JP3410802B2 - Interferometer device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer device capable of observing the surface shape of an object by means of interference fringes, and more specifically, a Fizeau interferometer in which the surface to be inspected and the reference surface are opposed to each other. It relates to the device.

[0002]

2. Description of the Related Art Interferometry is known as a method for accurately measuring the surface shape of a spherical surface or an aspherical surface.

In the shape measurement by the interferometric method, the degree of displacement of the surface to be inspected with respect to a highly accurate reference surface (interference standard) is determined by the interference fringes generated by interfering the light reflected from each surface. It seeks to be based on.

The interferometric method has an advantage that the shape accuracy of the entire surface can be instantly confirmed without contact. Among them, the interferometric method using the Fizeau interferometer device has a simple device configuration, and thus various surface profile measurements can be performed. Is used for.

As a general Fizeau interferometer device, a laser light source 101 and a laser beam 102 are shown in FIG.
To a divergent light 105, a beam splitter 106, a collimator lens 107, and a parallel light 10
A reference plate 109 for partially reflecting 8 by the semi-transparent mirror reference plane 109a arranged perpendicularly to this parallel light and transmitting the remaining parallel light 108 to irradiate the test surface 110a of the subject 110 with the reference plate 109. The TV camera 111 is provided for observing interference fringes formed by interference between the reference light reflected from the reference surface 109a and the object light reflected from the surface 110a to be tested.

[0006]

As described above, the Fizeau interferometer device has a structurally simple structure, but on the other hand, the object light and the reference light have the reference plane 109a and the surface 11 to be inspected.
Since the optical path difference is twice the distance of 0a, it is necessary to use a laser light source or the like capable of irradiating light with high coherence as a light source, and it is mainly opposite to the reference plane 109a of the reference plate 109. There is a problem that unnecessary interference fringes are likely to occur due to the reflected light from the side surface.

The present invention has been made in view of the above circumstances, and has the advantages of a Fizeau interferometer having a compact structure, and is clear even when using irradiation light having a very poor coherence. It is an object of the present invention to provide an interferometer device capable of obtaining high-frequency interference fringes.

[0008]

A first interferometer device of the present invention makes coherent light incident on a test surface of a subject through a reference surface of a reference plate, and In the interferometer device that forms an interference fringe caused by optical interference between the object light generated by the reflection on the surface to be inspected and the reference light generated by the reflection on the reference surface, the object light, The reference light superposed on the object light is separated from each other, and thereafter the reference light is more than the object light until the superposition of the object light and the reference light again, the distance between the reference surface and the surface to be inspected. An optical path difference adjusting optical system that guides the object light and the reference light so as to have an optical path length L corresponding to twice the optical path length L is disposed between the reference plate and the interference fringe formation surface. It is a feature.

Further, in the second interferometer device of the present invention, the optical path difference adjusting optical system of the first interferometer device transmits either one of the object light and the reference light, and the other side. A beam splitter for reflecting the reference beam emitted from the beam splitter, a first reflecting mirror for irradiating the reference beam emitted from the beam splitter and reflecting the reference beam in the direction of the beam splitter, and a first reflecting mirror for reflecting the reference beam by the first reflecting mirror. A second reflecting mirror, which is irradiated with the reference light emitted from the beam splitter and reflects the reference light in the beam splitter direction, is separated from each other on the optical path by an optical distance equal to 1/2 of the optical path length L. Before the reference light emitted from the beam splitter is reflected by the second reflecting mirror and is emitted from the beam splitter. Those characterized by comprising configured to overlap with the object beam.

Further, the third interferometer device of the present invention is
The second interferometer device, wherein at least one of the first and second reflecting mirrors is provided with a reflecting mirror vibrating means that is driven to vibrate in the beam splitter direction. Is.

In this specification, the optical path length means a length converted in vacuum.

[0012]

According to the first interferometer device described above, in the Fizeau interferometer device, the difference in optical path length between the reference light from the reference surface and the object light from the surface to be inspected is calculated based on the distance between the two surfaces. An optical path difference adjusting optical system for canceling is provided, whereby the emission timings of the object light and the reference light, which overlap each other on the interference fringe formation surface, at the light source become substantially equal to each other, and a light source that emits light whose coherence is not so good In the case of using, it is possible to form an interference fringe with excellent definition on the interference fringe formation surface.

Further, by canceling the difference in the optical path length, there is also an advantage that unnecessary interference fringes formed by the reflected light from the surface of the reference plate opposite to the reference surface are reduced.

In the second interferometer device, the beam splitter and the two reflecting mirrors detour the reference light by an optical path length equal to the difference in optical path length based on the surface distance between the reference surface and the test surface. After that, the characteristics of the Michelson-type interferometer device are incorporated into the Fizeau-type interferometer device, so to speak. The optical path difference is canceled.

In the third interferometer apparatus, one of two reflecting mirrors of an optical path difference adjusting optical system for canceling the optical path difference between the object light and the reference light is a reflecting mirror such as a piezo element. By attaching a vibrating means and vibrating the reflecting mirror in the beam splitter direction, the optical path difference between the object light and the reference light on the interference fringe formation surface is slightly changed to change the interference fringes. Apply the fringe scanning method (also called the phase shift method, which is to detect the unevenness of the test surface by observing in which direction the stripes move when the phase difference between the reference light and the object light is changed) It becomes possible. In the conventional Fizeau interferometer device, the test surface or the reference surface was vibrated in the optical axis direction in order to apply the fringe scanning method, so the drive mechanism for vibration became large and the device became large. However, since the same effect can be obtained by vibrating the path-matching reflecting mirror in the device of the present invention, it is possible to reduce the size of the reflecting-mirror vibrating means and the vibration driving mechanism, and thus the size of the device.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing an interferometer device according to an embodiment of the present invention. This interferometer device includes a light source 1 which emits light with a short coherence length, a collimator lens 3 which converts the light 2 emitted from the light source 1 into parallel light 4, and a polarized light which reflects the parallel light 4 in a direction rotated by 90 °. A beam splitter 5, a beam expander 6 that expands the beam diameter of the reflected parallel light 4, and a semi-transparent mirror plane with extremely high flatness that is vertically irradiated with the parallel light 7 that has the expanded beam diameter. A reference plate 8 having a reference plane 8a on its lower surface, and a subject 9 having a subject surface 9a on which the remaining parallel light 7 of the light reflected from the reference plane 8a is irradiated are mounted and held.
A table 10 that can be adjusted so that 9a is parallel to the reference plane 8a is provided, and is used for observing the minute surface shape of the test surface 9a that is substantially flat.

Further, the reference light reflected from the reference plane 8a and the object light reflected from the surface 9a to be inspected are overlapped with each other to form return light which is transmitted through the beam expander 6 and the polarization beam splitter 5. The image forming lens 11 for forming an image of this return light and the image forming lens 11
The TV camera 12 has a light receiving surface on its focal plane and is used to observe the interference fringes formed by the object light and the reference light.

An optical path difference adjusting optical system 15 including a beam splitter 13 and two reflecting mirrors 14a and 14b is provided between the polarization beam splitter 5 and the imaging lens 11. Further, a quarter wavelength plate 20 for converting linearly polarized light into circularly polarized light is inserted at a position before the beam splitter 13.

Further, a polarizing plate 16 for converting the parallel light 4 into linearly polarized light is provided after the collimator lens 3, and a 1/4 is provided on the beam expander 6 side of the polarization beam splitter 5.
A wave plate 17 is provided.

Next, the operation of the apparatus of this embodiment will be described.

That is, the light 2 from the light source 1 is converted into parallel light 4 and then linearly polarized by the polarizing plate 16 and reflected downward by the reflection surface of the polarization beam splitter 5. The collimated light 4 is made into a collimated light 7 whose beam diameter is expanded by the beam expander 6, and becomes a reference light and an object light by being reflected on the reference plane 8a and the test surface 9a.

After that, the object light and the reference light overlap with each other and enter the quarter wavelength plate 17 through the beam expander 6. With respect to the parallel light before entering the quarter-wave plate 17, the object light and the reference light emitted from the quarter-wave plate 17 have their polarization planes rotated by 90 °. The reference light and the reference light are then linearly polarized light that passes through the reflecting surface of the polarization beam splitter 5. Next, when the linearly polarized light enters the optical path difference adjusting optical system 15, the linearly polarized light is circularly polarized by the quarter-wave plate 20, and both the linearly polarized light that is transmitted and the linearly polarized light that is reflected are generated in the beam splitter 13.

By the way, as described above, the reference plane
The optical path difference between the object light and the reference light when they enter the beam expander 6 is L based on the distance between the surface 8a and the surface 9a to be inspected.

The optical path difference adjusting optical system 15 cancels the optical path difference L between the object light and the reference light to form an interference fringe image on the light receiving surface of the TV camera 12 even with light having a small coherence. Is possible.

That is, the object light and the reference light that have entered the beam splitter 13 are split on the half mirror surface thereof into light that is transmitted and light that is reflected in the direction of the first reflecting mirror 14a, and then the first light is split. The light reflected from the reflection mirror 14a returns to the beam splitter 13. After this, the first
A part of the light from the reflection mirror 14a of the second reflection mirror 14a
It is irradiated to b, reflected by the second reflecting mirror 14b, and returned to the beam splitter 13 again. A part of the light reflected from the second reflecting mirror 14b is reflected by the half mirror surface of the beam splitter 13 and emitted toward the TV camera 12. Then, the light that first passes through the beam splitter 13 in the direction of the TV camera 12 and is reflected by the two reflecting mirrors 14a and 14b, travels back and forth between these mirrors 14a and 14b, and then from the beam splitter 13 in the direction of the TV camera 12. It is set so as to have an optical path difference exactly equal to the optical path length L with the emitted light. Therefore, first the beam splitter
Of the light transmitted through the TV camera 12 direction 13 through the object light,
The reference light of the light emitted in the direction of the TV camera 12 from the beam splitter 13 after one round trip between the two mirrors 14a and 14b is the optical path difference caused by the distance between the reference plane 8a and the test surface 9a. Since L is canceled and the optical path difference becomes almost 0, it is possible to form an interference fringe with high definition (visibility) on the light receiving surface of the TV camera 12 even with light having a small coherence.

The optical path difference between the reference light and the object light in the optical path difference adjusting optical system 15 is, as shown in FIG. 2, the distance from the first reflecting mirror 14a to the beam splitter 13 is L ₁ ,
When the distance from the second reflection mirror 14b to the beam splitter 13 is L ₂ , the width of the beam splitter 13 is L ₃ , and the refractive index of the beam splitter 13 is n, 2 (L ₁ + L ₂ + nL
₃ ), and the optical system 15 is positioned so that it is equal to the optical path length L which is twice the distance between the reference plane 8a and the surface 9a to be tested.

As the light source 1, it is possible to use a light source which emits light having a small coherence, and therefore, for example, a white lamp, a xenon lamp, an LED or a super luminescence diode (SLD) can be used.

However, since the amount of light is attenuated while the light travels in the optical path difference adjusting optical system 15 using the beam splitter 13 having such a half mirror surface, it is necessary to form a clear interference fringe. The reflectance on the reference plane 8a for generating the reference light is set high, and the light amount of the reference light is made larger than the light amount of the object light in the initial stage.

It is desirable that at least one of the two reflecting mirrors 14a and 14b is movable so that the length of the optical path for canceling the optical path difference can be finely adjusted.

Further, the reflection mirror other than the movable mirror may be a vapor deposition mirror produced by vapor-depositing a metal such as Al on the case.

Further, in the apparatus of the above embodiment, as shown in FIG. 1, the piezo element 18 is attached to the first reflecting mirror 14a (or the second reflecting mirror 14b can be used), and at a predetermined timing. The piezo element 18 vibrates in response to a sawtooth voltage signal of a predetermined frequency applied from the piezo element driving power source 19, and the first reflection mirror 14a vibrates in the optical axis direction (direction of the beam splitter 13). The interference fringes move due to this vibration, and by using the fringe scanning method in which the interference fringe image at this time is input to a computer and automatically analyzed, it becomes possible to accurately recognize the uneven shape of the test surface.

In the apparatus of this embodiment, since the piezo element 18 is attached to the reflection mirror 14a of the optical path difference adjusting optical system 15 to slightly change the optical path difference, the piezo element is attached to the reference plate 8 or the subject 9. Compared with the prior art in which the optical path difference is slightly changed, the driving power of the piezo element may be small, and the mechanical part related to the actual vibration (for example, the mirror holding mechanism) becomes simple. Further, since the reference plate 8 is large, a single piezo element has a cantilever structure, and the conventional problem that the flatness is deteriorated does not occur.

The apparatus of the embodiment of the present invention is not limited to the above-mentioned embodiment, and various modifications can be made.

For example, the optical path difference adjusting optical system may be any one as long as it detours the reference light by an optical path length corresponding to the above-mentioned optical path length L with respect to the object light. Good.

The optical path difference adjusting optical system 115 shown in FIG. 3 splits the light into two systems by the beam splitter 113a, reflects the reflected light by the two reflecting mirrors 114a and 114b, and then makes it enter the beam splitter 113b. This beam splitter 113b
In (2), the split light of two systems is combined, and the light of the detoured system is set to be longer than the light of the system that goes straight as much as the optical path length L.

Further, an optical path difference adjusting optical system 215 shown in FIG.
Is split into two systems by the beam splitter 213, and the reflected light is directly transmitted through the imaging lens 211 to the TV camera 212.
On the other hand, the transmitted light reflects the reflection mirror 214a and the beam splitter 21.
3, via reflection mirror 214b, beam splitter 213,
The light enters each of the TV cameras 212 via the imaging lens 211. At this time, the light transmitted from the beam splitter 213 is configured to detour by the optical path length L (converted into a vacuum) with respect to the light reflected and incident on the TV camera 212 as it is.

Further, in the apparatus of the above embodiment, the light is split by the half mirror surface of the beam splitter 13 of the optical path difference adjusting optical system 15. In this case, however, the amount of light contributing to the interference is divided as described above. Therefore, the polarization beam splitter 13a can be used as the spectroscopic means to reduce the reduction rate of the light quantity, as shown in FIG. In this case, as shown in FIG. 5, the polarization beam splitter 13a and each reflection mirror 14a,
The quarter-wave plates 21a and 21b are inserted between the positions b to switch the transmission and reflection of polarized light by the polarization beam splitter 13a.

Further, a quarter wavelength plate 20 and a quarter wavelength plate 21c are provided in the front and rear stages of the polarization beam splitter 13a, respectively.
Is provided.

That is, the linearly polarized reference light and the object light that have passed through the beam splitter 5 are incident on the quarter-wave plate 20 and the plane of polarization is rotated by 45 °, and then are incident on the polarizing beam splitter 13a. , Linearly polarized light that is transmitted and linearly polarized light that is reflected. The linearly polarized light reflected by the polarization beam splitter 13a is reflected by the reflection mirror 14a and returned to the polarization beam splitter 13a.
Since it will pass through the / 4 wave plate 21a twice, the polarization plane
Linearly polarized light rotated by 90 ° Polarization beam splitter 13a
Will be transmitted.

After that, the linearly polarized light transmitted through the polarization beam splitter 13a is reflected by the reflection mirror 14b and returned to the polarization beam splitter 13a again, but during this time, the linearly polarized light passes through the quarter-wave plate 21b twice, so that the polarized light is polarized. The surface is rotated by 90 ° to be linearly polarized light, which is reflected by the polarization beam splitter 13a toward the TV camera 212 for observing interference fringes.

The linearly polarized light reflected by the polarization beam splitter 13a is superimposed on the linearly polarized light that has been transmitted through the polarization beam splitter 13a as it is. Both of these two linearly polarized lights are a combination of reference light and object light,
The optical path lengths of the object light of the linearly polarized light that reciprocates between the two reflection mirrors 14a and 14b and the reference light of the linearly polarized light that has passed through the polarization beam splitter 13a as they are are equalized.

However, since the polarization planes of the reference light and the object light whose optical path lengths have been adjusted are deviated from each other by 90 °, the polarization planes of the reference light and the object light are rotated by the 1/4 wavelength plate 21c, and the polarization components in the same direction are rotated. Is generated, and the reference light and the object light are caused to interfere with each other by this polarization component.

In the above-mentioned embodiment, the description has been made on the case of observing the surface shape of the substantially flat surface to be inspected. Even in the case of an elliptical surface, a hyperboloidal surface, a paraboloidal surface, a cylindrical surface, etc., it is applicable by disposing a reference plate according to the shape of the surface to be inspected at a predetermined position.

For example, in the case of observing a surface to be inspected, which has a substantially spherical concave shape, a device having a device configuration as shown in FIG. 6 is used. That is, as the reference lens 118, a lens having a positive refracting power with a reference surface formed of a concave surface facing the surface 119a to be tested is used.

The reference surface 118a is formed as a semi-transparent mirror spherical surface in which a highly accurate concave spherical surface is vapor-deposited with a reflective film having a predetermined reflectance.

Parallel light 4 incident on the reference lens 118
Is converted by this reference lens 118 into a light flux which is converged at the point O and then becomes divergent light and is incident on the surface 119a to be tested. It should be noted that the subject 11 having the test surface 119a formed of a substantially spherical concave surface
The reference numeral 9 is mounted and held on a table 10 capable of adjusting the alignment between the subject 119 and the reference lens 118.

The reference lens 118 and the subject 119 are
The reference surface 118a and the test surface 119a are arranged so as to be located on a spherical surface having the converging point O as a spherical center.

As a result, the divergent light emitted to the surface 119a to be inspected is reflected by the surface 119a to be inspected, becomes convergent light and converges at the point O, and then becomes divergent light and re-enters the reference surface 118a. To do.
That is, the light flux traveling from the reference surface 118a to the test surface 119a and the light flux reflected by the test surface 119a and traveling from the test surface 119a to the reference surface 118a (object light) overlap with each other, whereby the reference surface The reference light formed by the reflection at 118a and the object light re-incident on the reference surface 118a overlap each other, and the traveling directions of these two lights can be matched.

Also in this case, the optical path difference adjusting optical system 15
In the reference light, the reference light has a distance of 2 between the reference surface 118a and the surface 119a to be measured.
The TV camera is detoured by an optical path length L equivalent to double
On the light receiving surface of 12, the reference light and the object light whose optical path lengths are aligned form an interference fringe representing the surface shape of the surface 119a to be tested.

Further, in the above embodiment, the piezo element is used as the reflecting mirror vibrating means for vibrating the reflecting mirror, but other vibrating elements can be used.

[0052]

As described above, according to the interferometer device of the present invention, the Fizeau device is used to cancel the optical path difference between the light from the reference surface and the light from the surface to be inspected. Therefore, it is possible to obtain an interference fringe image with high definition even with light having a small coherence.

Further, if the reflecting mirror vibrating means is attached to the reflecting mirror of the optical path difference adjusting optical system and the optical path difference is slightly changed by vibrating this reflecting mirror, the fringe scanning method can be realized with a simple structure and a small driving power. Interference fringe analysis becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an interferometer device according to an embodiment of the present invention.

FIG. 2 is a schematic view showing an enlarged part of the interferometer device shown in FIG.

FIG. 3 is a schematic diagram showing a modification of an optical path difference adjusting optical system of the interferometer device shown in FIG.

FIG. 4 is a schematic view showing another modification of the optical path difference adjusting optical system of the interferometer device shown in FIG.

5 is a schematic view showing another modification of the optical path difference adjusting optical system of the interferometer device shown in FIG.

FIG. 6 is a schematic view showing an embodiment apparatus different from the embodiment apparatus of FIG.

FIG. 7 is a schematic diagram showing a conventional interferometer device.

[Explanation of symbols]

1,101 light source 2,102 light 5,13a, 106 Polarizing beam splitter 8,109 reference plate 8a, 109a Reference plane 9,110,119 subjects 9a, 110a, 119a Test surface 12,111,212 TV cameras 13, 113a, 113b, 213 Beam splitter 14a, 14b, 114a, 114b, 214a, 214b Reflection mirror 15, 115, 215 Optical path difference adjustment optical system 17, 20, 21a, 21b 1/4 wave plate 18 Piezo element 19 Piezo element drive power supply 118 Reference lens 118a Reference plane

Claims (3)

(57) [Claims] 1. Coherent light is made incident on a test surface of a subject through a reference surface of a reference plate, and object light generated by reflection of the coherent light on the test surface and the reference surface In an interferometer device that forms an interference fringe caused by optical interference with the reference light generated by reflection on a predetermined interference fringe forming surface, the object light and the reference light superposed with the object light are separated from each other, Thereafter, by the time the object light and the reference light are overlapped again, the reference light has an optical path length L corresponding to twice the distance between the reference surface and the test surface as compared with the object light.
An interferometer device characterized in that an optical path difference adjusting optical system for guiding the object light and the reference light is arranged between the reference plate and the interference fringe formation surface so as to have a long optical path length. 2. A beam splitter in which the optical path difference adjusting optical system transmits one of the object light and the reference light and reflects the other in a lateral direction, and the reference light emitted from the beam splitter. A first reflecting mirror that reflects the reference light toward the beam splitter, and the reference light that is reflected by the first reflecting mirror and is emitted from the beam splitter. A second reflecting mirror for reflecting in the direction of the beam splitter, and a second reflecting mirror arranged at positions separated from each other on the optical path by an optical distance equal to ½ of the optical path length L. The reference light reflected by the reflecting mirror and emitted from the beam splitter overlaps the object light emitted from the beam splitter. Interferometer according to claim 1, wherein that. 3. A reflecting mirror vibrating unit for driving at least one of the first and second reflecting mirrors so that the at least one reflecting mirror vibrates in the beam splitter direction. The interferometer device according to claim 2.