JP2000088549A - Aspherical prototype - Google Patents

Abstract

PROBLEM TO BE SOLVED: To calibrate the alignment error correction required for measuring the surface accuracy of a test surface by constituting an aspherical prototype with a first face being a nonspherical surface having a nonspherical surface axis to be a reference of a rotary asymmetric shape and a second face being a composite surface composed of a plane and spherical surface. SOLUTION: A plane wave 5a from an interferometer is incident on a first wave front conversion element 2 to convert into a desired null wave front 2a. The opposite side of an aspherical prototype 1 is a composite surface composed of a plane 1b and spherical surface 1c. A plane wave 5b is incident on a second wave front conversion element 3 to create a desired plane wave 3a and spherical wave 3b, the coincidence of the alignment deviation obtd. by the alignment error correction at the aspherical surface with the alignment deviation obtd. by the alignment error correction at the composite surface is utilized, an interference fringe formed with the planes 1b, 3c shows two shifts, and an interference fringe formed with the spherical surfaces 1c, 3d shows two alignment deviations formed by two shifts and two tilts in directions X, Y and the focus.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aspherical prototype used for interference measurement of the surface shape of a lens having an aspherical shape with reference to an aspherical wavefront generated by a null element.

[0002]

^2. Description of the Related Art For example, when the sectional shape is Z = X ² · [R {1 + {(1−κX ² / R ² )}] ⁻ 1+
Represented by _{^{C 04 X 4 + C 06 X}} 6 + C 08 X 8 + C 10 X 10, when the test surface having a higher-order aspherical shape which is based on secondary aspherical (including spherical), Shape error of the surface to be measured (geometric shape error based on the design value of the aspheric coefficient, and surface accuracy corresponding to a higher-order waviness component)
Is measured using an interferometer, for example, the method disclosed in
Null interferometry has been performed as disclosed in US Pat.

[0005] The apparatus is configured as shown in FIG. 5. An interferometer body (not shown), a Fizeau flat 4 having a reference reference surface 4 a for reflecting a plane wave 5 a emitted from the interferometer body, and a plane wave 5 a A nulling element 20 for converting into a null wavefront 20a having an aspherical shape equivalent to the aspherical surface 10a, which is the test surface of the test lens 10, is provided.

In the case where interference measurement is to be performed on an aspherical surface, it is very difficult to align the position of the test surface 10a with the null wavefront 20a, and the obtained interference fringes may look different depending on the alignment state. Because
For example, alignment error correction is performed as disclosed in Japanese Patent Application No. 9-0000703.

[0005]

However, in the case of aspherical measurement, the null wavefront has a distortion (a relative distortion between the coordinate system of the interference fringe imaging means and the coordinate system describing the shape of the surface to be measured). ) Is unavoidable, and especially if the amount of rotationally symmetric distortion is not accurately grasped, the correction value of the alignment error correction (hereinafter, referred to as
There is a problem that it is inevitable that an error also occurs in "alignment deviation".

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has been developed in the context of null interference measurement using an aspherical wave generated by a nulling element such as a null lens. Calibration of error correction is possible,
The purpose is to provide an aspheric prototype.

[0007]

As a first means for solving the problem, an aspherical prototype is constituted by at least two surfaces, and the first surface is a non-spherical axis having an aspherical axis serving as a reference of a rotationally symmetric shape. It is a spherical surface, and the second surface is a composite surface composed of a plane and a spherical surface. Providing a surface that only contributes to the alignment facilitates calibration. As a second means, the perpendicular drawn from the spherical center of the second surface to the same plane with respect to the aspheric prototype of the first means is made coincident with the aspheric axis of the first surface. . This makes calibration easier. As a third means, the aspherical prototype is composed of a plurality of optical elements, and serves as a reference for a rotationally symmetric shape.
An aspherical surface having an aspherical axis and a composite surface composed of a spherical surface having a plane and a rotationally symmetric axis were arranged on different optical elements. Thus, by individually forming the reference surface and the alignment surface, the labor for manufacturing can be largely saved, and the cost can be significantly reduced. As a fourth means, the reference aspherical surface and the composite aspherical surface are arranged on the outermost surface of the aspherical prototype of the third means. The outermost surface is, in a general expression, a surface corresponding to the first surface and the final surface of the optical system. Thus, the measurement can be easily realized with high accuracy. As a fifth means, the rotationally symmetric axis and the aspheric axis coincide with each other with respect to the aspherical prototype of the third or fourth means. This facilitates calibration for each stage.

The present inventor has found that the above problems can be solved by taking such means, and has accomplished the present invention.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 explains an embodiment of the first means of the present invention, and also serves as an explanatory diagram of the principle. First, creation of a null wavefront for measuring the aspheric surface 1a of the aspherical prototype 1 is the same as in the related art, and a plane wave 5a from an interferometer (not shown) is incident on the first wavefront conversion element 2, It is converted into a desired null wavefront 2a.

The opposite side of the aspherical prototype 1 has a flat surface 1b,
And a combined surface of the spherical surface 1c, and the incident plane wave 5b is converted into the second wavefront conversion element 3 similarly to the aspheric surface side.
Thus, a desired plane wave 3a and a desired spherical wave 3b are created. The simplest means to achieve this is the zone play.
A (grating-shaped diffraction grating) element, but the same applies to the case where a null lens composed of an ordinary spherical lens is used.

In this hardware configuration, the plane 1b and the spherical surface 1c can be calibrated with a very high precision in their geometrical shape and surface accuracy. This is to attempt calibration of error correction. That is, assuming that the aspherical prototype 1 causes a certain amount of alignment change from one state to another state, the alignment deviation obtained by the alignment error correction performed on the aspherical surface side and the alignment error correction performed on the composite surface side The fact that the calculated misalignment completely matches is used.

The misalignment includes two shifts in the XY directions (two directions perpendicular to the optical axis and orthogonal to each other), two tilts in the XY directions, and one shift in the Z (optical axis) direction. Shift (defocus) is a total of five values. However, since the tilt is defined on the basis of the vertex of the surface to be inspected, it should be noted that even if the object is tilted, a shift may occur.

The misalignment defined in this way is handled by the following surfaces on the composite surface side. That is, first, the interference fringes formed by the planes 1b and 3c represent two shifts. Next, the interference fringes formed by the spherical surfaces 1c and 3d (not shown) are two misalignments in the XY directions obtained by combining two shifts and two tilts,
And defocus. From these five values, each alignment shift can be separated.

It is necessary to convert the misalignment that can be calibrated as described above and the misalignment determined on the aspherical surface side because the positions of the vertices differ by the central thickness of the aspherical prototype. Not even. As a result of these measurements, when the values of the misalignment from both sides do not match, the value on the aspherical surface can be calibrated with the value on the composite surface as the true value. This is considered to be a rotationally symmetric distortion as one of the error factors, but the distortion of the plane or even spherical interference optical system is more accurately calibrated than the distortion of the aspherical interference optical system. Is based on what is possible.

FIG. 2 illustrates another example of the first embodiment of the present invention. FIG. 2 shows an example in which the unevenness of the spherical surface of the composite surface is reversed, unlike FIG. In this case, the plane portion is located at the center and the spherical surface is located at the outer peripheral portion thereof for reasons of manufacturing the optical element. Other operations are exactly the same as those in FIG. FIG. 4 shows an embodiment of the third means. The aspherical prototype 50 is composed of a lens 41 having a reference aspherical surface 1a and an aspherical axis 47 which is a rotationally symmetric axis thereof, and a lens 45 having a composite surface composed of a spherical surface 1c and a plane 1b and a rotationally symmetrical axis 48. . These lenses are held by a holder 46, for example. The positional relationship with the interferometer and the operation principle are the same as those of the first means. Although the number of components of the third means is increased as compared with the first means, by providing the reference aspheric surface and the composite surface for alignment on separate optical elements, the composite surface can be divided into individual reference surfaces. There is an advantage that it is not necessary to create each spherical surface.

Next, an example of actual use using the aspherical prototype according to the present invention will be described to clarify the effects of the present invention. First, a first embodiment of the present invention will be described with reference to FIGS. By using the aspherical prototype of the present invention, it is also possible to calibrate a null wavefront. As a calibration of this null wavefront, it is possible to perform a calibration that gives only an accurate amount of shift to the aspheric surface of the aspherical prototype with respect to the null wavefront, without using other displacement sensors. Become. Specifically, the tilt of the aspheric prototype is obtained by analyzing "interference fringes formed by planes 1b and 3c before and after shifting the aspheric surface with respect to the optical axis of the wavefront conversion element." At the same time as controlling the shift to zero, the original null by controlling the defocus of the aspherical prototype to zero, which is obtained by analyzing the "similarly, interference fringes formed by spherical surfaces 1c and 3d". This achieves the purpose of calibration of the wavefront, and the given shift amount can be obtained by analyzing "similarly, interference fringes formed by spherical surfaces 1c and 3d".
For this purpose, as shown in FIG. 3, as long as the interference fringes can be formed at least in the plane area, the interference fringes formed in the spherical area can be accurately replaced with other means. No problem. In this case, if the aspherical axis of the aspherical prototype is formed perpendicular to the plane, the following advantages will be obtained. That is, by observing the phase of the fringes on the plane side when the shift is given, it is possible to simultaneously control the defocus on the aspheric surface side in a practical range. As the given shift amount, a read value of a micrometer or the like may be used.

Next, a second embodiment of the present invention will be described with reference to FIGS. In any case of the first and third means, the deviation of the relative relationship between the optical axis of the wavefront conversion element on the aspherical surface side and the optical axis of the wavefront conversion element on the composite surface side can be determined as long as there is no drift. There was no problem in the purpose of using the optical element of the invention. However, as an actual optical element, the surface opposite to the aspherical surface (the composite surface side of the aspherical prototype) is also an optical surface, and the relative relationship between the two is very important in satisfying the performance of the optical element. Come. Specifically, it is eccentricity. Means for measuring this eccentricity is described in, for example, Japanese Patent Application No. 9-0007.
In general, the test object is moved so as to rotate the test object around the optical axis while performing interference measurement of the test object from both sides as disclosed in No. 03. In the present invention, the original purpose is "comparison measurement with a test surface using an aspherical prototype".
At the same time, or beforehand during the comparative measurement, this aspherical prototype is used to calibrate the deviation of the optical axis of the wavefront from the wavefront conversion element of the interference optical system on both sides, and The rotation of the object is omitted. In order to facilitate this, the optical axis of the composite surface of the aspherical prototype in FIGS. 1, 2 and 4 is made to coincide with the optical axis on the aspherical side of the aspherical prototype. (This corresponds to the second and fourth means corresponding to claims 2 and 4.) The optical axis of the composite surface can be defined by a perpendicular drawn from the spherical center of the spherical surface to a plane. Alternatively, it may be defined by the rotational symmetry axis of the spherical portion. However, if the coincidence of the two optical axes exceeds the allowable value, it is possible to correct the deviation for the purpose of using the optical element of the present invention. As the aspherical prototype for this purpose, the radius of curvature of the spherical surface, and the center thickness,
It is not possible to select freely, and it is desirable to keep it almost the same as the test object.

In the above embodiment, the measurement of the inclination of the plane, the shift of the center of the sphere, and the measurement of defocus are performed by the interference system in the above embodiment. Needless to say, the shift can be measured by an eccentricity measuring machine, and the defocus can be measured by an autofocus mechanism. On the composite surface side, two circumferences, a first circumference determined by a boundary area between a spherical surface and a plane and a second circumference of a measurement area determined by an outer diameter of the optical element, can be measured by interference. Therefore, the magnification of the interference fringe imaging means, which is difficult to calibrate in the interference measurement, is a so-called “mm, which is equivalent to what mm a pixel corresponds to when viewed from the Z direction on the surface to be measured when the imaging means is a CCD. / Pixel ").

Once the magnification of the interference optical system on the complex surface side can be calibrated, the magnification of the optical system up to the wavefront conversion element among the aspherical surface interference optical systems can be calibrated by shifting the magnification. Become. The arithmetic unit (not shown) of the interferometer system has a function of preliminarily inputting information on a measurement surface, calculating and storing coefficients required for the operation before measurement, and an interference fringe imaging unit (CCD camera) in the main body of the interferometer. A) converting the image information from the optical path difference data into optical path difference data, analyzing the optical path difference data based on the alignment error correction coefficient, calculating the geometrical shape error of the measurement surface, and the surface accuracy error; And a function of displaying a result.

The measurement surface is aligned by a holding and adjusting mechanism (not shown) so that the reflected light from the measurement surface and the reference reference surface of the interferometer form interference fringes. The alignment sends a signal from the interference fringe imaging means to a monitor (not shown),
This is performed while observing the interference fringes on the monitor. The alignment at this stage is merely an alignment to the state where the interference fringe can be analyzed, and the final alignment is guaranteed.
Alignment error correction by software analysis is required. The present invention guarantees the latter alignment.

[0021]

As described above, when the aspherical prototype according to the present invention is adopted, the surface accuracy of the surface to be measured in the null interference measurement using the aspherical wave generated by the nulling element such as a null lens. In addition to being able to calibrate the alignment error correction required for the measurement (Note: Distortion measurement is implied), it is also possible to easily measure the eccentricity of the aspherical surface and the opposite surface of the test object. It becomes possible.

[Brief description of the drawings]

FIG. 1 shows an embodiment of a first means according to the present invention, and usage example 1
FIG.

FIG. 2 is an explanatory diagram of another example of the mode of the first means according to the present invention, and a usage example 1.

FIG. 3 is a supplementary explanatory diagram of usage example 1 according to the present invention.

FIG. 4 is an explanatory view of an embodiment of a third means according to the present invention.

FIG. 5 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 1 Aspherical prototype of first means 1a Aspherical surface (higher order aspherical surface) 1b, 3c Plane 1c, 3d Spherical surface 2 First wavefront conversion element (null Element 2a Aspherical (higher order aspherical) wavefront 3 Second wavefront converting element (ZP element) 3a Plane wave 3b Spherical wave 4 Fizeau flat 4a ··· Reference reference surface 5a, 5b · Plane wave from interferometer 10 ··· Test object 10a ··· Test surface (higher order aspherical surface) 20 ··· Null element 20a · Null wavefront 41 Optical element having reference aspherical surface 47 Aspherical axis of 41 45 Optical element having composite surface 46 Holder 48 The rotationally symmetric axis of the spherical part of the aspherical prototype of the third means

Continuing on the front page (72) Inventor Kazushi Nomura 3-2-2, Marunouchi, Chiyoda-ku, Tokyo Nikon Corporation (72) Inventor Takashi Genma 2-3-2, Marunouchi, Chiyoda-ku, Tokyo Stock Company F term in Nikon (reference) 2F064 AA04 AA09 BB03 CC10 DD10 EE05 EE10 GG47 GG64 HH08 JJ01 2F065 AA20 AA45 CC21 DD19 EE08 FF51 FF61 JJ26 LL10 PP12 QQ23 QQ31 SS13

Claims (5)

[Claims] 1. An aspheric prototype, wherein the first surface is an aspheric surface having an aspheric axis, which serves as a reference for a rotationally symmetric shape, and a second surface. Is a composite surface composed of a plane and a spherical surface. 2. The aspherical prototype according to claim 1, wherein a perpendicular drawn from the spherical center of the spherical surface to the plane coincides with the aspherical axis. 3. A prototype of an aspheric surface, comprising a plurality of optical elements and comprising a non-spherical surface having an aspherical axis and a spherical surface having a plane and a rotationally symmetric axis, serving as a reference for a rotationally symmetric shape. Aspherical prototypes having surfaces on optical elements different from each other 4. The aspherical prototype according to claim 3, wherein the aspherical surface and the composite surface are outermost surfaces of the prototype. 5. The aspherical prototype according to claim 3, wherein the aspherical axis is coincident with the rotationally symmetric axis.