JP2016024060A - Measurement condition determination method and measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measurement condition determination method that is advantageous in simply suppressing reduction in measurement accuracy.SOLUTION: A method of determining a condition measuring a position of a measured surface by projection/reception of a light ray comprises: a first process S101 of projecting a light ray to the measured surface at a plurality of angles of incidence, and acquiring a measurement value by receiving reflection light from the measured surface for each angle of incidence; a second process S102 of obtaining information about coarseness of the measured surface using the measurement value acquired in the first process S101; and a third process S103 of determining a measurement condition satisfying target measurement accuracy using the information about the coarseness obtained in the second process S102.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a measurement condition determination method and a measurement apparatus.

  By measuring the position (three-dimensional coordinates) of the surface (test surface) of a workpiece using a precision instrument part such as a printer or camera or a mold used when manufacturing them as a workpiece (test object) A three-dimensional shape measuring apparatus that acquires a three-dimensional shape of a workpiece is known. Such a measurement apparatus has a measurement head for acquiring three-dimensional coordinates, and the measurement head has a contact-type or non-contact-type measurement probe using a light beam (laser) or the like. Among these, when a non-contact probe is used, the roughness of the test surface may affect the measurement accuracy. Therefore, as one method of satisfying the target accuracy, there is a method of averaging measurement results for different regions on the surface to be examined. Patent Document 1 discloses a method for improving measurement accuracy by using a signal obtained by integrating the acquired light amounts at a plurality of measurement points on a surface to be measured.

Japanese Patent No. 4216679

  In the technique disclosed in Patent Document 1, the method of determining the integration region is not considered, and the integration region for achieving the target accuracy is not optimized. In order to optimize the integration area, it is necessary to determine the size of the average area on the test surface that satisfies the target accuracy, and for this purpose, roughness information on the test surface is required. However, when actually measuring the roughness of the test surface in order to obtain the roughness information of the test surface, the measurement probe used for the shape measurement as described above is generally used for roughness measurement. Is unsuitable. This is because a spot diameter of sub μm to several μm is required for roughness measurement, but the minimum spot diameter of the measurement probe is limited and it is difficult to make such a small spot diameter. . On the other hand, it is conceivable to make the spot diameter of the measurement probe variable, or to configure the roughness measurement probe separately so that the roughness can be measured, but the apparatus configuration becomes complicated and expensive. .

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a measurement condition determination method that is advantageous for easily suppressing a decrease in measurement accuracy, for example.

  In order to solve the above problems, the present invention is a method for determining a condition for measuring the position of a test surface by projecting and receiving a light beam, projecting the light beam to the test surface at a plurality of incident angles, and A first step of receiving reflected light from the test surface for each angle and acquiring a measurement value, and a second step of obtaining information relating to the roughness of the test surface using the measurement value acquired in the first step And a third step of determining a measurement condition satisfying a target measurement accuracy using information on roughness obtained in the second step.

  According to the present invention, for example, it is possible to provide a measurement condition determination method that is advantageous for easily suppressing a decrease in measurement accuracy.

It is a figure which shows the structure of the measuring device which concerns on one Embodiment of this invention. It is a flowchart which shows the flow of the determination process of the measurement condition which concerns on one Embodiment. It is a graph which shows an example of the incident angle characteristic of AC intensity by actual measurement. It is a graph which shows an example of the incident angle characteristic and PSD characteristic of reflected light intensity. It is a flowchart which shows the flow of the production process of a correlation database. It is a figure explaining the relationship between the surface roughness of a workpiece | work, and measurement accuracy. It is a flowchart which shows the flow of the measurement condition derivation | leading-out process of step S103. It is a graph which shows an example of the measurement conditions which can be determined in Step S303.

  Hereinafter, a method for determining a measurement condition according to an embodiment of the present invention will be described with reference to the drawings and the like for a mode for carrying out the invention.

  The measurement condition determination method according to the present embodiment can be employed in, for example, a measurement apparatus using light (laser beam). Hereinafter, it is assumed that a measurement apparatus to which the measurement condition determination method according to the present embodiment can be applied is a three-dimensional shape measurement apparatus. The three-dimensional shape measuring apparatus is, for example, a part or a mold for manufacturing the part as a work (test object), and measures a position of the test surface that is the surface of the work in a non-contact manner. It is an apparatus which acquires the three-dimensional shape of a workpiece | work by repeating with respect to these measurement points.

  FIG. 1 is a schematic diagram illustrating a configuration of a measurement apparatus 100 according to the present embodiment. The measurement apparatus 100 can perform three-dimensional measurement by causing the measurement head 34 to project and receive light with respect to the workpiece W while relatively moving the measurement head 34 and the workpiece W. In each of the following drawings, an X axis and a Y axis orthogonal to each other are taken in a plane in a state where the workpiece W is placed, and a Z axis is taken perpendicular to the XY plane.

  The measurement apparatus 100 includes a surface plate 31, an XYZ stage 32 supported on the surface plate 31, a measurement head 34 held on the XYZ stage 32 via a rotation mechanism 33, and the control unit 5. The XYZ stage 32 enables the measurement head 34 to move to each axis of XYZ. The rotation mechanism 33 is a variable mechanism that changes the posture by rotating the measurement head 34. Although not shown, the rotation mechanism 33 includes a first rotation axis corresponding to the Y axis and a second rotation axis corresponding to the Z axis. Have two rotation axes. The measurement head 34 includes a non-contact probe 6, a galvanometer mirror 7, and an AC detector 8. As the non-contact probe 6, a point scanning method in which a spot-like light (spot light) 10 is incident on the surface to be measured and is scanned and measured, or a line-shaped light is incident on the surface to be examined. A probe such as a light cutting method that scans and measures the surface to be measured can be used. Hereinafter, in the present embodiment, the non-contact probe 6 will be described as adopting an interference length measurement method and a point scanning method. The galvanometer mirror 7 is a variable mechanism that varies the angle of the traveling direction of the light projected from the non-contact probe 6 or the light (reflected light) reflected by the test surface. The AC detector 8 detects interference light between reflected light and reference light reflected by a reference surface (not shown), and acquires an AC component of the interference light. The control unit 5 described later calculates the AC intensity of the interference light from the AC component obtained here. The AC detector 8 may be replaced with an interference light detector (not shown) in the non-contact probe 6 without being separated from the non-contact probe 6, or the AC intensity of the interference light may be directly measured. It may be detected. In the present embodiment, the XYZ stage 32 is a so-called portal type as shown in FIG. 1, but may be a stage having another configuration called an articulated type or an arm type.

  The control unit 5 is composed of, for example, an information processing device (computer), and performs calculation of drive amounts of each drive system in the measurement device 100 and drive control based thereon, measurement operation control of the measurement head 34, storage of measurement data, and the like. Run. In particular, the control unit 5 can execute the measurement condition determination method according to the present embodiment as a program, for example. In addition, about the process which concerns on calculation among the determination processes of measurement conditions, for example, it is performed with the information processing apparatus which is an apparatus different from the measuring apparatus 100, and the calculation result obtained as a result is applied to the control part 5. Also good.

  Next, a method for determining measurement conditions applicable to the measurement apparatus 100 according to the present embodiment will be described. In particular, in this embodiment, when determining the measurement conditions, the roughness of the test surface is taken into account, and when determining the roughness, the measurement head 34 in the measurement apparatus 100 is used. Hereinafter, when simply expressed as “roughness”, it means the roughness (surface roughness) of the test surface.

  FIG. 2 is a flowchart showing a flow of steps for determining the measurement conditions of the measurement apparatus 100 in the present embodiment. When starting the measurement condition determining step, the control unit 5 first measures the AC intensity as the first step (step S101). Specifically, the control unit 5 causes the measurement head 34 to make the measurement light incident while changing the incident angle at a position where roughness information on the surface to be obtained is desired to be obtained, and obtain reflected light. And the control part 5 specifies AC intensity | strength of interference light using the AC detector 8 grade | etc., From the interference light produced | generated based on reflected light. As a method of changing the incident angle of the measurement light, for example, the rotation mechanism 33 directly changes the angle of the measurement head 34 or the galvanometer mirror in the measurement head 34 while maintaining the angle of the measurement head 34. 7 has a method of changing. FIG. 3 is a graph showing an example of the AC intensity of interference light with respect to the incident angle of measurement light (hereinafter referred to as “incident angle characteristic of measured AC intensity”), which is a measurement value obtained by the measurement in step S101. is there.

  Next, as the second step, the control unit 5 uses the incident angle characteristic of the AC intensity obtained in the actual measurement obtained in step S101, and the power distribution for each unit frequency on the surface to be measured as information on roughness ( PSD) is determined (step S102). PSD is an abbreviation for Power Spectral Density. FIG. 4 is a graph showing characteristic information included in the correlation database referred to when the PSD is determined. 4A is a graph showing an example of the AC intensity of interference light (hereinafter referred to as “incident angle characteristic of reflected light intensity”) with respect to the incident angle of measurement light, which is acquired before actual measurement. . FIG. 4B is a graph showing an example of the PSD of the test surface with respect to the frequency (hereinafter referred to as “PSD characteristics”) acquired before actual measurement. Note that Legend 1 and Legend 2 shown in FIG. 4 (a) correspond to Legend 1 and Legend 2 shown in FIG. 4 (b), respectively. The control unit 5 acquires the incident angle characteristic and PSD characteristic of the reflected light intensity shown in FIG. 4 in advance as a correlation database, and collates the measured incident angle characteristic of the AC intensity with the correlation database. Determine the PSD.

  Here, a method of creating a correlation database (incident angle characteristics and PSD characteristics of reflected light intensity) referred to in step S102 will be described. FIG. 5 is a flowchart showing the flow of creating a correlation database. First, when starting the correlation database creation step, the controller 5 sets a plurality of test surface shapes to be assumed (step S201). Next, as a step of creating the incident angle characteristic of the reflected light intensity, the control unit 5 convolves the plurality of test surface shapes set in step S201 with the light amount distribution within the spot diameter of the non-contact probe 6. Is calculated. And the control part 5 calculates the complex amplitude of reflected light further by performing a Fourier-transform on the said calculation result (step S202). Next, the control unit 5 calculates the complex amplitude of the interference light from the complex amplitude of the reflected light obtained in step S202 and the complex amplitude of the reference light calculated separately (step S203). Thereby, the control part 5 can obtain | require the incident angle characteristic of reflected light intensity as shown to Fig.4 (a). On the other hand, as a step of creating the PSD characteristic, the control unit 5 performs Fourier transform on the test surface shape after step S201 (step S204). Thereby, the control part 5 can obtain | require a PSD characteristic as shown in FIG.4 (b). As a result, the control unit 5 can create a correlation database in which the incident angle characteristic of the reflected light intensity and the PSD characteristic are associated with each other.

  When calculating the complex amplitude of the reflected light or the complex amplitude of the reference light, not only convolution and Fourier transform, but also other scalar theories such as Fresnel diffraction using Maxwell's equations, various vector theories such as vector diffraction theory, etc. Techniques can be used. In particular, when calculating the complex amplitude of the reflected light, the material of the workpiece W may be taken into consideration and the refractive index and the absorption coefficient may be used in combination.

  In addition to creating the correlation database, there is a method that uses actually measured values in addition to the above method. In this case, first, work samples of various materials and roughness are prepared, and the control unit 5 acquires the incident angle characteristic of AC intensity obtained by actually measuring the surface of the work sample. Next, the control unit 5 refers to the roughness information of the work sample obtained in advance, and acquires PSD characteristics. And the control part 5 hold | maintains the incident angle characteristic and PSD characteristic of acquired AC intensity as a correlation database. Furthermore, there may be a method in which the correlation database is not prepared in advance. In this case, for example, the PSD may be directly calculated by various calculation methods from the incident angle characteristics of the AC intensity obtained in the actual measurement obtained in step S101.

  Furthermore, in step S102, PSD is used as information on roughness. However, the present invention is not limited to this. For example, arithmetic average roughness or complex amplitude of the test surface is calculated as information on other roughness. It can also be used.

  Returning to FIG. 2, next, as a third step, the control unit 5 derives a measurement condition that satisfies the target accuracy based on the PSD determined in step S102 (step S103). Specifically, the control unit 5 is a typical measurement condition related to the roughness of the surface to be measured, that is, the maximum incident angle and the size of the region (averaged region) where the measurement results are averaged and the time for averaging ( At least one of the averaging times) is determined. First, the relationship between the roughness of the test surface and the measurement conditions will be described. FIG. 6 is a diagram for explaining the relationship between the roughness of the test surface measured by the non-contact probe 6 (measurement head 34) and the measurement accuracy.

  FIG. 6A is a schematic plan view showing various dimensions at the time of measurement on the test surface. The non-contact probe 6 scans the spot 21 from the first measurement point 22 to the second measurement point 23 on the test surface. At this time, an averaged measurement result is obtained for the integrated area 24 on the test surface (the area to be averaged, which is scanned with the spot 21 displayed with diagonal lines in FIG. 6A), and the averaged result is obtained. The length 25 of the area is referred to as “averaged length”.

  FIG. 6B is a graph showing an example of speckle-induced error with respect to the average length at a certain spot diameter and incident angle. Here, the speckle-induced error is a length measurement that occurs due to speckles that occur when light irradiating a rough surface (corresponding to the test surface here) becomes reflected light with different phases and interferes with each other. Refers to errors. As shown in FIG. 6B, the speckle-induced error increases as the roughness of the test surface increases. Therefore, in order to obtain the target accuracy, an average length corresponding to the target accuracy is required. In the description here, the degree of speckle-induced error reduction is expressed by a length (average length 25) for simplicity, but strictly depends on the size of the average area.

  FIG. 6C is a graph showing an example of the phase detection error with respect to the averaging time at a certain spot diameter and incident angle. Here, the averaging time refers to the time during which the averaging length 25 is measured. As shown in FIG. 6C, the phase detection error increases as the roughness of the test surface increases. Therefore, in order to obtain the target accuracy, an averaging time corresponding to the target accuracy is required.

  As for the maximum incident angle with respect to the test surface, the rougher the test surface, the larger the error at the same incident angle, so the maximum incident angle that satisfies the same accuracy becomes smaller.

  FIG. 7 is a flowchart showing a detailed flow of the measurement condition derivation step in step S103. When starting the measurement condition deriving step, the controller 5 first acquires various information used in the subsequent steps (step S301). Information to be acquired includes user input information, device information, and work information. The user input information is information on the material and target accuracy of the workpiece W, for example, designated in advance by the user and stored in the measuring apparatus 100. The device information is information on the restriction range of the average length 25, and is stored in advance as device-specific information, for example. Here, the “restriction of the average length” means an allowable range of the average length determined by the resolution necessary for the non-contact probe 6 to reproduce the spatial frequency of the surface to be measured, an aliasing allowable error with respect to a high frequency component, and the like. Say. The work information is information such as the incident angle characteristic of the AC intensity measured in step S101 and the PSD determined in step S102.

  Next, based on the target accuracy information obtained in step S301, the control unit 5 determines an accuracy distribution value (allocation) between speckle-induced errors and phase detection errors in the target accuracy (step S302). ). As a method for determining the distribution value, for example, there are the following two methods. The first method is a method in which the control unit 5 determines in advance a distribution value serving as a reference for, for example, a target accuracy of 1 μm, and determines a proportional multiple of the target accuracy specified by the user. The second method is a method in which the relationship between the roughness and the distribution value is acquired in advance, and the distribution value is determined based on the PSD determined in step S102 and the relationship.

  Next, the control part 5 performs a calculation based on the information obtained in steps S301 and S302, and specifically determines measurement conditions (step S303). FIG. 8 is a graph showing an example of measurement conditions that can be determined in step S303.

  FIG. 8A shows an average with respect to the incident angle obtained using the roughness information obtained from the PSD characteristics obtained in step S301 and the information on the distribution value to the speckle-induced error determined in step S302. It is a graph which shows an example of conversion length. First, the control unit 5 uses the roughness information obtained from the PSD characteristics and the information of the distribution value to the speckle-induced error, with reference to FIG. 6B, the speckle at a certain incident angle and roughness. An average length that is less than or equal to the distribution value for the error is calculated. The control unit 5 executes such a calculation for each incident angle, and obtains an average length range. The range of the average length obtained here is a range (region) beyond the line 51 in FIG. In FIG. 8 (a), the line 51 is interrupted at the maximum incident angle θLmax. This is because the average length solution that is less than or equal to the distribution value to the speckle-induced error is obtained at an incident angle equal to or greater than θLmax. Because it is not possible. On the other hand, in step S301, the control unit 5 has already acquired information on the restriction range of the average length, and this restriction range is a range (area) sandwiched between the line 52 and the line 53 in FIG. ). Therefore, the range that can be taken by the incident angle and the average length satisfying the distribution value to the speckle-induced error is the painted portion 54 shown in FIG.

  FIG. 8B shows an average for the incident angle obtained using the roughness information obtained from the PSD characteristics obtained in step S301 and the information on the distribution value to the phase detection error determined in step S302. It is a graph which shows an example of time. First, the control unit 5 uses the roughness information obtained from the PSD characteristics and the information on the distribution value to the phase detection error, with reference to FIG. 6C, the phase detection error at a certain incident angle and roughness. An averaging time that is less than or equal to the distribution value is calculated. The control unit 5 executes such a calculation for each incident angle, and obtains an average time range. The range of the averaging time obtained here is a range (area) beyond the line 55 in FIG. 8B. In FIG. 8 (b), the line 55 is interrupted at the maximum incident angle θTmax. This is because an incident time equal to or greater than θTmax provides a solution for the averaging time that is less than or equal to the distribution value for the phase detection error. Because there is no. Therefore, the range that can be taken by the incident angle and the averaging time that satisfy the distribution value to the phase detection error is the filled portion 56 shown in FIG. In FIG. 8A and FIG. 8B, two maximum incident angles θLmax and θTmax appear, but the finally selected maximum incident angle is the smaller of θLmax or θTmax. .

  Thereby, in the measurement condition deriving step of step S103, a possible range of the incident angle satisfying the target accuracy and the average length or the average time is determined. Here, various optimization methods can be used to select one suitable value from the range of the determined incident angle and average length or average time. For example, when it is desired to increase the maximum incident angle, the distribution value between the speckle-induced error and the phase detection error may be changed in step S302 so that θLmax and θTmax are maximum and equal. When it is desired to shorten the total measurement time of the workpiece W, the measurement time of the workpiece W is measured based on the design shape data of the workpiece W with respect to the measurement condition in which the distribution value to the speckle-induced error and phase detection error is changed And the measurement condition that shortens the measurement time may be determined. When changing the distribution value between the speckle-induced error and the phase detection error, it is desirable that the square sum root thereof does not exceed the square sum root in the target accuracy.

  As described above, the measurement condition determined by the determination method according to the present embodiment takes into account the roughness of the surface to be measured in order to optimize the integration region for achieving the target accuracy. Therefore, by applying this measurement condition to the measurement apparatus 100 that measures the position of the test surface using light in particular, the occurrence of various errors in the measurement apparatus 100 can be suppressed, that is, the measurement accuracy can be maintained with high accuracy. It becomes possible. Furthermore, in this embodiment, when determining the measurement conditions, the roughness information of the test surface to be referenced is derived by measuring using the measurement probe (non-contact probe 6) of the measurement apparatus 100 that is actually applied. To do. Therefore, in particular, in order to directly measure the roughness of the surface to be measured, it is not necessary to modify the measurement probe so as to reduce the spot diameter, or to install a probe for measuring the roughness separately.

  As described above, according to the present embodiment, it is possible to provide a measurement condition determination method that is advantageous for easily suppressing a decrease in measurement accuracy. In addition, by applying the measurement condition determined by the determination method according to the present embodiment, it is possible to provide a measurement apparatus that is advantageous for easily suppressing a decrease in measurement accuracy.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

5 Control unit 6 Non-contact probe 7 Galvano mirror 33 Rotating mechanism 100 Measuring device

Claims (10)

A method for determining conditions for measuring the position of the surface to be measured by light projection and reception,
A first step of projecting the light beam to the test surface at a plurality of incident angles, receiving reflected light from the test surface for each incident angle, and obtaining a measurement value;
A second step of obtaining information on the roughness of the test surface using the measurement value acquired in the first step;
A third step of determining a measurement condition that satisfies a target measurement accuracy using the information on the roughness obtained in the second step;
A method for determining a measurement condition characterized by comprising:   The method for determining a measurement condition according to claim 1, wherein the information on the roughness obtained in the second step is a PSD (Power Spectral Density) of the test surface.   The measurement condition determination method according to claim 1, wherein the information on the roughness obtained in the second step is an arithmetic average roughness of the test surface.   The measurement condition determination method according to claim 1, wherein the information on the roughness obtained in the second step is a complex amplitude of the surface to be measured.   The measured value acquired in the first step is an AC intensity obtained by detecting interference light between the reflected light and the reference light reflected by a reference surface. A method for determining a measurement condition according to any one of the above items.   In the second step, the AC intensity obtained in the first step is based on a correlation database between the incident angle characteristics of the AC intensity of the interference light acquired in advance and the information on the roughness. 6. The method for determining a measurement condition according to claim 5, wherein information on the roughness is obtained by collating an incident angle characteristic with the correlation database.   The correlation database is created by an actual measurement value obtained by measuring the test surface of an actual test object or a calculation based on roughness of the test surface. A method for determining the described measurement conditions.   The measurement condition determination method according to claim 7, wherein the calculation based on the roughness is executed using a Maxwell equation.   The measurement condition determined in the third step is at least one of a maximum incident angle of the light beam with respect to the test surface, a size of a region for averaging the measurement results, and a time for averaging the measurement results. 9. The method for determining a measurement condition according to claim 1, wherein the measurement condition is determined. A measuring device that measures the position of a surface to be measured by light projection and reception,
A measurement probe for projecting and receiving the light beam;
A variable mechanism that scans the light beam and changes an incident angle of the light beam with respect to the test surface;
A control unit for controlling the operation of the measurement probe or the variable mechanism based on measurement conditions;
Have
By the measurement probe and the variable mechanism, the light beam is projected onto the test surface at a plurality of incident angles, and the reflected light from the test surface for each incident angle is received to obtain a measurement value,
The control unit obtains information on the roughness of the test surface using the measurement value, and determines a measurement condition that satisfies a target measurement accuracy using the information on the obtained roughness.
A measuring device characterized by that.

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