JP2020020695A - Interferometer and arrangement adjustment method - Google Patents

Abstract

To provide a technique for shortening the time needed for adjustment operation on an interferometer.SOLUTION: An interferometer 1 comprises: a light source 11 which irradiates an object surface and a reference surface with a light beam having low coherence; an acquisition part 131 which acquires an angle θ of the object surface based upon the optical axis of the light source 11; a detection part 12 which detects, by a plurality of detecting elements, intensities of an interference light beam generated as a measurement light beam reflected by the object surface and a reference light beam reflected by the reference surface interfere with each other; a distance specification part 132 which specifies a distance ΔX to an intersection of a position of a detecting element having detected a relatively large intensity of the interference light beam among intensities of the plurality of interference light beams detected by the plurality of detecting elements, with the optical axis; an analysis part 133 which specifies an optical path length difference ΔL between the measurement light beam and the reference light beam based upon the angle θ acquired by the acquisition part 131 and the distance ΔX; and a movement control part 134 which moves the object surface so that the optical path length difference ΔL decreases.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an interferometer and an arrangement adjustment method.

  2. Description of the Related Art An interferometer using a light source for irradiating low coherence light is known (for example, see Patent Document 1).

JP-A-2001-91223

  Before using a conventional interferometer, the worker identifies the position where interference light between the measurement light reflected from the object surface and the reference light reflected from the reference surface is detected before measuring the object surface I do. At this time, the operator needs to move the object surface by a distance shorter than the distance at which low-coherence light can interfere, and has to move the object surface many times. Thereafter, the operator specifies a position where the intensity of the interference light is the highest. At that time, the worker moves the object surface at a distance shorter than the distance moved when identifying the position where the interference light is detected, so that the worker checks the intensity of the interference light and moves the object surface. It had to be repeated many times. As described above, the operator takes time to adjust the interferometer.

  Therefore, the present invention has been made in view of these points, and an object of the present invention is to provide a technique for shortening the time required for an interferometer adjustment operation.

  An interferometer according to a first aspect of the present invention includes an object surface, a light source that irradiates low-coherence light to a reference surface, and an angle of the object surface with respect to an optical axis of the light source. An angle of the reference surface with respect to the optical axis, or an acquisition unit that obtains a relative angle of the object surface with respect to the reference surface, measurement light reflected from the object surface, and reflection from the reference surface A detection unit comprising an element for detecting the spatial intensity of the interference light due to the reference light and the distribution of the spatial intensity of the interference light detected by the element for detecting the spatial intensity; The position on the element that detects the spatial intensity corresponding to the distance specifying unit that specifies the distance between the reference point on the element that detects the spatial intensity, the angle acquired by the acquisition unit, and the distance Based on the distance specified by the specifying unit, An analysis unit that specifies an optical path length difference between the optical path length of the measurement light and the optical path length of the reference light; and a movement that moves at least one of the object surface or the reference surface so that the optical path length difference is reduced. A control unit.

  For example, the movement control unit moves one of the object surface and the reference surface along the optical axis of the light source, and specifies a position where the interference light is detected by the detection unit.

  The acquisition unit may acquire the angle based on the number of stripes of interference light detected by the element in a range of a predetermined length and the wavelength of the low coherence light.

  For example, the obtaining unit obtains an inclination angle based on the optical axis, and the movement control unit determines the angle of the object surface based on the optical axis or the reference surface based on the optical axis. Is inclined so as to have the above-mentioned inclination angle.

  For example, the obtaining unit obtains the relative angle that makes an interval between bright lines or dark lines of the interference light longer than a distance between two adjacent pixels of the element.

  An arrangement adjustment method according to a second aspect of the present invention includes a computer-implemented step of obtaining an inclination angle with respect to an optical axis of a light source that irradiates low-coherence light to an object surface and a reference surface. Tilting any of the angle of the object surface with respect to the optical axis or the angle of the reference surface with respect to the optical axis to be equal to the obtained tilt angle; and Irradiating the surface and the reference surface with the light; and moving, along the optical axis, a surface of the object surface or the reference surface, which is changed to be equal to the tilt angle; Detecting the spatial intensity of the interference light due to the measurement light reflected from the object surface and the reference light reflected from the reference surface by the element that detects the spatial intensity of the object, and detecting the spatial intensity by the element that detects the spatial intensity. The said interference From the intensity distribution, a position on the element for detecting the spatial intensity corresponding to the intensity of the relatively large interference light, and specifying a distance between a reference point on the element for detecting the spatial intensity, and Specifying an optical path length difference between the optical path length of the measurement light and the optical path length of the reference light based on the tilt angle and the distance; and at least one of the object surface and the reference surface is the optical path length. Moving so that the difference becomes small.

  According to a third aspect of the present invention, there is provided an arrangement adjusting method, comprising the steps of: irradiating a low coherence light to an object surface and a reference surface by a computer; and an element detecting a spatial intensity of the light. A step of detecting a spatial intensity of interference light caused by the measurement light reflected from the object surface and the reference light reflected from the reference surface; and, based on the interference light, a relative angle of the object surface with respect to the reference surface. Obtaining the relative angle that is, and from the intensity distribution of the interference light detected by the element for detecting the spatial intensity, on the element for detecting the spatial intensity corresponding to the intensity of the relatively large interference light A position, a step of specifying a distance between a reference point on the element for detecting the spatial intensity, and the relative angle, based on the distance, an optical path length of the measurement light and an optical path length of the reference light. Identify optical path length differences A method, comprising the steps of: moving to the optical path length difference of at least one of the object surface or the reference surface is reduced.

  ADVANTAGE OF THE INVENTION According to this invention, there exists an effect that the time which the adjustment work of an interferometer requires can be shortened.

FIG. 3 is a diagram for describing an outline of an interferometer. FIG. 4 is a diagram schematically illustrating a relationship between an optical path length difference and an interference fringe image. FIG. 7 is a diagram for describing an interference fringe image when an object surface is inclined with respect to a reference surface. FIG. 2 is a diagram illustrating a functional configuration of an interferometer. It is a figure which shows typically the graph which plotted the intensity | strength of the interference light which each of several pixel of the detection part detected with respect to each position of several pixel. FIG. 3 is a diagram for explaining a relationship between an angle and a distance of an object surface and an optical path length difference. 9 is a flowchart of a process for reducing an optical path length difference. FIG. 11 is a diagram for describing specifying an optical path length difference ΔL using a relative angle γ. FIG. 6 is a diagram for explaining that the interval Δd between interference fringes decreases as the relative angle γ increases. FIG. 8 is a diagram for describing a case where a position where interference light is detected is specified in a state where an object surface is parallel to a reference surface. FIG. 7 is a diagram for describing a case where a position at which interference light is detected is specified in a state where an object surface is inclined with respect to a reference surface. It is a flowchart of the process which specifies the position where interference light is detected. FIG. 14 is a diagram illustrating an outline of an interferometer according to a third modification. FIG. 14 is a diagram illustrating an outline of an interferometer according to a modification 4.

<First embodiment>
[Overview of Interferometer According to Embodiment]
FIG. 1 is a diagram for explaining an outline of the interferometer 1. The interferometer 1 includes a light source 11 that emits low-coherence light, a detection unit 12, a control unit 13, a movable stage 15, a lens 2 (2a, 2b, and 2c), a pinhole 3, A beam splitter 4, a reference surface 5, and an object surface 6 are provided. The reference plane 5 is arranged so as to be perpendicular to the optical axis of the light source 11. The object surface 6 is arranged so as to be substantially parallel to the reference surface 5 to such an extent that several or less interference fringes are observed.

  The light emitted from the light source 11 is enlarged and collimated by passing through the lens 2a, the pinhole 3, and the lens 2b. The collimated light is split at the beam splitter 4. One of the lights split by the beam splitter 4 goes straight and reaches the object surface 6, and the other light reaches the reference surface 5. The measurement light reflected on the object surface 6 and the reference light reflected on the reference surface 5 reach the lens 2c via the beam splitter 4, respectively.

  The optical path length L2 in FIG. 1 indicates the optical path length of the measurement light from the beam splitter 4 to the target object surface 6, reflected from the target object surface 6, and reaches the beam splitter 4 again. The optical path length L1 in FIG. 1 indicates the optical path length of the reference light from the beam splitter 4 to the reference surface 5 and reflected from the reference surface 5 to reach the beam splitter 4 again. After passing through the beam splitter 4, the measurement light and the reference light enter the detection unit 12 via the lens 2c. The detection unit 12 is, for example, a digital camera, and generates an interference fringe image based on the incident interference light.

  Hereinafter, the relationship between the optical path length difference ΔL between the optical path lengths L1 and L2 and the interference fringe image generated by the detection unit 12 will be described with reference to FIG.

  FIG. 2 is a diagram schematically illustrating the relationship between the optical path length difference ΔL and the interference fringe image. In FIG. 2, the arrow from left to right indicates that the optical path length difference ΔL increases. The image at the left end in FIG. 2 is an interference fringe image generated by the detection unit 12 when the optical path length difference ΔL is small (for example, ΔL = 0), and is an interference fringe image having a large difference in the lightness and darkness of the interference fringes.

  The second image from the left in FIG. 2 is an interference fringe image generated by the detection unit 12 when the optical path length difference ΔL is larger than the optical path length difference ΔL when the image at the left end is acquired. The third image from the left in FIG. 2 is an interference fringe image generated by the detection unit 12 when the optical path length difference ΔL is larger than the optical path length difference ΔL when the second image from the left is acquired. . The image at the right end in FIG. 2 is an image generated by the detection unit 12 when the optical path length difference ΔL is large and the optical path length difference ΔL is larger than the coherent distance Δl at which the measurement light and the reference light interfere with each other. In the image at the right end in FIG. 2, there is no difference in brightness and no interference fringes can be confirmed. As described above, in the interference fringe image generated by the detection unit 12, the difference between light and dark becomes larger as the optical path length difference ΔL is smaller, and the light and dark difference becomes the largest when the optical path length difference ΔL is 0.

  The interference fringe image generated by the detection unit 12 when the object surface 6 is arranged so as to be substantially parallel to the reference surface 5 so that several or less interference fringes are observed has been described. Next, an interference fringe image generated by the detection unit 12 when the target object surface 6 is arranged in a state inclined with respect to the reference surface 5 will be described.

  FIG. 3 is a diagram for describing an interference fringe image when the object surface 6 is inclined with respect to the reference surface 5. 3 shows the optical axis of the reference light generated by the reference surface 5. A point S on the dashed line is a point on the optical axis (z axis) where the optical path length difference ΔL between the optical path length L1 of the measurement light and the optical path length L2 of the reference light becomes zero.

  3A shows the optical axis as the Z axis, the axis perpendicular to the optical axis in the drawing as the X axis, and the axis perpendicular to the XZ plane as the Y axis. That is, it is a schematic view of the target object surface 6 inclined at an angle θ around the Y axis with respect to the reference surface 5 in the XZ plane. As shown in FIG. 3A, when the target object surface 6 is inclined with respect to the reference surface 5, the optical path length difference ΔL between the optical path length L1 of the measurement light and the optical path length L2 of the reference light is equal to the target object surface. 6, there are different optical path length differences at each of the plurality of positions. When the center of the object surface 6 and the point S match, the optical path length difference ΔL of each of the plurality of positions increases as the distance from the center of the object surface 6 increases.

  The image in which the XY plane on the right side of FIG. 3A is on the paper surface is such that the object surface 6 is inclined at an angle θ with respect to the reference surface 5 and the center of the object surface 6 and the point S coincide. 7 is an interference fringe image detected by the detection unit 12. As shown in FIG. 3A, in the interference fringe image generated by the detection unit 12 when the target object surface 6 is tilted, the brightness of the interference fringe differs depending on the position. Further, the difference between the light and dark of the interference fringe is largest at the center of the interference fringe image (the position where the optical path length difference ΔL is 0), and becomes greater from the center of the interference fringe image to the end of the interference fringe image (object surface 6 (farther from the center of 6).

  Next, a case where the position of the point S does not coincide with the center of the object surface 6 will be described with reference to FIG. When the position of the point S does not match the center of the object surface 6, the position where the optical path length difference ΔL becomes 0 moves from the center of the object surface 6. The position on the left side of FIG. 3B where the optical path length difference ΔL moved from the center of the object surface 6 becomes 0 is indicated by a point Q. The image on the right side of FIG. 3B shows an interference fringe image generated by the detection unit 12. As shown in FIG. 3B, the position where the difference between the light and dark of the interference fringe is the largest moves from the center of the interference fringe image. At this time, the distance ΔX from the center of the interference fringe image to the position where the difference between the light and dark of the interference fringe is the largest corresponds to the distance between the points S and Q.

  An outline of the operation of the interferometer 1 will be described based on the above description. The interferometer 1 simultaneously detects a plurality of interference lights having different optical path length differences ΔL in a state where the object surface 6 is inclined with respect to the reference surface 5. Next, based on the interference fringe images based on a plurality of detected interference light beams having different optical path length differences ΔL, the interferometer 1 calculates a distance ΔX from the center of the interference fringe image to a position where the difference between the light and dark of the interference fringes is largest. To identify. Then, the interferometer 1 specifies a position where the optical path length difference ΔL is 0 based on the angle θ of the target object surface 6 with respect to the reference surface 5 and the specified distance ΔX.

  By doing so, the interferometer 1 can detect a plurality of interference lights having different optical path length differences ΔL in one measurement. Therefore, the interferometer 1 can reduce the number of times of moving the target object surface 6, so that the adjustment time can be reduced.

[Functional configuration of interferometer 1]
FIG. 4 is a diagram illustrating a functional configuration of the interferometer 1. The interferometer 1 includes the configuration described with reference to FIG.

  The light source 11 irradiates the object surface 6 and the reference surface 5 with low coherence light via the optical system described with reference to FIG. The light source 11 emits light having a wavelength of 780 nm and a coherence length of 10 μm, for example.

  The detection unit 12 is, for example, a digital camera including a plurality of pixels. The detection unit 12 detects the spatial intensity of the interference light caused by the measurement light reflected from the target object surface 6 and the reference light reflected from the reference surface 5 incident on the detection unit 12. The spatial intensity is the intensity of the interference light at each of a plurality of positions on the space. The spatial intensity is, for example, the intensity of the interference light at each of a plurality of positions on a two-dimensional plane. Further, the detection unit 12 generates an interference fringe image in which the positions of the plurality of pixels, which are the minimum light receiving portions of the detection unit 12, are associated with the intensities of the interference lights detected by the plurality of pixels. Is also good.

  The storage unit 14 includes a storage medium such as a ROM (Read Only Memory) and a RAM (Random Access Memory). The storage unit 14 stores a program executed by the control unit 13.

  The movable stage 15 includes a pedestal on which the object surface 6 is arranged, and one or more actuators. The movable stage 15 can move the object surface 6 arranged on the pedestal in the optical axis direction by operating an actuator based on an instruction from the control unit 13. Further, the movable stage 15 can change the angle of the object surface 6 with respect to the reference surface 5.

  The control unit 13 is a calculation resource including a processor such as a CPU (Central Processing Unit) not shown. The control unit 13 implements the functions of the acquisition unit 131, the distance identification unit 132, the analysis unit 133, and the movement control unit 134 by executing a program stored in the storage unit 14.

  The acquisition unit 131 acquires the angle θ of the target object surface 6. For example, the acquisition unit 131 includes an angle sensor that measures the angle of the movable stage 15, and acquires the angle measured by the angle sensor with respect to the optical axis of the light source 11 as the angle θ of the target object surface 6.

  The distance specifying unit 132 specifies the position of the pixel at which the relatively large intensity of the interference light is detected, based on the spatial intensity distribution of the interference light detected by the detection unit 12. For example, the distance specifying unit 132 determines the relatively large intensity of the interference light based on the one-dimensional spatial intensity distribution among the spatial intensity distributions of the interference light on the two-dimensional plane detected by the detection unit 12. The position of the pixel that has detected is specified. Specifically, first, the distance identification unit 132 causes the detection unit 12 to detect the intensity of the interference light in a state where the target object surface 6 is inclined with respect to the reference surface 5. Subsequently, the distance specifying unit 132 extracts a plurality of peaks in the intensity of the interference light detected by each of the plurality of pixels arranged in one direction. Next, the distance specifying unit 132 calculates the difference between adjacent peaks or the ratio of the peaks among the plurality of extracted peaks, thereby specifying the position of the pixel at which the intensity of the relatively large interference light is detected. In the following description, the position of a pixel at which the intensity of the relatively large interference light is detected may be referred to as the maximum position. An example of a method for specifying the maximum position will be described with reference to FIG.

FIG. 5 is a diagram schematically illustrating a graph in which the intensity of the interference light detected by each of the plurality of pixels of the detection unit 12 is plotted with respect to each position of the plurality of pixels. The peak P shown in FIG. 5 indicates the peak of the intensity extracted by the distance specifying unit 132. The suffix k of the peak P indicates the order in which the intensity peaks were extracted. The distance specifying unit 132 calculates the absolute value of the difference between adjacent peaks P. For example, the distance specifying unit 132 calculates the absolute value of the difference between the peaks _Pk and _{Pk + 1} , the absolute value of the difference between the peaks _{Pk + 1} and _{Pk + 2,} and the absolute value of the difference between the peaks _{Pk + 2} and _{Pk + 3.} Are calculated respectively.

  The distance specifying unit 132 specifies the maximum position based on a set of peaks having the largest absolute value of the difference among the plurality of calculated absolute values of the difference. For example, the distance specifying unit 132 specifies, as the maximum position, the position of the pixel corresponding to the peak P at which the greater intensity is detected from the set of peaks having the largest absolute value of the difference. Further, the distance specifying unit 132 may specify, as the maximum position, the midpoint of the position of the pixel corresponding to each of the sets of peaks having the largest absolute value of the difference.

  Note that the distance specifying unit 132 may specify again a position included within a predetermined distance from the specified position as the maximum position. The predetermined distance is a distance determined by the configuration of the interferometer 1. The distance determined by the configuration of the interferometer 1 may be determined by a manufacturing error of the optical system of the interferometer 1, an error of the pixel of the detection unit 12, or an experiment. As described above, when specifying as the maximum position, the maximum position is roughly specified, which is effective when the optical path length difference ΔL does not need to be strictly set to zero.

  The distance specifying unit 132 specifies a reference point of the detection unit 12, and specifies an intersection between the reference point of the detection unit 12 and the optical axis. Since the optical axis is adjusted so that the optical axis passes through the center of the detection unit 12 when the interferometer 1 is manufactured, the distance specifying unit 132 sets the center of the detection unit 12 at the intersection between the reference point of the detection unit 12 and the optical axis. Identify that there is. The distance specifying unit 132 specifies the distance ΔX between the specified maximum position and the intersection between the reference point of the detection unit 12 and the optical axis. Specifically, the distance specifying unit 132 specifies the distance from the center of the detecting unit 12 to the specified position as the distance ΔX.

  The analysis unit 133 calculates the optical path length difference ΔL between the optical path length L1 of the measurement light and the optical path length L2 of the reference light based on the angle θ acquired by the acquisition unit 131 and the distance ΔX identified by the distance identification unit 132. To identify. Hereinafter, the relationship between the angle θ, the distance ΔX, and the optical path length difference ΔL will be described. FIG. 6 is a diagram for explaining the relationship among the angle θ of the object surface 6, the distance ΔX, and the optical path length difference ΔL. In FIG. 6, the reference plane 5 is perpendicular to the optical axis Z axis, and indicates that the object plane 6 is inclined about the point O by an angle θ with respect to the reference plane 5. Further, the description will be made on the assumption that the detection unit 12 has generated an interference fringe image.

6, the x-axis coordinate of the point 0 and _{X 0,} the Z-axis coordinate and _{Z 0.} The point M indicates a position on the object surface 6 corresponding to the position where the largest contrast difference is specified in the interference fringe image generated by the detection unit 12 in a state where the object surface 6 is inclined with respect to the reference surface 5. Show. The x-axis coordinate of the point M and _{X 1,} the Z-axis coordinate and _{Z 1.} Optical path length difference in Z ₁ is 0. The distance between Z ₀ and Z ₁ corresponds to the optical path length difference ΔL. At this time, the optical path length difference ΔL is expressed by Expression (1) using the angle θ and the distance ΔX.
ΔL = ΔX · tan θ (1)
The analysis unit 133 specifies the optical path length difference ΔL using Expression (1).

  The movement control unit 134 moves the object surface 6 such that the optical path length difference ΔL specified by the analysis unit 133 is reduced. Specifically, the movement control unit 134 moves the target object surface 6 by controlling the movable stage 15.

[Process for Reducing Optical Path Length Difference ΔL]
A flow of a process performed by the interferometer 1 to reduce the optical path length difference ΔL will be described. FIG. 7 is a flowchart of a process for reducing the optical path length difference ΔL. First, the control unit 13 causes the light source 11 to irradiate the object surface 6 and the reference surface 5 with low coherence light (step S1). When the object surface 6 and the reference surface 5 are irradiated with low coherence light, the detection unit 12 detects interference light due to the measurement light and the reference light.

  Subsequently, the acquisition unit 131 acquires the angle θ of the target object surface 6 measured by the angle sensor (Step S2). Next, based on the interference light detected by the detecting unit 12, the distance specifying unit 132 specifies a position where the difference between the light and dark of the interference fringes is the largest. Then, the distance ΔX to the intersection with is specified (step S3).

  When the acquisition unit 131 acquires the angle θ and the distance identification unit 132 identifies the distance ΔX, the analysis unit 133 identifies the optical path length difference ΔL based on the angle θ and the distance ΔX (step S4). Subsequently, the movement control unit 134 moves the target object surface 6 such that the optical path length difference ΔL becomes smaller (Step S5). Then, the movement control unit 134 tilts the target object surface 6 so that the target object surface 6 is substantially parallel to the reference surface 5 (Step S6).

[Effect of Interferometer 1 According to First Embodiment]
As described above, the interferometer 1 acquires the angle θ of the target object surface 6 and specifies the distance ΔX from the position where the difference between the light and dark of the interference fringes is relatively large to the intersection of the detection unit 12 and the optical axis. I do. Then, the interferometer 1 specifies the optical path length difference ΔL based on the angle θ and the distance ΔX. By doing so, the interferometer 1 can reduce the time required for specifying the optical path length difference ΔL. As a result, the interferometer 1 can reduce the time required for the operator to adjust the optical path length difference.

<Second embodiment>
The interferometer 1 according to the second embodiment does not include an angle sensor, and acquires a relative angle γ that is a relative angle of the target surface 6 with respect to the reference surface 5. The interferometer 1 specifies the optical path length difference ΔL between the reference surface 5 and the object surface 6 on the reference optical axis using the relative angle γ acquired by the process of specifying the relative angle γ described later. Hereinafter, the point of specifying the optical path length difference ΔL between the reference plane 5 and the target object plane 6 using the relative angle γ will be described on the points different from the first embodiment, and the same points will be omitted as appropriate.

  FIG. 8 is a diagram for describing specifying the optical path length difference ΔL using the relative angle γ. 8, the angle θ indicates the angle of the object surface 6 with respect to the optical axis. The angle θ-γ indicates the angle of the reference plane 5 with respect to the optical axis.

  The distance L3 is an optical path length difference corresponding to a position on the element where the intensity of the interference light is maximum. The distance L4 is an optical path length difference corresponding to the position of the reference point on the element. The optical path length difference ΔL on the reference optical axis is represented by the difference between the distance L3 and the distance L4. The difference between the distance L3 and the distance L4 is expressed by Expression (2) using the angle θ, the relative angle γ, and the distance ΔX.

  Here, a range of values that the angle θ can take will be described. Since the N / A of the imaging lens provided in the interferometer 1 is limited, if a special observation system is not used, the interferometer 1 cannot observe the interference light if the angle θ is large. Further, when the angle θ is not close to 0 degrees, errors caused by aberrations of the imaging lens increase. Therefore, in the interferometer 1, the angle θ of the object surface 6 with respect to the optical axis is set near 0 degrees. The range of the angle θ is desirably several degrees or less, and is often adjusted at an angle smaller than 1 °. In the following description, it is assumed that the angle θ is smaller than 1 °.

  Next, the range of values that the relative angle γ can take will be described. The relative angle γ is related to the interference fringe interval Δd, and the magnitude of the relative angle γ determines the interference fringe interval Δd. Specifically, the interval Δd between the interference fringes decreases as the relative angle γ increases. FIG. 9 is a diagram for explaining that the interval Δd between the interference fringes decreases as the relative angle γ increases. In FIG. 9, the angle of the reference surface 5 with respect to the optical axis is 90 degrees, and the relative angle of the object surface 6 with respect to the reference surface 5 is a relative angle γ.

The optical path length difference _Li is the optical path length difference of the interference light corresponding to the position of a certain bright line (i) in the interference fringe image. The optical path length difference _{Li + 1} is the optical path length difference of the interference light corresponding to the position of the bright line (i + 1) adjacent to the bright line (i) in the interference fringe image. The difference between the optical path length difference _Li and the optical path length difference _{Li + 1} is λ / 2. Therefore, the interval Δd of the interference fringes corresponding to the distance from the position of the bright line (i) to the position of the bright line (i + 1) is expressed by Expression (3) using the relative angle γ.
Δd = [λ / (2tanγ)] Equation (3)

  As shown in Expression (2), the interval Δd between the interference fringes is inversely proportional to tan γ, and therefore decreases as the relative angle γ increases. If the interval Δd between the interference fringes becomes smaller and becomes smaller than the resolution of the detector 12, the interferometer 1 cannot resolve the bright line (i) and the bright line (i + 1). In other words, the interferometer 1 cannot specify that the bright line (i) and the bright line (i + 1) are different bright lines. In this case, since the interferometer 1 cannot specify the bright line at which the intensity of the interference light becomes maximum, it becomes impossible to specify the position at which the intensity of the interference light becomes maximum. Therefore, the relative angle γ needs to be equal to or higher than the resolution of the detection unit 12.

  Therefore, the acquisition unit 131 acquires a relative angle γ at which the interval between the interference fringes is equal to or greater than the resolution of the detection unit 12. For example, the acquisition unit 131 acquires a relative angle γ that makes the distance between the bright lines or dark lines of the interference light longer than the distance between two adjacent pixels of the detection unit 12. The distance between two adjacent pixels is, for example, the distance from the center of the pixel to the center of the adjacent pixel. When the specific value of the relative angle γ is, for example, 0.02 °, the interval Δd between the interference fringes is 1.089 mm. The size of one pixel of the image sensor is generally 10 μm or less, and even if the optical system reduces the size of the target object surface to 1/100, the distance Δd between the interference fringes is determined by the distance between two adjacent pixels. Can be long.

When the angle θ and the relative angle γ are 1 ° or less, tan θ tan γ <3.05 × 10 ⁻⁴ . Therefore, [1-tan θ tan γ] can be approximated to 1. By using this approximation, the optical path length difference ΔL is expressed by Expression (4).

(Process for specifying relative angle γ)
The acquisition unit 131 specifies the relative angle γ based on the interference light detected by the detection unit 12. Specifically, the acquisition unit 131 calculates the relative angle γ based on the number of interference fringes in a predetermined length range of the interference fringe image generated by the detection unit 12 and the wavelength λ of the low coherence light. Identify. More specifically, when the number of interference fringes due to the light having the wavelength λ is t in the range w of the predetermined length in the interference fringe image, the acquisition unit 131 determines the relative angle γ using Expression (5). To identify.

The range of the predetermined length is, for example, a length from one end of the interference fringe image to the other end in a direction perpendicular to the interference fringes. In addition, the acquisition unit 131 may set a range including a pixel in which the intensity of the interference light is equal to or greater than a predetermined intensity as the range of the predetermined length. The predetermined intensity need only be equal to or higher than the intensity at which the number of interference fringes can be measured, and may be determined by the wavelength distribution of the light emitted from the light source 11.

  The acquisition unit 131 acquires the number t of interference fringes based on the detected interference light. For example, the acquisition unit 131 specifies the interference fringe pattern by analyzing the interference pattern image using a known image analysis method, and specifies the number t of the interference pattern included in the length range. By doing so, it is not necessary for the interferometer 1 to include an angle sensor, so that manufacturing costs can be reduced.

  Hereinafter, similarly to the first embodiment, the interferometer 1 specifies the optical path length difference ΔL using the relative angle γ. Then, the interferometer 1 moves the reference surface 5 or the object surface 6 such that the optical path length difference ΔL becomes small.

[Effects of Interferometer 1 According to Second Embodiment]
As described above, the interferometer 1 according to the second embodiment specifies the relative angle γ that is the relative angle between the reference surface 5 and the object surface 6 based on the number t of interference fringes. . Next, the interferometer 1 specifies an optical path length difference ΔL on the optical axis, which is a reference between the reference surface 5 and the object surface 6, based on the specified relative angle γ. Then, the interferometer 1 moves the reference surface 5 or the object surface 6 such that the optical path length difference ΔL becomes small.

  By doing so, the interferometer 1 does not need to include the angle sensor. Therefore, a company that manufactures the interferometer 1 can reduce the manufacturing cost of the interferometer 1. Further, since the interferometer 1 can determine the optical path length difference ΔL using the relative angle γ, it is not necessary to move the reference surface 5 or the target object surface 6 many times, and it is necessary to specify the optical path length difference ΔL. Time can be reduced. As a result, the interferometer 1 can reduce the time required for the operator to adjust the optical path length difference.

<Third embodiment>
In the first embodiment, it is assumed that the object surface 6 is arranged within a distance at which light emitted from the light source 11 can interfere. However, the position at which the object surface 6 is arranged on the interferometer 1 may not be included within the interferable distance at which the measurement light and the reference light interfere with each other. In this case, the worker needs to specify the position on the optical axis where the interference light is detected by the detection unit 12 while moving the object surface 6 along the optical axis.

  In the conventional interferometer, the position where the interference light is detected is specified in a state where the object surface 6 is arranged so as to be parallel to the reference surface 5. Therefore, the operator has to move the target object surface 6 by a distance shorter than the distance at which the measurement light and the reference light interfere with each other. Therefore, the operator needs to move the object surface 6 many times, and it takes time to specify the position. Hereinafter, the operation of specifying the position where the interference light is detected by the detection unit 12 will be specifically described.

  FIG. 10 is a diagram for describing a case where a position where interference light is detected is specified in a state where the object surface 6 is parallel to the reference surface 5. An interference range in which the measurement light and the reference light can interfere with each other is limited to an interference distance centered on a position where the optical path length difference ΔL becomes 0 on the optical axis. A range A0 in FIG. 10 indicates an interference possible range.

  FIG. 10A is a schematic diagram illustrating a state where the object surface 6 is not included in the interference possible range A0. FIG. 10B is a diagram illustrating a state in which the target object surface 6 has been moved by a distance a shorter than the distance at which interference can be made from the state of FIG. FIG. 10C is a diagram illustrating a state in which the target object surface 6 has been moved by a distance b longer than the interferable distance from the state of FIG. 10A. As described above, unless the operator moves the target object surface 6 by a distance shorter than the interferable distance, the operator cannot include the target object surface 6 in the interferable range A0. Therefore, an operator who uses the conventional interferometer has to move the object surface 6 by a shorter distance than the distance at which the interference light can be detected when specifying the position where the interference light is detected. It took time to identify the location where it was done.

  Next, an outline of an interferometer 1 according to a third embodiment will be described with reference to FIG. FIG. 11 is a diagram for describing a case where the position where the interference light is detected is specified in a state where the object surface 6 is inclined with respect to the reference surface 5. FIG. 11A is a schematic diagram showing that the object surface 6 is inclined with respect to the reference surface 5. The height difference Δh in FIG. 11 indicates the distance from one end of the object surface 6 to the other end on the optical axis. In FIG. 11A, the center S of the object surface 6 is outside the interference possible range A0, but the lower end of the object surface 6 is included in the interference possible range A0. Therefore, the detection unit 12 can detect the interference light.

  FIG. 11B is a schematic diagram showing that the target object surface 6 is inclined with respect to the reference surface 5, as in FIG. 11A. In FIG. 11B, the center S of the object surface 6 is outside the interference possible range A0, but the upper end of the object surface 6 is included in the interference possible range A0. Therefore, the detection unit 12 can detect the interference light. As described above, when the target object surface 6 is inclined with respect to the reference surface 5, the detection unit 12 sets the distance corresponding to the angle at which the target object surface 6 is inclined with respect to the reference surface 5 in the interference possible range A0. When a part of the object surface 6 is included in the added detectable range A1, the interference light can be detected. FIG. 11C is a diagram schematically illustrating a detectable range A1 in which interference light can be detected.

  The interferometer 1 according to the third embodiment has a detectable distance obtained by adding a distance corresponding to an angle at which the object surface 6 is inclined with respect to the reference surface 5 to a distance at which light emitted by the light source 11 can interfere. Then, the object surface 6 is moved. By doing so, the number of times the interferometer 1 moves the object surface 6 can be reduced, so that the time required for the detection unit 12 to specify the position where the interference light is detected can be reduced. Hereinafter, with respect to the functional configuration of the interferometer 1 according to the third embodiment, differences from the first embodiment will be described, and similar points will be omitted as appropriate.

[Functional Configuration of Interferometer 1 According to Third Embodiment]
The acquisition unit 131 acquires a tilt angle based on the optical axis as the angle θ at which the target object surface 6 is tilted. The acquisition unit 131 acquires, for example, an inclination angle input to the control unit 13 by an operator. Further, the obtaining unit 131 may obtain the inclination angle stored in the storage unit 14.

  The movement control unit 134 changes the angle θ of the target object surface 6 with respect to the optical axis so as to be the inclination angle acquired by the acquisition unit 131. The movement control unit 134 specifies a moving distance by which the target object surface 6 is moved based on the acquired inclination angle. For example, the movement control unit 134 sets the coherent distance Δl of the light emitted by the light source 11 to a height indicating the distance on the optical axis from one end to the other end of the object surface 6 determined by the inclination angle. The distance obtained by adding the difference Δh is specified as the movement distance.

  When the object surface 6 is inclined so that the m interference fringes are detected by the detection unit 12, the height difference Δh is expressed as m × λ / 2 using the wavelength λ of the light emitted from the light source 11. Is done. Utilizing this, the movement control unit 134 may tilt the target object surface 6 such that m interference fringes are detected in N pixels arranged in one direction. The movement control unit 134 specifies the height difference Δh based on the wavelength λ and the number m of interference fringes, and specifies a distance obtained by adding the specified height difference Δh as the movement distance.

  This will be described with specific numerical values. For example, when the number m of interference fringes is 100, the wavelength λ of the light source is 780 nm, and the coherence length Δl is 10 μm, the height difference Δh of the object surface 6 is 39 μm. At this time, the substantial interference possible range is 49 μm obtained by adding the coherence distance Δl (= 10 μm) to the height difference Δh (= 39 μm). The interferometer 1 can move the object surface 6 at a distance roughly five times as large as the coherent distance (10 μm).

  Subsequently, when low-coherence light is emitted from the light source 11 to the object surface 6 and the reference surface 5, the movement control unit 134 moves the object surface 6 along the optical axis of the light source 11. Just move. Then, the movement control unit 134 specifies the position where the interference light is detected by the detection unit 12 based on the intensity of the light detected by the detection unit 12.

(Process for specifying the position where interference light is detected)
Hereinafter, a flow of a process of specifying a position where interference light is detected will be described with reference to FIG. FIG. 12 is a flowchart of a process for specifying a position where interference light is detected. First, the acquisition unit 131 acquires an angle θ based on an optical axis that inclines the object surface 6 (Step S11). Subsequently, the movement control unit 134 tilts the target object surface 6 so as to have the angle θ (Step S12).

  When the movement control unit 134 tilts the object surface 6, the movement control unit 134 irradiates the object surface 6 and the reference surface 5 with light (step S13). Then, the movement control unit 134 determines whether or not the detection unit 12 has detected the interference light (Step S14). If the movement controller 134 determines that the detector 12 has not detected the interference light (No in step S14), the movement controller 134 adds a height difference Δh determined by the angle θ to the coherence distance Δl based on the wavelength λ of the light source 11. The object surface 6 is moved by the distance (step S15). The movement control unit 134 repeats steps S14 and S15 until it determines that interference light has been detected. Hereinafter, the processing from step S16 to step S19 is the same as the processing from step S3 to step S6 in the flowchart of the processing for reducing the optical path length difference ΔL in FIG.

[Effects of Interferometer 1 According to Third Embodiment]
As described above, the interferometer 1 according to the third embodiment specifies the position where the interference light is detected while the object surface 6 is inclined. By doing so, the interferometer 1 can move the object surface 6 at a distance longer than the distance at which the light emitted from the light source 11 can interfere, so that the position at which the interference light is detected can be determined in a short time. Can be identified.

  As described above, the present invention has been described using the embodiment, but the technical scope of the present invention is not limited to the scope described in the above embodiment, and various modifications and changes are possible within the scope of the gist. is there. For example, the acquisition unit 131 acquires the angle of the reference plane 5 with respect to the optical axis or the angle of the object plane 6 with respect to the optical axis using the same processing as the processing of acquiring the relative angle γ. be able to. Further, the specific embodiment of the distribution / integration of the apparatus is not limited to the above embodiment, and all or a part of the apparatus may be functionally or physically distributed / integrated in an arbitrary unit. Can be. Further, new embodiments that are generated by arbitrary combinations of the plurality of embodiments are also included in the embodiments of the present invention. The effect of the new embodiment caused by the combination has the effect of the original embodiment.

(Modification 1)
In the above description, the reference surface 5 is arranged to be perpendicular to the optical axis of the light source 11, but the present invention is not limited to this, and the reference surface 5 may be arranged on the movable stage. . In this case, the acquisition unit 131 acquires the angle θ of the reference surface 5 with respect to the optical axis or the relative angle γ that is the relative angle of the reference surface 5 with respect to the object surface 6. Then, the movement control unit 134 moves the reference surface 5 so that the optical path length difference ΔL becomes small. Also in this manner, the interferometer 1 can reduce the time required for the operation of reducing the optical path length difference ΔL.

(Modification 2)
Distance specifying unit 132, the difference between the bright and dark interference fringes based on the plurality of interference light may specify the position X ₁ to be the maximum. For example, the distance identification unit 132 uses the principle of the phase shift method in the interference fringe analysis, and based on a plurality of interference lights obtained by moving the object surface 6 in the optical axis direction with respect to the reference surface 5, difference in brightness of the interference fringes to specify positions X ₁ to be the maximum. For example, in the case of an interferometer such as a Fizeau interferometer or a Twyman-Green interferometer that measures the shape of the object surface 6 by reflected light, a half of the wavelength λ of the light source is divided into N and the object is divided. At this time, the distance specifying unit 132 specifies the amplitude distribution A (x) for each pixel in accordance with Expression (6).

Distance specifying unit 132, regardless of the period (spatial frequency) of light and dark interference fringes, it is possible to identify the amplitude distribution A (x) in units of pixels, and improved accuracy of specifying the position X _1, the optical path length difference ΔL can be specified more accurately.

(Modification 3)
The present invention can be applied to an interferometer that simultaneously detects a plurality of interference lights having different phases. FIG. 13 is a diagram illustrating an outline of an interferometer 1a according to the third modification. The interferometer 1 according to Modification 3 further includes a λ / 4 plate 7, a three-segment beam splitter 8, and a polarizing plate 9. Further, the interferometer 1 according to the third modification includes a polarization beam splitter 10 instead of the beam splitter 4. Distance specifying unit 132, the difference between the bright and dark interference fringes identifies the position X ₁ to be the maximum on the basis of a plurality of interference light obtained at the same time by a plurality of detector 12. In this way, the distance specifying unit 132 can specify the amplitude distribution A (x) without moving the target object surface 6 relatively to the reference surface 5.

(Modification 4)
The present invention can also be applied to a Fizeau interferometer used when measuring the shape of the object surface 6 relative to the reference surface 5 with high accuracy. FIG. 14 is a diagram showing an outline of an interferometer 1b using low coherence light in a Fizeau interferometer used when measuring the shape of the object surface 6 relative to the reference surface 5 with high accuracy. .

  The light emitted from the light source 11 is polarized and branched by the polarization beam splitter 101a. One light beam that is polarized and split in the polarization beam splitter 101a goes straight to the polarization beam splitter 101b, and the other light beam passes through the mirror 102 to the polarization beam splitter 101b. The distance between each of the polarizing beam splitters 101a and 101b and the mirror 102 is set at a distance La, and the optical path length difference between the two lights is 2La.

  Each of the two lights passes through various optical systems, enters the object surface 6 and the reference surface 5 arranged at the distance Lb, and is reflected by each surface. The optical path length difference between the optical path length of the measurement light reflected on the object surface 6 and the optical path length of the reference light reflected on the reference surface 5 is 2 Lb. The measurement light and the reference light are split into a plurality of lights by a three-segment beam splitter 8 via various optical systems. Each of the plurality of branched lights is detected by each of the plurality of detection units 12. By making the distances La and Lb equal, the interferometer 1b separates the S-polarized reference light reflected by the two polarization beam splitters 101a and 101b from the measurement light passing through the two polarization beam splitters 101a and 101b. , With different phase differences. By doing so, the interferometer 1b can instantaneously acquire a plurality of interference fringe images having different phases.

The interferometer 1b irradiates the object surface 6 and the reference surface 5 with light having low coherence, and the plurality of detection units 12 detect interference light. Next, the interferometer 1b acquires the angle θ of the reference surface 5 with respect to the optical axis or the angle θ of the target surface 6 with respect to the optical axis. Interferometer. 1b, the distance ΔX between the large amplitude position of the interference fringes contrast with the intersection of the reference point of the detector 12, the optical path length difference ΔL is a difference between L _a and L _b on the basis of the angle θ Identify. The interferometer 1b acquires a relative angle γ that is a relative angle of the object surface 6 with respect to the reference surface 5 instead of the angle θ, and specifies the optical path length difference ΔL based on the distance ΔX and the relative angle γ. May be. Then, the interferometer 1b adjusts the optical system of the interferometer 1b so that the optical path length difference ΔL becomes smaller. By doing so, the interferometer 1b can shorten the work of reducing the optical path length difference ΔL, as in the other embodiments or other modified examples.

REFERENCE SIGNS LIST 1 interferometer 2 lens 3 pinhole 4 beam splitter 5 reference surface 6 object surface 7 λ / 4 plate 8 three-segment beam splitter 9 polarizing plate 10 polarizing beam splitter 11 light source 12 detection unit 13 control unit 14 storage unit 15 movable stage 101 Polarization beam splitter 102 mirror 131 acquisition unit 132 distance identification unit 133 analysis unit 134 movement control unit

Claims (7)

A light source for irradiating the object surface and the reference surface with light of low coherence,
An acquisition unit that acquires an angle of the object surface with respect to the optical axis of the light source, an angle of the reference surface with respect to the optical axis, or a relative angle of the object surface with respect to the reference surface,
A measurement unit reflected from the object surface, and a detection unit including an element that detects a spatial intensity of interference light due to the reference light reflected from the reference surface,
From the distribution of the spatial intensity of the interference light detected by the element for detecting the spatial intensity, the position on the element for detecting the spatial intensity corresponding to the intensity of the relatively large interference light and the spatial intensity are detected. A distance specifying unit that specifies a distance from a reference point on the element to be
An analysis unit that specifies an optical path length difference between the optical path length of the measurement light and the optical path length of the reference light based on the angle obtained by the obtaining unit and the distance specified by the distance specifying unit. ,
A movement control unit that moves the object surface or at least one of the reference surfaces so that the optical path length difference is reduced.
Interferometer equipped with. The movement control unit moves one of the object surface or the reference surface along the optical axis of the light source, and specifies a position at which the interference light is detected by the detection unit.
The interferometer according to claim 1. The acquisition unit acquires the angle based on the number of fringes of interference light detected by the element in a range of a predetermined length and the wavelength of the low coherence light,
The interferometer according to claim 1. The acquisition unit acquires an inclination angle based on the optical axis,
The movement control unit tilts one of the angle of the object surface with respect to the optical axis or the angle of the reference surface with respect to the optical axis to be the tilt angle,
The interferometer according to claim 1. The acquisition unit acquires the relative angle to make the distance between the bright lines or the dark lines of the interference light longer than the distance between two adjacent pixels of the element,
The interferometer according to claim 1. Computer runs,
Obtaining an inclination angle with respect to the optical axis of the light source that irradiates the object surface and the reference surface with low coherence light,
Tilting any of the angle of the object surface with respect to the optical axis or the angle of the reference surface with respect to the optical axis to be equal to the obtained tilt angle,
Irradiating the light on the object surface and the reference surface,
Of the object surface or the reference surface, a step of moving the surface changed to be equal to the inclination angle along the optical axis,
By the element that detects the spatial intensity of light, the step of detecting the spatial intensity of the interference light by the measurement light reflected from the object surface and the reference light reflected from the reference surface,
From the intensity distribution of the interference light detected by the element for detecting the spatial intensity, a position on the element for detecting the spatial intensity corresponding to a relatively large intensity of the interference light, and an element for detecting the spatial intensity Determining the distance to the upper reference point;
Specifying the optical path length difference between the optical path length of the measurement light and the optical path length of the reference light based on the inclination angle and the distance;
Moving at least one of the object surface or the reference surface so that the optical path length difference is reduced,
An arrangement adjustment method having the following. Computer runs,
Irradiating the object surface and the reference surface with light of low coherence,
By an element that detects the spatial intensity of light, a step of detecting the spatial intensity of the interference light due to the measurement light reflected from the object surface and the reference light reflected from the reference surface,
Based on the interference light, obtaining a relative angle that is a relative angle of the object surface with respect to the reference surface,
From the intensity distribution of the interference light detected by the element for detecting the spatial intensity, a position on the element for detecting the spatial intensity corresponding to a relatively large intensity of the interference light, and an element for detecting the spatial intensity Determining the distance to the reference point above;
Specifying the optical path length difference between the optical path length of the measurement light and the optical path length of the reference light based on the relative angle and the distance;
Moving at least one of the object surface or the reference surface such that the optical path length difference is reduced,
An arrangement adjustment method having the following.