JP3795191B2 - Interferometer for transparent thin plate measurement - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an interferometer for interferometric measurement of the surface shape of a transparent thin plate such as liquid crystal glass, various optical filters and windows, and more particularly to a transparent thin plate measuring interferometer provided with a path match optical system that bypasses a part of measurement light. Is.
[0002]
[Prior art]
In general, in a Fizeau interferometer, it is necessary to increase the distance between the reference surface and the test surface to some extent in order to set the subject. Therefore, the light from the light source is at least twice the coherence. It is necessary to have a distance. However, for this reason, the interference fringes generated by the reflected light from the back of the subject whose optical path difference with the reflected light from the subject surface is twice the thickness of the subject are noise interference fringes on the original interference fringes. There is a problem of overlapping.
[0003]
As a measure for solving this problem, the applicant of the present application has already disclosed a transparent thin plate measuring interferometer including a path match optical system capable of preventing the generation of interference fringes due to reflected light from the back surface of a subject.
That is, this interferometer sets the coherence distance Lc of the output light from the light source to be smaller than twice the optical distance nt in the thickness direction of the transparent thin plate, and separates a part of the light toward the transparent thin plate. , After diverting to have an optical path length larger than the remaining light by a distance corresponding to twice the distance L (where L >> nt) between the reference surface and the test surface, A path-matching optical system for combining with light is provided.
[0004]
By adopting such a configuration, interference fringes are not generated except by the mutual interference between the reflected light from the test surface of the measurement light that has traveled straight without bypassing and the reflected light from the reference surface of the bypassed measuring light. As a result, it is possible to prevent the generation of interference fringes that are noises caused by the reflected light from the back surface of the subject.
[0005]
[Problems to be solved by the invention]
However, when performing observation and measurement of a transparent thin plate using this interferometer, there is a desire to improve the accuracy of interference fringe observation measurement by increasing the contrast of the interference fringe image formed on the imaging surface. There is. In order to satisfy this demand, it is a problem to prevent the above-described interferometer from superimposing reflected light that does not cause optical interference as background light on reflected light that causes optical interference.
[0006]
That is, in the interferometer, as described above, the interference fringes generated by the mutual interference between the reflected light from the test surface of the measurement light that has traveled straight without bypassing and the reflected light from the reference surface of the bypassed measurement light are generated. The surface to be measured is evaluated by observation and measurement. However, in the observation system, there are other reflected light from the reference surface and the back of the subject that has traveled straight without detouring, as well as reflected light from the test surface and the back of the subject that has been bypassed. Since it is incident and used as background light, the entire image becomes bright, and the contrast of interference fringes decreases. If the contrast of the interference fringes is lowered, the accuracy of observation measurement is lowered, and in particular, it becomes difficult to use an analysis apparatus that automatically analyzes the shape of the test surface from the interference fringes.
[0007]
The present invention has been made in view of such circumstances, and only the reflected light necessary for measuring the interference fringe image on the test surface is incident on the observation system and the other reflections are made. An object of the present invention is to provide a transparent thin plate measuring interferometer capable of preventing light from entering the observation system as background light of reflected light for measurement and obtaining an interference fringe image having a good contrast with a simple configuration. It is what.
[0008]
[Means for Solving the Problems]
In the interferometer for measuring a transparent thin plate of the present invention, the coherence distance of light emitted from the light source is set to be smaller than a distance corresponding to twice the thickness of the subject made of the transparent thin plate, and is directed to the subject. A part of the light is separated by the first light separation means, and the part of the light is larger than the remaining light by a distance corresponding to twice the distance between the reference surface and the test surface. A path-matching optical system that is detoured to have a length and then combined with the remaining light by a light combining means; a first reflected light that is a reflected light from the reference plane of the combined light; Transparent that measures the surface shape of the test surface based on interference fringes generated by optical interference of the combined light that has passed through the reference surface and second reflected light that is reflected light from the test surface A thin plate measuring interferometer,
In the optical path from the light source to the first light separating means, a first polarizing means that transmits only a polarized light component having a predetermined vibration surface out of the light from the light source is disposed,
A λ / 2 optical phase plate is disposed in the optical path of one of the part of light and the remaining light from the first light separating unit to the light combining unit;
A λ / 4 optical phase plate is disposed between the reference plate on which the reference surface is formed and the subject,
A second light separating means for guiding the first and second reflected lights to an observation system between the light source and the reference plate;
Between the second light separating means and the observation system, a predetermined vibration surface constituting the first reflected light by the part of light and the second reflected light by the remaining light is provided. The second polarizing means that transmits only the polarization component having the light is disposed.
[0009]
Further, the λ / 4 optical phase plate can be formed in close contact with the surface of the reference plate on the subject side.
The path matching optical system reflects a part of the light from the light source and transmits the remaining light, and the light that reflects the reflected light or transmitted light from the light separating means at least once. It is desirable to comprise a reflecting member and light combining means for combining the transmitted light or reflected light from the light separating means and the reflected light from the light reflecting member.
[0010]
Furthermore, it is more desirable to dispose a wavelength selection filter that selectively transmits light of a predetermined wavelength between the light source and the path match optical system.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic view showing an embodiment of the transparent thin plate measuring interferometer of the present invention. In this interferometer, the measurement light output from the light source 1 that outputs light having a wide-band wavelength component such as white light is converted into parallel light by the collimator lens 2 and the beam diameter is expanded by the condenser lens 8 and the collimator lens 10. Then, the transparent reference plate 11 and the thin glass object 12 are irradiated.
[0012]
The reflected light of the measurement light from the reference surface 11 a of the reference plate 11 and the test surface 12 a of the subject 12 interfere with each other, and is reflected at right angles by the half mirror surface 9 a of the half prism 9, and passes through the imaging lens 13. Then, an interference fringe image is formed on the CCD element in the imaging camera 14. An interference fringe image is displayed on an image display unit (not shown) such as a CRT based on the interference fringe image information photoelectrically converted by the CCD element.
By the way, an object of the present invention is to observe and measure the interference fringe image in a better contrast state. First, an interferometer having a path match optical system, which is the premise thereof, will be described.
[0013]
In general, when a subject 12 made of thin glass is measured using a Fizeau interferometer, the interval between the reference surface 11a and the subject surface 12a is inevitably increased for convenience of setting. Therefore, an interference fringe image is obtained using measurement light having a large interference distance. However, the reflected light from the test surface 12a has an optical path length twice as long as the optical distance nt in the thickness direction of the test object 12. The reflected light from the back surface 12b of the subject 12 not to interfere with the reflected light from the reference surface 11a and the reflected light from the test surface 12a. The interference fringes generated by the reflected light from the back surface 12b of the subject 12 are superimposed on the original interference fringes to reduce the measurement accuracy.
[0014]
However, in an interferometer having a path-matching optical system, it is possible to remove the interference fringes caused by the reflected light from the back surface of the subject and obtain only the original interference fringe image. In general, in such an interferometer, as described above, when the interference distance of the measurement light is large, noise interference fringes are generated.
Therefore, in order to prevent the occurrence of interference fringes due to the reflected light from the back surface of the subject, light having a coherence distance Lc of less than 2 nt is used as the measurement light (specifically, the light source 1 is halogenated). A lamp (coherence distance is 1 μm) is used.) On the other hand, in order to obtain the necessary interference fringes, the light reflected from the test surface and incident on the observation system and the light reflected from the reference surface are reflected in the observation system. It is necessary to make the difference in the optical path length from the light source to the observation system in each light incident on the light beam equal to or less than the coherence distance Lc.
[0015]
In order to make the optical path lengths of the two reflected lights equal, a path matching optical system is arranged. The path-matching optical system according to the present embodiment is an example of measurement light including two half mirrors 4 and 5 and two total reflection mirrors 6 and 7 in a parallel beam bundle region between the collimator lens 2 and the condenser lens 8. In the optical path that bypasses the part, a part of the measurement light is reflected by the half mirror 4 at a right angle and separated from the remaining measurement light, reflected by the total reflection mirror 6 at a right angle, and then reflected by the total reflection mirror 7 at a right angle, Further, the light is reflected at right angles by the half mirror 5 and recombined with the remaining measurement light. At this time, the optical path length L of the measurement light passing through the half mirror 4 from the half mirror 4 to the half mirror 5 with respect to the optical path length L ₁ between the half mirror 4 and the half mirror 5 is measured. ₂ (L ₂ = l ₁ + l ₂ + l ₃ ) is equal to the distance l ₁ + l ₃ (l because the distance L ₁ between the two half mirrors 4 and 5 and the distance between the two total reflection mirrors 6 and 7 are equal. ₁ = l ₃ ). These distances l ₁ + l ₃ correspond to twice the distance L between the reference surface 11a of the reference plate 11 and the test surface 12a of the subject 12, so that the measurement light travels straight between the two half mirrors 4 and 5. The optical path lengths of the reflected light from the test surface 12a and the reflected light from the reference surface 11a of the measurement light bypassing between the two half mirrors 4 and 5 are completely equal, and the two reflected lights interfere with each other. Arise.
[0016]
By the way, although the interferometer having such a path-matching optical system has improved the accuracy of observation measurement by removing the interference fringe image that becomes noise, the contrast of the interference fringe image formed on the imaging surface is more improved. There is a desire to increase the accuracy of observation and measurement of interference fringes. That is, in the interferometer, as described above, the interference fringes generated by the mutual interference between the reflected light from the test surface of the measurement light that has traveled straight without bypassing and the reflected light from the reference surface of the bypassed measurement light are generated. The observation surface is evaluated by observation measurement. In addition to this, the observation system also includes a reference beam of measurement light that has traveled straight without detouring, reflected light from each back surface of the subject, and measurement light subject to detouring. The next problem is that the reflected light from each of the inspection surface and the back surface of the subject is incident. Since these reflected lights have an optical path difference larger than the coherence distance of the measurement light with respect to the reflected lights causing mutual interference, noise interference fringes do not occur, but background light of interference fringe images. Therefore, the entire image becomes bright, and the contrast of interference fringes decreases. The contrast of the interference fringes affects the accuracy of observation and measurement. In particular, in order to use an analysis device that automatically analyzes the shape of the test surface from the interference fringes, it is desirable to obtain an image with good contrast.
[0017]
Therefore, in the present embodiment, first, the first polarizing plate 21 that transmits the P-polarized light is disposed between the light source 1 and the half mirror 4. The P-polarized measurement light transmitted through the polarizing plate 21 is separated by the half mirror 4, and part of the light is reflected at right angles by the half mirror 4 to bypass the path-matching optical system, and the remaining measurement light is transmitted from the half mirror 4 to the half mirror. Go straight to 5. The light traveling straight reaches the half mirror 5 with P polarization. On the other hand, since the λ / 2 optical phase plate 22 is inserted in the optical path of the path-matching optical system, the detoured light is converted into S-polarized light, and then reaches the half mirror 5. It is reflected at right angles and recombined with the remaining measurement light.
[0018]
The reflected light obtained by reflecting the straight P-polarized light and the S-polarized light passing through the path-matching optical system from the surfaces 11a and 12a will be considered. In the following, the optical path length from the half mirror 5 to the reference surface 11a of the reference plate 11 is set to L _3, the refractive index of the specimen 12 is n, the thickness is set to t.
[0019]
Here, the reflected light from the reference surface 11a has the following optical path length (however, the optical path length from the half mirror 4 to the reference surface 11a through the surfaces 11a and 12a; the same applies hereinafter) and the polarization component. This is a combined light of two reflected lights.
Reflected light of light transmitted through the half mirror 4-optical path length L ₁ + L ₃ -P-polarized light (1)
Reflected light of light reflected by the half mirror 4 —optical path length L ₂ + L ₃ —S-polarized light (2)
[0020]
Next, the reflected light from the test surface 12a is converted into S-polarized light and S-polarized light into P-polarized light by the λ / 4 optical phase plate 23 inserted between the reference plate 11 and the test plate 12, respectively. Since it is converted, it becomes a combined light of two reflected lights having the following optical path length and polarization component. Reflected light of light transmitted through the half mirror 4-optical path length L ₁ + L ₃ + 2L -S-polarized light (3)
Reflected light of the light reflected by the half mirror 4—optical path length L ₂ + L ₃ + 2L—P polarized light (4)
[0021]
Here, from the nature of the path-matching optical system,
L ₂ = L ₁ + 2L (5)
It is. Therefore, when the optical path lengths of the reflected lights are compared, the values of the reflected light (2) and the reflected light (3) are exactly the same, that is, from the reference surface 11a of the measuring light that bypasses between the two half mirrors 4 and 5. The optical path lengths of the reflected light from the test surface 12a of the measurement light that has traveled straight between the two half mirrors 4 and 5 are exactly the same, and the polarization components thereof coincide with each other.
[0022]
On the other hand, as is apparent from the optical path lengths of the reflected light (1) and the reflected light (4), the two reflected lights of P-polarized light are related to both the optical path difference from the other reflected lights and the coherence distance of the measuring light. Therefore, an interference fringe is not generated with other reflected light, and the contrast of the interference fringe image is simply lowered as background light. Therefore, in the apparatus of the present embodiment, the second polarizing plate 24 that transmits S-polarized light is disposed in front of the imaging camera 14 to block P-polarized light that is background light and to form a desired interference fringe image. Only the two S-polarized lights that contribute to the light are incident on the imaging camera 14.
[0023]
In this way, desired interference fringes with good contrast can be formed on the CCD element in the imaging camera 14, and the surface shape of the thin glass can be measured with high accuracy.
[0024]
In the above embodiment, the λ / 2 optical phase plate 22 is disposed in the path match optical path between the half mirrors 4 and 5, but instead of this, the light transmitted through the half mirror 4 as shown in FIG. It is also possible to arrange the λ / 2 optical phase plate 22a in this path.
In such a configuration, the reflected light carrying the necessary information of the interference fringe image (the reflected light (2), (3)) becomes P-polarized light, while the reflected light (background) Since the reflected lights (1) and (4)) are S-polarized light, in order to prevent the background light from entering the imaging camera, the second polarizing plate 24 is changed to a polarizing plate that transmits P-polarized light. It needs to be replaced.
[0025]
In the above description of the series of embodiments, a polarizing plate that transmits P-polarized light is used as the first polarizing plate 22. However, as a matter of course, a polarizing plate that transmits S-polarized light is used as the first polarizing plate 22. In this case, it is necessary to set the second polarizing plate 24 so as to have a transmission characteristic corresponding to the second polarizing plate 24.
[0026]
Further, in the embodiment shown in FIG. 1, the λ / 4 optical phase plate 23 is arranged between the reference plate 11 and the test plate 12, but as shown in FIG. A film-like λ / 4 optical phase plate 23a can be formed in close contact with the side. The film in this case can be formed using various known film forming techniques such as vapor deposition and sputtering.
By forming the λ / 4 optical phase plate 23a on the surface of the reference plate 11, there is no need to provide a special holding means for the phase plate 23a, and the alignment of the phase plate 23a can be adjusted when the optical system is assembled. It can be unnecessary.
[0027]
Further, since the thickness usually varies depending on the subject 12, in the path match optical system, the two total reflection mirrors 6 and 7 are finely moved integrally in the direction of the half mirrors 4 and 5 to slightly reduce the optical path length of the detour measurement light. It should be adjustable.
[0028]
In the present embodiment, the wavelength selection filter plate 3 is disposed between the collimator lens 3 and the half mirror 4.
As shown in FIG. 4, the wavelength selection filter plate 3 has a full-wavelength transmission part 3a, a red light selective transmission part 3b, a green light selective transmission part 3c, and a blue light selective transmission part 3d formed at 90 ° intervals on the turret plate. Thus, the desired color light can be selected as the measurement light by rotating the turret plate by a predetermined angle.
[0029]
This is useful when it is necessary to select light having a predetermined wavelength as measurement light, such as when the subject 12 is a dichroic mirror that reflects light having a predetermined wavelength.
Of course, when such a wavelength selection filter is not required at all, the filter plate 3 may be moved in the direction of arrow A in FIG.
[0030]
The transparent thin plate measuring interferometer of the present invention is not limited to the above embodiment, and various other modifications can be made. For example, in the path match optical system, instead of the two total reflection mirrors 6 and 7, a corner cube that can return the measurement light from the half mirror 4 toward the half mirror 5 can be used.
By adopting such a corner cube, when the optical path length of the detour measurement light is adjusted, the moving operation becomes easy.
Moreover, in the said embodiment, it is also possible to replace both so that the transmitted light of half-mirror-4 may be detour measurement light and the reflected light may be straight measurement light.
[0031]
The supporting means for the subject 12 may be structured to be fixedly held when supported on the test surface 12a side, but depending on the thickness of the subject 12 when supported on the back surface 12b side. It is desirable that the test object 12 has a structure that can move in the direction of the optical axis. Further, based on the thickness information of the test object 12 when obtaining interference fringes, the test surface 12a is moved to an appropriate position. It is desirable to automatically perform the movement operation of the subject 12.
Further, a half mirror can be used in place of the half prism 9, but astigmatism is improved by using the half prism 9 in the divergent beam as in this embodiment. Can do.
[0032]
Of course, each of the half mirrors 4 and 5 can be replaced with a half prism.
The subject of the interferometer of the present invention can be applied not only to a glass thin plate but also to various transparent thin plates such as a plastic plate and a quartz plate.
Further, in the above embodiment, the imaging camera 14 is used as an observation system. However, instead of this, an optical screen having a diffusing surface may be provided, and an interference fringe image may be formed on the screen and observed visually. It is.
[0033]
【The invention's effect】
As described above, according to the transparent thin plate measurement interferometer of the present invention, the transparent thin plate measurement is provided with a path match optical system that sets the coherence distance of the measurement light to be less than a predetermined length and bypasses a part of the measurement light. In the interferometer, two polarizing plates and two optical phase plates are arranged at predetermined optical path positions, and two light beams necessary for obtaining an interference fringe image of the test surface are either the first P or S first. In addition to the linearly polarized light component, the light beam that becomes background light other than this is used as the second linearly polarized light component of either P or S, and only the first polarized light component is incident on the observation system. Therefore, it is possible to obtain an interference fringe image with good contrast with a simple configuration. Thereby, the measurement of the surface shape of the test surface can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an interferometer for measuring a transparent thin plate according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing a modification of the embodiment shown in FIG. 1. FIG. 3 is an implementation shown in FIG. FIG. 4 is a schematic view showing a partial modification of the form. FIG. 4 is a front view showing the wavelength selective filter plate shown in FIG.
DESCRIPTION OF SYMBOLS 1 White light source 2,10 Collimator lens 3 Wavelength selection filter plate 4,5 Half mirror 6,7 Total reflection mirror 9 Half prism 11 Reference plate 11a Reference surface 12 Subject 12a Test surface 12b Back surface 14 Imaging camera 21 First polarization Plate 22, 22a λ / 2 optical phase plate 23, 23a λ / 4 optical phase plate 24 Second polarizing plate

Claims (4)

The coherence distance of the light emitted from the light source is set to be smaller than a distance corresponding to twice the thickness of the subject made of a transparent thin plate, and a part of the light traveling toward the subject is the first light. After separating by the separating means and diverting the part of the light so as to have an optical path length larger than the remaining light by a distance corresponding to twice the distance between the reference surface and the test surface, A path-matching optical system for combining with the remaining light by means, the first reflected light of the combined light reflected from the reference surface, and the combined light transmitted through the reference surface In the interferometer for measuring a transparent thin plate based on interference fringes generated by optical interference with second reflected light that is reflected light from the test surface,
In the optical path from the light source to the first light separating means, a first polarizing means that transmits only a polarized light component having a predetermined vibration surface out of the light from the light source is disposed,
A λ / 2 optical phase plate is disposed in the optical path of one of the part of light and the remaining light from the first light separating unit to the light combining unit;
A λ / 4 optical phase plate is disposed between the reference plate on which the reference surface is formed and the subject,
A second light separating means for guiding the first and second reflected lights to an observation system between the light source and the reference plate;
Between the second light separating means and the observation system, a predetermined vibration surface constituting the first reflected light by the part of light and the second reflected light by the remaining light is provided. An interferometer for measuring a transparent thin plate, characterized in that second polarizing means for transmitting only the polarization component having it is arranged. 2. The transparent thin plate measuring interferometer according to claim 1, wherein the λ / 4 optical phase plate is formed in close contact with the surface of the reference plate on the subject side. The path-matching optical system reflects part of the light from the light source and transmits the remaining light, and light reflection that reflects the reflected light or transmitted light from the light separating means at least once. 3. The transparent thin plate measuring interferometer according to claim 1, comprising: a member; and the light combining means for combining the transmitted light or reflected light from the light separating means and the reflected light from the light reflecting member. . The transparent thin plate measurement according to any one of claims 1 to 3, wherein a wavelength selection filter that selectively transmits light having a predetermined wavelength is disposed between the light source and the path match optical system. Interferometer.

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