JPH02259509A - Method and instrument for measuring surface shape or the like - Google Patents

Abstract

PURPOSE:To shorten the measuring time and to measure various shapes by concatenating surface shape data obtained by measuring respective divided partial areas. CONSTITUTION:When the shape of a body 7 to be measured is special and deviates greatly from a spherical surface, stripe intervals become too partially narrow to measure the whole shape at a time. In this case, the whole shape of the body 7 is divided by a computer 20 into small overlapping areas, whose data are stored and transferred to a stage driver 19 to move the body 7 so that the small areas of the body 7 can be measured. Then the curvature of a reference spherical surface is measured and phase differences between CCD picture elements are measured from an interference fringe pattern to calculate and store surface shape data in a memory. This measurement is repeated as to all the small areas and the surface shape data are concatenated to compose and display the whole shape of the body 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method and apparatus for measuring the shape of a three-dimensional object with high precision, for example, by using an interference effect using light from an object to be measured.

[Conventional technology] Conventionally, as a method for measuring special shapes such as asymmetric aspherical surfaces, cylindrical surfaces, and toric surfaces with high precision, the entire surface is measured by repeating point and spot measurements of the surface to be measured using a configuration as shown in Figure 4. It is known how to obtain the shape (Optics 12.1
983, pp. 450-454).

In addition, there is a method called the null test that measures the entire shape of the surface to be measured at once. This method uses a reference surface to perform shape measurement using normal interference fringes.

Furthermore, as a measurement method that combines both methods, a method is known in which an axially symmetric aspherical surface is moved in the direction of the optical axis and the measured shapes of each part of the surface to be measured, which produce coarse interference fringes, are connected to obtain the overall shape. (Optics 2.1983.296-300
(see page).

[Problems to be Solved by the Invention] However, among the conventional examples mentioned above, the point coordinate measurement method can measure objects of various shapes and is highly versatile, but on the other hand, it requires a large number of measurement points (as the measurement time becomes long). (There is a drawback that it becomes.

Further, the null test method requires one reference mirror or lens for one object shape to be measured, and has the drawback of low versatility (high measurement cost).

Furthermore, the method of connecting the coarse parts of the interference fringes to obtain the overall shape has the advantage of being versatile and requiring shorter measurement time than point coordinate measurement, but when joining, the coordinates of one point of the overlapped part are However, since they are connected assuming that they are the same, for example, a measurement error in the measurement data at one point in the overlapping portion becomes a measurement error in all the surface shape data after being connected. Furthermore, if there is movement δθ in the tilt direction of the object to be measured due to mechanical movement, a shape measurement error of r・δθ will occur, where r is the radial distance of the connecting part. I have the above problem.

In addition, in this conventional example, since the object to be measured is moved only in the optical axis direction, it is possible to measure only axisymmetric aspherical surfaces, and special shapes such as cylindrical and toric shapes cannot be measured.Therefore, the object of the present invention is to , In view of the above problems,
It is an object of the present invention to provide a method and device for measuring surface shapes, etc., which shortens measurement time and is capable of measuring various shapes, increasing versatility.

[Means for Solving the Problems] In order to achieve the above object, the present invention divides the object to be measured into a plurality of partial regions each having measurable overlapping portions, and measures each partial region. By connecting surface shape data, etc., the overall shape of the object to be measured is obtained.

Measurement of each partial area is performed by using interference effect using reflected light or transmitted light from this area, for example, to obtain surface shape data, etc.Furthermore, the interference measuring device of the present invention achieves the above object. , a stage that changes the relative distance between the interference optical system and the object to be measured in the optical axis direction, a stage that changes the interference measurement light irradiation area of the object to be measured in a direction perpendicular to the optical axis, and a stage that changes the relative distance between the interference optical system and the object to be measured in the optical axis direction. It has a stage that rotates the optical system around two axes perpendicular to the optical axis to provide tilt.

[Function] In the present invention having the above configuration, the object to be measured is arbitrarily divided in a plane perpendicular to the optical axis direction, each divided area is measured, and the measurement data is connected to determine the overall surface shape, etc. Because it uses a method of restoring data, the measurement time is relatively short and it is possible to measure objects with a variety of shapes, including cylindrical, toric, and axisymmetric aspheric surfaces, increasing its versatility.

Then, as a connection method, if the surface shape data etc. measured for each partial area are connected by statistically fitting the measurement data of the mutually overlapping parts, the occurrence of errors caused by the connection can be suppressed as much as possible. This prevents deterioration of accuracy.

[Embodiment 1] Figure 1 shows an embodiment of the present invention. In the figure, 1 is a laser as a light source, 2 is a beam expander that expands the beam diameter, and 3 is a beam expander that divides the large incident light into a P-polarized light component and an S-polarized light component. 4a and 4b are round quarter plates that rotate the polarization direction by 90 degrees by reciprocating, 5 is a plane mirror that is a reference surface, and 6 is a collimator lens that converts plane waves into spherical waves. , 7 is an object to be measured, 8 is a polarizing plate that causes two polarized light components perpendicular to each other to interfere with each other, 9 is an imaging lens, 10 is a photodetector such as a CCD element that observes interference fringes, and 11 is an object to be measured. 12, 13, and 14 are stages that give translational movement to the object to be measured 7; Z stage, Y stage, and X stage, respectively;
A stage, 15 is a piezo element for scanning interference fringes to perform fringe scanning measurement, and 16 is a piezo element in the optical axis direction of the stage (
17 is a PZT driver for driving the piezo element 15 in the optical axis direction; 18 is an image memory for temporarily storing data from the CCD element 10; 19 is a stage. A stage driver 20 for driving controls the entire system and also controls the CCD element 1.
It is a microcomputer that processes data from 0.

In the above configuration, the light emitted from the laser 1 is expanded to a predetermined diameter by the beam expander 2 and enters the polarizing beam splitter 3. Of this light, the S-polarized component is bent upward, passes through the λ/4 plate 4a, is reflected by the reference mirror 5, and passes through the λ/4 plate 4a again.

Polarizing beam splitter 3 with polarization angle rotated 90 degrees
Go back and go straight down. In this way, the light passes through the polarizing plate 8 and the imaging lens 9 and enters the CCD element 10 .

On the other hand, out of the light incident on the polarizing beam splitter 3, P
The polarized light component travels straight to the right, passes through the L/4 plate 4b, is converted into a suitable spherical wave by the collimator lens 6, and is sent to the object to be measured 7.
incident on . Then, it is reflected here, passes through the collimator lens 6 and the λ/4 plate 4b again, returns to the polarizing beam splitter 3 with the polarization angle rotated by 90 degrees, and is bent downward to pass through the polarizing plate 8 and the imaging lens 9. The light passes through and enters the CCD element 10.

At this time, if the position of the object to be measured 7 is adjusted and the shape of the object to be measured 7 and the shape of the spherical wave produced by the collimator lens 6 approximately match, there will be sufficient roughness of interference on the CCD element 10. Stripes are observed. The observed interference fringes are measured object 7.
It gives information on the deviation between the shape of the spherical wave and the shape of the spherical wave, that is, the wavefront aberration, and one stripe is exactly equal to a deviation of half the wavelength of the light from the laser 1.

Therefore, if the shape of the object to be measured 7 is close to a spherical surface, the roughness of the interference fringes will be appropriate over the entire area, and the overall shape of the object to be measured 7 can be determined at once by analyzing the interference fringe pattern. By performing fringe scanning measurement using the measurable piezo element 15, the overall shape can be measured with high precision by reading the phase difference between CCD pixels.

However, if the shape of the object 7 to be measured is special and deviates greatly from a spherical surface, if you try to measure the entire shape at once, the stripe spacing will be too small (so much so that it becomes impossible to measure).In principle, If the phase difference between the interference fringe patterns between adjacent pixels of the CCD on the CCD10 exceeds π, measurement becomes impossible.

However, if the shape of the object to be measured 7 is divided into several small parts, it is possible to make the interference fringe pattern observable in the small areas.

Figure 2 shows this situation, as shown in Figure 2(a).
Assuming that the area U has a relatively small radius of curvature, by bringing the object 7 closer to the collimator lens 6 and tilting it appropriately, the surface shape of the area U and the spherical wave will be approximately equal. In order to measure a region V with a slightly larger radius of curvature in the 0th order, where coarse stripes can be produced in that region, the object 7 is moved to Z as shown in FIG.
It is moved upward in the X direction and rightward in the X direction to move it a little further away from the collimator lens 6 and further tilt it appropriately. As a result, the surface shape of region V and the spherical wave with a slightly larger radius of curvature become almost the same (it is also possible to produce rough stripes in regions b!v). To measure W, move the object to be measured upward in the Z direction and to the right in the X direction and tilt it appropriately as shown in FIG. At this time, the object to be measured 7 is moved by the stages 12 and 13 shown in FIG.
, I4, and tilting is performed using the tilt stage 11 shown in the figure.

Since the data of each area u9v, w obtained by the above-mentioned operation is relative information indicated by the amount of deviation from the reference spherical surface with different curvature, the absolute value is corrected by taking into account the curvature of each reference spherical surface. Converted to shape data in scale.

The data converted to the shape on an absolute scale is
, When measuring W, it is necessary to restore the amount tilted by the tilt stage 11 and connect it together, but it is usually difficult to separately measure the amount of tilting with a precision higher than 0.1 seconds, and , Y, Z stage 14.13.12 Errors such as pitching, rolling, and yawing associated with translation usually exist for several seconds or more, so the error is not restored and connected by the amount of agitation.

Therefore, in order to realize highly accurate connection, shape data of the overlapping portion of the measurement data of each small area is used.

The following methods are available for this connection.

The first method is to select at least three arbitrary points that are not on the same straight line from among the shape data of one overlapping part, and determine the coefficients of the coordinate transformation matrix so that the coordinates of corresponding points in the overlapping area match. This method calculates v
Although the distance between f is short, if there is an error in the selected measurement point data, a large error will result when the shape data of each small area on an absolute scale is connected.

The second method uses all or part of the shape data of the overlapping parts to determine the coefficients of the coordinate transformation matrix that minimizes the sum of the squares of the data differences of corresponding points, and then calculates all the measured data of both parts. This is a method of fitting.

The third is the cross-correlation function R of the shape data of the overlapping parts.
In this method, the coefficients of the coordinate transformation matrix are determined so that o, etc. take the peak value or the maximum value, and all measured data of both parts are fitted.

The fourth is R2, R51nθ, Rcos when the shape data of the % overlapping parts are respectively expanded by Zernike polynomials.
In this method, the coefficients of the two parts representing the deviation of the radius of curvature and the surface inclination component are compared, the coefficients of the coordinate transformation matrix are determined, and all the measured data of the two parts are fitted.

The above methods have advantages and disadvantages in terms of calculation time and fitting accuracy, and it is necessary to select, combine, and set the number of data depending on the calculation processing capacity and required accuracy.

A flowchart of the entire measurement sequence is shown in FIG.

In the same figure, since the shape of the test object 7 set on the measurement table or stage should be approximately close to the design data, the sub-aperture, or small region, where the overall shape of the test object 7 overlaps is created based on this design data. The data is divided by the combiator 20, and this data is stored. This data is transferred to the drive unit, that is, the stage driver 19, and after the measurement of each area is completed, the driver 19 The test object 7 is moved appropriately.

Then, after fine adjustments are made so that each small area can be measured, the curvature of the reference sphere is measured, and the phase difference between COD pixels is measured from the interference fringe pattern, which is the absolute measure of the small area. Surface shape data is calculated and stored in memory.

After the above measurements have been performed over all the small areas, the surface shape data are connected in the manner described above, and the entire shape of the test object 7 is synthesized and appropriately displayed.

In the above embodiment, the object to be measured was moved, but since it is sufficient to change the relative positional relationship between the interference optical system and the object to be measured, if the object to be measured is large and heavy, the interference optical system All you have to do is move the system.

In addition, in the above embodiment, an example was taken in which the object to be measured has a concave surface, but in the case of a convex surface, the object to be measured 7 can be moved to the left side of the focal point P of the collimator lens 6 in FIG. In the case of a flat object to be measured, the measurement becomes possible by removing the collimator lens 6 shown in FIG. 1.

Furthermore, if the object to be measured has a special shape such as cylindrical or toric, the collimator lens 6 may be a lens system including a cylindrical lens, a toric lens, etc.

The reference wavefront and the shape of the test surface roughly match, making it possible to measure the entire surface with fewer connections.

[Effects of the Invention] As explained above, in the present invention, the shape of the surface to be measured is measured by dividing the surface into regions where interference fringes appear and can be measured by interference. By connecting the two, the measurement cost does not increase and the measurement time is relatively short (it is possible to measure and evaluate special shapes.

In addition, by using statistical means to connect the shape data of each region, it is possible to minimize the deterioration in accuracy caused by the connections.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one embodiment of the present invention, FIG. 2(a), (
b) and (c) are diagrams explaining the present invention in detail, FIG. 3 is a diagram showing a flowchart of the measurement sequence, and FIGS. 4 and 5.
The figure shows a conventional example. 1... Laser, 3... Polarizing beam splitter, 5...
...Reference mirror, 6...Collimator lens, 7...
Object to be measured, 1o... Image carrier element, 11... Tilt stage, 12.13.14... Z, Y, X stage, 1
5... Piezo element

Claims (1)

[Claims] In a method of measuring a one-dimensional shape, etc., an object to be measured is divided into a plurality of partial areas having measurable overlapping parts, and a surface measured for each partial area. A method for measuring a three-dimensional shape, etc., characterized in that the overall shape, etc. of an object to be measured is obtained by connecting shape data, etc. 2. The measuring method according to claim 1, wherein the measurement of each partial region is performed using interference effect using light from the partial region to obtain surface shape data and the like. 3. A surface measured for each partial area by selecting at least three points that are not on the same straight line in the overlapping areas as the number of measurement data, and performing coordinate conversion and fitting so that the coordinates of corresponding points in the overlapping partial areas match. 2. The measuring method according to claim 1, wherein shape data and the like are connected. 4. Surface shape data measured for each partial region by coordinate transformation and fitting of all of the other measurement data so that the sum of squares of the difference between the measurement data of one of the overlapping parts and the measurement data of the other is minimized 2. The measuring method according to claim 1, wherein the measuring method comprises connecting the above. 5. Surface shape data etc. measured for each partial region by coordinate transformation and fitting of all the other measurement data so that the correlation function between the measurement data of one of the overlapping parts and the measurement data of the other takes the maximum value. 2. The measuring method according to claim 1, wherein: 6. Among the coefficients obtained by expanding the measurement data of one of the overlapping parts into a Zernike polynomial, the coefficient of R^2, Rsinθ
2. The measuring method according to claim 1, wherein the surface shape data etc. measured for each partial region are connected by comparing the coefficients of R cos θ and the same coefficients of the other measurement data and performing coordinate transformation and fitting. 7. In an interferometric measurement device that measures three-dimensional shapes, etc., there is a stage that changes the relative distance between the interference optical system and the object to be measured in the optical axis direction, and a stage that changes the relative distance between the interference optical system and the object to be measured in the optical axis direction, and a stage that changes the relative distance between the interference optical system and the object to be measured in the direction perpendicular to the optical axis. A three-dimensional shape measuring device that has a stage that changes the shape of the object to be measured, and a stage that rotates the object to be measured about two axes orthogonal to the optical axis with respect to an interference optical system to give tilt.