JP2010060304A - Shape measurement method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure a surface shape of a specimen in a short period of time. <P>SOLUTION: A shape measuring method separates light emitted from a light source into object light and reference light and measures a surface shape of a specimen having a free curved surface by an interferometer for analyzing interference fringes obtained by a difference in the optical path between the object light and the reference light. The method includes: a first process for fixing a reference surface to a first position on the light axis of the reference light, applying the object light to the specimen, and acquiring a plurality of first interference fringes; a second process for fixing the reference surface to a second position different from the first position and acquiring a plurality of second interference fringes; a third process for fixing the reference surface to a third position different from the first and second positions and acquiring a plurality of third interference fringes; and a phase analysis process S3 for performing a phase analysis of interference fringes by the first, second, and third interference fringes. In at least one of the first, second, and third processes, the interferometer travels relative to the specimen so that the entire surface of the specimen is scanned, and a plurality of interference fringes are acquired intermittently at each travel of prescribed distance. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a shape measuring method for measuring a surface shape of a test object by analyzing interference fringes generated by light reflected on the surface of the test object, and more specifically, a test object having a free-form surface. The present invention relates to a surface shape measurement method.

  Conventionally, when measuring the surface shape of a large-area, high-inclined specimen using an interferometer, the surface of the specimen is divided into a plurality of measurement areas that can be measured by the interferometer, and measurement of each area is performed. A method of connecting the results and measuring the entire surface has been proposed (see, for example, Patent Document 1).

In the above method, in order to perform the shape measurement of each divided measurement region with high accuracy, it is necessary to perform a phase analysis by fringe scanning of the interference fringes on the surface of the test object. Therefore, normally, the test object is stopped for each measurement region, the reference surface of the interferometer is moved to a plurality of positions on the optical axis, and the measurement region is subjected to fringe scanning. The procedure of performing fringe scanning is taken.
JP 2003-57016 A

  However, when the surface shape is measured in the above-described procedure, the movement of the test object or interferometer is stopped and stopped for each measurement region, and the reference plane is moved for fringe scanning (minimum 3 positions, usually 4 positions or more). Therefore, a long time is required for measurement. This situation becomes more prominent as the number of measurement area divisions increases.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a shape measuring method capable of measuring the surface shape of a test object in a short time.

  The present invention separates light emitted from a light source into object light irradiated on a test object and reference light reflected on a reference surface, and the object light reflected on the surface of the test object and the reference light A shape measuring method for measuring a surface shape of the test object using an interferometer that analyzes an interference fringe obtained by an optical path difference of the test object, wherein the reference surface is fixed at a first position on the optical axis of the reference light. Then, the first step of irradiating the test object with the object light and acquiring the interference fringes as the plurality of first interference fringes each time the interferometer moves relative to the test object by a predetermined distance. The reference surface is fixed at a second position on the optical axis of the reference light different from the first position, the object light is irradiated onto the test object, and the interferometer is applied to the test object. A second step of acquiring the interference fringes as a plurality of second interference fringes for each relative movement relative to the predetermined distance; An illumination surface is fixed at a third position on the optical axis of the reference light different from the first position and the second position, and the object light is irradiated to the test object, and the interferometer is used for the test object. The interference fringes for each relative movement with respect to a predetermined distance are obtained as a plurality of third interference fringes, the first interference fringes obtained in the first step, and obtained in the second step. A phase analysis step of performing a phase analysis of the interference fringes using the second interference fringes obtained and the third interference fringes obtained in the third step, and the surface of the test object is predetermined In the at least one of the first step, the second step, and the third step, the interferometer is arranged on the surface of the test object based on the function. The surface of the test object is adjusted while adjusting the irradiation angle of the object light so that the object light is incident vertically. Wherein to scan the body is moved relative to the test object, wherein the interferometer is characterized intermittently a plurality of interference fringes are obtained each time moved by the predetermined distance relative.

  According to the shape measuring method of the present invention, in the state where the reference surface is at least one of the first position, the second position, and the third position, the interference fringes in each region of the surface of the object to be measured are detected by the interferometer. Since it is acquired intermittently while relatively moving so as to scan the surface of the inspection object, the number of times of moving the reference surface is drastically reduced.

  In the shape measuring method of the present invention, in any one of the first step, the second step, and the third step in which a plurality of interference fringes are obtained intermittently, the starting point of the relative movement of the interferometer The end point may be the same. In this case, it is possible to calculate an error of the surface shape at the start of relative movement and at the end of relative movement, and correct the measurement error of each region based on the error.

  The shape measurement method of the present invention may further include an integration step of acquiring the surface shape of the entire test object by connecting the phase analysis results obtained in the phase analysis step based on the predetermined distance. In this case, the surface shape of the entire test object can be measured in a short time.

  According to the shape measuring method of the present invention, the surface shape of the test object can be measured in a short time.

An embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing a main configuration of an interferometer 1 used in the shape measuring method of the present invention. The interferometer 1 is a known Twiman Green type interferometer. The principle is briefly described as follows.
The light L emitted from the light source 2 is reflected by the object light L1 irradiated on the surface of the test object S and the reference plane (reference surface) 3 and separated into the reference light L2 serving as a reference. The object light L1 reflected from the surface of the test object S interferes with the reference light L2, and becomes interference light L3. The interference light L3 that has passed through the beam splitter 4 forms an image on the CCD element (image conversion photoelectric element) 6 by the imaging lens 5, and interference fringes caused by the optical path difference between the object light L1 and the reference light L2 are obtained. A moving mechanism 3A for moving the reference plane 3 along the optical axis of the reference light L2 is attached to the reference plane 3. A piezo element or the like can be employed as the moving mechanism 3A.

  FIG. 2 is a block diagram showing the configuration of the shape measuring apparatus 11 that executes the shape measuring method of the present embodiment. The shape measuring device 11 includes the interferometer 1 described above. The interferometer 1 includes a control unit 7 that controls the operation of the entire interferometer 1, a moving mechanism 8 that moves the interferometer 1 relative to the test object, and a relative movement of the interferometer 1 relative to the test object. An encoder 9 for detecting the distance and relative position and an angle adjusting mechanism 10 for adjusting the irradiation angle of the irradiated object light by swinging the interferometer 1 are attached. The moving mechanism 8, the encoder 9, the angle adjusting mechanism 10, the CCD element 6, and the moving mechanism 3 </ b> A for moving the reference plane 3 are all connected to the control unit 7 and operate based on the control of the control unit 7. Is configured to do.

  In addition, the shape measuring device 11 includes a calculation unit 12 that performs phase analysis of the interference fringes acquired by the CCD element 6 and correction of the acquired partial surface shape data, and partial surface shape data corrected by the calculation unit 12. A data integration unit 13 that integrates and acquires surface shape data of the entire test object is provided.

  FIG. 3 is a perspective view showing an optical element S that is a test object. The surface of the optical element S is formed as a so-called free-form surface expressed by a predetermined function.

  A procedure for measuring the surface shape of the optical element S by the shape measuring apparatus 11 including the interferometer 1 configured as described above will be described below with reference to FIGS.

  FIG. 4 is a flowchart showing the flow of the shape measuring method of the present embodiment. In this shape measuring method, an interference fringe acquisition step S1 for acquiring interference fringes at a plurality of sites on the surface of the optical element S that is a test object, and a phase analysis using the acquired interference fringes are performed. The phase analysis process S3 which acquires surface shape data, and the integration process S4 which integrates each partial surface shape data and acquires the whole shape data of the surface of the optical element S are provided.

  First, the interference fringe acquisition process in step S1 will be described. The control unit 7 of the interferometer fixes the reference plane 3 to the first position P1, which is one of the positions of the reference plane 3 for acquiring the interference fringes, via the moving mechanism 8 via the moving mechanism 3A. The interferometer 1 is moved to a position facing the scanning start point A on the surface of the optical element S shown in FIG. Then, the control unit 7 operates the angle adjusting mechanism 10 so that the object light L1 irradiated from the interferometer 1 enters the surface of the optical element S perpendicularly as shown in FIG. The angle of the interferometer 1 is adjusted so that the interference fringes at the scanning start point A can be acquired by the meter 1.

  Subsequently, the control unit 7 operates the moving mechanism 8 to move the interferometer 1 to the scanning end point B so that the interferometer 1 scans the entire surface of the optical element S along a locus shown by an arrow in FIG. Move. At this time, the control unit 7 simultaneously operates the angle adjustment mechanism 10 to appropriately swing the interferometer 1 so that the object light L1 always enters the surface of the optical element S perpendicularly as shown in FIG. The angle of the interferometer 1 is adjusted. Such angle adjustment of the interferometer 1 can be easily performed by, for example, a method in which a setting function relating to the shape of the optical element S as shown in the following Equation 1 is input to the control unit 7 in advance.

While scanning the interferometer 1 as described above, the control unit 7 performs R1, R2 shown in FIG. 5 each time the interferometer 1 moves at a predetermined interval, for example, 5 millimeters (mm), according to the detection value of the encoder 9. , R3, and the image of a part of the surface of the optical element S is intermittently acquired by the CCD element 6. At this time, if all pixels of the CCD element 6 are set to perform synchronous readout, an image can be acquired at high speed. Further, the predetermined interval is set so that each acquired image R1 and the like has an overlapping area with at least one other image.
In this way, the interference fringes (first interference fringes) in each part of the surface of the optical element S when the reference plane 3 is located at the first position P1 are intermittently acquired, and the acquisition of the first interference fringes is performed. End (first step).

  After completion of the first step, whether or not the variable n input to the control unit 7 is 3 is verified in step S2. As shown in FIG. 4, since the initial value of n is 0, the determination in step S2 is NO at this point, and the process proceeds to step S5, where the control unit 7 operates the moving mechanism 3A (here, the piezo element PZT). Then, the reference plane 3 is moved to a different position P2 on the optical axis of the reference light L2 to prepare for the second step described later. Then, 1 is added to the variable n, and the process returns to step S1 again.

  After the reference plane 3 is fixed at the second position P2, the interferometer 1 is moved again to a position facing the scanning start point A. Then, at the same position as the first step described above and at the same position where the first interference fringe is acquired, the interference fringes (first fringes) at each part of the optical element S when the reference plane 3 is located at the second position P2. 2 interference fringes) are acquired intermittently (second step).

  Thereafter, in the same procedure, the reference plane 3 is moved to the third position P3 and the fourth position P4 on the optical axis of the reference light L2 behind the second position P2, and the third interference fringe and the fourth interference fringe are Each is acquired intermittently (third step, fourth step). That is, four types of interference fringes on the surface of the optical element S are obtained at each part, ie, first to fourth interference fringes.

  Since the value of the variable n is 3 at the end of the fourth interference fringe acquisition, the determination in step S2 is YES and the interference fringe acquisition step is ended. Then, all the acquired interference fringe data is sent from the CCD element 6 to the calculation unit 12.

  In the phase analysis step of step S3, the calculation unit 12 sets the reference plane at the first position P1, the second position P2, the third position P3, and the first position in a certain part from the interference fringe data sent from the CCD element 6. An image including four interference fringes obtained by being positioned at the four positions P4 is extracted, and these phase changes are analyzed to obtain partial surface shape data at the site. The calculation unit 12 performs the same operation at all the parts where the interference fringes are acquired, and obtains partial surface shape data of all parts of the surface of the optical element S. Each acquired partial surface shape data is sent to the data integration unit 13.

  In the integration step of step S4, the data integration unit 13 connects the partial surface shape data of each part acquired in step S3 using point sequence data in an overlapping area with another part (fitting) 1 The entire surface shape data of the optical element S is acquired by integrating the two pieces of data. Hereinafter, a specific procedure of the integration process will be described.

First, the data integration unit 13 selects a part including the scanning start point A as a reference part (fitting area), and uses a known shape extraction method or the like from an overlapping area with another part (fitting area) adjacent to the reference part. Then, an index shape that is an index of fitting is extracted.
As the index shape, for example, a fine scratch on the surface of the optical element S can be used. Although there is no restriction | limiting in particular about the shape, From a viewpoint of raising the precision of fitting, it is preferable that it is a specific shape. The user sets a desired condition in the data integration unit 13 in advance for the index shape to be extracted.

  Next, the data integration unit 13 extracts the surface shape data of the index shape portion from the partial surface shape data of the reference portion, performs fitting using the NURBS function shown in Equation 2, and obtains an approximate curved surface of the surface shape data of the index shape. The curve shown (approximate curve) is acquired.

  In the NURBS function, weighting can be performed for each coordinate by changing the value of ω. Therefore, when it can be determined that the value of a certain coordinate is clearly an incorrect value due to a measurement error or dust attached to the surface of the optical element S, no weight is given to the coordinate or the weight is smaller than other coordinates. By doing so, a more accurate approximate curve can be obtained.

For example, in the surface shape data DM of the index shape M shown in FIG. 8A, when the point sequence data C1 is an illegal value due to dust or the like, the fitting using a normal polynomial or the like is shown in FIG. 8B. As shown, an approximate curve A1 including noise of the point sequence data C1 is acquired. However, when the NURBS function is used, the influence of the point sequence data C1 is eliminated or corrected as shown in FIG. 8C. In addition, a more accurate approximate curve A2 can be obtained.
The determination as to whether or not the value is an illegal value may be made sequentially by the user looking at the surface shape data, or may be automatically determined by giving the data integration unit 13 conditions for the determination. For example, it is determined that an outlier that falls outside a predetermined range is an incorrect value, or when the difference between adjacent point sequence data is greater than or equal to a predetermined value, the point sequence data that is further away from the average value of the point sequence data Conditions such as judging an illegal value.

  After obtaining the approximate curve of the index shape, the data integration unit 13 takes out the surface shape data of the index shape in the surface shape data of the fitting area as point sequence data, and calculates the square sum of the difference between the approximate curve and the point sequence data. The amount of correction of the fitted region is calculated by the method of least squares so that is minimized. The correction amount is associated with, for example, translation amounts dx, dy, dz along the x-axis, y-axis, and z-axis, and tilt amounts da, db, dc around the respective axes, and a shift in the optical axis direction. There are seven types of similar magnification P for enlargement / reduction deformation.

  After calculating the correction amount, the data integration unit 13 converts all coordinate values of the surface shape data of the fitting region based on the correction amount, and performs fitting with the surface shape data of the fitting region. In this way, the two partial surface shape data are integrated.

  Subsequently, the data integration unit 13 uses the integrated partial surface shape data as a fitting region, and uses the partial surface shape data of a portion adjacent to the integrated region as a fitting region, and similarly extracts an index shape by the above-described procedure, and NURBS Fitting is performed by fitting with a function, calculating the correction amount by the least square method or the nonlinear programming method. When this is repeated up to the portion including the scanning end point B, all the partial surface shape data are integrated into one, and the entire surface shape data indicating the shape of the entire surface of the optical element S is acquired. Thus, the surface shape measuring method of this embodiment is complete | finished.

  According to the shape measurement method of the present embodiment, it is not necessary to move the reference plane 3 from the first position P1 to the fourth position P4 in each part where the interference fringes are acquired. Having surface shape measurements can be completed in a much shorter time.

  In addition, when it is desired to increase the measurement accuracy, it is only necessary to adjust so as to acquire a larger number of surface images by shortening the sampling period of the surface image by the CCD element 6, so that the shape can be easily formed without increasing the measurement time. Measurement accuracy can be increased.

  In the integration step of step S4, the data integration unit 13 acquires an approximate curve of the surface shape data of the index shape using the NURBS function, and performs the fitting of the surface shape data of each region based on the approximate curve. Therefore, impurities such as dust and incorrect values due to measurement errors can be eliminated, fitting based on a more accurate approximate curve can be performed, and highly accurate surface shape data can be obtained.

In the present embodiment, an example has been described in which an approximate curve of the surface shape data of the index shape in the fitting region is obtained, and fitting is performed by using a point sequence data of the surface shape of the fitting region and the least square method, etc. Also, with respect to the surface shape data of the index shape of the fitting region, an approximate curve may be obtained by the same method, and the fitting of both may be performed using the correlation coefficient.
Further, the weighting of the coordinates when obtaining the approximate curve may be performed using a so-called frequency filter that changes the weights of the coordinates corresponding to a specific frequency by increasing or decreasing the weight.
In addition, although the above-described NURBS function does not have the merit, fitting may be performed by obtaining an approximate curve using a general polynomial or the like.
Furthermore, in the surface shape measurement method of the present invention, since fitting in an approximate curve is not essential, integration of the partial surface shape data may be performed by a normal known fitting method.

Although one embodiment of the present invention has been described above, the technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above-described embodiment, the example in which the scanning start point A and the scanning end point B of the interferometer 1 on the surface of the optical element S are different from each other has been described, but instead, as in a modification example illustrated in FIG. The scanning trajectory may be set so that the interferometer returns to the scanning starting point A after the scanning is completed, that is, the scanning starting point A and the scanning end point B are at the same position. In this case, by calculating the amount of error between the first partial surface shape data acquired last and the last acquired partial surface shape data, and using this as the amount of deviation in the overall integration operation, the surface shape can be further corrected. Measurement accuracy can be improved.
Further, the scanning trajectory itself may be set to an arbitrary trajectory as long as it can cover the entire surface of the optical element S, not the trajectory like the so-called raster scanning as shown in FIGS.

In the above-described embodiment, the example in which the interferometer 1 is moved has been described. However, if the interferometer and the test object are relatively moved, the shape measuring method of the present invention can be executed. Instead, the optical element S as the test object may be moved by a moving mechanism or the like.
Further, in the above-described embodiment, an example in which a partial surface image of the test object including the interference fringe is acquired every time the interferometer moves relative to the test object for a predetermined distance based on the detection value of the encoder. However, instead of this, the interferometer may be moved at a constant speed, and partial surface images may be acquired at predetermined time intervals. In this case, since the encoder is not necessary, the shape measuring apparatus can be configured more simply.

  In the above embodiment, an example in which interference fringes are acquired while scanning and moving the interferometer 1 at all positions P1 to P4 on the reference plane has been described. However, the present invention is not limited to this, and reference is made. Interference fringes are intermittently acquired while scanning the interferometer 1 only at a part of the plane 3 (for example, P1). At the remaining positions, the interferometer 1 is moved by a predetermined distance as in the prior art. The interference fringes may be acquired in a state of being moved and stopped. Even in this case, the time required for obtaining interference fringes can be considerably reduced.

Furthermore, the position of the reference plane from which the interference fringes are acquired may be at least three, and may be five or more. In this case, the phase analysis process may be performed using a known phase analysis method that differs depending on the number of steps.
In addition, in the above-described embodiment, an example in which a Twiman Green type interferometer is used has been described. However, the present invention is intended to perform a fringe scan by advancing or retracting either the reference surface or the test object. For example, the present invention can be applied to a shape measuring method using an interferometer of another type such as a Fizeau type.

It is a figure which shows the structure of the interferometer used for the shape measuring method of one Embodiment of this invention. It is a block diagram which shows the structure of the shape measuring apparatus with which the same interferometer was attached. It is a figure which shows the optical element which is a test object of the same shape measuring method. It is a flowchart which shows the flow of the same shape measuring method. It is a figure which shows the scanning locus | trajectory of the interferometer in the optical element. It is a figure which shows the positional relationship of the same interferometer and the same optical element in an interference fringe acquisition process. It is a figure which shows operation | movement of the interferometer at the time of execution of the shape measuring method. (A) is a figure which shows an example of partial surface shape data, (b) and (c) are figures which show an example of fitting of the approximate function with respect to the partial surface shape data, respectively. It is a figure which shows the scanning locus | trajectory of the interferometer in the modification of the same shape measuring method.

Explanation of symbols

1 Interferometer 2 Light source 3 Reference plane (reference plane)
L1 Object light L2 Reference light P1 First position P2 Second position P3 Third position S3 Phase analysis step S4 Integration step S Optical element (test object)

Claims (3)

The light emitted from the light source is separated into the object light irradiated on the test object and the reference light reflected on the reference surface, and the optical path difference between the object light reflected on the surface of the test object and the reference light A shape measuring method for measuring the surface shape of the test object using an interferometer for analyzing the obtained interference fringes,
Each time the reference surface is fixed at a first position on the optical axis of the reference light, the object light is irradiated onto the object, and the interferometer moves relative to the object by a predetermined distance. A first step of acquiring the interference fringes as a plurality of first interference fringes;
The reference plane is fixed at a second position on the optical axis of the reference light different from the first position, the object light is irradiated onto the test object, and the interferometer is applied to the test object. A second step of acquiring the interference fringes for each relative movement of a predetermined distance as a plurality of second interference fringes;
The reference plane is fixed at a third position on the optical axis of the reference light different from the first position and the second position, the object light is irradiated onto the test object, and the interferometer is A third step of obtaining the interference fringes as a plurality of third interference fringes each time the relative movement relative to the inspection object is performed by a predetermined distance;
An interference fringe using the first interference fringe obtained in the first step, the second interference fringe obtained in the second step, and the third interference fringe obtained in the third step. A phase analysis process for performing phase analysis of
With
The surface of the test object is a free-form surface expressed by a predetermined function,
In at least one of the first step, the second step, and the third step, the interferometer is configured to cause the object light to vertically enter the surface of the test object based on the function. While adjusting the irradiation angle of the object light, it is moved relative to the test object so as to scan the entire surface of the test object, and each time the interferometer moves relative to the predetermined distance, a plurality of interferences are intermittently generated. A shape measuring method, wherein fringes are acquired.   The starting point and the ending point of the relative movement of the interferometer are the same in any one of the first step, the second step, and the third step in which a plurality of interference fringes are acquired intermittently. The shape measuring method according to claim 1.   The phase analysis result obtained in the phase analysis step is further connected, based on the predetermined distance, further comprising an integration step of acquiring the entire surface shape of the test object. Shape measurement method.

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