JP2016142715A - Interference measuring device and interference measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interference measuring device and an interference measuring method excellent in work efficiency.SOLUTION: The interference measuring device comprises: a light source 1; a beam splitting element 3 for splitting a beam L1 into a reference light beam L3 and a measurement light beam L4; a reference light beam side lens 8 for condensing the reference light beam to a reference surface 9; a measurement light beam side lens 12 for condensing the measurement light beam to a measurement target object 13; a reference light beam side diaphragm 7 disposed between the beam splitting element and the reference light beam side lens; a measurement light beam side diaphragm 11 disposed between the beam splitting element and the measurement light beam side lens; and a diaphragm adjusting mechanism for changing the aperture of the reference light beam side diaphragm and the aperture of the measurement light beam side diaphragm, respectively. The diaphragm adjusting mechanism changes the reference light beam side diaphragm and the measurement light beam side diaphragm for adjustment so that the aperture diameters become equal to each other.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an interference measuring apparatus and an interference measuring method for measuring an object using light interference.

  As a conventional interference measurement device, there is an interference measurement device for measuring an inner layer of an object to be measured based on a configuration of a linic interferometer using a white light source as measurement light (for example, Patent Document 1). reference).

  FIG. 12 is a diagram showing a conventional interference measuring apparatus described in Patent Document 1. In FIG. In FIG. 12, the conventional interference measuring apparatus includes a first arm 41 of an interferometer that is a signal side optical path and a second arm 44 of an interferometer that is a reference side optical path. A reference mirror 46 and an object 43 are installed on the second arm 44. A light beam 39 from the light source 38 is split by a beam splitter 40. In a state where the optical path lengths of the light beams after being split by the beam splitter 40 are equal on the first arm 41 side of the interferometer and the second arm 44 side of the interferometer, the light beams reflected by the respective optical paths Light interference occurs, and an interference image is obtained by the camera 48. Since a white light source is used as the light emission source 38, an interference fringe image is formed only in the vicinity of the position where the optical path lengths of the first arm 41 of the interferometer and the second arm 44 of the interferometer are equal. can get. Further, in order to minimize the optical difference between the first arm 41 of the interferometer and the second arm 44 of the interferometer, a lens 42 and a lens 45 having the same optical characteristics are provided in both optical paths. .

  In a general known white interferometer, usually, the object to be measured or the measurement optical system is scanned in the optical axis direction in the optical axis direction to obtain a position where the interference fringe intensity is maximum, and at that position, any object is detected. Alternatively, it is assumed that a reflection or scattering surface has been detected. Therefore, in order to search for a position where the interference fringe intensity is maximized, scanning at a position in the optical axis direction is performed at a distance sufficiently longer than a width (hereinafter referred to as a coherence gate) in which the interference fringe is generated in the optical axis direction. Must-have.

  Therefore, in the above-described conventional interferometer apparatus, the reference mirror 46 is vibrated sinusoidally in the optical axis direction by the piezoelectric ceramic 47, and the integral value of the interference fringe image signal is calculated for each camera pixel, so that the vicinity of the coherence gate is obtained. Since the interference fringe amplitude intensity at can be acquired, the scanning operation along the optical axis direction is unnecessary.

JP-T-2004-528586

  In general, it is known that when the numerical aperture of the objective lens (the lens 42 and the lens 45 in FIG. 12) is large, the optical resolution in the focal plane is improved. A larger number of lenses is not always better.

  However, in the conventional configuration, in order to change the numerical aperture of the objective lens, both the objective lens in the reference side optical path and the objective lens in the signal side optical path must be exchanged, and workability is significantly impaired. .

  The present invention solves the above-described conventional problems, and an object thereof is to provide an interference measurement apparatus and an interference measurement method with good workability.

In order to achieve the above object, an interference measurement apparatus according to one aspect of the present invention includes a light source,
A light beam splitting element that splits a light beam from the light source into reference light and measurement light;
A reference light side lens for condensing the reference light on a reference surface;
A measurement light side lens for condensing the measurement light on the object to be measured;
A reference light side stop disposed between the light beam splitting element and the reference light side lens;
A measurement light side stop disposed between the light beam splitting element and the measurement light side lens;
An aperture adjustment mechanism that changes the aperture of the reference light side aperture and the aperture of the measurement light side aperture, respectively,
The aperture adjustment mechanism adjusts the reference light side aperture and the measurement light side aperture by changing the aperture diameters to be equal to each other.

  In order to achieve the above object, an interference measurement method according to another aspect of the present invention measures the object to be measured using the interference measurement apparatus.

  As described above, according to the interference measuring apparatus and the interference measuring method according to the aspect of the present invention, it is possible to measure the surface from the surface layer to the inner layer of the object to be measured with high accuracy without exchanging the objective lens. Therefore, it is possible to provide an interference measurement device and an interference measurement method with good workability.

The figure which shows the structure of an objective lens in a high numerical aperture state by the interference measuring device in 1st Embodiment of this invention. The figure which shows the structure of an objective lens in a low numerical aperture state in the interference measuring device in 1st Embodiment of this invention. The figure which shows the large aperture diameter state of the variable aperture stop used as the aperture adjustment mechanism of the interference measuring device in 1st Embodiment of this invention. The figure which shows the small aperture diameter state of the variable aperture stop used as the aperture adjustment mechanism of the interference measuring device in 1st Embodiment of this invention. The figure which shows the relationship of the optical path length difference of an objective-lens center part at the time of observing a spherical to-be-measured object with an objective lens The flowchart which showed the 1st procedure which measures a to-be-measured object, changing the numerical aperture of an objective lens, and the position of a measurement target surface with the interference measuring device in 1st Embodiment of this invention. The flowchart which showed the 2nd procedure which measures a to-be-measured object, changing the numerical aperture of an objective lens, and the position of a measurement target surface with the interference measuring device in 1st Embodiment of this invention. The flowchart which showed the procedure which performs the shape filtering of the to-be-measured object by the numerical aperture change of the objective lens of the interference measuring device in 1st Embodiment of this invention. The figure which shows the state which measures the surface vicinity of a to-be-measured object when using a high numerical aperture objective lens in a general interference measuring device The figure which shows the state which is measuring the inner layer part of a to-be-measured object when using the objective lens of high numerical aperture in a general interference measuring device The figure which shows the state which is measuring the inner layer part of a to-be-measured object when using a low numerical aperture objective lens in a general interference measuring device The figure which shows the structure of the conventional interference measuring device described in patent document 1

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a diagram showing a high numerical aperture state of the interference measurement apparatus according to the first embodiment of the present invention. This interferometer has a structure in the form of a linic interferometer.

  This interference measuring apparatus is capable of adjusting at least the light source 1, the beam splitter 3, the reference light side lens 8, the measurement light side lens 12, the reference light side diaphragm 7 capable of adjusting the aperture diameter, and the aperture diameter. A measurement light side diaphragm 11 and diaphragm adjustment mechanisms 70 and 80 are provided. More specifically, the interference measurement apparatus includes a reference mirror stage 10 and a computer 21. Here, the beam splitter 3 is an example of a light beam splitting element. The computer 21 is configured by a personal computer as an example, and specifically includes a control unit 21a, a storage unit 21b, and an input unit 21c.

  The light source 1 is a white light source as an example. The light L 1 emitted from the light source 1 passes through the collimator lens 2 to become parallel light L 2 and enters the beam splitter 3.

  The parallel light L2 is split by the beam splitter 3 into a reference light side arm 4 and a measurement light side arm 5.

  The parallel light (reference light) L <b> 3 divided into the reference light side arm 4 is changed in direction by the mirror 6 and passes through the reference light side diaphragm 7. Thereafter, the parallel light (reference light) L3 is incident on the reference light-side objective lens 8 and converged, and is collected to the reference mirror 9 installed near the focal point of the reference light-side objective lens 8 (reference light-side lens). The light and the reflected light are returned to the parallel light L5 by the reference light side objective lens 8 again. Thereafter, the parallel light L <b> 5 passes through the reference light side diaphragm 7, changes its direction by the mirror 6, and enters the beam splitter 3. The reference mirror 9 is installed on the reference mirror stage 10, and the reference mirror 9 can be moved in a small amount in the optical axis direction by driving the reference mirror stage 10. The reference mirror 9 is an example of a reference surface.

  On the other hand, the parallel light (measurement light) L4 divided into the measurement light side arm 5 passes through the measurement light side diaphragm 11. Thereafter, the parallel light (measurement light) L4 enters the measurement light side objective lens 12 (measurement light side lens) and is converged and condensed on the measurement target surface 14. The collected light is scattered and reflected by the object to be measured 13, and part of the light L 6 is incident on the measurement light side objective lens 12 again, passes through the measurement light side stop 11, and enters the beam splitter 3. And return. The object to be measured 13 is installed on the Z stage 22 and can be moved in the optical axis direction of the measurement light side arm 5 to change the distance between the measurement light side objective lens 12 and the object to be measured 13. be able to.

  Lights L5 and L6 respectively returned from the reference light side arm 4 and the measurement light side arm 5 are overlapped by the beam splitter 3, enter the imaging lens 18, and form an image on the imaging element 20 of the camera 19. .

  Here, the light source 1 uses light that is not monochromatic light.

  Further, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3 (optical distance from the center of the half mirror surface 17 to the reflecting surface of the reference mirror 9), and the half mirror surface of the beam splitter 3 The stage for the reference mirror under the control of the control unit 21a so that the optical distance of the measurement light side arm 5 from the center of 17 (the optical distance from the center of the half mirror surface 17 to the measurement target surface 14) becomes equal to each other. 10 and aperture adjustment mechanisms 70 and 80 are respectively driven and adjusted. This adjustment is performed each time the apertures of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are adjusted by the diaphragm adjustment mechanisms 70 and 80.

  With this configuration, a so-called white interferometer is configured as the entire optical system, and thus an interference fringe image reflecting the state in the vicinity of the measurement target surface 14 is formed on the image sensor 20. The interference fringe image formed on the image sensor 20 is acquired as data by the control unit 21a and stored in the storage unit 21b.

  The control unit 21 a is connected to the camera 19, the reference mirror stage 10, the Z stage 22, and the aperture adjustment mechanisms 70 and 80, and drives the reference mirror stage 10, the Z stage 22, and the reference. The driving of the aperture adjusting mechanism 70 for the aperture diameter of the light side diaphragm 7 and the driving of the aperture adjusting mechanism 80 for the aperture diameter of the measurement light side diaphragm 11 are controlled independently.

  In the case of a general white light interferometer, by moving the DUT 13 in the optical axis direction (Z direction) using the Z stage 22, the inner layer deep portion from the surface of the DUT 13 to the DUT 13 is measured. Until then, the measurement target surface 14 is scanned in the direction of the optical axis over a distance sufficiently wider than the coherence gate. Along with this long distance scanning operation, the maximum amplitude position of the interference fringe intensity at each position from the surface of the object to be measured 13 to the inner layer deep part is calculated, and the inner layer of the object to be measured 13 is obtained with a three-dimensional array of the maximum amplitude positions. Measured data. However, in that case, the inner layer measurement data of the measurement object 13 can be obtained only after the measurement object 13 is scanned at a sufficiently long distance in the optical axis direction and the maximum amplitude position is calculated.

  Therefore, in the first embodiment, under the control of the control unit 21a, the reference mirror stage 10 connected to the control unit 21a is modulated by a minute amount in the optical axis direction for adjusting the optical distance. Similarly, the control unit 21a synchronizes with the imaging timing of the camera 19 connected to the control unit 21a. In this way, under the control of the control unit 21a, the reference mirror stage 10 can be modulated by a minute amount for adjusting the optical distance, and the length in the optical axis direction as in a normal white light interferometer can be increased. The inner layer measurement data of the object to be measured 13 (data of the inner layer state of the object to be measured 13) can be acquired without requiring the movement of the distance measurement target surface. The modulation operation may be a sine wave shape, a rectangular shape, a staircase shape, a sawtooth shape, or the like, as long as a known interference fringe amplitude calculation method can be performed.

  Note that the object to be measured 13 may be scanned with the measurement target plane 14 by moving the object to be measured 13 using the Z stage 22, but the present invention is not limited to this. It may be realized by fixing 13 and moving the entire other optical system with respect to the object to be measured 13. In short, it is only necessary to change the relative distance in the direction along the optical axis between the measurement target surface 14 constituted by the entire optical system and the DUT 13 as a result.

  The coherence length of the light source 1 is set to 0.1 μm or more and 20 μm or less. Further, the half width of the wavelength spectrum of the light source 1 is set to 20 nm or more and 5 μm or less. This interference measuring apparatus selectively extracts only the vicinity of the measurement target surface 14 as planar image data using optical interference. At that time, if the measurement is within the range of the coherence length and the wavelength spectrum half width of the light source 1 described above, the resolution in the depth direction along the optical axis when measuring the object to be measured 13 is 0.1 μm or more to 20 μm or less. Range. This resolution range is a value suitable for measurement of a typical object to be measured (for example, a resin or gel in suspension, a protein, a fine particle of starch, or a biological tissue).

  Note that the center wavelength of the light source 1 at this time is, for example, 800 nm.

  By such a method, it is not necessary to move the measurement target surface 14 with a long distance in the optical axis direction as in a normal white interferometer, and the inner layer state of the object 13 to be measured in the vicinity of the measurement target surface 14 can be measured. At this time, when the measurement target surface 14 is set to a deeper part of the object 13 to be measured and the inner layer state is to be measured, if the numerical aperture of the measurement light side objective lens 12 is large, the measurement light side objective lens 12 performs measurement. Since the object to be measured 13 exists up to the target surface 14, there is an optical path length difference between the light beam that passes through the central portion of the measurement light side objective lens 12 and the light beam that passes through the peripheral portion of the measurement light side objective lens 12. This causes a decrease in interference fringe signal intensity. Therefore, when measuring the deep inner layer of the object 13 to be measured, it is better to reduce the influence of the optical path length difference even if the numerical resolution of the measurement light side objective lens 12 is lowered and the optical resolution is somewhat sacrificed. Often results are obtained.

  Therefore, the aperture adjustment mechanisms 70 and 80 are used to set the aperture diameter of the reference beam side diaphragm 7 and the aperture diameter of the measurement beam side diaphragm 11 provided in the reference beam side arm 4 and the measurement beam side arm 5 so that the objective lens in FIG. It is made smaller than each set diameter in the case of several states.

  In addition, the to-be-measured object 13 is comprised by the flat structure | tissue 15 and the spherical structure 16 as an example. The spherical structure 16 is, for example, a spherical object having a diameter of 1 μm to 100 μm and no distortion. The flat tissue 15 is a disk-shaped object in which a sphere is crushed in one direction. The diameter (major axis) of the major axis of the disc is, for example, 1 μm to 100 μm, and the minor axis direction (minor axis: the diameter of the disc). (Thickness) is an object of 1/2 or less. Specific examples of the spherical tissue 16 and the flat tissue 15 include fine particles such as resin or gel in suspension, protein, or starch. As a more detailed example, the spherical tissue 16 includes epithelial cells present in the skin epidermis, and the flat tissue 15 includes squamous epithelial cells present in the skin epidermis.

  FIG. 2 is a diagram showing a low numerical aperture state of the interference measurement apparatus according to the first embodiment of the present invention. The aperture diameter of the reference light side diaphragm 7 and the aperture diameter of the measurement light side diaphragm 11 are adjusted by diaphragm adjustment mechanisms 70 and 80 described later in detail so that the state of FIG. 2 is smaller than the state of FIG. In addition, in the state of FIG. 2, the aperture diameter of the reference light side diaphragm 7 and the aperture diameter of the measurement light side diaphragm 11 are adjusted to be equal to each other. The reference light side objective lens 8 and the measurement light side objective lens 12 are provided by providing the reference light side diaphragm 7 and the measurement light side diaphragm 11 and adjusting them with the diaphragm adjusting mechanisms 70 and 80 so that the aperture diameters thereof are reduced. It is possible to reduce the substantial numerical aperture. Since the reference light side stop 7 and the measurement light side stop 11 function as a so-called aperture stop, a part of the visual field being measured is not limited by the opening / closing operation of the stop.

  However, if the aperture diameter of the reference light side diaphragm 7 and the aperture diameter of the measurement light side diaphragm 11 are not equal to each other, the reference light beam diameter on the reference light side arm 4 side and the measurement light beam diameter on the measurement light side arm 5 side are set. However, the optical conditions are not the same, and the optical interference signal is weakened or a phase shift occurs, resulting in a result different from the intended optical interference. For this reason, it is important to keep the aperture diameters of both stops equal. For the sake of explanation, FIG. 2 shows the light flux of the irradiation light from the light source 1 as being thin, but the actual state is not different from the state of FIG.

  That is, it is desirable that the distance between the reference light side diaphragm 7 and the reference light side objective lens 8 and the distance between the measurement light side diaphragm 11 and the measurement light side objective lens 12 are the same. This is to make the aperture diameters of both objective lenses coincide.

  In addition, the optical characteristics (transmittance, reflectance, etc.) of the reference light side diaphragm 7 and the optical characteristics (transmittance, reflectance, etc.) of the measurement light side diaphragm 11 are the same in order to further increase the degree of coincidence of the aperture diameters. It is desirable to be.

  Further, it is assumed that the objective lenses 8 and 12 used in the first embodiment are infinity correction system objective lenses. Usually, the aperture stop position in the optical design of the objective lens of the infinity correction system exists at the end of the objective lens barrel or inside the objective lens barrel. In any case, as shown in FIG. 1, it is desirable that the reference light side stop 7 and the measurement light side stop 11 are arranged at the aperture stop positions of the corresponding objective lenses 8 and 12, respectively. It can function as an almost ideal aperture stop without any problem. When the aperture stop adjusting function is realized by the external aperture adjusting mechanisms 70 and 80, a commercially available objective lens can be used. Therefore, the range of selection of the objective lens suitable for the object to be measured 13 is widened, and interference is achieved. The convenience of the measuring device is further increased.

  Further, in order to increase the accuracy of interference, the optical characteristics (transmittance, reflectance, refractive index, etc.) of the reference light side objective lens 8 and the optical characteristics (transmittance, reflectance, refractive index, etc.) of the measurement light side objective lens 12 are measured. Are preferably the same. More specifically, the reference light side objective lens 8 and the measurement light side objective lens 12 are preferably made of the same glass material.

  Further, as described below, when an iris diaphragm is used as each of the reference light side diaphragm 7 and the measurement light side diaphragm 11, the aperture diameter of the diaphragm can be changed substantially steplessly. As will be described in detail below, it can be operated in accordance with an instruction from the control unit 21a by combining with a known drive mechanism.

  The interference measurement apparatus further includes an optical path length adjustment mechanism that adjusts the optical path length of the reference light so as to correspond to the change in the aperture diameter. Specifically, the reference mirror stage 10 is also driven under the control of the control unit 21 a, and the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3 and the half of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of the mirror surface 17 is set to be equal to each other. That is, the reference mirror stage 10 and the control unit 21a constitute an optical path length adjusting mechanism. However, since the position of the reference light-side objective lens 8 may be changed, the optical path length adjustment mechanism may be constituted by the moving stage of the lens and the control unit 21a. In order to measure a desired position, the optical path length of the measurement light may be changed in the same manner as the reference light. As described above, the optical path length of the measurement light is adjusted and changed by the Z-axis stage 22 or the like.

  FIG. 3 is a diagram showing a state of the reference light side diaphragm 7 in a high numerical aperture state by the diaphragm adjusting mechanism 70 of the interference measuring apparatus according to the first embodiment of the present invention. FIG. 4 is a diagram showing a state of the reference light side diaphragm 7 in a low numerical aperture state by the diaphragm adjustment mechanism 70 of the interference measuring apparatus according to the first embodiment of the present invention. As shown in FIGS. 3 and 4, the aperture adjusting mechanism 70 moves the aperture diameter of an iris diaphragm 71 composed of a large number of diaphragm blades by a drive mechanism 72 such as a motor, thereby opening the aperture diameter of the reference light side diaphragm 7. To change. Note that the measurement light side diaphragm 11 of FIG. 1 or 2 also functions in the same manner with the diaphragm adjustment mechanism 80 having the same configuration. In order to increase the accuracy of interference, it is desirable that the optical conditions of the reference light side stop 7 and the measurement light side stop 11 are the same. The aperture adjusting mechanisms 70 and 80 can be operated independently according to instructions from the control unit 21a.

  As described above, when the numerical apertures of the objective lenses 8 and 12 of the interference measuring apparatus can be changed by the opening and closing operation of the apertures 7 and 11 by the aperture adjusting mechanisms 70 and 80 under the control of the control unit 21a, the numerical aperture adjustment can be performed. Therefore, it is not necessary to replace the objective lens each time, and workability is improved. In addition, since the numerical aperture can be changed while the DUT 13 is set, measurement with different numerical apertures can be performed at the same measurement position. In addition, by gradually changing the numerical aperture according to the measurement position in the depth direction of the object to be measured 13, measurement at a high resolution with a high numerical aperture (first numerical aperture) near the surface of the object to be measured 13, and In the inner layer portion of the object 13 to be measured, it is possible to perform measurement in which both improvement in measurement light penetration by a low numerical aperture (second numerical aperture) and improvement in image quality of an interference signal image by improvement in interference signal intensity are possible. However, the second numerical aperture is smaller than the first numerical aperture.

  Here, two procedures for changing the numerical aperture according to the measurement position in the depth direction of the DUT 13 will be described.

  In the first procedure, the aperture diameter can be set finely for each measurement position. For this reason, when the DUT 13 is a sample having a complex refractive index distribution that is not uniform depending on the measurement position, or a sample having a refractive index distribution that cannot change the optical path length depending on the measurement position. It is desirable to apply the first procedure.

  On the other hand, the second procedure is easy because the diaphragm system in the middle of the measurement is automatically determined if the operator determines the diaphragm diameter at the start and end of the measurement. Therefore, it is preferable to select the second procedure when the DUT 13 has a uniform refractive index or when the refractive index distribution at the measurement position of the DUT 13 is small.

  An example of a sample having a small refractive index distribution at the measurement position is a sample in which small fragments or particles are dispersed in a liquid or gel-like medium. In addition, examples of a sample having a refractive index distribution that cannot change the optical path length depending on the measurement position include a sample of a biological sample such as a cell tissue immersed in a culture solution, or a film body of a different material. A multilayer film sample in which a plurality of layers are stacked.

  First, the first procedure will be described.

  FIG. 6 is a flowchart illustrating a first procedure for measuring the object to be measured 13 while changing the numerical apertures of the objective lenses 8 and 12 and the position of the measurement target surface 14 of the interference measurement apparatus according to the first embodiment. .

  First, in step S1, the user determines the number of Z stage operation steps Nst, and the aperture diameters D (M) of the reference light side diaphragm 7 and the measurement light side diaphragm 11 at the Mth step (where M = 1, 2). ... Nst) is determined, the Z stage feed amount ΔZ is determined, and set in the control unit 21a using the input unit 21c. Here, in the setting of the stop diameter D (M) of the reference light side stop 7 and the measurement light side stop 11 and the feed amount ΔZ of the Z stage 22 at the M-th step, the position of the Z stage 22 is increased by the feed amount ΔZ. When setting to move to the inside of the object to be measured 13 sometimes, the aperture diameters D (M) of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are set so that D (N + 1) ≦ D (N). Set in the control unit 21a. That is, control is performed so that the aperture diameters D of the reference light side diaphragm 7 and the measurement light side diaphragm 11 monotonously decrease as the measurement target surface 14 moves further inside the device under test 13. Set in the part 21a. The procedure after step S2 is executed under the control of the control unit 21a.

  Next, in step S2, the user sets the variable N for internal count to 1 in the control unit 21a in a state where the DUT 13 is set on the Z stage 22 of the interference measuring apparatus. The position is set to the measurement start position Zst.

  Next, in step S3, the aperture adjusting mechanisms 70 and 80 are driven under the control of the control unit 21a, and the diameters of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are set to D (N), that is, D (1). Set to.

  At this time, the reference mirror stage 10 is also driven under the control of the control unit 21 a, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3, and the half mirror surface of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of 17 is set to be equal to each other.

  Next, in step S4, under the control of the control unit 21a, the Z stage 22 of the interference measuring apparatus and the camera 19 are operated to acquire the interference signal image Pic (N) by the control unit 21a and store it in the storage unit 21b. Store.

  Next, in step S5, the control unit 21a increments the variable N by 1 to obtain a new variable N.

  Next, in step S6, in the control unit 21a, if the new variable N in step S5 is equal to or less than Nst, the process proceeds to step S7. If the new variable N is greater than Nst, the flow ends.

In step S7 after the branch process in step S6, the control unit 21a changes the position Z of the Z stage 22 to Z = Zst + ΔZ (N−1) (Equation 1)
Set to. Here, under the control of the control unit 21a, the Z stage 22 is driven to move the position of the measurement target surface 14 relative to the object 13 to be measured by the Z stage feed amount ΔZ.

  Next, returning to step S3, the aperture adjusting mechanisms 70 and 80 are driven under the control of the control unit 21a, and the diameters of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are set to D (N).

  At this time, the reference mirror stage 10 is also driven under the control of the control unit 21 a, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3, and the half mirror surface of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of 17 is set to be equal to each other. Thereafter, the control unit 21a repeats the above-described processing after step S4 until the flow ends.

  In the control unit 21a, when the flow ends, the interference signal image Pic (N) (where N = 1, 2,... Nst) has a position Z in the Z direction of the DUT 13, Z = Zst + ΔZ. The interference signal image corresponding to (N-1) is stored in the storage unit 21b, and the entire interference signal image Pix (N) corresponds to each Z position of the object 13 to be measured. The three-dimensional measurement data is obtained by changing the number.

  Next, an example of another procedure in which the method for determining the aperture diameter is specified will be shown. FIG. 7 is a flowchart showing a second procedure for measuring the object to be measured 13 while changing the numerical apertures of the objective lenses 8 and 12 and the position of the measurement target surface 14 of the interference measuring apparatus according to the first embodiment. .

  First, in step S9, the user determines the maximum aperture diameter Dmax, the minimum aperture diameter Dmin, the Z stage operation step number Nst, and the Z stage feed amount ΔZ, and sets them in the control unit 21a using the input unit 21c. Here, the setting of the Z stage feed amount ΔZ is set so as to move to the inside of the DUT 13 when the position of the Z stage 22 is increased by the Z stage feed amount ΔZ. That is, the measurement target surface 14 is set so as to move further inside the device under test 13 as the measurement proceeds. The maximum aperture diameter Dmax and the minimum aperture diameter Dmin are set so that Dmax> Dmin. The procedures after step S10 are executed under the control of the control unit 21a.

Next, in step S10, in the control unit 21a, the aperture diameter change amount ΔD is set to ΔD = (Dmax−Dmin) / (Nst−1) (Expression 2)
To decide.

  Next, in step S11, in a state where the DUT 13 is set on the Z stage 22 of the interference measuring apparatus, the control unit 21a sets the variable N for internal count to 1, and sets the Z stage position to the measurement start position. Set to Zst.

Next, in step S12, the aperture adjusting mechanisms 70 and 80 are driven under the control of the control unit 21a, and the apertures D of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are set to D = Dmax−ΔD (N−1). (Formula 3)
Set to.

  At this time, the reference mirror stage 10 is also driven under the control of the control unit 21 a, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3, and the half mirror surface of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of 17 is set to be equal to each other.

  Next, in step S13, under the control of the control unit 21a, the Z stage 22 of the interference measurement device and the camera 19 are operated to acquire the interference signal image Pix (N) by the control unit 21a and store it in the storage unit 21b. Store.

  Next, in step S14, the control unit 21a increments the variable N by 1 to obtain a new variable N.

  Next, in step S15, in the control unit 21a, if the new variable N in step S14 is equal to or smaller than Nst, the process proceeds to step S16. If the new variable N is larger than Nst, the flow ends.

In step S16 after the branch process in step S15, the control unit 21a changes the position Z of the Z stage 22 to Z = Zst + ΔZ (N−1).
Set to. Here, under the control of the control unit 21a, the Z stage 22 is driven to move the position of the measurement target surface 14 relative to the object 13 to be measured by the Z stage feed amount ΔZ.

  Next, returning to step S12, in the control unit 21a, the apertures D of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are respectively set to D = Dmax−ΔD (N−1) in the same manner as described above. Thereafter, the control unit 21a repeats the above-described processing after step S13 until the flow ends.

  In the control unit 21a, when the flow ends, the interference signal image Pix (N) (where N = 1, 2,... Nst) includes the position Z of the DUT 13 in the Z direction, Z = Zst + ΔZ. The interference signal image corresponding to (N-1) is stored in the storage unit 21b, and the entire interference signal image Pix (N) corresponds to each Z position of the object 13 to be measured. The three-dimensional measurement data is obtained by changing the number.

  By utilizing the fact that the numerical apertures of the objective lenses 8 and 12 are variable according to the first procedure or the second procedure as described above, the interference signal image is improved by improving the permeability of the measurement light L3 and the interference signal strength. Measurements that achieve both improved image quality are possible.

  Next, it will be described that another effect can be obtained by making it possible to change the numerical apertures of the objective lenses 8 and 12 in the interference measurement apparatus of the first embodiment.

  The spot diameter Q in the focal plane in the microscope optical system using the objective lenses 8 and 12 is determined by the wavelength λ of the light to be used and the numerical aperture NA of the objective lens.

Q = 1.22 × λ / NA (Formula 4)
For example, the spot diameter Q is 4.1 μm when an objective lens having a numerical aperture of 0.3 with light having a wavelength of 1.0 μm is used. On the other hand, when the objective lens having a numerical aperture of 0.8 with a wavelength of 1.0 μm is used, the spot diameter Q is 1.5 μm. This spot diameter is also related to the resolution of the optical system, and a value that is half the spot diameter is the limit of the resolution. In other words, the optical system cannot distinguish and observe those having a spot diameter less than half.

  Here, as an example, consider a spherical object as the DUT 13. FIG. 5 is a diagram schematically showing the relationship between the spherical object to be measured 23 and the measurement optical system. In FIG. 5, components unnecessary for the description are omitted. The measurement light L4 irradiated to the spherical object to be measured 23 passes through the objective lens 24 (corresponding to the measurement light side objective lens 12 in FIG. 1) and is converged to the spot diameter Q. The central light beam 26 passing through the center of the objective lens 24 hits the spherical object to be measured 23 at the spot central portion 28, is reflected, passes again through the objective lens 24 and the beam splitter 3, and is referred to as reference light L5 (described in FIG. 5). 1), the light passes through the imaging lens 25 (corresponding to the imaging lens 18 in FIG. 1), and is condensed on the imaging element pixel 30 (corresponding to the imaging element pixel 19 in FIG. 1). Similarly, a peripheral ray 27 passing through the peripheral part of the objective lens 24 hits the spherical object to be measured 23 at the spot peripheral part 29, is reflected, passes through the objective lens 24 and the beam splitter 3 again, and is superimposed on the reference light L5. After that, the light passes through the imaging lens 25 and is condensed on the image sensor pixel 30. Since the measurement light L4 is applied to the spherical object to be measured 23, the spot central portion 28 and the spot peripheral portion 29 are different by a height u within the spot diameter Q. For this reason, the optical path length difference is generated by 2u between the peripheral ray 27 and the central ray 26 before the measurement light L4 is irradiated onto the spherical measurement object 23 and the reflected light reaches the image sensor pixel 30. Will be. When the measurement light wavelength is 1.0 μm and the numerical aperture is 0.3, the spot diameter Q is 4.1 μm. Then, if the diameter of the spherical object to be measured 23 is 15 μm, the optical path length difference 2u is 562 nm, which is almost half of the measurement light wavelength. Since the spot diameter Q is the limit of the optical resolution, in the image in which the reflected light from the spherical measured object 23 within the spot diameter Q is condensed on the image sensor pixel 30, the peripheral portion within the spot diameter Q The light beam 27 and the central light beam 26 cannot be distinguished from each other, and the total reflected light within the spot diameter Q is superimposed. For this reason, the peripheral ray 27 and the central ray 26 cancel each other on the image sensor pixel 30, and the interference fringe signal intensity detected by the image sensor pixel 30 decreases. On the other hand, when the numerical aperture of the objective lens 24 is 0.8, the spot diameter Q is 1.5 μm and the optical path length difference 2u is 78 nm, which is about 8% of the measurement light wavelength. For this reason, it is difficult for the interference fringe signal intensity to decrease when the numerical aperture is 0.3.

  Thus, it can be seen that when the object to be measured 23 is spherical, the interference fringe signal is lowered and difficult to observe under certain conditions due to a change in the numerical aperture of the objective lens 24. On the other hand, in the case of the measurement object 23 having a flat shape, the measurement light within the spot diameter is less likely to cancel out, so the ease of observation of the measurement object 23 is not much related to the numerical aperture of the objective lens. There is no. This can also be said that by changing the numerical aperture of the objective lens 24, only the measured object 23 having a specific shape can be filtered. Assuming that the measurement light wavelength is λ and the numerical aperture of the objective lens 24 is NA, the diameter φ of the spherical object to be measured 23 filtered by the interference optical system of the first embodiment is expressed by Equation 5.

φ = λ × (0.25+ [1.22 / NA] ² ) (Formula 5)
Actually, the wavelength of the measurement light λ is wide and the spherical object to be measured 23 is not an ideal sphere. Therefore, although it is not perfect filtering, the spherical object to be measured 23 filtered by the numerical aperture is not used. The approximate diameter can be defined by Equation 5. For this reason, the numerical aperture of the objective lens 24 can be determined in accordance with the diameter of the spherical object to be measured 23 to be filtered.

  In FIG. 1, since both the flat tissue 15 and the spherical tissue 16 constituting the object to be measured 13 can be observed with the interference measuring device, they are shown by solid lines. Similarly, in FIG. 2, the spherical tissue 16 is the objective lens. Since the numerical aperture is small and the filtering effect is difficult to observe, it is indicated by a broken line.

  As an example, the high numerical aperture state in the first embodiment means a numerical aperture of 0.5 or more, and the low numerical aperture state means a numerical aperture of less than 0.5. When the low numerical aperture state is 7 or more and the numerical aperture ranges from 0.4 to 0.1, the actual filtering effect can be sufficiently exerted.

  FIG. 8 is a flowchart showing an example of a procedure for performing shape filtering of the object to be measured 23 by changing the numerical aperture of the objective lens 24 of the interference measuring apparatus according to the first embodiment. The object to be measured 23 includes a flat tissue and a spherical tissue.

  First, in step S19, with the object to be measured 23 set in the interference measuring device, the diaphragm adjusting mechanisms 70 and 80 are driven under the control of the control unit 21a to set the reference light side diaphragm 7 and the measurement light side diaphragm 11 to each other. Open the interference measurement device to a high numerical aperture state. At this time, the reference mirror stage 10 is also driven under the control of the control unit 21, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3, and the half mirror of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of the surface 17 is set to be equal to each other.

  Next, in step S20, the Z stage 22 of the interference measuring device and the camera 19 are operated under the control of the control unit 21a, and the first interference signal image is acquired by the control unit 21a and stored in the storage unit 21b. Since the first interference signal image is an interference signal image acquired in a high numerical aperture state, tissues of all shapes are detected.

  Next, in step S21, the reference light side diaphragm 7 and the measurement light side diaphragm 11 are respectively squeezed by the diaphragm adjusting mechanisms 70 and 80 under the control of the control unit 21 to bring the interference measuring device into a low numerical aperture state. At this time, the reference mirror stage 10 is also driven under the control of the control unit 21, the optical distance of the reference light side arm 4 from the center of the half mirror surface 17 of the beam splitter 3, and the half mirror surface of the beam splitter 3. The optical distance of the measurement light side arm 5 from the center of 17 is set to be equal to each other.

  Next, in step S22, under the control of the control unit 21a, the Z stage 22 of the interference measurement device and the camera 19 are operated to acquire the second interference signal image by the control unit 21a and store it in the storage unit 21b. Since the second interference signal image is an interference signal image acquired in a low numerical aperture state, the signal of the spherical tissue is attenuated and a tissue other than the spherical tissue is detected.

  Next, in step S23, an interference signal difference image that is a difference image between the first interference signal image and the second interference signal image is acquired by the control unit 21a and stored in the storage unit 21b. In the first interference signal image, signals of all tissues are obtained, and in the second interference signal image, signals of tissues other than the spherical tissue are obtained. Therefore, in the interference signal difference image, a signal of only the spherical tissue is obtained. Become.

  In step S24, the control unit 21a evaluates the flat tissue in the object to be measured 13 using the second interference signal image. In step S25, the spherical structure in the object to be measured 13 is determined using the interference signal difference image. Evaluation is performed by the control unit 21a, and the procedure ends.

  In this way, using the fact that the tissue shape of the object to be measured 13 can be filtered by adjusting the diaphragms 7 and 11 of the interference measuring apparatus, in addition to the conventionally obtained interference fringe signal intensity and interference signal image shading. The shape information of the tissue can also be obtained.

  The reference light side stop 7 and the measurement light side stop 11 may be a stepless / stepped variable stop, or may be a form in which several kinds of stops having a fixed aperture diameter are prepared and used interchangeably. . In the first embodiment, the iris diaphragm 71 is used as the diaphragm adjusting mechanisms 70 and 80. However, the diaphragm diameter may be switched by a turret having a plurality of apertures, or a light-transmissive two-dimensional liquid crystal matrix element. The light transmitting part and the light shielding part may be configured.

  In FIG. 1, the aperture diameters of the reference light side diaphragm 7 and the measurement light side diaphragm 11 are sufficiently large so that the numerical aperture inherent in the reference light side objective lens 8 and the measurement light side objective lens 12 is not reduced. The opening diameter is set.

  Here, the reason for changing the numerical aperture will be described in detail using a general example.

  For example, when measuring the vicinity of the surface of the object to be measured with the interferometer, the coherence gate (position where the difference between the optical path length of the measurement light and the optical path length of the reference light is zero) is positioned near the surface of the object to be measured. As described above, the distance between the object to be measured and the objective lens of the interference measuring apparatus is adjusted. In this case, only a gas or medium liquid (when the object to be measured is measured by immersion) exists between the part to be measured and the objective lens. FIG. 9 shows a situation where the space between the objective lens 31 and the object to be measured 23 is filled with the medium liquid 33. The coherence gate center 34 is indicated by a broken line. In this case, the space from the distal end portion of the objective lens 31 to the measurement target region 35 is filled with an optically homogeneous object such as the liquid 33 of the medium. The same applies to a gas instead of the medium liquid 33. Therefore, in FIG. 9, in the space from the tip of the objective lens 31 to the measurement target portion 35, the optical conditions of the light beam 36 passing through the center of the objective lens 31 and the light beam 37 passing through the periphery of the objective lens 31 are the same. is there. In addition, the focal plane of the objective lens 31 is usually adjusted to coincide with the coherence gate center 34.

  On the other hand, when measuring the inside of the object to be measured, the distance between the object to be measured and the objective lens of the interference measuring device is adjusted so that the coherence gate is positioned inside the object to be measured. In this case, since the measurement target part is inside the object to be measured, the object constituting the object to be measured occupies a part of the space from the tip of the objective lens to the measurement target part. FIG. 10 shows a situation where the measurement target region 35 is set inside the device under test 23. The coherence gate center 34 is a measurement position of the interferometer optical system and coincides with the measurement target portion 35. On the other hand, when the light beam 36 passing through the center of the objective lens 31 reaches the measurement target region 35, the distance Lc that passes through the object constituting the object to be measured 23 and the light beam 37 passing through the periphery of the objective lens 31 is the measurement target region. The distance Ld is longer than the distance Ld that passes through the object constituting the DUT 23 when reaching 35. This becomes more noticeable as the objective lens has a larger numerical aperture so that the light rays 37 around the objective lens 31 can be bent at a larger angle.

  Normally, when the objective lens 31 is used in the air, the objective lens 31 is designed according to the refractive index of air. Further, when the objective lens 31 is used in a liquid immersion state as shown in FIG. 9 or FIG. 10, the objective lens 31 is designed in accordance with the refractive index of the liquid (usually water or dedicated oil). This is so that the highest optical performance can be obtained in a state where the objective lens 31 is filled with the assumed medium from the tip to the measurement site. However, in general, the refractive index of the DUT 23 is often different from the assumed refractive index of the medium. Therefore, when Ld> Lc as shown in FIG. 10, the distances passing through the DUT 23 having a refractive index different from that of the liquid 33 are different. Therefore, there is a difference in the optical path length until the light beam 36 passing through the center of the objective lens 31 and the light beam 37 passing through the periphery of the objective lens 31 reach the measurement target region 35, and the light beam 36 and the light beam 37 The optical conditions are different.

  In addition, the object to be measured 23 often has a variation in refractive index depending on its position. This is particularly noticeable when the object to be measured 23 is a biological sample such as skin or cells. This causes a difference in the optical path length between the light beam 36 and the light beam 37.

  For these reasons, the situation in which the optical conditions of the light beam 36 passing through the center of the objective lens 31 and the light beam 37 passing through the peripheral portion of the objective lens 31 are different from each other even if an objective lens having a large numerical aperture is used. May cause a drop. In particular, when the position behind the surface of the object to be measured 23 is set as the measurement target site, the decrease in the optical signal intensity returned from the object to be measured 23 or the decrease in the optical interference fringe signal intensity due to the influence of the optical path length difference. Cause. For this reason, in the conventional interference measuring apparatus, it becomes a hindrance in implement | achieving high resolution and a high-depth part measurement.

  Therefore, in the first embodiment, the numerical aperture of the objective lens 31 is changed in accordance with the position of the measurement target portion 35 with respect to the object to be measured 23. FIG. 11 shows a state in which the measurement target portion 35 is set inside the DUT 23 as in FIG. 10, but the numerical aperture of the objective lens 31 is smaller than that in FIG. 9 or FIG. It is a figure. The measurement target portion 35 is set to the same depth in the object to be measured 23 as in FIG. 10, but the numerical aperture of the objective lens 31 is small. For this reason, when the light beam 36 passing through the center of the objective lens 31 reaches the measurement target region 35, the distance Lc that passes through the object constituting the object to be measured 23 and the light beam 37 passing through the periphery of the objective lens 31 are measured. When the distance Ld that passes through the object constituting the object to be measured 23 is compared when reaching the part 35, the difference between the two is small as compared with the state of FIG. Furthermore, the position where the light beam 36 and the light beam 37 pass through the object to be measured 23 to the measurement target region 35 also passes through a position closer to that in the state of FIG. For this reason, it is in the state which is hard to generate | occur | produce the malfunction resulting from the optical path length difference of the light ray 36 and the light ray 37 mentioned above.

  As shown in FIG. 11, the optical resolution is somewhat disadvantageous by reducing the numerical aperture of the objective lens 31, but the light beam 36 and the light beam 37 are measured even when measuring a deeper inner layer of the object to be measured 23. Difficult to occur in the optical path length. For this reason, the trouble by the interference signal fringe intensity | strength fall and image quality fall reduces. Therefore, an interference optical system capable of obtaining a desired optical interference image can be configured.

  However, if the objective lens in the reference side optical path is replaced in order to change the numerical aperture, the objective lens in the signal side optical path must also be replaced, and workability is significantly impaired. In addition, since there is no guarantee that the same location can be measured by changing the position of the object to be measured before and after lens replacement, the measurement accuracy may decrease. Therefore, as in the first embodiment of FIG. 1, both the reference light side diaphragm 7 and the measurement light side diaphragm 11 are provided, and the control unit 21a controls the opening diameters of both so as to be equal to each other. Further, by adjusting and controlling the reference light side diaphragm 7 and the measurement light side diaphragm 11 with the diaphragm adjusting mechanisms 70 and 80, it is not necessary to exchange the objective lenses 8 and 12, so that workability can be improved and high measurement can be performed. It is possible to indicate accuracy.

  Therefore, it is possible to achieve both high resolution measurement and high depth measurement in the inner layer measurement in the interference measurement apparatus by configuring the optimum optical condition according to the observation position of the object to be measured 13. In other words, it is possible to provide an interference measuring apparatus and an interference measuring method that can measure the surface layer to the inner layer of the object to be measured 13 with high accuracy without exchanging the objective lenses 8 and 12 and have good workability.

  The control unit 21a, the storage unit 21b, and the input unit 21c in the computer 21 may be configured by one or more electronic circuits or processors including a semiconductor device, a semiconductor integrated circuit (IC), or an LSI. These functions or operations are executed by software processing. Such software may be recorded not only on a storage element mounted on an electronic circuit but also on a non-temporary recording medium such as a ROM, an optical disk, or an HDD.

  In addition, the effect which each has can be show | played by combining arbitrary embodiment or modification of the said various embodiment or modification suitably. In addition, combinations of the embodiments, combinations of the examples, or combinations of the embodiments and examples are possible, and combinations of features in different embodiments or examples are also possible.

  The interference measuring apparatus and the interference measuring method of the present invention can measure the surface layer to the inner layer of the object to be measured with high accuracy without exchanging the objective lens, and measure the details of the inner layer portion such as skin or cells in a non-invasive manner. It can be applied to uses such as biological measurement.

DESCRIPTION OF SYMBOLS 1 Light source 2 Collimator lens 3 Beam splitter 4 Reference beam side arm 5 Measurement beam side arm 6 Mirror 7 Reference beam side diaphragm 8 Reference beam side objective lens 9 Reference mirror 10 Reference mirror stage 11 Measurement beam side diaphragm 12 Measurement beam side objective lens DESCRIPTION OF SYMBOLS 13 Measured object 14 Measurement target surface 15 Flat structure 16 Spherical structure 17 Half mirror surface 18 Imaging lens 19 Camera 20 Imaging element 21 Computer 21a Control part 21b Storage part 21c Input part 22 Z stage 23 Measured object 24 Objective lens 25 Imaging lens 26 Central light beam 27 Peripheral light beam 28 Spot central portion 29 Spot peripheral portion 30 Image sensor pixel 31 Objective lens 33 Liquid 34 Coherence gate center 35 Measurement target region 36 Light beam 37 Light beam 38 Light source 39 Light beam 40 Beam splitter 41 First One arm Second lens 43 object 44 second reference mirror 47 of the arm 45 the lens 46 the piezoelectric ceramic 48 cameras 70 and 80 stop adjustment mechanism 71 iris diaphragm 72 drive mechanisms

Claims (8)

A light source;
A light beam splitting element that splits a light beam from the light source into reference light and measurement light;
A reference light side lens for condensing the reference light on a reference surface;
A measurement light side lens for condensing the measurement light on the object to be measured;
A reference light side stop disposed between the light beam splitting element and the reference light side lens;
A measurement light side stop disposed between the light beam splitting element and the measurement light side lens;
An aperture adjustment mechanism that changes the aperture of the reference light side aperture and the aperture of the measurement light side aperture, respectively,
The interference adjusting device is configured to adjust the reference light side diaphragm and the measurement light side diaphragm so that the aperture diameters are equal to each other.   The interference measurement apparatus according to claim 1, wherein optical characteristics of the measurement light side lens and the reference light side lens are the same.   The interference measurement apparatus according to claim 1, wherein a distance between the reference light side diaphragm and the reference light side lens and a distance between the measurement light side diaphragm and the measurement light side lens are the same.   The interference measurement apparatus according to claim 3, wherein optical characteristics of the reference light side diaphragm and the measurement light side diaphragm are the same.   The interference measurement apparatus according to claim 4, wherein the reference light side and the measurement light side diaphragm are disposed at respective aperture stop positions of the reference light side and the measurement light side lens.   The aperture adjustment mechanism has the first numerical aperture when measuring the surface of the object to be measured, and the second numerical aperture smaller than the first numerical aperture when measuring the inner layer of the object to be measured. The interference measurement apparatus according to any one of claims 1 to 5, wherein adjustment is performed by changing an aperture of the reference light side diaphragm and an aperture of the measurement light side diaphragm.   The interference measurement apparatus according to claim 1, further comprising an optical path length adjustment mechanism that adjusts an optical path length of the reference light so as to correspond to the change in the aperture diameter.   An interference measurement method for measuring the object to be measured using the interference measurement apparatus according to claim 1.

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