JPH0843014A - Interferometer - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide an interferometer in which the interference signal is reduced little with respect to the inclination of a plane moving mirror. A light source for supplying a light beam, a light splitting device for splitting the light beam into a reference light and a measurement light, a reference light reflecting device for reflecting the reference light, and a measurement light reflecting device for reflecting the measurement light. A photoelectric conversion means for receiving interference light between the reference light from the reference light reflection means and the measurement light from the measurement light reflection means, and the measurement based on a signal from the photoelectric conversion means. In an interferometer for detecting relative displacement of a light reflecting means, the measurement in a direction along a plane including a light ray incident on the measuring light reflecting means and a light ray reflected by the measuring light reflecting means. A beam diameter shaping means for reducing the diameter of light is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer such as a laser interferometer, and more particularly to an interferometer suitable for use in measuring the amount of movement of a stage of a precision measuring instrument with high accuracy.

[0002]

2. Description of the Related Art A conventional interferometer has a structure shown in FIG. However, here, the direction in which light is emitted from the laser light source unit is defined as the x-axis direction, and the direction perpendicular to the x-axis direction and in FIG.
In (a), the direction that is the vertical direction of the paper surface is the y-axis direction, and the direction perpendicular to both the x-axis direction and the y-axis direction is the z-axis direction. Further, FIG. 3A is a top view of the interferometer,
FIG. 3B is a side view of the interferometer as seen from below the sheet surface of FIG. 3A.

A laser beam emitted from the laser light source unit 1 enters a polarization splitting surface 12a of a polarization beam splitter 12 composed of two right-angle prisms 13 and 14 having different sizes, and a P-polarized component serving as a measurement light is The S-polarized component that is transmitted in the x-axis direction and serves as the reference light is reflected in the y-axis direction. The P-polarized light component that has passed through the polarization splitting surface 12a proceeds straight in the x-axis direction, passes through the quarter-wave plate 5a, and reaches the plane moving mirror 6. The laser beam that has reached the plane moving mirror 6 is reflected in the original direction, passes through the quarter-wave plate 5 a again, and again returns to the polarization splitting surface 1 of the polarization beam splitter 12.
It is incident on 2a. At this time, the polarization plane of the laser beam is 1
Since the light is transmitted through the quarter-wave plate 5a twice, it is S-polarized. Therefore, the laser beam incident on the polarization splitting surface 12a again is reflected by the polarization splitting surface 12a by 90 ° in the y-axis direction.

The laser beam reflected by the polarization splitting surface 12a is 90 ° in the x-axis direction on the reflecting surface 12b of the rectangular prism 14.
Reflected, transmitted through the quarter-wave plate 5b, and moved by the plane moving mirror 6
To reach again. However, it is a place different from when it is transmitted through the quarter-wave plate 5a. The laser beam that has reached the plane moving mirror 6 again is reflected in the original direction and again passes through the quarter-wave plate 5b. At this time, the plane of polarization is again P-polarized because it passes through the quarter-wave plate 5b twice. The P-polarized laser beam is again reflected by the right-angle prism 1.
The reflection surface 12b of No. 4 again reflects 90 ° in the y-axis direction and enters the polarization separation surface 12a three times. At this time, since the polarization plane of the laser beam is P-polarized, the polarization splitting surface 1
The light travels straight through 2a and enters the receiver 9.

On the other hand, the S-polarized light component emitted from the laser light source 1 and serving as the reference light is incident on the polarization splitting surface 12a of the polarization beam splitter 12, is reflected by 90 ° in the y-axis direction, and is directly incident on the receiver 9. In the receiver 9, the two laser beams have the same polarization direction and cause interference. The displacement of the plane moving mirror 6 can be obtained from the interference signal thus obtained.

[0006]

By the way, when the plane moving mirror 6 is actually horizontally moved in the x-axis direction, the plane moving mirror 6 is moved.
Tilts and vibrates slightly. Here, if
As shown in FIG. 4, when the plane moving mirror 6 tilts with the y-axis direction as the rotation axis, the angle difference θ between the wavefront WM1 of the measurement light and the wavefront WR1 of the reference light on the receiver 9 as shown in FIG. Occurs. When the angle difference θ is generated in this way, interference different from the above interference occurs, and the intensity of the interference signal decreases. Generally, in order to reduce this influence, it is preferable to reduce the diameter of the laser beam. However, if the diameter of the laser beam is reduced, the accuracy of rotation of the plane moving mirror 6 in FIG. Further, if the diameter of the laser beam is reduced, the influence of scratches, dust, etc. on each component forming the interferometer will increase.

[0007] In view of the above, the present invention provides an interferometer in which the interference signal is less reduced with respect to the inclination of the plane moving mirror.

[0008]

In order to solve the above problems, in the present invention, a light source for supplying a light beam, a light splitting device for splitting the light beam into reference light and measurement light, and reflection of the reference light are provided. Reference light reflection means, measurement light reflection means for reflecting the measurement light, and photoelectric conversion means for receiving interference light between the reference light from the reference light reflection means and the measurement light from the measurement light reflection means In the interferometer for detecting the relative displacement of the measurement light reflecting means based on the signal from the photoelectric conversion means, the light beam incident on the measurement light reflecting means and the incident light beam are the measurement light beams. There is provided an interferometer characterized by comprising beam diameter shaping means for reducing a diameter of the measuring light in a direction along a plane including a light ray reflected by the reflecting means.

Further, it is preferable that the following conditions are satisfied. | R · sin θ | ≦ λ / 2 where r is the radius of the light beam reduced by the beam diameter shaping unit, and θ is the light beam incident on the measurement light reflection unit and the incident light beam reflected by the measurement light reflection unit. Is the angle formed by the light rays reflected by the means, and λ is the wavelength.

[0010]

The operation will be described with reference to FIGS. 4 and 7. In a conventional interferometer, as shown in FIG. 4A, when a laser beam having a radius R is incident on a receiver with an inclination of an angle θ formed by a wavefront WM1 of measurement light and a wavefront WR1 of reference light, the center of the laser beam is On the other hand, in the vicinity, the interference signal decreased with a phase difference of φ.

However, for example, as shown in FIG. 4B, when the radius of the laser beam is set to R / 2, which is the half of the above, the angle θ between the wavefront WM2 of the measurement light and the wavefront WR2 of the reference light is changed. Even if the inclination is the same, the phase difference at the periphery is φ / 2. FIG. 7 is a diagram in which the above is viewed from the normal direction of the wavefront of the reference light, and will be further described with reference to FIG. FIG. 7 (a)
FIG. 4 is a diagram showing how wavefronts are overlapped on an incident surface of a receiver in the case of a conventional interferometer. However, in order to make the description easy to understand, the center of the wavefront WR1 of the reference light and the center of the wavefront WM1 of the measurement light are offset from each other. In the conventional interferometer, as shown by the hatched portion in FIG. 7A, the range in which the reference light and the measurement light interfere with each other is the overlapping portion of the circular cross sections of the reference light and the measurement light. If the measurement light and the reference light are tilted, the interference signal is reduced by an amount corresponding to the overlapping portion of the cross sections of the reference light and the measurement light.

However, according to the present invention, the diameter of the measuring light in the direction along the plane including the light ray incident on the measuring light reflecting means and the light ray reflected by the measuring light reflecting means is reduced. Thus, as shown in FIG. 7B, the overlapping portions of the cross sections of the reference light and the measurement light become
It is reduced as compared with the case of the conventional interferometer. (However, here, the reference light is also shaped to have the same cross section as that of the measurement light.) Incidentally, here, as in FIG. 7A, the measurement is performed with the center of the wavefront WR2 of the reference light. The center of the light wavefront WM2 is offset. Accordingly, even if the measurement light and the reference light are tilted, it is possible to reduce the reduction of the interference signal because the overlapping portions of the reference light and the measurement light in the cross section are smaller than in the conventional case.

In the above description, the cross sections of the reference light beam and the measurement light beam are shaped. However, even if the cross section of only one of the reference light beam and the measurement light beam is shaped, the reduction of the interference signal is reduced. It is possible. Further, in order to maximize the effect of the present invention, it is preferable that the conditional expression (1) is satisfied. The angle formed by the wavefront of the actual measurement light and the wavefront of the reference light (= the angle formed by the light ray incident on the measurement light reflection means and the light ray reflected by the measurement light reflection means).
Is very small, so r · sin θ≈φ. In other words, the conditional expression (1) indicates that the deviation is within λ / 2 within the wavefront around the center of the laser beam, which is within the allowable range. If the upper limit of conditional expression (1) is exceeded, the interference signal will be greatly reduced and it will not be suitable for practical use. When the upper limit of conditional expression (1) is set to λ / 4, an interferometer with almost no decrease in interference signal can be constructed.

[0014]

1 (a) and 1 (b) are views showing a first embodiment, and the first embodiment will be described with reference to FIGS. 1 (a) and 1 (b). . However, here, the direction in which light is emitted from the laser light source unit is the x-axis direction, and the direction perpendicular to the x-axis direction and that is the vertical direction on the paper surface in FIG. 1A is the y-axis direction. The direction perpendicular to both the y-axis direction and the y-axis direction is the z-axis direction. Further, FIG. 1A is a top view of the interferometer, and FIG. 1B is a side view of the interferometer when the interferometer is viewed from the lower side of the paper surface of FIG. 1A.

A laser beam emitted from the laser light source unit 1 which is a light source to be a parallel light by being emitted in the x-axis direction is incident on an anamorphic prism 103a which is a beam diameter shaping means and is directed in the z-axis direction of the laser beam. The diameter is compressed, and then the beam is incident on the anamorphic prism 103b which is the other beam diameter shaping means. Anamorphic prism 1
03b has a function of forming a pair with the preceding anamorphic prism 103a and converting the laser light emitted from the anamorphic prism 103b into parallel light. When exiting the anamorphic prism 103b, the optical path moves slightly in parallel in the z-axis direction, and the cross-sectional shape of the light beam is shaped into an elliptical shape compressed in the z-axis direction. The use of an anamorphic prism has the advantages of no aberration and easy adjustment. Anamorphic prisms 103a and 103b
The laser beam shaped in (1) further proceeds straight and enters the polarization splitting surface 12a of the polarization beam splitter 12 which is the light splitting means and is composed of right-angle prisms 13 and 14 having different sizes.

The P-polarized light component, which is the measurement light, is transmitted straight to the polarization splitting surface 12a. The P-polarized light component that has gone straight through the transmission further goes straight through the quarter-wave plate 5a,
It is incident on the plane moving mirror 6 which is the measuring light reflecting means. Since the incident light on the plane moving mirror 6 is vertical incidence, the laser beam reflected by the plane moving mirror 6 travels the same optical path as the incident light on the plane moving mirror 6 in the opposite direction. The laser beam reflected by the plane moving mirror 6 again passes through the quarter-wave plate 5a and enters the polarization splitting surface 12a again. At this time, the laser beam is 1 /
Since the light is transmitted through the four-wave plate 5a twice, the polarization plane is S-polarized, and the laser beam incident on the polarization separation surface 12a is reflected by 90 ° in the y-axis direction.

The light beam reflected by the polarization splitting surface 12a is reflected by 90 ° in the x-axis direction by the reflecting surface 12b of the rectangular prism 14 which constitutes the polarization beam splitter, and the quarter wavelength plate 5
After passing through b, it enters the plane moving mirror 6 again. The position where the laser beam is incident on the plane moving mirror 6 again is different from the position where the laser beam is incident on the plane moving mirror 6 for the first time, and is a position displaced in the y-axis direction. Since the incident light on the plane moving mirror 6 again is vertical incidence, the light ray reflected by the plane reflecting mirror 6 travels in the same optical path as the incident light and again becomes 1 / It transmits through the four-wave plate 5b. Again quarter wave plate 5b
The laser beam that has passed through is incident on the reflecting surface 12b of the rectangular prism 14 again, is reflected by 90 ° in the y-axis direction, and is incident on the polarization splitting surface 12a three times. At this time, the plane of polarization is 1 /
Since the light is transmitted through the four-wave plate 5b twice, it is P-polarized light and passes straight through the polarization separation surface 12a. Polarization splitting surface 12
The laser beam transmitted straight through a is finally incident on the receiver 9 which is a photoelectric conversion means.

On the other hand, the S-polarized light component serving as the reference light in the laser beam emitted from the laser light source unit 1 in the x-axis direction is in the y-axis direction on the polarization splitting surface 12a of the polarization beam splitter 12 which is also the reference light reflecting means. It is reflected at an angle of 90 ° and directly enters the receiver 9. In the receiver 9, the two polarization components of the measurement light and the reference light are aligned and cause interference.
The displacement of the plane moving mirror 6 can be obtained from the interference signal obtained from this interference.

Here, the rotation or inclination (rotation or inclination about the z-axis as the rotation axis) of the plane moving mirror 6 in the plane of FIG. 1 (a) is the lateral displacement of the measurement light with respect to the reference light, but FIG. FIG. 1 of the plane moving mirror 6 as shown in FIG.
The rotation or inclination (rotation or inclination about the y-axis as the rotation axis) in the plane of paper of (b) is the deviation of the incident angle of the reference light and the measurement light in the receiver 9. As shown in the present embodiment, by compressing the laser beam in the z-axis direction, the plane moving mirror 6 is rotated or tilted within the plane of FIG. 3b (rotation or tilt about the y axis as a rotation axis). , The reduction of the interference signal can be reduced.

Further, in the present embodiment, a helium-neon (He-Ne) laser is used as the laser light source unit 1 and, further, an original He-
It emits a frequency obtained by modulating the frequency of the Ne laser. That is, the laser light source unit 1 is a light source that emits the original frequency ν ₁ of the He—Ne laser and the frequency ν ₂ (= ν ₁ + Δν, Δν << ν ₁ ) modulated by the acoustooptic device. The planes of polarization of the frequencies ν ₁ and ν ₂ are orthogonal to each other. One of which is the reference light
It is polarized light, and the other is P polarized light which is the measurement light.
The reference light may be P-polarized light and the measurement light may be S-polarized light.

Then, these frequencies ν ₁ and ν ₂
In the receiver 9, a beat (groan) signal is detected by causing 45 ° polarization by using a polarizing plate and causing interference.
The configuration shown above is a so-called heterodyne interferometer.
When the heterodyne interferometer is configured as described above, it is generally possible to detect the displacement of the object to be inspected with considerably high accuracy. When the heterodyne interferometry is used, two different frequencies are not obtained by using the He-Ne laser and the acoustooptic device as described above.
There is also a method of obtaining two different frequencies using a Zeeman laser.

FIGS. 2 (a) and 2 (b) are views showing a second embodiment, and the second embodiment will be described with reference to FIGS. 2 (a) and 2 (b).
An example will be described. However, here, the direction in which light is emitted from the laser light source unit is the x-axis direction, and the direction that is perpendicular to the x-axis direction and that is the vertical direction on the paper surface in FIG. 2A is the y-axis direction. z in the direction perpendicular to the y-axis direction and both
Axial direction. 2 (a) is a top view of the interferometer, and FIG. 2 (b) is a side view of the interferometer as seen from below the sheet surface of FIG. 2 (a).

The parallel laser beam emitted from the laser light source unit 1 which is the light source in the x-axis direction is incident on the cylindrical lens 102a which is the beam shaping means, and the diameter of the laser beam in the z-axis direction is compressed. Then, the beam enters the cylindrical lens 102b which is the other beam shaping means. The cylindrical lenses 102a and 102b are y
The focal points exist only in the axial direction, and the respective focal points are arranged so as to coincide with each other. In the case of the present embodiment, the cylindrical lens 102a is a plano-concave negative cylindrical lens, and the cylindrical lens 102b is a plano-convex positive cylindrical lens. As described above, since the respective cylindrical lenses 102a and 102b are arranged so that their focal points coincide with each other, the cylindrical lens 102
When b is emitted, the cross-sectional shape of the light beam is shaped into an elliptical parallel light that is compressed in the z-axis direction. The use of the cylindrical lens has an advantage that the optical axis does not shift.

Cylindrical lenses 102a and 102
The laser beam shaped by b further goes straight, and is incident on the polarization splitting surface 12a of the polarization beam splitter 12 including the right-angle prisms 13 and 14 having different sizes as the light splitting means. The P-polarized light component, which is the measurement light, directly passes through the polarization splitting surface 12a as it is. The component of the P-polarized light that has gone straight through the transmission further goes straight through the quarter-wave plate 5a and enters the plane moving mirror 6 that is the measuring light reflecting means. Planar moving mirror 6
Since the incidence on the plane moving mirror 6 is vertical incidence, the laser beam reflected by the plane moving mirror 6 travels the same optical path as the light incident on the plane moving mirror 6 in the opposite direction. The laser beam reflected by the plane moving mirror 6 again passes through the quarter-wave plate 5a and enters the polarization splitting surface 12a again. At this time, since the laser beam is transmitted through the quarter-wave plate 5a twice, the polarization plane is S-polarized, and the laser beam incident on the polarization splitting surface 12a is reflected by 90 ° in the y-axis direction. .

The light beam reflected by the polarization splitting surface 12a is reflected by 90 ° in the x-axis direction by the reflecting surface 12b of the rectangular prism 14, passes through the quarter wavelength plate 5b, and enters the plane moving mirror 6 again. The position where the laser beam is incident on the plane moving mirror 6 again is different from the position where the laser beam is incident on the plane moving mirror 6 for the first time, and is a position displaced in the y-axis direction. Plane moving mirror 6 again
Since the incident light is also vertically incident, the light beam reflected by the plane reflecting mirror 6 travels in the same optical path as the incident light and is transmitted through the quarter-wave plate 5b again, as in the case of the first incidence on the plane moving mirror 6. To do. The laser beam transmitted through the quarter-wave plate 5b again enters the reflecting surface 12b of the rectangular prism 14 again, is reflected by 90 ° in the y-axis direction, and enters the polarization splitting surface 12a three times. At this time, the plane of polarization is P-polarized because it is transmitted through the quarter-wave plate 5b twice, and can be transmitted straight through the polarization separation surface 12a. The laser beam that has passed through the polarization splitting surface 12a and travels straight enters the receiver 9 that is the photoelectric conversion means.

On the other hand, the component of S-polarized light which is the reference light in the laser beam emitted from the laser light source 1 in the x-axis direction is 90 in the y-axis direction on the polarization splitting surface 12a of the polarization beam splitter 12 which is also the reference light reflecting means. It is reflected at an angle of ° and directly enters the receiver 9. In the receiver 9, the two polarization components of the measurement light and the reference light are aligned and cause interference. The displacement of the movable mirror 6 can be obtained from the interference signal obtained from this interference.

Here, the rotation and tilt of the plane moving mirror 6 in the plane of FIG. 2 (a) (rotation and tilt about the z axis as a rotation axis) are lateral displacements of the measurement light with respect to the reference light.
Rotation and inclination (rotation and inclination about the y-axis as a rotation axis) of the plane moving mirror 6 in the plane of FIG. 2B in the receiver 9 of the reference light and the measurement light as shown in FIG. Of the incident angle of. As shown in the present embodiment, by compressing the beam in the z-axis direction, the plane moving mirror 6 rotates and tilts (y in the plane of FIG. 2B as shown in FIG. 6B).
With respect to the rotation and the inclination about the axis as the rotation axis, the reduction of the interference signal can be reduced.

Although only one type of interferometer is shown in this embodiment, it goes without saying that the present invention may be applied to other types of interferometers such as the Michelson interferometer. . Further, the present invention is effective not only when applied to the heterodyne interferometry of the above-mentioned embodiment but also when applied to the homodyne interferometry.

[0029]

According to the present invention, it is possible to obtain an interferometer in which the interference signal is less reduced with respect to the inclination of the plane moving mirror.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a second embodiment of the present invention.

FIG. 3 is an explanatory diagram of a conventional interferometer.

FIG. 4 is an explanatory diagram of a shift amount of a wavefront.

FIG. 5 is a diagram when a plane moving mirror is tilted in a conventional interferometer.

FIG. 6 is a diagram when the plane moving mirror is tilted in the interferometer of the embodiment.

FIG. 7 is an explanatory diagram of overlapping of wavefronts.

[Explanation of main symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source part 5a, 5b 1/4 wavelength plate 6 Planar movement mirror 9 Receiver 12 Polarization beam splitter 102a, 102b Cylindrical lens 103a, 103b Anamorphic prism

Claims (4)

[Claims] 1. A light source that supplies a light beam, a light splitting device that splits the light beam into reference light and measurement light, reference light reflecting device that reflects the reference light, and measurement light reflection that reflects the measurement light. And photoelectric conversion means for receiving interference light between the reference light from the reference light reflection means and the measurement light from the measurement light reflection means, based on a signal from the photoelectric conversion means. In an interferometer for detecting the relative displacement of the measurement light reflecting means, in a direction along a plane including a light ray incident on the measurement light reflecting means and a light ray reflected by the measurement light reflecting means. An interferometer, characterized by comprising beam diameter shaping means for reducing the diameter of the measuring light. 2. The beam diameter shaping means is arranged between the light source and the light splitting means, and the beam diameter shaping means is
The interferometer according to claim 1, further comprising a pair of anamorphic prisms. 3. The beam diameter shaping means is arranged between the light source and the light splitting means, and the beam diameter shaping means is
The interferometer according to claim 1, further comprising a pair of cylindrical lenses. 4. The interferometer according to claim 1, wherein the following conditions are satisfied. | R · sin θ | ≦ λ / 2 where: r: radius of light beam reduced by the beam diameter shaping means θ: light ray incident on the measurement light reflecting means and light ray reflected by the measurement light reflecting means Angle formed by λ: Wavelength