JPH0447242B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the surface condition of a flat or curved surface, and particularly to a surface shape measuring method for effectively measuring curved surfaces such as toroidal surfaces and cylindrical surfaces.

BACKGROUND ART Conventionally, in order to measure the shape of an optically polished surface of a lens, mirror, etc., measurement means that utilize interference phenomena, such as a Newton prototype or various interferometers, have been used. These methods are mainly aimed at measuring the shape of planes and spherical surfaces, and if the surface to be measured has different curvatures for the generatrix and sagittal line, such as a rotating curved surface, measurement is impossible or is subject to significant limitations. It turns out. For example, when measuring the shape of a toroidal surface using a conventional Newton prototype, a Newton prototype with a spherical or cylindrical surface is usually used to inspect multiple locations on the cross section of the toroidal surface in the sagittal direction. Alternative methods are used. With this method, only one cross section can be inspected at a time, and if the Newton prototype is reapplied each time the measurement location is changed, the optical path length between the reference surface of the Newton prototype and the toroidal surface that is the surface to be measured is becomes unrelated to the previous measurement location. Therefore, even if the Newton rings in the sagittal direction cross section are recorded sequentially using photographs, etc., and these photographs are stitched together, the phases of the stripes 1 of the Newton rings are discontinuous with each other, as shown in Figure 1. However, it is difficult to know the two-dimensional shape of the toroidal surface. Even if interferometers such as those of Twyman and Fizeau are used in place of Newton's prototype, the same problem will inevitably occur.

One way to solve such problems is to
First, the cross-sectional shape of the central part in the generatrix direction is measured with an interferometer, and when the cross-section orthogonal to it, that is, the cross-sectional shape in the sagittal direction, is two-dimensionally joined, the interference is calculated based on the cross-sectional shape in the generatrix direction. One possible method is to correct the phase shift of the fringes. However, this method requires measurement in both the generatrix direction and the sagittal direction, and also requires correction of the phase shift of the interference fringes.
Furthermore, if the numerical aperture of the cross section in the generatrix direction of the measurement range is large and cannot be measured all at once, it becomes difficult to join interference fringes within the cross section in the generatrix direction, resulting in accurate measurement of the two-dimensional shape. You will not be able to do that.

As another method, the toroidal surface is placed on a turntable so as to be able to rotate around its axis of rotational symmetry, and while the turntable is rotated, the sagittal cross sections of the toroidal surface are successively measured using an interferometer. By measuring it, you can find out its two-dimensional shape. However, in this method, in order to keep the phase of the interference fringes in each cross section continuous, the optical path length difference between the reference surface of the interferometer and the measured surface is kept within a range sufficiently smaller than the wavelength of the light during measurement. It must be held constant, and a precise and expensive mechanism is required to protect the turntable's bearings and the entire measuring device from vibrations, and to prevent air fluctuations.

Still other means include using a Newtonian prototype having a toroidal surface, using an anamorphic optical system that generates a toroidal reference wavefront in an interferometer, or generating a toroidal reference wavefront with a hologram. Depending on the method,
It is conceivable to measure the entire toroidal surface two-dimensionally. However, this method also has the disadvantage that it is difficult to manufacture the Newton prototype, anamorphic optical system, hologram, etc., and the difficulty doubles when the numerical aperture of the surface to be measured becomes large, making the Newton prototype etc. less versatile. .

The purpose of the present invention is to solve the above-mentioned drawbacks of the conventional example, and to accurately measure the two-dimensional shape of not only continuous planes but also rotating continuous curved surfaces such as toroidal surfaces and cylindrical surfaces using an interferometer, and to The purpose is to provide a surface shape measurement method that can measure the shape of a rotating curved surface with a large numerical aperture of the surface to be measured, which cannot be measured in principle with an optical system. The splitter separates the first light beam into a first and second light beam, and the first light beam is divided into two by a splitting optical system and is made incident on two measured positions of a predetermined measurement region within the surface to be measured. forming an interferometer by superimposing the light beams from each of the measured positions with a second light beam passing through a reference optical path;
The shape of the measurement area is measured using the interferometer, and the shape measurement is performed while displacing the measurement area in the direction within the measurement surface by a predetermined amount corresponding to the interval on the measurement surface of the divided light beams within the measurement surface. Repeatedly, when measuring the shape of each part, the same measured position is illuminated in the same way by different divided light beams, so that at least part of the measured position of the measured part overlaps with part of the measured position of the different measured part. Then, compare the measured values obtained for the duplicated measurement positions that should be the same, find the amount of deviation between these measured values, and refer to the amount of deviation for the measured value of one measurement site. This surface shape measuring method is characterized in that information on the overall shape of the surface to be measured is obtained by correcting the shape of the surface to be measured and joining the shapes of the respective measurement parts obtained by the correction.

Next, the present invention will be explained in detail based on illustrated embodiments.

FIG. 2 shows an apparatus for implementing the method according to the present invention, and is a plan configuration diagram when measuring a sample S having a toroidal surface St, and FIG. 3 is a diagram showing the measurement configuration. Sample S is a lens or mirror, and
It is placed on a stage 10 having a stage and a tilting mechanism. Furthermore, this stage 10 is fixed on the turntable 20, and the stage 10 is connected to the turntable 2.
It is possible to rotate around the central axis l of 0. A Twyman interferometer 30 is installed facing the sample S, and an imaging device 40 for detecting interference fringes is attached to an observation window on its side. The output line of the image sensor 40 is connected to a memory circuit 50 for storing the intensity distribution of interference fringes, and the output of the memory circuit 50 is further input to a control calculation circuit 60 that controls and calculates the entire apparatus. Then, one output of the control calculation circuit 60 is sent to the stepping motor 70, so that the stepping motor 70 rotates the turntable 20 to an arbitrary position.

Prior to measurement, the toroidal surface St of the sample S is
The axis of rotational symmetry q is adjusted by the XY stage and tilting mechanism of the stage 10 so that it coincides with the axis of rotation l of the turntable 20.
However, if the toroidal surface St is processed by rotary polishing and can be rotated using the rotation axis during processing without removing it from the processing machine, the above adjustment can be omitted. In the interferometer 30, the parallel light beam L emitted from the laser light source 31 passes through the beam expander 3.
After widening the beam width by 2, half mirror 3
3, it is divided into a reflected light flux La and a transmitted light flux Lb. The light beam Lb transmitted through the half mirror 33 becomes two parallel light beams L1 and L2 at angles to each other by the double prism 34 and enters the objective lens 35. Therefore, the interferometer 30
Adjust the distance between and the toroidal surface St. However, toroidal surface St, stage 10, turntable 20
It is assumed that the relative positions between the comrades remain unchanged during the distance adjustment described above. Next, the double prism 34
By moving in the optical axis direction of the objective lens 35, the two light beams L after exiting the objective lens 35 are
1, without changing the imaging position of L2, the luminous flux L
The directions of the chief rays 1 and L2 are changed so that the intersection point of the chief rays is placed on the rotation axis l of the turntable 20.

Through the above procedure, the two light beams L1 and L2 incident on the toroidal surface St have their condensing directions directed toward the center of curvature p of the sagittal line of the toroidal surface St, and the principal rays of the two light beams L1 and L2 are each toroidal. It is directed toward the center of curvature q of the generatrix of the surface St. Therefore, the light beam reflected by the toroidal surface St is the second
In the plane of the figure, that is, in the plane including the generatrix of the toroidal surface St, only the principal ray travels in the opposite direction along the same optical path as the original, passes through the half mirror 33 and the imaging lens 36, and reaches the light-receiving surface of the image sensor 40. On the other hand, the third
In the plane of the figure, that is, the rotation axis l of the turntable 20
In the plane including the toroidal surface St, all the light beams reflected by the toroidal surface St travel in the opposite direction along the same optical path as before, are reflected by the half mirror 33, and reach the light receiving surface of the image sensor 40. A light beam La reflected by a half mirror 33 after being emitted from a laser light source 31 is reflected by a reference mirror 37, transmits through the half mirror 33, and is superimposed and interferes with the aforementioned principal ray reflected from the toroidal surface St. Therefore, as shown in FIG. 4, at a position corresponding to the light-receiving surface of the image sensor 40, the shapes of the two sagittal cross sections U and V on the toroidal surface St are represented for the two light beams L1 and L2. Interference fringes u and v appear. In this state,
By driving the stepping motor 70 and rotating the turntable 20, the shape of an arbitrary sagittal cross section on the toroidal surface St can be measured. As is well known, the interference fringes produced by a Twyman interferometer represent the contour lines of a wavefront in units of 1/2 of the wavelength of light, so by counting the contours of these interference fringes, the shape of the wavefront can be determined. , that is, the shape of the sagittal cross section of the toroidal surface St is known. However, if this continues as it is, discontinuity in the phase of the interference fringes will occur due to vibrations accompanying the rotation of the turntable 20 or disturbances, so the following procedure may be adopted.

In FIG. 5, at a certain point in time shown in a, the interference fringes h,
It is assumed that the intensity distribution of i is stored in the memory circuit 50. Next, the turntable 20 is rotated to change the measurement position and the measurement is performed again. At this time, the rotation angle of the turntable 20 is controlled so that one measurement position is the same as the previously measured position I, and
Two sagittal cross sections of J are measured to obtain interference fringes i' and j as shown in FIG. 5b. Here, the interference fringes i and i' are interference fringes that do not have the same pattern even though the same location I is measured, but are caused by vibrations due to the rotation of the turntable 20 mentioned above and a phase shift of the interference fringes due to disturbances. Normally, lateral slippage occurs because of this. Here, in Fig. 6, the horizontal axis is the position in the sagittal direction,
It is assumed that the vertical axis represents the cross-sectional shape of each measurement position and the height position, and that the interference fringes i and i' are shifted by Δ ₁ . Since positions I and J are ₁ at the same time,
The deviations of interference fringes i and j are the same amount, so the cross-sectional shape of interference fringe j is corrected by the deviation amount Δ ₁ obtained by comparing interference fringes i and i′, and a new cross-section is created. If the shape interference fringe j ₀ is used, this interference fringe j ₀ is one in which the effects of vibration and disturbance have been removed. Rotate the turntable 20 again and similarly measure the interference fringes j' and k at the position J and the new measurement position K as shown in Fig. 5c.
By measuring as shown in , performing the same comparison as in the case of i and i′, and correcting by finding the amount of deviation Δ ₂ , a shape k ₀ with continuity can be obtained for the position K as well. . These corrected interference fringes are stored in the memory circuit 50 and output to a display device or a recording device as required. By repeating the above operations, the shapes of the cross sections of each sagittal wire are seamlessly joined over the entire toroidal surface St as shown in Figure 7, and as a result, the two-dimensional shape of the toroidal surface St is The shape can be known, and similar observations can be made by observing spherical or flat interference fringes two-dimensionally using a conventional interferometer.

In order to obtain the phase shift of the interference fringes or the amount of lateral shift of the cross-sectional shape from two measured values at the same location, the control calculation circuit 60 performs a Fourier transform on each of the two measured values and compares the phase terms. You can use the following methods. In addition, since the measurement interval in the generatrix direction of the toroidal surface St in the above measurement is determined by the angle α formed by the principal rays of the two light beams L1 and L2 in FIG. The angle α of the double prism 34 may be made small to make the angle α small.
Furthermore, the rotation angle pitch α/ of the turntable 20 is
If n is set to a small number (n is an integer) and the same location is repeatedly measured every nth rotation, the measurement interval can be made n times closer.

In the above embodiment, the interferometer 30 is of the Twyman type, and the double prism 34 is used to generate the two light beams L1 and L2, but it is not necessarily limited to these, and for example, as shown in FIG. A Fizeau type interferometer can be used as shown, and various modifications are possible, such as combining two sets of interferometers as shown in FIG. 9 instead of using the double prism 34. Needless to say. In addition, in these FIGS. 8 and 9, the same reference numerals as in FIGS. 2 and 3 indicate the same members.

Applications of the present invention are not limited to the measurement of curved surfaces of revolution that have only one axis of rotational symmetry, such as toroidal surfaces and cylindrical surfaces. For example, the effectiveness of the present invention is demonstrated even for surfaces such as plane mirrors or spherical lenses for which simultaneous front surface measurement is impossible in principle due to the numerical aperture limit of conventional optical systems. Further, in the embodiment, two cross sections are measured at one time, but three or more locations may be measured at the same time.
Furthermore, it is also conceivable to compare the measured values not with one set of measured values but with a plurality of sets of measured values to accurately determine the amount of deviation. Also,
In the embodiment, the comparison of measured values to determine the amount of deviation is performed every time one measurement is completed, but all measured values are stored as they are in the memory circuit 50, and the comparison is performed sequentially after the measurement is completed. The measured value may be corrected by the control calculation circuit 60 to obtain a correct cross-sectional shape.

In addition, in the measurement when the sample S has a shape with a cylinder surface, the shape of the generatrix cross section is may be measured in the same manner as described above. Alternatively, it is also possible to measure the shape of the sagittal line cross section while linearly moving the cylinder surface using a stage that can linearly move the cylinder surface in a direction parallel to the generatrix of the cylinder surface.

As explained above, the surface shape measuring method according to the present invention obtains the amount of deviation between the measured values each time, and stitches the interference fringes corrected for this amount of deviation, so that an accurate cross-sectional shape can be obtained. This has an extremely large effect, especially on curved surfaces formed by generatrix lines and sagittal lines, as it enables measurements that were almost impossible in the past.

[Brief explanation of the drawing]

Figure 1 is a front view of the surface to be measured consisting of a set of interference fringes obtained by a method that does not correct for discontinuities;
The following figures show an embodiment of the surface shape measuring method according to the present invention, FIG. 2 is a plan configuration diagram of an apparatus for realizing this method, FIG. 3 is a side configuration diagram thereof, and FIG. 4 is a measurement method. Explanatory diagrams of interference fringes; Figures 5a, b, c, and 6 are explanatory diagrams of the procedure for correcting discontinuities in measured values; Figure 7 is an illustration of a set of interference fringes obtained by correcting discontinuities. 8 and 9 are configuration diagrams of other embodiments for implementing the method according to the present invention. 10 is an X-Y stage, 20 is a turntable, 30 is an interferometer, 31 is a laser light source, 32
is a beam expander, 33 is a half mirror, 3
4 is a double prism, 35 is an objective lens, 36 is an imaging lens, 37 is a reference mirror, 40 is an image sensor, 5
0 is a memory circuit, 60 is a control calculation circuit, 70 is a stepping motor, S is a sample, and St is a toroidal surface.

Claims (1)

[Claims] 1. A beam splitter separates the measurement light from a laser light source into a first and a second beam, and a splitting optical system divides the first beam into two to produce a predetermined beam on a surface to be measured. The light beams from each of the measurement positions are superimposed on the second light beam that has passed through the reference optical path to form an interferometer, and the interferometer is used to Measure the shape of the measurement site, repeat the shape measurement while displacing the measurement site in the direction of the measurement surface by a predetermined amount corresponding to the interval of the split light beams on the measurement surface, and measure the shape of each site. In this case, at least a part of the measurement position of the measurement part overlaps with a part of the measurement part of the different measurement part in such a way that the same measurement position is illuminated in the same way by different divided light beams, and the overlapped measurement part is Compare the measured values obtained for the positions that should be the same, find the amount of deviation between these measured values, and correct the measured value of one measurement site by referring to the amount of deviation. A method for measuring a surface shape, characterized in that information on the overall shape of the surface to be measured is obtained by piecing together the shapes of the respective measurement sites obtained. 2. When the surface to be measured is a curved surface formed by a generatrix and a sagittal line, the principal ray of the light beam emitted from the interferometer is aligned with the center line of curvature of the generatrix, and the focal position of the light beam is set at the center of curvature of the line. 2. The surface shape measuring method according to claim 1, wherein the measurement site is changed by rotating the surface to be measured about the center of curvature of the generatrix.