JPH11211417A - Method and device for measuring light wave interference - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cope with the high speed movement of an object without producing a nonlinear error by shifting the optical paths of two luminous fluxes incident on at least one of a fixed mirror and a moving mirror to measure the displacement of the moving mirror by the interference. SOLUTION: In this light wave interference measuring device, either one of the frequencies (f1-f2±Δf) and (f1-f2±Δf') detected by detectors 14, 16 is moved to high frequency side in both cases of a moving mirror MC leaving the detectors 14, 16 and approaching thereto. Thus, the measurement signals S2, S2' from the detectors 14, 16 are properly switched according to the moving direction of the moving mirror MC, whereby the moving mirror MD can be moved at a high speed regardless of the moving direction. Further, since this device is constituted so that the light coaxially emitted from a light source is separated/divided, shifted from the same axis, and directed to the moving mirror MC and a fixed mirror 10, and only the light to be detected by the detectors 14, 16 is set coaxial just before the detectors 14, 16, a measurement generating no linear error can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical interference measuring method and apparatus for measuring displacement of an object with high accuracy.

[0002]

2. Description of the Related Art As a typical example of a light wave interferometer for measuring the length, displacement, density, etc. of an object, a light wave interferometer using a heterodyne interferometry will be briefly described with reference to FIG. I do. FIG. 5 shows a schematic configuration of a conventional light wave interference measuring apparatus. In FIG. 5, a laser light source 2
No. 02 emits light of two frequencies f1 and f2 whose polarization directions are orthogonal to each other and are slightly different. Where the frequency f
It is assumed that the light of No. 1 has a polarization direction parallel to the paper surface and the light of frequency f2 has a polarization direction perpendicular to the paper surface. The two lights emitted from the laser light source 202 enter the polarization beam splitter 204 coaxially, and the light of the frequency f1 passes through the polarization beam splitter 204 as p-polarized light and travels on the measurement optical path (corner cube prism). ) Reflected by MX, the optical path is shifted, and the polarization beam splitter 204
Through. The light having the frequency f2 is reflected by the polarization beam splitter 204 as s-polarized light, reflected by a fixed mirror (corner cube prism) 206 on the reference optical path, shifted in the optical path, and reflected again by the polarization beam splitter 204.

The light of the frequency f1 reflected by the moving mirror MX on the measurement optical path and transmitted through the polarization beam splitter 204 and the light of the frequency f2 reflected by the polarization beam splitter 204 through the reference optical path are coaxial and have a detector 212. Incident on. In the detector 212, the polarization directions of the frequencies f1 and f2 are matched and then caused to interfere, and the interference light is received by a light receiving system and photoelectrically converted. The photoelectrically converted interference light is represented by (f1-f
2) Detected as a measurement signal S2 having a frequency (f1−f2 ± Δf) modulated by a frequency change ± Δf (Δf is a positive value) due to the Doppler effect caused by the movement of the movable mirror MX at the beat frequency of 2). From the comparator 212 to the phase comparator 1
Is input to On the other hand, the frequency f
(F1-) obtained by causing light of frequency 1 to interfere with light of frequency f2.
The signal having the beat frequency of f2) is input to the phase comparator 1 as the reference signal S1. Here, when the moving mirror MX moves in a direction away from the detector 212, the frequency changes.
When Δf occurs and moves in a direction approaching the detector 212, a frequency change + Δf occurs.

The phase difference between the reference signal S1 and the measurement signal S2 is
The output φ (phase difference) detected by the phase comparator 1 is sent to an integrator, and by integrating the change of the phase difference φ,
A signal P proportional to the displacement of the moving mirror MX is obtained. This will be described below using mathematical expressions. Moving mirror M
The change in frequency due to the displacement of X is Δf (t),

P = ∫dφ = ∫ (dφ / dt) dt = ∫ ± Δfdt

Here, from the Doppler effect, the moving mirror MX
Where v is the moving speed, λ is the wavelength of the measurement beam, and m is the number of optical path turns,

± Δf = ± 2 mv / λ Therefore, P = ± 2 m / λ∫vdt

[0008] Assuming that the displacement amount of the movable mirror MX is D, D = ± ∫vdt = ± λ / 2m · P

## EQU1 ## The electrical resolution of the phase comparator 1 is set to 1/256, and a He-Ne gas laser (λ = 633n) is used.
When m) is used as the light source, since the measurement optical path is m = 1 in the case of a single path as shown in FIG. 5, the displacement D is 633 nm / (2 × 256) = 1.236 nm.
It is possible to detect with a resolution of.

In this optical interference measuring apparatus, when the moving speed MX of the moving mirror MX in the direction away from the detector 212 increases, the frequency change Δ caused by the Doppler effect occurs.
As f increases, the value of the frequency (f1−f2−Δf) moves to the low frequency side, eventually becomes zero, and a state where the value becomes further negative may occur. However, since the detector 212 cannot make a positive / negative determination, the conventional light wave interference measurement apparatus has a problem that displacement measurement cannot be performed when the movable mirror MX is moved at a predetermined moving speed or higher.

On the other hand, Japanese Patent Application Laid-Open No. Sho 61-162714 discloses a light wave interference measuring device capable of improving the moving speed of a moving mirror MX. The optical interference measuring apparatus disclosed in the above publication will be described with reference to FIG. Components having the same functions and functions as the components shown in FIG. 5 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 6, light beams having frequencies f1 and f2 emitted coaxially from a laser light source 202 enter a beam splitter 302. In the beam splitter 302, the frequency f
Lights 1 and f2 are respectively split and travel to the measurement optical path and the reference optical path. The light having the frequencies f1 and f2 that have traveled to the measurement optical path is reflected by the movable mirror MX, shifted in the optical path, and is again split by the beam splitter 302 into transmitted light and reflected light.

The light having the frequencies f1 and f2 that have traveled to the reference light path passes through the quarter-wave plate 300, is reflected by the fixed mirror 206, is shifted in the optical path, and passes through the quarter-wave plate 300 again to form a beam. The light is split by the splitter 302 into reflected light and transmitted light. The light having the frequency f1 that has passed through the reference optical path has been rotated twice by 90 ° by passing through the quarter-wave plate twice, so that the light is shifted from the original polarization orientation parallel to the plane of the drawing to the direction perpendicular to the plane of the drawing. It is converted to a polarization direction. On the other hand, the light of frequency f2 that has passed through the reference optical path also has its polarization direction rotated by 90 ° by passing through the quarter-wave plate twice,
The polarization direction is converted from the original polarization direction perpendicular to the paper surface to the polarization direction parallel to the paper surface.

Light of frequencies f1 and f2 reflected by the moving mirror MX on the measurement optical path and transmitted through the beam splitter 302;
The polarization beam splitter 3 is coaxial with the light of frequencies f1 and f2 reflected by the beam splitter 302 through the reference optical path.
04. In the polarization beam splitter 304, the lights of frequencies f1 and f2 having the polarization directions parallel to the paper surface pass through the polarization beam splitter 304 as p-polarized light, and the frequencies f1 and f having the polarization directions perpendicular to the paper surface.
2 is converted into s-polarized light by the polarization beam splitter 30.
It is reflected at 4.

The lights of frequencies f1 and f2 transmitted through the polarizing beam splitter 304 are incident on a detector 306, and the interference light is received by a light receiving system and photoelectrically converted. The interference light that has been photoelectrically converted is moved to the moving mirror M at a beat frequency of (f1-f2).
The frequency (f1--f1) modulated by the frequency change. +-.. DELTA.f (.DELTA.f is a positive value) due to the Doppler effect caused by the movement of X.
f2 ± Δf) as the measurement signal S2
Are input to the phase comparator 1.

On the other hand, the light beams of the frequencies f1 and f2 reflected by the polarization beam splitter 304 are incident on a detector 308, and the interference light is received by a light receiving system and photoelectrically converted. The photoelectrically converted interference light has a frequency modulated by a frequency change ± Δf ′ (Δf ′ is a positive value) due to the Doppler effect caused by the movement of the movable mirror MX at a beat frequency of (f1−f2).

[0016]

(Equation 1)

The detector 30 as a measurement signal S2 'having
8 to the phase comparator 1. On the other hand, a signal having a beat frequency of (f1−f2) obtained by causing light of frequency f1 and light of frequency f2 to interfere with each other is input from laser light source 202 to phase comparator 1 as reference signal S1.
Here, when the movable mirror MX moves in a direction away from the detectors 306 and 308, the frequency f1 reflected by the movable mirror MX,
The f2 light undergoes frequency changes of -Δf and -Δf ', respectively. When the light moves in a direction approaching the detectors 306 and 308, the light of frequencies f1 and f2 reflected by the moving mirror MX has frequencies of + Δf and + Δf', respectively. Changes occur.

That is, in the optical interference measurement apparatus shown in FIG. 6, the detector 306 can be used regardless of whether the moving mirror MX moves away from or close to the detectors 306 and 308.
Frequency (f1−f2 ± Δf) detected at 308;

[0019]

(Equation 2)

Either one moves to the high frequency side. Therefore, if the measurement signals S2 and S2 'from the detectors 306 and 308 are appropriately switched according to the moving direction of the moving mirror MX, the moving mirror MX can be moved at high speed regardless of the moving direction. Become. Japanese Patent Application Laid-Open Nos. 9-138105, 9-138106, and 9-13881 disclose similar technologies.
No. 09 discloses a similar technique.

[0021]

By the way, in the light wave interference measuring apparatus having the structure shown in FIG.
Lights 1 and f2 pass through the measurement optical path coaxially, reach the polarization beam splitter 304, and are separated by the polarization beam splitter 304. However, this configuration cannot remove the nonlinear error. Here, the non-linear errors are the light of the frequency (f1 ± Δf) and the light of the frequency (f2 ± Δf ′) incident on the polarization beam splitter 304, and the frequencies f1 and f2 reflected by the fixed mirror 206.
Is a measurement error caused by light having a frequency that adversely affects the measurement being incident on the detectors 306 and 308 due to incomplete separation of the light by the polarization beam splitter 304. This measurement error is １ ／ or の of the wavelength of the measurement light depending on the movement of the movable mirror MX.
And so on.

Such a non-linear error results from the inability of the polarization beam splitter 304 to separate desired light. For example, when the arrangement of the polarization beam splitter 304 is inappropriate, that is, when the arrangement angle of the polarization beam splitter 304 (especially the angle around the optical axis) is inclined with respect to the polarization direction of the incident light, the polarization is mixed. Resulting in.
If the performance (extinction ratio) of the polarizing beam splitter 304 is low, p-polarized light and s-polarized light cannot be completely separated. However, the polarization beam splitter 30
Even if the arrangement and performance of 4 are perfect, the polarization state changes when the beam is reflected by moving mirror MX and fixed mirror 206, and furthermore, the pitching component or yawing component generated when moving mirror MX moves. The polarization state of the light reflected by the moving mirror MX changes.

As described above, in the conventional optical interference measuring apparatus, the movable mirror can be moved at high speed irrespective of the moving direction. However, the problem that the measurement accuracy is not deteriorated due to the nonlinear error cannot be prevented. have.

An object of the present invention is to provide a light wave interference measuring method and apparatus which can cope with a high-speed movement of an object without generating a nonlinear error.

[0025]

The above object will be described with reference to FIGS. 1 to 4 showing an embodiment of the present invention.
Each of the two light beams having different frequencies (f1, f2) is made incident on a fixed mirror (10, 46) and a movable movable mirror (MC, MP) which are fixedly arranged, and the two light beams In the light wave interference measurement method for measuring the displacement of the movable mirror (MC, MP) due to interference of light, the optical path of two light beams incident on at least one of the fixed mirror (10, 46) and the movable mirror (MC, MP) is shifted. The method is attained by a light wave interference measurement method including the steps of: Further, in the light wave interference measurement method of the present invention, the two light beams have a first polarization direction and a second polarization direction orthogonal to the first polarization direction and are emitted coaxially. I do. Further, the light wave interference measurement method of the present invention is characterized in that it includes a step of dividing two light beams and a step of dividing the two light beams according to a polarization state.

Further, the above-mentioned object is achieved at different frequencies (f1, f1,
f2) has a fixed mirror (10, 46) which is fixedly arranged to receive each of the two light beams, and a movable mirror (MC, MP) movable, and moves by interference of the two light beams In a light wave interferometer for measuring displacement of a mirror (MC, MP), a fixed mirror (10, 46) and a movable mirror (MC, MP)
This is achieved by a light wave interference measurement device characterized by including an optical system (6, 8, 20, 22, etc.) for shifting the optical path of two light beams incident on at least one of the above. In the light wave interference measuring apparatus according to the present invention, the two light beams are emitted coaxially with a first polarization direction and a second polarization direction orthogonal to the first polarization direction. Further, in the light wave interference measuring apparatus of the present invention, a splitting element (6, 8, 20, 22, etc.) for splitting two light beams and a polarizing element (4, 12, 26) for splitting two light beams according to the polarization state. Etc.).

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical interference measuring method and apparatus according to a first embodiment of the present invention will be described with reference to FIG.
In FIG. 1, a laser light source 2 emits light of two frequencies f1 and f2 whose polarization directions are orthogonal to each other and are slightly different. Here, the light of frequency f1 has a polarization direction parallel to the paper surface, and the light of frequency f2 has a polarization direction perpendicular to the paper surface. The two lights emitted from the laser light source 2 enter the polarization beam splitter 4 coaxially, and the light of frequency f1 passes through the polarization beam splitter 4 as p-polarized light, and the frequency f
The second light is reflected by the polarization beam splitter 4 as s-polarized light.

The light of frequency f1 transmitted through the polarization beam splitter 4 is further incident on the beam splitter 6 and split into two, and the frequency f1 transmitted through the beam splitter 6 is split.
Is reflected by a moving mirror (corner / cube / prism) MC on the measurement optical path, is shifted in optical path, and enters the polarization beam splitter 12. On the other hand, the light of the frequency f1 reflected by the beam splitter 6 is transmitted to a fixed mirror (corner / corner) on the reference optical path.
The light is reflected by a cube prism (10) and deviated from the optical path to enter a polarization beam splitter (12). At this time, FIG.
As shown in (1), each optical system is arranged such that the two light beams of the frequency f1 are displaced in the optical path on the polarization splitting surface of the polarization beam splitter 12.

On the other hand, the light of the frequency f2 reflected by the polarization beam splitter 4 is further incident on the beam splitter 8 and split into two, and the light of the frequency f2 reflected by the beam splitter 8 is converted to the frequency f1 on the measurement optical path. Travels along a different optical path, but parallel to the light, and enters the movable mirror MC on the measurement optical path,
The optical path is shifted by the moving mirror MC, and the polarization beam splitter 1
2 is incident. On the other hand, the light of the frequency f2 transmitted through the beam splitter 8 is parallel to the light of the frequency f1 on the reference optical path but travels a different optical path, is reflected by the fixed mirror 10 on the reference optical path, is shifted in the optical path, and is deflected. Incident on. At this time, as shown in FIG. 1, the frequency f passing through the measurement optical path is
The arrangement of the respective optical systems is adjusted so that the light of frequency 2 coincides with the light of frequency f1 that has passed through the reference optical path, and the light of frequency f2 that has passed through the reference optical path matches the light of frequency f1 that has passed through the measurement optical path. I have.

In the polarization beam splitter 12, the light of the frequency f1 ± Δf modulated with the movement of the movable mirror MC through the measurement optical path is polarized light splitter as p-polarized light because its polarization direction is parallel to the paper. The light of frequency f2 that has passed through the reference optical path and has passed through the reference optical path is reflected by the polarization beam splitter 12 as s-polarized light because its polarization direction is perpendicular to the plane of the paper. Incident on the vessel 14. The polarizer of the detector 14 is inclined by 45 ° with respect to the polarization direction of the light of the frequencies f1 and f2, and the light of the frequency f2 passing through the reference optical path and the light of the frequency f1 ± Δf passing through the length measuring optical path. After passing through the polarizer, the light interferes, and the interference light is received by the light receiving system and is photoelectrically converted. The photoelectrically converted interference light has a frequency (f1−f2 ±) modulated at a beat frequency of (f1−f2) at a frequency change ± Δf (Δf is a positive value) due to the Doppler effect generated with movement of the movable mirror MC. Measurement signal S2 having Δf)
From the detector 14 to the phase comparator 18.

On the other hand, in the polarization beam splitter 12, the light of the frequency f2 ± Δf ′ modulated by the movement of the movable mirror MC via the measurement optical path is converted into s-polarized light because its polarization direction is perpendicular to the paper. Polarizing beam splitter 1
2, the light of frequency f1 passing through the reference optical path is transmitted through the polarization beam splitter 12 as p-polarized light because its polarization direction is parallel to the plane of the paper, and both lights are coaxial and detected with a polarizer. Incident on the vessel 16. The polarizer of the detector 16 is also inclined by 45 ° with respect to the polarization directions of the lights of the frequencies f1 and f2, and the light of the frequency f1 that has passed through the reference optical path and the light of the frequency f2 ± Δf ′ that has passed through the length measurement optical path. The light interferes after passing through the polarizer, and the interference light is received by the light receiving system and photoelectrically converted. The photoelectrically converted interference light is represented by (f1-
The frequency modulated by the frequency change ± Δf ′ (Δf ′ is a positive value) due to the Doppler effect that occurs with the movement of the movable mirror MC at the beat frequency of f2)

[0032]

(Equation 3)

The detector 16 as a measurement signal S2 'having
Are input to the phase comparator 18. In addition, laser light source 2
Then, a signal having a beat frequency of (f1−f2) obtained by causing the light of the frequency f1 and the light of the frequency f2 to interfere with each other is input to the phase comparator 18 as the reference signal S1. Here, when the movable mirror MC moves away from the detectors 14 and 16, frequency changes −Δf and −Δf ′ occur. When the movable mirror MC moves closer to the detectors 14 and 16, the frequency change + Δ.
f, + Δf ′.

As described above, in the light wave interference measuring apparatus according to the present embodiment, the detector 1 can be used regardless of whether the movable mirror MC moves away from or close to the detectors 14 and 16.
Frequencies (f1−f2 ± Δf) detected at 4, 16;

[0035]

(Equation 4)

Either one moves to the high frequency side. Therefore, if the measurement signals S2 and S2 'from the detectors 14 and 16 are appropriately switched according to the moving direction of the moving mirror MC, the moving mirror MC can be moved at high speed regardless of the moving direction. Become. Further, the light emitted coaxially from the light source is separated / divided, decoupled from the coaxial light, and directed to the movable mirror MC and the fixed mirror 10, and the detectors 14, 1
Since only the light to be detected at 6 is configured to be coaxial just before the detectors 14 and 16, measurement without generating a non-linear error becomes possible.

Next, an optical interference measurement method and apparatus according to a second embodiment of the present invention will be described with reference to FIG. In the present embodiment, the movable mirror MC and the fixed mirror 10 of the optical interference measuring apparatus according to the first embodiment are
It is characterized in that a plane mirror is used instead of the prism. In FIG. 2, a laser light source 2 has two frequencies f1 and f2 whose polarization directions are orthogonal to each other and slightly different from each other.
To emit light. Here, the light of frequency f1 has a polarization direction parallel to the paper surface, and the light of frequency f2 has a polarization direction perpendicular to the paper surface. The two lights emitted from the laser light source 2 enter the beam splitter 30 coaxially and are split into two. Frequency f transmitted through beam splitter 30
The light of f1 and the light of f2 are incident on the polarization beam splitter 32 coaxially, and the light of the frequency f1 is transmitted as p-polarized light through the polarization beam splitter 32 and reflected by the moving mirror (plane mirror) MP in the measurement optical path. The light having the frequency f2 is reflected by the polarization beam splitter 32 as s-polarized light, and further reflected by the polarization beam splitter 38, and is parallel to the measurement optical path having a predetermined interval with respect to the measurement optical path of the light having the frequency f1. And reflected by the moving mirror MP.

A quarter-wave plate 34 is provided on the measurement optical path, so that light having a frequency f1 which enters the movable mirror MP and returns passes through the quarter-wave plate 34 twice. The polarization direction is rotated by 90 ° to become s-polarized light, and is reflected by the polarization beam splitter 32. The light of the frequency f1 reflected by the polarization beam splitter 32 is
The light is reflected by a fixed mirror (corner cube prism) 36, the optical path is shifted, the light is again incident on the polarization beam splitter 32, reflected, travels along the measurement optical path, and is reflected again by the movable mirror MP. The light having the frequency f1 that is incident on the movable mirror MP and returns passes through the quarter-wave plate 34 twice as described above, and its polarization direction is rotated by 90 ° to become p-polarized light. This time, the light passes through the polarization beam splitter 32. Frequency f1 ± Δ whose frequency changes with movement of movable mirror MP
The light of f transmits through the polarization beam splitter 32 and further transmits through the polarization beam splitter 54 and enters the detector 14.

On the other hand, the frequency f2 which travels along the parallel measurement optical path at a predetermined interval from the measurement optical path of the light of frequency f1
Also passes through the quarter-wave plate 34 twice and its polarization direction is rotated by 90 ° to become p-polarized light, so that it passes through the polarization beam splitter 38 and passes through a fixed mirror (corner cube prism). ) Reflected at 40, the optical path is shifted and transmitted again through the polarizing beam splitter 38, travels along the measuring optical path, and is reflected again by the moving mirror MP. The light having the frequency f2 that is incident on the movable mirror MP and returns is transmitted to the quarter wavelength plate 3 in the same manner as described above.
4 twice and its azimuth is rotated by 90 ° to become s-polarized light.
Reflect at 8. The light of the frequency f2 ± Δf ′, the frequency of which has changed with the movement of the movable mirror MP, is reflected by the polarization beam splitter 38, and further reflected by the polarization beam splitters 32 and 54.
Then, the light is incident on the detector 16.

The relative positions of the fixed mirrors 36 and 40 are shifted so that the width of the optical path shifted by the fixed mirror 40 is different from the width of the optical path shifted by the fixed mirror 36.
The frequencies f1, f separated by the polarization beam splitter 32
The two lights do not become coaxial again, and each detector 1
4 and 16 are incident.

The lights of frequencies f1 and f2 reflected by the beam splitter 30 are coaxial and polarized beam splitter 4
2, the light of frequency f1 passes through the polarization beam splitter 42 as p-polarized light and is reflected by a fixed mirror (plane mirror) 46 in the reference optical path, and the light of frequency f2 is polarized light as s-polarized light. After being reflected by the splitter 42, the light is further reflected by the polarization beam splitter 50, travels along a reference light path parallel to the reference light path of the light of the frequency f <b> 1 at a predetermined interval, and is reflected by the fixed mirror 46.

A quarter-wave plate 44 is provided on the reference optical path, so that light of frequency f1 which enters the fixed mirror 46 and returns passes through the quarter-wave plate 44 twice. The polarization direction is rotated by 90 ° to become s-polarized light, and is reflected by the polarization beam splitter 42. The light of the frequency f1 reflected by the polarization beam splitter 42 is
The light is reflected by a fixed mirror (corner cube prism) 48, the optical path is shifted, the light is again incident on the polarization beam splitter 42, reflected, travels along the reference light path, and is reflected again by the fixed mirror 46. The light of the frequency f1 that is incident on the fixed mirror 46 and returns is transmitted through the quarter-wave plate 44 twice as described above, and its polarization direction is rotated by 90 ° to become p-polarized light. This time, after passing through the polarization beam splitter 42, it further passes through the polarization beam splitter 54 and enters the detector 16.

On the other hand, the frequency f2 which travels along the reference light path parallel to the reference light path of the light having the frequency f1 at a predetermined interval from the reference light path.
Also passes through the quarter-wave plate 44 twice, and its polarization direction is rotated by 90 ° to become p-polarized light, so that it passes through the polarization beam splitter 50 and passes through a fixed mirror (corner cube prism). ) Reflected at 52, the optical path is shifted and transmitted again through the polarizing beam splitter 50, travels along the reference optical path, and is reflected again by the fixed mirror 46. The light having the frequency f2 that is incident on the fixed mirror 46 and returns is transmitted to the 波長 wavelength plate 4 in the same manner as described above.
4 is rotated twice by 90 ° to become s-polarized light, so that the polarization beam splitter 5
After being reflected at 0 and further reflected by the polarization beam splitters 42 and 54, the light enters the detector 14.

Here, since the relative positions of the fixed mirrors 48 and 52 are shifted so that the width of the optical path shifted by the fixed mirror 48 is different from the width of the optical path shifted by the fixed mirror 52,
The frequencies f1, f separated by the polarization beam splitter 42
The two lights are incident on the detector 14 or the detector 16 without being coaxial again.

From the polarization beam splitter 54, light having a frequency f1 ± Δf passing through a measurement optical path and light having a frequency f2 passing through a reference optical path enter the detector 14 coaxially, and are coaxially measured at a different position from these. F2 ± Δf '
And the light of frequency f1 passing through the reference optical path are coaxial and the detector 1
6 is incident. Subsequent configurations and operations are the same as in the first embodiment, and a description thereof will be omitted. As described above, also in the optical interference measuring apparatus according to the present embodiment, the moving mirror MP
Irrespective of whether the distance is far from or close to the detectors 14 and 16, the frequency (f1−f2 ± Δf) detected by the detectors 14 and 16;

[0046]

(Equation 5)

Either one moves to the high frequency side. Therefore, if the measurement signals S2 and S2 'from the detectors 14 and 16 are appropriately switched according to the moving direction of the moving mirror MP, the moving mirror MP can be moved at high speed regardless of the moving direction. Become. Further, the light emitted coaxially from the light source is separated / divided and decoupled from the coaxial light and directed to the movable mirror MP and the fixed mirror 46, and the detectors 14, 1
Since only the light to be detected at 6 is configured to be coaxial just before the detectors 14 and 16, measurement without generating a non-linear error becomes possible. Further, according to the present embodiment, since a double-pass configuration using a plane mirror as the movable mirror is used as compared with the first embodiment, the measurement resolution can be optically doubled. In addition, fixed mirrors 36, 40, 4
If the polarizer is arranged at an appropriate angle immediately after 8, 52, more accurate measurement is possible.

Next, an optical interference measurement method and apparatus according to a third embodiment of the present invention will be described with reference to FIG. In FIG. 3, a laser light source 2 emits light of two frequencies f1 and f2 whose polarization directions are orthogonal to each other and are slightly different. The light of the frequency f1 has a polarization direction parallel to the paper surface,
The light of frequency f2 has a polarization direction perpendicular to the plane of the drawing.
The two lights emitted from the laser light source 2 are incident on the beam splitter 20 and split respectively, and
The lights of frequencies f1 and f2 that have passed through 0 enter the polarization beam splitter 26, the light of frequency f1 passes through the polarization beam splitter 26 as p-polarized light, and the light of frequency f2 is a polarization beam as s-polarized light. The light is reflected by the splitter 26.

The light of frequency f1 transmitted through the polarization beam splitter 26 is reflected by the movable mirror MC on the measurement optical path, the optical path is shifted, and then enters the polarization beam splitter 26 again. On the other hand, the light of the frequency f2 reflected by the polarization beam splitter 26 is reflected by the fixed mirror 10 on the reference optical path, is shifted in the optical path, and enters the polarization beam splitter 26 again.
The frequency f whose frequency is modulated with the movement of the movable mirror MC
The light of 1 ± Δf passes through the polarization beam splitter 26 as p-polarized light, and the light of frequency f2 that has passed through the reference optical path is reflected by the polarization beam splitter 26 as s-polarized light. The light enters the detector 14 with a child. The polarizer of the detector 14 is inclined by 45 ° with respect to the polarization direction of the light of the frequencies f1 and f2, and the light of the frequency f2 passing through the reference optical path and the light of the frequency f1 ± Δf passing through the length measuring optical path. After passing through the polarizer, the light interferes, and the interference light is received by the light receiving system and is photoelectrically converted. The interference light that has been photoelectrically converted has a frequency change ± Δ due to the Doppler effect caused by the movement of the movable mirror MC at the beat frequency of (f1−f2).
f (Δf is a positive value) (f1-f2 ±
The measurement signal S2 having Δf) is input from the detector 14 to the phase comparator 18.

On the other hand, the lights of the frequencies f1 and f2 reflected by the beam splitter 20 are reflected by the mirror 22, and are parallel to the optical path of the lights of the frequencies f1 and f2 transmitted through the beam splitter 20 at a predetermined interval. Inject and １ ／ wavelength plate 2
4 is transmitted. Frequency f transmitted through half-wave plate 24
The lights of f1 and f2 are each rotated by 90 ° in the polarization direction, the light of frequency f1 is reflected by the polarization beam splitter 26 as s-polarized light, and the light of frequency f2 is reflected by the polarization beam splitter 26 as p-polarized light. To Penetrate.

The light of frequency f2 transmitted through the polarization beam splitter 26 is reflected by the movable mirror MC on the measurement optical path, is shifted in the optical path, and enters the polarization beam splitter 26 again. On the other hand, the light of the frequency f1 reflected by the polarization beam splitter 26 is reflected by the fixed mirror 10 on the reference optical path, is shifted in optical path, and enters the polarization beam splitter 26 again.
The frequency f whose frequency is modulated with the movement of the movable mirror MC
The light of 2 ± Δf ′ passes through the polarization beam splitter 26 as p-polarized light, and the light of frequency f1 that has passed through the reference optical path is reflected by the polarization beam splitter 26 as s-polarized light. The light enters the detector 16 with a polarizer.
The polarizer of the detector 16 is also inclined by 45 ° with respect to the polarization directions of the lights of the frequencies f1 and f2, and the light of the frequency f1 passing through the reference optical path and the frequency f2 ± Δ passing through the length measuring optical path.
The light of f 'interferes after passing through the polarizer, and the interference light is received by the light receiving system and is photoelectrically converted. The interference light that has been photoelectrically converted is moved to the moving mirror M at a beat frequency of (f1-f2).
A frequency modulated by a frequency change ± Δf ′ (Δf ′ is a positive value) due to the Doppler effect caused by the movement of C.

[0052]

(Equation 6)

The detector 16 as a measurement signal S2 'having
Are input to the phase comparator 18. In addition, laser light source 2
Then, a signal having a beat frequency of (f1−f2) obtained by causing the light of the frequency f1 and the light of the frequency f2 to interfere with each other is input to the phase comparator 18 as the reference signal S1. Here, when the movable mirror MC moves away from the detectors 14 and 16, frequency changes −Δf and −Δf ′ occur. When the movable mirror MC moves closer to the detectors 14 and 16, the frequency change + Δ.
f, + Δf ′.

As described above, in the lightwave interference measuring apparatus according to the present embodiment, the detector 1 can be used regardless of whether the moving mirror MC is far from or close to the detectors 14 and 16.
Frequencies (f1−f2 ± Δf) detected at 4, 16;

[0055]

(Equation 7)

Either one moves to the high frequency side. Therefore, if the measurement signals S2 and S2 'from the detectors 14 and 16 are appropriately switched according to the moving direction of the moving mirror MC, the moving mirror MC can be moved at high speed regardless of the moving direction. Become. Further, the light emitted coaxially from the light source is separated / divided, decoupled from the coaxial light, and directed to the movable mirror MC and the fixed mirror 10, and the detectors 14, 1
Since only the light to be detected at 6 is configured to be coaxial just before the detectors 14 and 16, measurement without generating a non-linear error becomes possible.

Next, an optical interference measurement method and apparatus according to a fourth embodiment of the present invention will be described with reference to FIG. In the present embodiment, the movable mirror MC and the fixed mirror 10 of the optical interference measurement apparatus according to the third embodiment are
It is characterized in that a plane mirror is used instead of the prism. In FIG. 4, a laser light source 2 has two frequencies f1 and f2 whose polarization directions are orthogonal to each other and slightly different from each other.
To emit light. Here, the light of frequency f1 has a polarization direction parallel to the paper surface, and the light of frequency f2 has a polarization direction perpendicular to the paper surface. The two lights emitted from the laser light source 2 enter the beam splitter 20 and are respectively split. The light having the frequency f1 that has passed through the beam splitter 20 passes through the polarization beam splitter 26 as p-polarized light and passes through the measurement optical path. The light is reflected by a moving mirror (plane mirror) MP. A 測定 wavelength plate 34 is provided on the measurement optical path, and
The light of the frequency f1 that is incident on the movable mirror MP and returns passes through the quarter-wave plate 34 twice and has a polarization direction of 90.
Since the light is rotated by ° and becomes s-polarized light, it is reflected by the polarization beam splitter 26.

The light of the frequency f1 reflected by the polarization beam splitter 26 is reflected by the fixed mirror 60, the optical path is shifted, the light is again incident on the polarization beam splitter 26, reflected, travels along the measurement optical path, and is again moved by the movable mirror MP. reflect. The light having the frequency f1 that is incident on the movable mirror MP and returns is 1 in the same manner as described above.
Since the light passes through the 波長 wavelength plate 34 twice and its polarization direction is rotated by 90 ° to become p-polarized light, it is transmitted through the polarization beam splitter 26 this time. The light of the frequency f1 ± Δf, the frequency of which changes with the movement of the movable mirror MP, passes through the polarization beam splitter 26 and then enters the detector 14.

On the other hand, the light of the frequency f2 is reflected by the polarization beam splitter 26 as s-polarized light, and is reflected by the fixed mirror 46 in the reference optical path. A quarter-wave plate 44 is provided on the reference optical path. Therefore, the light having the frequency f2 that is incident on the fixed mirror 46 and returns therethrough passes through the quarter-wave plate 44 twice and its polarization. Since the azimuth is rotated by 90 ° to become p-polarized light, the azimuth is transmitted through the polarization beam splitter 26. The light of frequency f2 transmitted through the polarizing beam splitter 26 is
The light is reflected by the fixed mirror 60, the optical path is shifted, the light passes through the polarization beam splitter 26 again, travels along the reference light path, and is reflected again by the fixed mirror 46. The light of the frequency f2 that is incident on the fixed mirror 46 and returns is transmitted through the quarter-wave plate 44 twice as described above, and its polarization direction is rotated by 90 ° to become s-polarized light. This time, the light is reflected by the polarization beam splitter 26 and enters the detector 14.

The lights of the frequencies f1 and f2 reflected by the beam splitter 20 are reflected by the mirror 22, and are parallel to the optical path of the lights of the frequencies f1 and f2 transmitted through the beam splitter 20 at a predetermined interval. Inject and １ ／ wavelength plate 2
4 is transmitted. Frequency f transmitted through half-wave plate 24
The lights of f1 and f2 are each rotated by 90 ° in the polarization direction, the light of frequency f1 is reflected by the polarization beam splitter 26 as s-polarized light, and the light of frequency f2 is reflected by the polarization beam splitter 26 as p-polarized light. To Penetrate. The light of the frequency f2 transmitted through the polarization beam splitter 26 is reflected by the moving mirror MP in the measurement optical path. 1/4 wavelength plate 3 on the measurement optical path
4 is provided, so that the light having the frequency f2 that is incident on the movable mirror MP and returns, passes through the quarter-wave plate
The light passes through the light beam once, and its polarization direction is rotated by 90 ° to become s-polarized light, so that the light is reflected by the polarization beam splitter 26.

The light of frequency f2 reflected by the polarization beam splitter 26 is reflected by the fixed mirror 60, the optical path is shifted, and is incident on the polarization beam splitter 26 again, reflected, travels along the measurement optical path, and is again moved by the movable mirror MP. reflect. The light of the frequency f2 that is incident on the movable mirror MP and returns is 1 in the same manner as described above.
Since the light passes through the 波長 wavelength plate 34 twice and its polarization direction is rotated by 90 ° to become p-polarized light, it is transmitted through the polarization beam splitter 26 this time. The light of the frequency f2 ± Δf ′, the frequency of which has changed with the movement of the movable mirror MP, passes through the polarization beam splitter 26 and then enters the detector 16.

On the other hand, the light of the frequency f1 is reflected by the polarization beam splitter 26 as s-polarized light and is reflected by the fixed mirror 46 in the reference optical path. A quarter-wave plate 44 is provided on the reference optical path. Therefore, the light having the frequency f1 that is incident on the fixed mirror 46 and returns is transmitted through the quarter-wave plate 44 twice and its polarization. Since the azimuth is rotated by 90 ° to become p-polarized light, the azimuth is transmitted through the polarization beam splitter 26. The light of frequency f1 transmitted through the polarizing beam splitter 26 is
The light is reflected by the fixed mirror 60, the optical path is shifted, the light passes through the polarization beam splitter 26 again, travels along the reference light path, and is reflected again by the fixed mirror 46. The light having the frequency f1 that is incident on the fixed mirror 46 and returns is transmitted through the quarter-wave plate 44 twice as described above, and its polarization direction is rotated by 90 ° to become s-polarized light. This time, the light is reflected by the polarization beam splitter 26 and enters the detector 16.

Accordingly, from the polarization beam splitter 26, the light of the frequency f1 ± Δf passing through the measuring optical path and the light of the frequency f2 passing through the reference optical path enter the detector 14 coaxially, and are made coaxial at different positions. Frequency f through measurement optical path
The light of 2 ± Δf ′ and the light of frequency f1 passing through the reference optical path enter the detector 16 coaxially. The subsequent configuration and operation are the same as in the third embodiment, and a description thereof will not be repeated.

As described above, also in the optical interference measuring apparatus according to the present embodiment, whether the moving mirror MP moves away from or near the detectors 14 and 16 is detected by the detectors 14 and 16. Frequency (f1-f2 ± Δf)
When,

[0065]

(Equation 8)

One of them moves to the high frequency side. Therefore, if the measurement signals S2 and S2 'from the detectors 14 and 16 are appropriately switched according to the moving direction of the moving mirror MP, the moving mirror MP can be moved at high speed regardless of the moving direction. Become. And also in this embodiment, the frequency f1, which is incident on the detector 14,
The optical path of the light of f2 and the frequency f1 incident on the detector 16;
Since the configuration is such that the optical path of the light of f2 is completely separated, it is possible to perform measurement without generating a nonlinear error. In addition, according to the present embodiment, a double-pass configuration using a plane mirror as the movable mirror is used as compared with the third embodiment, so that the measurement resolution can be optically doubled.

The present invention is not limited to the above embodiment, but can be variously modified. For example, in the above-described embodiment, only the configuration for measuring the displacement of the movable mirror has been described. However, an interference system for measuring a change in the refractive index of gas (air) in the measurement optical path has been added, and a more accurate movable mirror has been provided. Of course, it is also possible to carry out a displacement measurement.

In the second and fourth embodiments, each of the measurement optical path and the reference optical path has one
Although the quarter wave plates 34 and 44 are inserted, the present invention is not limited to this, and separate small quarter wave plates are used for each of the four light paths on the measurement light path and the reference light path. Of course you can.

The light wave interference measuring apparatus according to the above embodiment is a light exposure apparatus which is frequently used in a photolithography process in manufacturing a semiconductor device or a liquid crystal display device.
It is suitable for use in a stage-based positioning control system in which a wafer or a reticle is placed and two-dimensionally moved.

[0070]

As described above, according to the present invention, the moving mirror can be moved at a high speed regardless of the moving direction, regardless of whether the moving mirror moves away from the detector or approaches the detector. In addition, since the optical paths of the light having the frequencies f1 and f2 incident on the two detectors are completely separated from each other, it is possible to perform the measurement without generating the non-linear error.

[Brief description of the drawings]

FIG. 1 is a diagram showing an optical interference measuring apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an optical interference measuring apparatus according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an optical interference measuring apparatus according to a third embodiment of the present invention.

FIG. 4 is a diagram showing an optical interference measurement apparatus according to a fourth embodiment of the present invention.

FIG. 5 is a diagram showing a conventional light wave interference measurement device.

FIG. 6 is a diagram showing another conventional light wave interference measurement device.

[Explanation of symbols]

 2 Laser light source 4 Polarizing beam splitter 6, 8 Beam splitter 10 Fixed mirror 14, 16 Detector 18 Phase comparator MC, MP, MX Moving mirror

Claims (6)

[Claims] 1. A method according to claim 1, wherein each of the two light beams having different frequencies is made incident on a fixed mirror and a movable movable mirror which are fixedly arranged, and the displacement of the movable mirror is caused by interference of the two light beams. An optical interference measurement method for measuring, comprising: shifting an optical path of the two light beams incident on at least one of the fixed mirror and the movable mirror. 2. The light wave interference measuring method according to claim 1, wherein the two light beams have a first polarization direction and a second polarization direction orthogonal to the first polarization direction and are emitted coaxially. A light wave interference measurement method. 3. The light wave interference measurement method according to claim 2, further comprising: a step of splitting the two light beams; and a step of splitting the two light beams according to a polarization state. Method. 4. A movable mirror having a fixed mirror and a movable movable mirror, each of which receives two light beams having different frequencies, and the movable mirror is displaced by interference of the two light beams. A light wave interference measurement device, comprising: an optical system that shifts an optical path of the two light beams incident on at least one of the fixed mirror and the movable mirror. 5. The light wave interference measuring apparatus according to claim 4, wherein the two light beams have a first polarization direction and a second polarization direction orthogonal to the first polarization direction and are emitted coaxially. A light wave interference measurement device characterized by being performed. 6. The light wave interference measuring apparatus according to claim 5, further comprising: a splitting element for splitting the two light fluxes; and a polarizing element for splitting the two light fluxes according to a polarization state. Interferometer.