JP2011099787A - Apparatus and method for measuring shape - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an effect of a noise, and obtain a stable and accurate measurement result. <P>SOLUTION: An interference light intensity distribution image indicates an interference light intensity corresponding to each measurement position in a measurement plane of a to-be-measured object changed by an optical path length difference between a first optical path length from a light source 1 having a wide range spectrum to the to-be-measured object 4 and a second optical path from the light source 4 to a reference plane 5. While the optical path length difference is changed, the interference light intensity distribution image is sequentially stored in an image storage means 9. With regard to an interference light intensity sequence indicating a change in an interference light intensity due to the change in the optical path length difference at each measurement position of the interference light intensity distribution image, a predetermined threshold level is set in the positive and negative directions of an intensity axis from the center of the intensity. Peak position candidates are found as data of the interference light intensity exceeding the threshold level, and a peak value of the interference light intensity sequence is found as a barycenter of an area in which the peak position candidates are congested at the maximum. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a shape measuring machine and a shape measuring method for measuring the position of a measurement surface based on the interference intensity of light irradiated on a measurement surface from a light source having a broadband spectrum and light irradiated on a reference surface.

  Conventionally, various shape measuring apparatuses that measure the three-dimensional shape of an object to be measured in a non-contact manner using an optical system are known. For example, a white interferometer is known as a shape measuring apparatus capable of three-dimensional measurement of a minute object to be measured such as a micromachine or LSI. This white interferometer irradiates the object to be measured from the white light source and interferes with the white light reflected from the object to be measured and the white light irradiated from the white light source to the reference surface and reflected from the reference surface. The reference surface is moved in the optical axis direction to detect the reference surface position with the highest interference light intensity, and the height of the object to be measured in the optical axis direction is measured based on the reference surface position (Patent Document 1). ).

  Among such white interferometers, as a simplified calculation process, calculating a weighted average of the inspection positions weighted by the absolute value of the difference value of the interference fringe intensity before and after displacing the optical path length difference by a predetermined amount, A shape measuring apparatus is also known in which the value indicated by the weighted average is obtained as a peak position where the absolute value of the difference value is maximum (Patent Document 2).

International Publication No. 2006-068217 Japanese Patent No. 3220955

  However, since the apparatus disclosed in Patent Document 2 described above performs centroid calculation using the difference absolute value of the interference light intensity value as a coefficient, there is a problem that peak position detection is shifted due to the influence of noise. Thus, if it is easy to be influenced by noise, a stable and highly accurate measurement result cannot be obtained.

  The present invention has been made in view of these points, and an object of the present invention is to provide a shape measuring apparatus that can suppress the influence of noise and obtain a stable and highly accurate measurement result.

  A first shape measuring apparatus according to the present invention is a light source having a broadband spectrum, guides light from the light source to a measurement target and a reference surface, and combines light reflected from the measurement target and the reference surface. And corresponding to each measurement position in the measurement surface of the measurement target that varies depending on the optical path length difference between the first optical path length from the light source to the measurement target and the second optical path length from the light source to the reference surface. An optical system that generates an interference light intensity distribution image indicating the interference light intensity, an imaging unit that captures the interference light intensity distribution image output from the optical system, and an optical path having the first optical path length and the second optical path length. Optical path length difference changing means for changing the length difference, image storage means for sequentially storing an interference light intensity distribution image picked up by the imaging means and changing in accordance with the change in the optical path length difference, and stored in the image storage means The interference light intensity distribution image The peak value is obtained from the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position, and the peak value is the position in the optical axis direction at each measurement position of the measurement target. In the shape measuring apparatus comprising the calculating means obtained as: the calculating means sets a predetermined threshold level in the positive / negative direction of the intensity axis from the intensity center of the interference light intensity sequence, and the interference light intensity exceeding the threshold level The data is obtained as the peak position candidate, and the center of gravity of the region where the peak candidate points are most densely obtained is obtained as the peak value of the interference light intensity sequence.

  A second shape measuring apparatus according to the present invention is a light source having a broadband spectrum, and guides light from the light source to a measurement target and a reference surface, and combines light reflected from the measurement target and the reference surface. And corresponding to each measurement position in the measurement surface of the measurement target that varies depending on the optical path length difference between the first optical path length from the light source to the measurement target and the second optical path length from the light source to the reference surface. An optical system that generates an interference light intensity distribution image indicating the interference light intensity, an imaging unit that captures the interference light intensity distribution image output from the optical system, and an optical path having the first optical path length and the second optical path length. Optical path length difference changing means for changing the length difference, image storage means for sequentially storing an interference light intensity distribution image picked up by the imaging means and changing in accordance with the change in the optical path length difference, and stored in the image storage means The interference light intensity distribution image The peak value is obtained from the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position, and the peak value is the position in the optical axis direction at each measurement position of the measurement target. In the shape measuring apparatus including the calculating means obtained as follows, the calculating means applies the geometric element so that the amplitude is reproduced with respect to the interference light intensity sequence, and for each end point of the applied geometric element, the end point The end point position at which the sum of absolute values of the inclinations of the two geometric elements connected at the maximum is obtained as the peak value of the interference light intensity sequence.

  A third shape measuring apparatus according to the present invention guides light from a light source having a broadband spectrum and light from the light source to a measurement target and a reference surface, and combines light reflected from the measurement target and the reference surface. And corresponding to each measurement position in the measurement surface of the measurement target that varies depending on the optical path length difference between the first optical path length from the light source to the measurement target and the second optical path length from the light source to the reference surface. An optical system that generates an interference light intensity distribution image indicating the interference light intensity, an imaging unit that captures the interference light intensity distribution image output from the optical system, and an optical path having the first optical path length and the second optical path length. Optical path length difference changing means for changing the length difference, image storage means for sequentially storing an interference light intensity distribution image picked up by the imaging means and changing in accordance with the change in the optical path length difference, and stored in the image storage means The interference light intensity distribution image The peak value is obtained from the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position, and the peak value is the position in the optical axis direction at each measurement position of the measurement target. In the shape measuring apparatus comprising the calculating means to be obtained as the above, the calculating means thins out the data so as not to impair the shape of the interference light intensity sequence, and the thinned data is the most data. This is characterized in that a position having a high density is obtained as a peak value of the interference light intensity sequence.

  The first shape measuring method according to the present invention guides light from a light source having a broadband spectrum to a measurement target and a reference surface, combines light reflected from the measurement target and the reference surface, and the light source Interfering light intensity corresponding to each measurement position in the measurement surface of the measurement object, which varies depending on the optical path length difference between the first optical path length from the light source to the measurement object and the second optical path length from the light source to the reference surface An image of the interference light intensity distribution image is stored while the optical path length difference between the first optical path length and the second optical path length is changed by the optical path length difference changing means. From the interference light intensity sequence indicating the change in the interference light intensity associated with the change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means. Of the intensity axis from the center of A step of setting a predetermined threshold level in a direction, a step of obtaining interference light intensity data exceeding the threshold level as a peak position candidate, and a center of gravity of a region where the peak position candidates are most densely peaked. And a step of obtaining the obtained peak value as a position in the optical axis direction at each measurement position of the object to be measured.

  The second shape measurement method according to the present invention guides light from a light source having a broadband spectrum to a measurement target and a reference surface, combines light reflected from the measurement target and the reference surface, and supplies the light source. Interfering light intensity corresponding to each measurement position in the measurement surface of the measurement object, which varies depending on the optical path length difference between the first optical path length from the light source to the measurement object and the second optical path length from the light source to the reference surface An image of the interference light intensity distribution image is stored while the optical path length difference between the first optical path length and the second optical path length is changed by the optical path length difference changing means. A step of sequentially storing in the means, and an interference light intensity sequence indicating a change in the interference light intensity accompanying a change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, So that its amplitude is reproduced The step of applying any element and, for each end point of the fitted geometric element, the end point position where the absolute value sum of the inclinations of the two geometric elements connected at the end point is maximized is obtained as the peak value of the interference light intensity sequence. And a step of obtaining the obtained peak value as a position in the optical axis direction at each measurement position of the measurement object.

  The third shape measuring method according to the present invention guides light from a light source having a broadband spectrum to a measurement target and a reference surface, combines light reflected from the measurement target and the reference surface, and the light source Interfering light intensity corresponding to each measurement position in the measurement surface of the measurement object, which varies depending on the optical path length difference between the first optical path length from the light source to the measurement object and the second optical path length from the light source to the reference surface An image of the interference light intensity distribution image is stored while the optical path length difference between the first optical path length and the second optical path length is changed by the optical path length difference changing means. A step of sequentially storing in the means, and an interference light intensity sequence indicating a change in the interference light intensity accompanying a change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, So as not to damage its shape A step of thinning out the data, a step of obtaining a position having the highest data density as the peak value of the interference light intensity sequence for the thinned data, and the measured peak value of the measured data And a step of obtaining the position as the position in the optical axis direction at each measurement position of the object.

  According to the present invention, it is possible to obtain a stable and highly accurate measurement result while suppressing the influence of noise.

It is a block diagram which shows the structure of the shape measuring apparatus which concerns on 1st Embodiment of this invention. It is a figure for demonstrating the shape measuring method in the apparatus. It is a flowchart for demonstrating the shape measuring method in the apparatus. It is a flowchart which shows operation | movement of the arithmetic processing part in the same apparatus. It is a figure for demonstrating the shape measuring method in the apparatus. It is a flowchart for doing. It is a flowchart which shows operation | movement of the arithmetic processing part in the shape measuring apparatus which concerns on 2nd Embodiment of this invention. It is a figure for demonstrating the arithmetic processing method in the same apparatus. It is a flowchart which shows operation | movement of the arithmetic processing part in the shape measuring apparatus which concerns on 3rd Embodiment of this invention. It is a figure for demonstrating the arithmetic processing method in the same apparatus. It is a flowchart which shows operation | movement of the arithmetic processing part in the shape measuring apparatus which concerns on 4th Embodiment of this invention. It is a figure for demonstrating the arithmetic processing method in the same apparatus. It is a figure for demonstrating operation | movement of the shape measuring apparatus which concerns on 5th Embodiment of this invention.

[First Embodiment]
Next, the shape measuring apparatus and the shape measuring method according to the first embodiment of the present invention will be described in detail.

  FIG. 1 is a diagram illustrating a configuration of a white light interferometer that is a shape measuring apparatus according to the present embodiment. Although a Michelson interferometer is shown here, other equal optical path interferometers such as a Mirau type can also be used. Further, it may be used in combination with another optical measuring device such as an image measuring device.

  The light source 1 is a white light source having a broadband spectrum such as a halogen lamp, a xenon lamp, a mercury lamp, a metal halide lamp, or an LED. White light emitted from the light source 1 is collimated by the collimator lens 2 and divided in two directions by the beam splitter 3. One split light is applied to the measurement surface of the workpiece 4 to be measured, and the other split light is applied to the reference surface of the reference plate 5. White light reflected from the measurement surface and the reference surface is combined by the beam splitter 3, and interference light at that time is imaged by the CCD camera 8 via the imaging lens 7.

  The reference plate 5 is moved and scanned in the optical axis direction by driving means 6 such as a piezo element, and an interference image at each scanning position is sampled by the CCD camera 8 and stored in the image memory 9. The arithmetic processing unit 10 determines the height direction of the measurement surface of the workpiece 4 based on the intensity of interference light at each measurement position on the measurement surface of the workpiece 4 and the scanning position information of the reference plate 5 input from the encoder 14. Find the position. The input unit 11 inputs data necessary for measurement to the arithmetic processing unit 10. The output unit 12 outputs the measurement result obtained by the arithmetic processing unit 10. The display unit 13 displays information necessary for the input operation and measurement results.

  Next, a shape measuring method using this white interferometer will be described.

  White light from the light source 1 is reflected by the measurement surface of the workpiece 4 and the reference surface of the reference plate 5 and is combined by the beam splitter 3. The interference intensity at that time is determined by the optical path length difference between the first optical path length from the light source 1 to the workpiece 4 and the second optical path length from the light source 1 to the reference plate 5. When the first and second optical path lengths are equal, the interference light intensity is the highest. The interference light intensity is changed by moving and scanning the reference plate 5 in the direction of the optical axis with the piezo element 6. By using white light with less coherence, the range in which interference fringes are generated can be narrowed. Thereby, for example, as shown in FIG. 2, the change in the interference light intensity at each position on the measurement surface caused by the moving scanning of the reference surface occurs at a phase corresponding to the height (Z-direction position) of the measurement surface. Therefore, the scanning position of the reference surface where the peak value of the change in interference light intensity at each position on the measurement surface is observed can be obtained as the height of the corresponding portion of the measurement surface.

  FIG. 3 is a flowchart showing the shape measuring method.

  The reference plate 5 is moved by a predetermined amount in the optical axis direction (S1), and a two-dimensional distribution image of the interference light intensity on the measurement surface is stored in the image memory 9 (S2). This is repeated a predetermined number of samplings (S3), and when a predetermined number of distribution images are stored in the image memory 9, the interference light intensity associated with the change in optical path length difference at each measurement position on the measurement surface as shown in FIG. The peak position of the interference light intensity sequence indicating the change is detected (S4). Then, the detected peak position of each measurement position is displayed and output as the height at the measurement point (S5).

  FIG. 4 is a flowchart showing an example of processing for obtaining the peak position from the interference light intensity sequence at each position, and FIG. 5 is a diagram showing the interference light intensity sequence for explaining the processing. In this process, a predetermined geometric element (for example, a straight line or a curve) A is applied to the interference light intensity sequence obtained by moving and scanning the reference surface (S11). Here, the number of data of the interference light intensity sequence to which the geometric element is applied is a predetermined number or more. Specifically, for example, as shown in FIG. 5, fitting is performed on data in a section having a length substantially divided into two from the start to the end of the burst-like data string. This is because the average level cannot be detected if the number of data is too small. Next, the obtained geometric element A is shifted in the plus direction and minus direction of the intensity axis, respectively, and threshold levels B and C are set (S12). The interference light intensity exceeding the threshold levels B and C is obtained as a peak position candidate point (S13). Then, the center of gravity of the region where the peak position candidate points are most dense is obtained as the peak position P (S14). Alternatively, the peak position is set using the degree of deviation of the threshold range as an index within the region. By such processing, the peak position P can be obtained at high speed by reducing the number of processing points.

[Second Embodiment]
FIG. 6 is a flowchart showing a peak position detecting method in the shape measuring apparatus according to the second embodiment of the present invention, and FIG. 7 is a diagram showing an interference light intensity sequence for explaining the processing.

  In this embodiment, the interference light intensity sequence obtained by moving and scanning the reference surface is subjected to a smoothing process to obtain a smooth curve D as shown in FIG. 7 (S21). Next, the obtained smooth curve D is shifted in the positive and negative directions of the intensity axis, respectively, and threshold levels E and F are set (S22). The interference light intensity exceeding the threshold levels E and F is obtained as a peak position candidate point (S23). Then, the dispersed peak position candidate point areas are integrated into one area by, for example, Closing calculation (S24), and the center of gravity of the integrated area is obtained as the peak position P (S25). Here, the Closing calculation is an “expansion process” in which the point of interest is replaced with “1” if a point of “1” exists in the adjacent point of the point of interest, and a point of “0” in the adjacent point of the point of interest. If it exists, the “shrinkage process” for replacing the focus point with “0” is repeated several times, and the dispersed areas are gradually integrated. By such processing, the peak position P can be obtained at high speed by reducing the number of processing points.

[Third Embodiment]
FIG. 8 is a flowchart showing a peak position detecting method in the shape measuring apparatus according to the third embodiment of the present invention, and FIG. 9 is a diagram showing an interference light intensity sequence for explaining the processing.

  In this embodiment, geometric elements are fitted to the interference light intensity train obtained by moving and scanning the reference surface so that the amplitude is reproduced (S31). Here, in order not to be affected by noise, each data is weighted based on a distance from a geometric element that has been fitted in an initial state in advance (a geometric element that is smoother than a geometric element whose amplitude is reproduced). When fitting the geometric element whose amplitude is to be reproduced, a process such as performing a least-squares calculation considering the weight is performed. More specifically, a weight W [i] that becomes larger as the distance from the geometric element fitted in the initial state is closer is provided, and for each point X [i], Y [i],

[Equation 1]
Σ (W [i] * (a * X [i] + b * Y [i] + c) * (a * X [i] + b * Y [i] + c))-> min
Calculate a, b, and c. Next, for each end point of the fitted geometric element, the end point position (point H in FIG. 9) at which the absolute value sum of the inclinations of the two geometric elements connected at the end point is maximized is set as the peak position of the interference light intensity sequence. Obtained (S32).

[Fourth Embodiment]
FIG. 10 is a flowchart showing a peak position detection method in the shape measuring apparatus according to the fourth embodiment of the present invention, and FIG. 11 is a diagram showing an interference light intensity sequence for explaining the processing.

  In this embodiment, data is thinned out so as not to impair the shape of the interference light intensity sequence obtained by moving and scanning the reference surface (S41). Is determined as the peak position of the interference light intensity sequence (S42).

[Fifth Embodiment]
The peak position detection method described in the first to fourth embodiments can be applied to a case where a plurality of peak positions are detected such as film thickness measurement. For example, a region including one peak is extracted based on the above-described method, and the maximum position is calculated as the peak position based on each evaluation amount for each region. For example, as shown in FIG. 12, when the third embodiment is applied, areas where the absolute value of the slope of the fitted geometric element is greater than a certain threshold value are calculated, and the areas are connected and integrated by a closing operation or the like. Then, the peak positions P1 and P2 may be calculated from the center of gravity position of each integrated region.

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Collimator lens, 3 ... Beam splitter, 4 ... Work, 5 ... Reference board, 6 ... Drive means, 7 ... Imaging lens, 8 ... CCD camera, 9 ... Image memory, 10 ... Arithmetic processing part, DESCRIPTION OF SYMBOLS 11 ... Input part, 12 ... Output part, 13 ... Display part, 14 ... Encoder.

Claims (8)

A light source having a broadband spectrum;
The light from the light source is guided to the object to be measured and the reference surface, and the light reflected from the object to be measured and the reference surface is synthesized, and the first optical path length from the light source to the object to be measured and the light source An optical system for generating an interference light intensity distribution image indicating an interference light intensity corresponding to each measurement position in the measurement surface of the measurement target, which varies depending on a difference in optical path length from the second optical path length to the reference surface;
Imaging means for imaging the interference light intensity distribution image output from the optical system;
Optical path length difference changing means for changing an optical path length difference between the first optical path length and the second optical path length;
Image storage means for sequentially storing an interference light intensity distribution image picked up by the image pickup means and changing with a change in the optical path length difference;
A peak value is obtained from an interference light intensity sequence indicating a change in interference light intensity accompanying a change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, and the peak value is obtained. In a shape measuring apparatus comprising: a calculating means for obtaining a position in the optical axis direction at each measurement position of the measurement target;
The calculation means sets a predetermined threshold level in the positive / negative direction of the intensity axis from the center of intensity of the interference light intensity sequence, sets the interference light intensity data exceeding the threshold level as a peak position candidate, and the most peak candidate point is A shape measuring apparatus characterized in that the center of gravity of a dense area is obtained as a peak value of an interference light intensity sequence. The computing means is
Applying a geometric element to data exceeding a predetermined number in the interference light intensity sequence, and shifting the obtained geometric element in the positive and negative directions of the intensity axis of the interference light intensity sequence to set the threshold level. The shape measuring apparatus according to claim 1, characterized in that: The computing means is
A smoothing process is performed on the interference light intensity sequence to obtain a smoothing curve, and the obtained smoothing curve is shifted in the positive and negative directions of the intensity axis of the interference light intensity sequence to set the threshold level. The shape measuring apparatus according to claim 1. A light source having a broadband spectrum;
The light from the light source is guided to the object to be measured and the reference surface, and the light reflected from the object to be measured and the reference surface is synthesized, and the first optical path length from the light source to the object to be measured and the light source An optical system for generating an interference light intensity distribution image indicating an interference light intensity corresponding to each measurement position in the measurement surface of the measurement target, which varies depending on a difference in optical path length from the second optical path length to the reference surface;
Imaging means for imaging the interference light intensity distribution image output from the optical system;
Optical path length difference changing means for changing an optical path length difference between the first optical path length and the second optical path length;
Image storage means for sequentially storing an interference light intensity distribution image picked up by the image pickup means and changing with a change in the optical path length difference;
A peak value is obtained from an interference light intensity sequence indicating a change in interference light intensity accompanying a change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, and the peak value is obtained. In a shape measuring apparatus comprising: a calculating means for obtaining a position in the optical axis direction at each measurement position of the measurement target;
The calculation means applies a geometric element so that the amplitude is reproduced with respect to the interference light intensity sequence, and for each end point of the applied geometric element, an absolute value of an inclination of two geometric elements connected at the end point A shape measuring apparatus characterized in that an end point position where the sum is maximum is obtained as a peak value of the interference light intensity sequence. A light source having a broadband spectrum;
The light from the light source is guided to the object to be measured and the reference surface, and the light reflected from the object to be measured and the reference surface is synthesized, and the first optical path length from the light source to the object to be measured and the light source An optical system for generating an interference light intensity distribution image indicating an interference light intensity corresponding to each measurement position in the measurement surface of the measurement target, which varies depending on a difference in optical path length from the second optical path length to the reference surface;
Imaging means for imaging the interference light intensity distribution image output from the optical system;
Optical path length difference changing means for changing an optical path length difference between the first optical path length and the second optical path length;
Image storage means for sequentially storing an interference light intensity distribution image picked up by the image pickup means and changing with a change in the optical path length difference;
A peak value is obtained from an interference light intensity sequence indicating a change in interference light intensity accompanying a change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, and the peak value is obtained. In a shape measuring apparatus comprising: a calculating means for obtaining a position in the optical axis direction at each measurement position of the measurement target;
The arithmetic means thins out the data so as not to impair the shape of the interference light intensity sequence, and positions the highest density of data for the thinned data in the peak of the interference light intensity sequence. A shape measuring device characterized by being obtained as a value. The light from the light source having a broadband spectrum is guided to the measurement target and the reference surface, and the light reflected from the measurement target and the reference surface is synthesized, and the first optical path length from the light source to the measurement target is obtained. An interference light intensity distribution image indicating the interference light intensity corresponding to each measurement position in the measurement surface of the measurement target that changes due to the optical path length difference from the light source to the second optical path length from the light source to the reference surface is obtained by the imaging unit. Imaging step;
Sequentially storing the interference light intensity distribution image in the image storage means while changing the optical path length difference between the first optical path length and the second optical path length by the optical path length difference changing means;
From the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means, from the center of the intensity to the positive / negative direction of the intensity axis Setting a predetermined threshold level in
Obtaining interference light intensity data exceeding the threshold level as a peak position candidate;
Obtaining the center of gravity of the region where the peak position candidates are most dense as the peak value of the interference light intensity sequence;
Obtaining the obtained peak value as a position in the optical axis direction at each measurement position of the object to be measured. The light from the light source having a broadband spectrum is guided to the measurement target and the reference surface, and the light reflected from the measurement target and the reference surface is synthesized, and the first optical path length from the light source to the measurement target is obtained. An interference light intensity distribution image indicating the interference light intensity corresponding to each measurement position in the measurement surface of the measurement target that changes due to the optical path length difference from the light source to the second optical path length from the light source to the reference surface is obtained by the imaging unit. Imaging step;
Sequentially storing the interference light intensity distribution image in the image storage means while changing the optical path length difference between the first optical path length and the second optical path length by the optical path length difference changing means;
The amplitude is reproduced with respect to the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means. Applying a geometric element;
For each end point of the fitted geometric element, obtaining the end point position at which the sum of the absolute values of the inclinations of the two geometric elements connected at the end point is maximum as the peak value of the interference light intensity sequence;
Obtaining the obtained peak value as a position in the optical axis direction at each measurement position of the object to be measured. The light from the light source having a broadband spectrum is guided to the measurement target and the reference surface, and the light reflected from the measurement target and the reference surface is synthesized, and the first optical path length from the light source to the measurement target is obtained. An interference light intensity distribution image indicating the interference light intensity corresponding to each measurement position in the measurement surface of the measurement target that changes due to the optical path length difference from the light source to the second optical path length from the light source to the reference surface is obtained by the imaging unit. Imaging step;
Sequentially storing the interference light intensity distribution image in the image storage means while changing the optical path length difference between the first optical path length and the second optical path length by the optical path length difference changing means;
Data so as not to damage the shape of the interference light intensity sequence indicating the change in the interference light intensity accompanying the change in the optical path length difference at each measurement position of the interference light intensity distribution image stored in the image storage means. Decimation process,
For the thinned data, a step of obtaining a position having the highest data density as a peak value of the interference light intensity sequence;
Obtaining the obtained peak value as a position in the optical axis direction at each measurement position of the object to be measured.

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