JPH0611308A - Hologram interferometer - Google Patents

Abstract

PURPOSE:To eliminate the need of a reference optical element, such as a reference surface, etc., by directly diffracting reference light by using a hologram optical element at the time of making the reference light to interfere with reflected light from the surface to be measured of an object 12 to be measured by emitting laser light so that the constitution of the title interferometer can be simplified and, at the same time, to eliminate the influence of the formation error of the optical element on interference fringes by making the routes of the two kinds of light interfering with each other the same. CONSTITUTION:A laser beam emitted from a light source 1 is transformed into parallel rays through an optical system and a hologram optical element 9 has a first pattern which diffracts the parallel rays to a direction which is different from that of the surface 12a to be measured of an object to be inspected 12 as reference light 10 and second pattern which diffracts the parallel rays as measuring light 11 irradiating the surface 12a. The element 9 is constituted so that reflected light from the surface 12a can interfere with the reference light 10 through the element 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hologram interferometer for measuring a deviation of a measurement surface of a subject from an ideal shape, and more particularly to a hologram interferometer using a hologram optical element.

[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 this interferometry, how much the surface to be inspected is displaced with respect to the highly accurate reference surface (interference standard) is determined based on the interference fringes generated by interfering the light reflected from each surface. It is something to try.

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 computer-synthesized hologram is noticeable because it can measure a special shape other than a flat surface and a spherical surface. ing.
In general, this computer-synthesized hologram is one in which a photoresist coated on a glass substrate is scanned with an electron beam to expose a predetermined pattern (interference fringes) and then developed to make the interference fringes visible. is there.

Conventionally, as an interferometer using such a hologram, one configured as shown in FIG. 6 is known.

That is, in this interferometer, a laser beam 22 emitted from a laser light source 21 is reflected by a mirror 23, the beam diameter is narrowed down by a condenser lens 24, and divergent light 26 is formed by a pinhole 25 having a beam diverging function. It After this, this divergent light 26 passes through the half mirror 27, is reflected by the mirror 28, is converted into parallel light 30 by the collimator lens 29, and is incident on the reference lens 31. The reference lens 31 is a lens that acts as a convex lens, and converges the parallel light 30 to one point 0 on the optical axis, and then irradiates the measurement surface 33 with divergent light.

The measuring surface 33 is a mirror that acts as an aspherical concave mirror, and its position is adjusted so that the reflected light returns to the path of the incident light. Therefore, the reflected light reflected by the measurement surface 33 enters the half mirror 27 via the reference lens 31, the collimator lens 29 and the mirror 28.

Further, the reference surface 31a of the reference lens 31 facing the measurement surface 33 is provided with a special coating,
About half the amount of light is transmitted and the rest is reflected, so that the reflected light that is the reference light reflected by this reference surface 31a also returns to the path of the incident light that has entered this reference lens 31. Is formed in. These two reflected lights are reflected by the half mirror 27 at a constant ratio and the hologram 3
4, incident on the TV camera 39 through the field lens 35, the imaging lens 36, the mirror 37 and the spatial filter 38.

The hologram 34 has the function of converting a predetermined aspherical reflected light into a spherical reflected light. That is, the reference light reflected by the reference surface 31a is spherical reflected light, but the object light reflected from the measurement surface 33, which is an aspherical surface, is aspherical reflected light, and when these lights are directly interfered with each other. The obtained interference fringe becomes extremely fine according to the difference between these two reflecting surfaces, and its measurement becomes difficult. If the reference surface 31a is aspherical and the reference light is aspherical reflected light corresponding to the object light, rough interference fringes can be obtained, but there is a problem that the reference surface 31a is difficult to manufacture.
Therefore, the object light, which is aspherical reflected light, is passed through this hologram to be converted into spherical reflected light, and the spherical reflected light and the reference light, which is the spherical reflected light reflected from the reference surface 31a, are caused to interfere with each other to form a spherical surface. Coarse interference fringes of the reflected light are formed to facilitate observation with the TV camera 39.

When the measurement surface 33 is a convex surface, the hologram 34 at the position shown by the solid line is replaced with this hologram.
The hologram 34a is located at a position indicated by a broken line which is conjugate with the position where 34 is arranged.
To arrange.

In the above technique, even if the measurement surface is an aspherical surface, it is possible to make the reference surface a spherical surface instead of an aspherical surface by using a hologram, which facilitates manufacturing to some extent. I am trying.

[0011]

However, the hologram interferometer as described above requires a reference optical element having a reference surface for obtaining reference light, such as a spherical reference surface or a reference plane mirror, which is highly accurate. It is not always easy to manufacture a reference surface having a surface shape. Further, there is a problem that high positional accuracy is required even when the reference plane is incorporated in an interferometer, and accuracy control becomes difficult.

On the other hand, in the interferometer as described above, if the part where the object light reflected from the measurement surface and the reference light interfere with each other passes through different parts of the optical element such as the hologram optical element or the number of times of transmission is different. , The uniform distribution of the refractive index of the material of the optical element, or the interference state is changed by the influence of the formation error of the optical element such as surface accuracy, the measurement accuracy based on the observation of interference fringes It may decrease due to formation error.

In view of the above circumstances, the present invention aims at simplifying the structure by eliminating the need for a reference optical element such as a reference surface and eliminating the influence of an optical element forming error on the interference fringes. It is intended to provide.

[0014]

The hologram interferometer of the present invention for achieving the above object is provided by emitting laser light from a laser light source through an optical system to reflect a reference light and a measurement surface of an object. A method for measuring surface accuracy and the like of a measurement surface by causing light to interfere with each other through a hologram optical element,
The hologram optical element has a first pattern that diffracts laser light as reference light in a direction different from the measurement surface, and a second pattern that diffracts laser light to measurement light with which the measurement surface is irradiated. The reflected light reflected from the subject interferes with the reference light via the hologram optical element.

[0015]

According to the above construction, the laser light emitted from the laser light source through the optical system is diffracted in two directions by the hologram optical element having two patterns. The light diffracted by the first pattern has a second pattern after the light diffracted by the second pattern is reflected by the measurement surface of the subject when the measurement surface of the subject has an ideal shape. The reference light has the same characteristics as that of the light diffracted again, and the light diffracted by the second pattern becomes the measurement light with which the measurement surface of the subject is irradiated. The measurement light is reflected by the measurement surface of the subject. The reflected light, that is, the object light returns to the incident optical path and is incident on the hologram optical element again, is diffracted in the same direction as the reference light in the second pattern, and the two overlap and interfere with each other. When the beam is converged and made incident on a TV camera or a screen, interference fringes according to the surface shape of the subject can be observed.

[0016]

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

<Embodiment 1> FIG. 1 shows a schematic structure of a transmission type hologram interferometer according to this embodiment. This hologram interferometer includes a laser light source 1, a condenser lens 3 for narrowing down a laser beam 2 emitted from the light source 1, a pinhole 4 for converting the laser beam 2 into divergent light 5 from which a noise component is removed, and the divergent light. Beam splitter 6 for reflecting 5 in the direction rotated 90 °, and this reflected divergent light 5
A collimator lens 7 for converting the parallel light 8 into parallel light 8 and a reference light 10 formed by the parallel light obtained by reflecting and diffracting the parallel light 8 in the incident direction, and a measurement light 11 formed by the convergent light obtained by transmitting and diffracting the parallel light 8 And a hologram optical element 9 (HOE) having a pattern 9a, which is irradiated with the measurement light 11 on the measurement surface 12a of the subject 12.

The object light reflected from the measurement surface 12a of the subject 12 travels backward through the incident optical path, passes through the hologram optical element 9, is converted into parallel light, and passes through the collimator lens 7 together with the reference light. It reaches the beam splitter 6 and then passes through this beam splitter 6.

On the transmission side of the beam splitter 6, there is an image forming lens 14 for forming an image of both the reference light and the object light, and a light receiving surface on the focal plane of the image forming lens 14.
A TV camera 15 for observing interference fringes formed by the reference light and the object light is installed.

The hologram optical element 9 is a glass substrate.
The first pattern for obtaining the reference light by coating the 9b with aluminum for reflection, chromium, etc., and then coating the photoresist, exposing the two hologram patterns with an electron beam and developing, and then etching. And a flat optical element formed by simultaneously forming a second pattern for obtaining the measurement light.

That is, as shown in FIG. 2A, the first pattern of the hologram pattern 9a is a fringe for reflecting the parallel light 8 having the incident angle α in the incident direction as it is and diffracting the reference light 10. It has a pattern shape and has a striped pattern in which straight lines are arranged in parallel at equal intervals corresponding to the incident angle α and the like. The second pattern has a striped shape corresponding to the shape of the measurement surface 12a of the subject 12, and, for example, the measurement surface 12a of the subject 12 is a cylindrical surface (a convex surface or a concave surface shown by a chain line). In some cases, Figure 2 (B)
As shown in, in order to converge the parallel light 8 having the incident angle α to the focal length f, an interval according to the incident angle α and the focal length f, that is, a refraction angle, etc. Is smaller) is a striped pattern in which straight lines are arranged in parallel.

The beam splitter 6 is an optical element that splits incident light into transmitted light and reflected light, and reflects about half of the divergent light incident from the direction of the pinhole 4 toward the collimator lens 7, while It functions so that about half of the convergent light incident from the collimator lens 7 direction is transmitted to the TV camera 15 direction.

Next, the operation of this hologram interferometer will be described. The subject 12 is a cylindrical convex surface 12a.
The case of a reflecting mirror having is described.

The laser light 2 emitted from the light source 1 is converted from the condenser lens 3 into the parallel light 8 through the optical system of the collimator lens 7 and is incident on the hologram optical element 9, which is directed in the incident direction by the hologram optical element 9. It is diffracted into the reference light 10 that is reflected and the measurement light 11 that is transmitted and converged.

As described above, the measurement surface 12 of the subject 12 is
In the case where a is a reflecting mirror having a cylindrical surface, the pattern 9a has a striped pattern in which straight lines parallel to the central axis direction of the measurement surface 12a (direction perpendicular to the paper surface in FIG. 1) are arranged. Hologram pattern with straight stripes like this
When the parallel light 8 is applied to the beam 9a, the measurement light 11 that does not converge in the extending direction of the stripe and converges in the direction orthogonal to the stripe on the hologram surface is emitted. in this case,
The measuring light 11 is applied to the convex measuring surface 12a of the subject 12 before being converged on the central axis.

The object light reflected from the measurement surface 12a of the subject 12 enters the hologram optical element 9 so as to return to the optical path of the measurement light 11. The object light becomes parallel light by the second pattern of the hologram optical element 9, overlaps with the reference light 10 reflected as parallel light on the hologram optical element 9 and interferes with each other, and interferes with the light receiving surface of the TV camera 15. Forming stripes. That is, in the above embodiment, the hologram optical element 9 is used to convert the parallel light and the aspherical light corresponding to the cylindrical surface.

Since the interference fringes are generated as a result of the interference of parallel lights, they have a rough fringe pattern and can be detected. The detected striped pattern represents the distortion of the measurement surface 12a of the subject 12 from the ideal cylindrical surface.By measuring this striped pattern, the shape of the measurement surface 12a of the subject 12 is measured. Can be detected.

Further, since interference fringes can be efficiently obtained when the light quantities of the reference light and the object light are about the same, it is desirable to form a hologram pattern having a diffractive index corresponding to the reflectance of the subject 12. .

In the above embodiment, the measuring light 11 is formed so as to pass through the hologram optical element 9 and the optical path is secured even if the measuring surface 12a is a convex surface, which is particularly effective for a convex surface. Even when the measurement surface 13a of the subject 13 is a concave surface as shown by the chain line, the measurement light 11 once converges on the central axis of the cylindrical surface and then diverges and is irradiated on the concave measurement surface 13a of the subject 13. By doing so, similarly reflected object light returns along the incident optical path, and the same measurement can be performed.

<Embodiment 2> FIG. 3 shows a schematic configuration of a reflection type hologram interferometer according to this embodiment. This hologram interferometer has an optical system including a laser light source 1 similar to the previous example, a condenser lens 3, a pinhole 4, a beam splitter 6, and a collimator lens 7, and makes parallel light 8 incident on a hologram optical element 19.

The hologram pattern 19a of the hologram optical element 19 forms the reference light 10 by the parallel light obtained by reflecting and diffracting the parallel light 8 in the incident direction by the first pattern, and reflects the parallel light 8 by the second pattern. The measuring light 11 is formed by the diffracted convergent light, and the reflected measuring light 11 is applied to the measuring surface 13a (concave surface) of the subject 13.

The object light reflected from the measurement surface 13a of the subject 13 travels backward in the incident optical path, is reflected by the hologram optical element 19, is converted into parallel light, and is transmitted through the collimator lens 7 together with the reference light 10. Then, the reference light and the object light interfere with each other through the beam splitter 6 and are observed by the TV camera 15 via the imaging lens 14. The basic hologram pattern 19a of the hologram optical element 19 is the same as in the previous example, and the second pattern for obtaining the measurement light 11 is also formed of a reflective film.

In the case of the reflection type of this example, the object 12 having the convex measuring surface 12a as in the previous example is measured at the convergence point f of the measuring light.
If it is located further to the front side, this subject 12 may block the optical path of incident or reflected parallel light, and the measurable convex surface shape is limited.For the measurement of convex surfaces, the transmissive type of the previous example is suitable. .

<Third Embodiment> FIG. 4 shows an application example, in which a subject 12 is placed on a horizontal sample table 17 having an alignment mechanism 17a. Further, a light source, an optical system for obtaining parallel light, and an interferometer main body 16 having a built-in interference fringe observation unit are installed, and the main body is arranged vertically.
Hologram optical element in which parallel light 8 from 16 is placed horizontally
In order to enter at an incident angle α from 20 in an oblique direction, 2
Reflective mirrors 18a and 18b are installed.

Furthermore, in this example, the measurement surface 12a of the subject 12 is measured.
The axial direction of the (cylindrical surface) differs from that of the incident direction of the parallel light 8 by 90 ° from that of the first embodiment, and accordingly, the converging direction of the measurement light 11 by the hologram optical element 20 differs, and the hologram pattern for that is shown in FIG. It is formed like.

That is, as shown in FIG. 5 (A), the first pattern 20a for reflecting the reference light 10 of the parallel light in the incident direction is formed in a linear striped pattern at equal intervals as in the first embodiment. However, the second pattern 20b that transmits the converging measurement light 11 has curved lines arranged at equal intervals, as shown in FIG. Can also be measured.

In addition to the above embodiments, even if the measurement surface of the subject is a spherical or other aspherical surface, if the hologram optical element pattern is a predetermined curved stripe pattern. The shape can be measured by the hologram interferometer as described above.

Further, the hologram interferometer of the present invention is not limited to that of the above-mentioned embodiment, and various other modifications can be made.

For example, a half mirror may be used as the beam splitter, or the beam splitter may be arranged between the collimator lens and the hologram optical element.

Instead of the TV camera, a recording medium may be placed at that position to record the interference fringes, or the interference fringes may be directly observed with the eyes at that position.

[0041]

According to the hologram interferometer of the present invention, 2
Since the light diffracted by one pattern of the hologram optical element having two patterns is used as the reference light as it is, and the light diffracted by the other pattern is used as the measurement light for irradiating the measurement surface of the subject, the reference light is used. Reference optical elements such as a reference plane mirror and a spherical reference surface for generation are not required, and it is easy to simplify the structure and secure assembly accuracy by reducing the number of parts.

Since the pattern of the hologram optical element is formed on the same surface, the reference light and the object light which interferes with the reference light pass through the same portion of the hologram optical element the same number of times. Similarly, the influence of the uniformity of the material of the element, the plane accuracy, and the like is similarly exerted, and the influence on the interference result is eliminated, and the measurement accuracy is improved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a hologram interferometer according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a diffraction state of a hologram optical element in the example of FIG.

FIG. 3 is a schematic configuration diagram showing a hologram interferometer according to a second embodiment of the present invention.

FIG. 4 is a schematic configuration diagram showing a hologram interferometer according to a second embodiment of the present invention.

5 is an explanatory diagram showing an example of forming a hologram pattern in the example of FIG.

FIG. 6 is a schematic diagram showing a hologram interferometer according to the related art.

[Explanation of symbols]

 1 laser light source 2 laser beam 3 condenser lens 4 pinhole 5 divergent light 6 beam splitter 7 collimator lens 8 parallel light 9,19,20 hologram optical element 9a, 19a, 20a, 20b hologram pattern 10 reference light 11 measurement light 12,13 Subject 12a, 13a Measurement surface 15 TV camera

Claims (2)

[Claims] 1. A laser beam emitted from a laser light source through an optical system causes reference light and reflected light from a measurement surface of a subject to interfere with each other through a hologram optical element to improve surface accuracy of the measurement surface. A hologram interferometer for measurement, wherein the hologram optical element includes a first pattern for diffracting laser light as a reference light in a direction different from a measurement surface, and a first pattern for diffracting the laser light on a measurement light for irradiating the measurement surface. 2 pattern is formed, and the reflected light reflected from the subject is configured to interfere with the reference light via the hologram optical element. 2. The hologram interferometer according to claim 1, wherein the hologram optical element has a transmissive pattern.