JPS58173423A - Measuring method of face shape - Google Patents

Abstract

PURPOSE:To exactly observe a sectional shape of a rotating curved face of a troidal face, etc., by changing the positions of two sectional shapes, measuring them repeatedly by an interferometer, and joining an interference fringe which corrects a shift quantity of each measured value of each time. CONSTITUTION:A sample having a troidal face St is placed on an object placing plate having an X-Y stage, luminous fluxes L1, L2 from an interferometer 30 are irradiated, reflected light and a luminous flux La reflected by a half mirror 33 interfere with each other, and an interference fringe appears. For instance, interference fringes (h), (i) in tangential line sections H, I are stored in a memory circuit 50. Subsequently, sections I, J are measured by rotating a table 20. The position I in this case is made as same as the previous time. Between interference fringes i' of i' and J in this case and the previous (i), a shift of DELTA1 exists. By correcting it, a new interfernce fringe j0 is obtained. When this operation is repeated and the interference fringe of each tangential line section is joined, a two-dimensional shape of a troidal face, etc. can be observed exactly in the same way as a usual interference fringe.

Description

[Detailed description of the invention] The present invention is a method for measuring the surface condition of a flat or curved surface,
In particular, the present invention relates 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 such as a lens or mirror, measurement means that utilize interference phenomena such as a Newtonian 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. For example, when measuring the shape of a toroidal surface using a conventional Newtonian prototype, a Newtonian prototype with a spherical or cylindrical surface is usually used to measure multiple cross sections of the toroidal surface in the sagittal direction. Alternative methods of spot inspection are used. With this method, only one cross section can be inspected at a time, and if the Newtonian prototype is reapplied each time the measurement location is changed, the optical path length between the reference surface of the Newtonian prototype and the toroidal surface that is the surface to be measured is becomes unrelated to the previous measurement location. Therefore, even if Newton's rings in the sagittal direction cross-section are recorded sequentially using photographs, etc., and these photographs are stitched together, the phases of the Newton's ring fringes 1 are discontinuous with each other, as shown in Figure 1, and the toroidal It is difficult to know the two-dimensional shape of a surface. Even if a Twyman, Fizeau, etc. interferometer is used instead of the Newton prototype, the same problem will inevitably occur.

One way to solve such problems is to first measure the cross-sectional shape of the central part in the generatrix direction using an interferometer, and then measure the cross-sectional shape orthogonal to the cross-sectional shape, that is, the cross-sectional shape in the sagittal direction, two-dimensionally. A possible method is to correct the phase shift of the interference fringes based on the cross-sectional shape in the generatrix direction. However, this method requires measurement in both the linear 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 kept constant, and the turntable bearing and the entire measuring device must be earthquake-proofed.
Precise and expensive mechanisms are required to prevent air fluctuations.

Still other means include using a Newtonian prototype with a toroidal surface, or using an anamorphic optical system that generates an I-Roytal reference wavefront in the interferometer.
Alternatively, the entire toroidal surface may be measured two-dimensionally by a method such as generating a toroidal reference wavefront using a hologram. However, this method also has the disadvantage that it is difficult to produce Newtonian prototypes, anamorphic optical systems, holograms, etc., and the difficulty doubles when the numerical aperture of the surface to be measured becomes large, making the Newtonian 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 of this method is to provide a surface shape measurement method that makes it possible to 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. At least two or more cross-sectional shapes are repeatedly measured using an interferometer while changing the measurement position on the surface to be measured, and each measurement is performed at least once.
1 more than one measurement position is the same as the previous measurement position,
Compare the measured values obtained at the same position at 1st and 2nd measurement time - 9° to find the amount of deviation between these measured values, and use the measured value at one measurement as the standard. A method characterized in that information on the two-dimensional shape of the surface to be measured is obtained by correcting a plurality of measured values at the time of the other measurement and joining each cross-sectional shape obtained by the correction. be.

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 side configuration diagram thereof. The sample S is a lens or a mirror, and is placed on a stage 10 having an X-Y stage and a tilting mechanism. Furthermore, this stage 10 is fixed on a turntable 20 so that the stage 10 can rotate around the central axis 1 of the turntable 20.

A Twyman type interferometer 30 is installed opposite the sample S,
An imaging device 40 for detecting interference fringes is attached to the observation window on the side. The output line of this image sensor 40 is connected to a memory circuit 50 for storing the intensity distribution of interference fringes,
Furthermore, the output of the memory circuit 50 is input to a control calculation circuit 60 that controls the entire device and performs calculations. Then, one output of the control calculation circuit 60 is sent to the stepping motor 70, and the stepping motor 70 causes the turntable 2 to
Rotate 0 to any position.

Prior to measurement, the axis of rotational symmetry q of the toroidal surface St of the sample S is adjusted by the x-Y stage and tilting mechanism of the drying stand 10 so that it coincides with the axis of rotation g of the turntable 20.

However, the toroidal surface St is polished by rotary polishing,
If it can be rotated as it is using the rotating shaft at the time of machining 1 without removing it from the processing layer, the above-mentioned adjustment can be omitted. Interferometer 3
0, the parallel light beam emitted from the laser light source 31 is expanded in beam width by a beam ex-bang 32, and then reflected by a half mirror 33 into a light beam La.
and a transmitted light beam Lb. This half mirror
33 becomes two parallel light beams L1 and L2 having a transmission angle and enters the objective lens 35. Therefore, objective lens 3
The convergence position of the two light beams L1 and L2 that have passed through the toroidal surface St is an arc p formed by connecting the centers of curvature of the sagittal lines of the toroidal surface St.
The distance between the interferometer 30 and the toroidal surface St is adjusted so that the toroidal surface St is at the top. However, the toroidal surface St and the stage 10.
The relative position between the turntables 20 shall be kept unchanged during the distance adjustment described above. Next, by moving the double prism 34 in the optical axis direction of the objective lens 35, the objective lens 35 is Two light beams L1 after exiting.
The directions of the principal rays of the light beams Ll and L2 are changed to overlap the point at which the principal rays intersect on the rotation axis p of the turntable 20 without changing the imaging position of L2.

Through the above procedure, the two light beams Ll and L2 incident on the toroidal surface St are focused in the direction of the toroidal surface St.
toward the center of curvature P of the sagittal line, and two luminous fluxes Ll,
The principal rays of L2 each head toward the center of curvature q of the generatrix of the toroidal surface St. Therefore, in the light beam reflected by the toroidal surface St, within the plane of FIG. 2, that is, within 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.

The image sensor f- passes through the half mirror 33 and the imaging lens 36.
40 light receiving surfaces are reached. On the other hand, within the plane of FIG. 3, that is, within the plane containing the rotational axis load of the turntable 20,
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 are transferred to the image sensor 4.
It reaches the light receiving surface of 0. The light beam La reflected by the half mirror 33 after being emitted from the laser beam [31 is reflected by the reference mirror 37 and then transmitted through the half mirror 33, where it is superimposed with the aforementioned principal ray reflected from the toroidal surface St and crosses once. . Therefore, at a position corresponding to the light receiving surface of the image sensor 40, two light beams L
1 and L2, interference fringes U and ■ representing the shapes of the two sagittal cross sections U and V of the toroidal surface sth appear. In this state, if the stepping motor 70 is driven to rotate 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 are 1/2 of the wavelength of light.
Since it represents the contour line of the wavefront in units of , the shape of the wavefront, that is, the shape of the sagittal cross section of the toroidal surface St can be determined by counting the contour lines of this interference fringe. 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, it is assumed that the intensity distribution of the interference fringes and i at the two sagittal cross sections H and I on the toroidal surface St are stored in the memory circuit 50 at the time shown in (a). Rotate the table 20 to change the measurement position and measure again. At this time, one measurement position is the previously measured position l
The rotation angle of the turntable 20 was controlled so that the position was the same as the position shown in Fig. 5 (b).
), the interference fringes i and j are obtained. Here, the interference fringe i
, i' are interference fringes that do not have the same pattern even though the same point I is measured, but there is a lateral shift due to the vibration accompanying the rotation of the turntable 20 mentioned above and the phase shift of the interference fringes due to external disturbances. This is normal. Here, FIG. 6 shows the cross-sectional shape of each measurement position with the horizontal axis representing the sagittal direction position and the vertical axis representing the height position, and the interference fringes i and i' are Δ,
Assume that there is a misalignment. Since positions I and J are measured at the same time, the deviations of interference fringes i and j are also the same, and therefore the deviation amount Δ obtained by comparing interference fringes i and i' is
If the cross-sectional shape of the interference fringe j is corrected by the amount of l to obtain a new cross-sectional shape interference fringe j0, this interference fringe j0 will have the effects of vibration and disturbance removed. Rotate the turntable 20 again and measure interference fringes j' and k at position J and a new measurement position K in the same manner as shown in FIG. 5(c). By performing a comparison similar to the above, determining the amount of deviation Δ2, and performing correction, a shape ko having continuity can be obtained for the position INK 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 shape of each sagittal cross section is seamlessly joined over the entire surface of the toroidal surface St as shown in FIG. 7, and as a result, the two-dimensional shape of the toroidal surface St is known. 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. In addition, the measurement interval in the generatrix direction of the toroidal surface St in the above-mentioned M1 constant is the two luminous fluxes L1 in FIG.
, L2 is determined by the angle α formed by the principal rays of L2, so in order to make the measurement interval even closer, the angle α of the biprism 34 in FIG.
If the rotation angle pitch of the turntable 20 is made small to α/n (n is an integer) so that the same point is repeatedly measured every nth rotation, the measurement interval can be made n times closer. be able to.

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.

The application of the present invention is not limited to the measurement of rotational curved surfaces that have only one axis of rotational symmetry, such as toroidal surfaces and cylindrical surfaces. The effectiveness of the present invention is demonstrated even in areas where simultaneous measurement over the entire surface is theoretically impossible due to limitations. Furthermore, in the embodiment, two cross sections were measured at - times, 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. In addition, 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 after the measurement is completed, they are compared one after another. Alternatively, 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 measured while rotating the cylinder surface using a turntable that can rotate the cylinder surface around the axis of rotational symmetry of the cylinder surface. It can be measured in the same way as the method. Alternatively, it is also possible to measure the shape of the sagittal 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 is as follows:
The amount of deviation between the measured values for each measurement is determined, and the interference fringes corrected for this amount of deviation are stitched together, so that an accurate cross-sectional shape can be obtained. This is extremely effective, especially for curved surfaces formed by generatrix and sagittal lines, as it enables measurements that were previously almost impossible.

[Brief explanation of the drawing]

FIG. 1 is a front view of a surface to be measured consisting of a set of interference fringes obtained by a method that does not correct discontinuities; FIG. 3 is a plan configuration diagram of an apparatus for realizing this method, FIG. 3 is a side configuration diagram thereof, FIG. 4 is an explanatory diagram of interference fringes to be measured, and FIGS. 5(a) and (b). (c) and Fig. 6 are explanatory diagrams of the procedure for correcting discontinuities in measured values, Fig. 7 is a front view of the surface to be measured consisting of a set of interference fringes obtained by correcting discontinuities, and Fig. 8 , FIG. 9 is a block diagram of another embodiment for implementing the method according to the present invention. The code io is the X-Y stage, 20 is the turntable, 3
0 is a beam meter, 31 is a laser light source, 32 is a beam expander, 33 is a half mirror, 134 is a double prism, 35
is an objective lens, 36 is an imaging circuit, 0 is a memory circuit, 60 is a control calculation circuit, 7o is a stepping motor, S is a sample, St
is a toroidal surface. Patent applicant: Canon Co., Ltd. Figure 5 [Figure 7 Figure 8]

Claims (1)

[Claims] l, i which is the surface to be measured! At least two or more of the Ia side 1-
The cross-sectional shape of the surface to be measured is repeatedly measured using an L ladle meter while changing the measurement position on the surface to be measured, and in each measurement, at least one measurement position is the same as the previous measurement position, and the first and by comparing the measured values obtained for rotation and position during the second measurement to determine the deviation between these measured values, A method for measuring a surface shape, characterized in that information on the two-dimensional shape of the surface to be measured is obtained by correcting individual measured values and joining the respective cross-sectional shapes obtained by the correction. 2. When the surface to be measured is a curved surface formed by a generatrix and a sagittal line, align the - of the light beam emitted from the F interferometer with the white clay center point of the generatrix, and set the focus position of the light beam on the sagittal line. 2. The surface shape measuring method according to claim 1, wherein the measurement position is changed by rotating the surface to be measured about the center of curvature of the generatrix as an axis in accordance with the center of curvature.