JP5477183B2 - Measuring device - Google Patents

Description

  The present invention relates to a measuring apparatus, and more particularly to an apparatus for measuring a three-dimensional shape of an object.

Conventionally, various apparatuses and methods for measuring the three-dimensional shape of an object have been proposed.
For example, Patent Document 1 (Japanese Patent No. 2928548) discloses a method and apparatus for measuring a three-dimensional shape of an object according to a focusing method. According to this method, an image of the object is acquired while the sample stage on which the object is placed is moved in the vertical direction by the vertical mechanism. Thereby, a plurality of images having different focal planes are acquired. By executing predetermined image processing (specifically, contrast detection) on the plurality of images, a focal position is obtained for each pixel. The three-dimensional shape of the object is obtained from the focal position for each pixel.

  For example, Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-17401) discloses a method and an apparatus for easily obtaining a three-dimensional shape by one photographing with one photographing apparatus. According to this method, a subject is photographed through a photographing optical system in which the imaging distance differs for each wavelength of light due to axial chromatic aberration. By utilizing the axial chromatic aberration, it is possible to obtain subject images for a plurality of wavelengths that are focused on different portions of the surface of the subject. According to the above method, the plurality of images are acquired as stereoscopic image information.

  Further, for example, Patent Document 3 (Japanese Patent Application Laid-Open No. 2009-145279) discloses a measurement device that enables measurement of a three-dimensional shape of an object without mechanical movement. This apparatus includes a light source that can output light by changing the wavelength within a predetermined wavelength band. The light source changes the wavelength of light for irradiating the measurement object. Thereby, a plurality of images having different focal lengths are acquired by the imaging system, and information regarding the three-dimensional shape of the measurement object is acquired using the plurality of images.

Japanese Patent No. 2928548 Japanese Patent Laid-Open No. 2007-17401 JP 2009-145279 A

  According to the conventional technique for measuring the three-dimensional shape of an object, there is a problem that it is difficult to achieve both improvement in measurement accuracy and simplification of the configuration of the apparatus.

  For example, in the apparatus disclosed in Patent Document 1, a mechanism for moving the sample table up and down is required. Patent Document 1 describes that the objective lens may be moved up and down instead of the sample stage. However, when moving the objective lens up and down, a mechanism for moving the objective lens is required. According to the configuration disclosed in Patent Document 1, since a mechanism for moving the sample or the objective lens is required, the measuring apparatus is increased in size.

  For example, according to Patent Document 2, although the configuration of the measurement device can be simplified, it is not easy to increase measurement accuracy. In Patent Document 2, as a specific example of the photographing optical system, a camera that includes three imaging elements of red (R), green (G), and blue (B) and separates a color image by a prism is shown. By using this camera, three images corresponding to the respective colors can be obtained as images having different focal planes. In order to measure the three-dimensional shape of an object with high accuracy, it is preferable that the number of images with different focal planes is larger. However, it is difficult to increase the number of images with different focal planes in the camera. Furthermore, it is assumed that the image acquired by the camera is affected by the color of the object surface. According to the method disclosed in Patent Document 2, it is required to grasp in advance how the color of the surface of the object affects the measurement result prior to measurement of the three-dimensional shape. Furthermore, it is necessary to calibrate the measuring device based on the grasped result.

  For example, in order to execute the method disclosed in Patent Document 3, a wavelength tunable light source is necessary. However, such a wavelength tunable light source generally has a complicated configuration. For this reason, a general variable wavelength light source is large and expensive. Note that the wavelength tunable light source disclosed in Patent Document 3 uses polarized light to control the intensity of each wavelength and to perform wavelength sweeping. Specifically, it is a supercontinuum that is a high-intensity white point light source. The apparatus includes a nium light source, a diffraction grating, and a reflective liquid crystal element array. That is, the variable wavelength light source disclosed in Patent Document 3 has a complicated configuration for controlling polarization or intensity.

  An object of the present invention is to provide a measuring apparatus capable of measuring a three-dimensional shape with high accuracy with a simple structure.

  A measurement apparatus according to an aspect of the present invention includes a light source that irradiates a measurement object with light having a plurality of wavelength components, an optical system, and an interference image generation unit. The optical system is arranged so that light from the measurement object illuminated by the light source is incident, and the focal distance on the light incident side is varied for each wavelength component due to axial chromatic aberration. The interference image generation unit divides the light emitted from the optical system into first and second light beams having a variable optical path length difference, and generates an interference image by superimposing the first and second light beams. . The interference image has information on the intensity of the interference light generated by the interference between the first and second light beams. The measurement apparatus further includes a spectral image generation unit and a shape calculation unit. The spectral image generation unit generates a spectral image having information on the spectral spectrum of light from information on the intensity of interference light included in the interference image. The shape calculation unit calculates the three-dimensional shape of the measurement object by calculating the in-focus position in the spectral image from the spectral spectrum information of the spectral image.

  Preferably, the optical system includes a diffractive lens arranged so that light from a measurement object enters.

  Preferably, the first optical path length of the first light flux is a fixed length. The second optical path length of the second light flux is variable. The interference image generation unit includes an optical component for forming the second optical path length and a piezo drive device for moving the optical component.

  Preferably, the first optical path length of the first light flux is a fixed length. The second optical path length of the second light flux is variable. The interference image generating unit includes an optical component for forming the second optical path length, a vibrator coupled to the optical component and formed of a magnetic material, and an electromagnetic coil for controlling the amplitude of the vibrator. Including.

  According to the present invention, it is possible to realize a measuring device capable of measuring a three-dimensional shape with high accuracy with a simple structure.

It is the figure which showed the basic composition of the measuring device which concerns on embodiment of this invention. It is a flowchart for demonstrating the measuring method of the solid shape of the measurement object by the measuring apparatus 100 shown in FIG. It is the figure which showed the relationship between the optical path length difference d and the intensity | strength i (d) of the interference light received by the imaging part. It is the figure which showed the relationship between spectrum I ((nu)) and wave number (nu). It is a figure for demonstrating the relationship between the interference image acquired by the imaging part, and the spectral image produced | generated by the spectral image production | generation part. It is the figure which showed the relationship between a spectral image and a focal plane. It is the figure which showed typically the relationship between the focus measurement in a certain pixel in a spectral image, and the distance of az axis direction. It is the figure which showed the focus measurement at the time of irradiating a measurement object with the light of a single wavelength. It is a figure for demonstrating the method of calculating a focus measure by the differentiation of an image. It is the figure which contrasted the diffraction lens and the group lens. It is the figure which showed the other structural example of the optical path length variable part.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  FIG. 1 is a diagram showing a basic configuration of a measuring apparatus according to an embodiment of the present invention. With reference to FIG. 1, the measuring apparatus 100 measures the three-dimensional shape of the measurement object 50. Specifically, the measurement apparatus 100 includes a light source 1, a diffraction lens 2, an interference image generation unit 3, and a control unit 4.

  The light source 1 irradiates the measurement object 50 with light having a plurality of wavelength components. For example, the light source 1 is a white light source such as a halogen lamp, a xenon lamp, a white LED, a metal halide lamp, or an incandescent lamp.

The diffractive lens 2 is used in the measuring apparatus 100 as an optical system having axial chromatic aberration. The diffractive lens 2 is arranged so that light from the surface of the measurement object 50 is incident on the lens surface 5 of the diffractive lens 2. Since the diffractive lens 2 has axial chromatic aberration, the focal length on the lens surface 5 side of the diffractive lens 2 is varied according to the wavelength of light. Focal planes A, B, and C indicate focal planes of the diffraction lens 2 with respect to light having wavelengths λ ₁ , λ ₂ , and λ ₃ , respectively. Wavelengths λ ₁ , λ ₂ , and λ ₃ are wavelength components included in the light from the light source 1. In order to cause the axial chromatic aberration as described above, the lens surface 5 of the diffractive lens 2 is formed in, for example, a kinoform shape or a binary shape (step shape, step shape). However, the configuration of the diffractive lens 2 is not limited to the above configuration.

  In the above description, three wavelength components are exemplified, but the number of wavelength components included in the light emitted from the light source 1 is not particularly limited as long as it is plural. However, in order to measure the three-dimensional shape of the measurement object 50 with higher accuracy, it is preferable that the light emitted from the light source 1 includes more wavelength components.

  The interference image generation unit 3 separates the light L emitted from the diffraction lens 2 into light beams L1 and L2. Further, the interference image generation unit 3 changes the optical path length difference between the optical path of the light beam L1 (first optical path) and the optical path of the light beam L2 (second optical path). The interference image generation unit 3 generates an interference image by superimposing the light beams L1 and L2 having the optical path length difference. The interference image is composed of a plurality of pixels. Each pixel has information on the intensity of the interference light generated by the interference between the light beams L1 and L2. The interference image generation unit 3 generates an interference image every time the optical path length difference is changed. Therefore, a plurality of interference images respectively corresponding to a plurality of different optical path lengths are generated.

  Specifically, the interference image generation unit 3 includes a half mirror 11, a fixed mirror 12, an optical path length variable unit 13, a lens 14, and an imaging unit 15. The optical path length variable unit 13 includes a movable mirror 16 and a piezo drive device 17. The half mirror 11, the fixed mirror 12, and the optical path length varying unit 13 (movable mirror 16) constitute an interferometer. In the embodiment of the present invention, a Michelson interferometer is configured.

  The half mirror 11 splits the light L from the diffractive lens 2 into light beams L1 and L2. The light beam L1 is directed to the fixed mirror 12 by the half mirror 11. The light beam L 1 is reflected by the fixed mirror 12 and returns toward the half mirror 11. The light beam L1 returning toward the half mirror 11 passes through the half mirror 11 and travels toward the lens 14. The optical path from the half mirror 11 to the lens 14 via the fixed mirror 12 is the optical path of the light beam L1. On the other hand, the light beam L2 passes through the half mirror 11 and travels toward the movable mirror 16. The light beam L2 is reflected by the movable mirror 16 and returns toward the half mirror 11. The light beam L2 returning toward the half mirror 11 is reflected by the half mirror 11 and travels toward the lens 14. The optical path from the half mirror 11 to the lens 14 via the movable mirror 16 is the optical path of the light beam L2.

  The optical path length variable unit 13 changes the difference (optical path length difference) between the optical path length of the light beam L1 and the optical path length of the light beam L2. Specifically, the optical path length of the light beam L2 changes as the piezo drive device 17 moves the movable mirror 16. On the other hand, the optical path length of the light beam L1 is fixed. The piezo drive device 17 includes a piezo element (not shown) that expands and contracts when a voltage is applied. The piezoelectric element is moved by changing the voltage applied to the piezoelectric element. The movable mirror 16 moves using this movement.

  When light beams L1 and L2 having different optical path lengths travel toward the lens 14, they overlap each other. As a result, interference light is generated. The lens 14 is a lens for imaging the interference light on the imaging unit 15. The imaging unit 15 has an imaging surface 15B formed by a plurality of imaging elements 15A (for example, a CCD, a CMOS sensor, etc.) arranged two-dimensionally, and an image formed on the imaging surface 15B by the lens 14 (Interference image) is acquired. The imaging unit 15 sends the acquired interference image data to the control unit 4.

  The control unit 4 is realized by, for example, a computer that executes a predetermined program. The control unit 4 includes a spectral image generation unit 21 and a three-dimensional shape calculation unit 22.

  The spectral image generation unit 21 changes the voltage applied to the piezo drive device 17 in order to move the movable mirror 16. Furthermore, the spectral image generation unit 21 controls the imaging unit 15 so that the imaging unit 15 acquires an interference image.

  The spectral image generation unit 21 generates an image for each wave number using a plurality of interference images acquired by the imaging unit 15. An image generated for each wave number (which may be a wavelength) is hereinafter referred to as a “spectral image”. The spectral image generation unit 21 acquires a spectral image by performing Fourier transform on the intensity information of the interference light included in the interference image. Each pixel included in the spectral image has spectral spectrum information of light from the surface of the measurement object 50.

  The three-dimensional shape calculation unit 22 calculates a focal position for each pixel in the spectral image based on the spectral image generated by the spectral image generation unit 21. Furthermore, the three-dimensional shape calculation unit 22 acquires information regarding the three-dimensional shape of the measurement object 50 based on the in-focus position. This information may be information on the whole or part of the measurement object 50 or information on the height of an arbitrary position on the surface of the measurement object 50. The “height” in this description means a displacement in the z-axis direction from a reference position (for example, the position of the bottom surface of the measurement object 50) to the measurement target position.

  In the embodiment of the present invention, the three-dimensional shape of the measurement object is measured by the following principle. First, the measuring device 100 includes the diffractive lens 2 as an optical system having axial chromatic aberration. The diffractive lens 2 is arranged so that the focal length on the light incident side from the measurement object 50 is different for each wavelength component of the light.

  When the measurement object 50 is observed from the side of the interference image generation unit 3 through the diffraction lens 2, an image of the measurement object 50 is observed as follows. That is, when the measuring object 50 is illuminated with light of a certain wavelength, for example, an image of the measuring object 50 that is not in focus is observed. On the other hand, when the measurement object 50 is illuminated with light of another wavelength, for example, an image focused on a part of the surface of the measurement object 50 is observed. When the measurement object 50 is illuminated with light of another wavelength, for example, an image focused on another part of the surface of the measurement object 50 is observed.

  In the embodiment of the present invention, the measurement object 50 is illuminated by light having a plurality of wavelength components. Since the image of the measurement object 50 observed through the diffractive lens 2 is an image obtained by superimposing images corresponding to the respective wavelength components, the image is blurred as a whole. In the embodiment of the present invention, a spectral image is generated for each wavelength component (wave number corresponding to the wavelength) by performing two-dimensional Fourier transform on the image of the measurement object 50 observed through the diffraction lens 2. The spectral image generation means is not limited to Fourier transform, and other frequency analysis methods such as wavelet transform and maximum entropy method may be used.

  A plurality of spectral images corresponding to each wavelength component can be regarded as a plurality of images having different focal lengths in the z-axis direction of the diffractive lens 2. According to the embodiment of the present invention, the three-dimensional shape calculation unit 22 uses a plurality of spectral images to obtain a focal position at each pixel constituting the spectral image. By calculating the focal position for each pixel, the three-dimensional shape calculation unit 22 calculates the three-dimensional shape of the measurement object.

  In the embodiment of the present invention, a plurality of spectral images can be generated by processing in the computer (control unit 4) by applying the above-described imaging by two-dimensional Fourier spectroscopy. Therefore, imaging by two-dimensional Fourier spectroscopy is different from imaging for each color using an optical filter.

  FIG. 2 is a flowchart for explaining a method of measuring the three-dimensional shape of the measurement object by the measurement apparatus 100 shown in FIG. With reference to FIGS. 1 and 2, when the process is started, the movable mirror 16 changes the optical path length difference between the optical path length of the light beam L1 and the optical path length of the light beam L2 (step S1). As described above, the piezoelectric element expands and contracts by changing the voltage applied to the piezo driving device 17 by the spectral image generation unit 21. Thereby, since the movable mirror 16 moves, the optical path length difference changes. The relationship between the optical path length difference and the voltage applied to the piezo drive device 17 is determined in advance by experiments or the like. The spectral image generation unit 21 stores a table defining the relationship and changes the voltage applied to the piezo drive device 17 in accordance with the relationship.

  Next, the imaging unit 15 captures an interference image (step S2). The imaging unit 15 executes processing for capturing an interference image in response to a command from the spectral image generation unit 21 and transmits data of the interference image to the spectral image generation unit 21. The spectral image generation unit 21 receives interference image data and stores the data.

  Subsequently, the spectral image generation unit 21 determines whether or not the range of change in the optical path length difference has reached the specified range (step S3). The designated range is a moving range of the movable mirror 16 by the piezo drive device 17 and is stored in the spectral image generation unit 21 in advance. If it is determined that the range of change in the optical path length difference has not yet reached the specified range (NO in step S3), the process returns to step S1. On the other hand, when it is determined that the change range of the optical path length difference has reached the specified range (YES in step S3), the process proceeds to step S4. Steps S1 to S3 are repeated until the range of change in the optical path length difference reaches the specified range. Therefore, an interference image is captured every time the optical path length difference changes.

  Subsequently, the spectral image generation unit 21 generates a spectral image for each wave number by two-dimensional Fourier spectroscopy using the plurality of interference images obtained by the processes of steps S1 to S3 (step S4). A plurality of spectral images are generated by the process of step S4. Data of each spectral image is sent from the spectral image generation unit 21 to the three-dimensional shape calculation unit 22.

  The three-dimensional shape calculation unit 22 calculates a focus measure from a plurality of spectral images (step S5). In this specification, the in-focus measure is defined as a numerical value representative of the degree of focus in each pixel.

  Next, the three-dimensional shape calculation unit 22 calculates the in-focus position for all the pixels of the spectral image (step S6). Specifically, the three-dimensional shape calculation unit 22 calculates a focal position at which the value of the focus measure is maximum for each pixel in the spectral image. This position corresponds to the in-focus position.

  Subsequently, the three-dimensional shape calculation unit 22 obtains the three-dimensional shape of the measurement object 50 using the focal point position for each pixel calculated by the process of step S6 (step S7). As a method for calculating the three-dimensional shape of the measurement object 50 from the in-focus position, various known methods, for example, the method disclosed in Patent Document 1, can be applied, and detailed description thereof will not be repeated here.

  Further, the three-dimensional shape calculation unit 22 outputs information regarding the calculated three-dimensional shape (step S8). When the process of step S8 ends, the entire process ends.

<About two-dimensional Fourier spectroscopy>
Light from the surface of the measurement object 50 passes through the diffraction lens 2 and further passes through an interferometer constituted by the half mirror 11, the fixed mirror 12, and the optical path length variable unit 13 (the movable mirror 16 and the piezo drive device 17). In the following description, the optical path length difference between the light beams L1 and L2 is represented as d. The intensity i (d) of the interference light received by the imaging unit 15 is expressed according to the following equation (1).

  In the formula (1), I (ν) is a function indicating the spectral intensity of light from the surface of the measurement object 50, and ν is the wave number. A relationship of ν = 1 / λ is established between the wave number ν and the wavelength λ. The above equation (1) indicates that i (d) and I (ν) are Fourier transform pairs.

  FIG. 3 is a diagram illustrating a relationship between the optical path length difference d and the intensity i (d) of the interference light received by the imaging unit. FIG. 4 is a diagram showing the relationship between the spectrum I (ν) and the wave number ν. With reference to FIG. 3 and FIG. 4, spectrum I (ν) can be obtained by subjecting signal intensity i (d) to discrete Fourier transform with optical path length difference d. The signal intensity shown in FIG. 3 is the intensity of the interference light received by each imaging element of the imaging unit 15.

  In the processing of steps S1 to S3 in FIG. 2, the optical path length difference d is assumed to change by ΔL from −L to L. Each time the piezo driving device 17 moves the movable mirror 16 by ΔL, the imaging unit 15 captures an interference image. Each of the plurality of imaging elements 15A constituting the imaging unit 15 outputs a signal indicating the intensity of the interference light at the sampling interval ΔL. As shown in FIG. 3, the intensity of the signal output from the image sensor 15A changes according to the optical path length difference d.

  By performing discrete Fourier transform on the signal intensity i (d) with the optical path length difference d, a spectrum I (ν) is obtained at intervals of 1 / (2L) over a range from ν = 0 to ν = 1 / ΔL. As L is increased, that is, as the stroke of the movable mirror 16 is increased, 1 / (2L) is reduced, so that the resolution of the spectrum I (ν) is improved.

  In the embodiment of the present invention, the optical path length difference d is changed by the same amount in the positive direction and the negative direction around 0. However, the change amount in the positive direction and the change amount in the negative direction of the optical path length difference d need not be the same.

  As shown in FIG. 4, the wave number range of the spectrum I (ν) can be determined by ΔL. In the embodiment of the present invention, ΔL is determined from the spectrum of the light source 1, for example.

<About shape measurement>
FIG. 5 is a diagram for explaining the relationship between the interference image acquired by the imaging unit and the spectral image generated by the spectral image generation unit. FIG. 5 shows the processes of steps S1 to S4 in the flowchart of FIG.

  Referring to FIG. 5, the imaging unit 15 acquires an interference image every time the optical path length difference d changes from −L to L by ΔL. Thereby, a plurality (= 2L / ΔL) of interference images are acquired. FIG. 5 representatively shows interference images 31, 32, 33, 34, and 35 corresponding to cases where the optical path length difference d is −L, −L + ΔL, 0, L−ΔL, and L, respectively.

  The spectral image generation unit 21 acquires the luminance value of the pixel P1 at the position (x, y) of the interference image for all of the plurality of interference images. Thereby, the spectral image generation unit 21 acquires the spectrum (signal intensity) i (d) of the pixel P1.

  Next, the spectral image generation unit 21 performs a discrete Fourier transform on the spectrum i (d). Thereby, the spectrum I (x, y) at the position (x, y) is obtained. As described above, the spectral spectrum I (x, y) is a function of the wave number ν.

  The spectral image generation unit 21 calculates the spectrum i (d) for all the pixels in the interference image and calculates the spectral spectrum I (x, y) by discrete Fourier transform of the spectrum i (d). . Thereby, the spectral image generation unit 21 generates a spectral image for each wave number. As shown in FIG. 4, the wave number changes by ½ L within a range from 0 to 1 / ΔL. In FIG. 5, among the plurality of spectral images generated by the spectral image generation unit 21, representative spectral images 41 to 45 are shown. The pixel P2 is a pixel at a position (x, y) in the spectral image.

  FIG. 6 is a diagram showing the relationship between the spectral image and the focal plane. With reference to FIG. 6, an interference image is acquired by the diffraction lens 2 and the interference image generation unit 3. The control unit 4 generates a spectral image using the interference image. In FIG. 6, spectral images 41A to 41C are typically generated.

  In the embodiment of the present invention, by combining an optical system (diffractive lens 2) having axial chromatic aberration and two-dimensional Fourier spectroscopy, each of the plurality of spectral images is equivalent to an image having a different focal plane. The spectral images 41A to 41C are images corresponding to the focal planes A to C shown in FIG.

  The pixel P2 is a pixel at a position (x, y) in the spectral image. From the spectrum I (ν) of the pixel P2 and its surrounding pixels, the wave number at the time of focusing at the position of the surface of the measurement object 50 corresponding to the pixel P2 can be obtained. From the wave number at this time, the focal length of the diffractive lens 2 at the above position can be obtained. By obtaining the focal length for all the pixels in the spectral image, the three-dimensional shape of the measurement object 50 can be obtained.

  FIG. 7 is a diagram schematically showing the relationship between the focus measure and the distance in the z-axis direction at a certain pixel in the spectral image. Referring to FIG. 7, focus measure FM changes according to the distance in the z-axis direction. In the embodiment of the present invention, the position in the z-axis direction when the focus measure FM is the largest is defined as the focus position.

  FIG. 8 is a diagram showing a focus measure when a measurement object is irradiated with light of a single wavelength. Referring to FIG. 8, when a measurement object is irradiated with light having a single wavelength, a bright spot is generated in the spectral image only at a certain position on the z axis. Therefore, it is difficult to determine the in-focus position. In the embodiment of the present invention, the measurement object 50 is irradiated with light having a plurality of wavelength components. As a result, since the focus measure FM of the pixel in the spectral image can be changed according to the position in the z-axis direction, the focus position can be specified more accurately. By accurately specifying the in-focus position, the three-dimensional shape of the measurement object can be measured with higher accuracy.

  In the embodiment of the present invention, the method for the solid shape calculation unit 22 to calculate the focus measure is not particularly limited. For example, a method of calculating the focus measure from the contrast near the target pixel can be used. Specifically, a method for calculating a focus measure by differentiating an image, a method for calculating a focus measure by obtaining a luminance dispersion of a local region including a target pixel, and the like can be applied. Hereinafter, the former method will be described.

  FIG. 9 is a diagram for explaining a method of calculating a focus measure by image differentiation. Referring to FIG. 9, pixel PA is a pixel of interest in the spectral image. The coordinates of the pixel PA are indicated as (x, y).

  As an operator for image differentiation, a differentiation operator ML (x, y) shown in the following equation (2) is used. The differential operator ML (x, y) corresponds to the secondary differential of the interference image and represents the contrast near the pixel of interest (pixel PA).

  In Expression (2), I (x, y) represents the luminance value of the pixel PA. p indicates a pixel interval between the pixel PA to be compared with the pixel PA and the pixel PA. p is determined depending on the spatial frequency of the texture of the surface of the measurement object. FIG. 9 shows a case where p = 1. In this case, the pixels to be compared with the pixel PA are pixels adjacent to the pixel PA in the x-axis direction and pixels adjacent to the y-axis direction.

  The calculation result obtained by the differential operator ML (x, y) reacts sensitively to noise included in the spectral image. Therefore, as shown in Expression (3), the differential operator ML (x, y) for (2N + 1) × (2N + 1) (N is an integer of 1 or more) pixels centered on the target pixel (pixel PA). ) To average the differential operator ML (x, y). FIG. 9 shows a case where N = 1.

  FM (x, y) obtained by Expression (3) is defined as the focus measure of the pixel of interest (pixel PA). As shown in FIG. 7, the position where the value of the focus measure FM (x, y) is maximum is obtained as the focus position. However, the three-dimensional shape calculation unit 22 obtains the in-focus position after approximating the curve of the focus measure FM (x, y) to a Gaussian curve. Since the focus measure FM (x, y) is obtained discretely, the focus position can be calculated more accurately by approximating the curve of the focus measure FM (x, y) to a Gaussian curve. Further, even when noise is included in the interference image (as a result, noise is included in the spectral image), by approximating the curve of the focus measure FM (x, y) to a Gaussian curve, the in-focus position can be further increased. It can be calculated accurately.

  As described above, in the embodiment of the present invention, the three-dimensional shape of an object is measured by a combination of a light source that emits light having a plurality of wavelength components, an optical system having axial chromatic aberration, and two-dimensional Fourier spectroscopy. In the prior art disclosed in Patent Document 1, it is necessary to move the stage on which the measurement object is placed or to move the objective lens in order to change the focal length. For this reason, the mechanism for moving the sample stage or the objective lens becomes larger and complicated. On the other hand, in the embodiment of the present invention, the movable part may be only the movable mirror 16. In general, mirrors used for movable mirrors are smaller and lighter than objective lenses and stages. For this reason, a movable part can be reduced in size and the structure can be simplified. Therefore, according to the embodiment of the present invention, the measuring device can be miniaturized.

  In addition, according to the embodiment of the present invention, it is not necessary to select the wavelength of the light that irradiates the measurement object with the light source. For this reason, the structure of a light source can be simplified. Therefore, also from this point, according to the embodiment of the present invention, the measuring device can be downsized.

  Further, according to the embodiment of the present invention, the number of interference images and spectral images can be increased by increasing the change width (2L in the case of FIG. 3) of the optical path length difference d. By increasing the number of spectral images, the in-focus position can be accurately specified, so that the measurement accuracy of the shape of the measurement object can be increased.

<Configuration of optical system>
As described above, the measuring apparatus 100 according to the embodiment of the present invention includes the diffractive lens 2 as an optical system having axial chromatic aberration. As an optical system having axial chromatic aberration, a combined lens in which a plurality of lenses are combined can be applied to the measuring apparatus 100 instead of the diffractive lens 2. However, it is more preferable to apply the diffractive lens 2 to the measuring device 100 for the following reasons.

  In principle, the focal length of the diffractive lens 2 changes linearly with respect to the wave number. More specifically, the focal length f of the diffractive lens is expressed according to the following equation (4) using the wave number ν.

f = c × ν (4)
c is a proportionality constant. As described above, the wave number ν is the reciprocal of the wavelength λ (ν = 1 / λ). When the focal length f of the diffractive lens is differentiated by the wave number ν, df / dν = c. That is, in the case of a diffractive lens, the rate of change of the focal length f with respect to the wave number ν is constant.

  According to the embodiment of the present invention, the measurement accuracy can be kept constant within a predetermined measurement range by combining the diffractive lens having such characteristics and two-dimensional Fourier spectroscopy. As described above, in the embodiment of the present invention, a spectroscopic image is obtained for each constant wave number (1/2 L in the above description). Since the rate of change of the focal length f with respect to the wave number ν is constant, each of the plurality of spectral images can be said to be an image obtained every time the focal length f is changed by a certain amount. By obtaining an image when the focal length is changed by a certain amount, the measurement accuracy can be kept constant within a predetermined measurement range. The predetermined measurement range is a range designed in advance as a measurement range required for a product, for example.

  FIG. 10 is a diagram comparing a diffractive lens and a combined lens. FIG. 10A is a diagram showing a case where the diffraction lens 2 is applied to an optical system having axial chromatic aberration. FIG. 10B is a diagram showing a case where the combined lens 2A is applied to an optical system having axial chromatic aberration.

  Referring to FIG. 10A, an interference image is acquired by the diffraction lens 2 and the interference image generation unit 3. Further, a spectroscopic image is generated from the interference image by the control unit 4 (not shown in FIG. 10). The generated spectral image corresponds to an image sampled while changing the distance in the z-axis direction from the diffraction lens 2 and the interference image generation unit 3 (imaging unit 15) by an equal distance (distance a). Therefore, the measurement accuracy can be kept constant within the measurement range.

  Referring to FIG. 10B, when the group lens 2A is used instead of the diffraction lens 2, the focal length generally changes nonlinearly with respect to the wave number. For this reason, the spectral image obtained when the combined lens 2A is used is an image sampled while changing the position of the focal plane in the z-axis direction at unequal pitches (distances a1 to a6). That is, the measurement accuracy may change within the measurement range. Therefore, when using the group lens 2A, for example, the relationship between the focal length and the wave number is confirmed in advance by experiments or the like. The three-dimensional shape calculation unit 22 corrects the measurement result according to the relationship.

  Further, the configuration of the interference image generation unit 3 is not limited to the configuration shown in FIG. FIG. 11 is a diagram illustrating another configuration example of the optical path length variable unit. Referring to FIG. 11, the optical path length variable unit 13 </ b> A includes a coil driving device 17 </ b> A instead of the piezo driving device 17. The coil drive device 17A includes an electromagnetic coil 172, a drive circuit 171 for turning on and off the electromagnetic coil 172, and a cantilever 173.

  The drive circuit 171 energizes the electromagnetic coil 172 when a pulse signal is supplied from the control unit 4. The cantilever 173 is made of a magnetic material and is attracted to the energized electromagnetic coil 172. When the controller 4 stops supplying the pulse signal to the drive circuit 171, the drive circuit 171 stops energization of the electromagnetic coil 172, so that the cantilever 173 returns to its original position.

  The controller 4 repeats the supply and stop of the pulse signal to the drive circuit 171 so that the electromagnetic coil is repeatedly turned on and off. Thereby, the cantilever 173 vibrates. Since the movable mirror 16 is bonded to the cantilever 173, the position of the movable mirror 16 is changed with the vibration of the cantilever 173. For example, the amplitude of the cantilever 173 is controlled by changing the intensity of the pulse signal. Thereby, the optical path length difference d can be changed at a constant pitch.

  Note that the mirror surface may be formed by polishing the surface of the cantilever 173. This mirror surface can be used in place of the movable mirror 16. Although not shown, the cantilever 173 and the electromagnetic coil 172 shown in FIG. 17 may be replaced with a tuning fork vibrator. According to this configuration, the movable mirror 16 is attached to the tip of the tuning fork vibrator, and the optical path length difference d can be changed by vibrating the tuning fork vibrator.

  Further, the configuration for generating the interference image is not limited to the configuration shown in FIG. 1, but is changed from the configuration shown in FIG. 1 (for example, addition of optical components such as a mirror and a lens). May be. FIG. 1 shows a Michelson interferometer as an interferometer for generating an interference image. However, an interferometer other than the Michelson interferometer (for example, the Mirau interferometer) may be applied to the measurement apparatus according to the embodiment of the present invention.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Light source, 2 Diffraction lens, 2A group lens, 3 Interference image generation part, 4 Control part, 5 Lens surface, 11 Half mirror, 12 Fixed mirror, 13, 13A Optical path length variable part, 14 Lens, 15 Imaging part, 15A Imaging Element, 15B imaging surface, 16 movable mirror, 17 piezo drive device, 17A coil drive device, 21 spectral image generation unit, 22 solid shape calculation unit, 31-35, 41-45, 41A-41C spectral image, 50 measurement object, 100 measuring device, 171 drive circuit, 172 electromagnetic coil, 173 cantilever, AC focal plane, L, L1, L2 light, P1, P2, PA pixels.

Claims (4)

A light source for irradiating a measurement object with light having a plurality of wavelength components;
An optical system that is arranged so that the light from the measurement object illuminated by the light source is incident, and has an axial chromatic aberration that varies the focal length on the incident side of the light for each wavelength component;
Interference image generation for dividing the light emitted from the optical system into first and second light fluxes having a variable optical path length difference and generating an interference image by superimposing the first and second light fluxes The interference image has information on the intensity of interference light generated by the interference of the first and second light beams,
A spectral image generation unit configured to generate a spectral image having spectral spectrum information of the light from information on the intensity of the interference light included in the interference image;
A measurement apparatus further comprising: a shape calculation unit that calculates a three-dimensional shape of the measurement object by calculating a focal point position in the spectral image from information of the spectral spectrum of the spectral image.   The measurement apparatus according to claim 1, wherein the optical system includes a diffractive lens disposed so that the light from the measurement object is incident thereon. The first optical path length of the first luminous flux is a fixed length;
The second optical path length of the second light flux is variable,
The interference image generation unit
An optical component for forming the second optical path length;
The measurement device according to claim 2, further comprising a piezo drive device for moving the optical component. The first optical path length of the first luminous flux is a fixed length;
The second optical path length of the second light flux is variable,
The interference image generation unit
An optical component for forming the second optical path length;
A vibrator connected to the optical component and formed of a magnetic material;
The measurement apparatus according to claim 2, further comprising an electromagnetic coil for controlling the amplitude of the vibrator.

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