JP2000097658A - Aspherical reflex prototype - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately specify a measuring area even when reflex subtraction measurement using a null wavefront is used by using a reflect prototype which is set to the so-called 'base material' state against off-axis when a test surface is in the so-called off-axis state from the outside diameter. SOLUTION: The reflex surface 1a of a reflex prototype 1 is set at a size larger than the effective area of the test surface 2a of a work 2. When the surface 2a is an aspherical surface having the so-called off-axis shape, this condition is met by adopting the reflex prototype in the so-called base-material state as the reflex prototype 1. When the reflex prototype 1 is used, such rotation averaging as that shown by the arrow can be performed easily around the axis 3 of the aspherical surface at the time of measuring null interference. In addition, when eccentricity is required due to an optical surface formed on the rear surface of the work 2, the peculiar eccentricity (the eccentricity which is uniquely decided from the relative positional relation between the aspherical surface and rear surface and does not rely upon the outside diameter) of the aspherical surface having the off-axis shape can be measured with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for an interferometer for measuring spherical and aspherical surface shapes with high accuracy.
Reflex prototype.

[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.

As an interferometer that does not require a reference plane, FIG.
Based on the wavefront diffracted by the pinhole as shown in
Point-Diffraction-Interforometer is known. 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. Further, with this method, it was possible to measure the shape of the spherical surface with high accuracy, but it was not possible to measure the shape of the aspherical surface. Point-Diffraction-Interfo
When measuring an aspheric surface using a rometer, if the radius of curvature of the spherical wave diffracted by the pinhole and the radius of curvature of the aspheric surface to be measured match, interference fringes with coarse intervals will occur. Since the radius of curvature is different, the region where the interference fringe interval is coarse is only a part, and in the remaining region, the interference fringe interval is dense and measurement cannot be performed.

In order to solve the above problem, after measuring only the portion where the interference fringe interval is coarse, the positional relationship between the aspheric surface to be measured and the pinhole is changed (the arrow in FIG. 9 indicates this). A so-called "ring zone wavefront synthesis" is known in which the process of repeating the measurement after changing the portion where the interval becomes coarse is repeated, and the shape of the aspheric surface is measured by synthesizing the measurement results.

According to this method, the aspherical REF prototype is calibrated with high accuracy, but the measurement data is joined by wavefront synthesis, so that the measurement time becomes longer and mass productivity is poor. Therefore, in order to avoid this, as shown in FIG. 10, using the ref prototype 1 calibrated by this method, the test surface 2a of the work 2 is null-connected to the ref surface 1a of the ref prototype 1. A method has been filed which employs a so-called “Leff subtraction” in which comparison and subtraction are performed via a null wavefront generated by the element 4.

[0006]

However, when attempting to synthesize the annular wavefront, in addition to the absolute calibration of the reflex prototype surface, the consistency of each abscissa at the time of joining each measurement data is taken. I needed to. In addition, also in the case of using the reflex subtraction measurement based on the null wavefront, there is a problem that it is difficult to accurately specify the area measured by the interferometer.

Furthermore, in the case of a so-called "off-axis" external shape in which the outer diameter of the work (product) actually used is not symmetrical with respect to the aspherical axis, the consistency of the abscissas of the test surface and the reflex surface. There was a problem that it was difficult to take. SUMMARY OF THE INVENTION The present invention has been made in view of the above-described drawbacks of the related art, and has as its object to achieve high-precision matching of abscissas mainly used for a reflex surface of a reflex prototype used in an aspherical interferometer.

[0008]

According to the present invention, in order to achieve the above object, as a first means, "" a measuring light beam emitted from a light source is irradiated on a surface to be measured and reflected by the measuring light beam; A so-called interferometer for measuring a surface shape of the surface to be detected by causing a light beam and a reference light beam having a predetermined wavefront emitted from the light source to interfere with each other and detecting a state of interference fringes generated by the interference. Used in, a reflex prototype having a reflex surface having the same shape as the test surface, wherein the test surface is a so-called "off-axis" with respect to the outer diameter, the "off-axis" In contrast, a "reflection device characterized by being in a so-called" base material "state" was used. Thereby, even when using the reflex subtraction measurement by the null wavefront,
It is possible to accurately specify the area being measured by the interferometer.

The second means is composed of ““ a measuring light beam emitted from a light source onto a surface to be measured and reflected, and a reference light beam having a predetermined wavefront emitted from the light source. Have a reflex surface of the same shape as the test surface, which is used in a so-called interferometer that measures the surface shape of the test surface by detecting the state of interference fringes caused by the interference. A ref prototype, wherein a marking is provided on the ref surface, is used. Accordingly, when the wavefront synthesis is to be performed, in addition to the absolute calibration of the reference surface of the reflex, it is possible to take consistency of the respective abscissas when connecting the measurement data.

In the section of the means for solving the above-mentioned problems, which explains the configuration of the present invention, the drawings of the embodiments of the present invention are used for easy understanding of the present invention. However, the present invention is not limited to this.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS -Explanation of Reflex Prototype According to the Present Invention- Hereinafter, the reflex prototype of the present invention will be described with reference to FIG. Normally, in the reflex subtraction measurement using the null wavefront shown in FIG. 10, the refraction surface 1a of the reflex prototype 1 is set in consideration of the alignment error at the time of reflex subtraction and the lack of diffraction at the edge portion.
It must be set larger than the effective area of the test surface 2a of the work 2. The effective area is an area that satisfies the optical standard required for the work.

FIG. 2 shows a case where the work 2 is an aspherical surface having a so-called off-axis shape. In contrast, this condition is satisfied by adopting a so-called base material as the ref prototype 1. Things. FIG. 1 is a diagram illustrating the reflex prototype 1 and the workpiece 2 in FIG. 2 with the aspherical axis 3 being rotated, and shows the magnitude relationship between the base material and the off-axis. The use of the REF prototype in the state of the base material has an advantage that the rotation averaging around the aspherical axis 3 as indicated by an arrow in FIG.

Further, when eccentricity is required by forming an optical surface on the back surface, the inherent eccentricity of the off-axis aspherical surface (the eccentricity uniquely determined by the relative positional relationship between the aspherical surface and the back surface). , Independent of the outer diameter). -Explanation of the first embodiment of the ref prototype according to the present invention- The ref prototype of FIG. 3 is an example in which two areas covering the effective area of the work 2 shown in FIG. 1 are provided. The cover area 5 is set to be evenly thicker than the effective area of the work in consideration of the lack of the edge described above. As shown in the figure, the method of thickening may be changed for each area. FIG. 4
It is not necessary to limit the number to two as described above, and point symmetry may be given to the aspherical axis. FIG. 4 shows an example in which the cover area does not need to cover the aspherical axis.

This cover area may be formed by using a coat which can be easily removed, such as temporary Ag, in the process of creating a work or a ref prototype. The mask can be formed by providing a mask having an opening of a predetermined shape on the surface thereof and forming a region where a thin film is partially coated by a thin film forming method such as vacuum evaporation or sputtering (mask evaporation). -Description of Another Embodiment of Reflex Prototype According to the Present Invention- The reflex prototype 1 of FIG. 5 is used for the PDI annular wavefront synthesis shown in FIG. FIG. 5 shows an example of a reflex prototype used for synthesizing a total of two pieces of interference measurement data, one ring-shaped data at the center and one donut-shaped data at the periphery as ring data. The number of the donut-shaped data in the peripheral portion may increase depending on the aspherical amount of the aspherical shape of the workpiece (deviation from the optimal approximate spherical surface).

When these data are obtained by interference measurement, it is necessary to scan the surface of the ref prototype as shown in FIG. 9 in the optical axis direction in order to obtain a region having a low fringe density. Since the radius of curvature of the optimal approximation spherical surface of the two pieces of interferometric data obtained in this way is different in principle, even if the sine condition of the measurement wavefront is corrected, one pixel indicated by the interferometer data indicates The so-called “interferometer magnification,” which is defined by the size of the ref prototype, is different.

Therefore, when synthesizing these at least two data on the basis of the overlapping area, it is necessary to adjust the interferometer magnification as well as the alignment. In the present embodiment, the marking 6 is used when matching the abscissa. FIG. 5 shows an example in which at least two markings are applied to the overlapping area of each data for this magnification adjustment. The location of this marking is
Ideally, it should be applied point-symmetrically to the aspherical axis. Further, it may be groove-shaped.

This marking may be anything as long as it can be observed later by an interferometer. As a method of forming the marking, a region engraved by grinding, engraving, etching, or the like may be formed in the REF prototype before polishing, or a projecting region may be formed in the polishing stage. Further, at the stage when the ref prototype has been completed, the thin film may be formed by partially removing the thin film, and mask evaporation, lift-off, etching and the like can be used. Mask evaporation is performed according to the method described above. In lift-off, a mark is previously drawn with a photoresist or the like, a thin film is formed thereon, and the resist (and the thin film attached thereon) is removed later. In etching, after forming a thin film, a photoresist is applied to the entire substrate, the mark portion is exposed (in the case of a positive resist, reverse if negative) and developed, and wet etching or dry etching is performed using the resist as a mask. Do.

-Description of Second Embodiment of Reflex Prototype According to the Present Invention- FIG. 6 shows an example in which the number of markings is increased to three in a uniform arrangement on the circumference to improve the positioning accuracy. It is. FIG. 7 shows an example of marking when a reflex prototype having the same off-axis shape as the workpiece is employed. The number of markings is three at equal pitch on the circumference. In the case of this off-axis, point symmetry cannot be ensured. Therefore, if there is a rotationally symmetric distortion in the measurement wavefront of the PDI, at least three markings are required.

Further, the equal pitch property and the equal circumferential property of the marking are not necessarily required, and the connection is the same as long as the relative relationship between the amount of distortion received by the interferometer data and the marking is known in advance. It is possible. When identifying the location of this marking from interferometer data,
In order to use the centroid, it is desirable that the outer shape of the marking be circular. In addition, the distortion may be corrected in advance, and marking may be performed on the interferometer data so that a circle is secured.

Description of Third Embodiment of Reflex Prototype According to the Present Invention FIG. 8 shows an example in which the marking for wavefront synthesis is also used for setting the interferometer magnification for subtraction of null measurement reflex. As an example of setting the interferometer magnification for subtraction of the null measurement reflex, an example using a mask 7 as shown in FIG. 8 can be considered. The mask 7 is rotatable symmetrically with respect to the aspherical axis, for example, with reference to the outer shape of the ref prototype. In order to correlate interferometer data, the center is always visible at any rotation angle. , The aperture of the mask 7 is decentered.

If the movement distance (linear distance) of the center of gravity of the mask 7 is associated with the real space on the interferometer data using the mask 7, the interferometer magnification can be set with high accuracy.
If the marking for wavefront synthesis is used together, the accuracy can be improved. -Description of fourth embodiment of REF prototype according to the present invention-This is an example in which marking for wavefront synthesis and area setting for REF subtraction for off-axis are simultaneously applied to the REF prototype.

[0022]

As described above, the adoption of the REF prototype according to the present invention makes it possible to achieve high-accuracy consistency of the abscissa relating to aspherical interference measurement.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of a reflex prototype according to the present invention.

FIG. 2 is a diagram illustrating the principle of a reflex prototype according to the present invention.

FIG. 3 is an explanatory diagram of a first embodiment of a ref prototype according to the present invention.

FIG. 4 is an explanatory diagram of a first embodiment of a ref prototype according to the present invention.

FIG. 5 is a diagram illustrating the principle of a reflex prototype according to the present invention.

FIG. 6 is an explanatory diagram of a second embodiment of a ref prototype according to the present invention.

FIG. 7 is an explanatory diagram of a second embodiment of a ref prototype according to the present invention.

FIG. 8 is an explanatory diagram of a third embodiment of a ref prototype according to the present invention.

FIG. 9 is an explanatory diagram of PDI annular wavefront synthesis.

FIG. 10 is an explanatory diagram of Ref subtraction using a null wavefront.

[Explanation of symbols]

 1 Ref prototype 1a Ref surface 2 Work 2a Detected surface 3 Marking

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F064 AA09 CC10 DD10 EE02 EE05 JJ01 2F065 AA46 BB05 BB27 CC21 DD06 FF52

Claims (2)

[Claims] 1. A measurement light beam emitted from a light source is irradiated to a surface to be measured and reflected, and the measurement light beam reflected from the light source and a reference light beam having a predetermined wavefront emitted from the light source interfere with each other. By detecting the state of the interference fringes caused by measuring the surface shape of the test surface, used in a so-called interferometer, a reflex prototype having a reflex surface of the same shape as the test surface, A ref prototype, wherein when the surface to be inspected is a so-called off-axis with respect to an outer diameter, a so-called base material is used for the off-axis. 2. A measurement light beam emitted from a light source irradiating a surface to be measured and reflected, and the measurement light beam reflected from the light source and a reference light beam having a predetermined wavefront emitted from the light source interfere with each other. By detecting the state of the interference fringes caused by measuring the surface shape of the test surface, used in a so-called interferometer, a reflex prototype having a reflex surface of the same shape as the test surface, A reflex prototype, wherein a marking is provided on the reflex surface.