JP2000088547A - Shape measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the shape of an aspherical surface in a desired region at a high accuracy by analyzing a plurality of interference fringes obtd. by changing the positional relation between a spot light source disposed on a first reflective surface and a test surface and the change quantity of the positional relation. SOLUTION: A light from a laser light source 1 is condensed by a lens 2 to irradiate a pin hole mirror 3, irradiated as a measuring flux on a aspherical test surface 4, reflected by the aspherical test surface 4, and condensed again by the mirror 3. The measuring flux is reflected further by the mirror 3 to form an image on a light receiving face of a CCD 7 through a lens 6. On the CCD 7 light receiving face, a reference flux interferes with the measuring flux from the test surface to form an interference fringe which is then taken from the CCD 7 in a computer to analyze it and compute the shape of the aspherical test surface 4 from the condition of the interference fringe. A stage 5 moves the test surface 4 in the optical axis direction to change the spacing from the mirror 3 to move a part coincident with the radius of curvature along the asphyerical surface and the measurement in made once again. Results in different measuring regions are combined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shape measuring device for measuring an aspheric surface shape with high accuracy.

[0002]

2. Description of the Related Art Conventionally, a Fizeau interferometer or a Twyman-Green interferometer has been used for measuring a spherical shape.
These interferometers require a reference surface, and measure the spherical shape by comparison with the reference surface. Therefore, the measurement accuracy cannot exceed the surface accuracy of the reference surface. On the other hand, as an interferometer that does not require a reference plane, a Point-Diffraction-Interferometer (hereinafter referred to as PD) based on a wavefront diffracted by a pinhole is used.
I) are known. (Japanese Unexamined Patent Application Publication No. 2-228505) This interferometer realizes highly accurate measurement of a spherical shape using an ideal spherical wave generated by diffraction of a pinhole as a reference wavefront. Here, when the diameter φ of the pinhole satisfies the relationship of λ / 2 <φ <λr / 2a, the wavefront diffracted by the pinhole can be regarded as an ideal spherical wave. A part of the light is incident on the surface to be measured and the light reflected from the surface to be measured is reflected by the reflection surface provided near the pinhole, and interferes with a part of the spherical wave without using the reference surface. High precision measurement is realized. Here, λ is the used wavelength, r is the approximate radius of curvature of the surface to be measured, and a is the diameter of the pinhole.

[0003]

In general, when measuring a surface to be measured with an interferometer with high accuracy, the light receiving element CC
It is necessary to form interference fringes at intervals sufficiently coarser than the pixel interval of D. When the aspheric surface is measured using the above-described PDI, interference fringes with coarse intervals are generated if the radius of curvature of the spherical wave diffracted by the pinhole matches the radius of curvature of the surface to be measured. In the case of an aspherical surface, the radius of curvature varies depending on the location, so that only a part of the region where the interference fringe interval is coarse and can be measured is used, and in the remaining region, the interference fringe interval is extremely dense and measurement becomes impossible. Therefore, in the above-described conventional method, it was possible to measure the entire spherical shape with high accuracy, but it was not possible to measure the entire aspherical shape.

The present invention has been made in view of the above-mentioned problems, and has as its object to provide a shape measuring apparatus capable of measuring a shape in a desired area of an aspheric surface with high accuracy.

[0005]

According to claim 1 of the present invention,
While irradiating a part of the light beam emitted from the light source to the surface to be measured as a measurement light beam, the measurement light beam reflected on the surface to be measured and a part of the light beam emitted from the light source, and In the interferometer for measuring the aspherical shape of the surface to be measured by causing the reference light beam having the wavefront to interfere with each other and detecting the state of interference fringes generated by the interference, the measurement light beam and the reference light beam Is a light beam emitted from a point light source having a predetermined size disposed on a first reflection surface disposed between the light source and the surface to be measured, and the point light source and the surface to be measured. And a position measuring means for measuring a change amount of the positional relationship between the point light source and the surface to be measured. Different parts of the measured surface By analyzing a plurality of interference fringes representing the shape of a region and the amount of change in the positional relationship between the point light source and the surface to be measured, the shape of the surface to be measured in a desired region is measured. Provided is a shape measuring device.

According to a second aspect of the present invention, the position measuring means is a length measuring interferometer, which is installed near the first reflecting surface, and
Between one or more second reflecting surfaces considered to be integral with the reflecting surface and one or more third reflecting surfaces installed near the measured surface and considered to be integral with the measured surface 2. The shape measuring apparatus according to claim 1, wherein the length measurement is performed by using the method.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an optical path diagram showing a first embodiment of the present invention. In the interferometer of the present embodiment, the first reflecting mirror 3 is provided between the laser light source 1 and the surface 4 to be measured.
(Hereinafter referred to as a pinhole mirror). As shown in FIG. 2, the pinhole mirror 3 according to the present embodiment has a metal film deposited on the surface of a glass substrate 3b, and a pinhole 3c is provided in a substantially central portion of the metal film 3a by etching. The light emitted from the laser light source 1 is condensed by a lens 2 and is irradiated on a pinhole mirror 3.
The light is diffracted by the pinhole 3c and spreads as an ideal spherical wave. A part of the spherical wave is irradiated as a measuring light beam on the aspherical surface 4 to be measured, reflected on the surface 4 to be measured, and collected again on the pinhole mirror 3. The measurement light beam is further reflected by the pinhole mirror 3, becomes a parallel light beam by the lens 6, and reaches the light receiving surface of the CCD 7. Lens 6
Also serves to form an image of the surface 4 to be measured on the light receiving surface of the CCD 7, and is designed to suppress distortion in order to accurately know the shape of the surface 4 to be measured. By correcting the abscissa of the interference fringes using the designed or measured distortion values, the coordinates on the measured surface 4 and the coordinates on the CCD 7 can be accurately related.

The surface 4 to be measured is placed on a stage device 5 which can move in one direction. The optical axis of the surface 4 to be measured is adjusted to be parallel to the moving direction of the stage 5 and to pass through the center of the pinhole 3c. The stage 5 can perform fine movement and coarse movement in the movement direction. In this embodiment, a piezo element (not shown) is used for the fine movement operation. A part of the ideal spherical wave diffracted by the pinhole 3c is converted into a parallel light beam by the lens 6 as a reference light beam.
Reach D7.

When the surface 4 to be measured is aspherical, the light reflected on the surface to be measured does not converge at one point but spreads on the pinhole mirror 3. Assuming that the difference between the spherical wave applied to the measured surface 4 and the measured surface is W (h), the pinhole mirror 3
The radius of the spread above is r × dW (h) / dh. The maximum value of dW (h) / dh within the range to determine the surface shape by analyzing the interference fringes is dW
If max, the shape of the pinhole mirror 3 is radius r × dW
Within the range of max, it suffices to have accuracy enough to be regarded as a high-precision plane.

On the light receiving surface of the CCD 7, interference fringes occur due to interference between the reference light beam and the reflected light (measurement light beam) from the surface to be measured. The output from the CCD 7 is taken into a computer (not shown) and analyzed, and the shape of the measured surface 4 is calculated from the state of the interference fringes. As described above, when the surface to be measured is an aspherical surface, the interval between the interference fringes becomes coarse at a portion where the radius of curvature of the aspherical surface and the radius of curvature of the irradiated spherical wave match, but becomes denser away from that portion. The range in which the interference fringes are observed is limited by the pixel interval of the CCD camera 7. If the period of the interference fringes is shorter than the two pixels of the CCD camera, a phenomenon called aliasing occurs, and accurate measurement cannot be performed. In order to perform highly accurate measurement, it is desirable that the period of the interference fringes is at least four times or more the pixel interval of the CCD camera. When the shape of the surface to be measured has a large deviation from the spherical surface, only a part of the surface to be measured satisfies the above condition.

Next, the surface to be measured 4 is moved in the direction of the optical axis by the stage 5 to change the distance between the pinhole mirror 3 and the portion having the same radius of curvature along the aspherical surface. Take another measurement. At this time, the measurable area is different from the previous measurement area. By combining the results of these two measurements with a computer (not shown), it is possible to measure the shape of the surface to be measured in a wide area that cannot be measured by one measurement. Further, when the area of the measurement region is insufficient, these operations may be repeated. In order to combine the measurement results with high accuracy, it is preferable to increase the area of the part commonly measured in the two measurements before and after the measurement to some extent. Further, in order to synthesize the measurement results with high accuracy, it is necessary to accurately know the amount of movement of the surface to be measured with respect to the point light source and a change in the posture. The amount of movement and the movement are measured by measuring the change in the distance between the attached second reflecting surface 8 and the third reflecting surface 9 integrally formed on the outer periphery of the measured surface with three length measuring interferometers 10. , The inclination of the surface to be measured is measured with high accuracy. The orientation of the measured surface can be further calculated and corrected by alignment error correction software. The length measuring interferometer 10 only needs to be able to measure a change in the interval between two reflecting surfaces, and in the present embodiment, a commercially available length measuring interferometer system is used as it is.

In one measurement, in order to calculate the shape of the surface to be measured from the interference fringes, the interferometer of the present embodiment uses the fine movement mechanism of the stage 5 to move the surface to be measured 4 minutely in the optical axis direction. The surface accuracy of the surface to be measured is measured with high accuracy by moving (the conventional phase shift method). In this embodiment, the pinhole mirror 3 is fixed and the surface to be measured is movable by the stage. However, the surface 4 to be measured is fixed, and the pinhole mirror 3, the lens 6, and the CCD 7 are integrated. It may be configured to be movable.

Next, a second embodiment of the present invention is shown in FIG. An optical fiber is used in place of the pinhole mirror in the first embodiment. Optical fiber 11
The end face 11a corresponds to the reflection surface 3a of the pinhole mirror, and the optical transmission section 11c corresponds to the pinhole 3c. The reflection surface of the fiber end face is processed so that it can be regarded as a plane with a radius larger than the radius r × dWmax and sufficiently high precision. The second reflection surface 8 is integrally attached to the member 12 holding the fiber. The DUT 4 is mounted on the stage 5 via a holding member 13 having a third reflecting surface 9 integrally attached thereto. The three length measuring interferometers 10 are used to control the reflecting surface 8 and the reflecting surface 9. Interval changes are measured with high accuracy. The method of measuring the surface to be measured (aspheric surface) is the same as in the first embodiment.

Further, it is also possible to use an optical waveguide instead of the optical fiber. In this case, the optical transmission part of the optical waveguide corresponds to the pinhole 3c, and the end face of the optical waveguide is reflected by the pinhole mirror. This corresponds to the surface 3a.

[0015]

As described above, according to the present invention, it is possible to measure the shape of an aspherical surface with high accuracy, which is difficult with the conventional method.

[Brief description of the drawings]

FIG. 1 is an optical path diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a configuration of a pinhole mirror.

FIG. 3 is an optical path diagram showing a second embodiment of the present invention.

FIG. 4 is an enlarged view showing a tip portion of an optical fiber.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser light source for surface shape measurement 2 ... Condensing lens 3 ... Pinhole mirror 3a ... Metal film 3b ... Glass substrate 3c ... Pinhole 4 ... Surface to be measured 5 ··· Moving stage 6 ··· Lens 7 ··· CCD 8 ··· Reflecting surface 9 ··· Reflecting surface 10 ··· Length measuring interferometer 11 ··· Optical fiber 11a ··· Optical fiber end surface 11c ··· Optical fiber transmission unit 12 ・ ・ ・ Optical fiber holding member 13 ・ ・ ・ Measurement surface holding member

Claims (2)

[Claims] 1. A part of a light beam emitted from a light source is radiated to a surface to be measured as a light beam for measurement, and one of the light beam for measurement reflected by the surface to be measured and the light beam emitted from the light source. In the interferometer for measuring the shape of the surface to be measured by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference, the measurement light beam and the reference The luminous flux for use is a luminous flux emitted from a point light source having a predetermined size disposed on a first reflecting surface disposed between the light source and the surface to be measured, and A stage device for changing the positional relationship with the measurement surface, and position measuring means for measuring the amount of change in the positional relationship, wherein the positional relationship between the point light source and the surface to be measured is changed. Measured surface obtained by different By analyzing a plurality of interference fringes representing the shape of the partial region and the amount of change in the positional relationship between the point light source and the surface to be measured, a shape of a desired range on the surface to be measured is measured. Measuring device. 2. The position measuring means is a length measuring interferometer,
One or more second reflective surfaces installed near the first reflective surface and considered integral with the first reflective surface, and installed near the measured surface and considered integral with the measured surface The shape measuring apparatus according to claim 1, wherein an interferometric length is measured with one or more third reflecting surfaces.