JPH1047930A - Shape measuring method using interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable relative measurement of the shape of a surface to be measured with high accuracy even when the surface of a reflexion origin having reflectivity which is largely different from that of the surface to be measured is used by adjusting the contrast of interference fringes by inserting a damping plate between a reference surface and the surface to be measured and, at the same time, subtracting the transmitted wavefront aberration of the damping plate from measured data. SOLUTION: The interference between the surface 5a to be measured of an object 5 and the surface of a reflexion origin having reflectivity which is different from that of the surface 5a is measured by placing the surface of the reflexion origin at the position of the surface 5a and the error data of the Fizeau surface (reference surface) 1a of an obtained Fizeau flat 1 are subtracted from measured data. At the time of measuring the surface 5a or the surface of the reflexion origin having high reflectivity, a damping plate 3 is inserted between the surface 5a and the surface of the reflexion origin for damping the reflected light from the surface 5a and, at the time of measuring the surface 5a having low reflectivity, the interference is measured without inserting the plate 3 and the separately measured transmitted wavefront aberration of the plate 3 is subtracted from the measured data obtained when the surface 5a having high reflectivity is measured. Therefore, the shape of the surface 5a can be measured with high accuracy even when the surface of the reflexion origin having reflectivity which is different from that of the surface 5a is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an interferometer.
The present invention relates to a shape measuring method for measuring a highly accurate surface shape (plane, spherical surface, etc.) of an optical mirror or the like. In particular, the present invention relates to a shape measuring method capable of performing high-accuracy measurement even when the reflectance of a reference surface and the surface to be inspected (or the surface of a reflex prototype) is considerably different.

[0002]

2. Description of the Related Art A conventional technique will be described by taking a Fizeau interferometer as an example. FIG. 2 is a diagram for explaining the principle of the simplest Fizeau interferometer. In FIG. 2, a half mirror 1 called a Fizeau flat 1 and a measurement object 5 face each other at a certain distance. Fizeau Flat 1
Is a flat plate made of optical glass, and the right surface thereof (the surface on the side of the object to be measured) is a Fizeau surface 1a of a high-precision optical surface (a flat surface in this case). The Fizeau surface 1a is also called a reference surface. Left side of Fizeau Flat 1 (anti-measurement object surface 1
b) is usually provided with an anti-reflection coating.
The measurement object 5 has a surface 5a (in this case, a plane) to be measured. The test surface 5a is the Fizeau surface 1a
Is placed facing.

A coherent light 13a is incident on the Fizeau flat 1 from the left side of the figure (the side opposite to the object 5). Light 1
A laser beam 3a is usually adjusted so as to be perpendicular to the Fizeau surface 1a. The light 13a enters the Fizeau flat 1, and a part of the light 13a is reflected by the Fizeau surface 1a (light 13b). The reflectance of the Fizeau surface 1a is about 4% as a general value of optical glass. Therefore,
The remaining 96% of the light 13c passes through the Fizeau flat 1 and travels further right.

[0004] Next, the light 13c is reflected by the surface 5a to be inspected (light 13d). The reflectance of the test surface 5a differs depending on the material of the measurement object 5 and the properties of the same surface 5a. In the case of optical glass, the reflectance is about 4%, and in the case of a highly reflective metal surface, the reflectance is 90% or more. Light 1 reflected by the test surface 5a
3d passes through the Fizeau flat 1 (light 13e) and interferes with the light 13b reflected by the Fizeau surface 1a. By analyzing information on the interference between the two lights (usually the shape, number, and size of interference fringes), information on the relative shape difference between the Fizeau surface 1a and the test surface 5a can be obtained. If the Fizeau surface 1a can be considered as a high-precision plane, the interference information indicates the flatness error of the test surface 5a.

However, the Fizeau surface 1a (reference surface) also has an error. Therefore, in the case of performing particularly high-accuracy measurement, an operation of calibrating the accuracy of the Fizeau surface 1a with the prototype is performed. At this time, a prototype (or liquid) is used as the measurement object 5 in the figure, and the surface of the prototype (surface 5a, the liquid surface in the case of liquid) is defined as positive (or another method such as a wavefront creation extraction method). ), And the calibration of the Fizeau surface 1a is performed. In such a calibration measurement operation, the test surface 5a is referred to as a ref prototype surface (or reference surface).

In observing interference fringes in such a Fizeau interferometer, the reflected light 13b from the Fizeau surface 1a
Then, the intensity ratio of the reflected light 13e from the test surface 5a becomes a problem. When the intensity ratio is 1: 1, an interference fringe with the highest contrast is obtained. However, as the intensity ratio increases, the contrast decreases, and eventually interference fringes cannot be observed.

In FIG. 2, the reflectance of the Fizeau surface 1a is Rf,
Assuming that the reflectance of the test surface is Rw, the two lights 13b and 13b
The intensity ratio of e is Rf: (1-Rf) × Rw × (1-R
f), that is, Rf: (1−Rf) 2 × Rw.
Here, assuming that Rf = 0.04, the ratio is 0.04: 0.
92Rw. Further, assuming that Rw = 0.90 (high reflectance, mercury surface, etc.), the ratio is 0.04: 0.83 =
1:21, and the intensity of the two lights is generally too different, so that interference fringes cannot be observed and shape measurement cannot be performed.

In such a case, it is conceivable to reduce the intensity by passing strong reflected light through an attenuation plate (half mirror or pellicle). FIG. 1 is a diagram of an interference optical system having an attenuation plate 3 between a Fizeau surface 1a and a test surface 5a. That is, most of the incident light on the Fizeau flat 1 passes through the Fizeau flat 1 (11c), and further passes through the attenuation plate 3 (11c).
d), the light is reflected by the test surface 5a (11e), passes through the attenuation plate 3 (11f), and passes through the Fizeau flat 1 (11)
g).

Consider the light intensity in this case. The transmittance of the attenuation plate 3 is defined as Td. Fizeau surface 1a in FIG.
11b reflected from the surface 5a and reflected light 11g
Rf: (1-Rf) × Td × Rw × Td × (1-R
f), that is, Rf: (1−Rf) 2 × Td2 × Rw.

Here, Rf = 0.04, Rw = 0.90
In this case, the ratio is 0.04: 0.83 Td2. If Td is 0.2, the ratio is 0.04: 0.03
3. Since the intensities of both lights are close to each other and the contrast of the interference fringes is good, good interference measurement can be performed.

Next, the wavefront aberration measurement data in the interference system shown in FIG. 1 will be described. In FIG. 1, the wavefront aberration H1 observed at the time of measuring the test surface is as follows: H1 = F + D + W (1) Here, F is the shape error aberration of the Fizeau surface, D is the transmitted wavefront aberration of the attenuation plate, and W is the reflected wavefront aberration of the test surface. The wavefront aberration H2 observed when the measurement is performed by placing a reference ref prototype instead of the test surface is as follows: H2 = F + D + R (2) Here, R is the reflected wavefront aberration of the ref prototype.
When data processing is performed to calculate the difference between H1 and H2, H1−H2 = WR (3) is obtained. If R is negligible compared to W, then H1−H2 ≒ W (4)

[0012]

However, when measuring a test surface having a reflectance equivalent to that of a Fizeau surface with reference to a high-reflectivity ref prototype surface (for example, a mercury liquid surface), the ref prototype is required. If the attenuation plate is inserted only when measuring the surface and the attenuation plate is not inserted when measuring the surface to be measured, there is a difference between the presence and absence of the attenuation plate. The wavefront aberration H2 when measuring a high-reflection REF prototype is given by the above equation (2). On the other hand, when measuring a low-reflectance surface to be measured, the wavefront aberration H3 observed when the surface to be measured is measured is as follows: H3 = F + W (5) It is. The difference between H3 and H2 is obtained by subtracting equation (2) from equation (5), and H3−H2 = W− (D + R) (6). I get on.

Therefore, there is a problem that relative measurement of a low-reflectance test surface cannot be performed with reference to a high-reflection ref prototype surface. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional drawbacks, and is capable of performing high-precision relative measurement of a test surface even when using a reflex prototype surface having a significantly different reflectance with respect to the test surface. The aim is to provide a method.

[0014]

In order to solve the above-mentioned problems, a shape measuring method according to the present invention employs an interferometer that applies coherent light to a reference surface and a surface to be inspected and interferes with reflected light from both surfaces. In a method of measuring the shape of a test surface, an attenuating plate for reducing the intensity of light reflected from the test surface is inserted between the reference surface and the test surface to adjust the contrast of interference fringes,
The transmission wavefront aberration of the attenuation plate, which is separately measured, is subtracted from the measurement data.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A shape measuring method using an interferometer according to a first aspect of the present invention uses an interferometer that applies coherent light to a reference surface and a test surface and interferes with reflected light from both surfaces. A method for measuring the shape of a surface to be measured; wherein the measuring method includes a reference obtained by performing interference measurement at a position of the surface to be measured on a ref prototype surface having a reflectance different from that of the surface to be measured. Including the process of subtracting error data such as surfaces from measurement data,
When measuring the surface with the higher reflectivity, either the test surface or the reference surface of the ref, insert an attenuator to reduce the intensity of the light reflected from the surface, perform interference measurement, and measure the lower reflectivity. When measuring the surface, the interference measurement is performed without inserting the attenuation plate, and the transmitted wavefront aberration of the attenuation plate separately measured is subtracted from the high reflectance surface measurement data.

That is, if the transmitted wavefront aberration D of the attenuation plate is known, the relative wavefront aberration H4 of the test surface with respect to the ref prototype surface can be obtained by substituting into the equation (6): H4 = WR− W... (7) The transmitted wavefront aberration D of the attenuation plate is observed when the attenuation plate is inserted as shown in FIG. 1 and the wavefront aberration H2 obtained by measuring the reference surface of the reflex is measured when the attenuation plate is not inserted as shown in FIG. Of the wavefront aberration H5. H5
Is as follows: H5 = F + R (8) Therefore, from equations (2) and (8), the transmitted wavefront aberration D of the attenuation plate can be obtained from D = H2−H5 (9).

By substituting the transmitted wavefront aberration D of the attenuating plate into the equation (6), the shape of the surface to be measured can be measured. At this time, naturally, image processing for satisfying coordinate consistency such as magnification, distortion, and shift is required, but a known method can be used for this.

In the present invention, the reflectance on the reference surface (Fizeau surface) is Rf, and the transmittance of the attenuation plate is Td.
When a dummy test surface having a reflectance Rw 'that satisfies the following equation is used, 0.1Rf ≦ (1-Rf) 2 × Td2 × Rw ′ (A) (1-Rf ) 2 × Rw ′ ≦ 10Rf (B) The wavefront aberration when the attenuation plate is inserted between the reference surface and the dummy test surface and when it is not inserted are determined, and the difference between the two wavefront aberrations is obtained. Thus, the transmitted wavefront aberration of the attenuation plate can be obtained.

The present inventor has determined that if the intensity of the light reflected from the test surface is in the range of about 1/10 to 10 times the intensity of the light reflected from the reference surface, due to the noise of the CCD of the interferometer, etc. ,
Generally, interference measurement is possible, but when the intensity ratio is out of the range, the contrast is deteriorated due to the problem of the intensity ratio, and it has been found that measurement of interference fringes is generally difficult. Based on this knowledge, regardless of the presence or absence of a damping plate,
In each case, the above-described measurement of FIG. 1 and Expression (2) and the measurement of FIG. 2 and Expression (8) are performed using a dummy test surface capable of performing interference measurement, and the wavefront transmitted through the attenuation plate is obtained from the measurement results. Aberration can be determined.

In the shape measuring method using the interferometer according to the second aspect of the present invention, the coherent light is applied to the reference surface and the test surface, and the interferometer causes the reflected light from both surfaces to interfere with each other. A method for measuring a shape, wherein the measurement method includes measuring an error of a reference surface or the like obtained by performing interference measurement at a position of the test surface on a ref prototype surface having a reflectance different from that of the test surface. Includes the process of subtracting data from the measured data, and inserting an attenuator between the reference surface and the test surface and the reference surface of the ref. Where Rf is the reflectance on the reference surface, Td is the transmittance of the attenuating plate, Rw is the reflectance of the test surface, and Rr is the reflectance of the ref prototype surface. 1Rf ≦ (1-Rf) 2 × Td2 × Rw (C) 10Rf ≧ 1-Rf) and selects the Td such that the 2 × Td2 × Rr ········ (D).

In this case, by selecting an appropriate transmittance Td of the attenuating plate, it is possible to measure the surface to be inspected and the surface of the ref prototype having different reflectivities while inserting the same attenuating plate. By the method from 1) to the expression (4), highly accurate shape measurement of the test surface can be performed.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an embodiment of a Fizeau interferometer of the shape measuring method according to the first embodiment of the present invention will be described. The reflectance of the Fizeau surface and the surface to be inspected is set to the general reflectance of optical glass of 0.0.
4 and the reflectivity of the reflex prototype surface was assumed to be 0.9 on the mercury surface.
Set to 0. It is also assumed that the transmittance of the attenuation plate is fixed at 0.20. Then, an attenuation plate is inserted at the time of measuring the reference surface of the reflex, and the measurement is performed without inserting the attenuation plate at the time of measuring the surface to be measured.

At this time, the intensity ratio of the interference light when measuring the ref prototype surface with an attenuation plate inserted is: Fizeau surface reflection light: ref prototype surface reflection light = 0.04: (1−0.04) ) × 0.2 × 0.9 × 0.2 × (1−0.04) = 0.04: 0.033.

The intensity ratio of the interference light when the test surface was measured without the attenuating plate was: Fizeau surface reflection light: Test surface reflection light = 0.04: (1−0.04) × 0.04 × (1−0.04) = 0.04: 0.037.

In each case, the intensity ratio of the interference light is approximately 1: 1.
1, a good contrast of interference fringes can be obtained.

Next, a method for obtaining a suitable reflectance of the dummy test surface will be described. The above values of the reflectance and the like are applied to the above-described equations (A) and (B). 0.1Rf ≦ (1-Rf) 2 Td2 × Rw ′ (A) 0.1 × 0.04 ≦ (1−0.04) 2 (0.2) 2
× Rw ′ 0.11 ≦ Rw ′ (1-Rf) 2 × Rw ′ ≦ 10Rf (B) (1−0.04) 2 × Rw ′ ≦ 10 × 0.04 Rw '≦ 0.43 That is, a dummy test surface having a reflectivity of 0.11 to 0.43 was measured in two states, with and without an attenuating plate.
By applying the formulas (9) to (9), the transmitted wavefront aberration of the attenuation plate is obtained, and by subtracting it from the measurement data, highly accurate shape data of the test surface can be obtained.

Next, an embodiment of a Fizeau interferometer of the shape measuring method according to the second embodiment of the present invention will be described. As in the embodiment of the first aspect, the reflectance of the Fizeau surface and the surface to be inspected is set to 0.
04, and the reflectivity of the ref prototype surface is 0.9. Then, the transmittance Td of the attenuation plate is selected so that the measurement can be performed with the attenuation plate both when measuring the surface to be measured and when measuring the surface of the ref prototype.

Each value is substituted into the above equations (C) and (D). 0.1Rf ≦ (1-Rf) 2 × Td2 × Rw (C) 0.1 × 0.04 ≦ (1-0.04) 2 × Td2 ×
0.04 0.33 ≦ Td 10Rf ≧ (1-Rf) 2 × Td2 × Rr (D) 10 × 0.04 ≧ (1-0.04) 2 × Td2 × 0.
90 0.69 ≧ Td That is, 0.33 ≦ Td ≦ 0.69 is the transmittance range of the attenuation plate. The most preferable in this range is around the square root of the product of the upper and lower limits, and Td = (0.33 × 0.6
9) 1/2 = 0.48.

When Td = 0.48, the interference light intensity ratio at the time of measuring the reference surface of the reflex device is as follows. Fizeau surface reflected light: Test surface reflected light = 0.04: (1-
0.04) × 0.48 × 0.90 × 0.48 × (1-
0.04) = 0.04: 0.23 ≒ 1: 6 Further, the interference light intensity ratio at the time of measuring the test surface is as follows.

Reference surface reflected light: Test surface reflected light = 0.04:
(1-0.04) × 0.48 × 0.04 × 0.48 ×
(1−0.04) = 0.04: 0.0084 ≒ 5: 1 In any of these cases, interference measurement is sufficiently possible. According to the second aspect, since the interference measurement can be performed without inserting and removing the attenuation plate, the number of measurement steps is reduced.

[0031]

As is apparent from the above description, according to the present invention, even if a ref prototype having a large difference in reflectance with respect to the test surface is used, the relative measurement of the test surface can be performed with high accuracy. Becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram of an interference optical system having an attenuation plate between a Fizeau surface and a test surface.

FIG. 2 is a diagram for explaining the principle of the simplest Fizeau interferometer.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Fizeau flat (half mirror) 1a Fizeau surface (reference surface) 3 Attenuation plate 5 Measurement object 5a Surface to be inspected (reflective prototype surface) 11, 13 light

Claims (4)

[Claims] 1. A method of measuring the shape of a test surface using an interferometer that irradiates coherent light to a reference surface and a test surface and interferes reflected light from both surfaces; An interferometer characterized by inserting an attenuation plate for reducing the intensity of the reflected light from the test surface to adjust the contrast of interference fringes and subtracting the separately measured transmitted wavefront aberration of the attenuation plate from the measurement data. Shape measurement method. 2. A method of measuring the shape of a test surface using an interferometer that irradiates coherent light to a reference surface and a test surface and causes reflected light from both surfaces to interfere with each other; A process of subtracting error data of a reference surface or the like obtained by interferometrically measuring a reference surface of a ref having a different reflectance from the surface to be measured at a position of the surface to be measured from the measurement data, When measuring one of the original surfaces with the higher reflectivity, insert an attenuator to reduce the intensity of the reflected light from the surface, perform interference measurement, and measure the lower reflectivity surface. Is a method for measuring a shape using an interferometer, wherein interference measurement is performed without inserting an attenuation plate, and a transmitted wavefront aberration of the attenuation plate, which is separately measured, is subtracted from the high reflectance surface measurement data. 3. When the reflectance at the reference surface is Rf and the transmittance of the attenuating plate is Td, a dummy test surface having a reflectance Rw ′ satisfying the following expression is used: 0.1Rf ≦ (1−Rf) 2 × Td2 × Rw ′ (A) (1−Rf) 2 × Rw ′ ≦ 10Rf (B) Refer to the attenuation plate 2. A wavefront aberration in a case where the wavefront aberration is inserted and a wavefront aberration in a case where the wavefront aberration is not inserted between the dummy test surface, and a difference between both wavefront aberrations is obtained to obtain a transmitted wavefront aberration of the attenuation plate.
Or a shape measuring method using the interferometer according to 2. 4. A method for measuring the shape of a test surface using an interferometer that applies coherent light to a reference surface and a test surface and causes reflected light from both surfaces to interfere with each other; , Including a process of subtracting error data such as a reference surface obtained by interferometrically measuring a reference surface of a reflex device having a different reflectance from the surface to be measured at the position of the surface to be measured, from the measured data. And an attenuating plate for reducing the intensity of the reflected light from the surface to be inspected and the surface of the ref prototype is adjusted between the surfaces of the reference surface and the ref prototype to adjust the contrast of the interference fringes. When the transmittance of the attenuating plate is Td, the reflectance of the test surface is Rw, and the reflectance of the ref prototype surface is Rr, 0.1Rf ≦ (1-Rf) 2 × Td2 × Rw (C) 10Rf ≧ (1-Rf) 2 × Td2 × Rr (D) A shape measuring method using an interferometer, wherein Td is selected such that