JP2015099133A - Measurement method and measurement device for thickness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measurement method and device capable of highly quickly measuring the thickness of an inspected object in a non-destructive manner.SOLUTION: Light from a light source 10 is divided into reference light and inspected light, and the thickness of an inspected object 80 is measured by interference measurement causing interference between the inspected light transmitted through the inspected object and the reference light. A medium 70 having group refractive index equal to the group refractive index of the inspected object 80 with specific wavelength is arranged on the optical path of the inspected light and the reference light, and interference light causing interference between the inspected light transmitted through the inspected object 80 and the medium 70 and the reference light transmitted through the medium 70 is measured, and the specific wavelength is determined on the basis of the wavelength dependency of a phase difference between the inspected light and the reference light, and the group refractive index of the medium 70 corresponding to the specific wavelength is set to the group refractive index of the inspected object 80. The optical path length of the inspected object 80 corresponding to the specific wavelength is measured, and the thickness of the inspected object 80 is calculated by using the group refractive index corresponding to the specific wavelength and the optical path length corresponding to the specific wavelength.

Description

  The present invention relates to a thickness measuring method and measuring apparatus.

  Since the lens thickness affects the optical performance, a technique for measuring the lens thickness is required. In general, the lens thickness is measured by contact measurement using a probe. Contact measurement is inaccurate and may damage the lens. Therefore, there is a need for a technique for measuring the lens thickness with high accuracy in a nondestructive manner.

  In Patent Document 1, a gap between the transparent plate and the test object is measured with a low coherence interferometer in a state where the test object is arranged between the two transparent plates, and the measured value and the gap between the two transparent plates are measured. The method of calculating the thickness of the test object from the above is proposed. In Patent Document 2, a test object is immersed in two types of media, and the optical path length between the transparent plates is measured with a low coherence interferometer in each of the two types of media, and the measured value and the actual distance between the transparent plates are calculated. Therefore, a method for measuring the thickness of the test object is proposed.

JP-A-11-344313 JP 2012-083331 A

  In the method disclosed in Patent Document 1, when the test object is a lens having a curved surface, only a small interference signal can be obtained due to the influence of the power of the test object and the curvature of the reflecting surface, and the measurement accuracy is lowered. In the method disclosed in Patent Document 2, measurement errors of a total of six measurement values of four types of optical path lengths and two types of actual distances between two transparent plates are mixed, so that measurement accuracy is lowered.

  An object of the present invention is to provide a measurement method and a measurement apparatus that can measure the thickness of a test object with high accuracy in a nondestructive manner.

  The measurement method of the present invention divides light from a light source into test light and reference light, causes the test light to enter the test object, and transmits the test light and the reference light transmitted through the test object. A thickness measurement method for measuring the thickness of the test object by interferometry that causes interference between the test light and the reference with a medium having a group refractive index equal to the group refractive index of the test object at a specific wavelength. An interference light that is disposed on the optical path of the light and causes the test light that has passed through the test object and the medium to interfere with the reference light that has passed through the medium is measured, and the level of the test light and the reference light is measured. Group refraction that determines the specific wavelength based on the wavelength dependence of the phase difference and calculates the group refractive index of the medium corresponding to the specific wavelength as the group refractive index of the test object corresponding to the specific wavelength A rate measuring step, an optical path length measuring step for measuring the optical path length of the test object, Calculating a thickness of the test object based on a group refractive index of the test object corresponding to a specific wavelength and an optical path length of the test object corresponding to the specific wavelength. It is characterized by.

  The refractive index measuring apparatus of the present invention includes a light source, test light that divides light from the light source into test light and reference light, causes the test light to enter the test object, and transmits the test object. An interference optical system that causes interference between the test light and the reference light, a detection unit that detects interference light between the test light and the reference light, an optical path length measurement unit that measures an optical path length of the test object, and the detection unit A calculating means for calculating a refractive index of the test object using an output interference signal, and calculating a thickness of the test object using a refractive index of the test object and an optical path length of the test object; The test object is disposed in a medium having a group refractive index equal to a group refractive index of the test object at a specific wavelength, and the interference optical system includes the test object An optical system that causes the test light transmitted through the object and the medium to interfere with the reference light transmitted through the medium, The calculating means determines the specific wavelength based on the wavelength dependence of the phase difference between the test light and the reference light, and corresponds the group refractive index of the medium corresponding to the specific wavelength to the specific wavelength. Calculating the group refractive index of the test object, and using the group refractive index of the test object corresponding to the specific wavelength and the optical path length of the test object corresponding to the specific wavelength It is characterized by calculating the thickness of an object.

  ADVANTAGE OF THE INVENTION According to this invention, the measuring method and measuring apparatus which can measure the thickness of a to-be-tested object with high accuracy nondestructively can be provided.

It is a block diagram of the measuring device of Example 1 of the present invention. It is a flowchart which shows the procedure which calculates the thickness of a test object by the measuring apparatus of Example 1 of this invention. It is a figure which shows the relationship between the phase refractive index and group refractive index of each to-be-tested object and a medium. It is a figure which shows the interference signal obtained with the detector of the measuring device of Example 1 of this invention. It is a block diagram of the measuring device of Example 2 of the present invention. It is a block diagram of the measuring apparatus of Example 3 of this invention.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a block diagram of the measuring apparatus according to the first embodiment of the present invention. The measuring apparatus of the present embodiment is composed of a Mach-Zehnder interferometer. The measurement apparatus includes a light source 10, an interference optical system, a container 60 that can store a medium and a test object, a detector 90, and a computer 100, and measures the thickness of the test object 80.

  In this example, the group refractive index of a test object at a specific wavelength is obtained by immersing the test object in a medium (for example, oil) having a group refractive index equal to that of the test object at a specific wavelength. Measure with high accuracy. The thickness of the test object can be measured with high accuracy by combining the group refractive index of the test object at a specific wavelength and the optical path length of the test object. The test object is a refractive optical element such as a lens or a flat plate.

The refractive index includes a phase refractive index N _p (λ) with respect to a phase velocity v _p (λ) that is a moving velocity of the equiphase surface of light, and a light energy moving velocity (wave packet moving velocity) v _g ( There is a group refractive index N _g (λ) with respect to λ), which can be converted to each other according to Equation 2 described below.

  The light source 10 is a light source that emits light having a plurality of wavelengths (for example, a supercontinuum light source). The interference optical system divides the light from the light source 10 into light that does not pass through the test object (reference light) and light that passes through the test object (test light), and superimposes the reference light and the test light. The interference light is guided to the detector 90. The interference optical system includes beam splitters 20 and 21 and mirrors 30, 31, 40, 41, 50 and 51.

  The beam splitters 20 and 21 are constituted by, for example, cube beam splitters. The beam splitter 20 transmits part of the light from the light source 10 and reflects the rest at the interface (bonding surface) 20a. In this embodiment, the light transmitted through the interface 20a is the reference light, and the light reflected at the interface 20a is the test light. The beam splitter 21 reflects part of the reference light and transmits part of the test light at the interface 21a. As a result, the reference light and the test light interfere to form interference light, and the interference light is emitted to the detector 90.

  The container 60 contains a medium 70 and a test object 80. The optical path length of the reference light in the container and the optical path length of the test light preferably coincide with each other when the test object 80 is not arranged in the container. Therefore, it is desirable that the side surface (for example, glass) of the container 60 has a uniform thickness and refractive index, and both side surfaces of the container 60 are parallel. The container 60 includes a temperature adjustment mechanism (temperature control means), and can control the temperature rise and fall of the medium 70 and the temperature distribution of the medium 70.

  The refractive index of the medium 70 is measured by a medium refractive index measuring unit (not shown). The medium refractive index measuring means is composed of, for example, a temperature measuring means for measuring the temperature of the medium and a computer for converting the measured temperature into the refractive index of the medium. The medium refractive index measuring means includes a wavefront measuring sensor (wavefront measuring means) for measuring a transmitted wavefront of a glass prism (reference object) having a known refractive index and shape, and a medium based on the refractive index and shape of the transmitted wavefront and the glass prism. You may comprise from the computer which calculates the refractive index of this. The medium refractive index measuring unit may measure the phase refractive index or the group refractive index. When the phase refractive index is measured by the medium refractive index measuring means, the group refractive index can be calculated according to the relationship of Equation 2 described later.

  The mirrors 40 and 41 are, for example, prism type mirrors. The mirrors 50 and 51 are, for example, corner cube reflectors. The mirror 51 has a drive mechanism in the direction of the arrow in FIG. The drive mechanism of the mirror 51 is composed of, for example, a stage having a large drive range and a piezo stage having a high drive resolution. The driving amount of the mirror 51 is measured by a length measuring device (not shown) (for example, a laser displacement meter or an encoder). The drive of the mirror 51 is controlled by the computer 100. The optical path length difference between the reference light and the test light can be adjusted by the drive mechanism (adjustment unit) of the mirror 51.

  The detector 90 includes a spectroscope that separates interference light from the beam splitter 21 and detects the interference light intensity as a function of wavelength (frequency).

  The computer 100 is composed of a CPU or the like, and calculates the refractive index and optical path length of the test object 80 based on the interference signal output from the detector 90, and calculates the thickness of the test object 80. The function and the function as a control means which controls the drive amount of the mirror 51 are provided.

  The interference optical system is adjusted so that the optical path lengths of the reference light and the test light are equal in a state where the test object 80 is not disposed in the container. The adjustment method is as follows.

In the measurement apparatus of FIG. 1, the interference signal between the reference light and the test light is measured in a state where the test object 80 is not arranged in the container. At this time, the phase difference φ ₀ (λ) and the interference intensity I _φ0 (λ) between the reference light and the test light are expressed by Equation 1.

Where λ is the wavelength in the air, Δ ₀ is the difference in optical path length between the reference light and the test light, I ₀ is the sum of the reference light intensity and the test light intensity, and γ is the visibility (visibility). From Equation 1, when Δ ₀ is not zero, the interference intensity I ₀ (λ) is a vibration function. Therefore, in order to make the optical path lengths of the reference light and the test light equal, the mirror 51 may be driven to a position where the interference signal does not become a vibration function. At this time, delta ₀ becomes zero.

Here, the case where the optical path lengths of the test light and the reference light are adjusted to be equal (Δ ₀ = 0) has been described, but how much the current position of the mirror 51 is shifted from Δ ₀ = 0. If it is known, it is not necessary to make the optical path lengths of the test light and the reference light equal. The driving amount of the mirror 51 from the position where the optical path lengths of the test light and the reference light are equal (Δ ₀ = 0) can be measured by a length measuring device (for example, a laser length measuring device or an encoder) not shown.

  FIG. 2 is a flowchart showing a calculation procedure for calculating the thickness of the test object 80, and “S” is an abbreviation for Step.

  First, a medium 70 having a group refractive index equal to the group refractive index of the test object at a specific wavelength is filled in the container 60 (S10). At this time, the medium 70 and the test object 80 are arranged such that the test light passes through the test object 80 and the medium 70 and the reference light passes through the medium 70.

In general, since the ultraviolet absorption band of oil is closer to visible light than the ultraviolet absorption band of glass material, the slope of the refractive index dispersion curve in the visible light region is steeper for oil than for glass material. FIG. 3A is a diagram showing phase refractive index dispersion curves of the test object and the medium. FIG. 3B is a diagram illustrating group refractive index dispersion curves of the test object and the medium. The relationship between the phase refractive index n (λ) and the group refractive index n _g (λ) is expressed by Equation 2.

The group refractive index of the test object and the group refractive index of the medium are equal at the point where they intersect in FIG. The wavelength λ ₀ at the intersecting point in FIG. 3B corresponds to a specific wavelength. In a combination of a test object having a high phase refractive index and a medium having a low phase refractive index, a state called group refractive index matching is obtained. The medium also plays a role of reducing the effect of refraction on the surface of the test object.

Next, a specific wavelength λ ₀ is measured from the wavelength dependence of the phase difference between the reference light and the test light (S20). The interference signal in the spectral region measured by the detector 90 in FIG. 1 is as shown in FIG. The phase difference φ (λ) and the interference intensity I (λ) between the test light and the reference light are expressed by Equation 3.

Here, n ^sample (λ) is the phase refractive index of the test object, n ^medium (λ) is the phase refractive index of the medium, and L is the geometric thickness of the center of the test object. As can be seen from FIG. 4A and Equation 3, the interference signal is a vibration function reflecting the wavelength dependence of the phase difference φ (λ). In addition, the refractive index in a present Example means the relative refractive index with respect to air.

In FIG. 4A, λ ₀ indicates a wavelength at which the phase difference φ (λ) takes an extreme value. The inclination of the phase difference φ (λ) with respect to the wavelength, that is, the differential dφ (λ) / dλ of the phase difference is expressed by Equation 4.

However, _ng ^sample (λ) is the group refractive index of the test object, and _ng ^medium (λ) is the group refractive index of the medium. The wavelength λ _{0 at} which the phase difference φ (λ) takes an extreme value is a wavelength at which the differential dφ (λ) / dλ of the phase difference in Equation 4 is zero. In other words, the wavelength λ ₀ is a wavelength (specific wavelength) at which the group refractive index _ng ^sample (λ) of the test object is equal to the group refractive index _ng ^medium (λ) of the ^medium . Formula 5 represents the relationship between the group refractive index of the test object and the group refractive index of the medium at a specific wavelength λ ₀ . The specific wavelength λ ₀ can be determined by measuring the apex (extreme value) of the region where the vibration period of the interference signal in FIG. 4A is long (S20).

Next, the group refractive index _ng ^medium (λ) of the medium 70 is measured (S30). In the present embodiment, it is assumed that there is a medium refractive index measuring means constituted by a temperature measuring means for measuring the temperature of the medium and a computer 100 for converting the measured temperature into a refractive index of the medium. The phase refractive index n ₀ ^medium (λ) of the medium 70 at a certain reference temperature T ₀ and the temperature coefficient dn ^medium (λ) / dT of the refractive index of the medium 70 are known and are combined with the temperature measurement value T to Thus, the group refractive index _ng ^medium (λ) of the medium 70 is calculated.

The group refractive index _ng ^medium (λ ₀ ) of the medium 70 corresponding to the specific wavelength λ ₀ measured in S20 is calculated based on the relationship of Equation 6 (S40). As shown in Equation 5, the group refractive index n _g ^medium (λ ₀ ) of the medium 70 corresponding to the specific wavelength λ ₀ calculated in S40 is the group refractive index n of the test object 80 at the specific wavelength λ ₀ . _It is equal to _g ^sample (λ ₀ ).

  As described above, the method for measuring the group refractive index of the test object using Equation 5 does not depend on the thickness L of the test object because the group refractive index of the test object is measured via the group refractive index of the medium. . Therefore, the group refractive index of the test object can be measured even if the thickness L of the test object is unknown.

Incidentally, when the temperature difference between the measured value T and the reference temperature T ₀ of the temperature of the medium 70 is small, it may be using a look-up data table showing the data of the refractive index of each wavelength at a particular temperature.

Next, the optical path length of the test object 80 corresponding to the specific wavelength is measured in a state where the medium 70 is not disposed in the container 60 (S50). When the medium 70 is not disposed in the container 60, the phase difference φ ^empty (λ) between the reference light and the test light and the differential dφ ^empty (λ) / dλ of the phase difference are expressed as in Expression 7. The relational expression in the extreme value of the phase difference φ ^empty (λ) is expressed by Equation 8. Since λ is the wavelength in air, the refractive index of air is built into the wavelength. Here, it is assumed that the phase refractive index of air is equal to the group refractive index of air.

Wavelength extreme retardation φ ^empty (λ) is varied by delta ₀ value. Δ ₀ is adjusted so that the wavelength of the extreme value of the phase difference φ ^empty (λ) matches the specific wavelength λ ₀ . In the present embodiment, Δ ₀ = ( _ng ^sample (λ ₀ ) −1) L when the extreme wavelength coincides with a specific wavelength λ ₀ is defined as the optical path length of the test object 80. 4 (b) is an interference signal when the wavelength of the extreme matches a specific wavelength lambda _0.

  Finally, the thickness L of the test object 80 is calculated using the group refractive index of the test object corresponding to the specific wavelength and the optical path length of the test object corresponding to the specific wavelength (S60). The thickness L is expressed by Equation 9.

In this embodiment, the group refractive index of the test object is measured via the group refractive index of the medium at a specific wavelength λ ₀ where the group refractive index of the test object and the group refractive index of the medium are equal. Since the group refractive index of the medium is measured with high accuracy, the group refractive index of the test object is also measured with high accuracy. By combining the group refractive index of the test object measured with high accuracy and the optical path length of the test object at the specific wavelength λ ₀ , the thickness of the test object is also calculated with high accuracy.

In this example, a specific wavelength λ ₀ was measured from the oscillating interference signal. Instead, the specific wavelength measurement method may be a method in which the phase difference between the reference light and the test light is measured using the phase shift method, and the extreme value of the phase difference is directly obtained.

In this embodiment, after measuring the particular wavelength lambda _0, it was measured light path length of the object at a particular wavelength lambda _0. Instead, a method of measuring the specific wavelength λ ₀ after measuring the optical path length of the test object may be used. First, the test object 80 is arranged in a state where the medium 70 is not arranged in the container 60. By measuring the change of the extreme value of the phase difference φ ^empty (λ) with respect to the change of Δ ₀ , the optical path length of the test object 80 is measured as a function of wavelength (Formula 8). Then, the medium 70 is filled in the container 60, the specific wavelength lambda ₀ is measured. By substituting a specific wavelength λ ₀ for the optical path length (Equation 8) of the test object 80 as a function of the wavelength, the optical path length of the test object 80 corresponding to the specific wavelength is measured.

  In this embodiment, the inside of the container 60 is emptied (air), and the optical path length of the test object is measured. The optical path length of the test object may be measured using a medium (for example, water) having a refractive index different from that of the medium 70 instead of air.

  In this embodiment, a super continuum light source is used as the light source 10 that emits light of a plurality of wavelengths. Instead, a super luminescent diode (SLD), a halogen lamp, a short pulse laser, or the like may be used. Instead of a continuous spectrum light source, a discrete spectrum light source such as a multiline oscillation gas laser may be used.

  In this embodiment, interference light at a plurality of wavelengths is dispersed by the detector 90. Instead, a wavelength sweep method can be used. In the wavelength sweeping method, for example, a spectroscope is arranged immediately after a light source that emits light of a plurality of wavelengths, pseudo-monochromatic light is emitted, and an interference signal of that wavelength is measured by a detector such as a photodiode. In the wavelength sweeping method, the interference measurement of each wavelength is performed while wavelength scanning. Instead of a combination of a light source that emits light of a plurality of wavelengths and a spectroscope, a wavelength swept light source such as a tunable diode laser may be used.

  The wavelength sweeping method can be combined with heterodyne interferometry. The heterodyne interferometry is a temporal phase shift method in which a frequency difference is generated between the reference light and the test light by an acoustooptic device or the like and measured.

  The temperature distribution of the medium 70 is equivalent to the refractive index distribution of the medium 70. The refractive index distribution of the medium 70 gives an error to the calculated refractive index of the test object. Therefore, it is desirable to control by the temperature adjustment mechanism (temperature adjustment means) so that the temperature distribution of the medium 70 does not occur. An error due to the refractive index distribution of the medium 70 can be corrected if the amount of the refractive index distribution is known. Therefore, it is more desirable that a wavefront measuring device (wavefront measuring means) for measuring the refractive index distribution of the medium 70 is provided.

In this embodiment, the mirror 51 is adjusted so that the optical path lengths of the test light and the reference light are equal (Δ ₀ = 0) in a state where the test object 80 is not disposed in the container 60. Instead, it is sufficient to know how much the current position is shifted from Δ ₀ = 0. That is, the value of the current delta ₀ may be able to identify. In this case, the phase difference φ (λ) between the reference light and the test light may be replaced with a phase difference Φ (λ) as shown in Equation 10 instead of Equation 3.

  In this embodiment, a Mach-Zehnder interferometer is used, but a Michelson interferometer may be used instead. Further, in this embodiment, the phase difference and the refractive index are calculated as a function of wavelength, but may be calculated as a function of frequency instead.

FIG. 5 is a block diagram of the measuring apparatus according to the second embodiment of the present invention. An interferometer that measures the refractive index of the medium 70 is added to the measurement apparatus according to the first embodiment. In this embodiment, after measuring the optical path length of the test object as a function of wavelength, the specific wavelength λ ₀ is measured, and the optical path length at the specific wavelength λ ₀ is calculated. The test object is a lens with positive power. The same configurations as those in the first embodiment will be described with the same reference numerals.

  The light emitted from the light source 10 is split into transmitted light and reflected light by the beam splitter 22. The transmitted light travels to the interference optical system for measuring the thickness of the test object 80, and the reflected light is guided to the interference optical system for measuring the refractive index of the medium 70. The reflected light is further divided into transmitted light (medium reference light) and reflected light (medium test light) by the beam splitter 23.

  The medium test light reflected by the beam splitter 23 is reflected by the mirrors 42 and 52, passes through the side surface of the container 60 and the medium 70, is reflected by the mirror 33, and reaches the beam splitter 24. The medium reference light transmitted through the beam splitter 23 is reflected by the mirrors 32, 43, and 53, then passes through the compensation plate 61 and reaches the beam splitter 24. The medium reference light and the medium test light reaching the beam splitter 24 interfere with each other to form interference light, which is detected by a detector 91 configured by a spectroscope or the like. A signal detected by the detector 91 is sent to the computer 100.

  The compensation plate 61 plays a role of correcting the influence of refractive index dispersion due to the side surface of the container 60 and is made of the same material and the same thickness as the side surface of the container 60 (= thickness of the side surface of the container 60). The compensation plate 61 has the effect of equalizing the optical path length difference between the wavelengths of the medium reference light and the medium test light when the container 60 is empty.

  The mirror 53 has a drive mechanism similar to that of the mirror 51, and is driven in the direction of the arrow in FIG. The drive of the mirror 53 is controlled by the computer 100.

  The procedure for calculating the thickness of the test object 80 of this example is as follows.

First, the test object 80 is placed on the test light path. By measuring the change in the wavelength of the extreme value of the phase difference φ ^empty (λ) when Δ ₀ changes, the optical path length of the test object 80 is measured as a function of wavelength (Formula 8). A medium having a group refractive index equal to the group refractive index of the test object at a specific wavelength is disposed on the optical path of the reference light and the test light. Next, a specific wavelength is measured from the wavelength dependence of the phase difference between the reference light and the test light. In Example 1, a specific wavelength was measured from the interference signal of the reference light and the test light. In this embodiment, the specific wavelength is measured after measuring the phase difference between the reference light and the test light. The phase difference φ (λ) expressed by Equation 3 is calculated by the following phase shift method.

An interference signal is acquired while driving the mirror 51 minutely. The interference intensity I _k (λ) when the phase shift amount (= drive amount × 2π / λ) of the mirror 51 is δ _k (k = 0, 1,... M−1) is expressed by Equation 11.

The phase difference φ (λ) is calculated by Equation 12 using a least square algorithm. Guidelines to enhance the accuracy of calculation of phase difference phi (lambda) is to minimize the phase shift amount [delta] _k, is to maximize the number of drive steps M. The calculated phase difference φ (λ) is convolved with 2π. Therefore, an operation (unwrapping) for connecting 2π phase jumps is necessary.

A specific wavelength λ ₀ is measured from the wavelength corresponding to the extreme value of the phase difference φ (λ) calculated by Expression 12. The wavelength at which the differential dφ (λ) / dλ of the phase difference φ (λ) is zero is the specific wavelength λ ₀ .

  Since the phase difference φ (λ) is discrete data, the rate of change between the respective wavelength data is calculated as the differential phase difference dφ (λ) / dλ. In general, the operation of calculating the differential amount of data amplifies the influence of noise. In order to reduce the influence of noise, the derivative amount may be calculated after smoothing the original data. Alternatively, the differential data itself may be smoothed.

Next, the group refractive index _ng ^medium (λ) of the ^medium is measured. The phase difference φ ^medium (λ) between the medium reference light and the medium test light and the differential dφ ^medium (λ) / dλ of the phase difference are expressed by Equation 13.

Where Δ is the optical path length difference between the medium reference light and the medium test light when the medium 70 is not arranged in the container 60, and L ^tank is the distance between the side surfaces of the container 60 (the medium test light in the medium 70 Optical path length), a known amount. Similar to the method of measuring the phase difference φ (λ), the phase difference φ ^medium (λ) between the medium reference light and the medium test light is measured by the phase shift method using the driving of the mirror 53. When formula 13 is transformed, the group refractive index _ng ^medium (λ) of the ^medium is obtained.

As Equation 5, the group index _n ^{g medium} (λ ₀₎ of a medium corresponding to a particular wavelength lambda ₀ is equal to the group index _n ^{g sample} of the test object (lambda ₀₎ at a particular wavelength lambda _0. By substituting a specific wavelength λ ₀ for the optical path length (Equation 8) of the test object 80 as a function of the wavelength, the optical path length Δ _{0 of the} test object corresponding to the specific wavelength is calculated. Finally, the thickness L of the test object 80 is calculated by Equation 9 by associating the group refractive index of the test object 80 at the specific wavelength λ ₀ with the optical path length.

  FIG. 6 is a block diagram of the measuring apparatus according to the third embodiment of the present invention. The wavefront is measured using a two-dimensional sensor. In order to measure the refractive index of the medium, a glass prism (reference test object) having a known refractive index and shape is arranged on the test light beam in the medium. The same configurations as those in the first and second embodiments will be described with the same reference numerals.

  The light emitted from the light source 10 is split by the spectroscope 95 and enters the pinhole 110 as pseudo-monochromatic light. The wavelength of the pseudo-monochromatic light incident on the pinhole 110 is controlled by the computer 100. The light that has passed through the pinhole 110 and becomes divergent light is collimated into parallel light by the collimator lens 120. The collimated light is split by the beam splitter 25 into transmitted light (reference light) and reflected light (test light).

  The reference light transmitted through the beam splitter 25 passes through the medium 70 in the container 60, is reflected by the mirror 31, and reaches the beam splitter 26. The mirror 31 has a drive mechanism in the direction of the arrow in FIG. 6 and is controlled by the computer 100.

  The test light reflected by the beam splitter 25 is reflected by the mirror 30 and enters the container 60 that houses the medium 70, the test object 80, and the glass prism 130. Part of the test light passes through the medium 70 and the test object 80. Part of the test light passes through the medium 70 and the glass prism 130. The remaining light of the test light passes only through the medium 70. Each light transmitted through the container 60 interferes with the reference light in the beam splitter 26 to form interference light, and is detected by the detector 92 (for example, CCD or CMOS sensor) through the imaging lens 121. The interference signal detected by the detector 92 is sent to the computer 100. The detector 92 is disposed at a conjugate position with the position of the test object 80 and the glass prism 130 with respect to the imaging lens 121.

  The phase refractive index of the medium 70 is calculated by measuring the wavefront transmitted through the glass prism 130. The glass prism preferably has a phase refractive index approximately equal to the phase refractive index of the medium 70 so that the interference fringes between the light transmitted through the glass prism 130 and the reference light do not become too dense.

  The optical path lengths of the test light and the reference light are adjusted to be equal in a state where the test object 80 and the glass prism 130 are not arranged on the test light path.

  The procedure for calculating the thickness of the test object 80 of this example is as follows.

First, a medium having a group refractive index equal to the group refractive index of the test object at a specific wavelength is disposed on the optical path of the reference light and the test light. Next, the phase difference φ (λ) between the test light and the reference light and the refractive index n ^medium (λ) of the medium 70 are measured by wavelength scanning by the spectroscope 95 and a phase shift method using the driving mechanism of the mirror 31. Is done. A specific wavelength is calculated from the wavelength dependence (φ (λ) or dφ (λ) / dλ) of the phase difference. From the phase refractive index n ^medium (λ) of the medium 70, the group refractive index _ng ^medium (λ) of the medium 70 is measured using Equation 2. The group refractive index of the medium 70 corresponding to the specific wavelength is calculated as the group refractive index _ng ^sample (λ ₀ ) of the test object 80. The medium 70 is taken out from the container 60, and the optical path length of the test object corresponding to the specific wavelength is measured. Finally, the thickness L of the test object 80 is calculated using the group refractive index and the optical path length of the test object 80 at a specific wavelength.

  As described above, according to the measuring apparatus of each embodiment of the present invention, the thickness of the test object can be measured with high accuracy without destruction.

DESCRIPTION OF SYMBOLS 10 Light source 70 Medium 80 Test object 100 Computer (calculation means)

Claims (16)

The light from the light source is divided into the test light and the reference light, the test light is incident on the test object, and the test light transmitted through the test object is interfered with the reference light by the interference measurement. A thickness measurement method for measuring the thickness of a test object,
A medium having a group refractive index equal to the group refractive index of the test object at a specific wavelength is disposed on an optical path of the test light and the reference light, and the test light transmitted through the test object and the medium; Measures interference light that interferes with the reference light transmitted through the medium, determines the specific wavelength based on the wavelength dependence of the phase difference between the test light and the reference light, and corresponds to the specific wavelength A group refractive index measurement step of calculating the group refractive index of the medium as the group refractive index of the test object corresponding to the specific wavelength;
An optical path length measuring step for measuring an optical path length of the test object; and
A calculation step of calculating the thickness of the test object based on the group refractive index of the test object corresponding to the specific wavelength and the optical path length of the test object corresponding to the specific wavelength;
The thickness measuring method characterized by including.   In the optical path length measurement step, the phase difference between the test light and the reference light is measured in a state where the medium is not disposed on the optical path of the test light and the reference light, and the test light and the reference light The optical path length difference between the test light and the reference light is adjusted so that the wavelength corresponding to the extreme value of the phase difference matches the specific wavelength, and the optical path length difference between the adjusted test light and the reference light The thickness measurement method according to claim 1, wherein the optical path length of the test object corresponding to the specific wavelength is used.   The thickness measurement method according to claim 1 or 2, wherein, in the group refractive index measurement step, a wavelength corresponding to an extreme value of a phase difference between the test light and the reference light is determined as the specific wavelength. .   The group refractive index of the medium is calculated by measuring the temperature of the medium and converting the measured temperature of the medium into a group refractive index of the medium in the group refractive index measurement step. Item 4. The thickness measuring method according to any one of Items 1 to 3.   In the group refractive index measurement step, a reference test object having a known refractive index and shape is disposed in the medium, light is incident on the reference test object, and the transmitted wavefront of the reference test object is measured, 4. The group refractive index of the medium is calculated based on a refractive index and a shape of the reference specimen and a transmitted wavefront of the reference specimen. 5. Thickness measurement method.   In the group refractive index measurement step, the light from the light source is divided into medium test light and medium reference light, the medium test light is incident on the medium, and the medium test light and the medium transmitted through the medium 4. The group refractive index of the medium is calculated based on a phase difference between the medium reference light and the medium test light by measuring interference light obtained by causing interference with reference light. 5. The refractive index measuring device according to item 1.   The thickness measurement method according to claim 1, wherein in the group refractive index measurement step, a refractive index distribution of the medium is measured.   The thickness measurement method according to claim 1, wherein a temperature distribution of the medium is controlled in the group refractive index measurement step. A light source and interference optics that divides light from the light source into test light and reference light, causes the test light to enter the test object, and causes the test light transmitted through the test object to interfere with the reference light System, detection means for detecting interference light between the test light and the reference light, optical path length measurement means for measuring the optical path length of the test object, and the interference signal output from the detection means A thickness measuring apparatus that calculates a refractive index of a test object, and has a calculation unit that calculates the thickness of the test object using the refractive index of the test object and the optical path length of the test object,
The test object is disposed in a medium having a group refractive index equal to the group refractive index of the test object at a specific wavelength;
The interference optical system is an optical system that causes the test light transmitted through the test object and the medium to interfere with the reference light transmitted through the medium,
The calculation means determines the specific wavelength based on the wavelength dependence of the phase difference between the test light and the reference light, and sets the group refractive index of the medium corresponding to the specific wavelength to the specific wavelength. Calculated as the group refractive index of the corresponding test object, and using the group refractive index of the test object corresponding to the specific wavelength and the optical path length of the test object corresponding to the specific wavelength A thickness measuring apparatus for calculating a thickness of a specimen.   The optical path length measuring means includes an adjustment unit that adjusts an optical path length difference between the test light and the reference light, and the test light is not disposed on the optical path of the test light and the reference light. And the optical path length of the test light and the reference light so that the wavelength corresponding to the extreme value of the phase difference between the test light and the reference light matches the specific wavelength. The thickness measurement according to claim 9, wherein a difference is adjusted, and an optical path length difference between the adjusted test light and the reference light is set as an optical path length of the test object corresponding to the specific wavelength. apparatus.   The thickness measuring apparatus according to claim 9 or 10, wherein the calculation unit determines a wavelength corresponding to an extreme value of a phase difference between the test light and the reference light as the specific wavelength. Temperature measuring means for measuring the temperature of the medium;
The said calculating means calculates the group refractive index of the said medium by converting the temperature of the said medium measured by the said temperature measuring means into the refractive index of the said medium. Item 1. The thickness measuring apparatus according to item 1. A reference specimen with a known refractive index and shape; and
Having wavefront measuring means for measuring a transmitted wavefront of light incident on the reference specimen placed in the medium;
The calculation means calculates the group refractive index of the medium based on the refractive index and shape of the reference specimen and the transmitted wavefront of the reference specimen. The refractive index measuring device according to item 1. An interference optical system that splits light from the light source into medium test light and medium reference light, causes the medium test light to enter the medium, and causes the medium test light transmitted through the medium to interfere with the medium reference light When,
Detecting means for detecting interference light between the medium test light and the medium reference light;
12. The refractive index according to claim 9, wherein the calculation unit calculates a group refractive index of the medium based on a phase difference between the medium reference light and the medium test light. Measuring device.   The refractive index measuring device according to claim 9, further comprising: a wavefront measuring unit that measures a refractive index distribution of the medium.   The refractive index measuring apparatus according to claim 9, further comprising a temperature control unit that controls a temperature distribution of the medium.

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