JPH0660808B2 - Minute displacement measuring method and minute displacement measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring a minute displacement of an object using a diffraction grating and an apparatus therefor.

[Conventional technology]

With the high integration of LSI, in the ultraviolet or X-ray exposure device for exposing and transferring the integrated circuit pattern which becomes finer with high accuracy, or the electron beam exposure device for pattern drawing with high accuracy, etc. Advances in moving amount (displacement) measurement technology for positioning are indispensable.

Conventionally, as a method for measuring a minute displacement using a diffraction grating, a method utilizing optical heterodyne interference has been proposed. FIG. 5 shows a minute displacement measurement combining such a diffraction grating and optical heterodyne interference. An example of an apparatus is shown.
In FIG. 5, reference numeral 1 is a two-wavelength orthogonal polarization laser light source which emits monochromatic light of two wavelengths whose frequencies are slightly different from each other and whose polarization planes are orthogonal to each other, 2 is a beam splitter, 3a and 3b are condenser lenses, 4a. , 4b are polarizing plates, 5
a, 5b are photodetectors, 6a, 6b are preamplifiers, 7a,
7b, 7c and 7d are plane mirrors, 8 is a polarization beam splitter, 9 is a reflection type diffraction grating, 10 is a moving stage, 1
Reference numeral 1 is a stage drive unit, 12 is a detection signal processing unit, 13 is a displacement display unit, 14 and 15 are incident lights, and 16 is synthetic diffracted light.

In this device, a part of the laser light emitted from the two-wavelength orthogonal polarization laser light source 1 is extracted through the beam splitter 2, the laser light is condensed by the condenser lens 3a, and the optical heterodyne interference is generated by using the polarizing plate 4a. Then, it is detected by the photodetector 5a, passes through the preamplifier 6a, and is input to the detection signal processing unit 12 as a reference beat signal. On the other hand, a part of the light emitted from the two-wavelength orthogonal polarization laser light source 1 enters the polarization beam splitter 8 via the beam splitter 2 and the plane mirror 7a. The polarization beam splitter 8 splits the two monochromatic lights whose polarization planes are orthogonal to each other and whose frequencies are slightly different from each other, that is, P-polarized incident light 14 and S-polarized incident light 15, which are described later via plane mirrors 7b and 7c. Is incident on the reflection type diffraction grating 9 at a predetermined incident angle. Diffraction grating 9
The two-wavelength diffracted lights obtained from the above are optically combined, and the combined diffracted light 16 is passed through the plane mirror 7d, the condenser lens 3b, and the polarizing plate 4b, and optical heterodyne interference is caused to cause photodetector 5b.
And is input to the detection signal processing unit 12 through the preamplifier 6b as a diffracted light beat signal. The detection signal processing unit 12 detects the phase difference between the reference beat signal and the diffracted light beat signal corresponding to the displacement of the diffraction grating 9, converts the phase difference into a displacement amount, and the displacement display unit 13 displays the phase difference of the diffraction grating 9. Display the amount of displacement. Further, a control signal may be sent from the detection signal processing unit 12 to the stage driving unit 11 so that the phase difference becomes constant at a preset arbitrary value, and the diffraction grating 9 may be servo-controlled to a predetermined position.

Here, the relationship between the displacement amount ΔX of the diffraction grating 9 and the phase difference Δφ between the reference beat signal and the diffracted light beat signal will be described with reference to FIG. In FIG. 6, reference numeral 17 is a reflection type diffraction grating, 18a and 18b are incident light of wavelength λ ₁ , 1
Reference numerals 9a and 19b denote incident light of wavelength λ ₂ , and 20a and 20b denote combined diffracted light.

Incident lights 18a and 19a having wavelengths λ ₁ and λ ₂ whose polarization planes are orthogonal to each other and whose frequencies are slightly different from each other.
When incident on the diffraction grating 17 at, for example, point A at the n-th order diffraction angles θn ₁ and θn ₂ with respect to the normal direction of the diffraction grating 17, the normal direction of the diffraction surface from the point A of the diffraction grating 17 The nth-order diffracted lights of the incident lights 18a and 19a are emitted respectively. The n-th order diffracted lights are optically combined in the normal direction of the grating surface to form a combined diffracted light 20a, which enables optical heterodyne interference beat signal detection. here,
Displacement amount ΔX in the direction in which the diffraction grating 17 is orthogonal to the grating line
If point A moves to point A ', the incident light 1
The n-th order diffracted light is emitted to 8b and 19b,
They are optically combined to form combined diffracted light 20b.
When the grating pitch of the diffraction grating 17 is P, the nth-order diffraction angle θ
Between n ₁ and θn ₂ and wavelengths λ ₁ and λ ₂ , _sin θn ₁ = n · λ ₁ / P (n is a positive integer) (1) _sin θn ₂ = n · λ ₂ / P (〃 ) ... (2) Due to the displacement of the diffraction grating 17 by ΔX, the incident lights 18a and 18b have ΔX · _sin θn ₁ ,
An optical path length difference of −ΔX · _sin θn ₂ occurs between the incident lights 19a and 19b. Therefore, the synthetic diffracted light 20a
And 20b, the optical heterodyne interference beat signal has a phase difference Δφ, and this Δφ is Δφ = (2π · ΔX · _sin θn ₁ / λ ₁ ) + (2π · ΔX · _sin θn ₂ / λ ₂ ) ... (3) Substituting equations (1) and (2) into equation (3), Therefore, the phase difference Δφ changes with a period of P / 2n with respect to the displacement ΔX of the diffraction grating 17.

That is, when the synthetic diffracted light 16 from the diffraction grating 9 in FIG. 5 is a synthetic diffracted light of the nth-order diffracted light of the incident light 14 and 15, the reference beat signal detected by the photodetectors 6a and 6b and the diffracted light When the grating pitch of the diffraction grating 9 is P, the phase difference Δφ with the optical beat signal changes with the displacement amount P / 2n of the diffraction grating 9 as one cycle. Therefore, in the detection signal processing unit 12, for example, phase detection is performed and the phase difference is DC.
The displacement of the diffraction grating 9 can be measured with an accuracy of resolution P / 2n by converting the signal into a signal, generating a pulse signal from the DC signal for each 0 ° phase difference, and counting the pulse signal. Furthermore, by interpolating a DC signal change from 0 ° to 360 ° in phase difference to, for example, 1/360 and setting the detection resolution of the phase difference to 1 °, the displacement of the diffraction grating between the pulses is detected by the phase difference. , The displacement of the diffraction grating 9 can be detected with the resolution P / (2n · 360).
The direction of displacement of the diffraction grating 9 can be easily determined by discriminating whether the phase difference between the diffracted light beat signal and the reference beat signal is positive or negative.

[Problems to be solved by the invention]

However, the conventional small displacement measuring device described above has a drawback that it cannot detect a small displacement in the normal direction of the diffraction grating surface.

This will be described with reference to FIG. In FIG. 7, reference numeral 21 is a reflection type diffraction grating, 22a and 22b are incident lights of wavelength λ ₁ , 23a and 23b are incident lights of wavelength λ ₂ , and 24a and 24b are synthetic diffracted lights. Wavelength λ ₁ , λ
The incident angle of the _second laser beam is the nth diffraction angle θn ₁ ,
θn ₂ .

When the incident lights 22a and 23a are incident on the point B of the diffraction grating 21, the nth-order diffracted lights of the incident lights 22a and 23a are respectively emitted in the normal direction of the grating surface, and they are optically combined to be the combined diffracted light 24a. Becomes Here, when the diffraction grating 21 moves by the displacement amount ΔZ in the normal direction and the point B is displaced to the point B ′, similarly, the n-th order diffracted light is emitted to the incident lights 22b and 23b, respectively, and the combined diffracted light 24b is emitted. Becomes Incident light 22
-ΔZ (1 + cos θn ₁ ), incident light 2 in a and 22 b
Since an optical path length difference of −ΔZ (1 + cos θn ₂ ) is generated between 3a and 23b, a phase difference Δφ occurs in the optical heterodyne interference beat signal obtained from the combined diffracted lights 24a and 24b, and this Δφ is Δφ. = {2π · ΔZ (1 + cos θn ₂ ) / λ ₂ }-{2π · ΔZ (1 + cos θn ₁ ) / λ ₁ } = 2π · ΔZ · {(λ ₁ −λ ₂ ) + (λ ₁ cos θn ₂ −λ ₂ cos θn ₁ )} / λ ₁ λ ₂ (5)

However, the frequency difference Δf of the laser light having the wavelengths λ ₁ and λ ₂ , that is, the frequency of the beat signal is, for example, several tens KHz to several M.
It is about Hz, which is much smaller than the frequency of light, and | λ ₁ −λ ₂ | = (Δf / C) · λ ₁ · λ ₂ ≈0 (C is the speed of light) (6), and λ ₁ ≈ λ ₂ , θn ₁ ≈ θn _2, and (5)
The phase difference Δφ in the equation is always 0 regardless of the value of ΔZ.

As described above, the conventional micro-displacement measuring device cannot detect the micro-displacement ΔZ in the normal direction of the diffraction grating surface. Therefore, when it is indispensable to control the normal direction of the wafer surface such as the focus adjustment in the ultraviolet exposure apparatus or the mask-wafer gap setting in the X-ray exposure apparatus, the conventional apparatus described above cannot be applied. Met. For this reason, in the past, another measuring device had to be used to measure the displacement in two directions, and there was a problem that the device became large and complicated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method and an apparatus therefor capable of measuring a minute displacement of a diffraction grating surface in a normal direction.

[Means for solving problems]

The micro-displacement measuring method of the present invention synthesizes monochromatic lights of two wavelengths whose frequencies are slightly different from each other and causes optical heterodyne interference to generate a reference beat signal, and also disperses one of the monochromatic lights of the two wavelengths. The separated monochromatic lights are made to enter the diffraction grating fixed on the object from different directions respectively, and the other of the monochromatic lights of the two wavelengths are made incident on the diffraction grating to make each of the one monochromatic lights. And the diffracted light of the other monochromatic light are respectively combined to generate first and second optical heterodyne interference beat signals, and the reference beat signal and the first beat signal are generated.
By detecting the phase difference between the optical heterodyne interference beat signal and the phase difference between the reference beat signal and the second optical heterodyne interference beat signal, the grating line of the diffraction grating of the object from these phase differences. It is characterized by measuring a small displacement in a direction orthogonal to the direction and a small displacement in the normal direction of the diffraction grating.

Further, the micro-displacement measuring device of the present invention includes a diffraction grating fixed on an object, a light source for generating monochromatic light of two wavelengths whose frequencies are slightly different from each other, and a light source for emitting light from the light source.
First photosynthesis detecting means for synthesizing monochromatic light of wavelengths and causing optical heterodyne interference to generate a reference beat signal; spectroscopic means for spectrally splitting one of the monochromatic light of two wavelengths emitted from the light source; Incident angle adjusting means for respectively injecting the respective monochromatic light and the other monochromatic light into the diffraction grating from a direction having a predetermined angle, and diffracted light of each of the one monochromatic light spectrum and the other Generated by the second and third photosynthesis detecting means for respectively synthesizing the diffracted light of the monochromatic light to generate the first and second optical heterodyne interference beat signals, and the first and second photosynthesis detecting means, respectively. A displacement amount of the object in a direction orthogonal to the grating line of the diffraction grating by calculating a first phase difference signal from the reference beat signal and the first optical heterodyne interference beat signal. A first signal processing device for conversion, a first phase difference signal calculated and processed by the first signal processing device, and reference beat signals respectively generated by the first and third photosynthesis detecting means, a second And a second signal processing device for calculating a second phase difference signal from the optical heterodyne interference beat signal and converting it into a displacement amount of the object in the normal direction of the diffraction grating. I am trying.

[Action]

In the present invention, a total of three monochromatic lights are made incident on the diffraction grating by dispersing one of the monochromatic lights of two wavelengths, and the first and second optical heterodyne interference beat signals are generated from the diffracted lights. . Then, based on the first and second optical heterodyne interference beat signals and the reference beat signal,
The minute displacement of the diffraction grating is detected in two directions, that is, both the direction orthogonal to the grating line of the diffraction grating and the normal direction of the diffraction grating.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 4.

FIG. 1 shows a first embodiment of a small displacement measuring device according to the present invention. In FIG. 1, reference numeral 25 is a two-wavelength orthogonal polarization laser light source (light source), 26a and 26b are beam splitters,
27 is a polarization beam splitter, 28a, 28b, 28
c, 33a, 33b, 34a, 34b, 34c, 34d
Is a plane mirror, 29a, 29b and 29c are condenser lenses,
30a, 30b and 30c are polarizing plates, 31a, 31b and 3
1c is a photodetector, 32a, 32b and 32c are preamplifiers, 35 is a reflection type diffraction grating, 36 is a Z moving stage, 3
7 is an XY moving stage, 38 is an X displacement detection signal processing unit (first signal processing device), 39 is a Z displacement detection signal processing unit (second signal processing device), 40 is an X displacement display unit, and 41 is Z. Displacement display unit, 42 Z stage drive unit, 43 XY stage drive unit, 44, 45, 46 incident light, 47, 48
Is a synthetic diffracted light.

In the apparatus of the first embodiment, a part of the laser light emitted from the two-wavelength orthogonal polarization laser light source 25 is passed through the beam splitter 26a, the plane mirror 28a, the condenser lens 29a, and the polarizing plate 30a, and the photodetector 31a ( It is detected by the first photosynthesis detecting means) and inputted to the X displacement detection signal processing unit 38 and the Z displacement detection signal processing unit 39 as the reference beat signal of the optical heterodyne interference through the preamplifier 32a. On the other hand, the laser light emitted from the two-wavelength orthogonal polarization laser light source 25 enters the polarization beam splitter 27 via the beam splitter 26a, and is separated into two laser lights of P polarization and S polarization, and the P polarization component is a plane. Mirrors 33a, 3
The incident light 44 is incident on the reflection type diffraction grating 35 at a predetermined incident angle described later via 4b. The S-polarized component is further divided by the beam splitter 26b (spectroscopic means),
The light enters the reflection type diffraction grating 35 as incident light 45 via the plane mirror 34c and as incident light 46 via the plane mirrors 33b and 34a. Incident light 44, 45
The composite diffracted light 47 obtained from the diffraction grating 35 is detected by the photodetector 31b (second photosynthesis detection means) via the plane mirror 28b, the condenser lens 29b, and the polarizing plate 30b.
The first optical heterodyne interference beat signal is input to the X displacement detection signal processing unit 38 through the preamplifier 32b.
Further, the synthetic diffracted light 48 obtained from the diffraction grating 35 by the incident lights 44 and 46 is passed through the plane mirrors 34d and 28c, the condenser lens 29c, and the polarizing plate 30c, and the photodetector 31c.
(Third photosynthesis detecting means), and Z as a second optical heterodyne interference beat signal through the preamplifier 32c.
It is input to the displacement detection signal processing unit 39. In the X displacement detection signal processing unit 38, a direction orthogonal to the grating line direction in the diffraction grating plane (the horizontal direction in FIG. 1, hereinafter referred to as the X direction).
The phase difference between the reference beat signal corresponding to the displacement of the diffraction grating 35 and the first optical heterodyne interference beat signal is detected, the phase difference is converted into the displacement amount, and the X displacement display unit 4
A displacement amount of 0 is displayed. The Z displacement detection signal processing unit 39 detects the phase difference between the reference beat signal and the second optical heterodyne interference beat signal, and the X displacement detection signal processing unit 38 further detects the phase difference between the reference beat signal and the first optical heterodyne interference beat signal. By inputting a phase difference signal (first phase difference signal) with and adding processing, the displacement of the diffraction grating 35 in the normal direction of the diffraction grating surface (vertical direction in FIG. 1, hereinafter referred to as Z direction). A phase difference signal (second phase difference signal) corresponding to is generated, the phase difference is converted into a displacement amount and displayed on the Z displacement display unit 41. Further, a control signal is sent from the X and Z displacement detection signal processing units 38 and 39 to the Z and XY stage drive units 42 and 43 so that the phase difference becomes constant at a preset arbitrary value, and the diffraction grating 35 is determined. Servo control to the specified position.

In this device, when the incident lights 44 and 45 are respectively incident at the nth diffraction angle in the Z direction, the displacement amount ΔX of the diffraction grating 35 in the X direction, the reference beat signal, and the first optical heterodyne interference beat signal are The relationship with the phase difference Δφx is the same as in the case of the conventional apparatus described in FIGS. 6 and 7, In addition, the phase difference does not change with respect to the displacement amount ΔZ in the Z direction and remains constant.

On the other hand, the displacements ΔX and ΔZ of the diffraction grating 35 and the phase difference Δφzx between the reference beat signal and the second optical heterodyne interference beat signal have the following relationship, which is shown in FIGS. 2 and 3. It will be described with reference to FIG. In FIG. 2, reference numeral 49 is a reflection type diffraction grating, and 50a and 50b are wavelengths λ.
₁ incident light, 51a and 51b are incident light of wavelength λ ₂ , 52
Reference numerals a and 52b are synthetic diffracted lights.

Incident lights 50a and 51a having wavelengths λ ₁ and λ ₂ whose polarization planes are orthogonal to each other and whose frequencies are slightly deviated from each other have an n-th order diffraction angle θn ₁ and an angle 3θn ₂ that is three times the n-th order diffraction angle with respect to the Z direction. When the light enters the diffraction grating 49 at point A, for example,
From the point A of the diffraction grating 49 to the Z direction, the nth-order diffraction angle is 2
The + nth-order diffracted light of the incident light 50a and the −nth-order diffracted light of the incident light 51a are emitted in the direction of the double angle 2θn ₁ .
As described above, according to the equation (6), θn ₁ ≈θn ₂ , and
The + nth-order and -nth-order diffracted lights of the incident lights 50a and 51a are optically combined into a combined diffracted light 52a, which enables optical heterodyne interference beat signal detection.

Here, the diffraction grating 49 moves by the displacement amount ΔX and the point A becomes A ′.
When displaced to a point, similarly, the + n-order diffracted light and the −n-order diffracted diffracted light are optically combined with the incident lights 50b and 51b to become combined diffracted light 52b. Due to the displacement of the diffraction grating 49 by ΔX, the incident lights 50a and 50b have ΔX · ( _{sin
θn 1 - sin 2θn 1),} ΔX · in the incident light 51a and 51b _(sin 3θn ₂ - optical path length difference between the _{sin 2θn 2)} occurs, respectively. Thus the optical heterodyne interference beat signal obtained from the synthesis diffracted light 52a and 52 b, resulting phase difference Derutafai'zx, this Derutafai'zx is Δφ'zx = 2π · ΔX · {( sin θn 1 - sin 2θn 1) / lambda ₁ - a _{/ λ 2} ...... (8)} - (sin 2θn 2 sin 3θn 2). Where θn ₁ ≈ θn ₂ , _sin 3 θn ₂ ≈ 3 _sin θ
Since n ₂ , λ ₁ ≈λ ₂ , the equation (8) is Therefore, the phase difference Δφ′zx between the reference beat signal and the second optical heterodyne interference beat signal with respect to ΔX is the phase difference between the reference beat signal and the first optical heterodyne interference beat signal shown in equation (7). The phase shift direction is opposite to Δφx.

On the other hand, in FIG. 3, reference numeral 53 is a reflection type diffraction grating, 54
Reference numerals a and 54b are incident lights of wavelength λ ₁ , 55a and 55b are incident lights of wavelength λ ₂ , and 56a and 56b are synthetic diffracted lights. Incident light beams 54a and 55a having wavelengths λ ₁ and λ ₂ respectively have an n-th diffraction angle θn ₁ and an angle 3 which is three times the n-th diffraction angle with respect to the Z direction.
When incident on the diffraction grating 53, for example, at point B at θn ₂ , Z
The + nth-order diffracted light of the incident light 54a and the −nth-order diffracted light of the incident light 55a are emitted in the direction of an angle 2θn _{1 which} is twice the nth-order diffracted angle with respect to the direction to become the combined diffracted light 56a, and the optical heterodyne interference beat signal is detected. It will be possible. Here, when the diffraction grating 53 moves by the displacement amount ΔZ and the point B is displaced to the point B ′, similarly, the incident light beams 54b and 55b are respectively + n.
And the -nth order diffracted light are optically combined to form a combined diffracted light 56.
b. Due to the displacement of the diffraction grating 53 by ΔZ, −ΔZ · (cos θn ₁ + cos is generated between the incident lights 54a and 54b.
2θn ₁ ) and −ΔZ · (co
An optical path length difference of s 3θn ₂ + cos 2θn ₂ ) occurs. Therefore, the optical heterodyne interference beat signal obtained from the combined diffracted lights 56a and 56b has a phase difference Δφ ″.
zx occurs, Δφ "zx = 2π · ΔZ {(cos 3θn 2 + cos 2θn 2) / λ 2 - (cos θn 1 -cos 2θn 1) / λ 1} ...... becomes (10) .λ ₁ ≒ λ ₂ , θn ₁ considering the _{≒ θn 2, Δφ "zx ≒} 2π · ΔZ · (cos 3θn 1 -cos θn 1) / λ 1 ...... (11) _{next, ΔZ = λ 1 / (cos} 3θn 1 -cos θn 1 It can be seen that the phase changes in the cycle of.

Therefore, from the equations (9) and (11), the diffraction grating 3
The phase difference Δφzx between the reference beat signal and the second optical heterodyne interference beat signal with respect to the small displacements ΔX and ΔZ of 5 (see FIG. 1) is Δφzx = Δφ′zx + Δφ ″ zx = −Δφx + 2π · ΔZ · ( cos 3θn ₁ −cos θn ₁ ) / λ ₁ (12) That is, the Z displacement detection signal processing unit 39 detects the phase difference Δφzx between the reference beat signal and the second optical heterodyne interference beat signal. Then, by adding the phase difference Δφx between the reference beat signal detected by the X displacement detection signal processing unit 38 and the first optical heterodyne interference beat signal, as shown in the following equation (13), The phase difference signal Δφz corresponding to the minute displacement ΔZ of the grating 35 can be detected.

Δφz = Δφzx + Δφx = 2π · ΔZ · (cos 3θn ₁ −cos θn ₁ ) / λ ₁ (13) Therefore, in the detection signal processing units 38 and 39, for example, phase detection is performed and the phase difference signals Δφx and Δφz are DC signals. To a pulse signal from the DC signal at every phase difference of 0 °, and counting the pulse signal, the resolution P / 2n, λ ₁ / (cos 3θn ₁ −cos θn
_{In 1} ), the displacement measurement of the diffraction grating 35 in the X direction and the Z direction can be performed. Further, the DC signal change from the phase difference of 0 ° to 360 ° is complemented with, for example, 1/360 to set the detection resolution of the phase difference to 1 °, and the diffraction grating 3 is provided between the pulses.
By detecting the displacement of No. 5 by the phase difference, the displacements of the diffraction grating 35 in the X direction and the Z direction are resolved to P / 2n.3, respectively.
60, can be detected at _{λ 1 / (cos 3θn 1 -cos} θn 1) · 360. The directions of displacement in the X and Z directions can be easily determined by discriminating between the positive and negative phase differences of the first or second optical heterodyne interference beat signal with respect to the reference beat signal.

The first embodiment has been described above. Next, the second embodiment will be described with reference to FIG.

FIG. 4 shows a second embodiment of the minute displacement measuring apparatus according to the present invention. In FIG. 4, reference numeral 57 is a two-wavelength orthogonal polarization laser light source (light source), 58a and 58b are beam splitters, and 59a and 59b. , 59c, 59d, 65a, 6
5b and 65c are plane mirrors, 60a, 60b and 60c are condenser lenses, 61a, 61b, 61c and 61d are polarizing plates, 62a, 62b and 62c are photodetectors and 63a and 63.
b and 63c are preamplifiers, 64 is a half-wave plate, 66 is a reflective diffraction grating, 67 is a Z moving stage, 68 is an XY moving stage, 69 is a Z stage driving unit, 70 is an XY stage driving unit, and 71 is an X unit. Displacement detection signal processing unit (first signal processing device), 72 Z displacement detection signal processing unit (second signal processing device), 73 X displacement display unit, 74 Z displacement display unit, and 75a and 75b polarization. Beam splitter, 76
Reference numerals a and 76b denote incident light, and reference numerals 77a and 77b denote diffracted light.

In the device of the second embodiment, the incident direction of the laser beam on the diffraction grating 66 is partly opposite to that of the device of the first embodiment shown in FIG. 1, and the diffraction grating 66 has two wavelengths. The orthogonally polarized laser light enters as incident light 76a from the Z direction via the beam splitters 58a and 58b. Diffraction grating 66
The ± n-order diffracted lights 77a and 77b from are composed of two-wavelength P-polarized light and S-polarized light having polarization planes orthogonal to each other. S-polarized monochromatic light is extracted from the diffracted light 77a through the plane mirrors 65b and 59c and the polarization beam splitter 75a, and P-polarized monochromatic light is extracted from the diffracted light 77b through the plane mirror 65c and the polarization beam splitters 75b and 75a. The monochromatic lights of both polarizations are optically combined to cause optical heterodyne interference through the plane mirror 59b, the condenser lens 60b, and the polarizing plate 61b, and the photodetector (second photosynthetic detection means) 62b is used to generate the first optical heterodyne. Detects interfering beat signals.

On the other hand, the laser light split by the beam splitter 58b (spectroscopic means) is extracted as P-polarized monochromatic light through the plane mirror 59d and the polarizing plate 61d, and the plane of polarization is rotated by 90 ° by the ½ wavelength plate 64 to obtain a plane mirror. Incident light 76b is made incident on the diffraction grating 66 at an angle twice the nth diffraction angle with respect to the Z direction via 65a. The -nth order diffracted light of the incident light 76b is diffracted in the same direction as the diffracted light 77b, and the incident light 7b
Optical + heterodyne interference occurs with the + n-order diffracted light 77b of 6a and the S-polarized monochromatic light. The synthetic diffracted light of the -nth-order diffracted light of the incident light 76b and the S-polarized light of the diffracted light 77b is extracted through the plane mirror 65c and the polarization beam splitter 75b, and optical heterodyne interference is performed through the condenser lens 60c and the polarizing plate 61c. The photodetector 62c (third photosynthesis detecting means) detects the second optical heterodyne interference beat signal.

Further, a part of the laser light is extracted by the beam splitter 58a, and the plane mirror 59a, the condenser lens 60a,
Optical heterodyne interference is caused through the polarizing plate 61a, and the reference beat signal is detected by the photodetector 62a (first photosynthesis detecting means). The reference beat signal and the first and second optical heterodyne interference beat signals are respectively fed to the preamplifier 63a,
The reference beat signal and the first optical heterodyne interference beat signal are input to the X displacement detection signal processing unit 71 through 63b and 63c, and the reference beat signal and the second optical heterodyne interference beat signal are input to the Z displacement detection signal processing unit 72. Enter. In the detection signal processing units 71 and 72, as in the case of the first embodiment,
Phase detection is performed respectively to convert the phase difference Δφx and Δφz into displacement amounts ΔX and ΔZ of the diffraction grating 66, and a displacement display unit 73,
Display by 74. In addition, the detection signal processing units 71 and 72 respectively operate the stage driving unit 7 so that the phase difference becomes constant.
A control signal is sent to 0 and 69 to servo-control the diffraction grating 66 to a predetermined position.

In the second embodiment, it is clear that the following relational expression holds between the displacements ΔX and ΔZ of the diffraction grating 66 and the phase difference signals Δφx and Δφz by performing the same analysis as described above. Is.

Δφz = 2π · ΔZ · (cos 2θn ₁ −1) (θn ₁ is the nth diffraction angle) (15) Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Absent. For example, in the above-mentioned first and second embodiments, the two-wavelength orthogonal polarization laser light source is used as the two-wavelength monochromatic light source, but the light generated by using the acousto-optic device such as the Bragg cell is used as the two-wavelength monochromatic light. However, the same effect can be obtained. Further, in FIG. 1, the optical path system of either one of the two wavelengths of the monochromatic light is adjusted so that the directions of the polarization planes of the two wavelengths of the monochromatic light coincide with each other on the diffraction grating surface without using the polarizing plate. An optical heterodyne interference beat signal with good coherence can also be obtained by incorporating a two-wave plate, and the same effect can be obtained.

Further, in the above embodiment, the case where the direction of the incident light to the diffraction grating and the direction of the diffracted light from the diffraction grating are in the plane including the Z direction perpendicular to the diffraction grating surface and perpendicular to the direction of the grating line is described. However, monochromatic light of two wavelengths is made incident from a direction having a predetermined angle with respect to a plane that includes the Z direction and is perpendicular to the direction of the grating line, and diffracted light is detected in the direction having the predetermined angle. The same effect can be obtained by using a method of optically combining to generate an optical heterodyne interference beat signal.

The diffraction grating used in the present invention may be either an absorption type diffraction grating or a phase type diffraction grating, and is not limited to a binary diffraction grating, and various types such as a sinusoidal diffraction grating and a phrase diffraction grating may be used. It is also possible to use the diffraction grating of. It is also possible to use a transmissive diffraction grating in addition to the reflective diffraction grating.

Furthermore, in the above embodiment, the first-order diffracted lights of the laser lights of the two wavelengths having different frequencies are set to be the combined diffracted light in the two directions. Next-order diffracted light or a combined diffracted light of diffracted light of different orders may be used. In that case, only the period of the phase difference signal changes, and X is obtained by performing detection signal processing according to the phase difference signal. , Z-direction displacement can be detected, and similar effects can be obtained.

If two diffraction gratings are used and installed on an object so that their grating line directions are orthogonal to each other, it is possible to perform highly accurate microdisplacement measurement on three mutually orthogonal X, Y, and Z axes. Becomes

〔The invention's effect〕

As described in detail above, according to the micro-displacement measuring method of the present invention, one of the monochromatic lights of two wavelengths whose frequencies are slightly different from each other is split and incident on the diffraction grating,
Since the first and second optical heterodyne interference beat signals are generated by respectively combining the respective diffracted lights of the spectra and the diffracted light of the other monochromatic light, the minute directions in the two directions of the diffraction grating are generated. The displacement, that is, the small displacement in the direction perpendicular to the grating line of the diffraction grating and the small displacement in the normal direction of the diffraction grating can be measured as the phase change of the first and second optical heterodyne interference beat signals, respectively. Moreover, even if the diffracted light intensity fluctuates due to fluctuations in the intensity of the light source or the diffraction efficiency of the diffraction grating, the amplitude of the beat signal of the diffracted light only fluctuates and the phase change is not affected. Therefore, the phase difference detection can be performed with high stability and high accuracy. Therefore, it is possible to measure the minute displacements of the object in the two directions with high accuracy.

Further, the micro-displacement measuring device of the present invention can be configured to be compact and simple by integrating the detection optical system, and therefore it is effective for use in implementing the above method.

[Brief description of drawings]

1 to 4 are views showing an embodiment of the present invention.
1 to 3 show the first embodiment, FIG. 1 is a schematic configuration diagram of the apparatus of the first embodiment, and FIGS. 2 and 3 are enlarged views of the diffraction grating, respectively. Figure 4 is second
It is a schematic block diagram of the apparatus of an Example. 5 to 7 show a conventional minute displacement measuring apparatus. FIG. 5 is a schematic configuration diagram of this conventional apparatus, and FIGS. 6 and 7 are enlarged views of a diffraction grating, respectively. 25 ... 2-wavelength orthogonal polarization laser light source (light source), 26b ... Beam splitter (spectral means), 31a ... Photodetector (first photosynthesis detection means), 31b ... Photodetector (second photosynthesis detection) Means), 31c ... Photodetector (third photosynthesis detecting means), 33a, 33b, 34a, 34b, 34c ... Plane mirror (incident angle adjusting means), 35, 49, 53 ... Diffraction grating, 36 ... Z movement stage (object), 37 XY movement stage (object), 38 X displacement detection signal processing unit (first signal processing device), 39 Z displacement detection signal processing unit (second signal) Processing device), 57 ... 2-wavelength orthogonal polarization laser light source (light source), 58b ... beam splitter (spectral means), 63a ... photodetector (first photosynthesis detecting means), 63b ... photodetector (first) 2 photosynthesis detection ), 63c ... Photodetector (third photosynthesis detecting means), 59d, 65a ... Planar mirror (incident angle adjusting means) 66 ... Diffraction grating, 67 ... Z moving stage (object), 68 ... XY moving stage (object), 71 ... X displacement detection signal processing unit (first signal processing device), 72 ... Z displacement detection signal processing unit (second signal processing device).

Claims (2)

[Claims] 1. A reference beat signal is generated by synthesizing monochromatic lights of two wavelengths whose frequencies are slightly different from each other and causing optical heterodyne interference, and one of the monochromatic lights of the two wavelengths is spectrally dispersed to separate them. Light is incident on the diffraction grating fixed on the object from different directions, and the other of the monochromatic lights of the two wavelengths is incident on the diffraction grating to diffract each of the monochromatic lights of the respective spectra. Light and the diffracted light of the other monochromatic light are respectively combined to generate first and second optical heterodyne interference beat signals, and a phase difference between the reference beat signal and the first optical heterodyne interference beat signal, And a phase difference between the reference beat signal and the second optical heterodyne interference beat signal, respectively, to detect the diffraction pattern of the object from the phase difference. Minute displacement measuring method and measuring in a direction perpendicular to the grating lines small displacement and a normal direction of the minute displacement of the diffraction grating, respectively. 2. A diffraction grating fixed on an object, a light source for generating monochromatic light of two wavelengths having frequencies slightly different from each other, and monochromatic light of two wavelengths emitted from the light source are combined to cause optical heterodyne interference. First to generate a reference beat signal
Photosynthesis detecting means, a spectroscopic means for separating one of the two wavelengths of monochromatic light emitted from the light source, and the respective monochromatic light and the other monochromatic light which are split into a predetermined angle to the diffraction grating. Angle of incidence adjusting means for making incident light from different directions, and the first and second optical heterodyne interference by respectively combining the diffracted light of each of the one monochromatic light and the diffracted light of the other monochromatic light. A first phase difference from second and third photosynthesis detecting means for generating beat signals, and a reference beat signal and a first optical heterodyne interference beat signal generated by the first and second photosynthesis detecting means, respectively. A first signal processing device for calculating a signal to convert it into a displacement amount in a direction orthogonal to a grating line of the diffraction grating of the object, and a calculation processing by the first signal processing device. The second phase difference signal is calculated from the first phase difference signal, the reference beat signal respectively generated by the first and third photosynthesis detecting means, and the second optical heterodyne interference beat signal, and the object is calculated. (2) A second signal processing device for converting the displacement amount in the normal direction of the diffraction grating, to a small displacement measuring device.