System and method for correcting optical path length measurement errors - Patents.com

Abstract

To provide a system and a method that can correct an optical path length measurement error caused by a spectroscope.SOLUTION: A system includes: a first optical section that emits light to a measurement object, and receives first interference light from the measurement object; a second optical section that emits light to a reference object configured to make an optical path length constant in response to temperature fluctuations and receives second interference light from the reference object; a spectroscope that is connected to the first optical section and the second optical section and receives the first interference light and the second interference light; and a control section connected to the spectroscope. The control section calculates a change rate of a measurement optical path length relative to a reference optical path length under a predetermined temperature environment based on a measurement optical path length of the reference object calculated based on the second interference light incident on the spectroscope under the predetermined temperature environment and a reference optical path length of the reference object acquired in advance, and corrects, based on the change rate, the optical path length of the measurement object calculated based on the first interference light incident on the spectroscope under the predetermined temperature environment.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] Exemplary embodiments of the present disclosure relate to systems and methods for correcting optical path length measurement errors. [Background technology]

[0002] Patent Document 1 discloses a system that uses optical interference to appropriately measure the temperature of an object to be measured. The system includes a light source, a spectroscope, an optical transmission mechanism, an optical path length calculation unit, and a temperature calculation unit. The light source generates measurement light. The optical transmission mechanism emits reflected light from the front and back surfaces of the object to be measured to the spectroscope. The spectroscope measures the interference intensity distribution, which is the intensity distribution of the reflected light. The optical path length calculation unit performs a Fourier transform on the measured interference intensity distribution to calculate the optical path length. The temperature calculation unit calculates the temperature of the object to be measured based on the relationship between the optical path length and temperature. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2013-29487 A Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a system and method that can correct for optical path length measurement errors caused by a spectrometer. [Means for solving the problem]

[0005] A system according to an aspect of the present disclosure includes a light source, a first optical unit, a second optical unit, a spectrometer, and a control unit. The light source generates light. The first optical unit is configured to emit light to a measurement object and to receive a first interference light from the measurement object. The second optical unit is configured to emit light to a reference object configured to have a constant optical path length with respect to temperature fluctuations and to receive a second interference light from the reference object. The spectrometer is connected to the first optical unit and the second optical unit and configured to receive the first interference light and the second interference light. The control unit is connected to the spectrometer. The control unit calculates a fluctuation rate of the measured optical path length relative to the reference optical path length under a predetermined temperature environment based on a measured optical path length of the reference object calculated based on the second interference light incident on the spectrometer under a predetermined temperature environment and a reference optical path length of the reference object acquired in advance. Then, the control unit corrects the optical path length of the measurement object calculated based on the first interference light incident on the spectrometer under a predetermined temperature environment based on the fluctuation rate. Effect of the Invention

[0006] According to the present disclosure, it is possible to correct optical path length measurement errors caused by a spectrometer. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an example of a configuration of a system according to an embodiment. [Diagram 2] FIG. 2 is a diagram illustrating an example of the configuration of the interferometer illustrated in FIG. [Diagram 3] FIG. 2 is a diagram illustrating an example of a cross section of a reference object. [Figure 4] FIG. 13 is a diagram illustrating another example of a reference object. [Diagram 5] FIG. 1 is a diagram illustrating an error of a spectroscope. [Figure 6] 1A and 1B are diagrams illustrating the principle of correction of the optical path length. [Figure 7] 11 is a flowchart illustrating a method for correcting an optical path length. [Figure 8] 13A and 13B are diagrams illustrating modified examples of the second optical unit. [Figure 9]4A and 4B are diagrams for explaining the arrangement position of a second optical unit. [Figure 10] (A) and (B) are the simulation results. [Figure 11] FIG. 1 is a diagram illustrating an example of a configuration of a system according to an embodiment. [Figure 12] 13 is a graph showing the optical path length before and after correction. [Figure 13] FIG. 2 is a diagram illustrating an example of a cross section of a holder. [Figure 14] FIG. 13 is a diagram illustrating an example of a cross section of another holder. [Figure 15] 4A and 4B are diagrams for explaining the positional relationship between the second optical unit and a reference object. [Figure 16] 11 is a graph showing the optical path length and signal intensity measured for each relative horizontal position between the second optical unit and a reference object. [Figure 17] 13 is a graph showing the optical path length and signal intensity measured for each focal length of the second optical unit. [Figure 18] 13 is a graph showing the optical path length and signal intensity measured for each angle around the optical axis of the second optical unit. [Figure 19] 1 is a table showing the configuration of a reference object according to an embodiment. [Figure 20] 1 is a table showing evaluation results of examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Various exemplary embodiments are described below.

[0009] In an interferometer that uses a spectrometer to obtain a reflection spectrum from a measurement target (hereinafter, a spectroscopic interferometer), the temperature of the spectrometer mounted in the interferometer fluctuates due to the ambient temperature and heat generated in the interferometer. The spectrometer includes an array of multiple light receiving elements. If the temperature of the spectrometer fluctuates, the spacing or arrangement of the multiple light receiving elements may fluctuate due to thermal expansion or contraction. In this case, the spectrometer may not be able to properly measure the wavelength, and an error may occur in the measured optical path length. Therefore, the present disclosure provides a system and method that can correct optical path length measurement errors caused by the spectrometer.

[0010] A system according to an aspect of the present disclosure includes a light source, a first optical unit, a second optical unit, a spectrometer, and a control unit. The light source generates light. The first optical unit is configured to emit light to a measurement object and to receive a first interference light from the measurement object. The second optical unit is configured to emit light to a reference object configured to have a constant optical path length with respect to temperature fluctuations and to receive a second interference light from the reference object. The spectrometer is connected to the first optical unit and the second optical unit and configured to receive the first interference light and the second interference light. The control unit is connected to the spectrometer. The control unit calculates a fluctuation rate of the measured optical path length relative to the reference optical path length under a predetermined temperature environment based on a measured optical path length of the reference object calculated based on the second interference light incident on the spectrometer under a predetermined temperature environment and a reference optical path length of the reference object acquired in advance. Then, the control unit corrects the optical path length of the measurement object calculated based on the first interference light incident on the spectrometer under a predetermined temperature environment based on the fluctuation rate.

[0011] In this system, the first optical unit emits light to the measurement object and allows a first interference light from the measurement object to be incident. Then, the second optical unit emits light to a reference object configured to have a constant optical path length with respect to temperature fluctuations and allows a second interference light from the reference object to be incident. The first interference light and the second interference light are incident on the spectroscope. The control unit calculates a fluctuation rate of the measured optical path length with respect to the reference optical path length under a predetermined temperature environment based on the measured optical path length of the reference object calculated based on the second interference light incident on the spectroscope under a predetermined temperature environment and the reference optical path length of the reference object acquired in advance. Then, the optical path length of the measurement object calculated based on the first interference light incident on the spectroscope under a predetermined temperature environment is corrected based on the fluctuation rate. Since the optical path length of the reference object is constant with respect to temperature fluctuations, if the optical path length calculated from the measurement result of the spectroscope under a predetermined temperature environment fluctuates from the reference optical path length, it can be said that a measurement error caused by the temperature change of the spectroscope occurs. The measurement error caused by the temperature change of the spectrometer can be expressed as the optical path length fluctuation rate of the reference object. Therefore, the system can correct the optical path length of the measurement object based on the optical path length fluctuation rate so that the measurement error caused by the temperature change of the spectrometer is canceled. Therefore, the system can correct the optical path length measurement error caused by the temperature change of the spectrometer.

[0012] In one embodiment, the system may further include a circulator and an optical switch. The circulator is connected to the light source. The optical switch is connected to the circulator. The first optical unit may include a first optical fiber and a first optical element. The first optical fiber propagates light from the optical switch. The first optical element is configured to emit light to the measurement object and to input a first interference light from the measurement object. In one embodiment, the first optical element may be a focuser or a collimator.

[0013] In one embodiment, the second optical unit may include a second optical fiber and a second optical element. The second optical fiber propagates light from the optical switch. The second optical element is configured to emit light to the reference object and to receive second interference light from the reference object. In one embodiment, the second optical element may be a focuser or a collimator. In this case, the system can measure interference light from each of the measurement object and the reference object by switching the emission direction of the light by the optical switch.

[0014] In one embodiment, the reference object may be an etalon element. The second optical unit includes a third optical element and a fourth optical element. The third optical element emits light to the etalon element. The fourth optical element is configured to receive the transmitted light from the etalon element. In one embodiment, the third optical element and the fourth optical element may be a focuser or a collimator. In this case, the system can calculate the optical path length variation rate of the reference object based on the transmitted interference light from the etalon element.

[0015] In one embodiment, the reference object may include a pair of opposing parallel plates, hi one embodiment, a vacuum may be present in the space between the parallel plates.

[0016] A method according to another aspect of the present disclosure includes the steps of emitting light to a measurement object and inputting a first interference light from the measurement object to a spectroscope under a predetermined temperature environment, calculating an optical path length of the measurement object based on the first interference light, emitting light to a reference object configured to have a constant optical path length against temperature fluctuations and inputting a second interference light from the reference object to a spectroscope under a predetermined temperature environment, calculating a measured optical path length of the reference object based on the second interference light, acquiring a reference optical path length of the reference object, calculating a fluctuation rate of the measured optical path length relative to the reference optical path length under a predetermined temperature environment based on the measured optical path length of the reference object calculated based on the second interference light and the reference optical path length of the reference object, and correcting the optical path length of the measurement object based on the fluctuation rate. This method can correct an optical path length measurement error caused by a temperature change of the spectroscope, similar to the above-mentioned system.

[0017] Various embodiments will be described in detail below with reference to the drawings. In the following description and in each drawing, the same or corresponding elements are given the same reference numerals, and redundant description will not be repeated. The dimensional ratios of the drawings do not necessarily match those in the description. The terms "upper", "lower", "left" and "right" are based on the illustrated state and are for convenience.

[0018] (System Overview) FIG. 1 is a diagram illustrating an example of a system configuration according to an embodiment. The system 1 illustrated in FIG. 1 is a system that measures the optical path length of a measurement object SA by utilizing optical interference. The measurement object SA is, for example, a member formed so that both the front and back surfaces are parallel. The measurement object SA is transparent to the light used in the system 1. As an example, the measurement object SA is a polished single crystal silicon member. For example, the measurement object SA may be composed of at least one of Si (silicon), SiO2 (quartz), and Al2O3 (sapphire) for light of 1550 nm.

[0019] The system 1 includes an interferometer 10 and a first optical unit 14. The interferometer 10 generates measurement light. The interferometer 10 and the first optical unit 14 are connected by an optical fiber. The measurement light generated by the interferometer 10 is propagated to the first optical unit 14 by the optical fiber.

[0020] The first optical unit 14 is configured to emit light to the measurement object SA. The first optical unit 14 is, for example, a collimator or a focuser. The first optical unit 14 emits light adjusted as parallel rays or convergent light focused on the measurement object SA toward the surface of the measurement object SA. Reflected light from the measurement object SA (hereinafter also referred to as first interference light) is incident on the first optical unit 14. The first interference light includes not only reflected light from the surface but also reflected light from the back surface. The incident first interference light is propagated to the interferometer 10 via an optical fiber. As described later, the interferometer 10 calculates the optical path length of the measurement object SA based on the first interference light.

[0021] The system 1 has a configuration for measuring the optical path length of the reference object RE in order to cancel the temperature-dependent accuracy error of the interferometer 10. The reference object RE is configured so that the optical path length is constant with respect to temperature fluctuations. In other words, the reference object RE is configured so that the optical path length of the reference object RE hardly varies even when the environmental temperature fluctuates. For example, the reference object RE is an object whose temperature is strictly controlled. Alternatively, the reference object RE is an object whose optical path length has no temperature dependency or is extremely small. A specific example of these reference objects RE will be described later.

[0022] The system 1 has a second optical unit 15 for measuring the optical path length of the reference object RE. An optical switch 11 is provided between the interferometer 10 and the first optical unit 14. The optical switch 11 has a function of switching the propagation direction of light. The measurement light generated by the interferometer 10 is propagated to the optical switch 11 through an optical fiber, and is propagated to either the first optical unit 14 or the second optical unit 15 depending on the function of the optical switch 11. The optical switch 11 and the first optical unit 14 are connected by a first optical fiber 12, and the optical switch 11 and the second optical unit 15 are connected by a second optical fiber 13. In other words, the optical switch 11 is configured to be able to switch between the path of the first optical fiber 12 and the path of the second optical fiber 13.

[0023] The second optical unit 15 is configured to emit light to the reference object RE. The second optical unit 15 is, for example, a collimator or a focuser. The second optical unit 15 emits light adjusted as parallel rays or convergent light focused on the reference object RE toward the surface of the reference object RE. Reflected light from the reference object RE (hereinafter also referred to as second interference light) is incident on the second optical unit 15. The second interference light includes not only reflected light from the surface but also reflected light from the back surface. The incident second interference light is propagated to the interferometer 10 via the second optical fiber 13 and the optical switch 11. As will be described later, the interferometer 10 calculates the optical path length of the reference object RE based on the second interference light.

[0024] (Interferometer details) Fig. 2 is a diagram illustrating an example of the configuration of the interferometer shown in Fig. 1. The interferometer 10 includes a light source 100, an isolator 101, a circulator 102, a spectrometer 103, and a control unit 104. The light source 100, the isolator 101, the circulator 102, and the spectrometer 103 are connected by optical fibers.

[0025] The light source 100 generates measurement light. As the light source 100, an SLD (Super Luminescent Diode) is used, for example.

[0026] 1. Then, as described above, the first and second interference lights are acquired from the measurement object SA and the reference object RE, and are propagated to the optical switch 11. The first and second interference lights propagated from the optical switch 11 to the interferometer 10 are propagated from the circulator 102 to the spectrometer 103. The isolator 101 prevents the first and second interference lights from returning to the light source 100. A coupler may be used instead of the circulator 102.

[0027] The spectrometer 103 is connected to the first optical unit 14 and the second optical unit 15, and is configured to receive the first interference light and the second interference light. The spectrometer 103 has, for example, a spectroscopic mechanism unit and a light receiving unit. The spectroscopic mechanism disperses light at a predetermined dispersion angle for each wavelength. An example of the spectroscopic mechanism is a diffraction grating. The light receiving unit acquires the light dispersed by the spectroscopic mechanism. An example of the light receiving unit is a CCD (Charge Coupled Device). The number of light receiving elements is the number of samples. In addition, a wavelength span is defined based on the dispersion angle of the spectroscopic mechanism and the distance between the spectroscopic mechanism and the photoelectric element. The first interference light and the second interference light are each dispersed by wavelength in the spectrometer 103, and the light intensity for each wavelength is acquired. The spectrometer 103 converts the light intensity for each wavelength into digital data and outputs it to the control unit 104.

[0028] The control unit 104 is connected to the spectrometer 103, and performs calculations on digital data relating to the first interference light and the second interference light. The control unit 104 is physically configured as a computer system including a calculation device such as a CPU, a main storage device such as a RAM and a ROM, an auxiliary storage device such as a hard disk, and a communication interface such as a network card. The control unit 104 calculates the optical path length of the measurement object SA, calculates the optical path length of the reference object RE, generates parameters for canceling the measurement error of the spectrometer 103, and corrects the optical path length of the measurement object SA. Details will be described later. The control unit 104 may be provided outside the interferometer 10.

[0029] (Example of reference object) FIG. 3 is a diagram showing an example of a cross section of a reference object. The reference object RE shown in FIG. 3 is an object whose optical path length has no temperature dependency or is extremely small. The reference object RE has a pair of opposing parallel plates 21, 23. The parallel plate 21 is formed of a material that transmits the light of the interferometer 10. The parallel plate 21 is formed of, for example, sapphire, quartz, or the like. The front and back surfaces of the parallel plate 21 have optically polished, highly accurate flat surfaces. An anti-reflection layer is formed on the top surface 21a of the parallel plate 21. As a result, the light incident on the parallel plate 21 is reflected only on the back surface. The parallel plate 23 has a mirror-polished layer of a material that reflects the light of the interferometer 10 on its surface. The reflecting material is, for example, gold, aluminum, or the like. The parallel plate 23 is formed of, for example, sapphire, quartz, or the like. The front surface of the parallel plate 23 has an optically polished, highly accurate flat surface. The light incident on the parallel plate 23 is reflected only on its front surface. A spacer 22 is interposed between the pair of parallel plates 21, 23. This defines a space between the pair of parallel plates 21, 23. The spacer 22 is made of a material with a small linear expansion coefficient, such as quartz. The spacer 22 may be made of a glass material with an extremely small thermal expansion coefficient. This causes the distance between the parallel plates 21, 23 to barely change with temperature, making the temperature dependency of the optical path length extremely small. The space between the parallel plates 21, 23 may be a vacuum. In this case, the temperature dependency of the optical path length is further reduced.

[0030] FIG. 4 is a diagram showing another example of the reference object. The reference object REA shown in FIG. 4 is an object whose temperature is strictly controlled. The reference object REA has a member 24 formed so that both the front and back surfaces are parallel. The member 24 is transparent to the light used in the system 1. The member 24 is, for example, a polished single crystal silicon member. The reference object RE may be made of at least one of Si, SiO2, and Al2O3 for light of, for example, 1550 nm. The member 24 is accommodated in a temperature holder 25. The temperature holder 25 has a function of maintaining the temperature at a predetermined temperature by a heater, a heat exchanger, or the like. The temperature of the member 24 is maintained at a predetermined temperature by the temperature holder 25.

[0031] (Spectrometer Errors Due to Temperature) 5 is a diagram for explaining the error of a spectrometer. The spectrometer 103 has an array of photoelectric elements. In the example shown in FIG. 5, the photoelectric element group LM1 detects wavelengths λ0 to λ 511 The spectrometer 103 has photoelectric elements that receive the light split into three wavelengths. When the light SP is incident, the photoelectric elements measure the intensity for each wavelength of the light SP. The wavelength of the photoelectric element array of the spectrometer 103 is determined by the positional relationship of the optical elements such as the spectroscopic mechanism, mirrors, and photoelectric elements. For this reason, if the position of the optical elements changes due to temperature fluctuations, an error will occur in the wavelength of the spectrometer 103. For example, the entire photoelectric element array may shift left or right (photoelectric element group LM2), or the photoelectric elements may expand or contract (photoelectric element group LM3). If measurement is performed using the spectrometer 103 with the position of the optical elements changed, an accurate optical path length of the measurement object SA may not be obtained.

[0032] (Principle of optical path length correction) The control unit 104 calculates the optical path length of the measurement object SA. The control unit 104 performs Fourier transform, data interpolation, and center of gravity calculation on the optical spectrum, which is digital data of the first interference light, to calculate the optical path length of the measurement object SA. FIG. 6 is a diagram for explaining the optical path length in outline. The peak position of the optical spectrum shown in FIG. 6A indicates the measured optical path length L_SA of the measurement object SA. Similarly, the control unit 104 calculates the optical path length of the reference object RE. The control unit 104 performs Fourier transform, data interpolation, and center of gravity calculation on the optical spectrum, which is digital data of the second interference light, to calculate the optical path length of the reference object RE. The peak position of the optical spectrum shown in FIG. 6B indicates the measured optical path length L_RE of the reference object RE. The measured optical path length L_SA and the measured optical path length L_RE are calculated based on the interference light incident on the spectroscope 103 under a predetermined temperature environment. The predetermined temperature environment refers to a case where the temperature conditions at the time of measurement are predetermined conditions. That is, the first interference light and the second interference light are measured by the spectroscope 103 under the same temperature condition.

[0033] The optical path length of the reference object RE has no temperature dependency or is extremely small. Therefore, when the true value of the optical path length of the reference object RE (or the optical path length under a predetermined temperature environment) is taken as the reference optical path length L_REM, if the spectrometer 103 is accurate, the measured optical path length L_RE and the reference optical path length L_REM will match. However, if the measured optical path length L_RE and the reference optical path length L_REM do not match, it is considered that an error dependent on temperature occurs in the spectrometer 103 under the predetermined temperature environment. The control unit 104 calculates a parameter for canceling the difference between the measured optical path length L_RE and the reference optical path length L_REM. Specifically, the control unit 104 determines the value obtained by dividing the measured optical path length L_RE by the reference optical path length L_REM as the fluctuation rate of the measured optical path length L_RE with respect to the reference optical path length L_REM under the predetermined temperature environment. The control unit 104 corrects the measured optical path length L_RE of the measurement target SA based on the fluctuation rate. The control unit 104 can correct the optical path length of the measurement object SA, for example, by dividing the measurement optical path length L_RE of the measurement object SA by the fluctuation rate.

[0034] (Method of correcting optical path length) FIG. 7 is a flowchart showing a method for correcting the optical path length. The flowchart shown in FIG. 7 can be executed by the system 1. First, the first optical unit 14 emits light to the measurement object SA and causes the first interference light from the measurement object SA to enter the spectroscope 103 in a predetermined temperature environment (step S10). Next, the control unit 104 calculates the optical path length L_SA of the measurement object SA based on the first interference light (step S12, (A) of FIG. 6). Next, the second optical unit 15 emits light to the reference object RE and causes the second interference light from the reference object RE to enter the spectroscope 103 in a predetermined temperature environment (step S14). Next, the control unit 104 calculates the measurement optical path length L_RE of the reference object RE based on the second interference light (step S16, (B) of FIG. 6).

[0035] Next, the control unit 104 acquires the reference optical path length L_REM of the reference object RE (step S18, (B) of FIG. 6). The control unit 104 may read the true value of the reference optical path length of the reference object RE stored in a memory or the like, or may read the reference optical path length of the reference object RE measured at a predetermined timing and stored in a memory or the like.

[0036] Next, the control unit 104 calculates the variation rate of the measurement optical path length L_RE (step S20). The control unit 104 calculates the variation rate of the measurement optical path length relative to the reference optical path length under a predetermined temperature environment based on the measurement optical path length L_RE of the reference object RE calculated based on the second interference light and the reference optical path length L_REM of the reference object RE. The control unit 104 regards the value obtained by dividing the measurement optical path length L_RE by the reference optical path length L_REM as the variation rate of the measurement optical path length L_RE relative to the reference optical path length L_REM under a predetermined temperature environment.

[0037] Finally, the control unit 104 corrects the optical path length L_SA of the measurement object SA (step S22). The control unit 104 corrects the optical path length of the measurement object SA by dividing the optical path length L_SA of the measurement object SA by the fluctuation rate. This is the end of the flowchart showing the method for correcting the optical path length.

[0038] (Summary of the embodiment) In the system 1 and the method for correcting the optical path length, the first optical unit 14 emits light to the measurement object SA and causes the first interference light from the measurement object SA to be incident. Then, the second optical unit 15 emits light to a reference object RE configured to have a constant optical path length with respect to temperature fluctuations and causes the second interference light from the reference object RE to be incident. The first interference light and the second interference light are incident on the spectroscope 103. The control unit 104 calculates a fluctuation rate based on the measured optical path length L_RE of the reference object RE calculated based on the second interference light incident on the spectroscope 103 under a predetermined temperature environment and the reference optical path length L_REM of the reference object RE acquired in advance. The fluctuation rate is the fluctuation rate of the measured optical path length L_RE with respect to the reference optical path length L_REM under a predetermined temperature environment. Then, the optical path length L_SA of the measurement object SA calculated based on the first interference light incident on the spectroscope 103 under a predetermined temperature environment is corrected based on the fluctuation rate. Since the optical path length of the reference object RE is constant against temperature fluctuations, if the optical path length calculated from the measurement results of the spectrometer 103 in a specified temperature environment fluctuates from the reference optical path length L_REM, it can be said that a measurement error occurs due to the temperature change of the spectrometer 103. The measurement error caused by the temperature change of the spectrometer 103 can be expressed as a fluctuation rate of the optical path length of the reference object RE. Therefore, the system can correct the optical path length L_SA of the measurement target SA based on the fluctuation rate of the optical path length so that the measurement error caused by the temperature change of the spectrometer 103 is canceled. Therefore, this system can correct the optical path length measurement error caused by the temperature change of the spectrometer 103.

[0039] (Modification) It will be understood that various modifications of the disclosed embodiments may be made without departing from the scope and spirit of the present disclosure, and therefore the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the appended claims.

[0040] (Modification of the second optical section) FIG. 8 is a diagram for explaining a modified example of the second optical unit. As shown in FIG. 8, the second optical unit 15A is different from the second optical unit 15 in that it obtains transmitted light, not reflected light, as the second interference light. The second optical unit 15A has a circulator 150 and a pair of focusers or collimators 151, 152. A reference object REB is disposed between the pair of focusers or collimators 151, 152. The reference object REB includes an air gap therein and emits interference light according to the thickness of the air gap. As an example, the reference object REB is an etalon element.

[0041] FIG. 9 is a diagram for explaining the arrangement position of the second optical unit. As shown in FIG. 9, the second optical unit 15 may be provided at a position P1 between the isolator 101 and the circulator 102. In this case, the second optical unit 15 is built into the interferometer 10. Since the first interference light and the second interference light are acquired simultaneously, the optical path length may be set so that the FFT peak positions do not overlap. The second optical unit 15 may be provided at either a position P2 between the circulator 102 and the optical switch 11, or a position P3 between the circulator 102 and the spectrometer 103.

[0042] (simulation) A simulation was carried out to verify the effects of the present disclosure. As shown in FIG. 5, there are 512 photoelectric elements, and the reflection spectrum from silicon, which is the object to be measured, is received. 1550 nm±25 nm is equally divided by the 512 elements, and λ0 to λ 511 Using this wavelength as a reference, we verified how the optical path length changes when the photoelectric element is moved. The wavelength when the element is shifted as a whole (LM2 in Fig. 5) and the wavelength when the element is expanded or contracted (LM3 in Fig. 5) are called the fluctuation wavelengths (λ'0 to λ' 511) and calculated the reflectance at each of the fluctuating wavelengths. The thickness of the silicon to be measured was 775 μm, 675 μm, and 575 μm. Based on the calculated reflectance and wavelength, FFT processing was performed to calculate the optical path length. The optical path length calculated from the fluctuating wavelength was divided by the optical path length calculated using the reference wavelength, and this was used as the fluctuation rate of the optical path length. The amount of fluctuation in wavelength and the fluctuation rate of the optical path length were plotted for each thickness. The results are shown in (A) and (B) of FIG. 10. (A) and (B) of FIG. 10 are simulation results. (A) of FIG. 10 is a graph showing the relationship between the shift amount of the photoelectric element of the spectrometer and the fluctuation rate. (B) of FIG. 10 is a graph showing the relationship between the expansion / contraction amount (expansion amount of the wavelength range) of the photoelectric element of the spectrometer and the fluctuation rate. As shown in (A) and (B) of FIG. 10, the fluctuation rate of the optical path length due to the wavelength fluctuation of the spectrometer 103 was constant regardless of the thickness of the measurement object. This confirmed that by measuring one reference optical path length, effective correction can be performed for measurement objects having a plurality of different optical path lengths.

[0043] (Effectiveness check) FIG. 11 is a diagram showing an example of the configuration of a system according to an embodiment. The system 1A shown in FIG. 11 is different from the system 1 in FIG. 1 in that the interferometer 10 and the optical switch 11 are arranged inside the thermostatic chamber TA, the reference object RE is arranged in the temperature controller TB, and the measurement object SA is arranged in the temperature controller TC, and the rest is the same. That is, the environmental temperature of the interferometer 10 is made variable. The temperature range is set to 5°C to 40°C. The measurement object SA and the reference object RE are silicon substrates, and the temperature is controlled to a constant value. Since the temperature is constant, there is no increase or decrease in the refractive index due to thermal expansion or temperature, and the optical path length is constant. The optical path lengths of the respective objects when the environmental temperature is changed in the thermostatic chamber TA are measured, and the optical path length of the measurement object SA is evaluated in the cases where the correction is made with the fluctuation rate of the reference optical path length and where the correction is not made. The optical path length of the reference object RE is normalized with the optical path length at the start of measurement set to 1, and this is taken as the fluctuation rate of the reference optical path length. The optical path length of the measurement object SA measured at the same timing was divided by the reference optical path length fluctuation rate to obtain the corrected optical path length. The results are shown in Fig. 12. In Fig. 12, the horizontal axis represents the environmental temperature and the vertical axis represents the optical path length. As shown in Fig. 12, in the environmental temperature range of 5°C to 40°C, it was confirmed that the fluctuation range of the optical path length before correction was 3.3 μm, whereas the fluctuation range of the optical path length after correction was improved to 0.24 μm.

[0044] (holder) The system 1 may include a holder that holds the relative positions of the second optical unit 15 and the reference object RE. FIG. 13 is a diagram that shows an example of a cross section of the holder. As shown in FIG. 13, the holder 3 fixes the second optical unit 15 and the reference object RE. The holder 3 includes a housing 30. The housing 30 is a cylindrical body, and is, for example, a cylinder. The housing 30 may be made of a member having a small linear expansion coefficient. The housing 30 is formed of, for example, synthetic quartz. The housing 30 has an internal optical path 30c with a reduced diameter in the central part of the housing 30 in the vertical direction.

[0045] The upper surface of the housing 30 is opened. The second optical unit 15 is accommodated inside the housing 30 through the opening on the upper surface of the housing 30, and is arranged so that it can emit light to the internal optical path 30c and receive light from the internal optical path 30c. The lower surface of the second optical unit 15 is abutted against the first step surface 30b on which the upper end of the internal optical path 30c is formed. The abutted second optical unit 15 is sandwiched between the first holding member 31 and the first step surface 30b. The first holding member 31 is attached to the housing 30. For example, a screw thread is formed on the side surface of the first holding member 31 to screw into a screw groove formed on the inner surface 30a of the housing 30. A screw groove may be formed on the side surface of the first holding member 31 to screw into a screw groove formed on the inner surface 30a of the housing 30. The second optical unit 15 is fixed inside the housing 30 by screwing the first holding member 31 to the inner surface of the housing 30 with the second optical unit 15 sandwiched between the first holding member 31 and the first step surface 30b.

[0046] The lower surface of the housing 30 is opened. The reference object RE is accommodated inside the housing 30 through the opening of the lower surface of the housing 30, and is arranged so as to be able to receive light from the internal optical path 30c and reflect light to the internal optical path 30c. The upper surface 21a of the reference object RE is abutted against the second step surface 30d on which the lower end of the internal optical path 30c is formed. The abutted reference object RE is sandwiched between the second holding member 32 and the second step surface 30d. The second holding member 32 is attached to the housing 30. For example, a screw thread is formed on the side surface of the second holding member 32 to screw into a screw groove formed on the inner surface 30e of the housing 30. A screw groove may be formed on the side surface of the second holding member 32 to screw into a screw groove formed on the inner surface 30e of the housing 30. The reference object RE is fixed inside the housing 30 by screwing the second pressing member 32 to the inner surface of the housing 30 with the reference object RE sandwiched between the second pressing member 32 and the second step surface 30d.

[0047] In the holder 3, the housing 30 functions as a spacer that maintains the relative position between the second optical unit 15 and the reference object RE. This suppresses fluctuations in the incident angle of light and fluctuations in the measurement position, and as a result, suppresses decreases in signal strength. Therefore, the system 1 equipped with the holder 3 can more accurately correct optical path length measurement errors.

[0048] The housing 30 of the holder 3 is made of a material with a small linear expansion coefficient, which can reduce the effect of external temperature changes (environmental temperature changes) on the relative position between the second optical unit 15 and the reference object RE. In addition, since the components constituting the holder 3 are mechanically connected, the effect of external temperature changes on the relative position between the second optical unit 15 and the reference object RE can be reduced compared to when they are fixed with an adhesive or the like.

[0049] (Modification of holder) In the holder 3, the housing 30 functions as a spacer that maintains the relative position between the second optical unit 15 and the reference object RE, but the housing and the spacer may be separate bodies. FIG. 14 is a diagram that shows an example of a cross section of another holder. As shown in FIG. 14, the holder 4 fixes the second optical unit 15 and the reference object RE. The holder 4 includes a housing 40. The housing 40 is a cylindrical body, for example, a cylinder. Unlike the housing 30 shown in FIG. 13, the housing 40 does not function as a spacer that maintains the relative position between the second optical unit 15 and the reference object RE. For this reason, the housing 40 does not need to be made of a material with a small linear expansion coefficient, and is formed of, for example, aluminum or stainless steel.

[0050] 13, the second optical unit 15 and the reference object RE are housed inside the housing 40. A first spacer member 34 is disposed between the second optical unit 15 and the reference object RE. The first spacer member 34 may be made of a material with a small linear expansion coefficient. The first spacer member 34 is formed of, for example, synthetic quartz.

[0051] The second optical unit 15 and the reference object RE are sandwiched and fixed between the first holding member 41 and the second holding member 42. That is, the second optical unit 15 and the reference object RE are sandwiched between the first holding member 41 and the second holding member 42 while the relative positions are maintained by the first spacer member 34. A screw groove is formed on the inner surface of the housing 40 to screw with the screw formed on the outer periphery of the first holding member 41 and the second holding member 42. A screw may be formed on the inner surface of the housing 40 to screw with the screw groove formed on the outer periphery of the first holding member 41 and the second holding member 42. In this way, the first holding member 41 and the second holding member 42 are screwed to the housing 40 and sandwiched from above and below, thereby fixing the second optical unit 15 and the reference object RE.

[0052] In order to stably hold the second optical unit 15 and the reference object RE, a second spacer member 33 is provided between the first holding member 41 and the second optical unit 15. The second spacer member 33 may be made of a material with a small linear expansion coefficient. The second spacer member 33 may be made of a material with a small thermal conductivity. The second spacer member 33 is made of, for example, synthetic quartz.

[0053] An elastic member 43 is provided between the first holding member 41 and the second spacer member 33. Similarly, an elastic member 44 is provided between the second holding member 42 and the reference object RE. The elastic members 43, 44 are annular members, and are formed of resin, for example. This allows the relative position between the second optical unit 15 and the reference object RE to be maintained even if the housing 40 thermally expands / contracts due to external temperature fluctuations. Furthermore, the elastic members 43, 44 form an air layer between the housing 40 and components such as the second optical unit 15 and the reference object RE. For this reason, the holder 4 has a structure that makes it difficult for heat to be transferred from the housing 40 to the internal components.

[0054] The holder 4 may include at least one of the elastic members 43 and 44. Even with this configuration, when the housing 40 thermally expands / contracts, the holder 4 can maintain the relative position between the second optical unit 15 and the reference object RE. The holder 4 may include a third spacer member between the second holding member 42 and the reference object RE. The third spacer member is formed of, for example, synthetic quartz. By including the third spacer member in the holder 4, the structure is such that heat is less likely to be transferred from the housing 40 to the reference object RE.

[0055] The holders 3, 4 may be housed in a thermally insulating structure to suppress external temperature changes. The holders 3, 4 may hold a reference object REA instead of the reference object RE. Alternatively, the holders 3, 4 may hold a reference object REB.

[0056] (Modification of the reference object) The reference object may include a reflective film on at least one of the opposing inner surfaces of the parallel plates in order to increase the reflection intensity. For example, the reference object RE shown in Figs. 13 and 14 includes a pair of parallel plates 21 and 23. A reflective film is formed on the opposing inner surfaces 21b and 23a of the parallel plates 21 and 23. The reflective film may be a partially reflective film (a film that transmits a predetermined proportion of light and reflects the remaining light). The reflective film may be a multilayer dielectric film, for example. The reflective film 23a may be a totally reflective mirror film.

[0057] The parallel plates 21 and 23 may be made of single crystal silicon. When the parallel plates 21 and 23 are made of single crystal silicon, the reflection intensity from the reference object increases. Therefore, the reference object does not need to include a reflective film.

[0058] (Positional relationship between the second optical unit and the reference object) When the positional relationship between the second optical unit and the reference object is shifted, the influence of the shift on the optical path length and the signal intensity was confirmed. FIG. 15 is a diagram for explaining the positional relationship between the second optical unit and the reference object. As shown in FIG. 15, light was emitted from the second optical unit 15 to the reference object RE, and the return light was acquired. At this time, the measurement was performed while changing the measurement position and attitude of the second optical unit 15. In the measurement coordinate system, the horizontal position of the second optical unit 15 was set as the X and Y directions (the front is the origin 0), the focal length of the second optical unit 15 was set as the Z direction, and the optical axis rotation direction of the second optical unit was set as Θx and Θy.

[0059] FIG. 16 is a graph showing the optical path length and signal intensity measured for each relative horizontal position between the second optical unit and the reference object. The horizontal axis is the measurement horizontal position, the left vertical axis is the optical path length, and the right vertical axis is the signal intensity. The optical path length is plotted with black data points, and the signal intensity is plotted with white data points. It was confirmed that when the horizontal position of the second optical unit 15 is shifted by about 2 mm, an error in the optical path length occurs of about 0.2 μm. It was also confirmed that the signal intensity also varies slightly depending on the horizontal position of the second optical unit 15.

[0060] FIG. 17 is a graph showing the optical path length and signal intensity measured for each focal length of the second optical unit. The horizontal axis is the distance from the focal point, and the point where the signal intensity is maximum at the focal point is used as a reference point, and the case where the second optical unit and the reference object move away from each other is a positive value, and the case where the second optical unit and the reference object move closer to each other is a negative value. The left vertical axis is the optical path length, and the right vertical axis is the signal intensity. The optical path length is plotted with black data points, and the signal intensity is plotted with white data points. It was confirmed that when the position from the focal point of the second optical unit 15 deviates from the reference position, the optical path length and the signal intensity vary greatly depending on the position from the focal point of the second optical unit 15.

[0061] FIG. 18 is a graph showing the optical path length and signal intensity measured for each angle around the optical axis of the second optical unit. The horizontal axis is the angle around the optical axis, the left vertical axis is the optical path length, and the right vertical axis is the signal intensity. The optical path length is plotted with black data points, and the signal intensity is plotted with white data points. It was confirmed that an error of about 0.4 μm in the optical path length occurs when the rotation angle of the second optical unit 15 is shifted by about 2 degrees. It was also confirmed that the signal intensity changes depending on the rotation angle of the second optical unit 15.

[0062] 16 to 18, it was confirmed that the error in the optical path length becomes larger when the focal length is shifted than when the second optical unit 15 is shifted in the horizontal direction or the angle. This confirmed that when holding the relative position between the second optical unit 15 and the reference object RE, a configuration that does not cause positional shift in the Z direction is effective for reducing the error in the optical path length. In other words, it was confirmed that the holder shown in Figs. 13 and 14 is effective for reducing the error in the optical path length because it fixes the relative position with a structure that clamps from above and below.

[0063] (Evaluation of the reference object) Four reference objects were prepared, and the effect of the reflective film was evaluated. FIG. 19 is a table showing the configuration of the reference object according to the embodiment. As shown in FIG. 19, the embodiment 1 is configured such that there is no reflective film on the inner surface 21b (lower surface) of the upper parallel plate 21 and the inner surface 23a (upper surface) of the lower parallel plate 23. In the embodiments 2 and 3, the opposing inner surfaces 21b and 23a are coated with a reflective coating or a partially reflective coating to form a reflective film. In the embodiment 4, the parallel plates are made of single crystal silicon, and there is no reflective film on the opposing inner surfaces 21b and 23a.

[0064] For Examples 1 to 4, the signal intensity was measured with the external temperature change stabilized. Furthermore, measurements were performed multiple times, and the stability of the optical path length was evaluated at 6σ. Furthermore, the optical path length fluctuation rate was also measured when the temperature of the reference object RE was varied in the temperature range of 5 to 40°C, and when the temperatures of the reference object RE and the interferometer were varied in the temperature range of 5 to 40°C. The results are shown in FIG. 20.

[0065] Fig. 20 is a table showing the evaluation results of the examples. As shown in Fig. 20, it was confirmed that Examples 2 and 3 having a reflective film and Example 4 having a parallel plate made of single crystal silicon have significantly improved signal strength and optical path length stability compared to Example 1 having no reflective film. It was also confirmed that Examples 2 to 4 have a smaller rate of variation in optical path length compared to Example 1 even when the temperature is changed. This suggests that the use of a reference object having a reflective film on at least one of the opposing inner surfaces of the parallel plate allows accurate correction of optical path length measurement errors. [Explanation of symbols]

[0066] 1,1A...system, 3,4...holder, 11...optical switch, 12...first optical fiber, 13...second optical fiber, 14...first optical unit, 15,15A...second optical unit, 21,23...parallel plate, 100...light source, 102,150...circulator, 103...spectroscope, 104...control unit, 151,152...focuser or collimator, RE,REA,REB...reference object, SA...measurement object.

Claims

1. A light source that generates light; a first optical unit configured to emit the light to a measurement object and to receive a first interference light from the measurement object; a second optical unit configured to emit the light to a reference object configured to have a constant optical path length against temperature fluctuations and to receive second interference light from the reference object; a spectrometer connected to the first optical unit and the second optical unit and configured to receive the first interference light and the second interference light; A control unit connected to the spectrometer; Equipped with The control unit is calculating a rate of variation of the measured optical path length with respect to the reference optical path length under a predetermined temperature environment, based on a measured optical path length of the reference object calculated based on the second interference light incident on the spectrometer under a predetermined temperature environment and a reference optical path length of the reference object acquired in advance; correcting the optical path length of the object to be measured, which is calculated based on the first interference light incident on the spectroscope under the predetermined temperature environment, based on the rate of fluctuation; the reference object is an etalon element; the second optical unit includes a third optical element that outputs the light to the etalon element, and a fourth optical element configured to receive transmitted light from the etalon element; A system for correcting optical path length measurement errors.

2. a circulator connected to the light source; an optical switch connected to the circulator; The first optical unit is a first optical fiber that propagates light from the optical switch; a first optical element configured to emit the light to the measurement object and to receive the first interference light from the measurement object; 2. The system for correcting optical path length measurement errors of claim 1, comprising:

3. The system for correcting optical path length measurement errors according to claim 2 , wherein the first optical element is a focuser or a collimator.

4. The system for correcting optical path length measurement errors according to claim 1 , wherein the third optical element and the fourth optical element are a focuser or a collimator.

5. 5. The system for correcting an optical path length measurement error according to claim 1, wherein the reference object has a pair of opposing parallel plates.

6. 6. The system for correcting optical path length measurement errors according to claim 5, wherein a vacuum is present in the space between the parallel plates.

7. 7. The system for correcting an optical path length measurement error according to claim 5, wherein the reference object has a reflective film on at least one of the opposing inner surfaces of the parallel plate.

8. 7. The system for correcting an optical path length measurement error according to claim 5, wherein the pair of opposing parallel plates are made of single crystal silicon.

9. 6. The system for correcting an optical path length measurement error according to claim 1, further comprising a holder for holding a relative position between the second optical unit and the reference object.

10. A method for measuring a temperature of a measurement object by using a first optical unit, comprising the steps of: emitting light to a measurement object and allowing a first interference light from the measurement object to be incident on a spectrometer in a predetermined temperature environment; calculating an optical path length of the measurement object based on the first interference light; using a second optical unit, emitting the light to a reference object configured to have a constant optical path length against temperature fluctuations, and allowing a second interference light from the reference object to be incident on the spectrometer in the predetermined temperature environment; calculating a measured optical path length of the reference object based on the second interference light; obtaining a reference optical path length of the reference object; calculating a rate of variation of the measurement optical path length with respect to the reference optical path length under the predetermined temperature environment based on a measurement optical path length of the reference object calculated based on the second interference light and a reference optical path length of the reference object; correcting the optical path length of the measurement object based on the rate of variation; Including, the reference object is an etalon element; the second optical unit includes a third optical element that outputs the light to the etalon element, and a fourth optical element configured to receive transmitted light from the etalon element; A method for correcting optical path length measurement errors.

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