JP2000097661A - Interferometer calibrating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interferometer calibrating device which can calibrate the optical wavelength, distortion, etc., of an interferometer with high accuracy by performing arithmetic processing by correlating a measuring area by using two optical surfaces from a first interferometer which becomes a calibrating basis and a second interferometer which becomes an object to be calibrated. SOLUTION: Part of a measuring luminous flux 2 from a first interferometer 1 which becomes a basis is made incident on the test surface of a specimen 5 after passing through the reference surface 3a of a wavefront converting element 3 and passes through the surface 3a after reflection. The form error of the test surface is found on the basis of the surface 3a by analyzing formed interference fringes. Then the specimen 5 is replaced with another specimen 5 and interference is measured. A mask 4 placed in immediately front of the test surface of the specimen 5 decides the area of the test surface. The measuring luminous flux 20 from a second interferometer 10 which becomes an object to be calibrated also forms interference fringes together with the reflected luminous flux from the reference surface 30a of a wavefront converting element 30 and the reflected luminous flux from the surface to be inspected of the object 5. Although the measuring luminous flux 20 has a larger area than the luminous flux 2 has, a common measuring area can be set by means of the mask 4. At the time of replacing the object 5 with the object 50, the correlation between the surfaces to be inspected of the objects 5 and 50 is prevented from shifting at the first and second interferometers 1 and 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer calibration device for calibrating a second interferometer with reference to a first interferometer whose desired performance has been calibrated in advance.

[0002]

2. Description of the Related Art A conventional interferometer has a wavelength-stabilized He-Ne laser (λ ≒ 633 nm) as a laser light source.
Was used. Therefore, calibration of this wavelength itself was not usually required. On the other hand, recently, a CO2 laser (λ ≒ 1) for measuring a rough surface or the like has been used.
An infrared interferometer using an infrared light source typified by 0.6 μm) and a semiconductor laser interferometer using a semiconductor laser for measuring a parallel plane plate have been used. Calibration has become necessary in some cases.

[0003]

However, an ordinate (wavelength of a light source) obtained by measuring a sample having a step of a known depth, such as is generally employed in a microscope type interferometer using white light. Calibration of a normal interferometer that observes a large area is caused by a large area sample or phase jump, etc.
It was sometimes difficult to obtain reliable data.

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 provide an interferometer calibrating apparatus capable of calibrating an interferometer, for example, a light source wavelength and distortion with high accuracy.

[0005]

According to the present invention, in order to achieve the above object, a first interferometer serving as a calibration reference, a second interferometer serving as a calibration target, and an interferometer for calibration are provided. Use of an interferometer calibration device including at least two optical surfaces to be used, correlating means for correlating a measurement area of the optical surface, and a calculator for calculating and processing measurement data of the interferometer. And This makes it possible to easily calibrate the desired performance of the test interferometer using the reference interferometer regardless of the type of interferometer or the wavelength of the light source.

In the above interferometer calibration apparatus, at least one of the interferometers is a Fizeau interferometer, a reference surface of the interferometer also serves as the optical surface, and the reference surfaces form interference fringes with each other. By using a configuration in which the interferometer is arranged such that the interferometer can be used, the desired performance of the test interferometer can be calibrated with high accuracy without using a dummy optical surface and using a reference interferometer. become.

Although the drawings of the embodiments of the present invention are used to make the present invention easy to understand, the present invention is not limited to the embodiments.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of an interferometer calibration apparatus according to the present invention will be described below with reference to FIG. "Red" in Fig. 1
Is an interferometer to be calibrated and has a wavelength of 1.06 μm, for example.
m infrared interferometer. In addition, “OK” in FIG. 1 is an interferometer serving as a calibration reference, for example, a visible interferometer having a wavelength of 0.633 μm. "1" is the first work, "2"
Is a second workpiece, which represents two optical surfaces used for interferometry for calibration. In FIG. 1, both interferometers were assumed to be Fizeau interferometers.

At this time, the system error of the interferometer (shape error of Fizeau surface: Fizeau deviation), which cannot be avoided in the interferometer, can be obtained by subtracting the measurement data of the two works. 2 "will be offset. Therefore, this data obtained as the error shape can be obtained by using a correlating means (not shown) for correlating the measurement area of the optical surface so that the areas measured by the interferometer can be matched. Unless the abscissa is distorted,
They should exactly match. In other words, the ratio of the wavelength of the light source is determined (the ordinate is calibrated).

In practice, when both data are compared on a pixel basis due to variations in performing the interference measurement, this ratio should vary from pixel to pixel. For example, the average value of this ratio is calculated. The calculation is performed by a calculator (not shown). Calculating using the assumed wavelength, “magnification ≒ 1.67
5 ”, and the deviation from the design value is 0.6.
The wavelength shift is 1.06 μm with the wavelength of 33 μm as the true value. In the calculation, the shape error component may be developed into Zernike coefficients, and the ratio of the coefficients may be used, or optimal fitting using the least squares method may be appropriately performed.

FIG. 2 illustrates a correlating means for correlating a measurement area on an optical surface. The calibration procedure is
It is the same as FIG. First, in FIG. 2A, a part of a measurement light beam 2 emitted from a first interferometer 1 serving as a reference is a reference surface (Fizeau surface in FIG. 2) of a wavefront conversion element (Fizeau flat in FIG. 2). A part of the light is reflected from the reference surface 3a, passes through the reference surface 3a, enters the test surface 5a of the test object 5, and transmits through the reference surface 3a after being reflected by the test surface. These two measurement lights form interference fringes on the detector, and the interference fringes are analyzed by the calculator, whereby the shape error of the test surface 5a with respect to the reference surface 3a is obtained.

Then, as represented by a double arrow in the figure,
The test object 5 is replaced with the test object 50, and the test surface 50a of the test object 50 is subjected to interference measurement. In FIG. 2, the mask 4 is placed immediately before the surface to be measured in order to determine the area for measuring both surfaces to be measured. Next, in FIG. 2B, the measurement light beam 20 emitted from the second interferometer 10 to be calibrated also reflects the same light beam reflected from the reference surface 30 a of the wavefront conversion element 30 and the test object described above. The light beam reflected from the test surface 5a of 5 forms interference fringes. In the figure, the measurement light beam 20 has a larger area than the measurement light beam 2, but it can be seen that a common measurement region can be set by the mask 4.

However, even when the same test surfaces 5a and 50a are used, the correlation between the test surfaces when the first interferometer is used when exchanging the test object indicated by the double arrow is If the correlation between the test surfaces when using the second interferometer is deviated, an error occurs when calculating the wavelength magnification. Therefore, the positioning of the test object (the test surface) is performed using the measurement optical axis. Including alignment in the direction of rotation around,
It becomes important.

In FIG. 2, one mask 4 is used in common, but may be provided for each test surface or for each reference surface. At this time, if the aperture sizes of the two masks are different, the difference data for canceling the shape error of the reference surface becomes an aperture of a small mask, so that it is impossible to position the aperture reference. In order to avoid this, the aperture sizes of both masks may be matched. At this time, the completion of the positioning of the mask can be determined by the maximum number of CCD pixels having the ordinate value as the difference data.

In general, when an interferometer is provided with an anti-reflection film suitable for the conditions to be used and performs interference measurement on a surface to be measured,
An antireflection film is also formed on the reference surface so that the contrast of the interference fringes does not deteriorate. When such a reference surface is used, a vapor deposition film for adjusting the reflectance is also provided on the dummy surface so that the reflectance of the test surfaces 5a and 50a used as the dummy surface is optimal for each wavelength. It is necessary to apply or insert an ND filter (not shown) for adjusting the transmittance into the light beam for measurement.

[0016]

Embodiment 1 FIG. 3 shows a configuration in which a reference surface of an interferometer is directly used as a test surface, and two Fizeau interferometers are arranged facing each other. FIG. 3 shows an example of interference measurement between planes. However, the present embodiment can be applied to a case where a Fizeau lens that generates a spherical wave is directly opposed.

In FIG. 3A, a part of a measurement light beam 2 emitted from a first interferometer 1 serving as a reference is a reference surface (Fizeau flat in FIG. 3) of a wavefront conversion element (Fizeau flat in FIG. 3). After being reflected from the surface 3a and partially passing through the reference surface 3a and being incident on the reference surface 30a of the wavefront conversion element 30 of the second interferometer 10 to be calibrated and reflected by the reference surface 30a Is transmitted through the reference surface 3a. These two measuring beams form interference fringes on the detector, and the interference fringes are analyzed by the arithmetic unit, so that the reference surface 3a based on the reference surface 3a is obtained.
A shape error of 0a is obtained.

On the other hand, with respect to the measurement light beam 20 emitted from the second interferometer 10 to be calibrated, the reflected light beam from the same reference surface 30a of the wavefront conversion element 30 and the reference light beam of the wavefront conversion element 3 The light beam reflected from the surface 3a forms interference fringes. Further, as a mask as a correlating means for correlating the measurement area of the optical surface, a mask 4 is set on the reference surface 3a, and a mask 40 is set on the reference surface 30a. This mask may be operated so that positioning with respect to the reference surface is possible, or may be formed directly on the reference surface by vapor deposition. This applies to all embodiments of the present invention.

In this arrangement, the reference surface 3a and the reference surface 30a
Must face each other, and it is necessary to match the measurement optical axes defined by the central axes of the mask 4 and the mask 40.
This method may be performed by, for example, maximizing the number of pixels described above. FIG. 3B shows an example in which the measurement apertures are matched by inserting the mask 4 into a common light beam and sharing the light beam.

If the interference measurement is to be performed simultaneously in the arrangement shown in FIG. 3 as described above, one of the measurement light beams becomes noise in the other interferometer, so that it is necessary to provide a time lag in obtaining the interference fringe data. Becomes Further, in the case of a reference surface provided with an anti-reflection film suitable for measurement of the test surface as described above, the reflectance of the anti-reflection film of at least one of the reference surfaces is changed or the transmittance (not shown) is changed. It is necessary to insert an ND filter that adjusts for the common light flux.

[0021]

Embodiment 2 FIG. 4 shows an example in which a fold mirror 6 for deflecting a measurement light beam is introduced as an optical surface used for interference measurement for calibration, and is applied to the measurement example of FIG. is there. This is because the measurement posture of the test object 5 having the optical surface 5a used for the interference measurement for calibration as a dummy test surface is set perpendicular to the direction of gravity represented by "g" in the figure. It is a measurement arrangement | positioning figure when it must be. In this case, the shape error of the reflection surface 6a of the fold mirror 6 is also included in the Fizeau bias described in the explanation of the principle of FIG.

In FIG. 4, the mask 4 is set apart from the reference surface 3a. However, it goes without saying that the mask 4 may be provided on the reference surface.

[0023]

Embodiment 3 FIG. 5 shows the fold mirror 6 described above,
This is applied to the measurement example of FIG. In this figure, the second interferometer 10 to be calibrated is a downward measurement, and the measurement light beam 20 from the interferometer is horizontally deflected by the fold mirror 6 to be the reference interferometer 1. The light is vertically incident on the reference surface 3a. In this example, the mask 4 is provided for the common light beam and the mask 40 is provided on the reference surface 30a, so that versatility can be obtained.

In this embodiment, not only the gravitational deformation due to the posture of the test object is taken into consideration, but also the distortion of the measuring optical system of each interferometer can be calibrated. As a method of distortion calibration, a method is used in which a small alignment deviation is intentionally given, and an alignment error aberration generated at that time is analyzed, thereby making it possible to measure the distortion of the measurement optical system.

In FIG. 5, the misalignment is easily given by tilting the fold mirror 6 instead of tilting the other interferometer with respect to one interferometer. By using this arrangement to compare and calculate both interference fringe data with a 633 nm interferometer, the distortion of the reference interferometer can be calibrated with high accuracy. Then, the interferometer to be calibrated can be calibrated by using the reference interferometer that has been calibrated once with high accuracy.

Further, in order to enhance the distortion calibration accuracy, a shape error pattern for calibrating the abscissa of the interferometer may be provided on the reference surface 3a of the interferometer 1 as a reference. This is because the reference surface 30 is within the common beam.
a, or on the reflection surface 6 a of the fold mirror 6. The formation of this reference pattern is not limited to the present embodiment.

[0027]

As described above, if the interferometer calibration apparatus according to the present invention is employed, it becomes possible to calibrate the interferometer, for example, the light source wavelength and the distortion with high accuracy.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating the principle of an interferometer calibration device according to the present invention.

FIG. 3 is an explanatory diagram of Embodiment 1 of the interferometer calibration device according to the present invention.

FIG. 4 is an explanatory view of Embodiment 2 of the interferometer calibration device according to the present invention.

FIG. 5 is an explanatory view of a modification of the third embodiment of the interferometer calibration device according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 First interferometer 2 Measurement light beam 3 Wavefront conversion element 3a Reference surface 4 Mask 4a Measurement wavefront 5 Test object 5a Test surface 6 Fold mirror 6a Reflection surface

Claims (6)

[Claims] 1. A first interferometer to be a calibration reference, a second interferometer to be calibrated, at least two optical surfaces used for interferometry for calibration, and measurement of the optical surfaces An interferometer calibration apparatus comprising: a correlating means for correlating regions; and a calculator for calculating and processing measurement data of the interferometer. 2. The interferometer according to claim 1, wherein at least one of said interferometers is a Fizeau interferometer, a reference surface of said interferometer also serves as said optical surface, and said interferometers are arranged such that said reference surfaces form interference fringes with each other. The interferometer calibration apparatus according to claim 1, wherein the calibration is performed. 3. The interferometer calibration apparatus according to claim 1, wherein a folding mirror for deflecting the measurement light beam of the interferometer is used on the optical surface. 4. The interferometer calibration apparatus according to claim 3, wherein tilting of the fold mirror is enabled. 5. The interferometer calibration apparatus according to claim 1, wherein a vapor deposition film for adjusting the contrast of interference fringes formed by the interference measurement is provided on at least one of the optical surfaces. . 6. The interferometer calibration apparatus according to claim 1, wherein a shape error pattern for calibrating the abscissa of the interferometer is provided on at least one of the optical surfaces.