JPH07190737A - Shearing interference measuring method and shearing interferometer - Google Patents

Abstract

PURPOSE:To judge whether inclination is positive or negative when the inclination of a measured wave surface from a frequency peak is detected. CONSTITUTION:A reference light intensity signal is detected while the quantity of shear S is being altered, and the peak frequency fp of a frequency spectrum making the quantity of shear S a parameter is detected, so that profile information including the inclination W/ x of a measured light surface can be obtained based on the aforesaid peak frequency fp. The optical system of a shearing interferometer is set in such a way that the known specified quantity of frequency shift nu takes place in a frequency spectrum making the quantity of shear S a parameter, the profile information of a measured light surface is obtained using fp=k.(W/ x)+nu (where, k=2pi/lambda), and the quantity of frequency shift nu is set in such a way that the sign of k.(W/ x)+nu shall be positive or zero, or must be negative or zero in the all area of the measured light surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention utilizes the interference phenomenon of light waves to measure the phase distribution of light waves, that is, the shape of the light wave front, to measure the physical characteristics such as the shape of an object or the refractive index distribution. ) It relates to an interference measuring method and a shearing interferometer.

[0002]

2. Description of the Related Art A so-called interferometer utilizing the interference phenomenon of light waves is widely used in the field of high precision measurement because it can perform non-contact measurement with an accuracy of light wavelength, that is, submicron or more.

For example, Zygo's Mark IVxp
The interferometer called is He- with a wavelength λ = 632.8 nm.
Using a Ne laser as a light source, an interference fringe image analyzer is provided, and the shape of a mirror-finished object can be measured in a non-contact manner with a repeatability of λ / 300 or more.

However, as shown in FIG. 5, a conventional type interferometer such as the Mark IVxp manufactured by Zygo Co., which is widely used at present, has a highly precise and ideally shaped surface called a prototype 25. Since the measurement is performed by causing the light wave reflected by and the light wave reflected by the surface to be measured 26 to interfere with each other, it is impossible to perform measurement based on a shape in which it is difficult to create a highly accurate prototype 25.

The structure and operation of the interferometer of FIG. 5 (a) will be briefly described. The light emitted from the laser 21 is expanded by the expander lens 22 and is converted into parallel light by the collimator lens 24 via the semi-transparent mirror 23. , Is incident on the prototype 25. The prototype 25 has an ideally-shaped semi-transparent surface, and is shown in FIG.
As shown in (b) of the wavefront in the circle surrounded by the broken line in FIG. 5 (a), the wavefront 29 transmitted through the prototype 25 and reflected by the measured surface of the DUT 26 and the half of the prototype 25 The wavefront 28 reflected by the transparent surface is combined. The combined light reaches the semi-transparent mirror 23 through the collimator lens 24 in the opposite direction, is reflected there, and the interference fringes of both wavefronts 28 and 29 are observed on the observation surface 27.

As described above, the normal type interferometer requires a highly accurate prototype, but at present, the prototypes that can actually be produced are limited to flat and spherical surfaces. Therefore, for example, it is generally impossible to measure an aspherical surface with a normal type interferometer, except in a limited case where an aspherical amount (a difference from a spherical surface shape) is very small.

As a first method for solving this point and performing interferometric measurement of an aspherical surface, a computer for generating an optical wavefront having an ideal aspherical surface, as disclosed in, for example, Japanese Patent Laid-Open No. 2-24504. There is a method of creating a hologram and using this as a prototype. However, even this method cannot be a fundamental solution because it is not possible to verify with high accuracy whether or not the optical wavefront generated by the computer generated hologram has an ideal shape.

Then, as a second method for measuring the aspherical surface, for example, Japanese Patent Laid-Open Nos. 59-90009 and 63-210605, or "Optical Technology Contact", vol. 23, No. 12 (1985), p.
Shearing interferometers have been utilized that do not require a reference prototype, as shown at 889. Although the description will be given here based on the lateral shearing interferometry, the present invention described later is not limited to the lateral shearing interferometer.

The shearing interferometer, as shown in FIG. 6 showing the state of the wavefront, does not need a reference prototype in order to divide the optical wavefront of the measured light itself into two parts and shift them to interfere with each other. In FIG. 6, assuming that the measured optical wavefront is W (x) and the other wavefront that has been given the offset is W (x + S), the phase difference ΔW appearing in the interference fringes is ΔW = W (x + S) −W ( x) = W (x) + ∂W / ∂x · S−W (x) = ∂W / ∂x · S (1) To shift the wavefront in this way is called "shear".
ar) ”or“ shearing ”
Say An example of a specific optical means for providing shearing is shown in FIG. In this example, the incident wavefront 30 is divided into two wavefronts by a semi-transparent mirror 33, one of which is made to enter a right-angle prism 31 and the other is made to enter another right-angle prism 32 to be retroreflected and made to enter a semi-transparent mirror 33 again. It is an arrangement of a Twyman-Green interferometer that synthesizes both wavefronts into an output light wavefront 34, and by moving one right-angle prism 31 by S / 2 perpendicularly to the optical axis, one wavefront of S She is giving Shea.

The interference fringes obtained by the shearing interferometer are not the contours of the shape of the measured light wavefront itself, but the displacement of the measured light wavefront as shown in the above equation (1) shown in relation to FIG. It is the contour line of the differential coefficient along the direction. Therefore, it is difficult to intuitively understand the interference fringes as they are, and automatic interference fringe analysis such as the fringe scanning method is indispensable. Therefore, in the conventional shearing interferometry, for extracting phase information from interference fringes, for example, Japanese Patent Laid-Open No. 59-900.
09 publication or "optical technology contact" vo
l. 23, No. 12 (1985), p. 889, an interference fringe analysis technique called a fringe scanning method is often adopted.

This fringe scanning method will be described based on the shearing interferometer shown in FIG. In this interferometer, the light emitted from the laser 21 is the beam expander B.
The diameter of which is enlarged by E, enters the beam splitter BS through the mirror M, and the light reflected by the semi-transparent surface enters the surface under measurement of the object under measurement 26 through the converter lens 35 and is reflected there. The reflected light returns to the beam splitter BS through the converter lens 35 in the reverse direction, directly passes through the beam splitter BS, and enters the Twyman-Green type interferometer. That is, the incident light is split into two wavefronts by the semi-transparent mirror 33, one of which enters the corner cube prism 31 and the other of which enters the other corner cube prism 32, which are retroreflected and again enter the semi-transparent mirror 33. Both wavefronts are combined, and interference fringes are observed by a CCD (charge coupled device) camera 36. FIG. 8B shows the state of the wavefront that enters the interferometer and emerges therefrom. In this interferometer, one of the corner cube prisms 31 is moved perpendicularly to the optical axis to output the wavefront 34. Is given a certain amount of shear S. When the fringe scanning method is performed, for example, the other corner cube prism 32 is sequentially moved along the optical axis to give the two wavefronts while sequentially changing the optical path difference H.

For simplicity, consider a one-dimensional interference fringe image. Letting ΔW (x) be the phase difference distribution of the two wavefronts shifted by the shearing, the light intensity distribution I of the interference fringes
(X, H) is expressed by the following equation. x is the position in the X-axis direction, H
Is the optical path difference, and I ₀ is the sum of the intensities of the two optical paths.

I (x, H) = I ₀ [1 + γ cos {k (ΔW (x) −H}] (2) where k = 2π / λ (λ: wavelength of light wave, π: circular constant) ), Γ: a constant determined by the amplitude ratio When this equation (2) is further modified, I (x, H) = a ₀ + a ₁ · cos (kH) + a ₂ · sin (2kH) (3) a ₀ = I ₀ becomes _{··· (4) a 1 = I} 0 · γ · cos (2kΔW) ··· (5) a 2 = I 0 · γ · sin (2kΔW) ··· (6). Now, the corner cube prism 32 in FIG.
By changing the optical path difference in λ / 4 steps, and the interference fringe images obtained at that time are I ₁ , I ₂ , I ₃ , I ₄ (H = 0, λ / 4, 2λ / 4, 3λ / 4), I ₁ = I (x, 0) = a ₀ + a ₁ (7) I ₂ = I (x, λ / 4) = a ₀ + a ₂ (8) I ₃ = I (x, 2λ / 4) = a ₀ −a ₁ (9) I ₄ = I (x, 3λ / 4) = a ₀ −a ₂ (10) Therefore, the phase distribution ΔW (x) is obtained by ΔW (x) = ½k · arctan {(I ₁ −I ₃ ) / (I ₀ −I ₂ )} (11). 2 for arctangent
nπ (n is an integer) is optional, but there is no phase jump exceeding 2π between adjacent regions, that is,
Select ΔW (x) so that the phases are smoothly connected. Here, this is called phase connection.

Due to the phase connection, the detection of the interference fringe image is 1
It must be done with a resolution higher than the stripe. That is, there should be no phase jump of one period (2π) or more between the spatially adjacent detection pixels. Therefore, if the amount of aspherical surface becomes large and the interference fringes become too fine and dense and the detector cannot be resolved, measurement becomes impossible, which is an obstacle in aspherical surface measurement.

If a point detector having a sufficiently small aperture is spatially scanned to detect at a high resolution, a two-dimensional detector becomes unnecessary, but the number of analysis points is remarkably increased, which is unrealistic for analysis. The new problem arises that it takes a reasonable amount of time.

In summary, when the fringe scanning method is used,
When the interference fringes are fine and have a high density, it is difficult to extract the phase information from the interference fringes. Since a large amount of aspherical surface essentially means that the measured wavefront of the optical wave has large irregularities, it must be said that there is a big problem in using the fringe scanning method for extracting the phase information of the shearing interferometer. I don't get it.

Further, when the fringe scanning method is used, it is necessary to perform precise and minute positioning such that the corner cube prism 32 of FIG. 8 is moved by λ / 4, and a piezo positioner, a piezo actuator, a piezo stage, etc. There is a drawback in that an expensive device such as a positioning device using a piezoelectric element is required.

Therefore, as a means for solving the drawbacks of the shearing interferometry method using such fringe scanning, the inventors have
A new shearing interferometer and a shearing interferometer as shown in Japanese Patent Application No. 5-173046 are proposed. That is, the invention described in the specification of the application solves the drawbacks of the conventional shearing interferometer as described above, and does not use a precise optical path length changing means or the like, even with the interference light intensity signal detection of a coarse density, It is possible to accurately detect a large amount of aspherical surface and the like.

The above-mentioned inventors have already filed Japanese Patent Application No. 5-17304.
The new shearing interferometry method and interferometer, which is filed as No. 6, is the basis of the present invention and will be described in detail below.

The shearing interferometry method applied as the above-mentioned Japanese Patent Application No. 5-173046 divides the optical wavefront to be measured into two parts, and relatively gives a shear between the two divided wavefronts, and then causes the two wavefronts to interfere with each other. , In the shearing interference measurement method for obtaining the shape information of the measured light wavefront from the obtained interference fringes, the interference light intensity signal is detected while changing the amount of the shear, and based on the frequency analysis result of the detected interference signal. The shape information of the measured light wavefront is obtained.

In this case, the shear to be applied is lateral shear, the interference light intensity signal is detected while the amount S of shear is changed at a constant rate over time, and the peak frequency of the frequency spectrum with the amount S of shear of this signal as a variable. fp can be detected, and the shape information of the measured optical wavefront can be obtained from the peak frequency by the following equation.

Fp = k · (∂W / ∂x) (12) where x is the position coordinate along the shear direction and W is the shape of the measured wavefront.

In this case, a lateral shearing interferometer can be used as the shearing interferometer.

In the shearing interferometry and the shearing interferometer, which are applied as Japanese Patent Application No. 5-173046, the interference light intensity signal is detected while changing the amount of shear, and the frequency analysis result of the detected interference signal is obtained. Since the shape information of the optical wavefront to be measured is obtained based on, the phase information of a certain point in the space of the interference fringe image can be obtained independently of the adjacent area. Therefore, the amount of aspherical surface, which cannot be measured by the conventional shearing interferometry, is very large,
Even if the interference fringes are thin and have a high density and cannot be resolved by the detector, the aspherical shape can be measured by detecting the interference light intensity signal at a density lower than the density of the interference fringes. Further, unlike the fringe scanning shearing interferometry, the phase information can be detected from the interference fringe image without using an expensive optical path length changing means such as a positioning device using a piezoelectric element.

The principle of the shearing interferometry method and the shearing interferometer, which have been filed as Japanese Patent Application No. 5-173046, will be described below in more detail with reference to the drawings.

FIG. 9 shows the basic structure of a shearing interferometer based on the shearing interferometry of the prior application. The light emitted from the light source 41 that emits the coherent light partially passes through the semi-transparent mirror 42 and reaches the expander lens 43, and after being expanded to a desired size, the optical path is bent by the mirror 44.
The collimator lens 44 collimates the light, and the wavefront conversion lens 46 irradiates the measured object 47, for example, a lens with a wavefront shape that is as close as possible to the measured surface.

The light beam emitted from the light source 41 and reflected or transmitted from the object to be measured 47 is again in the opposite direction to the wavefront conversion lens 46, collimator lens 44 and mirror 4.
4, a part of the light is reflected by the semi-transparent mirror 42 through the expander lens 43, and is guided to the shearing interference means 55.

The shearing interference means 55 divides the light wavefront emitted from the light source 41 and reflected or transmitted from the DUT 47 into two.
Divided into two parts, and after giving a relatively desired shear, let them interfere,
Achieve shearing interference.

FIG. 9 shows one of the shearing interference means.
For example, the semi-transparent mirror 48 and the corner cube prism 4
9, 50, etc. and a shear amount control means 51. The light wavefront emitted from the light source 41 and reflected or transmitted from the DUT 47 is partially reflected by the semi-transparent mirror 48 to enter one corner cube prism 49, and the rest is transmitted to enter the other corner cube prism 50. . When the corner cube prism 50 is moved in the direction perpendicular to the incident optical axis by using the shear amount control means (stage in the figure) 51, the light emitted from the corner cube prism 50 moves with the movement of the corner cube prism 50. Since it moves in the same direction by twice the amount of movement of the corner cube prism 50, the other corner cube prism 49
When the mirror cubes are kept stationary, the reflected lights from the respective corner cube prisms are overlapped with each other by being shifted by twice the stage movement amount when returning to the semi-transparent mirror 48, and shearing interference is realized. The light reflected by the semi-transparent mirror 48 is
The optical path is bent by the mirror 32 and output from the shearing interference means 55.

The light emitted from the shearing interference means 55 is
It is guided to the interference fringe light intensity signal detection means 54 via the imaging lens 53. The interference fringes corresponding to the object to be measured are imaged on the detection surface of the interference fringe light intensity signal detection means 54.
The interference fringe light intensity signal detecting means 54 is, for example, a television camera for picking up an interference fringe, and the video signal from the interference fringe light intensity signal detecting means 54 is supplied to a computer 56, which will be described later by calculation processing in this computer 56. Thus, the wavefront shape is obtained from the light intensity of the interference fringes. Further, the computer 56 controls the shear amount control means 51 to change the shear amount S.

The shearing interference fringe generated on the detection surface of the interference light intensity signal detecting means 54 is given by the following equation.

I (x) = I ₁ (x) + I ₂ (x) +2 {I ₁ (x) · I ₂ (x)} ^1/2 × cos [k {∂W / ∂x) S + φ}] ... (13) where I ₁ and I ₂ is the intensity of the two light wave separated by the respective shearing unit, W is the shape of the optical wavefront to be measured, the amount of S is shifted that shear amount, phi two light waves of position The phase difference, k is the wave number of light (= 2π / λ, λ: wavelength of light).

As can be seen from the equation, when the light intensity signal I (x) is detected at equal intervals by changing the shear amount S, the light intensity signal I (x) has a frequency k with respect to the shear amount S.・ Vibrate at (∂W / ∂x). Prior invention (Japanese Patent Application No. 5)
No. 173,046) focuses on making effective use of this relationship.

That is, since the wave number k is known here, the differential ∂ of the shape of the light wavefront is calculated from the vibration frequency of the interference fringe light intensity time change signal at a certain point on the detection surface of the interference fringe light intensity signal detecting means 54. W / ∂x can be detected.

Specifically, the spectrum of the interference fringe light intensity I (x, S) when the shear amount S is changed is calculated with respect to the shear amount S, the peak frequency fp of the spectrum is detected, and the measured value is calculated by the following equation. Inclination of light wavefront (differentiation of shape) ∂W / ∂
x can be obtained. The details of the process of obtaining ∂W / ∂x will be described later. However, k is the wave number (2π /
λ). In addition, λ is the wavelength of light, and the red color He
For the Ne laser, it is known at 632.8 nm.

Fp = k · (∂W / ∂x) (14) Further, by integrating ∂W / ∂x over the entire area x, the shape W (x) of the measured wavefront can be obtained. .

In order to obtain the shape in the two-dimensional area, it is sufficient to adopt two directions that are not parallel as the direction of the shear and repeat the same measurement.

FIG. 10 shows the procedure for extracting the wavefront shape to be measured in this new shearing interferometry technique described above. This extraction procedure is roughly divided into three processes of measurement, interference fringe analysis, and integration, and is executed by the computer 56.

In the measuring process, first, the shear amount S is set to generate a shearing interference fringe (step 101), and an image of this shearing interference fringe is detected and recorded (step 102). This process is repeated while changing the shear amount S (step 104) until the detection / recording of the predetermined number N is completed (step 103). FIG. 11A shows the interference fringes generated in the measurement process.

Next, in the interference fringe analysis step, a change in interference fringes (change in light intensity) is obtained by focusing on one point in the measurement area (steps 104 and 105). FIG. 11B shows
7 is a graph showing a change in light intensity I with respect to the shear amount S.
Next, the spectrum of the change in the interference fringe at that point is obtained (step 106). This corresponds to the Fourier transform for the shear amount S. FIG. 11C is a graph showing the result of Fourier transform. Next, the peak frequency fp is detected from the result of the Fourier transform (step 107, FIG. 11).
(See (d)). Next, the slope of the wavefront is calculated based on the above equation (14) (step 108). Repeat the above steps 101 to 105 until the analysis is completed.
(Step 109). Completion of Analysis FIG. 11E is a graph showing changes in the differential coefficient ∂W / ∂x of the wavefront shape with respect to the position X.

In the final integration step, the result of the Fourier transform is integrated at the position coordinate (x) in the shear direction (step 110) to obtain the wavefront shape as shown in FIG. 11 (f).

Since the frequency of the interference light intensity signal at one point on the detection surface can be obtained independently of other places,
Even if the pixel pitch of the detector is coarser than the spatial frequency of the interference fringes, the measurement can be performed. This is the first feature of the new shearing interferometry technique of the prior application, which is different from the fringe scanning shearing interferometry. That is, in the case of the fringe scanning shearing interferometry, since the phase connection for solving the arbitrariness of 2nπ of the inverse trigonometric function must be performed, the detector must have fine and high density pixels sufficient to resolve the interference fringes. On the other hand, in the method of the shearing interferometry technology of this prior application, it is possible to perform measurement even by using a detector having pixels with a density higher than the spatial frequency of the interference fringes.

Due to this feature, even if the amount of aspherical surface becomes large and the interference fringes become fine and high in density, it is not necessary to perform high-density detection, and pixels having a density lower than that of the interference fringes are included. Shearing interferometry can also be performed with other detectors.

Further, since the control of the shear amount generally does not require the accuracy as high as the control of the optical path length in the fringe scanning method, an expensive optical path length control means such as a piezo stage becomes unnecessary. This is the second feature of this new shearing interferometry technology.

However, even in the shearing interferometry technique of the prior application, when the shape of the measured wavefront is close to a plane,
There is a problem that it is difficult to accurately measure the shape of the measured wavefront.

When the shape of the measured wavefront is deviated from the plane, as shown in FIG. 12B, the peak of the frequency distribution with respect to the shear amount S is at a position greatly separated from the frequency 0. Since it exists, the frequency of the peak to be detected can be easily separated from the DC (direct current) noise in the vicinity of the frequency 0. On the other hand, when measuring an optical wavefront having relatively small irregularities on the wavefront to be measured, which is close to a substantially flat surface, the following equation fp = k · (∂W / ∂x) ≒ 0 (however, ∂W / ∂x ≒
0), the frequency distribution for the shear amount S exists only near the frequency 0.

In a normal frequency analysis method such as FFT (Fast Fourier Transform), when the frequency peak is close to 0, the frequency component of interest becomes DC (direct current) noise as shown in FIG. It is buried and the accuracy of frequency analysis falls. In other words, a component close to a DC component whose frequency is close to zero does not include a sufficient number of cycles within the data collection period, and therefore accurate frequency analysis cannot be performed. For example, when one cycle is very long, there are cases where the data sampling period is less than one cycle, and when the frequency of such a component is estimated, the accuracy is usually very low.

Therefore, the earlier application (Japanese Patent Application No. 5-173046)
As a method of solving the problem of the shearing interferometry technology of No.), the inventors of the present invention have proposed a technique of introducing a technique called frequency shift and separating the detection target frequency from the vicinity of DC noise. I am proposing to the issue. Hereinafter, description will be made on the basis of a technique for solving the problem of the shearing interferometry technique of the prior application (Japanese Patent Application No. 5-173046).

As described above, the shearing interference fringes generated on the detection surface of the interference light intensity signal detecting means are, as described above, I (x) = I ₁ (x) + I ₂ (x) +2 { I ₁ (x) · I ₂ (x)} ^1/2 × cos [k {∂W / ∂x) S + φ}] (13) The expression (13) is an expression when the frequency shift technique described in detail below is not introduced.

Here, the phase term [k {∂W /
∂x) S + φ}] is introduced into the equation (15) by introducing a known amount of frequency shift ν.

I (x) = I ₁ (x) + I ₂ (x) +2 {I ₁ (x) · I ₂ (x)} ^1/2 × cos [{k (∂W / ∂x) + ν} S + k .Phi. "(15) In this equation (15), by appropriately setting the amount of frequency shift ν, as shown in FIG. 12 (b), the frequency peak of the detection target is separated from the DC noise. If it can be set to appear everywhere, the problem that it is difficult to accurately measure the shape of the measured wavefront when the measured wavefront has a shape close to a plane is solved. The shape W (x) of the light wavefront can be correctly obtained by subtracting the inclination corresponding to the frequency shift from the shape obtained by introducing the frequency shift.

The state of this frequency shift is shown in FIG.
When the measured wavefront has a shape close to a plane and frequency shift is not performed, as shown in FIG. 13A, the frequency of the peak to be detected ν _p (= k · (∂W / ∂x)) since the frequency components of the DC noise are close, but detected peak is hardly discriminated buried in the DC noise, [nu _sh
When only the frequency shift is performed, as shown in FIG. 13B, the frequency ν _p (= k · (∂W
Since / ∂x) + ν _sh ) and the frequency component of DC noise are close to each other, the DC noise and the peak to be detected are separated,
The peak to be detected can be accurately detected.

The following two can be considered as specific means for introducing the above frequency shift.

<Means 1> A first method of giving a frequency shift is to incline the optical wavefront to be measured by θ with respect to the direction perpendicular to the optical axis. At this time, the interference light intensity signal is expressed by the following equation. However, Ls = tan θ.

I (x) = I ₁ (x) + I ₂ (x) +2 {I ₁ (x) · I ₂ (x)} ^1/2 × cos [k {(∂W / ∂x + Ls) S + φ}] (16) When the measured optical wavefront is tilted with respect to the direction perpendicular to the optical axis of the interferometer, the differential coefficient of the wavefront at the focused position x is obtained from the following equation.

Fp = k (∂W / ∂x + Ls) (17) Here, fp represents the peak frequency of the spectrum for the shear amount S.

Since Ls = tan θ is a set value and is known, the differential coefficient of the wavefront to be measured can be obtained from this fp.
At this time, Ls = tan θ is appropriately selected so that the peak frequency fp is sufficiently separated from the DC noise.

<Means 2> A second method for providing the frequency shift will be described below. When the phase term of the equation (15) of shearing interference with frequency shift is modified, k {(∂W / ∂x + L) S + φ} is obtained. However, L = ν / k (ν is the frequency shift amount). Incidentally, in this proportional coefficient in the means 2, "L" is different from the coefficient "Ls" in the means 1. If this is further modified, k {(∂W / ∂x) S + L · S + φ} (18)

This corresponds to giving an optical path length change of L · S which is proportional to the shear amount S by the proportional coefficient L at the same time as the shear. That is, the optical path difference in the optical axis direction may be given simultaneously with the shear. To give the shear and the optical path difference at the same time, the means for giving the optical path difference and the means for giving the shear may be used together,
In the case of a lateral shearing interferometer, as shown in FIG. 14, the shearing direction can be easily achieved without tilting the direction perpendicular to the optical axis a little without adding an expensive device such as a piezo actuator. In this case, when the angle formed by the shearing direction and the direction perpendicular to the optical axis is θ, L = sin θ holds.

In this way, when the shearing direction is inclined with respect to the direction perpendicular to the optical axis of the interferometer, the differential coefficient of the wavefront at the target position x is obtained from the following equation.

Fp = k · (∂W / ∂x + L) (19) Here, fp represents the peak frequency of the spectrum for the shear amount S.

Since L is a set value and is known, this fp
Is the differential coefficient of the measured wavefront. As shown in FIG. 13B, the proportional coefficient L is such that the peak frequency fp is sufficiently DC.
It is properly selected according to the shape of the wavefront to be measured so as to be separated from noise.

In summary, even in the case of measuring an optical wavefront having a small unevenness and a nearly flat surface, the prior application Japanese Patent Application No. 5-17304
In addition to the shearing interferometry technique proposed in No. 6, the frequency component of interest is changed to D by applying the device proposed in Japanese Patent Application No. 5-261023, which is an improvement of this prior application.
By separating from C noise, accurate measurement can be performed.

[0064]

However, in the new shearing interferometry proposed in the above-mentioned two prior applications, when the inclination of the wavefront to be measured is detected from the frequency peak, the inclination of the inclination is detected only from the power spectrum. There was a problem that it was not possible to distinguish between positive and negative.

That is, as shown in FIG. 1, the inclination (∂W / ∂
Whether x) is positive or negative, the peak position of the power spectrum of the shear amount S, that is, the frequency, is the same, and therefore the positive / negative of the slope cannot be determined only from the power spectrum of the shear amount S.

Therefore, an object of the present invention is to make it possible to determine whether the inclination of the wavefront to be measured is positive or negative when detecting the inclination of the wavefront to be measured from the frequency peak.

[0067]

According to the present invention, an optical wavefront to be measured is divided into two, a shear is relatively applied in a lateral direction between the divided wavefronts, and then both wavefronts are caused to interfere with each other to generate an interference fringe. A shearing interference measuring method for obtaining the shape information of the measured light wavefront from the interference fringes, wherein the interference light intensity signal is detected while changing the shear amount S, and the shear amount S of this signal is detected.
The peak frequency fp of the frequency spectrum with the variable is detected, and the inclination ∂W / ∂x of the measured light wavefront from the peak frequency fp (where W is the shape of the measured light wavefront, and x is along the shear direction). The shape information including the component position coordinates) is obtained, and the optical system of the shearing interferometer is set so that a known constant amount of frequency shift ν is generated in the frequency spectrum having the shear amount S as a variable. Obtaining the shape information of the measured light wavefront, fp = k · (∂W / ∂x) + ν, where k = 2π / λ, λ: wavelength of light wave, π: circular ratio ν・ The sign of (∂W / ∂x) + ν must be positive or 0 everywhere in the measured wavefront, or the sign of k ・ (∂W + ∂x) + ν must be negative or 0 everywhere in the measured wavefront. It is characterized by setting to.

Further, the shearing interferometer of the present invention is a shearing interferometer which divides an optical wavefront to be measured into two and relatively gives a shear between the divided wavefronts and then interferes both wavefronts with each other. The shear amount control means for changing it dynamically,
An interference light intensity signal detecting means for detecting an interference light intensity signal at at least one specific point in the interference space according to a change in the shear amount, and a means for giving a frequency shift amount ν to the interference light intensity signal are provided. Let ν be k · (∂W /
A means to set the sign of ∂x) + ν to be positive or 0 everywhere in the measured wavefront, or to make the sign of k · (∂W / ∂x) + ν be negative or 0 everywhere in the measured wavefront. It is characterized by having.

[0069]

In order to determine the sign of the slope (∂W / ∂x) from the power spectrum, the present invention has made the following innovations. That is, the frequency shift amount ν is calculated as k · (∂W /
Make sure that the sign of ∂x) + ν is positive or 0 everywhere on the measured wavefront, or k ・ (∂W / ∂x)
Set the sign of + ν to be negative or zero everywhere in the measured wavefront. Under such a setting, the peak position fp of the measured power spectrum is | k ·
It corresponds to (∂W / ∂x) + ν |, or − | k ・ (∂
Whether it corresponds to W / ∂x) + ν | is obvious depending on the setting, and must be k · from the peak position of the power spectrum.
(∂W / ∂x) + ν can be obtained. Since the frequency shift amount ν is known from the above setting, (∂W / ∂
x) That is, the value of the slope of the measured wavefront can be obtained by including the positive and negative signs.

[0070]

【Example】

Example 1 An example in which the present invention is applied to a shearing interferometer for measuring the shape of an optical surface will be described with reference to FIG.

A light beam from a light source 1 composed of a He-Ne laser or the like passes through a semi-transparent mirror 2 and an expander lens 3, an optical path is bent by a mirror 4, and a collimator lens 5 and a wavefront convergence lens 6 are used to receive the light beam. The measurement object 7 (lens in this embodiment) is irradiated. DUT 7
The light beam reflected by is passed through the wavefront conversion lens 6, the collimator lens 5, the mirror 4, and the expander lens 3 in the opposite direction again, and a part of the light beam is reflected by the semitransparent mirror 2 and guided to the shearing interference unit 16.

The shearing interference section 16 divides the light wavefront emitted from the light source 1 and reflected from the object to be measured 7 into two, gives a relatively desired shear, and then causes interference to realize shearing interference.

The shearing interference section 16 is composed of a semi-transparent mirror 8, corner cube prisms 9-1 and 9-2, etc., and a rectilinear stage 10 as shear amount control means.
The light wavefront emitted from the light source 1 and reflected from the DUT 7 is guided to the inside of the shearing interference portion 16 by the semi-transparent mirror 2 and is then reflected by the semi-transparent mirror 8.
In the above, a part of the light is reflected to enter one corner cube prism 9-1 and the rest is transmitted to enter the other corner cube prism 9-2. Corner cube prism 9
-2 is moved in the direction perpendicular to the incident optical axis by using the rectilinear stage 10 which is the shear amount control means, the light emitted from the corner cube prism 9-2 moves as the corner cube prism 9-2 moves. Since it moves in the same direction by twice the amount of movement of the corner cube prism 9-2,
If the other corner cube prism 9-1 is kept stationary, the reflected lights from the respective corner cube prisms are shifted by twice the stage movement amount and overlap each other when returning to the semi-transparent mirror 8. Interference is realized. The light reflected by the semi-transparent mirror 8 has its optical path bent by the mirror 4, and is output from the shearing interference unit 16.

The light emitted from the shearing interference section 16 is guided to the television camera 12, which is an interference fringe light intensity signal detecting means, through the imaging lens 11. Shearing interference fringes corresponding to the object to be measured are formed on the image pickup surface of the television camera 12.

The video signal from the television camera 12 is supplied to the image input / processing device 14, and the processing result of the image input / processing device 14 is supplied to the personal computer 15. This personal computer 15
Goes straight through the stage controller 13
Control the position of 0.

[Measurement] As shown in FIG. 2A, the reflected light from the lens, which is the DUT 7, is reflected by the shearing interference part 1.
Shearing interference fringes are formed by being guided to 6, and this is observed by the TV camera 12. The optical axis of the lens 7 to be measured is tilted by θ with respect to the optical axis of the interferometer. The angle θ is Ls
= Tan θ, k · {(∂W / ∂x) + Ls}
Is always positive or 0, everywhere on the measured wavefront,
Alternatively, set the sign of k · (∂W / ∂x) + ν to be negative or zero everywhere in the measured wavefront.
It is difficult to accurately know the maximum inclination of the object to be measured before measurement, but it is sufficient to set θ to a large value with some allowance.
In the shearing interference section 16, the corner cube prism 9-2 is mounted on the rectilinear stage 10 that moves in a direction perpendicular to the optical axis. In this state, if the corner cube prism 9-2 is moved by a certain amount in the arrow direction,
Lateral shear is changed by a fixed amount on the measured wavefront,
The interference fringe changes as shown by the interference fringe light intensity I (x) in the following equation.

[0077]

[Equation 1] Here, the shear amount S is twice the stage movement amount. Every time the shear S is changed, the shearing interference fringe is displayed on the TV camera 1
2 to the computer 15. That is, the interference fringe image detection is performed at equal intervals with respect to the shear amount S while changing the shear amount S. The interference fringe image may be captured by using a one-dimensional image sensor such as a CCD sensor instead of the TV camera and scanning the two-dimensional image. FIG. 2B shows a one-dimensional image sensor 1
2A schematically shows a case of scanning an interference fringe image using 2a. The one-dimensional image sensor 12a is disposed so as to face the interference fringe image A, and the one-dimensional image sensor 12a
A two-dimensional image is obtained by electrically performing main scanning in the pixel arrangement direction of a, that is, in the longitudinal direction, and moving in the direction perpendicular to the pixel arrangement direction of the one-dimensional image sensor 12a and mechanically performing sub-scanning. take in.

<Stripe image analysis> Focusing on one point in the measurement region, the change in light intensity with respect to the change in the shear amount S at that point is obtained from the plurality of captured interference fringe images. From the change in the shear amount S, a frequency spectrum is obtained for the shear amount S, and based on the peak frequency fp obtained from the spectrum, the following equation showing the relationship between the peak frequency fp and the inclination (derivative coefficient) of the light wavefront.

[Equation 2] Based on the above, the slope at each point of the wavefront to be measured is obtained. Here, Ls is given by Ls = tan θ from the inclination θ of the lens under measurement set at the time of measurement. Next, the shape of the object to be measured can be obtained by integrating this.
Second Embodiment A second embodiment in which the present invention is applied to a shearing interferometer that measures the shape of an optical surface will be described with reference to FIG. The parts corresponding to those of the first embodiment shown in FIG. 2 are designated by the same reference numerals. In the second embodiment, the interference fringe image is captured by the point photodiode 12b, the corner cube prism 9-1 is mounted on the linear stage 17 that moves in the direction parallel to the optical axis, and The first point is that the measured lens is not tilted with respect to the optical axis.
Is different from the embodiment described above.

<Measurement> Reflected light from the object 7 to be measured, that is, the lens is guided to the shearing interference section 16 to form shearing interference fringes, which are observed with a dot-shaped photodiode 12b having one pixel. To do. In the shearing interference section 16, the corner cube prism 9-2 is
On the linear stage 10 that moves in the direction perpendicular to the optical axis,
The corner cube prism 9-1 is mounted on the rectilinear stage 17 that moves in a direction parallel to the optical axis. When the corner cube prisms 9-2 and 9-1 are simultaneously moved by a fixed amount in this state, a lateral shear and an optical path length change are simultaneously applied by a fixed amount to the wavefront to be measured, and the interference fringe light intensity signal I (x ) Changes like the following formula.

[0080]

[Equation 3] The ratio L represented by (amount of change in optical path / amount of shear) is k ·
Make sure that the sign of {(∂W / ∂x) + L} is positive or 0 everywhere in the measured wavefront, or k ·
Set so that the sign of (∂W / ∂x) + ν will always be negative or 0 anywhere on the measured wavefront. Before the measurement, it is difficult to know the ∂W / ∂x of the object to be measured, that is, the maximum slope of the local slope precisely, but with some margin, L always reaches (∂W / ∂x) + L. By the way, it may be set to be positive or zero, or negative or zero. Every time the shear amount S is changed by a constant amount, a shearing interference fringe is detected. That is, while changing the shear amount S, the interference fringe light intensity detection is performed at one point focusing on the shear amount S at equal intervals.

<Stripe image analysis> Focusing on one point in the measurement area, the change in light intensity with respect to the change in the shear amount S at that point is determined from the plurality of captured interference fringe images. From the change in the shear amount S, a frequency spectrum is obtained for the shear amount S, and based on the peak frequency fp obtained from the spectrum, the wavefront to be measured based on the equation fp = k · {(∂W / ∂x) + L} Find the slope at each point of. L is a ratio of (optical path length change amount) / (shear amount) set at the time of measurement. Next, the shape of the object to be measured can be obtained by integrating this. (See FIG. 10) <Spatial Scan> In order to measure the entire surface of the two-dimensional interference fringe image, the point photodiode 12b is raster-scanned within the two-dimensional measurement region, and the above measurement and analysis are repeated. The spatial detection interval during scanning may be coarser than the density (spatial frequency) of interference fringes. The state of raster scanning is shown in FIG. In FIG. 4, A shows an interference fringe image, and B shows a scanning path of the point photodiode 12b.

In the examples, the physical quantity to be measured was a surface shape, but the physical quantity to be measured may be any physical quantity such as a refractive index distribution that gives a phase difference to a light wave to change the shape of the light wave front. I do not care. Further, the light beam from the light source may be transmitted through the object to be measured or may be reflected on the surface of the object to be measured. In short, it is only necessary to select a method in which the physical quantity to be measured changes the shape of the light wavefront from the light source 1 and enables interference measurement.

[0083]

As is apparent from the above description, in the shearing interferometry method, the inclination of the wavefront to be measured is ∂W /
When ∂x, a known amount of shift is applied to the frequency peak for the shear amount S, and the frequency shift amount ν is k · (∂
Make sure that the sign of W / ∂x) + ν is always positive or 0 everywhere in the measured wavefront, or that the sign of k · (∂W / ∂x) + ν is always negative or 0 everywhere in the measured wavefront. As a result, it is possible to determine the value of the slope and the positive / negative from the power spectrum alone.

[Brief description of drawings]

FIG. 1 is a diagram showing that it is not possible to determine whether the inclination of a power spectrum is positive or negative.

FIG. 2 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 4 is a diagram showing how a dot photodiode is scanned in Example 2;

FIG. 5 is a diagram for explaining the configuration and operation of an example of a normal interferometer.

FIG. 6 is a diagram showing a wavefront state in a shearing interferometer.

FIG. 7 is a diagram showing an example of specific optical means for providing shearing.

FIG. 8 is a diagram for explaining the configuration and operation of a fringe scanning shearing interferometer.

FIG. 9 is a diagram showing a basic configuration of a new shearing interferometry method which is a basis of the present invention.

FIG. 10 is a flowchart showing a procedure for extracting a measured wavefront shape in a new shearing interferometry method which is a basis of the present invention.

FIG. 11 is an explanatory diagram showing a procedure for extracting a measured wavefront shape.

FIG. 12 is a diagram showing the relationship between frequency peaks and DC noise.

FIG. 13 is a diagram showing an effect of frequency shift.

FIG. 14 is a diagram showing a simple means for simultaneously applying a shear amount change and an optical path length change.

[Explanation of symbols]

1 ... He-Ne laser, 2 ... Semi-transparent mirror, 3 ... Expander lens, 4 ... Optical path bending mirror, 5 ... Collimator lens, 6 ... Wavefront conversion lens, 7 ... DUT (lens), 8 ... Semi-transparent mirror, 9-1, 9-2 ... Corner cube prism, 10 ... Linear stage, 11 ... Imaging lens, 12 ... TV camera, 13 ... Stage controller,
14 ... TV image input / processing device, 15 ... Personal computer, 16 ... Shearing interference unit

Claims (6)

[Claims] 1. An optical wavefront to be measured is divided into two, a shear is relatively applied in a lateral direction between the divided wavefronts, and then both wavefronts are caused to interfere with each other to generate interference fringes. A shearing interferometry method for obtaining surface shape information, which detects an interference light intensity signal while changing the amount S of the shear, and detects a peak frequency fp of a frequency spectrum having the amount S of the signal as a variable. From the peak frequency fp, the inclination of the wavefront of the measured light ∂W /
Shape information including ∂x (where W is the shape of the measured optical wavefront, and x is the position coordinate of the component along the shear direction) is obtained, and a known constant amount in the frequency spectrum with the shear amount S as a variable. The optical system of the shearing interferometer is set so as to generate the frequency shift ν of, and the shape information of the measured optical wavefront is obtained by the following equation: fp = k · (∂W / ∂x) + ν where k = 2π / λ, λ: wavelength of light wave, π: circular constant The frequency shift amount ν is a positive or zero sign of k · (∂W / ∂x) + ν everywhere in the measured wavefront, or k · (∂ An interference measuring method characterized in that the sign of W + ∂x) + ν is set to be negative or zero everywhere in the measured wavefront. 2. As a means for imparting a known fixed amount of frequency shift to the frequency spectrum, Ls = tan θ is obtained by inclining the measured light wavefront with respect to a direction orthogonal to the optical axis of the interferometer by a known amount of angle θ. The interference measuring method according to claim 1, wherein the known amount of frequency shift is given and the shape information of the light wave front is obtained from the peak frequency fp by the following equation. fp = k · {(∂W / ∂x) + Ls} 3. As a means for imparting a known and fixed amount of frequency shift to the frequency spectrum, a lateral shear is given to the wavefront of the light to be measured, and an optical path length change proportional to the lateral shear amount is given by a proportionality coefficient L to obtain a known L. The interference measurement method according to claim 1, wherein a frequency shift of the amount is given, and the shape information of the light wave front is obtained from the peak frequency fp by the following equation. fp = k · {(∂W / ∂x) + L} 4. A shearing interferometer in which a measured optical wavefront is divided into two, and a shear is relatively given between the divided two wavefronts, and then the two wavefronts interfere with each other. Means, an interfering light intensity signal detecting means for detecting an interfering light intensity signal at at least one specific point in the interference space according to a change in the shear amount, and means for giving a frequency shift amount ν to the interfering light intensity signal, In addition, the frequency shift amount ν is always positive or 0 where the sign of k · (∂W / ∂x) + ν is present in the measured wavefront, or the sign of k · (∂W / ∂x) + ν is present in the measured wavefront. By the way, a shearing interferometer characterized in that it is provided with a means for setting it to be always negative or zero. 5. The means for applying a frequency shift amount ν to the interference light intensity signal is a means for inclining the optical wavefront to be measured by a known angle θ with respect to the optical axis of the interferometer. The shearing interferometer according to Item 4. 6. The means for giving a frequency shift amount ν to the interference light intensity signal is a means for giving a lateral shear and an optical path length change proportional to the lateral shear amount to the optical wavefront to be measured. Shearing interferometer described.