JP2000097623A - Interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit the highly accurate calibration of chiefly a null wave front with a small amount of aspheric surface by alternately arrangiong a reflex primary standard with a spherical surface accurately calibrated separately instead of a test surface and measuring a wavefront for measurement from the reflex primary standard. SOLUTION: Instead of the test surface of a specimen, a reflex primary standard 6 with a spherical reflector surface accurately calibrated separately is set to perform interference measurement. An incident wavefront to a null element 4 is taken as a spherical wave from a Fizeau lens 7, and a Fizeau surface 7a is used as a reference surface. The Fizeau lens 7 may be a divergent system. The Fizeau surface as a reference surface with a plane-wave incident wavefront to the null element 4 is used. As a beam of light to be transformed by the null element 4 becomes a convergent beam of light, it becomes possible to measure not only a concave surface but also a convex surface. In the calibration of a null wavefront 4a in this case, a wavefront once converged and diverged at a concave reflector surface 6a is calibrated, and the shape of the null wave front 4a at a location in actual use is computed backward from the calibrated shape of the wavefront. Through the use of a pinhole interferometer for calibrating the concave reflector surface 6a, it becomes possible to perform accurate calibration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for measuring an aspherical wavefront shape with high accuracy.

[0002]

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

FIG. 5 is a conventional measurement arrangement diagram for null measurement using a null lens. The interference measurement here is a slightly modified version of the Fizeau interference measurement. That is, the plane wave 2 emitted from the interferometer 1 is reflected from the highly accurate Fizeau surface 3 a formed on the Fizeau plane plate 3. The plane wave transmitted through the Fizeau surface 3a is converted by the null element 4 into a measurement wavefront (null wavefront) 4a converted into a desired aspherical design shape at the measurement reference position, and the test object set at the reference position. The object 5 reaches the surface 5a to be inspected.
The light that has reached the test surface 5a is reflected from the test surface 5a and interferes with the light reflected from the Fizeau surface 3a to form a single-color interference fringe inside the interferometer 1. This interference fringe is detected by a detector such as a CCD (not shown), and the obtained signal is analyzed by an information processing system that processes information of the interferometer. Similar measurements are possible using a Twyman-Green interferometer.

[0004]

However, in order to accurately know the shape of the surface to be inspected, there must be no error in the null wavefront, so that a high-level technique is required for manufacturing the null element. Specifically, the optical characteristics of the optical element constituting the null element are measured in advance with high precision, and the shape of the null wavefront is calculated from the ray trace based on the measured values. Was hanging. Therefore, there is a problem that it takes time to measure the aspherical surface.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has as its object to enable a highly accurate and short-time calibration of a null wavefront having a small aspherical amount.

[0006]

According to the present invention, in order to achieve the above object, as a first means, a light beam emitted from a light source, which is converted into a desired shape by a null element and is applied to a surface to be inspected. Irradiated, the measurement wavefront reflected on the surface to be measured, and the reference wavefront reflected and illuminated on the reference surface is a light flux emitted from the light source,
By detecting the state of interference fringes caused by interference,
In the interferometer for measuring the surface shape of the surface to be measured, instead of the surface to be measured, a ref prototype having a spherical surface calibrated separately and separately is alternately arranged, and a measurement wavefront from the ref prototype is measured. By doing so, a wavefront formed by the null element is determined. This makes it possible to accurately know the shape of the null wavefront.

Further, in the present invention, as a second means, a light beam emitted from a light source is converted into a desired shape by a null element and is radiated to a surface to be measured, and is reflected by the surface to be measured. The measurement wavefront, the light flux emitted from the light source and the reference wavefront illuminated and reflected on the reference surface interfere with each other, and by detecting the state of interference fringes caused by the interference, the test surface In the interferometer that measures the surface shape of the above, the light source is a point light source of a predetermined size, which is emitted from the reflective surface and forms a spherical wave, and is separately provided instead of the test surface. A ref prototype having an accurately calibrated spherical surface is alternately arranged, and a wavefront for measurement from the ref prototype is reflected and folded back on the reflecting surface to interfere with the reference wavefront to measure interference fringes. By this, it is possible to form Determining the wavefront to provide an interferometer characterized by. This makes it possible to accurately know the shape of the null wavefront and the transmission characteristics of the null element.

The third means is a light beam emitted from a light source, which is converted into a desired shape by a null element and illuminated on a surface to be measured, and a wavefront for measurement reflected by the surface to be measured; The light beam emitted from the light source, and the reference wavefront illuminated and reflected on the reference surface interfere with each other, and the state of interference fringes generated by the interference is detected to measure the surface shape of the test surface. In the interferometer, instead of the surface to be detected, a point light source having a predetermined size, having a reflecting surface, and a point light source forming unit that forms a spherical wave is alternately arranged,
An interferometer is provided, wherein a wavefront formed by the null element is determined by measuring the wavefront from the point light source forming means through a null element. This makes it possible to accurately know the shape of the null wavefront without using a Ref prototype.

The fourth means is a light beam emitted from a light source, which is converted into a desired shape by a null element and radiated to a surface to be measured, and a wavefront for measurement reflected by the surface to be measured; The light beam emitted from the light source, and the reference wavefront illuminated and reflected on the reference surface interfere with each other, and the state of interference fringes generated by the interference is detected to measure the surface shape of the test surface. In the interferometer, the light source is
A point light source having a predetermined size, which is emitted from a reflecting surface to form a spherical wave, and is a point light source having a predetermined size instead of the surface to be detected, which has a reflecting surface. Further, an interferometer is provided, wherein point light source forming means for forming a spherical wave are alternately arranged to determine a wavefront formed by the null element. Thus, it is possible to accurately know the shape of the null wavefront and the transmission characteristics of the null element without using a ref prototype.

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 [Description of Interferometer According to the Present Invention] An interferometer according to the present invention will be described below with reference to FIG. Compared with the conventional example shown in FIG. 5, the interferometer according to the present invention sets a reflex prototype having a spherical reflex surface which is separately and accurately calibrated instead of the test surface (aspheric surface) of the test object. Then, interference measurement is performed.

The difference between the interferometer shown in FIG. 1A and the conventional example shown in FIG. 5 is that the wavefront incident on the null element is a spherical wave from a Fizeau lens, and the Fizeau surface is used as a reference surface. This is the point used. The Fizeau lens need not be limited to a convergent system as shown in the figure, but may be a divergent system. The interferometer shown in FIG. 1B is an example in which the wavefront incident on the null element is the same plane wave as the conventional example shown in FIG. 5 and a flat Fizeau surface is used as a reference surface. The difference from the conventional example shown in FIG. 5 is that the light beam converted by the null element is a convergent light beam, so that not only a concave surface but also a convex surface can be measured. As a method of calibrating the null wavefront in this case, the wavefront that converges and then diverges on the concave reflex surface is calibrated, and the shape of the null wavefront at the position actually used (thick line in the figure) is calibrated. Back calculation from the wavefront shape. For calibration of concave refraction surfaces, use a pinhole interferometer (Point Diffraction Interferometer, hereinafter PDI).
) Enables highly accurate calibration.

In the case of an aspherical surface having a small amount of aspherical surface, the whole surface can be collectively measured in the state shown in FIG. However, in the case of an aspheric surface that generates interference fringes exceeding the resolution of the CCD of the interferometer, the ref prototype is displaced in the optical axis direction relative to the null wavefront, and a plurality of annular wavefront data is obtained. Is measured, and the so-called wavefront synthesis method of joining the data so that the overlapping areas of the data are overlapped with each other without difficulty is applied, thereby similarly obtaining the entire data. [Explanation of First Embodiment of Interferometer According to the Present Invention] The embodiment shown in FIG. 2 uses the null element of the Fizeau interferometer shown in FIG. This is an embodiment in which measurement is performed using PDI.

The embodiment shown in FIG. 2A is a case where a diverging null element is used, and the embodiment shown in FIG.
This is a case where a converging system null element is used. The latter is adopted when calibrating a null wavefront for measuring a convex surface. FIG.
In these examples shown in Figure 3, the spherical wave incident on the null element is
Since it is an ideal spherical wave from a point light source, not only the shape of the null wavefront but also the transmission characteristics of the null element can be accurately known at the same time. [Explanation of Second Embodiment of Interferometer According to the Present Invention] The embodiment shown in FIG. 3 is applied to the measurement of a null element for generating a null wavefront of a convergent light beam shown in FIG. It is. Here, as the incident wavefront from the Fizeau surface to the null element, a spherical wave is used in the embodiment shown in FIG. 3A, and a plane wave is used in the embodiment shown in FIG. It is. It does not matter whether the spherical wave is a convergent light beam or a divergent light beam. Then, PDI is used instead of the calibration using the reflex surface. This PDI
Corresponds to the point light source forming means of the present invention.

For observation by PDI, a null wavefront of a convergent light beam is used, and its substantially converging point is matched with a point light source of PDI. As a result, the null wavefront reflected from the reflection surface around the pinhole and the ideal spherical wave form interference fringes. [Explanation of Third Embodiment of Interferometer According to the Present Invention] The embodiment shown in FIG. 4 uses two PDIs instead of the Fizeau lens for generating a spherical wave shown in FIG. P
The measurement light from the DI is used. In the arrangement shown in FIG. 3, there is a possibility that the measurement light from the PDI may become noise of the Fizeau interferometer, and in this case, it is preferable to use a polarizing element together to perform noise cut.

On the other hand, in the measurement arrangement shown in FIG.
There is an advantage that the pinhole forming the point light source of the second PDI acts as a noise cut. In addition, not only can accurate calibration of the null wavefront shape and the transmission characteristics of the null element be performed, but also the transmission characteristics from the normal and reverse directions can be calibrated by two PDIs, so that further improvement in accuracy can be achieved. Become.

In each of the above embodiments, when actually measuring the surface to be inspected, the measurement is carried out by removing the ref prototype, the point light source forming means and the PDI and replacing them with the original surface to be inspected and the light source.

[0018]

As described above, by employing the interferometer according to the present invention, it is possible to calibrate an aspherical null element with high accuracy and in a short time.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of an interferometer according to the present invention.

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

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

FIG. 4 is an explanatory diagram of a third embodiment of the interferometer according to the present invention.

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

[Explanation of symbols]

 Reference Signs List 1 interferometer 2 plane wave 3 Fizeau plane plate 3a Fizeau surface 4 null element 4a null wavefront 5 test object 5a test surface 6 ref prototype 6a reflex surface 7 Fizeau lens 7a Fizeau surface 10 light source 11 condenser lens 12 pinhole plate 12a Pinhole 12b Reflective surface 13 Imaging lens 14 Image sensor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F064 AA09 BB03 EE02 EE05 GG32 GG47 HH03 HH08 JJ01 2F065 AA54 CC21 DD06 FF52 FF61 GG12 HH03 JJ03 JJ26 LL10 LL30 LL32

Claims (6)

[Claims] 1. A light beam emitted from a light source, which is converted into a desired shape by a null element and is radiated to a surface to be measured, and a measurement wavefront reflected by the surface to be measured, and a light beam emitted from the light source. In the interferometer that measures the surface shape of the surface to be detected by causing the reference wavefront reflected and illuminated on the reference surface to interfere with each other, and detecting the state of the interference fringes generated by the interference, Instead of the test surface, alternately arrange a ref prototype having a spherical surface that is accurately calibrated separately, and determine a wavefront formed by the null element by measuring a measurement wavefront from the ref prototype. An interferometer. 2. The test surface is an aspherical surface, and a plurality of partial areas of the measurement wavefront obtained by changing a positional relationship between the measurement wavefront and the ref prototype. The interferometer according to claim 1, wherein a shape of a desired range of the measurement wavefront is measured by comprehensively analyzing interference fringes representing the shape. 3. A light beam emitted from a light source, which is converted into a desired shape by a null element and illuminated on a surface to be measured, and a measuring wavefront reflected by the surface to be measured, and a light beam emitted from the light source. In the interferometer that measures the surface shape of the surface to be detected by causing the reference wavefront reflected and illuminated on the reference surface to interfere with each other, and detecting the state of the interference fringes generated by the interference, The light source is a point light source of a predetermined size, which is emitted from a reflective surface and forms a spherical wave. Instead of the surface to be measured, a reflex element having a spherical surface calibrated separately and accurately. The null wave element is formed by alternately arranging instruments and reflecting the measurement wavefront from the reflex prototype on the reflection surface to interfere with the reference wavefront and measuring interference fringes. It is characterized by determining the wavefront Interferometer that. 4. The test surface is an aspherical surface, and a plurality of partial areas of the measurement wavefront obtained by changing a positional relationship between the measurement wavefront and the ref prototype. 4. The interferometer according to claim 3, wherein a shape of a desired range of the measurement wavefront is measured by comprehensively analyzing interference fringes representing the shape. 5. A light beam emitted from a light source, which is converted into a desired shape by a null element and illuminated on a surface to be measured, and a measurement wavefront reflected by the surface to be measured, and a light beam emitted from the light source. In the interferometer that measures the surface shape of the surface to be detected by causing the reference wavefront reflected and illuminated on the reference surface to interfere with each other, and detecting the state of the interference fringes generated by the interference, Instead of the surface to be inspected, a point light source of a predetermined size, having a reflecting surface, and a point light source forming means for forming a spherical wave are alternately arranged, and a wavefront from the point light source forming means is An interferometer characterized in that a wavefront formed by the null element is determined by performing interference measurement through a null element. 6. A light beam emitted from a light source, which is converted into a desired shape by a null element and illuminated on a surface to be measured, and a measuring wavefront reflected by the surface to be measured, and a light beam emitted from the light source. In the interferometer that measures the surface shape of the surface to be detected by causing the reference wavefront reflected and illuminated on the reference surface to interfere with each other, and detecting the state of the interference fringes generated by the interference, The light source is a point light source of a predetermined size, which is emitted from a reflective surface and forms a spherical wave, and is a point light source of a predetermined size instead of the surface to be measured, An interferometer, wherein point light source forming means having a reflecting surface and forming a spherical wave are alternately arranged, and a wavefront formed by the null element is determined.