JPH0540012A - Interferometer and its aligning method - Google Patents

Abstract

PURPOSE:To obtain a method which can easily adjust the relative inclination between a surface to be measured and reference surface so that the number of interference fringes formed on a light receiving surface can become a prescribed value. CONSTITUTION:This interferometer is provided with the first photoreceptor means which measures interference fringes formed by light to be detected from a surface 4 to be measured and reference light from a reference surface 5, condenser lens 9 which forms a light spot by condensing two reflected light rays, the second photoreceptor means 10 which receives two light spots P1 and P2 formed through the lens 9, and a mean 5 which polarizes at least either one of the light to be inspected and reference light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer having an alignment function for generating a number of interference fringes suitable for measurement on a light receiving surface when measuring the shape of a surface to be measured, and this interferometer. The present invention relates to the alignment method.

[0002]

2. Description of the Related Art As one of the methods for measuring the shape of an object on the order of light wavelength or less, there is a spatial fringe scanning method using an interferometer. In the spatial fringe scanning method, a light beam from a single light source is divided into two, reflected by a surface to be measured and a reference surface, and these reflected lights are combined to generate an interference fringe on the image pickup surface of a television camera. Then, one of the surface to be measured and the reference surface is moved in the optical axis direction to analyze the change in the amount of light for each pixel of the camera, and the surface shape of the surface to be measured is measured.

By the way, when the fringe analysis algorithm by the spatial fringe scanning method is used, in order to simplify this algorithm, the number of pixels n of the television camera is n / 4m (m = 1,
It is preferable to generate interference fringes (2, 3 ...). Therefore, the measured surface should be adjusted so that the number of interference fringes is suitable for measurement.
Alternatively, the reference plane is tilted to align the interferometer.

[0004]

Conventionally, in order to measure the number of interference fringes, the output of a television camera is displayed on a monitor, the number of interference fringes is counted by the human eye, and the predetermined number of interference fringes is counted. Since the inclination is adjusted so that the adjustment work is very complicated, there is a problem that the work efficiency is poor.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and the relative distance between the measured surface and the reference surface is set so that a predetermined number of interference fringes are formed on the light receiving surface. An object is to provide an interferometer whose tilt can be easily adjusted, and an alignment method thereof.

[0006]

In order to achieve the above object, an interferometer according to the present invention measures an interference fringe formed by a test light from a measurement surface and a reference light from a reference surface. A first light receiving means, a condenser lens that collects the test light and the reference light to form a light spot, and a second light receiving means that receives two light spots formed by the condenser lens, And a means for deflecting at least one of the test light and the reference light.

In the interferometer alignment method according to the present invention, the test light from the measured surface of the interferometer and the reference light from the reference surface are condensed to form light spots, and the light spots between the light spots are formed. At least one of the test light and the reference light is deflected so that the distance becomes a predetermined distance.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

[0009]

First Embodiment FIGS. 1 and 2 show a first embodiment of the present invention.

In the interferometer shown in FIG. 1, a light beam emitted from a light source 1 is collimated by a collimator lens 2 and is converted by a first half mirror 3 into a plane mirror 4 and a reference mirror 5 which are measured surfaces. Will be split towards. The light flux reflected by each mirror is superposed by the first half mirror 3,
It is incident on the second half mirror 6.

The component transmitted through the second half mirror 6 forms an image on the image pickup surface 8 of the CCD camera as the first light receiving surface arranged on the pupil plane conjugate with the pupil via the image forming lens 7, Form interference fringes. On the other hand, the component reflected by the half mirror 6 is converged by the condenser lens 9 and corresponds to each reflected light on the screen 10 as the second light receiving surface located on the Fourier plane.
Form two light spots P1 and P2.

The reference mirror 5 is provided so as to be tiltable with respect to the optical axis and has a function as a deflecting means for deflecting the reference light. By setting the inclination angle of the reference mirror 5 to an appropriate value, it is possible to adjust the number of interference fringes formed on the imaging surface 8 and the interval δ between the light spots on the screen 10.

Here, two light spots P1 and P2 on the screen 10 are
The relationship between the interval Δ and the number n of interference fringes formed on the image pickup surface 8 of the CCD camera will be described.

In order to generate n interference fringes, the light source 1
Is a monochromatic light source with an emission wavelength λ, and the diameter of the reflected light beam from the plane mirror 4 that is the object to be measured is D, it is necessary to set the inclination angle θ of the light beam due to the inclination of the reference mirror 5 according to the following equation. ..

[0015]

[Equation 1] θ = (nλ) / D [rad]

If the focal length of the condenser lens 9 is f, the interval δ between the light points P1 and P2 on the screen 10 is given by the following equation.

[0017]

[Equation 2] δ = fθ = (nλf) / D

Thus, the interval δ between the light spots is the number of interference fringes.
Since it becomes a function of n and the interval δ for generating a predetermined number of interference fringes is uniquely determined, the reference surface is inclined so that this interval δ becomes a set value δ0 according to the number of predetermined interference fringes. By doing so, n interference fringes can be formed on the imaging surface 8 of the CCD camera.

In the first embodiment, a diagram is displayed on the screen 10 to show the set values for generating a predetermined number n of interference fringes.
Cross-shaped indicators S1 and S2 as shown in 2 are provided. The distance between the centers of these indices is the set value δ0, and the light spots P1, P2
Reference mirror 5 to align the center of each indicator
By adjusting the angle of, a predetermined number of interference fringes are generated on the image pickup surface 8 of the CCD camera.

Although the second half mirror is fixedly provided in the optical path in the above example, it is set in the optical path only at the time of alignment and is inserted and removed so as to be retracted from the optical path at the time of measuring interference fringes. The configuration may be freely provided. With such a configuration, it is possible to reduce the light amount loss during measurement and make it less susceptible to noise. Furthermore, when the mirror is detachably provided, a total reflection mirror may be used instead of the half mirror 6.

[0021]

Second Embodiment FIGS. 3 and 4 show a second embodiment of the present invention.

In the above-mentioned first embodiment, the positions of the two light spots are visually confirmed directly on the screen, but in the second embodiment, the positions of the light spots are measured by a light receiving element such as a PSD or an image sensor. Further, the measurement surface and the reference surface are driven via an actuator based on the measurement result, and the light spot distance is automatically set to a predetermined distance.

The configuration of the optical system is almost the same as that of the first embodiment, and as shown in FIG. 3, the screen 10 is used as the second light receiving surface.
Instead of this, a light receiving element 11 such as a PSD (position sensing device) or an image sensor is used. PSD is
Since it is an element that detects the coordinates of the portion where the amount of light is the largest, in order to obtain the coordinates of the two light spots, two elements are required, one at each light collecting position. When an image sensor is used, one element can detect the coordinates of two light spots.

An electric signal including coordinate information of the light spot is input to the drive control circuit 12 by the light receiving element 11. Drive control circuit
The actuator 12 controls actuators 13 and 14 that drive the mirrors 4 and 5 based on this information.

FIG. 4 is a flow chart showing the procedure of measurement by the interferometer.

First, in step (denoted by S. in the figure) 1, a mirror to be measured is placed at a predetermined position, and in step 2, the distance between the light spots P1 and P2 is determined based on the output of the light receiving element 11. δ
To measure. In step 3, it is judged whether or not the measured interval δ is equal to the set value δ0 corresponding to the number of predetermined interference fringes.If they are not equal, in step 4 the actuators 13 and 14 are controlled so that they become equal. Then, the mirrors 4 and 5 are tilted, and the interval δ is measured again in step 2.
Steps 2, 3 and 4 are repeated until the interval δ becomes equal to the set value δ 0, and after it becomes equal, the interference fringe analysis is started in step 5.

According to the configuration of the second embodiment, the alignment of the interferometer can be automated.

[0028]

Third Embodiment FIG. 5 shows a third embodiment of the present invention. Example
1 and 2 are examples in which the present invention is applied to a Twyman-Green type interferometer, and Example 3 is an example in which the present invention is applied to a Mach-Zehnder type interferometer.

In the interferometer shown in FIG. 5, the light beam emitted from the light source 1 is collimated by the collimator lens 2 and is collimated by the collimator lens 2. The first half mirror 3 forms a plane-parallel plate 4'having a surface to be measured and a reference mirror 5. Will be split towards and. Plane-parallel plate
Reference light and test light transmitted through 4'and reflected by mirror 20
The reference light reflected by 5 is incident on the second half mirror 6.

The reference light that has passed through the second half mirror 6 and the component of the test light that has been reflected by the half mirror 6 are arranged via the imaging lens 7 in the first pupil plane that is conjugate with the pupil. An image is formed on the image pickup surface 8 of the CCD camera as a light receiving surface to form interference fringes. On the other hand, the reference light reflected by the half mirror and the component of the test light transmitted through the half mirror are converged by the condenser lens 9 and are reflected on the screen 10 as the second light receiving surface located on the Fourier plane. Form two light spots corresponding to the light.

The reference mirror 5 is provided so as to be tiltable with respect to the optical axis and has a function as a deflecting means for deflecting the reference light. By setting the tilt angle of the reference mirror 5 to an appropriate value, it is possible to adjust the number of interference fringes formed on the imaging surface 8 and the interval between the light spots on the screen 10.

As the deflecting means, in addition to the reference mirror 5, a mirror 20 may be rotatably provided.

[0033]

As described above, according to the present invention, the interference fringes are measured by converging the test light from the surface to be measured and the reference light from the reference surface and measuring the distance between the light spots. It is possible to know the number of the interference fringes, and it is possible to easily set the interference fringes to a predetermined number without performing an operation such as counting the number of the interference fringes.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an optical system showing the configuration of a first embodiment.

FIG. 2 is an explanatory diagram showing a screen of the device shown in FIG.

FIG. 3 is an explanatory diagram of an optical system showing the configuration of a second embodiment.

FIG. 4 is a flowchart showing the operation of the second embodiment.

FIG. 5 is an explanatory diagram of an optical system showing the configuration of Example 3.

[Explanation of symbols]

 1 ... Light source 4 ... Measured surface 5 ... Reference surface 8 ... Imaging surface (first light receiving surface) 10 ... Screen (second light receiving surface) 11 ... Light receiving element (second light receiving surface)

Claims (4)

[Claims] 1. A first light receiving unit for measuring an interference fringe formed by the test light from the surface to be measured and the reference light from the reference surface, and the test light and the reference light being condensed. A condensing lens that forms a light spot, a second light receiving unit that receives two light spots formed by the condensing lens, and a deflection that deflects at least one of the test light and the reference light. And an interferometer. 2. The second light receiving means is a screen for visually recognizing the light spot, and the screen is provided with an index at a position where the light spot should be located in order to set the interval to a predetermined value. The interferometer according to claim 1, wherein 3. The second light receiving means is a light receiving element that photoelectrically converts the coordinates of the light point to detect, and measures the interval between the light points based on the output of the light receiving element. A deflection control means for controlling the deflection means so that the distance becomes a predetermined value based on the distance.
The interferometer described in 1. 4. The light spot is formed by condensing the test light from the measured surface of the interferometer and the reference light from the reference surface, and the distance between the light points is set to a predetermined distance. An interferometer alignment method characterized in that at least one of the test light and the reference light is deflected.