JP2001349704A - Interferometer device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interferometer device capable of preventing bending of an optical axis of the whole device which makes difficult a design of a device housing or the like and adjustment of an optical system, by adopting a technique for removal of a noise caused by formation of a wedge shape to two optical members having the most probability of generating an interference fringe noise in the optical system, and by integrating skillfully the wedge directions of the two optical members. SOLUTION: When a reference plate 107 is formed in a wedged shape, bending of the optical axis by the rear surface 107b is inevitable. In this case, the optical axis bent by the rear surface 107b of the reference plate 107 is bent again in the reverse direction by a half mirror 104 having a wedged shape, to roughly collimate the optical axis from a test surface 108a to the reference plate 107 with the optical axis from a light source 101 to the half mirror 104. Hereby, the optical axis of the whole device can be regarded substantially as a single axis, to avoid difficulty for the design and the adjustment of the optical system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer apparatus for obtaining interference fringe information of an object using measurement light having a long coherence distance, and more particularly, to a Fizeau-type apparatus capable of removing interference fringe noise from the back surface of a reference plate. And the like.

[0002]

2. Description of the Related Art Conventionally, a Fizeau type interferometer has been widely used. In this Fizeau-type interferometer device, an interference fringe is formed on an imaging surface by an interference effect of a light beam reflected on a reference surface of a reference plate and a light beam transmitted through the reference plate and reflected on a test surface, The surface to be inspected is evaluated by observing and measuring the interference fringes.

By the way, in a Fizeau-type interferometer,
Generally, it is necessary to increase the distance between the reference surface and the test surface to some extent in order to set the object, and the light from the light source needs to have a coherence distance at least about twice this distance.

However, the interference fringes generated by the reflected light from the back surface of the subject whose optical path difference from the reflected light from the surface to be tested is twice the thickness of the subject causes noise on the original interference fringes. There is a problem of superimposition as interference fringes.

This will be described with reference to FIG.

That is, the measuring light output from the light source 1 is converted into diverging light by the diverging lens 2 and the pinhole plate 3, and is converted from P-polarized light to elliptically polarized light via the polarizing beam splitter 4 and the λ / 4 plate 5. The collimator lens 6 converts the light into parallel light and irradiates the reference plate 7 and the subject 8. Reference surface 7a of reference plate 7 and test surface 8 of subject 8
The two luminous fluxes reflected by both surfaces a interfere with each other and are converted from elliptically polarized light to s-polarized light by the λ / 4 plate 5. An interference fringe image carrying shape information is formed.

As described above, this interference fringe image is due to interference between the reflected light from the test surface 8a and the reflected light from the reference surface 7a, but the coherent distance of the measurement light from the light source 1 is large. For this reason, the reflected light from the test surface 8a also interferes with the reflected light from the back surface 7b of the reference plate 7, and the resulting interference light reaches the screen 9 along the same optical path. This generates interference fringe noise that is difficult to distinguish from fringes.

In order to solve such a problem, the front and back surfaces of the reference plate are made non-parallel to each other and are formed in a wedge shape.
There is known a conventional technique in which the reflected light from the reference plate back surface 7b deviates from the observation optical path, and as a result, interference fringe noise related to the reflected light from the reference plate back surface 7b is not formed on the observation screen 9 (for example, And a laser interferometer F-601 manufactured by Fuji Photo Optical Co., Ltd.).

[0009]

However, when the wedge-shaped reference plate 7 is provided, the light emission angle from the reference surface 7a and the light incidence angle with respect to the reference surface 7a are defined to be 0 °. Therefore, the measurement light from the light source 1 is incident on the back surface 7b of the reference plate 7 at a predetermined angle on both the outward path and the return path, and is refracted on this surface. That is, the optical axis of the entire device is bent at the reference plate back surface 7b. When the optical axis of the entire apparatus is bent in the middle as described above, since the entire length of such an interferometer apparatus is long, the constituent members as a whole are largely displaced in the optical axis width direction, and the apparatus casing and The design of the two-axis adjustment table becomes complicated. Further, the adjustment of the device optical system (from the light source to the screen) becomes complicated, and the design of the target for adjusting the optical axis becomes complicated.

The present invention has been made in view of such circumstances, and it is difficult to design the apparatus housing and the two-axis adjustment table and to adjust the apparatus optical system in the width direction of the optical axis of the entire apparatus. It is an object of the present invention to provide an interferometer device which can eliminate the influence of interference fringe noise due to light reflected from an optical surface other than the reference surface while preventing the interference.

[0011]

The interferometer device of the present invention comprises:
After the light emitted from the light source is converted into a parallel light beam, the light is irradiated onto the reference plate and the subject, and the reflected light from both the reference surface of the reference plate and the test surface of the subject interferes. In an interferometer apparatus which divides by a half mirror disposed therein and generates an interference fringe image carrying shape information of the test surface on a screen by using one of the half mirrors, the reference plate and the half mirror both have front and rear surfaces. Are configured to be non-parallel and wedge-shaped, and the wedges of these two members are configured so that the thickness and the thickness of the wedges are substantially opposite to each other.

It is preferable that an optical path bent by one wedge shape of the reference plate and the half mirror is formed so as to be compensated by the other wedge shape.

Here, "to be compensated" means that the compensated light path is substantially equal to the original light path at least at an optical angle, that is, substantially optically parallel. Is set as follows.

Further, it is preferable that the half mirror is disposed in a parallel light beam.

In this case, when the refractive index of the reference plate is N, the wedge angle is a, the refractive index of the half mirror is n, and the wedge angle is θ, the following conditional expressions are preferably satisfied. tanθ = {sin (ab + c) -nsind} / {cos (ab +
c) + ncosd} where d is the angle of incidence of the measurement light emitted from the light source on the half mirror surface, c is the emission angle of the measurement light from the half mirror surface, and the reference plate of the measurement light is c. Let b be the angle of incidence on the back surface of the sample, and a be the angle of refraction of the measurement light. Further, the measurement light is perpendicularly incident on and perpendicularly emitted from the reference surface, and the reflected light reflected by the half mirror and incident on the imaging surface is an optical path of the measurement light emitted from the light source. , And an angle of 90 degrees with respect to the optical path of the measurement light from the reference surface to the test surface.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an interferometer device according to the present embodiment.

In this interferometer device, measurement light output from a light source 101 that outputs light having a broadband wavelength component such as white light is converted into diverging light by a diverging lens 102 and a pinhole plate 103, and a collimator lens 106.
Is converted into parallel light, and is applied to the wedge-shaped reference plate 107 and the subject 108 via the wedge-shaped half mirror 104.

The reflected light of the measurement light from the reference surface 107a of the reference plate 107 and the test surface 108a of the test object 108 interfere with each other, are reflected laterally by the half mirror surface 104a of the half mirror 104, and are coupled. An interference fringe image is formed on the imaging surface (screen) 109 of the CCD device via the image lens 110 and the pinhole plate 111. Thereafter, based on the interference fringe image information photoelectrically converted by the CCD element, an interference fringe image is displayed on an image display unit (not shown) such as a CRT.

In the interferometer device of the present embodiment, both members of the half mirror 104 and the reference plate 107 have a predetermined wedge shape whose front and back surfaces are non-parallel, and the directions of the wedges are opposite to each other. In addition to eliminating the influence of interference fringe noise due to light reflected from optical surfaces other than the reference surface, it also prevents bending of the optical axis of the entire device, which makes it difficult to design and adjust the optical system. I have.

That is, the half mirror 104 and the reference plate 1
Reference numeral 07 denotes a wedge shape, and is interference light due to reflected light from both the test surface 108a and the reference surface 107a, such that only the light reflected on the half mirror surface 104a reaches the imaging surface 109. Even if the interference light is due to the reflected light from both the test surface 108a and the reference plate back surface 107b or the interference light due to the reflected light from both the test surface 108a and the reference surface 107a, the half mirror 10
The size of the wedge angle is set so that the light reflected by the back surface 104b of No. 4 is reflected in another direction and cannot reach the imaging surface 109.

If the reference plate 107 has a wedge shape, the optical axis is inevitably bent at the back surface 107b. However, in the present embodiment, the optical axis bent by the back surface 107b of the reference plate 107 is half. The optical axis of the entire apparatus is bent by being bent in the opposite direction by the mirror 104 so that the optical axis from the test surface 108a to the reference plate 107 is substantially parallel to the optical axis from the light source 101 to the half mirror 104. Can be regarded as substantially one, thereby avoiding difficulty in designing and adjusting the optical system.

This will be described with reference to FIG. Here, each symbol in FIG. 2 is defined as follows.

That is, the angle of incidence of the measurement light emitted from the light source 101 on the back surface 104b of the half mirror is f, the angle of refraction is e, the angle of incidence on the half mirror surface 104a is d, and the angle of emission is c, the back side of the reference plate 1
Let the angle of incidence on 07b be b and the angle of refraction be a. The refractive index of the reference plate 107 is N, the wedge angle is a, the refractive index of the half mirror 104 is n, and the wedge angle is θ. further,
The measurement light is assumed to be perpendicularly incident on and emitted from the reference surface 107a. The reflected light reflected by the half mirror surface 104a and incident on the imaging surface 109 is reflected by the light source 10
It is assumed that the angle is set to be 90 degrees with respect to the optical path of the measurement light emitted from 1 and the optical path of the measurement light from the reference surface 107a to the test surface 108a.

Under such a premise, since the angle a is already set, the value of b is determined from the following equation (1).

[0025]

(Equation 1)

Next, the value of c is determined from the following equation (2).

[0027]

(Equation 2)

Further, the value of d is determined from the following equation (3).

[0029]

(Equation 3)

Next, the value of e is determined from the following equation (4).

[0031]

(Equation 4)

Further, the value of f is determined from the following equation (5).

[0033]

(Equation 5)

As shown in FIG. 2, the following equation (6) is determined from the sum of the angles.

[0035]

(Equation 6)

Next, the following equation (7) is obtained from the above equations (4), (5) and (6).

[0037]

(Equation 7)

Therefore, when the above equation (7) is expanded using the following sine theorem, the following equation (8) is finally obtained.

[0039]

(Equation 8)

That is, the wedge angle θ of the half mirror 104
Is the value θ ′ represented by the arctangent function of the expression represented by the left side of the above expression (8), the optical axis from the test surface 108 a to the reference plate 107 is moved from the light source 101 to the half mirror 10.
It can be set so as to be substantially parallel to the optical axes up to 4.

That is, in the above-described embodiment, the wedge angle θ of the half mirror 104 is changed to the refractive index N of the reference plate 107,
It can be specified from the wedge angle a and the refractive index n of the half mirror 104.

However, since the adjustment range of the optical system is actually provided by a predetermined amount, the allowable wedge angle θ of the half mirror 104 is generally expressed by the following equation: 0.9 × θ ′ <θ <1.1 × θ ′ What is necessary is just to set it as satisfying.

It should be noted that the interferometer device of the present invention is not limited to the above-described embodiment, and various other modifications can be made. For example, in the above embodiment, the interference fringe image is formed on the imaging surface 109 of the CCD element. However, instead of this, an optical screen having a diffusion surface is provided, and the interference fringe image is formed on the screen. However, it is also possible to observe this visually.

[0044]

As described above, according to the interferometer apparatus of the present invention, not only the reference plate for transmitting the measurement light from the light source but also the half mirror for guiding the interference light to the side are provided. Since the front and back surfaces are configured to have a non-parallel wedge shape, the half mirror can be used not only for interference fringe noise due to light reflected from the back surface of the reference plate but also for interference light due to light reflected from the reference surface. Can be prevented from being generated on the screen.

The wedges of the two members are thick and thin, but are arranged so as to be substantially opposite to each other, so that the bending directions of the optical paths by the two wedges are opposite to each other, and the influence of the bending is reduced. This offsets each other, thereby preventing bending of the optical axis of the entire device.

That is, according to the interferometer apparatus of the present invention,
Conventionally known wedge-shaped noise removal technique is adopted for two optical members that are most likely to generate interference fringe noise in the optical system, and the wedge directions of these two optical members are skillfully combined. Thus, the effect of preventing the bending of the optical axis of the entire device, which makes the design of the device housing and the like and the adjustment of the optical system difficult, is obtained.

[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 partially enlarged view of the interferometer apparatus shown in FIG.

FIG. 3 is a schematic view showing a conventional general Fizeau interferometer apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 101 Light source 2, 102 Divergent lens 3, 103, 111 Pinhole plate 4 Polarization beam splitter 5 λ / 4 plate 6, 106 Collimator lens 7, 107 Reference plate 7a, 107a Reference surface 7b, 107b Reference plate back surface 8, 108 Subject 8a, 108a Test surface 9, 109 Imaging surface (screen) 104 Half mirror 104a Half mirror surface 104b Half mirror back surface 110 Imaging lens

Claims (4)

[Claims] 1. A method according to claim 1, wherein the light emitted from the light source is converted into a parallel light beam, and then irradiates the reference plate and the subject to cause reflected light from both the reference surface of the reference plate and the test surface of the subject to interfere. In the interferometer apparatus which divides the interference light by a half mirror arranged in an optical path and generates an interference fringe image carrying shape information of the test surface on a screen by one of the half mirrors, the reference plate and the half mirror , Both of which are configured so that the front and back surfaces are non-parallel and have a wedge shape, and that the thick and thin wedges of both members are arranged in substantially the opposite arrangement. . 2. The optical device according to claim 1, wherein an optical path bent by one wedge shape of the reference plate and the half mirror is formed so as to be compensated by the other wedge shape. Interferometer device. 3. The interferometer device according to claim 1, wherein the half mirror is arranged in a parallel light beam. 4. When the refractive index of the reference plate is N, the wedge angle is a, the refractive index of the half mirror is n, and the wedge angle is θ, the following conditional expression is satisfied. The interferometer device as described. tanθ = {sin (ab + c) -nsind} / {cos (ab +
c) + ncosd} where d is the angle of incidence of the measurement light emitted from the light source on the half mirror surface, c is the emission angle of the measurement light from the half mirror surface, and the reference plate of the measurement light is c. Let b be the angle of incidence on the back surface of the sample, and a be the angle of refraction of the measurement light. Further, the measurement light is perpendicularly incident and emitted perpendicularly to the reference plane, and the reflected light reflected by the half mirror and incident on the imaging surface is an optical path of the measurement light emitted from the light source. , And an angle of 90 degrees with respect to the optical path of the measurement light from the reference surface to the test surface.