JPH10221009A - Interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interference fringe excellent in contrast by simple operation even in the case where the reflection ratio of a test face is low by adjusting the incident angle of irradiation light for a parallel flat plate so that the parallel flat plate for light amount adjustment in which light permeability characteristics change in response to the incident angle of irradiation light is disposed at a prescribed position in an optical path, and the light amount rates of two pieces of return light from the test face and a reference face are substantially equal. SOLUTION: A parallel flat plate 8 is constituted of transparent material such as a glass plate, and a dichroic film in which light transmissivity is changed for irradiation light in response to the incident angle of light is formed on the face of the side of the beam splitter 3. The dichroic film has red color light transmissivity characteristics so that an irradiation light incident angle is 5-18 degrees and abruptly changes from 95% to 0%. The parallel flat plate 8 is inclined to adjust the incident angle so as to lessen an amount of light irradiating a reference face 5a, the light amounts of return light from a test face 4a and the reference face 5a are made substantially equal, and the contrast of an interference fringe is made good.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for measuring a surface shape of a surface to be inspected by using an interference fringe formed by an interference effect of light.

[0002]

2. Description of the Related Art Conventionally, various interferometer devices for measuring the surface shape of an object surface are known. In general, an interferometer apparatus irradiates irradiation light from a light source to a test surface and a reference surface, which are respectively arranged at predetermined positions, and causes return light reflected from the test surface and the reference surface to interfere with each other. By observing the interference fringes generated by the measurement, the surface shape of the test surface is measured.

In order to obtain interference fringes with good contrast in an interferometer device, it is necessary to adjust the amount of light reflected from the surface to be measured and the amount of light reflected from the reference surface to be substantially equal. Further, in recent years, the environment for measuring a test surface having a low reflectivity has been improved with the progress of external light shielding technology and optical noise reduction technology of the interferometer device itself. Attempts have been made to measure using an interferometer device.

[0004]

However, in the interferometer device, light interference is caused by light reflected from the surface to be measured and light reflected from the reference surface. In the case of the ratio, the light amount of the reflected light from the surface to be measured is very small, so that it is difficult to adjust the light amount of the reference light to this small light amount, and it is desired to easily obtain interference fringes having good contrast. I was

The present invention has been made in view of such circumstances, and when measuring the surface shape of a surface to be measured using an interferometer, the amount of light reflected from the surface to be measured and the amount of light reflected from the reference surface are measured. It is an object of the present invention to provide an interferometer device that can easily match the amount of reflected light.

In a so-called dual-arm type device such as a Michelson interferometer device, it is necessary that various optical effects on a light beam in a two-system optical path be substantially equal to each other. .

Therefore, the present invention provides a so-called dual-arm type device such as a Michelson interferometer device when the amount of reflected light from the surface to be measured and the amount of reflected light from the reference surface are adjusted to match. It is another object of the present invention to provide an interferometer device which can adjust various optical effects exerted on a light beam in a two-system optical path so as to be substantially equal to each other.

[0008]

The interferometer device of the present invention comprises:
Irradiation light from a light source is applied to a test surface and a reference surface arranged at predetermined positions, and the return light reflected from the test surface and the reference surface interfere with each other, and interference fringes generated by this interference are observed. In the interferometer device, the light beam irradiated on the surface to be inspected and the light beam irradiated on the reference surface do not overlap each other. A parallel plane plate for adjusting the amount of light with a transparent and uniform refractive index whose transmittance characteristic changes is provided, and the incident angle of the parallel plane plate is set so that the ratio of the amounts of light of the two return lights can be made substantially equal. An adjustment mechanism is provided.

Further, the interferometer apparatus of the present invention separates the irradiation light from the light source into two systems, a test surface irradiation system for irradiating the test surface, and a reference surface irradiation system for irradiating the reference surface. In the interferometer apparatus for causing the return light reflected from the test surface and the reference surface to interfere with each other in these two systems and observing interference fringes generated by the interference, at least one of the two systems includes: A transparent flat plate for adjusting the amount of light with a transparent and uniform refractive index whose light transmittance characteristic changes according to the incident angle of the separated irradiation light is provided, and the light amount ratio of the two return lights is made substantially equal. A mechanism for adjusting the angle of incidence of the plane-parallel plate is provided to obtain the angle.

Also, an optical compensator for disposing the light quantity adjusting parallel plane plate in one of the two systems, and compensating for the optical influence of the parallel plane plate in the other system. Is preferably arranged. Further, the optical compensator is transparent and has a refractive index that substantially matches the optical path length of the irradiation light of the other system with the optical path length of the irradiation light of the other system on which the parallel plane plate is disposed. Such a parallel plane plate for adjusting the optical path length or a compensating plate for compensating chromatic dispersion can be used.

It is preferable to provide a mechanism for changing the incident angle by interlocking the two planes so that the incident angles of the irradiation light to the parallel plane plate and the optical compensating plate substantially coincide with each other. . Further, it is preferable that an incident angle of the parallel light plate for adjusting the light amount in the initial state with respect to the irradiation light is set near an angle at which a change rate of the light transmittance characteristic is maximum.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing an interferometer apparatus according to one embodiment of the present invention, which is applied to a Michelson interferometer.

This Michelson interferometer has a light source 1,
It comprises a collimator lens 2, a beam splitter 3, an object 4, a reference plate 5, an imaging camera 7, a parallel plane plate 8 for adjusting a light amount, and a parallel plane plate 9 for adjusting an optical path length. I have.

The light source 1 is a light source that emits light having a short coherent distance. The light emitted from the light source 1 is collimated by a collimator lens 2 and is referred to as a test light by a beam splitter 3. It is divided into light and light. The test light and the reference light are applied to the test surface 5 of the test object 5.
After being reflected on the reference surface 4a of the reference plate 4 and the reference plate 4a, the beams are combined again by the beam splitter 3,
It is configured to form interference fringes on the imaging plane of D.

Incidentally, in the interferometer apparatus, the subject 5
If the reflected light from the test surface 5a and the reflected light from the reference surface 4a of the reference plate 4 are not adjusted to be substantially equal, it is difficult to obtain interference fringes with good contrast.
Therefore, in the present embodiment, a plane-parallel plate 8 for adjusting the amount of light is arranged between the beam splitter 3 and the reference plate 5.

The plane-parallel plate 8 is made of a transparent material having a uniform refractive index such as a glass plate, and its surface on the side of the beam splitter 3 receives the irradiation light in accordance with the incident angle of the light. A dichroic film for changing the light transmittance is formed, while an antireflection film is formed on the subject 5 side. In this embodiment, since the red light LED is used as the light source 1, the dichroic film also has the final transmission when the light of the wavelength of 633 nm has passed twice in total in the reciprocation with respect to the incident angle of the red light. Having a light transmittance characteristic as shown in FIG. That is,
This dichroic film has a red light transmittance of about 95% at about 0 ° to 5 °, and a red light transmittance of about 0% at about 18 ° or more, and rapidly changes from 95% to 0% at 5 ° to 18 °. Red light transmittance.

Therefore, when the test surface 4a of the subject 4 is an object surface having a low reflectivity and the amount of light is smaller than the light reflected from the reference surface 5a of the reference plate 5, this parallel flat plate 8
Is tilted so that the incident angle with respect to the incident light beam is increased, so that the amount of light irradiated on the reference surface 5a is reduced. Thereby, the contrast of the interference fringes can be improved by making the light amounts of the return light from these two surfaces 4a and 5a substantially equal. Since an antireflection film is formed on the parallel flat plate 8 on the subject 5 side, the return light from the reference surface 5a is not attenuated on this surface.

In the apparatus of this embodiment, a parallel flat plate 9 for adjusting the optical path length is provided between the beam splitter 3 and the subject 4. The plane-parallel plate 9 is made of a transparent and uniform-refractive-index plate material such as a glass plate having an anti-reflection film formed on both surfaces thereof. The product of the refractive index and the thickness is the same as that of the plane-parallel plate 8. It has become. Therefore, the amount of light does not attenuate when the irradiation light passes through the plane-parallel plate 9, but the same phase delay occurs as when the irradiation light passes through the plane-parallel plate 8. As a result, a phase delay similar to that caused by the insertion of the parallel plane plate 8 in the reference surface irradiation system also occurs in the test surface irradiation system. The parallel plane plate 9 for adjusting the optical path length compensates for a change in chromatic dispersion caused by a difference in the refraction angle depending on the wavelength when the parallel plane plate 8 for adjusting the light amount is inclined with respect to the optical axis. It also has the function of performing

The two parallel flat plates 8, 9
Are rotated and adjusted by a mechanism as shown in FIG. 3 so that they always have the same incident angle with respect to the irradiation light. That is, the parallel flat plate 8 is
The rotation center 8a of the gear 8 and the plane-parallel plate 9 are fixedly provided at the rotation center 9a of the gear 19 and substantially at the center thereof. Since these gears 18 and 19 have the same number of teeth and mesh with the driving gear 20, they rotate by the same angle in accordance with the rotation of the driving gear 20 (the direction of arrow A and the direction of arrow B). ), Whereby the two parallel flat plates 8 and 9 also rotate by the same angle with respect to each other.

Therefore, in the initial setting, these two
If the angles of incidence of the irradiation light on the two parallel plane plates 8 and 9 are adjusted to be the same, even if the driving gear 20 is thereafter rotated about the rotation center 20a, the parallel plane plates 8 and 9 are always maintained. Keeps the same incident angle with respect to the irradiation light, the optical path lengths of the two return lights can be strictly matched, and the chromatic dispersion caused by tilting the parallel plane plate 8 can be improved. It is possible to compensate.

Furthermore, if the initial setting positions of the parallel plane plates 8 and 9 are set as angular positions where the change in light transmittance is large, the transmittance can be largely changed by slightly rotating the parallel plane plates 8 and 9. It is preferable. In the present embodiment, since the parallel plane plate 8 has the light transmittance characteristic as shown in FIG. 2, the incident angle 1 at which the light transmittance becomes approximately 50% is obtained.
A position around 1.5 degrees may be set as the initial setting position.

The interferometer device of the present invention is not limited to the above-described embodiment, and for example, the positions of the two parallel plane plates can be interchanged. When the reflectance of the test surface is higher than the reflectance of the reference surface, it is effective to replace the two parallel flat plates in this way.

Also, the parallel plane plate for adjusting the light quantity may be any transparent and uniform refractive index whose light transmittance characteristics change according to the incident angle of the irradiation light. For example, a dichroic film or the like is provided. It is possible to use a transparent plate as it is.

The type of the interferometer to be applied can be applied to a Fizeau-type interferometer in addition to a so-called double-arm interferometer such as Michelson and Mach-Zehnder. However, in the Fizeau-type interferometer, a parallel flat plate for adjusting the light amount is inserted between the subject and the reference plate, and it is difficult to insert the parallel flat plate for adjusting the optical path length as described above. Therefore, it is preferable to provide an optical path correction plate for returning the optical path shift caused by the parallel plane plate for adjusting the light amount.

In the above embodiment, it is assumed that irradiation light having a short coherence distance is used, but it is of course possible to use irradiation light having a long coherence distance, such as laser light. Further, the wavelength of the irradiation light is not limited to the red light, and other color lights can be used. The dichroic film formed on the parallel flat plate for adjusting the amount of light depends on the incident angle of the irradiation color light. What is necessary is to select a material having such characteristics that the light transmittance characteristics change.

The mechanism for rotating the two parallel flat plates in conjunction with each other is not limited to the one described in the above embodiment. For example, when the driving gear is rotated by a motor rather than manually, For example, when it is desired to adjust the angle of the face plate with higher precision, the diameter of the driving gear can be made smaller than that of the planetary gear, and the number of teeth can be reduced. Further, instead of the gear, another driving force transmission mechanism such as a belt wheel, a cam mechanism, or a link mechanism can be used. When the reflectance of the surface to be measured is extremely small, it is useful to configure the beam splitter to adjust the transmission / reflection ratio.

[0027]

As described above in detail, according to the interferometer apparatus of the present invention, the light beam irradiated on the test surface and the light beam irradiated on the reference surface do not overlap each other. A transparent flat plate for adjusting the light amount, the light transmittance characteristic of which changes in accordance with the incident angle of the irradiation light, and the light amount ratio of the two return lights. The parallel plane plate is provided with an adjustment mechanism for adjusting the incident angle so that the parallel plane plate can be substantially equalized, and the amount of return light from the test surface and the reference surface can be reduced by simply rotating the parallel plane plate by a predetermined angle. Since they can be substantially the same, interference fringes with good contrast can be obtained by a simple operation even when the reflectance of the test surface is low. Further, in a dual-arm interferometer device such as a Michelson type, the parallel plane plate for adjusting the light amount is arranged in one of two separated irradiation systems, and the other is arranged in the other system. In one system, if an optical compensator for compensating the optical effect of the parallel plane plate on the light flux is arranged, an interference fringe image generated by inserting the parallel plane plate into the one system. Etc. can be prevented from being adversely affected.

[Brief description of the drawings]

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

FIG. 2 is a graph showing transmittance characteristics of a dichroic film used in the embodiment apparatus shown in FIG. 1 with respect to red light.

FIG. 3 is a schematic view showing a rotation driving mechanism of a plane-parallel plate used in the embodiment apparatus shown in FIG. 1;

[Explanation of symbols]

 Reference Signs List 1 light source 2 collimator lens 3 beam splitter 4 subject 4a test surface 5 reference plate 5a reference surface 7 imaging camera 8 parallel plane plate for adjusting light quantity 9 parallel plane plate for adjusting optical path length 18, 19, 20 Gear

Claims (7)

[Claims] An irradiation light from a light source is applied to a test surface and a reference surface arranged at predetermined positions, and return light reflected from the test surface and the reference surface interfere with each other. In the interferometer apparatus for observing the generated interference fringes, the light beam applied to the test surface and the light beam applied to the reference surface do not overlap with each other. A parallel flat plate for adjusting the amount of light with a transparent and uniform refractive index whose light transmittance characteristics change according to the angle is provided, and the parallel flat plate is arranged so that the light amount ratio of the two return lights can be made substantially equal. An interferometer device provided with a mechanism for adjusting the incident angle. 2. A test surface irradiation system for irradiating irradiation light from a light source to a test surface and a reference surface irradiation system for irradiating a reference surface.
An interferometer apparatus for separating return light reflected from the test surface and the reference surface from each other in these two systems and observing interference fringes generated by the interference, wherein at least one of the two systems A parallel flat plate for adjusting the amount of light with a transparent and uniform refractive index whose light transmittance characteristic changes according to the incident angle of the separated irradiation light, and the amount of light of the two return lights An interferometer device comprising a mechanism for adjusting the incident angle of the plane-parallel plate so that the ratio can be made substantially equal. 3. An optical compensator for disposing the light amount adjusting parallel plane plate in one of the two systems and for compensating an optical influence of the parallel plane plate in the other system. 3. The interferometer device according to claim 2, wherein 4. The optical compensator is transparent so that the optical path length of the irradiation light of the other system is substantially equal to the optical path length of the irradiation light of the other system in which the parallel plane plate is provided. The interferometer device according to claim 3, wherein the interferometer device is a parallel flat plate for adjusting the optical path length having a uniform refractive index. 5. The interferometer device according to claim 3, wherein the optical compensator is a compensator for compensating chromatic dispersion. 6. A mechanism for changing the angle of incidence by interlocking the parallel plane plate and the optical compensator so that the angles of incidence of the irradiation light on the parallel plane plate and the optical compensator substantially coincide with each other. The interferometer device according to any one of claims 3 to 5, further comprising: 7. An incident angle of the plane-parallel plate for adjusting the light amount in the initial state with respect to the irradiation light is set near an angle at which a change rate of the light transmittance characteristic is maximum. The interferometer device according to claim 1.