Method and apparatus for measuring surface shape

Abstract

To provide a surface shape measurement method and a surface shape measurement device which can shorten the measurement time.SOLUTION: A surface shape measurement method measures a surface shape of a measurement object by combining N stack images captured while scanning a measuring head in an optical axis direction. The method obtains, from an integration curve obtained by integrating a height-dependent signal based on a value of an N point indicating a change in a pixel value along the axial direction for a common position in the N stack images, a starting point side noise part straight line that approximates a starting point side noise part being a range where inclination is small in comparison to the vicinity of a measurement object surface on the starting point side relative to the measurement object surface, an ending point side noise part straight line that approximates an ending point side noise part being a range where inclination is small in comparison to the vicinity of the measurement object surface on the ending point side relative to the measurement object surface, and a surface vicinity straight line that approximates a surface vicinity part in the vicinity of the measurement object surface, and obtains a position in a Z-axis direction of the measurement object surface based on these straight lines.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a measurement method and a surface shape measurement device for measuring the surface shape of a measurement object by synthesizing a plurality of stack images captured while scanning a measurement head in the optical axis direction.

Background Art

[0002] Conventionally, a surface shape measurement device that precisely measures the surface shape of a measurement object using a plurality of stack images captured while scanning a measurement head in the optical axis direction is known.

[0003] As such a surface shape measurement device, for example, there is one that irradiates a measurement object with white light from a light source and uses the luminance information of interference fringes generated by light interference. In this surface shape measurement device, at the focus position where the optical path lengths of the reference optical path and the measurement optical path match, the peaks of the interference fringes of each wavelength overlap and the luminance of the synthesized interference fringes increases. Therefore, in the surface shape measurement device, while changing the optical path length of the reference optical path or the measurement optical path, an interference fringe image showing the two-dimensional distribution of the interference light intensity is photographed by an imaging element such as a CCD camera, and the focus position where the intensity of the interference light peaks at each measurement position within the photographed field of view is detected. Thus, the height of the measurement surface (that is, the surface of the measurement object) at each measurement position can be measured, and the surface shape of the measurement object can be measured.

[0004] In addition to those using the luminance information of interference fringes, there are also those that obtain the focusing position (height) from the change in contrast at each pixel position (for example, see Patent Document 2), those that project a periodic pattern onto the measurement object and obtain the position where the contrast of the stripe pattern is maximized (for example, see Patent Document 3), and so on.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] In surface shape measuring devices that use brightness information from interference fringes generated by the interference of light as described above, the brightness of the interference light changes with a period approximately equal to the wavelength of the light causing the interference. Therefore, it is necessary to repeatedly capture interference fringe images while changing the optical path length of the reference optical path or the measurement optical path at intervals sufficiently finer than the wavelength. The surface shape measuring device then analyzes several hundred to a thousand of the accumulated interference fringe images to determine the height of the measurement surface at each pixel position. This analysis includes a rough peak detection process that roughly detects the height at which the intensity of the interference light is maximum, and a fine peak detection process that determines the detailed height.

[0007] In the rough peak detection process, for each pixel position constituting the interference fringe image, a signal waveform showing the change in brightness value with respect to the height position is obtained. Signal processing such as squaring, integration, and smoothing differentiation is applied to the signal waveform, and then a peak search is performed to find the height at which the intensity of the interference light is maximum. In the fine peak detection process, the detailed height of the measurement surface is determined by focusing on the phase of the signal waveform at each pixel position constituting the interference fringe image.

[0008] Thus, conventional surface shape measuring devices required a significant amount of time for a single surface shape measurement because they had to capture numerous interference fringe images and then perform analysis on all of them. Furthermore, the analysis performed across a large number of accumulated images required a vast amount of work memory and high processing power.

[0009] Surface shape measuring devices that use a focus position detection method to determine the position (height) of focus from the change in contrast at each pixel position, and surface shape measuring devices that project a periodic pattern onto the object to be measured and determine the position where the contrast of the striped pattern is maximum, both require similar analytical processing to determine the surface shape of the object to be measured by synthesizing multiple images captured while scanning the measuring head in the optical axis direction, and thus face similar challenges.

[0010] Therefore, the present invention aims to solve the above problems and provide a surface shape measurement method and surface shape measuring apparatus that can suppress the work memory and computational processing power required for analysis processing, and consequently shorten the measurement time. [Means for solving the problem]

[0011] To solve the above problems, the surface shape measurement method according to the present invention measures the surface shape of an object to be measured by synthesizing N stacked images captured while scanning a measuring head in the optical axis direction. In this measurement method, for a common position in the N stacked images, a height-dependent signal based on the values ​​of N points indicating the change in pixel values ​​along the axis is integrated to obtain an integral curve consisting of the values ​​of N points. From this integral curve, a starting-side noise line is obtained that approximates the starting-side noise area, which is a range on the starting-point side of the object to be measured that has a smaller slope compared to the vicinity of the object to be measured; an ending-side noise line is obtained that approximates the ending-point noise area, which is a range on the ending-point side of the object to be measured that has a smaller slope compared to the vicinity of the object to be measured; and a surface vicinity line is obtained that approximates the surface vicinity area near the object to be measured. Based on the starting-side noise line, the ending-side noise line, and the surface vicinity line, the position of the object to be measured in the Z-axis direction is determined.

[0012] In the present invention, the measuring head may be an interferometer optical head that splits incoherent light emitted from a light source into reference light for a reference mirror and measurement light for the surface to be measured using a beam splitter, and acquires an interference fringe image generated by the optical path difference of the light reflected from each. In this case, the stacked image may be N (where N≧2) interference fringe images obtained by scanning the interferometer optical head with respect to the surface to be measured in the Z-axis direction along the optical axis of the interferometer optical head from the starting point to the ending point, and the height-dependent signal may be the square or absolute value of the interference signal consisting of N points that show the change in interference light intensity along the Z-axis direction.

[0013] Alternatively, in this invention, the measuring head may be an image optical head that captures a two-dimensional image of the object to be measured. In this case, the stacked image may be N (where N≧2) two-dimensional images obtained by scanning the image optical head with respect to the surface to be measured in the Z-axis direction along the optical axis of the image optical head from a starting point to an ending point, and the height-dependent signal may be a contrast curve consisting of N points showing the change in contrast along the Z-axis direction.

[0014] Alternatively, the present invention may further include a pattern projection unit that irradiates the surface of the object to be measured with projection light of a predetermined periodicity, and the measuring head may be an image optical head that captures a two-dimensional image of the object to be measured. In this case, the stacked image may be N (where N≧2) two-dimensional images obtained by scanning the image optical head with respect to the surface of the object to be measured in the Z-axis direction along the optical axis of the image optical head from a starting point to an ending point while the pattern projection unit is irradiating the surface of the object to be measured with projection light, and the height-dependent signal may be the square or absolute value of the N points that show the change in the intensity of the reflected light reflected from the surface of the object to be measured along the Z-axis direction.

[0015] In this invention, the nearest surface line is defined as the line with the maximum slope among the approximate lines for a predetermined number of consecutive points on the integral curve. For example, the approximate line can be found by applying the least squares method to all of the predetermined number of points. Alternatively, the approximate line can be defined as the line connecting the endpoints of a predetermined number of consecutive points.

[0016] In this invention, it is preferable to determine the starting noise line and the ending noise line under the constraint that the slopes of the starting noise line and the ending noise line are equal.

[0017] In this invention, it is preferable to determine the starting point noise line based on a predetermined number of points from the starting point of the integral curve, and to determine the ending point noise line based on a predetermined number of points from the ending point of the integral curve.

[0018] In this invention, the intersection point of an intermediate line having a slope obtained by averaging the slope of the starting noise line and the ending noise line, and an intercept obtained by averaging the intercepts of the starting noise line and the ending noise line, with a line near the surface is preferably set as the position in the Z-axis direction of the surface to be measured.

[0019] In this invention, after acquiring the first stacked image, analysis processing is performed on the stacked images up to M-1 while sequentially acquiring the Mth stacked image (where 2 ≤ M ≤ N), and after acquiring the Nth stacked image, analysis processing is performed on the stacked images from the first to the Nth image. The analysis process may include, for each position in the last acquired K stack image, an integral curve update process that finds the value of at least the Kth point from the starting point that constitutes the integral curve; a latest approximate line calculation process that finds an approximate line for a predetermined number of consecutive points that include the Kth point of the integral curve as the point closest to the endpoint; and a provisional surface proximity line update process that finds a provisional surface proximity line with the maximum slope among the approximate lines for a predetermined number of consecutive points up to the Kth point of the integral curve, and compares the provisional surface proximity line found for the K-1 points of the integral curve with the approximate line found in the latest approximate line calculation process, and sets the one with the larger slope as the new provisional surface proximity line. The provisional surface proximity line obtained in the provisional surface proximity line update process in the analysis process performed after acquiring the Nth stack image may be used as the surface proximity line.

[0020] Further, the surface shape measuring apparatus according to the present invention measures the surface shape of the measurement target surface of the object to be measured. The surface shape measuring apparatus divides the light irradiated from a light source that irradiates incoherent light into reference light to a reference mirror and measurement light to the measurement target surface by a beam splitter, and acquires an interference fringe image generated by the optical path difference of the light reflected from each by an image pickup device. An interferometer optical head, and an analysis unit that obtains the surface shape of the measurement target surface based on the interference fringe image acquired by the interferometer optical head. The interferometer optical head acquires N (where N≥2) interference fringe images while scanning the measurement target surface in the Z-axis direction along the optical axis of the interferometer optical head from the start point to the end point. The analysis means integrates the square or absolute value of an interference signal composed of N values indicating the change in the interference light intensity along the Z-axis direction for the common positions in the N interference fringe images acquired by the interferometer optical head, and obtains an integration curve composed of N values. From the integration curve, a starting point side noise portion straight line that approximates the starting point side noise portion, which is a range where no interference occurs on the starting point side of the measurement target surface, an end point side noise portion straight line that approximates the end point side noise portion, which is a range where no interference occurs on the end point side of the measurement target surface, and a surface vicinity straight line that approximates the interference portion, which is a range where interference occurs near the measurement target surface, are obtained, and based on the starting point side noise portion straight line, the end point side noise portion straight line, and the surface vicinity straight line, the position of the measurement target surface in the Z-axis direction is obtained.

Brief Description of the Drawings

[0021] [Figure 1] It is a perspective view showing the overall configuration of the surface shape measuring apparatus 1. [Figure 2] It is a schematic diagram showing the configuration of the interferometer optical head 152 together with the optical path. [Figure 3] It is a main part enlarged view showing the structure of the objective lens unit 22, the measurement optical path, and the reference optical path. [Figure 4] It is a block diagram showing the configuration of the computer main body 201. [Figure 5] FIG. 5(a) shows an example of an interference signal, and FIG. 5(b) shows an example of an integration curve w based on the interference signal. [Figure 6]This figure shows three straight lines (L1 to L3) that approximate the integral curve, an intermediate line L4 calculated from these lines, and the position Zcross of the surface being measured. [Figure 7] This flowchart shows an example of the procedure for measuring the surface shape in this embodiment. [Modes for carrying out the invention]

[0022] [First Embodiment] Hereinafter, a surface shape measuring device 1, which is a first embodiment of the surface shape measuring device 1 according to the present invention and combines an interference optical system and an image measuring device, will be described with reference to the drawings.

[0023] Figure 1 is a perspective view showing the overall configuration of a surface shape measuring device 1 according to the first embodiment. The surface shape measuring device 1 measures the surface shape of the object to be measured (workpiece) W. The surface shape measuring device 1 comprises a non-contact type image measuring machine 10 and a computer system 2 that drives and controls the image measuring machine 10 and performs necessary data processing. In addition to these, the surface shape measuring device 1 may also be appropriately equipped with a printer for printing out measurement results, etc.

[0024] The image measuring machine 10 comprises a stand 11, a sample stage 12, support arms 13a and 13b, an X-axis guide 14, and an imaging unit 15. As shown in Figure 1, the surface shape measuring device 1 is placed on a vibration isolation table 3 installed on the floor. The vibration isolation table 3 prevents vibrations from the floor from being transmitted to the surface shape measuring device 1 on the table. The vibration isolation table 3 may be either an active or passive type. The stand 11 is placed on the top plate of the vibration isolation table 3, and the stage 12 on which the workpiece W is placed is placed on it so that its upper surface is aligned with the horizontal plane as its base surface. In the following description, it will be assumed that the X and Y axes extend in directions parallel to the base surface of the stage 12, and the Z axis extends in a direction perpendicular to the base surface. The stage 12 is driven in the Y-axis direction by a Y-axis drive mechanism (not shown), and the workpiece W is movable in the Y-axis direction relative to the imaging unit. Support arms 13a and 13b, extending upward, are fixed to the center of both side edges of the mount 11, and an X-axis guide 14 is fixed so as to connect the upper ends of these support arms 13a and 13b. This X-axis guide 14 supports the imaging unit 15. The imaging unit 15 is driven along the X-axis guide 14 by an X-axis drive mechanism (not shown).

[0025] The imaging unit 15 includes an image optical head 151 for capturing a two-dimensional image of the workpiece W and an interferometer optical head 152 for measuring the surface shape of the workpiece W by optical interferometry. The workpiece W is measured at a measurement position set by the computer system 2 using either head. The measurement field of the image optical head 151 is usually set to be wider than that of the interferometer optical head 152, and the two heads can be switched between using control by the computer system 2. The image optical head 151 and the interferometer optical head 152 are supported by a common support plate to maintain a constant positional relationship, and are pre-calibrated to ensure that the measurement coordinate axes do not change before and after switching.

[0026] The image optical head 151, equipped with a CCD camera, illumination device, focusing mechanism, etc., captures a two-dimensional image of the workpiece W. The captured two-dimensional image data is input to the computer system 2.

[0027] Figure 2 is a schematic diagram showing the configuration of the interferometer optical head 152 along with its optical path. As will be described later, the interferometer optical head 152 acquires interference fringe images generated by the optical path difference of light reflected from the measurement optical path and the reference optical path using an image sensor. As shown in Figure 2, the interferometer optical head 152 is configured as a Michelson type interferometer and comprises a light emission unit 20, an illumination light guide unit 21, an objective lens unit 22, an imaging lens 24, an imaging unit 25, and a drive mechanism unit 26.

[0028] The light-emitting unit 20 is a light source that emits low-coherence (incoherent) light. Here, low-coherence light is, for example, light with a coherence length of about 100 μm or less. The light-emitting unit 20 has a wide bandwidth (for example, wavelengths of 500 to 800 nm) and outputs broadband light with low coherence that has many wavelength components. Examples of light-emitting units 20 include lamp light sources such as halogens, LEDs (Light Emitting Diodes), and SLDs (Super Luminescent Diodes). The light emitted by the light-emitting unit 20 is preferably white light, for example, but is not limited to this as long as it is low-coherence light.

[0029] The illumination light guide unit 21 includes a beam splitter 211 and a collimator lens 212. Light emitted from the light emission unit 20 is directed parallel to the beam splitter 211 via the collimator lens 212 from a direction perpendicular to the optical axis of the objective lens unit 22, and light is emitted from the beam splitter 211 along the optical axis, illuminating the objective lens unit 22 from above with a parallel beam.

[0030] The objective lens section 22 comprises an objective lens 221, a beam splitter 222, a reference mirror section 223, and the like. The reference mirror section 223 also includes a reference mirror 224 at a predetermined position. In the objective lens section 22, when a parallel beam is incident on the objective lens 221 from above, the incident light becomes focused light at the objective lens 221 and is incident on the reflective surface 222a inside the beam splitter 222.

[0031] The incident light is split by the beam splitter 222 into reflected light (reference light) traveling along the reference optical path in the reference mirror section 223 and transmitted light (measurement light) traveling along the measurement optical path where the workpiece W is placed. The reflected light is reflected by the reference mirror 224 and then reflected again by the reflective surface 222a of the beam splitter 222. Meanwhile, the transmitted light is focused and reflected by the workpiece W and passes through the reflective surface 222a of the beam splitter 222. The reflected light from the reference mirror 224 and the reflected light from the workpiece W are combined by the reflective surface 222a of the beam splitter 222 to form a composite wave.

[0032] The combined wave, formed at the reflective surface 222a of the beam splitter 222, becomes a parallel beam in the objective lens 221 and travels upward, passing through the illumination light guide 21 and entering the imaging lens 24 (dotted line in Figure 2). The imaging lens 24 focuses the combined wave and forms an interference fringe image on the imaging unit 25.

[0033] The imaging unit 25 is a CCD camera or the like consisting of a two-dimensional image sensor that constitutes the imaging means, and captures an interference fringe image of the composite wave (reflected light from the workpiece W and reflected light from the reference mirror 224) output from the objective lens unit 22. The interference fringe image captured by the imaging unit 25 corresponds to the stacked image in this invention.

[0034] Figure 3 is an enlarged view of the main part of the objective lens section 22. The drive mechanism section 26 corresponds to the optical path length variable means of the present invention, and moves the interferometer optical head 152 in the optical axis direction according to a movement command from the computer system 2. Figure 3 shows a state in which the optical path lengths of the reference optical path (dashed line) and the measurement optical path (solid line) are equal. During measurement, interference fringe images are taken while moving the interferometer optical head 152 in the optical axis direction (i.e., the Z axis direction), thereby acquiring many interference fringe images with different measurement optical path lengths. Interference occurs when the difference between the length of the measurement optical path and the length of the reference optical path is less than or equal to the coherent length from the light source, and the interference intensity is strongest (i.e., the contrast of the interference fringes is maximized) when the length of the measurement optical path matches the length of the reference optical path. In the above explanation, the case in which the interferometer optical head 152 is moved was used as an example, but the length of the measurement optical path may also be adjusted by moving the stage 12. Alternatively, the length of the reference optical path may be made variable by moving the reference mirror 224 in the optical axis direction (i.e., the left and right direction in Figure 3). Thus, in the interferometer optical head 152, the optical path length of either the reference optical path or the measurement optical path is made variable.

[0035] The interferometer optical head 152 is moved and scanned in the optical axis direction by the drive mechanism 26 under the control of the computer system 2, and the imaging unit 25 takes an image each time it moves a predetermined distance. The interference fringe images are sequentially transferred to and acquired by the computer system 2.

[0036] Returning to Figure 1, the computer system 2 comprises a computer main unit 201, a keyboard 202, a joystick box (hereinafter referred to as J / S) 203, a mouse 204, and a display 205. The computer system 2 determines the surface shape of the surface to be measured based on the interference fringe image acquired by the interferometer optical head 152. The computer system 2 functions as an analysis means in the present invention.

[0037] Figure 4 shows a block diagram illustrating the configuration of the computer main unit 201. As shown in Figure 4, the computer main unit 201 comprises a CPU 40 that forms the core of the control system, a storage unit 41, a work memory 42, interfaces (indicated as "IF" in Figure 5) 43, 44, 45, and 46, and a display control unit 47 that controls the display on the display 205.

[0038] Operator instructions entered via keyboard 202, J / S 203, or mouse 204 are input to CPU 40 via interface 43. Interface 44 is connected to image measuring machine 10 and supplies various control signals from CPU 40 to image measuring machine 10, and receives various status information and images from image measuring machine 10 and inputs them to CPU 40.

[0039] When the image measurement mode is selected, the display control unit 47 displays an image on the display 205 obtained from the image signal supplied from the CCD camera of the image optical head 151. When the optical interference measurement mode is selected, the display control unit 47 displays images captured by the interferometer optical head 152, surface shape data measured by the interferometer optical head 152, etc., on the display 205 as appropriate based on control by the CPU 40. The measurement results from the image optical head 151 and the interferometer optical head 152 can be output to a printer via the interface 45.

[0040] The work memory 42 provides a workspace for various processes performed by the CPU 40. The storage unit 41 is composed of, for example, a hard disk drive or RAM, and stores programs executed by the CPU 40, measurement results from the surface shape measuring device 1, etc. The programs executed by the CPU 40 include programs that perform the analysis processing described later.

[0041] Based on various input information via each interface, operator instructions, and programs stored in the memory unit 41, the CPU 40 performs various processes such as switching between the image measurement mode using the image optical head 151 and the optical interference measurement mode using the interferometer optical head 152, specifying the measurement range, moving the imaging unit 15 in the X-axis direction, moving the stage 12 in the Y-axis direction, capturing a two-dimensional image with the image optical head 151, measuring interference fringe images with the interferometer optical head 152, and calculating the surface shape.

[0042] When calculating the surface shape, the CPU 40 identifies the moving scan position where the interference fringe peak occurs for each pixel position in the interference fringe image, and sets this as the height (position in the Z-axis direction) at each pixel position in the interference fringe image.

[0043] Next, a method for determining the height at each pixel position in the interference fringe image using the surface shape measuring device 1 of this embodiment will be described. In the following, the interferometer optical head 152 is scanned in the Z-axis direction along the optical axis from a starting point (for example, the position closest to the workpiece W in the scanning range in the Z-axis direction) to an ending point (for example, the position furthest from the workpiece W in the scanning range in the Z-axis direction) to acquire N interference fringe images (where N≧2). Then, the height (Z-axis direction position) at each pixel position is determined based on the N interference fringe images acquired in this way. From the determined heights at each pixel position, the surface shape of the workpiece W can be determined.

[0044] In the method of this embodiment, for a common pixel position in N interference fringe images, a signal indicating the change in interference light intensity (pixel brightness value) at each imaging position along the Z-axis is defined as the interference signal (Figure 5(a)). The square of this interference signal or its absolute value is determined, and the height (Z-axis position) at each pixel position is determined from the integral curve obtained by integrating this value. The square or absolute value of the interference signal corresponds to the height-dependent signal in this invention.

[0045] In the method of this embodiment, such an integral curve is approximated by three straight lines, as shown in Figure 6: a starting noise line L1, an interference line L2, and an ending noise line L3. The starting noise line L1 approximates the starting noise area, which is the range where no interference occurs on the starting side of the measurement target surface. The ending noise line L3 approximates the ending noise area, which is the range where no interference occurs on the ending side of the measurement target surface. The interference line L2 approximates the interference area, which is the range where interference occurs in the vicinity of the measurement target surface. The interference area corresponds to the vicinity of the surface in this invention, and the interference line corresponds to the vicinity of the surface in this invention. Furthermore, the starting noise area and the ending noise area are ranges where the slope of the integral curve is smaller compared to the vicinity of the surface in the vicinity of the measurement target surface.

[0046] The starting noise line L1 can be determined based on a predetermined number of points (e.g., 10 points) from the starting point of the integral curve. The ending noise line L3 can be determined based on a predetermined number of points (e.g., 10 points) from the ending point of the integral curve. Furthermore, the starting noise line L1 and the ending noise line L3 can be determined under the constraint that their slopes are equal.

[0047] The interference line L2 can be defined as the line with the maximum slope among the approximate lines for a predetermined number of consecutive points on the integral curve. For example, the approximate line can be found by applying the least squares method to all of the predetermined number of consecutive points. Alternatively, the approximate line can be defined as the line connecting the endpoints of the predetermined number of consecutive points.

[0048] Then, the position (height) in the Z-axis direction of the surface to be measured is determined based on the starting noise line L1, the ending noise line L3, and the interference line L2. Specifically, an intermediate line L4 is determined that has a slope obtained by averaging the slopes of the starting noise line L1 and the ending noise line L3, and an intercept obtained by averaging the intercepts of the starting noise line and the ending noise line. Then, the intersection Z of the intermediate line L4 and the interference line L2 is determined. crossDetermine the value and set the position of this intersection as the position (height) in the Z-axis direction of the surface being measured.

[0049] Applying the above method to all pixels in the interference fringe image, the position Z in the Z-axis direction is determined. cross By determining this, the surface shape of the workpiece W can be obtained.

[0050] The position of the surface to be measured in the Z-axis direction is determined from the integral curve using the method described above. However, before all N points constituting the integral curve are aligned (i.e., before capturing N interference fringe images), the analysis process to determine the height at the pixel position can be started. Below, referring to the flowchart shown in Figure 7, a method is described in which at least part of the analysis process is performed using interference fringe images already stored in the computer system 2, while capturing and transferring interference fringe images with the interferometer optical head 152 to the computer system 2, before all N interference fringe images are aligned. This method can shorten processing time and reduce processing load.

[0051] As already explained, the surface shape measuring device 1 sequentially acquires N (where N≧2) interference fringe images by scanning in the Z-axis direction along the optical axis of the interferometer optical head 152 from the starting point to the ending point, and transmits them to the computer system 2. In this method, when the surface shape measuring device 1 starts the measurement, it first acquires the first interference fringe image (step S10). Subsequently, it sequentially acquires the M (where 2≦M≦N) interference fringe images while scanning the position of the interferometer optical head 152, and in parallel with this, the computer system 2 performs analysis processing on the interference fringe images up to M-1 (step S20).

[0052] The analysis process for interference fringe images up to the M-1th image includes an integral curve update process (step S21) that determines the value of the M-1th point from the starting point of the integral curve for each position in the M-1th interference fringe image acquired last. This integral curve update process determines the M-1th point on the integral curve by adding the squared luminance value to the integral value up to the M-2nd image for each position in the M-1th interference fringe image. Note that the initial value of the integral value (i.e., the integral value up to the M-2=0th image, to which the squared luminance value is added when M=2) is set to 0.

[0053] Furthermore, the analysis process for interference fringe images up to M-1 includes a process to calculate the latest approximate line (step S22) for a predetermined number of consecutive points that include the M-1 point of the integral curve as the point closest to the endpoint. If M-1 is less than the predetermined number, the process to calculate the latest approximate line does not need to be performed.

[0054] Furthermore, the analysis process for interference fringe images up to the M-1th image includes a provisional interference line update process (step S23) that finds the provisional interference line with the maximum slope among the approximate lines for a predetermined number of consecutive points up to the M-1th point of the integral curve. This provisional interference line update process compares the provisional interference line obtained for the M-2nd point of the integral curve with the approximate line obtained in the latest approximate line calculation process, and the one with the larger slope is adopted as the new provisional interference line. Note that if M-1 is less than the predetermined number, the provisional interference line update process does not need to be performed.

[0055] In this way, by acquiring the Mth interference fringe image while simultaneously performing analysis on the M-1th interference fringe images already acquired, it is possible to proceed with part of the process of determining the height of the surface to be measured before acquiring all N interference fringe images.

[0056] If the Nth interference fringe image has not yet been acquired after step S20 (step S30; No), the process returns to step S20 to acquire the next interference fringe image while simultaneously performing analysis on the acquired interference fringe images.

[0057] Step S20 is repeated until the Nth interference fringe image is obtained (Step S30; Yes), and then analysis processing (Step S40) is performed on the first to Nth interference fringe images. The analysis processing for the first to Nth interference fringe images includes an integral curve update process to find the value of the Nth point in the integral curve (Step S41), a latest approximate line calculation process (Step S42), and a provisional interference line update process (Step S43). The provisional interference line obtained in the provisional interference line update process in the analysis processing for the first to Nth interference fringe images is defined as the interference line.

[0058] In this way, once the Nth interference fringe image is acquired, the integral curve up to the (N-1)th point, which has already been acquired, and a provisional interference line based on the integral curve up to the (N-1)th point can be determined. Then, after acquiring the Nth interference fringe image, the entire integral curve can be determined simply by finding the Nth point (the last point) in the integral curve. Furthermore, by finding an approximate line that includes the Nth point and comparing it with the provisional interference line based on the integral curve up to the (N-1)th point, the final interference line can be obtained.

[0059] Furthermore, in the analysis process after acquiring the Nth interference fringe image, the starting point noise line L1 and the ending point noise line L3 are determined based on a predetermined number of points from the starting point of the integral curve and a predetermined number of points from the ending point of the integral curve (step S44).

[0060] If the constraint that the slopes of the starting noise line L1 and the ending noise line L3 are equal is not imposed, the starting noise line L1 may be determined in the analysis process for the interference fringe images up to M-1 when M-1 matches the number of points required to determine the starting noise line L1, and the ending noise line L3 may be determined in the analysis process after obtaining the Nth interference fringe image.

[0061] In the analysis process after obtaining the Nth interference fringe image, the intermediate line is then calculated from the starting noise line L1 and the ending noise line L3 (step S45). Furthermore, the intersection point of the intermediate line and the interference line is found, and the position Z of this intersection point is determined. cross This is defined as the position (height) in the Z-axis direction of the surface to be measured (step S46).

[0062] Thus, after determining the interference line, the height of the surface to be measured can be determined by a process that has a relatively low processing load and does not require a large amount of work memory, such as linear approximation with a relatively small number of points and calculation of intersection points between lines.

[0063] As described above, the surface shape measuring device 1 according to this embodiment can suppress the work memory and computational processing power required for analysis processing. Furthermore, by performing interference fringe image acquisition and analysis processing in parallel, the measurement time can be shortened.

[0064] [Second Embodiment] The surface shape measuring device 1B according to the second embodiment differs from the surface shape measuring device 1 of the first embodiment in that it performs so-called PFF (Point From Focus) measurement, which involves scanning the surface of a workpiece in the optical axis direction with a measuring head and using multiple stacked images to determine the height of the surface to be measured from the change in contrast at each pixel position in the stacked image. Below, the surface shape measuring device 1B will be described, focusing on the differences from the surface shape measuring device 1 of the first embodiment. Unless otherwise specified, the configuration should be understood to be the same as that of the surface shape measuring device 1 of the first embodiment.

[0065] The surface shape measuring device 1B, like the surface shape measuring device 1 of the first embodiment, comprises a non-contact type image measuring machine 10 and a computer system 2 that drives and controls the image measuring machine 10 and performs necessary data processing. The image measuring machine 10, like the one shown in Figure 1 with respect to the first embodiment, comprises a stand 11, a sample stage 12, support arms 13a and 13b, an X-axis guide 14, and an imaging unit 15.

[0066] In the second embodiment, the imaging unit 15 includes an image optical head 151 that captures a two-dimensional image of the workpiece W, and captures an image of the workpiece W at a measurement position set by the computer system 2. The image optical head 151 includes a CCD camera, an illumination device, a focusing mechanism, etc., and captures a two-dimensional image of the workpiece W. The data of the captured two-dimensional image is taken into the computer system 2.

[0067] When measuring surface shape using PFF measurement, the focal length is fixed at a predetermined distance by the focusing mechanism, and multiple stacked images are captured while scanning the measurement head in the optical axis direction (Z axis direction).

[0068] When calculating the height of the workpiece surface to be measured from the captured stacked image, the CPU 40 of computer system 2 determines a contrast curve for each pixel position in the stacked image that shows the degree of local focus according to the height at the time of imaging (scanning position of the measuring head), identifies the moving scan position where a peak occurs in this contrast curve, and sets this as the height (position in the Z-axis direction) at each pixel position in the stacked image.

[0069] In the surface shape measuring device 1 of the second embodiment, for each pixel position in the interference image, the square or absolute value of the interference signal, which indicates the intensity of interference according to the height at the time of imaging (scanning position of the measuring head), was used as the height-dependent signal. The height of the surface to be measured was then determined by analyzing the integral curve obtained by integrating this height-dependent signal.

[0070] In contrast, the surface shape measuring device 1B of the second embodiment uses the contrast curve as a height-dependent signal. That is, the integral curve is obtained by integrating the contrast curve, and the height of the surface to be measured can be determined by applying the same analysis method as in the first embodiment.

[0071] [Third Embodiment] The surface shape measuring device 1C according to the third embodiment differs from the surface shape measuring devices 1 and 1B of the first and second embodiments in that it performs measurement using the so-called structured illumination microscopy (SIM) method. In SIM measurement, projection light with a periodic pattern in a direction perpendicular to the optical axis is irradiated onto the workpiece, and while the measuring head is scanned in the direction of the optical axis, the surface of the workpiece to be measured is imaged, and the projection light reflected from the surface of the workpiece to be measured is imaged to obtain multiple stacked images. Then, using the obtained stacked images, the position (height) at which the pattern is in focus is determined from the change in contrast of the pattern at each pixel position in the stacked image, and this is taken as the height of the surface to be measured. The following description of the surface shape measuring device 1C will focus on the differences from the surface shape measuring devices 1 and 1B of the first and second embodiments. Unless otherwise specified, the configuration should be understood to be the same as that of the surface shape measuring devices 1 and 1B of the first and second embodiments.

[0072] The surface shape measuring device 1C, like the surface shape measuring device 1 of the first embodiment and the surface shape measuring device 1B of the second embodiment, comprises a non-contact type image measuring machine 10 and a computer system 2 that drives and controls the image measuring machine 10 and performs necessary data processing. The image measuring machine 10, like the one shown in Figure 1 with respect to the first embodiment, comprises a stand 11, a sample stage 12, support arms 13a and 13b, an X-axis guide 14, and an imaging unit 15. In the third embodiment, the imaging unit 15 includes an image optical head 151 that captures a two-dimensional image of the workpiece W, and captures an image of the workpiece W at a measurement position set by the computer system 2. The image optical head 151 includes a CCD camera, an illumination device, a focusing mechanism, etc., and captures a two-dimensional image of the workpiece W. The captured two-dimensional image data is input into the computer system 2.

[0073] In the third embodiment, the imaging unit 15 includes, in addition to the above, a pattern projection unit 153 that projects light of a predetermined periodic pattern onto the surface of the workpiece to be measured. The pattern projection unit 153 is, for example, a projector. The pattern projection unit 153 includes an illumination light source and a focusing mechanism, but these are independent of the illumination device and focusing mechanism of the image optical head 151. That is, when the image optical head is scanned in the direction of the optical axis, the pattern projection unit 153 does not move in conjunction with the scanning.

[0074] When measuring the surface shape using the SIM method, a predetermined pattern is projected by the pattern projection unit 153 so that it is in focus on the surface of the workpiece to be measured. Then, similar to the PFF measurement in the second embodiment, with the focal length fixed at a predetermined distance by the focusing mechanism of the image optical head 151, multiple stacked images are captured while scanning the measurement head in the optical axis direction (Z axis direction).

[0075] When calculating the height of the workpiece surface to be measured from the captured stacked image, the CPU 40 of computer system 2 determines the change in the intensity of reflected light corresponding to the height at the time of imaging (scanning position of the measuring head) for each pixel position in the stacked image, identifies the moving scan position where the peak of this change in the intensity of reflected light occurs, and sets this as the height (position in the Z-axis direction) at each pixel position in the stacked image.

[0076] The change in the intensity of the reflected light, which is reflected from the surface of the workpiece being measured, with respect to the Z-axis position, is a curve similar to the interference signal of the first embodiment shown in Figure 5(a). It is approximately constant at positions sufficiently far from the focus position, but increases or decreases near the focus position, with the amplitude being maximum at the focus position. Since this change in reflected light intensity is similar to the change in the interference signal in the first embodiment, the height of the surface being measured is determined using the same method as the surface shape measuring device 1 of the first embodiment. That is, in the first embodiment, for each pixel position in the interference image, the square or absolute value of the interference signal, which indicates the intensity of interference according to the height at the time of imaging (scanning position of the measuring head), was used as the height-dependent signal. Then, the height of the surface being measured was determined by analyzing the integral curve obtained by integrating this height-dependent signal.

[0077] Following this, in the surface shape measuring device 1C of the third embodiment, the square or absolute value of the change in the intensity of reflected light is used as the height-dependent signal. That is, the integral curve is obtained by integrating the square or absolute value of the change in the intensity of reflected light, and the height of the surface to be measured can be determined by applying the same analysis method as in the first embodiment.

[0078] According to the embodiments described above, it is possible to realize a surface shape measurement method and a surface shape measuring device that can suppress the work memory and computational processing power required for analysis processing, and consequently shorten the measurement time.

[0079] [Variations of the Embodiment] It should be noted that the present invention is not limited to the embodiments described above, and any modifications, improvements, etc., that can achieve the objectives of the present invention are included in the present invention.

[0080] For example, although the first embodiment described above used a Michelson interferometer as an example of an image measuring device, the present invention can also be applied to various measuring devices and microscopes that use interferometers other than image measuring devices. Furthermore, the present invention can also be applied to measuring devices that use Milleau type, Fizeau type, Twyman-Green type, and other isopath interferometers.

[0081] Furthermore, although the analysis processing was performed by the computer system 2 in each of the above embodiments, some or all of the analysis processing may be implemented by dedicated hardware using an ASIC or FPGA.

[0082] Furthermore, any modifications made by those skilled in the art to the aforementioned embodiments, including additions, deletions, or design changes to components, or combinations of features from each embodiment, are also included within the scope of the present invention, as long as they retain the essence of the present invention. [Industrial applicability]

[0083] By applying this invention to a surface shape measuring device, it is possible to suppress the work memory and computational processing power required for analysis, and consequently shorten the measurement time. [Explanation of symbols]

[0084] 1,1B,1C...Surface shape measuring device 2. Computer Systems 3. Vibration isolation table 10. Image measuring machine 11. Stand 12 stages 13a, 13b... Support arms 14. X-axis guide 15. Imaging Unit W....Object being measured (workpiece)

Claims

1. A measurement method for measuring the surface shape of an object to be measured by synthesizing N stacked images captured while scanning a measuring head in the optical axis direction, Regarding the common position in the N stacked images, By integrating a height-dependent signal based on the values ​​of N points that indicate the change in pixel values ​​along the Z-axis, an integral curve consisting of the values ​​of N points is obtained to approximate the starting-side noise portion, which is a range on the starting-point side of the measurement target surface with a smaller slope compared to the vicinity of the measurement target surface; an ending-side noise portion, which is a range on the ending-point side of the measurement target surface with a smaller slope compared to the vicinity of the measurement target surface; and a surface proximity line, which approximates the surface proximity portion near the measurement target surface. Based on the starting noise line, the ending noise line, and the surface-near-surface line, the intersection point of the intermediate line having a slope obtained by averaging the slope of the starting noise line and the slope of the ending noise line, and having an intercept obtained by averaging the intercepts of the starting noise line and the ending noise line, and the surface-near-surface line is determined as the position in the Z-axis direction of the surface to be measured. After obtaining the first stacked image, While sequentially acquiring the M-th (where 2 ≤ M ≤ N) stack image, analysis processing is performed on the stack images up to the M-1th image. After obtaining the Nth stacked image, the analysis process is performed on the stacked images from the first to the Nth image. The aforementioned analysis process performs at least the following for each position in the last acquired K-th stacked image: An integral curve update process that finds the value of the K-th point from the starting point that constitutes the integral curve, The latest approximation line calculation process finds an approximation line for a predetermined number of consecutive points that include the K-th point of the integral curve as the point closest to the endpoint, A provisional surface proximity line update process for finding a provisional surface proximity line with the maximum slope among the approximate lines for a predetermined number of consecutive points up to the K-th point of the integral curve, the provisional surface proximity line update process for finding a provisional surface proximity line for the (K-1)-th point of the integral curve and the approximate line found in the latest approximate line calculation process, and setting the one with the larger slope as the new provisional surface proximity line, Includes, A method for measuring surface shape, characterized in that the provisional surface vicinity line obtained in the provisional surface vicinity line update process in the analysis process performed after acquiring the Nth stacked image is used as the surface vicinity line.

2. The aforementioned measurement head is an interferometer optical head that splits incoherent light emitted from a light source into reference light for a reference mirror and measurement light for the surface to be measured using a beam splitter, and acquires an interference fringe image generated by the optical path difference of the light reflected from each. The stacked image is an N (where N≧2) interference fringe image obtained by scanning the interferometer optical head with respect to the measurement target surface in the Z-axis direction along the optical axis of the interferometer optical head from a starting point to an ending point. The surface shape measurement method according to claim 1, characterized in that the height-dependent signal is the square or absolute value of an interference signal consisting of N points representing the change in interference light intensity along the Z-axis direction.

3. The measuring head is an image optical head that captures a two-dimensional image of the object to be measured. The stacked image is a two-dimensional image obtained by scanning the image optical head with respect to the surface to be measured in the Z-axis direction along the optical axis of the image optical head from a starting point to an ending point, and N images are obtained in this manner. The surface shape measurement method according to claim 1, characterized in that the height-dependent signal is a contrast curve consisting of N point values ​​that show the change in contrast along the Z-axis direction.

4. The measurement target surface of the object to be measured is further provided with a pattern projection unit that projects light in a predetermined periodic pattern, The measuring head is an image optical head that captures a two-dimensional image of the object to be measured. The stacked image is a two-dimensional image obtained by scanning the image optical head with respect to the measurement target surface of the object to be measured from a starting point to an ending point in the Z-axis direction along the optical axis of the image optical head, while the pattern projection unit is irradiating the measurement target surface of the object to be measured with the projection light. The surface shape measurement method according to claim 1, characterized in that the height-dependent signal is the square or absolute value of the value of N points that indicate the change in the intensity of reflected light reflected by the projected light from the surface to be measured along the Z-axis direction.

5. The method for measuring surface shape according to any one of claims 1 to 4, characterized in that the line with the maximum slope among the approximate lines for a predetermined number of consecutive points in the integral curve is defined as the line near the surface.

6. The method for measuring surface shape according to claim 5, characterized in that an approximate straight line is obtained by applying the least squares method to all of a predetermined number of points.

7. The method for measuring surface shape according to claim 5, characterized in that a straight line connecting the endpoints of a predetermined number of consecutive points is used as an approximate straight line.

8. A method for measuring surface shape according to any one of claims 1 to 7, characterized in that the starting noise line and the ending noise line are determined under the constraint that the slopes of the starting noise line and the ending noise line are equal.

9. A method for measuring surface shape according to any one of claims 1 to 8, characterized in that a straight line of noise on the starting point side is determined based on a predetermined number of points from the starting point in the integral curve, and a straight line of noise on the ending point side is determined based on a predetermined number of points from the ending point in the integral curve.

10. A surface shape measuring device for measuring the surface shape of an object to be measured, An interferometer optical head that splits light emitted from a light source emitting incoherent light into reference light for a reference mirror and measurement light for the surface to be measured using a beam splitter, and acquires interference fringe images generated by the optical path difference of the light reflected from each using an image sensor, Analysis means for determining the surface shape of the surface to be measured based on the interference fringe image acquired by the interferometer optical head. Equipped with, The interferometer optical head acquires N interference fringe images (where N≧2) by scanning the surface to be measured in the Z-axis direction along the optical axis of the interferometer optical head from a starting point to an ending point. The aforementioned analysis means is Regarding the common position in the N interference fringe images acquired by the interferometer optical head, By integrating the square or absolute value of the interference signal consisting of N points representing the change in interference light intensity along the Z-axis, an integral curve consisting of N points is obtained, from which a starting-side noise line approximating the starting-side noise region, which is the range where no interference occurs on the starting-side side of the measurement target surface, an ending-side noise line approximating the ending-side noise region, which is the range where no interference occurs on the ending-side side of the measurement target surface, and an interference line approximating the interference region, which is the range where interference occurs in the vicinity of the measurement target surface, are obtained. Based on the starting noise line, the ending noise line, and the interference line, the intersection point of the intermediate line having a slope obtained by averaging the slope of the starting noise line and the slope of the ending noise line, and having an intercept obtained by averaging the intercepts of the starting noise line and the ending noise line, and the interference line is determined as the position in the Z-axis direction of the surface to be measured. After obtaining the first interference fringe image, While sequentially acquiring the M-th (where 2 ≤ M ≤ N) interference fringe image, analysis processing is performed on the interference fringe images up to M-1. After obtaining the Nth interference fringe image, the analysis process is performed on the first to Nth interference fringe images. The aforementioned analysis process performs at least the following for each position within the last acquired K-th interference fringe image: An integral curve update process that finds the value of the K-th point from the starting point that constitutes the integral curve, The latest approximation line calculation process finds an approximation line for a predetermined number of consecutive points that include the K-th point of the integral curve as the point closest to the endpoint, A provisional surface proximity line update process for finding a provisional surface proximity line with the maximum slope among the approximate lines for a predetermined number of consecutive points up to the K-th point of the integral curve, the provisional surface proximity line update process for finding a provisional surface proximity line for the (K-1)-th point of the integral curve and the approximate line found in the latest approximate line calculation process, and setting the one with the larger slope as the new provisional surface proximity line, Includes, The surface shape measuring device is characterized in that the analysis means uses the provisional surface vicinity line obtained in the provisional surface vicinity line update process in the analysis process performed after acquiring the Nth interference fringe image as the interference portion line.

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