning wavelength, and the position of the test surface must be measured with high accuracy. Can be difficult. Therefore, in the measuring apparatus 100 according to the first embodiment, the change in the refractive index calculated based on the environment information on the optical path of the test light and the change in the optical path length between the test light and the reference light are changed. The scanning wavelength is calculated based on the amount of change in the phase difference between the test light and the reference light before and after. Then, the measuring apparatus 100 calibrates the combined wavelength of the reference wavelength and the scanning wavelength using the calculated scanning wavelength. Hereinafter, a method for calibrating the synthetic wavelength will be described with reference to FIG. Here, the calibration of the synthetic wavelength lambda ₁₂ in the measurement apparatus 100 of the first embodiment, for example, devices such as shipment or device startup, it is assumed to be performed prior to measuring the position of the test surface. However, the calibration is not limited thereto, and the calibration may be performed while measuring the position of the test surface, for example. When the measurement apparatus of the present embodiment measures the position of the test object, the position is determined using the combined wavelength of the reference wavelength and the scanning wavelength with the surface of the test object as the test surface. By measuring the position of the surface of the test object, the shape of the surface of the test object can be obtained.

FIG. 3 is a flowchart showing a method for calibrating the combined wavelength of the reference wavelength and the scanning wavelength. The calculation of the scanning wavelength and the calibration of the combined wavelength are performed by the control unit 12. In S31, the control unit 12 controls the detection unit 11 to detect the phase difference between the test light and the reference light at the first wavelength λ ₁ (reference wavelength) and the second wavelength λ ₂ (scanning wavelength), respectively. To do. As described above, the phase difference in the first wavelength lambda ₁ is detected by the phase detector 11a, the phase difference in the second wavelength lambda ₂ is detected by the phase detector 11b. Information on the phase difference detected by the phase detectors 11a and 11b is supplied to the control unit 12 and stored therein. In S32, the control unit 12 changes the optical path length between the test light and the reference light by ΔL. The change in the optical path length changes, for example, the distance between the test surface 7 and the polarization beam splitter 8 (irradiation unit 10) or the distance between the reference surface 9 and the polarization beam splitter 8 (irradiation unit 10). Is done by letting In S33, the control unit 12 changes the phase difference between the test light and the reference light at the first wavelength λ ₁ and the second wavelength λ _{2 while} changing the optical path lengths of the test light and the reference light by ΔL. The detection unit 11 is controlled to detect each. Thereby, it is possible to obtain the amount of change in the phase difference between the test light and the reference light before and after the optical path length of the test light and the reference light is changed by ΔL. Here, the change amount of the phase difference at the first wavelength λ ₁ is ΔΦ ₁ , and the change amount of the phase difference at the second wavelength λ ₂ is ΔΦ ₂ . The change amount ΔΦ ₁ and the change amount ΔΦ ₂ obtained in this way are stored in the control unit 12.

In S34, the control unit 12 controls the environment measurement unit 13 to measure the environment on the optical path of the test light, and based on the information on the environment measured by the environment measurement unit 13, the first wavelength λ ₁ and The atmospheric refractive index at the second wavelength λ ₂ is calculated. The atmospheric refractive index is calculated by a known dispersion equation (for example, Edren's equation) using information (temperature, atmospheric pressure, and humidity) of the environment measured by the environment measurement unit 13. Edren's equation is an equation for calculating the atmospheric refractive index using the wavelength, temperature, pressure and humidity as parameters. Here, a variable indicating the atmospheric refractive index at the first wavelength λ ₁ is n ₁ , and a variable indicating the atmospheric refractive index at the second wavelength λ ₂ is n ₂ . The atmospheric refractive indexes n ₁ and n ₂ thus determined are stored in the control unit 12.

In S35, the control unit 12, second computing the wavelength lambda ₂ on the basis of each acquired parameter in S31 to S34. The change amount ΔL of the optical path length between the test light and the reference light is expressed by Expression (6). And based on Formula (6), 2nd wavelength (lambda) ₂ is represented by Formula (7). Here, since the first wavelength λ ₁ is a reference wavelength and is known, the second wavelength λ ₂ can be obtained by Expression (7).

In S36, the control unit 12 uses the second wavelength lambda _2, which is calculated by the equation (7), obtaining the first wavelength lambda ₁ and the synthetic wavelength lambda ₁₂ and the second wavelength lambda _2. The synthetic wavelength Λ ₁₂ can be obtained by the equation (8), and thereby the synthetic wavelength of the reference wavelength and the scanning wavelength can be calibrated.

As described above, the measuring apparatus 100 according to the first embodiment sets the scanning wavelength (second wavelength λ ₂ ) and the combined wavelength Λ ₁₂ according to the flowchart shown in FIG. 3 even when an error occurs in the scanning wavelength. By calibrating, the position of the test surface can be determined with high accuracy. Here, the flowchart shown in FIG. 3 shows a method of calibrating the composite wavelength Λ ₁₂ when the chromatic dispersion on the optical path of the test light is smaller than the target value, that is, when n ₂ / n ₁ = 1. Has been. However, there are actually cases where the wavelength dispersion on the optical path of the test light cannot be ignored (n ₂ / n ₁ ≠ 1). Therefore, a method for calibrating the combined wavelength Λ ₁₂ in consideration of the influence of chromatic dispersion on the optical path of the test light will be described below with reference to FIG. Figure 4 is a flow chart illustrating a method for calibrating the in consideration synthetic wavelength lambda ₁₂ an influence of chromatic dispersion in the optical path of the test light.

In S41, the control unit 12 controls the detection unit 11 to detect the phase difference between the test light and the reference light at the first wavelength λ ₁ (reference wavelength) and the second wavelength λ ₂ (scanning wavelength), respectively. To do. In S42, the control unit 12 changes the optical path length between the test light and the reference light by ΔL. In S43, the control unit 12, in a state of changing the optical path length of the reference beam and the test beam only [Delta] L, in the first wavelength lambda ₁ and the second wavelength lambda _2, the phase difference between test light and reference light The detection unit 11 is controlled to detect each. In S44, the control unit 12 controls the environment measurement unit 13 to measure the environment on the optical path of the test light, and based on the information on the environment measured by the environment measurement unit 13, the first wavelength λ ₁ and The atmospheric refractive index at the second wavelength λ ₂ is calculated. Here, S41 to S44 are the same as S31 to S34 in the flowchart shown in FIG.

In S45, the control unit 12 obtains the ratio _(n 2 / _{n 1)} with the atmosphere refractive index _{n 2} at atmospheric refractive index _{n 1} and the second wavelength lambda ₂ in the first wavelength lambda _1. Here, the case of performing S45 _initially, may be a _n 2 _/ n 1 = 1. In S46, the control unit 12, based on parameters obtained respectively at S41 to S45, the second computes the wavelength lambda ₂ using Equation (7). Since S46 is the same as S35 in the flowchart shown in FIG. 3, detailed description thereof is omitted. In S47, the control unit 12, the second wavelength lambda ₂ that has been calculated in S46 it is determined whether or not within the allowed range. If the second wavelength λ ₂ is not within the allowable range, the process returns to S44, and the second refractive index λ ₂ calculated in S46 is used to calculate the atmospheric refractive index n ₂ by a known dispersion equation (for example, Edren's equation). To do. Thus, it is possible by repeating S44～S47, error of the second wavelength lambda ₂ can be gradually reduced, fit the second wavelength lambda ₂ within the allowable range. Here, the allowable range refers to the second wavelength lambda ₂ in a range as not to affect the calculation result in the interference orders of the synthetic wavelength lambda ₁₂ of the first wavelength lambda ₁ and the second wavelength lambda _2. Interference order of the synthetic wavelength lambda ₁₂ is used in calculating the position of the test surface by a synthetic wavelength lambda _12. On the other hand, when the second wavelength lambda ₂ is within the allowable range, the process proceeds to S48. In S48, the control unit 12 uses the second wavelength lambda ₂ which falls within the allowable range, a first wavelength lambda ₁ and the synthetic wavelength lambda ₁₂ and the second wavelength lambda ₂ determined by Equation (8). Here, S48 is the same as S36 in the flowchart shown in FIG.

As described above, the measurement apparatus 100 according to the first embodiment acquires the amount of change in the phase difference between the test light and the reference light according to the change in the optical path length between the test light and the reference light, and the change. can be calculated synthetic wavelength Λ12 by the first wavelength lambda ₁ is a reference wavelength by using a quantity. As a result, even if the second wavelength λ ₂ is set by the Fabry-Perot etalon 2 having a high transmission spectrum dependency with respect to changes in the surrounding environment, the composite wavelength Λ ₁₂ can be obtained with high accuracy. That is, it is possible to reduce the error in the synthetic wavelength lambda _12, it is possible to measure the position of the test surface with high accuracy. Here, the measurement apparatus 100 according to the first embodiment may detect the phase difference before and after changing the optical path length between the test light and the reference light. Therefore, for example, even when the movement of the test surface is not a constant speed as in the interferometer used for controlling the substrate stage, the detected phase difference is not affected. That is, it the optical path length between the reference beam and the test light even when no change at a constant rate, which can be obtained with good synthetic wavelengths lambda ₁₂ precision, measures the position of the test surface with high accuracy it can. In the first embodiment, the gas cell is used as the first wavelength reference element. However, the gas cell is not limited thereto. For example, a Fabry-Perot etalon having a medium having a stable transmission spectrum may be used.

Next, a method for obtaining the combined wavelength Λ ₂₃ of the second wavelength λ ₂ and the third wavelength λ ₃ will be described. Similarly to the second wavelength λ ₂ , the third wavelength λ ₃ is the wavelength of light emitted from the light source 3, and is locked to a transmission spectrum different from the second wavelength λ ₂ in the Fabry-Perot etalon 2. Therefore, synthetic wavelength lambda ₂₃ and the second wavelength lambda ₂ and the third wavelength lambda _3, the first wavelength lambda ₁ and the interference order difference between the second wavelength lambda _2, the second wavelength lambda ₂ and the third wavelength lambda ₃ And the combined wavelength Λ ₁₂ of the first wavelength λ ₁ and the second wavelength λ ₂ . The combined wavelength Λ ₂₃ is the difference in etalon interference order between the first wavelength λ ₁ and the second wavelength λ ₂ to N ₁₂ , and the etalon interference order difference between the second wavelength λ ₂ and the third wavelength λ ₃ to N ₂₃ . Then, it can be expressed by equation (9). Here, the synthetic wavelength Λ ₂₃ represented by the equation (9) is based on the premise that the chromatic dispersion in the medium of the Fabry-Perot etalon 2 is sufficiently small and the FSR is constant regardless of the wavelength. That is, when the wavelength dispersion in the medium of the Fabry-Perot etalon 2 is small, for example, as shown by line 51 in FIG. 5, FSR is constant with respect to the interference order of the etalon, at that time, the synthetic wavelength lambda ₂₃ wherein It can be represented by (9).

On the other hand, there are cases where the wavelength dispersion in a Fabry-Perot etalon medium is large and cannot be ignored. In this case, the FSR is not constant with respect to the etalon interference order. In this case, the FSR with respect to the interference order is approximated as indicated by the line 52 in FIG. 5, and the slope (wavelength dispersion coefficient) when approximated is α, and a term for correcting chromatic dispersion is added to Equation (9). As a result, the combined wavelength Λ ₂₃ of the second wavelength λ ₂ and the third wavelength λ ₃ can be expressed by equation (10). Here, c is the speed of light, FSR ₁₂ is the average value of the FSR from the second wavelength λ ₂ to the first wavelength λ ₁ , and the FSR ₁₂ is expressed by Expression (11).

Further, the inclination α (wavelength dispersion coefficient α) can be obtained according to the following procedure. First, the resonator length d of the Fabry-Perot etalon 2 is obtained by Expression (12). Here, N ₁ is the interference order of the etalon at the first wavelength λ ₁ and is a known value. n _m1 is the refractive index of the medium of the Fabry-Perot etalon 2 at the first wavelength λ ₁ and can be obtained by a known dispersion equation. For example, the refractive index _nm1 can be obtained by Edren's equation when the etalon medium is air, and by the Selmeier dispersion equation when the etalon medium is glass.

As a result, FSR ₁ that is FSR at the first wavelength λ ₁ and FSR ₂ that is FSR at the second wavelength λ ₂ can be obtained by the equation (13), respectively, and the inclination α can be obtained by the equation (14). Can do. Here, n _{m @ 2} is the refractive index of the medium Fabry-Perot etalon 2 in the second wavelength lambda _2, as with the refractive index n _m1 at the first wavelength lambda _1, can be determined by known dispersion formula.

Second Embodiment
A measurement apparatus according to a second embodiment of the present invention will be described. In the measurement apparatus 100 of the first embodiment, the method of calibrating the synthetic wavelength Λ ₁₂ at the time of shipment of the apparatus or at the time of activation of the apparatus has been described. However, the method is not limited thereto, and the position of the test surface is measured. You may calibrate them in real time as you go. Therefore, in the second embodiment, the combined wavelength Λ ₁₂ is changed in real time when the test surface 7 is changed relative to the irradiation unit 10 (when the position of the test surface 7 is measured). A calibration method will be described with reference to FIGS. Here, since the device configuration in the measurement device of the second embodiment is the same as that of the measurement device 100 of the first embodiment, the description thereof is omitted here.

FIG. 6 is a flowchart showing a method for calibrating the synthetic wavelength in real time while measuring the position of the test surface 7. FIG. 7 is a graph showing the change of the phase difference Φ ₁ at the first wavelength λ ₁ detected by the detection unit 11 when the test surface 7 is moved, as a graph with respect to time. In S <b> 61, the control unit 12 controls the detection unit 11 to detect a phase difference between the reference surface 9 and the test surface 7. In S62, the control unit 12 accumulates information on the phase difference from the time t _n to the time t _n + Δ. Here, Δ is the accumulation time and can be arbitrarily set. For example, in the first wavelength lambda _1, the phase difference [Phi ₁ information from the time _{t n} to the time point _{t n} + delta is stored in the control unit 12, as shown in FIG. 7, from time _{t n} to the time point _{t n} + delta In addition, the phase difference changes from Φ ₁ (t _n ) to Φ ₁ (t _n + Δ). Also at the second wavelength λ ₂ , information on the phase difference from the time t _n to the time t _n + Δ is accumulated in the control unit 12 as in the first wavelength λ ₁ . In S63, the control unit 12 from the information of the accumulated phase difference in S62, to determine the time s to satisfy the condition of Formula (15), the phase difference [Phi _{(t n)} at time _{t n,} the time phase difference at t _n + s [Phi extracting a _(t n + s). Here, A is a threshold value in the amount of change in phase difference, and can be set arbitrarily. For example, in the first wavelength lambda _1, as shown in FIG. 7, from the information of the accumulated phase difference to the control unit, the phase difference [Phi ₁ at time t _n and _{(t n),} the phase difference at time t _{n +} s Φ ₁ (t _n + s) is extracted. Further, in the second wavelength lambda _2, similarly to the first wavelength lambda _1, the phase difference Φ _{2 (t n)} at time _{t n,} the phase difference [Phi ₂ at time _{t n + s (t n +} s) is extracted The

In S64, the control unit 12 calculates the second wavelength λ ₂ based on the phase difference Φ (t _n ) and the phase difference Φ (t _n + s) extracted in S63, based on the flowchart shown in FIG. The combined wavelength Λ ₁₂ is calibrated. In S65, the control unit 12 determines whether or not the measurement of the position of the test surface 7 has been completed. If it is determined that the measurement of the position of the test surface 7 is finished, the control unit 12 terminates the flow of calibrating a synthetic wavelength lambda _12. On the other hand, if it is determined that the measurement of the position of the test surface 7 does not end, the control unit 12 returns to S62, a step of calibrating again synthetic wavelength lambda _12. In this case, n 1 is added together to, in S62 in the second time, the control unit 12 stores the information of the phase difference at the time _{t n + 1} to time _{t n + 1 + Δ.} In this way, the phase difference of each wavelength is accumulated when measuring the position of the test surface 7 in the control unit 12, and the combined wavelength Λ of the first wavelength λ ₁ and the second wavelength λ ₂ in real time. By calibrating ₁₂ , the position of the test surface can be determined with high accuracy.

  As mentioned above, although preferred embodiment of this invention was described, it cannot be overemphasized that this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

Claims (11)

A measuring device for measuring the position of a surface to be tested,
A first wavelength set using the first wavelength reference element, and a second wavelength set using a second wavelength reference element whose wavelength dependency on environmental change is larger than the first wavelength reference element. An irradiation unit for irradiating the test surface and the reference surface with light;
A detector for detecting a phase difference between the test light from the test surface irradiated with the light and the reference light from the reference surface irradiated with the light;
A control unit that determines a position of the test surface using a combined wavelength of the first wavelength and the second wavelength;
Including
The control unit determines the wavelength of the second wavelength,
The amount of change in phase difference at the first wavelength and the amount of change in phase difference at the second wavelength before and after the change when the optical path length difference between the test light and the reference light is changed,
A refractive index on an optical path of the test light at the first wavelength;
A measurement apparatus characterized in that it is obtained based on a refractive index on the optical path at the second wavelength. The controller is configured to change the phase difference change ΔΦ ₁ at the first wavelength and the second wavelength at the second wavelength before and after the change in the optical path length difference between the test light and the reference light. The change amount ΔΦ ₂ of the phase difference is acquired by the detection unit,
When the variable indicating the refractive index on the optical path of the test light at the first wavelength is n ₁ , the variable indicating the refractive index on the optical path at the second wavelength is n ₂ , and the first wavelength is λ ₁ , Calculating the _second wavelength by the wavelength λ ₂ represented by the formula (1), and determining a combined wavelength of the first wavelength and the second wavelength;

The measuring apparatus according to claim 1. Wherein, when the synthetic wavelength and the second wavelength and the first wavelength is lambda _12, to determine the synthetic wavelength lambda ₁₂ according to equation (2),

The measuring apparatus according to claim 2. The light irradiated to the test surface and the reference surface by the irradiation unit includes a third wavelength set by the second wavelength reference element,
The control unit determines an order difference of the second wavelength reference element between the first wavelength and the second wavelength as N ₁₂ , and an order difference of the second wavelength reference element between the second wavelength and the third wavelength. N ₂₃ , where the combined wavelength of the second wavelength and the third wavelength is Λ ₂₃ , the combined wavelength Λ ₂₃ is calculated according to equation (3),

4. The measuring apparatus according to claim 2, wherein the position of the test surface is determined using the synthetic wavelength [Lambda] ₂₃ . The light irradiated to the test surface and the reference surface by the irradiation unit includes a third wavelength set by the second wavelength reference element,
The control unit determines the order difference of the second wavelength reference element between the first wavelength and the second wavelength as N ₁₂ , and sets the order difference between the second wavelength reference element as the second wavelength and the third wavelength as N ₁₂ . ₂₃ , a combined wavelength of the first wavelength and the second wavelength is Λ ₁₂ , an average value of a plurality of FSRs between the first wavelength and the second wavelength is FSR ₁₂ , and a wavelength in the second wavelength reference element the dispersion coefficient alpha, when the synthetic wavelength and the second wavelength and the third wavelength and lambda _23, calculates the synthetic wavelength lambda ₂₃ according to equation (4),

4. The measuring apparatus according to claim 2, wherein the position of the test surface is determined using the synthetic wavelength [Lambda] ₂₃ . The chromatic dispersion coefficient α is obtained according to the equation (5) when the FSR at the first wavelength is FSR ₁ and the FSR at the second wavelength is FSR ₂ .

The measuring apparatus according to claim 5.   The measurement apparatus according to claim 1, wherein the second wavelength reference element includes a Fabry-Perot etalon.   The measurement apparatus according to claim 1, wherein the first wavelength reference element includes a gas cell. It further includes an environment measurement unit that measures the environment on the optical path,
The measurement apparatus according to claim 1, wherein the control unit acquires the refractive index using information measured by the environment measurement unit. The information includes temperature, pressure and humidity on the optical path,
The measuring apparatus according to claim 9, wherein the control unit acquires the refractive index according to Edren's equation. The control unit is configured to change the second surface when a change amount of the phase difference detected by the detection unit exceeds a threshold value when the test surface is changed relative to the irradiation unit. The wavelength is calculated by the wavelength λ ₂ represented by the equation (1) using the first wavelength λ ₁ , and a combined wavelength of the first wavelength and the wavelength is determined. 7. The measuring device according to any one of 6 to 6.

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