JP2000097618A - Null interferometer computing method and storage medium storing the method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit the measurement of null interference independent of the accuracy of a coordinate measuring device by using both the data of the coordinate measuring device on a test surface and the data on the computation of a null wave front, performing simulation and computation on interference measurement in a computing unit, and feeding back the obtained results on the location of measurement along the optical axis. SOLUTION: First, through the use of alignment error aberration computed with respect to a wavefront of 'NULL 0', alignment error correction is performed, and a test surface is moved to the location of the result of defocusing in a computing unit. Then alignment error correction with respect to the location is performed on data on interference fringes virtually measured at the location, and obtained defocusing components are corrected. This procedure is repeated until the defocusing components due to alignment error correction are converged on zero to specify the location. As the shape of a test surface is approximately accurately measured in the case that higher-order components are not superimposed on the test surface as error, locational error along the optical axis at least on the computed null wave front is absent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating a null interferometer used for measuring a null interferometer for measuring an aspheric surface shape with high accuracy, and a recording medium on which the calculation method is recorded.

[0002]

2. Description of the Related Art Conventionally, a so-called null interference measurement using a null element has been performed for measuring an aspherical shape. As the null element, a null lens mainly composed of a spherical lens composed of a spherical surface, or a zone plate in which an annular diffraction grating is formed on a flat plate is used.

FIG. 4 is a measurement arrangement diagram of null measurement using a null lens, which is a modification of Fizeau interferometry. That is, the plane wave 2 emitted from the interferometer 1 is reflected by the high-precision Fizeau surface 3a formed on the Fizeau plane plate 3, and the plane wave 2 is converted by the null element 4 into a desired aspherical surface at a reference position for measurement. The measurement wavefront 4a converted into the shape is also
The test surface 5a of the test object 5 set at the reference position
Are reflected from each other, and each reflected light forms a single-color interference fringe inside the interferometer 1, the interference fringe is detected by a detector such as a CCD, and the obtained signal is processed by the information of the interferometer. This is analyzed by the information processing system. Similar measurements are possible using a Twyman-Green interferometer.

As a null interference measuring system using this null lens, the shape of a null wavefront is considered by dividing it into a rotationally symmetric component and a non-rotationally symmetric component, and the rotationally symmetric component is further divided into a lower-order component and a higher-order component. A measurement system (Japanese Patent Application No. 10-160027) has been proposed which accurately obtains each component and then adds the components to accurately obtain the aspherical shape of the test surface.

More specifically, the non-rotationally symmetric component is obtained by rotating and averaging the interferometer data measured by rotating the surface to be measured on a real machine in an arithmetic unit, so that the low-order component is separately measured by coordinates. High-order components are achieved by accurately measuring the manufacturing error of the null element and calculating the shape of the null wavefront using the value by accurately measuring the surface to be inspected using a device or the like. I was

This is because, depending on the accuracy of the coordinate measuring instrument, the high-order component cannot be accurately grasped, while the shape of the null wavefront may not accurately specify the low-order component from the grasping error of the manufacturing error.

[0007]

However, when using a coordinate measuring instrument that cannot accurately grasp higher-order components, if there is a large higher-order component error on the surface to be inspected, it is converted to a lower-order component error. There was a possibility. Conversely, when the error of the low-order component is large at a predetermined optical axis position of the null wavefront, the test surface whose low-order component is calibrated by the coordinate measuring instrument has a fringe with respect to the null wavefront having a large low-order error. Even if the measurement is performed at a position that is reduced as much as possible, since the measurement is performed at a position deviated from the test object setting position in the predetermined optical axis direction, an error of a higher-order component due to a change in the null wavefront occurs. There were also problems.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has as its object to enable null interference measurement independent of the accuracy of a coordinate measuring instrument.

[0009]

According to the present invention, in order to achieve the above object, a measuring light beam emitted from a light source is converted into a null wavefront having a predetermined aspherical shape by a null element.
The measurement light flux irradiated and reflected on the surface to be measured and the reference light flux having a predetermined wavefront emitted from the light source interfere with each other, and the state of interference fringes generated by the interference is detected, thereby detecting the interference. Measure the surface shape of the test surface, in the interferometry using a null interferometer, the coordinate measuring device data of the test surface,
By using both the calculation data of the null wavefront and the calculation data of the interference measurement in the calculator, the result of the measurement position in the optical axis direction is fed back to obtain the aspheric shape of the test surface. A null interferometer calculation method characterized by being capable of high-accuracy calibration is used. This makes it possible to calibrate the test surface and the null wavefront with high accuracy.

Although the drawings of the embodiments of the present invention have been used to make the configuration of the present invention easy to understand, the present invention is not limited to the embodiments. Further, in order to use the null interferometer calculation method of the present invention in an information processing apparatus used for an interferometer, the information processing apparatus is caused to execute a procedure for obtaining a surface shape of the test surface from the interference fringes. Recording medium on which a program for recording is recorded.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an alignment error correction by a null interferometer operation method according to the present invention will be described with reference to FIG. Alignment error correction means that when a test surface originally manufactured according to the aspheric design value is subjected to null interference measurement using a null wavefront of a null element manufactured similarly according to the design value, the test surface is Alignment error aberration (apparent aberration observed due to misalignment) occurs due to misalignment from the optimal position (alignment misalignment) with respect to the null wavefront. Therefore, this is an operation for obtaining the original shape error of the surface to be measured by removing the alignment error aberration from the measurement data. (In the case of the above assumption, the shape error is calculated as zero).

"NULL0" in FIG. 1 is a calculator for calculating the aspherical shape of the null wavefront formed by the null element based on the previously measured values such as the radius of curvature and the center thickness of the null element constituting members. (Calculated null wavefront). Furthermore, the waviness error due to the optical surface and internal homogeneity of each component is measured in advance, and is superimposed on the shape error. In the figure, an optimal position (a position where interference fringes are reduced as much as possible) is set as a reference position, where the test surface manufactured according to the design value is virtually set in the arithmetic unit with respect to the null wavefront shape. . This position may be obtained by performing the above-described alignment error correction on the basis of the wavefront of “NULL0”. On the other hand, “NULL1” in FIG. 1 is the shape of the surface to be inspected (in the figure, the reflex surface of the reflex prototype used for reflex subtraction) measured by the coordinate measuring device. FIG. 9 is a schematic diagram showing a case where only a low-order component is used ignoring a high-order component and the calculated null wavefront is similarly set at an optimum position. At this position, the above-described alignment error correction may be performed with reference to the wavefront of “NULL1”. However, this measurement position is obtained only after the alignment error correction is performed, but this position cannot be specified even if the alignment error aberration for performing the alignment error correction is calculated in advance. Therefore, first, "N
The alignment error is corrected by using the alignment error aberration calculated based on the UL0 "wavefront, the test surface is moved in the arithmetic unit to the position of the defocus result, and the interference virtually measured at that position By repeating the procedure of multiplying the fringe data by the position-based alignment error correction and correcting the obtained defocus component until the defocus component due to the alignment error correction converges to zero, Has been identified.

When the higher-order component is not superimposed as an error on the surface to be measured, its shape (lower-order component) is also measured almost exactly. There will be no position error. Furthermore, if there is no error in the null wavefront, the low-order component of the converged data should match the measurement result of the coordinate measuring instrument. However, since the higher-order component is superimposed on the actual test surface as an error and the lower-order component of the null wavefront may have an error, the coincidence may not be found. The correction at this time will be described with reference to FIG. First, the higher order components of the surface to be inspected are substituted by actual measurement data of the interferometer. At this time, if the null wavefront is manufactured so as to be “NULL0” at the reference position, the position where the interference measurement of the test surface is performed should be “NULL1” calculated by the arithmetic unit. Based on the above, the alignment error correction is applied to the measured data. Using the higher order component obtained as a result, together with the lower order component of the coordinate measuring instrument, the above-mentioned convergence operation is repeated (inner loop operation) as a more accurate shape of the test surface, and the converged data The low-order component matches the measurement result of the coordinate measuring instrument.

However, due to the recognition error of the low-order components of the null wavefront, these low-order components converge, but the convergence value is not 0. Accordingly, the shape of the null wavefront is corrected on the assumption that the convergence value (low-order component error) is a recognition error of the low-order component of the null wavefront. By repeating the above-described calculation based on this null wavefront shape (outer loop calculation), the above-described convergence value (low-order component error) can be made substantially zero. That is, the accurate specification of the reflex plane and the null wavefront is completed.

FIG. 2 shows the case where the low-order components of the coordinate measuring instrument which can be accurately measured are the second and fourth orders, and FIG. 3 shows the case where the low-order components are the second to sixth orders. Further, this null interferometer calculation method is programmed so as to be executable in an information processing device used for the interferometer, and the program is attached to a floppy disk, or a magneto-optical disk, a so-called hard disk, or an information processing device. ROM, RAM
By recording the information on a recording medium such as the above, the arithmetic method of the present invention can be stored and executed in the information processing apparatus.

[0016]

As described above, by employing the null interferometer calculation according to the present invention, it is possible to calibrate the test surface and the null wavefront with high accuracy.

[Brief description of the drawings]

FIG. 1 is an explanatory view of the principle of alignment error correction used in a null interferometer operation according to the present invention.

FIG. 2 is an explanatory diagram of an embodiment of a null interferometer operation according to the present invention.

FIG. 3 is an explanatory diagram of an embodiment of a null interferometer operation according to the present invention.

FIG. 4 is an explanatory diagram of the interference measurement used in the present invention.

[Explanation of symbols]

 1 ... interferometer 2 ... plane wave 3 ... Fizeau plane plate 3a ... Fizeau surface 4 ... null element 4a ... measurement wavefront 5 ... test object 5a ··· Test surface

Claims (2)

[Claims] 1. A measuring light beam emitted from a light source is converted into a null wavefront having a predetermined aspherical shape by a null element, and the measuring light beam irradiated and reflected on a surface to be measured and the light beam emitted from the light source are reflected. In the interference measurement using a null interferometer for measuring the surface shape of the test surface by causing the reference light beam having the predetermined wavefront to interfere with each other and detecting the state of interference fringes generated by the interference, By using both the coordinate measuring instrument data of the test surface and the calculated data of the null wavefront, the interference measurement is simulated and calculated in the calculator, and the result regarding the obtained measurement position in the optical axis direction is fed back. A method for calculating a null interferometer, characterized in that it is possible to calibrate an aspherical shape of a surface to be measured with high accuracy. 2. An information processing apparatus used for an interferometer using a null interferometer operation method according to claim 1, wherein a procedure for obtaining a surface shape of the surface to be detected from the interference fringes is performed by the information processing apparatus. Recording medium on which a program to be executed by a computer is recorded.