JP2015203580A - light interference measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light interference measurement device that is excellent in robustness.SOLUTION: A light interference measurement device comprises: a light source 201 that emits light with a center wavelength λc; a beam splitter 205 that splits the light from the light source 201 to make the split light incident upon a reference surface 211 and an object 209; a lens 210 that is disposed in an optical path of the light reaching the reference surface 211 from the beam splitter 205; a camera 212 upon which interference light in which light reflected or scattered upon the reference surface 211 interferes with light reflected or scattered upon the object 209 is incident; a movement mechanism 214 that causes the object 209 to move in a first direction in a field of view of the camera 212; a storage unit 215a that stores intensity of the interference light before and after the movement by the movement mechanism 214; and a processing unit 215b that generates an image of the object from a square of a difference between the interference light before and after the movement thereby.

Description

  The present invention relates to an optical interference measuring apparatus.

  2. Description of the Related Art A phase modulation type optical interference measuring apparatus that measures an object by applying phase modulation of about the wavelength of light to an optical path length difference between signal light and reference light is known. (Patent Document 1).

  FIG. 9 shows an optical interference measuring apparatus using the Linnik interferometer of Patent Document 1. Light from the light source 100 enters the beam splitter 101 and reaches each arm of the optical interferometer. A lens 103 and an object 104 are disposed on the first arm 102, and a lens 106 and a reference surface 107 are disposed on the second arm 105. A PZT element 108 is disposed on the reference surface 107 in order to give a fine amount of movement about the wavelength of light. The light sent to each arm is reflected from the object 104 and the reference surface 107 and enters the camera 109. In Patent Document 1, the phase of the interference light is continuously sinusoidally modulated by the PZT element 108.

JP-T-2004-528586

  However, the prior art has a problem related to robustness due to the mechanical vulnerability of the PZT element.

  Therefore, the present invention provides a phase modulation type optical interference measuring apparatus excellent in robustness.

  An optical interference measuring apparatus of the present invention includes a light source that emits light having a center wavelength of λc, a beam splitter that splits light from the light source and makes it incident on a reference surface and an object, and the beam splitter to the reference surface. A lens disposed in an optical path of the light reaching, a camera on which interference light obtained by interference of light reflected or scattered by the reference surface and light reflected or scattered by the object is incident, and within the field of view of the camera An image of the object is obtained from a moving mechanism that moves the object in the first direction, a storage unit that stores the intensity of the interference light before and after the movement by the movement mechanism, and the square of the difference in the intensity of the interference light before and after the movement. An arithmetic unit to be constructed, and the reference surface is provided with a first region and a second region adjacent to each other in the first direction within the field of view of the camera, and the movement amount by the moving mechanism is 1 area of the above The first region and the second region form a step having a height λc / 4n, where n is the refractive index of the medium between the lens and the reference surface. It is characterized by.

  According to the present invention, it is possible to provide a phase modulation type optical interference measuring apparatus excellent in robustness.

Schematic diagram of optical interference measurement apparatus in Embodiment 1 The perspective view of the reference surface in Embodiment 1 The figure of the relationship between the visual field of the camera in Embodiment 1, the 1st phase area, the 2nd phase area, and the axial direction of a mechanical stage Flow chart showing the measurement method in the first embodiment The figure of the positional relationship of the visual field of a camera and a target object in step S2 of Embodiment 1 The figure of the positional relationship of the visual field of a camera and a target object in step S4 of Embodiment 1 The figure of the periodic reference surface which is many examples of Embodiment 1 The figure of the lattice-shaped reference surface which is many examples of Embodiment 1 Figure of conventional optical interference measurement device

Hereinafter, embodiments will be described with reference to the drawings.
(Embodiment 1)
FIG. 1 is a schematic diagram of an optical interference measuring apparatus 200 in the first embodiment.

  The optical interference measuring apparatus 200 is a kind of linnic interferometer. The light source 201 is a unit that emits light, and includes a lamp 202, a diaphragm 203, and a collimating lens 204. The lamp 202 is a light source that emits low coherence light, such as a halogen lamp, a xenon arc lamp, a mercury lamp, or an LED. The light emitted from the lamp 202 has, for example, a center wavelength λc = 800 nm and a coherence length Lc = 2 μm.

  Light from the light source 201 enters the beam splitter 205. The beam splitter 205 may be a cube type or a plate type. The light incident on the beam splitter 205 is separated into signal light and reference light having the same amplitude. The signal light is sent to the first arm 206 of the interferometer and the reference light is sent to the second arm 207.

  A first objective lens 208 and an object 209 are disposed on the first arm 206. The focal plane of the signal light is adjusted inside the object 209 by the first objective lens 208. The signal light reflected or scattered by the focal plane returns to the beam splitter 205 through the first objective lens 208. Note that reflection includes Fresnel reflection, and scattering includes backscattering.

  On the other hand, the second objective lens 210 and the reference surface 211 are disposed on the second arm 207. The reference light is focused on the reference surface 211 by the second objective lens 210. The reference light reflected or scattered by the reference surface 211 returns to the beam splitter 205 through the second objective lens 210.

  When the optical path length of the signal light in the first arm 206 and the optical path length of the reference light in the second arm 207 coincide within the range of the coherence length Lc of the light source 201, the signal light and the reference light returned to the beam splitter 205 are Causes optical interference. The interfered light (interference light) is imaged by the imaging lens 213 that focuses on the camera 212.

  The camera 212 has a sensor surface composed of, for example, a CCD type two-dimensional pixel. The sensor surface and the object 209, and the sensor surface and the reference surface 211 are arranged so as to be optically conjugate with each other. That is, the camera 212 overlaps the object 209 and the reference surface 211 in the field of view. Note that it is desirable to image the object 209 and the reference surface 211 at the same magnification in order to reduce measurement errors due to optical system aberrations, and the magnification is adjusted using the same glass material in order to reduce measurement errors due to wavelength dispersion. It is desirable to do.

  The computer 215 includes a storage unit 215 a that stores data acquired by the camera 212, a calculation unit 215 b that calculates data to construct an image of the object 209, and an instruction unit 215 c that instructs the shutter of the camera 212 and the mechanical stage 214. It has. Note that the shutter of the camera 212 may be electronic or mechanical.

  Note that the object 209 is placed on the mechanical stage 214. The mechanical stage 214 is an example of a moving mechanism that moves the object 209 in the direction of the drive axis (first direction) within the field of view of the camera 212. In FIG. 1, the first direction refers to the left-right direction on the paper. The mechanical stage 214 may move the object 209 or the optical axis of the signal light. That is, the mechanical stage 214 may be configured to move the object 209 within the field of view of the camera 212.

  Here, FIG. 2 shows the structure of the reference surface 211.

  The reference surface 211 has a mirror surface. This mirror surface is divided into a first phase region 211a (first region) and a second phase region 211b (second region). This mirror surface is formed, for example, by depositing a metal film only on a specific portion of a mirror in which a metal such as aluminum is coated on a rectangular glass substrate. The first phase region 211a and the second phase region 211b form a step 211c, and have a function of giving the reference light a different phase difference for each region. This reference surface 211 can give a phase difference to the reference light without being driven by a PZT element. Therefore, it is possible to realize a phase modulation type optical interference measuring apparatus having excellent robustness. Incidentally, the first phase region 211a and the second phase region 211b are parallel.

  The method of creating each region on the reference surface 211 may be mechanical cutting or the like, but it is desirable to use vapor deposition because it is necessary to provide the step 211c in the order of the wavelength of light.

  The height d of the step 211c is designed to generate a π phase difference between the reference light reflected by the first phase region 211a and the reference light reflected by the second phase region 211b. This is to realize a phase modulation type optical interference measuring apparatus. The height d is expressed by Equation (1), where n is the refractive index of the medium between the second objective lens 210 and the reference surface 211 in FIG.

  Here, λc is the center wavelength of the light emitted from the light source 201. For example, when the center wavelength λc of the light source 201 is 800 nm and n = 1, d = 200 nm.

  The reference surface 211 is arranged in the field of view of the camera 212 as shown in FIG. FIG. 3 shows a first phase region 211 a and a second phase region 211 b that are reflected in the field of view of the camera 212. A white arrow in FIG. 3 indicates a direction (first direction) in which the object 209 is moved by the mechanical stage 214 within the field of view of the camera 212 in FIG. The reference surface 211 is provided with a first phase region 211 a and a second phase region 211 b so as to be adjacent to each other in the first direction within the field of view of the camera 212.

  Here, to simplify the description, the first direction is the X axis, the optical axis of the signal light emitted from the first objective lens 208 in FIG. 1 is the Z axis, and the imaging plane of the camera 212 is the XY plane. Each coordinate axis is defined. Here, since the coordinates are defined around the field of view of the camera 212, the optical axis of the reference light emitted from the second objective lens 210 is also the Z axis. In this case, the widths in the X-axis direction of the first phase region 211a and the second phase region 211b in the field of view of the camera 212 are respectively wa and wb as shown in FIG.

  A step 211c between the first phase region 211a and the second phase region 211b forms a straight line. This straight line is arranged so as to be orthogonal to the first direction, that is, along the Y axis.

  Here, a measurement method using the optical interference measurement apparatus 200 will be described with reference to FIGS. 1 and 4. FIG. 4 is a flowchart showing the measurement method.

  First, in step S1, the optical path length is adjusted. Specifically, by adjusting the reference surface 211 or the Z-direction stage (not shown) of the object 209, the optical path lengths of the signal light and the reference light are matched within the coherence length λc of the light from the light source 201. The Z-direction stage may be a mechanism that can be adjusted so that the optical path length difference is within the range of the coherence length λc, such as a stepping motor, and need not be a PZT element. Through this step, the signal light and the reference light interfere with each other, and the interference light enters the camera 212.

  In step S2, the camera 212 acquires a first interference light signal (interference signal). Here, FIG. 5 shows the positional relationship between the reference surface 211 and the object 209 in the field of view of the camera 212 when step S2 is performed. 5, it is assumed that the reference plane 211 and the field of view of the camera 212 in FIG. 1 coincide with each other, and a region illustrated as the reference plane 211 represents the field of view of the camera 212. The object 209 protrudes from the field of view of the camera 212. Here, in the field of view, the object 209 is shown only in the first phase region 211a and not in the second phase region 211b. At this time, only the interference signal corresponding to the first phase region 211a is stored in the storage unit 215a of FIG. An interference signal corresponding to the second phase region 211b may be acquired, but will not be used in the future.

  It should be noted that, at the time of storage, photographing may be performed a plurality of times, and the S / N ratio may be improved by integrating the plurality of images, or the S / N ratio may be increased by lowering the gain after photoelectric conversion of the CCD for a long time. May be improved.

  Next, in step S3 of FIG. 4, stage movement is performed. Specifically, the mechanical stage 214 moves in response to an instruction from the instruction unit 215c in FIG. This movement direction is the X-axis direction, and the amount of movement is equal to the width of the first phase region 211a in the field of view of the camera 212 in the X-axis direction, that is, wa in FIG. By this movement, the part of the object 209 that was in the first phase region 211a at the time of step S2 in the field of view moves to the second phase region 211b. Note that shooting by the camera 212 is not performed during movement.

  In step S4, a second interference light signal is acquired by the camera 212. FIG. 6 shows the positional relationship between the reference surface 211 and the object 209 in the field of view of the camera 212 at step S4. In the field of view, the part of the object 209 that was shown in the first phase region 211a at the time of step S2 has moved to the second phase region 211b. In step S4, the camera 212 acquires interference signals corresponding to the first phase region 211a and the second phase region 211b. Again, the S / N ratio may be improved by integration as in step S2.

  In step S5, using the interference signal in the first phase region 211a stored in step S2 and the interference signal in the second phase region 211b acquired in step S4, the calculation unit 215b in FIG. The image of the object 209 is constructed based on (Equation 4). The interference signal in the first phase region 211a (FIG. 2) acquired in step S2 and the interference signal in the second phase region 211b acquired in step S5 differ only in the phase obtained from the object having the same shape. Signal. The phase difference between these signals is given by the step of height d formed by the first phase region 211a and the second phase region 211b, and its value is π. That is, in step S5, interference light phase-modulated by π can be obtained from step S2. That is, the phase difference of the interference light at the same part of the object 209 before and after the movement by the movement mechanism is π. Thus, according to this method, it is possible to give a phase difference to the interference light without using a PZT element. In step S5, the interference signal corresponding to the first phase region 211a is stored in the storage unit 215a.

  Here, details of an image construction method using two interference signals whose phases are modulated will be described.

  When the intensity of the interference signal in the first phase region 211a is E1, and the intensity of the interference signal in the second phase region 211b is E2, it is expressed by (Expression 2).

Here, T is the number of integrations by the camera 212, and I ₁ and I ₂ are the luminance at each pixel of the camera 212.

Here, the three-dimensional reflectance distribution of the object 209 is R _sample (x, y, z), the reflectance of the reference surface 211 is R _ref , the coherence function of the light source 201 is γ ² (z), and the proportionality coefficient is K. In other words, since the phase difference between I ₁ and I ₂ is given by π due to the step of the reference surface 211, it is expressed by (Equation 3).

Here, the square of the difference between E1 and E2 is expressed by (Equation 4), the three-dimensional reflectance distribution R _sample (x, y, z) of the object 209, the coherence function γ ² (z), and a constant. Is given by the product of

  This (Expression 4) shows a signal at the z position where the coherence function becomes large in the object 209, that is, a signal on the XY plane where the optical path lengths of the signal light and the reference light coincide.

That is, an XY plane image of the object 209 can be constructed by acquiring E1 and E2 from the luminance at each pixel of the camera 212 and calculating (E1-E2) ² . That is, in the present embodiment, the calculation unit 215b (FIG. 1) constructs an image based on the square of the difference in the intensity of the interference signal before and after movement. Since the coherence function γ ² (z) is a product of a constant, the light source 201 needs to be a low coherence light source in order to obtain an image at a specific z position. Note that when light from the light source 201 passes through the object 209, a tomographic image can be constructed. The above is the details of constructing an image based on the phase-modulated interference signal.

  Now, the description returns to the flowchart of FIG. In step S6, it is determined whether or not the position is a predetermined position. If the predetermined position has been reached, the measurement is terminated. The predetermined position is an end portion of the object 209. If the predetermined position has not been reached, the process returns to step S2. Thereafter, by repeating a series of operations from step S2 to step S5 until the predetermined position is reached, the object 209 is moved in the X-axis direction, and an XY plane image is constructed.

  However, in the above repeating step, the interference signal corresponding to the first phase region 211a has already been acquired in step S5 immediately before step S2. Therefore, step S2 may be omitted in the above repeating step.

  In addition, it is preferable to make the width | variety of the X-axis direction of the 1st phase area | region 211a and the 2nd phase area | region 211b of FIG. 3 equal so that the area which can obtain an interference signal in step S5 is maximized. More specifically, the areas of the first phase region 211a and the second phase region 211b in the field of view of the camera 212 are configured to be equal.

  Note that the larger the wa, the larger the Z position error due to the low degree of straightness of the mechanical stage 214 in FIG. 1, and the greater the measurement noise. In order to prevent this, a periodic reference surface 216 shown in FIG. 7 may be employed instead of the reference surface 211. The periodic reference surface 216 has a stripe structure in which the first phase region 216a and the second phase region 216b are repeatedly arranged. Also in this case, the widths in the X-axis direction of the first phase region 216a and the second phase region 216b are made equal in the field of view of the camera 212. That is, it is desirable that the first phase region 216a and the second phase region 216b have a rectangular shape with a duty ratio of 50%. The first phase region 216a and the second phase region 216b form a step having a height d, which is given by (Equation 1).

  The periodic reference plane 216 maximizes the area where an interference signal can be obtained in step S5 in FIG. 4, and yet the amount of movement in step S3 can be reduced, so that the accuracy of the Z position with respect to movement in the X-axis direction can be increased.

  For the sake of simplicity, the movement in the X-axis direction has been described. However, the movement may be performed in a direction perpendicular to the step providing the phase difference. For example, when the lattice-like reference surface 217 shown in FIG. 8 is used instead of the reference surface 211, the phase of the interference light can be modulated even if it is moved in the Y-axis direction. On the lattice-shaped reference surface 217, the first phase region 217a and the second phase region 217b are arranged in a lattice shape so as to be distributed vertically and horizontally in the field of view of the camera 212. The first phase region 217a and the second phase region 217b form a step having a height d, which is given by (Equation 1). Accordingly, the phase of the interference light can be modulated by π by moving the object 209 also in the Y-axis direction orthogonal to the X-axis direction.

  The present invention is useful for measurement in the industrial field and measurement of the surface layer or inner layer of a living body in the chemical field or the medical field.

200 optical interference measuring apparatus 201 light source 202 lamp 203 stop 204 collimating lens 205 beam splitter 206 first arm 207 second arm 208 first objective lens 209 object 210 second objective lens 211 reference plane 212 camera 213 imaging Lens 214 Mechanical stage 215 Computer

Claims (5)

A light source that emits light having a center wavelength of λc;
A beam splitter that splits light from the light source and makes it incident on a reference surface and an object;
A lens disposed in an optical path of light from the beam splitter to the reference surface;
A camera on which interference light obtained by interference between light reflected or scattered by the reference surface and light reflected or scattered by the object is incident;
A moving mechanism for moving the object in a first direction within the field of view of the camera;
A storage unit for storing the intensity of interference light before and after movement by the moving mechanism;
A calculation unit that constructs an image of the object from the square of the difference in intensity of the interference light before and after the movement, and
The reference surface is provided with a first region and a second region adjacent to each other in the first direction in the field of view of the camera,
The amount of movement by the moving mechanism is the same as the width of the first region in the first direction,
An optical interference measuring apparatus, wherein the first region and the second region form a step having a height of λc / 4n, where n is a refractive index of a medium between the lens and the reference surface.   The optical interference measurement apparatus according to claim 1, wherein the first region and the second region have the same width in the first direction.   The optical interference measurement apparatus according to claim 1, wherein the first region and the second region are repeatedly arranged in the first direction.   The optical interference measuring apparatus according to claim 1, wherein the first region and the second region are arranged in a lattice pattern.   The optical interference measurement apparatus according to any one of claims 1 to 4, wherein a phase difference of interference light at the same portion of the object before and after movement by the movement mechanism is π.

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